Patent Publication Number: US-6904889-B2

Title: Engine for personal watercraft and crankshaft thereof

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to an engine for a personal watercraft and a crankshaft thereof. More particularly, the present invention relates to an engine mounted in a body for driving a propulsion device of a personal watercraft that ejects water rearward and planes on a water surface as the resulting action, and a crankshaft suitable for the engine. 
   In recent years, so-called jet-propulsion personal watercraft (PWC) have been widely used in leisure, sport, rescue activities, and the like.  FIG. 5  shows an example of such personal watercraft. 
   In a personal watercraft  81  in  FIG. 5 , A denotes a body of the watercraft. The body A comprises a hull H and a deck D covering the hull H from above. A line at which the hull H and the deck D are connected over the entire perimeter thereof is called a gunnel line G. L denotes a waterline in a state of the watercraft. 
   A straddle-type seat S is mounted onto an upper surface of the body A so as to extend substantially from a center portion to a rear portion over the deck D along the longitudinal direction of the watercraft  81 . Straddling the straddle-type seat S, the rider grips a steering handle B located forward of the seat S and steers the watercraft. Hereinafter, “forward” refers to the direction toward which the watercraft moves and “rearward” refers to the opposite side. 
   The personal watercraft  81  is configured to have a water jet pump  83  that pressurizes and accelerates water sucked from a water intake  82  generally provided on a hull bottom surface of the body A and ejects the water rearward through a steering nozzle  84 . Thereby, the body A is propelled. An engine  85  for driving the water jet pump  83  is contained in an engine room R located below the seat S. A propeller shaft  86  is connected to an output shaft of the engine  85  and an impeller  87  of the water jet pump  83  is attached on the propeller shaft  86 . 
   Commonly, a two-cycle engine is mounted in the personal watercraft as the engine  85 . Recently, in addition to this, the use of a four-cycle engine has been proposed. 
     FIG. 6  shows an example of the four-cycle engine. The engine  85  is a four-cylinder engine and is placed such that cylinders are arranged along the longitudinal direction of the body A (FIG.  5 ). Reference numeral  88  denotes a cylinder,  89  denotes a piston,  90  denotes a crankshaft,  91  denotes a crankcase,  92   a  denotes a cylinder head, and  92   b  denotes a head cover. A cam chain tunnel  93  is formed in a rear end portion of the engine  85 , and a generator  94  and a starter motor  95  are mounted in a front end portion of the engine. 
   A rear end portion of the crankshaft  90  of the engine  85  penetrates through a rear end wall  91   a  of the crankcase  91  and penetrates through the cam chain tunnel  93  so as to protrude outwardly in the rear end of the engine  85 . A coupling  86   a  for connecting the propeller shaft  86  ( FIG. 5 ) is mounted to the rear end of the crankshaft  90 . A sprocket  93   b  for a cam chain  93   a  is mounted to a position of the crankshaft  90  inside the cam chain tunnel  93 . Meanwhile, a front end portion of the crankshaft  90  penetrates through a front end wall  91   b  of the crankcase  91  and protrudes into a housing  94   a  covering the generator  94 . A rotor  94   b  of the generator  94  is mounted to the front end portion of the crankshaft  90 . A stator  94   c  of the generator  94  is fixed to the housing  94   a.    
   Since the above-structured engine has weight more than that of the two-cycle engine, it is desirable to position the engine in the personal watercraft so as to minimize the effect of the engine having more weight on cruising attitude of the watercraft. However, since placement of the engine is restricted by the space of the engine room of the personal watercraft, it is difficult to greatly change the position of the engine. For the purpose of improvement of the cruising attitude of the personal watercraft, it is desirable to locate a center of gravity of the four-cycle engine closest to the rear of the personal watercraft. 
   In order to obtain a power as high as that of the two-cycle engine in the four-cycle engine without an increase in displacement of the engine, increases in an engine speed and the number of cylinders are effective. 
   However, with the increase in the number of the cylinders, it becomes necessary to increase the length of the crankshaft  90 . It would be difficult to manufacture a longer crankshaft in the conventional manufacturing apparatus, and therefore, there arises a need for a manufacturing apparatus capable of manufacturing the longer crankshaft. But, the manufacturing apparatus of the crankshaft having a complex shape and requiring high accuracy would be complex and expensive. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above-described conditions, and an object of the present invention is to provide an engine having a center of gravity located rearward to be suitable for a personal watercraft, and a crankshaft suitable for the engine. 
