Patent Publication Number: US-2010123303-A1

Title: Gas generating system with thermal barrier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/199,567 filed on Nov. 18, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to gas generating systems and, more particularly, to gas generating systems employing a selectively applicable thermally insulative barrier for attenuating the effects of elevated temperatures on gas generating system components. 
     SUMMARY OF THE INVENTION 
     In one aspect of the embodiments of the present invention, a housing unit for a gas generating system is provided. The housing unit includes a housing and a thermally insulative barrier covering at least a portion of an exterior of the housing. 
     In another aspect of the embodiments of the present invention, a housing unit for a gas generating system is provided. The housing unit includes a housing and a thermally insulating barrier covering a portion of the housing so as to provide heating of an uncovered portion of the housing adjacent the covered portion at a greater rate than heating of the covered portion when heat from a heat source external to the housing impinges on the covered portion. 
     In another aspect of the embodiments of the present invention, a housing unit for a gas generating system is provided. The housing unit includes a longitudinal housing having an interior divided into first and second chambers, and a thermally insulating barrier covering a portion of the housing exterior of the first chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings illustrating embodiments of the present invention: 
         FIG. 1  is a cross-sectional side view showing the outer housing, an insulating thermal barrier applied to the outer housing, and the internal structure of a gas generating system in accordance with an embodiment of the present invention. 
         FIG. 2  is a side exterior view of the embodiment shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional end view of the gas generating system embodiment shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional end view of a gas generating system in accordance with an alternative embodiment, showing the gas generating system housing spaced apart from the insulative thermal barrier. 
         FIG. 5  is a cross-sectional side view showing the outer housing, an insulating thermal barrier applied to the outer housing, and the internal structure of a gas generating system in accordance with another embodiment of the present invention. 
         FIG. 6  is a side exterior view of the embodiment shown in  FIG. 5 . 
         FIG. 7  is a schematic representation of an exemplary vehicle occupant protection system incorporating a gas generating system including a thermal barrier or insulating structure in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a cross-sectional view of an outer housing and interior components, collectively designated  11 , of a gas generating system  10  incorporating an insulative thermal shield or barrier in accordance with one embodiment of the present invention. The gas generating system may constructed of components made from a durable metal such as carbon steel or iron, but may also include components made from tough and impact-resistant polymers, for example. One of ordinary skill in the art will appreciate various methods of construction for the various components of the gas generating system. U.S. Pat. Nos. 5,035,757, 6,062,143, 6,347,566, U.S. patent application Ser. No. 2001/0045735, WO 01/08936, and WO 01/08937 exemplify typical designs for the various gas generating system components, and are incorporated herein by reference in their entirety, but not by way of limitation. 
     In the embodiment shown in  FIG. 1 , the gas generating system includes a tubular housing  12  having a pair of opposed ends  14 ,  16  and a housing wall  18 . Housing  12  may be cast, stamped, extruded, or otherwise metal-formed. A plurality of gas exit apertures  20  are formed along housing wall  18  to permit fluid communication between an interior of the housing and an airbag (not shown). 
     A longitudinal gas generant enclosure  22  is inwardly radially spaced from housing  12  and is coaxially oriented along a longitudinal axis of the housing. Enclosure  22  has an elongate, substantially cylindrical body defining a first end  22   a,  a second end  22   b,  and an interior cavity for containing a gas generant composition  24  therein. Enclosure first end  22   a  is positioned to enable fluid communication between an igniter  26  and the enclosure interior cavity. Enclosure  22  is configured to facilitate propagation of a combustion reaction of gas generant  24  along the enclosure, in a manner described in greater detail below. 
     A plurality of gas generant tablets  24  are stacked side by side along the length of enclosure  22 . In the embodiment shown in  FIG. 1 , each tablet  24  has substantially the same dimensions. Other, alternative arrangements of gas generant material are also possible. Examples of gas generant compositions suitable for use in the present invention are disclosed in U.S. Pat. Nos. 5,035,757, 6,210,505, and 5,872,329, incorporated herein by reference. However, the range of suitable gas generants is not limited to those described in the cited patents. 
     A quantity of a known auto-ignition composition  28  is positioned at either end of the stack of gas generant material  24 . Enclosure  22  may be environmentally sealed at both ends with an aluminum tape (not shown) or any other effective seal, if desired. If desired, a quantity of a known booster compound (not shown) may be positioned in the housing so as to enable fluid communication between the booster compound and gas generant tablets  24  upon activation of the gas generating system. The booster compound facilitates ignition of the gas generant in a known manner. 
     An igniter  26  is mounted in the gas generating system such that the igniter is in communication with an interior of gas generant enclosure  22 , for activating the gas generating system upon occurrence of a crash event. In the embodiment shown, igniter  26  is positioned within an annular bore of an igniter closure  30 . Igniter  26  may be formed as known in the art. One exemplary igniter construction is described in U.S. Pat. No. 6,009,809, herein incorporated by reference. 
