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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
Introduction
The two-stroke engine is a spark-ignited inter-
nal combustion engine. An internal combustion
engine produces power inside the engine itself.
For example, a person may be holding a string
trimmer while the trimmer’s rotating string
cuts grass. The power to rotate the string comes
from the engine the operator is holding. The
term combustion refers to the process of burn-
ing fuel mixed with air. As an internal combus-
tion engine, the two-stroke engine burns the air
and fuel inside the engine itself.
An example of a non-internal combustion
engine would be a steam engine, where the
steam, the power for the engine, is produced in
an external boiler. The steam is then routed to
the engine to make the engine run.
A spark-ignition (SI) engine uses a spark to
ignite the fuel and air mixture. The time when
the spark plug sparks is called the spark event, or
just the spark. The presence of a spark plug iden-
tifies the two-stroke engine as a “SI” engine. The
spark plug provides a location for high-voltage
spark to take place in the engine. The voltage for
the spark is generated by the ignition coil, which
is connected to the spark plug. The spark plug in
the two-stroke engine in Figure 5-1 is at the top
of the cylinder.
The burning air-fuel mixture produced in
the engine is used to push a piston down in
the cylinder bore, causing the piston to stroke.
The piston is the solid cylindrical part of the
engine that slides up and down in the cylinder
bore. A stroke of the piston is one movement of
the piston, either up or down, in the cylinder.
The upward stroke (also called the up stroke)
of the piston begins when the piston is at the
bottom of the cylinder. The piston then moves
“up” towards the top of the cylinder, Figure 5-2.
During the downward stroke (down stroke), the
piston moves from the top of the cylinder to the
bottom of the cylinder. An internal combustion
engine is usually referred to by the number of
strokes the piston makes to produce power. A
two-stroke engine produces power during every
other stroke of the piston. A four-stroke engine
produces power during every fourth stroke of
the engine.
In this chapter we will discuss:
• The four events and two strokes a two-
stroke engine needs to produce power.
Goodheart-Willcox Publisher
Figure 5-1. The presence of a spark plug identi-
fies an engine as a spark ignition (SI) engine.
Spark plug
Cylinder
Piston
Goodheart-Willcox Publisher
Figure 5-2. The piston is at the bottom of the cyl-
inder and will move toward the top of the cylinder
on the “upward” or “up” stroke.