SERVICE MANUAL
ROBERT BOSCH MODEL MW INJECTION PUMP
Section 11
Supply Pump Operation
The supply pump plunger establishes and separates two
chambers within the supply pump, the suction chamber
and the pressure chamber (Figure 8). Two operating
strokes occur within the pump operating cycle. During
the intermediate stroke, the revolving camshaft eccentric
drives the roller tappet, pressure spindle and pump
plunger towards the plunger spring. A portion of the fuel
present in the suction chamber equal to the swept
volume or quantity of fuel or air delivered per stroke
flows through the pressure valve to the pressure
chamber. Towards the end of the intermediate stroke,
the spring loaded pressure valve closes.
As soon as the rotating eccentric has reached its
maximum travel or stroke toward the plunger spring, the
delivery and suction stroke starts. During this stroke, the
plunger spring drives the plunger and pressure spindle
toward the roller tappet. A portion of the fuel present in
the pressure chamber flows through the pressure valve
to the final filter and then to the injection pump housing.
Simultaneously during this stroke, fuel flows through the
pump suction into the suction chamber.
During the spring-actuated delivery and suction stroke,
all fuel swept out of the pressure chamber flows out the
discharge fitting. The fuel volume swept out of the
pressure chamber is slightly less than the fuel volume
entering the suction chamber. Actual fuel volume swept
out of the pressure chamber is the fuel volume swept out
of the suction chamber minus the volume of the pressure
spindle reaching into the pressure chamber.
Therefore during the intermediate stroke, a quantity of
fuel corresponding to the volume of the spindle is fed to
the injection pump. For example, if the fuel volume in
the pressure chamber were nine millimeters and the fuel
volume in the suction chamber were ten millimeters at a
given plunger travel, one millimeter would flow through
the pressure valve during the intermediate stroke.
Because the plunger spring provides the force to pump
the fuel and the camshaft eccentric and roller tappet
merely positions the plunger, supply pump pressure is
self-limiting. When supply pump pressure exceeds a
specified value, the plunger spring lifts the plunger only
INTERMEDIATE STROKE
DELIVERY AND SUCTION STROKE
Figure 8. - Supply Pump Operation
CGES-185-3
PRINTED IN UNITED STATES OF AMERICA
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