Abstract:
An improved structure of a gas sensor is provided which is designed to form an air-tight seal in a base end of a sensor body which is insensitive to heat. The gas sensor includes a seal assembly made up of an elastic seal and a heat-resistant support. The heat-resistant support is located closer to the top of the gas sensor than the elastic seal to protect the elastic seal from the heat transmitted to the base end from the top of the sensor body. The elastic seal is disposed within the base end of the sensor body and compressed constantly to produce a reactive force which serves to create the air-tight seal in the base end of the gas sensor through which lead wires pass.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to a gas sensor which may be installed in an exhaust pipe of automotive engines to measure the concentration of gas such O 2 , NOx, or CO, and more particularly to an improved structure of such a gas sensor which is designed to minimize thermal damage to a hermetic seal in a base end of a sensor body from which a lead wire extends.  
           [0003]    2. Background Art  
           [0004]    Gas sensors are know which are installed in an exhaust pipe of automotive engines for use in air-fuel ratio control of the engine.  
           [0005]    [0005]FIG. 14 shows one example of such a type of gas sensors which is taught in Japanese Patent First Publication No. 11-72472 (corresponding to U.S. Pat. No. 6,150,607, issued Nov. 21, 2000).  
           [0006]    The gas senor  9  has installed therein a sensor element (not shown) which is electrically connected to an external control device through leads  16  for transmitting a sensor output and receiving electric power therefrom. The gas sensor  9  has a seal member  97  made of rubber or resin which forms an air-tight seal between the leads  16  and an inner wall of an open base end of the gas sensor  9 . The seal member  97  is retained within an air cover  92  by crimping a side wall of the air cover  92 .  
           [0007]    In a case where the gas sensor  9  is installed in the exhaust pipe of the automotive engine, a top portion of the gas sensor is exposed to a hot exhaust gas, so that it is heated up. The heat is transmitted to the base end of the gas sensor  1 , which may result in thermal deformation or change in coefficient of elasticity of the seal member  97 . This may cause an air gap to be formed between the seal member  97  and the inner wall of the air cover, thus decreasing the degree of sealing therebetween and the leads  16  to be shifted in position or dislodged from the base end of the air cover  92 .  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.  
           [0009]    It is another object of the present invention to provide an improved structure of a gas sensor designed to form a hermetic seal in an end of a sensor body which is insensitive to heat.  
           [0010]    According to one aspect of the invention, there is provided a gas sensor which may be installed in an exhaust pipe of automotive engines to measure the concentration of a given component of exhaust gasses of the engine. The gas sensor has a length with a first and a second end opposed to the first end and comprises: (a) a hollow cylindrical housing having a length with a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor; (b) a sensor element retained in the housing, the sensor element having a length made up of a base portion and a sensing portion which works to measure a concentration of a given component of gasses; (c) a measurement gas side cylindrical cover joined to the first end of the housing to cover the sensing portion of the sensor element; (d) an atmosphere side cylindrical cover having a length with a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor, the atmosphere side cylindrical cover being installed at the first end thereof on the second end of the housing to cover the base portion of the sensor element; (e) an end cover formed on the second end of the atmosphere side cylindrical cover; and (f) a seal assembly working to retain a lead hermetically therewithin which is electrically connected to the sensor element for transmitting an output of the sensor element to or receiving electric power from an external device. The seal assembly is made up of an elastic seal and a heat-resistant support. The elastic seal includes a disc portion which has a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor and is placed in abutment of the second end thereof with the end cover and a cylindrical seal portion within which the lead is fitted and which has a length with a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor. The cylindrical seal portion extends from the disc portion in a longitudinal direction of the gas sensor and has an outer diameter decreasing from the second end to the first end thereof. The heat-resistant support is made up of a hollow cylindrical body and a disc-shaped support formed on the hollow cylindrical body, disposed within the atmosphere side cylindrical cover, and born by the housing through an inner support member. The hollow cylindrical body has an inner surface substantially contoured to conform with a contour of an outer surface of the cylindrical seal portion of the elastic seal so that the cylindrical seal portion is fitted within the hollow cylindrical body hermetically. The disc-shaped support is placed in abutment with the disc portion of the elastic seal. The elastic seal is compressed elastically between the end cover and the heat-resistant support in the longitudinal direction of the gas sensor to produce a reactive force which serves to ensure hermetic seals between the lead and the cylindrical seal portion and between the end cover and the elastic seal.  
