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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
Compression
The compression event reduces the volume of
the air-fuel charge to increase its density and
energy potential. The process of compressing
increases the temperature of the charge, which
helps any liquid gasoline in the combustion
chamber to vaporize.
The compression of the air-fuel charge in a
two-stroke engine takes place in two stages: pri-
mary compression and secondary compression.
The terms “primary” and “secondary” are not
equated to terms like “major” and “minor” or
“most important” and “less important.” Instead,
the terms refer to the two stages of compressing
the air-fuel charge. Primary compression refers
to the “first stage” of compressing. This takes
place in the crankcase. Secondary compres-
sion refers to the “second stage” of compressing.
This takes place in the combustion chamber
and is a higher compression than occurs in the
crankcase.
Primary compression takes place on the vol-
ume of air-fuel charge that entered the crank-
case during the intake event. The piston is
moving down in the cylinder and compresses
the large volume of air-fuel charge in the crank-
case. Crankcase pressure begins to increase.
See Figure 5-40.
In the combustion chamber end of the cylin-
der, as the piston moves down, the first port to
open is the exhaust port followed immediately
by tops of the transfer ports. The uncovered
transfer ports open the passage for the air-fuel
charge to transfer from the crankcase, but the
transfer does not begin immediately. It takes a
short time for the crankcase charge to pressur-
ize. It also takes a short time for the flow up the
transfer ports to begin. Primary compression
continues even though the tops of the transfer
ports are open and charge continues to trans-
fer. During primary compression is when a
pressure pulse is sent from the crankcase to the
carburetor.
When the piston reaches BDC, the primary
compression process ends. See Figure 5-41.
The flow of air-fuel charge continues up the
transfer ports and into the area above the pis-
ton. This causes the pressure in the crankcase
to decrease.
Secondary compression takes place above the
piston and as the piston is moving up in a sealed
chamber. The exhaust port is covered. The
Intake
port
closed
Top of
transfer
ports
closed
Trapped
air-fuel
charge is
compressed
Goodheart-Willcox Publisher
Figure 5-40. The piston is traveling down in the
cylinder. The skirt closes off the intake port. No new
air-fuel charge enters the crankcase. The intake
cycle ends and primary compression begins in the
crankcase. The piston continues to move down in
the cylinder compressing the air-fuel charge.
Compressed
air-fuel charge
fills cylinder
Top of transfer
ports open
Goodheart-Willcox Publisher
Figure 5-41. As the piston approaches BDC,
it uncovers the top of the transfer ports.
Compressed air-fuel charge from the crankcase
floods into the cylinder above the piston.