Patent Publication Number: US-2011057425-A1

Title: Side curtain airbag

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/186,656 filed Jun. 12, 2009, the contents of which are incorporated herein by reference thereto. 
     This application is a continuation in part of U.S. Non-Provisional patent application Ser. No. 11/190,499 filed Jul. 26, 2005; and this application is also a continuation-in-part of U.S. Non-Provisional patent application Ser. No. 12/256,224 filed Oct. 22, 2008, the contents each of which are incorporated herein by reference thereto. 
    
    
     BACKGROUND 
     Exemplary embodiments of the present invention relate generally to a side impact or rollover inflatable curtain airbag and more specifically to apparatus and methods for deploying the same. 
     Various side impact or rollover airbags (also referred to as side curtains or curtain airbags) provide a cushion between a side of a vehicle and the occupant. Side curtain airbags generally deploy downward from a stowed position within the roofline of vehicle and inflate between the occupant and the vehicle interior side structure, such as the side windows and the A, B and/or C pillars. 
     A side curtain airbag generally consists of two fabric panels either sewn or interwoven together to create a plurality of inflatable cells. These cells are inflated during a predetermined activation event wherein a signal is provided to inflate the side curtain airbag. A side curtain may have a plurality of cells in various arrangements and/or configurations. 
     Typical airbag curtain designs have an “open flow” between chamber cells. Open flow as described herein is characterized by the gas or fluid within a cell having open fluid communication with adjacent cells via a diffuser tube and/or fluid paths disposed about the diffuser tube proximate to adjacent cells. This configuration allows the gas to uniformly fill the entire airbag because the gas distributes among all or most of the airbag cells or inflated regions. An example of an open flow conventional airbag is disclosed in FIG. 2 of U.S. Pat. No. 6,481,743 to Tobe et al., the entire disclosure of which is herein fully incorporated by reference. 
     In some applications, it is desirable to provide a side impact or rollover restraint system having an inflatable curtain airbag that does not have “open flow” between chamber cells. Furthermore it is also desirable to provide an inflatable cushion or airbag with a low leak seam and method for providing such an inflatable cushion. 
     SUMMARY OF THE INVENTION 
     Thus in accordance with exemplary embodiments of the present invention there is provided an inflatable cushion for a side of a vehicle. The inflatable cushion has at least a first cushion section formed from a first material, the first cushion section having a plurality of separate inflatable cells each of which having an inlet opening for receipt of an inflation gas, wherein an internal passageway is formed in the first cushion section and the internal passageway fluidly couples each of the plurality of separate inflatable cells to an inflation gas via the inlet opening of each of the plurality of separate inflatable cells and a diffuser member is disposed in the internal passageway, the diffuser member is configured to supply the inflation gas to each of the plurality of separate inflatable cells, wherein the diffuser member consists essentially of a non-rigid fabric member formed from a permeable material and the permeable material of the non-rigid fabric member covers each inlet opening of each of the plurality of separate inflatable cells such that the inflation gas must pass through the permeable material. Means for restricting fluid flow between the plurality of inflatable cells is also provided. 
     In another exemplary embodiment, an inflatable cushion for a side of a vehicle is provided, the inflatable cushion having a first sheet of material; a second sheet of material, the first sheet of material being secured to the first sheet of material to define the inflatable cushion; wherein at least a portion of a peripheral edge of the inflatable cushion is defined by a seam wherein the first sheet is secured to the second sheet only by a plurality of stitches and the inflatable cushion maintains an internal pressure in a range of greater than 20 KPa and less than 50 KPa for at least 1.5 seconds during inflation of the inflatable cushion. 
     In another exemplary embodiment, an airbag module for a vehicle is provided, the airbag module having a side curtain inflatable cushion and inflator. The inflatable cushion comprising a first cushion section formed from a first material, the first cushion section having a plurality of separate inflatable cells each of which having an inlet opening for receipt of an inflation gas; an internal passageway formed in the first cushion section, the internal passageway linking and fluidly coupling to each of the plurality of separate inflatable cells via the inlet opening of each of the plurality of separate inflatable cells; a diffuser member disposed in the internal passageway, the diffuser member being configured to supply the inflation gas to each of the plurality of separate inflatable cells, wherein the diffuser member consists essentially of a non-rigid fabric member formed from a permeable material, the non-rigid fabric member is independent of the first material used to form the first cushion section and the permeable material of the non-rigid fabric member covers each inlet opening of each of the plurality of separate inflatable cells such that the inflation gas must pass through the permeable material; and means for restricting fluid flow between the plurality of inflatable cells. 
     In still another exemplary embodiment, a method of inflating an inflatable cushion is provided, the method comprises at least the step of controlling the flow rate of an inflation gas into the inflatable cushion by limiting an amount of surface area between an exterior surface of a non-rigid fabric diffuser member and an interior surface of an internal passageway formed in the inflatable cushion section, the internal passageway linking and fluidly coupling to each of a plurality of separate inflatable cells via an inlet opening of each of the plurality of separate inflatable cells, the inflatable cushion being formed from a first material and the diffuser member consists essentially of a non-rigid fabric member formed from a permeable material, the non-rigid fabric member is independent of the first material used to form the inflatable cushion and the permeable material of the non-rigid fabric member is located such that the inflation gas must pass through the permeable material, wherein the amount of surface area between the exterior surface of the non-rigid fabric diffuser member and the interior surface of the internal passageway is limited by applying a plurality of stitches through a first wall member and a second wall member of the inflatable cushion in order to secure the same together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a vehicle with an inflated side curtain airbag; 
         FIG. 2  is an elevational view of the airbag in  FIG. 1 ; 
         FIG. 3  is a cut-away view taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cut-away view taken along line  4 - 4  of  FIG. 2 ; 
         FIG. 5  is an elevational view of another embodiment of a side curtain airbag in accordance with the present invention; 
         FIG. 6  is a cut-away view taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a side view of an inflatable cushion constructed in accordance with an alternative exemplary embodiment of the present invention; 
         FIGS. 8A-8C  are partial views of inflatable cushions constructed in accordance alternative exemplary embodiments of the present invention; 
         FIG. 9A  is a partial view of an inflatable cushion constructed in accordance an alternative exemplary embodiment of the present invention; 
         FIG. 9B  is a partial view of an inflatable cushion without portions of an alternative exemplary embodiment of the present invention; 
         FIGS. 10-13C  illustrate inflatable cushions constructed in accordance various alternative exemplary embodiments of the present invention; and 
         FIG. 14  illustrates plots of an unsealed cushion not using exemplary embodiments of the present invention and plots illustrating exemplary embodiments of the present invention (cushion pressure vs. time). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to exemplary embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope and spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Referring to the drawings, and particularly to  FIG. 1 , a vehicle  10  is shown. Vehicle  10  includes an A-pillar  12 , a B-pillar  14 , and a C-pillar  16 . A side curtain airbag  18  in accordance with one exemplary embodiment of the present invention is illustrated and extends between A-pillar  12  and C-pillar  16 . In  FIG. 1 , airbag  18  is shown in an inflated state. In this regard, an inflator  20  provides a gas necessary to inflate airbag  18 . Inflators  20 ,  21  and  23  are shown in dashed lines to display alternative locations for the inflator. Thus, the inflator may be located in the B-pillar, in the C-pillar, in the roof, or in another suitable location within vehicle  10 . 
     Before airbag  18  is deployed, it may be stored within roof rail  22  of vehicle  10 . Optionally, tethers  24  and  26  may be used to restrain airbag  18 . In the embodiment shown in  FIG. 1 , tethers  24  and  26  attach at one end to airbag  18  and at the other end to the body of the vehicle. 
     Referring now to  FIG. 2 , further details of side curtain airbag  18  can be most easily explained. Airbag  18  includes a plurality of substantially isolated cells  28 ,  30 ,  32 ,  34 ,  36 ,  38 , and  40 . Cells  28 ,  30 , and  32  make up a rear bank of cells between B-pillar  14  and C-pillar  16 , while cells  34 ,  26 ,  38 , and  40  make up a front bank of cells between A-pillar  12  and B-pillar  14 . Area  42  is not inflated because an occupant is less likely to come into contact with that area. In some embodiments, however, area  42  may be a cell, or may inflate at a time later than the other cells. 
     Continuing to refer to  FIG. 2 , tabs  44  are provided in this embodiment to attach airbag  18  to roof rail  22 . Instead of tabs  44 , any suitable method of attachment may be used. A delivery tube  46  having a plurality of outlet orifices  48  is also provided. In the illustrated embodiment, outlet orifices  48  are formed as scoops. Orifices  48  open to cells  28 ,  30 ,  32 ,  34 ,  36 ,  38 , and  40 . To reduce turbulence within tube  46  and to better distribute the gas, outlet orifices  48  may be staggered about tube  48 . Such staggering can be seen in  FIG. 4 . Delivery tube  46  is sealed at  50 . Gas from inflator  20  enters delivery tube  46  at end  52  and is distributed into the cells through outlet orifices  48 . 
     Referring now to  FIGS. 2 ,  3 , and  4 , the mating of delivery tube  46  with airbag  18  can be described. In this embodiment, tube  46  is inserted into the top portion of airbag  18 . A top perimeter seam  54  runs along the top of airbag  18  and forms an upper restraint for tube  46 . A series of sewn ovals  56  are formed by stitching  57  ( FIG. 3 ) between the cells. The tops of ovals  56  form a substantially tight fit with the bottom of tube  46 . In this context, “tight” does not mean that no gas is able to flow between ovals  56  and tube  46 . Instead, “tight” refers to a close-fit that may be optimized to allow some gas flow between adjacent cells. Along with continuous seam  58  and seams  66 ,  68 ,  70 ,  72 , and  74 , ovals  56  form cells  28 ,  30 ,  32 ,  34 ,  36 ,  38 , and  40 . Any of the sewn seams may be single stitched, double stitched, or attached in another appropriate manner, depending on the strength and air-tightness requirements of the airbag. Airbag  18  has a thickness t ( FIGS. 3 and 4 ), which may vary over the cells. 
     Referring specifically to  FIG. 4 , outlet orifice  48 , formed as a scoop, can be seen. The orientation of orifice  48  in  FIG. 4  is somewhat staggered in that it is rotated at various angular positions around the tube periphery. The scoops preferably are rotated at various angular positions about tube  46  to better or more effectively capture and channel the gas flowing within the tube into the cell. 
       FIG. 5  shows a second embodiment of the present invention. In this embodiment, airbag  76  is inflated by inflator  78 , which is positioned in the B-pillar adjacent the center of deployed airbag  78 . Inflator  78  connects with delivery tube  80  at an intermediate location along its length, in this case near the longitudinal center. In this configuration, both of ends  82  and  84  are sealed, forcing gas into each of cells  86 ,  88 ,  90 ,  92 , and  94  upon inflation. Tethers  96  and  97  connect airbag  76  to the vehicle body. Depending on the vehicle or application, other configurations of the present invention may include embodiments with more than two banks of cells. 
     Continuing to refer to  FIG. 5 , it can be seen that a circle  98  is sewn into cell  88 . A seam  100  connects circle  98  to seam  102 , which forms part of the cell&#39;s perimeter. Seam  100  partially separates cell  88  and may be designed to reduce the volume and thickness of cell  88 . As can be seen in  FIG. 6 , a reinforcing layer  104  is included on one side of airbag  76  at circle  98 . Reinforcing layer  104  is another layer of fabric sized just larger than circle  98  in this embodiment, and is stitched together with the fabric forming airbag  76 . Each of ovals  56  also include a reinforcing layer  106  sized just larger than the oval. In some 30 liter airbag embodiments, a reinforcing layer that is ½ inch larger at each edge than the stitching has been used. In still further embodiments, a reinforcing layer has been used on either side of the airbag, yielding a structure with four layers in the area being reinforced. 
     Referring now to  FIG. 6 , the construction of reinforcing layer  104  may be described in more detail. First layer  108  and second layer  110  form respective sides of airbag  76 . Sewn circle  98  is formed by stitching  112 , which extends through first layer  108 , second layer  110 , and reinforcing layer  104 . The advantages of using reinforcing layer  104  will be discussed later. Note, however, that other configurations, including the use of multiple reinforcing layers on either or both sides of the airbag, are contemplated by the present invention. 
     The novel airbag disclosed herein is designed so the flow of gas between chamber cells is substantially reduced during the loading of the airbag. By creating a more reduced gas flow between the chamber cells during loading, the pressure within a given substantially isolated cell builds up greater than would otherwise occur with an open flow between the same cells. The increased pressure within the cell due to flow restriction or cross-cell flow restriction resists the occupant from striking through the cell to a greater extent than with conventional open flow between cells. With the gas restricted in its movement out of the cell, an increased resistance to occupant displacement or strike through is established. Therefore, the present invention provides airbags and methods of making airbags that restrict flow between chamber cells. 
