2-Stroke Cycle Engine
ratio. This compression of the fuel-air mixture
increases the pressure within the cylinder. Compressing
In the 2-stroke cycle engine, the same four events
the mixture in this way makes it more combustible, as
(intake, compression, power, and exhaust) take place in
not only does the pressure in the cylinder increase, but
only two strokes of the piston and one complete
the temperature of the mixture as well.
revolution of the crankshaft.
Power Stroke
A 2-stroke cycle diesel engine is shown in figure
3-6. Each time the piston moves down, it is on the
As the piston reaches top dead center at the end of
power stroke. Since this engine does the same thing in
the compression stroke and moves to the top of the
half as many strokes as the 4-stroke cycle engine, let's
cylinder, the compressed fuel-air mixture is ignited.
take a closer look at its construction. Instead of intake
The ignition system causes an electric spark to occur
valves, it has holes, called inlet ports, surrounding the
suddenly in the cylinder, and the spark ignites the
cylinder walls. It also has a blower to force the air into
fuel-air mixture. In burning, the mixture gets very hot
the cylinder when the inlet ports are uncovered by the
and tries to expand in all directions. The pressure rises
piston on its downward stroke. Since this engine is a
to about 600 or 700 pounds per square inch. Since the
diesel, it requires no spark plugs. A diesel engine
piston is the only thing that can move, the force
requires only a means of introducing fuel into the
produced by the expanding gases forces the piston
cylinder, where the fuel burns due to the high
down. This force, or thrust, is carried through the
temperature of the compressed air.
connecting rod to the crankshaft throw on the
Here is how one cycle occurs with only two strokes
crankshaft. The crankshaft is given a powerful push.
of the piston. The sequence of events is intake,
This is called the power stroke (fig. 3-5). This turning
compression, power, and exhaust. Intake begins when
effort, rapidly repeated in the engine and carried
the inlet ports are uncovered as the piston moves down
through gears and shafts, will turn the wheels of a
in the cylinder. It continues until the piston moves up,
vehicle and cause it to move.
covering the ports and blocking the airflow into the
Exhaust Stroke
cylinder. Very shortly after the inlet ports are covered,
the exhaust valve closes and the compression stroke
After the fuel-air mixture has burned, it must be
begins. During the compression stroke, the air is
cleared from the cylinder. This is done by opening the
compressed to ignite the fuel that is injected near the
exhaust valve just as the power stroke is finished and
end of the upward movement of the piston. The
the piston starts back up on the exhaust stroke (fig. 3-5).
pressure from combustion forces the piston down on
The piston forces the burned gases out of the cylinder
the power stroke.
past the open exhaust valve.
As the piston nears the intake ports, the exhaust
valve opens, starting the exhaust event and allowing the
ENGINE CYCLES
burned gases to start escaping. The continued
downward motion of the piston uncovers the inlet ports,
The four strokes (intake, compression, power, and
and then air (under pressure) is forced into the cylinder
exhaust) are continuously repeated as the engine runs.
and aids the exhaust event by pushing the burned gases
Now, with the basic knowledge you have of the parts
out of the cylinder. This cleaning action is referred to as
and the four strokes of the engine, let us see what
"exhaust gas scavenging."
happens during the actual running of the engine. To
produce sustained power, an engine must accomplish
4-Stroke Cycle Versus 2-Stroke Cycle
the same series of events--intake, compression, power,
and exhaust over and over again. This series of events is
A power stroke is produced every crankshaft
called a cycle. Remember that in a 4-stroke cycle
revolution within the 2-stroke cycle engine, whereas
engine, it takes four complete strokes of the piston to
the 4-stroke cycle engine requires two crankshaft
complete one engine cycle. Most engines that you will
revolutions for one power stroke.
deal with are of the 4-stroke cycle design.
3-5
