Patent Publication Number: US-2013233285-A1

Title: Filter device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2012-050340 filed on Mar. 7, 2012, and No. 2012-275822 filed on Dec. 18, 2012, the disclosures of which are incorporated herein by reference. 
     FIELD OF TECHNOLOGY 
     The present disclosure relates to a filter device for a fuel system of an engine mounted in a vehicle. 
     BACKGROUND 
     A pump module is known in the art, according to which a fuel pump is provided in a sub-tank of a fuel tank for sucking fuel from the sub-tank to supply the fuel to an engine of a vehicle. 
     The pump module has a jet pump, through which a part of the fuel sucked by the fuel pump from the sub-tank is returned to the sub-tank. The fuel in the fuel tank (outside of the sub-tank) is sucked by negative pressure generated by discharge of the fuel from the jet pump, so that the fuel in the fuel tank is supplied into an inside of the sub-tank. According to such a structure, the fuel is maintained in the sub-tank so as to stably supply the fuel to the engine, even when the fuel is moved to one of sides in the fuel tank at a parking on an inclined parking place or during vehicle turning operation. 
     However, an amount of fuel to be sucked by the fuel pump is larger than an amount of fuel consumed in the engine by such an amount of fuel to be discharged from the jet pump. Therefore, an amount of electrical power to be consumed by the fuel pump becomes larger. 
     According to another type of a fuel pump module, for example, as disclosed in Japanese Patent Publication No. 2011-149397, a filter device has a bag-shaped filter element for trapping extraneous material contained in the fuel to be sucked into the fuel pump. The filter device further has a fuel reserving portion of a conical shape, which extends from an outer periphery of the filter element in an upward and a radial inward direction. According to the filter device, an oil film is formed in the filter element by surface tension of the fuel, when the filter element is immersed in the fuel. Therefore, when the fuel pump is operated, the fuel is filled in an inside of the bag-shaped filter element and the fuel is reserved in an inside of the fuel reserving portion. Accordingly, it is possible to reserve the fuel in the inside of the fuel reserving portion without using the jet pump. 
     However, according to the above filter device (disclosed in JP No. 2011-149397), when the operation of the fuel pump is stopped, pressure of the fuel inside of the bag-shaped filter element and the pressure of the fuel in the fuel tank become equal to each other. Then, the fuel flows out from the inside of the bag-shaped filter element to the outside of the filter element. Therefore, it is not possible to keep the fuel in the inside of the fuel reserving portion, when the operation of the fuel pump is stopped. 
     A wall can be provided at an inner wall of the fuel tank surrounding the fuel reserving portion for preventing the movement of the fuel (the flow-out of the fuel), in order that the fuel is kept in the fuel reserving portion at the operation stop of the fuel pump. However, such a wall increases a cost for the filter device. 
     In addition, according to the above filter device, the inside of the bag-shaped filter element becomes negative pressure during the operation of the fuel pump. Therefore, when an upper-side filter element and a lower-side filter element are brought into contact with each other, an amount of the fuel to be sucked into the fuel pump may be decreased. When protectors for maintaining a shape of the bag-shaped filter element are inserted into the inside of the filter element, a number of parts and components is increased. A cost for the filter device is likewise increased. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure is made in view of the above problem. It is an object of the present disclosure to provide a filter device for a fuel system of an engine installed in a vehicle, according to which fuel can be reserved in an inside of a sub-tank when an operation of a fuel pump is stopped. 
     According to a feature of the present disclosure, filter elements are integrally formed with a sub-tank, wherein an upper-side filter element is fixed to an upper side of a bottom wall of the sub-tank and a lower-side filter element is fixed to a lower side of the bottom wall. 
     According to such a feature, a fuel reserving space is formed at least between the upper-side filter element and the bottom wall. More exactly, the fuel reserving space is formed in an inside of the sub-tank by an inner wall of a tubular portion of the sub-tank and a part of the bottom wall (which is located below a horizontal line of a communication hole formed in the bottom wall), when a fuel tank is inclined. The fuel is reserved in the fuel reserving space, even when an amount of the fuel in the fuel tank becomes smaller and the fuel tank is inclined. Therefore, oil film is formed in the upper-side filter element. In addition, oil film is formed in the lower-side filter element, so long as the lower-side filter element is immersed into the fuel. When the fuel pump is operated in such a condition, pressure in a space between the upper-side filter element and the bottom wall as well as pressure in a space between the lower-side filter element and the bottom wall becomes negative pressure. As a result, the fuel pump can suck the fuel from the inside and the outside of the sub-tank to supply the fuel to an engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic cross sectional view showing a filter device according to a first embodiment of the present disclosure; 
         FIG. 2  is a schematic top plan view when viewed in a direction of an arrow II in  FIG. 1 ; 
         FIG. 3  is a schematic cross sectional view, taken along a line III-III in  FIG. 4 , showing a sub-tank to which the filter device of the first embodiment is applied; 
         FIG. 4  is a schematic cross sectional view taken along a line IV-IV in  FIG. 3 ; 
         FIG. 5  is a schematic enlarged view showing relevant portions encircled by a dotted line V in  FIG. 1 ; 
         FIG. 6  is a schematic view showing one of conditions for the filter device provided in a fuel tank filled with fuel; 