     Igniter closure  30  is crimped or otherwise fixed to a first end  14  of housing  12 . A first endcap  32  is coaxially juxtaposed adjacent igniter closure  30  to form, in conjunction with igniter closure  30 , an inner housing for igniter  26 . First endcap  32  also provides a closure for gas generant enclosure  22 . A second endcap  34  is crimped or otherwise fixed to a second end  16  of housing  12 . Endcaps  32  and  34  and igniter closure  30  may be cast, stamped, extruded, or otherwise metal-formed. Alternatively, endcaps  32  and  34  may be molded from a suitable polymer. 
     A filter  36  may be incorporated into the gas generating system design for filtering particulates from gases generated by combustion of gas generant  24 . In general, filter  36  is positioned between gas generant  24  and apertures  20  formed along gas generating system housing wall  18 . In the embodiment shown in  FIG. 1 , filter  36  is positioned exterior of gas generant enclosure  22  intermediate enclosure  22  and housing wall  18 , and substantially occupies the annular space between gas generant enclosure  22  and housing wall  18 . In an alternative embodiment (not shown), filter  36  is positioned in the interior cavity of enclosure  22  between gas generant  14  and enclosure gas exit apertures  40  formed along enclosure  22 . The filter may be formed from one of a variety of materials (for example, a carbon fiber mesh or sheet) known in the art for filtering gas generant combustion products. 
     In accordance with the present invention, a plurality of gas exit apertures  40  is formed along enclosure  22 . If desired, apertures  40  may be spaced apart to tailor the rate of propagation of a combustion reaction of the gas generant  24  along the enclosure, as required by design criteria and as described in U.S. Pat. No. 7,080,854, incorporated herein by reference. Enclosure  22  may be roll formed from sheet metal and then perforated to produce apertures  40 . Enclosure apertures  40  may be environmentally sealed with an aluminum tape  42  or any other effective seal. 
     Referring again to  FIGS. 1-3 , in the embodiments of the present invention described herein, a gas generating system  10  includes housing and components  11  and an external insulating structure or thermal barrier  900  applied to at least a portion of the outer surface of housing  12 , to cover the portion of the housing. The thermally insulative barrier is a barrier containing one or more thermally insulating materials and which, by the performance of its thermal insulative function, permits transfer of heat and/or directs heat to a portion of the gas generating system in thermal communication with an auto-ignition material used for initiating combustion of a gas generant material located in the gas generating system, such that the auto-ignition material is ignited before the gas generating system housing becomes undesirably degraded or damaged. 
     Barrier  900  is designed to attenuate or mitigate the effects of elevated external housing temperatures (due, for example, to exposure to flame) on the covered portion(s) of the housing and/or any heat-sensitive internal gas generating system components residing on or within the covered portion of the housing. Barrier  900  may be selectively applicable so as to permit a high degree of control over the portion or portions of the housing covered by the barrier. This permits application of the barrier structure to selected portions of the housing while also permitting other portions of the housing in thermal contact with the auto-ignition material to remain exposed. Thus, sensitive portions of the housing and/or housing interior can be protected while still ensuring timely heat transfer to the auto-ignition compound, thereby permitting the auto-ignition compound to activate when desired. 
     Thermal insulating barrier  900  may be formed exclusively from one or more thermal insulating materials that impede heat transfer therethrough. Alternatively, thermal insulating barrier  900  may include both one or more thermal insulating materials and also materials containing one or more heat-reflective substance(s) that reflect heat away from the housing outer surface. Barrier  900  may also (or alternatively) include one or more flame retardant materials incorporated therein. For example, in one embodiment, harrier  900  includes a sheet or film of heat reflective material or flame retardant material which may possess minimal thermal insulative properties itself, but which can be applied over, under, or to the insulating material. One example of a suitable fire retardant material is Noxudol 999, available from Noxudol of North Hollywood, Calif. In a particular embodiment, the sheet or film is a pressure-sensitive, adhesive backed sheet or film comprising a polyimide material. One example of a heat reflective material suitable for the applications described herein is a sheet or film available under the designation “LG-1217”, available from LGI of Portland, Oreg. 
     Any of a variety of materials may be used for the insulating structure, including an suitable thermally-insulative polymers, films, coatings, structural ceramics, and/or other materials, which may be selected based on such factors as thermal insulation properties, flame-resistance or flame retardation, impact resistance, tear resistance and/or other factors, according to the requirements of a particular application. Also, layers of different materials having different thermal insulation properties may be overlaid or otherwise combined to achieve performance and manufacturing characteristics required for a particular application. 
     Materials are available for thermally insulating the housing in environments having a wide-range of external temperatures, depending on the requirements of a particular application. Insulation materials suitable for the purposes described herein include glass fiber matt or cloth, available from BGF Industries, Inc. of Greensboro, N.C. Other suitable insulation materials include carbon fiber cloth available from Jamestown Distributors of Bristol, R.I., and carbon fiber tubing, available from Highborn International Company. Other suitable insulation materials include glass-filled polymers, for example, glass-bead-filled polypropylene. Other suitable insulation materials include Kevlar® cloth and cloth composites available from BGF Industries, Inc. of Greensboro, N.C. Any other substance or combination of substances possessing thermal insulation properties suitable for the desired application may be used. 
