JP2915064B2 - Air-fuel ratio detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio measuring detector, and more particularly, to a detector for controlling an internal combustion engine, from a low air-fuel ratio (rich region) to a high air-fuel ratio (lean region). The present invention relates to a detector for measuring the air-fuel ratio suitable for use over a wide range.

[Conventional technology]

In general, an automobile combustion system using an air-fuel ratio measurement detector grasps the combustion state by measuring the concentration of oxygen and unburned gas in exhaust gas, and determines the fuel, that is, the gasoline supply amount and air amount. Is fed back to a circuit for controlling the air-fuel ratio, that is, the air-fuel ratio A / F.

The stoichiometric air-fuel ratio (A / F = 14.7) at which oxygen in the air reacts efficiently even with gasoline is the stoichiometric air-fuel ratio.

Conventionally, a stoichiometric sensor that detects a stoichiometric air-fuel ratio or a lean sensor that detects an air-fuel ratio in a region above the stoichiometric air-fuel ratio has a gas diffusion layer formed with a thickness of 50 to 450 μm by plasma spraying using magnesia spinel powder. It has a porosity of about 5 to 10% and an average pore diameter of about 20 to 50 nm as measured by a water root porosimeter.

In order to increase the combustion efficiency of an automobile, it is necessary to control the air-fuel ratio over a wide range from a low air-fuel ratio region with a large amount of fuel (called a rich region) to a high air-fuel ratio region with a relatively small amount of fuel (called a lean region).

However, in order to measure the air-fuel ratio in the rich region, it is necessary to make the diffusion resistance higher than that of a conventional gas diffusion layer. The reason will be described with reference to FIGS.

FIG. 3 is a diagram showing the relationship between the air-fuel ratio of exhaust gas and gas components, FIG. 4 is a diagram illustrating the principle of a general limit current type air-fuel ratio measuring detector, and FIG. FIG. 3 is a diagram illustrating a relationship with electrical characteristics.

As shown in FIG. 3, in the region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio, that is, in the lean region, the components in the exhaust gas are almost oxygen (O ₂ ), and the unburned gas carbon monoxide (CO), Hydrocarbon (HC) and hydrogen (H ₂ ) are extremely small.

In this case, as shown in FIG. 4, O ₂ passes through the gas diffusion layer 3 and is ionized by a catalytic reaction at the outer reaction electrode 2 b, and oxygen ions O ²⁻ move to the atmosphere side through the solid electrolyte 1. I do. At this time, it is necessary to control the rate of O ₂ passing through the gas diffusion layer. Here, the rate-determining means that O ₂ passing through the gas diffusion layer
Means to give conductance to the flow. The gas diffusion layer 3 is required to have a certain degree of denseness. O ₂ arriving at the reaction electrode 2b is ionized as described above, but since the oxygen concentration in the exhaust gas varies depending on the air-fuel ratio,
As shown in FIG. 5, the output shows a characteristic having a limit current value corresponding to each air-fuel ratio A / F.

In FIG. 5, the horizontal axis represents the inter-electrode voltage V, the vertical axis represents the pump current I _P (mA), and the solid line at which the pump current is constant corresponds to each air-fuel ratio A / F. Indicates the limit current value.

It is known that this limit current value is represented by the following theoretical formula (1).

F: Faraday constant R: Gas constant T: Absolute temperature of gas S: Equivalent cross-sectional area of holes in gas diffusion layer l: Thickness of gas diffusion layer αi: Conversion constant Di: Diffusion coefficient of molecule Pi: Gas partial pressure This The limit current value in FIG. 5 is determined by the value of each term in the expression (1). When the constants are collectively shown, the expression (1) is expressed as the expression (2).

C: constant That is, the limiting current _{Ip *} is determined by the equivalent sectional area S of the holes corresponding to the density of the gas diffusion layer and the thickness l of the gas diffusion layer.

If the thickness l of the gas diffusion layer is large, the limiting current _{Ip *} decreases, but if it is too large, the response and durability are affected. Therefore, the limiting current _{Ip *} depends on the equivalent cross-sectional area S of the holes in the gas diffusion layer, and the smaller the S is, that is, the denser the gas diffusion layer is, the smaller the Ip _* becomes. This is effective for detection control in a region.

