Patent Publication Number: US-8522744-B2

Title: Lubricating structure for an internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2009-088261 filed on Mar. 31, 2009 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an internal combustion engine having an oil chamber. 
     2. Description of Background Art 
     An internal combustion engine for a vehicle is known which includes a transmission case provided on one side of a crankcase for supporting a crankshaft and further includes a cover member provided on the transmission case for covering the sides while a transmission is accommodated by the transmission case and the cover member. 
     In an engine of the type described, an oil chamber is formed at a lower portion of the crankcase and oil in the oil chamber is pressure-fed to a cylinder or the like of the engine by an oil pump provided between the crankcase and the transmission case. See, for example, Japanese Patent Laid-Open No. 2007-170314. 
     However, according to the conventional structure, since oil fed into the cylinder or the like is heated and is pressure-fed to the cylinder or the like by the oil pump immediately after it returns into the oil chamber, although the oil radiates some heat through an outer surface of the crankcase only for a short period of time for which the oil stays in the oil chamber after returned, the amount of the radiated heat is small. Therefore, the oil is reserved normally in a state wherein the temperature thereof remains high. 
     In an air-cooled engine, although the engine itself is cooled by heat radiating fins provided on the cylinder, unless an oil cooler is provided separately, the oil is cooled very little. Accordingly, in a conventional engine which does not include an oil cooler, if the engine is operating, then the oil basically continues to be heated. 
     On the other hand, although it is a possible idea to add a large-sized oil cooler to achieve compulsory cooling of the oil, this increases the number of parts and gives rise to an increase in the cost and the weight. In addition, since it is necessary to assure also the arrangement space for the parts, where the arrangement space cannot be assured, the addition of the oil cooler is difficult. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The present invention has been made in view of the situation described above, where it is an object of an embodiment of the present invention to provide an internal combustion engine wherein the heat radiation amount of oil from an oil chamber can be increased. 
     In order to solve the problem described above, according to an embodiment of the present invention, an internal combustion engine with a transmission case which accommodates a transmission is provided on one side of a crankcase which supports a crankshaft and a crank side oil chamber is provided at a lower portion of the crankcase. A transmission side oil chamber partitioned from a transmission accommodation section is provided at a lower portion of the transmission case such that oil is circulated through the transmission side oil chamber. 
     According to an embodiment of the present invention, since the transmission side oil chamber partitioned from the transmission accommodation section is provided at the lower portion of the transmission case such that the oil is circulated through the transmission side oil chamber, the oil flow path in the oil chamber can be made long and the oil residence time can be made long. Thus, the oil heat radiation amount from the oil chamber can be increased. 
     In the configuration described above, the crank side oil chamber may have a first oil chamber and a second oil chamber separate from each other, and a first opening which communicates with the transmission side oil chamber may be provided in the first oil chamber while a second opening which communicates with the second oil chamber is provided in the transmission side oil chamber. With this configuration, the oil entering the first oil chamber of the crank side oil chamber can be fed past the transmission side oil chamber and the second oil chamber in order, and the oil heat radiation amount from the oil chamber can be increased efficiently. 
     In the configuration described above, a third opening which communicates with a strainer of an oil pump may be provided in the second oil chamber. With this configuration, since the oil in the first oil chamber and the transmission side oil chamber enters the strainer chamber past the second oil chamber, the oil from which heat has been radiated can be supplied to the strainer. 
     Further, in the configuration described above, the first oil chamber may be provided at a position to which returning oil from a cylinder section of the internal combustion engine drops and the second oil chamber may be provided rearwardly of the first oil chamber while the strainer chamber is provided forwardly of the second oil chamber below the first oil chamber. With this configuration, the returning oil from the cylinder section can be dropped into the first oil chamber with certainty so that heat radiation from the oil chamber can be carried out efficiently, and the first oil chamber and the strainer chamber can be disposed in an overlapping relationship with each other as viewed from above. Consequently, a limited space can be utilized efficiently to dispose the first oil chamber, second oil chamber and strainer chamber. 
     In the configuration described above, the second opening may be positioned lower than the first opening and the third opening is positioned lower than the second opening. With this configuration, the oil can be smoothly fed from the first oil chamber to the transmission side oil chamber and can be smoothly fed from the transmission side oil chamber to the second oil chamber by making use of gravity. 
     Further, in the configuration described above, the transmission side oil chamber may be positioned below the transmission chamber of the transmission case in which the transmission is accommodated. With this configuration, the oil heat radiation face of the transmission side oil chamber can be made wide. 
     Further, in the configuration described above, a guide member for guiding returning oil from a cylinder section of the internal combustion engine to the transmission side oil chamber may be provided on the inner side of the crankcase. With this configuration, the returning oil from the cylinder chamber can be guided smoothly to the transmission side oil chamber. 
     Further, in the configuration described above, the guide member may be provided so as to extend between left and right walls of the crankcase. With this configuration, the returning oil from the cylinder section can be guided to the transmission side oil chamber with a higher degree of certainty. 
     According to an embodiment of the present invention, since the transmission side oil chamber partitioned from a transmission accommodation section is provided at a lower portion of the transmission case such that the oil is circulated through the transmission side oil chamber, the oil flow path in the oil chamber can be made long and the oil residence time can be made long. Consequently, the heat radiation amount from the oil chamber can be increased and oil from which heat has been radiated is accumulated into an oil reservoir. 
     Further, since the crank side oil chamber has the first oil chamber and the second oil chamber separated from each other and the first opening which communicates with the transmission side oil chamber is provided in the first oil chamber while the second opening which communicates with the second oil chamber is provided in the transmission side oil chamber, the oil heat radiation amount from the oil chamber can be increased efficiently. 
     Further, since the third opening which communicates with the strainer of the oil pump is provided in the second oil chamber, and the oil in the first oil chamber and the transmission side oil chamber enters the strainer chamber past the second oil chamber, oil from which heat has been radiated can be supplied to the strainer. 
     Further, since the first oil chamber is provided at the position to which returning oil from the cylinder section of the internal combustion engine drops and the second oil chamber is provided rearwardly of the first oil chamber while the strainer chamber is provided forwardly of the second oil chamber below the first oil chamber, the returning oil from the cylinder section can be dropped into the first oil chamber with certainty so that heat radiation from the oil chamber can be carried out efficiently. Further, a limited space can be utilized efficiently to dispose the first oil chamber, second oil chamber and strainer chamber. 
     Further, since the second opening is positioned lower than the first opening and the third opening is positioned lower than the second opening, the oil can be smoothly fed from the first oil chamber to the transmission side oil chamber and can be smoothly fed from the transmission side oil chamber to the second oil chamber by making use of gravity. 
     Further, since the transmission side oil chamber is positioned below the transmission chamber of the transmission case in which the transmission is accommodated, heat radiation can be carried out using the outer surface of the transmission side oil chamber and the oil heat radiation face can be made wide. 
     Further, since the guide member for guiding returning oil from the cylinder section of the internal combustion engine to the transmission side oil chamber is provided on the inner side of the crankcase, the returning oil from the cylinder chamber can be guided smoothly to the transmission side oil chamber. 
     Further, since the guide member is provided so as to extend between the left and right walls of the crankcase, the returning oil from the cylinder section can be guided to the transmission side oil chamber with a higher degree of certainty. 
