Patent Publication Number: US-11035290-B2

Title: Internal combustion engine

Description:
FIELD OF THE INVENTION 
     The present invention relates to an internal combustion engine comprising: a crankcase supporting a crankshaft rotatably about a rotation axis of the crankshaft, the crankcase accommodating therein a multi-speed transmission; a cylinder block joined to the crankcase, and having at least one cylinder axis that is located in a vertical plane orthogonal to the rotation axis and rises relative to a horizontal plane; a main shaft incorporated in the multi-speed transmission and rotatably supported on the crankcase, the main shaft coaxially supporting thereon a primary driven gear, which is in mesh with a primary drive gear of the crankshaft, and a one-way clutch gear, and; a reduction gear including, at one end thereof, a small-diameter gear in mesh with the one-way clutch gear and at an opposite end thereof, a large-diameter gear in mesh with a drive gear of a starter motor 
     DESCRIPTION OF THE RELATED ART 
     Japanese Patent Application Laid-open No. 10-77936 discloses an engine unit (internal combustion engine) mounted on a body frame of a two-wheeled motor vehicle. The engine unit includes a crankcase supporting a crankshaft rotatably about a rotation axis thereof and accommodating a transmission device (multi-speed transmission) therein. Joined to a crankcase is a cylinder block having cylinder axes that are located in a vertical plane orthogonal to a rotation axis of the crankshaft and rise relative to a horizontal plane. 
     The transmission device has a main shaft rotatably supported on the crankcase, and coaxially supporting thereon a primary driven gear, which is in mesh with a primary drive gear of the crankshaft, and a one-way clutch gear. A reduction gear of the starter motor is in mesh with the one-way clutch on the main shaft so that a driving force of the starter motor can be transmitted to the crankshaft via the primary driven gear. 
     SUMMARY OF THE INVENTION 
     In the engine unit described in Japanese Patent Application Laid-open No. 10-77936, a reentrant space formed between an upper part of the crankcase and a rear wall of the cylinder block is left as a dead space. There is, accordingly, a desire to efficiently dispose components there. 
     The present invention has been achieved in view of the above-mentioned circumstances, and it is an object thereof to provide an internal combustion engine that makes it possible to efficiently dispose components in a reentrant space formed between a crankcase and a cylinder block. 
     According to a first aspect of the present invention, there is provided an internal combustion engine comprising: a crankcase supporting a crankshaft rotatably about a rotation axis of the crankshaft, the crankcase accommodating therein a multi-speed transmission; a cylinder block joined to the crankcase, and having a cylinder axis that is located in a vertical plane orthogonal to the rotation axis and rises relative to a horizontal plane; a main shaft incorporated in the multi-speed transmission and rotatably supported on the crankcase, the main shaft coaxially supporting thereon a primary driven gear, which is in mesh with a primary drive gear of the crankshaft, and a one-way clutch gear, and; a reduction gear including, at one end thereof, a small-diameter gear in mesh with the one-way clutch gear and at an opposite end thereof, a large-diameter gear in mesh with a drive gear of a starter motor, wherein the starter motor has a driveshaft disposed below a rotation axis of the reduction gear and within a width of the large-diameter gear as viewed in an axial direction. That is, the starter motor has the driveshaft with an axis thereof disposed below the rotation axis of the reduction gear and in a space sandwiched between a first vertical plane, which is parallel to the rotation axis of the crankshaft and circumscribes the large-diameter gear from one direction, and a second vertical plane, which is parallel to the first vertical plane and circumscribes the large-diameter gear from an other direction. 
     With the first aspect, the starter motor and the reduction gear are placed side by side in a longitudinal direction, and therefore the starter motor can be efficiently disposed in the reentrant space formed between the crankcase and the cylinder block. 
     According to a second aspect of the present invention, in addition to the first aspect, the driveshaft has an axis disposed within a width of the small-diameter gear as viewed in the axial direction. That is, the driveshaft has the axis disposed in a space sandwiched between a third vertical plane, which is parallel to the rotation axis of the crankshaft and circumscribes the small-diameter gear from one direction, and a fourth vertical plane, which is parallel to the third vertical plane and circumscribes the small-diameter gear from an other direction. 
