TITLE OF INVENTION: FUEL TANK FOR VEHICLE
TECHNICAL FIELD
[0001] The present invention relates to a fuel tank
for a vehicle provided in a vehicle.
BACKGROUND ART
[0002] In an engine-driven vehicle, a fuel tank
housing fuel such as gasoline is provided. The fuel
tank is configured by joining an upper tank and a
lower tank by welding, and fuel is housed in a closed
space formed by the upper tank and the lower tank.
On a bottom surface of an inner part of the lower
tank of the fuel tank, a sub-tank is generally
provided, and it is configured such that even if the
vehicle is inclined, a predetermined liquid level is
constantly maintained to prevent a suction failure of
fuel so that the fuel can be stably supplied to an
engine. The sub-tank is fixed to the lower tank by
spot welding in a state where a bottom surface of an
outer part thereof faces the bottom surface of the
inner part of the lower tank.
[0003] Regarding a fuel tank for a vehicle, due to a
vertical vibration during traveling, a weight of fuel
acts on a bottom surface of the fuel tank, and the
fuel tank vibrates because it moves up and down,
resulting in that a fatigue failure of welded portion
at which a lower tank and a sub-tank are joined is
caused, which is a problem. For this reason, a
reinforcement is made by providing a bead on a bottom
- 1 -
surface of the lower tank.
[0004] For example. Patent Literature 1 discloses a
technique in which a sub-tank is attached to a bottom
surface of a tank via a plate-shaped support to
reduce a stress concentration on a welded portion, to
thereby improve a flexural rigidity. Further, Patent
Literature 2 discloses a fuel tank for a vehicle in
which spot-welded portions at which a sub-tank and a
lower tank are fixed are changed, and concave beads
and a convex bead are linearly provided on a bottom
surface of the lower tank. Further, Patent
Literature 3 discloses a technique in which a stay is
provided to make a reinforcement for preventing a
separation between a sub-tank and a bottom surface of
a tank. Further, Patent Literature 4 discloses a
fuel tank for a vehicle provided with beads, on a
bottom surface of a tank main body, which extend in
different directions at a center portion in a
longitudinal direction of the tank main body and at
both side portions of an installation part of a subtank.
CITATION LIST
PATENT LITERATURE
[0005] Patent Literature 1: Japanese Laid-open
Patent Publication No. 10-44793
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2002-321537
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2002-67711
- 2 -
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2000-158956
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] However, when the additional member for
fixing the sub-tank to the lower tank is provided as
in the above-described Patent Literature 1 or 3, a
weight of the whole vehicle is increased, which goes
against a tendency to make a vehicle lighter.
Further, a cost is also increased due to the increase
in members, which is also a problem. Meanwhile, when
the plurality of beads are discontinuously provided "
on the lower tank 'as in the above-described Patent
Literature 2 or 4, a strength was proved to be
lowered at a discontinuous point of beads. At this
time, a sufficient rigidity cannot be obtained even
if the arrangement of spot welding is changed as in
Patent Literature 2, so that a fatigue failure of
welded portions at which the sub-tank and the lower
tank are joined cannot be effectively prevented.
[0007] Accordingly, the present invention has been
made in view of the above-described problems, and an
object of the present invention is to provide a new
and improved fuel tank capable of increasing a
rigidity of a tank and preventing a fatigue failure
of welded portions, at which a sub-tank and a lower
tank are joined, caused by a vertical vibration
during traveling.
SOLUTION TO PROBLEM
- 3 -
[0008] In order to solve the above-described
problems, according to a certain aspect of the
present invention, there is provided a fuel tank for
a vehicle characterized in that it includes: a tank
main body in which an upper tank and a lower tank are
mutually joined to form a closed space in which fuel
is housed; and a sub-tank fixed to a bottom su:^face
part of the lower tank by spot welding, in which a
plurality of rows of the spot welding are set along a
longitudinal direction of the lower tank with an
interval therebetween in a width direction of the
sub-tank, at least one bead positioned between the
rows of the spot welding and extending continudusly
along the longitudinal direction of the lower tank is
formed on the bottom surface part of the lower tank,
and a lower surface of the sub-tank has no portion
that is not brought into contact with the bottom
surface part of the lower tank except for the bead.
[0009] According to the present invention, on
approximately a center line of a length in a first
direction (width direction) of the sub-tank on the
bottom surface part of the lower tank, there is
formed at least one bead extending continuously in a
second direction (longitudinal direction) orthogonal
to the first direction, so that a natural frequency
in a secondary panel vibration mode of the fuel tank
for the vehicle can be improved. Accordingly, a
rigidity of the fuel tank for the vehicle can be
improved, and it becomes possible to prevent a
- 4 -
fatigue failure of welded portions, at which the subtank
and the lower tank are joined, caused by a
vertical vibration during traveling.
[0010] A length of the bead is set to a length being
80% or more of a length of a flat portion of the
bottom surface part of the lower tank in the
longitudinal direction. Accordingly, it is possibleto
sufficiently maintain the rigidity of the fuel
tank for the vehicle.
[0011] It is also possible that the bead is formed
continuously from the bottom surface part to a
sidewall part of the lower tank.
[0012] ' It is also possible to design such that the
plurality of rows of the spot welding are disposed to
be symmetric with respect to the bead formed on
approximately the center line in the width direction
of the sub-tank.
[0013] It is also possible that a width of the bead
is set to a length being 50% or more of the interval
of the rows of the spot welding which are adjacent
with the bead therebetween. Accordingly, it is
possible to sufficiently maintain the rigidity of the
fuel tank for the vehicle.
[0014] Each of embossed portions formed in a
vertical direction with respect to the bottom surface
part of the lower tank is provided between portions
formed by the spot welding and adjacent in the row
direction. Accordingly, it is possible to
sufficiently maintain the rigidity of the fuel tank
- 5 -
for the vehicle.
[0015] It is also possible that another bead is
formed on a flat portion between an end face in the
width direction of the sub-tank to the sidewall part
of the lower tank, on the bottom surface part of the
lower tank along the longitudinal direction of the
lower tank.
[0016] It is also possible that the bead is formed
as a meandering bead meandering in the width
direction or a width-changed bead whose width is
changed.
[0017] It is also possible that the tank main body
and the sub-tank are made of at least any one of
materials of a surface treated steel sheet, a
stainless steel, and an aluminum alloy, and the lower
tank and the sub-tank are formed of the same material.
