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TM 9-8000
2-1. Introduction.
2-2. Reciprocating Motion to Rotary Motion.
The force of the piston acting in a downward motion is of
little value if it is to turn the wheels of the vehicle. In
a. Because the most widely used piston engine is
order to utilize this straight line or reciprocating motion, it
the four-stroke cycle type, it will be used as the example
must be transformed into rotary motion. This is made
for section I, Engine Operation and as the basis for
possible through the use of a crankshaft.
comparison in section II, Comparison of Engine Types.
crankshaft, as the name implies, is a shaft connected to
the driving wheels of a vehicle through the drive train on
b. The operation of the piston engine can best be
one end. On the other end of the shaft is a crank with a
understood by comparing it to a simple cannon. In A,
crankpin offset from the shaft's center.  Figure 2-3
figure 2-1 a cannon barrel, charge of gunpowder, and a
illustrates how the piston and the crankshaft are
cannonball are illustrated. In B, figure 2-1 the gunpowder
connected through the connecting rod and the crankpin.
is ignited. The gunpowder burns very rapidly and as it
burns there is a rapid expansion of the resulting gases.
Figure 2-4 illustrates how reciprocating motion of the
This rapid expansion causes a tremendous increase in
piston is changed to rotating motion of the crankshaft.
pressure that forces the cannonball from the barrel.
2-3.  Intake and Exhaust.  If the engine is going to
In A, figure 2-2 the cannon barrel has been replaced by a
operate, the fuel and air mixture must be fed into the
cylinder and a combustion chamber. The cannonball has
combustion chamber.  The burnt gases also must be
been replaced by a piston. A mixture of vaporized fuel
exhausted after the fuel is burned. To accomplish this,
and air has replaced the gunpowder. In B, figure 2-2 the
there is a passage to the combustion chamber called the
gasoline is ignited. This time, the resulting force acts to
intake port and a passage from the combustion chamber
push the piston downward.
to the exhaust
Figure 2-1 . Piston Engine Principles


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