Patent Publication Number: US-7913817-B2

Title: Oil pump unit for internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2006-262505, filed in Japan on Sep. 27, 2006, the entirety of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oil pump unit for an internal combustion engine. 
     2. Background of the Invention 
     There is a case where an internal combustion engine is provided with an oil pump that is disposed in an engine case constituting an oil storage portion so as to improve the oil suction efficiency of the pump (see, for example, Japanese Patent Laid-open No. Sho 63-76952). 
     A lubricating device of the internal combustion engine disclosed in Japanese Patent Laid-open No. Sho 63-76952 is of a dry sump type and is such that an oil pump unit including respective united pump cases of a scavenging pump and a feed pump is disposed in a clutch chamber whose bottom portion serves as an oil storage portion. 
     The oil pump unit is configured differently from an engine case constituting the clutch chamber and is installed in the clutch chamber in such a manner that the pump case itself is provided with an oil intake port adapted to suck the oil collecting in the oil storage portion by the drive of the pump, and with an oil discharge port and the like. 
     Thus, the oil pump unit configured differently from the engine case is increased in the number of the component parts so that the pump case tends to be configured in a complicated manner to increase in size as well as in weight. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has been made and it is an object of the invention is to provide an oil pump unit for an internal combustion engine that can reduce the number of component parts, configure a pump case simply so as to be reduced in size and in weight. 
     To achieve the above object, according to a first aspect of the present invention, an oil pump unit for an internal combustion engine in which a case cover is put on the outside of a crankcase via a spacer and an oil tank chamber is formed by the inside surface of the case cover and the spacer, the spacer is formed inside an outer circumferential wall thereof with a division wall which partitions the oil tank chamber from a crank chamber and with a pump case half-body portion which is a portion of a pump case for an oil pump, a pump case half-body is joined to the pump case half-body portion to form the pump case, and the pump case half-body portion is formed in a lower portion thereof with an oil suction port communicating with a bottom portion of the oil tank chamber. 
     According to a second aspect of the present invention, the pump case half-body portion of the spacer is integrally formed with a pump discharge oil passage, which allows the oil pump to communicate with an oil filter. 
     According to a third aspect of the present invention, the pump case half body is used as a partition plate, the pump case half-body portion of the spacer is joined to one lateral surface of the partition plate to form the pump case of the oil pump, a second pump case half-body is joined to the other lateral surface of the partition plate to form a second pump case of a second oil pump, a portion of the pump case half-body which forms the second pump case together with the second pump case half-body and which protrudes from the pump case half-body portion is bored with a second oil pump discharge port of the second oil pump; and the division wall of the spacer is formed with a tank supply port communicating with the second oil pump discharge port. 
     According to the first aspect of the present invention, the spacer interposed between the crankcase and the case cover is integrally formed inside the outer circumferential wall thereof with the division wall which partitions the oil tank chamber from the crank chamber and with the pump case half-body portion forming the half-body of the pump case for the oil pump. Thus, the number of component parts of the oil pump unit can be reduced. 
     The pump case half-body portion of the spacer is formed in its lower portion with the oil suction port communicating with the bottom portion of the oil tank chamber. Thus, a connection pipe, a knock pin, an O-ring, etc. are not needed to simplify the suction oil passage of the oil pump from the oil tank chamber, simplifying the configuration of the pump case. Consequently, the oil pump unit can be reduced in size and in weight. 
     According to the second aspect of the present invention, the pump case half-body portion of the spacer is integrally formed with the pump discharge oil passage, which allows the oil pump to communicate with the oil filter. Thus, the number of component parts of the oil pump unit can further be reduced. 
     According to the third aspect of the present invention, the pump case half-body is shared as a division plate, an oil pump is formed on one lateral surface of the division plate and a second oil pump is formed on the other lateral surface thereof. Thus, the number of component parts of the oil pump unit can be reduced. 
     A portion of the pump case half-body as the partition plate, which protrudes from the pump case half-body portion, is bored with a second oil pump discharge port of the second oil pump. The division wall of the spacer is formed with the tank supply port communicating with the second oil pump discharge port. Thus, the discharge oil passage from the second oil pump to the oil tank can be reduced in the number of component parts thereof to simplify its configuration. 
