Abstract:
A gas generator for an airbag allows maintaining combustibility as originally designed. An ignition energy generated by an activation of a first igniter  21  flows via a first communication hole  24 , a first intermediate chamber  30  and second communication holes  43 , into a second combustion chamber  40 , to cause a gas generating agent  43  to ignite and burn. The ignition energy flows into the second combustion chamber  40  with making a detour, and therefore, the gas generating agent prevents from broken by the impact and the combustibility thereof is never changed.

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(e) on U.S. Provisional Application No. 60/547,810 filed on Feb. 27, 2004 and under 35 U.S.C. § 119(a) on patent application Ser. No. 2004-48999 filed in Japan on Feb. 25, 2005, the entire contents of which are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a gas generator for an air bag used in air bag systems for vehicles. 
   2. Description of the Related Art 
   In gas generators in which gas generating agents having a solid form (shaped as tablets, cylinders, etc.) are used, ignition energy (a high-temperature gas, a flame, etc.) generated by the activation of an igniter causes a gas generating agent to ignite and burn; or alternatively, a transfer charge is ignited by an igniter, and the released ignition energy causes a gas generating agent to ignite and burn. 
   In the gas generator, a large shockwave is created upon generation of the ignition energy due to the activation of the igniter; if this shockwave hits the gas generating agent directly, the gas generating agent may break as a result. The fragmentation of the gas generating agent results in an increased surface area of the same, which increases also the amount of gas generated during initial combustion, making thereby impossible to maintain combustibility as originally designed. 
   U.S. Pat. No. 5,345,875 discloses a gas generator in which a combustion chamber is divided into two by a shockwave barrier 60, one of the combustion chambers (first combustion chamber) faces the activation portion of an igniter 24, and the other combustion chamber (second combustion chamber) communicates with the first combustion chamber through a plurality of communication holes 64 formed in the barrier 60. 
   In U.S. Pat. No. 5,345,875, the igniter 24 and the two combustion chambers are arranged in series, so that the ignition energy advances directly into the first combustion chamber, and also into the second combustion chamber via the communication holes 64, which might result in a fragmentation of the gas generating agent by the shockwave. 
   In the constitution of conventional gas generators, the ignition energy generated by the activation of the igniter, in the form of a shockwave, collides directly with the gas generating agent, which may cause the fragmentation of the solid gas generating agent, increasing the surface area thereof and making thereby impossible to maintain combustibility as originally designed. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a gas generator for an air bag whose combustibility can be maintained as originally designed by preventing fragmentation, etc. of a gas generating agent caused by ignition energy. 
   As a solving means, the present invention provides a gas generator for an air bag comprising, inside a housing having a gas discharge port, an ignition means chamber including an ignition means, and a combustion chamber which is provided adjacent to the ignition means chamber and accommodates therein a gas generating agent adapted to be ignited and burned by the ignition means, wherein 
   the combustion chamber has a gas inflow space in which no gas generating agent exists, and which is surrounded, except for at least one side or one direction, by the housing inner wall surface and the ignition means chamber, 
   the ignition means chamber and the combustion chamber are in communication only through a communication hole(s), the communication hole faces the gas inflow space, and 
   an ignition energy generated by an activation of the ignition means inside the ignition means chamber flows into the gas inflow space via the communication hole, changes its flow direction by colliding against a housing inner wall surface, and causes the gas generating agent inside the combustion chamber to ignite and burn. 
   The gas generating agent in the gas generator for an air bag according to the present invention is molded into a desired shape, such as a tablet, a cylinder, a pellet, etc. The ignition means may comprise only an igniter or a combination of an igniter with a conventional transfer charge such as B/KNO 3  or a conventional gas generating agent (the gas generating agent acts mainly as a transfer charge, though its combustion gas may also be used to inflate the air bag). 
   The housing of the gas generator for an air bag according to the present invention is preferably cylindrical, though the cross-sectional shape in the width direction is not restricted to a circular shape, and may also be conical or polygonal. 
   The gas inflow space is part of the combustion chamber and is surrounded in three sides by the housing inner wall surface and the ignition means chamber, with the remaining one side communicating with the space inside the combustion chamber occupied by the gas generating agent. 
   In the gas generator for an air bag according to the present invention, the shockwave of the ignition energy generated by activation of the ignition means enters first into the gas inflow space unoccupied by the gas generating agent, changes its flow direction, and then collides with the gas generating agent. Thereby, the fragmentation, etc. of the gas generating agent is prevented, which allows maintaining the originally designed combustibility. 
   The combustion chamber of the gas generator for an air bag according to the present invention has a separating wall disposed inside the combustion chamber; herein, the gas inflow space may be surrounded, except a side in one direction, by the separating wall and the ignition means chamber. This is so designed to be adapted to changes in numerous types of the structure of the gas generator. 
   In the gas generator for an air bag according to the present invention, preferably, the size of a non-surrounded area and the dimension of the gas generating agent are preferably correlated so as to prevent the entry of the gas generating agent into the gas inflow space. 
   Since the gas inflow space is surrounded by the housing inner wall surface and the ignition means chamber, and communicates at least in one side with the space occupied by the gas generating agent in the combustion chamber, it is desirable that the size of the non-surrounded surface, serving as the communication portion, and/or the dimension of the smallest portion of the gas generating agent be adjusted so that the size of the non-surrounded area is smaller than the dimension of the smallest portion of the gas generating agent (the diameter of the gas generating agent, when it is cylindrical in shape). 
   In the gas generator for an air bag according to the present invention, preferably the communication hole directly opposes the housing inner wall surface or the separating wall. When they are arranged opposingly as the above, a direction of the ignition energy can be changed, and as a result, a more effective mist capture without hindering the flow of the ignition energy can be obtained. 
