Numerical Modeling of Diesel Engine Combustion and Emissions Under HCCI-Like Conditions With High EGR Levels

This study considers combustion processes in a heavy-duty diesel engine at various low emissions operating conditions. The start-of-injection timings varied from -20 to 5 ATDC while the EGR levels varied from 6% to 44%. At certain conditions, HCCI-like combustion characteristics were observed under which low emissions could be achieved. The numerical model used is an improved version of KIVA-3V that can simulate spray breakup and mixture autoignition over a wide range of conditions. The ignition and combustion processes were simulated using both detailed and standard (simplified) chemistry models. Model results show that engine combustion and emissions can be predicted reasonably well under the current conditions. The trends of NOx and soot emissions with respect to the injection timings and EGR levels were well captured. However, it was found that the model over-predicted the NOx emissions in certain early injection cases. The present model was also used for engine geometry design optimization in order to achieve low emissions. It was found that an open-chamber type geometry with high EGR and high swirl ratio was preferred to obtain HCCI-like combustion characteristics by creating a more homogeneous mixture in the cylinder.