Abstract:
A method for assembling a gas turbine engine including a core gas turbine engine, a low-pressure turbine, a starter, and a generator is provided. The method includes coupling a starter to the core gas turbine engine, and coupling a generator to the low-pressure turbine.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engines, and more specifically to a system configured to start the gas turbine engine and also configured to generate electrical power. 
         [0002]    At least some known gas turbine engines used with aircraft include a core engine having, in serial flow arrangement, a compressor which compresses airflow entering the engine, a combustor which burns a mixture of fuel and air, and low and high pressure turbines which extract energy from airflow discharged from the combustor to generate thrust. 
         [0003]    As aircraft accessory power demands have increased, there also has been an increased need to run the gas turbine engines at idle speeds that may be higher than other engines not subjected to increased power demands. More specifically, increasing the gas turbine engine idle speed enables the increased power demands to be met without sacrificing compressor stall margins. However, the increased idle speed may also generate thrust levels for the engine which are higher than desired for both flight idle decent operations and/or during ground idle operations. Over time, continued operation with increased thrust levels during such idle operations may increase maintenance costs and the increased fuel flow requirements may also increase aircraft operating expenses. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In one aspect, a method for assembling a gas turbine engine including a core gas turbine engine, a low-pressure turbine, a starter, and a generator is provided. The method includes coupling a starter to the core gas turbine engine, and coupling a generator to the low-pressure turbine. 
         [0005]    In another aspect, a starter/generator system for a gas turbine engine is provided. The system includes a starter coupled to a core gas turbine engine, and a generator coupled to a low-pressure turbine. 
         [0006]    In a further aspect, a gas turbine engine assembly is provided. The gas turbine engine assembly includes a core gas turbine engine, a low-pressure turbine, and a starter/generator system that includes a starter coupled to the core gas turbine engine, and a generator coupled to the low-pressure turbine. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a simplified schematic illustration of a gas turbine engine assembly including an exemplary starter/generator system; 
           [0008]      FIG. 2  is a simplified schematic illustration of the gas turbine engine assembly and starter/generator system shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a cross-sectional view of a radially outer portion of the starter/generator system shown in  FIGS. 1 and 2 ; 
           [0010]      FIG. 4  is a cross-sectional view of a radially inner portion of the starter/generator system shown in  FIGS. 1 and 2 ; and 
           [0011]      FIG. 5  is an end view of a portion of the starter generator system shown in  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  is a cross-sectional view of a gas turbine engine assembly  10  having a longitudinal axis  11 .  FIG. 2  is a simplified schematic illustration of gas turbine engine assembly  10  shown in  FIG. 1 . Gas turbine engine assembly  10  includes a fan assembly  12  and a core gas turbine engine  13 . Core gas turbine engine  13  includes a high pressure compressor  14 , a combustor  16 , and a high pressure turbine  18 . In the exemplary embodiment, gas turbine engine assembly  10  also includes a low pressure turbine  20 , and a multi-stage booster compressor  22 . Gas turbine engine assembly  10  has an intake side  28  and an exhaust side  30 . Fan assembly  12 , booster  22 , and low-pressure turbine  20  are coupled together by a first rotor shaft  31 , and compressor  14  and high-pressure turbine  18  are coupled together by a second rotor shaft  32 . 
         [0013]    In operation, air flows through fan assembly  12  and a first portion of the airflow is channeled through booster  22 . The compressed air that is discharged from booster  22  is channeled through compressor  14  wherein the airflow is further compressed and delivered to combustor  16 . Hot products of combustion (not shown) from combustor  16  are utilized to drive turbines  18  and  20 , and turbine  20  is utilized to drive fan assembly  12  and booster  22  by way of shaft  31 . Gas turbine engine assembly  10  is operable at a range of operating conditions between design operating conditions and off-design operating conditions. 
         [0014]    Gas turbine engine assembly  10  also includes a starter/generator system  100  that includes a starter  102  and a generator  104 . Although starter  102  is described herein as a device that is utilized to start the core gas turbine engine  13 , it should be realized, that starter  102  may also be driven by the core gas turbine engine  13  and function as a generator. Moreover, although generator  104  is described herein as an apparatus that is driven by low-pressure turbine  20  to generate electrical energy, it should be realized, that generator  104  may also drive low-pressure turbine  20  to facilitate restarting gas turbine engine assembly  10  during various operational conditions which will be discussed below. Gas turbine engine assembly  10  also includes a motor/generator  110 , a generator/motor  120 , and an accessory gearbox  130  that are discussed below. 