     With the second aspect, the starter motor and the reduction gear are placed side by side in the longitudinal direction to a maximum extent possible, and therefore the starter motor can be more efficiently disposed in the reentrant space formed between the crankcase and the cylinder block. 
     According to a third aspect of the present invention, in addition to the first aspect t, there is provided the internal combustion engine, further comprising: a clutch connected to the primary driven gear on the main shaft and switching between transmission and non-transmission of a driving force of the crankshaft, wherein the driveshaft has an axis disposed inside an imaginary cylindrical plane that is coaxial with the main shaft and circumscribes the clutch. 
     With the third aspect, the axis of the driveshaft of the starter motor is located in the space surrounded by the imaginary cylindrical plane that is coaxial with the main shaft and circumscribes the clutch, and therefore the starter motor can be disposed in a compact configuration in the reentrant space formed between the crankcase and the cylinder block. 
     According to a fourth aspect of the present invention, in addition to the third aspect, the rotation axis of the reduction gear is disposed outside the imaginary cylindrical plane. 
     With the fourth aspect, the starter motor and the reduction gear are placed side by side in the longitudinal direction, and therefore the starter motor can be efficiently disposed in the reentrant space formed between the crankcase and the cylinder block. 
     According to a fifth aspect of the present invention, in addition to the first aspect, the crankcase has an upper wall part, which covers the main shaft while bulging out along an imaginary cylindrical plane that is coaxial with the main shaft, whereby a reentrant space is formed between the crankcase and the cylinder block, and the starter motor is disposed in the reentrant space. 
     With the fifth aspect, the starter motor can be efficiently disposed in a dead space defined between the upper wall part of the crankcase and the cylinder block. 
     According to a sixth aspect of the present invention, in addition to the fifth aspect, the crankcase has a thickened portion surrounding a cylinder, which guides a piston, while bulging out toward the starter motor. 
     With the sixth aspect, the starter motor and the reduction gear are disposed in the longitudinal direction, and therefore a space sufficient to dispose the thickened portion can be left in the reentrant space formed between the crankcase and the cylinder block. 
     According to a seventh aspect of the present invention, in addition to the first aspect, there is provided the internal combustion engine, further comprising: a canister disposed above the starter motor at a location offset from the reduction gear in an axial direction of the main shaft. 
     With the seventh aspect, the starter motor and the reduction gear are disposed in the longitudinal direction, and therefore a space is created above the starter motor and behind the reduction gear and a canister having a sufficient volume can be efficiently disposed in the space. 
     According to an eighth aspect of the present invention, in addition to the first aspect, the starter motor has a flange outwardly extending in a horizontal direction from a cylindrical outer surface of the starter motor and fixed on the crankcase. 
     With the eighth aspect, the starter motor can be disposed closer to a bottom of the reentrant space compared with a case where a flange extends upward or downward in a vertical direction from the outer surface, and therefore the starter motor as a heavy component can be placed closer toward a center of the internal combustion engine, thereby making it possible to contribute to mass centralization. 
     The above and other objects, characteristics and advantages of the present invention will be clear from detailed descriptions of the preferred embodiment, which will be provided below while referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically depicting the overall configuration of a two-wheeled motor vehicle according to an embodiment. 
         FIG. 2  is an enlarged side view schematically depicting surroundings of an internal combustion engine as observed on a vertical section. 
         FIG. 3  is an enlarged sectional view of the internal combustion engine, which schematically depicts a structure as observed on a section including a rotation axis of a crankshaft and axes of a main shaft and a counter shaft. 
         FIG. 4  is a top view of the internal combustion engine, which includes in a part thereof a section taken along a horizontal plane. 
         FIG. 5  is an enlarged vertical sectional view along line  5 - 5  of  FIG. 4 . 
         FIG. 6  is an enlarged vertical sectional view along line  6 - 6  of  FIG. 4 . 
         FIG. 7  is an enlarged vertical sectional view along line  7 - 7  of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the attached drawings, an embodiment of the present invention will be described hereinafter. Here, the up/down, front/rear and left/right of a vehicle body are assumed to be defined based on the level of the eyes of a passenger riding a two-wheeled motor vehicle. 
