Patent Publication Number: US-6708652-B2

Title: Engine valve timing apparatus

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2000-403155 filed in Japan on Dec. 28, 2000, the entirety of which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an engine valve timing transmission apparatus, and more particularly, to an apparatus where a timing chain is arranged around a drive sprocket and a driven sprocket respectively fixed to a crankshaft and a valve camshaft. An end of a tensioner, press-contacted with an outer side surface of the timing chain to apply tension to the chain is swingably attached to an engine main body via a pivot pin. 
     2. Description of the Background Art 
     A similar engine valve timing transmission apparatus is described in Japanese Published Unexamined Patent Application No. Hei 7-71543. In this type of engine valve timing transmission apparatus, a pivot pin of a tensioner is held between junction surfaces of a crankcase divided in two halves to rotatably hold a crankshaft. 
     However, the pivot pin is too close to a timing chain, and the tensioner cannot be provided in an optimum position for tensioning of the timing chain. Further, since the pivot pin must be attached between the junction surfaces of the crankcase when the crankshaft is held between the two halves of the divided crankcase, assembly is difficult and unreliable. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings associated with the background art and achieves other advantages not realized by the background art. 
     An object of the present invention is to provide an engine valve timing transmission apparatus which increases freedom of attachment positions of the pivot pin. 
     A further object of the present invention is to enable placement of the chain tensioner in an optimum position for easy, reliable tensioning of the timing chain. 
     A further object of the present invention is to enable attachment of the pivot pin after assembly of the crankcase in order to improve the ease of assembly. 
     These and other objects are accomplished by an engine valve timing transmission apparatus comprising a timing chain engaging with a drive sprocket and a driven sprocket respectively fixed to a crankshaft and a valve camshaft; a chain tensioner, the chain tensioner having a first end press-contacted with an outer side surface of the timing chain to apply tension to the timing chain and swingably attached to an engine main body via a pivot pin; a timing chamber formed in a side wall of the engine main body for accommodating the timing chain; an opening having a diameter greater than the drive sprocket is provided on a side of the drive sprocket on an outer side wall of the timing chamber; and a lid plate for covering the opening is removably secured to the engine main body. 
     The pivot pin is held between opposite walls of the engine main body and the lid plate. The pivot pin can be reliably supported by the engine main body and the lid plate without special dropping preventing means. Further, the freedom of attachment position of the pivot pin in opposite walls of the engine main body and the lid plate increases, the pivot pin can be provided in a desired position, and the chain tensioner can be provided in an optimum position for tension of the timing chain. Furthermore, since the attachment of the pivot pin is made upon attachment of the lid plate, e.g. after assembly of the engine main body, ease of reliable assembly is improved. 
     Further, according to a second feature of the present invention, the engine main body is constructed with a cylinder block, a first crankcase half body connected to one end of the cylinder block, and a second crankcase half body connected to and in cooperation with the first crankcase half body and rotatably holding the crankshaft. The timing chamber is formed from the cylinder block to the second crankcase half body. The opening is provided over the first and second crankcase half bodies, and the pivot pin is held between opposite walls of the lid plate to close the opening and the second crankcase half body. 
     The pivot pin can be provided sufficiently away from the drive sprocket to the opposite side of the driven sprocket. Accordingly, a sufficient length of the chain tensioner can be ensured. The timing chain can be held under an approximately constant tension without influence by the extension of the chain, and the durability of the timing chain can be improved. 
     Further, according to a third feature of the present invention, a stator of a generator driven by the crankshaft is fixed to the lid plate. The lid plate also serves as a support base of the stator of the generator. This third feature permits a reduction in the number of parts. 
