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The octane rating of gasoline is measured in a test engine and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and normal heptane that would have the same anti-knocking capability as the fuel under test. The percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel.
In the United States and Canada, octane ratings are in AKI, commonly shown as "(R+M)/2". All states require gas pumps to be labeled with the correct octane level and nearly all states do regular testing to make sure gas stations are in compliance. A minimum 82 octane fuel is recommended for most vehicles produced since 1984.
Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. There are different conventions for expressing octane ratings, so the same physical fuel may have several different octane ratings based on the measure used. One of the best known is the research octane number (RON).
However, in engines that utilize compression rather than spark ignition, by means of very high compression ratios (14–25:1), such as the diesel engine or Bourke engine, high octane fuel is not necessary. In fact, lower-octane fuels, typically rated by cetane number, are preferable in these applications because they are more easily ignited ...
It was widely used as a fuel additive for much of the 20th century, first being mixed with gasoline beginning in the 1920s. This "leaded gasoline" had an increased octane rating that allowed engine compression to be raised substantially and in turn increased vehicle performance and fuel economy.
As more effort is put into maximizing an engine to take advantage of E85's higher octane rating, engines achieve greater power advantages. One car that has higher power on ethanol is the Koenigsegg CCXR, which on ethanol is the fifth-most powerful production car, with 20% more horsepower on E85 than on gasoline. According to the manufacturer ...
A fuel designated grade 130 would produce 130 percent as much power in an engine as it would running on pure iso-octane. During WWII, fuels above 100-octane were given two ratings, a rich and a lean mixture, and these would be called 'performance numbers' (PN). 100-octane aviation gasoline would be referred to as 130/100 grade. [43]
It has an octane rating (MON/RON) that is between 90 and 110 and an energy content (higher heating value—HHV) that is between 25.5 megajoules per litre (for pure propane) and 28.7 megajoules per litre (for pure butane) depending upon the actual fuel composition.