Patent Publication Number: US-7913587-B2

Title: Internal combustion engine and vehicle having the internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims priority to Japanese Patent Application No. 2007-063068, filed on Mar. 13, 2007, the entire contents of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates generally to an internal combustion engine and a vehicle including the internal combustion engine, and more specifically to an internal combustion engine including a pump part and a vehicle including the internal combustion engine. 
     BACKGROUND 
     Conventionally, motorcycles (or other types of vehicles) provided with an engine (internal combustion engine) including a coolant pump (pump part) are known. For example, Japanese Patent Document JP-B-3820970 discloses a lubricant cooling structure for an engine including: an oil pump drive gear (first gear) and a primary drive gear (second gear) that rotate together with a crankshaft; an oil pump idler gear meshed with the oil pump drive gear to rotate as the oil pump drive gear rotates; and a coolant pump gear (pump gear) meshed with the primary drive gear to rotate as the primary drive gear rotates. In this engine, the oil pump idler gear and the coolant pump gear are disposed inside a crankcase so as not to overlap each other as viewed from the extending direction of the crankshaft (from a side). 
     However, for this engine, because the oil pump idler gear and the coolant pump gear (pump gear) are disposed inside the crankcase so as not to overlap each other as viewed from the extending direction of the crankshaft (from a side), it is necessary to secure a space to be occupied by the oil pump idler gear and the coolant pump gear, which are respectively meshed with the oil pump drive gear (first gear) and the primary drive gear (second gear) which rotate together with the crankshaft inside the crankcase, in the direction perpendicular to the extending direction of the crankshaft. Therefore, the size of the engine (internal combustion engine) is increased in the direction perpendicular to the extending direction of the crankshaft. That is, the size of the internal combustion engine is increased as viewed from an end of the crankshaft. 
     SUMMARY 
     The present invention, in accordance with one or more embodiments, addresses the foregoing problem, and therefore for an embodiment provides an internal combustion engine that is not increased in size as viewed from an end of the crankshaft and a vehicle including the internal combustion engine. 
     In accordance with an embodiment, a first aspect of the present invention is directed to an internal combustion engine including: a crankcase; a crankshaft accommodated in the crankcase; a first gear and a second gear disposed inside the crankcase to rotate about the crankshaft; a third gear meshed with the first gear to rotate as the first gear rotates; and a pump part having a pump gear, in which the pump gear is meshed with the second gear to rotate as the second gear rotates; and the third gear and the pump gear are disposed to overlap each other as viewed from an end of the crankshaft. 
     According to the first aspect in accordance with an embodiment, the third gear and the pump gear are disposed to overlap each other as viewed from an end of the crankshaft as described above. Therefore, it is possible to reduce the space to be occupied by the third gear and the pump gear as viewed from an end of the crankshaft, unlike the case where the third gear and the pump gear are disposed so as not to overlap each other as viewed from an end of the crankshaft. This makes it possible to restrain an increase in the size of the internal combustion engine as viewed from an end of the crankshaft. 
     In the internal combustion engine according to the first aspect, in accordance with an embodiment, the pump part further includes a water pump unit having a coolant flow-in port and a coolant flow-out port. According to this configuration, coolant can be easily circulated by the coolant flow-in port and the coolant flow-out port. 
     In the internal combustion engine according to the first aspect, in accordance with an embodiment, the pump part further includes: a first rotary shaft that rotates together with the pump gear; and an impeller attached to the first rotary shaft, with the pump gear disposed between the impeller and the third gear. According to this configuration, the pump gear is disposed to overlap the third gear and the impeller as viewed from an end of the crankshaft, making it possible to restrain an increase in the size of the internal combustion engine as viewed from an end of the crankshaft. 
     In the internal combustion engine according to the first aspect, in accordance with an embodiment, the pump part further includes: a first rotary shaft that rotates together with the pump gear; and a first restriction member for restraining axial movement of the first rotary shaft. According to this configuration, the first restriction member can easily restrain axial movement of the first rotary shaft even if the first rotary shaft is applied with axial force. 
