‘Hot spots’ and gasoline engine knock

Abstract

When autoignition occurs in engines it does not do so uniformly in the end gas, but at localised centres. The reasons for this are discussed and, in particular, it is postulated that knock originates at ‘hot spots’, which are autoignition centres that have become critical. Critical radii for this transition have been computed from simple thermal explosion theory, for a one-step Arrhenius reaction. To determine engine operational regimes in which thermal explosion can occur, a size distribution of autoignition centres, arising from the integral length-scale of turbulence, is assumed. This enables the probability of a potential autoignition centre becoming critical and creating a thermal explosion at a hot spot, to be calculated over a range of chemical parameters.The overpressures generated at a hot spot are estimated and the conditions identified for two different modes of autoignition: the deflagration mode, in which a pressure wave attenuates as it propagates into the unburned charge, and the developing detonation mode, in which the pressure amplitude increases as the wave propagates and is coupled to the chemical reaction. The latter is damaging and it is shown that the regime in which there is a high probability of an autoignition centre becoming critical is also one which is prone to a developing detonation.