Octane rating or octane number is a standard measure of the
performance of a motor or aviation fuel. The higher the octane
number, the more compression the fuel can withstand before
detonating. In broad terms, fuels with a higher octane rating are
used in high-compression engines that generally have higher
performance. In contrast, fuels with low octane numbers (but high
cetane numbers) are ideal for diesel engines. Use of gasolines with
low octane numbers may lead to the problem of engine knocking.
Gasoline is a volatile, flammable liquid obtained from the
refinement of petroleum, or crude oil. It was originally discarded
as a byproduct of kerosene production, but its ability to vaporize
at low temperatures made it a useful fuel for many machines.
Gasoline is primarily a mixture of two volatile liquids, heptane
and isooctane. Pure heptane, a lighter fuel, burns so quickly that
it produces a great amount of knocking in an engine. Pure isooctane
evaporates slowly and produces virtually no knocking. The ratio of
heptane to isooctane is measured by the octane rating. The greater
the percentage of isooctane, the less knocking and the higher the
octane rating. For example, an octane rating of *7 is comparable to
a mixture of *7% isooctane and *3% heptane.Gasoline is one of the
products derived from distilling and refining petroleum. Compounds
of organic lead were added to gasoline in the past to reduce
knocking in engines, but due to environmental concerns this is no
longer common. Other chemicals are also added to gasoline to
further stabilize it and improve its color and smell in a process
called "sweetening."