Gas generator for an airbag module

The invention relates to a gas generator for an airbag module having a gas generator housing, in which a first chamber for receiving a propellant for igniting a main gas set in a second chamber, and an ignition device for the ignition of the propellant are provided. In order to simplify the production of the gas generator, the invention provides that the first chamber for receiving the propellant is limited by a pipe, on the first end of which the ignition device is disposed, and the second end of which reaches the gas generator housing at least approximately. The first and the second chamber are separated from each other such that no parts of the propellant may reach the second chamber via the second end of the pipe before the ignition of the propellant.

BACKGROUND

The invention relates to a gas generator for an airbag module.

For the activation of airbags gas generators are known which have a propellant with an ignition device in a gas generator housing. The propellant is arranged in a chamber. Outside of this chamber a main gas load is provided in the gas generator housing. In case of a crash the propellant is ignited at first which in turn ignites the main gas load which generates the gas flowing into the airbag.

The chamber for the propellant is formed by a cup-shaped sleeve in known gas generator as for instance disclosed in DE 198 12 221 A1. The sleeve is fixed to an ignition device with its open end and the closed end extends into the area of the gas generator housing. The sleeve has lateral breaches which are closed by a film before ignition of the propellant. After ignition of the propellant by the ignition device the film is destroyed by the hot gases of the propellant. The hot gases enter through the breaches into the room with the main gas load and ignite the same whereby the gases flowing into the airbag are generated.

The propellant is mostly present in form of pellets and the main gas load is present in form of tablets. By using a cup-shaped sleeve for the reception of the propellant an elaborate production process is required. Here, at first the pellets of the propellant has to be filled into the cup. Subsequently, the lower part of the gas generator housing with the ignition device has to be fitted onto the cup and has to be connected to the same. Afterwards the lower part has to be rotated around 180° so that the tablets can be filled in.

In order to guarantee good self-ignition properties of the gas generator for instance in case of a vehicle fire it is necessary to burn the main gas load in a controlled manner before its properties are negatively changed by a strong heat reaction. In order to guarantee a safe self-ignition a reproducible and good contact of the advanced ignition substance with the housing wall of the gas generator or sufficient stable heat bridges between the advanced ignition substance and the housing are indispensible. This is not always guaranteed when using a cup-shaped sleeve for the propellant since due to the production and assembly tolerances an air gap between the bottom of the sleeve and the generator housing can be present.

SUMMARY

The object of the present invention is to simplify the production of gas generators. Furthermore, the self-ignition properties and therefore the safety of the gas generator shall be improved in order to make the same in particular more unsusceptible towards assembly tolerances.

According to an exemplary embodiment of the invention at a gas generator for an airbag module with a gas generator housing, in which a first chamber for receiving a propellant for igniting a main gas set in a second chamber and an ignition device for the ignition of the propellant are provided, the first chamber for receiving the propellant is limited by a pipe, on the first end of which the ignition device is arranged and the second end of which reaches at the gas generator housing at least approximately, wherein the first and the second chamber are separated from each other such that all parts of the propellant may reach the second chamber via the second end of the pipe before the ignition of the propellant.

In this arrangement the hot gases of the propellant can reach from the first into the second chamber between the second end of the pipe and the gas generator housing.

The advantage of this arrangement is that the production process of the gas generator is simplified using the propellant and the tablets of the main gas set can be filled into the first or second chamber from the same side, e.g. a rotation of the housing after filling the propellant as by using a cup-shaped sleeve as limitation of the first chamber is nor required.

A further advantage is that breaches in the pipe can be omitted since the hot gases generated by ignition of the propellant can flow through the space between the pipe end and the gas generator housing from the first into the second chamber with the main gas set.

The invention can be implemented in different embodiments. The second end of the pipe can rest against the gas generator housing, which is deformable there by the pressure generated after ignition of the propellant in the pipe so that a space between the second pipe end and the gas generator housing is generated. Thereby, small gaps can be present before the ignition of the propellant between the generator housing and the resting second end of the pipe caused by production tolerances. It must be only guaranteed that no parts of the propellant reach the second chamber.

