This invention relates generally to a vehicle airbag collision protection system, and more particularly, to a novel inflator and gas-generating propellant useful in such systems. The invention also includes a method for igniting gas-generating propellant charges.

BACKGROUND INFORMATION

Air bag collision protection systems for motor vehicles generally utilize folded bags mounted on the steering wheel, dashboard or doors ofthe vehicle, which bags are rapidly inflated when the vehicle is involved in a collision, thereby protecting the driver and/or passenger(s). The airbag is inflated by a gas generator, or inflator, which in turn is activated by a signal from a collision sensor.

The inflator contains one or more gas generating materials, or propellants, typically in solid form. When ignited, the propellant generates gas, which inflates the airbag. Although the propellant is combustible, to initiate and accelerate ignition of the propellant, most inflators utilize ignition enhancing materials associated with the propellant, including, e.g., a pyrotechnic "ignition cord". The ignition enhancing material is in turn typically ignited by an "initiator", usually containing an explosive pyrotechnic material, which is activated by an electrical signal from the collision sensor.

The airbag must inflate rapidly enough, and to a sufficient extent, under a wide range of conditions, including, e.g., different temperature extremes, in order to protect the occupant from the effects of a collision. The optimum rate of airbag inflation can vary depending on the type of vehicle and its collision profile, and whether the airbag is designed for the driver or the passenger, or whether it is designed for a side-impact collision. For example, it is advantageous for a passenger side airbag to have a "soft- onset" inflation, i.e., to have gradual initial pressure rise, or inflation, followed by a more rapid inflation, in order to minimize risk of injury from an airbag expanding too rapidly into an out of position passenger, in particular, a child.

Various approaches have been used in the prior art to achieve controlled, rapid ignition of gas-generating propellants in airbag inflators.

Cunningham U.S. patent 4,950,458 utilizes a "rapid detonating cord", or ignition cord, further enhanced by an associated booster charge to achieve rapid ignition of propellant material.

Garner, et al., U.S. patent 4,246,051 relates to an ignition enhancer coating for propellants.

Chan, et al., U.S. patent 5,351,619 relates to a pyrotechnic ignitor film layered over a propellant, or alternatively, inserted into the central apertures of propellant disks so configured.

Pietz U.S. patent 4,758,287 relates to a porous propellant disk with a central cylindrical core designed to encase an igniting enhancer and a process for preparing the propellant disk.

Bender U.S. patent 5,101,730 relates to propellant disks containing a central aperture, arranged in a column within a combustion chamber so that an ignition channel is defined by the apertures. Bender requires outlet channels on the top and underside ofthe disks extending radially from the central aperture to the disk rim. All of these approaches to solving the problem of obtaining uniform, rapid combustion of propellant disks utilize ignition enhancing materials, in addition to an explosive initiator, to accelerate the combustion ofthe propellant.

The development ofthe field of airbag safety restraint systems has been driven, in part, by the need to increase the reliability and reduce the cost ofthe systems, so they could become standard, affordable safety items in a wide range of motor vehicles. The gas generator or inflator is typically the largest component of an airbag collision protection system. Therefore, an important factor influencing the cost and vehicle design compatibility of an airbag system is the inflator size. A smaller inflator also minimizes the vehicle design modifications needed to utilize the airbag system.

Thus, there is a need and a demand for an improved airbag inflating system that will provide desireable inflation characteristics and will be of maximum compactness. There is also a need and demand for an improved airbag inflating system with inflation parameters that can be easily manipulated depending on the proposed location ofthe airbag and the type of vehicle it will be installed in, without requiring extensive production modifications.

SUMMARY OF THE INVENTION

According to an embodiment ofthe present invention, there is provided an inflator for emitting gas into a vehicle airbag, utilizing propellant disks containing a central aperture. The propellant disks are aligned adjacent to each other in a column within a combustion chamber so that the central apertures ofthe disks form a central, axially symmetrical ignition channel extending through the propellant column. The diameter ofthe ignition channel is such that activation of an associated initiator propagates an initiating explosion substantially instantaneously along the ignition channel, resulting in a concomitant, substantially instantaneous ignition ofthe propellant disks. A preferred ignition channel diameter is about 1-5 millimeters.

