Abstract:
To provide a power unit mounted for a vehicle, such as a motorcycle, wherein the power unit is attached to a transmission actuating device or attached to a clutch actuating device, or attached to both actuating units. A power unit for a vehicle having an internal combustion engine mounted in a condition where a crank shaft is oriented in the longitudinal direction of the vehicle body, and having a transmission arranged behind the internal combustion engine, wherein one or both of a transmission actuating electric motor for driving an transmission actuating device of the transmission and a clutch actuating electric motor for driving a clutch actuating device of a clutch are arranged in a rear section of said power unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power unit attached to a transmission actuating device, a power unit attached to a clutch actuating device, or a power unit attached to both a transmission actuating device and a clutch actuating device, mounted on a vehicle such as a motorcycle. 
     2. Description of Related Art 
     A power unit for a vehicle having a crankshaft oriented in the longitudinal direction of the vehicle and having an internal combustion engine is disclosed in Japanese Patent Publication Laid-open No. Sho. 37-16106. 
     With such a vehicle power unit, a drive motor section for performing transmission actuation is provided inside the power unit, which means that it is difficult to carry out maintenance and it is difficult to adjust the drive motor section. 
     Also, since the drive motor section is provided linked to the transmission, a casing housing the drive motor section and the transmission is enlarged, and it is difficult to increase the space for an operator&#39;s legs. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The present invention relates to improvements to a power unit for a vehicle that solves these type of problems, and provides a power unit for a vehicle having an internal combustion engine mounted so that a crank shaft is oriented in the longitudinal direction of the vehicle body, and having a transmission arranged behind the internal combustion engine, wherein one or both of a transmission actuating electric motor for driving a transmission actuating device of the transmission and a clutch actuating electric motor for driving a clutch actuating device of a clutch are arranged in a rear section of the power unit. 
     In the present invention, since the electric motors are arranged in a rear portion of the power unit, maintenance and adjustment of the electric motors can be simply and easily carried out even in a case where the power unit is not disassembled. Also, operators comfort when riding can be improved by widening a space for an operator&#39;s legs. 
     Further, with the structure of the present invention, it is possible to effectively utilize a space between pivot portions of a pair of left and right rear forks, and it is possible to reliably protect the electric motors by means of the pivot portions of the rear forks. 
     Still further, with the structure of the present invention, the manufacture of rotary shafts of the electric motors and shafts coupled to these rotary shafts is carried out at the same time as the manufacture of rotary shafts of the transmission and clutch, and it is possible to improve productivity. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a left side elevation of a motorcycle, having a low slung floor of one embodiment of the present invention; 
     FIG. 2 is a plan view of the motorcycle, having a low slung floor, of one embodiment of the present invention; 
     FIG. 3 is a longitudinal section, taken along line III—III in FIG. 1, with a push rod omitted from the drawing; 
     FIG. 4 is a partial longitudinal section, taken along line IV—IV in FIG. 3, with a push rod omitted from the drawing; 
     FIG. 5 is a cross section, taken along line V—V in FIG. 4, with a rocker arm omitted from the drawing; 
     FIG. 6 is a cross section taken along line VI—VI in FIG. 7, showing part of a left side cylinder head cover of FIG.  5 . 
     FIG. 7 is an enlarged left side view of the left side cylinder head cover of FIG. 5, showing a partial cutaway; 
     FIG. 8 is an enlarged cross-sectional plan view of essential parts of FIG. 3; 
     FIG. 9 is a plan view of a second embodiment of the present invention; 
     FIG. 10 is a cross-sectional view of FIG. 9; 
     FIG. 11 is a left side cross-sectional view of FIG. 9; 
     FIG. 12 is a cross-sectional view of a geared transmission of the second embodiment; 
     FIG. 13 is a cross-sectional view of a relief valve and an oil filter of the second embodiment; 
     FIG. 14 is a view, observed from the direction shown by the line XIV—XIV of FIG. 11; 
     FIG. 15 is a view, observed from the direction shown by the line XV—XV of FIG. 11; 
     FIG. 16 is a view, observed from the direction shown by the line XVI—XVI of FIG. 11; 
     FIG. 17 is a view, observed from the direction shown by the line XVII—XVII of FIG. 11; and 
     FIG. 18 is an enlarged cross-sectional view of essential parts of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG.  1  and FIG. 2, a motorcycle  1  having a low slung floor  2  is provided with a horizontal opposed 2-cylinder internal combustion engine  3  positioned further towards the front of the vehicle than the low slung floor  2 . Each of left and right (upper and lower FIG. 2) cylinder heads  5   l ,  5   r  of the horizontal opposed 2-cylinder internal combustion engine  3 , looking in the travelling direction of the vehicle, are arranged in front of left and right step floors  2   l ,  2   r , and are covered by left and right cylinders ports  4   l ,  4   r , left and right cylinder head covers  6   l ,  6   r , and a front vehicle body cover  7   a  constituting a front section of a vehicle body cover  7 . 
