JP4137698B2 - Air-fuel ratio sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio sensor suitable for detecting an air-fuel ratio (mixing ratio of fuel and intake air) of an air-fuel mixture supplied to a combustion engine such as an automobile engine.
[0002]
[Prior art]
In general, in an automobile engine or the like, for example, an air-fuel ratio sensor (including an oxygen sensor) is provided in the middle of an exhaust pipe, and the oxygen concentration contained in the exhaust gas is detected using the air-fuel ratio sensor. Conventionally, as this type of technology, for example, those described in the following documents are known (see Patent Document 1).
[0003]
As this type of prior art air-fuel ratio sensor, for example, there is one configured as shown in the sectional view of FIG. 5A, each layer of the sensor element portion of the air-fuel ratio sensor is formed in a rod shape by, for example, curved surface printing. An alumina insulating layer 51, a first gas diffusion layer are provided on the outer periphery of an alumina rod 50 as a base. 52, a dense solid electrolyte layer 53 having oxygen ion conductivity is formed by being laminated. Further, an inner first electrode 54 is formed inside the first gas diffusion layer 52, and an outer electrode 55 is formed facing the inner first electrode 54 so as to sandwich the solid electrolyte layer 53. Further, an inner second electrode 56 is formed on the inner side of the first gas diffusion layer 52 adjacent to the inner first electrode 54. The reference electrode 57 is opposed to the inner second electrode 56 so as to sandwich the solid electrolyte layer 53. Is formed.
[0004]
The inner second electrode 56 is connected to a virtual ground (for example, a reference potential of about 1.5 V), and the reference electrode 57 is connected to an oxygen pumping power source 58 for supplying oxygen to the reference electrode 57.
[0005]
In such a configuration, when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio and there is a lot of oxygen in the exhaust gas, the oxygen partial pressure of the inner second electrode 56 and the oxygen partial pressure of the reference electrode 57 And the voltage of the reference electrode 57 is low. On the other hand, when the air-fuel ratio is richer than the stoichiometric air-fuel ratio and the oxygen concentration in the exhaust gas is low, the difference between the oxygen partial pressure of the inner second electrode 56 and the oxygen partial pressure of the reference electrode 57 becomes large, and the reference The voltage of the electrode 57 increases.
[0006]
The potential of the reference electrode 57 that changes in this way is supplied to the comparator 59 and is compared with a predetermined comparison voltage (a theoretical air-fuel ratio equivalent voltage). In the comparison result, when the potential of the reference electrode 57 is higher than the comparison voltage, a voltage lower than the voltage of the inner second electrode 56 is output from the comparator 59 as the pumping voltage, and the potential of the reference electrode 57 is compared. When the voltage is lower than the voltage, a voltage higher than the voltage of the inner second electrode 56 is output from the comparator 59 as a pumping voltage.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 61-194344
[Problems to be solved by the invention]
As described above, in the conventional air-fuel ratio sensor as shown in FIG. 5A, the inner first electrode 54 has a predetermined length in the depth direction (the element axis direction of the sensor element, the left-right direction in FIG. 5). It is formed. For this reason, as shown in FIG. 5B, there are many paths from the measured gas such as exhaust gas to the inner first electrode 54 through the first gas diffusion layer 52 from the tip of the sensor element. Will do.
[0009]
As a result, the gas diffusion resistance when the gas to be measured reaches the inner first electrode through the first gas diffusion layer 52 is likely to vary. Thus, in the characteristics of the diffusion limit (pump) current flowing between the reference electrode 57 and the inner first electrode 54 and the inner second electrode 56 via the solid electrolyte layer 53 when a pumping voltage is applied to the reference electrode 57, As shown in FIG. 6, flatness is impaired or hysteresis occurs. As described above, if the flatness of the diffusion limit current characteristic is impaired or hysteresis occurs in the diffusion limit current characteristic, the sensor output varies and becomes unstable, leading to a problem that the air-fuel ratio detection accuracy is lowered. It was.