     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 lateral view of an all terrain vehicle on which a power unit is mounted according to an embodiment of the present invention with a body cover and the like removed; 
         FIG. 2  is a plan view of the vehicle; 
         FIG. 3  is a front view of the power unit with an internal combustion engine partially omitted; 
         FIG. 4  is a cross-sectional view of a power transmission mechanism; 
         FIG. 5  is a cross-sectional view of the essential portion of a lubricating device (the cross-sectional view taken along line V-V of  FIGS. 6 and 7 ); 
         FIG. 6  is a front view of a spacer (crankcase extension member); 
         FIG. 7  is a rear view of the spacer; and 
         FIG. 8  is a front view of a front case cover. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the accompanying drawings, wherein the same reference numerals will be used to identify the same or similar elements throughout the several views. It should be noted that the drawings should be viewed in the direction of orientation of the reference numerals. 
     An embodiment of the present invention will hereinafter be described with reference to  FIGS. 1 through 8 . 
       FIG. 1  is a lateral view of an all terrain vehicle  1  on which a water-cooled internal combustion engine E according to the embodiment is mounted with a body cover or the like thereof removed.  FIG. 2  is a plan view of  FIG. 1 . 
     It is to be noted that “the front”, “the back or rear”, “the right”, and “the left” are determined based on the vehicle facing the direction of forward travel in this embodiment. 
     The all terrain vehicle  1  is a saddle-ride type four-wheeled vehicle and includes a pair of left and right front wheels FW and a pair of left and right rear wheels RW which are suspended by the front portion and rear portion, respectively, of a body frame  2 . Irregular ground-purpose lower pressure balloon tires are attached to the front wheels FW as well as to the rear wheels RW. 
     A body frame  2  is constructed by connecting a plurality of kinds of steel materials and includes a center frame portion  3 , a front frame portion  4  and a rear frame portion  5 . The center frame portion  3  mounts thereon a power unit P integrally composed of an internal combustion engine E and a transmission T in a crankcase  31 . The front frame portion  4  is joined to the front portion of the center frame portion  3  and suspends the front wheels WF. The rear frame portion  5  is connect to the rear portion of the center frame portion  3  and includes seat frames  6  supporting a seat  7 . 
     The center frame portion  3  is formed almost-rectangular as viewed laterally by connecting a pair of left and right upper pipes  3   a  each having front and rear parts bending downward to provide almost three sides, with a pair of left and right lower pipes  3   b  each providing the remaining one side. In addition, the left and right pipes are connected by cross members. 
     The lower pipe  3   b  bends and extends obliquely upwardly to form its rear portion to which a pivot plate  8  is fixedly connected. A swing arm  9  is swingably connected at its front end to the pivot plate  8 . A rear cushion  10  is interposed between the rear portion of the swing arm  9  and the rear frame portion  5 . A rear final reduction gear unit  19  is attached to the rear end of the swing arm  9 . The rear final reduction gear unit  19  suspends the rear wheel RW. 
     A steering column  11  is supported by a widthwise-central portion of a cross member spanned between the front ends of the left and right upper pipes  3   a . The steering column  11  steerably supports a steering shaft  12 . Steering handlebars  13  are joined to the upper end portion of the steering shaft  12 , which is connected at its lower end to a front wheel steering mechanism  14 . 
     The internal combustion engine E of the power unit P is a water-cooled single-cylinder 4-stroke internal combustion engine. This engine is mounted on the center frame portion  3  in the so-called longitudinally mounted posture with a crankshaft  30  oriented in the back and forth direction of the vehicle body. 
     The transmission T of the power unit P is disposed in a transmission chamber M on the left side (the right side in  FIG. 3 ) of the crank chamber C rotatably supporting the crankshaft  30  of the internal combustion engine E. An output shaft  15  projects forward and rearward from the transmission T close to the left side of the crank chamber C so as to be oriented in the back and forth direction. The rotational power of the output shaft  15  is transmitted from the front end of the output shaft  15  through a front drive shaft  16  and a front final reduction gear unit  17  to the left and right front wheels FW. In addition, the rotational power is transmitted to the left and right rear wheels RW through a rear drive shaft  18  and the rear final reduction gear unit  19 . 
