JPH11278066A - Fuel tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain durability of a wall high and restrain generation of noise caused fuel flow in a fuel storage chamber, by providing a stiffening means for increasing rigidity between an antinode part and a node part of a wave deformation of a formed film wall when it deforms with flow of fuel, and by restraining deformation of the film wall. SOLUTION: A fuel storage device 1 is equipped with a housing 4 consisting of a substantially bowl shaped upside portion 2 and a downside portion 3, and in the housing a fuel tank 6 partitioning a fuel storage chamber 5 for storing fuel is arranged. A plurality of ribs 30, 31 are provided in a region surrounding an upper wall 7 and a lower wall 8 of this fuel tank 6, number of the ribs 30, 31 is determined considering degree of bend deformation of the upper wall 7 or lower wall 8 to be restrained and likeliness of bend deformation to be secured. A stiffening means by these ribs is provided between a node part and an antinode part which are probably formed when fuel flows in the fuel storage chamber 5. Thus, the large bend deformation is restrained, and the generation of noise can be restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel tank for storing fuel.

[0002]

2. Description of the Related Art If there is a space above a fuel level in a fuel tank, evaporative fuel is generated, and this evaporative fuel may be released to the atmosphere. Therefore, a fuel storage chamber for storing fuel is formed by an expandable and contractible surrounding film and a housing to which the surrounding film is attached, and the surrounding film is expanded and contracted according to the amount of fuel in the fuel storage room, and the fuel liquid in the fuel storage room is formed. A fuel tank in which no evaporated fuel is generated above the surface is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 8-170568. The surrounding membrane includes an upper wall, a lower wall, and a side wall connecting the upper wall and the lower wall to each other. The surrounding membrane is attached at its lower wall to the lower wall of the housing. The side wall of the surrounding film has a bellows shape, and expands and contracts when the upper wall moves up and down in the housing according to the amount of fuel in the fuel storage chamber. This prevents a space from being formed above the fuel level in the fuel storage chamber.

[0003]

When the fuel tank is mounted on, for example, a vehicle, the fuel in the fuel storage chamber flows due to acceleration and deceleration of the vehicle, so that the side wall of the surrounding film expands or is depressed, causing a wave. It bends and generates noise when fuel strikes the upper or lower wall. In order to suppress the curving deformation of the side wall such as the wavy shape, the rigidity of the side wall may be increased. However, when the rigidity of the side wall is increased, a large stress is generated in the side wall due to the fuel flow in the fuel storage chamber, and the side wall may be damaged. Therefore, an object of the present invention is to suppress the generation of noise due to the fuel flow in the fuel storage chamber while maintaining the durability of the wall high in a fuel tank having a fuel storage chamber formed by a wall that can be bent and deformed.

[0004]

According to a first aspect of the present invention, there is provided a fuel storage chamber for storing fuel by means of a membrane wall, wherein the membrane wall is provided with a fuel in the fuel storage chamber. In a fuel tank which is deformable according to an amount, a film formed between an antinode portion and a node portion of a wave deformation of the film wall formed when the film wall is deformed by a flow of fuel in a fuel storage chamber. The wall is provided with reinforcing means for increasing the rigidity of the membrane wall. Thereby, the deformation of the membrane wall between the antinode part and the node part of the wave deformation of the deformable membrane wall is suppressed.

According to a second aspect of the present invention, in the first aspect, the membrane wall comprises a pair of polygonal walls and a connecting wall connecting the corresponding sides of the polygonal walls to each other. Wherein said reinforcing means is provided on said polygonal wall. Thereby, deformation of the membrane wall between the antinode part and the node part of the wave deformation of the deformable polygonal wall is suppressed.

According to a third aspect of the present invention, in the second aspect, the reinforcing means is a rib extending toward the center of the polygonal wall. Therefore, the deformation of the membrane wall between the antinode part and the node part of the wave deformation of the deformable polygonal wall is suppressed.

According to a fourth aspect of the present invention, in the third aspect, the rib extends from a peripheral portion of the polygonal wall. Thereby, deformation of the peripheral portion of the deformable polygonal wall is suppressed.

