GB2531021A

GB2531021A - System for performing staging control of a multi-stage combustor

Info

Publication number: GB2531021A
Application number: GB1417726.5A
Authority: GB
Inventors: Qin Yong
Original assignee: Rolls Royce Controls and Data Services Ltd
Current assignee: Rolls Royce Controls and Data Services Ltd
Priority date: 2014-10-07
Filing date: 2014-10-07
Publication date: 2016-04-13
Also published as: GB201417726D0

Abstract

A control system for performing staging control of a multi-stage combustor (15, figure 2) of a gas turbine engine (10). Fuel is fed to the combustor by a fuel supply system comprising: a fuel metering valve 34 having a position adjustable to control fuel flow rate to the combustor, a pressure drop control arrangement 36 regulating fuel pressure drop across the fuel metering valve, and a staging unit 44 which splits the fuel flow from the valve between the combustor stages. The control system includes a fuel pressure sensor 50 which senses the fuel pressure drop across the valve. The control system further includes a controller 52 which implements a control loop comparing a measurement signal indicating the position of the valve with a demanded fuel flow rate and issues a first command signal which adjusts the position of the fuel metering valve to achieve the demanded fuel flow rate. The controller further issues a second command signal to the staging unit to implement the calculated fuel split.

Description

I

SYSTEM FOR PERFORMING STAGING CONTROL OF A MULTI-STAGE COMBUSTOR

The present invention relates to a system for performing staging control of a multi-stage combustor of a gas turbine engine.

An engine fuel supply system is shown schematically in Figure 1. The fuel supply system has a pumping unit comprising a low pressure (LP) pumping stage which draws fuel from a fuel tan.k of the aircraft and supplies the fuel at boosted pressure via a fuel/oil heat exchanger (FOHE) and an LP filter 30 to the inlet of a high pressure (HP) pumping stage 32.

The LP stage typically comprises a centrifugal impeller pump while the HP pumping stage may comprise one or more positive displacement pumps, e.g. in the form of twin pinion gear pumps.

A hydro-mechanical unit (HMU) accepts fuel from the HP pumping stage 32 and feeds itto the combustor of the engine. The HMU comprises a fuel metering valve (FMV) 34 operable to control the rate at which fuel is allowed to flow to the combustor via a fuel manifold. The HMU further typically comprises: a pressure drop control arrangement (such as a spill valve and a pressure drop control valve 36) which is operable to maintain a substantially constant pressure drop across the FMV, a pressure relief valve 38, and a pressure raising and shut-off valve 40 at the fuel exit of the HMU whir..h ensures that a predetermined minimum pressure level is maintained upstream thereof for correct operation of any fuel pressure operated auxiliary devices (such variable inlet guide vane or variable stator vane actuators) that receive fuel under pressure from the HMU. Further details of such an HMU are described in EP 2339147 A, which is hereby incorporated by reference.

The engine electronic controller (EEC) 42 commands the HMU FMV to supply fuel at a given flow rate. In particular, to achieve a given thrust setting, a thrust control loop (major loop) in the EEC generates a fuel flow rate demand. An FMV control loop (minor loop) in the EEC compares the fuel flow rate demand from the major loop with an FMV position feedback, which is normally measured through an LVDT 46, to generate a command signal to a metering valve control valve (MVCV) 48. The MVCV then varies a servo flow to actuate the FMV until the LVDT measurement matches the fuel flow rate demand. This process relies on a constant pressure drop across the FMV, which is achieved through the operation of the spill valve and pressure drop control valve 36.

A fault in the system can compromise the operability of the spill valve and pressure drop control valve 36, which in turn can lose the capability to maintain a constant pressure drop across FMV 34. VThen this happens, the fuel flow rate indicated by the position measurement of the LVDT 16 may be significantly different from the actual fuel flow rate delivered to the combustor. which drives the actual engine thrust away from the specified thrust demand. The major loop picks up this thrust difference and re-adjusts the fuel flow rate demand in order to further actuate the FMV until the thrust demand is met. in this way, the fuel flow demand rate and the LVDT can he matched even when system faults occur.

