Engine knockingKnocking (also called pinking or pinging) in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug but burns too quickly, combusting completely before the optimum moment during the compression phase of the four-stroke cycle. The resulting shockwave collides with the rising piston, creating a characteristic metallic "pinging" sound. The fuel is normally ignited slightly before the point of maximum compression to allow a small time for the flame front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the point of maximum compression. It is only when this flame front arrives too early, for whatever reason, that the knocking effect occurs. If allowed to persist, knocking can cause vibration and damage to engine parts.
Knocking can typically be prevented by the use of higher octane gasoline, the addition of octane-increasing "lead or isooctane additives to the gasoline, reduction of in-cylinder temperatures (such as through cooling or compression ratio reduction), or retardation of spark plug ignition. Correct ignition timing is essential for optimum engine performance and efficiency. Modern automotive engines have sensors that can detect knock and delay the spark plug firing to prevent it, allowing cars to safely use gasoline of lower octane ratings, with the consequence of reduced power and efficiency.
Knocking is a different phenomenon from pre-ignition, which occurs when the air-fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by heat buildup in engine components or overheating of the air-fuel mixture during compression, and cannot be prevented by delaying spark plug firing. As such, if pre-ignition is allowed to continue for any length of time, severe engine damage can result.