Abstract:
An engine including a motorized throttle valve that can implement mutually neighboring disposition of a throttle body and a cylinder head and can implement a good opening and closing responsibility thereof. A swelling portion is formed on a side wall of a valve chamber in such a manner so as to bite between those of throttle bodies which neighbor with each other in order to accept a protrusion of part of a valve system in a radial direction of first and second camshafts. An electric motor is disposed at one side of the group of the throttle bodies. A speed reduction mechanism, for transmitting output power of the electric motor to a valve shaft, is disposed between those throttle bodies, which neighbor with each other at a different location in the group of the throttle bodies, in a neighboring relationship with the valve chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2013-198511 filed Sep. 25, 2013 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to improvement in or relating to an engine including a motorized throttle valve and further including a cylinder head joined to an upper end face of a cylinder block having three or more cylinders disposed in series to each other. A head cover is connected to an upper end face of the cylinder head with a valve system accommodated in a valve chamber defined by and between the cylinder head and the head cover and including first and second camshafts extending in parallel to each other. A plurality of throttle bodies are disposed at a side of the valve chamber and corresponding to the cylinders. An electric motor is connected to a valve shaft for throttle valves, which open and close intake paths of the throttle bodies, and are configured to drive the throttle valves to open or close. 
     2. Description of Background Art 
     An engine including a motorized throttle valve is disclosed in Japanese Patent No. 4476421. 
     In the engine including a motorized throttle valve disclosed in Japanese Patent No. 4476421, an electric motor is attached to one side face of an engine main body spaced from a throttle body such that output power of the electric motor is transmitted to a throttle valve through a control cable (Bowden wire). Therefore, while the throttle body and the cylinder head can be disposed in a neighboring relationship with each other without interfering with the electric motor, a transmission delay of the output power of the electric motor to the throttle valve occurs, due to the presence of the control cable (Bowden wire). Therefore, the opening and closing responsibility of the throttle valve is not good. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The present invention has been made in view of such a situation as described above. It is an object of an embodiment of the present invention to provide an engine including a motorized throttle valve which can implement mutually neighboring disposition of a throttle body and a cylinder head and can implement a good opening and closing responsibility of the throttle valve. 
     In order to achieve the object described above, according to an embodiment of the present invention, an engine having a motorized throttle valve includes a cylinder head joined to an upper end face of a cylinder block having three or more cylinders disposed in series to each other. A head cover is connected to an upper end face of the cylinder head with a valve system accommodated in a valve chamber defined by and between the cylinder head and the head cover and including first and second camshafts extending in parallel to each other. A plurality of throttle bodies is disposed at a side of the valve chamber and corresponds to the cylinders. An electric motor is connected to a valve shaft for throttle valves, which open and close intake paths of the throttle bodies, and are configured to drive the throttle valves to open or close. A speed reduction mechanism is configured to transmit output power of the electric motor to the valve shaft. A swelling portion is formed on a side wall of the valve chamber in such a manner so as to bite between those of the throttle bodies which neighbor with each other in order to accept a protrusion of part of the valve system in a radial direction of the first and second camshafts. The electric motor is disposed at one side of the group of the throttle bodies while the speed reduction mechanism is disposed between those throttle bodies, which neighbor with each other at a different location in the group of the throttle bodies, in a neighboring relationship with the valve chamber. It is to be noted that the first and second camshafts correspond to intake and exhaust camshafts  38  and  39 , respectively, in embodiments of the present invention to be described later. 
     According to an embodiment of the present invention, the engine including the motorized throttle valve is configured with the electric motor being disposed between the valve chamber and the group of the throttle bodies in an offset relationship from the swelling portion in an axial direction of the first and second camshafts. 
     According to an embodiment of the present invention, the valve system includes a timing transmission apparatus that transmits output power of a crankshaft to the first camshaft, and a synchronization transmission apparatus that rotates the first and second camshafts in synchronism with each other. 
     According to an embodiment of the present invention, the timing transmission apparatus is disposed in a timing transmission chamber formed on an engine outer side wall so as to protrude to the outer side of the group of the cylinders and is connected to one end portion of the first camshaft. 
     According to an embodiment of the present invention, the synchronization transmission apparatus is configured from a pair of synchronization gears fixedly mounted on the first and second camshafts and meshing with each other. 