       FIG. 1  schematically depicts an overall image of the two-wheeled motor vehicle as a saddle-ridden vehicle according to the embodiment of the present invention. The two-wheeled motor vehicle  11  includes a body frame  12  and a body cover  13  attached to the body frame  12 . The body cover  13  has a front cowl  14  and a tank cover  17 . The front cowl  14  covers the body frame  12  from the front thereof. The tank cover  17  continues forward from an outer surface of a fuel tank  15 , and is joined to a passenger&#39;s seat  16  at a rear of the fuel tank  15 . In the fuel tank  15 , fuel is stored. Upon operating the two-wheeled motor vehicle  11 , a passenger straddles the passenger&#39;s seat  16 . 
     The body frame  12  has a head pipe  18 , a pair of left and right main frames  21  extending rearwardly downward from the head pipe  18  and having pivot frames  19  at lower rear ends thereof, respectively, a down frame  22  extending, at a location below the main frames  21 , downwardly from the head pipe  18  and integrated with the main frames  21 , and left and right seat frames  23  extending rearwardly upward from curved areas  21   a  of the main frames  21 , respectively, and constructing a truss structure. The passenger&#39;s seat  16  is supported on the seat frames  23 . 
     A front fork  24  is steerably supported on the head pipe  18 . On the front fork  24 , a front wheel WF is supported rotatably about an axle  25 . A steering handlebar  26  is connected to an upper end of the front fork  24 . Upon operating the two-wheeled motor vehicle  11 , a rider holds grips on left and right ends of the steering handlebar  26 . 
     In a rear section of the two-wheeled motor vehicle  11 , a swingarm  28  is connected to the body frame  12  so that the swingarm  28  is swingable up and down about a pivot  27 . On a rear end of the swingarm  28 , a rear wheel WR is supported rotatably about an axle  29 . Between the front wheel WF and the rear wheel WR, an internal combustion engine  31  is mounted on the body frame  12  to generate a driving force which is to be transmitted to the rear wheel WR. The power of the internal combustion engine  31  is transmitted to the rear wheel WR via a power transmission device  69  (see  FIG. 3 ). 
     The internal combustion engine  31  includes a crankcase  32 , a cylinder block  33 , a cylinder head  34 , and a head cover  35 . The crankcase  32  is s disposed between the down frame  22  and the main frames  21 , is joined to and supported on the down frame  22  and the main frames  21 , respectively, and outputs power about a rotation axis Rx. The cylinder block  33  has cylinder axes C that are located in a vertical plane orthogonal to the rotation axis Rx and rise relative to a horizontal plane. The cylinder head  34  is joined to an upper end of the cylinder block  33 , and supports a valve mechanism. The head cover  35  is joined to an upper end of the cylinder head  34 , and covers the valve mechanism on the cylinder head  34 . 
     The two-wheeled motor vehicle  11  includes a canister  36 , which is disposed below the fuel tank  15 , above the crankcase  32  and behind the cylinder block  33 , is connected with the fuel tank  15 , and holds fuel vapor gas occurred from the fuel tank  15 . The canister  36  has a center axis Dx extending parallel to the rotation axis Rx in a vehicle width direction, and has a cylindrical body defining a space in which activated carbon is held. Therefore, the canister  36  has an external shape formed in a columnar shape. 
     As depicted in  FIG. 2 , the cylinder block  33  includes cylinders  38  formed therein to guide linear reciprocating motion of pistons  37  along the cylinder axes C, respectively. In this embodiment, the as many as four cylinders  38  are formed side by side along the rotation axis Rx in the cylinder block  33 , so that the internal combustion engine  31  has a so-called in-line four-cylinder configuration. Between the pistons  37  and the cylinder head  34 , combustion chambers  39  are defined, respectively. By the functions of intake valves  41   a  and exhaust valves  41   b  that open and close according to the rotation of the camshaft, an air-fuel mixture is introduced into the combustion chambers  39 , and subsequent to combustion, the resulting exhaust gas is discharged from the combustion chamber  39 . 
     As depicted in  FIG. 3 , a crankshaft  43  is supported rotatably about the rotation axis Rx on the crankcase  32 . The crankshaft  43  includes journals  44  connected to sliding bearings, respectively, and cranks  46  disposed between the adjacent journals  44 , respectively, and having crankpins  45  extending parallel to the rotation axis Rx and connecting associated crankwebs with each other. Connecting rods  47  extend from the pistons  37 , respectively, and are rotatably connected at enlarged end portions thereof to the associated crankpins  45 . The connecting rods  47  convert linear reciprocating motion of the associated pistons  37  to rotational motion of the crankshaft  43 . 