In Ritsuchi region, as shown in FIG. 3, the oxygen concentration in the exhaust gas is small, CO unburned gas, HC, H ₂ is large. Therefore,
These unburned gases pass through the gas diffusion layer 3 in FIG. 4, and the oxygen ions O ²⁻ pass through the solid electrolyte 1 from the atmosphere side and conversely on the outer electrode 2b as opposed to the case of the lean region. Reacts with combustion gases. However, since the size of the molecules of the unburned gas component is much smaller than that of oxygen molecules, the amount of gas passing through the gas diffusion layer cannot be limited by the conventional gas diffusion layer, and the control on the rich side cannot be performed. That is, in order to control the rich side, a gas diffusion layer having dense pores capable of controlling the diffusion of the unburned gas is required.

In consideration of these points, Japanese Patent Application Laid-Open No. Sho 53-116896 discloses that the gas diffusion layer has a two-layer structure having different densities by using a plasma spraying method. In this technology, the first layer of magnesia spinel is roughly 30
0 μm, the second layer is densely formed to a thickness of 2 mm.

On the other hand, a method for manufacturing a continuous porous sintered body using crystalline glass powder (glass ceramics) is disclosed in, for example,
-108179. In this technique, a crystalline glass powder is mixed with an inorganic salt to form a molded body, then fired, cooled, and then a soluble salt is extracted.The average pore diameter of the finally obtained molded body is 12 μm to 98 μm, porosity is 53% to 67%.

[Problems to be solved by the invention]

In the above prior art, the thickness and denseness of the gas diffusion layer,
No consideration was given to the relationship between heat resistance, productivity, and responsiveness, and there was a problem that the outer rough and thick layer was cracked by the thermal cycle. Furthermore, since the plasma spraying method is used, the production cost is high, and no consideration has been given to controlling the fineness.

In the latter technique, a continuous porous porous sintered body is formed using crystalline glass powder as in the present invention, but this is different from the use of the present invention. Moreover, the latter technology,
For example, even when applied to an air-fuel ratio measurement detector, it is considered difficult to apply it because the average pore diameter and the porosity greatly differ.

An object of the present invention is to provide a glass diffusion layer in which a ceramic glass powder having a melting point higher than that of a crystalline glass powder is mixed with the crystalline glass powder, alumina pores are impregnated into pores of the formed and fired film, and a fired gas diffusion layer is provided. The object of the present invention is to provide an air-fuel ratio measuring detector which can easily select the ratio and the average pore diameter, has excellent heat resistance, has good responsiveness, and has good productivity which can be widely applied from the lean region to the rich region. .

[Means to solve the problem]

In order to achieve the above object, the air-fuel ratio measuring detector according to the present invention is provided with a porous thin-film electrode on the inner and outer surfaces of a solid electrolyte element made of an oxygen ion conductive metal oxide, The outer electrode is formed of a gas diffusion layer made of a porous electrically insulating metal oxide, and ionizing oxygen in the atmosphere where the solid electrolyte element is placed by applying a constant voltage between the electrodes, In the detector for diffusing inside the solid electrolyte element and measuring the air-fuel ratio with respect to the flow rate of oxygen ions, a gas diffusion layer that should cover at least the entire surface of the outer electrode is made of a crystalline glass powder having a melting point higher than that of the crystalline powder glass. Is formed by mixing a ceramic fine powder having the following formula, forming and firing a thin film, impregnating pores in the thin film with alumina sol, and firing.

More preferably, the material of the ceramics fine powder is the same as that of the solid electrolyte or ZrO ₂ .

When attached, the ceramics fine powder was mixed with the crystalline glass powder, and the firing temperature of the formed thin film was 850 ° C to 1200 ° C.
C is preferred.

Further, the material to be impregnated into the fired thin film is not limited to alumina sol, for example, zirconia sol may be used,
Other types of sols and gel-like substances may be used as long as the coefficient of thermal expansion and the average particle diameter are the same. The method of impregnating the fired thin film may be a method of impregnating in a vacuum or a method of impregnating in a vacuum and then pressurizing a flow of the impregnating material from around the thin film using a hydraulic pressure or the like.

Optimally, the gas diffusion layer has an average pore size of 10 nm to 60 n
m, the film thickness is preferably 200 μm or less.

[Action]

The air-fuel ratio measurement detector configured as described above does not use plasma spraying, and forms fine particles such as alumina sol in a thin film that is formed after mixing ceramics fine powder with crystalline glass powder as a sintering inhibitor. Since the gas diffusion layer formed by impregnation and sintering has uniform pores, it has a small porosity despite its thin film thickness and exhibits a sufficient diffusion-controlling function.

By forming this layer thin, the thickness of the entire gas diffusion layer is reduced, the occurrence of thermal strain due to the difference in thermal expansion coefficient from the solid electrolyte element is reduced, and cracks are less likely to occur. Productivity is improved.