         FIG. 7  is a schematic view showing another condition for the filter device provided in the fuel tank filled with fuel; 
         FIG. 8  is a schematic view showing a further condition for the filter device provided in the fuel tank filled with fuel; 
         FIG. 9  is a schematic view showing a still further condition for the filter device provided in the fuel tank filled with fuel; 
         FIG. 10  is a schematic view showing one of conditions for the filter device provided in the fuel tank filled with fuel, according to a second embodiment of the present disclosure; 
         FIG. 11  is a schematic view showing another condition for the filter device provided in the fuel tank filled with fuel, according to the second embodiment of the present disclosure; 
         FIG. 12  is a schematic view showing a further condition for the filter device provided in the fuel tank filled with fuel, according to the second embodiment of the present disclosure; 
         FIG. 13  is a schematic view showing a still further condition for the filter device provided in the fuel tank filled with fuel, according to the second embodiment of the present disclosure; 
         FIG. 14  is a schematic view showing a still further condition for the filter device provided in the fuel tank filled with fuel, according to the second embodiment of the present disclosure; 
         FIG. 15  is a schematic view showing one of conditions for the filter device provided in the fuel tank filled with fuel, according to a third embodiment of the present disclosure; 
         FIG. 16  is a schematic view showing another condition for the filter device provided in the fuel tank filled with fuel, according to the third embodiment of the present disclosure; 
         FIG. 17  is a schematic view showing a further condition for the filter device provided in the fuel tank filled with fuel, according to the third embodiment of the present disclosure; 
         FIG. 18  is a schematic cross sectional view showing the filter device provided according to a fourth embodiment of the present disclosure; 
         FIG. 19  is a schematic top plan view when viewed in a direction of an arrow XIX in  FIG. 18 ; 
         FIG. 20  is a schematic cross sectional view showing the filter device provided according to a fifth embodiment of the present disclosure; 
         FIG. 21  is a schematic top plan view when viewed in a direction of an arrow XXI in  FIG. 20 ; 
         FIG. 22  is a schematic enlarged view showing relevant portions encircled by a dotted line XXII in  FIG. 20 ; 
         FIG. 23  is a schematic cross sectional view showing the filter device provided according to a sixth embodiment of the present disclosure; 
         FIG. 24  is a schematic top plan view when viewed in a direction of an arrow XXIV in  FIG. 23 ; 
         FIG. 25  is a schematic enlarged view showing relevant portions encircled by a dotted line XXV in  FIG. 23 ; and 
         FIG. 26  is a schematic view showing one of conditions for the filter device provided in the fuel tank filled with fuel, according to a comparison example. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure will be explained hereinafter by way of multiple embodiments. The same reference numerals are given to the same or similar portions and/or structures throughout the embodiments, for the purpose of eliminating repeated explanation. 
     First Embodiment 
     A filter device  1  according to a first embodiment of the present disclosure will be explained with reference to  FIGS. 1 to 9 . The filter device  1  is provided in a fuel tank  2  ( FIG. 6 ) for catching and trapping extraneous material contained in fuel, which is sucked into a fuel pump  3  from the fuel tank  2 . As shown in  FIGS. 1 and 2 , the filter device  1  is composed of a sub-tank  10 , an upper-side filter element  40 , a lower-side filter element  41 , an umbrella-type valve  50  working as a check valve, an upper-side fixing member  60 , a lower-side fixing member  70 , a connecting pipe  80  and so on. 
     The sub-tank  10  is made of, for example, resin and formed in a tub shape. The sub-tank  10  has an outer tubular portion  11 , a bottom wall  12 , multiple upper-side protectors  13 , multiple lower-side protectors  14 , a labyrinth wall  15 , a natural-flow port  20  ( FIG. 4 ) and so on. In  FIGS. 3 and 4 , only the sub-tank  10  is shown for the purpose of simplicity. 
     The sub-tank  10  has the bottom wall  12  of a circular shape at an axial lower end of the outer tubular portion  11  of a cylindrical shape. The bottom wall  12  has communication holes  21  in its thickness direction. A leg portion  22  is formed at a lower side of the bottom wall  12 . The leg portion  22  is placed on a bottom of the fuel tank  2 . 
     Each of the upper-side protectors  13  is formed in a flat plate shape and extends from the upper side of the bottom wall  12  toward the upper-side filter element  40 . In a similar manner, each of the lower-side protectors  14  is formed in a flat plate shape and extends from the lower side of the bottom wall  12  toward the lower-side filter element  41 . The multiple upper-side protectors  13  as well as the multiple lower-side protectors  14  are arranged in a radial fashion ( FIG. 4 ). A space  40   a  (an upper-side filter space) is formed by the upper-side protectors  13  between the bottom wall  12  and the upper-side filter element  40 , while another space  41   a  (a lower-side filter space) is formed by the lower-side protectors  14  between the bottom wall  12  and the lower-side filter element  41 . Each of the upper-side filter space  40   a  and the lower-side filter space  41   a  has such a size so as to be able to keep the fuel therein by oil films formed on a surface of the filter element by surface tension of the fuel. 
     The labyrinth wall  15  is composed of an inner tubular portion  16  and a blocking wall portion  17  ( FIG. 2 ). 
     The inner tubular portion  16  is provided inside of the outer tubular portion  11  and formed in a C-letter shape when viewed in an axial direction. The inner tubular portion  16  and the outer tubular portion  11  are coaxially formed with each other. A labyrinth passage  18  is formed between the inner tubular portion  16  and the outer tubular portion  11 . 
