GB1566015A

GB1566015A - Aircraft engine speed control apparatus

Info

Publication number: GB1566015A
Application number: GB2571877A
Authority: GB
Original assignee: Bodenseewerk Geratetechnik GmbH
Current assignee: Bodenseewerk Geratetechnik GmbH
Priority date: 1977-01-22
Filing date: 1977-06-20
Publication date: 1980-04-30
Also published as: DE2702564B2; DE2702564C3; DE2702564A1

Description

(54) AIRCRAFT ENGINE SPEED CONTROL APPARATUS (71) We, BODENSEEWERK GERATETECHNIK GmbH., a Germany Company, of 777 Uberlingen/Bodensee, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a device for controlling the rotary speed in turbo-jet engines for aircraft.

In a single axis jet engine, a compressor having a plurality of annular arrays of blades and a turbine are located one behind the other on a shaft, a combustion chamber being defined between the compressor and the turbine. Fuel is supplied continuously into the combustion chamber, the fuel flow being metered by a fuel control unit. Compression of the air sucked-in at the front of the engine is effected by the compressor, this air being heated up thereby. The fuel is injected into this compressed and heated air in the combustion chamber and is ignited. The hot propellent gases generated thereby are expelled at high velocity through a nozzle at the rear of the jet engine and generate the thrust of the jet engine. The energy for driving the compressor is taken from the propellent gas stream by the turbine.For better adaptation to the various operating states multiple axes jet engines are known, which comprise an inner shaft and one or two outer shafts designed as hollow shafts and enclosing the inner shaft. On each shaft, a compressor is located in front of the combustion chamber, and a turbing driving this compressor is located behind the combustion chamber. The compressors are arranged one axially behind the other, and also the associated turbines are located one behind the other with the order reversed. Each compressor therein is driven by the associated turbine at at least approximately optimum rotational speed.

The control of the rotary speed of such a jet engine presents certain problems.

Due to the inertia of the rotating engine rotor, the rotary speed of the engine rotor composed of the compressor, shaft and turbine varies relatively slowly as compared to the variations of the pressures and temperatures and of the mass flow, i.e. of the air flow delivered by the compressor, caused by disturbances. When the rotary speed of the jet engine is to be increased and, correspondingly, an increase of the metered fuel flow is effected, this increase does cause an immediate increase of pressure in the combustion chamber but, at first, does not cause a corresponding increase of the rotary speed. Hence, the compressor operates against an increased pressure but with, at first, unchanged rotary speed. The mass flow therefore drops.

The ratio of outlet to inlet pressures rises, at first, as the pressure at the outlet of the high-pressure compressor is increased without this increase of pressure becoming completely effective at the inlet of the high-pressure compressor.

With further decrease of the mass flow and, at first, invariable rotary speed, the pressure ratio will drop again.

This variation of the pressure ratio can also be considered as follows. With constant rotary speed of the high-pressure compressor, the ratio of outlet to inlet pressure rises at first with increasing mass flow. The rise in pressure across the highpressure compressor becomes the higher the more air is delivered by the high pressure compressor. From a maximum level which corresponds to the mass flow for which the high-pressure compressor has been designed as the optimum, the pressure ratio will drop again with increasing mass flow. The high-pressure compressor is not able further to compress the air sucked-in at the predetermined rotary speed, and it will then progressively operate no longer as a compressor but as a restrictor, across which the pressure drops again.

The variation of the pressure ratio as a function of the mass flow with constant rotary speed is illustrated in Fig. 1.

Reduction of the mass flow, for example beyond a point D in Fig. 1 with a corresponding pressure ratio derived from the curve of Fig. 1, results in an unstable state which may cause "surging" of the jet engine. Within the range between the points A and D of Fig. 1 the slope of the pressure ratio curve is positive. With decreasing mass flow the pressure ratio will become smaller. If at a point P of this range the mass flow of the high-pressure compressor drops temporarily due to some disturbance, for example because the air intake opening of the jet engine temporarily gets to the leeward side, this will be accompanied by a fall in the delivery pressure of the high-pressure compressor. If the pressure downstream of the high-pressure compressor, i.e. in the combustion chamber or in the last compressor stages, does not fall quickly enough, the flow will tend to reverse its direction.When this occurs, the pressure ratio drops rapidly. Meanwhile the pressure downstream of the compressor has fallen also, so that the compressor now begins to operate again. This cycle is then repeated at high frequency and results in heavy aerodynamic pulsation, or "surging", by which the jet engine can be destroyed within few seconds. Therefore such surging of the jet engine has to be prevented safely.