     The internal combustion engine E is erected so as to slightly slant leftwardly by putting a cylinder block  32 , a cylinder head  33  and a cylinder head cover  34  on a crankcase  31  in this order. 
     An air intake pipe  20  extends rearward from the cylinder head  33  and is connected to an air cleaner  22  via a throttle body  21 . An exhaust pipe  23  extends forward from the cylinder head  33 , bending leftward, extending rearward and passing by the left side of the air cleaner  22 , and connects with an exhaust muffler  24 . 
     A fuel tank  25  is supported above the power unit P by the center frame portion  3  of the body frame  2 . A fuel pump  26  is disposed below the front portion of the fuel tank  25 . The front frame portion  4  of the body frame  2  supports a radiator  27 . 
     The crankcase  31  forms the crank chamber C and transmission chamber M of the power unit P. In addition, the crankcase  31  has a front-rear-split structure composed of a front crankcase  31 F and a rear crankcase  31 R which are divided back and forth along a plane perpendicular to the crankshaft  30  which extends along the central axis of the cylinder bore of the cylinder block  32  and is oriented in the back and forth direction of the vehicle body. 
       FIG. 3  is a front view of the power unit P, illustrating a mating surface  31 Rf of the rear crankcase  31 R with the internal combustion engine E partially omitted. A cylinder sleeve  32   a  extends into the crankcase  31  from the cylinder block  32  in a fitting manner. A piston  35  is slidably fitted into the cylinder sleeve  32   a . A crank pin  37  is spanned between a pair of front and rear crank webs  30   w ,  30   w  of the crankshaft  30 . The crank pin  37  and a piston pin  36  attached to the piston  35  are connected by a connecting rod  38 . 
       FIG. 4  is a cross-sectional view of a power transmission mechanism of the internal combustion engine E and  FIG. 5  is a cross-sectional view of an essential portion of a lubricating device. Referring to  FIG. 4 , the crankshaft  30  is rotatably supported in front and rear of the crank webs  30   w ,  30   w  by the front, crankcase  31 F and the rear crankcase  31 R via main bearings  39 ,  39 . 
     A balancer shaft  40  is located on the right (on the left in  FIG. 3 ) of and slightly below the crankshaft  30  so as to be parallel to the crankshaft  30 . The balancer shaft  40  is rotatably supported at both ends thereof by the front crankcase  31 F and the rear crankcase  31 R via bearings  41 ,  41  as shown in  FIG. 5 . The balancer shaft  40  is formed with a balancer weight  40   w  at its central portion. A driven gear  42   b  is fixedly fitted to the balancer shaft  40  at its rear portion. The driven gear  42   b  meshes with a drive gear  42   a  (see  FIG. 4 ). 
     A cam shaft  43  of a valve system is located on the right of and obliquely above the crankshaft  30  so as to be parallel to the crankshaft  30 . The cam shaft  43  is rotatably supported at both ends thereof by the front crankcase  31 F and the rear crankcase  31 R. The lower end of a push rod  45  is in contact with cam lobes  43   a ,  43   b  of the cam shaft  43 . The push rod  45  is adapted to transmit a driving force to the valve system in the cylinder head  33 . 
     The transmission T is disposed on the left (on the right in  FIG. 3 ) of the crankshaft  30 . A main shaft  46 , a counter shaft  47  and an intermediate shaft  48  constitute a speed-change gear mechanism. A shift drum  49  is driven to execute shifting and power is transmitted to the output shaft  15 . 
     Referring to  FIG. 4 , a centrifugal start clutch  56  includes a clutch inner  56   i  serving as an input member, a bowl-like clutch outer  56   o  serving as an output member and a clutch shoe  56   s  serving as a centrifugal weight. The clutch inner  56   i  is rotated integrally with the crankshaft  30 . The outer clutch outer  56   o  surrounds the clutch inner  56   i  from the radial outside. The clutch shoe  56   s  is supported by the clutch inner  56   i  and comes into contact and engagement with the clutch outer  56   o  through radially external movement. A boss portion of the clutch outer  56   o  is spline-fitted to a cylindrical gear member  57  rotatably carried by the crankshaft  30 . 
