Patent Publication Number: US-8522741-B2

Title: Air-intake duct and air-intake structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2009-29919 filed on Dec. 29, 2009, which is hereby incorporated by reference in its entirety for all purposes. 
     BACKGROUND ART 
     1. Field of the Invention 
     The present invention relates to an air-intake duct and an air-intake structure for guiding air to a throttle device coupled to an engine. 
     2. Description of the Related Art 
     An engine mounted in a motorcycle and other vehicles includes a cylinder head having a combustion chamber. An air-intake structure forming an air-intake passage is coupled to an intake port of the combustion chamber to guide air and fuel to the combustion chamber. The air-intake structure typically includes an air cleaner box of an air cleaner, an air-intake duct, and a throttle body of a throttle device which are coupled to each other in this order from an upstream side in an air flow direction. The throttle body is provided with an injector for injecting a fuel. 
     Japanese Laid-Open Patent Application Publication No. 2006-90298 discloses a double-injector air-intake structure applied to a high power engine, in which an injector (upstream injector) is provided inside an air cleaner box in addition to the above injector (downstream injector), and an air inlet of an air-intake duct is disposed to face an injection port of the upstream injector so that air cleaned by the air cleaner and fuel injected from the upstream injector are efficiently guided to the throttle device. 
     The air-intake duct forming the double-injector air-intake structure has a coupling function for coupling in an air tight manner the air cleaner box to the throttle body and an air guiding function for guiding the cleaned air and the fuel to the throttle device. In the conventional air-intake structure disclosed in the above Publication, a coupling member for performing the coupling function and an air guiding member for performing the air guiding function are integrally formed of the same material, and therefore it is difficult to perform these functions in a well-balanced manner. 
     For example, the coupling member is desirably formed of an elastic rubber material to ensure air-tightness. If both the coupling member and the air guiding member are formed of the elastic rubber material, it is necessary to increase the wall thickness of the air guiding member to maintain its shape. This narrows an air passage and degrades the air guiding function. In addition, the weight of the air-intake duct increases because of an increase in a wall thickness of the air guiding member, thereby resulting in a reduced fuel efficiency. On the other hand, if both the coupling member and the air guiding member are formed of a material (synthetic resin, metal, etc) other than the elastic rubber material, then a seal member such as an O-ring is needed to ensure air-tightness in the coupling member. This reduces a mounting efficiency of the air-intake duct. 
     If the coupling member and the air guiding member are molded integrally using a die, the entire air-intake duct has a complex shape. For this reason, undercut frequently occurs, design flexibility is lessened, and manufacturing cost increases because of complexity of the die. 
     The air-intake performance of the air-intake passage can be controlled precisely by changing the length of the air guiding member. To this end, in the conventional structure, it is necessary to replace the entire air-intake duct. This task is burdensome. Therefore, it is not easy to control the air-intake performance of the air-intake passage. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above described conditions, and an object of the present invention is to provide an air-intake duct and air-intake structure which can perform a coupling function and an air guiding function in a well-balanced manner, can reduce weight to improve fuel efficiency, can improve design flexibility, can be manufactured without a cost increase, and can easily control an air-intake performance of an air-intake passage. 
     According to one aspect of the present invention, there is provided an air-intake duct which is disposed between an air outlet of an air cleaner box constituting an air cleaner and an air inlet of a throttle body constituting a throttle device and configured to guide air cleaned by the air cleaner to the throttle device, comprising: a tubular coupling member including an upstream coupling portion coupled in an air tight manner to the air outlet of the air cleaner box and a downstream coupling portion coupled in an air tight manner to the air inlet of the throttle body, the coupling member being entirely formed of an elastic rubber material; and an air guide member including a first air inlet configured to take in air therethrough from inside the air cleaner box, a first air outlet configured to discharge the air therethrough toward the throttle body, and a fitting portion fitted to the coupling member. 
     In accordance with this configuration, the coupling member and the air guide member can be manufactured individually as separate members. Therefore, the entire coupling member is formed of an elastic rubber material, and the entire or a part of the air guide member is formed to have a small wall thickness using a material other than the elastic rubber material, the material being lightweight and having a high stiffness, which makes it possible to maintain a shape of the air guide member, as compared to the elastic rubber material. As a result, the coupling member and the air guide member can be designed flexibly so that they can perform their respective functions in a well-balanced manner. In addition, fuel efficiency can be improved because of the reduced weight and a manufacturing cost of the air-intake duct can be reduced. 
     In addition, since the fitting portion of the air guide member is fitted to the coupling member to form the air-intake duct, only the air guide member can be changed easily without detaching the coupling member. Thus, air-intake performance of the air-intake passage can be controlled easily merely by changing the air guide member into one with a different length. 
     The air guide member may be formed of synthetic resin or metal. 
     In this configuration, the air guide member can be formed to have a small wall thickness and maintain its shape using synthetic resin or metal. 
     The air-intake duct may further comprise a second air inlet provided to open in a direction different from a direction in which the first air inlet opens and configured to take in the air therethrough from inside the air cleaner box, and a second air outlet configured to discharge the air therethrough toward the throttle body. 
     In accordance with this configuration, the air can be taken in from the inside of the air cleaner box through both the first air inlet and the second air inlet. 
     A portion of a downstream edge of the air guide member may be positioned inside the air cleaner box such that the portion of the downstream edge is apart from an upstream edge of the coupling member, and the second air inlet may be provided between the portion of the downstream edge and the upstream edge of the coupling member. 
