Patent Publication Number: US-2023135283-A1

Title: Drive unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2021-180365 filed Nov. 4, 2021. The entire contents of that application are incorporated by reference herein in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a drive unit. 
     BACKGROUND ART 
     Electric cars travel using an electric motor as a drive source. The electric cars move forward by forwardly rotating the electric motor and move backward by reversely rotating the electric motor. There has been proposed a type of electric car in which a torque converter is installed in order to amplify a torque outputted from the electric motor (e.g., Publication of Japan Patent No. 5370233). 
     It has been demanded that torque transmission is made efficiently in the electric car configured as described above. In view of this, it is an object of the present invention to provide a drive unit whereby torque transmission can be made efficiently. 
     BRIEF SUMMARY 
     A drive unit according to an aspect of the present invention includes an electric motor, a first torque transmission path, a second torque transmission path, a torque converter, a first gear train, a second gear train, and a first switch mechanism. The first torque transmission path is configured to transmit a torque outputted from the electric motor to an output unit. The second torque transmission path is provided parallel to the first torque transmission path. The second torque transmission path is configured to transmit the torque outputted from the electric motor to the output unit. The torque converter is disposed in the first torque transmission path. The torque converter is configured to amplify the torque outputted from the electric motor when the torque is directed in the first rotational direction. The first gear train is disposed in the first torque transmission path. The first gear train is disposed downstream with respect to the torque converter. The second gear train is disposed in the second torque transmission path. The first switch mechanism is configured to switch between the first torque transmission path and the second torque transmission path as a path for transmitting the torque outputted from the electric motor. 
     According to this configuration, when rotation of the torque converter is not required (e.g., when the electric motor is rotated in a second rotational direction), the path for transmission the torque outputted from the electric motor can be switched to the second torque transmission path by the first switch mechanism. Thus, when rotation of the torque converter is not required, torque transmission can be made without through the torque converter; hence, torque transmission can be made efficiently. 
     The second gear train can be made higher in reduction ratio than the first gear train. 
     The first gear train can be made higher in reduction ratio than the second gear train. 
     Preferably, the drive unit further includes a controller. The controller controls the electric motor and the first switch mechanism. 
     Preferably, the controller executes a first reverse moving mode. When executing the first reverse moving mode, the controller controls and causes the electric motor to be rotated in a second rotational direction. Besides, when executing the first reverse moving mode, the controller controls the first switch mechanism such that transmission of the torque outputted from the electric motor is made through the second torque transmission path. 
     Preferably, the controller executes a first forward moving mode. When executing the first forward moving mode, the controller controls and causes the electric motor to be rotated in the first rotational direction. Besides, when executing the first forward moving mode, the controller controls the first switch mechanism such that transmission of the torque outputted from the electric motor is made through the first torque transmission path. 
     Preferably, the controller executes a second forward moving mode. When executing the second forward moving mode, the controller controls and causes the electric motor to be rotated in the first rotational direction. Besides, when executing the second forward moving mode, the controller controls the first switch mechanism such that transmission of the torque outputted from the electric motor is made through the second torque transmission path. 
     Preferably, the drive unit further includes a third gear train and a second switch mechanism. The third gear train is disposed in the first torque transmission path. The third gear train is configured to receive the torque transmitted thereto from the torque converter. The second switch mechanism is configured to switch between the first gear train and the third gear train as the path for transmitting the torque after the torque is outputted from the torque converter. The controller controls the second switch mechanism. 
     Preferably, the third gear train is configured to output the torque in a reverse rotational direction to when the torque is outputted from the first gear train. 
     Preferably, the second switch mechanism is settable to a neutral position without selecting both the first and third gear trains so as to block transmission of the torque outputted from the torque converter. 
     Preferably, the controller controls and causes the second switch mechanism to be set to the neutral position when executing the second forward moving mode. 
     Preferably, the drive unit further includes a first clutch. The torque converter includes an input part and an output part. The input part is configured to receive the torque inputted thereto after the torque is outputted from the electric motor. The output part is configured to receive the torque transmitted thereto from the input part through a fluid. The first clutch is configured to allow and block transmission of the torque between the input part and the output part. 
     Preferably, the first clutch is a one-way clutch. The first clutch is configured to allow transmission of the torque outputted from the electric motor when the torque is directed in the second rotational direction and block transmission of the torque outputted from the electric motor when the torque is directed in the first rotational direction. 
     Preferably, the drive unit further includes a second clutch. The second clutch is disposed downstream with respect to the torque converter in the first torque transmission path. The second clutch is configured to allow and block transmission of the torque. 
     Preferably, the second clutch is a one-way clutch. The second clutch is configured to allow transmission of the torque outputted from the torque converter to a downstream side and block transmission of the torque from the downstream side to the torque converter. 
     Preferably, the drive unit further includes a fourth gear train. The fourth gear train is disposed upstream with respect to the torque converter in the first torque transmission path. 
     Preferably, the drive unit further includes a third torque transmission path and a third switch mechanism. The third torque transmission path branches off from the first torque transmission path at a position upstream of the torque converter and merges to the first torque transmission path at a position downstream of the torque converter. The third switch mechanism is configured to switch between the first torque transmission path and the third torque transmission path as the path for transmitting the torque. 
     Preferably, the torque converter includes a centrifugal lock-up clutch. 
     Preferably, the first switch mechanism includes a first torque output part, a first torque input part, a second torque input part, and a coupling part. The first torque output part is configured to output the torque outputted from the electric motor. The first torque input part is disposed in the first torque transmission path. The first torque input part is configured to receive the torque inputted thereto after the torque is outputted from the first torque output part. The second torque input part is disposed in the second torque transmission path. The second torque input part is configured to receive the torque inputted thereto after the torque is outputted from the first torque output part. The coupling part is settable to a first coupling state and a second coupling state. When set to the first coupling state, the coupling part couples the first torque output part and the first torque input part therethrough. When set to the second coupling state, the coupling part couples the first torque output part and the second torque input part therethrough. 
     The first torque output part, the first torque input part, and the second torque input part can be disposed along a rotational axis of the electric motor so as to be aligned from a side closer to the electric motor in an order of the first torque output part, the first torque input part, and the second torque input part. 