     The blocking wall portion  17  connects one circumferential end of the inner tubular portion  16  and one inner wall portion of the outer tubular portion  11 . The blocking wall portion  17  closes one end of the labyrinth passage  18  so as to block fuel flow through the labyrinth passage  18  in its circumferential direction. 
     An opening portion  19  is formed between one circumferential end and the other circumferential end of the inner tubular portion  16  for communicating the labyrinth passage  18  to an inside space surrounded by the inner tubular portion  16 . 
     The natural-flow port  20  is formed in the outer tubular portion  11  at such a portion, which is on an opposite side of the opening portion  19  across the blocking wall portion  17 . The natural-flow port  20  is an opening for communicating the labyrinth passage  18  to the outside of the sub-tank  10 . The natural-flow port  20  may be formed in the bottom wall  12 . 
     According to the above structure, it is possible to suppress flow-out of the fuel from an inside of the sub-tank  10  to an outside of the sub-tank  10  through the natural-flow port  20 , when the sub-tank  10  is inclined with respect to a horizontal line. 
     The upper-side filter element  40  is fixed to the sub-tank  10  at an upper side of the bottom wall  12  and traps extraneous material contained in the fuel, which is sucked into the fuel pump  3  from the inside of the sub-tank  10 . In a similar manner, the lower-side filter element  41  is fixed to the sub-tank  10  at a lower side of the bottom wall  12  and traps extraneous material contained in the fuel, which is sucked into the fuel pump  3  from the outside of the sub-tank  10  through the communication holes  21  formed in the bottom wall  12 . The upper-side and the lower-side filter elements  40  and  41  are made of, for example, bonded textile. When the filter elements  40  and  41  are immersed into the fuel, oil film is formed on each surface of the respective filter elements  40  and  41  due to the surface tension of the fuel. 
     The upper-side filter element  40  is preferably arranged in the fuel tank  2  at such a position lower than an E-point (that is, a position at which an empty alarm lamp is emitted), which is detected by a fuel gauge (not shown). 
     As shown in  FIG. 5 , the sub-tank  10  has an upper-side annular projection  23 , which is formed in the bottom wall  12  at a radial-outward peripheral side of the upper-side protectors  13 . The upper-side annular projection  23  is formed in a cylindrical shape and extends from the upper side of the bottom wall  12  in an axial upward direction. The upper-side fixing member  60  is formed in a ring shape and has an upper-side annular groove  61  at a position corresponding to the upper-side annular projection  23 . The upper-side fixing member  60  is fitted to an upper-side annular groove  24  formed at an inner wall of the inner tubular portion  16 . 
     An outer peripheral portion  401  of the upper-side filter element  40  is formed in a U-letter shape in its cross section. The outer peripheral portion  401  of the U-letter shape is interposed between the upper-side annular groove  61  of the upper-side fixing member  60  and the upper-side annular projection  23 . As a result, the upper-side filter element  40  is firmly fixed to the sub-tank  10  at the upper side of the bottom wall  12 . Since the outer peripheral portion  401  is compression-sealed, it is possible to prevent the extraneous material contained in the fuel from passing through the upper-side filter element  40  from the outside into the inside of the upper-side filter space  40   a  formed between the upper-side filter element  40  and the bottom wall  12 . 
     In a similar manner, the sub-tank  10  has a lower-side annular projection  25 , which is formed in the bottom wall  12  at a radial-outward peripheral side of the lower-side protectors  14 . The lower-side annular projection  25  is formed in a cylindrical shape and extends from the lower side of the bottom wall  12  in an axial downward direction. The lower-side fixing member  70  is formed in a ring shape and has a lower-side annular groove  71  at a position corresponding to the lower-side annular projection  25 . The lower-side fixing member  70  is fitted to a lower-side annular groove  27  formed at an inner wall of a guide wall portion  26 , which is formed in a cylindrical shape and extends from the bottom wall  12  in the axial downward direction. 
     An outer peripheral portion  411  of the lower-side filter element  41  is formed in a U-letter shape in its cross section. The outer peripheral portion  411  of the U-letter shape is interposed between the lower-side annular groove  71  of the lower-side fixing member  70  and the lower-side annular projection  25 . As a result, the lower-side filter element  41  is firmly fixed to the sub-tank  10  at the lower side of the bottom wall  12 . Since the outer peripheral portion  411  is compression-sealed, it is possible to prevent the extraneous material contained in the fuel from passing through the lower-side filter element  41  from the outside into the inside of the lower-side filter space  41   a  formed between the lower-side filter element  41  and the bottom wall  12 . 
     The upper-side annular projection  23  and the lower-side annular projection  25  are respectively or collectively referred to as an annular projection. The upper-side annular groove  61  and the lower-side annular groove  71  are respectively or collectively referred to as an annular groove. The upper-side fixing member  60  and the lower-side fixing member  70  are respectively or collectively referred to as a fixing member or an annular fixing member. 
     The umbrella-type valve (the check valve)  50  is provided at the communication holes  21  of the bottom wall  12 . The umbrella-type valve  50  has a shaft portion  51  and an umbrella portion  52 . The shaft portion  51  is fitted to the bottom wall  12 . 
