Abstract:
The invention relates to a gas generator for generating gases, comprising a housing which has an interior space and at least one combustion chamber filled with a predetermined quantity of solid propellant. The propellant consists of a plurality of solid propellant parts arranged in the combustion chamber in an irregular manner. The compressed gas is stored in a predetermined quantity in the combustion chamber and the combustion chamber constructed as a pressure chamber sealed hermetically toward outside in a non-activated state of the gas generator.

Description:
TECHNICAL FIELD 
     The invention relates to a gas generator. 
     BACKGROUND OF THE INVENTION 
     Gas generators which only operate with pyrotechnic solid propellant generate extremely hot gas. It is aimed that gas generators generate gases with a low temperature, in order to reduce the thermal stress of the vehicle occupant restraint system, which is arranged downstream of the gas generator. For this reason, so-called hybrid gas generators have been developed, in which in addition to solid propellant filling a combustion chamber also a separate chamber (not defining or co-defining the combustion chamber) is present which is filled with compressed gas. The hot gas and the compressed gas become mixed on activation of the gas generator, so that the hot gas is cooled. However, such hybrid gas generators require a very large structural space. These housings of the hybrid gas generators, constructed so as to be pressure-tight, are characterized by a high weight and unfavorable structural sizes owing to the high strength requirements. In addition, these pressure-tight housings require a number of gas-tight seals at component transitions, which requires relatively high manufacturing costs. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a gas generator occupying a small space, which with the same structural volume provides a higher output than a corresponding purely pyrotechnic gas generator. In addition to the reduced structural space, the gas generator provided according to the invention is reduced in its overall weight, i.e. is lighter and provides the possibility of manufacturing various gas generators within one production line and according to a modular concept. 
     This problem is solved in a gas generator which comprises a housing which has an interior space and at least one combustion chamber filled with a predetermined quantity of solid propellant. The propellant consists of a plurality of solid propellant parts, e.g. bodies in tablet form or extruded shaped bodies, which are arranged in the combustion chamber in an irregular, i.e. chaotic manner as the propellant parts are thrown or poured into the combustion chamber. The compressed gas is stored in a predetermined quantity in the combustion chamber and the combustion chamber is constructed as a pressure chamber sealed hermetically towards outside in a non-activated state of the gas generator. 
     With the invention, therefore, the combustion chamber of a gas generator actually only operating with solid propellant is used for accommodating additional compressed gas. Between the propellant units or parts which are usually present in the form of tablets or other pressed forms (e.g. extruded shaped bodies) in fact sufficient empty space is still present in order to be able to fill this with compressed gas. These empty spaces add up as a whole to such a high unused volume that additionally introduced compressed gas can actually lead to an increase in output by more than 10% with the same structural space and the same weight. In addition, the compressed gas can reduce the temperature of the outflowing gas mixture and possibly even lead to an afterburning. Through a suitable coordination of solid propellant and compressed gas, very low emissions of harmful substances can be achieved, which are much lower than those of the purely pyrotechnic systems. 
     In fact, hybrid gas generators already exist, in which the combustion chamber is arranged in a pressure chamber surrounding the combustion chamber and the combustion chamber is open to the pressure chamber and also compressed gas is present in the combustion chamber. In these systems, tie combustion chamber is only therefore open towards the pressure chamber, however, so that no resistance is offered to the pressure wave and the compressed gas which are generated on igniting of the solid propellant, on overflowing into the pressure chamber. This resistance would in fact be present if the combustion chamber were closed by a bursting membrane with respect to the pressure chamber, as has also already been proposed elsewhere in the prior art. 
     In the gas generator according to the invention, which is constructed in one or more stages, however, no pressure chamber containing gas is present around the combustion chamber. The solid propellant in the gas generator according to the invention generates more moles gas than moles compressed gas are stored in it. 
     According to an embodiment, the combustion chamber is constructed as a separate, gas-tight can which is able to be acted upon by pressure, which can is integrated in the interior space of the gas generator housing. This gas-tight can is provided as a separate component and can be integrated according to requirements as a combustion chamber into the respective gas generator series. 
