JPH067120B2 - Lean air-fuel ratio detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a lean region air-fuel ratio detection device for detecting the air-fuel ratio of an air-fuel mixture supplied to a combustion device, and particularly to oxygen using an oxygen ion conductive solid electrolyte. The present invention relates to a lean region air-fuel ratio detection device for detecting an air-fuel ratio in a lean region of an air-fuel mixture by using an air-fuel ratio detection device in which a concentration cell element and an oxygen pump element are arranged to face each other with a gap.

[Prior Art] Conventionally, for example, in combustion equipment such as internal combustion,
In order to improve fuel economy and emission, there is a so-called feedback control in which the oxygen concentration in the exhaust gas is detected and the air-fuel mixture burned in the combustion container is controlled near the stoichiometric air-fuel ratio. Then, as a 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, in addition to simply controlling the air-fuel ratio of the air-fuel mixture in the vicinity of the theoretical air-fuel ratio, by performing feedback control by changing the target air-fuel ratio in the lean region, for example, according to the operating state of the equipment, Although it is considered to improve the fuel economy and emission and improve the drivability of the equipment, in the above conventional oxygen sensor, since it is only possible to detect the stoichiometric air-fuel ratio of the air-fuel mixture, It was not possible to control the air-fuel mixture to the desired air-fuel ratio.

Therefore, recently, in order to realize the feedback control of the air-fuel ratio as described above, an element provided with electrode layers on both front surfaces of a plate-shaped oxygen ion conductive solid electrolyte is parallelized with two sheets spaced apart. An oxygen concentration battery element in which a gap is provided on the front side to fix the two elements, one element is an oxygen pump element, and the other element is activated by the difference in oxygen concentration between the ambient atmosphere and the gap. Oxygen sensor (air-fuel ratio sensor)
And so that the electromotive force of the oxygen concentration battery element becomes constant,
The oxygen concentration (lean region air-fuel ratio) is combined with a control circuit for adjusting the pump current of the oxygen pump in the direction of pumping oxygen from the gap and obtaining an oxygen concentration (air-fuel ratio) signal from the pump current at that time. A fuel ratio detection device has been proposed.

[Problems to be Solved by the Invention] However, in the case of this oxygen concentration detection device, not only when there is residual oxygen in the lean region of the air-fuel mixture, that is, in the exhaust gas,
Even in the rich region of the air-fuel mixture, that is, even when a small amount of residual oxygen is present in the exhaust gas, it reacts with CO, CO ₂ , H ₂ O, etc. in the exhaust gas, and a signal similar to that in the lean region is output in the rich region. Since the detection is performed according to the fuel ratio, two air-fuel ratios correspond to one output value (2
Value output problem).

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

That is, the lean air-fuel ratio detection device of the present invention includes an oxygen concentration battery element having a pair of electrodes permeable to oxygen gas on both sides of an oxygen ion conductive solid electrolyte and an oxygen ion conductive solid electrolyte on both sides. An oxygen pump element having a pair of electrodes permeable to oxygen gas is arranged so that the electrodes on one side face each other through a gap, and the gap is arranged with respect to the ambient atmosphere which is the gas to be measured. An air-fuel ratio detection element that is allowed to flow in a gas diffusion limited manner, and the pump current of the oxygen pump element is adjusted in the direction of pumping oxygen from the gap so that the electromotive force of the oxygen concentration cell element becomes a constant value. In a lean region air-fuel ratio detection device including a control circuit that obtains an air-fuel ratio signal from the pump current value at that time, an oxidation catalyst action of the electrode on the gap side of the pair of electrodes of the oxygen concentration battery element is provided. The other It is characterized in that the fixed value is selected to be a value smaller than the electromotive force generated in the rich region of the oxygen concentration battery element in which the catalytic action is set to be stronger than the oxidation catalytic action of the electrode.

[Operation] (1) The operation of the oxygen concentration battery element used in the present invention will be described.

