Patent Publication Number: US-11035450-B2

Title: Drive unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2019-074898, filed on Apr. 10, 2019. The contents of that application are incorporated by reference herein in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a drive unit. 
     BACKGROUND ART 
     In well-known electric cars, a torque, outputted from a motor, is transmitted to drive wheels through a reducer and a differential gear. For example, in an electric car disclosed in Japan Laid-open Patent Application Publication No. 2013-60996, the reducer is directly connected to the motor, and a torque is transmitted from the reducer to the drive wheels through the differential gear. 
     It has been demanded to enhance a driving force in such an electric car as described above. In view of this, it is an object of the present invention to provide a drive unit capable of enhancing a driving force. 
     BRIEF SUMMARY 
     A drive unit according to an aspect of the present invention includes a prime mover, a torque converter and a power transmission mechanism. The torque converter is a component to which a torque is inputted from the prime mover. The power transmission mechanism is disposed between the prime mover and the torque converter. Besides, the power transmission mechanism transmits the torque outputted from the torque converter therethrough toward a drive wheel. 
     According to this configuration, a torque is outputted from the prime mover toward the drive wheel through the torque converter. Hence, a driving force can be enhanced. Besides, the torque converter amplifies the torque inputted thereto from the prime mover and outputs the amplified torque to the power transmission mechanism. Hence, the torque converter is normally disposed between the prime mover and the power transmission mechanism. By contrast, in the present invention, the power transmission mechanism is disposed between the prime mover and the torque converter. In other words, attaching the torque converter is enabled without greatly changing the layout of the prime mover and the power transmission mechanism in well-known electric cars. It should be noted that in general, the prime mover is disposed on one side of the power transmission mechanism, whereas an unused space exists on the other side of the power transmission mechanism. Because of this, the unused space can be effectively utilized for disposing the torque converter in the present invention. 
     Preferably, the power transmission mechanism is a reducer. 
     Preferably, the drive unit further includes an output shaft and an input shaft. The output shaft outputs the torque outputted from the torque converter. The input shaft extends from the prime mover and inputs the torque from the prime mover therethrough to the torque converter. 
     Preferably, the output shaft extends from the torque converter toward the prime mover. 
     Preferably, the output shaft has a cylindrical shape. Besides, the input shaft extends in an interior of the output shaft. 
     Preferably, the torque converter includes a cover, an impeller and a turbine. The cover is a component to which the input shaft is fixed. The impeller is unitarily rotated with the cover. The turbine is opposed to the impeller. 
     Preferably, the impeller is disposed closer to the prime mover than the cover. 
     Preferably, the power transmission mechanism includes a planetary gear mechanism and a clutch. The planetary gear mechanism includes a sun gear, a planet gear, a planet carrier and a ring gear. The clutch is configured to brake rotation of the ring gear. The sun gear is unitarily rotated with the input shaft. The planet carrier is unitarily rotated with the output shaft. 
     Preferably, the clutch is a one-way clutch. The clutch is configured to make the ring gear rotatable in forward rotation of the input shaft and the output shaft. Besides, the clutch is configured to make the ring gear non-rotatable in reverse rotation of the input shaft and the output shaft. 
     Overall, according to the present invention, a driving force can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a drive unit. 
         FIG. 2  is a cross-sectional view of the drive unit. 
         FIG. 3  is a cross-sectional view of a torque converter. 
         FIG. 4  is a cross-sectional view of a type of impeller hub. 
         FIG. 5  is a cross-sectional view of another type of impeller hub. 
         FIG. 6  is a cross-sectional view of the drive unit shown for indicating a first cooling flow pathway. 
         FIG. 7  is a cross-sectional view of a sidewall portion of a type of cover. 
         FIG. 8  is a cross-sectional view of a sidewall portion of another type of cover. 
         FIG. 9  is a schematic diagram of a drive unit according to a modification. 
         FIG. 10  is a schematic diagram of a first one-way clutch according to another modification. 