[0010]
Therefore, the present invention has been made in view of the above, and an object of the present invention is to provide an air-fuel ratio sensor that stabilizes the characteristics of the diffusion limit current, stabilizes the sensor output, and improves the detection accuracy. It is to provide.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the invention described in claim 1 includes a first gas diffusion layer in which a gas to be measured diffuses inside, and an oxygen ion conductive solid electrolyte layer provided outside the first gas diffusion layer. And an outer electrode provided between the first gas diffusion layer and the solid electrolyte layer, the gas to be measured is guided through the first gas diffusion layer, and provided outside the solid electrolyte layer; And an inner electrode through which a diffusion limit current flows, and an air-fuel ratio sensor for measuring an oxygen concentration of the gas to be measured to detect an air-fuel ratio of the gas to be measured. A second gas diffusion layer is disposed between the gas atmosphere of the gas, and the second gas diffusion layer is formed by adding fiber carbon .
[0012]
In the above feature, the diffusion resistance of the gas to be measured in the path of the gas to be measured reaching the inner electrode is controlled by the second gas diffusion layer. As a result, it is possible to suppress variations in the gas diffusion resistance of the gas to be measured, and to stabilize the diffusion limit current characteristics. As a result, the sensor output is stable and the detection accuracy can be improved.
[0013]
The invention according to claim 2 is characterized in that the gas diffusion resistance of the second gas diffusion layer is set sufficiently larger than the gas diffusion resistance of the first gas diffusion layer.
[0014]
In the above feature, by setting the gas diffusion resistance of the second gas diffusion layer to be larger than the gas diffusion resistance of the first gas diffusion layer, the diffusion resistance of the measurement gas in the path of the measurement gas reaching the inner electrode is reduced. The second gas diffusion layer dominates. As a result, it is possible to suppress variations in the gas diffusion resistance of the gas to be measured, and to stabilize the diffusion limit current characteristics. As a result, the sensor output is stable and the detection accuracy can be improved.
[0015]
The invention according to claim 3 is the invention according to claim 2, wherein the first gas diffusion layer is formed by adding spherical carbon having an average particle diameter of about 4 to 6 μm to a ceramic material mainly composed of alumina. The second gas diffusion layer is formed by adding fiber carbon having an average wire diameter of about 1 μm and an average length of about 2 to 3 μm to a ceramic material mainly composed of alumina.
[0016]
In the above feature, the second gas diffusion layer having a gas diffusion resistance sufficiently larger than that of the first gas diffusion layer can be easily and reliably formed.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0018]
FIG. 1 is a diagram showing the configuration of an air-fuel ratio sensor according to the first embodiment of the present invention. The air-fuel ratio sensor of the first embodiment is characterized in that, in the conventional air-fuel ratio sensor shown in FIG. 5, the second gas diffusion is introduced into the gas path from the measurement gas introduction part to the inner electrode in the sensor element. In providing a layer.
[0019]
In FIG. 1, a first gas diffusion layer 2 (corresponding to the first gas diffusion layer 52 shown in FIG. 5) is laminated and formed on an insulating layer 1 laminated on a base of a sensor element of an air-fuel ratio sensor. The first gas diffusion layer 2 is formed by laminating an inner electrode 3 (corresponding to the inner first electrode 54 shown in FIG. 5). Further, adjacent to the first gas diffusion layer 2, a second gas diffusion layer 4 is laminated on the insulating layer 1 in a gas path reaching the inner electrode 3 from the measurement gas introduction part in the sensor element. ing. That is, the second gas diffusion layer 4 is provided between the measured gas atmosphere and the first gas diffusion layer 2.
[0020]
The first gas diffusion layer 2, the inner electrode 3, and the second gas diffusion layer 4 are formed by laminating a dense solid electrolyte layer 5 (corresponding to the solid electrolyte layer 53 shown in FIG. 5) having an oxygen ion conductivity. ing. Further, an electrode protective layer 6 for protecting an outer electrode (not shown) formed to face the inner electrode 3 with the solid electrolyte layer 5 interposed therebetween is laminated on the second gas diffusion layer 4 and the solid electrolyte layer 5. Is formed. Each of the insulating layer 1, the first gas diffusion layer 2, the inner electrode 3, the second gas diffusion layer 4, the solid electrolyte layer 5, and the electrode protective layer 6 is formed by curved surface printing around a rod serving as a base.
