NOx and soot reduction in diesel engine premixed charge compression ignition combustion: a computational investigation

The objective of the present work is to study soot-EGR and soot-NOx tradeoff for premixed charge compression ignition (PCCI) combustion at a light-load (4bar BMEP) operation of a medium-duty, large-bore, direct-injection diesel engine using multi-dimensional computational analysis. The simulations are performed using KIVA-3V computer code coupled with reduced chemical kinetics. Low (40 per cent) to heavy (70 per cent) EGR rates are applied to study the effects of reduced intake oxygen concentration on combustion and emissions. Model predictions of cylinder pressure and emissions are first validated against experimental data. Model-predicted temporal and spatial evolution of in-cylinder mixture in phi-T coordinates is then used to explain the fundamentals of PCCI combustion and emissions. Using computational analysis, it is shown that lower exhaust soot emissions for the lower EGR dilution cases (45 per cent) are due to higher soot oxidation rates, and lower exhaust soot emissions for very high EGR rates (70 per cent) are due to lower soot formation rates. The trend seen in NOx emissions are primarily attributable to the inert effect of EGR dilution and varied ignition delay caused by varied percentages of oxygen in the intake mixture.