What causes engine knocking?

Engine knocking is primarily caused by pre-ignition or detonation. This phenomenon occurs when the air-fuel mixture in the engine cylinder ignites prematurely or undergoes an uncontrolled combustion process, leading to a sudden increase in pressure and temperature. As a result, the knocking sound is produced when the piston is still in the upward motion of its stroke, causing a collision between the piston and the explosive force of the combustion event.

This condition can occur due to various factors including the use of low-octane fuel, which does not resist detonation well, or due to high engine temperatures and pressures that promote premature ignition. Understanding the effect of these conditions aids mechanics in diagnosing engine performance issues and provides insight into maintaining optimal fuel quality and engine conditions to avoid knocking.