Patent Publication Number: US-7717234-B2

Title: Lubricating device of internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2006-224637 filed on Aug. 21, 2006 the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a dry sump lubricating device for an internal combustion engine. 
   2. Description of Background Art 
   An internal combustion engine having a dry sump lubricating device includes an oil tank for storing oil pumped by a scavenge pump. The oil tank is generally configured by being defined in a crankcase, or is configured between a crankcase and a crankcase cover, if the crankcase cover is provided. See, for example, JP-A No. 2004-108257. 
   The lubricating device disclosed in JP-A No. 2004-108257 is configured in such a manner that the oil pumped by the scavenge pump of a trochoid type is supplied to the oil tank from a supply port for the tank located nearest to a front case cover (crankcase cover) through an oil cooler. The oil stored in the oil tank is sucked from a suction port which is open at a lower portion of the oil tank by a feed pump of a trochoid type. In addition, the oil discharged from a discharge port of the feed pump is supplied to respective regions for lubrication of the internal combustion engine. 
   In the dry sump lubricating device, when the oil is supplied to the oil tank from the supply port for the tank by the scavenge pump, bubbles are mixed in the oil. Further, bubbles are also mixed in the oil sucked by the feed pump. Thus, there is a high possibility that the feed pump causes cavitation. 
   A hydraulic pressure of the oil to be supplied to the respective regions for lubrication is not stabilized due to the cavitation of the feed pump. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   The present invention has been achieved in view of the foregoing. An object of an embodiment of the present invention is directed to providing a lubricating device of a power unit in which bubbles are prevented from being mixed in the oil sucked from an oil tank by a feed pump to reduce cavitation of the feed pump. Thus, hydraulic pressure of the oil to be supplied to respective regions for lubrication can be stabilized. 
   In order to achieve an object of an embodiment of the present invention, a lubricating device for an internal combustion engine is provided in which an oil tank chamber, defined by being separated from a crank chamber by a partition wall, is formed between a crankcase and a crankcase cover of the internal combustion engine. Oil pumped by a scavenge pump is supplied to a tank supply port which is open at an upper portion of the oil tank chamber. The oil stored in the oil tank chamber is sucked from a feed suction port which is open at a lower portion of the oil tank chamber to be supplied to respective regions for lubrication of the internal combustion engine by a feed pump, wherein an oil passage penetrating inside the oil tank chamber is formed below the tank supply port at an upper portion of the oil tank chamber. 
   An object of an embodiment of the present invention is to provide a lubricating device for an internal combustion engine wherein a relief valve is provided in order to return a part of the feed discharge oil to the feed suction oil passage when a discharge hydraulic pressure of the feed pump exceeds a predetermined value. 
   An object of an embodiment of the present invention is to provide the lubricating device for an internal combustion engine wherein the tank supply port is an opening formed in the crankcase and is directly coupled to a discharge port of the scavenge pump. The oil passage is a feed discharge oil passage formed integrally with the crankcase and is directly coupled to a discharge port of the feed pump. 
   An object of an embodiment of the present invention is to provide the lubricating device for an internal combustion engine wherein the tank supply port is formed above the oil passage while being adjacent thereto. 
   According to an embodiment of the present invention, the lubricating device for an internal combustion engine includes an oil passage that is formed and located below the tank supply port at an upper part of the oil tank chamber and which penetrates inside the oil tank chamber. Accordingly, bubbles which are mixed in an oil supplied by the scavenge pump to the tank supply port located at an upper portion of the oil tank chamber are prevented from traveling downward by the oil passage which is located below the tank supply port and which penetrates inside the oil tank chamber. Thus, air bleeding for the oil is prompted at an upper portion of the oil tank chamber. The bubbles are prevented from being mixed in the oil to be sucked from the feed suction port located at a lower portion of the oil tank chamber, so that cavitation of the feed pump can be reduced. Thus, the hydraulic pressure of the oil supplied to the respective regions for lubrication can be stabilized. 