     In the internal combustion engine in which the pump part includes the first rotary shaft and the first restriction member, in accordance with an embodiment, the first rotary shaft includes a first outer peripheral groove formed in an outer peripheral surface of the first rotary shaft, with the first restriction member including an engagement part to be engaged with the first outer peripheral groove of the first rotary shaft. According to this configuration, the first restriction member can be easily engaged with the first rotary shaft. 
     In this case, in accordance with an embodiment, the engagement part of the first restriction member includes: an engagement hole to be engaged with the first outer peripheral groove of the first rotary shaft; and an insertion hole formed to be continuous with the engagement hole and having a hole diameter larger than a diameter of the first rotary shaft. According to this configuration, the first restriction member can be further easily engaged with the first rotary shaft by inserting the first rotary shaft into the insertion hole of the engagement part and having the first outer peripheral groove of the first rotary shaft engaged with the engagement hole which is continuous with the insertion hole. 
     The internal combustion engine in which the pump part includes the first rotary shaft and the first restriction member, in accordance with an embodiment, further includes: a first bearing member disposed on a side of the crankcase; and a second rotary shaft that rotates together with the third gear, with the first bearing member configured to rotatably support the second rotary shaft of the third gear, and with the pump part further including a second bearing member for rotatably supporting the first rotary shaft of the pump gear. According to this configuration, the first bearing member disposed on a side of the crankcase allows the second rotary shaft to rotate stably, allowing the balancer gear to rotate stably. Also, the second bearing member of the pump part allows the first rotary shaft to rotate stably, allowing the pump gear to rotate stably. 
     In the internal combustion engine in which the pump part includes the first rotary shaft and the first restriction member, in accordance with an embodiment, the pump part further includes a second restriction member for restraining the pump gear from slipping off from the first rotary shaft, with the first rotary shaft further including a second outer peripheral groove formed in an outer peripheral surface of the first rotary shaft to be engaged with the second restriction member. According to this configuration, the second restriction member and the second outer peripheral groove can easily restrain the pump gear from slipping off from the first rotary shaft. 
     In the internal combustion engine according to the first aspect, in accordance with an embodiment, the pump gear and the third gear are configured to be positioned higher than an oil surface of oil reserved inside the crankcase. According to this configuration, it is possible to restrain the third gear and the pump gear from contacting the oil reserved in the oil pan, and thus to further restrain a loss of driving force due to such contact. 
     The internal combustion engine according to the first aspect, in accordance with an embodiment, further includes a crankcase cover provided to cover at least a part of a surface of the crankcase, with the pump part attached to the crankcase cover. According to this configuration, the pump gear included in the pump part can be easily disposed to overlap the third gear disposed inside the crankcase as viewed from an end of the crankshaft. 
     The internal combustion engine according to the first aspect, in accordance with an embodiment, further includes a second rotary shaft that rotates together with the third gear, with the third gear including a balancer gear, with an extension of the second rotary shaft of the balancer gear disposed within a radius of the pump gear, and with an extension of the first rotary shaft of the pump gear disposed within a radius of the balancer gear. According to this configuration, the balancer gear and the pump gear may be disposed to overlap each other to a greater extent as viewed from an end of the crankshaft, and thus the space to be occupied by the balancer gear and the pump gear may be reduced as viewed from an end of the crankshaft. 
     In the internal combustion engine according to the first aspect, in accordance with an embodiment, the second gear is formed such that a diameter of the second gear is smaller than that of the first gear. According to this configuration, the rotational speed of the pump gear can be reduced while restraining an increase in the size of the pump gear which is engaged with the second gear. 
     In accordance with an embodiment, a second aspect of the present invention is directed to a vehicle including the internal combustion engine with any one of the configurations described above. According to this configuration, it is possible to easily obtain a vehicle provided with an internal combustion engine that is not increased in size as viewed from an end of the crankshaft. 
