Patent Publication Number: US-8113168-B2

Title: Engine unit and vehicle including the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 USC 119 of Japanese patent application no. 2007-264682, filed on Oct. 10, 2007, which is incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an engine unit for a vehicle that has a V-type engine and a throttle body assembly. 
     2. Description of Related Art 
     Various types of throttle body assemblies for V-type engines are known. For example,  FIG. 12  illustrates a throttle body assembly  100  as disclosed in JP-A-2004-308536. 
     Throttle body assembly  100  includes a drum  102  attached to an input shaft  103 . A wire  101  is wound around drum  102 . Wire  101  is moved by operation of an acceleration grip (not shown) to rotate drum  102  and input shaft  103 . An accelerator position sensor  116  is provided at one end of input shaft  103 , which is also referred to as an accelerator position sensor (APS) shaft for this reason. The other end of input shaft  103  is connected to an output shaft  105  via a power transmission system  104 . A gear  104   a  of power transmission system  104  is connected with a driving motor  120  via gears  121  and  122 . 
     A base end of a first arm member  106  is fixed to a tip end of output shaft  105 . One end of a first link  107  is attached to a tip end of first arm member  106  in a swingable manner. The other end of first link  107  is attached to a front arm portion  108   a  of a second arm member  108  in a swingable manner. Second arm member  108  rotates about a front valve shaft  109 . A throttle valve  110  is attached to front valve shaft  109  in a front throttle portion  117 . Front throttle portion  117  is opened and closed by throttle valve  110 . 
     One end of a second link  111  is attached to a rear arm portion  108   b  of second arm member  108  in a swingable manner. The other end of second link  111  is attached to a tip end of a third arm member  112  in a swingable manner. A base end of third arm member  112  is fixed to a rear valve shaft  113 . A throttle valve  114  is attached to rear valve shaft  113  in a rear throttle portion  118 . Rear throttle portion  118  is opened and closed by throttle valve  114 . A throttle position sensor  115  is attached to rear valve shaft  113  and detects a throttle opening angle. 
     When accelerator grip is operated, wire  101  moves and drum  102  and input shaft  103  rotate. The rotational amount of input shaft  103  is detected by accelerator position sensor  116  as an accelerator opening angle. Then, according to the detected accelerator opening angle, driving motor  120  is driven. The rotation of driving motor  120  is transmitted to front valve shaft  109  and rear valve shaft  113  via gears  121  and  122 , power transmission system  104 , output shaft  105 , first arm member  106 , first link  107 , second arm member  108 , second link  111 , and third arm member  112 . As a consequence, front valve shaft  109  and rear valve shaft  113  rotate, thereby opening and closing throttle valves  110  and  114 . 
     As described in paragraph 50 of JP-A-2004-308536, input (APS) shaft  103  and output shaft  105  overlap with driving motor  120  in a vertical direction. Therefore, throttle body assembly  100  can be made compact and protrusion of throttle body assembly  100  from throttle portions  117  and  118  can be reduced. 
     As shown in  FIG. 12 , driving motor  120  is disposed between front throttle portion  117  and rear throttle portion  118 . Therefore, compared with a case in which driving motor  120  is disposed in front of front throttle portion  117  or at the rear of rear throttle portion  118 , a longitudinal length of throttle body assembly  100  is shortened. Nevertheless, since input (APS) shaft  103  and driving motor  120  are arranged one above the other in a vertical direction, it is difficult to make the height dimension of throttle body assembly  100  small enough. Accordingly, the use of throttle body assembly  100  makes it difficult to sufficiently reduce the size of the V-type engine. 
     SUMMARY OF THE INVENTION 
     The invention addresses this problem and achieves size reduction of an engine unit that includes a throttle body assembly. 
     An engine unit of the invention includes a throttle body assembly attached to a V-type engine. The V-type engine has a front cylinder connected to a front intake port and a rear cylinder connected to a rear intake port connected to the rear cylinder. 
     The throttle body assembly includes front and rear throttle bodies, an actuator and a second rotational shaft. A front cylinder of the front throttle body is connected to the front intake port. A front throttle valve opens and closes the front cylinder. A rear cylinder of the rear throttle body is connected to the rear intake port. A rear throttle valve opens and closes the rear cylinder. The actuator drives the front and rear throttle valves and has a first rotational shaft that extends in a widthwise direction. The actuator is disposed between center axes of the front and rear cylinders in a longitudinal direction. The shaft center of the second rotational shaft is located in front of or at the rear of the shaft center of the first rotational shaft. 
     A vehicle according to the invention includes the engine unit described above. 
     In the invention, the first and second rotational shafts are offset other in a longitudinal direction. Therefore, the throttle body assembly as well as the engine unit can be made compact. 
     Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view of a motorcycle according to the invention. 
         FIG. 2  is an enlarged right side view of an engine unit of the motorcycle. 
         FIG. 3  is a cross-sectional view of a throttle body assembly and an engine of the engine unit. 
         FIG. 4  is a plan view of the throttle body assembly. 
         FIG. 5  is a left side view of the throttle body assembly. 
         FIG. 6  is a right side view of the throttle body assembly. 
         FIG. 7  is a cross-sectional view of a second front throttle body. 
         FIG. 8  is a rear view of the throttle body assembly. 
         FIG. 9  is a cross-sectional view of the throttle body assembly illustrating a deceleration gear mechanism. 
