Enthalpy-based flamelet model for HCCI applied to a rapid compression machine

Homogeneous-Charge Compression Ignition (HCCI) engines have been shown to have higher thermal efficiencies and lower NO x and soot emissions than Spark Ignition engines. However, HCCI engines experience high levels of carbon monoxide (CO) and unburnt hydrocarbon (UHC) emissions. These pollutants are formed in regions of the cylinder where wall heat loss is significant. Improving CO and UHC emissions in HCCI engines requires a fundamental understanding of the heat loss, chemical kinetics, and transport between near wall regions and regions less affected by heat loss. In this study an enthalpy-based flamelet approach is introduced and applied in a simulation of a Rapid Compression Machine operated under HCCI conditions. This approach directly models transport between regions of higher and lower enthalpies. Results are compared to experimental data from Murase and Hanada [6]. The simulations correctly predict ignition timing trends as a function of initial mixture temperature. Additionally, the affect of modeled transport across enthalpies on ignition characteristics is quantified. It is demonstrated that this term is important and is of comparable magnitude to the chemical source term.