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level for the command E.P.R., then a signal is passed
to the torque motor to trim the fuel flow.
LOW PRESSURE FUEL SYSTEM
83. An L.P. system (fig.10-13) must be provided to
supply the fuel to the engine at a suitable pressure,
rate of flow and temperature, to ensure satisfactory
engine operation. This system may include an L.P.
pump to prevent vapour locking and cavitation of the
fuel, and a fuel heater to prevent ice crystals forming.
A fuel filter is always used in the system and in some
instances the flow passes through an oil cooler (Part
8). Transmitters may also be used to signal fuel
pressure, flow and temperature (Part 12).
FUEL PUMPS
84. There are two basic types of fuel pump, the
plunger-type pump and the constant-delivery geartype
pump; both of these are positive displacement
pumps. Where low pressures are required at the fuel
spray nozzles, the gear-type pump is preferred
because of its lightness.
Plunger-type fuel pump
85. The pump shown in fig. 10-14 is of the singleunit,
variable-stroke, plunger-type; similar pumps
may be used as double units depending upon the
engine fuel flow requirements.
86. The fuel pump is driven by the engine gear train
and its output depends upon its rotational speed and
the stroke of the plungers. A single-unit fuel pump
can deliver fuel at the rate of 100 to 2,000 gallons per
hour at a maximum pressure of about 2,000 lb. per
Fuel system
112
Fuel system
113
Fig. 10-14 A plunger-type fuel pump.
Fig. 10-14 A low pressure system.
square inch. To drive this pump, as much as 60
horsepower may be required.
87. The fuel pump consists of a rotor assembly
fitted with several plungers, the ends of which project
from their bores and bear on to a non-rotating
camplate. Due to the inclination of the camplate,
movement of the rotor imparts a reciprocating motion
to the plungers, thus producing a pumping action.
The stroke of the plungers is determined by the angle
of inclination of the camplate. The degree of
inclination is varied by the movement of a servo
piston that is mechanically linked to the camplate
and is biased by springs to give the full stroke
position of the plungers. The piston is subjected to
servo pressure on the spring side and on the other
side to pump delivery pressure; thus variations in the
pressure difference across the servo piston cause it
to move with corresponding variations of the
camplate angle and, therefore, pump stroke.
Gear-type fuel pump
88. The gear-type fuel pump (fig. 10-12) is driven
from the engine and its output is directly proportional
to its speed. The fuel flow to the spray nozzles is
controlled by recirculating excess fuel delivery back
to inlet. A spill valve, sensitive to the pressure drop
across the controlling units in the system, opens and
closes as necessary to increase or decrease the
spill.
FUEL SPRAY NOZZLES
89. The final components of the fuel system are the
fuel spray nozzles, which have as their essential
function the task of atomizing or vaporizing the fuel to
ensure its rapid burning. The difficulties involved in
this process can be readily appreciated when one
considers the velocity of the air stream from the
compressor and the short length of combustion
system (Part 4) in which the burning must be
completed.
90. An early method of atomizing the fuel is to pass
it through a swirl chamber where tangentially
disposed holes or slots imparted swirl to the fuel by
converting its pressure energy to kinetic energy. In
this state, the fuel is passed through the discharge
orifice which removes the swirl motion as the fuel is
atomized to form a cone-shaped spray. This is called
’pressure jet atomization’. The rate of swirl and
pressure of the fuel at the fuel spray nozzle are
important factors in good atomization. The shape of
the spray is an indication of the degree of
atomization as shown in fig. 10-15. Later fuel spray
nozzles utilize the airspray principle which employs
high velocity air instead of high velocity fuel to cause
atomization. This method allows atomization at low
fuel flow rates (provided sufficient air velocity exists)
thus providing an advantage over the pressure jet
atomizer by allowing fuel pumps of a lighter construction
to be used.
91. The atomizing spray nozzle, as distinct from the
vaporizing burner (Part 4), has been developed in
five fairly distinct types; the Simplex, the variable port
(Lubbock), the Duplex or Duple, the spill type and the
airspray nozzle.
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