         FIG. 11  is a schematic diagram of a drive unit according to yet another modification. 
     
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of a drive unit according to the present invention will be hereinafter explained with reference to drawings.  FIG. 1  is a schematic diagram of the drive unit according to the present preferred embodiment, whereas  FIG. 2  is a cross-sectional view of the drive unit according to the present preferred embodiment. It should be noted that in the following explanation, the term “axial direction” refers to an extending direction of a rotational axis O of a prime mover  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. Moreover, the term “forward rotation” refers to rotation in forward movement of a vehicle, whereas the term “reverse rotation” refers to rotation in backward movement of the vehicle. 
     [Drive Unit  100 ] 
     As shown in  FIGS. 1 and 2 , a drive unit  100  includes the prime mover  2 , the torque converter  3 , a reducer  4  (exemplary power transmission mechanism), an input shaft  5 , an output shaft  6 , a torque converter casing  7 , a hydraulic fluid sump  8  and a first cooling flow pathway  9   a . The drive unit  100  is installed in, for instance, an electric car. The drive unit  100  transmits a torque, outputted from the prime mover  2 , to drive wheels  101 . It should be noted that the torque converter  3 , the torque converter casing  7 , the hydraulic fluid sump  8  and the first cooling flow pathway  9   a  will be collectively referred to as a torque converter unit. 
     &lt;Prime Mover  2 &gt; 
     The prime mover  2  includes a prime mover casing  21 , a stator  22  and a rotor  23 . In the present preferred embodiment, the prime mover  2  is a motor. Detailedly, the prime mover  2  is a so-called inner rotor motor. The prime mover casing  21  is fixed to a vehicle body frame or so forth and is non-rotatable. 
     The stator  22  is fixed to the inner peripheral surface of the prime mover 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 . 
     &lt;Torque Converter  3 &gt; 
     The torque converter  3  is disposed at an interval from the prime mover  2  in the axial direction. The reducer  4  is disposed between the torque converter  3  and the prime mover  2 . The rotational axis O of the torque converter  3  is substantially matched with that of the prime mover  2 . The torque converter  3  is a device to which the torque, outputted from the prime mover  2 , is inputted. Additionally, the torque converter  3  amplifies the torque inputted thereto from the prime mover  2 , and outputs the amplified torque to the reducer  4 . 
     As shown in  FIG. 3 , the torque converter  3  includes a cover  31 , an impeller  32 , a turbine  33 , a stator  34 , a first one-way clutch  35  and a second one-way clutch  36 . Besides, the torque converter  3  further includes a centrifugal clutch  37 . 
     The torque converter  3  is disposed such that the impeller  32  faces the prime mover  2  (the left side in  FIG. 3 ) whereas the cover  31  faces opposite to the prime mover  2  (the right side in  FIG. 3 ). The torque converter  3  is accommodated in the interior of the torque converter casing  7 . 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 prime mover  2 , is inputted. The cover  31  is rotated by the torque inputted thereto from the prime mover  2 . The cover  31  is fixed to the input shaft  5  extending from the prime mover  2 . For example, the cover  31  includes a spline hole to which the input shaft  5  is spline-coupled. Because of this, the cover  31  is unitarily rotated with the input shaft  5 . 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 prime mover  2 . The disc portion  311  and the cylindrical portion  312  are provided as a single member. 
     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 member separated from the disc portion  311 . However, the cover hub  313  can be provided together with the disc portion  311  as a single member. 
     The cover hub  313  includes a first boss portion  313   a , a first flange portion  313   b  and a protruding portion  313   c . The first boss portion  313   a , the first flange portion  313   b  and the protruding portion  313   c  are provided as a single member. 
     The first boss portion  313   a  is made in the shape of a cylinder including a spline hole. The input shaft  5  is spline-coupled to the first boss portion  313   a . As shown in  FIG. 2 , the first boss portion  313   a  is rotatably supported by the torque converter casing  7  through a bearing member  102 . Because of this, the first boss portion  313   a  axially extends from the first flange portion  313   b  to the opposite side of the prime mover  2 . 
