Study on the combustion and emission characteristics of a diesel engine with multi-injection modes based on experimental investigation and computational fluid dynamics modelling

In this paper, experiments were carried out on a direct-injection diesel engine using a common-rail system, in order to study the effects of multi-injection modes on the combustion characteristics and pollutant emissions. A soot model was proposed for the post-injection mode, namely the Hiroyasu-Kodota averaged-reaction-rate soot model, which took into account both the chemical kinetics reaction and the turbulent mixing motion of the spray jet. Through integrating the revised soot model into a computational fluid dynamics (CFD) code, the combustion process and pollutants formation of the tested engine were simulated. The in-cylinder gas pressure and combustion heat release rate showed satisfactory agreement with measurements. The experimental data demonstrated that the pilot-injection mode was one of the most effective measures for reducing combustion noise. Meanwhile an optimum split-injection mode consisting of an appropriate pilot-injection fuel quantity combined with an optimal pilot-injection-main-injection interval could be achieved to decrease the nitrogen oxide (NOx) emission while not causing the particulate matter (PM) emission to deteriorate very much. Two innovative concepts of an active thermo-atmosphere and a passive inert atmosphere were presented from numerical simulation to discuss the effect of the pilot-injection mode on the combustion behaviour of the main injection. Regarding the post-injection mode, its prominent advantage was to decrease significantly the PM emission without an NOx emission penalty. Furthermore, by CFD modelling of the soot formation process, it can be observed that the turbulent mixing motion caused by the post-injection spray played a vital role in the soot oxidization process.