Page 109 - The Jet Engine

Fuel system

hydro-mechanical, and the acceleration and speed a balance of forces across the fuel pump servo
control and pressure ratio control systems, which are piston and ensuring a steady pressure to the throttle
mechanical. With the exception of the pressure ratio valve.
control system, which uses a gear-type pump, all the
systems use a variable-stroke, multi-plunger type 13. When the throttle is slowly opened, the
fuel pump to supply the fuel to the spray nozzles. pressure to the throttle valve falls and allows the
F.C.U. spill valve to close, so increasing the servo
8. Some engines are fitted with an electronic pressure and pump delivery. As the pressure to the
system of control and this generally involves the use throttle is restored, the spill valve returns to its
of electronic circuits to measure and translate sensitive or controlling position, and the fuel pump
changing engine conditions to automatically adjust stabilizes its output to give the engine speed for the
the fuel pump output. On helicopters powered by gas selected throttle position. The reverse sequence
turbine engines using the free-power turbine occurs as the throttle is closed.
principle (Part 5), additional manual and automatic
controls on the engine govern the free-power turbine 14. A reduction of air intake pressure, due to a
and, consequently, aircraft rotor speed. reduction of aircraft forward speed or increase in
altitude, causes the F.C.U. capsule to expand, thus
FUEL CONTROL SYSTEMS increasing the bleed from the F.C.U. spill valve. This
reduces fuel pump delivery until the fuel flow
9. Typical high pressure (H.P.) fuel control systems matches the airflow and the reduced H.P. pump
for a turbo-propeller engine and a turbo-jet engine delivery (throttle inlet pressure), allows the spill valve
are shown in simplified form in fig. 10-3, each to return to its sensitive position. Conversely, an
basically consisting of an H.P. pump, a throttle increase in air intake pressure reduces the bleed
control and a number of fuel spray nozzles. In from the spill valve and increases the fuel flow. The
addition, certain sensing devices are incorporated to compensation for changes in air intake pressure is
provide automatic control of the fuel flow in response such that fuel flow cannot be increased beyond the
to engine requirements. On the turbo-propeller pre-determined maximum permissible for static
engine, the fuel and propeller systems are co-
ordinated to produce the appropriate fuel/r.p.m. International Standard Atmosphere (I.S.A.) sea-level
conditions.
combination.
15. The engine speed governor prevents the engine
10. The usual method of varying the fuel flow to the
spray nozzles is by adjusting the output of the H.P. from exceeding its maximum speed limitation. With
increasing engine speed, the centrifugal pressure
fuel pump. This is effected through a servo system in
response to some or all of the following: from the fuel pump rotor radial drillings increases and
this is sensed by the engine speed governor
(1) Throttle movement. diaphragm. When the engine reaches its speed
(2) Air temperature and pressure. limitation, the diaphragm is deflected to open the
(3) Rapid acceleration and deceleration. governor spill valve, thus overriding the F.C.U. and
(4) Signals of engine speed, engine gas preventing any further increase in fuel flow. Some
temperature and compressor delivery pressure control systems employ a hydro-
pressure. mechanical governor (para. 23).

Pressure control (turbo-propeller engine) 16. The governor spill valve also acts as a safety
11. The pressure control system (fig. 10-4) is a relief valve. If the fuel pump delivery pressure
typical system as fitted to a turbo-propeller engine exceeds its maximum controlling value, the servo
where the rate of engine acceleration is restricted by pressure acting on the orifice area of the spill valve
a propeller speed controller. The fuel pump output is forces the valve open regardless of the engine
automatically controlled by spill valves in the flow speed, so preventing any further increase in fuel
control unit (F.C.U.) and the engine speed governor. delivery pressure.
These valves, by varying the fuel pump servo
pressure, adjust the pump stroke to give the correct Pressure control (turbo-jet engine)
fuel flow to the engine. 17. In the pressure control system illustrated in fig.
10-5, the rate of engine acceleration is controlled by
12. At steady running conditions, at a given air a dashpot throttle unit. The unit forms part of the fuel
intake pressure and below governed speed, the spill control unit and consists of a servo-operated throttle,
valve in the F.C.U. is in a sensitive position, creating which moves in a ported sleeve, and a control valve.

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