Patent Publication Number: US-10330043-B2

Title: Saddle-ride type vehicle

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a saddle-ride type vehicle in which a single-cylinder engine main unit is mounted on a body frame, the engine main unit having: a crankcase; a cylinder body rising upward from an upper portion of the crankcase; a cylinder head connected to an upper portion of the cylinder body; and a head cover connected to an upper portion of the cylinder head, the cylinder head includes a front wall connected to an exhaust system and a rear wall connected to an intake system. 
     Description of the Related Art 
     Such a saddle-ride type vehicle is disclosed in Japanese Patent No. 5913410 for example. 
     Incidentally, in the vehicle disclosed in Japanese Patent No. 5913410, output of an internal combustion engine is enhanced by devising arrangement of an intake system and an exhaust system of the internal combustion engine with respect to a body frame, however, further enhancement of the output is desired and a problem to be solved is to concentrate and compact mass of the internal combustion engine. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the above-mentioned circumstances, and it is an object thereof to provide a saddle-ride type vehicle that enhances output of an internal combustion engine and enables concentrating and compacting mass of the internal combustion engine. 
     In order to achieve the object, according to a first feature of the present invention, there is provided a saddle-ride type vehicle in which a single-cylinder engine main unit is mounted on a body frame, the engine main unit having: a crankcase; a cylinder body rising upward from an upper portion of the crankcase; a cylinder head connected to an upper portion of the cylinder body; and a head cover connected to an upper portion of the cylinder head, the cylinder head includes a front wall connected to an exhaust system and a rear wall connected to an intake system, wherein a joining surface of the cylinder head to the head cover is formed on a plane crossing a cylinder axis, the plane being inclined downward toward a front, rotation axes of camshafts are arranged on the plane, the rotation axes of camshafts configuring a part of a valve train opening/closing an intake valve and an exhaust valve respectively supported by the cylinder head, an axis of a valve stem of a butterfly throttle valve is arranged on the plane, the butterfly throttle valve being provided to a throttle device configuring a part of the intake system, a curve cover is formed on one side in a vehicle width direction of the head cover, the curve cover being curved to cover driven sprockets that are fixed to the camshafts so that a cam chain for transmitting power from a crankshaft to the camshafts is wound around the driven sprockets, and a drum housing box is arranged close to the curve cover, the drum housing box housing a throttle drum fixed to one end of the valve stem, the drum housing box being annexed to the throttle device. 
     With the first feature of the present invention, as the joining surface of the cylinder head to the head cover is formed on the plane crossing the cylinder axis, the plane being inclined diagonally downward toward the front, the rotation axis of the camshaft configuring a part of the valve train is arranged on the plane, the axis of the valve stem of the butterfly throttle valve provided to the throttle device configuring a part of the intake system is arranged on the plane, the curve cover curved to cover the driven sprocket fixed to the camshaft on one side in the vehicle width direction of the head cover is formed, and the drum housing box housing the throttle drum and annexed to the throttle device is arranged close to the curve cover, the throttle valve is arranged closer to the engine main unit so as to enable concentration and compactness of the intake system, the intake system can be made a greatly inclined downdraft type so that the output of the internal combustion engine can be enhanced, and besides, the handling of the throttle cables extended from the drum housing box is facilitated. 
     According to a second feature of the present invention, in addition to the first feature, the head cover includes a rear wall formed with a relief recess corresponding to the drum housing box. 
     With the second feature of the present invention, as the relief recess is formed on the rear wall of the head cover, the drum housing box can be brought closer to the head cover side and it is possible to achieve concentration and compactness of the intake system. 
     According to a third feature of the present invention, in addition to the second feature, a cam chain passage through which the cam chain is traveled is formed in the engine main unit, a lead-out pipe for leading out breather gas is installed in the rear wall of the head cover, and the relief recess is arranged between the cam chain passage and the lead-out pipe when viewed from a direction along the cylinder axis. 
     With the third feature of the present invention, as the relief recess of the head cover is arranged between the cam chain passage and the lead-out pipe, a dead space can be effectively utilized. 
     According to a fourth feature of the present invention, in addition to the first feature, the body frame includes: a head pipe that steerably supports a steering handlebar; a pair of right and left main frames extended backward from the head pipe; a pair of right and left pivot frames connected to rear ends of the main frames, the pair of pivot frames being extended downward; and a single down frame extended downward from the head pipe, the engine main unit is arranged so as to be encircled by the head pipe, the main frames, the pivot frames and the down frame as seen in a side view, and a pair of exhaust ports are mutually independently formed in the cylinder head so that the pair of exhaust ports are arranged on opposite sides of a center line of a vehicle body passing a center in the vehicle width direction of the down frame as seen in a plan view, the pair of exhaust ports respectively corresponding to a pair of exhaust valves openably and closably arranged in the cylinder head, the pair of exhaust ports being open to the front wall of the cylinder head. 
