Abstract:
A gas generator includes a housing, which has outflow openings for outflowing gas, and at least one destructible insulation foil which in a non-activated state of the gas generator closes at least one of the outflow opening so as to be moisture-tight. The insulation foil has a varying thickness. The varying thickness is provided by at least one thermally insulating foil layer with a varying thickness, which lies over a base layer.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to a gas generator with a housing which has outflow openings for outflowing gas.  
         BACKGROUND OF THE INVENTION  
         [0002]    A gas generator of this type normally has, in addition, at least one destructible insulation foil which in the non-activated state of the gas generator closes at least one of the outflow openings so as to be moisture-tight, the insulation foil having a varying thickness.  
           [0003]    A generic gas generator is known from DE 38 31 641 A1. The insulation foil closes off the outflow openings over a period of years, so as not to allow any moisture to reach the solid propellant. By insulation foil of varying thickness, the resistance of the insulation foil on opening can be adjusted and thereby the pressure inside the gas generator in the initial phase of combustion can be varied. The burning behavior is altered hereby. At high ambient temperatures of over 75° C. up to 90° C., propellant burns substantially faster than at low ambient temperatures of, for example, below −25° C. In order to keep the burning speed more constant over the entire temperature range of −40° C. to 90° C., it is intended to keep some outflow openings closed at lower temperatures and thereby to increase the pressure inside the gas generator.  
           [0004]    The invention provides a gas generator by which the fluctuations of the combustion chamber pressure in the above-mentioned temperature range are reduced.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    According to the invention, this is achieved in a gas generator with a housing, which has outflow openings for outflowing gas, and at least one destructible insulation foil which in a non-activated state of the gas generator closes at least one of the outflow opening so as to be moisture-tight. The insulation foil has a varying thickness. The varying thickness is provided by the insulation foil comprising a base layer and at least one thermally insulating foil layer with a varying thickness, which lies over said base layer. In the gas generator proposed, the insulation foil or the insulation foils is or are constructed having several layers. By means of the thermally insulating foil layer, it is achieved that the base layer lying under the foil layer, which is also a type of carrier layer, is heated more slowly and loses its strength more slowly. Thereby, the insulation foil in the region of its higher thickness is destroyed at least later than in the region of smaller thickness. A destruction of the insulation foil in the region of higher thickness, however, does not have to necessarily take place, it is even possible for the insulation foil not to be destroyed at all in this region at low ambient temperatures, whereby the associated outflow opening remains closed.  
           [0006]    Although according to the preferred embodiment, provision is made that an insulation foil is used which covers several outflow openings or even all outflow openings and has a varying thickness in the corresponding regions for various outflow openings, it is also possible to use several insulation foils of differing thickness, in order to thereby achieve the above-mentioned purpose.  
           [0007]    The thermally insulating foil layer can also be partially omitted, i.e. its thickness can be zero. In this region, the base layer is then very quickly exposed to the generated heat and is also destroyed more quickly than in the region with the thermally insulating foil layer.  
           [0008]    In this connection, the term “thermally insulating” means that the foil layer has a distinctly lower thermal conductivity than the base layer. The thermally insulating layer is, in particular, a plastic layer, whereas the base layer is preferably of metal. The thermally insulating layer is to lie on that side of the insulation foil which faces the gas flow, in order to prevent a direct flow onto the base layer from this side.  
           [0009]    Plastics have the characteristic that their strength decreases distinctly greater with increasing temperature in the range of −40° C. to 90° C., than is the case with metals. With an insulation of plastics, therefore, the static opening pressure will decrease in the temperature range of −40° C. to 90° C. On activation of the gas generator, this effect is further intensified, because the greater intensity of the combustion reaction at 90° C. heats the insulation foil more quickly than at −40° C. It is therefore highly efficient to coat the entire metallic base layer in order to lower the opening pressure of the insulation foil with increasing temperature. The effect becomes evident in FIG. 3 b  at the maximums of the combustion chamber pressures at room temperature (RT) and 85° C. The maximum at room temperature even lies over that of 85° C.  
           [0010]    It would therefore be conceivable for the base layer to be embedded between two thermally insulating foil layers, in order to delay a heating from both sides.  
           [0011]    For reasons of manufacturing technique, the base layer is preferably constructed so as to have a uniform thickness.  
           [0012]    A simple manufacture of the insulation foil can be achieved in that the thermally insulating layer is simply sprayed onto the base layer.  
           [0013]    As already indicated, the insulation foil in a preferred embodiment only partially has the thermally insulating foil layer, in order to cover at least one selected outflow opening with the additional thermally insulating layer, and to cover at least one selected outflow opening with only the base layer.  
           [0014]    At least one insulation foil should be coordinated with the output of the gas generator such that the generated gas exposes all outflow openings at an ambient temperature of greater than 75° C., in particular for instance 85° C.  
           [0015]    At low temperatures, i.e. at an ambient temperature of less than −25° C., in particular less than −30° C., not all the outflow openings are to be exposed, i.e. the insulation foil is not destroyed in these regions.  
