Abstract:
To provide a layout structure of a hydraulic control valve for a valve train in an internal combustion engine which can reduce the influence of heat from the cylinders and can avoid the impairment of the external appearance of the internal combustion engine. An internal combustion engine has a valve operation characteristics changing mechanism that changes the operation characteristics of at least one of an intake valve and an exhaust valve. A hydraulic control valve for a valve train controls the pressure of a hydraulic fluid supplied to the valve operation characteristics changing mechanism. The hydraulic control valve is located on the left side surface of right and left crankcases that support and cover a crankshaft at a position below cylinders.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 2005-251805 and 2006-120503, filed in Japan on Aug. 31, 2005 and Apr. 25, 2006, respectively. The entirety of each of the above-identified applications is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a layout structure of a hydraulic control valve for a valve train in an internal combustion engine.  
         [0004]     2. Description of Background Art  
         [0005]     An internal combustion engine has a valve train that has operation characteristics that can be changed by a hydraulic control valve. A layout structure of the hydraulic control valve is disclosed in Japanese Patent Laid-Open No. 2002-180812, for example.  
         [0006]     In Japanese Patent Laid-Open No. 2002-180812, the hydraulic control valve is located in a space below an induction system extending from V-banked cylinder heads of a V-type internal combustion engine toward the inside (valley) of the V bank. Accordingly, the hydraulic control valve does not interfere with any equipment including the induction system, and a hydraulic path can also be reduced.  
       SUMMARY OF THE INVENTION  
       [0007]     However, the space below the induction system of the V-type internal combustion engine is a space defined between V-banked cylinders. The heat from the cylinders directly influences this space. Furthermore, this space is confined both in the vertical direction and in the longitudinal direction (or the lateral direction) of a vehicle that includes the engine mounted therein. Therefore, the heat from the cylinders is prone to stay in this space. Accordingly, when the hydraulic control valve is located in this space, the influence of heat from the cylinders upon the hydraulic control valve is very large. As a result of the influence of heat upon the hydraulic control valve, the accuracy of the valve timing is reduced.  
         [0008]     Furthermore, the space between the V-banked cylinders of the V-type internal combustion engine is exposed to the outside of the internal combustion engine, depending on the vehicle. This space is an area where the functional beauty of the cylinders is expressed. Accordingly, when the hydraulic control valve is located in this area, there is a possibility that the external appearance of the internal combustion engine may be impaired.  
         [0009]     It is accordingly an object of the present invention to provide a layout structure of a hydraulic control valve for a valve train in an internal combustion engine that can reduce the influence of heat from the cylinders and can avoid the impairment of the external appearance of the internal combustion engine.  
         [0010]     According to a first aspect of the present invention, an internal combustion engine has a valve operation characteristics changing mechanism for changing the operation characteristics of at least one of an intake valve and an exhaust valve, and a hydraulic control valve for a valve train for controlling the pressure of a hydraulic fluid to be supplied to the valve operation characteristics changing mechanism. A layout structure of the hydraulic control valve for the valve train in the internal combustion engine is such that the hydraulic control valve is located on a first side surface of a crankcase for supporting and covering a crankshaft at a position below a cylinder.  
         [0011]     According to a second aspect of the present invention, the hydraulic control valve is located on the first side surface of the crankcase opposite to a second side surface of the crankcase along which an exhaust pipe is provided.  
         [0012]     According to a third aspect of the present invention, a rotating shaft cooperating with the crankshaft is provided in the crankcase, and the hydraulic control valve is located on the first side surface of the crankcase at a position lying on the axis of the rotating shaft.  
         [0013]     According to a fourth aspect of the present invention, oil discharged from the hydraulic control valve is returned through an axial hole formed in the rotating shaft into the crankcase.  
         [0014]     According to a fifth aspect of the present invention, the internal combustion engine is an OHV type internal combustion engine such that the valve operation characteristics changing mechanism is provided on a second side surface of the crankcase.  
         [0015]     According to the first aspect of the present invention, the hydraulic control valve is located on the first side surface of the crankcase for supporting and covering the crankshaft at a position below the cylinder. With this arrangement, the hydraulic control valve is located apart from the cylinder, so that the hydraulic control valve is hardly influenced by the heat from the cylinder. As a result, the hydraulic control valve can be operated properly to thereby maintain accurate valve timing. Furthermore, the hydraulic control valve is located on the first side surface of the crankcase. Therefore, the hydraulic control valve can be covered by any type of cover to avoid the impairment of the external appearance of the internal combustion engine.  
         [0016]     According to the second aspect of the present invention, the hydraulic control valve is located on the first side surface of the crankcase opposite to the second side surface of the crankcase along which the exhaust pipe is provided. With this arrangement, the hydraulic control valve is hardly influenced by the heat from the exhaust pipe.  
         [0017]     According to the third aspect of the present invention, the hydraulic control valve is located on the first side surface of the crankcase at a position lying on the axis of the rotating shaft. With this arrangement, the rotating shaft can be lubricated by utilizing the oil supplied to the hydraulic control valve.  
         [0018]     According to the fourth aspect of the present invention, the oil discharged from the hydraulic control valve is returned through the axial hole of the rotating shaft into the crankcase. With this arrangement, it is not necessary to provide any dedicated oil passage and oil recovery passage for lubrication of the rotating shaft, thereby simplifying a lubricating oil path.  
         [0019]     According to the fifth aspect of the present invention, the internal combustion engine is an OHV type internal combustion engine such that the valve operation characteristics changing mechanism is provided on the second side surface of the crankcase. With this arrangement, an oil passage from the hydraulic control valve located on the first side surface of the crankcase to the valve operation characteristics changing mechanism can be shortened.  