   According to an embodiment of the present invention, when the discharge hydraulic pressure of the feed pump exceeds a predetermined value, the oil returned to the feed suction oil passage is sucked by the feed pump again because the relief valve for returning a part of the feed discharge oil to the feed suction oil passage is provided. Accordingly, the amount of oil sucked from the feed suction port can be reduced, and the suction flow rate can also be decreased, thus resulting in further reduction of cavitation of the feed pump. 
   According to an embodiment of the present invention, the tank supply port is directly coupled to the discharge port of the scavenge pump so as to be formed in the crankcase. The oil passage is also directly coupled to the discharge port of the feed pump so as to be formed integrally with the crankcase. Accordingly, the number of components of the internal combustion engine can be cut down, and the internal combustion engine can be reduced in size and weight. 
   According to an embodiment of the present invention, the tank supply port is formed above the oil passage while being adjacent thereto. Accordingly, bubbles which are mixed in oil supplied to the supply port for the tank can be effectively prevented from traveling downward by the adjacent oil passage, air bleeding for the oil can be further prompted, and cavitation of the feed pump can be further reduced. 
   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 side view of an all terrain vehicle, in which a power unit is mounted, according to an embodiment of the present invention in a state where a body cover and the like are dismounted; 
       FIG. 2  is a plan view of the same; 
       FIG. 3  is a front view of the power unit in which an internal combustion engine is partially omitted; 
       FIG. 4  is a cross sectional view of a power transmission mechanism; 
       FIG. 5  is a cross sectional view taken along the line V-V of  FIG. 6  and  FIG. 7  of main components of a lubricating device; 
       FIG. 6  is a front view of a spacer (an extension member of a crankcase); 
       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 
   Hereinafter, an embodiment according to the present invention will be described on the basis of  FIGS. 1 to 8 . 
     FIG. 1  is a side view of an ATV (All Terrain Vehicle)  1 , in which a water-cooled internal combustion engine E is mounted, according to an embodiment in a state where a body cover and the like are dismounted.  FIG. 2  is a plan view of the same. 
   It should be noted that front, rear, left, and right directions of the vehicle are determined based on a state wherein the vehicle is directed in the forward direction in the embodiment. 
   The ATV  1  is a saddle-ride type four-wheeled vehicle in which a pair of left and right front wheels FW having low-pressure balloon tires for irregular terrain are mounted together with a pair of left and right rear wheels RW having the same kind of low-pressure balloon tires mounted that are suspended by a body frame  2  at its front and rear portions, respectively. 
   The body frame  2  is configured by coupling a plurality of kinds of steel materials to each other, and includes a center frame part  3  in which a power unit P that is integrally configured by an internal combustion engine E and a transmission T in a crankcase  31  is mounted. A front frame part  4  is continued to a front portion of the center frame part  3  to suspend the front wheels WF. A rear frame part  5  is continued to a rear portion of the center frame part  3  and includes seat rails  6  for supporting a seat  7 . 
   The center frame part  3  forms a substantially rectangular shape when viewed from its sides in such a manner that front and rear portions of a pair of left and right upper pipes  3   a  are bent downward to form substantially three sides with the rest of one side being configured by a pair of left and right lower pipes  3   b . Both left and right pipes are coupled to each other through a cross member. 
   A swing arm  9  is swingably provided to a pivot plate  8 , while its front end is journalled thereto, which is fastened to extension portions formed by bending rear portions of the lower pipes  3   b  diagonally upward. A rear shock absorber  10  is interposed between a rear portion of the swing arm  9  and the rear frame part  5 . The rear wheels RW are suspended by a rear final reduction gear unit  19  provided at a rear end of the swing arm  9 . 
   A steering column  11  is supported by a cross member, in the middle of its width direction, which is provided between front ends of the left and right upper pipes  3   a . A steering handlebar  13  is coupled to an upper end of a steering shaft  12  which is steerably supported by the steering column  11  with a lower end of the steering shaft  12  being coupled to a front wheel steering mechanism  14 . 
   The internal combustion engine E of the power unit P is a water-cooled, single-cylinder, four-stroke internal combustion engine that is mounted in the center frame part  3  in such a manner that a crankshaft  30  is directed in the front-rear direction of the vehicle body. More specifically, the crankshaft  30  is vertically placed. 