As has been explained hereinbefore, the jet engine may quickly get into the condition where its mass flow and pressure ratio is in the range between D and A of Fig. 1 in which the risk of "surging" exists, by increasing the metered fuel flow with, at first, still unchanged rotary speed, and this surging may be triggered by minor disturbances. Therefore it is necessary to limit the metered fuel flow, when accelerating the jet engine, such that the "surge line" or preferably a "surge line limit curve" extending at a safety distance therefrom, which separates the stable and the unstable ranges in the mass flow versus pressure ratio field, is not passed.

This situation will be explained in greater detail with reference to Fig. 2, wherein there is illustrated a family of curves comprising curves similar to the curve of Fig. 1 but associated with different rotary speeds. For each rotary speed there is a point on the respective curve corresponding to the point D of Fig. 1, at which point the unstable range begins. The locus of these points is the surge line. The surge line limit curve runs at a safety distance from this surge line.For acceleration of the jet engine to a higher rotary speed, for example from 0.6 to 1.0, which should be achieved as quickly as possible, just so much fuel should be metered that the surge line limit curve is just reached from point P1 in Fig. 2 along the "0.6"-curve and the mass flow and pressure ratio are increased with now increasing rotary speed just in such a way that the point characterising the state of the engine moves along the surge line limit curve until the "1 .0"-curve has been reached. At this rotary speed the jet engine changes over into its stable new operating state along the "l.0"-curve.

Such performance with overshooting being safely prevented is, however, difficult to achieve.

It has previously been proposed to provide a pressure ratio generator for generating a pressure ratio signal which represents the pressure ratio of the outlet and inlet pressures of the high-pressure compressor. Furthermore, there is a mass flow sensor for generating a mass flow signal representing the mass flow through the jet engine. A function generator provides the surge line limit curve in the form of a limit pressure ratio signal as a function of the mass flow signal. In order to avoid overshooting over the surge line limit curve, signal limiting means is provided for limiting the control device output signal supplied to the fuel control unit. This means limits the control output signal to a value which is determined by the difference between the pressure ratio signal provided by the pressure ratio generator and the limit pressure ratio signal.

In the proposed device the value to which the control device output signal is limited is a linear combination of the difference (surge line distance) and of its time derivative. As the fuel control unit has inertia, limiting of the control device output signal to a value proportional to the surge line distance only would result in overshooting over the surge line limit curve. It should be noted that with large control device output signals, since they occur when the jet engine is accelerated heavily, the metered fuel flow will be determined, at first, by the limitation only. To damp hunting, the time derivative of the surge line distance is applied.

With such an arrangement the actual distance from the surge line limit curve enters the limit in the form of a linear combination only. Therefore the signal limiting means never "knows" how far from the surge line limit curve the state of the jet engine, as characterised by mass flow and pressure ratio, actually is. The possibility of even damped overshooting occurring makes it necessary to keep the surge line limit curve at a respectful distance from the actual surge line. This, in turn, limits the acceleration with which the jet engine may be run up to a higher rotational speed, and thus to higher power.

It is an object of the invention to provide speed control apparatus for jet engines of aircraft, which permits an optimum rate of change of the rotational speed, but safely avoids any passing of the surge line limit curve.

According to the invention, apparatus for controlling the rotary speed in an aircraft turbo-jet engine comprises a rotary speed sensor which provides a rotary speed signal representing the rotary speed of the high-pressure compressor of the jet engine; a commanded value generator which provides a commanded value signal representing a commanded rotary speed; control means to which the rotary speed signal and the commanded value signal are applied and which is arranged to provide a control deviation signal derived from the difference between the rotary speed signal and the commanded value signal; a fuel control unit which is arranged to be controlled by the control deviation signal for metering the flow of fuel supplied to the jet engine; a pressure ratio generator for generating a pressure ratio signal which represents the ratio of the outlet and inlet pressures of the high-pressure compressor; a mass flow sensor for generating a mass flow signal representing the mass flow of the jet engine; a function generator which provides the surge line of the jet engine or a surge line limit curve extending at a safety margin therefrom in the form of a limit pressure ratio signal as a function of the mass flow signal; and signal limiting means between the control device and the fuel control unit, said means effecting limitation of the signal in accordance with the surge line distance obtained as difference of the limit pressure ratio signal and the pressure ratio signal, wherein the signal limiting means limits the time derivative of the fuel flow metered by the fuel control unit to a value substantially proportional to the surge line distance.