         [0020]     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  
       [0021]     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:  
         [0022]      FIG. 1  is a general left side view of a motorcycle according to a preferred embodiment of the present invention;  
         [0023]      FIG. 2  is a partially sectional, left side view of a V-type internal combustion engine;  
         [0024]      FIG. 3  is a top plan view of a rear cylinder head portion;  
         [0025]      FIG. 4  is a cross section taken along the line IV-IV in  FIG. 2 ;  
         [0026]      FIG. 5  is a cross section taken along the line V-V in  FIG. 2 ;  
         [0027]      FIG. 6  is a cross section taken along the line VI-VI in  FIG. 2 ;  
         [0028]      FIG. 7  is a top plan view of an inside portion of the V bank;  
         [0029]      FIG. 8  is a sectional view for illustrating the motion of a rear connecting rod;  
         [0030]      FIG. 9  is a perspective view of a front connecting rod;  
         [0031]      FIG. 10  is a perspective view of a rear connecting rod;  
         [0032]      FIG. 11  is an exploded side view of the rear connecting rod;  
         [0033]      FIG. 12  is an exploded sectional view of the rear connecting rod; and  
         [0034]      FIG. 13  is a perspective view of a rear connecting rod in an internal combustion engine according to another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     An embodiment of the present invention will now be described with reference to the accompanying drawings. The same reference numerals will be used to identify the same or similar elements throughout the several views. It should be noted that each of the drawings should be viewed in the direction of orientation of the reference numerals.  
         [0036]     A preferred embodiment of the present invention will now be described with reference to FIGS.  1  to  12 .  FIG. 1  is a general side view of a motorcycle  1  including an internal combustion engine  20  according to a preferred embodiment of the present invention. The motorcycle  1  has a head pipe  2  and a main frame  3  extending rearward from the head pipe  2 . A pair of right and left down tubes  4  extend downward from the head pipe  2  below the main frame  3  and further extend rearward from a lower bent portion of the down tubes  4 . A radiator  38  is supported between the right and left down tubes  4 .  
         [0037]     The main frame  3  is bifurcated at an intermediate portion thereof to form a pair of right and left rear portions. A pair of right and left pivot frames  5  are connected between the rear ends of the right and left rear portions of the main frame  3  and the rear ends of the right and left down tubes  4 , respectively.  
         [0038]     A handle  6  is pivotably supported at a lower end portion thereof to the head pipe  2 . The handle  6  extends upward from the head pipe  2  and further extends in the laterally opposite directions of the vehicle. A front fork  7  that is adapted to be steered by the handle  6  extends downward from the head pipe  2 . A front wheel  8  is supported on the lower ends of the front fork  7 . A rear fork  10  is pivotably supported at a front end thereof through a pivot shaft  9  to the pivot frames  5  so as to be vertically swingable. A rear wheel  11  is supported on the rear ends of the rear fork  10 . A pair of right and left brackets  12  are provided on the right and left rear portions of the main frame  3 , respectively. Right and left rear cushions  13  are interposed between the right and left brackets  12  and the right and left portions of the rear fork  10 , respectively.  
         [0039]     The internal combustion engine  20  is mounted in a space surrounded by the main frame  3 , the down tubes  4 , and the pivot frames  5  so as to be supported at front and rear portions thereof The internal combustion engine  20  has a pair of right and left crankcases  22  joined together. A pair of right and left transmission cases  27  that are joined together are formed on the rear side of the right and left crankcases  22  in an integral manner. A counter shaft  67  projects leftward from the left transmission case  27 . A drive gear  14  is fixedly mounted on the counter shaft  27 . A driven gear  15  is fixedly mounted on the axle of the rear wheel  11 . A drive shaft  16  connects the drive gear  14  and the driven gear  15 , thereby transmitting engine power to the rear wheel  11 .  
         [0040]     A fuel tank  17  is supported on the main frame  3  so as to straddle the main frame  3 . A rear fender  18  extends rearward from the upper portions of the pivot frames  5  so as to cover the upper side of the rear wheel  11 . A tandem seat  19  is provided between the fuel tank  17  and the rear fender  18  and on the upper side of the rear fender  18 .  
         [0041]     The internal combustion engine  20  is a V-type two-cylinder water-cooled four-stroke cycle internal combustion engine such that a crankshaft  21  extends horizontally in the lateral direction of the vehicle and a pair of front and rear bank cylinders  23   f  and  23   r  are inclined in the longitudinal direction of the vehicle so as to form a V-shape as viewed in side elevation (see  FIG. 2 ).  
         [0042]     The front and rear bank cylinders  23   f  and  23   r  extend obliquely upward from the right and left crankcases  22 . A pair of front and rear cylinder heads  24   f  and  24   r  are joined to the front and rear bank cylinders  23   f  and  23   r,  respectively. A pair of front and rear rocker arm holders  25   f  and  25   r  are joined to the front and rear cylinder heads  24   f  and  24   r,  respectively. A pair of front and rear cylinder head covers  26   f  and  26   r  are joined to the front and rear rocker arm holders  25   f  and  25   r,  respectively. The front and rear bank cylinders  23   f  and  23   r  are formed with water jackets  23   fw  and  23   rw  around the respective cylinder bores.  
         [0043]     Referring to  FIG. 4 , the right and left transmission cases  27  are formed on the rear side of the right and left crankcases  22  in an integral manner with right and left partition walls  22   a  formed therebetween, respectively. A clutch gear case  28  is connected to the right side of the right crankcase  22  and the right transmission case  27 . The clutch gear case  28  has an opening at a rear portion corresponding to the right transmission case  27 , and this opening is closed by a clutch cover  29 .  
         [0044]     Accordingly, a crank chamber  22 C is defined by the right and left crankcases  22  and the respective partition walls  22   a.  A transmission chamber  27 C is defined by the right and left transmission cases  27  and the respective partition walls  22   a.  Furthermore, a clutch chamber  28 C is formed so as to be partitioned from the crank chamber  22 C and the transmission chamber  27 C by the right crankcase  22  and the right transmission case  27  and to be covered by the clutch gear case  28  and the clutch cover  29 . A chain cover  30  covers the right side of a side wall of a front portion of the clutch gear case  28 . Furthermore, a side cover  36  (see  FIG. 1 ) and an ACG cover  31  are provided to cover the left side of the left crankcase  22 . A clutch drive mechanism cover  33  covers the left side of the left transmission case  27 .  
