JP2001153833A - Oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easily manufacturable inexpensive oxygen sensor by reducing the number of parts for casing structure in the oxygen sensor. SOLUTION: A casing is constructed of a main body metal fitting 9 holding an oxygen detection element 2 on the inside and a metallic cylindrical base body 14. In the cylindrical base body 14, a ceramic separator 18 allowing passage of a plurality of lead wires respectively is arranged, and by means of a metal elastic member 41, the ceramic separator 18 is energized backward so as to be elastically held inside the cylindrical base body 14. The casing structure is constructed of the cylindrical main body metal fitting 9 and the metallic cylindrical base body 14. Therefore, the number of parts is reduced, manufacture is simplified and a cost is lowered in comparison with a conventional structure having a casing constructed of a cylindrical main body metal fitting, an outer cylinder and a main cylinder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor for detecting an oxygen concentration in exhaust gas of an internal combustion engine or an oxygen sensor for detecting oxygen in a predetermined gas.

[0002]

2. Description of the Related Art In recent years, various oxygen sensors have been developed for use in controlling the air-fuel ratio of an internal combustion engine such as an automobile engine. In particular, environmental protection issues such as air pollution by exhaust gas have become a major issue, and the demand for oxygen sensors for detecting the oxygen concentration in exhaust gas for high-performance and long-life oxygen sensors has been increasing. .

As a typical example of such an oxygen sensor, houses an oxygen ion conductive solid electrolyte by the tip portion the oxygen sensing element formed in a hollow shaft shape closed such as ZrO ₂ in a tubular casing And contacting the detection surface on the outer surface of the tip of the oxygen detection element with the atmosphere to be detected, introducing air as a reference gas into the inner space thereof, and bringing the air into contact with the inner surface (reference surface). 2. Description of the Related Art A structure in which an oxygen concentration in an atmosphere to be detected is measured by detecting an electromotive force of an oxygen concentration cell generated due to a difference in oxygen concentration is widely used.

In the oxygen sensor, a ceramic separator is incorporated as a structure for drawing out a lead wire from an oxygen sensing element or a heating element for heating the oxygen sensing element to the outside, and each lead wire is inserted into an individual lead wire insertion hole. The thing which let a wire pass is used. By using such a ceramic separator, a short circuit between the lead wires or between the terminal portions following the lead wires can be prevented.

[0005]

Incidentally, a casing structure of a conventional oxygen sensor incorporating such a ceramic separator is disclosed in, for example, Japanese Patent Application Laid-Open No. H11-72463. A metal shell, a main cylinder connected to the metal shell, and an outer cylinder externally fitted to the main cylinder. A filter is externally fitted to the outer cylinder,
This is covered with a protective cover. In this configuration, after mounting the metal elastic member in the outer cylinder in advance,
The ceramic separator having the lead wire inserted therein is housed in the outer cylinder, and a unit is formed by the outer cylinder, the filter, and the ceramic separator. Thereafter, the outer cylinder is externally fitted to the main cylinder connected to the metal shell, and the front end of the ceramic separator is moved to the rear of the main cylinder by the urging force of the metal elastic member between the locking step surface formed on the outer cylinder and the ceramic separator. Elastic contact with the end, and in this state, it is fixed by caulking to the main cylinder. Thus, each member is assembled.

[0006] As described above, the conventional oxygen sensor casing structure is composed of a metal shell, a main cylinder, and an outer cylinder because of the premise of such assembling, and has a large number of parts and a complicated structure. Therefore, the production was troublesome and the cost was high.

An object of the present invention is to provide an oxygen sensor which is easy to manufacture and inexpensive by reducing the number of parts of the casing structure of the oxygen sensor.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a shaft-shaped oxygen sensing element, a short cylindrical metal shell holding the oxygen sensing element inside and projecting its detection surface, and a front end formed of a metal shell. A metal tubular base body that is tightly connected to the rear end and has a locking step on the inner surface of the rear section, and a rear side locked with the locking step and internally fitted into the cylindrical base body; The ceramic separator and each of the lead wires from the oxygen sensing element are respectively inserted into the plurality of lead wire insertion holes formed to penetrate in the direction, and are fitted inside the cylindrical base, and elastically contact the front side of the ceramic separator, A metal elastic member that presses and holds the rear surface side of the ceramic separator at a locking step; and an elastic seal member that is hermetically fitted to the rear end opening of the cylindrical base and hermetically inserts each lead wire. An oxygen sensor characterized in that: here,
In the axial direction of the oxygen sensing element, the side facing the tip is the front side, and the side facing the opposite direction is the rear side.

