Patent Publication Number: US-10775342-B2

Title: Gas sensor

Description:
This application claims the benefit of Japanese Patent Application No. 2017-207890, filed Oct. 27, 2017, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a gas sensor including a sensor element that is exposed to a subject gas and detects a specific gas component in the subject gas. 
     BACKGROUND OF THE INVENTION 
     One known gas sensor attached in use to, for example, an exhaust pipe of an automobile includes a sensor element which generates an electromotive force that changes with the concentration of a specific gas (e.g., NOx (nitrogen oxides) or oxygen) in exhaust gas or whose resistance value changes with the concentration. A sensing section for detecting the specific gas component is provided at the forward end of the sensor element. The sensing section is heated by, for example, a heater to detect the specific gas component. In the case where the sensing section of the sensor element is at high temperature, when water droplets contained in the exhaust gas adhere to the sensing section (the sensing section is wetted with water), the sensor element may break (for example, may crack) due to thermal shock. Therefore, a gas sensor in which the sensing section of the sensor element is covered with a protector to protect the sensor element from wetting with water has been developed (see, for example, Japanese Unexamined Publication No. 2009-115781). 
     As shown in  FIG. 5 , the gas sensor disclosed in Japanese Unexamined Publication No. 2009-115781 has a structure in which a sensor element  21  is inserted into and held within an insertion hole  320  of a ceramic holder  300  formed of an insulating material (ceramic such as alumina) and the ceramic holder  300  is disposed inside a metallic shell  1100 . A sensing section at the forward end of the sensor element  21  is covered with a protective layer  25 . 
     The metallic shell  1100  has a bore  1100   h  extending therethrough in a forward-rear direction, and a rear end portion  26  of the protective layer  25  of the sensor element  21  protruding forward from the ceramic holder  300  is accommodated within the bore  1100   h.  Metallic protectors  510  and  610  are attached to the forward end of the metallic shell  1100  so as to protect the sensor element  21 . Gas passage holes  560  and  670  of the protectors  510  and  610  are arranged in the circumferential direction so as to be point-symmetric with respect to the center of the protectors. 
     Problems to be Solved by the Invention 
     In the gas sensor in  FIG. 5 , when a gas under measurement is introduced through the gas passage holes  560  and  670  of the protectors  510  and  610 , as shown by an arrow, the gas under measurement impinges against portions of the sensor element  21 , which portions face the gas passage holes  560 , and most of the gas under measurement is discharged directly from a gas discharge hole  680  on the forward end side. Therefore, although the gas under measurement is not readily introduced into the bore  1100   h,  no problem arises so long as the sensing section is disposed forward of the gas passage holes  560 . 
     However, when the sensing section is disposed rearward of the gas passage holes  560 , the gas under measurement does not easily reach the sensing section, and the responsiveness of the sensor element may deteriorate. When the forward end of the sensor element is disposed rearward of the gas passage holes  560  for the purpose of reducing the size of the gas sensor, it is important to produce a flow of the gas under measurement toward the bore  1100   h.  However, in the protectors  510  and  610  in  FIG. 5 , since the gas passage holes having the same opening area are disposed so as to be point-symmetric, flows of the gas under measurement introduced collide against each other and are thereby disturbed, so it is difficult to produce a flow of the gas under measurement toward the bore  1100   h.  This may cause deterioration in responsiveness of the sensor element. 
     The present invention has been made in view of the above circumstances, and an object thereof is to provide a gas sensor that can be reduced in size without deterioration in responsiveness of the sensor element. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems 
     A gas sensor according to an one aspect of the present disclosure comprises a sensor element extending in an axial direction and including a sensing section which is disposed at a forward end of the sensor element and detects a specific gas component in a subject gas; a tubular metallic shell which circumferentially surrounds and holds the sensor element; and a protector that has a circumferential wall and a forward end wall disposed forward of the circumferential wall, the protector having a rear end portion fixed to a forward end of the metallic shell, the protector having gas introduction holes which are formed in the circumferential wall and through which the subject gas is introduced into the protector. The sensing section is disposed rearward of a rearmost one of the gas introduction holes. In a circular cross section of the protector formed along a plane which is orthogonal to the axial direction and passes through the gas introduction holes, one gas introduction hole and another gas introduction hole having an opening area different from that of the one gas introduction hole are arranged along an imaginary line passing through the center of the circular cross section such that the two gas introduction holes are located on opposite sides with respect to the center of the circular cross section. 