[0021]
The gas diffusion resistance value of the second gas diffusion layer 4 is set sufficiently larger than the gas diffusion resistance value of the first gas diffusion layer 2. Therefore, the gas diffusion resistance value of the first gas diffusion layer 2 is set sufficiently smaller than the gas diffusion resistance value of the second gas diffusion layer 4.
[0022]
Specifically, the first gas diffusion layer 2 is formed by adding spherical carbon having an average particle size of about 4 to 6 μm to a ceramic material mainly composed of alumina, for example. The second gas diffusion layer 4 is formed, for example, by adding fiber carbon having an average wire diameter of about 1 μm and an average length of about 2 to 3 μm to a ceramic material mainly composed of alumina. Further, the porosity of the first gas diffusion layer 2 is set to about 60 to 80%, and the porosity of the second gas diffusion layer 4 is set to about 15 to 20%. Furthermore, the gas diffusion distance of the second gas diffusion layer 4 is set to about 300 to 400 μm.
[0023]
In such a configuration, the measurement gas introduced into the sensor element first passes through the second gas diffusion layer 4, and the measurement gas that has passed through the second gas diffusion layer 4 passes through the first gas diffusion layer 2 to the inner electrode. 3 will be reached. That is, the gas to be measured must pass through the second gas diffusion layer 4 having a larger gas diffusion resistance value than the gas diffusion resistance value of the first gas diffusion layer 2 in order to reach the inner electrode 3. For this reason, in the flow of the gas to be measured until it reaches the inner electrode 3, the gas diffusion resistance of the gas to be measured is determined by the gas diffusion resistance of the second gas diffusion layer 4. That is, since the diffusion resistance value of the second gas diffusion layer 4 is set larger than the gas diffusion resistance value of the first gas diffusion layer 2, in the flow of the gas to be measured until it reaches the inner electrode 3, The gas diffusion resistance of the second gas diffusion layer 4 is governed by the gas diffusion resistance.
[0024]
As a result, in the flow of the gas to be measured until it reaches the inner electrode 3, the diffusion resistance of the gas becomes larger than in the conventional case, and it is possible to suppress the dispersion of the diffusion resistance in the gas flow path. Thereby, as shown in FIG. 2, the diffusion limit current characteristic can ensure flatness in a region where the characteristic should be flat, and can suppress the occurrence of hysteresis as compared with the conventional case. Therefore, as shown in FIG. 3, variations in sensor output are suppressed, the sensor output is stabilized, and air-fuel ratio detection accuracy can be improved.
[0025]
In addition, the porosity of the first gas diffusion layer 2 is not configured as a 100% space layer, and the porosity of the first gas diffusion layer 2 is set to about 60 to 80%, thereby overshooting and undershooting the sensor output. It is possible to improve the manufacturing yield when manufacturing the sensor.
[0026]
FIG. 4 is a diagram showing the configuration of an air-fuel ratio sensor according to the second embodiment of the present invention. In the air-fuel ratio sensor of the first embodiment shown in FIG. 1 described above, after the second gas diffusion layer 4 is formed, the solid electrolyte layer 5 is subsequently formed, and the outer electrode (not shown) is formed. Whereas the electrode protective layer 6 is formed and the second gas diffusion layer 4 is formed inside the solid electrolyte layer 5, the air-fuel ratio sensor of the second embodiment is characterized in FIG. As shown in FIG. 2, the second gas diffusion layer 4 is formed and arranged so as to cover the side surfaces of the first gas diffusion layer 2 and the solid electrolyte layer 5 before the electrode protective layer 6 is formed by curved surface printing.
