4-Cycle Engine Theory; Intake; Compression; Power - Tecumseh VLV Technician's Handbook

Tecumseh four-cycle engines require four strokes or cycles
to complete one power cycle.

Intake

Intake valve is open, exhaust valve is closed. Piston is
traveling downward creating a suction action, drawing the
fuel-air mixture from the carburetor into the cylinder area
above the piston.
1. Intake

Compression

As the piston reaches Bottom Dead Center, the IN-
TAKE valve closes. The piston then rises, compressing
the fuel and air mixture trapped in the combustion
chamber, because both valves are closed.
2. Compression
Some emission compliance engines may use an RCR
(Ramp Compression Relief) system. This system opens
the intake valve during the compression stroke allowing
a small amount of the intake charge back down the in-
take pipe. This eliminates unburned fuel going out the
exhaust during normal compression relief cycle typically
used on the exhaust valve.
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4-Cycle Engine Theory

NOTE

Power

Both valves remain closed. As the piston reaches the
Before Top Dead Center (BTDC) ignition point, the spark
plug fires, igniting the fuel-air mixture. In the time it
takes to ignite all the available fuel, the piston has
moved to TDC (Top Dead Center), ready to take the full
combustive force of the fuel for maximum power and
piston downward travel. The expanding gases force the
piston down.
3. Power

Exhaust

Exhaust valve opens. As the piston starts to the top of
the cylinder, the exhaust gases are forced out.
After the piston reaches Top Dead Center (TDC), the
four cycle process will begin again as the piston moves
downward and the intake valve opens.
4. Exhaust
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