Patent Publication Number: US-2022212740-A1

Title: Fuel tank and method for manufacturing fuel tank

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The disclosure of Japanese Patent Application No. 2021-000165 filed on Jan. 4, 2021, including specification, drawings and claims is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a fuel tank and a method for manufacturing the fuel tank. 
     A fuel tank of a straddle-type vehicle is provided with pipes such as a breather pipe and a drain pipe. A fuel tank of this type is known in which a pipe passes through the tank (for example, see Patent Literature 1). In the fuel tank disclosed in Patent Literature 1, a tank inlet is provided in an outer panel, and the pipe extends from the tank inlet to outside of the tank through a bottom wall of an inner panel. Since the pipe is hidden in the tank, an appearance of the fuel tank is maintained without covering the outside of the fuel tank with a tank cover, unlike a structure in which a pipe passes through outside of a tank. 
     Patent Literature 1: JP-A-2005-008023 
     SUMMARY 
     According to one advantageous aspect of the present invention, there is provided a fuel tank including: an outer component including an outer panel forming a tank outer shell and an inlet plate formed separately from the outer panel; an inner component including an inner panel forming a tank inner shell and an inner connection plate formed separately from the inner panel; and a pipe passing through inside of the outer panel and the inner panel and connecting the inlet plate and the inner connection plate. The inlet plate is joined to the outer panel, the inner connection plate is joined to the inner panel, and the inner panel is joined to the outer panel. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a left side view of a straddle-type vehicle according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of a fuel tank according to the present embodiment as viewed from above. 
         FIG. 3  is a perspective view of the fuel tank according to the present embodiment as viewed from below. 
         FIG. 4  is a cross-sectional view of the fuel tank taken along a line A-A in  FIG. 3 . 
         FIG. 5  is a front view of an inner connection plate according to the present embodiment. 
         FIGS. 6A to 6C  are transition views showing a method for manufacturing a fuel tank made of a steel plate according to a comparative example. 
         FIGS. 7A to 7D  are transition views showing the method for manufacturing the fuel tank made of the steel plate according to the comparative example. 
         FIGS. 8A to 8C  are transition views showing a method for manufacturing the fuel tank made of titanium according to the present embodiment. 
         FIGS. 9A to 9C  are transition views showing the method for manufacturing the fuel tank made of titanium according to the present embodiment. 
         FIGS. 10A to 10C  are transition views showing the method for manufacturing the fuel tank made of titanium according to the present embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLIFIED EMBODIMENT 
     In recent years, fuel tanks are manufactured using various materials. At the time of manufacturing the fuel tank disclosed in Patent Literature 1, brazing or the like is used for joining panels and pipes, but it may be difficult to manufacture the fuel tank due to manufacturing restrictions depending on the material and size. 
     The present invention has been made in view of the above, and an object thereof is to provide a fuel tank capable of maintaining an appearance of the fuel tank by allowing a pipe to pass through the tank even when there are manufacturing restrictions due to the material and size, and a method for manufacturing the fuel tank. 
     A fuel tank according to an aspect of the present invention includes an outer component in which an inlet plate is joined to an outer panel forming a tank outer shell, and an inner component in which an inner connection plate is joined to an inner panel forming a tank inner shell. The outer panel and the inlet plate are formed separately, and the inner panel and the inner connection plate are formed separately. Therefore, even when work is suitable for a material but has size restrictions, the work can be performed on the inlet plate and the inner connection plate. In this way, it is possible to flexibly select work that satisfies manufacturing restrictions due to the material and size. In addition, the outer component and the inner component are integrated, so that the fuel tank in which the pipe passes through the tank is manufactured. In order to maintain an appearance, a large tank cover that covers the entire tank is not required, and without a double structure, capacity of the fuel tank is not reduced. 
     Hereinafter, an embodiment will be described in detail with reference to the accompanying drawings.  FIG. 1  is a left side view of a straddle-type vehicle according to the present embodiment. In the following drawings, an arrow FR indicates a vehicle front side, an arrow RE indicates a vehicle rear side, an arrow L indicates a vehicle left side, and an arrow R indicates a vehicle right side. 
