Abstract:
A gas generator includes an ignition charge, at least one ignition element for igniting the ignition charge, a storage chamber containing a compressed gas, which storage chamber is closed by a closure element, and an impact element destroying the closure element under pressure. A piston is provided as the impact element, which piston, after the destruction of the closure element, forms an exit gap with the opening closed by the closure element. The cross-sectional area of which exit gap changes in according to the position of the piston.

Description:
This is a divisional application of U.S. Ser. No. 09/446,021, filed Mar. 8, 2000 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a gas generator, in particular a gas generator for air bags. 
     Gas generators for air bags have an ignitable ignition charge in a combustion chamber and a pressurized gas in a storage chamber. A gas or a solid fuel can be provided as ignition charge. Upon the initiation of the gas generator, an ignition element ignites the ignition charge. To open the storage chamber an impact element in the form of a piston is accelerated by the pressure produced during the combustion of the ignition charge, with the piston destroying a closure element of the storage chamber. In this way, the pressurized gas in the storage chamber can flow out through the opened outlet into the air bag. Moreover, the combustible gases produced as a result of the burning-away of the ignition charge flow into the storage chamber, where they mix with the pressurised gas. Therefore, after the destruction of the closure element, first of all cold pressurised gas and subsequently a mixture of pressurised gas and combustible gas flow into the air bag. In this way, hot combustible gases are prevented from the outset from arriving in the air bag. 
     The pressure characteristic of known gas generators is predetermined by the overall construction, with the air bag which is connected to the gas generator in each case being inflated quite suddenly. 
     The object of the invention is to create a gas generator which can be easily varied in order to achieve different pressure characteristics. 
     The gas generator in accordance with the invention has, for opening a closure element of the storage chamber, an impact element constructed as a tube and having several openings. The openings are arranged in such a way that, after the destruction of the closure element, at least one admission opening is arranged inside the storage chamber and at least one exit opening is arranged outside the storage chamber. In this way, after the destruction of the closure element, gas flows out of the storage chamber, through the admission opening into the tube and through the exit opening out of the tube into the air bag. The mass flow of the gas flowing through the openings can be influenced by the formation of one or more openings. Therefore, the mass flow can be influenced by the shape, the number and the arrangement of the inlet and/or exit openings. During the displacement of the tube upon the destruction of the closure element, a part of an entry opening or an exit opening can be closed and in the end position of the tube it can be completely open. In this way, the emergence of gas at the beginning of the inflation of the air bag is less. This avoids injuries to a driver or passenger by sudden inflation of the air bag. 
     By using differently shaped tubes as impact element, by varying a single component, gas generators can be manufactured with different pressure characteristics. Because the tube is a simple component, different formations of the tube can be produced easily and economically. Tubes with different openings which generate different pressure characteristics can be used in an otherwise unchanged gas generator. A change of the openings of the tube does not make necessary any changes on the rest of the assembly of the gas generator. 
     To change the temporal sequence of the pressure build-up, in a preferred embodiment of the invention the front of the tube which faces the closure element is closed. The exit openings are arranged on the circumference of the tube. Before the destruction of the closure element by the tube, the exit openings are arranged within the storage chamber. Upon the displacement of the tube for the destruction of the closure element, the exit openings are closed at least partially at the beginning of the displacement by a wall of the storage chamber. The further the tube exits from the storage chamber, the more the exit openings of the tube are opened. In this way, the pressure increase in the air bag at the beginning of the inflation is relatively small and increases continuously as the exit openings become larger. 
     In order to vary the temporal pressure characteristic, several exit openings can be staggered with respect to each other in the longitudinal direction of the tube. In particular, the exit opening can be constructed as a slot. 
     Several admission openings can be provided in the tube, so as to correspond with the exit openings, distributed on the circumference. The admission openings are preferably offset with respect to each other in the longitudinal direction in order to improve the flow performance upon the entry of the gas into the tube. 
     In another independent embodiment, a piston is provided in place of the tube, to control the pressure characteristic. The piston is formed in such a way that an exit gap is formed between the piston and the opening, closed by the closure element, after the destruction of the closure element, the cross-sectional area of which exit gap changes in dependence upon the position of the piston. If, for example, a tapering piston is used, the mass flow issuing from the gas generator increases with the displacement of the piston. The shape of the piston can be arbitrarily selected according to the desired pressure characteristic. 
     Both with the impact element constructed as a tube and with the impact element constructed as a piston, the pressure characteristic can be further optimized by the provision of a damping element. In this respect, the damping element can be deformable in an elastic or plastic manner. Thus, by means of an elastic damping element, for example a spring, the exit opening in the end phase of the inflation of the air bag may again be partially closed in order to reduce the strain on the air bag. 
