Page 113 - The Jet Engine

Fuel system

until the fuel flow matches the airflow. Conversely, an Flow control
increase in air intake pressure closes the spill valve 29. A flow control fuel system is generally more
to increase the fuel flow. compact than a pressure control system and is not
sensitive to flow effect of variations downstream of
23. H.P. compressor shaft r.p.m. is governed by a the throttle. The fuel pump delivery pressure is
hydro-mechanical governor which uses hydraulic related to engine speed; thus, at low engine speeds
pressure proportional to engine speed as its pump delivery pressure is quite low. The fuel pump
controlling parameter. A rotating spill valve senses output is controlled to give a constant pressure
the engine speed and the controlling pressure is difference across the throttle valve at a constant air
used to limit the pump stroke and so prevent over- intake condition. Various devices are also used to
speeding of the H.P. shaft rotating assembly. The adjust the fuel flow for air intake pressure variations,
controlling pressure is unaffected by changes in fuel idling and acceleration control, gas temperature and
specific gravity. compressor delivery pressure control.

24. At low H.P. shaft speeds, the rotating spill valve 30. A variation of the flow control system is the pro-
is held open, but as engine speed increases, portional flow control system (fig 10-7), which is more
centrifugal loading moves the valve towards the suitable for engines requiring large fuel flows and
which also enables the fuel trimming devices to
closed position against the diaphragm loads. This adjust the fuel flow more accurately. A small
restricts the bleed of fuel to the L.P. side of the valve controlling flow is created that has the same charac-
until, at governed speed, the governor pressure teristics as the main flow, and this controlling or pro-
deflects the servo control diaphragm and opens the portional flow is used to adjust the main flow.
servo spill valve to control the fuel flow and thereby
the H.P. shaft speed. 31. A different type of spill valve, referred to as a
kinetic valve, is used in this system. This valve
25. If the engine gas temperature attempts to consists of two opposing jets, one subjected to pump
exceed the maximum limitation, the current in the delivery pressure and the other to pump servo
L.P. speed limiter and temperature control solenoid is pressure, and an interrupter blade that can be moved
reduced. This opens the spill valve to reduce the between the jets (fig. 10-8). When the blade is clear
pressure on the pressure drop control diaphragm. of the jets, the kinetic force of the H.P. fuel jet causes
The flow control spill valve then opens to reduce the the servo pressure to rise (spill valve closed) and the
pump servo pressure and fuel pump output. fuel pump moves to maximum stroke to increase the
fuel flow. When the blade is lowered between the
26. To prevent the L.P. compressor from over- jets, the pressure jet is deflected and the servo
speeding, multi-spool engines usually have an L.P. pressure falls, so reducing the pump stroke and the
compressor shaft speed governor. A signal of L.P. fuel flow, When the engine is steadily running, the
shaft speed and intake temperature is fed to an blade is in an intermediate position allowing a slow
amplifier and solenoid valve, the valve limiting the bleed from servo and thus balancing the fuel pump
fuel flow in the same way as the gas temperature output.
control (para. 25).
32. All the controlling devices, except for the engine
27. The system described uses main and starting speed governor, are contained in one combined fuel
spray nozzles under the control of an H.P. shut-off control unit. The main parts of the control unit are the
valve. Two starting nozzles are fitted in the altitude sensing unit (A.S.U.), the acceleration
control unit (A.C.U.), the throttle and pressurizing
combustion chamber, each being forward of an valve unit, and the proportioning valve unit.
igniter plug. When the engine has started, the fuel
flow to these nozzles is cut off by the H.P. shut-off 33. At any steady running condition below governed
valve. speed, the fuel pump delivery is controlled to a fixed
value by the A.S.U. The spill valve in this unit is held
28. To ensure that a satisfactory fuel pressure to the in the controlling position by a balance of forces,
spray nozzles is maintained at high altitudes, a back spring force and the piston force. The piston is
pressure valve, located downstream of the throttle sensitive to the pressure difference across the
valve, raises the pressure levels sufficiently to sensing valve, the pressure difference being created
ensure satisfactory operation of the fuel pump servo by fuel flowing from the proportioning valve back to
system. the fuel pump inlet.

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