The thin film obtained by mixing the ceramic glass powder with the crystalline glass powder of the present invention and firing it has a relatively large pore diameter. Therefore, it is difficult to detect an air-fuel ratio in a wide range from a low air-fuel ratio (rich region) to a high air-fuel ratio (lean region), which is an object of the present invention. For this reason, the detection of the air-fuel ratio can be sufficiently performed within the range of only the high air-fuel ratio (lean region).

Further, by impregnating the fired thin film with fine particles of alumina sol or the like in a vacuum, a state is formed in which fine particles of alumina sol enter into large pores inside the thin film.
Thereafter, firing is performed in an electric furnace or the like.

The thin film formed as described above has fine and uniform holes. This is because fine particles such as alumina sol are very fine and uniform. Fine particles such as alumina sol change the bonding state between the particles by changing the firing temperature, so that it is easy to obtain an arbitrary pore diameter.

As described above, the method of applying the present invention having fine and uniform continuous holes in the gas diffusion layer of the detector for measuring the air-fuel ratio is a unique method completely different from the conventional example.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 and 2, and FIGS. 6 to 9.

First, one embodiment of the gas diffusion layer will be described with reference to FIG.

FIG. 6 is a sectional view of a main part of a gas diffusion layer of an air-fuel ratio measuring detector according to one embodiment of the present invention, which is used for controlling an automobile.

In FIG. 6, reference numeral 1 denotes a solid electrolyte element made of an oxygen ion conductive metal oxide (hereinafter simply referred to as a solid electrolyte).
In this example, the solid electrolyte 1 is zirconia partially stabilized by solid solution of yttrium oxide (yttrium).

Reference numeral 2 (collectively 2a and 2b) denotes a porous thin-film reaction electrode in which the inner and outer surfaces of the solid electrolyte 1 are plated with platinum. Since the outer reaction electrode 2b is related to the pore cross-sectional area S which affects the characteristics in the theoretical formula (1), the outer reaction electrode 2b is accurately formed by masking in the case of platinum plating.

3 is a gas diffusion layer made of an electrically insulating metal oxide formed so as to cover the outer reaction electrode 2b, 4 is a lead electrode,
Reference numeral 6 denotes a heater for heating the solid electrolyte 1.

More specifically, the lead electrode 4 connected to the outer reaction electrode 2b is formed of a platinum mask that has been masked at the same time, and is covered with a dense glass insulating layer 8 to completely block the reaction with the exhaust gas. On the outside, a gas diffusion layer 3 is formed by impregnating alumina sol into pores and sintering, with a skeleton of a film in which 29% ceramic fine powder is mixed with crystalline glass powder.

Next, FIG. 7 is a sectional view of a main part showing a gas diffusion layer of an air-fuel ratio measuring detector according to another embodiment of the present invention.

The difference between the embodiment of FIG. 7 and the embodiment of FIG. 6 lies in the formation state of the gas diffusion layer 3A. The gas diffusion layer 3A
It is not always necessary to cover the entire area outside the solid electrolyte 1, and the object of the present invention can be achieved if at least the entire surface of the outer reaction electrode 2b is covered.

The method of forming the gas diffusion layer in the embodiment shown in FIGS. 6 and 7 will be specifically described.

First, a solvent (for example, ethylene glycol monoethyl ether, etc.) and alumina balls are mixed with a fine powder mixed powder obtained by mixing 29% ceramics fine powder into a crystalline glass powder, and sufficiently mixed using a mixing mill such as a ball mill.
Stir to make a dispersion. The dispersion thus obtained is subjected to a defoaming treatment in a vacuum, and is applied to the element by dipping. Then, it was air-dried and baked at 1100 ° C. using an electric furnace to form a thin film having a thickness of about 70 μm.

FIG. 8 shows a thin film formed on the element in this manner. The pores 11 of the thin film are relatively large pores, and as a result of measurement using a mercury porosimeter, the average pore diameter is 1.
It is about 5 μm. The skeleton 10 of the thin film is formed by melting the fine ceramic powder so that the crystalline glass material surrounds the periphery. In this case, the ceramics fine powder to be mixed with the crystalline glass powder is a ceramics fine powder having an average particle size of 0.8 μm and a mixing ratio of 20 μm.
% To 40% is preferred. Further, as described above, since the ceramic fine powder is mixed as a sintering inhibitor, the mixing ratio of the ceramic fine powder to the crystalline glass powder changes depending on the particle size of the ceramic fine powder. Those having a smaller particle size, for example,
When mixing ultrafine particles, the mixing ratio may be reduced.