[0027]
In the first embodiment, the first gas diffusion layer 2 has a porosity of about 60 to 80%, and the second gas diffusion layer 4 has a porosity of about 15 to 20%. Further, the gas diffusion distance of the second gas diffusion layer 4 is set to about 300 to 400 μm, whereas in the second embodiment, the first gas diffusion layer 2 has its porosity. Is set to about 60 to 80%, the porosity of the second gas diffusion layer 4 is set to about 3 to 4%, and the gas diffusion distance of the second gas diffusion layer 4 is about 30 to 40 μm. Is set. Others are the same as in the first embodiment. Thus, in the configuration in which the second gas diffusion layer 4 is formed and arranged, the gas to be measured introduced into the sensor element is the same as in the first embodiment described above. First, the gas to be measured that has passed through the second gas diffusion layer 4 and passed through the second gas diffusion layer 4 passes through the first gas diffusion layer 2 and reaches the inner electrode 3. That is, the gas to be measured must pass through the second gas diffusion layer 4 having a larger gas diffusion resistance value than the gas diffusion resistance value of the first gas diffusion layer 2 in order to reach the inner electrode 3. For this reason, in the flow of the gas to be measured until it reaches the inner electrode 3, the gas diffusion resistance of the gas to be measured is determined by the gas diffusion resistance of the second gas diffusion layer 4. That is, since the diffusion resistance value of the second gas diffusion layer 4 is set larger than the gas diffusion resistance value of the first gas diffusion layer 2, in the flow of the gas to be measured until it reaches the inner electrode 3, The gas diffusion resistance value of the second gas diffusion layer 4 dominates the gas diffusion resistance.
[0028]
As a result, also in the second embodiment, the same effect as that obtained in the first embodiment described above can be achieved.
[0029]
Furthermore, technical ideas other than the claims that can be grasped from the above embodiment will be described below together with the effects.
[0030]
4. The air-fuel ratio sensor according to claim 1, wherein the first gas diffusion layer has a porosity of about 60% to 80%, and the second diffusion layer has The porosity is about 15% to 20%.
[0031]
In the above features, when manufacturing an air-fuel ratio sensor, sensor output overshoot and undershoot can be prevented, and the sensor manufacturing yield can be improved.
[0032]
The air-fuel consumption sensor according to any one of claims 1, 2, and 3, wherein the second diffusion layer is formed with a porosity of about 3 to 4%.
[0033]
In the above feature, it is possible to suppress variations in gas diffusion resistance in the gas flow path in the flow of the gas to be measured, the sensor output is stabilized, and the detection accuracy can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an air-fuel ratio sensor according to a first embodiment of the present invention.
FIG. 2 is a diagram showing diffusion limit current characteristics.
FIG. 3 is a diagram illustrating a relationship between an air-fuel ratio and a sensor output.
FIG. 4 is a diagram showing a configuration of an air-fuel ratio sensor according to a second embodiment of the present invention.
FIG. 5 is a diagram showing a configuration of a conventional air-fuel ratio sensor.
FIG. 6 is a diagram showing conventional diffusion limit current characteristics.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Insulating layer 2,52 ... 1st gas diffusion layer 3 ... Inner electrode 4 ... 2nd gas diffusion layer 5,53 ... Solid electrolyte layer 6 ... Electrode protective layer 50 ... Alumina rod 51 ... Alumina insulating layer 54 ... 1st electrode 55 ... outer electrode 56 ... second electrode 57 ... reference electrode 58 ... oxygen pumping power source 59 ... comparator

Claims (3)

A first gas diffusion layer in which the gas to be measured diffuses;
An oxygen ion conductive solid electrolyte layer provided outside the first gas diffusion layer;
Between the first gas diffusion layer and the solid electrolyte layer, the measurement gas is guided through the first gas diffusion layer, and between the outer electrode provided outside the solid electrolyte layer And an inner electrode through which the diffusion limit current flows,
In the air-fuel ratio sensor for measuring the oxygen concentration of the measurement gas and detecting the air-fuel ratio of the measurement gas,
A second gas diffusion layer is disposed between the first gas diffusion layer and the gas atmosphere of the gas to be measured;
The air-fuel ratio sensor, wherein the second gas diffusion layer is formed by adding fiber carbon . 2. The air-fuel ratio sensor according to claim 1, wherein the second gas diffusion layer has a gas diffusion resistance set sufficiently larger than a gas diffusion resistance of the first gas diffusion layer. The first gas diffusion layer is formed by adding spherical carbon having an average particle size of about 4 to 6 μm to a ceramic material mainly composed of alumina,
3. The second gas diffusion layer is formed by adding fiber carbon having an average wire diameter of about 1 [mu] m and an average length of about 2 to 3 [mu] m to a ceramic material mainly composed of alumina. Air-fuel ratio sensor.

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