Abstract:
The fuel tank structure includes: a fuel tank that is configured to contain fuel inside; a liquid level detection sensor arranged in a vertical orientation inside the fuel tank and configured such that a capacitance of the liquid level detection sensor varies on the basis of a contact range in which the fuel is in contact with the liquid level detection sensor; a tubular element extending vertically while laterally surrounding the liquid level detection sensor and configured to allow the fuel to enter from a lower portion of the tubular element to an inside of the tubular element and to exit from the inside to the lower portion; and a fuel storage member that communicates with the inside of the tubular element and the inside of the fuel tank through a fuel input/output port and configured to store the fuel inside the fuel tank.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a fuel tank structure. 
         [0003]    2. Description of Related Art 
         [0004]    A fuel tank for an automobile is desired to accurately detect the liquid level of fuel contained. For example, Japanese Patent Application Publication No. 2-087022 (JP 2-087022 A) describes a liquid level measuring device that includes first to third tubular elements extending through the top and bottom of a sub-tank and that forms a measuring electrode portion and a reference electrode portion from these tubular elements. In the liquid level measuring device, the reference electrode portion is filled with fuel in the sub-tank, and a liquid level is detected by the measuring electrode portion that communicates with a main tank. 
         [0005]    Incidentally, a fuel having a different capacitance property (for example, a fuel having a different mixture ratio of gasoline and ethanol) may be fed into a fuel tank. In a structure for detecting a liquid level on the basis of the capacitance of a capacitance sensor, when such a fuel having a different capacitance property contacts the capacitance sensor, it may be difficult to accurately detect the liquid level. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides a fuel tank structure that is able to reduce an error of liquid level detection even when a fuel having a different capacitance property is fed. 
         [0007]    An aspect of the invention provides a fuel tank structure. The fuel tank structure includes: a fuel tank that is configured to contain fuel inside; a liquid level detection sensor arranged in a vertical orientation inside the fuel tank and configured such that a capacitance of the liquid level detection sensor varies on the basis of a contact range in which the fuel is in contact with the liquid level detection sensor; a tubular element extending vertically while laterally surrounding the liquid level detection sensor and configured to allow the fuel to enter from a lower portion of the tubular element to an inside of the tubular element and to exit from the inside to the lower portion; and a fuel storage member that communicates with the inside of the tubular element and the inside of the fuel tank through a fuel input/output port and configured to store the fuel inside the fuel tank. 
         [0008]    With this fuel tank structure, it is possible to detect the liquid level of the fuel inside the fuel tank from the capacitance of the liquid level detection sensor. 
         [0009]    The liquid level detection sensor is laterally surrounded by the tubular element in the vertical direction; however, the fuel is allowed to enter from the lower portion of the tubular element to the inside of the tubular element or to exit from the inside to the lower portion. Furthermore, the fuel tank structure includes the fuel storage member that communicates with the inside of the tubular element and the inside of the fuel tank, and the fuel inside the fuel tank is stored in the fuel storage member. 
         [0010]    In the case where the fuel tank is refueled, when the fed fuel flows into the fuel storage member, the fuel stored in the fuel storage member (the fuel inside the fuel tank before being fed) moves into the tubular element. Even when a fuel (hereinafter, referred to as “different-type fuel”) having a property different from that of the fuel remaining in the fuel tank is fed, rapid introduction of the fuel having a different property into the tubular element is suppressed. Therefore, it is possible to reduce an error of the liquid level detected by the liquid level detection sensor. 
         [0011]    In the above aspect, a fuel storage volume of the fuel storage member may be larger than an internal volume of a portion of the inside of the tubular element, in which the liquid level detection sensor is present. 
         [0012]    In this way, the fuel storage volume of the fuel storage member is larger than the internal volume of the portion of the inside of the tubular element, in which the liquid level detection sensor is present. Thus, even when a different-type fuel is fed, it is possible to suppress contact of the different-type fuel with the liquid level detection sensor as a whole. 
         [0013]    In the above aspect, the fuel tank structure may further include a property detection sensor arranged inside the fuel tank and configured such that a capacitance of the property detection sensor varies on the basis of a property of the fuel. 
         [0014]    In the property detection sensor, the capacitance varies on the basis of the property of the fuel, so it is possible to correct the liquid level detected by the liquid level detection sensor on the basis of the detected capacitance, so further accurate liquid level detection is possible. 
         [0015]    In the above aspect, the fuel tank structure may further include a sub-cup provided inside the fuel tank and configured to contain the fuel inside the fuel tank, the property detection sensor being provided inside the sub-cup. 
         [0016]    The fuel is contained and the property detection sensor is provided inside the sub-cup, so, in comparison with a configuration without such a sub-cup, it is possible to further reliably keep a state where the fuel inside the sub-cup is in contact with the property detection sensor even in a state where a fuel liquid surface is inclined. 
         [0017]    A fuel pump for feeding the fuel to the outside may be provided inside the sub-cup. In this case, it is possible to reliably draw the fuel inside the sub-cup with the use of the fuel pump. In addition, in comparison with a structure that the property detection sensor is provided outside the sub-cup, it is possible to acquire the property of the fuel at a location close to the fuel pump. 
         [0018]    In the above aspect, the fuel tank structure may further include a fuel introduction device configured to introduce the fuel inside the sub-cup into the tubular element. 
         [0019]    After the fuel tank is refueled, by introducing the fuel inside the sub-cup into the tubular element with the use of the fuel introduction device, it is possible to bring the fuel inside the tubular element close to a uniform state, so further accurate liquid level detection is possible. 
         [0020]    In the above aspect, the fuel introduction device may include a communication portion that communicates an upper portion of the sub-cup with an upper portion of the tubular element and a pressure pump configured to feed the fuel inside the fuel tank into the sub-cup under pressure. 