It is furthermore possible that an intermediate space is provided between the gas generator housing and the second end of the pipe which is blocked by a film which is torn open by the pressure in the first chamber after ignition of the propellant.

By using a cup-shaped gas generator as common in case of driver airbags, the pipe extends between the ignition device extending from the bottom of the gas generator into the gas generator and the cap of the gas generator.

It is provided in a second exemplary embodiment that a chamber for arranging a pyrotechnical load is provided, furthermore a housing which at least partially encloses the chamber; a pipe which restricts a section of the chamber; a part movable relatively to the pipe, which restricts the chamber towards the housing; means for exerting a restoring force on the movable part in order to keep it in thermal contact with the housing such that the chamber is in thermal contact with the surrounding of the housing via the movable part and the housing.

The means for exerting a restoring force provide that the chamber and therefore a pyrotechnical load arranged in the same is in good thermal contact with the surrounding of the gas generator housing so that a premature ignition of the pyrotechnical load occurs when the gas generator is exposed to a high temperature, which is for instance generated by an external heat source (for instance a fire close to the gas generator).

By a premature ignition of the pyrotechnical load its controlled burning is in particular achieved and it is thereby prevented that a pressure builds up in the chamber, which causes a fragmentation of the gas generator. Furthermore, due to a premature ignition it is prevented that components (in particular the housing) of the gas generator are weakened in its stability by the high temperature before the pyrotechnical load ignites.

According to an exemplary embodiment of the invention the good thermal contact is achieved by keeping the movable part, which restricts the chamber towards the housing, in good thermal contact to the housing by the means for exerting a restoring force and the appearance of a gap between the movable part and the housing (e.g. an inner surface of the housing) is avoided. In a variant the pyrotechnical load is an advanced ignition load, which serves the ignition of a further pyrotechnical load (the actual gas set for producing a gas which is used for inflating the airbag). Such an advanced ignition load for instance consists of an advanced ignition means, which has a lower self-ignition temperature than the actual pyrotechnical gas set. In another embodiment the advanced ignition load has a (conventional) booster load to which an advanced ignition means is mixed. Due to the good thermal contact between the chamber and the surrounding of the gas generator it is guaranteed that the advanced ignition means of the advanced ignition load ignites fast enough and therefore, a premature ignition is possible and therefore a controlled burning of the actual gas set occurs.

The housing can be for instance formed by an upper part and a lower part, which are connected to each other in particular by welding. Thereby, the lower part is pressed against the upper part before connecting, whereby the case can occur that immediately before connecting the lower part is again pushed away from the upper part by the gas generator parts (for instance elastic filter elements) arranged in the housing, and a gap is formed between the chamber and the housing (lower part or upper part). The formation of a gap during connecting the housing parts is in particular largely prevented by the means for exerting a restoring force.

The means for exerting a restoring force guarantee for instance that the movable part is kept in a position in which it rests against an inner side of the housing. Furthermore, the means for exerting a restoring force on the movable part can be designed such that they preload the movable part against the housing.

In a further exemplary variant of the invention the pipe extends with one end in direction of the inner surface of the housing, e.g. the end faces the inner surface of the housing. Here, the pipe can at least sectionally have a hollow cylindrical form whereby one end of the pipe faces the inner surface of the gas generator housing and is arranged in the area of the inner surface of the housing.

In a further exemplary embodiment of the invention the movable part is formed flange-like, whereby a first section of the movable part extends along an inner circumference of the pipe and a second section covers at least partially the opening which is formed in one end of the pipe. In this variant the movable part forms therefore a cover-like sealing for sealing the first chamber towards the housing.

The second section of the movable part can in particular rest with one side against an inner surface of the housing. The first section projects for instance into the opening of the end of the pipe facing the inner surface of the housing so that a movement of the movable part is carried out through the pipe. Hereby, the movable part is essentially only movable in one direction towards the inner surface of the housing. For instance, the movable part is inserted via the first section into the opening of the pipe such that its movement can essentially occur exclusively along an axis of the pipe.