     In this gas sensor, since one gas introduction hole and another gas introduction hole having an opening area different from that of the one gas introduction hole are arranged on opposite sides with respect to the center of the circular cross section, even when the gas under measurement introduced through the gas introduction hole located on one side and the gas under measurement introduced through the gas introduction hole located on the other side collide against each other, a flow of the gas under measurement toward a bore of the metallic shell can be produced, because the amount of the gas under measurement introduced through the gas introduction hole having a larger opening area is larger than the amount of the gas under measurement introduced through the gas introduction hole having a smaller opening area. 
     The gas sensor according to the one aspect of the present disclosure may be configured such that, when the circular cross section is bisected into first and second divisional regions by a straight line which passes through the center of the circular cross section and does not pass through the gas introduction holes, the total opening area of the gas introduction hole located in the first divisional region is smaller than the total opening area of the gas introduction hole located in the second divisional region. 
     In this gas sensor, even when the gas under measurement introduced through the gas introduction hole located in the first divisional region collides with the gas under measurement introduced through the gas introduction hole located in the second divisional region, a strong flow of the gas under measurement toward the bore of the metallic shell can be produced, because the amount of the gas under measurement introduced through the gas introduction hole located in the second divisional region and having a larger total opening area is larger than the amount of the gas under measurement introduced through the gas introduction hole located in the first divisional region. 
     The gas sensor according to the one aspect of the present disclosure may be configured such that a forward end of the sensor element is located rearward of the rearmost one of the gas introduction holes. 
     In this gas sensor, the gas under measurement introduced through the gas introduction hole in the second divisional region collides with a portion of the inner wall of the protector, which portion faces the gas introduction hole, without collision with the sensor element. Therefore, it is possible to produce a strong flow of the gas under measurement toward the bore of the metallic shell without disturbing the gas flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a cross-sectional view of a gas sensor according to an embodiment of the present invention. 
         FIG. 2  is an enlarged illustration of part of  FIG. 1 . 
         FIG. 3  is a circular cross-sectional view in a first embodiment. 
         FIG. 4  is an illustration showing a gas flow in the first embodiment. 
         FIG. 5  is a partial cross-sectional view of a conventional gas sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Gas sensors according to embodiments of the present invention will be described in detail with reference to  FIGS. 1 to 3 . Each of the gas sensors is a full-range air-fuel-ratio gas sensor that detects the concentration of oxygen in exhaust gas. The general structure of the gas sensor  1  will first be described, and then the components thereof and their structures will be described in detail. 
     As shown in  FIG. 1 , the gas sensor (full-range air-fuel-ratio gas sensor)  1  includes a sensor element  21 , a ceramic holder  30  having an insertion hole  32  into which the sensor element  21  is inserted, and a metallic shell  11  that surrounds the radial circumference of the ceramic holder  30 . 
     A forward end portion of the sensor element  21  in which a sensing section  22  is formed protrudes forward from a forward-facing surface  30   a  of the ceramic holder  30  (see  FIG. 2 ). A seal member (talc in the present embodiment)  41  disposed rearward (on the upper side in the figures) of the ceramic holder  30  is compressed in a forward-rear direction through a sleeve  43  formed of an insulating material and a ring washer  45 . Thus, the sensor element  21  passing through the insertion hole  32  is fixed inside the metallic shell  11 , while gas-tightness in the forward-rear direction is maintained. A rear end portion of the sensor element  21  including its rear end  29  (hereinafter, the rear end portion will be referred to as the “rear end  29  side portion”) protrudes rearward beyond the sleeve  43  and the metallic shell  11 . Metallic terminals  75  disposed at the forward ends of lead wires  71  extending to the outside through a seal member  85  are in pressure-contact with and electrically connected to respective electrode terminals  24  formed in the rear end  29  side portion of the sensor element  21 . The rear end  29  side portion of the sensor element  21 , including the electrode terminals  24 , is covered with a protective tube  81 . The gas sensor  1  will be described in more detail. 