         [0021]    The upper portion of the sub-cup and the upper portion of the tubular element communicate with each other via the communication portion. Therefore, when the fuel inside the fuel tank is fed into the sub-cup under pressure by the pressure pump, the entire or part of the fuel overflowed from the sub-cup flows into the tubular element through the communication portion. With a simple structure that the communication portion and the pressure pump are provided, it is possible to introduce the fuel inside the sub-cup into the tubular element. 
         [0022]    In the above aspect, the fuel storage member may extend along a periphery of the sub-cup. 
         [0023]    The fuel storage member does not excessively project outward of the sub-cup, so mounting the sub-cup on the fuel tank becomes easy. 
         [0024]    With the above configuration according to the aspect of the invention, even when a fuel having a different capacitance characteristic is fed, it is possible to reduce an error of liquid level detection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0026]      FIG. 1  is a front view that shows a fuel tank structure according to a first embodiment of the invention together with an engine and a fuel supply tube; 
           [0027]      FIG. 2  is a schematic perspective view that shows a fuel pump module that constitutes the fuel tank structure according to the first embodiment of the invention; 
           [0028]      FIG. 3  is a cross-sectional view taken along the line in  FIG. 2 , showing the fuel pump module that constitutes the fuel tank structure according to the first embodiment of the invention together with part of the fuel tank; 
           [0029]      FIG. 4  is a cross-sectional view taken along the line IV-IV in  FIG. 2 , showing the fuel pump module that constitutes the fuel tank structure according to the first embodiment of the invention; 
           [0030]      FIG. 5  is a front view that partially shows a capacitance sensor unit that is used in the fuel tank structure according to the first embodiment of the invention; 
           [0031]      FIG. 6  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state before refueling in the fuel tank structure according to the first embodiment of the invention; 
           [0032]      FIG. 7  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state immediately after refueling in the fuel tank structure according to the first embodiment of the invention; 
           [0033]      FIG. 8  is a cross-sectional structure that shows a fuel pump module that constitutes a fuel tank structure according to a comparative embodiment together with part of a fuel tank; 
           [0034]      FIG. 9  is a graph that shows a capacitance of a liquid level detection sensor after refueling the fuel tank and a capacitance ratio between the liquid level detection sensor and a property detection sensor in the case of each of the first embodiment of the invention and the comparative embodiment. 
           [0035]      FIG. 10  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state after a lapse of a predetermined period of time from refueling in the fuel tank structure according to the first embodiment of the invention; 
           [0036]      FIG. 11  is a cross-sectional view taken along the same line as  FIG. 4 , showing a state after a lapse of a predetermined period of time from refueling in the fuel tank structure according to the first embodiment of the invention. 
           [0037]      FIG. 12  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state before refueling in the fuel tank structure according to the first embodiment of the invention; 
           [0038]      FIG. 13  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state immediately after refueling in the fuel tank structure according to the first embodiment of the invention; 
           [0039]      FIG. 14  is a cross-sectional view taken along the same line as  FIG. 3 , showing a state after a lapse of a predetermined period of time from refueling in the fuel tank structure according to the first embodiment of the invention; and 
           [0040]      FIG. 15  is a front view that partially shows a capacitance sensor unit that is used in a fuel tank structure according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0041]      FIG. 1  shows a fuel tank structure  12  according to a first embodiment of the invention together with a fuel supply tube  52  for supplying fuel to an engine  20 .  FIG. 2  is a perspective view that shows a fuel pump module  22  (a sub-cup  24  and its surroundings) used in the fuel tank structure  12 . 
         [0042]    The fuel tank structure  12  includes a fuel tank  14  that is able to contain fuel inside. The fuel tank  14  has a substantially rectangular parallelepiped shape as a whole. Particularly, in the present embodiment, the volume of the fuel tank  14  is configured to be variable as a bottom wall  14 B and an upper wall  14 U approach or move away from each other. 
         [0043]    A full-tank, level HL and an alarm level LL are set for the fuel tank  14 . The full-tank level HL is a liquid level that is set such that, as the liquid level reaches the full-tank level HL when fuel is fed into the fuel tank  14 , fuel cannot be fed any more. Thus, normally, the liquid level in the fuel tank  14  does not exceed the full-tank level HL. In addition, the alarm level LL is a liquid level that is set such that, when fuel inside the fuel tank  14  is consumed, an alarm, or the like, is issued and refueling is prompted by the time when the liquid level reaches the alarm level LL. 
         [0044]    The upper wall  14 U of the fuel tank  14  has an insertion port  16 . The fuel pump module  22  is allowed to be inserted through the insertion port  16 . The insertion port  16  is closed by a lid member  18  from the outer side of the fuel tank  14 . 
         [0045]    The fuel pump module  22  arranged inside the fuel tank  14  is able to feed fuel inside the fuel tank  14  to the engine  20 . As shown in  FIG. 2  in detail, the fuel pump module  22  has the substantially cylindrical sub-cup  24  of which the upper face is open. The upper face of the sub-cup  24  is covered with a sub-cup lid  32 . 
         [0046]    One or a plurality of (two in the present embodiment) guide rods  34  extend downward from the lid member  18 , and are inserted in guide cylinders of the sub-cup  24 . Thus, even when the bottom wall  14 B and the upper wall  14 U approach or move away from each other, the position and orientation of the sub-cup  24  are kept stably. Particularly, compression coil springs are respectively mounted on the guide rods  34 , and urge the guide cylinders downward with respect to the lid member  18 . With this urging force, it is possible to keep a state where a bottom wall  24 B of the sub-cup  24  contacts the bottom wall  14 B of the fuel tank  14 . 
