Patent Publication Number: US-11378178-B2

Title: Straddle vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2019-123350, filed on Jul. 2, 2019, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a straddle vehicle such as a motorcycle. 
     Description of the Related Art 
     Japanese Patent No. 5013813 discloses a vehicle power unit in which a main clutch disposed in a power transmission route from a crankshaft of an engine to a transmission is actuated by a hydraulic clutch actuator. This clutch actuator is mounted on an outer side surface of an upper portion of a clutch cover in such a manner that the clutch actuator overlaps a cylinder block in a side view of the vehicle. 
     In a configuration where a hydraulic actuator, namely an oil control valve unit, is mounted on a cover of a crankcase, the oil control valve unit obstructs the work of mounting and dismounting the cover of the crankcase, thus diminishing the efficiency of maintenance operations. If the oil control valve unit is mounted on a vehicle body frame, the oil control valve unit will be so far from the main clutch that the hydraulic oil passage will be long. 
     SUMMARY OF THE INVENTION 
     A straddle vehicle according to one aspect of the present disclosure includes: a prime mover; a hydraulically actuated main clutch disposed in a power transmission route between the prime mover and a drive wheel; an oil control valve unit that controls flow of a hydraulic oil supplied to the main clutch; and a power unit case including a case body and a cover removably mounted on the case body, the power unit case accommodating at least the main clutch, wherein the oil control valve unit is secured to the case body. 
     With the above configuration, since the oil control valve unit is secured to the case body rather than to the cover, the oil control valve unit does not obstruct the work of mounting and dismounting the cover, and the efficiency of maintenance operations is improved. Additionally, since the oil control valve unit is secured to the case body rather than to a vehicle body frame, the hydraulic oil passage from the oil control valve unit to the main clutch can be shortened. 
     In an exemplary configuration, the main clutch may include an inlet port into which the hydraulic oil is supplied, each of the case body and the cover may include a hydraulic oil passage, and the oil coming out of the oil control valve unit may flow through the hydraulic oil passage of the case body and then through the hydraulic oil passage of the cover to enter the inlet port of the main clutch. 
     With this configuration, the hydraulic oil passage from the oil control valve unit to the main clutch can be formed with a small number of components. 
     In an exemplary configuration, the prime mover may include a drive shaft having an axis extending in a vehicle width direction of the straddle vehicle, and the power unit case may include a main chamber accommodating the drive shaft and a subsidiary chamber accommodating the main clutch, the subsidiary chamber being defined between the case body and the cover and located on one side in the vehicle width direction with respect to the main chamber. 
     With this configuration, the power system can be made compact in the vehicle width direction, and an increase in vehicle width can be prevented. 
     In an exemplary configuration, at least a part of the oil control valve unit may be at the same location in the vehicle width direction as the subsidiary chamber. 
     With this configuration, the power system can be made compact in the vehicle width direction, and an increase in vehicle width can be prevented. 
     In an exemplary configuration, the oil control valve unit may be secured to a lower front portion of the case body. 
     With this configuration, interference of the oil control valve unit with other components can be avoided, and the oil control valve unit can be disposed in a compact manner. 
     In an exemplary configuration, when the oil control valve unit and the case body are viewed in the vehicle width direction, an upper end of the oil control valve unit may be located above a lower end of the case body, and a rear end of the oil control valve unit may be located rearwardly of a front end of the case body. 
     With this configuration, the length over which the oil control valve unit as viewed in the vehicle width direction projects from the case body can be reduced. 
     In an exemplary configuration, the case body may include a side wall portion defining the main chamber and a frame-shaped wall portion projecting from the side wall portion and defining the subsidiary chamber, the frame-shaped wall portion may include a lower edge portion, a front edge portion, and an inclined portion extending obliquely forward and upward from the lower edge portion to the front edge portion, the side wall portion may have a mounting surface located forwardly of and below the inclined portion when viewed in the vehicle width direction, and the oil control valve unit may be secured to the mounting surface of the side wall portion. 
     With this configuration, a size increase of the power unit case can be prevented, and at the same time the length over which the oil control valve unit projects from the power unit case can be reduced. 
     In an exemplary configuration, the mounting surface may be located inwardly of an outer end face of the frame-shaped wall portion in the vehicle width direction, and an outer end of the oil control valve unit in the vehicle width direction may be located inwardly of an outer end of the power unit case in the vehicle width direction. 
     With this configuration, the contact of the oil control valve unit with the ground can be reduced when the vehicle overturns. 
     In an exemplary configuration, the oil control valve unit may be secured to the mounting surface by fastening a fastener element onto the mounting surface from outside to inside in the vehicle width direction. 
     With this configuration, mounting and dismounting of the oil control valve unit can easily be performed by access from outside in the vehicle width direction. 
     In an exemplary configuration, the oil control valve unit may have an elongated shape, and the oil control valve unit may be disposed to extend longitudinally in a forward/rearward direction. 
     With this configuration, components other than the oil control valve unit can easily be mounted on the case body in such a manner that the other components are adjacent to the oil control valve unit in the vehicle width direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a motorcycle according to an embodiment. 
         FIG. 2  is a schematic diagram of a power system of the motorcycle of  FIG. 1 . 
         FIG. 3  is a cross-sectional view showing key components of the power unit of  FIG. 2 . 
         FIG. 4  is a schematic diagram of various shafts and other components of the power system of the motorcycle of  FIG. 1  as viewed in the vehicle width direction. 
         FIG. 5  is a right rear perspective view of the power unit of the motorcycle of  FIG. 1  with covers removed from the power unit. 
         FIG. 6  is a left rear perspective view of the power unit of  FIG. 5 . 
         FIG. 7  is a right side view of the power unit of  FIG. 5 . 
         FIG. 8  is a right side view of the power unit of  FIG. 7  with the covers mounted thereon. 
         FIG. 9  is a longitudinal sectional rear view of an extended portion of a crankcase and the vicinity of the extended portion in the power unit of  FIG. 5 . 