     The insulation material may be in the form of layers which may be folded over or wrapped around the housing or a portion thereof. One example of a such a wrappable insulation material is Superwool® available from Thermal Ceramics Inc. of Augusta, Ga. In another embodiment, a thermal insulating material is sprayed onto the housing or a portion thereof. An example of a suitable material is Mega-Temp™ Insulation, available from Mega-Temp™ of Las Vegas, Nev. 
     In one embodiment, the insulating material is a moldable or formable material applicable directly to the housing using a caulking gun or pump. The material is then cured or dried to form an insulating layer which adheres to the housing material. Alternatively, the housing can be insert-molded into shroud or sleeve of suitable insulating material. Alternatively, the insulating material may be injected into a mold or otherwise formed into a jacket or receptacle into which the housing or a portion thereof is inserted. One example of a suitable material is ZIRCAR Alumina Insulation Type SALI Moldable available from ZIRCAR Ceramics, Inc. of Florida, N.Y. Other examples of suitable materials are castable ceramic compounds such as those available from Cotronics Corp. of Brooklyn, N.Y. Alternatively, a flexible insulative sleeving may be formed from a ceramic or other suitable material for receiving therein the housing or a portion thereof. Such sleeving is available from Cotronics Corp. of Brooklyn, N.Y. The material or materials forming the barrier may be amenable to printing of information thereon. 
     In another embodiment, an adhesive is applied to the housing exterior prior to application of the barrier structure. The adhesive secures one or more components of the insulating structure to the gas generating system housing. In a particular embodiment, the adhesive securing the barrier structure to the housing has thermal insulative thermal properties such that it augments the thermal insulative properties of the thermal barrier. Suitable high-temperature adhesives are available from various vendors, for example, Cotronics Corp. of Brooklyn, N.Y. 
     In another embodiment, a thermally insulative adhesive is applied to one or more layers or components of the insulating barrier, to secure these layers or components together. The adhesive may be applied and the barrier components bonded together such that the components are spaced apart and connected to each other only across the adhesive. 
     In one embodiment, a space or passage is provided between the insulating structure and the gas generating system housing. This provides an additional degree of thermal insulation between the exterior of the insulating structure and the gas generating system housing. In one particular embodiment, thermally insulating cells or voids are formed into the insulating material itself. One example of such a material is a closed-cell, cross linked polyolefin foam such as Thermobreak, available from Sekisui Pilon of Sydney, Australia. 
     The insulating structure may comprise a single layer of material or multiple layers of material. For example, in one particular embodiment, the gas generating system outer housing (or a suitable portion thereof) is covered with an inner layer having favorable insulative properties, while an outer layer having enhanced flame resistance and/or heat reflectivity encloses or covers the inner layer. In this manner, the performance of the insulative structure may be optimized to meet design requirements, with regard to such factors as material costs and material properties. 
     In a particular embodiment, the insulating structure covers most of the housing outer surface, as shown in  FIGS. 2 and 3 . In this embodiment, a portion of the housing proximate or in thermal communication with the auto-ignition material inside the housing may be left uncovered by the insulating structure, to expose this portion of the housing to externally-applied heat as previously described. 
     After application of insulating barrier  900  to housing  12 , any of housing gas exit apertures  20  residing along a portion of the housing covered by the barrier must be either open or openable to expel generated gases. Any of a variety of methods may be employed for ensuring that apertures  20  will be capable of receiving generated gases therethrough. For example, through holes in a pattern conforming to the pattern of gas exit apertures along housing  12  may be pre-formed or molded into the barrier prior to application of the barrier to the housing. Alternatively, through holes in the barrier structure coincident with the gas exit aperture locations may be made after formation of the barrier structure. In another alternative embodiment, dowel pins may be inserted into apertures  20  prior to application of the barrier structure to the housing. This may facilitate application of the barrier structure by a molding, spraying or other process, if so desired. The pins may then be extracted from the holes after the barrier structure has been applied. 
     In the embodiment shown in  FIGS. 2 and 3 , insulating structure  900  is applied directly to (or is in substantially direct contact with) the gas generating system outer housing, and gas-exit openings  902  formed in the insulating structure are substantially aligned with gas-exit openings formed in gas generating system housing  12  to facilitate flow of gases from the housing. 
     In another embodiment (not shown), a space or passage is provided between the insulating structure and the gas generating system housing, and openings in insulating structure  900  are out of alignment with openings in the system housing. This forces gases exiting the system housing to flow along a passage prior to exiting openings  902 , thereby facilitating cooling of the gases. In a particular embodiment (shown in  FIG. 4 ), the gas-exit openings in the housing are spaced apart and non-aligned with the openings formed in the insulating structure, so that the generated gases are forced to flow along a tortuous path prior to exiting the insulating barrier. This enhances cooling of the gases prior to expulsion from the insulating barrier. 