         [0047]    As shown in  FIG. 3 , a fuel pump  40  is provided inside the sub-cup  24 . A fuel suction port  42  is provided below the fuel pump  40 . Fuel is allowed to be drawn through the fuel suction port  42 . By driving the fuel pump  40 , fuel inside the sub-cup  24  is drawn through the fuel suction port  42 . Fuel inside the sub-cup  24  is allowed to be fed toward the engine  20  (see  FIG. 1 ) through a fuel feed tube  44 . 
         [0048]    A fuel filter  46  is attached to the fuel suction port  42  of the fuel pump  40 . The fuel filter  46  is formed in a bag shape from a mesh member, and the fuel suction port  42  is located inside the fuel filter  46 . The fuel filter  46  has the function of removing foreign matter in fuel at the time when fuel GS inside the sub-cup  24  is drawn through the fuel suction port  42 . 
         [0049]    Part of fuel inside the fuel tank  14  is stored in the sub-cup  24 . Thus, even when the fuel GS is inclined and unevenly distributed with respect to the fuel tank  14 , it is possible to inhibit a phenomenon (so-called shortage of fuel) that part of fuel stored in the sub-cup  24  separates from the fuel filter  46 . 
         [0050]    As is apparent from  FIG. 2  and  FIG. 4 , a recess  24 D formed by partially curving a peripheral wall  24 S inward is formed at the lower portion of the peripheral wall  24 S of the sub-cup  24 . A jet pump  48  is arranged in the recess  24 D. 
         [0051]    An introduction tube  54  is connected to the jet pump  48 . Part of fuel drawn by the fuel pump  40  is introduced into the jet pump  48  via the introduction tube  54  as return fuel without being delivered to the outside. A negative pressure is generated inside the jet pump  48  due to return fuel introduced from the introduction tube  54 . The jet pump  48  has the function of drawing fuel GS from the outside of the sub-cup  24  (inside of the fuel tank  14 ) through a suction port  48 B because of the negative pressure and feeding (feeding under pressure) fuel into the sub-cup  24  through a through-hole  24 H formed at the recess  24 D. 
         [0052]    As shown in  FIG. 3  and  FIG. 4 , a partition wall  24 P is provided upright from the bottom wall  24 B inside the sub-cup  24 . The partition wall  24 P surrounds the through-hole  24 H together with part of the peripheral wall  24 S, and is formed so as to be lower than the height of the peripheral wall  24 S. A temporary containing portion  24 T is formed between part of the peripheral wall  24 S and the partition wall  24 P. Fuel introduced from the jet pump  48  via the through-hole  24 H is temporarily contained in the temporary containing portion  24 T. Fuel overflowed from the temporary containing portion  24 T flows beyond the partition wall  24 P and is contained in the sub-cup  24  (region other than the temporary containing portion  24 T). Hereinafter, a simple phrase “inside the sub-cup  24 ” or “the inside of the sub-cup  24 ” means a region other than the temporary containing portion  24 T in the sub-cup  24 . 
         [0053]      FIG. 3  is a cross-sectional view taken along the line in  FIG. 2 . The line is also shown in  FIG. 4 , and indicates a cross-sectional position. 
         [0054]    As shown in  FIG. 2  and  FIG. 3 , the fuel pump module  22  includes a tubular element  38  located on the outer side of the sub-cup  24 . The tubular element  38  is formed so as to extend to a position higher than the full-tank level HL of the fuel tank  14 . In the present embodiment, as is apparent from  FIG. 4 , the tubular element  38  has a substantially rectangular shape in horizontal cross section, and is present at part of the outer periphery of the sub-cup  24  in plan view. Part of the tubular element  38  is shared with the peripheral wall  24 S of the sub-cup  24 . 
         [0055]    A fuel input/output port  56  is formed at the lower portion of the tubular element  38  (near the bottom wall  14 B). Furthermore, a fuel storage member  58  that communicates with the inside of the tubular element  38  through the fuel input/output port  56  is provided inside the fuel tank  14 . Particularly, in the present embodiment, as is apparent from  FIG. 4 , the fuel storage member  58  is formed in a substantially annular shape extending along the peripheral wall  24 S of the sub-cup  24 , and an end portion at the opposite side with respect to the fuel input/output port  56  serves as an opening  56 H that opens at the lower portion (near the bottom wall  14 B) inside the fuel tank  14 . Thus, the fuel storage member  58  communicates with both the inside of the tubular element  38  and the inside of the fuel tank  14 . 
         [0056]    The volume of the fuel storage member  58 , that is, the amount of fuel (fuel storage volume) storable in a region from the opening  56 H to the fuel input/output port  56 , is larger than or equal to the volume of a portion of the tubular element  38 , in which a liquid level detection sensor  26 L (described later) is present. 
         [0057]    Fuel inside the fuel tank  14  enters into or exits from the inside of the tubular element  38  via the fuel storage member  58  and the fuel input/output port  56 . Therefore, the liquid level in the fuel tank  14  is substantially equal to the liquid level in the tubular element  38 . 
         [0058]    The upper face of the sub-cup  24  is closed by the sub-cup lid  32 ; however, a portion of the upper face near the tubular element  38  is open, and a fuel introduction wall  62  facing the tubular element  38  extends upward so as to surround the open portion. The fuel introduction wall  62  and the tubular element  38  form a fuel introduction passage  64  therebetween. When the jet pump  48  is driven, part of fuel overflowed from the inside of the sub-cup  24  (however, outflow of fuel into the fuel tank  14  is suppressed by the sub-cup lid  32 ) passes through the fuel introduction passage  64  and flows into the tubular element  38  from above as indicated by the arrow F 1 . A fuel introduction device  60  according to the invention includes the fuel introduction passage  64  and the jet pump  48 . 
