Detonation

An explosion or uncontrolled burning inside the cylinder of a reciprocating engine. Detonation occurs when the pressure and temperature inside the cylinder become higher than the critical pressure and temperature of the fuel, and may be caused by using fuel that has a lower octane rating or performance number than is specified for the engine. The pressure rise inside the cylinder caused by the fast-moving flame front can heat and compress the unburned fuel-air mixture enough for it to explode, or release its energy faster than the engine can accept it. This causes a rapid rise in cylinder pressure, excessive cylinder head temperature, and a decrease in engine power. Detonation is often confused with preignition but is not the same. See preignition.