     During operation, as explained previously, the barrier  900  attenuates or mitigate the effects of elevated external housing temperatures (due, for example, to exposure to flame) on the covered portion(s) of the housing and/or any heat-sensitive internal gas generating system components residing on or within the covered portion of the housing, while still ensuring timely heat transfer to the auto-ignition compound, thereby permitting the auto-ignition compound to activated when desired. This prevents thermally-induced damage to the housing and/or any heat-sensitive internal components. 
     The auto-ignition composition  28  previously described may be positioned in thermal contact with housing  12  such that heat transfer between the housing and the auto-ignition composition is facilitated when a portion of the housing not covered by thermal shield  900  is exposed to elevated exterior temperatures. For example, the auto-ignition composition may be placed in direct contact with the housing, or the housing and the auto-ignition composition may be thermally coupled by a heat-conductive material joined to both the auto-ignition composition and the housing. Numerous other alternatives modes of thermal connection between the housing and the auto-ignition composition are also contemplated. 
     In one particular embodiment, barrier  900  covers substantially the entire exterior of the housing. In another particular embodiment, barrier  900  covers only a portion (or multiple portions) of the housing. In both of these embodiments, however, a thermal path is provided from an exterior of the housing to portion(s) of the housing in thermal contact with the auto-ignition compound are left uncovered so that heat sufficient to ignite the auto-ignition material may be transferred to the material in a timely manner. 
     In a first embodiment (shown in  FIGS. 1-3 ), thermal barrier  900  covers a substantial portion of the exterior of the housing. As seen in  FIGS. 1 and 2 , one or more end portions of the housing are in thermal communication with auto-ignition material  28  and are therefore left uncovered by the insulative barrier  900  so that externally generated heat sufficient to ignite an auto-ignition material residing within the housing proximate the exposed end portion(s) may be transferred to the auto-ignition material in a timely manner. In addition, in particular embodiments, it is believed that a quantity of heat impinging on covered portions of the housing may be deflected along the thermal barrier toward an exposed portion of the housing exterior which is in thermal communication with the auto-ignition material. 
     In another particular embodiment (shown in  FIGS. 5 and 6 ), a thermal barrier  901  in accordance with the present invention is applied to a dual-chamber gas generating system  110 . In combination, housing  112  and thermal barrier  901  form a thermally-shielded housing unit  113 . 
     Referring to  FIGS. 5 and 6 , gas generating system  110  includes an elongate, substantially cylindrical housing  112 , such as is well known in the art. Housing  112  has a first end  114  and a second end  116 . A plurality of gas discharge apertures  190  are spaced along housing  112  to enable fluid communication between an interior of the housing and an exterior of the housing, the exterior of the housing being configured so as to enable fluid communication with an airbag (not shown) or other inflatable element of a vehicle occupant restraint system after activation of the gas generating system. Housing  112  also has a longitudinal central axis A, a wall  113  extending between ends  114  and  116 , and openings formed at both ends of housing  112 . The housing may be roll-formed, extruded, cast, or otherwise metal formed and may be made from aluminum, low carbon steel, or any other metal/alloy that is not gas permeable and that does not fragment during the burning of the gas generant enclosed therein. 
     A bulkhead  155  divides the interior volume of housing  112  into two portions, a first combustion chamber  110   a  and a second combustion chamber  110   b  arranged in a side-by-side configuration. Bulkhead  155  prevents fluid communication between first chamber  110   a  and second chamber  110   b.  Bulkhead  155  may be formed from the same material as housing  112 , or from another suitable material. Bulkhead  155  may be positioned within housing  112  and secured therein, for example, by crimps formed along housing  112  on either side of the bulkhead. The positioning of bulkhead  155  along the interior of housing  112  may be adjusted such that chambers  110   a  and  110   b  are of different sizes, enabling a different quantity of gas generant composition to be positioned in each chamber, as shown in  FIG. 5 . 
     Bulkhead  155 , along with filters  150   a,    150   b  (described in greater detail below) also prevents sympathetic ignition within the gas generating system. Sympathetic ignition is defined herein as the ignition of a gas generant in one of combustion chambers  110   a ,  110   b  resulting from heat generated by the burning of gas generant in the other one of combustion chambers  110   a,    110   b.  Sympathetic ignition would occur, for example, when a gas generant  142   a  is deliberately ignited in combustion chamber  110   a  by a first igniter  119 , and where the heat and energy associated with the burning of gas generant  142   a  ignites gas generant  142   b  in second combustion chamber  110   b.  Bulkhead  155  and filters  150   a,    150   b  absorb the heat from the burning of gas generants  142   a  and  142   b  to prevent sympathetic ignition. 
