Patent Publication Number: US-2002000377-A1

Title: Oxygen sensor

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an oxygen sensor suitable for detecting oxygen concentration in an exhaust gas emitted from an engine of a vehicle.  
       [0002] Generally, when a vehicular engine with a turbo charger is operated at a rich side air-fuel ratio relative to a stoichiometric air-fuel ratio, a temperature of an exhaust gas emitted from the engine is approximately 280° C. Oxygen sensors are usually activated at approximately 350° C. The engine with the turbo charger, therefore, uses an oxygen sensor including a ceramic heater for heating the sensing element.  
       [0003] Japanese Patent Application First Publication No. 10-282050 discloses an oxygen sensor for detecting oxygen concentration in an exhaust gas emitted from a vehicular engine. The oxygen sensor includes a tubular casing, a sensing element disposed at an end of the casing, and a heater attached to the sensing element for heating the sensing element. The sensing element is made of ceramic materials such as zirconia, and provided with electrodes on the opposed surfaces. The electrodes are connected to a control unit via signal output terminals extending through the casing. The heater is connected to the control unit via electric energy supply terminals extending through the casing. The heater is fixed to the sensing element by sintering or adhering so that the heater and the sensing element form a laminated integral body. Hermetically sealed spaces are formed on both sides of the sensing element, one of which is opposed surfaces of the sensing element and the heater. The sensing element is activated to detect an electromotive force generated between the electrodes due to a difference between the oxygen concentration in the exhaust gas flowing into one of the hermetically sealed spaces and the oxygen concentration in the atmosphere flowing into the other of the hermetically sealed spaces. The sensing element transmits a signal output indicative of the electromotive force to the control unit. The control unit conducts a feedback control of air-fuel ratio depending on the signal output from the sensing element. If a temperature of the sensing element is low upon starting of the engine, the heater is operated by the control unit to heat the sensing element up to approximately 350° C. and bring the sensing element into the active state. As a result, the feedback control is earlier started.  
       [0004] Japanese Patent Application First Publication No. 9-26409 discloses an oxygen sensor including a zirconia-based sensing element, an alumina plate overlapping on the sensing element and a heater fixed to the alumina plate. The sensing element and the alumina plate are bonded to each other by sintering to form a laminated integral body.  
       [0005] In the above-described earlier techniques, the sensing element and the heater are bonded together by sintering or through an adhesive layer disposed therebetween. When the sensing element is heated by the heater, the heater tends to peel off from the sensing element due to the thermal stress caused therebetween upon heating. The peeling of the heater will cause damage, such as crack, to the sensing element.  
       [0006] Further, it is likely that the crack caused due to the peeling of the heater from the sensing element reaches the hermetically sealed space between the heater surface and the sensing element surface to thereby deteriorate air-tightness of the hermetically sealed space and reduce accuracy of detection of the oxygen concentration.  
       [0007] Furthermore, in the above-described earlier techniques, the production process of the oxygen sensor is complicated because the heater must be sintered or adhered to the sensing element for formation of the hermetically sealed space. Further, since the electrodes of the sensing element are embedded in the laminated body, two output terminals for external electrical-connection of the electrodes are required. This results in increase in the production cost.  
       [0008] In addition, if the laminated body is formed by sintering multiple ceramic layers different in composition, crack will occur during the sintering in the boundary between the ceramic layers due to a difference in thermal expansion therebetween.  
       SUMMARY OF THE INVENTION  
       [0009] It is an object of the present invention to provide an oxygen sensor capable of being prevented from suffering from the thermal stress caused between the heater and the sensing element and from being damaged due to the thermal stress, and capable of being improved in its reliability and durability.  
       [0010] According to one aspect of the present invention, there is provided an oxygen sensor useable for sensing oxygen concentration in an exhaust gas, comprising:  
       [0011] an elongated casing including an exhaust gas inlet through which the exhaust gas enters thereinto;  
       [0012] a sensing element including first and second planar surfaces opposed to each other, the first planar surface being opposed to the exhaust gas inlet, and first and second electrodes disposed on the first and second planar surfaces, respectively, the sensing element being disposed within the casing; and  
       [0013] a heater separably disposed on the second planar surface of the sensing element.  
       [0014] According to a further aspect of the present invention, there is provided an oxygen sensor useable for sensing oxygen concentration in an exhaust gas, comprising:  
       [0015] an elongated casing including an exhaust gas inlet through which the exhaust gas enters thereinto;  
       [0016] a sensing element including first and second planar surfaces opposed to each other in a direction substantially crossing a longitudinal direction of the casing, the first planar surface being opposed to the exhaust gas inlet; and  
       [0017] a heater forced to be in contact with the second planar surface of the sensing element within the casing. 