     As shown in  FIG. 1 , a straddle-type vehicle  1  is configured by mounting various components such as an engine  14  and an electrical system on a twin-spar type vehicle body frame  10  formed by aluminum casting. The vehicle body frame  10  includes a pair of main frames  11  branching from a head pipe (not shown) to left and right sides and extending rearward, and a pair of down frames (not shown) branching from the head pipe to the left and right sides and extending downward. A rear portion of the engine  14  is supported by the pair of main frames  11 , and a front portion of the engine  14  is supported by the pair of down frames. Since the engine  14  is supported by the vehicle body frame  10 , a rigidity of the entire vehicle is ensured. 
     A front side portion of the main frame  11  is a tank rail  12  located above the engine  14 , and a fuel tank  30  is supported by the tank rail  12 . A rear side portion of the main frame  11  is a body frame  13  located behind the engine  14 , and a swing arm  15  is swingably supported at a substantially intermediate position of the body frame  13  in an upper-lower direction. A seat rail (not shown) and a backstay (not shown) extend rearward from an upper portion of the body frame  13 . A rider seat  21  and a pillion seat  22  are supported on the seat rail behind the fuel tank  30 . 
     A pair of front forks  23  are steerably supported by the head pipe via a steering shaft (not shown). A front wheel  24  is rotatably supported by lower portions of the front forks  23 , and an upper portion of the front wheel  24  is covered with a front fender  25 . The swing arm  15  extends rearward from the body frame  13 . A rear wheel  26  is rotatably supported at a rear end of the swing arm  15 , and the rear wheel  26  is covered with a rear fender  27  from above. The engine  14  is coupled to the rear wheel  26  via a chain drive type transmission mechanism, and power from the engine  14  is transmitted to the rear wheel  26  via the transmission mechanism. 
     Such a straddle-type vehicle is often equipped with a fuel tank made of a steel plate for reasons such as productivity and cost, but may be equipped with a fuel tank made of a non-ferrous metal or a resin, which is advantageous in terms of specific gravity. Most of non-ferrous metal fuel tanks are made of aluminum, and many of resin fuel tanks are made of polyethylene (PE) or nylon (PA). All of them have manufacturing methods different from those of steel plate fuel tanks, and require special manufacturing equipment and molds. Recently, manufacture of a fuel tank made of titanium, which has properties close to those of iron as compared with aluminum, as a nonferrous metal, has been studied. 
     One of the properties of titanium is a property of easily deteriorating due to reacting with oxygen, and thus manual brazing (torch brazing) in atmosphere, which has been performed at the time of manufacturing a fuel tank made of a steel sheet, cannot be performed. Brazing is employed in a fuel tank that requires airtightness, as a welding method for a portion where a working range is narrow, such as joining of pipes, in order to fill a gap between members by a capillary action without melting a base material. When in-furnace brazing is performed, the brazing can be performed in an oxygen-free environment, but a large component such as a panel component of a fuel tank cannot be put into a furnace, and pipes cannot be directly welded to the panel component. 
     In a case of a fuel tank made of a steel plate, since brazing work can be performed in the atmosphere, pipes can be manually brazed to a tank inlet of an outer panel, and the pipes can be manually brazed directly to an inner panel while passing through the tank. In a case of a fuel tank made of titanium, it is necessary to perform all welding in an oxygen-free environment, but since pipes cannot be brazed to a large inner panel in a furnace as described above, it is difficult for the pipes to pass through the tank. For this reason, in a general fuel tank made of titanium, a rubber hose is extended from a tank inlet to outside of the tank. 
     In order to maintain an appearance of the fuel tank, it is necessary to cover the fuel tank with a tank cover so as to hide the rubber hose outside the tank. When the tank cover is attached to the fuel tank, an advantage of reducing a weight due to titanium is reduced, and the fuel tank has a double structure, resulting in reduction of capacity. Therefore, in the fuel tank  30  according to the present embodiment, connection plates for pipes are separated from an inner panel  61  in a size that allows in-furnace brazing (see  FIG. 4 ). Thereby, even in a case of the fuel tank  30  made of titanium, which is subject to manufacturing restrictions due to the material and size, it is possible to maintain an appearance of the fuel tank  30  by allowing the pipes to pass through the tank. 