Oxygen ion conductive solid electrolytes are solidified from a place where the oxygen gas partial pressure is high on the surface of the solid electrolyte to a place where the oxygen gas partial pressure is low under appropriate temperature conditions (for example, 400 ° C or higher when the solid electrolyte is zirconia). Oxygen ions move in the electrolyte. Therefore, by attaching an oxygen gas permeable electrode to the solid electrolyte, the difference in oxygen gas partial pressure between the electrodes can be taken out as a voltage (electromotive force).

Here, if one of the electrodes has a strong catalytic action for the oxidation reaction and the other has a weak catalytic action as in the present invention, the following phenomenon occurs.

(A) That is, in a state where unburned hydrocarbons and carbon monoxide are present in the exhaust gas to be measured, that is, in a so-called rich region, the oxidation reaction is promoted and non-equilibrium free oxygen is consumed in the electrode having a strong catalytic action. By the equilibration, the partial pressure of free oxygen gas becomes almost 0, but in the other electrode with weak catalytic action, the partial pressure of free oxygen is almost the same as the partial pressure of non-equilibrium free oxygen gas of the exhaust gas. . For this reason,
A voltage is output as oxygen ions try to move from the electrode side having a weak catalytic action to the electrode side having a strong catalytic action.

(B) On the other hand, in the state where there is no or a small amount of unburned hydrocarbons and carbon monoxide, that is, in the so-called lean region, there is no component or a small amount of components that undergo the oxidation reaction, so the oxidation reaction no longer has a strong catalytic action. It has nothing to do with it. In other words, since oxygen is hardly consumed regardless of the strength of the catalytic action, the partial pressure of oxygen gas does not change at either electrode, which is different from the case of an oxygen concentration cell device having a pair of electrodes with the same catalytic action. Similarly, no voltage is output.

(2) Next, the function of the oxygen pump element used in the present invention will be described.

As described above, the oxygen ion conductive solid electrolyte can extract the difference in oxygen gas partial pressure as voltage (electromotive force). On the contrary, the oxygen ion conductive solid electrolyte is one in which oxygen ions move in the solid electrolyte when a voltage is applied.By attaching an oxygen gas permeable electrode to the solid electrolyte, it is used as an oxygen pump. can do. That is, oxygen ions can be moved from the lower potential of the voltage applied to the electrode to the higher potential, the oxygen gas partial pressure is reduced on the side of low electrode potential, and conversely on the side of high electrode potential. The partial pressure increases. 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.

(3) Next, the air-fuel ratio detection element in which the oxygen concentration cell element and the oxygen pump element are arranged with a gap, that is, the electrode having a strong catalytic action for the oxidation reaction of the oxygen concentration cell element is directed toward the gap side. The operation of the lean region air-fuel ratio detecting device of the present invention using the air-fuel ratio detecting elements that are arranged opposite to each other will be described.

(A) First, when the air-fuel mixture is in the lean region, the oxygen partial pressure in the electrode atmosphere of the oxygen concentration battery element becomes irrelevant to the strength of the catalytic action of the electrodes as described above. Similar to the case of the conventional air-fuel ratio detecting device in which the strength of action does not change, the air-fuel ratio signal of the lean region air-fuel ratio detecting device of the present invention is determined corresponding to only the mixture ratio of the air-fuel mixture. Therefore, the operation is the same as that of the conventional device and is as follows.

That is, by applying a positive voltage to the outer electrode and a negative voltage to the inner electrode of the oxygen pump element by the control circuit,
Oxygen ions move from the inner electrode to the outer electrode in the solid electrolyte of the oxygen pump element, and oxygen gas existing in the gap between the oxygen pump element and the oxygen concentration cell element is pumped to the outside of the oxygen pump element. When oxygen gas is pumped out from the gap as described above, a difference in oxygen gas concentration occurs due to the oxygen diffusion limiting action outside the oxygen concentration battery element, that is, between the ambient atmosphere and the gap. Due to this concentration difference, an electromotive force V _S is generated in the oxygen concentration battery element.