     
    
    
     [0018] The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019]FIG. 1 is a longitudinal cross-section of an oxygen sensor of a first embodiment, according to the present invention;  
     [0020]FIG. 2 is an enlarged view of an important part of FIG. 1, showing an end portion of the oxygen sensor;  
     [0021]FIG. 3 is an exploded perspective view of the end portion of the oxygen sensor;  
     [0022]FIG. 4 is a diagram showing a relationship between a position on a heater surface and a temperature of the heater surface; and  
     [0023]FIG. 5 is a view similar to FIG. 2, but showing a second embodiment of the oxygen sensor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0024] Referring now to FIGS.  1 - 4 , an oxygen sensor, according to the present invention, of a first embodiment is explained. The oxygen sensor of this embodiment is applicable to engines of automobiles.  
     [0025] As illustrated in FIG. 1, the oxygen sensor includes an elongated cylindrical casing  1 . The casing  1  is constituted of a stepped tube-shaped element holder  2 , a tubular insulator  3  whose one end portion is received in the element holder  2 , and a cylindrical cap  4  substantially enclosing the insulator  3 . The element holder  2  is made of an electrically conductive metal material. The element holder  2  has an element-receiving portion  2 A at one end thereof into which a sensing element  14  is mounted. The element holder  2  also has an exhaust gas inlet  2 B located on a distal end of the element-receiving tube portion  2 A. An exhaust gas emitted from the engine (not shown) is introduced to the sensing element  14  through the exhaust gas inlet  2 B. A threaded portion  2 C is formed on an outer circumferential surface. The oxygen sensor is mounted to an exhaust pipe (not shown) of the engine by screwing the threaded portion  2 C into the exhaust pipe in such a manner that the sensing element  14  is exposed to the inside of the exhaust pipe. A positioning portion  2 D is formed on an inner circumferential surface of the element-receiving portion  2 A. The positioning portion  2 D is formed by an annular recess around a center axis of the element holder  2 . The positioning portion  2 D is disposed adjacent to the exhaust gas inlet  2 B in the axial direction of the element holder  2 . The element holder  2  has a stepped inner periphery that is disposed axially adjacent to the positioning portion  2 D and steppedly increased in diameter than the positioning portion  2 D. A cover mount portion  2 E is disposed on a radial outside of the exhaust gas inlet  2 B, to which a protection cover  20  is mounted.  
     [0026] The insulator  3  is disposed in concentric with the element holder  2 . As shown in FIG. 1, the insulator  3  has an axially extending bore  3 A into which a tube  10  enclosing lead wires  8 ,  8  and  9  is inserted. The insulator  3  has a stepped outer periphery substantially corresponding to the stepped inner periphery of the element holder  2 . The insulator  3  is made of a suitable ceramic material such as alumina.  
     [0027] The cap  4  has a cylindrical side wall extending along outer peripheral surfaces of the insulator  3  and the element holder  2 , and an annular end wall that is connected with the side wall and opposed to an opposite end of the insulator  3 . The cap  4  is fixed at one end thereof to a base portion of the element holder  2  by a suitable joining method such as welding, and extends in the axial direction of the insulator  3 .  
     [0028] A disk spring  5  is installed inside the end wall of the cap  4  and interposed between the end wall of the cap  4  and the opposite end of the insulator  3 . The disk spring  5  biases the insulator  3  toward the positioning portion  2 D of the element holder  2  to retain the sensing element  14  and a heater  18  explained later at the positioning portion  2 D. The cap  4  and the disk spring  5  cooperate to retain the sensing element  14  and the heater  18  between the opposite end of the insulator  3  and the positioning portion  2 D of the element holder  2  as best shown in FIG. 2. The cap  4  holds the sensing element  14  and the heater  18  in place in the axial direction.  
     [0029] A tubular seal  6  is mounted to an axial end of the casing  1  through an outer cap  7  in such a manner as to be opposed to the opposite end of the insulator  3 . The outer cap  7  is fixed to the side wall of the cap  4  near the end wall of the cap  4  by welding the outer periphery thereof (so-called welding-all-around). The tubular seal  6  is made of an insulating material and receives the lead wires  8 ,  8  for supplying electric energy to the heater  18  and the lead wire  9  for transmitting a signal output from the sensing element  14 . Each of the lead wires  8  and  9  has one end extending from the casing  1  and the other end extending through the tube  10  toward the element-receiving portion  2 A of the element holder  2 . The lead wires  8 ,  8  are connected to a power source side of the automobile and an earth side thereof. The lead wire  9  is connected to a control unit (not shown) for controlling the engine of the automobile.  
