Abstract:
A lubrication structure for an internal combustion engine including an oil pump provided at a first end of an oil pump shaft arranged parallel to a crankshaft. Oil passageways are formed from a side surface of a pump chamber of the oil pump to another end of the oil pump shaft in parallel with the oil pump shaft. An oil filter is provided facing towards the oil passageways. A communicating passageway extending vertically from the oil passageways to the oil filter is also provided. This arrangement provides a lubrication structure for an internal combustion engine with a high degree of freedom with respect to oil filter arrangement with a simple oil passageway that is easy to manufacture, requires relatively small space for arrangement, and thus makes it possible to reduce the size of the internal combustion engine.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a lubrication structure for an internal combustion engine, and more particularly to a lubrication structure permitting optimum arrangement of lubrication system components and size reduction of an internal combustion engine for a vehicle.  
           [0003]    2. Background Art  
           [0004]    Generally, a lubrication system for an internal combustion engine guides oil from a pump chamber arranged near an inner side of the internal combustion engine to the oil filter arranged near the outer side of the internal combustion engine. An example of a similar arrangement is shown in Japanese Patent Laid-open Publication No. Hei. 9-88538.  
           [0005]    [0005]FIG. 8 is a cross-sectional view of a section of an internal combustion engine taken in the vicinity of an oil pump shaft as found in the conventional art. A clutch cover  02 ′ is joined to a right surface of a right crankcase  01 ′. An oil pump shaft  04 ′ is pivotally supported between the right crankcase  01 ′ and the clutch cover  02 ′ oriented in a lateral direction parallel to a crankshaft  03 ′.  
           [0006]    An oil pump  05 ′ is provided at an end of this oil pump shaft  04 ′ at the right crankcase  01 ′ side, and is connected to an oil filter  06 ′ provided in a manner projecting from the clutch cover  02 ′ by an oil passageway  07 ′.  
           [0007]    The main oil passageway  07 ′ is formed having an oil passageway  7   c ′ passing an oil passageway way  07   b ′ curving upwards from an oil passageway  07   a ′ extending to the left from the side surface of the pump chamber of the oil pump at the right crank case  01 ′ side. The main oil passageway  07 ′ curves vertically and extends in a lateral direction, the oil passageway  07   c ′ communicating with an oil passageway  07   d ′ on the clutch cover  02 ′ side. At the end of the oil passageway  07   d ′ one branch passageway  07   e ′ branches upwards to an oil filter  6 ′, and another branch passageway  07   f ′ is connected to an oil relief valve arranged between an oil pump shaft  04 ′ and the oil passageway  07   d′.    
           [0008]    Since the oil filter  06 ′ is provided on the clutch cover  02 ′ at the outside of the internal combustion engine, the degree of freedom of arrangement of the oil filter  06 ′ is low, and oil passageways  07   a ′,  07   b ′,  07   c ′ guiding oil from the oil pump  05 ′ provided at the right crank case  01 ′ inside the internal combustion engine to the oil filter  06 ′ detour in an open-ended square shape, making the passageway complicated and difficult to manufacture.  
           [0009]    As the oil passageways  07   a ′,  07   b ′,  07   c ′,  07   d ′ form an open-ended square while detouring around the oil relief valve  08 ′, a wide space is required to arrange the oil distribution route. Furthermore, this causes the oil filter  06 ′ to be arranged in a further extended condition. These conditions make it further difficult to reduce the size of the internal combustion engine.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention overcomes the shortcomings associated with the related art and achieves other advantages not realized by the related art.  
           [0011]    An aspect of the present invention is to provide a lubrication structure for an internal combustion engine that permits a reduction in size of the internal combustion engine.  
           [0012]    An additional aspect of the present invention is to provide a high degree of freedom with respect to oil filter arrangement that benefits from a simple oil passageway that is easy to process and requires relatively small spaces for arrangement.  
