GB2051379A

GB2051379A - Air-fuel ratio detecting apparatus

Info

Publication number: GB2051379A
Application number: GB8017292A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1979-05-25
Filing date: 1980-05-27
Publication date: 1981-01-14
Also published as: JPS55156854A; FR2457487A1; JPS6034062B2; DE3019825C2; DE3019825A1; FR2457487B1; GB2051379B

Abstract

Air-fuel ratio detecting apparatus, comprises an electronically conductive wire (12), an oxygen ion conductive solid electrolyte layer (13) directly contacting with the surface of the electronically conductive wire, an electronically conductive layer (14) directly contacting with the surface of the electrolyte layer, and a voltage sensor (17) for sensing an electromotive force developed between the electronically conductive wire and layer. <IMAGE>

Description

SPECIFICATION Air-fuel ratio detecting apparatus This invention relates to apparatus for detecting air-fuel ratio of a gas by using an oxygen ion conductive solid electrolyte.

It is well known in the field of automotive internal combustion engines and other various combustion apparatus, to detect air-fuel ratio (the ratio between air and fuel) by using an oxygen ion conductive solid electrolyte in order to control the condition under which combustion takes place.

Various structures of such air-fuel ratio detecting apparatus have been proposed and developed.

One of these is shown in Fig. 1, in which an oxygen sensor section is constructed by an electrically nonconductive base plate of rectangular shape in plan, an electronically conductive layer 2 laid on the base plate 1, an oxygen ion conductive solid electrolyte layer 3 laid on the layer 2, and an electronically conductive solid electrolyte layer 4 laid on the layer 3, thereby forming the oxygen sensor section generally into a plate shape as a whole. Additionally, a DC power source 6 is provided to compulsorily cause DC current to flow to both electronically conductive layers 2 and 4 through leads 5. A voltage sensing device 7 is provided to sense an electromotive force developed across the both electronically conductive layers 2 and 4.

The thus arranged air-fuel ratio detecting apparatus is advantageous in that its oxygen sensor section is of simple and compact construction, but has encountered the following drawbacks: (1) The oxygen sensor section is formed into the flat shape as a whole since it is constructed by successively stacking the electronically conductive layer 2, the solid electrolyte layer 3, and the electronically conductive layer 4. Accordingly, the oxygen sensor section exhibits different sensing characteristics depending upon the directional location thereof relative to a gas flow to be detected.In other words, the sensing or response characteristics are best when a gas flow is directed at right angles to the stacked surface as indicated by an arrow A in Fig. 1, whereas they are worst when the gas flow is directed at the stacked surface of the oxygen sensor in the opposite direction to the arrow A i.e.

in the direction of an arrow B. In this regard, it is necessary to sufficiently take the directional location of the oxygen sensor into consideration when the oxygen sensor section is installed. (2) The sensing characteristics of the oxygen sensor are limited to a relatively low value since a structural base member is formed of an electrically nonconductive material. This will be explained as follows: according to the principle of the oxygen sensor, an electromotive force is developed only by the combination of the solid electrolyte layer 3 and the electronically conductive layers 2 and 4 disposed on the both sides of the layer 3, and accordingly it is enough that only these three layers have excellent response characteristics to temperature and to the atmosphere of the gas to be detected.However, actually the structural base member is necessary to ensure the construction of the oxygen sensor. For this purpose, the electrically nonconductive base member formed of alumina etc. is used, and this unavoidably limits the heat capacity and therefore the response characteristics of the oxygen sensor. (3) Since the rectangular electrically nonconductive base plate is used as a structural base mernber, the oxygen sensor becomes larger in size. In other words, it is difficult to install the oxygen sensor section in a very narrow space when the apparatus shown in Fig. 1 is used, while a thermocouple used for measuring temperature is installable in a much narrower space.

