JPH0641931B2 - Air-fuel ratio detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an air-fuel ratio detection device for detecting an air-fuel ratio of a fuel mixture supplied to various combustion devices based on the oxygen concentration in exhaust gas.

[Prior Art] As an air-fuel ratio sensor conventionally used in this type of device, for example, Japanese Patent Application No. 59-261314 or Japanese Patent Application No. 60
No. 137586, two detection elements provided with porous electrodes on both sides of a plate-like oxygen ion conductive solid electrolyte, one electrode of each detection element is a measurement gas in which diffusion of exhaust gas is restricted. An air-fuel ratio sensor in which the electrode on one side of the detection element that is not in contact with the measurement gas chamber is in contact with the reference oxygen chamber whose inside is maintained at a predetermined oxygen concentration. There is.

In this type of sensor, the detection element on the side in contact with the reference oxygen chamber is usually operated as an oxygen concentration battery element, and the other detection element is operated as an oxygen pump element, so that the voltage generated across the oxygen concentration battery element becomes a predetermined value. A detection circuit configured to bidirectionally control the current flowing through the oxygen pump element, detect the current, and output the detection result as an air-fuel ratio signal representing the air-fuel ratio is provided.

That is, if the oxygen concentration in the reference oxygen chamber is kept constant by introducing the atmosphere or by introducing a constant current to the oxygen concentration battery element to introduce oxygen in the measurement gas chamber, the oxygen concentration battery element will be A voltage is generated according to the ratio between the oxygen partial pressure and the oxygen partial pressure in the measurement gas chamber, so that the oxygen pump element is adjusted so that this voltage becomes constant, that is, the oxygen partial pressure in the measurement gas chamber becomes a predetermined value. If oxygen is exchanged between the exhaust gas and the measurement gas chamber by controlling the flowing current bidirectionally, the current flowing in the oxygen pump element changes according to the partial pressure of oxygen in the exhaust gas, and this current value changes to the air-fuel ratio. The corresponding air-fuel ratio signal can be obtained.

Therefore, the air-fuel ratio signal obtained by this type of air-fuel ratio detecting device continuously changes according to the oxygen concentration in the exhaust gas, that is, the air-fuel ratio, as shown in FIG. It can be used for an air-fuel ratio control device that controls the air-fuel ratio from a lean to rich desired value according to the operating state of the device.

[Problems to be Solved by the Invention] By the way, an air-fuel ratio control device of this type is usually provided in order to improve exhaust gas purification efficiency by a three-way catalyst like an air-fuel ratio control device for an internal combustion engine for a vehicle. The air-fuel ratio is controlled to the stoichiometric air-fuel ratio, and during light load operation that does not require much engine output or high load operation that requires large engine output, the air-fuel ratio is controlled to a specified value on the lean or rich side accordingly. Some are configured to. This type of control device requires an air-fuel ratio detection device that not only can detect the air-fuel ratio over a wide range but also has a high theoretical air-fuel ratio detection system.

However, in the above conventional air-fuel ratio detection device, the obtained air-fuel ratio signal continuously changes uniformly from the lean region to the rich region of the air-fuel ratio, so it is not possible to improve the detection accuracy of the theoretical air-fuel ratio. In order to perform such air-fuel ratio control satisfactorily, it was necessary to use a λ sensor made of titania or the like, the detection characteristics of which greatly change depending on the theoretical air-fuel ratio.

Therefore, the present invention not only provides an air-fuel ratio signal that continuously changes according to the change in the air-fuel ratio as described above,
The present invention has been made for the purpose of providing an air-fuel ratio detection device that can obtain a theoretical air-fuel ratio signal whose characteristics greatly change with the theoretical air-fuel ratio.