     In accordance with this configuration, since the portion of the downstream edge of the air guide member and the upstream edge of the coupling member form together the second air inlet, it is not necessary to form the second air inlet only in one of the air guide member and the coupling member. As a result, the structure of the air guide member and the structure of the coupling member can be simplified, and the air-intake duct can be manufactured without a cost increase. 
     The second air inlet may be a hole formed on a side surface of the air guide member. 
     In accordance with this configuration, since the second air inlet is formed by the hole formed on the side surface of the air guide member, the opening area of the second air inlet can be determined correctly. 
     The coupling member may have a stepped portion on at least a portion of an inner peripheral surface thereof, and at least a portion of a downstream end surface of the air guide member being engageable with the stepped portion. The fitting portion may be fitted to a portion of the inner peripheral surface of the coupling member which is located upstream of the stepped portion. A portion of the inner peripheral surface of the coupling member which is located downstream of the stepped portion may be continuous with an inner surface of the air guide member without a level difference. 
     In accordance with this configuration, the air guide member can be positioned correctly with respect to the coupling member by engaging at least the portion of the downstream end surface of the air guide member with the stepped portion. In addition, since the portion of the inner peripheral surface of the coupling member which is located downstream of the stepped portion is continuous with the inner surface of the air guide member without a level difference, the air can flow through these regions smoothly. 
     The coupling member may include an air guide portion configured to guide the air from inside the air cleaner box to the second air inlet. 
     In accordance with this configuration, the air guide portion provided at the coupling member can efficiently guide the air from the inside of the air cleaner box to the second air inlet. 
     According to another aspect of the present invention, there is provided an air-intake structure including an air-intake duct structure which is disposed between an air outlet of an air cleaner box constituting an air cleaner and an air inlet of a throttle body constituting a throttle body and configured to guide air cleaned by the air cleaner to the throttle device, the air-intake duct structure comprising: a tubular coupling member including an upstream coupling portion coupled in an air tight manner to the air outlet of the air cleaner box and a downstream coupling portion coupled in an air tight manner to the air inlet of the throttle body, the coupling member being entirely formed of an elastic rubber material; and an air guide member including a first air inlet configured to take in air therethrough from inside the air cleaner box, a first air outlet configured to discharge the air therethrough toward the throttle body, and a fitting portion coupled to the coupling member, wherein an injector is disposed inside the air cleaner box and includes an injection port configured to inject a fuel therethrough, and the first air inlet is positioned to face the injection port. 
     This configuration relates to the air-intake structure including the air-intake duct structure using the air-intake duct, the injection port of the injector communicates with the inner space of the air cleaner box, and the first air inlet of the air guide member is positioned to face the injection port. Therefore, the air cleaned by the air cleaner and the fuel injected through the injection port of the injector can be guided efficiently toward the throttle device. 
     The air-intake duct structure may include a second air inlet which is provided to open in a direction different from a direction in which the first air inlet opens and configured to take in the air therethrough from inside the air cleaner box, and a second air outlet configured to discharge the air therethrough toward the throttle body. 
     In accordance with this configuration, the air can be taken in from inside the air cleaner box through both the first air inlet and the second air inlet. 
     The air-intake duct structure may include an air guide unit configured by coupling a plurality of air guide members to each other and at least one fastening member configured to fasten the air guide unit to the air cleaner box. 
     In accordance with this configuration, since each of the plurality of air guide members constituting the air guide unit can be reinforced by the other air guide member, the shape of the air guide member can be maintained invariably. In addition, since the air guide unit including the plurality of air guide members can be fastened to the air cleaner box, as one component, higher fastening stiffness can be obtained with fewer fastener members as compared to a configuration in which the plurality of air guide members are fastened individually to the air cleaner box. 
     The above and further objects and features of the invention will more fully be apparent from the following detailed description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view of a construction of an entire motorcycle including an air-intake structure including an air-intake duct structure according to Embodiment 1. 
         FIG. 2  is a cross-sectional view showing a configuration of the air-intake structure according to Embodiment 1. 
         FIG. 3  is a cross-sectional view showing the air-intake structure according to Embodiment 1. 
         FIG. 4  is a perspective view showing a part of the air-intake duct structure including air-intake ducts according to Embodiment 1. 
         FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 4 . 
         FIG. 6  is an exploded perspective view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 1. 
         FIG. 7  is an exploded cross-sectional view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 2. 
         FIG. 8  is a cross-sectional view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 3. 
         FIG. 9  is a cross-sectional view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 4. 
         FIG. 10  is an exploded perspective view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 5. 
         FIG. 11  is a perspective view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 6. 
         FIG. 12  is an exploded perspective view showing a part of an air-intake duct structure including air-intake ducts according to Embodiment 7. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. The stated directions are referenced from the perspective of a driver straddling a motorcycle, unless otherwise explicitly noted. 
     Embodiment 1 
     [Construction of Motorcycle] 
       FIG. 1  is a left side view of a construction of an entire motorcycle  12  including an air-intake structure  15  including an air-intake duct structure  10  according to Embodiment 1.  FIG. 2  is a cross-sectional view showing a configuration of the air-intake structure  15 . 
