Patent Publication Number: US-10767568-B2

Title: Dual spool power extraction with superposition gearbox

Description:
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. 
     A high pressure turbine drives the high pressure compressor through an outer shaft to form a high spool, and the low pressure turbine drives the low pressure compressor through an inner shaft to form a low spool. The fan section may also be driven by the low pressure turbine through the inner shaft. 
     The engine is typically started by driving the high spool through a tower shaft through an accessory gearbox. Once the high spool is up to speed, the low spool follows and the engine is brought to an idle condition. When the engine is operating, the accessory gearbox is driven through the same tower shaft to drive accessory components such as hydraulic pumps and electric generators. The loads from the accessory gearbox on the high spool reduce efficiency. 
     Turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies. 
     SUMMARY 
     A turbofan engine according to an exemplary embodiment of this disclosure includes, among other possible things, a first spool including a first turbine; a second spool including a second turbine disposed axially forward of the first turbine; a first tower shaft engaged to the first spool; a second tower shaft engaged to the second spool; a superposition gearbox including a sun gear, a plurality of intermediate gears engaged to the sun gear and supported in a carrier and a ring gear circumscribing the intermediate gears, wherein the second tower shaft is engaged to drive the sun gear; a first clutch for selectively coupling the first tower shaft to the ring gear; a second clutch for selectively coupling the sun gear to the carrier; and an accessory gearbox driven by an output of the superposition gearbox. 
     In a further embodiment of the foregoing gas turbine engine, the output of the superposition gearbox comprises a lay shaft coupled to the carrier. 
     In a further embodiment of any of the foregoing gas turbine engines, the first turbine comprises a low pressure turbine and the second turbine comprises a high pressure turbine. 
     In a further embodiment of any of the foregoing gas turbine engines, the first tower shaft and the second tower shaft are concentric about a common axis. 
     In a further embodiment of any of the foregoing gas turbine engines, the first tower shaft and the second tower shaft are disposed about different axes. 
     In a further embodiment of any of the foregoing gas turbine engines, the first clutch and the second clutch comprise one-way mechanical clutches. 
     In a further embodiment of any of the foregoing gas turbine engines, the first clutch couples the first tower shaft to the ring gear and the second tower shaft drives the sun gear in a first operating condition such that both the first tower shaft and the second tower shaft combine to drive the output. 
     In a further embodiment of any of the foregoing gas turbine engines, the second clutch couples the sun gear to the carrier. The lay shaft drives the carrier and thereby the second tower shaft and the second spool in a starting operating condition. 
     In a further embodiment of any of the foregoing gas turbine engines, the first clutch couples the first tower shaft to the ring gear and the ring gear drives the carrier to drive the output in a forward wind milling operating conditions. 
     In a further embodiment of any of the foregoing gas turbine engines, both the first clutch and the second clutch are disengaged in a rear wind milling operating condition such that rotation of either the first spool and the second spool is not communicated to the output. 
     In a further embodiment of any of the foregoing gas turbine engines, the superposition gearbox is not fixed to a static structure of the engine. 
     An auxiliary gearbox drive system for a turbofan engine, according to an exemplary embodiment of this disclosure includes, among other possible things, a sun gear, wherein the sun gear is configured to be coupled to a second tower shaft of the turbofan engine; a plurality of intermediate gears engaged to the sun gear and supported in a carrier; a ring gear circumscribing the intermediate gears; a first means for selectively coupling the ring gear to a first tower shaft of the turbofan engine; a second means for selectively coupling the sun gear to the carrier; and an output to an auxiliary gearbox. 
     In a further embodiment of the foregoing auxiliary gearbox drive system for a turbofan engine, the output comprises a lay shaft coupled to the carrier. 
     In a further embodiment of any of the foregoing auxiliary gearbox drive systems, both the first means for selectively coupling the ring gear to the first tower shaft and the second means for selectively coupling the sun gear to the carrier comprise a one-way mechanical clutch. 
     In a further embodiment of any of the foregoing auxiliary gearbox drive systems, the first means couples the first tower shaft to the ring gear and the second tower shaft drives the sun gear in a first operating condition such that both the first tower shaft and the second tower shaft combine to drive the output. 
     In a further embodiment of any of the foregoing auxiliary gearbox drive systems, the second means couples the sun gear to the carrier and the lay shaft drives the carrier and thereby the second tower shaft and a high speed spool in a starting operating condition. 
