Patent Publication Number: US-6209505-B1

Title: Four-cycle engine for vehicle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a four-cycle engine for a vehicle, in which a cylinder bore is disposed in such a manner that the axial line of the cylinder bore extends substantially in the horizontal direction. Furthermore, a cam shaft is disposed between an intake valve and an exhaust valve which have operational axial lines crossing each other, forming an approximately V-shape. 
     2. Background Art 
     In a prior art four-cycle engine including an intake valve and an exhaust valve having operational axial lines crossing each other in an approximately V-shape, a diameter of the intake valve is larger than that of the exhaust valve. Accordingly, in order to make the squish area of the combustion chamber on the intake side and that on the exhaust side equal to each other, as disclosed in Japanese Patent Publication No. Hei 1-14406, an angle formed between the operational axial line of an exhaust valve and the axial line of the cylinder bore is set to be larger than an angle formed between the operational axial line of the intake valve and the axial line of the cylinder bore. 
     In the four-cycle engine for a vehicle in which the axial line of the cylinder bore extends substantially in the horizontal direction and the intake valve and the exhaust valve are disposed on the upper and lower sides of the cylinder head respectively, since a distance between the axial line of the cylinder bore and the outer end portion of the exhaust valve becomes large, both the cylinder head and the head cover protrude substantially downwardly. This results in the height of the engine from the road surface on which the vehicle is grounded not being able to be reduced. In particular, for a four-cycle engine mounted on a motorcycle of a type in which the axial line of a cylinder bore extends in the width direction of the motorcycle, since the position of the outer end portion of the exhaust valve exerts a large effect on the bank angle of the motorcycle, the mounting position of the engine to the body frame must be made relatively high. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a four-cycle engine for a vehicle, which is capable of making the position of the outer end portion of an exhaust valve as close to the axial line of a cylinder bore as possible, thereby making the mounting position of the engine as low as possible. 
     To achieve the above object, according to a first aspect of the present invention, there is provided a four-cycle engine for a vehicle, in which a cylinder head is connected to a cylinder block including a cylinder bore having an axial line extending substantially in the horizontal direction in such a manner that a combustion chamber is formed between the cylinder head and a piston slidably fitted in the cylinder bore. An intake valve for taking air in the combustion chamber is openably/closably supported at an upper portion of the cylinder head. An exhaust valve for discharging exhaust gas from the combustion chamber is openably/closably supported at a lower portion of the cylinder head. Operational axial lines of the intake valve and the exhaust valve cross each other including the axial line of the cylinder bore, forming an approximately V-shape on a projection plane perpendicular to the axial line of a crank shaft. Furthermore, a cam shaft common to the intake valve and the exhaust valve has an axial line parallel to the crank shaft and is disposed between the intake valve and the exhaust valve. The four-cycle engine includes a cam shaft disposed above the axial line of the cylinder bore, and on the projection plane, an angle formed between the axial line of the cylinder bore and the operational axial line of the intake valve is set to be larger than an angle formed between the axial line of the cylinder bore and the operational axial line of the exhaust valve. 
     With this configuration, since the cam shaft is disposed above the axial line of the cylinder bore, and an angle formed between the operational axial line of the exhaust valve and the axial line of the cylinder bore is set to be smaller than the angle formed between the operational axial line of the intake valve and the axial line of the cylinder bore, it is possible to make the outer end portion of the exhaust valve as close to the axial line of the cylinder bore as possible, and hence to make the mounting position of the engine as low as possible while ensuring sufficient ground clearance. This makes it possible to lower the center of gravity of the vehicle and hence to improve the steering of the vehicle. 
     According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, on the projection plane, a crossing point of the operational axial lines of the intake valve and the exhaust valve is disposed under the axial line of the cylinder bore. With this configuration, it is possible to easily ensure a squish area on the side of the intake valve having a diameter larger than that of the exhaust valve, and hence to make the squish area on the intake valve side nearly equal to that on the exhaust side. 
     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 a motorcycle to which the present invention is applied; 
     FIG. 2 is an enlarged sectional view taken on line  2 — 2  of FIG. 1; 
     FIG. 3 is a sectional view taken on line  3 — 3  of FIG. 2; 
     FIG. 4 is an enlarged view of an essential portion shown in FIG. 2; 
     FIG. 5 is a sectional view taken on line  5 — 5  of FIG. 3; 
     FIG. 6 is an enlarged sectional view taken on line  6 — 6  of FIG. 3; 
     FIG. 7 is an enlarged sectional view of a cylinder block taken on line  7 — 7  of FIG. 3; 
     FIG. 8 is a view of FIG. 2 seen along an arrow  8 ; 
     FIG. 9 is an enlarged sectional view of a cylinder head taken on line  9 — 9  of FIG. 3; 
     FIG. 10 is a sectional view taken on line  10 — 10  of FIG. 9; 
     FIG. 11 is an enlarged sectional view taken on line  11 — 11  of FIG. 2; 
     FIG. 12 is a sectional view taken on line  12 — 12  of FIG. 11; and 
     FIG. 13 is an enlarged view of an essential portion of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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: 
     Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings. Referring first to FIG. 1, there is shown a low floor type motorcycle on which a power unit P composed of a horizontally-opposed type two-cylinder/four-cycle engine E and a transmission M is mounted. 
     A body frame F includes a pair of right and left main frames  11  extending downwardly, rearwardly from the front side of the motorcycle in the running direction of the motorcycle. A steering handle  13  is steerably supported by a head pipe  12  commonly provided at the front ends of a pair of the main frames  11 . A front wheel WF is suspended from a front fork  14  turnable together with the steering handle  13 . 
     The rear ends of both of the main frames  11  are connected to a transmission case  15  of the transmission M of the power unit P. The transmission case  15  constitutes a part of the body frame F. 
     Front ends of a pair of right and left rear frames  16  extending to the rear side of the motorcycle are connected to the transmission case  15 . The front end of a rear fork  17  is vertically swingably connected to the transmission case  15 . A rear wheel W, is rotatably supported by the rear end of the rear fork  17 . A cushion unit  18  is provided between a rear portion of the rear fork  17  and each of the rear frames  16 . A drive shaft (not shown) for transmitting the output of the transmission M to the rear wheel WR is contained in the rear fork  17 . The drive shaft is connected to an output member of the transmission M via a universal joint. 
     The entire body frame F is covered with a body cover  20  made from a synthetic resin. A tunnel portion  20   a  for covering the power unit P is formed at an intermediate portion of the body cover  20  in the longitudinal direction. A seat  21  on which a driver is to be seated is provided on the body cover  20  at a position behind the tunnel portion  20   a , and steps  20   b  on which the driver is to rest her/his foot are provided on the right and left sides of the tunnel portion  20   a . A fuel tank  22  is mounted on the rear frames  16  in such a manner as to be located under the seat  21  and to be covered by the body cover  20 . An air cleaner  23  is mounted on the main frames  11  in such a manner as to be located above the engine E. A pair of right and left radiators  24  are mounted on the main frames  11  between the air cleaner  23  and the engine E. The air cleaner  23  and the radiators  24  are also covered by the body cover  20 , and openings (not shown) through which running wind is introduced to the air cleaner  23  and the radiators  24  are formed in the front end portion of the body cover  20 . 
