JPH10325825A - Concentration detector for nitride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride concentration meter for detecting NOx concentration accurately through a simple circuit. SOLUTION: The NOx sensor comprises a first measuring chamber 20 conducting through a diffusion control layer to the side of gas being measured, and a second measuring chamber 26 conducting through diffusion limit layers 6d, 22d to the first measuring chamber 20. The quantity of oxygen flowing into the second measuring chamber 26 is controlled constant by controlling a first pump current IP1 such that the output from a Vs cell 6 is equal to a reference voltage VCO. The NOx concentration is detected from the detection signals VIP1, VIP2 of first and second pump currents IP1, IP2 by applying a constant voltage to a second pump cell 8 to decompose the NOx component in the second measuring chamber 26 and pumping oxygen. The electrode 6b on the first measuring chamber 20 side of the Vs cell 6 is formed on the circumferential fringe of the diffusion limit layer 6d or above the diffusion limit layer 6d. According to the arrangement, the quantity of oxygen flowing from the first measuring chamber 20 into the second measuring chamber 26 can be detected accurately and the NOx concentration can be detected from the VIP1, VIP2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide concentration detector for detecting the concentration of nitrogen oxide, which is a harmful component, discharged from various combustion devices such as internal combustion engines.

[0002]

2. Description of the Related Art Conventionally, as a nitrogen oxide concentration detector (NOx sensor) for detecting a nitrogen oxide concentration (NOx) in a gas to be measured, for example, European Patent Application Publication No. 0678740A1 and SAE paper
No. 960334 P137-142 1996 and the like, a first measurement chamber communicated with the measured gas side via a first diffusion-controlling layer, and a first diffusion-controlling layer connected to the first measurement chamber via a second diffusion-controlling layer. The communicating second measurement chamber is formed by an oxygen ion-conductive solid electrolyte layer, and the first measurement chamber is provided with a first oxygen pumping cell and an oxygen concentration by sandwiching the solid electrolyte layer between porous electrodes. Forming a measuring cell and
Further, a second measurement chamber in which a second oxygen pumping cell is formed by sandwiching a solid electrolyte layer between porous electrodes is also known.

In this type of NOx sensor, the first oxygen pumping cell is energized so as to pump oxygen from the first measuring chamber so that the output voltage from the oxygen concentration measuring cell becomes a preset constant value. Thus, by controlling the oxygen concentration in the first measurement chamber to a constant concentration and applying a constant voltage to the second oxygen pumping cell to pump more oxygen from the second measurement chamber, the second oxygen pumping can be performed. The NOx concentration in the gas to be measured can be detected from a current flowing through the cell (hereinafter, referred to as a second pump current).

[0004] That is, since other gas components such as oxygen, carbon monoxide, carbon dioxide, etc., in addition to NOx, are present in the exhaust gas from the internal combustion engine or the like, which is the gas to be measured, First, a current (hereinafter, referred to as a first pump current) is supplied to the first oxygen pumping cell to extract most of the oxygen in the first measurement chamber, and further, the second measurement chamber into which the measured gas from which the oxygen has been extracted flows. On the side, the catalytic function of the second oxygen pumping cell causes the NOx
Is decomposed into nitrogen and oxygen, and is operated to extract oxygen from the second measurement chamber, so that the gas to be measured is not affected by the other gas components in the gas to be measured from the second pump current. It is designed to detect the concentration of NOx therein.

In order to accurately detect the NOx concentration using the NOx sensor, it is necessary to heat the NOx sensor to a predetermined activation temperature (for example, 800 ° C. or higher) to activate each cell. In some cases, a heater for heating is provided in the NOx sensor and the amount of current supplied to the heater is controlled to control the temperature of the NOx sensor to a predetermined temperature.

[0006]

In the above-described NOx sensor, it is necessary to correct the NOx concentration obtained from the second pump current in order to secure the detection accuracy of the NOx concentration. It turned out that there was a problem that it became complicated and increased the cost of the detection device.
Hereinafter, this problem will be described.

First, a conventional NOx sensor pumps oxygen from a first measurement chamber using a first oxygen pumping cell,
If the gas to be measured in the first measurement chamber is controlled to an extremely low oxygen concentration containing almost no oxygen, the gas to be measured flowing into the second oxygen pumping cell becomes substantially only the NOx component, and the gas to be measured is converted into the second gas. It has been developed in accordance with the technical idea that if the catalyst is decomposed into nitrogen and oxygen by the oxygen pumping cell, the NOx concentration can be detected from the second pump current flowing through the second oxygen pumping cell.

However, in practice, when the first oxygen pumping cell is controlled so that the oxygen concentration in the first measurement chamber becomes substantially zero (theoretically, about 10 ⁻⁹ atm), the NOx concentration is calculated from the second pump current. In order to detect NOx concentration with high sensitivity using a conventional NOx sensor,
It is necessary to control the first oxygen pumping cell so that the oxygen concentration in the measurement chamber becomes a low oxygen concentration of about 1000 ppm.

The reason why good detection sensitivity cannot be obtained when the oxygen concentration in the first measurement chamber is controlled so as to be substantially zero is that by controlling the first pump current, the measured current in the first measurement chamber is controlled. It is considered that the nitrogen monoxide component in the gas is decomposed, and the gas to be measured corresponding to the NOx component does not flow into the second measurement chamber.