     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing the entire structure of a motorcycle provided with an engine in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross sectional view of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 3  is a cross sectional view of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 4  is a cross sectional view showing the vicinity of a water pump unit of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 5  is a partial cross sectional view showing a crankcase of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 6  is a side view of the water pump unit of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 7  is a plan view of a circlip of the water pump unit of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 8  is a plan view of a plate member of the water pump unit of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 9  is a perspective view of the plate member of the water pump unit of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
         FIG. 10  is a cross sectional view showing the vicinity of a water pump shaft of the engine provided in the motorcycle in accordance with the embodiment shown in  FIG. 1 . 
     
    
    
     The description of various reference numerals and symbols in the drawings may be set forth in accordance with one or more embodiments, for example, as follows:  1 : motorcycle,  15 : engine (internal combustion engine),  22 : crankcase,  31 : crankshaft (crankshaft),  32 : first crank gear (first gear),  33 : second crank gear (second gear),  34 : balancer shaft (second rotary shaft),  35 : balancer gear (third gear),  43 : bearing (first bearing member),  53 : crankcase cover,  54 : water pump part (pump part),  55 : water pump shaft (first rotary shaft),  55   b : engagement groove (second outer peripheral groove),  55   c : engagement groove (first outer peripheral groove),  56 : water pump gear (pump gear),  57 : impeller,  59 : bearing (second bearing member),  60 : plate member (first restriction member),  60   a : engagement part,  60   b:  engagement hole,  60   c:  insertion hole,  62 : circlip (second restriction member),  63   a:  coolant flow-in port,  63   b:  coolant flow-out port, and  64 : water pump unit. 
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
       FIG. 1  is a side view showing the entire structure of a motorcycle (e.g., a type of vehicle) provided with an engine (internal combustion engine) in accordance with an embodiment of the present invention.  FIGS. 2 to 10  illustrate in detail, in accordance with one or more embodiments, the structure of the engine in accordance with the embodiment shown in  FIG. 1 . In this embodiment, a motorcycle is described as an example of a vehicle for an embodiment of the present invention. In the drawings, the arrow FWD indicates the forward running direction of the motorcycle. First, an engine  15  in accordance with this embodiment and a motorcycle  1  provided with the engine  15  are described with reference to  FIGS. 1 to 10 . 
     In the motorcycle  1  provided with the engine (internal combustion engine)  15  in accordance with the embodiment of the present invention, as shown in  FIG. 1 , a main frame  3  extending longitudinally is disposed at the rear of a head pipe  2 . The main frame  3  has an upper frame  3   a  extending rearward from above and a lower frame  3   b  extending downward and then rearward. A backstay  5  is connected between the upper frame  3   a  and the rear part of a seat rail  4 . The head pipe  2 , the main frame  3 , the seat rail  4 , and the backstay  5  constitute a vehicle body frame. 
     Handlebars  6  are rotatably attached to the top of the head pipe  2 . A pair of front forks  7  having a suspension for absorbing vertical shock is disposed below the handlebars  6 . A front wheel  8  is rotatably mounted at the lower end of the front forks  7 . A front fender  9  is disposed above the front wheel  8 . A radiator  10  is disposed at the rear of the front fork  7 . 
     The front end of a swing arm  11  is attached to the rear end of the main frame  3  via a pivot shaft  3   c . A rear wheel  12  is rotatably mounted at the rear end of the swing arm  11 . A driven sprocket  13  is attached to the rear wheel  12  so as to rotate together with the rear wheel  12 . A drive chain  14  is meshed with the driven sprocket  13 . The drive chain  14  is configured to be driven by a drive sprocket  48  of the engine  15  to be discussed later. The engine  15  is mounted as interposed between the upper frame  3   a  and the lower frame  3   b  of the main frame  3 . The engine  15  is an example of the “internal combustion engine” in accordance with an embodiment of the present invention. A muffler  16  is connected to the engine  15 . A fuel tank  17  is disposed on top of the main frame  3 . A seat  18  is disposed on top of the seat rail  4 . 