[Means for Solving Problems] That is, the structure of the present invention as means for solving the above problems is an air-fuel ratio for controlling the air-fuel ratio of a fuel mixture supplied to a combustion device to a desired air-fuel ratio. It is used in a fuel ratio control device, and is formed by contacting two detection elements each having a pair of porous electrodes laminated on both surfaces of an oxygen ion conductive solid electrolyte and one porous electrode of each of the detection elements. Of the measurement gas chamber whose diffusion is sinusoidal, and the reference oxygen chamber which is formed in contact with the porous electrode on the opposite side of the gas diffusion limiting chamber of one of the detection elements and whose inside is maintained at a predetermined oxygen concentration, An air-fuel ratio sensor configured, and of the two detection elements, the detection element on the side in contact with the reference oxygen chamber is operated as an oxygen concentration battery element, and the other detection element is operated as an oxygen pump element, and both ends of the oxygen concentration battery element are operated. The voltage generated on the electrodes of A pump current control means for bidirectionally controlling a current flowing through the oxygen pump element so as to obtain a voltage, and an air-fuel ratio signal output means for detecting the controlled pump current and outputting it as an air-fuel ratio signal representing an air-fuel ratio. In an air-fuel ratio detection device provided with the current control stop means for stopping the operation of the pump current control means when the air-fuel ratio control apparatus controls the air-fuel ratio to the stoichiometric air-fuel ratio, and the current control stop means is the control means. When the operation of is stopped, a constant current is supplied to the oxygen pump element in the direction in which oxygen is pumped into the measurement gas chamber, and the voltage generated at the electrodes at both ends of the oxygen pump element at that time is adjusted to a lean rich air-fuel ratio. The gist is an air-fuel ratio detection device characterized by including a theoretical air-fuel ratio signal output means for outputting as a theoretical air-fuel ratio signal represented.

As the oxygen ion conductive solid electrolyte used in the detection element here, a solid solution of zirconia and yttria, or a solid solution of zirconia and calcia is typical, and other cerium dioxide, thorium dioxide, and hafnium dioxide A solid solution, a perovskite type oxide solid solution, a trivalent metal oxide solid solution and the like can also be used.

As the porous electrodes provided on both sides of the solid electrolyte, platinum or rhodium, which has a catalytic action for the oxidation reaction, may be used. Examples include a method in which powder of a ceramic material is mixed and made into a paste, which is printed by using a thick film technique and then sintered and formed, or a method in which a thin film technique such as flame spraying, chemical plating or vapor deposition is used. . Since the porous electrode is in direct contact with the measurement gas, that is, exhaust gas, the electrode layer is further provided with alumina,
It is preferable to form a porous protective layer of spinel, zirconia, mullite or the like using a thick film technique.

Next, the above-mentioned measurement gas chamber is a chamber into which exhaust gas is introduced in a diffusion-restricted manner, and two detection element portions are arranged opposite to each other with a gap or between the two detection element portions. A hollow spacer made of Al ₂ O ₃ , spinel, forsterite, steatite, zirconia, etc. is sandwiched, and a hole for communicating the surrounding measurement gas atmosphere with the measurement gas chamber is provided as a gas diffusion limiting part in this spacer. It can also be formed.

Further, the reference oxygen chamber is for storing a gas having a predetermined oxygen concentration, and as described in the section of the prior art, the atmosphere is introduced, or a predetermined current is applied to the detection element in contact with the reference oxygen chamber. Can be realized by introducing oxygen from the measurement gas chamber.

When introducing oxygen into the measurement gas chamber, it is necessary to form a leak resistance part for leaking a fixed amount of oxygen in the reference oxygen chamber to the outside in order to keep the oxygen concentration inside constant. That is, a constant amount of oxygen is introduced from the measurement gas chamber by applying a constant current to the detection element, and the introduced oxygen is leaked to the outside at a constant rate through the leakage resistance section, so that oxygen in the measurement gas chamber is released. The density can be maintained at a predetermined value.

[Operation] In the air-fuel ratio detection device of the present invention configured as described above,
When the air-fuel ratio control device is performing air-fuel ratio control that controls the air-fuel ratio to a lean region or a rich region that is different from the stoichiometric air-fuel ratio, the pump current control means makes contact with the reference oxygen chamber of the air-fuel ratio sensor. The detection element on the side is operated as an oxygen concentration cell element, and the other detection element is operated as an oxygen pump element, so that the voltage generated at the electrodes at both ends of the oxygen concentration cell element becomes a predetermined voltage, that is, the oxygen partial pressure in the measurement gas chamber is The pump current flowing through the oxygen pump element is bidirectionally controlled so as to have a predetermined value. The controlled pump current is detected by the air-fuel ratio signal output means and output as an air-fuel ratio signal. That is, as described in the section of the prior art,
By controlling the pump current as described above and detecting the controlled pump current by the air-fuel ratio signal output means, an air-fuel ratio signal that continuously changes according to the air-fuel ratio can be obtained.

