Efficient Airbag Module

This application is related to U.S. patent application Ser. No. 08/550,217 entitled "Airbag System" filed Oct. 30, 1995, U.S. patent application Ser. No. 08/247,763 entitled "Plastic Film Airbag" filed May 23, 1994 (now U.S. Pat. No. 5,505,485) and U.S. patent application Ser. No. 08/539,676 entitled "Airbag System With Self Shaping Airbag", filed Oct. 5, 1995, (now U.S. Pat. No. 5,653,464) all of which are incorporated by reference herein.

This invention is in the field of inflator devices for inflating airbag occupant restraints mainly for the protection of occupants of automobiles and trucks although it also is applicable to the protection of occupants of other vehicles and for inflating other inflatable objects. In particular, by means of the present invention, a more efficient utilization of the energy in a propellant is attained resulting in the need for a lower amount of propellant than in currently existing inflators, and thus a smaller inflator, to inflate a given size inflatable object. This is accomplished in part through a more efficient aspirating nozzle design and an improved geometry of a gas generator which houses the propellant.

Most airbag modules in use today are large, heavy, expensive, and inefficient. As a result, airbags are now primarily only used for protecting the passenger and driver in a frontal impact, although at least three automobile manufacturers currently offer a small airbag providing limited protection in side impacts. The main advantage of airbags over other energy absorbing structures is that they utilize the space between the occupant and vehicle interior surfaces to absorb the kinetic energy of the occupant during a crash, cushioning the impending impact of the occupant with the vehicle interior surfaces. Airbags have been so successful in frontal impacts that it is only a matter of time before they are effectively used for side impact protection, protection for rear seat occupants and in place of current knee bolsters. Substantial improvements, however, must be made in airbags before they assume many of these additional tasks

A good place to start describing the problems with current airbags is with a calculation of the amount of energy used in a typical airbag inflator and how much energy is required to inflate an airbag. By one analysis, the chemical propellant in a typical driver's side inflator contains approximately 50,000 foot pounds (68,000 joules) of energy. A calculation made to determine the energy required to inflate a driver's side airbag yields an estimate of about 500 foot pounds (680 joules). A comparison of these numbers shows that approximately 99% of the energy in a chemical propellant is lost, that is, generated but not needed for inflation of the airbag. One reason for this is that there is a mismatch between the output of a burning propellant and the inflation requirements of an airbag. In engineering this is known as an impedance mismatch. Stated simply, propellants naturally produce gases having high temperatures and high pressures and low gas flow rates. Airbags, on the other hand, need gases with low temperatures and low pressures and high gas flow rates.

In view of this impedance mismatch, inflators are, in theory at least, many times larger then they would have to be if the energy of the propellant contained within the inflator were efficiently utilized. Some attempts to partially solve this problem have resulted in a so called "hybrid" inflator where a stored pressurized gas is heated by a propellant to inflate the airbag. Such systems are considerably more energy efficient, however, they also require a container of high pressure gas and means for monitoring the pressure in that container. Other systems have attempted to use aspiration techniques, but because of the geometry constraints of current car inflator designs and mounting locations, and for other reasons, currently used aspiration systems are only able to draw up to about 30% of the gas needed to inflate an airbag from the passenger compartment. Theoretical studies have shown that as much as 90% or more of the gas could be obtained in this manner.

Furthermore, since inflators are large and inefficient, severe restrictions have been placed on the type of propellants that can be used since the combustion products of the propellant must be breathable by automobile occupants. It is of little value to save an occupant from death in an automobile accident only to suffocate him from an excessive amount of carbon dioxide in the air within the passenger compartment after the accident. If inflators operated more efficiently, then alternate, more efficient but slightly toxic propellants could be used. Also, current inflators are made from propellants, namely sodium azide, which are not totally consumed. Only about 40% of the mass of sodium azide propellants currently being used, for example, enters the airbag as gas. This residual mass is very hot and requires the inflator to be mounted away from combustible materials further adding to the mass and size of the airbag system.

It is a persistent problem in the art that many people are being seriously injured or even killed today by the airbag itself. This generally happens when an occupant is out-of-position and against an airbag module when the airbag deploys. In order to open the module cover, sometimes called the deployment door, substantial pressure must first build up in the airbag before enough force is generated to burst open the cover. This pressure is even greater if the occupant is in a position which prevents the door from opening. As a result, work is underway to substantially reduce the amount of energy required to open the deployment doors and devices have been developed which pop off the deployment door or else cut the deployment door material using pyrotechnics, for example.

One reason that this is such a significant problem is that the airbag module itself is quite large and, in particular, the airbags are made out of thick, heavy material and packaged in a poor, folded geometry. The airbag, for example, which protects the passenger is housed in a module which is typically about one third as long as the deployed airbag. All of this heavy airbag material must be rolled and folded inside this comparatively small module, thus requiring substantial energy to unfold during deployment. This situation could be substantially improved if the airbag module were to have an alternate geometry and if the airbag material were substantially lighter and thinner and, therefore, less massive and folded mainly parallel to the inflator. Even the time to deploy the airbag is substantially affected by the mass of the airbag material and the need to unfold an airbag with a complicated folding pattern. Parallel folding, as used herein, means that the airbag material is folded with the fold lines substantially parallel the axis of the inflator without being folded over lengthwise as is now done with conventional airbag folding patterns.

Devices are under development which will monitor the position of the occupant and prevent the airbag from deploying if the occupant is dangerously close to the module where he or she can be seriously injured by the deployment. Some systems will also prevent deployment if the seat in connection with which the airbag operates is unoccupied. An alternate approach is to move the deployment doors to a location away from normal occupant positions. One such location is the ceiling of the vehicle. One problem with ceiling mounted airbags is that the distance required for the airbag to travel, in some cases, is longer and therefore a larger airbag is needed with greater deployment time. With the use of light airbag materials, such as thin plastic film, as disclosed in the above referenced patent applications Ser. Nos. 08/247,763 and 08/539,676, and the use of more efficient inflators, both of these problems can be solved especially for the front and rear seat passengers. The driver poses a different problem since it would be difficult to position a ceiling mounted airbag module where the airbag would always be projected properly between the occupant and the steering wheel.

This problem for the driver's airbag system is not the concept of mounting the airbag on the ceiling, but the design of the steering wheel and steering column. These designs come from the time when the only way of steering an automobile was through mechanical linkages. The majority of vehicles manufactured today have power assisted steering systems and, in fact, most drivers would have difficulty steering a car today if the power steering failed. If servo power steering were used, the need for a mechanical linkage between a steering wheel, or other such device, and the power steering system would no longer be necessary. Servo power steering for the purposes here will mean those cases where the linkage between the manually operated steering device, which regardless of what that device is, will herein be called a steering wheel, is done with a servo system either electrically or hydraulically and the system does not have an operative mechanical connection between the steering wheel and the steering mechanism which moves the wheels.

The problem of educating the general population, which has become secure in the feeling of a steering wheel and steering column, might be insurmountable if it were not for the substantial safety advantage resulting from substituting servo power steering for conventional steering systems and using a non-steering wheel mounted airbag module for the driver.

The steering wheel and steering column are among the most dangerous parts of the vehicle to the occupant. Small people, for example, who are wearing seatbelts can still be seriously injured or killed in accidents as their faces slam into the steering wheel hubs. The problem of properly positioning an airbag, when the comfort and convenience features of telescoping and tilting steering columns are considered, results in substantial safety compromises. Deployment induced injuries which result when a small person is close to the steering wheel when the airbag deploys have already caused several deaths and numerous serious injuries. Future vehicles, therefore, for safety reasons should be constructed without the massive steering wheel and steering column and substitute therefor a servo steering assembly. With this modification, a ceiling mounted airbag module, such as discussed herein, becomes feasible for the driver as well as the other seating positions in the vehicle.

The front seat of the vehicle today has an airbag for the passenger and another for the driver. In some accidents, an occupant, and particularly a center seated occupant, can pass between the two airbags and not receive the full protection from either one. If a ceiling mounted airbag system were used, a single airbag could be deployed to cover the entire front seat greatly simplifying the airbag system design.

One method of partially solving many of these problems is to use an efficient aspirated airbag system. There have been numerous patents granted on designs for airbag systems using aspirated inflators. In these patents as well as in the discussion herein the term "pumping ratio" is used. The pumping ratio as used in the art is defined as the ratio of the mass of gas aspirated from the environment, either from inside or outside of the vehicle, to the mass of gas generated by burning the propellant. A brief description of several pertinent patents, all of which are included herein by reference, follows:

U.S. Pat. No. 2,052,869 to Coanda illustrates the manner in which a fluid jet is caused to change direction, although no mention is made of its use in airbags. This principle, the "Coanda effect", is used in some implementations of the instant invention as well as in U.S. Pat. No. 3,909,037 to Stewart discussed below. It's primary contribution is that when used in inflator designs, it permits a reduction in the length of the nozzle required to efficiently aspirate air into the airbag. No disclosure is made of a pumping ratio in this system and in fact it is not an object of Coanda to aspirate fluid.

U.S. Pat. No. 3,204,862 to Hadeler also predates the invention of vehicular airbags but is nonetheless a good example of the use of aspiration to inflate an inflatable structure. In this device, an inflating gas is injected into an annular converging-diverging nozzle and some space efficiency is obtained by locating the nozzle so that the flow is parallel to the wall of the inflatable structure. No mention is made of a pumping ratio of this device and furthermore, this device is circular.

U.S. Pat. No. 3,632,133 to Hass provides a good example of a nozzle in a circular module with a high pumping ratio in an early construction of an airbag. Although analysis indicates that pumping ratios of 4:1 or 5:1 would be difficult to achieve with this design as illustrated, nevertheless, this reference illustrates the size and rough shape of an aspirating system which is required to obtain high pumping ratios using the prior art designs.

U.S. Pat. No. 3,909,037 to Stewart provides a good example of the application of the Coanda effect to airbag aspirating inflators. Stewart, nevertheless, still discards most of the energy in the propellant which is absorbed as heat in the inflator mechanism. Most propellants considered for airbag applications burn at pressures in excess of about 1000 psig. Stewart discloses that the maximum efficiency corresponding to a 5:1 pumping ratio occurs at inflator gas pressures of about 5 to about 45 psig. In order to reduce the pressure, Stewart utilizes a complicated filtering system similar to that used in conventional inflators. Stewart requires the use of valves to close off the aspiration ports when the system is not aspirating. Through the use of the Coanda effect, Stewart alludes to a substantial reduction in the size of the aspiration system, compared to Hass for example. Also, Stewart shows only a simple converging nozzle through which the burning propellant is passed.

