2
CYCLE PRINCIPLES
OF
OPERATION
I
Transfer
Transfer
Port
31
SCAVENGING
COMPRESSION/
INTAKE
Lawn-Boy
"M"
Series engines are 2 cycle design. They are piston ported loop scavenged
design. There are only 2 strokes
of
the piston to complete the entire power cycle in one
revolution of the crankshaft. Four major events occur during each revolution of the crankshaft.
They are: Power, Exhaust, Scavenging, and Compression/Intake Ignition of the air-fuel
mixture will occur during compression. During
the
1st Cycle (first 1/2 revolution) Power and
Exhaust occur. During the 2nd Cycle (second 1/2 revolution) Scavenging and Compression/
Intake occur. Near Top Dead Center Ignition occurs and a fresh charge of air-fuel mixture is
drawn into the crankcase.
1st CYCLE
Down
Stroke
Power
The burning air-fuel mixture expands, pushing the piston and connecting
exhaust port, power will continue. The intake port has dosed.
Exhaust
When the exhaust port begins to open, the intake port has closed.
As
the
piston continues to move down, the exhaust port is opened and
the
burnt gases flow out
the
exhaust port.
At
the
same time the volume
of
the crankcase is decreasing, the air-fuel mixture in the crankcase
is
increasing in pressure until the piston uncovers the transfer ports.
rod down, causing the crankshaft to rotate. Until the piston opens the
2nd
CYCLE
Up
Stroke
Scavenging
Near the bottom
of
the down stroke and during the beginning of the up
stroke, the transfer ports open. The pressure is higher in the crankcase
than in the combustion chamber, the air-fuel mixture in the crankcase
moves from the crankcase through the transfer ports into the combustion
chamber. This action purges the remaining exhaust gases from the
combustion chamber replacing it with fresh air-fuel mixture.
Compression/
As
the piston moves upward
it
closes the transfer ports and then the
Intake
exhaust port. With transfer and exhaust
ports
dosed,
the volume of the
combustion chamber decreases and the air-fuel mixture is compressed.
Just before top dead center, ignition of the air-fuel mixture occurs to
begin the next complete cycle. At the same time the piston skirt opens
the intake port and the volume
of
the crankcase is increasing. With higher
air pressure outside the carburetor than in the crankcase, the air rushes
in mixing with fuel in the carburetor and moves into the crankcase.
1-3
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