A GAS DIFFUSER DEVICE FOR CAR SAFETY

The present invention relates to the field of car safety and relates more particularly to a unit for protecting the occupants of a vehicle.

In order to minimize the risks of injury to the occupants of a motor vehicle in the event of a collision, it is known to install a protection unit inside the vehicle. The unit generally comprises a gas generator and an airbag.

In the event of an impact, the bag is inflated by the gas coming from the generator and becomes interposed between the walls of the vehicle and the body or the face of the occupant, so as to prevent the occupant from being thrown against said walls.

Such a protection unit is already known in which the gas generator is positioned inside the bag, so that the gas diffuses directly into the bag. Under such circumstances, the diffusion of gas into the inside of the airbag depends very precisely on the gas delivery- rate of the generator. Accompanying Figure 1 shows how the total accumulated inflation flow D varies inside the airbag as a function of time t:.

The dashed-line curve shows the results obtained with the above-mentioned protection unit. It can be seen that the accumulated flow D increases very quickly inside the airbag on the gas generator being triggered. Consequently, the bag is inflated very quickly in the first instants of operation.

However, even though the airbag needs to be deployed in an extremely short length of time, of the order of 30 milliseconds (ms) to 40 ms, it is nevertheless preferable for deployment not to be too fast during the first few milliseconds of operation, in order to minimize stresses on the vehicle occupant that the bag is supposed to protect.

This applies in particular either for the driver who is located very close to the protection device housed in the steering wheel, or for the front passenger who is not necessarily in a normal sitting position. Thus, when the accident occurs, the passenger might be leaning somewhat forwards or sideways, e.g. in order to pick up an object that has been dropped.

It is therefore desirable to inflate the airbag progressively, firstly in order to decrease its deployment speed towards the body of the vehicle occupant, and secondly in order to straighten up and/or center the occupant's body before the bag is fully inflated.

In order to do this, it is appropriate to modify the curve plotting accumulated inflation flow so that it presents a shape that is known to the person skilled in the art as an "S-shaped" curve. The ideal theoretical curve is drawn as a continuous line in Figure 1.

In other words, the inflation flow rate into the inside of the airbag ought to be relatively slow during the first ten or even twenty milliseconds of inflation, after which it ought to increase rapidly so as to finish off inflating the bag and reach the volume needed to damp the impact . British patent document No. 2 323 568 describes a device comprising a gas generator and an airbag between which a rigid diffuser chamber is interposed. That chamber is provided with two orifices, one of which is shut and opens only beyond a certain pressure threshold in order to regulate the inflation of the bag.

However, such a diffuser chamber is cumbersome.

US patent No. 6 361 067 has also disclosed a protection unit comprising a gas generator and an airbag, with a diffuser bag disposed inside it to diffuse the gas .

The diffuser bag is provided with at least two side openings that serve to deflect the flow of gas escaping

axially from the generator so as to send it sideways, thus unfolding the airbag initially sideways, and subsequently frontally. As a result, the airbag is not deformed excessively towards the vehicle occupant. US patent document No. 2002/0033590 discloses a similar device, in which the diffuser bag, provided with a plurality of openings, acts as a buffer volume enabling the airbag to be filled quickly, while protecting it from possible damage. In addition, the number and positioning of the openings in the diffuser bag can be adapted to deflect the flow of gas in a selected direction.

Finally, US patent No. 6 283 499 describes a car safety device comprising a gas generator, an airbag, and a diffuser bag disposed inside said airbag and made of a flexible material. The diffuser bag presents a plurality of inflation gas flow openings formed by stitching or heat sealing said flexible material.

The diffuser bag acts as a heat shield. It receives the flow of hot gas coming from the generator and redirects it towards certain portions of the airbag, while retaining any particles that might be present in the gas. The positions of the openings in the diffuser bag also serves to modify the flow direction of the gas inside the airbag. The airbag is thus subjected to a smaller amount of stress, thereby reducing the risk of it bursting.

Nevertheless, none of the three above-mentioned documents describes a device enabling the rate at which the airbag is inflated to be adjusted so as to obtain an S-shaped curve.

An object of the invention is to solve this problem. To this end, the invention relates to a gas diffuser device for car safety, the device comprising a gas generator and a diffuser chamber for diffusing the gas delivered by the generator, said gas generator being suitable for being triggered in the event of an impact in order to inflate an airbag for protecting the occupants

of a motor vehicle, the airbag being initially in a folded state, and said gas diffuser chamber being connected to said gas generator and being provided with one or more orifices, with at least one "main" orifice being open to allow gas to be exhausted into said airbag as soon as the gas generator is triggered, said orifice (s) defining a gas exhaust passage having an initial area.