     The crankshaft  43  projects out at one end thereof from a left side surface of the crankcase  32 . An alternating current generator (ACG)  48  is connected to the one end of the crankshaft  43 . On the left side surface of the crankcase  32 , an ACG cover  49  is joined to accommodate the ACG  48  between the crankcase  32  and the ACG cover  49 . The ACG  48  has a stator  51  fixed on the ACG cover  49 , and a rotor  52  connected to the one end of the crankshaft  43  projecting out from the crankcase  32 , so that the rotor  52  and the crankshaft  43  are incapable of rotation relative to each other. The stator  51  has a plurality of coils  51   a  arrayed in a peripheral direction about the crankshaft  43  and wound on stator cores, respectively. The rotor  52  has a plurality of magnets  52   a  arrayed in a peripheral direction along an annular track that surrounds the stator  51 . When the crankshaft  43  rotates, the magnets  52   a  undergo relative displacement to the coils  51   a  so that the ACG  48  generates electricity. 
     The crankshaft  43  projects out at an opposite end thereof from a right side surface of the crankcase  32 . To the opposite end of the crankshaft  43 , a cam drive mechanism  53  is connected to transmit power to a camshaft. The cam drive mechanism  53  includes a drive cam gear  53   a  coaxially fixed on the crankshaft  43 , a driven cam gear (not depicted) fixed on the camshaft, and a cam gear train  53   b  constructed of a plurality of gears and configured to mesh with the drive cam gear  53   a  and the driven cam gear sequentially in this order to transmit power from the drive cam gear  53   a  to the driven cam gear. On the right side surface of the crankcase  32 , a cam drive mechanism cover  54  is joined to accommodate the drive cam gear  53   a  between the crankcase  32  and the cam drive mechanism cover  54 . The ACG cover  49  and the cam drive mechanism cover  54  cover an outer surface of the crankcase  32  to define a crank chamber  55  that accommodates the crankshaft  43  therein. The cam drive mechanism  53  may include a drive sprocket, driven sprocket and cam chain in place of the drive cam gear  53   a , driven cam gear and cam gear train  53   b.    
     A multi-speed transmission of the dog clutch type (hereinafter “the transmission”)  56  is incorporated in the internal combustion engine  31 . The transmission  56  is accommodated in a transmission case  57  defined in the crankcase  32  and continuing from the crank chamber  55 . The transmission  56  includes a main shaft  58  and a counter shaft  59 , which have axes parallel to the axis of the crankshaft  43 . The main shaft  58  and counter shaft  59  are rotatably supported on the crankcase  32  via rolling bearings  61   a ,  61   b ,  62   a , and  62   b.    
     On the main shaft  58  and counter shaft  59 , a plurality of transmission gears  63  are supported. The transmission gears  63  supported on the main shaft  58  are disposed between the bearings  61   a  and  61   b , while the transmission gears  63  supported on the counter shaft  59  are disposed between the bearings  62   a  and  62   b . The transmission gears  63  are accommodated in the transmission case  57 . The transmission gears  63  include rotation gears  63   a  coaxially supported for relative rotation on the main shaft  58  and the counter shaft  59 , respectively, stationary gears  63   b  fixed on the main shaft  58  for non-rotation relative to each other and meshable with the corresponding rotation gears  63   a , and shift gears  63   c  supported on the main shaft  58  and the counter shaft  59 , respectively, for non-rotation relative to each other and for axial displacement and meshable with the corresponding rotation gears  63   a . Axial displacement of the rotation gears  63   a  and stationary gears  63   b  is restricted. When the shift gear  63   c  is connected to the corresponding rotation gear  63   a  due to axial displacement, relative rotation between the rotation gear  63   a  and the main shaft  58  or counter shaft  59  is restricted. When the shift gear  63   c  meshes with the corresponding stationary gear  63   b , rotating power is transmitted between the main shaft  58  and the counter shaft  59 . When the shift gear  63   c  is connected to the rotation gear  63   a  meshing with the corresponding stationary gear  63   b , rotating power is transmitted between the main shaft  58  and the counter shaft  59 . Through meshing of particular transmission gears  63  between the main shaft  58  and the counter shaft  59  as described above, rotating power is transmitted at a specified reduction ratio from the main shaft  58  to the counter shaft  59 . 
