GB2128771A - Fuel control system for a gas turbine engine - Google Patents

Abstract

A fuel control system for a gas turbine engine shows an arrangement for preventing overfuelling when a rapid deceleration is immediately followed by a rapid acceleration. This arrangement depends upon the fact that the relationship between the speeds of the various spools of the engine changes during deceleration. A scheduling device (36) calculates a theoretical steady state value of one rotational speed from the actual value of another rotational speed, and the difference between the theoretical and actual values is used to reset the acceleration control unit (30) to cause it to demand less fuel on re-acceleration than it would from the steady state condition, thus avoiding surging when rapid acceleration follows a rapid deceleration of the engine. <IMAGE>

Description

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

The fuel control systems presently used for gas turbine engines are by and large very satisfactory. They are reliable and accurate when dealing with steady state conditions, and are able to cope effectively with most transient conditions. There are, however, certain conditions which are difficult to deal with even today.

One such condition arises in the carrying out of the so-called 'Bodie' manoeuvre. A 'Bodie' manoeuvre comprises a rapid deceleration of the engine followed by a rapid acceleration. When these manoeuvres are carried out, particularly at high altitude, there is a tendency for the fuel control unit instantaneously to overfuel the engine on commencement of the acceleration, leading to the engine surging.

The present invention provides a way in which the rapid deceleration forming the first part of a Bodie manoeuvre may be detected and used to prevent over-fuelling during a subsequent acceleration.

According to the present invention a fuel control system for a gas turbine engine comprises an acceleration control unit, transducers for producing signals indicative of the rotational speeds of at least two spools of the engine, comparator means for comparing the actual relationship between these speeds with the theoretical relationship, a delay device through which the comparator output passes, and reset means in the acceleration control unit and operated by the comparator output for reducing the fuel flow to the engine from that normally permitted on acceleration.

A further delay device may be used to avoid rapid increases in fuel flow from that demanded by the deceleration control to that demanded by the reset acceleration control.

The reset may be arranged to be inoperative below a predetermined altitude.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawing which diagrammatically illustrates a gas turbine engine and its fuel control system in accordance with the invention.

In the drawing, the gas turbine engine 10 is of the type known as a front fan, three shaft engine. It comprises a fan 11, intermediate and high pressure compressors 12 and 13, a combuster 14, and high, intermediate and low pressure turbines 15, 16 and 17 respectively. Drive shafts 18, 19 and 20 interconnect the compressors and turbine 13 and 15, 12 and 16 and 11 and 17 respectively, each unit of compressor (or fan), shaft and turbine comprising one spool or rotor. Operation of the engine in general is conventional and well-known, and will not be further described in this specification.

Fuel for the engine flows through a feed pipe 21 under the control of a fuel valve 22 which is in turn controlled by a fuel control system generally indicated at 23. Various transducers are installed in the engine to measure various of the operating parameters. Thus sensors 24 and 25 in the engine inlet measure the inlet pressure and temperature P1 and T1 respectively, while tachometer sensors 26 and 27 measure the rotational speeds N2 and N3 of the intermediate pressure and high pressure spools of the engine. It would of course be possible, and may be desirable, to measure many additional parameters but for proper illustration of the present invention these parameters suffice.

The fuel control system 23 includes three main units; a steady static control unit 28, a deceleration control unit 29 and an acceleration control unit 30. Various of the engine parameters, including P1, T1, N2 and N3 are fed into these units which then produce output of control parameters related to the fuel flow which their particular control laws demand. Various methods may be used to ensure that the correct one of the three control outputs is chosen as the actual controlling parameter. In the present embodiment the acceleration control output and the steady state control output are compared in at 'least wins' unit 31 so that the lower of the two control functions is selected, and this is then compared with the deceleration control function in a 'highest wins' unit 32.This combination ensures that during an acceleration the acceleration control unit prevents substantial overfuelling, while during deceleration the deceleration control unit prevents underfuelling. Of course, as mentioned above the combination of a rapid deceleration followed by a rapid deceleration (a Bodie manoeuvre) can sometimes produce transient overfuelling and surge.

The present system is therefore provided with an arrangement to prevent this overfuelling. This relies on the fact that during a deceleration the relationship between the rotational speeds N2 and N3 of the intermediate and high pressure speeds of the engine differs from that in the steady state condition. Thus a graph of N2 against N3 (or more correctly the non-dimensional parameter (N2IVT; against N3/+ ) will comprise a particular curve when the engine is in a steady state condition, but upon rapid deceleration the graph will comprise a rather similar curve but displaced in the N2 or N3 direction.

