Originally Posted by boosted hybrid

Knock and Pre-Ignition

Knock is the most important abnormal combustion phenomenon. Knock is created as the flame front travels across the combustion chamber, and the unburnt mixture ahead of the flame called end gas, is compressed. The compression of the end gas causes its pressure, temperature and density to increase. The product of this event is that the unburnt gases rapidly release its energy up to 25 times that of a normal combustion event. This in turn causes high frequency pressure oscillations inside the cylinder, that in turn produces a sharp metallic noise called knock.

The presence, or absence of knock reflects the outcome of a race between the advancing flame front and the precombustion reactions in the unburned end gas. Knock will not occur if the flame front consumes the end gas before these reactions have time to cause the fuel-air mixture to auto ignite. Knock will occur if the precombustion reactions produce auto ignition before the flame front arrives.

The other significant abnormal combustion phenomenon is pre-ignition, or surface ignition. Surface ignition is ignition of the fuel-air charge by overheated valves or spark plugs, by glowing combustion-chamber deposits, or by any other hot spot in the engine combustion chamber: it is ignition by any source other than normal spark after normal ignition (post ignition). It may produce a single flame, or many flames. Uncontrolled combustion is most evident and its effects most severe after the spark plug fires (post ignition), the spark discharge no longer has complete control over the combustion process. Surface ignition may result in knock. Knock occurs following normal spark ignition is called spark knock to distinguish if from knock which has been preceded by surface ignition.