ADVANTAGEOUS EFFECTS OF INVENTION
[0018] As described above, according to the present
invention, it is possible to provide the fuel tank
capable of increasing the rigidity of the tank and
preventing the fatigue failure of the welded portions,
at which the sub-tank and the lower tank are joined,
caused by the vertical vibration during traveling.
BRIEF DESCRIPTION OF DRAWINGS
[0019] [Fig. 1] Fig. 1 is a perspective view
illustrating an external appearance of a fuel tank
for a vehicle according to a first embodiment of the
present invention.
[Fig. 2] Fig. 2 is a perspective view
- 6 -
illustrating an inner part of a lower tank, of the
fuel tank for the vehicle according to the first
embodiment of the present invention,
[Fig. 3] Fig. 3 is a plan view of Fig. 2.
[Fig. 4] Fig. 4 is an explanatory diagram
illustrating a secondary panel vibration mode when a
length LB of a bead is set to a length L of a flat
portion.
[Fig. 5] Fig. 5 is an explanatory diagram
illustrating the secondary panel vibration mode when
the length LB of the bead is set to the length L of
the flat portion.
[Fig. 6] Fig. 6 is a plan view illustrating a
shape of a lower tank when the length LB of the bead
illustrated in Fig. 3 is set to a length being 48% of
the length L of the flat portion.
[Fig. 7] Fig. 7 is an explanatory diagram
illustrating a secondary panel vibration mode when
the length LB of the bead is set to the length being
48% of the length L of the flat portion.
[Fig. 8] Fig. 8 is an explanatory diagram
illustrating the secondary panel vibration mode when
the length LB of the bead is set to the length being
48% of the length L of the flat portion.
[Fig. 9] Fig. 9 is an explanatory diagram
illustrating a shape of a lower tank when a bead
width WB of the bead has a length being 66% of a spot
welding interval Ws.
[Fig. 10] Fig. 10 is an explanatory diagram
- 7 -
illustrating a shape of a lower tank when the bead
width WB of the bead has a length being 19% of the
spot welding interval Ws.
[Fig. 11] Fig. 11 is an explanatory diagram
illustrating a shape of a lower tank when each of
sub-beads is formed on a sub-tank side within a width
WA in which the sub-bead can be disposed.
[Fig. 12] • Fig. 12 is an explanatory diagram
illustrating a shape of a lower tank when each of the
sub-beads is form.ed on a side surface side of the
lower tank within the width WA in which the sub-bead
can be disposed.
[Fig. 13] Fig. 13 is an explanatory diagram
illustrating a shape of a lower tank when three
discontinuous beads are formed on a bottom surface
part of the lower tank,
[Fig. 14] Fig. 14 is an explanatory diagram
illustrating a secondary panel vibration mode of a
fuel tank having the lower tank in which the
discontinuous beads are formed in Fig. 13.
[Fig. 15] Fig. 15 is an explanatory diagram
illustrating the secondary panel vibration mode of
the fuel tank having the lower tank in which the
discontinuous beads are formed in Fig. 13.
[Fig. 16] Fig. 16 is a perspective view
illustrating an inner part of a lower tank having
other beads in a periphery of extension of bead, as a
comparative example with respect to the present
invention.
- 8 -
[Fig. 17] Fig. 17 is a plan view of the lower
tank in Fig. 2.
[Fig. 18] Fig. 18 is an explanatory diagram
illustrating a secondary panel vibration mode in the
lower tank having the other beads.
[Fig. 19] Fig. 19 is an explanatory diagram
illustrating the secondary panel vibration mode in
the lower tank having the other beads.
[Fig. 20] Fig. 20 is a perspective view
illustrating an inner part of a lower tank having a
portion with which a lower surface of a sub-tank is
not brought into contact except for a bead, as a
comparative example with respect to the present
invention.
[Fig. 21] Fig. 21 is a plan view of the lower
tank in Fig. 20.
[Fig. 22] Fig. 22 is a sectional view taken
along a line I-I in Fig. 21,
[Fig. 23] Fig. 23 is an explanatory diagram
illustrating a secondary panel vibration mode in the
lower tank having the portion with which the lower
surface of the sub-tank is not brought into contact
except for the bead.
[Fig. 24] Fig. 24 is an explanatory diagram
illustrating the secondary panel vibration mode in
the lower tank having the portion with which the
lower surface of the sub-tank is not brought into
contact except forthebead.
[Fig. 25] Fig. 25 is a plan view illustrating
- 9 -
one configuration of a fuel tank according to a
second embodiment of the present invention.
, [Fig. 26] Fig. 26 is a plan view illustrating
another configuration of the fuel tank according to
the second embodiment of the present invention.
[Fig. 27] Fig. 27 is • a . sectional view
illustrating one example of a shape of a lower tank
when a bead formed on the lower tank is set to have a
convex shape.
[Fig. 28] Fig. 28 is a sectional view
illustrating one example of a shape of a lower tank
when a bead formed on the lower tank is set to have a
convex shape.
[Fig. 29] Fig. 29 is a plan view of a lower tank
having a meandering bead, as a modified example of
the present invention.
[Fig. 30] Fig. 30 is a plan view of a lower tank
having a width-changed bead, as a modified example of
the present invention.
DESCRIPTION OF EMBODIMENTS
[0020] Hereinafter, preferred embodiments of the
present invention will be described in detail while
referring to the attached drawings. Note that in the
present specification and the drawings, components
having practically the same functional configuration
are denoted by the same reference numerals to omit
repeated explanation.
[0021] (First Embodiment)
• [1-1. Example of external appearance of fuel
- 10 -
tank]
First, explanation will be made on a schematic
configuration of a fuel tank for a vehicle 100
according to a first embodiment of the present
invention. Note that Fig. 1 is a perspective view
illustrating an external appearance of the fuel tank
for 'the vehicle 100 according to the present
embodiment. Fig. 2 is a perspective view
illustrating an inner part of a lower tank 120 of the
fuel tank for the vehicle 100 according to the
present embodiment. Fig. 3 is a plan view of Fig. 2.
Note that in the description hereinbelow, explanation
will be made by setting a longitudinal direction of
the fuel tank 100 as a traveling direction of a
vehicle.