By detecting this change in relationship it is therefore possible to arrange a reset on the acceleration control unit to avoid any subsequent overfuelling. In detail, the inputs from the transducers reading N1, N3 and T1, are connected to dividers 33 and 34 and a square root calculator 35. In these devices T1 is operated on to find its square root T, and the result is divided into N2 and N3 to produce an output representative of the non-dimensional parameters N2/+ and N3/.

The latter output is applied to a scheduling type of calculating device 36 where a notional steady state value of N3/T; is calculated from the actual value of N3/T; and the known, steady state relationship between N3f; and N2f;. This notional steady state value for N2/iT; is compared with the actual value in a differential device 37. If the engine is in a steady state condition the output will be zero, but if a rapid deceleration is taking place there will be an output whose size will depend upon the rate of deceleration.

This output forms the basis of the reset which, in the present invention, reduces the fuel flow upon acceleration immediately following the deceleration. The output, proportional to error in N2/Vs fed to the acceleration control unit 30 via a muting device 38, a multiplier 39 and a delay device 40. The muting device 38 has an additional input representing the input pressure P1, and is arranged to allow the error signal to pass only when this pressure is below a predetermined limit. The muting device is an optional addition, and its effect is to allow the resetting of the acceleration control unit 30 only when the engine is operating above a predetermined altitude.

This may be advantageous in some cases since the problem of surge during the acceleration part of a Bodie manoeuvre is worse at high altitudes than at low. Therefore, it may be possible to leave the unmodified fuel system operative at low altitudes and to modify it only at high altitudes; use of the muting device 38 allows this.

The other devices acting on the signal before it reaches the acceleration control unit are the multiple 39 and delay device 40, which together delay the signal by an amount dependant upon the size of the error signal. The purpose of the delay is to ensure that the reset on the acceleration control unit is operative after the deceleration has ceased and when acceleration has commenced; it would clearly not be helpful if the reset were only operative during the deceleration itself.

The signal from the delay device 40 finally enters the acceleration control unit 40, where it operates a reset device which may be one of a number of known expedients. The effect of the reset is to move the schedule of engine parameters against acceleration control function which controls fuel flow, the movement being in the direction of reduced fuel flow. In this way the fuel flow on acceleration immediately after a rapid deceleration is reduced by the amount of reset of the acceleration control unit.

Even with the added sophistication of the reset device described above, there is still the possibility of the fuel system calling for a step change in fuel flow from the low level required by the deceleration control unit to the higher level demanded by the acceleration control unit, even when reset. This is not desirable, since a sharp change in fuel flow can be deleterious to engine handling.

The illustrated embodiment is therefore provided with features which avoid this possibility. The error in N2/iT; is again used, the signal representative of this error being taken from the differential device 37. The error is again multiplied in a unit 41 and operates a delay device 42. The delay device 42 operates on the output signal from the unit 43 which produces a demanded fuel flow signal from the output of the highest wins device 32. Output from the delay 42 operates the fuel valve 22.

The delay device 42 performs a Laplace transformation of the input and will therefore have the effect of delaying the full onset of rapid change in fuel flow so that instead of a step change of fuel flow the demand will be for a more gradual change.

The system described above will therefore avoid any of the deleterious effects of the overfuelling which can happen after rapid changes of throttle setting such as occur in Bodie manoeuvres. It will be appreciated that a number of modifications of this system are possible; in particular the delay device 42 and/or the muting device 38 could be omitted.

It will also be understand that the invention could be applied to a variety of fuel system stages ranging from hydromechanical to fully electronic digital systems, and that the engine could be of the two-shaft rather than the three-shaft variety.

Claims (5)

1. A fuel control system for a gas turbine engine comprising an acceleration control unit, transducers for producing signals indicative of the rotational speeds of at least two spools of the engine, comparator means for comparing the actual relationship between these speeds with the theoretical relationship, a delay device through which the comparator output passes, and reset means in the acceleration control unit and operated by the delayed comparator output to reduce the fuel flow of the engine from that normally permitted on acceleration.

2. Afuel control system is claimed in claim 1 and comprising a multing device adapted to allow said comparator output to reach said reset means only when the ambient air pressure is below a predetermined level.

3. Afuel control system as claimed in claim 1 or claim 2 and comprising a further delay device operated by the output of said comparator to prevent step changes in fuel flow up to that deamnded by said acceleration control unit.

4. A fuel control system substantially as hereinbefore particularly described with reference to the accompanying drawings.

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