Patent Publication Number: US-2020300180-A1

Title: Variable transmission driven fuel pump for a gas turbine engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/821,032 which was filed on Mar. 20, 2019. 
    
    
     BACKGROUND 
     A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines. 
     Fuel supplied to the combustor is provided by a mechanical pump driven by a rotating shaft of the engine. The mechanical pump is reliable and supplies fuel in proportion to engine speed. The minimum capacity of the mechanical pump is sized such that sufficient fuel is provided for high power conditions and/or engine starting. Excess fuel not needed is recirculated within the fuel system or back to the fuel tank. The fuel is further utilized as a coolant for other systems of the engine. Recirculation of fuel increases the temperature of the fuel and thereby reduces the available capacity to absorb heat from other systems. The capacity of the fuel to absorb heat from other systems is further limited by the characteristics of the fuel. At a certain temperature, the fuel begins to degrade and create deposits in the fuel system that can degrade engine performance. Reducing the amount of fuel that is recirculated during engine operation may improve the capacity of the fuel to absorb heat from other systems. 
     Turbine engine manufacturers continuously seek improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies. 
     SUMMARY 
     A fuel system for a gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, a fuel pump to provide fuel flow during engine operation, and a transmission system that includes an output shaft coupled to drive the fuel pump, and an input shaft driven through a mechanical link to a shaft of the gas turbine engine. The output shaft drives the fuel pump at a variable speed that is independent of a rotational speed of the input shaft. 
     In a further embodiment of the foregoing fuel system for a gas turbine engine, the transmission system comprises a hydraulic drive driven by the mechanical link. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the output shaft is coupled to the hydraulic drive. A hydraulic control valve controls the hydraulic fluid flow to the hydraulic drive for controlling a speed of the output shaft. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the hydraulic drive comprises a constant speed drive. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the transmission comprises a continuously variable transmission with first shaft coupled to the input shaft. A second shaft is coupled to the output shaft and a flexible link coupling the first shaft to the second shaft. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the first shaft includes a primary pulley and the second shaft includes a secondary pulley with the flexible link disposed between the primary pulley and the secondary pulley. Each of the primary pulley and the secondary pulley include halves split relative to the axis of rotation and the flexible link comprises a V-shape in cross-section. A distance between the halves of each of the primary pulley and the secondary pulley is variable to change a drive ratio between input shaft and the output shaft. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the variable speed of the output shaft is controlled responsive to a fuel flow demand of the gas turbine engine. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, the transmission system comprises a variable drive that is controlled to provide a speed of the output shaft independent of the speed of the input shaft. 
     In another embodiment of any of the foregoing fuel systems for a gas turbine engine, a heat exchanger is included for transferring heat into a fuel flow generated by the fuel pump. 
     A gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, a fan rotatable within a fan nacelle and a core engine that includes a compressor communicating compressed air to a combustor where compressed air is mixed with fuel and ignited to generate a high-energy gas flow expanded through a turbine. A fuel system includes a fuel pump to provide fuel flow during engine operation and a transmission system that includes an output shaft coupled to drive the fuel pump and an input shaft driven through a mechanical link to a shaft of the gas turbine engine. The output shaft drives the fuel pump at a variable speed that is independent of a rotational speed of the input shaft. 
     In a further embodiment of the foregoing gas turbine engine, the transmission system comprises a hydraulic drive driven by the mechanical link. 
     In another embodiment of any of the foregoing gas turbine engines, the output shaft is coupled to the hydraulic drive and a hydraulic control valve controls the hydraulic fluid flow to the hydraulic drive for controlling a speed of the output shaft. 
     In another embodiment of any of the foregoing gas turbine engines, the hydraulic drive comprises a constant speed drive. 
     In another embodiment of any of the foregoing gas turbine engines, the transmission comprises a continuously variable transmission with a first shaft coupled to the input shaft. A second shaft is coupled to the output shaft and a flexible link coupling the first shaft to the second shaft. 
     In another embodiment of any of the foregoing gas turbine engines, the first shaft includes a primary pulley and the second shaft includes a secondary pulley with the flexible link disposed between the primary pulley and the secondary pulley. Each of the primary pulley and the secondary pulley include halves split relative to the axis of rotation and the flexible link comprises a V-shape in cross-section. A distance between the halves of each of the primary pulley and the secondary pulley is variable to change a drive ratio between input shaft and the output shaft. 
     In another embodiment of any of the foregoing gas turbine engines, the transmission system comprises a variable drive that is controlled to provide a speed of the output shaft independent of the speed of the input shaft. The variable speed of the output shaft is controlled responsive to a fuel flow demand of the gas turbine engine. 
