Better knowledge of the instantaneous soot and nitrogen oxides (NO x ) emissions in Diesel engines would allow better choices of injection parameters and thus lead to better raw emissions than, for instance, with classical averaged measures, like smoke maps. Unfortunately, while for most regulated emissions fast measurements are possible and reasonably fast sensors are available, the same is not true for soot. Against this background, this paper proposes a real time, delay-free soot model based on the pressure trace information. The model essentially maps changes of the pressure trace into changes of the raw soot emissions. The key elements of the approach are the design of experiments, segmentation of the pressure trace and the use of the principal component analysis technique to extract the essential information. The model is based only on data; that is, it needs an initial calibration with working points measured with a standard device, such as an AVL Microsoot or Opacimeter. This paper describes the method and then shows validation results with an additional amount of pilot injection in one cylinder which confirms the delay-free soot estimation.
If emissions control systems and engine control devices experience a failure, the engine system may malfunction causing exhaust emissions to exceed the standard levels of the regulated emissions. Thus, the functional status of each of all primary control devices needs to be monitored for detecting possible failures, so that the proper action can be taken to warn the vehicle operator. This article reviews a soot sensor currently developed for on-board diagnostics to monitor a diesel particulate filter and detect the failure. Original equipment manufacturers have selected a resistive soot sensor for the on-board diagnostics application because of its commercial feasibility in terms of functionality and cost. The sensor accumulates soot particles in exhaust gas on the sensing element. Structure, design, and function of the cumulative sensor including the signal processing techniques are described. Since the sensor signal contains noises and fluctuations, the post-diesel particulate filter soot concentration is repeatedly measured over 100 times and averaged in order to ensure the measurement accuracy, so that the diesel particulate filter failures can be detected. In addition to the resistive soot sensor, this article reviews the status of currently developed continuous soot sensors such as an electrostatic sensor, an electrical charging sensor, and a radio frequency–based sensor. These sensors have major issues of packaging and cost to be resolved prior to application to production vehicles. This review article is based primarily on scientific literatures most of which were published a few years ago, but the development of new soot sensor concepts has been continuing in the industry. Three new soot sensor concepts all of which are under development are also reported.
The use of a knock intensity threshold set at a relatively high level is effective for identifying knocking cycles, but results in high Type II (false negative) classification errors. Many cycles, although classified as non-knocking cycles (i.e. below the threshold), are actually operating in an undesirably high knock rate region. A traditional controller is therefore likely to advance the spark further into this region, when the correct response would be to retard the spark. In this article, a new dual-threshold knock controller is presented in which a second threshold is introduced in order to identify non-knocking cycles more clearly. This enables the advance gain to be increased without adversely affecting other aspects of the response, thereby improving the transient and steady-state performance characteristics of the controller. The threshold values can also be optimized so as to minimize the total Type I and Type II misclassification errors, resulting in a significant improvement in most aspects of the controller response.
This study presents detailed characterization of the chemical and physical properties of particulate matter emitted by a 2.0-L BMW lean-burn turbocharged gasoline direct injection engine operated under a number of combustion strategies that include lean homogeneous, lean stratified, stoichiometric, and fuel-rich conditions. We characterized particulate matter number concentrations, size distributions, and the size, mass, compositions, and effective density of fractal and compact individual exhaust particles. For the fractal particles, these measurements yielded fractal dimension, average diameter of primary spherules, and number of spherules, void fraction, and dynamic shape factors as function of particle size. Overall, the particulate matter properties were shown to vary significantly with engine operation condition. Lean stratified operation yielded the most diesel-like size distribution and the largest particulate matter number and mass concentrations, with nearly all particles being fractal agglomerates composed of elemental carbon with small amounts of ash and organics. In contrast, stoichiometric operation yielded a larger fraction of ash particles, especially at low speed and low load. Three distinct forms of ash particles were observed, with their fractions strongly dependent on engine operating conditions: sub-50 nm ash particles, abundant at low speed and low load, ash-containing fractal particles, and large compact ash particles that significantly contribute to particulate matter mass loadings.
This article employs a mixed lubrication model to investigate the performance of the textured surface. The Jakobsson–Floberg–Olsson model is used to obtain the hydrodynamic support of the textured conjunction, while the calculation of the asperity contact load is based on the load-sharing concept. Based on the simulated Stribeck curves of the smooth surface and the textured surface, comparisons are conducted to study the effect of texturing under different lubrication regimes. It appears that the transition of lubrication regimes is influenced by the texturing parameters and the convergence degrees of conjunction. The presence of textures delays the appearance of the mixed lubrication regime and the boundary lubrication regime.
This two-part article presents a model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two components: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development and validation of the homogeneous charge compression ignition model for use under a broad range of operating conditions. Using computational fluid dynamics simulations of the negative valve overlap valve events typical of homogeneous charge compression ignition operation, it is shown that there is no noticeable reaction progress from low-temperature heat release, and that ignition is within the high-temperature regime (T > 1000 K), starting within the highest temperature cells of the computational fluid dynamics domain. Additional parametric sweeps from the computational fluid dynamics simulations, including sweeps of speed, load, intake manifold pressures and temperature, dilution level and valve and direct injection timings, showed that the assumption of a homogeneous charge (equivalence ratio and residuals) is appropriate for ignition modelling under the conditions studied, considering the strong sensitivity of ignition timing to temperature and its weak compositional dependence. Use of the adiabatic core temperature predicted from the adiabatic core model resulted in temperatures within ±1% of the peak temperatures of the computational fluid dynamics domain near the time of ignition. Thus, the adiabatic core temperature can be used within an auto-ignition integral as a simple and effective method for estimating the onset of homogeneous charge compression ignition auto-ignition. The ignition model is then validated with an experimental 92.6 anti-knock index gasoline-fuelled homogeneous charge compression ignition dataset consisting of 290 data points covering a wide range of operating conditions. The tuned ignition model predictions of
In this work, we study the effects of injector nozzle inclusion angle, injection pressure, boost, and swirl ratio on gasoline compression ignition combustion. Closed-cycle computational fluid dynamics simulations using a 1/7th sector mesh representing a single cylinder of a four-cylinder 1.9 L diesel engine, operated in gasoline compression ignition mode with 87 anti-knock index (AKI) gasoline, were performed. Two different operating conditions were studied—the first is representative of idle operation (4 mg fuel/cylinder/cycle, 850 r/min), and the second is representative of a low-load condition (10 mg fuel/cylinder/cycle, 1500 r/min). The mixture preparation and reaction space from the simulations were analyzed to gain insights into the effects of injection pressure, nozzle inclusion angle, boost, and swirl ratio on achieving stable low-load to idle gasoline compression ignition operation. It was found that narrower nozzle inclusion angles allow for more reactivity or propensity to ignition (determined qualitatively by computing constant volume ignition delays) and are suitable over a wider range of injection timings. Under idle conditions, it was found that lower injection pressures helped to reduce overmixing of the fuel, resulting in greater reactivity and ignitability (ease with which ignition can be achieved) of the gasoline. However, under the low-load condition, lower injection pressures did not increase ignitability, and it is hypothesized that this is because of reduced chemical residence time resulting from longer injection durations. Reduced swirl was found to maintain higher in-cylinder temperatures through compression, resulting in better ignitability. It was found that boosting the charge also helped to increase reactivity and advanced ignition timing.
Size-resolved particle mass and number concentrations were obtained from different operating conditions using a spark-ignition direct-injection engine and a heavy-duty diesel engine. Particle mass versus mobility diameter results obtained for the engines showed weak dependence on the operating condition. The particle mass–mobility data enabled the use of an integrated particle size distribution method to estimate the particulate matter mass concentration in the exhaust stream. Average mass concentrations determined with the integrated particle size distribution method were
This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*,
Referring to spark-ignition engines, the downsizing, coupled to turbocharging and variable valve actuation systems are very common solutions to reduce the brake-specific fuel consumption at low-medium brake mean effective pressure. However, the adoption of such solutions increases the complexity of engine control and management because of the additional degrees of freedom, and hence results in a longer calibration time and higher experimental efforts. In this work, a twin-cylinder turbocharged variable valve actuation spark-ignition engine is numerically investigated by a one-dimensional model (GT-Power™). The considered engine is equipped with a fully flexible variable valve actuation system, realizing both a common full-lift strategy and a more advanced early intake valve closure strategy. Refined sub-models are used to describe turbulence and combustion processes. In the first stage, one-dimensional engine model is validated against the experimental data at full and part load. The validated model is then integrated in a multipurpose commercial optimizer (modeFRONTIER™) with the aim to identify the engine calibration that minimizes brake-specific fuel consumption at part load. In particular, the decision parameters of the optimization process are the early intake valve closure angle, the throttle valve opening, the turbocharger setting and the spark timing. Proper constraints are posed for intake pressure in order to limit the gas-dynamic noise radiated at the intake mouth. The adopted optimization approach shows the capability to reproduce with good accuracy the experimentally identified calibration. The latter corresponds to the numerically derived Pareto frontier in brake mean effective pressure–brake specific fuel consumption plane. The optimization also underlines the advantages of an engine calibration based on a combination of early intake valve closure strategy and intake throttling rather than a purely throttle-based calibration. The developed automatic procedure allows for a ‘virtual’ calibration of the considered engine on completely theoretical basis and proves to be very helpful in reducing the experimental costs and the engine time-to-market.
In internal combustion engines, cycle-to-cycle and cylinder-to-cylinder variations of the combustion process have been shown to negatively impact the fuel efficiency of the engine and lead to higher exhaust emissions. The combustion variations are generally tied to differences in the composition and condition of the trapped mass throughout each cycle and across individual cylinders. Thus, advanced engines featuring exhaust gas recirculation, flexible valve actuation systems, advanced fueling strategies, and turbocharging systems are prone to exhibit higher variations in the combustion process. In this study, the cylinder-to-cylinder variations of the combustion process in a dual-fuel internal combustion engine leveraging late intake valve closing are investigated and a model to predict and address one of the root causes for these variations across cylinders is developed. The study is conducted on an inline six-cylinder heavy-duty dual-fuel engine equipped with exhaust gas recirculation, a variable geometry turbocharger, and a fully flexible variable intake valve actuation system. The engine is operated with late intake valve closure timings in a dual-fuel combustion mode in which a high reactivity fuel is directly injected into the cylinders and a low reactivity fuel is port injected into the cylinders. The cylinder-to-cylinder variations observed in the study have been associated with the maldistribution of the port-injected fuel, which is exacerbated at late intake valve timings. The resulting difference in indicated mean effective pressure between the cylinders ranges from 9% at an intake valve closing of 570° after top dead center to 38% at an intake valve closing of 620° after top dead center and indicates an increasingly uneven fuel distribution. The study leverages both experimental and simulation studies to investigate the distribution of the port-injected fuel and its impact on cylinder-to-cylinder variation. The effects of intake valve closing as well as the impact of intake runner length on fuel distribution were quantitatively analyzed, and a model was developed that can be used to accurately predict the fuel distribution of the port-injected fuel at different operating conditions with an average estimation error of 1.5% in cylinder-specific fuel flow. A model-based control strategy is implemented to adjust the fueling at each port and shown to significantly reduce the cylinder-to-cylinder variations in fuel distribution.
Variable geometry turbine is a technology that has been proven on diesel engines. However, despite the potential to further improve gasoline engines’ fuel economy and transient response using variable geometry turbine, controlling the variable geometry turbine during transients is challenging due to its highly non-linear behaviours especially on gasoline applications. After comparing three potential turbocharger transient control strategies, the one that predicts the turbine performances for a range of possible variable geometry turbine settings in advance was developed and validated using a high-fidelity engine model. The proposed control strategy is able to capture the complex transient behaviours and achieve the optimum variable geometry turbine trajectories. This improved the turbocharger response time by more than 14% compared with a conventional proportional–integral–derivative controller, which cannot achieve target turbocharge speed in all cases. Furthermore, the calibration effort required can be significantly reduced, offering significant benefits for powertrain developers. It is expected that the structure of this transient control strategy can also be applied to complex air-path systems.
It is of interest for engine combustion modeling to quantify the evaporation behaviors of fuel spray impinging on a wall as the fuel atomization, evaporation, and mixing with oxygen in the combustion chamber usually dominate the subsequent combustion processes. In this study, the vapor and liquid mass distributions in diesel-like fuel sprays were quantified using the ultraviolet-visible laser absorption scattering imaging technique. The sprays were injected from a single-hole nozzle with a common-rail injection system and impinged on a flat wall at an ambient pressure of 4 MPa and an ambient temperature of 833 K. The mass of the total fuel vapor, the spray volume covered by the vapor phase, and the air mass entrained into the spray were characterized. The results indicate that the time evolution of these parameters until shortly after the end of injection can be expressed by a power-law function, Yi = ki·ts1.5, where Yi represents the parameter like vapor mass and so on, ts is the time after start of injection, and ki is the coefficient corresponding to Yi. The physics behind this power-law function was analyzed and discussed based on the theory of atomization and evaporation, and verified using measurement data obtained under different conditions of injection quantity.
Ever tighter restrictions on pollutant emissions, energy security and a continuous drive for improving fuel economy have extended the range of application for direct injection in spark ignition engines and promoted the use of alternative fuels. Direct injection features higher soot formation compared to external mixture preparation, and therefore, intensive research is performed for understanding the processes related to this pollutant category. This study looked into the effect of injection timing in a wall-guided direct injection spark ignition engine when gasoline was completely replaced with n-butanol. Thermodynamic measurements were coupled with optical investigations that provided improved insight into local distribution of diffusive flames during late combustion stages. These data were correlated with exhaust gas measurements of CO, HC and NOx, as well as opacity. The optimum setting for injection timing was found to be a compromise between intake airflow velocity and piston positioning that influenced wall impingement. Late injection resulted in reduced soot but higher HC emissions, as well as lower performance compared to the optimum point. Early fuel delivery had roughly the same effect on indicated mean effective pressure and stability, with the downside of increased opacity. These observations were detailed with data obtained through cycle-resolved imaging that showed different integral luminosities with respect to injection phasing and confirmed that fuel impingement on the piston crown is the main factor of influence for soot formation. Ultraviolet–visible spectroscopy in the late combustion phase was also applied in repetitive mode in order to provide better insight into cyclic variability of the emission intensity in the range specific for carbonaceous structures.
Gasoline direct injection (GDI) shows advantages compared with port fuel injection (PFI) regarding efficiency and specific power. Due to stricter regulations for fuel consumption (via the regulation of carbon dioxide), GDI engines are becoming increasingly favourable compared with PFI engines. Therefore the share of GDI engines, especially in combination with turbocharging, is increasing in most of the markets with CO2 regulations. Challenging for GDI engines is the mixture formation process due to the short time between fuel injection and the start of combustion. Thus, the injector needs to provide a fine fuel atomization in a considerably short time. The generated spray pattern thereby interacts with the in-cylinder charge motion to generate an appropriate air–fuel mixture. Because of this challenging mixture formation process, the formation of soot in local fuel-rich areas is possible. Thus GDI engines emit more particles compared with PFI engines and need special attention on the mixture formation process. To understand the reasons for the increased particle number (PN) emissions, a project concerning the cause of particle emissions was started at Institute of Internal Combustion Engines (IFKM) in Karlsruhe in 2011. During the project, different causes for PN emissions were identified. This article discusses the possible reasons for particle emissions under high engine load and low engine speed and shows some possible solutions to reduce the emission of particles. Discussed possible solutions to enhance the mixture formation process are the generation of a large-scale charge motion (tumble and swirl), the reduction of the hydraulic flow of the multi-hole, solenoid-activated injector and an increase of the rail pressure up to 50 MPa. The reduction of the hydraulic flow and the increase of the injection pressure lead to smaller average droplets and thus to a faster evaporation. An implementation of a large-scale charge motion enhances the mixture formation process and leads to a reduction of the emitted PN concentration at high engine load. This is shown for wide open throttle (WOT) as well as for boosted operation. The reduction of the hydraulic flow of the injector by reducing the bore hole diameter at constant number of holes and spray targeting of the injector leads to smaller droplets. By increasing the injection pressure, the injection duration as well as the average droplet size can be reduced and leads to a better homogenization and a further reduced PN concentration.
The objective of this study was to develop knock criteria for aviation diesel engines that have experienced a number of malfunctions during flight and ground operation. Aviation diesel engines have been vulnerable to knock because they use cylinder wall coating on the aluminum engine block, instead of using steel liners. This has been a trade-off between reliability and lightweighting. An in-line four-cylinder four-stroke direct-injection high-speed turbocharged aviation diesel engine was tested to characterize its combustion at various ground and flight conditions for several specially formulated Jet A fuels. The main fuel property chosen for this study was cetane number, as it significantly impacts the combustion of the aviation diesel engines. The other fuel properties were maintained within the MIL-DTL-83133 specification. The results showed that lower cetane number fuels showed more knock tendency than higher cetane number fuels for the tested aviation diesel engine. In this study, maximum pressure rise rate, or Rmax, was used as a parameter to define knock criteria for aviation diesel engines. Rmax values larger than 1500 kPa/cad require correction to avoid potential mechanical and thermal stresses on the cylinder wall coating. The finite element analysis model using the experimental data showed similarly high mechanical and thermal stresses on the cylinder wall coating. The developed diesel knock criteria are recommended as one of the ways to prevent hard knock for engine developers to consider when they design or calibrate aviation diesel engines.
This article discusses an approach to exceeding current peak exergy efficiencies of approximately 50% for transportation-scale engines. A detailed model was developed for an internal combustion engine and a fuel cell, where the internal combustion engine is operated under fuel-rich conditions to produce a hydrogen-rich exhaust gas as a fuel for the fuel cell. The strategy of using combustion and electrochemical energy conversion processes has been shown to reduce reaction-related exergy losses while providing the balance of plant necessary to achieve efficient thermal management. Prior approaches which used internal combustion engines downstream of the fuel cell have shown exergy efficiencies near 70%. The system architecture developed for this article, in addition to achieving exergy efficiencies near 70%, provides further advantages. The internal combustion engine, producing work in addition to generating synthesis gas, enables a quick-start approach to this mixed strategy and the ability to use a range of fuels. Therefore, the proposed architecture supplies a very efficient starting point for the development of a quick-start, hybridized system for transportation-scale applications.
In this work, a simple methodology was implemented to predict the onset of knock in spark-ignition engines and quantify the benefits of two practical knock mitigation strategies: cooled exhaust gas recirculation and syngas blending. Based on the results of this study, both cooled exhaust gas recirculation and the presence of syngas constituents in the end-gas substantially improved the knock-limited compression ratio of the engine. At constant load, 25% exhaust gas recirculation increased the knock-limited compression ratio from 9.0 to 10.8:1 (0.07 compression ratio per 1% exhaust gas recirculation) due to lower end-gas temperature and reactant (fuel and oxygen) concentrations. At exhaust gas recirculation rates above 43%, higher intake temperature outweighed the benefits of lower end-gas reactant concentration. At constant intake temperature, cooled exhaust gas recirculation was significantly more effective at all exhaust gas recirculation rates (0.10 compression ratio per 1% exhaust gas recirculation), and no diminishing returns or optimum was observed. Both hydrogen and carbon monoxide were also predicted to improve knock by reducing end-gas reactivity, likely through the conversion of high-reactivity hydroxy-radicals to less reactive peroxy-radicals. Hydrogen increased the knock-limited compression ratio by 1.1 per volume percent added at constant energy content. Carbon monoxide was less effective, increasing the knock-limited compression ratio by 0.38 per volume percent added. Combining 25% cooled exhaust gas recirculation with reformate produced from rich combustion at an equivalence ratio of 1.3 resulted in a predicted increase in the knock-limited compression ratio of 3.5, which agreed well with the published experimental engine data. The results show the extent to which syngas blending and cooled exhaust gas recirculation each contribute separately to knock mitigation and demonstrate that both can be effective knock mitigation strategies. Together, these solutions have the potential to increase the compression ratio and efficiency of spark-ignition engines.
This work presents a scalable model of a naturally aspirated gasoline engine forecasting the effective efficiency map for varying cylinder displacements. Engine test bench measurements and a global nonlinear hybrid optimization method were used to calibrate the engine model. The validation showed a good prediction of engine efficiency by the scaling model with a mean error of 2% compared with the measurements. A pure scaling of the cylinder displacement led to overall small changes in the effective engine efficiency map. In addition to the development of a scalable engine model, a forward-looking hybrid vehicle simulation model was used in order to evaluate the impact of different engine cylinder displacements on fuel consumption. For this purpose, simulations for varying cylinder displacements were performed in a series–parallel hybrid drivetrain of an A-class vehicle in two driving cycles. The simulation results showed a small influence of different engine cylinder displacements on fuel consumption for the given configuration.
This two-part article presents a combustion model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two parts: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development of the combustion model which is algebraic in form and is based on the key physical variables affecting the combustion process. The model is fit with experimental data collected from 290 discrete automotive homogeneous charge compression ignition operating conditions with moderate stratification resulting from both the direct injection and negative valve overlap valve events. Both the ignition model from part 1 and the combustion model from this article are implemented in GT-Power and validated against experimental homogeneous charge compression ignition data under steady-state and transient conditions. The ignition and combustion model are then exercised to identify the dominant variables affecting the homogeneous charge compression ignition and combustion processes. Sensitivity analysis reveals that ignition timing is primarily a function of the charge temperature, and that combustion duration is largely a function of ignition timing.
