Abstract:
A vehicle occupant restraint system comprises an inflatable knee airbag ( 10 ). The knee airbag ( 10 ) includes an airbag sheath ( 12 ) and at least one vent hole ( 18 ) formed therein. The vent hole ( 18 ) is arranged in the upper half of the knee airbag ( 10 ) and is provided on a side of the knee airbag ( 10 ) facing away from the vehicle occupant which in the operating position of the knee airbag ( 10 ) is directly opposed to a fixed component ( 28 ) in the vehicle interior. A method of protecting a vehicle occupant comprises the following steps of: Inflating a knee airbag ( 10 ) with gas so that the knee airbag ( 10 ) adopts an operating position; and adapting the gas discharge behavior in accordance with the constitution, especially the body height of the vehicle occupant, in that a vent hole ( 18 ) formed in an upper section ( 36 ) of the knee airbag ( 10 ) is pressed against a fixed component ( 28 ) in the vehicle interior by the knee ( 38 ) of a normal size vehicle occupant moving forward and is closed thereby, or is not pressed against a fixed component ( 28 ) in the vehicle interior by the knee ( 38 ) of a small size vehicle occupant moving forward.

Description:
RELATED APPLICATIONS 
     This application corresponds to PCT/EP2012/002377, filed Jun. 5, 2012, which claims the benefit of German Application No. 10 2011 105 032.2, filed Jun. 8, 2011, the subject matter, of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a vehicle occupant restraint system comprising an inflatable knee airbag. The invention further relates to an automotive vehicle comprising such vehicle occupant restraint system. Finally the invention also relates to a method of protecting a vehicle occupant. 
     The inflation and discharge behavior of airbags is tested by crash test dummies in impact tests. Crash test dummies are life-size dummies simulating the human body. In order to duly take the different anatomic factors of real persons into account during simulations, the tests are usually carried out with three different dummies. The “50 percent male” having a body height of 175 cm and a weight of 78 kg is supposed to generally correspond to a vehicle occupant of “normal” size, the “95 percent male” having a body height of 188 cm and a weight of 101 kg is supposed to correspond to a tall vehicle occupant and the “5 percent female” having a body height of 152 cm and a weight of 54 kg is supposed to correspond to a small vehicle occupant. 
     From DE 10 2004 009 013 A1 a side impact restraint device for protecting an occupant being seated on a vehicle seat in a vehicle comprising an airbag is known which adapts to the build of the vehicle occupant. The airbag deploys between a side structure of the vehicle and the vehicle occupant and includes a first vent hole arranged in a zone of the airbag facing the vehicle occupant. The vertical position of the vent hole is selected so that it is not covered by the upper part of the body of a small size vehicle occupant but is definitely covered by a large size vehicle occupant. Thus for a small size vehicle occupant a lower internal pressure is resulting than for a large size vehicle occupant. 
     In EP 1 279 565 A1 a generic occupant protection device comprising a knee airbag is shown in which a vent hole is provided in an upper end portion. The vent hole interacts with a passenger airbag disposed above the knee airbag. When the legs of the vehicle occupant strike the deployed knee airbag and the vent hole initially is not yet covered by the passenger airbag, gas may flow out of the vent hole. In this way the impact exerted on the vehicle occupant is damped. However, as soon as the deployed passenger airbag covers the vent hole, the gas discharge from the vent hole is restricted so that a minimum internal pressure of the knee airbag is maintained for a quite long period of time. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to enable the protective effect of an inflated knee airbag to be adapted to the build of a vehicle occupant. 
     Said object is achieved by a vehicle occupant restraint system comprising the features of claim  1 . Advantageous and expedient configurations of the vehicle occupant restraint system according to the invention are described in the corresponding subclaims. 
     The vehicle occupant restraint system according to the invention comprises an inflatable knee airbag having an airbag sheath and at least one vent hole formed therein. The vent hole is arranged in the upper half of the knee airbag. In accordance with the invention, the vent hole is provided on a side of the knee airbag facing away from the vehicle occupant which, in the operating position of the knee airbag, is directly opposed to a fixed component in the vehicle interior. 
     The invention is based on the finding that an improved protective effect can be obtained by adaptation of the internal pressure of the knee airbag to the physical constitution of the respective vehicle occupant. Compared to expensive “active” solutions that require e.g. an electronic occupant qualification and a multi-stage inflator, the “passive” solution according to the invention can be realized at definitely lower cost. 