U.S. Pat. No. 4,833,996 to Hayashi et al. describes a gas generating apparatus for inflating an airbag which is circular and allegedly provides an instantaneous pumping ratio of up to 7:1 although analysis shows that this is unlikely in the illustrated geometry. The average pumping ratio is specified to be up to 4:1. This invention is designed for the driver side of the vehicle where unrestricted access to the aspirating port might be difficult to achieve when mounted on a steering wheel. The propellant of choice in Hayashi et al. is sodium azide which requires extensive filtering to remove particulates. No attempt has been made in this design to optimize the nozzle geometry to make use of a converging-diverging nozzle design, for example. Also, the inflator has a roughly conventional driver side shape. It is also interesting to note that no mention is made of valves to close off or restrict flow through the aspiration port during deflation. Since most aspiration designs having even substantially smaller pumping ratios provide for such valves, the elimination of these valves would be a significant advance in the art. Analysis shows, however, that the opening needed for the claimed aspiration ratios would in general be far too large for it also to be used for exhausting the airbag during a crash. Since this is not discussed, it should be assumed that valves are required but not illustrated in the figures.

U.S. Pat. No. 4,877,264 to Cuevas describes an aspirating/venting airbag module assembly which includes a circular gas generator and contemplates the use of conventional sodium azide propellants or equivalent. The aspiration or pumping ratio of this inflator is approximately 0.2:1, substantially below that of Hayashi et al., but more in line with aspiration systems in common use today. This design also does not require use of aspiration valves which is more reasonable for this case, but still unlikely, since the aspiration port area is much smaller. Again, no attempt has been made to optimize the nozzle design as is evident by the short nozzle length and the low pumping ratio.

U.S. Pat. No. 4,909,549 to Poole et al. describes a process for inflating an airbag with an aspiration system but does not discuss the aspiration design or mechanism and merely asserts that a ratio as high as 4:1 is possible but assumes that 2.5:1 is available. This patent is significant in that it discloses the idea that if such high pumping ratios are obtainable (i.e., 2.5:1 compared with 0.2:1 for inflators in use), then certain propellants, which would otherwise be unacceptable due to their production of toxic chemicals, can be used. For example, the patent discloses the use of tetrazol compounds. It is interesting to note that there as yet is no commercialization of the Poole et al. invention which raises the question as to whether such high aspiration ratios are in fact achievable with any of the prior art designs. Analysis has shown that this is the case, that is, that such large aspiration ratios are not achievable with the prior art designs.

U.S. Pat. No. 4,928,991 to Thorn describes an aspirating inflator assembly including aspiration valves which are generally needed in all high pumping ratio aspiration systems. Sodium azide is the propellant used. Pumping ratios of 1:1 to 1.5:1 are mentioned in this patent which by analysis is possible. It is noteworthy that the preamble of this patent discloses that the state of the art of aspirating inflators yields pumping ratios of 0.1:1 to 0.5:1, far below those specified in several of the above referenced earlier patents. Once again, little attempt has been made to optimize the nozzle design.

U.S. Pat. No. 5,004,586 to Hayashi et al. describes a sodium azide driver side inflator in which the aspirating air flows through a series of annular slots on the circumference of the circular inflator in contrast to the earlier Hayashi et al. patent where the flow was on the axis. Similar pumping ratios of about 4:1 are claimed however, which by analysis is unlikely. Once again, aspiration valves are not shown and the reason that they can be neglected is not discussed. An inefficient nozzle design is again illustrated. The lack of commercial success of these two Hayashi patents is probably due to the fact that such high pumping ratios as claimed are not in fact achievable in the geometries illustrated.

U.S. Pat. No. 5,060,973 to Giovanetti describes the first liquid propellant airbag gas generator wherein the propellant burns clean and does not require filters to trap solid particles. Thus, it is one preferred propellant for use in the instant invention. This system however produces a gas which is too hot for use directly to inflate an airbag. The gas also contains substantial quantities of steam as well as carbon dioxide. The steam can cause burns to occupants and carbon dioxide in significant quantities is toxic. The gas generator is also circular. Aspirating systems are therefore required when using the liquid propellant disclosed in this patent, or alternately, the gas generated must be exhausted outside of the vehicle.

U.S. Pat. No. 5,129,674 to Levosinski describes a converging-diverging nozzle design which provides for more efficient aspiration than some of the above discussed patents. Nevertheless, the airbag system disclosed is quite large and limited in length such that the flow passageways are quite large which requires a long nozzle design for efficient operation. Since there is insufficient space for a long nozzle, it can be estimated that this system has a pumping ratio less than 1:1 and probably about 0.2:1. Once again a sodium azide based propellant is used.

U.S. Pat. No. 5,207,450 to Pack, Jr. et al. describes an aspirated air cushion restraint system in which no attempt was made to optimize the nozzle design for this sodium azide driver side airbag. Also, aspiration valves are used although it is suggested that the exhaust from the airbag can be made through the aspirating holes thereby eliminating the need for the flapper valves. No analysis, however, is provided to prove that the area of the aspiration holes is comparable to the area of the exhaust holes normally provided in the airbag. Although no mention is made of the pumping ratio of this design, the device as illustrated appears to be approximately the same size as a conventional driver side inflator. This, coupled with an analysis of the geometry, indicates a pumping ratio of less than 1:1 and probably less than 0.2:1. The statement that the aspiration valves are not needed also indicates that the aspiration ratio must be small. Large inlet ports which are needed for large aspiration ratios are generally much larger than the typical airbag exhaust ports.

U.S. Pat. No. 5,286,054 to Cuevas describes an aspirating/venting motor vehicle passenger airbag module in which the principal of operation is similar to the '264 patent discussed above. Once again the aspiration pumping ratio of this device is 0.15:1 to 0.2:1 which is in line with conventional aspirated inflators. It is interesting to note that this pioneer in the field does not avail himself of designs purporting to yield higher pumping ratios. Again the nozzle design has not been optimized.

Other U.S. patents which are relevant to the instant invention but which will not be discussed in detail are: U.S. Pat. No. 3,158,314 to Young et al., U.S. Pat. No. 3,370,784 to Day, U.S. Pat. No. 5,085,465 to Hieahim, U.S. Pat. No. 5,100,172 to Van Voorhies et al., U.S. Pat. No. 5,193,847 to Nakayama, U.S. Pat. No. 5,332,259 to Conlee et al. and U.S. Pat. No. 5,423,571 to Hawthorn.

None of the prior art inflators contain the advantages of the combination of (i) a linear inflator having a small cross section thereby permitting an efficient nozzle design wherein the length of the nozzle is much greater than the aspiration port opening, (ii) a non sodium azide propellant which may produce toxic gas if not diluted with substantial quantities of ambient air, and (iii) an inflator where minimal or no filtering or heat absorption is required.

It is interesting to note that in spite of the large aspiration pumping ratios mentioned and even claimed in the prior art references mentioned above, and to the very significant advantages which would result if such ratios could be achieved, none has been successfully adapted to an automobile airbag system. One reason is that pumping ratios which are achievable in a steady state laboratory environment are more difficult to achieve in the transient conditions of an actual airbag deployment.

None of these prior art designs have resulted in a thin linear module which permits the space necessary for an efficient nozzle design as disclosed herein. In spite of the many advantages claimed in the prior art patents, none have resulted in a module which can be mounted within the vehicle headliner trim, for example, or can be made to conform to a curved surface. In fact, the rigid shape of conventional airbag modules has forced the vehicle interior designers to compromise their designs since the surface of such modules must be a substantially flat plane.

It is an object of the present invention to provide a new and improved airbag module in which the disadvantages and problems in the prior art airbag module designs are substantially eliminated.

It is another object of the present invention to provide a new and improved airbag module in which the propellant is utilized more efficiently than in prior art constructions, i.e., more of the energy generated by the burning propellant is used to inflate the airbag with less waste of generated energy.

It is yet another object of the present invention to provide a new and improved airbag module in which the propellant is utilized more efficiently than in prior art constructions so that a smaller amount of propellant can be used thereby enabling a wider variety of propellants, which may generate toxic gases, to be used since only small quantities of such toxic gases will be generated.

It is still another object of the present invention to provide a new and improved airbag module which is designed so that if an occupant is out-of-position and leaning against the module, the module will still enable deployment of the airbag to protect the occupant.

It is another object of the present invention to provide a new and improved airbag module in which a more efficient aspirating system is used, i.e., one having a larger pumping ratio.

Other principle objects and advantages of this invention are:

1. To provide a long, thin airbag module which can be conveniently mounted on the ceiling or almost any other surface of the vehicle for the protection of vehicle occupants in frontal collisions.

2. To provide a highly efficient airbag module which uses the minimum amount of propellant.

3. To provide a rapidly deploying airbag module causing minimal injury to out-ofposition occupants.

4. To provide an airbag module where a single module can be used to protect both a front and a rear passenger of the vehicle in side impacts.

5. To provide a cover system for the module which is easily released thereby reducing the risk of deployment induced injuries to the vehicle occupants.

6. To provide an airbag module which can be conveniently mounted on the instrument panel, or knee bolster support structure, to provide knee and leg protection for the vehicle occupant as well as for an occupant, such as a child, lying on the seat.

7. To provide a driver protection airbag system for use with vehicles having servo power steering and thereby promote the elimination of the massive steering column and steering wheel.

8. To provide an airbag module which is displaced from its mounting surface during the initial stage of airbag deployment to thereby opening aspirating channels.

9. To provide a highly efficient aspirated airbag module.

10. To provide a long thin airbag module where the module is approximately the same length as the inflated airbag thereby simplifying the folding, and unfolding during deployment, of the airbag and permit more rapid airbag deployment.

11. To provide an airbag module which is easily replaced when the vehicle is repaired after an accident.

12. To provide a ceiling mounted airbag module for protection of rear seated occupants.

13. To provide an aspirated airbag system where the aspiration inlet ports are also used as the venting ports for the airbag, including an automatic adjustment system to reduce the vent area, thereby simplifying the airbag design by removing the requirement for vents within the airbag material.