In this configuration, the metal elastic member is press-fitted between the locking projection provided on the cylindrical base and the front side of the ceramic separator, and the front end is supported by the locking projection. By elastically contacting the rear end with the front side of the ceramic separator, the rear side of the ceramic separator can be constituted by a washer such as a corrugated washer that presses and holds the rear side of the ceramic separator at the locking stage. In this configuration, the front end of the washer can be reliably held by the locking projection.

Further, a metal elastic member is press-fitted into a cylindrical base, and is positioned by engaging teeth formed on a peripheral edge with the inner surface of the cylindrical base of the ceramic separator, and the inner peripheral edge is positioned on the front side of the ceramic separator. It may be constituted by a toothed washer that makes elastic contact. This configuration has the advantage that it is not necessary to form a locking projection on the tubular base.

Here, an annular filter is fitted around the outer cylinder so as to surround the gas introduction hole formed at the rear end of the cylindrical base, and the gas from the gas introduction hole is applied to the reference surface of the oxygen sensing element. Secure the air flow path to be introduced.

In the basic configuration of the present invention, the ceramic separator is urged rearward by the metal elastic member to stably hold the ceramic separator. For this reason, an outer cylinder or the like for holding the ceramic separator is not required, and the casing structure is composed of a cylindrical metal shell and a metal cylindrical base. Therefore,
The number of parts is smaller than in the past.

In the assembly of the casing structure, a corrugated washer and teeth are inserted through a front end opening in a state where a ceramic separator having a lead wire connected to an oxygen sensing element or the like inserted therein is housed in a metal cylindrical base. A metal elastic member such as a washer is press-fitted and arranged in a compressed state between the ceramic separator and the locking projection, whereby the ceramic separator is elastically held in the locking step by its restoring elasticity.
Thereafter, the metal cylindrical base is externally fitted to the rear end of the metal shell and is consolidated by means such as press fitting or welding. As described above, the ceramic separator is held simply by press-fitting the metal elastic member into the cylindrical base, whereby the casing structure including the ceramic separator is completely unitized, so that the assembling step to the metal shell is easy. Becomes

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described based on embodiments shown in the drawings. The oxygen sensor 1 shown in FIG.
It comprises an oxygen sensing element 2 which is a hollow shaft-shaped solid electrolyte member having a closed end, and a heating element 3 which is a shaft-shaped ceramic heater, and furthermore, is constituted by a casing structure incorporating these.

Here, the oxygen detecting element 2 is constituted by a solid electrolyte having oxygen ion conductivity. A typical example of such a solid electrolyte is ZrO _{2 in} which Y ₂ O ₃ or CaO is dissolved, and a solid solution of an oxide of another alkaline earth metal or a rare earth metal and ZrO ₂ is used. May be used. Also, the base ZrO
₂ may contain HfO ₂ .

The oxygen sensing element 2 is inserted into a short metal shell 9 made of metal, and its tip is protruded and held. The oxygen sensing element 2 exposes a sensing surface 2a in contact with a gas to be detected (exhaust gas) on the peripheral surface. are doing. Insulators 6 and 7 formed of insulating ceramic and ceramic powder 8 formed of talc between the insulators 6 and 7 are provided between the oxygen sensing element 2 and the inner surfaces of the short cylindrical metal shell 9. The oxygen sensing element 2 is penetrated and held while being electrically insulated from the metal shell 9. The peripheral surface of the metal shell 9 has a screw portion 9b for mounting the oxygen sensor 1 to a mounting portion such as an exhaust pipe. In addition, a caulking projection 10 formed at the rear end of the metal shell 9 is caulked to the rear end side of the insulator 6 so that each member inside the metal shell 9 is tightly held.

An electrode layer is provided on the detection surface 2a on the surface of the oxygen sensing element 2 protruding from the metal shell 9 and the reference surface 2b on the inner surface so as to cover almost the entire surface. Each of these inner and outer electrode layers is reversible with respect to a dissociation reaction of oxygen molecules for injecting oxygen into the solid electrolyte constituting the oxygen sensing element 2 and a recombination reaction of oxygen for releasing oxygen from the solid electrolyte. It is configured as a porous electrode having an excellent catalytic function (oxygen dissociation catalytic function), for example, a Pt porous electrode.

