Abstract:
A knee airbag for a motor vehicle is described. This knee airbag extends in its installed state along a longitudinal direction (X) from a rear end to a front end. The airbag skin ( 5 ) of the knee airbag exhibits a front section ( 20 ) and a rear section ( 22 ) and a connecting line for contouring of the airbag is located on the rear section ( 22 ). In order to simplify the assembly of the knee airbag, the connecting line mainly extends transverse to the longitudinal direction (X) and shortens the rear section ( 22 ) in longitudinal direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Patent Application PCT/EP2008/007978 filed Nov. 7, 2009, and German Patent Application 10 2008 060 858.0 filed Dec. 9, 2008. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a knee airbag for a motor vehicle which in an installed state extends along a longitudinal direction from a rear end to a front end. 
     BACKGROUND OF THE INVENTION 
     Knee airbags are becoming increasingly widespread in the automotive industry and have two basic tasks: on the one hand they should prevent the knee-shin-areas of the vehicle occupant to be protected from coming into direct contact with the lower area of the instrument panel in case of an accident. Furthermore, knee airbags should prevent the pelvic area of the occupants from being displaced forwards; this is particularly important in the case of occupants who are not wearing seat belts. 
     The knee airbag is arranged in a housing which is located in the lower area of the instrument panel. It is often desired to locate this housing as far as possible in the direction of the footwell, therefore in the front area of the instrument panel which faces away from the occupant, or in the transition area between the instrument panel and the footwell. However, as the instrument panel is domed in a convex fashion, the knee airbag also has to follow this domed shape, thus showing a “banana shaped” cross-section. In principal this can be achieved through the attachment of outer tethers, which is, however, often not desired. 
     Generic patent application DE 10 2007 005 304 A1 therefore suggests providing a longitudinal fold and several further folds arranged within this longitudinal fold in a rear section of the airbag skin, in other words in the section which is turned towards the instrument panel, whereby the further folds are secured by means of seams. These further folds achieve a shortening of the ridge of the first fold. When the rear section is under tensile stress—which is the case when the airbag is filled—it assumes a concave shape because of the shortening, leading to assumption of the desired “banana shape”. A knee airbag manufactured in this way basically functions well. However, a disadvantage is that several folds have to be provided which are positioned relatively precisely in relation to each other, which on the one hand makes it difficult to automate manufacture, and on the other hand generally requires a precise final inspection. 
     SUMMARY OF THE INVENTION 
     Starting from this basis, the present invention sets itself the problem of improving a knee airbag of the generic type in such a way that it can be manufactured in a more rational way. 
     This task is fulfilled by means of a knee airbag having a rear section of the airbag skin that exhibits a connecting line for contouring of the airbag. The airbag skin is connected with itself along this connecting line, so that a fold is created. The connecting line basically extends transverse to the longitudinal direction of the knee airbag, whereby by longitudinal direction a direction is meant which, when the airbag is in installed and expanded state lies in a vertically standing longitudinal vehicle plane. In other words, the connecting line extends basically perpendicular to the longitudinal direction of the vehicle when in installed and expanded state. 
     As a rule, one single such connecting line is sufficient, which is located in the vicinity of the fixing points of the knee airbag. 
     In a preferred embodiment, the rear section of the airbag skin exhibits an oblong-shaped opening, which is closed by the connecting line. This has two advantages: on the one hand it is prevented that the inflated airbag is pushed away from the internal structure by the material of the fold, which would counteract the desired effect and in the worst case would compensate it partially. On the other hand, this measure makes it easily possible to provide a tether which connects the rear with the front section of the knee airbag precisely at the position of the connecting line. 
     In most applications it will be preferable to form the connecting line as a seam, but it is also possible to implement the connecting line in the form of a weld seam or bonded connection. 
     Further preferred embodiments result from the embodiment described below with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIG. 1  shows a pre-cut piece for an airbag skin in unsewn condition, 
         FIG. 2  shows a first tether to be sewn to the pre-cut piece of  FIG. 1 , 
         FIG. 3  shows a second tether to be sewn to the pre-cut piece of  FIG. 1 , 
         FIG. 4  shows a third tether to be sewn to the pre-cut piece of  FIG. 1 , 
         FIG. 5  shows the finished sewn knee airbag in a view corresponding to  FIG. 1 , 
         FIG. 6  shows a schematic view of a section along section line A-A of  FIG. 5 , and 
         FIG. 7  shows the knee airbag mounted in a vehicle and inflated in a view mainly corresponding to the section shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a pre-cut piece for a knee airbag,  FIGS. 2 ,  3  and  4  each show a tether  30 ,  34 ,  38 , which are sewn together with pre-cut piece  10  from  FIG. 1  to form a knee airbag. The positions of the seams are indicated in  FIGS. 1 to 4  and designated with the reference numbers  40   a ,  42   a ,  44   a ,  46   a ,  48   a ,  50   a  and  52   a.    