Further, the material of the ceramic fine powder is preferably the same as the material of the solid electrolyte from the viewpoint of the coefficient of thermal expansion, and ZrO ₂ and
Al ₂ O ₃ may be used.

Regarding the firing conditions, it is preferable to fire at a temperature higher than the crystallization temperature of the crystalline glass powder, and the appropriate range varies depending on the components of the crystalline glass powder.
850 ° C to 1200 ° C.

In addition, the application method of the dispersion liquid is not limited to the dipping method,
Various methods such as brushing, spraying, and spin coating are used.

The average pore diameter of the thin film thus formed is preferably 3000 nm or less, and the smaller the average pore diameter, the better. The porosity is preferably 5% to 20%. Moreover, the scope finally also obtained Te cowpea to impregnate the alumina sol in the next step is outside of the average pore diameter and porosity, may be a film quality capable of diffusion control of the H ₂ gas.

Thin film formed by using the method described above, since the pore diameter is relatively large, it is difficult to diffusion control small gas such as H ₂ gas, sufficient oxygen gas to the diffusion control pore size Therefore, if the range of only the high air-fuel ratio (lean region) is controlled, there is also a feature that can be used sufficiently.

As a next step, the thin film thus formed was impregnated with alumina sol into holes in the thin film in a vacuum, dried, and then fired at 1100 ° C. using an electric furnace or the like.

FIG. 9 shows a detailed diagram. The voids in the skeleton 10 composed of crystalline glass powder and ceramic fine powder are filled with a sintered body 12 of alumina sol having an average particle diameter of 50 nm.
Generally, if only a thin film is dipped and applied before impregnation with alumina sol, the alumina sol adheres only to the surface. However, by performing the impregnation treatment of the present invention, it becomes possible to fill the pores in the thin film with the impregnation material.

As the impregnation method, there is a vacuum impregnation performed in a vacuum, and a method in which the impregnation material is pressurized from around the thin film using a hydraulic pressure or the like after the vacuum impregnation.

Further, as the impregnating material, not only alumina sol but also, for example, zirconia sol may be used, and other types of sols and gel-like substances may be used as long as the thermal expansion coefficient and the average particle diameter are the same.

The sintering temperature after the impregnation is similar to the sintering temperature of the crystalline powder glass and the skeletal portion 10 made of the ceramics fine powder, or a temperature somewhat lower (from 50 ° C to 100 ° C).
C.).

The thin film obtained by firing the impregnated material has fine and uniform pores. This is because fine particles such as alumina sol are very fine and uniform. In addition, fine particles such as alumina sol change the firing temperature to change the bonding state between the particles,
It is easy to obtain an arbitrary pore size.

After the first impregnation process, even if the pores of the thin film are not completely seen by the impregnating material, the impregnation material is used several times until the pores are seen by the impregnating material. May go. In this case, there is an advantage that the basic film thickness does not change.

The optimum average pore diameter of the gas diffusion layer thus formed is
The thickness is 10 nm to 60 nm, the porosity is 4% to 20%, and the film thickness is 200 μm or less.

In the gas diffusion layer of this embodiment, not only the cost reduction but also the conventional plasma sprayed film is formed by impregnating alumina sol into the pores of a thin film obtained by mixing a ceramic glass fine powder with a crystalline glass powder to form a dense film. In addition, the film thickness was reduced, and the durability and responsiveness to thermal strain were improved.

Next, the overall configuration and output characteristics of the limiting current type air-fuel ratio measuring detector having such a gas diffusion layer will be described in the first section.
This will be described with reference to FIG.

FIG. 1 is a longitudinal sectional view of a limit current type air-fuel ratio measuring detector according to one embodiment of the present invention, and FIG. 2 is an output characteristic diagram obtained by the detector of FIG.

In FIG. 1, the solid electrolyte 1 is fixed to a plug 5. An outer cylinder 7 for protecting the gas diffusion layer 3 from impurities in the exhaust gas as described in each embodiment is provided at the end of the plug 5, and an element is provided inside the solid electrolyte 1. Is heated to 600 to 700 ° C., and a heater 6 for making zirconia of the element material an electrolyte is built in. Further, lead wires 9a, 9b, 9c for extracting electric signals and applying voltages to the inner reaction electrode 2a, the outer reaction electrode 2b, and the heater 6, respectively, are connected.

