JP2009180669A - Air-fuel ratio sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein a responsiveness is decreased when a catalyst layer is formed on the surface of a sensor element to improve the detection accuracy of an air-fuel ratio sensor. <P>SOLUTION: This air-fuel ratio sensor comprises: a solid electrolyte 12 having a first surface 12a on which sensor element section 11 faces the atmosphere side and a second surface 12b on which it faces the exhaust side; an atmosphere-side electrode 14 arranged on the first surface 12a of the solid electrolyte 12; an exhaust side electrode 15 arranged on the second surface 12b so as to face the atmosphere-side electrode 14 with respect to the solid electrolyte 12; a diffusion resistive layer 16 formed so as to cover exhaust side electrode 15 and the second surface 12b of the solid electrolyte 12; and a catalyst layer 18 formed on the surface of the diffusion resistive layer 16 except for a region 16c for introducing exhaust air to the exhaust side electrode 15 via the diffusion resistive layer 16. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio sensor for detecting an air-fuel ratio that is a ratio of air and fuel supplied to an internal combustion engine based on oxygen concentration in exhaust gas.

  With the tightening of emission regulations regarding harmful substances contained in exhaust gas discharged from internal combustion engines mounted on vehicles, oxygen sensors that detect the oxygen concentration in exhaust gas and the ratio of intake air to fuel for internal combustion engines Further improvements in detection accuracy of air-fuel ratio sensors and the like are being studied. As one of countermeasures, Patent Document 1 proposes a technique in which a catalyst layer is formed on the surface of a sensor element facing the exhaust side. This catalyst layer is configured to change the hydrogen gas that causes a decrease in the detection accuracy of the oxygen sensor into a harmless oxide compound, thereby preventing a decrease in the detection accuracy of the oxygen sensor.

JP 2002-181769 A

  When the technique disclosed in Patent Document 1 is applied to an air-fuel ratio sensor, the surface of the diffusion resistance layer of the air-fuel ratio sensor is covered with a catalyst layer, so that the gas (exhaust gas) passing through the diffusion resistance layer Equilibration can be achieved. In addition, since hydrogen molecules, hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust gas that may adversely affect the detection work of the air-fuel ratio sensor are rendered harmless, reliable and stable detection can be performed. it can. On the other hand, since the surface region of the diffusion resistance layer located on the opposite side of the electrode is covered with the catalyst layer, a catalytic reaction occurs in this portion, resulting in a decrease in detection responsiveness.

  An object of the present invention is to provide an air-fuel ratio sensor capable of improving detection accuracy without causing a decrease in responsiveness.

  According to a first aspect of the present invention, there is provided a solid electrolyte body having a first surface facing the atmosphere side and a second surface facing the exhaust side, and an atmosphere side electrode disposed on the first surface of the solid electrolyte body; An exhaust-side electrode disposed on the second surface so as to face the atmosphere-side electrode across the solid electrolyte body, and to cover the exhaust-side electrode and the second surface of the solid electrolyte body And a sensor element unit including a catalyst layer formed on the surface of the diffusion resistance layer except for a region for guiding exhaust gas to the exhaust-side electrode through the diffusion resistance layer. The air-fuel ratio sensor is characterized by the following.

  In the present invention, the region for guiding the exhaust to the exhaust side electrode is not covered by the catalyst layer, and no catalytic reaction occurs here. Exhaust gas reaches the exhaust side electrode through the diffusion resistance layer from the region for guiding it to the exhaust side electrode.

  In the air-fuel ratio sensor according to the first aspect of the present invention, the solid electrolyte body may have a cylindrical shape with a closed tip.