     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 hereinafter 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 scooter type motorcycle according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  in FIG. 1; 
     FIG. 3 is a side view taken along arrows  3 — 3  in FIG. 2; 
     FIG. 4 is a side view corresponding to the view of FIG. 3 shown without a radiator cover; 
     FIG. 5 is a cross-sectional view taken along line  5 — 5  in FIG. 3; 
     FIG. 6 is a side view corresponding to the view of FIG. 3 shown without a radiator and a generator; 
     FIG. 7 is a cross-sectional view taken along line  7 — 7  in FIG. 2; and 
     FIG. 8 is a cross-sectional view taken along line  8 — 8  in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described with reference to the accompanying drawings. FIG. 1 is a side view of a scooter type motorcycle according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line  2 — 2  in FIG.  1 . FIG. 3 is a side view taken along arrows  3 — 3  in FIG.  2 . FIG. 4 is a side view corresponding to the view of FIG. 3 shown without a radiator cover. FIG. 5 is a cross-sectional view taken along line  5 — 5  in FIG.  3 . FIG. 6 is a side view corresponding to the view of FIG. 3 shown without a radiator and a generator. FIG. 7 is a cross-sectional view taken along line  7 — 7  in FIG.  2 . FIG. 8 is a cross-sectional view taken along line  8 — 8  in FIG. 6. A working example according to an embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. 
     In FIG. 1, a vehicle body frame F of a scooter type motorcycle V having a front wheel Wf steered by a steering handle  11 , and a rear wheel Wr driven by a swing type power unit P, is divided into three parts: a front frame  12 , a center frame  13  and a rear frame  14 . The front frame  12  includes an aluminum alloy casted member integrally provided with a head pipe  12   a , a down tube  12   b  and a step floor  12   c.  The center frame  13 , on which the power unit P can be upwardly-and-downwardly swung via a pivot pin  15 , also includes an aluminum alloy casted member. The center frame  13  is connected to a rear end of the front frame  12 . The rear frame  14 , extending at a rear and upper position from the power unit P, includes a ring-shaped pipe member. 
     A fuel tank  16  is supported by the rear frame  14  such that the tank is surrounded by the rear frame  14 . A helmet case  17  is supported on an upper surface of the center frame  13 , and the helmet case  17  is openably/closably covered with a lid  19  integrally provided with a seat  18 . 
     The power unit P has a water-cooling, monocylinder 4-cycle engine E and a belt-type continuously variable transmission T extending from a left side surface of the engine E toward the rear of the vehicle body. A rear upper surface of the continuously variable transmission T is connected to a rear end of the center frame  13  via a rear cushion (shock absorber)  20 . An air cleaner  21  is supported on an upper surface of the continuously variable transmission T, a muffler  22  is supported on a right side surface of the continuously variable transmission T, and a main stand  23  which can be set upright and tilted downward is supported on a lower surface of the engine E. 
     In FIG.  2  through FIG. 4, an engine main body  25  of the engine E has an engine block  32  and a rear crankcase half body  33   b  divided by a dividing surface extending in upward and rearward directions along an axial line of the crankshaft  31 . The engine block  32  is integrally provided with a cylinder block  32   a  having a cylinder bore  41 , and a front crankcase half body  33   a . The front crankcase half body  33   a  and the rear crankcase half body  33   b  form the crankcase  33 . A cylinder head  34  is connected to a front end of the engine block  32 , and a head cover  35  is connected to a front end of the cylinder head  34 . 
     This engine main body  25  is mounted on the vehicle body frame F approximately along frontward and rearward directions of the vehicle body frame F, with a front part of an axial line L of the cylinder bore  41  being slightly raised off a centerline. A bracket  27  provided in an upper part of the engine block  32  is swingably connected to the pivot pin  15  fixed to the center frame  13  of the vehicle body frame F via a rubber mount  28 . 
     The continuously variable belt-type transmission T has a right casing  37  and a left casing  38  connected to each other. A front right side surface of the right casing  37  is connected to a left side surface of the front and rear crankcase half bodies  32  and  33 . Further, a deceleration casing  39  is connected to a rear right side surface of the right casing  37 . 
     A piston  42  slidably engaged in the cylinder bore  41  of the engine block  32  is connected to the crankshaft  31  via a connecting rod  43 . A camshaft  44  is rotatably supported in the cylinder head  34 . An intake valve and an exhaust valve (not shown) provided in the cylinder head  34  are opened/closed by the camshaft  44 . 