     The umbrella portion  52  opens the communication holes  21  when the fuel pressure in the lower-side filter space  41   a  between the bottom wall  12  and the lower-side filter element  41  is higher than the fuel pressure in the upper-side filter space  40   a  between the bottom wall  12  and the upper-side filter element  40 , so that the fuel is allowed to flow from the lower-side filter space  41   a  to the upper-side filter space  40   a  through the communication holes  21 . On the other hand, the umbrella portion  52  closes the communication holes  21  when the fuel pressure in the lower-side filter space  41   a  is lower than that in the upper-side filter space  40   a , so as to block the fuel flow from the upper-side filter space  40   a  to the lower-side filter space  41   a.    
     An inlet port  4  of the fuel pump  3  is connected to the upper-side filter element  40  via the connecting pipe  80 . The fuel in the upper-side filter space  40   a  as well as the fuel in the lower-side filter space  41   a  is sucked into the inlet port  4  of the fuel pump  3  via the connecting pipe  80 . 
     It is possible to change a ratio of an upper-side flow rate and a lower-side flow rate, when the positions and inner diameters of the communication holes  21  are changed. The upper-side flow rate is a flow rate of the fuel, which is sucked into the fuel pump  3  from the inside of the sub-tank  10 . The lower-side flow rate is a flow rate of the fuel, which is sucked into the fuel pump  3  from the outside of the sub-tank  10  (that is, a lower side of the sub-tank  10 ). When the communication holes  21  are located at positions closer to the connecting pipe  80  and the inner diameter of the respective communication holes  21  is made larger, the lower-side flow rate becomes larger than the upper-side flow rate. 
     An operation of the filter device  1  will be explained with reference to  FIGS. 6 to 9 . The fuel in the fuel tank  2  is indicated by one-dot-chain lines in  FIGS. 6 to 9 . 
     As shown in  FIG. 6 , when the fuel is filled into the fuel tank  2 , the fuel flows into the inside of the sub-tank  10  through the natural-flow port  20  and the labyrinth passage  18 . When the fuel pump  3  is operated in this condition, the fuel in the sub-tank  10  is sucked into the inlet port  4  of the fuel pump  3  through the upper-side filter element  40  and the connecting pipe  80 . In addition, the fuel in the fuel tank  2  outside of the sub-tank  10  is sucked into the inlet port  4  of the fuel pump  3  through the lower-side filter element  41  and the connecting pipe  80 . The fuel discharged from the fuel pump  3  is supplied to an engine (not shown). The fuel (the return fuel), which is not consumed in the engine, returns to the inside of the sub-tank  10  through a return pipe (not shown). 
     As shown in  FIG. 7 , when the lower-side flow rate is larger than the upper-side flow rate, the amount of the fuel inside of the sub-tank  10  becomes larger than that outside of the sub-tank  10 . More exactly, the liquid level of the fuel in the inside of the sub-tank  10  becomes higher than that in the outside of the sub-tank  10 . Even when the lower-side flow rate is smaller than the upper-side flow rate, the amount of the fuel inside of the sub-tank  10  also becomes larger than that outside of the sub-tank  10  (the liquid level in the sub-tank  10  becomes higher than that in the outside of the sub-tank  10 ), if the amount of the return fuel is larger than a difference between the upper-side flow rate and the lower-side flow rate. An inner diameter of the natural-flow port  20  is made to be such a value that a flow rate for the natural-flow port  20  is smaller than a flow rate for increasing the fuel amount in the sub-tank  10 . 
     When the operation of the fuel pump  3  is stopped in the condition that the fuel tank  2  is in a horizontal position, the fuel flows out through the labyrinth passage  18  and the natural-flow port  20 , so that the liquid level of the fuel in the sub-tank  10  and the liquid level of the fuel outside the sub-tank  10  become equal to each other. 
     As shown in  FIG. 8 , during the operation of the fuel pump  3  and at a time point when the operation of the fuel pump  3  is stopped in the condition that the fuel tank  2  is inclined with respect to the horizontal line, the fuel amount inside the sub-tank  10  is larger than that outside the sub-tank  10  (the liquid level in the sub-tank  10  is higher than that outside the sub-tank  10 ) by the return fuel because of the labyrinth wall  15  and the umbrella-type valve  50 . 
     As shown in  FIG. 9 , during the operation of the fuel pump  3  and at the time point when the operation of the fuel pump  3  is stopped, the fuel is reserved in a fuel reserving space of the inside of the sub-tank  10  by the labyrinth wall  15  and the umbrella-type valve  50  even in a condition that the fuel amount in the fuel tank  2  becomes smaller and the fuel tank  2  is inclined with respect to the horizontal line. The fuel reserving space corresponds to such a space formed in the inside of the sub-tank  10  above the upper-side filter space  40   a . In other words, the inside of the sub-tank  10  includes the upper-side filter space  40   a  and the fuel reserving space. 
     In the condition of  FIG. 9 , the oil film is formed on the surface of the upper-side filter element  40  due to the surface tension of the fuel inside the sub-tank  10 . The oil film is also formed on the surface of the lower-side filter element  41 , so long as the lower-side filter element  41  is immersed into the fuel in the fuel tank  2  (outside the sub-tank  10 ). The space between the upper-side filter element  40  and the lower-side filter element  41  is partitioned by the bottom wall  12  and the umbrella-type valve  50  (namely, divided into the upper-side and the lower-side filter spaces  40   a  and  41   a ). As already explained above, each of the upper-side filter space  40   a  and the lower-side filter space  41   a  has such a size so as to be able to keep the fuel therein by oil films formed on a surface of the filter element by surface tension of the fuel. Therefore, the air may not enter the upper-side and the lower-side filter spaces  40   a  and  41   a  during the operation of the fuel pump  3  and even after the operation stop of the fuel pump  3 . Accordingly, the fuel can be also reserved in the upper-side and the lower-side filter spaces  40   a  and  41   a , in addition to a portion of the fuel reserving space (which is indicated by a reference  40   b  and hereinafter referred to as a fuel holding portion  40   b ). As a result, the fuel pump  3  can suck the fuel from the inside of the sub-tank  10  and from the outside of the sub-tank  10 , when the fuel pump  3  is operated. 