         FIG. 10  is a front view of the power unit of  FIG. 5  with the covers mounted thereon. 
         FIG. 11  is a perspective view of the power unit of  FIG. 6  with a drive motor removed therefrom. 
         FIG. 12  is an enlarged left side view of the drive motor and its vicinity in the power unit of the  FIG. 6 . 
         FIG. 13  is a block diagram illustrating an oil passage of the power unit of  FIG. 5 . 
         FIG. 14  is a bottom view of the power unit of  FIG. 8 . 
         FIG. 15  is a longitudinal sectional view of a bypass passage in the oil passage of the power unit of  FIG. 14 . 
         FIG. 16  is a longitudinal sectional view of a secondary filter and its vicinity in the power unit of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment will be described with reference to the drawings. 
       FIG. 1  is a side view of a motorcycle  1  according to the embodiment.  FIG. 2  is a schematic diagram of a power system of the motorcycle  1  of  FIG. 1 . As seen from  FIG. 1 , the motorcycle  1  is an example of a straddle vehicle on which the rider is seated in a straddling position. The motorcycle  1  is configured as a hybrid vehicle. The motorcycle  1  includes a front wheel  2 , a rear wheel  3  (drive wheel), a vehicle body frame  4 , a front suspension  5  connecting the front wheel  2  to a front portion of the vehicle body frame  4 , and a rear suspension  6  connecting the rear wheel  3  to a rear portion of the vehicle body frame  4 . The front suspension  5  is coupled to a bracket  7  spaced from the front suspension  5  in the upward/downward direction. A steering shaft connected to the bracket  7  is supported by a head pipe  4   a  constituting a part of the vehicle body frame  4 , and the steering shaft is angularly movable. On the steering shaft is mounted a handle  8  to be held by the hands of the rider. A fuel tank  9  is disposed rearwardly of the handle  8 , and a seat  10  on which the rider sits is disposed rearwardly of the fuel tank  9 . On the vehicle body frame  4  is mounted a power unit  11  serving as a drive source for travel, and the power unit  11  is located between the front and rear wheels  2  and  3 . 
     As shown in  FIGS. 1 and 2 , the power unit  11  includes an engine E and a drive motor M as prime movers. The engine E is an internal combustion engine, and the drive motor M is an electric motor. A transmission  12  is disposed rearwardly of the engine E. The transmission  12  includes an input shaft  12   a , an output shaft  12   b , and a plurality of gear trains  12   c  having different reduction ratios. The transmission  12  is configured to transmit power from the input shaft  12   a  to the output shaft  12   b  through the gear trains  12   c  and configured to perform speed change with a selected one of the gear trains  12   c . For example, the transmission  12  is a dog clutch transmission. The engine E includes a crankshaft Ea, one end of which is connected to a primary gear  17  so as to be capable of power transmission to the primary gear  17 . The other end of the crankshaft Ea is connected to an integrated starter generator ISG so as to be capable of power transmission to the integrated starter generator ISG. 
     The primary gear  17  is disposed around the input shaft  12   a  and located between the main clutch  13  and the gear trains  12   c  in the axial direction of the input shaft  12   a . The primary gear  17  is rotatable relative to the input shaft  12   a . The primary gear  17  transmits rotational power from the crankshaft Ea to the main clutch  13 . The primary gear  17  is connected to the input shaft  12   a  of the transmission  12  via the main clutch  13  so as to be capable of power transmission to the input shaft  12   a . The main clutch  13  is mounted on one end of the input shaft  12   a  and functions to establish and break the power transmission route from the crankshaft Ea to the input shaft  12   a . The main clutch  13  is actuated by hydraulic pressure. The main clutch  13  is, for example, a multi-plate clutch. 
     In the vicinity of the input shaft  12   a  there are disposed a clutch-dedicated pump P 1  and an engine-dedicated pump P 2  which are mechanically linked to, and driven by, rotation of the input shaft  12   a . The respective driven shafts Pa of the clutch-dedicated pump P 1  and engine-dedicated pump P 2  are coaxial. The crankshaft Ea of the engine E, the input and output shafts  12   a  and  12   b  of the transmission  12 , the main clutch  13 , the clutch-dedicated pump P 1 , and the engine-dedicated pump P 2  are accommodated in a crankcase  14  (power unit case). The crankshaft Ea, input shaft  12   a , output shaft  12   b , and driven shafts Pa are parallel to one another and extend in the vehicle width direction of the motorcycle  1  (leftward/rightward direction). The vehicle body frame  4  supports a swing arm  15  supporting the rear wheel  3  and extending in the forward/rearward direction, and the swing arm  15  is angularly movable. The rotational power of the output shaft  12   b  of the transmission  12  is transmitted to the rear wheel  3  through an output transmission member  16  (e.g., a chain or belt). 
     Between the primary gear  17  and the gear trains  12   c  is disposed a sprocket  18  (rotary member), which is mounted around the input shaft  12   a  and rotates together with the input shaft  12   a . The drive motor M includes a motor housing Ma and a motor drive shaft Mb projecting from the motor housing Ma, and a sprocket  19  is mounted on the motor drive shaft Mb to rotate together with the motor drive shaft Mb. Gears or pulleys may be used as the rotary members instead of the sprockets  18  and  19 . A chain  20  (power transmission member) is connected to both the sprocket  18  mounted on the input shaft  12   a  and the sprocket  19  mounted on the motor drive shaft Mb. Thus, the drive power of the drive motor M is transmitted to the input shaft  12   a  through the sprocket  18 . That is, the combination of the sprocket  19 , chain  20 , and sprocket  18  is an example of a power transmission mechanism  40  that transmits power from the motor drive shaft Mb to the input shaft  12   a.    