     As shown in  FIG. 3 , the first flange portion  313   b  extends radially outward from the first boss portion  313   a . Detailedly, the first flange portion  313   b  extends radially outward from the prime mover  2 -side end of the first boss portion  313   a . The disc portion  311  is fixed to the outer peripheral end of the first flange portion  313   b.    
     The protruding portion  313   c  axially extends from the first flange portion  313   b . The protruding portion  313   c  extends toward the prime mover  2 . The protruding portion  313   c  extends from the outer peripheral end of the first flange portion  313   b . The protruding portion  313   c  has a cylindrical shape. The protruding portion  313   c  includes a plurality of through holes  313   d . The hydraulic fluid is discharged from the torque converter  3  through the through holes  313   d.    
     The impeller  32  is rotated unitarily 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 , an impeller hub  323  and a plurality of supply flow pathways  324 . 
     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 together with the impeller shell  321  as a single member but can be provided as a member separated from the impeller shell  321 . 
     The impeller hub  323  includes a second boss portion  323   a  and a second flange portion  323   b . The second boss portion  323   a  has a cylindrical shape and axially extends. The second boss portion  323   a  is rotatably supported by the torque converter casing  7  through a bearing member  103  (see  FIG. 2 ). A stationary shaft  104  axially extends in the interior of the second boss portion  323   a . It should be noted that the stationary shaft  104  has a cylindrical shape and the output shaft  6  axially extends in the interior of the stationary shaft  104 . Besides, the stationary shaft  104  extends from, for instance, a reducer casing  42  or the torque converter casing  7 . The stationary shaft  104  is non-rotatable. 
     The supply flow pathways  324  are provided in the impeller hub  323 . Detailedly, the supply flow pathways  324  are provided in the second flange portion  323   b . The supply flow pathways  324  extend radially outward from the inner peripheral surface of the impeller hub  323 . Additionally, the supply flow pathways  324  are opened to the interior of a torus T. It should be noted that the torus T is a space enclosed by the impeller  32  and the turbine  33 . 
     The supply flow pathways  324  are axially closed. In other words, the supply flow pathways  324  are through holes radially extending in the impeller hub  323 . As shown in  FIG. 4 , the supply flow pathways  324  extend in a radial shape. The supply flow pathways  324  slant opposite to a forward rotational direction, while extending radially outward. In other words, the supply flow pathways  324  slant in a reverse rotational direction (counterclockwise in  FIG. 4 ), while extending radially outward. It should be noted that the extending shape of each supply flow pathway  324  is not limited to a straight shape. For example, as shown in  FIG. 5 , each supply flow pathway  324  can extend in a curved shape. 
     As shown in  FIG. 3 , the turbine  33  is disposed in opposition to the impeller  32 . Detailedly, the turbine  33  is axially opposed to the impeller  32 . The turbine  33  is a component to which a 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 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 at least one rivet. In the present preferred embodiment, the turbine hub  333  is provided as a member separated from the turbine shell  331 . However, the turbine hub  333  can be provided together with the turbine shell  331  as a single member. 
     The output shaft  6  is attached to the turbine hub  333 . Detailedly, the output shaft  6  is spline-coupled to the turbine hub  333 . The turbine hub  333  is unitarily rotated with the output shaft  6 . 
     The turbine hub  333  includes a third boss portion  333   a  and a third flange portion  333   b . The third boss portion  333   a  and the third flange portion  333   b  are provided as a single member. 
     The third boss portion  333   a  has a cylindrical shape and includes a spline hole. The output shaft  6  is spline-coupled to the third boss portion  333   a . The third boss portion  333   a  axially extends from the third flange portion  333   b  to the opposite side of the prime mover  2 . In other words, the third boss portion  333   a  axially extends from the third flange portion  333   b  toward the cover hub  313 . 