         FIG. 10  is a block diagram of a control block of the motorcycle. 
         FIG. 11  is a left side view of a throttle body assembly according to a modified embodiment of the invention. 
         FIG. 12  is a perspective view of a throttle body assembly of the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention is now described with reference to a motorcycle  1  ( FIG. 1 ). However, the invention is not restricted to a motorcycle and may be any vehicle including a V-type engine, including four-wheeled and straddle-type vehicles. A straddle-type vehicle is a vehicle on which a rider straddles a seat (saddle) and may include an all terrain vehicle (ATV) and the like in addition to a motorcycle. Furthermore, the motorcycle is not restricted to a so-called American-type motorcycle and may be other types of motorcycles, a moped, a scooter, an off-road vehicle and the like. Moreover, a motorcycle includes a vehicle with multiple wheels that rotate together with at least one of the front and rear wheels, and that is tilted to change a traveling direction. 
     In the following description, the longitudinal and horizontal directions are from the perspective of a rider seated on a seat  14 . 
     (Overall Structure of Motorcycle  1 ) 
     As shown in  FIG. 1 , motorcycle  1  has a vehicle body frame  10 , a vehicle body cover  13  and a seat  14 . A part of vehicle body frame  10  is covered by vehicle body cover  13 . Seat  14  is disposed on the top of vehicle body frame  10 . 
     Vehicle body frame  10  has a main frame  11  and a rear frame  12 . Main frame  11  includes left and right frame portions  11   a  and  11   b  that extend to the rear from a head pipe  15 . Head pipe  15  is rotatably attached to main frame  11 . A handle  16  is fixed to an upper end portion of head pipe  15  by a handle holder (not shown) and is provided with a throttle grip  17  as a throttle operator. Throttle grip  17  is connected to an accelerator position sensor (APS)  51  by a throttle wire  18 . Therefore, when throttle grip  17  is operated by a rider, throttle wire  18  is moved and the amount of operation of throttle grip  17  is detected by accelerator position sensor  51  as an accelerator opening angle. 
     A front fork  20  with forks to the left and right is fixed to head pipe  15  and extends obliquely downward to the front. A front wheel  21  is rotatably attached to a lower end portion of front fork  20 . A pivot shaft  22  is attached to a rear end portion of vehicle body frame  10 . A rear arm  23  is attached to pivot shaft  22  in a swingable manner. A rear wheel  24  is rotatably attached to a rear end portion of rear arm  23 . Rear wheel  24  is connected with an output shaft of an engine unit  30  by a power transmission mechanism such as a drive shaft. Power from engine unit  30  is thereby transmitted to rear wheel  24  and rotates rear wheel  24 . 
     As shown in  FIGS. 1 and 2 , engine unit  30  is suspended from main frame  11 . Engine unit  30  includes a V-type engine  31 , a throttle body assembly  50 , a clutch, a transmission mechanism and the like. Throttle body assembly  50  is disposed on engine  31  between left and right frame portions  11   a  and  11   b  in a plan view ( FIG. 4 ). 
     An insulator  48  is disposed between engine unit  30  and throttle body assembly  50 . Insulator  48 , engine  31 , and throttle body assembly  50  are mutually fixed by cross members  82   a  and  82   b  arranged at both sides of the vehicle in a widthwise direction. As shown in  FIG. 3 , insulator  48  is provided with connecting channels  48   a  and  48   b  that connect intake ports  42   a  and  42   b  of engine  31  to respective cylinders  55  and  56  of throttle body assembly  50 . 
     As shown in  FIG. 2 , an air cleaner  49  that serves as an intake system part is arranged on and supplies outside air to throttle body assembly  50 . As an alternative to air cleaner  49 , an air chamber may be arranged as the intake system part. 
     As shown in  FIG. 1 , a fuel tank  19  is disposed at the rear of engine  31 . Fuel tank  19  is connected with a fuel nipple  82  of throttle body assembly  50  ( FIG. 4 ) by a fuel supply hose. Fuel stored in fuel tank  19  is supplied to throttle body assembly  50  through the fuel supply hose. Air and fuel supplied to throttle body assembly  50  are mixed in throttle body assembly  50 , thereby creating an air-fuel mixture that is supplied to engine  31 . 
     As shown in  FIG. 4 , in a space enclosed by main frame  11  in a plan view, a battery  47  that supplies power to engine unit  30  and to throttle body assembly  50  is installed at the immediate rear of throttle body assembly  50 . 
     (Engine  31 ) 
     Engine  31  is now described, mainly with reference to  FIGS. 1-3 . In this embodiment, engine  31  is a water-cooled 4-stroke V-type 4-cylinder engine. However, engine  31  is not particularly restricted as long as it is a V-type engine and may be, for example, an air-cooled engine or a 2-stroke engine. Furthermore, engine  31  may be a V-type engine with three cylinders or less or five cylinders or more. 
     “V-type engine” as used herein refers to an engine having a front cylinder and a rear cylinder that are arranged in such a manner as to form a V-bank. That is, the front and rear cylinders are arranged such that a center axes of the front and rear cylinders diagonally intersect with each other with a shaft center of a crankshaft being the center of the intersection. 
