JPH067118B2 - Air-fuel ratio sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to an air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture supplied to a combustion device, and particularly to a lean air-fuel mixture using an oxygen ion conductive solid electrolyte. The present invention relates to an air-fuel ratio sensor capable of detecting an air-fuel ratio in a rich region (a state in which fuel is in excess of the stoichiometric air-fuel ratio) to a rich region (state in which air is in excess of the stoichiometric air-fuel ratio).

[Prior Art] Conventionally, for example, in a combustion device such as an internal combustion engine, in order to improve fuel efficiency and emission, the oxygen concentration in exhaust gas is detected and the air-fuel mixture burned in the combustion container is close to the stoichiometric air-fuel ratio. There is one that executes so-called feedback control, such as controlling to. And as an oxygen sensor used in this type of control device to detect the oxygen concentration in the exhaust gas,
For example, it is configured by depositing a porous electrode layer on an ion conductive solid electrolyte, and detects the combustion state near the stoichiometric air-fuel ratio by the change in electromotive force caused by the difference between the oxygen partial pressure of exhaust gas and the oxygen partial pressure of air. In general, an oxygen sensor such as an oxygen sensor in which the output voltage changes in a switching manner with the stoichiometric air-fuel ratio of the air-fuel mixture as a boundary is known.

By the way, in recent years, by not only controlling the air-fuel ratio of the air-fuel mixture to near the stoichiometric air-fuel ratio but also changing the target air-fuel ratio according to the operating state of the equipment and executing feedback control, it is possible to improve fuel economy and emissions. Although it is considered to improve the operability of the equipment as well as improve it, the conventional oxygen sensor described above can only detect the stoichiometric air-fuel ratio of the air-fuel mixture. The fuel ratio could not be controlled.

On the other hand, in recent years, in order to realize the feedback control of the air-fuel ratio as described above, a chamber for forming a sealed space including one electrode surface of the solid electrolyte oxygen pump element is provided, and a minute diffusion hole is formed in the wall of the chamber. A voltage is applied between the electrode surfaces so that the gas components in the gas to be measured are diffused and introduced into the chamber as much as possible through this hole, so that the atmosphere in the chamber is uniformly made into an oxygen-deficient state including a so-called chemical equivalence point state. A method for measuring the gas component concentration in the gas to be measured by measuring the amount of current flowing sometimes, that is, a method for measuring the gas component concentration by the so-called diffusion limiting current method is disclosed in JP-A-52-72286 and JP-A-53-66292. Has been proposed in the issue.

Further, in Japanese Patent Application Laid-Open No. 59-190652, a sensor based on the same principle and method as the above-mentioned gas component concentration measuring method is basically a diffusion limiting current method, but as a chemical equivalent point detecting means. A solid electrolyte oxygen concentration battery element using air as a reference oxygen concentration gas is provided separately from the solid electrolyte oxygen pump element, and these elements form a space chamber having diffusion pores, and the oxygen concentration battery is formed. The electromotive force of the element is approximately equal to the electromotive force value (about 500 mV) generated when the atmosphere in the space is in the state of chemical equivalence point (that is, the diffusion rate through the pores reaches the limit value). A sensor for measuring the gas component concentration in the gas to be measured by the pump current value when the pump current of the oxygen pump element is adjusted to show the value and the atmosphere in the space is brought into a state of chemical equivalence point uniformly. It has been mounting.

However, in these so-called diffusion limiting current type sensors, there is a demand for measurement over a wide range of gas component concentrations, and by making the gas component concentrations in the space chamber as uniform as possible, it is possible to achieve the characteristics specific to diffusion limiting current type sensors. While maintaining a rapid change characteristic of Vs (hereinafter referred to as a limiting current characteristic) in a relationship curve of the electromotive force Vs of the oxygen concentration battery element and the pump current Ip of the oxygen pump element under a constant gas component concentration in the measurement gas In order to satisfy it, the diameter of the pores had to be made extremely small, and as a result, the responsiveness was not practically satisfactory.