     Referring to FIGS. 2 and 3, a main body of the engine E includes a first cylinder block  25   1  disposed on the right side when the motorcycle is directed forwardly in the running direction; a second cylinder block  25   2  disposed on the left side when the motorcycle is directed forwardly in the running direction; a crank case  26  commonly connected to the cylinder blocks  25   1  and  25   2 ; a first cylinder head  27   1  connected to the first cylinder block  25   1  on a side opposite to the crank case  26 ; and a second cylinder head  27   2  connected to the second cylinder block  25   2  on the opposed side to the crank case  26 . 
     The crank case  26  is formed by connecting a front case half  26   a  on the front side in the longitudinal direction of the motorcycle to a rear case half  26   b  on the rear side in the longitudinal direction of the motorcycle. A crank shaft  28  having a substantially horizontal axial line in the longitudinal direction of the motorcycle is rotatably supported by the crank case  26 . First and second cylinder bores  29   1  and  29   2 , which extend in opposite directions from each other at 180° with respect to the axial line of the crank shaft  28 , are provided in the first and second cylinder blocks  25   1  and  25   2  in such a manner that the axial lines of the cylinder bores  29   1  and  29   2  are directed substantially in the horizontal direction. 
     A piston  31   1 , which forms a combustion chamber  30   1  between the first cylinder head  27   1  and the same, is slidably fitted in the first cylinder bore  29   1 . A piston  31   2 , which forms a combustion chamber  30   2  between the second cylinder head  27   2  and the same, is slidably fitted in the second cylinder bore  29   2 . Both of the pistons  31   1  and  31   2  are commonly connected to the crank shaft  28  via connecting rods  32   1  and  32   2 , respectively. The first and second cylinder blocks 25 , and  25   2  are connected to the crank case  26  in such a manner that the axial line of the first cylinder bore  29   1  is offset by an offset amount L 1  from the axial line of the cylinder bore  29   2  onto one side in the axial direction of the crank shaft  28 , more specifically, on the front side in the longitudinal direction of the motor cycle in this embodiment. 
     An intake port  33   1  (or  33   2 ) in communication with the combustion chamber  30   1  (or  30   2 ) is opened in an upper surface portion of the first cylinder head  27   1 , (or second cylinder head  27   2 ). An exhaust port  34   1  (or  34   2 ) in communication with the combustion chamber  30   1  (or  30   2 ) is opened in a lower surface portion of the cylinder head  27   1  (or  27   2 ). 
     Referring particularly to FIG. 4, the first cylinder head  27   1  has an intake valve  35   1  for opening/closing the intake port  33   1  in communication with the combustion chamber  30   1  thereby taking air in the combustion chamber  30   1 , and an exhaust valve  36   1  for opening/closing the exhaust port  34   1  in communication with the combustion chamber  30   1  thereby exhausting air from the combustion chamber  30   1 . The intake valve  35   1  and exhaust valve  36   1  are openably/closably operated. The intake valve  35   1  and the exhaust valve  36   1  are arranged in such a manner as to have operational axial lines L I  and L O  crossing each other into an approximately V-shape on a projection plane perpendicular to the axial line of the crank shaft  28  and including the axial line of the first cylinder bore  29   1  (see FIG.  4 ). Furthermore, on the projection plane, an angle α I  formed between the axial line L C  of the first cylinder bore  29   1  and the operational axial line L I  of the intake valve  35   1  is larger than an angle α O , formed between the axial line L C  of the first cylinder bore  29   1  and the operational axial line L O  of the exhaust valve  36   1  (α I &gt;α O ). Furthermore, the intake valve  35   1  and the exhaust valve  36   1  are arranged in the first cylinder head  27   1  in such a manner that a crossing point P C1  at which the operational axial lines L I  and L O  of the intake valve  35   1  and the exhaust valve  36   1  cross each other on the projection plane is lower than the axial line L C  of the first cylinder bore  29   1 . 
     An intake valve  35   2  for opening/closing the intake port  33   2  in communication with the combustion chamber  30   2  thereby taking air in the combustion chamber  30   2 , and an exhaust valve  36   2  for opening/closing the exhaust port  34   2  in communication with the combustion chamber  30   2  thereby taking air in the combustion chamber  30   2  are arranged in the second cylinder head  27   2  in accordance with the same angular and positional relationship as that for the intake valve  35   1  and the exhaust valve  36   1  arranged in the first cylinder head  27   1 . 
     A first head cover  37   1  (or second head cover  37   2 ), which forms a first valve system chamber  38   1  (or second valve system chamber  38   2 ) between the first cylinder head  27   1  (or  27   2 ) and the same, is connected to the first cylinder head  27   1  (or second cylinder head  27   2 ). A first valve system mechanism  39   1  for opening/closing the intake valve  35   1  and the exhaust valve  36   1  is contained in the first valve system chamber  38   1 , and a second valve system mechanism  39   2  for opening/closing the intake valve  35   2  and the exhaust valve  36   1  is contained in the second valve system chamber  38   2 . 
     The first valve system mechanism  39   1  includes a first cam shaft  40   1  having an axial line parallel to the axial line of the crank shaft  28 , an intake side rocker arm  41  for converting the rotational motion of the cam shaft  40   1  into the linear opening/closing motion of the intake valve  35   1 , and an exhaust side rocker arm  42  for converting the rotational motion of the first cam shaft  40   1  into the linear opening/closing motion of the exhaust valve  36   1 . 
     The first cam shaft  40   1  is located above the axial line L C  of the first cylinder bore  29   1  and between the intake valve  35   1  and the exhaust valve  36   1 . The first cam shaft  40   1  is rotatably supported by the first cylinder head  27   1  and a holder  43  connected to the first cylinder head  27   1 . 
     The first cam shaft  40   1  has an intake side cam  44  corresponding to the intake valve  35   1  and an exhaust side cam  45  corresponding to the exhaust valve  36   1 . The intake side and exhaust side rocker arms  41  and  42  are respectively swingably supported by supporting shafts  46  and  47  which have axial lines parallel to the first cam shaft  40   1  and are supported by the holder  43 . One-sided ends of the intake side and exhaust side rocker arms  41  and  42  are slidably in contact with the intake side and exhaust side cams  44  and  45 , respectively. Tappet screws  48  and  49  are fittingly screwed in the other ends of the intake side and exhaust side rocker arms  41  and  42 , respectively. The intake valves  35   1  and  36   1 , which are biased in the valve closing direction by valve springs  50  and  51  provided between the first cylinder head  27   1  and the same, are in contact with the tappet screws  48  and  49 , respectively. 
     A second valve system mechanism  39   2  contained in a valve system chamber  38   2  provided between the second cylinder head  27   2  and the second head cover  37   2  has a second cam shaft  40   2  and is configured like the first valve system mechanism  39   1 . 
     Referring particularly to FIG. 5, in the front case half  26   a  of the crank case  26 , the first and second cylinder block  25   1  and  25   2  and the first and second cylinder heads  27   1  and  27   2 , a cam chain chamber  52  for communicating both of the valve system chambers  38   1  and  38   2  with the crank case  26  is provided on the offset side of the axial line of the first cylinder bore  29   1  from the axial line of the second cylinder bore  29   2 , i.e., on the front end side of the motorcycle in the longitudinal direction. 
