Abstract:
The present invention relates to a gas generator, in particular for airbag modules in motor vehicles, comprising two generator stages each of which includes in its pressure housing at least one igniter, at least one propellant charge and at least one combustion chamber. An inner generator stage is arranged at least substantially inside an outer generator stage and the two generator stages are arranged at least partly in a common filter housing together with a common filter unit arranged outside a pressure housing of the outer generator stage.

Description:
TECHNICAL FIELD 
     The present invention relates to a gas generator, in particular for airbag modules in motor vehicles, comprising two generator stages each of which includes in its own pressure housing at least one igniter, at least one propellant charge and at least one combustion chamber. 
     BACKGROUND OF THE INVENTION 
     Such multi-stage gas generators are already known from the prior art. One or more generator stages can be ignited depending on the respective demands. The same gas generator can thus be used in vehicle applications for different airbag modules and different vehicle types. However, with the aid of a corresponding control, a decision can also be made with a gas generator installed with an airbag module in a vehicle in dependence on the magnitude of the impact, on different accident conditions or on the situation of use, e.g. the manner of seat occupation, which generator stages are ignited at which time. 
     With such multi-stage gas generators, it must be prevented by the geometrical arrangement and the design of the individual generator stages that, when one generator stage is ignited, the propellant charge of the other generator is also unintentionally ignited (sympathetic ignition). The pressure housings of the individual generator stages must therefore be correspondingly insulated from one another, with the weight of the gas generator, however, simultaneously being kept as low as possible. Since such gas generators are mass products produced in very high volumes, the manufacture of the generator should moreover be as simple as possible despite these aforementioned demands. As few different parts as possible should in particular be used. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a gas generator of the initially named kind which is as light and as compact as possible in which the aforesaid problem of cross-ignition does not occur and which can nevertheless be produced in as simple and as cost-favorable manner as possible in high volumes. 
     This object is satisfied in accordance with the invention in that an inner generator stage is arranged at least substantially inside an outer generator stage and in that the two generator stages are arranged at least partly, in a common filter housing together with a common filter unit arranged outside a pressure housing of the outer generator stage. 
     The arrangement of the two generator stages inside one another (so-called “stage-in-stage” design) saves room and permits an arrangement overall which is as symmetrical as possible. An additional chamber, which the gases have to flow through, can be provided outside the combustion chambers at the interior of the filter housing due to the arrangement of the two generator stages in a common filter housing including the filter unit. Said gases are not only filtered in this process, so that a gas as free of particles as possible can enter into the airbag, but can also cool down. Since the two generator stages each have their own pressure housing and a common housing with a separating wall is, for instance, not provided, the gas generator can be made up in modular form of different housing modules. For example, a pressure housing of always the same type for the outer generator housing can be combined with different pressure housings for the inner generator stage so that the costs for the manufacture of different gas generators can be reduced. Due to the arrangement of the filter unit outside the outside pressure housing, the latter can be made relatively small, whereby material and weight are saved. It was recognized that it is not necessary to arrange the filter unit inside the outer pressure housing, but that it is rather sufficient to provide a comparatively thin-walled filter housing which can then moreover advantageously be used to satisfy specific additional functions. 
     Advantageous embodiments of the invention are described in the dependent claims and in the description in conjunction with the enclosed drawings. 
     In accordance with an embodiment of the invention preferred due to its simplicity, the pressure housing of the outer generator stage is cylindrical and the igniters of the two generator stages are arranged at least approximately symmetrically with respect to a central axis of this pressure housing. This arrangement of the igniters permits an at least largely symmetrical weight distribution inside the pressure housing. The cylindrical shape of the outer pressure housing moreover permits a rotationally symmetrical outflow of the gases generated in the generator. 
     The two generator stages are preferably cylindrical. 
     An outflow behavior which is symmetrical to a high degree can be obtained when the central axes of the two igniters and the central axes of the housings and of the filter unit lie in a common plane. 
