In case of spark ignition engines, cooling of the combustion chamber inhibits the spontaneous ignition of the air-fuel mixture. Since spark ignition engines have an essentially homogeneous mixture of fuel and air, then spontaneous ignition can affect a significant quantity of mixture, and the subsequent rapid pressure rise or so called detonation, generates the characteristic ‘knocking’ sound.

Effects of detonation: Noise: when the intensity of knock is high, a loud pulsating noise is created because of high intensity pressure wave which vibrates back and forth across the cylinder. This noise is like a bell noise. The high vibrating motion of the gases causes, crankshaft vibrations also and engine runs rough.

Mechanical damage: knocking creates very high pressure wave (200 bar) of large amplitude. This increases the rate of wear of almost all mechanical parts like piston, cylinder head and valves. The frequency of this wave is as large as 5000CPS. The engine parts are also subjected to very high temperatures (2000-5000oC) due to auto-ignition and the piston is damaged by overheating. Increase in heat transfer rate: when the engine is knocking, more heat is lost to the coolant as the dissipating rate increases. The major reason of increase in heat transfer rate during knocking is, the boundary layer of gas near the wall is removed because of high vibration of gas molecules.

Power output: the knocking has little effect on power output. The efficiency falls by 1 or 2% due to increased heat transfer rate to water and more heat is carried away with exhaust as its temperature is higher. It is observed that slightly retard spark develops more power under knocking condition. This may be due to rapid burning of the last part of the charge and retard spark may be optimum under knocking.

Preignition: preignition is defined as an ignition of charge as it comes in contact with hot surface, in the absence of the spark plug. Auto ignition may be overheat the spark plug and exhaust valve and remains so hot that its temperature is sufficient to ignite the charge in the next cycle during the compression stroke before the spark occurs and this causes the pre ignition of the charge.

This process destroys the thermal boundary layer, and can lead to overheating of components and ensuring damage.

There are three ways in which overheating can affect the mechanical performance of an engine.

Firstly, overheating can lead to a loss of strength. For example, aluminum alloys soften at temperatures over about 200 ℃ and the piston ring grooves can be deformed by a creep mechanism. Furthermore, if the spontaneous ignition is sufficiently severe, then the piston can be eroded in the top-land region. This is usually the hottest region of the piston, and it also coincides with the end-gas region, the region where spontaneous ignition most frequently occurs.

Secondly, the top piston ring groove temperature must also be limited to about 200 ℃ if the lubrication is to remain satisfactory. Above this temperature lubricants can degrade, leading to both a loss of lubrication, and packing of the piston ring grooves with products from decomposed oil.

Finally, failure can result through thermal strain. Data for material failure are usually expressed in terms of stress, either for a single load application, or alternatively as fatigue data where the number of load application, or alternatively as fatigue data where the number of load applications also has to be specified. Such data can also be considered in terms of the strain that would cause failure: the thermal strain is directly proportional to the temperature gradient. Failure is not likely from a single occurrence, but as a consequence of thermal fatigue.