In accordance with the invention, said gas diffuser chamber is a small bag of flexible material disposed inside the airbag and the area of said exhaust passage increases to reach a final value greater than the value of the initial area as soon as the pressure inside said diffuser chamber exceeds a predetermined threshold value, such that the curve plotting the accumulated inflation flow into the airbag as a function of time presents an S- shape .

For simplification purposes, this inflation curve is referred to below as an "S-shaped" curve. According to other characteristics of the invention that are advantageous but not limiting, taken singly or in combination:

^• the dimensions of the main orifice of the diffuser chamber increase to reach a final area greater than its initial area as soon as the pressure inside said chamber exceeds a predetermined threshold value;

^• the main orifice of the diffuser bag is bordered at least in part by stitching suitable for tearing, or by a zone of heat sealing suitable for breaking, when the pressure inside said diffuser bag exceeds the threshold value so as to increase the dimensions of said main orifice; the diffuser chamber includes at least one

"secondary" exhaust orifice that is in the closed state when the gas generator is triggered and that passes to the open state only once the pressure inside said chamber exceeds a predetermined threshold value;

the diffuser chamber presents a volume that is less than or equal to about one-third the volume of the airbag; the main orifice is situated on the axis of the gas generator; and the value of the threshold pressure inside the diffuser chamber corresponds substantially to twice the value of the pressure inside the airbag at the end of its deployment . The invention also provides a gas diffuser chamber suitable for being mounted inside a motor vehicle, between a gas generator and an airbag for protecting the occupants of said vehicle, said gas generator being suitable for being triggered in the event of an impact against said vehicle, and the airbag which is initially folded, being suitable for being inflated by the gas delivered by the generator.

In accordance with the invention, the chamber consists of a small bag of flexible material for placing inside the airbag, this bag is suitable for being connected to said gas generator to be in gaseous fluid communication therewith, and it is provided with one or more exhaust orifices, with at least one "main" orifice being open to allow gas to be exhausted into said airbag as soon as the gas generator is triggered, said orifice (s) defining a gas exhaust passage having an initial area, and the chamber includes means for increasing the area of said exhaust passage to a final value greater than the value of the initial area as soon as the pressure inside said diffuser chamber exceeds a predetermined threshold value, such that the curve plotting accumulated inflation flow into the airbag as a function of time presents an S-shape.

Finally, the invention provides a protection unit comprising the above-specified device together with an airbag.

Other characteristics and advantages of the invention appear from the description given below with reference to the accompanying drawings which show one possible embodiment by way of non-limiting indication. In the drawings :

Figure 1 is a graph showing variation in the accumulated inflation flow D inside an airbag of a unit for protecting the occupants of a motor vehicle, plotted as a function of time t:; ^■ Figure 2 is a diagrammatic perspective view of a first embodiment of the diffuser chamber in accordance with the invention;

Figures 3A to 3C are diagrams showing different stages in the deployment of the airbag in a protection unit fitted with the diffuser chamber of Figure 2;

Figure 4 is a longitudinal section of a second embodiment of a diffuser chamber in accordance with the invention;

• Figure 5 is a cross-section view of the diffuser chamber shown in Figure 4;

Figures 6A to 6C are diagrams showing different stages in the deployment of the airbag of a protection unit fitted with the diffuser chamber of Figure 4;

Figure 7 is a graph plotting variation in the accumulated inflation flow D inside an airbag as a function of time t, for various different protection units provided with different diffuser devices; and

Figure 8 is a graph plotting variation in the pressure P inside the diffuser chamber as a function of time t, for the various different diffuser devices of Figure 7.

The structure and the general operating principle of the unit for protecting the occupants of a motor vehicle in accordance with the invention are described below with reference to the diagram of Figure 3A.

The protection unit comprises a gas diffuser device 1 and an airbag 4 for protecting the occupants of the vehicle .

The gas diffuser device 1 comprises a gas generator 2 and a gas diffuser chamber 3.

In the description below and the claims, the term

"gas generator" is used to mean any device suitable for being triggered in the event of an impact against said motor vehicle, in order to release a gas and thus inflate the airbag 4.

The gas generator may equally well be a device comprising a tank or cartridge of gas under pressure, or a device containing a pyrotechnic charge which releases gas by combustion. The airbag 4 is initially in the folded state (see Figure 3A) , and it is then unfolded and inflated by the gas released by the generator 2 on being triggered (see Figures 3B and 3C) .

