JPS6258160A - Air/fuel ratio detection element - Google Patents

Abstract

PURPOSE:To simplify the constitution of the titled detection element, by combining an oxygen concn. cell element having a pair of electrodes different in the intensity of catalytic action in oxidizing reaction with an oxygen pump element. CONSTITUTION:An oxygen pump element 2 and an oxygen conc. cell element 3 are assembled to constitute an integrated air/fuel ratio detection element 1 having a gap part 9. At this time, a spacer 11 having the same outer shape as each of both elements 2, 3 is interposed between both elements 2, 3 and the part thereof corresponding to the electrodes 5, 6 of both elements 2, 3 is punched so as to form the gap part 9. The spacer 11 has a hole 12 at the leading end side thereof and is laminated to both elements 2, 3, for example, by a heat resistant inorg. adhesive so as to be held between both elements 2, 3. Further, the element 1 controls the current flowing to the element 2, for example, in order to constantly control the output voltage of the element 3 and used in a measuring apparatus constituted so as to take out the air/fuel ratio signal corresponding to the air/fuel ratio of a gaseous mixture. Therefore, the constitution of the apparatus can be simplified.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an air-fuel ratio detection element for detecting the air-fuel ratio of an air-fuel mixture supplied to combustion equipment, and particularly relates to an oxygen concentration battery using an oxygen ion-conducting solid electrolyte. The present invention relates to an air-fuel ratio detection element capable of detecting the air-fuel ratio of an air-fuel mixture, in which an element and an oxygen pump element are disposed opposite to each other with a gap therebetween, and the gap is communicated with a surrounding gas to be measured.

[Prior Art] Conventionally, for example, in combustion equipment such as internal combustion engines, in order to improve fuel efficiency and emissions, the oxygen concentration in the exhaust gas has been detected and the air-fuel mixture combusted in the combustion vessel has been adjusted to near the stoichiometric air-fuel ratio. There are devices that perform so-called feedback control. An oxygen sensor used in a rice seed control device to detect oxygen IIrti in the exhaust gas is composed of, for example, an ion-conductive solid electrolyte coated with a porous electrode layer, and is used to detect oxygen partial pressure in the exhaust gas and oxygen in the air. Generally, oxygen sensors whose output voltage changes in a switching manner around the stoichiometric air-fuel ratio of the air-fuel mixture are well-known, such as oxygen sensors that detect combustion conditions near the stoichiometric air-fuel ratio based on changes in electromotive force caused by differences in partial pressure. It is being

By the way, in recent years, it has become possible to not only simply control the air-fuel ratio of the air-fuel mixture to near the stoichiometric air-fuel ratio, but also to execute feedback control by changing the target air-fuel ratio according to the operating status of equipment, thereby improving fuel efficiency and emissions. Although it is considered that the conventional oxygen sensor described above can only detect the stoichiometric air-fuel ratio of the air-fuel mixture, it is difficult to adjust the air-fuel mixture to the desired air-fuel ratio. It was not possible to control the fuel ratio.

On the other hand, in recent years, in order to realize feedback control of the air-fuel ratio as described above, two elements each having electrode layers provided on both sides of the front side of a plate-shaped oxygen ion conductive solid electrolyte are arranged in parallel at intervals. A gap portion communicating with the surrounding gas to be measured is provided on the tip side, the pixel element is fixed, and one element is lll! An oxygen sensor has been proposed in which one element is a pump element and the other element is an M cumulative concentration battery element which operates based on the oxygen concentration difference between the surrounding atmosphere and the gap, and the sensor generates a signal according to the air-fuel ratio at least in the lean range of the air-fuel mixture. A device configured to be able to detect is being considered.

Problems to be Solved by the Invention However, in the case of a sensor, this oxygen is present not only in the lean region of the mixture, that is, when residual oxygen exists in the exhaust gas, but also in the rich region of the mixture, that is, when residual oxygen exists in the exhaust gas. Even when oxygen is present in only a trace amount, it reacts with 00% CO2, H2O, etc. in the exhaust gas, and a signal similar to that in the lean range is detected depending on the air-fuel ratio in the rich range, so it can be detected for one output value. It has been found that there is a problem in that the two air-fuel ratios correspond to each other.

[Means for Solving the Problems] The present invention employs the following technical means as a configuration of the invention in order to solve the above problems.