           [0013]    These and other aspects of the present invention are accomplished by a lubrication structure for an internal combustion engine comprising an oil pump provided at a first end of an oil pump shaft, the oil pump shaft arranged in parallel with a crankshaft; a plurality of oil passageways extending from a side surface of a pump chamber of the oil pump to a second end of the oil pump shaft, the oil passageways arranged in parallel with the oil pump shaft; an oil filter positioned facing towards the oil passageways; and an oil communicating passageway extending vertically from a first oil passageway to the oil filter.  
           [0014]    These and other aspects of the present invention are accomplished by a lubrication structure for an internal combustion engine, the internal combustion engine including a crankcase divided into a left crankcase and a right crankcase along a longitudinal centerline, and having a crankshaft arranged transversely therein, the lubrication structure comprising an oil pump having a pump chamber provided at a first end of an oil pump shaft, the oil pump shaft arranged in parallel with the crankshaft; a plurality of oil passageways extending from a side surface of the pump chamber of the oil pump to a second end of the oil pump shaft, the oil passageways arranged in parallel with the oil pump shaft; an oil filter positioned facing towards the oil passageways; an oil introduction passageway extending vertically from a first oil passageway to the oil filter; and an oil discharge port forming an oil supply passageway from an oil pan positioned in a lower portion of the crankcase.  
           [0015]    With this simple oil passageway arrangement having an oil passageway formed from a side surface of the pump chamber of the oil pump to another end of the oil pump shaft parallel to the oil pump shaft, and a communicating passageway extending to the oil filter towards the oil passageway vertically without bypassing the oil passageway, the lubrication system is easy to manufacture, the space for arrangement can be reduced, and the protrusion of the oil filter from the internal combustion engine can be suppressed. Therefore the size of the internal combustion engine can be reduced.  
           [0016]    As it is capable of extending the communicating passageway vertically from any position of the oil passageway to connect to the oil filter, the degree of freedom of oil filter arrangement is increased.  
           [0017]    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  
       [0018]    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 intended to limit the present invention to the embodiments shown, and wherein:  
         [0019]    [0019]FIG. 1 is a side view of a vehicle having an internal combustion engine according to an embodiment of the present invention;  
         [0020]    [0020]FIG. 2 is a cross-sectional side view of an internal combustion engine and belt-type automatic transmission according to an embodiment of the present invention;  
         [0021]    [0021]FIG. 3 is a cross-sectional view taken along line III-III of FIG. 1;  
         [0022]    [0022]FIG. 4 is a right side view of an internal combustion engine having a case cover removed according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 5 is a cross-sectional view taken along line V-V and line V 1 -V 1  of FIG. 4;  
         [0024]    [0024]FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. 4;  
         [0025]    [0025]FIG. 7 is a frontal view of lower parts of the internal combustion engine; and  
         [0026]    [0026]FIG. 8 is a cross-sectional view of a section of an internal combustion engine taken in the vicinity of an oil pump shaft as found in the conventional art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The present invention will now be described with reference to FIG. 1 to FIG. 7. FIG. 1 is a side view of a vehicle having an internal combustion engine according to an embodiment of the present invention. FIG. 2 is a cross-sectional side view of an internal combustion engine and belt-type automatic transmission according to an embodiment of the present invention. FIG. 3 is a cross-sectional view taken along line III-III of FIG. 1. FIG. 4 is a right side view of an internal combustion engine having a case cover removed according to an embodiment of the present invention. FIG. 5 is a cross-sectional view taken along line V-V and line V 1 -V 1  of FIG. 4. FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. 4. FIG. 7 is a frontal view of lower parts of the internal combustion engine.  
         [0028]    An internal combustion engine in a preferred embodiment is applied to a scooter-type motorcycle. A side view of a scooter-type motorcycle is shown in FIG. 1.  
         [0029]    In the vehicle frame of the scooter-type motorcycle  1 , a pair of left and right main pipes  3 ,  3  extend from an upper part of head pipe  2  diagonally downwards and rearward, and in a straight line when viewed from the side. A pair of support pipes  4 ,  4  extending substantially horizontal from the head pipe  2  are connected to the main pipes  3 ,  3 , and support front parts of the main pipes  3 ,  3 .  
