Overview of the main mechanisms triggering low-speed pre-ignition in spark-ignition engines

Low-speed pre-ignition has become of great concern since it represents a strong limit to any further downsizing of gasoline engines. The increase of pressure and temperature inside the combustion chamber at high loads can indeed lead to a premature auto-ignition of the mixture and to severe engine damages. Several hypotheses have been formulated but there is no consensus to explain and demonstrate the cause of low-speed pre-ignition. This article provides an overview of the mechanisms that might lead to low-speed pre-ignition. A general discussion and a synthesis of the favourable conditions for auto-ignition are first introduced. Fundamentals of auto-ignition in homogeneous gaseous phase are discussed and confronted to concrete experimental observations of low-speed pre-ignition. The real operating conditions of modern gasoline engines complicate the analysis of auto-ignition because spatial and cycle-to-cycle fluctuations are linked to various physicochemical phenomena involved in the mixture preparation process. At the same time, the increase in temperature and pressure with load enhances the mixture reactivity and any perturbation at the end of the compression can unbalance the mixture and lead to uncontrolled low-speed pre-ignition. A classification of these mechanisms into three groups is proposed to categorize pre-ignitions resulting from auto-ignitions in the gaseous phase, or from auto-ignitions resulting from interactions between either a liquid or a solid phase and the gaseous mixture. Finally, the impact of mixture dilution with burned gases, mixture temperature, liquid films and deposits is examined for different engine settings, fuels and charge motions to illustrate each of these three groups.