     The main shaft  58  projects out at one end thereof from the right side surface of the crankcase  32 . Outside the crankcase  32 , a primary driven gear  65  and a one-way clutch gear  66  are coaxially supported for relative rotation on the one end of the main shaft  58 . The primary driven gear  65  is in mesh with a primary drive gear  64  of the crankshaft  43 , and the one-way clutch gear  66  is connected to the primary driven gear  65 . The primary drive gear  64  is formed, for example, integrally with the crank  46  of the crankshaft  43 . The one-way clutch gear  66  applies a rotational force to the primary driven gear  65  upon rotation in one direction according to an external force acting from its gear teeth, but rotates relative to the primary driven gear  65  and remains in a stationary state on the main shaft  58  upon rotation of the primary driven gear  65  according to a drive force from the crankshaft  43 . 
     On the main shaft  58 , a friction clutch  67  (as a clutch) is connected to the primary driven gear  65 . To the right side surface of the crankcase  32 , a clutch cover  68  is joined to accommodate the friction clutch  67  between the crankcase  32  and the clutch cover  68 . The friction clutch  67  includes a clutch outer  67   a  and a clutch hub  67   b . The primary driven gear  65  is connectable to the clutch outer  67   a . According to the operation of a clutch lever, connection and disconnection are switched between the clutch outer  67   a  and the clutch hub  67   b  in the friction clutch  67 . 
     To the counter shaft  59 , a drive sprocket  69   a  of the power transmission device  69  disposed outside the crankcase  32  is connected. A drive chain  69   b  is wrapped around the drive sprocket  69   a . The drive chain  69   b  transmits rotating power of the drive sprocket  69   a  to the rear wheel WR. 
     As depicted in  FIG. 2 , the crankcase  32  has an upper wall part  72  forming a reentrant space  71  between the upper wall part  72  and the cylinder block  33 , the upper wall part  72  covering the main shaft  58  while bulging out along an imaginary cylindrical plane that is coaxial with the main shaft  58 . In the reentrant space  71 , a starter motor  73  is disposed below the canister  36 . 
     The starter motor  73  includes a cylindrical housing  73   a  having a central axis in parallel with the rotation axis Rx. The housing  73   a  accommodates a rotor and a stator, the rotor being connected to a driveshaft having an axis on the central axis, and the stator surrounding the rotor. The housing  73   a  has a pair of flanges  75  outwardly extending in horizontal directions from a cylindrical outer surface. The flanges  75  are fixedly secured on the crankcase  32  by bolt members having axes that are parallel to the central axis. 
     On the crankcase  32 , a base  76  is formed, and a mating face  76   a  which is mated with the cylinder block  33  is formed on the base  76 . The base  76  defines cylindrical cavities for receiving cylinder liners which guide linear reciprocating motion of the associated pistons  37 . The base  76  has a thickened portion  76   b  that bulges out toward the starter motor  73  side while surrounding the cylinders  38 . The thickened portion  76   b  continues from the upper wall part  72 , and a thickness of the thickened portion  76   b  becomes larger than a thickness of the upper wall  72  in going toward the mating face  76   a.    
     As depicted in  FIG. 4 , the driveshaft  77  of the stator motor  73  is connected to the one-way clutch  66  via a reduction gear  78 . The reduction gear  78  includes a shaft body  78   a  supported rotatably about a rotation axis Gx of the reduction gear  78  on the crankcase  32 . As depicted in  FIG. 5 , a small-diameter gear  79  is fixed on one end of the shaft body  78   a , coaxially with shaft body  78   a , and in mesh with the one-way clutch gear  66  outside the crankcase  32 . Above a horizontal plane Hr in which a rotation axis of the one-way clutch gear  66  is included, the small-diameter gear  79  is accommodated between a perpendicular plane PL 1 , in which the rotation axis of the one-way clutch gear  66  is included, and a perpendicular plane PL 2 , which circumscribes the one-way clutch gear  66  from the front. 
