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An improved parameter called “Homogeneity Factor (HF) of in-cylinder charge” has been introduced as a measure to quantify the quality of the air-fuel mixing process in diesel engines. For this purpose, a CFD simulation has been performed to evaluate the effects of Homogeneity Factor on different injection strategies and its correlation with pre mixing process in a common rail DI diesel engine. The results showed a higher Homogeneity Factor will result in higher rate of air-fuel mixing and more complete combustion process. However, the careful adjustment must be made for ideal reduction for both NOx and soot emissions. It was also found when the dwell delay between injection pulses becomes longer, it leaves more time for the air-fuel mixing and initial combustion process of first injection pulse and therefore, the increase of Homogeneity Factor takes place at a later stage and it can caused a reduction of NOx formation.

As the impacts of global warming have become increasingly severe, Oxy-Fuel Combustion (OFC) has been widely considered as a promising solution to reduce Carbon Dioxide (CO2) for achieving net-zero emissions. In this study, a one-dimensional simulation was carried out to study the implementation of OFC technology on a practical turbocharged 4-cylinder Gasoline Direct Injection (GDI) engine with economical oxygen-fuel ratios and commercial gasoline. When the engine is converted from Conventional Air-fuel Combustion (CAC) mode to OFC mode, and the throttle opening, oxygen mass fraction, stoichiometric air-fuel ratio (lambda = 1) are kept constant, it was demonstrated that compared to CAC mode, θF gets a remarkable extension whereas θC is hardly affected. θF and θC are very sensitive to the ignition timing, and Brake Specific Fuel Consumption (BSFC) would benefit significantly from applying Maximum Brake Torque (MBT) ignition timing.

The mechanism of NOx and soot reduction using different pilot and multiple injection strategies has been computationally studied in a heavy duty DI Diesel engine. A designed set of advanced injection schemes with various variables and exhaust gas recirculation rate (up to 10%) have been analyzed. The CFD model was firstly calibrated against experimental data for a part load operation at 1600 rpm. The computational models used were found to predict the correct trends obtained in the experiment. The study demonstrated the potential and explained the mechanism of the combination of EGR and multiple injection to reduce both soot and NOx emissions together with improved fuel economy.

Oxyfuel combustion and nitrogen-free combustion coupled with Carbon Capture and Storage (CCS) techniques have been recently proposed as an efficient method to achieve carbon free emissions and to improve the combustion efficiency in diesel engines. In this study, a 3-D computational fluid dynamics model has been used to evaluate the influence of oxyfuel-HCCI combustion on engine operating conditions and combustion characteristics in a HSDI diesel engine. Investigations have conducted using four different diluent strategies based on the volume fraction of pure oxygen and a diluent gas (carbon dioxide). The first series of investigations has performed at a constant fuel injection rating at which 4.4 mg of fuel has injected per cycle. In the second part of analysis, the engine speed was maintained at 1500 rev/min while the engine loads were varied by changing the fuel injection rates in the range of 2.8 to 5.2 mg/cycle.

Diesel homogeneous charge compression ignition (HCCI) engines with early injection can result in significant spray/wall impingement which seriously affects the fuel efficiency and emissions. In this paper, the spray/wall interaction models which are available in the literatures are reviewed, and the characteristics of modeling including spray impingement regime, splash threshold, mass fraction, size and velocity of the second droplets are summarized. Then three well developed spray/wall interaction models, O'Rourke and Amsden (OA) model, Bai and Gosman (BG) model and Han, Xu and Trigui (HXT) model, are implemented into KIVA-3V code, and validated by the experimental data from recent literatures under the conditions related to diesel HCCI engines. By comparing the spray pattern, droplet mass, size and velocity after the impingement, the thickness of the wall film and vapor distribution with the experimental data, the performance of these three models are evaluated.

The effects of Air/Fuel (A/F) ratios and Exhaust Gas Re-Circulation (EGR) rates on Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane have been experimentally investigated. The experiments were carried out in a single-cylinder, 4-stroke and variable compression-ratio engine equipped with a port fuel injector. Investigations concentrate on the HCCI combustion of n-heptane at different A/F ratios, EGR rates and their effects on knock limit, engine load, combustion variability, and engine-out emissions such as NOx, CO, and unburned HC. Variations of auto-ignition timings and combustion durations in the two-stage combustion process are analyzed in detail. Results show that HCCI combustion with a diesel type fuel can be implemented at room temperature with a conventional diesel engine compression-ratio. However, its knock limit occurs at very high A/F ratios, although high EGR rates can be tolerated.

Diesel engine emissions are directly influenced by the air fuel mixture within the cylinder chamber. Increasing concern over the environment impacts of the exhaust pollutants has enforced the setting of emissions legislation since the 1960s. In the last decades emissions legislations have become stricter which resulted to the introduction of multiple injection strategies and exhaust gas recirculation (EGR) in the cylinder in order to abate emissions produced. In this study, the effect of injection rate for double in-cylinder injection in combination with various EGR and bio-diesel fuel rates has been studied using CFD simulations. Taguchi orthogonal arrays have been used for reducing the number of simulations for possible combinations of different rates of injection quantities, EGR composition and bio-diesel quantities. Oneway analysis of variance technique (ANOVA) has been used to estimate the importance of the above factors to the emissions output and performance of the engine.