  According to a second aspect of the present invention, a solid electrolyte body having a first surface and a second surface facing the exhaust side, and the solid electrolyte body are formed integrally with the first surface of the solid electrolyte body. A shielding member that shields the first surface from the exhaust side so as to face the atmosphere side; an atmosphere-side electrode disposed on the first surface of the solid electrolyte body surrounded by the shielding member; and the solid An exhaust side electrode disposed on the second surface so as to face the atmosphere side electrode across the electrolyte body, and the exhaust side electrode and the second surface of the solid electrolyte body are covered. A diffusion resistance layer; a masking layer formed so as to cover a surface of the diffusion resistance layer excluding a region for guiding exhaust gas to the exhaust side electrode; and surfaces of the solid electrolyte body, the shielding member, and the masking layer With a catalyst layer formed to cover In the air-fuel ratio sensor, characterized in that it comprises an element portion.

  Also in the present invention, as in the first embodiment, the region for guiding the exhaust to the exhaust side electrode is not covered by the catalyst layer, and no catalytic reaction occurs here. Exhaust gas reaches from the region of the diffusion resistance layer not covered with the masking layer to the exhaust electrode through this region.

  In the air-fuel ratio sensor according to the second aspect of the present invention, the solid electrolyte body may have a flat plate shape.

  In the air-fuel ratio sensor according to the first or second aspect of the present invention, it is preferable that the catalyst layer is for accelerating the oxidation of hydrogen gas in the exhaust to an oxidized compound.

  The heater may be further provided so as to face the second surface of the solid electrolyte body and for heating the sensor element portion.

  An outer casing having a communication hole that communicates with the exhaust side, and an inner casing that is accommodated inside the outer casing and that communicates with a gap defined between the outer casing and the outer casing. The sensor element part may be accommodated in the inner casing.

  According to the air-fuel ratio sensor of the present invention, since the catalyst layer is formed on the surface of the diffusion resistance layer except for the region for guiding the exhaust gas to the exhaust side electrode via the diffusion resistance layer, the detection by the air-fuel ratio sensor is adversely affected. Potential hydrogen gas can be turned into harmless oxide compounds. As a result, both the detection accuracy by the air-fuel ratio sensor and its reliability can be improved. In addition, it becomes possible to guide the exhaust gas from the diffusion resistance layer region for guiding exhaust gas to the exhaust side electrode to the exhaust side electrode through this diffusion resistance layer, and suppress the deterioration of the response of the air-fuel ratio sensor, The same level of responsiveness as a conventional one not formed can be maintained. In addition, it is not necessary to form an expensive catalyst layer over the entire surface of the sensor element, and the amount of valuable catalyst material used can be suppressed.

  An outer casing having a communication hole that communicates with the exhaust side, and an inner casing that is accommodated inside the outer casing and that communicates with a gap defined between the outer casing and the outer casing. In the case where the sensor element portion is accommodated in the inner casing, the following effects can be obtained. That is, while the exhaust gas flows into the inner casing through the gap between the outer casing and the inner casing, the exhaust gas becomes more balanced, and the detection accuracy by the air-fuel ratio sensor and its reliability are further improved. Can do.

  An embodiment in which an air-fuel ratio sensor according to the present invention is applied to a stacked type will be described in detail with reference to FIGS. However, the present invention is not limited to such an embodiment, but can be applied to a tube type as needed, or can be applied to any other technique belonging to the spirit of the present invention.

  The external appearance of the sensor element part of the air-fuel ratio sensor in this embodiment is shown in FIG. 1, its cross-sectional structure is shown in FIG. 2, its III-III arrow cross-sectional structure and IV-IV arrow cross-sectional structure are shown in FIGS. Respectively. That is, the sensor element portion 11 constituting the tip side of the air-fuel ratio sensor in the present embodiment includes a solid electrolyte body 12, a shielding member 13, an atmosphere-side electrode 14, an exhaust-side electrode 15, and a diffusion plate. A resistance layer 16, a masking layer 17, a catalyst layer 18, and a heater 19 are provided.

  The solid electrolyte body 12 formed of ceramics such as zirconia has a first surface 12a that communicates with the atmosphere side and a second surface 12b that faces the exhaust side.