Then, the first oxygen concentration in the first measurement chamber is reduced to about 1000 ppm as described above.
When the NOx concentration is detected from the second pump current while controlling the oxygen pumping cell, the second measurement chamber includes:
Not only NOx (nitrogen oxide) in the gas to be measured,
Since the oxygen in the measurement chamber also flows, the second pump current changes according to the NOx concentration in the gas to be measured, but is also affected by the oxygen concentration in the gas to be measured. As a result, conventionally, the detection result of the NOx concentration may be affected by the oxygen concentration in the gas to be measured around the sensor, and the detected NOx concentration may not correspond to the actual value.

Such a problem can be solved by using a current flowing in the first oxygen pumping cell (hereinafter, referred to as a first pump current). That is, the first pump current and the second pump current are measured, and the nitrogen oxide concentration is detected from the two values, so that even if the oxygen concentration in the gas to be measured around the sensor fluctuates, the nitrogen gas is accurately measured. Oxide concentration can be detected.

However, the first pump current and the second
Even if the nitrogen oxide concentration is detected from the pump current,
If a relatively simple relationship is not established between these two values and the nitrogen oxide concentration, the correction means becomes complicated and causes an increase in cost. The present invention has been made in view of such a problem. In detecting the NOx concentration using the NOx sensor, when detecting the NOx concentration from the first pump current and the second pump current, these two values and NO
An object of the present invention is to make it possible to detect the NOx concentration with high accuracy by connecting the x concentrations in a simple relationship.

[0013]

Means for Solving the Problems In order to achieve the above object, a nitrogen oxide concentration detector according to the present invention (described in claim 1) comprises an oxygen ion conductive solid electrolyte layer formed of a porous electrode. A first measurement chamber having a first oxygen pumping cell and an oxygen concentration measurement cell interposed therebetween, and a first measurement chamber communicated with the gas to be measured via a first diffusion-controlling layer; A second oxygen pumping cell sandwiched between the electrodes, and a second measuring chamber communicated with the first measuring chamber via a second diffusion-controlling layer. The output voltage of the oxygen concentration measuring cell is The first oxygen pumping cell pumps oxygen from the first measurement chamber so as to have a constant value, and applies a constant voltage to the second oxygen pumping cell in a direction to pump oxygen from the second measurement chamber. By doing
In a nitrogen oxide concentration detector for detecting nitrogen oxides in a gas to be measured from a current flowing in the first oxygen pumping cell and a current flowing in the second oxygen pumping cell, the oxygen concentration measuring cell is connected to the first measurement cell. Without being affected by the oxygen distribution in the room, the oxygen concentration of the gas to be measured flowing from the first measurement chamber through the first diffusion-controlling layer to the second measurement chamber side can be detected. Features.

[0014]

As described above, in the nitrogen oxide concentration detector (NOx sensor) according to the present invention, the oxygen concentration measuring cell is not affected by the oxygen distribution in the first measuring chamber.
It is arranged at a position where the amount of oxygen of the gas to be measured flowing from the first measurement chamber to the second measurement chamber through the first diffusion-controlling layer can be detected.

In the conventional NOx sensor, the oxygen concentration measuring cell arranged so as to detect the oxygen concentration in the first measuring chamber can be used to detect the amount of oxygen flowing from the first measuring chamber to the second measuring chamber. In order to accurately detect the oxygen concentration of the measurement gas flowing into the second measurement chamber and to control the first oxygen pumping cell so that the oxygen concentration becomes a predetermined value, the first oxygen pumping cell can be controlled. is there.

That is, as described above, in the conventional NOx sensor, the oxygen concentration in the first measurement chamber is substantially zero (theoretically, 1).
0 ^-9 Controlling the first oxygen pumping cell so atm or so) and consists, since it was not possible to detect the NOx concentration from the second pumping current, hypoxia about the oxygen concentration in the first measurement chamber 1000ppm The first so that the concentration
The oxygen pumping cell was controlled. The cause of this is that if the first oxygen pumping cell is controlled so that the oxygen concentration in the first measurement chamber becomes substantially zero, the nitrogen monoxide component in the gas to be measured in the first measurement chamber is decomposed, It is considered that the measured gas corresponding to the NOx component does not flow into the second measurement chamber.

The present inventors have investigated and found that the conventional N
In the Ox sensor, the first measurement chamber side electrode of the oxygen concentration measurement cell is formed relatively large in order to detect the oxygen concentration in the first measurement chamber with the oxygen concentration measurement cell.
Although the average oxygen concentration in the measurement chamber can be detected,
It was found that the oxygen concentration of the gas to be measured flowing from the measurement chamber to the second measurement chamber could not be detected. That is, an external gas to be measured flows into the first measurement chamber through the first diffusion-controlling layer, and oxygen is extracted from the gas to be measured by the first oxygen pumping cell.
The oxygen distribution in the first measurement chamber is not uniform. As a result, in the conventional NOx sensor, the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber is accurately measured by the oxygen concentration measurement cell. Could not be detected. As a result, the first
The relationship between the two values of the pump current and the second pump current and the nitrogen oxide concentration became complicated, making the detection difficult.

Therefore, in the present invention, the oxygen concentration measuring cell is arranged at a position where the amount of oxygen flowing from the first measuring chamber to the second measuring chamber can be detected instead of the oxygen concentration in the first measuring chamber. In the concentration measurement cell, the amount of oxygen flowing from the first measurement chamber to the second measurement chamber can be accurately detected.

As a result, according to the present invention, by controlling the energization of the first oxygen pumping cell, a current is caused to flow to the first oxygen pumping cell to such an extent that the NOx component in the first measuring chamber is decomposed, Thus, the amount of oxygen flowing into the second measurement chamber can be sufficiently reduced, and the NOx concentration can be accurately detected from the first pump current and the second pump current.