[0008] According to a fifth aspect of the present invention, in the second aspect, the reinforcing means is a rib extending from a central portion to a peripheral portion of the polygonal wall. Thereby, the deformation of the central portion of the deformable polygonal wall is suppressed.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A fuel tank according to a first embodiment of the present invention will be described with reference to the drawings. The fuel tank of the present invention is used, for example, as a tank for storing fuel to be supplied to an internal combustion engine. Of course, it can also be used simply as a tank for storing fuel. Referring to FIG. 1, the fuel storage device 1 of the first embodiment includes a housing 4 including an upper part 2 and a lower part 3 having a substantially bowl shape. The upper part 2 and the lower part 3 are connected to each other at flanges 2a, 3a formed on the periphery thereof. Housing 4
A fuel container or a fuel tank 6 defining a fuel storage chamber 5 for storing fuel is disposed therein.

Referring to FIG. 2, the fuel tank 6 of the first embodiment has a pair of substantially rectangular upper and lower walls 7 and 8, and corresponding sides 719a to 719d and 819 of the upper and lower walls 7.
a to 819d (note that sides 819b and 819c are not shown).
And The side walls 9a to 9d are connected at both ends to the ends of the adjacent side walls 9a to 9d, and
A connecting portion is formed between the positions 9 to 9d. Therefore, the fuel tank 6 has a substantially rectangular parallelepiped shape, and the fuel storage chamber 5 is formed therein. The upper wall 7, the lower wall 8, and the side walls 9a to 9
d corresponds to the membrane wall forming the fuel storage chamber, and is formed of, for example, resin and has substantially rigidity. The area of the upper wall 7 or the lower wall 8 is larger than the area of one side wall, and the rigidity of the upper wall 7 and the lower wall 8 is smaller than the rigidity of the side walls 9a to 9d.

The upper wall 7 and the lower wall 8 are not limited to rectangular walls, but may be any pair of polygonal walls. Also the side wall 9
Also, a to 9d are not limited to rectangular walls, and may be any polygonal wall that connects the upper wall 7 and the lower wall 8 to each other and connects each other. Therefore, the upper wall 7 and the lower wall 8 correspond to polygonal walls, and the side walls 9a to 9d correspond to connecting walls connecting the upper wall 7 and the lower wall 8. The shapes of the upper wall 7, the lower wall 8, and the side walls 9a to 9d are appropriately selected according to the shape of the space in which the fuel tank 6 is to be installed.

A plurality of ribs 30 are provided in a region around the upper wall 7.
Is provided. These ribs 30 extend along a line (hereinafter, a connecting line) L connecting a central point or centroid (hereinafter, central portion) C of the upper wall 7 and a point P on the side 719a of the upper wall 7, for example. To the central part C of the upper wall 7. Also rib 30
Extends upward perpendicular to the upper wall 7. The central part C
A part of the connection line connecting the point P on the side 719a of the upper wall 7 has been described, but the same applies to the sides 719b to 719d.

By the way, in the fuel tank 56 having no ribs as shown in FIG. 4, when the fuel in the fuel storage chamber 55 flows in the direction of arrow A, a part of the upper wall 57 is curved and deformed so as to expand. At the same time, the other portion of the upper wall 57 is curved and deformed so as to be depressed (see the solid line in FIG. 4). Next, when the fuel in the fuel storage chamber 55 flows in the direction of arrow B, a part of the upper wall 57 that has been curved and deformed so as to expand is curved and deformed so as to be concave, and the upper wall that has been curved and deformed so as to be concave. The other portion 57 is curved and deformed so as to expand (see the broken line in FIG. 4). The curved deformation of the upper wall 57 is repeated, and the upper wall 57
Is deformed in a wave-like manner (hereinafter referred to as wave deformation). At this time, the upper wall 57 has a portion having the largest degree of bending deformation or displacement, that is, a portion corresponding to the antinode of the wave deformation of the upper wall 57 (hereinafter, an antinode portion) and a portion having the lowest degree of bending deformation or displacement, that is, A portion corresponding to a node of the wave deformation of the upper wall 57 (hereinafter, a node portion) is formed.

