Combustion modelling for a diesel engine with multi-stage injection using a stochastic combustion model

Abstract

This study presents the development of a phenomenological combustion model to simulate the combustion processes in diesel engines with multi-stage fuel injection. A newly developed zero-dimensional spray propagation model and a model of spray-to-spray interaction were combined with a stochastic combustion model, which had been developed for the calculation of diesel combustion in the case of single-stage injection. In this model, the combustion chamber is divided into an ambient air zone and several spray zones, where the spray formed by each injection is treated as a spray zone. The turbulent mixing, the fuel evaporation, the heat loss and the chemical reactions are calculated in each spray zone separately. A zero-dimensional spray propagation model including the spray evolution after the end of injection and a model of interaction between the sprays from sequential injections are developed to describe the spray behaviour for the case of multi-stage injection. Then the developed combustion model is validated against the experimental data from a single-cylinder direct-injection diesel engine with two-stage pilot–main injection, in which the pilot injection conditions are varied with a fixed main-injection timing. Based on the analysis of the heat release rate, the entrainment rate and the microscopic information inside the spray, such as the probability density function of the equivalence ratio, the effects of the wall impingement and the interaction between adjacent sprays on the fuel–air mixing rate and the entrainment rate are formularized and employed to reproduce the measured histories of the heat release rate. The reduction in the fuel–air mixing rate is considered when the spray flows into the squish region after wall impingement, which is effective in obtaining the measured decrease in the heat release of the pilot spray with advancing pilot injection timing. The effects of the wall impingement of the main spray and the interaction between adjacent sprays are modelled to reproduce the heat release rate during the initial part and later part of the mixing-controlled combustion. After these improvements, the heat release rates of the test engine when varying the pilot injection conditions were successfully predicted.