Abstract:
An internal reinforcement structure of a plastic fuel tank resists deformation of opposing walls of the fuel tank and provides an integral, and directionally sensitive, stress relief feature when pre-determined forces are exceeded. The stress relief feature is contained within a fuel chamber of the fuel tank defined by the opposing walls. Each wall has an inward projecting indentation of the structure which engage one-another at their distal ends or bottom portions, preferably, via a welded plastic engagement area. The indentations have a consistent wall thickness which has a higher cross-sectional area than the stress relief feature causing the stress relief feature to tear as opposed to the tank walls thereby assuring fuel tank integrity.

Description:
REFERENCE TO RELATED APPLICATION 
   Applicants claim priority of German patent application Serial. No. 10104511.5, filed Jan. 31, 2001. 
   FIELD OF THE INVENTION 
   This invention relates to a fuel tank, and more particularly to a fuel tank having a reinforcing structure with an integral stress relief feature. 
   BACKGROUND OF THE INVENTION 
   For safety purposes, fuel tanks must withstand forces produced by predetermined internal and external pressure differentials, transients and stresses. This is particularly true for tanks made of plastic or high density polyethylene, HDPE. Pressure transients are typically caused by environmental temperature changes. For example, a temperature rise of the tank, or the fuel contained therein, will cause the internal tank pressure to rise and deflection or deformation of the shell of the tank to occur. Uncontrolled deformation and/or expansion of the tank must be avoided to prevent the tank shell from contacting the vehicle body, which could lead to the transmission of noise to the passenger compartment of the vehicle or to damage of the tank shell and ensuing fuel leakage. The weight of the fuel contained within the tank may also lead to a deformation of the shell contour. One method to ensure the shape integrity of the tank is to use retainer straps externally clamping the tank shell. Unfortunately, this causes an increase of the assembly and mounting labor or effort and also increases materials costs, all of which ultimately increases the total production costs. Moreover, such measures provide no or only limited protection against external forces or vacuum or sub-atmospheric pressure conditions inside the tank. 
   A further known method utilizes one or multiple kiss-off members, or reinforcing structures inside the tank. The structures typically have two opposing indentations projecting inwardly and molded into respective opposing walls of the tank. The indentations “kiss” or engage and are welded to each other at their distal ends thereby decreasing deflection of the shell and increasing the shape stability of the tank. This increases tank rigidity, however, it tends to increase the opportunity of tank wall tears causing fuel tank leaks when internal pressure within the tank is excessive or external forces exerted upon the tank are extreme. 
   The distal ends of the opposing indentations are engaged by a spot-like or essentially circular weld. Desirably, the engagement area serves not only as a structural feature but also would serve as a yield feature which tears upon excessive forces so that the tank wall or shell does not otherwise tear. The engagement area, however, is difficult to control and/or define in production. Experiments have shown that with this type of point-like spot weld it is very difficult to obtain the desired yield behavior, since the effective wall thickness is larger at the weld than in the surrounding region. Thus, it is observed that often it is the surrounding wall region and not the weld area that yields, resulting in leakage from the tank. 
   SUMMARY OF THE INVENTION 
   An internal reinforcing structure of a plastic fuel tank resists deformation and tearing of opposing walls of the fuel tank and provides an integral, and directionally sensitive, stress relief feature when predetermined forces are exceeded. The stress relief feature is contained within a fuel chamber of the fuel tank defined by the opposing walls. Each wall has an inward projecting indentation of the structure which engage one-another at their distal ends or bottom portions, preferably, via a welded plastic engagement area. The indentations have a consistent wall thickness which has a higher cross-sectional area than the stress relief feature causing the stress relief feature to tear or separate as opposed to the walls thereby assuring fuel tank integrity and avoiding fuel leakage. 
   Preferably, the stress relief feature includes the engagement area located between bottom portions of the opposing indentations. The weld area is preferably annular in shape and encircles a void carried between the two bottom portions. Preferably, the tear or separation of the annular engagement area begins at an opening which lies in the same imaginary plane as the weld engagement area and communicates between the void and the chamber. 
