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How are octane numbers related to engine knocking?

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structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

The octane number is a measure of a fuel's ability to resist knocking within an internal-combustion engine.12

Knocking occurs when a portion of the air-fuel mixture in the engine cylinder ignites prematurely and burns uncontrollably, creating high-frequency pressure waves that cause the engine to vibrate and produce an audible sound. This phenomenon often happens at high compression ratios and can lead to engine damage, such as overheating and surface erosion.3

The octane number functions as an indicator of antiknock quality: a higher number signifies a greater resistance to premature ignition, meaning the fuel is less likely to cause knocking. The rating is determined by comparing the fuel's performance against a mixture of iso-octane—which resists knocking—and heptane, which knocks easily. Using gasoline with a higher octane number is the most common method to prevent the inefficient and potentially harmful effects of engine knocking.213