Abstract:
An airbag inflator ( 10 ) has an inflator housing ( 20 ) with an internal chamber ( 11 ); a pyrotechnic gas generant ( 8 ) stored in the internal chamber inside the housing ( 20 ) for generating inflation gases upon ignition; a strainer ( 50 ) with openings through which the inflation gases pass prior to inflating the airbag; a primary nozzle wall ( 40 ) positioned between the strainer and the gas generator; a plenum chamber ( 12 ) between the primary nozzle wall ( 40 ) and the strainer ( 50 ). The primary nozzle wall ( 40 ) has a plurality of nozzles ( 40   a,    40   b ) with nozzle openings ( 42, 44 ) oriented radially about the wall which directs gas flow tangentially spiraling laterally onto the strainer while preventing the inflation gas from flowing radially. The tangentially flowing gases impinge an internal face of the strainer laterally causing gas generant particles to recirculate and burn internally and residual debris of a size greater than the openings of the strainer ( 50 ) to be swept and settle in the plenum chamber ( 12 ) of the housing.

Description:
FIELD OF THE INVENTION 
     This invention relates to airbag inflators generally, more specifically to an improvement in inflator burn efficiency and filtration. 
     BACKGROUND OF THE INVENTION 
     Current pyrotechnic inflators for vehicle airbags contain filters to reduce the size of generant particles that are ejected from the inflator and to normalize the temperature of the exit gas. 
     Such filters can be made of spiral wraps of perforated steel plate. Because particulate builds up on such filters blocking the gas flow, a larger than practical flow area may be required or the perforation hole size may be bigger than desirable. A large portion of generant may be retained unburned in the depth of the filter reducing the inflator&#39;s efficiency and increasing its size. 
     Such compromise means that burning particles of generant ejected from the inflator as projectiles may cause direct damage to the airbag and may also elevate the temperature of the exit gases. It is often necessary to include expensive heat resistant cloth with the airbag or a separate metal heat shield or deflector with the inflator to protect the airbag from such damage. 
     It is therefore an objective to limit the absolute size of any solid particle ejected with generant gas to less than 20 micron spherical size, to improve burn efficiency of the inflator so that less generant is needed for a given performance, and to reduce performance variability. It is a further objective to be able to modify existing inflators and match their performance for the same or lower cost without requiring depth filtration. 
     These and other improvements over prior art inflators are achieved by the invention described hereinafter. 
     SUMMARY OF THE INVENTION 
     An airbag inflator has an inflator housing with an internal chamber; a pyrotechnic gas generant stored in the internal chamber inside the housing for generating inflation gases upon ignition; a strainer with openings through which the inflation gases pass prior to inflating the airbag; a primary nozzle wall positioned between the strainer and the gas generator; a plenum chamber between the primary nozzle wall and the strainer. The primary nozzle wall has a plurality of nozzles with nozzle openings oriented radially. Each opening lies in a radial plane generally perpendicular to the wall about the wall which directs gas flow tangentially spiraling laterally onto the strainer while preventing the inflation gas from flowing radially. The tangentially flowing gases impinge an internal face of the strainer laterally causing gas generant particles to recirculate and burn internally and residual debris of a size greater than the openings of the strainer to be swept and settle in the plenum chamber of the housing. 
     Preferably, the inflator housing is a circular short cylindrical pancake shaped structure with a plurality of exit openings for inflation gases to exit. The strainer is a short circular hollow cylinder with an inside diameter larger than the diameter of the primary nozzle wall and the gas generant. The strainer extends internally adjacent the housing to a height at least equal to the size of the exit opening of the housing. The inflation gases must pass through the strainer prior to exiting the housing openings. 