     Because of the restricted flow between adjacent cells, the pressure delivery from the inflator to the cells may require a more precise design of the gas delivery system to provide each substantially isolated cell with its required operating pressure. For example, one such delivery method to achieve gas delivery to the individual cells may utilize an elongated tube (for example, delivery tube  46 ) extending within the upper portion of a side curtain airbag. The tube may be fabricated with the appropriate size, quantity, and location of outlet orifices to sufficiently deliver the appropriate amount of gas to each cell. Additionally, scoops configured to channel flow into a given cell may be utilized to further control the desired gas delivery. The scoops may be advantageous in areas of the delivery tube where the gas flowing in the tube tends to substantially pass by outlet orifices in the delivery tube due to the dynamics of the supersonic gas flow within the tube. When the tube has a scoop, which is essentially an indented section of the tube including an opening, the gas flowing within the tube is channeled into a particular cell. 
     The scoops may be staggered or staged along the delivery tube to achieve the required channeling of gas into each cell. For example, one cell may have two or more scoops but would preferably not have them directly “in-line” with each other along the longitudinal axis of the delivery tube. Instead, the scoops are preferably staggered to better or more effectively capture and channel the gas flowing within the tube into the cell. 
     The restrictive flow of the present invention may be characterized using the term “diode.” The term “diode” is generally used in electronics to refer to a device that freely passes electrical current in one direction but not in the opposite direction. In the present invention, “diode” is used to refer generally to the restriction of flow between airbag cells. This “diode effect” is due in part to differences in pressures; during initial filling of the cells the gas pressure within the delivery tube is very high, yet the gas pressure within each pressurized cell is comparatively much lower after inflation. Thus, the gas flow into each cell occurs quite quickly, while the gas flow out of the cells and into adjacent cells, into the delivery tube, or to the atmosphere is comparatively slow. 
     One way to keep the gas from escaping the cell is to appropriately design how the airbag fits around the delivery tube between cells. The following example is characteristic of the fit between the delivery tube and the airbag between cells. The delivery tube is inserted between the top perimeter seam and the ovals. The ovals assist in isolating the cells or regions from each other. Given a ⅝ inch delivery tube outer diameter, the distance between the top perimeter seam and the oval&#39;s seam may be in the range of 1.02 inches to 1.10 inches (with the fabric sewn flat-no pressure). The ⅝ inch delivery tube outer diameter would have a circumference of 1.96 inches nominal. A distance of 1.02 to 1.10 inches between the oval&#39;s seam and the top perimeter seam yields an inner circumference of about 2.04 inches to 2.20 inches when expanded by the tube insertion. The clearance between the tube and the fabric opening provides enough clearance to allow for installation. However, the fit between the delivery tube and the airbag still provides enough flow restriction between the tube outer diameter and the airbag fabric to restrict the gas flow between chambers (e.g., cells). 
     The difference in effective flow area between cells in a conventional open flow airbag and that of an airbag in accordance with the present invention is shown in the following example. A probable effective flow area from the delivery tube into a diode cell to meet initial side impact fill requirements may have an effective flow area in the range of 0.05 square inches to 0.15 square inches for a 30 liter side airbag. This effective flow area from the tube orifices is in addition to the effective flow area occurring from the clearance between the delivery tube outer diameter and the fabric layers sewn around the delivery tube in areas between the cells. Various clearances between the delivery tube outer diameter and sewn fabric layers surrounding the tube between cells may be used, yielding different effective flow areas. For example, with a 2.04 inch fabric circumference around the tube, the maximum possible flow area between a ⅝ inch tube outer diameter and the fabric circumference would be about 0.025 square inches. With the same tube and a 2.2 inch fabric circumference, the maximum flow area would be 0.078 square inches. Given the larger 2.2 inch fabric circumference and the larger of 0.15 square inch flow area from the tube orifices, the maximum effective flow area between adjacent diode cell might be 0.23 square inches. 
     In a conventional airbag, the flow area, even if restricted by a 1 inch diameter opening between cells is around 0.8 square inches. A 1 inch diameter opening between cells is actually toward the much more restrictive end of current conventional airbags; many, if not all, have even larger open flow effective flow areas. The effective flow area difference in this example would yield 3 to 4 times more open flow area in the conventional design as compared to the diode design discussed above. The range of effective flow areas given for the diode design of the present invention is only an example for illustrational purposes and is not intended to limit designs into that range. Depending on cell sizes, cell volumes, or even timing requirements for filling the cells, the effective flow area requirements may change. Thus, for different sized airbags, different effective flow areas may prove effective. 
     The diode airbag was tested with varying fits or clearances between the tube outer diameter and the fabric between the cells. When the effective flow area between the tube outer diameter and the fabric between the cells went beyond 0.3 square inches, the chances for strike-through increased. The flow area became too great between cells, thus not as effectively increasing the pressure within the cell during loading of the cell by the mass. Therefore, it was determined that anything under an effective flow area of 0.5 square inches (flow area between fabric and tube plus the flow area from the orifices) between the adjacent cells may provide effective protection in some 30 liter airbag applications. Under 0.25 square inches flow area proved to be even more ideal. 
     It should be noted that cells not on the ends of the air bag may have almost double the maximum flow area of end cells, since flow can escape from a loaded interior cell into adjacent cells on both the left and the right. Because of this, the cells at the end of the airbag (the cells with only one adjacent cell) may become stiffer than interior cells (the cells with two adjacent cells) during occupant loading. The extra stiffness of the end cells should be taken into account in designing the airbag. 
     The gas pressures within the inflated cells of the present invention are substantially low in comparison to the pressures within the delivery tube during inflation of the cells from the inflator. Typically, a side airbag inflates during approximately the first 25 milliseconds after being triggered. The occupant interaction with the airbag may initiate around 30 milliseconds in some applications or as late as approximately 55 milliseconds or more in others. Thus, by the time the occupant is loading or interacting with the airbag cell the pressure within the delivery tube has dropped substantially. By this time, the pressure in the delivery tube may actually come close to or equalize to the pressures within the airbag cells. For example, the pressure within the cells may be between 20 to 40 kpa while the pressure within the delivery tube may be 500 kpa to 1500 kpa during initial cell filling from the inflator. 
     As the occupant loads the cell and increases the cell&#39;s pressure, the gas within the cell may use the delivery tube outlet orifices as a cell “vent”. By the gas flowing back through the delivery tube holes and essentially being vented back to other cells in the airbag, the cell is kept from becoming excessively hard. The general goal is to have delivery tube outlet orifices sized with a large enough total effective flow area to achieve the required fill timing for a given cell while being small enough to restrict the backflow, along with the flow restrictions from the fit between the delivery tube and airbag and seam optimization, to achieve desired cell pressure increases during occupant loading. 
     Due to the advantages of the present invention, lower cell operating pressures may be utilized with the present invention compared to the pressures needed in similar cell inflated cross-sections using a conventional open flow construction between airbag cells. The operating pressure is the pressure the inflator must deliver to the airbag prior to occupant interaction (cell loading) to effectively dissipate the occupant&#39;s energy prior to striking through the airbag. The lower operating pressure requirement offered by the diode design is advantageous over previous art since a reduced inflator output can now achieve similar overall occupant protection performance. For example, an airbag without the more restrictive flow design of the present invention would require a higher output inflator (larger size) to fill the cells to a higher required operating pressure. A reduced output inflator or smaller size inflator required with the present invention may offer the advantages of lower cost, lower weight, and less space required to package the inflator within the vehicle. 
     Another possible advantage of a diode airbag is the ability to reduce the overall volume of each cell while retaining desired occupant protection properties. Reducing the cell inflated thickness of a conventional airbag will achieve this lowering of the volume, but will require an increased cell operating pressure over that of a thicker inflated cell to achieve similar occupant protection. Because of the smaller cell volume that may be required with the present invention, faster fill times and faster in-position times may be possible. This can be achieved since it typically takes less time to fill a smaller cell volume. 
     In the case of a diode design using a thicker cell cross-section (say 4-5 inches), the pressure could be approximately 20 kpa to meet current impact requirements. Reducing the cell volume long with the cell inflated thickness to about 2.5 to 3 inches would require an increased operating pressure of around 60 kpa. The same size inflator, however, could be used for each approach. The advantage of the 20 kpa approach is that it may apply less stress to the seams and thus, reduce overall lower airbag leakage. The approach with the 60 kpa and lower cell thickness/volume could give the advantage of faster in-position times for the same inflator output. Depending on the specific application requirements and goals, either approach may be implemented. 
     Alternatively, a soft or flexible delivery tube may be utilized instead of a rigid or solid tube. Even a delivery tube constructed from fabric with appropriately staged outlet holes may be utilized for appropriate gas distribution to the individual cells. While more versatility and tunabilty may be allowed by using a rigid delivery tube (due to the ability to shape the tubing wall contour with scoops), the use of a more collapsible (flat lying) tube may have packaging benefits for some applications. 
     With a solid delivery tube, the scoops which are utilized to channel gas flow into a particular cell region can more efficiently achieve a high flow rate of gas into the cell without disrupting the more efficient laminar flow within the delivery tube. In some solid delivery tubes used for gas delivery into conventional airbags, the outlet orifices are created by perforating the tubing wall. In these cases, the perforated tube wall creates an obstruction within the tubing internal diameter. In addition to restricting the effective flow area to the remainder of the tube, the perforations create a more turbulent gas flow within the tube. Turbulent flow compared to a more laminar flow is known to create increased pressure losses. The more turbulent the flow within a delivery tube, the more these pressure losses may add up, which may lead to inefficient use of the gas energy delivered from the inflator. Thus, more effectively optimizing the delivery tube with appropriately positioned scoops for each individual cell, will use the inflator&#39;s gas energy more efficiently. 
     While a diode-type airbag could be used with any of a number of inflators known in the art, an extended output inflator may work better than some other inflators in roll-over applications. An example of an extended output inflator is shown and discussed in U.S. Pat. No. 6,543,806 to Fink, the entire disclosure of which is herein incorporated fully by reference. One of the aspects giving the extended output inflator a performance advantage is the use of a gas mixture contained therein. One gas with a small molecular size, such as helium, in combination with a gas with a larger molecular size, such as argon, nitrogen, carbon dioxide, nitrous oxide, etc., may be utilized. 
     A smaller molecule gas, such as helium, may be utilized because it has been shown to more rapidly fill an airbag. This likely is because helium has a lower molecular weight of 4. For example, argon is a higher molecular weight gas with a molecular weight of almost 40. The helium molecules flow more quickly through a given flow area than a larger gas molecule, such as argon. 
     The initial inflation of the airbag cells from the stowed state to the inflated state typically needs to occur within 15 to 25 milliseconds (in-position time) after a signal is received from a crash sensor. In-position time is the time required for the airbag to deploy from the stowed state within the roofline of a vehicle to a substantially unfolded and inflated position covering the vehicle&#39;s interior side structure. Thus, helium in a gas mixture may give the pressurized gas mix the ability to quickly flow into the airbag to meet the required in-position timing. Helium however, due to its small molecular size, will have a greater tendency to leak through any potential leak paths in the airbag than would a higher molecular size gas. Therefore, a higher molecular size gas within the pressurized gas mixture, such as argon, gives this pressurized gas mix the characteristic of a slower leak rate through any airbag leak paths. A gas mixture can therefore be optimized to meet both demands, fast in-position time and low leakage, by utilizing the best mixture percentage scenario to meet particular application requirements. 
     It has been found that a cold gas inflator containing only a higher molecular size gas, such as argon, may not achieve the required 15-20 millisecond in-position fill timing alone. In a cold gas inflator, the gas within the bottle undergoes decompression during inflation and cools rapidly. The larger size gas molecules become more sluggish compared to a smaller size molecule when cooled. This sluggishness has to do with each gas&#39;s critical temperature. The larger the gas molecule, the higher the gas&#39;s critical temperature. The closer a gas comes to its critical temperature during cooling from decompression, the slower the random movement of the molecules becomes. Thus, a higher molecular weight gas will become more sluggish as it is cooled than will a lower molecular weight gas. Therefore, in general, the flow rate of a higher molecular weight gas will be lower through a given outlet area as compared to a smaller molecular weight gas. 