     Power is transmitted from a primary drive gear  57   a  of the cylindrical gear member  57  to the transmission T. The main shaft  46  of the transmission T includes a first main shaft  46   a  and a second main shaft  46   b  that is partially rotatably fitted to the outer circumference of the first main shaft  46   a . The second main shaft  46   b  is rotatably supported by the front crankcase  31 F via a bearing  85 . The first main shaft  46   a  is rotatably supported at its rear end by the rear crankcase  31 R via a bearing  86 . 
     An input sleeve  80  is rotatably fitted onto the first main shaft  46   a  so as to be next to and in front of the second main shaft  46   b . A disk plate  81  is fixedly fitted to the central portion of the input sleeve  80 . A primary driven gear  82  carried on the outer circumference of the disk plate  81  meshes with the primary drive gear  57 . 
     A first shift clutch  91  and a second shift clutch  92  are disposed in front and rear, respectively, of the disk plate  81  formed integrally with the primary driven gear  82 . The first and second shift clutches  91 ,  92  are hydraulic multi-disk friction clutches having the same structure. 
     The first shift clutch  91  on the font side is located adjacently to the start clutch  56  on the rear side. The bowl-like clutch outer  91   o  opening forward is integrally fixedly fitted to the front portion of the input sleeve  80 . A clutch inner  92   i  is integrally fixedly fitted to the first main shaft  46   a.    
     On the other hand, the second shift clutch  92  on the rear side is such that a clutch outer  92   o  formed like a bowl to be open rearward is integrally fixedly fitted to the rear portion of the input sleeve  80  and a clutch inner  92   i  is integrally fixedly fitted to a portion of the second main shaft  46  extending forward from the bearing  85 . 
     In this way, if the first shift clutch  91  is brought into engagement and the second shift clutch  92  into disengagement, power inputted to the driven gear  83  ( 82 ) is transmitted to the first main shaft  46   a  via the first shift clutch  91 . In contrast, if the first shift clutch  91  is brought into disengagement and the second shift clutch  92  into engagement, the power is transmitted to the second main shaft  46   b  via the second shift clutch  92 . 
     The counter shaft  47  rotatably supported by bearings  95 ,  96  is disposed parallel to respective portions, of the first main shaft  46   a  and second main shaft  46   b , extending in the transmission chamber M. A shift gear train group T 1  that is the assemblage of gear trains setting shift stages is constructed between the portions mentioned above and the counter shaft  47  (and the intermediate shaft  48 ). 
     The gear trains of the first main shaft  46   a  via the first shift clutch  91  constitute first-speed, second-speed, and fifth-speed shift stages. The gear trains of the second main shaft  46   b  via the second shift clutch  92  constitute second-speed, fourth-speed and reverse shift stages. 
     A drive gear  97  is fixedly fitted to the rear end of the counter shaft  47 , which projects rearward from the rear crankcase  31 R. The drive gear  97  meshes with a driven gear  98  fixedly fitted to the output shaft  15  disposed parallel to the counter shaft  47 . Thus, the power reduced in speed is transmitted to the output shaft  15 . 
     The shift drum  49  is turnably spanned between the front crankcase  31 F and the rear crankcase  31 R. The shift pins of shift forks  50   a ,  50   b ,  50   c  slidably carried by the guide shaft  50  are fitted into three shift grooves formed on the outer circumferential surface of the shift drum  49 . The shift drum  49  is turned to axially move the shift fork  50   a  by being guided by the shift grooves. The shift fork  50   a  moves the gears on the main shaft  46  and the shift forks  50   b ,  50   c  move the gears on the counter shaft  47 . Thus, a set of meshing shift gears is changed. 