     The left and right step floors  2   l ,  2   r  are respectively formed as plates extending in a longitudinal direction of the vehicle, with the widths becoming wider from the front of the vehicle to the rear. Portions in front of the left and right step floors  2   l ,  2   r  constitute left and right front inclined portions  7   dl ,  7   dr  ( 7   dr  is not shown in the drawing) formed in the front vehicle body cover  7   a , with these left and right front inclined portions  7   dl ,  7   dr  and in the left and right step floors  2   l ,  2   r  together forming the low slung floor  2 . The left and right cylinder heads  5   l ,  5   r , as has been described above, are covered by the front vehicle body cover  7   a  and are arranged so as to face the left and right front inclined portions  7   dl ,  7   dr.    
     A crankcase  8  of the horizontal opposed 2-cylinder internal combustion engine  3  is sandwiched between the left and right cylinders  4   l ,  4   r  and arranged in the center, widthwise, of the vehicle, while a transmission case  9  adjacent to the crankcase  8  is arranged inside a saddle shaped floor tunnel formed by the floor vehicle body cover  7   b  constituting the floor section of the vehicle body cover  7 , and these two cases are united by being connected together. 
     The crankcase  8  and transmission case  9  that have been united by connecting together as described above constitute part of the frame of the motorcycle  1  (a central part), and they are respectively connected, at upper left and right side wall portions in the vicinity of connecting portions of the cases, to lower end portions of a main frame  10  that runs diagonally downwardly from the upper front of the vehicle, and are fixed by bolts. This position is a position in front of the left and right step floors  2   l ,  2   r . The main frame  10  does not extend as far as a rear fork  17 , which will be described later, which means that spaces of the left and right step floors  2   l ,  2   r  can be made wider. 
     The main frame  10  branches to the left and to the right from a head pipe  11  to form left and right main frame sections  10   l ,  10   r , and these left and right main frame sections  10   l ,  10   r  respectively extend diagonally downwardly to upper left and right side wall portions in the vicinity of connecting portions of the crankcase  8  and the transmission case  9 , and are connected and fixed there as described above. 
     A steering stem  13  which is integral with a front fork  12  is inserted into the head pipe  11 , as is well known, and the steering direction of a front wheel  15  is changed by a handlebar  14  that turns integrally with the steering stem  13 . 
     At rear end parts of the transmission case  9 , on left and right sides, left and right support arms  16   r ,  16   l  are provided so as to extend towards the rear, and front ends of left and right arm portions  17   r ,  17   l  of a rear fork  17  are respectively rotatably connected to left and right sides of the left and right support arms  16   r ,  16   l.    
     A left side arm portion  17   l  of the rear fork  17  is formed as a comparatively large size cylinder. An output shaft  54  (refer to FIG. 3) of the transmission protruding from the transmission case  9  and a shaft connected via a universal joint pass through the cylinder (i.e. the rear fork  17 ). A bevel gear is integrally attached to an end of this shaft, and meshes with a bevel gear attached to a rotation shaft side of the rear wheel  18 . As a result, power generated by the horizontal opposed 2-cylinder internal combustion engine  3  is transferred to the rear wheel  18  (this is not shown in the drawing). The rear end of the right side arm portion  17   r  of the rear fork  17  supports a support axis of the rear wheel  18 . 
     The front end of the rear fork  17  is caught between left and right step floors  2   l ,  2   r  and is positioned at an inner side of these step floors, which means that the front end of the rear folk  17  does not interfere with the left side step floor  2   l . Particularly, the universal joint does not interfere with the left step floor  2   l . It is therefore possible to widen spaces of the left and right step floors  2   l ,  2   r.    
     Upward and downward rocking of the transmission case  9  with respect to the rear fork  17  is absorbed by a shock absorber  19  suspended between the rear fork  17  and the transmission case  9  in a central portion in a widthwise direction of the vehicle. The front end of the shock absorber  19  is pivoted at a corner above the rear end of the transmission case  9 . A rear end of the shock absorber  19  is rotatably connected by a pin at one end of an L-shaped link  20 . The L-shaped link  20  having a central portion supported at a point slightly closer to a front end than a central portion, in a longitudinal direction, of the rear fork  17 , and the other end of the L-shaped link  20  is rotatably connected by a pin to the other end of a link  21  having one end pivoted below the rear end of the transmission case  9 . 