         [0045]     In the V-type internal combustion engine  20 , the cylinder axis of the front bank cylinder  23   f  and the cylinder axis of the rear bank cylinder  23   r  lie on the same plane perpendicular to the axis of the crankshaft  21 . That is, the front and rear bank cylinders  23   f  and  23   r  are not offset from each other in the lateral direction of the vehicle.  
         [0046]     A piston  40   f  having a piston pin  40   fp  is provided to reciprocate in the front bank cylinder  23   f,  and a front connecting rod  41  is connected at its small end to the piston pin  40   fp  of the piston  40   f.  Similarly, a piston  40   r  having a piston pin  40   rp  is provided to reciprocate in the rear bank cylinder  23   r,  and a rear connecting rod  45  is connected at its small end to the piston pin  40   rp  of the piston  40   r.  The big ends of the front and rear connecting rods  41  and  45  are connected to a common crankpin  21   p  connecting a pair of crank webs  21   w  of the crankshaft  21 .  
         [0047]     As shown in  FIG. 9 , the front connecting rod  41  has an integral small end  41   s  and a big end  41   b.  The big end  41   b  has a split structure such that it can be split into two parts in a plane perpendicular to the rod axis of the front connecting rod  41 . That is, the big end  41   b  of the front connecting rod  41  is composed of a semicircular rod end portion  41   bh  and a semicircular connecting rod cap  42  joined to the rod end portion  41   bh  by means of two connecting rod bolts  43 , wherein the crankpin  21   p  is interposed between the rod end portion  41   bh  and the connecting rod cap  42 .  
         [0048]     On the other hand, the rear connecting rod  45  is different in shape from the front connecting rod  41  as shown in FIGS.  10  to  12 . While the rear connecting rod  45  has an integral small end  45   s  like the front connecting rod  41 , the big end of the rear connecting rod  45  is bifurcated into a pair of big ends  45   b  opposed to each other. Furthermore, each big end  45   b  has a split structure such that it can be split into two parts in a plane perpendicular to the rod axis of the rear connecting rod  45 .  
         [0049]     That is, the pair of big ends  45   b  of the rear connecting rod  45  are composed of a pair of semicircular rod end portions  45   bh  opposed to each other and a pair of semicircular portions  46   bc  joined to the rod end portions  45   bh  by means of four connecting rod bolts  47 . The pair of semicircular portions  46   bc  project radially outward (in a centrifugal direction) at central portions thereof The projecting portions are connected by a connecting portion  46   c,  thus forming an integral connecting rod cap  46 . That is, the connecting rod cap  46  is composed of the pair of semicircular portions  46   bc  and the connecting portion  46   c  connecting these semicircular portions  46   bc.    
         [0050]     The pair of big ends  45   b  of the rear connecting rod  45  are supported on the crankpin  21   p  in such a manner that the crankpin  21   p  is interposed between the rod end portions  45   bh  and the connecting rod cap  46 . The big end  41   b  of the front connecting rod  41  is interposed between the big ends  45   b  of the rear connecting rod  45  in the axial direction of the crankpin  21   p  (see  FIG. 4 ). In the rear connecting rod  45 , the connecting portion  46   c  of the connecting rod cap  46  that constitutes the pair of big ends  45   b  projects downward to a large extent (see  FIGS. 2 and 8 ). Thus, the big end  41   b  of the front connecting rod  41  and the pair of big ends  45   b  of the rear connecting rod  45  are supported to the common crankpin  21   p  in such a manner that the big end  41   b  is interposed between the pair of big ends  45   b.  With this arrangement, coupling vibrations can be prevented.  
         [0051]     In the mounted condition of the pair of big ends  45   b  of the rear connecting rod  45  and the big end  41   b  of the front connecting rod  41  to the crankpin  21   p,  the connecting portion  46   c  of the connecting rod cap  46  of the rear connecting rod  45  straddles the connecting rod cap  42  of the front connecting rod  41  so as to avoid the interference with each other. Furthermore, the cylinder axes of the front and rear bank cylinders  23   f  and  23   r  lie on the same plane perpendicular to the axis of the crankshaft  21 . Specifically, the front and rear bank cylinders  23   f  and  23   r  are not offset from each other in the lateral direction of the vehicle. Accordingly, the width of the internal combustion engine  20  in the lateral direction of the vehicle can be reduced.  
         [0052]     The crankshaft  21  adapted to be rotationally driven through the connection of the front and rear connecting rods  41  and  45  to the common crankpin  21   p  is supported through a pair of bearings  35  to the right and left crankcases  22 . The crankshaft  21  projects rightward into the clutch chamber  28 C formed on the right side of the right crankcase  22 , and a sleeve  50  is engaged with this projecting portion of the crankshaft  21 . Furthermore, a cap  51  with a drive sprocket  52  at an outer end thereof is engaged with the outer end of the projecting portion of the crankshaft  21 , and a bolt  53  is tightened to fix the sleeve  50  and the cap  51  together to the projecting portion of the crankshaft  21 .  
         [0053]     The outer end of the cap  51  with the drive sprocket  52  projects rightward from an opening of the clutch gear case  28 . A primary gear  55  is mounted on the sleeve  50 , and the rotation of the crankshaft  21  is transmitted through a damper mechanism  56  to the primary gear  55 .  
         [0054]     The crankshaft  21  projects leftward from the left crankcase  22 . An AC generator  57  is provided on the projecting portion of the crankshaft  21 . A driven gear  58  in a starting mechanism is mounted through a one-way clutch  59  on the projecting portion of the crankshaft  21 .  
