SERVICE MANUAL
ROBERT BOSCH MODEL MW INJECTION PUMP
Section 11
Delivery Valve and Snubber Valve
The delivery valve, which is held on its seat by spring
pressure, prevents all the fuel from draining out of the
high pressure line between pumping cycles. Just prior to
the time the delivery valve seats, the delivery valve
allows a specified amount of fuel, called the retraction
volume, to flow out of the injection line. The retraction
volume is sufficient to establish a residual line pressure
which is low enough to prevent pressure waves within
the line from causing secondary injection.
Within the delivery valve holder is another component
called the snubber valve which affects fuel injection. The
snubber valve is an integral part of the holder and
essentially consists of a disc with a hole in the center of
the disc (Figure 14). The disc is free to move
Figure 14. - Delivery Valve and Snubber ValveCGES-
185-3
within the holder. During injection, fuel flows around the
disc and to the nozzle. When the helix aligns with the
charging/high pressure spill port, injection ends and a
small amount of fuel flows back towards the plunger.
Fuel then flows through the center of the disc which
results in a delay in fuel pressure decay within the line
which minimizes secondary injection and prevents line
pressure from going below zero which would cause line
cavitation.
Metering Positions
Metering (providing the proper amount of fuel to meet
engine requirements at various loads and speeds) is
accomplished by rotating the plunger in its barrel to
change the effective pumping stroke. The effective
stroke is the distance traveled upward by the plunger
from the time the charging/high pressure spill port is
covered until it is again uncovered by the helix. A short
effective stroke means a small amount of fuel is injected.
As effective stroke increases, the amount of fuel injected
increases.
The metering function can be described by observing the
relationships of plunger helix to barrel charging port
under various metering positions (Figure 15).
Starting
For starting, plunger is rotated by control rack to a point
where
the
plunger
retard
notch
aligns
with
the
charging/high pressure spill port. The effective plunger
stroke is nearly the maximum during starting. The
maximum effective plunger stroke occurs just after the
plunger rotates "past" the retard notch.
Engine Operating (Metered Delivery)
At engine operating speeds from idle through maximum
governed speed, the quantity of fuel delivered is
determined by the effective stroke (port closing to port
opening) as controlled by the position of the throttle lever
and governor.
CGES-185-3
PRINTED IN UNITED STATES OF AMERICA
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