The ribs 30 of the first embodiment are shown in FIGS.
As shown in (1), it extends along the connecting line L from the outer end of the connecting line L by a quarter of the length of the connecting line L. That is, the rib 30 is provided between a node portion and an antinode portion which may be formed when fuel flows in the fuel storage chamber 5. One-fourth of the length of the connection line L corresponds to one-fourth of the frequency of the wave deformation of the upper wall caused by the fuel flow in the fuel storage chamber 5 which causes noise. The ribs 30 prevent the upper wall 7 between the knot portion and the belly portion from being greatly curved and deformed. Therefore, the ribs 30 correspond to reinforcing means for increasing the rigidity of the upper wall 7. For this reason, the upper wall 7 does not expand or dent largely due to the fuel flow in the fuel storage chamber 5. Therefore, no noise is generated in the fuel tank 6.

The ribs 30 do not suppress the bending deformation of the antinode portion where the degree of bending deformation is greatest when the amount of fuel is large. Therefore, no large stress is generated in the antinode portion due to the fuel flow in the fuel storage chamber 5, and the upper wall 7 is not damaged. Further, the height of the rib 30 is lower as it is closer to the central portion C or the belly portion. Therefore, the effect of suppressing the bending deformation of the upper wall 7 is smaller as it is closer to the belly portion, and the bending deformation is more likely.
Therefore, no large stress is generated in the antinode portion due to the fuel flow in the fuel storage chamber 5, and the upper wall 7 is not damaged. In the first embodiment, a rib 31 similar to the rib 30 provided on the upper wall 9 is also provided on the lower wall 8. The ribs 30, 3
The number of 1 is determined in consideration of the degree of the curved deformation of the upper wall 7 or the lower wall 8 to be suppressed and the ease of the curved deformation of the upper wall 7 or the lower wall 8 to be secured. Also in the case where the upper wall 7 and the lower wall 8 are polygonal walls other than a rectangle, the points on the sides of the polygonal wall are connected to the center points or centroids of the polygonal wall as in the first embodiment. What is necessary is just to provide a rib along a connection line.

When fuel is supplied into the fuel tank 6 and the amount of fuel in the fuel tank 6 exceeds the amount of fuel that can be stored in a rectangular parallelepiped shape (hereinafter, a predetermined amount), the upper wall 7 and the lower wall 8 separate from each other. As shown in FIG. 5, the side walls 9a to 9d are curved and deformed so as to bulge outward. That is, in the first embodiment, when the fuel amount in the fuel tank 6 exceeds a predetermined amount, the upper wall 7 is displaced upward, the lower wall 8 is displaced downward, and the side walls 9a to 9d are displaced laterally and inward. . Thus, the amount of fuel that can be stored in the fuel tank 6 gradually increases. Since the degree of bending deformation in the peripheral portions of the upper wall 7 and the lower wall 8 provided with the ribs 30 and 31 is relatively small, the ribs 30 and 31 do not hinder the bending deformation of the upper wall 7 and the lower wall 8. Absent. The deformation of the upper wall 7 and the lower wall 8 is smaller than the deformation of the side walls 9a to 9d. Therefore, the upper wall 7 and the lower wall 8 correspond to large deformation surfaces, and the side walls 9a to 9d correspond to small deformation surfaces.

On the other hand, fuel is discharged from the fuel tank 6,
When the amount of fuel in the fuel tank 6 becomes smaller than a predetermined amount, the upper wall 7 and the lower wall 8 approach each other and bend and deform inward, and the side walls 9a to 9d approach each other and bend so as to indent inward. Deforms (see FIG. 6). That is, in the first embodiment, when the fuel amount in the fuel tank 6 becomes smaller than a predetermined amount, the upper wall 7 is displaced downward, the lower wall 8 is displaced upward, and the side walls 9a to 9d are displaced laterally and inward. . Thus, the amount of fuel that can be stored in the fuel tank 6 gradually decreases. Here, the degree of the bending deformation in the peripheral portions of the upper wall 7 and the lower wall 8 provided with the ribs 30 and 31 is relatively small, so that the ribs 30 and 31 inhibit the bending deformation of the upper wall 7 and the lower wall 8. Never.