On the other hand, in the above-mentioned Japanese Patent Laid-Open No. 59-190652, the space between the reference oxygen concentration battery element and the oxygen pump element is not closed, but the electrodes of both elements are simply a minute gap (0.
(075 mm) so that the diffusion is controlled so as to face each other, and the electromotive force value of the oxygen concentration battery element is the electromotive force value that should be shown when it is assumed that the space chamber is in the state of chemical equivalence point uniformly. (Around 500 mV) A pump current is adjusted so as to show an electromotive force value almost equal to (about 500 mV), and the gas component concentration in the gas to be measured is measured by the pump current value at that time, that is, a diffusion-limited sensor using a minute air gap. Is also disclosed. However, this sensor
At least in practically required size, that is, the sensor is housed in a metallic housing with a screw and mounted on an exhaust pipe of an internal combustion engine, and the width of the element is 10 mm. When it is attempted to carry out in a small size which is less than the above, a large amount of component gas diffuses in from the open end of the periphery of the gap, and this diffuses in the void with diffusion restriction over the whole area,
Oxygen concentration partial pressure difference is extremely large in the direction along the opposing electrode surfaces, and as a result, the limiting current characteristic hardly occurs (in that sense, this method is hard to say as the diffusion limiting current method), and the measurement accuracy of the sensor is In addition, it is unavoidable that a large oxygen concentration partial pressure difference occurs between the two electrodes facing each other in the facing direction. Therefore, the pump current value exceeds the allowable value and the durability is extremely poor, which is not practical.

[Problems to be Solved by the Invention] Diffusion limiting method using pores (diffusion limiting current method)
A very useful full-range air-fuel ratio sensor that overcomes the problems of the diffusion-limited air-fuel ratio sensor with small air gaps and has excellent responsiveness and a sufficiently clear limiting current. With the goal.

[Means for Solving the Problems] That is, the air-fuel ratio sensor of the present invention includes an oxygen pump element having a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte medium, and an oxygen ion conductive solid electrolyte medium. An oxygen concentration battery element having a pair of electrodes on its surface and exposing one electrode of the pair of electrodes to a reference oxygen concentration gas such as the atmosphere; and one electrode of the oxygen pump element and the oxygen. Gas diffusion between the space formed between the two elements by arranging the two elements in parallel so that the other electrode of the concentration cell element faces each other, and the gas formed and measured gas. A gas diffusion chamber communicated via a limiting means, and the electromotive force of the oxygen concentration battery element always holds an electromotive force value substantially equal to the electromotive force value when the atmosphere in the diffusion chamber is in the chemical equivalence point state. In the air-fuel ratio sensor in which the pump current flowing in the oxygen pump element is adjusted based on the feedback signal, and the pump current value at that time is the air-fuel ratio sensor output, the diffusion limiting means is a wall of the diffusion chamber. The first diffusion limiting portion for limiting diffusion by the pores provided so that the diffusion chamber communicates with the gas atmosphere to be measured, and the dimension of the space in the diffusion chamber in the thickness direction is 0.2 mm or less. The second that limits diffusion by the minute voids formed by making it over 01 mm
And a diffusion limiting unit of.

The oxygen concentration cell element and the oxygen pump element are, for example, Y ₂ O.
1 on the front and back of a solid electrolyte plate such as _3- ZrO ₂ solid solution
It is formed by providing a pair of porous electrodes.

As a material of the solid electrolyte plate, a solid solution of zirconia with yttria or calcia is typical,
In addition, cerium dioxide, thorium dioxide, hafnium dioxide solid solutions, perovskite type oxide solid solutions, trivalent metal oxide solid solutions and the like can be used.

As a material for the porous electrode, platinum, gold or the like can be used, which may be formed by pasting raw material powder as a main component into a paste using a thick film technique, followed by sintering, and forming a frame. It may be formed using a thin film technique such as spraying, chemical plating or vapor deposition.

Further, both the oxygen concentration battery element and the oxygen pump element may be provided on one solid electrolyte plate, and the oxygen pump element may be provided on the other one solid electrolyte plate. The capability of discharging and inhaling oxygen gas in the diffusion chamber of the device, which will be described later, is improved, and the partial pressure control of oxygen gas in the vicinity of the oxygen concentration battery device electrode becomes easier. However, in any case, most of the large surface of one of the diffusion chambers should be the electrode of the pump element. It is usually necessary that the electrode area of the pump element is at least 5 mm ² .