According to the present invention, there is provided apparatus for detecting the air-fuel ratio of a gas, comprising a wire made of an electronically conductive riiatçrial; a first layer made of an oxygen ion conductive solid electrolyte and directly contacting the wire; a second layer made of an electronically conductive material and directly contacting said first layer, said second layer being contactable with the gas; and voltage sensing means for sensing an electromotive force developed between said wire and said second layer.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, wherein:- Fig. 1 is a schematic vertical sectional view of a conventional air-fuel ratio detecting apparatus; Fig. 2 is a schematic vertical sectional view of an embodiment of an air-fuel ratio detecting apparatus in accordance with the present invention; Fig. 3 is a schematic vertical sectional view of another embodiment of an air-fuel ratio detecting apparatus in accordance with the present invention; Fig. 4 is a schematic vertical sectional view of a further embodiment of an air-fuel ratio detecting apparatus in accordance with the present invention;; Fig. 5 is an enlarged sectional view of an oxygen sensor section of an embodiment of an airfuel ratio detecting apparatus according to the present invention, which is produced in accordance with Example 1; Fig. 6 is an enlarged sectional view of the apparatus in which the oxygen sensor section of Fig. 5 is used; Fig. 7 is a fragmentary sectional view of a part of the apparatus including the oxygen sensor section of Fig. 5; Fig. 8 is a graph showing the output voltage characteristics of the apparatus according to Example 1, in terms of time and output voltage; Fig. 9 is a graph showing the output voltage characteristics of the apparatus according to Example 2, in terms of time and output voltage; Fig. 10 is a graph showing the output voltage variation on rich side air-fuel ratios;; Fig. 11 is a graph similar to Fig. 10, but showing the output voltage variation on lean side air-fuel ratios; Fig. 1 2A is a cross-sectional view explaining the production process of an oxygen sensor section in Example 3; Fig. 1 2B is a cross-sectional view of the oxygen sensor section produced according to Example 3; Figs. 1 3A to 1 3E are schematic illustrations showing the production process of a conventional oxygen sensor section as described in the Comparative Example; Figs. 1 4A to 1 4C are graphs showing the response characteristics of embodiments of the air-fuel ratio detecting apparatus according to the present invention and conventional apparatus; and Fig. 1 5 is a graph illustrating the rate of change in output voltage, in terms of air excess ratio and output voltage.

Referring now to Fig. 2 of the drawings, there is shown an embodiment of an air-fuel ratio detecting apparatus in accordance with present invention. The apparatus comprises an oxygen sensor section (no numeral) which includes an electronically conductive wire 12 in such a manner as to cover the end section including an extreme end and to directly contact the surface of the wire 12. An electronically conductive layer 14 is formed on the surface of the solid electrolyte layer 13 so as to directly contact the surface of the solid electrolyte 13. A voltage measuring device or voltmeter 1 7 is electrically connected through leads 1 5 to the electronically conductive wire 12 and the electronically conductive layer 14 to sense an electromotive force developed across the wire 12 and the layer 14.

With this arrangement, an oxygen partial pressure at the interface between the oxygen ion conductive solid electrolyte layer 13 and the wire 12 serves as a reference. Accordingly, for example, in case the gas to be detected is exhaust gas of an automotive internal combustion engine in which rich gas (fuel proportion is larger than in stoichiometric level) low in oxygen partial pressue and lean gas (air proportion is larger than in stoichiometric level) high in oxygen partial pressure flow alternately to contact with the oxygen sensor section detection of the stoichiometric air-fuel ratio of the exhaust gas can be accomplished by sensing an electromotive force developed at a time period where the rich gas flow is replaced with the lean gas flow and vice versa.

Fig. 3 illustrates another embodiment of the airfuel ratio detecting apparatus in accordance with the present invention, in which a DC power source 1 6 is connected to the electronically conductive wire 12 and the electronically conductive layer 14 so as to compulsorily cause electric current to flow across the wire 12 and the layer 14. Thus, in this case, the DC power source, preferably a constantcurrent DC power source, is connected to the electrically conductive wire 1 2 and the electronically conductive layer 14 so as to compulsorily cause flow of oxygen ions within the solid electrolyte 13, therefore the oxygen partial pressure at the interface between the solid electrolyte layer 13 and the electronically conductive wire 12 becomes more constant, by which stable characteristics of output voltage can be obtained.