The means for exerting a restoring force onto the movable part can for instance comprise an elastic element (of rubber or plastic) and/or a spring-like element. The elastic element, which is provided as a means for exerting a restoring force, is in particular temperature resistant up to a temperature, which is above the ignition temperature of the pyrotechnical load arranged in the chamber.

In an exemplary embodiment of the invention the elastic element is arranged between the end of the pipe and the inner surface of the housing, for instance in form of a ring (i.e. O-ring) of an elastic material. In case the movable part is formed flange-like as described above the elastic element can be arranged between the end of the pipe (e.g. between an edge of this end facing the inner surface of the housing) and an overhang of the second section of the movable part. Thereby, the elastic element exerts a restoring force onto the movable part if it is pushed into the direction of the pipe so that the cover-like second section of the movable part remains in thermal contact with the housing inner surface, for instance rests with one surface against it.

Although the elastic element can be shaped in form of a ring, it does not necessarily have a circular cross section, but can also have for instance an oval cross section. Between the elastic element and the flange-like formed part can in particular exist a clearance fit. Examples of materials for forming the elastic element are heat resistant elastomers as for instance FPM or FFPM (according to DIN/ISO 1629).

In a further exemplary improvement of the invention the chamber with the pyrotechnical load presents a first chamber of the gas generator and the pyrotechnical load a first pyrotechnical load (for instance in form of an advanced ignition load), whereby the housing has beside the first chamber a second chamber for receiving a second pyrotechnical load (for instance in form of a main gas set, which can be ignited by hot gas generated by ignition of the advanced ignition set). In this variant an elastic element, which is formed as a means for exerting a restoring force on the part, can simultaneously seal the first chamber against the second chamber, e.g. can in particular before ignition of the first pyrotechnical load prevent an overflowing of gas from the first chamber into the second chamber or vice versa or can at least counteract the overflowing.

In a exemplary further improvement the movable part is designed such that it releases under influence of a gas pressure generated by igniting the pyrotechnical load an outflow opening from which the gas can flow out of the chamber. In particular, in case the gas generator has a first and a second chamber gas can flow via the outflow opening from the first chamber of the gas generator into the second chamber so that the gases generated by the first pyrotechnical load in the first chamber can effect an ignition of the second pyrotechnical load in the second chamber.

It is pointed out that the gas generator cannot only have one but multiple main chambers (second chambers). For instance, an ignition chamber (first chamber) is arranged to each of the main chambers. In a variant the gas generator has two ignition chambers which in each case are assigned to one main chamber.

In a further exemplary embodiment of the invention, the movable part has pre-determined breaking point, which breaks when the gas pressure in the chamber exceeds a determined value so that the movable part forms (releases) in the area of the pre-determined breaking point an outflow opening.

In a further exemplary embodiment the first chamber for receiving the propellant is restricted by a pipe at which first end an ignition device is arranged and which second end is in close contact to an elastic gas guiding plate that rests at least sectionally inside against the gas generator housing.

Due to the specific adjustment of the stiffness and elastic resilience of the gas guiding plate a good contact with the gas generator housing and therefore good advanced ignition properties are guaranteed. It is practical that for adjusting the stiffness and elastic resilience at least one circular bead is provided in the bottom of the gas guiding plate. Production and assembly tolerances in axial direction can be compensated by a corresponding contact length of the bead with the gas generator housing.

The close contact of the second end of the pipe with the gas guiding plate can be guaranteed in such that the gas guiding plate has a cup-shaped section into which the second end of the pipe engages whereby the second end rests closely against the side wall of the cup-shaped section without being connected to it by auxiliary means. The pipe rests preferably in the area of its first end with its inner wall in a pressed fit against a receptacle for an ignition device and in the area of its second end with its outer wall in press fit against a side wall of the cup-shaped section. The press fit on the receptacle of the ignition device and in the cup-shaped section differs preferably in its strength.

The bottom of the cup-shaped recess rests at least sectionally closely against the generator housing.