         [0030]    From the middle part of the support pipe  4 ,  4 , a pair of down pipes  5 ,  5  extend laterally and steeply diagonally downwards to form front side vertical sections  5   a ,  5   a . The down pipes  5 ,  5  then each bend rearward at a lower end to form central horizontal sections  5   b ,  5   b , and bend upwards at the rear end to form rear side sloping sections  5   c ,  5   c.    
         [0031]    The rear ends of the main pipes  3 , 3  are connected to lower parts of the rear side sloping sections  5   c ,  5   c , and a reinforcing pipe  6  is interposed between the main pipe  3  and the down pipe  5 , forming a substantially triangular shape in side view.  
         [0032]    Seat rails  7 ,  7 , the front ends of which are fixed to sections slightly rearward from the center of the main pipes  3 ,  3 , extend slightly diagonally upwards, nearly horizontal, and rearwards to the rear part of the vehicle body. Upper ends of the rear side sloping sections  5   c ,  5   c  of the down pipes  5 , 5  are connected to the center parts of the seat rails  7 ,  7 , and support the seat rails  7 , 7  from underneath.  
         [0033]    A head pipe  2  pivotally supports a steering shaft  11 . Handlebars  12 , 12  are formed spreading out laterally above the head pipe  2 . A front fork  13  extends underneath, and a front wheel is pivotally supported at the lower end thereof.  
         [0034]    At the upper part and lower part of the rear side sloping section  5   c  of the down pipe  5 , support brackets  5   d  and  5   e  are provided protruding rearwards. The internal combustion engine  20  is suspended from a respective pair of support brackets  5   d  and  5   e  on both the left side and right side of the vehicle.  
         [0035]    The internal combustion engine  20  is a four cycle, two-cylinder internal combustion engine having a crankcase  21  arranged rearward of the rear side sloping section  5   c  of the down pipe  5 , and a cylinder block  22 , cylinder head  23 , and cylinder head cover  24  sequentially stacked together with a crankcase  21 . The cylinder block  22 , cylinder head  23 , and cylinder head cover  24  are provided in a position forward of the rear side sloping section  5   c  and tilting significantly to the front.  
         [0036]    The cylinder block  22 , cylinder head  23 , and cylinder head cover  24  are arranged in the triangular shape formed by the rear side sloping section  5   c , the rear part of the main pipe  3 , and the front part of the seat rail  7  on both the left side and right side when viewed from a side of the vehicle. Each of the mounting brackets  21   a  provided at the upper part, and the mounting bracket  21   b  provided at the front part of the crankcase  21  in a protruding state are supported by the support brackets  5   d ,  5   e  via support shafts  8  and  9 . Accordingly, the internal combustion engine  20  is suspended from the vehicle frame.  
         [0037]    The front part of a belt-type automatic transmission  50  is mounted on the crankcase  21  of the internal combustion engine  20 . The transmission  50  extends rearwards and pivotally supports a rear wheel  15  at a rear portion thereof.  
         [0038]    Intake pipes  31 ,  31 , extending upwards from each cylinder of the cylinder head  23  at a forward-bending position of the internal combustion engine, are curved rearwards and are connected to carburetors  32 ,  32  provided parallel to each other on the crankcase  21 . Further, the carburetors  32 ,  32  are connected to an air cleaner  33  provided rear of the carburetors.  
         [0039]    The air cleaner  33  is provided between the seat rails  7 ,  7  on left side and right side. A helmet storage box  34  is suspended and supported by the seat rails  7 ,  7  in a position above the air cleaner  33 .  
         [0040]    A driver&#39;s seat is provided that is capable of opening and closing covers over the internal combustion engine  20  and carburetor  32 . A pillion seat  36  is also provided that is capable of opening and closing covers over the helmet storage box  34  and the rear part.  
