WO2008034805A1

WO2008034805A1 - Pyrotechnic generator with two combustion chambers

Info

Publication number: WO2008034805A1
Application number: PCT/EP2007/059801
Authority: WO
Inventors: Roger Andry; Frédéric Burlot; Renaud Cherel; François Peremarty; Cyrille Poindron
Original assignee: Autoliv Development Ab
Priority date: 2006-09-20
Filing date: 2007-09-17
Publication date: 2008-03-27
Also published as: FR2905918B1; DE112007002110B4; DE112007002110T5; FR2905918A1

Abstract

The invention relates to a pyrotechnic gas generator for inflating a motor vehicle airbag, the generator comprising at least a pyrotechnic charge (5) in the combustion chamber (C1) adjacent to an initiator (6) is formed by juxtaposing pellets, each presenting a central hole, thus constituting a longitudinal channel (CL); outlet orifices (33) of the inner tube (3) are situated facing the combustion chamber (C2) that contains a charge (5') presenting a shorter duration of combustion than the charge (5) in the combustion chamber (C1); a plenum and filter chamber (T) containing a filter (F) in register with both combustion chambers (C1, C2),- and an outer tube (2) presents orifices (23) for allowing gas to escape to the outside substantially at each end of a cylindrical wall (20).

Description

PYROTECHNIC GENERATOR WITH TWO COMBUSTION CHAMBERS

This invention relates to a pyrotechnic gas generator for inflating an airbag of a motor vehicle.

Document FR-A-2 863 986 describes a pyrotechnic gas generator for inflating an airbag of a motor vehicle, which comprises:

- two blind and coaxial cylindrical tubes, namely an internal tube and an external tube, with the internal tube delimiting two combustion chambers longitudinally separated by a partition forming a nozzle and each containing a pyrotechnic charge;

- a pyrotechnic initiator placed in the extension of the internal tube, near its opening;

- wherein the pyrotechnic charges present in each chamber have different combustion times; - and the internal tube has, only opposite one of said chambers (i.e. in register with one of said chambers), discharge ports or "nozzles" for discharging gases and particles generated by the combustion of the charge toward a so-called plenum and filtrering chamber delimited by the space separating the two tubes, while the internal tube also has, on its cylindrical wall, discharge ports for discharging gases to the outside, in order to inflate the airbag, opposite the combustion chamber farthest from the initiator.

The use of this generator presents two types of problems .

First, a low-pressure combustion occurs at the end of operation. Indeed, once the "high-speed" propellant has finished burning, the "low-speed" propellant is alone in the chamber, of which all of the discharge nozzles are open, which causes a significant drop in pressure.

This results in incomplete combustions and high toxicity values.

Second, there is significant breakage of the propellant pellets on ignition. This causes an increase in the surface exposed to combustion, and therefore variability in the performance of the generator during the first milliseconds (modification of the combustion speed) .

This invention is intended to solve these problems by proposing a generator comprising two pyrotechnic charges that have different operation times and are perfectly controlled.

It therefore relates to a pyrotechnic gas generator for inflating an airbag of a motor vehicle, which comprises:

- a pyrotechnic initiator placed in the extension of the internal tube, near its opening; - wherein the pyrotechnic charges present in each chamber have different combustion times and different combustion gas delivery rates;

- and the internal tube has, only opposite one of said chambers, i.e. in register with one of said chambers, discharge ports or "nozzles" for discharging gases and particles generated by the combustion of the charge toward a so-called plenum and filtrering chamber delimited by the space separating the two tubes, while the internal tube also has, on its cylindrical wall, discharge ports for discharging gases to the outside, in order to inflate the airbag, characterised in that:

- at least the pyrotechnic charge of the combustion chamber adjacent to the initiator is formed by a juxtaposition of pellets having a central hole, thus constituting a longitudinal channel; the discharge ports of the internal tube are located opposite the combustion chamber, i.e. in register with the chamber which contains the charge having the shortest combustion time; the plenum and filtrering chamber contains a filter that extends opposite the two combustion chambers; the external tube has discharge ports for discharging gases to the outside, substantially at each end of the cylindrical wall.

Owing to this combination of features, a controlled combustion of the two charges is obtained, without breakage of pellets, and the emission of gases resulting from the combustion of the charge "with a long combustion time" in some way takes over the emission of gases resulting from the combustion of the charge "with a short combustion time". Thus, the gas leakages and heat losses in the airbag are properly compensated.

