50 Auto Fundamentals
In the fi rst two chapters, you designed and
constructed a basic engine and studied engine parts
and their uses. This chapter studies the various
methods in which basic engine theory and parts
are applied in multi-cylinder engines. Methods
of engine classification as well as different kinds
of engines will be discussed. Internal combustion
engines can be classified in many ways. The most
common methods used to classify engines are by
cycle, valve location, cylinder arrangement, type of
fuel used, cooling, and number of cylinders. This
chapter will discuss each classification.
Cycle Classifi cation
Engines are often classified according to cycle.
Most internal combustion piston engines use a
two- or four-stroke cycle. The basic engine, which
was discussed in the fi rst two chapters, is a four-
stroke cycle engine. Figure 3-1 illustrates that it
takes two complete crankshaft revolutions to com-
plete the cycle. As a result, each cylinder receives
only one power stroke per cycle. All modern vehi-
cle engines use the four-stroke cycle engine.
Two-Stroke Cycle Engine
The two-stroke cycle engine performs the
intake, compression, fi ring, and exhaust sequence
in one revolution of the crankshaft. This is done
by eliminating the poppet valves used in the
four-stroke cycle engine. In place of the valves,
two ports enter the cylinder wall. One is used
for the intake of fuel and the other for exhaust.
A passage connects the intake port to
the crankcase interior. The air-fuel mixture is
drawn into the crankcase through a reed valve
or a rotary valve. Oil is usually mixed with the
gasoline to provide lubrication. Some two-stroke
cycle engines use an oil injection system. Upon
entry into the crankcase, the oil provides ample
lubrication for the bearings. Two-stroke cycle
engines are used in chain saws, lawn mowers,
marine outboard motors, go-carts, and other
small engine applications. Due to emission con-
trol problems, gasoline powered two-stroke
cycle engines are not used in road vehicles.
Two-Stroke Cycle Operation
Imagine that a charge of air and fuel is in
the cylinder and that the piston is at the bottom
of its stroke. As the piston travels up, it closes off
both intake and exhaust ports. As it continues
upward, it compresses the air-fuel charge while
forming a vacuum in the airtight crankcase.
The vacuum pulls open a small fl ap-like reed
valve and a mixture of air and fuel enters the
crankcase. When the piston reaches the top of its
stroke, it has compressed the charge in the cyl-
inder and fi lled the crankcase with an air-fuel
Intake valve
open
Exhaust
closed
Exhaust
closed
Exhaust
open
Intake
closed
Intake
closed
Intake
closed
Exhaust valve
closed
Spark plug fires
Intake stroke Compression stroke Power stroke Exhaust stroke
Figure 3-1.
Four-stroke cycle principle. This series of events requires two complete revolutions (720° of rotation) of the crankshaft.
Note that there is one power stroke per cycle.
TRW, Inc.
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