     The third boss portion  333   a  is disposed at a radial interval from the protruding portion  313   c . In other words, the protruding portion  313   c  is disposed radially outside the third boss portion  333   a . The first one-way clutch  35  is disposed between the third boss portion  333   a  and the protruding portion  313   c . It should be noted that without installation of the first one-way clutch  35 , the outer peripheral surface of the third boss portion  333   a  and the inner peripheral surface of the protruding portion  313   c  are opposed to each other. 
     A flow pathway is provided between the cover hub  313  and the distal end of the third boss portion  333   a  such that the hydraulic fluid flows therethrough. In the present preferred embodiment, the third boss portion  333   a  is provided with a plurality of cutouts  333   c  on the distal end thereof. The cutouts  333   c  radially extend on the distal end of the third boss portion  333   a . The hydraulic fluid is discharged from the torque converter  3  through the cutouts  333   c  and the through holes  313   d.    
     The third flange portion  333   b  extends radially outward from the third boss portion  333   a . Detailedly, the third flange portion  333   b  extends radially outward from the prime mover  2 -side end of the third boss portion  333   a . The turbine shell  331  is fixed to the outer peripheral end of the third flange portion  333   b  by the at least one rivet or so forth. 
     The stator  34  is configured to regulate the flow of the hydraulic fluid (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 the stationary shaft  104  through the second one-way clutch  36 . The stator  34  is disposed axially between the impeller  32  and the turbine  33 . 
     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 first one-way clutch  35  is disposed between the cover  31  and the turbine  33 . The first one-way clutch  35  makes the cover  31  rotatable relative to the turbine  33  in the forward rotational direction. In other words, when the prime mover  2  is forwardly rotated to move the vehicle forward, the first one-way clutch  35  is configured such that the cover  31  is rotated relative to the turbine  33 . Because of this, in forward movement of the vehicle, the first one-way clutch  35  does not transmit a torque from the cover  31  to the turbine  33 . 
     By contrast, the first one-way clutch  35  makes the cover  31  rotate unitarily with the turbine  33  in the reverse rotational direction. In other words, when the prime mover  2  is reversely rotated to move the vehicle backward, the first one-way clutch  35  is configured such that the cover  31  is rotated unitarily with the turbine  33 . Because of this, in backward movement of the vehicle, the first one-way clutch  35  transmits a torque from the cover  31  to the turbine  33 . 
     The second one-way clutch  36  is disposed between the stationary shaft  104  and the stator  34 . The second one-way clutch  36  is configured to make the stator  34  rotatable in the forward rotational direction. By contrast, the second one-way clutch  36  makes the stator  34  non-rotatable in the reverse rotational direction. The torque is transmitted from the impeller  32  to the turbine  33 , while being amplified by the stator  34 . 
     The centrifugal clutch  37  is attached to the turbine  33 . The centrifugal clutch  37  is unitarily rotated with the turbine  33 . The centrifugal 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 . Detailedly, the centrifugal 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 centrifugal 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 while being radially movable. It should be noted that the centrifugal elements  371  are disposed while being 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 centrifugal 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 centrifugal clutch  37  is turned to an on state, and the torque outputted from the cover  31  is transmitted to the turbine  33  through the centrifugal clutch  37 . It should be noted that even when the centrifugal clutch  37  is turned to the on state, the hydraulic fluid is capable of flowing through the centrifugal 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 centrifugal clutch  37  is turned to an off state, and the torque outputted from the cover  31  is not transmitted to the turbine  33  through the centrifugal clutch  37 . In other words, the torque outputted from the cover  31  is transmitted to the impeller  32  and is then transmitted to the turbine  33  through the hydraulic fluid. 
     &lt;Reducer  4 &gt; 
     As shown in  FIG. 2 , the reducer  4  is disposed axially between the prime mover  2  and the torque converter  3 . The reducer  4  transmits a torque, outputted from the torque converter  3 , to the drive wheel  101  side. Detailedly, the reducer  4  amplifies the torque outputted from the torque converter  3  and transmits the amplified torque to the drive wheel  101  side through a differential gear  109 . It should be noted that the reducer  4  includes a plurality of gears  41  and the reducer casing  42  accommodating the respective gears  41 . It should be also noted that one of the plural gears  41  is fixed to the output shaft  6 . In the present preferred embodiment, one of the gears  41  is provided together with the output shaft  6  as a single member. 