Fig. 4 illustrates a further embodiment of the air-fuel ratio control apparatus in accordance with the present invention, in which an electronically conductive coiled wire 14' is disposed around the solid electrolyte layer 13 so as to contact with the layer 13. This coiled wire 14' is used in place of the electronically conductive layer 14 in Figs. 2 and 3. With this arrangement, it is unnecessary to directly connect leads to the electronically conductive layer 14 as shown in Figs. 2 and 3.

As described above the electronically conducive wire 12 of circular or rectangular shape in section is used as the structural base member, and additionally the solid electrolyte layer 13 and the electronically conductive layer 14 are disposed around the wire 12. Accordingly, the oxygen sensor exhibits excellent sensing characteristics for any gas flow in radial directions relative to the cylindrical oxygen sensor.

Furthermore, since the apparatus is not provided with an electrically nonconductive mass base member, the heat capacity of the oxygen sensor can be considerably lowered, and therefore its response characteristics to a gas atmosphere and to temperature are improved so as to attain an excellent response in the oxygen sensor section. In this regard, the sectional area of the electronically conductive wire 12 is small so far as the strength as the structural base member is maintained, so that the diameter of the wire 12 is preferably 0.5 mm or less in the case of the wire 1 2 having a circular section. Additionally, the thickness of the solid electrplyte layer 13 is preferably 0.1 mm or less.Such an arrangement makes the oxygen sensor section of the oxygen sensor very small, and, therefore the thus constructed oxygen sensor section becomes applicable to wide uses corresponding to those to which a conventional thermocouple is applicable.

The electronically conductive wire 1 2 is preferably formed of a metallic thin wire made of a material which does not form its oxide, such as Au, Ag, Pt, or Pd. Alternatively, the material of the metallic thin wire may be an alloy such as Ag-Pd, or Au-Pd. The material of the oxygen ion conductive solid electrolyte layer 1 3 is preferably ZrO2 stabilized with, for example, CaO, Y2O3, SrO, MgO, ThO2, WO3, orTa205; or Bi2O3 stabilized with Nb20s, SrO, W03, Ta205 orY2O3;orTh02-Y2O3 system or Ca0-Y203 system. The layer 13 of solid electrolyte can be formed onto the surface of the electronically conductive wire 12, for example, by a physical evaporation method such as sputtering, ion-plating, an electrochemical method, or a method of firing a solid electrolyte paste applied onto the surface of wire 12.

The electronically conductive layer 14 is made of a material which does not exhibit catalytic activity, such as Au, Ag, or SiC; an oxide semiconductor such as TiO2, CoO, or LaCrO3; a platinum group metal which exhibits catalytic activity such as Ru, Pd, Rh, Os, Ir, or Pt, including alloy of these platinum group metals; or an alloy of a platinum group metal and a base metal. This electronically conductive layer 1 4 is formed onto the surface of the solid electrolyte layer 13 by an evaporation method such as sputtering, ionplating, an electrochemical method such as metal plating, or a method of firing a paste applied onto the surface of the solid electrolyte layer 13.

Otherwise, the thin wire 14' is spirally wound around the solid electrolyte layer 13 as shown in Fig. 4.

Additionally, it is also preferable to form a protective layer on the surface of the oxygen sensor section which is formed as described above. The protective layer may be formed of CaO-ZrO2 (calcium zirconate), Awl203 (alumina), or MgAl2O4 (spinel), which layer is adhered on the surface of the oxygen sensor by way of firing a coating (by dipping) thereof, or plasm spraying thereof. As will be appreciated, the condutive degree of oxygen ion of the solid electrolyte becomes lower or deteriorates at a low temperature. In this regard, it is preferable to provide an electronically conductive member which is capable of generating heat, within the protective layer, or to locate the oxygen sensor section in the atmosphere which can be heated.

EXAMPLE 1 This EXAMPLE is directed to the embodiment shown in Figs. 5, 6 and 7.