     [0030] A generally cylindrical bushing  11  is concentrically mounted into the one axial end portion of the insulator  3 . The bushing  11  is made of an insulating ceramic material and has a radially outwardly extending flange  11 A shown in FIG. 2. A seal ring  12  is interposed between the flange  11 A of the bushing  11  and the one axial end portion of the insulator  3 . As shown in FIG. 2, the bushing  11  has a terminal-insertion through-hole  11 B extending in a direction of a center axis of the bushing  11 , into which a terminal  17 A of an electrode plate  17  is inserted. Two terminal-insertion through-holes  11 C,  11 C extend substantially parallel to the terminal-insertion through-hole  11 B, into which terminals  19 A,  19 A of a contact plate  19  are inserted.  
     [0031] As best shown in FIG. 2, a washer  13  is disposed on the positioning portion  2 D of the element holder  2 . The washer  13  is made of an electrically conductive metal material and has a generally annular shape. The sensing element  14 , the electrode plate  17 , the heater  18  and the contact plate  19  are disposed between the washer  13  and the bushing  11  within the element-receiving portion  2 A of the element holder  2  and arranged in a laminated state.  
     [0032] The sensing element  14  is mounted onto the washer  13  in such a manner that an outer planar surface  14 A is opposed to the exhaust gas inlet  2 B of the element holder  2  and thus exposed to the inside of the exhaust pipe of the engine. The sensing element  14  is made of a suitable ceramic material, zirconia in this embodiment. As shown in FIG. 3, the sensing element  14  has a generally thin disk shape having a diameter D 1 . The outer electrode  15  is disposed on the outer planar surface  14 A of the sensing element  14  and an inner electrode  16  is disposed on an inner planar surface  14 B of the sensing element  14 . The sensing element  14  is operated to detect an electromotive force (voltage signal) generated between the outer and inner electrodes  15  and  16  in response to a difference between the oxygen concentration in the exhaust gas flowing on the outer planar surface  14 A and the oxygen concentration in the atmosphere present on the inner planar surface  14 B. The sensing element  14  then transmits a signal output indicative of the electromotive force to the control unit.  
     [0033] The outer and inner electrodes  15  and  16  are in the form of a generally circular thin film made of an electrically conductive paste containing platinum. The outer electrode  15  substantially covers the outer planar surface  14 A of the sensing element  14 . An outer circumferential portion of the outer electrode  15  is in contact with the washer  13  so that the outer electrode  15  is grounded to the exhaust pipe of the engine through the washer  13  and the element holder  2 . The inner electrode  16  substantially covers the inner planar surface  14 B of the sensing element  14 . The inner electrode  16  is connected with the lead wire  9  through the electrode plate  17 .  
     [0034] The electrode plate  17  is in the form of a generally disk shape made of an electrically conductive metal material. The electrode plate  17  is disposed on the sensing element  14  in contact with the inner electrode  16 . As best shown in FIG. 2, the terminal  17 A is connected to a central portion of the electrode plate  17  and extends into the tube  10  within the insulator  3  through a terminal-insertion through-hole  18 A of the heater  18  and the through-hole  11 B of the bushing  11 . A distal end of the terminal  17 A is connected with the lead wire  9  within the tube  10 .  
     [0035] The heater  18  is arranged to be separably associated with the sensing element  14 . Namely, the heater  18  is formed as a separate body from the sensing element  14  without being bonded thereto. The heater  18  is forced by the disk spring  5  against the inner planar surface  14 B of the sensing element  14  to be in contact therewith through the electrode plate  17  interposed between the heater  18  and the sensing element  14 . The heater  18  having a generally disk shape is disposed between the bushing  11  and the washer  13  in a concentric relation to the sensing element  14  and the electrode plate  17 . The heater  18  has a diameter D 2  larger than the diameter D 1  of the sensing element  14  as shown in FIG. 3. The heater  18  has opposed planar surfaces each having an area greater than the area of each of the outer and inner planar surfaces  14 A and  14 B of the sensing element  14 . The heater  18  covers the entire inner planar surface  14 B of the sensing element  14 . The heater  18  is formed of a burnt or fired ceramic material, such as burnt or fired alumina. As illustrated in FIG. 3, the heater  18  has a pair of diametrically opposed sector-shaped electrodes  18 B,  18 B on the planar surface facing the contact plate  19 . Heater patterns  18 C,  18 C are embedded between the electrodes  18 B,  18 B. The heater patterns  18 C,  18 C extend over the sectorial areas between the electrodes  18 B,  18 B in a wavy or zigzag manner. The heater  18  is supplied with electric energy via the contact plate  19  to thereby heat the sensing element  14  through the electrode plate  17 .  