   The transmission T of the power unit P is arranged in a transmission chamber M located on the left side (the right side in  FIG. 3 ) of a crank chamber C to which the crankshaft  30  of the internal combustion engine E is journalled. An output shaft  15 , that is directed in the front-rear direction, projects to the front and rear from the transmission M located nearer the left side. The rotational power of the output shaft  15  is transmitted to the left and right front wheels FW from a front end of the output shaft  15  through a front drive shaft  16  and a front final reduction gear unit  17 , and is transmitted to the left and right rear wheels RW from a rear end of the output shaft  15  through a rear drive shaft  18  and the rear final reduction gear unit  19 . 
   The internal combustion engine E is configured in such a manner that a cylinder block  32 , a cylinder head  33 , and a cylinder head cover  34  are sequentially laminated in the crankcase  31  so as to be provided in an erect manner while being slightly inclined to the left. 
   A suction pipe  20 , which extends to the rear from the cylinder head  33 , is connected to an air cleaner  22  through a throttle body  21 . An exhaust pipe  23 , which extends to the front from the cylinder head  33 , is bent to the left to extend to the rear, and then extends rearward on the left side of the air cleaner  22  to be connected to an exhaust muffler  24 . 
   A fuel tank  25  is supported, above the power unit P, by the center frame part  3  of the body frame  2  with a fuel pump  26  being arranged on the front lower side of the fuel tank  25 , and a radiator  27  being supported by the front frame part  4  of the body frame  2 . 
   The crankcase  31  configuring the crank chamber C and the transmission chamber M of the power unit P has a structure including a front crankcase  31 F and a rear crankcase  31 R which are divided into front and rear portions by a face orthogonal to the crankshaft  30  that passes through a center axis line of a cylinder bore of the cylinder block  32  and that is directed in the front-rear direction of the vehicle body. 
     FIG. 3  is a front view of the power unit P illustrating a mating face  31 Rf of the rear crankcase  31 R while partially omitting the internal combustion engine E. 
   A cylinder sleeve  32   a  is fitted into the crankcase  31  from the cylinder block  32 , and a piston  35  is swingably fitted into the cylinder sleeve  32   a.    
   A crank pin  37 , which is provided between a pair of front and rear crank webs  30   w  and  30   w  of the crankshaft  30 , is coupled to a piston pin  36  provided in the piston  35  through a connecting rod  38 . 
     FIG. 4  shows a cross-sectional view of a power transmission mechanism of the internal combustion engine E, and  FIG. 5  is a cross-sectional view of the main components of a lubricating device. 
   As shown in  FIG. 4 , the crankshaft  30  is journalled to the front crankcase  31 F and the rear crankcase  31 R in front of and in rear of the crank webs  30   w  and  30   w  through main bearings  39  and  39 . 
   A balancer shaft  40  which is parallel to the crankshaft  30  is located slightly below the right side (the left side in  FIG. 3 ) of the crankshaft  30 , and both ends of the balancer shaft  40  are journalled to the front crankcase  31 F and the rear crankcase  31 R through bearings  41  and  41 , as shown in  FIG. 5 . 
   A balancer weight  40   w  is formed in the middle of the balancer shaft  40 , and a driven gear  42   b  is fitted into a rear portion of the balancer shaft  40  so as to be meshed with a drive gear  42   a  (see  FIG. 4 ) fitted into the crankshaft  30 . 
   A cam shaft  43  of a valve system which is parallel to the crankshaft  30  is located diagonally above the right side of the crankshaft  30 , and both ends of the cam shaft  43  are journalled to the front crankcase  31 F and the rear crankcase  31 R. 
   A lower end of a push rod  45  which transmits a drive power to a valve mechanism inside the cylinder head  33  abuts on cam lobes  43   a  and  43   b  of the cam shaft  43 . 
   The transmission T is arranged on the left side (the right side in  FIG. 3 ) of the crankshaft  30 , and a main shaft  46 , a counter shaft  47 , and an intermediate shaft  48  configure a speed-change gear mechanism by which a shift drum  49  is driven for speed-change to transmit the changed speed to the output shaft  15 . 