     At a rear part of the transmission case  9  being higher than the rear fork  17 , rear frame pipe assembly  22  includes a plurality of bent pipes and straight pipes attached in an unswingable manner to the transmission case  9  at upper surface connection points  23  (2 positions, one left and one right) of the transmission case  9  and lower connections points  24  (2 positions, one left and one right) further back than the connection points  23 . 
     A battery  26  and a fuel tank  27  are provided one behind the other in the longitudinal direction at a lower part of a double seat  25  supported at the rear frame pipe assembly  22 , and storage shelves for a helmet  28  are provided on both left and right sides of the rear wheel  18 . The double seat  25  is also supported by the rear frame pipe assembly  22 . 
     A rear half section of the rear frame pipe assembly  22  is covered by a rear vehicle body cover  7   c  continuing from the floor vehicle body cover  7   b . The rear vehicle body cover  7   c  can be opened and closed by pivoting around a pivot portion, not shown in the drawings, and the helmet  28  can be taken out or stowed by holding the upper part of the rear vehicle body cover  7   c  and opening it outwards. 
     The intake system of the horizontal opposed 2-cylinder internal combustion engine  3  has an air cleaner case  29  and a carburetor  30 , and these are arranged inside a front vehicle body cover  7   a  above the horizontal opposed 2-cylinder internal combustion engine  3 . 
     The air cleaner case  29  is arranged inside the front vehicle body cover  7   a  above the horizontal opposed 2-cylinder internal combustion engine  3 , inside a space A enclosed by left and right main frame sections  10   l ,  10   r  of the main frame  10  and the front vehicle body cover  7   a , and forming a triangular shape widening towards the front and sloping downwardly towards the rear when viewed from the side. 
     The front part of the space A is a comparatively wide space leading to a head pipe  11  which is inclined from the side, and the air cleaner case  29  arranged inside the space A can be made as large as the volume of the space A. 
     Inside the front vehicle body cover  7   a , at an upper part of the front wheel  15 , radiators  31   l ,  31   r . are provided on both the left and right sides of the front fork  12 , and an oil cooler  33  is provided forward of the radiators at a central portion, in a widthwise direction of the vehicle, in a space between a fender of the front wheel  15  and a headlight  32 . The radiators  31   l ,  31   r  and the oil cooler  33  are respectively connected to the left and right cylinders  4   l ,  4   r  of the horizontal opposed 2-cylinder internal combustion engine  3  and the crankcase  8  by pipelines not shown in detail in the drawings. 
     Core portions of the pair of radiators  31   l ,  31   r  are arranged in a sloping fashion such that they slope downwardly and outwardly from the front to the rear of the vehicle, and a core portion of the oil cooler  33  is arranged along the width direction of the vehicle, sloping slightly upwardly from the front to the rear of the vehicle. Both the radiators  31   l ,  31   r  and the oil cooler  33  are effectively cooled by air that flows in during travel from an air intake port  34  formed in a front end of the front vehicle body cover  7   a  and a central opening running forwardly and downwardly from the front vehicle body cover  7   a  (a portion partially housing the front wheel  15 ). 
     One end of each of a pair of exhaust pipes  35   l ,  35   r , that will be described later, is connected to rear parts of the left and right cylinder heads  5   l ,  5   r  of the horizontal opposed 2-cylinder internal combustion engine  3 , and the other ends of the exhaust pipes  35   l ,  35   r  are merged together and connected to a muffler  36 . A stand  37  is provided for supporting the motorcycle. 
     In this embodiment, of the left and right cylinder heads, the right cylinder head  5   r  is arranged further forward than the left cylinder head  5   l , and this makes it easier to make the lengths of the exhaust pipes  35 R and  35 L that meet at the right side of the vehicle coincide. Also, a crossing part of the left exhaust pipe to the right side of the vehicle contributes to reducing the protrusion of the exhaust pipe by passing downwardly through a casing housing the transmission and maintaining minimum ground clearance. 
     The arrangement and structure of the horizontal opposed 2-cylinder internal combustion engine  3  will be further described. 
     Upper rear wall surfaces (upper halves) of the cylinder heads  5   l ,  5   r  and the left and right cylinder head covers  6   l ,  6   r , respectively form downward inclines B that are high at the front and low at the rear, as shown in FIG.  1  and FIG.  7 . Lower rear wall surfaces (lower halves), upper front wall surfaces and lower front wall surfaces of the left and right cylinder heads  5   l ,  5   r  and left and right cylinder head covers  6   l ,  6   r  respectively form upward or downward slopes orthogonal to the respective directions. 