     The drive power of the engine E is transmitted to a first one-way clutch  21  through the primary gear  17 . The drive power of the drive motor M is transmitted to a second one-way clutch  22  through the sprocket  18 . The first one-way clutch  21  and second one-way clutch  22  are connected to the driven shafts Pa of the clutch-dedicated pump P 1  and engine-dedicated pump P 2  via a power transmission mechanism  23  so as to be capable of power transmission to the driven shafts Pa. That is, the first one-way clutch  21  and second one-way clutch  22  constitute a pump power transmitter  30  that transmits the drive power of the engine E or drive motor M to the clutch-dedicated pump P 1  and engine-dedicated pump P 2 . 
     An oil discharged from the clutch-dedicated pump P 1  is supplied as a hydraulic oil to the main clutch  13  through an oil control valve unit  24 . The oil control valve unit  24  opens and closes a flow passage through which the oil flows from the clutch-dedicated pump P 1  to the main clutch  13 . An oil discharged from the engine-dedicated pump P 2  is supplied as a lubricating oil to the engine E and the transmission  12 . An electronic control unit (ECU)  25  controls the engine E. Specifically, the ECU  25  controls a throttle device T, a fuel injection device F, and an ignition device I. The ECU  25  further controls engagement and disengagement of the main clutch  13  by controlling the opening and closing of the oil control valve unit  24 . 
       FIG. 3  is a cross-sectional view showing key components of the power unit  11  of  FIG. 2 . As shown in  FIG. 3 , the crankcase  14  includes: a case body  27  including a main body portion  31  and an extended portion  32  ( FIG. 5 ) which are described in detail below; and a first cover  28  removably mounted on a lateral side of the case body  27 . A second cover  29 , which is described in detail below, is further mounted on the case body  27 . Inside the case body  27  is formed a main chamber MR (crank chamber). The main chamber MR accommodates the crankshaft Ea and the transmission  12 . The main body portion  31  of the case body  27  includes a side wall portion  31   a  provided with a hole H, inside which a bearing  33  is mounted. The bearing  33  rotatably supports the input shaft  12   a . The side wall portion  31   a  is located between the primary gear  17  and the gear trains  12   c.    
     Between the case body  27  and the first cover  28  (and the second cover  29 ) is formed a subsidiary chamber SR. One end of the input shaft  12   a  projects into the subsidiary chamber SR. Although not illustrated in  FIG. 3 , the driven shafts Pa (see  FIG. 4 ) of the clutch-dedicated pump P 1  and engine-dedicated pump P 2  also project into the subsidiary chamber SR. The subsidiary chamber SR accommodates one end of the crankshaft Ea, the one end of the input shaft  12   a , the main clutch  13 , the primary gear  17 , the pump power transmitter  30 , and the driven shafts Pa of the oil pumps P 1  and P 2 . 
     The main clutch  13  includes an outer case (not illustrated), an inner case (not illustrated), and a multi-plate unit (not illustrated). When the multi-plate unit is brought into a pressed state, the inner case becomes unable to rotate relative to the outer case, and a clutch-engaged state is established. When the multi-plate unit is brought into a non-pressed state, the inner case becomes able to rotate relative to the outer case, and a clutch-disengaged state is established. The outer case of the main clutch  13  is coupled to the primary gear  17  to rotate together with the primary gear  17 . The inner case of the main clutch  13  is coupled to the input shaft  12   a  to rotate together with the input shaft  12   a . The inner diameter of the primary gear  17  is larger than the outer diameter of the input shaft  12   a , and the inner peripheral surface of the primary gear  17  is spaced radially outward from the outer peripheral surface of the input shaft  12   a.    
     The input shaft  12   a  is hollow. Inside the input shaft  12   a  are formed a first flow passage S 1  and a second flow passage S 2  which are divided from each other. The first flow passage S 1  opens to the outside at one end of the input shaft  12   a , while the second flow passage S 2  opens to the outside at the other end of the input shaft  12   a . The input shaft  12   a  includes a first opening  12   d  through which the first flow passage S 1  communicates with an inlet port  13   a  of the main clutch  13 . The input shaft  12   a  includes a second opening  12   e  through which a lubricating oil is supplied from the second flow passage S 2  to the main clutch  13 . The first cover  28  includes a fitting portion  28   a  tightly fitted on the one end of the input shaft  12   a.    
     In the first cover  28  is formed a hydraulic oil passage  28   b  communicating with the oil control valve unit  24  (see  FIG. 2 ). The hydraulic oil passage  28   b  communicates with the first flow passage S 1  at the inside of the fitting portion  28   a . When a hydraulic pressure larger than a predetermined pressure is applied from the hydraulic oil passage  28   b  of the first cover  28  to the main clutch  13  through the first flow passage S 1  of the input shaft  12   a , the main clutch  13  is brought into an engaged state. When the hydraulic pressure transmitted from the hydraulic oil passage  28   b  of the first cover  28  to the main clutch  13  through the first flow passage S 1  of the input shaft  12   a  falls below the predetermined pressure, the main clutch  13  is brought into a disengaged state. 
     The pump power transmitter  30  is configured to receive power transmitted from the crankshaft Ea of the engine E through the primary gear  17  and power transmitted from the drive motor M through the input shaft  12   a  and sprocket  18  and configured to appropriately transmit the power of the crankshaft Ea and the power of the drive motor M to the driven shafts Pa of the clutch-dedicated pump P 1  and engine-dedicated pump P 2 . The pump power transmitter  30  includes the first one-way clutch  21 , the second one-way clutch  22 , and a common gear  34  (tubular member). A sprocket or pulley may be used as the tubular member instead of the common gear  34 . 
     The sprocket  18 , to which the drive power of the drive motor M is transmitted, is located closer to the gear trains  12   c  than is the primary gear  17 . Specifically, the sprocket  18  is disposed between the primary gear  17  and the bearing  33  mounted in the side wall portion  31   a  of the case body  27 . Thus, the chain  20  (see  FIGS. 2 and 5 ) connected to the sprocket  18  is prevented from being located outwardly of the main clutch  13  in the vehicle width direction, and an increase in vehicle width can be prevented. 