Reduction of harmful emissions (CO and NOX) from aircraft engines may be achieved by the use of staging valves to control the delivery of fuel to the engine, in such arrangements, the staging valves can operate to determine whether fuel is delivered to the engine just through pilot nozzles or whether it is delivered through the pilot nozzles and mains nozzles.

A staging unit containing the staging valves can he under the control of the engine electronic controller (EEC), which also has a task of controlling the total fuel flow to the engine. A problem may then arise, however, in that fuel flow rate from the HMV may be one of the parameters used to generate the demand to actuate the staging unit. As it can be difficult to measure fuel flow rates directly, particularly during fast transients, an option is to use the EEC's fuel flow demand or the LVDT position measurement to indicate the actual fuel flow rate delivered to the combustor. But as explained above, mitigation of fuel system faults through thrust control oop fuel flow demand re-adjustment can result in either a fuel flow rate demand or an LVDT measurement that does not give a correct indication of the actual fuel flow rate delivered to the combustor. Use of such degraded fuel flow rate signals can induce an incorrect demand to split the fuel between pilot and mains injectors, with risks of causing engine flame out, surge, visible smoke generation., or even loss of thrust control.

Accordingly, in a first aspect the present invention provides a control system for performing staging control of a multi--stage combustor of a gas turbine engine, fuel being fed to the combustor by a fuel supply system comprising: a fuel metering valve having a position which.

is adjustable to control the rate at which fuel passes to the comnbustor, a pressure drop control arranqement which regulates a fuel pressure drop across the fuel metering valve, and a staging unit which splits the fuel flow from the fuel metering valve between the combustor stages, wherein the control system includes: a fuel pressure sensor which senses the fuel pressure drop across the fuel metering valve, and a controller which implements a control loop which compares a measurem ent signal indicating the position of the fuel metering valve with a demanded fuel flow rate, and on the basis of the comparison issues a first command signal which adjusts the position of the fuel metering valve to achieve the demanded fuel flow rate; wherein the controller further repeatedly: calculates a fuel split based on (a) the demanded fuel flow rate and/or the measurement signal and (b) the fuel pressure drop sensed by the fuel pressure sensor, and issues a second command signal to the staging unit to implement the calculated fuel split.

Advantageously, by sensing the fuel pressure drop across the fuel metering valve, the controller can compensate for faults n the fuel system when calculating the fuel split for the staging unit. in particular, even if a fault in the pressure drop control arrangement results in incorrect regulation of the fuel pressure drop across the fuel metering valve, and the controller reacts by re-adjusting the fuel flow rate demand until the thrust demand is met, the sensed pressure drop can be used to ensure that the correct fuel split is set.

In a second aspect, the present invention provides a luel supply system for feeding fuel to a multi-stage car.bustor of a gas turbine engine, the fuel supply system including: a fuel metering valve having a position which is adjustable to control the rate at which fuel passes to the combustor, a pressure drop control arrangement which regulates a fuel pressure drop across the fuel metering valve, a staging unit which splits the fuel flow from the fuel metering valve between the combustor stages, and a control system according to the first aspect for performing staging control of the combustor.

In a third aspect, the present invention provides the controller of the control system of the first aspect.

In a fourth aspect, the present invention provides a gas turbine engine (e.g. an aircraft engine) having the fuel supply system of the second aspect.

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

Generally, the controller calculates the demanded fuel flow rate to achieve a desired engine thrust level.

The controller may be an engine electronic controller.

The fuel supply system may further comprise a displacement transducer which measures the position of the fuel metering valve and supplies the measurement signal to the controller.

The fuel supply system may further comprise a control-servo valve which receives the first command signal and is operable to adjust the position of the fuel metering valve.