     As shown in  FIG. 2 , the engine  15  in accordance with this embodiment includes a cylinder  19 , a cylinder head  20 , a cylinder head cover  21 , and a crankcase  22 . A piston  23  is disposed in the cylinder  19  so as to be slidable along its inner peripheral surface. One end of a connecting rod  24  is rotatably attached to the piston  23 . The cylinder head  20  is disposed to block an opening of the cylinder  19 . The cylinder head  20  is formed with an intake port  20   a  and an exhaust port  20   b . An intake valve  25 , made of titanium for example, and an exhaust valve  26 , made of steel for example, are disposed in the intake port  20   a  and the exhaust port  20   b , respectively. The area of an umbrella part  25   a  of the intake valve  25  is larger than that of an umbrella part  26   a  of the exhaust valve  26 . A throttle body  27  is connected to the intake port  20   a . An injector  28  for injecting fuel into the intake port  20   a  is attached to the throttle body  27 . The intake port  20   a  is provided to supply a mixture of air and fuel to a combustion chamber  19   a  of the cylinder  19 . The exhaust port  20   b  is provided to exhaust a residual gas after combustion from the combustion chamber  19   a . The muffler  16  (see  FIG. 1 ) is connected to the exhaust port  20   b  via an exhaust pipe (not shown). A drain port  20   c  is formed in the rear of the cylinder head  20 . The drain port  20   c  returns coolant warmed by the engine  15  to the radiator  10  (see  FIG. 1 ). 
     The cylinder head cover  21  is disposed on top of the cylinder head  20 . The cylinder head cover  21  is attached to the cylinder head  20  so as to cover a pair of camshafts  29 . The camshafts  29  are each provided with a cam  29   a  for actuating the intake valve  25  and the exhaust valve  26 , respectively. As shown in  FIG. 3 , a cam chain  30  is meshed with a gear  29   b  of the camshaft  29 . The cam chain  30  is meshed with a gear  31   a  of a crankshaft  31  to be discussed later. The camshafts  29  are configured to rotate as the crankshaft  31  rotates. 
     As shown in  FIG. 2 , the crankcase  22  is attached to the bottom of the cylinder  19 . Inside the crankcase  22  are disposed: a crankshaft  31 ; a first crank gear  32  and a second crank gear  33  that rotate about the crankshaft  31 ; a balancer shaft  34 ; a balancer gear  35  that rotates about the balancer shaft  34 ; a main shaft  36 ; a driven gear  37  that rotates about the main shaft  36 ; and a clutch mechanism  38  attached to the main shaft  36 . The crankshaft  31  is an example of the “crankshaft” in accordance with an embodiment of the present invention, and the first crank gear  32  is an example of the “first gear” in accordance with an embodiment of the present invention. The second crank gear  33  is an example of the “second gear” in accordance with an embodiment of the present invention. The balancer shaft  34  is an example of the “second rotary shaft” in accordance with an embodiment of the present invention, and the balancer gear  35  is an example of the “third gear” in accordance with an embodiment of the present invention. 
     The other end of the connecting rod  24  is rotatably attached to the crankshaft  31 . In other words, the crankshaft  31  is configured to rotate as the piston  23  slides relative to the cylinder  19 . As shown in  FIG. 3 , the crankshaft  31  is disposed in the crankcase  22  so as to be perpendicular to the traveling direction of the vehicle (the direction of the arrow FWD) (see  FIGS. 1 and 2 ). The crankshaft  31  is rotatably supported by a pair of bearings  39  mounted in the crankcase  22 . The second crank gear  33  is fixed to the crankshaft  31  in the vicinity of one end thereof (in the direction of the arrow R), and configured to rotate together with the crankshaft  31 . The first crank gear  32  is fixed to the crankshaft  31  adjacent to the second crank gear  33  in the direction of the arrow L, and configured to rotate together with the crankshaft  31 . The second crank gear  33  is configured such that the diameter of the second crank gear  33  is smaller than that of the first crank gear  32 . 
