Patent Publication Number: US-2018045119-A1

Title: Geared turbofan with low spool power extraction

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-speed 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. 
     The 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 inner shaft. A direct drive gas turbine engine includes a fan section driven by the low spool such that the low pressure compressor, low pressure turbine and fan section rotate at a common speed in a common direction. 
     The engine is typically started by driving the high spool through a tower shaft with an electric motor. 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 electric motor is driven through the same tower shaft to generate electric power. The tower shaft driven by the high spool may also drive an accessory gear box. The loads placed on the high spool can decrease overall engine efficiency. 
     Turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies. 
     SUMMARY 
     In a featured embodiment, a geared turbofan engine includes a first spool including a first compressor and a first turbine. The first compressor is immediately before a combustor and the first turbine is immediately after the combustor. A second spool includes at least a second turbine disposed axially aft of the first turbine. A first tower shaft is engaged to drive the high speed spool. A second tower shaft engaged to be driven by the second spool. A starter is engaged to drive the first tower shaft. An accessory gear box is driven by the second tower shaft. A first clutch is disposed between the first tower shaft and the starter. The first clutch is configured to enable the starter to drive the high speed spool. A second clutch is disposed between the second tower shaft and the accessory gear box, the second clutch configured to enable the second spool to drive the accessory gear box. 
     In another embodiment according to the previous embodiment, includes a fan driven by a speed reduction device. The speed reduction device is driven by the second spool. 
     In another embodiment according to any of the previous embodiments, includes a windmill oil system configured to supply lubricant to the speed reduction device responsive to windmilling of the fan. 
     In another embodiment according to any of the previous embodiments, the second spool includes a second compressor axially forward of the first compressor. The second compressor includes an operating pressure less than that of the first compressor. 
     In another embodiment according to any of the previous embodiments, the starter includes a starter/generator operable to drive the first tower shaft and be driven by the second tower shaft. 
     In another embodiment according to any of the previous embodiments, the starter generator is driven through a gear set of the accessory gearbox. 
     In another embodiment according to any of the previous embodiments, the accessory gearbox includes at least one mechanical pump. 
     In another embodiment according to any of the previous embodiments, the first clutch and the second clutch are one-way mechanical clutch devices. 
     In another featured embodiment, a gas turbine engine includes a fan including a plurality of fan blades rotatable about an axis. A compressor section includes a first compressor. A combustor is in fluid communication with the first compressor. A turbine section is in fluid communication with the combustor. The turbine section includes a first turbine and a second turbine. A geared architecture is driven by the second turbine for rotating the fan about the axis. A first shaft couples the first turbine to the first compressor. A second shaft couples the second turbine to the geared architecture. A first tower shaft is coupled to the first shaft. A second tower shaft is coupled to the second shaft. An accessory gear box includes gear system for driving at least one accessory component. The gear system is coupled to both the first tower shaft and the second tower shaft. A starter generator is coupled to the gear system. A first clutch is configured to control torque transfer between the starter and the first tower shaft. The first clutch enables the starter to drive the first shaft through the first tower shaft. A second clutch is configured to control torque transfer between the second tower shaft and the accessory gearbox. The second clutch enables the second shaft to drive the second tower shaft. 
     In another embodiment according to the previous embodiment, the first clutch is configured to disengage torque transfer from the first shaft to the starter generator. 
     In another embodiment according to any of the previous embodiments, the second clutch is configured to disengage torque transfer from the gear system to the second shaft. 
     In another embodiment according to any of the previous embodiments, includes a windmill oil system configured to supply lubricant to the geared architecture responsive to rotation of the fan without the engine operating. 
     In another embodiment according to any of the previous embodiments, the accessory gearbox includes at least one mechanical pump. 
     In another featured embodiment, a method operating a gas turbine engine includes driving a first spool with a starter through a first tower shaft and a first clutch to start the engine. An accessory gear box drives through a second clutch with a second tower shaft coupled to a second spool once the engine is started. The starter is decoupled from the first spool once the first spool reaches an engine idle speed. 
