enclosed end of a cylinder has two openings. One of the
openings, or ports, permits the mixture of air and fuel to
enter, and the other port permits the burned gases to
escape from the cylinder. The two ports have valves
assembled in them. These valves, actuated by the
camshaft, close off either one or the other of the ports,
or both of them, during various stages of engine
operation. One of the valves, called the intake valve,
opens to admit a mixture of fuel and air into the cylinder.
The other valve, called the exhaust valve, opens to allow
the escape of burned gases after the fuel-and-air mixture
has burned. Later you will learn more about how these
valves and their mechanisms operate.
The following paragraphs explain the sequence of
actions that takes place within the engine cylinder: the
intake stroke, the compression stroke, the power stroke,
and the exhaust stroke. Since these strokes are easy to
identify in the operation of a four-cycle engine, that
engine is used in the description. This type of engine is
called a four-stroke-Otto-cycle engine, named after Dr.
N. A. Otto who, in 1876, first applied the principle of
The first stroke in the sequence is the intake stroke
(fig. 12-4). During this stroke, the piston is moving
downward and the intake valve is open. This downward
movement of the piston produces a partial vacuum in
the cylinder, and air and fuel rush into the cylinder past
the open intake valve. This action produces a result
similar to that which occurs when you drink through a
straw. You produce a partial vacuum in your mouth, and
the liquid moves up through the straw to fill the vacuum.
When the piston reaches bottom dead center at the
end of the intake stroke (and is therefore at the bottom
of the cylinder) the intake valve closes and seals the
upper end of the cylinder. As the crankshaft continues
to rotate, it pushes the connecting rod up against the
piston. The piston then moves upward and compresses
the combustible mixture in the cylinder. This action is
known as the compression stroke (fig. 12-4). In gasoline
engines, the mixture is compressed to about one-eighth
of its original volume. (In a diesel engine the mixture
may be compressed to as little as one-sixteenth of its
original volume.) This compression of the air-fuel
mixture increases the pressure within the cylinder.
Compressing the mixture in this way makes it more
combustible; not only does the pressure in the cylinder
go up, but the temperature of the mixture also increases.
As the piston reaches top dead center at the end of
the compression stroke (and is therefore at the top of the
cylinder), the ignition system produces an electric spark.
The spark sets fire to the fuel-air mixture. In burning,
the mixture gets very hot and expands in all directions.
The pressure rises to about 600 to 700 pounds per square
inch. Since the piston is the only part that can move, the
force produced by the expanding gases forces the piston
down. This force, or thrust, is carried through the
connecting rod to the crankpin on the crankshaft. The
crankshaft is given a powerful twist. This is known as
the power stroke (fig. 12-4). This turning effort, rapidly
repeated in the engine and carried through gears and
shafts, will turn the wheels of a vehicle and cause it to
move along the highway.
After the fuel-air mixture has burned, it must be
cleared from the cylinder. Therefore, the exhaust valve
opens as the power stroke is finished and the piston starts
back up on the exhaust stroke (fig. 12-4). The piston
forces the burned gases of the cylinder past the open
exhaust valve. The four strokes (intake, compression,
power, and exhaust) are continuously repeated as the
Now, with the basic knowledge you have of the parts
and the four strokes of the engine, let us see what
happens during the actual running of the engine. To
produce sustained power, an engine must repeatedly
complete one series of the four strokes: intake,
compression, power, and exhaust. One completion of
this series of strokes is known as a cycle.
Most engines of today operate on four-stroke
cycles, although we use the term four-cycle engines to
refer to them. The term actually refers to the four strokes
of the piston, two up and two down, not the number of
cycles completed. For the engine to operate, the piston
continually repeats the four-stroke cycle.
In the two-cycle engine, the entire series of strokes
in two piston strokes.
power, and exhaust) takes place