     A generation device  40  is attached to the other end of the crankshaft  31  (in the direction of the arrow L). The generation device  40  is configured to generate electricity along with the rotation of the crankshaft  31 . A starter gear  41  is fixed to the crankshaft  31  adjacent to the generation device  40  in the direction of the arrow R. The starter gear  41  is connected to a starter motor  42  (see  FIG. 2 ) via a plurality of intermediate gears (not shown). A gear  31   a  is provided on the crankshaft  31  at a side of the starter gear  41  in the direction of the arrow R (see  FIG. 3 ). The cam chain  30  is meshed with the gear  31   a.    
     In accordance with an embodiment, as shown in  FIG. 2 , the balancer gear  35  is configured to be meshed with the first crank gear  32 . As shown in  FIG. 4 , the balancer gear  35  is fixed to the balancer shaft  34 . The balancer shaft  34  is rotatably supported by a bearing  43  mounted in the crankcase  22  and provided on the side of the crankcase  22  (in the direction of the arrow L). The bearing  43  is an example of the “first bearing member” in accordance with an embodiment of the present invention. The balancer gear  35  is configured such that the diameter of the balancer gear  35  is larger than that of the first crank gear  32 . A balancer  35   a , made of steel for example, is attached to the balancer gear  35 . As shown in  FIG. 2 , the balancer  35   a  has an arcuate shape, and is fixed to a flat part of the balancer gear  35  by three screw members  70 . The balancer  35   a  is attached to the balancer gear  35  so as to be positioned at the lower part of the flat part of the balancer gear  35  when the piston  23  is positioned at the top dead center. Since the balancer gear  35  rotates a half turn as the first crank gear  32  rotates a half turn, the balancer  35   a  is moved to be positioned at the upper part of the flat part of the balancer gear  35  when the piston  23  is positioned at the bottom dead center. 
     The driven gear  37  is meshed with the second crank gear  33 . The driven gear  37  is configured to have a larger diameter than that of the second crank gear  33 . That is, the driven gear  37  is configured such that the rotational speed of the driven gear  37  is lower than that of the second crank gear  33 . The driving force transmitted to the driven gear  37  is transmitted to the main shaft  36  via the clutch mechanism  38 . As shown in  FIG. 3 , the main shaft  36  is rotatably supported by a pair of bearings  44  mounted in the crankcase  22 . The driving force transmitted to the main shaft  36  is transmitted to a drive shaft  46  via a transmission mechanism  45 . The drive shaft  46  is rotatably supported by a pair of bearings  47   a  and  47   b  mounted in the crankcase  22 . A drive sprocket  48  is attached to an end of the drive shaft  46  in the direction of the arrow L. This allows the driving force transmitted to the drive shaft  46  to be transmitted to the rear wheel  12  (see  FIG. 1 ) via the drive chain  14 . 
     As shown in  FIG. 2 , an oil pump drive gear  49  is disposed on the main shaft  36  so as to rotate together with the main shaft  36 . An oil pump gear  51  is connected to the oil pump drive gear  49  via an idler gear  50 . The oil pump gear  51  is disposed in an oil pump unit  52 . The oil pump unit  52  is configured to be driven by rotation of the oil pump gear  51 . 
     In accordance with an embodiment, as shown in  FIG. 3 , a crankcase cover  53  is attached to a surface of the crankcase  22  in the direction of the arrow R so as to cover the surface of the crankcase  22 . As shown in  FIGS. 4 and 5 , a water pump part  54  is attached to the crankcase cover  53 . The water pump part  54  has a function of circulating coolant through the inside of the engine  15 . The water pump part  54  is an example of the “pump part” in accordance with an embodiment of the present invention. 