Here, the nitrogen oxide concentration detector of the present invention controls each cell to a predetermined activation temperature and secures the detection accuracy of the NOx concentration, similarly to the above-mentioned conventional NOx sensor. It is desirable to provide a heater for heating the heater. When the heater for heating and temperature control is provided in this manner, the first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell are different from each other. By forming the solid electrolyte layer in the form of a thin plate, and laminating each solid electrolyte layer through a predetermined gap with the solid electrolyte layers forming the first and second oxygen pumping cells outside, the gaps are formed in the gaps. Along with forming the first measurement chamber and the second measurement chamber, a thin plate-like heater substrate provided with a heater for heating is disposed on both sides of each solid electrolyte layer in the stacking direction via a predetermined gap, and further, It is desirable to form a first diffusion-controlling layer at a position of the solid electrolyte layer in which the first oxygen pumping cell is formed, facing a central portion of the heater formed on the heater substrate.

That is, when the nitrogen oxide concentration detector of the present invention is configured in this manner, the solid electrolyte layer in which the oxygen concentration measurement cell is formed has the first oxygen pumping cell and the second oxygen pumping cell formed therein. Since the heater substrate is disposed between the solid electrolyte layers and further on both sides in the stacking direction, it is easy to control each cell to a predetermined temperature by controlling the power supply to the heater. First
The gas to be measured flowing into the measurement chamber and thus into the second measurement chamber can also be heated to a predetermined temperature by the heater. Therefore, according to the second aspect of the present invention, the temperature variation of each cell in the sensor body is less likely to occur, and each cell is less likely to be affected by the temperature of the gas to be measured. Can be further improved.

In this case, as described in claim 3,
If the second diffusion-controlling layer is formed so as to overlap at least a part of the first diffusion-controlling layer when the nitrogen oxide concentration detector is projected from the stacking direction of each solid electrolyte layer, the first diffusion-controlling layer can be controlled by the heater control. The temperature of the gas to be measured flowing from the measurement chamber to the second measurement chamber can be reliably controlled by the target temperature, and the detection accuracy of the NOx concentration can be further improved.

Next, according to the present invention (claim 1),
To arrange the oxygen concentration measuring cell at a position where the oxygen amount of the gas to be measured flowing from the first measuring chamber to the second measuring chamber can be detected without being affected by the oxygen distribution in the first measuring chamber, As described in Item 4, the oxygen concentration measurement cell may be formed on the periphery of the second diffusion-controlling layer. In this case,
In particular, as described in claim 5, the second diffusion-controlling layer is constituted by a porous solid electrolyte layer capable of controlling the gas to be measured by diffusion, and the first measurement chamber side electrode of the oxygen concentration measuring cell is formed by the second solid-state electrode. The first measurement chamber side electrode of the oxygen concentration measurement cell may be formed on the diffusion control layer or the solid electrolyte layer and the first diffusion control layer forming the first measurement chamber as described in claim 6. It is good to form between. That is, with this configuration, the oxygen concentration measurement cell can more accurately detect the oxygen amount of the gas to be measured flowing from the first measurement chamber to the second measurement chamber, and the above effect can be more reliably achieved. Can be achieved.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an exploded perspective view showing a configuration of a nitrogen oxide concentration detector (NOx sensor) according to an embodiment to which the present invention is applied, and FIG.
FIG. 1 is a configuration diagram showing a configuration of an entire nitrogen oxide concentration detection device that detects a NOx concentration using the NOx sensor.

As shown in FIG. 1, the NOx sensor 2
Oxygen pumping cell (hereinafter referred to as first pump cell)
4, an oxygen concentration measurement cell (hereinafter, referred to as Vs cell) 6, a second oxygen pumping cell (hereinafter, referred to as second pump cell)
8, and a pair of heaters 12, 1 for heating these cells
4 is provided.

The first pump cell 4 has rectangular porous electrodes 4 on both sides of a plate-shaped solid electrolyte layer 4a.
b, 4c and their lead portions 4bl, 4cl are formed.
By forming a round hole in the solid electrolyte layer 4a so as to penetrate through the central portions of the porous electrodes 4b and 4c, and filling the round hole with a porous filler such as alumina, for example,
This is one in which a diffusion controlling layer 4d is formed.

The Vs cell 6 is provided on both sides of a solid electrolyte layer 6a having the same shape as the solid electrolyte layer 4a of the first pump cell 4.
Circular porous electrodes 6b, 6c and their lead portions 6bl, 6cl are formed, respectively, and further, a round hole is formed in the solid electrolyte layer 6a so as to penetrate through the center of the porous electrodes 6b, 6c. The diffusion-controlling layer 6d is formed by filling the round hole with a porous filler such as alumina.

The porous electrode 6 of the Vs cell 6
b, 6c and the porous electrodes 4b, 4c of the first pump cell 4 have substantially the same center positions on the solid electrolyte layers 4a, 6a, and when the Vs cell 6 and the first pump cell 4 are stacked, each diffusion The rate limiting layers 6d and 4d are configured to face each other. The circular porous electrodes 6b and 6c formed in the Vs cell 6 are arranged around the diffusion-controlling layer 6d, and
Rectangular porous electrodes 4b, 4 formed in the pump cell 4
It is smaller than c. In particular, a pair of porous electrodes 6
b, 6c, the porous electrode 6b on the first pump cell 4 side
Is formed only in a very narrow range around the diffusion-controlling layer 6d in order to accurately detect the amount of oxygen flowing into the diffusion-controlling layer 6d.