           [0011]    In the preferred mode of the invention, the gas sensor may further comprise a heat-resistant elastic member disposed between the heat-resistant support and the inner support member. As already discussed in the introductory part of this application, in the case where the gas sensor is installed in the exhaust pipe of the engine, the top of the gas sensor is the highest in temperature. Installation of the heat-resistant elastic member closer to a heat source than the seal assembly serves to protect the hermetic seals against the heat.  
           [0012]    The end cover has formed therein a hole through which the lead passes. The hole has a cross section in a direction perpendicular to the longitudinal direction of the gas sensor which is substantially similar in shape to an opening formed in the second end of the cylindrical seal portion of the elastic seal. This results in a maximized area of contact between the end cover and the cylindrical seal portion, thus increasing an area of the end cover pressing the cylindrical seal portion to enhance the degree of sealing between the end cover and the cylindrical seal portion.  
           [0013]    The second end of the disc portion of the elastic seal may have a flat surface abutting with the end cover, thereby resulting in a decrease in production cost of the elastic seal and also minimizing thermal damage or breakage of the elastic seal.  
           [0014]    The end cover has an annular protrusion around a periphery of the hole thereof which presses a periphery of an opening formed in the second end of the cylindrical seal portion of the elastic seal, thereby enhancing the degree of sealing between the end cover and the cylindrical seal portion.  
           [0015]    The portion of the elastic seal may have an annular recess formed in the second end thereof around the opening of the cylindrical seal portion. The annular protrusion of the end cover is fitted within the annular recess hermetically.  
           [0016]    The cylindrical seal portion of the elastic seal may have an annular protrusion formed around an opening formed in the second end thereof. The annular protrusion is fitted in an opening formed in the end cover hermetically, thereby enhancing the degree of sealing between the end cover and the cylindrical seal portion.  
           [0017]    The end cover has an inner end surface placed in abutment with the second end of the disc portion of the elastic seal. The disc portion is placed in abutment of the first end with the second end of the disc-shaped support of the heat-resistant support. The disc-shaped support is placed in abutment of the first end thereof with an end of the inner support member facing the second end of the gas sensor. The end cover may have formed on the inner end surface thereof a rib which projects to the second end of the disc portion of the elastic seal to establish a hermetic seal between the inner end surface of the end cover and the second end surface of the disc portion.  
           [0018]    The rib may be circular in shape. The disc portion of the elastic seal has formed in the second end thereof an annular groove within which the rib is fitted hermetically.  
           [0019]    The heat-resistant support may be made of an electric insulating material, thereby avoiding leakage of current from the head to any parts of the gas sensor and dielectric breakdown causing a failure in operation of the gas sensor.  
           [0020]    The cylindrical seal portion may have a rib formed on an inner surface which abuts the lead to establish a hermetic seal therebetween.  
           [0021]    According to the second aspect of the invention, there is provided a gas sensor which has a length with a first and a second end opposed to the first end. The gas sensor comprises: (a) a hollow cylindrical housing having a length with a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor; (b) a sensor element retained in the housing, the sensor element having a length made up of a base portion and a sensing portion which works to measure a concentration of a given component of gasses; (c) a measurement gas side cylindrical cover joined to the first end of the housing to cover the sensing portion of the sensor element; (d) an atmosphere side cylindrical cover having a length with a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor, the atmosphere side cylindrical cover being installed at the first end thereof on the second end of the housing to cover the base portion of the sensor element; (e) an end cover formed on the second end of the atmosphere side cylindrical cover; and (f) a seal assembly working to retain a lead hermetically therewithin which is electrically connected to the sensor element for transmitting an output of the sensor element to or receiving electric power from an external device. The seal assembly is made up of an elastic seal and a heat-resistant support. The elastic seal includes a disc portion which has a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor and is placed in abutment of the second end thereof with the end cover and a cylindrical seal portion within which the lead is fitted and which has a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor. The cylindrical seal portion extends from the disc portion in a longitudinal direction of the gas sensor. The heat-resistant support includes a hollow cylindrical body, disposed within, and born by the atmosphere side cylindrical cover from a side of the second end of the gas sensor. The heat-resistant support has a first end facing the first end of the gas sensor and a second end facing the second end of the gas sensor. The heat-resistant support is placed in abutment of the second end thereof with first end of the cylindrical seal portion of the elastic seal. The elastic seal is compressed elastically between the end cover and the heat-resistant support in the longitudinal direction of the gas sensor to produce a reactive force which serves to ensure hermetic seals between the lead and the cylindrical seal portion and between the end cover and the elastic seal.  