     In accordance with an embodiment, as shown in  FIG. 4 , the water pump part  54  includes: a water pump shaft  55 ; a water pump gear  56  attached to the water pump shaft  55 ; an impeller  57  attached to the water pump shaft  55 ; a seal member  58 ; a bearing  59  for rotatably supporting the water pump shaft  55 ; and a plate member  60  engaged with the water pump shaft  55 . The water pump shaft  55  is an example of the “first rotary shaft” in accordance with an embodiment of the present invention, and the water pump gear  56  is an example of the “pump gear” in accordance with an embodiment of the present invention. The bearing  59  is an example of the “second bearing member” in accordance with an embodiment of the present invention, and the plate member  60  is an example of the “first restriction member” in accordance with an embodiment of the present invention. 
     The water pump shaft  55  in accordance with an embodiment is inserted into an insertion hole  53   a  of the crankcase cover  53 . The water pump gear  56  is attached to a side of the water pump shaft  55  in the direction of the arrow L. Specifically, as shown in  FIGS. 4 and 6 , a pin hole  55   a  is formed in the water pump shaft  55 , and a pin  61  (see  FIG. 4 ) is inserted into the pin hole  55   a . As shown in  FIG. 4 , the pin  61  is formed such that its length is longer than the diameter of the water pump shaft  55  so that both ends  61   a  of the pin  61  project from the pin hole  55   a . The water pump gear  56  is formed with a groove  56   a  engageable with the both ends  61   a  of the pin  61 . The water pump shaft  55  is rotated together with the water pump gear  56  with the both ends  61   a  of the pin  61  engaged with the groove  56   a . The water pump gear  56  is meshed with the second crank gear  33 , and configured to rotate as the second crank gear  33  rotates. 
     In accordance with an embodiment, as shown in  FIGS. 4 and 6 , an engagement groove  55   b  is formed in the outer peripheral surface of the water pump shaft  55  in the vicinity of an end thereof in the direction of the arrow L. The engagement groove  55   b  is an example of the “second outer peripheral groove” of the present invention. As shown in  FIG. 4 , a circlip (C-ring)  62  is engaged with the engagement groove  55   b . As shown in  FIG. 7 , the circlip  62  is formed to project from the outer peripheral surface of the water pump shaft  55  (see  FIG. 6 ), and has a function of restraining the water pump gear  56  from slipping off from the water pump shaft  55 . The circlip  62  is an example of the “second restriction member” in accordance with an embodiment of the present invention. 
     In accordance with an embodiment, as shown in  FIGS. 4 and 6 , an engagement groove  55   c  is formed in the outer peripheral surface of the water pump shaft  55  at a side of the pin hole  55   a  in the direction of the arrow R. The engagement groove  55   c  is an example of the “first outer peripheral groove” in accordance with an embodiment of the present invention. As shown in  FIG. 4 , the plate member  60  is engaged with the engagement groove  55   c . Specifically, as shown in  FIGS. 8 and 9 , the plate member  60  is provided with an engagement part  60   a . The engagement part  60   a  is formed with an engagement hole  60   b  and an insertion hole  60   c . The engagement hole  60   b  has such a predetermined hole diameter that allows engagement with the engagement groove  55   c  of the water pump shaft  55  (see  FIG. 6 ). The insertion hole  60   c  has a hole diameter larger than the diameter of the water pump shaft  55  (see  FIG. 6 ) so as to enable the water pump shaft  55  to be inserted into the engagement part  60   a . The insertion hole  60   c  is formed to be continuous with the engagement hole  60   b . This allows the water pump shaft  55  to be inserted into the insertion hole  60   c  and then moved to the engagement hole  60   b  which is formed continuously with the insertion hole  60   c , enabling the engagement hole  60   b  of the plate member  60  to be engaged with the engagement groove  55   c  of the water pump shaft  55 . A pair of screw insertion holes  60   d  is formed in the plate member  60 . As shown in  FIGS. 4 and 10 , a screw member  71  is inserted into each screw insertion hole  60   d  to screw the plate member  60  to the crankcase cover  53 . This makes it possible to restrain movement of the water pump shaft  55 , which is engaged with the engagement part  60   a  (see  FIG. 8 ) of the plate member  60 , in the vehicle width direction (in the direction of the arrow L and the arrow R). 