     The sensor element  21  extends in the direction of an axial line O, has a strip shape (plate shape), and includes the sensing section  22  that is disposed at the forward end (the lower end in the figure) exposed to the measurement target and includes detection electrodes (not shown) etc. for detection of a specific gas component in the subject gas. The sensor element  21  has a rectangular transverse cross section whose size is constant in the forward-rear direction, is composed mainly of a ceramic (such as a solid electrolyte), and formed as a long and narrow member. The sensing section  22  of the sensor element  21  is covered with a porous protective layer  25  made of alumina, spinel, etc. In a region in which the protective layer  25  is formed, the sensor element  21  has a transverse cross section which is larger than other transverse cross sections thereof by an amount corresponding to the thickness of the protective layer  25  (e.g., 0.5 to 0.6 mm) (the thickness is exaggerated in the figure). The sensor element  21  itself is the same as those known in the art. A pair of detection electrodes constituting the sensing section  22  are disposed in a forward end portion of the solid electrolyte (member), and a pair of electrode terminals  24  extending from the detection electrodes are formed in a rear end portion of the solid electrolyte such that the electrode terminals  24  are exposed to the outside. A pair of lead wires  71  for outputting detection output are connected to the electrode terminals  24 . In the present embodiment, the sensor element  21  has a heater (not shown) which is provided in a forward end portion of a ceramic member stacked on the solid electrolyte (member). Another pair of electrode terminals  24  to which another pair of lead wires  71  are connected are formed in a rear end portion of the ceramic member such that the electrode terminals  24  are exposed to the outside. A voltage is applied to the heater though the second pair of electrode terminals  24 . Although not illustrated, these electrode terminals  24  each have a rectangular shape elongated vertically and are provided in the rear end  29  side portion of the strip-shaped sensor element  21  such that two or three electrode terminals are arranged laterally on each of opposite main faces of the sensor element  21  which are winder than the remaining surfaces thereof. 
     The metallic shell  11  includes concentric tubular portions arranged in the forward-rear directions and having different diameters. Specifically, a small-diameter cylindrical annular portion (hereinafter may be referred to also as a cylindrical portion)  12  is formed at the forward end of the metallic shell  11 , and protectors  51  and  61  described later are externally fitted and fixed to the cylindrical annular portion. A thread  13  for fixation to an exhaust pipe of an engine is formed on the outer circumferential surface of a portion rearward (upward in the figure) of the cylindrical portion  12  and having a larger diameter than the cylindrical portion  12 . A polygonal portion  14  used to screw the thread  13  of the sensor  1  into the exhaust pipe is provided rearward of the thread  13 . A cylindrical portion  15  is provided rearward of the polygonal portion  14 , and the protective tube (outer tube)  81  that covers a rear portion of the gas sensor  1  is fitted externally to and welded to the cylindrical portion  15 . A thin-walled cylindrical portion  16  having a smaller outer diameter than the cylindrical portion  15  and used for crimping is provided rearward of the cylindrical portion  15 . In  FIG. 1 , the cylindrical portion  16  for crimping has been crimped and bent inward. A gasket  19  for sealing by screwing is attached to the lower surface of the polygonal portion  14 . 
     As shown in  FIG. 2 , the metallic shell  11  has a bore  18  extending therethrough in the direction of the axial line O. The bore  18  includes a small-diameter bore  18   a  disposed on the forward end side; and a large-diameter bore  18   b  disposed rearward of the small-diameter bore  18   a  and having a larger diameter than the small-diameter bore  18   a.  The metallic shell  11  further has a rearward-facing surface  17   b  that connects a wall surface  17   a  of the small-diameter bore  18   a  to a wall surface  17   c  of the large-diameter bore  18   b.  In the present embodiment, the rearward-facing surface  17   b  is tapered toward the forward end side. The wall surface  17   a , the rearward-facing surface  17   b,  and the wall surface  17   c  are collectively referred to as an inner circumferential surface  17  of the metallic shell  11 . 
     The ceramic holder  30  made of an insulating ceramic (e.g., alumina) and generally having the shape of a short cylinder is disposed inside the large-diameter bore  18   b  of the metallic shell  11 . As shown in  FIG. 2 , the forward-facing surface  30   a  of the ceramic holder  30  includes: an outer forward-facing surface  30   a   2  tapered toward the forward end side; and a flat inner forward-facing surface  30   a   1  disposed inward of the outer forward-facing surface  30   a   2 . An outer portion of the outer forward-facing surface  30   a   2  is engaged with the rearward-facing surface  17   b,  and the ceramic holder  30  is thereby placed in position within the metallic shell  11  so as to be loose-fitted therein. 