In producing an oxygen sensor 20 shown in Fig. 5, an end section (a range of 2 mm from the extreme end) of a platinum wire having a diameter of 0.2 mm and a length of 30 mm was dipped in a paste of a solid electrolyte, which wire served as an electronically conductive wire 12. The paste of a solid electrolyte was previously prepared by mixing and blending 5 mol% Y203-ZrO2 powder and lacquer in a weight ratio of 1:1, and thereafter by adjusting the viscosity of the resultant mixture to about 80,000 centipoise with thinner. After the above-mentioned dipping, the platinum wire 12 was dried at 1 000C for 1 hour.

The thickness of the resultant solid electrolyte layer 1 3 (not yet fired) was about 50 ,um.

Subsequently, a platinum paste as the electronically conductive layer 14 was attached to the surface of the solid electrolyte layer 13 in such a manner that the platinum paste did not directly contact the platinum wire 12, and thereafter the film of the platinum paste was dried at 1 000C for 1 hour after a platinum wire 18 having a diameter of 0.2 mm and a length fo 30 mm was contacted with the film of the platinum paste so that the platinum wire 1 8 served as a lead. Thereafter, the above-mentioned solid electrolyte paste film and the platinum paste were fired at 1 4000C for 3 hours, in which the rate of heating was 60 C/hr.

within a range from room temperature to 14000 C.

The film thickness of the resultant solid electrolyte layer 1 3 was about 30 4m, and the film thickness of the electronically conductive layer (platinum) 14 was about 7-8 ym. Furthermore, a layer of CaO--ZrO, (calcium zirconate) was formed on the surface of the electronically conductive layer 14 and the lead 18 as the protective layer 1 9 by way of plasma spraying as shown in Fig. 5 to obtain the oxygen sensor section 20. The thickness of the resultant layer 1 9 was about 50 ym.

Figs. 6 and 7 show an example in which the resultant oxygen sensor section 20 was assembled as an air-fuel ratio detecting apparatus.

As shown, the oxygen sensor section 20 was disposed in a hood 21 so as to prevent the oxygen sensor section 20 from being diretly subjected to a gas to be detected. The hood 21 was formed with through holes 21 a through which the gas to be detected entered the inside of the hood 21 to contact the oxygen sensor section 20 and thereafter discharged through the other hole 21 a.

The oxygen sensor section 20 was cemented or securely connected to an electrically nonconductive pipe member 23 was formed with with a ceramic adhesive 22. The adhesive 22 served also as a gas sealing member. The noncoductive pipe member 23 was formed with elongate holes 23a and 23b in which the platinum wires 12 and 1 8 were disposed, respectively, in order to prevent a short-circuit between the platinum wires 12 and 1 8. The nonconductive pipe member 23 was covered with a holder 24 made of stainless steel in order to prevent the breakage of the nonconductive pipe member 23. The hood 21 was securely connected through a ring 25 made of stainless steel to the holder 24 by means of welding.

The platinum wires 12 and 1 8 were connected at their welding portions 26, 26 to nickel wires 27 and 27, respectively. The elongate holes 23a and 23b were filled with ceramic adhesive, respectively, so as to prevent gas from entering therein.

Additionally, the holder 24 was securely connected to another holder 28 by means of welding, the holder 28 being provided therein with alumina powder 29 which prevents a short-circuit between the nickel wires 27 and 27.

The holder 28 was connected to a stainless steel pipe 31 by forming depressed sections 30.

The holder 28 was provided therein with a separator 33 made of silicon rubber in order to prevent the alumina powder 29 from coming off and additionally to prevent the short-circuit between the nickel wires 27 and between copper wires 32. The copper wires 32, 32 were connected to the nickel wires 27, 27, respectively, at sections 34, 34 where silver solder was used.

The copper wires 32, 32 were prevented from short-circuit therebetween by silicon rubber 35 around which a shield wire layer 36 was formed.

The shield wire layer 36 was fixed to the stainless steel pipe 31 by means of depressed sections 37.