[0022] The fuel tank for the vehicle 100 according
to the present embodiment is formed by joining an
upper tank 110 and the lower tank 120, as illustrated
in Fig. 1. Each of the upper tank 110 and the lower
tank 120 according to the present embodiment is
formed of a bottom surface part and a sidewall part,
and a tank main body is configured by mutually
joining the upper tank 110 and the lower tank 120, by
making opening portions, namely, flanges formed on
edge portions of the sidewall parts of the upper tank
110 and the lower tank 120 face each other.
Accordingly, a closed space in which fuel is housed
can be formed. A joint portion between the bottom
surface part and the sidewall part of each of the
- 11 -
upper tank 110 and the lower tank 120 is formed as a
curved portion having an R shape.
[0023] In the closed space, a sub-tank 130 is fixed
to a bottom surface part 124 of the lower tank 120,
as illustrated in Fig. 2. The sub-tank 130 is fixed
by spot welding. A portion at which the spot welding
is conducted is indicated as a spot-welded portion
150. The lower tank 120 and the sub-tank 130 of the
present embodiment are fixed by six spot-welded
portions 150a to 150f.
[0024] On the bottom surface part 124 of the lower
tank 120, there is formed a jointless continuous bead
142 in a longitudinal direction (y direction) on
approximately a center line of a lower tank width WL
being a length of the sub-tank 130 in a width
direction (x direction). As illustrated in Fig. 3,
the bead 142 is formed on the lower tank 120 of the
present embodiment so that a center of width of the
bead 142 is positioned on the center line of the
lower tank width Wi,, but, the bead 142 does not have
to be formed exactly on the center line of the lower
tank width WL . In this case, it is desirable that the
bead 142 is formed on the center line of the lower
tank width WL. Further, on both sides of the bead 142,
two side beads 144 and 146 are formed approximately
in parallel to the bead 142.
[0025] Each of the upper tank 110, the lower tank
120 and the sub-tank 130 that form the fuel tank 100
is formed of, for example, a surface treated steel
- 12 -
sheet obtained by performing surface treatment such
as plating and painting, a stainless steel, an
aluminum alloy or the like. Note that since the
lower tank 120 and the sub-tank 130 are fixed by the
spot welding, they are formed of the same material.
[0026] Here, the fuel tank for the vehicle 100
according to the present embodiment is characterized
in that the bead 142 extending continuously along the
longitudinal direction on approximately the center
line of the lower tank width WL of the sub-tank 130 is
formed on the bottom surface part 124 of the lower
tank 120. As described above, a bead has been
conventionally provided on the lower tank 120 to
improve the rigidity of the fuel tank 100, but, the
fatigue failure of the spot-welded portions, at which
the sub-tank and the lower tank are joined, caused by
the vertical vibration during traveling, has not been
effectively prevented.
[0027] As a result of earnest studies, the inventors
of the present application found out that in the fuel
tank 100 in which the sub-tank 130 is attached to the
bottom surface part 124 of the lower tank 120, a
secondary panel vibration mode of the bottom surface
part 124 of the lower tank 120 is a main cause of
separating the spot-welded portions 150 fixing the
lower tank 120 and the sub-tank 130. Specifically,
in the fuel tank 100 according to the present
embodiment, it is important to effectively improve
the rigidity (natural frequency) with respect to the
- 13 -
secondary panel vibration mode of the bottom surface
part 124 of the lower tank 120, and there is a need
to form a bead corresponding to such a mode on the
lower tank 120. Further, it was proved that, by
forming the jointless continuous bead 142 in the
longitudinal direction on approximately the center
line of the lower tank width WL of the sub-tank 130,
the rigidity (natural frequency) with respect to the
secondary panel vibration mode of the bottom surface
part 124 of the lower tank 120 can be effectively
improved.
[0028] Hereinafter, a shape of the bead 142 formed
on the lower tank 120 of the fuel tank 100 according
to the present embodiment, and a shape of the subbeads
144 and 146 provided to further increase the
rigidity of the fuel tank 100, will be described in
detail.
[0029] [1-2. Shape of bead]
(A. Length of bead)
First, a length LB in the longitudinal direction
of the bead 142 formed on the lower tank 120 will be
described based on Fig. 3 to Fig. 8. It is desirable
to set the length LB of the bead 142 to a length being
about 80% or more of a length L of a flat portion
being a portion in which an R shape of the bottom
surface part 124 of the lower tank 120 is not formed.
By setting the length LB of the bead 142 to such a
length, it is possible that the reduction in the
natural frequency in the secondary panel vibration
- 14 -
mode is suppressed to 10% at the maximum, resulting
in that the rigidity of the fuel tank 100 can be
sufficiently maintained. Accordingly, the fatigue
failure of the spot-welded portions 150, at which the
sub-tank 130 and the lower tank 120 are joined,
caused by the vertical vibration during traveling,
can be prevented over a period of time during which
the fuel tank 100 is in service.
On the contrary, it was found out that, when the
natural frequency in the secondary panel vibration
mode is reduced by more than 10%, the rigidity of the
fuel tank 100 becomes insufficient, resulting in that
the fatigue failure of the spot-welded portions 150
frequently occurs during the period of time in which
the fuel tank 100 is in service.
[0030] An effect provided by setting the length of LB
of the bead 142 to the length being about 80% or more
of the length L of the flat portion, was verified by
a simulation using a finite element method. As
conditions of the simulation, a length, a width, and
a height as a size of the lower tank 120 were set to
600 mm, 450 mm, and 120 mm, respectively, and a
length, a width, and a height as a size of the subtank
130 were set to 200 mm, 160 mm, and 90 mm,
respectively. Further, it was set such that the bead
142 and the sub-beads 144 and 146 are formed on the
bottom surface part 124 of the lower tank 120 as
illustrated in Fig. 3, and each of these beads has a
width of 40 mm and a depth of 7 mm. The lower tank
- 15 -
120 and the sub-tank 130 are set to be fixed by spotwelded
portions 150a to 150c provided in the
longitudinal direction between the bead 142 and the
sub-bead 144, and spot-welded portions 150d to 150f
provided in the longitudinal direction between the
bead 142 and the sub-bead 146.
[0031] Further, a ratio of the length LB of the bead
142 to the length L of the flat portion of the lower
tank 120 is changed, and a ratio of a natural
frequency after changing the length LB of the bead 142
to a natural frequency when the length LB of the bead
142 is the length L of the flat portion (also
referred to as a "first reference natural frequency")
was calculated.
[0032] The following Table 1 and Fig. 4 to Fig. 8
present results of the above-described simulation.