     As shown in FIG. 2, and FIG.  5  through FIG. 8, a timing chamber  40  is formed in one side wall of the crankcase  33 , cylinder block  32   a  and the cylinder head  34 . The timing chamber  40  accommodates an endless timing chain  45 , placed around a drive sprocket  46  provided on the crankshaft  31  and a driven sprocket  47  provided on the camshaft  44 . The drive sprocket  46 , the driven sprocket  47  and the timing chain  45  forms a timing transmission apparatus Ti to reduce the rotation of the crankshaft  31  by half and to transmit this rotational force to the camshaft  44 . The camshaft  44  then opens/closes the intake and exhaust valves (not shown) by its controlled rotation. 
     Further, a chain guide  48  for guiding the running of the timing chain  45  on the tension side, and a chain tensioner  49  for applying tension to the timing chain  45  on the loose side, are also provided in the timing chamber  40 . The chain guide  48  is bent into an arc shape. One end of the chain guide is received by a shelf-shaped support  113  on an inner wall of the rear crankcase half body  33   b  and the other end of the chain guide is swingably attached to the cylinder block  32   a  with a bolt  36  such that the chain guide is in sliding contact with the outer side surface of the timing chain  45  on the tension side, approximately over the entire length. 
     The chain tensioner  49  is also bent to have an arc shape, with a curvature greater than that of the chain guide  48 . The chain tensioner  49  mainly presses a central portion of the outer side surface of the timing chain  45  on the loose side. A bush  50  connected to one end of the tensioner via a ring-shaped elastic member  51  is rotatably supported by the pivot pin  112  provided in a position away from the drive sprocket  46  to the opposite side to the driven sprocket  47  (an attaching structure of the pivot pin will be described later), and the other end of the chain tensioner  49  is a free end. A lifter  52  to press a central portion rear surface of the chain tensioner  49  against the timing chain  45  side by a constant pressing force is provided in the cylinder block  32   a . Thus, the timing chain  45  is provided with a constant tension from the chain tensioner  49 . 
     The front and rear crankcase half bodies  33   a  and  33   b  are provided with an opening  53  having a diameter sufficiently greater than the drive sprocket  46  around the crankshaft  31  in a portion corresponding to an outer side wall of the timing chamber  40 . The timing chain  45  is secured in a position around the drive sprocket  46  through the opening  53 . A lid plate  73  fixed to the front and rear crankcase half bodies  33   a  and  33   b  with a plurality of bolts  74  closes the opening  53 . 
     An O-ring  110  is inserted between junction surfaces of both crankcase half bodies  33   a ,  33   b  and the lid plate  73 . An oil seal  111  in tight contact with an outer peripheral surface of the crankshaft  31  is applied to a through hole  73   a  of the lid plate  73  formed through the crankshaft  31 . Accordingly, the timing chamber  40  is securely maintained with an oil seal. 
     The attachment structure of the pivot pin  112  supporting the chain tensioner  49  will be described hereinafter. As clearly shown in FIG.  6  through FIG. 8, a pair of support bosses  125  and  126  with bottomed holes  125   a  and  126   a  opposite to each other in desired position of the pivot pin  112  are integrally projected from opposite walls of the lid plate  73  and the crankcase  33 . The bush  50  of the chain tensioner  49  is provided between both support bosses  125  and  126 , and both ends of the pivot pin  112  rotatably inserted through the bush  50  are engaged with the bottomed holes  125   a  and  126   a  of the support bosses  125  and  126 . These bottoms prevent movement of the pivot pin  112  in an axial direction. 
     Accordingly, the pivot pin  112  is reliably supported by the crankcase  33  and the lid plate  73  without special dropping prevention means. Further, as the crankcase  33  and the lid plate  73  are arranged in positions opposite to each other in a wide range around the drive sprocket  46 , the freedom of arrangement for the support bosses  125  and  126  supporting the pivot pin  112  increases. The pivot pin  112  can be supported at a desired position, and the chain tensioner can be provided in an optimum position for tensioning of the timing chain. 
     As in the illustrated example, if the support bosses  125  and  126  are provided on opposite walls of the rear crankcase half body  33   b  and the lid plate  73 , the pivot pin  112  supported by the bosses can be provided sufficiently away from the drive sprocket  46  to the opposite side to the driven sprocket  47 . As a result, a sufficient length of the chain tensioner  49 , e.g. a sufficient distance between the pivot pin  112  and the lifter  52 , is ensured. The timing chain  45  can be held under an approximately constant tension without influence by the extension of the chain, thus the durability of the timing chain  45  can be improved. 