     Finding an optimum mixture of high and low molecular weight gases is important to the functionality of an airbag. The ideal gas mixture will depend on the application or, more specifically, on the internal volume of the airbag and the fill timing requirements. It has been found that airbags of a smaller internal volume, for example around 25-35 liters, may allow for a higher concentration of argon in a helium-argon mixture while meeting required fill times or in-position timing. As discussed above, providing the airbag with as high a concentration of the larger gas molecule as appropriate will achieve better gas pressure retention over time in the airbag. In particular, higher pressure retention over time is desired when rollover protection is a concern. 
     In larger airbag volumes, the concentration of argon may need to be reduced to assist in meeting required in-position timing for the larger volume being filled. Typical gas mixtures for smaller airbags may range from 60% helium/40% argon to 75% helium/25% argon. For the larger airbag volumes (40 L and up), the ratio of helium may need to be increased. Typical ratios found effective may range from 65% helium/35% argon to 80% helium/20% argon. These ratios are typical of ranges found effective with conventional open flow airbags. 
     As disclosed herein, the diode airbag designs allow for higher concentrations of argon due to techniques achieving faster in-position times more independent of the gas mix ratio. Again, these possible higher concentrations of the larger gas molecule will further enhance airbag pressure retention. Mixtures have been used with a cold gas extended output inflator or even in a single chambered cold gas inflator in the range of 50% helium/50% argon. This particular mixture provided in-position times in the 40 millisecond range. Thus, depending on fill timing requirements (longer in-position time requirements allow increased argon ratios), the concentration of the larger gas molecule, such as argon, may range from 10% to 100%. 
     Effusion is the rate at which a gas will pass through a porous barrier/hole/orifice or any mall potential leak path or opening. Effusion, as it applies to airbags, relates to the tendency of smaller gas molecule, such as helium, to leak through the airbag leak paths to the atmosphere at higher rate than a larger gas molecule, such as argon. 
     Once in the airbag, the larger gas molecule within the gas mixture may effectively act as a “blocker” to restrict the leakage of some of the helium molecules through the seam openings or other leak paths. If the larger argon molecule were not also randomly escaping through the leak paths, the helium molecules would escape more unrestricted or more freely through the leak paths. The helium molecules may now collide with the larger argon molecules, thus diverting a path which would have otherwise met directly with the atmosphere. In effect, the overall gas leakage is reduced. 
     Another airbag characteristic that has an influence on the required inflator gas mix ratio is the airbag&#39;s operating pressure. A distinct advantage of the lower operating pressure diode airbag of the present invention is that the gas mixture ratio may allow for a higher concentration of the larger gas molecule, such as argon. A diode airbag may be inflated to operating pressures of about 20-40 kpa as opposed to conventional open flow designs requiring around 60-120 kpa. Because a diode airbag has a lower operating pressure, a smaller amount of gas in a smaller inflator is necessary. 
     Additionally, a lower airbag operating pressure allows for reduced seam leakage and reduced stress to the seams. Also, it is easier for an extended output inflator to effectively maintain a lower operating pressure over an extended period of time than it would be for the inflator to maintain a higher required operating pressure. Therefore, the combination of an extended output inflator and a diode airbag design can offer significant system level advantages. 
     Another advantage of a lower operating pressure airbag may be reduced injury to out-of-position occupants. In some situations, an occupant may be in a position very close to a deploying airbag. Airbags are required to deploy at extremely fast rates and have been known to cause injury to occupants who intrude on the deployment path of an inflating airbag. One aspect having an influence on potential injury to the out of position occupant is the internal airbag pressure. The diode airbag of the present invention would effectively reduce the force experienced by the out-of-position occupant because of its lower internal pressure over a given surface area. 
     Additionally, as an airbag “tuning” benefit, the diode airbag may be designed to deliver a relatively higher pressure to only some of the cells within an airbag. It is possible to achieve a higher pressure in a selected cell(s) over the initial filling/occupant interaction event, approximately the first 20-60 milliseconds. If, for instance, certain cells would perform better with higher pressure over the initial occupant impact, tuning these cells may be advantageous. For example, cells known to interacted with an occupant during a vehicle or pole impact may be tuned to a higher pressure. Once the selected cells receive the higher pressure to meet the requirements for the initial side impact, the pressures within that particular cell may equalize with the remaining cells as the gas is gradually transferred back through the delivery tube outlet orifices. 
     A further performance “tuning” advantage with the diode approach is to have particular cells receive gas at a faster rate than other cells. Delivering gas at a faster rate to selected cells may yield faster in-position timing. This will allow the selected cells to pull the remainder of the airbag down and be in the fully deployed in-position state faster than if all the cells received equal amounts of the initial gas delivery. The cells targeted to be the cells to receive the higher pressure can also be the same selected to cells to have the faster filling times. These two objectives of higher cell pressure and a faster gas delivery rate work well together. 
     Another option with the diode approach is to create particular cell(s) or cell regions that inflate over a longer period of time. These cells could be cells that are not required during the initial side impact, but are needed in time for a rollover type event. This may be achieved by having delivery tube outlet orifices with an effective flow area substantially smaller than that of the tube orifices used for filling the cells needed for the initial side impact. These smaller outlet orifices could be in direct communication with the slower filling cells. Instead of these slower filling cells filling in the 15-25 millisecond time frame, they could fill relatively slower, 100-500 milliseconds or even longer, for example. 
     By using the slower filling cell option, less inflator output may be required for the initial side impact requirement because less total volume is required to be filled. Then, the cells that fill over the longer period of time need to fill only to a lower pressure compared to the initially faster filled cells, as much as half the pressure or less. Thus, the total inflator output requirement is reduced by staging the inflation of selected cells. Yet, the total protection area is provided as needed, when and where it is needed. 
     In particular, slower filling cells could be cells which fill to provide protection on the roof area or ceiling of the vehicle. These cells would not necessarily be required during an initial side impact event, but would provide benefit during a rollover event. The time requirement for these cells to fill may be relatively much later in time to that of the cells utilized during the side impact event. 
     With the inflatable cells filling for roof protection, there may not be as much room between an occupant&#39;s head and the roof. Especially with larger or taller occupant&#39;s, this will be the case. For the cells intended to cover a larger surface area within the roof liner, the airbag can alternatively be stowed in an unfolded condition within the roof liner. This would allow for essentially immediate or pre-existing in-position timing and reduce concerns about airbag positioning in cases with taller occupants. 
     Another application for slower filling cells could be to expand the inflated airbag cell coverage area over the side structure of the vehicle. This provides expanded protection in a vehicle rollover, as the occupant may be tossed around and come into contact with areas of the vehicle not typical of non-rollover events. 
     It is possible that areas within a particular airbag which were not intentionally designed to fill with gas may fill gradually over time. The reason for these additional unintended cells may be the fact that seams used to close-off these un-inflated airbag areas actually allow leakage through the seams and into the unintended cell area. As an extended output inflator continues to supply pressure to an airbag, these unintended cells fill with gas. Depending on the degree of leakage in the seam, the time it takes for the unintended cells to fill may vary. In one particular airbag, the unintended cell filled in approximately 1 second, as viewed on video monitoring of the deployment. Optionally, the offending seams creating the unintended cells may be strategically opened, creating slower filling cells. Furthermore, the total tension in the airbag may be further increased by the expansion of these slower filling cells over time. 
     Occupant containment is another demand required of an airbag. Occupant containment is the ability of a deployed airbag to keep the occupant within the vehicle, preventing possible ejection of the occupant through a window opening. Airbag tension over a window opening has an effect on the degree of containment or occupant displacement beyond the window opening. With an airbag in accordance with the present invention, it is likely that less displacement of the cell at the airbag cross-section level will occur. This may translate into less total occupant excursion when compared to a similar cell inflated cross-section at the same pressure level using conventional open flow between cells, which likely would deform to a greater extent. 
     Yet another option that could be added to an airbag in accordance with the present invention is inflatable straps. Straps are often used to anchor an airbag to the vehicle. Having the straps inflate will decrease their length as compared to their un-inflated state, thereby creating tension within the straps and the airbag to which they are affixed. Configuring the straps to inflate after the initial inflation of the airbag could advantageously add tension to the airbag at a time when it would otherwise be losing tension due to pressure loss. 
     The following example highlights some of the advantages of the diode design of the present invention by comparison to a conventionally designed airbag. In this particular comparison example, a two-row coverage (A to C pillar) airbag with an approximate volume of 30 liters is used. The airbags in this comparison example were both cut and sewn airbag constructions without seam sealing and used a similar fabric. 
     The conventional open flow airbag has been found through dynamic pole testing to require an operating pressure of around 60-70 kpa to prevent occupant head strike through. The internal volumes of the airbags were essentially held constant at 30 liters. The airbags were each subjected to energy absorbing tests where a fixed mass is dropped from a predetermined height into each airbag in the same location/area. Also, similar inflated cross-section thicknesses were utilized. The mass used was a 6.5 inch diameter shape, which approximately simulates the surface area of an occupant head. Tests of both types of airbags revealed that the conventional open flow airbag required approximately two to three times more pressure to prevent the same mass energy from striking through an airbag as compared to the pressure required by a diode airbag in accordance with the present invention. 
     This difference in the operating pressure requirement allows a diode airbag to use a substantially smaller inflator. In the example above, simulating side impact protection only (non rollover), the inflator requirement in the conventional open flow airbag to yield 60 kpa requires a 2.3 molar output cold gas inflator (He/Ar). The inflator required by the diode airbag to meet an approximate 22 kpa operating pressure is a 1.5 molar output cold gas inflator (He/Ar). Thus, the conventional airbag requires an approximate 50% higher molar output inflator than the diode airbag. This is due in part to the lower operating pressure requirement of the diode airbag, but also to the reduced airbag stress allowed by the lower operating pressures (lower airbag leakage). 
     Using the same sewn/unsealed airbag examples, airbag/inflator combinations were then evaluated as they relate to rollover protection. It was determined that the conventional open flow airbag would require approximately 15 kpa at 5 seconds to sufficiently meet containment requirements, given a 5 second containment objective. Given that fixed 5 second objective, a diode airbag will perform similarly on containment objectives with an airbag pressure of around 10 kpa at 5 seconds. This pressure value for a diode airbag is estimated from the reduction in displacement within the loaded cell cross-section on a diode airbag compared to the conventional airbag. Through testing, it was determined that the conventional airbag required a 3.5 molar output cold gas Extended Output Inflator (EOI) to meet the approx. 15 kpa at 5 seconds. Preliminary testing also found that the lower operating pressure diode airbag to requires a 2.5 molar output cold gas EOI to meet the 10 kpa at 5 seconds criteria. 
     Seam sewing to reduce leak paths for enhanced pressure retention in unsealed airbags can also affect the performance of an airbag. Airbag leakage can be broken down into several key leak paths. First, leakage may occur through the base fabric, which is more commonly referred to as fabric permeability. The permeability is the rate of gas leakage through the fabric structure or through the thread weaves. Several methods can be employed to reduce or effectively eliminate this potential leak path. One conventional method is to coat the fabric with a gas impermeable substrate such as silicon. Other coatings such as neoprene, polyurethane, polyester, etc. or others may be used. Another commonly known method is the use of laminates. Yet another method to reduce fabric permeability is dipping a fabric in a solution that penetrates and bonds/adheres to the fabric creating a barrier to leakage. 
     In the fabrics coated with silicon, it has been found that higher levels of coating help to reduce airbag leakage. A typical coating level used in a popular 420 denier nylon fabric is 0.7 ounce per square yard of coating. At this coating level, the permeability may be substantially reduced compared to a non-coated fabric. However, some permeability is present, especially as the pressures are increased. 