   According to the present invention, there is provided an engine of a personal watercraft having a water jet pump at a rear end thereof, comprising: a crankcase having a front end wall and a rear end wall; a crankshaft provided inside the crankcase; a generator provided at an output end portion of the crankshaft, the generator including a rotor fixed to the crankshaft; a starter motor provided on an output end side of the crankshaft outside the crankcase; and a clutch provided between the starter motor and the rotor of the generator, for connecting/disconnecting the starter motor to/from the rotor. 
   With such a structure, since the generator, the starter motor, the clutch and the like of the engine, which have relatively large weight, are located on the output end side of the engine, a center of gravity of the engine is advantageously located rearward in the engine so as to be suitable for the watercraft. Since the generator and the starter motor are placed on the output end side of the engine, the clutch is correspondingly placed on the output end side of the engine. As defined herein, the output end side refers to the rear side of the engine. 
   Preferably, the engine may further comprise: a cam chain tunnel formed in a front portion of the front end wall of the crankcase on an opposite side of the output end side of the crankshaft; and a sprocket for driving a cam mechanism of the engine, the sprocket being mounted to an end portion of the crankshaft passing through a penetrating hole formed in the front end wall of the crankcase and protruding into the cam chain tunnel. 
   With such a structure, the diameter of the sprocket can be selected irrespective of the diameter of output end side of the crankshaft adapted to transmit an output of the crankshaft to the propeller shaft. This reduces the diameter of the sprocket. As a result, a lightweight and compact engine is achieved. Therefore, for the watercraft, it is preferable to place the cam chain on the opposite side (front end side of the engine) of the output side of the crankshaft. 
   Preferably, the engine may further comprise: a generator chamber formed in a rear portion of the rear end wall of the crankcase, for containing the generator, the output end portion of the crankshaft may pass through a penetrating hole formed in the rear end wall of the crankcase and protrudes into the generator chamber, and a seal member may be provided in the penetrating hole of the crankcase to seal between an inside of the crankcase and the generator chamber. In this structure, since an oil inside the crankcase is prevented from flowing into an inside of the generator chamber, the loss of a rotational energy of the rotor of the generator which would be caused by viscosity of the oil is avoided. 
   Preferably, the engine may further comprise a water jacket formed in a housing constituting the generator chamber to allow cooling water to flow therethrough. With this structure, the generator and its vicinity are cooled more effectively. 
   The engine may comprise a generator chamber formed in a rear portion of the rear end wall of the crankcase, for containing the generator; a cam chain tunnel communicating with an inside of the generator chamber; and a sprocket for driving a cam mechanism, the sprocket being mounted to a portion of the crankshaft between the rotor and the penetrating hole in the generator chamber, and the rotor is mounted to the output end portion of the crankshaft passing through a penetrating hole formed in the rear end wall of the crankcase and protruding into the generator chamber. Such a structure is advantageous in that the generator and its vicinity are efficiently cooled by the oil dropping through the cam chain tunnel although a slight loss of the rotational energy of the rotor of the generator occurs due to the oil. 
   According to the present invention, there is provided a crankshaft of an engine for a personal watercraft, comprising: a crankshaft body; a first extended shaft member connected to one end portion of the crankshaft body so as to be rotatable integrally with and separable from the crankshaft body; and a rotor-mounting portion provided at an end portion of the crankshaft on the first extended shaft member side, for mounting a rotor of a generator of the engine, and the first extended shaft member constitutes an output end part of the engine. 
   With such a structure, the crankshaft body is shortened. Consequently, manufacturing the crankshaft becomes easy. In addition, the generator can be easily placed in the output end side of the crankshaft. 
   Preferably, the rotor-mounting portion may be formed integrally with the first extended shaft member. With this structure, the rotor of the generator is easily placed on the output end side of the crankshaft. 
   Preferably, the crankshaft body may comprise an extended shaft mounting portion for connecting the first extended shaft member and the rotor-mounting portion in this order as seen from the one end portion of the crankshaft body, and the first extended shaft member may be mounted to the extended shaft mounting portion to thereby allow the rotor of the generator to be fixed to the rotor-mounting portion. With this structure, the rotor of the generator is easily placed on the output end side of the crankshaft. 