14. To provide an inflator design wherein the propellant is a single solid material attached to the inside of the inflator housing thereby simplifying the inflator design and providing a definable burning surface.

15. To provide a method of ignition of a propellant through a coating placed on the surface of a propellant containing an igniter material such as BKNO3.

16. To provide a thin airbag module which can be mounted substantially on a surface of the passenger compartment such as the ceiling, instrument panel, or seat back without penetrating deeply within the surface.

17. To provide for a thin airbag module which can be made to conform to a curved surface.

18. To provide an inflator design wherein the propellant is a liquid substantially filling the inside of the inflator housing thereby simplifying the inflator design and providing a definable burning surface.

These and other objects and advantages will become apparent from the discussion below.

Briefly, the airbag module in accordance with the invention is long and thin and can conveniently be made in any length and bent into almost any generally linear shape. This module is typically mounted on or slightly below a surface in the passenger compartment such as the ceiling, instrument panel, seat or knee bolster support structure. When the deployment of the module is initiated, a cover is released and a thin, preferably film, airbag is inflated using a highly aspirated inflator using a clean propellant which if undiluted might be toxic to humans.

More particularly, in accordance with the invention, the airbag module comprises an elongate housing having a length in the longitudinal direction which is substantially larger than a width or thickness thereof in a direction transverse to the longitudinal direction, an airbag situated within the housing, inflator means arranged in the housing for producing pressurized gas to inflate the airbag, mounting means for mounting the module in the passenger compartment, initiation means for initiating the inflator means to produce the pressurized gas in response to the crash of the vehicle, and the housing comprises cover means for releasably retaining the airbag. Preferably, the length of the housing is at least ten times larger than the width or the thickness of the housing thereby permitting mounting of the module with minimal penetration below a mounting surface of the passenger compartment. In one embodiment, the inflator means and airbag extend in the longitudinal direction of the housing and the inflator means are elongate and have a length which is more than half the length of the airbag measured in the longitudinal direction when the airbag is inflated.

The inflator means may comprise a gas generator having a length substantially in the longitudinal direction of the housing exceeding ten times a width or thickness of the gas generator in a direction transverse to the longitudinal direction.

Furthermore, the housing includes an elongate support base mounted to a surface of the passenger compartment by the mounting means and which has a catch at each longitudinal side,. In this embodiment, the cover means comprise a tab engaging with the catch to retain the cover means, the tab being released from the catch during deployment of the airbag.

In another embodiment, the inflator means comprise an elongate gas generator including a propellant for producing pressurized gas to inflate the airbag which has a length at least ten times its width or thickness. The mounting means are then structured and arranged to mount the module substantially onto a peripheral surface of the passenger compartment while the initiation means are structured and arranged to initiate the gas generator to produce the pressurized gas in response to the crash of the vehicle. The cover means cover the airbag prior to the production of the pressurized gas and the housing further includes removal means for enabling removal of the cover means to permit the deployment of the airbag. The surface of the passenger compartment to which the module is mounted is a back surface of a front seat of the vehicle, an instrument panel in the vehicle, possibly in such a position as to afford protection to the knees of a front seat occupant during the crash of the vehicle, or the ceiling of the vehicle, e.g., at a location in front of a front seat of the vehicle and suitable for mounting the module for protecting occupants of the front seat in a frontal impact or at a location along a side of the vehicle and suitable for mounting the module for protecting occupants of both front and rear seats in a side impact or at a location behind a front seat of the vehicle.

In yet another embodiment, the inflator means comprises an elongate gas generator for producing the pressurized gas to inflate the airbag and therefore, the initiation means are structured and arranged to initiate the gas generator to produce the pressurized gas in response to the crash of the vehicle. The module further includes aspiration means for combining gas from the passenger compartment with the pressurized gas from the gas generator and directing the combined gas into the airbag. Such aspiration means may comprise a linear nozzle leading from a combustion chamber in the gas generator and having a converging section followed by a diverging section and ending at a mixing chamber in the module, whereby the pressurized gas flows from the combustion chamber through the linear nozzle into the mixing chamber, and means defining at least one aspiration inlet port such that gas from the passenger compartment flows through the aspiration inlet port into the mixing chamber. The mixing length is at least fifty times the minimum thickness of a jet of the pressurized gas from the gas generator within the nozzle. The mixing chamber comprises nozzle means defining a converging section and a diverging section arranged after the converging section in a direction of flow of the combined pressurized gas and gas from the passenger compartment. The dimensions of the converging-diverging nozzle and aspiration inlet port(s) are selected so that the gas entering the airbag is at least 80 percent from the passenger compartment. The aspiration means may also comprise a pair of nozzle walls extending in the longitudinal direction at a respective side of the gas generator, such that the gas generator is situated between the nozzle walls and the pressurized gas from the gas generator is directed into a mixing chamber defined in part between the nozzle walls, and springs for connecting the nozzle walls to the support base. The springs have a first position in which the nozzle walls are proximate to the support base and a second position in which the nozzle walls are spaced apart from the support base. The springs are extended to the second position during production of the pressurized gas to separate the nozzle walls from the support base to define aspiration inlet ports between the nozzle walls and the support base. In this case, the module also includes support shields connected to and extending between the nozzle walls. The support shields define the mixing chamber prior to deployment of the airbag and are forced outward to define a second converging-diverging nozzle leading from the mixing chamber to the airbag through which the pressurized gas flows.

The gas generator preferably comprises an elongate plastic housing having a length at least 10 times its thickness or width, propellant dispersed in an interior of and substantially along the length of the gas generator housing, igniter means for initiating burning of the propellant; and gas generator mounting means for mounting the gas generator in the passenger compartment. The gas generator housing comprises at least one opening through which gas passes from the interior of the gas generator into the airbag. The opening(s) has/have a variable size depending on the pressure of the gas in the interior of the gas generator housing.

The present invention also relates to an occupant protection system for a vehicle including an airbag module and having power steering means comprising a steering wheel opposed to a driver side portion of a front seat, a servo control system and means for connecting the steering wheel to the control system, e.g., an deformable support member. The mounting means mount the module in the passenger compartment apart from the steering wheel of the vehicle. In this case, the occupant protection system comprises yieldable steering wheel support means for enabling the steering wheel to be displaced away from a position opposed to a driver when situated in the driver side portion of the front seat. The module is thus structured and arranged such that the airbag after deployment cushions the driver from impact with surfaces of the passenger compartment. As such, it is possible to provide a single airbag module to provide protection for the entire front seat.

It is also envisioned that a single airbag module can provide protection for both a front seat occupant and a rear seat occupant on the same side of the vehicle, In this case, the mounting means mount the airbag module to a surface of the passenger compartment such that it extends in a horizontal direction from a front portion of the passenger compartment toward a rear portion of the passenger compartment adjacent a side of the vehicle.

Furthermore, it will be appreciated by those skilled in the art, and as explained below, that it is ideal to vary the size of the nozzle of the gas generator through which gas generated by the burning propellant flows in response to variations in the pressure in the chamber in which the propellant is burning. To this end, the present invention includes a gas generator having a housing, propellant dispersed in an interior thereof, igniter means for initiating burning of the propellant, and gas generator mounting means for mounting the gas generator housing to the support base and spaced from the support base to define a nozzle therebetween and which comprise elastic support brackets arranged in the nozzle between the gas generator housing and the support base or strips of deformable material, which deforms as a function of temperature variation, arranged in the nozzle between the gas generator housing and the support base., In yet another embodiment, the airbag module comprises an airbag, an inflator for producing pressurized gas to inflate the airbag and which comprises a housing, a gas generator arranged therein, and a variable exit opening from the housing through which gas from the gas generator flows to inflate the airbag. The size of the variable exit opening is controlled by the pressure within the housing. The remaining structure of this module may be as described above.

The following drawings are illustrative of embodiments of the invention and are not meant to limit the scope of the invention as encompassed by the claims.

FIG. 1 is a perspective view, with certain parts removed, of a preferred implementation of the airbag module in accordance with the invention shown mounted on a ceiling of a vehicle passenger compartment for deployment to protect rear seat occupants.

FIG. 2A is a cross section view of the airbag module of FIG. 1 prior to deployment of the airbag.

FIG. 2B is a view of the apparatus of FIG. 2A after the initial stage of deployment where the airbag module has been displaced from the mounting surface to open aspirating ports.

FIG. 2C is a cross section view similar to FIG. 2A with the airbag fully deployed taken along lines 2C--2C of FIG. 1.

FIG. 2D is a cross section view similar to FIG. 2C after the airbag has deployed showing the substantial closure of the aspirating ports.

FIG. 2E is an enlarged view of the high pressure gas generator nozzle taken within circle 2E of FIG. 2A.

FIG. 2F is a perspective view of the apparatus of FIG. 2A, with the airbag and other parts cut away and removed.

FIG. 2G is a cross section view of the apparatus of FIG. 2A with parts cut away and removed illustrating an alternate configuration of the invention wherein a slow burning propellant in the form of a thin flat sheet is used.

FIG. 2H is a perspective view of the apparatus as shown in FIG. 2F wherein a separate inflator is used in place of the propellant in the tube of FIG. 2A and the tube is used here as a method of dispensing the output from the inflator to the aspirating nozzle design of this invention, FIG. 3A is a cross section view of an alternate embodiment of the airbag module in accordance with the invention where sufficient space is available for the aspirating ports without requiring movement of the module toward the occupant.

FIG. 3B is a cross-sectional view of the embodiment of FIG. 3A with the airbag deployed.

FIG. 4 is a perspective view of a preferred embodiment of the airbag module in accordance with the invention used for knee protection shown in the deployed condition.

FIG. 5 is a view of another preferred embodiment of the invention shown mounted in a manner to provide head protection for a front and a rear seat occupant in side impact collisions and to provide protection against impacts to the roof support pillars in angular frontal impacts.

FIG. 6 illustrates still another preferred embodiment of the invention used to provide protection for all front seat occupants in a vehicle which incorporates servo power steering.

FIG. 7 is a view as in FIG. 6 showing the flow of the inflator gases into the passenger compartment when occupants begin loading the airbag.

