GB2092677A - Fuel control system for a gas turbine engine - Google Patents

Abstract

The system has a main control unit 22 and an overspeed governor 25 which is operative to prevent overspeed of the rotors of the multi- spool engine. The governor comprises a flyweight governor 38 mechanically connected to the high pressure spool of the engine and an electronic governor 44 which applies a variable bias to the mechanical flyweight governor via servo 46, reaction plate 47 and spring 37 in response to the speed of the low pressure spool. Stops 48, 49 limit the influence of the electronic governor. In this way a basically mechanical governor is provided which responds to the speed of the two spools and which is not catastrophically affected by failure of the relatively less reliable electronic components. <IMAGE>

Description

SPECIFICATION Fuel control system for a gas turbine engine This invention relates to a fuel control system for a gas turbine engine.

In recent years designers have been introducing electronic main fuel control systems for gas turbine engines. These systems have various advantages over the commonly used hydromechanical systems but they have not as yet demonstrated the high degree of reliability which hydro-mechanical systems have shown. It is therefore vital that fully electronic main fuel control systems should have some form of limiting device which will prevent an uncontrolled runaway of the engine speed should a component fail.

Existing fuel systems include such a component in the form of an overspeed governor which is often a mechanical flyweight governor. The governor is normally connected to rotate at a speed proportional to the rotational speed of the high pressure rotor of the engine. This is quite satisfactory in the case of single shaft engines and in some two shaft engines. However, when a fan engine is being controlled the straightforward use of the rotational speed of the high pressure rotor is not sufficient. Thus it is possible in some circumstances for the fan to operate at excessive speeds while the high pressure rotor speed is still within the controlled limits, and some fan designs are prone to fan flutter at speeds only a few percent above the normal maximum.

It is therefore desirable to provide an overspeed limiter which retains the basic reliability of the hydro-mechanical system but which is enabled to take into account the speed of the fan or low pressure shaft in addition to that of the high pressure system. The present invention provides a fuel control system having an overspeed limiter which satisfies this requirement.

According to the present invention a fuel control system for a gas turbine engine comprises a main fuel control unit which controls fuel flow to the engine and an overspeed goveror which prevents the fuel flow being such as to allow an overspeed of a rotor of the engine, the overspeed governor comprising a mechanical flyweight governor connected to be driven at a speed proportional to the rotational speed of a high pressure rotor of the engine and to affect the fuel flow accordingly and an electric governor connected to receive a signal proportional to the rotational speed of the low pressure rotor of the engine and to bias the flyweight governor in accordance with the degree of overspeed of the low pressure rotor, stop means being provided whereby the effect of the electronic governor on the flyweight governor is limited.

The bias is preferably applied by way of a spring or springs, and the motion of the part of the flyweight governor affected by springs may be limited by said stop means.

The invention will now be particularly described merely by way of example with reference to the accompanying drawing which is: A diagrammatic view of a gas turbine engine and its fuel control system in accordance with the invention.

In the drawing there is shown a gas turbine engine of the type comprising a fan 10, an intermediate pressure compressor 11, a high pressure compressor 12, a combustion chamber 13, a high pressure turbine 14, an intermediate pressure turbine 15, and a low pressure turbine 16. A fuel manifold 1 7 conveys fuel to the combustion chamber 1 3 to be burnt. Operation of the engine is conventional in that air is taken into the fan 10 and compressed. Part of the compressed air provides propulsive thrust while the remainder provides the intake air for the intermediate pressure compressor 11. The air is then compressed in the successive compressors 11 and 12 and enters the combustion chamber 13 where it is mixed with fuel and burnt.The resulting hot gases drive the turbines 14, 1 5, and 1 6 in turn and finally exhaust to atmosphere to provide propulsive thrust. The turbines are drivingly interconnected with their respective compressors to drive these. The high pressure compressor 12 carries at its forward end a drive gear 1 8 which engages with an output governor 1 9 on a drive shaft 20 so that the rotational speed of the drive shaft 20 is proportional to the rotational speed NH of the high pressure rotor.

Fuel is shown as flowing through a duct 21 to the manifold 1 7 and to control the flow of fuel a main fuel control unit 22 is provided. This unit will normally have a number of inputs of various engine operating parameters and is arranged only to allow sufficient fuel to pass through the ducts 21 to give a desired thrust from the engine. The various inputs are not in general shown in the drawing but one sensor 23 and inter-connection 24 are shown as examples.

It is becoming more desirable to arrange that the fuel control unit 22 is an electronic unit. These devices can cope with greater complication than hydro-mechanical devices and they can be cheaper to manufacture. However, they have not demonstrated the same high degree of reliability as that of hydro-mechanical systems and steps must be taken to cater for possible catastrophic failure of the main fuel control which would allow an unrestricted flow of fuel through the duct 21 and thus an uncontrolled overspeed of the engine.

An overspeed governor shown generally at 25 is therefore provided.