     &lt;Input Shaft  5 &gt; 
     The input shaft  5  extends from the prime mover  2 . The input shaft  5  extends toward the torque converter  3 . The rotational axis of the input shaft  5  is substantially matched with that of the prime mover  2  and that of the torque converter  3 . 
     The input shaft  5  inputs the torque, outputted from the prime mover  2 , to the torque converter  3 . The input shaft  5  is attached at the distal end thereof to the cover hub  313  of the torque converter  3 . The input shaft  5  is unitarily rotated with the rotor  23  of the prime mover  2 . The input shaft  5  extends through the interior of the output shaft  6 . The input shaft  5  is solid. The input shaft  5  includes a communicating pathway  51  in the distal end thereof. The communicating pathway  51  extends in the axial direction. Besides, the communicating pathway  51  is opened toward the first cooling flow pathway  9   a.    
     &lt;Output Shaft  6 &gt; 
     The output shaft  6  outputs the torque outputted from the torque converter  3 . The output shaft  6  outputs the torque, outputted from the torque converter  3 , to the reducer  4 . The output shaft  6  extends from the torque converter  3  toward the prime mover  2 . 
     The output shaft  6  has a cylindrical shape. The input shaft  5  extends through the interior of the output shaft  6 . The output shaft  6  is attached at one end (the right end in  FIG. 2 ) to the turbine  33  of the torque converter  3 . On the other hand, the output shaft  6  is rotatably supported at the other end (the left end in  FIG. 2 ) by the reducer casing  42  through a bearing member  105 . 
     &lt;Torque Converter Casing  7 &gt; 
     As shown in  FIG. 6 , the torque converter casing  7  accommodates the torque converter  3 . In the present preferred embodiment, the torque converter casing  7  is provided together with the reducer casing  42  as a single member. However, the torque converter casing  7  can be provided as a member separated from the reducer casing  42 . 
     The torque converter casing  7  includes a side wall portion  71 , an outer wall portion  72  and a plurality of heat dissipation fins  73 . The sidewall portion  71  is disposed in opposition to the cover  31  of the torque converter  3 . The sidewall portion  71  is disposed orthogonal to the rotational axis O. 
     The torque converter  3  is disposed on one axial side (the left side in  FIG. 6 ) of the sidewall portion  71 . On the other hand, the sidewall portion  71  makes contact at the other side (the right lateral surface in  FIG. 6 ) with external air. In other words, a member, functioning as a heat source, is not disposed on the other side of the sidewall portion  71 . 
     The cover  31  is rotatably attached to the middle part of the sidewall portion  71  through the bearing member  102 . The sidewall portion  71  is made of a material, having a high specific heat and a high thermal conductivity, so as to quickly absorb a large amount of heat from the hydraulic fluid flowing through the first cooling flow pathway  9   a  and release the absorbed heat to the atmosphere. For example, the sidewall portion  71  is made of magnesium, aluminum or so forth. 
     The outer wall portion  72  is disposed in opposition to the outer peripheral surface of the torque converter  3 . The outer wall portion  72  is provided together with the sidewall portion  71  as a single member. However, the outer wall portion  72  can be provided as a member separated from the sidewall portion  71 . The outer wall portion  72  extends toward the prime mover  2  from the outer peripheral end of the sidewall portion  71 . The outer wall portion  72  extends substantially in parallel to the rotational axis O. It should be noted that the distal end (the prime mover  2 -side end) of the outer wall portion  72  slants radially inward. The outer wall portion  72  can be made of a similar material to the sidewall portion  71 . 