FIG. 8 shows the application of a preferred implementation of the invention for mounting on the rear of front seats to provide protection for rear seat occupants.

FIG. 9 is a perspective view of a typical module cover design as used with the embodiment of FIGS. 2A-FIG. 2H.

Referring to the accompanying drawings wherein the same reference numerals refer to the same or similar elements, an airbag module constructed in accordance with the teachings of the invention and adapted for mounting, e.g., on a ceiling 105 in a passenger compartment 195 of a motor vehicle to protect rear seat occupants in collisions and particularly frontal collisions, is shown generally at 100 in FIG. 1. The airbag module 100 is elongate and includes an inflator module 120 and an airbag 110 made substantially of plastic film which is coupled to the inflator module 120. The airbag module 100 is attached to the ceiling 105 by fastening means 107. The airbag module 100, and more particularly the inflator module 120, is also coupled to a sensor and diagnostic module 180 which receives input data and determines if an accident involving the vehicle is of such severity as to require deployment of the airbag 110. If so, the sensor and diagnostic module 180 sends a signal to the inflator module 120 to start the process of deploying the airbag, i.e., by initiating the burning of a propellant housed within a gas generator portion of the inflator module 120 as described in more detail below.

FIG. 2A is a cross-sectional view of the airbag module 100 prior to inflation of the airbag 110. As shown in FIG. 2A, the airbag module 100 includes a protective cover 150 which partially defines a housing of the airbag module 100 and as such, encloses the airbag 110 and an interior portion of the airbag module 100 and prevents contaminated particles from entering the interior of the airbag module 100. Shortly after the inflator module 120 is directed to initiate burning of the propellant, the protective cover 150 is released as a direct result of the burning propellant and the folded airbag 110 begins to inflate using a combination of gases from both the gas generator portion of the inflator module 120 and, through aspiration, from the passenger compartment 195 of the vehicle.

The term "airbag" as used herein means either the case where the airbag module 100 contains a single airbag, as in most conventional designs, or where the airbag module 100 contains a plurality of airbags, or where the airbag module 100 contains a single airbag having a plurality of compartments which deploy in concert to protect an occupant. The term "inflator" as used herein means the gas generator plus all other parts required to deliver gas to the airbag including the aspiration system if present. The term "gas generator", on the other hand, refers only to the propellant, its housing and all other parts required to generate gas. In nonaspirated implementations, the inflator and the gas generator are the same. The terms "propellant" and "gas generator" are used here as equivalents. The term "cover" as used herein means any type of covering for enclosing an interior portion of the airbag module 100, or at least for overlying the airbag 110 per se, to protect the same and may even constitute a simple covering on an outermost region of the airbag 110. Thus, it will be appreciated by those skilled in the art that the cover may be the material which forms the outer peripheral surface of the passenger compartment, e.g., fabric, without deviating from the objects of the invention. Alternatively, the covering may actually be a surface of the airbag itself coated to appear like a cover.

FIG. 2B is a view of the airbag module 100 of FIG. 2A after the initial stage of airbag inflation where the initial gas from the gas generator has generated sufficient pressure within the interior of the airbag module 100 to force the release of the cover 150. FIG. 2C shows the airbag 110 in its inflated condition and is a view taken along lines 2C--2C of FIG. 1. In the embodiment illustrated in FIGS. 2A, 2B and 2C, the airbag module 100 is substantially elongate and the inflator module 120 comprises an elongate gas generator made from an approximately rectangular-cross-section housing, such as a tube 121, with at least one opening 122 therein for outflow of gas generated thereby. A propellant 127 in the form of a solid material is affixed to the inner surface of the walls of tube 121 in a significant portion of the interior of tube 121. The surface of the propellant 127 not engaging a wall of the tube 121 is coated with a layer of pyrotechnic igniter mix 128 such as a coating made from nitrocellulose and BKNO3, or other suitable materials, to aid in starting burning of the propellant 127. This layer of igniter mix 128 also serves to seal the propellant 127 from the environment, i.e., the atmosphere. If the material of the igniter mix 128 is made at least partially from nitrocellulose or another appropriate sealant, then another seal to isolate the propellant from the atmosphere (?) is not required and the propellant 127 is effectively hermetically sealed. A screen member 129 is also positioned within tube 121 in a position spaced from the layer of igniter mix 128 covering the propellant 127 and adjacent to the opening(s) 122 to prevent any particulate matter from leaving the tube 121. A chamber 125 is thus defined between the screen member 129 and the layer of igniter mix 128. Although any screen member 129 will inherently provide some initial cooling of the gas from the propellant 127, this is simply an ancillary benefit. On the other hand, for cases where the selected propellant 127 is known to burn at too high a temperature, the screen member 129 can be made thicker so as to also serve as a heat sink, i.e., its size can be regulated to affect the temperature of the gases generated by the burning propellant 127 and expelled from the gas generator 120. Upon receiving a signal from the sensor and diagnostic module 180, an electronic module (not shown) ignites a squib at one end of the inflator module 110 (FIG. 1) which ignites the igniter mix 128 which in turn ignites the propellant 127. The propellant 127 burns in a direction from the surfaces coated by the igniter mix 128 toward wall 123 (FIG. 1), which is opposed to the wall of the tube 121 having the opening(s) 122 until the propellant 127 is totally consumed. It will be appreciated by those skilled in the art that the size of the tube 121 can be regulated, i.e., elongated or widened, depending on the propellant used and ideally minimized based on appropriate selection of the propellant and the required gas output parameters of the gas generator without deviating from the scope of the invention.

The airbag module 100 also includes, external of the tube 121 and in fluid communication with the opening(s) 122, a mixing chamber 130 in which the gas from the gas generator 120 resulting from the burning of the propellant 127 and gas from the passenger compartment 195, e.g., air, entering through an aspiration nozzle or inlet ports or inlet slits 135 are combined, and from which the combined gases are delivered to the airbag 110 through a port or nozzle, i.e., converging-diverging nozzle 115 (FIGS. 2B and 2C).

The airbag module 100 also comprises elongate U-shaped nozzle walls 160 (which define the nozzle 115 therebetween), base 114 which is mounted to the ceiling 105 by the fastening members 107 (FIG. 1) and support springs 145. The gas generator 120 is attached to base 114 by brackets 170 which will be described in more detail below (FIG. 2F). Each of the nozzle walls 160 has two leg portions 160a1, 160a2 extending from opposite end regions of a base portion 160b. At least one of the springs 145, two in the illustrated embodiment, is attached to leg portion 160a1 of each nozzle wall 160 and, as shown in FIG. 2A, prior to airbag inflation are maintained in a compressed state so that the walls 160 are proximate to the base 114, i.e., the aspiration inlet ports 135 are substantially closed. Attached to the base portion 160b of each nozzle wall 160 is a spring shield 155 which supports and protects the material of the airbag 110 during the initial inflation period, as shown in FIG. 2B, prior to the start of the aspiration when the gases are hot, keeping the airbag 110 from blocking the inlet to the inflator module 120 from the passenger compartment 195. Prior to inflation of the airbag 110, the spring shields 155 exert pressure against the folded airbag 110 to force the same against the cover 150. During inflation, the spring shields 155 are designed to be forced outward by the expulsion of gases from the gas generator 120 and help start the airbag deployment process since the cover 150 no longer acts to restrain the airbag 100, and then the support shields 155 form the converging and diverging portions of the low pressure part of the aspiration nozzle 115. The ends of the airbag 110 are connected to leg portions 160a2.

In view of this construction, the airbag 110 is not circular but rather is elongate as shown in FIG. 1 Some of the advantages of this non-circular airbag 110 is its ease of manufacture from flat plastic film sheets as described in copending patent application Ser. No. 08/539,676 and its ease in parallel folding the airbag into the module 100. In this regard, it is noted that in view of the elongated shape of the airbag 110, it can be folded lengthwise in the airbag module 100.

In operation, shortly after the propellant 127 has been ignited by the igniter mix 128, high pressure gas begins to flow through screen 129, through opening(s) 122 and out through a converging-diverging nozzle 117,118,119, also referred to as a convergent-divergent nozzle. The nozzle 117,118,119 extends along the longitudinal sides of the inflator module 120. This nozzle has the effect of causing a jet of the combustion gases to achieve a high supersonic velocity and low pressure and to spread to rapidly fill the mixing chamber 130 formed by an outer wall of tube 121, nozzle walls 160 and spring shields 155. This causes a low pressure to occur in the mixing chamber 130 causing substantial amounts of gas to flow through aspirator inlet ports 135, which are opened by the expansion of springs 145 forcing nozzle walls 160 to move away from base 114 as shown, e.g., in FIGS. 2B and 2C. The converging portion 117 of the nozzle is constructed so that its cross-sectional area gradually decreases until throat 118. After throat 118, the cross-sectional area of the diverging portion 119 of the nozzle gradually increases toward exit 132. Thus, after the throat 118, there is a significant continuation of the nozzle to provide for the diverging portion 119. An approximate analysis of an aspirating system similar to that of this invention appears in the Appendix.

The pressure then begins to build in the mixing chamber 130 until sufficient pressure is obtained to expel or release the cover 150 causing springs 145 to expand even more, support shields 155 to be opened to fully open the nozzle 115 and airbag 110 to be released. Prior to release, the cover 150 is retained by a catch 152. Upon pressurization of the mixing chamber during airbag deployment, a tab 153 on cover 150 is pulled from under catch 152 releasing the cover. Since the pressure builds at the end of the module which is initially ignited by the squib, not shown, the tab 153 is initially released at that end and then is rapidly pulled out from under catch 152 progressing to the end furthest away from the squib. This process can be facilitated by removal of either the tab 153 or catch 152 at the squib end of the airbag module 100. In this manner, the cover 150 is easily released yet retains the airbag 110 during normal vehicle operation. One important feature of the invention is that since the flow out of the high pressure nozzle is supersonic, the pressure rise needed to expel the cover 150 will not affect the flow through the nozzle. This is true as long as the flow remains supersonic which, in the preferred design, is set to permit a ten fold pressure rise to expel the cover 150 over that which should be required. Since relatively little pressure is required to expel the cover 150, if an object is loading the cover 150 in one location in the longitudinal direction of the tube 121, the pressure will be released by flowing out to the sides of the obstruction, i.e., at other longitudinal locations. This design is unique in that the pressure buildup never reaches the point that it will cause injury to an out-of-position occupant. Even if the entire cover 150 is restrained, which is virtually impossible, the cover 150 will release the gas to the sides.