         [0055]     A pair of front and rear balancer shafts  60  and  61  for canceling primary vibrations are provided at symmetrical positions with respect to the crankshaft  21 . The balancer shafts  60  and  61  laterally extend in parallel to the crankshaft  21  in such a manner that the axes of the balancer shafts  60  and  61  lie on the same horizontal plane as that containing the axis of the crankshaft  21 , thus constituting a biaxial balancer mechanism.  
         [0056]     The front balancer shaft  60  is rotatably supported through bearings  62  to the right and left crankcases  22  (see  FIG. 5 ). The rear balancer shaft  61  is also rotatably supported through bearings  63  to the right and left crankcases  22  (see  FIG. 4 ). The balancer shafts  60  and  61  respectively have balancer weights  60   w  and  61   w,  which are adapted to rotationally pass through the space between the pair of crank webs  21   w  of the crankshaft  21 .  
         [0057]     The balancer shafts  60  and  61  project through the respective right bearings  62  and  63  fitted to the right crankcase  22  into the clutch chamber  28 C. A pair of front and rear balancer gears  64  and  65  are mounted on the right projecting end portions of the balancer shafts  60  and  61 , respectively. The front and rear balancer gears  64  and  65  mesh with the primary gear  55  mounted on the crankshaft  21 . Each of the balancer gears  64  and  65  has the same diameter as that of the primary gear  55 , so that the balancer shafts  60  and  61  are rotated at the same speed as that of the crankshaft  21  in concert with the rotation of the crankshaft  21  in a direction opposite to the rotational direction of the crankshaft  21 .  
         [0058]     A main shaft  66  and a counter shaft  67  are supported to the right and left transmission cases  27  on the rear side of the rear balancer shaft  61 . The counter shaft  67  is located on the rear upper side of the main shaft  66 , and is supported at its opposite ends through bearings to the right and left transmission cases  27 . A gear shift mechanism  75  is located above the main shaft  66 . The left end of the counter shaft  67  projects from the transmission chamber  27 C. The drive gear  14  is mounted on the left projecting end of the counter shaft  67 .  
         [0059]     The right end of the main shaft  66  projects from the transmission chamber  27 C, and a multiplate clutch  77  is provided in the right projecting end of the main shaft  66 . The multiplate clutch  77  has a clutch housing  77   h  rotatably supported to the main shaft  66  and a main gear  78  integrated with the clutch housing  77   h.  The main gear  78  meshes with the balancer gear  65  mounted on the rear balancer shaft  61 . An oil pump gear  79  is also integrated with the main gear  78 .  
         [0060]     A clutch operating shaft  81  extends through an axially extending center hole of the main shaft  66 . The right end of the clutch operating shaft  81  is connected to a pressure plate  77   p  of the multiplate clutch  77 . The left end of the clutch operating shaft  81  is inserted in a clutch drive mechanism  82  provided in the clutch drive mechanism cover  33 . Accordingly, when the crankshaft  21  is rotated during the operation of the internal combustion engine  20 , the primary gear  55  is rotated through the damper mechanism  56 . The rotation of the primary gear  55  is transmitted through the front and rear balancer gears  64  and  65  meshing with the primary gear  55  to the front and rear balancer shafts  60  and  61 .  
         [0061]     The rotation of the rear balancer gear  65  is transmitted to the main gear  78  meshing with the rear balancer gear  65 , thereby rotating the clutch housing  77   h  of the multiplate clutch  77 . When the multiplate clutch  77  is engaged, the rotation of the clutch housing  77   h  is transmitted to the main shaft  66 .  
         [0062]     The meshing of gears on a main gear train and on a counter gear train is selectively effected by the gear shift mechanism  75 . The rotation of the main shaft  66  is transmitted through the selected gear pair to the counter shaft  67 . The rotation of the counter shaft  67  is transmitted through the drive gear  14  mounted on the counter shaft  67  and through the drive shaft  16  to the driven gear  15 , thereby rotationally driving the rear wheel  11 . An oil pump  85  is located on the front lower side of the main shaft  66  in the transmission chamber  27 C (see  FIG. 2 ).  
         [0063]     As shown in  FIG. 6 , the oil pump  85  is composed of a crank chamber scavenging pump  86 , a feed pump  87 , and a clutch chamber scavenging pump  88  arranged in this order from the left side. The inner rotors in these pumps  86 ,  87 , and  88  are mounted on a common rotating shaft  89  extending in the lateral direction of the vehicle.  
         [0064]     The clutch chamber scavenging pump  88  has a pump case  88   c.  A bearing portion  88   cb  is formed at the center of the right side wall of the pump case  88   c.  The bearing portion  88   cb  of the pump case  88   c  projects rightward to be inserted into a circular hole of the right transmission case  27 . The right end of the rotating shaft  89  is inserted through the bearing portion  88   cb  to project into the clutch chamber  28 C. A pump drive gear  90  is fixed to the right projecting end of the rotating shaft  89  by a bolt  91 . The pump drive gear  90  meshes with the oil pump gear  79 . Accordingly, the rotation of the crankshaft  21  is transmitted through the primary gear  55  and the balancer gear  65  to the main gear  78 , thereby rotating the oil pump gear  79  integrated with the main gear  78 . As a result, the pump drive gear  90  meshing with the oil pump gear  79  is rotated to thereby drive the oil pump  85 .  
         [0065]     The bearing portion  88   cb  inserted from the transmission chamber  27 C into the circular hole of the right transmission case  27  is sealed by an oil seal  92  interposed between the outer circumference of the bearing portion  88   cb  and the inner circumference of the circular hole, thus making the seal between the transmission chamber  27 C and the clutch chamber  28 C (see  FIG. 6 ).  
         [0066]     A starter motor  95  is located behind a joining portion between the crankcases  22  and the rear bank cylinder  23   r.  A drive shaft  96  of the starter motor  95  projects leftward. As shown in  FIG. 2 , the drive shaft  96  is formed with a drive gear  96   g,  which meshes with a large-diameter gear  97   b  on a reduction gear shaft  97 . A small-diameter gear  97   s  on the reduction gear shaft  97  meshes with a large-diameter intermediate gear  98   b  on an intermediate shaft  98 . A small-diameter intermediate gear  98   s  on the intermediate shaft  98  meshes with the driven gear  58  on the crankshaft  21 .  