The heating element 3 is usually a ceramic heater, for example, a ceramic rod mainly composed of alumina as a core material, and a heating section 4 formed of, for example, a meandering resistance wire on the surface of the ceramic rod. And a heater terminal 5
Through a lead wire extending from (e.g., FIG. 1), power is supplied for heat generation. Such a heat generating portion 4 is provided to be biased toward the front end side of the heat generating member 3 and comes into contact with the oxygen detecting element 2 to locally generate heat at the front end portion. As a result, the heat generated in the heat generating portion 4 is quickly transmitted to the oxygen detecting element 2 by heat conduction based on the contact and heats the oxygen detecting element 2, and the heat radiation of a locally heated portion of the heat generating portion 4 near the contact portion is generated. Also heats the oxygen sensing element 2. Then, the synergistic heat transfer due to the heat conduction and the heat radiation rapidly heats the oxygen sensing element 2 and shortens the rise time to the activation temperature.

In one opening 9a of the metal shell 9, a protector 11 is provided so as to cover the distal end side of the oxygen sensing element 2 with a predetermined space therebetween. A transmission port 12 is formed so that oxygen in the exhaust gas can come into contact with the detection surface 2 a on the front end surface of the oxygen detection element 2. Around the other caulking protrusion 10 of the metal shell 9, a metal cylindrical base 14 according to the main part of the present invention is fixed.

The metal cylindrical base 14 has a length from the rear end of the metal shell 9 to the lead-out end of the lead wire. The diameter of the rear portion is reduced, and a locking step 15 is formed on the inner surface. Then, a linking flange 19 provided around the ceramic separator 18 fitted inside the metal cylindrical base 14 is locked by the locking step 15 to restrict the rearward movement of the ceramic separator 18. An elastic seal member 17 made of rubber or the like is hermetically fitted to the rear end opening of the metal tubular base 14.

The lead wires 20 and 21 of the oxygen sensing element 2 and the lead wire (not shown) of the heating element 3 are drawn out so as to penetrate the ceramic separator 18 and the elastic seal member 17.

The one lead wire 20 connected to the oxygen sensing element 2 has a terminal fitting 24 and a lead wire section 25 following the terminal fitting 24.
(Although it is covered with the insulating tube 25a, it may be omitted.) And the internal electrode connection portion 26 of the terminal fitting 23, and electrically connect with the electrode layer of the inner reference surface 2b of the oxygen sensing element 2 described above. It is connected to the. On the other hand, the other lead 21
Is electrically connected to the electrode layer on the detection surface 2 a outside the oxygen detection element 2 via another terminal fitting 34 and a lead wire 35 following the terminal fitting 34. In addition, a pair of heater terminals 5 on the plus side and the minus side for supplying electricity to the heating element 3 described above.
Is fixed to the base end (upper end in FIG. 1) of the heating element 3, and is supplied with electric current through the heater terminals 5 to a heating resistance circuit embedded in the heating element 3, which will be described later. I have. Note that a lead wire (not shown) for a heating element is connected to each of the pair of heater terminal portions 5.

At the front part of the linking flange 19 of the ceramic separator 18 locked by the locking step 15, a locking protrusion 40 is formed in the metal cylindrical base 14 in the circumferential direction so as to protrude inward. A metal elastic member 41 is provided between the locking projection 40 and the linking flange 19. As a result, the link flange 19 elastically contacts the locking step 15, and the ceramic separator 18 is stably held. The ceramic separator 18 is formed with a plurality of lead wire insertion holes 72 through which the lead wires 20 and 21 are inserted in the axial direction.

The locking projection 40 may be formed by projecting the cylindrical base 14 inwardly or partially in a protruding manner, or a screw 60 or the like from the outside of the cylindrical base 14 as shown in FIG. , And various modes can be proposed.