   With reference to  FIG. 4 , a centrifugal starting clutch  56  includes a clutch inner  56   i , as an input member, which is rotated integrally with the crankshaft  30 , a bowl-shape clutch outer  56   o , as an output member, which encircles the clutch inner  56   i  outside in the diameter direction, and a clutch shoe  56   s , as a centrifugal weight, which is pivoted by the clutch inner  56   i  and which is brought into contact with and connected to the clutch outer  56   o  by being swung outside in the diameter direction. A boss part of the clutch outer  56   o  is spline-fitted to a cylindrical gear member  57  which is rotatably journalled to the crankshaft  30 . 
   A power is transmitted from a driven gear  57   a  of the cylindrical gear member  57  to the transmission T. 
   [0032] 
   The main shaft  46  of the transmission T is configured by a first main shaft  46   a  and a second main shaft  46   b  which is partially fitted into an outer circumference of the first main shaft  46   a  in a rotatable manner. The second main shaft  46   b  is journalled to the front crankcase  31 F through a bearing  85 , and a rear end of the first main shaft  46   a  is journalled to the rear crankcase  31 R through a bearing  86 . 
   [0033] 
   An input sleeve  80  provided side-by-side with the second main shaft  46   b  is rotatably fitted into the front side of the first main shaft  46   a , a disk-shape disk plate  81  is fitted into a middle portion of the input sleeve  80 , and a driven gear  82  provided on an outer circumference of the disk plate  81  is meshed with the drive gear  57   a.    
   A first speed-change clutch  91  and a second speed-change clutch  92  are arranged ahead of and behind the disk plate  81 , respectively. 
   [0034] 
   The first speed-change clutch  91  and the second speed-change clutch  92  are hydraulic multi-disk friction clutches with the same structure. 
   The first speed-change clutch  91  located on the front side is adjacent to the rear side of the starting clutch  56 , and is configured in such a manner that a bowl-shape clutch outer  91   o  which is open to the front is integrally fitted into the front side of the input sleeve  80 , and a clutch inner  91   i  is integrally fitted into the first main shaft  46   a.    
   On the other hand, the second speed-change clutch  92  located on the rear side is configured in such a manner that a bowl-shape clutch outer  92   o , which is open to the rear is integrally fitted into the rear side of the input sleeve  80 , and a clutch inner  92   i , is integrally fitted into a portion extending forward relative to the bearing  85  of the second main shaft  46   b.    
   Accordingly, if the first speed-change clutch  91  is placed into an engaged state and the second speed-change clutch  92  is placed into a disengaged state, a power input to a driven gear  83  is transmitted to the first main shaft  46   a  through the first speed-change clutch  91 . On the contrary, if the first speed-change clutch  91  is placed into a disengaged state and the second speed-change clutch  92  is placed into an engaged state, the power input to the driven gear  83  is transmitted to the second main shaft  46   b  through the second speed-change clutch  92 . 
   Between the counter shaft  47  (and the intermediate shaft  48 ) which are arranged parallel to extension portions of the first main shaft  46   a  and the second main shaft  46   b  in the transmission chamber M and which are journalled through bearings  95  and  96 , there is configured a speed change gear train group T 1  which is an assembly of gear trains for setting a speed change gear. 
   The gear trains of the first main shaft  46   a  through a first speed-change clutch  251  configure speed change gears of the first-speed, third-speed and fifth-speed, and the gear trains of the second main shaft  46   b  through a second speed-change clutch  252  configure speed change gears of the second-speed, fourth-speed and the reverse. 
   A drive gear  97  is fitted into a rear end of the counter shaft  47  which extends to the rear from the rear crankcase  31 R. A driven gear  98 , which is fitted into the output shaft  15  arranged parallel to the counter shaft  47 , is meshed with the drive gear  97 , so that the power of a reduced speed is transmitted to the output shaft  15 . 
   The shift drum  49  is rotatably provided between the front crankcase  31 F and the rear crankcase  31 R, and respective shift pins of shift forks  50   a ,  50   b , and  50   c  which are swingably supported by a guide shaft  50  are fitted into shift grooves of three stripes formed on an outer circumferential face of the shift drum  49 . The shift fork  50   a  which is guided by the shift groove by rotation of the shift drum  49  to be moved in the axis direction allows the gears on the main shaft  46  to be moved. In addition, the shift forks  50   b  and  50   c  allow the gears on the counter shaft  47  to be moved, so that a combination of speed change gears to be meshed are changed. 