     The primary gear  17  includes a tubular portion  17   a  projecting toward the sprocket  18  from an inner peripheral portion of the primary gear  17 . The first one-way clutch  21  is fitted around the tubular portion  17   a  of the primary gear  17 . The sprocket  18  includes: a tubular portion  18   a  which is an inner peripheral portion fitted around the input shaft  12   a  so that the sprocket  18  rotates together with the input shaft  12   a ; an annular plate portion  18   b  projecting radially outward from the tubular portion  18   a ; and a power receiving portion  18   c  which is a teeth portion disposed on the outer periphery of the annular plate portion  18   b  and having the chain  20  connected thereto. The tubular portion  18   a  projects toward the primary gear  17  from the annular plate portion  18   b . The portions of the sprocket  18  need not be integrally formed; for example, the tubular portion  18   a  and the annular plate portion  18   b  may be separately formed and secured to each other. 
     The second one-way clutch  22  is fitted around the tubular portion  18   a  of the sprocket  18 . The second one-way clutch  22  is located closer to the sprocket  18  than is the first one-way clutch  21 . The first one-way clutch  21  and second one-way clutch  22  are disposed around the input shaft  12   a  and adjacent to each other in the direction of the axis X of the input shaft  12   a . The rotational power of the primary gear  17  is transmitted to the first one-way clutch  21 , and the rotational power of the input shaft  12   a  is transmitted to the second one-way clutch  22 . 
     The first one-way clutch  21  and second one-way clutch  22  are fitted inside the common gear  34 . The first one-way clutch  21  transmits power in the direction from the primary gear  17  toward the common gear  34 , but does not transmit power in the direction from the common gear  34  toward the primary gear  17 . The second one-way clutch  22  transmits power in the direction from the sprocket  18  toward the common gear  34 , but does not transmit power in the direction from the common gear  34  toward the sprocket  18 . Since the two adjacent one-way clutches  21  and  22  are fitted inside the common gear  34 , an increase in the number of required components can be prevented, and a reduction of the size of the pump power transmitter  30  in the direction of the axis X can be achieved. The common gear  34  is connected to the driven shafts Pa of the clutch-dedicated pump P 1  and engine-dedicated pump P 2  so as to be capable of power transmission to the driven shafts Pa, and this connection is made via a gear serving as the power transmission mechanism  23 . 
     The primary gear  17  includes a recessed portion  17   b  recessed toward the main clutch  13  in the direction of the axis X. The recessed portion  17   b  is located radially outward of, and adjacent to, the tubular portion  17   a . A part of the first one-way clutch  21  and a part of the common gear  34  are placed in the recessed portion  17   b . In other words, the first one-way clutch  21  and the common gear  34 , as viewed in the radial direction of the primary gear  17 , overlap the primary gear  17 . Thus, the power system can be made compact in the vehicle width direction. 
     In the sprocket  18 , the power receiving portion  18   c  is offset with respect to the annular plate portion  18   b  in a direction away from the side wall portion  31   a ; namely, the power receiving portion  18   c  is offset outwardly in the vehicle width direction. As such, even when the sprocket  18  is configured to have a large diameter, the side wall portion  31   a  of the case body  27  is unlikely to interfere with the sprocket  18 . Thus, the flexibility in shape design of the case body  27  can be increased. The sprocket  18  includes a recessed portion  18   d  located between the tubular portion  18   a  and the power receiving portion  18   c  and recessed in a direction away from the main clutch  13  along the axis X (a direction toward the main chamber MR). The recessed portion  18   d  is disposed radially outward of, and adjacent to, the tubular portion  18   a . A part of the second one-way clutch  22  and a part of the common gear  34  are placed in the recessed portion  18   d . In other words, the second one-way clutch  22  and the common gear  34 , as viewed in the radial direction of the sprocket  18 , overlap the sprocket  18 . Thus, the power system can be made compact in the vehicle width direction. 
     The sprocket  18  is larger in diameter than the gears of the gear trains  12   c . The sprocket  18  is larger in diameter than the common gear  34 . The sprocket  18  is disposed in the subsidiary chamber SR in which the main clutch  13  having a relatively large diameter is disposed, rather than in the main chamber MR in which the transmission  12  is disposed. Thus, even when the sprocket  18  has a large diameter, a size increase of the crankcase  14  can be prevented. Additionally, since the sprocket  18  is disposed in the subsidiary chamber SR, the sprocket  18  can be configured to have a large diameter such that the speed of rotation produced by the drive power transmitted from the drive motor M to the input shaft  12   a  is reduced at a sufficient reduction ratio. This can prevent a size increase of the drive motor M. Hence, in the hybrid motorcycle  1  in which the drive power of the drive motor M is transmitted to the input shaft  12   a  of the transmission  12 , a size increase of the power system can be prevented. 
       FIG. 5  is a right rear perspective view of the power unit  11  of the motorcycle  1  of  FIG. 1  with the covers removed from the power unit  11 .  FIG. 6  is a left rear perspective view of the power unit  11  of  FIG. 5 .  FIG. 7  is a right side view of the power unit  11  of  FIG. 5 .  FIG. 8  is a right side view of the power unit of  FIG. 7  with the covers mounted thereon.  FIG. 9  is a longitudinal sectional rear view of the extended portion  32  of the crankcase  14  and the vicinity of the extended portion  32  in the power unit  11  of  FIG. 5 .  FIG. 10  is a front view of the power unit  11  of  FIG. 5  with the covers mounted thereon. As shown in  FIGS. 5 to 8 , the power unit  11  includes the engine E and the drive motor M. The engine E includes a cylinder Eb extending upwardly from a front portion of the crankcase  14 . The crankcase  14  includes the main body portion  31  projecting rearwardly from a lower portion of the cylinder Eb. The drive motor M is disposed rearwardly of the cylinder Eb and mounted on the top surface of the main body portion  31 . That is, the drive motor M is aligned with the main body portion  31  in the upward/downward direction. 