     Further, since the attachment of the pivot pin  112  is performed upon attachment of the lid plate  73  to the front and rear crankcase half bodies  33   a  and  33   b , after connection of the front and rear crankcase half bodies holding the crankshaft  31 , assembly can be easily made. 
     In FIG. 8, a bearing boss  127  projecting toward the timing chamber  40  side is integrally formed with the lid plate  73  and rotatably supports a rotor shaft  129  of an oil pump  128 . In this manner, the lid plate  73  also serves as a support member of the rotor shaft  129 , which contributes to a reduction of the number of required parts and assembly steps of the oil pump  128 . The rotor shaft  129  is driven from the crankshaft  31  via a large-diameter gear  130  fixed to the rotor shaft and a small-diameter gear  131  integrally connected to one end of the drive sprocket  46 . 
     In FIG. 5, a rotor  69  is fixed to the right end side of the crankshaft  31 . A stator  70 , forming an alternator  68  in combination with the rotor  69 , is fixed to the lid plate  73  with plural bolts  74 . The rotor  69  surrounds the stator  70 . Accordingly, the lid plate  73  also serves as an attachment base of the stator  70 , and contributes to the reduction of the number of parts. 
     A cooling fan  71  is fixed to a right end of the crankshaft  31  in a position exterior to the alternator  68 . A radiator  72  is provided so as to hold the cooling fan  71  between the radiator  72  and the alternator  68 . The radiator  72  is attached to the engine main body  25  via a shroud  81  surrounding the cooling fan  71 . 
     The radiator  72  is constructed with upper and lower tanks  77  and  78 , provided at an interval, and a radiating core  79  connecting these tanks  77  and  78  while mutually communicating with the inside of these tanks. The radiating core  79  is made of metal having high radiation qualities. Respective pairs of connection projection pieces  101 ,  101 ;  102 ,  102  are projected leftward and rightward from both upper and lower ends. The upper connection projection pieces  101 ,  101  are swaged with both left and right ends of the upper tank  77  having its lower surface opened, with seal members  103 ,  103  held therebetween. The lower connection projection pieces  102 ,  102  are swaged with both left and right ends of the lower tank  78  having its upper surface opened, with seal members  104 ,  104  held therebetween. The upper and lower tanks  77  and  78  are formed of synthetic resin. 
     Connection flanges  105  and  106  are integrally formed with the upper and lower tanks  77  and  78 , and one end of the shroud  81  of elastic material such as synthetic resin is fixed to these flanges with a plurality of rivets  107 . A connection flange  81   a  is integrally formed with the other end of the shroud  81 , and the connection flange  81   a  is fixed to the engine main body  25  with a plurality of bolts  108 . 
     The outer periphery of the radiator  72  is covered with a radiator cover  75  of synthetic resin fixed to the shroud  81  with a plurality of screws  109 . A grille  75   a  integrally formed with the radiator cover  75  is provided in a position opposite to a front surface of the radiating core  79 . A cooling wind is introduced from the outside through the grille  75   a  to the radiating core  79 . 
     Referring to FIG.  6  and FIG. 8, plural discharge ports  76  are provided in the shroud  81  at the side of the cooling fan  71 . Upon actuation of the cooling fan  71 , air introduced from the grille  75   a  is passed through the radiating core  79  of the radiator  72 , and the radiating core  79  is cooled down. The air is discharged from the discharge ports  76  to the outside. Thus, cooling water in the radiator  72  is also cooled. 
     The radiator  72  forms a part of a cooling device  83  to circulate cooling water in a water jacket  82  provided in the cylinder block  32   a  of the engine block  32  and the cylinder head  34  in the engine main body  25 . The cooling device  83  includes a water pump  84  for supplying cooling water to the water jacket  82 . The radiator  72  is inserted between the water jacket  82  and an intake port of the water pump  84 . A thermostat  85  selects an operating state for either restoring the cooling water from the water jacket  82  to the water pump  84  (thereby avoiding the radiator  72 ) or restoring the cooling water from the water jacket  82  through the radiator  72  to the water pump  84  in accordance with the cooling water temperature. 
