Pre-ignition and ‘super-knock’ in turbo-charged spark-ignition engines

Abstract

Earlier studies of pre-ignitions at hot surfaces are first reviewed. The concept of a critical radius of a hot pocket of gas, closely related to the laminar flame thickness, that is necessary to initiate a propagating flame, has been used successfully to predict relative tendencies of different fuel–air mixtures to pre-ignite. As the mixture is compressed, the thickness of potential laminar flames decreases, and when this becomes of the order of the thermal sheath thickness at the hottest surface, pre-ignition can occur there, creating a propagating flame. Measured engine pre-ignition ratings are shown to correlate well with laminar flame thicknesses. Predictions are made concerning the effects of changes in intake temperature and pressure on the pre-ignition of different fuels.

A growing current concern is occasional gas-phase, autoignitive, pre-ignitions that can occur in turbo-charged engines, giving rise to very severe autoignition and knock. It is concluded from the evidence of engine pressure records and autoignition delay times of the mixtures that such pre-ignitions have not arisen from autoignition of the fuel, but of a mixture with a smaller autognition delay time than stoichiometric n-heptane–air. One possibility is that autoignition occurs at hot spots containing some lubricating oil. It is shown that such pre-ignitions, particularly with catalytic enhancement, could initiate a propagating flame, rather than autoignitive propagation.

In the later, much more severe autoignition arising after pre-ignition, autoignitive propagation velocities at a hot spot are estimated from computed values of the ignition delay times and assumed reactivity gradients in the fuel–air mixture at the hot spot. The severity of the associated pressure pulse is dependent upon the ratios $\xi$ , of the acoustic speed to the localised autoignitive velocity, and $\epsilon$ , of the residence time of the acoustic wave in the hot spot to the short excitation time in which most of the chemical energy is released. The regime in which a localised detonation can be generated in the hot spot is defined by a peninsula on a plot of $\xi$ against $\epsilon$ . A locus is plotted on this figure corresponding to the growing measured intensities of the engine knock as the pressure increases. This is based on computed autoignition delays and excitation times, for an appropriate surrogate fuel. These changes are characterised by $\xi$ tending to unity and $\epsilon$ tending to ever-higher values, with increasingly intense, localised, developing detonations.