The detector for measuring the limiting current type air-fuel ratio thus manufactured is attached to the exhaust pipe of the automobile, the heater 6 is energized to heat the solid electrolyte 1 of the element body to about 700 ° C., and a voltage is applied to the element. As shown in FIG. 2, the output characteristic of the air-fuel ratio measuring detector according to the present embodiment shows a linear output from the stoichiometric air-fuel ratio (A / F = 14.7) to the rich region side, as indicated by the output voltage V indicated by the solid line in FIG. As a result, the air-fuel ratio can be detected. As shown by the broken line, the characteristics of the conventional diffusion film can only be detected up to A / F = 12 in the rich region, and the output drops sharply in the rich region where the fuel concentration is higher. Have been.

If this is replaced with drivability, normal driving on flat ground (40-60km / h) will be economical driving by lean area control,
When traveling uphill on a mountain road or the like, the output is improved by the rich region control, and the drivability can be improved as a whole.

Also, with the current engine that performs three-way feedback control (CO, HC, NO control in exhaust gas) with an oxygen sensor (stoichiometric sensor), the air-fuel ratio A / F is 9 at the time of cold start or rapid acceleration. The air-fuel ratio measurement detector according to the present embodiment can be used not only for lean burn engines (high air-fuel ratio, lean-burn control engines) but also for wide-range air-fuel ratio control in current engines. It can be used, improving fuel efficiency, improving drivability,
Further, there is a ripple effect that is effective for improving safety and the like.

As described above, the present invention relates to a detector for measuring an air-fuel ratio used for fuel injection control, and particularly has a feature in a gas diffusion layer of a detecting element, and has a pore of a thin film obtained by mixing a ceramic glass fine powder with a crystalline glass powder. The point of the present invention lies in that a dense gas diffusion layer is formed by impregnating an alumina sol into the gas.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, it is equipped with the dense gas diffusion layer which has the optimal gas diffusion-controlling function, is excellent in heat resistance, has good responsiveness, and has good productivity which can be widely applied from the lean region to the rich region. A detector for measuring a fuel ratio can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of a limit current type air-fuel ratio measuring detector according to an embodiment of the present invention, FIG. 2 is an output characteristic diagram obtained by the detector of FIG. 1, and FIG. , A diagram showing the relationship between the air-fuel ratio of the exhaust gas and the gas components, FIG. 4 is a diagram illustrating the principle of a general limit current type air-fuel ratio measurement detector, and FIG. FIG. 6 is a sectional view of a main part of a gas diffusion layer of an air-fuel ratio measuring detector according to one embodiment of the present invention, and FIG. 7 is an air-fuel ratio according to another embodiment of the present invention. FIG. 8 and FIG. 9 are cross-sectional views of main parts showing the structure of the gas diffusion layer of the present invention. 1 ... solid electrolyte, 2a ... inner reaction electrode, 2b ... outer reaction electrode, 3, 3A ... gas diffusion layer, 10 ... thin film skeleton, 11 ... voids, 12 ... sintered body.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Norio Ichikawa 2520 Oji Takaba, Katsuta City, Ibaraki Prefecture Inside Sawa Plant, Hitachi, Ltd. (72) Inventor Seining Ueno 2520 Oji Takaba Katsuta City Ibaraki Prefecture Stock Company (56) References JP-A-1-227955 (JP, A) JP-A-2-80946 (JP, A) JP-A-2-130460 (JP, A) JP-A-2-287251 (JP, A) JP-A-64-88148 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/41, 27/409

Claims (3)

(57) [Claims] 1. A solid electrolyte element made of an oxygen ion conductive metal oxide is provided with a porous thin-film electrode on the inner and outer surfaces thereof, and the outer electrode of the solid electrolyte element is made of a porous electrically insulating metal oxide. The solid electrolyte element is covered by a gas diffusion layer, and oxygen in the atmosphere in which the solid electrolyte element is placed is ionized by applying a constant voltage between the electrodes, and diffused inside the solid electrolyte element. In the air-fuel ratio measuring detector for measuring the air-fuel ratio by obtaining the limiting current value, at least a gas diffusion layer to cover the entire surface of the outer electrode, a crystalline glass powder, a melting point higher than the crystalline glass powder The alumina impregnated in a vacuum is impregnated into the pores of the thin film obtained through the molding and firing process, followed by firing. An air-fuel ratio measuring detector formed by repeating several times. 2. The air-fuel ratio measuring detector according to claim 1, wherein the mixing ratio of the ceramic fine powder mixed with the crystalline glass powder is 40% or less. 3. The detector for measuring an air-fuel ratio according to claim 1, wherein the gas diffusion layer finally obtained has an average pore diameter of 10 to 60 nm, a porosity of 4 to 20%, and a film thickness of 200 μm or less. .