That is, the air-fuel ratio detection element of the present invention comprises an oxygen ion conductive solid electrolyte and a pair of electrodes that are permeable to oxygen gas and have different oxidation reaction catalytic strengths on the front and back surfaces of the oxygen ion conductive solid electrolyte. an oxygen pump element having a pair of electrodes that are permeable to oxygen gas on both sides of an oxygen ion conductive solid electrolyte; The present invention is characterized in that the electrodes are disposed facing toward the oxygen pump element, and the gap is communicated with the surrounding gas to be measured via a diffusion control section made of holes or porous material.

Oxygen concentration battery elements are manufactured under appropriate temperature conditions of an oxygen ion conductive solid electrolyte (for example, when the solid electrolyte is zirconia,
00℃ or higher), oxygen ions move in the solid electrolyte from a place on the surface of the solid electrolyte where the oxygen gas partial pressure is high to a place where the oxygen gas partial pressure is low, and an oxygen gas permeable electrode is attached to the solid electrolyte. This method takes advantage of the property that the difference in oxygen gas partial pressure between electrodes can be extracted as a voltage (electromotive force).

Here, if one of the electrodes of the oxygen concentration battery element is an electrode with a strong catalytic effect (cathode) for oxidation reactions and the other is an electrode with a weak catalytic effect (anode), unburned hydrocarbons will be present in the measured exhaust gas. In the presence of carbon monoxide and carbon monoxide (so-called rich conditions), the oxidation reaction is promoted at the cathode and oxygen is consumed, resulting in an oxygen gas partial pressure almost close to O.However, at the anode, the oxygen gas partial pressure is almost O. The pressure is almost the same as the exhaust gas above. Therefore, oxygen ions move from the anode side to the cathode side, and a voltage is output. Conversely, in a state where there is no or little unburned hydrocarbons and carbon monoxide (so-called lean), almost no oxygen is consumed at the cathode, so the oxygen gas partial pressure remains the same at both electrodes. , no voltage is output.

Oxygen pump elements utilize the property of an oxygen ion conductive solid electrolyte that oxygen ions move through the solid electrolyte by applying a voltage. Can be used as a pump.

In other words, since oxygen ions move from the lower electrode potential to the higher electrode potential, the oxygen gas partial pressure decreases on the lower electrode potential side, and conversely, the oxygen gas partial pressure increases on the higher electrode potential side. do. Furthermore, since the charge carrier of the solid electrolyte is oxygen ions, the amount of movement of oxygen ions can be adjusted by adjusting the amount of current flowing between the electrodes.

Furthermore, the electrode in the oxygen pump element of the air-fuel ratio detection element of the present invention that does not face the oxygen concentration cell element comes into contact with the surrounding atmosphere. This simplifies the structure of the air-fuel ratio detection element.

The solid electrolyte forming the oxygen pump element and oxygen concentration battery element must have the properties of an oxygen ion conductor, and a typical solid electrolyte is a solid solution of zirconia with itria or calcia; Solid solutions of cerium, thorium dioxide, and hafnium dioxide,
Perovskite oxide solid solutions, trivalent metal oxide solid solutions, and the like can be used as oxygen ion conductive solid electrolytes.

In addition, as a combination of electrode materials with different strengths of catalytic action for oxidation reactions, Pt has a strong catalytic action and A has a weak catalytic action.
u, similarly Pt and Ag, Pt and Pt +Au (
1-20%), Pt and Pt + Ru (1-20%>, p
Examples include t+Rh (1 to 15%) and Pt, pt and catalyst-poisoned Pt, and Pt with a semiconductive metal oxide added. These may be formed by making a paste with raw material powder as the main component, printing using thick film technology, and then sintering, or may be formed using thin film technology such as flame bombardment, chemical plating, or vapor deposition. However, in that case, it is more preferable to provide a porous protective layer of alumina, spinel, zirconia, mullite, etc. over the electrode using thick film technology.

Further, in order to impart oxidation reaction catalytic action to the protective layer of the electrode, Pt, Ril, etc. may be dispersed in alumina, spinel, zirconia, mullite, etc. used as the protective layer.

Communication between the gap and the surrounding gas to be measured can be achieved by making the gap a closed space that communicates with the surrounding gas to be measured only through a hole that is a diffusion restriction part, or by using the hole or the opposite end between the oxygen pump element and the oxygen concentration battery element. This is done by filling the end portion, which is open to the surrounding gas to be measured, with a porous material as a diffusion restriction portion.