           [0022]    In the preferred mode of the invention, the end cover has formed therein a hole through which the lead passes. The hole has a cross section in a direction perpendicular to the longitudinal direction of the gas sensor which is substantially similar in shape to an opening formed in the second end of the cylindrical seal portion of the elastic seal.  
           [0023]    The second end of the disc portion of the elastic seal may have a flat surface abutting with the end cover.  
           [0024]    The end cover has an annular protrusion around a periphery of the hole thereof which presses a periphery of an opening formed in the second end of the cylindrical seal portion of the elastic seal.  
           [0025]    The disc portion of the elastic seal may have an annular recess formed in the second end thereof around the opening of the cylindrical seal portion. The annular protrusion is fitted within the annular recess to establish an air-tight seal therebetween.  
           [0026]    The cylindrical seal portion of the elastic seal may have an annular protrusion formed around an opening formed in the second end thereof. The annular protrusion is fitted in an opening formed in the end cover, thereby creating an air-tight seal therebetween.  
           [0027]    The cover has an inner end surface placed in abutment with the second end of the disc portion of the elastic seal. The disc portion is placed in abutment of the first end with the second end of the hollow cylindrical body of the heat-resistant support. The hollow cylindrical body is supported at the first end thereof by the atmosphere side cylindrical cover and urging the first end of the disc portion of the elastic seal through a hollow cylinder to compress the elastic seal, thereby producing the reactive force. The end cover has formed on the inner end surface thereof a rib which projects to the second end of the disc portion of the elastic seal to establish a hermetic seal between the inner end surface of the end cover and the second end surface of the disc portion.  
           [0028]    The rib may be circular in shape. The disc portion of the elastic seal has formed in the second end thereof an annular groove within which the rib is fitted hermetically to establish an air-tight seal therebetween.  
           [0029]    The heat-resistant support may be made of an electric insulating material.  
           [0030]    The cylindrical seal portion may have a rib formed on an inner surface which abuts the lead to establish a hermetic seal therebetween. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0032]    In the drawings:  
         [0033]    [0033]FIG. 1 is a longitudinal sectional view which shows a gas sensor according to the first embodiment of the invention;  
         [0034]    [0034]FIG. 2 is a partially sectional view which shows a structure of a hermetic seal in an open end of the gas sensor of FIG. 1;  
         [0035]    [0035]FIG. 3 is an exploded sectional view which shows a seal assembly installed in the open end of the gas sensor, as illustrated in FIG. 2;  
         [0036]    [0036]FIG. 4( a ) is a top view which shows an elastic seal of the seal assembly, as illustrated in FIG. 3;  
         [0037]    [0037]FIG. 4( b ) is a side view of FIG. 4( a );  
         [0038]    [0038]FIG. 5( a ) is a top view which shows an end cover of the gas sensor, as illustrated in FIG. 1;  
         [0039]    [0039]FIG. 5( b ) is a side view of FIG. 5( a );  
         [0040]    [0040]FIG. 6( a ) is a top view of a washer installed in the gas sensor, as illustrated in FIG. 1;  
         [0041]    [0041]FIG. 6( b ) is a side view of FIG. 6( a );  
         [0042]    [0042]FIG. 7 is a longitudinal sectional view which shows a gas sensor according to the second embodiment of the invention;  
         [0043]    [0043]FIG. 8( a ) is a top view which shows an elastic seal installed in the gas sensor, as illustrated in FIG. 7;  
         [0044]    [0044]FIG. 8( b ) is a vertical sectional view which shows the elastic seal of FIG. 8( a );  
         [0045]    [0045]FIG. 8( c ) is a vertical sectional view which shows a heat-resistant support installed in the gas sensor, as illustrated in FIG. 7;  