     According to an embodiment of the present invention, the synchronization transmission apparatus is configured from a pair of sprocket wheels fixedly mounted on the first and second camshafts, and an endless transmission belt extending between and around both of the sprocket wheels. 
     According to an embodiment of the present invention, the electric motor and the speed reduction mechanism are disposed such that a straight line which connects center axes of the valve shaft and a rotor shaft of the electric motor to each other is inclined in a direction in which the rotor shaft side of the straight line comes near to the upstream side of the intake path with respect to a center axial line of the intake path. 
     According to an embodiment of the present invention, the electric motor is disposed between the throttle body positioned at an intermediate location of the group of the throttle bodies and the valve chamber. 
     According to an embodiment of the present invention, the swelling portion is formed on the side wall of the valve chamber in such a manner as to bite between those of the throttle bodies which neighbor with each other in order to accept a protrusion of part of the valve system in a radial direction of the first and second camshafts. Therefore, the neighboring disposition of the throttle bodies with the valve chamber is permitted without interference by the swelling portion. Further, the electric motor is disposed at one side of the group of the throttle bodies while the speed reduction mechanism, which transmits output power of the electric motor to the valve shaft, is disposed between those throttle bodies, which neighbor with each other at a different location in the group of the throttle bodies, in a neighboring relationship with the valve chamber. Therefore, the neighboring disposition of the throttle body group with the valve chamber is permitted while mutual interference between the speed reduction mechanism and the swelling portion is avoided. Therefore, compactification around the cylinder head can be achieved. In addition, the output torque of the electric motor is transmitted to the intermediate portion of the valve shaft through the speed reduction mechanism without a delay and then transmitted from the intermediate portion of the valve shaft toward the opposite end portions of the valve shaft. Consequently, twist deformation of the valve shaft can be prevented or reduced to achieve tuning of all of the throttle valves. Accordingly, a balance in the output characteristics of all cylinders can be secured. 
     According to an embodiment of the present invention, the electric motor is disposed between the valve chamber and the group of the throttle bodies in an offset relationship from the swelling portion in an axial direction of the first and second camshafts. Therefore, the electric motor can be disposed between the valve chamber and the throttle body group without interference by the swelling portion, and compactification around the cylinder head can be implemented. 
     According to an embodiment of the present invention, the valve system includes the timing transmission apparatus which transmits output power of the crankshaft to the first camshaft, and the synchronization transmission apparatus which rotates the first and second camshafts in synchronism with each other. Therefore, the number of such driven sprocket wheels of the timing transmission apparatus which have a maximum diameter in the valve system may be only one. In addition, the synchronization gears can be formed with a smaller diameter than that of the driven sprocket wheel, and consequently, the distance between the axes of the intake and exhaust camshafts can be reduced. As a result, compactification of the valve chamber for accommodating the intake and exhaust camshafts therein and hence compactification of the cylinder head can be implemented. 
     According to an embodiment of the present invention, the timing transmission apparatus is disposed in the timing transmission chamber formed on the engine outer side wall so as to protrude to the outer side of the group of the cylinders. Therefore, the neighboring disposition of the throttle body group with the valve chamber is permitted without interference by the timing transmission apparatus. Consequently, compactification around the cylinder head can be implemented. 
     According to an embodiment of the present invention, the synchronization transmission apparatus is configured from the paired synchronization gears fixedly mounted on the first and second camshafts and meshing with each other. Therefore, the synchronization transmission apparatus can be configured from a minimum number of parts, and simplification of the structure can be implemented. 
     According to an embodiment of the present invention, the synchronization transmission apparatus is configured from the paired sprocket wheels fixedly mounted on the first and second camshafts, and the endless transmission belt extending between and around both of the sprocket wheels. Therefore, the synchronization sprocket wheels can be formed with a smaller diameter than that of the synchronization gears. Consequently, compactification of the valve chamber can be implemented. 
     According to an embodiment of the present invention, the electric motor and the speed reduction mechanism are disposed such that the straight line that connects the center axes of the valve shaft and the rotor shaft of the electric motor to each other is inclined in the direction in which the rotor shaft side of the straight line comes near to the upstream side of the intake path with respect to the center axial line of the intake path. Therefore, a small space between the valve chamber and the throttle body group can be effectively utilized as an installation space for the electric motor and the speed reduction mechanism. Consequently, that can contribute to compactification around the cylinder head. 