     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 elevational view of a motorcycle to which an embodiment of the present invention is applied; 
         FIG. 2  is a view showing an internal structure of an engine of the motorcycle as viewed from the right side of a vehicle body; 
         FIG. 3  is a view showing a section taken along line III-III of  FIG. 2 ; 
         FIG. 4  is a view showing a section taken along line IV-IV of  FIG. 2 ; 
         FIG. 5  is a view showing a crankshaft of the engine together with peripheral elements; 
         FIG. 6  is a view of a right crankcase as viewed from the inner side (left side); 
         FIG. 7  is a view of the right crankcase as viewed from the outer side (right side); 
         FIG. 8  is a view of a left crankcase as viewed from the inner side (right side); 
         FIG. 9  is a view of a transmission case as viewed from the right crankcase side (left side); 
         FIG. 10  is a view of the engine as viewed from the lower side; 
         FIG. 11  is a view showing a gear damper together with peripheral elements; 
         FIG. 12(A)  is a side elevational view of a final gear; 
         FIG. 12(B)  is a view showing a section of the final gear taken along line A 1 -A 1 ; 
         FIG. 13(A)  is a side elevational view of a damper holding member; 
         FIG. 13(B)  is a view showing a section of the damper holding member taken along line A 2 -A 2 ; and 
         FIG. 14  is a view illustrating a modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an embodiment of the present invention is described with reference to the drawings. 
     It is to be noted that, in the following description, unless otherwise specified, the representations of the directions such as forward, backward, leftward, rightward and upward, downward directions are the same as the directions of a vehicle. Further, an arrow mark F in the figures indicates the forward direction of the vehicle body, another arrow mark R the rightward direction of the vehicle body, and a further arrow mark U the upward direction of the vehicle body. 
       FIG. 1  is a side elevational view of a motorcycle  1  to which an embodiment of the present invention is applied. 
     A vehicle body frame  2  of the present motorcycle  1  includes a head pipe  3  at a front portion of the vehicle body, a single main frame  4  extending in an obliquely downward direction toward the rear from the head pipe  3 , a pair of left and right pivot brackets  5  secured to a rear portion of the main frame  4  and extending downwardly, a pair of left and right seat rails  6  extending obliquely upwardly toward the back from the securing position of the pivot brackets  5  at a rear portion of the main frame  4 , bent midway and coming to rear ends thereof, and a pair of left and right reinforcing frames  7  for the reinforcement between the pivot brackets  5  and middle portions of the seat rails  6 . 
     A rider&#39;s seat  8  is provided above the pair of left and right seat rails  6  of the vehicle body frame  2 , and an accommodation section (accommodating box)  9  is provided below the rider&#39;s seat  8 . A steering bar  10  supported for pivotal motion on the head pipe  3  is provided at an upper portion of a front portion of the vehicle body, and a pair of front forks  11 ,  11  extending below the steering bar  10  and a front wheel  12  is supported for rotation at lower ends of the front forks  11 ,  11 . A rear fork  14  is supported at a front end thereof for rocking motion on a pivot shaft  13  at the pivot brackets  5  in the middle of the vehicle body and extends rearwardly, and a rear wheel  15  is supported for rotation at a rear end portion of the rear fork  14 . A pair of left and right rear shock absorbers  16  are interposed between a rear portion of the rear fork  14  and the seat rails  6 . 
     An engine (also referred to as power unit)  20  in the form of an internal combustion engine is suspended below the main frame  4  forwardly of the pivot brackets  5 . A support bracket  17  provided vertically at a central portion of the main frame  4  is suspended above the engine  20 , and the engine  20  is secured at two portions thereof to the pivot brackets  5 . In other words, the engine  20  is supported in a manner wherein it is suspended downwardly at a rear portion of the main frame  4 . Further, the vehicle body frame  2  is covered with a vehicle body cover  18  made of a synthetic resin and divided into several portions. 
     The engine  20  is a single-cylinder 4-cycle air-cooled engine and is formed as a horizontal engine wherein a cylinder section  22  is inclined forwardly by a great amount from the front face of a crankcase  24  to such a state proximate to a substantially horizontal state. Therefore, the vehicle body can be formed such that the center of gravity thereof is positioned low, and the main frame  4  can be made as low as seen from  FIG. 1  so that a stepping over portion M over which a driver steps when he/she gets on the motorcycle  1  can be set low and the facility in getting on and off can be enhanced. Further, a generator cover  25  is attached to a front portion of the left side face of the crankcase  24 . The vehicle body cover  18  has a cover shape for covering the vehicle body up to a position in the proximity of an outer edge of the crankcase  24  as viewed in a side elevation of the vehicle body while the side face of the crankcase  24  including the generator cover is exposed to the outside. 
     An intake pipe  26  is connected to the upper side of the cylinder section  22  of the engine  20 . This intake pipe  26  extends upwardly and is connected to a throttle body  27  and an air cleaner  28  supported on the main frame  4 . An exhaust pipe  29  is connected to the lower side of the cylinder section  22 . This exhaust pipe  29  extends downwardly and is bent such that it extends rearwardly until it is connected to a muffler  30  disposed on the right side with respect to the rear wheel  15 . 
     Meanwhile, an output power shaft  31  of the engine  20  is supported for rotation at a rear portion of the left side face of the crankcase  24  such that an end thereof is exposed. A driving sprocket wheel  32  is attached to the end of the output power shaft  31 , and a power transmitting chain  34  (refer to  FIG. 1 ) extends between and around the driving sprocket wheel  32  and a driven sprocket wheel  33  provided integrally on the rear wheel  15  to configure a chain driving mechanism. Accordingly, rotation of the output power shaft  31  of the engine  20  is transmitted to the rear wheel  15  through the chain driving mechanism. It is to be noted that this chain driving mechanism functions also as a secondary speed reducing mechanism for setting the reduction ratio (secondary reduction ratio) between the output power shaft  31  and the rear wheel shaft depending upon the ratio of tooth numbers of the sprocket wheels  32  and  33 . A cover  35  is provided for covering the chain driving mechanism. 
     A step bar  36  which extends in the left and right directions of the vehicle body is attached to a lower portion of the crankcase  24 , and a pair of steps  36 A,  36 A on which the driver is to place his/her feet are attached to the opposite ends of the step bar  36 . 
     Further, in the present motorcycle  1 , a kick member (starting system member)  37  which configures part of a kick type starting apparatus  140  for starting the engine  20  is disposed on the left sidewardly of the crankcase  24 . More specifically, the kick member  37  includes a kick arm  39  attached to a kick shaft  38  supported for rotation on the crankcase  24  with an end thereof exposed, and a kick pedal  40  attached for rotation at an end portion of the kick arm  39 . The driver can step the kick pedal  40  to rotate to the kick shaft  38  to start the engine  20 . 
     Furthermore, the motorcycle  1  also includes a starter motor  41  for starting the engine that is disposed in addition to the kick type starting apparatus  140 . This starter motor  41  is attached to a front portion of the upper face of the crankcase  24 . The engine  20  can be started by operating the starter motor  41 . In other words, the present motorcycle  1  is configured such that the engine  20  can be started by any of the kick type method and the starter motor type method. 
       FIG. 2  is a view showing an internal structure of the engine  20  as viewed from the right side of the vehicle body and shows the position of principal rotary shafts of the power transmission system and the starting system. Also a cylinder axial line L 1  is shown. Meanwhile,  FIG. 3  is a view showing a section taken along line III-III. 
     As shown in  FIGS. 2 and 3 , the cylinder section  22  of the engine  20  includes a cylinder block  22 A connected to the front face of the crankcase  24 , a cylinder head  22 B connected to the front face of the cylinder block  22 A, and a head cover  22 C for covering the front face of the cylinder head  22 B. On the cylinder head  22 B, a combustion chamber  22 D and an intake port and an exhaust port, not shown, are connected to the combustion chamber  22 D. An ignition plug  23  is disposed such that an end thereof faces the combustion chamber  22 D. The intake pipe  26  is connected to an entrance of the intake port. The exhaust pipe  29  is connected to an exit of the exhaust port. Further, in  FIG. 2 , a heat radiating fin  22 F is provided on the cylinder section  22 , and the cylinder section  22  is air-cooled by the heat radiating fins  22 F. 
