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Injection Pump

Injection Pump

The Roosa Master fuel injection pump is described as an opposed plunger, inlet metering, distributor-type pump. Simplicity, the prime advantage of this design, contributes to greater ease of service, low maintenance cost, and greater dependability. Before describing the injection pump components and operation, let’s familiarize ourselves with the model numbering system. For example, model number DB2833JN3000 breaks down like this:

D—Pump series



8—Number of cylinders

33—Abbreviation of plunger diameter; 33, 0.330 in.

JN—Accessory code that relates to special pump options

3000—Specification number


For information on the accessory code and the specification number for a particular pump, always refer to the manufacturer’s service manual.

The main components of the DB2 fuel injection pump are the drive shaft, distributor rotor, transfer pump, pumping plungers, internal cam ring, hydraulic head, end plate, governor, and housing assembly with an integral advance mechanism. The rotating members that revolve on a common axis include the drive shaft, distributor rotor, and transfer pump.

DRIVE SHAFT (fig. 5-12)—The drive shaft is the driving member that rotates inside a pilot tube pressed into the housing. The rear of the shaft engages the front of the distributor rotor and turns the rotor shaft. Two lip type seals prevent the entrance of engine oil into the pump and retain fuel used for pump lubrication.

DISTRIBUTOR ROTOR (fig. 5-13)—The distributor rotor is the drive end of the rotor, containing two pumping plungers located in the pumping cylinder. Slots in the rear of the rotor provide a place for two spring-loaded transfer pump blades. In the rotor, the shoe, which provides a large bearing surface for the roller, is carried in guide slots. The rotor shaft rotates with a very close fit in the hydraulic head. A passage through the center of the rotor shaft connects the pumping cylinder with one charging port and one discharging port The hydraulic head in which the rotor turns has a number of charging and discharging port, based on the number of engine cylinders. An eight-cylinder engine will have eight charging and eight discharging ports. The governor weight retainer is supported on the forwarded end of the rotor.

TRANSFER PUMP (fig. 5-14)—The transfer pump is a positive displacement, vane-style unit, consisting of a stationary liner with spring-loaded blades that ride in slots at the end of the rotor shaft. The delivery capacity of the transfer pump is capable of exceeding both pressure and volume requirements of the engine, with both varying in proportion to engine speed. A pressure regulator valve in the pump end plate controls fuel pressure. A large percentage of the fuel from the pump is bypassed through the regulating valve to the inlet side of the pump. The quantity and pressure of the fuel bypassed increases, as pump speed increases.

The operation of the model DB2 injection is similar to that of an ignition distributor. However, instead of the ignition rotor distributing high-voltage sparks to each cylinder in firing order, the DB2 pump distributes pressurized diesel fuel as two passages align during the rotation of the pump rotor, also in firing order. The basic fuel flow is as follows:

  • Fuel is drawn from the fuel tank by a fuel lift pump (mechanical or electrical) through the primary and secondary filters before entering the transfer pump.
  • As fuel enters the transfer pump, it passes through a cone-type filter and on into the hydraulic head assembly of the injection pump.
  • Fuel under pressure is also directed against a pressure regulator assembly, where it is bypassed back to the suction side should the pressure exceed that of the regulator spring.
  • Fuel under transfer pump pressure is also directed to and through a ball-check valve assembly and against an automatic advance piston.
  • Pressurized fuel is also routed from the hydraulic head to a vent passage leading to the governor linkage area, allowing any air and a small quantity of fuel to return to the fuel tank through a return line which self-bleeds air from the system. Fuel that passes into the governor linkage compartment is sufficient to fill it and lubricate the internal parts.
  • Fuel leaving the hydraulic head is directed to the metering valve, which is controlled by the operator throttle position and governor action. This valve controls the amount of fuel that will be allowed to flow on into the charging ring and ports.
  • Rotation of the rotor by the drive shaft of the pump aligns the two inlet passages of the rotor with the charging ports in the charging ring, thereby allowing fuel to flow into the pumping chamber (fig. 5-15).
  • The pumping chambers consists of a circular cam ring, two roller, and two plungers. As the rotor continues to turn, the inlet passages of the rotor will move away from the charging ports, allowing fuel to be discharged, as the rotor registers with one of the hydraulic head outlets.
  • With the discharge port open (fig. 5-16), both rollers come in contact with the cam ring lobes, which forces them toward each other. This causes the plungers to pressurize the fuel between them and sending it on up to the injection nozzle and into the combustion chamber. The cam is relieved to allow a slight outward movement of the roller before the discharge port is closed off. This action drops the pressure in the injection line enough to give sharp cutoff injection and to prevent nozzle dribbling.

The maximum amount of fuel that can be injected is limited by maximum outward travel of the plungers. The roller shoes, contacting an adjustable leaf spring, limit this maximum plunger travel. At the time the charging ports are in register, the rollers are between the cam lobes; therefore, their outward movement is unrestricted during the charging cycle except as limited by the leaf spring.

To prevent after-dribble and therefore unburnt fuel at the exhaust, the end of injection must occur crisply and rapidly. To ensure that the nozzle valve does, in fact, return to its seat as rapidly as possible, the delivery valve (fig. 5-18). located in the drive passage of the rotor, acts to reduce injection line pressure. This occurs after fuel injection and the pressure is reduced to a value lower than that of the injector nozzle closing pressure. The valve remains closed during charging and opens under high pressure, as the plungers are forced together. Two small grooves are located on either side of the charging port or the rotor near its flange end. These grooves carry fuel from the hydraulic head charging posts to the housing. This fuel flow lubricates the cam, the rollers, and the governor parts. The fuel flows through the entire pump housing, absorbs heat, and is allowed to return to the supply tank through a fuel return line connected to the pump housing cover, thereby providing for pump cooling.

In the DB2 fuel pump, automatic advance is accomplished in the pump by fuel pressure acting against a piston, which causes rotation of the cam ring, thereby aligning the fuel passages in the pump sooner (fig. 5-18). The rising fuel pressure from the transfer pump increases the flow to the power side of the advance piston. This flow from the transfer pump passes through a cut on the metering valve, through a passage in the hydraulic head, and then by the check valve in the drilled bottom head locking screw. The check valve provides a hydraulic lock, preventing the cam from retarding during injection. Fuel is directed by a passage in the advance housing and plug to the pressure side of the advance piston. The piston moves the cam counterclockwise (opposite to the direction of the pump rotation). The spring-loaded side of the piston balances the force of the power side of the piston and limits the maximum movement of the cam. Therefore, with increasing speed, the cam is advanced and, with decreasing speed, it is retarded.

We know that a small amount of fuel under pressure is vented into the governor linkage compartment. Flow into this area is controlled by a small vent wire that controls the volume of fuel returning to the fuel tank, thereby avoiding any undue fuel pressure loss. The vent passage is located behind the metering valve bore and leads to the governor compartment by a short vertical passage. The vent wire assembly is available in several sizes to control the amount of vented fuel being returned to the tank. The vent wire should NOT be tampered with, as it can be altered only by removing the governor cover. The correct wire size would be installed when the pump assembly is being flow-tested on a pump calibration stand.


For information concerning removal, installation, and servicing the injection pump, always refer to the manufacturer’s service manual.

Figure 5-12.—Drive shaft.

Figure 5-13.—Distributor rotor.

Figure 5-14.—Transfer pump.

Figure 5-15.—Rotor in charge position.

Figure 5-16.—Rotor in discharge position.

Figure 5-18.—Delivery valve.

Figure 5-18.—Speed advance operation

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
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