According to other advantageous and non-limiting features : - the internal tube has the same diameter over its entire length;

- the pyrotechnic charge present in the combustion chamber adjacent to the initiator has a combustion time greater than that of the charge present in the other chamber;

- the pyrotechnic charge present in the combustion chamber adjacent to the initiator has a combustion time lower than that of the charge present in the other chamber; - the pyrotechnic charges of the combustion chambers are formed by a juxtaposition of pellets having a central hole, thus constituting a longitudinal channel;

- the generator has a length/width ratio greater than 4; - the number and/or sizes of the ports present at each end of the cylindrical wall are chosen so as to distribute the gas flow between these two groups of ports according to a ratio between 20/80 and 80/20;

- it comprises three combustion chambers, with the chamber farthest from the initiator being that of which the charge has the longest combustion time.

Other features and advantages of the invention will appear on reading the following detailed description of a specific embodiment. This non-limiting description will be provided in reference to the appended figures, in which: figure 1 is a cross-section view, along a longitudinal median plane, of a generator according to this invention;

- figure 2 is a graph showing, for a generator such as that of figure 1, the mass flow rates of the gas generated by each of the combustion chambers, as well as the total mass flow rate;

- figure 3 is a curve showing the inflation power of figure 1, which is expressed in the form of a cumulative gas flow, taking the temperature into account;

- figure 4 shows a view similar to that of figure 1, with an alternative of a generator with three combustion chambers .

As mentioned above, the generator 1 according to the invention, as shown in the appended figure 1, has two coaxial cylindrical tubes, namely an internal tube 3 of which the internal volume longitudinally delimits two combustion chambers Ci and C2, and an external tube 2.

These two tubes are, for example, made of a thin metal.

According to the preferred embodiment, the tubes have a circular cross-section and have the special feature of both being blind, with their closed ends being arranged on the same side. However, it is possible to use other tube cross-section shapes, such as a square-shaped cross-section .

References 20 and 30 designate their cylindrical walls, while references 22 and 32, respectively, designate their blind ends. Their respective open ends, opposite the blind ends, are referenced 21 and 31, respectively. Purely by way of indication, the external tube 2 has a length between 150 and 280 millimetres, while that of the internal tube is between 130 and 260 millimetres.

Also by way of indication, the values of their respective internal diameters are between 10 and 18 millimetres .

The generator according to the invention preferably has a length/width ratio strictly greater than four.

The use of blind tubes provides the advantage, by comparison with the generators of the prior art, of not having to close, by any means, the corresponding end of the tube, which has the disadvantage of requiring additional assembly operations, adding additional assembly time and requiring the management of additional parts.

The blind end 32 of the internal tube 3 is in contact with that of the external tube referenced 22.

Due to the difference in length between the tubes, there is an area inside the tube that communicates directly with its opening 21, in which the internal tube 3 is not present.

It is in this area that a pyrotechnic initiator 6 is placed, in the immediate extension of the internal tube 3, capable of initiating the combustion of a pyrotechnic charge 5 placed in a first combustion chamber Ci.

The initiator is itself positioned, at the level of its electrodes, on a base 60, with the assembly thus formed being enclosed in a support that is formed by two identical half-shells 61 and 62. The external tube 2 is crimped on this support 61/62 at the level of an annular area 26. The initiator assembly can include means for focusing the gas in the direction of the internal channel of the adjacent pyrotechnic charge, as will be seen below. These means are, for example, deflectors. The initiator is partially engaged inside the tube 3, at a predetermined distance from the pyrotechnic charge placed in the vicinity. This distance is chosen by a person skilled in the art on the basis of the best compromise between sufficient ignition of the pyrotechnic charge and lack of stress of this charge.

As indicated above, the internal volume of the internal tube longitudinally delimits two combustion chambers Ci and C₂. These are separated by a partition 7, with the tube 3 at this level being crimped or welded on the latter. It is pierced with a cylindrical axial opening 70 forming a nozzle.

The area of the wall 30 of the tube 3 that is located opposite the combustion chamber Ci (in register with this chamber), i.e. on the left-hand side of the partition 7 when looking at figure 1, is continuous, which means that it has no opening.