     The drive motor M includes the motor housing Ma which is approximately in the shape of a circular tube, and the motor drive shaft Mb projects to one side (toward the subsidiary chamber SR) in the vehicle width direction. The crankcase  14  further includes the extended portion  32  projecting upwardly from the main body portion  31  so as to be located lateral to the motor housing Ma. Thus, the extended portion  32  as viewed from the one side in the vehicle width direction overlaps the drive motor M. The upper end of the extended portion  32  is at a lower level than the upper end of the drive motor M. One end of the motor drive shaft Mb is inserted into the extended portion  32 . 
     The case body  27  includes a frame-shaped wall portion  31   b  projecting outwardly (toward the main clutch  13 ) in the vehicle width direction from the side wall portion  31   a  of the main body portion  31  defining the main chamber MR. The frame-shaped wall portion  31   b  has an opening  31   c  facing outwardly in the vehicle width direction. The first cover  28  is removably secured to the outer end face of the frame-shaped wall portion  31   b  in the vehicle width direction so as to close the opening  31   c . The extended portion  32  has an opening  32   d  facing outwardly in the vehicle width direction, and the second cover  29  is removably secured to the frame-shaped outer end face of the extended portion  32  in the vehicle width direction so as to close the opening  32   d . The case body  27  includes a bridge portion  37  dividing the opening  31   c  from the opening  32   d  and connecting the end face of the frame-shaped wall portion  31   b  to the end face of a frame-shaped wall portion  32   b.    
     A space surrounded by the side wall portion  31   a , frame-shaped wall portion  31   b , and first cover  28  and a space surrounded by the extended portion  32  and second cover  29  constitute the subsidiary chamber SR. The subsidiary chamber SR includes a first region R 1  defined by the main body portion  31  and a second region R 2  defined by the extended portion  32 . The second region R 2  communicates with the first region R 1  in the upward/downward direction. The first cover  28  closes the first region R 1  from outside in the vehicle width direction. The second cover  29  closes the second region R 2  from outside in the vehicle width direction. Since the cover closing the subsidiary chamber SR is divided into the first cover  28  and second cover  29  which are separate from each other, the individual covers  28  and  29  can be prevented from having a large area even when the size of the subsidiary chamber SR is increased by the presence of the extended portion  32 . Thus, noise due to vibration of the covers can be reduced. 
     As shown in  FIG. 9 , the motor housing Ma of the drive motor M includes a projecting tubular portion Mc into which the motor drive shaft Mb is inserted. The extended portion  32  of the crankcase  14  includes a side wall portion  32   a  facing the motor housing Ma and a frame-shaped wall portion  32   b  projecting outwardly in the vehicle width direction from the periphery of the side wall portion  32   a  and continuous with the main body portion  31 . The side wall portion  32   a  of the extended portion  32  is provided with an insertion hole  32   c . The projecting tubular portion Mc of the motor housing Ma is inserted into the insertion hole  32   c . An annular sealing member  35  is interposed between the inner peripheral surface of the insertion hole  32   c  and the outer peripheral surface of the projecting tubular portion Mc. The motor drive shaft Mb projects outwardly in the vehicle width direction from the projecting tubular portion Mc. That is, the motor drive shaft Mb extends through the insertion hole  32   c  and projecting tubular portion Mc and leads to the second region R 2  defined inside the extended portion  32 . Thus, the motor housing Ma and the extended portion  32  are easily connected by fitting. An annular sealing member  36  is disposed in a gap between the motor drive shaft Mb and the projecting tubular portion Mc. 
     The sprocket  19  is located in the second region R 2  of the subsidiary chamber SR and mounted on the outer end of the motor drive shaft Mb. The chain  20  wound around both the sprocket  19  mounted on the motor drive shaft Mb and the sprocket  18  mounted on the input shaft  12   a  is accommodated in the subsidiary chamber SR and lies partly in the second region R 2  and partly in the first region R 1 . 
     Since the sprockets  18  and  19  are disposed in the subsidiary chamber SR which can be opened by removing the first and second covers  28  and  29 , the work of mounting and dismounting the chain  20  on and from the sprockets  18  and  19  can easily be performed. Additionally, since the power transmission mechanism  40  which transmits drive power from the motor drive shaft Mb to the input shaft  12   a  is entirely accommodated in the crankcase  14 , a size increase of the power system can be prevented, and lubrication of the power transmission mechanism  40  can easily be accomplished along with lubrication of the main clutch  13 . 
     As shown in  FIG. 7 , the chain  20  is held between a front chain guide  38  and a rear chain guide  39  in the forward/rearward direction and guided by these guides. A chain tensioner  41  is mounted on the rear surface of the case body  27  of the crankcase  14 , and the chain tensioner  41  biases the rear chain guide  39  toward the chain  20 . This prevents loosening of the chain  20 . On the rear surface of the cylinder Eb is mounted a chain tensioner  42  that prevents loosening of a chain (not illustrated) mechanically connecting a valve actuator (not illustrated) disposed above the cylinder Eb to the crankshaft Ea. 
     As shown in  FIGS. 6 and 9 , the end face of the frame-shaped wall portion  32   b  of the extended portion  32  is inclined to extend inwardly in the vehicle width direction from bottom to top. Thus, in a rear view, the chain tensioner  42  protrudes outwardly of the extended portion  32  in the vehicle width direction. As such, even when the extended portion  32  and the chain tensioner  42  are at least partly at the same location in the upward/downward direction, the chain tensioner  42  is easily accessible from the rear. 
     As shown in  FIG. 8 , the crankcase  14  includes an engine lubricating oil passage  43  through which flows a lubricating oil for lubrication of the engine E and a motor power system lubricating oil passage  44  branching from the engine lubricating oil passage  43 . The motor power system lubricating oil passage  44  is formed in the extended portion  32 . In particular, the motor power system lubricating oil passage  44  is formed partly in the case body  27  and partly in the second cover  29 . The motor power system lubricating oil passage  44  includes an upstream passage  44   a  formed in the frame-shaped wall portion  32   b  of the extended portion  32  and a downstream passage  44   b  formed in the second cover  29  and communicating with the upstream passage  44   a . The upstream passage  44   a  and downstream passage  44   b  communicate at a contact plane between the respective end faces of the frame-shaped wall portion  32   b  and second cover  29 . 