Further, in the present invention, when the air-fuel ratio control device shifts to the air-fuel ratio control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio, the operation of the pump current control means is stopped by the current control stopping means, and the oxygen is supplied by the theoretical air-fuel ratio signal output means. A predetermined current is supplied to the pump element in the direction of drawing oxygen into the measurement gas chamber. Then, at this time, the voltage generated at the electrodes on both ends of the oxygen pump element is output as a theoretical air-fuel ratio signal representing lean rich of the air-fuel ratio.

That is, when oxygen is pumped into the measurement gas chamber of the oxygen pump element by applying a predetermined current to the oxygen pump element, the oxygen partial pressure in the pumped measurement gas chamber is used as a reference for the electrodes at both ends of the oxygen pump element. As a result, an electromotive force is generated according to the partial pressure of oxygen in the surrounding exhaust gas, and its voltage is large on the rich side of the air-fuel ratio with little oxygen in the exhaust gas and small on the lean side of the air-fuel ratio with many oxygen in the exhaust gas. Therefore, the voltage at both ends changes stepwise on the lean side and the rich side of the air-fuel ratio with the stoichiometric air-fuel ratio as the boundary, and by outputting this voltage as the air-fuel ratio signal, the air-fuel ratio becomes lean. It becomes possible to accurately detect whether it is rich or rich.

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

First, FIGS. 2 and 3 show the structure of the air-fuel ratio sensor of this embodiment. FIG. 2 is a partially cutaway perspective view thereof, and FIG. 3 is an exploded perspective view thereof.

As shown in FIG. 2 and FIG. 3, the air-fuel ratio sensor of this embodiment includes an oxygen pump element 4 formed by stacking porous electrodes 2 and 3 on both surfaces of a solid electrolyte plate 1, and a solid electrolyte plate 5 as well. An oxygen concentration battery element 8 formed by laminating porous electrodes 6 and 7 on both sides, and laminated between these detection elements 4 and 8,
By the spacer 9 in which the hollow portion 9a is formed in the facing porous electrodes 3 and 6 of each of the detection elements 4 and 8, and the shield 10 laminated on the porous electrode 7 side of the oxygen concentration battery element 8. It is configured.

First, the spacer 9 is composed of the porous electrode 3 and the porous electrode 6.
Is to form a measurement gas chamber in which the diffusion of the measurement gas is restricted, and the hollow portion 9a is used as the measurement gas chamber. Further, the spacer 9 is provided with notches T at three locations around the hollow portion 9a so that the surrounding measurement gas can be introduced into the hollow portion 9a.

Next, the shield 10 uses the porous electrode 7 of the oxygen concentration battery element 8 as an internal reference oxygen source as the reference oxygen chamber.
The purpose is to shield the porous electrode 7 from the external measuring gas. In addition, the porous electrode 7 covered with the shield 10
Is a porous insulator Z made of, for example, alumina or the like, a through hole H, so that oxygen generated inside when used as an internal reference oxygen source can leak into the measurement gas chamber, that is, the hollow portion 9a. Via the lead portion 6l of the porous electrode 6
Connected with. That is, the porous insulator Z, the through hole H, and the lead portion 6l of the porous electrode 6 are formed as a leakage resistance portion, and oxygen generated in the porous electrode 7 is allowed to pass through the leakage resistance portion and the hollow portion 9a. It leaks into the inside and the inside of the porous electrode 7 can be maintained at a constant oxygen concentration.

Further, the electrode terminals of the porous electrodes 2, 3, 6, 7 of the oxygen pump element 4 and the oxygen concentration cell element 8 are formed on the outer wall surface of the air-fuel ratio sensor. That is, the oxygen pump element 4
Since the porous electrode 2 of is formed to be exposed to the outside,
In the porous electrode 3 of the oxygen pump element 4 or the porous electrodes 6 and 7 of the oxygen concentration cell element 8 which are laminated inside, the lead portion 2l is directly used as an electrode terminal, and the lead portion 3l or 6l and 7l , Solid electrolyte plate 1
Alternatively, it is formed by electrically connecting the electrode terminals 3t or 6t and 7t respectively laminated on the outer wall surface of the shield 10 through the through holes 3h or 6h and 7h.

The air-fuel ratio sensor of this embodiment configured as described above has the first
As shown in FIG. 2A, the exhaust pipe 1 of the internal combustion engine is sealed via a fixing portion 15 that seals the porous electrode layer 7 so that oxygen does not leak to the outside and fixes the air-fuel ratio sensor S, and a screw portion 16.
7 and is operated by the detection circuit 20 shown in FIG. In the figure, in order to facilitate understanding of the attachment state of the air-fuel ratio sensor S, the lead portion and electrode terminal of each porous electrode are omitted. The air-fuel ratio sensor S and the detection circuit 20 are connected by connecting the terminals P1, P2, P3 and P4 shown in the figure.