     A driven sprocket  53   1  is fixed to one end portion, on the cam chain chamber  52  side, of the first cam shaft  40   1  of the first valve system mechanism  39   1 , and a driven sprocket  53   2  is fixed to one end portion, on the cam chain chamber  52  side, of the second cam shaft  40   2  of the second valve system mechanism  39   2 . In the cam chain chamber  52   1  a drive sprocket  54   1  corresponding to the driven sprocket  53   1  and a drive sprocket  54   2  corresponding to the driven sprocket  53   2  are fixed to the crank shaft  28 . An endless cam chain  55   1  is wound around the drive sprocket  54   1  and the driven sprocket  53   1  for transmitting the rotational power of the crank shaft  28  reduced into half to the first cam shaft  40   1 . An endless cam chain  55   2  is wound around the drive sprocket  54   2  and the driven sprocket  53   2  for transmitting the rotational power of the crank shaft  28  reduced into half to the second cam shaft  40   2 . 
     In accordance with the offset of the axial line of the first cylinder bore  29   1  from the axial line of the second cylinder bore  29   2  by the offset amount L 1  in the axial direction of the crank shaft  28 , the combination of the drive sprocket  54   1 , the driven sprocket  53   1  and the cam chain  55   1  is offset from the combination of the drive sprocket  54   2 , the driven sprocket  53   2 , and the cam chain  55   2  by an offset amount L 2  in the axial direction of the crank shaft  28 . In this case, in order to miniaturize the engine main body in the axial direction of the crank shaft  28 , the offset amount L 2  is set to be smaller than the offset amount L 1  (L 2 &lt;L 1 ). 
     The crank shaft  28  is rotated in the rotational direction shown by an arrow  58  in FIG. 5. A chain tensioner  59   1  is elastically, slidably in contact with the forward running portion, i.e., the upper running portion of the cam chain  55   1  in the direction from the drive sprocket  54   1  to the driven sprocket  53 . A chain guide  60   1  is slidably in contact with the backward running portion, i.e., the lower running portion of the cam chain  55   1  in the direction from the driven sprocket  53   1  to the drive sprocket  54   1 . 
     One end portion of the chain tensioner  59   1  is turnably supported by the crank case  26 . A tensioner lifter  61   1 , which is in contact with an intermediate portion of the chain tensioner  59   1  in the longitudinal direction and presses the chain tensioner  59   1  to the cam chain  55   1 , is mounted in the upper portion of the first cylinder block  25   1 . 
     A chain tensioner  59   2  is elastically, slidably in contact with the forward running portion, i.e., the lower running portion of the cam chain  55   2  in the direction from the drive sprocket  54   2  to the driven sprocket  53   1 . A chain guide  60   1  is slidably in contact with the backward running portion, i.e., the upper running portion of the cam chain  55   1  in the direction from the driven sprocket  53   2  to the drive sprocket  54   2 . 
     One end portion of the chain tensioner  59   2  is turnably supported by the crank case  26 . A tensioner lifter  61   2 , which is in contact with an intermediate portion of the chain tensioner  59   2  in the longitudinal direction and presses the chain tensioner  59   2  to the cam chain  55   2 , is mounted in the lower portion of the second cylinder block  25 . 
     The front case half  26   a  of the crank case  26  has an opening  62  at the front end in the longitudinal direction of the motorcycle. A case  64  for a power generator  63  coaxially connected to the crank shaft  28  in the cam chain chamber  52  is fastened to the front case half  26   a  in such a manner as to close the opening  62 . 
     Referring particularly to FIGS. 6 and 7, breather chamber  65  is provided for the second cylinder block  25   2 , the second cylinder head  27   2 , and the front case half  26   a  of the crank case  26  in such a manner as to be located between the cam chain chamber  52  and the second cylinder bore  29   2 . 
     A through-hole  66  extending in parallel to the axial line of the second cylinder bore  29   2  is provided in the lower portion of the second cylinder block  25   2  in such a manner as to be located between the cam chain chamber  52  and the second cylinder bore  29   2 . A through-hole  67  extending in parallel to the axial line of the second cylinder bore  29   2  is provided in the upper portion of the second cylinder block  25   2  in such a manner as to be located between the cam chain chamber  52  and the second cylinder bore  29   2 . A partition wall  68  is interposed between the through-hole  66  and the through-hole  67 . 
     The breather chamber  65  is composed of a first chamber  65   a  formed between the second cylinder block  25   2  and the crank case  26 , a second chamber  65   b  formed in one through-hole  66  of the through-holes  66  and  67 , a third chamber  65   c  formed between the second cylinder block  25   2  and the second cylinder head  27   2 , and a fourth chamber  65   d  formed in the other through-hole  67  of the through-holes  66  and  67 . 
     A through-hole  69  for communicating the first chamber  65   a  into the crank case  26  is provided in the front case half  26   a  of the crank case  26 . A lubricating oil passage  72  is formed between a projecting portion  70  and a rising portion  71 . The projecting portion  70  is provided in the through-hole  67  in such a manner as to be integrated with a portion, near the crank case  26 , of the second cylinder block  25   2 . The rising portion  71  is provided on the crank case  26  in such a manner as to be matched with the projecting portion  70 . The through-hole  69  is provided in the crank case  26  at a position which is lower than the rising portion  71  to the through-hole  66 . A gasket  73  is provided between the crank case  26  and the second cylinder block  25   2  for blocking communication between the first chamber  65   a  and the fourth chamber  65   d . The gasket  73  has an opening  74  for communicating the first chamber  65   a  to the second chamber  65   b . A gasket  75  is provided between the second cylinder block  25   2  and the second cylinder head  27   2 . The gasket  75  has an opening  76  for commonly communicating the second and fourth chambers  65   b  and  65   d  to the third chamber  65   c.    
     Accordingly, the first chamber  65   a  is in communication with the crank case  26 ; the second chamber  65   b  formed in one through-hole  66  is in communication with the first chamber  65   a ; the third chamber  65   c  is in communication with the second chamber  65   b ; and the fourth chamber  65   d  formed in the other through-hole  67  is in communication with the third chamber  65   c  but is blocked from communicating with the first chamber  65   a . A breather gas outlet  77  in communication with the fourth chamber  65   d  is provided in the upper portion of the second cylinder block  25   2 . 
     Referring particularly to FIG. 8, an intake manifold  80  is connected to the intake ports  33   1 , and  33   2  of the first and second cylinder heads  27   1  and  27   2 . The intake manifold  80  is composed of an intake pipe  81   1 , an intake pipe  81   2 , and a common pipe portion  82 . One end of the intake pipe  81   1  is connected to the intake port  33   1  of the first cylinder head  27   1  and the other end of the intake pipe  81   1  is connected to the common pipe portion  82 . One end of the intake pipe  81   2  is connected to the intake port  33   2  of the second cylinder head  27   2  and the other end of the intake pipe  81   2  is connected to the common pipe portion  82 . The common pipe portion  82  is connected to the air cleaner  23  (see FIG. 1) via a throttle body (not shown). 