     A thermostat case  86  accommodating the thermostat  85  is connected to a right side surface of the cylinder head  34 . The water pump  84  is provided on a right end of the camshaft  44  and is accommodated in a space surrounded by the cylinder head  34  and the thermostat case  86 . 
     An upwardly-extending water supply port tube  87  is integrally provided with one end of the upper tank  77  along the frontward and rearward directions of the vehicle body frame F (in this embodiment, a rear end). A water supply cap  88  opened/closed by rotational operation is attached to an upper end of the water supply port tube  87 . Further, a forwardly-projecting connection pipe  89  is integrally provided with the other end of the lower tank  78  along the frontward and rearward directions of the vehicle body frame F (in this embodiment, a front end). 
     This radiator  72  is attached to the engine main body  25  as described above in a position tilted at an angle a to the axial line L of the cylinder bore  41  of the engine main body  25 . When the engine main body  25  is mounted on the vehicle body frame F, the radiator  72  is frontwardly tilted at an angle b to a horizontal plane. The water supply cap  88  is provided in the highest position in the cooling device  83  and the connection pipe  89  is provided in the lowest position in the cooling device  83 . 
     The foregoing arrangement avoids increases in cost by forming the radiator  72  in a special shape and providing a water supply cap in a tank connected to the radiator  72  and provided aside from the radiator  72 , attains a comparatively large head difference in the cooling device  83  upon water supply from the water supply port tube  87 , and improves air removal characteristics and water supply performance from the water supply port tube  87 . 
     Further, in the case where the radiator  72  is tilted at an angle a to the axial line L of the cylinder bore  41  as described above, the radiator  72  can be provided to avoid the pivot pin  25  to support the engine main body  25  on the vehicle body frame F. Further, adequate space to hold an exhaust pipe  90  connected to an exhaust port of the cylinder head  32  is ensured in the rear of the radiator  72  and the freedom of arrangement of the exhaust pipe  90  can be improved. 
     One end of a flexible first conduit  91  comprising a rubber hose or the like, to guide the cooling water in the radiator  72  to the thermostat  85  side, is connected to the connection pipe  89  of the radiator  72 . The other end of the first conduit  91  is connected to the thermostat case  86 . 
     The radiator  72  is provided in a position where at least a part (a front part in this embodiment) of the upper tank  77  is overlapped with the cylinder block  32   a  of the engine main body  25 , in a side view. A connection hole  115  connected to the inside of the upper tank  77  and a connection hole  116  connected to an exit  82  in an upper part of the water jacket  82  are provided in the upper tank  77  and the cylinder block  32   a , within a region where the upper tank  77  and the cylinder block  32   a  overlap with each other (as seen in a side view). 
     Both ends of a second conduit  92  comprising a metal pipe or the like having rigidity are engaged with these connection holes  115  and  116  along a fastening direction of the bolts  108  via seal members  117  and  118 , e.g. O-rings. The second conduit  92  is provided so as to be inserted through a through hole  119  provided in the shroud  81  in a non-contact state. Further, a gap to allow swing movement of the second conduit  92  at a slight angle while elastically deforming the seal members  117  and  118  is provided in an engagement portion between the second conduit  92  and the connection holes  115  and  116 . 
     Further, one end of a flexible third conduit  93  comprising a rubber hose or the like, to guide the cooling water from the water pump  84 , is connected to the thermostat case  86 . The other end of the third conduit  93  is connected to an entrance  82   i  in a lower part of the water jacket  82  projected from a lower surface of the cylinder block  32   a.    
     A pipe line (not shown) to guide the cooling water from the water jacket  82  so as to humidify a carburetor  95  is connected to the carburetor  95  connected to an inlet port of the cylinder head  32 . A flexible fourth conduit  96  comprising a rubber hose or the like, to guide the cooling water, which humidified the carburetor  95  to the thermostat  85 , is connected to the thermostat case  86 . 