     As shown in  FIG. 2 , engine  31  has a crankcase  32  that houses a crankshaft. Crankcase  32  is attached with a front cylinder body  33  and a rear cylinder body  35 . Front cylinder body  33  and rear cylinder body  35  are arranged in a V-shape having the crankshaft as a center thereof in a side view. A front cylinder head  36  is provided on front cylinder body  33 , and a front head cover  38  is provided on the top of front cylinder head  36 . Similarly, a rear cylinder head  37  is provided on the top of rear cylinder body  35 , and a rear head cover  39  is provided on top of rear cylinder head  37 . 
     As shown in  FIG. 3 , a front cylinder  34  formed in a substantially cylindrical shape is provided in front cylinder body  33 , and a rear cylinder  29  formed in a substantially cylindrical shape is provided in rear cylinder body  35 . Front cylinder  34  and rear cylinder  29  are arranged to form a V-bank. Specifically, front cylinder  34  is disposed to extend obliquely upward to the front, while rear cylinder  29  is disposed to extend obliquely upward to the rear. The degree of an angle θ 0  formed by center axes of front and rear cylinders  34  and  29  ( FIG. 1 ) is set such that cylinders  34  and  29  do not positionally interfere with each other in consideration of engine noise caused by engine  31 , characteristics to be obtained by engine  31 , and the like. The angle θ 0  is normally in the range of 10-170 degrees, preferably in the range of 30-150 degrees, and more preferably in the range of 45-100 degrees. 
     As shown in  FIG. 3 , front cylinder  34  and rear cylinder  29  respectively house connecting rods  40   a  and  40   b  that are connected to respective crankshafts. Pistons  41   a  and  41   b  are attached to the tip end portions of connecting rods  40   a  and  40   b . Pistons  41   a  and  41   b , cylinders  34  and  29 , and cylinder heads  36  and  37  define and form combustion chambers  47   a  and  47   b.    
     Front cylinder head  36  and rear cylinder head  37  are provided with intake ports  42   a  and  42   b  and exhaust ports  43   a  and  43   b , respectively. Intake ports  42   a  and  42   b  are provided with intake valves  44   a  and  44   b  that open and close intake ports  42   a  and  42   b . Intake valves  44   a  and  44   b  are driven by intake cams  46   a  and  46   b  disposed on the top face of intake valves  44   a  and  44   b . Similarly, exhaust ports  43   a  and  43   b  are provided with exhaust valves  45   a  and  45   b  that open and close exhaust ports  43  and are driven by exhaust cams. 
     (Throttle Body Assembly  50 ) 
     —Front Throttle Body  53  and Rear Throttle Body  54 — 
     Throttle body assembly  50  is now described in detail with reference mainly to  FIGS. 4-9 . Throttle body assembly  50  includes a first front throttle body  53   a  and a second front throttle body  53   b . In the following descriptions, front throttle bodies  53   a  and  53   b  may be collectively called front throttle bodies  53 . 
     Front throttle bodies  53   a  and  53   b  are arranged in the vehicle width direction. First front throttle body  53   a  is provided with a first front cylinder  55   a  formed in a substantially cylindrical shape, and second throttle body  53   b  is provided with a second front cylinder  55   b  formed in a substantially cylindrical shape. Front cylinders  55   a  and  55   b  extend in a vertical direction, respectively. In the following description, front cylinders  55   a  and  55   b  may be collectively called front cylinders  55 . 
     Front throttle bodies  53   a  and  53   b  have front throttle valves  57   a  and  57   b , respectively. In the following description, front throttle valves  57   a  and  57   b  may be collectively called front throttle valves  57 . Front throttle valve  57   a  is connected with front throttle valve  57   b  by a valve shaft  65 . When valve shaft  65  is rotated by a motor  60 , front throttle valves  57   a  and  57   b  move simultaneously to open and close front cylinders  55   a  and  55   b.    
     A first rear throttle body  54   a  and a second rear throttle body  54   b  are arranged at the rear of front throttle bodies  53   a  and  53   b . In the following description, rear throttle bodies  54   a  and  54   b  may be collectively called rear throttle bodies  54 . Rear throttle bodies  54   a  and  54   b  are arranged in the vehicle width direction. First rear throttle body  54   a  is disposed approximately to the rear of first front throttle body  53   a  and second rear throttle body  54   b  is disposed approximately to the rear of second front throttle body  53   b . However, due to the arrangement of connecting rods  40   a  and  40   b , front throttle bodies  53   a  and  53   b  are arranged slightly offset with respect to rear throttle bodies  54   a  and  54   b  in the vehicle width direction. 
     In the embodiment, upper ends of front throttle bodies  53   a  and  53   b  and upper ends of rear throttle bodies  54   a  and  54   b  are located at the same height. 
     First rear throttle body  54   a  is provided with a first rear cylinder  56   a  formed in a substantially cylindrical shape. Meanwhile, second rear throttle body  54   b  is provided with a second rear cylinder  56   b  formed in a substantially cylindrical shape. In the following description, rear cylinders  56   a  and  56   b  may be collectively called rear cylinders  56 . 
     Rear throttle bodies  54   a  and  54   b  have rear throttle valves  58   a  and  58   b , respectively. Hereafter, rear throttle valves  58   a  and  58   b  may be collectively called rear throttle valves  58 . Rear throttle valve  58   a  is connected with rear throttle valve  58   b  by a valve shaft  66 . When valve shaft  66  is rotated by motor  60 , rear throttle valves  58   a  and  58   b  move simultaneously to opens and closes rear cylinders  56   a  and  56   b.    
