TECHNICAL FIELD 
       [0001]    The present disclosure relates to a propulsion system for a vehicle and more particularly to a continuously variable transmission using a dual rotor switched reluctance machine. 
       BACKGROUND 
       [0002]    Internal combustion engines can be used to power a generator and create electric energy which is stored and in turn drives an electric motor to propel a vehicle. This technique has been used effectively in vehicles such as locomotives and hybrid automobiles. Typically this process involves a generator that produces AC power. The AC power can be converted to DC power using, in some cases using a rectifier, but more popularly uses an AC-DC converter. The DC power can then be stored and a DC-AC inverter can be used to supply 3-phase power to an AC motor used to drive the vehicle. 
         [0003]    This arrangement, while relatively efficient, has several drawbacks. One involves the cost of the high power components used in the AC-DC converter and the DC-AC inverter. Another is the loss of inertia provided by the flywheel of a standard engine-clutch-transmission drivetrain that allows smoother performance in the event of momentary engine power changes. 
         [0004]    WO0034066 (“the &#39;066 patent”) discloses a dual rotor machine with a stator surrounding an output rotor and an input rotor surrounded by the output rotor. The stator and input rotor each have windings, the output rotor has embedded permanent magnets that interact with fields generated at the stator and input rotor. The input rotor requires slip rings to carry current to the input rotor windings. The permanent magnets of the &#39;066 patent impose a substantial cost penalty on the system. The slip rings of the &#39;066 patent also have a cost impact due to the elaborate construction requirements and also create a reliability weakness at the contacts between the slip rings and the shaft. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    In a first aspect, an energy conversion machine has a stator having a cylindrical shape with an inner circumference and an outer circumference. The stator is fixedly mounted and has poles extending radially between the inner circumference and the outer circumference. Each pole has an electrical winding. The energy conversion machine also includes an input rotor rotatably mounted adjacent to one circumference of the stator, such as an outer circumference of the stator. The input rotor is free of windings and may be free of permanent magnets other than magnets used for position sensing. The energy conversion machine can also include an output rotor rotatably mounted adjacent to the other circumference of the stator, for example, an inner circumference. The output rotor is also be free of windings and permanent magnets. The energy conversion machine also includes a controller that selectively energizes the stator electrical windings to transfer torque developed between the input rotor and the stator to the output rotor. 
         [0006]    In another aspect, a method of converting energy from a power source to a mechanical load can include providing a switched reluctance machine with a first rotor and a second rotor, each rotor overlapping a stator, the stator having a cylindrical shape and fixedly mounted with respect to the first rotor and the second rotor, receiving power from the power source at the first rotor, and energizing a first stator pole during a stoke angle of the second rotor. Concurrent with the energizing of the first stator pole, the method can include energizing a second stator pole once for each pole of the first rotor that passes the second stator pole, the second stator pole adjacent to the first stator pole and transmitting torque to the mechanical load via the second rotor. 
