Octane Number Definition and Example

What Octane Rating Means

Octane number reflects resistance for engine knock.
Octane number reflects resistance for engine knock. PM Images, Getty Images

The octane number is a value used to indicate the resistance of a motor fuel to knock. Octane number is also known as octane rating. Octane numbers are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock). The higher the octane number, the more compression required for fuel ignition. Fuels with high octane numbers are used in high performance gasoline engines. Fuels with low octane number (or high cetane numbers) are used in diesel engines, where fuel is not compressed.

Octane Number Example

A gasoline with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane.

Why the Octane Number Matters

In a spark-ignition engine, using a fuel with too low an octane rating can lead to pre-ignition and engine knock, which can cause engine damage. Basically, compressing the air-fuel mixture may cause fuel to detonate before the flame front from the spark plug reaches it. The detonation produces higher pressure than the engine may be able to withstand.