     The first torque output part, the first torque input part, and the second torque input part can be disposed along the rotational axis of the electric motor so as to be aligned from the side closer to the electric motor in an order of the second torque input part, the first torque output part, and the first torque input part. 
     The first torque output part, the first torque input part, and the second torque input part can be disposed along the rotational axis of the electric motor so as to be aligned from the side closer to the electric motor in an order of the first torque output part, the second torque input part, and the first torque input part. 
     Overall, according to the present invention, torque transmission can be made efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram showing torque transmission paths in a drive unit. 
         FIG.  2    is a schematic diagram of the drive unit. 
         FIG.  3    is a closeup view of a first switch mechanism and the periphery thereof. 
         FIG.  4    is a closeup view of the first switch mechanism and the periphery thereof. 
         FIG.  5    is a cross-sectional view of a torque converter. 
         FIG.  6    is a block diagram showing torque transmission paths in a drive unit according to a modification. 
         FIG.  7    is a block diagram showing torque transmission paths in a drive unit according to another modification. 
         FIG.  8    is a block diagram showing torque transmission paths in a drive unit according to still another modification. 
         FIG.  9    is a block diagram showing torque transmission paths in a drive unit according to yet another modification. 
         FIG.  10    is a closeup view of a second switch mechanism and the periphery thereof. 
         FIG.  11    is a block diagram showing torque transmission paths in a drive unit according to further still another modification. 
         FIG.  12    is a block diagram showing torque transmission paths in a drive unit according to further yet another modification. 
         FIG.  13    is a closeup view of a first switch mechanism and the periphery thereof according to still yet another modification. 
         FIG.  14    is a schematic diagram of a drive unit according to the still yet another modification. 
         FIG.  15    is a closeup view of a first switch mechanism and the periphery thereof according to further still yet another modification. 
         FIG.  16    is a schematic diagram of a drive unit according to the further still yet another modification. 
     
    
    
     DETAILED DESCRIPTION 
     A drive unit according to the present preferred embodiment will be hereinafter explained with reference to drawings.  FIG.  1    is a block diagram showing torque transmission paths in the drive unit, whereas  FIG.  2    is a schematic diagram of the drive unit. It should be noted that in the following explanation, the term “axial direction” refers to an extending direction of a rotational axis O for both an electric motor  2  and a torque converter  3 . On the other hand, the term “circumferential direction” refers to a circumferential direction of an imaginary circle about the rotational axis O, whereas the term “radial direction” refers to a radial direction of the imaginary circle about the rotational axis O. Besides, the term “upstream” and “downstream” mean an upstream side and a downstream side in a direction of transmitting a torque. 
     [Drive Unit] 
     As shown in  FIGS.  1  and  2   , the drive unit ( 100 ) is configured to drive an output unit  101 . The output unit  101  includes a final gear train  102 , a differential gear  103 , a pair of drive shafts  105 , and a pair of drive wheels  110 . It should be noted that the output unit  101  can include only the drive wheels  110 . 
     The pair of drive shafts  105  extends from the differential gear  103  to the pair of drive wheels  110 , respectively. The pair of drive shafts  105  extends parallel to the rotational axis O. When described in detail, the pair of drive shafts  105  extends parallel to first and second torque transmission shafts  41   a  and  42   a  (to be described). Besides, the pair of drive shafts  105  extends to be offset (displaced) from the first and second torque transmission shafts  41   a  and  42   a.    
     The differential gear  103  is disposed in the middle of the space between the pair of drive wheels  110  in the extending direction of the pair of drive shafts  105 . In other words, the pair of drive shafts  105  is substantially equal in length to each other. 
     The drive unit  100  includes the electric motor  2 , the torque converter  3 , first and second torque transmission paths  4   a  and  4   b , a first switch mechanism  5 , a first clutch  6 , a first gear train  7 , a second gear train  72 , and a controller  9 . The drive unit  100  is installed in, for instance, an electric car. 
     &lt;Electric Motor&gt; 
     The electric motor  2  (hereinafter simply referred to as “motor”) includes a motor casing  21 , a stator  22 , a rotor  23 , and an output shaft  24 . In the present preferred embodiment, the motor  2  is a so-called inner rotor motor. The motor  2  is configured to be rotatable in a first rotational direction and a second rotational direction. It should be noted that the second rotational direction is a rotational direction reverse to the first rotational direction. 
     The motor casing  21  is non-rotatable, while being fixed to a body frame of the vehicle or so forth. The stator  22  is fixed to the inner peripheral surface of the motor casing  21 . The stator  22  is non-rotatable. The rotor  23  is rotated about the rotational axis O. The rotor  23  is disposed radially inside the stator  22 . The output shaft  24  extends from the rotor  23  in the axial direction. The output shaft  24  is unitarily rotated with the rotor  23 . 
     &lt;First and Second Torque Transmission Paths&gt; 
     As shown in  FIG.  1   , the first and second torque transmission paths  4   a  and  4   b  are configured to transmit a torque, outputted from the motor  2 , to the output unit  101 . Besides, the first and second torque transmission paths  4   a  and  4   b  are provided parallel to each other. 
     The first torque transmission path  4   a  includes the first and second torque transmission shafts  41   a  and  42   a . As shown in  FIG.  2   , the torque, outputted from the motor  2 , is inputted to the first torque transmission shaft  41   a  through the first switch mechanism  5 . The first torque transmission shaft  41   a  extends in the axial direction. The first torque transmission shaft  41   a  extends to be coaxial to the output shaft  24  of the motor  2 . The first torque transmission shaft  41   a  extends toward the torque converter  3 . The rotational axis of the first torque transmission shaft  41   a  is substantially matched with that of the motor  2  and that of the torque converter  3 . 
     The first torque transmission shaft  41   a  transmits the torque, outputted from the motor  2 , to the torque converter  3 . The first torque transmission shaft  41   a  is attached at the downstream-side end thereof (the right-side end thereof in  FIG.  2   ) to a cover hub  313  (see  FIG.  5   ) of the torque converter  3 . The first torque transmission shaft  41   a  is solid. 