     The insertion hole  32  is formed at the center of the ceramic holder  30  and has a rectangular opening having substantially the same size as a transverse cross section of the sensor element  21  so that a portion of the sensor element  21  that is rearward of the protective layer  25  passes through the opening with almost no gap therebetween. 
     The ceramic holder  30  has a circular recess  35  having a larger diameter than the insertion hole  32  and located at the forward end of the insertion hole  32 . The circular recess  35  extends from the inner forward-facing surface  30   a   1  of the ceramic holder  30  toward the rear end side and is in communication with the forward end of the insertion hole  32 . In the present embodiment, the circular recess  35 , which is larger in diameter than the insertion hole  32 , has a flat bottom surface  35   b  (located at the forward end of the insertion hole  32 ). In the present embodiment, an inner circumferential surface  35   i  of the circular recess  35  is parallel to the axial line O. A forward edge  35   e  is formed at a position at which the inner circumferential surface  35   i  of the circular recess  35  is connected to the inner forward-facing surface  30   a   1 . A portion of the inner forward-facing surface  30   a   1  on the forward edge  35   e  side is chamfered. 
     The sensor element  21  is inserted into the insertion hole  32  of the ceramic holder  30 , and the forward end of the sensor element  21  protrudes forward beyond the forward-facing surface  30   a  of the ceramic holder  30  and the forward end  12   a  of the metallic shell  11 . A rear end portion  26  of the protective layer  25  is accommodated within the circular recess  35 . When the sensor  1  is assembled by inserting the sensor element  21  into the insertion hole  32  of the ceramic holder  30 , the protective layer  25  may be damaged if the protective layer  25  collides with the wall surface of the insertion hole  32 . To prevent this, it is preferable that the rear end portion  26  of the protective layer  25  is spaced apart forward from the forward end of the insertion hole  32  (the bottom surface  35   b ). The protective layer  25  is formed such that the axial length of the rear end portion  26  located within the circular recess  35  is shorter than the axial length of a forward end portion located outside the circular recess  35 . This can prevent a reduction in detection accuracy of the sensor element  21 . 
     The inner circumferential surface  35   i  of the circular recess  35  is spaced apart from the outer circumferential surface of the protective layer  25  accommodated within the circular recess  35 . The entire inner circumferential surface  35   i  of the circular recess  35  is located radially inward of the wall surface  17   a  of the small-diameter bore  18   a  of the metallic shell  11  and an inner circumferential surface  51   a  of the inner protector  51 , which is the innermost one (directly facing the element) of the protectors  51  and  61 . 
     In the present embodiment, the forward end portion of the sensor element  21  is covered with the closed-end cylindrical protectors (protective covers)  51  and  61  having gas passage holes  56  and  67 , respectively, and forming a double-layer structure. The rear end of the inner protector  51  is externally fitted and welded to the cylindrical portion  12  of the metallic shell  11 . Two gas passage holes  56 , for example, are provided in a rear end portion of a circumferential wall of the protector  51  so as to be symmetric in the circumferential direction. Meanwhile, four discharge holes  53 , for example, are provided in a forward end portion of the protector  51  and arranged in the circumferential direction. The outer protector  61  is externally fitted to the inner protector  51  and welded to the cylindrical portion  12  together with the inner protector  51 . Eight gas passage holes  67 , for example, are provided in a forward end portion of a circumferential wall of the outer protector  61  and arranged in the circumferential direction, and a discharge hole  69  is provided at the center of the bottom of the protector  61  located on the forward end side thereof. 
     As shown in  FIG. 1 , the metallic terminals  75  disposed at the forward ends of the lead wires  71  extending to the outside through the seal member  85  resiliently come into pressure-contact with the respective electrode terminals  24  formed in the rear end  29  side portion of the sensor element  21 . Thus, electrical contact between the metallic terminals  75  and the electrode terminals  24  is established. In the gas sensor  1  of the present embodiment, the metallic terminals  75 , including their pressure contact portions, are disposed in respective accommodation spaces of a metallic terminal holder  91  in a facing arrangement. The metallic terminal holder  91  is formed of an insulating material and disposed inside the protective tube (metallic tube)  81  having tubular portions with different diameters. The metallic terminal holder  91  is prevented from moving radially and forward by an annular support member  80  fixedly provided inside the protective tube (metallic tube)  81 . A forward end portion (large-diameter portion)  82  of the protective tube  81  is externally fitted and welded to the cylindrical portion  15  at the rear end of the metallic shell  11 . Thus, a rear end portion of the gas sensor  1  is a gas-tightly covered. The lead wires  71  extend to the outside through the seal member (e.g., rubber)  85  disposed inside a small-diameter tubular portion  83  at the rear end of the protective tube  81 . The small-diameter tubular portion  83  is crimped and reduced in diameter to compress the seal member  85 , thereby maintaining the gas-tightness of this portion. 
