knock

Knock is audible, often metallic pinging or rattling that occurs when fuel self-ignites inside a spark ignition engine cylinder before the spark plug fires. The flame front from premature combustion collides with the normal flame front ignited by the spark, creating shock waves that strike the cylinder walls and produce the distinctive sound. This is not a design feature or harmless noise; it indicates abnormal combustion that reduces power output and can damage engine internals.

The root cause is fuel octane rating too low for the engine's compression ratio and operating conditions. Higher compression ratios demand higher octane fuel. A 10:1 compression engine may run 87-octane fuel without knock, while a 12:1 engine needs 91-octane or higher. Ambient temperature, engine load, spark timing, and fuel composition all influence knock tendency. Ethanol content raises octane, so E10 fuel (10% ethanol) has higher knock resistance than pure gasoline at the same research octane number.

Modern engines use knock sensors, typically piezoelectric devices screwed into the block, to detect vibrations at knock frequency (5 to 10 kHz depending on engine size). The engine control unit monitors these sensors and retards spark timing in real time if knock is detected, sacrificing some performance to prevent damage. This adaptive strategy allows manufacturers to run higher compression ratios and lighter ignition timing under normal conditions while protecting the engine during temporary knock conditions caused by poor fuel or abnormal driving.

Extended knock damages ring lands, pistons, and bearing surfaces through mechanical stress and localized overheating. A single engine shutdown followed by knockless restart causes no harm. Chronic knock from consistently using undersized octane fuel or a malfunctioning knock sensor can score cylinder walls and score piston crowns within thousands of miles. Turbo and supercharged engines are far more knock-sensitive than naturally aspirated ones at the same stated compression ratio, because boost pressure effectively increases compression.

The term originates directly from the sound: the metallic knock audible to a driver or mechanic with the hood open. In the early carbureted era, drivers could hear knock and manually advance or retard spark timing with a lever or dial. Today knock is detected electronically and corrected automatically, but the name persists unchanged. Diesel engines do not experience knock in the gasoline sense, although they exhibit diesel knock, a completely different phenomenon related to ignition delay.