     The second torque transmission shaft  42   a  receives the torque inputted thereto from the torque converter  3 . The second torque transmission shaft  42   a  outputs the torque, inputted thereto from the torque converter  3 , to the first gear train  71 . The second torque transmission shaft  42   a  extends axially toward the motor  2  from the torque converter  3 . 
     The second torque transmission shaft  42   a  has a cylindrical shape. The first torque transmission shaft  41   a  extends through the interior of the second torque transmission shaft  42   a . The second torque transmission shaft  42   a  is attached at the upstream-side end thereof (the right-side end thereof in  FIG.  2   ) to a turbine  33  (see  FIG.  5   ) of the torque converter  3 . On the other hand, the second torque transmission shaft  42   a  is rotatably supported at the downstream-side end thereof (the left-side end thereof in  FIG.  2   ) by, for instance, a transmission casing  40  or so forth through a bearing member and/or so forth. 
     The second torque transmission path  4   b  is configured to transmit the torque, outputted from the motor  2 , to the output unit  101  without through the torque converter  3 . 
     &lt;First Switch Mechanism&gt; 
     The first switch mechanism  5  is configured to switch between the first torque transmission path  4   a  and the second torque transmission path  4   b  as a path for transmitting the torque outputted from the motor  2 . It should be noted that the first switch mechanism  5  is controlled by the controller  9 . The first switch mechanism  5  is disposed in the interior of the transmission casing  40 . 
     As shown in  FIGS.  3  and  4   , the first switch mechanism  5  includes a first torque output gear  51  (exemplary first torque output part), a first torque input gear  52  (exemplary first torque input part), a second torque input gear  53  (exemplary second torque input part), and a first ring gear  54  (exemplary coupling part). Besides, the first switch mechanism  5  further includes an actuator  55  causing the first ring gear  54  to move in the axial direction. The actuator  55  is controlled by the controller  9 . 
     The first torque output gear  51  is configured to output the torque, outputted from the motor  2 , to the first or second torque transmission path  4   a ,  4   b . The first torque output gear  51  is attached to the distal end of the output shaft  24 . The first torque output gear  51  is unitarily rotated with the output shaft  24 . The first torque output gear  51  can be provided as a different member separated from the output shaft  24 , or alternatively, can be provided as a single member integrated with the output shaft  24 . The first torque output gear  51  is disposed to be rotatable about the rotational axis O. The first torque output gear  51  includes a plurality of teeth on the outer peripheral surface thereof. 
     The first torque input gear  52  is disposed in the first torque transmission path  4   a . The first torque input gear  52  is configured to receive the torque that is inputted thereto after outputted from the first torque output gear  51 . The first torque input gear  52  is disposed to be rotatable about the rotational axis O. 
     The first torque input gear  52  is attached to the first torque transmission shaft  41   a . When described in detail, the first torque input gear  52  is attached to the upstream-side end (the left-side end in  FIG.  3   ) of the first torque transmission shaft  41   a . The first torque input gear  52  is unitarily rotated with the first torque transmission shaft  41   a . The first torque input gear  52  can be provided as a different member separated from the first torque transmission shaft  41   a , or alternatively, can be provided as a single member integrated with the first torque transmission shaft  41   a . The first torque input gear  52  is approximately equal in diameter to the first torque output gear  51 . The first torque input gear  52  includes a plurality of teeth on the outer peripheral surface thereof. 
     The second torque input gear  53  is disposed in the second torque transmission path  4   b . The second torque input gear  53  is configured to receive the torque that is inputted thereto after outputted from the first torque output gear  51 . The second torque input gear  53  is disposed to be rotatable about the rotational axis O. The second torque input gear  53  is approximately equal in diameter to the first torque output gear  51 . The second torque input gear  53  includes a plurality of teeth on the outer peripheral surface thereof. The second torque input gear  53  is supported by the output shaft  24 , while being rotatable relative thereto. 
     The first torque output gear  51  is disposed axially between the first and second torque input gears  52  and  53 . The second torque input gear  53 , the first torque output gear  51 , and the first torque input gear  52  are disposed along the rotational axis O, while being aligned in this order from a side closer to the motor  2 . The first torque output gear  51 , the first torque input gear  52 , and the second torque input gear  53  are disposed to be rotatable relative to each other. 
     The first ring gear  54  includes a plurality of teeth on the inner peripheral surface thereof. The first ring gear  54  is constantly meshed with the first torque output gear  51  and is unitarily rotated therewith. In other words, the first ring gear  54  is unitarily rotated with the output shaft  24 . 
     The first ring gear  54  is disposed to be movable in the axial direction. When controlled by the controller  9 , the first ring gear  54  is moved in the axial direction. When described in detail, as described above, the actuator  55 , controlled by the controller  9 , causes the first ring gear  54  to move in the axial direction. When moved in the axial direction, the first ring gear  54  is settable to a first coupling state and a second coupling state. It should be noted that  FIG.  3    shows the first ring gear  54  set to the first coupling state, whereas  FIG.  4    shows the first ring gear  54  set to the second coupling state. 
     As shown in  FIG.  3   , when set to the first coupling state, the first ring gear  54  couples the first torque output gear  51  and the first torque input gear  52  therethrough. When described in detail, the first ring gear  54  is meshed with the first torque output gear  51 , while being meshed with the first torque input gear  52 . 
     When the first ring gear  54  is thus meshed with both the first torque output gear  51  and the first torque input gear  52 , the first torque output gear  51  and the first torque input gear  52  are coupled to be unitarily rotated with each other. In other words, the output shaft  24  and the first torque transmission shaft  41   a  are unitarily rotated with each other. As a result, the first switch mechanism  5  can set the first torque transmission path  4   a  as the path for transmitting the torque outputted from the motor  2 . It should be noted that when the first ring gear  54  is set to the first coupling state, the torque outputted from the motor  2  is not transmitted to the second torque transmission path  4   b.    
     As shown in  FIG.  4   , when set to the second coupling state, the first ring gear  54  couples the first torque output gear  51  and the second torque input gear  53  therethrough. When described in detail, the first ring gear  54  is meshed with the first torque output gear  51 , while being meshed with the second torque input gear  53 . 
