Chevrolet Light Duty Truck 1973 Service Manual page 354

6 -1 8 ENGINE
Fig. 5D—Piston Power Stroke
compression pressure (input) means at least three times
as great an increase in combustion pressure (output).
Valve Timing
As in most 4-stroke cycle engines, the intake valve begins
to open before the piston reaches the top, and the
exhaust valve remains open until after top dead center.
This means that both valves are open for a short period
of time. This condition is called valve overlap (Fig. 7D).
The valve timing is arranged this way to use the inertia
of the gas in evacuating and in filling the cylinders.
When the air-fuel mixture and exhaust gases move in or
out of the cylinder, its weight gives it momentum in the
established direction. When a valve opens, the initial air
flow is slow. Valve timing allows for this lag in starting
and stopping in the flow. In order to pack the maximum
air-fuel mixture into the cylinder, each valve opens
earlier and closes later than would be necessary if the
mixture were weightless.
On the intake stroke, the exhaust valve stays open a little
after top center to take advantage of the momentum of
the exhaust gases rushing out through the valve, even
though the piston has started down. With the exhaust
Fig. 6D—Piston Exhaust Stroke
TOP DEAD CENTER
INTAKE VALVE-
OPENS
INTAKE
VALVE
CLOSES
Fig. 7D—Typical Valve Overlap (720-degree spiral)
EXHAUST VALVE
CLOSES
EXHAUST
VALVE
OPENS
BOTTOM DEAD CENTER
LIGHT DUTY TRUCK SERVICE MANUAL