The octanes of gasoline

When you go to the gas station, you choose gasoline of 92 octanes, or 89, or 87... What do those octanes mean?

The octane rating is a way to measure the gasoline's resistance to knocking. Theknocking is a rapid pinging or knocking sound which comes from an engine when it's pushed to produce a lot of power quickly. Usually occurs when a car is accelerating, especially while going uphill.

Gasoline is a blend of more than a hundred hydrocarbons (compounds with only carbon-hydrogen and carbon-carbon bonds). The more highly branched a hydrocarbon, the greater its tendency to burn smoothly and to resist knocking. A combination of the hydrocarbon 2,2,4-trimethylpentane (mistakenly named 'isooctane' or even simply 'octane') and heptane is used to evaluate octane ratings. Because of its high ability to burn smoothly and to resist knocking, 2,2,4-trimethylpentane ('octane') is assigned an octane rating of 100; heptane, with its great tendency to knock, receives an octane rating of 0. Mixtures of the two are given octane ratings equal to the percentage of the octane they contain.

To determine the octane rating of any particular gasoline, they compare the knocking tendencies of that particular blend with those of mixtures of "octane" and heptane. If, for example, a particular gasoline has knocking tendencies identical to those of a mixture of 92% "octane" and 8% heptane, under standard test conditions, they assign that blend of gasoline an octane rating of 92.

In summary, the higher the octane rating of a gasoline, the lower knocking tendency, because it contains more percentage of highly branched hydrocarbons.