     A flexible fifth conduit  97  comprising a rubber hose or the like, to remove air from the water pump  84 , is connected to an upper part of the thermostat case  86 . The fifth conduit  97  and a conduit (not shown) connected to the upper part of the cylinder block  32   a  to remove air from an upper part in the water jacket  82  are connected to a flexible sixth conduit  98  comprising a rubber hose or the like. The sixth conduit  98  is connected to a rear side upper part of the upper tank  77  in the radiator  72 . 
     Further, one end of a flexible seventh conduit  100  comprising a rubber hose or the like is connected to the water supply port tube  87 , and the other end of the seventh conduit  100  is connected to a reservoir (not shown), opened in atmosphere and provided aside from the radiator  72 . When the temperature of the cooling water in the radiator  72  becomes high and the water expands, excessive cooling water overflows to the reservoir. When the temperature of the cooling water in the radiator  72  becomes low, the cooling water is restored from the reservoir to the radiator  72 . By this movement of cooling water between the radiator  72  and the reservoir, air stored in the water supply port tube  87  is discharged to the reservoir. That is, adequate air removal from the cooling device  83  can be achieved even when the engine E is running. 
     Then, in a status where warming up of the engine E is complete, the cooling water discharged from the water pump  84  driven by the camshaft  44  is supplied through the thermostat case  86  and the third conduit  93  to the water jacket  82  in the engine block  32  and the cylinder head  34 . When the cooling water passes through the water jacket  82 , it cools the engine E. The cooling water then is sent via the second conduit  92  to the upper tank  77  of the radiator  72 . Then, the cooling water, the temperature of which has been lowered when the water flowed from the upper tank  77  via the cooling core  79  to the lower tank  78 , is taken into the water pump  84  via the first conduit  91  and the thermostat  85 . 
     On the other hand, when the engine E is warmed up and the temperature of the cooling water is low, the thermostat  85  is actuated to circulate the cooling water while avoiding the radiator  72 . The cooling water is circulated, without passing through the radiator  72 , through the water jacket  82 , the carburetor  95  and the water pump  84 , in order to raise the temperature rapidly. 
     Since the upper and lower tanks  77  and  78  of the radiator  72  are made of light-weight synthetic resin, the weight of the radiator  72  can be greatly reduced. Further, since the shroud  81  to guide the cooling wind that passed through the radiator  72  to the outside from the discharge ports  76  is made of elastic material and the radiator  72  is attached to the engine main body  25  via the shroud, the shroud  81  absorbs engine E vibration by its own elasticity, and prevents additional vibration from being transferred from the engine E to the radiator  72 . 
     The shroud  81  serves a role of vibration isolation by blocking transmission of vibration from the engine E to the radiator  72  in addition to its original function to guide the cooling wind from the radiator  72 . Accordingly, specialized vibration isolation means for the radiator  72  is unnecessary, and simplification of the structure and subsequent cost reduction can be attained. 
     Further, since the radiator  72  is light weight as described above, the load capacitance of the shroud  81  can be reduced. Accordingly, the thickness of the shroud  81  can be reduced, and by extension, further improvement in vibration isolation function and weight reduction can be attained. Since the radiator  72  is attached to the engine E in the power unit P, which connected to the vehicle body frame F via the pivot pin  15  and supported via the rear cushion  20 , and which swings upwardly and downwardly with the rear wheel Wr, the above-described weight reduction of the radiator  72  and the shroud  81  reduces spring load and contributes to improvement in driving feeling. 
     Again referring to FIG. 2, a drive pulley  54  is provided at a left end of the crankshaft  31  projecting inside the right casing  37  and the left casing  38 . The drive pulley  54  has a fixed pulley half body  55  fixed to the crankshaft  31  and a movable pulley half body  56  approachable/withdrawable to/from the fixed pulley half body  55 . The movable pulley  56  is pressed, by a centrifugal weight  57  moving radial-outwardly in correspondence with increments in the number of revolutions of the crankshaft  31 , in a direction approaching to the fixed pulley half body  55 . 