     The primary nozzle wall is in the form of an annular ring spaced from the strainer. The space between the primary wall and the strainer defines the plenum chamber. The primary nozzle wall is made of sheet metal or a hollow cylindrical tube. The nozzle openings are formed by stamping the sheet metal or hollow cylindrical tube to create the nozzles by cutting and forming scoop shaped depressions or bulges, each having an opening transverse or perpendicular to the wall. The primary nozzle wall has ends joined to form a tubular ring for encircling the gas generant. The ends can be welded, riveted or otherwise fastened together. The openings of the scoop shaped nozzles are oriented parallel to radial lines extending from an axis of the primary nozzle wall when formed as a ring. The primary nozzle wall has a height extending to an upper and a lower surface of the housing thereby sealing the gas generant in the internal chamber wherein the inflation gas must pass through the nozzle openings into the plenum chamber. 
     The plurality of nozzle openings is arranged in one or more rows around the circumference of the primary nozzle wall to create a cyclonic flow vortex. In a preferred embodiment, the plurality of nozzle openings is arranged in at least two rows of equally spaced nozzle openings, an upper row and a lower row, each row having at least four nozzle openings. The one or more rows extend about the circumference surface of the primary nozzle wall. The nozzle openings of each row are equal in number and equally spaced between openings. 
     The openings within one row are aligned with openings in each adjacent row or alternatively can be staggered. In a most preferred embodiment, the primary nozzle wall has one or more upper rows of nozzle openings facing in a first direction for receiving inflation gases and one or more lower rows facing in a second opposite direction for receiving gas flows. The gas flows tangentially into an upper portion of the strainer in the first direction and tangentially in a second opposite direction into a lower portion of the strainer thereby creating two oppositely directed cyclonic flow vortexes. This creates a gravity uplift region in the mid-center of the inflator which assists in burning generant particles. 
     The strainer is made of one or more layers of wire mesh. The wire mesh of the strainer has openings sized to 20 microns. Preferably, the strainer is a single layer of fine wire mesh formed into an annular ring. An internal coarse strainer can also be used inside the primary nozzle wall to limit the size of generant particles projected from the inner chamber through the nozzle openings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of the preferred embodiment inflator of the present invention. 
         FIGS. 1A and 1B  show further details of the present invention. 
         FIG. 2  is a perspective view of a subassembly of the primary nozzle wall with an internal coarse strainer and the external flow wash strainer. 
         FIG. 3  is an upper or top portion of the housing shown in a perspective view. 
         FIG. 4  is a perspective view of the preferred primary nozzle wall shown formed in a ring with two rows of nozzles. 
         FIG. 5  is a perspective view of the strainer for covering the exit openings of the housing. 
         FIG. 6  is a perspective view of an internal coarse wire mesh strainer with a plurality of gas generant pellets. 
         FIG. 7  is a cross-sectional view of an inflator with a conventional depth filter. 
         FIGS. 8A, 8B and 8C  are views of an alternative embodiment primary nozzle wall having a single row of nozzles with nozzle openings. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the invention the filter  500  of a current standard inflator  100  as shown for example in  FIG. 8 , is replaced by a primary nozzle wall  40  having two sets of nozzles  40   a  and  40   b , as shown for example in  FIG. 1 , which surrounds an internal coarse mesh strainer  60 , with inter-mesh openings  62  located between the nozzle wall  40  and the generant. The strainer cages the generant and permits only generant particles that are small enough to pass through the mesh to enter to the nozzles as they exit the coarse strainer  60 . 
     The strainer  60  can also provide a spring action or is collapsible to reduce the initial volume available to the generant pellets  8  and restrict them from rattling. 
     The nozzles  40   a  and  40   b  in the primary nozzle wall  40  cause particles of the generant to flow in a broadly tangential direction as each enters the annular or outer plenum chamber  12  formed between the nozzle wall  40  and the inflator housing  20 . Such flow tends to centrifuge any particles onto the housing  20  wall since they are much denser than the surrounding gas. The exit strainer flanges  51 ,  53  are purposefully positioned radially inwards of this housing  20  wall. The housing wall  20  includes a plurality of openings  21  disposed circumferentially thereabout. 