   As another solving means, the present invention provides a gas generator for an air bag comprising, inside a housing having a gas discharge port, an ignition means chamber including an ignition means, a combustion chamber accommodating therein a gas generating agent ignited and burned by the ignition means, and an intermediate chamber formed between the ignition means chamber and the combustion chamber, wherein 
   the ignition means chamber and the intermediate chamber are in communication only by a first communication hole(s), and the intermediate chamber and the combustion chamber are in communication only by a second communication hole(s), 
   an ignition energy generated by an activation of the ignition means inside the ignition means chamber flows into the intermediate chamber via the first communication holes and into the combustion chamber via the second communication holes to cause the gas generating agent to ignite and burn, and 
   the first communication hole and the second communication hole are disposed such that the ignition energy generated in the ignition means chamber flows into the combustion chamber not directly but by making a detour. 
   The gas generating agent in the gas generator for an air bag according to the present invention is molded into a desired shape, such as a tablet, a cylinder, a pellet, etc. The ignition means may comprise a combination of an igniter and a conventional transfer charge such as B/KNO 3  or a conventional gas generating agent (the gas generating agent acts mainly as a transfer charge though its combustion gas may also be used to inflate the air bag). 
   The housing of the gas generator for an air bag according to the present invention is preferably cylindrical, though the cross-sectional shape in the width direction is not restricted to a circular shape, and may also be conical or polygonal. 
   In the gas generator for an air bag according to the present invention, the shockwave of the ignition energy generated by the activation of the ignition means flows into the combustion chamber not directly but by making a detour, thus not colliding directly with the gas generating agent, thereby preventing the fragmentation, etc. of the gas generating agent and maintaining the originally designed combustibility. 
   In the gas generator for an air bag according to the present invention, the opening directions of the first communication hole and second communication hole are preferably different, and more preferably the opening directions of the first communication hole and second communication hole are perpendicular to each other. The first communication holes are arranged in a direction different to, and particularly preferably in a direction perpendicular to, the direction in which the ignition energy advances. 
   With such a arrangement of the first communication hole and the second communication hole, the direction in which the ignition energy passes through the first and second communication holes is different, so that the ignition energy flows into the combustion chamber not advancing directly but by making a detour, whereby the effect of the present invention can be obtained. 
   In the gas generator for an air bag according to the present invention, the first communication hole or second communication hole preferably face a housing inner wall surface or an intermediate chamber inner wall surface. 
   The high-temperature gas (ignition energy) contains in some cases a mist (solid components, e.g., metal components included in the gas generating agent and released by the combustion thereof). If the first or second communication holes are arranged to face the housing inner wall surface or the intermediate chamber inner wall surface, the high-temperature gas collides reliably against the housing inner wall surface, whereby the mist component is captured by adhering to the lower-temperature inner wall surface and solidifies. As a result, an amount of mist component decreases. 
   In the gas generator for an air bag according to the present invention, preferably, at least one of a first partitioning means for separating the ignition means chamber and the intermediate chamber, and second partitioning means for separating the intermediate chamber and the combustion chamber is preferably movable, so that volumes of the ignition means chamber, the intermediate chamber, and the combustion chamber can be modified by moving at least one of the first and second partitioning means. 
   Modifying the volume of each chamber allows adjusting charging amounts of the gas generating agent inside the combustion chamber and of the gas generating agent or transfer charge inside the ignition means chamber, thereby affording an easy regulation of the gas generator output. 
   In the gas generator for an air bag according to the present invention, the first partitioning means for separating the ignition means chamber and the intermediate chamber has preferably a protruding portion; alternatively, the second partitioning means for separating the intermediate chamber and the combustion chamber has preferably a recess portion, wherein the protruding portion or the recess portion abuts against an opposite partitioning means. 
   An intermediate chamber having a predetermined volume can be easily formed by abutting the protruding portion of the first partitioning means against the flat surface of the second partitioning means, or by abutting the recess portion of the second partitioning means against the flat surface of the first partitioning means, and additionally by abutting both of the protruding portion and the recess portion against the flat surface. Accordingly, a moving passage for the ignition energy from the first communication hole to the second communication hole can be obtained easily. The height of the protruding portion and the depth of the recess portion are set in accordance with a desired volume of the intermediate chamber. 
   In the gas generator for an air bag according to the present invention, preferably, the cross-section area of the intermediate chamber in the axial direction of the housing is larger than the total opening area of the second communication hole(s). 
   These relationships allow the ignition energy to flow easily into the combustion chamber, thereby increasing the combustibility of the gas generating agent. 
   In the gas generator for an air bag according to the present invention, preferably, an igniter and a first gas generating agent (as explained above, mainly serving as a transfer charge) are accommodated inside the ignition means chamber, while a second gas generating agent is accommodated in the combustion chamber, such that a combustion temperature of the first gas generating agent is higher than a combustion temperature of the second gas generating agent. 
   A gas generating agent having a combustion temperature of 1700 to 3000° C. may be used as the first gas generating agent, with for instance a composition comprising nitroguanidine as fuel, strontium nitrate as an oxidizer, etc. 
   A gas generating agent having a combustion temperature of 1000 to 1700° C. may be used as the second gas generating agent, with for instance a composition comprising guanidine nitrate as fuel, a basic copper oxide as an oxidizer, etc. 
   Among the first and second gas generating agents, the first gas generating agent has a better ignitability and a higher combustion temperature, and generates a larger ignition energy. Thus, even when the ignition energy generated by the combustion of the first gas generating agent flows into the combustion chamber by making a detour, the ignitability of the second gas generating agent is not lowered in comparison with the case in which the ignition energy directly flows, by not making a detour, into the combustion chamber. 
   The gas generator for an air bag according to the present invention may be applicable to both a pyrotechnic inflator in which only a high-temperature combustion gas is used as an air bag inflating means and a hybrid inflator in which a high-temperature combustion gas and a pressurized gas such as helium, argon, nitrogen, etc. are used. 
   The gas generator for an air bag according to the present invention may also be suitably used as both of a single-type gas generator, comprising only a single set of an ignition means and a combustion chamber, or a dual type gas generator comprising two sets of an ignition means and a combustion chamber. 
   The gas generator for an air bag according to the present invention may be applicable to conventional gas generators for an air bag, such as an air bag inflator for a driver side, an air bag inflator for a front passenger side, a side air bag inflator, a curtain inflator, etc. 
