SERVICE MANUALROBERT BOSCH MODEL MW INJECTION PUMPSection 11Figure 12. - Plunger ConstructionPlunger Positions During Injection Cycle The injectioncycle can be described by observing the plunger in itsfour principle positions in the barrel (Figure 13). Atbottom dead center the plunger is held down on thelowest point of the cam lobe by the plunger spring andthe top of the plunger is below the barrel charging/highpressure spill port. Fuelat supply pump pressure flows through from the injectionpump housing fuel gallery through the barrelcharging/high pressure spill port to fill the area above theplunger.Port closure begins when the rising plunger covers thecharging/high pressure spill port and fuel is trappedabove the plunger.Start of injection occurs as the plunger continues upwardand causes high pressure fuel to unseat the deliveryvalve. Fuel then flows past the delivery valve, throughthe injection pipe, and unseats the nozzle valve and fuelis injected into the cylinder.End of injection occurs when the plunger moves up tothe point where the charging/ high pressure spill port isuncovered by the helix. High fuel pressure above theplunger is relieved through the helix, through the barrelcharging/high pressure spill port into the injection pumpfuel gallery. This pressure drop allows the delivery valveand injection nozzle to seat, thus ending injection.Figure 13. - Plunger Positions During Injection CycleCGES-185-3PRINTED IN UNITED STATES OF AMERICAPage 11
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