         [0059]    Furthermore, the fuel pump module  22  includes a capacitance sensor unit  26 . As shown in  FIG. 2  in detail, the capacitance sensor unit  26  includes a sensor circuit unit  26 C mounted on the upper face of the sub-cup lid  32  and a sensor element unit  26 S extending downward from the sensor circuit unit  26 C through the sub-cup lid  32 . 
         [0060]    As shown in  FIG. 5 , the sensor element unit  26 S has a base  28  that is formed in a substantially long shape as a whole from a foldable insulator, such as a resin film. The distal end of the base  28  is branched off in a bifurcated shape, and has a first base portion  28 A and a second base portion  28 B. 
         [0061]    As shown in  FIG. 2  and  FIG. 3 , the first base portion  28 A is inserted in the tubular element  38  from above, and its distal end reaches a portion near the lower portion of the tubular element  38 . The second base portion  28 B is inserted in the sub-cup  24 , and its distal end reaches a portion near the bottom wall  24 B of the sub-cup  24 . 
         [0062]    A plurality of electrodes  30  are arranged on the surface of the first base portion  28 A at set intervals in the longitudinal direction of the base  28 , thus forming the liquid level detection sensor  26 L. The highest position of the liquid level detection sensor  26 L is higher than the full-tank level HL of the fuel tank  14 . The first base portion  28 A is inserted in the tubular element  38 , so the tubular element  38  surrounds the liquid level detection sensor  26 L. 
         [0063]    A plurality of electrodes  30  are also arranged on the surface of the second base portion  28 B at set intervals in the longitudinal direction of the base  28 , thus forming a property detection sensor  26 R. However, the property detection sensor  26 R is shorter than the liquid level detection sensor  26 L, and is formed at only the distal end portion of the second base portion  28 B. The distal end of the second base portion  28 B reaches a portion near the bottom wall  24 B of the sub-cup  24 . 
         [0064]    The plurality of electrodes  30  that constitute the liquid level detection sensor  26 L and the property detection sensor  26 R have different capacitances between a portion that is in contact with fuel and a portion that is not in contact with fuel. In addition, the capacitance also varies depending on the property of fuel with which each electrode  30  is in contact. By using the difference in capacitance, it is possible to output a signal based on whether the contact range in which fuel is in contact with the capacitance sensor unit  26  is wide or narrow. 
         [0065]    An output signal from the property detection sensor  26 R and an output signal from the liquid level detection sensor  26 L are transmitted to the sensor circuit unit  26 C. Furthermore, information about a fuel property and a fuel level is transmitted to an engine control unit  70 , and fuel injection, and the like, in the engine  20  are controlled. 
         [0066]    Here, in a normal state, fuel is fed by the jet pump  48  into the sub-cup  24  such that the fuel liquid level in the sub-cup  24  reaches the upper end position of the sub-cup  24  (the inside of the sub-cup  24  is filled up). Therefore, the entire property detection sensor  26 R is immersed in fuel. The property detection sensor  26 R is able to detect the property of fuel inside the fuel tank  14  by utilizing the fact that the capacitance varies on the basis of the property of fuel with which the property detection sensor  26 R is in contact. 
         [0067]    In contrast to this, the liquid level detection sensor  26 L is arranged in a vertical orientation inside the fuel tank  14 . Therefore, the length of the portion immersed in fuel varies on the basis of the amount of fuel inside the fuel tank  14 , and the capacitance also takes a different value. It is possible to detect the amount of fuel inside the fuel tank  14  by utilizing this phenomenon. 
         [0068]    In the present embodiment, the property detection sensor  26 R and the liquid level detection sensor  26 L are formed on the single base  28 . In other words, the property detection sensor  26 R and the liquid level detection sensor  26 L are integrated to constitute the capacitance sensor unit  26 , so an increase in the number of components is suppressed. 
         [0069]    As shown in  FIG. 3 , the bottom wall  24 B of the sub-cup  24  has a fuel inflow hole  66 . Furthermore, a one-way valve  68  is provided in the fuel inflow hole  66 . The one-way valve  68  allows movement of fuel from the inside of the fuel tank  14  to the inside of the sub-cup  24 , and blocks movement of fuel in the opposite direction. For example, when the fuel tank  14  is initially refueled (the fuel tank  14  is refueled in a state where there is no fuel inside the fuel tank  14  at all), fuel inside the fuel tank  14  flows into the sub-cup  24  from the fuel inflow hole  66 , so the liquid level of fuel is equal between the fuel tank  14  and the sub-cup  24 . In contrast to this, when the liquid level in the fuel tank  14  decreases, fuel inside the sub-cup  24  does not flow out into the fuel tank  14  through the fuel inflow hole  66 . Fuel fed by driving the jet pump  48  is held inside the sub-cup  24 , so the fuel liquid level is kept at the upper end position of the sub-cup  24 . 
         [0070]    Next, the operation of the fuel tank structure  12  according to the present embodiment will be described. 
         [0071]    With this fuel tank structure  12 , it is possible to feed fuel stored in the sub-cup  24  to the engine, or the like, through the fuel feed tube  44  by driving the fuel pump  40 . 
         [0072]    Even in a state where the amount of fuel inside the fuel tank  14  is small, fuel is present inside the sub-cup  24 . Thus, even when fuel GS inclines and is unevenly distributed inside the fuel tank  14 , the fuel GS inside the sub-cup  24  is held near the fuel suction port  42 . Therefore, it is possible to inhibit a phenomenon (so-called shortage of fuel) that the fuel OS separates from the fuel filter  46  and, as a result, an oil film of the fuel filter  46  runs out. In addition, it is easy to keep a state where the fuel OS inside the sub-cup  24  is in contact with the property detection sensor  26 R. 
