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Strict NOx and soot emission regulations for Diesel engines have created an interest in low-temperature partially-homogeneous combustion regimes in both the US and Europe. One strategy, Homogeneous-Charge Late-Injection (HCLI) combustion utilizes 55% or more cooled external Exhaust Gas Recirculation (EGR) with a single Direct Injection strategy to control ignition timing. These engines are operated at low temperatures to ensure near zero NOx emissions, implying that fuel in the thermal boundary layers will not reach sufficient temperature to fully oxidize, resulting in Unburnt Hydrocarbon (UHC) and CO emissions. Of particular interest to HCLI engines are the UHC's that are not fully oxidized by the Diesel Oxidation Catalyst (DOC). Experimental measurements reveal that at average equivalence ratios greater than 0.8, methane is the single largest tailpipe-out UHC emission.

Homogeneous-Charge Compression Ignition (HCCI) engines have been shown to have higher thermal efficiencies and lower NOx 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.