     Hereinafter, the fuel tank will be described with reference to  FIGS. 2 to 5 .  FIG. 2  is a perspective view of the fuel tank according to the present embodiment as viewed from above.  FIG. 3  is a perspective view of the fuel tank according to the present embodiment as viewed from below.  FIG. 4  is a cross-sectional view of the fuel tank taken along a line A-A in  FIG. 3 .  FIG. 5  is a front view of the inner connection plate according to the present embodiment. 
     As shown in  FIG. 2 , a tank body  31  of the fuel tank  30  is formed by joining outer edges of an outer panel  41  forming a tank outer shell and the inner panel  61  forming a tank inner shell (see  FIG. 3 ). The outer panel  41  and the inner panel  61  form a fuel storage space S (see  FIG. 4 ). The outer panel  41  is formed in a substantially dome shape such that symmetrical panel members  42 L,  42 R are joined along a center line in a front-rear direction and rise from a panel rear portion  43  toward a panel front portion  44 . A surface of the panel front portion  44  of the outer panel  41  is lowered by one step as a whole, and a tank cover  59  (see  FIG. 10C ) is formed to be attachable to the panel front portion  44 . 
     A circular opening  45  is formed in a center of the panel front portion  44  of the outer panel  41 , and a tank inlet  46  is joined to the circular opening  45 . The tank inlet  46  includes a bottomed cylindrical inlet plate  47  and an inlet pipe  49  attached to a bottom wall of the inlet plate  47 . A tank cap (not shown) is attached to the inlet plate  47 , and a fuel filling port of the inlet pipe  49  is opened and closed by the tank cap. When a fuel filling nozzle (not shown) is inserted into the inlet pipe  49  with the tank cap open, fuel is supplied from the fueling nozzle to the storage space S of the tank body  31 . 
     An upper breather pipe  51  and an upper drain pipe  52  as outer pipes are joined to the bottom wall of the inlet plate  47 . The upper breather pipe  51  and the upper drain pipe  52  extend from the bottom wall of the inlet plate  47  into the tank (particularly, see  FIG. 4 ). An upstream side of a breather pipe is formed by the upper breather pipe  51 , and an upstream side of a drain pipe is formed by the upper drain pipe  52 . The inlet plate  47  is slightly recessed around a joint portion of the upper drain pipe  52 , and liquid droplets on the inlet plate  47  are easily collected at an inlet of the upper drain pipe  52 . 
     Screw holes  54  for three nuts  53  (see  FIG. 4 ) are exposed from the bottom wall of the inlet plate  47 . The tank cap is attached to the tank inlet  46  by screwing the tank cap into these screw holes  54 . Seat surfaces  55 L,  55 R for a cover bracket (not shown) are formed on both left and right sides of the panel front portion  44  of the outer panel  41 . When the cover bracket is joined to the seat surfaces  55 L,  55 R, the tank cover  59  (see  FIG. 10C ) is attached to the panel front portion  44  of the outer panel  41  via the cover bracket. A periphery of the tank cap is covered with the tank cover  59  so as to hide the tank inlet  46 . 
     As shown in  FIG. 3 , the inner panel  61  includes a panel rear portion  63  formed in a planar shape, and is formed in a substantially dome shape so as to rise from a front side of the panel rear portion  63  toward a panel front portion  64 . A circular opening  65  is formed in a center of a bottom surface of the panel rear portion  63  of the inner panel  61 , and an upper side of a fuel pump (not shown) is inserted into the tank through the circular opening  65 . A plurality of screw holes  66  are formed around the circular opening  65 , and a flange portion on a lower side of the fuel pump is screwed into the screw holes  66 . The fuel in the tank is pumped out by the fuel pump and sent to a fuel injector (not shown) of the engine  14  (see  FIG. 1 ). 
     An oval opening  67  is formed next to the circular opening  65  (on a left side in the present embodiment), and an inner connection plate  71  is joined to the opening  67 . The inner connection plate  71  is formed in a dome shape bulging into the tank, and a lower breather pipe  73  and a lower drain pipe  74  as inner pipes are joined to an upper bottom wall of the inner connection plate  71 . The lower breather pipe  73  and the lower drain pipe  74  pass through the upper bottom wall of the inner connection plate  71  (particularly, see  FIG. 4 ). A downstream side of the breather pipe is formed by the lower breather pipe  73 , and a downstream side of the drain pipe is formed by the lower drain pipe  74 . 