  The shielding member 13 is formed integrally with the solid electrolyte body 12, and shields the first surface 12a from the exhaust side so that the first surface 12a of the solid electrolyte body 12 faces the atmosphere side. In other words, the shielding member 13 includes an atmosphere communication path 20 whose base end side communicates with the atmosphere side, and a chamber 21 that communicates with the end of the atmosphere communication path 20 and accommodates the atmosphere side electrode 14. It is defined between the first surface 12a. Although the shielding member 13 in the present embodiment is formed of the same ceramic as the solid electrolyte body 12 such as zirconia, it is not necessary to be porous.

  The atmosphere side electrode 14 formed of platinum or the like is disposed together with the lead wire 22 on the first surface 12 a of the solid electrolyte body 12 facing the chamber 21 defined between the shielding member 13.

  Similarly to the atmosphere side electrode 14, the exhaust side electrode 15 formed of platinum or the like has its lead wire on the second surface 12 b of the solid electrolyte body 12 so as to face the atmosphere side electrode 14 with the solid electrolyte body 12 interposed therebetween. 23.

  Regardless of the flow rate of the exhaust gas flowing in the exhaust flow path, the diffusion resistance layer 16 for adjusting the passage speed of oxygen molecules in the exhaust gas reaching the exhaust side electrode 15 to be substantially constant includes the exhaust side electrode 15 and the solid electrolyte body. It is formed so as to cover the 12 second surfaces 12b. In the present embodiment, the diffusion resistance layer 16 has a trapezoidal cross-sectional shape perpendicular to the longitudinal direction, and the masking layer 17 is bonded to the upper base, that is, the surface 16a defining the short side. That is, the surface 16 b that defines the long side is joined to the second surface 12 b of the solid electrolyte body 12. Oxygen molecules in the exhaust are guided to the exhaust-side electrode 15 from the surfaces 16c that define a pair of hypotenuses. It is effective that the diffusion resistance layer 16 is a porous body for allowing oxygen molecules to pass through. In the present embodiment, the diffusion resistance layer 16 is formed of the same ceramic as the solid electrolyte body 12 such as zirconia.

  The masking layer 17 that is formed of ceramics such as alumina and prevents the passage of gas is formed on the surface 16a that defines the short side of the diffusion resistance layer 16 having a trapezoidal cross section as described above. That is, the masking layer 17 is not formed in the region for guiding the exhaust to the exhaust-side electrode 15, that is, the surface 16 c defining the pair of oblique sides of the solid electrolyte body 12.

  The catalyst layer 18 formed of platinum-rhodium or the like is formed so as to cover the surfaces of the solid electrolyte body 12 exposed to the outside, the shielding member 13, and the masking layer 17, and has a trapezoidal cross section like the masking layer 17. The catalyst layer 18 is not formed on the surface 16c defining the pair of hypotenuses of the layer 16. As is well known, when carbon, carbon compounds, nitrogen oxides or sulfur oxides are contained in the exhaust gas, these react with each other to generate hydrogen gas, which is detected by the air-fuel ratio sensor. This causes a detection error. The catalyst layer 18 is for accelerating the change of hydrogen gas in the exhaust gas into a harmless oxide compound by its catalytic function.

  The heater 19 is disposed so as to face the second surface 12b of the solid electrolyte body 12, and is used to quickly raise the temperature of the sensor element unit 11 in a low temperature state to a temperature at which it normally functions. In the present embodiment, the heater 19 is embedded in the shielding member 13 together with the power supply line 24 so as to face the atmosphere side electrode 14, but the heater 19 is placed in the chamber 21 that faces the first surface 12a. It is also possible to arrange them.