     A method of operating an auxiliary gearbox for a turbofan engine according to an exemplary embodiment of this disclosure includes, among other possible things, coupling a first tower shaft to engage a first spool; coupling a second tower shaft to engage a second spool; coupling a sun gear of a superposition gearbox to the second tower shaft, wherein the superposition gearbox includes the sun gear, a plurality of intermediate gears engaged to the sun gear and supported in a carrier and a ring gear circumscribing the intermediate gears; and selectively coupling the first tower shaft to the ring gear and/or the sun gear to the carrier to generate a driving output to an accessory gearbox or a driving input to the second tower shaft. 
     In a further embodiment of the foregoing method of operating an auxiliary gearbox for a turbofan engine, the first tower shaft is coupled to the ring gear such that the first tower shaft drives the ring gear. The second tower shaft drives the sun gear in a first operating condition such that both the first tower shaft and the second tower shaft combine to drive the output. 
     In a further embodiment of any of the foregoing methods of operating an auxiliary gearbox for a turbofan engine, the sun gear is coupled to the carrier and driving the carrier and thereby the second tower shaft and the second spool in a starting operating condition. 
     In a further embodiment of any of the foregoing methods of operating an auxiliary gearbox for a turbofan engine, the first tower shaft is coupled to the ring gear such that the ring gear drives the carrier to drive the output in a forward wind milling operating conditions. 
     In a further embodiment of any of the foregoing methods of operating an auxiliary gearbox for a turbofan engine, both the first tower shaft is decoupled from the ring gear and the sun gear is decoupled from the carrier in a rear wind milling operating condition such that rotation of either the first spool and the second spool is not communicated to the output. 
     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 embodiment. 
         FIG. 2  is a schematic representation of the example accessory drive system. 
         FIG. 3  is a schematic illustration of the accessory drive system in a first operating condition. 
         FIG. 4  is a schematic illustration of the accessory drive system in a starting operating condition. 
         FIG. 5  is a schematic illustration of the example accessory drive system in a forward wind milling operating condition. 
         FIG. 6  is a schematic illustration of the auxiliary drive system in an aft wind milling operating condition. 
     
    
    
     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 various bearing systems  38  at various locations may alternatively or additionally be provided, 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. 
     The example engine  20  includes an accessory drive system  62  that receives power from both the high speed spool  32  and the low speed spool  30 . The accessory drive system  62  drives an accessory gearbox  68  that includes accessory component  72  and lubricant pump  74 . The accessory component  72  may include pumps, generators and other devices driven to enable operation of different engine and aircraft systems. The accessory gearbox  68  is also coupled to a starter  70 . The starter  70  is capable of driving the accessory drive system  62  to start the engine  20 . In this example, a tower shaft assembly  64  is coupled to both the low speed spool  30  and the high speed spool  32  to distribute power extraction between the two spools  30 ,  32 . 
     Excessive power extraction from a single spool, such as the high speed spool  32 , can limit operation and degrade overall performance and engine efficiency. Accordingly, the example accessory drive system  62  extracts power from both the low speed spool  30  and the high speed spool  32  to meet the overall power demands of the engine  20  and the aircraft associated with the engine. 
     Referring to  FIG. 2 , with continued reference to  FIG. 1 , the example accessory drive system  62  includes a superposition gearbox  66  that is coupled between accessory gearbox  68  and the tower shaft assembly  64 . The superposition gearbox  66  is an epicyclic gearbox that includes a sun gear  102  that rotates about an axis  112 . A plurality of intermediate gears  104  are engaged with the sun gear  102  and supported by a carrier  106 . A ring gear  108  circumscribes and is engaged with the plurality of intermediate gears  104 . The example superposition gearbox  66  is not coupled to a static structure of the engine  20  and, therefore, is operated by various input combinations to provide the desired distribution of power. 
     In the disclosed example, the tower shaft assembly  64  includes a first tower shaft  76  that is driven by a gear  82  disposed on the low speed spool  30 . A first gear  86  on the tower shaft  76  is coupled to the gear  82 . A second gear  88  is disposed on a second end of the tower shaft  76  and engages a drive gear  90  disposed on a ring gear shaft  92 . 