     In another embodiment of any of the foregoing gas turbine engines, a heat exchanger is included for transferring heat into a fuel flow generated by the fuel pump. 
     A method of supplying fuel to a combustor of a gas turbine engine according to an exemplary embodiment of this disclosure includes, among other possible things, driving an input shaft of a transmission system with a mechanical link to a shaft of the gas turbine engine at a varying input speed. A fuel pump is driven with an output shaft of the transmission system at a varying output speed that is independent of the varying input speed. The varying output speed drives the fuel pump at a speed that provides fuel flow corresponding with an operating condition of the engine determined to minimize excess fuel flow. 
     In a further embodiment of the foregoing method of supplying fuel to a combustor of a gas turbine engine, the transmission system comprises a hydraulic drive powered by a hydraulic fluid flow from a hydraulic pump. The hydraulic pump is driven by the mechanical link. The output shaft is coupled to the hydraulic drive and a hydraulic control valve controls the hydraulic fluid flow to the hydraulic drive for controlling the varying output speed. 
     In another embodiment of any of the foregoing methods of supplying fuel to a combustor of a gas turbine engine, the transmission comprises a continuously variable transmission with first shaft coupled to the input shaft. A second shaft is coupled to the output shaft and a flexible link coupling the first shaft to the second shaft. The first shaft includes a primary pulley and the second shaft includes a secondary pulley with the flexible link disposed between the primary pulley and the secondary pulley. Each of the primary pulley and the secondary pulley include halves split relative to the axis of rotation and the flexible link comprises a V-shape in cross-section, and a distance between the halves of each of the primary pulley and the secondary pulley is variable to change a drive ratio between input shaft and the output shaft. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example gas turbine engine. 
         FIG. 2  is a schematic view of an example fuel system embodiment. 
         FIG. 3  is a schematic view of another example fuel system embodiment. 
         FIG. 4  is a schematic view of an example drive system embodiment in a first operating condition. 
         FIG. 5  is a schematic view of the example drive system embodiment in a second operating condition. 
         FIG. 6  is a schematic view another example fuel system embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a nacelle  18 , and also drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that the various bearing systems  38  may alternatively or additionally be provided at different locations, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects, a first (or low) pressure compressor  44  and a first (or low) pressure turbine  46 . The inner shaft  40  is connected to a fan section  22  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive fan blades  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a second (or high) pressure compressor  52  and a second (or high) pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  58  of the engine static structure  36  may be arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  58  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  58  includes airfoils  60  which are in the core airflow path C. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of the low pressure compressor  44  and the fan blades  42  may be positioned forward or aft of the location of the geared architecture  48  or even aft of turbine section  28 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and less than about 5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFCT’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second). 
     The example gas turbine engine includes the fan section  22  that comprises in one non-limiting embodiment less than about 26 fan blades  42 . In another non-limiting embodiment, the fan section  22  includes less than about 20 fan blades  42 . Moreover, in one disclosed embodiment the low pressure turbine  46  includes no more than about 6 turbine rotors schematically indicated at  34 . In another non-limiting example embodiment, the low pressure turbine  46  includes about 3 turbine rotors. A ratio between the number of fan blades  42  and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine  46  provides the driving power to rotate the fan section  22  and therefore the relationship between the number of turbine rotors  34  in the low pressure turbine  46  and the number of blades  42  in the fan section  22  disclose an example gas turbine engine  20  with increased power transfer efficiency. 
     Fuel is delivered to the combustor  56  by a fuel system  62 . The example fuel system  62  includes a main fuel pump  66  and a boost fuel pump  68 . The boost pump  68  pumps fuel from a fuel tank  64  and communicates the fuel to the main pump  66 . The main pump  66  increases a pressure of the fuel flow to a pressure suitable for communication to the combustor  56 . The disclosed fuel system  62  tailors a flow of fuel  75  to the combustor  56  based on engine operating conditions. Instead of simply providing a fuel flow that provides for extremes of operating demands, the disclosed fuel system  62  varies the flow of fuel  75  according to a demand for fuel. By tailoring the flow of fuel through the main fuel pump  66  to engine operating demand, excess fuel flow through a fuel recirculation loop  72  can be reduced and/or eliminated. 
     Fuel is utilized as a heat sink to cool other flows within the engine such as lubricant and air flows. In this example, a heat fuel/oil heat exchanger  74  cools a flow of lubricant generated by a lubricant system  76 . Recirculation of fuel results in an increased temperature of the fuel and thereby a reduced capability to accept heat from other engine systems, such as the example lubricant system  76 . 