         [0073]    As the fuel pump  40  is driven, part of fuel is introduced into the jet pump  48  through the introduction tube  54 . Thus, the jet pump  48  is driven, so the fuel GS is fed to the temporary containing portion  24 T. Fuel overflowed from the temporary containing portion  24 T flows beyond the partition wall  24 P and is contained in the sub-cup  24  (region other than the temporary containing portion  24 T). 
         [0074]    Here, the case where the fuel tank  14  according to the present embodiment is refueled is assumed. Particularly, in the present embodiment, the case where the fuel tank  14  is refueled with a plurality of types of fuels having different specific gravities. 
         [0075]    Hereinafter, high specific gravity fuel HF having a relatively high specific gravity and low specific gravity fuel LF having a relatively low specific gravity are distinguished from each other. An example of the low specific gravity fuel LF may be gasoline (fuel not mixed with ethanol, or the like), an example of the high specific gravity fuel HF may be ethanol fuel (fuel obtained by mixing ethanol with gasoline at a predetermined ratio, fuel formed of only ethanol, or the like). 
         [0076]    Initially, a state where the high specific gravity fuel HF is present in the fuel tank  14  (see  FIG. 6 ) and a case where the fuel tank  14  is refueled with the low specific gravity fuel LF in this state (see  FIG. 7 ) will be described. 
         [0077]    When the engine  20  is driven before refueling, the fuel pump  40  is driven, and the jet pump  48  is driven by return fuel through the fuel supply tube  52 . Therefore, the high specific gravity fuel HF inside the fuel tank  14  is introduced into the sub-cup  24 . 
         [0078]    Even when the fuel pump  40  and the jet pump  48  are stopped by stopping the engine  20  in this state, a liquid level L 2  in the tubular element  38  coincides with a liquid level L 1  in the fuel tank  14 . In addition, the high specific gravity fuel HF is stored in the sub-cup  24  up to the upper end position of the partition wall  24 P. Furthermore, the high specific gravity fuel HF is stored in the fuel storage member  58 . 
         [0079]    Here, when the fuel tank  14  is refueled with the low specific gravity fuel LF, the low specific gravity fuel LF is located above the high specific gravity fuel HF immediately after refueling and two layers are temporarily formed as shown in  FIG. 7  (the high specific gravity fuel HF and the low specific gravity fuel LF are mixed with each other with time). 
         [0080]    Part of the high specific gravity fuel HF flows into the fuel storage member  58  through the opening  56 H as indicated by the arrow F 2  in  FIG. 4 , so the high specific gravity fuel HF stored in the fuel storage member  58  moves into the tubular element  38  as indicated by the arrow F 3 . Inside the tubular element  38 , the liquid level L 2  of the high specific gravity fuel HF rises, and coincides with the liquid level L 1  in the fuel tank  14 . Particularly, a fuel storage volume of the fuel storage member  58  is larger than the volume of a portion of the inside of the tubular element  38 , in which the liquid level detection sensor  26 L is present. Thus, even when the low specific gravity fuel LF is fed up to the full-tank level HL, the low specific gravity fuel LF does not flow into the tubular element  38 , and the high specific gravity fuel HF contacts all the range of the liquid level detection sensor  26 L. 
         [0081]    In addition, the state where the high specific gravity fuel HF is stored in the sub-cup  24  is kept, so the high specific gravity fuel HF is in contact with the property detection sensor  26 R. 
         [0082]    That is, with the fuel tank structure  12  according to the present embodiment, even when the low specific gravity fuel LF is fed into the fuel tank  14  in which the high specific gravity fuel HF remains, a fuel of the same type (high specific gravity fuel HF) is in contact with both the liquid level detection sensor  26 L and the property detection sensor  26 R immediately after refueling. Particularly, the entire property detection sensor  26 R is immersed in the high specific gravity fuel HF. In addition, the high specific gravity fuel. HF is in contact with part or the entire liquid level detection sensor  26 L on the basis of the liquid level L 2  in the tubular element  38 ; however, a state where the low specific gravity fuel LF is not in contact with the liquid level detection sensor  26 L is achieved. 
         [0083]    In order to actually detect the liquid level in the fuel tank  14 , initially, the property of fuel is detected by the property detection sensor  26 R. That is, the property detection sensor  26 R takes a different capacitance on the basis of the type of fuel with which the property detection sensor  26 R is in contact, so it is possible to determine whether the contact fuel is the low specific gravity fuel LF or the high specific gravity fuel HF using the capacitance (in the case of the present embodiment, it is possible to determine that the type of fuel is the high specific gravity fuel HF). 
         [0084]    Subsequently, the capacitance of the liquid level detection sensor  26 L is measured. That is, the capacitance of the liquid level detection sensor  26 L varies on the basis of the contact range in which fuel is in contact with the liquid level detection sensor  26 L, so it is possible to acquire the liquid level L 2  in the tubular element  38  and further acquire the liquid level L 1  in the fuel tank  14  from the capacitance. 
         [0085]    In the present embodiment, as described above, even when the low specific gravity fuel LF is fed into the fuel tank  14  in which the high specific gravity fuel HF remains, the high specific gravity fuel HF that is a fuel of the same type as the fuel that is in contact with the property detection sensor  26 R is in contact with the liquid level detection sensor  26 L, and contact of the low specific gravity fuel LF is inhibited. The capacitance detected by the property detection sensor  26 R is used as a reference, and the liquid level is obtained from the capacitance detected by the liquid level detection sensor  26 L. Thus, it is possible to further accurately detect the liquid level. Therefore, further accurate liquid level detection is possible. This point will be described below in more detail. 
