| (of gasoline) a designation of antiknock quality, numerically equal to the percentage of isooctane by volume in a mixture of isooctane and normal heptane that matches the given gasoline in antiknock characteristics. |
| octane number n. A numerical representation of the antiknock properties of motor fuel, compared with a standard reference fuel, such as isooctane, which has an octane number of 100. Also called octane rating. |
octane number
measure of the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an internal-combustion engine. The octane number is determined by comparing, under standard conditions, the knock intensity of the fuel with that of blends of two reference fuels: iso-octane, which resists knocking, and heptane, which knocks readily. The octane number is the percentage by volume of iso-octane in the iso-octane-heptane mixture that matches the fuel being tested in a standard test engine. See also knocking
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