Figure 7-8.—The fuel supply system of a typical diesel engine.
operated by a separate cam. The intake valve is
held open during the intake stroke, and the
exhaust valve is opened during the exhaust stroke.
Since two revolutions of the crankshaft are
necessary to complete a cycle, the camshaft of
these engines turns only half as fast as the
crankshaft.
COMPRESSION IGNITION SYSTEM
In the four-stroke cycle engine, air enters the
cylinders through intake valves. As each piston
moves downward on the intake stroke, the volume
in the combustion chamber increases and the
pressure decreases. The normal atmospheric
pressure then forces the air into the cylinder
through the intake valve.
Since the two-stroke cycle engine does not go
through an intake stroke, a means must be
provided to force air into the cylinders. The air
enters through intake ports (uncovered when the
piston approaches the bottom of the power
stroke). (See fig. 7-5.) Since the exhaust valves are
open when the intake ports are being uncovered,
the incoming air forces the burned gases out
through the exhaust valves and fills the cylinder
with a supply of fresh air.
7-8
