Regulations on emissions from diesel and gasoline fuelled engines are becoming more stringent in all parts of the world. Hence there is a great deal of interest in developing advanced combustion systems that offer the efficiency of a diesel engine, but with low PM and NOx. One promising approach is that of Partially-Premixed Compression Ignition (PPCI) or Low Temperature Combustion (LTC). Using this approach, PM can be reduced in compression ignition engines by promoting the mixing of fuel and air prior to combustion.
This paper describes the application of an advanced combustion simulator for fuels, combustion and emissions to analyze the key processes which occur in PPCI combustion mode. A detailed chemical kinetic model with advanced PM population balance sub-model is employed in a PPCI engine context to examine the impact of ignition resistance on combustion, mixing, ignition and emissions. The ignition and combustion of a diesel-like fuel (n-heptane) and low octane gasoline-like fuel (84PRF) are compared using the model highlighting how the diesel-like fuel ignites at very rich equivalence ratios whereas the gasoline-like fuel ignites on the lean side. Sources of exhaust gas emissions are also identified.
For the first time, a computational model is employed to demonstrate the trade-off between low PM emissions and “over-mixing” (sensitivity to cycle-to-cycle variations and combustion instability) for a full range of fuels with increasing ignition resistance. These results are then discussed noting that conventional hydrocarbon fuels which fulfill either a conventional diesel or gasoline standards are not necessarily consistent with those required to run an engine operating at it's optimal point in terms of PM emissions and combustion stability.