What is knock and why it matters

Knock is rapid, uncontrolled combustion which results in a correspondingly extremely fast increase in cylinder pressure. Knock is often accompanied by extremely high instantaneous combustion temperatures. Knock earned its name from the knocking or rattling noise that is audible from pressure waves smacking pieces of engine.

Knock matters because it breaks things. Peak cylinder pressures during knocking can be tens to hundreds of times higher than normal controlled combustion. The violent changes in pressure almost always exceed design parameters of the engine. Things break - headgaskets, pistons, connecting rods, crankshafts, rodbolts, headstuds, more. The intense heat associated with uncontrolled combustion can torch valves, melt holes in pistons and cylinder heads. Knocking at extremely light loads can sometimes be survived but it is a good idea to not operate engines while knocking.

Knock is typically detected by in-cylinder pressure transducers, ion sensing or acoustic sensors.

It is the job of engine management to take corrective action when knock is detected. This usually takes the form of retarding ignition timing.