     The substantial requirement for adapting the internal pressure within the knee airbag to the build of the respective vehicle occupant is the particular arrangement of the vent hole on the side facing away from the vehicle occupant directly opposed to a fixed component in the vehicle interior, especially the lower part of the instrument panel of the vehicle. It is possible in this way to automatically control closing and releasing the vent hole by interaction of the vehicle occupant with the knee airbag: The vent hole is closed when the respective section of the airbag in which the vent hole is formed is pressed against the component by the knees of the vehicle occupant moving forward. On the other hand, the vent hole remains open when the section including the vent hole is not pressed against the component. 
     In this way, irrespective of the constitution of the vehicle occupant, always the same amount of gas can be used for inflating the knee airbag, because the internal pressure can adapt automatically. Occupant qualification or a multi-stage inflator is not necessary for the solution according to the invention. 
     In particular, the knee airbag is arranged at its operating position (i.e. in the completely inflated condition) so that the vertical section of the airbag in which the vent hole is formed is opposed to the knee of a normal size vehicle occupant. This ensures that in the case of a normal size vehicle occupant the vent hole is pressed against the component in the vehicle interior and is thus closed so that the maximum internal pressure is permanently available for restraining the vehicle occupant. 
     Accordingly, the knee airbag should be arranged at its operating position also in such manner that the vertical section of the airbag in which the vent hole is formed is not opposed to the knee of a small size vehicle occupant. This ensures that in the case of a small size vehicle occupant the knees of the latter do not close the vent hole and gas can flow out of the knee airbag. The internal pressure is reduced and a better damped restraint behavior is permitted. 
     In this context, the invention is based on the fact that the various vehicle occupants (large size, normal size, small size) can be differentiated with sufficient reliability by way of the vertical knee position. The knee position relates to the seating position of appropriate dummies common in crash tests. 
     Due to the anatomic differences and the contingent variation of the knee height of the vehicle occupants, it is most favorable when in a knee airbag which in its operating position, especially by tethers inside the knee airbag, is subdivided into plural vertical sections the vent hole is formed in an upper, preferably the uppermost section. 
     In the case of a knee airbag of a vehicle occupant restraint system according to the invention designed for large size, normal size and small size vehicle occupants a vent hole having a diameter between 20 mm and 40 mm turns out to be ideal. 
     In accordance with a more expensive approach of the invention, in a special embodiment plural vertically superimposed vent holes are provided. Such arrangement allows for adapting the internal pressure of the airbag more accurately to the actual constitution of the vehicle occupant. In this way, a normal size occupant will press more vent holes against the opposite fixed component and thus close them than a somewhat smaller size occupant. Depending on the number and the distance of the vent holes, different variations are possible. 
     In order to fulfill the basic functionality of the knee airbag according to the invention it is provided in the afore-mentioned embodiment that at least a large part of the vent holes is arranged between the knee impact positions of a normal size vehicle occupant but above the knee impact positions of a small size vehicle occupant. 
     Preferably the superimposed vent holes have a larger horizontal width compared to their vertical height. By way of such gill-type structure a plurality of vent holes can be realized within small space (in the vertical direction) so that a finer adjustment of the internal pressure of the airbag can be obtained. 
     In the embodiment including the superimposed vent holes preferred adjustments of the internal pressure of the airbag depending on the build of the vehicle occupant are resulting from a vertically centered arrangement of the vent holes and/or from the fact that the width of the vent holes decreases with increasing height. 
     In order to direct the gas flowing out of the knee airbag away from the vehicle occupant a vent passage starting from the vent hole can be provided on the outside of the knee airbag. The vent passage ensures that the possibly hot gas provided in the knee airbag and containing particles does not impinge on the vehicle occupant. 
     According to the preferred embodiment of the vent passage, the latter is formed by a cover that is mounted on the airbag sheath in a way not completely circumferentially around the vent hole and can bulge under the pressure of the outflowing gas. On the one hand, this allows for a space-saving design as the cover needs hardly any additional space in the folded airbag package. On the other hand, the bulging ensures that the vent passage can reach and maintain a predetermined size, even if the inflated airbag (without the pressure exerted by a vehicle occupant moving forward) is in close contact with the instrument panel. 
