pressure inside the barrel is released and the check valve
is seated by its spring.
Within the check valve assembly is a reverse flow
check valve that opens when fuel pressure in the
injection line remains above 1,000 psi and closes as
soon as the fuel pressure drops to 1,000 psi. This will
keep the fuel lines filled with fuel at 1,000 psi and ready
for the next injection. This provides for a consistent and
smooth engine power curve.
TRANSFER PUMP.With the introduction of
the scroll metering fuel system, the gear-type fuel
transfer pump that had been used for years by
Caterpillar was superseded by the use of a piston-type
transfer pump. Current scroll metering fuel systems use
a single-piston, double-acting pump with three one-way
The transfer pump is bolted to the low side of the
injection pump housing. It is capable of delivering up to
51 gallons of fuel per hour at 25 psi. There is no need for
a relief valve in this transfer pump due to the fact that
maximum pressure is controlled automatically by the
force of the piston return spring.
The transfer pump is activated by an eccentric (a
device that converts rotary motion into reciprocating
motion) on the injection pump camshaft, causing the
pushrod to move in and out, as the engine is running.
This action causes the piston to move down against the
force of the piston return spring inside the transfer pump
housing. The downward movement of the piston will
cause the inlet check valve and the outlet check valve to
close, while allowing the pumping check valve to open
to allow fuel below the piston to flow into the area
immediately above the downward piston.
As the injection pump camshaft eccentric rotates
around to its low point, the transfer pump spring pushes
the piston up inside its bore, causing the pumping check
valve to close, and both the out and inlet valves are
forced open. Fuel above the piston will be forced
through the outlet check valve and the pump outlet port
at approximately 35 psi. As this occurs, fuel will also
flow through the pump inlet port and the inlet check
valve to fill the area below the piston and the pump will
repeat the cycle.
GOVERNOR.The governor assembly used
with the scroll metering fuel system is a hydra-
mechanical servo-type unit. The reason for using a
servo-valve is to provide a "boost" to the governor.
Without the servo-valve, both the governor spring and
flyweights would have to be very large and heavy. With
the use of the servo assist, little force is required to move
both the accelerator and the governor control lever.
Basically, the governor assembly consists of three
The mechanical components of the governor,
such as the weights, springs, and linkage.
The governor servo that provides hydraulic
assistance through the use of pressurized engine
oil to provide rapid throttle response and to
reduce overall size requirement of the
flyweights and springs.
The dashpot assembly that is designed to
provide stability to the governor during rapid
FUEL INJECTOR NOZZLE.The fuel injector
nozzle, used with the scroll metering fuel system, is a
multiple-hole design, inward-opening, non-leakoff
type. There are minor changes between the earlier
nozzles and current models. Older nozzles are
identified by the use of a color-coded black or blue
washer, while the newer ones use a copper washer.
The nozzle is a multiple-hole design since it is used
in direct injection engines only. The number and size of
the holes will vary between different series of engines.
For example, the 3306 engine nozzle uses a nine-hole
tip, while the nozzle in the 3406 uses a six-hole tip.
These different nozzles cannot be intermixed in the
same engine or switched from one series engine to
The nozzle is designed for injection pressures of
15,000 psi and short injection duration to prevent a loss
in fuel economy due to stringent EPA emission
requirements. The nozzle incorporates a carbon dam on
the lower end of the pencil part of the body and a seal
washer on the upper end. The carbon dam prevents
carbon blow-by into the nozzle bore in the cylinder
head, while the upper seal prevents compression
leakage from the cylinder. Injector nozzle operation is
The nozzle receives high-pressure fuel from the
fuel pump through the inlet passage and filter
screen and into the fuel passage.
When fuel pressure is high enough, the injector
valve is lifted against the force of the return
spring and fuel is injected through the multiple
holes in the spray tip. This causes an increase in
fuel pressure and the fuel to be finely atomized
spray for penetration of the compressed air in the