A gas introducing hole 50 is formed in the rear peripheral surface of the metal cylindrical base 14, and an annular filter 53 is fitted around the gas introducing hole 50 so as to surround the gas introducing hole 50. The filter 53 has a cylindrical shape that is connected substantially coaxially to the metal cylindrical base 14 from the rear outside and has a holding cylinder 5 on the outside.
4 are provided around. An auxiliary gas introduction hole 55 through which outside air passes is formed on a main surface of the holding cylinder 54 surrounding the filter 53. The upper edge of the holding cylinder 54 extends in a reduced diameter along the peripheral surface of the base 14, and is connected to the cylindrical base 14 by caulking at the peripheral surface of the reduced diameter surface 56. Further, the surface surrounding the filter 53 is also swaged to secure the connection with the filter 53. Further, a protective tube may be further attached from the outside of the holding tube 54. In this case, it is necessary to form a gas introduction hole also in the protection cylinder. Further, the protective cylinder can be extended up and down and caulked and fixed to the tubular base 14 at the extended portion.

Here, the filter 53 is made of, for example, a porous material obtained by stretching an unsintered molded body of polytetrafluoroethylene (hereinafter referred to as PTFE) in a uniaxial or more direction at a heating temperature lower than the melting point of PTFE. The porous fiber structure (trade name: Gore-Tex (Japan Gore-Tex Co., Ltd.)) prevents the transmission of liquids mainly composed of water such as water droplets and allows the transmission of gas such as air and / or water vapor. The filter is configured as a water-repellent filter. Thus, the air (outside air) as a reference gas is introduced into the metal cylindrical base 14 from the auxiliary gas introduction hole 55 through the filter 53 through the gas introduction hole 50, and the liquid state such as water droplets is formed by the filter 53. Water is a metal cylindrical substrate 14
Intrusion into the inside is prevented.

Since the outer surface of the filter 53 is in close contact with the inner surface of the holding cylinder 54, the filter 5
It is difficult for dust and oil to enter between the holding cylinder 3 and the holding cylinder 54, and thus, a decrease in oil repellency or water repellency on the outer surface side of the filter 53 is prevented or suppressed. For example, even when the reference gas temperature becomes high, Sensor output is unlikely to drop.

The metal elastic member 41 is a spring washer, for example, as shown in FIG.
Are arranged in a compressed state between the front surface of the linking flange portion 19 of the ceramic separator 18 (the front surface side of the ceramic separator 18) and the locking projection 40. Thereby, the metal elastic member 41 makes the ceramic separator 18 resiliently contact the locking step 15 and stabilizes the holding thereof. Also, the metal elastic member 41
Has excellent heat resistance because its constituent material is metal,
The rattling prevention effect of the ceramic separator can be favorably maintained over a long period of time.

Further, as shown in FIG. 4, the metal elastic member 41 is deformed in the thickness direction, and the main ring portion 41a which can be elastically shrunk in the thickness direction and the outer engagement with the locking projection 40. It comprises a plurality of engaging projections 41b projecting in the direction. The metal elastic member 41 is press-fitted after the ceramic separator 18 is mounted from the front end opening of the cylindrical base 14. In this press-fitting, the curvature of the engagement projection 41b allows the annular engagement projection 40 to pass through. And, by this press-fitting, the main ring 41
“a” is elastically contracted in the thickness direction, and the ceramic separator 18 elastically contacts the locking step 15 as described above due to the restoring elasticity. Thus, the ceramic separator 18 is held only by this press-fitting.
Is completely unitized.
For this reason, the process of assembling the metal shell 9 is facilitated.

Incidentally, the above-mentioned Japanese Patent Application Laid-Open No.
In the configuration disclosed in No. 3, the ceramic separator is urged in the protruding direction by the metal elastic member to be in a non-holding state. Must be pressed against the main cylinder, and caulking must be performed while maintaining this state, and the assembly is troublesome.

Further, since the relative position between the outer cylinder and the main cylinder is displaced by the elasticity of the metal elastic member, it is difficult to fix the outer cylinder to the main cylinder at a constant position. On the other hand, in the present invention, the ceramic separator 18 is positioned at the locking step 15, and the mounting gap of the metal elastic member 41 is determined between the locking projection 40 and the linking flange 19. Accordingly, the ceramic separator can be elastically held at a fixed position with respect to the tubular base 14 at all times.

