SERVICE MANUAL
ROBERT BOSCH MODEL MW INJECTION PUMP
Section 11
Figure 12. - Plunger Construction
Plunger Positions During Injection Cycle The injection
cycle can be described by observing the plunger in its
four principle positions in the barrel (Figure 13). At
bottom dead center the plunger is held down on the
lowest point of the cam lobe by the plunger spring and
the top of the plunger is below the barrel charging/high
pressure spill port. Fuel
at supply pump pressure flows through from the injection
pump
housing
fuel
gallery
through
the
barrel
charging/high pressure spill port to fill the area above the
plunger.
Port closure begins when the rising plunger covers the
charging/high pressure spill port and fuel is trapped
above the plunger.
Start of injection occurs as the plunger continues upward
and causes high pressure fuel to unseat the delivery
valve. Fuel then flows past the delivery valve, through
the injection pipe, and unseats the nozzle valve and fuel
is injected into the cylinder.
End of injection occurs when the plunger moves up to
the point where the charging/ high pressure spill port is
uncovered by the helix. High fuel pressure above the
plunger is relieved through the helix, through the barrel
charging/high pressure spill port into the injection pump
fuel gallery. This pressure drop allows the delivery valve
and injection nozzle to seat, thus ending injection.
Figure 13. - Plunger Positions During Injection Cycle
CGES-185-3
PRINTED IN UNITED STATES OF AMERICA
Page 11