     Each of chambers  110   a  and  110   b  has the same basic arrangement of gas generating system components; thus, in general, the following discussion of the components in one of the chambers also applies to the components in the other chamber. Gas discharge apertures  190  may be covered with a foil  156  such as aluminum or stainless steel foil to prevent the incursion of water vapor into gas generating system housing  112 . The foil  156 , sometimes referred to as “burst foil” is typically of a thickness of from 0.01 to about 0.20 mm. The foil  156  is typically adhered to the interior surface of the housing  112  through the use of an adhesive. 
     A pair of substantially concentric baffle tubes  122   a,    124   a  is positioned and secured within combustion chamber  110   a,  substantially centered about housing longitudinal axis A. Similarly, a pair of substantially concentric baffle tubes  122   b,    124   b  is positioned and secured within combustion chamber  110   b,  also substantially centered about housing longitudinal axis A. 
     Baffle tubes  122   a,    124   a,    122   b,    124   b  form, in conjunction with housing  112 , a series of annular passages  126   a,    128   a,    126   b,  and  128   b  through which combustion gases propagate to discharge apertures  190  from interior portions of inner baffle tubes  122   a ,  122   b.  As is known in the art, baffle passages  126   a,    128   a,    126   b,    128   b  are designed to cool the combustion products and to reduce or eliminate flaming of the combustion products prior to the products exiting the gas generating system through apertures  190 . In alternative embodiments (not shown), more than two baffle tubes may be employed in one or more of combustion chambers  110   a,    110   b  to further enhance cooling of the generated gases. 
     A plurality of gas discharge apertures  123   a  is spaced circumferentially around an end portion of inner baffle tube  122   a  to enable fluid communication between an interior of baffle tube  122   a  and an exterior of the baffle tube. Similarly, a plurality of gas discharge apertures  125   a  is spaced circumferentially around an end portion of outer baffle tube  124   a  to enable fluid communication between an interior of baffle tube  124   a  and an exterior of the baffle tube. 
     In addition, a plurality of gas discharge apertures  123   b  is spaced circumferentially around an end portion of inner baffle tube  122   b  to enable fluid communication between an interior of baffle tube  122   b  and an exterior of the baffle tube. Similarly, a plurality of gas discharge apertures  125   b  is spaced circumferentially around an end portion of outer baffle tube  124   b  to enable fluid communication between an interior of baffle tube  124   b  and an exterior of the baffle tube. 
     Endcaps  115 ,  120  are secured at respective first and second ends  114   116  of housing  112  to seal the openings provided in the housing ends. End caps  115 ,  120  may be stamped, cast, or otherwise metal formed and may be made from carbon steel or stainless steel, for example. End caps  115 ,  120  may be crimped, welded or clamped to housing  112  in a manner sufficient to ensure a gas tight seal between endcaps  115 ,  120  and housing  112 , and in a manner sufficient to resist elevated internal housing pressures experienced during burning of the gas generant. In the embodiment shown in  FIGS. 5-6 , end portions of housing  112  are crimped over shoulders formed in end caps  115 ,  120 . 
     A cavity may be formed in endcap  115  to accommodate an igniter  119  secured therein, thereby forming an igniter end cap assembly  116  as described below. Similarly, a cavity may be formed in endcap  120  to accommodate an igniter  121  secured therein, thereby foaming an igniter end cap assembly  127  as described below. Endcap  115  has an annular step portion  115   a  formed along an outer surface thereof for receiving a silicon sealing compound  101  therealong, as described in greater detail below. Similarly, endcap  120  has an annular step  120   a  portion formed along an outer surface thereof for receiving a silicon sealing compound  101  therealong. Step portions  115   a  and  120   a  are configured so as to provide a cavity between each of endcaps  115 ,  120  and housing  112  for receiving the silicon sealing compound  101  therein when the endcaps are crimped in position within housing  112 . 
     Hermetic seals are formed between endcaps  115 ,  120  and housing  112  by using a two-part quick-cure silicon compound  101 . Silicone compound  101  forms a seal at each end of gas generating system  110  when end portions of housing  112  are crimped to secure endcaps  115 ,  120  in position. The silicone compound may include an additive causing it to fluoresce when exposed to an ultraviolet light. This enables a relatively low-cost vision system to be used during gas generating system assembly to inspect for the presence of the silicone prior to crimping of the housing to secure the endcaps. Silicone sealants as contemplated for use in the present invention are commercially available from, for example, Electro Insulation Corporation of Arlington Heights, Ill. 
     Referring again to  FIGS. 5 and 6 , gas generating system  110  also includes first and second igniters  119 ,  121  for igniting the gas generant in respective ones of chambers  110   a  and  110   b.  Igniter  119  is secured to housing  112  such that the igniter is in communication with an interior of combustion chamber  110   a  and also with an exterior of the housing. Igniter  121  also is secured to housing  112  such that the igniter is in communication with an interior of combustion chamber  110   b  and also with an exterior of the housing. In the embodiment shown, igniter  119  is incorporated into an igniter end cap assembly  116  that includes igniter  119  and end cap  115 . Similarly, igniter  121  is incorporated into an igniter end cap assembly  127  that includes igniter  121  and end cap  120 . Igniter end cap assemblies  116  and  127  are positioned along central axis A to seal openings provided in the end portions of housing  112 . Igniters  119  and  121  may be formed as known in the art. One exemplary igniter construction is described in U.S. Pat. No. 6,009,809, herein incorporated by reference. Igniters  119  and  121  may be twisted or screwed into respective endcaps  115  and  120 . Other contemplated means of attaching the igniters to their respective endcaps include crimping, welding, and the like. 