     The second cover  29  includes a discharge outlet  44   c  through which the lubricating oil flowing in the downstream passage  44   b  is discharged into the second region R 2 . Through the discharge outlet  44   c , the oil is ejected toward meshing portions of the sprocket  19  and chain  20 . In this configuration, part of the lubricating oil flowing through the engine lubricating oil passage  43  for lubrication of the engine can be used for lubrication of the sprocket  19  and chain  20  in the second region R 2 . Thus, the lubrication structure can be simplified. Additionally, since the lubricating oil is discharged from the second cover  29  rather than from the case body  27 , the flexibility in setting the location of discharge of the lubricating oil is increased. Beneath the crankcase  14  is mounted an oil pan  45  retaining the oil falling from the main chamber MR and the subsidiary chamber SR. 
     As shown in  FIGS. 5, 7, 8, and 10 , the oil control valve unit  24  which controls flow of the hydraulic oil supplied to the main clutch  13  is secured to the case body  27  of the crankcase  14 . Since the oil control valve unit  24  is secured to the case body  27  rather than to the first cover  28 , the oil control valve unit  24  does not obstruct the work of mounting and dismounting the first cover  28 , and the efficiency of maintenance operations is improved. Additionally, since the oil control valve unit  24  is secured to the case body  27  rather than to the vehicle body frame  4 , the hydraulic oil passage from the oil control valve unit  24  to the main clutch  13  can be shortened. In particular, the oil control valve unit  24  is secured to a lower front portion of the case body  27 . As such, interference of the oil control valve unit  24  with other components can be avoided, and the oil control valve unit  24  can be disposed in a compact manner. The oil control valve unit  24  has an elongated shape and is disposed to extend longitudinally in the forward/rearward direction. Thus, the power system can be made compact in the vehicle width direction. 
     As seen from  FIG. 7 , when the power unit is viewed in the vehicle width direction, the upper end of the oil control valve unit  24  is located above the lower end of the case body  27 , and the rear end of the oil control valve unit  24  is located rearwardly of the front end of the case body  27 . This provides a reduction in the length over which the oil control valve unit  24  as viewed in the vehicle width direction projects from the case body  27 . The frame-shaped wall portion  31   b  of the main body portion  31  of the case body  27  includes a lower edge portion  31   ba , a front edge portion  31   bb , and an inclined portion  31   bc  extending obliquely forward and upward from the lower edge portion  31   ba  to the front edge portion  31   bb . The side wall portion  31   a  of the main body portion  31  has a mounting surface  31   g  (see  FIG. 10 ) located forwardly of and below the inclined portion  31   c  when viewed in the vehicle width direction. 
     As shown in  FIG. 10 , the mounting surface  31   g  is located inwardly of the outer end face of the frame-shaped wall portion  31   b  in the vehicle width direction. The oil control valve unit  24  is secured to the mounting surface  31   g . The rear end portion of the oil control valve unit  24  is at the same location in the vehicle width direction as the subsidiary chamber SR. The outer end of the oil control valve unit  24  in the vehicle width direction is located inwardly of the outer end of the crankcase  14  in the vehicle width direction. Thus, the length over which the oil control valve unit  24  projects from the crankcase  14  is so small that the power unit  11  can be made compact and that when the vehicle overturns, the contact of the oil control valve unit  24  with the ground can be reduced. The oil control valve unit  24  is secured to the mounting surface  31   g  by fastening fastener elements B onto the mounting surface  31   g  from outside to inside in the vehicle width direction. As such, mounting and dismounting of the oil control valve unit  24  can easily be performed by access from outside in the vehicle width direction. 
     As shown in  FIGS. 3, 7, and 8 , the main clutch  13  includes the inlet port  13   a  into which the hydraulic oil is supplied. The first cover  28  includes the hydraulic oil passage  28   b  into which flows the oil coming out of the oil control valve unit  24 . The oil coming out of the oil control valve unit  24  flows in the hydraulic oil passage  28   b  of the first cover  28  and passes through the first flow passage S 1  of the input shaft  12   a  to enter the inlet port  13   a  of the main clutch  13 . Thus, the hydraulic oil passage from the oil control valve unit  24  to the main clutch  13  can be formed with a small number of components. The flow outlet of the oil control valve unit  24  and the flow inlet of the hydraulic oil passage  28   b  may be directly connected to each other, may be connected via a flow passage formed in the main body portion  31 , or may be connected via a tube. 
     As shown in  FIG. 10 , the second cover  29  is located inwardly of the first cover  28  in the vehicle width direction. Thus, the size of the crankcase  14  is effectively reduced to allow another component (such as a portion  4   b  of the vehicle body frame  4 ) to be disposed in a space outward of the second cover  29  in the vehicle width direction. An increase in vehicle width can therefore be prevented. The bottom surface of the main body portion  31  of the case body  27 , as viewed from the front, is provided with a recessed portion  31   e  recessed upwardly. An exhaust pipe  46  connected to an exhaust port of the cylinder Eb passes through the recessed portion  31   e  from front to rear. 
       FIG. 11  is a perspective view of the power unit  11  of  FIG. 6  with the drive motor M removed therefrom.  FIG. 12  is an enlarged left side view of the drive motor M and its vicinity in the power unit  11  of  FIG. 6 . As shown in  FIGS. 11 and 12 , the main body portion  31  of the crankcase  14  includes an upper wall portion  31   f , and the upper wall portion  31   f  includes front mount portions  47 , rear mount portions  48 , and a case top surface  49 . Each of the front and rear mount portions  47  and  48  is, for example, a base provided with a bolt hole, and projects upwardly from the upper wall portion  31   f  of the main body portion  31 . The front of the motor housing Ma is secured to the front mount portions  47  from above by fastener elements B. The rear of the motor housing Ma is secured to the rear mount portions  48  from above by fastener elements B. That is, the electric motor M is supported by the front and rear mount portions  47  and  48  of the crankcase  14 . 