The electromotive force V _S is the oxygen partial pressure in the gap portion in which the amount of oxygen gas that diffuses and flows into the inside from the end portion of the gap portion in a limited manner and the oxygen gas pumped out from the gap portion to the outside by the oxygen pump element are in equilibrium. It will be constant when it reaches.

Therefore, if the amount of current (pump current) supplied to the oxygen pump element side is changed so that the electromotive force V _S is maintained at a predetermined constant value V ₀ , the amount of current at constant temperature is equal to that in the surrounding atmosphere. Since it can be made almost linearly proportional to the oxygen gas content, the oxygen gas concentration in the lean region can be obtained.

(B) Next, when the air-fuel mixture is in a rich region, both electrodes of the oxygen concentration battery element have strong and weak catalytic action as described above. Therefore, the oxygen pump element is activated to induce the oxygen gas partial pressure difference. Even without it, the electromotive force V _S is generated between both electrodes of the oxygen concentration battery element.

In the present invention, the constant value V ₀ (which is a target value for control) is smaller than the value V _S0 of the electromotive force V _S generated when the oxygen concentration battery element is exposed alone in the rich region. Has been selected for.

Therefore, when the air-fuel mixture is in the rich region, in order to bring the oxygen concentration battery element to the above-mentioned constant value V ₀ , the direction of the pump current flowing through the oxygen pump element is reversed (direction in which oxygen is pumped into the gap). This must be done, but this is different from the direction of the pump current that can flow from the control circuit (the direction in which oxygen is pumped out of the gap), so the pump current does not actually flow.

Further, when the air-fuel mixture has the stoichiometric air-fuel ratio or an air-fuel ratio near the stoichiometric air-fuel ratio, the electromotive force V _S and the constant value V ₀ become equal to each other, so that the pump current becomes zero.

(C) As described above, the pump current flowing in the pump element side is maintained in a predetermined direction (from the gap portion) so that the electromotive force of the oxygen concentration battery element of the air-fuel ratio detecting element is maintained at a constant value V ₀ predetermined according to the present invention. When adjusted in the direction of pumping oxygen),
The pump current stops flowing in the rich region (and near the stoichiometric air-fuel ratio point). On the other hand, in the lean region, the pump current always correctly corresponds to only the air-fuel ratio, and the problem of binary output is solved.

(4) The solid electrolyte forming the oxygen pump element and the oxygen concentration cell element in order to exhibit the above-mentioned action needs to have the property of an oxygen ion conductor, and a solid solution of zirconia with yttria or calcia. Is typical, other 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 oxygen ion conductive solid electrolytes.

Further, as a combination of electrode materials having different catalytic action strengths of oxidation reaction, Pt having a strong catalytic action and A having a weak catalytic action are used.
u, and the like below, Pt and Ag, Pt and Pt + Au (1-2
0%), Pt and Pt + Ru (1 to 20%), Pt + Rh
(1 to 15%) and Pt, Pt and Pt poisoned with catalyst, Pt
And Pt etc. ignited a semiconductive metal oxide. These may be formed by pasting raw material powder as a main component, printing using a thick film technique, and then sintering, or may be formed using a thin film technique such as flame spraying or chemical plating or vapor deposition. However, in that case, it is more preferable to provide a porous protective layer of alumina, spinel, zirconia, mullite or the like on the electrode by using a thick film technique.

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

[Embodiment] An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of an air-fuel ratio detecting element in a lean region air-fuel ratio detecting device of an embodiment, FIG. 2 is a plan view thereof, FIG. 3 is a rear view thereof, and FIG. . Here, 1 is a lean region air-fuel ratio detection device of the present invention, 2 is an oxygen pump element, and 3 is an oxygen concentration cell element.