     Referring again to  FIGS. 5 and 6 , an elongated propagation tube  134   a  is provided for channeling combustion products formed by ignition of igniter  119  down the length of combustion chamber  110   a,  thereby producing longitudinal propagation of gas generant combustion toward bulkhead  155 . Similarly, an elongated propagation tube  134   b  is provided for channeling combustion products formed by ignition of igniter  121  down the length of combustion chamber  110   b,  thereby producing longitudinal propagation of gas generant combustion toward bulkhead  155 . Propagation tube  134   a  has an elongate, substantially cylindrical body defining a first end  139 - 1 , a second end  139 - 2 , and an interior cavity. Propagation tube  134   a  also includes a plurality of apertures (not shown) spaced along a length thereof to enable fluid communication between igniter combustion products flowing along tube  134   a  and a quantity of gas generant composition  142   a  positioned in combustion chamber  110   a  alongside tube  134   a.    
     Propagation tube  134   b  also has an elongate, substantially cylindrical body defining a first end  140 - 1 , a second end  140 - 2 , and an interior cavity. Propagation tube  134   b  also includes a plurality of apertures (not shown) spaced along a length thereof to enable fluid communication between igniter combustion products flowing along tube  134   b  and a quantity of gas generant composition  142   b  positioned in combustion chamber  110   b  alongside tube  134   b.    
     Propagation tubes  134   a,    134   b  may be extruded or roll formed from sheet metal and then perforated, In the embodiment shown in  FIGS. 5 and 6 , propagation tubes  134   a  and  134   b  are positioned within housing  112  to extend along central axis A of the housing. First end  139 - 1  of tube  134   a  is positioned to enable fluid communication between igniter  119  and the interior cavity of tube  134   a.  First end  140 - 1  of tube  134   b  is positioned to enable fluid communication between igniter  121  and the interior cavity of tube  134   b.  The elongate shapes of tubes  134   a  and  134   b  provide for combustion of gas generants  142   a  and  142   b  that propagates substantially from respective tube first ends  139 - 1 ,  140 - 1  toward respective tube second ends  139 - 2 .  140 - 2 . In an alternative embodiment (not shown), tubes  134   a  and  134   b  are omitted from the gas generating system. 
     Referring again to  FIGS. 5 and 6 , a cup  152   a  coupled to propagation tube  134   a  may enclose igniter  119  to define a fluid-tight interior portion of the cup in communication with the interior cavity of tube  134   a  and igniter  119 . In addition, a cup  152   b  coupled to propagation tube  134   b  may enclose igniter  121  to define a fluid-tight interior portion of the cup in communication with the interior cavity of tube  134   b  and igniter  121 . Cups  152   a  and  152   b  are positioned proximate respective propagation tube first ends  139 - 1  and  140 - 1 . During activation of gas generating system  110 , cups  152   a  and  152   b  can each accommodate a resident interim gas pressure, facilitating ignition of respective gas generants  142   a  and  142   b.  A quantity of booster propellant (not shown) may also be positioned in the interior portions of either of cups  152   a  and  152   b  to facilitate combustion of respective gas generants  142   a  and  142   b,  in a manner known in the art. Cups  152   a  and  152   b  may be formed integral with respective propagation tubes  134   a  and  134   b,  and may be stamped, cast, or otherwise metal formed and may be made from carbon steel or stainless steel, for example. Alternatively, cups  152   a  and  152   b  may be formed separately from tubes  134   a  and  134   b , then attached to respective ones of tubes  134   a  and  134   b  (for example, by welding or adhesive attachment) prior to assembly of the gas generating system. 
     Suitable gas generant compositions are disclosed, for example, in Applicant&#39;s U.S. Pat. No. 7,094,296, incorporated herein by reference. Also, other gas generants that should be incorporated by reference in the application include, but are not limited to those described in U.S. Pat. Nos. 5,035,757, and 5,872,329, also incorporated herein by reference. In the embodiment shown in  FIGS. 5 and 6 , gas generant  142   a  is in the form of a plurality of annular wafers stacked along tube  134   a  to substantially enclose tube  134   a  along a portion of its length. Similarly, gas generant  142   b  is in the form of a plurality of annular wafers stacked along tube  134   b  to substantially enclose tube  134   b  along a portion of its length. Each of the gas generant wafers has a cavity formed therein for receiving a portion of a corresponding propagation tube therethrough, if desired. 
