Evinrude 4906B 4HP 1969 Owner's Manual page 44

is ignited; and the power stroke begins.
reed valve was open because as the piston
moved upward, the crankcase volume increased,
which reduced the crankcase pressure to less
than the outside atmosphere.
As the piston moves downward
power stroke, the combustion chamber is filled
with burning gases.
uncovered, the gases, which are under great
pressure, escape rapidly through the exhaust
ports. The piston continues
movement. Pressure
increases, closing the reed valves against their
seats.
The crankcase then becomes a sealed chamber.
The-fuel mixture is compressed
delivery the combustion chamber. As the piston
continues to move downward the intake port is
uncovered. Fresh fuel rushes through
intake port into the combustion chamber strik-
ing the top of the piston where it is deflected
along the cylinder wall.
closed until the piston moves upward again.
When the piston begins to move upward on
the compression stroke, the reed valve opens
because the crankcase
increased, reducing crankcase pressure to less
than the outside atmosphere.
exhaust ports are closed and the fresh fuel
charge is compressed
chamber.
DEFLECTOR
INTAKE
Complete piston cycle
The
on the
As the exhaust port is
its downward
within the crankcase
ready for
the
The valve remains
volume has been
The int.ake and
inside the combustion
COMPRESS ION
of
a two-cycle
Pressure in the crankcase decreases as the
piston moves upward and a fresh charge of air
flows through the carburetor picking up fuel.
As the piston approaches top dead center, the
spark plug ignites the air-fuel mixture,
power stroke begins and one complete cycle is
completed.
Cross Fuel Flow Principle
OMC pistons are a deflector dome type.
The design is necessary
charge up and around the combustion chamber.
The fresh fuel mixture enters the combustion
chamber through the intake ports and flows
across the top of the piston. The piston design
contributes to clearing the combustion chamber,
because the incoming fuel pushes the burned
gases out the exhaust ports.
Loop Scavenging
Some powerheads have what is commonly
known as a loop scavenging system. The piston
dome is relatively flat on top with just a small
amount of crown.
crankcase is forced up through the skirt of the
piston and out through irregular shaped open-
ings cut in the skirt. Mer the fuel is forced out
the piston skirt openings
ward through long deep grooves molded in the
cylinder walls. The fuel then enters the com-
IGNITION POWER STROKE
engine, depicting intake, power, and exhaust.
INTRODUCTION
to deflect the fuel
Pressurized fuel in the
it is transferred
EXHAUST
3-3
the
up-
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