Knock onset, knock intensity, superknock and preignition in spark ignition engines

Knock is an abnormal and stochastic combustion phenomenon which limits the efficiency of spark ignition engines. It occurs because of autoignition initiated locally in hot spots in the fuel/air mixture ahead of the advancing flame front. The onset of knock is governed by chemical kinetics and is determined by the pressure and temperature history of the hot spot and the anti-knock quality of the fuel. Knock intensity is governed by the evolution of the pressure wave set off by the initial autoignition. As engine designers seek higher efficiency through downsizing and turbocharging, occasional extremely intense knock, 'superknock', is found to occur, even though operating conditions are chosen to avoid knock. Superknock is a manifestation of developing detonation which can be triggered by autoignition of the fuel/air mixture at high pressures and temperatures. Another stochastic phenomenon, preignition, is necessary but not sufficient to enable the pressures and temperatures that could cause superknock. These phenomena are discussed in this review.