Acceleration control in multi spool gas turbine engine

A gas turbine engine system comprises a first compression stage; a second compression stage; a combustor; a controller; a first sensor for sensing the speed of the first compression stage and providing a first indication of the sensed speed to the controller; and a second sensor for sensing the speed of the second compression stage and providing a second indication of the sensed speed to the controller, wherein the controller is operable to control the supply of fuel to the combustor in dependence upon the first indication received from the first sensor and the second indication received from the second sensor. This arrangement is particularly useful in controlling the acceleration of an aero-engine from minimum idle.

Embodiments of the present invention relate to the controlled acceleration of a gas turbine multi-spool engine. In particular, they relate to the controlled acceleration of a multi-spooled gas turbine aero-engine from minimum idle.

An aero-engine must have rapid acceleration from low power to high power. In particular it must be able to accelerate from approach (high) idle and from minimum (low) idle within specified minimum times. Approach (high) idle is the minimum level of thrust used during the landing phase and minimum (low) idle is the minimum level of thrust used at all other flight phases. It is lower than approach (high) idle.

One trend in modern aircraft is towards lowering the minimum level of thrust at idle. Another trend is towards increasing the acceleration rate from idle. A consequence of this is that greater and greater acceleration demands are being placed upon aero-engines.

It is important not only to accelerate the engine quickly but to do so in a controlled manner. Over powering the high pressure compressor (HPC) of a multi-spool engine can cause over pressure and surge. During surge, the flow in the compressor becomes unstable and breaks down and the engine does not work.

FIG. 1illustrates a present-day closed loop speed derivative acceleration control system1suitable for controlling the acceleration of a multi-spool engine4from approach and/or minimum idle to a predetermined engine thrust within a predetermined time. The system1provides consistent acceleration times while avoiding surge.

The system1includes a controller2, a fuel supply controller6and an engine4. The controller2has a first input node10, a second input node11, a third input node12and an output node21. The first, second and third input nodes are connected to the engine4. The output node21is connected to the fuel supply controller6, which controls the fuel input22to the engine4. The closed loop controls the rate of change of the HPC shaft speed by modulating the fuel supplied to the engine's combustor using the fuel supply controller6.

The first input node10receives a value P2that represents the engine inlet stagnation pressure divided by the sea level reference pressure. The second input node11receives a value T2that represents the engine inlet stagnation temperature divided by the sea level reference temperature. The third input node12receives a value NH that represents the instantaneous speed of the high pressure compressor (HPC) shaft.

The controller2uses a predetermined schedule8, which schedules rate of change of HPC speed against engine power level and flight conditions. A corrected acceleration of the HPC shaft is scheduled against an instantaneous corrected speed of the HPC shaft. This schedule is designed so that a predetermined engine thrust can be achieved within a predetermined time from idle. The scheduled corrected acceleration is given by NHdotS/P2, where NHdotS is the scheduled rate of change of NH. The corrected speed of the HPC shaft is given by NH/√T2. The schedule may be defined as NHdotS/P2=f(NH/√T2).

The square root of T2is a turbo machinery correction that takes account of the speed of sound, which is proportional to the square root of temperature. P2is a measure of the amount of air going through the engine. As the altitude of the aircraft increases P2decreases and more work is required to produce the same amount of mass flow by the engines. As the aircraft speed increases the pressure increases and less work is required for the same mass flow.

In the controller2, T2is square rooted30and the HPC instantaneous speed NH is divided31by the square root of T2to create a corrected instantaneous HPC speed. The corrected instantaneous speed is input to the schedule8. The schedule outputs the scheduled corrected acceleration value NHdotS/P2. This is multiplied35by P2to produce the scheduled acceleration NHdotS. Meanwhile, the instantaneous speed of the HPC shaft NH is differentiated33with respect to time to find the actual acceleration NHdot of the HPC shaft. The actual acceleration NHdot is subtracted34from the scheduled acceleration NHdotS to produce an error signal NHdotE, which represents how much the acceleration of the engine is off-schedule. The error signal NHdotE is output from the output node21to the fuel controller6.

The schedule8is designed to maintain a surge margin between a surge line and the transient working line excursion for the HPC on acceleration from minimum idle while still achieving the predetermined engine thrust within the predetermined time. The surge margin takes into account that the surge line lowers as the engine ages.

It would be desirable to improve the control of the acceleration of a multi-spool aero-engine from idle.

According to one aspect of the present invention there is provided a gas turbine engine system comprising: a first compression stage; a second compression stage; a combustor; a controller; a first sensor for sensing the speed of the first compression stage and providing a first indication of the sensed speed to the controller; and a second sensor for sensing the speed of the second compression stage and providing a second indication of the sensed speed to the controller, wherein the controller is operable to control the supply of fuel to the combustor in dependence upon the first indication received from the first sensor and the second indication received from the second sensor. The controller may or may not be incorporated as part of the gas turbine engine.