     As shown in  FIG. 4 , the upper breather pipe  51  and the upper drain pipe  52  are substantially L-shaped, and the lower breather pipe  73  and the lower drain pipe  74  are straight. 
     A downstream end of the upper breather pipe  51  and an upstream end of the lower breather pipe  73  are connected to each other via a first breather hose (pipe)  81 , and a downstream end of the upper drain pipe  52  and an upstream end of the lower drain pipe  74  are connected to each other via a first drain hose (pipe)  82 . The inlet plate  47  and the inner connection plate  71  are connected to each other by the first breather hose  81  and the first drain hose  82  passing inside the outer panel  41  and the inner panel  61 . 
     A second breather hose  83  is connected to a downstream end of the lower breather pipe  73  outside the tank, and a second drain hose  84  is connected to a downstream end of the lower drain pipe  74  outside the tank. In this way, the upper breather pipe  51 , the first breather hose  81 , the lower breather pipe  73  and the second breather hose  83  form a breather pipe that guides a gas component in the tank inlet  46  to outside of the fuel tank  30 . The upper drain pipe  52 , the first drain hose  82 , the lower drain pipe  74  and the second drain hose  84  form a drain pipe that discharges liquid droplets in the tank inlet  46 . 
     The oval opening  67  of the inner panel  61  is formed along an outer peripheral edge  75  of the inner connection plate  71 , and an opening edge  68  of the inner panel  61  protrudes to the outside of the tank over the entire periphery. The outer peripheral edge  75  of the inner connection plate  71  protrudes from the opening  67  to the outside of the tank, and a lower end surface of the outer peripheral edge  75  of the inner connection plate  71  is aligned with a lower end surface of the opening edge  68  of the inner panel  61 . Since the opening edge  68  of the inner panel  61  and the outer peripheral edge  75  of the inner connection plate  71  protrude to the outside of the tank, the outer peripheral edge  75  of the inner connection plate  71  is easily and reliably joined to the opening edge  68  of the inner panel  61  from the outside of the tank. 
     At this time, the lower breather pipe  73  and the lower drain pipe  74  pass through the upper bottom wall of the inner connection plate  71  substantially perpendicularly. When the inner connection plate  71  is viewed from a front side, that is, when the inner connection plate  71  is directly faced, the downstream end of the lower breather pipe  73  and the downstream end of the lower drain pipe  74  are located inward than the outer peripheral edge  75  of the inner connection plate  71  (see  FIG. 5 ). Therefore, when the inner connection plate  71  and the inner panel  61  are joined by welding, the lower breather pipe  73  and the lower drain pipe  74  protruding to the outside of the tank do not interfere with a welding torch. 
     When the fuel tank  30  is manufactured, an outer component  32  including the outer panel  41  and outer related components, and an inner component  33  including the inner panel  61  and inner related components are individually manufactured. After the first breather hose  81  and the first drain hose  82  are connected, the outer component  32  and the inner component  33  are integrated to manufacture the fuel tank  30 . The outer component  32  and the inner component  33  are each made of titanium or titanium alloy, and members are welded to each other in an oxygen-free environment, so that a weight of the fuel tank  30  is reduced and weather resistance and rust resistance are improved. 
     As described above, since it is necessary to perform in-furnace brazing in the oxygen-free environment for joining the pipes and the plates, the inlet plate  47  and the inner connection plate  71  are formed in a size that can be accommodated in a furnace. Since the outer panel  41  and the inlet plate  47  are formed separately in this manner, the inlet plate  47 , the upper breather pipe  51  and the upper drain pipe  52  are brazed in the furnace. Similarly, since the inner panel  61  and the inner connection plate  71  are formed separately, the inner connection plate  71 , the lower breather pipe  73  and the lower drain pipe  74  are brazed in the furnace. 
     A method for manufacturing the fuel tank will be described with reference to  FIGS. 6A to 10C .  FIGS. 6A to 7D  are transition views showing a method for manufacturing a fuel tank made of a steel plate according to a comparative example.  FIGS. 8A to 10C  are transition views showing a method for manufacturing the fuel tank made of titanium according to the present embodiment. The fuel tank made of the steel plate according to the comparative example is different from the fuel tank according to the present embodiment in that each of a breather pipe and a drain pipe is formed of one pipe, and an inner connection plate is not formed. 