     In accordance with a further advantageous embodiment of the invention, the inner generator stage almost completely fills the outer generator stage. This provides an optimum space utilization since a region above the inner stage and belonging to the outer generator stage would not be utilized ideally for technical flow reasons. The probability is moreover reduced that, on an ignition of only the outer generator stage, the inner generator stage is unintentionally ignited as well, since at least the upper wall of the inner generator stage is not directly heated by the combusting propellant charge. 
     The utilization of the space present can furthermore be optimized in that the filter unit and the two generator stages at least substantially fully fill the filter housing. 
     In accordance with a further preferred embodiment of the invention, the filter unit is in ring shape and surrounds the outer generator stage. Such an arrangement is again characterized by its symmetry. A ring-shaped filter unit is moreover comparatively simple to assemble. 
     The filter ring preferably has a smaller axial length than the outer generator stage. Installation space and weight can thereby be saved overall when it is not necessary due to the respectively desired filter effect for a filter ring to extend over the whole axial length of the outer generator stage. 
     To guide the gas from the outer generator stage through the filter and, finally, out of the gas generator into the airbag to be inflated, in accordance with a preferred embodiment of the invention, outflow openings are provided in the pressure housing of the outer generator stage which are arranged at the level of the lower end of the filter unit and open into a space receiving the filter unit. This space is bounded from the outside by the filter housing and does not have to be completely filled by the filter unit. The generated gas inside this space can cool inside the same due to the expansion before exiting the gas generator. Outflow openings arranged at the level of the lower end of the filter unit in conjunction with skillfully arranged outflow openings of the filter housing permit a relatively long filter path and thus a particularly reliable filtering and a particularly effective cooling of the gas. 
     It is particularly advantageous for the mentioned outflow openings of the pressure housing of the outer generator stage to be lower than outflow openings of the filter housing. The gas flowing out of the outer generator stage through the lower lying outflow openings of the pressure housing in this case flows upwardly through the filter before exiting the filter housing. Since it is generally more advantageous to arrange the outflow openings through which the gas exits the gas generator as far upwardly as possible, a long filter path can be realized in a particularly elegant manner with such an arrangement, without the dimensions of the gas generator being unnecessarily enlarged. 
     In order to generate a flow path for the gas flowing out of the inner generator stage which is as long as possible, outflow openings of the inner pressure housing are preferably disposed approximately at the level of the outflow openings of the filter housing. With outflow openings of the filter housing arranged relatively far upwardly and with outflow openings of the outer pressure housing disposed comparatively far downwardly, the gas of the inner generator stage must therefore first move downwardly from the outflow openings of the inner generator stage to the outflow openings of the outer generator stage, from where it then again moves upwardly through the filter and then exits the filter housing approximately at the level of the outflow openings of the inner generator stage. 
     The outflow openings of the inner generator stage are preferably likewise aligned radially. 
     In accordance with a further preferred embodiment of the invention, the filter housing has a radially widened portion at the level of the filter unit. Overall, the filter housing can fit snugly substantially from the outside to the pressure housing of the outer generator stage in order to minimize the space requirement, with the said radially widened portion being able to be made to receive the filter unit of in particular ring shape in the region in which the filter unit is arranged. 
     The igniters of the two generator stages can be secured to a common base plate, which simplifies the manufacture of the gas generator. The mechanical stability of the total generator is moreover increased. 
     It is very particularly advantageous to form weld connections between the components forming the pressure housings in each case by the same welding process in order to simplify the manufacturing process. In particular capacitor discharge welding, laser welding or friction welding can be considered. The use of the same welding process for all connections saves time and thus costs in the manufacture of the gas generator in accordance with the invention. Depending on the process used and on the geometry of the generator stages, optionally even a plurality of weld connections can be formed in one workstep. 
     The filter housing is preferably secured to the previously manufactured assembly of the two generator stages with the aid of a reshaping operation, for example, a rolled joint or a beaded joint. Such a fastening can be realized in a simple manner. Alternatively or additionally, the filter housing can, however, also be welded to the base plate and/or to the generator stages. 
     The filter housing is preferably made as an outer housing of the generator. An additional outer housing of the generator is therefore not necessary, which saves material and thus costs, weight and installation space. 