         [0041]    The exhaust pipes  37 ,  37 , each extend downwards from the cylinder head  23 , and extend in front of the crankcase  21  along its right side to the rear. The exhaust pipes combine into a single pipe at a position to the rear of the crankcase. The single exhaust pipe then extends diagonally upwards on the right side of the vehicle body and is connected to the muffler  38  supported at the right side of the rear wheel  15 .  
         [0042]    A fuel tank  39  surrounded by four pipes, namely two left and right main pipes  3 , 3  above and two left and right down pipes  5 , 5  in front and below, is suspended and supported in front of the internal combustion engine  20 .  
         [0043]    The configuration of a scooter-type motorcycle  1  according to a preferred embodiment is as described hereinabove. The structure of the internal combustion engine  20  and the belt-type automatic transmission mounted on a crankcase  21  will now be described hereinafter.  
         [0044]    The divided crankcase  21  includes a combination of left and right crankcases  21 L,  21 R, respectively. As shown in FIG. 3, an outer rotor  29   a  of the AC generator  29  is attached to the right end of the crankshaft  25  oriented horizontally in a lateral direction in the crankcase  21 . A case cover  28  is fixed to the right crankcase  21 R and covers the side. An inner stator  29   b  of the AC generator  29  is supported by the case cover  28 .  
         [0045]    Pistons  26 ,  26  each reciprocate inside the  2  cylinder sleeves  30  of the cylinder block  22 . The pistons  26  are connected to the crank pins of the crankshaft  25  via connecting rods  27  , 27 . Both of the crank pins have a phase angle of 360 degrees.  
         [0046]    A valve gear mechanism  40  is provided on the cylinder head  23 . A timing chain  44  is suspended between cam chain sprockets  42 ,  42  fitted to the right ends of the upper and lower cam shafts  41 ,  41  oriented horizontally in a lateral direction. A drive chain sprocket  43  is fitted to the base part of crankshaft  25  protruding from the right crankcase  21 R, to transmit the driving power.  
         [0047]    The timing chain  44  passes through the cam chain chambers  22   a ,  23   a  arranged on the right side of the cylinder block  22  and the cylinder head  23 . Cam shafts  41 , 41  drive the intake valve  45  and exhaust valve  46  at the prescribed timing.  
         [0048]    The belt-type automatic transmission  50  is mounted on the crankcase  21  of the internal combustion engine  20 .  
         [0049]    A case cover  28 , blocking off the right opening of the right crankcase  21 R and covering the AC generator  29 , has an opening coaxial with the crankshaft  25 . A rotating shaft  55  is provided to the right in the opening via a bearing  54  so as to protrude from the right side of the crankcase  21 R. The base end section  51   a  of the right side transmission case  51  of the belt-type automatic transmission case  50  is also fitted into this protruding section.  
         [0050]    The right side transmission case  51  has a connection section  51   b  which runs from the base end section  51   a  circling around along the rear surface of the right crankcase  21 R and to an inner side.  
         [0051]    A mounting boss section  51   c  protrudes rearwards from respective upper and lower positions on the rear surface of the connection section  51   b . A left side joining surface of the front end of the right fork member  53  is joined to the right side joining surface of the mounting boss section  51   c , and the two upper and lower positions are screwed together with the bolt  56  to integrally connect the right fork member  53  to the right side transmission case  51 , and extend rearward.  
         [0052]    The left end of the crankshaft  25  protrudes leftward piercing through the left crankcase  21 L. A drive pulley  60  with a centrifugal transmission mechanism is provided at the protruding section.  
         [0053]    An annular support member  57  is fixed around the crankshaft  25  on the outer surface pierced by the crankshaft  25  of the left side crankcase  21  L. A base end section  52   a  of the left side transmission case  52  is swingably supported by the annular support member  57  via a bearing  58 .  
         [0054]    The left side transmission case  52  has a connection section  52   b , which runs from the base end section  52   a  circling around along the rear surface of the left crankcase  21 L and into the inner side. A left fork section  52   c  extends further rearward.  