Next, the detection circuit 20 supplies a predetermined current to the oxygen concentration battery element 8 of the air-fuel ratio sensor S to draw oxygen in the hollow portion 9a into the porous electrode 7 to keep the oxygen concentration on the porous electrode 7 side constant. The pump current control means bidirectionally controls the pump current flowing through the oxygen pump element 4 so that the voltage generated at the electrodes 6 and 7 at both ends of the oxygen concentration battery element 8 becomes constant at a predetermined value while being held. A current control circuit 21,
An air-fuel ratio signal output circuit 22 as the air-fuel ratio signal output means for detecting the controlled pump current and outputting it as an air-fuel ratio signal, and an air-fuel ratio control device for an internal combustion engine to which the air-fuel ratio detecting device is attached. It is switched by a switching signal output when the fuel ratio is controlled to the stoichiometric air-fuel ratio, and the pump current control of the oxygen pump element 4 by the current control circuit 21 is stopped to control the pump current at a constant level. And a switching circuit 23.

Here, first, the current control circuit 21 applies a predetermined voltage Vb (for example, 10 V) to the porous electrode 7 in order to apply a current to the oxygen concentration battery element 8 to pump oxygen into the porous electrode 7.
The resistance R that limits the current flowing to the other porous electrode 6 side to which the reference voltage Va is applied and the electrodes 6 and 7 at both ends of the oxygen concentration battery element 8 are generated, and the reference voltage Va (for example, 5 V) is applied. A buffer circuit 31 configured by using the operational amplifier OP1 for detecting the raised voltage, and a non-inverting amplifier circuit 32 configured by the operational amplifier OP2 for amplifying the detected voltage output from the buffer circuit 31. The input signal and a preset reference voltage Vc are compared in magnitude, and when the input signal is larger than the reference voltage Vc, the input signal is gradually decreased by a predetermined integration constant, and when the input signal is larger than the reference voltage Vc, a predetermined integration constant is used. Output a control voltage as shown in FIG. 4, which gradually increases,
Comparison / integration circuit 3 constructed using operational amplifier OP3
3 and an operational amplifier OP4 that outputs the reference voltage Va.
And a buffer circuit 34 configured by using the output signal from the non-inverting amplifier circuit 32.
Is input to the comparison / integration circuit 33 when the air-fuel ratio control device does not control the air-fuel ratio to the stoichiometric air-fuel ratio. Then, in this case, the pump current flowing through the oxygen pump element 8 is bidirectionally controlled according to the output signal from the non-inverting amplifier circuit 32, and the air-fuel ratio sensor S causes the oxygen partial pressure in the hollow 9a to change to the internal combustion engine. It is operated so that the partial pressure of oxygen in the exhaust gas is generated when the engine is operated at the stoichiometric air-fuel ratio. Therefore, the pump current in this case becomes 0 when the air-fuel ratio is the theoretical air-fuel ratio, and the flowing directions are reversed in the lean region and the rich region of the air-fuel ratio.

Further, the air-fuel ratio signal output circuit 22 is the current control circuit 21.
Current detection resistor Ri provided in the current path of the oxygen pump element 4 in order to detect the pump current controlled by
And the reference voltage V generated across the current detecting resistor Ri.
An output circuit 3 configured by using an operational amplifier OP5 that outputs the voltage raised by a as the air-fuel ratio signal Vλ.
It is composed of 5 and. This air-fuel ratio signal output circuit 3
Since the air-fuel ratio signal Vλ output from 2 changes according to the current flowing through the oxygen pump element 4, when the current control circuit 21 bidirectionally controls the pump current as described above, as shown in FIG. Therefore, the air-fuel ratio of the fuel-air mixture supplied to the internal combustion engine can be detected from the air-fuel ratio signal Vλ.

Next, the switching circuit 23 normally connects the non-inverting amplifier circuit 32 of the current control circuit 21 and the comparison / integration circuit 33 to bidirectionally control the pump current, and the stoichiometric air-fuel ratio control is performed from the air-fuel ratio control device. When the switching signal is input, the non-inverting amplifier circuit 32 and the comparison / integration circuit 33 are disconnected from each other to stop the bidirectional control of the pump current, and the comparison / integration circuit 33 is used to form a hollow portion in the oxygen pump element 8. A constant current is supplied in the direction of drawing oxygen into 9a.