     In accordance with a preferred further development of the invention, the filter housing can have a securing flange to attach the gas generator to an airbag module. The filter housing can thus be used as an outer housing of the generator without an additional connection piece being necessary for the attachment to the airbag module. 
     It is particularly advantageous for outflow openings of the filter housing to be provided in an upper region of the filter unit. Such an arrangement is in particular to be preferred when the filter housing also forms the outer housing of the generator, since the gas should flow out of the gas generator as far toward the top as possible, but still in a radial direction, for an optimum unfolding of the airbag. 
     In accordance with a further preferred embodiment of the invention, the filter housing can be expanded by the gas pressure generated by means of the generator stages so that it can assist a pressure buffer function. The pressure of the gas flowing out can be reduced by such an expansion of the filter housing such that the airbag is inflated with a lower force than with a filter housing of a less resilient design. This is above all of particular importance at high environmental temperatures, in comparison with lower environmental temperatures, which have the consequence of a higher maximum pressure which would have a full effect on the inflation behavior of the airbag without a pressure buffer. The dilatability of the filter housing can be set such that the inflation behavior is less dependent on the environmental temperature, that is such that the airbag does not behave too “aggressively” in summer and behaves sufficiently “dynamically” in winter. It must also be taken into account here that current regulations require a problem-free function and a simultaneous observation of safety requirements over a temperature range from −35° C. to +85° C., i.e. the gas generator must also be designed for very low temperatures. The pressure buffer function of the filter housing in particular ensures that the increased pressure development does not result in a bursting of the filter housing at very high temperatures. 
     In accordance with an advantageous further development of the invention, this filter housing can be made so that it surrounds the generator stages, the filter unit and a base plate common to the generator stages like a clamp. This clamp form can be established, for example, by a roll at the lower side of the filter housing. The stability of the total gas generator is increased and the filter housing provided with an additional function by such an arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be explained in more detail in the following with reference to a preferred embodiment and to the two enclosed Figures. The Figures show in detail: 
         FIG. 1  is an axial section through a gas generator in accordance with the invention; and 
         FIG. 2  is a radial section through the gas generator of  FIG. 1  in a partly simplified representation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  illustrates the basic design of the two-stage gas generator which has a substantially cylindrical shape overall. An inner generator stage  20 , which is bounded by a cylindrical pressure housing  26 , is arranged asymmetrically inside a larger cylindrical outer generator stage  30 . 
     The interior space of the pressure housing  26  of the inner generator stage  20  forms a combustion chamber  24  in which a propellant charge not shown in the Figures is stored in the form of pressed fuel pellets. As can better be recognized in  FIG. 1 , the outer generator stage  30  has a cylindrical pressure housing  36  whose inner space forms a combustion chamber  34  in which a second propellant charge, which is likewise not shown, of the same type is disposed. The outer pressure housing  36  has a much larger wall thickness than the pressure housing  26  of the inner generator stage  20  since, due to the lower wall surface of the pressure housing  26  of the inner generator stage  20 , a lower wall thickness is sufficient to withstand the pressures which occur. 
     As is easily recognizable in  FIG. 2 , two igniters  22 ,  32  each having a circular cross-section are arranged approximately symmetrically to a central axis of the pressure housing  36  of the outer generator stage  30 . The igniter  32  disposed at the right in the two Figures belongs to the outer generator stage  30 , whereas the igniter  22  shown at the left belongs to the inner generator stage  20  and is arranged centrally in its pressure housing  26 . 
     The inner generator stage  20  substantially lies in the half of the gas generator or of the outer generator stage  30  disposed at the left in the Figures. The pressure housing  26  of the inner generator stage  20  extends in height, as can be recognized in  FIG. 1 , almost up to the upper side of the pressure housing  36  of the outer generator stage  30 . 
     A collection grating  42  extending at a low spacing from the side wall of the pressure housing  36  of the outer generator stage  30  lies regionally between the inner generator stage  20  and the pressure housing  36 . The fuel pellets of the propellant charges are prevented by the collection grating  42  from clogging outflow openings  38  formed in the pressure housing  36  and described further below. 