         [0046]    FIGS.  9 ( a ) and  9 ( b ) are partially sectional views which shows sequential steps of installation of a seal assembly in the gas sensor, as illustrated in FIG. 7;  
         [0047]    [0047]FIG. 10 is a partially vertical sectional view which shows a structure of a hermetic seal in a base end of a gas sensor according to the third embodiment of the invention;  
         [0048]    [0048]FIG. 11 is a partially vertical sectional view which shows a structure of a hermetic seal in a base end of a gas sensor according to the fourth embodiment of the invention;  
         [0049]    [0049]FIG. 12 is a partially vertical sectional view which shows a structure of a hermetic seal in a base end of a gas sensor according to the fifth embodiment of the invention;  
         [0050]    [0050]FIG. 13 is a top view of FIG. 12; and  
         [0051]    [0051]FIG. 14 is a partially vertical sectional view which shows a structure of a hermetic seal formed in a base end of a conventional gas sensor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to FIG. 1, there is shown a gas sensor  1  according to the first embodiment of the invention which may be employed in automotive air-fuel ratio control systems to measure O 2 , HC, CO, or NOx contained in exhaust gasses of an internal combustion engine.  
         [0053]    The gas sensor  1  generally includes a gas sensor element  19  working to measure a preselected component of gasses, a hollow cylindrical housing  10  retaining therein the gas sensor element  19 , a protective cover assembly  11  installed on a head end of the housing  10  to cover a head portion (i.e., a sensing portion) of the gas sensor element  19 , a cylindrical air cover  4  installed on a base end of the housing  10  to cover a base portion of the gas sensor element  19 , and a seal assembly  17  fitted hermetically within an open end of the air cover  4 . The seal assembly  7  also works to retain leads  16  connecting electrically between the gas sensor element  19  and an external sensor controller (not shown) to transmit a sensor output to and receive electric power from the sensor controller.  
         [0054]    The seal assembly  17  consists of an elastic seal  2  and a heat-resistant support  3 . The elastic seal  2  is, as clearly shown in FIG. 3, made up of disc-shaped support  21  (will also be referred to as a flange below) and four hollow cylindrical lead seals  22 . Each of the lead seals  22  extends in a longitudinal direction of the gas sensor  1  and has an outer diameter which decreases from a base end (i.e., an upper end, as viewed in FIG. 3) joined to the flange  21  to a top end (i.e., a lower end, as viewed in FIG. 3). Each of the lead seals  22  retains therein one of the leads  16  hermetically.  
         [0055]    The heat-resistant support  3  is made up of a disc-shaped support  31  (will also be referred to as a flange below) and a cylindrical body  32 . The cylindrical body  32  has formed therein four cylindrical through holes  333  within which the lead seals  22  of the elastic seal  2  are to be fitted. The heat-resistant support  3  is, as can be seen from FIGS. 1 and 2, disposed within a small-diameter portion of the air cover  4  and born by an inner cover  42  supported indirectly by the housing  10 . The heat-resistant support  3  may be made of an electric insulating material in order to avoid leakage of current from the heads  16  to any parts of the gas sensor  1  and dielectric breakdown causing a failure in operation of the gas sensor  1 .  
         [0056]    The circular flange  31  has a base end surface  311  which abuts a top end surface  212  of the circular flange  21  of the elastic seal  2 . The cylindrical body  32  has inner side surfaces  322  each of which is substantially contoured to conform with the contour of one of the lead seals  22  so that the lead seal  22  is fitted within the through hole  333  hermetically.  
         [0057]    The air cover  4  includes, as shown in FIG. 1, an end cover  45  which abuts to a base end surface  211 , as shown in FIG. 3, of the circular flange  21  of the elastic seal  2 .  