     A driven pulley  59 , provided on an output shaft  58  supported between a rear part of the right casing  37  and the deceleration casing  39 , has a fixed pulley half body  60  rotatably supported relatively to the output shaft  58 . A movable pulley half body  61  approachable/withdrawable to/from the fixed pulley half body  60 , and the movable pulley half body  61  is pressed by a spring  62  toward the fixed pulley half body  60 . Further, a take-off clutch  63  is provided between the fixed pulley half body  60  and the output shaft  58 . An endless V-belt  64  is placed around the drive pulley  54  and the driven pulley  59 . 
     An intermediate shaft  65  and a vehicle axle  66  parallel to the output shaft  58  are supported between the right casing  37  and the deceleration casing  39 . A deceleration gear array  67  is provided among the output shaft  58 , the intermediate shaft  65  and the vehicle axle  66 . The rear wheel Wr is spline-engaged with a right end of the vehicle axle  66  projecting rightward through the deceleration casing  39 . 
     Therefore, the rotation power of the crankshaft  31  is transmitted to the drive pulley  54 . The rotational energy is then transmitted from the drive pulley  54 , via the V belt  64 , the driven pulley  59 , the take-off clutch  63  and the deceleration gear array  67 , to the rear wheel Wr. 
     Upon low-speed revolution of the engine E, as a centrifugal force acting on the centrifugal weight  57  of the drive pulley  54  is small, a groove width between the fixed pulley half body  60  and the movable pulley half body  61  is reduced by the spring  62  of the driven pulley  59 . The transmission gear ratio is LOW in this arrangement. From this state, if the number of revolutions of the crankshaft  31  is increased, the centrifugal force acting upon the centrifugal weight  57  increases and a groove width between the fixed pulley half body  55  and the movable pulley half body  56  of the drive pulley  54  decreases. The groove width between the fixed pulley half body  60  and the movable pulley half body  61  of the driven pulley  59  increases. Accordingly, the transmission gear ratio continuously varies from LOW toward TOP (HIGH). 
     The present invention is not limited to the above described embodiments, but various design changes can be made without departing from the scope of subject matter of the present invention. For example, the present invention is applicable to various vehicles such as an automatic three-wheeled vehicle other than the above motorcycle V or scooter type vehicle. 
     As described above, according to the first feature of the present invention, in an engine valve timing transmission apparatus, a timing chain is placed around a drive sprocket and a driven sprocket respectively fixed to a crankshaft and a valve camshaft. An end of a tensioner, press-contacted with an outer side surface of the timing chain to apply tension to the chain, is swingably attached to an engine main body via a pivot pin. An opening having a diameter greater than the sprocket is provided on the side of the drive sprocket on an outer side wall of a timing chamber accommodating the timing chain, formed in one side wall of the engine main body. A lid plate to close the opening is removably fixed to the engine main body. 
     The pivot pin is held between opposite walls of the engine main body and the lid plate. The pivot pin can be reliably supported by the engine main body and the lid plate without special dropping preventing means. Further, the freedom of attachment position of the pivot pin in opposite walls of the engine main body and the lid plate increases, the pivot pin can be provided in a desired position, and the chain tensioner can be provided in an optimum position for tension of the timing chain. Furthermore, since the attachment of the pivot pin is made upon attachment of the lid plate, e.g. after assembly of the engine main body, ease of reliable assembly is improved. 
     Further, according to a second feature of the present invention, the engine main body is constructed with a cylinder block, a first crankcase half body connected to one end of the cylinder block, and a second crankcase half body connected to and in cooperation with the first crankcase half body and rotatably holding the crankshaft. The timing chamber is formed from the cylinder block to the second crankcase half body. The opening is provided over the first and second crankcase half bodies, and the pivot pin is held between opposite walls of the lid plate to close the opening and the second crankcase half body. 
     The pivot pin can be provided sufficiently away from the drive sprocket to the opposite side of the driven sprocket. Accordingly, a sufficient length of the chain tensioner can be ensured. The timing chain can be held under an approximately constant tension without influence by the extension of the chain, and the durability of the timing chain can be improved. 
     Further, according to a third feature of the present invention, a stator of a generator driven by the crankshaft is fixed to the lid plate. The lid plate also serves as a support base of the stator of the generator. This third feature permits a reduction in the number of parts. 
     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.