     With the fourth feature of the present invention, as the pair of exhaust ports respectively corresponding to the pair of exhaust valves are formed in the cylinder head so as to be arranged on opposite sides of the center line of the vehicle body passing the center line in the vehicle width direction of the down frame, the curve of the exhaust port is reduced so that the output can be enhanced, and the cylinder head can be arranged close to the down frame while easily avoiding interference with the exhaust pipes connected to the pair of exhaust ports. 
     According to a fifth feature of the present invention, in addition to the fourth feature, exhaust pipes configuring a part of the exhaust system and respectively connected to the pair of exhaust ports integrally includes: a curved pipe portion having an upstream end connected to each exhaust port, the curved pipe portion being bent in a U shape beside the down frame; and an inclined linear pipe portion having an upstream end connected to a downstream end of the curved pipe portion, the inclined linear pipe portion being extended backward while inclined to be located inside in the vehicle width direction as going toward the rear, the inclined linear pipe portions are arranged so as to pass between the pair of right and left pivot frames, and an inclined face along the inclined linear pipe portion of one of the pair of exhaust pipes is formed in a protruded portion of a cam chain tensioner protruded from the rear wall of the cylinder head, the cam chain tensioner being provided to the rear wall of the cylinder head so as to adjust tension of the cam chain. 
     With the fifth feature of the present invention, as the exhaust pipe is integrally provided with the curved pipe portion bent in the U shape beside the down frame and the inclined linear pipe portion having the upstream end connected to the downstream end of the curved pipe portion, the inclined linear pipe portion being extended backward while inclined so as to be located inside in the vehicle width direction as going toward the rear, the inclined linear pipe portion passing between the pair of right and left pivot frames, and the inclined face along one of the pair of inclined linear pipe portions is formed in the protruded portion of the cam chain tensioner protruded from the rear wall of the cylinder head, the compact arrangement of the pair of exhaust pipes is enabled, avoiding interference with the cam chain tensioner. 
     According to a sixth feature of the present invention, in addition to the fifth feature, a rotor of a generator is fixed to one end of the crankshaft, a cam chain-driving sprocket for engaging the cam chain therewith is provided to an other end side of the crankshaft in a portion where a cylinder bore of the cylinder body is sandwiched between the portion and the generator, and a power transmission mechanism for starting that transmits power for starting from a starter motor installed in the crankcase to the crankshaft is arranged between the generator and the cylinder bore. 
     With the sixth feature of the present invention, as the generator and the cam chain-driving sprocket are separately arranged in the direction along the axis of the crankshaft and the power transmission mechanism for starting is arranged on the generator side, the sideway overhang of the cam chain passage from the cylinder bore is inhibited, the exhaust pipe can be arranged closer to the center side of the vehicle body beside the cylinder body and the cylinder head, and the more compact arrangement of the exhaust pipe is enabled. 
     According to a seventh feature of the present invention, in addition to the sixth feature, a front end of a swing arm is swingably supported by the pivot frames via a pivot, a rear end of the swing arm pivotally supporting a rear wheel, a rear cushion unit that buffers a swing of the swing arm is extended up and down and the rear cushion unit is arranged between the pair of right and left pivot frames, the pair of right and left exhaust pipes pass respectively between the pair of right and left pivot frames and the rear cushion unit and are extended backward, and exhaust mufflers are respectively connected to downstream ends of the exhaust pipes. 
     With the seventh feature of the present invention, as the exhaust pipe is arranged between the rear cushion unit that buffers the swing of the swing arm and each of the pair of right and left pivot frames, and the right and left exhaust pipes are similar and symmetrical, a part of members configuring the exhaust pipes can be shared between the right and left exhaust pipes. 
     According to an eighth feature of the present invention, in addition to the sixth feature or the seventh feature, the down frame is provided with a pair of side walls that are inclined so as to approach each other as going to the rear in a longitudinal direction of the vehicle while respectively facing the curved pipe portions of the exhaust pipes. 
     With the eighth feature of the present invention, as the down frame is provided with the pair of side walls inclined to approach each other as going toward the rear in the longitudinal direction of the vehicle, the space for arranging the exhaust pipes can be readily secured on opposite sides of the down frame, the bend of the exhaust pipe is reduced so that exhaust performance can be enhanced. 
     According to a ninth feature of the present invention, in addition to the fourth feature or the fifth feature, a center in the vehicle width direction of the down frame, the cylinder axis and a center of an intake port open to the rear wall of the cylinder head are arranged on the center line of the vehicle body as seen in the plan view. 
     With the ninth feature of the present invention, as the center of the down frame, the cylinder axis and the center of the intake port are located on the center line of the vehicle body as seen in a plan view, the influence of bias in a right-left direction of intake and exhaust is inhibited to be smaller so that the output can be enhanced. 
