Chevrolet Light Duty Truck 1973 Service Manual page 352

6-16 ENGINE
Intake Stroke
The intake valve is opened as the piston moves down in
the cylinder (Fig. 3D). The piston traveling downward in
the cylinder creates an area of pressure lower than that
of the atmosphere surrounding the engine. Atmospheric
pressure will cause air to flow into this low pressure area.
By directing the air flow through the carburetor, a
measured amount of vaporized fuel is added. When the
piston reaches the bottom of the intake stroke, the
cylinder is filled with air and vaporized fuel. The exhaust
valve is closed during the intake stroke.
Compression Stroke
When the piston starts to move upward, the compression
stroke begins (Fig. 4D). The intake valve closes, trapping
the air-fuel mixture in the cylinder. The upward
movement of the piston compresses the mixture to a
fraction of its original volume; exact pressure depends
principally on the compression ratio of the engine.
Power Stroke
The power stroke is produced by igniting the compressed
air-fuel mixture. When the spark plug arcs, an explosion
does not occur. Instead, the mixture ignites and burns
very rapidly during the power stroke. The extremely high
temperature expands the gases, creating a very high
pressure on the top of the piston which drives the piston
Fig. 2D—Sectional View of Six-Cylinder Engine
down. This downward motion of the piston is
transmitted through the connecting rod and is converted
to rotary motion by the crankshaft. Both the intake and
exhaust valve are closed during the power stroke (Fig.
5D).
Exhaust Stroke
The exhaust valve opens just before the piston completes
the power stroke (Fig.
this time causes the exhaust gas to rush into the exhaust
manifold. The upward movement of the piston on its
exhaust stroke expels most of the remaining exhaust gas.
As the piston pauses momentarily at the top of the
exhaust stroke, the inertia of the exhaust gas tends to
remove any remaining gas in the combustion chamber;
however, a small amount of exhaust gas always remains
to be mixed with the incoming mixture - this unexpelled
gas is captured in the clearance area between the piston
and the cylinder head.
Combustion
The power delivered from the piston to the crankshaft is
the result of a pressure increase in the gas mixture above
the piston. This pressure increase occurs as the mixture is
heated, first by compression, and then — on the down
stroke — by burning. The burning fuel supplies heat that
raises temperature and at the same time also raises
D). Pressure in the cylinder at
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LIGHT DUTY TRUCK SERVICE MANUAL