72Lucas DPA type pump

The high cost of the in-line pump relative to that of a small engine was the incentive for the development by Lucas of the DPA rotary distributor type injection pump. A diagram showing the overall arrangement of a fuel system for such a pump is shown in Fig. 7.1, and the pump is illustrated in Fig. 7.2.

Immediately inside one end of the housing, and mounted on the drive shaft, is the governor. To the right of the governor, as viewed in Fig. 7.2, is an articulated splined muff coupling connecting it to the end of the drive

Fig. 7.1 Fuel system with DPA pump and mechanical governor
Dpa Fuel Injection Pump
Fig. 7.2 DPA Pump

shaft to the rotor. This rotor, which houses the diametrically opposed plungers, has an integral extension serving as the fuel injection distributor. Surrounding the plunger rotor is a cam ring. Interposed between it and the plungers are cam followers in the form of shoes sliding in radial slots in the rotor, Fig. 7.3. Mounted on an extension of the right-hand end of the rotor is the vane type transfer pump.

This whole rotor assembly is in a steel housing termed the hydraulic head, in which is a ported cylinder carrying the distributor portion of the rotor. A controlled degree of leakage passes from the hydraulic head into the cam box, in which the maximum pressure is limited by the pressurising valve which, in Figs 7.2 and 7.7, projects vertically upwards from the rear, or driven, end of the pump. The hydraulic head is spigoted into the rear end of the cam box, the top of which is closed by an inverted bath tub-shaped casting that houses the governor control springs and linkage. Controls actuated by the driver are linked to levers on the upper ends of vertical spindles rotating in bearings in the top of this cover. Levers on the lower ends of these spindles are connected to the governor controls.

Maximum travel, and therefore maximum delivery is adjusted on the production line. Lugs extend outwards from the ends of the shoes, Fig. 7.3, to register in cam-shaped slots in two side plates, which are clamped to the rotor by screws passing through slotted holes in them. The screws are loosened and the plates rotated relative to the rotor, to cause the shoes to ride up, or down, in the cam-shaped slots to the appropriate maximum delivery setting. The screws are then tightened again. Actual delivery is determined by the driver, and modified by the governor. To perform its function, the governor varies the rotational setting of a restrictor. This restrictor, termed the metering valve, limits the quantity of fuel that can flow to the plungers in the time available, and therefore their outward strokes. The delivery pressure of the pump increases with speed up to a maximum determined by the setting of a pressure limiting valve in the pump end plate.

As the rotor turns, it opens each inlet port in the distributor sleeve one after the other. Fuel from the transfer pump is delivered into a radial hole in the rotor, and then through an axial hole towards the end of the rotor, where it enters the space between the plungers. Transfer pump pressure forces the plungers outwards while, at the same time, filling the space between them. To avoid dribbling at the nozzles at the end of injection, the cut-off must be sharp, so the cam profiles include what are termed retraction profiles, Fig. 7.17. These allow the plungers to retract a short distance outwards, before

Dpa Injector Pump Cam Profile Action

A Cam-shaped slot

B Lug on roller shoe

C Roller shoe

D Roller

E Plunger

F Adjusting plate

G Locking screw hole in shoe carrier

Fig. 7.3 The method used to adjust the maximum travel of the plungers

A Cam-shaped slot

B Lug on roller shoe

C Roller shoe

D Roller

E Plunger

F Adjusting plate

G Locking screw hole in shoe carrier

Fig. 7.3 The method used to adjust the maximum travel of the plungers the fuel begins to enter to drive them further outwards on their normal induction stroke. Therefore, at the end of injection, the delivery pressure is instantly reduced although, between the injection phases, some residual pressure is maintained in the line.

When the plungers are driven inwards by a pair of cams, the fuel between them is forced out at very high pressure through first the axial hole and then a single radial delivery hole. As the rotor turns, the delivery hole is aligned at regular timed intervals with ports in the distributor sleeve. The shots of fuel pass along ducts in the hydraulic head, through the delivery valves, into the pipes serving each of the cylinders in turn. So the porting and flow sequence for delivery is exactly the inverse of that for the incoming fuel to the plungers. Accuracy of spacing of the delivery ports and the cams is essential for obtaining precise timing intervals between injections.

To keep the engine speed constant regardless of variations in load, the rate of delivery of fuel to the plungers is regulated by the rotary metering valve. Since the quantity of fuel delivered by the transfer pump increases with speed, it is necessary for the rollers to contact the plungers at points dependent on movements of both the governor linkage and the accelerator. Consequently, the return spring for the governor arm is connected by an arm and linkage to the accelerator pedal, Fig. 7.1, and the rotary metering valve is actuated by a link between it and the governor arm.

As load is reduced, so also are the outward movements of the plungers. Consequently, they make their initial contacts further up the profiles of the cams and therefore later. Therefore, if the injection timing is set for maximum load and speed, it must be progressively retarded as the speed falls. The mechanism for doing this is illustrated in Fig. 7.4.

Screwed radially into the periphery of the cam ring is a short ball-ended lever projecting downwards into a hole in the plunger of a hydraulic servo, which is aligned tangentially relative to the cam ring. Axial displacement of the plunger therefore rotates the cam ring. A coil spring pushes the plunger to one end of its cylinder, to retard injection, while fuel transfer pressure advances it by pushing the plunger back against the load exerted by the spring. A non-return valve in the delivery from the transfer pump to the

plunger prevents the timing from being retarded by the impacts of the roller followers on the cams.

For starting and low speed operation, when the transfer pressure is low, the plunger is, of course, in the fully retarded position. However, with some high speed engines, further retardation could occur owing to the previously mentioned impact between the plungers and cams. However, this is avoided by the incorporation of a light load advance device, to be described in the next section.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Responses

  • orlaith
    How to increase speed on a dpa pump?
    1 year ago

Post a comment