In our discussion of the distributor fuel injection system, we will use the DPA type for our example because of its wide use on Navy small craft. The DPA pump is a compact unit that is lubricated throughout by fuel and requires no separate lubrication system. It contains no ball or roller bearings, gears, or highly stressed springs. Sensitive speed control is maintained by a governor that is either mechanically or hydraulically operated within the pump itself.

Design and Components

In the DPA distributor type of injection pump, the fuel is pumped by a single element. The fuel charges are distributed in the correct firing order by a rotary distributor that is integral with the pump. Therefore, equality of delivery to each nozzle is an inherent feature of the pump. Since the timing interval between injection strokes is determined by the accurate spacing of the distribution ports and the operating cams do not have to be adjusted, accurate timing of delivery is also an inherent feature of the pump.

Figure 9-15.-Plunger types.

There is a central rotating steel member known as the pumping and distributing rotor. The rotor is a close fit in a stationary steel cylindrical body, called the hydraulic head. The pumping section of the rotor has a transverse bore containing two opposed pump plungers. These components rotate inside a cam ring in the pump housing,and operate through rollers and shoes sliding in the rotor. The cam ring has as many internal lobes as the engine has cylinders. The opposed plungers have no springs but are moved outward hydraulically by fuel pressure.

The pumping and distributing rotor is driven by splines from a drive shaft. At its outer end, the rotor carries a vane-type fuel transfer pump. With a piston-type regulating valve housed in the end plate, the transfer pump serves to raise the pressure of the fuel to an intermediate level, known as the transfer pressure.

Operation

The DPA pump is driven at half engine speed. As the rotor turns, a charging port in the rotor aligns with the metering port in the hydraulic head. Fuel at metered pressure then flows into the central passage in the rotor and forces the plungers apart. The amount of plunger displacement is determined by the amount of fuel that can flow into the element while the ports are aligned. The fuel inlet port closes as rotation continues. As the rotor turns, the fuel remains isolated into the rotor. As the single distributor port in the rotor comes into alignment with one of the outlet ports in the hydraulic head, the plungers are forced quickly together by the action of the cam. At this point, high pressure is generated, and the pressurized fuel passes via a high-pressure line to an injector. From the injector, the fuel passes to the engine combustion chamber. This entire cycle of operation is repeated once for each engine cylinder per pump revolution. Typical DPA system for a 4-cylinder engine.