     The above explained first embodiment has the following advantages: 
     (1) In the first embodiment, the upper-side filter element  40  is provided at the upper side of the bottom wall  12 , while the lower-side filter element  41  is provided at the lower side of the bottom wall  12 . The sub-tank  10  has the labyrinth wall  15 . The umbrella-type valve  50  is provided at the communication holes  21  formed in the bottom wall  12 . According to the above structure, the fuel can be reserved in the inside of the sub-tank  10  during the operation of the fuel pump  3  and when the operation of the fuel pump  3  is stopped, even in the case that the fuel amount in the fuel tank  2  becomes smaller and the fuel tank  2  is inclined with respect to the horizontal line. As a result, it is possible for the fuel pump  3  to suck the fuel from the inside and the outside of the sub-tank  10 . 
     Even when the lower-side filter element  41  is no longer immersed into the fuel outside of the sub-tank  10  (in the fuel tank  2 ), the fuel is still held in the inside of the sub-tank  10  (in the upper-side filter space  40   a  and the fuel holding portion  40   b ) so that the oil film in the upper-side filter element  40  is not broken. In this case, the fuel pump  3  sucks the fuel from the inside of the sub-tank  10 . 
     (2) In the first embodiment, the upper-side filter space  40   a  is formed between the bottom wall  12  and the upper-side filter element  40  by the multiple upper-side protectors  13  extending from the bottom wall  12  in the upward direction. In the similar manner, the lower-side filter space  41   a  is formed between the bottom wall  12  and the lower-side filter element  41  by the multiple lower-side protectors  14  extending from the bottom wall  12  in the downward direction. According to such a structure, each of the upper-side and the lower-side filter elements  40  and  41  is prevented from being stuck to the bottom wall  12 . It is, thereby, possible to ensure the necessary flow amount of the fuel to be sucked into the fuel pump  3 . 
     In addition, the upper-side and the lower-side protectors  13  and  14  are integrally formed with the sub-tank  10  so as to reduce a number of working processes and thereby the manufacturing cost for the filter device  1 . 
     (3) In the first embodiment, the filter device  1  has the umbrella-type valve (the check valve)  50 . It is, thereby, possible to prevent the fuel in the inside of the sub-tank  10  from flowing out to the outside of the sub-tank  10  through the communication holes  21 , when the operation of the fuel pump  3  is stopped. 
     In addition, it is possible to prevent the fuel from flowing out from the upper-side and the lower-side filter spaces  40   a  and  41   a  to the outside of the sub-tank  10 , when the operation of the fuel pump  3  is stopped. 
     (4) In the first embodiment, the sub-tank  10  has the labyrinth wall  15 . It is, thereby, possible to suppress the flow-out of the fuel from the inside to the outside of the sub-tank  10 , when the fuel tank  2  is inclined with respect to the horizontal line. 
     (5) In the first embodiment, the outer peripheral portion  401  of the upper-side filter element  40  is fixed to the bottom wall  12  by the upper-side fixing member  60 , while the outer peripheral portion  411  of the lower-side filter element  41  is fixed to the bottom wall  12  by the lower-side fixing member  70 . As above, the upper-side and the lower-side filter elements  40  and  41  can be fixed to the bottom wall  12  in a simple manner, without using adhesion or bonding process. 
     COMPARISON EXAMPLE 
     A filter device according to a comparison example will be explained with reference to  FIG. 26 . 
     A filter device  100  of the comparison example has a first filter  101  of a bag shape provided inside of a sub-tank  110  and a second filter  102  of a bag shape provided outside of the sub-tank  110 . An inlet port of a fuel pump  105  is connected to the first filter  101  through a first connecting pipe  103 . The second filter  102  is connected to the first connecting pipe  103  via a second connecting pipe  104 . 
     According to the filter device  100  of the comparison example, the fuel pump  105  cannot suck the fuel when either one of the first and second filters  101  and  102  is no longer immersed into the fuel, because the oil film is broken on a filter element of such filter. In the condition shown in  FIG. 26 , since the second filter  102  is not immersed into the fuel (namely, since the oil film is no longer formed in the filter element of the second filter  102 ), the fuel pump  105  cannot suck the fuel even from the first filter  101 . 
     In another case that the fuel is not reserved in the sub-tank  110  and thereby the first filter  101  is not immersed into the fuel in the sub-tank  110 , the fuel pump  3  cannot suck the fuel, either, even when the second filter  102  is immersed into the fuel in the fuel tank  2 . 
     Second Embodiment 
     A filter device according to a second embodiment will be explained with reference to  FIGS. 10 to 14 . 
     In the filter device of the second embodiment, a valve corresponding to the umbrella-type valve  50  of the first embodiment is not provided. 