In a diesel engine, diesel particulate filter is used to reduce particle matter emissions. Diesel particulate filter requires periodic regenerations under high-temperature conditions in the exhaust pipe in order to oxidize the accumulated soot. A common strategy to produce high exhaust gas temperature is to inject late post-injections after the main injection. However, this practice may dilute the engine oil, causing engine wear. Biofuel addition to petroleum diesel may increase oil dilution even more. This is related to the fuel spray characteristics, the post-injection control and the vaporization process of fuel in engine oil. In this study, spray properties of late post-injection were studied with petroleum diesel and two types of transport biofuel blends containing 30% either fatty acid methyl ester or hydro-treated vegetable oil. Three different late post-injection timings were investigated. Image sequences of the main spray flame as well as the non-combusting late post-injection spray were extracted. In order to verify oil dilution during regeneration cycle and late post-injection, oil samples from six-cylinder test engine were analyzed. According to the present experiments, differences in the spray characteristics are not significant with the tested fuels. However, higher oil dilution rates were observed with fuel blend composed of 30% fatty acid methyl ester. All the studied late post-injection timings were noted to lead to the unwanted cylinder spray/wall interaction and wall-wetting consequently diluting the engine oil. The spray/wall interaction is thoroughly explained by introducing a theoretical/computational framework which characterizes any spray/wall interaction in terms of a phase diagram for any considered operation conditions. The novelty of this study arises from (1) first comparison of fatty acid methyl ester and hydro-treated vegetable oil blends in an optical engine, (2) strong evidence on the phenomena related to post-injection phase in six-cylinder and single-cylinder optical engine configurations and (3) the development of a single-droplet model showing inevitable wall-wetting.
Low-temperature combustion offers an attractive combination of high thermal efficiency and low NO x and soot formation at moderate engine load. However, the kinetically-controlled nature of low-temperature combustion yields little authority over the rate of heat release, resulting in a tradeoff between load, noise, and thermal efficiency. While several single-fuel strategies have achieved full-load operation through the use of equivalence ratio stratification, they uniformly require retarded combustion phasing to maintain reasonable noise levels, which comes at the expense of thermal efficiency and combustion stability. Previous work has shown that control over heat release can be greatly improved by combining reactivity stratification in the premixed charge with a diffusion-limited injection that occurs after low-temperature heat release, in a strategy called direct dual fuel stratification. While the previous work has shown how the heat release control offered by direct dual fuel stratification differs from other strategies and how it is enabled by the reactivity stratification created by using two fuels, this paper investigates the effects of the diffusion-limited injection. In particular, the influence of fuel selection and the pressure, timing, and duration of the diffusion-limited injection are examined. Diffusion-limited injection fuel type had a large impact on soot formation, but no appreciable effect on performance or other emissions. Increasing injection pressure was observed to decrease filter smoke number exponentially while improving combustion efficiency. The timing and duration of the diffusion-limited injection offered precise control over the heat release event, but the operating space was limited by a tradeoff between NO x and soot.
This article describes the use of a Lagrangian discrete droplet model to evaluate the liquid fuel impingement characteristics on the internal surfaces of an early injection gasoline direct injection engine. This study focuses on fuel impingement on the intake valve and cylinder liner between start of injection and 20° after start of injection using both a single- and a multi-component fuels. The single-component fuel used was iso-octane and the multi-component fuel contained fractions of iso-pentane, iso-octane and n-decane to represent the light, medium and heavy fuel fractions of gasoline, respectively. A detailed description of the impingement and liquid film modelling approach is also provided. Fuel properties, wall surface temperature and droplet Weber number and Laplace number were used to quantify the impingement regime for different fuel fractions and correlated well with the predicted onset of liquid film formation. Evidence of film stripping was seen from the liquid film formed on the side of the intake valve head with subsequent ejected droplets being a likely source of unburned hydrocarbons and particulate matter emissions. Differences in impingement location and subsequent location of liquid film formation were also observed between single- and multi-component fuels. A qualitative comparison with experimental cylinder liner impingement data showed the model to well predict the timing and positioning of the liner fuel impingement.
Numerical prediction of cycle-to-cycle variability in spark ignition engines is extremely challenging for two key reasons: (1) high-fidelity methods such as large eddy simulation are required to accurately capture the in-cylinder turbulent flow field and (2) cycle-to-cycle variability is experienced over long time scales, and hence, the simulations need to be performed for hundreds of consecutive cycles. In this study, a methodology is proposed to dissociate this long time-scale problem into several shorter time-scale problems, which can considerably reduce the computational time without sacrificing the fidelity of the simulations. The strategy is to perform multiple parallel simulations, each of which encompasses two to three cycles, by effectively perturbing the simulation parameters such as the initial and boundary conditions. The proposed methodology is validated for the prediction of cycle-to-cycle variability due to gas exchange in a motored transparent combustion chamber engine by comparing with particle image velocimetry measurements. It is shown that by perturbing the initial velocity field effectively based on the intensity of the in-cylinder turbulence, the mean and variance of the in-cylinder flow field are captured reasonably well. Adding perturbations in the initial pressure field and the boundary pressure improves the predictions. It is shown that this new approach is able to give accurate predictions of the flow field statistics in considerably less time than that required for the conventional approach of simulating consecutive engine cycles.
Low-temperature compression ignition combustion can result in nearly smokeless combustion, as indicated by a smoke meter or other forms of soot measurement that rely on absorbance due to elemental carbon content. Highly premixed low-temperature combustion modes do not form particulate matter in the traditional pathways seen with conventional diesel combustion. Previous research into reactivity controlled compression ignition particulate matter has shown, despite a near zero smoke number, significant mass can be collected on filter media used for particulate matter certification measurement. In addition, particulate matter size distributions reveal that a fraction of the particles survive heated double-dilution conditions. This study summarizes research completed at Oak Ridge National Laboratory to date on characterizing the nature, chemistry and aftertreatment considerations of reactivity controlled compression ignition particulate matter and presents new research highlighting the importance of injection strategy and fuel composition on reactivity controlled compression ignition particulate matter formation. Particle size measurements and the transmission electron microscopy results do show the presence of soot particles; however, the elemental carbon fraction was, in many cases, within the uncertainty of the thermal–optical measurement. Particulate matter emitted during reactivity controlled compression ignition operation was also collected with a novel sampling technique and analyzed by thermal desorption or pyrolysis gas chromatography mass spectroscopy. Particulate matter speciation results indicated that the high boiling range of diesel hydrocarbons was likely responsible for the particulate matter mass captured on the filter media. To investigate potential fuel chemistry effects, either ethanol or biodiesel were incorporated to assess whether oxygenated fuels may enhance particle emission reduction.
Soot onset in n-dodecane sprays is investigated both experimentally, by means of high-speed imaging data from the Sandia spray combustion vessel, and numerically, using the conditional moment closure combustion model and an integrated two-equation soot model in a Reynolds-averaged Navier–Stokes framework. Five operating conditions representative of modern diesel engines are studied at constant density (22.8 kg/m3) with variations in ambient oxygen concentration and temperature. The reference case at 15% O2 and 900 K is compared with measurements in terms of the evolving soot mass distribution and spatiotemporal distributions of formaldehyde and polycyclic aromatic hydrocarbons obtained by 355-nm laser-induced fluorescence (polycyclic aromatic hydrocarbons represented by C2H2 in simulation) and soot optical thickness (KL) signal obtained by diffused back-illumination extinction imaging. All operating points are validated in terms of ignition delay and lift-off length, soot onset time and location, soot mass evolution, and peak location. Measurements show that time lag between ignition and soot onset is considerably increased by a reduction in ambient oxygen or temperature. The trend of this time lag is captured very well by the simulations, as is the evolving axial distribution of soot, despite the simple soot model employed. Building on the good agreement between spatiotemporal distributions in experiment and simulation, further results from the latter are extracted to provide insight into relevant processes. The advancing soot tip lags behind the fuel–vapor spray tip due to soot oxidation. Tracking the Lagrangian time history of notional fluid particles from the soot onset location back to the injector orifice reveals that their trajectories evolve along rich conditions ( > 1.5) throughout the entire path. Overall, novel insights obtained from experiments with respect to soot and soot precursor evolutions are complemented by simulations using the integrated conditional moment closure/soot modeling approach, showing encouraging results for prediction and understanding of transient soot processes in high-pressure diesel sprays.
A generalized approach, based on linear algebra, is described for processing exhaust gas analyser data. Systematic methods of deriving useful relationships from arbitrary data are proposed and used to produce several novel and useful results, as well as to show how existing relationships may be derived in forms that involve no approximations. The methods developed lend themselves to automatic real-time assessment of the consistency of gas analyser data, and in the case of inconsistencies, identifying plausible reasons. The approach is also used to develop methods to examine storage and release mechanisms within after-treatment devices, such as oxygen storage/release in three-way catalysts, soot oxidation in particle filters and water condensation/evaporation.
Advanced diesel combustion, accomplished via a single pulse fuel injection and high levels of exhaust gas recirculation, is shown to be a path to reduce oxides of nitrogen and particulate matter simultaneously. This is well established in the literature. Less established is how well such dilute combustion processes influence soot formation and affect soot that is emitted from diesel engines under such combustion modes. This work focuses on characterization of the nanostructure and oxidative reactivity of soot generated by a light-duty turbodiesel engine operating under a dilute, low-temperature combustion process referred to as high-efficiency clean combustion. The high-efficiency clean combustion soot samples are shown to have a fullerenic nanostructure, characterized by high levels of tortuosity of the fringe layers as seen in transmission electron micrograph images and as quantified using an image processing algorithm. Thermogravimetric analysis of the high-efficiency clean combustion soot samples shows that they have higher rates of oxidation than soot samples from a conventional diesel combustion mode. The linkage between the nanostructure of the high-efficiency clean combustion soot samples and their oxidative reactivity reinforces and supports the structure–property relationship for soot that greater curvature on the soot nanostructure leads to significant increases in oxidative reactivity.
The aim of this article is to assess the impact of turbulence and cavitation models on the prediction of diesel injector nozzle flow. Two nozzles are examined, an enlarged one, operating at incipient cavitation, and an industrial injector tip, operating at developed cavitation. The turbulence model employed includes the re-normalization group k–, realizable k– and k– shear stress transport Reynolds-averaged Navier–Stokes models; linear pressure–strain Reynolds stress model and the wall adapting local eddy viscosity large eddy simulation model. The results indicate that all Reynolds-averaged Navier–Stokes and the Reynolds stress turbulence models have failed to predict cavitation inception due to their limitation to resolve adequately the low pressure existing inside vortex cores, which is responsible for cavitation development in this particular flow configuration. Moreover, Reynolds-averaged Navier–Stokes models failed to predict unsteady cavitation phenomena in the industrial injector. However, the wall adapting local eddy viscosity large eddy simulation model was able to predict incipient and developed cavitation, while also capturing the shear layer instability, vortex shedding and cavitating vortex formation. Furthermore, the performance of two cavitation methodologies is discussed within the large eddy simulation framework. In particular, a barotropic model and a mixture model based on the asymptotic Rayleigh–Plesset equation of bubble dynamics have been tested. The results indicate that although the solved equations and phase change formulation are different in these models, the predicted cavitation and flow field were very similar at incipient cavitation conditions. At developed cavitation conditions, standard cavitation models may predict unrealistically high liquid tension, so modifications may be essential. It is also concluded that accurate turbulence representation is crucial for cavitation in nozzle flows.
A fully predictive model of a Common Rail fuel injection apparatus, which includes a detailed simulation of rail, pump, piping system, injectors and rail pressure control system, is presented and discussed. The high-pressure pump and injector sub-models have been validated separately and then coupled to the rail and pressure control system sub-models. The complete predictive model has been validated and applied to investigate the effects of the dynamics of each component of the injection apparatus on the rail pressure time history. Variable timing of the high-pressure pump delivery phases has also been considered, and the influence of this parameter on the injection performance has been analysed for both single- and multiple-injection events. Furthermore, the injection system dynamics during the transients between steady-state working conditions has been investigated in order to highlight the role played by the dynamic response of the pressure control system on the rail pressure time history.
Road vehicles account for a substantial portion of energy consumed by transportation. A large amount of this energy is lost to overcome friction within vehicle engines, in which the piston compression rings are a major source of such parasitic losses. The internal losses of engines increase several times in the case of unfavorable warm-up conditions. Recent developments in surface modification showed promising results in improving the frictional behavior of piston rings. Analyses are often idealized, such as isothermal conditions and unrealistic engine operating conditions. This study presents a numerical investigation of the frictional behavior of mixed-hydrodynamic interaction in a textured piston compression ring–cylinder liner during the warm-up process. The transient Reynolds equation is solved with a mass-conserving cavitation algorithm, realistic oil rheology, and practical engine operating conditions. Several multigrade and monograde oils are considered to draw comprehensive conclusions. The results show that ring surface texturing substantially reduces energy loss during the entire warm-up phase.
Closed-loop combustion control in gasoline engines can improve efficiency, calibration effort, and performance using different fuels. Knowledge of in-cylinder pressures is a key requirement for closed-loop combustion control. Adaptive cylinder pressure reconstruction offers a realistic alternative to direct sensing, which is otherwise necessary as legislation requires continued reductions in CO2 and exhaust emissions. Direct sensing, however, is expensive and may not prove adequately robust. A new approach is developed for in-cylinder pressure reconstruction on gasoline engines. The approach uses time-delay feedforward artificial neural networks trained with the standard Levenberg–Marquardt algorithm. The same approach can be applied to reconstruction via measured crank kinematics obtained from a shaft encoder or measured engine cylinder block vibrations obtained from a production knock sensor. The basis of the procedure is initially justified by examination of the information content within measured data, which is considered to be equally important as the network architecture and training methodology. Key hypotheses are constructed and tested using data taken from a three-cylinder (direct injected spark ignition) engine to reveal the influence of the data information content on reconstruction potential. The findings of these hypotheses tests are then used to develop the methodology. The approach is tested by reconstructing cylinder pressure across a wide range of steady-state engine operation using both measured crank kinematics and block accelerations. The results obtained show a very marked improvement over previously published reconstruction accuracy for both crank kinematics and cylinder block vibration–based reconstruction using measurements obtained from a multi-cylinder engine. This article shows that by careful processing of measured engine data, a standard neural network architecture and a standard training algorithm can be used to very accurately reconstruct engine cylinder pressure with high levels of robustness and efficiency.
Factors determining the success or failure of combustion initiation using a glow plug have been investigated through experimental work on a single cylinder, common rail diesel engine with a geometric compression ratio of 15.5, and a quiescent combustion bomb with optical access. A glow plug was required to avoid engine misfires when bulk gas temperature at the start of injection was less than 413 °C. The distance between the glow plug and the spray edge, the glow plug temperature, and the bulk gas temperature were important factors in meeting two requirements for successful ignition: a minimum local temperature of 413 °C and a minimum air/fuel vapour equivalence ratio of 0.15–0.35.
Low temperature, highly premixed compression ignition strategies have proven to produce high efficiency and low soot emissions, but struggle to reach high loads within normal operating constraints. Recent research has suggested that a mixed mode combustion strategy using a premixed main heat release followed by a mixing controlled load extension injection can retain the part-load thermal efficiency and emissions reduction potential of premixed compression ignition strategies, while enabling high load operation. However, soot emissions under this type of mixed mode combustion strategy have been shown to be problematic. This work investigates soot formation and mitigation methods using a combination of detailed engine experiments and computational fluid dynamics modeling. A premixed compression ignition combustion event was achieved using a premixed charge of gasoline and n-heptane to control combustion phasing, and a load extension injection of gasoline was added near top dead center. The experiments showed negligible engine out soot under the premixed compression ignition operating conditions (i.e. without the load extension injection). When the load extension injection was added, soot increased by several orders of magnitude. Detailed experiments were used to isolate the effects of injection timing, injection pressure, charge conditions (e.g. air–fuel ratio), and fuel type. Computational fluid dynamics modeling considering polycyclic aromatic hydrocarbon chemistry up to pyrene was then used to explain the experimentally observed soot trends. As expected, the soot emission results showed a strong impact of oxygen concentration and injection pressure for injection timings near top dead center; however, as the load extension injection event was delayed beyond the end of the premixed compression ignition heat release, the soot formation decreased and became independent of oxygen concentration. At these conditions, the computational fluid dynamics modeling showed that soot formation is dependent solely on temperature. The results identify a pathway to enable premixed compression ignition load extension, while minimizing soot emissions.
The combination of in-cylinder thermal environment and fuel ignition properties plays a critical role in the homogeneous charge compression ignition engine combustion process. The properties of fuels available in the automotive market vary considerably and display different auto-ignition behaviors for the same intake charge conditions. Thus, in order for homogeneous charge compression ignition (HCCI) technology to become practically viable, it is necessary to characterize the impact of differences in fuel properties as a source of ignition/combustion variability. To quantify the differences, 15 gasolines composed of blends made from refinery streams were investigated in a single-cylinder homogeneous charge compression ignition engine. The properties of the refinery stream blends were varied according to research octane number, sensitivity (S = research octane number – motor octane number) and volumetric contents of aromatics and olefins. Nine fuels contained 10% ethanol by volume, and six more were blended with 20% ethanol. Pure ethanol (E100) and an un-oxygenated baseline fuel (RD3-87) were included too. For each fuel, a sweep of intake temperature at a consistent load and engine speed was conducted, and the combustion phasing given by the crank angle of 50% mass fraction burned was tracked to assess the sensitivity of auto-ignition to fuel chemical kinetics. The experimental results provided a wealth of information for predicting the HCCI combustion phasing from the given properties of a fuel. In this study, the original octane index correlation proposed by Kalghatgi based solely on fuel research octane number and motor octane number was found to be insufficient for characterizing homogeneous charge compression ignition combustion of refinery stream fuels. A new correlation was developed for estimation of auto-ignition properties of practical fuels in the typical HCCI engine. Fuel composition, captured by terms indicating the fraction of aromatics, olefins, saturates and ethanol, was added to generate the following formula:
Theoretical and experimental methodologies have been proposed and illustrated to determine the transfer function between the injected flow-rate and the rail pressure for common rail injection systems. An analytical transfer function has been calculated in the frequency domain, utilizing a previously developed lumped parameter model of the overall hydraulic layout of a common rail system. The predicted transfer function has been compared, in a Bode diagram, with an experimental estimation of the transfer function, based on the measured rail pressure and injected flow-rate time histories that were acquired at the hydraulic rig for different working conditions. The experimental estimation of the transfer function has been worked out by applying a selective spectral technique in order to reduce the effects of measurement noise on the rail pressure and injected flow-rate time histories. The accuracy of the model-derived transfer function has been improved significantly by integrating a pressure control system sub-model, which includes the action of the electronic control unit on the rail pressure time history through the pressure regulator, in the hydraulic model of the common rail circuit. Finally, the time histories of the rail pressure, predicted by means of the complete injection apparatus model, have been compared with the corresponding experimental traces at different working conditions and a very satisfactory agreement has in general been found. The methodologies proposed for the accurate evaluation of the transfer function between the injected flow-rate and the rail pressure time histories can be applied to diesel engines in order to implement innovative closed-loop strategies for the injected mass control.
Particulate number count is an important consideration for engine developers due to changes in emissions legislation. Changes are driven by an increasing body of evidence that particulate number, particularly smaller particles, have a deleterious effect on human health. This article presents the results of an investigation into the key factors influencing particulate number emissions measurement repeatability during dynamometer-based testing of a gasoline direct-injection engine. At the outset of this work, a review of literature summarises some of the current discussion concerning particulate formation, evolution and measurement to identify the key factors that influence these three things. Having established what these factors are a number of engine experiments are undertaken to determine how sensitive particulate number measurements are to change in these factors and therefore how great an influence they are on measurement repeatability in engine experiments of a similar type. The investigation highlights a number of important results, showing that particular regard ought to be given to the pre-conditioning of engine internal surfaces which begins when the engine is started. In addition, the effect of coolant temperature (including the dynamics of the control system) is observed and highlighted as another key source of variation as is intake air temperature.
We investigate the turbulent multiphase flow inside a nine-hole common rail Diesel injector during a full injection cycle of ISO 4113 diesel fuel into air by implicit large-eddy simulation (LES). The simulation includes a prescribed needle movement obtained from a one-dimensional multi-domain simulation. The injector geometry is represented by a conservative cut-element-based immersed boundary method with subcell resolution, which has been developed for the application in the context of cavitating liquid flows. We employ a barotropic two-phase two-fluid model, where all components (i.e. air, liquid diesel, gaseous diesel) are represented by a homogenous mixture approach. The cavitation model is based on a thermodynamic equilibrium assumption. Compressibility of all phases enables full resolution of collapse-induced pressure wave dynamics. The analysis of the turbulent flow field reveals that the opening and closing phase are dominated by small-scale turbulence, while in the main injection phase large vortical structures are formed in the needle volume and reach into the nozzle holes. Violent collapse events of cavitation structures are detected during the closing phase in the nozzle holes and after closing in the sac hole region. A comparison with LES results with a fixed injector needle at different lift positions shows a good agreement for large needle lifts, while the needle movement has significant effects on important flow features at low needle lifts.
