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What do the octane numbers of gasoline mean?

Find out how an octane number is determined and how it affects gasoline.

A gasoline octane rating refers to the rate at which the fuel incinerates in an engine. [©Jupiter Images, 2008]
©Jupiter Images, 2008
A gasoline octane rating refers to the rate at which the fuel incinerates in an engine.

The octane number is a measure of the gasoline's ability to resist engine knock (pinging caused by premature ignition).

Two test fuels, normal heptane and isooctane, are blended for test results to determine octane number. Normal heptane has an octane number of zero and isooctane a value of 100. Gasolines are then compared with these test blends to find one that makes the same knock as the test fuel.

The octane rating of the gasoline under testing is the percentage by volume of isooctane required to produce the same knock. For example, if the test blend has 85 percent isooctane, the gasoline has an octane rating of 85.

The octane rating that appears on gasoline pumps is an average of research octane determined in laboratory tests with engines running at low speeds, and motor octane, determined at higher speeds.

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