     The pressure-tight gas can is constructed for example from a gas-tight foil or from a thin sheet metal material. The gas-tight can follows in its outer shape the inner shape of the gas generator housing. 
     With the use of combustion chambers in the form of gas-tight cans, a bursting means which is arranged in the overflow zone of the gases in the gas generator housing, is dispensed with, because the gas-tight can has a sufficiently thin wall and breaks open in the region of the overflow openings. 
     The gas-tight can stores compressed gas with a pressure in the range of 10 to 20 bar. 
     The construction of the combustion chamber in the form of a gas-tight can offers the advantage that in a relatively simple manner a solid propellant can be surrounded by a fixedly defined gas mixture. 
     The optional equipping of gas generators with separate combustion chambers in can form with or without additional gas within a production line offers the advantage of high flexibility with manufacture of various gas generators using as many identical parts as possible. 
     Preferably, the compressed gas is a gas mixture containing oxygen. The oxygen opens up the possibility of a subsequent combustion. 
     If, in addition, the solid propellant in the gas generator is underbalanced with respect to oxygen, then the oxygen can make possible an oxidation of the resulting CO and H 2  to CO 2  or water with, at the same time, very low NO x  values. 
     The compressed gas is stored with a pressure of more than 10 bar, even of approximately 200 bar in the combustion chamber. With the use of combustion chambers in the form of gas-tight cans, the compressed gas is stored at a pressure of 10 to 20 bar, i.e. a pressure which lies below the usual pressure of 240 bar for hybrid gas generators. Thereby, the strength requirements for the combustion chamber wall, constructed as pressure chamber wall, are substantially lower than in hybrid gas generators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section through a schematically illustrated driver&#39;s gas generator according to the invention; 
     FIG. 2 shows a cross-section through a schematically illustrated passenger&#39;s gas generator according to the invention; 
     FIG. 3 shows a cross-section through a schematically illustrated side gas generator according to the invention; 
     FIG. 4 shows a cross-section through a further embodiment of the gas generator according to the invention, which is constructed in two stages; and 
     FIG. 5 shows a cross-section through a further embodiment of the gas generator according to the invention with a combustion chamber constructed as a can. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 a driver&#39;s gas generator is illustrated, which has an outer housing  3  which is gas- and pressure-tight. In the housing  3  an igniter device  5  is accommodated. Except for the igniter device  5 , the entire interior space of the housing  3  forms a combustion chamber  7  which is filled with solid propellant  9  consisting of a plurality of solid propellant parts in tablet form or in the form of extruded shaped bodies  9 ′. The combustion chamber  7  is the part of the gas generator filled with propellant, here solid propellant. The propellant parts fill the combustion chamber in an irregular, i.e. chaotic manner. On the side lying opposite the igniter device  5 , a screen  11  is arranged and downstream therefrom a bursting membrane  13  is arranged. A diffuser is designated by  15 . 
     Compressed gas, containing oxygen, with a pressure of approximately 20 bar is accommodated in the combustion chamber  7 . As solid propellant a propellant based on 5-aminotetrazol/KNO 3  is used. With a combustion chamber volume of 45 cm 3 , a propellant mass of 55 g, a packing density for the propellant of 0.6 g/cm 3 , in the driver&#39;s gas generator shown in FIG. 1, with a pressure of 200 bar 0.17 mole compressed gas can be accommodated. The total moles of propellant together with the compressed gas can be increased from 0.90 to 1.07 through the introduction of the compressed gas compared with an identically constructed gas generator without compressed gas. Thereby, an increase in the gas yield by just under 20% results compared with a corresponding purely pyrotechnic gas generator. 
     The mode of operation of the gas generator according to the invention is explained hereinbelow. After the activation of the igniter unit  5 , the solid propellant  9  is also ignited and generates hot gas, including also CO and H 2 . This gas can oxidize to CO 2  and water with the oxygen contained in the compressed gas. As the quantity of solid propellant is underbalanced, low NO x  values are produced. 