         [0055]    The connection section  51   b  of the right side transmission case  51  circling clockwise along the rear surface of the crankcase  21 , and the connection section  52   b  of the left side transmission case  52  circling counterclockwise along the rear surface of the crankcase  21 , are joined to each other on their connection joining surfaces. The left and right side transmission cases  51 , 52  are screwed to each other with bolts  59  (four bolts  59  in a preferred embodiment), and the left fork member  52   c  and right fork member  53  face each other and are integrally connected.  
         [0056]    The connected right side transmission case  51  is swingably supported by the bearing  54  on the crankshaft  25 , and the left side transmission case  52  is swingably supported by a bearing  58  on the crankshaft  25 . Therefore the left fork member  52   c  and right fork member  53  facing each other are integrally supported swingably up and down on the crankshaft  25 .  
         [0057]    A rear section of the left fork member  52   c  of the left side transmission case  52  includes a transmission chamber. A driven shaft  64  is rotatably supported, and the driven pulley  62  is pivotally supported on the driven shaft  64  via a central clutch  63 . A V-belt  61  is suspended between the driven pulley  62  and the drive pulley  60  and constitutes a belt-type automatic transmission mechanism.  
         [0058]    In the transmission chamber at the rear section of the left fork member  52   c , a reduction gear includes a set of transmission gears  65   a , in which drive force is transmitted from a driven shaft  64  to an axle  66  via an intermediate shaft  65 .  
         [0059]    The axle  66  is rotatably suspended between the left fork section  52   c  and the right fork member  53 . The rear wheel  15  is supported by the axle  66  between the left fork section  52   c  and the right fork member  53 .  
         [0060]    As a result, the left and right side transmission cases  51  and  52  supporting the belt-type automatic transmission pivotally support the crankshaft  25  with the left fork section  52   c , right fork member  53  and the rear wheel  15  swingable/pivotable up and down.  
         [0061]    The center of the swingable rear wheel  15  is set coaxially with the crankshaft  25 . Therefore, the length between the internal combustion engine  20  and the rear wheel  15  in a longitudinal direction can be shortened, and the overall length of the vehicle in the longitudinal direction can be shortened.  
         [0062]    A rear cushion  67  is interposed between the rear end of the left side transmission case  52  and the rear end of the seat rail  7 .  
         [0063]    A belt cover  68  blocks off the left side opening of the left side transmission case  52  which stores the belt-type automatic transmission  50 , and covers the belt-type automatic transmission from its left side.  
         [0064]    The internal combustion engine  20  has a pair of balancer shafts  71  and  72  above and below the crankshaft  25 . The balancer driven gears  74  and  75 , each being respectively attached to each of the balancer shafts  71  and  72 , mesh with the drive gear  73  fitted to the crankshaft  26  along the inner side surface of the bearing section of the right crankcase  21 R at the same time. The rotation of the crankshaft  25  makes the two balancer shafts  71 ,  72  rotate in opposite directions.  
         [0065]    Above the upper side balancer shaft  71 , a mounting bracket  21   a  is mounted on the crankcase  21  so as to protrude therefrom. The starter motor  78  is arranged in front of the mounting bracket  21   a . The starter motor  78 , mounting bracket  21   a  and the upper side balancer shaft  71 , are arranged centrally (refer to FIG. 4).  
         [0066]    A pump drive shaft  80  is horizontally suspended diagonally forward under the lower side balancer shaft  72 . A chain  82  is suspended between the drive sprocket  72 , which is fitted to the right end of the lower side balancer shaft  72  protruding from the right crankcase  21 R, and the driven sprocket  80   a , which is fitted to the right end of the pump drive shaft  80  (refer to the FIG. 4 and FIG. 5).  
         [0067]    Accordingly, the rotation of the crankshaft  25  causes rotation of the pump drive shaft  80  via the balancer shaft  72 .  
         [0068]    The two ends of the pump drive shaft  80  are arranged at an outer position of the two crank weight sections of the crankshaft  25 . The oil pump  85  is provided between the right crankcase  21 R of the pump drive shaft  80  and the driven sprocket  81  at the right end. The water pump  86  is provided at the part of the pump drive shaft  80  protruding from the left crankcase  21 L.  