Referring again to FIG. 1, a fuel supply pipe 10 for supplying fuel into the fuel tank 6 is connected to substantially the center of the lower wall 8 of the fuel tank 6. The pipe wall of the fuel supply pipe 10 has a bellows shape, and is extendable and contractible and bendable. On the other hand, an evaporative fuel discharge pipe 11 for discharging the gas in the fuel tank 6, particularly the evaporative fuel, to the outside of the fuel tank 6 is connected to a substantially center of the upper wall 7 of the fuel tank 6 via a shutoff valve 12. The fuel vapor discharge pipe 11 has flexibility. The shutoff valve 12 shuts off the evaporated fuel discharge pipe 11 when the fuel level in the fuel tank 6 reaches the shutoff valve 12. This prevents the fuel from leaking out of the fuel tank 6. The evaporative fuel discharge pipe 11 is connected to an intake passage (not shown) of the internal combustion engine via a charcoal canister (not shown) for temporarily absorbing the evaporative fuel. The evaporated fuel adsorbed on the charcoal canister is discharged into the intake passage according to the operating state of the internal combustion engine. A check valve 13 is attached to the fuel vapor discharge pipe 11. The check valve 13 opens when the pressure in the evaporated fuel discharge pipe 11 between the check valve 13 and the shut-off valve 12 exceeds a predetermined positive pressure, and becomes lower than the predetermined positive pressure. Closes when This prevents gas from entering the fuel tank 6 after the shutoff valve 12 is once shut off.

Below the lower wall 8 of the fuel tank 6, there is arranged an auxiliary fuel tank 14 for temporarily storing the fuel in the fuel tank 6. A fuel pump 16 for delivering fuel in the auxiliary fuel tank 14 to a fuel injection valve (not shown) of the internal combustion engine via a fuel feed pipe 15 is disposed in the auxiliary fuel tank 14. The fuel pump 16 sucks the fuel through a filter 17 for filtering dust and the like contained in the fuel and sends the fuel to a fuel injection valve (not shown) of the internal combustion engine.
In addition, a dividing wall 18 extends substantially vertically upward from the lower wall 8 of the auxiliary fuel tank 14. The dividing wall 18 is the auxiliary fuel tank 14
Is tilted to prevent the fuel around the filter 17 from being depleted when the fuel level in the auxiliary fuel tank 14 is tilted. Therefore, the dividing wall 18 corresponds to a fuel depletion prevention means. The auxiliary fuel tank 14 is connected to the fuel tank 6 via a fuel introduction pipe 20. The pipe wall of the fuel introduction pipe 20 has a bellows shape, and is extendable and contractible and bendable.

Further, the upper surface of the upper wall 52 of the auxiliary fuel tank 14 conforms to the shape of the lower wall 8 of the fuel tank 6 when the lower wall 8 of the fuel tank 6 expands downward. Therefore, the upper wall 52 of the auxiliary fuel tank 14 is displaced or bent downward in the lower wall 8 of the fuel tank 6, and the upper wall 5 of the auxiliary fuel tank 14 is
2, the further displacement or curved deformation of the lower wall 8 of the fuel tank 6 is suppressed. A part of the upper surface of the lower portion 3 conforms to the shape of the lower wall 8 of the fuel tank 6 when the lower wall 8 of the fuel tank 6 expands downward. Therefore, the lower portion 3 causes the lower wall 8 of the fuel tank 6 to be displaced or curved downward, and when the lower wall 3 comes into contact with the upper surface of the lower portion 3, further displacement or curved deformation of the lower wall 8 of the fuel tank 6 occurs. Suppress. For this reason, the lower wall of the fuel tank 6 is not extremely bent and deformed beyond the allowable value, and the lower wall of the fuel tank is prevented from being damaged. Since the upper wall 52 of the auxiliary fuel tank 14 is used to suppress the curved deformation of the lower wall of the fuel tank 6, the size of the entire fuel storage device 1 can be reduced. In addition, since the fuel supply pipe 10 is extendable and contractible and bendable, the fuel supply pipe 10 does not suppress the downward displacement or the curved deformation of the lower wall 8 of the fuel tank 6. Of course, the fuel supply pipe 10 does not suppress the upward displacement or the curved deformation of the lower wall 8 of the fuel tank 6.