   The crankshaft may further comprise a coupling for taking out an output of the engine, the coupling being provided at a rear end portion of the first extended shaft member. Thereby, the output of the engine can be easily taken out. 
   Preferably, the crankshaft may further comprise a second extended shaft member connected to the other end portion of the crankshaft body to be rotatable integrally with and separable from the crankshaft body; and a sprocket-mounting portion for mounting a cam-driving sprocket on the second extended shaft member. With this structure, the crankshaft body is shortened, and therefore manufacturing the crankshaft becomes easy. The crankshaft is applied to an engine having a cam chain tunnel at a front portion thereof. 
   The crankshaft may comprise: a sprocket-mounting portion formed at the one end portion of the crankshaft body adjacent the first extended shaft member, for mounting a cam-driving sprocket of the engine. Such a crankshaft is suitably used in the engine having the cam chain tunnel communicating with an inside of the generator. 
   Preferably, in the crankshaft, a connecting hole may be formed in an end portion of the first extended shaft member for connecting the crankshaft body by inserting the one end portion of the crankshaft body thereinto, an outer peripheral face of the one end portion of the crankshaft body and an inner peripheral face of the connecting hole of the first extended shaft member may respectively have a cylindrical fitting face for centering, and an engagement portion for preventing circumferential relative rotation between the crankshaft body and the first extended shaft member. A center axis of the fitting face of the crankshaft body corresponds with a rotational center axis of the crankshaft and a center axis of the fitting face of the first extended shaft member corresponds with a center axis of the first extended shaft member. With such a structure, the crankshaft body can be made shorter. 
   According to the present invention, there is provided an engine of a personal watercraft comprising: a crankcase having a front end wall and a rear end wall; a crankshaft provided inside the crankcase; a generator provided at an output end portion of the crankshaft, the generator including a rotor fixed to the crankshaft; a starter motor provided on an output end side of the crankshaft outside the crankcase; a clutch provided between the starter motor and the rotor of the generator, for connecting/disconnecting the starter motor to/from the rotor, the crankshaft includes: a crankshaft body; a first extended shaft member having a tip end portion and a base end portion and connected to an end portion of the crankshaft body so as to be rotatable integrally with and separable from the crankshaft body; and a rotor-mounting portion provided at an end portion of the crankshaft on the first extended shaft member side, for mounting the rotor of the generator, and the first extended shaft member constitutes an output end part of the engine. With this structure, the center of gravity of the engine is located rearward to be suitable for the personal watercraft, and manufacturing the engine becomes easy. 
   The above and further objects and features of the invention will be more fully apparent from the following detailed description with accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view showing a portion including a crankshaft of an engine for a personal watercraft according to an embodiment of the present invention; 
       FIG. 2  is a longitudinal sectional view showing a portion including a crankshaft of an engine for a personal watercraft according to another embodiment of the present invention; 
       FIG. 3  is a longitudinal sectional view showing a portion including a crankshaft of an engine for a personal watercraft according to another embodiment of the present invention; 
       FIG. 4  is a longitudinal sectional view showing a portion including a crankshaft of an engine for a personal watercraft according to another embodiment of the present invention; 
       FIG. 5  is a partially cross-sectional side view showing an example of a personal watercraft capable of mounting the engine of the present invention or the conventional engine; and 
       FIG. 6  is a longitudinal sectional view showing the conventional engine of the personal watercraft. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of an engine of a personal watercraft of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  is a longitudinal sectional view showing main components including a crankshaft of an engine of a personal watercraft according to an embodiment of the present invention. 
   An engine  1  of this embodiment is a multi-cylinder (for example, four-cylinder) four-cycle engine. The engine  1  is mounted in the watercraft such that cylinders  2  are arranged along the longitudinal direction of a body of the watercraft (not shown) like the conventional engine  85  (see FIG.  6 ). Therefore, a crankshaft  5  of the engine  1  is placed along the longitudinal direction of the body and an output end portion thereof is connected to a propeller shaft of the watercraft (not shown). The propeller shaft functions to drive a water jet pump provided at a rear portion of the watercraft. The propeller shaft extends rearwardly from the output end portion of the crankshaft  5  and is connected to the water jet pump. 