The backward flow of the hot burned gas surrounding a diesel flame was found to be one of the factors reducing the set-off length (also called the lift-off length), that is, the distance from a nozzle exit into which a diffusion flame cannot intrude. In the combustion chamber of an actual diesel engine, the entrainment of the surrounding gas into a spray jet injected from a multi-hole nozzle is restricted by the combustion chamber walls and the adjacent spray jets, thus inducing the backward flow of the surrounding gas toward the nozzle exit. The emergence of this backward flow was measured by particle tracking velocimetry in the non-combusting condition. A new momentum theory for calculating the backward flow velocity was established by extending Wakuri’s momentum theory. Shadowgraph imaging in an optical engine successfully visualized the backward flow of the hot burned gas. The hot burned gas is re-entrained into the spray jet in the region of the set-off position and shortens the set-off length in comparison to that of a single free-spray flame which does not induce the backward flow.
Internal combustion engine research on alternative fuels has gained momentum because of growing awareness about energy security and environmental issues worldwide. Biodiesel offers an ideal solution to these problems and is an excellent partial replacement to mineral diesel. In this study, Karanja biodiesel blended with mineral diesel has been investigated for macroscopic spray characterization vis-à-vis baseline mineral diesel by varying fuel injection pressures (50, 100 and 150 MPa). Spray developed with relatively narrower spray angle for KB20. Injector needle movement for energizing and real injection durations were also compared for diesel and KB20 at fuel injection pressures of 50, 100 and 150 MPa. Needle movement was slightly higher for KB20 because of its relatively higher viscosity. However, with increasing fuel injection pressure, the difference reduced and showed quite similar results. A 2.2 L common rail direct injection sport utility vehicle EURO-IV diesel engine was used for the experiments. Engine performance, emissions and combustion characteristics of KB20 were compared with baseline mineral diesel at (1) the rated engine speed (2500 r/min) with varying engine loads as well as (2) at the rated load at varying engine speeds (1500–3500 r/min). Brake thermal efficiency of KB20 was lower than mineral diesel. Brake-specific carbon monoxide and carbon dioxide emissions decreased with increasing brake mean effective pressure and showed increasing trend with increasing engine speeds. KB20 showed emission of higher number of particles compared to mineral diesel at all engine operating conditions. Higher oxygen content of biodiesel resulted in shorter ignition delay and slightly higher peak cylinder pressure. KB20 showed relatively longer combustion duration compared to mineral diesel at 2500 r/min engine speed.
In order to minimize the short-circuiting of the intake charge in a poppet-valved two-stroke engine, a "step structure" on the intake side of the pent-roof was employed to shroud the bulk flow at the upper periphery of the intake valves at low valve lifts. Several intake-port geometries and pent-roof angles were evaluated on their charging and reversed tumble generating abilities using a steady flow test rig built in computational fluid dynamics, among which top-entry ports and relatively small pent-roof angle were selected. The scavenging processes of the two-stroke engine at high load and 3000 r/min condition with different intake valve seat heights were simulated and evaluated. The local flow patterns and composition distribution were also studied. The results show that lowering the intake valve seat height achieves higher delivery ratio and charging efficiency, but marginally better scavenging efficiency and slightly lower trapping efficiency. The scavenging process experiences a slow displacement period due to low flow momentum for the high valve seat cases and flow interaction at the exhaust exits for the low valve seat cases. Reversed tumble is the dominant flow pattern during the scavenging process. The lower the valve seat, the higher the tumble ratio in the early stage and at the end of the scavenging period. For all valve seat configurations, the least scavenged regions are located near the engine head wall and between the intake and the exhaust valves, while the regions rich in fresh charge are located along the major path of the reversed tumble. Low intake valve seat height is effective in increasing the overall in-cylinder air proportion. Further reducing it can result in higher fresh charge proportion below the exhaust valves and near the head wall.
Fuel injection into retained residuals and the resulting exhaust-fuel reactions appear to be an effective method for enhancing mixture reactivity in residual effected homogeneous charge compression ignition engines. Varying the extent of reactions preceding the main combustion event enables the control of auto-ignition timing and heat release. Although fuel injection during the negative valve overlap period is widely utilised, there is still insufficient amount of data on its chemical effects. This study experimentally explores the species formation as a result of negative valve overlap exhaust-fuel reactions. To collect experimental data, a single-cylinder engine with negative valve overlap and direct gasoline injection was utilised. Valving strategy of the engine was strategically set to achieve a high rate of backflows at the end of the intake process. Negative valve overlap gas, diluted by intake air, was sampled from the intake runner and passed to a Fourier transform infrared analytical system. A dedicated procedure was applied to compute degrees of fuel conversion into species from the diluted samples. The experiments were designed to acquire comprehensive data on the effects of different fuel injection timings on species formation during negative valve overlap. The results showed that exhaust-fuel reactions could produce high quantities of methane and light unsaturated hydrocarbons. At early negative valve overlap fuel injection, up to 25% of fuel was converted into species, whereas chemical changes were negligible for late fuel injection. Additionally, the effects of excess air and amounts of fuel injected during negative valve overlap were investigated.
The starved lubrication is common in engine. However, there are only few studies on starved lubrication problem for textured surfaces. In particular, for the textured surface, a two-dimensional solution is required to study their performances clearly, by considering the pattern and distribution of textured cells. This article presents a new analysis method for the two-dimensional starved lubrication problems with the textured surfaces. In the starved model, the inlet boundary condition of oil film is required and needs to be updated over the process of calculation. In order to obtain the inlet boundary condition, the flow conservation equations at the inlet region are built to determine the position of the attaching point. Meanwhile, as the two-dimensional code is expensive in the computational cost, the multigrid method is employed to accelerate the convergence of pressure. The inlet boundary condition should be transferred between the coarse grids and the fine grids. In this way, solutions for the starved lubrication are offered for an efficient implementation. The starved lubrication model is successfully applied in the ring–liner system. The effects of the surface features on the inlet boundary, wear and friction loss are discussed. It is found that the surface texturing can improve the performance of the ring–liner system under the staved lubrication.
The honeycomb monolith is the most prevalent geometry in automotive exhaust aftertreatment, with applications including oxidation catalysts, partial-flow and wall-flow filters. Particle collection mechanisms at the leading edge of honeycomb monoliths and in open channels are usually neglected by engineers of these devices. Under specific conditions however, these phenomena can make an appreciable contribution to overall particle collection and deposits on channel walls may affect catalyst performance. In the current study, experimental measurements of deposit loading, capture efficiency and pressure drop at constant flow conditions are presented for three flow-through monoliths with different cell densities and lengths, while particle distribution is varied in an additional case. The processed results reveal the evolution of deposit distribution at the leading edge, the entry zone inside the channel and the remaining part of the monolith. Next a one-dimensional transient mathematical model is proposed, which takes into account interception at the edge and coupled diffusion–interception inside the channel. After fitting the model parameters, all experimental cases are predicted with sufficient accuracy with the exception of loadings extended beyond 12 hours. Finally, further application of the model reveals the negative effect of flow rate on collection efficiency and the potential to maximize or minimize collection efficiency using various cell designs.
The introduction of fuel economy and CO2 emission legislations for passenger cars in many countries and regions has spurred the research and development of more efficient gasoline engines. The pumping loss at part-load operations is a major factor for the higher fuel consumption of spark-ignition gasoline engines than the diesel engines. Various approaches have been identified to reduce the pumping loss at part-load operations, leading to improved fuel economy, including early intake-valve closing, positive valve overlap and controlled auto-ignition combustion. On the other hand, in order to reduce the CO2 emissions from the fossil fuel, ethanol produced from renewable resources is becoming widely used in the gasoline engine. In this article, the performance, combustion and emissions were measured, analysed and compared between gasoline and its mixture with ethanol (E15 and E85) at a typical part-load condition when a direct-injection gasoline engine was operated with the controlled auto-ignition combustion by means of the negative valve overlap and spark-ignition combustion by means of the intake throttled, early intake-valve closing and positive valve overlap. An electro-hydraulic actuated camless system enabled the engine to be operated with controlled auto-ignition combustion and spark-ignition combustion of different valve timings and durations at the same load. The results showed that the controlled auto-ignition combustion reduces nitrogen oxide emissions by more than 90%. The positive valve overlap results in better mixture preparations and improved combustion efficiency and best fuel economy compared to all the other modes. The early intake-valve closing operation led to a moderate improvement in the fuel conversion efficiency over the throttled spark-ignition operation, but it was characterised by the slowest combustion and worst hydrocarbon and carbon monoxide emissions. Fewer and smaller particle numbers were detected in early intake-valve closing using E0 and E15 fuel blends. Using ethanol blends reduces the knocking combustion in controlled auto-ignition modes by about 50%. The use of E85 resulted in an increased number of particulate emissions in early intake-valve closing but increased indicated specific fuel consumption in all the modes. The particulate emission results showed that soot is the dominant particle in the exhaust.
Modern engine design has challenging requirements toward maximum power output, fuel consumption and emissions. For engine combustion development programs, the injection system has to be able to operate reliable under a variety of operating conditions. Today’s legislations for quieter and cleaner engines require multiple-injection strategies, where it is important to understand the behavior of the system and to measure the effect of one injection on subsequent injections. This study presents a methodology for zero-dimensional modeling of the mass flow rate and the rail pressure of a common rail system, constructed from a set of experimental measurements in engine-like operating conditions, for single- and multiple-injection strategies. The model is based on mathematical expressions and correlations that can simulate the mass flow rate obtained with the Bosch tube experiment, focusing on the shape and the injected mass, using few inputs: rail pressure, back pressure, energizing time and so on. The model target is to satisfy two conditions: lowest computational cost and to reproduce the realistic injected quantity. Also, the influence of the rail pressure level on the start of injection is determined, especially for multiple-injection strategies on the rate shape and injected mass. Good accuracy was obtained in the simulations. The results showed that the model error is within the 5%, which corresponds at the same time to the natural error of the injector and to the accuracy of the measures which had been done. The benefits of the model are that simulations can be performed quickly and easily for any operation points, and, on the other hand, that the model can be used in real-time on the engine test bench for mass estimations when doing additional experiments or calibration activities.
Time-resolved particle image velocimetry and Mie-scattering of fuel droplets at 16 kHz were used to capture simultaneously the temporal evolution of the in-cylinder flow field and spray formation within a direct-injection spark-ignition engine. The engine was operated in stratified combustion mode, with stratified mixtures created by a triple injection late in the compression stroke. The impact of geometric variation of the intake port on in-cylinder flow and flow–spray interactions was investigated, focusing on the second injection, since it provides ignitable mixtures at the time of ignition and is subject to strong fluctuations, rather than the first injection, which is very reproducible. Flow field statistics conditioned on the spray shape of the second injection revealed regions with macroscopic cycle-to-cycle flow variations, which correlated with the spray for all recorded cycles. The flow–spray interaction was traced back to before the first injection using correlation analysis, which revealed that cycle-to-cycle fluctuations of the large-scale tumble vortex had a big impact on the spray shape of the second injection, while the first injection was unaffected. This indicates that the origin of the spray fluctuations may be during intake. Despite significant flow modifications due to the intake port geometry variation, fluctuation levels of the second injection were the same for both geometries, that is, spray fluctuations were not sensitive to the geometric change.
A reduced chemical kinetic mechanism for tri-propylene glycol monomethyl ether has been developed and applied to computational fluid dynamics calculations for predicting combustion and soot formation processes. The reduced tri-propylene glycol monomethyl ether mechanism was combined with a reduced n-hexadecane mechanism and a poly-aromatic hydrocarbon mechanism to investigate the effect of fuel oxygenation on combustion and soot emissions. The final version of the tri-propylene glycol monomethyl ether–n-hexadecane–poly-aromatic hydrocarbon mechanism consists of 144 species and 730 reactions and was validated with experiments in shock tubes as well as in a constant-volume spray combustion vessel from the Engine Combustion Network. The effects of ambient temperature, varying oxygen content in the tested fuels on ignition delay, spray lift-off length and soot formation under diesel-like conditions were analyzed and addressed using multidimensional reacting flow simulations and the reduced mechanism. The results show that the present reduced mechanism gives reliable predictions of the combustion characteristics and soot formation processes. In the constant-volume spray combustion vessel simulations, two important trends were identified. First, increasing the initial temperature in the constant-volume spray combustion vessel shortens the ignition delay and lift-off length and reduces the fuel–air mixing, thereby increasing the soot levels. Second, fuel oxygenation introduces more oxygen into the central region of a fuel jet and reduces residence times of fuel-rich area in active soot-forming regions, thereby reducing soot levels.
Advanced combustion engine concepts, such as the homogeneous charge compression ignition engine, rely on chemical kinetics for their proper operation. Accurate prediction and control of auto ignition time scales are therefore key issues. Real-time prediction and control of ignition using detailed chemical kinetic models is difficult due to the large size of such mechanisms that calls for high computational costs. Ignition models in the form of correlations can overcome these challenges, as long as they are able to predict the time scales that would be obtained using a detailed chemical kinetic model. In this work, a correlation approach based on detailed chemical models is used to simplify the determination of chemical time scales associated with kinetically controlled combustion events. Detailed combustion mechanisms for biodiesel surrogate (methyl decanoate), gasoline surrogates (isooctane/n-heptane/toluene), bio-alcohol (n-octanol) and ethanol/gasoline surrogate blends from the literature are used to generate ignition databases for correlation development, taking into account complex ignition behavior such as pressure-dependent limits of negative temperature coefficient kinetics. We further employ the Livengood–Wu integral method to assess the ability of the resulting correlations to predict ignition in cases where the temperature and pressure are changing prior to ignition. The integral method is compared with results of a single-zone adiabatic homogeneous charge compression ignition engine model simulation using the detailed chemical kinetic model. It is found that the simplified correlations accurately reproduce the predictions of the detailed chemical kinetic models at an insignificant computational cost relative to the detailed simulations. The correlation approach presented here can also be applied to experimental ignition data, as far as these are obtained in a manner that covers a wide range of the relevant parameters with minimal experimental uncertainties. This work enables the incorporation of realistic chemical kinetic effects in the computational analysis and control of kinetically controlled combustion systems.
This study concerns a quantitative analysis of late-cycle soot oxidation in diesel engines that focuses on two-dimensional KL factor images obtained by the two-color method. The spatially integrated KL factor was converted into the in-cylinder soot mass using a new formula of diesel soot emissivity. This methodology was applied to two combustion systems: a heavy-duty optical engine which was tuned for a higher fuel–air mixing capability and a rapid compression and expansion machine which had a lower mixing performance. The in-cylinder soot mass history during the last stage of soot oxidation phase was converted into a normalized soot mass history and was used for comparison with simulated soot mass history. A model calculation of in-cylinder soot mass history which was based on oxidation of a primary soot particle was performed with the surface-specific soot oxidation rate as a parameter. A value of the surface-specific soot oxidation rate was specified from the curve fitting approach between the experimental and simulated in-cylinder soot mass traces. The resultant soot oxidation rates plotted on the Arrhenius diagram were found to lie in domains with different oxidation mechanisms. The reason for the scattered plots was discussed referring to model predictions of soot oxidation in the literature, and it was concluded that the higher oxidation rates could be attributed to well-mixed soot oxidizer structure.
The renewable biodiesel fuel is considered as one of the most promising alternative fuels to compression ignition engines. As there are more than 350 oil-bearing crops with wide variation in their compositional characteristics, it is indispensable to study the combustion characteristics of biodiesel fuels to adopt them as alternative fuels for diesel engines. This article presents a multi-zone phenomenological model for predicting the combustion characteristics of non-edible karanja and jatropha biodiesel fuels in a compression ignition engine. The various thermo-physical properties of the biodiesel fuels needed for the combustion modeling are evaluated based on their methyl ester composition. The model predictions in terms of combustion characteristics of diesel and biodiesel fuels are validated with the experimental results at different engine speed and load conditions. The model predictions are observed to match well with the experimental values within the maximum prediction error of 8.6%. A comparative analysis of combustion characteristics between diesel and biodiesel fuel is also pursued. It is predicted that the peak pressure and spray average peak temperature of karanja and jatropha biodiesel are higher compared to those of diesel fuel which corroborate with the several observations reported in the literature.
Many research studies have shown that low temperature combustion in compression ignition engines has the ability to yield ultra-low NOx and soot emissions while maintaining high thermal efficiency. To achieve low temperature combustion, sufficient mixing time between the fuel and air in a globally dilute environment is required, thereby avoiding fuel-rich regions and reducing peak combustion temperatures, which significantly reduces soot and NOx formation, respectively. It has been demonstrated that achieving low temperature combustion with diesel fuel over a wide range of conditions is difficult because of its properties, namely, low volatility and high chemical reactivity. On the contrary, gasoline has a high volatility and low chemical reactivity, meaning it is easier to achieve the amount of premixing time required prior to autoignition to achieve low temperature combustion. In order to achieve low temperature combustion while meeting other constraints, such as low pressure rise rates and maintaining control over the timing of combustion, in-cylinder fuel stratification has been widely investigated for gasoline low temperature combustion engines. The level of fuel stratification is, in reality, a continuum ranging from fully premixed (i.e. homogeneous charge of fuel and air) to heavily stratified, heterogeneous operation, such as diesel combustion. However, to illustrate the impact of fuel stratification on gasoline compression ignition, the authors have identified three representative operating strategies: partial, moderate, and heavy fuel stratification. Thus, this article provides an overview and perspective of the current research efforts to develop engine operating strategies for achieving gasoline low temperature combustion in a compression ignition engine via fuel stratification. In this study, computational fluid dynamics modeling of the in-cylinder processes during the closed valve portion of the cycle was used to illustrate the opportunities and challenges associated with the various fuel stratification levels.
Many journal bearings exist in an engine. Therefore, the effects of operation and design parameters of a journal bearing on power loss are the important information. In this study, simplified estimation method of bearing friction is proposed by means of a regression equation. Dimensional analysis on friction of a journal bearing is performed for the operation and design parameters using hydrodynamic lubrication under steady load. Based on the dimensional analysis, the effect of the parameters is evaluated and the regression equation is proposed. The regression equation is fitted for two cases; one is based on calculated frictions with hydrodynamic lubrication under steady load and the other is based on experimental frictions obtained using a bearing test apparatus. In the bearing test apparatus, a rotor is supported by a pair of test journal bearings and friction torque of the test journal bearings is measured based on angular deceleration of the rotor. Power loss of journal bearing is numerically calculated with hydrodynamic lubrication under dynamic load of an engine and it is compared with the power losses estimated with the regression equations. Finally, the suitability for the simplified estimation method is discussed.
A novel online wavelet least-squares support machine fuzzy predictive control for engine lambda regulation is presented in this article. The prediction model of the proposed online wavelet least-squares support machine fuzzy predictive control is built and updated with a newly proposed modelling algorithm, namely, online wavelet least-squares support machines. The proposed online wavelet least-squares support machine adopts wavelet function that can inherit the local analysis ability and feature extraction from the wavelet transformation, as well as a novel online incremental and decremental updating procedure that can maintain the built prediction model to be accurate, sparse and updated without losing the generalization by continually adding the latest useful data and pruning out the outdated data. Besides, an advanced fuzzy optimizer is proposed to determine the optimal control signal for the online wavelet least-squares support machine fuzzy predictive control, which is faster than the traditional optimizers. The proposed online wavelet least-squares support machine fuzzy predictive control was implemented on a real performance test car and compared with the latest lambda control techniques based on various modelling algorithms, optimizers, updating procedure and support vector kernel for evaluating the effectiveness. The experimental results show that the proposed online wavelet least-squares support machine fuzzy predictive control is a promising scheme for lambda regulation.
This article is aimed at exploring syngas as an alternative fuel for the modern gas engines. It presents the experimental results on the effect of start of injection on combustion, performance and emissions of a direct-injection spark-ignition engine powered by syngas of H2/CO composition. The engine was operated with wide open throttle at minimum advance to achieve maximum brake torque. Two different start of injections were selected to represent before and after inlet valve closing and the excess air ratio () was set at 2.3. The engine operation at start of injection = 120° before top dead center was found to be best for combustion and performance at speed up to 2100 r/min. At engine speed higher than 2100 r/min, this start of injection does not permit maximum performance due to injection duration limitation. Hence, early injection at start of injection = 180° before top dead center was adopted at higher speed with better combustion and performance. Therefore, best performance of syngas in direct-injection spark-ignition engine could be attained by setting start of injection at 120° before top dead center for lower speeds and at 180° before top dead center for speed greater than 2100 r/min. Even though fast combustion of syngas suggested late injection for better combustion, performance and emissions, its lower calorific value resulted in operational limitations for direct-injection system particularly at higher speeds maintaining air–fuel ratio close to the stoichiometry.