         [0067]     Accordingly, the rotation of the drive shaft  96  of the starter motor  95  is transmitted through the drive gear  96   g,  the large-diameter gear  97   b,  the small-diameter gear  97   s,  the large-diameter intermediate gear  98   b,  the small-diameter intermediate gear  98   s  to the driven gear  58  in this order, thereby rotationally driving the crankshaft  21  through the one-way clutch  59  and thus starting the internal combustion engine  20 .  
         [0068]     An intake and exhaust system will now be described. The internal combustion engine  20  is of an OHV (overhead valve) type such that intake valves  101   f  and  101   r  and exhaust valves  102   f  and  102   r  are provided in the cylinder heads  24   f  and  24   r,  respectively. A valve operating cam mechanism  160  is provided on the right crankcase  22 . The intake valves  101   f  and  101   r  are provided so that they can open and close openings of intake ports  103   f  and  103   r  formed in the cylinder heads  24   f  and  24   r  to combustion chambers  105   f  and  105   r,  respectively. Similarly, the exhaust valves  102   f  and  102   r  are provided so that they can open and close openings of exhaust ports  104   f  and  104   r  formed in the cylinder heads  24   f  and  24   r  to the combustion chambers  105   f  and  105   r,  respectively.  
         [0069]     As shown in  FIG. 4 , two spark plugs  117   r  are mounted in the cylinder head  24   r  so as to be exposed to the combustion chamber  105   r.  Although not shown, two spark plugs are similarly mounted in the cylinder head  24   f  so as to be exposed to the combustion chamber  105   f.    
         [0070]     In the front cylinder head  24   f,  the intake port  103   f  extends rearward from the combustion chamber  105   f  to an intake pipe  110   f  provided in the space between the front and rear cylinder heads  24   f  and  24   r  (see  FIG. 2 ). The exhaust port  104   f  extends frontward from the combustion chamber  105   f  to an exhaust pipe  115  (see  FIG. 1 ).  
         [0071]     On the other hand, in the rear cylinder head  24   r,  the intake port  103   r  extends frontward from the combustion chamber  105   r  to an intake pipe  110   r  provided in the space between the front and rear cylinder heads  24   f  and  24   r  (see  FIG. 2 ). The exhaust port  104   r  extends rearward from the combustion chamber  105   r  to an exhaust pipe  116  (see  FIG. 1 ).  
         [0072]     As shown in  FIG. 7 , the intake pipe  110   f  extending rearward from the front cylinder head  24   f  and the intake pipe  110   r  extending frontward from the rear cylinder head  24   r  are joined together at the center of the space between the front and rear cylinder heads  24   f  and  24   r,  and further joined to an upstream intake pipe  110   c  extending rightward. Thus, the intake pipes  110   f  and  110   r  and the upstream intake pipe  110   c  are joined together to constitute an integral intake pipe  110 . In other words, the intake pipes  110   f  and  110   r  are branch pipes of the intake pipe  110 .  
         [0073]     A throttle body  120  is connected to the upstream end of the upstream intake pipe  110   c.  An air cleaner  130  is located upstream of the throttle body  120  on the right side thereof The air cleaner  130  is provided so as to cover the space between the front and rear banks on the right side thereof along the right side surfaces of the front and rear cylinders  23   f  and  23   r,  the cylinder heads  24   f  and  24   r,  and the cylinder head covers  26   f  and  26   r.    
         [0074]     A pair of fuel injection valves  111   f  and  111   r  are respectively mounted on the intake pipes  110   f  and  110   r  so as to project obliquely upward. The upper ends of the fuel injection valves  111   f  and  111   r  are engaged in a common socket  112 . A connection pipe  113  projects leftward from the socket  112 . A fuel pipe (not shown) is connected to the connection pipe  113 .  
         [0075]     As shown in  FIG. 1 , the exhaust pipe  115  extends frontward from the front cylinder head  24   f  and then bends downward to extend rearward along the right side surface of the right crankcase  22 . The downstream end of the exhaust pipe  115  is connected to a muffler  115   m  extending along the right side of the rear wheel  11  in the longitudinal direction of the vehicle. Furthermore, the exhaust pipe  116  extends rearward from the rear cylinder head  24   r  and then bends downward to extend rearward along the right side surface of the right transmission case  27 . The downstream end of the exhaust pipe  116  is connected to a muffler  116   m  extending along the right side of the rear wheel  11  above the muffler  115   m  in parallel relationship thereto.  
         [0076]     The valve train mechanism will now be described. Referring to  FIG. 3 , there is shown a top plan view of a rear cylinder head portion  24   ra.  The rocker arm holder  25   r  is mounted on the upper surface of the cylinder head  24   r  including the above-mentioned intake and exhaust system. The rocker arm holder  25   r  rotatably holds an intake rocker arm shaft  150   r  and an exhaust rocker arm shaft  155   r  inside the cylinder head cover  26   r.    
         [0077]     Referring to  FIGS. 3 and 4 , a rocker arm  151   r  for operating the intake valve  101   r  and a rocker arm  152   r  adapted to be operated by an intake push rod  153   r  are mounted on the intake rocker arm shaft  150   r  at the inner position in the rear bank. Similarly, a rocker arm  156   r  for operating the exhaust valve  102   r  and a rocker arm  157   r  adapted to be operated by an exhaust push rod  158   r  are mounted on the exhaust rocker arm shaft  155   r  at the outer position in the rear bank.  
         [0078]     The upper ends of the intake push rod  153   r  and the exhaust push rod  158   r  are in abutment against the rocker arms  152   r  and  157   r  in the cylinder head cover  26   r,  respectively. The lower ends of the intake and exhaust push rods  153   r  and  158   r  are in cooperative engagement with the valve operating cam mechanism  160  provided in the clutch gear case  28  on the right side of the right crankcase  22 . The elongated stem portions of the intake and exhaust push rods  153   r  and  158   r  except their upper and lower ends extend along the right side surfaces of the cylinder  23   r  and the cylinder head  24   r.    
