PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 1
unburned part of the mixture to the point of selfignition. This secondary
wavefront collides with the normal wavefront, making an audible knock or
ping. It is an uncontrolled explosion, causing the unconfined gases in the
combustion chamber to rap against the cylinder head walls. Detonation may
harm an engine or hinder its performance in several ways. In extreme cases,
pistons have been shattered, rings broken, or heads cracked. Detonation
also may cause overheating, excessive bearing wear, loss of power, and high
fuel consumption.
d. Octane Rating.
(1) The ability of a fuel to resist detonation is measured by its
octane rating. The octane rating of a fuel is determined by matching it
against mixtures of normal heptane and isooctane in a test engine, under
specified test conditions, until a pure mixture of hydrocarbons is found
that gives the same degree of knocking in the engine as the gasoline being
tested. The octane number of the gasoline then is specified as the percent
of isooctane in the matching isooctane normal heptane mixture. For
example, a gasoline rating of 75 percent octane is equivalent in its
knocking characteristics to a mixture of 75 percent isooctane and 25
percent normal heptane. Thus, by definition, normal heptane has an octane
rating of 0 and isooctane has an octane of 100 percent.
(2) The tendency of a fuel to detonate varies in different engines,
and in the same engines under different operating conditions. The octane
number has nothing to do with starting qualities, potential energy,
volatility, or other major characteristics. Engines are designed to operate
within a certain octane range. Performance is improved with the use of
higher octane fuels within that operational range. Engine performance will
not be improved if a gasoline with an octane rating higher than the
operational range is provided.
(3) Tetraethyl lead is the most popular of the compounds added to
gasoline to raise its octane rating. The introduction of catalytic
converters, however, has created a need for a higher octane leadfree
gasoline produced by more careful refining processes and numerous
substitutes for lead. Leadfree gasolines to date, however, do not have the
antiknock qualities of leaded ones.
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