   A rear mating face of the front crankcase  31 F is combined with a front mating face  31 Rf of the rear crankcase  31 R shown in  FIG. 3  so as to be tightened together. In addition, the crankcase  31  is configured by accommodating therein the crank web  30   w  of the crankshaft  30 , the balancer weight  40   w  of the balancer shaft  40 , the cam lobes  43   a  and  43   b  of the cam shaft  43 , and the speed change gear train group T 1 . 
   The front side of the front crankcase  31 F is covered with a front case cover  100  through a spacer  70 . 
   The spacer  70  is an extension member formed in such a manner that a marginal portion of a front face of the front crankcase  31 F is extends forwardly. In addition, an oil pump unit  60  of a dry sump lubrication system is configured in the spacer  70  and a part of an oil tank  120  is formed therein. 
     FIG. 6  shows a front view of the spacer  70 , and  FIG. 7  is a rear view of the same. 
   The spacer  70  is used for linking the front crankcase  31 F and the front case cover  100 , and is a circular-shape member having front and rear mating faces  70   f  and  70   r  which are parallel to each other on a peripheral wall  71  and having a larger width in the left-right direction than that in the up-down direction. The inside of the peripheral wall  71  is partitioned by a circular arc-shape partition wall  73  along a bent right side portion (a left side portion in  FIG. 6 ) of the peripheral wall  71 , so that a large cavity  72  which is a part of the crank chamber C is defined on the left side (the right side in  FIG. 6 ) of the partition wall  73 . 
   A right side portion of the peripheral wall  71  and the partition wall  73  are coupled through a vertical wall  74 . An elongated concave part  121   r  forms a rear portion of an oil tank chamber  121  in a circular arc-shape portion formed by being encircled by the peripheral wall  71 , the partition wall  73 , and the vertical wall  74  serving as a bottom wall. 
   The crankshaft  30  and the main shaft  46  penetrate the left-side cavity  72  which is substantially partitioned by the partition wall  73  of the spacer  70 . The first speed-change clutch  251  and the second speed-change clutch  252  provided on the main shaft  46  are accommodated in the cavity  72 . The partition wall  73  is formed in a circular arc-shape so as to be along a clutch outer  202  of a starting clutch  200  provided at a front end of the main shaft  46 . 
   Thus, the elongated concave part  121   r  which forms a rear portion of the oil tank chamber  121  and which is located between the peripheral wall  71  and the partition wall  73  vertically extends with a circular arc shape from an upper portion of the peripheral wall  71  to a lowermost portion thereof while being separated from the cavity  72  (crank chamber C) by the partition wall  73 . 
   Another circular arc-shape concave part is formed opposite to the elongated, circular arc-shape concave part  121   r  of the spacer  70  in the front case cover  100  with which the front side of the spacer  70  is covered. The oil tank chamber  121  is configured by coupling both circular arc-shape concave parts. 
   Oil inside the oil tank chamber  121  smoothly flows downward along inclined inner faces of outer peripheral walls of the circular arc-shape concave parts. 
   The vertical wall  74  projects to the cavity  72 , and a part of the vertical wall  74  configures a front oil pump case  61   f  of the oil pump unit  60 . 
   More specifically, on the right side of the spacer  70 , the concave part  121   r  which is a rear portion of the oil tank chamber  121  is formed ahead of the vertical wall  74 , and the front oil pump case  61   f  of the oil pump unit  60  is formed behind the vertical wall  74 . 
   As shown in  FIG. 5 , the oil pump unit  60 , formed in an inclined elongated shape, is configured in such a manner that a rear portion of the front oil pump case  61   f  is covered with a rear oil pump case  61   r  while sandwiching a partition wall  61   a . The cases are tightened to bolts (see  FIG. 7 ). 