     As shown in  FIG. 2 , the upper end portions of front cylinders  55  and rear cylinders  56  are connected to air cleaner  49 . The lower ends of front cylinders  55  rear cylinders  56  are connected to intake ports  42   a  and  42   b , as shown in  FIG. 3 . By this structure, air taken from air cleaner  49  is supplied to engine  31  via throttle body assembly  50 . 
     —Injectors  75  and  76  and Fuel Supply Pipe  81 — 
     As mainly shown in  FIG. 8 , front throttle bodies  53   a  and  53   b  are provided with front injectors  75   a  and  75   b , respectively, and rear throttle bodies  54   a  and  54   b  are provided with rear injectors  76   a  and  76   b , respectively. In the following description, front injectors  75   a  and  75   b  may be collectively called front injectors  75 . and rear injectors  76   a  and  76   b  may be collectively called rear injectors  76 . 
     As shown in  FIGS. 2 and 3 , upper end portions of front injectors  75  and rear injectors  76  are connected to a fuel supply pipe  81 . As shown in  FIG. 4 , fuel supply pipe  81  extends between front and rear cylinders  55  and  56  in the vehicle width direction. More specifically, a center axis A 2  of fuel supply pipe  81  is located at the center of center axes A 4  and A 5  of front cylinders  55  and center axes A 6  and A 7  of rear cylinders  56  in the longitudinal direction. Furthermore, in relation to the vertical direction, fuel supply pipe  81  is disposed at a position that is lower than the upper ends of front and rear throttle bodies  53  and  54  and higher than the lower ends of throttle bodies  53  and  54 . Note that, when the upper ends of front throttle bodies  53  and the upper ends of rear throttle bodies  54  are different in height, which is not the case in this embodiment, fuel supply pipe  81  is preferably disposed at a position lower than the upper ends of front throttle bodies  53  or the upper ends of rear throttle bodies  54 , whichever is higher. 
     As shown in  FIG. 4 , fuel supply pipe  81  is connected with a fuel nipple  82  that extends to the rear from fuel supply pipe  81  between rear cylinders  56   a  and  56   b . Fuel nipple  82  is connected to fuel tank  19  ( FIG. 1 ) by a fuel supply pipe (not shown). The fuel in fuel tank  19  is thereby supplied to front and rear injectors  75  and  76  via the fuel pipe, fuel nipple  82  and fuel supply pipe  81 . 
     As shown in  FIGS. 4 and 8 , a pulsation damper  83  is attached to fuel supply pipe  81 . Pulsation damper  83  is located at the rear of and slightly obliquely downward from fuel supply pipe  81 . Pulsation damper  83  suppresses pulsation of fuel supplied to front and rear injectors  75  and  76 . 
     A nozzle  73  provided at the tip ends of front injectors  75 , as shown in  FIG. 3 , is adjusted such that fuel injected from front injectors  75  is injected centering on the center axis direction of front cylinders  55 . Similarly, a nozzle  74  provided at the tip ends of rear injectors  76  is adjusted such that fuel is injected centering on the center axis direction of rear cylinders  56 . 
     As shown in  FIGS. 6 and 8 , front injectors  75   a  and  75   b  include injector main bodies  68   a  and  68   b  and first front connectors  77   a  and  77   b . Rear injectors  76   a  and  76   b  include injector main bodies  69   a  and  69   b  and first rear connectors  78   a  and  78   b . Hereafter, injector main bodies  68   a  and  68   b  may be collectively called injector main bodies  68 , first front connectors  77   a  and  77   b  may be collectively called front connectors  77 , injector main bodies  69   a  and  69   b  may be collectively called injector main bodies  69 , and first rear connectors  78   a  and  78   b  may be collectively called rear connectors  78 . 
     Connectors  77  and  78  are connected to an electronic control unit (ECU)  80  shown in  FIG. 10 . A control signal is sent from ECU  80  to injectors  75  and  76  via connectors  77  and  78 , thereby controlling fuel injection from injectors  75  and  76 . Note that, although  FIG. 6  is a right side view of throttle body assembly  50 , a right fixing plate  88   a  shown in  FIG. 4  is omitted from  FIG. 6  for convenience in illustrating connectors  77  and  78 . 
     As shown in  FIG. 8 , injector main bodies  68  and  69  extend in the longitudinal direction in a plan view. On the other hand, connectors  77  and  78  extend obliquely in relation to the longitudinal direction in the plan view. To be specific, front connectors  77   a  and  77   b  extend obliquely to the rear in mutually opposite directions in the vehicle width direction. More specifically, front connectors  77   a  and  77   b  extend obliquely to the rear and outward in the vehicle width direction. Rear connectors  78   a  and  78   b  extend obliquely to the rear in mutually opposite directions in the vehicle width direction. To be specific, rear connectors  78   a  and  78   b  extend obliquely to the rear and outward in the vehicle width direction. 