   Objects, features, and advantages of this invention include providing a fuel tank with a reinforcing structure capable of flexing and separating when extreme forces are exerted upon the tank so the external walls do not tear which would lead to fuel tank leakage, has a limited number of parts, and provides a relatively simple, low cost, rugged, durable, and reliable fuel tank. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description, appended claims and accompanied drawings in which: 
       FIG. 1  is a partial cross section view of a fuel tank illustrating a reinforcing structure of the present invention; 
       FIG. 2  is a cross section of the reinforcing structure illustrating a stress relief feature, and taken along line  2 — 2  of  FIG. 1 ; 
       FIG. 3  is a partial cross section of a blow molding tool for forming the reinforcing structure; 
       FIG. 4  is an enlarged cross section of the reinforcing structure of  FIG. 1 ; 
       FIG. 5  is a second embodiment of a reinforcing structure; and 
       FIG. 6  is a third embodiment of a reinforcing structure. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring in more detail to the drawings,  FIG. 1  illustrates a fuel tank  10  made of a high density polyethylene (HDPE) plastic or a multi-layered plastic shell utilizing a blow molding process. The tank  10  has mutually opposed and substantially parallel walls  12 ,  14  having respective interior surfaces  16 ,  18  which substantially face one-another defining a primary fuel chamber  20  between them. Walls  12 ,  14  unitarily form respective deep indentations  22 ,  24  which project into the fuel chamber  20  toward one-another to form a support or reinforcing structure or kiss-off member  26 . As best shown in  FIG. 2 , interior surface  16  adheres to interior surface  18  at the distal ends or bottom portions  28 ,  30  of the respective indentations  22 ,  24  via a weld thereby forming an annular engagement area  32  of a stress relief feature  33  which will yield or separate upon the exertion of excessive shear forces before wall  12  or wall  14  tear themselves. The engagement area  32  is substantially evenly annular, so that the width does not vary appreciably along its circumference. This favorably influences the yield, tearing or separation characteristics through the welded annular engagement area  32 . Yielding of the weld or annular engagement area  32 , instead of the walls of the fuel tank shell, assures that the fuel tank  10  and/or permeation barriers thereof will not leak or permeate fuel vapor as a result of a vehicle accident. 
   The interior surfaces  16 ,  18  enclosed by the engagement area  32  and carried by the bottom portions  28 ,  30  define a substantially hollow sphere or void  34 . In other words, bottom portions  28 ,  30  of respective indentations  22 ,  24  resemble minor reverse indentations or dome portions  31  projecting in an outward direction with reference to the fuel tank  10 . When manufacturing a plastic fuel tank  10  made by a blow molding process, the void  34  is created by the use of tooling  35  (as best shown in  FIG. 3 ) which subjects the walls  12 ,  14  to a vacuum in the direction of arrows  36 ,  38 . The tool  35  is divided into two halves each forming one of the indentations  22 ,  24  and having an annular portion  39  that corresponds to the annular engagement area  32  and a semispherical recess  41  forming one of the domed portions  31 . The vacuum assures that an essentially constant wall thickness  40  is attained in the region of the indentations  22 ,  24  and is dependent on the ratio of the diameter of the annular area  32  to the volume of the hollow region or spherical void  34 . By controlling the height and diameter of the semispherical contour the essentially constant wall thickness  40  in the region of the reinforcing structure is achieved. 
   If the engagement area  32  were of a spot-like or solid weld, without the void  34 , or if the annular engagement area  32  was too large, it is likely that the welded area engagement  32  would not yield, and instead a tear through either wall  12 ,  14  designated by the arrows  37 ,  39  in the region of the indentations  22 ,  24  would occur causing a fuel leak from the tank  10 . To prevent this tearing, a criterium for the dimension of the annular engagement area  32  is desirable. The area of the annular engagement  32  is thus smaller than the total cross sectional area of the reinforcing structure  26 , and must be smaller than a minimum cross sectional area A S  of either tank wall  12 ,  14  which would otherwise represent the location of an undesired tank wall tear. Referring to  FIG. 4 , the tear area A S  is calculated from the inner diameter  42  of the annular engagement  32  and the minimum wall thickness  40  of either wall  12 ,  14  in the region of the annular engagement. The equation is as follows:
 
