In a normal spark-ignition engine, the air-fuel mixture is heated due to being compressed and is then triggered to burn rapidly by the spark plug. During the combustion process, if the unburnt portion of the fuel in the combustion chamber is heated (or compressed) too much, pockets of unburnt fuel may self-ignite (detonate) before the main flame front reaches them. Shockwaves produced by detonation can cause much higher pressures than engine components are designed for, and can cause a "knocking" or "pinging" sound. Knocking can cause major engine damage if severe.
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