     In side curtain applications, the airbag operating pressures are well above atmospheric pressure. A typical pressure could be 50 kpa to 120 kpa and beyond. At these higher pressures and with the large surface areas required to make up a side curtain, the impact of the fabric permeability more substantially affects the total airbag leak rate. This is especially true as the time requirements for maintaining an inflated airbag increase. The fabric permeability may be low compared to uncoated fabrics but even small amounts can surely add up when considered in totality over the entire surface area of the airbag. 
     A second leak path exists in areas where a seam is used to join fabric panels. One commonly practiced method to create a seam is to sew the fabric panels together using stitching (sewn seams). With a sewn seam, several potential leak paths exist. One leak path exists between the fabric panels as the panels are sandwiched together by the sewn seam. That is, leakage may occur between the fabric layers through the perimeter opening of the fabric. Increasing the density of the stitching may also reduce leakage. 
     Yet another leak path exists where a needle hole is created during the stitching process as the needle thread is passed through each panel fabric layer. As the stitching thread passes through the created needle holes, it will assist in blocking some gas flow. However, some degree of leakage will still exist. 
     Another method used to reduce airbag leakage has been termed “Seal and Sew”. This method utilizes an adhesive or sealant that is applied to fabric panels in all the areas that are needed to create the pattern and shape of the inflated airbag. Then, for increased strength and integrity, a sewn stitch is added in the center of the sealant bead. While this method has been found to reduce airbag leakage and be a potential option for increased pressure retention, drawbacks exist. The adhesive/sealants required are quite expensive. The application process has been deemed “messy” and time consuming. The needle passing through the adhesive bead can pick up contaminants from the sealant bead, which may then negatively affect the sewing process. A cure time is also required after applying the adhesive bead prior to sewing. Another substantial drawback with the “seal and sew” method is that the seam requires an increased package size when the airbag is folded and stored in the stowed position in the vehicle roof line area. 
     A fabric type known for low permeability is disclosed in Published U.S. Patent Application 2004/0242098 A1 (the &#39;098 application) to Bass, published on Dec. 2, 2004 and incorporated herein by reference. Such a fabric has advantages for reduced leakage due to its extremely low permeability, while also displaying favorable leak prevention characteristics at the seam level. The treated fabric has been found to have reduced leakage at the needle hole leak path as compared to other fabric alternatives. Such a fabric also appears to more effectively mold around the thread and create a better seal against gas escaping through the needle hole. 
     Another common seam construction method is called OPW or One Piece Woven. This process weaves the fabric panels together to create an interconnected seam as the fabric is passed through a loom. This method yields fewer leaks because no thread is used. However, OPW is still susceptible to seam stress from inflation, and leak paths may be created in these seam areas. 
     Because a smaller needle size generally yields less leakage, several combinations of thread sizes and needle types/sizes were explored. Three different thread sizes were used (#138, #92 and #69 thread sizes). The most commonly used thread size in automotive airbags is the #138 size in nylon. After matching each thread size with the best needle to efficiently deliver the thread without breakage or seam inconsistencies, the thread size/needle combinations were tested to comparatively evaluate seam leak rates. 
     To more effectively evaluate seam leakage and compare multitudes of variables (such as seam density, thread size, needle size, needle point type, thread type, thread brand, bobbin thread tension, needle thread tension, and fabric type), fabric swatches were sewn together to create multitudes of small inflatable square shaped “pillows.” The test specimen pillows were used to evaluate seam leakage at the outer pillow perimeter seam along with a circular seam sewn in the center. Four fabrics were used for comparisons with the varying seam constructions—fabric disclosed in the &#39;098 application in 315 and 420 denier and silicon coated 315 and 420 denier fabric (both having approximately 1 oz/sq yd of coating). 
     The test pillows were pressurized to pressures of both 20 kpa and then 60 kpa and maintained at each of those levels during leak evaluations. Three methods were used to evaluate the leakage—submersion of the pillows with visual observation, spray of the seam with a bubble leak check solution and lastly, electronically monitoring the pressure decay after shutting off the supply pressure. 
     The seam in the center was chosen to simulate a higher stressed type seam that is typical of many side curtain airbag patterns. These types of higher stressed seams are those that have the airbag inflatable areas pulling up on the seam around substantially the entire seam perimeter. In addition, these seams are typically of relatively small size so as to not add too large of an un-inflated area within a required protection zone of the airbag. Therefore, these higher stressed seams have a relatively small seam length (circumference) exposed to some of the highest forces occurring within the inflated airbag. These higher stressed seams generally occur in the inner areas of the inflated side curtain. It is these higher stressed seam areas which generally develop the most notable degree of leakage under pressurization. Thus, finding the best solutions to reduce leakage on these highly stressed seams will prove quite beneficial for enhanced airbag pressure retention. 
     Through testing, it was discovered that by adding an additional small fabric layer to the exterior side(s) of the airbag, substantially reduced leakage resulted. This is advantageous in areas that undergo much higher stresses during inflation and pressurization. The reinforcements could be small circular cutouts covering the circular center seam. 
     Externally positioned reinforcements serve a dual purpose. First, they add additional fabric strength to the highly stressed seam area, thus effectively reducing the spreading apart of the fabric weave. Second, the fabric layer takes on a gasketing effect to reduce the leakage which would otherwise flow more easily through the needle holes. 
     Comparative tests were also conducted with the reinforcing layer(s) positioned on the inside of the pillow surfaces or within the pressure boundary. Positioning on the inside showed substantially less promise in reducing leakage. Some minor benefit was realized by adding strength or resistance to the spreading of the outer fabric weave, but the overall leakage in these stress point areas were still substantially higher than with the fabric reinforcing layer sewn on the outside surface or exterior sides of the pillow. 
     Therefore, in higher stressed areas of an airbag, externally positioned reinforcing layers can improve pressure retention. Both the silicon coated fabric reinforcing layers and the &#39;098 application fabric reinforcing layers were found effective when positioned externally. The &#39;098 application fabric showed an edge over the silicon coated fabric. The silicon fabric displayed better results with the coated surface facing the airbag. 
     The results related to seam density along the perimeter showed that in all the fabric types a higher seam density provided lower overall leakage. A seam density of 18 to 22 stitches per inch with a #92 thread being preferable. As the density was increased beyond approximately 24 stitches per inch, no appreciable reduction in leakage was seen with any of the thread sizes. A #92 thread allows for approximately 50% more thread length to be wound onto the bobbin case/spool compared to a #138 thread. This allows for reduced change-over times when replacing the bobbin spools. Also, the smaller #92 thread cross-section reduces the seam thickness, which has advantages when the airbag is folded. 
     It was also discovered that too low a bobbin tension resulted increased leakage. The lower bobbin tension is not as effective in bunching together the fabric to better restrict leakage in the seam. So, a combination of an appropriate higher bobbin tension combined with a needle tension of approximately 2-4 times more than the bobbin tension have been found quite effective in reducing leakage. Also, a polyester thread with silicon additive has been found to process through the machine/needle more effectively than even a same thread size #92 in nylon. 
     Utilizing a cold gas inflator for the inflation of the airbag allows for the use of a polyester thread. Conventionally, many airbags utilized inflators with hotter gas outputs, and, therefore, required nylon thread, or in some cases even threads with higher temperature resistance such as brands of Kevlar or Nomex. In the case of using a cold gas inflator, the use of a polyester thread type is possible. 
     A combination found most effective is the use of a polyester Coats brand, #92 thread size of bonded construction, with a silicon additive. The needle found most effective is a 100-16. The needle point type found most effective is an RG. This type of needle will produce less cutting or abrasion of the fabric during sewing, while not adversely affecting the leak rate. The tension for the bobbin thread found most effective is 6-9 ounces. A corresponding effective tension for the needle thread was found to be 18-25 ounces. 
     Additionally, the elongation of the different thread types were found to have an effect on leakage. Generally, a stiffer thread yields a lower leakage rate during airbag pressurization. Too stiff a thread may adversely affect the stitch breaking strength, but a thread such as a bonded polyester has been found quite effective. A thread utilizing a higher content of silicon coating or treatment could prove to be even further advantageous for reducing sewn seam leakage. 
     Several techniques in sewing the actual patterns into the airbag have also uncovered advantages for reduced leakage. Some of these methods can also improve upon process time while providing favorable seam uniformity and consistency. In sewing the smaller circular, oval, oblong, etc. shaped patterns, utilizing a programmed machine can provide superior seam uniformity. These program machines move the fabric in the circular pattern without the need for turning the entire airbag through a 360 rotation. 
     It has also been noted that the utilization of a single seam to connect a centrally located seam (circular for example) back to the airbag perimeter seam also may reduce leakage. The number of needle holes created for this connecting seam is reduced by about half when compared to a conventional dual seam. 
     Another feature found helpful for reducing leakage is to fold the airbag at the bottom perimeter as opposed to using a seam and two separate fabric panels. This technique is known in the industry as a “taco fold.” 
     Referring now to  FIGS. 7-10A  another alternative exemplary embodiment of the present invention is provided. In this embodiment an inflatable curtain airbag or inflatable cushion  210  for a side of a vehicle is provided. The inflatable cushion has a first cushion section  212  formed from a first material, the first cushion section having a plurality of separate inflatable cells  214  each of which having an inlet opening  216  for receipt of an inflation gas during inflation of the inflatable cushion. 
     The inflatable cushion also has an internal passageway  218  formed in the first cushion section. The internal passageway traverses along an upper edge of the inflatable cushion linking and fluidly coupling each of the plurality of separate inflatable cells by providing a path to each inlet opening of each of the plurality of separate inflatable cells. Although the internal passageway is shown traversing along an upper edge of the inflatable cushion alternative embodiments contemplate the internal passageway being located at other locations (e.g., bottom, sides or portions thereof). 
     In accordance with exemplary embodiments of the present invention the inflatable cushion may comprise only first section  212  or first section  212  and a second section  220  each having a plurality of separate inflatable cells wherein each of the inflatable cells has an inlet opening proximate to the internal passageway. 
     Other embodiments contemplate an inflatable cushion with a single and/or multiple inflatable sections with non-inflatable sections secured thereto and/or between the inflatable sections. In order to provide inflation gas from an inflator  222  to the inflatable cushion, a diffuser member  224  is disposed in the internal passageway. In accordance with an exemplary embodiment of the present invention the diffuser member is configured to supply an inflation gas from the inflator to each of the plurality of separate inflatable cells. In one embodiment, the diffuser member is shaped as a tube or any other equivalent structure. In still another embodiment, the tube or member  224  consists essentially of a non-rigid fabric tube or member (hereinafter referred to as tube) formed from a permeable material wherein the tube is sealed to an output of the inflator. The non-rigid fabric tube is independent of a first material used to form the first cushion section and the permeable material of the non-rigid fabric tube covers each or in some instances (e.g., the outermost cells at the ends of the cushion) a portion of the inlet opening of each of the plurality of separate inflatable cells such that the inflation gas must pass through the permeable material in order to inflate the inflatable cushions. In other words, the non-rigid fabric tube does not have any inflation openings and in order to inflate the inflatable cushion the inflation gas passes through the fabric of the non-rigid fabric tube. In essence, the permeability of the fabric of the tube defines the air passages that the inflation gases pass through thus, the fabric tube is formed without any intentionally added openings or fabricated openings in the fabric tube. 
     In one embodiment the non-rigid fabric tube is a woven non-rigid fabric tube and the woven non-rigid fabric tube is formed from a material that is more permeable than the first material forming the first cushion section. In still another embodiment portions of the non-rigid fabric tube are more permeable than other portions of the non-rigid fabric tube, wherein fluid flow of the inflation gas to the plurality of inflatable cells is varied. In other words, the more permeable sections are aligned with the openings of the inflatable cells. In still another alternative embodiment the less permeable portions of the non-rigid fabric tube are provided by a double walled non-rigid fabric tube (e.g., a tube within a tube) while the more permeable portions are provided by a single walled non-rigid fabric tube. Typically this double walled section can be over the first several inches directly at an output of the inflator where the stresses from the out flowing gas are the highest. This provides added strength in that area and also reduces the permeability that otherwise would occur from the highly stressed fabric tube due to the violent outflow of gas in this section. Thus, sections of the tube are in some embodiments provided with a double wall section while others are a single wall section. 