On the front and back surfaces of the Vs cell 6, current leakage from the lead portions 6bl and 6cl can be prevented, and the amount of oxygen flowing into the diffusion-controlling layer 6d can be accurately detected by the porous electrodes 6b and 6c. To do so, the lead portions 6bl, 6
An insulating film 6 made of zirconia or the like so as to cover cl from the outside
e is formed between the lead portions 6bl and 6cl, and a leakage resistance portion 6f for leaking a part of oxygen pumped into the porous electrode 6c by the energization control described later to the porous electrode 6b. Are formed.

The first pump cell 4 and the Vs cell 6 thus formed are stacked via the solid electrolyte layer 18 having the same shape as the solid electrolyte layers 4a and 6a. A rectangular hole larger than the porous electrode 4c is formed in the solid electrolyte layer 18 at a position facing each of the porous electrodes 4c and 6b, and the hole functions as the first measurement chamber 20. I do.

The Vs cell 6 also has a porous electrode 6c side.
The solid electrolyte layers 22 having the same shape as the solid electrolyte layers 4a and 6a are stacked. In the solid electrolyte layer 22, a round hole of the same size is formed at the same position as the diffusion-controlling layer 6d of the Vs cell 6, and the round hole is filled with a porous filler such as alumina. Thereby, a diffusion-controlling layer 22d is formed.

On the other hand, similarly to the first pump cell 4, the second pump cell 8 has rectangular porous electrodes 8b and 8c and lead portions 8bl and 8cl on both sides of the solid electrolyte layer 8a formed in a plate shape. Is formed. Then, the second pump cell 8 is stacked on the solid electrolyte layer 22 stacked on the Vs cell 6 via a solid electrolyte layer 24 formed exactly like the solid electrolyte layer 18. As a result, the rectangular hole formed in the solid electrolyte layer 24 functions as the second measurement chamber 26.

On both sides of the stacked body of the first pump cell 4, the Vs cell 6, and the second pump cell 8 thus stacked, that is, outside the first pump cell 4 and the second pump cell 8, a spacer 28, 29, the heaters 12, 14 are stacked at a predetermined interval. The heaters 12 and 14 have heater substrates 12a, 12c and 14a, 14c having the same shape as the solid electrolyte layers 4a, 6a,.
Between the heater substrates 12a and 12c and the heater substrate 1
Heater wirings 12b and 14b, which are respectively sandwiched between 4a and 14c and buried in each heater substrate;
It is composed of the leads 12bl and 14bl. The spacers 28 and 29 are arranged such that the heaters 12 and 14 are connected to the porous electrodes 4 b of the first pump cell 4 and the second pump cell 8.
And 8c, and are disposed between the heaters 12, 14 and the first pump cell 4 and the second pump cell 8, respectively, so as to face each other with a gap therebetween.

Here, each of the solid electrolyte layers 4a, 6a,
As a solid electrolyte constituting ..., a solid solution of zirconia and yttria or a solid solution of zirconia and calcia is typical. In addition, a solid solution of hafnia, a perovskite-type oxide solid solution, and a trivalent metal oxide solid solution are also used. it can. For the porous electrodes provided on the surfaces of the solid electrolyte layers 4a, 6a, 8a, it is preferable to use platinum, rhodium, or an alloy thereof having a catalytic function. As a forming method, for example, a thick film forming method in which a mixture of platinum powder and a powder of the same material as that of the solid electrolyte layer is made into a paste, screen-printed on the solid electrolyte layer, and then sintered, There is known a method of forming a film by using the method described above. Further, it is preferable that the diffusion controlling layers 4d, 6d, and 22d use ceramics having a small through hole or porous ceramics.

On the other hand, the heater wiring 12 of the heaters 12 and 14
b and 14b are made of a composite material of ceramics and platinum or a platinum alloy, and the lead portions 12bl and 14bl are preferably made of platinum or a platinum alloy in order to reduce the resistance value and reduce the electric loss in the lead portion. preferable. Also, the heater substrate 1
2a, 12b, 14a, 14c and spacers 28, 29
For example, alumina, spinel, forsterite, steatite, zirconia, or the like can be used.

In particular, when zirconia is used for the material of the heater substrate and the spacer, the heater and each pump cell can be simultaneously integrated and sintered, so that NO
This is suitable for producing the x sensor 2. In this case, an insulating layer is provided between the heater wiring 12b and its lead portion 12bl and the heater substrates 12a and 12c, and between the heater wiring 14b and its lead portion 14bl and the heater substrates 14a and 14c, respectively. (Made of alumina or the like).

When alumina is used for the heater substrate, a porous body is used as a spacer in order to prevent the occurrence of cracks due to a difference in contraction rate and a difference in thermal expansion coefficient between each pump cell and the other during sintering. Good to use. In addition, the heater and the pump cells can be separately sintered and then joined by using an inorganic material such as cement as a joining material also serving as a spacer.

Next, as shown in FIG. 2, the nitrogen oxide concentration detecting device for detecting the NOx concentration using the NOx sensor 2 constructed as described above has a first pump cell 4 and a Vs A drive circuit 40 for energizing the cell 6 and switching the energization path, a detection circuit 45 for detecting a current (first pump current) IP1 flowing through the first pump cell 4 via the resistor R4, and a NOx sensor 2 A detection circuit 42 for applying a constant voltage to the second pump cell 8 and detecting a current (second pump current) IP2 flowing at that time, and a pair of heaters 1 provided in the NOx sensor 2.
A heater energizing circuit 44 for energizing the cells 2, 14 and heating the cells 4, 6, 8; a driving circuit 40 and a heater energizing circuit 4;
An electronic control circuit (hereinafter referred to as an ECU) composed of a microcomputer that drives and controls the microcomputer 4 and calculates the NOx concentration in the gas to be measured based on the detection signal VIP1 from the detection circuit 45 and the detection signal VIP2 from the detection circuit 42. ) 5
0.

