very quickly. A low volatility gasoline vaporizes
slowly. A good gasoline should have the right volatility
for the climate in which the gasoline is used.
If the gasoline is too volatile, it will vaporize in the
fuel system. The result will be a condition called
VAPOR LOCK. Vapor lock is the formation of vapor in
the fuel lines in a quantity sufficient to prevent the flow
of gasoline through the system. Vapor lock causes the
vehicle to stall from lack of fuel. In the summer and in
hot climates, fuels with low volatility lessen the
tendency toward vapor lock.
In modern high compression gasoline engines, the
air-fuel mixture tends to ignite spontaneously or to
explode instead of burning rather slowly and uniformly.
The result is a knock, a ping, or a detonation. For this
reason, gasoline refiners have various ways to make
gasoline that does not detonate easily.
Agasoline that detonates easily is called low octane
gasoline. A gasoline that resists detonation is called
high octane gasoline.
The octane rating of a gasoline is a measurement of
the ability of the fuel to resist knock or ping. A high
octane rating indicates the fuel will NOT knock or ping
easily. It should be used in a high compression or turbo-
charged engine. A low octane gasoline is suitable for a
low compression engine.
Octane numbers give the antiknock value of
gasoline. A higher octane number (91) will resist ping
better than a gasoline with a low octane number (83).
Each manufacturer recommends an octane number for
For proper combustion and engine performance,
the right amounts of air and fuel must be mixed together.
If too much fuel or too little fuel is used, engine power,
fuel economy, and efficiency are reduced.
For a gasoline engine, the perfect air to fuel ratio is
15:1 (15 parts air to 1 part fuel by weight). Under
constant engine conditions, this ratio can help assure
that all fuel is burned during combustion. The fuel
system must change the air-fuel ratio with the changes
in engine-operating conditions.
Lean Air-Fuel Mixture
A lean air-fuel mixture contains a large amount of
air. For example, 20:1 would be a very lean mixture. A
slightly lean mixture is desirable for high gas mileage
and low exhaust emissions. Extra air in the cylinder
ensures that all the fuel will be burned; however, too
lean of a mixture can cause poor engine performance
(lack of power, missing, and even engine damage).
Rich Air-Fuel Mixture
A rich air-fuel mixture contains a little more fuel
mixed with the air. For gasoline, 8:1(8 parts air to 1 part
fuel) is a very rich mixture. A slightly rich mixture tends
to increase power; however, it also increases fuel
consumption and exhaust emissions. An overly rich
mixture will reduce engine power, foul spark plugs, and
cause incomplete burning (black smoke at engine
For gasoline or any other fuel to burn properly, it
must be mixed with the right amount of air. The mixture
must then be compressed and ignited. The resulting
combustion produces heat, expansion of the gases, and
Normal gasoline combustion occurs when the spark
plug ignites the fuel and burning progresses smoothly
through the fuel mixture. Maximum cylinder pressure
should be produced after a few degrees of crank rotation
after the piston passes TDC on the power stroke.
Normal combustion only takes about 3/1,000 of a
This is much slower than an explosion.
Dynamite explodes in about 1/50,000 of a second.
Under some undesirable conditions, however, gasoline
can be made to bum quickly, making part of the
combustion like an explosion.
Abnormal combustion occurs when the flame does
NOT spread evenly and smoothly through the
combustion chamber. The lean air-fuel mixture, high-
operating temperatures, low octane, and unleaded fuels
used today make abnormal combustion a major
problem that creates unfavorable conditions, such as the