     In the first embodiment two or more rows of nozzles  40   a  and  40   b  point in opposite directions both clockwise and counter clockwise feeding into this annular plenum chamber  12  to balance reaction forces and to increase generant gas swirl across the exit strainer  50  in this plenum chamber  12 . This positioning of the nozzles is shown in  FIG. 1A  which is a cutaway section view showing the upper portion  22  with exit openings  21  and lower portion  24  welded together at the seam  23 . The strainer  50  is positioned over and spaced inward of the exit openings. The inner coarse mesh strainer  60  is shown inside primary wall  40  with the openings  62 . For the purpose of illustration only a section of the strainer  50  shown in  FIGS. 1A and 1B . 
     The nozzle openings  42  and  44  are oriented in a radial plane so the particles projected in a straight path outwardly must strike the nozzle wall  40  prior to being redirected by the nozzle depression or bulge  41 ,  43  out the respective opening  42 ,  44 . This flow redirection causes the gases to spiral out in the clockwise or counterclockwise in a cyclonic rotation. This rotation washes particle debris from the strainer  50  causing the debris to fall to the bottom of the plenum chamber  12  as the gases escape out the exit openings  21 . The outlets or openings  42 ,  44  from these rows of nozzles are arranged so that they cause a swirling flow across the surface of the exit strainer  50  to wash it clean of particles that are too large to pass through it. The swirling flow across the exit strainer  50  from each of the sets of nozzle openings  42 ,  44  is shown by arrows  242  and  244 . A portion of the exit strainer  50  has been removed in  FIG. 1B  to show the swirling flow pattern that is located behind the strainer  50 . The nozzles  40   a  and  40   b  also create a flow pattern urging some of the particulates in the gas stream to move away from the exit strainer  50 . This flow pattern is shown by arrows  250 . The exit strainer  50  provides a simple surface with little depth or geometry that could trap or jam such particles. The strainer  50  limits the absolute size of particles in generant gas exiting the inflator  10  and would quickly block if not continuously washed clean by such internal swirling flow. Any strainer micron size can be chosen but something of the order of 20 microns might be needed. Without the flow washing concept this would require an unacceptably large surface area. Arrows  252  show the inflation gas exiting the openings  21  in the inflator housing  20 . 
     The swirl associated with each radial nozzle opening  42 ,  44  tends to generate a local cyclonic/vortex that accelerates and mixes the gases and causes a centrifugal force on particulate proportional to its mass times velocity squared. The swirl velocity is proportional to the square root of the pressure drop across the first tangential nozzles. So such solid particles tend to be thrown away from the exit and its protective strainer  50  and circulate around the outer housing wall of the annular plenum chamber  12  driven by the velocity of gas exiting the tangential nozzles. Heavier particles are thus preferentially trapped inside the inflator  10  until they are burned small enough to pass these second radial nozzles with exiting generant gas. 
     The total flow path pressure drop is controlled by the nozzle cross-sectional area and is broadly similar to that of current inflators. But because there are two (sets of) nozzles in parallel, the pressure drop across each set of nozzles will be governed by their respective total cross-sectional areas in the flow. In this design, which is intended to be a minimum change from the current prior art inflator  100 , the upstream nozzles have approximately four times the flow area and therefore can be expected to drop only 1/16th of the total pressure. 
     Of course any distribution of pressure drop can be implemented by design. If approximately one half the total pressure is dropped across each of these nozzle sets, then the gas exit velocity can be reduced to: 1/SQRT(2)=70.7% of the velocity of a current standard inflator for the same volume flow rate. The swirl velocity in the second chamber will also be increased. 
     Current inflators  100  as shown in  FIG. 7  may leave a large percentage of the generant  8  unburned which may in some part be due to trapping in the depth of the filter  500 . Particulates in the present invention circulate continuously in the gas stream, and are exposed on all sides and free to burn providing for a more efficient use of generant, less functional variation and therefore require less generant for a given useful output energy. 