         [0007]    In yet another aspect, a system for propelling a vehicle can include an engine and an energy conversion machine having an input rotor coupled to the engine and an output rotor. The energy conversion machine can also include a stator having a cylindrical shape with an inner circumference and an outer circumference, the stator fixedly mounted and having stator poles extending between the inner circumference and the outer circumference, with each pole having an electrical winding. The input rotor can be rotatably mounted adjacent to one circumference of the stator, the input rotor free of windings and permanent magnets other than position sensing magnets. The output rotor can be rotatably mounted adjacent to the other circumference of the stator, the output rotor being free of windings and permanent magnets other than position sensing magnets. The energy conversion machine can also include a controller that selectively energizes the stator electrical windings to transfer torque developed between the input rotor and the stator to the output rotor. The system can include a driveshaft coupled to the output rotor and a propulsion device that converts torque from the output rotor received via the driveshaft to propel the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an illustration of a simplified and exemplary prior art vehicle using a separate generator and motor; 
           [0009]      FIG. 2  is an illustration of a simplified and exemplary vehicle in accordance with the current disclosure; 
           [0010]      FIG. 3  is a cutaway view of a dual rotor machine; 
           [0011]      FIG. 4  is a cross-section view of the dual rotor machine; 
           [0012]      FIG. 5  is a cross-section view of the dual rotor machine depicting activation of a stator pole; 
           [0013]      FIG. 6  is a cross-section view of the dual rotor machine depicting activation of adjacent stator poles; 
           [0014]      FIG. 7  is a schematic diagram of an embodiment of control electronics; 
           [0015]      FIG. 8  is a flow chart of a method of operating the dual rotor machine; 
           [0016]      FIG. 9  is a prior art control diagram for a switched reluctance motor; 
           [0017]      FIG. 10  is a control diagram for the dual rotor machine; 
           [0018]      FIGS. 11   a - 11   b  are input and output torque diagrams for the dual rotor machine in Quadrant 1 with an output angle of 5 degrees; 
           [0019]      FIGS. 12   a - 12   b  are instantaneous torque input and output charts for Quadrant 1 operation of the dual rotor machine; 
           [0020]      FIGS. 13   a - 13   b  are input and output torque diagrams for the dual rotor machine in Quadrant 4 with an output angle of 5 degrees; 
           [0021]      FIGS. 14   a - 14   b  are instantaneous torque input and output charts for Quadrant 4 operation of the dual rotor machine; and 
           [0022]      FIG. 15  is a definition of operating regimes a vehicle powered by a dual rotor machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  illustrates a vehicle  140  powered by an engine  142 , a gear box  144  and a driveshaft  146 , similar to the vehicle shown in  FIG. 1 . In some embodiments, the gear box  144  may not be used. A dual rotor machine  148  can include an input rotor  150  coupled to the drive shaft  146 , a stator  152 , an output rotor  154 , and control electronics  156  used to time energizing pole windings in the stator  152 . The output rotor  154  can be coupled to a driveshaft  158 , and in prior-art fashion transmit torque via axle  160  and propulsion device  162 . 
         [0024]      FIG. 2  is a cutaway view of an embodiment of a dual rotor machine  148 , also known as a dual rotor switched reluctance machine. The dual rotor machine  148  can include, as discussed above, an input rotor  150 , an output rotor  154 , and a stator  152 . The input rotor  150  can be coupled to the driveshaft  146  so as to receive power from or return power to the engine  142 . The output rotor  154  can be coupled to the driveshaft  158 . Bearings, such as bearing  170 , may be used to support rotating devices. For example, the driveshaft  146  may be supported by mounts  178  and associated bearings. 
         [0025]    The stator  152  can be mounted by supports  180  and provide support to both the input rotor  150  and the output rotor  154  via respective bearings. As will be appreciated, numerous variations exist for providing the mechanical mounting between the stator and the two rotors. 
         [0026]    As will be discussed in more detail below, the stator  152  includes numerous poles  172  having windings  174  that can be coupled to the control electronics  156  via leads  176 . The dual rotor machine  148  can also include an input rotor position sensor  182  and an output rotor position sensor  184 . The actual location and construction of the sensors  182  and  184  may vary in known fashion but can include optical or Hall Effect sensors and may be used by the control electronics  156  to determine the position of input rotor poles and output rotor poles with respect to stator poles. 
         [0027]      FIG. 3  is a cross-section view of the dual rotor machine  148  shown at view ‘X’ of  FIG. 3 . This view shows the input rotor  150 , the stator  152 , and the output rotor  154 . This view also shows a first stator pole  172  and corresponding windings  174  as well as a second stator pole  200  and its corresponding windings  202 . Also illustrated is one of a plurality of input rotor poles  204 . In this exemplary embodiment, the input rotor has 12 poles, the stator  152  has six poles, and the output rotor  154  has four poles. In common fashion, opposite stator poles are energized concurrently, so in the following discussions only the upper half of the dual rotor machine  148  are discussed. 
