JPH0115022B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oxygen concentration detection element (so-called O ₂ sensor) for measuring the oxygen concentration in exhaust gas of an engine to detect an air-fuel ratio, and in particular for detecting a wide range of air-fuel ratios including the stoichiometric air-fuel ratio. Regarding things that can be detected.

Conventionally, such oxygen concentration detection elements (O ₂
As shown in FIG. 1, the sensor) has a solid electrolyte tube 1 conductive to oxygen ions such as ZrO ₂ -Y ₂ O ₃ with one end closed and the other end communicated with the atmosphere. An internal electrode layer 2 and an external electrode layer 3 made of a conductive material having a catalytic effect such as Pt are formed on the inner and outer walls of the solid electrolyte tube 1, and the atmosphere is used as a reference gas inside the solid electrolyte tube 1, and the gas to be measured is placed outside the solid electrolyte tube 1. By flowing the exhaust gas as a gas, the inner and outer electrode layers 2,
It is widely known that the oxygen concentration in the exhaust gas is detected by measuring the electromotive force generated during the three-hour period.

However, in the conventional oxygen concentration detection element described above, the electromotive force changes with respect to the air-fuel ratio as shown by the solid line in Figure 2, and the electromotive force changes in an ON-OFF manner near the stoichiometric air-fuel ratio. Although suitable for detecting the stoichiometric air-fuel ratio, it has been almost impossible to detect air-fuel ratios on the lean side and rich side other than the stoichiometric air-fuel ratio.

By the way, there is a need to control the air-fuel ratio of the engine at the optimum position depending on the operating conditions (for example, operating the engine in the combustion range with the highest fuel efficiency, obtaining optimal combustion at high speeds, and controlling the air-fuel ratio during cold starts). A wide range of information including the stoichiometric air-fuel ratio is required due to the need to use it as a sensor in a comprehensive control system that takes into consideration everything in the powertrain system, such as obtaining the optimum combustion), ignition timing, exhaust gas recirculation rate, and transmission gear ratio. If you want to detect the air-fuel ratio, it is necessary to have a characteristic in which the electromotive force changes linearly with the air-fuel ratio, as shown by the broken line in Figure 2. development was requested.
Furthermore, from the viewpoint of practicality, it is necessary that this linear oxygen concentration detection element has excellent quick response and is easy to manufacture.

Therefore, in view of the above, the inventors of the present invention have conducted extensive research, and as a result of the above-mentioned conventional oxygen concentration detection element using oxygen ions as a carrier, the present inventor has made the external electrode an electrode with semi-catalytic performance, and the external electrode layer. It was discovered that by forming a film made of a specific material on the surface of the oxygen concentration sensor, an element with excellent linear characteristics could be obtained.As a result, the oxygen concentration sensing element of the present invention, which is revolutionary and can be used for a wide range of air-fuel ratio control, was developed. This is what led to the development of this.

That is, the present invention has an oxygen ion conductive solid electrolyte tube with one end closed and the other end communicated with the atmosphere, and an inner electrode layer and an outer electrode layer made of a conductive material on the inner and outer walls of the solid electrolyte tube. At least the external electrode of the oxygen concentration detection element formed with the above is an electrode having semi-catalytic performance, and the surface of the external electrode layer is further made of SnO ₂ that adsorbs reducing gas.
It is characterized by forming a film consisting of:

Here, considering the reason why the electromotive force exhibits a characteristic that the electromotive force is almost linear with respect to the air-fuel ratio in the present invention, it is found that the exhaust gas contains O ₂ , NOx, CO,
Contains HC, _H2 , among which CO, HC,
This is thought to be due to the action of unburned components of _H2 , so-called reducing gases. In other words, a film made of SnO ₂ as an oxide that easily adsorbs these reducing gases is formed on the surface of the external electrode layer of the detection element, so when this detection element is exposed to exhaust gas, a large amount of the film is formed. , _{the O 2 concentration} at the interface between the coating and the external electrode layer is lower than it actually is. Then, O ₂ reacts with the adsorbed gas (mainly CO) on the external electrode layer due to catalytic action (oxidation action), and the O ₂ concentration further decreases. As a result, even in the lean side range of the air-fuel ratio, O ₂ is considered to be insufficient, so
The electromotive force becomes higher. This electromotive force exhibits a characteristic that it decreases as the air-fuel ratio increases due to the relationship between the adsorption amount of reducing gas and the O ₂ concentration. Furthermore, in the rich side range of the air-fuel ratio, since the original amount of O ₂ is very small, there is almost no influence from CO, HC, and H ₂ , and it is thought that it is unlikely to appear as a change in electromotive force. Therefore, a substantially linear electromotive force characteristic as shown by the broken line in FIG. 5 is obtained.