The main body of the oxygen pump element 2 is a rectangular sintered plate-shaped body of an oxygen ion conductive solid electrolyte. Pump element 2 front side 2
The electrodes 4 and 5 made of a heat-resistant metal layer are provided in a square shape on the surface a at positions facing each other on the front and back sides and slightly away from the three edges on the front side. A lead wire 4a made of a heat-resistant metal layer from one of the two corners of the rectangular electrode 4 in the original direction is provided in a strip shape that extends straight to the original side 2b of the plate-shaped body. Similarly, of the two corners of the other rectangular electrode 5 in the direction of the original side, a lead wire 5a is provided in a strip shape extending straight from the corner opposite to the electrode 4 to the original side 2b of the plate-shaped body. The lead wire 5a is electrically connected to the take-out portion 5b on the opposite side through a through hole 5d penetrating the front and back of the plate-shaped body on the base side 2b. The lead wire 4a forms a lead-out portion 4b on the original side 2b, and as a result, lead-out portions 4b, 5 of the two electrodes 4, 5 are formed on the same surface.
b will be provided.

Like the pump element 2, the oxygen concentration cell element 3 is also mainly composed 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 having a strong catalytic action for the oxidation reaction is provided on the back side at a position facing the front and back sides, and a catalytic action for the oxidation reaction is weak on the front side. Each electrode (anode) 7 made of an electrode material is provided in a rectangular shape. A lead wire 6a made of a heat-resistant metal layer from one of the two corners of the cathode 6 in the direction of the original side is provided in a strip shape extending straight to the original side 3b of the plate-shaped body. Similarly, a lead wire 7a is provided in a strip shape extending straight from the corner opposite to the cathode 6 among the two corners of the anode 7 toward the base side 3b of the plate-shaped body. The lead wire 6a is electrically connected to the take-out portion 6b on the opposite side through a through hole 6d penetrating the front and back of the plate-shaped body on the original side 1b. The lead wire 7a forms the lead-out portion 7b on the original side 3b, and as a result, the lead-out portions 6b and 7b of the two electrodes 6 and 7 are arranged on the same surface.

A solid solution of zirconia and yttria was used here as the solid electrolyte forming each plate of the oxygen pump element 2 and the oxygen concentration battery element 3.

Cathode 6 and anode 7 formed on the surface of each plate
Electrodes 4 and 5, other than lead lines 4a, 5a, 6a and 7a
The take-out portions 4b, 5b, 6b and 7b are made of a heat-resistant metal layer. Here, a paste mainly made of Pt is print-printed on a green solid electrolyte sheet, and then is integrally sintered by a method of simultaneous integral sintering. did. Further, as the material of the cathode 6 and the anode 7 formed on the front and back surfaces of the oxygen concentration cell device, here, Pt was used as the cathode 6 and Pt + Au was used as the anode 7.

These may be manufactured for each element, but generally, in consideration of productivity, a large number of metal pastes for electrodes and the like are simultaneously printed and printed on a large raw ceramic sheet of a solid electrolyte before sintering, and thereafter, It is advantageous to employ a method of cutting and firing each element.

Next, in order to assemble the two elements 2 and 3 into the integrated lean region air-fuel ratio detection device 1 having the gap portion 9, a heat-resistant inorganic adhesive agent or the like is used on the original sides 2b and 3b of both elements 2 and 3, respectively. It is made by laminating in parallel at regular intervals.

The following method, for example, is used to bond the two elements in parallel at a constant interval. First, an appropriate amount of a heat-resistant inorganic adhesive such as a ceramic adhesive is applied to the original adhesive surface of one element. Next, a spacer with a uniform thickness, for example, a gauge or a paper of 0.01 to 0.05 mm in thickness, vinyl, aluminum foil, etc., is laid one on top of the other and is pressed against the front side of the two elements with an adhesive. The elements are pressed and spread over the entire bonding surface on the original side, excess adhesive that has run off is removed, and then heat treatment is performed to harden the adhesive.