     In accordance with a preferred configuration of the vent passage, the length thereof is at least 1.5 times the diameter of the vent hole, measured from the center thereof. 
     In order to protect the vehicle occupant against the gas escaping from the knee airbag the vent passage should not be directed toward the occupant, but it should extend e.g. to one side, preferably in the direction of a vehicle side structure. 
     Alternatively, the vent passage can also extend substantially upwards or downwards. 
     In the latter case (vent passage extending upwards) an embodiment in which the cover has a central downwardly facing extension not mounted on the airbag sheath is of special advantage. A vehicle occupant whose body height and weight range between that of a normal size vehicle occupant and a small size vehicle occupant presses only said extension against the fixed component. Thus the cross-section of the vent passage is reduced, but not to zero. Especially in this case a two-part vent passage is formed by the central arrangement of the extension, with each of the partial passages facing obliquely downwards. 
     Being tailored to this course of the partial passages, the vent hole preferably includes one or more bulges extending equally obliquely downwards. 
     The object of the invention is also achieved by a method of protecting a vehicle occupant comprising the steps of:
         Inflating a knee airbag with gas so that the knee airbag adopts an operating position;   Adapting the gas discharge behavior in accordance with the constitution, especially the body height of the vehicle occupant, in that a vent hole formed in an upper section of the knee airbag       

     a) is pressed against a fixed component in the vehicle interior by the knee of a normal size vehicle occupant moving forward and is closed thereby, or 
     b) is not pressed against a fixed component in the vehicle interior by the knee of a small size vehicle occupant moving forward. 
     In this case, too, a seating position usually adopted by appropriate crash test dummies in impact tests is assumed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention are evident from the following description and from the enclosed drawings which are referred to and in which: 
         FIG. 1  shows a top view onto a spread knee airbag for a vehicle occupant restraint system according to the invention in accordance with a first embodiment; 
         FIG. 2  shows a perspective view of a cut-out of the knee airbag from  FIG. 1  in the inflated condition; 
         FIG. 3  shows interaction of a normal size vehicle occupant with the inflated knee airbag from  FIG. 1  in a vehicle; 
         FIG. 4  shows interaction of a small size vehicle occupant with the inflated knee airbag from  FIG. 1  in a vehicle; 
         FIG. 5  is a top view onto a spread knee airbag for a vehicle occupant restraint system according to the invention in accordance with a second embodiment; 
         FIG. 6  is a top view of a spread knee airbag for a vehicle occupant restraint system according to the invention in accordance with a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a knee airbag  10  for a vehicle occupant restraint system according to a first embodiment comprising an airbag sheath  12  having a connecting zone  14  for an inflator and connecting points  16  for one or more tethers. Furthermore the airbag sheath  12  includes a vent hole  18  which is substantially circular and preferably has a diameter between 20 mm and 40 mm. 
     The vent hole  8  is covered by a cover  20  in the form of a flexible fabric layer or another not dimensionally stable material which, with the exception of a venting section  22  around the vent hole  18 , is fastened to the airbag sheath  12 . The cover  20  is fastened so that a passage portion  24  of the cover  20  located above the vent hole  18  can bulge. Thus said passage portion  24  together with the part of the airbag sheath  12  located there below can define a vent passage for the gas provided inside the knee airbag  10  in the inflated condition of the knee airbag  10 , as shown in  FIG. 2 . For this purpose, the cover  20  can also be made of expandable material. The length of the vent passage is at least 1.5 times the diameter of the vent hole  18 , measured from the center thereof. 
     It is evident from  FIGS. 3 and 4  that the knee airbag  10  is mounted in a receptacle  26  behind the lower part of the instrument panel  28  of the vehicle so that in the case of restraint upon inflation by the inflator it deploys out of an aperture in the instrument panel  28 . In the shown operating position the knee airbag  10  adopts an orientation substantially corresponding to the orientation of the spread but not inflated knee airbag  10  illustrated in  FIG. 1 . 
     This means that in the inflated condition of the knee airbag  10  the vent passage extends horizontally in the direction of a vehicle side structure starting from the vent hole  18 . Also in the mounted, not deployed condition of the knee airbag  10  the vent passage extends in this direction. However, in other embodiments also extensions deviating by up to 30° or even up to 45° from the horizontal are possible. 