As shown in FIG. 2, the first NOx sensor 2
The porous electrodes 4c and 6b of the pump cell 4 and the Vs cell 6 on the first measurement chamber 20 side are grounded via the resistor R1, and the other porous electrodes 4b and 6c are connected to the drive circuit 40. I have. The driving circuit 40 has a resistor R2 connected to the porous electrode 6c of the Vs cell 6 at one end through the open / close switch SW1 and a resistor R2 to the other end through the open / close switch SW1. The differential amplifier A is connected to the porous electrode 6c of the Vs cell 6 and one end of the capacitor Cp, the reference voltage VCO is applied to the + input terminal, and the output terminal is connected to the porous electrode 4b of the first pump cell 4.
And a control unit 40a including an MP. The other end of the capacitor Cp is grounded.

The control unit 40a includes an open / close switch SW1
Operates as follows when is in the ON state. First, oxygen in the first measurement chamber 20 is pumped into the porous electrode 6c side of the Vs cell 6 by flowing a constant minute current iCP to the Vs cell 6 via the resistor R2. This porous electrode 6c is
The closed space in the porous electrode 6c has a constant oxygen concentration due to the passage of the minute current iCP because it is closed by the solid electrolyte layer 22 and communicates with the porous electrode 6b through the leak resistance portion 6f. , Functions as an internal oxygen reference source.

As described above, the porous electrode 6 of the Vs cell 6
When the c side functions as an internal oxygen reference source, an electromotive force is generated in the Vs cell 6 according to the ratio of the oxygen concentration in the first measurement chamber 20 to the oxygen concentration on the internal oxygen reference source side, and the porous electrode 6
The c-side voltage Vs is a voltage corresponding to the oxygen concentration in the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26.
Since this voltage is input to the differential amplifier AMP, the differential amplifier AMP outputs a voltage corresponding to a deviation (VCO−input voltage) between the reference voltage VCO and the input voltage. Is the porous electrode 4 of the first pump cell 4
b.

As a result, a current (first pump current) IP1 flows through the first pump cell 4, and the first pump current
By P1, the electromotive force generated in the Vs cell 6 is controlled to be constant. That is, the control unit 40a performs the first control so that the amount of oxygen flowing from the first measurement chamber 20 to the second measurement chamber 26 via the diffusion control layers 6d and 22d as the second diffusion control layer becomes a predetermined amount. Control for pumping out oxygen from the measurement chamber 20 to the outside is executed.

In this embodiment, the amount of oxygen flowing from the first measuring chamber 20 to the second measuring chamber 26 is sufficient (theoretically, 1).
In order to reduce the current (to about 0 ^-9 atm), the first pump cell 4 is caused to flow an electric current to such an extent that the NOx component in the first measuring chamber 20 is decomposed. For example, a value of about 450 mV is set as the reference voltage VCO for determining the amount of oxygen flowing from the first measurement chamber 20 to the second measurement chamber 26). However, it is not always necessary to set the reference voltage VCO to 450 mV,
It suffices if it is within the range of mV to 450 mV.

Next, the drive circuit 40 includes the control unit 40
a constant current circuit 40b connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW2 and flowing a constant current between the porous electrodes 6b-6c in a direction opposite to the minute current iCP; A constant current circuit 40 which is connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW3, and allows a constant current to flow between the porous electrodes 6b-6c in the same direction as the minute current iCP.
c is provided.

Each of these constant current circuits 40b and 40c
This is for detecting the internal resistance RVS of the s cell 6.
Then, the voltage Vs on the porous electrode 6c side is input to the ECU 50 so that the internal resistance RVS of the Vs cell 6 can be detected by the ECU 50 by applying the constant current. The constant currents flowing through the constant current circuits 40b and 40c are set to the same current value except for the direction of the current. This current value is larger than the minute current iCP supplied to the Vs cell 6 via the resistor R2.

The open / close switches SW1 to SW3 provided between the control unit 40a, the constant current circuits 40b and 40c, and the porous electrode 6c of the Vs cell 6, respectively, are connected to the ECU 50.
In the normal state where the operation is performed by the control signal from the controller and the detection operation of the NOx concentration is performed, only the opening / closing switch SW1 is turned on and the control unit 40a operates to detect the internal resistance RVS of the Vs cell 6. , Open / close switch SW
1 is turned off, and the open / close switches SW2 and SW3 are sequentially turned on.

On the other hand, the second pump cell 8 of the NOx sensor 2
A constant voltage VP2 is applied between the porous electrodes 8b and 8c via a resistor R3 as a constant voltage applying means constituting the detection circuit 42. The application direction of this constant voltage VP2 is
In the second pump cell 8, the porous electrode 8c side is a positive electrode and the porous electrode 8 is so configured that an electric current flows from the porous electrode 8c to the 8b side and oxygen in the second measurement chamber 26 is pumped out.
The b side is set to be the negative electrode. The constant voltage VP2 is supplied from the first measurement chamber 20 to the diffusion-controlling layers 6d and 22d.
The voltage is set to a voltage that can decompose the NOx component in the gas to be measured in the second measurement chamber 26 flowing through d and pump out the oxygen component, for example, 450 mV.

