Abstract:
The hybrid gas generator to inflate airbags comprises two combustion chambers ( 14, 16 ), in each of which a solid material charge ( 15, 17 ) is placed, and a storage chamber ( 11 ) containing a pressurized storage gas. The storage chamber ( 11 ) is connected to diffusion chamber ( 30 ) by a closing member ( 29 ) which can be perforated. The first solid material charge ( 15 ), and the second solid material charge ( 17 ) can, for instance, can be sequentially ignited by separate igniter elements ( 18, 19 ), thereby determining the pressure accumulation.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a hybrid gas generator which contains an ignitable, gas-generating solid charge and a storage chamber containing a gas. 
     A hybrid gas generator for air bags is known from EP 0 616 578 B1, which contains an ignitable solid charge in a combustion chamber and a pressurized gas in a storage chamber. Upon the initiation of the gas generator, an ignition element ignites the solid charge. A hollow piston is driven by the combustion gases which are generated, with the hollow piston pushing through a closure element which closes the storage chamber, and with the pressurized gas contained in the storage chamber being able to flow out to the outlet. Moreover, the combustion gases generated by the solid charge flow into the storage chamber, where they mix with the compressed gas. When the closure element is destroyed, first of all cold compressed gas flows to the outlet. This prevents the hot combustion gases from arriving in the air bag first. Subsequently, a mixture of cold gas and combustion gas flows into the air bag. 
     Moreover, a hybrid gas generator is known from EP 0 669 231 A2, where a mixing chamber is arranged between a combustion chamber containing the solid charge and a storage chamber containing a compressed gas. The combustion chamber and the storage chamber are sealed by sealing discs which are destroyed by the combustion gases of the solid charge. In this respect, the combustion gas and the cold gas penetrate from opposite directions into the mixing chamber, with the flowing-out being impeded as a result. Moreover, the hot combustion gas loaded with harmful substances flows out initially because the sealing disc of the combustion chamber is the first one to be destroyed. 
     The known hybrid gas generators have a pressure buildup behaviour determined by their construction, with the maximum pressure and the pressure increase gradient being specified. In the process, the air bag connected to the gas generator is inflated very quickly and in one go. In the process, the passengers can be injured by the air bag if they are “out of position”. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to create a hybrid gas generator which makes it possible to control the course of the pressure build-up with time in order to effect greater safety for the passengers. 
     With the gas generator in accordance with the invention, two solid charges are provided in different combustion chambers, with the second solid charge being ignitable in a time-delayed manner in relation to the first solid charge. In this way, the gas generation takes place with two different pressure increase gradients, with preferably first of all a relatively flat pressure increase and then a steeper pressure increase being realized. The instant of the steeper pressure increase can be determined very accurately by the ignition delay. The two solid charges can also be of different sizes or can provide different quantities of gas. The percentage pressure distribution of the entire charge to the two individual charges makes a purposeful influencing of the pressure characteristic possible. Within the scope of the invention, it is even possible to provide, in addition to the two solid charges, at least one further solid charge. 
     The hybrid gas generator in accordance with the invention makes the realization of different types of pressure build-up characteristics possible, indeed not only by means of the purposeful distribution of the charges and the control of the ignition procedures, but also by means of other structural modifications, for example the dimensioning of the cross section of the outflow openings. 
     According to a preferred development of the invention, the first solid charge and the closure element are arranged at opposite ends of the storage chamber, and the pushing element extends through the storage chamber. In this respect, the length of the pushing element can be varied in order to realize different pressure build-up characteristics. A further parameter is the dwell time of the combustion gases in the storage chamber before the closure element is opened. In this way, the heat transmission between the combustion gas and the storage gas can be influenced, this affecting the maximum pressure and the pressure increase gradient. 
     Further advantages and features of the invention are evident from the following description of various exemplifying embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, exemplifying embodiments of the invention will be explained in more detail with reference to the drawings. 
     FIG. 1 shows a first embodiment of the gas generator in the stand-by state. 
