Patent Publication Number: US-2009236350-A1

Title: Ventilating device for fuel tank

Description:
This application claims the benefit of and priority from Japanese Application No. 2008-71046 filed Mar. 19, 2008, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ventilating device for a fuel tank, adapted to prevent fuel from spilling from the fuel tank to the outside. 
     2. Description of the Related Art 
     One known technology proposed for a fuel tank ventilating device of this kind is that disclosed in JP-A 2005-172161, for example. Specifically, the fuel tank ventilating device is provided with a fuel cutoff valve (rollover valve) and a full tank regulating valve connected to a canister, these being disposed in the upper part of the fuel tank; through appropriate opening and closing of the valves at prescribed fuel levels, venting of the fuel tank to the outside may be assured while at the same time preventing the liquid fuel from spilling to the outside. Despite enhanced sealing ability provided by a valve body of rubber or the like, the fuel cutoff valve in the closed state will not be able to prevent outflow of fuel to the canister, and slight leakage will occur. In order to prevent leaking fuel from reaching the canister, means such as extending the length of the pipe, situating a section of the pipe at an elevated location, or employing a vapor-liquid separating device have been employed. 
     However, a problem with such means is the complexity of design entailed with complicated and elongated piping arrangement, or with a piping arrangement having elevated section produced through a bending process. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide with a fuel tank ventilating device of simple design, that does not require a complicated and elongated piping arrangement or a piping arrangement having an elevated section produced through a bending process. 
     The present invention is addressed to attaining the above objects at least in part according to the following modes of the invention. 
     A first mode of the present invention provides a fuel tank ventilating device comprising: a fuel cutoff device for opening and closing a passage to a canister according to fuel level in a fuel tank; and a connector pipe connecting the fuel cutoff device with the canister. The fuel cutoff device includes: (i) a first fuel cutoff valve adapted to open and close a first connecting passage through rise and fall of a first float mechanism according to the fuel level in the fuel tank; (ii) a second fuel cutoff valve adapted to open and close a second connecting passage through rise and fall of a second float mechanism according to the fuel level in the fuel tank; and (ii) a positive pressure valve connected to the second connecting passage and adapted to open when tank internal pressure rises above a prescribed valve opening pressure, wherein the first and second fuel cutoff valves are arranged to have a first submersion state and second submersion state when the fuel tank are oriented at a tilted position between a first maximum tilt angle and a second maximum tilt angle representing reverse maximum tilts respectively. The first submersion state is a state in which the first fuel cutoff valve is submerged and the second fuel cutoff valve communicates a headspace of the fuel tank with the canister, and the second submersion state is a state in which the second fuel cutoff valve is submerged and the first fuel cutoff valve communicates the headspace of the fuel tank with the canister. The connector pipe includes: (i) a first flow pipe connected to the first connecting passage; (ii) a second flow pipe connected to the second connecting passage via the positive pressure valve and connected to the first flow pipe by a connecting part; and (iii) a third flow pipe connecting the connecting part with the canister. The fuel tank ventilating device is configured such that when in the first submersion state, fuel that has flowed out from the fuel tank into the first flow pipe via the first fuel cutoff valve, the fuel is returned to the fuel tank by a differential pressure between a first pressure and a second pressure, the first pressure being a tank internal pressure which has been regulated to a level at or below the prescribed valve opening pressure of the positive pressure valve, the second pressure being a pipe internal pressure bearing on the fuel inside the first flow pipe; and when in the second submersion state and at the second maximum tilt angle, fuel that has leaked from the fuel tank into the second flow pipe via the second fuel cutoff valve, the fuel does not reach the connecting part. 
     According the fuel tank ventilating device taught in this first mode, even where the vehicle has a full tank and has assumed a condition tilted as far as either the first or second maximum tilt angle, either the first fuel cutoff valve or the second fuel cutoff valve will open to ensure venting of the fuel tank to the outside. 