         [0086]      FIG. 8  shows a fuel pump module  122  of a fuel tank structure  112  according to a comparative embodiment. In the comparative embodiment, the tubular element  38  and the fuel storage member  58  according to the first embodiment are not provided, and the partition wall  24 P is also not formed inside the sub-cup  24 . The liquid level detection sensor  26 L is arranged on the outer side of the peripheral wall  24 S of the sub-cup  24 . 
         [0087]    Thus, when the low specific gravity fuel LF is fed into the fuel tank  114  according to the comparative embodiment in a state where the high specific gravity fuel HF remains, because the high specific gravity fuel HF is located at a relatively low side, the specific gravity of fuel that is in contact with the liquid level detection sensor  26 L gradually becomes lower from a lower part of the contact portion toward an upper part thereof. 
         [0088]      FIG. 9  shows an example of a capacitance of the liquid level detection sensor  26 L and a value (capacitance ratio) obtained by dividing the capacitance of the liquid level detection sensor  26 L by a capacitance of the property detection sensor  26 R in the case of each of the present embodiment and the comparative embodiment. 
         [0089]    In this example, both in the present embodiment and in the comparative embodiment, an actual liquid level in the fuel tank is 40 mm for the sake of convenience of description. In addition, the capacitance of the property detection sensor  26 R with which the high specific gravity fuel HF is in contact is a constant value (5000 pF). 
         [0090]    When a uniform fuel (in the example of the graph, the high specific gravity fuel HF) is in contact with the liquid level detection sensor  26 L as in the case of the first embodiment, the capacitance of the liquid level detection sensor  26 L is directly proportional to the liquid level L 2  (contact area of the fuel GS) as indicated by the continuous line C 11 . Because the capacitance of the property detection sensor  26 R is a constant value, the capacitance ratio is directly proportional to the liquid level L 2  as indicated by the solid line C 12  in  FIG. 9 , and is on a target value indicated by the dashed line C 01 . 
         [0091]    Generally, where the area of each of two electrodes is S, the distance between the electrodes is d and the dielectric constant is ∈, the capacitance C is expressed by C=∈×(S/d). The capacitance ratio is (C 26L /C 26R ) where the capacitance of the liquid level detection sensor  26 L is C 26L  and the capacitance of the property detection sensor  26 R is C 26R . 
         [0092]    Particularly, in the graph shown in  FIG. 9 , the area S of each electrode and the distance d between the electrodes in the liquid level detection sensor  26 L and the property detection sensor  26 R are adjusted such that the capacitance ratio becomes 1 in a state where the liquid level detection sensor  26 L is immersed in fuel up to the upper end of the liquid level detection sensor  26 L (liquid level=100 mm). 
         [0093]    When the liquid level is 40 mm, the capacitance of the liquid level detection sensor  26 L is 2000 pF, so the capacitance ratio is 2000 pF/5000 pF=0.4. Because the capacitance ratio where the liquid level is 100 mm is set to 1, an actual liquid level is calculated as 100 mm×0.4=40 mm. That is, in the present embodiment, because the capacitance ratio (C 26L /C 26R ) is directly proportional to the liquid level, it is possible to easily and accurately acquire the liquid level L 2 . 
         [0094]    In contrast to this, with the fuel tank structure  112  according to the comparative embodiment, when the liquid level rises through feeding of the low specific gravity fuel LF, both the high specific gravity fuel HF and the low specific gravity fuel LF contact the liquid level detection sensor  26 L, so the capacitance of the liquid level detection sensor  26 L is not directly proportional to the liquid level in the fuel tank, and takes a value smaller than the continuous line C 11  with a rise in liquid level as indicated by the alternate long and two-short dashes line. In the comparative embodiment, the capacitance ratio also becomes smaller than an actual value as indicated by the dashed line C 32 . For example, when the liquid level is 40 mm, the capacitance of the liquid level detection sensor  26 L according to the comparative embodiment is 900 pF. When the liquid level in the fuel tank  114  is calculated using the above-described mathematical expression (1) using this capacitance, the liquid level is 18 mm, so the liquid level is calculated to be lower by 22 mm than the actual liquid level. 
         [0095]    In this way, in the present embodiment, it appears that occurrence of an error in the liquid level obtained on the basis of the capacitance of the liquid level detection sensor  26 L as in the case of the comparative embodiment is suppressed. 
         [0096]    As a predetermined period of time elapses after refueling, the high specific gravity fuel HF and the low specific gravity fuel LF mix with each other. Hereinafter, the mixed fuel is termed composite fuel MF. In the example shown in  FIG. 10 , the fed low specific gravity fuel LF mixes with the high specific gravity fuel HF present in the fuel tank  14 , and the composite fuel MF is present at the lower portion in the fuel tank  14 . 
         [0097]    As the fuel pump  40  and the jet pump  48  are driven by driving the engine  20 , the composite fuel MF inside the fuel tank  14  is fed by the jet pump  48  into the sub-cup  24  as indicated by the arrow F 4 . In this way, the composite fuel MF of which the property is uniformed contacts the property detection sensor  26 R, so the detection accuracy of the property detection sensor  26 R for the property of fuel is high. 
         [0098]    Furthermore, when the jet pump  48  is driven, the composite fuel MF flows beyond the partition wall  24 P, passes through the fuel introduction passage  64  from the inside of the sub-cup  24  and flows into the tubular element  38  from above as indicated by the arrow F 1 . Fuel in the tubular element  38  is replaced with the composite fuel MF of which the property is uniformed, and the composite fuel MF contacts the liquid level detection sensor  26 L. Fuel having the same mixture ratio contacts the upper portion and lower portion of the liquid level detection sensor  26 L, so the detection accuracy for the liquid level also increases. Fuel inside the tubular element  38  flows through the inside of the fuel storage member  58  toward the opening  56 H as indicated by the arrow F 5  in  FIG. 11 , and is returned to the inside of the fuel tank  14  through the opening  56 H as indicated by the arrow F 6 . 