     A depth filter  500  as used in the prior art, by its very nature, blocks some percentage of the particles presented to it and consequently to some extent blocks flow. This blockage is random and therefore characterized by variance which could affect the functional performance of the inflator. 
     Additionally, the smaller the particle size that is blocked by a traditional filter, the larger its filter area and volume become. So a compromise is reached where unacceptably large particles are allowed to pass out of the inflator in order for its output not to be blocked. The flow washed filter/strainer of the present invention can block particles by design while reducing the inflator size. 
     As mentioned the present pyrotechnic airbag inflator  10  incorporates a strainer/filter that prevents generant and other particles of unacceptable size exiting with generant gas. Gas flow within the inflator  10  is made to swirl freely in a way that continuously moves blocked particles from the strainer&#39;s surface thus permitting it to have a practical flow area and size. A circulating gas flow creates an artificial gravity that preferentially diverts particles of generant away from the exit and permits them to continue to burn. 
     Returning to the prior art, inflator  100  shown in  FIG. 7 . This inflator  100  has a housing structure  200  having an upper or top portion  202  and a bottom or lower portion  204  welded together to form the small circular shaped inflator  100 . Connected to the bottom portion  204  is an igniter assembly  300  with a squib  302  that reacts to an electrical charge to fire and ignite a small propellant charge  400 . The flame from that ignites gas generant pellets  8  to rapidly produce inflation gases under pressure. These inflation gases pass through a ring shaped depth filter  500  designed to prevent particles from exiting the housing through the plurality of exit openings  201 . Each opening has a burst disk covering it to prevent moisture from damaging the generant pellets. These burst disks rupture once the internal pressure builds up allowing the inflation gases to escape to inflate an airbag. 
     As shown, the top of depth filter  500  has a deflector plate  600  extending like a skirt blocking the exit opening  201  of the housing. This helps prevent hot particles from being expelled as an added safety precaution. The filter  500  can clog and absorb unburned generant. This adversely affects inflation performance. The present invention described hereinafter prevents these problems. 
     Returning to  FIG. 1 , the inflator  10  further has a housing  20  with an upper portion  22  and a lower portion  24  welded together along a seam weld  23 . The bottom portion  24  has an igniter assembly  30  of a general design with a squib  31  to ignite a pyrotechnic charge  33  stored in an igniter housing  34 . Products of combustion then exit a series of openings  35  in the igniter housing which then ignite gas generant pellets  8  stored in the primary chamber  11 . All of these igniter components are the same as or similar to those currently found in this style inflator, commonly referred to as a “pancake” or disk shaped inflator. The upper portion  22  of the housing  20  has the exit openings  21  aligned in a row and equally sized and spaced. The housing construction with the row of openings is also commonly found in such inflators. The prior art depth filter  500  used in the prior art is replaced in the present invention inflator  10  and in its place is a combination of components that greatly improves the reliability and performance of these types of inflators. 
     As previously mentioned the preferred embodiment has a primary nozzle wall  40  in the shape of a ring with a plurality of nozzles  40   a  and  40   b , with nozzle openings  42 ,  44  arranged in two rows. These nozzle openings  42  and  44  are equally sized and spaced circumferentially around the wall  40 . The wall  40  extends generally from the top to the bottom of the housing  20  and may be connected with an upper  47  and lower  49  spacer and the space inside the wall  40  defines the primary chamber  11  which holds the generant pellets  8 . Inside and adjacent the wall  40  is a coarse mesh strainer  60 . The coarse mesh strainer  60  limits the size of generant that can pass to the nozzle openings  42 ,  44 . The coarse mesh openings  62  are sized to restrict the size of the pellet  8  that can impinge the openings  62  insuring no blockages can occur. On the exterior or outer side of the primary nozzle wall  40  is a second space or plenum chamber  12 . This space  12  receives inflation gases and burns debris and unburned generant particles as they pass through the nozzle openings  42 ,  44 . Once in the plenum chamber  12 , these gases must impinge on a strainer  50 , the strainer  50  is preferably made of a single layer of fine mesh  52  having a mesh opening sized to limit the size of any debris allowed to pass to the airbag on inflation. In the preferred embodiment, the mesh  52  is sized to 20 microns. The reason such fine sized openings in a single layer are possible is due to the unique directional flow that the inflation gases have as they impinge the strainer  50 . As shown, the strainer  50  has upper and lower flanges  51 ,  53  affixed to the housing upper portion  22  spanning above and below the exit openings  21 . The mesh layer  52  is affixed to the flanges  51 ,  53  to form the strainer  50 . All inflation gases must pass through the strainer  50  prior to leaving the housing opening  21  to inflate the airbag (not shown). 