     These left and right cylinder heads  5   l ,  5   r , as has already been described, are arranged in front of the left and right step floors  2   l ,  2   r  so as to face left and right front inclined portions  7   dl ,  7   dr  of the front vehicle body cover  7   a  continuous with the left and right step floors  2   l ,  2   r . This means that these left and right front inclined portions  7   dl ,  7   dr  run along the above described inclines B formed at the rear upper wall portions of the left and right cylinder heads  5   l ,  5   r , and can be made to slope greatly towards the front. In this way, spaces in front of the left and right step floors  2   l ,  2   r  can be enlarged, legroom on the floor portions can be increased and riding feel can be improved. 
     As shown in FIG. 3, left and right valve systems are housed in the left and right cylinder heads  5   l ,  5   r , and left and right intake and exhaust valve systems  38   l ,  38   r ,  39   l ,  39   r  of the left and right valve systems are arranged in the directions of the intake and exhaust systems (direction of the intake valve system  38   l  with respect to the exhaust system  39   l , and the direction of the intake valve system  38   r  with respect to the exhaust valve system  39   r ) have a horizontal included angle α. Rear walls of the sections housing the left and right valve systems, namely the rear walls of the left and right cylinder heads  5   l ,  5   r , are respectively formed sloping downwards so that they are high at the front and low at the rear. 
     Because of the above described arrangement of the left and right intake and exhaust systems  38   l ,  38   r ,  39   l ,  39   r , the left and right exhaust and intake ports  41   l ,  41   r  are formed in the left and right cylinder heads  5   l ,  5   r  and respectively run downwardly from the left and right cylinder heads  5   l ,  5   r , and end parts of the left and right exhaust and intake ports  41   l ,  41   r  are connected to one end of the exhaust pipes  35   l ,  35   r.    
     The exhaust pipes  35   l ,  35   r  also go further downwardly from that point, and after that are fed orthogonal to the vehicle body, as described above. 
     The left and right intake ports  40   l ,  40   r  formed in the left and right cylinder heads  5   l ,  5   r  extend upwardly from the front of respective left and right cylinder heads  5   l ,  5   r , and one end of the left and right intake manifolds  42   l ,  42   r  projecting from the carburetor  30  are connected to ends of the respective left and right intake ports  40   l ,  40   r  (refer to FIG. 1, FIG.  3  and FIG.  5 ). 
     Because of the above arrangement of the left and right exhaust and intake ports  41   l ,  41   r  and the arrangement of the left and right intake ports  40   l ,  40   r , it becomes possible to lower the upper height of the left and right cylinder heads  5   l ,  5   r , and there is no longer any need to insert spaces close to the left and right cylinder heads  5   l ,  5   r  so as to arrange the intake system, which means that left and right front inclined portions  7   dl ,  7   dr  can be made to incline further towards the front. Thus, the spaces in front of the left and right step floors  2   l ,  2   r  are enlarged and the legroom of the floor space is widened and the riding feel is further improved. 
     With respect to the left and right intake and exhaust systems  38   l ,  38   r ,  39   l ,  39   r , reciprocating rocker arms  43   l ,  43   r ,  44   l ,  44   r  are driven by push rods  45   l ,  45   r ,  46   l ,  46   r  ( 46   r  is not shown in the drawing), and the ends at the other side of these push rods  45   l ,  45   r ,  46   l ,  46   r  are arranged below the crankcase  8 , and respectively engage with corresponding portions of a cam shaft  49  rotatably driven by a crankshaft  47  via a chain  48  (refer to FIG. 4 to FIG.  7 ). 
     In FIG.  3  and FIG. 5, a starting clutch  50  is provided at a rear portion of the crankshaft  47 , and a gear  51  is integral with the output shaft  50   a  of the starting clutch  50  and interlocks with an input gear  52   b  integral with a casing  52   a  of a swash plate type hydraulic pump  52  of a hydraulic transmission HFT. The swash plate type hydraulic pump  52  and a swash plate type hydraulic motor  53  are connected at a gear ratio according to an inclined state of a swash plate  53   a  of the swash plate type hydraulic motor  53 , and an output gear  53   c  is integral with an output shaft  53   b  of the swash plate type hydraulic motor  53  meshed with a gear  54   a  integral with an output shaft  54 . Rotation of the crankshaft  47  is produced by the starting clutch  50  and the output shaft  54  through the swash plate type hydraulic motor  53  and the swash plate type hydraulic pump  52  of the hydraulic transmission HFT. 