     In accordance with an embodiment, as shown in  FIG. 4 , the bearing  59  is disposed at a side of the plate member  60  in the direction of the arrow R. The bearing  59  is fitted into a bearing fitting hole  53   b  of the crankcase cover  53 . The bearing  59  rotatably supports the water pump shaft  55 . 
     As shown in  FIGS. 4 and 6 , the impeller  57 , made of resin for example, is attached to an end of the water pump shaft  55  in the direction of the arrow R. The water pump shaft  55  is fitted into the impeller  57  so as to rotate the impeller  57  together therewith. As shown in  FIG. 4 , the seal member  58 , made of rubber for example, is disposed on the water pump shaft  55  at a side of the impeller  57  in the direction of the arrow L. The seal member  58  is inserted into a seal member insertion hole  53   c  of the crankcase cover  53 . The seal member  58  makes it possible to restrain coolant flowing through in a side of the water pump part  54  in the direction of the arrow R (outer side) with respect to the seal member  58  from entering into a side thereof in the direction of the arrow L (inner side) with respect to the seal member  58 . The seal member  58  also makes it possible to restrain oil splashing in the side of the water pump part  54  in the direction of the arrow L (inner side) with respect to the seal member  58  from entering into the side thereof in the direction of the arrow R (outer side) with respect to the seal member  58 . 
     A cover member  63  is attached at an outer side of the impeller  57  (in the direction of the arrow R) by a screw member  72 . A coolant flow-in port  63   a  is formed in the cover member  63 . The water pump part  54  is configured to take coolant cooled by the radiator  10  (see  FIG. 1 ) into the water pump part  54  from the coolant flow-in port  63   a . A coolant flow-out port  63   b  is formed in the upper area of the impeller  57 . Coolant drawn by the impeller  57  is fed through the inside of the engine  15  from the coolant flow-out port  63   b . Coolant flow paths  53   d  and  22   a  are provided to extend from the coolant flow-out port  63   b . The coolant flow paths  53   d  and  22   a  allow the coolant to flow through the inside of the engine  15 . The coolant flow-in port  63   a  and the coolant flow-out port  63   b  constitute a water pump unit  64 . 
     In accordance with an embodiment, as shown in  FIG. 2 , the water pump gear  56  is disposed to overlap the balancer gear  35  as viewed from an end of the crankshaft  31 . Specifically, as shown in  FIGS. 2 and 4 , an extension of the balancer shaft  34  which rotates together with the balancer gear  35  is configured to be disposed within the radius of the water pump gear  56  as viewed from an end of the crankshaft  31 . Also, an extension of the water pump shaft  55  (see  FIG. 4 ) which rotates together with the water pump gear  56  is configured to be disposed within the radius of the balancer gear  35 . 
     In accordance with an embodiment, the diameter of the second crank gear  33  is smaller than that of the first crank gear  32  as described above, and therefore the rotational speed of the water pump gear  56  can be reduced while restraining an increase in the diameter of the water pump gear  56 . By restraining an increase in the diameter of the water pump gear  56 , it is possible to restrain the lower end of the water pump gear  56  from being positioned in the lower area of the crankcase  22 . 
     In accordance with an embodiment, as shown in  FIG. 2 , an oil pan  22   b  for reserving oil is provided at the bottom of the crankcase  22 . The oil pan  22   b  is filled with oil with the oil surface F 1  lower than the oil pump gear  51 . The water pump gear  56  and the balancer gear  35  are provided higher than the oil pump gear  51 , and thus disposed higher than the oil surface F 1 . 
     In accordance with an embodiment, the balancer gear  35  and the water pump gear  56  are disposed to overlap each other as viewed from an end of the crankshaft  31  as described above. Therefore, it is possible to reduce the space to be occupied by the balancer gear  35  and the water pump gear  56  as viewed from an end of the crankshaft  31 , and thus to restrain an increase in the size of the engine  15  as viewed from an end of the crankshaft  31 , unlike the case where the balancer gear  35  and the water pump gear  56  are disposed so as not to overlap each other as viewed from an end of the crankshaft  31 . 