  The sensor element unit 11 in the present embodiment is accommodated in a double-structure casing including an outer casing 25 and an inner casing 26. A plurality of communication holes 25a are formed in the outer casing 25 disposed so as to face an exhaust passage of an internal combustion engine or the like so as to communicate the inside with the exhaust passage. Similarly, the inner casing 26 is formed with a plurality of communication holes 26 a that communicate with the annular gap 27 defined between the inner casing 26 and the inner casing 26. In the present embodiment, the communication hole 25a formed in the outer casing 25 and the communication hole 26a formed in the inner casing 26 are formed unevenly at positions where they face each other at an interval of approximately 180 degrees. . Thereby, it is possible to efficiently balance the exhaust led to the inside of the inner casing 26 in which the sensor element unit 11 is accommodated.

  The basic structure of the air-fuel ratio sensor other than the sensor element unit 11 described above may be the same as the known structure disclosed in Japanese Patent Application Laid-Open No. 2003-202316.

  Therefore, when this air-fuel ratio sensor is mounted in the middle of the exhaust pipe of the internal combustion engine, a part of the exhaust flowing in the exhaust passage formed in the exhaust pipe is communicated with the outer casing 25 from the communication hole 25a of the outer casing 25. The air is guided into an annular gap 27 between the inner casing 26 and further flows from the gap 27 into the inner casing 26 through the communication hole 26 a of the inner casing 26. The air / fuel ratio of the exhaust gas flowing into the inner casing 26 in a balanced state is detected by the sensor element unit 11. In this case, hydrogen gas in the exhaust gas, which is a detection error by the air-fuel ratio sensor, is rendered harmless by the catalyst layer 18 as an oxidized compound, and a highly reliable detection result can be obtained. Further, exhaust gas is directly guided to the exhaust-side electrode 15 from the surfaces 16c defining the pair of oblique sides of the diffusion resistance layer 16 without the catalyst layer 18, and the response delay accompanying the detection can be prevented.

  The exhaust gas that has flowed into the inner casing 26 is sucked into the exhaust passage by the venturi effect from the outer casing 25 and a discharge hole (not shown) formed at the tip of the inner casing 26.

  In the above-described embodiment, the laminated air-fuel ratio sensor using the solid electrolyte body 12 having an elongated plate shape has been described. However, as described above, a so-called solid electrolyte body 12 having a cylindrical shape with a closed tip is used. The present invention can also be applied to a tube (cup) type air-fuel ratio sensor.

  FIG. 5 shows a cross-sectional structure of another embodiment of the air-fuel ratio sensor according to the present invention. Elements having the same functions as those of the previous embodiment are designated by the same reference numerals, and redundant description is omitted. Shall. That is, the sensor element portion 11 of the air-fuel ratio sensor in the present embodiment has a cylindrical shape with a closed tip, and has a first surface facing the atmosphere side, that is, an inner peripheral surface 12a and a second surface facing the exhaust side, That is, the solid electrolyte body 12 having the outer peripheral surface 12b is provided. The annular atmosphere-side electrode 14 is disposed along with the lead wire 22 on the inner peripheral surface of the solid electrolyte body 12. The exhaust-side electrode 15 having an annular shape is disposed together with the lead wires 23 on the outer peripheral surface of the solid electrolyte body 12 so as to face the atmosphere-side electrode 14 with the solid electrolyte body 12 interposed therebetween. The diffusion resistance layer 16 is formed so as to cover the outer peripheral surface of the solid electrolyte body 12 including the exhaust-side electrode 15. The catalyst layer 18 is formed on the surface of the diffusion resistance layer 16 except for a region for guiding exhaust gas to the exhaust side electrode 15 via the diffusion resistance layer 16. The region for guiding the exhaust gas to the exhaust side electrode 15 in the present embodiment is an annular region 16 d where the outer peripheral surface of the diffusion resistance layer 16 faces the exhaust side electrode 15. The heater 19 in which the power supply line 24 is incorporated is accommodated in the solid electrolyte body 12 so as to face the atmosphere side electrode 14 and heats the sensor element unit 11.

