Abstract:
An electromagnetically variable transmission includes an outer rotor and an inner rotor. The inner rotor is independently rotatable within a center aperture of the outer rotor. The outer rotor is independently rotatable about the inner rotor. One of the rotors has a plurality of permanent magnets configured in pairs and facing an air gap disposed between the outer rotor and the inner rotor. The other rotor has a plurality of slots spaced about a magnetically permeable core having embedded windings. The outer inner rotors are simultaneously rotatable in one direction. In response to rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the permanent magnet pairs, the air gap, the outer rotor core and the inner rotor portion core, to induce electrical power in the windings, which transfers power between the inner rotor portion and the outer rotor portion.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a method and apparatus for transferring power between rotating shafts of an engine, and more specifically to an Electromagnetic Variable Transmission (EVT) for transferring torque and power directly from one rotating shaft with operating at one speed to another rotating shaft operating at a different speed than the first shaft. 
       BACKGROUND OF THE INVENTION 
       [0002]    A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power. 
         [0003]    The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers. 
         [0004]    It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power. 
         [0005]    Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly. 
         [0006]    Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than rely solely on the HP engine spool to drive the electrical generators. The LP engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. The boost cavity of gas turbine engines has available space that is capable of housing an inside out electric generator, however, the boost section rotates at the speed of the LP engine spool. 
         [0007]    However, extracting this additional mechanical power from an engine when it is operating at relatively low power levels (e.g., at or near idle descending from altitude, low power for taxi, etc.) may lead to reduced engine operability. Traditionally, this power is extracted from the high-pressure (HP) engine spool. Its relatively high operating speed makes it an ideal source for mechanical power to drive electrical generators that are attached to the engine. However, it is desirable at times to increase the amount of power that is available on this spool, by transferring torque and power to it via some other means. 
         [0008]    Another source of power within the engine is the low-pressure (LP) spool, which typically operates at speeds much slower than the HP spool, and over a relatively wider speed range. Tapping this low-speed mechanical power source without transformation result in impractically large generators. Many solutions to this transformation have been proposed, including various types of conventional transmissions, mechanical gearing, and electromechanical configurations. One solution is a turbine engine that utilizes a third, intermediate-pressure (IP) spool to drive a generator independently. However, this third spool is also required at times to couple to the HP spool. The means used to couple the IP and HP spools are mechanical clutch or viscous-type coupling mechanisms. 
         [0009]    U.S. Pat. No. 6,895,741, issued May 24, 2005, and entitled “Differential Geared Turbine Engine with Torque Modulation Capacity”, discloses a mechanically geared engine having three shafts. The fan, compressor, and turbine shafts are mechanically coupled by applying additional epicyclic gear arrangements. The effective gear ratio is variable through the use of electromagnetic machines and power conversion equipment. 
         [0010]    Unlike the conventional electrical machine having a rotor or rotating portion, and a stator or stationary portion, the present invention includes two rotating portions. Further, in the conventional electrical machine, power is converted either from electrical to mechanical or from mechanical to electrical. By contrast, the present invention is used to transfer mechanical power from one rotating shaft to another without any electrical power output or input. This is also a major distinction between the present invention and previous variable transmissions. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to an electromagnetically variable transmission for transferring power between a pair of independently rotating shafts. The electromagnetically variable transmission includes a hollow cylindrical outer rotor portion and a hollow cylindrical inner rotor portion, the inner rotor portion being disposed within a center aperture of the outer rotor portion and independently rotatable within the outer rotor portion. The outer rotor portion is independently rotatable circumferentially about the inner rotor portion. A first one of the outer rotor portion and the inner rotor portion has a plurality of permanent magnets pairs spaced about a first surface. The magnets are configured in pairs and facing an air gap. The air gap is disposed between the outer rotor portion and the inner rotor portion. The other one of the outer and inner rotor portions has a plurality of slots spaced about a magnetically permeable core portion. Some of the slots have windings embedded therein. The outer rotor portion and the inner rotor portion are simultaneously rotatable in one direction. In response to co-rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the plurality of permanent magnet pairs, the air gap and the inner rotor portion core. The magnetic flux path induces electrical power in the windings and causes mechanical power to be transferred between the inner rotor portion and the outer rotor portion. 