     [0036] The contact plate  19  is interposed between the bushing  11  and the heater  18 . The contact plate  19  is formed by a pair of the generally sector-shaped contact plates  19 ,  19  as shown in FIG. 3. The contact plates  19 ,  19  are arranged in contact with the electrodes  18 B,  18 B of the heater  18 , respectively. Each of the contact plates  19 ,  19  is made of an electrically conductive metal material. The terminals  19 A,  19 A are connected to inner radial peripheries of the contact plates  19 ,  19 , respectively. The terminals  19 A,  19 A extend into the tube  10  within the insulator  3  through the through-holes  11 C,  11 C of the bushing  11  as shown in FIG. 2. Distal ends of the terminals  19 A,  19 A are connected with the lead wires  8 ,  8  within the tube  10 , respectively.  
     [0037] As best shown in FIG. 2, the domed protection cover  20  for protecting the sensing element  14  is caulked at the cover mount portion  2 E of the element holder  2 . The protection cover  20  is formed with a plurality of windows  20 A circumferentially spaced from each other, through which the exhaust gas is introduced into the exhaust gas inlet  2 B of the element holder  2 .  
     [0038] An operation of the thus-constructed oxygen sensor now is explained.  
     [0039] When the exhaust gas emitted from the engine during the engine operation passes through the exhaust pipe and enters into the exhaust gas inlet  2 B of the element holder  2  of the oxygen sensor, the sensing element  14  is activated to detect an electromotive force generated between the outer and inner electrodes  15  and  16  due to a difference between the oxygen concentration in the exhaust gas present on the outer planar surface  14 A side and the oxygen concentration in the atmosphere present on the inner planar surface  14 B side. The sensing element  14  transmits a signal output indicative of the electromotive force to the control unit via the electrode plate  17  and the lead wire  9 . The control unit carries out a feedback control of air-fuel ratio on the basis of the signal output transmitted. If the temperature of the sensing element  14  is lower than a level sufficient to activate the sensing element  14  upon the engine starting, the control unit operates the heater  18  so as to be supplied with electric energy via the lead wires  8 ,  8  and the contact plates  19 ,  19 . The heater  18  is thus actuated to heat the electrode plate  17  and the sensing element  14 . When the sensing element  14  is heated up to a predetermined temperature, for instance, approximately 350° C., the sensing element  14  is activated. The activation of the sensing element  14  is facilitated by the heating by the heater  18 , so that the feedback control of air-fuel ratio can be early commenced.  
     [0040] As is appreciated from the above explanation, the sensing element  14  and the heater  18  are separably associated with each other, i.e., formed as the separate bodies from each other. With the arrangement, one of the sensing element  14  and the heater  18  can be prevented from suffering from thermal stress caused on the other during the heating because the two elements can be separately expanded and shrunk. The sensing element  14  and the heater  18  can be prevented from being damaged due to the peeling of the heater  18  which will occur during the heating in the oxygen sensors of the earlier techniques. This can enhance reliability and durability of the oxygen sensor. In addition, the sintering and/or adhering process for bonding the sensing element and the heater together as proposed in the earlier techniques can be omitted. This can avoid the occurrence of cracks in the sintering process and serve for improving the production efficiency and reducing the production cost.  
     [0041] Further, as described above, the sensing element  14  is arranged in concentric with the heater  18  having the greater surface area than that of the sensing element  14 . This arrangement can provide good temperature distribution on the planar surface of the heater  18  as shown in FIG. 4, serving for reducing the thermal stress acting on the heater  18 .  
