Page 115 - The Jet Engine

Fuel system

41. The pressure drop control spill valve closes to the fuel output in the same way as the gas
increase the fuel pump delivery and maintains the temperature control.
controlling pressure difference across the plunger.
The fuel flow, therefore, progressively rises as airflow 48. An idling speed governor is often fitted to
through the compressor increases. The degree of ensure that the idling r.p.m. does not vary with
overfuelling can be automatically changed by the air changing engine loads. A variation of idling r.p.m.
switch, which increases the pressure signal on to the causes the rocker arm to move and alter the propor-
capsule. The full value of compressor delivery tional flow, and the A.S.U. adjusts the pump delivery
pressure is now passed on to the A.C.U. capsule until the correct idling r.p.m. is restored.
assembly, thus increasing the opening rate of the 49. On some engines, a power limiter is used to
metering plunger.
prevent overstressing of the engine. To achieve this,
42. As the controlled overfuelling continues, the compressor delivery pressure acts on the power
pressure difference across the throttle valve limiter capsule. Excess pressure opens the power
increases. When it reaches the controlling value, the limiter atmospheric bleed to limit the pressure on the
A.S.U. takes over due to the increasing proportional A.C.U. capsule and this controls the fuel flow through
flow and again gives a steady fuel flow to the spray the metering plunger.
nozzles.
50. To enable the engine to be relit and to prevent
43. The engine speed governor can be of the flame-out at altitude, the engine idling r.p.m. is made
pressure control type described in para. 15, or a to increase with altitude. To achieve this, some
hydro-mechanical governor as described in para. 23. engines incorporate a minimum flow valve that adds
a constant minimum fuel flow to that passing through
44. The control of servo pressure by the hydro- the throttle valve.
mechanical governor is very similar to that of the
pressure control governor, except that the governor Combined acceleration and speed control
pressure is obtained from pump delivery fuel passing 51. The combined acceleration and speed control
through a restrictor and the restricted pressure is system (fig. 10-9) is a mechanical system without
controlled by a rotating spill valve; this type of small restrictors or spill valves. It is also an all-speed
governor is unaffected by changes in fuel specific governor system and therefore needs no separate
gravity. governor unit for controlling the maximum r.p.m. The
controlling mechanism is contained in one unit,
45. At low engine speeds, the rotating spill valve is usually referred to as the fuel flow regulator (F.F.R.).
held open; however, as engine speed increases, An H.P. fuel pump (para. 85) is used and the fuel
centrifugal loading moves the valve towards the pump servo piston is operated by H.P. fuel on one
closed position against the diaphragm loads. This side and main spray nozzle (servo) pressure on the
restricts the bleed of H.P. fuel to the L.P. side of the spring side.
drum until, at governed speed, the governor
pressure deflects the diaphragm and opens the fuel 52. The F.F.R. is driven by the engine through a
pump servo pressure spill valve to control the gear train and has two centrifugal governors, known
maximum fuel flow and engine speed. as the speed control governor and the pressure drop
control governor. Two sliding valves are also rotated
46. If the engine gas temperature exceeds its by the gear train. One valve, known as the variable
maximum limitation, the solenoid on the proportion- metering sleeve, has a triangular orifice, known as
ing valve unit is progressively energized. This causes the variable metering orifice (V.M.O.), and this sleeve
a movement of the rocker arm to increase the is given axial movement by a capsule assembly. The
effective flow area of one restrictor, thus increasing V.M.O. sleeve moves inside a non-rotating governor
the proportional flow and opening the A.S.U. spill sleeve that is moved axially by the speed control
valve to reduce servo pressure. The fuel flow is thus governor. The other valve, known as the pressure
reduced and any further increase of gas temperature drop control valve, is provided with axial movement
is prevented. by the pressure drop control governor and has a
triangular orifice, known as the pressure drop control
47. To prevent the L.P. compressor from over- orifice, and a fixed-area rectangular orifice. The
speeding, some twin-spool engines have an L.P. speed control governor is set by the throttle lever
shaft r.p.m. governor. A signal of L.P. shaft speed is through a cam, a spring and a stirrup arm inside the
fed to an amplifier and solenoid valve, which limits regulator.

105