   In the engine  1 , a generator  3 , a starter motor  4 , and a clutch  6  for connecting/disconnecting the starter motor  4  to/from the crankshaft  5  are located in the vicinity of the rear end of the engine  1 . Here, “front” refers to the direction toward which the body of the watercraft moves and “rear” refers to the opposite direction. More specifically, a coupling  18  for connecting the propeller shaft mentioned later is located on the “rear” side. 
   In  FIG. 1 , reference numeral  7  denotes a crankcase,  8  denotes a generator chamber that contains the generator  3 ,  9  denotes a cam chain tunnel,  10  denotes a cylinder block, and  11  denotes a piston. A cylinder head and a cylinder head cover provided over the cylinder block  10  are not illustrated in FIG.  1 . The clutch  6  is accommodated in the generator chamber  8  together with a rotor  12  of the generator  3  and a stator  13  of the engine. The generator chamber  8  is comprised of a body portion  8   a  formed integrally with the crankcase  7  so as to open outside at a rear end thereof, and a first lid member  8   b  removably attached to the body portion  8   a  to close the open part of the body portion  8   a.    
   A cam chain tunnel  9  that contains a cam chain  26  is provided in a rear portion of a rear end wall  7   a  of the crankcase  7 . The cam chain tunnel  26  extends from the body portion  8   a  of the generator chamber  8  to the cylinder head (not shown) through an inside of the cylinder block  10 . The cam chain  26  serves to transmit rotation of the crankshaft  5  to a camshaft provided on the cylinder head. The crankshaft  5  comprises a crankshaft body  5 A (expressed as colored in  FIG. 1 ) having a crank web  5 B, a crank pin  5 C and the like, and a first extended shaft member  15  as an output end part of the crankshaft  5 . 
   A rear end portion  5   a  of the crankshaft body  5 A passes through a penetrating hole  14  formed in a rear end wall  7   a  of the crankcase  7  so as to be rotatably supporting the crankshaft  5  and protrudes into an inside of the generator chamber  8 . The rear end portion  5   a  has a screw portion  16 . The screw portion  16  is screwed into an inner thread formed in a connecting hole  15   a  of the first extended shaft member  15  such that the first extended shaft member  15  can rotate integrally with the crankshaft body  5 A. Alternatively, the first extended shaft member  15  may be connected to the crankshaft body  5 A so as to be integrally rotatable by any suitable means, including by a spline connection. In this embodiment, in the portion where the rear end portion  5   a  is fitted to the first extended shaft member  15 , the screw portion  16  is formed on a rear end side of the rear end portion  5   a  and a centering location portion  17  is formed adjacent a front end side of the screw portion  16  (on the opposite side of the rear end side). In the centering location portion  17 , an outer peripheral face of the rear end portion  5   a  is fitted to a corresponding inner peripheral face of the connecting hole  15   a  of the first extended shaft member  15  without substantial clearance. The outer peripheral face of the rear end portion  5   a  and the corresponding inner peripheral face of the first extended shaft member  15  are respectively formed concentrically with a rotational center axis of the crankshaft  5 . 
   The first extended shaft member  15  passes through a penetrating hole  8   c  formed in the first lid member  8   b  of the generator chamber  8  and protrudes outwardly. The coupling  18  for connecting the propeller shaft (not shown) is connected by a spline to a rear end portion (output end portion) of the outwardly protruding first extended shaft member  15 . The portion where a spline groove is formed constitutes a coupling mounting portion. The rotor  12  of the generator  3  is mounted to an outer periphery of a portion of the first extended shaft member  15  which is located inside the generator chamber  8  so as to be integrally rotatable. More specifically, an outwardly extending flange  19  of a circular-ring shape is formed at an outer periphery of a front end of the first extended shaft member  15 . The rotor  12  is of a cylinder shape having a bottom and is mounted to the flange  19  by means of a rivet  20 . An opening  12   a  is formed in the bottom of the rotor  12  to allow the rear end portion  5   a  of the crankshaft body  5 A to be inserted thereinto. A magnet  12   c  is fixed to the inner peripheral face of a cylindrical body portion  12   b  of the rotor  12 . 