That is, the switching circuit 23 is generated at both ends of the current detection resistor Ri,
The output signal from the non-inverting amplifier circuit 32 is normally input to the comparison / integration circuit 33 and the switching signal is input from the air-fuel ratio control device, as well as the voltage dividing resistors R1 and R2 that divide the voltage raised by the reference voltage Va. When input, voltage dividing resistors R1 and R
It is composed of the analog switch S1 for inputting the voltage divided by 2 to the comparison / integration circuit 33, and the current detection resistor R
The voltage generated at both ends of i and raised by the reference voltage Va is converted by the voltage dividing resistors R1 and R2 into a value corresponding to the reference voltage Vc, and the air-fuel ratio control device executes the theoretical air-fuel ratio control. When it is present, this is input to the comparison / integration circuit 33, and a voltage for matching the divided voltage with the reference voltage Vc is output from the comparison / integration circuit 33.

Therefore, when the air-fuel ratio control device is executing the theoretical air-fuel ratio control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio, the comparison / integration circuit 33 is operated as a constant current source, and the current flowing through the pump element 8 becomes a constant value. A certain amount of oxygen is pumped into the hollow portion 9a.

By the way, when a certain amount of oxygen is pumped into the hollow portion 9a in this way, an electromotive force is generated in the oxygen pump element 8 in accordance with the ratio of the oxygen partial pressure in the hollow portion 9a and the oxygen partial pressure in the exhaust gas. As for the voltage, the oxygen partial pressure in the exhaust is small and the hollow portion 9a is
It is large in the rich region of the air-fuel ratio where the ratio with the internal oxygen partial pressure is large, and is small in the lean region of the air-fuel ratio where the oxygen partial pressure in the exhaust gas is large and the ratio with the oxygen partial pressure in the hollow portion 9a is small. Therefore, the voltage output from the comparison / integration circuit 33 changes as shown in FIG. 6, and in this embodiment, this output voltage is the theoretical air-fuel ratio signal V representing the theoretical air-fuel ratio.
It is output as λ1.

That is, for example, the current value Ip flowing through the oxygen pump element 8 is 2 [m
A], reference voltage Va is 5 [V], and element resistance Rs is 300
[Ω], the current detection resistor Ri is 200 [Ω], and the electromotive force Vp generated when the air-fuel ratio is rich is 0.8 [V], the comparison / integration circuit when the air-fuel ratio is in the lean range. Three
3 to Va-Ip (Rs + Ri), that is, about 4.0.
When the voltage [V] is output and the air-fuel ratio is in the rich region, Va-Ip (Rs + Ri) -Vp, that is, about 3.2.
The voltage [V] is output, and the theoretical air-fuel ratio can be satisfactorily detected by outputting this voltage as the theoretical air-fuel ratio signal Vλ1.

At this time, the value of the current flowing through the oxygen pump element 8 is
The current value may be about one fifth of the current value required for bidirectionally controlling the pump current in the atmosphere using the current control circuit 21.

As described above, in this embodiment, when the air-fuel ratio control device is performing the theoretical air-fuel ratio control, the oxygen pump element 8 is operated as an oxygen concentration battery element, and the electromotive force generated in the oxygen pump element 8 is used. It is configured to output a theoretical air-fuel ratio signal Vλ1 whose characteristics greatly change with the theoretical air-fuel ratio. Therefore, the air-fuel ratio control device can accurately detect the stoichiometric air-fuel ratio by reading the stoichiometric air-fuel ratio signal Vλ1 when the stoichiometric air-fuel ratio control is executed, and the stoichiometric air-fuel ratio control can be executed well. Further, in this embodiment, when the air-fuel ratio control device is not executing the stoichiometric air-fuel ratio control, the air-fuel ratio signal output circuit 22 continuously changes the air-fuel ratio signal from the lean region to the rich region of the air-fuel ratio as in the conventional case. Is output, it is possible to provide the information necessary for the air-fuel ratio control device to control the air-fuel ratio to the desired air-fuel ratio in the lean range or the rich range.