The resistor R3 applies a second pump current I flowing through the second pump cell 8 by applying the constant voltage VP2.
This is for converting P2 into a voltage VIP2 and inputting it to the ECU 50 as a detection signal of the second pump current IP2. In the nitrogen oxide concentration detection device of this embodiment configured as described above, if the open / close switch SW1 in the drive circuit 40 is turned on and the open / close switches SW2 and SW3 are turned off, the operation of the control unit 40a causes The gas to be measured in the first measurement chamber 20 flowing through the diffusion-controlled layer (first diffusion-controlled layer) 4d flows into the second measurement chamber 26 through the diffusion-controlled layer (second diffusion-controlled layer) 6d and 22d. Since the amount of oxygen at the time of cleaning can be sufficiently suppressed (theoretically to about 10 ⁻⁹ atm), the first pump current IP1 flowing through the first pump cell 4 and the second pump current IP2 flowing through the second pump cell 8 can be accurately determined. And the NOx concentration can be measured. That is, the detection signal VIP1 of the first pump current IP1 and the second pump current I
By reading the detection signal VIP2 of P2 and executing predetermined arithmetic processing, the detection signal VIP1 (in other words, the first
The pump current IP1) and the detection signal VIP2 (in other words, the second
The NOx concentration in the gas to be measured can be detected from the pump current IP2).

To ensure the detection accuracy of the NOx concentration, the temperature of each of the cells 4, 6 and 8, particularly the temperature of the Vs cell 6 for detecting the oxygen concentration in the first measurement chamber 20, is controlled to be constant. For this purpose, it is necessary to control the amount of current supplied from the heater power supply circuit 44 to each of the heaters 12 and 14 so that the temperature of the Vs cell 6 becomes the target temperature. Therefore, the ECU 50 normally operates the open / close switch SW1
Is turned on, the on / off switches SW2 and SW3 are turned off, and the detection signal VIP1 of the first pump current IP1 and the detection signal VIP2 of the second pump current IP2 are read to detect the NOx concentration in the gas to be measured. The NOx concentration detection process is executed. At a predetermined time T0, the switch temperature is detected from the internal resistance RVS of the Vs cell 6 by turning off the open / close switch SW1 and turning on / off the other open / close switches SW2 and SW3. And the temperature reaches a target temperature (for example, 850).
° C) from the heater energizing circuit 44
A heater energization control process for controlling the amount of energization to the heaters 2 and 14 is also executed.

That is, as shown in FIG. 3, when starting the heater energization control process (time t1), the ECU 50 first reads the voltage Vs on the porous electrode 6c side of the Vs cell 6, and turns off the open / close switch SW1. Open / close switch SW
2, a constant current is applied to the Vs cell 6 in a direction opposite to the small current iCP, and then at a time t2 after a lapse of a predetermined time T1 (for example, 60 μsec.), The porous electrode of the Vs cell 6 is again turned on. 6c side voltage Vs is read. Further, at the time point t3 when a predetermined time T2 (for example, 100 μsec.) Has elapsed after the start of the control, the open / close switch SW2 is turned off and the open / close switch SW3 is turned on, so that the Vs cell 6 has the same direction as the minute current iCP (ie A constant current is supplied in a direction in which oxygen in the chamber 20 is pumped into the closed space side. Then, at the time t4 when a predetermined time T3 (for example, 200 μsec.) Has elapsed after the start of the control, the open / close switch SW3 is turned off.
After a lapse of a predetermined time T4 (for example, 500 μsec.), The open / close switch SW1 is turned on, and the process returns to the NOx concentration detection process.

Also, during execution of the heater energization control process,
The ECU 50 calculates the internal resistance RVS of the Vs cell 6 from the deviation ΔVs of the porous electrode 6c side voltage Vs detected at the time t1 and the time t2, respectively, and sets the heater energizing circuit 44 so that the internal resistance RVS becomes a target value. Controls the amount of current supplied to the heaters 12 and 14 from. As a result, the heaters 12, 14
Is controlled so that the internal resistance RVS (and, consequently, the sensor temperature) of the Vs cell 6 becomes constant, and the NOx sensor 2 is maintained at the target temperature.

As described above, in the nitrogen oxide concentration detecting apparatus of the present embodiment, the first
By forming the porous electrode 6b on the side of the pump cell 4 only within a very narrow range around the diffusion-controlling layer 6d, the amount of oxygen flowing from the first measurement chamber 20 to the second measurement chamber 26 in the Vs cell 6 can be accurately determined. When the NOx concentration is detected, the output voltage from the Vs sensor 6 becomes the reference voltage VCO, and the amount of oxygen flowing from the first measurement chamber 20 to the second measurement chamber 26 becomes a predetermined value. Next, the first pump current IP1 is controlled.

Therefore, according to the present embodiment, the NOx concentration can be accurately detected from the first pump current IP1 and the second pump current IP2. Here, in the above embodiment, the diffusion-controlling layer (first diffusion-controlling layer) 4 d is formed at the center of the electrodes 4 b and 4 c constituting the first pump cell 4,
Although opposed to the heater 12 (specifically, the center of the heater wiring 12b), for example, as shown in FIGS. 4 and 5, the solid electrolyte layer 18 forming the first measurement chamber 20 is diffused as a first diffusion-controlling layer. The gas to be measured may be taken in from the side surface of the NOx sensor 2 by forming the rate controlling layers 20a and 20b.