Hence the metered fuel flow is not limited directly but the time derivative thereof is. The smaller the surge line distance becomes, the slower will the change of the metered fuel flow be. When the surge line limit curve has been reached, the rate of change of the metered fuel flow becomes zero, i.e. the metered fuel flow remains constant. The increase of the rotary speed following with a time delay, the increase of the metered fuel flow, results in an increase of the mass flow and, because of the constant fuel flow, in a reduction of the combustion chamber temperature, and thus tends again to displace the operating pressure ratio from the surge line limit curve. With reasonable application of the surge line distance, any passing of the surge line limit curve is safely prevented.The "control operation described, comprising increasing of the metered fuel flow until the surge line limit curve is approached and constancy of the metered fuel flow is achieved; increasing of the rotary speed and thereby of the mass flow, following with a lag; and the displacement of the operating pressure ratio from the surge line limit curve; is then repeated, so that the state of the jet engine eventually travels along the surge line limit curve (as in Fig. 2), until the selected rotary speed (for example 1.0) has been reached. Then such a state at a distance from the surge line limit curve arises that the metered fuel flow is no longer determined by the limitation but by the control device output signal itself. The state of the jet engine then travels along the "1.0"curve in Fig. 2, as illustrated therein by the dashed line, to the new stable state at the point P2.

Certain problems arise in obtaining accurately and, above all, without lag a signal which represents the time derivative of the metered fuel flow.

In a modification of the invention, the limitation is effected in such way that the control device output signal is differentiated with a lag, and is then applied to the fuel control unit integrated with a lead corresponding to this lag; the transient response of the lag is selected equal to the transient response of the fuel control unit; and the signal limiting means effects limitation of the control device output signal differentiated with lag.

With such an arrangement and with normal control operation without limitation, i.e. with sufficient distance of the operating pressure ratio from the surge line limit curve, the control device output signal is applied directly to the fuel control unit, as the differentiation with lag having a transfer function 1+Ts and the integration with lead having a transfer function 1+Ts s cancel each other. Limitation is effected between this differentiation and the integration, where in any case, whether the limitation is effective or not, the time derivative MREG of the metered fuel flow is available free from lag, as will be shown hereinbelow.The signal limited to a value proportional to the surge line distance, with the limitation becoming effective, is integrated with the lead and is applied to the fuel control unit, which supplies, with a lag according to a transfer function, a fuel flow M proportional to the applied signal to the jet engine. When the time constant of the lead network and the time constant of the lag network is equal to the time constant of the fuel metering unit (T=TFCu), the transfer function between the limited signal and the output M of the fuel control unit is l+Ts I 1 s T+Ts s i.e. the limited signal represents the undelayed time derivative M of the metered fuel flow M.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a graph showing, for ajet engine with a predetermined rotary speed nH of the high-pressure compressor, the variation of the pressure ratio (outlet to inlet pressures of the high-pressure compressor) P2/P as a function of the mass flow, Fig. 2 shows a family of curves similar to Fig. I for various rotary speeds of the high-pressure compressor, the positions of the surge line and of the surge line limit curve, and the variations of the engine states during the acceleration of the engine to a higher rotary speed, and Fig. 3 shows schematically a jet engine and a block diagram of apparatus for controlling rotary speed in accordance with the invention.

In Fig. 3 a known jet engine 10 with three axes is illustrated schematically. An inner casing 14 is arranged coaxially in the front portion of an outer casing 12. An annular chamber 16 is defined between the inner and the outer casings 14 and 12, respectively. A solid inner shaft 18 is mounted coaxially within the inner casing 14.

The inner shaft carries a low-pressure compressor 20 on its front end in front of the inner casing 14 and a turbine 22 on its rear end within the inner casing 14. The lowpressures compressor 20 extends across the front opening of the inner casing and across the front opening of the annular chamber 16. Thus the low-pressure compressor 20 generates both an air stream through the inner casing 14 ("hot" stream) and a relatively large air stream around the inner casing 14 through the annular chamber 16 ("cold" stream).

A first outer shaft 24 in the form of a hollow shaft is mounted coaxially with respect to the inner shaft I8. This first outer shaft 24 carries a medium-pressure compressor 26 on its front end within the inner casing 14 and downstream of the low-pressure compressor 20. A turbine 28 is located on the rear end upstream of the turbine 22 of the inner shaft 18. A second outer shaft 30 in the form of a hollow shaft is mounted coaxially around the first outer shaft 24. The second outer shaft 30 carries a high-pressure compressor 32 on its front end downstream of the mediumpressure compressor 26, and a turbine 34 on its rear end in front of the turbine 28 of the first outer shaft 24.

