What petroleum-industry rating measures gasoline’s resistance to engine knocking?

The octane rating measures gasoline’s resistance to engine knocking.

Knocking is abnormal combustion that can produce a sharp metallic sound and potentially damage an engine. Gasoline with a higher octane rating can withstand greater compression before spontaneously igniting, making the rating important for engines designed with higher compression ratios or turbocharging.

The scale originated from laboratory comparison fuels. Iso-octane was assigned a value of 100, while n-heptane was assigned a value of 0. A fuel’s rating indicates how its knocking behavior compares with mixtures of those reference compounds under specified test conditions.

Octane rating is often confused with energy content. A higher number does not automatically mean that gasoline contains more energy or delivers more power in every vehicle. Cetane number serves a related but different purpose for diesel fuel, measuring ignition quality rather than gasoline’s resistance to knock.