According to another aspect of the present invention there is provided a multi-spool gas turbine engine comprising: a first spool; a second spool; a combustor; a controller; a first sensor for sensing the speed of the first spool and providing a first indication of the sensed speed to the controller; and

a second sensor for sensing the speed of the second spool and providing a second indication of the sensed speed to the controller, wherein the controller is operable to control the supply of fuel to the combustor in dependence upon the first indication received from the first sensor and the second indication received from the second sensor. The controller may or may not be incorporated as part of the gas turbine engine.

According to a further aspect of the invention there is provided a method of controlling the acceleration of an aero-engine from idle, comprising the steps of: a) sensing the speed of the first compression stage; b) sensing the speed of the second compression stage; and c) controlling the supply of fuel in dependence upon steps a) and b).

According to a still further aspect of the present invention there is provided an acceleration controller for a gas turbine engine comprising: a first input for receiving an indication of the speed of a first compression stage; a second input for receiving an indication of the speed of a second compression stage; and processing means operable to control the supply of fuel to the engine in dependence upon the indications received at the first and second inputs.

The inventor has realised that at low speeds the HPC does most of the compression work, and cannot sustain a rapid acceleration rate without a large HPC working line excursion. The HPC working line excursion arises at low speeds, because at low engine speeds the HPC does most of the compression work and more fuel is required to hit a scheduled acceleration target. The HPC is therefore hard to accelerate. A corollary is that less compression work is done by other spools and they are easier to accelerate. As the speed of the engine increases, the other spools do more of the compression work. The inventor has realised the importance of taking into account the different and varying behaviour of the multiple spools during acceleration from idle.

The inventor has realised the importance of taking into account the contribution the spool(s) other than the HPC spool make to the acceleration of the engine from idle. The kinetic energy of the other spool(s) may be taken into account by using a composite speed parameter as the speed in a closed loop speed derivative acceleration control system. Thus the working line excursion of the HPC is substantially reduced on acceleration from minimum idle and therefore less surge margin has been designed into the engine. As a consequence, the engine can be run at higher pressure and with greater efficiency.

TheFIGS. 2 and 3illustrate a gas turbine engine system comprising: a first compression stage (HPC54); a second compression stage (IPC53); a combustor (55); a controller (102); a first sensor (82) for sensing the speed of the first compression stage and providing a first indication (NH) of the sensed speed to the controller (102); and a second sensor (84) for sensing the speed of the second compression stage and providing a second indication (NI) of the sensed speed to the controller (102), wherein the controller (102) is operable to control the supply of fuel to the combustor in dependence upon the first indication (NH) received from the first sensor (82) and the second indication (NI) received from the second sensor (84).

The kinetic energy of a multi-spool engine may be approximated to the sum of the kinetic energies of the high pressure compressor (HPC) spool and the intermediate compressor (IPC) spool.
Kinetic Energy (KE)=0.5*JI*ωI2+0.5*JH*ωH2=0.5*JI*(2π/60*NI100*NI/100)2+0.5*JH*(2π/60*NH100*NH/100)2(1)
where

JHis the moment of inertia of the HPC spool,

NH100is the maximum speed of the HPC spool (rpm),

NH is the instantaneous (percentage) HPC spool speed, expressed as a percentage of NH100

JIis the moment of inertia of the IPC spool,

NI100is the maximum attainable speed of the IPC spool (rpm),

NI is the instantaneous (percentage) IPC spool speed, expressed as a percentage of NI100

A composite speed parameter, NMIX, is defined such that:
KE=0.5*JH*(2π/60*NH100*NMIX)2+0.5*JI*(2π/60*NI100*NMIX)2(2)
equating (1) with (2) gives
NMIX=SQRT((K*NI2+NH2)/(K+1))
where the inertia weighting constant K is such that:
K=(JI/JH)*(NI100/NH100)2

For a typical large modern turbo fan
K=(112/396)*(8300/12200)2=1120/398≈1.3114

The derivative of the composite speed NMIX, the composite-speed derivative NMIXdot, can be used to control the engine using a closed-loop composite-speed derivative acceleration control system101. One such system is schematically illustrated inFIG. 2.

FIG. 2illustrates a closed loop composite-speed derivative acceleration control system101suitable for controlling the acceleration of a multi-spool engine104from approach and/or minimum idle to a predetermined speed within a predetermined time. The system101provides a consistent acceleration times while avoiding surge.

The system101includes a controller102, a fuel supply controller106and an engine104. The controller102has a first input node110, a second input node111, a third input node112, a fourth input node113, a fifth input node114and an output node121. The first, second, third and fourth input nodes are connected to the engine104. The output node121is connected to the fuel supply controller106, which controls the fuel input122to the engine104. The closed loop controls the rate of change of a composite speed parameter NMIX by modulating the fuel supplied to the engine's combustor using the fuel supply controller6.

The first input node110receives a value P2that represents the engine inlet stagnation pressure divided by the sea level reference pressure. The second input node111receives a value T2that represents the engine inlet stagnation temperature divided by the sea level reference temperature. The third input node112receives a value NH that represents the instantaneous speed of the high pressure compressor (HPC) shaft. The fourth input node113receives a value NI that represents the instantaneous speed of the intermediate pressure compressor (IPC) shaft. The fifth input node receives a value of the inertia weighting constant K or the values for calculating K.

