Effects of flame propagation speed and chamber size on end-gas autoignition

End-gas autoignition has direct relevance to engine knock and thereby has been extensively studied. However, in the literature there are still some contradictions on how different factors affect end-gas autoignition and knock intensity. Specifically, there is contradictory literature on (1) whether faster combustion may promote or inhibit end-gas autoignition and engine knock, and (2) whether knock intensity increases or decreases with burned mass fraction (BMF). To answer these two questions, one-dimensional flame propagation and end-gas autoignition in a closed cylindrical chamber are investigated and the effects of flame propagation speed and chamber size on end-gas autoignition are examined in this study. In the transient numerical simulation, two fuels, hydrogen and iso-octane, are studied; and detailed chemistry is considered. It is shown that if the flame propagation is fast enough or the chamber is small enough, end-gas autoignition and knock can be prevented; otherwise, the knock intensity may increase as the flame propagation speed increases or as the chamber size decreases. The maximum pressure is found to change non-monotonically with the BMF as well as the flame propagation speed and chamber size. This helps to explain why there is contradictory literature on those two questions mentioned above. The answers to these two questions depend on the amount of unburned mixture at the moment of end-gas autoignition: if there is enough unburned mixture before end-gas autoignition, the maximum pressure increases with the flame propagation speed and BMF; otherwise, the opposite trend occurs. Besides, comparison between the results for hydrogen and iso-octane indicates that end-gas chemical reaction and heat release occurring before autoignition can greatly reduce the maximum pressure.