However, the second area of the wall 30, which is located opposite the combustion chamber C₂, i.e. on the right-hand side of the partition 7 when looking at figure 1, comprises a series of small openings 33 forming nozzles, which enable communication between this chamber and the annular space located between the two tubes 2 and 3, which constitutes a settling and filtration chamber T, as will be seen later in the description. We will also later describe the benefit of the presence of openings 33 in register with chamber C₂, and of their absence opposite chamber Ci. The charge 5 contained in this chamber Ci is constituted by identical connected pellets, preferably of propellant, which have an internal channel CL that gives them an annular shape. In this way, there is, in the longitudinal direction of the chamber Ci, in the axis of the charge 5, a passage free of material. This passage allows the combustion and ignition gases to circulate. The channel CL is aligned with the nozzle 70 of the partition 7. The external diameter of the pellets is equal, with a close clearance, to the internal diameter of the tube 3.

The charge 5' that is present in the second combustion chamber C₂ has substantially the same shape characteristics as the charge 5. However, in an alternative embodiment not shown, the pellets that constitute it may not have a material-free passage, so that the channel CL described above is not present.

The external tube 2 has, on its cylindrical wall 20, discharge ports 23 for discharging gases toward the bag portion of an airbag (which is not shown here, for the sake of clarity of the figure) . They are distributed in two distinct groups, one opposite combustion chamber Ci, and the second opposite combustion chamber C₂ •

Pins for attachment to a support are designated by reference 4.

In the plenum (or settling) chamber T, means F for filtering hot gases and particles from the combustion chambers are provided.

The filtering means be a metal fibre filter, a filament winding or a compacted metal filament winding.

That is the general structure of a generator according to this invention. We will now describe its operation.

The composition, mass, volume and shape of the pyrotechnic charges are chosen so that the charge 5 of the chamber Ci has a long combustion time and generates a low instantaneous gas flow, and the charge 5' of the chamber C2 has a shorter combustion time and generates a higher instantaneous gas flow.

In reference to figure 1, it is noted that the volume occupied by the charge 5 of chamber Ci is lower than that occupied by the charge 5' of chamber C2.

When the initiator 6 is activated, the gases and particles escaping from it first cause the combustion of the charge 5 of the adjacent chamber Ci.

The presence of the channel CL facilitates the propagation of gases over the entire length of the charge and, similarly, its combustion.

In addition, when focusing means are present, they contribute to directing gases and particles resulting from the activation of the initiator toward the channel CL.

The fact that the wall 30 of the tube 3 is free of openings opposite the chamber Ci makes it possible to provide a high level of pressure therein, since the combustion gases can be discharged only through the nozzle 70 of the partition 7. Thus, a good functional compromise is obtained while maintaining a reasonable pressure level and preventing any risk of bursting.

Some of the gases and particles transmitted by the initiator 6 and those transmitted by the charge 5 rush into chamber C₂, which causes the combustion of the charge 5' present therein. The presence of the channel CL facilitates the propagation of the combustion of the charge 5'. However, it is not absolutely essential. It is necessary only in the charge 5 adjacent to the initiator.

The gases are then discharged from the tube 3 through the openings 33 present only opposite chamber C₂. They rush into the chamber T in which they are cooled and filtered by the filtering means F.

Finally, they leave the tube 2 to rush into the inflatable airbag (not shown), through the openings 23.

Figure 2 shows, in the form of curves, the mass flow rates of the gas transmitted by the combustion of each of the charges, as a function of time.

The curve Di is corresponds to charge Ci, and curve D₂ corresponds to charge C₂, and DT is the resulting curve showing the total mass flow rate of the gas as a function of time. It is noted that the combustions are initiated at substantially the same time, and that the maximum flow rate values are reached at substantially the same time.

However, the flow rate according to curve D₂ is zero by t = 20 ms, whereas it still has a notable value according to curve Di, even well after this time.

This shows that the gases transmitted by the combustion of the charges of the two chambers contribute to the initial inflation of the bag. However, given the charge of chamber Ci, having a longer combustion time, the gases generated "at the end of operation" help to maintain the inflation over time, compensating for losses due to venting.

This data is confirmed by that provided in figure 3. Indeed, a regular and significant increase in the gas flow rate to t = around 12-13 ms is noted. Beyond this time, the gas flow rate continues to increase, but more slowly. Therefore, we note here the additional "relay" function of the gases generated by the combustion chamber that has the longest operation time.

In an embodiment not shown, it is possible to use pyrotechnic charges having different combustion speeds. Thus, in the configuration of figure 1, charge 5 would have a slower combustion speed than charge 5' .

