JPH1010081A - Oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen sensor which can reduce activation time without damaging a structure. SOLUTION: A cylindrical base 111 formed from ZrO2 and having a closed end is provided with a projecting part 111a by cutting work or the like, and an electric heater 117 is directly installed on the upper surface of the projecting part 111a via an insulator 116. Therefore, not only the base and inside and outside electrodes 112, 113 provided respectively on the inner and outer surfaces of the base, but also exhaust gas passing through a coating layer 115 and a frame-sprayed layer 114 and making contact with an external electrode are heated by the electric heater, resulting in reduced activation time. Further, since the insulator and the base are almost equal in heat conductivity, cracking between them is also prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor, and more particularly to an oxygen sensor for detecting the concentration of residual oxygen in exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art It is known to use a catalytic converter to suppress nitrogen oxides, hydrocarbons and carbon monoxide contained in exhaust gas emitted from an internal combustion engine for automobiles from being emitted into the atmosphere. However, in order to maintain a high purification rate, it is necessary to control the air-fuel mixture supplied to the internal combustion engine to approximately the stoichiometric air-fuel ratio.

[0003] In order to control the air-fuel mixture to the stoichiometric air-fuel ratio, the concentration of oxygen remaining in the exhaust gas correlated with the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is detected, and the fuel supply amount is determined according to the intake air amount. Is generally controlled. An oxygen sensor that is currently widely used is a zirconia oxygen sensor has a structure in which a platinum electrode is coated on the inner and outer surfaces of the zirconia solid electrolyte of cylindrical tip is closed (Z _r O _2).

[0004] The oxygen sensor is inserted into an exhaust pipe to expose the outer surface to exhaust gas having a low oxygen partial pressure and to introduce an atmosphere having a high oxygen partial pressure into the inner surface, whereby oxygen ions flow through the zirconia solid electrolyte to cause the inner and outer surfaces to flow. A voltage corresponding to the residual oxygen concentration is generated between the platinum electrodes, but in order to function as an oxygen sensor, the zirconia solid electrolyte and the platinum electrodes on the inner and outer surfaces are maintained in an activated state at a predetermined temperature (650 ° C.) or higher. There is a need to.

In order to shorten the time required for the oxygen sensor to be activated after the internal combustion engine is started, an electric heater is inserted into the hollow portion inside the oxygen sensor to heat the zirconia solid electrolyte and the platinum electrodes on the inner and outer surfaces. Normal. FIG. 2 is an external view of a conventionally used air-fuel ratio sensor and an enlarged sectional view of a portion surrounded by a broken line in the external view. An electric heater 22 is inserted into a hollow portion of a cylindrical air-fuel ratio sensor 21. ing.

[0006] The air-fuel ratio sensor 21 is formed of a zirconia solid electrolyte 211 having a thickness of several millimeters, and the inner and outer surfaces are coated with platinum electrodes 212 and 213.
Then, in order to protect the outer electrode 213, the outer electrode 21
A thermal sprayed layer 214 and a coating layer 215 of about 500 micrometers are formed on the outside of 3. However, since the oxygen sensor according to the above configuration is heated from the inside of the oxygen sensor, it takes time for the outer electrode to reach a predetermined temperature. There is a limit to shortening the activation time of the sensor.

FIG. 3 is a graph showing the change over time in the amount of hydrocarbons discharged after the internal combustion engine is started and the integrated value thereof. The amount of hydrocarbons discharged during about 50 seconds after the start of the internal combustion engine is extremely large. However, it can be understood that the discharge amount after 50 seconds is small. That is, in order to satisfy the stricter emission regulations, it is necessary to activate the oxygen sensor within a few seconds after the internal combustion engine is started and to purify the exhaust gas by the catalytic converter.

Therefore, the electric heater is changed from aluminum oxide (Al ₂ O ₃ ), which has been used conventionally, to silicon nitride (Si _3).
An oxygen sensor with N ₄ ) has already been proposed. However, when the oxygen sensor is rapidly activated, not only the electric heater and the zirconia solid electrolyte may be broken by a thermal shock, but also the inner electrode may be deteriorated by thermal condensation.

In order to enable rapid activation of the oxygen sensor without destruction of the electric heater and the zirconia solid electrolyte and deterioration of the electrode, a heater covered with an insulating layer is formed on a part of the inside of the outer electrode. An oxygen sensor has already been proposed (see Japanese Patent Application Laid-Open No. Hei 4-370758).

[0010]

In the oxygen sensor according to the above proposal, an electric heater is installed outside the zirconia solid electrolyte to rapidly heat the zirconia solid electrolyte and the electrodes coated on the inner and outer surfaces of the zirconia solid electrolyte. Therefore, the activation time of the oxygen sensor can be reduced to some extent.

However, in the oxygen sensor according to the above proposal, it is impossible to prevent low-temperature exhaust gas from directly hitting the outer electrode immediately after the start of the internal combustion engine, so that the activation time of the oxygen sensor is sufficiently reduced. Can not. The present invention has been made in view of the above problems, and has as its object to provide an oxygen sensor capable of shortening an activation time without damaging a structure.

[0012]

An oxygen sensor according to a first aspect of the present invention includes a base material exhibiting a characteristic change according to an oxygen concentration in a contacting gas, and electrodes provided on inner and outer surfaces of the base material. , Wherein the substrate has a convex portion formed by the substrate on the outside thereof, and an electric heater is provided above the convex portion via an insulating layer.

According to the oxygen sensor, not only the base member and the electrodes provided on the inner and outer surfaces of the base member are heated by the electric heater, but also the exhaust gas in contact with the outer electrode is heated. The oxygen sensor according to claim 2 further includes a gas-permeable covering layer covering both the electrode and the electric heater provided on the outer surface.

