Octane Number

A rating of the 'knock' characteristics of motor fuels. The octane number is equal to the numerical value of the percentage by volume of iso-octane, C₈H₁₈, in a mixture of iso-octane and heptane, C₇H₁₆, having the same knock characteristics as the fuel being tested.

The higher the octane number or rating, the greater are the antiknock qualities of the petrol.

See also: Engine Knock.