     In still other embodiments less permeable portions of the soft fabric tube may be provided by selectively coating certain sections of the fabric diffuser tube with a sealant while leaving other sections uncoated thus providing more permeable sections where the more permeable sections are aligned with the inlet openings of the inflatable cells. In the embodiment where woven fabric tubes are utilized the weave of the fabric tube may be varied along its length such that a larger weave pattern is provided in the more permeable sections while a tighter weave pattern is provided in a less permeable sections. Other ways of varying the permeability of the fabric tube include providing non-uniform weave in the more permeable sections and a uniform weave in the less permeable sections. Accordingly, different areas have different weave patterns to produce different flow rates through the fabric of the diffuser tube. In still another embodiment, the tubes are formed by sewing flat cushion fabric into a tubular structure (e.g., end to end securement while leaving at least one end open to allow for securement of the inflator thereto) instead of a woven fabric tube. Thus, numerous ways to tune and control fluid flow into the inflatable cells of the cushion is provided. Moreover, and as will be discussed herein, further tuning is provided by locating stitching around the diffuser tube after it is inserted into the internal passageway of the inflatable cushion. 
     During inflation of the inflatable cushion when an inflation gas is delivered from the inflator through the diffuser tube and into the plurality of separate inflatable cells the diffuser tube expands from an un-inflated state to an inflated state wherein the tube expands from a generally flat configuration into an expanded tubular or inflated state thus limiting fluid flow between the cells other than fluid flow through the diffuser tube. 
     During the inflation of the inflatable cushion and diffuser tube it is desirable to restrict fluid flow between an exterior portion  226  of the diffuser tube and an interior surface  228  of the internal passageway disposed proximate to the edges of the inlet openings of the plurality of inflatable cells such that fluid flow between the isolated cells is limited and fluid flow from the diffuser tube into the cell through the fabric of the diffuser tube is provided. As used herein edge of the inlet openings refers to the opening defined by the sewing patterns or portions of the cushion for example, the edges of the dividing walls illustrated as top edge portion  248 . Of course, this is but one non-limiting example. 
     In order to do this a first side (e.g., inboard side) and a second side (e.g., outboard side) of the first cushion section or the first cushion section and the second cushion section are secured to each other proximate to the areas in which the diffuser tube passes through the internal passageway and also at the edges of the inlet openings of the inflatable cells such that upon inflation, inflation gases cannot traverse or the inflation gas flow is limited from one inflatable cell to another inflatable cell through an air gap between an exterior surface of the diffuser tube and an interior surface of the internal passageway proximate to the inlet openings of the inflatable cells. 
     For example, and referring now to  FIG. 9A  a portion of an upper edge portion of the inflatable cushion is illustrated. Here the soft diffuser tube is illustrated passing through the internal passageway and across an inlet opening  216  of an inflatable cell. As shown, the inflatable cushion has an upper edge portion  230 , seam  232  (e.g., wherein the two fabric portions comprising the inflatable cushion are sewn together), stitching  234  which also secures the two fabric portions comprising the inflatable cushion together to define sidewalls  236  of each of the inflatable cells. In addition, and in order to define the rest of the perimeter of the inflatable cells a lower edge portion  238  of the inflatable cushion also has a seam or stitching  240  ( FIG. 7 ) securing the two fabric portions together to form a lower edge of the inflatable cushion such that inlet opening  216  is the only inflation opening for each of the plurality of inflatable cells. 
     As shown in  FIG. 9A , the inlet opening  216  of each of the plurality of inflatable cells is covered or traversed by a portion  242  of the non-rigid fabric diffuser tube. In accordance with an exemplary embodiment of the present invention, the portion  242  is permeable such that as the diffuser tube inflates inflation gases illustrated by arrows  244  will pass through portion  242  of the diffuser tube into the inflatable cell via inlet opening  216 . In order to provide each inflation opening  216 , as illustrated in  FIG. 9A , a plurality of stitches  246  are provided. The plurality of stitches  246  secure the two side portions of the inflatable cushion together such that fluid flow between each of the inflatable cells through a gap between a top edge portion  248  (the end of the stitching  234  defining sidewalls  236 ) and an exterior surface  226  of the diffuser tube is restricted or limited since the two walls of the inflatable cushion are now secured to each other. Moreover and since the plurality of stitches  246  are in a circular configuration the size of the inlet opening can be varied by the configuration of the plurality of stitches  246  or location of the circular or other configurations of stitches  246 . In essence, the placement of location of the circles is one method of adjusting the inlet opening size. Furthermore, and since the stitching defines a closed loop, circle or oval, the two sides of the inflatable cushion are secured to each other on either side of the stitching forming sidewalls  236 . 
     In addition and in one alternative embodiment, an upper edge portion  250  of the plurality of stitches is located proximate to the location of the exterior surface of the diffuser tube when it is in the internal passageway in the inflated state. Located vertically above the plurality of stitches  246  is an upper set of plurality of stitches  252 . Alternatively, the stitches forming seam  232  are used in conjunction with upper edge portion  250  to limit fluid flow between an exterior surface of the diffuser tube and the interior surface  228  of the internal passageway. The upper set of stitches  252  are independent and distinct from the plurality of stitches  246 . In this embodiment and as illustrated, the plurality of stitches  252  are arranged in a semicircular or other curved configuration wherein a lower edge portion  254  is located proximate to the location of the exterior surface of the diffuser tube when it is in the internal passageway in the inflated state. Accordingly, upper edge portion  250  and lower edge portion  254  define a dimension or height  256  of the internal passageway proximate to the edge of the inlet opening of each of the plurality of inflatable cells such that during inflation, gases will pass through the fabric of the diffuser tube (portion  242 ) in the directions of arrows  244  and leakage of inflation gases between each of the inflatable cells through a gap between the exterior surface of the diffuser tube and an interior surface of the internal passageway since the dimension of the internal passageway is now limited to dimension  256  which corresponds closely to the external dimension of the diffuser tube. In an exemplary embodiment, dimension  256  is similar to the corresponding dimension of the inflated diffuser tube such that fluid flow between the exterior surface of the diffuser tube and the interior surface of the internal passageway proximate to the edges of the of the inlet openings of the inflatable cells is limited. 
     During manufacture of the inflatable cushion one method is that stitches  246  and  252  are not provided until the diffuser tube has been inserted into the internal passageway since the lack of stitches  246  and stitches  252  allow the internal passageway to have a dimension larger than a dimension of the diffuser tube such that the same can be easily inserted into the opening (e.g., pulled and/or pushed into the opening). In still another alternative exemplary embodiment, the diffuser tube is laid upon one of these sections of the inflatable cushion prior to the other one being secured thereto then the stitching is applied to the inflatable cushion. Accordingly and once the diffuser tube is properly located stitches  246  and  252  are applied to reduce the dimension down to dimension  256 . Alternatively the soft tube may be inserted after all sewing has been completed. A rigid rod assembly tool may be used to push the soft tube into its proper location. 
     Also shown proximate to top edge portion  248  is a sew pattern  249 , which is the overlap of the stitching pattern  246  (e.g., either ends of the pattern of stitches  246 ). This overlapping design has shown lower leakage rates than if the two ends terminated parallel to each other or if the ends terminated within the pressurized boundary of the inflated cell. 
     In another alternative exemplary embodiment the diffuser tube  224  may comprise a solid or rigid (e.g., steel, plastic, etc.) diffuser tube inserted into the internal passageway after the application of stitches  246  and  252 . Of course and in applications where a solid or rigid tube is used the same will require inflation openings aligned with opening  216  since the solid or rigid tube is not permeable. In still other alternative exemplary embodiments, and as will be discussed herein, the inflatable cushion will comprise a non-rigid fabric diffuser tube for a portion of the inflatable cushion and a solid or rigid diffuser tube inserted in other sections of the inflatable cushion. 
     In one embodiment and in order to properly align the location of stitches  246  and  252  locating features  258  are provided. In one non-limiting exemplary embodiment the locating features are slits cut in the fabric portions of the inflatable cushion. 
     In one embodiment and in order to provide reinforcement and reduced leakage to the plurality of stitches  246 , a piece of fabric  260  is sewn to the exterior surface of the inflatable cushion via plurality of stitches  246  such that upon inflation the inflation forces do not tear stitches  246 . In one non-limiting exemplary embodiment, the fabric portions are formed from materials similar to those used for the inflatable cushion. 
     Referring now to  FIG. 9B , the same section of the inflatable cushion is illustrated without stitches  246  and  252 . As illustrated, flow paths or passageways  262  and  264  are located between an exterior surface of the diffuser tube and an interior surface of the internal passageway. Accordingly and during inflation, inflation gases will be able to pass from one inflatable cell to the other via these flow paths or passageways unless stitches  246  and  252  are applied. 
     Referring back now to  FIGS. 8A-8C  each of the inlet openings  216  of each of the inflatable cells may be varied by locating the stitching  234  defining the sidewalls  236  at different locations in order to provide different sized openings. The main way to control the flow into the cells is by the location of the same sized circles (or any other configuration having a curved surface)  246 , some being spaced further apart than others. In addition to or as an alternative to varying the location of the stitching  234  the configuration of stitching  246  may be varied for example, oval stitching  270  may be provided in conjunction with or as an alternative to a more circular type of stitching  272  ( FIG. 8B ). Thus, the oval stitches traverses in a greater horizontal direction (as view in the figures) than the circular stitches. In still yet another alternative embodiment, sewn lines securing the two sides of the inflatable cushion together extend into inflation opening  216  such that the size of opening  216  is now further reduced ( FIG. 8C ). In accordance with still another alternative exemplary embodiment and in order to vary the flow rate through the permeable material of the non-rigid fabric diffuser tube, the permeability of the portions of the diffuser tube corresponding to the inflation openings of the inflatable cells may vary by for example, using two layers of fabric material such that the inflation gas must now pass through two layers of fabric material prior to it passing into the inflatable cell. Of course, other methods of limiting the fluid flow through the material of the fabric diffuser tube may be employed by for example, varying the density or permeability of the same. 
     Accordingly, it is understood that any of the aforementioned embodiments can be used alone or in combination with others in accordance with exemplary embodiment of the present invention. 
     Referring now to  FIG. 10  an inflatable cushion  210  with a first inflatable cushion section  212  and a second inflatable cushion section  220  is illustrated. In this embodiment the diffuser tube is entirely a soft non-rigid fabric tube fluidly coupled to the inflator and disposed in the internal passageway with the plurality of stitches or a means for dividing or separating flow into the desired cells between an exterior portion of the diffuser tube and an interior surface of the internal passageway disposed between each inlet opening of the plurality of inflatable cells as described herein. In addition, the inflatable cushion illustrated in  FIG. 10  comprises non-inflatable sections  274  disposed between and/or along the periphery of the inflatable sections. In addition, the inflatable cushion may further comprise tethers  276  and  278  extending from forward and rearward ends of the inflatable cushion. 
     Referring now to  FIG. 11  still another alternative exemplary embodiment of the present invention is illustrated, here a portion of the diffuser tube comprises a rigid (e.g., steel, other metals or plastic) tube  280  (illustrated by dashed lines) and another portion is a non-rigid fabric inserted in the internal passageway. In this embodiment, the rigid tube will, of course, have inflation openings aligned with the openings of the inflatable cells. In this embodiment, the non-rigid fabric portion of the diffuser tube will be the only means for providing inflation gas to some of the plurality of inflatable cells while the rigid diffuser tube will be the only means for providing inflation gas to the remainder of the plurality of inflatable cells of the inflatable cushion. Also shown in  FIG. 11  is a third inflatable cushion section. It being understood that numerous inflatable cushion sections or a single inflatable cushion section may be used in any of the aforementioned embodiments and it is understood that the inflatable cushion of exemplary embodiments of the present invention may be configured to cover vehicles of various sizes (e.g., 1, 2, 3 row vehicles). 