     One means for preventing outflow of fuel to the canister through the second flow pipe in the second submersion state with the second fuel cutoff valve submerged would be to dispose the connecting part at a location whereby the second flow pipe is longer than the first flow pipe, rather than the first flow pipe and the second flow pipe being of equal length. With this arrangement, even if the first flow pipe and the second flow pipe are routed on the horizontal, if the fuel tank should tilt, to the extent that the second flow pipe is longer it will be easier for the connecting part to be positioned above the fuel level in the fuel tank, that is, it will be easier to produce a pipe arrangement that inhibits outflow of fuel to the canister. Consequently, there will be no need for a complicated and elongated line or a line having an elevated section produced through a bending process as described in the prior art, thus affording a simpler design. 
     An arrangement whereby the connecting part is situated at a location such that the first flow pipe is shorter than the second flow pipe will make it more difficult, owing to the short length of the first flow pipe, for the connecting part to be situated at a high position when the tank is tilted, and will make it easier for leaked fuel to reach the connecting part where the first fuel cutoff valve is submerged; however, means such as the following can be adopted to avoid such problems. Specifically, firstly, the sealing ability of the first fuel cutoff valve may be enhanced to reduce the amount of fuel leakage; and secondly, there may be employed means adapted to return fuel that has collected in the first flow pipe back to the fuel tank by utilizing a differential pressure arising between pipe internal pressure in the first flow pipe versus the tank internal pressure, caused by cyclical negative pressure of the tank internal pressure. Such avoidance means will be simple in design, as it involves merely improving sealing ability of the first fuel cutoff valve used conventionally. 
     These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a fuel tank having an installed fuel tank ventilation device according to an embodiment of the present invention; 
         FIG. 2  is a sectional view depicting a first fuel cutoff valve; 
         FIG. 3  is a sectional view depicting a fuel cutoff valve in exploded view; 
         FIG. 4  is a cross sectional view depicting a second fuel cutoff valve; 
         FIG. 5  is an enlarged cross sectional view depicting the vicinity of a pressure regulating valve mechanism; 
         FIG. 6  is a sectional view depicting a positive/negative pressure valve; 
         FIG. 7  shows the operation of the fuel tank ventilating device in a first submersion state; 
         FIG. 8  shows the operation of the fuel tank ventilating device in a second submersion state; and 
         FIG. 9  shows a fuel tank having an installed fuel tank ventilation device according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (1) Overall Configuration of Fuel Tank Ventilating Device 
       FIG. 1  is an illustration depicting a fuel tank FT having an installed fuel tank ventilation device according to an embodiment of the present invention. The fuel tank FT is a flattened tank adapted to ensure a roomy passenger cabin in the vehicle, and is formed split into two parts, an upper half part FTa and a lower half part FTb which are joined together. The fuel tank ventilation device is disposed on the lower wall FTc and to the upper of the fuel tank FT. The fuel tank ventilation device has a fuel cutoff device  10 ; a connecting pipe CP that connects the fuel cutoff device  10  to a canister CN; and a positive/negative pressure valve  90 . The fuel cutoff device  10  also includes a first fuel cutoff valve  20  and a second fuel cutoff valve  40  respectively situated to either side of the upper section of the fuel tank FT. The first fuel cutoff valve  20  and the second fuel cutoff valve  40  ensure venting of the fuel tank FT to the outside, as well as preventing fuel from spilling to the outside in the event that the vehicle tilts. 
     The connecting pipe CP includes a first flow pipe CP 1  that connects with the first fuel cutoff valve  20 ; a second flow pipe CP 2  that connects with the second fuel cutoff valve  40  via a positive pressure valve  70  and that connects with the first flow pipe CP 1  by a connecting part CP-J; a third flow pipe CP 3  that connects the connecting part CP-J and the positive/negative pressure valve  90 ; and a fourth flow pipe CP 4  that connects the positive/negative pressure valve  90  and the canister CS. The connecting part CP-J is disposed at a location whereby the second flow pipe CP 2  is longer than the first flow pipe CPU 1 , rather than the first flow pipe CP 1  and the second flow pipe CP 2  being of equal length. The arrangements of the various parts of the fuel tank ventilation device will be discussed below. 
     (2) First Fuel Cutoff Valve  20   
       FIG. 2  is a sectional view depicting the first fuel cutoff valve  20 . The first fuel cutoff valve  20  includes a casing  21 ; a first float mechanism  30  housed in a valve chamber  20 S inside the casing  21 ; and a spring  34 . The first fuel cutoff valve  20  is attached to the upper wall of the fuel tank FT via a mounting bracket BK 1 . 