Each of Fig. 4 and Fig. 5 is an explanatory diagram
illustrating a secondary panel vibration mode when
the length LB of the bead 142 is set to the length L
of the flat portion. Fig. 6 is a plan view
illustrating a shape of the lower tank 120 when the
length LB of the bead 142 is set to a length being 48%
of the length L of the flat portion. Each of Fig. 7
and Fig. 8 is an explanatory diagram illustrating a
secondary panel vibration mode when the length LB of
the bead 142 is set to the length being 48% of the
length L of the flat portion. Note that in Figs. 4,
5, 7 and 8, a portion with a deeper color indicates a
portion with a larger amplitude in the fuel tank 100
- 16 -
in the up and down directions (z direction).
[0033]
[Table 1]
LENGTH OF BEAD [%] | NATURAL FREQUENCY [%]
100 ~ 100
8^^ 90 .
64 l_e
48 -. 70
[0034] From the results in Table 1, it can be
understood that as the length LB of the bead 142 is
set to be shorter than the length L of the flat
portion, the ratio of the natural frequency to the
first reference natural frequency is lowered.
Therefore, when the length LB of the bead 142 becomes
too small, the rigidity of the fuel tank 100 cannot
be sufficiently secured.
[0035] Further, when the secondary panel vibration
mode when the length LB of the bead 142 is set to the
length L of the flat portion is seen, among the spotwelded
portions arranged in two rows in the
longitudinal direction, each row having three spotwelded
portions, an amplitude in each of the spotwelded
portions 150a, 150c, 150d and 150f close to a
sidewall part 122 of the lower tank 120, is larger
than that of another portion, as illustrated in Fig.
4 and Fig. 5. Specifically, as illustrated in Fig. 5,
it can be understood that the lower tank 120 vibrates
in a vibration mode in which the spot-welded portions
150a, 150c, 150d and 150f are set to antinodes, and
- 17 -
the spot-welded portions 150b and 150e are set to
nodes.
[0036] Meanwhile, when the secondary panel vibration
mode of the lower tank 120 when the length LB of the
bead 142 is set to the length being 48% of the length
L of the flat portion illustrated in Fig. 6 is seen,
an amplitude in the up and down directions (z
direction) in the vicinity of both ends of the bead
142 is increased, as illustrated in Fig, 7 and Fig. 8.
At this time, it can be understood that, when
compared to a case where the length LB of the bead 142
is set to the length L of the flat portion, a
magnitude of amplitude in the up and down direction's
is also increased, resulting in that the lower tank
120 is largely vibrated, and the sufficient rigidity
is not maintained.
[0037] From the results of simulation as above, it
is judged that a sufficient rigidity as the lower
tank 120 is maintained in a state up to when the
reduction in the natural frequency from the first
reference natural frequency is suppressed to about
10%, and accordingly, the length LB of the bead 142
was defined as 80% or more of the length L of the
flat portion. Note that it is also possible -that the
length LB of the bead 142 exceeds the length L of the
flat portion of the lower tank 120, and the bead is
formed continuously to reach the sidewall part 122,
[0038] (B. Bead width)
Next, explanation will be made on a bead width WB
- 18 -
in an x direction of the bead 142, based on Fig. 9
and Fig. 10. The bead width Wg of the bead 142 formed
on the lower tank 120 according to the present
embodiment is desirably set to have a length being
about 50% or more of an interval between the two rows
of spot-welded portions 150a to 150c and 150d to 150f
adjacent in the x direction (also referred to as a
"spot welding interval") Ws- By setting the bead
width WB to have the length being about 50% or more of
the spot welding interval Ws, it is possible to
suppress the reduction in the natural frequency in
the secondary panel vibration mode to about 10%,
resulting in that the rigidity of the fuel tank 100
can be sufficiently maintained.
[0039] An effect provided by setting the bead width
WB to have the length being about 50% or more of the
spot welding interval Ws, was verified by a simulation
using a finite element method. Here, a length, a
width, and a height as a size of the lower tank 120
were set to 600 mm, 4 5 0 mm, and 120 mm, respectively,
and a length, a width, and a height as a size of the"
sub-tank 130 were set to 200 mm, 160 mm, and 90 mm,
respectively. Further, it was set such that the bead
142 and the sub-beads 144 and 146 are formed on the
bottom surface part 124 of the lower tank 120 as
illustrated in Fig. 3, and each of these beads has a
depth of 7 mm. The lower tank 120 and the sub-tank
130 are set to be fixed by the spot-welded portions
150a to 150c provided in the longitudinal direction
- 19 -
between the bead 142 and the sub-bead 144, and the
spot-welded portions 150d to 150f provided in the
longitudinal direction between the bead 142 and the
sub-bead 146. The spot welding interval Ws was set to
85 mm, and a bead width of each of the sub-beads 144
and 146 was set to 40 mm.
[0040] Further, a ratio of the bead width WB of the
bead 142 to the spot welding interval Ws is changed,
and a ratio of a natural frequency after changing the
bead width WB of the bead 142 to a natural frequency
when the bead width WB of the bead 142 has a length
being 66% of the spot welding interval Ws (also
referred to as a "second reference natural
frequency") was calculated. Note that the length
being 65% of the spot welding interval Ws is a maximum
value of the bead width WB of the bead 142 capable of
being obtained in the manufacture in which a space
required at the time of performing the spot welding
operation is taken into consideration (refer to Fig,
9) .
[0041] The following Table 2 presents results of the
above-described simulation. Further, Fig. 9
illustrates a shape of the lower tank 120 when the
bead width WB of the bead 142 has a length being 66%
of the spot welding interval Ws, and Fig. 10
illustrates a shape of the lower tank 120 when the
bead width WB of the bead 142 has a length being 19%
of the spot welding interval Ws.
[0042] [Table 2]
- 20 -
BEAD WIDTH [%] NATURAL FREQUENCY [%]
66 100
47 92
28 84
19 . 8^^
[0043] From the results in Table 2, it can be
understood that as the bead width WB of the bead 142
becomes smaller, the ratio of the natural frequency
to the second reference natural frequency is lowered.
Specifically,» as the bead width WB of the bead 142
becomes smaller, the vibration in the up and down
directions of the lower tank 120 is increased. From
the results of the simulation, it is judged that a
sufficient rigidity as the lower tank 120 is
maintained in a state up to when the reduction in the
natural frequency from the second reference natural
frequency is suppressed to about 10%, and accordingly,
the bead width WB of the bead 142 was defined as 50%
or more of the spot welding interval Ws.