It is advisable that the generator housing and the gas guiding plate are deformable in the area of the second end of the pipe by the pressure generated after ignition of the propellant in the pipe such that a space between the first end of the pipe and the gas guiding plate and therefore a connection between the first and the second chamber is provided, or a space between the first end of the pipe and the ignition device and therefore a connection between the first and the second chamber is provided.

It can also be alternatively advisable that the gas generator housing is deformable on the side of the first end of the pipe or the ignition device by the pressure wave generated after ignition of the propellant in the pipe such that a space between the first end of the pipe and the ignition device and therefore a connection between the first and the second chamber is present or a space between the second end of the pipe and the gas guiding plate and therefore a connection between the first and the second chamber is present.

The hot gases generated by ignition of the propellant can flow through this space from the first into the second chamber in which the main gas set is located. Through this, breaches in the pipe can be omitted.

The pipe extends when using a cup-shaped gas generator preferably between the ignition device extending from the bottom of the gas generator into the same and the cover of the gas generator.

DETAILED DESCRIPTION

The gas generator according toFIG. 1has a bottom1and a cover2, which form together the housing of the gas generator. An ignition device3projecting into the gas generator is connected with the bottom1. A first end5of a pipe4, which encloses a first chamber6for the receiving of a propellant7, is connected with the ignition device3. The connection between the pipe4and the ignition device corresponds to a known arrangement between a cup-shaped sleeve and the ignition device. The propellant7is present in form of pellets.

The second end8of the pipe4reaches up to the cover2and thereby seals the first chamber6upwardly and against a second chamber9for a main gas set10, which is provided there in form of tablets. The sealing between the end8of the pipe4and the cover2does not have to be gas tight. In fact, these parts can be produced with the usual tolerances so that small gaps are present. It has only to be prevented that pellets of the propellant reach into the chamber9before its ignition.

After ignition of the propellant7overpressure is built up inside the chamber6due to the gases generated thereby, which deforms the cover2of the generator housing outwardly as shown inFIG. 2. Thereby, a gap11is formed between the second end8of the pipe4and the cover2through which the hot gases flow from the chamber6into the chamber9in order to ignite the main gas set10there. This delivers the gases for the inflation of the airbag.

Due to the formed gap11breaches in pipe4are not required so that the production is simplified compared to a cup-shaped sleeve.

But also the production process of the gas generator is simplified by the use of pipe4.

As shown inFIG. 3the gas generator has to be turned at least once during the production process. At first, a cup-shaped sleeve12was filled with the pellets of the propellant7. Afterwards, the bottom1of the gas generator was placed on the sleeve12and connected to the ignition device3. In order to insert the main gas set10from above into the chamber9the bottom1connected with the sleeve12had to be pivoted around 180°. In this position a filter ring13was additionally inserted.

By using the pipe4as a container for the propellant this production process is simplified as can be seen fromFIG. 4. The pipe4is inserted into the bottom1pointing upwards with the opening to which the ignition device3as well as the casing of chamber9is fixed and is connected to the ignition device3. Subsequently, the propellant7as well as the main gas set10can be inserted from above. The turning of the bottom1can therefore be omitted.

In the embodiment according toFIG. 5a pipe14, which is arranged instead of the pipe4of the first embodiment, does not reach with its second end15up to the cover2. The gap16thus formed between this and the end15is in this case sealed with a film17so that before ignition of the gas generator the propellant7cannot reach into the chamber9.

As a result of the overpressure generated after ignition of the propellant7in the chamber6the film17is destroyed so that now the hot gases of the propellant7can enter through the gap16into the chamber9.

Also in this second embodiment the production process is simplified as shown inFIG. 4.

The gas generator201ofFIG. 7has a first chamber202which is surrounded by a housing203whereby the housing203is formed by a lower housing part231(cup) which is connected to an upper housing part232(base). Adjacent to the first chamber202a second chamber208is provided which surrounds the first chamber202in a circular manner. The first chamber202contains a first pyrotechnical load in form of an advanced ignition load291(for instance in form of pellets) which has a conventional booster load mixed with an advanced ignition means. The advanced ignition load291serves for the ignition of a second pyrotechnical load in form of a main gas set292of the second chamber208. In turn, the gas set292serves the generation of gas with which an airbag (not shown) shall be inflated. The advanced ignition means contained in the advanced ignition load291is characterized by a lower ignition temperature compared to the main gas set292, for instance approximately 160° C. while the main gas set has for instance an ignition temperature of approximately 260° C.