     (2)- 1  Casing  21   
       FIG. 3  is a sectional view depicting the fuel cutoff valve in exploded view. In  FIG. 3 , the casing  21  has a main casing body  22 ; a base plate  26  installed at the bottom of the main casing body  22 ; and a cover  28  installed at the top of the main casing body  22 . The main casing body  22  includes an upper wall  23 ; and a side wall  24  of round tubular contours that extends downward from the outside peripheral edge of the upper wall  23 , thereby defining a cup-shaped valve chamber  20 S that is enclosed by the upper wall  23  and the side wall  24  and whose bottom constitutes a lower opening  22   a.  A float passageway-defining projection  23   a  is formed in the center of the upper wall  23  and projects downward therefrom. The float passageway-defining projection  23   a  is perforated by a first connecting passage  23   b;  and the first connecting passage  23   b  on the valve chamber  20 S thereof constitutes a float seal part  23   c.  A vent hole  24   a  that connects the fuel tank FT interior with the valve chamber  20 S is formed in the upper part of the side wall  24 , and a mating recess  24   b  is formed in the lower part. The mating recess  24   b  is employed for attaching the base plate  26 . 
     The base plate  26  is a member adapted to close off the lower opening  22   a  of the main casing body  22 , and is attached so as to provide closure to the lower opening  22   a  of the main casing body  22  through engagement of a mating hook  26   a  formed at its outside edge mating recess  24   b  within the mating recess  24   b.  The base plate  26  is perforated by a through-hole  26   b.  A spring support part  26   c  adapted to support the lower end of the spring  34  is formed on the upper face of the base plate  26 . 
     The cover  28  includes an upper wall  28   a  of circular disk shape; a side wall  28   b  of round tubular contours that projects downward from the outside peripheral part of the upper wall  28   a;  and a flange portion  28   c  that extends peripherally outward from the lower part of the side wall  28   b.  A sideward-projecting pipe body  28   d  is formed on the side wall  28   b  of the cover  28 . A pipe passage  28   e  is defined inside the pipe body  28   d;  this pipe passage  28   e  connects at a first end to the valve chamber  20 S via the first connecting passage  23   b,  and at its other end to the canister. A cover-side welding part  28   f  is formed on the lower face of the flange portion  28   c  of the cover  28 . The cover-side welding part  28   f  is attached through laser welding or the like to an annular welding part  22   b  of the casing  21 . 
     (2)-2 First Float Mechanism  30   
     The first float mechanism  30  includes a float  31 , and an upper part valve body  32  made of rubber situated in the upper part of the float  31 . The float  31  is designed with receptacle shape having an upper wall  31   a  and a tubular side wall  31   b  formed to the lower side from the outside periphery of the upper wall  31   a;  its interior space functions as a buoyancy chamber  31 S for producing buoyancy. Guide ribs  32  are formed on the outside peripheral part of the float  31 . The guide ribs  31   c  have rib contours and project in the vertical direction at eight equidistant locations in the circumferential direction from the side wall of the float  31 . The float  31  is supported on the spring  34  by the spring support part  26   c  of the base plate  26 . The upper part valve body  32  is a valve body made of rubber and including a mounting part  32   a  adapted to be supported on a valve support portion  31   d  that projects up from the center of the upper wall  31   a,  specifically, by being pressure-fit onto the valve support portion  31   d;  and a seal part  32   b  of circular disk shape formed from the upper part of the mounting part  32   a.  The seal part  32   b  is adapted to deflect when seated in the float seal part  23   c  so as to exhibit good sealing ability. The spring  34  is positioned inside the buoyancy chamber  31 S, and is adapted to urge the float  31  upward by being interposed between one end of the float  31  and the spring support part  26   c  of the base plate  26 . 
     (2)-3 Operation of the First Float Mechanism  30   
     As depicted in  FIG. 2 , if the fuel level in the fuel tank FT should reach a prescribed level FL 1  due to tilting or rocking motion of the vehicle, fuel will inflow to the valve chamber  20 S through the through-hole  26   b  in the base plate  26 . Accordingly, the first float mechanism  30  will give rise to buoyancy and ascend (shown by double dot-and-dashed lines), with the seal part  32   b  of the upper part valve body  32  of the first float mechanism  30  becoming seated in the float seal part  23   c  and blocking off the first connecting passage  23   b  so that fuel does not flow out to the canister. At this point, the seal part  32   b  of the upper part valve body  32  will undergo deflection when it seats in the float seal part  23   c  so as to attain good sealing ability. 