     The above and other objects, characteristics and advantages of the present invention will be clear from detailed descriptions of the preferred embodiment which will be provided below while referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view of a two-wheeled motor vehicle. 
         FIG. 2  is a left side view of an internal combustion engine. 
         FIG. 3  is a right side view of the internal combustion engine. 
         FIG. 4  is a sectional view viewed along a line  4 - 4  in  FIG. 2 . 
         FIG. 5  is a sectional view viewed along a line  5 - 5  in  FIG. 2 . 
         FIG. 6  is a view of a cylinder head when viewed in a direction of an arrowed line  6 - 6  in  FIG. 2 . 
         FIG. 7  is a longitudinal sectional view of the cylinder head and a head cover which is viewed along a line  7 - 7  in  FIG. 6 . 
         FIG. 8  is a sectional view viewed along a line  8 - 8  in  FIG. 7 . 
         FIG. 9  is a perspective view of a drum housing box and its vicinity when viewed from the right diagonal upside. 
         FIG. 10  is a cross sectional plan view viewed along a line  10 - 10  in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to attached  FIGS. 1 to 10 , an embodiment of the present invention will be described below. In the following description, up-down, front-rear and right-left directions shall be directions viewed from a rider straddling a two-wheeled motor vehicle. 
     First as shown in  FIG. 1 , a body frame F of a two-wheeled motor vehicle for traveling on an uneven road is provided with a head pipe  13 , a pair of right and left main frames  14 , a down frame  15 , a pair of right and left lower frames  16 , a pair of right and left pivot frames  17 , a pair of right and left seat rails  18 , and a pair of right and left rear frames  19 . The head pipe  13  steerably supports a front fork  11  pivotally supporting a front wheel WF and a steering handlebar  12 . The main frames  14  are extended downward to the rear from the head pipe  13 . The down frame  15  is extended downward to the rear from the head pipe  13  at a steeper angle than each main frame  14 . The lower frames  16  are connected to a lower end of the down frame  15  and extended backward. The pivot frames  17  each have an upper end integrally connected to a rear end of each main frame  14  and are extended downward, the pivot frames  17  each having a lower end connected to a rear end of each lower frame  16 . The seat rails  18  are connected to the rear end of each main frame  14  and extended backward. The rear frames  19  each have a front end coupled to an intermediate portion in an up-down direction of each of the pivot frames  17  and are extended upward to the rear, the rear frames  19  each having a rear end coupled to each pivot frame  17 . 
     An engine main unit  20  of a single-cylinder internal combustion engine E is mounted on the body frame F in a state encircled by the head pipe  13 , the main frames  14 , the down frame  15 , the lower frames  16 , and the pivot frames  17  as seen in a side view. An axle  21  of a rear wheel WR as a drive wheel is pivotally supported on a rear end of a swing arm  22  and a front end of the swing arm  22  is supported swingably in the up-down direction by a lower portion of the pivot frame  17  via a pivot  23  in the body frame F. 
     A link mechanism  25  is provided between a bracket  24  provided to the lower portion of the pivot frame  17  in the body frame F and the swing arm  22  and an up-down extended rear cushion unit  27  is provided between a link member  26  configuring a part of the link mechanism  25  and an upper portion of the pivot frame  17 . 
     A transmission  52  (see  FIG. 4 ) is housed in a crankcase  28  configuring a part of the engine main unit  20 , an output shaft  32  of the transmission  52  is protruded leftward from the crankcase  28 , and an endless drive chain  35  is wound onto a drive chain-driving sprocket  33  provided to the output shaft  32  and a driven sprocket  34  provided to the axle  21  of the rear wheel WR. 
     Besides, a fuel tank  36  is provided on both main frames  14  above the engine main unit  20 , and a riding seat  37  is arranged at the back of the fuel tank  36  with the riding seat  37  supported by the seat rails  18 . 
     As also shown in  FIGS. 2 and 3 , the engine main unit  20  is provided with the crankcase  28 , a cylinder body  29 , a cylinder head  30  and a head cover  31 , and the engine main unit  20  is mounted on the body frame F in such posture that a cylinder axis C is slightly inclined forward. The cylinder body  29  is connected to a front-side upper portion of the crankcase  28  in a state rising upward from the crankcase  28 . The cylinder head  30  is connected to an upper portion of the cylinder body  29 . The head cover  31  is connected to an upper portion of the cylinder head  30 . Moreover, a joining surface  38  of the cylinder head  30  to the head cover  31  is located on a second plane PL 2  crossing the cylinder axis C, the second plane PL 2  being inclined downward to the front so that a forward inclined angle thereof is slightly larger than that of a first plane PL 1  perpendicular to the cylinder axis C and inclined so as to be located upward as going rearward in a longitudinal direction of a vehicle. 