     The case top surface  49  is formed between the front mount portions  47  and the rear mount portions  48  and has a circular arc shape conforming to the outer peripheral surface of the motor housing Ma. The motor housing Ma is located in proximity to the case top surface  49  and slightly spaced from the case top surface  49 . The drive motor M is disposed in such a manner that a lower portion of the motor housing Ma is held between the front mount portions  47  and the rear mount portions  48 . As such, the drive motor M can be stably supported by the crankcase  14 . 
     The case body  27  of the crankcase  14  includes a breather portion  50  projecting upwardly from the main body portion  31  accommodating the crankshaft Ea and the transmission  12 . The breather portion  50  defines a breather chamber BR into which blowby gas is introduced from the main chamber MR. For example, the breather chamber BR has a labyrinth structure and separates oil mist coming from the main chamber MR into air and oil. The breather portion  50  is formed between the front of the case top surface  49  and the main body portion  31  by making use of the circular arc shape of the case top surface  49  which is downwardly recessed. 
     In particular, the breather portion  50  has a facing surface  50   a  facing the outer peripheral surface of the motor housing Ma, and the facing surface  50   a  as viewed in the vehicle width direction has a circular arc shape. In other words, the facing surface  50   a  of the breather portion  50  constitutes a part of the case top surface  49 . The drive motor M is disposed rearwardly of the cylinder Eb of the engine E and separated from the cylinder Eb by a gap G. The breather portion  50  is disposed in the gap G and located rearwardly of the cylinder Eb and forwardly of the drive motor M. The front mount portions  47  are spaced rearwardly from the cylinder Eb of the engine E. 
     With this configuration, the breather portion  50  can be efficiently arranged even though the drive motor M is disposed rearwardly of the cylinder Eb of the engine E and mounted on the top surface of the crankcase  14 . As such, the engine E, the crankcase  14 , and the drive motor M can be closely arranged, and a size reduction of the motorcycle  1  can be achieved. Additionally, heat transfer from the cylinder Eb to the drive motor M can be reduced. Further, since the cylinder Eb and the front mount portions  47  are independent of each other, a process such as machining can easily be performed. 
     The top portion of the breather portion  50  is provided with a connection port  50   b  to which a breather tube  51  is connected. The connection port  50   b  is at the same location in the forward/rearward direction as the front mount portions  47 . Specifically, the front mount portions  47  (two front mount portions  47  in  FIG. 11 ) are aligned in the vehicle width direction. The connection port  50   b  is disposed between the pair of front mount portions  47  spaced from each other in the vehicle width direction. To the connection port  50   b  is connected one end of the breather tube  51 . The other end of the breather tube  51  is connected to an air cleaner box  52  disposed above the drive motor. The air cleaner box  52  forms an intake passage for introduction of intake air into the cylinder Eb and purifies the intake air. The blowby gas from which the oil has been separated in the breather chamber BR is delivered into the air cleaner box  52  through the breather tube  51 . 
     The breather tube  51  extends upwardly from the breather portion  50  and passes through the gap G toward the air cleaner box  52 . Thus, the breather tube  51  can be efficiently arranged by making use of the gap G between the cylinder Eb of the engine E and the drive motor M, and the space efficiency is therefore increased. Additionally, since the connection port  50   b  is at the same location in the forward/rearward direction as the front mount portions  47  spaced rearwardly from the cylinder Eb, the breather tube  51  can easily be attached to the connection port  50   b  while the breather tube  51  is maximally spaced from the cylinder Eb. 
       FIG. 13  is a block diagram illustrating the oil passage of the power unit  11  of  FIG. 5 . As shown in  FIG. 13 , the oil retained in the oil pan  45  is sucked to a strainer  60  under the action of sucking pressures generated by the clutch-dedicated pump P 1  and engine-dedicated pump P 2 , and the sucked oil is filtered through a primary filter  61  attached to the strainer  60  and drawn into the clutch-dedicated pump P 1  and engine-dedicated pump P 2 . 
     The oil discharged from the clutch-dedicated pump P 1  passes through a relief valve  62 , then is filtered through a secondary filter  63 , and reaches the oil control valve unit  24 . Once the pressure of the oil flowing from the clutch-dedicated pump P 1  to the oil control valve unit  24  exceeds a predetermined relief pressure, the relief valve  62  operates to discharge the oil into a bypass passage  64 . The bypass passage  64  communicates with a flow passage between the primary filter  61  and the clutch-dedicated pump P 1  (and the engine-dedicated pump P 2 ). 
     When the flow outlet of the oil control valve unit  24  is closed, the relief valve  62  is brought into a relief state once the hydraulic pressure applied to the relief valve  62  exceeds the relief pressure. While the relief valve  62  is in the relief state, the oil delivered from the clutch-dedicated pump P 1  is discharged from the relief valve  62  into the bypass passage  64  and returned to a point upstream of the clutch-dedicated pump P 1  (and the engine-dedicated pump P 2 ). In this manner, the amount of the oil sucked from the oil pan  45  through the primary filter  61  can be reduced, and thus the energy loss due to the passing of the oil through the primary filter  61  can be reduced. 
     Once the flow outlet of the oil control valve unit  24  is opened, the relief valve  62  is brought into a normal state, where the oil delivered from the clutch-dedicated pump P 1  passes through the relief valve  62 , then is filtered through the secondary filter  63 , and supplied as the hydraulic oil to the main clutch  13  (in particular its inlet port  13   a ) through the oil control valve unit  24 . The oil discharged from the engine-dedicated pump P 2  is filtered through a secondary filter  65  and supplied to a main gallery  67  in the engine lubricating oil passage  43  of the engine E. 
