Abstract:
An improved structure of a gas sensor is provided which is designed to minimize thermal damage to a water-repellent filter installed in a base end portion of the gas sensor. The gas sensor has an elastic seal installed in an opening of the base end portion of the gas sensor and an air cover assembly made up of a main cover and a filter cover. The filter cover is crimped to form at least two necks which establish joints of the filter cover to the main cover through the water-repellent filter. At least one of the necks is used to retain the elastic seal within the main cover. This structure locates the water-repellent filter farther away from the top of the gas sensor exposed to intense heat, thereby minimizing the thermal deformation or deterioration of the water-repellent filter.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   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 water-repellent filter. 
   2. Background Art 
   Gas sensors are known which are installed in an exhaust pipe of automotive vehicles for use in air-fuel ratio control of engines. 
     FIG. 7  shows one example of such a type of gas sensor which is taught in Japanese Patent First Publication No. 11-72464 (corresponding to U.S. Pat. No. 6,395,159 B2, issued May 28, 2002). 
   The gas sensor  9  includes a gas sensor element working to measure the concentration of a given component contained in gasses, a housing within which the gas sensor element is disposed through an insulation porcelain, a gas sensor element protective cover joined to a top end of the housing, and an atmosphere side cover  91  joined to a base end of the housing. 
   The atmosphere side cover  91  is made up of a main cover  911  and a filter cover  912 . The main cover  911  includes a large-diameter portion which is exposed to air and welded to a side wall of the base end of the housing and a small-diameter portion surrounded by the filter cover  912 . The filter cover  912  is crimped to form necks which works to joint the filter cover  912  to the small-diameter portion of the main cover  911  through a cylindrical water-repellent filter  23 . 
   An elastic seal  17  is installed hermetically within an open end of the small-diameter portion of the main cover  911 . The elastic seal  17  is retained firmly, as clearly shown in the drawing, through the necks of the main cover  911  and the filter cover  912 . Leads  16  extend hermetically through holes  170  formed in the elastic seal  17  outside the gas sensor  9  for transmitting a sensor output to and receiving electric power from an external sensor controller. 
   When the gas sensor  19  is installed in the exhaust pipe of the automotive engine, a top end portion (not shown) of the housing is exposed to the exhaust gasses, so that it is exposed to intense heat. The heat is transmitted to the atmosphere side cover  91 , thus causing a top end portion of the main cover  911  to be elevated in temperature. The main cover  911  is, as described above, exposed to air, so that the temperature thereof drops gradually from the top end portion to a base end portion thereof (i.e., an upper portion, as viewed in the drawing). 
   The small-diameter portion of the main cover  911  is surrounded by the water-repellent filter  23  and the filter cover  912 , so that the heat stays at the small-diameter portion. 
   The water-repellent filter  23  is usually made of a porous resin material, so that it may, like the elastic seal  171 , undergo a thermal deformation or a change in coefficient of elasticity, thus causing the joint of the water-repellent filter  23  to the main cover  911  to be loosened. The thermal deformation of the water-repellent filter  23  also causes pores thereof to be occluded or closed, thus resulting in disturbance of a flow of the air from outside the filter cover  912  to inside the main cover  911  through air vents  913  and  914 . This results in a lack of oxygen within the gas sensor  9 , which leads to a failure in operation of the gas sensor  9 . 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the present invention to avoid the disadvantages of the prior art. 
   It is another object of the present invention to provide an improved structure of a gas sensor which provides a portion of the gas sensor around a water-repellent filter which is less sensitive to heat. 