The staging unit may comprise an arrangement of valves which splits the fuel flow between fuel manifolds corresponding to respective combustor stages.

The fuel supply system may include a pressure raising valve (such as a pressure raising and shut off valve) downstream of the fuel metering valve, the pressure raising valve ensuring that the fuel pressure upstream thereof is maintained above a predetermined minimum level.

The pressure drop control arrangement may maintain in normal operation a substantially constant fuel pressure drop across the metering valve. The pressure drop control arrangement may comprise a spill valve and a pressure drop control valve. The spill valve and pressure drop control valve may be separate components or may be combined in a single valve.

The controller may further compare the fuel pressure drop sensed by the fuel pressure sensor with a reference fuel pressure drop (which is typically a constant fuel pressure drop maintained by the pressure drop control arrangement), and issue a system fault alert if the fuel pressure drop comparison indicates that a difference between the fuel pressure drop sensed by the fuel pressure sensor and the reference fuel pressure drop is greater than a predetermined threshold. Early maintenance can then be scheduled to fix the fuel system before the fault significantly disrupts the engine operation.

The controller may condition the fuel pressure drop sensed by the fuel pressure sensor to remove fuel pump pressure ripples. Such ripples tend to be a characteristic of positive displacement pumps of the type commonly used in the HP pumping stage of engine fuel supply systems. By removing the ripples, the controller can ensure that they do not affect the fuel split calculation.

Further optional features are discussed below.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically an engine fuel supply system; Figure 2 shows a longitudinal cross-section through a ducted fan gas turbine engine; and Figure 3 shows schematically an engine fuel supply system according to the present invention.

With reference to Figure 2, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

The combustion equipment 15 includes a multi-stage combustor. Figure 3 shows schematically a system according to the present invention for supplying fuel to the combustor and for performing staging control thereof. Elements of the fuel supply system of Figure 3 which are same as those of Figure 1 have the same reference numbers as used in Figure 1.

The pumping stages and many elements of the HMU are the same or similar as between the fuel supply systems of Figures 1 and 3, and accordingly these elements have the same reference numbers in Figures i and 3.

However, in the system of Figure 3. the EEC 52 has an enhanced functionality, as explained more fully below. Further, on leaving the HMU. the metered fuel arrives at a staging unit 44 (typically an arrangement of vaNies), which splits the fuel under the control of the EEC into two flows: one for the pilot manifold and the other for one or more mains manifolds. The pilot manifold feeds pilot nozzles of a number of fuel injectors of the combustor. The mains manifold(s) feeds mains nozzles of the fuel injectors. By varying the fuel split between the manifolds, the EEC can thus perform staging control of the engine. Also, a differential pressure sensor 50 senses the fuel pressure drop across the FMV 34 and sends the sensed pressure drop to the EEC. The sensor may be digital or analogue. The sensor may measure the fuel pressure drop directly, or t may measure the fuel pressure drop ndirectly e.g. by measuring the absolute pressures at both sides of the FMV 34 and deriving Ihe pressure drop from the difference between the two absolute pressures.

The EEC 52 implements a thrust control oop (major loop) which generates a fuel flow rate demand corresponding to a given thrust setting, and an FMV control loop (minor loop) which compares (e.g. via a look up table, calibration factors, graphs, algorithms or other approaches known to the skilled person) the fuel flow rate demand from the major loop with an FMV position feedback measured through the LVDT 46 and generates a command signal to the MVCV 48. As discussed above, the major loop re-adjusts the fuel flow rate demand in order to further actuate the FMV 34 until the thrust demand is met.