   According to the gas generator for an air bag of the present invention, the shockwave of the ignition energy generated by the activation of the igniter does not collide directly against the gas generating agent in the combustion chamber; as a result, the gas generating agent does not break nor is damaged. Thus, the gas generating agent can preserve its originally designed combustibility, which increases the reliability of the product. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial cross-sectional view of a gas generator for an air bag. 
       FIG. 2  is a partial cross-sectional view of  FIG. 1 . 
       FIG. 3  is an axial cross-sectional view of another embodiment of a gas generator for an air bag. 
       FIG. 4  is a partial cross-sectional view of  FIG. 3 . 
       FIG. 5  is an axial cross-sectional view of another embodiment of a gas generator for an air bag. 
       FIG. 6  is a partial axial cross-sectional view of the embodiment of a gas generator for an air bag of  FIG. 5 , partially modified. 
       FIG. 7  is a partial axial cross-sectional view of the embodiment of a gas generator for an air bag of  FIG. 5 , partially modified. 
       FIG. 8  is a partial axial cross-sectional view of the embodiment of a gas generator for an air bag of  FIG. 5 , partially modified. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   (1) First Embodiment 
   A first embodiment will be described below with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is an axial cross-sectional view of a gas generator, and  FIG. 2  is a partial cross-sectional view of  FIG. 1 . The gas generator  10  of  FIG. 1  is a dual-type pyrotechnic inflator. 
   In one end of a cylindrical housing  11 , there are provided a first ignition means chamber  20  and a first combustion chamber  40 , adjacent to each other in the axial direction of the housing. 
   The first ignition means chamber  20  is enclosed by a first ignition means chamber housing  22  serving as a partitioning means, and accommodates therein a first igniter  21 . The first igniter  21  is fixed together with a collar  29  by crimping an end portion periphery  15   a  of the cylindrical housing  11 . Reference numeral  28  denotes an O-ring  28  for securing moisture-proof. 
   The first ignition means chamber housing  22  is fixed by an annular crimped portion  17   a  of the cylindrical housing  11  and the first igniter  21 , so that it cannot move in the axial direction of the housing  11 . If the annular crimped portion  17   a  is not provided, the first ignition means chamber housing  22  can be press-inserted into the cylindrical housing  11  to be moved optionally in the axial direction of the housing. 
   The first ignition means chamber  20  accommodates therein a first gas generating agent  23  having a combustion temperature of 1700 to 3000° C. The ignition portion  25  of the first igniter  21  is covered by an aluminum cup  26  so that the ignition portion  25  and the first gas generating agent  23  are not in contact with each other. 
   The first combustion chamber  40  is enclosed by the first ignition means chamber housing  22 , the cylindrical housing  11  and a first partition wall  18   a . The first combustion chamber  40  accommodates therein a second gas generating agent  43  having a combustion temperature of 1000 to 1700° C. 
   In part of the first combustion chamber  40 , an annular gas inflow space  16   a  is defined, being surround by a housing inner wall surface  13  (crimped portion  17   a ), the outer peripheral surface  22   a  of the ignition means chamber housing  22 , and a flange portion  22   b . And only one side of the annular gas inflow space  16   a  communicates with a space where the second gas generating agent  43  inside the first combustion chamber  40  exists (a space where the gas generating agent exists). 
   As shown in  FIG. 2  dimension of the width W of the communicating portion (the non-surrounded portion of the annular gas inflow space  16   a ) between the gas inflow space  16   a  and the space where the gas generating agent exists is set to be smaller than a diameter of the cylindrical second gas generating agent  43 , which prevents the second gas generating agent  43  from entering the gas inflow space  16   a . As a result, the charging operability of the gas generating agent does not become impaired. 
   The first ignition means chamber  20  and the first combustion chamber  40  are only communicated through a plurality of first communication holes  24 . The plurality of first communication holes  24  faces the gas inflow space  16   a  and faces directly the inner wall surface  13  of the cylindrical housing  11 . 
   Due to the position of the first communication hole  24  and presence of the gas inflow space  16   a  and the inner wall surface  13 , the ignition energy, generated by the activation of the first igniter  21  in the first ignition means chamber  20 , passes through the first communication hole  24 , flows in the first gas inflow space  16   a , strikes against the inner wall surface  13  of the housing to change its direction and then ignites and burns the second gas generating agent  43  in the first combustion chamber  40 . 
   In the other end of the cylindrical housing  11  there are provided a second ignition means chamber  50  and a second combustion chamber  70 , adjacent to each other in the axial direction of the housing. 
   The second ignition means chamber  50  is surrounded by a second ignition means chamber housing  52  as a partitioning means, and accommodates therein a second igniter  51 . The second igniter  51  is fixed together with a collar  59  by crimping an end portion periphery  15   b  of the cylindrical housing  11 . Reference numeral  58  denotes an O-ring for securing moisture-proof. 
   The second ignition means chamber housing  52  is fixed by an annular crimped portion  17   b  of the cylindrical housing  11  and the second igniter  51 , so that it cannot move in the axial direction of the housing  11 . If the annular crimped portion  17   b  is not provided, the second ignition means chamber housing  52  can be press-inserted into the cylindrical housing  11  to be moved optionally in the axial direction of the housing. 
   The second ignition means chamber  50  accommodates therein a first gas generating agent  53  having a combustion temperature of 1700 to 3000° C. The igniting portion  55  of the second igniter  51  is covered by an aluminum cup  56  so that the ignition portion  55  and the first gas generating agent  53  are not in contact with each other. 
   The second combustion chamber  70  is surrounded by a second ignition means chamber housing  52 , the cylindrical housing  11  and a second partition wall  18   b . The second combustion chamber  70  accommodates therein a second gas generating agent  73  having a combustion temperature of 1000 to 1700° C. 
   In part of the second combustion chamber  70 , an annular gas inflow space  16   b  is defined, being surround by a housing inner wall surface  13  (crimped portion  17   b ), the peripheral surface  52   a  of the ignition means chamber housing  52 , and a flange portion  52   b . And only one side of the annular gas inflow space  16   b  communicates with a space where the second gas generating agent  73  inside the second combustion chamber  70  exists (a space where the gas generating agent exists). 
