TRUCK SERVICE MANUAL
During last part of compression stroke and early part of
power stroke, a small metered charge of fuel is injected
into combustion chamber.
Almost immediately after fuel charge is injected into
combustion chamber, fuel is ignited by the existing hot
Cummins Diesel Cycle
Cummins Diesel Engines have a high compression ratio;
the charge taken into combustion chamber during the
intake stroke consists of air only with no fuel mixture.
During the beginning of the power stroke, the piston is
Cummins injectors receive low-pressure fuel from the
pushed downward by the burning and expanding gases;
fuel pump and deliver it into individual combustion
both intake and exhaust valves are closed. As more fuel
chambers at the proper time, in equal quantity and
is added and burns, gases get hotter and expand more
atomized condition for burning. Ignition of fuel is caused
to further force piston downward and thus adds driving
by heat of compressed air in the combustion chamber.
force to crankshaft rotation.
It is easier to understand the function of engine parts if it
during each of the four piston strokes of the cycle. The
four strokes and order in which they occur are: Intake
valves are open, and piston is on upstroke.
Upward travel of piston forces burned gases out of
combustion chamber through open exhaust valve ports
In order for the four strokes to function properly, valves
and into the exhaust manifold.
and injectors must act in direct relation to each of the
Proper engine operation depends upon two things first,
four strokes of the piston. The intake valves, exhaust
compression for ignition; and second, that fuel be
valves and injectors are camshaft actuated, linked by
measured and injected into cylinders in proper quantity at
tappets or cam followers, push rods, rocker levers and
valve crossheads. The camshaft is gear driven by the
crankshaft gear, thus rotation of the crankshaft directs
the action of the camshaft which in turn controls the
The PT fuel system is used exclusively on Cummins
opening and closing sequence of the valves and the
injection timing (fuel delivery).
The identifying letters, "PT," are an abbreviation for
"pressure-time." The operation of the Cummins PT Fuel
System is based on the principle that the volume of liquid
During intake stroke, the piston travels downward; intake
flow is proportionate to the fluid pressure, the time
valves are open, and exhaust valves are closed. The
allowed to flow and the passage size through which the
downward travel' of the piston allows air from the
liquid flows. To apply this simple principle to the
atmosphere to enter the cylinder. On turbocharged
Cummins PT Fuel System, it is necessary to provide:
engines the intake manifold is pressurized as the
turbocharger forces more air into the cylinder through the
1. A fuel pump.
intake manifold. The intake charge consists of air only
2. A means of controlling pressure of the fuel being
with no fuel mixture.
delivered by the fuel pump to the injectors so individual
cylinders will receive the right amount of fuel for the
power required of the engine.
At the end of the intake stroke, intake valves close and
3. Fuel passages of the proper size and type so fuel will
piston starts upward on compression stroke.
be distributed to all injectors and cylinders with equal
exhaust valves remain closed. At end of compression
pressure under all speed and load conditions.
stroke, air in combustion chamber has been forced by
4. Injectors to receive low-pressure from the fuel pump
piston to occupy a smaller space (depending upon
and deliver it into the individual combustion chambers at
engine model about one-fourteenth to one-sixteenth as
the right time, in equal quantities and proper condition to
great in volume) than it occupied at beginning of stroke.
Thus, compression ratio is the direct proportion in the
amount of space the air occupies in the combustion
chamber before and after being compressed.
Compressing air into a small space causes temperature
of that air to rise to a point high enough for ignition of