In additon, the EEC 52 reads the LVDT measurement signal and feeds it to a look-up table which converts the measurement signal to a fuel flow rate. However, as well as accepting the measurement signal as an input, the look-up table also accepts the sensed pressure drop across the FMV 34. The sensed pressure drop is used to calibrate the look-up table so that even if a fault in the fuel system results in a change to what should be the constant pressure drop across the FMV, the correct fuel flow rate can be calculated from the measurement signal. In sonic instances, it may not be necessary to use a look-up table, and instead the corrected fuel flow rate can be calculated from the LVDT measurement signal and sensed pressure drop using suitable calibration factors. Other suitable approaches for calculating the corrected fuel flow rate from the LVDT measurement signal and sensed pressure drop, such as the use of graphs or algorithms, are known to the skiUed person.

The EEC then calculates a fuel spht based on the correct fuel flow rate and sends a corresponding command signal to the staging unit 44. In this way, even if the functionaUty of the spEll valve and pressure drop control valve 3$ is compromised and the EECs thrust control oop compensates by re-adjusting the fuel flow rate demand, a correct fuel split can be implemented, thereby reducing or avoiding the risks associated with incorrect fuel splits.

As an alternative to the LVDT measurement signal, the EEC 52 can calculate a fuel split from the demanded fuel flow rate and the sensed pressure drop e g. using a look-up table, calibration factors, graphs, algorithms etc. Indeed, the two approaches can be combined such that the calculated fuel split is based on all of the LVDT measurement signal, the demanded fuel flow rate and the sensed pressure drop.

To ensure that pressure ripples caused by a displacement pump of the HP pumping stage 32 do not affect the fuel spht calculatEon, the EEC 52 can appropriately filter the sensed pressure drop before using it to calibrate the look-up table.

In addition, the sensed pressure drop can be used by the EEC 52 to indicate faults in the fuel supply system. In particular, the EEC can compare the sensed pressure drop with the value for the constant pressure drop which should be maintained by the spill valve and pressure drop control valve 36. If the comparison indicates that a difference between the pressure drops is greater than a predetermined threshold, the EEC can issue a system fault alert to prompt maintenance of the system.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disdosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limitEng. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

C LA MS1 A control system for performing staging control of a multi-stage combustor of a gas turbine engine, fu& being fed to the combustor by a fuel supply system comprising: a fuel metering valve (34) having a position which is adjustable to control th.e rate at which fuel passes to the combustor, a pressure drop control arrangement (36) which regulates a fuel pressure drop across the fuel metering valve, and a staging unit (44) which splits the fuel flow from the fuel metering valve between the combustor stages, wherein the control system includes: a fuel pressure sensor (50) which senses the fuel pressure drop across the fuel metering valve, and a controHer (52) which implements a control loop which compares a measurement signal indicating the position of the fuel metering valve with a demanded fuel flow rate, and on the basis of the comparison issues a first command signal which adjusts the position of the fuel metering valve to achieve the demanded fuel flow rate; wherein the controller further repeatedly: calculates a fuel split based on (a) the demanded fuel flow rate and/or the measurement signal and (b) the fuel pressure drop sensed by the fuel pressure sensor, and issues a second command signal to the staging unit to implement the calculated fuel split.
2. A control system according to claim 1, wherein the controller further: compares the fuel pressure drop sensed by the fuel pressure sensor with a reference fuel pressure drop, and issues a system fault alert if the fuel pressure drop comparison indicates that a difference bebveen the fuel pressure drop sensed by the fuel pressure sensor and the reference fuel pressure drop is greater than a predetermined threshold.
3. A control system according to claim I or 2, wherein the controller conditions the fuel pressure drop sensed by the fuel pressure sensor to remove fuel pump pressure ripples.
4. A fuel supply system for feeding fuel to a multi-stage combustor of a gas turbine engine, the fuel supply system including: a fuel metering valve having a position which is adjustable to control the rate at which fuel passes b the combustor, a pressure drop control arrangement which regulates a fuel pressure drop across the hiS metering valve, a staging unit which splits the fuel flow from the fuel metering valve between the combustor stages, and a control system according to any one of the previous claims for performing staging control of the combustor.
5. The controller of the control system of any of claims I to 3.
6. A gas turbine engine having the fuel stçply system of claim 4.