     Referring again to FIG. 4, the intake pipe  81   1  includes a first straight pipe portion  83 , a second straight pipe portion  84 , and a bent pipe portion  85 . The first straight portion  83  extends along a first straight center line CL 1  and has a downward end connected to the intake port  33   1 . The second straight pipe portion  84  extends along a second straight center line CL 2  crossing the first center line CL 1 . The bent pipe portion  85  is formed into a circular-arc shape while connecting the upstream end of the first straight pipe portion  83  to the downstream end of the second straight pipe portion  84 . The upstream end of the second straight pipe portion  84  is connected to the common pipe portion  82 . A fuel injection valve  86   1  for injecting fuel to the intake port  33   1  side is held between a portion, near the intake port  33   1 , of the intake pipe  81   1  and a mounting member  87   1  fastened to the intake pipe  81   1 . 
     A mounting flange  88  projecting outwardly is provided on an intermediate portion of the fuel injection valve  86   1 . A fitting hole  89  in which the leading end of the fuel injection valve  86   1  is to be fitted is provided in the intake pipe  81   1 , and a seat  90  for receiving the mounting flange  88  is formed around an outer end portion of the fitting hole  89 . In this case, the fitting hole  89  and the seat  90  are located in the intake pipe  81   1  at a portion which is closer to the intake port  33   1  than a straight line  91  which connects a crossing point P C2  where the first and second center lines CL 1  and CL 2  cross each other and a curved center C C  of the bent pipe portion  85 . 
     A pair of fastening portions  92  and  93  are provided on the mounting member  87   1 . Both of the fastening portions  92  and  93  of the mounting member  87   1 , in which the outer end of the fuel injection valve  86   1  is fitted, are fastened to a pair of fastening seats  94  and  95  provided on the intake pipe  81   1  by means of a pair of bolts  96  and a pair of bolts  97 , respectively. Both of the fastening seats  94  and  95  are provided at such a position as to hold the straight line  91  between the seat  90  and the same. The fastening portions  92  and  93  are formed in parallel with the seat  90 . 
     A fuel passage  98   1 , which extends in a direction tilting at an acute angle formed with respect to the second center line CL 2  of the second straight pipe portion  84  and which is in communication with the outer end of the fuel injection valve  86   1 , is formed in the mounting member  87   1 . 
     The intake pipe  81   2  connected to the intake port  33   2  of the second cylinder head  27   2  is configured like the intake pipe  81   1 . A fuel injection valve  86   2  is held between the intake pipe  81   2  and a mounting member  87   2  mounted to the intake pipe  81   2 . The fuel injection valve  86   2  is mounted to the intake pipe  81   2  in accordance with basically the same the structure as that for mounting the fuel injection valve  86   1  to the intake pipe  81   1 . Like the fuel passage  98   1  formed in the mounting member  87   1  a fuel passage  98   2  in communication with the fuel injection valve  86   2  is formed in the mounting member  87   2 . 
     The fuel passages  98   1  and  98   2  of both of the mounting members  87   1  and  87   2  are in communication with each other via a fuel conduit  99  disposed along the second straight pipe portions  84  of the intake pipes  81   1  and  81   2 . A fuel feed pipe  101 , to which fuel having been pumped from the fuel tank  22  by the fuel pump  100  (see FIG. 1) is fed from the fuel pump  100 , is connected to one mounting member  87   1  of both of the mounting members  87   1  and  87   2 . The other mounting member  87   1  is additionally provided with a regulator  102  for regulating a fuel pressure in the fuel passages  98   1  and  98   2  and the fuel conduit  99 . A fuel return pipe  103  for returning excess fuel to the fuel tank  22  is connected to the regulator  102 . 
     An exhaust manifold  106  is connected to the exhaust ports  34   1  and  34   2  of the first and second cylinder heads  27   1  and  27   2 . The exhaust manifold  106  includes an exhaust pipe  107   1  having one end connected to the exhaust port  34   1  of the first cylinder head  27   1  and an exhaust pipe  107   2  having one end connected to the exhaust port  34   2  of the second cylinder head  27   2 . The other ends of the exhaust pipes  107   1  and  107   2  are connected to each other on the right side of the transmission case  15  when the motorcycle is directed forwardly in the running direction, and extend to the rear side of the motorcycle. 
     An ignition plug  108   1  (or  108   2 ) having a leading end protruding into the combustion chamber  30   1  (or  30   2 ) is provided in the rear side, along the longitudinal direction of the motorcycle, of the cylinder head  27   1  (or  27   2 ) in such a manner as to be gradually tilted onto the cylinder block  25   1  (or  25   2 ) in the direction toward the outer end side of the ignition plug  108   1  (or  108   2 ). A mounting hole  109   1  (or  109   2 ) for mounting the ignition plug  108   1  (or  108   2 ) is provided in the cylinder head  27   1  (or  27   2 ) in such a manner as to be opened rearwardly in the longitudinal direction of the motorcycle. Since the mounting hole  109   1  (or  109   2 ) for mounting the ignition plug  108   1  (or  108   2 ) is opened rearwardly, it is possible to prevent water, mud and the like splashed up upon running of the motorcycle from permeating into the mounting hole  109   1  (or  109   2 ) as much as possible, and hence to eliminate the necessity of provision of a plug cap and the like and also eliminate the necessity of forming a drain opening in communication with the mounting hole  109   1  (or  109   2 ) in the cylinder head  27   1  (or  27   2 ). 
     Referring particularly to FIGS. 9 and 10, a secondary air feed passage  110  for feeding secondary air to exhaust gas flowing in the exhaust port  34   2  is provided in the second cylinder head  27   2 . The secondary air feed passage  110  is composed of a first passage portion  111  and a second passage portion  112 . The first passage portion  111  extends in a straight line with one end opened to a portion, near the exhaust valve  36   1 , of the inner surface of the exhaust port  34   2  towards the downstream side of the flowing direction of exhaust gas. The second passage portion  112 , which has a straight axial line bent from the axial line of the first passage portion  111  to the second cylinder block  25   2  side, is connected to an intermediate portion of the first passage portion  111 . To be more specific, the first passage portion  111  is formed by piercing the second cylinder head  27   2  in straight line from the upper surface of the second cylinder head  27   2  to the exhaust port  34   2 . The outer end portion of the first passage portion  111  is blocked with a plug  113 . One end of the second passage portion  112  is in communication with the intermediate portion of the first passage portion  111 , and the other end of the second passage portion  112  is opened to the connection plane of the second cylinder head  27   2  to which the first cylinder block  25   2  is connected. 
     A valve case  114  for a reed valve  115   2  is mounted on the upper surface of the second cylinder block  25   2  at a position near the second cylinder head  27   2 . A communication passage  116  for communicating the reed valve  115   2  to the second passage portion  112  of the secondary air feed passage  110  is provided in the second cylinder block  25   2 . A connection pipe portion  117  is integrally provided with the valve case  114 , and is connected to a control valve (not shown). 
     Like the second cylinder head  27   2 , the first cylinder head  27   1  is provided with a secondary air feed passage (not shown) in communication with the exhaust port  34   1 , and a reed valve  115   1  connected to the second air feed passage is mounted on the upper surface of the first cylinder block  25   1 . 
     A first cooling jacket  118   1  is provided in the first cylinder block  25   1  and the first cylinder head  27   1 , and a second cooling jacket  118   2  is provided in the second cylinder block  25   2  and the second cylinder head  27   2 . 