     Since the primary filter  61  is used for both the clutch-dedicated pump P 1  and the engine-dedicated pump P 2 , an increase in the number of required components and an increase in size of the space required for arrangement of the components can be prevented. Additionally, since the clutch-dedicated pump P 1  and the engine-dedicated pump P 2  are independent of each other and are not used for the same purpose, the hydraulic oil can, even at the start-up of the engine E, be supplied quickly to the main clutch  13  before the main gallery  67  is filled with the oil. As such, the time required for the main clutch  13  to become operable can be shortened even at the start-up of the engine E. 
       FIG. 14  is a bottom view of the power unit  11  of  FIG. 8 . As shown in  FIG. 14 , a frame-shaped mounting seat  79  on which the oil pan  45  is mounted is disposed on the bottom surface of the crankcase  14  (in particular its case body  27 ). The frame-shaped mounting seat  79  surrounds a region of the bottom surface of the crankcase  14 , and the strainer  60  with the primary filter  61  attached thereto and the secondary filter  63  are disposed in the region surrounded by the mounting seat  79  and are covered from below by the oil pan  45 . With the oil pan  45  removed from the mounting seat, the strainer  60  and the secondary filter  63  as viewed from below are exposed to the outside. Thus, both the primary filter  61  and the secondary filter  63  are made easily accessible by removing the oil pan  45 . This improves the ease of maintenance of the two filters  61  and  63 . 
     The strainer  60  is connected to a flow inlet  70  opening at the bottom surface of the crankcase  14 . The oil sucked to the strainer  60  and filtered through the primary filter  61  flows into the flow inlet  70 . The crankcase  14  includes a first flow passage  71  in which the oil entering the crankcase  14  through the flow inlet  70  flows toward the suction inlet of the clutch-dedicated pump P 1  (and the suction inlet of the engine-dedicated pump P 2 ). The crankcase  14  includes a second flow passage  72  in which the oil discharged from the clutch-dedicated pump P 1  flows toward the secondary filter  63 . The second flow passage  72  includes a portion located close to the first flow passage  71  and extending parallel to the first flow passage  71 . The crankcase  14  includes a third flow passage  73  in which the oil filtered through the secondary filter  63  flows toward the flow inlet of the oil control valve unit  24 . 
       FIG. 15  is a longitudinal sectional view of the bypass passage  64  in the oil passage of the power unit  11  of  FIG. 14 . As shown in  FIGS. 14 and 15 , the bottom surface of the crankcase  14  is provided with an opening  71   a  communicating with the first flow passage  71  between the primary filter  61  and the clutch-dedicated pump P 1  and facing toward the oil pan  45 . The bottom surface of the crankcase  14  is further provided with an opening  72   a  communicating with the second flow passage  72  between the clutch-dedicated pump P 1  and the secondary filter  63  and facing toward the oil pan  45 , and the relief valve  62  is connected to the opening  72   a . The opening  72   a  is formed in that portion of the second flow passage  72  which is located close to the first flow passage  71  and extends parallel to the first flow passage  71 . 
     The bottom surface of the crankcase  14  is provided with a surrounding wall portion  74  which, as viewed from below, projects downwardly to surround both the opening  71   a  and the relief valve  62 . The lower edge of the surrounding wall portion  74  faces the oil pan  45 . The bottom surface of the crankcase  14 , the inner peripheral surface of the surrounding wall portion  74  of the crankcase  14 , and the top surface of the oil pan  45  define a communication chamber  75 . While the relief valve  62  is in the relief state, the oil discharged from the relief valve  62  is returned to the first flow passage  71  through the communication chamber  75  and the opening  71   a . That is, the communication chamber  75  serves the function of the bypass passage  64 . 
     In this configuration, the bypass passage  64  can easily be formed by mounting the oil pan  45  on the crankcase  14 . A sealing member may be disposed between the lower end face of the surrounding wall portion  74  and the oil pan  45 . The surrounding wall portion  74  may be configured to project from the oil pan  45  rather than from the crankcase  14  or may be configured to have opposing portions which project from the crankcase  14  and the oil pan  45 , respectively. 
       FIG. 16  is a longitudinal sectional view of the secondary filter  63  and its vicinity in the power unit  11  of  FIG. 14 . As shown in  FIGS. 14 and 16 , the bottom surface of the crankcase  14  (in particular its case body  27 ) is provided with a recessed portion  76  recessed upwardly. The second flow passage  72  opens into the recessed portion  76 , and the third flow passage  73  also opens into the recessed portion  76 . Thus, the second flow passage  72  communicates with the third flow passage  73  through the recessed portion  76 . 
     The top surface of the oil pan  45  is provided with a recessed portion  77  recessed downwardly. With the oil pan  45  mounted on the crankcase  14 , the recessed portion  76  and the recessed portion  77  are combined to form an accommodation chamber  78 . The secondary filter  63  is accommodated in the accommodation chamber  78 . The accommodation chamber  78  for the secondary filter  63  can easily be formed merely by mounting the oil pan  45  on the crankcase  14 . The oil pan  45  need not necessarily be provided with the recessed portion  77  as long as the crankcase  14  and the oil pan  45  are shaped to form together the accommodation chamber  78 . 
     The present invention is not limited to the embodiment described above, and modifications, additions, or omissions can be made to the configuration of the above embodiment. For example, while in the above embodiment the clutch-dedicated pump P 1  and the engine-dedicated pump P 2  are mechanical pumps driven by power of the engine E, at least one of the clutch-dedicated pump P 1  and the engine-dedicated pump P 2  may be an electrically-driven pump. While in the above embodiment the crankcase is presented as an example of the power unit case, a case accommodating the drive motor may serve as the power unit case when the vehicle does not include the engine E but only the drive motor as a prime mover. While in the above embodiment the two covers, namely the first cover  28  and second cover  29 , are provided as the covers closing the subsidiary chamber SR, a single large cover may be used to close the entire subsidiary chamber SR. The straddle vehicle is not limited to motorcycles but may be another type of vehicle such as a motor tricycle.