   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 end and a second end opposed to the first end and comprises: (a) a hollow cylindrical housing having 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 disposed within the housing, the sensor element having a base portion projecting from the first end of the housing and a sensing portion projecting from the second end of the housing, the sensing portion working to measure a concentration of a given component of gasses; (c) a cylindrical measurement side cover joined to the second end of the housing to define a gas chamber which is filled with the gasses and to which the sensing portion of the sensor element is exposed; (d) an elastic seal; (e) a cylindrical water-repellent filter; (f) a cylindrical atmosphere side cover assembly joined to the housing to define an air chamber to which the base portion of the sensor element is exposed; and (g) air vents. The atmosphere side cover assembly includes a main cover and a filter cover. The main cover 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 joined at the second end thereof to the first end of the housing. The main cover has a first end portion which is crimped to form a first neck working to retain the elastic seal within the first end portion. The filter cover is crimped to form a second neck closer to the first end of the main cover than the first neck and a third neck closer to the second end of the main cover than the first neck. The second and third necks establish joints of the filter cover to an outer side wall of the first end portion of the main cover through the water-repellent filter and define a cavity therebetween around the first neck of the main cover within which the water-repellent filter is disposed. The air vents are formed in the main cover and the filter cover of the atmosphere side cover assembly which face the water-repellent filter disposed within the cavity. The air vents work to create a flow of air from outside the atmosphere side cover assembly to inside the air chamber through the water-repellent filter. 
   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 heated by exhaust gasses flowing in the exhaust pipe, so that the housing is elevated in temperature. The above structure retains the water-repellent filter at a location where it faces the elastic seal. In other words, the water-repellent filter is located farther away from the top of the gas sensor exposed to intense heat than the conventional structure, as illustrated in  FIG. 7 , thereby resulting in a decreased rise in temperature of the water-repellent filter, which minimizes thermal deformation or deterioration of the water-repellent filter. 
   The formation of the first neck on the main cover facilitates ease of forming the larger volume cavity between the main cover and the filter cover within which the water-repellent filter is disposed. The air vents and are formed inside and outside the water-repellent filter in the main cover and the filter cover. This structure facilitates flow of air from outside the filter cover toward the main cover. 
   In the preferred mode of the invention, the air vent of the main cover of the atmosphere side cover assembly has an end closer to the second end of the main cover than an end of the elastic seal, thereby facilitating the flow of air into the air chamber. 
   The main cover of the atmosphere side cover assembly is made of a material which is higher in hardness than that of the filter cover. 
   According to the second aspect of the invention, there is provided a gas sensor which has a length with a first end and a second end opposed to the first end. The gas sensor comprises: (a) a hollow cylindrical housing having 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 disposed within the housing, the sensor element having a base portion projecting from the first end of the housing and a sensing portion projecting from the second end of the housing, the sensing portion working to measure a concentration of a given component of gasses; (c) a cylindrical measurement side cover joined to the second end of the housing to define a gas chamber which is filled with the gasses and to which the sensing portion of the sensor element is exposed; (d) an elastic seal; (e) a cylindrical water-repellent filter; (f) a cylindrical atmosphere side cover assembly joined to the housing to define an air chamber to which the base portion of the sensor element is exposed; and (g) air vents. The atmosphere side cover assembly includes a main cover and a filter cover. The main cover 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 joined at the second end thereof to the first end of the housing. The filter cover is crimped to form a first neck closer to the first end of the main cover and a second neck closer to the second end of the main cover. The first and second necks establish a first and a second joint of the filter cover to an outer wall of the main cover, respectively, and define a cavity therebetween within which the water-repellent filter is disposed. The first joint works to retain the elastic seal within the main cover. The second joint works to retain the water-repellent filter in the cavity along with the first joint. The air vents are formed in the main cover and the filter cover of the atmosphere side cover assembly which face the water-repellent filter disposed within the cavity. The air vents work to create a flow of air from outside the atmosphere side cover assembly to inside the air chamber through the water-repellent filter. This structure retains the water-repellent filter at a location where it faces the elastic seal. In other words, the water-repellent filter is located farther away from the top of the gas sensor exposed to intense heat than the conventional structure, as illustrated in  FIG. 7 , thereby resulting in a decreased rise in temperature of the water-repellent filter, which minimizes thermal deformation or deterioration of the water-repellent filter. 