     Referring now to  FIGS. 12-13C  still other alternative exemplary embodiments of the present invention are illustrated, here a portion of the diffuser tube comprises a rigid (e.g., steel, other metals or plastic) tube  280  (illustrated by dashed lines) and another portion is a non-rigid fabric inserted in the internal passageway. In this embodiment, the rigid tube will, of course, have inflation openings aligned with the openings of the inflatable cells. In this embodiment, the non-rigid fabric portion of the diffuser tube is used only to connect the two sections of rigid tubes (yet may also be the only means for providing inflation gas to some of the plurality of inflatable cells or in this specific embodiment it only connects and likely would even be a sealed fabric tube) while the rigid diffuser tube will be the only means for providing inflation gas to the remainder of the plurality of inflatable cells of the inflatable cushion.  FIG. 13B  also shows that the location of the rigid and/or the non-rigid section provides packaging benefits (e.g., lack of rigid portions allows the side curtain air bag to be folded up into a smaller configuration that allows for ease of shipping).  FIGS. 12-13C  also show different inflatable cushion section configurations wherein some sections are connected to each other via the diffuser tube (rigid and/or non-rigid) and non inflatable sections comprising a single layer of fabric or in some applications only the diffuser tube (rigid and/or non-rigid) are the means for securing separate inflatable cushion sections together. 
     In one non-limiting exemplary embodiment, the diffuser tube and means for restricting fluid flow between each cell by limiting fluid flow between an exterior portion of the diffuser tube and an interior surface of the internal passageway at the junction between each cell, the inflatable cushion requires only a single inflator in order to inflate the inflatable cushion during an activation event. Of course, the usage of multiple inflators is also contemplated in accordance with exemplary embodiments of the present invention. 
     Also provided herein is a method of controlling the flow rate of an inflation gas into the inflatable cushion by limiting an amount of surface area between an exterior surface of a non-rigid fabric diffuser tube and an interior surface of an internal passageway formed in the inflatable cushion section as well as controlling the amount of and permeability of the surface area of an exterior surface of the non-rigid fabric diffuser tube positioned across an inflation opening of each or a portion of the plurality of cells in an inflatable cushion section or sections. As discussed herein and in one exemplary embodiment, the inflatable cushion is formed from a first material and the diffuser tube consists essentially of a non-rigid fabric tube formed from a permeable material, the non-rigid fabric tube is independent of the first material used to form the inflatable cushion and the permeable material of the non-rigid fabric tube covers each inlet opening of each of the plurality of separate inflatable cells such that the inflation gas must pass through the permeable material. 
     Further various embodiments seek to add enhancements and construction to the aforementioned diode/low leak cushion designs and methods for construction of low leak sewn unsealed cushions. Additionally, alternative embodiments for gas delivery while retaining all the advantages of the diode. These embodiments generally relate to various sewn unsealed low leak side-curtain cushion constructions used in side-impact and rollover accidents. In addition, these embodiments relate to side-curtain cushion designs that manage gas flow between cushion cells during occupant loading. However, the designs may also be useful in other airbag application types. The cushions of these embodiments are cut and sewn constructions capable of meeting all performance requirements and expectations without the need for seam sealing using fabrication and assembly techniques, which allow for reduced overall cushion costs. 
     These embodiments are methods of using at least two fabric panels or layers sewn together without the need for seam sealing. In other words, the low leakage performance of the cushion is obtained without adding any additional type of a sealing component to or between the fabric layers such as sealant, adhesive, glue, filler or the like, only fabric and thread. Accordingly and as used herein an unsealed seam refers to an airbag or inflatable cushion with at least two fabric panels or layers sewn together without adding any additional type of a sealing component to or between the fabric layers such as sealant, adhesive, glue, filler or the like, only fabric and thread. 
     As discussed, herein side-curtain airbags or inflatable cushions deploy downward from a stowed position within the roofline of the vehicle and inflate between the occupant and the vehicle interior side structure, such as the side windows and the A, B, C and/or D pillars. 
     A side-curtain airbag generally consists of two fabric panels either sewn or interwoven together in such a fashion to create “cells” which are inflated during an accident to provide inflatable side restraint. A typical side-curtain may have a plurality of cells in various arrangements. 
     One type of prior art cushion construction method commonly referred to in the industry is called OPW or One Piece Woven. This process weaves the fabric panels together to create a seam as the fabric is passed through a loom. The costs for these cushions are known to be high due to the capital equipment investments for the looms and the amount of coating and/or specialty coating chemistry required to reduce cushion leakage. 
     Another cushion/type construction is known as seal and sew. This method utilizes an adhesive or a sealant that is applied to the fabric panels in all the areas that are needed to create a pattern and shape to the inflated curtain. Then, for acceptable minimum strength and integrity, a sewn stitch is added in the center of the sealant bead. While this method has been found to reduce airbag leakage, drawbacks also exist. The sealant(s) required are expensive. The application and curing process is considered “messy” and time consuming. The needle passing through the sealant bead can pick up chemicals/particles from the sealant bead which can act as contaminants, negatively affect the sewing process. A cure time is also required after applying the sealant bead prior to sewing. The overall costs to produce these seal and sew cushions are therefore also relatively high. 
     Exemplary embodiments of the present invention will eliminate the need for large investments in looms used in OPW constructions or the need for heavily coated or expensively formulated coatings cushions. The disclosed embodiments will also eliminate the costly and time consuming need to add sealant to the sewn seams. Accordingly, the disclosed method utilizes cut and sewn fabric panels constructed to reduce leakage and reduce overall costs without the need for adding a seam sealing component. 
     The sewn cushion constructions disclosed herein can also be used in conjunction with an extended output inflator to further increase the pressure retention over time. Further, the cushions can be configured in a diode cell arrangement as discussed above to reduce the total operating pressure required to meet performance objectives and reduce cushion stresses typically induced by high inflation pressures. 
     An airbag and method of making an airbag with a low leak seam is provided. In an exemplary embodiment, the low leak seam is provided by securing at least two fabric panels or layers with a plurality of stitches without the need or use of seam sealing (e.g., an unsealed seam). In one exemplary embodiment the stitches are lock stitches wherein a rotary hook sewing machine or equivalent thereof is used to provide the lock stitches. A non-limiting example of a rotary hook sewing machine is found in U.S. Pat. No. 4,009,670 the contents of which are incorporated herein by reference thereto. Of course any other equivalent machine for providing lock stitches in accordance with the desired ranges is contemplated for use in exemplary embodiments of the present invention. 
     In other words, the low leakage performance of the cushion is obtained without adding any additional type of a sealing component to or between the fabric layers such as sealant, adhesive, glue, filler or the like, only fabric and thread. 
     In one alternative embodiment and wherein un-inflated portions of the inflatable cushion are desired, a segmented cushion is provided wherein the inflatable portions have unsealed seams joining together at least two fabric panels or layers and the segmented construction feature consists of less expensive fabrics used in areas of the inflatable cushion (e.g., side-curtain) that do not require inflation. One of these un-inflated regions may be an area between the rows of seating in the vehicle. Other areas may be at the front and possibly the rear of the inflatable curtain, otherwise known as sail panels. The fabric in these areas has no requirement for containing pressurized gas so it can therefore be a lower cost material, and even uncoated if desired. This lower cost material may be a fabric or alternatively may be any flexible structural material with sufficient strength to help retain an occupant within the vehicle. These areas may also utilize only one layer of fabric to further reduce cost. 
     In still another embodiment and for providing strength the unsealed seal a 3rd exterior layer of fabric is sewn to the cushion proximate to the unsealed seams that are exposed to pressure from the inflation of the cushion, which have been found to help further reduce leakage. It has also been noted that the fabric layer positioned on the needle side during the sewing process typically has a higher leak rate than the fabric layer positioned on the bobbin side during sewing. Therefore, in the segmented arrangement as discussed above, positioning the un-inflated fabric section to be sewn together as a 3rd layer when the inflatable panels are sewn will be advantageous for increased pressure retention. Furthermore, sewing this 3rd exterior layer of fabric on the needle side of the cushion is most favorable for decreasing cushion leakage. This also eliminates the need for an additional seam to attach the inflatable cushion fabrics to the un-inflated fabric or flexible structural material sections as they are all joined together in the same seam. 
     An additional embodiment taking advantage of this 3rd exterior layer to reduce leakage may utilize a 3rd layer on selected seam areas. For example, lower cost polyester fabric panels with “strips” of polyester fabric used as a 3rd external layer. Tests have found that adding this 3rd exterior layer to seams on the needle side substantially increased the cushion pressure retention over time in a 420 denier polyester with otherwise inadequate leakage performance. It should also be understood that having this 3rd (or a 4th exterior layer if using exterior layers on both sides of the cushion) exterior layer added to the seam on the bobbin side of the cushion further reduces overall leakage, albeit not to the same degree as an exterior layer added to the needle side. 
     As described above the use of a polyester thread for the seams exposed to pressure from filling the cushion has shown advantages for processing and pressure retention. In recent cushion builds comparing the pressure performance of a nylon thread to that of a polyester thread of the same size has shown a substantial difference in pressure retention. This advantage in pressure performance was obtained with all the sewing machine settings being the same for each build, only the thread being different, polyester vs. nylon. The differences in pressure when monitored over a 5 second time range found the polyester to have a greater retention of pressure compared to the nylon in the same build. 
     Like the cushion leak test “pillow” samples described above, a slightly larger test sample was then utilized to simulate a pair of inflatable cells that are actually used in a vehicle cushion design. This particular test sample was derived from the inflatable area in the second row of an existing cushion design. These new samples for testing were given the term “leak samples”. These “leak samples” are constructed with fixed dimensions and have the same sewing pattern or cell configuration for each test sample fabricated. The changes that are made to the seam can be those such as needle tension, bobbin tension, seam density, thread size, thread type, needle size, needle type or point, and various fabric constructions or coatings. These changes are used to compare performance from one variation to the next. The leak samples are then pressurized using shop air until they reach a pressure of 40 kpa. The pressure input ceases via activation of a shut off valve while the pressure decay is electronically monitored over a period of at least 6 seconds. 
     In one embodiment a method providing a seam having at least two fabric panels sewn together without the need for adding a seam sealing component where the cushion pressure at 6 seconds after deployment has a pressure of at least 5 kpa. 
     A further embodiment may utilize a hybrid fabric approach. The inflatable curtain sections may use two different fabrics. One panel that has been found to exhibit low leakage can be used in combination with a less expensive fabric panel. The second lower cost panel would still require sufficient low leak characteristics but add the advantage of lower overall cost to the cushion as compared to the approach using the same higher cost low leak fabric. An example embodiment of this may be a 315 denier nylon silicon coated fabric combined with a 420 denier polyester urethane coated fabric. Combinations of the hybrid approach along with the segmented feature may also be advantageous for overall cost and leakage reductions. 
     One ancillary benefit of the unsealed seam cushion constructions is the capability of the cushion to have sufficient pressure during a roll-over event, yet further, inherently leak down to a lower or no gauge pressure later in time is advantageous for an occupant who needs to escape, or be extracted from the vehicle following an accident. This benefit may be further realized using an extended output inflator in combination with the unsealed seam cushion construction. The extended output inflator continues to deliver gas for several seconds after impact, which keeps the cushion pressure elevated during the required crash or rollover event. Since the unsealed seams of the cushion inherently leaks gas and the cushion is no longer being replenished with gas from the inflator, the cushion pressure will more quickly go to zero pressure thus allowing an occupant to more easily lift or move the deployed cushion away from the door or window opening. 
     In OPW or seal and sew cushion constructions the cushion needs to be sealed very well in order to retain sufficient pressure during the entire crash event. Thus, inherently in these designs, the pressure following the accident may still be substantially high within the cushion. A deployed cushion with as little as 5 kpa gauge pressure at a time an occupant needs to be removed from the vehicle has the potential to appreciably hinder the ability for escape or extraction. 
