ROBERT BOSCH MODEL MW INJECTION PUMP
After the specified plunger lift to port closure is
established for number one barrel and plunger, the
discharge order "port close" every sixty degrees.
Fuel is retained around the barrel and within the pump
housing by an "O" ring and oval "O" ring. Located on the
barrel body and approximately three-eighths inches
below the barrel flange is an "O" ring groove. This
groove contains an oval "O" ring which seals against the
upper housing bore and retains fuel within the upper
housing barrel cavity. An "O" ring located at the bottom
of the injection pump housing cavity and retained by a
spacer seals against the lower end of the barrel. The
spacer is pressed into the housing and can be replaced if
it is damaged.
When the barrel is installed into the pump housing, the
barrel flange with the adjustment notch is positioned
opposite the control rack. Pump balance, which is the
term used to describe equal delivery per cylinder, is
adjusted during production pump calibration by rotating
the barrel using the adjustment notch (Figure 11).
Figure 11. - Barrel Construction
Two fuel passages in the barrel allow fuel to flow from
the pump housing cavity and into/ out of the plunger
barrel. The upper barrel passage is the charging and
high pressure spill port and the lower passage is the
leakoff port. The leakoff port indexes with a groove on
the plunger and allows high pressure fuel which has
leaked past the upper plunger/barrel to return to the
pump housing cavity.
To protect the pump housing from erosion when high
pressure fuel spills back into the pump cavity at the end
of the pumping cycle, a metal impact cap and retaining
ring covers the upper barrel duct. This cap has six holes
and is installed so the holes do not align with the
charging port in the barrel.
Each barrel and plunger is a matched metal to metal fit
to each other and produces peak injection pressure of
approximately 9000 pounds per square inch. Camshaft
rotation causes the roller tappet to lift the plunger a
constant amount or stroke on each pumping cycle. Fuel
delivery per stroke is varied by rotating the plunger
through a control sleeve, common control rack,
governor and control lever to change the effective
At the top of the plunger, a retard start notch in
conjunction with the charging/high pressure spill port in
the barrel aid engine starting by retarding injection timing
(Figure 12). After the engine starts, effective plunger
stroke is determined by the relationships between the
plunger helix and the charging/high pressure spill port.
The accelerator pedal must be fully depressed to allow
the control rack to move into the start fuel position.
High pressure fuel that leaks past the upper plunger and
barrel area is returned to the pump housing cavity
through the plunger leakoff groove and leakoff port in the
barrel. Additional smaller grooves located above the
leakoff groove and 180 degrees from the helix allow fuel
to flow to the leakoff groove.
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