         [0028]    Other configurations of poles are also viable. Further, the input and output rotors may be reversed so that the input rotor is along an inner circumference of the stator  152  and the output rotor is mounted along an outer circumference of the stator  152 . These alternate configurations are enabled as long as the relationship between input rotor and output rotor timing with respect to the stator  152  discussed below is preserved. 
         [0029]      FIG. 4  and  FIG. 5  depict in simplified fashion operation of the dual rotor machine  148  at two points of time in an operating sequence. This operation is in so-called Quadrant 1, that is forward direction and accelerating. Other quadrants include Quadrant 2, forward operation/decelerating, Quadrant 3, reverse operation/accelerating, and Quadrant 4, reverse operation/decelerating. 
         [0030]      FIG. 4  illustrates the dual rotor machine  148  with a first stator  172  having its windings  174  energized. In this example, both rotors are rotating counterclockwise. As the windings  174  of the first stator  172  are energized, the outer, or input rotor pole  204  is pulled toward the stator pole  172 . The adjacent poles are inactive. In an embodiment this represents an input torque of about 2.0 KiloNewton-meters (KNm). Similarly, the output rotor  154  is pulled toward the stator  172  with a torque of about 1.6 KNm. 
         [0031]      FIG. 5  illustrates the dual rotor machine  148  slightly later in the cycle, denoted by the rotation of the input rotor  150  from reference A to reference B. the inner, or output rotor  154  has not significantly changed position. 
         [0032]    Stator pole  174  is energized so that the output pole of rotor  154  is still pulled toward the first stator pole  172 . Stator pole  200  is also energized. The output pole that is fully aligned with stator pole  200  has no net torque. However, because the stator poles  172  and  200  are energize, the breaking magnetic field connection between the input poles and the stator poles  172  and  200  increases the stored energy in their respective magnetic fields. This increased field at stator pole  172  has a direct impact on the torque of the output rotor and a neutral effect on the output rotor pole aligned with stator pole  200 . In both cases, electrical power can be returned to the capacitor  216 . In an exemplary embodiment, the output torque imparted to the output rotor  154  can be about 2.2 KNm while the energy returned to the dual rotor machine  148  by the input rotor  150  can be about −4.8 KNm. 
         [0033]    This effect is discussed in more detail below, but at a high level, energy from the magnetized input rotor pole builds the magnetic field at the stator pole as it breaks away from the respective stator poles and results in a net increase in energy at the dual rotor machine  148 . That energy is then transferred to the output rotor. 
         [0034]      FIG. 6  illustrates one embodiment of control electronics  156  suitable for use with the dual rotor machine  148  of  FIG. 3 . Recalling that opposite pairs of stator poles can be operated together, the control electronics includes drivers for three sets of stator pole windings depicted by inductors  174 ,  202 , and  218 . Each of the drive circuits are the same and include a low side drive transistor  208  a high side drive transistor  214  and a pair of diodes  210  and  212 . A capacitor  216  can be used to store electricity used to drive the stator pole windings and/or filter the DC ripple generated by the dual rotor machine  148 . A battery (not depicted) may be connected to the capacitor  216  to supplement the capacitor  216 , for example, during startup. 
         [0035]    A controller  206  receives position information from position sensors on the input rotor and the output rotor, or corresponding driveshafts. The controller  206  also includes output drivers for each of the paired transistors that drive the stator pole windings. The transistors  208  and  214  can be insulated gate bipolar transistors (IGBT) known for their high current capacity and fast switching speed. In operation, when both transistors  214  and  208  are turned on current flows through winding  174  (and it&#39;s paired pole) and builds up a magnetic field. When the transistors  208  and  214  are turned off at an appropriate point during the rotation of the respective rotors  150  and  154 , the collapsing magnetic field generates electric current that is transmitted via diodes  212  and  210  back to the capacitor  216 . 
