Autoignition and detonation development from a hot spot inside a closed chamber: effects of end wall reflection
The advancement of highly boosted internal combustion engines (ICEs) with high thermal efficiency is mainly constrained by knock and super-knock respectively due to the end gas autoignition and detonation development. At the end of the compression stroke, the pressure wave propagation and reflection in a small confined space may greatly influence the end gas autoignition, leading to different autoignition characteristics from those in a large or open space. The present study investigates the transient autoignition process in an iso-octane/air mixture inside a closed chamber under engine-relevant conditions. The emphasis is given to the assessing effects of the pressure wave-wall reflection and the mechanism of extremely strong pressure oscillations typical for super-knock. It is found that the hot spot induced autoignition in a closed chamber can be greatly affected by shock/pressure wave reflection from the end wall. Different autoignition modes respectively from the hot spot and end wall reflection are identified. A non-dimensional parameter quantifying the interplay between different length and time scales is introduced, which helps to identify different autoignition regimes including detonation development near the end wall. It is shown that detonation development from the hot spot may cause super-knock with devastating pressure oscillation. However, the detonation development from the end wall can hardly produce pressure oscillation strong enough for the super-knock. The obtained results provide a fundamental insight into the knocking mechanism in engines under highly boosted conditions.