         [0058]    A reactive force produced by compressive presses, as indicated by arrows F in FIG. 2, acting on the circular flange  211  of the elastic seal  2  in opposite directions parallel to the longitudinal center line of the gas sensor  1  between the end cover  45  and the flange  31  of the heat-resistant support  3  works to form air-tight seals between the leads  16  and the cylindrical lead seals  22  and between the flange  211  and the end cover  45  of the air cover  4 .  
         [0059]    The gas sensor  1  is installed, for example, in a wall of an exhaust pipe joining to an automotive engine to determine an air-fuel ratio for use in air-fuel ratio control of the engine. In the installation of the gas sensor  1 , an end surface  108  of a flange  100  of the housing  10 , as illustrated in FIG. 1, is placed in abutment to an outer surface of the exhaust pipe through a spring  107 . The spring  107  works to provide hermetic sealing between the end surface  108  and the outer surface of the exhaust pipe.  
         [0060]    When the engine is running, a lower portion of the gas sensor  1  below a broken line Min FIG. 1, is exposed to exhaust gasses flowing through the exhaust pipe and heated thereby. An upper portion of the gas sensor  1  above a broken line L is exposed to the atmospheric air. The temperature of the gas sensor  1 , thus, decreases gradually from the broken line L to the base end of the gas sensor  1  (i.e., the upper end, as viewed in FIG. 1).  
         [0061]    The protective cover assembly  11  is of a double-walled structure and made up of an outer cylindrical cover and an inner cylindrical cover disposed within the outer cover coaxially with each other. The outer and inner covers, as shown in FIG. 1, have gas holes  112  through which the exhaust gasses pass and enter inside a gas chamber defined in the inner cover. The gas sensor element  19  has a head portion (i.e., a sensing portion) exposed to the exhaust gasses in the inner cover. The protective cover assembly  11  may alternatively be of a single- or multi-walled (more than two) structure.  
         [0062]    The gas sensor element  19  is retained within the housing  10  through the insulation porcelain  12 . Gas-tight seals are formed between the insulation porcelain  12  and the housing  10  and between the insulation porcelain  12  and the gas sensor element  19 .  
         [0063]    The insulation porcelain  13  is disposed within the air cover  4  in alignment with the insulation porcelain  12 . The insulation porcelain  13  has formed therein a cavity  130  within which a base portion of the gas sensor element  19  is disposed. The insulation porcelain  13  has formed in a base end thereof holes  131  leading to the cavity  130 .  
         [0064]    The gas sensor element  19  connects with leads  16  through terminals  191  and connectors  192  such as clamp contacts for transmitting an output of the gas sensor element  19  to and receiving electric power from an external sensor controller (not shown). The terminals  191  pass through the holes  131  and extend into an air chamber formed inside a base portion of the air cover  4  above the insulation porcelain  13 . Within the air chamber, the terminals  191  are joined electrically to the leads  16  through the connectors  192 . The leads  16  extend through the seal assembly  17  and connect with the external sensor controller.  
         [0065]    A disc spring  151  is placed on the base end of the insulation porcelain  13 . The disc spring  151  is urged elastically by the insulation porcelain  13  and a shoulder of the inner cover  42  to produce a spring pressure.  
         [0066]    The elastic seal  2 , as can be seen in FIG. 3, includes the circular flange  21  and the cylindrical lead seals  22 . The lead seals  22  extend from the flange  21  in parallel to the longitudinal center line of the gas sensor  1 . Each of the lead seals  22  has an outer wall which tapers off to the top end thereof (i.e., the lower end as viewed in the drawing). Each of the lead seals  22  has two annular ribs  2220  formed on an inner wall  222  thereof which work to retain the leads  16  hermetically.  
         [0067]    Each of the lead seals  22  also has an annular boss  290  which extends upward, as viewed in the drawing, from the flange  21  toward the end cover  45 . Each of the lead seals  22  has formed therein, as clearly shown in FIG. 4( a ), an opening  291  for insertion of one of the leads  16 .  