     (3) Second Fuel Cutoff Valve  40   
     (3)-1 General Arrangement of Second Fuel Cutoff Valve  40   
       FIG. 4  is a cross sectional view depicting the second fuel cutoff valve  40 . The second fuel cutoff valve  40  is substantially identical in design to the first fuel cutoff valve  20  in respect of being adapted to shut at the prescribed level FL 1 ; however, it differs in design in that an additional sealing mechanism and pressure regulating valve mechanism  60  are provided. Specifically, the second fuel cutoff valve  40  includes a casing  41 ; and a second float mechanism  50  and a pressure regulating valve mechanism  60  which are respectively housed in a valve chamber  41 S inside the casing  41  and in an upper valve chamber  60 S. The second float mechanism  40  includes a valve part  51   a  of substantially conical shape projecting up from the upper part of the float  51 . The valve part  51   a  is designed to contact with or release from a float seal part  43   d  in association with ascent or descent of the second float mechanism  50 , thereby opening or closing a second connecting passage  43   b.  Because the valve part  51   a  is made of resin, its sealing ability will be somewhat inferior to that of the upper part valve body  32  of the first fuel cutoff valve  20 . 
     (3)-2 Pressure Regulating Valve Mechanism  60   
       FIG. 5  is an enlarged cross sectional view depicting the vicinity of the pressure regulating valve mechanism  60 . The pressure regulating valve mechanism  60  includes the positive pressure valve  70  and a relief valve  80 , which are housed within the upper valve chamber  60 S. The positive pressure valve  70  is a valve adapted to open when internal pressure in the fuel tank has exceeded a prescribed valve opening pressure Pp, and includes a first passageway-defining projection  71  of round tubular shape projecting from the top center part of the upper wall  23  of the main casing body  22 . The first passageway-defining projection  71  defines a first pressure regulating valve chamber  70 S that connects the inner space to the second connecting passage  43   b.  A first pressure regulating valve seal portion  71   c  facing into the second connecting passage  43   b  is formed on the upper part of the upper wall  23 . 
     The positive pressure valve  70  includes a first pressure regulating valve body  72  that is urged in the closed direction by the spring  76 . A valve part  73  is formed in the lower part of the first pressure regulating valve body  72 . The valve part  73  alternately contacts or separates from the first pressure regulating valve seal portion  71   c  to open or close the second connecting passage  43   b.  With this design of the positive pressure valve  70 , when upward force bearing on the valve part  73  due to pressure through the second connecting passage  43   b  overcomes the urging force of the spring  76  and the weight of the first pressure regulating valve body  72 , the first pressure regulating valve body  72  will move upward and the second connecting passage  43   b  will open, whereby fuel vapors in the fuel tank may communicate with the canister via the valve chamber  41 S, the second connecting passage  43   b,  the first pressure regulating valve chamber  70 S, the upper valve chamber  60 S, and the pipe passage  28   e.    
     The relief valve  80  is a valve adapted to reduce pressure inside the fuel tank by opening when the tank internal pressure exceeds a pressure value higher than a prescribed valve opening pressure Pp for the relief valve  80 ; it includes a second passageway-defining projection  81  of round tubular shape, a second pressure regulating valve body  82 , and a spring  85 . The second passageway-defining projection  81  defines a second pressure regulating valve chamber  80 S that connects the inner space to the valve chamber  41 S via a relief passage  81   a.  A second pressure regulating valve seal portion  81   c  is formed facing into the relief passage  81   a.  The second pressure regulating valve body  82  is a ball valve. With this design of the relief valve  80 , when upward force bearing on the second pressure regulating valve body  82  due to pressure through the relief passage  81   a  overcomes the urging force of the spring  85  and the weight of the second pressure regulating valve body  82 , the second pressure regulating valve body  82  will move upward and the relief passage  81   a  will open, whereby the interior of the fuel tank may communicate with the canister via the valve chamber  41 S, the relief passage  81   a,  the second pressure regulating valve chamber  80 S, and the upper valve chamber  60 S, etc. 
