How does detonation differ from pre-ignition in an aircraft engine?

Detonation and pre-ignition both pertain to abnormal combustion processes in an aircraft engine, but they represent different phenomena. Detonation occurs when the fuel-air mixture in the cylinders ignites spontaneously due to excessive pressure and temperature before the normal spark event, resulting in a rapid increase in pressure that can harm the engine. This phenomenon is characterized by a knock or pinging sound and is often unexpected.

In contrast, pre-ignition refers to the ignition of the fuel-air mixture before the intended spark event, typically caused by hot spots in the combustion chamber, such as glowing carbon deposits or overheated spark plugs. This premature ignition can lead to reduced engine performance and possible damage, but it occurs on its own before the spark plug fires.

Therefore, the correct option accurately summarizes the essence of these two issues. Detonation is the rapid explosion occurring after the normal ignition, whereas pre-ignition is the more problematic early ignition of the fuel-air mixture. This differentiation is crucial for diagnosing engine issues and maintaining engine health.