     As shown in  FIG. 10 , when the fuel is filled into the fuel tank  2 , the fuel flows into the inside of the sub-tank  10  through the communication hole  21 , the natural-flow port  20  and the labyrinth passage  18 . The liquid level of the fuel inside of the sub-tank  10  and the liquid level of the fuel outside of the sub-tank  10  become equal to each other. 
     When the fuel pump  3  is operated in the above condition, the liquid level of the fuel inside of the sub-tank  10  becomes higher than that outside the sub-tank  10 , as in the same manner to the first embodiment. 
     As shown in  FIG. 11 , when the fuel amount in the fuel tank  2  is reduced and the operation of the fuel pump  3  is stopped, the fuel flows out from the inside of the sub-tank  10  to the outside of the sub-tank  10  through the communication hole  21  and the natural-flow port  20 . As a result, the liquid level of the fuel inside of the sub-tank  10  and the liquid level of the fuel outside of the sub-tank  10  become equal to each other. The oil film is formed on the surface of the upper-side filter element  40  by the surface tension of the fuel in the upper-side filter space  40   a . The lower-side filter element  41  is immersed into the fuel in the fuel tank  2 . 
     When the fuel pump  3  is operated in this condition, the air in the upper-side filter space  40   a  above the liquid level of the fuel is sucked into the fuel pump  3 , while the air is prevented from entering the upper-side filter space  40   a  from the outside of the upper-side filter element  40  because of the oil film formed on the surface of the upper-side filter element  40 . As a result, as shown in  FIG. 12 , the upper-side filter space  40   a  is filled with the fuel from the fuel tank  2  (from the outside of the upper-side and lower-side filter spaces  40   a  and  41   a ). As above, the fuel pump  3  sucks the fuel from the inside and outside of the sub-tank  10 . After the operation of the fuel pump  3 , the fuel amount in the sub-tank  10  is increased because of the return fuel. 
     As shown in  FIG. 13 , when the fuel amount in the fuel tank  2  is reduced and the operation of the fuel pump  3  is stopped in a condition that the fuel tank  2  is inclined, the fuel flows out from the inside of the sub-tank  10  to the outside of the sub-tank  10  through the communication hole  21  and the natural-flow port  20 . A part of the fuel, however, is maintained in the inside of the sub-tank  10  (in the fuel holding portion  40   b ) by the labyrinth wall  15  and a portion of the bottom wall  12 , which is located below the communication hole  21  in the vertical direction in the inclined condition. Therefore, the oil film is formed on the surface of the upper-side filter element  40  by the surface tension of the fuel maintained in the inside of the sub-tank  10  (in the fuel holding portion  40   b ). The oil film is also formed on the surface of the lower-side filter element  41  by the surface tension of the fuel in the fuel tank  2  (the fuel outside of the sub-tank  10 ), so long as the lower-side filter element  41  is immersed into the fuel in the fuel tank  2 . 
     When the fuel pump  3  is operated in this condition, the air in the upper-side filter space  40   a  and the lower-side filter space  41   a  is sucked into the fuel pump  3 , while the air is prevented from entering the upper-side filter space  40   a  from the outside of the upper-side and the lower-side filter elements  40  and  41 , because of the oil films formed on the respective surfaces of the upper-side and the lower-side filter elements  40  and  41 . As a result, as shown in  FIG. 14 , the upper-side and the lower-side filter spaces  40   a  and  41   a  are filled with the fuel from the fuel tank  2  (from the outside of the upper-side and lower-side filter spaces). As above, the fuel pump  3  can suck the fuel from the inside and outside of the sub-tank  10 . 
     The second embodiment has the following advantages: 
     (1) According to the second embodiment, it is possible to reserve the part of the fuel in the fuel holding portion  40   b  of the inside of the sub-tank  10  surrounded by the labyrinth wall  15  and the portion of the bottom wall  12  below the communication hole  21 , even when the fuel amount is reduced and the operation of the fuel pump  3  is stopped in the condition that the fuel tank  2  is inclined with respect to the horizontal line. The oil films are, therefore, formed on the surfaces of the upper-side and the lower-side filter elements  40  and  41 . When the fuel pump  3  is operated in such a situation, the upper-side and the lower-side filter spaces  40   a  and  41   a  become negative pressure, so that the fuel pump  3  sucks the fuel from the inside and the outside of the sub-tank  10  to supply the fuel to the engine. 
     As explained above, in the filter device  100  of the comparison example shown in  FIG. 26 , the fuel pump  105  cannot suck the fuel when either one of the filters  101  and  102  is not immersed into the fuel. For example, when the fuel is not reserved in the sub-tank  110  and the first filter  101  is not immersed into the fuel, the fuel pump  105  cannot suck the fuel. 
     According to the second embodiment, however, it is possible to keep the part of the fuel in the fuel holding portion of the inside of the sub-tank  10 , so that the fuel pump  3  can suck the fuel from the inside and the outside of the sub-tank  10 . 
     (2) According to the second embodiment, the umbrella-type valve  50  is not provided. Since suction resistance is smaller in the second embodiment, it is easily possible to make the lower-side flow-rate larger than the upper-side flow-rate. 
     Third Embodiment 
     A filter device according to a third embodiment will be explained with reference to  FIGS. 15 to 17 . 
     In the third embodiment, a structure corresponding to the labyrinth wall  15  of the first embodiment is not provided. The other structure of the third embodiment is the same to that of the first embodiment. 