This propellant design and its configuration in the inflator eliminates the need for an ignition cord or similar ignition enhancer to ignite the propellant (although one can be used, if desired), and permits a smaller inflator which is cheaper and safer to manufacture, while at the same time, maintaining or reducing the time to first pressure performance characteristics ofthe inflator and enhancing the ability to custom tailor inflation parameters depending on the type of vehicle or the location ofthe airbag. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding ofthe invention and its advantages, an embodiment ofthe invention is described in the accompanying drawings and detailed description ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, in cross section, illustrating the geometry of one embodiment of applicants' novel propellant.

FIG. 2 is a top view ofthe disk shown in FIG. 1. FIG. 3 is a side view, in cross section, illustrating one embodiment of an inflator assembly employing applicants' propellant disks.

FIG. 4 is a side view, in cross section, illustrating applicants' current commercial inflator assembly utilizing a standard ignition propellant cord.

FIG. 5-7 are "Tank" curves comparing the performance ofthe embodiment of applicants' novel inflator depicted in FIG. 3 to applicants' commercial inflator depicted in FIG.4, at temperatures of -40°C, 23 °C and 80°C, respectively.

FIG. 8 is a top view of an automobile with applicants' novel inflator assembly.

DETAD ED DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE OF THE INVENTION

FIGS. 1-2 show one embodiment of applicants' novel propellant. This embodiment shows applicants' propellant in the form of a disk. The shape of the propellant can be otherwise configured if desired. The diameter ofthe central aperture 2 of applicants' disk 1 can vary depending on the desired combustion speed and propellant composition and is sufficiently narrow to cause substantially instantaneous propagation of an initiating explosion along the ignition channel formed by the apertures when the disks are stacked in a column in a combustion chamber.

"Substantially instantaneously" is used here to mean rapidly enough to ignite the propellant disks so that an airbag inflation rate is achieved which is suitable to help protect the vehicle occupants from the effects of a collision.

The preferred central aperture diameter is about 1-5 millimeters in diameter, and most preferably, about 3.18 millimeters. The outside diameter 3 of the disk will also vary depending on desired combustion speed, but preferably is about 7-15 millimeters. As illustrated in FIG. 1 , in its preferred embodiment the propellant disk has a "stepped" configuration, so that the central portion 4 ofthe disk is thinner than its outer portion 5, creating air spaces within the propellant column. The height, or thickness 6, ofthe outer portion ofthe disk may vary depending on, e.g., desired combustion speed, but preferably is about 1-10 millimeters. The design of applicants' propellant disks, and their configuration within applicants' inflator eliminates the need for radially extending outlet channels in the disks, although those could be included if desired. Applicants' novel propellant can be composed of a variety of gas-generating compositions having the desired biαrning rate, non-toxicity and flame temperature. A preferred gas generating composition contains about eighty-five weight percent sodium azide and about fifteen weight percent potassium perchlorate. The propellant

disks are manufactured conventionally, such as, pressmolding in dies corresponding to the desired configuration ofthe disks.

FIG. 3 shows an inflator assembly 7 according to the invention. The inflator assembly includes a generally cylindrical casing or housing 8, enclosing a combustion chamber 9. The combustion chamber 9 has a first end 10 and a second end 11 closed in a sealing manner. A plurality of apertures or ports 12 are formed on the housing 8 for discharging gas from the inflator. A generally cylindrical screen 13 is positioned adjacent to the inner wall ofthe housing 8, which screen filters and cools the gas generated by the inflator. A layer of rupturable aluminum foil 14, or other suitable material is provided between the housing 8 and the screen 13, which foil hermetically seals the inflator against moisture and other potentially deleterious constituents ofthe environment to which it is exposed. The foil 14 is designed to rupture when the internal pressure of the inflator increases due to the combustion ofthe propellant disks contained therein. A generally cylindrical ignition tube 15, with a first end 16 and a second end

17, is centrally positioned within the annular space ofthe combustion chamber 9, aligned axially to the housing 8. Stacked within the ignition tube 15, are a plurality of applicants' novel propellant disks 1. The propellant disks 1 are arranged adjacent to each other in a column, such that the central apertures ofthe disks form a relatively linear ignition channel 18. A retainer 19 is located at the second end ofthe ignition tube 15 to hold the propellant disks 1 in place in the ignition tube. The ignition tube 15 contains a plurality of ports or apertures (not shown), for discharging gas from the propellant contained therein.

An electroexplosive ignition device, or initiator assembly 20, such as that manufactured by OEA, Inc., P.O. Box 10488, Denver, Colorado 80210, is mounted at the first end ofthe combustion chamber, centrally thereof, and is held in place by closure 21. The initiator 20 includes a conventional electric squib internally connected with a resistance wire and associated explosive ignition material, and is electrically connected by wires 22 to a collision sensor (not shown). The initiator 20 is arranged at the first end 16 ofthe ignition tube 15, and in operative relationship with the propellant disks 1, contained therein.

An auto ignitor 23 is disposed centrally at the second end ofthe combustion chamber, under the retainer 19. In the event of a fire, either during storage or shipping, or when installed in the vehicle, the auto ignitor 23 is designed to ignite and burn the gas generating materials and any additional pyrotechnic material in the inflator at a temperature which is lower than the natural ignition temperatures of these materials, to prevent an explosion. A preferred autoignition compound is comprised of about fifty-eight weight percent potassium perchlorate, about twenty-one weight percent 5-aminotetrazol and about twenty-one weight percent 2,4-dinitrophenyl- hydrazine. Also contained within the combustion chamber 9 ofthe inflator, and placed around the ignition tube 15, are a plurality of secondary propellant pellets 24. In the embodiment shown, the secondary propellant pellets 24 are "randomly packed" inside the combustion chamber 9, as opposed to being axially aligned and stacked in a column, as with applicants' novel propellant disks 1 contained within the ignition tube 15. The secondary propellant pellets 24 are held in place within the combustion chamber by spacer element 25. The secondary propellant pellets 24 can be composed of a variety of gas generating compounds. In one embodiment of applicants' invention, the composition ofthe secondary propellant 24 is selected to burn more slowly than applicants' novel propellant disks 1. The secondary propellant 24 is preferably comprised of about sixty-two weight percent sodium azide, about thirty-six weight percent iron oxide and about two weight percent potassium nitrate. The secondary propellant can also manufactured conventionally, usually by pressmolding. Functioning ofthe inflator 7 begins with an electrical signal from the collision sensor to the initiator 20. This signal causes the explosive material in the initiator 20 to detonate. The detonation propagates substantially instantaneously along the ignition channel 18 formed by the internal apertures ofthe central propellant disks 1, igniting the propellant disks. Without being bound by any theory, it is believed that the narrow diameter ofthe ignition channel 18, preferably about 1-5 millimeters, channels the initiating explosion into a supersonic Shockwave that allows for substantially instantaneous propagation of heat and energy along the length ofthe central propellant disks 1 from the point of initial ignition, eliminating the need to

utilize an ignition cord or other additional ignition enhancing material to achieve that result.

As the hot gas generated by the ignition ofthe central propellant disks 1 passes through the ports ofthe ignition tube 15, it ignites the secondary propellant 24. The gas passes through the filter screen 13, and when the pressure within the combustion chamber 9 reaches a threshold value, the foil 14 ruptures, discharging gas into, and beginning the inflation ofthe airbag.

FIG. 4 shows the previous commercial embodiment of an inflator 26 utilized by applicants in their airbag collision protection systems. This inflator comprises a combustion chamber 27 with a perforated central ignition tube 28 filled with propellant 29, with secondary propellant 30 packed around the ignition tube. The preferred chemical compositions ofthe two propellants are the same as those utilized in applicants' novel gas generator. However, as illustrated in FIG. 4, the central propellant 29 is not axially aligned but is instead randomly packed around an ignition cord 31, in operative relation with an initiator 32.

The functioning ofthe inflator 26 depicted in FIG. 4 also begins with an electrical signal from a collision sensor to the initiator 32. The signal causes the explosive ignition material in the initiator 32 to detonate. However, the propellant 29 in the ignition tube 28 is not configured to form an ignition channel to propagate the igniting detonation. Instead, the initiator 32 ignites the ignition cord 31 which in turn ignites the central propellant 29. The hot gas generated by the ignition ofthe central propellant 29 ignites the secondary propellant 30.

FIGS. 5-7 graphically illustrate a comparison ofthe performance at three different temperature extremes, ofthe FIG.4 inflator (depicted as a solid line), and the gas inflator according to the embodiment ofthe invention depicted in FIG. 3 utilizing applicants' novel propellant disks (depicted as a dashed line). The curves show the pressure rise in KiloPascals in a 60 liter test tank when the inflator is activated, plotted against time in milliseconds. "Time to first pressure" is the elapsed time between initiation or activation ofthe inflator, and the first registration of pressure in the tank, and is illustrated in FIGS. 5-7 as the beginning ofthe curve. Time to first pressure is an important indicator ofthe performance characteristics of an airbag inflator.

Generally, the shorter the time to-first pressure, the more reliable the inflator system will be in timely inflating the airbag after a collision, especially at low temperatures.

As illustrated in FIGS. 5-7, even though applicants' novel gas generator does not utilize an ignition cord or other ignition enhancer to ignite the central propellant, its time to first pressure is significantly shorter than that ofthe standard inflator shown in FIG. 4, even during cold ignition. As illustrated in FIG. 5, at a temperature of - 40°C, applicants' novel inflator achieves a time to first pressure of about 4 milliseconds, as compared to about 8 milliseconds for the FIG. 4 inflator using an ignition cord. Comparable time to first pressure enhancement at ambient and elevated temperatures is shown in FIGS. 6 and 7.

FIG. 8 shows an automobile 33 containing applicants' novel inflator 7. Removing the need for an ignition enhancer, such as an ignition cord, from an inflator, results in a substantial cost saving per unit. The configuration of applicants' propellant disks in the ignitor tube also results in a more efficient reaction allowing a reduction in the amount ofthe propellant contained therein, while still maintaining the same overall gas output. Thus, applicants' invention provides an inflator which can provide desired inflation characteristics with minimal ignition material per volume of gas generant. Applicants' inflator also exhibits enhanced time to first pressure per volume of ignition material. Additionally, because of high packing density achieved by stacking the central propellant as opposed to random packing, the embodiment of applicants' novel inflator shown in FIG. 3 is 210 mm in length, compared to 254 mm for the FIG. 4 standard inflator, a more than 13% reduction in length. Furthermore, applicants' design eliminates the need for outlet channels on the propellant disks to achieve adequate gas conductance into the airbag, further decreasing production costs. Still another advantage of applicants' inflator system follows from the improved ability to vary the onset and rate of inflation. This ability is important to allow for tailoring an airbag collision protection system depending on whether the system will be utilized to protect the driver, a passenger, or for protection from a side impact. The preferred onset and rate ofinflation can also vary depending on the particular collision profile or "crash pulse" ofthe vehicle. The onset and inflation rate of a particular inflator is graphically illustrated by the shape ofthe "tank" curves

shown in FIGS. 5-7. Varying the amount of gas produced in the early part ofthe propellant burn to that produced in the later part ofthe burn will change the curve shape and vary the inflator's onset and inflation rate.

Applicants' novel inflator allows a manufacturer to easily modify the curve shape to fit a particular inflation requirement without the need to vary the amount of an ignition enhancer or to vary the propellant composition, and without substantially reconfiguring the inflator design. Applicants' preferred embodiment, as described in FIG. 3, allows extensive flexibility in curve shaping simply by varying the ratio ofthe fast burning central propellant to the slower burning main propellant. Extensive curve shaping can also be achieved by "ballistic tailoring", i.e., by varying reaction velocity ofthe propellants utilized in applicants' inflator by changing the thickness or diameter ofthe propellant disks and/or by varying the diameter ofthe propellant ignition channel, to vary the burn surface area. The inflation parameters of an embodiment of applicants' gas generator which utilizes only the central propellant (not shown) can also be changed by manipulating the thickness ofthe central propellant disks and the diameter ofthe ignition channel.

It is not intended that the scope of this invention be limited to the specific embodiments that have been described and illustrated. Rather, the scope ofthe invention is determined by the scope ofthe appended claims.