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Multidimensional computations were made of combustion of natural gas engine via the KIVA-II code to evaluate the combustion and emission characteristics. In the combustion submodel, a two-step kinetic reaction mechanism is employed to account for oxidation of methane. The first of the two global rate equations controls the disappearance of methane, and the second, the oxidation of carbon monoxide. Four types of combustion chamber and a two-spark-plug geometry are considered to achieve quick flame propagation of the lean air-methane mixture. The effect of spark plug locations on the combustion processes is discussed. The calculated results show that the more effective burning process with lower NOx emission could be achieved by proper design of the geometry of the piston bowl and the arrangement of the spark plug by matching of the flame front development with the in-cylinder gas flow.

In the effort to improve combustion of a Light-load Stratified-Charge Direct-Injection (LSCDI) combustion system, CFD modeling, together with optical engine diagnostics and single cylinder engine testing, was applied to resolve some key technical issues. The issues associated with stratified-charge (SC) operation are combustion stability, smoke emission, and NOx emission. The challenges at homogeneous-charge operation include fuel-air mixing homogeneity at partial load operation, smoke emission and mixing homogeneity at low speed WOT, and engine knock tendency reduction at medium speed WOT operations. In SC operation, the fuel consumption is constrained with the acceptable smoke emission level and stability limit. With the optimization of piston design and injector specification, the smoke emission can be reduced. Concurrently, the combustion stability window and fuel consumption can be also significantly improved.

Multidimensional computations were carried out on a spark-ignition natural gas engine with a bowl-in-piston combustion chamber. The engine in-cylinder flow distributions and their effects on combustion are examined. The fact that the engine swirl would speed up the combustion process is confirmed due to the enhancement of turbulent diffusion process. The engine squish increases both the mean velocity and the turbulence intensity of the gas flow and, therefore, quickens the combustion process. The computations indicate further that the engine swirl impacts the engine in-cylinder flow fields more profoundly than the engine squish does. When the piston bowl is offset, the in-cylinder gas motion can be enhanced considerably. Computations were also made to study the sensitivity of the computed cylinder pressure history to initial values of the selected thermodynamic parameters and chosen initial turbulence conditions.

Charge cooling due to fuel evaporation in a direct-injection spark-ignition (DISI) engine typically allows for an increased compression ratio relative to port fuel injection (PFI) engines. It is clear that this results in a thermal efficiency improvement at part load for homogenous-charge DISI engines. However, very little is known regarding the effect of compression ratio on stratified charge operation. In this investigation, DISI combustion data have been collected on a single cylinder engine equipped with a variable compression ratio feature. The results of experiments performed in stratified-charge direct injection (SCDI) mode show that despite its over-advanced phasing, thermal conversion efficiency improves with higher compression ratios. This benefit is quantified and dissected through an efficiency analysis. Furthermore, since the engine was equipped with both wall-guided DI and PFI systems, direct comparisons are made at part load for fuel consumption and emissions.

Multidimensional computations were carried out to simulate the in-cylinder fuel/air mixing process of a direct-injection spark-ignition engine using a modified version of the KIVA-3 code. A hollow cone spray was modeled using a Lagrangian stochastic approach with an empirical initial atomization treatment which is based on experimental data. Improved Spalding-type evaporation and drag models were used to calculate drop vaporization and drop dynamic drag. Spray/wall impingement hydrodynamics was accounted for by using a phenomenological model. Intake flows were computed using a simple approach in which a prescribed velocity profile is specified at the two intake valve openings. This allowed three intake flow patterns, namely, swirl, tumble and non-tumble, to be considered. It was shown that fuel vaporization was completed at the end of compression stroke with early injection timing under the chosen engine operating conditions.

An integrated numerical model has been developed for diesel engine computations based on the KIVA-II code. The model incorporates a modified RNG k-ε, turbulence model, a ‘wave’ breakup spray model, the Shell ignition model, the laminar-and-turbulent characteristic-time combustion model, a crevice flow model, a spray/wall impingement model that includes rebounding and breaking-up drops, and other improved submodels in the KIVA code. The model was validated and applied to model successfully different types of diesel engines under various operating conditions. These engines include a Caterpillar engine with different injection pressures at different injection timings, a small Tacom engine at different loads, and a Cummins engine modified by Sandia for optical experiments. Good levels of agreement in cylinder pressures and heat release rate data were obtained using the same computer model for all engine cases.

The sources of unburned hydrocarbon (UHC) emissions in direct injection stratified charge engines are presented. Whereas crevices in the combustion chamber are the primary sources of UHC emissions in homogeneous charge engines, lean quenching and liquid film layers dominate UHC emissions in stratified charge operation. Emissions data from a single cylinder engine, operating in stratified charge mode at a low speed / light load condition is summarized. This operating point is interesting in that liquid film formation, as evidenced by smoke emissions, is minimal, thus highlighting the lean quenching process. The effects of operating parameters on UHC emissions are demonstrated via sweeps of spark advance, injection timing, manifold pressure, and swirl level. The effects of EGR dilution are also discussed. Spark advance is shown to be the most significant factor in UHC emissions. A semi-empirical model for UHC emissions is presented based on the analysis of existing engine data.