The lower surface of the upper portion 2 conforms to the shape of the upper wall 7 of the fuel tank 6 when the upper wall 7 of the fuel tank 6 expands upward. Therefore, the upper portion 2 displaces or bends the upper wall 7 of the fuel tank 6 upward, and suppresses further displacement or curved deformation of the upper wall 7 of the fuel tank 6 when it comes into contact with the lower surface of the upper portion 2. . Evaporated fuel discharge pipe 1
Since 1 has flexibility, the evaporated fuel discharge pipe 11 does not suppress the displacement or the curved deformation of the fuel tank 6 above the upper wall 7 of the fuel tank 6. Of course, the fuel vapor discharge pipe 11 is
It does not suppress downward displacement or curved deformation of the upper wall 7.

Further, the upper wall 7 of the fuel tank 6 is
When the lower wall 8 of the fuel tank 6 contacts the upper surface of the lower portion 3 and the upper surface of the upper wall 52 of the auxiliary fuel tank 14, no more fuel is supplied into the fuel tank 6.

A gap 21 having a width substantially equal to the height of the side walls 9 a to 9 d of the fuel tank 6 is formed by the upper part 2 and the lower part 3 in the periphery of the housing 4. The space 21 accommodates the side walls 9 a to 9 d of the fuel tank 6 and a portion 22 (hereinafter, upper and lower wall portions) of the upper wall 7 and the lower wall 8 connected to the side walls 9 a to 9 d. In the first embodiment, the upper and lower wall portions 22 are regarded as a part of the side walls 9a to 9d, and hereinafter, the upper and lower wall portions 22 and the side walls 9a to 9d are collectively referred to as side wall equivalent portions. Thus, the fuel tank 6 has a portion corresponding to the side wall in the upper portion 2.
The housing 4 is sandwiched between the
Supported by That is, the upper portion 2 and the lower portion 3 form a holding portion 23 for holding a portion corresponding to the side wall.

In the first embodiment, when the upper wall 7 and the lower wall 8 are displaced, the portion corresponding to the side wall can be displaced by sliding in the lateral direction and inward. Therefore, the holding portion 23 does not suppress the displacement of the upper wall 7 and the lower wall 8. Side walls 9a to 9d when the upper wall 7 and the lower wall 8 are displaced upward and downward.
The displacement of the part 22 of the upper wall 7 and the lower wall 8 connected to the For this reason, the holding portion 23 includes the upper wall 7 and the lower wall 8.
It does not suppress upward and downward displacement of.

Therefore, in the first embodiment, the fuel tank 6
Is supported by the housing 4 at a portion corresponding to the side wall, so that the upward and downward displacement of the upper wall 7 and the lower wall 8 due to the increase in the amount of fuel supplied into the fuel tank 6 is suppressed by the displacement of the fuel tank 6. If the fuel amount in the fuel tank 6 does not reach the fuel amount that can be stored in the fuel tank 6 (maximum storage amount) when the fuel amount is not set, the fuel amount is not suppressed. Therefore, the maximum amount of fuel can be stored in the fuel tank 6 and the fuel can be easily introduced into the fuel tank 6.

Next, a fuel tank according to a second embodiment of the present invention will be described. In the second embodiment, as shown in FIG.
A plurality of ribs 32 are provided in a central region of the plurality. These ribs 32 extend from the central portion C of the upper wall 7 along the connecting line L to the upper wall 7.
Extending towards the periphery of the. The rib 32 extends upward perpendicular to the upper wall 7. The rib 32 extends from the inner end of the connection line L along the connection line L by a quarter of the length of the connection line L. That is, the rib 32 is provided between the node portion and the antinode portion. The ribs 32 prevent the upper wall 7 between the knot portion and the belly portion from being greatly curved and deformed. Therefore, the ribs 32 correspond to reinforcing means for increasing the rigidity of the upper wall 7. For this reason, the upper wall 7 does not expand or dent largely due to the fuel flow in the fuel storage chamber 5. Therefore, no noise is generated in the fuel tank.

The ribs 32 do not suppress the bending deformation of the antinode part where the degree of the bending deformation is greatest. Therefore, no large stress is generated in the antinode portion due to the fuel flow in the fuel storage chamber 5, and the upper wall 7 is not damaged. Further, the height of the rib 32 is lower as it is closer to the peripheral portion or the antinode. Therefore, the effect of suppressing the bending deformation of the upper wall 7 is smaller as it is closer to the belly portion, and the bending deformation is more likely. Therefore, the fuel storage chamber 5
Since no large stress is generated in the antinode portion in accordance with the fuel flow inside, the upper wall 7 is not damaged. In the second embodiment, a rib similar to the rib 32 provided on the upper wall 9 is also provided on the lower wall 8. The number of the ribs 32 is determined in the same manner as in the first embodiment. In addition, the other configuration and operation and effect of the second embodiment are the same as those of the first embodiment, and thus description thereof is omitted.