Although the airbag 110 is stored in a compact arrangement as shown in FIG. 2A, when it deploys, the aspiration inlet ports 135 and the converging-diverging nozzle 115 become quite large especially when compared with the size of the high pressure nozzle 117,118,119. It is because of this geometry that very high aspiration pumping ratios are achievable in the invention compared to the prior art. Representative dimensions for the high pressure nozzle are about 0.054 inches for the converging portion 117 of the nozzle, about 0.0057 for the minimum opening or throat 118, to about 0.1 inches at the exit 132 of the nozzle in the diverging portion 119. Representative dimensions for the aspirating nozzle on the other hand are about 1 inch at the inlet ports 135, about 2 inches at the minimum double clearance, i.e., the minimum distance between spring shields 155. The length of the mixing portion of the nozzle is about 2 inches for the illustrated design. It is the ratio of the minimum high pressure gas jet thickness, here about 0.0057 to the length on the mixing channel, here about 2 inches, which is only made possible by the design disclosed herein where the gas jet is very long and thin. The dimensions provided here are illustrative only and the actual dimensions will vary according to the particular application and the particular gas generate used.

FIG. 2D shows the state of the airbag module 100 when the propellant 127 has completed its burning cycle and the pressure has dropped in the module 100 and the springs 145 are acting to move the walls 160 in a direction toward the base 114, i.e., toward their initial position in which the inlet port 135 are substantially closed. The springs 145 do not return the walls 160 completely to their initial position but rather maintain a sufficient opening between base 114 and walls 160 to permit the gases to vent from the airbag 110 as it is loaded by an occupant, i.e., so that gases will flow in an opposite direction through inlet port 135 than the direction of gas flow during inflation of the airbag 110. Springs 145 are typically made from flat strips of spring steel.

In the nozzle, the gas flow initially converges to a very thin cross section which in one preferred design is about 0.005 inches. It then expands becoming supersonic and emerges from the high pressure nozzle as a sheet canted or slanted at an angle with respect to the incoming aspirated air. As shown in FIG. 2C, the gas from the gas generator 120 flows in a direction F2 whereas the gas from the passenger compartment 195 flows from the aspirating inlet ports 135 in a direction F1 which is at an angle a to the direction F2. This interaction between the two planar flows at an angle promotes efficient mixing of the two gas flows as the flow downstream and into the airbag. In most cases, the angles of the two flows can be adjusted in the design to assure this efficient mixing. In some cases, additional measures are implemented such as varying the directions of the combined flows to further promote mixing before the gas mixture enters the airbag 110. The design illustrated in the figures provides ample space for the gas flows to mix after their initial impact.

In accordance with the invention, the length of the gas generator 120 is measured in a horizontal or longitudinal direction perpendicular to the direction of the flow of the gas and is the longest dimension of the device. The length of the nozzle 117,118,119, on the other hand, is measured in the direction of the gas flow and perpendicular to the length of the gas generator 120. A distinctive feature of the inflator module of this invention is that its length is much longer that its width or thickness and the length of the mixing chamber is much longer than the minimum thickness of the high pressure jet. It is this ratio which governs the completeness of the mixing of the gases generated by the propellant 127 and the gases from the passenger compartment 195 which in turn governs the pumping ratio and thereby permits the large pumping ratios achieved here in contrast to the constructions disclosed in the prior art patents discussed above. This is achieved by using a very long and thin jet of high pressure gas which is achieved by the elongate airbag module 100 disclosed herein. It is known in the art that the volume of gas flowing from a gas generator is proportional to the cross-sectional area of the jet times its length, but the ability of the jet to mix rapidly with the aspirating air is determined by the surface area of the jet. By using the thin linear geometry disclosed herein, the ratio of surface area to cross section area is maximized which in turn maximizes the amount of air which can be pumped and thus the pumping ratio. Other geometries can achieve high pumping ratios only by increasing the mixing length. In most implementations, however, this is not practical since there is insufficient space in the vehicle. This is the main reason that current inflators are limited to pumping ratios of substantially less than 1:1. In the case described above, the ratio of the mixing length to the minimum jet diameter is greater than 200:1. In most cases, in the implementation of this invention this ratio will exceed 100:1 and in all cases 50:1. Similarly, the ratio of the length of the gas generator 120 to the minimum thickness of the gas jet in the case described in FIG. 1 and FIGS. 2A-2F is greater than 4000:1. In most cases, this ratio will be greater than 1000:1 and in all cases it will be greater than 100:1.

Referring now to FIG. 2F, it can be seen that the nozzle walls 160 are solid and extend in the longitudinal direction of the tube 121. Similarly, spring shields 155 are connected to the walls 160 over substantially the entire length of the walls 160. However, springs 145 are thin members which are connected only at discrete locations to the walls 160 such that upon release of the springs 145 during airbag inflation, gas from the passenger compartment 195 can flow around and between the springs 145 into the mixing chamber 130. Although two springs 145 are shown, it is of course possible to have a single spring or more than two springs. Further, it is important to note that the length of the gas generator 120 does not have to be the same as the length of the nozzle walls and the module 100.

Further, it is a known property or characteristic of propellants, e.g., propellant 127 situated in the tube 121, that their burn rate is dependent on the surrounding pressure, in this case the pressure in chamber 125 in the tube 121. The gas flow rate out of chamber 125 depends on the flow resistance through the opening(s) 122 and the clearance at the throat 118 between the outer wall of the tube 121 and the base 114. In FIG. 2F, this clearance is nominally set by supporting brackets 170 which are connected to the tube 121 at one end region and to the base 114 at an opposite end region. The brackets 170 are designed to hold the tube 121 at a certain distance from base 114. These brackets 170 can be designed to operate in three different ways: (i) as a fixed support, (ii) as a flexible support, or (iii) as a support which changes with temperature. In general, brackets 170 serve to fix the minimum clearance at throat 118 between the gas generator 120 as a unit and the support base 114.

If brackets 170 operate as fixed supports, then the inflator will have a response which varies with temperature as is the case with all conventional inflator designs. This greatly increases the total amount of propellant which is required as discussed below.

If support brackets 170 are made elastic or flexible, supports 126 are arranged in the converging portion 117 of the nozzle defined by the tube 121 and the support base 114 (FIG. 2E). The brackets 170 then spring load the tube 121 against the supports 126 so that the tube 121 can lift off of supports 126 when the pressure is sufficient to overcome the spring force of supports which act in the compression direction to keep tube 121 close to the base 114. In this case, the clearance at throat 118 between the tube 121 and base 114 increases, which reduces the flow restriction which in turn reduces the pressure within chamber 125 resulting in a nearly constant propellant burn rate and thus an inflator which operates relatively independent of temperature. The supports 126 are in the form of projections which are arranged at a plurality of spaced apart discrete locations between the base 114 and the gas generator tube 121 and fix the minimum clearance for the case where the support brackets 170 are elastic or flexible.

Alternately, the brackets 170 can be made from strips of bimetallic material and, through a proper choice of materials and geometry, the brackets 170 will deform over temperature to vary the clearance at throat 118 also with the ambient temperature in order to increase the clearance at throat 118 with temperature thus reducing the flow resistance and reducing the pressure in chamber 125 with temperature providing partial compensation for the variation in the combustion rate of the gas generated as a function of temperature.

In general, the burn rate of propellants increases with ambient temperature and also with pressure, the so-called pressure exponent. The techniques described here are used to control the pressure in chamber 125 to offset the often uncontrollable changes in ambient temperature. By keeping the pressure in chamber 125 relatively constant through the techniques described above, or even decreasing the pressure in chamber 125 with temperature, the propellant burn rate is kept approximately constant. This serves to reduce the variation in the inflator gas output as a function of temperature. At cold temperatures, when the propellant tends to burn slowly, the clearance at throat 118 will be reduced and the pressure will build up increasing the propellant burn rate until the flow of gas out of chamber 125 is sufficient to relieve the pressure. Similarly, at high temperatures, when the propellant tends to burn at a higher rate, the pressure will be relieved reducing the pressure and slowing down the burn rate. In this manner, the invention provides a sort of self-correcting system for providing a substantially constant propellant burn rate, based on adjustment in pressure in the chamber 125, regardless of the ambient temperature.

Conventional inflators, which do not have this pressure adjusting mechanism, produce higher gas flow rates at high ambient temperature than at low temperature since the resistance to the gas flow rate out of the inflator ports is constant. Therefore, a greater gas generating capacity is required at cold temperatures than at high temperatures and the inflators must be designed with sufficient propellant to handle the cold temperature case. Thus, a larger quantity of propellant is needed for conventional inflators by as much as a factor of two than would be the case in the inflator described above.

A further advantage of the elastic pressure adjustment system described above is that since the inflator nozzle 117,118,119 opens as a function of the pressure in the chamber 125, it would not be possible for the inflator module 120 to explode in a fire which can be a problem in conventional designs. Thus, in general, a device called a "match" or auto-igniter which is used in conventional inflators to start the propellant burning in the case where the vehicle is on fire, for example, is not required for the gas generator described herein particularly when the elastic brackets 170 are used. It is also in general not required since the total amount of propellant used is small and it is distributed along a significant length. Thus, the confinement pressures required for the propellant to detonate do not occur in this design rendering this design inherently safer than the design of conventional inflators.

Different propellants have different rates of combustion which has a significant effect on the geometry of the gas generator. For propellants with slow burn rates, the ratio of the burning surface width to the thickness of the solid propellant will have to increase. For slow burning propellants, therefore, the width of the tube 121 in a direction parallel to the igniter mix 128 may be much larger than the thickness of propellant 127. In other cases, the width of the tube 121 might become significantly less than the thickness of the propellant 127. This design therefore can accommodate a wide variety of propellant chemistries, and particularly those which produce small amounts of toxic gas which have heretofore been unusable, and therefore design is not limited to any particular propellant.