     A pair of right and left exhaust ports  40  open to a front wall  30   a  of the cylinder head  30  are provided to the cylinder head  30 , and an exhaust system  41  of the internal combustion engine E is provided with a pair of exhaust pipes  42  and a pair of right and left exhaust mufflers  43  as plainly shown in  FIG. 1 . Upstream ends of the exhaust pipes  42  are connected to the exhaust ports  40  and the exhaust pipes  42  extend respectively around the right and left sides of the engine main unit  20 . The exhaust mufflers  43  are respectively connected to downstream ends of the exhaust pipes  42  and arranged above the rear wheel WR. 
     In addition, an intake connecting pipe  45  forming a single intake port  44  is protruded backward and diagonally upward from a rear wall  30   b  of the cylinder head  30 , and an intake system  46  of the internal combustion engine E is provided with a throttle body  47 , an insulator  48 , a connecting tube  49 , and an air cleaner  50  (see  FIG. 1 ). The throttle body  47  is a throttle device for regulating air volume supplied to the intake port  44 . The insulator links the intake connecting pipe  45  and the throttle body  47 . A downstream end of the connecting tube  49  is connected to an upstream end of the throttle body  47 . The air cleaner  50  is connected to an upstream end of the connecting tube  49  and arranged below the riding seat  37 . 
     As shown in  FIG. 4 , the crankcase  28  is configured by connecting a left case half  28 L arranged on the left side in a vehicle width direction and a right case half  28 R arranged on the right side in the vehicle width direction, and in the crankcase  28 , a crankshaft  51  having an axis extended in the vehicle width direction is rotatably supported. 
     In the crankcase  28 , a crank chamber  53  housing a principal part of the crankshaft  51  and a transmission chamber  54  housing the transmission  52  are formed with a partition wall  55  interposed between both chambers, and in the cylinder body  29 , a cylinder bore  56  communicating with the crank chamber  53  is formed. 
     A generator  58  driven by the crankshaft  51  and a clutch  59  interposed between the crankshaft  51  and the transmission  52  are arranged respectively on opposite sides in an axial direction of the crankshaft  51 , and in this embodiment, the generator  58  is arranged on the left side in the axial direction of the crankshaft  51 , and the clutch  59  is arranged on the right side in the axial direction of the crankshaft  51 . 
     Also referring to  FIG. 5 , the generator  58  is provided with a rotor  60  fixed to the crankshaft  51  and a stator  61  arranged in the rotor  60 , and the stator  61  is fixed to a crankcase cover  62  fastened to the left case half  28 L of the crankcase  28  with the crankcase cover  62  covering the generator  58 . 
     Paying attention to  FIG. 4 , a power transmission mechanism  64  for starting that transmits torque for starting from a starter motor  63  to the crankshaft  51  is arranged between the cylinder bore  56  of the cylinder body  29  and the generator  58  in the axial direction of the crankshaft  51 , and the power transmission mechanism  64  for starting is covered with the crankcase cover  62  together with the generator  58  from the opposite side to the clutch  59 . 
     The starter motor  63  is arranged in the upper portion of the crankcase  28  at the back of the cylinder body  29  in the longitudinal direction of the vehicle, and the power transmission mechanism  64  for starting is provided with a driving gear  66 , a first intermediate gear  67 , a second intermediate gear  68 , a third intermediate gear  69 , and a driven gear  70 . The driving gear  66  is provided to a motor shaft  65  of the starter motor  63 , the first intermediate gear  67  is engaged with the driving gear  66 , the second intermediate gear  68  is turned together with the first intermediate gear  67  and arranged coaxially with the first intermediate gear  67 , the third intermediate gear  69  is engaged with the second intermediate gear  68 , and the driven gear  70  is engaged with the third intermediate gear  69 . A one-way clutch  71  is interposed between the driven gear  70  and the rotor  60  of the generator  58 , the driven gear  70  being coaxially arranged with the crankshaft  51 , and a needle bearing  72  is inserted between the crankshaft  51  and the driven gear  70 . 
     The second intermediate gear  68  is formed to have a smaller diameter than that of the first intermediate gear  67  and integrated with an intermediate rotating shaft  73  having axial opposite ends rotatably supported by the left case half  28 L of the crankcase  28  and the crankcase cover  62 . The first intermediate gear  67  is fixed to the intermediate rotating shaft  73  in a position sandwiching the second intermediate gear  68  between the first intermediate gear  67  and the left case half  28 L. The third intermediate gear  69  is turnably supported by a supporting shaft  74  having one end press-fitted to and supported on the left case half  28 L of the crankcase  28 , and a disc-shaped regulating plate  75  sandwiching the third intermediate gear  69  between the regulating plate  75  and the left case half  28 L of the crankcase  28  is fixed to the other end of the supporting shaft  74 . 
