Combustion and ignition modeling for IC engines

HCCI engines potentially offer an efficient and low emissions alternative to conventional Spark Ignition (SI) and Compression Ignition (CI) engines. Though significantly more homogeneous than conventional CI engines, HCCI engines often include thermal and mixture composition inhomogeneities. Two combustion limits can be identified for HCCI; the homogeneous reactor regime and the propagating diffusive fronts regime. An enthalpy and mixture fraction-based auto-ignition model that is valid for both limits was developed. Enthalpy and mixture fraction were chosen as the phase-space variables because they are the main parameters that affect local changes in auto-ignition behavior. Since these two variables are not statistically independent, a new modified enthalpy variable was defined to remove the mixture fraction dependence, resulting in a model with two independent phase-space variables. Closure models for application of the model in CFD engine simulations were also developed.; The combustion model was validated in three steps. First, a one-dimensional enthalpy-based combustion model, which is a special limit of the general model, was validated using DNS data for lean H2 auto-ignition with temperature inhomogeneities. The second step of the model validation was the application of the mathematical model together with closure models for transport equations solved in three-dimensional CFD simulations of a Rapid Compression Machine (RCM) experiment. The RCM was operated with a spatially homogeneous mixture composition and was compressed at a rate typical of HCCI engines. The final validation step was to apply the full two-dimensional model to an HCLI engine experiment. The model was validated using six experimental cases, comparing simulation results to pressure traces and CO and UHC emissions measurements. The model was in excellent agreement with the experimental pressure traces. A detailed investigation into the source of individual UHC species was performed.; In order to analyze the relative importance of auto-ignition versus flames in HCCI engines, a criterion was developed based on the ratio of the time scales associated with ignition fronts and deflagration fronts. The results indicate that some HCCI engines are operated in the propagating diffusive fronts regime, and therefore an appropriate approach must consider both diffusive mixing and chemical kinetics. The model presented in this work takes these affects into account and is capable of modeling combustion across the different combustion regimes observed in HCCI.