         [0068]    The outer diameter R1 of the elastic seal  2  (i.e., the flange  21 ), as shown in FIG. 4( a ), is 13 mm. The center-to-center pitch R2 of two of the openings  291  diametrically opposed to each other is 5.4 mm. The minimum outer diameter R3, as shown in FIG. 4( b ), of the cylindrical lead seals  22  is 3 mm. The maximum outer diameter R4 of the cylindrical lead seals  22  (i.e., the base end just beneath the flange  21 ) is 4 mm. The total height R5 of the elastic seal  2  is 5.4 mm.  
         [0069]    The heat-resistant support  3  is, as shown in FIG. 3, made up of the flange  31  and the cylindrical body  32 .  
         [0070]    The flange  31  has the base end surface  311  which abuts the top end surface  212  of the flange  21  of the elastic seal  2  and the cylindrical body  32  which has the four through holes  333  within which the cylindrical lead seals  22  are fitted, respectively. The cylindrical body  32  has cylindrical inner surfaces  322  which define the through holes  333  and are contoured to conform with the contour of the lead seals  22  of the elastic seal  2  for guiding insertion of the lead seals  22  when the elastic seal  2  and the heat-resistant support  3  are assembled. The lead seals  22  have the outer diameter slightly greater than the diameter of the through holes  333  which establishes a press fit of the lead seals  22  within the through holes  333  to enhance the degree of sealing between the lead seals  22  and the through holes  333 .  
         [0071]    The main cover  41  of the air cover  4 , as clearly shown in FIGS.  5 ( a ) and  5 ( b ), has the end cover  45  formed integrally. The end cover  45  has an outer end surface  451  exposed outside the gas sensor  1  and an inner end surface  452  which abuts to the base end surface  211  of the flange  21  of the elastic seal  2 . The end cover  45  has formed therein four lead insertion holes  459  which coincide with the openings  291  of the elastic seal  2 , respectively, when installed within the air cover  4  and annular protrusions  450  each of which extends around the periphery of one of the lead insertion holes  459 . The holes  459  are circular, like the openings  291  of the elastic seal  2 , thereby resulting in a maximized area of the end cover  45  working to press the elastic seal  2  elastically with aid of the inner cover  42  to create the air-tight seal between the end cover  45  and the elastic seal  2   
         [0072]    The elastic seal  2 , as described above, has the annular bosses  290  which are, as shown in FIG. 2, press-fitted within the lead insertion holes  459  to form air-tight seals between the leads  16  and the lead insertion holes  450 .  
         [0073]    The seal assembly  17  is, as already described, made up of the elastic seal  2  and the heat-resistant support  3 . The elastic seal  2  is disposed within the main cover  41  of the air cover  4  and urged elastically between the end cover  45  and the base end of the inner cover  42  through the heat-resistant support  3  and the washers  461  and  462 , thereby enhancing the degree of air-tight sealing between the leads  16  and the lead seals  22  of the elastic seal  2  and between the end cover  45  and the elastic seal  2 . The washers  461  and  462  may be made of a heat-resistant material such as a stainless steel in order to protect the seal assembly  17  from the heat transmitted from the top of the gas sensor  1  when installed in the exhaust pipe of the automotive engine, thereby ensuring the sealing ability of the seal assembly  17 . Instead of the washers  461  and  462 , disc springs or leaf springs may be employed.  
         [0074]    The heat-resistant support  3  is held by the housing  10  through the insulator porcelains  12  and  13  and the inner cover  42 . The heat-resistant support  3  works to urge the elastic seal  2  elastically into constant abutment with the end cover  45 .  
         [0075]    In a case where the gas sensor  1  is installed in the exhaust pipe of the automotive engines, the temperature of the gas sensor  1  drops from the top end (i.e., the protective cover assembly  11 ) to the base end (i.e., the end cover  45 ) thereof. In other words, the base end of the gas sensor  1  is located farther away form a heat source. The heat-resistant support  3  is located closer to the heat source than the elastic seal  2 , thereby protecting the air-tight seals produced by the elastic seal  2  against the heat.  
         [0076]    FIGS.  7  to  9 ( b ) show the gas sensor  1  according to the second embodiment of the invention.  