     To dampen the tube or the piston, it can, moreover, plunge into a recess. In this respect, the intensity of the damping depends on the gap width between the recess and the tube or piston. 
     A further possibility of changing the pressure characteristic is the arrangement of at least one additional ignition composition in the ignition charge. The additional ignition composition or compositions can be ignited in a time-staggered manner with respect to the first ignition composition which activates the gas generator. The ignition of another ignition composition effects an increase of the pressure in the storage chamber and therefore a change of the pressure characteristic in the air bag. The ignition of further ignition compositions can also be made dependent on external parameters, for example the vehicle speed or similar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following with reference to preferred embodiments and with reference to the drawings. 
     FIG. 1 shows a longitudinal section of a first embodiment of a gas generator with a tube. 
     FIG. 2 shows the gas generator shown in FIG. 1 after the initiation. 
     FIGS. 3-8 show partial sections of different embodiments of gas generators with a tube before and after the initiation. 
     FIGS. 9 and 10 show a partial section of a gas generator with a piston before and after the initiation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     All the exemplifying embodiments have, corresponding to FIG. 1, a tubular housing  10 , the interior of which forms the storage chamber  11 . One end of the housing  10  is connected to a charge head  12  and the opposite end is connected to a diffusor head  13 . 
     An ignition charge  16  is arranged in a housing  14  of the charge head  12 , the ignition charge consisting of two ignition compositions  17 ,  17   a.  The ignition compositions  17 ,  17   a  each have a solid fuel charge  18 ,  18   a  which are each arranged in a combustion chamber  19 ,  20 . Each ignition composition  17 ,  17   a  has, moreover, an ignition element  21 ,  22  for igniting the solid fuel charges  18 ,  18   a.  The ignition elements  21 ,  22  can be ignited electrically by a control component (not shown). They each contain an ignition charge which burns away and thereby ignites the associated ignition composition  17 ,  18 . 
     The combustible gas produced upon the burning-away of the solid fuel charge  18  in the combustion chamber  19  flows into the intermediate chamber  25 . To avoid ignition of the second solid fuel charge  18   a  by the combustible gas present in the intermediate chamber  25 , the combustion chamber  20  is closed by a cover  26 . In this way, the solid fuel charge  18   a  can be ignited by the ignition element  22  at a later time, with the cover  26  being destroyed by the pressure produced in the combustion chamber  20  upon the combustion of the solid fuel charge  18   a.    
     The housing  14  of the charge head  12  is connected to a cover  30  of the housing  10  by an edge of the housing  14  which points in the direction of the cover  30  being edged into a groove  32  of the cover. The cover  30  is welded to the housing  10  in order to tightly close the storage chamber  11 . 
     A head part  33  of a tube  34  used as impact element is arranged in the cover  30 . The head part  33  is connected to the cover  30  in such a way that, with adequate gas pressure in the intermediate chamber  25 , predetermined breaking points  35  of the head part  33  break. In the process, the head part  33  is pressed into the storage chamber  11  by the gas pressure prevailing in the intermediate chamber  25 . Together with the head part  33 , the tube  34  firmly connected thereto is moved to the right in FIG.  1  and the combustible gas flows out of the intermediate chamber  25  into the storage chamber  11 . In this respect, a closure element  38  is destroyed by the right end of the tube  34  in FIG.  1 . The closure element  38  is arranged in an opening  39  of a cover  40  of the housing  10 . The cover  40  is welded to the tubular housing  10  in order to seal the storage chamber  11  corresponding to the cover  30 . 
     In the embodiment shown in FIGS. 1 and 2 the tube  34  has admission openings  41 ,  42  which are staggered with respect to each other with regard to the longitudinal axis of the tube. The front end  43  of the tube  34  is open and is therefore used as exit opening  44 . 
     In the position of the tube  34  shown in FIG. 2, a stop  45  abuts against the inner side of the cover  40  so that a situation is prevented where the tube  34  emerges too far out of the housing  10  and, for example, where the entry opening  41  is covered by the cover  40 . In this way, the combustible gas flows out of the intermediate chamber  25 , through the opening  36  of the cover  30  into the storage chamber  11 , and on the opposite side compressed gas flows in the direction of the arrows out of the storage chamber  11 , through the admission openings  41 ,  42  into the tube interior and through the exit opening  44  into the diffusor head  13 . 
     The diffusor head  13  is securely connected to the cover  40  of the housing by means of a flanged connection  48 . The gas flowing into the diffusor head  13  flows through openings  49 , which are arranged on the circumference of the diffusor head  13 , into the air bag which is not shown. 
     The part of the gas generators which is not shown in FIGS. 3 to  10  corresponds to FIGS. 1 and 2. 