     (4) Positive/Negative Pressure Valve  90   
       FIG. 6  is a sectional view depicting the positive/negative pressure valve  90 . The positive/negative pressure valve  90  connects to the third flow pipe CP 3  and to the fourth flow pipe CP 4  of the canister CS (see  FIG. 1 ), and is composed of a positive pressure valve  92  and a negative pressure valve  96  which are housed within a valve chamber  90 S of a casing  91 . The positive pressure valve  92  includes a positive pressure valve body  93 , and a coil spring  94  adapted to urge the positive pressure valve body  93 . Meanwhile, the negative pressure valve  96  includes a negative pressure valve body  97  and a coil spring  98 . Adjustment of pressure inside the fuel tank by the positive pressure valve  92  and the negative pressure valve  96  takes place through the following operation. As pressure in the third flow pipe CP 3  increases to the point that the differential pressure bearing on the positive pressure valve body  93  of the positive pressure valve  92  goes above a prescribed pressure, the positive pressure valve body  93  will move upward in opposition to the urging force of the coil spring  94  and the positive pressure valve  92  will open. On the other hand, if tank pressure drops to the point that the differential pressure bearing on the negative pressure valve body  97  of the negative pressure valve  96  goes above a prescribed pressure, the negative pressure valve body  97  will move downward and the negative pressure valve  96  will open. That is, when the pressure in the third flow pipe CP 3  goes to positive pressure or negative pressure with respect to atmospheric pressure, and the value thereof has reached or exceeded a prescribed pressure, the positive pressure valve  92  and the negative pressure valve  96  will open and adjust the pressure to within a prescribed range with respect to atmospheric pressure. 
     (5) Operation of Fuel Tank Ventilating Device 
     (5)-1 Operation of Fuel Tank Ventilating Device with Vehicle in a Horizontal Attitude 
     As depicted in  FIG. 1 , with the vehicle in a horizontal attitude and the fuel tank FT in a horizontal attitude, neither the first fuel cutoff valve  20  nor the second fuel cutoff valve  40  are submerged. At this time, through rise and fall of the first float mechanism  30  of the first fuel cutoff valve  20  according to the fuel level FL in the fuel tank FT, venting of the fuel tank FT to the canister CS or shutoff thereof will be assured. Here, even if the second fuel cutoff valve  40  is in the open state, the positive pressure valve  70  will not open as long as the prescribed pressure Pp is not exceeded, and thus venting to the canister CS through the second flow pipe CP 2  will not take place. The positive pressure valve  70  opens when tank internal pressure reaches the prescribed pressure Pp, while the relief valve  80  functions as an emergency escape valve that opens when the tank internal pressure exceeds the prescribed pressure Pp by a sufficiently large extent. 
     (5)-2 Operation of Fuel Tank Ventilating Device with Vehicle in a Tilted Attitude 
     (a) First Submersion State 
     Let it be assumed that the vehicle is at a stop in a titled attitude, with the wheels on one side driven up over the curb for example. In this instance, the fuel tank ventilation device is adapted to prevent fuel from spilling to the outside at the angle of tilt of the fuel tank FT lying between a first maximum tilt angle and a second maximum tilt angle that represent maximums, namely, between −20° and 20°.  FIG. 7  depicts a first submersion state in which the first fuel cutoff valve  20  is submerged, while the head space of the fuel tank FT communicates with the canister CS through the second fuel cutoff valve  40 . At this time, the first fuel cutoff valve  20  will be situated at a position lower than the first flow pipe CP 1  and the second flow pipe CP 2 . In this first submersion state, since the fuel level FL in proximity to the first fuel cutoff valve  20  exceeds the prescribed level FL 1  (see  FIG. 2 ), the first float mechanism  30  will be in the uplifted position, closing off the first connecting passage  23   b.  In this instance, since the first float mechanism  30  employs the upper part valve body  32  made of rubber ( FIG. 3 ), it will be difficult for fuel to leak; however, even if there is slight leakage into the first flow pipe CP 1 , this will not reach the connecting part CP-J. On the other hand, since the fuel level in proximity to the second fuel cutoff valve  40  does not exceed the prescribed level FL 1  ( FIG. 4 ), the second float mechanism  50  will be in the descended position and the second connecting passage  43   b  will be open. In this case, owing to its connection to the positive pressure valve  70 , the second connecting passage  43   b  will open when the positive pressure valve  70  has exceeded the prescribed pressure Pp. Since the prescribed pressure Pp has been set to a small value, venting of the fuel tank FT to the canister CS will be assured even where tank pressure is only slightly positive pressure. 
