Figure 2-2.Piston Engine Operation.
system called the exhaust port. A simplified arrangement
to each of these ports and these valves are called the
Is shown in figure 2-5. By putting openings in the
intake and exhaust valves. A simplified arrangement is
combustion chamber, another problem is created. The
shown in figure 2-6.
problem is that the force of the burning fuel and air
The intake and the exhaust valves are opened and
mixture will be lost through these openings rather than
closed in a timed sequence by the valve train. The valve
pushing down the piston. To solve this problem, there
train will be discussed in paragraph 2-9.
must be something that opens and closes the intake and
exhaust ports to the combustion chambers.
accomplish this, a valve is added
2-4. Action in the Cylinder. When the piston is at its
highest point in the cylinder, it is in a position called top
dead center (tdc). When the piston is at its lowest point
in the cylinder, it is in a position called bottom dead
center (bdc). As the piston moves from top dead center
to bottom dead center or vice versa, the crankshaft
rotates exactly one-half of a revolution, as shown in
Each time the piston moves from top dead center to
bottom dead center, or vice versa, it completes a
movement called a stroke.
Therefore, the piston
completes two strokes for every full crank-shaft
revolution. There are four definite phases of operation
that an engine goes through in one 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. 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
Figure 2-3 . Piston and Crankshaft .
each operating cycle.