     As shown in  FIG. 3 , the crankcase  24  of the engine  20  is formed to the left and right with two-divisional part structure including a left crankcase  24 A and a right crankcase  24 B. A crankshaft  51  is supported laterally for rotation at a front portion of the crankcase  24  through a pair of left and right bearings (roll bearings)  45 ,  45  supported on the left and right crankcases  24 A and  24 B such that an axis C 1  thereof extends perpendicularly to the advancing direction of the vehicle. 
     This crankshaft  51  includes a crank journal  51 A serving as the center of rotation, a crank web  51 B formed with a diameter greater than that of the crank journal  51 A and a crank pin (eccentric shaft)  51 C supported through the crank web  51 B. The crank web  51 B and the crank pin  51 C are positioned between the pair of left and right bearings  45 ,  45 . Further, a balance weight (hereinafter referred to as weight)  51 D for establishing a balance in rotation is provided on the crank web  51 B. 
     A piston  21 A disposed for sliding movement along the cylinder axial line L 1  in the cylinder section  22  is connected to the crank pin  51 C of the crankshaft  51  through a connecting rod  21 B. Further, in  FIG. 3 , a sprocket wheel  55 A is provided on the crankshaft  51  with a sprocket wheel  55 B provided on a camshaft  55 C provided in the head cover  22 C of the cylinder section  22 . The sprocket wheels  55 A and  55 B are connected to each other through a cam chain  55 D. Consequently, a valve motion wherein the camshaft  55 C is rotated in response to rotation of the crankshaft  51  to push or move intake and exhaust valves not shown provided in the cylinder head  22 B. 
     A belt type continuously variable transmission  60  is provided on the right side (one side) of the crankshaft  51  while a generator  180  is provided on the left side (other side) of the crankshaft  51 . 
     More specifically, the left end of the crankshaft  51  extends leftwardly in the left crankcase  24 A and extends to a position in the proximity of the generator cover  25  attached so as to cover the left side opening (outside opening) of the left crankcase  24 A, and the generator  180  is accommodated in a space surrounded by the generator cover  25  and the left crankcase  24 A. The generator  180  includes a rotor  181  secured to the crankshaft  51  and a stator  182  disposed in the rotor  181  with the stator  182  being secured to the generator cover  25 . 
     The belt type continuously variable transmission  60  is a power transmission mechanism of the dry type wherein lubrication by engine oil is not carried out, and is accommodated in a transmission accommodation section  61  provided on the right side (one side) of the crankshaft  51 . The transmission accommodation section  61  forms a chamber having no oil and is different from that of the crankcase  24  which is lubricated by engine oil. The transmission accommodation section  61  is formed to the left and right as a two-divisional part structure of a transmission case  61 A which forms a body portion of the transmission accommodation section  61 . A transmission cover (cover member)  61 B for covering the outer side opening (right side opening) of the transmission case  61 A is provided. 
     More particularly, the right end of the crankshaft  51  extends to the right through the right crankcase  24 B and then extends through the transmission case  61 A connected to the right side of the right crankcase  24 B by bolts. The right end of the crankshaft  51  further extends to a position in the proximity of the transmission cover  61 B provided contiguously to the transmission case  61 A, and a right end portion thereof is used as a driving pulley shaft (driving shaft)  51 R of the belt type continuously variable transmission  60 . A driving pulley  63  is attached to the driving pulley shaft  51 R. 
     At a rear portion of the crankcase  24 , a driven pulley shaft (driven shaft)  64  of the belt type continuously variable transmission  60  is supported laterally for rotation such that an axis C 2  thereof extends perpendicularly to the advancing direction of the vehicle. The driven pulley shaft  64  is positioned to the rear of and in parallel to the driving pulley shaft  51 R and is supported for rotation through a pair of left and right bearings (roller bearings)  65 ,  65  supported by the right crankcase  24 B and the transmission accommodation section  61  (transmission case  61 A). 
     A driven pulley  67  is attached to the driven pulley shaft  64 , and a V belt  68  extends between and around the driving pulley  63  and the driven pulley  67  such that rotation of the driving pulley  63  is transmitted to the driven pulley  67 . It is to be noted that seal members  69 A and  69 B for preventing engine oil on the crankcase  24  side from entering the transmission accommodation section  61  are interposed between the transmission accommodation section  61  and the pulley shafts  51 R and  64  and the transmission accommodation section  61  is sealed from the crankcase  24 . 
     The driving pulley  63  has a fixed half  63 A which rotates together with the driving pulley shaft  51 R and a movable half  63 B, and the fixed half  63 A is secured to the driving pulley shaft  51 R while the movable half  63 B is fixed for movement in an axial direction on the left side with respect to the fixed half  63 A. The movable half  63 B rotates together with the crankshaft  51  and slidably moves in the axial direction by an action of a weight roller  70  which moves in a centrifugal direction by centrifugal force toward or away from the fixed half  63 A to vary the wrapping diameter of the V belt  68  sandwiched between the two pulley halves  63 A and  63 B. 
     The driven pulley  67  of the belt type continuously variable transmission  60  has a fixed half  67 A which rotates together with the driven pulley shaft  64  and a movable half  67 B. The fixed half  67 A is fixed on the left side with respect to the movable half  67 B. The movable half  67 B is disposed for movement in the axial direction through an annular slider  71  at a right end portion of the driven pulley shaft  64  and is biased to the left (toward the fixed half  67 A side) by a biasing member  72  in the form of a coil spring. Therefore, if the wrapping diameter of the V belt  68  sandwiched between the two halves  63 A and  63 B of the driving pulley  63  increases, then the distance between the two halves  67 A and  67 B of the driven pulley  67  conversely increases against the biasing force of the biasing member  72  thereby to decrease the wrapping diameter of the V belt  68 . Consequently, a continuous speed variation is carried out automatically. 
     The driven pulley shaft  64  transmits power to a power transmission mechanism  81  disposed in the crankcase  24  through a centrifugal clutch  80  disposed in a space (clutch chamber R 1  hereinafter described) formed between the right crankcase  24 B and the transmission case  61 A. 
     The centrifugal clutch  80  is a clutch of the wet type wherein lubrication and cooling of components are carried out by engine oil and includes a clutch inner race  83  spline-fitted with the driven pulley shaft  64  and a clutch outer race  85  connected to a clutch output gear  84  provided for relative rotation at a left end portion of the driven pulley shaft  64 . A clutch weight  87  is provided on each of a plurality of support shafts  86  provided in a projecting manner on the outer circumferential end side of the clutch inner race  83 . Therefore, if the speed of rotation of the driven pulley shaft  64  exceeds a predetermined speed, then the clutch weight  87  which moves in a centrifugal direction by centrifugal force is engaged with the clutch outer race  85  and rotates the clutch outer race  85  integrally with the driven pulley shaft  64  to rotate the clutch output gear  84 . 
     It is to be noted that a clutch reinforcing plate  88  is provided for suppressing the clutch outer race  85  from expanding in the centrifugal direction. A retainer  90  is disposed between the clutch output gear  84  and the driven pulley shaft  64 . The retainer  90  has two roller trains of bearing rollers disposed in a spaced relationship from each other in a circumferential direction such that the roller trains extend in the axial direction. The two roller trains allow the clutch output gear  84  to make relative rotation to the driven pulley shaft  64 . 
     The power transmission mechanism  81  is a mechanism which carries out power transmission between the belt type continuously variable transmission  60  and the output power shaft  31  of the engine  20  and also functions as a primary speed reduction mechanism. The power transmission mechanism  81  includes an intermediate gear shaft (speed reduction gear shaft)  91  provided between the driven pulley shaft  64  and the output power shaft  31  for reducing the speed of rotation of the clutch output gear  84  provided on the driven pulley shaft  64  at a predetermined speed reduction ratio and transmitting the rotation of the reduced speed to the output power shaft  31 . It is to be noted that the intermediate gear shaft  91  includes an axis C 3 , and the output power shaft  31  includes an axis C 4 . 