         [0015]      FIG. 3  is a cross-sectional view of a radially outer portion of starter/generator system  100  shown in  FIGS. 1 and 2 . In the exemplary embodiment, starter  102  includes motor/generator  110 , a motor shaft  112  that is coupled to, and driven by motor/generator  110 , and a drive pinion  114  that is coupled to drive shaft  112 . Generator  104  includes generator/motor  120 , a generator shaft  122  that is coupled to, and driven by generator/motor  120 , and a drive pinion  124  that is coupled to drive shaft  122 . In the exemplary embodiment, motor/generator  110  and generator/motor  120  are each coupled to accessory gearbox  130  that is coupled to core gas turbine engine  13 . 
         [0016]    Starter/generator system  100  also includes first drive shaft  150  that is coupled to motor/generator  110  and a second drive shaft  152  that is coupled to generator/motor  120 . First drive shaft  150  has an inside diameter  154  and second drive  152  has an outside diameter  156 . In the exemplary embodiment, outside diameter  156  is less than inside diameter  154  such that first drive shaft  150  substantially circumscribes second drive shaft  152 . Specifically, second drive shaft  152  has an outside diameter  156  that is less than the inside diameter  154  of first drive shaft  150  such that second drive shaft  152  may be positioned inside first drive shaft  150 . 
         [0017]    First drive shaft  150  includes a first end  160  and a pinion  162  that is coupled to first end  160 . In the exemplary embodiment, drive pinion  114  and pinion  162  are each bevel gears configured such that drive pinion  114  is intermeshed with pinion  162  and such that rotating drive pinion  114  causes pinion  162  to rotate. Second drive shaft  152  includes a first end  164  and a pinion  166  that is coupled to first end  164 . In the exemplary embodiment, drive pinion  124  and pinion  166  are each bevel gears configured such that drive pinion  124  is intermeshed with pinion  166  and such that rotating drive pinion  124  causes pinion  166  to rotate. 
         [0018]      FIG. 4  is a cross-sectional view of a radially inner portion of starter/generator system  100  shown in  FIGS. 1 ,  2 , and  3 .  FIG. 5  is an end view of the radially inner portion of starter/generator system  100  shown in  FIG. 4 . 
         [0019]    In the exemplary embodiment, first drive shaft  150  also includes a second end  170  and a pinion  172  that is coupled or splined to second end  170 . Moreover, gas turbine engine assembly  10  also includes a ring gear  174  that is coupled or splined to second rotor shaft  32 . In the exemplary embodiment, pinion  172  and ring gear  174  are each bevel gears configured such that pinion  172  is intermeshed with ring gear  174  and such that rotating shaft  32  causes ring gear  174  to rotate and thus causes pinion  172  to rotate. Second drive shaft  152  also includes a second end  180  and a pinion  182  that is coupled or splined to second end  180 . Moreover, gas turbine engine assembly  10  also includes a ring gear  184  that is coupled or splined to first rotor shaft  31 . In the exemplary embodiment, pinion  182  and ring gear  184  are each bevel gears configured such that pinion  182  is intermeshed with ring gear  184  and such that rotating shaft  31  causes ring gear  184  to rotate and thus causes pinion  182  to rotate. As shown in  FIGS. 3 and 4 , shafts  150  and  152  may be fabricated to include one or more shaft portions that are coupled together to form a single respective shaft, and thus simplify assembly. Optionally, shafts  150  and  152  may also be fabricated as unitary components without affecting the scope of the invention described herein. 
         [0020]    As shown in  FIG. 4 , starter generator system  100  also includes a plurality of bearing assemblies to facilitate maintaining shafts  150  and  152  in the proper position within gas turbine engine assembly  10 . Specifically, starter/generator system  100  includes a first differential bearing  190  and a second differential bearing  192  that are each coupled between shafts  150  and  152 . Although, the invention described herein is described with respect to a pair of differential bearings  190  and  192 , it should be realized that a single differential bearing  190  may be utilized when shafts  150  and  152  are fabricated as unitary components. Differential bearings  190  and  192  each include a rotating outer race  200  that is secured to shaft  150  and a rotating inner race  202  that is secured to shaft  152 . Each differential bearing  190  and  192  also includes a plurality of rolling elements  204  that are positioned between outer and inner races  200  and  202  respectively. Differential bearings  190  and  192  each facilitate maintaining shaft  152  in a substantially fixed orientation with respect to shaft  150 , while allowing shaft  152  to rotate independently from shaft  150 . 
         [0021]    Starter/generator system  100  includes a third bearing  210  that is coupled between shaft  150  and a structural component of gas turbine engine assembly  10  to support shaft  150  while still allowing shaft  150  to rotate with respect of the structural component. Third bearing  210  includes a rotating inner race  212  that is coupled to shaft  150 , a stationary outer race  214  that is secured to the structural component. Third bearing  210  also includes a plurality of rolling elements  216  that are positioned between inner and outer races  212  and  214  respectively. 
         [0022]    Starter/generator system  100  includes a fourth bearing  220  that in the exemplary embodiment is a roller bearing configured to maintain shaft  150  in a substantially fixed orientation within gas turbine engine assembly  10 . Fourth bearing  220  includes a rotating inner race  222  that is coupled to shaft  150 , a stationary outer race  224  that is secured to the structural component, and a plurality of rolling elements  226  that are positioned between inner and outer races  222  and  224  respectively. 
         [0023]    During assembly, the starter  102  is coupled to the core gas turbine engine  13  and the generator  104  is coupled to the low-pressure turbine  20 . Specifically, starter  102  and generator  104  are each coupled to the accessory gearbox  130 . The second drive shaft  152  is inserted into the first drive shaft  150 . The first drive shaft  150  is then coupled between the starter  102  and the core gas turbine engine  13 , and the second drive shaft  152  is coupled between the generator  104  and the low-pressure turbine  20 . 
         [0024]    During operation, starter  102  is activated to start the core gas turbine engine  13 . Specifically, activating starter  102  causes shaft  150  to rotate and thus causes the core gas turbine engine  13  to rotate and start as is known in the art. In the exemplary embodiment, starting the core gas turbine engine  13  causes the low-pressure turbine  20  to rotate, thus causing generator  104  to generate electrical energy. Additionally, during flight or other operations, generator  104  may be utilized to restart the gas turbine engine. Specifically, since generator  104  includes a generator/motor  120 , supplying electrical power to generator  104  causes generator  104  to function as a starter. Specifically, since generator  104  is coupled to low-pressure turbine  20  via shaft  152 , operating generator  104  as a motor causes shaft  152  to rotate the low-pressure turbine  20  and thus restart the gas turbine engine assembly  10 . Moreover, during selected operating conditions, core gas turbine engine  13  may be utilized to drive starter  102  thus causing starter  102  to function as a generator to facilitate generating additional electrical energy that may be utilized by the aircraft to peak power load conditions. Specifically, gas turbine engine assembly  10  also includes a clutch  230  that is coupled between starter  102  and shaft  32  such that during selected operating conditions, clutch  230  may be disengaged such that shaft  32  drives starter  102  to produce electrical energy. For example, in the exemplary embodiment, clutch  230  is an overrunning clutch such that clutch  230  is engaged during engine startup. Optionally, a control signal is transmitted to clutch  230  when desired such that clutch  230  does not overrun and such that starter  102  functions as a generator to produce electrical energy. 
         [0025]    Described herein is a gas turbine engine assembly that is configured to extract relatively large amounts of power from the engine while operating the engine at low thrust conditions. The starter/generator system described herein is configured to extract power from both the core gas turbine engine and the low-pressure turbine simultaneously in order to share the load requirements. For example, during ground start, the starter engages the high-pressure compressor, thus rotating the high-pressure compressor in order to establish airflow in the core gas turbine engine before lighting the combustor. Optionally, during a windmill start, the generator, which is coupled to the low-pressure turbine, can produce power that is used to motor the high-pressure compressor to speed using its generator/starter-motor in order to achieve light off of the combustor. 
         [0026]    As a result, additional energy is extracted from the low-pressure turbine and fan assembly to support ever increasing electrical demands. Specifically, newer aircraft are designed to require an atypically large amount of electrical power driven by the generator on the engine accessory gearbox. The power requirements during idle conditions thus require the engine to run at idle speeds that are higher than desirable in order to maintain adequate compressor stall margin. This results in thrust levels for the engine that are higher than desired for both flight idle descent points and ground idle conditions, which has both maintenance cost implications for aircraft brakes and excess fuel burn penalties for typical short range missions. 
         [0027]    Whereas the system described herein, takes power off both shafts simultaneously in order to share the load requirements. As a result, the system described herein is relatively simple to install, and also provides a low weight solution to this problem. Moreover, the system described herein, allows for reduced thrust during ground idle conditions to reduce aircraft brake maintenance, reduced dirt ingestion, and reduced flight idle thrusts for an improved flight profile and improved short range fuel burn while still maintaining adequate compressor stall margin during high power extraction conditions. 
         [0028]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.