     The rear mating surface of the front crankcase  31 F is superposed on and fastened to a front mating surface  31 Rf of the rear crankcase  31 R shown in  FIG. 3 . The crank webs  30   w  of the crankshaft  30 , the balancer weight  40   w  of the balancer shaft  40 , the cam lobes  43   a ,  43   b  of the cam shaft  43  and the shift gear train group T 1  are housed inside, thus, constructing the crankcase  31 . A front case cover  100  is put on the front case  31 F from the front via a spacer  70 . 
     The spacer  70  is an extending member obtained by forwardly extending the front surface circumferential edge portion of the front crankcase  31 F. This spacer  70  is formed with the oil pump unit  60  of the dry sump type lubricating system and with part of the oil tank  120 . 
       FIG. 6  is a front view of the spacer  70  and  FIG. 7  is a rear view of the spacer  70 . The spacer  70  is adapted to connect the front crankcase  31 F with the front case cover  100 . In addition, the spacer  70  is an annular member, which has front and rear mating surfaces  70   f ,  70   r  on its outer circumferential wall  71  and which has a left-right width greater than an up-down width. The outer circumferential wall  71  is internally partitioned by an arcuate partition wall  73  extending along the right portion (the left portion in  FIG. 6 ) of the curved outer circumferential wall  71  to define a large cavity, which is a portion of the crank chamber C on the left side (the right side) of the partition wall  73 . 
     A division wall  74 , which is a vertical wall, connects the right portion of the outer circumferential wall  71  with the partition wall  73 . The division wall  74  is adapted to partition the crank chamber from an oil tank chamber  121 . A recess portion  121   r  is defined by the outer circumferential wall  71  and the partition wall  73  so as to be formed arcuately elongate and be open forwardly. Thus, a rear portion of the oil tank chamber  121  is formed by the recess portion  121   r  and the division wall  74  used as a bottom wall. 
     The spacer  70  is substantially partitioned by the partition wall  73  to provide a left side cavity  72 . The crankshaft  30  and main shaft  46  pass through the cavity  72  and in particular the first and second shift clutches  91 ,  92  carried by the main shaft  46  are housed in the cavity  72 . The partition wall  73  is formed almost arc-circular so as to extend along the clutch outer  56   o  of the start clutch  56  installed on the front end of the main shaft  46 . 
     In this way, the elongate recess portion  121   r  is defined between the outer circumferential wall  71  and partition wall  73  to form the rear portion of the oil tank chamber  121 . In addition, the recess portion  121   r  extends upwardly-downwardly arcuately from the upper portion of the outer circumferential wall  71  to the lowermost portion while being partitioned from the cavity  72  (the crank chamber C) by the partition wall  73 . 
     The front case cover  100  covered on the spacer  70  from the front is formed with an arcuate recess portion opposed to the elongate arcuate recess portion  121   r  of the spacer  70 . Thus, both the arcuate recess portions are joined together to form the oil tank chamber  121 . The oil in the oil tank chamber  121  smoothly flows downwardly along the inclining inner surface of the outer circumferential wall of the arcuate recess portion. The division wall  74  along with a portion thereof protruding to the cavity  72  constitutes a front pump case half-body portion  61   f  of the oil pump unit  60 . 
     That is to say, the right portion of the spacer  70  is formed forward of the division wall  74  with a recess portion  121   r , which is a rear portion of the oil tank chamber  121 , and rearward of the division wall  74  with the crank chamber C. In addition, the right portion of the spacer  70  constitutes a front pump case half-body portion  61   f  of the partial oil pump unit  60 . 
     As shown in  FIG. 5 , the obliquely elongate oil pump unit  60  is configured such that a partition plate  61   a  or a pump case half-body is disposed rearward of the front pump case half-body portion  61   f , covered by the rear pump case half-body  61   r , put between the front pump case half-body portion  61   f  and the rear pump case half-body  61   r  and fastened thereto with bolts (see  FIG. 7 ). 
     On the side of the cavity  72  extending along the partition wall  72  of the oil pump unit  60 , a pump drive shaft  63  passes, in the back and forth direction, through the front pump case half-body portion  61   f , the partition plate  61   a  and the rear pump case half-body  61   r  and is rotatably supported coaxially with the balancer shaft  40 . The pump drive shaft  63  has a rear end, which further passes through the front crankcase  31 F and is integrally and rotatably connected to the balancer shaft  40  (see  FIG. 5 ). 