     According to an embodiment of the present invention, the electric motor is disposed between the throttle body positioned at the intermediate location of the group of the throttle bodies and the valve chamber. Therefore, the electric motor is entirely accommodated between the valve chamber and the throttle body group and the electric motor can be protected from an obstacle. Therefore, there is no necessity to take special protection means such as a cover. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a left side elevational view of a motorcycle according to a first embodiment of the present invention; 
         FIG. 2  is a left side elevational view of an engine of the motorcycle; 
         FIG. 3  is a right side elevational view of the engine of the motorcycle; 
         FIG. 4  is a view as viewed in a direction indicated by an arrow mark  4  of  FIG. 2  depicting the engine in a state in which a head cover is removed; 
         FIG. 5  is a sectional view taken along line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 4 ; 
         FIG. 7  is a sectional view taken along line  7 - 7  of  FIG. 4 ; and 
         FIG. 8  is a view depicting a second embodiment of the present invention and corresponding to  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention are described with reference to the drawings. 
     A first embodiment of the present invention is described with reference to  FIGS. 1 to 7 . In  FIGS. 1 and 2 , a vehicle body frame F of a motorcycle M is configured from a main frame  2  having a head pipe  1  at a front end thereof, a center frame  3  connected to a rear end of the main frame  2 , and a rear frame  4  connected to a rear end of the center frame  3 . A front fork  5  is attached for providing a steering motion to the head pipe  1  and supports a front wheel Wf thereon, and a rear fork  6  is attached for upward and downward rocking motion to the center frame  3  and supports a rear wheel Wr thereon. Further, a series multi-cylinder engine E is attached to the main frame  2  and the center frame  3  and has a forwardly inclined cylinder block  9  thereon, and a riding seat  7  is attached to an upper portion of the rear frame  4 . 
     A speed change gear is accommodated in a crankcase  8  of the engine E. Output power of an output power shaft  13  of the speed change gear is transmitted to the rear wheel Wr through a chain transmission apparatus  14  disposed on the left side of the crankcase  8 . 
     Referring to  FIGS. 3 to 5 , the cylinder block  9  has four cylinders  15   a  to  15   d  juxtaposed in series with a cylinder head  10  is joined to an upper end of the cylinder block  9 . A head cover  11  is joined to an upper end of the cylinder head  10 . Further, an intake port  16  is opened to the rear face of the cylinder head  10  and continues to each of the four cylinders  15   a  to  15   d . Meanwhile, an exhaust port  17  is opened to the front face of the cylinder head  10  and continues to each of the four cylinders  15   a  to  15   d . Further, four throttle bodies  18   a  to  18   d  are connected to the rear face of the cylinder head  10  through heat insulation cylinders  20 . Each of the throttle bodies  18   a  to  18   d  has an intake path  19  continuing to the intake port  16 . 
     Funnels  21  are each connected to an upstream end of each of the throttle bodies  18   a  to  18   d  and accommodated in an air cleaner  22  (refer to  FIG. 2 ), which is disposed at an upper portion of the main frame  2 . A first fuel injection valve  23  is mounted on each of the throttle bodies  18   a  to  18   d  and injects fuel toward the intake port  16 . A second fuel injection valve  24  is provided on the air cleaner  22  and injects fuel toward the funnels  21 . 
     Exhaust pipes  25  are connected to the front face of the cylinder head  10  and continue to the exhaust ports  17 , and the exhaust pipes  25  are coupled together and connected at the downstream side thereof to an exhaust muffler  26  disposed on the right side of the motorcycle M. 
     As depicted in  FIGS. 4 and 5 , a pair of intake valves  30  (only one is depicted in  FIG. 5 ) for opening and closing the intake ports  16  and a pair of exhaust valves  31  (only one is depicted in  FIG. 5 ) for opening and closing the exhaust ports  17  are provided in the cylinder head  10 . The intake valves  30  and the exhaust valves  31  have an intake valve spring  32  and an exhaust valve spring  33  mounted thereon for biasing them in a closing direction, respectively. 
     A valve chamber  35  is defined between the cylinder head  10  and the head cover  11 . A timing transmission chamber  36  is formed in an outer side wall of the engine E so as to extend from the crankcase  8  to the cylinder head  10  such that it communicates with one end portion of the valve chamber  35 . A valve system  37  is disposed so as to extend from the timing transmission chamber  36  to the valve chamber  35 . 