   In the preferred mode of the invention, the main cover of the atmosphere side cover assembly is made of a material which is higher in harness than that of the filter cover. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
     In the drawings: 
       FIG. 1  is a longitudinal sectional view which shows a gas sensor according to the first embodiment of the invention; 
       FIG. 2  is a partially enlarged sectional view which shows installation of an elastic seal and a water-repellent filter in a gas sensor in the first embodiment; 
       FIG. 3  is a partially sectional view which shows a production step of crimping an air cover to retain an elastic seal; 
       FIG. 4  is a partially sectional view which shows a second production step following the step as illustrated in  FIG. 3 ; 
       FIG. 5  is a partially sectional view which shows a third production step following the step as illustrated in  FIG. 4 ; 
       FIG. 6  is a partially enlarged sectional view which shows installation of an elastic seal and a water-repellent filter in a gas sensor according to the second embodiment of the invention; and 
       FIG. 7  is a partially enlarged sectional view which shows installation of an elastic seal and a water-repellent filter in a conventional gas sensor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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. The gas sensor  1  may also be installed in an exhaust pipe of automotive engines and work as an air-fuel ration sensor. 
   The gas sensor  1  generally includes a hollow cylindrical housing  10 , a sensor element  19  disposed within the housing  10 , a double-walled protective cover assembly  3  secured to a head end of the housing  10  to cover a sensing portion of the gas sensor element  19 , and a hollow cylindrical air cover  2  joined to a base end of the housing  10 . The sensor element  19  works to output a signal as a function of the concentration of a given component of gasses. 
   The 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. 
   The air cover  2  is made up of a cylindrical main cover  21  and a cylindrical filter cover  22 . The main cover  21  is welded directly to a side wall of a base portion of the housing  10 . The filter cover  22  is secured to an outer surface of a small-diameter portion of the main cover  21  and crimped to retain a water-repellent filter  23  on the periphery of the main cover  21 . The main cover  21  and the filter cover  22  have formed therein air vents  210  and  220  through which air is admitted into the air chamber defined inside the small-diameter portion of the main cover  21 . The air vents  210  and  220  face the water-repellent filter  23 . 
   An elastic seal  17  is retained firmly within an open base end of the main cover  21  to create an air-tight seal in the base end of the main cover  21 . The retaining of the elastic seal  17  is achieved, as clearly shown in  FIG. 2 , by crimping a base end portion (i.e., the small-diameter portion) of the main cover  21  to form a first annular shriveled portion  251  which will also be referred to as a neck below. The first neck  251  has an outer surface facing the water-repellent filter  23 . 
   The filter cover  22  is joined to the base end portion of the main cover  21  by crimping it to form second and third annular shriveled portions  252  and  253  which will also be referred to as necks below. The second neck  252  is located closer to the base end of the filter cover  22  than the first neck  251 , while the third neck  253  is located closer to a top end (i.e., a lower side, as viewed in  FIG. 2 ) of the filter cover  22  than the first neck  251 . Specifically, the second and third necks  252  and  253  are located out of coincidence with the first neck  251  in a radius direction of the air cover  2 . 
   The gas sensor  1  is installed, for example, in a wall of an exhaust pipe joining to the 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  102  of a flange  100  of the housing  10 , as illustrated in  FIG. 1 , is placed in abutment to an outer surface of the wall of the exhaust pipe through a spring  103 . The spring  103  works to provide hermetic sealing between the end surface  102  and the outer surface of the exhaust pipe. 
   When the engine is running, a lower portion of the gas sensor  1  below a broken line L in  FIG. 1 , is exposed to exhaust gasses flowing within the exhaust pipe and heated thereby. An upper portion of the gas sensor  1  above the 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 ). 
   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 have gas holes through which the exhaust gasses pass and enter inside a gas chamber defined in the inner cover. The gas sensor  1  has a head portion (i.e., the 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. 
   The sensor element  19  is retained within the housing  10  through a cylindrical 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 sensor element  19 . 