     It will be appreciated that other, alternative arrangements of the gas generant composition may be used. For example, either (or both) of combustion chambers  110   a  and  110   b  may be partially or completely filled with a gas generant in granulated or tablet form. In addition, as stated previously, the position of bulkhead  155  may be adjusted to permit different amounts of gas generant to be positioned in chambers  110   a  and  110   b , thereby enabling the inflation profile to be tailored according to design requirements. 
     Referring again to  FIGS. 5 and 6 , one or more spring members  220   a  are positioned intermediate endcap  115  and gas generant  142   a  for exerting a force on the gas generant to maintain the wafers or tablets comprising the gas generant in contact with each other. Force is applied by spring members  220   a  through an endplate  230   a  movable along cup  152   a  to press against gas generant  142   a.  Similarly, one or more spring members  220   b  are positioned intermediate endcap  120  and gas generant  142   b  for exerting a force on the gas generant to maintain the wafers or tablets comprising the gas generant in contact with each other. Force is applied by spring members  220   b  through an endplate  230   b  movable along cup  152   b  to press against gas generant  142   h.  Spring members  220   a  and  220   b  and endplates  230   a  and  230   b  may be formed from steel or other suitable metal alloys. 
     A filter  150   a  is incorporated into the gas generating system design for filtering particulates from gases generated by combustion of gas generant  142   a.  In general, filter  150   a  is positioned at an end of combustion chamber  110   a,  proximate bulkhead  155  and aligned with apertures  123   a  of inner baffle  122   a  to help ensure that inflation gas passes through the filter before exiting inner baffle  122   a.  Similarly, a filter  150   b  may be incorporated into the gas generating system design for filtering particulates from gases generated by combustion of gas generant  142   b.  The filters also act as a heat sink to reduce the temperature of the hot inflation gas. In general, filter  150   b  is positioned at an end of combustion chamber  110   b,  proximate bulkhead  155  and aligned with apertures  123   b  of inner baffle  122   b  to help ensure that inflation gas passes through the filter before exiting inner baffle  122   b.  Filters  150   a  and  150   b  may be formed from compressed knitted metal wire which is commercially available from vendors such as Metex Corp. of Edison, N.J. Alternative filter compositions and structures (not shown) are also contemplated. 
     A quantity of a known auto-ignition material  128  as previously described is positioned proximate an end of the stack of gas generant material  142   a  so as to enable fluid communication between the auto-ignition material and the gas generant  142   a  before and/or after ignition of the auto-ignition material. Similarly, a quantity of a known auto-ignition material  128  as previously described is positioned proximate an end of the stack of gas generant material  142   b  so as to enable fluid communication between the auto-ignition material and the gas generant  142   b  before and/or after ignition of the auto-ignition material. As in the previously described embodiment, auto-ignition material  128  is also positioned so as to be in thermal communication with housing  112  such that heat transfer between the housing and the auto-ignition composition is enabled when a portion of the housing not covered by a thermal shield or barrier  901  (described below) is exposed to elevated exterior temperatures. Auto-ignition material  128  is ignited by heat transmitted from an exterior of housing  112  to the interior of the housing due to an elevated external temperature condition (produced, for example, by a fire). 
     Referring again to  FIGS. 5 and 6 , a thermal barrier  901  covers a portion of housing  112  exterior of the first chamber  110   a.  As in the previously described embodiment, portion(s) of the housing in thermal communication with the auto-ignition compound are left uncovered so that heat sufficient to ignite the auto-ignition material may be transferred to the material in a timely manner. 
     In the multi-chamber embodiment shown in  FIGS. 5 and 6 , chamber  110   a  is longer and contains more gas generant  42  than chamber  110   b.  As stated previously, auto-ignition material  128  is positioned proximate housing ends  114  and  116  and is in thermal communication with the housing at these ends. As the length of housing  112  is increased, the average distance of points along the housing from either end of the housing increases. In addition, as the length of either of gas generant stacks  142   a  and  142   b  increases, the average distance of the gas generant in the stack from either end of the housing increases. Thus, for a relatively longer gas generant stack and/or housing (and depending on such factors as where the externally-generated heat impinges upon the housing), a relatively greater length of time may be required for an externally-generated heat source to heat the end(s) of the housing to ignite the auto-ignition material, due to the greater potential separation distance between the heat source and the housing end portion. This separation distance can result in relatively longer-term exposure of a more central portion of the housing to elevated temperatures while the housing ends are heating to a temperature sufficient to ignite the auto-ignition material. Such exposure may cause undesirable damage to the housing or other gas generating system components before the auto-ignition material has been heated sufficiently to ignite. An insulative thermal barrier in accordance with an embodiment of the present invention aids in shielding the covered portion of the housing while sufficient heat is transferred to the auto-ignition material to produce ignition of the auto-ignition material. Thus, the protection afforded housing  112  by the thermal insulation provides additional time for heat received by the uncovered housing portions to raise the temperature of the housing to a point where the auto-ignition material is activated. 