         [0099]    In this case as well, the same fuel contacts the property detection sensor  26 R and the liquid level detection sensor  26 L, so the fuel property detected by the property detection sensor  26 R may be used as a reference for detecting the liquid level with the use of the liquid level detection sensor  26 L. That is, with the use of the single property detection sensor  26 R, it is possible to not only simply detect the property of fuel but also determine the reference in liquid level detection. 
         [0100]    In the above description, the case where the low specific gravity fuel LF is fed into the fuel tank  14  in which the high specific gravity fuel HF remains is illustrated. Hereinafter, on the other hand, the case (see  FIG. 13 ) in which the high specific gravity fuel HF is fed into the fuel tank  14  in which the low specific gravity fuel LF remains (see  FIG. 12 ) will be described. 
         [0101]    In this case, when the engine  20  is driven before refueling, the fuel pump  40  and the jet pump  48  are driven, so the low specific gravity fuel LF inside the fuel tank  14  is introduced into the sub-cup  24 . 
         [0102]    Even when the fuel pump  40  and the jet pump  48  are stopped by stopping the engine  20 , the liquid level L 2  in the tubular element  38  coincides with the liquid level L 1  in the fuel tank  14 . In addition, inside the sub-cup  24 , the low specific gravity fuel LF is stored up to the upper end position of the partition wall  24 P. The low specific gravity fuel LF is stored in the fuel storage member  58 . 
         [0103]    Here, when the high specific gravity fuel HF is fed into the fuel tank  14 , the high specific gravity fuel HF is located below the low specific gravity fuel LF and two layers are temporarily formed as shown in  FIG. 13  (the high specific gravity fuel HF and the low specific gravity fuel LF mix with each other with time). 
         [0104]    Part of the high specific gravity fuel HF flows into the fuel storage member  58  through the opening  56 H, so the low specific gravity fuel LF stored in the fuel storage member  58  moves into the tubular element  38 . Thus, inside the tubular element  38 , the liquid level L 2  of the low specific gravity fuel LF rises, and coincides with the liquid level L 1  in the fuel tank  14 . Even when the high specific gravity fuel HF is fed up to the full-tank level HL, the high specific gravity fuel HF does not flow into the tubular element  38 , and the low specific gravity fuel LF contacts all the range of the liquid level detection sensor  26 L. 
         [0105]    Because the state where the low specific gravity fuel LF is stored in the sub-cup  24  is kept, the low specific gravity fuel LF is in contact with the property detection sensor  26 R. 
         [0106]    That is, with the fuel tank structure  12  according to the present embodiment, even when the high specific gravity fuel HF is fed into the fuel tank  14  in which the low specific gravity fuel LF remains, a fuel of the same type (low specific gravity fuel LF) is in contact with both the liquid level detection sensor  26 L and the property detection sensor  26 R immediately after refueling. In addition, the low specific gravity fuel LF is in contact with part of or the entire liquid level detection sensor  26 L on the basis of the liquid level L 2 ; however, a state where the high specific gravity fuel HF is not in contact with the liquid level detection sensor  26 L is achieved. Therefore, further accurate liquid level detection is possible. 
         [0107]    Particularly, when the high specific gravity fuel HF is fed into the fuel tank  14  in which the low specific gravity fuel LF remains, the high specific gravity fuel HF is located at a relatively low layer, so, with a structure having no fuel storage member  58  (for example, sec the structure shown in  FIG. 8  as the comparative embodiment), there is a high possibility that the high specific gravity fuel HF contacts the liquid level detection sensor  26 L. However, in the present embodiment, it is possible to inhibit contact of the fed high specific gravity fuel HF with the liquid level detection sensor  26 L. That is, when the high specific gravity fuel HF is fed into the fuel tank  14  in which the low specific gravity fuel LF remains, the invention significantly contributes in the viewpoint of further accurate liquid level detection. 
         [0108]    After refueling (after a lapse of a predetermined period of time), as shown in  FIG. 14 , the high specific gravity fuel HF and the low specific gravity fuel LF mix with each other, and become composite fuel MF. As the fuel pump  40  and the jet pump  48  are driven by driving the engine  20 , the composite fuel MF inside the fuel tank  14  is fed by the jet pump  48  into the sub-cup  24 . The composite fuel MF of which the property is uniformed contacts the property detection sensor  26 R, so the detection accuracy of the property detection sensor  26 R for the property of fuel is high. 
         [0109]    Furthermore, when the jet pump  48  is driven, the composite fuel MF flows beyond the partition wall  24 P, passes through the fuel introduction passage  64  from the inside of the sub-cup  24  and flows into the tubular element  38  from above. Fuel inside the tubular element  38  is replaced with the composite fuel MF of which the property is uniformed, and the composite fuel MF contacts the liquid level detection sensor  26 L. Fuel having the same mixture ratio contacts the upper portion and lower portion of the liquid level detection sensor  26 L, so the detection accuracy for the liquid level also increases. 
         [0110]    In this case as well, the same fuel contacts the property detection sensor  26 R and the liquid level detection sensor  26 L, so the fuel property detected by the property detection sensor  26 R may be used as a reference for detecting the liquid level with the use of the liquid level detection sensor  26 L. That is, with the use of the single property detection sensor  26 R, it is possible to not only simply detect the property of fuel but also determine the reference in liquid level detection. 
         [0111]    In the first embodiment, the location of the property detection sensor  26 R is not limited to the inside of the sub-cup  24 ; however, when the property detection sensor  26 R is arranged inside the sub-cup  24 , it is possible to detect the property of fuel that is fed to the engine  20  by driving the fuel pump  40 . Instead, the property detection sensor  26 R may be arranged at the lower portion inside the tubular element  38 . With this arrangement, it is possible to detect the property of fuel near the liquid level detection sensor  26 L. 