The interval between the above two elements depends on the element shape and the electrode area, but by appropriately selecting the output of the oxygen concentration battery element,
For example, in the case of 100 mm ² square electrode, 0.01-0.5 mm
It is possible to use the pump current of the oxygen pump element in a relatively large current range of 1mA to 100mA, and it is rather wide.
When used in a relatively current range of 0.1 mA to 10 mA, it can be changed as necessary, such as making it slightly narrower. Here, the gap size was 0.1 mm. The oxygen concentration that can be detected by these current regions can be from 0.1% or less to about 10%. Therefore, the air-fuel ratio detecting element of the present embodiment has a feature that the element spacing can be easily designed in the manufacturing process to a desired width by simply changing the thickness of the spacer.

According to the present invention, the air-fuel ratio detecting element 1 adjusts the magnitude of the pump current in a predetermined direction flowing to the oxygen pump element 2 in order to control the output voltage of the oxygen concentration cell element 3 at a constant level, and the current value at that time is a mixture gas. It is used in combination with a control circuit designed to extract an air-fuel ratio signal according to the air-fuel ratio of.

FIG. 5 shows a control circuit of the apparatus of this embodiment, which has terminals a, b,
c and d are connected to the anode 7 and cathode 6 of the oxygen concentration battery element 3 and the electrodes 5 and 4 of the oxygen pump element 2, respectively. Then, the voltage V _S output from the oxygen concentration cell element 3 according to the oxygen concentration in the exhaust gas or CO, CO ₂ , H ₂ O in the exhaust gas is detected through the resistor R1, and the operational amplifier (hereinafter, referred to as
An amplifier 4 which is mainly composed of OP1)
The voltage V _S is amplified by 1 to obtain the voltage nV _S amplified by a predetermined multiple n (for example, 5 times). Next, this voltage nV _S is input to the integrating circuit 42 mainly composed of the operational amplifier OP2, and the voltage determined by the Zener diode D1 is
Transistor TR1 in pump current control circuit 43 including next transistors TR1 and TR2 is integrated as shown in FIG. 6 with reference to a predetermined voltage V ₀ obtained by dividing the voltage by 1 / n using variable resistor VR1. base voltage of, i.e. is taken out as a control voltage V _B of the pump current in a predetermined direction (way to pump oxygen from the gap). Incidentally, + B shown in the drawing represents the battery voltage.

Here, as the predetermined constant voltage value V ₀ , a value of 40 mV, which is smaller than the electromotive force value V _S0 (normally 100 mV) obtained when the oxygen concentration battery element is exposed to the atmosphere in the rich region, is selected.

When the pump current flowing through the oxygen pump element 2 is controlled in this manner, the concentration of oxygen existing in the gap between the oxygen concentration cell element 3 and the oxygen pump element 2 is controlled, and the oxygen concentration in the gap between the stoichiometric air-fuel ratio and the lean region is controlled. The output voltage of the oxygen concentration battery element 3 can be controlled to be substantially constant (40 mV).

Therefore, by extracting the pump current of the oxygen pump element 2 as the air-fuel ratio signal V ₁ via the resistor R2, as shown in FIG. 7, the air-fuel ratio corresponding to the air-fuel ratio is increased from approximately the theoretical air-fuel ratio to the lean region. The fuel ratio signal V ₁ is output.

That is, in the rich region, the pump current no longer flows in the direction of pumping oxygen from the gap,
The sensor output is almost uniquely determined for the air-fuel ratio from the theoretical air-fuel ratio to the lean range. In FIG. 7, λ = 1 represents the theoretical air-fuel ratio.

Further, by providing a heating element for heating the air-fuel ratio detecting element of the lean region air-fuel ratio detecting device of the present invention, temperature compensation can be performed during the air-fuel ratio measurement, and more precise and accurate measurement becomes possible.

[Effects of the Invention] As described in detail above, the lean region air-fuel ratio detection device of the present invention includes the oxygen concentration battery element and the oxygen pump element having the pair of electrodes having different catalytic strengths of the oxidation reaction. Control that adjusts only the magnitude of the pump current of the oxygen pump element in a predetermined direction so that the electromotive force of the oxygen concentration cell element is maintained at a constant value by using the air-fuel ratio detection element combined in the arrangement. Since the air-fuel ratio signal is obtained from the pump current by controlling the circuit, the lean region air-fuel ratio is uniquely and always accurately determined from the air-fuel ratio signal.