     In another embodiment according to the previous embodiment, driving the accessory gear box includes driving a generator through the accessory gear box. 
     In another embodiment according to any of the previous embodiments, decoupling the first clutch includes rotating the second tower shaft at a speed greater than that of the starter. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure 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 accessory gear box embodiment. 
         FIG. 3  is a schematic view of the example accessory gear box and tower shafts. 
         FIG. 4  is a schematic view of accessory gear box operation during a starting process. 
         FIG. 5  is a schematic view of accessory gear box operation during engine operation. 
     
    
    
     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 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a nacelle  15 , while the compressor section  24  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 fan  42 , a first (or low) pressure compressor  44  and a first (or low) pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive the fan  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  is 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 combustor section  26  or even aft of turbine section  28 , and fan section  22  may be positioned forward or aft of the location of gear system  48 . 
     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. 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 (10.67 km). The flight condition of 0.8 Mach and 35,000 ft (10.67 km), with the engine at its best fuel consumption —also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—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 m/second). 
     The example gas turbine engine includes the fan  42  that comprises in one non-limiting embodiment less than about twenty-six (26) fan blades. In another non-limiting embodiment, the fan section  22  includes less than about twenty (20) fan blades. Moreover, in one disclosed embodiment the low pressure turbine  46  includes no more than about six (6) turbine rotors schematically indicated at  34 . In another non-limiting example embodiment the low pressure turbine  46  includes about three (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 a first tower shaft  64  that is engaged to drive the high speed spool  32 . The engine  20  further includes a second tower shaft  66  that is engaged to be driven by the low speed spool  30 . The low speed spool  30  includes a gear  70  and the high speed spool  32  includes a gear  68 . The gear  68  is engaged to the tower shaft  64  and the gear  70  is engaged to the tower shaft  66 . Each of the tower shafts  64  and  66  drive portions of an accessory gearbox  62 . In one disclosed embodiment the gears  68  and  70  are bevel gears and engage corresponding bevel gears on the corresponding tower shaft  64 ,  66 . 
     Referring to  FIG. 2  with continued reference to  FIG. 1 , the example gearbox  62  includes a gear engagement with both the first tower shaft  64  and the second tower shaft  66 . The tower shafts  64 ,  66  are supported within a common accessory gearbox  62  and enable the use of the low speed spool  30  to drive the accessory components within the accessory gearbox  62 . 
     As the core components of the engine  20  become more efficient, larger fans will be utilized and smaller core components such as the high speed spool  30  will become smaller and more efficient. As the core engine components such as the high speed. As the high speed spool  32  components become smaller, the drag caused by the loads that accompany driving the accessory gearbox can alter compressor surge margins and other performance characteristics that detract from the overall engine deficiency. 
     Starting of an engine requires driving of the high speed spool up to an starting or engine idle speed and therefore a tower shaft is provided to drive the high speed spool. Because the tower shaft is already provided for starting purposes, it was traditionally more expedient to drive the accessory components through the same tower shaft. Accordingly, prior engines included a single tower shaft that was utilized both to drive the high speed spool during starting operations and then to have the high speed spool drive the accessory components while the engine was operating. However, as efficiencies are gained that enable the high speed spool to become smaller, it becomes less desirable to drive the accessory components with the high speed spool. 
     Accordingly, the example gas turbine engine includes the second tower shaft  66  that is driven by the low speed spool  30  and utilized to drive the accessory components. 
     Referring to  FIG. 3  with continued reference to  FIG. 2 , the example gearbox  62  includes a first clutch  72  that is engaged to the first tower shaft  64  coupled to the high speed spool  32 . A second clutch  74  is disposed on the second tower shaft  66  driven by the low speed spool  30 . Each of the clutches  74  and  72  provide for the transmission of torque in a single direction. The accessory gearbox  62  is engaged to a starter generator  76  that drives a gear  78  that meshes to drive both the first tower shaft  64  and the second tower shaft  66 . The starter/generator  76  can be driven to provide electrical power to electric accessories including a fuel pump  80 , oil pump  82  and a hydraulic pump  84 . The accessory gearbox  62  may also be geared and include power takeoff gearing to provide driving force for mechanical systems or mechanical pumps. 