Further, it is necessary to use an electrode having semi-catalytic performance as the external electrode. This is due to the following reason.

As described above, the present invention allows the oxygen concentration on the lean side to be detected to be lower than the actual concentration. Therefore, if the electromotive force on the rich side is relatively low and the electromotive force changes slowly from the rich side to the lean side, the electromotive force on the lean side should be raised as shown by the broken line in Figure 5. It is possible to generate an approximately straight line that is continuous with the electromotive force on the rich side.

However, as shown by the dashed line in Figure 5, if an electromotive force with perfect drooping characteristics is generated by an electrode with high catalytic performance, in order to raise the electromotive force on the lean side to the level of the electromotive force on the rich side, it is necessary to The adsorption amount of HC, CO, and H ₂ in exhaust gas must be extremely increased. However, HC, CO, _H2 in exhaust gas
This is impractical in consideration of the amount of water, the thickness of the coating, and the responsiveness.

Furthermore, it can be easily inferred that the object of the present invention cannot be achieved with an electrode having a non-catalytic performance that exhibits the electromotive force characteristics shown by the two-dot chain line in FIG.

It should be noted that electrodes with semi-catalytic performance can be easily formed by selecting materials such as Ag and Au, but even with Pt, which is generally said to have high catalytic performance, there are problems with particle size and film thickness. Semi-catalytic performance can be easily achieved by adjusting the .

In addition, it is possible to know whether SnO ₂ that adsorbs the reducing gas has that property or not based on the change (increase) in conductivity when it is set in a reducing gas atmosphere.

The following various conventional related technologies have been proposed for the oxygen concentration detecting element of the present invention, but none of them detract from the innovativeness and uniqueness of the present invention, and the basics thereof. The ideas are completely different. For example, the device disclosed in Japanese Unexamined Patent Publication No. 51-89497 is a so-called semiconductor device, which contains TiO 2 , TiO ₂ ,
Although ZnO, PhO ₂ , etc. are used, changes in the resistance of the element are detected depending on how the semiconductor itself is used, and therefore it is not possible to detect anything other than the stoichiometric air-fuel ratio. In addition, the one disclosed in Japanese Patent Application Laid-Open No. 55-156855 changes the O ₂ partial pressure at the reaction interface between the electrolyte, electrode, and gas by electrolysis, and detects the air-fuel ratio on the rich side and lean side. However, because the electromotive force characteristics are approximately S-shaped and the gradient straight line is not continuous, there are two air-fuel ratio values for the same electromotive force, which makes it difficult to detect the air-fuel ratio and also Therefore, responsiveness is poor, and there are limits to actual engine air-fuel ratio control. Furthermore, Tokko Akira
The product published in No. 53-34077 is a one-to-one mixture of Pb, S, P, As, halides, etc. and Pt in order to eliminate the catalytic action of the external electrode, but the gradient of the electromotive force characteristics is Small (electromotive force is 100 to 300mV)
range), the detection accuracy is poor, and it is difficult to obtain a characteristic line with a slope in the lean side region of the air-fuel ratio, making detection in the lean region difficult. Furthermore, the method disclosed in JP-A-54-46598 uses an oxide having a predetermined O ₂ partial pressure range in the third layer, but since the O ₂ partial pressure is not fixed, the characteristics Variations occur in the oxide layer, the detection accuracy is poor, and the function is completely different from that of the oxide layer of the present application. Furthermore, JP-A No. 52-44689 discloses forming an oxide film such as CuO on the surface of an external electrode, but the purpose of this oxide film is to deteriorate the catalytic performance of the electrode. The objective of this invention is different from that of the present invention, and since this oxide film uses a highly conductive substance, it is different from the present invention.
In addition to being different from SnO ₂ , as is clear from the specification and drawings, this property is maintained by using an electrode that exhibits electromotive force characteristics with excellent drooping characteristics. Both of these are different from the present invention.