In the present invention, the ribs 30, 31, and 32 are provided between the joint portion and the antinode, but the rib may be provided in the antinode. In this case, the upper wall 7, the lower wall 8 and the rib can withstand the stress generated in the upper wall 7, the lower wall 8 and the rib due to the fuel flow in the fuel storage chamber 5, and the upper wall 7, the lower wall 8 and the curved deformation of the upper wall 7 and the lower wall 8 can be prevented. The upper wall 7
And the rigidity of the lower wall 8 and the dimensions of the length, height and width of the ribs. In the present invention, the ribs are provided in the peripheral region of the upper wall 7 and the lower wall 8 in the first embodiment, and the ribs are provided in the central region of the upper wall 7 and the lower wall 8 in the second embodiment. Can be combined.
For example, a part of the upper wall 7 or the lower wall 8 may be provided with a rib in a peripheral area, and another part of the upper wall 7 or the lower wall 8 may be provided with a rib in a central area. In this case, no rib should be provided on the antinode to suppress the generation of stress at the antinode.

In the present invention, a rib having a length of a quarter of the frequency of the wave deformation of the upper wall 7 and the lower wall 8 is provided, but a rib having a quarter or less may be provided. Also in this case, ribs should not be provided on the antinode to suppress the generation of stress in the antinode. Further, the present invention can be applied to a fuel tank having a part of a wall that can be bent and deformed by fuel flow. If the deformation of the fuel tank is not hindered, ribs may be provided in the fuel chamber.

[0030]

According to the first to fifth aspects of the present invention, the deformation of the membrane wall between the antinode part and the node part of the wave deformation of the deformable wall is suppressed. For this reason, deformation of the wall, such as a wave, is suppressed, and generation of noise from the fuel tank is suppressed. However, the deformation of the antinode part having the highest degree of deformation is not suppressed. Therefore, generation of a large stress on the antinode portion of the membrane wall is suppressed, and the durability of the membrane wall can be maintained high.

[Brief description of the drawings]

FIG. 1 is a partial sectional view of a fuel storage device according to a first embodiment of the present invention.

FIG. 2 is a perspective view of a fuel tank according to the first embodiment of the present invention.

FIG. 3 is a sectional view of the fuel tank taken along line III-III in FIG. 2;

FIG. 4 is a cross-sectional view of a fuel tank similar to FIG. 3, but without ribs.

FIG. 5 is a perspective view of the fuel tank of the first embodiment of the present invention in an expanded state.

FIG. 6 is a perspective view of the fuel tank according to the first embodiment of the present invention in a depressed state.

FIG. 7 is a perspective view of a fuel tank according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Fuel storage device 6 ... Fuel tank 7 ... Upper wall 8 ... Lower wall 9a-9d ... Side wall 14 ... Auxiliary fuel tank 30, 31, 32 ... Rib

Claims (5)

[Claims] 1. A fuel tank, wherein a fuel storage chamber for storing fuel is formed by a membrane wall, wherein the membrane wall is deformable according to the amount of fuel in the fuel storage chamber. A reinforcing means for increasing the rigidity of the membrane wall is provided on the membrane wall between the antinode part and the node part of the wave deformation of the membrane wall formed when the membrane is deformed by the flow of the fuel. And the fuel tank. 2. The method according to claim 1, wherein the membrane wall includes a pair of polygonal walls and a connecting wall that connects corresponding sides of the polygonal walls to each other,
The fuel tank according to claim 1, wherein the reinforcing means is provided on the polygonal wall. 3. The fuel tank according to claim 2, wherein the reinforcing means is a rib extending toward a center of the polygonal wall. 4. The fuel tank according to claim 3, wherein the rib extends from a periphery of the polygonal wall. 5. The fuel tank according to claim 2, wherein said reinforcing means is a rib extending from a central portion to a peripheral portion of said polygonal wall.