     Referring now to  FIG. 1 , the motorcycle  12  includes a main frame member  16 , a head pipe  18  provided at the front portion of the main frame member  16  and a pair of right and left pivot frame members  20  provided at the rear portion of the main frame member  16 . A steering shaft (not shown) is rotatably inserted into the head pipe  18 . A front fork  22  and a steering handle  24  are attached to the steering shaft. A pair of right and left swing arms  26  are attached to the pivot frame members  20 , respectively. A front wheel  28  is mounted to the lower end portion of the front fork  22 . A rear wheel  30  is mounted to the rear end portions of the swing arms  26 . A fuel tank  32  and a seat  34  are arranged at the upper portion of the main frame member  16  such that the fuel tank  32  is disposed forward relative to the seat  34 . An engine E is mounted at the center portion in a space defined by the main frame  16  below the fuel tank  32 . 
     As shown in  FIG. 1 , the engine E includes a cylinder head  36 , a cylinder block  38 , and a crankcase  40 . Although not shown, a combustion chamber is formed inside the cylinder head  36 . A cylinder and a piston are accommodated in the cylinder block  38 . A crankshaft driven to rotate by the piston is accommodated in the crankcase  40 . In this embodiment, the engine E is an inline four-cycle four-cylinder reciprocating engine. The four cylinders and four combustion chambers are arranged in a rightward and leftward direction, i.e., a width direction of the motorcycle  12 . Exhaust pipes  44  are respectively coupled to exhaust ports  42  respectively corresponding to the four combustion chambers and configured to exhaust air therethrough. An air-intake structure  15  constituting an air-intake passage  14  is coupled to intake ports  46  respectively corresponding to the four combustion chambers to suction an air-fuel mixture containing air and fuel. 
     As shown in  FIGS. 1 and 2 , the air-intake structure  15  includes an air cleaner box  54  of an air cleaner  48  positioned between the engine E and the fuel tank  32 , four air-intake ducts  50  (air-intake ducts  50 A,  50 B,  50 C and  50 D), and a throttle body  70  of a throttle device  52  disposed behind the engine E and below the air cleaner  48 , which are coupled to each other in this order from an upstream side in the air flow direction. As shown in  FIG. 3 , a portion of the air-intake structure  15  which is located downstream of the air cleaner  48  branches to form four branch passages, illustrated at  14   a  in  FIG. 2 , respectively corresponding to the four combustion chambers and the four cylinders. 
     [Configuration of Air Cleaner] 
     As shown in  FIG. 2 , the air cleaner  48  is configured to take in air from outside by utilizing a ram pressure, clean the air and distribute the air to the four branch passages  14   a , and includes the air cleaner box  54  constituting a part of the air-intake structure  15  and an air cleaner element  56  for cleaning the air flowing through the inside of the air cleaner box  54 . 
     The air cleaner box  54  includes a lower case  58  formed of synthetic resin and an upper case  60  formed of synthetic resin. The lower case  58  and the upper case  60  are joined to each other to form the air cleaner box  54  of a box shape. The air cleaner element  56  is disposed in the vicinity of a boundary between a space (inner space: dirty side) S 1  formed inside the lower case  58  and a space (inner space: clean side) S 2  formed inside the upper case  60 . 
     At least one (in this embodiment, one) air inlet  58   a  is formed at the front portion of the lower case  58  forming the inner space Si to open in a forward direction. Four air outlets  58   b  respectively corresponding to the four branch passages  14   a  are formed at the rear portion of the lower case  58  forming a part of the inner space S 2  to open in a downward direction such that the four air outlets  58   b  are aligned in the rightward and leftward direction. Annular fitting protrusions  62  are formed at the peripheral edges of the upstream end portions of the four air outlets  58   b , respectively, such that the fitting protrusions  62  protrude into the inner space S 2  of the air cleaner box  54 . The air-intake ducts  50  are fitted to the fitting protrusions  62 , respectively. A portion (front portion)  60   a  of the upper case  60  which is opposite to the air cleaner element  56  is tilted such that its inner surface increases in height toward a center portion  60   b . A portion (rear portion)  60   c  of the upper case  60  which is opposite to the air outlets  58   b  is formed such that its inner surface is lower than the inner surface of the center portion  60   b . Such a structure allows the air cleaned by the air cleaner element  56  to be guided smoothly to the respective four air outlets  58   b  along the inner surface of the upper case  60 . Recesses  64  having through-holes  64   a  are formed at the rear portion  60   c  of the upper case  60 . Tubular fuel guides  64   b  are formed at the peripheral portions of the through-holes  64   a , respectively, such that the fuel guides  64   b  protrude into the inner space S 2  of the air cleaner box  54 . A plurality of (in this embodiment, six) fastening portions  66  are formed inside at least either the lower case  58  or the upper case  60  (in this embodiment, the lower case  58 ) to fasten the air-intake ducts  50 , respectively. 
     The tip end portions of the upstream injectors  68  are accommodated into the recesses  64  of the upper case  60 , respectively. The upstream injectors  68  are configured to inject the fuel into the air-intake passage  14  in the inner space S 2  of the air cleaner box  54 . Injection ports  68   a  formed at the tip end portions of the upstream injectors  68  are fitted to the through-holes  64   a  of the recesses  64 , respectively. The injection ports  68   a  communicate with the air-intake passage  14  (inner space S 2 ) through the fuel guides  64   b , respectively. Therefore, in the air-intake passage  14  (inner space S 2 ), the air cleaned by the air cleaner element  56  is mixed with the fuel injected through the injection ports  68   a , and the resulting air-fuel mixture is distributed to the four air-intake passages  14   a  through the air-intake ducts  50  provided at the four air outlets  58   b , respectively. 