A nut 38 was freely movably disposed around the shield wire layer 36, which nut was movable in the direction of an arrow to contact with the ring 25 in order to securely connect this air-fuel ratio control apparatus to the wall of an exhaust pipe (not shown) through which engine exhaust gas passes.

Subsequently, an evaluation test was carried out on the air-fuel ratio detecting apparatus arranged as mentioned above, in which the variation in output voltage of the oxygen sensor section 20 was measured by the voltage measuring device 1 7 under conditions in which rich gas (oxygen partial pressure: about 10-20 atm.) and lean gas (oxygen partial pressure: about 10-3 atm.) alternately flow to contact with the oxygen sensor section 20 at intervals of 20 seconds at a temperature of 6000 C. In this test, the positive terminal of the voltage measuring device 1 7 was connected to the electronically conductive layer 14. The result of the test is shown in Fig. 8.

In this test, when exhaust gas to contact the oxygen sensor section 20 was maintained in a rich gas state where the oxygen concentration is less than in the stoichiometric state, the oxygen partial pressure in the exhaust gas was equal to the oxygen partial pressure at the interface between the electronically conductive wire 1 2 and the solid electrolyte layer 13. Accordingly, the output voltage became zero level.However, the instance that exhaust gas in a lean gas state where the oxygen concentration is greater than in the stoichiometric state subsequently flowed into the air-fuel detecting apparatus to contact the oxygen sensor section 20, an output voltage of 740 mV was developed as being expressed by the following Nernst equation: RT 10-3 E = In = 740 mv 4F 10-20 At this time, since the lean gas permeated the interface between the electronically conductive wire 12 and the solid electrolyte layer 13 through the electronically conductive layer 14 and the solid electrolyte layer 13, the oxygen partial pressures at the above-mentioned interface and in the exhaust gas became equal to each other, so that the output voltage approached zero level.

Then, the instant that the rich gas flowed into the air-fuel ratio detecting apparatus to contact the oxygen sensor section 20, an output voltage of -740 mV was developed as shown in the following equation: RT 10-20 In --740 mv 4F 10-3 Then, the rich gas reached to the abovementioned interface and was diffused therein and accordingly the output voltage approached zero level, exhibiting the output voltage characteristics shown in Fig. 8.

Thus, when the rich gas was changed to the lean gas so that the air-fuel ratio varied beyond the stoichiometric level, a positive electromotive force was generated, while when the lean gas was changed to the rich gas so that the air-fuel ratio varied beyond the stoichiometric level, a negative electromotive force was generated. Otherwise, if the positive and negative terminals of the voltage measuring device 17 were reversely connected, the positive and negative characteristics were reversed relative to each other. It will be understood that, any way, detecting stoichiometric air-fuel ratio became possible in such ways.

EXAMPLE 2 The evaluation test of this Example was carried out for the air-fuel ratio detecting apparatus which was the same as that in the Example 1 with the exception that the electronically conductive wire 1 2 and the electronically conductive layer 14 were connected to a DC power source (as illustrated in Fig. 3) so as to compulsorily cause current flow through them. In this case, a constant-current DC power source was used as the DC power source 16, in which the negative terminal of the DC power source was connected to the electronically conductive layer 14 side, and accordingly the positive terminal of the same was connected to the electronically conductive wire 12 side.

With the thus arranged testing device, rich and lean gases were alternately supplied to flow into the air-fuel ratio detecting apparatus at intervals of 20 seconds as in Example 1, maintaining always a constant-current of 5 yA at a temperature of 6000 C, in order to detect the output characteristics of the air-fuel ratio detecting apparatus. In this case, the electronically conductive wire 12 side was connected to the positive terminal of the voltage measuring device 17, in which the measured impedance was 1 MQ.

The result of this test is shown in Fig. 9.