     The disclosed fuel system  62  includes utilizes a transmission system  82  that receives a varying input through a shaft  88  or other mechanical link one of the inner and outer shafts  40 ,  50  of the engine  20 . In one disclosed example, an accessory gearbox  78  is driven by a tower shaft  80  coupled to the outer shaft  50 . The accessory gearbox  78  drives the shaft  88  that drives the transmission system  82 . The transmission system  82  drives an output shaft  86  coupled to drive the main pump  66 . The transmission system  82  produces a variable speed output through the shaft  86  to tailor operation of the main pump  66 . The input shaft  88  varies with a speed of the outer shaft  50 . The transmission system  82  provides an output speed through the output shaft  86  that varies independent of the input speed of the input shaft  88 . Accordingly, regardless of the input speed provided by the input shaft  88 , the output speed  86  is tailored to operate the main fuel pump  66  to provide the fuel flow  75  to the combustor  56  to more closely match a demand for fuel such that excess flow through a recirculation passage  72  is reduced and/or eliminated. Varying the flow of fuel  75  based on demand to reduce and/or eliminate the recirculation of fuel enables an increase the ability of the fuel to accept heat from other engine systems. 
     Referring to  FIG. 2 , with continued reference to  FIG. 1 , the transmission system  82  in one disclosed example embodiment comprises a hydraulic drive  90  driven by the input shaft  88  to provide an output through a gear system  95 . The accessory gearbox  78  drives the hydraulic drive  90  through the input shaft  88 . The hydraulic drive  90  utilizes hydraulic fluid to drive an output shaft  85  that drives a gear system  94 . Control of fluid flow through the hydraulic drive  90  controls a speed of the output shaft  85  relative to the speed of the input shaft  88 . A hydraulic fluid reservoir  92  provides hydraulic fluid to the hydraulic drive  94 . The disclosed hydraulic drive  90  is a pump that may be a dedicated system for driving the main fuel pump  66  or may be part of a hydraulic fluid system utilized to power other devices and actuators of the engine and aircraft. 
     The example hydraulic drive  90  is supplied hydraulic fluid from the reservoir  92  during operation. The relationship between the speed of the input shaft  88  and the speed of the output shaft  85  is varied by control of the flow of hydraulic fluid through the drive  90 . In one disclosed example, a hydraulic control valve  96  is provided that governs the flow of hydraulic fluid to the drive  90 . Depending on the desired speed of the output shaft  85 , the control valve  96  will communicate fluid to the drive  90 . An increase in hydraulic fluid flow will provide an increase of the speed of the output shaft  85  relative to the speed of the input shaft  88 . A decrease in hydraulic fluid flow provides a decrease in the speed of the output shaft  85  relative to the speed of the input shaft  88 . Excess hydraulic fluid flow is routed back to the reservoir  92  through a bypass passage  97 . 
     The accessory gearbox  78  may provide a varying input speed through the input shaft  88 . The hydraulic drive  90  provides an output that also varies based on the needs of the fuel pump  66 . The disclosed hydraulic drive  90  is driven at a variable speed through the input shaft  88 . In the disclosed example, the control valve  96  is controlled by a controller  84  to control hydraulic fluid flow to the drive  90  to control the output speed of the output shaft  85 . The output shaft  85  drives the gear system  85  that in turn drive the shaft  86  that drives the fuel pump  66 . The example control valve  96  is a proportional valve that proportionally controls fluid flow to the drive  90  as required to provide the desired output speed of the output shaft  85 . The speed of the output shaft  85  therefore may be controlled independent of the input speed of the input shaft  88 . 
     It should be appreciated that although the example control valve  96  is controlled by a controller  84 , the control valve  96  may be a mechanically actuated valve that proportions fluid flow to the drive  90  to provide a fixed output speed through the output shaft  85 . When a mechanical control valve is utilized, excess flow is directed through the bypass passage  97  based on the input speed of the input shaft  88  to provide the desired output speed. 
     The disclosed hydraulic drive  90  may be part of an integrated drive system provided in a common housing, schematically indicated at  95 . The hydraulic drive  90  may comprise a constant speed drive capable of providing an output speed that is fixed or that varies independent of a speed of an input shaft. It should be appreciated, that although a specific hydraulically driven transmission is disclosed by way of example, other hydraulic systems capable of providing a tailored output speed to drive the main fuel pump  66  are within the contemplation and scope of this disclosure. 