     The second cooling jacket  118   2  is composed of a cylinder side cooling water passage  119   2  provided in the second cylinder block  25   2  in such a manner as to surround the second cylinder bore  29   2 , and a head side cooling water passage  120   2  provided in the second cylinder head  27   2  in such a manner as to be in communication with the cylinder side cooling water passage  119   2 . 
     Referring to FIG. 7, the second cylinder block  25   2  is provided with a partition wall  121  which extends in parallel to the axial line of the second cylinder bore  29   2  and which partitions the cylinder side cooling water passage  119   2 . A water inlet  122   2  in communication with the cylinder side cooling water passage  119   2  on one side of the partition wall  121  is provided beneath the second cylinder block  25   2 . 
     On the other hand, as shown in FIG. 9, a pair of communication passages  123  and  124  for communicating the cylinder side cooling water passage  119   2  to the head side cooling water passage  120   2  on the other side of the partition wall  121  are provided in the second cylinder head  27   2 . A water outlet  125 , which is in communication with the head side cooling water passage  120   2  on the side being substantially opposed to the communication passages  123  and  124  with respect to the combustion chamber  302 , is provided at the upper portion of the second cylinder head  27   2 . 
     To be more specific, both of the communication passages  123  and  124  allow the cylinder side cooling water passage  119   2  to communicate with the head side cooling water passage  120   2  via an opening (not shown) provided in the gasket  73  provided between the second cylinder block  25   2  and the second cylinder head  27   2 . Both of the communication passages  123  and  124  are provided in the second cylinder head  27   2  in proximity to each other in such a manner that the one communication passage  124  is disposed substantially corresponding to the ignition plug  108   2 . 
     The first cooling jacket  118   1  includes a cylinder side cooling water passage  119   1  provided in the first cylinder block  25   1  in such a manner as to surround the first cylinder bore  29   1 , and a head side cooling water passage  120   1  provided in the first cylinder head  27   1  in such a manner as to be in communication with the cylinder side cooling water passage  119   1 . The first cooling jacket  118   1  is configured like the second cooling jacket  118   2 . A water inlet  121   1  in communication with the cylinder side cooling water passage  119   1  is provided in a lower portion of the first cylinder block  25   1 , and a water outlet (not shown) in communication with the head side cooling water passage  120   1  is provided on an upper portion of the first cylinder head  27 . 
     Referring to particularly to FIGS. 11 and 12, a single water pump  128  is mounted to the crank case  26  in such a manner as to be located under the lowermost portions of the first and second cooling jackets  118   1  and  118   2  and between both of the cooling jackets  118   1  and  118   2 . 
     A pump housing  129  of the water pump  128  includes a pump body  130  for rotatably supporting a pump shaft  132   1  and a pump cover  131  fastened to the pump body  130  in such a manner as to cover an impeller  133  fixed to the pump shaft  132 . 
     The pump body  130  is fastened to the front case half  26   a  of the crank case  26  in such a manner that a supporting cylinder portion  130   a  integrated with the pump body  130  air-tightly protrudes into the front case half  26   a . The pump cover  131  is fastened to the pump body  130 , to form a circular pump chamber  134  coaxial with the pump shaft  132  between the pump body  130  and the pump cover  131 . 
     The pump shaft  132  is liquid-tightly and rotatably supported by the supporting cylinder portion  130   a  of the pump body  130  in a state in which one end thereof protrudes into the pump chamber  134 . The impeller  133  disposed in the pump chamber  134  is fixed to the other end of the pump shaft  132 . 
     An upper discharge passage  135  and a lower discharge passage  136  are formed in the pump housing  129 . The upper discharge passage  135  is connected to an upper end portion of the pump chamber  134  and extends obliquely, upwardly therefrom along the tangential direction of the outer edge of the pump chamber  134 . The lower discharge passage  136  is connected to a lower end portion of the pump chamber  134  and extends obliquely, downwardly therefrom along the tangential direction of the outer edge of the pump chamber  134 . A first connection pipe  137  extending in a straight line from the upper discharge passage  135  and a second connection pipe  138  extending in a straight line from the lower discharge passage  136  are integrally provided on the pump body  130  of the pump housing  129  in such a manner that the inner ends of the first and second connection pipes  137  and  138  are in communication with the upper and lower discharge passages  135  and  136 , respectively. Furthermore, the first and second discharge ports  139  and  140  are formed at the outer ends of the first and second connection pipes  137  and  138 , respectively. 
     Referring again to FIG. 2, the first discharge port  139  formed at the outer end of the first connection pipe  137  is connected to the water inlet  122   1  formed in the first cooling jacket  118   1  for the first cylinder block  25   1  and the first cylinder head  27   1  through the first conduit  141 . The first discharge port  140  formed at the outer end of the second connection pipe  138  is connected to the water inlet  122   2  formed in the second cooling jacket  118   2  for the second cylinder block  25   2  and the second cylinder head  27   2  through the second conduit  142 . The length of the first conduit  141  is set to be shorter than the length of the second conduit  142 . In other words, the difference in length between the first and second conduits  141  and  142  is determined such that the flow resistance corresponding to the difference in pump head between the first and second discharge ports  139  and  140  of the water pump  128  is allowed to occur on the second conduit  142  side. 
     The pump cover  131  has first and second suction ports  143  and  144  in communication with the pump chamber  134 . The first suction port  143  is connected to a thermostat (not shown) and the second suction port  144  is connected to the radiators  24  (see FIG.  1 ). 
     If the temperature of cooling water is low before warming of the engine E, the thermostat is operated to return cooling water discharged from the water pump  128  to the first suction port  143  by way of only the first and second cooling jackets  118   1  and  118   2 , i.e., not by way of the radiators  24 . However, if the temperature of cooling water becomes high after warming of the engine E, the thermostat is operated to return cooling water discharged from the water pump  128  to the second suction port  144  by way of not only the first and second cooling jackets  118   1  and  118   2  but also the radiators  24 . 
     A trochoid type oil pump  146  for feeding lubricating oil to portions to be lubricated of the engine E is provided on the inner surface, on the transmission case  15  side, of the rear case half  26   b  of the crank case  26  in such a manner as to be coaxial with the water pump  128 . 
     A pump housing  147  of the oil pump  146  is composed of a pump body  148  integrally formed on the rear case half  26   b  and a pump cover  149  fastened to the pump body  148 . A pump shaft  150  coaxial with the pump shaft  132  of the water pump  128  is rotatably supported by the pump housing  147 . A pinion  151  is fixed to the pump shaft  150  in the pump housing  147 , and an inner gear  152  meshed with the pinion  151  is rotatably supported by the pump housing  147 . A strainer  154  is connected to a suction port  153  of the oil pump  146 . 
     One end of the pump shaft  150  of the oil pump  146  faces to the other end of the pump shaft  132 , projecting from the supporting cylinder portion  130   a , of the water pump  128 . An engagement plate  156  provided on the one end of the pump shaft  150  is engaged with an engagement recess  155  provided on the other end of the pump shaft  132 . That is to say, both of the pump shafts  132  and  150  are connected to each other with relative rotation thereof prevented. 
     The other end of the pump shaft  150  of the oil pump  146  projects from the pump housing  147  and is located in the transmission case  15 , and a driven sprocket  157  is fixed to the other end of the pump shaft  150 . 