           [0016]    However, it can also be contemplated that even at low temperatures all the outflow openings or at least one outflow opening, which are closed by the thicker insulation foil, are opened. However, then it is possible by means of the insulation foil with a varying thickness, that a variable time delay can be set, until the associated outflow opening is opened. Here, the outflow opening or openings which are closed with a thicker insulation foil are to be exposed at an ambient temperature of less than −25° C. with a time delay, compared to the outflow opening with thin insulation foil, which is greater at least by the factor four than the time delay which exists at an ambient temperature of greater than 75° C. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows a cross-sectional view through an embodiment of the gas generator according to the invention,  
         [0018]    [0018]FIG. 2 shows an insulation foil which is able to be used in the gas generator according to the invention,  
         [0019]    [0019]FIGS. 3 a  and  3   b  show the combustion chamber pressure profiles and can pressure profiles of a gas generator equipped with a conventional insulation foil (FIG. 3 a ) compared to the combustion chamber pressure profiles and can pressure profiles in the gas generator according to the invention (FIG. 3 b ), and  
         [0020]    [0020]FIG. 4 shows a second embodiment of an insulation foil that can be used. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    In FIG. 1 a gas generator  10  is illustrated, which has a housing  12 , the housing  12  having walls which define the outer housing and the inner housing. The gas generator has a combustion chamber  14  which is filled with solid propellant  16 . The solid propellant  16  can be ignited by an igniter  18 . A section of the housing  12  defines the combustion chamber  14 ; this section is named the combustion chamber wall  20 . The combustion chamber wall  20  has on its periphery several uniformly distributed outflow openings  22 , which preferably all have the same diameter. An insulation foil  24 , which closes all the outflow openings  22 , is glued onto the inner side of the combustion chamber wall.  
         [0022]    The housing  12  has in addition an outer wall  26  which is likewise provided with outflow openings  28 . On the inner side of the outer wall  26 , an insulation foil  24  can likewise be provided, this insulation foil  24  being provided additionally or alternatively to the insulation foil  24  lying on the inner side of the combustion chamber wall  20 . The insulation foil  24  prevents the entry of moisture into the combustion chamber  20 .  
         [0023]    In FIG. 2 the insulation foil  24  is illustrated in spread-out state. The insulation foil  24  consists of several layers, namely a base layer  32  of metal having a large area, and a thermally insulating foil layer  34  of plastic applied onto the base layer, which is applied onto the base layer  32  by spraying. As can be seen from FIG. 2, the foil layer  34  is, however, only partially applied onto the base layer  32 . The illustrated holes  36  symbolize the positions of the outflow openings  22  which are closed by the insulation foil  24 . The base layer  32  and the foil layer  34  each have a uniform thickness across their extent, so that the entire insulation foil  24  has the greatest thickness in the region of the foil layer  34 . The insulation foil  24  is fastened to the combustion chamber wall  20  such that the foil layer  34  directly faces the solid propellant  16 .  
         [0024]    After the igniting of the solid propellant  16 , the latter is burned and hot gas is produced in the combustion chamber  14 , which strikes directly onto the base layer  32  or, where present, onto the foil layer  34 . The heat development combined with the pressure development provides for a local destruction of the insulation foil  24  in the region of the outflow openings  22 .  
         [0025]    The individual layers of the insulation foil  24  are constructed here such that at an ambient temperature of the gas generator (temperature of the gas generator before ignition) of greater than 75° C., in particular of approximately 85° C., all the outflow openings  22  are opened, i.e. also the foil layer  34  is destroyed.  
         [0026]    At low ambient temperatures of less than −25° C., in particular less than −30° C., the foil layer  34  insulates the section of the base layer  32  lying under it, so that the insulation foil  24  in this region is either not destroyed at all and the associated outflow openings  22  thereby remain closed, or these outflow openings would be opened with a distinctly greater time delay compared to the outflow openings  22  covered only by the base layer  32  than is the case with an ambient temperature of greater than 75° C. The time delay here should be greater by at least the factor four than the time delay which occurs at the ambient temperature of greater than 75° C.  
         [0027]    In FIG. 4, the construction of a second insulation foil  124  is shown, which has a metallic base layer  32  which in three regions has insulation foils of different thickness applied thereon. FIG. 3 b  shows the corresponding combustion chamber and can pressure curves with this coated insulation compared to a generator with uniformly thick metal foil as insulation (FIG. 3 a ).  
         [0028]    The six larger openings  122  are covered with the thinnest foil layer section  134  and open in the entire temperature range of −40° C. to +90° C. At 85° C., in addition all the more intensively covered eight smaller openings  124  and  126  will open. At 23° C., only the four smaller openings  124  are opened, in the region  136  of which the foil layer has a medium thickness. The bores  126  in a region  138 , in which the foil layer has the greatest thickness, remain closed at 23° C.  
         [0029]    Through a corresponding graduation of the layer thickness of the foil layer over all the small openings  124 ,  126 , also even a refinement of the opening behavior over the entire temperature range would be possible.  
         [0030]    [0030]FIGS. 3 a  and  3   b  show a comparison of a gas generator with a uniformly thick metal foil as insulation (FIG. 4 a ) and with the use of the insulation according to FIG. 4. The combustion chamber pressure and also the so-called can pressure are illustrated. The can pressure is the pressure inside a metal container of standardized size, in which the gas generator is ignited in the laboratory, and with which the pressure inside a gas bag is simulated. With the aid of the comparison of FIGS. 3 a  and  3   b , it becomes clear that in the gas generator according to the invention, the combustion chamber pressure profile depends less on the ambient temperature than in a conventional one. Furthermore, the can pressure also varies less over the temperature range than with a uniformly thick insulation foil. In order to keep the combustion chamber pressure at 85° C. as low as possible, the housing provided with the insulation foil according to the invention is to have a large outflow area which can be greater than in the gas generator, the pressure profile of which can be seen in FIG. 3 a.