     FIGS. 3 and 4 show another embodiment before and after the initiation of the gas generator. The front end  43  of the tube  34  is tightly closed by a flush closure  50 . The tube  34  therefore has exit openings  51 ,  52  which are arranged close to the front end  43  of the tube on the circumference, so that the exit openings  51 ,  52 , after the closure element  38  has been pushed through, are arranged in the diffusor head  13  or outside the storage chamber  11  (FIG.  4 ). 
     Corresponding to the embodiment shown in FIGS. 1 and 2, the tube  34  has a stop  45  and staggered admission openings  41 ,  42 . In this way, the gas flows out of the storage chamber  11 , after the destruction of the closure element  38 , in accordance with the arrows shown in FIG. 4, through the admission openings  41 ,  42  into the tube interior and through the exit openings  51 ,  52  into the diffusor head  13 . The diffusor head  13  is constructed corresponding to the embodiment described with reference to FIGS. 1 and 2. 
     In the embodiment shown in FIGS. 5 and 6 the front end of the tube  34  is likewise closed by a closure  55 . The closure  55  is set up in an arched manner in the interior of the tube  34 . Exit openings  56 ,  57  are provided on the circumference of the tube  34 , the spacing of which exit openings from the front  43  of the tube  34  is greater than the spacing of the exit openings  51 ,  52  in the embodiment shown in FIGS. 3 and 4. The tube  34  has, in turn, corresponding admission openings  41 ,  42 . 
     Upon the initiation of the gas generator, the tube  34 , as described with reference to FIG. 1 and 2, is moved in the direction of a diffusor head  13   a.  In this respect, the closure element  38  is destroyed by the front end  43  of the tube  34 . In this embodiment the  35  diffusor head  13   a  has a cylindrical recess  58 . The diameter of the recess  58  is slightly larger than the outer diameter of the tube  34 , so that when the tube  34  plunges into the recess  58  a gap  59  is formed between the tube  34  and the recess  58 . When the tube  34  plunges into the recess  58  the air present in the recess  58  must escape through the gap  59 . In this way, the displacement of the tube  34  is dampened. 
     As soon as the exit openings  56 ,  57  have passed the cover  40  at least partially, gas flows in the direction of the arrows shown in FIG. 6, out of the storage chamber  11 , through the admission openings  41 ,  42  into the tube  34  and through the exit openings  56 ,  57  into the diffusor head  13   a.  During the dampened displacement of the tube  34  the exit openings  56 ,  57  are partially closed by the cover  40 , with the result that the mass flow into the air bag is limited. 
     In the embodiment shown in FIGS. 7 and 8 an indentation  60  which is deformable in a plastic or elastic manner is arranged opposite the closure element  38  as damping for the tube  34  in a diffusor head  13 b. The front end  43  of the tube  34  is provided with a closure  61  which can likewise be deformable in a plastic or elastic manner. Moreover, the tube  34  has exit openings  62 ,  63  arranged close to the front end  43  as well as admission openings  41 ,  42 . After the destruction of the closure element  38 , gas flows out of the storage chamber  11 , in the direction of the arrows, through the tube  34  into the diffusor head  13 . 
     The embodiment shown in FIGS. 9 and 10 has a piston  65  in place of the tube  34 . The piston  65  consists of a piston rod  66  and a piston foot  67 , the diameter of which is larger than the diameter of the piston rod  66 . The piston  65  has a transition region  68 , in the shape of a truncated cylinder, between the piston foot  67  and the piston rod  66 . Moreover, the piston  65  is connected at the end opposite the piston foot  67  to a piston head, not shown, which corresponds to the head part  33  (FIG. 1) of the tube  34 . Upon the initiation of the gas generator, the piston  65  is moved to the right in FIG.  9 . As a result, the piston foot  67  destroys the closure element  38  and meets the indentation  60  of the diffusor head  13   b  which serves as damping means. Both the indentation  60  and the piston foot  67  can be deformable in an elastic or plastic manner. 
     In place of the indentation  60 , the recess  58  described with reference to FIGS. 5 and 6 can also be provided in the diffusor head  13   b  for the damping of the piston  65 . 
     As soon as the closure element  38  is destroyed, gas flows out of the storage chamber  11 , in the direction of the arrows shown in FIG. 10, into the diffusor head  13   b.  As a result of the truncated part  68  of the piston  65 , the mass flow through a gap  69  formed between the truncated part  68  and the opening  39  of the cover  40  is small at the beginning of the displacement. With increasing displacement of the piston  65  to the right in FIG. 10, the gap  69  becomes larger and the mass flow out of the storage chamber  11  accordingly increases. 
     The details described in the individual embodiments can also be meaningfully combined with each other for the further control of the temporal pressure characteristic. In this way, for example, the tube  34  can have an outer contour corresponding to the shape of the piston  65 .