     An angle formed by the center axis of injector main body  68   a  located on the outer side of the vehicle in the vehicle width direction and an extending direction of first front connector  77   a  in the plan view, and an angle formed by the centerline of injector main body  69   b  and an extending direction of second rear connector  78   b  in the plan view are both equally set to be θ 1 . Meanwhile, an angle formed by the center axis of injector main body  68   b  located on the inner side of the vehicle in the vehicle width direction and an extending direction of second front connector  77   b  in the plan view, and an angle formed by the center axis of injector main body  69   a  and an extending direction of first rear connector  78   a  in the plan view are both equally set to be θ 2 . θ i  and θ 2  are set within a range that does not cause positional interference between connectors  77  and  78 . A preferable range of θ 1  and θ 2  is between 5 and 180 degrees. 
     —Motor  60 — 
     Throttle body assembly  50  has a motor  60 . As shown in  FIG. 9 , motor  60  has a rotational shaft  60   a  as a first rotational shaft. A shaft center A 1  of rotational shaft  60   a  extends in the vehicle width direction. Rotational shaft  60   a  is provided with a motor pinion gear  61 . Motor pinion gear  61  is engaged with a transmission gear mechanism  62  that includes three idle gears  63   a ,  63   b  and  63   c  and two counter gears  64   a  and  64   b . Counter gear  64   a  is fixed to valve shaft  65  and counter gear  64   b  is fixed to valve shaft  66 . Motor pinion gear  61  is engaged with counter gear  64   a  via one idle gear  63   a . On the other hand, since motor pinion gear  61  and counter gear  64   b  are located relatively apart from each other, motor pinion gear  61  is engaged with counter gear  64   b  via two idle gears  63   b  and  63   c . By this structure, when motor  60  is driven and motor pinion gear  61  rotates, counter gears  64   a  and  64   b  are rotated and valve shafts  65  and  66  are rotated in the same direction. As a result, front throttle valves  57   a  and  57   b  and rear throttle valves  58   a  and  58   b  ( FIG. 4 ) are rotated, and thus cylinders  55  and  56  are opened and closed in synchronization. In this embodiment, motor  60  and transmission gear mechanism  62  are collectively called a throttle valve drive mechanism  59 . 
     As shown in  FIG. 8 , in the plan view, motor  60  as an actuator is disposed in an area enclosed by the center axis A 4  of first front cylinder  55   a , center axis A 5  of second front cylinder  55   b , center axis A 6  of first rear cylinder  56   a , and center axis A 7  of second rear cylinder  56   b . As  FIG. 9  illustrates, in relation to the vertical direction, motor  60  is disposed at a position that is lower than the upper ends and higher than the lower ends of throttle bodies  53  and  54 . That is, motor  60  is disposed in a space enclosed by the four throttle bodies, namely, front throttle bodies  53   a  and  53   b  and rear throttle bodies  54   a  and  54   b.    
     As shown in  FIG. 9  and  FIG. 4 , motor  60  is offset with respect to fuel supply pipe  81  in the longitudinal direction. Specifically, shaft center A 1  of rotational shaft  60   a  as first rotational shaft of motor  60  and center axis A 2  of fuel supply pipe  81  are located at different positions in the longitudinal direction. More specifically, shaft center A 1  is located in front of center axis A 2  of fuel supply pipe  81 . That is, as  FIG. 9  illustrates, motor  60  is disposed such that shaft center A 1  is located, in the longitudinal direction, between center axis A 2  of fuel supply pipe  81  and center axes A 4  and A 5  of front cylinders  55 . 
     —Casing  70 — 
     As shown in  FIGS. 4 and 8 , motor  60  and transmission gear mechanism  62  are housed in a casing  70 . As  FIG. 8  illustrates, valve shafts  65  and  66  connected to transmission gear mechanism  62  pass through casing  70 . 
     Casing  70  has a first casing portion  71  and a second casing portion  72  that face each other in the vehicle width direction. Casing portions  71  and  72  are fixed to each other by a bolt, rivet or the like. First casing portion  71  is disposed closer to transmission gear mechanism  62  and is made of a metal such as iron or an alloy such as aluminum and stainless steel. In this embodiment, first casing portion  71  is made of die cast aluminum. 
     First casing portion  71  is fixed to first front throttle body  53   a  and first rear throttle body  54   a . Specifically, a portion of casing  70  that houses transmission gear mechanism  62  and is penetrated by valve shafts  65  and  66  is directly fixed to throttle bodies  53   a  and  54   a.    
     Second casing portion  72  is located closer to motor  60  and is made of a resin such as polybutylene terephthalate (PBT) or the like. The resin that forms second casing portion  72  may include, for example, a glass fiber. In addition, second casing portion  72  may be made of a metal like first casing portion  71 . 
     Second casing portion  72  is fixed to second rear throttle body  54   b  via a metal stay  67  ( FIG. 8 ). To be more specific, stay  67  is fastened by a bolt to a top part of a portion of second casing portion  72  that houses motor  60 . Stay  67  is also fastened by a bolt to second rear throttle body  54   b.    
     —Connecting Member  85 — 
     As shown in  FIG. 4 , front throttle bodies  53   a  and  53   b  and rear throttle bodies  54   a  and  54   b  are fixed to each other by a connecting member  85 . Connecting member  85  includes two inner connecting pipes  86   a  and  86   b , two outer connecting pipes  87   a  and  87   b , right fixing plate  88   a , and a left fixing plate  88   b.    
