Evinrude 4906B 4HP 1969 Owner's Manual page 43

3-2 POWERHEAD
First, it creates
incoming charge of fuel enters the combus-
tion chamber. This turbulence
more complete burning of the fuel than if
the piston top were flat.
of the deflector-type
force the exhaust gases from the cylinder
more rapidly.
This system of intake and exhaust is in
marked contrast to individual valve arrange-
ment employed on four-cycle engines.
Lubrication
A two-cycle engine is lubricated by mix-
ing oil with the fuel.
parts are lubricated
passes through the crankcase and the cylin-
der. Four-cycle engines have a crankcase
containing oil. This oil is pumped through a
circulating system
crankcase to begin the routing again.
Power Stroke
The combustion
engine has four distinct phases.
1- Intake
2- Compression
3- Power
4- Exhaust
Three phases of the cycle are accom-
plished with each stroke of the piston,
the fourth phase, the power stroke occurs
ATMOSPHERIC
Air flow principle for a modern carburetor.
turbulence when the
results
The second effect
piston crown is to
Therefore, various
as the fuel mixture
and returned to the
cycle of a two-cycle
and
••
\
AIR PRESSURE
with each
Compare this system with a four-cycle
gine. A stroke of the piston is required to
in
accomplish each phase of the cycle and the
power stroke occurs on every other revolu-
tion of the crankshaft.
two revolutions
crankshaft are required to complete one full
cycle, the four phases.
Physical Laws
The two-cycle engine is able to function
because of two very simple physical laws.
One: Gases will flow from an area of
high pressure to an area of lower pressure.
A tire blowout is an example of this princi-
ple. The high-pressure air escapes rapidly if
the tube is punctured.
Two: If a gas is compressed into a smal-
ler area, the pressure increases, and if a gas
expands into a larger area, the pressure is
decreased.
If these two laws are kept in mind, the
operation of the two-cycle
easier understood.
Actual Operation
Beginning with the piston approaching
top dead center on the compression stroke:
The intake and exhaust ports are closed by
the piston; the reed valve is open; the spark
plug fires; the compressed fuel-air
Adding OMC approved oil into the [uel tank.
revolution of the crankshaft.
Stated another way,
of the four-cycle
engine will be
en-
engine
mixture