         [0112]    Next, a second embodiment of the invention will be described. The second embodiment differs from the first embodiment in the structure of a capacitance sensor unit  76 ; however, the overall configuration of a fuel tank structure according to the second embodiment is the same as that of the first embodiment, so the fuel tank structure according to the second embodiment is not shown separately. 
         [0113]      FIG. 15  shows the capacitance sensor unit  76  for the fuel tank structure according to the second embodiment. The capacitance sensor unit  76  includes the base  28 , and a liquid level detection sensor  76 L substantially similar to that of the first embodiment is provided at the first base portion  28 A. A liquid level detection sensor  76 M, instead of the property detection sensor  26 R according to the first embodiment, is provided at the second base portion  28 B. 
         [0114]    The liquid level detection sensor  76 M has substantially the same height as the liquid level detection sensor  26 L. The upper end portion of the liquid level detection sensor  76 M has substantially the same or larger width than that of the liquid level detection sensor  26 L; however, the width gradually reduces downward, and has an inverted triangular shape as a whole. 
         [0115]    In the liquid level detection sensor  76 L, the capacitance and the liquid level are directly proportional to each other, and there is no difference in sensitivity due to the liquid level. In contrast to this, in the liquid level detection sensor  76 M, the sensitivity is lower at a low liquid level (when the remaining level of fuel GS is low). 
         [0116]    Even when the property of fuel GS changes, the capacitance ratio ( 76 M/ 76 L) becomes a value close to a target value (dashed line C 01 ) when compared with the capacitance ratio (see the dashed line C 32  in  FIG. 9 ) according to the comparative embodiment. In the second embodiment, the capacitance ratio ( 76 M/ 76 L) is referenced, so it is possible to detect an accurate liquid level. 
         [0117]    In the second embodiment as well, when fuel of a type different from fuel remaining in the fuel tank  14  is fed into the fuel tank  14 , fuel stored in the fuel storage member  58  moves to the tubular element  38 . Thus, fuel of the same type contacts all the range of the liquid level detection sensor  26 L. Therefore, the accuracy of liquid level detection increases. 
         [0118]    In the second embodiment, the liquid level detection sensor  76 M may be arranged inside the sub-cup  24  or may be arranged inside the tubular element  38 . 
         [0119]    In each of the above-described embodiments, the description is made on the example in which the fuel storage volume of the fuel storage member  58  is larger than the internal volume of the portion of the inside of the tubular element  38 , in which the liquid level detection sensor  26 L is present. Thus, even when all the fuel stored in the fuel storage member  58  moves into the tubular element  38 , it is possible to keep the state where fuel of the same type reliably contacts all the range of the liquid level detection sensor  26 L. 
         [0120]    The structure (shape) of the fuel storage member  58  is not limited to an annular shape in which the sub-cup  24  is surrounded as described above. For example, the structure (shape) of the fuel storage member  58  may extend radially outward in a cylindrical shape when the sub-cup  24  is viewed in plan. When the fuel storage member  58  is formed in an annular shape in which the sub-cup  24  is surrounded, a projection at the time when the sub-cup  24  is viewed in plan reduces, and the sub-cup  24  and the fuel storage member  58  are easily mounted inside the fuel tank  14 . 
         [0121]    The description is made on the example in which the sub-cup  24  is provided; however, a structure with no sub-cup  24  is applicable. In this case, the property detection sensor  26 R according to the first embodiment and the liquid level detection sensor  76 M according to the second embodiment may be arranged inside the tubular element  38  as described above. 
         [0122]    Furthermore, a structure with no property detection sensor  26 R in the first embodiment or a structure with no liquid level detection sensor  76 M in the second embodiment is applicable. That is, when fuel of a type different from the type of fuel remaining in the fuel tank  14  is fed into the fuel tank  14 , fuel stored in the fuel storage member  58  moves to the tubular element  38 , so the fuel of the same type contacts all the range of the liquid level detection sensor  26 L, and the accuracy of liquid level detection increases. 
         [0123]    In each of the above-described embodiments, a structure with no fuel introduction device  60  is applicable. That is, when the engine  20  is driven, even with a structure that fuel is not introduced from the upper portion of the tubular element  38 , it is advantageous in improving the accuracy of liquid level detection after a different-type fuel is fed. In the case where the fuel introduction device  60  is provided, when the engine  20  is driven (when the jet pump  48  is driven), it is possible to introduce the composite fuel into the tubular element  38 , so further accurate liquid level detection is possible. The fuel introduction device  60  includes the fuel introduction passage  64  and the jet pump  48 ; however, in order to introduce fuel into the tubular element  38 , a structure with the jet pump  48  is desirable. Thus, only by additionally providing the fuel introduction passage  64 , the fuel introduction device  60  may be formed. In addition, when a structure with no fuel introduction passage  64  is provided, it is possible to eventually achieve a structure with no fuel introduction device  60 . 
         [0124]    In the structure with no fuel introduction passage  64 , by also omitting the sub-cup lid  32  (or providing a fuel outflow hole), fuel overflowed from the sub-cup  24  just needs to be returned into the fuel tank  14 . 
         [0125]    The liquid level detection sensor  26 L and the property detection sensor  26 R each are a sensor having such a structure that the capacitance varies on the basis of the length of the contact portion of fuel or the property of fuel as described above; however, a sensor having such a structure that outputs a variation in amount other than capacitance as a signal is also applicable. For example, a sensor of a type that an electric resistance varies on the basis of the length of the contact portion of fuel or the property of fuel is also applicable.