[Effect] The operation of the air-fuel ratio detection element of the present invention will be explained.

First, when the air-fuel mixture is in a lean range, the air-fuel ratio detection element is placed in the exhaust gas, and a positive electrode is connected to the atmospheric side electrode of the oxygen pump element.
By applying a negative voltage to the electrode on the gap side, when the @ elemental gas present in the gap between the oxygen pump element and the oxygen concentration battery element is pumped out, it is connected to the outside of the oxygen concentration battery element, that is, the surrounding atmosphere. A difference in oxygen gas concentration occurs due to the oxygen diffusion limiting action of the portion communicating with the gap. This concentration difference generates an electromotive force in the oxygen concentration battery element.

The electromotive force becomes constant when the amount of oxygen gas that diffuses and flows into the gap in a limited manner and the amount of oxygen gas that is pumped out from the gap by the oxygen pump element reach an equilibrium. Then, by changing the amount of current (pump current) flowing through the oxygen pump element so that this electromotive force is maintained at a predetermined constant value, at a constant temperature, the amount of current changes depending on the content of oxygen gas in the surrounding atmosphere. The oxygen gas concentration can be determined from the pump current.

Next, when the air-fuel ratio detection element is put into the exhaust gas when the air-fuel mixture is on the rich side, the oxygen concentration battery element operates the oxygen pump element between the two electrodes to increase the partial pressure of oxygen gas. Since electromotive force is generated even without causing a difference,
In order to keep the electromotive force of the oxygen concentration battery element constant, the pump current flowing through the oxygen pump element is very small, or rather the direction of the pump current is reversed. That is, at the cathode of the oxygen concentration cell element, oxygen is consumed by unburned hydrocarbons and carbon monoxide in the exhaust gas, so the difference in oxygen gas partial pressure between the cathode side and the anode side becomes too large. As a result, the electromotive force becomes larger than a predetermined value. Therefore, in order to maintain the electromotive force at a predetermined value, it is necessary to send oxygen into the gap using an oxygen pump element. At this time, the pump current is in the opposite direction to the pump current in the lean region, and the pump current corresponds to the amount of unburned hydrocarbons and carbon monoxide in the exhaust gas. Therefore, in the rich region, the pump current corresponds to the air-fuel ratio.

That is, when adjusting the pump current flowing to the pump element so that the electromotive force of the oxygen ill cell element of the air-fuel ratio detection element is maintained at a predetermined constant value, the pump current corresponds to the air-fuel ratio.

Note that when the air-fuel ratio detection element is used by a control circuit that controls the pump current only in one direction, the pump current corresponds to the air-fuel ratio in the lean region, and no pump current is output in the rich region. That is, it is possible to eliminate signals that do not correspond to the air-fuel ratio in the rich region.

Furthermore, the air-fuel ratio can be determined from the electromotive force when the pump current is kept constant. Here, the direction of the pump current is defined as positive when oxygen is pumped out from the gap.

When the pump current is O, the point at which the value of the electromotive force changes rapidly is approximately the stoichiometric air-fuel ratio (A/F-14°6).

Furthermore, when the pump current is negative, that is, when oxygen is supplied to the gap, the change point moves to the rich region.

Furthermore, when the pump N flow is positive, the change in electromotive force is smoother than when the pump current is 0 or negative, but the point of change moves to the lean region.

The amount of movement of this change point corresponds to the pump current.

[Example] A first embodiment of the present invention will be described using the drawings.

Fig. 1 is an exploded perspective view of the air-fuel ratio detection element of this embodiment;
The figure shows its end view. Here, 1 is an air-fuel ratio detection element of this embodiment, 2 is an oxygen pump element, and 3 is an oxygen concentration battery element.

The main body of the oxygen pump element 2 is a rectangular sintered plate-like body made of an oxygen ion conductive solid electrolyte. Tip side 2 of pump element 2
In a, there are electrodes 4 made of a heat-resistant metal layer at opposing positions on the front and back surfaces and at a position slightly away from the three edges on the tip side.
．． 5 is provided in a rectangular shape. A drawer 1!4a made of a heat-resistant metal layer is provided in a band shape extending straight toward the base side 2b of the plate-like body from one of the two corners in the base side direction of one of the rectangular electrodes 4. Similarly, out of the two corners of the other rectangular electrode 5 in the direction toward the base, a lead wire 5a is provided in the shape of a band extending straight from the corner opposite to the electrode 4 toward the base side 2b of the plate-like body. The lead wire 5a is electrically connected to the lead-out portion 5b on the opposite side of the base side 2b through a through hole 5d penetrating the front and back sides of the plate-like body. The lead wire 4a forms a lead-out portion 4b on the base side 2b, and as a result, the lead-out portions 4b, 5 of the two electrodes 4.5 are formed on the same surface.
b will be placed.