     As shown in  FIG. 5 , a feed pump  64  and a scavenge pump  65  are provided on the pump drive shaft  63  in front and rear, respectively, of the partition plate  61   a . The partition plate  61   a  is a pump case half-body shared by the feed pump  64  and the scavenge pump  65 . The front pump case half-body portion  61   f  of the spacer  70  and the partition plate  61   a  form a pump case for the feed pump  64 . The rear pump case half-body  61   r  and partition plate  61   a  form a pump case for the scavenge pump  65 . 
     The spacer  70  is formed with the front pump case half-body portion  61   f  of the feed pump  64  and the partition plate  61   a  serves as the pump case half-body shared by the feed pump  64  and the scavenge pump  65 . Thus, the number of component parts of the oil pump unit  60  can significantly be reduced. 
     Between the rear pump case half-body  61   r  and partition plate  61   a , an oil pumping passage S 2  is formed to extend obliquely below the scavenge pump  65  and a tank supply oil passage S 3  is formed to extend above the scavenge pump  65 . The oil pumping passage S 2  extending obliquely below the scavenge pump has a lower end opening rearward, which communicates with the oil pumping passage S 1  in the lower portion of the front crankcase  31 F through a connection pipe  68 . An oil strainer  67  is interposed between the oil pumping passage S 1  and an oil sump chamber S 0  below the oil pumping passage S 1 . 
     The tank supply oil passage S 3  extends upward and communicates with a scavenge pump discharge port  61   aa  bored in an upper end portion, of the partition plate  61   a , protruding from the front pump case half-body portion  61   f . The scavenge pump discharge port  61   aa  is connected to and communicates with a tank supply port S 4  formed at a corresponding portion of the division wall  74  (the bottom wall of the recess portion  121   r  of the oil tank chamber  121 ) of the spacer  70 . 
     The tank supply port S 4  is open at the upper portion of the oil tank chamber  121 . Thus, the scavenge pump  65  is driven to pump the oil collecting in the oil sump chamber S 0  corresponding to the bottom portion of the crank chamber C through the oil pumping passages S 1 , S 2 , discharges it to the tank supply oil passage S 3  and then supplies it to the oil tank chamber  121  through the tank supply port S 4 . 
     Alternatively, the discharge oil from the scavenge pump  65  may be supplied from the tank supply port S 4  to the oil tank chamber  121  through auxiliary equipment such as an oil cooler. 
     As described above, the scavenge pump  65  of the oil pump unit  60  is such that the partition wall  61   a  which is a pump case half-body is bored with the scavenge pump discharge port  61   aa  and the division wall  74  of the spacer  70  is formed with the tank supply port S 4  communicating with the scavenge pump discharge port  61   aa . Thus, the discharge oil passage extending from the scavenge pump  65  to the oil tank chamber  121  is reduced in the number of the component parts for simple configuration. 
     On the other hand, between the front pump case half-body portion  61   f  of the spacer  70  and the partition plate  61   a , an feed suction oil passage F 1  is formed to extend obliquely below the feed pump  64  and a feed discharge oil passage F 2  is formed to extend obliquely upward from the right of the feed pump  64 . 
     A feed suction port F 0  is formed at the lower portion, extending obliquely downward, of the front pump case half-body portion  61   f  constituting the feed suction oil passage F 1 . In addition, the feed suction port F 0  is open at the bottom portion of the oil tank chamber  121  for communication therewith. 
     Thus, since a connecting pipe, a knock pin, an O-ring and the like are not needed, the suction oil passage extending from the oil tank chamber  121  to the feed pump  64  can be simplified to configure the simplified pump case of the feed pump  64 . This can reduce the size and weight of the oil pump unit  60  including the pump case of the scavenge pump  65 . 
     A cylindrical portion  61   fa  is formed to project forward from a portion, of the pump case half-body portion  61   f  of the spacer  70 , adjacent to the downside of the tank supply port S 4 . The feed discharge oil passage F 2  extends obliquely upward, bending forward, and communicates with the cylindrical portion  61   fa  (see  FIG. 5 ). 