     [Configuration of Throttle Device] 
     As shown in  FIG. 2 , the throttle device  52  is configured to control the amount of air-fuel mixture supplied to the combustion chambers (not shown), and includes throttle bodies  70  constituting a part of the air-intake structure  15 , downstream throttle valves  72  for controlling the flow rate of the air-fuel mixture inside the throttle body  70 , and upstream throttle valves  74  for controlling the flow rate of the air-fuel mixture inside the throttle body  70 . 
     The throttle bodies  70  are tubular members configured to guide the air-fuel mixture supplied from the air cleaner  48  through the air-intake ducts  50 , to the combustion chambers. In this embodiment, the four throttle bodies  70  are aligned in the rightward and leftward direction. Each throttle body  70  includes a downstream tubular portion  76  coupled to the intake port  46  and an upstream tubular portion  78  coupled to the associated air-intake duct  50 . The upstream tubular portion  78  has a larger inner diameter than the downstream tubular portion  76 . The downstream tubular portion  76  is provided on its outer surface with a recess  80  having a through-hole  80   a . The downstream throttle valve  72  is provided inside the downstream tubular portion  76 . The upstream throttle valve  74  is provided inside the upstream tubular portion  78 . The tip end portion of the downstream injector  82  is accommodated into the recess  80 . 
     The downstream throttle valve  72  is a main throttle valve configured to be directly operated by the driver. An accelerator grip (not shown) is coupled to the downstream throttle valve  72  via a throttle wire (not shown). According to the driver&#39;s operation amount of the accelerator grip, the opening degree of the downstream throttle valve  72  is controlled. In contrast, the upstream throttle valve  74  is a sub-throttle valve actuated in an auxiliary manner by a control unit (ECU) or the like. A drive motor (not shown) is coupled to the upstream throttle valve  74 . The control unit (ECU) drives the drive motor to actuate the upstream throttle valve  74 , thereby controlling the opening degree of the upstream throttle valve  74 . Therefore, even when the driver operates the accelerator grip rapidly to change the opening degree of the downstream throttle valve  72  rapidly, the upstream throttle valve  74  operates to change the flow rate of the air smoothly, thereby enabling the engine speed of the engine E to change smoothly. 
     The downstream injector  82  is configured to inject the fuel to the air-intake passage  14  in an inner space S 3  of the throttle body  70 . An injection port  82   a  formed at the tip end portion of the downstream injector  82  is fitted to the through-hole  80   a  of the recess  80  and communicates with the air intake passage  14  (inner space S 3 ). Therefore, in each branch passage  14   a  (inner space S 3 ) of the air-intake passage  14 , the air or air-fuel mixture delivered from the air cleaner  48  through the air-intake duct  50  is mixed with the fuel injected through the injection port  82   a , and the resulting air-fuel mixture is supplied to the combustion chamber through the intake port  46 . The fuel injection amount of the upstream injector  68  and the fuel injection amount of the downstream injector  82  are controlled according to a load state of the engine E. For example, in a state where the engine E is under a low-load state, i.e., running at a low engine speed, only the downstream injector  82  injects the fuel, while in a state where the engine E is under a high-load state, i.e., running at a high engine speed, both the upstream injector  68  and the downstream injector  82  inject the fuel. 
     [Configuration of Air-Intake Duct Structure] 
       FIG. 3  is a cross-sectional view showing the air-intake structure  15  including the air-intake duct structure  10 .  FIG. 4  is a perspective view showing a part of the air-intake duct structure  10  including the air-intake ducts  50 .  FIG. 5  is a cross-sectional view showing a part of the air-intake duct structure  10 .  FIG. 6  is an exploded perspective view showing a part of the air-intake duct structure  10 . 
     As shown in  FIGS. 2 and 3 , the air-intake duct structure  10  constitutes the air-intake structure  15 , and includes the air-intake ducts  50  (air-intake ducts  50 A,  50 B,  50 C and  50 D).  FIGS. 3 ,  4  and  6  show the configuration of the air-intake duct structure  10  as viewed from the front. The right and left in these drawings are reverse of the right and left from the perspective of the driver straddling the motorcycle  12 . 
     Referring to  FIG. 3 , the air-intake duct structure  10  is provided between the air outlets  58   b  of the air cleaner box  54  and air inlets  70   a  (upstream opening portions of the upstream tubular portions  78 ) of the throttle bodies  70  and is configured to guide the air cleaned by the air cleaner  48  to the throttle device  52 . The air-intake duct structure  10  includes the four fitting protrusions  62  formed at the peripheral edges of the four air outlets  58   b  of the air cleaner box  54 , respectively, the four air-intake ducts  50 A,  50 B,  50 C and  50 D mounted to the four air outlets  58   b , respectively, and six fastening portions  66  formed inside the lower case  58 . In  FIG. 3 , four fastening portions  66  are depicted and the remaining two fastening portions  66  are positioned forward relative to the cross-section and are invisible. 
