Effect of Heat Release Pattern of Flame during Propagation on Auto-Ignition Process of End-Gas

Knock is a factor hindering enhancement of the thermal efficiency of spark ignition engines, and is an unsteady phenomenon that does not necessarily occur each cycle. In addition, the heat release history of the flame also fluctuates from cycle to cycle, and the auto-ignition process of the unburned mixture (end-gas), compressed by the global increase in pressure due to release of chemical energy, is affected by this fluctuation. Regarding auto-ignition of the end-gas, which can be the origin of knock, this study focused on the fluctuation of the flame heat release pattern, and used a zero-dimensional (0D) detailed chemical reaction calculation in an attempt to analyze and examine the consequence on the end-gas compression and auto-ignition process of changes in the i) start of combustion, ii) combustion duration and iii) center of heat release of the flame. The results showed that the compression speed of the end-gas varies according to the heat release pattern of the flame, and the ignition delay is affected by the compression speed. Furthermore, when the compression speed is slow, the temperature at which the end-gas starts heat release decreases and the combustion-phase advance limit deteriorates. In addition, enhancement of the combustion speed of the flame (shortening the combustion duration) was confirmed to enable completion of combustion before the end-gas auto-ignites, and also to shorten the end-gas compression time, shift the heat release start temperature to the high-temperature side, and improve the knock limit. Analysis was also performed on the impact of varying the flame heat release pattern for different initial pressures and air-fuel mixture equivalence ratios. It was found that there exists an optimum heat release pattern for avoiding knock effectively and obtaining increased engine power.