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 TM 9-8000
that is contained within the PSB unit. Fuel is delivered
two crankshaft revolutions corresponding
to
the
to the PSB pump from the fuel tank by the fuel delivery
requirements of a four-stroke cycle diesel.
pump at a regulated pressure of approximately 20 psi
(137.9 kPa). The low pressure fuel supply enters the
(4) A hole called a spill port is drilled through
pump chamber through the inlet port when the plunger is
the lower portion of the pump plunger. The spill port is
retracted fully. As the plunger begins its delivery stroke,
connected to the pump chamber by another drilled
the fuel inlet passage is blocked, trapping fuel in the
passage. The spill port is covered by a plunger sleeve
pump chamber. The delivery stroke of the plunger then
whose position is adjusted by the control lever through
pushes the charge of fuel out of the chamber through the
an eccentrically mounted pin.
delivery passage. The high-pressure fuel charge then
unseats the delivery valve, allowing it to flow into the
(5) The movement of the control lever controls
distribution chamber.
the up and down position of the plunger sleeve. The
position of the control lever is determined by the
(2) The pump plunger has a spool like recess
governor (para 5-25). When the sleeve is in its extreme
in its diameter about halfway down its sides that, in
downward position, the spill port is immediately
conjunction with the pump cylinder, forms the distribution
uncovered as the plunger begins its delivery stroke.
chamber. A slot is cut into the plunger at the top of the
This causes all of the pressure from the pump chamber
distribution chamber.
As it reciprocates, the plunger
to bleed off to the pump return. In this position, there
also is rotated through a quill gear. As it rotates, the slot
will be no fuel delivery to the injectors.
lines up with equally spaced passages around the inside
of the plunger bore. Each passage is connected to a
(6) When the plunger sleeve is in the extreme
upward position, the spill port is covered until the plunger
fuel injector. The reciprocating and rotating motion are
timed so that the plunger will go through a delivery stroke
almost reaches the end of the delivery stroke.
This
as the slot lines up with each injector passage. This
position will deliver maximum fuel to the injectors. As
enables the PSB injector pump to deliver a fuel charge to
the plunger moves upward, the pressure developed in
each consecutive injector every time the plunger makes
the pump chamber unseats the delivery valve.
Fuel
one complete revolution.
flows into the distribution chamber and is sent to
whatever injector is scheduled to receive it by the slot in
(3) The PSB pump is geared to the engine so
the plunger.
that the camshaft rotates at crankshaft speed. The cam
contains half as many lobes as the engine has cylinders
(7) The amount of fuel delivered by each
(there would be three cam lobes if the engine had six
injection charge will increase proportionately as the
cylinders). The pump plunger is geared to rotate at one-
plunger sleeve is moved from its extreme downward to
half of camshaft speed. This arrangement allows the
its extreme upward position. The higher the plunger
PSB pump to deliver a charge of fuel to each injector for
sleeve, the longer the effective pump stroke (plunger
every
movement before the spill port is uncovered).
Section IV. FUEL SUPPLY PUMPS
c. Without a supply pump, the system would lose
5-20. General.
Fuel injection pumps must be
its prime whenever the pump is in the no- delivery mode.
supplied with fuel under pressure for the following
reasons:
The supply pumps in use generally are of the positive
displacement type with a performance that is
a. The injection pumps lack the suction ability to
independent of any reasonable variations in viscosity,
draw fuel from the tank by themselves.
pressure, or temperature of the fuel. In a majority of the
equipment, the fuel supply pump is built into the injection
b. It is important to supply fuel to the injection pump
pump unit.
This cuts down on fuel tubing and the
In excess so that fuel may be used to cool and lubricate
complexity of the
the system before bypassing it back to the tank.
5-28
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