     When the first ring gear  54  is thus meshed with both the first torque output gear  51  and the second torque input gear  53 , the first torque output gear  51  and the second torque input gear  53  are coupled to be unitarily rotated with each other. In other words, the output shaft  24  and the second torque input gear  53  are unitarily rotated with each other. As a result, the first switch mechanism  5  can set the second torque transmission path  4   b  as the path for transmitting the torque outputted from the motor  2 . It should be noted that when the first ring gear  54  is set to the second coupling state, the torque outputted from the motor  2  is not transmitted to the first torque transmission path  4   a.    
     &lt;Torque Converter&gt; 
     As shown in  FIG.  1   , the torque converter  3  is disposed in the first torque transmission path  4   a . The torque converter  3  is configured to amplify the torque outputted from the motor  2  when the torque is directed in the first rotational direction. It should be noted that when directed in the second rotational direction, the torque outputted from the motor  2  is not amplified by the torque converter  3 . The torque converter  3  output the amplified torque to the first gear train  71 . 
     As shown in  FIG.  2   , the rotational axis O of the torque converter  3  is substantially matched with that of the motor  2 . When torque transmission is made through the first torque transmission path  4   a  by the first switch mechanism  5 , the torque outputted from the motor  2  is transmitted to the torque converter  3 . The torque converter  3  is disposed axially apart from the motor  2  at an interval. 
     The first switch mechanism  5  is disposed between the torque converter  3  and the motor  2 . The motor  2 , the first switch mechanism  5 , and the torque converter  3  are axially aligned in this order. Besides, the first and second gear trains  71  and  72  are also disposed between the torque converter  3  and the motor  2 . 
     As shown in  FIG.  5   , the torque converter  3  includes a cover  31 , an impeller  32 , the turbine  33 , a stator  34 , and a one-way clutch  36 . Besides, the torque converter  3  further includes a lock-up clutch  37  of a centrifugal type. It should be noted that the cover  31  and the impeller  32  correspond to an input part in the present invention, whereas the turbine  33  corresponds to an output part in the present invention. 
     The torque converter  3  is disposed such that the impeller  32  faces the motor  2  (the left side in  FIG.  5   ), whereas the cover  31  faces opposite to the motor  2  (the right side in  FIG.  5   ). The torque converter  3  is accommodated in the interior of a torque converter casing  30  (see  FIG.  2   ). Hydraulic fluid is supplied to the interior of the torque converter  3 . The hydraulic fluid is, for instance, hydraulic oil. 
     The cover  31  is a component to which the torque, outputted from the motor  2 , is inputted. The cover  31  is rotated by the torque inputted thereto from the motor  2 . The cover  31  is fixed to the first torque transmission shaft  41   a . For example, the cover  31  includes a spline hole to which the first torque transmission shaft  41   a  is spline-coupled. Because of this, the cover  31  is unitarily rotated with the first torque transmission shaft  41   a . The cover  31  is disposed to cover the turbine  33 . 
     The cover  31  includes a disc portion  311 , a cylindrical portion  312 , and a cover hub  313 . The disc portion  311  includes an opening in the middle thereof. The cylindrical portion  312  extends from the outer peripheral end of the disc portion  311  toward the motor  2 . The disc portion  311  and the cylindrical portion  312  are provided as a single member integrated with each other. 
     The cover hub  313  is fixed to the inner peripheral end of the disc portion  311 . In the present preferred embodiment, the cover hub  313  is provided as a different member separated from the disc portion  311 . However, the cover hub  313  can be provided as a single member integrated with the disc portion  311 . 
     The cover hub  313  includes the spline hole to which the first torque transmission shaft  41   a  is spline-coupled. The cover hub  313  is rotatably supported by the torque converter casing  30  through a bearing member (not shown in the drawings). 
     The impeller  32  is unitarily rotated with the cover  31 . The impeller  32  is fixed to the cover  31 . The impeller  32  includes an impeller shell  321 , a plurality of impeller blades  322 , and an impeller hub  323 . 
     The impeller shell  321  is fixed to the cover  31 . The plural impeller blades  322  are attached to the inner surface of the impeller shell  321 . 
     The impeller hub  323  is attached to the inner peripheral end of the impeller shell  321 . It should be noted that in the present preferred embodiment, the impeller hub  323  is provided as a single member integrated with the impeller shell  321 , but alternatively, can be provided as a different member separated from the impeller shell  321 . 
     The turbine  33  is disposed in opposition to the impeller  32 . When described in detail, the turbine  33  is axially opposed to the impeller  32 . The turbine  33  is a component to which the torque is transmitted from the impeller  32  through the hydraulic fluid. 
     The turbine  33  includes a turbine shell  331 , a plurality of turbine blades  332 , and a turbine hub  333 . The plural turbine blades  332  are fixed to the inner surface of the turbine shell  331 . 
     The turbine hub  333  is fixed to the inner peripheral end of the turbine shell  331 . For example, the turbine hub  333  is fixed to the turbine shell  331  by rivets. In the present preferred embodiment, the turbine hub  333  is provided as a different member separated from the turbine shell  331 . However, the turbine hub  333  can be provided as a single member integrated with the turbine shell  331 . 
     The second torque transmission shaft  42   a  is attached to the turbine hub  333 . When described in detail, the second torque transmission shaft  42   a  is spline-coupled to the turbine hub  333 . The turbine hub  333  is unitarily rotated with the second torque transmission shaft  42   a . In other words, the torque converter  3  outputs the amplified torque to the second torque transmission shaft  42   a.    
     The stator  34  is configured to regulate the flow of the hydraulic oil returning from the turbine  33  to the impeller  32 . The stator  34  is rotatable about the rotational axis O. For example, the stator  34  is supported by a stationary shaft  104  through the one-way clutch  36 . The stator  34  is disposed axially between the impeller  32  and the turbine  33 . 
     It should be noted that the stationary shaft  104  axially extends through the interior of the impeller hub  323 . The stationary shaft  104  has a cylindrical shape and the second torque transmission shaft  42   a  axially extends through the interior of the stationary shaft  104 . Besides, the stationary shaft  104  extends from, for instance, the transmission casing  40  or the torque converter casing  30 . The stationary shaft  104  is non-rotatable. 