[0044] (C. Position of sub-bead)
On the lower tank 120 according to the present
embodiment, the sub-beads 144 and 146 are formed on
both sides of the jointless continuous bead 142
formed in the longitudinal direction on approximately
the center line of the lower tank width WL of the subtank
130. The sub-beads 144 and 146 are formed in an
auxiliary manner to further increase the rigidity of
the lower tank 120. Each of the sub-beads 144 and
146 is only required to be formed on a flat portion
- 21 -
from an end face of the sub-tank 130 to an end of an
R shape of a curved portion of the lower tank 120
(also referred to as a '"width WA in which the sub-bead
can be disposed") in the width direction of the lower
tank 120. For example, each of the sub-beads 144 and
146 can also be formed on the sub-tank 130 side as
illustrated in Fig. 11, or can also be formed on a
side surface side of the lower tank 120 as
illustrated in Fig. 12, within the width WR in which
the sub-bead can be disposed.
[0045] A simulation regarding how much of the
rigidity of the fuel tank 100 is changed depending on
the positions at which the sub'-beads 144 and 146 are
formed, was conducted. In the simulation, tanks
having the same shapes as those of the lower tank 120
and the sub-tank 130 set in the studies regarding the
length of bead described above, are assumed, and a
change in the natural frequency when the installation
positions of the sub-beads 144 and 146 are changed on
the flat portion from the end face of the sub-tank
130 to the end of the R shape of the lower tank 120,
was verified. As a result of this, even if the
installation positions of the sub-beads 144 and 146
are changed within the above-described range, a value
of the natural frequency is changed by 10% or less
with respect to the first reference natural frequency,
and no large change in the natural frequency caused
by the change in the installation positions of the
sub-beads 144 and 146 was observed.
- 22 -
[0046] Therefore, each of the sub-beads 144 and 146
is only required to be formed in the width WA in which
the sub-bead can be disposed, on the flat portion
from the end face of the sub-tank 130 to the end of
the R shape of the curved portion of the lower tank
120. Accordingly, the reduction in the natural
frequency in the secondary panel vibration mode can
be suppressed to about 10%, and it is possible to
sufficiently maintain the rigidity of the fuel tank
100.
[0047] [1-3. Verification of effect obtained by
forming continuous bead]
The lower tank 120 according to the present
embodiment suppresses a large reduction in the
natural frequency in the secondary panel vibration
mode by forming the jointless continuous bead 142
formed in the longitudinal direction on approximately
the center line of the lower, tank width Wi, of the subtank
130. Here, there was conducted a simulation of
verifying an effect provided by continuously forming
the bead 142 in the longitudinal direction on the
bottom surface part 124 of the lower tank 120, by
comparing the tank with a fuel tank with a
conventional configuration,
[0048] In the present simulation, regarding a case
where the bead 142 is formed continuously in the
longitudinal direction on the bottom surface part 124
of the lower tank 120 illustrated in Fig. 3
(configuration according to the present embodiment)
- 23 -
and a case where three discontinuous beads 147 to 149
are formed in the longitudinal direction on
approximately the center line of the lower tank width
WL of the sub-tank 130 on the bottom surface part 124
of the lower tank 120 illustrated in Fig. 13
(conventional configuration), natural frequencies in
the secondary panel vibration mode were compared.
Note that the present simulation was conducted by
assuming tanks having the same shapes as those of the
lower tank 120 and .the sub-tank 130 set in the
studies regarding the length of bead described above.
[0049] On the bottom surface part 124 of the lower
tank 120 illustrated in Fig. 13, there are provided
the bead 148 formed on the bottom surface part of the
sub-tank 130 on approximately the center line of the
lower tank width WL of the sub-tank 130, and the beads
147 and 149 formed, by being adjacent to the bead 148,
in the longitudinal direction. Discontinuous
portions of bead exist between the bead 147 and the
bead 148, and between the bead 148 and the bead 149.
As a result of the simulation, a natural frequency in
the secondary panel vibration mode of such a lower
tank 120 was proved to be largely lowered by about
30%, compared to the natural frequency (first
reference natural frequency) of the lower tank 120
illustrated in Fig. 3.
[0050] When the secondary panel vibration mode of
the lower tank illustrated in Fig. 13 is seen, it can
be understood that the rigidity is locally lowered in
- 24 -
the discontinuous portions of bead existed between
the bead 147 and the bead 148, and between the bead
148 and the bead 149, and at the portions, the
largest displacement of vibration occurs, as
illustrated in Fig. 14 and Fig. 15. As described
above, it can be understood that, when compared to a
case where the continuous bead 142 is formed in the
longitudinal direction, the magnitude of amplitude in
the up and down directions is also increased,
resulting in that the lower tank 120 is largely •
vibrated and the sufficient rigidity is not
maintained.
[0051] From the results of the present simulation,
it can be recognized that, by forming,the bead 142
continued in the longitudinal direction on
approximately the center line of the lower tank width
WL of the sub-tank 130 on the bottom surface part 124
of the lower tank 120, it is possible to effectively
improve the natural frequency in the secondary panel
vibration mode, when compared to a case where the
discontinuous beads 147 to 149 are formed.
[0052] [1-4. Relation with beads in different
direction and the like]
(A. Area in periphery of extension in
longitudinal direction of bead)
Further, in the lower tank 120 according to the
present embodiment, it is extremely effective not to
form beads in a different direction in a periphery of
extension in the longitudinal direction of the bead
- 25 -
formed on the bottom surface part of the sub-tank 130,
for securing the rigidity. Here, the periphery of
extension in the longitudinal direction of the bead
means a periphery of area with an extent including
the bead itself and an extension in the longitudinal
direction of the bead, and positioned on the outside
of the sub-tank 130.
Specifically, when there is no bead with a
sufficient length in the longitudinal direction of
the lower tank 120, if beads in the different
direction are disposed on the extension in the
longitudinal direction of the bead, an effect of
preventing the reduction in the natural frequency
cannot be practically obtained.
On the other hand, when the bead in the
longitudinal direction of the lower tank 120 has a
sufficient length, namely, when there is a bead
having a length being 80% or more of the length in
the longitudinal direction of the flat portion on the
bottom surface of the lower tank 120, even if.beads
in the different direction or in the same direction
are disposed on a very small portion on the extension
of the bead, there is no influence due to the
disposition, namely, no change in the natural
frequency is caused.