     A second tower shaft  78  is coupled to a drive gear  84  that is driven by the high speed spool  32 . The second tower shaft  78  includes a first gear  94  driven by a gear  84  on the high speed spool  32 . A second gear  96  of the second tower shaft  78  is engaged to drive gear  98  disposed on a sun gear shaft  100 . In this example, the first tower shaft  76  and the second tower shaft  78  are disposed concentrically about a common axis  80 . Moreover, the axis  80  is disposed at an angle relative to the engine longitudinal axis A and an axis  112  of the superposition gearbox  66 . It should be appreciated that although the specific disclosed embodiment includes concentric tower shafts  76 ,  78 , other configurations and orientations of the tower shafts are within the contemplation and scope of this disclosure. 
     First tower shaft  76  is coupled to the ring gear shaft  92  that is selectively coupled to the ring gear  108 . The second tower shaft  78  is coupled to the sun gear shaft  100  that is coupled to drive the sun gear  102 . The sun gear shaft  100  is directly coupled to the sun gear  102  and is not selectively engaged to the sun gear  102 . 
     The superposition gearbox  66 , therefore, has a first input provided by the first tower shaft  76  through the ring gear shaft  92  to drive the ring gear  108  and a second input provided by the second tower shaft  78  to drive the sun gear shaft  100  and, thereby, the sun gear  102 . An output from the superposition gearbox  106  is provided by a lay shaft  110  that is coupled to the carrier  106 . The lay shaft  110  drives the accessory gearbox  68  in the disclosed example embodiment. The accessory gearbox  68  includes another gear system or plurality of gears as is required to the drive accessory components schematically illustrated at  72  and  74 . Moreover, the accessory gearbox  68  is coupled to the starter  70  to provide a driving input through the lay shaft  110  to the superposition gearbox  66  to drive the high speed spool  32  during starting operation. 
     Referring to  FIG. 3 , with continued reference to  FIG. 2 , the example superposition gearbox  66  includes a first clutch assembly  114  that selectively couples the ring gear shaft  92  to drive the ring gear  108 . The first clutch  114  selectively couples driving input from the low speed spool  30  through the first tower shaft  76  to the ring gear shaft  92  and ring gear  108 . 
     A second clutch assembly  116  selectively couples the sun gear  102  to the carrier  106 . The second clutch assembly  116  is shown uncoupled such that the sun gear  102  and the carrier  106  may rotate at different relative speeds. Coupling of the sun gear  102  to the carrier  106  enables the second tower shaft  78  to directly drive the carrier  106 . Accordingly, through a selective coupling of the first clutch assembly  114  and the second clutch assembly  116  different inputs from the high speed spool  32  and the low speed spool  30  can be input through the superposition gearbox  66  to drive the lay shaft  110  and, thereby, the accessory gearbox  68 . 
     The first clutch assembly  114  and the second clutch assembly  116  are one-way mechanical clutches that do not require control or separate independent actuation. Each of the clutch assemblies  114 ,  116  are automatically engaged and disengaged depending on the speed and direction of torque input. The selective actuation of the mechanical clutches  114 ,  116  enables both the low speed spool  30  and the high speed spool  32  to provide torque input to drive the lay shaft  110 . 
     In the configuration for operation shown in  FIG. 3 , the first clutch  114  is engaged to couple the ring gear shaft  92  to the ring gear  108 . Accordingly, the first tower shaft  76  driven by the low speed spool  30  is coupled to drive the ring gear  108 . The second clutch  116  is not engaged and is free running. The second tower shaft  78  drives the sun shaft  100  to drive the sun gear  102 . Accordingly, the low speed spool  30  is driving the ring gear  108  and the high speed spool  32  is driving the sun gear  102 . The driving inputs are both in the same rotational direction and result in an overall output through the carrier  106  to drive the lay shaft  110 . 
     Accordingly, a rotational input, schematically indicated at  120 , of the low speed spool  30  combined with a rotational input  122 , schematically shown by the rotational arrows, combined within the superposition gearbox  66  generate an output, schematically shown at  118 , to drive the accessory gearbox  68 . 
     Differing rotational inputs provided by  120  and  122  may be of differing speeds and torques. The superposition gearbox  66  receives and automatically distributes the torques to provide the driving input  118  through the lay shaft  110  to drive the accessory gearbox  68 . 