     Inner connecting pipes  86   a  and  86   b  and outer connecting pipes  87   a  and  87   b  extend in the vehicle width direction. As is illustrated by  FIG. 6 , inner connecting pipes  86   a  and  86   b  are disposed in different positions to outer connecting pipes  87   a  and  87   b  in the vertical direction. Specifically, inner connecting pipes  86   a  and  86   b  are disposed approximately at the same position in the vertical direction as the upper end portions of throttle bodies  53  and  54 . On the other hand, outer connecting pipes  87   a  and  87   b  are disposed approximately at the same position in the vertical direction as the center portions of throttle bodies  53  and  54 . 
     As shown in  FIGS. 4 and 6 , inner connecting pipes  86   a  and  86   b  are disposed between center axes A 4  and A 5  of front cylinders  55  and center axes A 6  and A 7  of rear cylinders  56 . Inner connecting pipe  86   a  is fixed to front throttle bodies  53   a  and  53   b  to the rear of center axes A 4  and A 5  of front cylinders  55 . Inner connecting pipe  86   b  is fixed to rear throttle bodies  54   a  and  54   b  to the front of center axes A 6  and A 7  of rear cylinders  56 . Inner connecting pipes  86   a  and  86   b  are mutually fixed at two points in the widthwise direction by two fixing members  89 ; In the following description, inner connecting pipes  86   a  and  86   b  and fixing members  89  may be collectively called inner connecting member  91 . 
     Outer connecting pipe  87   a  is fixed to front throttle bodies  53   a  and  53   b  to the front of center axes A 4  and A 5  of front cylinders  55 . On the other hand, outer connecting pipe  87   b  is fixed to rear throttle bodies  54   a  and  54   b  to the rear of center axes A 6  and A 7  of rear cylinders  56 . 
     As described above, front throttle bodies  53   a  and  53   b  are securely fixed to each other by being sandwiched by inner connecting pipe  86   a  and outer connecting pipe  87   a . Furthermore, rear throttle bodies  54   a  and  54   b  are securely fixed to each other by being sandwiched by inner connecting pipe  86   b  and outer connecting pipe  87   b.    
     In addition, as shown in  FIGS. 4 and 5 , front throttle bodies  53   a  and  53   b  and rear throttle bodies  54   a  and  54   b  are fixed to each other by right fixing plate  88   a  that serves as a right fixing member and left fixing plate  88   b  that serves as a left fixing member. More specifically, as shown in  FIG. 5 , left fixing plate  88   b  is fixed by four points, namely, the upper and lower portions of second front throttle body  53   b  and the upper and lower portions of second rear throttle body  54   b . Right fixing plate  88   a  is fixed by four points, namely, the upper and lower portions of first front throttle body  53   a  and the upper and lower portions of first rear throttle body  54   a.    
     As described above, front throttle bodies  53   a  and  53   b  and rear throttle bodies  54   a  and  54   b  are fixed to each other by right fixing plate  88   a , left fixing plate  88   b , and inner connecting member  91 . In the plan view, as a connecting member for mutually fixing front throttle bodies  53   a  and  53   b  and rear throttle bodies  54   a  and  54   b , inner connecting member  91  only is disposed in an area enclosed by center axes A 4  and A 5  and center axes A 6  and A 7 . In the area enclosed by center axes A 4  and A 5  and center axes A 6  and A 7 , no connecting members that mutually fix front throttle bodies  53   a  and  53   b  with rear throttle bodies  54   a  and  54   b  are disposed below fuel supply pipe  81 . 
     —Accelerator Position Sensor  51  and Throttle Position Sensor  52 — 
     As shown in  FIG. 4 , throttle body assembly  50  is provided with accelerator position sensor  51  and a throttle position sensor  52 . Throttle position sensor  52  is disposed to the left of second front throttle body  53   b . Throttle position sensor  52  is connected to valve shaft  65 . Throttle position sensor  52  detects a throttle opening angle by detecting rotation of valve shaft  65 . 
     Accelerator position sensor  51  is connected to the right end portion of APS shaft  90 , which serves as the second rotational shaft. As  FIG. 5  illustrates, a shaft center A 3  of APS shaft  90  is located at a position lower than the upper ends of throttle bodies  53  and  54 . Note that, when the upper ends of throttle bodies  53  and  54  are different in height, which is not the case in this embodiment, APS shaft  90  is preferably disposed at a position lower than the upper ends of front throttle bodies  53  or than the upper ends of rear throttle bodies  54 , whichever is higher. 
     As shown in  FIGS. 4 and 5 , in the plan view, motor  60  is disposed in the area enclosed by center axes A 4  and A 5  of front cylinders  55  and center axes A 6  and A 7  of rear cylinders  56 . Meanwhile, APS shaft  90  is disposed outside the area. Specifically, in relation to the longitudinal direction, APS shaft  90  is disposed such that center axis A 3  of APS shaft  90  is located to the front of center axes A 4  and A 5  of front cylinders  55 . More specifically, as shown mainly in  FIG. 2 , APS shaft  90  is disposed between front head cover  38  and air cleaner  49  in the side view. In this manner, APS shaft  90  is offset with respect to motor  60  in the longitudinal direction. 
     As shown in  FIG. 4 , a pulley  92  is attached to APS shaft  90 . Throttle wire  18  ( FIG. 1 ) is wound around pulley  92 . Therefore, when throttle grip  17  is operated, throttle wire  18  moves, thereby rotating APS shaft  90 . Accelerator position sensor  51  detects an accelerator opening angle by detecting rotation of APS shaft  90 . 