It is to be noted that, as in Examples described later, it is necessary to make the areas of the electrodes of the oxygen pump element and the oxygen concentration cell element facing each other to be substantially the same as shown in FIGS. 1 and 2. This is one of the preferable modes because the oxygen concentration cell element can detect the change quickly when the gas component concentration in the gas changes, and thus the response is good.

Further, at least the surface of the oxygen concentration battery element which is not in contact with the diffusion chamber needs to be in contact with the reference oxygen concentration gas, and for example, a passage for guiding the atmosphere is provided by a known method. For example, a passage forming body including a U-shaped stress relaxation layer and a plate-shaped support may be joined to the surface of the solid electrolyte that is not in contact with the diffusion chamber to form the air introduction passage.

As the gas diffusion limiting means by the pores, a hole parallel to the void connecting the diffusion chamber and the measurement gas atmosphere can be used. This hole may have a relatively large diameter, and one or two or more holes may be provided and the inside thereof may be filled with a porous material to further increase the diffusion resistance. This hole is easily formed by providing a notch that connects the diffusion chamber and the measurement gas to a spacer that forms the diffusion chamber described later. In the air-fuel ratio sensor of the present invention, the gas diffusion restriction is performed not only by the gas diffusion restriction means but also in the diffusion chamber as described later. That is, the diffusion of the gas is limited by the hole forming a part of the gas diffusion limiting means and the diffusion chamber, and also by the porous material when the hole is filled with the porous material.

The diffusion chamber is joined by sandwiching a spacer having a predetermined thickness having a void serving as a diffusion chamber between a solid electrolyte plate provided with an oxygen concentration battery element and a solid electrolyte plate provided with an oxygen pump element. It is formed by Particularly, it is preferable to dispose one layer of granulated particles produced by a spray dryer or the like having a diameter substantially equal to that of the diffusion chamber before firing, because this diffusion chamber is prevented from being deformed during firing.

The thickness of this diffusion chamber, that is, the distance between the electrode surfaces of both elements, is preferably 0.01 to 0.2 mm in order to perform the diffusion limiting action in the direction along the surface of the electrode in the diffusion chamber, and particularly, It is preferably 0.05 to 0.1 mm.
If this thickness is less than 0.01 mm, the effect of limiting the diffusion of oxygen gas by the diffusion chamber itself is too large, and the limiting current characteristics of the air-fuel ratio sensor will not sufficiently occur, resulting in poor measurement accuracy and responsiveness. There is a problem in that it deteriorates and is easily deformed during manufacturing, it becomes difficult to maintain electrical insulation between the oxygen concentration battery element and the oxygen pump element, and it is difficult to produce a product of uniform quality. On the other hand, if the thickness is larger than 0.2 mm, the partial pressure difference of the component gas in the facing direction in the diffusion chamber, especially between the two electrodes facing each other, becomes large when the responsiveness is secured, and the pump current becomes oxygen. It becomes larger than the capacity of the pump element, and on the other hand, as long as the pump current is kept low, the response becomes poor. That is, in the conventional diffusion limiting current type sensor, the output voltage of the oxygen concentration battery element during the measurement operation is about 500 mV (450 to 5
50 mV) to make a uniform chemical equivalence point state in the diffusion chamber, which was considered preferable from the viewpoint of responsiveness and limiting current characteristics (measurement accuracy), but in that case, responsiveness was still insufficient, and this Practically satisfactory level (350-400m
When trying to select a large pump current to keep the value of (sec or less), a partial pressure difference occurs in the space chamber in the electrode facing direction, which is a problem from the viewpoint of durability of the oxygen pump cell. Chamber thickness 0.2mm
If it is made larger, the same problem will occur.

[Operation] The operation of the air-fuel ratio sensor of the present invention will be described.