     The intermediate gear shaft  91  includes a penetrating shaft portion  91 A supported for rotation on a pair of left and right bearings (roller bearings)  92 ,  92  supported on the left and right crankcases  24 A and  24 B and extending through a wall portion of the right crankcase  24 B. To the penetrating shaft portion  91 A, an intermediate shaft driven gear (speed reduction gear)  93  of a large diameter which meshes with the clutch output gear  84  provided on the driven pulley shaft  64  is secured, and an intermediate shaft driving gear  94  of a smaller diameter which meshes with a final gear  95  secured to the output power shaft  31  is secured in a space between the left and right crankcases  24 A and  24 B. Consequently, rotation of the clutch output gear  84  positioned on the outer side of the crankcase  24  is transmitted at a predetermined speed reduction ratio to the final gear  95  of the output power shaft  31  positioned in the crankcase  24  through the intermediate gear shaft  91 . 
     The output power shaft  31  is supported by a pair of left and right bearings (roll bearings)  96 ,  96  supported on the left and right crankcases  24 A and  24 B. The final gear  95  is provided for rotation on the output power shaft  31 , and rotation of the final gear  95  is transmitted to the output power shaft  31  through a gear damper  97 . 
     More specifically, a space (hereinafter referred to as clutch chamber R 1 ) surrounded by the right crankcase  24 B and the transmission case  61 A is formed in the neighborhood on the right of a space (hereinafter referred to as crank chamber R 0 ) surrounded by the left and right crankcases  24 A and  24 B. In other words, the transmission case  61 A is connected to the right crankcase  24 B so that it serves also as a clutch case member which forms a clutch case. 
     The crank chamber R 0  and the clutch chamber R 1  are chambers in which lubrication and cooling by engine oil are carried out, and an oil reserving section is formed at a lower portion of the crankcase  24  and a lower portion of the transmission case  61 A. 
     Further, a space (hereinafter referred to as transmission chamber R 2 ) is formed between the transmission case  61 A and the transmission cover  61 B in the neighborhood on the right of the clutch chamber R 1 , and the transmission chamber R 2  is a chamber in which lubrication and cooling by engine oil are not carried out. In other words, in the present engine  20 , the chamber in which engine oil is used and the chamber in which no engine oil is used are partitioned definitely in the vehicle widthwise direction. 
     Now, the kick type starting apparatus  140  is described. 
       FIG. 4  is a view showing a section taken along line IV-IV and shows a mechanical portion of the kick type starting apparatus  140  together with peripheral elements. The kick type starting apparatus  140  is accommodated in a lower portion of the engine  20  (principally in a lower portion of the crankcase  24 ). 
     The kick shaft  38  is disposed at a position forwardly and downwardly of the driven pulley shaft  64  at which the kick shaft  38  does not overlap with the driven pulley  67  formed in a large diameter as viewed in a side elevation (refer to  FIG. 2 ) and is supported for rotation by bearing portions (in the present example, plain bearings each formed from a through-hole)  141  and  142  formed in the left and right crankcases  24 A and  24 B. A left end portion of the kick shaft  38  extends through the bearing portion  141  formed in a wall portion of the left crankcase  24 A and projects to the left, and the kick arm  39  having an end to which the kick pedal  40  is attached is secured at a base end portion thereof to the penetrating shaft portion  38 A. Further, a seal member  143  which fills up a gap between the left crankcase  24 A and the kick shaft  38  is provided on the left crankcase  24 A. In this crankcase  24 , a return spring  145  for biasing the kick shaft  38  in a reverse direction to the kicking direction and a stopper  146  for stopping the kick shaft  38  which is rotated by biasing force of the return spring  145  at a kicking operation starting position are disposed at a right side portion of the kick shaft  38 , and a kick drive gear  147  of a large diameter positioned adjacent the bearing portion  141  is provided at a left side portion of the kick shaft  38 . 
     A kick intermediate shaft  150  for transmitting rotation of the kick shaft  38  to the crankshaft  51  is disposed between the kick shaft  38  and the crankshaft  51 . The kick intermediate shaft  150  of the present configuration has a two-shaft configuration and includes a first kick intermediate shaft  151  driven to rotate by the kick shaft  38  and a second kick intermediate shaft  155  for transmitting the rotation of the first kick intermediate shaft  151  to the crankshaft  51 . In  FIG. 2 , the axis of the kick shaft  38  is denoted by reference character K 1  and the axis of the first kick intermediate shaft  151  is denoted by reference character K 2  while the axis of the second kick intermediate shaft  155  is denoted by reference character K 3 . 
     The first kick intermediate shaft  151  is disposed laterally at a position below an intermediate position between the driven pulley shaft  64  and the crankshaft  51  at which the first kick intermediate shaft  151  overlaps with the driven pulley  67  formed in a large diameter as viewed in side elevation as seen in  FIG. 2 . Further, the first kick intermediate shaft  151  is supported for rotation by a pair of left and right bearing portions (in the present example, plain bearings each formed from a non through-hole)  161  and  162  provided on the left and right crankcases  24 A and  24 B as seen in  FIG. 4 . With the first kick intermediate shaft  151 , a first kick intermediate shaft driven gear (kick driven gear)  163  of a small diameter which is accommodated fully in the crankcase  24  and meshes with the kick drive gear  147  is formed integrally. Further, a first kick intermediate shaft driving gear (first idle gear)  164  of a larger diameter than that of the first kick intermediate shaft driven gear  163  is secured to the first kick intermediate shaft  151  rightwardly of and adjacent the first kick intermediate shaft driven gear  163 . 
     As shown in  FIG. 2 , the second kick intermediate shaft  155  is disposed laterally at a position rearwardly and downwardly of the crankshaft  51  at which the second kick intermediate shaft  155  does not overlap with the driven pulley  67  formed in a large diameter as viewed in side elevation. As shown in  FIG. 4 , the second kick intermediate shaft  155  is supported for rotation by a pair of left and right bearing portions (in the present example, plain bearings each formed from a non through-hole)  166  and  167  provided on the left crankcase  24 A and the transmission case  61 A. This second kick intermediate shaft  155  is formed as a shaft longer than the first kick intermediate shaft  151  such that it extends through and outwardly from an opening  24 B 1  formed in the wall portion of the right crankcase  24 B in a state wherein it is supported at a left end portion thereof by the left crankcase  24 A. The extension shaft portion  155 A of the second kick intermediate shaft  155  extends over the space (clutch chamber R 1 ) between the crankcase  24  and the transmission case  61 A and is supported for rotation by the transmission case  61 A. At a shaft portion of the second kick intermediate shaft  155  in the crankcase  24 , a second intermediate shaft driven gear (second idle gear)  168  of a small diameter which meshes with the first kick intermediate shaft driving gear  164  of the first kick intermediate shaft  151  is formed integrally. A dive gear mechanism  170  is disposed at the extension shaft portion  155 A of the kick intermediate shaft  155  outside the crankcase  24 . 
     The dive gear mechanism  170  is formed as a mechanism which includes a dive gear  171  is positioned between the right crankcase  24 B and the transmission case  61 A and is provided for movement with respect to the second kick intermediate shaft  155 . A biasing member  173  for biasing the dive gear  171  to a retracted position at which the dive gear  171  does not mesh with a kick starting driven gear  172  is provided on the crankshaft  51 . A friction spring  174  is wrapped around the dive gear  171  and is supported on the transmission case  61 A wherein the dive gear  171  is slidably moved to the left side by rotation of the second kick intermediate shaft  155  upon kicking until it is brought into meshing engagement with the kick starting driven gear  172 . It is to be noted that, while, in the example shown, a coil spring is used as the biasing member  173 , a spring other than the coil spring such as a leaf spring or a coned disk spring may be used. 
     Accordingly, if the kick pedal  40  is stepped down and the kick shaft  38  is rotated against the biasing force of the return spring  145 , then the rotation of the kick shaft  38  is transmitted through the gear train of the first kick intermediate shaft  151  and the second kick intermediate shaft  155  to move the dive gear  171  in a direction in which the dive gear  171  is brought into meshing engagement with the kick starting driven gear  172  so that the crankshaft  51  can be rotated compulsorily to start the engine  20 . 