     As can easily be recognized in  FIG. 2 , a filter housing  56  is arranged coaxially around the pressure housing  36  of the outer generator stage  30 . Both generator stages  20 ,  30  lie completely inside the filter housing  56  which is provided at the top and bottom with a relatively large opening to save material and weight and thus practically represents a housing ring which radially completely surrounds and upwardly and downwardly engages around the arrangement of pressure housing  36  and a base plate  46  described in more detail in the following. The space  54 , which is bounded outwardly by the filter housing  56  and inwardly by the pressure housing  36  of the outer generator stage  30 , serves i.a. for the reception of a filter unit  50  explained in the following. 
     The filter unit  50 , which is not shown in  FIG. 2  for reasons of simplicity, is arranged in ring-shape between the pressure housing  36  of the outer generator stage  30  and the filter housing  56 . The filter ring  50  extending around the pressure housing  36  does not extend beyond the total height of the pressure housing  36 , but rather has an axial extent which only amounts to approximately two thirds of the total axial extent of the gas generator. 
     The filter housing  56  having a substantially lower wall thickness than the pressure housing  36  of the outer generator stage  30  fits snugly to it from the outside, as shown in  FIG. 1 , with a radially widened portion being formed in the region of the filter unit  50  which extends, like the filter unit  50 , around the pressure housing  36  in the manner of a ring and serves for the reception of the filter unit  50 . The contour of the filter housing  56  thus substantially corresponds to the outer contour of the pressure housing  36  with the filter ring  50  surrounding it. 
     As in particular  FIG. 2  shows, the central axes of the housings  26 ,  36 ,  56 , of the ring-shaped filter unit  50 , of the collection grating  42  and of the igniters  22 ,  32  lie in a common plane, with the housings  36 ,  56 , the filter unit  50  and the collection grating  42  being arranged concentrically and their central axes consequently coinciding. 
     It can be recognized in  FIG. 1  that the outer generator stage  30  is fitted with a booster container  40  which is pushed over the igniter  32  and likewise has a cylindrical shape. This booster container  40 , just like the remaining space of the combustion chamber  34 , is filled with fuel pellets and only opens when a specific threshold pressure has built up such that a very high pressure can be built up very quickly in the combustion chamber  34 . Due to the lower volume of the inner generator stage  20 , it can do without such a booster container; optionally, however, a booster container can likewise be pushed over the igniter  22  of the inner generator stage  20 . 
     Volume compensation pieces  25  and  35  are provided in both the smaller inner generator stage  20  and in the outer generator stage  30  respectively. They prevent a movement of the fuel pellets of the propellant charges so that these cannot be damaged by friction which occurs and so that an unwanted noise development is avoided. 
     The pressure housing  26  of the inner generator stage  20  terminates at its upper side at a cover  23  having a circular cross-section which has a peripheral double fold  21  U-shaped in section at its outer rim. This improves the sealing of the pressure housing  26  at its upper rim and gives the cover  23  a certain flexibility. After ignition of the inner generator stage  20 , the cover  23  is presses upwardly by the pressure prevailing in the pressure housing  26  until it abuts the pressure housing  36  of the outer generator stage  30  after overcoming the axial intermediate space which is present. This movement of the cover  23  is made possible by its double fold  21  providing the required path length, without the sealing of the housing  26  being impaired due to the upwardly pressed cover  23 . In contrast, on an ignition of the outer generator stage  30 , the cover  23  is pressed onto the pressure housing  26  of the inner generator stage  20  by the pressure acting from the outside in particular due to the axial intermediate space so that the inner generator stage  20  is or remains reliably sealed. 
     The pressure housing  26  of the inner generator stage  20  has radial outflow openings  28  which are arranged in an upper region of the pressure housing  26 . The outflow openings  28  are covered by an insulating film  29  at the inner side of the pressure housing  26 . The insulating film  29  can either be made as a film ring and extend around the total periphery of the pressure housing  26 , or every outflow opening  28  is covered by a separate insulation film  29 . The insulation film  29  tears at a certain minimum pressure and thus has a similar effect to the booster container  40  described above: only when a certain minimum pressure has been built up can the gas flow out of the inner generator stage  20 . The outflow openings  28  are covered by a reverse flow protection  29   a  from the outside. It can, for example, be a steel band which prevents a flowing of gas from the outer generator stage  30  into the inner generator stage  20  (reverse flow protection). It is avoided in this way that, after an ignition of the outer generator stage  30 , the inner generator stage  20  is unintentionally ignited (“sympathetic ignition”). 
