Octane number of a fuel

The octane number of a fuel describes the tendency of a fuel to detonate.  Detonation is the undesirable combustion of fuel inside and engine.  The combustion occurs when the piston is not in the power stroke.  The detonation produces a pinging sound and can damage the engine components.

Detonation is chiefly caused by octane.

The octane number is the percentage of octane in a mixture of iso-octane and iso-heptane.

To calculate the octane number of a fuel, an engine is run with the fuel.  Trials are conducted in which detonation occurs in the engine.

The engine is then run with different mixtures of iso-octane and iso-heptane in the same conditions.  The trial is repeated with different mixtures.  The conditions at which knocking occurs are noted.

The combination of iso-octane and iso-heptane which causes knocking at the same conditions as that of the fuel is identified.

For instance, let us say that a combination of 65% iso octane and 35% iso heptane causes knocking in the same conditions as that of the fuel.

Then, the fuel is said to have an octane number of 65.