         [0036]    Compared to a prior art implementation that uses separate AC-DC converters and DC-AC inverters, the current embodiment uses a single converter to receive excess electrical energy from the stator winding  174  as well as to deliver drive current to the stator  174 , improving both the cost and reliability of the system because of the decreased number of components. 
       INDUSTRIAL APPLICABILITY 
       [0037]    A dual rotor machine  148  can be employed in any application where power from an engine needs to be converted to drive a vehicle or other mechanical device. The dual rotor machine  148  is particularly well-suited to continuously variable transmission (CVT) applications where a diesel or gasoline engine  142  may be operated at a nearly constant speed to improve its efficiency and the CVT is responsible for drive speed and direction. 
         [0038]    Because the input rotor  150  and output rotor  154  share a common stator  152 , magnetic energy can be directly transmitted from the input rotor  150  to the output rotor  154  without externally storing electricity as an intermediate step. This eliminates the prior art the requirement that all magnetic energy be converted to AC electrical energy in a generator set, stored or filtered, and converted to DC electrical energy before being applied as magnetic energy in the motor portion of the set. 
         [0039]    Further, the use of a shared stator  152  eliminates at least one stator from the hardware associated with separate generator-motor sets, including the windings which are generally made from relatively expensive copper wires. Overall, the sharing of a stator, elimination of the DC-AC inverter, and elimination of the other associated mounting hardware and duplicate control electronics can result in a CVT that may be half the size and weight of a similar horsepower generator-motor set. 
         [0040]    Because neither the input rotor nor the output rotor requires windings or permanent magnets for operation, slip rings and expensive magnets can be eliminated, improving both cost and reliability over motor-generator sets or other prior art dual rotor devices. 
         [0041]      FIG. 7  is a flow chart of a method  220  of operating a dual rotor machine  148 . At block  222 , a switched reluctance machine can be provided with a first rotor  150  and a second rotor  154 , each rotor overlapping a common stator  152 . The stator  152  may have a cylindrical shape and be fixedly mounted with respect to the first rotor  150  and the second rotor  154 . The first rotor  150  may have an integer multiple of poles of both the stator  152  and the second rotor  154 . For example, the second rotor  154  can have four poles and the first rotor  150  can have 12 poles in a ratio of 3:1. The stator  152  can have 6 poles with a ratio of 2:1 input poles to stator poles. In an embodiment, The first rotor  150  can have 12 poles, the second rotor  154  can have four poles and the stator  152  can have six poles. Both the first rotor  150  and the second rotor  154  may be free of windings or permanent magnets other than position sensing magnets. 
         [0042]    The stator  152  can have poles, e.g., poles  172 ,  200 , extending from an inner circumference to an outer circumference of the cylindrical shape, each stator pole  172 ,  202  electrically energized with a corresponding, axially opposite stator pole. A vehicle associated with the dual rotor machine  148  may have a gearbox  144  between the power source  142  and the first rotor  150  that maintains a rotational speed of first rotor  150  above a rotational speed of second rotor  154  during operation. While not necessary, there may be advantages to maintaining this speed relationship when convenient so that input rotor poles always have multiple stator pole crossings compared to output rotor poles crossing of a stator pole. 
         [0043]    At a block  224 , power can be received from the power source at the first rotor  150 . For example, the power source may be a diesel or gasoline engine  142  and the first rotor  150  (or input rotor) can be turned by the engine either directly or via a gearbox  144 . 
         [0044]    At block  226 , a first stator pole  172  can be energized during a stoke angle of the second rotor  154 . A stoke angle is that range of angles of an output rotor pole  155  over which a particular stator pole can effectively cause motion in a desired direction. Output stoke angle can be calculated as 360 degrees/(output rotor poles*stator phases). In the illustrated embodiment, the six stator poles are paired so that there are 3 stator phases. Therefore, the output stoke angle of the embodiment can be calculated as 360 degrees/(4*3)=30 degrees. 