The elastic seal member 17 is elastically fitted inside the rear end opening of the cylindrical base 14 and is fitted in a sealed manner, and the respective lead wires 20, 21 and the like are inserted therethrough. And a seal between the outer surfaces of the lead wires 20 and 21 and the inner surface of the filter holding part 51 so that the respective lead wires 20 and 21 are inserted in an airtight manner.

On the other hand, a gas introduction hole 75 is formed on the peripheral surface of the ceramic separator 18 so as to coincide with the gas introduction hole 50, and gas from the gas introduction hole 50 is supplied to the gas introduction hole 75.
Through the lead wire insertion hole 72 into the cylindrical base 14. In addition, separately from the lead wire insertion hole 72, the gas introduction hole 7 is provided.
A ventilation through-hole communicating with 5 may be separately provided, and air may be introduced into cylindrical base 14 from the through-hole.

On the other hand, since the lead wires 20, 21 and the like are inserted into the elastic seal member 17 and the ceramic separator 18 at different pitch circle diameters,
Each lead wire necessarily bends between the elastic seal member 17 and the ceramic separator 18. However,
An appropriate gap 77 is formed between the elastic seal member 17 and the ceramic separator 18, and the lead wires 20, 21 and the like can be bent relatively gently in the gap 77. This makes it difficult for the lead wire to be strongly bent and damaged or broken at the time of assembling the oxygen sensor 1 or the like.

In the assembling of such a configuration, the ceramic separator 1 is previously provided in the metal cylindrical base 14 with the electrodes inside and outside the oxygen sensing element 2 and the terminal fitting 23.
The ceramic separator 18 is mounted in the metal cylindrical base 14 by connecting the respective lead wires through which the wires 8 have been inserted. Further, the elastic sealing member 17 is fitted into the opening at the rear end of the metal cylindrical base 14, and the filter 53 is fitted together with the holding cylinder 54, and the filter 53 is integrated by caulking. Then, a metal elastic member 41 such as a corrugated washer is press-fitted from the front end opening of the cylindrical base 14 and is arranged in a compressed state between the linking flange 19 and the locking projection. Is brought into contact with the locking step 15.

Thereafter, the metal cylindrical base 14 is fitted to the rear end of the metal shell 9 by press fitting. Alternatively, it is welded and consolidated. Thereby, each member is connected, and the metal shell 9
The oxygen detecting element 2 is housed in a casing structure composed of the oxygen sensing element 2 and the cylindrical base 14.

For this reason, an outer cylinder or the like for holding the ceramic separator is not required, and the casing structure is constituted by the cylindrical metal shell 9 and the metal cylindrical base 14. Therefore, the number of parts is reduced as compared with the related art.

The above structure employs a metal elastic member 41 made of a corrugated washer or the like, the front end of which is engaged with and supported by the locking projection 40, and the rear end of which is in elastic contact with the front side of the ceramic separator 18. However, as shown in FIGS. 5 and 6, the use of the metal elastic member 80 made of toothed washers eliminates the need for the locking projection 40.

That is, as shown in FIG. 6, the metal elastic member 80 has an annular shape and a frusto-conical shape on the side surface, and a plurality of external teeth 81 are formed radially around its periphery. Then, with the external teeth 81 facing forward, the cylindrical base 1
4, the inner peripheral edge 82 on the rear side is pressed against the front surface of the linking flange 19, and the metal elastic member 80 is elastically contracted in the thickness direction. At this position, the tips of the external teeth 81 bite into the inner surface of the tubular base 14 and engage with the restoring force in the radial direction. Thus, the metal elastic member 80 is positioned in an elastically contracted state, and its inner peripheral edge 82 elastically contacts the front surface of the linking flange portion 19, and the ceramic separator 18 is brought into engagement with the locking step 1.
5 so as to stabilize its holding.

According to this structure, the formation of the locking projections 40 becomes unnecessary, the number of processing steps is reduced, and the same ease of assembly and stable holding of the ceramic separator as described above can be provided.