         [0077]    The gas sensor  1  consists essentially of the gas sensor element  19 , the hollow cylindrical housing  10  retaining therein the gas sensor element  19 , the protective cover assembly  11  installed on the head end of the housing  10 , the cylindrical air cover  4  installed on the base end of the housing  10 , and the seal assembly  17  fitted hermetically within the open end of the air cover  4 . The seal assembly  7 , like the first embodiment, works to retain the leads  16  connecting electrically between the gas sensor element  19  and an external sensor controller (not shown) to transmit a sensor output to and receive electric power from the sensor controller and to establish the hermetic seals between the leads  16  and the elastic seal  2  and between the elastic seal  2  and the end cover  45  of the air cover  4 .  
         [0078]    The air cover  4  includes the cylindrical main cover  41  welded to the side wall of the base end of the housing  10  and the cylindrical filter cover  44 . The main cover  41  is welded directly to the side wall of the base portion of the housing  10 . The filter cover  44  is secured to the outer surface of the small-diameter portion of the main cover  41  and crimped to retain the water-repellent filter  43  on the periphery of the main cover  41 . The main cover  41  and the filter cover  44  have formed therein air vents through which air is admitted into the air chamber defined inside the small-diameter portion of the main cover  41 .  
         [0079]    The seal assembly  17  is disposed between the base end  419  of the main cover  41  and the end cover  45  formed integrally with the filter cover  44 . The seal assembly  17  is made up of the elastic seal  2  and the heat-resistant support  5 .  
         [0080]    The elastic seal  2  includes, as clearly shown in FIGS.  8 ( a ) and  8 ( b ), a disc  21  and cylindrical lead seals  290 . The lead seals  290  extend through the disc  21  in a direction parallel to the longitudinal center line of the gas sensor  1  and work to retain the leads  16  hermetically.  
         [0081]    The heat-resistant support  5  is, as shown in FIG. 8( c ), a cylindrical member which has four cylindrical through holes  32  formed therein and a flange  595 . The holes  32  have openings  391  which communicate with openings  292  of the lead seals  22 , respectively when the heat-resistant support  5  and the elastic seal  2  are assembled, as illustrated in FIG. 7.  
         [0082]    The heat-resistant support  5  is, as shown in FIG. 7, disposed within the air cover  4  and born by the housing  10  through the main cover  41 . Specifically, the main cover  41  bears the heat-resistant support  5  in abutment of the base end  419  thereof with the top end surface  592  of the heat-resistant support  5 .  
         [0083]    The heat-resistant support  5  has the base end surface  591  abutting to the top end surfaces  252  of the lead seals  22  of the elastic seal  2 .  
         [0084]    The heat-resistant support  5  has a shoulder  54  formed on an outer side wall thereof. A metallic sleeve  55  is disposed within the main cover  41  in abutment of a lower end thereof to the shoulder  54  of the heat-resistant support  5 . The metallic sleeve  55  has a flange  551  extending in a radius direction of the gas sensor  1  which abuts to the end surface  212  of the disc  21  of the elastic seal  2 . Specifically, the metallic sleeve  55  works as a support member which supports the disc  21  of the elastic seal  2  elastically to create an air-tight seal between the inner surface of the end cover  45  and the end surface of the disc  21 .  
         [0085]    The filter cover  44 , as already described, has the end cover  45  which abuts the end surface  211  of the disc  21  of the elastic seal  2 .  
         [0086]    A total outer diameter S1, that is, a distance between peripheries of diametrically opposed two of the lead seals  291  is 10 mm. A center-to-center pitch S2 of the openings  292  of the lead seals  291  is 5.4 mm.  
         [0087]    Installation of the seal assembly  17  is achieved in the following steps.  
         [0088]    First, the elastic seal  2  is, as shown in FIG. 9( a ), disposed on the heat-resistant support  5  within the filter cover  44  with the disc  21  placed between the end cover  45  and the flange  551  of the metallic sleeve  55 . The heat-resistant support  5  is born by the base end  419  of the main cover  41 . The water-repellent filter  43  is disposed between the side wall of the filter cover  44  and the side walls of the metallic sleeve  55 , the heat-resistant support  5 , and the main cover  41 . The water-repellent filter  43  is separate at the lower end thereof from the shoulder of the main cover  41  through an air gap  601 . The metallic sleeve  55  is separate at the lower end thereof from the shoulder  54  of the heat-resistant support  5  through an air gap  602 . Although not illustrated, in this step, the leads  16  are inserted into the elastic seal  2  and the heat-resistant support  5 .  