In this instance, since the current was compulsorily caused to flow, oxygen ions always flowed through the solid electrolyte layer 13 to the interface between the electronically conductive wire 12 and the solid electrolyte layer 13, so that the oxygen partial pressure at the interface became higher. As a result, a higher output voltage was generated when the oxygen partial pressure in the exhaust gas was lower as in the rich gas, whereas a lower output voltage was generated when the oxygen partial pressure in the exhaust gas was higher as in the lean gas. In addition, the result shown in Fig. 10 was obtained by the tests conducted to measure the output voltages by varying air-fuel ratio of the rich gas.

The connections of the terminals of the constant-current DC power source were then reversed relative to those mentioned above, in which the positive and negative terminals of the DC power source were connected to the electronically conductive layer 1 4 side and the electronically conductive wire 1 2 side respectively, in order to carry out the test. In this instance, a higher output voltage was generated when the oxygen partial pressure in the exhaust gas was higher as in the lean gas, whereas a lower output voltage was generated when the oxygen partial pressure in the exhaust gas was lower as in the rich gas. Furthermore, the result shown in Fig.

11 was obtained by the tests conducted to measure the output voltages by varying the airfuel ratio of the lean gas.

EXAMPLE 3 The oxygen sensor section 20 of an air-fuel ratio detecting apparatus made according to this Example is shown in Figs. 1 2A and 1 2B.

In producing the oxygen sensor section 20, an end section (a range of 2 mm from the extreme end thereof) of a platinum wire having a diameter of 0.2 mm and a length of 30 mm was dipped in a solid electrolyte paste, which wire served as the electronically conductive wire 1 2. The paste of the solid electrolyte was previously prepared by mixing and blending 5 mol% Y2O3-ZrO2 powder and lacquer in a weight ratio of 1:1, and thereafter by adjusting the viscosity of the resultant mixture to about 80,000 centipoise.

After the above-mentioned dipping, the resultant platinum wire 12 coated with the paste was dried at 1000C for 1 hour. The film thickness of the resultant solid electrolyte layer 1 3 (not yet fired) was about 50 Mm as shown in Fig. 1 2A.

Subsequently, an end section of a platinum wire having a diameter of 0.2 mm was wound around the solid electrolyte layer 1 3 to form an coiled state electronically conductive layer 14'. In winding the one end section of the platinum wire, the free end of the platinum wire 1 2 may be bent to turn in a reverse direction and wound around the solid electrolyte 13, and thereafter cut to separate the wound wire section serving as the electronically conductive layer 14'.

Subsequently, a firing was carried out at 1 4000C for 3 hours to sinter the solid electrolyte layer 13 in which the rate of heating was 60 /hr.

from room temperature to 14000 C. The resultant solid electrolyte layer 1 3 after the firing was about 30 ,um in film thickness and so porous that oxygen gas passed through the layer 13.

Thereafter, CaO-ZrO2 was attached by way of plasma spraying to form a protective layer 1 9 for surrounding the platinum wire 14', the solid electrolyte layer 1 3, and an exposed section of the platinum wire 12, in order to produce the oxygen sensor section 20 as shown in Fig. 128.

An evaluation test of the resultant oxygen sensor section 20 was carried out under conditions in which the sensor section 20 was disposed within the hood as shown in Fig. 7 and the electronically conductive wire 12 formed of platinum wire and the electronically conductive layer 14' are connected to the nickel wires 27, 27, respectively. As a result, the voltage output characteristics as shown in Fig. 8 were obtained by the same test as described with reference to Example 1 in which rich and lean gases were alternately supplied to flow at intervals of 20 seconds at a temperature of 6000 C. Additionally, the voltage output characteristics as shown in Fig.

9 were obtained by the same test as described with reference to Example 2 in which rich and lean gases were alternately supplied to flow and, a DC power source was connected to the electronically conductive wire 12 and the electronically conductive layer 14'.

COMPARATIVE EXAMPLE A flat plate type oxygen sensor section as illustrated in Fig. 1 was prepared by a conventional production process shown in Figs.