     As shown in  FIG. 2 , an oil pump  100  for supplying engine oil to the components of the engine  20  is provided in the crankcase  24  of the engine  20 . This oil pump  100  is provided at a forwardly obliquely downward position with respect to the crankshaft  51  and is driven by rotating force of the crankshaft  51  by a cam chain driving to discharge the engine oil so that the engine oil is supplied to the bearings such as the bearings  45 ,  45  for supporting the crankshaft  51 , a valve motion (not shown) of the cylinder section  22 , the centrifugal clutch  80  and the power transmission mechanism  81  and so forth. 
     Further, an extension  106  which extends outwardly from the engine  20  is provided on the engine  20 , and heat radiating fins are formed on the extension  106  and an oil path (oil passage)  108  is formed in the engine  20  to carry out cooling of the oil. 
     More specifically, the extension  106  extends from the transmission case  61 A which forms the body portion of the transmission accommodation section  61  to the front side of the vehicle body substantially along the cylinder axial line L 1 . An oil path cover  107  is connected to the extension  106  by means of bolts. The substantially annular oil path  108  is formed and heat radiating fins are provided between the extension  106  and the oil path cover  107  such that oil flowing in the oil path  108  is cooled efficiently by running wind through the heat radiating fins. Further, the section modulus of the extension  106  and the oil path cover  107  becomes high and the rigidity is sufficiently assured. In other words, the extension  106  and the oil path cover  107  function as a small-sized oil cooler  105  (refer to  FIGS. 2 and 3 ) of the type integrated with the engine. 
     In the present configuration, oil pressured-fed from the oil pump  100  is branched, and the oil in one system passes along an oil path (not shown) connecting to the cylinder section  22  to lubricate the components of the cylinder section  22  and then returns to the oil reserving section at a lower portion of the crankcase  24  by natural dropping. In the other system, the oil passes through the oil cooler  105  and then passes an oil path  110  illustrated in  FIG. 3  to lubricate the components of the crankshaft  51 , whereafter it naturally drops and returns to the oil reversing section. It is to be noted that naturally the oil pressure-fed from the oil pump  100  may be branched after it passes through the oil cooler  105 . 
     As shown in  FIG. 5 , in the present configuration, the oil path  110  along which oil from the oil pump  100  is pressure-fed is configured so as to supply oil between the bearing  45  on the right side from between the pair of left and right bearings  45  which support the crankshaft  51  and a seal member  69 C which seals between the crankshaft  51  and the right crankcase  24 B. 
     The oil flowing out from the oil path  110  enters the crankcase  24  past an oil passage groove  51 M formed between the right bearing  45  and the crankshaft  51  and then is supplied to a large end portion of the connecting rod  21 B along an oil path not shown formed in the crank pin  51 C. 
     More specifically, in the present configuration, since the oil passage groove  51 M which cooperates with the bearing  45  to form a gap therebetween such that oil is passed to the crank pin  51 C side is formed on the outer circumferential face of the crankshaft  51 , oil can be supplied to the sliding face of the connecting rod  21 B and so forth without forming an oil path in the crankshaft  51 . It is to be noted that a plurality of such oil passage grooves  51 M may be formed in a spaced relationship from each other in the circumferential direction of the crankshaft  51  or a single oil passage groove  51 M may be formed if sufficient lubrication is assured. 
     Further, as shown in  FIG. 5 , in the present configuration, an O-snap ring  175  is not disposed on an inner circumference of the kick starting driven gear  172 , but is disposed on an inner circumference of a collar  172 A which is inserted until it is abutted with an end face of the kick starting driven gear  172  after the kick starting driven gear  172  is fitted on the crankshaft  51 . If the kick starting driven gear  172  and the collar  172 A are formed from a single integrated part, then since the O-snap ring  175  is disposed on an inner circumference of the part, it is necessary to pay attention so that the position of the O-snap ring upon assembly of the same may not be displaced. 
     In contrast, in the present configuration, since the kick starting driven gear  172  and the collar  172 A are formed as separate members from each other and the O-snap ring  175  is disposed between the members, the collar  172 A may be fitted on the crankshaft  51  after the O-snap ring  175  is assembled to its assembly position of the crankshaft  51 . Accordingly, the O-snap ring  175  can be assembled readily without any positional displacement, and the assembly performance of the O-snap ring  175  is improved. 
     In this instance, the gap on the inner circumference side of the collar  172 A (gap between the collar  172 A and the crankshaft  51 ) is sealed with the O-snap ring  175  and the gap on the outer circumference side of the collar  172 A (gap between the collar  172 A and the transmission case  61 A) is sealed with the seal member  69 A, the sealing performance between the transmission accommodation section  61  and the crankcase  24  can be sufficiently assured. 
     In addition, in the case of the configuration wherein oil pressure-fed to and heated by the cylinder or the like is pressure-fed to the cylinder section  22  or the like by the oil pump  100  immediately after it returns to the oil chamber serving as an oil reserving section, the oil is cooled a little in the oil chamber, and even if the oil path  108  described above is provided, in an environment of use wherein the average temperature is high, an increase in the oil cooling capacity is demanded. On the other hand, with the method wherein a large-sized oil cooler separate from an engine is additionally provided to increase the oil cooling capacity, not only the number of parts increases to increase the cost and the weight, but in a small-sized vehicle like the present vehicle, it is difficult to assure an arrangement space for the large-sized oil cooler. 
     Therefore, in the present engine  20 , the oil heat radiation amount from an oil chamber is configured such that a crank side oil chamber RA which functions as an oil reserving section of the crankcase  24  is divided into a first oil chamber RO 1  and a second oil chamber RO 2  and oil heated by the cylinder section  22  or the like flows from the first oil chamber RO 1  to a transmission side oil chamber RB which functions as an oil reserving section of the transmission case  61 A and then flows from the transmission side oil chamber RB to the second oil chamber, whereafter the oil is sucked out by an oil pump so that the oil heat radiation amount from the oil chamber is increased. In the following, this oil chamber structure is described in detail. 
       FIG. 6  is a view of the right crankcase  24 B as viewed from the inner side (left side), and  FIG. 7  is a view of the right crankcase  24 B as viewed from the outer side (right side).  FIG. 8  is a view of the left crankcase  24 A as viewed from the inner side (right side) and  FIG. 9  is a view of the transmission case  61 A as viewed from the right crankcase  24 B side (left side). 
     On the inner side of the right crankcase  24 B, a up/down partitioning rib  191  for partitioning a bottom side space of the right crankcase  24 B upwardly and downwardly and a front/rear partitioning rib  192  for partitioning an upper side space partitioned by the up/down partitioning rib  191  forwardly and backwardly as seen in  FIG. 6  are provided. Also on the inner side of the left crankcase  24 A, an up/down partitioning rib  193  for partitioning the bottom side space of the left crankcase  24 A upwardly and downwardly is provided so as to connect to the up/down partitioning rib  191  as shown in  FIG. 7 , and a front/rear partitioning rib  194  for partitioning the upper side space partitioned by the up/down partitioning rib  193  of the left crankcase  24 A forwardly and backwardly is provided so as to connect to the front/rear partitioning rib  192 . 
     In particular, the left and right up/down partitioning ribs  191  and  193  and the left and right front/rear partitioning ribs  192  and  194  are formed in a leftwardly and rightwardly symmetrically shaped relationship with a parting plane of the left and right crankcases  24 A and  24 B and extend along the left and right walls of the crankcase  24 . Therefore, the inside of the crankcase  24  is partitioned forwardly and backwardly by the front/rear partitioning ribs  192  and  194 , and the front side chamber is formed as the first oil chamber RO 1  and the rear side chamber is formed as the second oil chamber RO 2 . 
