PART TWO. ENGINES

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 2-2.Piston Engine Operation.

TM 9-8000

PART TWO

ENGINES

CHAPTER 2

PISTON ENGINE CHARACTERISTICS

Section I. ENGINE OPERATION

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.

The

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In A, figure 2-2 the cannon barrel has been replaced by a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been replaced by a piston.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

2-1