That is, the value V _S0 of the electromotive force V _S generated between both electrodes when an oxygen concentration battery element having a pair of electrodes different in the strength of the catalytic action of the oxidation reaction is placed in a rich atmosphere and the control target set value in the control circuit. as long as the relationship to V ₀ is maintained at V _S0> V _0, also happened by the use deterioration such as the value of V _S0 responsible for such strength of the oxidation catalytic action of the electrode changes varies The output of the air-fuel ratio signal in the rich region is suppressed without being affected by that, and the air-fuel ratio which always corresponds to the lean region air-fuel ratio is always accurate in the lean region except near the stoichiometric air-fuel ratio. There is an advantage that a signal is output.

In addition, a sensor having a complicated structure for distinguishing the lean region and the rich region, a special circuit, and software are unnecessary.
That is, the control circuit has an advantage that the conventional lean region air-fuel ratio detecting device having a relatively simple structure can be used as it is.

Further, since the air-fuel ratio detecting element does not need to be provided with a reference oxygen partial pressure source, the size is the same as that of the conventional one, and the structure is simple and the manufacturing is easy.

[Brief description of drawings]

FIG. 1 is a perspective view of an air-fuel ratio detecting element of a lean region air-fuel ratio detecting device of an embodiment, FIG. 2 is its plan view, FIG. 3 is its rear view, and FIG. 4 is its 1-1 cross-sectional view. FIG. 5 is a control circuit diagram of the lean air-fuel ratio detecting device of the embodiment, and FIG. 6 is an integrating circuit 4
2 is a voltage waveform diagram showing the control voltage V _B for controlling the pump current, which is output from No. 2, and FIG. 7 is a graph showing the air-fuel ratio signal V ₁ detected by the lean region air-fuel ratio detecting device of the embodiment. 1 ... Air-fuel ratio detecting element 2 ... Oxygen pump element 3 ... Oxygen concentration battery element 4, 5 ... Electrode 6 ... Electrode (cathode) 7 ... Electrode (anode)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryoki Kawaji 14-18 Takatsuji-cho, Mizuho-ku, Nagoya, Aichi Nihon Special Ceramics Co., Ltd. (56) Reference JP-A-58-153155 (JP, A) ) JP-A-52-130391 (JP, A) JP-A-59-67455 (JP, A) JP-A-58-27055 (JP, A)

Claims (3)

[Claims] 1. An oxygen concentration cell device having a pair of electrodes permeable to oxygen gas on both sides of a solid electrolyte having oxygen ion conductivity.
An oxygen pump element having a pair of electrodes permeable to oxygen gas is provided on both sides of an oxygen ion conductive solid electrolyte, and the electrodes of each one are arranged so as to face each other with a gap portion between them, and
An air-fuel ratio detecting element that is allowed to flow through the gap in a gas diffusion limited manner with respect to the ambient atmosphere as the gas to be measured, and a pump of the oxygen pump element so that the electromotive force of the oxygen concentration battery element becomes a constant value. A lean region air-fuel ratio detection device comprising a control circuit for adjusting an electric current in a direction of pumping oxygen from the gap and obtaining an air-fuel ratio signal from a pump current value at that time, wherein a pair of electrodes of the oxygen concentration battery element Of the electrode on the side of the gap is stronger than that of the electrode on the other side, and is smaller than the electromotive force generated in the rich region of the oxygen concentration battery element in which the catalytic action is set in such a manner. A lean region air-fuel ratio detection device characterized in that the above-mentioned constant value is selected as the value. 2. The lean air-fuel ratio detection device 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 lean region air-fuel ratio detection device according to claim 2, wherein the catalytic action of the oxidation reaction is strong and weak only in the protective layer of the electrode in the oxygen concentration battery element.