Further, in the above embodiment, the first Vs cell 6 is constituted so that the Vs cell 6 can accurately detect the amount of oxygen flowing from the first measurement chamber 20 to the second measurement chamber 26. Although the porous electrode 6b on the pump cell 4 side is formed on the periphery of the diffusion-controlling layer 6d, the porous electrode 6b may be formed on the diffusion-controlling layer 6d, for example, as shown in FIG. As shown in FIG. 7, the diffusion-controlling layer 6d and the first
You may form between each solid electrolyte layer 6a which comprises the measurement chamber 20.

However, when the porous electrode 6b is formed on the diffusion-controlling layer 6d (the upper surface on the first measuring chamber 20 side), the porous electrode 6b corresponds to the amount of oxygen flowing into the second measuring chamber 26 between the electrodes 6b and 6c. In order to generate such a voltage, the diffusion-controlling layer 6d needs to be formed of a porous solid electrolyte (such as zirconia) capable of controlling the diffusion of the gas to be measured.

The NOx sensor shown in FIGS. 4 to 7 has the same configuration as that of the NOx sensor 2 shown in FIGS. 1 and 2 except for the above-described parts, and thus a detailed description is omitted. Next, it is measured how much the detection accuracy of the NOx concentration is improved by forming the porous electrode 6b of the Vs cell 6 on the periphery of the diffusion-controlling layer 6d or on the diffusion-controlling layer 6d. The following describes the results of the experiment.

In this experiment, the NOx sensor of the first embodiment shown in FIGS. 1 and 2 (that is, the NOx sensor in which the first diffusion-controlling layer is opposed to the heater 12) was used. And the NOx sensor of the second embodiment (that is, the NOx sensor in which the first diffusion-controlling layer is provided on the side surface of the sensor). In addition, in the experiments, the size (diameter) of the porous electrode 6b was set to 2 mm and 1 mm for each of these types of NOx sensors, and the porous electrode 6b was set to the second type as shown in FIG. When a sensor formed on the diffusion-controlling layer is manufactured, and each of these NOx sensors (a total of six types of NOx sensors) is attached to the exhaust pipe of the internal combustion engine, respectively, when the internal combustion engine is accelerated and decelerated, The detection result of the NOx concentration (that is, the second pump current IP
This was performed by averaging the time ΔT until 2) became stable (see FIG. 8).

As a result, as shown in Table 1 below, the porous electrode 6 was used in any type of NOx sensor.
When b is formed on the periphery of the diffusion-controlling layer 6d, the smaller the porous electrode 6b is, the smaller the N changes with respect to the transient change of the gas to be measured (in this case, both the temperature and the oxygen concentration change).
The time ΔT required to stabilize the Ox concentration becomes shorter, and the time when the porous electrode 6b is formed directly on the diffusion-controlling layer 6d is more stable than when the porous electrode 6b is formed on the periphery of the diffusion-controlling layer 6d. It was found that ΔT could be shortened. this is,
The closer the porous electrode 6d is to the diffusion-controlling layer 6d, the more the first
This is because the amount of oxygen flowing from the measurement chamber 20 to the second measurement chamber 26 can be accurately detected, and the relationship between the two values of the first pump current and the second pump current is simplified, and the accuracy is improved.

Further, the first diffusion-controlling layer is formed by the N diffusion layer of the second embodiment.
Rather than being provided on the side of the sensor like an Ox sensor,
It was found that facing the heater 12 like the NOx sensor of the embodiment can shorten the time ΔT until the NOx concentration stabilizes with respect to the transient change of the gas to be measured. This is because if the first diffusion-controlling layer faces the heater 12, the first
This is because the gas to be measured flowing into the measurement chamber 20 can be heated by the heater 12, and the rate at which the temperature change of the gas to be measured is affected by the detection result of the NOx concentration can be reduced.

[0060]

[Table 1]

Note that N in the first embodiment used in the above experiment was used.
The dimensions of each part of the Ox sensor are as shown in FIG.
The first pump cell 4, the Vs cell 6, the second pump cell 8, the first measuring chamber 20, the diffusion-controlling layer 22d, and the solid electrolyte layers 4a, 6a, 8a, 18, 22,
24 and the thickness A of the heaters 12 and 14 are each 0.25 m.
m, and the thickness B of the spacers 28, 29 for determining the gap between the heaters 12, 14 and the first and second pump cells 4, 8.
Is 0.1 mm. Therefore, the thickness D of the NOx sensor main body excluding the heaters 12 and 14 is 1.5 mm, and the thickness C of the second diffusion control layer composed of the diffusion control layers 6d and 22d is 0.5.
mm.

The diameter of each of the diffusion-controlling layers 4d, 6d, 22d constituting the first and second diffusion-controlling layers is 0.5 mm.
The width F of the first measurement chamber 20 and the second measurement chamber 26 in the lateral direction of the NOx sensor is 2.5 mm, and the width G of the NOx sensor is 3.5 mm. Further, as shown in FIG. 9B, the width I of the porous electrodes 4b and 4c formed around the diffusion-controlling layer (first diffusion-controlling layer) 4d is 2.4 mm, and is in the sensor longitudinal direction. The length H is 3.0 mm. Further, as shown in FIG. 9C, the rectangular hole formed in the solid electrolyte layer 18 to form the first measurement chamber 20 is K (K = 0.5 mm) from the upper end of the sensor, and the left and right sides. J (J
= 0.5 mm), and when the porous electrode 6b is formed on the periphery of the diffusion-controlling layer 6d, the diameter L is 2 mm.
mm and 1 mm. When the porous electrode 6b was formed on the diffusion-controlling layer 6d, the diameter of the porous electrode 6b was 0.4 mm.

