Intake Stroke

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Figure 2-4. Piston to Crankshaft Relationship

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 2-8. Four Stroke Cycle Operation.

TM 9-8000

power stroke, the exhaust valve opens, beginning the

2-5. Intake Stroke (A, Fig. 2-8). The Intake stroke

exhaust stroke. As the piston moves upward toward top

begins at top dead center. As the piston moves down,

dead center, it pushes the burnt gases from the fuel and

the intake valve opens. The downward movement of the

air mixture out of the combustion chamber through the

piston creates a vacuum in the cylinder. The vacuum

exhaust port. As the piston reaches top dead center,

causes a fuel and air mixture to be drawn through the

ending the exhaust stroke, the exhaust valve closes. As

intake port Into the combustion chamber. As the piston

the exhaust valve closes, the intake valve opens to begin

reaches bottom dead center, the Intake valve closes.

the Intake stroke in the next cycle.

2-6.

Compression Stroke (B, Fig.

2-8).

The

2-9. Valve Train. It is obvious in paragraphs 2-5 thru 28

compression stroke begins with the piston at bottom

that it is very important to operate the valves in a timed

dead center. Both the intake and the exhaust valves

sequence. If the exhaust valve opened In the middle of

remain closed. As the piston moves toward top dead

the intake stroke, the piston would draw burnt gases into

center, the amount of space in the upper cylinder gets

the combustion chamber with a fresh mixture of fuel and

smaller. The fuel and air mixture is compressed and the

air. As the piston continued to the power stroke, there

potential energy in the fuel is concentrated.

The

would be nothing in the combustion chamber that would

compression stroke ends when the piston reaches top

burn. The engine is fitted with a valve train to operate the

dead center.

valves.

2-7. Power Stroke (C, Fig. 2-8). As the piston reaches

A simplified valve train is illustrated in A, figure 2-9. The

top dead center, ending the compression stroke, the

camshaft is made to rotate with the crankshaft through

spark plug (para 15-3) ignites the compressed fuel and

the timing gears. The raised piece on the camshaft is

air mixture. Because both valves are closed, the force of

called a cam lobe. As illustrated in view B, the valve

the resulting explosion pushes the piston down, giving a

spring Is designed to hold the valve closed.

powerful driving thrust to the crankshaft. The power

stroke ends as the piston reaches bottom dead center.

The cam lobe contacts the bottom of the lifter as it

rotates with the camshaft, as shown in view C. As

2-8. Exhaust Stroke (D, Fig. 2-8). As the piston

reaches bottom dead center, ending the

Change 1 2-4