     Referring to  FIG. 4  again, the transmission  52  is provided with gear trains of a plurality of gear shift stages capable of being selectively established, for example first to fifth speed gear trains G 1 , G 2 , G 3 , G 4 , G 5 , between an input shaft  77  having an axis parallel to the crankshaft  51  and rotatably supported by the left and right case halves  28 L,  28 R and the output shaft  32  having an axis parallel to the input shaft  77  and rotatably supported by both case halves  28 L,  28 R, and the drive chain-driving sprocket  33  is fixed to a protruded end of the output shaft  32  protruded from the left case half  28 L. 
     Respective right ends in the vehicle width direction of the crankshaft  51  and the input shaft  77  are protruded from the right case half  28 R of the crankcase  28 , the clutch  59  for switching connection/disconnection of power between the crankshaft  51  and the input shaft  77  is mounted on the right end of the input shaft  77 , and rotational power of the crankshaft  51  is transmitted to a clutch outer  78  of the clutch  59  via a primary speed-reduction device  79  and a damper  80 . The primary speed-reduction device  79  is configured with a primary driving gear  81  fixed to the right end of the crankshaft  51  and a primary driven gear  82  engaged with the primary driving gear  81  and relatively turnably supported by the input shaft  77 , and the primary driven gear  82  is coupled to the clutch outer  78  relatively turnably supported by the input shaft  77  via the damper  80 . 
     A cam chain-driving sprocket  84  for engaging a cam chain  83  therewith is arranged between the cylinder bore  56  and the clutch  59  in the axial direction of the crankshaft  51 , in this embodiment, between the primary speed-reduction device  79  and the cylinder bore  56 , and the cam chain-driving sprocket  84  is integrated with the crankshaft  51 . 
     A right crankcase cover  86  having an opening  85  opposite to a portion of the clutch  59  is fastened to the right case half  28 R so as to cover a portion of the primary speed-reduction device  79 , and a clutch cover  87  covering the clutch  59  is fastened to the right crankcase cover  86  so as to close the opening  85 . 
     The clutch  59  is provided with the clutch outer  78 , plural friction plates  88 , a clutch inner  89 , plural friction plates  90 , a pressure receiving plate  91 , a pressing member  92 , a clutch spring  93 , and a lifter  95 . The clutch outer  78  is relatively turnably supported by one end of the input shaft  77 , the plural friction plates  88  are engaged with the clutch outer  78 , the clutch inner  89  is relatively non-turnably connected to the input shaft  77 , the plural friction plates  90  are arranged alternately with the friction plates  88  and engaged with the clutch inner  89 , the pressure receiving plate  91  is opposite to the axially inside friction plate out of the alternately arranged friction plates  88 ,  90 , the pressure receiving plate  91  being integrated with the clutch inner  89 , the pressing member  92  holds the alternately arranged friction plates  88 ,  90  between the pressing member  92  and the pressure receiving plate  91 , the clutch spring  93  biases the pressing member  92  in a direction where the friction plates  88 ,  90  are fixed by applying pressure between the clutch spring  93  and the pressure receiving plate  91 , and the lifter  95  is coupled to an inner peripheral portion of the pressing member  92  via a release bearing  94 . 
     The input shaft  77  of the transmission  52  is formed in the shape of a cylinder having a hollow inside, a pressing rod  96  having one end abutting against the lifter  95  is axially movably inserted into the input shaft  77 , and the clutch  59  disconnects transmission of power between the clutch outer  78  and the clutch inner  89  when the lifter  95  is pressed by the pressing rod  96 . 
     The pressing rod  96  is axially driven by turning a release rod  98  of a clutch release lever  97  (see  FIG. 2 ) and the release rod  98  coupled to the other end of the pressing rod  96  via a cam mechanism  99  is turnably supported by the crankcase cover  62 . 
     As also shown in  FIGS. 6 to 8 , the cylinder head  30  and the cylinder body  29  are fastened to the crankcase  28  by a plurality of through bolts  118  inserted through the cylinder head  30  and the cylinder body  29  and screwed into the crankcase  28 . Besides, in the cylinder head  30 , there are disposed a pair of intake valves  100  arranged in the vehicle width direction and a pair of exhaust valves  101  arranged in the vehicle width direction in front of the intake valves  100  in the longitudinal direction of the vehicle so that the intake valves  100  and the exhaust valves  101  can be opened and closed, and a valve chamber  103  for housing a valve train  102  that opens/closes the intake valve  100  and the exhaust valve  101  is formed between the cylinder head  30  and the head cover  31 . 
     The pair of exhaust ports  40  respectively correspond to the pair of exhaust valves  101  and are formed in the cylinder head  30  mutually independently so as to open to a front wall  30   a  of the cylinder head  30 , the pair of exhaust ports  40  being arranged on opposite sides of a center line of a vehicle body BC passing the center in the vehicle width direction of the down frame  15  as seen in a plan view. 
