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A gasoline's ability to resist detonation is called its "octane" or
anti-knock rating. Gasoline from asphaltic base crude oil produces
less knock than one from paraffinic base crude. Cracked gas has less
tendency to knock than straight run gas. All marketed gasolines are a
blend of straight run and cracked gasolines, so unless their blending
is controlled, the anti-knock qualities will vary.
A mixture of iso-octane, which has a very high anti-knock rating, and
heptane, which makes a pronounced knock, is used as a reference fuel
to establish an anti-knock standard. The anti-knock value or octane
number is represented by the percentage of volume of iso-octane that
must be mixed with normal heptane in order to duplicate the knocking
of the gasoline which is being tested. These ratings range from 50 in
third grade gasolines to 110 in aviational fuels. The rating of 100
means a fuel having an anti-knock value equal to that of iso-octane.
If the octane rating of a gasoline is naturally low, the fuel will
detonate as it burns and power will be applied to the pistons in
hammer-like blows. The ideal power is that which pushes steadily on
the pistons, rather than hammer against them. The octane rating of a
gasoline can be raised by treating it with a chemical which is not a
fuel. The best chemical known is tetra-ethyl lead compound, which is
added to the gasoline.
Tetra-ethyl lead is a liquid which mixes thoroughly with gasoline and
vaporizes completely. Ethylene dibromide prevents the tetra-ethyl
lead from forming lead oxide deposits on spark plugs and on valve
seats and stems. Red dye is added to identify an ethyl treated
gasoline and to warn against its being used as anything but an engine
fuel. In 1975, it became illegal to use a leaded gasoline except in
cars built prior to this time. With the addition of the catalytic
converter, it is undesirable to burn leaded fuel, because leaded fuel
will clog the converter and increase the back-pressure of the
exhaust.
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