First, when the air-fuel mixture is in the lean range, the air-fuel ratio sensor is put in the exhaust gas, and the positive electrode is placed on the atmosphere side electrode of the oxygen pump element.
By applying a negative voltage to the electrode on the diffusion chamber side, oxygen ions move in the solid electrolyte of the oxygen pump element to the opposite side of the diffusion chamber, and oxygen gas in the diffusion chamber is pumped out.

When oxygen gas is pumped out of the diffusion chamber as described above, a difference in oxygen gas concentration occurs between the atmosphere side of the oxygen concentration battery element and the diffusion chamber due to the oxygen diffusion limiting action of the diffusion limiting means. Due to this concentration difference, an electromotive force is generated in the oxygen concentration battery element. Then, for example, when the amount of current (pump current) flowing to the oxygen pump element side is changed and adjusted so that the electromotive force E is maintained at a predetermined constant value, the amount of current is determined by the content of oxygen gas in the measurement gas. The rate can be made to be almost linearly proportional to the rate, and the oxygen gas concentration can be obtained.

Next, when the air-fuel ratio sensor is put into the exhaust gas when the air-fuel mixture is on the rich side, the oxygen concentration cell element is activated without causing an oxygen gas partial pressure difference by operating the oxygen pump element between both electrodes. Since electric power is generated, the direction of the pump current flowing through the oxygen pump element is reversed to keep the electromotive force of the oxygen concentration battery element constant. That is, in the diffusion chamber side electrode portion of the oxygen concentration battery element, since oxygen is consumed by unburned hydrocarbons and carbon monoxide in the exhaust gas, the difference in oxygen gas partial pressure between the diffusion chamber side and the atmosphere side is large. It becomes too much, and the electromotive force becomes larger than a predetermined value. Therefore, it is necessary to send oxygen into the diffusion chamber by the oxygen pump element so as to maintain the electromotive force at a predetermined value. At this time, the pump current is opposite to the pump current in the lean region, and the magnitude of the required pump current corresponds to the amount of unburned hydrocarbons and carbon monoxide in the exhaust gas. Therefore, the pump current corresponds to the air-fuel ratio in the rich region.

That is, when the pump current flowing through the oxygen pump element is adjusted so that the electromotive force of the oxygen concentration cell element of the air-fuel ratio sensor is maintained at a predetermined constant value, the pump current corresponds to the air-fuel ratio. The pattern of this relationship is shown in FIG.

Here, it is easily known that the oxygen gas partial pressure in the diffusion chamber is lower and the closer to the chemical equivalence state, the better the responsiveness and the limiting current characteristic of the air-fuel ratio sensor. As a flat chamber, the diffusion limitation due to the minute voids is performed together with the diffusion limitation due to the pores, and one limiting diffusion limitation is performed, so that the oxygen concentration cell element and the oxygen pump element in the diffusion chamber face each other. In both electrodes, a difference in component gas partial pressure is produced in the direction along the electrode surface, but the difference in component gas partial pressure in the opposing direction between the two electrodes does not become excessive, and therefore durability is not impaired. Also, while the deterioration of the limit electrode characteristics, that is, the deterioration of the measurement accuracy is advantageously suppressed, the ratio of the diffusion flow rate of the component gas (that is, the pump current amount) to the volume of the diffusion chamber is increased to make the responsiveness at a practical level ( For example 40 Up to ~350msec within) is significantly improved.

It should be noted that, here, it is determined whether the partial pressure difference between the component gases in the opposite direction is excessive or not by determining the output voltage Vs of the oxygen concentration battery element.
Is a predetermined value, that is, an electromotive force value generated between both electrodes of the oxygen pump element when it is constantly maintained at about 500 mV (value obtained by correcting the voltage drop due to the internal resistance with respect to the applied voltage Vp). And the difference between the above Vs value is at least 700mV
Within, preferably within 500 mV, more preferably 20
It can be easily judged from the fact that it should be within 0 mV. When this value is excessively large, the oxygen partial pressure value becomes 10 ⁻²⁴ atm or less in the portion of the electrode of the oxygen pump element facing the diffusion chamber where the oxygen partial pressure is smallest.
The solid electrolysis at the interface of (gas)-(electrode)-(solid electrolyte) is reduced and deteriorated.