     The valve system  37  includes an intake camshaft  38  and an exhaust camshaft  39  supported on the cylinder head  10  in the valve chamber  35  and extending in parallel to the direction in which the cylinders  15   a  to  15   d  are arrayed. The intake camshaft  38  is disposed at the side of the throttle bodies  18   a  to  18   d  with respect to the exhaust camshaft  39 . 
     The intake camshaft  38  has an intake cam  38   a  for opening and closing each of the intake valves  30  through an intake cam follower  40 . Meanwhile, the exhaust camshaft  39  has an exhaust cam  39   a  for opening and closing each of the exhaust valves  31  through an exhaust cam follower  41 . The intake cam follower  40  and the exhaust cam follower  41  are supported for rocking motion on the cylinder head  10 . 
     In the group of the cylinders  15   a  to  15   d , the cylinders are referred to as first to fourth cylinders  15   a  to  15   d  from the right side in  FIG. 4 , respectively. Further, in the group of the throttle bodies  18   a  to  18   d , the throttle bodies are similarly referred to as first to fourth throttle bodies  18   a  to  18   d  from the right side in  FIG. 4 , respectively. The timing transmission chamber  36  is formed, in the example depicted, in an engine outer side wall at the first cylinder  15   a  side such that it protrudes to the outer side of the first throttle body  18   a . A timing transmission apparatus  42  is disposed in the timing transmission chamber  36  and configured from a driving sprocket wheel  43 , a driven sprocket wheel  44  and a timing chain  45 . The driving sprocket wheel  43  is fixedly mounted at one end portion of a crankshaft  29 . The driven sprocket wheel  44  is fixedly mounted at one end portion of the intake cam shaft  38 . The timing chain  45  extends between and around the two sprocket wheels  43  and  44 . The timing transmission apparatus  42  transmits rotation of the crankshaft  29  at a speed reduced to ½ to the intake camshaft  38 . 
     Further, as depicted in  FIGS. 4 and 7 , in the valve chamber  35 , the intake camshaft  38  and the exhaust camshaft  39  are connected to each other through a synchronization transmission apparatus  46 . The synchronization transmission apparatus  46  is configured from synchronization gears  47  and  48  formed integrally at intermediate positions of the intake camshaft  38  and the exhaust camshaft  39  corresponding to each other between the third and fourth cylinders  15   c  and  15   d  and held in meshing engagement with each other. The synchronization gears  47  and  48  are formed with a diameter smaller than that of the driven sprocket wheel  44  of the timing transmission apparatus  42 . 
     In addition, the synchronization gears  47  and  48  are configured such that, from a relationship wherein they protrude outwardly in a radial direction from the intake and exhaust camshafts  38  and  39 , a swelling portion  35   a  is formed on a side wall of the valve chamber  35 . The swelling portion  35   a  accepts protruding portions of the synchronization gears  47  and  48  therein. The swelling portion  35   a  is disposed such that it bites between the third and fourth throttle bodies  18   c  and  18   d.    
     As depicted in  FIGS. 4 and 6 , a valve shaft  49  is supported on the throttle bodies  18   a  to  18   d . The valve shaft  49  extends in a direction in which the throttle bodies  18   a  to  18   d  are arrayed in such a manner so as to traverse the intake paths  19  of the throttle bodies  18   a  to  18   d . A throttle valve  50  for opening and closing each of the intake paths  19  is provided on the valve shaft  49 . An electric motor  51  is disposed between the second throttle body  18   b  and the valve chamber  35 . In particular, the electric motor  51  is disposed in an offset relationship from the swelling portion  35   a  along an axial direction of the intake and exhaust camshafts  38  and  39 . A rotor shaft  51   a  of the electric motor  51  is connected to the valve shaft  49  through a speed reduction mechanism  52  disposed between the second and third throttle bodies  18   b  and  18   c.    