     In accordance with an embodiment, the water pump part  54  is provided with the water pump shaft  55  that rotates together with the water pump gear  56 , and the plate member  60  for restraining axial movement of the water pump shaft  55  (in the direction of the arrow L and the arrow R). The plate member  60  can easily restrain axial movement of the water pump shaft  55  (in the direction of the arrow L and the arrow R) even if the water pump shaft  55  is applied with axial force (in the direction of the arrow L and the arrow R). 
     In accordance with an embodiment, the engagement groove  55   c  is formed in the outer peripheral surface of the water pump shaft  55 , and the plate member  60  is provided with the engagement part  60   a  for engagement with the engagement groove  55   c  of the water pump shaft  55 . The plate member  60  can be easily engaged with the water pump shaft  55 , further restraining axial movement of the water pump shaft  55 . 
     In accordance with an embodiment, the engagement part  60   a  of the plate member  60  is formed with the engagement hole  60   b  for engagement with the engagement groove  55   c  of the water pump shaft  55 , and the insertion hole  60   c  formed to be continuous with the engagement hole  60   b  and having a hole diameter larger than the diameter of the water pump shaft  55 . Therefore, the plate member  60  can be further easily engaged with the water pump shaft  55  by inserting the water pump shaft  55  into the engagement part  60   a  and having the engagement groove  55   c  of the water pump shaft  55  engaged with the engagement hole  60   b  which is continuous with the insertion hole  60   c.    
     In accordance with an embodiment, the water pump part  54  is provided with the circlip  62  for restraining the water pump gear  56  from slipping off from the water pump shaft  55 , and the engagement groove  55   b  for engagement with the circlip  62  is formed in the outer peripheral surface of the water pump shaft  55  in the vicinity of one end thereof (in the direction of the arrow L). The circlip  62  and the engagement groove  55   b  can easily restrain the water pump gear from slipping off from the water pump shaft  55 . 
     In accordance with an embodiment, an extension of the balancer shaft  34  of the balancer gear  35  is disposed within the radius of the water pump gear  56 , and an extension of the water pump shaft  55  of the water pump gear  56  is disposed within the radius of the balancer gear  35 . Therefore, the balancer gear  35  and the water pump gear  56  can be disposed to overlap each other to a greater extent as viewed from an end of the crankshaft  31 , and thus the space to be occupied by the balancer gear  35  and the water pump gear  56  can be reduced as viewed from an end of the crankshaft  31 . 
     It should be understood that the embodiments disclosed herein are construed to be illustrative in all respects rather than restrictive. The scope of the present invention is defined by the scope of the claims rather than by the description of the above embodiments, and includes all modifications falling within the scope of the claims and equivalents thereof. 
     For example, in an above embodiment, a motorcycle is described as an example of the vehicle including an internal combustion engine. However, the present invention is not limited thereto, and may be applied to vehicles provided with an internal combustion engine other than motorcycles, such as bicycles, tricycles, and ATVs (all terrain vehicles). 
     In an above embodiment, the balancer gear is disposed to overlap the water pump gear as viewed from an end of the crankshaft. However, the present invention is not limited thereto, and any gear other than the balancer gear may be disposed in an overlapping manner. 
     In an above embodiment, an extension of the balancer shaft is disposed within the radius of the water pump gear, and an extension of the water pump shaft is disposed within the radius of the balancer gear. However, the present invention is not limited thereto, and an extension of the balancer shaft may not be disposed within the radius of the water pump gear, or an extension of the water pump shaft may not be disposed within the radius of the balancer gear. 
     In an above embodiment, the circlip (C-ring) is provided to restrain the water pump gear from slipping off from the water pump shaft. However, the present invention is not limited thereto, and any member other than the circlip such as a nut may be used to restrain the water pump gear from slipping off from the water pump shaft. 
     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.