The pursuit of flexibility is a recurring theme in engine design and development. Engines that are able to switch between the two-stroke operating cycle and four-stroke operation promise a great leap in flexibility. Such 2S-4S engines could then continuously select the optimum operating mode - including HCCI/CAI combustion - for fuel efficiency, emissions or specific output. With recent developments in valvetrain technology, advanced boosting devices, direct fuel injection and engine control, the 2S-4S engine is an increasingly real prospect. The authors have undertaken a comprehensive feasibility study for 2S-4S gasoline engines. This study has encompassed concept and detailed design, design analysis, one-dimensional gas dynamics simulation, three-dimensional computational fluid dynamics, and vehicle simulation. The resulting 2/4SIGHT concept engine is a 1.04 l in-line three-cylinder engine producing 230 Nm and 85 kW.

Effects of split injection with different EGR rate on combustion process and pollutant emissions in a DI diesel engine have been evaluated with CFD modeling. The model was validated with experimental data achieved from a Caterpillar 3401 DI diesel engine and 3D CFD simulation was carried out from intake valve closing (IVC) to exhaust valve opening (EVO). Totally 12 different injection strategies for which two injection pulses with different fuel amount for each pulse (up to 30% for the second pulse) and different separation between two pulses (up to 30° CA) were evaluated. Results show that adequate injection separation and enough fuel amount of the second pulse could form a separate 2nd stage of heat release which could reduce the peak combustion temperature and improve the oxidation of soot formed in the first heat release stage.

An investigation has been carried out to examine the influence of re-entrant combustion chamber geometry on mixture preparation, combustion process and engine performance in a high-speed direct injection (HSDI) four valves 2.0L Ford diesel engine by CFD modeling. The computed cylinder pressure, heat release rate and soot and NOx emissions were firstly compared with experimental data and good agreement between the predicted and experimental values was ensured the accuracy of the numerical predictions collected with the present work. Three ITs (Injection Timing) at 2.65° BTDC, 0.65° BTDC and 1.35° ATDC, all with 30 crank angle pilot separations were also considered to identify the optimum IT for achieving the minimum amount of pollutant emissions.

Tumble and swirl motions in the cylinder of a four-valve SI engine with production type cylinder head were investigated using a cross-correlation digital Particle Image Velocimetry (PIV). Tumble motion was measured on the vertical symmetric plane of the combustion chamber. Swirl motion was measured on a plane parallel to the piston crown with one of intake ports blocked. Large-scale flow behaviours and their cyclic variations were analysed from the measured two-dimensional velocity data. Results show that swirl motion is generated at the end of the intake stroke and persists to the end of the compression stroke. Tumble vortex is produced in the early stage of the compression stroke and distorted in the late stage of the stroke. The cyclic variation of swirl motion is noticeable. The cyclic variation in tumble dominated flow field is much greater.

Owing to the potentials for low NOx and soot emissions, diesel PCCI combustion has been widely studied over last 10 years. However, its control is still the main barrier to constrain it to be applied on production engines. As there are a number of variables which affect the mixing and combustion process, it is difficult to develop control strategies with adequate functions but simple control order for implementing them. In the current research, a reformed Homogeneity Factor (HF) of in-cylinder charge has been explored as a control medium for simplifying the control model structure. Based on multi-pulse injection, the effects of operating parameters on the Homogeneity Factor and the relationship between Homogeneity Factor and mixing, combustion processes, emissions were investigated in a four-valve, direct-injection diesel engine by CFD simulation using KIVA-3V code coupled with detailed chemistry.

Multiple-injection strategy for Premixed Charge Compression Ignition (PCCI) combustion was investigated in a four-valve, direct-injection diesel engine by CFD simulation using KIVA-3V code [ 1 ] coupled with detailed chemistry. The effects of fuel splitting proportion, injection timing, included spray angles, injecting velocity, and the combined effects of injection parameters and EGR rate and boost pressure were examined. The mixing process and formations of soot emission and NO x were investigated as the main concern of the research. The results show that the fuel splitting proportion and the injection timing significantly impacted the combustion and emissions due to the considerable changes of the mixing process and fuel distribution in cylinder. The soot emission and unburned HC (UHC) were affected by included spray angles since the massive influences of the fuel distribution resulted from the change in spray targeting point on piston bowl.

The multi-zone model has been attracting growing attention as an efficient and accurate numerical model for homogeneous charge compression ignition (HCCI) combustion simulations. In this paper, a comparative study was carried out to clarify the effect of various sub-models on the prediction capability of the multi-zone model. The influences of the distribution of zones, heat transfer from the wall, mass and heat exchange between zones and boundary layer thickness on HCCI combustion and emissions were discussed based on the experimental data. The results indicate dividing the colder region into more zones can improve the emissions prediction, however, more zones in the hotter region has little effect on the predictions. The improved Woschni model significantly improves the prediction of heat transfer.