         [0012]    In another aspect, the present invention is directed to a gas turbine engine. The gas turbine engine includes at least one compressor, a combustor, a high pressure turbine and a low pressure turbines arranged in serial flow communication and disposed about a longitudinal shaft of the engine within an annular outer casing. The compressor is driven by the high pressure and low pressure turbines and compressor air during operation. An electrical generator is disposed within the annular outer casing; and an electromagnetically variable transmission is provided for transferring power between a pair of independently rotating shafts, one of the independent rotating shafts being attached to the HP turbine, and the other independently rotating shaft being attached to the LP turbine. The electromagnetically variable transmission includes a hollow cylindrical outer rotor portion and a hollow cylindrical inner rotor portion, the inner rotor portion being disposed within a center aperture of the outer rotor portion and independently rotatable within the outer rotor portion. The outer rotor portion is independently rotatable circumferentially about the inner rotor portion. A first one of the outer rotor portion and the inner rotor portion has a plurality of permanent magnets pairs spaced about a first surface. The magnets are configured in pairs and facing an air gap. The air gap is disposed between the outer rotor portion and the inner rotor portion. The other one of the outer and inner rotor portions has a plurality of slots spaced about a magnetically permeable core portion. Some of the slots have windings embedded therein. The outer rotor portion and the inner rotor portion are simultaneously rotatable in one direction. In response to co-rotation of the outer rotor portion and the inner rotor portion, a magnetic flux path is generated between the plurality of permanent magnet pairs, the air gap, the outer rotor core and the inner rotor portion core. The magnetic flux path induces electrical power in the windings and causes mechanical power to be transferred between the inner rotor portion and the outer rotor portion. 
         [0013]    An advantage of the present invention is torque transfer between concurrently rotating shafts is achieved through a rotating electromagnetic field without any mechanical connection between the two shafts. Induced field current in the winding is all that is required to generate electromagnetic fields in the air gap to interact with electromagnetic fields driven by the permanent magnets on the other rotor to transfer torque and power from the PM rotor, on which the permanent magnets are fixed to, to the induction rotor, the rotor with windings. Since no electric power flow occurs into or out of the EVT, there is no requirement for a power converter and associated control that are typically provided in conventional power transfer devices, e.g. electrical motors and generators. 
         [0014]    Another advantage of the present invention is increased fuel efficiency, reliability and fault tolerance. 
         [0015]    A further advantage of the present invention is the ability to transfer power from low speed LP turbine shaft to the high speed turbine shaft, with a variable speed ratio for transferring power over the entire speed range of the engine. Using electromagnetic techniques mechanical power is transferred without creating a mechanical linkage between the LP turbine shaft and the HP turbine shaft. Also there is no audible noise related to a mechanical gear due to its absence. 
         [0016]    Yet another advantage of the present invention is that an external electrical power source is not required, and the control circuit for the internally-generated field currents is uncomplicated. 
         [0017]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional schematic view of one embodiment of an electromagnetically variable transmission of the present invention. 
           [0019]      FIG. 1A  is a cross-sectional schematic view of one embodiment of an electromagnetically variable transmission of the present invention. 
           [0020]      FIGS. 2-5  are various interconnection diagrams for the rotor windings, which interconnections can be used in the configurations of  FIG. 1  (inner rotor windings) and  FIG. 1A  (outer rotor windings). 
           [0021]      FIG. 6  is a schematic circuit diagram for a torque control circuit using silicon-controlled rectifiers (SCRs). 
           [0022]      FIG. 6A  is a schematic circuit diagram for a torque control circuit using insulated-gate bipolar transistor (IGBT) units in reverse series. 
           [0023]      FIG. 7  is a schematic circuit diagram for the auxiliary control system. 
           [0024]      FIG. 8  is an alternate embodiment of the control circuit for torque control. 
           [0025]      FIG. 9  is an alternate embodiment of the control circuit option for torque control. 
           [0026]      FIG. 10  is a preferred embodiment of the present invention corresponding to the EVT arrangement of  FIG. 1 . 
           [0027]      FIG. 11  is an alternate embodiment of the present invention corresponding to the EVT arrangement of  FIG. 1A   
           [0028]      FIG. 12  is a schematic diagram of a PM induction EVT of the present invention in an aircraft engine. 
       
    
    
       [0029]    Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring to  FIG. 1 , an electromagnetically variable transmission (EVT)  10  includes two rotating components, an inner rotor  12  and an outer rotor  14 . Both the inner rotor  12  and the outer rotor  14  rotate in the same direction around a common axis  16 . The outer rotor  14  has multiple permanent magnet pole pairs  18  facing the outer surface  34  of the inner rotor  12 . The magnets of the pole-pairs  18  are oriented in alternating fashion, such that one magnet of the pair has its north pole directed radially outwards and the adjacent magnet has its south pole directed radially inwards. An optional pole cap  24  may be attached on the top of each magnet segment  22  to reduce losses induced in the magnets due to flux slot harmonics inside the magnets  22  when there is a large difference between the rotational velocity of the inner rotor  12  and the outer rotor  14 . The pole caps  24  may be laminated stack, soft magnetic composite material, or other magnetically permeable material suitable to form a magnetic path. Claps  26  are positioned between the magnets  22  to secure the magnets  22  and pole caps  24  to the solid rotor core  28 . The rotor core  28  is preferably made of solid steel or a laminated stack of steel plates. The rotor core  28  is similar in construction to a permanent magnet (PM) rotor in an inside-out PM electrical machine. 