The chamber202is restricted by a pipe4(booster pipe) and towards the lower housing part203by a movable part in form of a sealing205. More precisely, the tube-like first part204extends towards a housing inner surface of the lower housing part231so that an end241of the pipe204is arranged in the area of the inner surface of the housing.

The sealing205has a first section251, which is formed hollow cylindrical and extends along an inner circumference of the pipe204, and a second section252, which covers an opening242in the end241of the first part204cap-like and is connected in one part to the first section251.

The first section251is bent from the second cap-like section252such that an overhang521is formed which sticks out from the first section251and protrudes from the opening242. Between the end241of the pipe204(more precisely between a side of an edge of the end241which faces the housing part231) and the inner wall of the housing part231ameans for exerting a restoring force onto the sealing205in form of an elastic element is arranged. In the embodiment ofFIG. 7a plastic ring206forms the elastic element, which encloses an outer circumference of the first section251of the sealing205.

The plastic ring206is elastically formed and arranged such that it keeps the sealing205in position relatively to the lower housing part231so that a side of the second section251rests against the inner surface of the housing part231and a gap between the first chamber (e.g. between the section251) and the housing part231is avoided as far as possible.

The plastic ring206allows in particular that the formation of a gap between the chamber202and the housing part231is avoided, if the housing part231is connected (welded) to the upper housing part232. Due to the restoring force exerting onto the sealing205it is achieved that it moves with the lower housing part231, if this is pushed away by the upper housing part232through exertion of a force by elastic components inside of the gas generator, for instance by elastic filters207before welding.

Such a pushing apart of the housing parts231,232can occur although between the sections of the two housing parts a frictional force is effective. The frictional force between the two housing parts can in particular be reduced during the welding of the two housing parts as a result of the temperature increase so that a pushing apart of the housing parts231,232does not occur indeed before the welding but during the welding. Also, in this case the construction according to the invention avoids the formation of a gap between the first chamber202and the housing part232due to the movable sealing205and the elastic plastic element206.

The gas generator201has for ignition of the advanced ignition load291in the first chamber202an ignition device210which is arranged on a receptacle101. The receptacle101restricts a second end243of the pipe204which is opposite to the first end241. The second end243is for instance mounted onto the receptacle101by press fit.

This has the consequence that by impact of a high temperature onto the gas generator the advanced ignition load291is ignited premature which leads to a premature ignition of the main gas set292. The premature ignition effects a controlled burning whereby an excessive inner pressure is avoided in particular in the second chamber208.

In order to ignite the main gas set292gas from the first chamber202has to reach the second chamber208. For this reason the part205has a predetermined breaking point which breaks by the impact of a pressure in the first chamber202and releases an outflow opening over which gas from the first chamber202can flow into the second chamber208. This is shown inFIG. 8.

It is pointed out that the invention is of course not restricted to gas chambers with two chambers. It extends also for instance to gas generators which have only one chamber, e.g. working without a separate advanced ignition load. Furthermore, a gas generator according to the invention can have for instance two first chambers with an advanced ignition load in each case and two further chambers containing a main gas set.

FIG. 8relates to the situation that an ignition of the advanced ignition load291occurs after the impact of outside heat. The ignited advanced ignition load291releases gas whereby the gas pressure generated in the chamber202bends the lower housing part231away from pipe204and the part205is moved away from the pipe204(downwards) by the inner pressure existing in the chamber202.