     As shown in  FIGS. 5 and 6 , the fitting protrusion  62  is an annular protrusion formed at a peripheral edge N of the upstream end portion of the air outlet  58   b  to protrude into the inner space S 2  of the air cleaner box  54 . The peripheral edge N of the air outlet  58   b , including the fitting protrusion  62  has a cross-sectional shape of a substantially L-shape. The axial length of the inner peripheral surface of the air outlet  58   b  is equal to a dimension which is a sum of the thickness of the air cleaner box  54  and the height of the fitting protrusion  62 . This structure allows the air outlet  58   b  to contact the associated one of the air-intake ducts  50 A,  50 B,  50 C and  50 D with a larger area. 
     As shown in  FIG. 3 , the first, second, third and fourth air-intake ducts  50 A,  50 B,  50 C and  50 D are configured such that the second air-intake duct  50 B and the third air-intake duct  50 C located at the center of the air cleaner box  54  are different in shape from the first air-intake duct  50 A and the fourth air-intake duct  50 D which are located at the right and left sides of the second air-intake duct  50 B and the third air-intake duct  50 C, respectively. The first air-intake duct  50 A and the second air-intake duct  50 B which are different in shape are coupled to each other to form a first air-intake duct unit  90 A. The third air-intake duct  50 C and the fourth air-intake duct  50 D which are different in shape are coupled to each other to form a second air-intake duct unit  90 B. Since the first air-intake duct unit  90 A and the second air-intake duct  90 B are configured symmetrically in the rightward and leftward direction, only the first air-intake duct unit  90 A will be described hereinafter, and the description of the second air-intake duct unit  90 B will be omitted. 
     [Configuration of Air-Intake Duct Unit] 
     As shown in  FIGS. 4 and 6 , the first air-intake duct unit  90 A includes the first air-intake duct  50 A, the second air-intake duct  50 B, a coupling portion  92  coupling the first air-intake duct  50 A to the second air-intake duct  50 B, and three fastening portions  94 . 
     As shown in  FIGS. 5 and 6 , the first air-intake duct  50 A includes a tubular coupling member  96  which is entirely formed of an elastic rubber material (rubber, elastomer, etc) and an air guide member  98  which is entirely formed of a material (synthetic resin, metal, etc) which is other than the elastic rubber material. 
     Turning back to  FIG. 2 , the coupling member  96  is a tubular member configured to perform a coupling function for coupling in an air tight manner the air cleaner box  54  to the throttle body  70 . As shown in  FIG. 5 , the coupling member  96  includes a tubular peripheral wall portion  100  inserted into the air outlet  58   b , an upstream coupling portion  102  formed at the upstream end portion of the peripheral wall portion  100  and coupled in an air tight manner to the air outlet  58   b , and a downstream coupling portion  104  formed at the downstream end portion of the peripheral wall portion  100  and coupled in an air tight manner to the air inlet  70   a  of the throttle body  70 . The upstream coupling portion  102  protrudes into the inner space S 2  of the air cleaner box  54 , while the downstream coupling portion  104  protrudes into an outside space S 4  of the air cleaner box  54 . 
     The upstream coupling portion  102  includes a flanged upper engagement portion  102   a  extending radially outward from the upstream end portion of the peripheral wall portion  100  and configured to cover the tip end surface of the fitting protrusion  62 , an annular seal portion  102   b  extending from the outer peripheral edge of the upper engagement portion  102   a  toward the inner surface of the lower case  58  and configured to contact the outer surface of the fitting protrusion  62 , a flanged lower engagement portion  102   c  extending radially outward from the outer peripheral surface of the peripheral wall portion  100  and configured to contact the outer surface of the lower case  58 . The peripheral edge portion N of the air outlet  58   b , including the fitting protrusion  62 , is fitted to a bag-like portion defined by the upper engagement portion  102   a , the seal portion  102   b  and the lower engagement portion  102   c.    
     The downstream coupling portion  104  includes an annular protrusion  104   a  protruding radially inward from the inner peripheral surface of the peripheral wall portion  100 . A downstream end surface  98   a  of the air guide member  98  is in contact with the upstream end surface  106   a  of the protrusion  104   a . The end surface of the air inlet  70   a  is in contact with the downstream end surface  106   b  of the protrusion  104   a . In this embodiment, the protrusion  104   a  forms upper and lower stepped portions V 1  and V 2  over the entire circumference of the inner peripheral surface of the coupling member  96  (peripheral wall portion  100 ). The downstream end surface  98   a  of the air guide member  98  is in contact with the upstream stepped portion V 1 , while the end surface of the air inlet  70   a  is in contact with the downstream stepped portion V 2 . 
     Turning back to  FIG. 2 , the air guide member  98  is a tubular member configured to perform the air guiding function for guiding the air cleaned by the air cleaner element  56  and the fuel injected from the upstream injector  68  to the throttle device  52 . The air guide member  98  is formed integrally using the material other than the elastic rubber material, such as synthetic resin, metal, etc. As shown in  FIGS. 5 and 6 , the air guide member  98  includes a tubular peripheral wall portion  110  formed of the material other than the elastic rubber material (in this embodiment, synthetic resin), a first air inlet  112  configured to take in the air therethrough from inside the air cleaner box  54 , a first air outlet  114  configured to discharge the air therethrough toward the throttle body  70 , a fitting portion  116  fitted to the coupling member  96 , a second air inlet  118  ( FIG. 5 ) which is provided to open in a direction different from the direction in which the first air inlet  112  opens and configured to take in air therethrough from inside the air cleaner box  54 , and a second air outlet  120  configured to discharge the air therethrough toward the throttle body  70 . The first air inlet  112 , the first air outlet  114 , the fitting portion  116 , the second air inlet  118  and the second air outlet  120  are integral with the peripheral wall portion  110 . 