In particular, FIG. 2G illustrates a case where a very slow burning propellant 127 is used and thus a wide thin geometry has been chosen for the inflator module 120. In this case, a tube is not used and the inflator module comprises the propellant 127 which is attached by an adhesive to a piece of formed metal 161. This geometry has the significant advantage of simplicity as can be readily seen in the illustration. It has another advantage in that propellants having slow burn rates can be used which heretofore could not be used. Slow burn rate propellants require very thin structures with a large surface area. If such propellants are formed into tablets as is conventionally done in conventional inflators, they would lack sufficient structural strength and be prone to breakage. If the tablets break, the surface area is increased and the propellant burns faster. This would be an uncontrollable property of these tablets such that the burn rate of various inflators depends on how many tablets break. Since this is unacceptable, there is a practical lower limit for the burn rates for propellants used in conventional inflator designs. This, therefore, restricts that classes of propellants which can be used. This restriction does not apply to the present invention since the propellant 127 is supported by metal piece 161 and therefore can be made very thin without fear of breakage. The metal piece 161 is also provided with appropriate curved sections to define a convergingdiverging nozzle 117,118,119 with the support base 114. Also in this embodiment, a layer of igniter mix 128 may be coated onto the propellant 127.

Although in the preferred implementations of the invention described above, the propellant is placed within the tube 121, in some cases it is desirable to place the gas generator in another location and to use the tube geometry described above to distribute the gas to the high pressure nozzle. Thus, it will be appreciated by those skilled in the art that it is not required to generate gas within the inflator housing tube per se but it is possible to generate the gas in an auxiliary structure and direct the generated gas into the tube which then merely distributes the gas. Such an embodiment is illustrated schematically in FIG. 2H where an inflator 280 is illustrated in a position connected to tube 121 by a conduit through which gas generated in inflator 280 would be conducted into the tube 121. The inflator 280 is shown here for illustrative purposes only and is not meant to indicate the actual size or location of the inflator. Naturally, if a conventional inflator were used, it would be considerably larger than 280. Such an inflator 280 could be placed adjacent to, or in the vicinity of, the tube 121 or at a more remote location with the gas being transmitted to tube 121 through any type of tube. Such an arrangement is particularly useful when the chosen propellant does not burn clean and the effluent must be filtered. Although the inflator now feeds gas to the tube from one end, the remainder of the operation of the module is the same as described above.

As should now be evident, the aspirated inflator of the present invention has significant advantages over other aspirated inflators currently used in conventional airbag systems. In particular, large openings are provided in the form of the inlet slits ports 135 on either side of the aspirating nozzles. These ports 135 permit the flow of gas from the passenger compartment 195 into the aspirating nozzle through ports 135 with a short, low flow resistance, flow path. Once the cover 150 is removed from the airbag system, the module pops open and the inlet ports 135 are open to allow the entrance of the passenger compartment gas. The outlet passage from the inflator passes through a converging-diverging nozzle as described above which causes a smooth developed supersonic flow pattern in the emerging gas. The gas leaves the nozzle 117,118,199 at exit 132 and enters the mixing chamber 130 at a position near the throat of the aspirating nozzle 115 defined by walls 160 and shields 155 after which the aspirating nozzle 115 diverges, see FIGS. 2C and 2D, to create the minimum pressure at the throat to aid in drawing in the maximum amount of gas from the passenger compartment 195 through inlet ports 135. Using this design, total pumping ratios of up to 10:1 of passenger compartment to generated gas result and instantaneous ratios of up to 20:1 have been proven feasible.

The deployment of the airbag 110 is timed so that, as shown in FIG. 2D, after the airbag 110 is fully inflated and the gas generator 120 has stopped generating gas, i.e., all of the propellant 127 has been burned, the occupant begins to press against the deployed and inflated airbag 110. As a result of the impact of the occupant against the inflated airbag 110, the gas in the airbag 110 begins to flow back through the nozzle 115 defined by spring shields 155 and walls 160 and back out into the passenger compartment 195 through the aspiration inlet ports 135, which thus function as outlet ports at this juncture. In view of absence of gas flow from the gas generator 120, the nozzle 115 defined by spring shields 155 and walls 160 is smaller than during the flow of gas from the gas generator 120 and, the support springs 145 pull walls 160 closer to the base 114 thereby reducing the size of aspiration inlet ports 135. By this method and design, the flow resistance of the aspiration inlet ports 135 for this return flow will, by design of the support springs 145, be optimum regardless of the particular propellant used or the particular airbag geometry.

Thus, after the propellant 127 in the tube 121 has finished burning, as a result of a pressure difference between the area proximate and within the gas generator 120 and the interior of the airbag 110, the gas inside the airbag 110 will be caused to flow back through the aspiration inlet ports 135, in a direction opposite to that during inflation of the airbag 110, gradually exhausting the gas from the airbag 110. This only happens, however, when the pressure in the gas generator 120 drops indicative of the end of the burning process. In this manner, the gases produced by the gas generator 120 are always cooled by the aspirated air through inlet ports 135 and there is no need for cooling screens inside of the gas generator 120. If an occupant interacts with the airbag 110 during its initial inflation phase before the cover 150 is expelled, the pressure in the chamber 130 will rise, the flow of gas into the airbag 110 in preparation for deployment will be stopped, and gas from the gas generator 120 will flow out through aspiration inlet ports 135. This construction prevents injury to an occupant who is loading the cover 150, i.e., resting against the same, prior to inflation of the airbag 110 in the manner described above. In current airbag module designs, if an occupant loads the airbag cover or casing, the pressure will continue to build up behind the cover until thousands of pounds of force are available to force the cover open and thereby injure or kill the "out-ofposition" occupant.

An additional advantage to using the aspirating ports 135 as the exhaust ports for an "out-of-position" occupant is that gas does not start flowing out of the airbag 110 until the gas generator 120 stops producing gas. In contrast, in current airbag module designs the gas begins flowing out of the airbag immediately as the airbag is being inflated. The design that is described herein, therefore, conserves inflator gas and permits the use of a smaller amount of propellant in the inflator. The combination of this effect and pressure compensation effect described above can reduce the propellant required to inflate the airbag by a factor of two or more, thus again substantially reducing the size and cost of the inflator and the quantity of toxic gases which are exhausted into the passenger compartment and widening the class of propellants available for use with airbags. The total compartment pressure rise which results from the deployment of multiple airbags is also substantially reduced.

As discussed above, in some situations with conventional inflators, but not with the inflator of this invention, the occupant may be so out-of-position as to be already leaning against the airbag module when the sensor and diagnostic module signals that the airbag should be deployed. This is a particularly serious situation since deceleration of the vehicle may cause the occupant to exert a significant force against the airbag cover preventing it from opening. The inflator module will begin producing gas and if the flow out of the inflator module is resisted, the pressure in the inflator module will increase even exceeding about 1000 psi if necessary until the resistance is overcome and the cover opens. This can result in very large forces against the head or chest of the occupant and result in serious injury or even death.

Several inflator designers and manufacturers are experimenting with variable output inflators where the gas flow from the inflator is reduced. However, this will not solve this problem but only delay the pressure buildup for a few milliseconds with the same eventual catastrophic results. The design used herein eliminates this problem since even though the occupant may load one portion of the cover 150 at one location in the longitudinal direction of the cover 150 preventing it from opening by the release of latch 152, and even if he were able to entirely block the removal of the cover 150, the build-up of gas in the module 100 will cause a slight bulge in the cover 150 causing it to pop free of the base 114 and the gas will begin flowing out through inlet ports 135 as soon as the pressure exceeds a design value which is significantly below that required to oppose the force of an occupant leaning against the airbag module cover 150. Note, however, for many of the preferred mounting locations of the airbag module 100 of this invention, such as on the ceiling of the vehicle, it is very difficult for the occupant to get into a position where he/she is against the module cover 150.

In conventional airbag module designs, the cover is cut open during the deployment process. The expulsion method used here has the advantage of simplicity since no cutting of the material is necessary and it also permits a rapid opening of the aspiration inlet ports which is important to the inflator design disclosed herein. If properly designed, the cover release mechanism requires little force to release the cover and yet is very difficult to detach from outside the module. Thus, the cover is released before significant pressures occur in the module, reducing the danger of deployment-induced injuries to the occupants.

Another preferred embodiment of the invention in which there is sufficient space for the formation of aspirating channels behind the mounting surface on which the airbag module is mounted is illustrated in FIGS. 3A and 3B. Aside from the direction of movement of the airbag module 100 to open aspirating channels, the operation of this embodiment is essentially the same as the embodiment discussed above. However, in contrast to the embodiment in FIGS. 2A-2H, in this case, the gas generator 120 and base 114 are displaceable and are moved away from a mounting surface 190 in one direction during airbag deployment whereas the airbag 110 is deployed in an opposite direction. To enable this relative movement of the gas generator 120 and base 114 relative to the fixed mounting surface 190, the leg portion 160a2 of each of the walls 160 is attached to the mounting surface 190. As shown in FIG. 3A, the base 114 is mounted in engagement with the mounting surface 190 on one side thereof, which is the side to which the walls 160 extend. On the opposite side of the mounting surface 190, the latches 152 are mounted so that the cover 150 is detachably connected to the mounting surface 190 by tabs 153 engaging with the latches 152. Thus, in this embodiment, instead of connecting the base 114 to, e.g., the ceiling of the vehicle as in FIG. 1, the walls 160 are connected to the mounting location, such as the instrument panel or knee bolster structure, of the vehicle and would not be substantially displaced during deployment of the airbag 110. In this context, the instrument panel of the vehicle is defined so as to include the knee bolster area of the vehicle from which airbags used as knee bolsters would be deployed.

In an alternative embodiment, the inflator module 110 can be mounted in a position with the aspirating ports 135 open providing there is sufficient space available and thus, this invention is not limited to the preferred embodiments whereby the inflator module 110 expands on deployment.

One particular application for the design of FIG. 3A is for use as a knee protection airbag as shown in FIG. 4. FIG. 4 illustrates a preferred embodiment of the present invention used as a knee protection device for the front driver and passenger occupants. The airbag module is shown deployed generally at 200 in FIG. 4 and includes an airbag 210. The airbag 210 is designed to interact with the driver's knees, not shown, and with the body of an occupant who may be lying down, also not shown. In this manner, the airbag 210 protects not only the knees of the driver from injury but also protects a child lying on the front seat, for example.

Knee airbags have heretofore been commercially used only as part of the front passenger airbag system where they have been inside of and in conjunction with the passenger airbag and inflated by the passenger airbag inflator. Current front passenger airbag systems are all mid or high mount systems where it is no longer convenient to mount a knee airbag inside of the passenger airbag or to use the same inflator. The exemplifying embodiment shown in FIG. 4 uses a separate airbag system with its own inflator. This is only now made practical by the low cost efficient airbag module design disclosed herein.

FIG. 5 illustrates another possible mounting location for the airbag module, shown generally at 300, in accordance with the invention and including a deployable airbag 310. In this embodiment, the airbag 310 will be deployed from the ceiling in the event of a side impact of sufficient severity that an occupant's head would otherwise be injured. This implementation is significant since the airbags for the front and rear seats are combined which results in significantly greater protection in side impacts when the windows are broken. The airbags are less likely to project outside of the windows if they are restrained by the B-pillar 320 and other vehicle structures such as the A-pillar 322 as shown in FIG. 1. This support is achieved since the module extends forward almost to the windshield and is mounted adjacent to but somewhat away from the side of the vehicle. When the airbag deploys, therefore, it is partially restrained by the A-pillar further aiding the retention of the occupant's head within the vehicle.

As discussed above, the steering wheel and steering column are among the most dangerous objects in the automobile. Even with airbags and seatbelts many people are still injured and killed by an impact with a steering wheel rim, spokes or hub, or by the airbag as it is deployed from the steering wheel hub. The steering column also significantly interferes with the operation of many knee bolster designs causing significant leg and knee injuries. With today's technology, neither the steering wheel nor steering column are necessary and only bring harm to drivers. Naturally, a substantial educational program is necessary to wean people away from the false feeling of security of a substantial steering wheel and steering column. However, if it can be shown to the population that a vehicle with a servo electronic steering system (called steer-by-wire) is considerably safer, then the battle can be won. Such a system is common in commercial aircraft although a steering wheel is still usually used.

One implementation of a driver protection airbag system which is deployed from the ceiling 490 coupled with a steer-by-wire system is illustrated at 400 in FIG. 6 which is a partial cutaway view of a driver in an automobile. The conventional steering wheel and column have been eliminated from this vehicle and a thinner, lighter steering wheel 460, which projects from an instrument panel 470, is provided instead. The steering wheel 460 is attached to a deformable column 480 which is supported by the instrument panel 470. In the event of an accident, the steering column 480 easily bends at the connection point 482 with the instrument panel 470 and permits the steering wheel 460 to be displaced or to be rotated out of the way, i.e., to make room for the deploying airbag as illustrated in FIG. 7. In this embodiment, a single airbag 410 is deployed downward from the ceiling 490 to protect all occupants in the front seat of the vehicle, and thus is elongate extending substantially across the entire width of the passenger compartment of the vehicle.

If some energy absorption is desired, the steering wheel support 480 can be made in the form of an elastic spring which has the property that it will provide a nearly constant force versus deflection relationship which can be designed to aid the energy absorption of the airbag. The steering wheel and support in FIG. 6 is shown with the airbag wrapped around which somewhat reduces the energy absorption effects of the airbag. Other implementations are to pull the steering wheel into the instrument panel space using pyrotechnics or to release the support so that the airbag itself moves the steering wheel out of the way. Naturally, if alternate steering systems can be sold to the public, the steering wheel and support can be eliminated entirely and replaced by a device mounted onto or between the seats or on the floor, for example. Many other steering systems which do not interfere with the airbag will now be evident to those skilled in the art.

FIG. 7 illustrates the positions of front and rear seat airbags as well as of the steering wheel and steering support after the airbags have deployed and the occupants have begun moving forward.

FIG. 8 illustrates another implementation of the airbag module of this invention. In this case the module 510 is mounted to the rear of the front seats 525 of the vehicle and is designed for those cases where a ceiling mounted system is not desired or practical. In this case, two airbag modules 510 are provided, one in the back of each of the front seats 525. Naturally, a single airbag can be used for vehicles with bench seats.

A primary advantage of the linear, elongated module disclosed herein is that it can be mounted on the surface of the ceiling, instrument panel, seat back, or other appropriate surface. In some cases, the module is literally attached to the mounting surface while more commonly it is recessed so that the surface of the module is approximately flush with the surrounding surfaces prior to deployment. In most cases, however, the depth of penetration into the mounting surface will be small and less than 1/5 of the module length and in most cases less than 1/10 of the module length. For the purposes of this disclosure, therefore, mounting on a surface will mean mounting so that the penetration into the surface will be less than 1/5 of the module length.

The preferred material for the gas generator housing of this invention is steel in order to withstand the pressure and temperature from the burning propellant. It is possible to make the inflator housing from a high temperature plastic, however, the propellant tube in this case will be considerably thicker. Plastic can be used in the inflator of this invention since the propellants generally used burn completely in a very short time period. Thus, there is little time for the heat to penetrate into the plastic housing. For this same reason, the inflator of this invention can be mounted adjacent to combustible materials without fear of starting a fire.

In the preferred embodiment illustrated in FIGS. 1 and 2 of the airbag module of this invention, the length of the module was approximately the same as the length of the airbag. This permits the airbag, especially if made of film, to be easily rolled or folded with the portions of the airbag which project beyond the module easily accommodated without the special endwise folding required in conventional inflators. This results in a uniform geometry and symmetry for the airbag module and permits the module to be easily made in any convenient length. Additionally, the long thin design permits the module to be bent somewhat so as to conform to the surface of the location where it is mounted. This geometry also permits the airbag to unfold much more easily and in considerably less time than with conventional designs. Thus, the airbag system of this invention can be deployed in less time with less force and thus with less danger of deployment induced injuries than with conventional designs.

In a particular example used in this application, the cover is mechanically pushed off by the expansion of the airbag, or the displacement of the module, progressively from one end to the other much like a zipper. In other applications it may be required to pyrotechnically cut or eject the cover which would require separate pyrotechnic devices. Also, in the examples illustrated herein, the module cover has been pushed off and removed from the module. Although this is the preferred method, other designs could remove the cover by cutting an opening in the material which covers the module. In such cases, the existence of the module could be completely hidden through the use of a seamless covering and only cut open when it is required for deployment of the airbag. For the purposes herein, therefore, removal of the cover will include any method by which an opening is provided to permit the airbag to deploy.

The propellant and gas generator assembly has been shown with an approximate rectangular cross section so that once the propellant begins burning the surface area neither decreases or increases as the inflator propellant is consumed. In some cases, it may be desirable to vary the burn rate of the propellant by changing the surface area which is burning. If the cross section area of the inflator, and thus of the propellant, were made triangular, for example, with a wider base and narrower top, the rate of gas generation would increase as the propellant bums. Conversely, if the base of the propellant were narrower then the top, the opposite would occur and the propellant will begin burning fast and slow down with time. Naturally, the shape of the inflator housing can be infinitely varied to achieve any reasonable variation in propellant burn rate with time desired. For complicated shapes it is necessary to cast the propellant in place in the tube which also helps the propellant to adhere to the surfaces of the gas generator housing.

It is also noted that U.S. Pat. No. 5,060,973 discloses the use of a liquid propellant patent. Central to this patent is the method of injecting the liquid from its container into a combustion chamber. A liquid propellant has an important advantage that many such propellants, and the particular one disclosed in this patent, burn without producing solid particles which could clog the high pressure nozzle or burn holes in a film airbag. The purpose of the injection system is to control the burning rate of the fuel. In solid fuel inflators, this is done by shaping the surface of the propellant as discussed above.

The burning surface can also be controlled in the geometry of the inflator in accordance with the present invention by increasing the viscosity of the liquid through an emulsifying process or, alternately, by placing a solid matrix within the tube which is non-combustible such as one made from glass fibers. These fibers therefore serve to hold the liquid propellant in a position where the burning surface area is known and thus the burning rate controlled. Naturally, other methods of controlling the liquid burn rate without resorting to an injection system will now become apparent to those skilled in the art.

In addition, the liquid propellant can be used in a separate inflator housing such as disclosed in the discussion above with reference to FIG. 2H. This invention is not limited to the use of liquid or solid propellants but also contemplates the use of stored gas, hot gas, hybrid or other designs.

A perspective view of a preferred cover design is shown generally as 150 in FIG. 9. It comprises a semi-rigid molded plastic backing material 542 covered with a foam 544 and skin 546 or other combinations of materials to be compatible with the vehicle roof liner, instrument panel or other mounting location. Tabs 153 are placed in the cover to interlock with corresponding catches 152 on the module housing base. When pressure builds beneath the airbag, it causes the airbag to bulge pulling tabs 152 out of engagement with catches 152 in the progressive manner as described above. The cover is then free to move and will in general be projected downward by the deploying airbag. It has a low density, however, and will not cause injury even if it impacts an occupant.

In the example shown in FIG. 9, the cover 150 can be attached to the airbag by an adhesive or other suitable means. When the airbag deploys the cover adheres to the airbag on the side away from the occupant thereby generally preventing interaction between the occupant and the cover.

With the development of the film airbag as described in the copending patent applications referenced above, and the inflator design described herein, a very thin airbag module is possible which can be made in any length. Typically the module length will exceed 10 to 20 times the width or thickness of the module, and in all cases at least 5 times. The length of the gas generator will typically be 40 to 80 times its thickness and in all cases at least 10 times. This shape permits the module to be easily mounted in many locations and to be bent or curved to match the interior shape of the vehicle. For example, one could be positioned so as to conform to the ceiling to protect rear seat occupants. Another one could stretch the length of the car on each side to protect both front and rear occupants from head injuries in side impacts. A similar system can be used for a deployable knee bolster, and eventually a single module can be used for both the passenger and driver in frontal impacts when used in conjunction with an servo electronic steer-by-wire system, for example. With the economies described above, airbags of this type would be very inexpensive, perhaps one-fifth to one-tenth the cost of current airbag systems offering comparable, or inferior, protection.

The airbags described herein would be easily and inexpensively replaceable. They would require only a single connection to the vehicle safety system. Although the bags themselves would not be reusable, in some cases the airbag covers could be.