     FIG. 2 shows the gas generator according to FIG. 1 after the burning-away of the solid charges. 
     FIG. 3 shows a further embodiment of the gas generator with an annular second solid charge. 
     FIG. 4 shows an exemplifying embodiment where the solid charges are coupled to each other pyrotechnically, so that the ignition takes place with only one ignition element. 
     FIG. 5 shows an exemplifying embodiment with separate pushing elements which are allocated to the individual solid charges. 
     FIG. 6 shows a further exemplifying embodiment with a valve which protects the second solid charge against back-igniting when the first solid charge burns away. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With the gas generator according to FIGS. 1 and 2 a tubular housing  10  is provided, the interior of which forms the storage chamber  11 . At the one end the housing  10  is sealed by a charge head  12  and at the opposite end by a diffuser head  13 . 
     The charge head  12  contains, next to each other, a first combustion chamber  14  with a first solid charge  15  and a second combustion chamber  16  with a second solid charge  17 . The solid charges  15 ,  17  are pyrotechnic charges. The solid charge  15  is allocated a first ignition element  18  and the solid charge  17  is allocated a second ignition element  19 . The ignition elements  18 ,  19  are accommodated in the charge head  12  and they project into the respective combustion chamber. The ignition elements can be electrically ignited by a control part (not shown). They contain a respective ignition charge which burns away and thereby ignites the associated solid charge  15  or  17 . 
     The combustion chamber  14  has an outflow opening  20  leading into the storage chamber  11 , and the combustion chamber  16  has an outflow opening  21  which likewise leads into the storage chamber  11 . A perforated disc  22  or  23  which keeps back the solid charge is arranged in front of the respective outflow opening  20  or  21  in the interior of the respective combustion chamber. The perforated discs  22 ,  23  are to keep back the solid particles when the solid charges burn away, until complete combustion has taken place. Moreover, the outflow opening  21  is sealed by a membrane or bursting disc  24  which is likewise destroyed or opened by way of the gas pressure. 
     The storage chamber  11  contains a pressurized storage gas. A pushing element  25  in the form of a rod extends through the storage chamber, the pushing element being provided at the end facing the combustion chamber  14  with a cap  26  covering the outflow opening  20  and having a plunger  27  at the opposite end. The plunger  27  is guided in a tube guide  28  of the diffuser head  13 . The tube guide  28  is sealed by a closure element  29  which the plunger  27  can push through or push off in order to open the passage from the storage chamber  11  to the diffuser chamber  30  contained in the diffuser head  13 . Outflow openings  31  lead from the diffuser chamber  30  into the area surrounding the housing  10 , with the area being connected to the air bag (not shown). 
     In the event of initiation, first of all the ignition element  18  is electrically ignited, which for its part ignites the first solid charge  15 . As a result of the pressure of the combustion gases, the pushing element  25  is axially moved in order to push open the closure element  29 . The hot combustion gases flow from the combustion chamber  15  into the storage chamber  11 , where they mix with the cold storage gas. The gas mixture flows into the diffuser chamber  30  and out of the latter through the outflow openings  31 . After the initiation of the first ignition element  18  the second ignition element  19  is electrically initiated, even before the pressure in the storage chamber  11  has declined. A steep pressure increase is produced by way of the burning-away of the second solid charge  17 , with the steep pressure increase being attributable to the fact that both solid charges  15 ,  17  now burn away simultaneously. The pressure characteristic and the value of the maximum pressure reached and the instant of the steeper pressure increase are determined by means of the two solid charges  15 ,  17 , the quantities of which are coordinated with each other, and by means of the time-delayed ignition of the solid charges. The instant when the closure element  29  is pushed open is determined by the length of the pushing element  25 . Furthermore, the heat transmission between the combustion gas and the storage gas is influenced by the residence time of the hot combustion gas of the storage chamber  11  before the closure element  29  is pushed open, this affecting the maximum pressure and the pressure increase gradient. 
