Fuel Metering

Fig. 7 Phases of the Plunger Stroke:

The fuel delivery of injection pumps is a dynamic process. It consists of several stroke phases. The pressure required for injection is produced by the injection pump.
With the plunger at bottom dead center the inlet ports are left open by the pump plunger. The fuel which is under pressure generated by the supply pump flows from the suction gallery through the inlet ports into the high pressure chamber. As it moves up in the barrel, the plunger closes the inlet ports. This phase of the plunger stroke is termed the prestroke. In the further course of the plunger stroke the fuel pressure is increased. This causes the delivery valve to lift slightly from its seat without opening completely. This phase of plunger travel following the prestroke is known as the retraction stroke. After the retraction stroke comes the actual effective stroke. The pressure generated in the stroking motion of the plunger now opens the delivery valve completely and the fuel flows through the fuel injection tubing to the nozzle holder and to the injection nozzle. The effective stroke is completed when the helix of the pump plunger opens the spill port/inlet port. From this point in time no more fuel is delivered to the injectors. During the remaining stroke of the plunger to top dead center, the residual stroke, the fuel is forced back into the suction gallery through the vertical groove linking the plunger chamber and the suction gallery. After the reversal of direction at top dead center the fuel initially flows back into the pump barrel through the vertical groove until the helix again closes the inlet. As the plunger moves further down in the barrel a vacuum is created. It is only after the inlet ports have been opened by the top edge of the plunger that the fuel, under the pressure from the supply pump, flows from the suction gallery into the high pressure chamber. The high pressure chamber above the pump plunger is again filled with fuel. The individual stroke and delivery phases are shown in Fig. 7.