   The dimension of the width (which is the same as W in  FIG. 2 ) of the communicating portion (the non-surrounded side of the annular gas inflow space  16   b ) between the gas inflow space  16   b  and the second combustion chamber  70  where the gas generating agent exists is set to be smaller than a diameter of the cylindrical second. gas generating agent  73 , which prevents the second gas generating agent  73  from entering the gas inflow space  16   b . As a result, the charging operability of the gas generating agent does not become impaired. 
   The second ignition means chamber  50  and the first combustion chamber  70  are only communicated through a plurality of second communication holes  54 . The plurality of second communication holes  54  faces the gas inflow space  16   b  and faces directly the inner wall surface  13  of the cylindrical housing  11 . 
   Due to the position of the first communication hole  54  and presence of the gas inflow space  16   b  and the inner wall surface  13 , the ignition energy, generated by the activation of the first igniter  51  in the first ignition means chamber  50 , passes through the first communication hole  54 , flows in the first gas inflow space  16   b , strikes against the inner wall surface  13  of the housing to change its direction and then ignites and burns the second gas generating agent  73  in the first combustion chamber  70 . 
   In the central portion of the cylindrical housing  11 , there is provided a filter chamber  80  sandwiched between a first partition wall  18   a  and a second partition wall  18   b . The first partition wall  18   a  and the second partition wall  17   b  are welded and fixed to the cylindrical housing  11  at welding portions  19   a  and  19   b.    
   A third communication hole  86   a  is provided in the first partition wall  18   a ; this third communication hole  86   a  is covered, from the first combustion chamber  40  side, by a cap  91  having a plurality of vent holes  92 . The cap  91  is provided to prevent the second gas generating agent  43  from getting into the third communication hole  86   a  thereby blocking it. The cap  91  is fixed to the first partition wall  18   a  by welding a flange  93 . 
   A fourth communication hole  86   b  is provided in the first partition wall  18   a ; this fourth communication hole  86   b  is covered, from the second combustion chamber  70  side, by a cap  94  having a plurality of vent holes  95 . The cap  94  is provided to prevent the second gas generating agent  73  from getting into the fourth communication hole  86   b  thereby blocking it. The cap  94  is fixed to the first partition wall  18   b  by welding a flange portion  96 . 
   In the filter chamber  80 , is provided a cylindrical filter  82 , and a space is provided between the outer surface thereof and the inner wall surface  13  of the cylindrical housing  11 . In the wall surface of the cylindrical housing  11 , opposing the cylindrical filter  82 , there is provided a plurality of gas discharge ports  84 . The plurality of gas discharge ports  84  is closed by a seal tape made of aluminum etc. as a moisture-proof measure. 
   The operation of the gas generator  10 , when incorporated in an automobile air bag system, is described next with reference to  FIGS. 1 and 2 . Below is also explained the case when there is a slight time lag between the activations of the first igniter  21  and the second igniter  51 . 
   The first igniter  21  is activated upon a vehicle collision, causing the first gas generating agent  23  to ignite and burn. Since the first gas generating agent  23  has a combustion temperature of 1700 to 3000° C., it has an excellent ignitability that affords a large ignition energy. 
   The ignition energy (high-temperature gas and flame) generated by this combustion is jetted from the first communication holes  24 , in the diametrical direction of the housing, into the gas inflow space  16   a , where it hits against the inner wall  13  of the cylindrical housing. A mist is captured then by adhering and solidifying to the inner wall  13 , which weakens slightly the intensity of the ignition energy impact. 
   The flow of ignition energy colliding against the inner wall  13  of the cylindrical housing, after making a 90° turn, comes into contact with the second gas generating agent  43  of the first combustion chamber  40 , triggering its ignition and combustion. 
   The ignition energy, generated thus by the combustion of the first gas generating agent  23 , comes into contact with the second gas generating agent  43  of the first combustion chamber  40  not directly but by making a detour, which decreases the likelihood of damages such as fragmentation, etc. of the second gas generating agent  43 , caused by the shockwave of the ignition energy. 
   The combustion temperature of the second gas generating agent  43  ranges from 1000 to 1700° C., which intrinsically corresponds to a low ignitability; however, its effective ignitability is excellent thanks to the large ignition energy released by the combustion of the first gas generating agent  23 , which has a high combustion temperature. Thus, ignitability does not decrease by the roundabout flow of the ignition energy as it comes into contact with the second gas generating agent  43  in the first combustion chamber  40 . 
   The high-temperature gas released in the combustion of the second gas generating agent  43  passes though the vent holes  92  and into the cap  91 , and flows into the filter chamber  80  though the third communication hole  86   a . The gas is cooled then by passing through the cylindrical filter  82 , where combustion residues are filtered; the gas is then discharged out of the gas discharge ports  84 , by bursting the seal tape, in order to inflate the air bag. 
   If the second igniter  51  is activated after a time lag, an identical action causes high-temperature gas to be discharged through the gas discharge ports  84 , further inflating the air bag. 
   (2) Second Embodiment 
   Another embodiment is explained next with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is an axial cross-sectional view of a disk-shaped gas generator, and  FIG. 4  is a partial cross-sectional view of  FIG. 3 . In this embodiment, the effect of the present invention is achieved only in the second combustion chamber. 
   An outer shell container of a gas generator  100  is constituted by a housing  111  formed by joining a diffuser shell  112  and a closure shell  113  which forms an internal storage space together with the diffuser shell  112 . The diffuser shell  112  and the closure shell  113  are welded at a welding portion  114 . Another black-shaded parts of  FIG. 3  indicate other welding portions. 
   Gas discharge ports  117 ,  118  are provided in the circumferential direction of the diffuser shell  112  in a required number, and these gas discharge ports  117 ,  118  are closed by an aluminum seal tape  175 . The gas discharge ports  117  and  118  may have identical or different diameters. 