   A stator  13  of the generator  3  is removably mounted to the first lid member  8   b  of the generator chamber  8 . The penetrating hole  8   c  formed in the first lid member  8   b  has an increased diameter in a rear portion thereof and a bearing  21  is mounted in the increased-diameter portion for rotatably supporting the first extended shaft member  15 . A concave portion  22  is formed in an outer face of the first lid member  8   b  to allow cooling water to flow therethrough. A second lid member  23  is removably attached to close the concave portion  22 . In this embodiment, the concave portion  22  is circularly formed in the first lid member  8   b  around the bearing  21 , and constitutes a water jacket for introducing the cooling water. The water jacket serves to cool the generator  3 , the bearing  21 , an oil dropping from the cam chain tunnel  9 , and the like. A seal member  24  is provided in a penetrating hole  23   a  of the second lid member  23 , through which the first extended shaft member  15  penetrates. 
   The water jacket of this embodiment is formed in the rear end wall of the first lid member  8   b  to surround the bearing  21 . Instead, the water jacket may be provided at an outer peripheral wall of the generator chamber  8  or the like, as necessary. 
   The clutch  6  is accommodated in the generator chamber  8  in such a manner that both ends of a shaft  6   a  of the clutch  6  are supported by support holes  50   a ,  50   b  formed in the crankcase  7  and the first lid member  8   b , respectively. An output shaft  4   a  of the starter motor  4  mounted to an outer face of the side wall at the rear portion of the crankcase  7  protrudes into the generator chamber  8 . A gear  4   b  provided on the output shaft  4   a  is in mesh with a first gear  6   b  provided on the shaft  6   a  of the clutch  6 . A second gear  6   c  is provided on the shaft  6   a  of the clutch  6  so as to be axially movable. A gear  25  with/from which the second gear  6   c  engages/disengages is provided on the outer peripheral face of the rotor  12  (on the cylindrical portion  12   b  of the rotor  12 ). When the rotation of the starter motor  4  is transmitted to the clutch  6  and causes the shaft  6   a  of the clutch  6  to rotate, the second gear  6   c  moves to engage with the gear  25  of the rotor  12 . The rotational force transmitted in this manner is transmitted to the crankshaft  5  through the rotor  12 . 
   It should be appreciated that gears in the starting mechanism provided between the starter motor and the crankshaft may be always in mesh with each other, and a one-way clutch is adapted to connect/disconnect transmission between the starter motor and the crankshaft. Alternatively, a stator motor containing a built-in clutch may be used. In brief, the clutch needs to be adapted to connect/disconnect transmission of the power between the stator motor and the crankshaft. In this embodiment, the gear  25  as an output end gear in a reduction gear train of the starting mechanism is provided on the outer periphery of the rotor  12 . But, the starting mechanism is not intended to be limited to the above, so long as the mechanism is adapted to rotate the crankshaft during the starting of the engine. For example, the starter motor may be installed at the rear portion of the engine in such a manner that the output end gear in the reduction gear train is attached to the bottom portion of the rotor  12 , the outer periphery of a member adapted to rotate in mesh with the crankshaft, or the crankshaft. 
   A sprocket  27  around which the cam chain  26  is installed is formed in the rear end portion  5   a  of the crankshaft body  5 A protruding into the generator chamber  8  in a portion between the first extended shaft member  15  and the rear end wall  7   a  having the penetrating hole  14  of the crankcase  7  such that the sprocket  27  is integral with the crankshaft  5 . 
   A front end portion  5   b  of the crankshaft body  5 A is rotatably supported by a penetrating hole  28  formed in the front end wall  7   b  of the crankcase  7  and protrudes through the penetrating hole  28 . The protruding portion of the front end portion  5   b  is covered by a sensor case  29   b  formed in the outer face of the front end wall  7   b  of the crankcase  7 . A rotation sensor  29   a  is attached to a sensor case  29   b , for detecting a member  29   c  mounted to the front end portion  5   b . According to the detection by the rotation sensor  29   a , ignition timing or the like of the engine is controlled. 
   According to the above-mentioned structure, the generator  3  is placed at a position corresponding to the rear end portion  5   a  of the crankshaft  5  of the engine  1 , the starter motor  4  is placed on the outer face of the rear portion of the side wall of the crankcase  7 , and the clutch  6  for connecting/disconnecting the starter motor  4  to/from the crankshaft  5  is placed in the generator chamber  8 . This follows that the components  3 ,  4 ,  6  having relatively large weight are located on the rear end side of the engine  1 , and the center of gravity of the engine  1  is thereby located at the rear portion of the engine  1 . This is advantageous when the four-cycle engine is mounted in the personal watercraft. 