     Since the first oil chamber RO 1  is formed on the front side in the inside of the crankcase  24 , it serves as an oil chamber into which not only oil having lubricated the portions of the cylinder section  22  enters but also oil having lubricated the portions of the crankshaft  51 , that is, oil heated by the components of the engine  20 , enters first. 
     Here, the front/rear partitioning ribs  192  and  194  which partition the rear end of the first oil chamber RO 1  are provided rearwardly and downwardly of the crankshaft  51  as shown in  FIGS. 6 and 8 . More particularly, the front/rear partitioning ribs  192  and  194  extend upwardly and downwardly below the second kick intermediate shaft  155  (axis K 3 ) positioned rearwardly and downwardly of the crankshaft  51 . Consequently, return oil from the cylinder section  22  and the crankshaft  51  side does not directly enter the second oil chamber RO 2  exceeding the front/rear partitioning ribs  192  and  194  but enters the first oil chamber RO 1  with certainty. 
     Meanwhile, the second oil chamber RO 2  is, in the right crankcase  24 B, partitioned upwardly and downwardly by a rib  195  which extends rearwardly from a lower end of the up/down partitioning rib  191  and forms a wall portion which is projecting upwardly and then connects to the opening  24 B 1  of the right crankcase  24 B as seen in  FIG. 6 . However, in the left crankcase  24 A, such a rib like the rib  195  which partitions the second oil chamber RO 2  upwardly and downwardly as seen in  FIG. 8  does not exist. Consequently, the second oil chamber RO 2  continues upwardly and downwardly in such a manner so as to extend across the rib  195  upwardly and downwardly in the left crankcase  24 A. 
     Further, the left and right up/down partitioning ribs  191  and  193  extend forwardly in the left and right crankcases  24 A and  24 B and connect at the front ends thereof to bottom plates  24 A 1  and  24 B 1  of the crankcases  24 A and  24 B (refer to  FIGS. 6 and 7 ). Consequently, the up/down partitioning ribs  191  and  193  partition the first oil chamber RO 1  and a space portion  196  below the first oil chamber RO 1  fully from each other. 
     This lower space portion  196  extends between the left and right crankcases  24 A and  24 B and forms part of the second oil chamber RO 2 . Sidewardly (rightwardly) of the space portion  196 , that is, on the opposite side with respect to the side wall of the right crankcase  24 B, a strainer chamber  101  which forms a strainer from which oil is sucked out by the oil pump  100  is formed as seen in  FIG. 7 . This strainer chamber  101  and the space portion  196  which forms part of the second oil chamber RO 2  are in communication with each other through an opening (hereinafter referred to as third opening)  197  formed in the side wall of the space portion  196 . It is to be noted that a suction path  102  extending downwardly from the oil pump  100  positioned above the strainer chamber  101  is in communication with the strainer chamber  101 , and another strainer (filter)  103  is disposed below the suction path  102 . 
     Further, in the present structure, the transmission side oil chamber RB serving as an oil reserving section is formed also at a lower portion of the transmission case  61 A. More particularly, a lower portion of the transmission case  61 A is depressed to the right side farther than a portion (for example, a portion of the side wall of the transmission case  61 A through which the driving pulley shaft  51 R shown in  FIG. 3  extends) which projects most to the left side (crankcase  24  side). The space between the side wall of the transmission case  61 A including this depressed portion  198  (refer to  FIG. 9 ) and the left crankcase  24 A functions as the transmission side oil chamber RB. 
     As shown in  FIG. 9 , a strainer chamber covering portion  199  for covering a sideward opening of the strainer chamber  101  formed in the right crankcase  24 B is formed integrally on the transmission case  61 A. Consequently, the transmission side oil chamber RB is configured so as not to communicate directly with the strainer chamber  101 . It is to be noted that, on the strainer chamber covering portion  199 , a depressed portion  199 A which is depressed in the widthwise direction in such a manner as to expand the space of the strainer chamber  101  is formed, and the strainer chamber  101  is expanded to the transmission case  61 A side by the depressed portion  199 A. 
     Meanwhile, an oil receiving rib  201  which projects from the side wall of the transmission case  61 A in such a manner so as to extend in the forward and backward direction is formed on the transmission case  61 A. This rib  201  extends below and over the parts (crankshaft  51 , second kick intermediate shaft  155 , driven pulley shaft  64  and gears and the centrifugal clutch  80  which are supported on the crankshaft  51 , second kick intermediate shaft  155  and driven pulley shaft  64 ) disposed between the transmission case  61 A and the crankcase  24 , and receives oil having lubricated the parts and allows the oil to drop into the transmission side oil chamber RB through a hole  201 A formed at a predetermined location. Since oil having lubricated the parts is collected into and dropped through the hole  201 A provided at the predetermined location in this manner the appearance of bubbles in the oil can be suppressed. 
     It is to be noted that, as shown in  FIG. 7 , also on the transmission side oil chamber RB side of the right crankcase  24 B, an oil receiving rib  203  and a hole  203 A formed in a substantially leftwardly and rightwardly symmetrical shape with the oil receiving rib  201  and the hole  201 A with respect to the parting plane between the right crankcase  24 B and the transmission case  61 A are provided. Oil from the parts is received by the left and right oil receiving ribs  201  and  203  and dropped from the predetermined location into the transmission side oil chamber RB. 
     The transmission side oil chamber RB extends forwardly and backwardly over a lower portion of the transmission case  61 A and is provided at a position at which it overlaps with the first oil chamber RO 1  and the second oil chamber RO 2  in the crankcase  24  as viewed in side elevation. 
     Further, as shown in  FIG. 6 , in the side wall of the right crankcase  24 B, first openings  211 ,  212  and  213  for communicating the first oil chamber RO 1  with the transmission side oil chamber RB are formed and a second opening  215  for communicating the transmission side oil chamber RB with the second oil chamber RO 2  is formed. Consequently, oil entering the first oil chamber RO 1  flows into the transmission side oil chamber RB through the first openings  211  to  213 , and the oil in the transmission side oil chamber RB flows into the second oil chamber RO 2  through the second opening  215 . Then, the oil entering the second oil chamber RO 2  flows into the strainer chamber  101  (refer to  FIG. 7 ) through the third opening  197  and sucked out by the oil pump  100 . 
     Therefore, describing the direction of a flow of oil briefly, oil entering the first oil chamber RO 1  first moves to the right of the engine  20  through the first openings  211  to  213  and enters the transmission side oil chamber RB. In the transmission side oil chamber RB, the oil moves in the rearward direction (backward direction) of the engine  20  and then moves in the leftward direction of the engine  20  through the second opening  215  until it enters the second oil chamber RO 2 . In the second oil chamber RO 2 , the oil moves in the forward direction of the engine  20  and the moves in the rightward direction of the engine  20  through the third opening  197  until it enters the strainer chamber  101 . 
     Since the engine oil flows into the strainer chamber  101  along a circulating path along which the engine oil circulates in the engine widthwise directions and forward and backward directions in the engine  20  in this manner, the oil flowing path in the oil chamber (crank side oil chamber RA and transmission side oil chamber RB) can be made long and the oil residence time can be made long. Consequently, the oil heat radiation amount can be increased as much. In addition, since the circulating path of the oil chamber is formed over the crankcase  24  and the transmission case  61 A, heat radiation of the oil can be carried out making use of the outer surfaces of both of the crankcase  24  and the transmission case  61 A. The heat radiation amount of the oil can be increased also by increase of the heat radiation face for the oil. 
     In addition, since, in the present configuration, returning oil heated by various portions such as the cylinder section  22  and the crankshaft  51  enters the first oil chamber RO 1  as described above, returning oil having passed through a high temperature portion of the engine  20  can be cooled efficiently through the longest oil flow path. 