[Embodiment] A first embodiment of the present invention will be described with reference to a partially broken perspective view of FIG. 1 and an explanatory view of FIG.

In this embodiment, one oxygen concentration cell element 2 and one oxygen pump element 3 are arranged to face each other with the diffusion chamber 1 interposed therebetween.

The oxygen concentration battery element 2 is made of 7 × 45 × 0.6 mm Y ₂ O ₃
-Y _{2 on} both surfaces of the solid electrolyte plate 4 made of ZrO ₂ solid solution
Electrodes 5 and 6 made of platinum containing 5 wt% of O ₃ -ZrO ₂ solid solution are provided by a thick film technique, and a mixed sintering of Al ₂ O ₃ and ZrO ₂ is performed on a surface of the solid electrolyte 4 which is not in contact with the diffusion chamber 1. The body is a thickness of 1.0 mm, the outer shape is 7 × 45 mm, the inner shape is 5 × 43 mm, and the U-shaped stress relaxation layer 7 and Al ₂ O _{3 are} 7 × 45.
Passage forming body 9 formed by a support body 8 of 0.8 mm
It becomes by providing. The atmosphere is introduced into the electrode 5 of the oxygen concentration cell element 2 through the passage 10 formed by the passage forming body 9. Further, a heating element 11 is provided on the passage 10 side of the support 8.

Like the oxygen concentration battery element 2, the oxygen pump element 3 includes a solid electrolyte plate 12, electrodes 13, 14, a stress relaxation layer 15, and a passage forming body 17 including a support body 16. The atmosphere is introduced into the electrode 13 of the oxygen pump element 3 by the passage 18 formed by the passage forming body 17. A heating element 19 is provided on the support 16.

The diffusion chamber 1 is a mixed sintered body of Al ₂ O ₃ and ZrO ₂ and has a thickness of 0.1 mm, an outer shape of 7 × 45 mm, and an inner shape of 3 × 9 mm, and has a substantially U-shaped cross section with a cross section of 0.1 × 0. 1mm diffusion limiter 30
The diffusion chamber and the diffusion limiting portion forming body 20 for forming the above are sandwiched and joined by the solid electrolyte plate 4 provided with the oxygen concentration battery element 2 and the solid electrolyte plate 12 provided with the oxygen pump element 3. Formed by. It is also possible to further increase the diffusion resistance by filling the pores as the diffusion limiting portion with a porous material made of, for example, a particle-bonded body of alumina. At that time, the porosity of the porous material may be relatively large, and therefore there is less risk that the characteristics will be changed due to clogging, and it is easy to manufacture.

In the air-fuel ratio sensor of this embodiment, since the diffusion chamber 1 is a flat chamber and the pump electrode area can be made larger than the volume of the diffusion chamber 1, the concentration of the component gas in the diffusion chamber can be adjusted quickly and the response is good. Moreover, since the pump current is small, it is possible to reduce the difference in the partial pressures of the component gases in the opposing directions between the electrodes of both elements, and the durability of the electrodes can be maintained.

In addition, the stress relief layers 7 and 15 are formed on the passage forming bodies 9 and 17 as A
Since the mixed sintered body of l ₂ O ₃ and ZrO ₂ is used, it is possible to prevent the warp of the air-fuel ratio sensor during use and the damage due to the difference in thermal expansion coefficient. In particular, the warp during use is almost completely canceled out because it is configured to be substantially plane-symmetrical about the diffusion chamber 1.

Further, since the heating elements 11 and 19 are provided, temperature compensation can be easily performed.

FIG. 3 is a diagram showing characteristics in use of this embodiment.
As described above, FIG. 3 shows the relationship between the pump current and the air-fuel ratio when the output voltage of the oxygen concentration battery element 2 is kept constant. In the above embodiment, the electrode of the oxygen pump element on the side opposite to the gas diffusion chamber was formed with a passage so as to contact the atmosphere, but this was simply exposed to the exhaust gas so that oxygen was extracted from the oxygen or oxygen-containing component in the exhaust gas. It goes without saying that it is okay.

