SERVICE MANUALROBERT BOSCH MODEL MW INJECTION PUMPSection 11Delivery Valve and Snubber ValveThe delivery valve, which is held on its seat by springpressure, prevents all the fuel from draining out of thehigh pressure line between pumping cycles. Just prior tothe time the delivery valve seats, the delivery valveallows a specified amount of fuel, called the retractionvolume, to flow out of the injection line. The retractionvolume is sufficient to establish a residual line pressurewhich is low enough to prevent pressure waves withinthe line from causing secondary injection.Within the delivery valve holder is another componentcalled the snubber valve which affects fuel injection. Thesnubber valve is an integral part of the holder andessentially consists of a disc with a hole in the center ofthe disc (Figure 14). The disc is free to moveFigure 14. - Delivery Valve and Snubber ValveCGES-185-3within the holder. During injection, fuel flows around thedisc and to the nozzle. When the helix aligns with thecharging/high pressure spill port, injection ends and asmall amount of fuel flows back towards the plunger.Fuel then flows through the center of the disc whichresults in a delay in fuel pressure decay within the linewhich minimizes secondary injection and prevents linepressure from going below zero which would cause linecavitation.Metering PositionsMetering (providing the proper amount of fuel to meetengine requirements at various loads and speeds) isaccomplished by rotating the plunger in its barrel tochange the effective pumping stroke. The effectivestroke is the distance traveled upward by the plungerfrom the time the charging/high pressure spill port iscovered until it is again uncovered by the helix. A shorteffective stroke means a small amount of fuel is injected.As effective stroke increases, the amount of fuel injectedincreases.The metering function can be described by observing therelationships of plunger helix to barrel charging portunder various metering positions (Figure 15).StartingFor starting, plunger is rotated by control rack to a pointwhere the plunger retard notch aligns with thecharging/high pressure spill port. The effective plungerstroke is nearly the maximum during starting. Themaximum effective plunger stroke occurs just after theplunger rotates "past" the retard notch.Engine Operating (Metered Delivery)At engine operating speeds from idle through maximumgoverned speed, the quantity of fuel delivered isdetermined by the effective stroke (port closing to portopening) as controlled by the position of the throttle leverand governor.CGES-185-3PRINTED IN UNITED STATES OF AMERICAPage 12
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