[0053] For example, it is set that in the lower tank
120, the sub-beads 144 and 146 are formed on both
sides of the bead 142 formed in the longitudinal
direction on the bottom surface part of the sub-tank
- 26 - , ,
130, as illustrated in Fig. 16 and Fig. 17. In this
case, a case where two beads 140 are formed on both
sides of the sub-tank 130 along a direction
orthogonal to the longitudinal direction of the bead
142 as beads in the different direction on
discontinuous portions of the bead 142, is set as a
comparative example. Further, an influence given by
a model with such a configuration, namely, a case
where the beads 140 in the different direction being
a direction orthogonal to the longitudinal direction
of the bead 142, are formed in the periphery of
extension in the longitudinal direction of the bead
142, was verified by a simulation.
[0054] In this model, a natural frequency in the
secondary panel vibration mode of the model and the
first reference natural frequency were compared, and
as a result of calculation, it was proved that the
natural frequency in the secondary panel vibration
mode of the present model is reduced by about 15%,
compared to the first reference natural frequency.
Fig. 18 and Fig. 19 present results of the
simulation of the secondary panel vibration mode in
this model. As illustrated in these drawings, the
rigidity is locally lowered in the discontinuous
portions in which the beads 140 in the direction
orthogonal to the longitudinal direction exist as the
beads in the different direction, and at the portions,
the largest displacement of vibration occurs,
resulting in that the rigidity is lowered when the
- 27 -
beads 140 in the different direction are formed in
the periphery of extension in the longitudinal
direction of the bead 142. Therefore, it is suitable
that the beads in the different direction are not
formed in the periphery of extension in the
longitudinal direction of the bead 142.
[0055] (B. Lower surface area of sub-tank)
Further, in the lower tank 120 according to the
present embodiment, it is extremely effective that
the lower surface of the sub-tank 130 has no portion
that is not brought into contact with the bottom
surface part of the lower tank 120 except for the
bead, for securing a failure strength^ of the spotwelded
portions.
[0056] For example, it is set that in the lower tank
120, sub-beads 141 and 144 are formed on both sides
of the bead 142 formed in the longitudinal direction
on the bottom surface part of the sub-tank 130, as
illustrated in Fig. 20 to Fig. 22. In this case, a
case where there exists a portion at which the lower
surface of the sub-tank 130 is not brought into
contact with the bottom surface part of the lower
tank 120 due to, other than the bead 142, the subbead
141 being one of the sub-beads, as illustrated
also in the example of the aforementioned Patent
Literature 4 (Fig. 10), is set as a comparative
example. Further, an influence given by a model with
such a configuration, namely, a case where the lower
surface of the sub-tank 130 has a portion that is not
- 28 -
brought into contact with the bottom surface part of
the lower tank 120 other than the bead 142, was
verified by a simulation.
[0057] In this model, a natural frequency in the
secondary panel vibration mode of the model and the
first reference natural frequency were compared, and
as a result of calculation, it was proved that the
natural frequency in the secondary panel vibration
mode of the present model is lowered by about 15%,
compared to the first reference natural frequency.
Fig. 23 and Fig. 24 present results of the
simulation of the secondary panel vibration mode in
this model. As illustrated in these drawings, since
there exists a portion of the sub-bead 141 at which
the lower surface of the sub-tank 130 and the bottom
surface of the lower tank 120 are not brought into
contact with each other, the joining of the sub-tank
130 becomes unstable, and in the secondary panel
vibration mode to be a problem, the arrangement of
beads becomes nonuniform, resulting in that a
configuration in which a load is concentrated on
specific spot-welded portions on one side (the spotwelded
portions 150 between the bead 142 and the subbead
144) is provided, which causes a failure of the
spot-welded portions. Therefore, it is suitable that
the lower surface of the sub-tank 130 has no portion
that is not brought into contact with the bottom
surface part of the lower tank 120 except for the
bead 142.
- 29 -
[0058] The fuel tank for the vehicle 100 according
to the first embodiment of the present invention has
been described as above. By forming the bead 142
continued in the longitudinal direction on
approximately the center line of the lower tank width
WL of the sub-tank 130, there is no chance that the
rigidity is locally lowered, resulting in that the
fatigue failure of the spot-welded portions 150 being
the joint portions between the lower tank 120 and the
sub-tank 130 caused by the vertical vibration during
traveling of an automobile can be effectively
prevented. Further, in that case, it is extremely
effective that the beads in the different direction
are not formed in the periphery of extension in the
longitudinal direction of the bead 142, and the lower
surface of the sub-tank 130 has no portion that is
not brought into contact with the bottom surface part
of the lower tank 120 except for the bead 142, in
order to secure the rigidity and the failure strength.
[0059] (Second Embodiment)
Next, a fuel tank for a vehicle 100 according to
a second embodiment of the present invention will be
described based on Fig. 25 and Fig. 26. Note that
Fig. 25 is a plan view illustrating one configuration
of the fuel tank 100 according to the present
embodiment. Fig. 26 is a plan view illustrating
another configuration of the fuel tank 100 according
to the present embodiment.
[0060] In the fuel tank 100 according to the present
- 30 -
embodiment, the bead 142 continued in the
longitudinal direction is formed on approximately the
center line of the lower tank width WL of the sub-tank
130 on the bottom surface part 124 of the lower tank
120, and embossed portions 160a to 160d are formed by
embossing among spot-welded portions 150a to 150f at
which the sub-tank 130 is fixed to the lower tank 120.
The embossed portions 160a to 160d function in a
similar manner to the sub-beads 144 and 146 formed on
the bottom surface part 124 of the lower tank 120 of
the fuel tank for the vehicle 100 according to the
first embodiment, and are provided in an auxiliary
manner to improve the rigidity in the secondary panel
vibration mode of the fuel tank 100.
[0061] For example, it is set that the bead 142
extending continuously in the longitudinal direction
on approximately the center line in the width
direction of the sub-tank 130, and the sub-beads 144
and 146 adjacent to the bead 142 in the width
direction, are formed on the bottom surface part 124
of the lower tank 120, as illustrated in Fig. 25,
Further, the sub-tank 130 is fixed to the lower tank
120 by three spot-welded portions 150a to 150c and
three spot-welded portions 150d to ISOf on both sides
of the bead 142. Further, in the lower tank 120
according to the present embodiment, four embossed
portions 160a to 160d are formed among the spotwelded
portions 150a to 150c and 150d to 150f within
an installation area of the sub-tank 130.
