110
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
valve engine because the carburetor is mounted
directly to the crankcase.
On a reed valve engine, there is a reed assem-
bly between the carburetor and the crankcase.
See Figure 5-4. The reed valve itself is a thin
metal plate of spring steel that forms a one-way
check valve between atmospheric pressure and
crankcase pressure. It is located between the
carburetor and the crankcase.
Just like the piston ported engine, the intake
event begins with the piston at bottom dead
center. Then, the piston moves up in the cylin-
der to create a low pressure area in the crank-
case. When the pressure difference between
atmospheric pressure and crankcase pressure
is great enough to overcome the value of the
spring, atmospheric pressure outside the engine
causes air to rush into the crankcase, unseating
the valve. The air passes through the carburetor
first and picks up premixed gasoline and oil at
the correct ratio.
The air moving into the crankcase speeds up
as it passes through the carburetor and picks up
premixed fuel. Once the piston passes top dead
center, the reed valve closes and traps the air-
fuel charge in the crankcase. Primary compres-
sion begins in the crankcase as the piston moves
down in the cylinder.
Because of the mechanical nature of reed
valves, they usually limit engine speed to less
than 10,000 rpm. To compensate for lower
engine speeds, manufacturers generally change
the design of the crankshaft so it produces more
torque.
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