End-gas autoignition and detonation development in a closed chamber

Abstract

It is generally accepted that knock in spark ignition engines might be caused by end-gas autoignition. However, the detailed mechanism for autoignition-induced pressure oscillation and detonation development is still not well understood. This work studied end-gas autoignition and detonation development in a closed chamber using 1D simulation. Stoichiometric hydrogen/air mixture at different initial temperatures and pressures was considered and detailed chemistry was included in simulation. The objectives were to identify possible modes of end-gas combustion and to understand the mechanism of autoignition-induced pressure wave and detonation development. Depending on the chamber length as well as the initial temperature and pressure, there are three modes of end-gas combustion: normal flame propagation without autoignition, autoignition without detonation development, and detonation development. The amplitude of pressure oscillation was found to be determined by the mode of end-gas autoignition: autoignition can induce high amplitude of pressure oscillation similar to conventional knock; and detonation development can cause extremely high amplitude of pressure oscillation similar to super-knock. It was shown that autoignition and detonation development can be induced by increasing the initial temperature, initial pressure, or chamber length. The evolution of states of different flow particles was tracked and the combustion mode was found to switch from constant-pressure to constant-volume when autoignition occurs. The coupling between pressure wave and chemical reaction was analyzed and the mechanism for autoignition front acceleration and detonation development was investigated. Moreover, autoignition in end-gas with different values of ignition progress was simulated. It was demonstrated that high reactivity of end-gas promotes autoignition and detonation development.