         [0031]    The outer rotor  14  and the inner rotor  12  are separated by an air gap  30 . The inner rotor  12  is constructed of steel laminations and windings similar to a conventional induction machine rotor. Slots  32  are located on the outer surface  34  of the inner rotor lamination  36 . The slots  32  may be open, half-closed, or closed. Multiple-phase windings (not shown) are disposed within the slots  32 . The multiple-phase windings form multiple pole pairs of the inner rotor  12 . The number of pole pairs on the outer rotor  14  is the same as the number of pole pairs of the inner rotor  12 . 
         [0032]    Referring to  FIGS. 2 through 5 , several exemplary interconnections for the rotor windings  38  [either the inner rotor winding in the embodiment of  FIG. 1 , or the outer rotor winding in the embodiment of FIG.  1 A.] are shown.  FIG. 2  shows the rotor windings  38  as three single-phase connections with switches  40  wired in series with each phase winding  38 . The rotor windings in  FIG. 3  are configured in a wye connection with switches  40  in two of the three legs of the wye connection, which is all that are required to switch off the current flowing in the wye circuit, although another switch  40  could be connected in the third phase. In  FIG. 4 , a delta-connected configuration is used for the rotor windings  38 , and a single switch  40  is used to switch off current flowing in the delta circuit arrangement. In  FIG. 5 , there are multiple parallel circuits shown, which are multiple parallel combinations of the delta circuit  42  and the wye-connections  44  of  FIGS. 3 and 4 . Note that other interconnection configurations may also be employed within the scope of the present invention, as the configurations shown in  FIGS. 2 through 5  are intended as examples and not to limit the various configurations that will be readily understood by those persons skilled in the art. Although the circuits shown in  FIGS. 2 through 5  are preferably configured as 3-phase windings, any number of electrical phases can be used. The switches  40  are preferably a pair of silicon-controlled rectifiers (SCRs) connected in reverse parallel, or any other power devices having current control capability in both directions. 
         [0033]      FIG. 6  shows an exemplary control circuit for controlling the torque and power transferred between the inner rotor  12  and the outer rotor  14 . The circuit of  FIG. 6  controls how much torque and power is transferred and when to transfer torque and power between the two rotors to satisfy the engine power requirement. An exemplary rotor coil  46  could represent a single coil  38 , as in  FIG. 2 , or the rotor coil  46  could represent multiple coils  38 . e.g. three series connected coils  38 , as in  FIG. 4 . Also, switch  40  in  FIGS. 2 through 5  could be either switch  40  in  FIG. 6  or the switch  40  in  FIG. 6A . The V, L, and R are a net effect of the coil(s) in the circuits embodied in  FIGS. 2 through 5 . An exemplary rotor coil  38  is represented as an inductance  46   a  (L) and resistance  46   b  (R). A voltage V is induced in the winding  38  due to the variation of flux linked by the winding. The flux is driven by the magnets on the opposite rotor, while the flux variation is due to the relative speed of the two rotors. A pair of power devices  40   a  and  40   b  is arranged in reverse parallel. Preferably the power devices are silicon-controlled rectifiers (SCRs). Switch  40  can also consist of two insulated-gate bipolar transistor (IGBT) units  41   a ,  41   b  in  FIG. 6A . The IGBT units  41   a ,  41   b  are connected in reverse series. Each of the IGBT units  41   a ,  41   b  consists of at least one IGBT and at least one diode that is in reverse parallel with the IGBT(s). 
         [0034]    Referring to  FIG. 7 , power for the torque control circuit in  FIG. 6  may be provided through a power supply  50  that is energized by an auxiliary coil  52  on the rotor  14  driven by the HP spool  54 . The whole control system  48  may be located on the HP spool  54 , and powered by the HP spool  54 , or alternately, may be powered by the LP spool. The control circuit  48  controls the current in the windings  38 . Control circuit  48  must be located on the same induction rotor on which the windings  38  are located, to avoid wire connections between two rotating parts. For the same reason, the control circuit  48  must be powered by the same induction rotor. The induction rotor could be configured on either the inside or outside, as shown in  FIGS. 1 and 1A . A control signal indicated by a bi-directional arrow  56  may be transmitted wirelessly to the control unit  58  by an external stationary control unit  60 . 