1 3A to 13E. In this production process, a platinum paste for forming the electronically conductive layer 2 (indicated by an oblique-lined section) was printed as shown in Fig. 1 3B on the surface of an alumina base plate 1 having dimentions of 5 x 4 x0.6 mm shown in Fig. 1 3A. The alumina base plate 1 printed with the platinum paste was then dried at 1000C for 1 hour, and thereafter fired at 1 3000C for 1 hour in atmospheric air. The film thickness of the resultant electronically conductive layer 2 was 5-6 Mm. Subsequently, a solid electrolyte paste was printed as indicated by an oblique-lined section of Fig. 1 3C to form a solid electrolyte layer 3.The base plate thus printed with the electrolyte paste was then dried at 1 000C for 1 hour, and thereafter fired at 1 4000C for 3 hours. The film thickness of the resultant solid electrolyte layer 3 was about 50 clam.

Furthermore, a platinum paste was printed as indicated by an oblique-lined section in Fig. 1 3D to form the electronically conductive layer 4. The resultant base plate 1 printed as shown in Figs.

1 3B to 1 3D was dried at 1 OBOC for 1 hour, and thereafter fired at 1 3000C for 1 hour. The film thickness of the resultant electronically conductive layer 4 was 5-6 pim. In addition, leads 5 were attached as shown in Fig. 1 3E and thereafter the resultant oxygen sensor section was disposed in a protective tube made of alumina to produce an air-fuel ratio detecting apparatus of a conventional type, on which an evaluation test was conducted in comparision with an embodiment of the air-fuel ratio detecting apparatus according to the present invention.

The evaluation test was carried out to measure the response characteristics in output voltage of the conventional air-fuel ratio detecting apparatus having the plate type oxygen sensor section in comparison with an embodiment of the air-fuel ratio detecting apparatus according to the present invention under conditions in which the exhaust gas was abruptly changed from its lean gas state (oxygen proportion is larger than in stoichiometric state) its rich gas state (oxygen proportion is smaller than in stoichiometric state). It is hereinafter to be noted with reference to Figs. 1 4A to 1 4C, that the reference character "a" represents an embodiment of the air-fuel ratio detecting apparatus according to the present invention, and reference numeral "b" represents a conventional apparatus prepared acording to the process of Figs. 1 3A to 1 3E and tested by directing a gas (to be detected) flow at right angles to the stacked surface thereof as indicated by the arrow A in Fig. 1, and the reference numeral "c" represents another conventional apparatus prepared in the same way as "b" and tested by directing a gas flow in the opposite direction to the arrow A as indicated by the arrow B in Fig, 1.

In all cases, the negative terminal of the constantcurrent DC power source 16 or 6 was connected to the exhaust gas side electronically conductive layer 14 or 4 so as to supply a constant-current of 5 yA to the electronically conductive layer. The results of the tests are shown in Figs 1 4A to 14C, in which Fig. 1 4A is at a temperature of 7000C, Fig. 14B is at a temperature of 5000C, and Fig.

1 4C is at a temperature of 350"C. In the graphs of Figs. 1 4A to 1 4C, the -abscissa represents time, and the ordinate represents rate F of change in output voltage. The rate F of output voltage was determined by the following equation: VV, F ------ x x100(%Y VRVL in which, as shown in Fig.15, VR is the stable output voltage in an atmosphere of the rich gas, V, is the stable output voltage in an atmosphere of the lean gas, and V is the output voltage at a time point after the lapse of a certain time period.

As apparent from Figs. 1 4A to 1 4C, the response characteristics improved as the temperature rose, both with respect to the apparatus a and the conventional apparatus b and c.

However, under the same temperature conditions, the response characteristics of the apparatus a was considerably better than those of the conventional apparatus b and c. Additionally, with respect to the conventional air-fuel ratio detecting apparatus having the flat plate type oxygen sensor section, the response characteristics were considerably different between the apparatus a and b, and good response characteristics could not be obtained unless the apparatus was so used that the gas flows in the direction of the arrow A as shown in Fig. 1.

As appreciated from the above, the embodiments of the air-fuel ratio detecting apparatus according to the present invention do not use a special electrically nonconductive base plate as a structural base member, and accordingly it is simple in construction and small in size. This allows the air-fuel ratio detecting apparatus to be locatable in a very narrow space, similar to a conventional thermocouple, and therefore the apparatus is widely usable.