A combustion chamber 36 is defined between the high pressure compressor 32 and the associated turbine 34 within the inner casing 14, a metered fuel flow being supplied continuously to said chamber by a fuel control unit 38 through nozzles 40.

The known mode of operation of such a jet engine is as follows: The air sucked into the inner casing 14 and compressed by the low-pressure compressor 20 is further compressed by the medium-pressure compressor 26 and the high-pressure compressor 32, whereby it is heated up. Fuel is injected into the heated and compressed air in the combustion chamber 36. This fuel is ignited, and the hot combustion gases emerge with high velocity from the rear end of the inner casing 14. In a nozzle chamber 42 they mix with the air stream, which is directed by the low-pressure compressor 20 around the inner casing through the annular chamber 16. The air and combustion gas stream expelled with high velocity backwards from the nozzle chamber generates the thrust of the jet engine.The energy for driving the compressors 20, 26 and 32 is taken from the combustion gas stream by means of the turbines 22, 28 and 34, respectively. By using three separate shafts 18, 24 and 30 for three series-connected compressors, the rotary speeds of the compressors may be made different and may be adapted best to the respective compression requirements.

The following designations will be used in the present specification: total pressure at the inlet of the high-pressure compressor 32, p10=static pressure at the inlet of the high-pressure compressor 32, p2=total pressure at the outlet of the high-pressure compressor 32, Ap=different of total pressure p1 and static pressure p10 at the inlet of the highpressure compressor 32, rotary speed of the high-pressure compressor 32, nH50"=commanded rotary speed of the high-pressure compressor 32, T=filter time constants, TFcu=time constant of the fuel control unit 38, s=variable of the Laplace transform, control device output signal, M=metered fuel flow, M=time derivative of the metered fuel flow, m=mass flow of the high-pressure compressor 32, (p2/p1)pG=limit pressure ratio, i.e. the ratio of p2 and p1 corresponding to the surge line limit curve for selected nH and m, A=surge line distance (p2/p1)0-p2/j,1, k=proportionality factor.

The jet engine 10 has sensors which provide signals indicative of the various operating parameters.

An inlet pressure sensor 44 provides an inlet pressure signal which represents the total pressure p1 at the inlet of the high-pressure compressor 32, and an outlet pressure sensor 46 provides a signal which represents the total pressure p2 at the outlet of the high-pressure compressor 32. A sensor 48 provides a signal which represents the difference Ap of total pressure p, and static pressure prO at the inlet of the high-pressure compressor 32. A rotary speed sensor 50 provides a signal which represents the rotary speed nH of the high-pressure compressor.

A commanded value generator 52 provides a signal which represents a commanded rotary speed nH50" for the high-pressure compressor. The signal nH from the rotary speed sensor 50 and nHSO,, from the commanded value generator 52 are applied to a control device 54. This device provides an output signal fi which, when applied to the fuel control unit, causes a fuel flow to be fed to the engine so that the rotary speed nH is made equal to the commanded rotary speed nHSO".

In order to prevent surging of the jet engine due to too high a fuel flow M with respect to the respective rotary speed nH, the control device output signal fi is not applied directly to the fuel control unit 38. Instead, differentiation of the signal P with lag is effected by a filter 56 having a transfer function s 1+Ts The signal thus obtained (if signal limiting means 58 is disregarded at first) is subsequently integrated with lead by a filter 60 in accordance with a transfer function 1+Ts s the time constant T of the lead network in the filter 60 being equal to that of the lag network of the filter 56.The resultant transfer function of the two filters is s 1+Ts =1, l+Ts s so that without the limitation the control device output signal fi is applied to the fuel control unit 38 unchanged, and controls the metered fuel flow M. This is the normal control loop.

The signal Ap from the sensor 48 is applied to a divider 62 together with the inlet pressure signal p, from the inlet pressure sensor 44. The output signal Bplp, of the divider 62 provides a measure of the mass flow m of the high-pressure compressor 32. This output signal m is applied to a function generator 64, which at an output 66 provides a limit pressure ratio (P2/P1)PG associated with the mass flow m.

The outlet and inlet pressure signals p2 and p, from the outlet pressure sensor 46 and the inlet pressure sensor 44, respectively, are applied to a divider 68 which forms the operating pressure ratio P/P2. In a subtractor 70 the difference Aa=(p2/p1 )0-p2/p1 of the limit pressure ratio and of the actual operating pressure ratio, i.e. the surge line distance, is formed. This surge line distance is multiplied by a factor k at 72 and the resultant is applied to the signal limiting means 58.