         [0079]     Thus, the valve train in this preferred embodiment is of an OHV type such that the valve operating cam mechanism  160  is provided on the right side of the right crankcase  22  and the intake valve  101   r  and the exhaust valve  102   r  provided in the cylinder head  24   r  are operated by the valve operating cam mechanism  160  through the push rods  153   r  and  158   r,  respectively. Although not shown, a similar structure is provided for the front cylinder head  24   f.    
         [0080]     The valve operating cam mechanism  160  in the clutch gear case  28  includes a camshaft  161   f  (not shown) for the front bank and a camshaft  161   r  for the rear bank located above the right end portion of the crankshaft  21 . The camshafts  161   f  and  161   r  are rotatably supported to the right crankcase  22  and the clutch gear case  28  so as to extend in parallel to the crankshaft  21 .  
         [0081]     As shown in  FIG. 4 , the right end of the camshaft  161   r  projects through the side wall of the clutch gear case  28 . A driven sprocket  162   r  is mounted on the projecting right end of the camshaft  161   r.  As previously mentioned, the drive sprocket  52  is fixed to the right end of the cap  51  mounted on the right end of the crankshaft  21  projecting rightward from the opening of the clutch gear case  28 . Although not shown, a similar driven sprocket  162   f  is mounted on the camshaft  161   f  for the front bank. An endless chain  163  is wrapped among the drive sprocket  52 , the front driven sprocket  162   f,  and the rear driven sprocket  162   r.  The driven sprocket  162   r  has twice the number of teeth of the drive sprocket  52 . Accordingly, the rotation of the crankshaft  21  is transmitted from the drive sprocket  52  through the chain  163  to the driven sprocket  162   r,  so that the camshaft  161   r  is rotated at a speed half that of the crankshaft  21 .  
         [0082]     As shown in  FIG. 4 , the camshaft  161   r  is formed with a pair of high-speed and low-speed cams for the intake valve  101   r  and a pair of high-speed and low-speed cams for the exhaust valve  102   r.  The rollers of four rocker arms  164   r  (two high-speed rocker arms  164   r  and two low-speed rocker arms  164   r ) are in abutment against the four cams of the camshaft  161   r,  respectively. The four rocker arms  164   r  are pivotably supported to a common rocker arm shaft  165   r.  Two elastic biasing devices  166   r  are provided to bias the rollers of the two high-speed rocker arms  164   r  against the two high-speed cams.  
         [0083]     The rocker arm shaft  165   r  is formed with a hydraulic passage for supplying oil pressure to a link mechanism formed between the high-speed rocker arm  164   r  and the low-speed rocker arm  164   r  for each pair. The operation ends of the two low-speed rocker arms  164   r  receive the lower ends of the intake push rod  153   r  and the exhaust push rod  158   r,  respectively.  
         [0084]     Accordingly, when no oil pressure is applied to the link mechanism, the high-speed rocker arm  164   r  and the low-speed rocker arm  164   r  of each pair are not engaged with each other to pivotally move independently of each other, so that the intake push rod  153   r  and the exhaust push rod  158   r  received at their lower ends by the operation ends of the low-speed rocker arms  164   r  are lifted and lowered according to the low-speed cams. The intake valve  101   r  and the exhaust valve  102   r  are operated at the low-speed valve timing.  
         [0085]     In contrast, when oil pressure is applied to the link mechanism, the high-speed rocker arm  164   r  and the low-speed rocker arm  164   r  of each pair are engaged with each other to pivotally move together, so that the intake push rod  153   r  and the exhaust push rod  158   r  are lifted and lowered according to the high-speed cams. The intake valve  101   r  and the exhaust valve  102   r  are operated at the high-speed valve timing.  
         [0086]     Although not shown, the camshaft  161   f  for the front bank has a structure similar to that of the camshaft  161   r  for the rear bank, and is provided with similar members including four rocker arms, a rocker arm shaft, elastic biasing means, a link mechanism, an intake push rod, and an exhaust push rod. When no oil pressure is applied to the link mechanism, the intake valve  101   f  and the exhaust valve  102   f  are operated at the low-speed valve timing, whereas when an oil pressure is applied to the link mechanism, the intake valve  101   f  and the exhaust valve  102   f  are operated at the high-speed valve timing.  
         [0087]     A hydraulic control valve  168  for controlling an oil pressure to the link mechanism is mounted on the side wall of the left crankcase  22  at a position corresponding to a left extension of the front balancer shaft  60  (see  FIG. 5 ). The hydraulic control valve  168  mounted on the left crankcase  22  is covered by the side cover  36  (see  FIG. 1 ).  
         [0088]     A lubricating system will now be described. Referring to  FIG. 8 , the right and left crank webs  21   w  of the crankshaft  21  and the connecting portion  46   c  of the two big ends  45   b  of the rear connecting rod  45  interposed between the crank webs  21   w  are turned in the crank chamber  22 C. The bottom of the crankcase  22  is formed with an arcuate oil reservoir  170  along the locus described by the outermost point of the connecting portion  46   c  during its turn. An oil discharge opening  171  is formed adjacent to the oil reservoir  170  on the rear side thereof. The oil discharge opening  171  is in communication with a small oil chamber  172  formed adjacent to the oil reservoir  170  on the rear side thereof.  
         [0089]     The oil discharge opening  171  is provided with a discharge guide rib  171   a  formed by projecting a part of the bottom wall of the oil reservoir  170  into the small oil chamber  172  so as to be inclined slightly upward. An oil outlet A 1   a  is formed at a rear portion of the small oil chamber  172  along the bottom surface thereof The discharge guide rib  171   a  projecting into the small oil chamber  172  also functions to prevent the return of oil from the small oil chamber  172  to the oil reservoir  170 . The oil outlet A 1   a  is an opening of an oil passage A 1  that is in communication with an oil pipe A 2  located in the transmission chamber  27 C. The oil pipe A 2  is in communication with a suction oil passage A 3  through which the oil is sucked into the crank chamber scavenging pump  86  in the oil pump  85 .  