The controller102uses a predetermined schedule108, which schedules the rate of change of the composite speed parameter against engine power level and flight conditions. A corrected rate of change of the composite speed parameter (corrected composite acceleration) is scheduled against a corrected instantaneous composite speed parameter. This schedule is designed so that a predetermined engine thrust can be achieved within a predetermined time from idle. The corrected composite acceleration is given by NMIXdotS/P2, where NMIXdotS is the scheduled rate of change of the composite sped parameter NMIX. The corrected composite speed parameter is given by NMIX/√T2. The schedule may be defined as NMIXdotS/P2=f(NMIX/√T2).

In the controller102, T2is square rooted130. The instantaneous composite speed parameter NMIX is divided131by the square root of T2to create a corrected instantaneous composite speed parameter. The corrected instantaneous speed parameter is input to the schedule108. The schedule108outputs the scheduled corrected acceleration value NHdotS/P2. This is multiplied132by P2to produce the scheduled acceleration NMIXdotS. Meanwhile, the instantaneous composite speed parameter NMIX is differentiated133with respect to time to find the actual composite acceleration NMIXdot. The actual composite acceleration NMIXdot is subtracted134from the scheduled composite acceleration NMIXdotS to produce an error signal NMIXdotE, which represents how much the acceleration of the engine is off-schedule. The error signal NMIXdotE is output from the output node121to the fuel controller106which directly controls the amount of fuel supplied to the engine104and consequently the rate of change of the composite speed NMIX. Thus a closed loop control system is formed using composite-speed derivative control that gives consistent acceleration times.

The controller102creates the composite speed parameter NMIX from the values provided to its third, fourth and fifth input nodes as follows. A first parameter (NH2) is formed by squaring140the HPC instantaneous speed NH received at the third input node112. A second parameter (NI2) is formed by squaring141the IPC instantaneous speed NI received at the fourth input node113. The second parameter is multiplied142by the inertia weighting constant K and the product is added143to the first parameter. The resultant sum is divided144by an inertia weighting denominator (K+1) created by adding145one to the inertia weighting constant K. The result of the division is square rooted146to produce the composite speed parameter NMIX.

The controller102may be, for example, a programmed microprocessor or a micro-controller.

FIG. 3illustrates a sectional side view of the upper half of a multi-spool aero-engine104that incorporates a closed loop composite-speed derivative acceleration control system101as described inFIG. 2. The aero-engine104comprises, in axial flow series, an air intake51, a propulsive fan52, an intermediate pressure compressor (IPC)53, a high pressure compressor (HPC)54, a combustor55, a turbine arrangement comprising a high pressure turbine56, an intermediate pressure turbine57and a low pressure turbine58and an exhaust nozzle59. The aero-engine104further comprises interconnecting shafts60.

The aero-engine104operates in a conventional manner so that air entering into the air intake51is accelerated by the propulsive fan52which produces two air flows: a first air flow into the intermediate pressure compressor53and a second air flow which provides propulsive thrust. The intermediate pressure compressor53compresses air flow directed into it for delivering that air to the high pressure compressor54where further compression takes place. The compressed air exhausted from the high pressure compressor54is directed into the combustor55where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand and thereby drive the high, intermediate and low pressure turbines56,57,58before being exhausted through the nozzle59to provide additional propulsive thrust. The high, intermediate and low pressure turbines56,57,58respectively drive the high and intermediate pressure compressors54,53and the propulsive fan52by suitable interconnecting shafts60. The high pressure turbine56, the high pressure compressor54and their interconnecting shaft form a first spool. The intermediate pressure turbine57, the intermediate pressure compressor53and their interconnecting shaft form a second spool. The low pressure turbine58, the propulsive fan52and their interconnecting shaft form a third spool.

The aero-engine110additionally comprises a probe80located in the air intake510, a first speed sensor82coupled to the first spool and a second speed sensor84coupled to the second spool. In this embodiment the entirety of the acceleration control system101is located at the engine and the engine104also comprises the controller102and the fuel supply controller106. The acceleration control system101is capable of consistently controlling the acceleration of the multi-spool engine104from approach and/or minimum idle to a predetermined speed within a predetermined time as described with relation toFIG. 2.

The probe80measures the inlet stagnation pressure P2and inlet stagnation temperature T2and provides them respectively as inputs to the first input node110and second input node112of the controller102. The first speed sensor82measures the speed NH of the first, high pressure compressor (HPC), spool and provides this as an input to the third input node112of the controller102. The second speed sensor84measures the speed NI of the second, intermediate pressure compressor (IPC), spool and provides this as an input to the fourth input node113of the controller102. The output node121of the controller102provides a control input signal to the fuel supply controller106, which modulates the amount of fuel provided to combustor55.

The value of the inertia weighting constant K is provided to the fifth input node114of the controller102. This value is programmable.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, although the above described implementation takes account of two spools in the calculation of the composite speed parameter, in other implementations more than two spools may be taken into account.