   The crankshaft  5  of this embodiment is comprised of the crankshaft body  5 A and the first extended shaft member  15 , which are connected to each other to be rotatable integrally with and separable from each other. This advantageously reduces the length of the crankshaft body  5 A of a complex shape so as to be easily manufactured. In addition, since the rotator  12  of the generator  3  is formed integrally with the first extended shaft member  15  having the output shaft end, the generator  3  is easily placed at the rear end portion of the crankshaft, and assembly is facilitated. 
     FIG. 2  shows an engine  31  according to another embodiment. The engine  31  is identical in structure to the engine  1  in  FIG. 1  except that a cam chain tunnel  32  is provided on the front end side of the engine  31 . Therefore, the same components as those in the engine  1  in  FIG. 1  are identified by the same reference numerals and will not be further described. 
   A front end portion  33   b  of a crankshaft body  33 A of a crankshaft  33  of the engine  31  is rotatably supported by a penetrating hole  35  formed in a front end wall  34   b  of a crankcase  34 . A second extended shaft member  36  is located in a front chamber  32 A that defines a cam chain tunnel  32  formed in a front portion of the front end wall  34   b  and is connected to a tip end of the front end portion  33   b  in such a manner that the second extended shaft member  36  is rotatable integrally with and separable from a crankshaft body  33 A. A sprocket  37  for cam chain is mounted to the second extended shaft member  36 . A front end face of the crankshaft body  33 A is substantially coplanar with an outer face of the front end wall  34   b  of the crankcase  34 . A convex portion  33   c  for fitting is formed in the front end face of the crankshaft body  33 A. A fitting hole  36   a  fitted over the convex portion  33 C, is formed in a rear end face of the second extended shaft member  36 . The convex portion  33   c  and the fitting hole  36   a  constitute so-called centering-location portion, where an outer peripheral face of the convex portion  33   c  is fitted to the corresponding inner peripheral face of the fitting hole  36   a  without substantial clearance. The fitting face is formed concentrically with a rotational center axis of the crankshaft  33 . 
   The second extended shaft member  36  is fitted to the front end of the crankshaft body  33 A and secured thereto by means of a bolt  38   a . To prevent the second extended shaft  36  from rotating relatively to the crankshaft body  33 A, a pin  38   b  for preventing rotation is attached to the front end face of the crankshaft body  33 A. 
   A sprocket-mounting portion for mounting the cam sprocket  27  in  FIG. 1  is not provided at a rear end portion  33   a  of the crankshaft body  33 A, but instead, the first extended shaft member  15  is exteriorly fitted to a portion of the crankshaft body  33 A outside a penetrating hole  14  of a crankcase  34 . The rotor  12  is mounted to the first extended shaft member  15  as in the crankshaft  5  in FIG.  1 . For sealing between the generator chamber  8  and an inner portion  34   b  of the crankcase  34 , a seal member  39  is attached to the penetrating hole  14  in the rear end wall  34   a  of the crankcase  34 . The sealing prevents an oil in the inner portion  34   b  of the crankcase  34  from flowing into the inside of the generator chamber  8 . As a result, loss of rotational energy of the generator  8  which would be caused by viscosity of the oil is avoided. 
   As should be appreciated, since the sprocket-mounting portion is not provided on the rear end portion  33   a  of the crankshaft body  33 A, the crankshaft  33  is shorter than the crankshaft  5  of the engine  1  in  FIG. 1  substantially by difference between the length of the sprocket-mounting portion and the length of the seal member  39 . 
   Reference numeral  49   a  denotes a sprocket for driving a scavenging pump  49   b  or a feed pump  49   c  by means of a chain  49   d.    
     FIG. 3  shows an engine  41  according to another embodiment. The engine  41  is identical in structure to the engine  31  in  FIG. 2  except for a mounting mechanism of a rotor  42 . Therefore, the same components as those in  FIG. 2  are identified by the same reference numerals and will not be further described. 
   As shown in  FIG. 3 , a screw portion  45   a  for connecting a first extended shaft member  44  and a centering-location portion  45   b  for causing a rotational center axis of the first extended shaft member  44  to conform to a rotational center axis of a crankshaft body  43 A are formed in a rear end portion  43   a  of the crankshaft body  43 A. The first extended shaft member  44  is provided with a connecting hole  44   a  for connecting the crankshaft body  43 A. The connecting hole  44   a  has an inner thread and a fitting portion inside thereof. 
