Numerical Investigation of Unburnt Hydrocarbon Emissions in a Homogeneous-Charge Late-Injection Diesel-Fueled Engine

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. In this study, a four-cylinder 2.15 L Diamler (om646) engine is simulated with Fluent coupled to a 2D combustion code. The combustion model is formulated to account for thermal and mixture composition inhomogeneities in addition to turbulent mixing in the engine. The simulation results show good agreement with experimental pressure traces, and with engine-out CO emissions. The simulation indicates that while the majority of CO and UHC emissions are produced in fuel-lean mixtures, nearly all methane emissions originate from rich regions of incomplete mixing.