The operative part of the governor 25 consists of a bypass passage 26, through which the output from the main fuel pump 27 may be allowed to recirculate round the pump and therefore not to proceed to the engine. The passage 26 is provided with a valve 28 consisting of an obturating member 29 which can slide as a close fit within the passage 26 up to stops 30. Depending upon how far the obturator 29 is pushed into the passage 26 there will be a varying degree of recirculation permitted around the pump 27.

To control the position of the obturating member 29 a sealed chamber 31 behind the member 29 is fed with a servo pressure through a line 32. The pressure in this line is determined by the position of a spool valve 33. The spool 33 may be moved axially so as to allow a high pressure from the supply duct 34 or a low pressure from the supply duct 35 to feed to the passage 32. The axial position of the spool valve 33 is determined by the position of the thrust plate 36 which forms part of a mechanical flyweight governor. The governor consists of a normal combination of the thrust plate which is spring biased by a spring 37 and a pair of flyweights 38 pivoted at 39 and having operating arms 40 engaging with the thrust plate 36. A pinion 41 drives the thrust plate from a driving gear 42 connected in this instance to the shaft 20.

It will be seen that the thrust plate 36 and flyweights 38 will rotate about their axis at a speed proportional to the rotational speed NH of the high pressure rotor of the engine. Should this speed become excessive centrifugal loads on the bob-weights 38 will cause them to fly outwards pushing the thrust plate 36 upwards against the spring pressure and aliowing low pressure servo fluid to enter the passage 32, to retract the obturator 29 and allow an increased bypass flow through the passage 26 round the pump 27. Any overspeed of the high pressure rotor of the engine may thus be detected and corrected. However, because of the independence of the high pressure and low pressure rotors of the engine it is clearly possible for the low pressure rotor to overspeed in conditions where the high pressure rotor is still within its operating limits.Such an overspeed is not likely to be very considerable because the rotors are in fact aerodynamically linked together and their speeds are therefore dependent upon one another. Nevertheless, some small overspeed of the fan might be possible and this might be detrimental.

Therefore the governor 25 is provided with means for taking into account the speed of the low pressure system. A pick-up shown at 43 detects the speed of the low pressure system and a signal proportional to this speed is fed from the pick-up into an electronic overspeed detector 44. If the speed detected is greater than preset limits the detector 44 will produce an output signal which will cause a torque motor 44 to operate. The motor actuates a servo 46 which in turn moves a spring reaction plate 47. The reaction plate 47 provides the back plate for the spring 37, therefore any movement of the plate 47 will vary the bias applied by spring 37 to the plate 36 and hence will provide an input to the hydro-mechanical governor which takes into account the speed of the low pressure system.

Since the detector 44 is an electronic device it is again important that arrangements are made such that catastrophic failure of the device cannot have correspondingly catastrophic effects on the operation of the engine. Therefore the position of the reaction plate 47 is only allowed to be varied between two stops 48 and 49 whose position is chosen so as to allow only a limited degree of bias to be applied to hydro-mechanical governor.

It can be seen therefor that this system provides the desired combination of a reliable overspeed governor which will in normal circumstances govern on a combination of the high pressure and low pressure rotor speeds but which in the case of catastrophic electronic failure will continue to operate albeit in a less efficient manner.

It will be appreciated that a number of variations would be possible. Thus for instance the way in which the bias is applied to the hydro mechanical governor could be altered and it will be appreciated that the fan engine involved could easily be a two shaft engine.

Claims (8)

1. A fuel control system for a gas turbine engine comprising a main fuel control unit which controls fuel flow to the engine and an overspeed governor which prevents the fuel flow being such as to allow an overspeed of a rotor of the engine, the overspeed governor comprising a mechanical flyweight governor connected to be driven at a speed proportional to the rotational speed of the high pressure rotor of the engine and to affect the fuel flow accordingly and an electronic governor connected to receive a signal proportional to the speed of the low pressure rotor of the engine and to bias the flyweight governor in accordance with the degree of overspeed of the low pressure rotor, stop means being provided whereby the effect of the electronic governor on the flyweight governor is limited.

2. A fuel control system as claimed in claim 1 and in which said biasing is carried out by varying the spring pressure which counter-balances the effect of centrifugal force on the flyweights of the governor.

3. A fuel control system as claimed in claim 2 and in which said stop means limit the degree of bias applied by said springs.

4. A fuel control system as claimed in claim 2 or claim 3 and which there is a servo device controlled by said electronic governor and which applies said bias via said springs.

5. Afuel control system as claimed in any one of the preceding claims and in which said overspeed governor controls the fuel flow to said engine by opening or closing a bypass passage around the pump which delivers fuel to the engine.

6. A fuel control system as claimed in any one of the preceding claims and in which said low pressure rotor includes a fan.

7. A fuel control system substantially as hereinbefore particularly described with reference to the accompanying drawing.

8. A gas turbine engine having a fuel control system as claimed in any one of the preceding claims.