     Referring to  FIG. 4 , another schematic illustration of the disclosed drive system  62  is schematically shown configured for a starting operation configuration. In the starting configuration, starter  70  provides input torque  124  to drive the accessory gearbox  68  and the lay shaft  110  to rotate high speed spool  32 . In this position, the first clutch assembly  114  is not coupled because the ring gear  108  due to the direction of torque input and rotation. Accordingly, the first tower shaft  76  is not back driven by the superposition gearbox  66 . The second clutch  116  is engaged to couple the carrier  106  to the sun gear shaft  100 . Accordingly, rotation of the carrier  106  by the lay shaft  110  drives the sun gear shaft  100  and, thereby, the second tower shaft  78 . Driving the second tower shaft  78  rotates the high speed spool  32  to provide a rotational input, schematically shown at  126 . None of the torque or rotational input provided by the lay shaft  110  is transmitted to the low spool  30 . The high speed spool  32  is driven until the engine starts and begins spinning under its own power as is known and understood by those skilled in gas turbine engine structure and operation. 
     Referring to  FIG. 5 , with continued reference to  FIG. 2 , a forward wind milling operating condition is schematically shown and includes an input of torque on the low speed spool  30  from the fan section  22 . Airflow schematically shown at  25  passing through the fan  22  when the engine is not operating will cause rotation of the low spool  30 . Rotation of the low spool  30  in turn causes rotation of various structures in the engine. Forward rotation of the engine requires lubricant to be supplied. In this example embodiment, rotation of the low spool  30  causes rotation of the geared architecture  48  ( FIGS. 1 and 2 ) and therefore creates a need to provide lubricant to rotating components. Moreover, other components of the engine such as the support bearings as well of the superposition gearbox  66  require lubricant when rotated. Accordingly, the disclosed accessory drive system  62  drives the lay shaft  110  to drive the accessory gearbox  68  that in turn drives a lubricant pump  74 . The lubricant pump  74  drives lubricant through a system of conduits schematically shown at  134  of the lubrication system schematically indicated at  132  that provides lubricant to engine components as indicated at  136  and the superposition gearbox  66 . The example lubrication system  132  is shown schematically and is contemplated to include features that distribute lubricant throughout the engine  20  as would be understood by one skilled in turbine engines. 
     Operation of the accessory drive system  62  in the illustrated forward wind milling operating condition includes rotation of the low speed spool  30  caused by airflow  25  through the fan section  22 . Rotation of the low speed spool  30  drives the first tower shaft  76 . The first clutch  114  is coupled such that rotation of the first tower shaft  76  rotates the ring gear shaft  92  and the ring gear  108 . The high speed spool  32  is stationary and does not provide an input to the second tower shaft  78  and the sun gear  102 . The sun gear  102  is therefore held stationary. The sun gear  102  rotates with the carrier  106  because the second clutch  116  does not allow the carrier  116  to rotate faster than the sun gear  102 . Accordingly, because the high speed spool  32  is attached to the sun gear  102 , the high speed spool  32  becomes a parasitic drag on the system to slow or prevent rotation of the sun gear  102 . Rotation of the ring gear  102  in combination with the sun gear  102  drives the intermediate gears  104  about the axis  112 . Rotation of the intermediate gears  104  drives the carrier  106  and thereby the lay shaft  110 . Accordingly, the input schematically indicated at  128  from the low speed spool  30  into the superposition gearbox  66  generates an output  130  to drive the accessory gearbox  68 . Driving of the gearbox  68  in turn drives the pump  74  to operate the lubrication system  132 . 
     Referring to  FIG. 6 , an aft wind milling operating condition is schematically shown where airflow  25  from aft of the engine is driving rotation of the fan  22 . Airflow  25  from the aft direction causes the fan section  22  and thereby the low speed spool  30  to rotate in about the axis A in a direction that does not cause coupling between the ring shaft  92  and the ring gear  108 . Accordingly, neither the torque from the low speed spool  30  or the high speed spool  32  is transmitted through the superposition gearbox  66 . In the reverse wind milling operating condition, rotation of the high speed spool  32  is not possible and therefore lubricant flow is not necessary. Accordingly, the first clutch  114  does not couple the ring shaft  92  to the ring gear  108  and no torque is transferred to the accessory gearbox  68 . 
     The example accessory drive system  62  includes a superposition gearbox  66  that automatically distributes input driving torque between the low speed spool  30 , the high speed spool  32  and the accessory gearbox  68  as required during engine operation. The selective operation of the superposition gearbox  66  is enabled by first and second one-way clutches that provide different combinations of inputs and outputs that automatically couple based engine operating conditions. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that this disclosure is not just a material specification and 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.