     As shown in  FIG. 15 , when the fuel is filled into the fuel tank  2 , the fuel flows into the inside of the sub-tank  10  through the natural-flow port  20 . The liquid level of the fuel inside of the sub-tank  10  and the liquid level of the fuel outside of the sub-tank  10  become equal to each other. 
     As shown in  FIG. 16 , when the fuel amount in the fuel tank  2  is reduced and the operation of the fuel pump  3  is stopped, the fuel flows out from the inside of the sub-tank  10  (except for the upper-side filter space  40   a ) to the outside of the sub-tank  10  through the natural-flow port  20 . In this situation, the oil film is formed on the surface of the upper-side filter element  40  by the surface tension of the fuel in the inside of the sub-tank  10  (that is, in the upper-side filter space  40   a ). The lower-side filter element  41  is immersed into the fuel in the fuel tank  2 . The upper-side filter space  40   a  and the lower-side filter space  41   a  are partitioned by the bottom wall  12  and the umbrella-type valve  50 . The air is not allowed to enter the upper-side or the lower-side filter space  40   a  or  41   a  and the fuel is reserved in the upper-side and the lower-side filter spaces  40   a  and  41   a  between the upper-side and the lower-side filter elements  40  and  41 . 
     When the fuel pump  3  is operated in this condition ( FIG. 16 ), the fuel pump  3  can suck the fuel from the inside and the outside of the sub-tank  10 . The fuel amount is increased after the start of the operation of the fuel pump  3  because of the return fuel. 
     As shown in  FIG. 17 , when the fuel amount in the fuel tank  2  is reduced and the operation of the fuel pump  3  is stopped in a condition that the fuel tank  2  is inclined, the fuel flows out from the inside of the sub-tank  10  (except for the upper-side filter space  40   a ) to the outside of the sub-tank  10  through the natural-flow port  20 . In this situation, since the oil films are formed on the surfaces of the upper-side and the lower-side filter elements  40  and  41 , the fuel can be reserved in the upper-side and the lower-side filter spaces  40   a  and  41   a.    
     When the fuel pump  3  is operated in this condition ( FIG. 17 ), the fuel pump  3  can suck the fuel from the inside and the outside of the sub-tank  10 . The fuel amount is increased thereafter in the inside of the sub-tank  10  because of the return fuel. 
     In the third embodiment, since the filter device  1  has the umbrella-type valve  50 , it is possible to prevent the fuel from flowing out from the inside of the sub-tank  10  (more exactly, from the upper-side filter space  40   a ) to the outside of the sub-tank  10  through the communication hole  21 , when the operation of the fuel pump  3  is stopped. It is, therefore, possible to prevent the fuel from flowing out from the upper-side and the lower-side filter spaces  40   a  and  41   a  to the outside of the sub-tank  10 . 
     Fourth Embodiment 
     A filter device according to a fourth embodiment will be explained with reference to  FIGS. 18 and 19 . 
     According to the fourth embodiment, the outer peripheral portion  401  of the upper-side filter element  40  is fixed by welding to an upper surface of the upper-side annular projection  23 , which extends from the bottom wall  12  in the upward direction. In a similar manner, the outer peripheral portion  411  of the lower-side filter element  41  is fixed by welding to a lower surface of the lower-side annular projection  25 , which extends from the bottom wall  12  in the downward direction. 
     A welding process is carried out by well-known various kinds of welding methods, such as a heat-plate welding process, a high-frequency welding process and so on. 
     According to the fourth embodiment, it is possible to fix the upper-side and the lower-side filter elements  40  and  41  to the bottom wall  12  of the sub-tank  10  without using the fixing members  60  and  70  of the first to third embodiments. Therefore, a number of parts and components can be decreased to thereby reduce the manufacturing cost for the filter device. 
     In addition, in the fourth embodiment, each of the upper-side and the lower-side filter elements  40  and  41  is welded not to the surface of the bottom wall  12  but to the upper surface and the lower surface of the respective annular projections  23  and  25 . It is, therefore, possible to concentrate heat energy of the thermal welding on the upper surface of the upper-side annular projection  23  and the lower surface of the lower-side annular projection  25 . It is, thereby, possible to surely fix the upper-side and the lower-side filter elements  40  and  41  respectively to the upper-side annular projection  23  and the lower-side annular projection  25 . 
     Fifth Embodiment 
     A filter device according to a fifth embodiment will be explained with reference to  FIGS. 20 to 22 . 
     According to the fifth embodiment, an upper-side step portion  231  is formed at a radial inside of the upper-side annular projection  23 , wherein the upper-side step portion  231  is recessed in the downward direction. Adhesive material is applied to the upper-side step portion  231  and the outer peripheral portion  401  of the upper-side filter element  40  is stuck to the upper-side step portion  231 . The upper-side filter element  40  is thus adhered to the upper-side annular projection  23 . 
     In a similar manner, a lower-side step portion  251  is formed at a radial inside of the lower-side annular projection  25 , wherein the lower-side step portion  251  is recessed in the upward direction. Adhesive material is applied to the lower-side step portion  251  and the outer peripheral portion  411  of the lower-side filter element  41  is stuck to the lower-side step portion  251 . The lower-side filter element  41  is thus adhered to the lower-side annular projection  25 . 
     During a step for applying the adhesive material to the lower-side step portion  251 , the sub-tank  10  may be turned upside down in order to prevent dripping of the adhesive material. 