 A   S =(π) (inner diameter  42 ) (minimum wall thickness  40 ), or 
 
 A   S   =πD   42   T   40  
 
where D 42  is the inner diameter  42 , and T 40  is the minimum wall thickness in the annular engagement  32  region. In a similar manner, the area of the annular engagement  32  can be calculated from its inner diameter  42  and outer diameter  44 , as follows:
 
Area  32 =[(π)/(4)][(outer diameter  44 ) 2 −(inner diameter  42 ) 2 ], or 
 
 A   32 =(π/4)( D   44   2   −D   42   2 ) 
 
where D 44  is the outside diameter of the annular engagement  32 . Experiments have shown that a dependable yield or separation of the welded annular engagement area  32  is obtained when the engagement area  32  is not more than seventy five percent of A S , i.e. A 32 ≦¾A S . Making engagement area  32  even smaller with respect to A S  introduces a greater safety margin for the yielding of the engagement area  32 .
 
   As best illustrated in  FIGS. 1 and 2 , the pressure between the void  34  and the chamber  20  remains equal during the manufacturing cooling process via an opening  46  of the stress relief feature  33  which extends there between. The annular engagement area  32  is therefore not a closed ring, but one interrupted by at least one opening  46 . Opening  46  further supports interior cooling of the void  34  which, along with equalized pressure, leads to a constant wall thickness  40  and an increase in shape stability of the walls  12 ,  14  during removal of the tank  10  from the mold. 
   The opening  46  of the stress relief feature  33  further provides a deliberate, directional, weakening of the annular engagement area  32 . The opening  46  extends radially through and is co-planar to the engagement area  32 , lying in the same imaginary plane. The circumferential orientation of the opening  46  is determined theoretically or empirically and generally extends in the direction of the expected problematic internal or external forces exerted upon the tank  10  during a vehicle accident. The opening  46  thereby forms a starting point for a bust-tear through the annular area  32  when a critical force is exceeded. If multi-directional forces are expected, then more than one such opening  46  may be provided for pressure relief or propagation separation. When a force is sufficient to cause a tear through the reinforcing structure  26 , acting in the direction of the pressure relief opening  46 , an even tear occurs through the engagement area  32  only, and without adverse tears through the walls  12 ,  14 , which could lead to leaks from the tank  10 . 
   Referring to  FIG. 5 , a second embodiment of a reinforcing structure  26 ′ is shown wherein the annular engagement area  32  and the opening  46  of the stress relief feature  33  of the first embodiment is replaced with a plastic stress relief bar  32 ′ with a groove  46 ′ providing a stress relief feature  33 ′. The bar  32 ′ is engaged at both ends to respective plastic fuel tank walls  12 ′,  14 ′ via tear resistant welds or adhesives. The bar  32 ′ is preferably injection molded and is placed within the plastic parison while blow molding the fuel tank and before the blow molding tooling  35 ′ is closed. The stress relief bar  32 ′ carries the lateral groove  46 ′ disposed approximately at mid-section. Groove  46 ′ provides the starting point for a bust-tear through the bar  32 ′ when a predetermined internal or external pressure or force is exceeded. The bar  32  may have a variety of shapes in lateral cross section including circular, oval and retangular. However, the lateral cross section of the bar  32 ′ at the groove  46 ′ is substantially smaller than the cross section of wall  12 ′ or wall  14 ′ or any indentation formed therein. Similar to the first embodiment, the lateral cross section of the bar  32 ′ at the groove  46 ′ is seventy five percent or less the cross section of either indentation of wall  12 ′ or wall  14 ′ substantially near the respective weld of the bar  32 ′. 
   Referring to  FIG. 6 , a third embodiment of a reinforcing structure  26 ″ is shown wherein the annular engagement area  32  of the first embodiment is replaced with a solid rectangular or square engagement area  32 ″. An indentation  22 ″ has a bottom portion or hollow protrusion  28 ″ which, unlike the first embodiment, projects further into a fuel chamber  20 ″ defined by a tank  10 ″. A distal end  50  of the protrusion  28 ″ is carried by an interior surface  16 ″ of a wall  12 ″ which unitarily forms the indentation  22 ″, and is rectangular in shape and thus defines the shape of the engagement area  32 ″ which provides the engagement to an opposing indentation  24 ″. Indentation  24 ″ has a consistent wall thickness which is greater than a minimum wall thickness  40 ″ of the indentation  22 ″ located at an acute juncture  52  disposed between the protrusion  28 ″ and the remaining indentation  22 ″. 
   Unlike the first and second embodiments, when an internal or external force is applied to the reinforcing structure  26 ″ a tear occurs through the wall  12 ″ at the minimum wall thickness  40 ″ of the indentation  22 ″. A plug or welded plate  54  engaged sealably to an exterior surface  56  of the wall  12 ″ prevents leakage of fuel out of the tank  10 ″. Any fuel leakage through wall  12 ″ is contained within a secondary chamber  58  carried between the exterior surface  56  at the indentation  16 ″ and the plug  54 . 
   While the forms of the invention herein disclose constitute presently preferred embodiments, many others are possible. For instance, the fuel tank and reinforcing structure need not be plastic, but can be made of metal or any other variety of materials. Moreover, adherence of the engagement area  32  can be achieved via an adhesive in place of the weld. It is not intended herein to mention all the equivalent forms or ramifications of the invention, it is understood that the terms used herein are merely descriptive rather than limiting and that various changes may be made without departing from the spirit or scope of the invention.