Carburetion - Johnson SKEE-HORSE 25-201R Service Manual

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3-2
POWER STROKE - DOWN
COMBUSTION OF
FUEL-AIR
MIXTURE
CONNECTING COUNTER-
ROD BALANCE
AXIS OF
ROTATION
F igure 3-1
171 3 3
FUEL INTAKE AND EXHAUST
LEAF VALVES
EXHAUST
PORT OPEN
Figure 3-2
17134
COMPRESSION STROKE - UP
FUEL FROM
EXHAUST
PORT CLOSED
Figure 3-3
17 135
TWO CYCLE ENGINE THEORY
An internal combustion engine is one in which fuel is burned inside the
engine: a charge of fuel is introduced into a combustion chamber
(cylinder) within the engine and ignited. The energy released by the
expansion of the burning fuel is converted to torque by the piston, con-
necting rod, and crankshaft.
Internal combustion engines are classified as either four-cycle or
two-cycle engines. The "four" and the "two" refers to the number of
piston strokes required to complete a power cycle of intake, compres-
Sion, power , and exhaust. A piston stroke is piston travel in one direc -
tion only; up is one stroke, down is another. In a four - cycle engine,
two crankshaft revolutions, or four strokes, are required for each power
cycle. In a two-cycle engine only one crankshaft revolution is required
per power cycle.
In
a two-cycle engine , the ignition of the fuel-air mixture occurs as
the piston reaches the top of each stroke. The expansion of gases drives
the piston downward (see Figure 3-1). Toward the end of the downward
stroke, ports which lead from the cylinder to the exhaust system are
uncovered. The expanding exhaust gases flow into these ports, reducing
pressure in the cylinder. Immediately after, intake ports are opened.
These ports connect the cylinder with the crankcase where a mixture
of fuel and air has been developed by carburetion. The downward motion
of the piston compresses this mixture and forces it through the intake
ports into the cylinder. See Figure 3-2.
The inrushing charge of the fuel-air mixture helps to eject (scavenge)
the last of the exhaust gases from the cylinder. At this point, the mo-
mentum of the flywheel is required to return the piston to the top of the
cylinder . As the piston begins its up-stroke, it closes the intake and
exhaust ports and begins to compress the fuel-air mixture trapped in the
cylinder. See Figure 3-3. The upward motion of the piston also re -
duce s the pressure in the crankcase. The resulting crankcase suction
opens leaf valves which admit a fresh charge of air and fuel from the
carburetor into the crankcase, thus preparing for the next power cycle.
Near the top of the piston stroke, the compressed fuel-air mixture is
ignited, the piston is driven downward, and the power cycle is repeated.
At full throttle, this cycle may be repeated more than five thousand
times every minute.
CARBURETION
The system which controls the intake of the fuel-air mixture in the
two cycle engine consists of a set of leaf valves which serve the same
purpose as the intake valves on a four cycle engine. The leaf valves
are thin, flexible metal strips mounted between the carburetor intake
manifold and crankcase.
When the piston is on the up-stroke, it creates a partial vacuum in
the crankcase. Atmospheric pressure forces the leaves away from the
body (see Figure 3-4), opening the passage between the carburetor and
crankcase. When the piston is on the down-stroke, it compresses the
crankcase charge, forcing the leaves against the passage opening, and
sealing off the crankcase from the carburetor. Since the opening and
clOSing may occur in excess of five thousand times per minute, the
leaves must be thin and flexible.
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