     The upper-side step portion  231  and the lower-side step portion are respectively or collectively referred to as an annular step portions. 
     According to the fifth embodiment, since the upper-side step portion  231  is formed at the upper-side annular projection  23  and the adhesive material is applied to the upper-side step portion  231 , it is possible to hold the adhesive material on the upper-side step portion  231  by surface tension of the adhesive material. In a similar manner, the lower-side step portion  251  is formed at the lower-side annular projection  25  and the adhesive material is applied to the lower-side step portion  251 . It is possible to hold the adhesive material on the lower-side step portion  251  by surface tension of the adhesive material. As a result, it is possible to surely fix (adhere) the upper-side filter element  40  to the upper-side annular projection  23  and to fix (adhere) the lower-side filter element  41  to the lower-side annular projection  25 . 
     Sixth Embodiment 
     A filter device according to a sixth embodiment will be explained with reference to  FIGS. 23 to 25 . 
     According to the sixth embodiment, the upper-side step portion  231  is formed at the upper-side annular projection  23 , as in the same manner to the fifth embodiment ( FIGS. 20 to 22 ). In addition, an upper-side annular claw portion  232  is formed at the upper-side annular projection  23 , wherein the upper-side annular claw portion  232  extends from the upper-side step portion  231  in the upward direction before being bent in a radial inward direction. 
     The upper-side filter element  40  is fixed to the upper-side annular projection  23  in the following manner. The outer peripheral portion  401  of the upper-side filter element  40  is positioned on the upper-side step portion  231 . The upper-side annular claw portion  232  is heated and bent in the radial inward direction, so that the outer peripheral portion  401  of the upper-side filter element  40  is interposed between the upper-side step portion  231  and the upper-side annular claw portion  232 . As a result, the outer peripheral portion  401  of the upper-side filter element  40  is compression-sealed by thermal caulking between the step portion  231  and the annular claw portion  232 . 
     In a similar manner, the lower-side step portion  251  is formed at the lower-side annular projection  25 . In addition, a lower-side annular claw portion  252  is formed at the lower-side annular projection  25 , wherein the lower-side annular claw portion  252  extends from the lower-side step portion  251  in the downward direction before being bent in the radial inward direction. 
     The lower-side filter element  41  is fixed to the lower-side annular projection  25  in the following manner. The outer peripheral portion  411  of the lower-side filter element  41  is positioned on the lower-side step portion  251 . The lower-side annular claw portion  252  is heated and bent in the radial inward direction, so that the outer peripheral portion  411  of the lower-side filter element  41  is interposed between the lower-side step portion  251  and the lower-side annular claw portion  252 . As a result, the outer peripheral portion  411  of the lower-side filter element  41  is compression-sealed by thermal caulking between the step portion  251  and the annular claw portion  252 . 
     During the above process for fixing the lower-side filter element  41  to the sub-tank  10 , the sub-tank  10  may be turned upside down in order to prevent the lower-side filter element  41  from falling down from the lower-side annular projection  25 . 
     The upper-side annular claw portion  232  and the lower-side annular claw portion  252  are respectively or collectively referred to simply as a claw portion. 
     In the sixth embodiment, the outer peripheral portions  401  and  411  of the upper-side and the lower-side filter elements  40  and  41  are respectively fixed to the upper-side and the lower-side annular projections  23  and  25  by thermal caulking. It is not necessary to use adhesive material and therefore the manufacturing cost can be reduced. 
     In addition, since the outer peripheral portions  401  and  411  of the upper-side and the lower-side filter elements  40  and  41  are respectively compression-sealed, it is possible to surely fix the upper-side and the lower-side filter elements  40  and  41  to the upper-side and the lower-side annular projections  23  and  25 , respectively. As a result, it is possible to surely trap the extraneous material contained in the fuel. 
     Further Embodiments and/or Modifications 
     (1) In the above embodiments, each of the upper-side and the lower-side filter elements is made of bonded textile. However, the filter element may be made of fabric cloth, porous resin, resin or metal of a net-like fashion and so on. 
     (2) In the above first to third embodiments, the upper-side annular projection  23  is formed on the upper side of the bottom wall  12  and the upper-side annular groove  61  is formed in the upper-side fixing member  60 . In the same manner, the lower-side annular projection  25  is formed on the lower side of the bottom wall  12  and the lower-side annular groove  71  is formed in the lower-side fixing member  70 . 
     However, it may be modified in the following manner: An annular groove (recessed in the downward direction) is formed on the upper side of the bottom wall and an annular projection (corresponding to the annular groove and projecting in the downward direction) is formed in the upper-side fixing member. In addition, an annular groove (recessed in the upward direction) is formed on the lower side of the bottom wall and an annular projection (corresponding to the annular groove and projecting in the upward direction) is formed in the lower-side fixing member. 
     (3) In the above fourth embodiment, the upper-side filter element is welded to the upper surface of the upper-side annular projection and the lower-side filter element is welded to the lower surface of the lower-side annular projection. 
     However, it may be modified in the following manner: The upper-side filter element is directly welded to the upper surface of the bottom wall of the sub-tank, while the lower-side filter element is directly welded to the lower surface of the bottom wall of the sub-tank. 
     The present disclosure should not be limited to the above embodiments and/or modifications, but can be modified in various manners without departing from the spirit of the present disclosure.