     In other instances there may be a need to push on or squeeze the cushion to further release gas in order to sufficiently clear an exit path. Even a cushion with only several kilo Pascals of pressure still remaining may act as an obstruction. Again, with the unsealed seam cushion, inherently it will allow for further gas to much more easily be pushed out as the cushion is forced upward and out of the way. However, with the OPW and seal and sew cushions this can not simply be done due to the fact that the pressure will only increase in the cushion instead of finding the inherent leak paths as with the sewn unsealed cushion constructions of the present invention. 
     In one non-limiting exemplary embodiment, the unsealed seam will allow for greater than 20 kpa for up to 1.5 seconds and less than 2 kpa after 15 seconds. It can also allow for the 6 second pressure to be around 6 kpa or more while still allowing the pressure at 15 seconds to be less than 2 kpa. 
     As previously discussed, the various types of fabric construction used for sewing together the 1st and 2nd layers or panels used to create the side-curtain have shown to be influential in the performance related to pressure retention over time. Fabric denier, composition, coating weight, coating formulation, and weave count, for example, all have an appreciable effect on pressure retention. 
     Fabric denier is the weight in grams of 9000 meters of yarn. Fabrics used for the construction of the side-curtain airbags may be characterized by denier. Some of the more common deniers used are 315 d, 420 d, 525 d and 630 d in nylon with 420 denier being the most common for the side curtain applications. 
     It has been found that the weave count of the fabric can play a significant role in the pressure retention capability in the sewn unsealed curtain construction. The higher weave count fabrics have enabled higher pressure retentions over time in the constructions for the low leak unsealed seam cushions. 
     Tests were performed keeping all the seam sewing variables constant and only changing the fabric weave count. For example, using a 420 denier fabric with a weave count average with a warp and weft of around 51×51 or more has proven to be advantageous for pressure retention in the sewn unsealed seam side-curtain applications when compared with warp and weft constructions of 49×49 or 46×46. Another way to categorize a “weave count average” is by the total weave count over a fixed coverage area. The 51×51 weave count has a total weave count of 2600 over a one inch squared area. The weave count will vary due to manufacturing variables such as machine variability, fabric shrinkage, lot differences, etc. Within the same fabric category or part number, for example, the fabric weave count may have a 51×48, or a 51×51, or a 50×52, etc. Therefore, the total weave count over the one square inch area will vary over the different warp and weft counts within the same fabric type or part number. It has been found for example, that a 420 denier fabric with a total weave count of 2400/sq in or more has provided the desirable pressure retention for the sewn unsealed curtain. Fabric deniers in the range of 396 to 499 denier with a weave count total of around 2400/sq in or more exhibit the best desired pressure retention over time. Further, fabrics with deniers both lower and higher than the 420 d also exhibit better pressure retention as the total weave count is increased. This includes fabrics around the 315 denier, the 520 denier and even 630 denier. 
     Higher weave count fabrics may also allow for reduced coating weights to the fabric to retain similar pressure retention qualities as lower weave count fabrics with higher coating weights. Adding coatings to fabrics has been known to increase the product costs rather substantially due to the high costs of silicon coating for example. Therefore, achieving increased pressure retention characteristics by increasing the weave count as opposed to adding more silicon coating has shown to be advantageous. An example of this was done where two different fabrics were compared. The first fabric had a weave count of 48×48 and a coating weight of 45 grams/sq meter. The second fabric had a weave count of 53×53 and coating weight of only 15 grams/sq meter, although, the pressure retention performance over time was similar. 
     Further, having the higher weave count or the fabrics having this higher total weave count such as in the 420 d of over the 2400/sq in, allows for any orientation of the fabric panels during sewing with-negligible change in pressure retention. In fabrics with lower weave count totals the orientation of the fabric panels may play more a part in degradation in pressure retention. Examples of this is where the warp may run more parallel to a pressure seam or the weft or where the weft or warp may be at a 45 degree bias to the pressure seam and pressure retention is effected depending upon the bias of the fabric. This has shown not to be the case with the higher weave count totals as discussed above. 
     Conventional weave counts for 420 denier fabrics are in the weave count range of around 46×46 or a total weave count of around 2100/sq in. The highest weave counts in the more conventional 420 denier fabrics may be up to 49×49 or a total weave count of around 2400/sq in. 
     As previously discussed, it was also found that the higher the seam thread density the better the pressure retention over time. Therefore, using the higher thread seam densities along with the higher weave count fabrics allows for enhanced pressure retention. The other factors discovered allowing for enhanced pressure retention in the sewn unsealed cushions were to have a higher bobbin tension then used in prior seams with a needle tension around 2 to 4 times higher than that of the bobbing tension and higher than those previously used. A bobbin tension of 6 ounces or more is one preferable range. For instance, a manufacturing tolerance may have a bobbin tension tolerance range of 7-10 ounces and a needle tension from a possible low of 14 ounces to a high of 40 ounces, although, a more appropriate corresponding range for the needle tension for more controlled sewing may fall in the midrange of 3 times more and could be around 21 to 30 ounces. 
     The tensions are measured with a force gauge pulling the needle thread through the machine mechanics including a tensioning spring and through the needle eyelet. The bobbin tension is measured pulling the thread up through the base plate as it comes out of the bobbin case where the tension is set. 
     In typical sewing applications the bobbin tensions are much lower than 6 ounces and in many instances special attention and adjustments are required in order to get to the higher 6 ounces or greater tension from conventional bobbin cases. In some instances a special bobbin case spring is required in order to reach the higher bobbin tension settings. The other variable shown to improve pressure retention is the use of a polyester thread for the seam. 
     Using a #92 thread size (92txt or T-90 which designates the same thread size), for example, with seam densities of 18 to 24 stitches per inch have been found to be effective along with densities upwards to 30 or more stitches per inch also giving further increased pressure retention. The higher densities may require longer sewing times in some cases where automated sewing equipment is used but these higher densities have also been successfully sewn using manual sewing machines that allow for much faster revolutions per minute and therefore are capable of sewing within reasonable and competitive process times. 
     Furthermore and with the more restrictive flow of gas between adjacent cells in the diode cushion configuration, as discussed herein, the pressure increase within the cell impacted increases to a much greater degree than in a conventional curtain airbag. During testing of occupant interaction using conventional cushion designs with the essentially free flow of gas between cells during the occupant interaction, there were relatively small pressure increases to the impacted cell when the cushion was impacted. 
     In these tests the cushion is impacted by a Free Motion Head-form (FMH). A FMH (Free Motion Head-form) is an instrumented dummy head used to simulate a vehicle occupant&#39;s head impacting the curtain during a side impact collision. Typical tests are performed with a FMH speed of 18 mph. Under this test using the FMH, the conventional cushion had a pressure increase in the cushion (the impacted cell) on the order of 10-30% greater than before impacted. In the diode cushion by restricting the flow area through which gas can pass between adjacent cells, as explained in the original specification, the dynamics of occupant interaction with the curtain significantly changes and the pressure increase in a single impacted cell is significantly higher than before impacted, on the order of 100-200%. As the cell is loaded by the FMH it displaces volume of the cell and with the restrictive flow of the diode design the gas is not able to freely flow into adjacent cells so in turn the pressure within the cell increases significantly from the pressure before impact. In order for a pole test to be successful or deemed a “pass”, the FMH must not strike through the cushion. 
     As discussed above, the diode cushion may be of a cut and sewn unsealed construction, a one-piece woven construction or of a seal and sew construction. The diode cushion may also be constructed using a welded seam construction for example of an ultra sonic welded type seam or a dielectric welding. Any other form of a welded seam may also work for the cushion construction. Further, any type of conventional construction or alternative method may be used to form the cells while keeping with the intent and spirit of the diode design with restrictive flow between adjacent cells during occupant loading. 
     Alternative gas delivery methods may also be utilized other than a tubular delivery tube to deliver gas to the isolated diode cells. For example, U.S. patent application Ser. No. 12/256,224 filed Oct. 22, 2008 and U.S. Provisional Patent Application Ser. No. 61/178,755 filed May 15, 2009, the contents each of which are incorporated herein by reference thereto, disclose alternative inflation gas delivery mechanisms or apparatus. 
     Another alternate embodiment for gas delivery to the diode cells is to use a plenum chamber that is coupled directly to the inflator outlet(s). This type of arrangement may allow for the inflator to be directly inserted into the mouth of the cushion without the need for additional plumbing to couple the inflator to a delivery tube. The plenum chamber is positioned in the mouth of the cushion to directly accept the inflator output and redirect it into various isolated cells of the diode cushion. For example, the plenum chamber may be constructed out of a higher denier coated fabric such as a 630 d for added strength to handle the more violent output typical at the inflator outlet. 
     The plenum chamber collects the inflator gas output and channels it into the various isolated diode cells as shown in the attached FIGS. The gas flows into the individual cells quite rapidly due to the high pressure within the plenum chamber but will not be able to flow back into the plenum chamber as rapidly due in part to the more restrictive openings used going into each isolated cell and also because the pressure within the individual diode cells is much less than what it was when flowing from the plenum chamber into the cells. The pressure within a cell may be around 20 to 40 kpa operating pressure or can increase to 80 to 100 kpa+during occupant loading, but this is still much less than the plenum pressure during filling from the inflator. 
     Pressure 
     Pressure versus time for side impact and rollover—the present unsealed cushion allows for greater than 20 kPa, after initial inflation and over the first second and a half and less than 2 kPa after 15 seconds. Cushion Pressure at 1½ sec higher than 50 ms pressure. 
     Operating pressure at 5 seconds—Cushion Pressure at 5 seconds after deployment has a pressure of at least 50% operating pressure. This allows for the unsealed cushion to meet rollover performance objectives (containment testing, pole impact testing, ground strike, etc. 
     Slower Filling Cells/Cell Tuning 
     As also discussed herein, slower filling cells may be utilized by “strategically” opening the cells&#39; seam to a faster filling adjacent cell. This allows for gas to flow into that slower filling cell. Depending on the size of this opening (effective flow area) into that cell, the speed at which the cell will fill and pressurize can be controlled. 
     In addition to controlling the speed to which the slower filling cell will inflate, the size of the seam opening or the effective flow area into that cell from the faster filling cell can also influence the pressure within the faster filling cell during occupant interaction. As an occupant loads the faster filling cell it will displace that cell&#39;s volume and increase the pressure within that cell. This faster filling cell may be one of the substantially isolated cells within the airbag. By having this communication opening between the faster filling cell to the slower filling cell, the pressure dynamic in the impacted cell can be further tuned to advantageously affect occupant performance. This communication opening can be used to reduce the peak pressure within the impacted cell. By reducing the peak pressure within the impacted cell it has been found that the measured injury levels such as peak acceleration (G&#39;s) and Head Injury Criteria (HIC) can be reduced. During the occupant interaction the opening between the cells essentially acts as a vent allowing the higher impacted cell pressure to vent into the lower pressure slow fill cell. This “venting” is not to atmosphere but to the lower pressure differential within the slow fill cell. This “venting” can keep the impacted cell from becoming too undesirably stiff. The size of the opening can thus be tuned to control this venting between the cells to achieve desired performance. 
     Another option as discussed herein for controlling the pressure dynamic within an impacted cell is through the clearances between the delivery tube and the airbag fabric between isolated cells. The larger the clearances the more “venting” there will be between cells during the occupant interaction. The possible advantage of more “venting” between cells is a softer cell allowing for lower injury measurements. The drawback to more “venting” between cells is less resistance to strike through. Accordingly, it is desirable to provide a balancing between providing sufficient strike through resistance within the impacted cell and enough “venting” to allow for acceptable lower injury numbers. At lower impact speeds the isolated cell can be quite stiff and still provide acceptable injury levels, say 17 mph and lower. At higher impact speeds of approx. 20 mph the isolated cell can become excessively stiff resulting in higher peak G&#39;s and HIC&#39;s. At these higher speeds the need for increased “venting” of the impacted cell pressure can help to reduce the injury level peaks. 
     Some non-limiting examples of exemplary embodiments are provided below and in some instances are compared to current ranges. As discussed herein the unsealed seam provides desirable results wherein a combination of the at least some of the following parameters stitch count, thread type, thread tension, speed of sewing machine, fabric selection (weave and coating), needle size and type of sewing machine are in the below ranges. 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                   
                 UNSEALED SEAM OF 
               