Reactivity-controlled compression ignition is a low-temperature engine combustion strategy that utilizes in-cylinder blending of fuels with different autoignition characteristics to produce low NOx (oxides of nitrogen) and particulate matter emissions while maintaining high thermal efficiency. This study investigates reactivity-controlled compression ignition combustion in a light-duty, multi-cylinder, compression ignition engine over steady-state and transient operating conditions with both petroleum and bio-derived fuels. The engine experiments consisted of in-cylinder fuel blending with port fuel injection of gasoline or E20 and early-cycle, direct injection of ultra-low sulfur diesel or B20. Performance and emissions results were compared at steady-state and over an up-load change between 1 and 4 bar brake mean effective pressure at 1500 r/min. The results under steady-state operation showed that E20 offered reduced hydrocarbon emissions from the lower port fuel injection mass fraction. Port fuel injection mass fraction is defined as the mass fraction of the port fuel injection injected fuel compared to the total fuel injected, as calculated by
Enhancing the predictive quality of engine models, while maintaining an affordable computational cost, is of great importance. In this study, a phenomenological combustion and a tabulated NOx model, focusing on efficient modeling and improvement of computational effort, is presented. The proposed approach employs physical and chemical sub-models for local processes such as injection, spray formation, ignition, combustion, and NOx formation, being based on detailed tabulated chemistry methods. The applied combustion model accounts for the turbulence-controlled as well as the chemistry-controlled combustion. The phenomenological combustion model is first assessed for passenger car application, especially with multiple pilot injections and high exhaust gas recirculation ratios for low-load operating points. The validation results are presented for representative operating conditions from a single-cylinder light-duty diesel engine and over the entire engine map of a heavy-duty diesel engine. In the second part of this study, a novel approach for accurate and very fast modeling of NO formation in combustion engines is proposed. The major focus of this study is on the development of a very fast-running NO mechanism for usage in the next generation of the engine control units. This approach is based on tabulation of a detailed chemical kinetic mechanism and is validated against the detailed chemical reaction mechanism at all engine-relevant conditions with the variation in pressure, temperature, and air–fuel ratio under stationary and ramp-type transient conditions in a perfectly stirred reactor. Using this approach, a very good match to the results from calculations with the detailed chemical mechanism is observed. Finally, the tabulated NOx kinetic model is implemented in the combustion model for in-cylinder NOx prediction and compared with the experimental engine measurement data.
This work investigates the role of the intake generated thermal stratification in the temperature field evolution during the compression stroke using direct numerical simulation. The analysis compares two direct numerical simulations during the compression stroke, from which one is initialized with a homogeneous temperature distribution and the other one with a stratified temperature field resulting from a precursor direct numerical simulation of the intake stroke. All other initial and boundary conditions are identically imposed. The results show that the thermal situation at bottom dead center has nearly no impact on the evolution of the temperature field and the wall heat transfer during compression. Dominating mechanism for the temperature field evolution is found to be the convective transport of cold gases from the boundary layers toward the cylinder center. As a consequence, the wall temperature and the flow field are the main influencing parameters controlling the evolution of the temperature distribution during compression. This finding can assist practical engine experiments, since it points out which mechanisms are promising to affect the temperature field during the compression stroke. In addition, it explains why the stratification of the temperature field during intake by varying the gas temperatures in the two intake channels showed only a minor impact on the thermal situation at top dead center.
The Livengood–Wu correlation has been widely used to predict the state of auto-ignition in internal combustion engines, although its application to two-stage ignition processes remains unresolved. In this study, the original Livengood–Wu integral is extended to such two-stage ignition process and applied to simulations of typical operations within homogeneous charge compression ignition engines. Specifically, based on recent understanding of the global and detailed kinetics of low-temperature chemistry leading to ignition, simplified Arrhenius-based global reaction expressions were developed for both stages of constant-state auto-ignition. It is shown that the original Livengood–Wu integral works well for the first-stage ignition delay, as demonstrated in previous studies. Furthermore, by also accurately describing the cool flame temperature and pressure increment at the end of the first-stage ignition, the second-stage ignition delay can in addition be coupled with the first-stage ignition and predicted satisfactorily with a second integral. This formulation is then applied to extensive homogeneous charge compression ignition engine operation conditions, showing satisfactory predictive capability.
Computational optimizations of dual-fuel reactivity controlled compression ignition combustion and gasoline compression ignition combustion were performed using a novel adaptive dual-fuel injector capable of direct injecting both gasoline and diesel fuel in a single cycle. Optimization used the Engine Research Center KIVA code coupled with a multiobjective genetic algorithm. Model validation was performed by comparing simulation results to conventional diesel, reactivity controlled compression ignition, and gasoline compression ignition combustion, and the validated model was used to develop an optimum reactivity controlled compression ignition–gasoline compression ignition combustion strategy. The reactivity controlled compression ignition optimization results showed that by direct injecting gasoline and diesel fuel, the gasoline quantity can be held at a high percentage across the range of loads considered. In this study, the mode weighted gasoline percentage was 91%. At the lightest load point, direct injecting the gasoline gave optimum results, whereas for the other load points, premixing the gasoline yielded the optimum results. The optimized results were compared with conventional diesel combustion, and it was seen that reactivity controlled compression ignition combustion gives a cycle-averaged improvement of 33% in gross indicated efficiency over conventional diesel combustion. The cycle-averaged NOx and soot emissions were reduced by 95% and 75%, respectively. To demonstrate operation over the entire operating map, an optimization was performed at a high-speed–high-load (16 bar, 2500 r/min) condition. Optimization results showed that a gross indicated efficiency of 46.4% with near zero NOx and soot emissions could be achieved using gasoline compression ignition at this load point.
Water was directly injected into the cylinder with an injection pressure of 5 MPa to investigate its effect on engine performance and emissions in a gasoline engine. The test engine was a 1.6-L naturally aspirated prototype engine consisted of water direct injection and port fuel injection systems. The engine featured a compression ratio of 13.5. Commercial gasoline direct injection injectors were used to inject the water. The water was injected at a fixed timing of –120 crank angle degrees after top dead center. The addition of water showed potential to mitigate the knock occurrence at part-load condition where the knock initially started to occur due to the high compression ratio. It allowed a further advance of spark timing; thus, the brake-specific fuel consumption was improved. The effects of water injection were further investigated under full-load condition within the engine speed range of 1500–3000 r/min. The water effectively reduced the in-cylinder temperature and the exhaust gas temperature; therefore, charge cooling through over-fueling (fuel enrichment) was eliminated with reduced brake-specific fuel consumption. Increase in the injected water mass resulted in further spark advance without the knock occurrence and provided room for further brake-specific fuel consumption reduction. An optimum water mass existed because too much water deteriorated the combustion efficiency, burn duration, and cycle efficiency. The positive effects of water injection were dulled with increased engine speed because the knocking resistance was already high intrinsically with the higher engine speed.
Full-cycle computational fluid dynamics simulations with gasoline chemical kinetics were performed to determine the impact of breathing and fuel injection strategies on thermal and compositional stratification, combustion and emissions during homogeneous charge compression ignition combustion. The simulations examined positive valve overlap and negative valve overlap strategies, along with fueling by port fuel injection and direct injection. The resulting charge mass distributions were analyzed prior to ignition using ignition delay as a reactivity metric. The reactivity stratification arising from differences in the distributions of fuel–oxygen equivalence ratio (
A predicted ignitability index for diesel combustion (the predicted diesel ignitability index) has been established with multiple regression analysis of parameters related to the bond structures in hydrocarbons as explanatory variables and the cetane numbers as a response variable. There were 116 hydrocarbons with known cetane numbers and molecular structures used for the calculations. The numbers of carbon atoms for the seven categories—CM (carbon in a main-chain), CSL (carbon in a side-chain longer than five atoms), C1A (carbon in a single-benzene ring), C2A (carbon in a double-benzene ring), CNA (carbon in a naphtheno-benzene ring), C1N (carbon in a single-saturated six-membered ring), and C2N (carbon in a double-saturated six-membered ring)—were included. The predicted diesel ignitability index was expressed with these seven parameters in the following equation
Diesel engines exhibit highly efficient, environmentally sound performance under good operational control; however, because of the demand of controlling multiple actuators under various environmental conditions, the conventional experimental method for controlling diesel engines has become increasingly difficult. Therefore, diesel combustion models with less calculation loads were ultimately developed with the aim of implementing such model-based controllers for engine control unit in the future. To achieve the on-board application of the diesel combustion model, the in-cylinder state of a single cycle was discretized into several representative phases such as a valve-opening and valve-closing phase, ignition phase, and maximum pressure phase. Temperature and oxygen quantities in residual gas were considered as the state variables of the system because they have a critical effect on combustion and induce cyclic coupling. The model could take account of the effect of actuators in diesel engines, and the states in each phase were calculated by fundamental thermodynamic equations and some empirical equations. The model was validated against experimental results and had a good agreement with in-cylinder pressures and temperatures at each phase. In addition, the calculation times of the model were confirmed to be capable of on-board application. Furthermore, as a demonstrative example and to show the added value of the model, it was used to synthesize controllers to enable multi-input/multi-output control of a diesel engine in simulation.
The Reitz and Diwakar and the KHRT atomization models are widely used for high-pressure diesel-fuel spray. The constants in both models must be calibrated to correctly predict the injection process based on the nozzle geometry, injection conditions, and fuel. Calibration can be significantly time-consuming given the four constants in both models. This paper suggests a strategy to assess the impact of models’ constants on spray tip penetration and mean droplet-diameter predictions on a reference case, with an injection pressure of 700 bar, to characterize the influence of the atomization model’s calibration. The assessment used a design of experiment method (DOE), which demonstrated the important interaction between constants on the results. Obtained calibrations were used for comparing the models’ performances qualitatively and quantitatively by accounting for spray and air-entrainment characteristics. Both models gave similar results, but the KHRT model yielded a better spray shape. Finally, based on DOE results, a method is proposed to modify the model’s constants for higher pressures (900 and 1300 bar).
Heuristic methods have been a successful tool for optimizing engine parameters in both simulation and experimental testing. An improved hybrid method applying both the particle swarm optimization method and genetic algorithm was developed, tested, and compared with a basic particle swarm method for improving engine emissions and performance. A computational comparison between the particle swarm optimization–genetic algorithm hybrid, basic particle swarm optimization, and basic genetic algorithm was done using standard test problems. Computational results indicated improvements in both the efficiency and effectiveness of the present hybrid method. Engine testing was performed under steady-state conditions at 1400 r/min at 4.15 bar brake mean effective pressure. The basic particle swarm optimization and the hybrid particle swarm optimization–genetic algorithm method were applied to the test apparatus and used to locate the optimum neighborhood of the engine operation. A single-objective function representing NOx, particulate matter, hydrocarbon, CO, and fuel consumption was used in this application. The hybrid method was able to locate a narrow window of operation which showed 27% lower NOx emissions and 60% lower particulate matter emissions than the standard particle swarm optimization method. The hybrid method was able to locate the improvements using similar dynamometer time, indicating that the hybrid method is more efficient and more effective. Trends relating combustion characteristics and input parameters were observed and are discussed with regard to future improvements to heuristic methods for optimizing diesel engine performance.
Adapting turbocharger performance maps to a form suitable for dynamic simulations is challenging for the following reasons: (1) the amount of available data is typically limited, (2) data are typically not provided for the entire operating range of the compressor and turbine and (3) the performance data are non-linear. To overcome these challenges, curve fits are typically generated using the performance data individually for each device. The process, however, can take un-economical amounts of effort to implement for a range of compressors and turbines. This article introduces a method to implement non-dimensional performance maps thereby allowing a range of turbochargers to be modeled from the same performance data, reducing the effort required to implement models of different sizes. The non-dimensional maps seek to model the performance of compressor and turbine families in which the geometry of the rotor and housing are similar and allow the turbocharger to be scaled for simulation in much the same way used to design customized sizes of turbochargers. A method to match the non-dimensional compressor map to engine performance targets by selecting the compressor diameter is presented, as well as a method to match the turbine to the selected compressor.
The fuel carbon pathway for the cold-start first cranking cycle in a gasoline direct injection engine is characterized quantitatively. The engine is fired for a single cycle in one cylinder at a specified cranking speed and at a coolant temperature of 20 °C. The fuel carbon is accounted for from measurements of the exhaust carbon (CO2, CO, and hydrocarbon). The remaining carbon is assumed to go into the oil and crankcase. The parameters studied are the amount of injected fuel, the injection timing, the intake pressure, the injection pressure, and the cranking speed. Substantial fuel enrichment is needed to produce stable combustion in the first cycle, with significant residual fuel that goes into preparing the mixture of the second cycle and into the oil and crankcase. The first cycle hydrocarbon emissions as a fraction of the fuel are not sensitive to the fuel enrichment, the manifold absolute pressure, and the injection pressure.
A nano-aerosol monitor, which is a newly developed particle number (PN) measurement system having no volatile particle remover, was evaluated as an alternative to the particle measurement programme (PMP) methodology. The nano-aerosol monitor is a combination of a newly developed sheathless classifier and an electrometer. The nano-aerosol monitor is quite compact compared with other conventional PMP systems and can be used under vibrating conditions. Furthermore, omitting the volatile particle remover results in lower electricity usage. These features will allow the nano-aerosol monitor to be applied to PN portable emission measurement system. It was confirmed that the nano-aerosol monitor classified particles as expected, using 23-nm sodium chloride particles. The exhaust measurements from a gasoline direct injection passenger car and diesel trucks equipped with a diesel particulate filter were conducted using the nano-aerosol monitor and the PMP methodology. The results obtained with the nano-aerosol monitor were similar to those obtained with the PMP methodology for all vehicles and in all test modes, despite the fact that the nano-aerosol monitor does not have a volatile particle remover and hence detects both solid and volatile particles. These results indicated that the nano-aerosol monitor has a performance comparable to the PMP methodology. Simultaneously, the results suggested the possibility that the ratio of volatile particles in the exhausts from modern vehicles is constant. If this feature is applied to various conditions, it will be possible to obtain data comparable to those from the PMP methodology, using a particle measurement system without a volatile particle remover.
An experimental study of partially premixed compression ignition combustion with low octane fuel was conducted on a single-cylinder engine. The effects of the external exhaust gas recirculation and intake boost on this partially premixed compression ignition combustion and emissions were investigated. During the experiments, a trade-off relationship between the NOx and smoke emissions was observed in this partially premixed compression ignition combustion. However, heavy exhaust gas recirculation usage has the potential to decrease NOx and soot emissions simultaneously at the expense of the fuel economy. It is determined that at an increased intake port pressure, the maximum in-cylinder pressure increases, and the ignition timing of the high-temperature combustion is retarded. Also, the peak value of the low-temperature combustion is slightly depressed, the peak value of the high-temperature heat release decreases significantly, and the maximum value of the diffusing burn increases to some extent. Compared to natural aspirated condition, the partially premixed compression ignition combustion with intake boost has the capability of simultaneously reducing NOx and soot emissions to ultra-low levels, that is, the intake boost could be an important strategy for the combustion and emission improvement in this advanced engine combustion mode.
The correlation between the autoignition characteristics of n-butane/di-methyl ether/air mixtures and the knock intensity in a rapid compression and expansion machine was studied. The propagation velocity was obtained from the images of propagating flames at the moment of autoignition by a high-speed camera. The dimensionless parameters, and , were used in the investigation, which are based on the theories of Zeldovich and Bradley. To obtain these dimensionless parameters, the numerical calculation using CHEMKIN-PRO with AramcoMech1.3 was carried out. The smaller gradient of autoignition delay time for n-butane with the higher amounts of di-methyl ether addition caused the higher flame propagation velocity and the resultant higher knock intensity, while significant differences of excitation times for autoignition heat release could not be seen. As a result, the gradient of autoignition delay time was concluded to have a strong influence on the knock intensities.
Fuel efficient thermal management of diesel engine aftertreatment is a significant challenge, particularly during cold start, extended idle, urban driving, and vehicle operation in cold ambient conditions. Aftertreatment systems incorporating NOx-mitigating selective catalytic reduction and diesel oxidation catalysts must reach ~250 °C to be effective. The primary engine-out condition that affects the ability to keep the aftertreatment components hot is the turbine outlet temperature; however, it is a combination of exhaust flow rate and turbine outlet temperature that impact the warm-up of the aftertreatment components via convective heat transfer. This article demonstrates that cylinder deactivation improves exhaust thermal management during both loaded and lightly loaded idle conditions. Coupling cylinder deactivation with flexible valve motions results in additional benefits during lightly loaded idle operation. Specifically, this article illustrates that at loaded idle, valve motion and fuel injection deactivation in three of the six cylinders enables the following: (1) a turbine outlet temperature increases from ~190 °C to 310 °C with only a 2% fuel economy penalty compared to the most efficient six-cylinder operation and (2) a 39% reduction in fuel consumption compared to six-cylinder operation achieving the same ~310 °C turbine out temperature. Similarly, at lightly loaded idle, the combination of valve motion and fuel injection deactivation in three of the six cylinders, intake/exhaust valve throttling, and intake valve closure modulation enables the following: (1) a turbine outlet temperature increases from ~120 °C to 200 °C with no fuel consumption penalty compared to the most efficient six-cylinder operation and (2) turbine outlet temperatures in excess of 250 °C when internal exhaust gas recirculation is also implemented. These variable valve actuation-based strategies also outperform six-cylinder operation for aftertreatment warm-up at all catalyst bed temperatures. These benefits are primarily realized by reducing the air flow through the engine, directly resulting in higher exhaust temperatures and lower pumping penalties compared to conventional six-cylinder operation. The elevated exhaust temperatures offset exhaust flow reductions, increasing exhaust gas-to-catalyst heat transfer rates, resulting in superior aftertreatment thermal management performance.
Following the single-fuel initiative, the US Army is transitioning its power plants from running on various fuels to a single fuel—JP-8. Due to its low vapor pressure, JP-8 could not be used to cold start or run gasoline engines without extensive retrofit. The feasibility of running a low compression ratio, spark ignition engine with direct injection of heated JP-8 was investigated. A preliminary study found that a small piston-type spark ignition engine would not start or run on JP-8 unless JP-8 was heated to certain temperature. JP-8’s thermal decomposition, or coking, was found to be a serious issue that must be addressed if fuel heating method would be used. A flash heater concept was proposed to solve problems of JP-8’s low vapor pressure on running low compression ratio spark ignition engines, as well as JP-8’s coking issue at high temperature. A flash vapor fuel injector was designed and tested to be capable of heating up JP-8 from 26.7 °C to its vaporization temperature of >154.4 °C under one-tenth of a second at the required flow rate. The longest duration heating test (>1 million injections) did not show any coking sign with flash heating. Ignition test results show equivalent or superior ignition characteristics of pre-heated JP-8 (with heater temperature at 260.0 °C or above) provided by the developed flash vapor fuel injector, compared with non-heated aviation gasoline.
There is a strong motivation to decrease the production and release of harmful substances such as oxides of nitrogen (NOX) and particulate matter from internal combustion engines. Simultaneously, there are on-going efforts to increase fuel efficiency to curb usage of natural resources and emission of carbon. In general, improvements in one of these areas come at the cost of the other; however, the results of a previous computational study have indicated that emissions can be decreased while simultaneously increasing efficiency through the application of low heat rejection techniques to low temperature combustion. The goal of this study is to experimentally confirm these findings using a light-duty, multi-cylinder diesel engine. Low temperature combustion is realized through high levels of exhaust gas recirculation and retarded injection timings while different degrees of low heat rejection are achieved by means of higher coolant temperatures which should serve to decrease the temperature gradients across the cylinder walls. By applying low heat rejection techniques to diesel low temperature combustion operation, the undesirable side effects of low temperature combustion such as lower combustion and energy conversion efficiencies were found to be mitigated. Specifically, the emissions of carbon monoxide and unburned hydrocarbons were reduced and the loss in fuel conversion efficiency was also diminished. NOX and smoke (an indicator of particulate matter) emissions did increase but they remained at acceptably low levels and below those of conventional combustion. While the full potential of improvements in low temperature combustion was not explored, these results point to the viability of further research into low heat rejection–low temperature combustion concepts.
The engine lubrication system is a vital element for engine health but causes a parasitic load on the engine which increases the fuel consumption: this load can be reduced by matching the oil flow to lubricating requirements using a variable displacement oil pump. In a first stage, two variable displacement oil pumps were installed on a 2.4-L diesel engine; experiments over the New European Drive cycle showed reductions in fuel consumption of up to 3.4% and up to 5.8% over the urban phase of the cycle. A variable displacement oil pump was subsequently installed on an instrumented engine capturing over 100 metal and fluid temperatures within the engine structure. This showed that reducing oil flows resulted in lower oil temperature by up to 4 °C during cold-start New European Drive cycle but hotter cylinder liner temperatures by up to 6 °C. The higher cylinder wall temperatures caused an increase of 3% in oxides of nitrogen emissions but a reduction of 3%–5% in carbon monoxide and hydrocarbon emissions. Finally, an energy flow analysis showed that the variable displacement oil pump can reduce oil pump energy consumption by 160 kJ (32%) but that this led to a 400-kJ reduction in friction and accessory work. These findings highlight the need for a system-level rather than a component-level approach to engine lubrication design to capture key thermal interactions.
Recently, several promising biomass-derived fuels for diesel engines have been identified, produced, and tested. Diesel engine experiments confirmed very low soot and low nitrogen oxide emissions. With regard to further improvements of the combustion system, it is desirable to complement the diesel engine experiments with numerical simulations. To date, this is hindered by the lack of suitable chemical reaction mechanisms for these novel fuels. Therefore, a surrogate approach is presented here and applied in computational fluid dynamics simulations. Combustion and pollutant formation is simulated using the representative interactive flamelet model. Ignition, combustion, and pollutant formation are described in a consistent manner by inclusion of detailed reaction chemistry. Different mixtures of n-heptane, toluene, ethanol, dimethylether, ethane, and phenol are employed to describe the combustion chemistry of the biofuels. The compositions of the surrogate fuels are compiled according to hydrogen/carbon ratio, oxygen content, cetane rating, and molecular properties of the experimental fuels. Spray, injection, and evaporation properties of the experimental fuels, as obtained from spray vessel experiments, are included in the computational fluid dynamics simulations. By systematic comparison of experimental and numerical results, the surrogate methodology is validated and an improved understanding of the limitations of the current surrogates is achieved. Thus, a methodology for the fast adoption of novel fuels for simulations is proposed that can be used regardless of the availability of specific chemical reaction mechanisms.