Also in an embodiment not shown, the area of the internal tube provided with openings 23 is that opposite the combustion chamber Ci. In this configuration, it is that charge with the shortest combustion time (and/or the highest combustion speed) that occupies this chamber.

The generator shown in figure 4 has substantially the same structure as that described in reference to figure 1. Thus, the same numeric references have been used to designate identical or similar elements. Focusing means not shown are provided in the initiator 6.

It is noted however that in this case the tube 3 contains three chambers Ci, C2 and C3, separated two-by- two by partitions 7 and 7' pierced respectively with openings 70, 70' forming a nozzle.

Chamber C₂, arranged closest to the initiator, has the shortest operation time. It is therefore opposite this chamber that the tube 3 has openings 33 at the level of its wall 30.

Chamber C₃, which is farthest from the initiator, has the longest operation time. The intermediate chamber Ci, arranged between the previous two, has an operation time between those of chambers Ci and C2. It is important that at least chambers Ci and C2 comprise a central channel CL, so as to transport the combustion gases to chamber C₃. In an alternative embodiment not shown, the distribution of chambers is different. However, again, the charges of the two chambers closes to the initiator are provided with a channel CL.

The generators described above are particularly suitable for inflating "side airbags", "curtain airbags" and "knee airbags".

Claims

1. A pyrotechnic gas generator for inflating a motor vehicle airbag, the generator comprising:

^• two blind and coaxial cylindrical tubes, namely an internal tube (3) and an external tube (2), with the internal tube (3) delimiting at least two combustion chambers (C₁, C₂) longitudinally separated by a nozzle- forming partition (7), each combustion chamber containing a pyrotechnic charge (5, 5 ' ) ; • a pyrotechnic initiator (6) placed in the extension of the internal tube (3), near its opening (31);

^• the pyrotechnic charges (5, 5') present in each of the combustion chambers (C₁, C₂) having different combustion times and different combustion gas delivery rates; and ^• the internal tube (3) presenting, only opposite one, i.e. in register with one of said combustion chamber (C₁, C₂) , discharge ports or "nozzles" (33) for discharging gas and particles towards a so-called plenum and filtering chamber (T) delimited by the space between the two tubes (2; 3), while the internal tube (2) has also, on its cylindrical wall (20), discharge ports (23) for discharging gases to the outside, in order to inflate the airbag; the generator being characterized by the fact that: • at least the pyrotechnic charge (5) of the combustion chamber (C₁) adjacent to the initiator (6) is formed a juxtaposition of pellets, each presenting a central hole, thus constituting a longitudinal channel (CL) ;

^• the discharge ports (33) of the internal tube (3) located opposite, i.e. facing the combustion chamber (C₂) that contains the charge (5') having the shortest combustion time;

• the plenum and filtering chamber (T) contains a filter (F) that extends opposite, i.e. in register with both combustion chambers (C₁, C₂) ; and ^• the external tube (2) has discharge ports (23) for discharging gas to the outside, substantially at each end of the cylindrical wall (20) .

2. A generator according to claim 1, characterized by the fact that the internal tube (3) has the same diameter over its entire length.

3. A generator according to claim 1 or claim 2, having two combustion chambers (C₁, C₂) , and characterized by the fact that the pyrotechnic charge (5) present in the combustion chamber (C₁) adjacent to the initiator has a combustion time greater than that of the charge (5') present in the other chamber (C₂) .

4. A generator according to claim 1 or claim 2, having two combustion chambers (C₁, C₂) , and characterized by the fact that the pyrotechnic charge present in the combustion chamber (C₁) adjacent to the initiator has a combustion time lower than that of the charge (5') present in the other chamber (C₂) .

5. A generator according to any preceding claim, characterized by the fact that the pyrotechnic charges (5, 5') of the combustion chambers (C₁, C₂) are formed by juxtaposition of pellets, having a central hole, thus constituting a longitudinal channel (CL) .

6. A generator according to any preceding claim, characterized by the fact that it presents a length/width ratio that is greater than 4.

7. A generator according to any preceding claim, characterized by the fact that the number and/or the sizes of the ports (23) present at each end of the cylindrical wall (20) are choosen so as to distribute the gas flow between these two groups of ports in a ratio between 20/80 to 80/20.

8. A generator according to any one of claims 1, 2, and 5 to 7, characterized by the fact that it comprises three combustion chambers (C₁, C₂, C₃) , with the chamber farthest from the initiator (6) being that of which charge has the longest combustion time.