Embodiments of the present invention will be described below based on the drawings. The main structural parts of the present invention use the conventional oxygen concentration detection element shown in FIG. 1, so the same parts as in FIG. 1 are given the same reference numerals and detailed explanation thereof will be omitted. .

That is, as shown in FIGS. 3 and 4,
The oxygen concentration detection element A of the present invention has internal electrodes made of a conductive material such as Pt on the inner and outer walls of a solid electrolyte tube 1 made of oxygen ion conductive material such as ZrO ₂ , which is closed at one end and communicated with the atmosphere at the other end. A layer 2 and an external electrode layer 3 are formed, and a reducing gas is adsorbed on the surface of the external electrode layer 3 of the oxygen concentration detection element Ao in which at least the external electrode is an electrode having semi-catalytic performance.
It is formed by forming a coating 4 made of SnO ₂ ,
By flowing the atmosphere as a reference gas inside the solid electrolyte tube 1 and the exhaust gas as a gas to be measured to the outside, the gap between the inner and outer electrode layers 2 and 3 is proportional to the difference in oxygen concentration between the reference gas and the gas to be measured. The air-fuel ratio is detected by measuring the electromotive force generated.

Next, a specific example will be described.

Example As the above-mentioned current oxygen concentration detection element Ao, the solid electrolyte tube 1 is made of ZrO ₂ -8 mol% Y ₂ O ₃ , and the inner and outer electrode layers 2 and 3 are made of Pt particles mixed with a solvent and made into a paste. (Product name manufactured by Tanaka Matsuse Co., Ltd.;
Platinum Paste N758) was coated with a brush and then fired in an electric furnace to a thickness of 25μ.
A layer formed of a Pt layer and having semi-catalytic properties as shown by the solid line in Figure 5 was used.

Then, this detection element Ao was immersed in a solution in which SnO ₂ powder and a solution of ethyl silicate condensate were mixed at a ratio of 5:2, air-dried in the air, and then heated in an electric furnace at 700 to 800 °C. By heating and baking at a temperature of 30 minutes, a SnO ₂ film 4 having a thickness of 60 μm was formed on the surface of the external electrode layer 3, thereby obtaining an oxygen concentration sensing element of the present invention.

Next, a bench test was performed using the above example in a reciprocating engine, and the air-fuel ratio was varied from 11 to 20 while maintaining the exhaust gas temperature near the detection element at 550°C while adjusting the rotation speed and load. When the electromotive force characteristics were measured, the results shown in FIG. 6 were obtained. As is clear from Figure 6, the air-fuel ratio
It can be seen that the characteristics are almost linear over a wide range of 11 to 20, and that a wide range of air-fuel ratios including the stoichiometric air-fuel ratio can be detected with high accuracy, satisfying the requirements of the present invention. Further, the above results were obtained with good stability and excellent quick response.

Note that in the above examples, no protective layer was provided on the outer surface of the coating, but even if a protective layer is provided, the above performance will not be impaired.

As explained above, the oxygen concentration detection element of the present invention exhibits an almost linear electromotive force characteristic with respect to the air-fuel ratio with good responsiveness, and can accurately detect a wide range of air-fuel ratios including the stoichiometric air-fuel ratio. In addition, it is easy to manufacture and can be provided at low cost, so it can be widely used as a sensor for controlling the air-fuel ratio of the engine at the optimal position depending on the driving conditions and for comprehensive control systems for automobiles. It can be used for many purposes and has significant practical effects.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing a conventional example, Fig. 2 is a graph showing electromotive force characteristics of the conventional example, Fig. 3 is a sectional view showing an embodiment of the present invention, and Fig. 4 is an enlarged view of the main part of Fig. 3. Figure 5 is a graph showing the electromotive force characteristics of the current oxygen concentration detection element that constitutes the main structural part used in the present invention, and Figure 6 is a measurement result diagram showing the electromotive force characteristics of an example of the present invention. be. 1... Solid electrolyte tube, 2... Internal electrode layer, 3...
... External electrode layer, 4... Coating.

Claims (1)

[Claims] 1 An element having an oxygen ion conductive solid electrolyte tube with one end closed and the other end communicated with the atmosphere, and an internal electrode layer and an external electrode layer made of a conductive material formed on the inner and outer walls of the solid electrolyte tube. An oxygen concentration detection element characterized in that at least the external electrode is an electrode having semi-catalytic performance, and a coating made of SnO ₂ that adsorbs reducing gas is formed on the surface of the external electrode layer. .