     Further, in the present configuration, since the second opening  215  for communicating the transmission side oil chamber RB with the second oil chamber RO 2  is provided at a position lower than that of the first openings  211  to  213  which communicate the first oil chamber RO 1  with the transmission side oil chamber RB and the third opening  197  for communicating the second oil chamber RO 2  with the strainer chamber  101  is provided at a position lower than that of the second opening  215  as shown in  FIGS. 6 and 7 , the oil can be fed along the first oil chamber RO 1 →transmission side oil chamber RB→second oil chamber RO 2 →strainer chamber  101  making use of the gravity, and a situation in which the oil flows in the opposite direction to that of the flow described above can be eliminated. 
     Further, in the present configuration, since the first openings  211  to  213  and the second opening  215  are formed in a spaced relationship from each other in the forward and backward direction and also the second opening  215  and the third opening  197  are formed in a spaced relationship from each other in the forward and backward direction, the distance of movement in the oil chamber in the forward and backward direction of the engine can efficiently be made long. More particularly, at least by forming the first opening  211  at the front end of the bottom portion of the crankcase  24  and forming the second opening  215  at a rear end of the bottom portion of the crankcase  24  and further forming the third opening  197  on the front side of the bottom portion of the crankcase  24 , the distance of movement in the forward and backward direction of the engine can be made long and the oil heat radiation amount can efficiently be increased. 
     Further, in the present configuration, since a plurality of (in the present example, three) first openings  211  to  213  are formed in a spaced relationship from each other in the forward and backward direction, a wide oil path can be assured from the first oil chamber RO 1  to the transmission side oil chamber RB, and oil entering several locations of the front side, an intermediate portion in the forward and backward direction and a rear portion of the first oil chamber RO 1  can be introduced readily and immediately to the transmission side oil chamber RB side. Since it is considered that, during driving of the engine, the temperature of the transmission case  61 A is lower than that of the crankcase  24 , if oil in the crankcase  24  is fed immediately into the transmission case  61 A, then heat radiation of the oil can efficiently be carried out. Also by this, the oil heat radiation amount can be increased. 
     Further, since the first opening  213  is formed along a corner portion formed by the up/down partitioning rib  191  (and  193 ) and the front/rear partitioning rib  192  (and  194 ) as seen in  FIG. 6 , return oil flowing along the ribs  191  to  194  can be introduced smoothly into the transmission side oil chamber RB through the first opening  213 . In other words, it is possible to cause the up/down partitioning ribs  191  and  193  and the front/rear partitioning ribs  192  and  194  to function as guide members for guiding return oil from the cylinder section  22  into the transmission side oil chamber RB. 
     Further, in the present configuration, the transmission side oil chamber RB is formed over the right crankcase  24 B and the transmission case  61 A and a partition wall  217  for partitioning in the upward and downward direction is provided between the first openings  211  to  213  and the second opening  215  on the right crankcase  24 B side while no such partition wall is provided on the transmission case  61 A side. Therefore, oil entering the transmission side oil chamber RB through the first openings  211  to  213  does not flow rearwardly of the engine  20  and enter the second opening  215 , but the rearward flow of the oil is stopped by the partition wall  217  and flows in the rightward direction of the engine  20 . Consequently, the oil enters the second opening  215  bypassing the partition wall  217 . Consequently, the oil flow path in the transmission side oil chamber RB can be made long, and the oil heat radiation amount can be further increased. 
     Further, in the present configuration, since the first oil chamber RO 1  and the second oil chamber RO 2  are formed over the overall width of the crankcase  24 , heat of the oil can be radiated not only through the bottom plates  24 A 1  and  24 B 1  of the crankcase  24  but also through the opposite side walls. Further, heat of the oil can be discharged to the outside also through the bottom plate and the side walls of the transmission case  61 A by the transmission side oil chamber RB. Accordingly, a wide oil heat radiation face can be assured, and the oil heat radiation amount can be further increased. 
     As described above, in the present embodiment, since the transmission side oil chamber RB partitioned from the transmission accommodation section  61  is provided at a lower portion of the transmission case  61 A such that oil is circulated through the transmission side oil chamber RB, the oil flow path in the oil chamber can be made long and the oil residence time can be made long, and the oil heat radiation amount from the oil chamber can be increased. Accordingly, oil after heat radiation is reserved in the oil reservoir. Consequently, even if a large-sized oil cooler is not provided additionally, the oil cooling amount by the air-cooled engine can be enhanced. 
     In addition, in the present configuration, the crank side oil chamber RA formed in the crankcase  24  is divided into the first oil chamber RO 1  and the second oil chamber RO 2  and the first openings  211  to  213  which communicate with the transmission side oil chamber RB formed in the transmission case  61 A are provided in the first oil chamber RO 1  while the second opening  215  which communicates with the second oil chamber RO 2  is provided in the transmission side oil chamber RB, the oil flow path in the oil chamber can be made long efficiently and the oil residence time can be made long efficiently. Accordingly, the oil heat radiation amount from the oil chamber can be increased efficiently. 
     Further, since the third opening  197  which communicates with the strainer chamber  101  from which oil is sucked out by the oil pump  100  is provided in the second oil chamber RO 2 , oil in the first oil chamber RO 1  and the transmission side oil chamber RB enters the strainer chamber  101  past the second oil chamber RO 2 . Therefore, in comparison with the alternative configuration wherein return oil directly enters the strainer chamber  101 , the oil flow path and the oil residence time in the oil chamber can be made long and the oil heat radiation amount from the oil chamber can be increased. Consequently, oil whose heat has been radiated can be supplied to the strainer  103 . 
     Further, since the first oil chamber RO 1  is provided at a position to which return oil from the cylinder section  22  drops and the second oil chamber RO 2  is provided rearwardly of the first oil chamber RO 1  while the strainer chamber  101  is provided forwardly of the second oil chamber RO 2  below the first oil chamber R 01 , return oil from the cylinder section  22  can be dropped into the first oil chamber RO 1  with certainty so that heat radiation from the oil chamber can efficiently be carried out. Further, the first oil chamber RO 1  and the strainer chamber  101  can be disposed in an overlapping relationship with each other as viewed from above, and a limited space can be utilized efficiently to dispose the first oil chamber RO 1 , second oil chamber RO 2  and strainer chamber  101 . 
     Further, since the second opening  215  is positioned lower than the first openings  211  to  213  and the third opening  197  is positioned lower than the second opening  215 , oil can be introduced smoothly from the first oil chamber RO 1  to the transmission side oil chamber RB making use of the gravity and oil can be introduced smoothly from the transmission side oil chamber RB to the second oil chamber RO 2  making use of the gravity. 
     Further, since a lower portion of the transmission case  61 A is depressed toward the transmission chamber R 2  side to form the transmission side oil chamber RB, the transmission side oil chamber RB is positioned below the transmission chamber R 2 . If the transmission side oil chamber RB is positioned below the transmission chamber R 2 , then also the outer surface of the transmission side oil chamber RB on the transmission chamber R 2  side can be made function as an oil heat radiation face, and since the oil heat radiation face increases, the oil heat radiation amount can be increased as much. 
     Furthermore, since, in the present configuration, the up/down partitioning ribs  191  and  193  and the front/rear partitioning ribs  192  and  194  are provided so as to extend between the left and right walls of the crankcase  24  and the first opening  213  is provided at a position at which return oil from the cylinder section  22  which flows along the ribs  191  to  194  is introduced to the transmission side oil chamber RB, the ribs  191  to  194  can be caused to function as guide members for guiding return oil smoothly to the transmission side oil chamber RB. In this instance, since the guide members are provided so as to extend between the left and right walls of the crankcase  24 , return oil from the cylinder section  22  can be guided to the transmission side oil chamber RB with a higher degree of certainty. 
     Hereinafter, a wind guide structure of the belt type continuously variable transmission  60  is described. 
     Into the transmission chamber R 2 , that is, into the transmission accommodation section  61 , external air is introduced such that the belt type continuously variable transmission  60  is cooled with the thus introduced external air. 
