Monday, January 23, 2012

Fuel control system

Fuel system

hydro-mechanical, and the acceleration and speed control and pressure ratio control systems, which are mechanical. With the exception of the pressure ratio control system, which uses a gear-type pump, all the systems use a variable-stroke, multi-plunger type fuel pump to supply the fuel to the spray nozzles.

8. Some engines are fitted with an electronic system of control and this generally involves the use of electronic circuits to measure and translat changing engine conditions to automatically adjust the fuel pump output. On helicopters powered by gas turbine engines using the free-power turbine principle (Part 5), additional manual and automatic controls on the engine govern the free-power turbine and, consequently, aircraft rotor speed.


9. Typical high pressure (H.P.) fuel control systems for a turbo-propeller engine and a turbo-jet engine are shown in simplified form in fig. 10-3, each basically consisting of an H.P. pump, a throttle control and a number of fuel spray nozzles. In addition, certain sensing devices are incorporated to provide automatic control of the fuel flow in response to engine requirements. On the turbo-propeller engine, the fuel and propeller systems are co- ordinated to produce the appropriate fuel/r.p.m. combination.
10. The usual method of varying the fuel flow to the spray nozzles is by adjusting the output of the H.P. fuel pump. This is effected through a servo system in response to some or all of the following:

(1) Throttle movement.
(2)  Air temperature and pressure.
(3)  Rapid acceleration and deceleration.
(4)  Signals of engine speed, engine gas temperature and compressor deliverypressure.

Pressure control (turbo-propeller engine) 11. The pressure control system (fig. 10-4) is a typical system as fitted to a turbo-propeller engine where the rate of engine acceleration is restricted by a propeller speed controller. The fuel pump output is automatically controlled by spill valves in the flow control unit (F.C.U.) and the engine speed governor. These valves, by varying the fuel pump servo pressure, adjust the pump stroke to give the correct fuel flow to the engine.
12. At steady running conditions, at a given air intake pressure and below governed speed, the spillvalve in the F.C.U. is in a sensitive position, creatinga balance of forces across the fuel pump servo
piston and ensuring a steady pressure to the throttle
13. When the throttle is slowly opened, the pressure to the throttle valve falls and allows the F.C.U. spill valve to close, so increasing the servo pressure and pump delivery. As the pressure to the throttle is restored, the spill valve returns to its sensitive or controlling position, and the fuel pump stabilizes its output to give the engine speed for the selected throttle position. The reverse sequence occurs as the throttle is closed.

14. A reduction of air intake pressure, due to a reduction of aircraft forward speed or increase in altitude, causes the F.C.U. capsule to expand, thus increasing the bleed from the F.C.U. spill valve. This reduces fuel pump delivery until the fuel flow matches the airflow and the reduced H.P. pump delivery (throttle inlet  ressure), allows the spill valve to return to its sensitive position. Conversely, an increase in air intake pressure reduces the bleed from the spill valve and increases the fuel flow. The compensation for changes in air intake pressure is such that fuel flow cannot be increased beyond the pre-determined maximum permissible for  tatic International Standard Atmosphere (I.S.A.) sea-level conditions. 

15. The engine speed governor prevents the engine from exceeding its maximum speed limitation. With increasing engine speed, the centrifugal pressure from the fuel pump rotor radial drillings increases and this is  sensed by the engine speed governor diaphragm. When the engine reaches its speed limitation, the  diaphragm is deflected to open the governor spill valve, thus overriding the F.C.U. and preventing any further increase in fuel flow. Some pressure control systems employ a hydro-mechanical governor (para. 23).

16. The governor spill valve also acts as a safety relief valve. If the fuel pump delivery pressure exceeds its maximum controlling value, the servo pressure acting on the orifice area of the spill valve forces the valve open regardless of the engine speed, so preventing any further increase in fuel delivery pressure.

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