   An annular disc spring  122  is disposed on a base end of the insulation porcelain  12  and covered with a press cup  121 . The press cup  121  is so fitted on a side wall of the housing  10  as to press the disc spring  122 . The disc spring  122 , thus, produces a reactive force oriented in a direction parallel to a longitudinal center line of the gas sensor  1  which urges the insulation porcelain  12  into constant engagement with a tapered inner shoulder  105  of the housing  10 . 
   A hollow cylindrical insulation porcelain  13  is mounted on the press cup  121 . The insulation porcelain  13  has an inner cavity  130  to which a base portion of the sensor element  19  is exposed. The insulation porcelain  13  has formed in a base end thereof holes  131  which establish communication between the inner cavity  130  and the atmosphere. 
   The sensor element  19  connects with leads  16  through terminals  191  and connectors  192  such as clamp contacts for transmitting an output of the 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  2  above the insulation porcelain  13 . Within the air chamber, the terminals  191  are joined electrically to the leads  16  through the connectors  192 . 
   A cylindrical press cup  151  is fitted on a shoulder of the base end of the insulation porcelain  13  and urged by an inner shoulder of the main cover  21  downward. An annular plate  152  is installed on a base end of the press cup  151  which works to apply pressures to the base end of the press cup  151  inwardly in a radius direction of the press cup  151  to hold the base end from expanding outward. The press cup  151  and the annular plate  152  work to hold the insulation porcelain  13  firmly within the air cover  2  without any play. 
   The air cover  2  is, as clearly shown in  FIG. 2 , made up of the main cover  21  and the filter cover  22  crimped to hold the water-repellent filter  23  between itself and the base end portion of the main cover  22 . The main cover  21  and the filter cover  22  are made of cylindrical stainless steel (e.g., SUS304). The main cover  21  has a Vickers hardness of 230. The filter cover  22  has a Vickers hardness of 140. 
   The water-repellent filter  23  is made of a porous material such as tetrafluoroethylene which has higher air permeability. The main cover  21  is made up of the small-diameter portion and the large-diameter portion which is welded directly, as shown in  FIG. 1 , to the side wall  100  of the housing  10 . 
   The elastic seal  17  is installed in the open end of the main cover  21  in elastic abutment with an inner wall of a portion of a lap of the main cover  21  and the filter cover  22 . The installation is, as described above, achieved by crimping the base end portion of the main cover  21  to grasp the elastic seal  17  elastically at the first neck  251 . 
   The filter cover  22  is fitted on the base end portion of the main cover  21  outside the first neck  251  and jointed to the outer wall of the main cover  21  at the second and third necks  252  and  253  located across the first neck  251 . The joining of the filter cover  22  to the main cover  21  is achieved by putting a hollow cylinder of uniform diameter on the base end portion (i.e., the small-diameter portion) of the main cover  21  and crimping it to form a neck  254  in addition to the second and third necks  252  and  253 . 
   The main cover  21  and the filter cover  22  have formed therein the air vents  210  and  220  through which air is admitted into the air chamber defined inside the small-diameter portion of the main cover  21 . The air vents  210  are located at regular intervals around the first neck  251  of the main cover  21 . Similarly the air vents  220  are located at regular intervals around the filter cover  22 . The air vents  210  and  220  face the water-repellent filter  23 . 
   Each of the air vents  201  of the main cover  21  extends vertically, as viewed in the  FIGS. 1 and 2 , to have a top end (i.e., a lower end as viewed in the drawings) located closer to the top end of the gas sensor  1  than the top end surface (i.e. a lower end surface as viewed in the drawings) of the elastic seal  17 , thereby establishing communication between the air vents  220  and the air chamber within the main cover  21  through the water-repellent filter  23 . Specifically, the air entering the air vents  220  of the filter cover  22  passes through the water-repellent filter  23  and the first neck  251  (i.e., the air vents  210 ), thereby forming an air flow oriented diagonally down to inside the main cover  21  (i.e., the air chamber) from the air vents  220  of the filter cover  22 . 