     In the method for manufacturing the fuel tank made of the steel plate shown in the comparative example, individual components of the fuel tank are manufactured. For example, various panels are press-formed by manufacturing equipment such as a press machine. Next, as shown in  FIGS. 6A to 6C , work of manufacturing an outer component  91  is performed. First, the work of manufacturing the outer panel  92  shown in  FIG. 6A  is performed. In this case, a shield gas is blown onto mating surfaces of panel members  93 L,  93 R, and the mating surfaces of the panel members  93 L,  93 R are plasma-welded in an oxygen-free environment. An inlet related component such as a cover bracket is spot-welded to the outer panel  92 . 
     Next, work of joining a breather pipe  95  and a drain pipe  96  to an inlet plate  94  shown in  FIG. 6B  is performed. In this case, in a state where the breather pipe  95  and the drain pipe  96  are inserted into the inlet plate  94 , the breather pipe  95  and the drain pipe  96  are manually brazed to the inlet plate  94  in atmosphere. Next, work of joining the inlet plate  94  to the outer panel  92  shown in  FIG. 6C  is performed. In this case, an outer peripheral edge of the inlet plate  94  is welded to an opening edge of the outer panel  92  by metal active gas (MAG) welding while blowing the shield gas. 
     As shown in  FIG. 7A , work of manufacturing an inner component  99  is performed. In this case, an attachment component to a fuel pump and an inner-related component such as a bracket for a vehicle body frame are spot-welded to an inner panel  97 . As shown in  FIG. 7B , work of integrating the outer component  91  and the inner component  99  is performed. In this case, an outer peripheral edge of the outer panel  92  and an outer peripheral edge of the inner panel  97  are aligned with each other, and the breather pipe  95  and the drain pipe  96  are inserted into through holes of the inner panel  97 . Then, the outer peripheral edge of the outer panel  92  and the outer peripheral edge of the inner panel  97  are seam-welded to integrate the outer component  91  and the inner component  99 . 
     As shown in  FIG. 7C , work of joining the breather pipe  95  and the drain pipe  96  to the inner panel  97  is performed. In this case, the breather pipe  95  and the drain pipe  96  are manually brazed to the inner panel  97  in the atmosphere. As shown in  FIG. 7D , surface treatment work on the outer panel  92  is performed. In a fuel tank  90  made of the steel plate, coating treatment is selected as surface treatment on the outer panel  92 . By coating a surface of the outer panel  92 , an appearance, weather resistance and rust resistance of the outer panel  92  are improved. Then, a tank cover  98  is attached to the outer panel  92  to manufacture the fuel tank  90 . 
     In the fuel tank made of titanium shown in the present embodiment, since properties of titanium are close to those of iron as compared with aluminum, some manufacturing equipment and the like are used in common with the fuel tank made of the steel plate. First, individual components of the fuel tank are manufactured. For example, the outer panel  41  and the inner panel  61  are press-formed. At this time, in press forming of the outer panel  41  and the inner panel  61 , which are the most expensive, the manufacturing equipment such as a die or a press machine for the fuel tank  90  made of the steel plate can be used. The manufacturing equipment of the fuel tank  90  made of the steel plate can also be used for manufacturing some peripheral components of the outer panel  41  and the inner panel  61 . 
     As shown in  FIGS. 8A to 8C , work of manufacturing the outer component  32  (a step of manufacturing the outer component) is performed. First, work of forming the outer panel  41  shown in  FIG. 8A  is performed. In this case, a shield gas is blown onto mating surfaces of the panel members  42 L,  42 R, and the mating surfaces of the panel members  42 L,  42 R are plasma-welded in the oxygen-free environment. An inlet related component such as a cover bracket is spot-welded to the outer panel  41 . In the welding, the manufacturing equipment of the fuel tank  90  made of the steel plate can be used. In spot welding, since joint surfaces of the members are blocked from the atmosphere, an influence of oxygen on the outer panel  41  made of titanium is minimized. 
