PRIN. OF INTERNAL COMBUSTION ENGINES - OD1619 LESSON 1/TASK 1
complete operating cycle. Each one of these operating phases is completed in one
piston stroke. Because of this, each operating phase is also referred to as a
stroke and, because there are four strokes of operation, the engine is referred to
as a four stroke cycle engine. The four strokes are intake, compression, power,
and exhaust. Because there are four strokes in one operating cycle, it may be
concluded that there are two complete crankshaft revolutions in each operating
cycle.
(a) Intake Stroke (figure 7 on page 9). The intake stroke begins at top dead
center. As the piston moves down, the intake valve opens. The downward movement
of the piston with the exhaust valve closed creates a vacuum in the cylinder. The
vacuum causes a fuel and air mixture to be drawn through the intake port into the
combustion chamber. As the piston reaches bottom dead center, the intake valve
closes.
(b) Compression Stroke. The compression stroke begins with the piston at
bottom dead center. Both the intake and the exhaust valves remain closed. As the
piston moves toward top dead center, the amount of space in the upper cylinder gets
smaller. The fuel and air mixture is compressed and the potential energy in the
fuel is concentrated. The compression stroke ends when the piston reaches top dead
center.
(c) Power Stroke. As the piston reaches top dead center, ending the power
stroke, the spark plug ignites the compressed fuel and air mixture. Because both
valves are closed, the force of the resulting explosion pushes the piston down,
giving a powerful driving thrust to the crankshaft. The power stroke ends as the
piston reaches bottom dead center.
(d) Exhaust Stroke. As the piston reaches bottom dead center, ending the
power stroke, the exhaust valve opens, beginning the exhaust stroke. As the piston
moves upward toward top dead center, it pushes the burnt gases from the fuel and
air mixture out of the combustion chamber through the exhaust port. As the piston
reaches top dead center, ending the exhaust stroke, the exhaust valve closes. As
the exhaust valve closes, the intake valve opens to begin the intake stroke in the
next cycle.
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