   Crimping of the main cover  21  and the filter cover  22  is accomplished in the following steps. 
   First, the elastic seal  17  is, as shown in  FIG. 3 , put in the open end of the main cover  21 . The pressure F 1  is applied to the periphery of a smaller-diameter portion of the main cover  21  in a radius direction of the main cover  21  to shrivel the side wall to form, as shown in  FIG. 4 , the first neck  251 . Simultaneously, pressure is applied to a larger-diameter portion of the main cover  21  to shrivel it to form a neck  219 . 
   Next, the water-repellent filter  23  and the filter cover  22  are fitted on the smaller-diameter portion of the main cover  21  in abutment of a lower end of the water-repellent filter  23  and an inner shoulder of the filter cover  22  with a shoulder of the main cover  21  formed above the neck  219 . Specifically, the water-repellent filter  23  is placed around the elastic seal  7 . 
   Finally, pressures F 2 , F 3 , and F 4  are, as shown in  FIG. 5 , applied to the side wall of the filter cover  22  to form the necks  252 ,  253 , and  254 , as illustrated in  FIG. 2 , thereby establishing a firm joint of the filter cover  22  to the main cover  21  and retaining the elastic seal  17  within the open end of the main cover  21  hermetically. 
   As apparent from the above discussion, firm fixing of the elastic seal  17  within the open end of the main cover  21  is achieved by forming the first neck  251  on the main cover  21 . The firm joint of the filter cover  22  to the main cover  21  is achieved by forming the second and third necks  252  and  253  on the filter cover  22 . The water-repellent filter  23  is placed around the first neck  251  and retained by the second and third necks  253  and  253 . Specifically, the water-repellent filter  23  is disposed at a location where it faces the elastic seal  17 . In other words, the water-repellent filter  23  is located farther away from the top end of the gas sensor  1  exposed to intense heat than the conventional structure, as illustrated in  FIG. 7 , thereby resulting in a decreased rise in temperature of the water-repellent filter  23 , which minimizes thermal deformation or deterioration of the water-repellent filter  23 . 
   The formation of the first neck  251  on the main cover  21  facilitates ease of forming a larger volume cavity between the main cover  21  and the filter cover  22  within which the water-repellent filter  23  is disposed. The air vents  210  and  220  are formed inside and outside the water-repellent filter  23  in the main cover  21  and the filter cover  22 . This structure facilitates flow of air from outside the filter cover  22  toward the main cover  21 . 
   Each of the air vents  201  of the main cover  21  extends vertically, as viewed in the  FIGS. 1 and 2 , to have the top end located closer to the top end of the gas sensor  1  than the top end surface of the elastic seal  17 , thereby creating a flow of a required amount of air to inside the main cover  21 . 
     FIG. 6  shows the gas sensor  1  according to the second embodiment of the invention. 
   The air cover  2  consists of the main cover  21  joined to the base end of the housing  10  (not shown in  FIG. 6 ) and the filter cover  22  joined to the base end portion of the main cover  21  through the cylindrical water-repellent filter  23 . 
   The air vents  210  of the main cover  21  coincide with the air vents  220  of the filter cover  22  through the water-repellent filter  23 . 
   The main cover  21  and the filter cover  22  have upper necks  261  and lower necks  262 . The upper necks  261  work to retain the elastic seal  17  firmly within the open end of the main cover  21  and also join the filter cover  22  to the main cover  21  firmly together with the lower necks  262 . The upper and lower necks  261  and  262  also work to hold the water-repellent filter  23  therebetween. 
   The water-repellent filter  23  is, like the first embodiment, disposed at a location where it faces the elastic seal  17 . In other words, the water-repellent filter  23  is located farther away from the top end of the gas sensor  1  exposed to intense heat than the conventional structure, as illustrated in  FIG. 7 , thereby resulting in a decreased rise in temperature of the water-repellent filter  23 , which minimizes thermal deformation or deterioration of the water-repellent filter  23 . 
   Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
   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.