     The valve train  102  is provided with an intake side camshaft  104 , an exhaust side camshaft  105 , a pair of intake side rocker arms  106 , and a pair of exhaust side rocker arms  107 . The intake side camshaft  104  and the exhaust side camshaft  105  are extended in the vehicle width direction in positions apart in the longitudinal direction of the vehicle, the pair of intake side rocker arms  106  are swung according to turning of the intake side camshaft  104  so as to open and close the intake valve  100 , and the pair of exhaust side rocker arms  107  are swung according to turning of the exhaust side camshaft  105  so as to open and close the exhaust valve  101 . 
     The intake side camshaft  104  and the exhaust side camshaft  105  are turnably supported by a shaft supporting portion  108  integrated with the cylinder head  30  and a cam holder  109  fastened to the shaft supporting portion  108 . The cam holder  109  is fastened to the shaft supporting portion  108  of the cylinder head  30  by plural, for example, eight cam holder joining bolts  110 . Rotational axes CI, CE of the intake side camshaft  104  and the exhaust side camshaft  105  respectively turnably supported by the shaft supporting portion  108  and the cam holder  109  are located on the same plane as the joining surface  38  of the cylinder head  30  to the head cover  31 , that is, on the second plane PL 2 . 
     A base of the intake side rocker aim  106  is arranged in a slit  111  formed in the shaft supporting portion  108  of the cylinder head  30  in front of the intake side camshaft  104  in the longitudinal direction of the vehicle and swingably supported by an intake side rocker shaft (not shown) fixed to the shaft supporting portion  108 . Besides, a base of the exhaust side rocker arm  107  is arranged in a slit  112  formed in the shaft supporting portion  108  of the cylinder head  30  in front of the exhaust side camshaft  105  in the longitudinal direction of the vehicle and swingably supported by an exhaust side rocker shaft (not shown) fixed to the shaft supporting portion  108 . 
     Driven sprockets  113 ,  114  are fixed to respective protruded ends of the intake side camshaft  104  and the exhaust side camshaft  105  respectively protruded from a right end in the vehicle width direction of the cam holder  109 , and the cam chain  83  that transmits rotational power to the intake side camshaft  104  and the exhaust side camshaft  105  is wound onto the driven sprockets  113 ,  114 . In addition, a cam chain passage  115  through which the cam chain  83  is traveled is formed in the crankcase  28 , the cylinder body  29  and the cylinder head  30 . Additionally, a curve cover  116  curved to cover the driven sprockets  113 ,  114  is formed on the right side in the vehicle width direction of the head cover  31 . 
     A plug hole  120  for inserting thereinto an ignition plug (not shown) attached to the cylinder head  30  is provided so as to straddle between the cylinder head  30  and the head cover  31  connected to the cylinder head  30 . The plug hole  120  is configured with a plug opening  121 , a cam holder side tube portion  122 , a head cover side tube portion  123  and a through hole  124 . The plug opening  121  is formed in the shaft supporting portion  108  of the cylinder head  30 , the cam holder side tube portion  122  is integrated with the cam holder  109  so as to be connected to the plug opening  121 , and the head cover side cylinder portion  123  is integrally provided in the head cover  31  so that a gasket  125  is interposed between the head cover side tube portion  123  and an upper end of the cam holder side tube portion  122 , and the through hole  124  is provided to a ceiling wall  31   a  of the head cover  31  so as to be connected to the head cover side tube portion  123 . 
     A portion of the gasket  125  interposed between the cam holder side tube portion  122  and the head cover side tube portion  123  is integrally connected to a portion interposed between the cylinder head  30  and the head cover  31 . 
     A stepped portion  120   a  opposite to the head cover  31  side is formed in a portion of the plug hole  120  corresponding to the cam holder side tube portion  122 . The two cam holder joining bolts  110  arranged between the intake side camshaft  104  and the exhaust side camshaft  105  out of the eight cam holder joining bolts  110  for fastening the cam holder  109  to the shaft supporting portion  108  of the cylinder head  30  are each inserted into the stepped portion  120   a.    
     A breather plate  127  is fastened to the head cover  31 , a breather chamber  128  encircling the plug hole  120  is formed between the breather plate  127  and the ceiling wall  31   a  of the head cover  31 , and a lead-out pipe  129  for leading out breather gas from the breather chamber  128  is installed in a rear wall  31   b  of the head cover  31 . 
     The throttle body  47  is provided with a butterfly throttle valve  130  and an axis of a valve stem  131  of the throttle valve  130  is arranged on the second plane PL 2  on which the joining surface  38  of the cylinder head  30  to the head cover  31  is arranged. In addition, a throttle drum  132  is fixed to one end on the right side in the vehicle width direction of the valve stem  131 , the throttle drum  132  is housed in a drum housing box  135  configured with a body  133  of the throttle body  47  and a lid member  134  fastened to the body  133  from the right side in the vehicle width direction, the drum housing box  135  is arranged close to the curve cover  116  of the head cover  31 , and throttle cables  137 ,  138  wound onto the throttle drum  132  are extended upward from the drum housing box  135 . 