   On the cavity  72 -side along the partition wall  73  of the oil pump unit  60 , a pump drive shaft  63  penetrates the front oil pump case  61   f , the partition wall  61   a , and the rear oil pump case  61   r  in the front-rear direction, is journalled coaxially with the balancer shaft  40 . A rear end thereof further penetrates the front crankcase  31 F to be coupled to the balancer shaft  40  while being integrally rotatable therewith (see  FIG. 5 ). 
   As shown in  FIG. 5 , a feed pump  64  and a scavenge pump  65  are provided ahead of and behind the partition wall  61   a  of the pump drive shaft  63 . 
   Between the rear oil pump case  61   r  and the partition wall  61   a , an oil pumping passage S 2  is formed in an extended manner diagonally below the scavenge pump  65 . An oil supply passage S 3  for tank is formed in an extended manner above the scavenge pump  65 . 
   A lower end of the oil pumping passage S 2  extending diagonally downwardly has an opening at its rear side, and is in communication with an oil pumping passage S 1  located at a lower portion of the front crankcase  31 F through a coupling pipe  68 . An oil strainer  67  is interposed between the oil pumping passage S 1  and an oil storage chamber S 0  located therebelow. 
   The oil supply passage S 3  for the tank extending upwardly is open at an upper end of the partition wall  61   a , and is in communication with a supply port S 4  for a tank of a corresponding portion of the vertical wall  74  (the bottom wall of the concave part  121   r  of the oil tank chamber  121 ) of the spacer  70 . 
   The supply port S 4  for tank is open at an upper portion of the oil tank chamber  121 . 
   Thus, by driving the scavenge pump  65 , oil stored in the oil storage chamber S 0  corresponding to a bottom portion of the crank chamber C is pumped by passing through the oil pumping passages S 1  and S 2  via the oil strainer  67 , and is discharged to the oil supply passage S 3  for tank so as to be supplied from the supply port S 4  for tank to the oil tank chamber  121 . 
   The oil discharged from the scavenge pump  65  may be supplied from the supply port S 4  for tank to the oil tank chamber  121  through, for example, auxiliaries such as an oil cooler. 
   On the other hand, between the front oil pump case  61   f  and the partition wall  61   a , a feed suction oil passage F 1  is formed in an extended manner diagonally below the feed pump  64 . A feed discharge oil passage F 2  is formed while being extended diagonally upwardly from a right portion of the feed pump  64 . 
   A lower end of the feed suction oil passage F 1  extending diagonally downwardly serves as a feed suction port F 0  formed in the front oil pump case  61   f , and is open at a lower portion of the oil tank chamber  121 . 
   A cylindrical part  74   a  is formed below the supply port S 4  for the tank while being adjacent thereto and while projecting forward from the vertical wall  74  (the bottom wall of the oil tank chamber  121 ). The feed discharge oil passage F 2  extending diagonally upwardly is bent forward so as to be in communication with the cylindrical part  74   a.    
   With reference to  FIG. 5 , a filter case of an oil filter  110  is formed on a right side wall of the front case cover  100  with which the front side of the spacer  70  is covered, and a cylindrical part  111   a  which forms an oil inflow passage A 1  extending rearwardly from the filter case  111  is coupled to the cylindrical part  74   a  nearer the spacer  70  through a coupling pipe  69 . 
   The cylindrical part  74   a  and the cylindrical part  111   a  coupled through the coupling pipe  69  penetrate inside the oil tank chamber  121  in the front-rear direction. 
   Below the feed pump  64 , a relief valve  66  is fitted into a valve accommodation part  74   b  formed by swelling the vertical wall  74 , and a rear end of the relief valve  66  penetrates the partition wall  61   a  to be fitted into a valve upstream chamber R 3  defined in the rear oil pump case  61   r.    
   A relief oil passage R 1  which is formed by extending a part of the feed discharge oil passage F 2  downwardly and the valve upstream chamber R 3  are in communication with each other through a through-hole R 2  formed by drilling a hole in the partition wall  61   a.    
   A relief exit R 4 , formed in the valve accommodation part  74   b  on the valve downstream-side of the relief valve  66 , is open to the feed suction oil passage F 1 . 