Through this, the portion of the hollow cylindrical first section251of the sealing205, which does not extend into the pipe204, is elongated. In this portion of the first section251apre-determined breaking point522is provided, which breaks when the inner pressure in the first chamber202exceeds a certain value whereby the breaking of the pre-determined breaking point522releases an opening523, which is formed by a passage between the end241of the pipe204and the plastic ring206and by an opening in the section251in the area of the broken pre-determined breaking point522. Gas can flow via the opening523from the first chamber202into the second208in order to ignite the main gas set292there. The gas generated by the ignited main gas set292in the second chamber208flows via the filter207and spaces271respectively through an outflow opening272into an airbag (not shown).

FIG. 9illustrates the position of a gas generator according to the invention in a variant of the so called bonfire test for examining the heat resistance of the gas generator, namely the so called position of the gas generator “from fire”. In this position an outside heat of a heat source600affects mainly the lower housing part231of the gas generator201.

The gas generator shown inFIG. 10has a bottom301and a cap302. Furthermore, an ignition device303and a receptacle for the ignition device303aare provided which both extend into the gas generator. A first end313of the pipe312is connected by press fit to the receptacle of the ignition device303a. The pipe312encloses the first chamber307for receiving the propellant308. The propellant308is present in form of pellets. In the area of the cap shaped elements can be arranged in the chamber307. Here, it is dealt with specific fuels which due to their characteristics guarantee an ignition at low temperatures. These shaped elements are not shown in the Figures.

In the gas generator an elastic deformable gas guiding plate315is arranged which has a cup-shaped section316which rests inside against the cap302. Furthermore, a circular section317is present which can also rest inside against the cap302. A circular bead318extends between the sections316and317. This bead supports the elasticity of the gas guiding plate315in the upper area302aof the cap302and therefore a tight rest of the section316against the upper area302a.

Due to the specific adjustment of stiffness and elastic resilience of the gas guiding plate315by the bead318a good contact to the cap302of the generator housing and therefore a good advanced ignition property is guaranteed. Furthermore, geometrical tolerances of the single components as well as such resulting during assembly are compensated due to the elastic resilience of the gas guiding plate.

The second end314of the pipe312extends into the cup-shaped section316and is connected to it by press fit so that a mixing of the propellant308from the first chamber307with the main gas set310in the second chamber309is suppressed. Additionally, a good thermal conductivity between the pipe312and the gas guiding plate315is provided. A lateral section319of the gas guiding plate315continues in a distance to side wall302bof the cap302.

InFIG. 10the gas generator is shown before ignition of the propellant308. TheFIGS. 11aand11bshow the gas generator after ignition of the propellant308. It is recognizable that the cup-shaped section316and the upper area302aof the cap302is buckled due to the pressure generated after ignition of the propellant308in the first chamber307. Through this, two embodiments result for connecting the first chamber307with the second chamber309.

The first possibility is shown inFIG. 11a. In the embodiment ofFIG. 11athe press fit between the pipe312and the receptacle for the ignition device303ais stronger than between the pipe312and the side wall of the cup-shaped section316. Through this it is guaranteed that the pipe312is only released on its second end314from its fixing so that a space320between the second end314and the cup-shaped section316is formed through which the gas generated in the first chamber307can flow into the second chamber309in order to ignite the main gas set310.

The second embodiment is shown inFIG. 11b. In the embodiment ofFIG. 11bthe press fit between the pipe312and the side wall of the cup-shaped section316is stronger than the press fit between the pipe312and the receptacle for the ignition device303a. Through this, it is guaranteed that the pipe312is released only at its first end313from the receptacle for the ignition device303a. Thus, a space321between the first end313and the receptacle for the ignition device303ais generated through which the gas generated in the first chamber307flows into the second chamber309in order to ignite the main gas set310.

Alternatively, the gas generator housing can be deformable on the side of the first pipe end313or the ignition device303by the pressure wave generated after the ignition of the propellant308in the pipe such that the space between the first end313of the pipe312and the ignition device303and therefore the connection between the first and the second chamber307or309is formed or that the space between the second end314of the pipe312and the cup-shaped section316and therefore the connection between the first and the second chamber307or309is formed. Although this alternative deformation is not shown in theFIGS. 11aand11b, spaces are generated there corresponding to the spaces320and321.