     As depicted in  FIG. 6 , a large-diameter gear  83  is formed on an opposite end of the shaft body  78   a , coaxially with the shaft body  78   a , and in mesh with a drive gear  82  of the starter motor  73  in a gear compartment  81 . The drive gear  82  is cut, for example, on the driveshaft  77  of the starter motor  73 . Rotation of the driveshaft  77  is reduced at the reduction gear  78 , and then transmitted to the one-way clutch gear  66 . The starter motor  73  generates a driving force that forcedly rotates the crankshaft  43 . As depicted in  FIG. 4 , the canister  36  is disposed above the starter motor  73  at a location offset from the gear compartment  81  in an axial direction of the main shaft  58 . 
     In this embodiment, the driveshaft  77  of the starter motor  73  is disposed below the rotation axis Gx of the reduction gear  78  and within a width of the large-diameter gear  83  as viewed in the axial direction. In other words, the driveshaft  77  of the starter motor  73  has the axis  77   a  disposed below the rotation axis Gx of the reduction gear  78  and in a space sandwiched between a first vertical plane VP 1 , which is parallel to the rotation axis Rx of the crankshaft  43  and circumscribes the large-diameter gear  83  from one direction (the front), and a second vertical plane VP 2 , which is parallel to the first vertical plane VP 1  and circumscribes the large-diameter gear  83  from an other direction (the rear). In addition, the axis  77   a  of the driveshaft  77  is disposed within the width of the small-diameter gear  79  as viewed in the axial direction. In other words, the axis  77   a  of the driveshaft  77  is located in a space sandwiched between a third vertical plane VP 3 , which is parallel to the rotation axis Rx of the crankshaft  43  and circumscribes the small-diameter gear  79  from one direction (the front), and a fourth vertical plane VP 4 , which is parallel to the third vertical plane VP 3  and circumscribes the small-diameter gear  79  from an other direction (the rear). 
     As depicted in  FIG. 7 , the primary driven gear  65  has an external diameter formed greater than an external diameter of the one-way clutch gear  66  that is in mesh with the small-diameter gear  79  at external teeth of both the gears. The rotation axis Gx of the reduction gear  78  is disposed outside an imaginary cylindrical plane Cv, which is coaxial with the main shaft  58  and circumscribes the friction clutch  67 . In this embodiment, the rotation axis Gx of the reduction gear  78  is also disposed outside an imaginary cylindrical plane Cq, which is coaxial with the main shaft  58  and circumscribes the primary driven gear  65 . The driveshaft  77  of the starter motor  73  has the axis  77   a  disposed inside the imaginary cylindrical plane Cv. 
     The operation of this embodiment will next be described. When electric power is supplied to the starter motor  73  upon starting the internal combustion engine  31 , the driveshaft  77  rotates in a specified direction about the axis  77   a . The rotation of the driveshaft  77  is transmitted to the large-diameter gear  83  of the reduction gear  78 . As the large-diameter gear  83  has a number of teeth significantly greater than the drive gear  82 , the reduction gear  78  rotates at a lower speed than the driveshaft  77 . The rotation of the reduction gear  78  is transmitted from the small-diameter gear  79  to the one-way clutch gear  66 . As the one-way clutch gear  66  has a significantly greater number of teeth than the small-diameter gear  79 , the one-way clutch gear  66  rotates at a lower speed than the reduction gear  78 . 
     The one-way clutch gear  66  rotates together with the primary driven gear  65  on the main shaft  58 . The rotation of the primary driven gear  65  is transmitted from the primary drive gear  64  to the crankshaft  43 . The crankshaft  43  is forcedly rotated in a specified direction. Combustion starts in the combustion chambers  39 , and the pistons  37  start linear reciprocating motion in the cylinders  38 . The operation of the starter motor  73  is now ended. 
     When the linear reciprocating motion of the pistons  37  is started according to the combustion operation, rotational motion is transmitted from the primary drive gear  64  to the clutch outer  67   a  via the primary driven gear  65 . The rotation of the primary driven gear  65  is not transmitted to the one-way clutch gear  66 , and therefore the one-way clutch gear  66  remains in a stationary state on the main shaft  58 . Hence, loading on the starter motor  73  is avoided during operation of the internal combustion engine  31 . 