         [0069]    As shown in FIG. 5, an intake connection pipe  87  protrudes forward from the left space in the center of the impeller  86   a  of the water pump  86 . An exhaust connection pipe  88  protrudes upwards from the side of the impeller  86   a . The exhaust connection pipe  88  and the connection pipe  91 , which is provided at the cooling water inlet provided on the left side surface of the cylinder block  22  in a protruding state, are connected together with a hose  89  (refer to the FIG. 2).  
         [0070]    An oil intake passageway toward the oil pan at the lower part of the crankcase  21  is formed at the intake port  85   a , which is provided on the left surface of the pump chamber of the oil pump. An oil strainer  93  is interposed at the mid-point of the oil intake passageway  92  (refer to the FIG. 5).  
         [0071]    As shown in FIG. 6, the oil passageway  94  is formed parallel to the pump shaft  80  to the left of the exhaust port  85   b  which is provided separately from the intake port  85   a  on the left side surface of the pump chamber of the oil pump  85  and from the right crankcase  21 R to the left crankcase  21 L in front of the water pump  86 .  
         [0072]    An oil introduction path  95 , which bends forward vertically at the left end of the oil passageway  94 , is a connection passageway for introducing oil into the oil filter  96  provided protruding forward from the lower part of the front wall of the left crankcase  21 L.  
         [0073]    The oil discharge path  97  extends rearward from the center of the jointing surface on the rear surface of the oil filter  96 . The oil introduction path  95  is arranged above the oil discharge path  97  in the center (refer to FIG. 4 and FIG. 7).  
         [0074]    The oil discharge path  97  extends straight rearward to the vicinity of the lower side of the crankshaft, and is connected to the oil supply path  98  which supplies oil to the various bearing sections, etc. of the internal combustion engine  20 .  
         [0075]    A relief communication passageway  99  protrudes rearward from the lower side of the oil discharge path  97  in the center of the rear surface of the oil filter  96  to the lower side of the oil passageway  94  (refer to FIG. 4 and FIG. 7), and bends rightward horizontally at the rear end. A relief passageway  100  parallel to the oil passageway  94  is formed therein.  
         [0076]    The oil passageway  94  and the relief passageway  100  are formed at the upper and lower position of the oil discharge path  97  parallel to each other, and can be arranged while reducing the width in the longitudinal direction, which contributes to the reduction of the length of the internal combustion engine  20  in the longitudinal direction.  
         [0077]    [0077]FIG. 5 shows a cross-sectional view along line V-V and line V 1 -V 1  of FIG. 4. The right end of the relief passageway  100 , which is arranged at the left crankcase while referring to the cross-section cut along the line V 1 -V 1 , is opened in the vicinity of the joining surface against the right crankcase  21 R. A relief valve  101  is arranged fitted to the opening from its right side, and is attached coaxially to the relief passageway  100 .  
         [0078]    A protuberance  102 , which holds the relief valve  101  from its right side, protrudes from the right crankcase  21 R.  
         [0079]    The configuration of the oil lubrication structure is as described above. When the oil pump  85  is driven by the rotation of the pump shaft  80 , the oil pump  85  takes in the oil accumulated in the oil pan  103  at the lower part of the crankcase  21  via the oil strainer  93 , and discharges the oil to the discharge port  85   b.    
         [0080]    The oil discharged to the discharge port  85   b  flows leftwards in the oil passageway  94 , and enters the oil filter  96  via the oil introduction path  95 .  
         [0081]    The oil enters the outer side of the filter element of the oil filter  96 , gets filtered and enters the inner side. Oil in the inner side of the filter  96  then flows out rearward via the oil discharge path  97 , and is then supplied to the various bearing sections, etc. of the internal combustion engine  20  via the oil supply path  98 .  