   Thus, by driving the feed pump  64 , oil inside the oil tank chamber  121  is sucked by passing through the feed suction oil passage F 1  from the feed suction port F 0  which is open at a lower portion of the oil tank chamber  121 , and is discharged to the feed discharge oil passage F 2  to reach the oil filter  110  by passing through the oil inflow passage A 1  inside the cylindrical parts  74   a  and  111   a  which penetrate inside the oil tank chamber  121  in the front-rear direction. 
   When a discharge hydraulic pressure of the feed pump  64  exceeds a predetermined value, the relief valve  66  is open, and a part of the discharged oil returns to the feed suction oil passage F 1  from the feed discharge oil passage F 2  through the relief oil passage R 1 , the valve upstream chamber R 3 , and the relief exit R 4 . 
   As shown in  FIG. 6  and  FIG. 7 , the bottom wall of the spacer  70  is inclined downwardly from the left and right sides toward the middle, and a bolt boss part  75  is formed, while swelling in the oil tank chamber  121 , on the bottom wall at the lowermost position in the middle. A bolt hole to which a drain bolt  77  is fastened by screwing from the lower side is vertically provided in the bolt boss part  75 , and a drain hole  76  penetrates the bolt boss part  75  in the front-rear direction so as to intersect with the bolt hole, and a bottom portion of the oil tank chamber  121  is accordingly in communication with the oil storage chamber S 0  of a bottom portion of the crank chamber C (see  FIG. 5 ). 
   Thus, when the drain bolt  77  is fastened to the bolt boss part  75  by screwing from the lower side of the spacer  70 , the bottom wall can be closed while partitioning between a bottom portion of the oil tank chamber  121  and a bottom portion of the crank chamber C. When the drain bolt  77  is unscrewed, oil can be simultaneously drained from the both of the oil tank chamber  121  and the crank chamber C. 
   In the front case cover  100  with which the front side of the spacer  70  is covered, a front wall  101  inside a circular-shape mating face opposite to the front mating face  70   f  of the spacer  70  is formed while being swelled forward. On the right side of the front case cover  100 , the circular arc-shape concave part configuring the oil tank chamber  121  is formed as described above, and the starting clutch  56 , the first speed-change clutch  91 , and the like are accommodated in a swelled space except the circular arc-shape concave part (see  FIG. 4 ). 
   As shown in  FIG. 4 , on the front wall  101  of the front case cover  100 , a bearing hole  101   a  to which a front end of the crankshaft  30  is journalled through a bearing  106 , and a bearing cylindrical part  102  to which a front end of the first main shaft  46   a  is journalled through a bearing  87  are formed while projecting to the inner side. 
   The inside of an outer cylindrical part  103  formed by extending the bearing cylindrical part  102  outwardly and the inside of the bearing cylindrical part  102  are partitioned by a division wall  102   a , an opening of a front end of the outer cylindrical part  103  is closed by a lid member  104 , and the inner space is partitioned into a front chamber  103   a  and a rear chamber  103   b  by a partition member  105 . 
   On the other hand, a shaft hole  106  is provided from a front end of the first main shaft  46   a  to a position corresponding to the second speed-change clutch  92  by drilling a hole in a front portion of the first main shaft  46   a . A long conduction inner pipe  107  which penetrates the partition member  105  from the front chamber  103   a  to be inserted into the shaft hole  106  is arranged so as to reach an intermediate position between the first speed-change clutch  91  and the second speed-change clutch  92 . A rear end thereof is supported by the shaft hole  106  by using a seal member  107   a.    
   A short conduction outer pipe  108  which is coaxially arranged around the outer circumference of the long conduction inner pipe  107  is inserted into the shaft hole  106  with a front end thereof fitted into the division wall  102   a . A rear end thereof is supported by the shaft hole  106  by using a seal member  108   a.    
   To each of the front chamber  103   a  and the rear chamber  103   b  of the outer cylindrical part  103 , hydraulic pressure is supplied from a hydraulic pressure control valve unit  160 . 
   When hydraulic pressure is supplied to the rear chamber  103   b , the pressured oil passes between the short conduction outer pipe  108  and the conduction inner pipe  107 , and is supplied to the first speed-change clutch  91  from the front of the seal member  107   a  so as to allow the first speed-change clutch  91  to be in an engaged state. 