     Next, work of joining the upper breather pipe  51  and the upper drain pipe  52  to the inlet plate  47  shown in  FIG. 8B  (a sub-step of in-furnace brazing) is performed. In this case, the upper breather pipe  51  and the upper drain pipe  52  are accommodated in the furnace in the state where the upper breather pipe  51  and the upper drain pipe  52  are inserted into the inlet plate  47 , and the upper breather pipe  51  and the upper drain pipe  52  are brazed to the inlet plate  47  in the furnace in the oxygen-free environment. Since the inlet plate  47  is formed to have a size that can be accommodated in the furnace, in-furnace brazing having size restrictions can be performed. 
     Next, work of joining the inlet plate  47  to the outer panel  41  shown in  FIG. 8C  (a sub-step of TIG welding) is performed. In this case, an outer peripheral edge of the inlet plate  47  is welded to an opening edge of the outer panel  41  by tungsten inert gas (TIG) welding while blowing the shield gas. By partially modifying welding equipment of the fuel tank  90  made of the steel sheet, not only the MAG welding but also the TIG welding can be performed. For this reason, the manufacturing equipment of the fuel tank  90  made of the steel plate can also be used for welding the outer panel  41  and the inlet plate  47 . 
     As shown in  FIGS. 9A to 9C , work of manufacturing the inner component  33  (a step of manufacturing the inner component  33 ) is performed. First, work of forming the inner panel  61  shown in  FIG. 9A  is performed. In this case, an attachment component to a fuel pump and an inner-related component such as a bracket for a vehicle body frame are spot-welded to the inner panel  61 . In the welding, the manufacturing equipment of the fuel tank  90  made of the steel plate can be used. 
     Next, work of joining the lower breather pipe  73  and the lower drain pipe  74  to the inner connection plate  71  shown in  FIG. 9B  (a sub-step of in-furnace brazing) is performed. In this case, the lower breather pipe  73  and the lower drain pipe  74  are accommodated in the furnace in a state where the lower breather pipe  73  and the lower drain pipe  74  are inserted into the inner connection plate  71 , and the lower breather pipe  73  and the lower drain pipe  74  are brazed to the inner connection plate  71  in the furnace in the oxygen-free environment. Since the inner connection plate  71  is formed to have a size that can be accommodated in the furnace, in-furnace brazing having size restrictions can be performed. 
     Next, work of joining the inner connection plate  71  to the inner panel  61  shown in  FIG. 9C  (a sub-step of TIG welding) is performed. In this case, the outer peripheral edge of the inner connection plate  71  is welded to the opening edge of the inner panel  61  by tungsten inert gas (TIG) welding while blowing the shield gas. As described above, by partially modifying the welding equipment of the fuel tank  90  made of the steel sheet, not only the MAG welding but also the TIG welding can be performed. For this reason, the manufacturing equipment of the fuel tank  90  made of the steel plate can also be used for welding the inner panel  61  and the inner connection plate  71 . 
     As shown in  FIGS. 10A and 10B , work of integrating the outer component  32  and the inner component  33  (a step of integrating) is performed. First, work of connecting the first breather hose  81  and the first drain hose  82  to the pipes shown in  FIG. 10A  (a sub-step of connecting) is performed. In this case, the upper breather pipe  51  of the outer component  32  and the lower breather pipe  73  of the inner component  33  are connected to each other via the first breather hose  81 . The upper drain pipe  52  of the outer component  32  and the lower drain pipe  74  of the inner component  33  are connected to each other via the first drain hose  82 . A rubber hose or a resin hose having flexibility and oil resistance is used for the first breather hose  81  and the first drain hose  82 . 
     Next, work of joining the outer panel  41  and the inner panel  61  shown in  FIG. 10B  (a sub-step of seam welding) is performed. In this case, an outer peripheral edge of the outer panel  41  and an outer peripheral edge of the inner panel  61  are aligned with each other, and the outer peripheral edge of the outer panel  41  and the outer peripheral edge of the inner panel  61  are seam-welded. Also for seam welding, the manufacturing equipment of the fuel tank  90  made of the steel plate can be used. In the seam welding, since joint surfaces of the outer panel  41  and the inner panel  61  are blocked from the atmosphere, an influence of oxygen on the outer panel  41  and the inner panel  61  made of titanium is minimized. 