     The heat dissipation fins  73  are provided on the sidewall portion  71 . The heat dissipation fins  73  extend from the sidewall portion  71  to the opposite side (rightward in  FIG. 6 ) of the torque converter  3 . The heat dissipation fins  73  are attached to the sidewall portion  71  in order to efficiently dissipate the heat of the hydraulic fluid flowing through the first cooling flow pathway  9   a . The thermal conductivity of the heat dissipation fins  73  is preferably set to be equivalent to or higher than that of the sidewall portion  71  but is not particularly limited to this setting. The heat dissipation fins  73  are made of, for instance, magnesium, aluminum, copper or so forth. 
     &lt;First Cooling Flow Pathway  9   a&gt;   
     The first cooling flow pathway  9   a  is a flow pathway for cooling the hydraulic fluid discharged from the torque converter  3 . The first cooling flow pathway  9   a  extends in the interior of the torque converter casing  7 . In the present preferred embodiment, the first cooling flow pathway  9   a  is provided only in the upper half of the torque converter casing  7  (see  FIG. 2 ). 
     The first cooling flow pathway  9   a  extends from the middle part to the outer peripheral part in the interior of the sidewall portion  71  and axially extends therefrom beyond the torque converter  3  in the interior of the outer wall portion  72 . The first cooling flow pathway  9   a  is communicated with the hydraulic fluid sump  8 . 
     As shown in  FIG. 7  or  FIG. 8 , the first cooling flow pathway  9   a  includes a plurality of paths in the interior of the sidewall portion  71 . In the present preferred embodiment, the first cooling flow pathway  9   a  is divided into two paths in the interior of the sidewall portion  71 . In the interior of the sidewall portion  71 , the first cooling flow pathway  9   a  extends from the middle part to the outer peripheral part not in a straight shape but in a winding shape. 
     The first cooling flow pathway  9   a  can include a plurality of paths in the interior of the outer wall portion  72  as well. In the present preferred embodiment, the first cooling flow pathway  9   a  is divided into, for instance, three paths in the interior of the outer wall portion  72 . The first cooling flow pathway  9   a  axially extends in a straight shape in the interior of the outer wall portion  72 . Alternatively, the first cooling flow pathway  9   a  can extend in a winding shape in the interior of the outer wall portion  72 . 
     [Hydraulic Fluid Sump  8 ] 
     As shown in  FIG. 6 , the hydraulic fluid sump  8  is disposed to axially interpose the torque converter  3  together with the sidewall portion  71  therebetween. In other words, the hydraulic fluid sump  8 , the torque converter  3  and the sidewall portion  71  are axially aligned in this order. The hydraulic fluid sump  8  is disposed in the interior of the reducer casing  42 . The hydraulic fluid sump  8  is disposed above the rotational axis O. 
     The hydraulic fluid sump  8  contains the hydraulic fluid to be supplied to the torque converter  3  in the interior thereof. The hydraulic fluid sump  8  is provided with a supply port  81  in the bottom surface thereof. The hydraulic fluid, discharged from the supply port  81 , is supplied to the torque converter  3  through a flow pathway  106  provided between the stationary shaft  104  and the second boss portion  323   a  of the impeller hub  323 . 
     Specifically, a centrifugal force is generated in rotation of the impeller  32  of the torque converter  3 , whereby the hydraulic fluid residing in the interior of the flow pathway  106  is supplied to the interior of the torus T through the supply flow pathways  324 . Then, the hydraulic fluid, discharged from the torque converter  3 , flows to the first cooling flow pathway  9   a  through the communicating pathway  51 . Subsequently, the hydraulic fluid, cooled while flowing through the first cooling flow pathway  9   a , is returned to the hydraulic fluid sump  8 . 
     [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. 
     Modification 1 
     For example, as shown in  FIG. 9 , the torque converter unit can further include a second cooling flow pathway  9   b . The second cooling flow pathway  9   b  extends through the interior of a compartment  107  of a vehicle into which the torque converter unit is installed. The hydraulic fluid, discharged from the torque converter  3 , flows through the second cooling flow pathway  9   b . The hydraulic fluid, flowing through the second cooling flow pathway  9   b , is cooled while dissipating heat thereof into the compartment  107 . 