     In the first submersion state, in the event that the temperature in the vicinity of the fuel tank FT has dropped, fuel vapors inside the fuel tank FT will become liquid and the tank internal pressure will go to negative pressure. In the event that the tank internal pressure goes to negative pressure in this way, the positive pressure valve  70  will shut; and moreover because the pipe internal pressure Pa in the first flow pipe CP 1  is being maintained at a higher level by the positive/negative pressure valve  90 , a differential pressure will arise between internal pressure Pa in the first flow pipe CP 1  and the tank internal pressure Pb. This differential pressure will bear on the fuel that has collected in the first flow pipe CP 1  and in such a way as to push down the first float mechanism  30  of the first fuel cutoff valve  20 , to slightly open the first fuel cutoff valve  20 . Thus, the fuel inside the first flow pipe CP 1  will return to the fuel tank FT through the connecting passage  23   b,  and the fuel level in the first flow pipe CP 1  will drop. In this way, on the basis of a differential pressure, fuel which has collected in the first flow pipe CP 1  will be returned to the fuel tank FT without reaching the connecting part CP-J, thus avoiding outflow to the canister CS. Here, in the course of daily temperature changes, since nighttime temperatures are lower than daytime temperatures, fuel vapors in the fuel tank will become liquid and tank internal pressure will go to negative pressure in cyclical fashion. Owing to such cyclical negative pressure of tank internal pressure, any fuel that has collected in the first flow pipe CP 1  will be reliably returned to the fuel tank FT. 
     The action of returning fuel that has collected in the first flow pipe CP 1  to the fuel tank FT based on the aforementioned differential pressure will take place even if the first fuel cutoff valve  20  is not in the submersion state, or if the fuel level is such that the first fuel cutoff valve is shut. 
     (b) Second Submersion State 
       FIG. 8  depicts a second submersion state in which the second fuel cutoff valve  40  is submerged and the headspace of the fuel tank FT is in communication with the canister CS through the first fuel cutoff valve  20 . In the second submersion state, since the fuel level FL in proximity to the second fuel cutoff valve  40  exceeds the prescribed level FL 1  ( FIG. 4 ), the second float mechanism  50  will be in the uplifted position, closing off the second connecting passage  43   b.  On the other hand, since the fuel level in proximity to the first fuel cutoff valve  20  does not exceed the prescribed level FL 1  ( FIG. 2 ), the first float mechanism  30  will be in the descended position and the first connecting passage  23   b  will be open. Venting and shutoff to the canister CS by the first fuel cutoff valve  20  will be ensured thereby. Because the second fuel cutoff valve  40  does not have higher sealing ability than the first fuel cutoff valve  20 , in this instance more fuel will leak into the second flow pipe CP 2  than in the first submersion state. However, with a full tank and the fuel tank FT tilted to the second maximum tilt angle, the connecting part CP-J will be positioned at a location above the fuel level in the fuel tank FT, and thus fuel leaking into the second flow pipe CP 2  will not reach the connecting part CP-J and will not outflow to the canister CS. 
     (c) Return to Horizontal Attitude 
     Where fuel has collected in the first flow pipe CP 1  or the second flow pipe CP 2  which have assumed the first submersion state or the second submersion state, when the fuel tank FT returns to the horizontal attitude, since the first fuel cutoff valve  20  is open, the fuel which has collected in the first and second flow pipes CP 1 , CP 2  will return to the fuel tank FT. 
     (6) Effects and Advantages of the Embodiment 
     The embodiment described above affords the following effects and advantages. 
     (6)-1 Even where the vehicle has a full tank and has assumed a condition tilted as far as either the first or second maximum tilt angle, in the fuel tank ventilating device, either the first fuel cutoff valve  20  or the second fuel cutoff valve  40  will open so that venting of the fuel tank FT to the outside can be assured. 