The reduction of the legislative emission limits has led to an increased complexity of the engine control unit (ECU) calibration. Integrating feedback of the Particulate Matter (PM) and NOx emissions into the engine management could make fulfilment of legislation easier and reduce the complexity of the necessary calibration process. Due to the fact that production type PM sensors for raw emission feedback will not be available, or will be exceedingly expensive in the near future, in part 1 of this work, a virtual soot sensor (VSS) has been developed. Along with the very good steady state behaviour, the VSS is able to predict PM emissions in transient engine operation with a sufficient precision. This has been approved with different changes in torque demand at constant engine speed. Though an overestimation of the soot occurs during the first cycles of the step in torque demand, the behaviour of the engine out soot was well captured and compared with measurements from a photo-acoustic soot sensor (PASS) and characteristic end values of the representative in-cylinder soot trace (measured by multi-colour pyrometry). The performance of the control structure with integrated VSS is demonstrated on the new European driving cycle and an Urban Dynamometer Driving Schedule. Furthermore, a change in set point value demonstrates the opportunity of changing the raw emission strategy on-line. These results offer the opportunity to expand this cylinder-pressure-based VSS approach to other pollutants (primarily NOx) as well.
A breakup model has been developed and optimized for diesel spray simulation with a large-eddy simulation. Previously, diesel spray analysis has been carried out using a breakup model such as the Kelvin–Helmholtz and Rayleigh–Taylor model. The Kelvin–Helmholtz and Rayleigh–Taylor models are used for high Weber number conditions. However, even when fuel is injected at high pressures, the downstream region of spray corresponds to relatively low Weber number conditions. Hence, a hybrid breakup model that combines the Kelvin–Helmholtz and modified Taylor analogy breakup models (Kelvin–Helmholtz–modified Taylor analogy breakup) has been developed. The Kelvin–Helmholtz and modified Taylor analogy breakup models are used to model the primary and secondary breakup, respectively. The modified Taylor analogy breakup model is more suitable than the Rayleigh–Taylor model to describe secondary breakup. Spray simulations under non-evaporative and evaporative conditions were performed to validate the Kelvin–Helmholtz–modified Taylor analogy breakup model. It is found that the simulation results of Kelvin–Helmholtz–modified Taylor analogy breakup are in good agreement with experimental measurements of non-evaporative and evaporative spray.
A single-cylinder engine was used to assess the sensitivity of emissions, noise and fuel consumption to variation in cetane number when operated in low temperature combustion both with and without the application of compensation strategies. Without compensation, changes in cetane caused greater variability in all of the parameters studied when operated in low temperature combustion as compared to conventional diesel combustion. Correcting combustion phasing was explored by adjusting three different parameters: start of injection timing, burnt gas fraction and intake manifold temperature. None of these methods were able to achieve substantially equivalent combustion and emissions across all fuels evaluated. Large variations in hydrocarbon and particulate emissions were observed when start of injection was adjusted to correct combustion phasing. Adjusting the burnt gas fraction caused large differences in nitrogen oxides and noise, particularly with low cetane fuels. Finally, intake manifold temperatures low enough to reach the target combustion phasing with the higher cetane fuel were unachievable, suggesting that this method would not be viable as a production solution. It was concluded that the most effective way to ensure robust combustion and emissions in low temperature combustion is to reduce ignition delay variability by refining the fuel specification, either through tighter control over the cetane number range or by shifting the fuel specification toward higher cetane number, where ignition delay is less sensitive to changes in cetane rating.
The maldistribution of urea/NH3 at the selective catalytic reduction inlet and the resulting NH3/NOx ratio variation have been reported to have negative effects on the NOx conversion efficiency and NH3 slip in heavy-duty diesel applications. Maldistribution is caused by incomplete mixing and decomposition of the injected urea water solution within the exhaust flow in the aftertreatment system. How NH3 and the NH3/NOx ratio are distributed at the selective catalytic reduction inlet in an engine aftertreatment system and how the resultant maldistribution affects the system performance and the kinetics are presented in this combined experimental and modeling study. The distribution profiles of the selective catalytic reduction inlet NH3 and NH3/NOx ratio were determined through engine experiments. The NH3 maldistribution effects on the selective catalytic reduction performance and kinetics were quantified by using a multi-channel one-dimensional model that takes into consideration the NH3 maldistribution. It was determined that the NH3 maldistribution is a major factor causing the difference in catalyst performance in the engine exhaust as compared to the laboratory reactor environment, and these results explain specific differences in the model calibrations determined from the laboratory reactor and engine data. Improving the selective catalytic reduction inlet NH3 uniformity can improve the NOx conversion efficiency by up to 7% and reduce NH3 slip by 10–20 ppm. A single-channel one-dimensional selective catalytic reduction model calibrated to the engine data that accounts for the NH3 maldistribution phenomena can be applied for the development of on-board diagnostic and control strategies.
Spark-ignition direct-injection engines operating in a stratified, lean-burn regime offer improved engine efficiency; however, seemingly random fluctuations in stratified combustion that result in partial-burn or misfire prevent widespread implementation. Eliminating these poor combustion events requires detailed understanding of engine flow, fuel delivery, and ignition, but knowing the dominant cause is difficult because they occur simultaneously in an engine. This study investigated the variability in fuel–air mixture linked to fuel injection hardware in a near-quiescent pressure vessel at high-temperature conditions representative of late, stratified-charge injection. An eight-hole spark-ignition direct-injection spray was interrogated using high-speed schlieren and Mie-scatter imaging from multiple, simultaneous views to acquire the vapor and liquid envelopes of the spray. The mixture fraction of vaporized sections of the spray was then quantified at a plane between plumes using Rayleigh scattering. Probability contours of the line-of-sight vapor envelope showed little variability between injections, whereas probability contours derived from planar, quantitative mixing measurements exhibit greater amounts of variability for lean-combustion-limit charge. The mixture field between plumes was characterized by multi-hole and end-of-injection dynamics that attract the plumes to each other and toward the injection axis, resulting in a liquid-fuel-droplet-dense merged central jet in the planar measurements. Supplemental long-working distance microscopy imaging showed the existence of fuel droplets far downstream in the region of the planar laser measurements.
Recent work has shown the utility of using simplified models with prescribed burn rates to assess the potential of advanced combustion strategies to increase engine efficiency. However, this approach can be improved by incorporating knock and flammability limits. This work incorporates such limits using a combination of simplified conceptual models that are based on theoretical understanding of knock and flame phenomenon and calibration with experimental results. Using this method, the ideal (unconstrained) and feasible (constrained by knock and flammability) potential of a high efficiency gasoline and E85 engine are compared against a baseline naturally aspirated gasoline engine. Turbocharging, dilution with EGR, and higher compression ratios are used to increase the efficiency potential of the high efficiency gasoline and E85 engines. Results demonstrate the benefit of using this simplified approach in modeling high efficiency engines: the high efficiency gasoline engine is most limited by knock while the E85 engine is limited much less; also increased EGR can be used for the E85 engine due to the higher flame speeds of ethanol. Fuel economy maps are created for each engine/fuel strategy and evaluated in a vehicle model to obtain fuel economy results. Results comparing feasible engines show that peak brake thermal efficiency (BTE) is increased by 11.4% for the high efficiency gasoline engine and 17.8% for the E85 engine, as compared to the baseline gasoline engine. Projected vehicle fuel economy (energy equivalent) improvements are 30.1% for the high efficiency gasoline, and 40.9% for the E85 engine relative to baseline.
Low-speed pre-ignition has become of great concern since it represents a strong limit to any further downsizing of gasoline engines. The increase of pressure and temperature inside the combustion chamber at high loads can indeed lead to a premature auto-ignition of the mixture and to severe engine damages. Several hypotheses have been formulated but there is no consensus to explain and demonstrate the cause of low-speed pre-ignition. This article provides an overview of the mechanisms that might lead to low-speed pre-ignition. A general discussion and a synthesis of the favourable conditions for auto-ignition are first introduced. Fundamentals of auto-ignition in homogeneous gaseous phase are discussed and confronted to concrete experimental observations of low-speed pre-ignition. The real operating conditions of modern gasoline engines complicate the analysis of auto-ignition because spatial and cycle-to-cycle fluctuations are linked to various physicochemical phenomena involved in the mixture preparation process. At the same time, the increase in temperature and pressure with load enhances the mixture reactivity and any perturbation at the end of the compression can unbalance the mixture and lead to uncontrolled low-speed pre-ignition. A classification of these mechanisms into three groups is proposed to categorize pre-ignitions resulting from auto-ignitions in the gaseous phase, or from auto-ignitions resulting from interactions between either a liquid or a solid phase and the gaseous mixture. Finally, the impact of mixture dilution with burned gases, mixture temperature, liquid films and deposits is examined for different engine settings, fuels and charge motions to illustrate each of these three groups.
High boost and direct injection are the main tendency of gasoline engine technology. However, pre-ignition/super-knock tends to occur at low-speed high-load conditions, which is the main obstacle for improving power density and fuel economy. This work distinguished the relationship between super-knock and pre-ignition by experimental investigation and numerical simulation. The experiment was conducted on a turbocharged gasoline direct injection engine with compression ratio of 10. The engine was operated at an engine speed of 1750 r/min and the brake mean effective pressure of 2.0 MPa under stoichiometric conditions. Super-knock is the severe engine knock triggered by pre-ignition. Pre-ignition may lead to super-knock, heavy-knock, slight-knock, and non-knock. Significantly advancing spark timing can only simulate pre-ignition, not super-knock. Although knock intensity tends to increase with earlier pre-ignition timing, higher unburned mixture fraction at start of knock, and higher temperature and pressure of the unburned mixture at start of knock, knock intensity cannot be simply correlated to any of the parameters above. A one-dimensional model is set up to numerically simulate the possible combustion process of the end-gas after pre-ignition. Two distinct end-gas combustion modes are identified depending on the pressure and temperature of the mixture: deflagration and detonation. Hot-spot in the mixture at typical near top dead center pressure and temperature condition can only induce deflagration. Hot-spot in the unburned end-gas mixture at temperature and pressure conditions above ’’deto-curve’’ may induce detonation. The mechanism of deto-knock may be described as hot-spot-triggered pre-ignition followed by hotspot- induced deflagration to detonation.
This article addresses the design and evaluation of virtual NO x sensors for heavy-duty off-road diesel engines based on static polynomial black box modeling. Three approaches, differing in the chosen sets of regressors, are analyzed regarding their NO x prediction capability. As regressor sets only quantities available on standard production-type electronic control units, features extracted from the in-cylinder pressure trace via singular value decomposition extended by the engine speed as well as geometric values from the pressure trace and heat release curves are utilized, respectively. It is shown that while each of the approaches alone has its drawbacks, a systematic combination, especially of the first two methods, results in high-accuracy NO x models while at the same time keeping the number of regressors low.
Low-pressure loop exhaust gas recirculation systems are effective means of simultaneously reducing the NOx emissions and fuel consumption of diesel engines. Further lower emission levels can be achieved by adopting a system that combines low-pressure loop exhaust gas recirculation with a NOx storage and reduction catalyst. However, this combined system has to overcome the issue of combustion fluctuations resulting from changes in the air–fuel ratio due to exhaust gas recirculation from rich operating conditions. The aim of this research was to reduce combustion fluctuations by developing low-pressure loop exhaust gas recirculation control logic. In order to control the combustion fluctuations caused by low-pressure loop exhaust gas recirculation, it is necessary to estimate the recirculation time. First, recirculation delay was investigated, and a model was developed. A good correlation was found between actual measurements and the recirculation delay estimated by this model. Next, the control logic for low-pressure loop exhaust gas recirculation was studied. The recirculation gas under rich operating conditions was detected by an air–fuel ratio sensor to examine a method of controlling the exhaust gas recirculation valve in accordance with the timing for the rich gas to reach the exhaust gas recirculation valve actually. Thus, fluctuations in torque and combustion noise were improved.
A detailed study of the sulfation and desulfation phenomena of a NOx storage reduction catalyst using synthetic gas bench is presented. The experimental observations and results were put together, in order to develop a model, which can simulate the NOx storage reduction catalyst behavior during the sulfation and desulfation conditions. The target is to use the model for simulation of the lean/rich desulfation strategies, in order to comprehend the complicated phenomena and ultimately predict the sulfur speciation at the outlet of the NOx storage reduction catalyst. Sulfation tests in the presence and in the absence of O2 at different temperatures were conducted, in order to identify the different sulfation sites. Desulfation tests with constantly rich conditions and lean/rich oscillation, using H2 as the reducing gas, were performed in order to investigate the reaction pathways of the decomposition of the sulfates. Additionally, some preliminary investigation was performed using CO and decane in the reducing gas mixture, in order to evaluate the NOx storage reduction behavior under more realistic gas composition for diesel exhaust gas. The important role of the accurate prediction of sulfur axial distribution at the desulfation onset was identified. It was found that cerium oxides play a critical role in the sulfation process by increasing the sulfur capacity of the NOx storage reduction catalyst. Finally, it was clarified that the oxygen stored on cerium oxides strongly affects the selectivity of the desulfation products by oxidizing H2S and controlling H2 availability.
This article presents a combination of theoretical and experimental investigations for determining the main heat fluxes within a turbocharger. These investigations consider several engine speeds and loads as well as different methods of conduction, convection, and radiation heat transfer on the turbocharger. A one-dimensional heat transfer model of the turbocharger has been developed in combination with simulation of a turbocharged engine that includes the heat transfer of the turbocharger. Both the heat transfer model and the simulation were validated against experimental measurements. Various methods were compared for calculating heat transfer from the external surfaces of the turbocharger, and one new method was suggested. The effects of different heat transfer conditions on the heat fluxes of the turbocharger were studied using experimental techniques. The different heat transfer conditions on the turbocharger created dissimilar temperature gradients across the turbocharger. The results show that changing the convection heat transfer condition around the turbocharger affects the heat fluxes more noticeably than changing the radiation and conduction heat transfer conditions. Moreover, the internal heat transfers from the turbine to the bearing housing and from the bearing housing to the compressor are significant, but there is an order of magnitude difference between these heat transfer rates.
More and more stringent emission regulations and the desire to reduce fuel consumption lead to an increasing demand for efficient and reliable modelling tools in the automotive industry. When conventional physical modelling is not possible due to the lack of precise, formal knowledge about the system, black-box- and grey-box-oriented nonlinear system identification procedures are a widely used concept to create models based on measured input and output data of the process. In this context, local model networks are an established approach for nonlinear dynamic system identification as they provide not only accurate but also interpretable models and therefore allow a better understanding of the true system than pure black-box models. As a consequence, local model networks provide a basis for the development of systematic approaches to stability analysis and nonlinear controller design. In this article, local model network–based dynamic NOx emission modelling is presented. A robust and efficient local model network training algorithm is described, and the proposed concepts are validated using real measurement data. An important advantage of the architecture of local model networks is their good interpretability which is an important advantage for the design of controllers or observers. Additionally, stability analysis of both the nonlinear open- and closed-loop system is possible based on Lyapunov stability theory.
In this article, a new in-cylinder turbulence modeling approach aims at the improvement of quasi-dimensional simulations for modern spark ignition engines with fully variable valvetrains. Within the derived quasi-dimensional turbulence model, the turbulent production term can physically react on a change of engine operation (e.g. intake valve lift, intake valve timing, engine speed and boost pressure). Moreover, the approach offers access to detailed charge motion quantities for the first time in quasi-dimensional calculations. Hence, it is able to satisfy qualitative and quantitative turbulence descriptions within the entire operating range of the engine.
Model-based calibration of steady-state engine operation is commonly performed with highly parameterized empirical models that are accurate but not very robust, particularly when predicting highly nonlinear responses such as diesel smoke emissions. To address this problem, and to boost the accuracy of more robust non-parametric methods to the same level, GT-Power was used to transform the empirical model input space into multiple input spaces that simplified the input-output relationship and improved the accuracy and robustness of smoke predictions made by three commonly used empirical modeling methods: Multivariate Regression, Neural Networks and the k-Nearest Neighbor method. The availability of multiple input spaces allowed the development of two committee techniques: a ‘Simple Committee’ technique that used averaged predictions from a set of 10 pre-selected input spaces chosen by the training data and the "Minimum Variance Committee" technique where the input spaces for each prediction were chosen on the basis of disagreement between the three modeling methods. This latter technique equalized the performance of the three modeling methods. The successively increasing improvements resulting from the use of a single best transformed input space (Best Combination Technique), Simple Committee Technique and Minimum Variance Committee Technique were verified with hypothesis testing. The transformed input spaces were also shown to improve outlier detection and to improve k-Nearest Neighbor performance when predicting dynamic emissions with steady-state training data. An unexpected finding was that the benefits of input space transformation were unaffected by changes in the hardware or the calibration of the underlying GT-Power model.
In residual-effected homogeneous charge compression ignition, substantial exhaust-gas-recirculation is applied to limit the high rate of pressure rise from rapid combustion process. However, significant dilution, especially at low load, may lead to incomplete combustion and unstable operation. To improve the low-load-limit behavior, direct-fuel-injection during negative valve overlap was suggested by many researchers. Due to the high temperature and pressure conditions during negative valve overlap, the injected fuel experiences chemical reactions with trapped exhaust, referred to as recompression reaction. In this study, the possibility of the low-load-limit extension using this injection strategy during negative valve overlap is demonstrated from our previous experiments using isooctane fuel, and simulation with cycle-coupled, zero-dimensional homogeneous charge compression ignition engine model is conducted to understand the effects of recompression reaction. There are three major conclusions from our simulation. First, thermal consequence of the recompression reaction depends on the leftover oxygen in the trapped exhaust gas. Two equivalence ratios, 0.85 and 0.95, are tested in our simulation, and the overall recompression reaction is net-exothermic at 0.85, while net-endothermic at 0.95. Both conditions show significant fuel pyrolysis and reforming reactions during negative valve overlap, which are mostly endothermic, but sufficient oxidation reaction at 0.85 overcompensates for the endothermicity, leading to net-exothermicity in the end. Second, during the main compression stroke, the resultant fuel mixture from recompression reaction has overall higher specific heat ratio than the fresh fuel mixture in comparable operating conditions, which achieves higher mixture temperature rise at a given compression ratio and reaches ignition temperature earlier. Finally, in order to understand the chemical effect of the recompression reaction, ignition delays of the fuel mixture with or without recompression reaction are compared. The results show that the former has overall shorter ignition delay than the latter, which demonstrates overall improved ignitability of the fuel mixture by recompression reaction.
Diesel engine combustion and emission formation are highly nonlinear and thus create a challenge related to engine diagnostics and engine control with emission feedback. This article describes the development of neuro-fuzzy models for prediction of transient NOX and soot emission from a diesel engine. The modeling techniques are motivated by the idea of divide and conquer the input–output space. The complex problem is divided into multiple simpler subproblems, which are then identified using simpler class of models. This article explores two different choices of local models, specifically polynomial and neural networks. The modeling technique is augmented with input relevance algorithm to select the most relevant input regressors. Two algorithms, namely, orthogonal least square and automatic relevance determination, are introduced. The models are data driven, and an advanced experimental setup incorporating a medium duty diesel engine and fast emission analyzers for soot and NOX is used to generate training data. The choice of local models and input relevance algorithm is validated with instantaneous emission recorded during transient schedules different from those used in development. High prediction accuracy, both qualitatively and quantitatively, is demonstrated with low computational cost.
The integral method proposed by Livengood and Wu has been traditionally used to predict the occurrence of knock on spark-ignition engines. Due to its simplicity and low computational demand, this is a method of great interest for the prediction of another autoignition phenomenon, such as the onset of combustion in compression ignition or homogeneous charged compression ignition engines. However, the simplicity of the method is a consequence of the restrictive assumptions considered during its development, which may limit the applicability of the equation. In this study, the validity of the correlation proposed by Livengood and Wu has been evaluated at different initial operative conditions under pure homogeneous charged compression ignition combustion mode for fuels with practical interest (hydrogen, methane, ethanol and n-heptane). The integral method has shown very good prediction capability for the fuels, which do not present two-stage heat release (hydrogen, methane and ethanol) except in those cases when the onset of combustion is very delayed. When cool flames appear (as in the case of n-heptane), the integral method overpredicts the autoignition times since it does not consider the first stage of heat release. In these cases, the prediction of the integral method may be improved if the whole combustion process is considered as two individual processes. This approach shows fairly good prediction capacity although it is unpractical since the simulation of the second-stage combustion requires the previous calculation of the composition of the mixture and the temperature increase at the end of the first stage. Finally, two alternatives to the original integral method are tested which keep its simplicity and universality while taking into account both first and second heat release, one of them showing better results than the original Livengood and Wu equation.