The diffuser chamber 3 is in gaseous fluid communication both with the generator 2 and with the airbag 4. Thus, the gas released by the gas generator 2 is expelled towards the airbag 4, passing through the diffuser chamber 3.

For this purpose, the diffuser chamber 3 includes a gas inlet opening connected to the outlet orifice of the generator 2 or to its combustion chamber, and at least one gas exhaust orifice that opens out to the inside of the airbag 4.

The diffuser chamber 3 may either be interposed between the gas generator 2 and the airbag 4, or it may be directly located inside said airbag. When inside the airbag, the chamber 3 is disposed over the inflation inlet of the airbag 4.

In accordance with the invention, the gas diffuser chamber 3 may be either in the form of a rigid enclosure 31, or else in the form of a bag 32 of flexible material.

The first embodiment corresponding to the rigid enclosure 31 is described in greater detail below with reference to Figure 2.

The enclosure 31 is in the form of a hollow cylinder of longitudinal axis X-X' . The cylinder is advantageously made of metal.

The enclosure 31 has a cylindrical wall 310 closed at one of its ends by a circular end wall 311, and partially closed at its opposite end by a wall 312 having an inlet fitting 313 forming a gas access opening.

The inlet fitting 313 is connected to the outlet from the gas generator 2 which likewise extends along the axis X-X' .

The cylindrical wall 310 is also pierced by two gas exhaust orifices, one of which, referenced 314, is said to be a "main" orifice, while the other one, referenced 315, is said to be a "secondary" orifice.

The main orifice 314 is continuously open, so as to enable gas to be exhausted into the inside of the airbag 4 as soon as the gas generator 2 is triggered.

In contrast, the secondary orifice 315 is closed while the car safety device is not in use and it remains closed temporarily for a short length of time after the gas generator 2 has begun to operate. This secondary orifice 315 changes to the "open" state only when the pressure inside the enclosure 31 exceeds a predetermined threshold value.

Advantageously, the secondary orifice 315 is closed by a material 316 that is fusible or suitable for breaking, being suitable for melting or breaking or tearing once the above-mentioned threshold value is reached (see Figures 3A and 3B) .

By way of example, this material may be polyethylene or polypropylene. Other variant embodiments may be envisaged without going beyond the ambit of the invention.

Thus, the enclosure 31 may present some other geometrical shape, for example it may be in the form of a rectangular parallelepiped.

It is also possible to have a plurality of main orifices 314 and secondary orifices 315, or to have an enclosure 31 in which the main orifice 314 is partially closed by a material suitable for breaking that breaks above a certain pressure threshold, as described below for the diffuser bag variant. Finally, unlike that which is shown in Figure 2, the main orifice 314 could be situated on the axis X-X' of the gas generator 2.

The main orifice is located relative to the outlet axis X-X' of the gas generator in such a manner as to minimize aggression against the airbag 4, and in particular minimize damage being inflicted thereto by a flow of hot gas .

The shape, the disposition, and the relative dimensions of the main orifice (s) and the secondary orifice (s) can be adapted as a function of the inflation profile desired for the airbag 4, providing that enables the above-described S-shaped inflation curve to be obtained and providing there is always at least one main orifice 314 that is permanently open. The way in which the diffuser device 1 operates is described in greater detail with reference to Figures 3A to 3C.

So long as the gas diffuser device 1 is not in use, as shown in Figure 3A, the main orifice 314 is open, the secondary orifice 315 is closed, and the airbag 4 is folded.

As soon as an impact against the motor vehicle or some other sudden deceleration thereof is detected, the gas generator device 2 is triggered and generates gas. The gas penetrates into the inside of the enclosure 31 from which it is exhausted towards the airbag 4 via the

open main orifice 314. The airbag begins to unfold, as shown in Figure 3B.

This main orifice 314 defines a passage for exhausting the gas towards the airbag 4, which passage has an initial area Ai .

In the embodiment shown in Figures 2 and 3, this initial area Ai corresponds to the area of the orifice 314.

When there are several main orifices 314, the initial area Ai corresponds to the sum of the areas of those orifices.

Finally, when the pressure inside the enclosure 31 reaches a predetermined threshold value, the fusible or

"suitable for breaking" material 316 melts or breaks so as to open the secondary orifice 314, as shown in

Figure 3C.

The gas exhaust passage then presents a final area Af that is greater than the initial area Ai, and in the embodiment of Figures 2 and 3 the final area corresponds to the sum of the areas of the orifices 314 and 315.