     The stator  34  includes a stator carrier  341  having a disc shape and a plurality of stator blades  342  attached to the outer peripheral surface of the stator carrier  341 . 
     The one-way clutch  36  is disposed between the stationary shaft  104  and the stator  34 . The one-way clutch  36  is configured to make the stator  34  rotatable in the first rotational direction. By contrast, the one-way clutch  36  makes the stator  34  non-rotatable in the second rotational direction. The torque is transmitted from the impeller  32  to the turbine  33 , while being amplified by the stator  34 . 
     The centrifugal lock-up clutch  37  is attached to the turbine  33 . The lock-up clutch  37  is unitarily rotated with the turbine  33 . The lock-up clutch  37  is configured to couple the cover  31  and the turbine  33  to each other by a centrifugal force generated in rotation of the turbine  33 . When described in detail, the lock-up clutch  37  is configured to transmit the torque from the cover  31  to the turbine  33  when the rotational speed of the turbine  33  becomes greater than or equal to a predetermined value. 
     The lock-up clutch  37  includes a plurality of centrifugal elements  371  and a plurality of friction materials  372 . The friction materials  372  are attached to the outer peripheral surfaces of the centrifugal elements  371 , respectively. The centrifugal elements  371  are disposed to be radially movable. It should be noted that the centrifugal elements  371  are disposed to be circumferentially immovable. Because of this, the centrifugal elements  371  are rotated together with the turbine  33  and are moved radially outward by centrifugal forces. 
     When the rotational speed of the turbine  33  becomes greater than or equal to the predetermined value, the lock-up clutch  37  is configured such that the centrifugal elements  371  are moved radially outward and the friction materials  372  are engaged by friction with the inner peripheral surface of the cylindrical portion  312  of the cover  31 . As a result, the lock-up clutch  37  is turned to an on state, and the torque inputted to the cover  31  is transmitted therefrom to the turbine  33  through the lock-up clutch  37 . It should be noted that even when the lock-up clutch  37  is turned to the on state, the hydraulic fluid is flowable through the lock-up clutch  37 . 
     When the rotational speed of the turbine  33  becomes less than the predetermined value, the centrifugal elements  371  are moved radially inward, whereby the friction materials  372  and the inner peripheral surface of the cylindrical portion  312  of the cover  31 , engaged by friction, are disengaged from each other. As a result, the lock-up clutch  37  is turned to an off state, and the torque inputted to the cover  31  is not transmitted therefrom to the turbine  33  through the lock-up clutch  37 . In other words, the torque inputted to the cover  31  is transmitted therefrom to the impeller  32  and is then transmitted to the turbine  33  through the hydraulic fluid. 
     &lt;First Clutch&gt; 
     The first clutch  6  is configured to allow and block torque transmission between the input part and the output part in the torque converter  3 . When described in detail, the first clutch  6  is configured to allow and block torque transmission between the cover  31  and the turbine  33 . 
     The first clutch  6  is disposed between the cover  31  and the turbine  33 . The first clutch  6  is a one-way clutch. 
     The first clutch  6  blocks transmission of the torque outputted from the motor  2  when the torque is directed in the first rotational direction. In other words, when the torque, outputted from the motor  2  so as to be directed in the first rotational direction, is inputted to the first clutch  6 , the first clutch  6  makes the cover  31  rotatable relative to the turbine  33 . Because of this, when the motor  2  is rotated in the first rotational direction, the first clutch  6  does not allow torque transmission from the cover  31  to the turbine  33 . As a result, the torque, outputted from the motor  2  so as to be directed in the first rotational direction, is sequentially transmitted to the cover  31 , the impeller  32 , and the turbine  33  in this order. 
     By contrast, the first clutch  6  transmits the torque outputted from the motor  2  when the torque is directed in the second rotational direction. In other words, when the torque, outputted from the motor  2  so as to be directed in the second rotational direction, is inputted to the first clutch  6 , the first clutch  6  makes the cover  31  unitarily rotate with the turbine  33 . Because of this, when the motor  2  is rotated in the second rotational direction, the first clutch  6  allows torque transmission from the cover  31  to the turbine  33 . The torque, outputted from the motor  2  so as to be directed in the second rotational direction, is transmitted without through the impeller  32  and the hydraulic fluid. 
     &lt;First and Second Gear Trains&gt; 
     As shown in  FIG.  2   , the first and second gear trains  71  and  72  are disposed axially between the motor  2  and the torque converter  3 . Besides, the first torque output gear  51 , the first torque input gear  52 , and the second torque input gear  53  in the first clutch mechanism  5  are disposed axially between the first and second gear trains  71  and  72 . The first and second gear trains  71  and  72  are accommodated in the transmission casing  40 . 
     As shown in  FIGS.  1  and  2   , the first and second gear trains  71  and  72  output the torque toward the output unit  101 . When described in detail, the first and second gear trains  71  and  72  output the torque to the drive wheels  110  through the final gear train  102 , the differential gear  103 , and the drive shafts  105 . 
     The first and second gear trains  71  and  72  are configured to output the torque, outputted from the motor  2  so as to be directed in the first rotational direction, as a forward rotation directional torque. In other words, the first and second gear trains  71  and  72  are configured to output the torque, outputted from the motor  2  so as to be directed in the second rotational direction, as a reverse rotation directional torque. Because of this, when the motor  2  is rotated in the first rotational direction and the torque is outputted therefrom to the drive wheels  110  through the first or second gear train  71 ,  72 , the vehicle is moved forward. By contrast, when the motor  2  is rotated in the second rotational direction and the torque is outputted therefrom to the drive wheels  110  through the first or second gear train  71 ,  72 , the vehicle is moved backward. 
     The first gear train  71  is disposed in the first torque transmission path  4   a . The first gear train  71  is disposed downstream with respect to the torque converter  3 . Specifically, the first gear train  71  is attached to the second torque transmission shaft  42   a.    
     As shown in  FIG.  3   , the first gear train  71  includes a first gear  71   a  and a second gear  71   b  that are meshed with each other. The first gear  71   a  is attached to the downstream-side end of the second torque transmission shaft  42   a . The first gear  71   a  is unitarily rotated with the second torque transmission shaft  42   a.    