     The first air inlet  112  includes, at the upstream end portion of the peripheral wall portion  110 , an opening portion  112   a  configured to open toward the inner space S 2  of the air cleaner box  54  and a rear wall  112   b  extending upward from the rear portion of the opening portion  112   a . The opening portion  112   a  is shaped as a funnel to take in the air smoothly. The surface of the rear wall  112   b  is shaped to be smooth to guide the air to the opening portion  112   a  smoothly. As shown in  FIG. 3 , in the air-intake duct structure  10 , the opening portion  112   a  is positioned closer to the lower case  58  than the tip end portion of the fuel guide  64   b , while the tip end portion of the rear wall  112   b  is positioned closer to the upper case  60  than the tip end portion of the fuel guide  64   b . Therefore, a space is ensured between the opening portion  112   a  and the tip end portion of the fuel guide  64   b . The air and fuel flowing into the space are guided along the rear wall  112   b  and suctioned into the throttle body  70  efficiently through the opening portion  112   a.    
     The second air inlet  118  is located below the first air inlet  112  and serves to take in the air flowing in a direction different from the direction in which the air flows through the first air inlet  112 . The second air inlet  118  has an opening portion  118   a  extending from a region of the front portion of the peripheral wall portion  110  which is in the vicinity of the axial center, to its downstream end portion. The opening portion  118   a  has a shape formed by cutting a part of the cylindrical peripheral wall portion  110  from the downstream end portion toward its upstream side. In other words, the opening portion  118   a  is not a hole surrounded by the peripheral wall portion  110  over the entire periphery, but is formed by cutting a portion of the peripheral wall portion  110  to open toward its downstream side. A part of the downstream edge  98   b  of the air guide member  98  (peripheral wall portion  110 ) forms a part of the inner peripheral edge of the opening portion  118   a . As shown in  FIG. 5 , in a state where the air guide member  98  is joined to the coupling member  96  to form the first air-intake duct  50 A of  FIGS. 4 and 6 , a part of the downstream edge  98   b  of the air guide member  98  is positioned in the inner space S 2  of the air cleaner box  54  to be distant from the upstream edge  96   a  of the coupling member  96 , and the second air inlet  118  of a hole shape is formed between a part of the downstream edge  98   b  and the upstream edge  96   a.    
     As described above, in this embodiment, since a part of the downstream edge  98   b  of the air guide member  98  and the upstream edge  96   a  of the coupling member  96  form the second air inlet  118  together, it is not necessary to form the second air inlet  118  only in one of the air guide member  98  and the coupling member  96 . Thus, the structure of the air guide member  98  and the structure of the coupling member  96  can be simplified, and as a result, a manufacturing cost does not increase. 
     The fitting portion  116  is fitted to a portion of the inner peripheral surface of the coupling member  96  which is located upstream of the stepped portion V 1 . In this embodiment, the fitting portion  116  is a portion of a substantially semicylinder shape which is located at the downstream end portion of the peripheral wall portion  110 . Therefore, the protruding amount of the air-intake duct  50 A which protrudes into the inner space S 2  of the air cleaner box  54 , and the position of the first air inlet  112  are determined by the length (length in the direction in which the air-intake duct A protrudes) of the portion of the peripheral wall portion  110  which is located upstream of the fitting portion  116 . 
     The first air outlet  114  and the second air outlet  120  share an opening portion  98   c  formed at the downstream end portion of the air guide member  98  (peripheral wall portion  110 ). The air and fuel taken in through the first air inlet  112  are discharged through the opening portion  98   c  (first air outlet  114 ), while the air and the fuel taken in through the second air inlet  118  are discharged through the opening portion  98   c  (second air outlet  120 ). In other words, inside the peripheral wall portion  110 , there are a main passage W 1  from the first air inlet  112  to the opening portion  98   c  and a sub-passage W 2  from the second air inlet  118  to the opening portion  98   c  (second air outlet  120 ). The main passage W 1  and the sub-passage W 2  are joined to each other inside the peripheral wall portion  110 . 
     As shown in  FIG. 6 , the opening portion  118   a  of the peripheral wall portion  110  is formed by cutting a portion of the peripheral wall portion  110  and a part of the opening portion  118   a  opens toward its downstream side. The second air inlet  118 , the first air outlet  114  and the second air outlet  120  are continuous. As shown in  FIG. 5 , in the first air-intake duct  50 A of  FIGS. 4 and 6  since the second air inlet  118 , the first air outlet  114  and the second air outlet  120  are defined by the coupling member  96 , the air and the fuel flowing through the main passage W 1  and the sub-passage W 2  can be discharged through the air outlets  114  and  120  smoothly. To enable the main passage W 1  of a larger passage length to rectify the flow effectively, it is desirable to set the passage cross-sectional area of the sub-passage W 2  smaller than the passage cross-sectional area of the main passage W 1 . 
     As shown in  FIGS. 4 and 6 , the second air-intake duct  50 B includes the tubular coupling member  96  which is entirely formed of an elastic rubber material (rubber, elastomer, etc) and an air guide member  128  which is entirely formed of a material (synthetic resin, metal, etc) which is other than the elastic rubber material. Since the constituents of the coupling member  96  of the second air-intake duct  50 B are identical to those of the coupling member  96  of the first air-intake duct  50 A, they will not be described, respectively. 