Like the pump element 2, the oxygen concentration battery element 3 also mainly consists of a rectangular sintered plate-like body of an oxygen ion conductive solid electrolyte. On the front side 3a of the oxygen concentration battery element 3, an electrode (cathode) 6 made of an electrode material with a strong oxidation reaction catalytic effect is placed on the back side, and an electrode with a weak oxidation reaction catalytic effect on the front side, at opposing positions on the front and back sides. Each electrode (anode) 7 made of a material is provided in a rectangular shape. A lead wire 6a made of a heat-resistant metal layer from one of the two corners in the original direction of the cathode 6
is provided in a band shape extending straight toward the base side 3b of the plate-like body. Similarly, out of the two corners of the anode 7 in the direction toward the base side, a lead wire 7a is provided in a band shape extending straight from the corner opposite to the cathode 6 toward the base side 3b of the plate-like body.

The lead wire 6a is electrically connected to the lead-out portion 6b on the opposite side of the base side 1b through a through hole 6d penetrating the front and back sides of the plate-like body. The lead wire 7a forms a take-out part 7b on the base side 3b, and as a result, on the same plane,
Two take-out portions 6b and 7b of the electrodes 6.7 are provided.

The solid electrolyte forming each plate-like body of the oxygen pump element 2 and the oxygen concentration battery element 3 must have the properties of an oxygen ion conductor, and a typical example is a solid solution of zirconia with ittria or calcia. In addition, solid solutions of cerium dioxide, thorium dioxide, hafnium dioxide, perovskite oxide solid solutions, trivalent metal oxide solid solutions, etc. can be used as oxygen ion conductive solid electrolytes.

Cathode 6 and 7 nodes 7 formed on the surface of each plate-shaped body
N poles other than 4.5, lead wires 4a, 5a, 5a,
7a and the take-out parts 4b, 5b, 6b, and 7b are made of heat-resistant metal layers, mainly Pt, RU, Pd, Rh, and I.
A paste of R, Au, Ag, etc. is deposited using methods such as printing sintering, flame spraying, chemical plating or vapor deposition. In addition, the materials of the cathode 6 and anode 7 formed on the front and back surfaces of the oxygen concentration battery element are Pt for the cathode 6 and PT+ for the anode 7.
Although Au was used, Pt and A! It, Pi and All,
pt and Pt +Ru, Pt +Rh and Pt, Pt
Combinations of Pt with catalyst poisoning, Pt with Pt and semiconductive metal oxide added, etc. are also formed in the same manner as the other electrodes.

These may be manufactured for each element, but in general, in consideration of productivity, a large number of metal pastes for electrodes etc. are printed at the same time on a large raw ceramic sheet of solid electrolyte before sintering, and then It is advantageous to adopt a method in which each element is cut out and fired.

Next, in order to assemble the pixel element 2.3 into an integrated air-fuel ratio detection element 1 having a gap 9, the outer shape is the same as that of the pixel element, and the electrode 5.6 of the pixel element is connected to the pixel element 2.3. A corresponding part is punched out as a gap 9, and a hole 12 is formed on the tip side.
This is done by bonding the spacer 11 having the following properties to the pixel elements using, for example, a heat-resistant inorganic adhesive so that the spacer 11 is sandwiched between the pixel elements.

The air-fuel ratio detection element 1 of this embodiment is, for example, an oxygen pump element 2 in order to control the output voltage of an oxygen concentration battery element 3 at a constant level.
The present invention can be used in an air-fuel ratio measuring device configured to control the pump current flowing through the air-fuel ratio and extract an air-fuel ratio signal according to the air-fuel ratio of the air-fuel mixture.

FIG. 5 shows the relationship between the air-fuel ratio signal obtained from the pump current and the air-fuel ratio when used in the above air-fuel ratio measuring device. With an appropriate circuit configuration, it is possible to measure the air-fuel ratio in the air-fuel ratio range from lean to rich (approximately A/F-10>) as shown in FIG.