     After the burning of a portion of the propellant, the pressure in the combustion chamber  7  becomes so high that the bursting membrane  13  breaks and the resulting gas mixture arrives via the then exposed opening into the diffuser  15  and from there into a gas bag restraint system. The screen  11  prevents an outflow of hot particles which arise with the burning of the propellant. 
     The embodiment illustrated in FIG. 2 corresponds in its function to that illustrated in FIG.  1 . The housing  3 , however, is constructed here as a tubular housing  3 ′. Otherwise, the parts already explained in connection with FIG. 1 bear the reference numbers which have already been introduced. 
     The side gas generator illustrated in FIG. 3 differs from that in FIG. 2 in that it takes up a smaller volume and its housing  3 ″ is more elongated in construction. Also in the embodiments shown in FIGS. 2 and 3, the combustion chamber is at the same time a pressure chamber and contains the entire compressed gas which is accommodated in the gas generator. 
     The gas generator illustrated in FIG. 4 corresponds, with regard to its construction, substantially to that shown in FIG. 1; however, it is constructed as a two-stage gas generator. For this, an inner wall  103  separates the combustion chamber according to FIG. 1 into two combustion chambers  7 ′ and  7 ″, which are each hermetically sealed and do not have any flow connection with respect to each other. Also, the two chambers  7 ′ and  7 ″ are filled accordingly with compressed gas. Each stage has its own igniter  5  and also its own outlet with the filter  11 , the bursting membrane  13  and the diffuser  15 . The two stages can be ignited independently of each other. The pressure of the gas inside the combustion chambers  7 ′ and  7 ″ can be identical or different; however it amounts to at least  10  bar. 
     Also in this embodiment, for each stage the quantity of the solid propellant  9  is coordinated with the quantity of the compressed gas contained in the corresponding combustion chamber  7 ′,  7 ″, and the quantity of the generated moles gas from the solid propellant is greater than the quantity of moles compressed gas in the corresponding combustion chamber  7 ′,  7 ″. 
     The gas generator illustrated in FIG. 5 corresponds as regards its structure substantially to the single-stage gas generator shown in FIG.  1 . The gas generator has a housing  3  which is constructed so as to be gas-tight. In the housing  3  an ignition device  5  is accommodated. A can  16  is integrated into the interior space of the housing  3 . The can  16  is constructed as a separate component and is axially clamped in the housing in the region of the ignition device  5  with recessing thereof. The interior space of the can  16  forms the combustion chamber  7 ′″ of the gas generator. In the can  16 , the pyrotechnic propellant  9  is accommodated, surrounded by a fixedly defined gas mixture. 
     The can  16  is produced from thin sheet metal which merely has one joint site  17  which is closed so as to be tight with respect to helium. The wall thickness of the gas-tight can  16  is approximately 0.3 to 1 mm. The outer contour of the can  16  follows the inner contour of the housing  3  of the gas generator, so that the combustion chamber  7 ′″ formed by the can  16  completely fills the housing interior space of the housing  3 . A rupture membrane in the region between filter  11  and diffuser  15  is not necessary. The can, which at the same time forms the combustion chamber  7 ′″ as well as the pressure chamber, receives compressed gas with a pressure of 10 to 20 bar. Owing to the pressure conditions of smaller dimensions within the can  16  in the range of 10 to 20 bar, it is no longer necessary to construct the gas generator housing so as to be pressure-tight. Welding technique in the manufacture of these gas generator housings can be dispensed with and other forms of connection such as screwing or flanging can be used. Advantageous in addition to this is the fact that a gas-tight sealing of component transitions on the housing is no longer necessary owing to the small excess pressure in the range of &gt;10 bar to 20 bar, which is received by the can  16 . In the can  16 , in the region of the radial overflow openings  21 , a filter  23  is provided in the form of a circular ring, which lies against the inner wall of the can  16 . On the periphery of the wall of the can  16 , several indentations  25  are provided, which serve for fixing the filter  23  in position. 
     Also in this embodiment, the quantity of solid propellant  9  is coordinated with the quantity of compressed gas contained in the can  16 , and the quantity of the generated moles gas from the solid propellant is greater than the quantity of moles compressed gas in the can  16 .