     Turning back to  FIG. 2 , the air guide member  128  is a tubular member configured to perform the air guiding function like the air guide member  98 . As shown in  FIG. 6 , the air guide member  128  includes a tubular peripheral wall portion  130  formed of the material other than the elastic rubber material, such as synthetic resin, metal, etc. In this embodiment, the air guide member  128  is formed of synthetic resin. A first air inlet  132  configured to take in the air therethrough from inside the air cleaner box  54 , a first air outlet  134  configured to discharge the air therethrough toward the throttle body  70  ( FIG. 3 ), a fitting portion  136  fitted to the coupling member  96 , a second air inlet  138  which is oriented to open in a direction different from the direction in which the first air inlet  132  opens and configured to take in the air therethrough from inside the air cleaner box  54 , and a second air outlet  140  configured to discharge the air therethrough toward the throttle body  70 , are integral with the peripheral wall portion  130 . 
     The first air inlet  132  of the air guide member  128  is different in structure from the first air inlet  112  of the first air-intake duct  50 A. The first air outlet  134 , the fitting portion  136 , the second air inlet  138  and the second air outlet  140  of the second air-intake duct  50 B are identical in structure to the first air outlet  114 , the fitting portion  116 , the second air inlet  118  and the second air outlet  120  of the first air-intake duct  50 A. Therefore, only the structure of the first air inlet  132  will be described and the structure of other constituents will not be described repetitively. 
     As shown in  FIG. 6 , the first air inlet  132  includes, at the upstream end portion of the peripheral wall portion  130 , an opening portion  132   a  formed to open toward the inner space S 2  of the air cleaner box  54 . The opening portion  132   a  funnels such that its front portion is lower than its rear portion. The rear portion of the opening portion  132   a  is substantially equal in height to the tip end portion of the rear wall  112   b  of the first air-intake duct  50 A. In other words, as shown in  FIG. 3 , the rear portion of the opening portion  132   a  is closer to the upper case  60  than the tip end portion of the fuel guide  64   b . Therefore, the tip end portion of the fuel guide  64   b  is disposed below the opening portion  132   a  inside the peripheral wall portion  130 , and the fuel injected through the tip end portion of the fuel guide  64   b  is discharged efficiently from the first air outlet  134  through the main passage W 1 . 
     Thus, in this embodiment, since the protruding amount of the second air-intake duct  50 B (peripheral wall portion  130 ) which protrudes into the inner space S 2  of the air cleaner box  54  is set larger than the protruding amount of the first air-intake duct  50 A (peripheral wall portion  110 ) which protrudes into the inner space S 2  of the air cleaner box  54 , the air-intake properties of the first and second air-intake ducts  50 A and  50 B are compensated as a whole in the air-intake duct structure  10 , and an engine torque is stabilized. 
     As shown in  FIG. 6 , the air guide member  98  of the first air-intake duct  50 A is coupled to the air guide member  128  of the second air-intake duct  50 B via the coupling portion  92  to form an air guiding unit  150  as one component. The coupling portion  92  has a cross-section of a substantially L-shape and includes a center coupling portion  92   a  and a rear coupling portion  92   b . One fastening portion  94  is formed integrally with the coupling portion  92   a  located at the center. The remaining two fastening portions  94  are formed integrally with the right and left end portions of the air guide unit  150 . The fastening portions  66  of the lower case  58  have holes  66   b  provided with female threads  66   a  on their inner peripheral surfaces. The fastening portions  94  of the air guide unit  150  have holes  94   b  provided with bolt engagement portions  94   a  on their bottom portions, respectively. Each fastening portion  66  of the lower case  58  is joined to the associated fastening portion  94  of the air guide unit  150  by inserting and threading a fastener bolt  152  into these fastening portions  66  and  94 . 
     [Manufacturing Method of Air-Intake Duct Structure And Advantages] 
     The manufacturing method of the air-intake duct structure  10  will be described with reference to  FIGS. 5 and 6 . Initially, the upstream coupling portions  102  of the coupling members  96  are fitted to the air outlets  58   b  of the air cleaner box  54 , and the downstream coupling portions  104  of the coupling members  96  are fitted to the air inlets  70   a  of the throttle bodies  70 . Then, the fitting portion  116  of the air guide member  98  constituting the air guide unit  150  and the fitting portion  136  of the air guide member  128  constituting the air guide unit  150  are fitted to the inner peripheral surfaces of the coupling members  96 , respectively, and the fastening portions  94  of the air guide unit  150  are fastened to the fastening portions  66  formed inside the lower case  58 , using the fastener bolts  152 , respectively. Then, the upper case  60  is joined to the lower case  58 , thereby completing the air cleaner box  54 . 
     In this embodiment, the coupling members  96 , and the air guide members  98  and  128  are manufactured individually as separate members. Therefore, the entire coupling member  96  is formed of the elastic rubber material and the entire or a part of the air guide members  98  and  128  can be manufactured to have a small wall thickness using synthetic resin, metal, etc, which is lightweight and makes it possible to maintain the shape of the air guide members  98  and  128 , as compared to the elastic rubber material. 