     The already mentioned outflow openings  38  of the pressure housing  36  of the outer generator stage  30  are likewise radially aligned and lie approximately at a level with the lower end of the filter ring  50  in a lower region of the pressure housing  36 . As can be seen in  FIG. 2 , the outflow openings  38  of the pressure housing  36  are distributed at uniform intervals over the total periphery of the cylindrical pressure housing  36 . The outflow openings  38  are covered on the inner side of the pressure housing  36  by an insulating film  39  which has the same effect as the insulating film  29  described above for the outflow openings  28  of the inner generator stage  20 . 
     The filter housing  56  likewise has outflow openings  58  which are located in the region of the upper end of the filter  50  and thus clearly above the outflow openings  38  of the pressure housing  36 . The outflow openings  28  of the pressure housing  26  of the inner generator stage  20  are located approximately at the same level as the outflow openings  58  of the filter housing  56 . 
     The two generator stages  20 ,  30  have a common, circular base plate  46  having cut-outs, likewise circular, intended to receive the igniters  22 ,  32 . The igniters are pushed into these cut-outs and can be welded along its periphery, for example by means of a laser welding process, to the base plate  46 . 
     The filter housing  56  engaging around the pressure housing  36  at the upper side moreover engages around the base plate  46  and thus forms an outer envelope surrounding the total gas generator like a clamp. The rim engaging around the base plate  46  can be established by a reshaping such as a rolling process or a flanging. 
     A flange  44  which serves for the attachment of the gas generator to an airbag module, is attached to the filter housing  56 , which also forms the outer housing of the gas generator in the embodiment shown, whereby the filter housing  56  satisfies a further additional function. 
     The components which form the pressure housings of the two generator stages  20 ,  30  are welded together, with the same welding process being able to be used for all welding spots  60 . The outer pressure housing  36 , the collection grating  42  an the pressure housing  26  of the inner generator stage  20  are welded to the base plate  46 . First, the pressure housing  26  of the inner generator stage  20  is preferably filled with a propellant charge and welded to the base plate  46  already provided with the igniters  22 ,  32 . An assembly is formed by welding the collection grating  42  and the pressure housing  36  of the outer generator stage  30  to the base plate  46  over which the filter unit  50  and the filter housing  56  are then pushed. The filter housing  56  is secured to the base plate  46  in a last step by reshaping, for example by rolling. Alternatively or additionally, it can also be welded to the base plate  46 . A connection of the components by means of capacitor discharge welding, laser welding or friction welding is in particular especially advantageous 
     Different ignition sequences are now feasible for the described two-stage gas generator. On the one hand, only one stage, namely the outer generator stage  30 , can be ignited, which is for example advantageous when it is a question of a driver airbag for a comparatively small driver sitting closely behind the steering wheel. Alternatively, both generator stages  20 ,  30  can be ignited offset in time or simultaneously. The time span between the first ignition and the second ignition can in particular lie in the range from 0 to 10 ms. The time for the maximum degree of filling of the airbag and the rate of the pressure increase can be matched to different conditions by a corresponding choice of this time span. Generally, with the described arrangement, the outer generator stage  30  is ignited first and then the inner generator stage  20 . It would also be feasible first to ignite the inner generator stage  20 , with the outer generator stage  30  then likewise being ignited shortly thereafter by the gas flowing in via the outflow openings  28  as a consequence of the heat development. However, the individual components can be strained comparatively greatly due to the pressure relationships prevailing in this case so that the reverse ignition order is preferred. 
     In particular on the ignition of both generator stages  20 ,  30 , the common expansion space outside the pressure housing  36  serves not only for filtering, but also for the cooling down and mixing of the gases generated by the two generator stages  20 ,  30 .