     As shown in  FIG. 10C , surface treatment work (a step of roughening) is performed on the outer panel  41 . In the fuel tank  30  made of titanium, shot treatment is selected as surface treatment on the outer panel  41 . Since titanium has excellent weather resistance and rust resistance, physical properties do not deteriorate even when coating treatment is not performed, but a surface of the outer panel  41  has processing marks such as press scratches and beads. The shot treatment removes the processing marks, and the exposed metal surface improves the appearance. Then, the tank cover  59  is attached to the outer panel  41  to manufacture the fuel tank  30 . As the tank cover  59 , a cover the same as that of the fuel tank  90  made of the steel plate can be used. 
     As described above, according to the present embodiment, the outer panel  41  and the inlet plate  47  are formed separately, and the inner panel  61  and the inner connection plate  71  are formed separately. Therefore, even in a case of in-furnace brazing suitable for titanium but having size restrictions, in-furnace brazing can be used for joining pipes to the inlet plate  47  and the inner connection plate  71 . In this way, it is possible to flexibly select work that satisfies manufacturing restrictions due to the material and size. By integrating the outer component  32  and the inner component  33 , the fuel tank  30  in which the pipes passes through the tank is manufactured. In order to maintain the appearance, a large tank cover that covers the entire tank is not required, and without a double structure, capacity of the fuel tank  30  is not reduced. 
     The fuel tank according to the present embodiment can also be appropriately applied to other straddle-type vehicles such as a buggy-type motor tricycle. Here, the straddle-type vehicle is not limited to general vehicles in which a rider rides in a posture of straddling a seat, but also includes a scooter-type vehicle in which a rider rides without straddling a seat. 
     In the present embodiment, the method for manufacturing the fuel tank made of titanium has been described, but this manufacturing method can also be applied to manufacture of fuel tanks made of other materials. In particular, the present invention is effective for manufacturing a fuel tank using a material that requires joining in an oxygen-free environment or a low-oxygen environment. 
     In the present embodiment, the breather pipe is formed by four components including the upper breather pipe, the first breather hose, the lower breather pipe and the second breather hose, but the breather pipe may be formed by one component. Similarly, in the present embodiment, the drain pipe is formed by four components including the upper drain pipe, the first drain hose, the lower drain pipe and the second drain hose, but the drain pipe may be formed by one component. 
     In the present embodiment, the outer component and the inner component are made of titanium, but some components of the outer component and the inner component may be made of different materials. For example, the upper breather pipe, the lower breather pipe, the upper drain pipe and the lower drain pipe may be made of iron. 
     In the present embodiment, joining methods such as plasma welding, spot welding, in-furnace brazing, TIG welding and seam welding are used, but other joining methods that have less influence of oxygen on a joining target may be adopted. 
     As described above, a fuel tank ( 30 ) according to the present embodiment includes: an outer component ( 32 ) including an outer panel ( 41 ) forming a tank outer shell and an inlet plate ( 47 ) formed separately from the outer panel; an inner component ( 33 ) including an inner panel ( 61 ) forming a tank inner shell and an inner connection plate ( 71 ) formed separately from the inner panel; and a pipe (the first breather hose  81  and the first drain hose  82 ) passing through inside of the outer panel and the inner panel and connecting the inlet plate and the inner connection plate. The inlet plate is joined to the outer panel, the inner connection plate is joined to the inner panel, and the inner panel is joined to the outer panel. According to this configuration, the outer panel and the inlet plate are formed separately, and the inner panel and the inner connection plate are formed separately. Therefore, even when work is suitable for a material of each member but has size restrictions, the work can be performed on the inlet plate and the inner connection plate. In this way, it is possible to flexibly select work that satisfies manufacturing restrictions due to the material and size. In addition, the outer component and the inner component are integrated, so that the fuel tank in which the pipe passes through the tank is manufactured. In order to maintain an appearance, a large tank cover that covers the entire tank is not required, and without a double structure, capacity of the fuel tank is not reduced. 
     In the fuel tank according to the present embodiment, the outer component and the inner component are made of titanium or titanium alloy. According to this configuration, members can be joined to each other in an environment suitable for titanium or titanium alloy, whereby a weight of the fuel tank can be reduced and weather resistance and rust resistance can be improved. 
