10.9.
Lucas In-line Injection Pumps
Lucas manufactures in- line pumps of minimec, majormec and maximec models, all basically of similar design. All of these are driven from the end remote from the governor and differ mainly in size. Minimec is available for three, four, six and eight cylinder engines and is produced with camshaft lifts of 7 and 9 mm. Majormec, available for six and eight cylinder engines, has lifts of 10 and 11 mm. Maximec is used only for twelve cylinder engines.
Description and Construction of the Pump.
The in-line injector pump (Fig. 10.12), a multi-pumping-element unit, has one element for each engine cylinder. Each element contains a plunger, a barrel, and a delivery valve all placed in a single row in a casing made in two parts. One part is a steel T-shaped body containing the fuel passages and pumping elements. The second is an aluminium-alloy cam-box and governor housing, which contains the camshaft and bearing, cam followers, control rod, governor assembly, and operating linkage.
The camshaft creates the plunger movement for each pumping element occuring in the firing order and at the correct point in the engine’s cycle of operation. The followers relay the cam-lobe lift to the plungers, and the governor senses the changes in engine speed and, in conjunction with the accelerator-lever position, automatically regulates the amount of fuel to be injected into the combustion chamber.
Plunger-and-barrel Pumping Action.
Towards the end of the down ward movement of the plunger in the barrel both the inlet and spill ports are uncovered, and the inlet port opens slightly before the spill port. Fuel at lift-pump pressure is forced through both ports and it fills up the space above the plunger (Fig. 10.13A).
Fig. 10.13. Cycle of operation. A. Filling. B. Cut-off point (beginning of injection). C. Spill over.
As the plunger moves up, it first cuts off the spill port. During its movement it pushes back a small quanÂtity of fuel through the inlet port until the top edge of the plunger cuts off this port from the space above the plunger (Fig. 10.13B), which is completely filled with fuel. The rising plunger pressurises the trapped fuel and forces open the delivery valve situated above the plunger. The plunger then transmits rising pressure through the existing fuel in the pipeline to the injector-nozzle needle (Fig. 10.12). Eventually this pressure build-up lifts the needle off its seat, and fuel is sprayed out into the combustion chamber.
Further upward movement of the plunger forces fuel past the delivery valve until the upper edge of the plunger helical groove is exposed to the barrel spill port (Fig. 10.13C). As fuel escapes down the axial spill pasÂsage, across to the helical groove, and out through the spill port the fuel pressure suddenly collapses. The spring-loaded delivery valve is then closed, due to which fuel delivery and injection stop.
Fig. 10.14. Pumping element.
Fig. 10.15. Output control. A. Stop. B. Half load. C. Full load.
Output Control.
The plunger stroke is always constant as it is controlled by the cam-lobe, but the part of the stroke, which actually pumps varies. During the upward travel of the plunger the point at which the spill occurs is altered by partially rotating the plunger relative to the barrel. The device used for this operation consists of an arm attached to the bottom of the
plunger. This engages a box-shaped control-arm mounted on a square-sectioned control-rod (Fig. 10.14). The control-rod is located to one side of the pumping elements and is positioned parallel to the camshaft. It links up all the plunger control-arms due to which any to-and-fro movement of this control-rod rotates each plunger an equal amount. The amount of fuel injected by each plunger is thus determined before the spill port is uncovered.
When the plunger is partially rotated in the barrel, the position of the plunger helical spill groove varies relative to the fixed barrel spill port, so that the effective pumping stroke of the plunger either increases or decreases. If the helical groove aligns with the spill port earlier (Fig. 10.15B) in the plunger upstroke, the amount of fuel injected is reduced. For a late spill to increase the delivered output, the effective plunger stroke is lengthened (Fig. 10.15C), which indicates the full-load position. For the shut-off position i.e. no delivery, the plunger is rotated until the helical groove uncovers the spill port in the barrel for the entire stroke (Fig. 10.15A) so that the fuel cannot be trapped and compressed within the barrel.
Delivery.valve Action.
The function of the delivery valve is to provide (a) residual pipeline pressure, so that each successive pumping stroke immediately
actuates the injector, (6) rapid fuel cut-off, to eliminate injector-nozzle dribble, and (c) positive continuous air purging or bleeding.
Fig. 10.16. Delivery-valve action. A. Closed. B. Injection. C. End of injection.
When the plunger rises on its injection stroke, the pressurised fuel lifts the delivery valve, so that fuel is displaced towards the injector (Fig. 10.16B). Further upward movement of the plunger aligns the helical groove and the spill port, so the injection stops. The delivery valve is then immediately closed by the return-spring. This causes the remaining fuel in the pipeline to be subjected to a residual pressure, which is about 2945 kPa. In the process of closing the delivery valve, the piston portion of the valve initially cuts off the fuel pipeline above the valve from the barrel-pumping chamber (Fig. 10.16C) and then sweeps further down to rest on its seat (Fig. 10.16A). The delivery valve also assists in taking air out of the system, because the fuel volume between the delivery valve and the nozzle does not add to the pump clearance volume.
The plunger action of the close-fitting piston increases the total volume of the fuel trapped between the valve and the injector, and this causes a rapid pressure drop, which unloads the injector line pressure to below the working pressure. Consequently the injector needle is able to snap on to its seat, thus providing a clean cut-off without nozzle dribble.
A cylindrical shouldered piece of low-carbon steel with a hole through the centre and a transverse slot across the enlarged shouldered end is called volume reducer, which fits inside the delivery-valve holder. Its acts as a spring-guide and also reduces the effective volume and compressibility of the fuel between the delivery valve and the injector (Fig. 10.16B). In the absence of the volume reducer, the effective volume is increased and the sweeping action of the
delivery-valve piston is insufficient to reduce the line pressure. This almost certainly promotes injection dribble.