     The seal member  85  is disposed so as to press forward the rear end of the metallic terminal holder  91 , thereby enhancing the installation stability of the metallic terminal holder  91  and the metallic terminals  75  disposed therein. The metallic terminal holder  91  has a flange  93  formed on its outer circumference, and the flange  93  is supported on the annular support member  80  fixedly provided inside the protective tube  81 . Thus, the compressive force of the seal member  85  is borne by the annular support member  80 . 
     The first embodiment will be described with reference to  FIG. 3 .  FIG. 3  shows a circular cross section (an A-A cross section of  FIG. 2 ) obtained by cutting the gas sensor  1  along a plane orthogonal to the axial direction and passing through the gas passage holes  56 . The inner protector  51  has gas passage holes  56   a  and  56   b  which are spaced from each other in the circumferential direction. In other words, the gas passage holes  56   a  and  56   b  are formed symmetrically in the circumferential direction with respect to the center of the inner protector  51 . In  FIG. 3 , the protective layer  25  is not illustrated. 
     The circular cross section of the gas sensor  1  is divided into first and second divisional regions by a straight line AL which passes through the center axis of the inner protector  51  and does not pass through any gas passage holes. The gas passage hole  56   a  is provided in the first divisional region. The gas passage hole  56   b  is provided in the second divisional region. Since the gas passage hole  56   b  is perfect circular and has a diameter of 1.0 mm, its opening area is about 0.785 mm 2 . Since the gas passage hole  56   a  is perfect circular and has a diameter of 1.5 mm, its opening area is about 1.766 mm 2 . Specifically, the opening area of the gas passage hole  56   b  located in the second divisional region is smaller than the opening area of the gas passage hole  56   a  located in the first divisional region. In other words, the opening area of the gas passage hole  56   a  located in the first divisional region differs from the opening area of the gas passage hole  56   b  located in the second divisional region. Namely, the total opening area of the gas passage hole located in the first divisional region is smaller than the total opening area of the gas passage hole located in the second divisional region. 
       FIG. 4  is an illustration showing the flow of gas in the first embodiment. The gas flow GS is indicated by an arrow. As shown in  FIG. 4 , the gas under measurement introduced through the gas passage holes  56   a  and  67  of the protectors  51  and  61  collides with the gas under measurement introduced through the gas passage hole  56   b  located on the side opposite the gas passage hole  56   a.  However, the gas under measurement introduced through the gas passage holes  56   a  having a larger opening area flows toward the bore  18 , while using, as a stepping-stone, the gas under measurement introduced through the gas passage holes  56   b.  The gas under measurement then flows toward the discharge holes  53 , whereby the gas flow GS is formed. The sensing section  22  is present in the path of the gas flow GS. Therefore, a sufficient amount of the gas under measurement can reach the sensing section  22  of the sensor element  21 . 
     The gas sensor of the present invention can be embodied with its structure and configuration appropriately modified, so long as the modifications do not go beyond the scope of the present invention. 
     Specifically, in the above embodiments, the sensor element has a strip shape with a rectangular transverse cross section. However, the sensor element used in the gas sensor of the present invention may have a square transverse cross section or another cross-sectional shape. In the above embodiments, the gas sensor of the present invention is embodied as a full-range air-fuel-ratio gas sensor. However, the gas sensor of the present invention may be embodied as another gas sensor. The shape of the gas passage holes is not limited to a perfect circle, and the gas passage holes may have, for example, a rectangular shape. The gas passage holes may have different opening areas. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1  gas sensor 
       11  metallic shell (housing) 
       18  bore 
       21  sensor element 
       22  sensing section 
       25  protective layer 
       30  ceramic holder 
       32  insertion hole of ceramic holder 
       35  circular recess 
       51  inner protector 
       56 ,  67  gas passage hole 
       61  outer protector 
     O axial line