A greater quantity of gas then reaches the inside of the airbag 4 so it inflates quickly, such that the curve plotting the accumulated inflation flow in the airbag as a function of time presents an S-shape. A second embodiment corresponding to a diffuser bag 32 of flexible material is described in greater detail below with reference to Figure 4.

The bag 32 is placed inside the airbag 4.

The bag 32 is generally tubular in shape, with its middle portion being wider than its two ends, both ends being open.

The longitudinal axis of the bag 32 carries the reference X-X' .

The bag 32 is advantageously made of fabric, e.g. polyamide fabric.

At its bottom end, it presents a circular access opening 321 for connection to the gas generator 2, and its opposite end it presents a main exhaust orifice 322.

The opening 321 and the orifice 322 may be in alignment on the axis X-X' of the gas generator 2, as shown in Figures 4 to 6C.

Nevertheless, the exhaust orifice (s) 322 could also be placed perpendicularly to the axis X-X' so as to limit any risk of the jet of hot gas delivered by the generator 2 heating the airbag 4 in its zone that comes into contact with the head of the vehicle occupant.

The main orifice 322 is of a shape that flares outwards to a small extent.

This orifice 322 is bordered at least in part by a fusible heat-seal zone that is suitable for melting as soon as the temperature inside the diffuser bag 32 reaches a predetermined threshold, or by a heat-seal zone that is suitable for breaking as soon as the pressure that exists inside the diffuser bag 32 exceeds a predetermined threshold value.

The expression " zone suitable for breaking" is used to mean stitching or a zone of the diffuser chamber that presents weakness.

As shown in Figures 4 and 5 by way of example, the two edges 323 and 325 of the outlet 322, situated on diametrically opposite sides thereof, are pinched together with adhesive 327 being applied between the two facing faces 324 & 324' or 326 & 326' of each of the two pinched-together edges. The adhesive 327 is applied over a certain height.

When the pressure inside the diffuser bag 32 reaches the threshold value, the two facing areas 324 & 324' of the pinched zone 323, and likewise 326 & 326' of the pinched zone 325 become unstuck so that the dimensions of the exhaust orifice 322 are enlarged.

In a variant that is not shown in the figures, the two pinched-together edges 323 and 325 could equally well

be held together by stitching that is suitable for tearing due to its constituent thread breaking when a threshold pressure value is exceeded.

Under such circumstances, the material of the thread used for stitching the "suitable for breaking" zone is different in kind from the material used for stitching the sides of the diffuser bag 32 that extend between the inlet opening 321 and the edges 323, 325 of the outlet 322. In other variants, the diffuser bag 32 may be of some other shape and may be provided with a plurality of outlet orifices 322.

The operating principle of the diffuser device is described in greater detail below with reference to Figures 6A to 6C.

When the gas diffuser device 1 is not in use, as shown in Figure 6A, the airbag 4 and the diffuser bag 32 are folded, but the exhaust orifice 322 is open.

As soon as an impact against the motor vehicle or a sudden deceleration thereof is detected, the gas generator device 2 is triggered and generates gas. The gas penetrates into the inside of the diffuser bag 32 from which it is exhausted towards the airbag 4 via the main orifice 322 that is open. The diffuser bag 32 unfolds completely and the airbag 4 begins to unfold, as can be seen in Figure 6B.

The main orifice 322 defines a passage for exhausting gas towards the airbag 4 through an initial area Ai. This corresponds to the area of the orifice 322. Finally, once the pressure inside the diffuser bag 32 has reached a predetermined threshold value, the fusible or "suitable for breaking" material or the stitching beside the orifice 322 melts or breaks, so as to open the main orifice 322 wider (see Figure 6C) . The gas exhaust passage 322 then presents a final area Af that is greater than the initial area Ai.

A greater quantity of gas then reaches the inside of the airbag 4 and it inflates quickly, so that the curve plotting accumulated inflation flow into the airbag as a function of time presents an S-shape. Compared with the rigid enclosure 31, the bag 32 has the advantages of being lighter and also more compact since it is placed directly inside the airbag, resulting in a saving of space.

Furthermore, the inflation of the bag 32 allows deployment to begin, e.g. by lifting the lid of the airbag module, and this is not possible with a rigid enclosure since its volume is constant.

In the two above-described embodiments, the final area Af of the gas exhaust passage continues at all times to present a finite size and the gas must always pass through one or more orifices in order to reach the airbag

4.

