Formation of Unburned Hydrocarbons in Low Temperature Diesel Combustion

Low temperature combustion is a promising way to reach low NOx emissions in Diesel engines but one of its drawbacks, in comparison to conventional Diesel combustion is the drastic increase of Unburned Hydrocarbons (UHC). In this study, the sources of UHC of a low temperature combustion system were investigated in both a standard, all-metal single-cylinder Diesel engine and an equivalent optically-accessible engine. The investigations were conducted under low load operating conditions (2 and 4 bar IMEP). Two piston bowl geometries were tested: a wall-guided and a more conventional Diesel chamber geometry. Engine parameters such as the start of injection (SOI) timing, the level of charge dilution via exhaust gas re-circulation (EGR), intake temperature, injection pressure and engine coolant temperature were varied. Furthermore, the level of swirl and the diameter of the injector nozzle holes were also varied in order to determine and quantify the sources of UHC. It was found that for the wall-guided combustion chamber geometry, the formation of liquid films and their subsequent emission was a significant source of UHC. In the conventional Diesel combustion system liquid films did not seem to exist under warmed-up, steady-state engine operation and therefore are not believed to be a significant source of UHC emissions. Overall the major source of UHC in low temperature combustion appeared to be bulk quenching in over-lean regions rather than fuel partial oxidation in locally over-rich zones. Bulk quenching is characterized by incomplete fuel oxidation in regions of the combustion chamber where the combination of local temperature, equivalence ratio and charge dilution is inadequate for fast oxidation reactions. Indeed, the local temperature is too low to allow complete oxidation despite the excess of oxygen. In contrast, partial oxidation occurs in over-rich zones where fuel oxidation is limited due to a lack of available oxygen. A detailed analysis of the experimental data revealed that the level of UHC emissions correlated well with the heat release peak. Finally, a conceptual representation of the bulk-quenching mechanism is proposed which is primarily based on consideration of the effects of mixing. This conceptual model is useful for understanding the various phenomena involved in bulk quenching and the impact of engine settings and configurations on UHC emissions.