     Pre-cut piece  10  basically exhibits mirror symmetry in relation to fold line  25 , around which pre-cut piece  10  is folded during manufacture of the knee airbag. Fold line  25  separates and divides rear section  22  from front section  20 . An edge seam is located along the edge of pre-cut piece  10 ; the position of the edge seam is designated with reference number  40   a . Following closing of the edge seam, front section  20  and rear section  22  are joined directly with one another along their edges, whereby the edges are formed by the edge seam  40   b  and the fold line  25 . When front section  20  and rear section  22  are joined together by means of the edge seam, the areas enclosed by the edge seam first lie on top of one another congruently (with exception of the hole  12 ) and flat. 
     There are the following deviations from the mirror symmetry of front section  20  and rear section  22 : rear section  22  exhibits side ears  23 , which are bent over and sewn with the edge seam  40   b  when the airbag is sewn together, so that they protect the edge seam at the side. Furthermore, rear section  22  exhibits an ignition connector opening  14  at its rearmost end, two bolt holes  16 ,  16 ′ and a cut  18 . A gas generator can be inserted through this cut  18  and can be connected with the interior structure of the vehicle by means of bolts which extend through bolt holes  16 ,  16 ′. An ignition cable is connected to the gas generator through ignition connector opening  14 . 
     Finally, rear section  22  exhibits an opening  12  which is formed in an oblong shape, and whose long axis stands parallel to fold line  25 , in other words perpendicular to the longitudinal direction X of the pre-cut piece. The position  46   a  of the shortening seam extends around the opening  12 . The shortening seam is at the same time the third tether seam. 
     The knee airbag is manufactured of the components described above as follows: as a first step tethers  30 ,  34 ,  38  are sewn along positions  52   a ,  48   a  and  44   a  with the front section  30  of pre-cut piece  10 . After this, the pre-cut piece  10  is folded along fold line  25  and initially the third tether  38  is sewn along position  50   a  and then the first tether  30  is sewn along position  42   a  with rear section  22 . At this state of manufacture, front and rear sections  20 , 22  lie flat and congruently on top of one another, so that edge seam  40   b  is very easy to create. Following creation of edge seam  40   b , finally opening  12  is closed by means of the shortening seam and third tether seam  46   b , whereby the corresponding end area of second tether  46   a  is held between the edges of opening  12 . 
     First and third tether  30 ,  38  are double T-shaped with side cutouts through which gas can flow. The side facing the rear section  22  of the second tether  34  is concave and follows the curvature of the oblong opening  12 . 
       FIG. 5  shows the finished sewn knee airbag. Edge seam  40   b  can be seen, along with first tether seam  42   b , the shortening seam and the third tether seam  46   b  and the fifth tether seam  50   b.    
       FIG. 6  is a schematic section along Plane A-A through  FIG. 5 . In this view, it can be seen particularly clearly how tethers  30 ,  34 ,  38  are sewn to the airbag skin  5  by means of tether seams  42   b ,  44   b ,  46   b ,  48   b ,  50   b ,  52   b . In particular it can be seen how the rear end of second tether  34  is held between the edges of opening  12  by means of the shortening third tether seam  46   b .  FIG. 6  also shows a gas guide element  60  with additional reinforcement layer  62 , which can be additionally provided. Gas guide element  60  and reinforcement layer  62  are not shown in the other figures. 
       FIG. 7  shows the installed situation when the knee airbag is fully inflated. The knee airbag extends from a rear end R to a front end F basically in the direction X. Here the effect can be seen which is caused by shortening seam  46   b . Because of this shortening third tether seam  46   b , rear section  22  is shortened in X direction in comparison with the front section  20 , which causes the airbag filled with gas to bend away at this location, and therefore comes to rest against a lower area  68  of the instrument panel with its support surface  22   a , which is part of rear section  22 , and follows this lower area of the instrument panel, so that a banana-shaped cross-section results. The impact surface  20   a  lies opposite support surface  22   a.    
     Alternatively to the embodiment shown, the airbag skin can be assembled of two pre-cut pieces, whereby one pre-cut piece forms the front section and the other pre-cut piece forms the rear section and a completely surrounding edge seam is provided. 
     The invention is not limited to the illustrated embodiment, but rather includes all means functioning identically within the meaning of the invention. The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.