by Octane number
The most commonly used measure of a gasoline's ability to burn
without knocking is its octane number. Octane numbers compare
a gasoline's tendency to knock against the tendency of a blend
of two hydrocarbons – heptanes and 2,2,4-trimethylpentane, or
isooctane – to knock. Heptanes (C7H16) are a long straight-chain
alkane, which burns unevenly and produce a great deal of knocking.
Highly branched alkanes such as 2,2,4-trimethylpentane are more
resistance to knocking. Gasoline's that match a blend of 87% isooctane
and 13% heptanes are given an octane number of 87.
There are three ways of reporting octane number. Measurement made
at high speed and high temperature is reported as motor octane
numbers. Measurements taken under relatively mild engine conditions
are known as research octane number. The road-index octane number
reporting on gasoline pumps are an average of these two. Road-index
octane numbers for a few pure hydrocarbons are given in the table
below.
By 1922 an umber of compounds had been discovered
that could increase the octane number of gasoline. Adding oF
gasoline, for example, can increase the octane number by 15 to 20
units. This discovery gave rise to the first '' ethyl'' gasoline,
and enabled the petroleum industry to produce aviation gasolines
with octane number greater than 100.