     The second gear  71   b  is supported by a drive shaft  70 . The second gear  71   b  is unitarily rotated with the drive shaft  70 . The second gear  71   b  outputs the torque, inputted thereto from the first gear  71   a , to the drive shaft  70 . 
     The second gear train  72  is disposed in the second torque transmission path  4   b . The second gear train  72  includes a third gear  72   a  and a fourth gear  72   b  that are meshed with each other. The number of gears in the second gear train  72  is equal to that in the first gear train  71 . 
     The third gear  72   a  is unitarily rotated with the second torque input gear  53 . The third gear  72   a  is integrated with the second torque input gear  53 . The third gear  72   a  can be provided as a single member integrated with the second torque input gear  53 , or alternatively, can be provided as a different member separated from the second torque input gear  53 . The third gear  72   a  is supported by the output shaft  24 , while being rotatable relative thereto. 
     The fourth gear  72   b  is supported by the drive shaft  70 . The fourth gear  72   b  is unitarily rotated with the drive shaft  70 . The fourth gear  72   b  outputs the torque, inputted thereto from the third gear  72   a , to the drive shaft  70 . 
     The first gear train  71  is different in reduction ratio from the second gear train  72 . When described in detail, the first gear train  71  is higher in reduction ratio than the second gear train  72 . 
     &lt;Controller&gt; 
     As shown in  FIG.  1   , the controller  9  is configured to control the motor  2  and the first switch mechanism  5 . For example, a computer (e.g., microcomputer), including a CPU (Central Processing Unit), a ROM (Read Only Memory), and so forth, is provided as the controller  9 . The ROM stores programs for various computations. The CPU executes the programs stored in the ROM. 
     The controller  9  executes any of a first forward moving mode, a second forward moving mode, a first reverse moving mode, and a second reverse moving mode. When the controller  9  executes the first or second forward moving mode, the drive unit  100  is actuated to move the vehicle forward. By contrast, when the controller  9  executes the first or second reverse moving mode, the drive unit  100  is actuated to move the vehicle backward. 
     When executing the first forward moving mode, the controller  9  controls and causes the motor  2  to be rotated in the first rotational direction. Besides, when executing the first forward moving mode, the controller  9  controls the first switch mechanism  5  such that transmission of the torque outputted from the motor  2  is made through the first torque transmission path  4   a.    
     When described in detail, the controller  9  controls the actuator  55  such that the first ring gear  54  is set to the first coupling state. As a result, the first ring gear  54  couples the first torque output gear  51  and the first torque input gear  52  to each other, whereby the torque, outputted from the motor  2 , is transmitted to the first torque transmission path  4   a.    
     It should be noted that when the controller  9  controls and causes the motor  2  to be rotated in the first rotational direction, the first clutch  6  blocks torque transmission. As a result, the torque, outputted from the motor  2  so as to be directed in the first rotational direction, is transmitted via the torque converter  3 . It should be noted that when the rotational speed of the turbine  33  in the torque converter  3  becomes greater than or equal to a predetermined value, the lock-up clutch  37  is turned to a lock-up on state, whereby torque transmission is made. In other words, torque transmission is directly made from the cover  31  to the turbine  33  without through the hydraulic fluid. 
     When executing the second forward moving mode, the controller  9  controls and causes the motor  2  to be rotated in the first rotational direction. Besides, when executing the second forward moving mode, the controller  9  controls the first switch mechanism  5  such that transmission of the torque outputted from the motor  2  is made through the second torque transmission path  4   b.    
     When described in detail, the controller  9  controls the actuator  55  such that the first ring gear  54  is set to the second coupling state. As a result, the first ring gear  54  couples the first torque output gear  51  and the second torque input gear  53  to each other, whereby the torque, outputted from the motor  2 , is transmitted to the second torque transmission path  4   b.    
     In the first forward moving mode, torque amplification is enabled by the torque converter  3 . Besides, in the first forward moving mode, torque transmission is made through the first gear train  71  higher in reduction ratio than the second gear train  72 . Therefore, in the first forward moving mode, the drive unit  100  is enabled to output a high drive force during traveling at low speed. In other words, the first forward moving mode is suitable for traveling at low speed. 
     By contrast, in the second forward moving mode, torque transmission is directly made without through the torque converter  3 ; hence, torque transmission can be made efficiently. Besides, in the second forward moving mode, torque transmission is made through the second gear train  72  lower in reduction ratio than the first gear train  71 ; hence, the motor  2  can be reduced in rotational speed. The second forward moving mode is suitable for traveling at high speed. 
     When executing the first reverse moving mode, the controller  9  controls and causes the motor  2  to be rotated in the second rotational direction. Besides, when executing the first reverse moving mode, the controller  9  controls the first switch mechanism  5  such that transmission of the torque outputted from the motor  2  is made through the second torque transmission path  4   b . It should be noted that the method of controlling the first switch mechanism  5  is identical to that in the second forward moving mode described above. 
     When executing the second reverse moving mode, the controller  9  controls and causes the motor  2  to be rotated in the second rotational direction. Besides, when executing the second reverse moving mode, the controller  9  controls the first switch mechanism  5  such that transmission of the torque outputted from the motor  2  is made through the first torque transmission path  4   a . It should be noted that the method of controlling the first switch mechanism  5  is identical to that in the first forward moving mode described above. 
     Torque transmission is made through the first clutch  6  in the first torque transmission path  4   a . In other words, transmission of the torque directed in the second rotational direction is made without through the torque converter  3  in the first torque transmission path  4   a.    
     &lt;Actions&gt; 
     In the drive unit  100  configured as described above, the controller  9  executes either the first forward moving mode or the second forward moving mode when the vehicle is moved forward. It should be noted that either the first forward moving mode or the second forward moving mode can be selected in accordance with operation by a driver or can be selected by the controller  9  based on at least one traveling condition or so forth. 
     The controller executes either the first reverse moving mode or the second reverse moving mode when the vehicle is moved backward. In similar manner to the above, either the first reverse moving mode or the second reverse moving mode can be selected in accordance with operation by the driver or can be selected by the controller  9  based on at least one traveling condition or so forth. 
