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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
been pushing down on the piston is instead sent
out the exhaust port, and the power event ends.
During the downstroke portion of the power
event, the force pushing down on the piston
is transmitted through the connecting rod to
the crankshaft, counterweights, and flywheel
mass, causing it to accelerate. The rotation of
this mass continues in the form of inertia. The
inertia of the flywheel, crankshaft, and coun-
terweights carries the piston down through pri-
mary compression and up through secondary
compression.
Exhaust
The exhaust event begins when the piston is
going down from TDC and the crown of the pis-
ton uncovers the exhaust port. Exhaust gases
exit the exhaust port and enter the muffler. See
Figure 5-43. Exhaust gases enter the exhaust
port at combustion chamber pressure. As the
exhaust flow continues, the remaining exhaust
gas follows the flow out the exhaust port,
decreasing the combustion chamber pressure.
The cross-sectional dimensions of the exhaust
port are engineered for the operating speed and
displacement of the engine. Each volume of
exhaust must be removed from the combustion
chamber before the piston covers the exhaust
port for the next secondary compression event.
The initial wave of high-pressure exhaust
gases passes through the exhaust port and
enters the muffler. Baffles inside the muffler
redirect and break up sound waves for noise
suppression. The muffler also allows the gases to
expand, creating a low pressure area in the muf-
fler. This helps draw in the next high-pressure
exhaust pulse. The size and shape of the inside
of the muffler is engineered to work with the
specific displacement of the engine and its oper-
ating speed. The muffler is “tuned” to the fre-
quency of the exhaust events.
The exhaust event is the longest of the four
events and spans both the downstroke and the
upstroke of the piston. It overlaps primary com-
pression, intake, and transfer. After the exhaust
port is uncovered, the tops of the transfer ports
are uncovered and the piston continues moving
down to perform primary compression. During
primary compression, the air-fuel charge trans-
fers up the transfer ports and into the area
above the piston. The piston continues to bot-
tom dead center. The piston starts up from bot-
tom dead center, creating the low pressure in the
crankcase to start the intake event. The upward
movement also pulls down the contents of the
transfer ports, causing transfer to end. The
charge transferred into the combustion cham-
ber swirls in a pre-engineered pattern designed
to keep unburned hydrocarbons from exiting
through the exhaust port. All this takes place
while the exhaust port is open. Once the exhaust
gases begin moving through the exhaust port,
they will continue to flow during the downward
travel and then the upward travel of the piston.
As the piston moves up in the cylinder, it first
covers the tops of the transfer ports. This stops
the flow of the air-fuel charge into the combus-
tion chamber. The last port covered by the pis-
ton is the exhaust port. This ends the exhaust
event.
Intake Event for a Reed Valve
Engine
The primary compression, secondary com-
pression, power, exhaust, and transfer events
are the same for both piston ported and reed
valve engines. Intake is different on the reed
Exhaust
port open
Claire McAdams/Shutterstock.com, Goodheart-Willcox Publisher
Figure 5-43. Heat and exhaust gases leave the
combustion chamber during the exhaust event.