A numerical study of the effects of boost pressure and exhaust gas recirculation ratio on the combustion process and exhaust emissions in a diesel engine

Abstract

Experimental studies carried out at the New ACE Institute revealed that a combination of high-pressure fuel injection, high-pressure supercharging, and high exhaust gas recirculation (EGR) ratio was very effective in reducing both emissions of NO x and soot in a heavy-duty diesel engine. However, it is difficult to clarify the mechanisms of emission reduction under such operating conditions by experiments alone because the mixture formation and combustion processes are very complicated. The main purpose of this study was to investigate the effects of boost pressure and EGR ratio on the combustion process and exhaust emissions in a direct injection diesel engine by means of a three-dimensional numerical analysis. The combustion processes under various conditions of boost pressure and EGR ratio in a research diesel engine have been numerically simulated using the authors' engine computational fluid dynamics code (GTT code). By comparing the calculated results with the experimental ones, it has been confirmed that the cylinder gas pressure, the rate of heat release, and the NO emission can be predicted reasonably well and the soot emission tendency can be predicted qualitatively. Furthermore, soot and NO formation characteristics have been examined under various EGR ratio and boost pressure conditions on the basis of φ- T (equivalence ratio-temperature) maps where the calculated local gas states in the combustion regions and the soot and NO peninsulas obtained by zero-dimensional numerical simulation have been plotted.