     Referring again to FIG. 3, a drive sprocket  158  corresponding to the driven sprocket  157  is fixed to the crank shaft  28  in the transmission case  15 . An endless chain  159  is wound around the drive sprocket  158  and the driven sprocket  157  for transmitting the rotational power of the crank shaft  28  to the oil pump  146  and the water pump  128 . 
     Referring to FIG. 13, the crank shaft  28  passes through a bearing hole  161  provided in the rear case half  26   b  of the crank case  26  and projects towards the transmission case  15  side. A cylindrical bearing  162  is provided between the outer surface of the crank shaft  28  and the inner surface of the bearing hole  161 . 
     On the outer side of the rear case half  26   b  of the crank case  26 , i.e., on the transmission case  15  side, a drive gear  163  is fixed on a portion, near the rear case half  26   b , of the crank shaft  28 . An over-running clutch  164  is mounted on the crank shaft  28  at a position between the drive gear  163  and the drive sprocket  158 . 
     The drive gear  163  is meshed with a driven gear (not shown) provided on a balancer shaft  165  (see FIG. 2) having an axial line parallel to the crank shaft  28  and rotatably supported by the crank case  26 . 
     The over-running clutch  164  is used for transmitting power from a starter motor  166  (see FIG. 3) mounted to the transmission case  15  to the crank shaft  28 , while blocking the power transmission from the crank shaft  28  to the starter motor  166  side. The over-running clutch  164  includes a clutch inner race  168  for coaxially surrounding the crank shaft  28  with a roller bearing  167  interposed between the crank shaft  28  and the same, a ring-shaped clutch outer race  169  for coaxilly surrounding the clutch inner race  168 , and a plurality of rollers  170  provided between the clutch inner race  168  and the clutch outer race  169 . 
     An output member  171 , which is spline-connected to the crank shaft  28  in such a manner as to face toward the drive gear  163 , is connected to the clutch outer race  169  by means of a plurality of bolts  173 . An input member  172  is fixed to the clutch inner race  168  with the clutch outer race  169  located between the output member  171  and the input member  172 . A driven gear  174  is provided on the outer periphery of the input member  172 . A first intermediate gear  175  meshed with the driven gear  174  is rotatably supported by the transmission case  15 . A second intermediate gear  176  integrated with the first intermediate gear  175  is meshed with a drive gear  177  (see FIG. 3) provided on an output shaft of the starter motor  166 . 
     Lubricating oil is fed from an oiling passage  178  provided in the rear case half  26   b  of the crank case  26  to the bearing  162 . The bearing  162  has a plurality of through-holes  179  extending from the inner surface to the outer surface of the bearing  162 . Accordingly, the lubricating oil fed from the oiling passage  178  is uniformly fed between the outer surface of the bearing  162  and the rear case half  26   b  and between the inner surface of the bearing  162  and the outer surface of the crank shaft  28 . On the other hand, an oil passage  180  having one end in communication with the through-holes  179  is provided in the crank shaft  28 . The oil passage  180  functions to introduce lubricating oil into a connection portion between the crank shaft  28  and the connecting rod  32   2 . 
     A projecting portion  181   1  which projects radially inwardly from the end portion, on the transmission case  15  side, of the bearing hole  161 , is integrally provided on the rear case half  26   b  of the crank case  26 . An annular discharge port  182  for discharging lubricating oil fed to the bearing  162  onto the output member  171  side of the over-running clutch  164  is formed between the projecting portion  181  and the outer surface of the crank shaft  28 . 
     The output member  171  has introducing holes  183  for introducing lubricating oil discharged from the annular discharge port  182  into the over-running clutch  164 . The introducing holes  183  are provided at a plurality of positions spaced from each other in the peripheral direction of the output member  171 . 
     The drive gear  163 , which is disposed between the annular discharge port  179  and the output member  171 , is fixed on the crank shaft  28  and is substantially integrated with the output member  171 . Accordingly, a plurality of introducing holes  184  individually corresponding to the introducing holes  183  of the output member  171  are provided in the drive gear  163 . With this configuration, lubricating oil discharged from the annular discharge port  182  is introduced in the over-running clutch  164  through the introducing holes  183  and  184  without obstruction by the drive gear  163 . 
     The function of this embodiment will be described below. As described above, in the horizontally-opposed type four-cycle/two-cylinder engine E, the first cam shaft  40   1  (or second cam shafts  40   2 ) is disposed above the axial line L C  of the first cylinder bore  29   1  (or second cylinder bore  29   2 ); and on the projection plane perpendicular to the axial line of the crank shaft  28  including the axial line of the first cylinder bore  29   1  (or second cylinder bore  29   2 ), an angle α I  formed between the axial line L C  of the first cylinder bore  29   1  (or second cylinder bore  29   2 ) and the operational axial line L I  of the intake valve  35   1  (or  35   2 ) is larger than an angle α O  formed between the axial line L C  of the first cylinder bore  29   1  (or the second cylinder bore  29   2 ) and the operational axial line L O  of the exhaust valve  36   1  (or  36   2 ) 
     With this configuration, the outer end of the exhaust valve  35   1  (or  35   2 ) can be disposed in such a manner as to be made as close to the axial line of the cylinder bore  29   1  (or  29   2 ) as possible. Accordingly, it is possible to avoid restriction of the bank angle of the motorcycle at the outer ends of the exhaust valves  35   1  and  35   2 , and hence to make the mounting position of the engine E as low as possible while ensuring the ground clearance of the motorcycle. This is effective to make the center of gravity of the motorcycle lower and also to improve the steering of the motorcycle. 
     Furthermore, on the projection plane perpendicular to the axial line of the crank shaft  28  including the axial line of the first cylinder bore  29   1  (or the second cylinder bore  29   2 ), the crossing point P C1  at which the operational axial lines L I  and L O  of the intake valve  35   1  (or  35   2 ) and the exhaust valve  36   1  (or  36   2 ) cross each other is located lower than the axial line L C  of the cylinder bore  29   1  (or  29   2 ). Accordingly, it is possible to easily ensure a squish area of the combustion chamber  30   1  (or  30   2 ) on the intake valve  35   1  (or  35   2 ) side having a diameter larger than the exhaust valve  36   1  (or  36   2 ), and hence to make the squish area on the intake side nearly equal to that on the exhaust side. 
     The first and second cylinder blocks  25   1  and  25   2  are commonly connected to the crank case  28  in such a manner that the axial line of the first cylinder bore  29   1  of the first cylinder block  25   1  is offset from the axial line of the second cylinder bore  29   2  of the second cylinder block  25   2  onto one side along the axial line of the crank shaft  28 . Furthermore, on one side along the axial line of the crank shaft  28 , the cam chain chamber  52  is provided for the crank case  26 , the cylinder blocks  25   1  and  25   2  and the cylinder heads  27   1  and  27   2 . Accordingly, a relatively large space is formed between the second cylinder bore  29   2  and the cam chain chamber  52 , so that a breather chamber  65  can be provided for the crank case  26 , the second cylinder block  25   2  and the second cylinder head  27   2  by making effective use of the space. As a result, it is possible to form the breather chamber  65  having a relatively large capacity while avoiding enlargement of the size of the entire engine, and hence to improve the breather performance. 
