Copyright Goodheart-Willcox Co., Inc.
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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
Summary
• A two-stroke engine is a spark-ignition, internal combustion engine.
• A two-stroke engine produces power during one of every two strokes of
the engine.
• The power cycle is a sequence of four operational events required to
produce power.
• The four events in the power cycle are intake, compression, power, and
exhaust.
• Two-stroke handheld engines can be classified into piston ported and
reed valve models, depending on where the carburetor is mounted and
the method of controlling the intake of air and fuel.
• The main components of a two-stroke engine include the crankcase,
cylinder, piston, crankshaft, connecting rod, flywheel, ignition coil, and
spark plug.
• The intake event begins when the piston is at bottom dead center. As
the piston moves up in the cylinder, low pressure is created in the
crankcase. The air-fuel charge from the carburetor, which is at a higher
pressure, rushes into the crankcase.
• The compression event occurs in two stages: primary compression and
secondary compression. Primary compression begins when the piston
is at TDC. As the piston moves down, it compresses the air-fuel charge
in the crankcase. As the piston clears the tops of the transfer ports, part
of the crankcase air-fuel charge is transferred to the space above the
piston. Primary compression ends when the piston reaches BDC, but
air-fuel transfer continues. Secondary compression begins during the
upstroke, when the piston seals the exhaust port, and continues until
the piston reaches TDC.
• Before the piston reaches TDC, the spark plug fires and ignites the air-
fuel charge. The power event begins when the piston reaches TDC. The
expanding combustion gases push the piston downward. The power
event ends when the crown of the piston moves below the top edge of
the exhaust port.
• The exhaust event begins as the piston’s crown moves below the top
edge of the exhaust port. The pressure difference between the muf-
fler and the cylinder causes the exhaust gases to rush out through the
exhaust port. The exhaust event is the longest of the four events in the
operating cycle and continues until the piston moves up to block the
exhaust port.
Chapter Questions
Answer the following questions using the information provided in this chapter.
1. A two-stroke engine gets its name because it completes one operating
_____ in two strokes of the piston.
2. In a piston-ported two-stroke engine, the intake port is sealed by the
_____ during primary compression.
3. In a(n) _____ engine, a thin metal or composite diaphragm allows air-
fuel mixture to enter the crankcase during intake, and seals the intake
port during primary compression.