     The hydraulic fluid is supplied to the second cooling flow pathway  9   b  from the communicating pathway  51 . Additionally, the hydraulic fluid is returned to the hydraulic fluid sump  8  through the second cooling flow pathway  9   b.    
     The torque converter unit further includes a selector mechanism  11 . The selector mechanism  11  is configured to select either the first cooling flow pathway  9   a  or the second cooling flow pathway  9   b  as a cooling flow pathway for supplying the hydraulic fluid discharged from the torque converter  3 . 
     Modification 2 
     As shown in  FIG. 10 , the torque converter  3  can further include a plurality of elastic members  38 . The elastic members  38  are disposed circumferentially between the first one-way clutch  35  and the cover  31 . The elastic members  38  transmit a torque, applied in the reverse rotational direction from the cover  31 , to the first one-way clutch  35 . It should be noted that, when the cover  31  is rotated with respect to the first one-way clutch  35  by more than a predetermined angle in the reverse rotational direction, first stopper surfaces  314  of the cover  31  make contact with second stopper surfaces  351  of the first one-way clutch  35 . As a result, the torque, applied from the cover  31 , is directly transmitted to the first one-way clutch  35 . 
     In reverse rotation as described above, the torque, applied from the cover  31 , is firstly transmitted to the first one-way clutch  35  through the elastic members  38 , whereby massive and sudden torque transmission can be eased. 
     It should be noted that the elastic members  38  can be disposed circumferentially between the first one-way clutch  35  and the turbine  33 . In this case, the elastic members  38  transmit a torque, applied from the first one-way clutch  35  in the reverse rotational direction, to the turbine  33 . 
     Modification 3 
     As shown in  FIG. 11 , the present power transmission mechanism can include a planetary gear mechanism  400  and a clutch  401 . The planetary gear mechanism  400  includes a sun gear  402 , a plurality of planet gears  403 , a planet carrier  404  and a ring gear  405 . 
     The sun gear  402  is attached to the input shaft  5 . The sun gear  402  is unitarily rotated with the input shaft  5 . The planet carrier  404  is attached to the output shaft  6 . The planet carrier  404  is unitarily rotated with the output shaft  6 . 
     The clutch  401  is disposed between a non-rotatable member (e.g., the reducer casing  42  or the prime mover casing  21 ) and the ring gear  405 . Besides, the clutch  401  is configured to brake rotation of the ring gear  405 . 
     The clutch  401  is, for instance, a one-way clutch. The clutch  401  makes the ring gear  405  rotatable in forward rotation of the input shaft  5  and the output shaft  6 . By contrast, the clutch  401  makes the ring gear  405  non-rotatable in reverse rotation of the input shaft  5  and the output shaft  6 . 
     According to this configuration, when the input shaft  5  and the output shaft  6  are forwardly rotated, in other words, when the vehicle is forwardly moved, the ring gear  405  is being rotated without being fixed, whereby an amplifying action does not work in the planetary gear mechanism  400 . Because of this, the torque, outputted from the prime mover  2 , is transmitted to the drive wheels  101  through the torque converter  3  and the reducer  4 . 
     By contrast, when the input shaft  5  and the output shaft  6  are reversely rotated, in other words, when the vehicle is backwardly moved, the clutch  401  makes the ring gear  405  non-rotatable, whereby the amplifying function works in the planetary gear mechanism  400 . Because of this, the torque, outputted from the prime mover  2 , is transmitted to the drive wheels  101  through the reducer  4 , while being amplified by the planetary gear mechanism  400 . 
     REFERENCE SIGNS LIST 
     
         
           2  Prime mover 
           3  Torque converter 
           31  Cover 
           32  Impeller 
           33  Turbine 
           4  Reducer 
           400  Planetary gear mechanism 
           401  Clutch 
           402  Sun gear 
           403  Planet gear 
           404  Planet carrier 
           5  Input shaft 
           6  Output shaft 
           100  Drive unit