     The breather chamber  65  is composed of the first chamber  65   a  in communication with the interior of the crank case  26 , the second chamber  65   b  in communication with the first chamber  65   a , the third chamber  65   c  in communication with the second chamber  65   b , and the fourth chamber  65   d  in communication with the third chamber  65   c , but is blocked from the first chamber  65   a ; and the breather gas outlet  77  in communication with the fourth chamber  65   d  is provided in the second cylinder block  25   2 . Accordingly, since the breather chamber  65  has a labyrinth structure, it is possible to effectively separate oil mist from breather gas in the breather chamber  65  and hence to further improve the breather performance. 
     The intake pipe  81   1  (or  81   2 ) in communication with the intake port  33   1  (or  33   2 ) of the first cylinder head  27   1  (or the second cylinder head  27   2 ) includes the first straight pipe portion  83  extending along the first straight center line CL 1 , the second straight pipe portion  84  extending along the second straight center line CL 2  crossing the first center line CL 1 , and the bent pipe portion  85  formed into a circular-arc shape while connecting the upstream end of the first straight pipe portion  83  to the downstream end of the second straight pipe portion  84 ; and the fuel injection valve  86   1  (or  86   2 ), for injecting fuel to the intake port  33   1  (or  33   2 ), is held between the intake pipe  81   1  (or  81   2 ) and the mounting member  87   1  (or  87   2 ) fastened to the intake pipe  81   1  ( 81   2 ). Furthermore, the seat  90  for receiving the fuel injection valve  86   1  (or  86   2 ) is provided in the intake pipe  81   1  (or  81   2 ) at a portion which is closer to the intake port  33   1  (or  33   2 ) than the straight line  91  which connects the crossing point P C2  where the first and second center lines CL 1  and CL 2  cross each other and the curved center C C  of the bent pipe portion  85 . As a result, it is possible to suppress the projecting amount of the fuel injection valve  86   1  (or  86   2 ) from the outer end of the cylinder head  27   1  (or  27   2 ) and hence to make the entire engine including the fuel injection system compact. 
     The fastening seats  94  and  95  for fastening the mounting member  87   1  (or  87   2 ) are provided on the intake pipe  81   1  (or  81   2 ) with the straight line  91  located between the seat  90  and the same. As a result, the fastening seats  94  and  95  of the mounting member  87   1  (or  87   2 ) are provided on the second straight pipe portion  84  side while the outer end of the fuel injection valve  86   1  (or  86   2 ) is disposed at a position relatively far away from the first center line CL 1 , so that a space for disposing the mounting member  87   1  (or  87   2 ) can be relatively largely ensured. 
     Since the seat  90  and the fastening seats  94  and  95  are formed in parallel to each other, it becomes easy to mount the fuel injection valve  86   1  (or  86   2 ) to the intake pipe  81   1  (or  81   2 ), and also it is possible to improve the mounting reliability. 
     The fuel passage  98   1  (or  98   2 ), which extends in a direction tilting at an acute angle formed with respect to the second center line CL 2  and which is connected to the fuel injection valve  86   1  (or  86   2 ), is formed in the mounting member  87   1  (or  87   2 ), so that the fuel conduit  99  connected to the fuel passage  98   1  (or  98   2 ) can be disposed along the second straight pipe portion  84  of the intake pipe  81   1  (or  81   2 ). Accordingly, it is easy to ensure a space for disposing the fuel conduit  99  and to protect the fuel conduit  99 . This is advantageous in preventing occurrence of vapor gas due to vibration of the fuel conduit  99 . 
     The secondary air feed passage  110  for feeding secondary air to exhaust gas flowing in the exhaust port  34   1  (or  34   2 ) is provided in the first cylinder head  27   1  (or second cylinder head  27   2 ). The secondary air feed passage  110  is composed of the first passage portion  111  and the second passage portion  112 . The first passage portion  111  extends in a straight line having one end opened in the inner surface of the exhaust port  34   1  (or  34   2 ) onto the downstream side of the flowing direction of exhaust gas. The second passage portion  112 , which has a straight axial line bent from the axial line of the first passage portion  111  onto the cylinder block  25   1  (or  25   2 ) side, is connected to the first passage portion  111 . 
     The shape of the secondary air feed passage  110  causes the secondary air to be sucked from the secondary air feed passage  110  into the exhaust port  34   1  (or  34   2 ) by the flow of exhaust gas in the exhaust port  34   1  (or  34   2 ). This makes it possible to prevent the permeation of exhaust gas into the secondary air feed passage  110  as mush as possible. Furthermore, the second passage portion  112  is in communication with the first passage portion  111  in such a manner as to be bent from the first passage portion  111 . Accordingly, even if exhaust gas permeates in the first passage portion  111  of the secondary air passage  110 , it is possible to prevent the exhaust gas thus permeated in the first passage portion  111  from further permeating into the second passage portion  112  side, and hence to shorten the length of the secondary air feed passage  110 . 
     Since the reed valve  115   1  (or  115   2 ) connected to the secondary air feed passage  110  is mounted on the outer surface of the cylinder block  25   1  (or  25   2 ), it is possible to avoid the enlargement of the entire engine accompanied by arrangement of the reed valve  115   1 (or  115   2 ). 
     The first cooling jacket  118   1  is provided in the first cylinder block  25   1  and the first cylinder head  27   1 , and the second cooling jacket  118   2  is provided in the second cylinder block  25   2  and the second cylinder head  27   2 . The cooling jacket  118   1  (or  118   2 ) is composed of the cylinder side cooling water passage  119   1  (or  119   2 ) provided in the cylinder block  25   1  (or  25   2 ) in such a manner as to surround the cylinder bore  29   1  (or  29   2 ). The head side cooling water passage  120   1  (or  120   2 ) is provided in the cylinder head  27   1  (or  27   2 ) in such a manner as to be in communication with the cylinder side cooling water passage  119   1  (or  119   2 ). The cylinder block  25   1  (or  25   2 ) is provided with a partition wall  121  which extends in parallel to the axial line of the cylinder bore  29   1  (or  29   2 ) for partitioning the cylinder side cooling water passage  119   1  (or  119   2 ). The water inlet  122   1  (or  122   2 ) in communication with the cylinder side cooling water passage  119   1  (or  119   2 ) is provided in the cylinder block  25   1  (or  25   2 ) on one side of the partition wall  121 . The cylinder head  27   1  (or  27   2 ) has a pair of communication passages  123  and  124  on the other side of the partition wall  121  for communicating the cylinder side cooling water passage  119   1  (or  119   2 ) to the head side cooling water passage  120   1  or ( 120   2 ). 
     Accordingly, cooling water which has been fed from the water inlet  122   1  (or  122   2 ) to the cylinder side cooling water passage  119   1  (or  119   2 ) on the one side of the partition wall  121 , flows in the cylinder side cooling water passage  119   1  (or  119   2 ) toward the other side of the partition wall  121  in such a manner as to substantially go around the cylinder bore  29   1  (or  29   2 ). The cooling water is then introduced to the head side cooling water passage  120   1  (or  120   2 ) via the communication passages  123  and  124 . 
     On the other hand, the cylinder head  27   1  (or  27   2 ) has the water outlet  125  which is located substantially opposite to the communication passages  123  and  124  with respect to the combustion chamber  30   1  (or  30   2 ) in such a manner as to be in communication with the cylinder side cooling water passage  120   1  (or  120   2 ). Accordingly, the cooling water having been introduced in the head side cooling water passage  120   1  (or  120   2 ) flows in the head side cooling water passage  120   1  (or  120   2 ) toward the water outlet  125  disposed substantially opposite to the communication passages  123  and  124  with respect to the combustion chamber  30   1  (or  30   2 ). 
