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octane number,figure of merit representing the resistance of gasolinegasoline
light, volatile mixture of hydrocarbons for use in the internal-combustion engine and as an organic solvent, obtained primarily by fractional distillation and "cracking" of petroleum, but also obtained from natural gas, by destructive distillation
..... Click the link for more information. to premature detonation when exposed to heat and pressure in the combustion chamber of an internal-combustion engine. Such detonation is wasteful of the energy in the fuel and potentially damaging to the engine; premature detonation is indicated by knocking or pinging noises that occur as the engine operates. If an engine running on a particular gasoline makes such noises, they can be lessened or eliminated by using a gasoline with a higher octane number. The octane number of a sample of fuel is determined by burning the gasoline in an engine under controlled conditions, e.g., of spark timing, compression, engine speed, and load, until a standard level of knock occurs. The engine is next operated on a fuel blended from a form of isooctane that is very resistant to knocking and a form of heptane that knocks very easily. When a blend is found that duplicates the knocking intensity of the sample under test, the percentage of isooctane by volume in the blended sample is taken as the octane number of the fuel. Octane numbers higher than 100 are found by measuring the amount of tetraethyl lead that must be added to pure isooctane to duplicate the knocking of a sample fuel. At present three systems of octane rating are used in the United States. Two of these, the research octane and motor octane numbers, are determined by burning the gasoline in an engine under different, but specified, conditions. Usually the motor octane number is lower than the research octane. The third octane rating, which federal regulations require on commercial gasoline pumps, is an average of research octane and motor octane. Under this system a regular grade gasoline has an octane number of about 87 and a premium grade of about 93. Most American-made cars that were built in the 1971 model year or later can use regular gasoline. To prevent knocking, premium grade gasoline must be used in many cars built before 1971 and in some new cars that have high-performance engines.
an arbitrary quantitative characterization of the antiknock qualities of motor fuels used in carburetor internal-combustion engines. The octane number is determined by comparing the fuel being tested with secondary standard fuels. The primary standards are isooctane, whose high antiknock value is arbitrarily assumed to be 100 on the octane scale, and n-heptane, whose antiknock value is assumed to be zero. The antiknock value of the fuel being tested is characterized by determining the composition of the mixture of isooctane and n-heptane that has an equivalent antiknock value. Quantitatively, the octane number indicates the percentage of isooctane (by volume) in the reference mixture.
The octane number is determined under standardized conditions using special, small one-cylinder engines according to one of three methods (the motor method, the research method, or the temperature method), which differ in the operational parameters of the engine. The motor method is used for evaluating aviation gasolines and automotive gasolines with octane numbers 65–100; the research method, for automotive gasolines; and the temperature method, in testing high-octane aviation gasolines and their high-octane components with octane numbers 90–115, and also in determining the quality grade of gasolines. The octane number characterizes the fuel under conditions of a lean mixture (excess air ratio, 0.9–1.1).