     As shown in  FIG. 2 , an external air intake port  115  is provided at a front upper portion of the transmission case  61 A which corresponds to a position above the driving pulley  63  while an external air exhaust port  116  is provided at a rear upper portion of the transmission case  61 A which corresponds to a position above the driven pulley  67 . The external air intake port  115  and the external air exhaust port  116  have duct portions  115 A and  116 A which are provided in a spaced relationship from each other in the forward and backward direction and extend rearwardly upwardly in parallel to each other. The external air intake port  115  and the external air exhaust port  116  are formed integrally with the transmission case  61 A. A duct not shown is connected to an upper end portion of each of the external air intake port  115  and the external air exhaust port  116  such that external air can be communicated through the ducts. It is to be noted that in  FIG. 2  a drainage hole  62  is provided for discharging water therethrough in the transmission case  61 A (in the transmission chamber R 2 ). 
     Blowing fins  63 C for allowing the driving pulley  63  to function as a blower fan are provided on the fixed half  63 A of the driving pulley  63  disposed in the transmission accommodation section  61 . If the blowing fins  63 C revolve by rotation of the driving pulley  63 , then external air is taken into the transmission chamber R 2  from the external air intake port  115 . 
     Further, blowing fins  67 C for allowing the driven pulley  67  to function as a blower are provided also on the fixed half  67 A of the driven pulley  67  in the transmission accommodation section  61 . By revolution of the blowing fins  67 C, external air taken in from the external air intake port  115  can be taken in to the driven pulley  67  side in the transmission chamber R 2  and can be exhausted from the external air exhaust port  116 . By this, a flow of external air from the driving pulley  63  side toward the driven pulley  67  side is produced in the transmission chamber R 2 , and the belt type continuously variable transmission  60  is forcibly air-cooled. 
     It is to be noted that, in  FIG. 2 , the direction of rotation of each of the driving pulley  63  and the driven pulley  67  is indicated by an arrow mark. Both of the driving pulley  63  and the driven pulley  67  rotate in the clockwise direction as viewed in right side elevation so that external air can be taken in smoothly from the external air intake port  115  and the taken-in external air can be exhausted smoothly from the external air exhaust port  116 . 
       FIG. 10  is a view of the engine  20  as viewed from the lower side. As described hereinabove, in the present engine  20 , the crankcase  24  is composed of the left crankcase  24 A and the right crankcase  24 B and the transmission case  61 A is connected to the right side of the right crankcase  24 B, and this transmission case  61 A functions also as a clutch case for covering the centrifugal clutch  80 . Since an oil reserving section is formed also at a lower portion of the transmission case  61 A, the lower face of the crankcase  24  and the lower face of the transmission case  61 A become lower faces of the oil reserving sections (crank side oil chamber RA and transmission side oil chamber RB) and lie almost in flush with each other (refer to  FIG. 2 ). 
     In the present configuration, a pair of front and rear boss portions (step bar supporting portions)  36 B which project downwardly are provided in the oil reserving section (crank side oil chamber RA) of the crankcase  24  while a pair of front and rear boss portions (step bar supporting portions)  36 B which project downwardly are provided also on the oil reserving section (transmission side oil chamber RB) at the lower portion of the transmission case  61 A, and flange bolts not shown for attaching the step bar  36  extending in the leftward and rightward directions of the vehicle body are fastened to the boss portions  36 B. 
     By the configuration, the distance between the supporting portions of the step bar  36  in the leftward and rightward direction can be assured wider than that in an alternative case wherein the step bar  36  is supported otherwise only by the crankcase  24 . 
     Hereinafter, the gear damper  97  is described. 
       FIG. 11  is a view showing the gear damper  97  provided on the output power shaft  31  together with peripheral elements. 
     A damper holding member  98  is provided adjacent the right side of the final gear  95  on the output power shaft  31 . This damper holding member  98  is secured to the output power shaft  31  by force fitting so that it rotates integrally with the output power shaft  31 . 
     The final gear  95  is held for rotation on the output power shaft  31 , and an increased diameter portion  31 A serving as a spring receiving portion is provided integrally on the output power shaft  31  on the left side of the final gear  95 . A spring member  99  (in the present example, a plurality of disk springs) is interposed between the increased diameter portion  31 A and a left end face of the final gear  95  such that the final gear  95  is biased toward the damper holding member  98  side by elastic force of the spring members  99 . 
       FIG. 12(A)  is a side elevational view of the final gear  95  and  FIG. 12(B)  is a view showing a section taken along line A 1 -A 1  of the final gear  95 .  FIG. 13(A)  is a side elevational view of the damper holding member  98  and  FIG. 13(B)  is a view showing a section taken along line A 2 -A 2  of the damper holding member  98 . 
     As shown in  FIGS. 12(A) to 13(B) , a plurality of (in the example shown, three) depressed cams  95 A are formed at intervals of an equal angle on a face of the final gear  95  on the damper holding member  98  side. Projecting cams  98 A for meshing with the depressed cams  95 A are formed on a face of the damper holding member  98  on the final gear  95  side. 
     Where driving torque acts from the engine  20  side and torque in the opposite direction to the driving direction (so-called back torque) does not act from the driving wheel side (rear wheel  15  side), the depressed cams  95 A of the final gear  95  and the projecting cams  98 A of the damper holding member  98  mesh with each other, and the output power shaft  31  is driven to rotate by the driving torque from the engine  20  side so that the rear wheel  15  serving as a driving wheel is driven. 
     On the other hand, if back torque acts from the driving wheel side (rear wheel  15  side), then the projecting cams  98 A of the damper holding member  98  circumferentially slip with respect to the depressed cams  95 A of the final gear  95  against the elastic force of the spring members  99  acting upon the final gear  95  thereby to moderate transmission of back torque to the engine  20  side. A gear damper of the cam type which absorbs from the driving wheel side by this action is disposed in the crankcase  24 . 
     While the present invention has been described in connection with an embodiment thereof, the present invention is not limited to this. For example, while, in the embodiment described above, the driven pulley shaft (driven shaft)  64  is supported by the pair of left and right bearings  65 ,  65  individually disposed in the right crankcase  24 B and the transmission case  61 A, the supporting configuration for the driven pulley shaft  64  is not limited to this. As shown in  FIG. 14  in which an example is shown, the left end of the crankcase  24  is extended to the left through the right crankcase  24 B until it is supported by one of the bearings  65  disposed on the left crankcase  24 A. With the present configuration, since the clutch output gear  84  provided on the driven pulley shaft  64  is disposed in the left and right crankcases  24 A and  24 B, the intermediate shaft driven gear (speed reduction gear)  93  which meshes with the clutch output gear  84  is positioned in the left and right crankcases  24 A and  24 B, and the necessity for a member for preventing letting off of the intermediate shaft driven gear  93  is eliminated. 
     Further, in the configuration shown in  FIG. 14 , instead of the fact that the gear damper  97  is not provided for the output power shaft  31  of the engine  20 , an output power shaft gear  31 X which meshes with the intermediate shaft driving gear  94  for transmitting rotation of the intermediate shaft driven gear  93  to the output power shaft  31  is force-fitted with or spline-coupled to the output power shaft  31  so that the output power shaft  31  is driven to rotate. In this manner, presence or absence of the gear damper  97 , the supporting position of the driven pulley shaft (driven shaft)  64  and so forth can be readily changed in design. 
     Further, while, in the embodiment described above, the present invention is applied to a single-cylinder engine, the application of the present invention is not limited to this, and the present invention may be applied to a so-called V-type engine wherein different cylinders are disposed so as to form a predetermined angle of nip therebetween or a parallel type engine wherein different cylinders are disposed in parallel to each other. 
     Further, while, in the embodiment described above, the present invention is applied to an internal combustion engine for a motorcycle, the application of the present invention is not limited to this, and it is possible to apply the present invention to an internal combustion engine which is used in other vehicles than the motorcycle. 
     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.