Due to their ability to simultaneously reduce fuel consumption and NOx emissions, controlled autoignition or homogeneous charge compression ignition combustion processes have been extensively researched over the last decade and adopted on prototype gasoline engines. Despite the fact that they were initially achieved on conventional two-stroke ported gasoline engines, there have been much fewer studies carried out on controlled autoignition combustion in two-stroke engines because of the inherent problems associated with the conventional two-stroke engine with intake and exhaust ports. In the meantime, engine downsizing has been actively researched and developed as an effective means to improve the vehicle’s fuel economy by operating the engine at higher load and more efficient regions and by reducing the number of cylinders. But the aggressive engine downsizing of the current four-stroke gasoline engine is limited by the knocking combustion and high peak cylinder pressure. As an alternative approach to engine downsizing, the boosted two-stroke cycle in a poppet valve engine is proposed and researched. It has been shown that the controlled autoignition combustion in the two-stroke cycle could be readily achieved and it led to significant reduction in carbon monoxide and unburned hydrocarbon emissions than the spark ignition combustion. In this study, extensive engine experiments have been performed to determine the optimum boosting for minimum fuel consumption in a single-cylinder gasoline direct injection camless engine operating in the two-stroke cycle. In order to minimise the air short-circuiting rate, the intake and exhaust valve timings were adjusted. Lean boost was applied to the engine operation, which was found to extend the range of controlled autoignition combustion, result in higher combustion and thermal efficiencies and significantly lower carbon monoxide and hydrocarbon emissions. By means of the cycle-resolved in-cylinder measurements and heat release analysis, the improvement in combustion and thermal efficiencies was attributed to the improved in-cylinder mixture, optimised autoignition and combustion phases.
The performance and emissions of modern automotive diesel engines are highly dependent on the application of pilot injection technology. This technology also appears well suited for application to low-temperature combustion strategies. In this study, the first results of a new quantitative planar laser-induced fluorescence equivalence ratio measurement technique of pilot injections inside an optically accessible diesel engine are presented using 1-methylnaphthalene as a tracer in a mixture of the diesel primary reference fuels, n-hexadecane (cetane) and 2,2,4,4,6,8,8-heptamethylnonane (iso-cetane). This combination overcomes the shortcomings of mismatched fuel volatility and density associated with commonly used toluene/n-heptane/iso-octane planar laser-induced fluorescence techniques. A tracer characterization in a flow cell and a calibration in the internal combustion engine are performed. The internal combustion engine measurements illustrate the mixture formation process for a pilot injection. Even at low injection mass of 3 mg, a strong penetration of the pilot is observed; fuel hits the piston bowl wall and is redirected upward to the cylinder head. Small amounts of fuel are also found to have penetrated into the bottom of the piston bowl. At top dead center, the pilot injection is still not completely homogeneously distributed in the piston bowl, and local equivalence ratios of > 1 are found in the bowl.
Aiming at direct ignition-timing control of homogeneous charge compression ignition in low-load conditions, pulsed flame jet, which is the jet of burning gas issuing from a small cavity facing a combustion chamber, was utilized. The characteristics of homogeneous charge compression ignition combustion initiated by pulsed flame jet were investigated using rapid compression expansion machine, which realizes single compression and expansion strokes and thus the measurement of indicated work. Higher indicated mean effective pressure was obtained using pulsed flame jet without increasing NOx emission drastically, and it was shown that pulsed flame jet has potential to control the onset of homogeneous charge compression ignition combustion appropriately. The validity of pulsed flame jet utilization was shown through pressure and NOx measurements.
The piston ring/cylinder liner conjunction can experience various regimes of lubrication during piston strokes inside the engine cylinder. In the current engines, the nature of lubrication usually remains hydrodynamic at mid-stroke, while a mixed regime of lubrication may be experienced at and near reversals. The direct contact between the tips of some of the asperities of opposing surfaces leads to mixed (partial) regime of lubrication. A model proposed by Greenwood and Tripp can be used to predict asperity-level contribution to the total piston friction. At the same time, Reynolds equation can be employed to predict the portion of load carried by the lubricant trapped between the asperities. Friction between the asperity tips is usually proportional to the load that they support, stated in terms of a proportionality factor, that is, coefficient of friction. The surfaces are usually furnished with hard wear-resistant coatings and in parts by solid lubricants. Both the piston rings and cylinder liner surfaces are usually coated. These coatings change the friction characteristics of the counterfaces because of their surface topography as well as material mechanical properties. Atomic force microscope is used to obtain surface topographical parameters in contact tapping mode. The corresponding surface topographical parameters are obtained from representative regional areas of the contacting solid surfaces, using a Talysurf. The combination of topography and coating characteristics are used to develop the necessary parameters for a boundary friction model. A numerical model of the top compression ring to cylinder liner is developed based on mixed-hydrodynamic regime of lubrication. The results for friction and the effect of coating on the power loss and wear of the conjunction are discussed in this article.
An experimental study was conducted to investigate the effect of engine operating parameters on exhaust particulate size–number distribution in a homogeneous charge compression ignition engine fueled with gasoline and n-butanol. In this investigation, portfuel injection was done for preparing homogeneous charge, and intake air preheating was used for auto-ignition of the charge. Engine exhaust particle sizer was used for measuring size–number distribution of particulate matter emitted from the homogeneous charge compression ignition engine. Experiments were conducted at different engine speeds by varying intake air temperature and air–fuel ratio of the charge. Effect of engine operating parameters on particulate size–number distribution, size–surface area distribution, and total particulate number concentration was investigated. Most significant particle numbers were in the range of 6–150 nm mobility diameter for all test conditions. n-Butanol showed relatively higher peak number concentration and lower mobility diameter corresponding to the peak concentrations as compared to baseline gasoline. On increasing intake air temperature, mobility diameters corresponding to peak number concentration of particles moved towards lower mobility diameters. Count mean diameter of particles was in the range of 35–80 nm and 20–65 nm for gasoline and n-butanol, respectively, for all test conditions in homogeneous charge compression ignition operating range.
A comparison of large eddy simulation and Reynolds-averaged Navier–Stokes combustion models was performed using simulations of a diesel–methanol dual-fuel engine. The two models share the assumption that the reaction rate for each species is determined by the chemical kinetic processes as well as the relative magnitude of mixing and reaction effects, which are characterized by a kinetic timescale and a turbulence timescale. The main difference in the two models lies in their formulations of the turbulence timescale. The turbulence timescale in the Reynolds-averaged Navier–Stokes model is proportional to the eddy turnover time, while the turbulence timescale in the large eddy simulation model is the time needed for the mixture to reach perfectly mixed conditions based on the scalar dissipation rate. The models were tested using data from experiments on a modified heavy-duty diesel engine. Test cases using low methanol ratios, high methanol ratios and different initial temperatures were examined. In general, the ignition delay increases as the methanol ratio increases. In the models, this was found to occur due to the chemical interaction of the methanol and the diesel fuel. In addition, methanol auto-ignition occurs when the initial temperature is over 425 K. The large eddy simulation model and Reynolds-averaged Navier–Stokes model give similar predictions for the low methanol ratio cases. The large eddy simulation model shows improved capability to predict the high methanol ratio cases and methanol auto-ignition cases, represented by good agreement with experimental results.
The direct-injection-premixed charge compression ignition–based combustion process with a high exhaust gas recirculation ratio and early injection timing is simulated using a Reynolds-averaged Navier–Stokes–based commercial computational fluid dynamics code with a nonhomogeneous mixture auto-ignition combustion model. In addition, the formation processes of nitrogen oxide (NO), carbon monoxide (CO), and unburnt hydrocarbon are calculated. The calculation results are compared with the experimental data. According to the calculation results, the formation processes of NO, CO, and hydrocarbon are discussed in relation to the spray development and ignition point. Furthermore, the combustion process of two-stage injection is also calculated. The result shows that the combustion process is described well by this model except in the case where auto-ignition occurs in the squish area. Additionally, the relationship between the combustion process and the mixture distribution is clarified. The main origins of the unburnt hydrocarbon and CO emissions are located in the center region of the combustion chamber, where the mixture becomes excessively lean and low in temperature.
A diode laser–based sensor system, utilizing absorption spectroscopy, has been developed to provide high-speed (5 kHz) simultaneous measurements of temperature and water vapor concentration in the intake manifold of a diesel engine. A fiber-coupled 1.38 µm diode laser was used to probe absorption transitions of water vapor for the high-speed gas temperature and water vapor concentration measurements. Water vapor readily absorbs in the near-infrared region, and distributed feedback diode lasers as well as optics for near-infrared region are readily available because of their use in telecommunications. Fresh charge and combustion products are the only sources of water vapor in an engine’s gas exchange path; therefore, water vapor concentration at various locations in the intake manifold is a useful measure of the recirculated exhaust gas distribution in the intake manifold. Measurements were performed on a six-cylinder Cummins diesel engine using compact fiber optic–coupled connectors. The chosen water vapor absorption transitions provided good absorption strength even without exhaust gas recirculation and water vapor concentration of as low as 0.7 vol.% could be measured with a signal-to-noise ratio of ~35 leading to very good spectral fits. The sensor output was within 2% of the thermocouple readings and within 10% of the water vapor concentration derived from mean CO2 analyzer measurements for steady-state engine operation. For transient engine operation, the time response of the diode laser sensor was shown to be vastly superior to that of the installed thermocouple and the gas analyzer system.
There exist two fundamental tools for modeling and simulating wave action of internal combustion engines. The first is a nonlinear time domain solution of the Euler equations using a space–time meshing. The second is a frequency domain solution of the linear wave equations. These two methods exist with a wide range of complexity and sophistication. Hybrid coupled methods also exist; they attempt to bridge the gap between the two techniques while maintaining the overall goal of engine simulation in mind. This work deals with the frequency characterization of a complex intake element, the charge air cooler. First, a transfer matrix for a simple tube is defined and measured. Two identical versions of the previous tube serve for identifying the transfer matrix of the charge air cooler directly on an operating four-cylinder turbocharged engine. Once the transfer matrix is measured, it is coupled to GT-Power as four transfer functions coded into Simulink. The final validation comprises two tubes in GT-Power with measured boundary conditions of pressure and mass flow with the Simulink model in between. Results are presented in the time and frequency domains with future objectives and perspectives as well.
Low-temperature combustion has shown the potential to provide solutions for future clean and efficient powertrain systems. Traditional approaches using the first law of thermodynamics have been established for describing energy flows within engine systems and comparing losses between low-temperature and traditional combustion modes. An augmented approach, using the second law of thermodynamics, can be utilized to gain insight into the exergy flows within the system and thus identify areas of irreversibilities and inefficiencies. The present article aims at introducing the required framework for the second law analysis of low-temperature combustion concepts and demonstrating its application to boosted homogeneous charge compression ignition engines. The framework consists of a combination of the first law and second law expressions combined with the University of Michigan homogeneous charge compression ignition combustion model and was applied on a modeled light-duty four-cylinder boosted homogeneous charge compression ignition engine. It was found that combustion irreversibilities in homogeneous charge compression ignition were ~25% more than traditional spark ignition or diesel engines and increased with dilution. On the other hand, exergy transfer to the walls was reduced with low-temperature combustion. It was also found that the combination of negative valve overlap and turbocharging is not beneficial for high-load operation. Exergy analysis of the exhaust system revealed that turbine useful work was lower than 50% of exergy at the exhaust ports and indicated that boosting performance may be improved by manipulating exergy transfer in the exhaust manifold. Results from the present study are focused on boosted homogeneous charge compression ignition, but the conclusions reached are applicable to other low-temperature combustion concepts as well.
This article investigates the possibility of utilizing the standard knock sensor for an in situ injector calibration. The goal is to estimate the actual injection duration by means of the structure-borne sound emission during the injection process. Since the sound signals are highly nonstationary and contain various transients, a time–frequency analysis is applied. Based on the findings of the signal analysis, a method is presented for detecting the beginning and the end of injection by applying the theory of the change-point problem.
The piston of a heavy-duty single-cylinder research engine was instrumented with 11 fast-response surface thermocouples, and a commercial wireless telemetry system was used to transmit the signals from the moving piston. The raw thermocouple data were processed using an inverse heat conduction method that included Tikhonov regularization to recover transient heat flux. By applying symmetry, the data were compiled to provide time-resolved spatial maps of the piston heat flux and surface temperature. A detailed comparison was made between conventional diesel combustion and reactivity-controlled compression ignition combustion operations at matched conditions of load, speed, boost pressure, and combustion phasing. The integrated piston heat transfer was found to be 24% lower, and the mean surface temperature was 25 °C lower for reactivity-controlled compression ignition operation as compared to conventional diesel combustion, in spite of the higher peak heat release rate. Lower integrated piston heat transfer for reactivity-controlled compression ignition was found over all the operating conditions tested. The results showed that increasing speed decreased the integrated heat transfer for conventional diesel combustion and reactivity-controlled compression ignition. The effect of the start of injection timing was found to strongly influence conventional diesel combustion heat flux, but had a negligible effect on reactivity-controlled compression ignition heat flux, even in the limit of near top dead center high-reactivity fuel injection timings. These results suggest that the role of the high-reactivity fuel injection does not significantly affect the thermal environment even though it is important for controlling the ignition timing and heat release rate shape. The integrated heat transfer and the dynamic surface heat flux were found to be insensitive to changes in boost pressure for both conventional diesel combustion and reactivity-controlled compression ignition. However, for reactivity-controlled compression ignition, the mean surface temperature increased with changes in boost suggesting that equivalence ratio affects steady-state heat transfer.
This article discusses some specific data-driven model structures suitable for prediction of NOx and soot emissions from a diesel engine. The model structures can be described as local linear regression models where the regression parameters are defined by two-dimensional lookup tables. It is highlighted that this structure can be interpreted as a B-spline function. Using the model structure, models are derived from measured engine data. The smoothness of the derived models is controlled by using an additional regularization term, and the globally optimal model parameters can be found by solving a linear least squares problem. Experimental data from a five-cylinder Volvo passenger car diesel engine is used to derive NOx and soot models, using a leave-one-out cross-validation strategy to determine the optimal degree of regularization. The model for NOx emissions predicts the NOx mass flow with an average relative error of 5.1% and the model for soot emissions predicts the soot mass flow with an average relative error of 29% for the measurement data used in this study. The behavior of the models for different engine management system settings regarding boost pressure, amount of exhaust gas recirculation, and injection timing has been studied. The models react to the different engine management system settings in an expected way, making them suitable for optimization of engine management system settings. Finally, the model performance dependence on the selected model complexity and on the number of measurement data points used to derive the models has been studied.
This article presents a diagnostic technique in which nonintrusive measurements are used with the aim of indirect characterization of the combustion process of an internal combustion diesel engine. The developed technique is based on the vibration signal coming from a mono-axial accelerometer placed in a selected location of the engine block. Such a location is able to guarantee high sensitivity to vibration caused by forces directly linked to the combustion process and low sensitivity to all the other excitation sources. The technique is applied to the signals acquired during two series of experimental tests, carried out on the same kind of engine (multi-cylinder diesel engine, equipped with common rail injection system), in two separate engine test facilities in order to test the engine stand-alone and the engine dressed up with the integrated automatic transmission, aimed at reproducing its real operation condition (it is mainly employed in mini-car sector application). The obtained results suggest the potential applicability of the technique both in the laboratory, during the tuning between the injection parameter settings and the engine, and in the regular running condition of the engine for combustion process diagnosis.
The reduction of the emission limits has lead to an increased complexity of the ECU calibration process and to the need for expensive aftertreatment methods in order to fulfil the legislated limits. Integrating feedback of the Particulate Matter (PM) and NO x emissions into the engine management could make fulfilment of legislation easier and reduce the complexity of the necessary calibration process. Since production type PM sensors for raw emission feedback are not available, a virtual soot sensor (VSS) has been developed. The VSS is a mean value soot model which provides predictions for the PM in real-time (cycle resolved). Its inputs are ECU variables and characteristic values of the heat release rate which are obtained on-line from in-cylinder pressure measurement. The structure of the VSS has been derived from optical kL-measurement data, i.e. from representative, crank angle resolved evolutions of the in-cylinder PM (3-color pyrometry). The model is structured into three consecutive phases which represent the in-cylinder PM evolution and is calibrated with measurements of the exhaust PM concentration of a standard engine operating map only. The three phases correspond to an initial phase where formation of PM dominates, a phase when formation and oxidation are roughly in balance, and a phase during which oxidation dominates. For steady state experiments, the VSS shows an excellent correlation with the exhaust gas soot (respectively elementary carbon) concentration that has been measured with a photo-acoustic soot sensor (PASS). In addition a reasonable ratio between soot formation and soot oxidation is reproduced.
Soot dynamics during diesel combustion is a complex process and consists mainly of the competing mechanisms soot formation and soot oxidation. For future crank angle–based closed loop combustion control design it is not sufficient to focus only on the soot measured with standard sensors in the tailpipe, which is similar to the soot amount in the combustion chamber at exhaust valve opening, but to understand how soot is formed and oxidized during combustion. Hence, this article focuses on analyzing soot dynamics inside the combustion chamber with an optical sensor and how the analyzed effects can be captured with crank angle–based simulation models. An optical sensor was installed on the engine to measure the actual soot concentration and to analyze it together with the results of a thermodynamic engine process calculation based on the measured cylinder pressure. Based on the measured results, two observed effects are analyzed in detail and hypotheses to explain the behavior are sketched. First, it is shown that the soot formation after start of combustion must be subdivided into three phases. The second hypothesis shows that the impact of increased swirl level is mainly present during the soot oxidation phase and that it almost has no impact during the formation phase. As a main innovation, data-based methods were used to underline the formulated hypotheses. Additionally, the crank angle–based soot model equation structure according to Hiroyasu was extended with suitable inputs to capture these effects. Locally, the extended structure is able to catch the effects well with only one set of parameters.
Previous study shows that the constant volume sampler incorrectly measures some of the exhaust gas when testing a plug-in hybrid electric vehicle in the cold start condition when comparing the CO2 results from constant volume sampler and fuel flow meter. The main reason is likely associated with the exhaust left in the vehicle tailpipe and constant volume sampler sampling line. Other factors, such as fuel line expansion and water condensation in the exhaust system, are also considered to have contributions. This article evaluates these issues quantitatively by testing a Toyota Prius hybrid electric vehicle on the industry standard constant volume sampler system combined with both a fuel flow meter measurement and an electronic control unit record for fuel consumption. Cold start test cycles and test cycles with a system pre-purge event show that the constant volume sampler has a significant delay in measuring the exhaust, and the estimated exhaust losses for the test car are 15 g CO2. Tests with a purge event at the end of the driving cycle show that there are approximately 7 g of CO2 trapped in the exhaust system and the constant volume sampler sampling line, and the possible reasons for the discrepancy of the above two points (15 and 7 g) are evaluated. The expansion and air bubble influence the fuel flow meter, and the impact of water condensation on CO2 and CO appears to be negligible.
In this article, a novel identification framework is proposed to capture a class of nonlinear dynamic relationships that link several input signals (and their past values) to a single output. The class of relationships is the one in which the single output to be identified may be any monotonic nonlinear function of a linear regressor that may be built up with the input signals and their past values. This obviously recalls the known Wiener identification structure with differences that are underlined in the article. The whole framework is validated using the difficult problem of deriving real data–based dynamic model of emissions (including NOx and particulate matter) of a diesel engine. The compelling feature of the proposed approach lies in the fact that the underlying optimization problem to be solved is a constrained quadratic programming problem and this, despite the nonlinear character of the identified relationship.
Thin-film thermocouples were used to measure the instantaneous temperature at 100 points on all the combustion chamber wall surfaces in a naturally aspirated direct-injection diesel engine. Instantaneous heat flux at each measured point was also obtained through heat transfer analysis with the measured instantaneous wall surface temperature applied as a boundary condition. In addition, the instantaneous mass-averaged gas temperature in the combustion chamber was calculated through the equation of state of an ideal gas. As a result, the local and overall heat transfer coefficients were evaluated using the corresponding wall surface temperatures and heat fluxes. The overall heat transfer coefficients thus obtained were compared with those calculated with Eichelberg’s and Woschni’s empirical equations for five ignition timings and three engine speeds. As a result, it was revealed that an overall average heat transfer coefficient obtained through the authors’ experiments has characteristics different from those of the heat transfer coefficients calculated from the empirical equations proposed by Eichelberg and Woschni.
Soot, mono-nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbon emissions are investigated as functions of fuel oxygen and engine load in a single-cylinder diesel engine operated in a premixed low-temperature combustion mode by isolating fuel oxygen from other fuel properties and removing or minimizing variations in engine combustion parameters. It is found that at high loads (high global equivalence ratios), soot is reduced as fuel oxygen increases. Over the load range tested, CO and hydrocarbon emissions are not affected by fuel oxygen fraction. NOx emission increases as load is decreased because intake composition changes. NOx emission is not directly affected by fuel oxygen. The difference in NOx emission among the different oxygen fuels is indirectly caused by different exhaust composition affecting intake composition. Combustion efficiency and indicated thermal efficiency are similar for the test fuels with different oxygen fractions.
A gasoline direct injection engine typically operates in multiple fuel preparation modes. In general, at higher loads, a homogeneous mixture is favored, whereas a stratified mixture is preferred at part- and low-load conditions. This is usually achieved by altering the injection parameters and injection strategy with respect to load and speed and through appropriate cylinder and piston geometries. In this article, a new injector concept has been proposed to aid attaining multiple modes in a gasoline direct injection engine. This is achieved by shaping the spray structure to suit the required mixture preparation. Detailed simulations are performed to assess the mixing process in a gasoline direct injection engine using the new injector. The charge preparation at the onset of ignition is studied for different injection modes of the same injector. The results indicate a significant improvement in the mixing process for different modes of operation.
