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Fue/ Injection System-cont/nll~d
Page S4
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As soon as the helicat edge of the annular
groovc on the pump plunger uncovcrs the port
in the pump barrel the pressurc of the oil below
the dclivery valve falls and that valvc is promptly
closed by its spring. As it falls the effects of the
withdrawal of the piston part of the valvc within
tbc seat causes a drop in pressurc in the pipe to
tbc atomiser, causing the nozzle valve to snap
on to its seat sharply, cutting off tbe fuel supply,
so tbat there is na dribble.
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A steel peg is used to centralise the upper end
or the valve spring and also to reduce the capacity
of the holder, thereby improving injection control.
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against the resistance of the light spring shown
disposed within the right-hand chamber.
It will be appreciated that as the butterfly valve
is closed the vacuum increases and the diaphragm
moves to the light.
The centte of the diaphragm is coupled to the
control rod of the fuel pump.
Movement of this
rod towards the governor tends to reduce the
supply of fuel to the atomisers, as explained on
page S.3.
Thus, closing the butterfly valve increases the
vacuum in the chamber, moves the diaphragm
to the light, and reduces the supp!y of Cue! to the
engine.
The butterfly valve is operated by the ordinary
throttle valve levers and in that way the speed
of the engine is controlled.
Now here is a point of the very greatest im
portance.
Any leaks in the joints of the pipe, 29, in the
pipe itself, or in the diaphragm, will diminish
the degree of vacuum and cause the engine to
run raster than it should and id!e erratically.
Hence the instructions, given later, urging cara
in ensuring that there are no air leaks in the
system.
If the pipe, 29, were to become detached, or
be !eft uncoupled, and the engine run, it wou!d
race and very smous damage be caused.
Hence the warning, repeated later, that the
engine must never be run unless the venturi
control unit, pipe and induction manifold are il1
position and all joints are tight.
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open
closed
Pneumaöc Governor.
Thc position of tbc control rack, by mcans of
\'thich the quantity of fucl delivered to tbc engine
is rcgulated as described above, is determined by
a pneumatic governor. This govemor, part of
\'thich is in tbe farm of a venturi in the enginc
induction system, operates the con trol rack
according to tbc degrec of vacuum in tbc vcnturi
passagc.
Tberc is a buttertly valve in the venturi. When
that valve is closed, or nearly closed, tbe suction
on tbc engine side of it is considerabie, lessening
as tbc valvc is opened.
A pipe, 29, connects this venturi passagc witb
a governor casing which is mounted on tbc fucl
pump. See Fig. S.ll.
Reference to the drawing of tbc pneumatic
govemor, Fig. S.ll, will enable its operation to
be understood.
Tbc important elcmcnt is tbe tlexible leather
diaphragm, 28, which divides the space inside
tbc govcrnor casing into two compartments, one
of which, that on tbe right, is airtight.
This airtight compartment is in communication
with tbc venturi by means of a pipe, 29. The
vacuum in tbc vcnturi is thus cómmunicated to
tbe airtight compartmcnt and tbc right-hand side
of tbc diaphragrn.
Tbc chambcr, to the left of the diaphragrn,
is not subject to vacuum.
Tbc diaphragrn, thereforc, is constantly subject
to a pressurc tending to move it to the right,
FIG. 54. Delivery Valve.
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