According to the oxygen sensor, the exhaust gas passing through the air-permeable coating layer is warmed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in order to quickly activate an oxygen sensor after starting an internal combustion engine, the following two methods are required.
The point is important. 1. Heating the contact surface (three-layer interface) between the outer electrode, the solid electrolyte and the exhaust gas as quickly as possible. 2. To prevent the three-layer interface from being cooled by the low temperature exhaust gas immediately after starting.

Therefore, the following three types of structures were examined for their activation time and damage to the oxygen sensor structure. That is, FIG. 4 is an enlarged cross-sectional view of the investigated oxygen sensor. FIG. 4A shows an insulator (for example, Al) outside the outer electrode 213 in order to directly heat the three-layer interface.
It has a structure in which established the electric heater (e.g., tungsten) 117 via the ₂ O ₃₎ 116.

(B) has a structure in which an electric heater 117 is provided outside the outermost coating layer 115 for heating low-temperature exhaust gas. (C) heats both the three-layer interface and the low-temperature exhaust gas.
4 has a structure in which an electric heater 117 is provided via an insulator 116. As a result, the following matters became clear.

In the structure (a), although the temperature at the three-layer interface is rapidly increased, it is unavoidable that the three-layer interface is cooled by low-temperature exhaust gas, and the activation time of the oxygen sensor is slightly increased. It is only resolved. In the structure of (b), the low-temperature exhaust gas is heated because the low-temperature exhaust gas is heated.
Although the cooling of the layer interface is avoided, the distance from the heater to the three-layer interface is large, and the temperature rise at the three-layer interface is delayed, and the activation time of the oxygen sensor becomes longer.

In the structure (c), not only the temperature of the three-layer interface is rapidly raised, but also the low-temperature exhaust gas is heated. The activation time of the sensor is reduced. However, it was difficult to prevent the occurrence of cracks between the insulator 116 and the solid electrolyte 111 due to the difference in the coefficient of thermal expansion between the insulator 116 and the outer electrode 113.

The present invention has been made to solve these problems.
It has the following structure. That is, FIG. 1 is an external view and an enlarged cross-sectional view of the oxygen sensor according to the present invention.
11 is subjected to a cutting process using, for example, a lathe, and a convex portion 111a is formed.
Is provided. It is desirable that the height of the projection 111a be approximately 200 micrometers. Then, an electric heater (for example, tungsten) 117 is provided on the upper surface of the projection 111a via an insulator (for example, Al ₂ O ₃ ) 116. Note that an outer electrode 113 is provided on the outer surface of the solid electrolyte 111 other than the convex portion 111a as in the related art.

FIG. 5 shows an example of the configuration of the projection 111a formed by processing the solid electrolyte 111. The height of the projection 211a is exaggerated. That is, (a) is a ring shape,
(B) is a spiral, and (c) is an island-shaped projection 211.
However, the oxygen sensor according to the present invention may have at least one protrusion on the outer surface of the solid electrolyte 111.

That is, in the oxygen sensor according to the present invention, since the exhaust gas reaching the outer electrode 113 through the three-layer interface and the coating layer 115 and the sprayed layer 114 is heated by the electric heater 117, Activation time is reduced. Furthermore, the flow of the exhaust gas in the axial direction of the oxygen sensor is prevented by the convex portion 111a provided on the outer surface of the solid electrolyte 111, and the cooling of the three-layer interface is suppressed.

Further, since the electric heater 117 is attached to the direct solid electrolyte 111 having substantially the same thermal conductivity as the insulator 116 via the insulator 116, the occurrence of cracks, that is, the occurrence of mechanical damage is prevented. Is done. FIG. 6 is a graph showing the activation time of the oxygen sensor according to the present invention. The activation time of the oxygen sensor having the structure shown in FIGS. As can be understood from this graph, the oxygen sensor according to the present invention is activated about 6 seconds after the start of the internal combustion engine, and the shortest activation time of the oxygen sensors having the configurations shown in FIGS. The activation time is almost the same as that of (c).

[0024]

According to the oxygen sensor of the present invention, not only the electrodes provided on the inner and outer surfaces of the substrate but also the external electrodes by the heater provided on the upper surface of the convex portion of the substrate via an insulator. Since the exhaust gas contacting the gas is also heated, the activation time of the oxygen sensor can be shortened. Further, since the electric heater is directly attached to the base material via the insulator, it is possible to suppress the occurrence of cracks between the insulator and the base material.

[Brief description of the drawings]

FIG. 1 is an external view and an enlarged sectional view of an oxygen sensor according to the present invention.

FIG. 2 is an external view and an enlarged cross-sectional view of a conventionally used oxygen sensor.

FIG. 3 is a graph showing the amount of hydrocarbons discharged after the start of the internal combustion engine and its integrated value over time.

FIG. 4 is an enlarged cross-sectional view of the investigated oxygen sensor.

FIG. 5 is a configuration example of a convex portion.

FIG. 6 is a graph showing activation time.

[Description of Signs] 11 ... Oxygen sensor 111 ... Solid electrolyte 111a ... Protrusion 112 ... Inner electrode 113 ... Outer electrode 114 ... Sprayed layer 115 ... Coating layer 116 ... Insulator 117 ... Electric heater

Claims (2)

[Claims] 1. An oxygen sensor comprising: a base material exhibiting a characteristic change according to the oxygen concentration in a contacting gas; and electrodes provided on an inner surface and an outer surface of the base material, wherein the base material is An oxygen sensor having a convex portion formed of the base material on the outer side thereof, and an electric heater provided above the convex portion via an insulating layer. 2. The oxygen sensor according to claim 1, further comprising a gas-permeable covering layer covering both the electrode provided on the outer surface and the electric heater.