               
                   
                 CONVENTIONAL 
                 EXEMPLARY 
               
               
                 SEAM PARAMETER 
                 PREFERRED 
                 EMBODIMENTS 
               
               
                   
               
             
            
               
                 Stitch setting - needle 
                 Not typically monitored - 
                 25 oz. (2x amount of 
               
               
                 tension 
                 estimated at about 12 oz. 
                 conventional) 
               
               
                 Stitch setting - bobbin 
                 2 oz. 
                 8.5 oz. (4-10x of 
               
               
                 tension 
                   
                 conventional) 
               
               
                 Ratio of needle/bobbin 
                 Typically ratio of &gt;5 
                 Ratio of &lt;3 
               
               
                 tension 
               
               
                 Bobbin spring 
                 Standard 
                 Custom design 
               
               
                 Thread size 
                 138/138 
                 92/92 
               
               
                 Thread - some property 
                 High initial elongation (nylon) 
                 Low initial elongation 
               
               
                   
                   
                 (polyester) 
               
               
                 Needle size 
                 160/23 
                 100/16 
               
               
                 Needle point 
                 “R” point 
                 Medium ball point 
               
               
                 Stitch count 
                 12 spi 
                 25 spi 
               
               
                 Stitch type 
                 Lock 
                 Lock 
               
               
                 Sewing Machine model 
                 Adler 767 
                 Pfaff 5487 
               
               
                 Sewing machine 
                 Typical for airbag 
                 Atypical for airbag; 
               
               
                   
                   
                 used in non-airbag, industrial 
               
               
                   
                   
                 sewing 
               
               
                 Sewing speed 
                 1800 spm 
                 4000 spm 
               
               
                 Sewing accessory 
                 N/A 
                 Air cooling for needle 
               
               
                 Sewing feed method 
                 Compound feed - fabric 
                 Drop feed - fabric moves 
               
               
                   
                 moves while needle is in the 
                 only while needle is NOT in 
               
               
                   
                 fabric 
                 the fabric 
               
               
                 Fabric weight (denier) 
                 420 d 
                 420 d 
               
               
                 Fabric weave count 
                 49 × 49 
                 53 × 53 
               
               
                 (threads/in) 
               
               
                 Fabric coating 
                 35 gsm 
                 35 gsm 
               
               
                   
               
               
                 Note: 
               
               
                 custom designed springs devised for bobbin casings to achieve unusually high bobbin tension - not otherwise possible. 
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 reduce 
                 reduce 
               
               
                   
                 leakage 
                 leakage 
               
               
                   
                 between 
                 through 
               
               
                   
                 2 fabric 
                 seam 
               
               
                 Parameters 
                 layers 
                 stitching 
               
               
                   
               
             
            
               
                 increase coating weight 
                 better 
                 Better 
               
               
                 increase fabric weave count 
                 negligible 
                 better 
               
               
                 increase seam stitches/inch 
                 better 
                 worse 
               
               
                 reduce seam stitches/inch 
                 worse 
                 better 
               
               
                 increase thread tension 
                 better 
                 worse 
               
               
                 increase needle thread tension 
                 better 
                 worse 
               
               
                 increase bobbin thread tension to 
                 better 
                 negligible 
               
               
                 conventional limits 
               
               
                 increase bobbin thread tension beyond 
                 worse 
                 better 
               
               
                 conventional limits 
               
               
                 Properly balance needle and bobbin tension 
                 better 
                 better 
               
               
                 reduce thread size 
                 worse 
                 better 
               
               
                 reduce needle size 
                 worse 
                 better 
               
               
                 properly balance needle size and thread size 
                 better 
                 better 
               
               
                 Thread elongation/tensile = conventional 
                 limited 
                 limited 
               
               
                   
                 benefit 
                 benefit 
               
               
                 Thread elongation/tensile and bobbin tension 
                 better 
                 better 
               
               
                 combination of balancing 3 items in yellow 
                 better 
                 better 
               
               
                 Needle type &amp; size 
               
               
                 Sewing equipment? Non-conventional 
               
               
                 for airbag 
               
               
                   
               
            
           
         
       
     
     One Non-Limiting Embodiment: 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Variable 
                 Preferred Setting 
               
               
                   
                   
               
             
            
               
                   
                 Stitch count 
                 25 spi 
               
               
                   
                 Thread type 
                 92 polyester 
               
               
                   
                 Thread Tension 
               
               
                   
                 needle 
                 25 ounces 
               
               
                   
                 bobbin 
                 8.5 ounces 
               
               
                   
                 Speed of sewing machine 
                 4000 spm 
               
               
                   
                 Fabric selection (weave and coating) 
                 53 × 53 420 d 35 g sq/m 
               
               
                   
                 Needle size 
                 100/16 
               
               
                   
                 Needle point 
                 Medium ball 
               
               
                   
                 Type of sewing machine 
                 drop feed 
               
               
                   
                   
               
            
           
         
       
     
     Other Non-Limiting Embodiments in Ranges: 
       
     
       
         
           
               
               
             
               
                   
               
               
                 Variable 
                 Preferred Range 
               
               
                   
               
             
            
               
                 Stitch count 
                 22-30 spi (not a manufacturing 
               
               
                   
                 tolerance) 
               
               
                 Thread type 
                 92 polyester 
               
               
                 Thread Tension 
                 needle 21-30 ounces - bobbin 6-10 
               
               
                   
                 ounces 
               
               
                 Speed of sewing machine 
                 4000 spm 
               
               
                 Fabric selection (weave and coating) 
                 51 × 51 to 53 × 53 420 d 
               
               
                   
                 20 g sq/m 
               
               
                 Needle size 
                 100/16 
               
               
                 Type of sewing machine 
                 drop feed 
               
               
                   
               
            
           
         
       
     
     Exemplary embodiments disclosed herein utilize much higher spi than current sewn seams. It would not be obvious to significantly increase the spi and expect increased pressure retention since leakage occurs through the needle holes created in the fabric during sewing. Having many more of these holes due to the higher spi it would not be obvious to have better pressure retention. Instead this would be an unexpected result different from conventional airbag wisdom. 
     No sealing means of any kind are used in the unsealed embodiments disclosed herein. Typically side-curtains constructed to retain pressure for rollover, utilize some type of sealant bead or adhesive tape, glue etc. 
     In addition, the disclosed embodiments use a smaller sized thread than typically used and the thread is also polyester instead of the conventional nylon. Multiple tests were conducted which showed the polyester thread had a noticeable improvement over the conventional nylon with other parameters held constant. It would not be obvious to use a polyester thread to achieve higher pressure retention when the industry uses nylon exclusively. This was also an unexpected result going away from the conventional wisdom of using nylon thread to sew the airbag. 
     The bobbin tension has been increased well beyond the conventional and in some cases requires special adjustments and even custom designed springs to the bobbin case in order to achieve these higher tensions. This bobbin tension of 6-10 ounces is set higher with a corresponding needle tension of around 25 ounces. As discussed herein, if the bobbin tension is below this range the bobbin thread can be pulled through to the needle side of the fabric and the pressure retention is reduced. It was not obvious to adjust the bobbin tension to 4 to 10× more than conventional in order to provide the best performance with a corresponding appropriate needle tension. However, this unexpected result was discovered. 
     Higher weave count fabrics have been found to work especially well with the above sewing parameters, better than the conventional weave counts of 46×46. The higher weave also allows for reduced coating weight. Utilizing a fabric with reduced coating weight would conventionally be expected to provide decreased pressure retention so it would not be obvious to make the change to the higher weave count with reduced coating to have increased pressure retention. 
     Unconventional sewing machines to the airbag industry are utilized for this commercial embodiment. Due to the smaller thread size than the conventional 138 size, machines typically used in the garment industry can be used which run at much faster revolutions. Switching to the smaller thread size opened the door to explore sewing machines that were more suited to the 92 thread and brought with it sewing speeds that were significantly faster than the conventional airbag machines. Conventional airbag machines ran at a rate of 1800 stitches per minute (spm) while the machines used to make our commercial embodiment run at 4000 spm. This allows for us to sew the higher spi and still obtain competitive sewing times. 
     As shown in the above chart, multiple levels of balancing were required with the sewing parameters to achieve the desired results of the lower leakage. Arriving at this balance would not have been an obvious but instead were arrived at through extensive testing and trials with multitudes of different combinations. These ranges were not simply settling on optimum values of a result effective variable. These ranges were discovered only after creating a design contrary to conventional airbag methods and determining the ranges where the contrary conventional approach was already believed to provide a safe performance advantage. The ranges disclosed in the present invention would not have been found under conventional air bag design wisdom. 
       FIG. 14  is a graph that illustrates a plot of an unsealed cushion not using exemplary embodiments of the present invention (plots  300  and  305 ) and plots  310 ,  320  and  330  illustrating exemplary embodiments of the present invention (cushion pressure vs. time). 
     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.