Lesson 1/Learning Event 1
By the time the rising piston reaches TDC, the camshaft has turned far enough to allow the exhaust valve to
close. At this same time, the intake valve cam lobe has moved into position to open the intake valve. This has
the piston and valves in the proper position to begin another intake stroke and to repeat all four strokes again.
Engine Operation - Four-Stroke Cycle
The crankshaft rotates half a turn to complete each stroke of the four piston strokes of intake, compression,
power, and exhaust. By adding four half turns, you will find the crankshaft must rotate two turns to complete
all four strokes. This is called one cycle of engine operation.
While the crankshaft is turning around twice (completing one cycle), it is receiving power for about half a turn
or one-fourth of the time.
On every power stroke, when the fuel-air mixture is ignited, the crankshaft will receive a hard twist. Then with
the aid of the flywheel, it will coast for one and one-half turns through the remaining strokes.
One cycle of engine operation requires that each valve be opened and closed one time. With a cam on the
camshaft under each valve, one turn of the camshaft opens and closes both valves as needed.
Therefore, the crankshaft must rotate two turns and the camshaft one turn for each cycle of engine operation.
Remember the crankshaft timing gear has half as many teeth as the camshaft timing gear to cause a difference
in rotations of the shafts.
Engine speed and power output on gasoline engines are controlled by a throttle valve placed in the fuel-air
mixture inlet. Closing this valve restricts or limits the amount of mixture flowing to the cylinder. This reduces
engine speed and power output. Of course, opening the valve allows more mixture to be drawn or sucked into
the cylinder, therefore increasing engine speed and power.