         [0082]    The outer side of the filter element of the oil filter  96  is connected to the relief passageway  100  via the relief communicating passageway  99 . When the fluid pressure at the outer side of the filter element becomes higher than a prescribed value due to the filter element being blocked/clogged, the relief valve  101  provided at the relief passageway  100  opens and discharges the oil to the crankcase  21 .  
         [0083]    The oil passageway  94  is arranged from the discharge port  85   b  on the side surface of the oil chamber of the oil pump  85  at the right end of the oil pump shaft  80  toward the left end parallel to the oil pump shaft  80 . The oil introduction path  95  extends vertically without bypassing the oil passageway  94 , and forms a simple oil passageway reaching the oil filter  96 . The crankcase  21  is divided into a left part and a right part. Therefore, the oil passageway can be manufactured easily.  
         [0084]    Also, the relief passageway  100  arranged at the left crankcase  21 L extends rightward horizontally, and has an opening for attaching the relief valve  101  in the vicinity of the dividing surface. Therefore this relief passageway  100  can also be manufactured easily.  
         [0085]    The oil passageway  94  is in the vicinity of the pump shaft  80 . Therefore the oil introduction path  95  can be shortened, and the size of the space required for passageway arrangement can be reduced.  
         [0086]    As a result, the position for mounting the oil filter  96  can be arranged closer to the pump shaft  80  and can be prevented from protruding forward, and therefore the size of the internal combustion engine can be reduced.  
         [0087]    The relief passageway  100  is oriented horizontally in the lateral direction in the vicinity of the lower side of the oil passageway. Since the relief valve  101  is attached coaxially, it does not require as much space for the arrangement of the relief passageway  100  and the relief valve  101 . This further contributes to the reduction of the size of the internal combustion engine.  
         [0088]    The oil introduction path  95  can be extended from any part of the oil passageway  94  oriented horizontally in the lateral direction and can be connected to the oil filter  96 . Therefore, the degree of freedom provided for arranging the oil filter  96  in the lateral direction can be increased.  
         [0089]    In this embodiment, the exhaust pipe  37  is arranged at the right side of the vehicle body, and the oil filter is arranged offset to the left side of the center of the engine, which is the opposite side of the oil pump  85 .  
         [0090]    The exhaust pipes  37 , extending from the lower surface of the cylinder head  23  in a steeply forward bending position, are arranged parallel to each other between the crank weights on the left and right side in the crankshaft  25 , and extend substantially downward (refer to FIG. 7).  
         [0091]    As shown in FIG. 7 and FIG. 4, the left side exhaust pipe  37 L, which extends from the lower surface of the left side of the cylinder head  23 , detours around the oil filter  96  arranged in a protruding condition offset to the left side of the center of the engine. The left side exhaust pipe  37 L bends rightward at the same height as its front part, extends rightward along the front surface of the right crankcase  21 R, then circles around rearward along the side surface at the lower part of the case cover  28 . The left side exhaust pipe  37 L then extends rearward along the outer side surface of the case cover  28 .  
         [0092]    The right side exhaust pipe  37 R, which extends from the lower surface on the right side of the cylinder head  23 , extends downward in front of the left side exhaust pipe  37 L extending rightward on the front surface of the right crankcase  21 R. The right side exhaust pipe  37 R circles around into an indentation  21 Ra at the lower right corner of the right crankcase  21 R, then extends rearward.  
         [0093]    As shown in FIG. 7 and FIG. 4, the lowest point of the right side exhaust pipe  37 R, which is arranged at a lower position of the left side exhaust pipe  37 L, is at substantially the same height as the lowest point of the crankcase  21 . Therefore, the exhaust pipe  38  arrangement does not affect the minimum ground clearance of the vehicle body.  
         [0094]    The oil filter  96  protruding forward from the lower part on the front surface of the crankcase  21  has a high degree of freedom with respect to arrangement in the lateral direction. Therefore, the two exhaust pipes  37 R,  37 L, which extend from the cylinder head  23  on the front surface of the crankcase  21 , and the oil filter  96 , can be arranged closer to each other without overlapping and protruding. Therefore, the size of the vehicle can be reduced.  
         [0095]    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.