     As also shown in  FIGS. 9, 10 , a relief recess  136  corresponding to the drum housing box  135  is formed on the rear wall  31   b  of the head cover  31  and the relief recess  136  is arranged between the cam chain passage  115  and the lead-out pipe  129  when viewed from a direction along the cylinder axis C. 
     The exhaust pipe  42  is configured with a first pipe member  140  and a second pipe member  141 . The first pipe member  140  has an upstream end connected to the exhaust port  40  of the cylinder head  30 , and the second pipe member  141  is connected to a downstream end of the first pipe member  140  via a joint  142  and extended backward, the second pipe member  141  having a downstream end connected to the exhaust muffler  43 . 
     The first pipe member  140  of the exhaust pipe  42  is integrally provided with a curved pipe portion  140   a  and an inclined linear pipe portion  140   b.  The curved pipe portion  140   a  has an upstream end connected to the exhaust port  40  and is bent in a U shape beside the down frame  15 . The inclined linear pipe portion  140   b  has an upstream end connected to a downstream end of the curved pipe portion  140   a  and is extended backward so as to be inclined to be located inside in the vehicle width direction as going toward the rear. The inclined linear pipe portion  140   b  and the second pipe member  141  are arranged in such a manner that they pass between the pair of right and left pivot frames  17 . 
     In addition, a cam chain tensioner  143  that adjusts tension of the cam chain  83  is provided to the rear wall  30   b  on the right side in the vehicle width direction of the cylinder head  30 , and an inclined face  143   a  along the inclined linear pipe portion  140   b  of the exhaust pipe  42  on the right side in the vehicle width direction out of the pair of exhaust pipes  42  is formed in a protruded portion of the cam chain tensioner  143  protruded from the rear wall  30   b  of the cylinder head  30 . 
     Besides, the second pipe members  141  of the pair of right and left exhaust pipes  42  are extended backward, passing respectively between the pair of right and left pivot frames  17  and the rear cushion unit  27 , and the exhaust muffler  43  is individually connected to the downstream end of the exhaust pipe  42 , that is, to the downstream end of the second pipe member  141 . 
     Moreover, the down frame  15  has a pair of side walls  15   a  that are inclined so as to approach each other as going toward the rear in the longitudinal direction of the vehicle, while being opposite to the curved pipe portion  140   a  of the exhaust pipe  42 , and the center of the down frame  15 , the cylinder axis C and the center of the intake port  44  are arranged on the center line of the vehicle body BC as seen in the plan view. 
     Next, the operation of this embodiment will be described. The joining surface  38  of the cylinder head  30  to the head cover  31  in the single-cylinder engine main unit  20  mounted on the body frame F is formed on the second plane PL 2  that crosses the cylinder axis C and is inclined downward toward the front, and the rotation axes CI, CE of the intake side and exhaust side camshafts  104 ,  105  respectively configuring a part of the valve train  102  that opens and closes the pair of intake valves  100  and the pair of exhaust valves  101  respectively supported by the cylinder head  30  are arranged on the second plane PL 2 . Besides, the axis of the valve stem  131  of the butterfly throttle valve  130  provided to the throttle body  47  configuring a part of the intake system  46  connected to the rear wall  30   b  of the cylinder head  30  is arranged on the second plane PL 2 , the curve cover  116  curved to cover the driven sprockets  113 ,  114  that are respectively fixed to the intake side and exhaust side camshafts  104 ,  105  so that the cam chain  83  for transmitting power to the intake side and exhaust side camshafts  104 ,  105  from the crankshaft  51  is wound onto the driven sprockets  113 ,  114  is formed on one side in the vehicle width direction of the head cover  31 , and the drum housing box  135  that houses the throttle drum  132  fixed to one end of the valve stem  131  and is annexed to the throttle body  47  is arranged close to the curve cover  116 . For the reasons described above, the throttle valve  130  can be brought closer to the engine main unit  20 , concentration and compactness of the intake system  46  can be achieved, the intake system  46  can be made a greatly inclined downdraft type, accordingly, the output of the internal combustion engine E can be enhanced, and besides, the handling of the throttle cables  137 ,  138  extended from the drum housing box  135  can be also facilitated. 
     Besides, as the relief recess  136  corresponding to the drum housing box  135  is formed on the rear wall  31   b  of the head cover  31 , the drum housing box  135  can be brought closer to the head cover  31  side and further concentration and compactness of the intake system  46  can be achieved. 