In the oxygen sensor 1, as described above, the air as the reference gas is introduced through the filter 53, and the air is introduced into the reference surface 2b of the oxygen detecting element 2 by the air passage secured inside. Is done. On the other hand, the exhaust gas introduced through the gas permeation port 12 of the protector 11 comes into contact with the outer detection surface 2 a of the oxygen detection element 2. Therefore, an oxygen concentration cell electromotive force is generated in the oxygen sensing element 2 according to the difference in oxygen concentration between the inner and outer surfaces. The oxygen concentration cell electromotive force is extracted from the inner and outer electrode layers via the lead wires 21 and 20 as a detection signal of the oxygen concentration in the exhaust gas. This makes it possible to detect the oxygen concentration. This oxygen sensor 1 is used for air-fuel ratio control and the like in an internal combustion engine such as an automobile engine. Here, when the temperature of the exhaust gas is sufficiently high, the oxygen sensing element 2 is heated and activated by the exhaust gas. However, when the temperature of the exhaust gas is low, such as when starting the engine, the oxygen detection element 2 is activated. Is activated by forcibly heating with the heating element 3.

[0042]

According to the present invention, as described above, the casing is constituted by the metallic shell for holding the oxygen sensing element inside and the metallic cylindrical base, and the ceramic separator through which a plurality of lead wires are inserted. In addition to being provided on the cylindrical base, a metal elastic member is press-fitted into the cylindrical base to hold the ceramic separator in press contact with the locking step, and the casing structure includes a cylindrical metal shell and a metal cylindrical body. And a substrate. Therefore, unlike the conventional configuration in which the casing is composed of a cylindrical metal shell, an outer cylinder, and a main cylinder, there are excellent effects such as a reduced number of parts, simplification of manufacturing, and low cost. .

Further, the ceramic separator is held simply by press-fitting the metal elastic member into the cylindrical base, whereby the casing structure including the ceramic separator is completely unitized. Therefore, the step of assembling the metal shell is completed. It will be easier.

[Brief description of the drawings]

FIG. 1 is a vertical sectional side view of an oxygen sensor 1 according to the present invention.

FIG. 2 is an enlarged sectional view of a main part.

3 is a perspective view of a corrugated washer used as a metal elastic member 41. FIG.

FIG. 4 is an enlarged vertical sectional side view of the embodiment in which the locking projection 40 is formed by a projecting end of a screw 60.

5 is an enlarged vertical sectional side view of a main part using a metal elastic member 80. FIG.

6A and 6B show a metal elastic member 80, wherein A is a bottom view and B is a side view.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oxygen sensor 2 Oxygen detecting element 2a Measurement surface 2b Reference surface 9 Metal shell 14 Metallic cylindrical base 15 Engagement step 17 Elastic seal member 18 Ceramic separator 40 Engagement protrusion 41 Metal elastic member 50 Gas introduction hole 53 Ring filter 73 Coupling flange 80 Metal elastic member 81 External teeth

Claims (4)

[Claims] 1. A shaft-shaped oxygen sensing element, a short cylindrical metal fitting holding the oxygen sensing element inside and projecting its detection surface, and a front end closely fitted to a rear end of the metal fitting. A metal tubular base member connected and having a locking step on a rear inner peripheral surface; and a rear surface side locked by the locking step, fitted inside the cylindrical base member, and formed to penetrate in the axial direction. A ceramic separator in which each lead wire from the oxygen sensing element is inserted into a plurality of lead wire insertion holes; a ceramic separator fitted inside the cylindrical base and elastically contacting the front side of the ceramic separator to engage the rear side of the ceramic separator; An oxygen sensor comprising: a metal elastic member that is pressed against and held at a stop; and an elastic seal member that is hermetically fitted to the rear end opening of the cylindrical base and hermetically inserts each lead wire. . 2. A metal elastic member is press-fitted between a locking projection provided on a cylindrical base and a front side of a ceramic separator, and a front end is supported by the locking projection and a rear end is formed of ceramic. 2. The oxygen sensor according to claim 1, wherein the washer is a spring washer that elastically contacts a front surface of the separator to press and hold a rear surface of the ceramic separator at a locking step. 3. A metal elastic member is press-fitted into a cylindrical base, and is positioned by engaging external teeth formed on a peripheral edge with an inner surface of the cylindrical base of the ceramic separator. 2. The oxygen sensor according to claim 1, wherein said oxygen sensor is a toothed washer which is elastically contacted with said oxygen sensor. 4. An annular filter is fitted around the cylindrical base so as to surround a gas inlet formed in the peripheral surface of the rear end portion of the cylindrical base, and gas from the gas inlet is used as a reference for the oxygen sensing element. 4. The oxygen sensor according to claim 1, wherein an air flow path introduced into the surface is secured.