         [0090]     Referring to  FIG. 8 , the motion of the connecting portion  46   c  of the big ends  45   b  of the rear connecting rod  45  is shown. Near the bottom dead center of the piston  40   r  in the rear bank cylinder  23   r,  the connecting portion  46   c  starts to be immersed into the oil gathered in the oil reservoir  170 . When the connecting portion  46   c  is moved toward the oil discharge opening  171  in concert with the upward movement of the piston  40   r,  a flat rear surface  46   cb  of the connecting portion  46   c  functions to push the oil gathered in the oil reservoir  170  as shown by two-dot chain lines  46   c ′ and  46   c ″, thus effectively discharging the oil from the oil discharge opening  171  (the condition of the oil surface at the time corresponding to the position  46   c ′ is shown by a two-dot chain line). Accordingly, the oil in the oil reservoir  170  can be efficiently discharged into the small oil chamber  172 .  
         [0091]     The connecting portion  46   c  thus functioning to push out the oil in the oil reservoir  170  is preferably formed on the big ends  45   b  of the rear connecting rod  45  rather than on the big end  41   b  of the front connecting rod  41 , because the connecting rod  46   c  of the big ends  45   b  can push the oil toward the oil discharge opening  171  more effectively, thereby discharging the oil from the oil reservoir  170  more efficiently. The rear surface  46   cb  of the connecting portion  46   c  may be formed as a concave surface, so as to increase the discharge amount of oil per stroke of the connecting portion  46   c.    
         [0092]     The big ends  45   b  of the rear connecting rod  45  have a split structure such that each big end  45   b  can be split into two parts in a plane perpendicular to the rod axis of the rear connecting rod  45 . Accordingly, the pair of semicircular portions  46   bc  of the connecting rod cap  46  can be integrally formed, so that the connecting rod cap  46  can be easily formed and mounted. The pair of big ends  45   b  are formed by joining the pair of semicircular portions  46   bc  of the connecting rod cap  46  to the pair of semicircular rod end portions  45   bh  and tightening the connecting rod bolts  47  at four positions on the front and rear sides of the connecting portion  46   c.  Accordingly, the shape of the connecting portion  46   c  is not limited by the connecting rod bolts  47 , and it can be provided easily and flexibly so as to efficiently push out the oil.  
         [0093]     As mentioned above, the oil gathered in the oil reservoir  170  of the crank chamber  22 C is pushed by the connecting portion  46   c  of the rear connecting rod  45  and is thereby discharged from the oil discharge opening  171  into the small oil chamber  172 . The oil thus discharged into the small oil chamber  172  is sucked from the oil outlet A 1   a  through the oil passage A 1 , the oil pipe A 2 , and the oil passage A 3  into the crank chamber scavenging pump  86  in the oil pump  85  (see  FIG. 2 ).  
         [0094]     A discharge pipe  175  extends upward from the crank chamber scavenging pump  86 . The oil discharged from the crank chamber scavenging pump  86  splashes from the discharge pipe  175  toward the main gear train and the counter gear train that mesh with each other in the transmission chamber  27 C.  
         [0095]     As shown in  FIGS. 2 and 5 , the bottom wall of the clutch gear case  28  is formed with an oil passage B 1  extending rearward from a position below the primary gear  55  to a position behind the rear balancer gear  65 . The oil passage B 1  has a communication hole B 1   a  opening to the position below the primary gear  55  and communicating with the clutch chamber  28 C. The rear end of the oil passage B 1  is in communication with a strainer (not shown) provided in the clutch chamber  28 C. A suction oil passage is connected through the strainer to the clutch chamber scavenging pump  88  in the oil pump  85  (see  FIG. 6 ). Accordingly, the oil in the clutch chamber  28 C is sucked through the oil passage B 1  and the strainer into the clutch chamber scavenging pump  88  by the operation thereof.  
         [0096]     As shown in  FIGS. 2 and 6 , the oil gathered at the bottom of the transmission chamber  27 C is sucked through a strainer  185  and a suction pipe C 1  into the feed pump  87  in the oil pump  85  by the operation thereof. The oil from the feed pump  87  is discharged into a discharge oil passage C 2 . The discharge oil passage C 2  is connected to an oil inlet of an oil filter  186  mounted on the transmission case  27  at its rear lower portion. The oil from the oil filter  186  is fed into a main gallery M.  
         [0097]     The main gallery M includes a main oil passage M 1  extending frontward from the oil filter  186 , a main oil passage M 2  expanded into the small oil chamber  172  adjacent to the crank chamber  22 C on the rear side thereof and extending laterally so as to communicate with the main oil passage M 1 , and a pair of main oil passages M 3  bent from the right and left ends of the main passage M 2  and extending frontward along the side walls of the right and left crankcases  22 . A pair of oil passages D 1  are branched from the main oil passages M 3  at positions below right and left journal portions of the crankshaft  21  (see  FIGS. 2 and 5 ).  
         [0098]     Referring to  FIG. 2 , the front end of the left main oil passage M 3  that extends frontward in the left crankcase  22  so as to pass through the branch point with respect to the left oil passage D 1  is in communication with an oil passage E 1  that extends upward and is bent frontward to reach the hydraulic control valve  168 . An oil passage F 1  is formed in the left crankcase  22  so as to extend rightward from the hydraulic control valve  168  toward the front balancer shaft  60  (see  FIG. 5 ). An oil passage E 2  is also formed in the left crankcase  22  so as to extend upward from the hydraulic control valve  168  (see  FIG. 2 ). The hydraulic control valve  168  can switch the communication of the oil passage E 1  with either the oil passage E 2  which extends upwardly or the oil passage F 1  which leads to the front balancer shaft  60 .  