   An inner cylinder  115  having a substantially cylindrical form is disposed inside the housing  111 . The upper-end peripheral edge of the inner cylinder  115  is joined to a ceiling surface  112   a  of the diffuser  112 , and the lower end peripheral edge thereof is joined to a base surface  113   a  of the closure shell  113 . Thus, a first combustion chamber  120  which accommodates a first gas generating agent  120   b  is formed in the space inside the housing and outside the inner cylinder  115 . A single collar  133 , to which a first igniter  131  and a second igniter  132  are fixed, is provided in the lower side opening of the open inner cylinder  115 , to close the interior of the inner cylinder  115 . 
   A partition wall  140  is provided in the inner cylinder  115  for dividing the space in the interior of the inner cylinder  115  into upper and lower sections. The partition wall  140  is formed in a flat circular form comprising a skirt portion  141 , which surrounds the periphery of the second igniter  132 , and a first through hole  152  formed within the part of the partition wall  140  that is surrounded by the skirt portion  141 ; the partition wall  140  is fitted, from the lower side, into a stepped notch portion  116  of the inner cylinder  115 . 
   The fitting of the partition wall  140  into the stepped notch portion  116  of the inner cylinder  115  prevents the pressure generated upon activation of the first igniter  131  from traveling upwards. Moreover, since the inner diameter of the skirt portion  141  is set to be substantially identical to the outer diameter of the ignition part of the igniter  132 , such that the skirt portion  141  surrounds the ignition part in an airtight fashion, the flame that is generated by an activation of the second igniter  132  advances directly only in the direction of the first communication hole  152 . 
   The first igniter  131  and a first gas generating agent  120   a  charged into an aluminum cup  135  are disposed in a first ignition means chamber  190  which is a space defined outside the skirt portion  141  but inside the lower space partitioned by the partition wall  140  in the inner cylinder  115  (i.e. the space closed by the collar  133 ). The first combustion chamber  120  communicates with the first ignition means chamber  190  through a first through hole  151 . 
   A space occupied by the second igniter  132  inside the skirt portion  141  is a first space  191 , and a space surrounded by the partition wall  140  and a retainer  123  is a second space  121 ; the first space  191  and the second space  121  form a second ignition means chamber. The second space  121  accommodates therein a second gas generating agent  125   a.    
   By disposing the partition wall  140  having the skirt portion  141  in the inner cylinder  115 , a second combustion chamber  125  is separated from the two igniters, and the first igniter  131  is separated from the second igniter  132 . Thus, the combustion energy released by the activation of the first igniter  131  flows into the first combustion chamber  120  but is prevented from entering the first space  191 . 
   The gas generating agent  120   a  charged in the aluminum cup  135  is positioned directly above the first igniter  131 , and the first through hole  151  provided in a lower portion of a side wall of the inner cylinder  115  is positioned substantially directly opposing the axial center of the aluminum cup  135 , in a position that does not directly face the direction of advancement of the flame generated by an activation of the first igniter  131 . An aluminum or stainless steel seal tape  160  is adhered to the first through hole  151  from the inside. 
   Further, the first through hole  151  is provided in the lower portion of the inner cylinder  115 , and a shielding plate  166  is provided outside the first combustion chamber  120 , at a position directly facing the first through hole  151  inside a cylindrical filter  165  which is provided to face the peripheral wall surface of the housing. 
   The shielding plate  166  comprises a tube portion  192  and an outward flange-shaped circular portion  193  formed integrally with one side (the lower side in  FIG. 3 ) of the tube portion  192 ; the shielding plate  166  is formed such that the circular portion  193  abuts against the base surface  113   a  and the tube portion  192  covers a predetermined range of the lower portion of the cylindrical filter  165  (a height range of approximately ½ to ⅔ of the entire height of the cylindrical filter  165 ). 
   The shielding plate  166  may be positioned in relation to the housing by making the outer peripheral edge portion of the circular portion  193  abut against a curved portion  194  of the housing. Filter positioning during assembly may be performed by making the inner peripheral surface of the filter  165  abut against the lower-side outer peripheral surface of the tube portion  192 . The tube portion  192  is disposed to secure an annular gap  171  between the tube portion  192  and the inner peripheral surface of the filter  165 . 
   The space enclosed by the cylinder  115  and the retainer  123  that is provided on the partition wall  140  forms a second combustion chamber  125 . In the second combustion chamber  125 , is charged a second gas generating agent  125   b . The second gas generating agent  125   a  and the second gas generating agent  125   b  have the same combustion temperature relationship as in the first embodiment. 
   In part of the second combustion chamber  125 , an annular gas inflow space  124  is defined, being surrounded by the inner wall surface  115   a  of the inner cylinder  115  (partition wall), a peripheral surface  123   b  and a flange portion  123   c  of the retainer  123 . And only one side of the annular gas inflow space  124  communicates with a space where the second gas generating agent  125   b  inside the second combustion chamber  125  exists. As shown in  FIG. 4 , when a width of the gas inflow space  124  is largest at a communication portion between the space occupied by the second gas generating agent  125  and the gas inflow space  124 , it can desirably afford a better gas flow. However, such width may also be made uniform, as in the case of  FIG. 1 . 
   The dimension of the width W of the communicating section between the second combustion chamber  125  occupied by the second gas generating agent  125   b  and the gas inflow space  124  is set to be smaller than the diameter of the cylindrical second gas generating agent  125   b , which prevents the second gas generating agent  125   b  from entering the gas inflow space  124 . As a result, the charging operability of the gas generating agent does not become impaired. 
   The second space  121  and the second combustion chamber  125  are only in communication with each other through a plurality of second communication holes  122 . The plurality of second communication holes  122  faces the gas inflow space  124  and faces directly the inner wall surface  115   a.    
   The volumes of the second space  121  and the second combustion chamber  125  may be adjusted by moving the retainer  123  optionally in the axial direction of the housing. Alternatively, the retainer  123  may be fixed, but another movable retainer having a plurality of vent holes may be disposed inside the second combustion chamber  125  to adjust the volume of the second combustion chamber  125 . 
   In the gas generator  100  according to the present embodiment, the activation of the second igniter  132  inside the first space  191  causes the ignition and combustion of the second gas generating agent  125   a , thus filling with ignition energy the first space  191  and the second space  121 . 
