What phenomenon occurs when an air-fuel mixture ignites prematurely in an engine?

Detonation is the correct answer because it refers to the situation in a combustion engine where the air-fuel mixture ignites in an uncontrolled manner, creating a rapid and violent explosion instead of a smooth burn. This premature ignition leads to a sharp rise in cylinder pressure and temperature, which can cause significant engine damage if not addressed.

The phenomenon typically occurs due to factors such as high compression ratios, low octane fuel, or excessive heat in the combustion chamber. When detonation occurs, it can lead to performance issues, including loss of power, engine knocking sound, and potential damage to pistons and valves.

Understanding detonation is crucial for technicians, as it helps in diagnosing engine performance issues and guiding appropriate corrective measures, such as adjusting the fuel mixture, improving cooling, or using higher-octane fuel to mitigate the problem.