A detailed understanding of the air–fuel mixing process in gasoline direct injection engines is necessary to avoid soot formation that might result from charge inhomogeneities or liquid fuel impingement on the cylinder walls. Within this context, the use of multidimensional models might be helpful to better understand how spray evolution in cylinder charge motions and combustion chamber design affects the mixture quality at spark-timing. In this work, the authors developed and applied a computational fluid dynamics methodology to simulate gas exchange and air–fuel mixture formation in gasoline direct injection engines. To this end, a suitable set of spray submodels was implemented into an open-source code to properly describe the evolution of gasoline jets emerging from multihole atomizers. Furthermore, the complete liquid film dynamics was also considered. For a proper assessment of the approach, a gasoline direct injection engine running at full load was simulated and effects of spray targeting and engine speed were studied. A detailed postprocessing of the computed data of liquid film mass, homogeneity index and equivalence ratio distributions was performed and correlated with experimental data of particulate emissions. Satisfactory results were achieved, proving the effectiveness of the proposed methodology in predicting the effects of injection system and operating conditions on soot formation.
For the auto-ignition phenomena such as homogeneous charge compression ignition combustion and knocking of spark ignition engines, it is thought that combustion products in burned gas affect ignition timing. However, those compounds are not clarified because it is difficult to separate compounds by using one-dimensional gas chromatography which is the most common method for gas analysis. In this study, a comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometer was employed to analyze combustion products in exhaust gas emitted by spark ignition combustion. In the exhaust gas analysis of spark ignition combustion which used regular gasoline and 10W-30 lubricating oil, a lot of mass spectrometer signals which include many compounds produced by lubricating oil were detected. In addition to the influence of the oil, the large number of detected substances about fuel and exhaust gas makes analysis of relation between reactants and products difficult. Therefore, n-heptane as fuel and poly-α-olefin as lubricating oil were applied for engine experiments. By these changes, the number of detected substances was decreased and influence of lubricating oil became weak. From the analysis of each detected substance, it was suggested that nitrogen compounds, aromatic compounds and cyclic compounds existed in exhaust gas of n-heptane spark ignition combustion. In addition, existence of many types of oxygen-containing hydrocarbons was suggested. From the analysis focusing on linear and branched oxygen-containing hydrocarbons with seven carbon atoms, it was suggested that many types of oxygen-containing hydrocarbons which were not considered in detailed reaction model existed in exhaust gas.
Physical models of NOx formation are becoming more and more interesting in the area of combustion feedback control. The fact that cylinder pressure sensors are made available on the market enables fast and accurate calculations of heat release, which is an essential part of every physical NOx formation model. This article describes such a zero-dimensional model for a diesel engine using crank angle–resolved cylinder pressure to determine heat release. The model also incorporates the thermal effect of exhaust gas recirculation that is proven to have a major effect on NOx formation rates. The reaction mechanisms used to describe NOx formation rates are given by the well-known Zeldovich mechanism. The model output results given in this article show an average deviation of about 12.0% from acquired measured NOx data. The least squares interpolation approach indicates a negligible difference from the original model with an average deviation of 1.2% in 25 measurement points.
The effects of fuel properties including ignitability, volatility, and compositions on operational range and combustion characteristics of premixed diesel combustion with various high volatility model fuels and an ordinary diesel fuel were examined in a direct injection diesel engine. The indicated mean effective pressure was limited by knocking with high-intake oxygen concentrations and by unstable combustion or significant increases in CO and total hydrocarbon emissions with low-intake oxygen concentrations regardless of fuels. The fuel volatility has little effect on the combustion characteristics and the stable operational range in premixed diesel combustion. With increasing octane number, the combustion phasing is retarded, and higher intake oxygen concentrations can be employed within the tolerance limits of rapid combustion, expanding the stable premixed diesel combustion indicated mean effective pressure range. The operational range of premixed diesel combustion with normal heptane and toluene blend fuels shifts to higher intake oxygen concentrations when compared with primary reference fuels with the same research octane numbers, showing lower ignition characteristics than primary reference fuel. The silent, low-NOx, and smokeless operation with high thermal efficiency was possible with both primary reference fuel and normal heptane and toluene blend fuel when the intake oxygen concentration is optimized corresponding to indicated mean effective pressure.
Swirling flow fields in combustion chambers can be determined based on swirl ratio and a velocity profile specified along some path to the vortex center. A method is presented whereby flow fields can be constructed by applying the continuity equation in a streamline coordinate system and imposing irrotationality about the symmetry axis of the vortex ring. The swirl ratio may be specified at the vortex core, along with a velocity profile along any semi-axis of the vortex cross section.
An approach to the analysis of the performance of the diesel oxidation catalysts is presented in this article. A modeling methodology is proposed, whose main characteristics are the usage of a limited set of input data for the execution of the simulations, the minimal effort required for the preliminary calibration of the models and the reduced computational time. The developed diesel oxidation catalyst model structure is described, and its predictive capability is shown by means of a comparison with experimental measurements. The model is based on quasi-zero-dimensional and quasi-steady-state approaches, which ensure a reasonable compromise between practicality of usage (including faster than real-time computational time) and quality of the results. Thanks to the quick execution and the accuracy of the results, the proposed modeling approach can be used not only for the development of the diesel powertrains but also for the optimization of the related control and calibration strategies. This process is particularly effective when the models for the aftertreatment systems are coupled with models for the prediction of the engine-out quantities and with software tools for the virtual calibration of the main engine and exhaust system control parameters. A specific example of the effectiveness of this kind of analysis is also given in this article, with focus on the assessment of the system robustness.
In-cylinder swirl flow is well known to influence emissions behaviour in diesel engine combustion. The post-oxidation part of the combustion stands for typically 25 - 45 % of total Heat Release and is of paramount importance for engine out particulate matter (PM) emissions, especially during an engine transient, at low . To investigate the link between in-cylinder flow and engine out emissions, single-cylinder and optical engine measurements were performed. Injection pressure, swirl and tumble were varied, and emission data, together with high speed photography of in-cylinder flow field during the injection- and the post-oxidation events, were measured. Particle image velocimetry (PIV) software was used to evaluate the flame luminescence images and to calculate the flow field in the cylinder, crank angle resolved during combustion. The glowing soot structures from the combustion were used as tracers. Single cylinder tests with an active valve train were used, which gave a controlled variation in swirl number, 0.4 to 6.7, and tumble number, 0.5 to 4.0. The main findings is that the injection pressure strongly affects the flow field in the cylinder, both before and during the post-oxidation period. Correlations between measured engine out soot emissions and changes in in-cylinder flow has been found and was coupled to the changes in swirl and injection pressure. The observed swirl vortex in the post-oxidation period deviates strongly from solid body rotation. The unsymmetrical rotation was found to be a function of injection pressure. This deviation is concluded to affect the soot oxidation in form of increased turbulence during the post-oxidation period.
In-cylinder strategies to reduce soot emissions have demonstrated the potential to lessen the burden on, and likely the size and cost of, exhaust aftertreatment systems for diesel engines. One in-cylinder strategy for soot abatement is the use of close-coupled post injections. These short injections closely following the end of the main injection can alter soot-formation and/or oxidation characteristics enough to significantly reduce engine-out soot. Despite the large body of literature on post injections for soot reduction, a clear consensus has not yet been achieved regarding either the detailed mechanisms that affect the soot reduction, or even the sensitivity of the post-injection efficacy to several important engine operating parameters. We report that post injections reduce soot at a range of close-coupled post-injection durations, intake-oxygen levels, and loads in an optical, heavy-duty diesel research engine. Maximum soot reductions by post injections at the loads and conditions tested range from 40% at 21% intake oxygen (by volume) to 62% at 12.6% intake oxygen. From a more fundamental fluid-mechanical perspective, adding a post injection to a constant main-injection for conditions with low dilution (21% and 18% intake oxygen) decreases soot relative to the original main injection, even though the load is increased by the post injection. High-speed visualization of natural combustion luminosity and laser-induced incandescence of soot suggest that as the post-injection duration increases and the post injection becomes more effective at reducing soot, it interacts more strongly with soot remaining from the main injection.
The tabulated diffusion flamelet model approximated diffusion flame-presumed conditional moment is here adapted to the Reynolds-averaged Navier–Stokes simulation of diesel engines. The first model modification concerns the effects of variable pressure, which are necessary to retrieve the chemical species concentrations during the expansion stroke. They are accounted for following an approach similar to the variable volume tabulated homogeneous chemistry approach. The second model modification concerns the local fresh gases temperature stratification modeling that needs to be included due to the liquid injection and is based on the transport equation for the fresh gases enthalpy conditioned in the air. The resulting model is called engine approximated diffusion flames and is able to account for the auto-ignition of the diffusion flame, the local mixture fraction heterogeneity through a presumed probability density function, complex chemistry effects, variable pressure, and temperature stratification. As a first validation, an ideal homogeneous adiabatic engine is computed and successfully compared with the reference solution of the same case obtained with a kinetic solver. Then, six diesel engine operating points at various loads, engine speeds, and dilutions are simulated and compared with experimental measurements. It is shown that the proposed model correctly reproduces the mean pressure evolution and gives a correct estimation of the CO mass fraction. Furthermore, coupled to the NO relaxation approach model, relatively accurate NO predictions are obtained. Finally, different simplified formulations of engine approximated diffusion flames are evaluated, showing that all model components are necessary to correctly estimate the pressure evolutions and pollutant emissions.
In diesel engines, long ignition delay due to cold in-cylinder conditions has been shown to lead to high cycle-to-cycle variability, as well as result in pressure oscillations due to rapid localised pressure rise rates from the resulting premixed combustion. These pressure oscillations appear as superimposed pressure waves on the engine indication graph, with an oscillation frequency corresponding to the first radial vibration mode. In the current study, the influences of pressure oscillations on heat release rate and the progress of in-cylinder soot concentration are investigated. Results showed that cycles where pressure oscillations occur reach a higher peak pressure than average or low pressure oscillation cycles, as a result of increased diffusion combustion rate and apparent mixing rate. Additionally, using in-cylinder soot pyrometry, cycles with high pressure oscillations were shown to exhibit increased soot oxidation rates. The combination of the two above-mentioned observed effects leads to the conclusion that pressure oscillations in direct injection diesel engines result in more rapid mixing due to increased turbulent intensity.
We have developed an accelerated multi-zone model for engine cycle simulation (AMECS) of homogeneous charge compression ignition (HCCI) combustion. This model incorporates chemical kinetics and is intended for use in system-level simulation software. A novel methodology to capture thermal stratification in the multi-zone model is proposed. The methodology calculates thermal stratification inside the cylinder based on a single computational fluid dynamics (CFD) calculation for motored conditions. CFD results are used for tuning zone heat loss multipliers that characterize wall heat loss from each individual engine zone based on the assumption that these heat loss multipliers can then be used at operating conditions different from those used in the single CFD run because the functional form of thermal stratification is more dependent on engine geometry than on operating conditions. The model is benchmarked against detailed CFD calculations and fully coupled HCCI CFD chemical kinetics calculations. The results indicate that the heat loss multiplier approach accurately predicts thermal stratification during the compression stroke and (therefore) HCCI combustion. The AMECS model with the thermal stratification methodology and reduced gasoline chemical kinetics shows good agreement with boosted gasoline HCCI experiments over a range of operating conditions, in terms of in-cylinder pressure and heat release rate predictions. The computational advantage of this method derives from the need for only a single motoring CFD run for a given engine, which makes the method very well suited for rapid HCCI calculations in system-level codes such as GT-Power, where it is often desirable to evaluate consecutive engine cycles.
A new method for optimizing the knock threshold is presented and shown to significantly improve the closed loop performance of a standard knock controller. Traditional approaches assume that in order to control potentially damaging knock events, it is necessary to use thresholds set to detect such events. The proposed new method takes a more stochastic view and sets the threshold such that it maximizes the sensitivity to changes in the knock intensity distribution. The behavior of a standard knock controller in response to different threshold and gain values is investigated and illustrated using experimental and simulation data. In particular, it is shown that optimizing the threshold and controller parameters in the manner proposed results in a controller with fast transient response, improved mean spark advance, and reduced cyclic dispersion. With no modifications other than optimizing the parameters of a standard controller, it is therefore possible to operate closer to the knock limit, thereby improving fuel efficiency, emissions, and output torque.
The effect of fuel properties and fuel temperature on the behaviour of the internal nozzle flow, atomization and cyclic spray fluctuations is examined for a three-hole direct injection spark ignition injector by combining numerical simulation of the nozzle flow with macroscopic and microscopic spray visualization techniques. A dominant influence of the liquid fuel viscosity on the highly unsteady, cavitating nozzle flow and spray formation was observed. A reduced viscosity (or larger Reynolds number) increases the flow velocity, turbulence and cavitation in the nozzle and leads to a slim spray with a reduced width but increased spray penetration. Furthermore, the spray cone angle is larger for lower Reynolds numbers due to the changed internal nozzle flow profile as predicted by the numerical calculation. The shot-to-shot fluctuations of the sprays were found to have their origin in the highly unsteady, cavitating nozzle flow. Larger cyclic spray fluctuations were observed at low Reynolds numbers although the predicted vapour formation in the nozzle is weaker. This can be explained by flow instabilities at low Reynolds numbers leading to large fluctuations in the nozzle flow.
Cylinder-to-cylinder variation in a multi-cylinder diesel engine was found to increase substantially when transitioning to a low-temperature combustion mode. This study was started to investigate the potential influence this effect could have on the emissions levels. Initial testing showed an imbalance in the fuel distribution that prompted this article to focus on data from before and after swapping two injectors under both conventional and low-temperature combustion modes. A significant improvement is observed in cylinder variation based both on visual heat release inspection and on mean effective pressure variation. This is likely a result of a changing combination of exhaust gas recirculation and fuel distribution such that less cylinder-to-cylinder variation is present (e.g. high dilution and low fuel, switched to low dilution and low fuel).
Interestingly, despite the reduced cylinder-to-cylinder variation, the results show that the emissions levels are actually not affected. Despite the lack of influence on emissions results, the cylinder-to-cylinder variation in low-temperature combustion modes is still a critical factor that could impact its ability to be implemented in a commercial setting. Further cylinder balancing was attempted and achieved by introducing small (microsecond) adjustments to each cylinder start of injection and injection duration. The balancing is effective, but due to exhaust gas recirculation imbalance, a single adjustment setting does not apply to both conventional and low-temperature combustion modes. Additionally, day-to-day ambient conditions also negate the effectiveness. This supports the idea that some type of consumer-based real-time automatic balancing system may be needed in the future.
The new fuel property "particulate matter (PM) index" suggested by Aikawa et al. in their previous report, which reflects particulate matter emission potential of gasoline, has been shown to have a significant correlation with PM in port fuel injection engine. However, the particulate matter index applicability to direct-injection gasoline engines has not been well verified. The purpose of this study is to confirm the particulate matter index applicability to direct-injection gasoline engines. Results verified good correlation between particulate matter index and particulate matter emissions of a direct-injection gasoline engine, just as with the port fuel injection engine. The verification of particulate number emissions of the direct-injection gasoline engine on the US FTP-75 cycle indicated a high correlation with the particulate matter index. In particular, a significantly high correlation (R2 = 0.9644) was observed in the FTP-75 cold-start phase (Phase 1) in which the fuel influences are considered most evident. The filter PM (PM mass) was simultaneously measured, and its correlation with PM index was verified. The correlation with PM mass was slightly lower than with particulate number, but it also had a good correlation with PM index. This indicated that the PM index will apply well to DI gasoline engines.
This article reports continuing research into operating a free-piston internal combustion engine at extremely high compression ratios. In particular, the use of a homogeneous charge compression ignition combustion strategy with a premixed, stoichiometric methane–air mixture is investigated. A stoichiometric mixture was chosen to correspond with potential use of a three-way catalyst for meeting emissions regulations. To achieve correct autoignition phasing at very high compression ratios, methods for cooling rather than heating the intake charge are discussed, including intercooling and water injection. Results of a set of experiments designed to represent an intercooling approach are reported for effective compression ratios up to 80:1. Ignition phasing slightly after the minimum volume point was achieved for all cases. Indicated efficiency approached 53%. CO, NOx, and hydrocarbon emissions are also reported for equivalence ratios in the range of 0.96–1.04. The measured emissions profiles were suitable for use with a three-way catalyst. Results are reported from a second set of experiments in which the injection of water during compression is used to evaporatively cool the gas. The desired ignition phasing was achieved for compression ratios up to 60:1; however, the amount of water required was far greater than predicted. A dramatic reduction in pressure ringing resulting from combustion was noted with the water injection as compared to the experiments without water injection.
Dimensional modeling, GT-Power in particular, has been used for two related purposes—to quantify and understand the inaccuracies of transient engine flow estimates that cause transient smoke spikes and to improve empirical models of opacity or particulate matter used for engine calibration. It has been proposed by dimensional modeling that exhaust gas recirculation flow rate was significantly underestimated and volumetric efficiency was overestimated by the electronic control module during the turbocharger lag period of an electronically controlled heavy duty diesel engine. Factoring in cylinder-to-cylinder variation, it has been shown that the electronic control module estimated fuel–Oxygen ratio was lower than actual by up to 35% during the turbocharger lag period but within 2% of actual elsewhere, thus hindering fuel–Oxygen ratio limit–based smoke control. The dimensional modeling of transient flow was enabled with a new method of simulating transient data in which the manifold pressures and exhaust gas recirculation system flow resistance, characterized as a function of exhaust gas recirculation valve position at each measured transient data point, were replicated by quasi-static or transient simulation to predict engine flows. Dimensional modeling was also used to transform the engine operating parameter model input space to a more fundamental lower dimensional space so that a nearest neighbor approach could be used to predict smoke emissions. This new approach, intended for engine calibration and control modeling, was termed the "nonparametric reduced dimensionality" approach. It was used to predict federal test procedure cumulative particulate matter within 7% of measured value, based solely on steady-state training data. Very little correlation between the model inputs in the transformed space was observed as compared to the engine operating parameter space. This more uniform, smaller, shrunken model input space might explain how the nonparametric reduced dimensionality approach model could successfully predict federal test procedure emissions when roughly 40% of all transient points were classified as outliers as per the steady-state training data.
This study compares conventional diesel combustion and reactivity controlled compression ignition combustion in a light-duty engine at NOx levels equivalent to US Tier 2 Bin 5 and proposes a simple method to account for the added fluid consumption required to meet NOx constraints using aftertreatment. Reactivity controlled compression ignition and conventional diesel combustion are compared assuming that the conventional diesel combustion mode uses selective catalytic reduction to meet NOx constraints. The results show that reactivity controlled compression ignition is capable of meeting cycle-averaged NOx targets (equivalent to Tier 2 Bin 5) without NOx aftertreatment. In addition, efficiency comparisons show that reactivity controlled compression ignition offers a 4% improvement in fuel consumption and a 7.3% improvement in total fluid consumption (fuel + diesel exhaust fluid) over conventional diesel combustion with selective catalytic reduction. The fuel consumption improvement is due primarily to lower heat transfer losses. Additionally, it was found that the efficiency of reactivity controlled compression ignition can be further improved by careful selection of operating conditions and the combustion chamber configuration. The modeling shows that over 52% gross indicated efficiency can be achieved in the light-duty engine while meeting NOx targets in-cylinder.
Alkali metal atoms show an intense natural fluorescence in the burned gas region of internal combustion engines. This fluorescence offers great opportunity for spectroscopic combustion analysis in internal combustion engines without the requirement of laser excitation or image intensifiers. To quantify this fluorescence intensity, spectroscopic and thermodynamic properties of the alkali metals lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs) and their oxidation products and ions were analyzed. Collisional energy transfer and reabsorption effects (including temperature- and pressure-dependent lineshapes) were calculated over the range of engine environments. Three compounds containing Li, Na and K, respectively, were selected as fuel additives for engine experiments. The experiments were conducted on an optical, single-cylinder, spark-ignited, direct-injection research engine, and the fluorescence of the three alkali components was recorded simultaneously using three complimentary metal-oxide semiconductor high-speed cameras. The two-component fluorescence intensity ratios (Na/K, Li/K and Na/Li) are shown to depend on temperature, pressure and equivalence ratio. However, the three-component ratio Na·Li/K2 is nearly independent of pressure and equivalence ratio in the tested range of operating conditions and can serve as a direct marker for burned gas temperature. Subsequently, equivalence ratio can be determined from any of the bicomponent fluorescence ratios.
Homogeneous charge compression ignition offers the possibility to reduce the fuel consumption of gasoline passenger car engines. However, the combustion strategy is limited to low loads due to pressure oscillations at higher loads. A strategy for extending the homogeneous charge compression ignition load range is charge stratification, using, for example, late direct injection to prolong the combustion duration and reduce the rate of pressure rises and pressure oscillations. In this study, local temperatures and fuel concentrations near top dead centre in a gasoline engine operating in homogeneous charge compression ignition mode were measured using two-wavelength planar laser-induced fluorescence, and the following combustion was analysed using high-speed video to investigate the effects of fuel and temperature stratification on combustion in order to explain the ringing inhibiting effect of charge stratification for fuels displaying single-stage ignition. The extent of spatial distribution of combustion timing correlated well with the extent of fuel and temperature stratification. Furthermore, the gas was leaner and hotter in early igniting regions, while it was richer and colder in late igniting regions. The dampening effects of charge stratification on the combustion speed and pressure oscillations are probably due to rich conditions in the latest burning regions (where combustion is usually most intense) slowing down combustion, which explains why the strategy only works when the global air-to-fuel ratio is not excessively lean.
