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OPERATION OF HSK, HXL840-850 MODELS.
A low pressure area is created in the crankcase as the
piston moves upward to compress the air / fuel mixture
in the cylinder. Then the piston moves far enough to
uncover the intake port, the air / fuel mixture from the
carburetor flows into the engine crankcase due to higher
pressure atmospheric air. Just before the piston reaches
top dead center (TDC), the spark plug ignites the
air / fuel mixture in the cylinder.
The expanding combustion gases force the piston down.
The downward piston travel causes a pressure buildup
in the crankcase. The piston uncovers the exhaust port
first followed by the transfer ports. The exhaust flows
out the exhaust port while the pressurized air / fuel
mixture enters the cylinder from the crankcase through
the transfer ports. As the piston travels upward the
sequence is repeated.
OPERATION OF TVS, TVXL840
This model uses a loop scavenge system with a reed
valve.
The following illustrates the loop scavenge design which
uses a vacuum-pressure activated reed valve. Here the
ports are located on three sides of the cylinder; the intake
ports are on two sides opposite each other, and the
exhaust ports are illustrated by the three holes just above
the head of the piston.
As the mixture flows into the combustion chamber
through the transfer ports, it collides and is directed to
the top of the combustion chamber looping when it
strikes the cylinder head, thus forcing all spent gases
out through the open exhaust ports.
TERMS.
Listed are common terms often referred to on
2-cycle engines.
EXHAUST or SCAVENGE PHASE.
The burned gases
must be cleared out of the combustion chamber and
replaced by a fresh charge of fuel-air mixture through
the intake ports. The exhaust passes out through the
exhaust ports into the outside air.
PORTS
. Openings in the cylinder allow gases to pass
into and out of the combustion chamber. The ports are
opened or closed by the upward and downward
movement of the piston.
EXHAUST PORTS.
Allow the burned gases to pass out
of the combustion chamber.
THIRD PORT (Sometimes called piston port).
A third
port is for entry of the fuel-air and oil mixture to the
crankcase. From the crankcase the fuel-air mixture
enters the combustion chamber through the intake ports.
The third port is controlled by the piston skirt.
LUBRICATION.
Tecumseh 2-cycle engines are
lubricated by a gas oil mix. The correct mix ratio of oil
and gas combines with air in the venturi and enters the
crankcase. During engine operation the oil clings to all
the internal moving parts for lubrication.
NOTE: ALWAYS MIX AT OUR RECOMMENDED
RATIO AND USE ONLY CERTIFIED OILS OR
WARRANTY IS VOID.
REED VALVE.
A reed valve is activated by crankcase
pressure or vacuum. A decrease in crankcase pressure
opens the valve allowing the fuel air and oil mixture to
enter the crankcase. Increased crankcase pressure
closes the valve, preventing escape of the fuel-air and
oil mixture back through the carburetor.
4.
3.
2.
1.
2 CYCLE THEORY OF OPERATION
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