The signal limiting means 58 selects the smaller one of the two signals applied thereto, namely the output from the filter 56 and the signal k åt, and connects this signal through the filter 60 to the fuel control unit 38. Thus if k åa is sufficiently large and the jet engine is sufficiently far away from the surge line, there will be the described normal control of the rotary speed under the control of the signal A. As the surge line limit curve is approached, however, the signal k åt will become effective as the controlling signal, since it will become the smaller of the two signals.

T=TFCu has been selected, i.e. the time constants T of the filters 56 and 60 are equalised with the time constant TFCU of the fuel control unit. Then

or, because of TFCu=T k å . M, s or 1 #α=. s . M, k or transformed back: 1 b--M.

k The limitation therefore causes the time derivative M of the fuel flow M metered by the fuel control unit 38 to become proportional to the surge line distance å.

The signal processing can be effected with analog signals. Preferably, however, the signals are digitized and are processed digitally with known means.

As will as in the three-axis jet engine described above as an example, the invention can be used also, for example, in a single axis jet engine.

The determination of the quantity M is also possible with another transfer function of the FCU.

WHAT WE CLAIM IS: 1. Apparatus for controlling the rotary speed of an aircraft turbo-jet engine, comprising a rotary speed sensor which provides a rotary speed signal representing the rotary speed of the high-pressure compressor of the jet engine; a commanded value generator which provides a commanded value signal representing a commanded rotary speed; control means to which the rotary speed signal and the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. so that without the limitation the control device output signal fi is applied to the fuel control unit 38 unchanged, and controls the metered fuel flow M. This is the normal control loop. The signal Ap from the sensor 48 is applied to a divider 62 together with the inlet pressure signal p, from the inlet pressure sensor 44. The output signal Bplp, of the divider 62 provides a measure of the mass flow m of the high-pressure compressor 32. This output signal m is applied to a function generator 64, which at an output 66 provides a limit pressure ratio (P2/P1)PG associated with the mass flow m. The outlet and inlet pressure signals p2 and p, from the outlet pressure sensor 46 and the inlet pressure sensor 44, respectively, are applied to a divider 68 which forms the operating pressure ratio P/P2. In a subtractor 70 the difference Aa=(p2/p1 )0-p2/p1 of the limit pressure ratio and of the actual operating pressure ratio, i.e. the surge line distance, is formed. This surge line distance is multiplied by a factor k at 72 and the resultant is applied to the signal limiting means 58. The signal limiting means 58 selects the smaller one of the two signals applied thereto, namely the output from the filter 56 and the signal k åt, and connects this signal through the filter 60 to the fuel control unit 38. Thus if k åa is sufficiently large and the jet engine is sufficiently far away from the surge line, there will be the described normal control of the rotary speed under the control of the signal A. As the surge line limit curve is approached, however, the signal k åt will become effective as the controlling signal, since it will become the smaller of the two signals. T=TFCu has been selected, i.e. the time constants T of the filters 56 and 60 are equalised with the time constant TFCU of the fuel control unit. Then or, because of TFCu=T k å . M, s or 1 #α=. s . M, k or transformed back:

1 b--M.

commanded value signal are applied and which is arranged to provide a control deviation signal derived from the difference between the rotary speed signal and the commanded value signal; a fuel control unit which is arranged to be controlled by the control deviation signal for metering the flow of fuel supplied to the jet engine; a pressure ratio generator for generating a pressure ratio signal which represents the ratio of the outlet and inlet pressures of the high-pressure compressor; a mass flow sensor for generating a mass flow signal representing the mass flow of the jet engine; a function generator which provides the surge line of the jet engine or a surge line limit curve extending at a safety margin therefrom in the form of a limit pressure ratio signal as a function of the mass flow signal; and signal limiting means between the control device and the fuel control unit, said means effecting limitation of the signal in accordance with the surge line distance obtained as difference of the limit pressure ratio signal and the pressure ratio signal, wherein the signal limiting means limits the time derivative of the fuel flow metered by the fuel control unit to a value substantially proportional to the surge line distance.

2. Apparatus as claimed in Claim 1, wherein the output signal from the control device is differentiated with a lag and is then applied to the fuel control unit integrated with a lead corresponding to this lag; wherein the transient response of the lag is selected equal to the transient response of the fuel control unit; and wherein the signal limiting means effects limitation of the control device output signal differentiated with lag.

3. Apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.