     In this embodiment, the driveshaft  77  of the starter motor  73  is disposed below the rotation axis Gx of the reduction gear  78  and within the width of the large-diameter gear  83  as viewed in the axial direction. Specifically, the driveshaft  77  of the starter motor  73  has the axis  77   a  disposed in the space sandwiched between the first vertical plane VP 1 , which is parallel to the rotation axis Rx of the crankshaft  43  and circumscribes the large-diameter gear  83  from one direction (the front), and the second vertical plane VP 2 , which is parallel to the first vertical plane VP 1  and circumscribes the large-diameter gear  83  from an other direction (the rear). The starter motor  73  and the reduction gear  78  are placed side by side in the longitudinal direction, and therefore the starter motor  73  is efficiently disposed in the reentrant space  71  formed between the crankcase  32  and the cylinder block  33 . 
     In particular, the axis  77   a  of the driveshaft  77  is disposed within the width of the small-diameter gear  79  as viewed in the axial direction. That is, the axis  77   a  of the driveshaft  77  is located in the space sandwiched between the third vertical plane VP 3 , which is parallel to the rotation axis Rx of the crankshaft  43  and circumscribes the small-diameter gear  79  from one direction (the front), and the fourth vertical plane VP 4 , which is parallel to the third vertical plane VP 3  and circumscribes the small-diameter gear  79  from an other direction (the rear). The starter motor  73  and the reduction gear  78  are, therefore, placed side by side in the longitudinal direction to a maximum extent possible, and the starter motor  73  is more efficiently disposed in the reentrant space  71  formed between the crankcase  32  and the cylinder block  33 . 
     Further, the axis  77   a  of the driveshaft  77  is disposed inside the imaginary cylindrical plane Cv, which is coaxial with the main shaft  58  and circumscribes the friction clutch  67 . The axis  77   a  of the driveshaft  77  is located in the space surrounded by the imaginary cylindrical plane Cv as described above, so that the starter motor  73  is disposed in a compact configuration in the reentrant space  71  formed between the crankcase  32  and the cylinder block  33 . 
     In this embodiment, the rotation axis Gx of the reduction gear  78  is disposed outside the imaginary cylindrical plane Cv. As the starter motor  73  and the reduction gear  78  are placed side by side in the longitudinal direction, the starter motor  73  is efficiently disposed in the reentrant space  71  formed between the crankcase  32  and the cylinder block  33 . 
     The crankcase  32  has the upper wall part  72 , which covers the transmission gear  63  on the main shaft  58  while bulging out along the imaginary cylindrical plane that is coaxial with the main shaft  58 , and which forms the reentrant space  71  between the upper wall part  72  and the cylinder block  33 . The starter motor  73  is disposed in the reentrant space  71 . The starter motor  73  is efficiently disposed in the dead space defined between the upper wall part  72  of the crankcase  32  and the cylinder block  33 . 
     The crankcase  32  has the thickened portion  76   b  that bulges out toward the starter motor  73  side while surrounding the cylinders  38  that guide the associated pistons  37 . The starter motor  73  and the reduction gear  78  are disposed in the longitudinal direction, and therefore a space sufficient to dispose the thickened portion  76   b  therein is left in the reentrant space  71  formed between the crankcase  32  and the cylinder block  33 . 
     In the two-wheeled motor vehicle  11  according to this embodiment, the canister  36  is disposed above the starter motor  73  at the location offset from the gear compartment  81  of the reduction gear  78  in the axial direction of the main shaft  58 . As the starter motor  73  and the reduction gear  78  are disposed in the longitudinal direction, the space is formed above the starter motor  73  and behind the gear compartment  81  and the canister  36  which has a sufficient volume is efficiently disposed in the space. 
     The starter motor  73  has the flanges  75 , which outwardly extend in the horizontal directions from the cylindrical outer surface and are fixed on the crankcase  32 . The starter motor  73  is disposed closer to the bottom of the reentrant space  71  compared with a case where flanges extend upward or downward in the vertical direction from the outer surface, and therefore the starter motor  73  as a heavy component is placed closer toward a center of the internal combustion engine  31 , thereby contributing to mass centralization.