ASE Engine Performance (A8) Practice Test

Detonation, often referred to as "knocking," occurs when the air-fuel mixture in the combustion chamber ignites spontaneously and produces a rapid increase in pressure that can lead to engine damage. This phenomenon typically occurs after the spark plug has fired and is a result of the mixture igniting before the spark or in places other than intended, which can create shock waves within the cylinder.

In contrast, pre-ignition is a phenomenon that happens before the spark plug fires. It occurs when the air-fuel mixture ignites due to heat from hot spots in the combustion chamber or from excessive compression, leading to premature combustion. This can also be harmful to the engine, as it can cause a loss of power and increased engine temperatures.

The key distinction between the two concepts lies in the timing and cause of combustion: detonation is a rapid and uncontrolled explosion after the intended ignition, while pre-ignition is an uncontrolled ignition that happens before the intended time. This clear differentiation highlights why option B accurately describes these two types of combustion events in an engine.