Section 2 Basics of Engine Operation
90
1. When atoms that make up tiny molecules
of air and fuel are squeezed closer together,
heat energy is created. Each molecule of fuel
is heated very close to its fl ash point (point
at which fuel will ignite spontaneously).
When combustion does occur, it is practically
instantaneous and complete for the entire
air-fuel mixture.
2. The force of combustion is increased
because tightly packed molecules are highly
activated and are striving to move apart. This
energy, combined with expanding energy
of combustion, provides tremendous force
against the piston.
Power Stroke
During the power stroke, both valves remain
in the closed position. See Figure  5-10C. As the
piston compresses the charge and reaches the top
of the cylinder, an electrical spark jumps the gap
Note
I It is p p po ssible s to t run u an engine on uncompressed I I i ib l i d
m m mi xtu ur es, s s but pow we r loss pr ro duces a very inefficient
engin en n ne .
large boost to the air-fuel induction process. With
nature balancing unequal pressures in this man-
ner, it follows that the larger the diameter of the
cylinder and the longer the stroke of the piston, the
greater the volume of air entering the cylinder on
the intake stroke.
Bear in mind that the intake valve performs
several key functions. These key functions are as
follows:
1. It must open at the correct instant to permit
intake of air-fuel mixture.
2. It must close at the correct time and seal
during compression.
3. Its shape must be streamlined, so the fl ow of
gases into the combustion chamber will not
be obstructed.
The intake valves do not get as hot as the
exhaust valves. The incoming air-fuel mixture
tends to cool the intake valve during operation.
Compression Stroke
The compression stroke occurs as the piston
moves upward in the cylinder. See Figure 5-10B.
During this stroke, the valves are tightly closed.
As the piston moves upward, the air-fuel
mixture is compressed into a smaller space. This
increases the force of combustion for two reasons:
Goodheart-Willcox Publisher
Figure 5-10.
Sequence of events in a four-stroke engine, requiring two revolutions of the crankshaft and one power stroke out
of four.
A B C D
Intake
stroke
First Rotation Second Rotation
Compression
stroke
Power
stroke
Exhaust
stroke
Copyright Goodheart-Willcox Co., Inc.
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