Further, in this embodiment, when the pump current is bidirectionally controlled, the oxygen partial pressure in the hollow portion 9a of the air-fuel ratio sensor S is controlled to be the oxygen partial pressure in the exhaust gas when the internal combustion engine is operated at the stoichiometric air-fuel ratio. Therefore, at this time, almost no oxygen exists in the hollow portion 9a, and the oxidation of the electrode on the hollow portion 9a side of the oxygen pump element 8 can be suppressed. Also, since there is no rich gas, adsorption of rich gas can be prevented. Therefore, the air-fuel ratio control device enters the theoretical air-fuel ratio control, and when the oxygen pump element 8 operates as an oxygen concentration battery, the electromotive force generated at the electrodes at both ends always changes according to the oxygen partial pressure in the exhaust gas. The detection accuracy of the air-fuel ratio signal can always be kept good.

[Effects of the Invention] As described in detail above, according to the air-fuel ratio detection device of the present invention, not only the air-fuel ratio signal that changes according to the air-fuel ratio is obtained, but also the air-fuel ratio control device used is When the stoichiometric air-fuel ratio control for controlling the fuel ratio to the stoichiometric air-fuel ratio is started, a stoichiometric air-fuel ratio signal that changes stepwise at the stoichiometric air-fuel ratio is obtained. Therefore, the stoichiometric air-fuel ratio can be accurately detected by the air-fuel ratio control device, and the air-fuel ratio control can be satisfactorily executed only by this air-fuel ratio detection device.

[Brief description of drawings]

FIG. 1 shows the overall configuration of the air-fuel ratio control system of the embodiment,
(A) is a sectional view showing a state in which the air-fuel ratio sensor of this embodiment is attached to an exhaust pipe of an internal combustion engine, (b) is an electric circuit diagram showing a circuit configuration of a detection circuit, and FIG. 2 is a part of the air-fuel ratio sensor. FIG. 3 is a broken perspective view, FIG. 3 is an exploded perspective view thereof, FIG. 4 is a diagram showing a control voltage for controlling the pump current output from the comparison / integration circuit, and FIG. 5 is output from the air-fuel ratio signal output circuit. 6 is a diagram showing the air-fuel ratio signal, and FIG. 6 is a diagram showing the theoretical air-fuel ratio signal obtained from the voltage signal output from the comparison / integration circuit. 4 ... Oxygen pump element 7 ... Porous electrode (reference oxygen chamber) 8 ... Oxygen concentration battery element 9a ... Hollow part (measurement gas chamber) 20 ... Detection circuit 21 ... Current control circuit (pump current control means) ) 22 ... Air-fuel ratio signal output circuit (air-fuel ratio signal output means) 23 ... Switching circuit (current control stop means)

Front Page Continuation (72) Inventor Akira Suzuki 14-18 Takatsuji-cho, Mizuho-ku, Aichi Prefecture Nihon Special Ceramics Co., Ltd. (56) Reference JP-A-60-183548 (JP, A) JP-A-62 -103565 (JP, A)

Claims (1)

[Claims] 1. An air-fuel ratio control device for controlling an air-fuel ratio of a fuel mixture supplied to a combustion device to a desired air-fuel ratio, wherein a pair of porous electrodes are laminated on both surfaces of an oxygen ion conductive solid electrolyte. Two detection elements, a measuring gas chamber formed in contact with one porous electrode of each of the detection elements and having exhaust diffusion limited, and the gas diffusion limiting chamber of one detection element opposite The air-fuel ratio sensor, which is formed in contact with the porous electrode on the side and has a reference oxygen chamber whose inside is maintained at a predetermined oxygen concentration; The detection element is operated as an oxygen concentration battery element, and the other detection element is operated as an oxygen pump element, and the current flowing through the oxygen pump element is bidirectionally controlled so that the voltage generated at the electrodes at both ends of the oxygen concentration battery element becomes a predetermined voltage. With pump current control means An air-fuel ratio signal output means for detecting the controlled pump current and outputting as an air-fuel ratio signal representing an air-fuel ratio, wherein the air-fuel ratio control device controls the air-fuel ratio to a stoichiometric air-fuel ratio. When the current control stopping means stops the operation of the pump current control means, when the current control stopping means stops the operation of the control means, in the direction of pumping oxygen into the measurement gas chamber to the oxygen pump element. And a theoretical air-fuel ratio signal output means for supplying a constant current and outputting a voltage generated at the electrodes across the oxygen pump element at that time as a theoretical air-fuel ratio signal representing lean rich of the air-fuel ratio. Air-fuel ratio detection device.