     (Control Block of the Motorcycle  1 ) 
     A control block of motorcycle  1  is shown in  FIG. 10 . Electronic control unit (ECU)  80  is provided as a controller and is connected to various types of sensors including accelerator position sensor  51 , throttle position sensor  52 , a vehicle speed sensor  94  and the like. Accelerator position sensor  51  outputs an accelerator opening angle to ECU  80 . Throttle position sensor  52  outputs a throttle opening angle to ECU  80 . Vehicle speed sensor  94  outputs a vehicle speed to ECU  80 . 
     ECU  80  is connected to and controls engine  31  based on the input accelerator opening angle, throttle opening angle, vehicle speed, and the like. In addition, ECU  80  is connected to throttle body assembly  50 . Specifically, ECU  80  is connected to motor  60  and injectors  75  and  76 . ECU  80  drives motor  60  based on the input accelerator opening angle, throttle opening angle, vehicle speed, and the like. As motor  60  is driven, valve shaft  65  and valve shaft  66  rotate accordingly. As a consequence, throttle valves  57  and  58  move, thereby opening and closing front cylinders  55  and rear cylinders  56 . As a result, air taken from air cleaner  49  is introduced into cylinders  55  and  56 . 
     At the same time, ECO  80  controls the amount of fuel supplied from injectors  75  and  76  based on the input accelerator opening angle, throttle opening angle, vehicle speed, and the like. Fuel injected from injectors  75  and  76  is mixed with air supplied from air cleaner  49  to create an air-fuel mixture that is supplied to intake ports  42   a  and  42   b  ( FIG. 3 ). 
     (Operation and Effects) 
     As is described above, in the embodiment, as shown in  FIGS. 4 and 5 , motor  60  and APS shaft  90  which serves as the second rotational shaft are offset from each other in the longitudinal direction. Therefore, when compared with a case in which motor  60  and APS shaft  90  are arranged in the vertical direction, the height of throttle body assembly  50  can be suppressed. 
     Moreover, by disposing motor  60 , which normally has a larger volume than accelerator position sensor  51 , in the area enclosed by center axes A 4  and A 5  of front cylinders  55  and center axes A 6  and A 7  of rear cylinders  56  in a plan view, a longitudinal length of throttle body assembly  50  can be shortened. Therefore, the size of throttle body assembly  50  can be reduced and downsizing of engine unit  30  can be achieved. 
     Furthermore, since the size of engine unit  30  can be reduced, the capacity of air cleaner  49  which serves as the intake member disposed on throttle body assembly  50  can be increased. Accordingly, intake noise can be reduced. 
     Moreover, since the longitudinal length of throttle body assembly  50  can be reduced, the V-bank angle θ 0  of engine  31  can also be decreased. 
     In addition, by reducing the size of engine unit  30 , a space for installing battery  47  is increased. Accordingly, battery  47  can be installed even though it is large. 
     In the embodiment, APS shaft  90  is described as disposed to the front of center axes A 4  and A 5  of front cylinders  55  in the longitudinal direction. However, APS shaft  90  may be disposed to the rear of center axes A 4  and A 5  of front cylinders  55  in the longitudinal direction. In this case, size reduction of throttle body assembly  50  is still achieved. 
     Furthermore, in the embodiment, the second rotational shaft does not need to be APS shaft  90 . That is, a rotational shaft other than APS shaft  90  may be arranged offset with respect to motor  60  in the longitudinal direction. 
     Moreover, in the embodiment, as shown in  FIG. 9 , motor  60  which serves as an actuator is disposed such that its upper end is located at a position lower than the upper ends of throttle bodies  53  and  54 . Therefore, the height dimension of throttle body assembly  50  can be reduced more effectively. As a result, the height dimension of engine unit  30  can be reduced more effectively. 
     Note that, when the upper end of front throttle body  53  and the upper end of rear throttle body  54  are different in height, the aforementioned effects can be achieved by locating the upper end of motor  60  at a position lower than the upper end of front throttle body  53  or the upper end of rear throttle body  54 , whichever is higher. 
     As shown in  FIG. 5 , APS shaft  90  which serves as the second rotational shaft is disposed such that center axis A 3  of APS shaft  90  is located at a position lower than the upper ends of throttle bodies  53  and  54 . Therefore, the height dimension of throttle body assembly  50  can be reduced more effectively. As a result, the height dimension of engine unit  30  can be reduced more effectively. 
     When the upper ends of throttle bodies  53  and  54  are different in height, the aforementioned effects can be achieved by disposing APS shaft  90  such that its center axis A 3  is located at a position lower than the upper end of front throttle body  53  or the upper end of rear throttle body  54 , whichever is higher. 
     Meanwhile, since engine unit  30  is a source of vibration, a clearance of a predetermined distance or more needs to be provided between air cleaner  49  and engine unit  30 , as shown in  FIG. 2 . Specifically, front head cover  38  must be disposed apart from air cleaner  49 . In the embodiment, APS shaft  90  and accelerator position sensor  51  are arranged in a space between front head cover  38  and air cleaner  49 . Accordingly, by effectively using the space between front head cover  38  and air cleaner  49 , the height dimension of throttle body assembly  50  can be reduced, and overall size reductions can be achieved with respect to air cleaner  49 , throttle body assembly  50  and engine unit  30 . 