     Furthermore, it is evident from  FIGS. 3 and 4  that the vent hole  18  is formed in the upper half of the knee airbag  10 . In the shown embodiment the knee airbag  10  is subdivided by a constriction  30  caused by the arrangement and the length of one or more inner tethers  32  into a lower section  34  which is closer to the receptacle  26  and an upper section  36  in which the vent hole  18  is provided. When the knee airbag  10  includes plural vertical sections, the vent hole  18  is preferably formed in the uppermost of said sections. In any case the vent hole  18  is formed on the side of the knee airbag  10  facing away from the vehicle occupant and thus is immediately opposed to the instrument panel  28 . 
     The arrangement of the knee airbag  10  including the vent hole  18  is selected such that a normal size vehicle occupant, who can be simulated by a “50 percent male” in crash tests, upon moving forward impacts with his/her knee  38  on the inflated knee airbag  10  so that the upper section  34  of the knee airbag  10 , and especially the vent hole  18  formed in the same, is pressed against the instrument panel  28 . In this closed condition of the vent hole  18  represented in  FIG. 3  no gas or only very little gas can escape from the knee airbag  10  through the vent hole  18  so that the knee airbag  10  is adapted to maintain a high internal pressure and thus exert a protective function sufficient for the normal size vehicle occupant. 
     Intentional suppression of the gas discharge is permitted by the fact that the upper airbag section  36  including the vent hole  18  is opposed to the knee  38  of the normal size vehicle occupant. The knee airbag  10  shows a similar behavior in the case of a large size vehicle occupant. 
     On the other hand, the knee airbag  10  behaves differently in the case of a small size vehicle occupant who can be simulated by a “5 percent female”. As is evident from  FIG. 4 , the small size vehicle occupant impacts with his/her knee  38  most largely on the lower section  34  of the knee airbag  10 . The knee airbag  10  is designed so that in this case the vent hole  18  is spaced from the instrument panel  28  so that gas can escape from the knee airbag  10  through the vent hole  18 . This behavior of the knee airbag  10  is assisted by the fact that the cover  20  bulging by the gas exiting through the vent hole  18  can be supported on the instrument panel  28  and thus keeps the vent hole  18  at a distance from the instrument panel  28 . 
     In this way the impact of the knees  38  of the small size vehicle occupant on the fully inflated knee airbag  10  is damped. However, the amount of gas flowing into the knee airbag  10  as well as the vent hole  18  and the vent passage are dimensioned such that an internal pressure of the airbag sufficient for a small size vehicle occupant is ensured. The vent passage directs the outflowing gas to the side into an area that is uncritical to the vehicle occupant. 
     The design and the arrangement of the knee airbag  10  are geared to the knee positions of the respective crash test dummies (50 percent male, 95 percent male and 5 percent female) as they are predetermined by the common seating position of the dummies during impact tests (no OOP positions as they are called). 
     Also embodiments are possible in which the vent hole  18  is not opposed to the instrument panel  28  but to another fixed component in the vehicle interior. What is important is that the vent hole  18  can be pressed against said component by a normal size vehicle occupant moving forward so that thereby the outflow of gas is hindered. 
     As a matter of course, also a plurality of said vent holes  18  and pertinent vent passages can be provided. 
       FIG. 5  illustrates a knee airbag  10  for a vehicle occupant restraint system according to a second embodiment. As in  FIG. 1 , also in this case the airbag  10  is shown from the opposite view of the vehicle occupant. In contrast to the first embodiment, here plural vent holes  18  are provided. The vent holes  18  are provided, as in the first embodiment, on the side of the knee airbag  10  facing away from the vehicle occupant, i.e. they are directly opposed to the instrument panel  28  or any other fixed component. All of the vent holes  18  are also arranged in the upper half, more exactly speaking in the upper third, of the knee airbag  10 . 
     The vent holes  18  are superimposed at small distances and, compared to their vertical height, have a definitely larger horizontal width. Especially the vent holes  18  can have an oval or slit-like shape. The vent holes  18  are arranged to be vertically centered with the width of the vent holes  18  decreasing with increasing height. 