     Moreover, as the cam chain passage  115  through which the cam chain  83  is traveled is formed in the engine main unit  20 , the lead-out pipe  129  for leading out breather gas is installed in the rear wall  31   b  of the head cover  31 , and the relief recess  136  is arranged between the cam chain passage  115  and the lead-out pipe  129  when viewed from the direction along the cylinder axis C, a dead space can be effectively utilized. 
     In addition, as the engine main unit  20  is arranged in such a manner that it is encircled by the head pipe  13 , the main frames  14 , the pivot frames  17  and the down frame  15  as seen in the side view, and the pair of exhaust ports  40  respectively corresponds to the pair of exhaust valves  101  that are arranged in the cylinder head  30  so as to be able to be opened and closed, the pair of exhaust ports  40  being open to the front wall  30   a  of the cylinder head  30  and mutually independently formed in the cylinder head  30  so that the exhaust ports  40  are arranged on opposite sides of the center line of the vehicle body BC passing the center in the vehicle width direction of the down frame  15  as seen in the plan view, a curve of the exhaust port  40  is reduced so that the output is enhanced, and the cylinder head  30  can be arranged close to the down frame  15 , readily avoiding interference with the exhaust pipes  42  connected to the pair of exhaust ports  40 . 
     Further, the upstream ends of the exhaust pipes  42  respectively connected to the pair of exhaust ports  40  while configuring a part of the exhaust system  41  are connected to the exhaust ports  40 , and each of the exhaust pipes  42  is integrally provided with the curved pipe portion  140   a  bent in the U shape beside the down frame  15  and having the upstream end connected to the exhaust port  40  and the inclined linear pipe portion  140   b  having the upstream end connected to the downstream end of the curved pipe portion  140   a,  the inclined linear pipe portion  140   b  being extended backward and inclined so as to be located inside in the vehicle width direction as going toward the rear. Besides, the inclined linear pipe portions  140   b  are arranged in such a manner as passing between the pair of right and left pivot frames  17  and the inclined face  143   a  along the inclined linear pipe portion  140   b  of one of the pair of exhaust pipes  42  is formed in the protruded portion of the cam chain tensioner protruded from the rear wall  30   b  of the cylinder head  30 , the cam chain tensioner  143  being provided to the rear wall  30   b  of the cylinder head  30  so as to adjust the tension of the cam chain  83 . For the reasons described above, compact arrangement of the pair of exhaust pipes  42  is enabled, avoiding interference with the cam chain tensioner  143 . 
     Furthermore, as the rotor  60  of the generator  58  is fixed to one end of the crankshaft  51 , the cam chain-driving sprocket  84  for engaging therewith the cam chain  83  is provided on the other end side of the crankshaft  51  in a portion where the cylinder bore  56  of the cylinder body  29  is sandwiched between the portion and the generator  58 , and the power transmission mechanism  64  for starting that transmits, to the crankshaft  51 , power for starting from the starter motor  63  attached to the crankcase  28  is arranged between the generator  58  and the cylinder bore  56 , a sideway overhang of the cam chain passage  115  from the cylinder bore  56  is inhibited, the exhaust pipes  42  can be arranged closer to the center side of the vehicle body beside the cylinder body  29  and the cylinder head  30 , and the more compact arrangement of the exhaust pipes  42  is enabled. 
     Furthermore, as the front end of the swing arm  22  that has the rear end pivotally supporting the rear wheel WR is swingably supported by the pivot frames  17  via the pivot  23 , the rear cushion unit  27  that buffers the swing of the swing aim  22  is extended up and down and arranged between the pair of right and left pivot frames  17 , the pair of right and left exhaust pipes  42  are extended backward, respectively passing between the pair of right and left pivot frames  17  and the rear cushion unit  27 , and the exhaust muffler  43  is individually connected to the downstream end of each of the exhaust pipes  42 , the right and left exhaust pipes  42  are made similar and symmetrical, and a part of materials configuring the exhaust pipes  42  can be shared for the right and left exhaust pipes  42 . 
     Furthermore, as the down frame  15  is provided with the pair of side walls  15   a  that are inclined so as to approach each other as going toward the rear in the longitudinal direction of the vehicle while being opposite to the curved pipe portion  140   a  of the exhaust pipe  42 , a space for arranging the exhaust pipes  42  can be readily secured on opposite sides of the down frame  15 , and the curve of the exhaust pipe  42  is reduced so that exhaust performance can be enhanced. 
     Furthermore, as the center in the vehicle width direction of the down frame  15 , the cylinder axis C and the center of the intake port  44  open to the rear wall  30   b  of the cylinder head  30  are arranged on the center line of the vehicle body BC as seen in the plan view, an influence of bias in the right-left direction of intake and exhaust is inhibited to be small so that the output can be enhanced. 
     The embodiment of the present invention has been described above, however, the present invention is not limited to the above-mentioned embodiment and various design variations can be made without deviating from the gist of the present invention.