Detonation development in H2, CH4, and PRF-air mixtures under engine-relevant conditions: From a chemical perspective

Abstract

The progression of highly boosted internal combustion engines (ICEs) faces impediments due to the super-knock phenomenon primarily associated with detonation development during end gas autoignition. Although it is well known that hot spot potentially triggers super-knock, quantitative prediction of super-knock and detonation development across diverse states and fuels remains a challenge. This study numerically investigates the transient autoignition process induced by a hot spot, considering different initial states and fuels (including alternative fuels H2, CH4, and conventional PRFs), with an emphasis on the chemical effects. It is found that the intense exothermic stage (IES) in a stoichiometric H2/air mixture greatly differs from those in hydrocarbon/air (CH4/air and PRF/air) and lean or diluted H2/air mixtures. Through sensitivity analysis, key elementary reactions dominating the autoignition are identified, attributing their impact on the IES primarily to their influence on the radical pool rather than on the heat release rate. It is found that only reactions H + O2⇌O+OH and H2O2(+M)⇌OH+OH(+M) significantly affect both excitation time and ignition delay time, suggesting that the kinetic pathway controlling the IES differs somewhat from that governing the overall ignition. For the autoignition induced by a hot spot, two types of detonation feature are identified: one corresponds to stoichiometric H2/air with broad detonation regime and short excitation time, while the other is associated with other fuel/air mixtures exhibiting relatively narrow detonation regime and prolonged excitation time. These findings highlight that the excitation time predominantly governs the detonation development. All hydrogen/air mixtures (stoichiometric, lean, and diluted) manifest a suppressed detonation intensity compared to hydrocarbon/air mixtures. Moreover, due to its significantly higher reactivity in the IES, stoichiometric H2/air has an additional normal detonation sub-mode for hot spot with larger temperature gradient. This sub-mode greatly expands the detonation regime and thereby results in a notably higher detonation propensity for the stoichiometric H2/air mixture.