         [0035]    Referring next to  FIG. 8 , there is another exemplary control circuit for torque control. In this embodiment a switch  62  controlled by centrifugal force of the rotors  12  and  14   a  controls the speed at which to transfer torque between LP and HP spools. Switch  62  must be on the same induction rotor as the windings  38 , and the induction rotor could be located on either the inside or the outside as shown in  FIGS. 1  and  1 A. Rotor coil  46  has a characteristic inductance L and resistance R, and an induced voltage V. Centrifugal switch  62  is closed when the rotor  12  or  14   a  is rotating at low speed and opens when the rotor  12  or  14   a  exceeds a predetermined speed, in response to the centrifugal force applied by the rotational speed. Optionally, if necessary, a resistor having a resistance R NTC  with negative temperature coefficient may be included. The resistor R NTC  limits the current pulse that occurs when the centrifugal switch  62  closes. Transfer of torque occurs between the LP spool  64  and the HP spool  54 , when the centrifugal switch  62  is in the closed position, and the HP and LP spools  64 ,  54  are disengaged when the centrifugal switch is open. 
         [0036]    In  FIG. 9 , there is another exemplary control circuit option for torque control. In the configuration shown in  FIG. 9 , a control switch is not required. Torque transfer is controlled by the slip frequency, or the speed difference between two shafts. During aircraft cruise or taking off, the HP and LP spools rotate at higher speed and the speed difference between two spools or between the two rotors of the EVT is smaller. Therefore the slip frequency or the frequency of the current induced in the windings is lower. During aircraft landing or idle descending, the LP spool speed is reduced more than HP spool speed change and the slip frequency is larger. Based on induction machine principle, there will be more torque and power transferred between the two EVT rotors at a larger slip frequency than at a small slip frequency. The gear ratios of gearboxes  66  and  68  in  FIG. 10  are selected such that the desired slip frequency is achieved at both the high and low speed range. The characteristic inductance L and resistance R of the rotor coil  46  are designed, e.g. by adjusting turns and conductor cross-section area, using skin effect for high slip frequency, or by selecting material in such a way that the desired torque can be transferred from LP spool to HP spool when it is needed at or near idle descent, while the torque transferring is minimized when it is not required during cruise and taking off. 
         [0037]    According to another embodiment of the present invention, there are one or more auxiliary coils  52  in the rotor slots shown in  FIG. 1 . The induced electrical power in the coil or coils  52  can be used to supply power for the control circuit of the switch or switches in the circuits that are located on the rotor. 
         [0038]    The SCR switches  40 , control unit  58  and power supply  50  are preferably mounted on the same rotor as the main rotor windings  38 , so slip rings are not required to electrically connect a stationary portion to the rotating windings  38 . Signals required to control the SCR switches  40  can be transmitted wirelessly to the inner rotor  12  (See  FIG. 7 ). 
         [0039]      FIG. 1A  shows an alternate embodiment of the present invention. In this configuration, the rotors are arranged as the reverse of FIG.  1 —the outer rotor  14   a  has slots  32   a  with windings is now outside, and the inner rotor  12   a  has permanent magnet PM poles  22   a.    
         [0040]    In either of the configurations in  FIG. 1  or  FIG. 1A , the magnetic flux is driven by the permanent magnets and link the rotor winding  38  across the air gap  30 , as with conventional PM machines. When the PM rotor  14 ,  12   a  rotates, a rotating flux field (not shown) is induced in the air gap  30 . Based on conventional induction machine principles, voltage and current is induced in the coils when the PM rotor  14  or  12   a  is rotating at a different speed than the induction rotor  12  or  14   a . When the PM rotor  14  or  12   a  is rotating faster than the induction rotor  12  or  14   a , torque is transferred from the PM rotor  14  or  12   a  to the induction rotor  12  or  14   a . When the PM rotor  14  or  12   a  is rotating lower than the induction rotor  12  or  14   a , torque is transferred from the induction rotor  12  or  14   a  to the PM rotor  14  or  12   a.    