A mixed-flow turbine with pivoting nozzle vanes was designed and tested to actively adapt to the pulsating exhaust flow (called the active control turbocharger). The turbine was tested at an equivalent speed of 48,000 r/min with inlet flow pulsation of 40 and 60 Hz, which corresponds to a four-stroke diesel engine speed of 1600 and 2400 r/min, respectively. The nozzle vane operating schedules for each pulse period are evaluated experimentally in two general modes: natural opening and closing of the vanes due to the pulsating flow and the forced sinusoidal oscillation of the vanes to match the incoming pulsating flow. The turbine energy extraction as well as efficiency is compared for the two modes to formulate its effectiveness. In addition, a one-dimensional commercial code was implemented, matching an active control turbocharger to an engine with equivalent characteristics to the one simulated in the laboratory. The results obtained represented an improvement over the experimental data with the engine power increasing by between 3.58% and 7.76% between 800 and 1400 r/min; the actual turbocharger power recovery required to achieve this increase in engine power was far higher and typically exceeded 20% throughout the lower half of the engine speed range while remaining higher than 10% for most of the rest. The aim of this article is to demonstrate the potential of active control turbocharger in relation to current turbocharging practice. It has shown strong potentials to improve engine performance in parts of the operational envelope, which need to be further harnessed for real-life applications.
A chemical reaction mechanism has been developed for modeling the combustion process and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. A reduced n-heptane/polyaromatic hydrocarbon mechanism was applied and updated to better predict the formation of polyaromatic hydrocarbon up to four rings (A4) in ethylene and n-heptane premixed flames. In addition, a reduced toluene mechanism was updated and combined with the n-heptane/polyaromatic hydrocarbon mechanism to predict the combustion and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. The final mechanism consists of 71 species and 360 reactions. This mechanism was validated with experimental ignition delay data in shock tubes, premixed flame species concentration profiles, homogeneous charge compression ignition combustion and direct injection spray combustion data. A practical multistep soot model was integrated with the polyaromatic hydrocarbon kinetic model to predict soot emissions of diesel and n-heptane/toluene direct injection engine data. Constant-volume combustion vessel simulations were also conducted and the effects of combustion parameters, such as temperature and equivalence ratio, together with the n-heptane/toluene ratio on polyaromatic hydrocarbon and soot formation are discussed. The results show that the present mechanism provides promising agreement in terms of polyaromatic hydrocarbon prediction for various fuels in premixed flames and highlights the importance of aromatics on the polyaromatic hydrocarbon formation and soot emissions. Homogeneous charge compression ignition combustion and direct injection spray combustion simulation results confirm that the present mechanism gives reliable predictions of combustion and soot emissions for both diesel and n-heptane/toluene fuels under various conditions.
In this study, 10 premixed diesel low-temperature combustion engine operating conditions were chosen based on engine intake pressure (1.2–1.6 bar), intake oxygen concentration (10%, 11%, and 12%), and injection timing (–24° after top dead centre in all test conditions). At each intake oxygen concentration, the effects of intake pressure on combustion parameters and emission measurements (carbon monoxide, hydrocarbons, nitrogen oxides, particulate matter mass concentration, and particle size distributions) were analyzed. Although increased intake pressure resulted in higher in-cylinder charge air density that improved fuel/air premixing and late-cycle oxidation quality, higher intake pressure also advanced the start of combustion and thereby decreased the time available for fuel and air premixing. But even with the decrease in premixing time available before start of combustion, increased intake pressure caused significant decreases in carbon monoxide, hydrocarbon, particulate matter mass, and particle number emissions. Particle size distribution measurements allowed greater understanding of how higher intake pressure decreased the particulate matter mass concentrations with respect to particle size. To further investigate the experimental results, a zero-dimensional engine heat release code was used to analyze combustion temperatures, and a one-dimensional free spray model was used to estimate the relative levels of liquid fuel spray impingement on the piston surface and maximum local equivalence ratio at start of combustion for each test case. Therefore, though the premixing time was shortened by higher intake pressures, the decreased emissions were understood by combined effects of enhanced fuel and air premixing quality and improved late-cycle oxidation near the end of combustion.
Wall flow–type particulate filters are used in diesel vehicle engines to reduce particulate emissions below the limits established in regulations Euro 5 and Euro 6. The soot accumulated in the trap is eliminated during regeneration processes, often combining passive strategies with active ones. Active regeneration is conducted by modifications of the engine control parameters with respect to those set for normal vehicle operation. In this work, three of these parameters were modified to look for an optimized regeneration strategy, considering fuel consumption, gaseous emissions and rate of regeneration. The selected parameters were injection timing (affecting all injection events), exhaust gas recirculation and amount of postinjected fuel. The fuel tested was considered as an additional variable, and therefore, tests were performed with three different fuels: an ultra-low sulfur diesel fuel, a biodiesel fuel produced from animal fat and a gas-to-liquid fuel from low-temperature Fischer–Tropsch process. It was proved that eliminating the gas recirculation is the optimal option for a fast regeneration and reduced fuel consumption and that the late postinjection is essential to keep the temperature conditions needed for an efficient regeneration. Some limits were also proposed for these parameters to prevent from excessively fast or uncontrolled regeneration, to avoid excessive increase in fuel consumption and to reduce the probability of fuel dilution. Finally, the fuel properties were proved to be very relevant to the regeneration process, and therefore, both biodiesel and gas-to-liquid fuels (especially the former) showed un-optimized regeneration processes under the conditions set for regeneration process optimized with diesel fuel.
A three-step phenomenological soot model and a nitric oxide emission model have been developed by applying the current understanding of conceptual models for direct-injection diesel engine combustion processes. The three-step soot model incorporates the physical processes of fuel pyrolysis, soot inception and soot oxidation. The nitric oxide model is governed by the Zeldovich (thermal) mechanism and N2O intermediate mechanism. With the local information provided by the previously developed multi-zone thermodynamic diesel engine combustion model, the emission models can be successfully applied via specific detection concerning where and when each of these reactions mainly occur within the diesel fuel jet evolution process. The simulation was completed for a 4.5-L, inline four-cylinder diesel engine. The results demonstrated that this method, which incorporates emission models into the developed multi-zone diesel engine combustion model, has the potential to qualitatively predict the effects of various engine parameters on the engine-out soot and nitric oxide emissions. The results showed that advanced injection timing and higher injection pressure lead to the increase of nitric oxide concentration because of not only the increased residence time but also the higher entrainment rate of fresh gas into combustion products. Soot formed in-cylinder decreases with increasing injection pressure and advanced start of injection timing mainly due to the extension of the lift-off length and lower local equivalence ratio. In spite of decreased ambient oxygen concentration, the extended lift-off length and the reduced combustion temperature contribute to the reduction of soot formation under heavy exhaust gas recirculation levels.
Biodiesel is a diesel fuel alternative which is produced from renewable and domestically available sources. The use of biodiesel generally lowers carbon dioxide, carbon monoxide and particulate matter emissions. However, there are certain challenges associated with the use of biodiesel, mainly (1) lower fuel energy density, (2) increased nitrogen oxide (NOx) emissions and (3) fuel variability due to feedstock and processing differences. In prior efforts, the authors have demonstrated that the first two of these challenges can be overcome for different blend fractions of soy-based biodiesel by using a control algorithm incorporating energy-based fueling for torque control and combustible oxygen mass fraction control for NOx regulation. However, in addition to overcoming these combustion-related challenges, in this work, the authors consider the extension of these techniques to biodiesel generated from oils/fats of varying composition. The type of oil/fat from which the biodiesel is derived will impact the fuel properties via variation in the fuel’s fatty acid composition. The fuel’s fatty acid composition can also be altered by an additional processing done in order to change certain fuel properties. For example, the saturation level of biodiesel can be reduced in order to lower the fuel cloud point, making it suitable for colder climates. The effect of variation in the fuel fatty acid structure on the previously developed control algorithm is studied in this work. It is shown both theoretically and experimentally that the proposed control algorithms are robust to variation in the fatty acid composition of biodiesel due to the fact that biodiesels with very different fatty acid compositions exhibit minor changes in heating values and fuel oxygen mass fraction. As such, the control technique is suitable for use with variable blend fractions of biodiesel produced from different feedstocks as well as fuel processed to improve cold weather operation.
In-cylinder velocity measurements were acquired to study the bulk fluid motion in two geometrically scaled, two-valve, optically accessible, and single-cylinder research engines. Different port geometries (two), different port orientations (two), and both shrouded and nonshrouded intake valves were tested to vary the intake-generated flow. The engines were motored at speeds ranging from 300 to 1200 r/min for the larger engine and from 600 to 1800 r/min for the smaller engine. Prior to testing on the engines, the different head configurations were tested on a steady flow bench. Particle image velocimetry data were taken on a single plane, parallel to the piston surface, in the engines to characterize the large-scale flow phenomena. The mean location of the swirl center and the mean angular velocity were determined by fitting a solid-body profile to the flow. The results showed that the swirl center location was relatively insensitive to engine speed for both engines, but did change position throughout the cycle. The swirl center locations, scaled by the cylinder radii, were found to be in nearly the same location for the two-scaled engines in the same nominal configuration, indicating that the swirl center motion was deterministic in nature. Normalizing the best-fit solid-body angular velocity by the engine rotation rate was found to collapse the data from the multiple engine speeds nearly onto a single curve for a given engine configuration. The angular velocity was found to decrease with crank angle due to wall friction, which was higher for the small engine because of the higher surface-to-volume ratio.
Turbocompounding is the process of recovering a proportion of an engine’s fuel energy that would otherwise be lost in the exhaust process and adding it to the output power. This was first seen in the 1930s and is carried out by coupling an exhaust gas turbine to the crankshaft of a reciprocating engine. It has since been recognised that coupling the power turbine to an electrical generator instead of the crankshaft has the potential to reduce the fuel consumption further with the added flexibility of being able to decide how this recovered energy is used. The electricity generated can be used in automotive applications to assist the crankshaft using a flywheel motor generator or to power ancillaries that would otherwise have run off the crankshaft. In the case of stationary power plants, it can assist the electrical power output. Decoupling the power turbine from the crankshaft and coupling it to a generator allows the power electronics to control the turbine speed independently in order to optimise the specific fuel consumption for different engine operating conditions. This method of energy recapture is termed ‘turbogenerating’.
This paper gives a brief history of turbocompounding and its thermodynamic merits. It then moves on to give an account of the validation of a turbogenerated engine model. The model is then used to investigate what needs to be done to an engine when a turbogenerator is installed. The engine being modelled is used for stationary power generation and is fuelled by an induced biogas with a small portion of palm oil being injected into the cylinder to initiate combustion by compression ignition. From these investigations, optimum settings were found that result in a 10.90% improvement in overall efficiency. These savings relate to the same engine without a turbogenerator installed operating with fixed fuelling.
The key driving forces in engine development are fuel efficiency and emission levels. These aspects are particularly poignant under vehicle idling or low crawling motions in typical city driving. Under these conditions, the parasitic frictional losses are exacerbated and the emission levels are especially high. A key engine sub-system is the valve-train system. Although it accounts for only 2–3% of the overall engine losses, it is the highest loaded conjunction in the engine, thus limiting the opportunity for lowering the lubricant bulk viscosity. The paper presents detailed tribology of the cam–tappet contact, subjected to a mixed thermo-elastohydrodynamic regime of lubrication. In particular, the frictional behaviour of the conjunction is investigated under the stringent North American emission testing city cycle. Such a comprehensive approach has not hitherto been reported in the literature. The predictions show good conformance with vehicle frictional assessments in industry. It further demonstrates that under the aforementioned cycle, the highest power losses occur mainly as the result of lubricant film viscous shear at low sliding speeds and below the lubricant limiting Eyring shear stress.
Fault detection and isolation have become one of the most important aspects of automobile design. A new fault detection and isolation scheme is developed for automotive engines in this paper. The method uses an independent radial basis function neural network model to model engine dynamics, and the modelling errors are used to form the basis for residual generation. Furthermore, another radial basis function network is used as a fault classifier to isolate occurred fault from other possible faults in the system by classifying fault characteristics embedded in the modelling errors. The performance of the developed scheme is assessed using an engine benchmark, the mean value engine model with Matlab/Simulink. Five faults have been simulated on the mean value engine model, including three sensor faults, one component fault and one actuator fault. The three sensor faults considered are 10–20% changes superimposed on the measured outputs of manifold pressure, manifold temperature and crankshaft speed sensors; the component fault considered is air leakage in the intake manifold; the actuator fault considered is the malfunction of fuel injector. The simulation results show that all the simulated faults can be clearly detected and isolated in dynamic conditions throughout the engine operating range.
The pursuit for higher efficiency and ultra-low exhaust emissions from diesel engines requires the combustion process to be precisely controlled so as to minimize departures from the intended engine operation. The combustion control system must be able to perform corrective actions on a cycle-by-cycle basis, with a robust feedback on the combustion process. The combustion phasing, commonly represented by the crank angle of 50% heat release and derived from the measured cylinder pressure data, shows a strong correlation to the efficiency and the engine-out nitrogen oxide emissions. To accurately estimate the combustion phasing from the derived heat-release rate, the authors previously introduced and experimentally validated a computationally efficient diesel pressure departure ratio algorithm, against selected cases of boost, engine load and exhaust gas recirculation. In this work, the formulation of the pressure departure ratio algorithm is presented in detail along with its implementation to enable combustion control during both transient and steady-state engine operations. Engine tests demonstrate that the algorithm was effective in stabilizing the combustion process on a cycle-by-cycle basis for a range of engine speeds, load and exhaust gas recirculation, which included conventional and low-temperature diesel combustion modes.
Research was conducted on a means of responding to the issue of sticking of exhaust gas recirculation valves resulting from the occurrence of high concentrations of hydrocarbons when the exhaust gas recirculation operating range is expanded. The chemical components responsible for the phenomenon were identified, and exhaust gas and deposits were analyzed. From the tendency of deposition and the results of analyses of deposits, it was conjectured that the type of hydrocarbons present, and their dew points, were the major factors in the sticking of exhaust gas recirculation valves. In order to confirm that the dew points of the hydrocarbons were the major factor, tests were conducted to synthesize deposits in a batch furnace. When the hydrocarbons involved in deposit-forming reactions reached their dew points and did liquefy, the chemical reactions were checked. The conditions in which valve sticking occurs in engines were determined to correlate with the dew points of the hydrocarbons in question. The relationship between the dew points of the hydrocarbons and exhaust gas recirculation valve wall temperature was studied, and a method of avoiding valve sticking by applying control to maintain the temperature of the valve walls above the dew points was proposed.
The characteristics of ambient gas motion induced by a single diesel spray were measured quantitatively by using a laser-induced fluorescence–particle image velocimetry technique under non-evaporating quiescent conditions. The effects of fuel injection pressure, ambient gas density and nozzle hole diameter on the ambient gas mass flow rate into the spray through the whole spray periphery (spray side periphery and tip periphery) were investigated quantitatively according to the gas flow velocity measurements. The results show that the captured gas mass flow rate through the spray tip periphery is prominent in the whole periphery and the proportion of the gas entrainment through the spray side periphery increases with spray development. The higher injection pressure significantly enhances the total gas mass flow rate through the whole periphery; however, the increase in the ratio of ambient gas and fuel mass flow rate becomes moderate gradually with the increase in the injection pressure. The higher ambient gas density results in a slight increase in ambient gas flow velocity along the spray side periphery and the tip periphery and a reduction of the spray volume; however, the ambient gas mass flow rate was apparently enhanced. The smaller nozzle hole diameter results in a significant decrease in the ambient gas mass flow rate and an increase in the ratio of the gas and fuel mass flow rate. Numerical simulation results provide more understanding of the spray-induced gas flow field and validate the measurement accuracy of the laser-induced fluorescence–particle image velocimetry results.
An experimental and computational study of an increasingly used third-generation common-rail injection system with a piezo actuator has been carried out. A complete characterization of the different elements of the system, both geometrically and hydraulically, has been performed in order to describe its behaviour. The information obtained through the characterization has been used to create a one-dimensional model that has been implemented in the commercial software AMESim and extensively validated against experimental data. The results of the validation demonstrate the model ability to predict the injection rate of the injector with a high level of accuracy, therefore, constituting a powerful tool in order to carry out further studies of this type of injection system.
In order to meet the ever more stringent emission standards, significant efforts have been devoted to the research and development of internal combustion engines. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high-pressure pipes and fuel injectors, must be thoroughly examined and compensated for since the combustion and the formation of pollutants in direct-injection engines are directly influenced by the spatial and temporal distribution of the injected fuel within the combustion chamber. This study investigated the hydraulic effects of two-stage fuel injection on diesel combustion and emissions. The fuel-injection system was characterised for all the tested strategies through the measurement of the fuel-injection rate and quantity. In particular, the interaction between the two injection events was identified. The effects of two-stage injection, dwell angle and the interactions between two consecutive injection events on the combustion process and the emissions were investigated in a high-speed direct-injection single-cylinder optical diesel engine using heat-release analysis and high-speed fuel spray and combustion visualisation techniques. The results indicated that the two-stage injection strategy has the potential for simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions. The results suggested that an optimum fuel quantity in the first injection exists, 0–30%, with which simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions can be achieved with the added benefits of improved engine performance, fuel economy and combustion noise. However, higher soot emissions were produced, mainly due to the interaction between the two consecutive fuel-injection events whereby the fuel sprays during the second injection were injected into burning regions, as well as reduced soot oxidation due to the continuation of the combustion into the expansion stroke.
A misfire detection and re-ignition control device based on an ion current signal is designed, and a method for calculating the threshold level of an ion current integral signal is proposed to judge a misfire event. Based on the combustion cycle control strategy, start-up combustion cycle misfire detection and re-ignition control are investigated in a TSDI-Two Stage Direct Injection gasoline engine using ion current signal feedback. The results show that it is feasible for the ion current integral signal to be used for misfire detection and re-ignition control. In the experiment, when the ion current integral signal is larger than the misfire threshold level (Umisfire > 0.5 V), the ion current exists in the combustion chamber, the in-cylinder fuel–air mixture combusts, and the re-ignition event cannot occur. When the ion current integral signal is smaller than the misfire threshold level (Umisfire < 0.5 V), the ion current is close to zero, the misfire event occurs, and re-ignition takes place. As far as hydrocarbon emissions are concerned, the level of unburnt hydrocarbons under successful ignition is lowest, the level of unburnt hydrocarbons under successful re-ignition is higher, and the level of unburnt hydrocarbons under unsuccessful ignition and re-ignition is the highest. Hence, the start-up combustion cycle misfire detection and re-ignition control based ion current signal feedback strategy is favorable for reducing unburnt hydrocarbon emissions.
In recent years, paraffinic fuels have attracted attention because of their potential for reducing diesel exhaust emissions, mainly smoke or particulate matter emissions. One of the paraffinic fuels, a Fischer–Tropsch diesel fuel, was selected to demonstrate the lower exhaust emissions while improving fuel economy in this study. To examine the detailed effects of fuel specifications on diesel combustion and emissions, preliminarily tests for three Fischer–Tropsch fuels and a baseline diesel fuel were carried out with three diesel engines having different engine displacements. In addition, differences in combustion phenomena between Fischer–Tropsch fuels and the baseline diesel fuel were observed by means of a single-cylinder engine with optical access. From these findings, one of the tested engines was modified to improve both exhaust emissions and fuel consumption, simultaneously, dedicated to the use of neat Fischer–Tropsch fuels. The conversion efficiency of an oxides of nitrogen reduction catalyst has also been improved. The desirable properties of Fischer–Tropsch fuels for diesel combustion, namely high cetane number and absence of poly-aromatic hydrocarbon contents, have been fully utilized to enhance the conventional diesel combustion limits to show the possibility to achieve very low exhaust emissions with substantial improvement in fuel economy. The results of this study indicate not only the superior emission characteristics of the Fischer–Tropsch fuels, but also evidence that higher exhaust gas recirculation and lower excess air ratios will be a key concept of both engine and aftertreatment optimization for further fuel consumption improvement.
Extensive engine experiments were carried out on controlled auto-ignition combustion, also known as homogeneous charge compression ignition, in a four-stroke gasoline engine, by internally recycling burned gases through positive valve overlap, variable compression ratio, intake temperature control and boosting. The operational range of controlled auto-ignition combustion was determined for a range of compression ratios and intake air temperatures at wide-open throttle conditions. This was followed by further engine experiments with boosted intake and external exhaust gas recirculation in order to evaluate their effect on the operational range of controlled auto-ignition combustion and the engine’s performance and emissions. It has been found that the controlled auto-ignition operational region could be extended to the higher load region by boosting and running with leaner mixtures, whilst the use of external exhaust gas recirculation allowed the engine to operate controlled auto-ignition combustion optimized in the region between naturally aspirated and lean boosted controlled auto-ignition operational regions. The highest fuel conversion efficiency was obtained when the exhaust heat could be utilized to supply the intake air heating.