     As depicted in  FIG. 6 , the speed reduction mechanism is configured from a pinion gear  53 , a first large diameter gear  54 , a small diameter gear  55  and a second large diameter gear  56  of the sector type. The pinion gear  53  is fixedly mounted on the rotor shaft  51   a  of the electric motor  51 . The first large diameter gear  54  is held in meshing engagement with the pinion gear  53 . The small diameter gear  55  is formed coaxially with and integrally on the first large diameter gear  54 . The second large diameter gear  56  is fixedly mounted on the valve shaft  49  and is held in meshing engagement with the small diameter gear  55 . The speed reduction mechanism  52  transmits rotation of the rotor shaft  51   a  to the valve shaft  49  at a speed reduced by two stages. 
     A speed reduction case  57  that accommodates the speed reduction mechanism  52  is configured from case halves  57   a  and  57   b  formed integrally with the second and third throttle bodies  18   b  and  18   c , respectively, and joined to each other. An intermediate shaft  59  is supported on the speed reduction case  57  and supports the first large diameter gear  54  and the small diameter gear  55 , and the electric motor  51  is attached to the case half  57   a  at the second throttle body  18   b  side. The electric motor  51  and the speed reduction mechanism  52  are disposed such that the straight line L interconnecting the center axes of the rotor shaft  51   a  and the valve shaft  49  is inclined such that the rotor shaft  51   a  side thereof approaches the upstream side of the intake path  19 . 
     A throttle sensor  58  is attached to an outer side wall of the first throttle body  18   a  or the fourth throttle body  18   d  at the outermost side from among the throttle bodies  18   a  to  18   d . The throttle sensor  58  detects the angle of rotation of the valve shaft  49  as an opening of the throttle valve  50 . 
     Now, the operation of the present embodiment is described. 
     During the operation of the engine E, the rotation of the crankshaft  29  is transmitted to the intake camshaft  38  at a speed reduced to ½ by the timing transmission apparatus  42 . The rotation of the intake camshaft  38  is transmitted in synchronism to the exhaust camshaft  39  by the synchronization transmission apparatus  46 . Consequently, the intake cams  38   a  of the intake camshaft  38  cooperate with the intake valve springs  32  to open and close the intake valves  30 , and the exhaust cams  39   a  of the exhaust camshaft  39  cooperate with the exhaust valve springs  33  to open and close the exhaust valves  31 . In this manner, the driving system for the intake and exhaust camshafts  38  and  39  is divided into the timing transmission apparatus  42  for driving one of the camshafts, in the example depicted, the intake camshaft  38 , from the crankshaft  29  and the synchronization transmission apparatus  46  for synchronously connecting the both camshafts  38  and  39  to each other. Therefore, the number of such driven sprocket wheels  44  of the timing transmission apparatus  42  which have a maximum diameter in the driving system may be only one. In addition, the synchronization gears  47  and  48  are formed with a diameter smaller than that of the driven sprocket wheel  44 . Consequently, the distance between axes of the intake and exhaust camshafts  38  and  39  can be reduced. As a result, compactification of the valve chamber  35  in which the intake and exhaust camshafts  38  and  39  are accommodated and hence compactification of the cylinder head  10  can be anticipated. 
     Further, the timing transmission apparatus  42  is disposed in the timing transmission chamber  36  formed in the outer side wall of the engine E in such a manner so as to protrude to the outer side of the first throttle body  18   a . Therefore, the group of the throttle bodies  18   a  to  18   d  can be disposed in a neighboring relationship with the valve chamber  35  without interference with the timing transmission apparatus  42 . Thus, compactification around the cylinder head  10  can be anticipated. 
     Further, the swelling portion  35   a  of the valve chamber  35  that accepts the protrusion of the synchronization transmission apparatus  46  is disposed in such a manner as to bite between the adjacent third and fourth throttle bodies  18   c  and  18   d . Therefore, the group of the throttle bodies  18   a  to  18   d  can be disposed in a neighboring relationship with the valve chamber  35  without interference with the swelling portion  35   a . Thus, further compactification around the cylinder head  10  can be anticipated. 
     Since the synchronization transmission apparatus  46  can be configured from the synchronization gears  47  and  48  in pair, it can be configured from a minimum number of parts, which can contribute to simplification of the structure. 
     Meanwhile, the electric motor  51  is controlled by an electronic controlling unit not depicted based on the operation amount of the accelerator operation member, opening of the throttle valve  50 , engine speed and so forth. The electric motor  51  opens and closes the throttle valves of the throttle bodies  18   a  to  18   d  through the speed reduction mechanism  52  and the valve shaft  49 . 