     In the embodiment shown in  FIGS. 5 and 6 , the thermal barrier is shown applied to the exterior of the portion of the housing containing the relatively longer gas generant stack  142   a.  In particular embodiments and depending on the nature of the thermal barrier and the nature of the heat source, heat may also be reflected away from the barrier. In addition, in particular embodiments of the present invention, it is believed that heat impinging on a covered portion of the housing is deflected along the barrier toward one or more exposed ends  114 ,  116  of the housing. This reflection and/or deflection of the incident heat greatly reduces or eliminates the potential for damage to the housing and/or system components prior to ignition of the auto-ignition material  128 . 
     The exposed or uncovered portions of the housing contain second gas generant chamber  110   b  housing the relatively shorter gas generant stack  142   b,  and the average distance from the exposed portion of the housing to the nearest end of the housing is relatively short. Correspondingly, the distance that external heat impinging on this exposed portion of the housing must travel along the housing (via conduction and/or convection) to an end of the housing proximate the auto-ignition material  128  is relatively short. Thus, because of the relatively shorter length of the second chamber  110   b  and the associated gas generant stack  142   b,  this portion of the housing exterior may be left uncovered if desired in the embodiment shown in  FIGS. 5 and 6 . 
     Thus, in the manner described above, the insulative thermal barriers  900 ,  901  attenuate or mitigate the effects of elevated external housing temperatures (due, for example, to exposure to flame) on the covered portion(s) of the housing and/or any heat-sensitive internal gas generating system components residing on or within the covered portion of the housing, while still ensuring timely heat transfer to the auto-ignition compound, thereby permitting the auto-ignition compound to activated when desired. This prevents thermally-induced damage to the housing and/or any heat-sensitive internal components while the uncovered portion(s) of the housing is being heated to a temperature sufficient to ignite the associated auto-ignition material. 
     In a particular embodiment, a “thermal conduit” may be provided extending through or around the insulating structure to the housing. This thermal conduit provides means for enabling thermal communication between the housing unit exterior and an auto-ignition material and/or between the housing unit exterior and a portion of the gas generating system in thermal communication with the auto-ignition material. The auto-ignition material is positioned inside the housing either in operative communication with a gas generant material, or so as to enable operative communication with the gas generant material after activation of the gas generating system. 
     In one embodiment, the conduit is a thermally-conductive material providing thermal communication between the exterior of the insulating structure and the gas generating system housing, enabling heat from an external source to be transmitted to the portion of the housing proximate the auto-ignition material. Alternatively, the thermal conduit may extend from the exterior of the housing through the housing wall and into the housing interior to permit direct contact with the auto-ignition material inside the housing. Alternatively, the thermal conduit may be an opening formed in the insulating barrier and extending through the harrier from the barrier exterior to a portion of the housing exterior surface proximate and/or in thermal communication with the auto-ignition material. 
     Use of the thermal conduit obviates the need to position the auto-ignition material in thermal communication with an uncovered portion of the housing, and enhances flexibility in the positioning of the auto-ignition material within the housing. 
     If desired, at least a portion of the thermal conduit may be thermally insulated so that heat conducted along the conduit is not conducted or otherwise transmitted to a body other than the auto-ignition material in physical contact with the conduit. 
     In another embodiment, a portion of the thermal conduit is not in direct contact with the housing but is in thermal communication with an exterior of the housing proximate the uncovered portion of the housing, so that heat impinging on the uncovered portion of the housing also impinges on the conduit. This heat is then transmitted along the conduit to an auto-ignition material inside the housing, to ignite the auto-ignition material. 
     A thermally-conductive material connecting the exterior of the insulation and the gas generating system housing may be molded or formed into the insulating structure, if desired. The thermal conduit may be formed from any suitable thermally-conductive material, for example, copper or a copper-containing alloy. 
     Referring now to  FIG. 7 , a gas generating system  10 ,  110  in accordance with one of the embodiments described herein may be incorporated into a vehicle occupant restraint system  200 . Vehicle occupant protection system  200  includes at least one airbag  202  and a gas generating system  10 ,  110  in accordance with the present invention and coupled to airbag  202  so as to enable fluid communication with an interior of the airbag. Vehicle occupant protection system  200  may be in operative communication with a crash event sensor  211  which communicates with a known crash sensor algorithm that signals actuation of vehicle occupant restraint system  200  via, for example, activation of airbag gas generating system  10 ,  110  in the event of a collision. 
     Although the embodiments of the present invention are described herein with reference to a gas generating system having a cylindrically-shaped housing, it will be understood that embodiments of the thermal barrier described herein can be applied to any of a wide variety of alternative housing shapes and configurations. For example, embodiments of the thermal barrier described herein may be applied to gas generating systems having housing formed from a base and cap, rather than a cylindrical tube. Embodiments of the thermal barrier described herein may be also applied to gas generating systems having multiple combustion chambers. Application of embodiments of the thermal barrier to numerous other types and structures of gas generating systems is also contemplated. 
     It will be understood that the foregoing description of the present invention is for illustrative purposes only, and that the various structural and operational features herein disclosed are susceptible to a number of modifications, none of which departs from the spirit and scope of the present invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.