     A moving end  55   a  of the swash plate driving mechanism  55  is connected to the swash plate  53   a  of the swash plate type hydraulic motor  53  with a pin  55   b , and is screwed to a rotating shaft  55   c . The rotating shaft  55   c  meshes with an output pinion  56   a  of the swash plate driving motor  56  through a reducing gear group  55   d . The swash plate driving mechanism  55  is driven by rotation of the swash plate driving motor  56 . The swash plate  53   a  of the swash plate type hydraulic motor  53  is inclined to a desired angle, and the hydraulic transmission HFT is set to a gear ratio according to this inclined angle. 
     A potentiometer  57  is connected via gear  57   a  to a pinion  55   e  of the rotating shaft  55   c  of the swash plate driving mechanism  55 . The potentiometer  57  detects an inclination angle of the swash plate  53   a  of the swash plate driving mechanism  55 , as well as the gear ratio of the hydraulic transmission HFT by detecting a rotation angle of the rotating shaft  55   c  of the swash plate driving mechanism  55 . 
     The swash plate driving motor  56  and the potentiometer  57  are arranged between left and right support arms  16   r ,  16   l  behind the transmission case  9 . The left and right support arms  16   r ,  16   l  are support sections of the rear fork  17 , and are installed on a rear end wall  9   a  of the transmission case  9 , so as to be detachable through the rear portion of the vehicle. 
     A starter motor  64  is connected via a chain  48  to the crankshaft  47 , and an alternating current generator  59  is provided on the crankshaft  47 . 
     In FIG. 4, the rotation of the crankshaft  47  is transmitted to a cooling water pump  62  and an oil pump  63  through a chain transmission mechanism  61 . The cooling water pump  62  is a magnet type pump. The two pumps are contained so as to extend into a rearwardly opening space of the crankcase  8  in connecting portions of the transmission case  9  and the crankcase  8 . A breather chamber  65  is illustrated in FIG.  3 . 
     Because the embodiment shown in FIG. 1 to FIG. 8 has the structure as described above, it is possible to attach the swash plate driving motor  56  and the potentiometer  57  to a rear wall  9   a  of the transmission case  9  from the rear of the vehicle, attachment and detachment of the swash plate driving motor  56  and potentiometer  57  is simplified, and maintenance, inspection and adjustment can be accomplished efficiently and easily. 
     Also, since the horizontal opposed 2-cylinder internal combustion engine  3  and the hydraulic transmission HFT are arranged longitudinally, the width of the power unit becomes narrow, the left and right step floors  2   l ,  2   r  become wide and operator comfort is improved. 
     Further, the swash plate driving motor  56  and the potentiometer  57  are sandwiched between the left and right support arms  16   r ,  16   l , and are provided on a rear end wall  9   a  of the transmission case  9 . This means that the swash plate driving motor  56  and the potentiometer  57  are sufficiently protected from the outside. 
     Upper rear wall surfaces (upper halves) of the cylinder heads  5   l ,  5   r  and the left and right cylinder head covers  6   l ,  6   r , are formed sloping downwardly so that they are respectively high at the front and low at the rear. Since the height of the upper region of the intake and exhaust systems is low because of the arrangement having an included angle α of the left and right intake and exhaust valve systems  38   l ,  38   r ,  39   l ,  39   r , it is possible to form left and right front inclined portions  7   dl ,  7   dr  continuously with the left and right step floors  2   l ,  2   r  and to be sloping strongly forward. As a result, the space in front of the left and right step floors  2   l ,  2   r  is enlarged, leg room in the floor part is widened and it is possible to significantly improve the feel of the vehicle during operation. 
     Since the exhaust pipes  35   l ,  35   r  protrude respectively downwardly to the rear of the vehicle body, there is absolutely no impediment to the formation of the forward sloping, left and right front inclined portions  7   dl ,  7   dr . The formation of these portions is easier, and the left and right step floors  2   l ,  2   r  are not heated up by the heat of exhaust gas. 
     The rear lower wall surfaces (upper halves) of the left and right cylinder heads  5   l ,  5   r  and the left and right cylinder head covers  6   l ,  6   r , and front upper wall surfaces and front lower wall surfaces are oriented in respective directions and are formed sloping upwardly or downwardly, margins occur in spaces facing these inclined wall surfaces and the layout of devices, machinery and parts is made easier. 