   The ignition energy flows from the second communication holes  122  into the gas inflow space  124 , and collides with the inner wall surface  115   a . A mist is captured by adhering and solidifying to the inner wall  115   a , which weakens slightly the intensity of the ignition energy impact. 
   The flow of ignition energy colliding against the inner wall  115   a  of the cylindrical housing, after making a 90° turn, comes into contact with the second gas generating agent  125   b  in the second combustion chamber  125 , triggering its ignition and combustion. 
   The ignition energy generated thus by the combustion of the second gas generating agent  125   a  comes into contact with the second gas generating agent  125   b  inside the second combustion chamber  125 , not directly but by making a detour, which decreases the likelihood of damage such as breaking of the second gas generating agent  125   b  caused by the shockwave of the ignition energy. 
   The high-temperature gas generated in the second combustion chamber  125  bursts the seal tape  158  and flows into the first combustion chamber  120  via the second through holes  180 , passes then through the filter  165  and a space  172  defined between the filter  165  and the housing  111 , and is discharged through the gas discharge ports  117 ,  118 , thus further inflating the air bag. 
   (3) Third Embodiment 
   A third embodiment will be described below with reference to  FIG. 5 .  FIG. 5  is an axial cross-sectional view of a gas generator. The gas generator of  FIG. 5  is a dual-type pyrotechnic inflator. 
   In one end of a cylindrical housing  11 , there are provided a first ignition means chamber  20 , a first intermediate chamber  30  and a first combustion chamber  40 . 
   The first ignition means chamber  20  is enclosed by a first ignition means chamber housing  22  serving as a partitioning means, and accommodates therein a first igniter  21 . The first igniter  21  is fixed together with a collar  29  by crimping an end portion periphery  15   a  of the cylindrical housing  11 . The numeral  28  denotes an O-ring  28  for securing moisture-proof. 
   The first ignition means chamber housing  22  is fixed by an annular crimped portion  17   a  of the cylindrical housing  11  and the first igniter  21 , so that it cannot move in the axial direction of the housing  11 . 
   The first ignition means chamber  20  accommodates therein a first gas generating agent  23  having a combustion temperature of 1700 to 3000° C. The ignition portion  25  of the first igniter  21  is covered by an aluminum cup  26  so that the ignition portion  25  and the first gas generating agent  23  are not in contact with each other. 
   The first intermediate chamber  30  is surrounded by a first retainer  32 , the cylindrical housing  11  and a first ignition means chamber housing  22 . 
   The first retainer  32  is fitted to be capable of moving in the axial direction of the cylindrical housing  11 , such that the movement of the first retainer  32  in the axial direction of the housing allows adjusting the volumes of the first intermediate chamber  30  and of the first combustion chamber  40 . 
   A protruding portion  27  is provided at the top of the first ignition means chamber housing  22 , so that even if the first retainer  32  moves towards the first ignition means chamber  20  acted upon by an external force, it is stopped at the position where the top surface of the first retainer  32  hits against the protruding portion  27 . Thereby, such a risk is eliminated that the first intermediate chamber  30  disappears and the flow of the ignition energy is blocked. 
   The first combustion chamber  40  is surrounded by the first retainer  32 , the cylindrical housing  11  and a first partition wall  18   a . The first combustion chamber  40  accommodates therein a second gas generating agent  43  having a combustion temperature of 1000 to 1700° C. 
   The first ignition means chamber  20  and the first intermediate chamber  30  are in communication through a plurality of first communication holes  24  provided in the peripheral wall of the first ignition means chamber housing  22 . The first communication holes  24  are directly opposite to the inner wall surface  13  of the cylindrical housing  11 . 
   The first intermediate chamber  30  and the first combustion chamber  40  are in communication through a plurality of second communication holes  34  provided in the top surface (flat side) of the first retainer  32 . The second communication holes  34  do not directly face the inner wall surface  13  of the cylindrical housing  11 . 
   Part or all of the second communication holes  34  may also be provided in the vicinity of the central portion of the top surface of the first retainer  32  (around a position directly facing the protruding portion  27 ). By providing these holes in the central portion of the top surface, the circumventing distance of the ignition energy increases, further buffering its impact. 
   The first communication holes  24  and the second communication holes  34  are thus arranged such that the directions of their openings are perpendicular to each other. Moreover, the cross-section area of the first intermediate chamber  30  in the axial direction of the housing (the axial cross-section area of the portion shown as A in the drawing) is larger than the total opening area of the second communication holes  34 . 
   In the other end of the cylindrical housing  11 , there are provided a second ignition means chamber  50 , a second intermediate chamber  60 , and a second combustion chamber  70 . 
   The second ignition means chamber  50  is surrounded by a second ignition means chamber housing  52  as a partitioning means, and accommodates therein a second igniter  51 . The second igniter  51  is fixed together with a collar  59  by crimping an end portion periphery  15   b  of the cylindrical housing  11 . The numeral  58  denotes an O-ring for securing moisture-proof. 
   The second ignition means chamber housing  52  is fixed by an annular crimped portion  17   b  of the cylindrical housing  11  and the second igniter  51 , so that it cannot move in the axial direction of the housing  11 . 
   The second ignition means chamber  50  accommodates therein a first gas generating agent  53  having a combustion temperature of 1700 to 3000° C. The igniting portion  55  of the second igniter  51  is covered by an aluminum cup  56  so that the ignition portion  55  and the first gas generating agent  53  are not in contact with each other. 
   The second intermediate chamber  60  is surrounded by a second retainer  62 , the cylindrical housing  11  and a second ignition means chamber housing  52 . 
   The second retainer  62  is fitted to be capable of moving in the axial direction of the cylindrical housing  11 , such that the movement of the second retainer  62  in the axial direction of the housing allows adjusting the volumes of the second intermediate chamber  60  and of the second combustion chamber  70 . 
   A protruding portion  57  is provided at the top of the second ignition means chamber housing  52 , so that even if the second retainer  62  moves towards the second ignition means chamber  50  acted upon by an external force, it is stopped at the position where the top surface of the second retainer  62  hits against the protruding portion  57 . Thereby, such a risk is eliminated that the second intermediate chamber  60  disappears and the flow of the ignition energy is blocked. 