         [0041]      FIG. 10  shows a preferred embodiment of the present invention corresponding to the EVT arrangement of  FIG. 1 , in which an EVT  10  includes the outer PM-type rotor  14  connected to the LP spool  64 , and the inner induction-type rotor  12  connected to the HP spool  54 . The interconnected windings of the inner rotor  12  are indicated by loops  37 . The LP spool speed N 4  is stepped up by gearbox 1:Y to speed N 3 . In order to transfer torque from the LP spool  64  to the HP spool  54 , the rotational speed N 3  of the outer or PM rotor  14  has to be higher than the speed N 2  of the inner or induction rotor  12 . A first gearbox  68  having a gear ratio of 1:X is used to couple the HP spool  54  to the inner rotor  12 , and a second gearbox  66  having a gear ration of 1:Y is used to couple the LP spool  64  to the outer rotor  14 . The gearboxes  66 ,  68  are used to match the speed N 1  of the HP spool  54  and the speed N 4  of the LP spool  64  to correspond to the desired EVT rotor speeds, N 2  and N 3 , respectively. Depending on the engine spool operating speeds and EVT rotor speeds, one of the gearboxes  66 ,  68  may be omitted. Outer rotor speed N 3  is greater than the speed N 2  of the inner rotor  12  so that torque and power will be transferred to the shaft  55  at speed N 2  based on the induction principle. A variable gear ratio may be employed on one or both of the gearboxes  66 ,  68 . In the configuration of  FIG. 1 , with LP spool rotating at speed N 4  and the HP spool rotating at speed N 1 , the speed conversions are related by Equation 1: 
         [0000]        N 1 *X=N 2 &lt;N 3 =N 4 /Y   Equation 1       where N 4 &lt;N 1           
         [0043]      FIG. 11  shows an alternate embodiment of the present invention corresponding to the EVT arrangement of  FIG. 1A , in which an EVT  10  is connected to the LP spool  64  and the HP spool  54  of an aircraft engine through gearboxes  66 ,  68 . In order to transfer torque from the LP spool  64  to the HP spool  54 , the rotational speed N 3  of the outer induction-type rotor  14   a  (with winding interconnections indicated by loops  37 ) has to be lower than the speed N 2  of the inner PM-type rotor  12   a . A first gearbox  68  having a gear ratio of 1:X is used to couple the LP spool  64  to the inner rotor  12   a , and a second gearbox  66  having a gear ratio of 1:Y is used to couple the HP spool  54  to the outer rotor  14   a . The gearbox  66  is used to match the outer rotor speed N 3  of the EVT  10  to speed N 4  of the HP spool  54 . Depending on the engine spool operating speeds and EVT rotor speeds, one of the gearboxes  66 ,  68  may be omitted. In the configuration of  FIG. 11 , with HP spool rotating at N 4  and the LP spool rotating at N 1 , the speed conversions are related by Equation 2: 
         [0000]        N 1 *X=N 2 &gt;N 3 =N 4 /Y   Equation 2       wherein N 4 &gt;N 1 ; and   the gear ratio X&gt;=1         
       Optionally, a variable gear ratio may be applied. 
       [0046]    As an option, either of the gearboxes  66 ,  68  described above could have variable gear ratio to reduce the speed range of two rotors in the EVT, therefore reducing the maximum speed and associated mechanical stresses when either of the LP spool or HP spool speed range is too great. 
         [0047]    Referring next to  FIG. 12 , an exemplary arrangement of the PM induction EVT in an aircraft engine  110  has a core engine  138  including in serial, axial flow relationship, a low pressure compressor or booster compressor  120 , a high pressure compressor  114 , a combustor or burner  124 , a high pressure turbine  116  and a low pressure turbine  118 . Core engine  138  is downstream from an inlet  122  and a fan  112 . Fan  112  is in serial, axial flow relationship with core engine  138  and a bypass duct and a bypass nozzle (not shown). Fan  112 , compressor  114 , and low pressure turbine  118  are coupled by a first shaft  64 , and compressor  114  and turbine  116  are coupled with a second shaft  54 . A portion of airflow entering inlet  122  is channeled through the bypass duct and exhausted through bypass nozzle, and remaining airflow passes through core engine  138  and is exhausted through a core engine nozzle. The EVT  10  is coupled to LP shaft or spool  64  through gearbox  66  and shaft  57 . The output shaft  57  of the EVT  10  is connected to the HP shaft or spool  54  through gearbox  68 . A pair of starter/generators  130  is coupled to HP spool  54  through a primary gearbox, to receive power from or to provide power to the HP spool, depending whether the pair of starter/generators  130  is operated as starters or generators.  FIG. 12  is just one example out of many possible starter/generator  130  and primary gearbox  132  configurations that may be used with the present invention for sharing torque between the LP shaft  64  and the HP shaft  54 , as will be readily appreciated by those skilled in the art. Also, the EVT  10  may be located either internally or externally of the engine envelope. 
         [0048]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.