- 31 -
[0062] The embossed portion 160a is formed between
the spot-welded portions 150a and 150b, and the
embossed portion 160b is formed between the spotwelded
portions 150b and 150c. Further, the embossed
portion 160c is formed between the spot-welded
portions 150d and 150e, and the embossed portion 160c
is formed between the spot-welded portions 150e and
150f. An embossed width in a width direction (x
direction), an embossed length in a longitudinal
direction (y direction), and an embossed depth in a
depth direction (z direction) of each of these
embossed portions 160a to 150d can be appropriately
set. In an example illustrated in Fig. 25, 'the
embossed width is set to a size smaller than an
interval between the adjacent spot-welded portions
and in which the embossing can be performed, and the
embossed length is set so that the embossed portion
is formed between an end face extending in the
longitudinal direction of the -bead 142 to an end face
of the sub-tank 130. Further, the embossed depth can
be set to the same depth as that of the bead 142, and
the sub-beads 144 and 146, for example.
[0063] By forming the embossed portions 160a to 160d
among the spot-welded portions 150a to 150f as
described above, the natural frequency in the
secondary panel vibration mode of the fuel tank 100
can be further improved, and the rigidity of the fuel
tank 100 can be sufficiently maintained.
[0064] Further, as another example, it is also
- 32 -
possible to form, on the bottom surface part 124 of
the lower tank 120, the bead 142 that continues in
the longitudinal direction on approximately the
center line of the lower tank width WL of the sub-tank
130, and four embossed portions 160a to 160d provided
among the spot-welded portions 150a to 150c, and 150cl
to 150f, as illustrated in Fig. 26. In the lower
tank 120 of the present example, the sub-beads 144
and 146 are not formed on the bottom surface-part 124
of the lower tank 120, compared to the shape of the
lower tank 120 illustrated in Fig. 25. Accordingly,
in order to suppress the reduction in the natural
frequency in the secondary panel vibration mode of
the fuel tank 100, the embossed width of each of the
embossed portions 160a to 160d is enlarged, as
illustrated in Fig. 26.
[0065] The embossed portion 160a is formed between
the spot-welded portions 150a and 150b, and the
embossed portion 160b is formed between the spotwelded
portions 150b and 150c. Further, the embossed
portion 160c is formed between the spot-welded
portions 150d and 150e, and the embossed portion 160c
is formed between the spot-welded portions 150e and
150f. These embossed portions 160a to 160d are
formed in the width direction from the end face
extending in the longitudinal direction of the bead
142 to the end of the R shape of the curved portion
of the lower tank 120. Accordingly, even if the subbeads
144 and 146 are not provided, the reduction in
- 33 -
the rigidity at the spot-welded portions 150a, 150c,
150d and 150f can be prevented when the fuel tank 100
vibrates in the secondary panel vibration mode,
resulting in that the fatigue failure o£ the spotwelded
portions 150a to 150f can be'" prevented.
[0066] The fuel tank for the vehicle 100 according
to the second embodiment of the present invention has
been described as above. In the fuel tank 100
according to the present embodiment, there are formed,
on the bottom surface part 124 of the lower tank 120,
the bead 142 extending continuously in the
longitudinal direction on approximately the center
line in the width direction of the sub-tank 130, and
the embossed portions 160a to 160d provided among the
spot-welded portions 150a to 150c, and 150d to 150f.
Accordingly, it is possible to suppress the•reduction
in the natural frequency in the secondary panel
vibration mode of the fuel tank 100, and the fatigue
failure of the spot-welded portions 150a to 150f can
be effectively prevented.
[0067] Note that in the present embodiment, the
shape of each of the embossed portions 160a to 160d
is approximately a quadrangular shape, but, the
present invention is not limited to such an example,
and it is also possible to form the embossed portions
160a to 160d each having approximately a circular
shape, for example.
[0068] The preferred embodiments of the present
invention have been described in detail above with
- 34 -
reference to the attached drawings, but, the present
invention is not limited to such examples. It is
apparent that a person having common knowledge in the
technical field to which the present invention
belongs is able to devise various variation or
modification examples within the range of technical
ideas described in the claims, and it should be
understood that such examples belong to the technical
scope of the present invention as a matter of course.
[0069] For example, in each of the above-described
embodiments, each of the bead 142 and the sub-beads
144 and 146 is formed as a convex bead projecting
toward the outside of the fuel tank 100, but, the
present invention is not limited to such an example.
For example, each of the beads may also be formed as
a concave bead projecting toward the inner part of
the fuel tank 100. The bead 142 of the lower tank
120 according to each of the above-described
embodiments is formed as a convex bead formed by
making the bottom surface part 142 project toward a
negative direction of z-axis from an inner space in
which the sub-tank 130 is provided, as illustrated in
Fig. 27. At this time, the sub-tank 130 is spotwelded
to the lower tank 120 at a flat portion on
both sides of the bead 142.
[0070] Meanwhile, as illustrated in Fig. 28, for
example, areas of the lower tank 120 corresponding to
both sides of approximately the center line in the
width direction of the sub-tank 130 are projected to
- 35 -
the inner space side of the lower tank 120, to
thereby form convex line portions 142a and 142b.
Each flat surface on the inner space side of each of
the line portions 142a and 142b is utilized as a flat
area required for spot-welding the sub-tank 130 to
the lower tank 120, Further, by forming the line
portions 142a and 142b, there is formed a convex bead
142 projected in the negative direction of z-axis
from the flat surfaces of the line portions 142a and
142b, as illustrated in Fig. 28.
Note that the sub-beads 144 and 145, and the
embossed portions 160a to 160d formed on the bottom
surface part 124 of the' lower tank 120 can be formed
in a convex shape or a concave shape.
[0071] Further, in each of the above-described
embodiments, the sub-beads 144 and 146 are formed on
both sides of the bead 142, but, the present
invention is not limited to such an example, and it
is also possible to form one or a plurality of subbead{
s) on the bottom surface part 124 of the lower
tank 120. The sub-bead is formed continuously in the
longitudinal direction of the fuel tank 100 to be
approximately parallel to the bead 142, as described
in the above-described embodiments.