     Referring to  FIG. 3  with continued reference to  FIG. 1 , the example fuel system  62  is driven by a continuously variable transmission  100  with a first shaft  102  coupled to the input shaft  88  and a second shaft  14  coupled to drive the output shaft  86  and a flexible link  110  coupling the first shaft  102  to the second shaft  14 . The example variable transmission  100  is continuously variable transmission that uses split pulleys on each of the first and second shafts to adjust a speed ratio between the first and second shafts  102 ,  104 . 
     The first shaft  106  includes a first pulley  106  and the second shaft  104  includes a second pulley  18 . The flexible link  110  is a V-shaped belt disposed between the first and second pulleys  106 ,  108 . The first and second pulleys  106 ,  108  include radially ramped surfaces that expand and contract depending on a rotational speed. The V-shaped belt  110  transmits rotational torque and speed between the shafts at relative speed that are based on a varying radial location on each of the first and second pulleys  106 , 108 . 
     Referring to  FIGS. 4 and 5  with continued reference to  FIGS. 1 and 3 , in this disclosed example, primary pulley and the secondary pulley include halves split relative to the axis of rotation and the flexible link comprises a belt  110  having a V-shape in cross-section. The first pulley  106  include a first half  106 A and a second half  106 B that are spaced axially apart by a distance  120 . The second pulley  108  includes a first half  108 A and a second half  108 B that are spaced an axial distance  122  apart. 
     An inner surface  116  of the first pulley  106  and an inner surface  118  of the second pulley  108  are ramped such that the belt  110  will ride on each of the pulleys  106 ,  108  at different radial positions depending on the corresponding axial distance  120 ,  122 . The axial distance  120 ,  122  between halves of each pulley  106 ,  108  will vary depending on a speed of the corresponding shaft  102 ,  104  to provide a continuously varying speed ratio. The speed ratio between the first shaft  102  and the second shaft  104  may thereby provide a variable output speed to the main fuel pump  66  by adjusting the axial spacing  120 ,  122  for each of the first and second pulleys  106 ,  108 . 
     The axial spacing  120 ,  122  can be arranged to automatically adjust to provide a defined output speed of the output shaft  86  to the main fuel pump  66 . The axial spacing  120 ,  122  may also be controlled by the controller  84  based on sensed conditions. In one disclosed example embodiment, the controller  84  receives rotational speed information from a first sensor  112  sensing a speed of the first shaft  102  and a second sensor  114  sensing a speed of a second shaft  104 . The controller  84  uses the different speed information to adjust the transmission  100  to provide a desired output speed to drive the main fuel pump  66 . The controller  84  adjusts the speed reduction ratio based on sensed or otherwise determined engine operating conditions. Based on the specific engine operating condition, the controller  84  adjusts the transmission  100  to provide a speed at the output shaft  86  that is determined to tailor the fuel flow generated by the main fuel pump  66  to match the demand for fuel of the gas turbine engine. 
     Referring to  FIG. 6  with continued reference to  FIG. 1 , the example fuel system  62  is schematically shown and is driven by a continuously variable transmission  125  that includes a variable drive  128  that provides an output speed of an output shaft  130  that varies relative to an input speed of the input shaft  88  driven by the accessory gearbox  78 . The speed sensors  112  and  114  provide information to the controller  84  indicative of current speeds of the corresponding input shaft  88  and output shaft  140 . The example variable drive  128  can be controlled to provide a speed of the output shaft  130  independent of the speed of the input shaft  88 . The variable drive may also be controlled to provide a speed of the output shaft  130  that proportional to the speed of the input shaft  88 . 
     In one example embodiment, the controller  84  controls the variable drive  128  to drive the output shaft  130  at a speed independent of the speed of the input shaft  88 . The controller  84  adjusts the variable drive to provide a desired speed of the fuel pump  66  based on fuel demand during engine operation. The variable drive  128  may be of any configuration that enables control of an output speed of an output shaft independent of or in proportion to an input speed of an input shaft. In one disclosed embodiment, the variable drive  128  is a toroidal continuously variable transmission that utilizes rollers between discs to transit power. It should be appreciated that although several continuously variable transmission systems have been disclosed by way of example, any transmission system that enables control on an output speed of an output shaft independent of the speed of an input shaft could be utilized to drive the fuel pump  66  and are within the contemplation and scope of this disclosure. 
     Accordingly, the disclosed fuel systems provide a varied flow to match engine demand during operation that enables an increased heat acceptance capacity of the fuel while maintaining operation with a secondary drive system to assure uninterrupted fuel flow. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure.