Abstract:
A method for gas turbine engine off-load starting includes using a starter connected to a rotor of a gas turbine engine to start the gas turbine engine while terminating or reducing torque transmission from the starter to accessory devices until a gas turbine engine starting cycle is completed and the gas turbine engine is operating under a self-sustaining condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a gas turbine engine used for aircraft, and more particularly to a method and a system for gas turbine engine starting. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engine starting typically requires the use of a powerful electric or pneumatic starter, which is usually mounted in an accessory gear box, to drive the compressor/turbine rotor, accessories connected to the accessory gear box, such as generators, pumps, air conditioning, etc., and engine system components integrated into the gear box, during the start sequence of the gas turbine engine. The starting cycle is completed when the gas turbine engine reaches a self-sustaining operative condition. In conventional practice, the electric or pneumatic starters provide drive torque during the start cycle to overcome the drag from the compressor and turbine rotors, and loads resulting from all of the accessories connected to the accessory gearbox drive train driven by the starter. 
     The power source for actuating a starter motor of a gas turbine engine used in aircraft generally includes electric power stored in batteries or pressurized fluid provided by an APU (Auxiliary Power Unit). The energy provided by those power sources is limited and therefore it is desirable to reduce the torque requirement for gas turbine engine starting. 
     Clutches are well known in the art and are used in gear trains of gas turbine engines for selectively establishing or terminating torque transmission through the gear trains. U.S. Pat. No. 5,201,798, issued to Hogan on Apr. 12, 1993 describes one example of the use of one way clutches in the gear train of a gas turbine engine. A multiple function integrated power unit for use aboard aircraft includes two turbine engines operable under different conditions, an integrating gear box receiving power from the two engines selectively, and a plurality of accessory devices receiving shaft power from either of the two engines through the gear box. One clutch is used to connect a starter to the gear train to drive the first engine. After attaining self-sustaining speed, the first engine accelerates under its own power to its operating speed, and the flow of pressurized air to the starter is discontinued. The clutch ensures that the starter is not driven by the first engine. The first engine provides shaft power to the gear train within the gear box through a sprag clutch. The multiple integrated power unit also includes a second engine which is connected to the gear train within the gear box through another sprag clutch so that when the accessories are driven by either one of the two engines the other engine will not be driven. 
     U.S. Pat. No. 4,257,281, issued to Bunger on Mar. 24, 1981 also describes the use of an over-running clutch incorporated into an engine starter and accessory drive system, so that the clutch is engaged to transfer torque from the starter to the engine and the accessory devices during the engine starting, and the clutch is disengaged to prevent torque from being transferred to the starter during engine operation. 
     Nevertheless, those systems generally prevent torque from being transmitted to the starter or other rotator during engine operation, and does not improve engine starting. Therefore, there is a need for a method and a system for improved engine starting. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a method for gas turbine engine starting with a reduced load. 
     Another object of the present invention is to provide a gas turbine engine starting system which can start the gas turbine engine with a reduced load. 
     In accordance with one aspect of the present invention, the method of gas turbine engine starting comprises using a starter connected to a rotor of a gas turbine engine to start the gas turbine engine while terminating or reducing torque transmission from the starter to accessory devices until a gas turbine engine starting cycle is completed and the gas turbine engine is operating under a self-sustaining condition. 
     A clutch is preferably used for completely or partially disconnecting a drive system of the accessory devices from the rotor of the gas turbine engine before the gas turbine engine starting cycle begins. The drive system is used for distribution of torque from the rotor of the gas turbine engine to the accessory devices during gas turbine engine operation. Partial re-engagement of accessory drives can be accomplished to provide necessary drive to fuel pumps and oil pumps during the start sequence as required. When the gas turbine engine starting cycle is completed and the gas turbine engine is operating in the self-sustaining condition, the drive system of the accessory devices is fully reconnected to the rotor of the gas turbine engine. In one embodiment of the present invention, the clutch is a magnetic fluid clutch and it is controlled by an electronic controller of the gas turbine engine, or the air craft. 
     In accordance with another aspect of the present invention, a system is provided for starting a gas turbine engine. The system comprises a starter coupled to a rotor of the gas turbine engine by means of a torque transmitting mechanism. A clutch is provided to couple a drive system of accessory devices with the torque transmitting mechanism for controlling torque transmission from the torque transmitting mechanism to the drive system of the accessory devices while maintaining the torque transmission from the starter to the rotor of the gas turbine engine. Thus, torque required for the gas turbine engine starting can be reduced by completely or partially declutching the clutch during a gas turbine engine starting cycle. 
     The clutch is preferably a magnetic fluid clutch and is controlled by an electronic controller which is incorporated into the engine control system or the aircraft control system. The drive system of the accessory devices preferably comprises an accessory gear box for distribution of torque from the rotor of the gas turbine engine to the accessory devices during gas turbine engine operation. The magnetic fluid clutch according to one embodiment of the present invention includes a rotating shaft having a first end and a second end. A casing rotatably surrounds the rotating shaft and contains a magnetic fluid therein. The first end of the rotating shaft is coupled to the starter and the second end of the rotating shaft is connected to the rotor of the gas turbine engine. The casing is coupled to a gear train of the accessory gear box. 
     The magnetic fluid clutch according to another embodiment of the present invention, includes a stationary casing containing the magnetic fluid. A first rotating shaft and a second rotating shaft are provided. The respective rotating shafts extend oppositely and outwardly from the inside of the casing, and are rotatable relative to each other and relative to the casing. Each rotating shaft has a plate affixed thereto. The two plates are axially spaced apart and are disposed in the magnetic fluid contained within the casing. An electric coil is provided for applying an electric-magnetic field to the magnetic fluid. The electric coil is controlled by an electronic controller which could be incorporated into the control system of the gas turbine engine or the control system of the aircraft. The first shaft of the magnetic fluid clutch is coupled to the starter and is also connected to the rotor of the gas turbine engine. The second rotor shaft of the magnetic fluid clutch is connected to a gear train of the accessory gear box. 
     In the system of the present invention, torque transmission between the starter and the rotor of the gas turbine engines is ensured by the structure, and torque transmission from the engine rotor/starter system to the accessory devices is controlled by the magnetic fluid clutch. Thus, the accessory devices can be completely or partially disconnected from the engine rotor/starter system during a starting cycle to reduce a total amount of torque provided by the starter for engine starting. 
     Reduced load from accessories, is especially beneficial during starting at cold temperatures. Additionally, the clutch can disengage accessories during engine shut down, thereby avoiding injection of excess fuel and oil into the engine. 
     Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the present invention, reference will now be made to the accompanying drawings, showing by way of illustration the preferred embodiments thereof, in which: 
     FIG. 1 is a schematic illustration of a gas turbine engine starting system according to one embodiment of the present invention; 
     FIG. 2 is a schematic illustration of a gas turbine engine starting system according to another embodiment of the present invention; and 
     FIG. 3 is a schematic illustration of an accessory gear box in a side view thereof, incorporated with the embodiment shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a gas turbine engine starting system, generally indicated by numeral  10  includes an electric or pneumatic starter motor  12  which is actuated by electric power stored in batteries or by pressurized air stored in pressure containing chambers or is provided by an Auxiliary Power Unit (neither are shown). The starter motor  12  has a torque output shaft  14  on which a gear  16  is affixed in order to rotate together with the torque output shaft  14  of the starter motor  12 . The gear  16  is engaged with gear  18  which is affixed to a rotating shaft  20 . The rotating shaft  20  is coupled to the rotor  22  of the gas turbine engine (not shown). The rotor  22  generally includes compressors and turbines of the gas turbine engine which rotate at high speeds during engine operation powered by the torque power generated by the gas turbine engine. However, input torque is needed to drive the rotor  22  for rotation when the engine is being started, and until the rotor  22  reaches a rotational speed at which the engine can operate in a self-sustaining condition. The torque output shaft  14 , gears  16 ,  18  and the rotating shaft  20  in combination form a torque transmitting mechanism to transmit torque generated by the starter motor  12  to the rotor  22  during a gas turbine engine starting cycle. 
     The torque output shaft  14  of the starter motor  12  is coupled to a torque input shaft  24  of a magnetic fluid clutch  26 . The magnetic fluid clutch  26  includes a casing  28  supported in an accessory gear box (not shown) of the gas turbine engine. The torque input shaft  24  extends outwardly from the inside of the casing  28  and is rotatable relative to the casing  28 . A first plate  30  is attached to the inner end of the torque input shaft  24  and is rotatable together with the shaft  24 . The magnetic fluid clutch  26  further includes a torque output shaft  32  which extends, oppositely with respect to the torque input shaft  24 , outwardly from the inside of the casing  28  and is rotatable relative to the casing  28 . A second plate  34  is attached to an inner end of the torque output shaft  32  and is rotatable together with the shaft  32 . The torque input and output shafts  24 ,  32  are positioned co-axially and the first and second plates  30 ,  34  are axially spaced apart from each other. 
     The magnetic fluid clutch  26  further includes a first core material  36  and a coil  38  wound around the first core material  36 . The two ends of the coil  38  are connected to an electronic controller  40  which is incorporated into a control system of the gas turbine engine or the control system of the aircraft which is equipped with the gas turbine engine. A second core material  42  like-wise has a coil  44  wound therearound. The two ends of the coil  44  are connected to the electronic controller  40  which is the same one controlling the coil  38 , but is illustrated separately for convenience of illustration. 
     Both the first and second plates  30 ,  34  are immersed in a magnetic fluid medium which is generally designated  46  and is contained within the casing  28 . Preferably, the core materials  36 ,  42  are positioned within the casing  28  and the magnetic fluid medium  46  is contained by an interior surface of the core materials  36 ,  42 . Alternatively, an inner casing (not shown) may be provided to protect coils  38  and  44 . The first and second plates  30 ,  34  and the first and second core materials  36 ,  42  preferably have a circular cross-section. The size of the first and second plates  30 ,  34  and the volume of the magnetic fluid medium  46  are determined by the specific properties of the magnetic fluid medium chosen, as well as the performance specifications of the magnetic fluid clutch  26 . 
     The torque input shaft  24  and the torque output shaft  32  pass through axially aligned openings  48 ,  50  defined by core materials  36 ,  42 . Seals  52 ,  54  are also preferably placed between openings  48 ,  50  and the magnetic fluid medium  46 , to prevent leakage thereof. The electronic controller  40  provides current through coils  38 ,  44  in a controlled manner in order to apply a magnetic field to the magnetic fluid medium  46 . 
     A gear  56  is attached to an outer end of the torque output shaft  32  and is rotatable together with the shaft  32 . The gear  56  engages a corresponding gear  58  affixed on a shaft  60  which is part of a drive system such as an accessory gear box, similar to that illustrated in FIG. 3, for distribution of torque from the rotor  22  of the gas turbine engine to the accessory devices (not shown) during gas turbine engine operation. 
     The magnetic fluid clutch  26  provides a magnetically controlled fluid coupling between the first and second plates  30 ,  34 . Magnetic fluid medium  46  contains magnetically polarized particles. When a magnetic field which can be generated and controlled by current through the coils  38 ,  44 , is applied to the magnetic fluid medium  46 , particle chains form. In effect, magnetic fluid medium  46  changes from a free flowing state (steady-state liquid phase) to a highly viscose state (steady-state quasi-solid phase) when current is steadily increased through coils  38 ,  44 . Various intermediate levels of viscosity can be obtained by varying the magnetic field applied to the magnetic fluid medium  46 . Advantageously, the response time for magnetic fluid medium  46  to change between a steady-state quasi-solid phase to a steady-state liquid phase is in the millisecond range. Therefore, torque transfer control changes can be performed very quickly. 
     During operation, before the gas turbine engine starting cycle begins, the electronic controller  40  supplies no current or only a small amount of current through the coils  38 ,  44  in order to maintain the magnetic fluid medium  46  in a substantially steady-state liquid phase. The starter motor  12  is then actuated to drive the rotor  22  of the gas turbine engine through the drive system formed by torque output shaft  14 , gears  16 ,  18  and the rotating shaft  20 . The first plate  30  of the magnetic fluid clutch  26  is also rotated by the starter motor  12  through the coupling of the torque output shaft  14  of the starter motor  12  and the torque input shaft  24  of the clutch  26 . However, the plate  34  which is coupled to the drive system  62  of the accessory devices through output shaft  32  and gear  56 , remains substantially immobile because the friction between the respective plates  30 ,  34  and the magnetic fluid medium  46  in the steady-state liquid phase is not enough to transmit torque from the first plate  30  to the second plate  34  in order to drive the accessory devices. 
     When the rotor  22  of the gas turbine engine reaches a rotational speed required for injection of fuel and oil into the engine, partial re-engagement of the clutch can be accomplished to provide the necessary drive to fuel and oil pump systems. When the engine reaches a steady self-sustaining condition and begins acceleration under its own power, the control system of the gas turbine engine or the control system of the aircraft stops the energy supply (current or pressurized fluid) to the starter motor  12  and the controller  40  increases the supply of current to the coils  38 ,  44  in order to produce and increase the electric-magnetic field applied to the magnetic fluid medium  46  so that the viscosity of the magnetic fluid medium  46  increases and the magnetic fluid medium  46  becomes a steady-state quasi-solid. The friction between the respective plates  30 ,  34  and the magnetic fluid medium  46  in steady-state quasi-solid phase is enough to allow torque transmission from plate  30  to plate  34 , thereby providing the torque required to drive all required accessory devices. Thus, the torque generated by the rotor  22  of the gas turbine engine is transmitted through the magnetic fluid clutch  26  to the drive system  62  of the accessory devices after the gas turbine engine starting cycle is completed and the rotor  22  of the gas turbine engine is operating under the self-sustaining condition. 
     It is noted that various intermediate levels of viscosity of the magnetic fluid medium  46  can be obtained by varying the electric-magnetic field applied to the magnetic fluid medium  46  so that the first and second plates  30 ,  34  can be partially declutched. In the partially declutched state, rotation slippage occurs between the first and second plates  30 ,  34  when the torque to be transmitted is greater than a selected level and thereby only a reduced amount of torque is transmitted to the accessory drive system  62 . This provides an option in a gas turbine engine starting cycle, to partially de-clutch the magnetic fluid clutch  26  in order to provide a reduced amount of torque to the accessory devices when it is required. 
     In one embodiment of the present invention the entire torque generation capacity of the starter motor  12  can be used to start the engine rotor  22  from its stationary state at the very beginning of the engine starting cycle by completely declutching the magnetic fluid clutch  26 . After a moment, the magnetic fluid clutch  26  can be partially declutched to allow only a reduced torque level to be transmitted so that fuel pumps and oil pumps for example, can work in a partial load condition to provide a small percentage of the full fuel and oil supply to the combustor and bearings of the engine during the gas turbine engine starting cycle. When the gas turbine engine operates in its self-sustaining condition and generates torque to drive the accessory devices, the magnetic fluid clutch  26  is completely clutched so that the accessory devices, including the fuel and ail pumps work in their full load conditions. 
     FIGS. 2 and 3 illustrate another embodiment of the present invention in which parts similar to those in FIG. 1 are indicated by similar numerals and will not be redundantly described. In this embodiment of the present invention, a magnetic fluid clutch  64  which has a configuration different from that of the magnetic fluid clutch  26  in FIG. 1, is used in the gas turbine engine starting system  10 ′ as an alternative to the magnetic fluid clutch  26  used in the gas turbine engine starting system  10  illustrated in FIG.  1 . 
     The magnetic fluid clutch  64  includes a rotating shaft  66  rotatably supported by bearings  68  in a gear box  70 , and a cylindrical casing  72  rotatably surrounding the rotating shaft  66 . The casing  72  is also rotatably supported by bearings  68  in the gear box  70 . The rotatable casing  72  contains the magnetic fluid medium (not indicated) therein. The torque input end of the rotating shaft  66  is coupled to the torque output shaft  14  of the starter motor  12  and the torque output end of the rotating shaft  66  is connected to gear  16  which engages gear  18  on the rotating shaft  20  which is itself coupled to the rotor  22  of the gas turbine engine (not shown). The torque output gear  56  is affixed to the external periphery of the rotatable casing  72  and engages the gear  58  in the accessory gear box  70 . The accessory gear box  70  supports the starter motor  12 , and houses the magnetic fluid clutch  64 , and gear trains forming the drive system  62  of the accessory devices, and can be in a configuration different from that shown in FIG.  3 . Rotating members  74  and  76  are provided to the respective rotatable casing  72  and the rotating shaft  66 , respectively. The rotating members  74 ,  76  are both immersed in the magnetic fluid medium within the rotatable casing  72  in order to increase the frictional contact of the respective rotatable casing  72  and the rotating shaft  66  with the magnetic fluid medium. For example, the rotating member  74  can comprise a plurality of axially extending blades attached to the interior surface of the rotatable casing  72  and circumferentially spaced apart from one another. Similarly, the rotating member  76  can also comprise a plurality of axially extending blades attached to the rotating shaft  66  and circumferentially spaced apart from one another. The rotating members  74 ,  76  are radially spaced apart to prevent interference when they rotate respectively. 
     A pair of coils  78  which are shown with broken lines as a general schematic illustration, are provided to apply an adjustable magnetic field to the magnetic fluid medium within the rotatable casing  72 , and are controlled by the electronic controller  40 . It is understood that the electric coil  78  is wound around core materials (not shown) and can be positioned either inside or outside of the casing  72 . The coils  78  as shown in FIG. 2 are placed outside and at opposite ends of the rotatable casing  72 , which allows the coils  78  to be supported on a stationary structure of the gear box  70 . When the coils  78  are disposed within the rotatable casing  72 , and are supported by and rotated together with either the rotatable casing  72  or the rotating shaft  66 , special electrical connectors must be provided for electrical connection between the rotating coils  78  and the electronic controller  40 . 
     In operation, the torque transmission between the starter motor  12  and the rotor  22  of the gas turbine engine is ensured by the mechanical linkage of the torque output shaft  14  of the starter motor  12 , the rotating shaft  66  of the magnetic fluid clutch  64 , the gears  16 ,  18  and the rotating shaft  20 . Although torque transmission between the starter motor  12  and the rotor  22  of the gas turbine engine is conducted through the rotating shaft  66  of the magnetic fluid clutch  64 , the clutch operation does not affect such torque transmission. However, torque transmitted from the starter motor  12  to the drive system  62  associated with the accessory gear box  70  can be controlled through the clutch operation. During the gas turbine engine starting cycle, the electronic controller  40  ensures the steady-state liquid phase of the magnetic fluid medium in the rotatable casing  72  in order to declutch the magnetic fluid clutch  64  so that the rotatable casing  72  does not rotate in response to the rotation of the rotating shaft  66 . Thus, torque transmission from the starter motor  12  to the drive system  62  associated with the accessory gear box  70  is terminated. Alternately, if a partial workload for the drive system  62  associated with the accessory gear box  70  is required, the electronic controller  40  can change the viscosity of the magnetic fluid medium in the rotatable casing  72  to a selected one of various intermediate levels between the steady-state liquid phase and the steady-state quasi-solid phase in order to partially declutch the magnetic fluid clutch  64  so that torque can be transmitted from the starter motor  12  to the drive system  62  of the various accessory devices, but the amount of transmitted torque is reduced in contrast to that required for a full load of the accessory devices. 
     When the gas turbine engine starting cycle is completed and the gas turbine engine is operating under a self-sustaining condition, the electronic controller  40  completely clutches the magnetic fluid clutch  64  by increasing current through the coils  78  to generate and maintain a steady-state quasi-solid phase of the magnetic fluid contained within the rotatable casing  72  so that the rotatable casing  72  rotates in response to the rotation of the rotating shaft  66  which is now driven by the rotor  22  of the gas turbine engine through the rotating shaft  20  and the gears  16 ,  18 . Thus, torque is transmitted from the rotor  22  of the gas turbine engine to the various accessory devices which are operated under full workload conditions during the gas turbine engine operation. 
     In this embodiment of the present invention, the magnetic fluid clutch  64  couples the respective starter motor  12  and the rotor  22  of the gas turbine engine at the opposite ends of the rotating shaft  66  and couples the drive system  62  of the accessory devices at the middle of the magnetic fluid clutch  64  through the torque output gear  56 . This configuration provides convenience by allowing the starter motor  12  to be disposed along with the magnetic fluid clutch  64 , within the accessory gear box  70 . Gear  16  is conveniently disposed within or outside of the accessory gear box  70  for connection to the rotor  22  of the gas turbine engine. 
     The magnetic fluid clutches  26  and  64  described with reference to FIGS. 1 and 2 are used as examples to illustrate the present invention. Magnetic fluid clutches having configurations different from those of the magnetic fluid clutches  26 ,  64  illustrated in FIGS. 1 and 2 can also be used to implement the system according to the present invention. As a matter of fact, any type of clutch which does not use magnetic fluid but is controllable by a controller between a clutching state and a declutching state can be used to implement the gas turbine engine starting system according to this invention. 
     Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.