     [Modifications] 
     One preferred embodiment of the present invention has been explained above. However, the present invention is not limited to the above, and a variety of changes can be made without departing from the gist of the present invention. It should be noted that modifications to be described, excluding part thereof, are applicable simultaneously. 
     (a) In the preferred embodiment described above, the first gear train  71  is higher in reduction ratio than the second gear train  72 . However, the configuration for the first and second gear trains  71  and  72  is not limited to the above. For example, as shown in  FIG.  6   , the second gear train  72  can be higher in reduction ratio than the first gear train  71 . 
     (b) As shown in  FIG.  7   , the drive unit  100  may not include the first clutch  6 . Besides, the torque converter  3  may not include the lock-up clutch  37 . 
     (c) As shown in  FIG.  8   , the drive unit  100  can include a second clutch  7 . The second clutch  7  is disposed in the first torque transmission path  4   a . The second clutch  7  is disposed downstream with respect to the torque converter  3 . The second clutch  7  is configured to allow and block torque transmission. 
     When described in detail, the second clutch  7  is a one-way clutch. The second clutch  7  is configured to allow transmitting downstream the torque outputted from the torque converter  3  and block transmitting the torque from downstream to the torque converter  3 . 
     (d) As shown in  FIG.  9   , the drive unit  100  can include a third gear train  73  and a second switch mechanism  5   a . The third gear train  73  is disposed in the first torque transmission path  4   a . In other words, two gear trains, composed of the first and third gear trains  71  and  73 , are installed in the first torque transmission path  4   a . Besides, the first and third gear trains  71  and  73  are disposed parallel to each other. 
     The third gear train  73  is configured to receive the torque transmitted thereto from the torque converter  3 . The third gear train  73  is attached to the second torque transmission shaft  42   a . The third gear train  73  is configured to output the torque toward the output unit  101 . When described in detail, the third gear train  73  outputs the torque to the drive wheels  110  through the final gear train  102 , the differential gear  103 , and the drive shafts  105 . 
     The third gear train  73  outputs the torque in a reverse rotational direction to when the torque is outputted from the first gear train  71 . When described in detail, the third gear train  73  is configured to output the torque, outputted from the motor  2  so as to be directed in the first rotational direction, as a reverse rotation directional torque. In other words, the third gear train  73  is configured to output the torque, outputted from the motor  2  so as to be directed in the second rotational direction, as a forward rotation directional torque. Because of this, when the motor  2  is rotated in the first rotational direction and the torque is outputted therefrom to the output unit  101  through the third gear train  73 , the vehicle is moved backward. By contrast, when the motor  2  is rotated in the second rotational direction and the torque is outputted therefrom to the output unit  101  through the third gear train  73 , the vehicle is moved forward. 
     The controller  9  is enabled to execute a third reverse moving mode in addition to the first and second reverse moving modes. When executing the third reverse moving mode, the controller  9  controls and causes the motor  2  to be rotated in the first rotational direction. Besides, when executing the third reverse moving mode, the controller  9  controls the first switch mechanism  5  such that transmission of the torque outputted from the motor  2  is made through the first torque transmission path  4   a . Moreover, when executing the third reverse moving mode, the controller  9  controls the second switch mechanism  5   a  such that transmission of the torque amplified by the torque converter  3  is made through the third gear train  73 . 
     As shown in  FIG.  10   , the third gear train  73  includes fifth to seventh gears  73   a  to  73   c . The fifth gear  73   a  is supported by the second torque transmission shaft  42   a , while being rotatable relative thereto. By meshing of a second ring gear  54   a  (to be described) in the second switch mechanism  5   a , the fifth gear  73   a  is unitarily rotated with the second torque transmission shaft  42   a.    
     The sixth gear  73   b  is meshed with the fifth gear  73   a . The sixth gear  73   b  is supported by a countershaft (not shown in the drawings). The sixth gear  73   b  can be rotated unitarily with or relative to the countershaft. 
     The seventh gear  73   c  is meshed with the sixth gear  73   b . The seventh gear  73   c  is supported by the drive shaft  70 . The seventh gear  73   c  is unitarily rotated with the drive shaft  70 . The seventh gear  73   c  outputs the torque, transmitted thereto from the fifth gear  73   a , to the drive shaft  70 . 
     Besides, in the present modification, in similar manner to the fifth gear  73   a , the first gear  71   a  is supported by the second torque transmission shaft  42   a , while being rotatable relative thereto. 
     The second switch mechanism  5   a  is configured to switch between the first gear train  71  and the third gear train  73  as the path for transmitting the torque after the torque is outputted from the torque converter  3 . Besides, the second switch mechanism  5   a  is settable to a neutral position without selecting any of the first and third gear trains  71  and  73  so as to block transmission of the torque outputted from the torque converter  3 . For example, when executing the second forward moving mode described above, the controller  9  controls and causes the second switch mechanism  5   a  to be set to the neutral position. 
     The second switch mechanism  5   a  includes a second torque output gear  51   a , a third torque input gear  52   a , a fourth torque input gear  53   a , the second ring gear  54   a , and a second actuator  55   a . The second actuator  55   a  is controlled by the controller  9 . When controlled by the controller  9 , the second actuator  55   a  is caused to axially move the second ring gear  54   a.    
     The second torque output gear  51   a  is attached to the second torque transmission shaft  42   a . The second torque output gear  51   a  is unitarily rotated with the second torque transmission shaft  42   a . The second torque output gear  51   a  can be provided as a single member integrated with the second torque transmission shaft  42   a , or alternatively, can be provided as a different member separated from the second torque transmission shaft  42   a . The second torque output gear  51   a  includes a plurality of teeth on the outer peripheral surface thereof. 
     The third torque input gear  52   a  is supported by the second torque transmission shaft  42   a , while being rotatable relative thereto. When meshed with the second ring gear  54   a , the third torque input gear  52   a  is unitarily rotated with the second torque transmission shaft  42   a . The third torque input gear  52   a  is unitarily rotated with the first gear  71   a  in the first gear train  71 . It should be noted that the third torque input gear  52   a  can be provided as a single member integrated with the first gear  71   a , or alternatively, can be provided as a different member separated from the first gear  71   a.    