         [0099]     The oil passage E 2  is bent rightward at an upper end thereof to communicate with an oil passage E 3  that extends laterally to the right crankcase  22 . As shown in  FIG. 2 , the oil passage E 3  is in communication with an oil passage E 4  extending rearward horizontally in the side wall of the right crankcase  22 . The oil passage E 4  is in communication with an oil passage E 5  that extends obliquely upward to the rear side and communicates with the hydraulic passage formed in the rear rocker arm shaft  165   r  of the valve operating cam mechanism  160 . Furthermore, an oil passage E 6  branches from the oil passage E 5  so as to extend obliquely upward to the front side and communicates with the hydraulic passage formed in the front rocker arm shaft  165   f.    
         [0100]     The hydraulic passages in the front and rear rocker arm shafts  165   f  and  165   r  are in communication with the link mechanisms for the rocker arms  164   f  and  164   r,  respectively, to apply oil pressure to the link mechanisms. Accordingly, when the hydraulic control valve  168  communicates between the oil passage E 1  and the oil passage E 2 , the oil pressure is applied to the link mechanisms, so that the intake valves  101   f  and  101   r  and the exhaust valves  102   f  and  102   r  are operated at the high-speed valve timing. Conversely, when the hydraulic control valve  168  communicates between the oil passage E 1  and the oil passage F 1 , no oil pressure is applied to the link mechanisms, so that the intake valves  101   f  and  101   r  and the exhaust valves  102   f  and  102   r  are operated at the low-speed valve timing.  
         [0101]     As shown in  FIG. 5 , the oil flowing into the oil passage F 1  is supplied to the left bearing  62  for the front balancer shaft  60 , and is discharged through the center hole  60   a  of the front balancer shaft  60  into the crank chamber  22 C at the right end opening of the center hole  60   a.  Thus, the hydraulic control valve  168  is located on the left side surface of the left crankcase  22  at a left position of the front balancer shaft  60 , which is at the same level as that of the crankshaft  21 . That is, the hydraulic control valve  168  is located apart from the cylinders  23   f  and  23   r,  so that the hydraulic control valve  168  is not directly influenced by the heat from the cylinders  23   f  and  23   r.  Accordingly, the hydraulic control valve  168  can be operated properly to thereby switch the valve timing accurately.  
         [0102]     Furthermore, the exhaust pipe  115  connected to the exhaust port  104   f  pointed obliquely rightward in the front cylinder head  24   f  extends downward and further extends rearward along the right side surface of the right crankcase  22 . To the contrary, the hydraulic control valve  168  is provided on the left side surface of the left crankcase  22 , so that the hydraulic control valve  168  is hardly influenced by the heat from the exhaust pipe  115 .  
         [0103]     The front balancer shaft  60  can be lubricated by utilizing the oil supplied to the hydraulic control valve  168 . The oil flowing from the hydraulic control valve  168  can be discharged into the crank chamber  22 C through the center hole  60   a  of the front balancer shaft  60  as an oil passage. Accordingly, it is not necessary to provide any dedicated oil passage and oil recovery passage for lubrication of the balancer shaft, thereby simplifying a lubricating oil path.  
         [0104]     Furthermore, the valve operating cam mechanism  160  using an OHV-type valve train is provided on the right crankcase  22 . Accordingly, the total length of the oil passages E 2 , E 3 , E 4 , and E 5  from the hydraulic control valve  168  provided on the left crankcase  22  to the link mechanism for applying an oil pressure to the valve operating cam mechanism  160  can be reduced, thereby further simplifying the lubricating oil path.  
         [0105]     Furthermore, the hydraulic control valve  168  is mounted on the side wall of the left crankcase  22  and is covered by the side cover  36 . Accordingly, the hydraulic control valve  168  is not exposed to the outside of the internal combustion engine, so that the external appearance of the internal combustion engine can be kept in a good condition.  
         [0106]     While the single rear connecting rod  45  connected at its small end to the piston pin  40   rp  of the piston  40   r  reciprocating in the rear bank cylinder  23   r  has a bifurcated big end in this preferred embodiment, two rear connecting rods may be provided instead.  
         [0107]      FIG. 13  shows such another preferred embodiment, wherein a pair of rear connecting rods  215  and  216  symmetrical in shape respectively have a pair of small ends  215   s  and  216   s  supported to the common piston pin  40   rp  and a pair of big ends  215   b  and  216   b  each splittable in a plane perpendicular to the rod axis of each rear connecting rod. The pair of big ends  215   b  and  216   b  of the rear connecting rods  215  and  216  are respectively composed of a pair of semicircular rod end portions  215   bh  and  216   bh  opposed to each other and a pair of semicircular portions  217   bc  joined to the rod end portions  215   bh  and  216   bh  by means of four connecting rod bolts  218 . The pair of semicircular portions  217   bc  project radially outward (in a centrifugal direction) at their central portions, and these projecting portions are connected by a connecting portion  217   c,  thus forming an integral connecting rod cap  217 . The connecting rod cap  217  itself has substantially the same shape as that of the connecting rod cap  46  shown in  FIG. 10 , so the connecting rod cap  46  may be utilized.  
         [0108]     Although not shown, a front connecting rod used in this preferred embodiment is the same as the front connecting rod  41 . A connecting rod cap of a big end thereof is secured to a rod end portion by means of two connecting rod bolts as in  FIG. 9 .  
         [0109]     The rear connecting rods  215  and  216  having the connecting portion  217   c  perform the same motion as that of the rear connecting rod  45  shown in  FIG. 8 . That is, the rear surface of the connecting portion  217   c  functions to effectively push out the oil gathered in the oil reservoir  170  toward the oil discharge opening  171 . Accordingly, the oil in the oil reservoir  170  can be efficiently discharged from the oil discharge opening  171  into the small oil chamber  172 , and the lubricating oil can be smoothly circulated regardless of an operating condition.  
         [0110]     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.