   The second combustion chamber  70  is surrounded by the second retainer  62 , the cylindrical housing  11  and a second partition wall  18   b . The second combustion chamber  70  accommodates therein a second gas generating agent  73  having a combustion temperature of 1000 to 1700° C. 
   The second ignition means chamber  50  and the second intermediate chamber  60  are in communication through a plurality of second communication holes  54  provided in the peripheral wall of the second ignition means chamber housing  52 . The second communication holes  54  are directly opposite to the inner wall surface  13  of the cylindrical housing  11 . 
   The second intermediate chamber  60  and the second combustion chamber  70  are in communication through a plurality of second communication holes  64  provided in the top surface (flat side) of the second retainer  62 . The second communication holes  64  do not directly face the inner wall surface  13  of the cylindrical housing  11 . 
   The first communication holes  54  and the second communication holes  64  are thus arranged so that the directions of their openings are perpendicular to each other. Moreover, the cross-section area of the second intermediate chamber  60  in the axial direction of the housing is larger than the total opening area of the second communication holes  64 . 
   In the central portion of the cylindrical housing  11 , there is provided a filter chamber  80  sandwiched between a first partition wall  18   a  and a second partition wall  18   b . The first partition wall  18   a  and the second partition wall  18   b  are welded and fixed to the cylindrical housing  11  by welding portions  19   a ,  19   b.    
   A third communication hole  86   a  is provided in the first partition wall  18   a ; this third communication hole  86   a  is covered, from the first combustion chamber  40 , by a cap  91  having a plurality of vent holes  92 . The cap  91  is provided to prevent the second gas generating agent  43  from getting into the third communication hole  86   a  thereby blocking it. The cap  91  is fixed to the first partition wall  18   a  by welding a flange portion  93 . 
   A fourth communication hole  86   b  is provided in the first partition wall  18   b ; this fourth communication hole  86   b  is covered, from the second combustion chamber  70 , by a cap  94  having a plurality of vent holes  95 . The cap  94  is provided to prevent the second gas generating agent  73  from getting into the fourth communication hole  86   b  thereby blocking it. The cap  94  is fixed to the first partition wall  18   b  by welding a flange portion  96 . 
   In the filter chamber  80 , is provided a cylindrical filter  82 , and a gap is provided between the outer surface of the cylindrical filter  82  and the inner wall surface  13  of the cylindrical housing  11 . In the wall surface of the cylindrical housing  11  opposing the cylindrical filter  82 , there is provided a plurality of gas discharge ports  84 . As a moisture-proof measure, the plurality of gas discharge ports  84  are closed by a seal tape made of aluminum etc. 
   The operation of the gas generator  10 , when incorporated in an automobile air bag system, is described next with reference to  FIG. 5 . Below is also explained the case when there is a slight time lag between the activations of the first igniter  21  and the second igniter  51 . 
   The first igniter  21  is activated upon vehicle collision, causing the first gas generating agent  23  to ignite and burn. The first gas generating agent  23 , which has a combustion temperature of 1700 to 3000° C., has an excellent ignitability that affords a large ignition energy. The ignition energy (high-temperature gas and flame) generated by this combustion is jetted from the first communication holes  24  in the diametrical direction of the housing, into the first intermediate chamber  30 . At this time, combustion energy hits against the inner wall  13  of the cylindrical housing, and a mist is captured by adhering and solidifying to the inner wall  13 . 
   The ignition energy flowing into the first intermediate chamber  30  changes its course and is then jetted from the second communication holes  34  into the first combustion chamber  40 , where it triggers the ignition and combustion of the second gas generating agent  43 . 
   The ignition energy thus generated by the combustion of the first gas generating agent  23  enters the first combustion chamber  40 , not directly but by making a detour, which decreases the likelihood of damage such as fragmentation, etc., of the second gas generating agent  43 , caused by the shockwave of the ignition energy. 
   The combustion temperature of the second gas generating agent  43  ranges from 1000 to 1700° C., which intrinsically corresponds to a low ignitability; however, its effective ignitability is excellent thanks to the large ignition energy released by the combustion of the first gas generating agent  23  which has a high combustion temperature. Thus, ignitability does not decrease by the roundabout flow of the ignition energy to enter in the first combustion chamber  40 . 
   The high-temperature gas released in the combustion of the second gas generating agent  43  passes through the vent holes  92  and into the cap  91 , and flows into the filter chamber  80  though the third communication hole  86   a . The gas is cooled then by passing through the cylindrical filter  82 , where combustion residues are filtered; the gas is then discharged out of the gas discharge ports  84 , by bursting the seal tape, in order to inflate the air bag. 
   When the second igniter  51  is operated after a time lag, an identical action causes high-temperature gas to be discharged through the gas discharge ports  84 , further inflating the air bag. 
   (4) Fourth to Sixth Embodiments 
   Another embodiments of the present invention are described with reference to  FIGS. 6 to 8 .  FIGS. 6 to 8  are partial cross-sectional view of differing embodiments of the gas generator  10  of  FIG. 5 . The basic structure of them all is identical to that of  FIG. 5 , with differences only as regards the structure and the configuration of the first ignition means chamber, the intermediate chamber, and the combustion chamber. In all cases, only the structures in one end will be described since the other end is identical, like the embodiment of  FIG. 5 . 
   Gas Generator of  FIG. 6   
   A first ignition means chamber  20  is surrounded by a first ignition means chamber housing  22  as a partitioning means, and a cylindrical housing  11 , and accommodates therein a first igniter  21 . The first igniter  21  is fixed together with a collar  29  by crimping an end portion peripheral edge  15   a  of the cylindrical housing  11 . Reference numeral  28  denotes an O-ring  28  for securing moisture-proof. 
   As shown in the drawing, the first ignition means chamber housing  22 , having a two-step shape, abuts against a first igniter  21  at the opening, and has an outer peripheral surface  22   a , corresponding to the first step, that abuts against the inner wall surface  13 , and an outer peripheral surface  22   b , corresponding to the second step, where there are provided a plurality of first communication holes  24 ; in addition, a protruding portion  27  is provided on the central portion of the top surface  22   c  of the first ignition means chamber housing  22 . The first communication holes  24  directly oppose the inner wall surface  13 . 