[0072] Further, in each of the above-described
embodiments, the sub-tank 130 is fixed to the lower
tank 120 by the six spot-welded portions 150a to 150f,
but, the present invention is not limited to such an
example. The number and the welded positions of the
- 36 -
spot-welded portions 150 can be appropriately
determined in accordance with the size of the subtank
130 with respect to the lower tank 120, and the
like.
[0073] Further, although each of the above-described
embodiments is explained using an example of
illustration in which each of the bead 142 and the
sub-beads 144 and 146 formed on both sides of the
bead 142 exhibits a linear shape (refer to Fig. 3 and
the like), it is also possible that the bead 142 is
meandered in the width direction within a range of
interval between the spot-welded portions 150 in the
width direction (x direction) of the sub-tank 130 as
illustrated in Fig. 29, for example.
Alternatively, it is also possible to change the
width of the bead 142 within the range of interval
between the spot-welded portions 150 in the width
direction of the sub-tank 130 as illustrated in Fig.
30.
[0074] Here, there is a case where supporting
members for piping, a baffle plate and the like are
collaterally attached to the inner part of the fuel
tank 100 by spot welding. In such a case, if no
measure is taken, the spot-welded portions 150 formed
by the spot welding and the bead 142 sometimes
interfere with each other. In order to avoid the
interference, it is effective to employ the shape of
bead in which the bead is meandered or the width of
the bead is changed as described above (the
- 37 - '
meandering bead or the width-changed bead).
[0075] Note that when a range of meandering of the
meandering bead or a change width of the widthchanged
bead falls within the range of interval
between the spot-welded portions 150, the reduction
in the natural frequency can be suppressed to about
10%, compared to the linear bead, and the sufficient
rigidity is maintained.
[0076] Further, although an example in which the
bead 142 is formed on the bottom surface part 124 of
the lower tank 120 is described, it is also possible
to form the bead as a bead 142A formed continuously
from the bottom surface part 124 to the sidewall part
122, as additionally illustrated in Fig. 2.
By extending the bead 142A to the area of the
sidewall part 122 as described above, a threedimensional
structure is constructed by the bead 142A
along the bottom surface part 124 and the sidewall
part 122, and accordingly, the rigidity as a whole
can be increased.
INDUSTRIAL APPLICABILITY
[0077] According to the present invention, a fuel
tank for a vehicle capable of increasing a rigidity
of a fuel tank, capable of effectively preventing a
fatigue failure of welded portions between a sub-tank
and a lower tank caused by a vertical vibration
during traveling of a vehicle, and having extremely
excellent durability, reliability and the like, is realized.
[Claim 11 A fuel tank for a vehicle, comprising:
a tank main body in which an upper tank and a
lower tank are mutually joined to form a closed space
in which fuel is housed; and
a sub-tank fixed to a bottom surface part of the
lower tank by spot welding,-wherein:
a plurality of rows of the spot welding are set
along a longitudinal direction of the lower tank with
an interval therebetween in a width direction of said
sub-tank;
at least one bead positioned between the rows of
the spot welding and extending continuously along the
longitudinal direction of the lower tank is formed on
the bottom surface part of the lower tank; and
a lower surface of said sub-tank has no portion
that is not brought into contact with the bottom
surface part of the lower tank except for the bead.
[Claim 21 The fuel tank for the vehicle according to
claim 1, wherein
a length of the bead is set to a length being 80%
or more of a length of a flat portion of the bottom
surface part of the lower tank in the longitudinal
direction.
[Clacim 31 The fuel tank for the vehicle according to
claim 2, wherein
the bead is formed continuously from the bottom
surface part to a sidewall part of the lower tank.
[Claim 41 The fuel tank for the vehicle according to
c l a i m 1, w h e r e i n
t h e p l u r a l i t y of rows of t h e s p o t w e l d i n g a r e
d i s p o s e d t o be symmetric w i t h r e s p e c t t o t h e bead
formed on a p p r o x i m a t e l y a c e n t e r l i n e i n t h e w i d t h
d i r e c t i o n o f s a i d s u b - t a n k .
[Claim 51 The f u e l t a n k f o r t h e v e h i c l e a c c o r d i n g t o
c l a i m 4 , w h e r e i n
a w i d t h of t h e bead is set t o a l e n g t h b e i n g 50%
o r more of t h e i n t e r v a l of t h e rows of t h e s p o t
w e l d i n g which a r e a d j a c e n t w i t h t h e bead t h e r e b e t w e e n .
[Claim 61 The f u e l t a n k f o r t h e v e h i c l e a c c o r d i n g t o
c l a i m 4 , w h e r e i n
" e a c h of embossed p o r t i o n s formed i n a v e r t i c a l
d i r e c t i o n w i t h r e s p e c t t o t h e bottom s u r f a c e p a r t of
t h e l o w e r t a n k is p r o v i d e d between p o r t i o n s formed by
t h e s p o t w e l d i n g and a d j a c e n t i n t h e row d i r e c t i o n .
[Claim 71 The f u e l t a n k f o r t h e v e h i c l e a c c o r d i n g t o
c l a i m 1, w h e r e i n
a n o t h e r bead i s formed on a f l a t p o r t i o n between
an end f a c e i n t h e w i d t h d i r e c t i o n of said s u b - t a n k
t o a s i d e w a l l p a r t of t h e l o w e r t a n k , on t h e b o t t o m
surface p a r t of t h e lower t a n k a l o n g t h e l o n g i t u d i n a l
d i r e c t i o n of t h e lower t a n k .
[Claim 81 The f u e l t a n k f o r t h e v e h i c l e a c c o r d i n g t o
c l a i m 4 , w h e r e i n
t h e bead i s formed a s a m e a n d e r i n g b e a d
m e a n d e r i n g i n t h e w i d t h d i r e c t i o n o r a w i d t h - c h a n g e d
bead whose w i d t h is changed.
[Claim 91 The f u e l t a n k f o r t h e v e h i c l e a c c o r d i n g t o
c l a i m 1, w h e r e i n : J
s a i d t a n k main body and s a i d s u b - t a n k a r e made of
' a t l e a s t any one of m a t e r i a l s of a s u r f a c e t r e a t e d
, s t e e l s h e e t , a stainless s t e e l , and an aluminum
a l l o y ; and
t h e lower t a n k and s a i d s u b - t a n k a r e formed.of
t h e same m a t e r i a l .