Chevrolet Light Duty Truck 1973 Service Manual page 353

Fig. 3D—Piston Intake Stroke
pressure. Actually, about 75 per cent of the mixture in
the cylinder is composed of nitrogen gas that does not
burn but expands when heated by the burning of the
combustible elements, and this expanding nitrogen
supplies most of the pressure on the piston.
The fuel and oxygen must burn smoothly within the
combustion chamber to take full advantage of this
heating effect. Maximum power would not be delivered
to the piston if an explosion took place, because the
entire force would be spent in one sharp hammer-like
blow, occurring too fast for the piston to follow.
Instead, burning must take place evenly as the flame
moves across the combustion chamber. Burning must be
completed by the time the piston is about half-way down
so that maximum pressure will be developed in the
cylinder at the time the piston sends its greatest force to
the crankshaft. This will be when the mechanical
advantage of the connecting rod and crankshaft is at a
maximum.
At the beginning of the power stroke, as the piston is
driven downward by this pressure the volume above the
piston increases, which would normally allow the
pressure in the cylinder to drop. However, the
Fig. 4D—Piston Compression Stroke
combustion process is still occurring and this continues to
raise the temperature of the gases, expanding them and
maintaining a continuous pressure on the piston as it
travels downward. This provides a smooth application of
power throughout the effective part of the power stroke
to make the most efficient use of the energy released by
the burning fuel.
An internal combustion engine actually runs on heated
air, the air being composed mainly of inert nitrogen.
The fuel is used, not to cause explosions, but to cause
high pressure within the cylinder to push the piston
down smoothly during the power stroke.
Compression Ratio
The compression ratio is a comparison of the volume of
the cylinder and combustion chamber when the piston is
all the way down, to the volume remaining when the
piston is all the way up.
The main advantage of a
it enables the engine to develop more power from a
given charge of fuel. The combustion pressure exerted
downward on the piston is always 3 or 4 times as great as
the compression pressure. Consequently, an increase in
EN G IN E 6 -1 7
compression ratio is that
high
LIGHT DUTY TRUCK SERVICE MANUAL