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

Abstract

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 (NO _x ) 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 NO _x 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.

Keywords

direct-injection diesel engine multi-injection mode combustion noise nitrogen oxide emission particulate matter emission computational fluid dynamics simulation active thermo-atmosphere passive inert atmosphere Hiroyasu—Kodota averaged-reaction-rate soot model

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