         [0045]    Referring briefly to  FIG. 8 , a prior art control diagram  240  for a 6/4 (6 stator poles and 4 rotor poles) switched reluctance motor is shown. The control diagram  240  shows a control signal  242  and corresponding control current  244  (Ia, Ib, Ic) for each stator pole pair. As illustrated, successive stator poles are energized for 30 degrees of mechanical rotation (Ro) of the output rotor  154 . 
         [0046]    Returning to  FIG. 7 , at a block  228 , concurrent with the energizing of the first stator pole, a second stator pole is energized once for each pole of the first rotor that passes the second stator pole. The second stator pole can be adjacent to the first stator pole. For forward and accelerating operation (Quadrant 1) the second stator pole energized can be adjacent to the first stator pole in a downstream direction of rotation of the first rotor. 
         [0047]    Turning briefly to  FIG. 9 , a control diagram  246  for the dual rotor machine  148  is shown. In order to create the desired motion in the forward (in this case, counterclockwise) direction, the stator poles with respect to the output rotor  150  are excited as shown in  FIG. 8 . In addition, the adjacent stator pole pair is excited once for each input pole that passes the adjacent stator pole. In the illustration, when the B stator is excited with current Ib, the C stator pole is excited once for each input rotor pole that passes the C stator pole. As discussed above, energizing the stator pole during the overlap of an input rotor pole causes energy from the input rotor  150  to build the magnetic field at the adjacent stator pole when the input pole breaks free of the energized stator pole. This transfers energy from the input rotor to the stator  152  and subsequently to the output rotor. In the illustrated embodiment, during the Ib output rotor phase, the Ic stator pole is pulsed during input rotor pole crossings. Similarly, during the Ia output rotor phase, the Ib stator pole is pulsed; and during the Ic output rotor phase, the Ia stator pole is pulsed. 
         [0048]    Returning to  FIG. 7 , at a block  230  torque from the output rotor (i.e, the second rotor) is transmitted to the mechanical load, such as a wheel or track that drives the vehicle. 
         [0049]      FIG. 14  illustrates one definition of operating regimes for a vehicle powered by a dual rotor machine  148  and will be referred to in the following discussion. Quadrant 1  340  is defined as forward and accelerating. Quadrant 2  342  is defined as reverse and braking. Quadrant 3 is defined as reverse and accelerating. Quadrant 4 is defined as forward and braking. As described for quadrants 1 and 2, each quadrant deals with either positive or negative torque between the engine  142  and the dual rotor machine  148  as well as positive or negative torque between the dual rotor machine  148  and the propulsion devices  162 . 
         [0050]      FIGS. 10   a  and  10   b  are input and output torque diagrams for the dual rotor machine in Quadrant 1 with a fixed output rotor angle of 5 degrees. In  FIG. 10   a , a torque diagram  250  shows input side torque over 25 degrees (5 degrees to 30 degrees) of input rotor angle. The torque diagram  250  shows a first torque curve  252  with only one stator pole (“B”) energized and a second torque curve  254  with the B stator pole and an adjacent stator pole (“C”) active. Referring to Table 1, below, a stator energizing scheme is summarized for Quadrant 1 operation. 
         [0000]    
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Forward and accelerating (Quadrant 1) 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0051]    The output rotor angle of 5 degrees is highlighted in the first column of Table 1 corresponding to the fixed output rotor angle illustrated in  FIGS. 10   a  and  10   b . Following the column down at 5 and 10 degrees, only the B stator is energized and the actual torque at the input will follow curve  252 . At 15 degrees, the C stator is also energized and the actual torque will follow line  254  from 15 degrees to 30 degrees. Solid reference lines  256  track the torque path. 
         [0052]    Referring briefly to  FIG. 11   a , input torque diagram  270  illustrates instantaneous input torque  272  over a range of output rotor angles, beyond the fixed angle of  FIG. 10   a . The average input torque  274  is negative, showing that power from the engine  142  is converted to torque at the dual rotor machine  148 . 