In the NOx sensor according to the second embodiment, the length of the diffusion-controlling layers (first diffusion-controlling layers) 20a and 20b formed on the side surface of the sensor along the sensor longitudinal direction is set to 1 mm. The porous electrodes 4b and 4c that constitute one pump cell were 2.4 mm × 3.0 mm in size and formed over the entire surface without forming a hollow portion. The other dimensions were exactly the same as those of the NOx sensor of the first embodiment.

[Brief description of the drawings]

FIG. 1 is an exploded perspective view illustrating a configuration of a NOx sensor according to an embodiment.

FIG. 2 is a schematic configuration diagram illustrating a configuration of an entire nitrogen oxide concentration detection device according to an embodiment.

FIG. 3 is a time chart illustrating an operation of a heater energization control process executed to control a sensor temperature in the ECU of the embodiment.

FIG. 4 shows a NO having a first diffusion limiting layer provided on a side surface of a sensor.
FIG. 3 is an exploded perspective view illustrating a configuration of an x sensor.

FIG. 5 is a cross-sectional view illustrating an electrode arrangement of the NOx sensor shown in FIG.

FIG. 6 is a cross-sectional view illustrating an electrode arrangement of a NOx sensor in which an electrode on a first measurement chamber side constituting a Vs cell is formed on a second diffusion layer.

FIG. 7 is a cross-sectional view illustrating an electrode arrangement of a NOx sensor in which an electrode on a first measurement chamber side constituting a Vs cell is formed between a second diffusion layer and a solid electrolyte layer.

FIG. 8 is a time chart showing changes of a first pump current and a second pump current with respect to a transient change of a gas to be measured.

FIG. 9 is an explanatory diagram illustrating dimensions of each part of the NOx sensor used in the experiment.

[Explanation of symbols]

2 NOx sensor (nitrogen oxide concentration detector) 4 1st pump cell (first oxygen pumping cell) 6 Vs cell (oxygen concentration measuring cell) 8 2nd pump cell (second oxygen pumping cell) 1
2, 14 ... heaters 4a, 6a, 8a, 18, 22, 24 ... solid electrolyte layers 4b, 4c, 6b, 6c, 8b, 8c ... porous electrodes 4d, 20a, 20b ... diffusion control layers (first diffusion control layers) 6d, 22d... Diffusion controlling layer (second diffusion controlling layer) 6e
... insulating film 6f ... leakage resistance parts 12a, 12c, 14a, 14c
... heater boards 12b and 14b ... heater wiring 20 ... first measurement chamber
26 ... second measurement room

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Noboru Ishida 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Prefecture Inside Japan Specialty Ceramics Co., Ltd. No. 18 Japan Special Ceramics Co., Ltd.

Claims (6)

[Claims] A first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, and the first oxygen pumping cell and the oxygen concentration measuring cell communicate with the gas to be measured via the first diffusion-controlling layer. And a second oxygen pumping cell in which an oxygen ion-conductive solid electrolyte layer is sandwiched between porous electrodes, and is connected to the first measurement chamber via a second diffusion-controlling layer. A second measurement chamber, wherein oxygen is pumped from the first measurement chamber by the first oxygen pumping cell so that the output voltage of the oxygen concentration measurement cell becomes a constant value, and the second oxygen pumping is performed. By applying a constant voltage to the cell in a direction in which oxygen is pumped from the second measurement chamber, the nitrogen oxides in the gas to be measured can be calculated from the current flowing in the first oxygen pumping cell and the current flowing in the second oxygen pumping cell. Detect In the elemental oxide concentration detector, the oxygen concentration measurement cell is passed through the first diffusion-controlling layer from the first measurement room without being affected by the oxygen distribution in the first measurement room. A nitrogen oxide concentration detector which is arranged at a position where the amount of oxygen of the gas to be measured flowing to the side can be detected. 2. The first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell are respectively formed on mutually different thin plate-shaped solid electrolyte layers, and each of the solid electrolyte layers is formed by the first and second solid electrolyte layers. The first and second measurement chambers are formed in the gap by laminating the solid electrolyte layer forming the second oxygen pumping cell on the outside with a predetermined gap between the solid electrolyte layers. A thin plate-like heater substrate provided with a heater for heating is disposed on both sides in the stacking direction of the layers with a predetermined gap therebetween, and the heater of the solid electrolyte layer in which the first oxygen pumping cell is formed is further provided. The nitrogen oxide concentration detector according to claim 1, wherein the first diffusion-controlling layer is formed at a position facing a central portion of a heater formed on a substrate. 3. The method according to claim 1, wherein the second diffusion-controlling layer is formed by projecting the nitrogen oxide concentration detector from the stacking direction.
The nitrogen oxide concentration detector according to claim 2, wherein the nitrogen oxide concentration detector is formed so as to overlap at least a part of the diffusion-controlling layer. 4. The nitrogen oxide concentration detector according to claim 1, wherein said oxygen concentration measurement cell is formed on a periphery of said second diffusion-controlling layer. 5. The second diffusion-controlling layer is constituted by a porous solid electrolyte layer capable of controlling the gas to be measured by diffusion, and the first measurement chamber side electrode of the oxygen concentration measuring cell is connected to the second diffusion-controlling layer. The nitrogen oxide concentration detector according to claim 4, wherein the nitrogen oxide concentration detector is formed on a rate-controlling layer. 6. The first measuring chamber side electrode of the oxygen concentration measuring cell is formed between a solid electrolyte layer constituting the first measuring chamber and the second diffusion-controlling layer. Item 6. A nitrogen oxide concentration detector according to Item 4.