   The first intermediate chamber  30  is surrounded by a first retainer  32 , the cylindrical housing  11  and a first ignition means chamber housing  22 . The peripheral edge of the first retainer  32  fits against a step  14  provided in the wall surface of the cylindrical housing  11 . A plurality of second communication holes  34  not facing the inner wall surface  13  are provided in the vicinity of the peripheral edge of the first retainer  32 , except in the central portion thereof. 
   Neither the first ignition means chamber housing  22  nor the first retainer  32  can move in the axial direction of the cylindrical housing  11 , since the protruding portion  27  in the first ignition means chamber housing  22  abuts against the first retainer  32 , while the peripheral edge of the first retainer  32  fits against the step  14  and the opening of the first ignition means chamber housing  22  abuts against the first igniter  21 . 
   Thus, in the gas generator of  FIG. 6 , the first communication holes  24  and the second communication holes  34  are arranged such that the directions of their openings are perpendicular to each other. Moreover, the cross-section area of the first intermediate chamber  30  in the axial direction of the housing (the cross-section area of the narrowest section, corresponding to A in  FIG. 5 ) is larger than the total opening area of the second communication holes  34 . 
   Part of all of the second communication holes  34  may also be provided in the vicinity of the central portion of the top surface of the first retainer  32  (around a position directly facing the protruding portion  27 ). By providing the holes in the vicinity of the central portion of the top surface, the circumventing distance of the ignition energy increases, further buffering its impact. 
   Gas Generator of  FIG. 7   
   A first ignition means chamber  20  is surrounded by a first ignition means chamber housing  22  as a partitioning means, and a cylindrical housing  11 , and accommodates therein a first igniter  21 . The first igniter  21  is fixed together with a collar  29  by crimping end portion peripheral edge  15   a  of the cylindrical housing  11 . Reference numeral  28  denotes an O-ring  28  for securing moisture-proof. 
   As shown in the drawing, the first ignition means chamber housing  22 , having a two-step shape, abuts against a first igniter  21  at the opening portion, and has an outer peripheral surface  22   a , corresponding to the first step, that abuts against the inner wall surface  13 , and an outer peripheral surface  22   b , corresponding to the second step, where there are provided a plurality of first communication holes  24 . The first communication holes  24  directly oppose the inner wall surface  13 . 
   The first intermediate chamber  30  is surrounded by a first retainer  32 , the cylindrical housing  11  and a first ignition means chamber housing  22 . The peripheral edge of the first retainer  32  fits against a step  14  provided in the wall surface of the cylindrical housing  11 ; in addition, a recess  32   a  (a protrusion in the direction of the first igniter  21 ) is also provided in the central portion of the first retainer  32 . A plurality of second communication holes  34  not facing the inner wall surface  13  is provided in the vicinity of the peripheral edge of the first retainer  32 , except in the central portion thereof. 
   Neither the first ignition means chamber housing  22  nor the first retainer  32  can move in the axial direction of the cylindrical housing  11 , since the recess  32   a  of the first retainer  32  abuts against the top surface  22   c  of the first ignition means chamber housing  22 , while the edge of the first retainer  32  fits against the step  14  and the opening portion of the first ignition means chamber housing  22  abuts against the first igniter  21 . 
   Thus, in the gas generator of  FIG. 7 , the first communication holes  24  and the second communication holes  34  are arranged such that the directions of their openings are perpendicular to each other. Moreover, the cross-section area of the first intermediate chamber  30  in the axial direction of the housing (the cross-section area of the narrowest section, corresponding to A in  FIG. 5 ) is larger than the total opening area of the second communication holes  34 . 
   Gas Generator of  FIG. 8   
   A first ignition means chamber  20  is surrounded by a first ignition means chamber housing  22  as a partitioning means, and a cylindrical housing  11 , and accommodates therein a first igniter  21 . The first igniter  21  is fixed together with a collar  29  by crimping end portion peripheral edge  15   a  of the cylindrical housing  11 . Reference numeral  28  denotes an O-ring  28  for securing moisture-proof. 
   The first ignition means chamber housing  22 , as shown in the drawing, abuts against a first igniter  21  at the opening portion, and has an outer peripheral surface  22   a  that abuts against the inner wall surface  13 , and a top surface  22   b  where there are provided a plurality of first communication holes  24 ; in addition, a protruding portion  27  is provided on the central portion of the top surface  22   b  of the first ignition means chamber housing  22 . The first communication holes  24  herein do not directly face the inner wall surface  13 . 
   The first intermediate chamber  30  is surrounded by a first retainer  32 , the cylindrical housing  11  and a first ignition means chamber housing  22 . 
   As shown in the drawing, the first retainer  32  has a stepped shape and the peripheral edge thereof fits against a step  14  provided in the wall surface of the cylindrical housing  11 . A plurality of second communication holes  34 , directly facing the inner wall surface  13 , are provided in the outer peripheral surface  34   a  of the step portion of the first retainer  32 . 
   Neither the first ignition means chamber housing  22  nor the first retainer  32  can move in the axial direction of the cylindrical housing  11 , since the protruding portion  27  of the first ignition means chamber housing  22  abuts against the top surface  32   b  of the first retainer  32 , while the edge of the first retainer  32  fits against the step  14  and the opening portion of the first ignition means chamber housing  22  abuts against the first igniter  21 . 
   Thus, in the gas generator of  FIG. 8 , the first communication holes  24  and the second communication holes  34  are arranged such that the directions of their openings are perpendicular to each other. Moreover, the cross-section area of the first intermediate chamber  30  in the axial direction of the housing (the cross-section area of the narrowest section, corresponding to A in  FIG. 5 ) is larger than the total opening area of the second communication holes  34 . 
   The operation of the gas generator for an air bag of  FIG. 6 to 8  is identical to the operation of the gas generator  10  for an air bag of  FIG. 5 .