     [0042] In FIG. 4, a characteristic curve of the temperature distribution on the planar surface of the heater  18  is indicated by the solid line. The broken line indicates a characteristic curve of the temperature distribution on the heater surface in a case where the planar surface of the heater  18  has the same area as that of the planar surface of the sensing element  14 . In other words, the broken line denotes the characteristic curve of the temperature distribution on the heater surface when the diameter of the heater  18  is equivalent to the diameter D 1  of the sensing element  14 . As illustrated in FIG. 4, the temperature at the central portion, namely, the inner radial periphery near around the terminal-insertion through-hole  18 A, of the heater surface is higher than the temperature at the outer radial periphery thereof. Temperature gradient of the heater surface is expressed by a temperature change ΔT 1  and ΔT 2  of the heater surface relative to a radial position change ΔD on the heater surface. The temperature gradient ΔT 2 /ΔD of the heater surface having the greater area than the area of the planar surface of the sensing element  14  is smaller than the temperature gradient ΔT 1 /ΔD of the heater surface having the same area as the area of the planar surface of the sensing element  14 . Therefore, if the heater  18  having a relatively greater surface area is used, the temperature gradient of the heater surface can be lowered and the thermal stress acting on the heater  18  corresponding to the temperature gradient of the heater surface can be reduced. The heater  18  having such the greater surface area can be prevented from being damaged due to the thermal stress. This results in improving the oxygen sensor in the reliability and durability.  
     [0043] Further, the heater  18  can substantially evenly heat the sensing element  14  over the entire area thereof. In addition, the heater  18  can be heated at higher temperature to thereby heat the sensing element  14  more quickly. The sensing element  14  can be activated within a shorter period during the engine starting operation, so that the detection of the oxygen concentration can be promoted.  
     [0044] Furthermore, as explained above, the washer  13  is located between the sensing element  14  and the element holder  2 , and the electrode plate  17  is interposed between the sensing element  14  and the heater  18 . The sensing element  14  is grounded through the washer  13  and the element holder  2  and permitted to transmit the signal output to the control unit through the electrode plate  17 . With this arrangement, the single lead wire  9  for transmitting the signal output can be connected with the electrode plate  17 , and another,output terminal or lead wire as used in the earlier technique can be omitted. This can simplify the structure of the oxygen sensor, serving for the cost-saving.  
     [0045] Further, the casing  1  is constituted of the element holder  2  having the positioning portion  2 D, the insulator  3  received in the element holder  2 , and the cap  4  securing the insulator  3  to the element holder  2 . The element holder  2 , the insulator  3  and the cap  4  cooperate to support the sensing element  14  and the heater  18  between the positioning portion  2 D and the axial end portion of the insulator  3 . Furthermore, the disk spring  5  is installed into the cap  4  and biases the insulator  3  toward the positioning portion  2 D to force the sensing element  14  and the heater  18  against the positioning portion  2 D. With this arrangement, the sensing element  14  and the heater  18  which are separably overlapped, can be held in place within the element-receiving portion  2 A of the element holder  2  without displacement. In addition, the spring force of the disk spring  5  acts onto the outer and inner planar surfaces  14 A and  14 B of the sensing element  14 , so that the electrodes  15  and  16  can be kept in contact with the washer  13  and the electrode plate  17 , respectively. This can assure transmission of the signal output from the sensing element  14 . In addition, the electrodes  18 B,  18 B of the heater  18  can be kept in contact with the contact plates  19 ,  19  by the spring force of the disk spring  5 . Therefore, the electric energy supply to the heater  18  through the contact plates  19 ,  19  can be stably conducted.  
     [0046] Referring to FIG. 5, a second embodiment of the oxygen sensor of the invention will be explained hereinafter, which differs in arrangement of the heater  118  from the above-described first embodiment. Like reference numerals denote like parts, and therefore, detailed explanations therefor can be omitted.  
     [0047] As illustrated in FIG. 5, the heater  118  has substantially the same diameter as that of the sensing element  14 . Namely, the area of the planar surface of the heater  118  is substantially the same as the area of the planar surface of the sensing element  14 .  
     [0048] The second embodiment can perform substantially the same effects as those of the first embodiment. Namely, since the heater  118  and the sensing element  14  are separably arranged, the thermal stress caused between the heater  118  and the sensing element  14  can be reduced. The heater  118  and the sensing element  14  can be prevented from being adversely affected by the thermal stress. This can contribute to improvement in the reliability and durability of the oxygen sensor and in the production efficiency and to reduction of the production cost. Further, since the electrode plate  17  is connected with the single lead wire  9 , the second embodiment can provide the simple structure of the oxygen sensor and contribute to the are cost-saving. Furthermore, the heater  118  and the sensing element  14  can be held in place and surely retained within the element-receiving portion  2 A of the element holder  2 .  
     [0049] The entire contents of basic Japanese Patent Application No. 2000-197056 filed on Jun. 29, 2000, inclusive of the specification, claims and drawings, are herein incorporated by reference.  
     [0050] Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.