     Since one of the air guide members  98  and  128  constituting the air guide unit  150  is reinforced by the other, the shape of the air guide members  98  and  128  can be maintained surely. Since the air guide unit  150  including the two air guide members  98  and  128  can be fastened as one component to the air cleaner box  54 , great fastening stiffness can be attained with fewer fastener bolts  152  and the air guide members  98  and  128  can be fastened to the air cleaner box  54  more efficiently than a case where the air guide members  98  and  128  are individually fastened to the air cleaner box  54 . 
     Furthermore, since the air-intake duct  50  is assembled by fitting the fitting portion  116  of the air guide member  98  and the fitting portion  136  of the air guide member  128  to the coupling members  96 , respectively, only the air guide members  98  and  128  can be changed by pulling out the air guide members  98  and  128  from the coupling members  96 . 
     (Embodiment 2) 
       FIG. 7  is an exploded perspective view showing a part of an air-intake duct structure  160  including air-intake ducts  50  according to Embodiment 2. Although in the air-intake duct structure  10  according to Embodiment 1, the two air guide members  98  and  128  are coupled to each other to form one air guide unit  150 , the four air guide members  98  and  128  may be coupled to each other or may be formed independently of each other. 
     In the air-intake duct structure  160  of Embodiment 2, the four air guide members  98  and  128  are formed independently of each other and one fastening portion  94  is provided for each of the four air guide members  98  and  128 . 
     (Embodiment 3) 
       FIG. 8  is a cross-sectional view showing a part of an air-intake duct structure  170  including air-intake ducts  50  according to Embodiment  3 . Although in the air-intake duct structure  10  of Embodiment 1, all of the four air guide members  98  and  128  are fastened to the lower case  58 , a part or all of them may be fastened to the upper case  60 . 
     In the air-intake duct structure  170  of Embodiment 3, all of the four air guide members  98  and  128  are fastened to the upper case  60 , and the fastening portions  66  are formed inside the upper case  60 . 
     (Embodiment 4) 
       FIG. 9  is a cross-sectional view showing a part of an air-intake duct structure  180  including air-intake ducts  182  according to Embodiment 4. Although in the air-intake duct structure  10  of Embodiment 1, the annular protrusion  104   a  forms the stepped portion V 1  extending over the entire circumference of the inner peripheral surface of the coupling member  96 , the stepped portion V 1  may be formed only a part of the entire circumference of the inner peripheral surface of the coupling member  96 . 
     In the air-intake duct structure  180  of Embodiment 4, the stepped portion V 1  may be formed on only a part (rear portion) of the entire circumference of the inner peripheral surface of the coupling member  96  such that the stepped portion V 1  has a height equal to the thickness of the air guide member  98 , and the fitting portion  116  is fitted to a portion of the inner peripheral surface which is located upstream of the stepped portion V 1 . In this structure, a portion  184   a  of the inner peripheral surface of the coupling member  96  which is located downstream of the stepped portion V 1  is continuous with an inner surface  184   b  of the air guide member  98  without a level difference and there is no stepped portion V 1  in the sub-passage W 2 . This prevents the air flow from being disordered by the level difference. 
     (Embodiment 5) 
       FIG. 10  is an exploded perspective view showing a part of an air-intake duct structure  190  including air-intake ducts  192 A and  192 B according to Embodiment 5. In the air-intake duct structure  10  of Embodiment 1, a part of the downstream edge  98   b  of the air guide member  98  and the upstream edge  96   a  of the coupling member  96  form the second air inlet  118  together, while in the air-intake duct structure  190  of Embodiment 5, a hole  110   a  is formed on the peripheral wall portion  110  of the air guide member  98  and a hole  130   a  is formed on the peripheral wall portion  130  of the air guide member  128 , and the holes  110   a  and  130   a  are used as second air inlets  194  and  196 , respectively. Thus, in Embodiment 5, the opening areas of the second air inlets  194  and  196  can be determined accurately so that the flow rate of the air flowing through the sub-passage W 2  can be made invariable. 
     (Embodiment 6) 
       FIG. 11  is a perspective view showing a part of an air-intake duct structure  200  (air-intake ducts  202 A and  202 B) according to Embodiment 6. Funnel-shaped air guide portions  204  are formed at upstream edges  96   a  of the coupling members  96  constituting the second air inlets  118  and  138 , respectively. Therefore, in Embodiment 6, the air guide portions  204  enable the air to be guided from the air cleaner box  54  efficiently to the second air inlets  118  and  138 , respectively. 
     (Embodiment 7) 
       FIG. 12  is an exploded perspective view showing a part of an air-intake duct structure  210  including air-intake ducts  212 A and  212 B according to Embodiment 7. In the above embodiments, the air is taken in from inside the air cleaner box  54  through the second air inlets  118  and  138 , whereas in the air-intake duct structure  210  according to Embodiment 7, the second air inlets  118  and  138  are not provided but the air is taken in from inside the air cleaner box  54  only through the first air inlets  112  and  132 . In this embodiment, a design change is easily accomplished in such a manner that other guide members  214  and  216  may be fitted to the coupling members  96  used in Embodiments 1 to 5. 
     As should be appreciated from the above, the air-intake duct and air-intake structure of the present invention can perform a coupling function and an air guiding function in a well-balanced manner, can reduce weight to improve fuel efficiency, can improve design flexibility, can be manufactured without a cost increase, can easily control an air-intake performance of an air-intake passage, and are widely applicable to vehicles such as motorcycles and personal watercraft (PWC) which can achieve these advantages. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.