ENGINE
Piston Ported Engine
When the piston opens the exhaust port, high-pressure
exhaust gases exit the exhaust port. (See Figure 68.)
Further movement of the piston towards the crankcase
uncovers the transfer port, which allows a fresh charge
of oil/fuel mixture to enter the combustion chamber. As
the exhaust gases continue to exit the exhaust port, the
engine is readied for another cycle. (See Figure 69.)
In a piston ported engine, there are no reed valves.
The fuel/air mixture enters the crankcase through a
third port that is opened and closed by the piston. (See
Figure 66.) All other functions are the same as a reed
valve engine.
Figure 66
A. Intake Ports Closed C. Third Port Closed
B. Compression D. Partial Vacuum
Figure 68
A. Intake Ports Closed C. Third Port Open
B. Exhaust Port Opening D. Pressure Building Up
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Both Piston Ported and Reed Valve
Engines
At about 28 degrees before top dead center (BTDC),
the spark plug fires. When the engine is functioning
properly, the air/fuel mixture ignites, causing a burn
that occurs evenly through the power stroke. (If the
compression ratio is too high, the air/fuel mixture
actually explodes, which can be heard and is identified
as detonation.) The burn continues as the piston
changes direction and begins its travel towards the
crankcase. (See Figure 67.)
Figure 69
A. Intake Ports Open C. Fresh Fuel Change
B. Exhaust Port Open D. Third Port Closed
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The 2-cycle engine is always well lubricated as long as
the correct oil/fuel mixture is maintained in the fuel
tank. Oil suspended in the fuel vapor adheres to all
moving parts, keeping them continually coated,
regardless of operating angle.
Figure 67
A. Intake Ports Closed C. Third Port Open
B. Ignition D. Partial Vacuum
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