     The vent holes  18  are covered by a cover  20  made of a flexible fabric layer or any other not dimensionally stable material. As in the first embodiment, the cover  20  is fastened, with the exception of a venting section  22  around the vent holes  18 , to the airbag sheath  12  so that a passage section  24  of the cover  20  located above the vent holes  18  can bulge and thus define a vent passage. However, the cover  20  is arranged such that the venting section  22  faces upwards, i.e. the vent passage extends vertically upwards. 
     In  FIG. 5  also the knee impact points  40  for a normal size vehicle occupant and the lower knee impact points  42  for a small size vehicle occupant are indicated. In the case of a normal size vehicle occupant, practically all or at least a larger part of the vent holes  18  are located between the knee impact points  40  (on the opposite side of the knee airbag  10 ), wherein in the case of a small size vehicle occupant practically all or at least a larger part of the vent holes  18  are located above the knee impact points  42 . 
     If, upon moving forward, a normal size vehicle occupant impacts with his/her knees  38  on the inflated knee airbag  10  and presses the upper section  34  of the knee airbag  10  including the vent holes  18  against the instrument panel  28  (cf.  FIG. 3 ), all or at least a larger part of the vent holes  18  are closed thereby. In the case of a small size vehicle occupant, on the other hand, all or at least a larger part of the vent holes  18  are available, as the vent holes  18  are spaced apart from the instrument panel  28  (cf.  FIG. 4 ). In this case, part of the gas can flow out upwards through the vent passage. 
     For vehicle occupants whose body height and/or weight ranges from that of a 5 percent dummy to that of a 50 percent dummy more or fewer vent holes  18  are accordingly released or blocked. Thus the internal pressure of the knee airbag  10  individually adapts to the constitution of each vehicle occupant. 
       FIG. 6  illustrates a knee airbag  10  for a vehicle occupant restraint system according to a third embodiment. In this case a single vent hole  18  arranged in the same zone of the knee airbag  10  as the vent holes  18  of the second embodiment is provided. The vent hole  18  is also covered by a cover  20 . 
     As can be inferred from the enlarged detail in  FIG. 6 , the vent hole  18  has a special shape here. On the basis of an oval basic shape  18   a , plural bulges  18   b  extend on both sides of a centerline M obliquely downwards, viz. away from the centerline M. 
     The cover  20  in turn delimits a vent passage which in this case faces downwards, i.e. in the direction of the floor space of the vehicle. Moreover the cover  20  is shaped so that in the center (related to the centerline M) it has an extension  20   a  and optionally on both sides of the centerline M it has indentations  20   b.    
     As in the foregoing embodiments, a normal size vehicle occupant will press the knee airbag  10  with his/her knees  38  against the instrument panel such that the vent hole  18  is closed by the cover  20 . In the case of a small size occupant, on the other hand, whose knees  38  most largely do not impact on the lower section  34  of the knee airbag  10  the vent hole  18  is not pressed to be closed. 
     Even in the case of a somewhat taller vehicle occupant (body height/weight between the 5 percent dummy and the 50 percent dummy), gas can escape from the knee airbag  10 , as in this case only the extension  20   a  of the cover  20  is pressed against the airbag sheath  12 . The vent passage is thus subdivided into two smaller partial passages on both sides of the centerline M. The bulges  18   b  of the vent hole  18  are appropriately tailored to the cover  20  including the extension  20   a  thereof and possibly the indentations  20   b  thereof. 
     If gas escapes through the vent hole  18 , the gas flows out substantially to the bottom, wherein the main venting direction(s) is/are predetermined by the occupant-dependent shape of the vent passage (a large downwardly directed passage or two smaller oblique partial passages). 
     As a matter of course, the afore-described embodiments or particular features of the individual embodiments may be appropriately combined with each other. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  knee airbag 
           12  airbag sheath 
           14  connecting zone 
           16  connecting points 
           18  vent hole 
           18   a  basic shape 
           18   b  bulges 
           20  cover 
           20   a  extension 
           20   b  indentations 
           22  venting section 
           24  passage section 
           26  receptacle 
           28  instrument panel 
           30  constriction 
           32  tether 
           34  lower airbag section 
           36  upper airbag section 
           38  knee 
           40  knee impact points (normal size vehicle occupant) 
           42  knee impact points (small size vehicle occupant)