     A small vehicle (motorcycle)  1  having the low slung floor  2  of the present embodiment is provided with the horizontal opposed 2-cylinder internal combustion engine  3 , but is not thus limited, and can be provided with a horizontal opposed internal combustion engine having more cylinders. Also, the embodiment is not limited to a motorcycle, and can be used with respect to a small vehicle having a pair of rear wheels. 
     Next, a second embodiment of the present invention as illustrated in FIG. 9 to FIG. 18 will be described. 
     Parts that are similar to parts having reference numerals in the first embodiment shown in FIG. 1 to FIG. 8 have the same reference numerals attached hereto. 
     As shown in FIG. 9, the horizontal opposed 2-cylinder internal combustion engine  3  of the second embodiment is divided into left and right cylinders  4   l ,  4   r , left and right cylinder heads  5   l ,  5   r  and left and right cylinder head covers  6   l ,  6   r , substantially the same as the horizontal opposed 2-cylinder internal combustion engine  3  of the first embodiment. 
     However, differing from the valve system of the horizontal opposed 2-cylinder internal combustion engine  3  of the first embodiment is a push rod type OHV system. In the push rod type OHV system, intake and exhaust valve systems  38  and  39  are formed having a horizontal included angle α running along the longitudinal horizontal surface. The valve system of the horizontal opposed 2-cylinder internal combustion engine  3  of the second embodiment is a rocker arm type OHC system in which rocker arms  43  and  44  are pivoted on rocker arm shafts  66  oriented in a longitudinal direction and the intake and exhaust valve systems (not shown in the drawings) are provided so as to form a desired pinch angle running along left and right perpendicular surfaces. Another difference is that a chain sprocket  100  is wound around a driven sprocket  68  integral with a rocker arm shaft  66  and a drive sprocket  67  of a front part of the crankshaft  47 . 
     In the first embodiment, an alternating current generator  59  was arranged further back relative to the horizontal opposed 2-cylinder internal combustion engine  3 , but in the second embodiment the alternating current generator  59  is arranged further forward than the horizontal opposed 2-cylinder internal combustion engine  3 . 
     In the first embodiment, the crankcase  8  is divided laterally, but in the second embodiment the crankcase  8  is longitudinally divided into a front crankcase  8   a  and a rear crankcase  8   b.    
     Also, in the first embodiment, the transmission case  9  is divided in two in the longitudinal direction, but in the second embodiment the transmission case  9  is divided into three, namely a front transmission case  9   b , a middle transmission case  9   c  and a rear transmission case  9   d , while the output shaft  50   a  of the starting clutch  50  is connected to a gear transmission  70  through the gear  51  and the multiple plate friction clutch  69 . 
     Still further, the gear  51 , integral with the output shaft  50   a  of the starting clutch  50 , meshes with an input gear  69   b  integral with a outer clutch  69   a  of the multiple plate friction clutch  69 , a number of friction plates  69   d  are interposed between the outer clutch  69   a  and a inner clutch  69   c  of the multiple plate friction clutch  69 , and a pressure plate  69   e  is arranged in front of the friction plates  69   d . If a clutch lifter rod  69   f  penetrating through a main shaft  70   a  of the above described gear transmission  70  is pushed in a forward direction against the spring force of a plate spring  69   g , engagement of the outer clutch  69   a  and the inner clutch  69   c  is disconnected. 
     Here the hydraulic transmission HFT of the first embodiment is replaced with a gear transmission  70  in the second embodiment. 
     The gear transmission  70  comprises a main shaft  70   a  oriented in a longitudinal direction, a counter shaft  70   b , a transmission gear group  70   c  in which gears are fitted in splines of the shafts  70   a  and  70   b , and are mutually meshed, a shift fork shaft  70   d  oriented in the longitudinal direction, a shift fork  70   e  engaged with the shift fork shaft  70   d  and a shift drum  70   f  oriented in the longitudinal direction. If the shift drum  70   f  is relatively driven by the transmission actuating motor  72 , the shift fork  70   e  that is engaged with a peripheral groove of the shift drum  70   f  moves in an axial direction to selectively mesh with one of the transmission gear group  70   c , so as to set a desired transmission ratio. 
