Section 2 Basics of Engine Operation
86
In Figure 5-2, the surface area of the wick in
the lighter is small. Vapor from the surface of the
liquid, combined with oxygen, is what burns readily.
If the surface of the liquid is small, relatively little
vapor will be given off to provide combustion.
Since the liquid must change to vapor before it is
burned, it would take considerable time to use up
the fuel at this rate.
By placing the same amount of fuel in a shallow,
wide container, more surface area will contact air,
more vapor will be given off, and the fuel will burn
more rapidly. See Figure 5-3.
To produce the rapid burning required in an
engine, gasoline must be broken up into tiny droplets
and mixed with air. This is called atomization.
Once the entire surface of each droplet of the
air-fuel mixture is exposed to the surrounding
air, a huge burning area becomes available. Given
a spark, the entire amount of gasoline will fl ash
into fl ame almost instantly. In effect, atomization
causes a sudden release of heat energy. See
Figure 5-4.
Combustion Force Must Be
Contained
To perform useful work, the force caused by
the rapidly burning gas must be contained and
controlled. To illustrate this point, imagine that a
metal lid is suspended on a string and held several
inches above the ground. If a mixture of gasoline
and air (atomized) were sprayed under it and
ignited, the lid would be raised a short distance by
the force of the combustion. See Figure 5-5.
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Figure 5-1.
A—An external combustion engine burns fuel outside the engine. B—An internal combustion engine burns fuel
within the engine.
A B
Steam
boiler
Fuel burning outside of engine Steam engine
Steam pressure
Piston
Flywheel
Fuel burning
within engine
Piston
Cylinder
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Figure 5-2.
A small flame is produced, due to a small area of
exposed fuel.
Fuel
Small
flame
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Figure 5-3.
A large flame is produced by a large area of exposed fuel.
Fuel
(gasoline)
Large flame
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