Mazda Training manual — part 231

2 – BASIC OPERATION

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Piston Engine Fundamentals

TC010-05-01S

THE FOUR-STROKE CYCLE

Most automotive engines use a four-stroke cycle to complete the
combustion process. A stroke is the movement of the piston from its
highest point in the cylinder to its lowest point, or from its lowest point to
the highest point.

The piston’s highest point in the cylinder is called top dead center, or TDC.
The lowest point is called bottom dead center, or BDC. As Figure 4 shows,
a full stroke of the piston takes one half-turn of the crankshaft, or 180
degrees.

The four-stroke cycle requires four up-and-down movements of the piston
to complete the cycle. Figure 5 shows the stages of the four-stroke cycle,
including the:

1. Intake stroke

2. Compression stroke

3. Power (combustion) stroke

4. Exhaust stroke

FIGURE 4. A stroke
is the distance a
piston travels from
top dead center to
bottom dead center,
or from bottom dead
center to top dead
center.

Top dead center
(TDC)
One stroke

Bottom dead
center (BDC)

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FIGURE 5. The
four-stroke cycle
includes intake,
compression,
power, and
exhaust strokes.

Intake valve

Exhaust valve

Intake port

Exhaust port

Intake Stroke

The four-stroke cycle begins with the intake stroke. The rotating crankshaft
pulls the piston down from TDC. The exhaust valve is closed, and the
intake valve is open.

As the piston moves down in the cylinder, it creates a partial vacuum,
which draws the air-fuel mixture through the intake valve into the cylinder.
The closed exhaust valve prevents the mixture from escaping through the
exhaust port.

When the piston is at BDC, the intake stroke is completed. However, the
intake valve may be held open slightly longer to let the air-fuel mixture fill
the cylinder more completely.

Intake Stroke

Compression

Stroke

Power Stroke

Exhaust Stroke

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Piston Engine Fundamentals

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Compression Stroke

As the piston passes BDC and starts up again, the compression stroke begins. The intake
valve closes, and the exhaust valve stays closed. With both valves closed, the air-fuel
mixture is trapped in the cylinder. As the piston pushes up into the cylinder, it squeezes, or
compresses, the air-fuel mixture into a very small volume between the piston and cylinder
head.

Compression of the air-fuel mixture is very important for developing power. The greater the
compression, the more pressure the mixture will create when it burns. Compression also
“pre-heats” the mixture to help it burn better. Because compression is so important, the
valves and piston rings must seal the cylinder perfectly, or power will be reduced.

Power Stroke

Just before the piston reaches TDC, a spark from the spark plug ignites the air-fuel mixture,
and the power stroke begins. The burning gases expand rapidly, creating very high pressure
on top of the piston. Both valves remain tightly closed, so all the force is directed down onto
the piston, which is pushed down in the cylinder and turns the crankshaft. The greatest push
on the piston occurs in the first half of the power stroke.

By the time the piston approaches BDC, most of the pressure in the cylinder is used up. At
this point, the exhaust valve begins to open, relieving any remaining pressure so the piston
can move back up into the cylinder during the final stroke.

Exhaust Stroke

During the exhaust stroke, the exhaust valve remains open, and the rotating crankshaft
pushes the piston back up the cylinder. The motion of the piston pushes the burned gases
out through the exhaust valve.

As the piston passes TDC, the four-stroke cycle begins again with the intake stroke. The
exhaust valve stays open momentarily at the beginning of the intake stroke, allowing the
momentum of the gases to empty the cylinder completely.

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Piston Engine Fundamentals

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Summary

We have illustrated the four-stroke cycle in only one cylinder. Remember, though, that these
four strokes are continuously repeated in all the cylinders.

The four strokes of the cycle — intake, compression, power, and exhaust — require two full
rotations of the crankshaft. However, the piston receives direct combustion pressure only
during one stroke, or about one quarter of the cycle.

When you realize that no power is being generated during 3/4 of the four-stroke cycle, you
can begin to see why the flywheel is so important. The flywheel “stores” the energy that is
generated, and uses that stored energy to keep the pistons moving smoothly when they are
not receiving combustion pressure.



REVIEW EXERCISE 1

Fill in the words that correctly complete these sentences. Check your answers with the
answer key on page 15.

1. In an engine, power is generated when an air-fuel mixture is ignited in the free space

at the top of the ____________________.

2. To convert the up-and-down motion of the piston into rotary motion, the piston is

attached to a connecting rod, which is connected to the _______________________.

3. The heavy plate connected to the crankshaft that stores the energy generated during

combustion is called the _______________________.