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Octane number:

An octane number is the laboratory measurement of gasoline's ability to prevent detonation (spark knock) during combustion. A single-cylinder four-stroke engine, designed with an adjustable compression ratio, is used to determine the octane rating. Two pure hydrocarbons: isooctane, rated 100(a very high knock-resistant), and heptane rated 0 (a very poor knock-resistant) are blended together to match the performance of the gasoline being tested. The octane number is the percentage of isooctane added with the heptane. Once tested, the equation (R+M)/2 is used to find the average percentage rating. An eighty eight rating on the gas pump would indicate that an average of 88% isooctane blend was need to match the gasoline's performance in the test engine. To achieve a gasoline with a rating over 100 octane, an additive must be added to pure isooctane.