  Accordingly, a part of the exhaust gas flowing in the exhaust passage of the internal combustion engine is guided into the annular gap 27 between the outer casing 25 and the inner casing 26 from the communication hole 25 a of the outer casing 25, and from this gap 27. It flows into the inner casing 26 through the communication hole 26 a of the inner casing 26. Thus, the air-fuel ratio of the exhaust gas flowing into the inner casing 26 in a balanced state is detected by the sensor element unit 11. In this case as well, as in the previous embodiment, hydrogen gas in the exhaust gas, which becomes a detection error by the air-fuel ratio sensor, is rendered harmless by the catalyst layer 18 and is rendered harmless, and a highly reliable detection result can be obtained. . Further, the exhaust gas is guided to the exhaust side electrode 15 from the annular region 16d of the diffusion resistance layer 16 without the catalyst layer 18, and the response delay accompanying the detection can be prevented.

  It should be noted that the present invention should be construed only from the matters described in the claims, and in the above-described embodiment, all the changes and modifications included in the concept of the present invention are other than those described. Is possible. That is, all matters in the above-described embodiment are not intended to limit the present invention, and include any configuration not directly related to the present invention. To get.

It is a three-dimensional projection figure showing the external appearance of the part of the sensor element in one Embodiment which applied the air-fuel ratio sensor by this invention to the laminated type thing. It is sectional drawing of the air fuel ratio sensor shown in FIG. FIG. 3 is a cross-sectional view taken along arrow III-III in FIG. 2. It is IV-IV arrow sectional drawing in FIG. It is sectional drawing of the part of the sensor element in one Embodiment which applied the air fuel ratio sensor by this invention to a tube type thing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Sensor element part 12 Solid electrolyte body 12a 1st surface 12b 2nd surface 13 Shielding member 14 Atmosphere side electrode 15 Exhaust side electrode 16 Diffusion resistance layer 16a Surface which defines short side 16b Surface which defines long side 16c Surface defining the oblique side 16d Annular region 17 Masking layer 18 Catalyst layer 19 Heater 20 Atmospheric communication path 21 Chamber 22, 23 Lead wire 24 Feed line 25 Outer casing 25a Communication hole 26 Inner casing 26a Communication hole 27 Gap

Claims (3)

A solid electrolyte body having a first surface facing the atmosphere side and a second surface facing the exhaust side;
An atmosphere-side electrode disposed on the first surface of the solid electrolyte body;
An exhaust-side electrode disposed on the second surface so as to face the atmosphere-side electrode across the solid electrolyte body;
A diffusion resistance layer formed so as to cover the exhaust-side electrode and the second surface of the solid electrolyte body;
An air-fuel ratio sensor comprising a sensor element portion including a catalyst layer formed on the surface of the diffusion resistance layer except for a region for guiding exhaust gas to the exhaust side electrode through the diffusion resistance layer . A solid electrolyte body having a first surface and a second surface facing the exhaust side;
A shielding member that is integrally formed with the solid electrolyte body and shields the first surface from the exhaust side so that the first surface of the solid electrolyte body faces the atmosphere;
An atmosphere-side electrode disposed on the first surface of the solid electrolyte body surrounded by the shielding member;
An exhaust-side electrode disposed on the second surface so as to face the atmosphere-side electrode across the solid electrolyte body;
A diffusion resistance layer formed so as to cover the exhaust-side electrode and the second surface of the solid electrolyte body;
A masking layer formed so as to cover the surface of the diffusion resistance layer except for a region for guiding exhaust to the exhaust side electrode;
An air-fuel ratio sensor comprising: a sensor element portion including: the solid electrolyte body, the shielding member, and a catalyst layer formed so as to cover surfaces of the masking layer. An outer casing having a communication hole that communicates with the exhaust side inside;
An inner casing which is accommodated inside the outer casing and has a communication hole communicating with a gap defined between the outer casing and the sensor element portion is accommodated in the inner casing. The air-fuel ratio sensor according to claim 1 or 2, characterized by the above.

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