     The fourth torque input gear  53   a  is supported by the second torque transmission shaft  42   a , while being rotatable relative thereto. When meshed with the second ring gear  54   a , the fourth torque input gear  53   a  is unitarily rotated with the second torque transmission shaft  42   a . The fourth torque input gear  53   a  is unitarily rotated with the fifth gear  73   a  in the third gear train  73 . It should be noted that the fourth torque input gear  53   a  can be provided as a single member integrated with the fifth gear  73   a , or alternatively, can be provided as a different member separated from the fifth gear  73   a.    
     The second ring gear  54   a  includes a plurality of teeth on the inner peripheral surface thereof. The second ring gear  54   a  is constantly meshed with the second torque output gear  51   a  and is unitarily rotated therewith. In other words, the second ring gear  54   a  is unitarily rotated with the second torque transmission shaft  42   a . The second ring gear  54   a  is disposed to be movable in the axial direction. 
     The second ring gear  54   a  is meshed with the second torque output gear  51   a  and is also capable of being turned to a state of engagement with the third torque input gear  52   a . When the second ring gear  54   a  is meshed with the second torque output gear  51   a  and the third torque input gear  52   a  as described above, the torque, transmitted from the second torque transmission shaft  42   a , is outputted through the first gear train  71 . 
     On the other hand, the second ring gear  54   a  is meshed with the second torque output gear  51   a  and is also capable of being turned to a state of engagement with the fourth torque input gear  53   a . When the second ring gear  54   a  is meshed with the second torque output gear  51   a  and the fourth torque input gear  53   a  as described above, the torque, transmitted from the second torque transmission shaft  42   a , is outputted through the third gear train  73 . 
     By contrast, when the second switch mechanism  5   a  is set to the neutral position, the second ring gear  54   a  is turned to a state of meshing with only the second torque output gear  51   a . When the second ring gear  54   a  is meshed with only the second torque output gear  51   a  without being meshed with both the third torque input gear  52   a  and the fourth torque input gear  53   a , torque transmission can be blocked between the drive shaft  70  and the second torque transmission shaft  42   a . With this configuration, the torque converter  3  and so forth can be prevented from being rotated in conjunction with another component, while the drive unit  100  is driven in the second forward moving mode. 
     The second switch mechanism  5   a  is controlled by the controller  9 . When controlled by the controller  9 , the second ring gear  54   a  is moved in the axial direction. The axial movement of the second ring gear  54   a  results in meshing with the second torque output gear  51   a  and the third torque input gear  52   a , meshing with the second torque output gear Ma and the fourth torque input gear  53   a , or meshing with only the second torque output gear  51   a.    
     (e) As shown in  FIG.  11   , the drive unit  100  can further include a fourth gear train  74 . The fourth gear train  74  is disposed in the first torque transmission path  4   a . The fourth gear train  74  is disposed upstream with respect to the torque converter  3 . The fourth gear train  74  changes the speed of a torque inputted thereto and outputs the torque changed in speed. The fourth gear train  74  is, for instance, a planetary gear mechanism. 
     When the drive unit  100  includes the fourth gear train  74  as described above, the first gear train  71  can be set to be equal in reduction ratio to the second gear train  72 . For example, the first and third gears  71   a  and  72   a  can be set to be equal in diameter and can be meshed with a single gear attached to the drive shaft  70 . 
     (f) As shown in  FIG.  12   , the drive unit  100  can further include a third torque transmission path  4   c  and a third switch mechanism  5   b . The third torque transmission path  4   c  branches off from the first torque transmission path  4   a  at a position upstream of the torque converter  3  and merges to the first torque transmission path  4   a  at a position downstream of the torque converter  3 . 
     The third switch mechanism  5   b  is configured to switch between the first torque transmission path  4   a  and the third torque transmission path  4   c  as the path for transmitting the torque. It should be noted that the third switch mechanism  5   b  can be configured in similar manner to the first switch mechanism  5  or the second switch mechanism  5   a.    
     (g) In the preferred embodiment described above, the second torque input gear  53 , the first torque output gear  51 , and the first torque input gear  52  are disposed along the rotational axis O, while being aligned in this order from the side closer to the motor  2 . However, the configuration of the first switch mechanism  5  is not limited to this. 
     For example, as shown in  FIG.  13   , the first torque output gear  51 , the first torque input gear  52 , and the second torque input gear  53  can be disposed along the rotational axis O, while being aligned in this order from the side closer to the motor  2 . 
     The drive unit  100 , employing the first switch mechanism  5  configured as described above, is applicable to, for instance, a motorcycle as shown in  FIG.  14   . In the motorcycle, an output unit does not include any differential gear, but instead, includes a belt  108 . 
     (h) As shown in  FIG.  15   , the first torque output gear  51 , the second torque input gear  53 , and the first torque input gear  52  can be disposed along the rotational axis O, while being aligned in this order from the side closer to the motor  2 . 
     The drive unit  100 , employing the first switch mechanism  5  configured as described above, is applicable to, for instance, a four-wheel drive car as shown in  FIG.  16   . 
     (i) In the preferred embodiment described above, the first and second gear trains  71  and  72  are disposed between the motor  2  and the torque converter  3 . However, the configuration of the drive unit  100  is not limited to this. For example, as shown in  FIG.  15   , the first and second gear trains  71  and  72  can be disposed on the opposite side of the motor  2  with reference to the torque converter  3 . In other words, the torque converter  3  can be disposed between the motor  2  and both the first and second gear trains  71  and  72 . 
     REFERENCE SIGNS LIST 
     
         
           2 : Electric motor 
           3 : Torque converter 
           4   a : First torque transmission path 
           4   b : Second torque transmission path 
           4   c : Third torque transmission path 
           5 : First switch mechanism 
           5   a : Second switch mechanism 
           5   b : Third switch mechanism 
           6 : First clutch 
           7 : Second clutch 
           9 : Controller 
           37 : Lock-up Clutch 
           51 : First torque output gear 
           52 : First torque input gear 
           53 : Second torque input gear 
           54 : First ring gear 
           71 : First gear train 
           72 : Second gear train 
           73 : Third gear train 
           74 : Fourth gear train 
           100 : Drive unit 
           101 : Output unit