         [0089]    Next, pressure is applied, as indicated by arrows Fin FIG. 9( b ), to the end cover  45  to compress it until the air gaps  601  and  602  disappear and reactive forces are created which act on the metallic sleeve  55  and the water-repellent filter  43 . The reactive force acting on the metallic sleeve  55  causes the diameter of the inner side walls  222  of the lead seals  22  to be decreased, thereby enhancing the degree of sealing between the inner wide walls  222  and the leads  16 .  
         [0090]    Finally, the side walls of the filter cover  44  and the main cover  41  are, as shown in FIG. 9( b ), crimped to retain the water-repellent filter  43  and to keep the above reactive forces at constant levels.  
         [0091]    The structure of the gas sensor  1  of this embodiment, like the first embodiment, has the elastic seal  2  located farther away from the heat source than the heat-resistant support  5  to protect the air-tight seals produced by the elastic seal  2  against the heat. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0092]    [0092]FIG. 10 shows the gas sensor  1  according to the third embodiment of the invention which is different from the second embodiment only in that the end cover  45  has a flat inner surface  452 , and the disc  21  of the elastic seal  2  has a flat surface  211  placed in abutment with the inner surface  452  of the end cover  45 . Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0093]    The structure of this embodiment provides the simplicity of shape of the elastic seal  2 , thus resulting in ease of machining of the elastic seal  2  and also minimizing thermal damage or breakage of the elastic seal  2 .  
         [0094]    [0094]FIG. 11 shows the gas sensor  1  according to the fourth embodiment of the invention which is a modification of the third embodiment.  
         [0095]    The elastic seal  2  has annular grooves  2910  formed around the openings  291 . The end cover  45  of the air cover  4  has annular protrusions  4500  fitted firmly within the annular grooves  2910 , respectively, thereby increasing the degree of sealing between the end cover  45  and the disc  21  of the elastic seal  2 . The annular grooves  2910  may alternatively be omitted, while the annular protrusions  4500  may press the flat end surface  211  of the disc  21  to create an air-tight seal between the inner surface of the end cover  45  and the end surface  211  of the disc  21 .  
         [0096]    Other arrangement are identical with those in the third embodiment, and explanation thereof in detail will be omitted here.  
         [0097]    [0097]FIGS. 12 and 13 show the gas sensor  1  according to the fifth embodiment of the invention which is a modification of the third embodiment.  
         [0098]    The disc  21  of the elastic seal  2  has, as clearly shown in FIG. 13, an annular seal groove  2100  formed in the flat end surface  211  coaxially with the outer periphery of the disc  21 . The end cover  45  of the air cover  4  has formed on the inner surface  452  an annular seal rib (i.e., a protrusion)  4521  which is fitted within the annular groove  2100  to form an air-tight seal between the end surface  211  of the disc  21  of the elastic seal  2  and the end cover  45  of the air cover  4 .  
         [0099]    The annular seal grooves  2100  may alternatively be omitted, while the annular seal rib  4521  may press the flat end surface  211  of the disc  21  to create an air-tight seal between the inner surface  452  of the end cover  45  and the end surface  211  of the disc  21 .  
         [0100]    Other arrangements are identical with those in the third embodiment, and explanation thereof in detail will be omitted here.  
         [0101]    The structures of the end cover  45  and the elastic seal  2  in the third to fifth embodiment may also be used in the first embodiment as illustrated in FIGS.  1  to  6 .  
         [0102]    The gas sensor element  19  may be made of a laminated plate such as one taught in U.S. Pat. No. 5,573,650, issued Nov. 12, 1996 to Fukaya et al., disclosure of which is incorporated herein by reference. The gas sensor element  19  may alternatively be made of a known cup-shaped sensor element.  
         [0103]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.