This embodiment uses oxygen in the surrounding atmosphere as the oxygen source for the oxygen pump element. Therefore, in the rich region, approximately A/
It can only measure up to about F=10. However, in normal internal combustion engine operation, the air-fuel ratio is about A/F-13 even when the accelerator is depressed, which is the richest. Therefore, the air-fuel ratio detection element of this embodiment can measure the entire practical air-fuel ratio range.

Further, in the air-fuel ratio detection element of this embodiment, when the pump current is 0, the air-fuel ratio is richer than the stoichiometric air-fuel ratio point, and the oxygen concentration in the measured gas is extremely low, and the combustible gas increases rapidly. The electromotive force of the oxygen concentration cell element 3 increases stepwise, and the current dyeing can be used as a means for detecting the stoichiometric air-fuel ratio point. By changing the pump current, it is possible to move the stepwise changing point, which means that the air-fuel ratio, which is the center of feedback control, can be shifted from the stoichiometric air-fuel ratio point by changing the pump current. means.

FIG. 4 is a diagram showing how the relationship between the air-fuel ratio (A/F) and the electromotive force E changes depending on the pump current 1p. However, the direction of the pump current is defined as the direction in which oxygen is pumped out from the gap 9.

When the air-fuel ratio detection element 1 is used as described above, it can be used in an air-fuel ratio control device that uses a conventional oxygen sensor whose output changes rapidly at the stoichiometric air-fuel ratio point, without changing its configuration, and can be used to What! ! 111611 can be performed.

If a heating element for heating the element is provided near the air-fuel ratio detecting element 1 of this embodiment, temperature compensation can be performed when measuring the air-fuel ratio, allowing more precise and accurate measurement.

Furthermore, this embodiment is easy to manufacture because it has a simple groove structure in which a spacer is sandwiched between two solid-core solute plates.

[Effects of the Invention] As detailed above, the air-fuel ratio detection element of the present invention has an IS! 1
It is a combination of a cumulative concentration battery element and an oxygen pump element.

This air-fuel ratio detection element is designed to keep the electromotive force of the oxygen concentration cell element constant! When used in an air-fuel ratio measuring device configured to control the pump current of a pump element and obtain an air-fuel ratio signal from the pump current, the air-fuel ratio is uniquely determined from the air-fuel ratio output signal. Therefore, a sensor and a circuit for determining the lean range and the rich range are unnecessary, and the configuration of the air-fuel ratio detection device can be made very simple.

In addition, when used in an air-fuel ratio measuring device configured to obtain an air-fuel ratio signal from the electromotive force of an oxygen concentration cell element while keeping the pump current of the oxygen pump element constant, the air-fuel ratio determined by the pump current may be measured. It is possible to output a clear air-fuel ratio signal with a boundary between

[Brief explanation of drawings]

Fig. 1 is an exploded perspective view of an air-fuel ratio detection element according to a first embodiment of the present invention, Fig. 2 is an end port thereof, and Fig. 3 is a diagram of the relationship between the air-fuel ratio signal obtained from the pump current and the air-fuel ratio. , FIG. 4 is a relationship diagram showing changes in electromotive force and air-fuel ratio due to the pump current. 1... Air-fuel ratio detection element 2... Oxygen pump element 3... Oxygen concentration battery element 4.5... Electrode 6... Electrode (cathode) 7... Electrode (anode) 9... gap area

Claims (1)

[Scope of Claims] 1. An oxygen concentration battery element having a pair of electrodes that are permeable to oxygen gas on the front and back surfaces of an oxygen ion conductive solid electrolyte and have different strengths of oxidation reaction catalytic action on the front and back surfaces. an oxygen pump element having a pair of electrodes that are permeable to oxygen gas on both sides of an oxygen ion conductive solid electrolyte; An air-fuel ratio detection element characterized in that the air-fuel ratio detection element is arranged facing toward the element side and the gap is communicated with the surrounding gas to be measured via a diffusion control section made of holes or porous material. 2. The air-fuel ratio detection element according to claim 1, wherein the electrodes of the oxygen concentration battery element and the oxygen pump element are composed of two layers: a conductive layer and a protective layer. 3. The air-fuel ratio detection element according to claim 2, in which only the protective layer of the electrode in the oxygen concentration battery element has a strength or weakness of oxidation reaction catalytic action.