     Referring to  FIG. 5 , a filter case  111  of an oil filter  110  is formed on the right side wall of the front case cover  100  covered on the spacer  70  from the front. A cylindrical portion  111   a  forms an inflow oil passage A 1  extending rearward from the filter case  111  and is connected to a cylindrical portion  74   a  on the side of the spacer  70  via a connection pipe  69 . The cylindrical portion  74   a  and cylindrical portion  111   a  connected to each other through the connection pipe  69  passes through the oil tank chamber  121  in the back and forth direction. 
     As described above, the cylindrical portion  61   fa  of the feed discharge oil passage F 2 , which causes the feed pump  64  to communicate with the oil filter  110  is formed integral with the pump case half-body portion  61   f  of the spacer  70 . Therefore, the number of component parts of the oil pump unit  60  can further be reduced. 
     A valve storage portion  61   fb  is disposed below the feed pump  64  and formed by forward protruding a portion of the pump case half-body portion  61   f . A relief valve  66  is fitted into the valve storage portion  61   fb . The rear end of the relief valve  66  passes through the partition plate  61   a  and extends into a valve upstream chamber R 3  defined in the rear pump case half-body  61   r.    
     A relief oil passage R 1  is formed by downward extending a portion of the feed discharge oil passage F 2 . The relief oil passage R 1  communicates with the valve upstream chamber R 3  via a through-hole R 2  bored in the partition plate  61   a . A relief outlet R 4  is formed at a portion of the valve storage portion  61   fb  on the downstream side of the relief valve  66  to open in the feed suction oil passage F 1 . 
     Thus, the feed pump  64  is driven to cause the oil in the oil tank chamber  121  to flow from the feed suction port F 0  opening at the lower portion of the oil tank chamber  121 , passing the feed suction oil passage F 1 , and be sucked therein. Then, the oil sucked is discharged to the feed suction oil passage F 2 , passing the inflow oil passage A 1  in the cylindrical portions  61   fa ,  111   a  passing through the oil tank chamber  121  in the back and forth direction, and reaches the oil filter  110 . 
     If the discharged oil pressure exceeds a predetermine value, the relief valve  66  is opened to allow a portion of the discharged oil to return from the feed discharge oil passage F 2  to the feed suction oil passage F 1  through the relief oil passage R 1 , the valve upstream chamber R 3  and the relief outlet R 4 . 
     Referring to  FIGS. 6 and 7 , the bottom wall of the spacer  70  is inclined obliquely downwardly from the left and right to the center thereof. A bolt boss portion  75  is formed at the lowest position of the center of the bottom wall so as to protrude in the oil tank chamber  121 . The bolt boss portion  75  is vertically bored with a bolt hole adapted to receive a drain bolt  77  threaded thereto from below. In addition, the bolt boss portion  75  is bored with a drain hole  76  which passes therethrough in the back and forth direction so as to intersect the bolt hole and to cause the bottom portion of the oil tank chamber  121  to communicate with the oil sump chamber S 0  of the bottom portion of the crank chamber C. 
     Thus, if being threaded to the bolt boss portion  75  from the downside of the spacer  70 , the drain bolt  77  can close the bottom wall while partitioning the bottom portion of the oil tank chamber  121  from the bottom portion of the crank chamber C. If the drain bolt  77  is removed, the oil can be drained from both the oil tank chamber  121  and the crank chamber C. 
     The front case cover  100  covered on the spacer  70  from the front includes a front wall  101 , which is disposed inside the annular mating surface opposed to the front mating surface  70   f  of the spacer  70 , so as to be formed to protrude forward. The arcuate recess portion forming the oil tank chamber  121  as described above is formed on the right side portion of the front wall  101 . The start clutch  56 , the first shift clutch  91  and the like are accommodated in the protruding space excluding the arcuate recess portion (see  FIG. 4 ). 
     As shown in  FIG. 4 , the front wall  101  of the front case cover  100  is formed projecting inwardly with a bearing hole  101   a , a bearing cylindrical portion  102 , etc. The bearing hole  101   a  rotatably supports the front end of the crankshaft  30  via a bearing  106 . The bearing cylindrical portion  102  rotatably supports the front end of the first main shaft  46   a  via a bearing  87 . 