     To be more specific, cooling water smoothly flows from the water inlet  122   1  (or  122   2 ) to the water outlet  125  by way of the cylinder side cooling water passage  119   1  (or  119   2 ), the communicating passages  123  and  124 , and the head side cooling water passage  120   1  (or  120   2 ). As a result, it is possible to effectively cool the cylinder blocks  25   1  and  25   2  and the cylinder heads  27   1  and  27   2 . 
     The ignition plug  108   1  (or  108   2 ) having a leading end protruding in the combustion chamber  30   1  (or  30   2 ) is provided in the cylinder head  27  (or  27   2 ) in such a manner as to be tilted onto the cylinder block  25   1  (or  25   2 ) in the direction toward the outer end side of the ignition plug  108   1  (or  108   2 ). One communication passage  124  of both of the communication passages  123  and  124  is disposed at a position substantially corresponding to the ignition plug  108   1  (or  108   2 ). As a result, the flow area of a portion, corresponding to the communication passage  124 , of the head side cooling water passage  120   1  (or  120   2 ) becomes inevitably small, so that it is possible to improve the cooling performance of the cylinder head  27   1  (or  27   2 ) in the vicinity of the ignition plug  108   1  (or  108   2 ) by increasing the flow rate of cooling water at the above portion of the head side cooling water passage  120   1  (or  120   2 ). 
     The single water pump  128  commonly used for the first and second cooling jackets  118   1  and  118   2  is disposed under the lowermost portions of both of the cooling jackets  118   1  and  118   2  and between both of the cooling jackets  118   1  and  118   2 . The first and second discharge ports  139  and  140  of the water pump  128  are connected to the water inlets  122   1  and  122   2  of both of the cooling jackets  118   1  and  118   2 , respectively. 
     The pump housing  129  of the water pump  128  contains a circular pump chamber  134  for rotatably containing the impeller  133 ; the upper discharge passage  135  connected to the upper end of the pump chamber  134  and extending obliquely, upwardly therefrom; and the lower discharge port  136  connected to the lower end of the pump chamber  134  and extending obliquely, downwardly therefrom. The first and second connection pipes  137  and  138  with their outer end openings taken as the first and second discharge ports  139  and  140  are arranged continuously to the pump housing  129  in such a manner that they extend in straight lines from the upper and lower discharge passages  135  and  136  and the inner ends thereof are in communication with the upper and lower discharge passages  135  and  136 , respectively. 
     Accordingly, the path from the upper end of the pump chamber  134  to the first discharge port  139  at the outer end of the first connection pipe  137  by way of the upper discharge passage  135  extends obliquely in such a manner that the first discharge port  139  is located at the highest position, while the path from the lower end of the pump chamber  134  to the second discharge port  140  at the outer end of the second connection pipe  138  by way of the lower discharge passage  136  extends obliquely in such a manner that the lower end of the pump chamber  134  is located at the highest position. To be more specific, the path from the second discharge port  140  to the first discharge port  139  by way of the lower discharge passage  136 , the pump chamber  134 , and the upper discharge passage  135  does not have any portion in which air remains. As a result, it is possible to eliminate the necessity of providing a structure specialized for ventilation such as an air vent bolt and to easily extract air from the water pump  128 . 
     The first discharge port  139  is connected to the first cooling jacket  118   1  by means of the first conduit  141 , and the second discharge port  140  is connected to the second cooling jacket  118   2  by means of the second conduit  142 . Furthermore, the length of the first conduit  141  is set to be shorter than that of the second conduit  142  in order that the flow resistance corresponding to the difference in pump head between the first and second discharge ports  139  and  140  of the water pump  128  is allowed to occur on the second conduit  142  side. As a result, since the difference in pump head between the first and second discharge ports  139  and  140  of the water pump  128  is balanced with the flow resistance occurring at the first and second conduits  141  and  142 , it is possible to uniformly feed cooling water from the common water pump  128  to the first and second cooling jackets  118   1  and  118   2 . 
     The over-running clutch  164  including the input member  172  to which power is inputted from the starter motor  166  and the output member  171  connected to the crank shaft  28  is mounted on the crank shaft  28  in the transmission case  15 . The bearing hole  161  allowing the crank shaft  28  to pass therethrough is provided in the rear case half  26   b  of the crank case  26 , and the bearing  162  is provided between the inner surface of the bearing hole  161  and the outer surface of the crank shaft  28 . 
     Furthermore, the over-running clutch  164  is mounted to the crank shaft  28  with its output member  171  disposed on the bearing  162  side, and the annular discharge port  182  is formed between the projecting portion  181  and the outer surface of the crank shaft  28 . The projecting portion  181  is integrally provided on the rear case half  26   b  of the crank case  26  in such a manner as to project radially inwardly from the end, on the transmission case  15  side, of the bearing hole  161 . The oiling passage  178  for feeding lubricating oil to the bearing  162  is provided in the rear case half  26   b  of the crank case  26 . Accordingly, the lubricating oil fed to the bearing  162  is discharged from the annular discharge port  182  onto the output member  171  side of the over-running clutch  164 . 
     The output member  171  has introducing holes  183  for introducing the lubricating oil discharged from the annular discharge port  182  into the over-running clutch  164 . 
     Accordingly, when the lubricating oil fed to the bearing  162  is discharged from the annular discharge port  182  onto the output member  171  of the over-running clutch  164 , the lubricating oil is correspondingly introduced from the introducing holes  183  of the output member  171  rotated together with the crank shaft  28  into the over-running clutch  164 . As a result, it is possible to eliminate the necessity of forming lubricating oil feed holes in the over-running clutch  164 , and hence to reduce the number of processing steps and the manufacturing cost. Furthermore, since the lubricating oil fed from the bearing  162  is used for lubricating the over-running clutch  164 , it is possible to miniaturize the oil pump  146  without decreasing the amount of lubrication oil discharged from the oil pump  146 . 
     While the embodiment of the present invention has been described, the present invention is not limited thereto, and it is to be understood that various changes in design may be made without departing from the spirit or the scope of the claims. 
     For example, although the present invention is applied to a horizontally-opposed type four-cycle/two-cylinder engine E in the above-described embodiment, the present invention can be applied not only to four-cycle engines for motorcycles but also to four-cycle engines for automobiles. 
     As described above, according to the first aspect of the present invention, since the cam shaft is disposed above the axial line of the cylinder bore, and an angle formed between the operational axial line of the exhaust valve and the axial line of the cylinder bore is set to be smaller than the angle formed between the operational axial line of the intake valve and the axial line of the cylinder bore, it is possible to make the outer end portion of the exhaust valve as close to the axial line of the cylinder bore as possible, and hence to make the mounting position of the engine as low as possible while ensuring sufficient ground clearance. This makes it possible to lower the center of gravity of the vehicle and hence to improve the steering of the vehicle. 
     According to the second aspect of the present invention, it is possible to easily ensure a squish area on the side of the intake valve having a diameter larger than that of the exhaust valve, and hence to make the squish area on the intake valve side nearly equal to that on the exhaust side. 
     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.