     Furthermore, vehicle width and height are severely restricted for a straddle-type vehicle, particularly a motorcycle. Therefore, the installation space for throttle body assembly  50  and engine unit  30  is severely restricted. In particular, in a motorcycle which has throttle body assembly  50  disposed between left and right frame portions  11   a  and  11   b  in the plan view, the installation space for throttle body assembly  50  and engine unit  30  is even more severely restricted. As a consequence, the present invention, which allows size reduction of throttle body assembly  50 , is effective for straddle-type vehicles, particularly for motorcycles. 
     In the embodiment, in a plan view, motor  60  is disposed in the area enclosed by center axes A 4  and A 5  of front cylinders  55  and center axes A 6  and A 7  of rear cylinders  56 . APS shaft  90 , which serves as the second rotational shaft, is located outside the area. Positional interference between APS shaft  90  and motor  60  is thereby reliably suppressed. As a result, the degree of freedom in the arrangement of motor  60  and accelerator position sensor  51  attached to APS shaft  90  is increased. Accordingly, the degree of freedom in design of throttle body assembly  50  is increased. 
     Furthermore, by disposing APS shaft  90  and accelerator position sensor  51  to the front of center axes A 4  and A 5  of front cylinders  55  or to the rear of center axes A 6  and A 7  of rear cylinders  56 , throttle bodies  53   a ,  53   b ,  54   a  and  54   b  can be arranged relatively close to each other. As a result, the V-bank angle of engine  31  can also be reduced. 
     Specifically, in the embodiment, APS shaft  90  is disposed to the front of center axes A 4  and A 5  of front cylinders  55  in the longitudinal direction. Therefore, throttle grip  17  and APS shaft  90  can be connected easily. Specifically, the length of winding of throttle wire  18  can be reduced and positional interference of throttle wire  18 , front cylinders  55  and the like can be avoided. Therefore, the winding of throttle wire  18  becomes easy. 
     In the embodiment, as shown in  FIGS. 3 and 6 , the upper end portions of injectors  75  and  76  are connected with fuel supply pipe  81 . Therefore, positional interference between injectors  75  and  76  and fuel supply pipe  81  does not occur and an angle formed by injectors  75  and  76  can be decreased. As a result, throttle bodies  53  and  54  can be arranged close to each other in the longitudinal direction. Therefore, the V-bank angle θ 0  of engine  31  can be made smaller. 
     Particularly, in the embodiment, fuel supply pipe  81  is shared by front injector  75  and rear injector  76 . Therefore, compared with a case in which a fuel supply pipe is separately provided for each of injectors  75  and  76 , the size of throttle body assembly  50  can be reduced. For instance, compared with a case in which two fuel supply pipes are arranged in the longitudinal direction, a distance between front and rear throttle bodies  53  and  54  can be reduced. As a result, the V-bank angle θ 0  of engine  31  can be made smaller. Also, for example, compared to a case in which two fuel supply pipes are arranged in the vertical direction, the height dimension of throttle body assembly  50  can be reduced. 
     Moreover, in the embodiment, fuel supply pipe  81  is disposed at a position lower than the upper ends of throttle bodies  53  and  54 . Therefore, in relation to the vertical direction, injectors  75  and  76  can be accommodated between the upper ends and lower ends of throttle bodies  53  and  54 . Accordingly, the overall height of throttle body assembly  50  can be reduced. 
     In the embodiment, connectors  77  and  78  are arranged in such a manner as to extend obliquely with respect to the longitudinal direction. Accordingly, positional interference between connectors  77  and  78  is suppressed. As a result, an angle between injectors  75  and  76  can be reduced. Consequently, throttle bodies  53  and  54  can be arranged close to each other in the longitudinal direction. As a consequence, the V-bank angle θ 0  of engine  31  can be made smaller. 
     In the embodiment motor  60  is offset with respect to fuel supply pipe  81  in the longitudinal direction. Specifically, a location of shaft center A 1  of rotational shaft  60   a  at which the height dimension of motor  60  is at its highest is offset in the longitudinal direction with respect to center axis A 2  of fuel supply pipe  81 . Accordingly, motor  60  and fuel supply pipe  81  can be arranged close to each other in the height direction. Therefore, the height dimension of throttle body assembly  50  can be reduced. That is, motor  60  is disposed between front throttle body  53  and rear throttle body  54  in the longitudinal direction, and motor  60  and fuel supply pipe  81  are offset from each other in the longitudinal direction. By this structure, both the longitudinal and height dimensions of throttle body assembly  50  can be reduced. As a result, both the longitudinal and height dimensions of engine unit  30  can be reduced. 
     MODIFIED EXAMPLE 
     In the aforementioned embodiment, shaft center A 3  of APS shaft  90  is described as located to the front of center axes A 4  and A 5  of front cylinders  55   a  and  55   b . However, the invention is not restricted to this structure. For example, as shown in  FIG. 11 , shaft center A 3  of APS shaft  90  may be located to the rear of center axes A 6  and A 7  of rear cylinders  56   a  and  56   b.    
     Furthermore, in the embodiment, APS shaft  90  is described as offset with respect to rotational shaft  60   a  of motor  60 . That is, the case in which the second rotational shaft is shaft  90  has been explained. However, in the invention, the second rotational shaft is not restricted to APS shaft  90 .