Westerbeke 33a Owner's Manual download pdf (Page 5)

POWER STROKE

During the power stroke, the piston is pushed downward with both the intake and exhaust

valves closed. By the time the piston reaches the end of the compression stroke, the

burning fuel causes a further increase in the pressure above the piston. As more fuel is

added and burns, the gases get hotter and expand more to push the piston downward and

to add momentum to the crankshaft rotation.

EXHAUST STROKE

During the exhaust stroke, the intake valves are closed, the exhaust valves are opened,

and the piston is on its upstroke. Burned gases are forced out of the combustion chamber

through the open exhaust valve ports by the upward travel of the piston and by the

pressure differential at the exhaust outlet.

Proper engine operation depends upon two things; first, compression for ignition and

second, that fuel be measured and injected into the cylinder in the proper quantity and at

the proper time.

It takes two revolutions of the crankshaft to complete a cycle. Therefore, as piston one

starts its power stroke, it turns the crankshaft through 90° or one quarter of a revolution

(for an eight cylinder engine) before the second piston in the firing order starts its power

stroke. The third piston follows the second piston, and so on until after two full

crankshaft revolutions all pistons have started or completed a power stroke.

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Westerbeke 33a Owner's Manual Page 5