     A clutch actuating motor  71  for driving the multiple plate friction clutch  69 , as shown in FIG.  10  and FIG. 18, is attached to a clutch actuating motor base plate  73 . The clutch actuating motor base plate  73  is attached to a rear end wall  9   a  of the transmission case  9 , refer to the lower part of FIG. 18, so as to be detachable. A reduction gear group  74  is pivoted at the rear end wall  9   a  of the transmission case  9  and the clutch actuating motor base plate  73 . An input gear  74   a  of the reduction gear group  74  meshes with a pinion  71   a  of the clutch actuating motor  71 , and an output gear  74   b  of the reduction gear group  74  meshes with a lifter gear  75 . The lifter gear  75  is engaged with a push shaft  77  through a bearing  76 . The rotational output of the lifter gear  75  is not transmitted to the push shaft  77 , but the push shaft  77  moves in an axial direction in response to movement of the lifter gear  75  in an axial direction. 
     A support shaft  78  is attached to the clutch actuating motor base plate  73  by being positioned to the rear of the lifter gear  75 , and a ball  81  is interposed between a cam holding plate  80  that is fixed to the clutch actuating motor base plate  73  by a pin  79  by engagement with the support shaft  78 , and a rear cam surface  75   a  of the lifter gear  75 . If the lifter gear  75  is rotatively driven by the clutch actuating motor  71  the lifter gear  75  and the push shaft  77  are moved in an axial direction by the rear cam surface  75   a  of the lifter gear  75 , the clutch lifter rod  69   f  of the multiple plate friction clutch  69  is pushed forward by the push shaft  77 , and the multiple plate friction clutch  69  is disengaged. 
     A clutch actuating motor rotation angle sensor  82  for detecting a rotation angle of the clutch actuating motor  71  is provided in the reduction gear group  74  of the gear transmission  70 , and as shown in FIG. 10, FIG.  15  and FIG. 18. A detector  83  is pivoted at a front crankcase  9   b  relative to a tip of the clutch lifter rod  69   f  of the multiple plate friction clutch  69 , and a clutch disengage sensor  84  is connected to the detector  83 . When the multiple plate friction clutch  69  is put into the disengaged state by the clutch lifter rod  69   f  being pushed forwardly, the detector  83  rotates because the tip of the clutch lifter rod  69   f  hits against a cut end face  83   a  of the detector  83 , as shown in the left side portion of FIG.  18 . The clutch disengage sensor  84  detects that the multiple plate friction clutch  69  is disengaged. 
     By providing the clutch disengage sensor  84  in this way, the effects caused by the play of the clutch actuating system are reduced, and it is possible to carry out a smooth transmission actuation while reducing the time required for transmission actuation. 
     As shown in FIG. 12, the transmission actuating motor  72  for driving the gear transmission  70  is attached to a transmission actuating motor base plate  85 , the transmission actuating motor base plate  85  is detachably fixed to the rear end wall  9   a  of the transmission case  9 , and a reduction gear group  86  is pivoted at the rear end wall  9   a  of the transmission case  9  and the transmission actuating motor base plate  85 . An input gear  86   a  of the reduction gear group  86  meshes with a pinion  72   a  of the transmission actuating motor  72  and an output gear  86   b  of the reduction gear group  86  meshes with a spindle shaft  87 . The spindle shaft  87  is connected to the shift drum  70   f  of the gear transmission  70  through a shift drum feed mechanism  88 , and if the transmission actuating motor  72  rotates by only a desired number of rotations the shift drum  70   f  is driven by a desired angle of rotation and the gear transmission  70  is set to a desired transmission ratio by movement of the specific shift drum  70   e  in the axial direction. 
     A shift drum rotation angle detecting sensor  89  is provided on a rear end of the spindle shaft  87 , and a shift position sensor  90  positioned below the shift drum rotation angle detecting sensor  89  is linked to the shift drum  70   f . A shift position of the gear transmission  70  is detected by the shift position sensor  90 , and the shift position of the gear transmission  70  is displayed on an instrument panel, not shown in the drawings, arranged at the front of the motorcycle  1 . 
     As shown in FIG. 11, a balancer drive gear  91 , positioned in front of the one-way starter switch  58 , is engaged with the crankshaft  47 , a balancer  92  positioned above the crankshaft  47  is pivoted at the crankcase  8  so as to rotate parallel to the crankshaft  47 , a balancer driven gear  93  integral with the balancer  92  meshes with the balancer drive gear  91 , and the balancer  92  is driven to rotate in a direction opposite the rotational direction of the crankshaft  47 . 
     Lubricating oil remaining in the front transmission case  9   b  of the transmission case  9  is sucked in to an oil pump  63  through a strainer  9  and, as shown in FIG. 13, is supplied to an outer oil chamber  60   a  of an oil filter  60  through a discharge path  95 . Lubricating oil that has been filtered in the oil filter  60  is supplied to the respective lubricating sections of the horizontally opposed 2-cylinder internal combustion engine  3 , the multiple plate friction clutch  69 , and the gear transmission  70  through lubricating oil paths  96 . 
