Numerical study of wall-impinging ignition at different wall distances for cold start of heavy-duty diesel engine

For diesel engines, the distance between the wall and the spray nozzle is an important feature for the matching of spray and combustion chamber geometry. In this study, the effects of the cold wall with different wall distance on the spray and ignition characteristics at low-temperature conditions are systematically analyzed. CFD simulations are used to reproduce and explain the phenomena in optical experiments, and the chemical kinetics analysis is used to investigate the microscopic influence of the cold wall on the chemical reactions. The results show that the cold wall has little influence on the ignition when the wall distance is larger than a critical distance which is determined by the proportion of vapor phase in the impinged fuel and the chemical reaction degree at the time of impingement. As the wall distance reduces, the fuel evolution process is more inclined to the route of low temperature and low concentration, causing the delay of chemical reactions, and the misfire occurs when the time required for chemical reactions exceeds the time of fuel dissipation. Moreover, varying wall distance changes the timing of the local fuel cooling, which leads to diverse effects. During the transition from low temperature reaction to high-temperature reaction, earlier the fuel cooling leads to a more substantial inhibition effect. The fuel cooling after the start of the high-temperature reactions has little impact on the chemical reaction.