     In the disclosed example, the clutches  72  and  74  are sprag clutches that only allow torque to be transmitted in one direction. When torque is reversed meaning that the driving member becomes the driven member, the clutch will slip and allows the shaft to over speed and rotate independent of the gearbox  62 . In this example, the second clutch  74  will allow the low rotor to drive the gearbox  62  and the starter/generator  76  but does not allow the gearbox  62  to drive low speed spool  30 . In this example, the clutch  74  is located within the accessory gearbox  62 , however, the clutch  74  may be located wherever practical to provide the selective application of torque between the starter/generator, accessory gearbox  62  and the low speed spool  30 . 
     The clutch  72  is configured to allow the starter/generators  76  to drive the high speed spool  32  but not to allow the high speed spool  32  to drive the gearbox  62 . In this example, because the high speed spool  32  will rotate much faster than the starter/generator  76 , the clutch  72  is configured such that the high speed spool  32  may over speed past the speed of the starter/generator  76  and not transmit torque to the accessory gearbox  62  through the tower shaft  64 . 
     Referring to  FIG. 4 , the example accessory gearbox  62  is shown during an engine starting operation. In this schematic illustration, the starter/generator  76  is shown driving gear  78  within the accessory gearbox  62  that in turn drives the clutch  72  and thereby the tower shaft  64  to drive the high speed spool  30  up to a speed required for starting of the engine. The same gear  78  driven by the starter/generator is also driving the second clutch  74  that is engaged to the second tower shaft  66  driven by the low speed spool  30 . However, in this instance, the clutch  74  is not transmitting torque to the low speed spool  30 . Accordingly, in the configuration schematically illustrated in  FIG. 4 , only the high spool  32  is turning. 
     Once the high speed spool  30  has been spun up to operating conditions, it will attain a speed that is much greater than that input by the starter/generator  76  and the tower shaft  64 . The tower shaft  64  will continue to rotate in a direction originally provided by the starter/generator  76 , however, the high speed spool driven tower shaft  64  is rotating at a much higher speed and therefore spin past the speed input by the starter/generator  76 . The clutch  72  will not allow the transmission of this higher torque from the high speed spool  32  into the accessory gearbox  62 . 
     Once the high speed spool  32  has become operational, the low speed spool  30  will also begin to turn. Rotation of the high speed spool will result in turning of the second tower shaft  66 . The second tower shaft  66  will in turn, turn the gear  78  through the clutch  74  which will drive the starter/generator  76 . Because the one way clutch  74  is orientated and configured to enable the low speed spool  30  to drive the tower shaft  74  and in turn drive the starter/generator  76 , the accessory components engaged to the gearbox  62  along with the starter/generator  76  are turned and operated by the low speed spool  30 . 
     Accordingly, once the engine is running, the starter/generator  76  may produce electric power to drive any number of accessory units, such as the fuel pump  80 , hydraulic pump  84  and/or oil pump  82  as illustrated in  FIG. 3 . Moreover, once the engine is operational, the accessory components no matter if they are electrically powered or mechanically powered are driven by the low speed spool  30 . 
     The accessory gearbox  62  may also mechanically drive a pump system for a windmill lubricant system as shown schematically at  86  in  FIG. 3 . A windmill lubricant system  86  provides lubricant flow to the geared architecture when the fan is rotating due to wind and airflow without the engine operating. If the engine is not running and the fan is running, the geared architecture still requires lubricant flow and therefore the example windmill lubricant system provides lubricant flow in the absence of engine operation. The example windmill lubricant system is configured to provide lubricant flow regardless of the direction that the fan is turning. 
     Accordingly, the example accessory gearbox enables the use of compact high speed spool systems to maximize overall engine efficiencies. 
     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.