   When hydraulic pressure is supplied to the front chamber  103   a , the pressurized oil passes through the long conduction inner pipe  107 , and is supplied to the second speed-change clutch  92  from the shaft hole  106  located rearwardly relative to the seal member  107   a  so as to allow the second speed-change clutch  92  to be in an engaged state. 
   The speed change gears of the first-speed, third-speed and fifth-speed of the gear trains of the first main shaft  46   a  through the first speed-change clutch  91 , and the speed change gears of the second-speed, fourth-speed and the reverse of the gear trains of the second main shaft  46   b  through the second speed-change clutch  252  are alternately shifted by the control of the hydraulic pressure control valve unit  160 . Thus, a speed change is smoothly performed. 
   As shown in  FIG. 5 , the oil filter  110  is configured in such a manner that a filter element  113  is inserted into the filter case  111  formed on a right side wall of the front case cover  100 , and the filter case  111  is covered with the filter cover  112  from the right side. 
   Oil discharged from the feed pump  64  flows in from the oil inflow passage A 1  extending rearwardly behind the filter case  111 , and an oil outflow passage A 2  extends to the left along the front wall  101  of the front case cover  100  from the middle of a bottom wall of the filter case  111 . 
   As shown in  FIG. 8 , the oil outflow passage A 2  is in communication with the bearing hole  101   a  of the front wall  101  to which a front end of the crankshaft  30  is journalled through the bearing  106  for allowing the bearing  106  to be lubricated. Further, an oil supply passage A 3  which is continued to the oil outflow passage A 2  extends diagonally upwardly from the bearing hole  101   a , and is in communication with the hydraulic pressure control valve unit  160  arranged on the left upper side of the front wall  101  so as to supply oil. 
   In addition, an oil supply passage B 1  is branched from the mid-course of the oil outflow passage A 2 , and extends upwardly to supply oil to the cylinder head  32 . 
   The hydraulic pressure control valve unit  160  is adjacent to the outer cylindrical part  103  and coaxial with the bearing cylindrical part  102  to which the main shaft  46  having thereon the first and second speed-change clutches  91  and  92  provided is journalled for controlling hydraulic pressure to be supplied to the front chamber  103   a  and the rear chamber  103   b  of the outer cylindrical part  103  which controls engagement/disengagement of the first and second speed-change clutches  91  and  92 . 
   As described above, in the lubricating device of the internal combustion engine E, the feed discharge oil passage F 2  of the cylindrical part  74   a  is formed which is located below the supply port S 4  for the tank while being adjacent thereto at an upper part of the oil tank chamber  121  and which penetrates inside the oil tank chamber  121 . Accordingly, bubbles which are mixed in oil supplied by the scavenge pump  65  to the supply port S 4  for the tank located at an upper portion of the oil tank chamber  121  are prevented from traveling downward by the feed discharge oil passage F 2  which is located below the supply port S 4  for the tank while being adjacent thereto and which penetrates inside the oil tank chamber. Air bleeding for the oil is prompted at an upper portion of the oil tank chamber  121 . Thus, bubbles are prevented from being mixed in the oil to be sucked from the feed suction port F 0  located at a lower portion of the oil tank chamber  121 , so that cavitation of the feed pump  64  can be reduced, and the hydraulic pressure of the oil supplied to the respective regions for lubrication can be stabilized. 
   When the discharge hydraulic pressure of the feed pump  64  exceeds a predetermined value, the oil returned to the feed suction oil passage F 1  is sucked by the feed pump  64  again because the relief valve  66  for returning a part of the feed discharge oil to the feed suction oil passage F 1  is provided. Accordingly, the amount of oil sucked from the feed suction port F 0  can be reduced, and the suction flow rate can be also decreased, thus resulting in a further reduction of cavitation of the feed pump. 
   The supply port S 4  for the tank is directly coupled to the discharge port of the scavenge pump  65 , and is formed in the spacer  70  which is an extension member of the crankcase  31 . The feed discharge oil passage F 2  is also directly coupled to the discharge port of the feed pump  64  so as to be formed integrally with the spacer  70 . Accordingly, the number of components of the internal combustion engine E can be cut down, and the internal combustion engine E can be reduced in size and weight. 
   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.