     (6)-2 As the means for preventing outflow of fuel to the canister CS through the second flow pipe CP 2  in the second submersion state with the second fuel cutoff valve  40  submerged as depicted in  FIG. 8 , the connecting part CP-J is disposed at a location whereby the second flow pipe CP 2  is longer than the first flow pipe CP 1 , rather than the first flow pipe CP 1  and the second flow pipe CP 2  being of equal length. With this arrangement, even if the first flow pipe CP 1  and the second flow pipe CP 2  are routed on the horizontal, if the fuel tank FT should tilt, to the extent that the second flow pipe CP 2  is longer it will be easier for the connecting part CP-J to be positioned above the fuel level in the fuel tank FT, that is, it will be easier to produce a pipe arrangement that inhibits outflow of fuel to the canister CS. Consequently, there will be no need for a complicated and elongated line or a line having a section which has been made higher through a bending process as described in the prior art, thus affording a simpler design. 
     (6)-3 An arrangement whereby the connecting part CP-J is disposed at a location such that the first flow pipe CP 1  is shorter than the second flow pipe CP 2  will, owing to the shorter length of the first flow pipe CP 1 , make it more difficult for the connecting part CP-J to be situated at a high position when the fuel tank FT is tilted; and will make it easier for leaked fuel to reach the connecting part CP-J where the first fuel cutoff valve CP 1  is submerged; however, means such as the following can be adopted to avoid such problems. Specifically, as depicted in  FIG. 7 , firstly, the upper part valve body  32  of the first fuel cutoff valve  20  is made of rubber to enhance its sealing ability and reduce the amount of fuel leakage; and secondly, there is employed means adapted to return fuel that has collected in the first flow pipe back CP 1  to the fuel tank FT by utilizing the differential pressure arising between pipe internal pressure in the first flow pipe CP 1  and tank internal pressure caused by cyclical negative pressure of the tank internal pressure. Such avoidance means is simple in design, as it involves merely improving sealing ability of the first fuel cutoff valve  20  used conventionally, and appropriately setting the valve opening pressure for the positive pressure valve  70  and the positive/negative pressure valve  90 . 
     (7) Other Embodiments 
     The present invention is not limited to the embodiment set forth hereinabove, and may be reduced to practice in various other modes without departing from the scope of the invention, such as in the following possible modified embodiments for example. 
     (7)-1 While the preceding embodiment described an example in which the fuel cutoff device  10  of the fuel tank ventilating device is situated in the interior of the fuel tank, no limitation is implied thereby, and arrangements wherein device is instead situated on the top part of the fuel tank or protrudes partway into the fuel tank would be acceptable as well. An arrangement whereby the fuel cutoff device is mounted directly onto the top wall of the fuel tank would be acceptable, as would an arrangement whereby it is supported by a support member situated inside the tank. 
     (7)-2 While the fuel cutoff device  10  was described as having an arrangement with two valves, i.e. the first fuel cutoff valve  20  and the second fuel cutoff valve  40 , disposed to either side of the fuel tank FT, no limitation is implied thereby, and three or more valves could be provided depending on the shape of the fuel tank FT. 
     (7)-3 While the first flow pipe CP 1  and the second flow pipe CP 2  differ in length due to the positioning of the connecting part CP-J, no limitation is implied thereby, and it would be acceptable to instead increase the pipe diameter of the first flow pipe CP 1  or to provide a reservoir for example, to keep the amount of leakage from the first fuel cutoff valve  20  within a prescribed cycle to a lower level than the amount of fuel that can collect in the first flow pipe CP 1 . 
     (7)-4  FIG. 9  shows a fuel tank FT having an installed fuel tank ventilation device according to another embodiment of the present invention. This embodiment involves a piping arrangement in which a first flow pipe CP 1 -B and a second flow pipe CP 2 -B are situated inclined by a prescribed angle with respect to the horizontal, specifically, with a first fuel cutoff valve  20 B positioned below a second fuel cutoff valve  40 B. According to this embodiment, fuel that has collected in a connecting pipe CP-B can be discharged more smoothly to the fuel tank FT by the first fuel cutoff valve  20 B. 
     The foregoing detailed description of the invention has been provided for the purpose of explaining the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. The foregoing detailed description is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Modifications and equivalents will be apparent to practitioners skilled in this art and are encompassed within the spirit and scope of the appended claims.