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Description  |
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BACKGROUND OF THE INVENTION
One type of passive restraint for safeguarding automotive passengers from
injury is the inflatable envelope or cushion which, in the event of a
collision, is filled by an inert gas generated through the controlled
combustion of a solid chemical.
Whereas relatively compact gas-generating units are used for restraints
facing the driver in the case of an accident, larger cylindrical
generators are used for the passenger-side which utilize means for
diffusing the generated gas in advance of inflating the envelope.
The generator typically comprises a main chemical generant or propellant
in, for example, pressed pellet form, packed in a cylindrical canister and
surrounding a booster charge. An igniter responds to a signal from a
collision-sensor and causes an ignition cord within the booster to fire
the booster which, in turn, ignites the propellant. The gas generated by
the propellant is filtered, cooled, and passed through ports in a
surrounding gas diffuser into the restraint cushion, typically a fine-mesh
nylon bag.
The volume, pressure and temperature of the gas at various stages during
the very brief time interval between the sensing of a crash and the full
deployment of the cushion, as well as rates of ignition, burning, and
gas-generation may be tailored by means well-known within the art.
It is known that, in general, as the ambient temperature increases, the
burn-rate of a solid propellant tends to increase. The effect of this
increase in burn-rate is that the gases are produced at a much higher
volumetric rate. Consequently, at the higher ambient temperatures, the
system operates at higher pressures, and the deployment velocity of the
cushion increases.
Additionally, as the propellant and filter become heated, the temperature,
and hence the volume, of the delivered gas is increased. It is therefore
desirable that the tailored operation of the gas generator proceed as
uniformly as possible, whether under hot or cold ambient temperatures,
i.e. with a minimal spread between resulting hot and cold operating
conditions.
Although it is known, as for example through U.S. Pat. No. 4,191,392 to
Barnett, to provide a vent at one end of the gas generator in order to
prevent gas pressure build-up at that end, such vent merely redirects
pressure, so that it still enters the cushion, but through the vent at the
end of the generator rather than through the ports in the diffuser.
Such venting does not, however, contemplate the varying conditions of
ambient temperature and gas flow rate which may obtain at the particular
moment when firing takes place.
SUMMARY OF THE INVENTION
This invention solves the above problem by providing a gas-generator having
temperature-compensating means for directing an increasing fraction of
gas-volume overboard, i.e., outside the cushion, as the ambient
temperature increases. There is accordingly provided on the generator a
cap or cone whose flexibility varies with temperature, and which opens
increasingly in response to pressure with a rise in temperature. In this
manner, the performance variation between hot and cold operation is
minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side view of the front seat of an automobile, showing
the generator, with the associated inflated cushion in place.
FIG. 2 is a cross-sectional view of a portion of the gas-generator, at the
instant before ignition.
FIG. 3 is a view similar to FIG. 2, but showing the venting action by the
end cone of the invention upon the generator firing.
FIG. 4, is a view similar to FIG. 2, but with the vent cone of the
invention in the process of reseating itself.
FIG. 5 is a graph comparing the vented device of the invention with an
unvented control device.
DETAILED DESCRIPTION
FIG. 1 shows generally the environment of the present invention, the
generator 10 shown in cross-section within the front compartment of a
vehicle 12, with the associated cushion 14 in inflated operative
condition.
As shown in FIG. 2, the generator 10 comprises a generally cylindrical
inflator 16 lodged within a diffuser 18, both of a length generally
several times greater than their diameter.
Extending axially through the center of the inflator 16 is an ignition cord
20 which may be fired by the ignitor or squib 24, upon activation through
electric wires 22. The ignitor is separated from cord 20 by a plate 21.
Surrounding the ignition cord 20 is a booster 26 comprised of pellets 28
of a rapidly-burning chemical housed within a perforated booster casing or
tube 30. A booster tube-end cap 31 and a grommet 33 locate the end of cord
20.
The main propellant comprises pellets 32 packed around the booster tube 30
and within a tubular propellant canister 36. Typically, such propellant
pellets will be pressed from granules of a gas-generating composition
comprised for example of sodium azide and metal oxide. Between propellant
canister 36 and an outer inflator housing 38, perforated as at 48, is
located an annular filter 40 for screening undesirable materials from the
combustion products produced during firing. Such filters are well-known
and may include combinations of perforated metal, screening, glass fibers,
steelwool, etc.
The ends of the inflator housing 38, generally of light metal, are turned
radially inwardly as at 46, sealingly engaging an inflator housing cap 50,
which receives ignitor housing 52 and simultaneously seals the parts of
the inflator 16.
Surrounding the inflator housing 38 is a cylindrical metal diffuser 18
having perforations 54 spaced circumferentially and longitudinally
thereof, and giving access to the interior of the cushion 14, which may
comprise a fine-mesh material, and is sealed by means not shown against
the outer surface of the diffuser near its end 60.
At one end 60 of the diffuser 18, its edge 62 is engaged by a vent cone or
cap 64. The cone 64 comprises a radial disk or web portion 66 apertured at
68 to fit around a portion of the inflator housing cap 50 and retained
between inflator housing cap 50 and ignitor housing 52. The cone 64 is
made of a material whose flexibility increases with temperature.
The disk 66 terminates at its outer radial edge in an annular rim portion
74. The portion of the rim facing the diffuser cylinder is notched axially
and radially as at 76 to sealingly accommodate the diffuser edge 62.
Radially inwardly of the notch 76 is an annular lip 78 which fits
slidingly between the radially inner surface of the diffuser 18 and the
radially outer surface of the inflator housing 38.
At the opposite end of the generator, the end 61 of the diffuser 10 may be
drawn against a portion of the support housing 100 by means of stud 102
and nut 104 on inflator housing 38, also trapping and sealing the wall of
cushion 14. The major portion of the generator 10 will thus be located
internally of the cushion 14, with the vent cone 64 extending outside the
cushion.
Additional stud-nut combinations, not shown, may help anchor the generator.
OPERATION
A crash sensor will, by means of wires 22, igniter 24 and ignition cord 20,
ignite the booster charge 28 which, in turn, fires the main propellant 32.
The gases created by the ignition and subsequent burning flow outwardly
through filter 40, where undesirables are removed. The cleansed gas
emerging from filter 40 exits the inflator housing 38 through perforations
48 and flows into chamber 49 between the inflator housing 38 and the
diffuser 18.
The gas within chamber 49 will seek to escape through the diffuser
perforations 54, while also acting axially against the vent cone lip 78
and the cone-web 66.
Where the device is working in the higher temperature range, with the
attendant increased propellant burn-rate and increased volumetric rate of
gas-production, such higher temperature will increase the flexing tendency
of the cone-web 66, and allow the increased pressure in chamber 49 to move
the notch 76 of the rim 74 axially from its normally sealed position
against the diffuser edge 62.
So long as the pressure within the diffuser 18 remains greater than the
ratio between the force on the vent cone lip 78 and its exposed annular
area, the vent cone 64 will remain open as in FIG. 3. Once the pressure
becomes equal to or smaller than this ratio, the cone will close as seen
in FIG. 4.
In this manner a certain fraction of the increased gas volume is caused to
vent out of generator, outside the cushion 14, while the remainder exits
the perforations 54 to fill the cushion.
The material of the cone 64 and its physical dimensions are so chosen as to
provide a venting device which is more easily displaced from its seat at
higher temperatures than at low ones. Accordingly, at lower temperatures,
the cone remains sealed in place against edge 62 but, in the nature of a
pressure safety relief valve, opens to vent gas in increasing amounts at
higher and higher temperatures.
FIG. 5 is a graph showing the traces of actual tests comparing the
performance of the vent cone inflators of the invention, shown in the
full-line curves, with that of unvented inflators, shown in the
dashed-line curves. The devices were placed in turn in a 10.3 ft..sup.3
(292,000 cm.sup.3) test-tank, simulating the cushion, with instrumentation
to trace the gas-pressure in kilo-pascales (kPa) from the instant of
firing well beyond 100 milli-seconds (ms). While the entire unvented
device was located inside the test-tank, the vented device was placed with
the diffuser end 60 outside the test-tank. It should be noted that 140 kPa
represents the tank pressure which is considered equivalent to full
cushion deployment.
In order to show the superiority of the invention, each device to be fired
was first stabilized for a minimum of four hours at a specific
temperature: those compared in the A-curves were cooled down to a
temperature of -20.degree. F. (-28.9.degree. C.); the B-curve devices were
brought to a room temperature of 70.degree. F. (21.1.degree. C.); and
those of the C-curve were heated to 180.degree. F. (82.degree. C.).
The three curves of the inventive devices will be seen to track within a
very narrow band. With a vented cold device, the time, from firing until a
tank gas-pressure of 140 kPa is reached, varies by only 12 ms from that
for a vented hot device; the unvented device, on the other hand, exhibits
a 26 ms variation.
It will be seen that, for the inventive generator, operating pressures are
reached within approximately 42 ms in a hot device, and in about 54 ms in
a cold device. In the prior art control device, the 140 kPa level is
reached in about 27 ms when hot, but not until 53 ms when cold.
For the unvented device, it will be seen from the curves of FIG. 5 that the
maximum hot-to-cold output pressure difference occurring at about the 40
ms point, is about 100 kPa, which represents approximately 40% of the
maximum tank pressure of 246 kPa at room temperature. For the vented
device of the invention, the ratio of the maximum difference between hot
and cold pressures, to the maximum tank pressure at room temperature, is
seen to be less than half of the above percentage. Since the ideal system
would exhibit no pressure difference between hot and cold operation, and
thus a 0 percentage, the lower percentage of the device of the invention
indicates an improvement.
A typical device tested comprised a unit measuring about 55 centimeters
(cm) in length and about 8 cm in diameter.
The ignition system consists of an igniter cord 20 such as manufactured by
Explosives Technology, Fairfield, Calif., under the designation ITLX; the
cord is fired by, for example, a titanium potassium chlorate igniter or
squib 24 such as manufactured by Imperial Chemicals Industries. The
booster mixture comprised sodium azide and potassium perchlorate, formed
into pellets 28, and contained within a perforated envelope 30. The
propellant 32, also in pellet form, comprised sodium azide and iron oxide.
The vent cone 64 was made of nylon whose web 66 had a thickness of about 6
millimeters (mm); the annular rim portion 74 has a thickness of about 7
mm. The cone sealing portion which faces the axial end 62 of the diffuser
18 has an area of about 10 cm.sup.2, compared to a total area of all
diffuser vents of about 15 cm.sup.2. When the gas pressure thus rises to a
point where it moves the vent cone from its seat, the vent area will be
increased by about 67%.
The vent cone may be made of other materials such as, for example,
polyester; it will be seen that different venting characteristics may be
had by varying the material, the shape, or the dimensions of the cone; as
well as by varying the pressures delivered by the inflator through changes
in ignition and propellant chemistry, and in filter-, canister-, housing-,
and diffuser-design.
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