     The bearing cylindrical portion  102  extends outwardly to form an external cylindrical portion  103 . The external cylindrical portion  103  is internally partitioned from the inside of the bearing cylindrical portion  102  by the partition wall  102   a . The external cylindrical portion  103  has a front end opening, which is closed by a lid member  104  to define an internal space. This internal space is partitioned by a partitioning member  105  into a front chamber  103   a  and a rear chamber  103   b.    
     On the other hand, the first main shaft  46   a  is bored in its front portion with a shaft hole  106  extending from the front end thereof to a position corresponding to the second shift clutch  92 . A long conduction inner tube  107  is inserted from the front chamber  103   a  into the shaft hole  106  so as to pass through the partition member  105 . The conduction inner tube  107  is disposed to reach an intermediate position between the first shift clutch  91  and the second shift clutch  92 . In addition, the rear end of the conduction inner tube  107  is supported in the shaft hole  106  by the seal member  107   a.    
     A short conduction outer tube  108  is disposed coaxially with the conduction inner tube  107  and on the outer circumference of the conduction inner tube  107 . The conduction outer tube  108  is fitted at its front end into the partition wall  102   a , is inserted into the shaft hole  106 , and is supported at its rear end by a seal member  108   a.    
     Hydraulic pressure is supplied to the front chamber  103   a  and rear chamber  103   b  of the external cylindrical portion  103  from a hydraulic control unit  160 . 
     If being supplied to the rear chamber  103   b , the hydraulic pressure passes between the shorter conduction outer tube  108  and the conduction inner tube  107  and is supplied to the first shift clutch  91  from the front of the seal member  107   a , thereby bringing the first shift clutch  91  into engagement. 
     If being supplied to the front chamber  103   a , the hydraulic pressure passes through the longer conduction inner tube  107  and is supplied to the second shift clutch  92  from the shaft hole  106  rearward of the seal member  107   a , thereby bringing the second shift clutch  92  into engagement. 
     Shifting is smoothly executed by the hydraulic control valve unit  160  controlled to alternately switch between the shift stages of the first-speed, third-speed and fifth-speed of the gear train on the first main shaft  46   a  via the first shift clutch  91  described above and the shift gears of the second-speed, fourth-speed and reverse of the gear train on the second main shaft  46   b  via the second shift clutch  92 . 
     As shown in  FIG. 5 , a filter element  113  is inserted into the filer case  111  formed on the right-side wall of the front case cover  100  and is covered by the filter cover  112  from the right, thus constituting the oil filter  110 . 
     The oil discharged from the feed pump  64  is allowed to flow into the filter case  111  from the suction oil passage A 1  extending rearward from the filter case  111 . An outflow oil passage A 2  extends from the center of the bottom wall of the filter case  111  along the front wall  101  of the front case cover  100 . 
     Referring to  FIG. 8 , the outflow oil passage A 2  communicates with the bearing hole  101   a  to lubricate the bearing  110 . The bearing hole  101   a  is adapted to rotatably support the front end of the crankshaft  30  via the bearing  110 . An oil supply passage A 3  continuous with the outflow oil passage A 2  extends obliquely upward from the bearing hole  101   a  and communicates with the hydraulic control valve unit  160  disposed on the upper-left portion of the front wall  101  for oil supply. 
     In addition, another oil supply passage B 1  braches from the intermediate portion of the outflow oil passage A 2  and extends upward for supplying oil to the cylinder head  32 . 
     The hydraulic control valve unit  160  is disposed adjacently to the external cylindrical portion  103  coaxial with the bearing cylindrical portion  102  which rotatably supports the main shaft  46  carrying the first and second shift clutches  91 ,  92  thereon. The hydraulic control valve unit  160  controls hydraulic pressure to be supplied to the front chamber  103   a  and rear chamber  103   b  of the external cylindrical portion  103  which controls the engagement and disengagement of each of the first and second shift clutches  91 ,  92 . 
     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.