The pressure threshold value inside the diffusion enclosure 31 or the diffuser bag 32 preferably corresponds to substantially twice the pressure that exists inside the airbag 4 at the end of deployment.

Preferably, the volume of the enclosure 31 or of the diffuser bag 32 is less than or equal to one-third the volume of the airbag 4. Tests have been performed with a diffuser device of the kind shown in Figures 4 to 6.

The pressure inside the diffuser bag 32 was measured and the gas flow rate through the exhaust orifice 322 of the diffuser bag 32 (i.e. at the inlet to the airbag 4) was calculated as a function of time by simulation. The tests were performed with a "passenger" type airbag having a volume of 120 liters (L) , and a diffuser bag having a volume of 25 L.

The initial diameter of the exhaust orifice was 60 millimeters (mm) (giving an area of about 28 square centimeters (cm ² ) ) , and its final diameter was 80 mm (area

about 50 cm ² ) . The pressure threshold for enlarging the diameter of the opening was set at 0.2 megapascals (MPa) .

The results obtained are shown by the continuous- line curve in Figure 1 which plots the accumulated inflation flow D into the airbag (expressed in kmol.K) as a function of time (expressed in ms) .

The spot represents the pressure threshold beyond which the exhaust orifice of the diffuser bag enlarges.

The continuous-line curve presents the S-shape that is characteristic of the invention. During the first 25 ms, approximately, the diameter of the exhaust orifice remains at 60 mm, after which it opens suddenly and the airbag 4 inflates quickly and reaches practically complete deployment at 40 ms . Comparative tests have also been performed with diffuser bags of different values, presenting different initial and final diameters for the gas exhaust orifice and different pressure threshold values.

The results are given in Figures 7 and 8 where curves a to f correspond to the parameters set out in the following table:

Figure 7 shows that the accumulated inflation flow D into the airbag (expression in kmol.K) as a function of time (expressed in ms) , while Figure 8 shows the pressure (expressed in MPa) that exists inside the diffuser bag. As can be seen in Figure 7, the device of the invention enables the airbag 4 to be inflated progressively.

During a short period of time, the airbag is inflated to a small extent, thus avoiding deployment taking place too quickly, and making it possible where necessary, to lift the vehicle occupant into a more upright position, after which the airbag is inflated very quickly in order to reach full deployment.

Other tests have been performed with a smaller diffuser bag. Thus, with an airbag of the "driver" type having a volume of 80 L and a diffuser bag with a volume of 1.3 L, the pressure threshold at which the exhaust orifice opened was 0.3 MPa and the pressure inside the airbag was 0.02 MPa. In general, for equivalent performance, i.e. for identical S-shaped curves, the smaller the volume of the diffuser chamber, the greater the threshold pressure above which the area of the exhaust passage should increase, and conversely, the greater said volume, the lower the threshold pressure.

The various parameters have an effect on the shape of the S-shaped curve.

Thus, the volume of the diffuser bag or chamber influences the first portion of the S-shaped curve. The greater this volume, the slower the increase in flow into the airbag, and the lower the pressure in the airbag, so the exhaust orifice of the diffuser chamber or bag opens later, other things remaining equal. The S-shaped curve is flatter and the slope of its middle zone is steeper. The initial area Ai of the exhaust passage also has an influence on the first portion of the S-shaped curve. The smaller the initial area Ai, the smaller the

inflation flow rate into the airbag, so the pressure threshold is reached more quickly. In other words, the S-shaped curve is flatter and its second portion is shorter. The value of the pressure threshold beyond which the area of the gas exhaust passage increases has an influence on the shape of the second portion of the S- shaped curve. The greater this threshold pressure, the faster the flow into the airbag increases and the steeper the slope of the middle portion of the S-shaped curve (it approaches the vertical) .

Similarly, the greater the final area Af of the gas exhaust passage, the faster the rate at which the flow into the airbag increases . The selected volume for the diffuser chamber, areas for the initial area Ai and final area Af of the gas exhaust passage, and pressure threshold value beyond which the area of the gas exhaust passage enlarges, serves to adjust the shape of the S-shaped curve for inflation rate to match the requirements of the car manufacturer. These requirements depend in particular on the desired displacement performance for the airbag and the space available in the vehicle.

The nature of the fusible or "suitable for breaking" material is selected as a function of the temperature or pressure threshold at which it is desired that it melts or breaks .

Finally, it should be observed that it is possible to have a single diffuser chamber, whether it is an enclosure or a bag, with a main exhaust orifice of dimensions that enlarge and a secondary orifice that opens above a certain pressure threshold.