     In addition, the speed reduction mechanism  52  is disposed between the second and third throttle bodies  18   b  and  18   c  and is offset from the swelling portion  35   a  of the valve chamber  35  along the axial direction of the intake and exhaust camshafts  38  and  39 . Therefore, the group of the throttle bodies  18   a  to  18   d  can be disposed in a neighboring relationship with the valve chamber  35  while mutual interference of the speed reduction mechanism  52  and the swelling portion  35   a  is prevented. Consequently, compactification around the cylinder head  10  can be anticipated. Further, since the speed reduction mechanism  52  is connected to a central portion of the valve shaft  49 , the output torque of the electric motor  51  is transmitted to the central portion of the valve shaft  49  through the speed reduction mechanism  52  without delay. Further, the torque is transmitted from the central portion of the valve shaft  49  to the opposite ends of the valve shaft  49 . Consequently, twist deformation of the valve shaft  49  is prevented or reduced and synchronization of all of the throttle valves  50  can be achieved. Accordingly, the balance in output characteristic of all of the cylinders  15   a  to  15   d  can be assured. 
     The electric motor  51  is disposed in an offset relationship from the swelling portion  35   a  along the axial direction of the intake and exhaust camshafts  38  and  39  between the valve chamber  35  and the group of the throttle bodies  18   a  to  18   d . Therefore, the electric motor  51  can be disposed between the valve chamber  35  and the group of the throttle bodies  18   a  to  18   d  without interfering with the swelling portion  35   a . Further, since the electric motor  51  disposed at one side of the second throttle body  18   b  at an intermediate position, the electric motor  51  is fully accommodated between the valve chamber  35  and the group of the throttle bodies  18   a  to  18   d . Consequently, the electric motor  51  can be protected from an obstacle, and there is no necessity to use special protection means such as a cover. 
     Further, the electric motor  51  and the speed reduction mechanism  52  are disposed such that the straight line L interconnecting the center axes of the valve shaft  49  and the rotor shaft  51   a  of the electric motor  51  is inclined with respect to the center axial line Y of the intake path in a direction in which the rotor shaft  51   a  of the straight line L approaches the upstream side of the intake path  19 . Therefore, the small space between the valve chamber  35  and the group of the throttle bodies  18   a  to  18   d  can be utilized effectively as an installation space for the electric motor  51  and the speed reduction mechanism  52 . This can contribute to compactification around the cylinder head  10 . 
     Now, a second embodiment of the present invention depicted in  FIG. 8  is described. 
     In the present second embodiment, the synchronization transmission apparatus  46  is configured from a pair of synchronization sprocket wheels  60  and  61  fixedly provided on the intake camshaft  38  and the exhaust camshaft and a chain  62  extending between and around the synchronization sprocket wheels  60  and  61 . The configuration of the other part of the motorcycle is similar to that of the preceding embodiment. Therefore, elements in  FIG. 8  corresponding to those of the preceding embodiment are denoted by like reference symbols and overlapping description of them is omitted herein to avoid redundancy. 
     With the present second embodiment, the synchronization sprocket wheels  60  and  61  can be formed with a smaller diameter than the synchronization gears  47  and  48  in the preceding embodiment. Thus, compactification of the valve chamber  35  can be anticipated as much. 
     The embodiments of the present invention have been described. However, the present invention is not limited to the embodiments but can be modified in various manners without departing from the subject matter of the present invention. For example, the synchronization transmission apparatus  46  can be disposed also at a corresponding position between the first and second cylinders  15   a  and  15   b . Further, if the speed reduction mechanism  52  is disposed between the first and second throttle bodies  18   a  and  18   b  or between the third and fourth throttle bodies  18   c  and  18   d  while the synchronization transmission apparatus  46  is disposed at a corresponding position between the first and second cylinders  15   a  and  15   b , then also it is possible to avoid interference between the swelling portion  35   a  of the valve chamber  35  and the speed reduction mechanism  52 . Also it is possible to provide a single chain extending between a driving sprocket wheel fixedly mounted on the crankshaft  29  and a pair of synchronization sprocket wheels fixedly mounted on the intake and exhaust camshafts  38  and  39  to integrate the timing transmission apparatus  42  and the synchronization transmission apparatus  46  with each other. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims