Fuel Octane Rating

In recent years, car manufacturers have been requiring or recommending premium gasoline (a high-octane grade of fuel) for use in more of their vehicle models. The difference in prices between premium and lower octane grades has also increased. As a result, more people are curious about what octane is and what those octane rating numbers on gas pumps mean.

Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is actually the average of two different octane rating methods—motor octane rating (MOR) and research octane rating (RON)—that differ primarily in the specifics of the operating conditions. The higher an octane number, the more stable the fuel.

Of the 18 isomers of normal octane (C₈H₁₈), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C₇H₁₆) molecule is the 0 octane reference fuel.

Gasoline is a complex mixture of over 500 hydrocarbons that may have 5 to 12 carbons. Smaller amounts of alkane cyclic and aromatic compounds are present. Virtually no alkenes or alkynes are present in gasoline. Gasoline is most often produced by the fractional distillation of crude oil. The crude oil is separated into fractions according to different boiling points of hydrocarbons of varying chain lengths.