     The lubricating oil sump  98  in FIG.  1  and FIG. 14 is covered by a lower part of a driven sprocket  61   a  of the chain transmission mechanism  61 , and lubricating oil remaining in the lubricating oil sump  98  is used to lubricate the chain transmission mechanism  61 . 
     As shown in FIG. 13, a relief valve  97  is provided in a branch path  95   a  of the discharge path  95 . If the pressure inside the discharge path  95  exceeds a relief pressure, lubricating oil is discharged from the relief valve  97 . 
     As shown in FIG. 13, a relief valve case  97   a  of the relief valve  97  is engaged with an attachment lug  97   b  that are integral with a rear crankcase  8   b . If a front crankcase  8   a  and the rear crankcase  8   b  are combined, the relief valve case  97   a  is pushed forward by the rear crankcase  8   b  and fixed by the attachment lug  97   b  so that it cannot be pulled out. 
     As shown in FIG.  15  and FIG. 16, a number of breather chambers  65  are formed in the front transmission case  9   b , middle transmission case  9   c , and rear transmission case  9   d , along the length of the transmission case  9 . The transmission cases and breather chambers are mutually connected by opening portions  65   a.    
     In this way, the integral attachment lug  97   b  holds the relief valve  97  and at the same time acts as an oil distribution path for the oil filter  60 . 
     A transmission actuation control device, not shown in the drawings, is provided in the vehicle body cover  7  positioned above the horizontal opposed 2-cylinder internal combustion engine  3 , and a shift-up push-button switch and a shift-down push-button switch (neither of which are shown in the drawings) are provided adjacent to the handlebar grip. If the shift up push-button switch is pressed, the clutch actuating motor  71  is operated by a control signal of the transmission actuation control device to disconnect the multiple plate friction clutch  69  (the transmission actuating motor  72  is operated by a disengage signal from the clutch disengage sensor  84  so as to cause the gear transmission  70  to shift up one stage), the multiple plate friction clutch  69  is reconnected by a shift up signal of the shift drum rotation angle detecting sensor  89 , and the shift up operation is completed. 
     The shift down operation is also carried out by a similar operation. 
     To carry out multiple stage shift up or shift down, the shift up push-button switch or shift down push-button switch is pressed again after the shift up or shift down has been completed for one stage, and this can be repeated. 
     As has been described above, the embodiment shown in FIG. 9 to FIG. 17 can carry out automatic disengaging and engaging of the multiple plate friction clutch  69 , and shift up and shift down of the gear transmission  70 , simply by pressing a shift up push-button switch or a shift down push-button switch, which means that the rider can travel on the motorcycle  1  in extreme comfort. 
     Also, the clutch actuating motor  71 , the power transmission system for the clutch actuating motor  71 , and the clutch actuating motor rotation angle sensor  82  are provided on the rear end wall  9   a  of the transmission case  9 . The transmission actuating motor  72 , the power transmission system for the transmission actuating motor  72 , the shift drum rotation angle detecting sensor  89  and the shift position sensor  90  are also provided on the rear end wall  9   a  of the transmission case  9 . This means that even if the motorcycle  1  is in an assembled state with the horizontal opposed 2-cylinder internal combustion engine  3 , multiple plate friction clutch  69  and gear transmission  70  attached, these components can be checked from the rear of the vehicle frame, and maintenance and adjustment can be easily carried out by simply pulling them out from the rear of the vehicle frame. 
     Further, since the clutch actuating motor  71 , transmission actuating motor  72  and power transmission sensors for the clutch actuating motor  71  and transmission actuating motor  72  are provided in the rear end wall  9   a  of the transmission case  9 , and the external parts thereof are covered by the vehicle body cover  7 , they are sufficiently protected stones, etc . . . flying in from the front of the vehicle. The endurance and reliability are significantly improved. 
     Since the power transmission system of the motorcycle  1  is constituted by the multiple plate friction clutch  69  and the gear transmission  70 , fuel consumption is improved regardless of the riding comfort. 
     If the front crankcase  8   a  and the rear crankcase  8   b  are divided longitudinally it is easy to remove the relief valve  97 , and maintenance, checking and adjustment of the relief valve  97  can be carried out easily. 
     Moreover, since the plurality of breather chambers  65  are formed using spaces inside the transmission case  9 , there is no need to enlarge the volume of the crankcase  8  of the horizontal opposed 2-cylinder internal combustion engine  3 , and the horizontal opposed 2-cylinder internal combustion engine  3  can be reduced in size. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.