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of the scroll metering fuel system, the gear-type fuel transfer pump that had been used for years by Caterpillar was superseded by the use of a piston-type transfer pump.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 check valves.

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 separate components:

1. The mechanical components of the governor, such as the weights, springs, and linkage.

2. 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.

3. The dashpot assembly that is designed to provide stability to the governor during rapid load/throttle changes.

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 another.

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 as follows:

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 combustion chamber.