   A small-diameter portion  47  is formed with a step portion  46  at the rear end portion  43   a  of the crankshaft body  43 A. The screw portion  45   a  and the centering location portion  45   b  are formed in the small-diameter portion  47 . A portion  45   c  of the small-diameter portion  47  between the centering location portion  45   b  and the step portion  46  is tapered with a diameter increasing toward the step portion  46 . The portion  45   c  is also expressed as a taper portion  45   c . A rotor  42  of the generator  3  is cylindrical with a bottom. A cylindrical boss  42   d  having an inner peripheral face  42   a  to which the taper portion  45   c  is fitted is formed at a bottom portion of the rotor  42 . The inner peripheral face  42   a  is tapered to allow fitting to the taper portion  45   c . The rotor  42  is first fitted to the taper portion  45   c , and the first extended shaft member  44  is screwed to the screw portion  45   a  of the small-diameter portion  47 , thereby causing the rotor  42  to be pressed toward the large-diameter side of the taper portion  45   c . As a result, the inner peripheral face  42   a  of the cylindrical boss  42   d  is fitted to the taper portion  45   c , and thus, the rotor  42  is fixed to the taper portion  45   c  of the crankshaft body  43 A. A magnet  42   c  is fixed to an inner peripheral face of a cylindrical body portion  42   b  like the rotor  12  in  FIGS. 1 and 2 . 
   A seal member mounting portion  14   a  is formed in a rear portion of the penetrating hole  14  formed in the rear end portion  48   b  of the crankcase  48 . Between an inner peripheral face of the seal member mounting portion  14   a  and an outer peripheral face of the cylindrical portion  42   d  of the rotor  42 , a seal member  39  is provided for sealing between an inside of the crankcase  48  and the generator  8 . 
   The crankshaft  43  is shorter than the crankshaft  33  of the engine  31  in  FIG. 2  substantially by difference between the length of the rivet  20  ( FIG. 2 ) and the thickness of the flange  19  (FIG.  2 ). Also, since the rotor  42  is independent of the first extended shaft member  44 , the first extended shaft member  54  is simply shaped. 
     FIG. 4  shows an engine  51  according to another embodiment. The same components as those in the engine  41  in  FIG. 3  are identified by the same reference numerals and will not be further described. In the engine  51 , the rotor  42  is fixed to a crankshaft body  53 A by mounting a first extended shaft member  54  to a crankshaft body  53 A in such a manner that a rear end portion  53   a  of the crankshaft body  53 A is inserted into a connecting hole  54   a  of the first extended shaft member  54 . The engine  51  does not have the second extended shaft member  36 ( FIG. 3 ) unlike the engine  41  in FIG.  3 . In addition, the centering location portion is not formed in the portion where the rear end portion  53   a  of the crankshaft body  53 A is fitted to the first extended shaft member  54 . 
   The rear end portion  53   a  is connected to the first extended shaft member  54  by means of a screw portion  55 . A fitting hole  56   a  is formed on a rear end face of the crankshaft body  53 A and a fitting hole  56   b  is formed on the bottom of a connecting hole  54   a  provided on a front end of the first extended shaft member  54 . A shaft  57  for allowing a center axis of the crankshaft body  53 A to conform to the center axis of the first extended shaft member  54  penetrates into the fitting holes  56   a ,  56   b . The fitting hole  56   a  is concentric with the crankshaft body  53 A and the fitting hole  56   b  is concentric with the center axis of the first extended shaft member  54 . Specifically, a centering-location portion for centering the first extended shaft member  54  and the crankshaft body  53 A is formed between an outer peripheral face of the shaft  57  and inner peripheral faces of the fitting holes  56   a ,  56   b . By fittingly inserting the shaft  57  into the fitting holes  56   a ,  56   b , the crankshaft body  53 A is caused to be coaxial with the first extended shaft member  54 . 
   With the above-mentioned structure, the crankshaft body  53 A is shorter than the crankshaft body  43 A in  FIG. 3  substantially by the length of the centering-location portion in the rear end portion  43   a.    
   Although the present invention has been described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the invention, they should be construed as being included therein.