         [0053]    Turning to  FIG. 10   b , an output torque diagram  260  for the same fixed output rotor angle of 5 degrees shows output torque as a function of input rotor angle. A first torque curve  262  shows torque with only the B stator pole energized. The second torque curve  264  shows torque with both the B and C stator poles energized. Again referring to Table 1, the actual torque follows solid lines  266  as the B stator pole is energized from 5 to 10 degrees and the B and C stator poles are energized from 15 degrees to 30 degrees. 
         [0054]    Referring to  FIG. 11   b , chart  276  illustrates instantaneous output rotor torque  278  and average output rotor torque  280 . As can be seen, all the average output rotor torque values  280  are positive, indicating positive (in this case, forward) torque is being generated.  FIGS. 10   a ,  10   b ,  11   a , and  11   b  demonstrate operation where power from the engine  142  is captured at the dual rotor machine  148  and positive torque is generated at the output. 
         [0055]    While operation in each quadrant will not be discussed in detail, the following figures illustrate operation in Quadrant 2, reverse and braking. 
         [0056]      FIG. 12   a  shows another instance of the input torque diagram  250  of  FIG. 10   a , with B stator pole only curve  252  and B and C stator pole curve  254 . However, referring to Table 2, below, use of a different excitation pattern causes different torque profiles to be generated. From 5 degrees to 15 degrees, both the B and C stator poles are energized, from 20 to 25 degrees only the B stator pole is energized, and at 30 degrees, again both the B and C stator poles are energized. 
         [0000]    
       
         
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Reverse and Braking (Quadrant 4) 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0057]    Referring to  FIG. 13   a , a torque diagram  310  shows instantaneous torque  312  as a function of output rotor angle for Quadrant 4 operation. The average torque  314  is positive, meaning that torque is being returned to the engine in a form of engine braking. 
         [0058]    Similarly,  FIG. 12   b  shows another instance of the output rotor torque diagram  260  with B stator pole torque curve  262  and B and C stator pole torque curve  264  for an output rotor angle of 5 degrees. Using the energizing pattern highlighted in Table 2, the actual output torque follows solid lines  300  as the energizing scheme is implemented over the increasing input rotor angles. 
         [0059]      FIG. 13   b  shows a torque diagram  316  with instantaneous output torque  318  as a function of output rotor angle. The average power is signified by each solid bar  320 .  FIGS. 12   a ,  12   b ,  13   a , and  13   b  demonstrate operation where power from the propulsion device  162  (e.g., tires) is captured at the dual rotor machine while the vehicle is in reverse and braking. In turn, the dual rotor machine  148  transmits positive torque to the engine, causing the engine to attempt to increase in speed. 
         [0060]    Operation in Quadrants 3 and 4 are similarly embodied by selecting stator pole activation sequences to positive or negative input torque and positive or negative output torque. The variations in torque shown in  FIGS. 11   a ,  11   b  and  13   a ,  13   b  is called torque jitter and is a known side effect in switched reluctance motors and generators. The mass of the input rotor  150  and output rotor  154  provides a certain flywheel effect and can smooth torque jitter. In some embodiments, the moment of inertia of the input rotor  150  is greater than that of the output rotor, providing engine-side inertia while allowing the output rotor to be more responsive to changes in speed and/or direction.
A dual rotor switched reluctance machine with a fixed stator and separate input and output rotors on either side of the fixed stator is used to transmit power between a power source such as an gas engine and a mechanical drive unit such as wheels or tracks. A switched reluctance motor configuration, such as a 6/4 motor can be combined with a with a higher pole count switched reluctance generator to allow simultaneous operation of the dual rotor machine as both a generator and a motor. Because the fixed stator has the only set of windings the control electronics are greatly simplified over separate motor-generator configurations.