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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
• Two types of two-stroke engines.
• The components of a two-stroke engine.
• How the two-stroke engine operates.
Cycles and Strokes
The two-stroke engine is a two-stroke cycle
engine. The phrase two-stroke cycle means that
the engine will use two strokes of the piston
to produce one “cycle” of power. One operat-
ing cycle of the engine consists of four distinct
events: taking in air plus fuel, compressing it,
igniting it to produce a power push on the pis-
ton, and then getting rid of the exhaust gases.
After getting rid of the exhaust gases, the engine
begins the next operating cycle.
In a two-stroke engine, there is an order in
which the four events must take place. Intake
must happen before compression. The power
event ignites the air and fuel, but then the engine
must get rid of the burned air-fuel gases before
the engine can take in the next mixture of fresh
air and fuel. Even though there is a sequence for
the four events—intake, compression, power, and
exhaust—more than one event is occurring during
each stroke of the two-stroke engine’s piston.
The Four Events in a Two-Stroke
Engine
During the intake event, the engine brings in
air from outside the engine and forces the air
to travel through a carburetor before entering
the engine. The carburetor is mounted on the
engine and sends fuel into the engine, depend-
ing on the amount of air entering the engine.
The carburetor adds the proper amount of fuel
to the airstream to create a highly combustible
air-fuel mixture.
The compression event reduces the volume
of the air-fuel mixture in the engine’s cylinder
in order to improve the engine’s ability to create
power. In a two-stroke engine, the air-fuel mix-
ture goes through two stages of compression to
increase its ability to create power. We will cover
these two stages in more detail later in the chapter.
The power event occurs when the air-fuel mix-
ture is ignited to produce expanding gases. The
expanding gases push the piston with force. The
engine uses this force to perform “work,” like
rotating a string on a string trimmer, driving a
blade on a chain saw, or blowing leaves with a
blower.
During the exhaust event, the exhaust gases
are expelled from the engine. The by-products
from burning the air-fuel mixture during the
power event are called the exhaust gases. When
these gases leave the engine, they are referred to
as the “exhaust.”
The Two Strokes in a Two-Stroke
Engine
The two-stroke engine gets its name from the
fact that it uses only two strokes of the piston to
perform the four events in the operating cycle.
One stroke occurs when the piston moves toward
the top of the engine and the other occurs when
the piston moves down toward the bottom of
the engine. During the upward stroke of the pis-
ton, the engine is performing the first part of
the intake event, the second half of the exhaust
event, and the last of two stages of the compres-
sion event. During the downward stroke of the
piston, the engine is performing the second part
of the intake event, the complete power stroke,
and the first of the two stages of the compres-
sion event.
Types of Two-Stroke Engines
The location of the carburetor on a two-stroke
engine determines how the two-stroke engine is
classified. The two major classifications of two-
stroke handheld engines are:
• Piston ported engine.
• Reed valve engine.
The Piston Ported Engine
When the carburetor is mounted on the cylinder
of the engine, the engine is identified as a piston
ported engine. See Figure 5-3 . The cylinder of
a two-stroke engine has cooling fins on the out-
side to carry off engine heat during engine oper-
ation and contains the piston on the inside. Air
for the engine enters through the carburetor, is
mixed with fuel, and is sent into the engine. The
“port” in the cylinder through which the air-
fuel charge enters is called the intake port. The
piston itself controls the process of the air-fuel
charge entering the engine. As the piston moves
up in the cylinder, it opens the intake port. Later,
as the piston moves back down in the cylinder,
it seals off the intake port. Thus, an engine with
this design is called “piston ported.”