The designs illustrated herein are simple and because of their small cross-section can be easily mounted to conform to interior vehicle surfaces. They are very efficient, in some cases requiring less than 1/10 of the amount of propellant which is required by competitive conventional systems. The particular designs are also easily manufactured. Since they use less propellant, the noise and problems with high pressures when multiple airbags are deployed are greatly reduced. They also offer protection in cases such as the sleeping child which has heretofore not been available. These designs as disclosed herein, therefore, provide all of the objects and advantages sought.

Although several preferred embodiments are illustrated and described above, there are possible combinations using other geometries, materials and different dimensions for the components that perform the same functions. This invention is not limited to the above embodiments and should be determined by the following claims. For example, although the airbag is described as being a film airbag, it must be understood that this is only a preferred embodiment and that the airbag can be made of any other material, even though this may detract from efficient operation of the airbag module. Lastly, a variety of different processes for inflating an airbag so as to provide a high pumping ratio, i.e., a high ratio of gas from the passenger compartment to pressurized gas from the inflator means, as well as to achieve the numerous objects mentioned above, are also within the scope of the invention disclosed herein.

Gas at high pressure and temperature is generated in the chamber of the inflator. This gas flows through a convergent-divergent nozzle, to emerge near the throat of a larger convergent-divergent nozzle. Both nozzles are two-dimensional and symmetric about the center plane. Passenger compartment air enters the larger nozzle and begins to mix with the inflator gas where the inflator gas emerges from its nozzle. At the end of a mixing section the two streams are substantially uniformly mixed. The mixing section is followed by a divergent section where some of the velocity of the mixed stream is converted back into pressure sufficient to fill the airbag.

The inflator gas emerges from its nozzle as a supersonic jet at high speed and relatively low static pressure and temperature. As the jet mixes with the surrounding air it entrains this air, imparting some of its high kinetic energy to the combined stream. The techniques required for analysis of this process are contained in the book The Theory of Turbulent Jets by G. N. Abramovich (The M.I.T. Press, Cambridge Mass., 1963). The basic technique is to reduce the full three- or four-dimensional equations of transient fluid flow by integrating them over the cross-section. This is supplemented by additional approximations justified by theory and experiment.

The equations of fluid flow may be expressed as conservation of mass, momentum, and energy. Let

x be the distance along the central plane from the exit of the inflator gas nozzle, in the direction of flow,

y be the distance from the central plane,

b be the value of y at the boundary of the jet,

C.sub.p be the fluid heat capacity at constant pressure,

u be the time-averaged fluid speed in the x-direction,

p be the fluid static pressure,

T be the (absolute) static temperature of the fluid,

h be the fluid enthalpy,

r be the fluid mass density,

R be the gas constant,

g be the specific heat ratio of the fluid,

m be the fluid viscosity,

k be the fluid thermal conductivity,

mc/ be the mass flow rate per unit width (per unit z) of the inflator,

qc/ be the heat input rate per unit width of the inflator nozzle,

n be the aspiration ratio, the ratio of the mass flow rate of air to the mass flow rate of the inflator gas,

f be the fanning friction factor,

h.sub.e be the heat transfer coefficient, based on the adiabatic saturation temperature,

Pr be the Prandtl number,

Re be the Reynolds number,

St be the Stanton number.

The following subscripts are used:

m for the central plane (inside the jet),

H for the fluid outside the jet (air),

w for the fluid adjacent to the wall of the outer nozzle, or for the wall of the inner nozzle,

0 for the cross-section where the supersonic inflator gas jet emerges (the beginning of the mixing section),

2 for the end of the mixing section,

3 for the exit from the air nozzle (airbag entrance),

t for total (stagnation) properties,

a for ambient conditions, inside the vehicle,

i for pressure and temperature inside the inflator, before expansion,

* for the throat of the convergent-divergent inflator exit nozzle,

as for adiabatic saturation (temperature),

ref for properties or numbers evaluated at the reference temperature, for fluid properties inside the airbag.

Note that b.sub.0 is the half-thickness of the supersonic inflator gas nozzle at its exit.

Then after integrating over the cross-section the conservation equations for steady flow become ##EQU1##

The approximations have been made that the conditions (velocity, temperature, pressure) in the two streams are each substantially uniform at the cross-section where they meet,

that time-averaged fluid velocity components in the cross-directions are much smaller than in the axial direction, and that the friction and heat transfer at the wall in the outer nozzle may be neglected. These approximations may be relaxed, and will be in later studies, but should have a secondary effect on the results of interest.

Abramovich provides expressions for the velocity and total temperature profiles in the jet in terms of the values on the mid-plane and in the surrounding air. These can be substituted in the conservation equations, along with expressions for the air properties in terms of the pressure, to provide 3 equations for the axial distribution of mid-plane velocity, mid-plane temperature, and pressure. This calculation will be necessary to determine the effect of the finite length of the mixing section and to find the optimum length. An approximation to the overall performance of the aspirator can be found more simply if the flow is assumed to be fully mixed and substantially uniform at the end of the mixing length. Further approximations in this preliminary analysis are that in the air nozzle and in the mixing section the fluids are perfect gases with constant specific heats, that the two streams have the same properties, those of air, ##EQU2## that the air flow up to the point where mixing begins is isentropic, so ##EQU3## and that y.sub.w is constant in the mixing region, from x=0 to the section where the flow is substantially uniform again, say at section 2. Then the 3 conservation equations may be written

If the gas temperature in the inflator is relatively low, say less than 2000.degree. F., then equations similar to Eqs. (6) may be used for the inflator nozzle in the preliminary analysis, without introducing errors that cannot be adjusted for later. Sufficient aspiration can be achieved so that the airbag temperature is not excessive. When the inflator temperature is much higher, though, then the inflator gas must be cooled. The inflator gas exit nozzle is, as will be seen, quite thin (small b), and as a result the heat transfer, and, to a lesser extent, the wall fluid friction is significant unless the nozzle is very short. Thus the nozzle is a natural place to cool the hot gases from the inflator.

Flow in the nozzle will be assumed to be turbulent. The fluid flow and heat transfer equations are taken to be (see, for example, F. Kreith, Principles of Heat Transfer, 3rd ed. (Harper & Row, New York, 1973), Chapters 6 and 8, and M. Jakob, Heat Transfer, vol. 1 (John Wiley, New York, 1949), . . . ##EQU4## Some of these may be combined to yield the following equations for the rates of change of velocity and total temperature ##EQU5## The temperature dependence of the specific heat of air was taken from NACA Report 1135, Equations, Tables, and Charts for Compressible Flow (U.S. Govt. Printing Office, Washington, 1953) p. 15 (thermally perfect); the temperature dependencies of the viscosity and thermal conductivity were found by fitting Sutherland-type equations (S. Chapman & T. G. Cowling, The Mathematical Theory of Non-Uniform Gases (CUP, Cambridge, 1961), pp. 224 and 241) to data from Kreith p. 636: ##EQU6## Now suppose that the pressure and temperature in the inflator, p.sub.i and T.sub.i, and the ambient pressure and temperature, p.sub.a and T.sub.a, are specified, along with the airbag pressure p.sub.bag, the desired airbag temperature T.sub.bag, the airbag inflated volume V.sub.bag, the airbag fill time, and the width of the inflator nozzle. The airbag pressure, temperature, volume, and fill time allow the total mass flow rate m to be calculated, and dividing this by the width gives the required mc/. If heat loss from the inflator gas is ignored then the required aspiration ratio is given by ##EQU7## If the inflator temperature is too high, then the inflator exit nozzle may be designed to achieve some exit total temperature T.sub.mt0 and the aspiration ratio becomes ##EQU8## Then m.sub.m c/=mc//(1+n) and m.sub.H c/=n m.sub.m c/.

The inflator exit nozzle may be designed to produce any reasonable pressure p.sub.m0 at its exit, regardless of the air pressure at that point.

When the inflator temperature is relatively low, the inflator exit nozzle design is approximated by assuming isentropic flow. Equations similar to Eqs. (6) are used to calculate the density and velocity of the inflator gas at the nozzle exit, section 0. When the inflator temperature is high the inflator nozzle is designed by integrating Eqs. (9) numerically, using the relations of Eqs. (8) and (10). For this the nozzle wall temperature, and an entrance loss factor for the pressure drop, must be specified. The shape was determined by requiring a constant rate of pressure drop with axial distance, until the divergence angle reached a preset maximum and then the divergence angle was held at that value. Different pressure drop rates were tried until the desired exit total temperature was reached. This procedure produces the inflator gas density and velocity at the exit, along with the total temperature.

An air pressure at the start of the mixing section, p.sub.H0, may be chosen. Equations (6) then allow T.sub.H0 and u.sub.H0 to be calculated, along with the air nozzle size y.sub.w -b.sub.0.

Now Equations (7) may be used to find the properties at the end of the mixing section. The first of Equations (7) allows the product r.sub.2 u.sub.2 to be calculated, and the third of Equations (7) yields ##EQU9## and when this is used in the second of Equations (7) that equation becomes a quadratic in u.sub.2 with all other quantities known. The two roots of this quadratic correspond to the two possibilities of either subsonic or supersonic flow at section 2. In practice the flow here will be expected to be subsonic (that is, shocks will occur in the supersonic stream). Once u.sub.2 is found, p.sub.2 is calculated with Equation (13) and p.sub.2t with ##EQU10## When the gas mixture comes to rest in the airbag its temperature will be T.sub.bag =T.sub.2t (neglecting heat lost to the airbag material).

In the divergent portion of the main nozzle, after section 2, some of the kinetic energy at section 2 is converted into an increase in static pressure. If y.sub.w3 is the half-thickness at the exit from the divergent portion, and p.sub.3 the pressure at the exit, then ##EQU11## p.sub.3 will be the airbag pressure p.sub.bag.

If p.sub.bag and y.sub.w3 are specified then the last series of calculations may be performed for different p.sub.H0 until the correct value is found for p.sub.H0. This will be the value that causes Equation (15) to be satisfied. Alternatively, if p.sub.2t is greater than p.sub.bag, then the necessary value of y.sub.w3 may be computed.

When Equations (9) are integrated numerically then the thickness at the minimum section of the inflator nozzle will be evident. When the gas flow in the inflator nozzle is assumed to be isentropic, then the half-thickness at the throat of the convergent-divergent inflator exit nozzle is given by ##EQU12##