     With reference to  FIG. 2 , a perspective view of the primary nozzle wall  40 , the strainer  50  and the internal coarse mesh strainer  60  and some exemplary gas generant pellets  8  are shown in the space of the internal primary chamber  11 . 
       FIG. 3  shows the upper housing portion  22  with the exit openings  21 . 
       FIG. 4  depicts a perspective view of the primary nozzle wall  40  with nozzle openings  42  and  44 , however the strainer  50  has been removed compared to  FIG. 2 . As shown, the openings  42 ,  44  are formed in shallow nozzle depressions or bulges  41 ,  43  formed in the tubular shaped wall by stamping or die pressing. The openings  42  are aligned in a circumferential extending row, equally sized and spaced all facing in a first direction such that gases passing through the openings  42 ,  44  of the nozzles  41  are flowing in a counterclockwise spiral swirling in a cyclonic action. On the lower row of openings  44  in the nozzles  43 , the openings  44  are equally sized and spaced, but facing opposite those of the upper row openings  42 . In this arrangement, the gas flow is spirally moving clockwise creating an opposite cyclonic flow. Where the two flows meet, the cyclonic flows cancel and a mixing flow occurs which allows the unburned gas generant to burn. This cyclonic flow pattern, as previously shown in  FIG. 1B , further assists in moving the inflation gases laterally onto the fine mesh  52  which allows the particles to impinge the mesh, knocking debris clear and allowing it to fall to the bottom of the plenum chamber  12 . As shown, the two rows of openings  42 ,  44  are oppositely oriented and this is believed preferred because it cancels torsional forces. However, it is possible to make the primary nozzle wall  40  with both rows  42 ,  44  oriented the same way or to only use one row of similarly oriented openings  42  or  43  to increase the cyclonic action for washing debris from the strainer  50 . In such a case the inflator mounting must be sufficiently strong to absorb any resultant thrust. 
     As shown in  FIG. 5 , the strainer  50  is shown with the annular flanges  51 ,  53  of a “z or n” type shape on each side of the fine mesh layer  52 . The flanges  51 ,  53 , which are better shown in  FIGS. 2A and 2B , abut the inside wall of the upper housing portion  22 . The strainer  50  when inserted has to be sufficiently wide to cover the row of exit openings in the upper housing  22 . 
     In  FIG. 6  the coarse mesh inner strainer  60  is shown having mesh openings  62  to control the generant  8  particle size that can pass through to the nozzle openings  42 ,  44 . 
       FIGS. 8A, 8B and 8C  are views of an alternative embodiment primary nozzle wall  40  having a single row of nozzles  41 A with nozzle openings  42 A. In this embodiment, all the components of the inflator  10  are the same, but the two-row nozzle wall  40  is replaced with the nozzle wall  40 A having a single row of nozzles  41 A with nozzle openings  42 A. In this embodiment, the nozzles  41 A are cut out along three sides and flared out from the wall. This alternative construction is a different way of creating the nozzle effect of the present invention. In any single row construction, the depression or bulge nozzle  41 ,  43  are equally suitable in a single row as previously shown and discussed. In this embodiment as shown, the flow will be counterclockwise. Alternatively, by oppositely facing the nozzles a clockwise flow can be achieved. 
     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.