     In the fuel tank according to the present embodiment, an opening ( 67 ) along an outer peripheral edge ( 75 ) of the inner connection plate is formed in the inner panel and the outer peripheral edge of the inner connection plate protrudes from the opening to outside of the tank, and an opening edge ( 68 ) of the inner panel protrudes to the outside of the tank and the outer peripheral edge of the inner connection plate is joined to the opening edge of the inner panel. According to this configuration, the outer peripheral edge of the inner connection plate can be easily and reliably welded to the opening edge of the inner panel from the outside of the tank. 
     In the fuel tank according to the present embodiment, the outer component includes an outer pipe (the upper breather pipe  51  and the upper drain pipe  52 ) extending from the inlet plate into the tank and the inner component includes an inner pipe (the lower breather pipe  73  and the lower drain pipe  74 ) penetrating the inner connection plate, and the pipe is connected to the outer pipe and the inner pipe, the outer pipe is joined to the inlet plate and the inner pipe is joined to the inner connection plate. According to this configuration, the inlet plate and the outer pipe are formed separately, and the inner connection plate and the inner pipe are formed separately. Therefore, a joining method suitable for the material and size of the inlet plate and the outer pipe and a joining method suitable for the material and size of the inner connection plate and the inner pipe can be used. 
     In the fuel tank according to the present embodiment, an end portion protruding from the inner pipe to the outside of the tank is located inward than the outer peripheral edge of the inner connection plate in a state of directly facing the inner connection plate. According to this configuration, when the inner connection plate is joined to the inner panel, the end portion protruding from the inner pipe to the outside of the tank does not interfere. 
     A method for manufacturing the fuel tank according to the present embodiment includes: a step of manufacturing an outer component by joining an inlet plate formed separately from an outer panel forming a tank outer shell to the outer panel; a step of manufacturing an inner component by joining an inner connection plate formed separately from an inner panel forming a tank inner shell to the inner panel; and a step of connecting the inlet plate and the inner connection plate by a pipe passing through the tank, and joining and integrating the outer panel and the inner panel. According to this configuration, the outer panel and the inlet plate are formed separately, and the inner panel and the inner connection plate are formed separately. Therefore, even when work is suitable for a material of each member but has size restrictions, the work can be performed on the inlet plate and the inner connection plate. In this way, it is possible to flexibly select work that satisfies manufacturing restrictions due to the material and size. In addition, the outer component and the inner component are integrated, so that the fuel tank in which the pipe passes through the tank is manufactured. In order to maintain an appearance, a large tank cover that covers the entire tank is not required, and without a double structure, capacity of the fuel tank is not reduced. 
     In the method for manufacturing the fuel tank according to the present embodiment, the step of manufacturing the outer component includes a sub-step of brazing an outer pipe to the inlet plate in a furnace and a sub-step of TIG-welding the inlet plate to the outer panel, the step of manufacturing the inner component includes a sub-step of brazing an inner pipe to the inner connection plate in the furnace and a sub-step of TIG-welding the inner connection plate to the inner panel, and the step of integrating includes a sub-step of connecting the outer pipe and the inner pipe by the pipe and a sub-step of seam-welding the outer panel and the inner panel. According to this configuration, even if each component is formed of the material that is difficult to be welded in atmosphere, the fuel tank can be manufactured satisfactorily. 
     In the method for manufacturing the fuel tank according to the present embodiment, the outer component and the inner component are made of titanium or titanium alloy, and the method further includes: a step of roughening a surface of the outer panel joined to the inner panel by performing shot treatment on the surface of the outer panel. According to this configuration, a processing mark such as a scratch or a welding mark on the surface of the outer panel can be eliminated. It is not necessary to cover the surface of the outer panel with a coating film or the like, and a metal surface made of titanium or titanium alloy can be exposed to the outside to improve the appearance. In addition, a weight of the fuel tank can be reduced, and weather resistance and rust resistance can be improved. 
     Although the present embodiment has been described, the above-described embodiment and modifications may be combined entirely or partially as another embodiment. 
     The technique of the present invention is not limited to the above-described embodiment, and various changes, substitutions and modifications may be made without departing from the spirit of the technical idea of the present invention. Further, the present invention may be implemented using other methods as long as the technical idea can be implemented by the methods through advance of the technique or other derivative technique. Accordingly, the claims cover all embodiments that may be included within the scope of the technical idea.