EFFECTS OF THE INVENTION The present invention provides an oxygen pump element having a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte medium, and a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte medium. Of one of the electrodes of the oxygen concentration battery element in which one electrode is exposed to a reference oxygen concentration gas such as the atmosphere, and one electrode of the oxygen pump element and the other electrode of the oxygen concentration battery element face each other. A space formed between the two elements by arranging them in parallel with each other, and a gas diffusion chamber formed by the space and communicating with the gas to be measured through gas diffusion limiting means. The pump current supplied to the oxygen pump element is controlled so that the electromotive force of the oxygen concentration cell element is almost equal to the electromotive force value when the atmosphere in the diffusion chamber is in the chemical equivalence point state. In the air-fuel ratio sensor that adjusts the amount based on the feedback signal and uses the pump current value at that time as the air-fuel ratio sensor output, the diffusion limiting means includes the diffusion chamber and the measurement gas atmosphere in the wall of the diffusion chamber. A first diffusion limiting portion that limits diffusion by pores provided so as to communicate with the micropores formed by setting the dimension of the space of the diffusion chamber in the thickness direction to 0.2 mm or less and 0.01 mm or more. The second that limits diffusion by a gap
Of the oxygen concentration cell element and the oxygen pump element, a difference in oxygen partial pressure is produced in the direction along the electrode surface between the two electrodes facing each other. The pressure difference does not become excessive, and therefore the durability of the oxygen pump element is not impaired, and while the deterioration of the limiting current characteristics, that is, the measurement accuracy is also advantageously suppressed, the diffusion flow rate of the component gas with respect to the volume of the diffusion chamber is suppressed. (Pump current amount)
The responsiveness is remarkably improved to a practical level by increasing the ratio.

According to the present invention, as described above, the practically sufficient limiting current characteristics, responsiveness, and durability are obtained together, and as a result, the size of the sensor for satisfying all of them is increased. It became possible for the first time to provide a small-sized full-range air-fuel ratio sensor of a practical level without the need for it.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of an embodiment of the present invention, FIG. 2 is a development explanatory view thereof, and FIG. 3 is a characteristic view during use thereof. DESCRIPTION OF SYMBOLS 1 ... Diffusion chamber 2 ... Oxygen concentration battery element 3 ... Oxygen pump element 10, 18 ... Passage 30 ... Diffusion restriction part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yutaka Adachi 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Nihon Special Ceramics Co., Ltd. No. 18 in Nihon Special Ceramics Co., Ltd. (56) Reference JP-A-60-1336 (JP, A) JP-A-60-14161 (JP, A) JP-A-59-190652 (JP, A) JP-A 59-3252 (JP, A) JP-A-59-43348 (JP, A) JP-A-59-67455 (JP, A) Actual development Sho-59-78968 (JP, U)

Claims (2)

[Claims] 1. An oxygen pump element having a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte medium, and a pair of electrodes on the surface of an oxygen ion conductive solid electrolyte medium as well as the pair of electrodes. An oxygen concentration battery element in which one of the electrodes is exposed to a reference oxygen concentration gas such as the atmosphere, and one electrode of the oxygen pump element and the other electrode of the oxygen concentration battery element are opposed to each other. A space formed between both elements by arranging both elements in parallel, and a gas diffusion chamber formed by the space and communicating with the gas to be measured via gas diffusion limiting means,
Based on a feedback signal, a pump current flowing through the oxygen pump element is always maintained so that the electromotive force of the oxygen concentration cell element is almost equal to the electromotive force value when the atmosphere in the diffusion chamber is in the chemical equivalence point state. In the air-fuel ratio sensor that adjusts and adjusts the pump current value at that time to be the output of the air-fuel ratio sensor, the diffusion limiting means connects the diffusion chamber and the measured gas atmosphere to the wall of the diffusion chamber. The first diffusion limiting portion for limiting diffusion by the pores provided in the above, and the microscopic voids formed by setting the dimension of the space of the diffusion chamber in the thickness direction to 0.2 mm or less and 0.01 mm or more Second to do
And an air-fuel ratio sensor. 2. The air-fuel ratio sensor according to claim 1, wherein the pores of the first diffusion limiting portion are formed by filling the inside of the openings with a porous material.