     FIG. 2 shows the end state of the hybrid gas generator after the burning-away of both solid charges. The plunger  27  with the closure element  29  is now located in the diffuser chamber  30 , and all outflow openings  20 ,  21  and the tube guide  28  are open. 
     Whereas in the first exemplifying embodiment the two solid charges are arranged next to each other, in the exemplifying embodiment of FIG. 3 the second solid charge  17  is contained in an annular combustion chamber  16  which surrounds the centrally arranged combustion chamber  14 . The outflow opening  20  of the first combustion chamber  14  is arranged along the central axis of the storage chamber  11 . The annular second combustion chamber  16  has outflow openings  33  which do not lead into the storage chamber  11 , but are directed radially to the outside so that the escaping hot gas flows directly into the area surrounding the housing  10  and mixes there with the remaining gas mixture which has issued from the diffuser chamber  30 . Filters  34  in front of the radial outflow openings  33  are to keep back the solid particles of the solid charge until complete combustion has taken place. 
     As a result of gas pressure, the combustion gas of the solid charge  15  separates the connection between the container  10  and the cap  26  at a predetermined breaking point, so that the combustion gases enter into the storage chamber  11 . Here they mix with the storage gas, and the gas mixture flows into the diffuser chamber  30 . After the ignition of the first solid charge  15 , the second solid charge  17  is ignited in a time-delayed manner. 
     In the exemplifying embodiment of FIG. 4 only the first solid charge  15  is coupled to an ignition element  18 . The two combustion chambers  14 ,  16  are connected to each other by way of a bridge  34 . The bridge  34  can contain a pyrotechnic delay line, a heat-transmitting wall part or a wall which can be destroyed when the solid charge  15  is burned away. During the burning-away of the first solid charge  15 , the thermal or pyrotechnic ignition of the second solid charge  17  takes place by way of the bridge  34 . The advantage of this variant lies in that only an electrical signal has to be emitted to the gas generator. The ignition element  19  in FIG. 4 can therefore be dispensed with. However, if the ignition element  19  is present, optionally the one or the other ignition element  18 ,  19  can be activated first of all, so that, depending on the ignition element which is actuated, another characteristic of the pressure distribution is obtained. 
     In the exemplifying embodiment of FIG. 5, two solid charges  15 ,  16  of different sizes are provided, each of which has its own ignition element  18  or  19 . As in the first exemplifying embodiment, the first solid charge  15  drives a pushing element  25 , and the second solid charge  17  is allocated a further pushing element  35  which leads through the storage chamber  11  and covers the outflow opening  21  with a cap  36 . At the end of the second pushing element  35  there is a plunger  37  for pushing open a closure element  39  which seals the storage chamber  11  against the diffuser chamber  30 . Either of the two solid charges  15 ,  16  can be the first one to be ignited, and the other solid charge is thereafter ignited after a certain delay. The solid charge which ignites first opens the relevant closure element  29 ,  39  of the storage chamber  11 . The gas mixture is additionally heated by the solid charge which is ignited last. 
     In the exemplifying embodiment of FIG. 6, likewise each of the two solid charges  15 ,  17  is allocated its own ignition element  18 ,  19 . The outflow openings  20 ,  21  lead into a collecting chamber  40  which is connected to the storage chamber  11  by way of an outflow opening  41 . The outflow opening  41  is covered by the cap  26  which is connected to the pushing element  25 . As a consequence of the collecting chamber  40  it is possible, by using a single pushing element  25 , to ignite either of the two solid charges  15 ,  17  first as a matter of choice. Because the explosive charges have different sizes and/or different structures, the pressure generation characteristics can be selected by means of suitable activation of the ignition elements  19 ,  20 . 
     The collecting chamber  40  contains a valve  42  in the form of a shutter, which can be pivoted about an axis  43 . If the solid charge  15  is ignited first, the valve  42  in front of the outflow opening  21  of the other solid charge  17  shuts, in order to prevent this solid charge  17  from being ignited by the collecting chamber  40 . If, on the other hand, the solid charge  17  is ignited first, the valve  42  in front of the outflow opening  20  of the other solid charge  15  shuts.