Additionally, the heat capacity of the apparatus is smaller than that of the conventional apparatus which is provided with an electrically nonconductive base plate. Furthermore, it is possible to remove the difference in performance characteristics depending on directional orientation relative to a gas flow. Accordingly, it is unnecessary to select the directional orientation for installation of the air-fuel ratio detecting apparatus.

Claims

1. Apparatus for detecting the air-fuel ratio of a gas, comprising a wire made of an electronically conductive material; a first layer made of an oxygen ion conductive solid electrolyte and directly contacting said wire; a second layer made of an electronically conductive material and directly contacting said first layer, said second layer being contactable with the gas; and voltage sensing means for sensing an electromotive force developedFbetween said wire and said second layer.

2. Apparatus as claimed in Claim 1, in which said second layer is in the form of a film.

3. Apparatus as claimed in Claim 1, in which said second layer is in the form of a coil wound around the surface of said first layer.

. .

4.'Apparatus as claimed in any one of Claims 1-3, further comprising means for supplying a direct current to said wire and said second layer.

5. Apparatus as claimed in Claim 4, in which said direct current supply means is a constantcurrent DC power source.

6. Apparatus as claimed in any one of Claims 1-5, further comprising means for heating said first layer.

7. Apparatus as claimed in Claim 6, further comprising a third layer formed on said second layer to protect said second layer, first layer, and wire from being damaged, oxygen gas being permeable through said third layer.

8. Apparatus as claimed in Claim 7, in which said heating means includes a heater disposed in said third layer to generate heat within said third layer.

9. Apparatus as claimed in any one of Claims 1-8, in which said first layer is formed on the surface of an end section of said wire to cover the extreme end of said wire.

10. Apparatus as claimed in Claim 9, in which said second layer is formed on the surface of said first layer to cover the extreme end of said wire covered with first layer.

11. Apparatus as claimed in any one of Claims 1-10, in which said voltage sensing means includes a voltmeter connected to said wire and said second layer.

12. Apparatus as claimed in any one of Claims 1-11, in which said wire has a diameter of 0.5 mm or less in a case in which said wire is circular in section.

13. Apparatus as claimed in any one of Claims 1-1 2, in which said second layer has a thickness of 0.1 mm or less.

14. Apparatus as claimed in any one of Claims 1-13, in which said wire is made of a material selected from a group consisting of Au, Ag., Pt Pd, and an alloy thereof.

1 5. Apparatus as claimed in any one of Claims 1-14, in which said first layer is made of a material selected from the group consisting of ZrO2, Bi2O3,ThO2-Y2O3, and CaO--Y,O,.

1 6. Apparatus as claimed in Claim 15, in which said ZrO2 is stabilized with a material selected from the group consisting of CaO, Y2O3,SrO,MgO, ThO2, WO3, and Ta205.

1 7. Apparatus as claimed in Claim 16, in which said Bi2O3 is stabilized with a material selected from the group consisting of Nub205, SrO, WO3, Ta2O5, and Y2O3.

18. Apparatus as claimed in any one of Claims 1-17, in which said second layer is made of a material selected from the group consisting of Au, Ag, and SiC.

1 9. Apparatus as claimed in any one of Claims 1-17, in which said second layer is made of a material selected from the group consisting of TiO2, CoO, and LaCrO3.

20. Apparatus as claimed in any one of Claims 1-17, in which said second layer is made of a material selected from the group consisting of Ru, Pd, Rh, Ir, Pt, and an alloy thereof.

21. Apparatus as claimed in Claim 7, in which said third layer is made of a material selected from the group consisting of CaO-ZrO2,Al2O3, and MgAl2O4.

22. Apparatus for detecting the air-fuel ratio of a gas, substantially as hereinbefore described with reference to, and as illustrated in, Fig. 2, 3, 4, 5, 6, 7,. or 1 2A and 12B, of the accompanying drawings.