JP3431493B2 - Nitrogen oxide concentration detector - Google Patents

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 equipment such as an internal combustion engine.

[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 or SAE paper.
No. As disclosed in 960334 P137-142 1996 and the like, a first measurement chamber communicated with the measured gas side through a first diffusion rate controlling layer, and a second diffusion rate controlling layer in the first measurement chamber. The communicating second measurement chamber is formed by an oxygen ion conductive solid electrolyte layer, and the first measurement chamber has a solid electrolyte layer sandwiched between porous electrodes to form a first oxygen pumping cell and an oxygen concentration. Forming a measuring cell,
Further, in the second measurement chamber, it is known that a solid electrolyte layer is sandwiched between porous electrodes to form a second oxygen pumping cell.

In this type of NOx sensor, the first oxygen pumping cell is energized to pump out oxygen from the first measuring chamber so that the output voltage from the oxygen concentration measuring cell becomes a preset constant value. Accordingly, while controlling the oxygen concentration in the first measurement chamber to a constant concentration, a constant voltage is applied to the second oxygen pumping cell so that oxygen is further pumped out from the second measurement chamber. The NOx concentration in the measured gas can be detected from the current flowing in the cell (hereinafter referred to as the second pump current).

That is, in the exhaust gas from the internal combustion engine, which is the gas to be measured, there are other gas components such as oxygen, carbon monoxide, carbon dioxide, etc. in addition to NOx. First, a current (hereinafter referred to as a first pump current) is made to flow through 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 oxygen has been extracted flows. On the side, the catalytic function of the second oxygen pumping cell causes NOx in the gas to be measured.
Is decomposed into nitrogen and oxygen, and is operated so as to extract oxygen from the second measurement chamber, so that the gas to be measured is not affected by other gas components in the gas to be measured from the second pump current. The NOx concentration in the inside can be detected.

Further, 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. 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 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, and the signal processing system for that purpose is required. It has been found that there is a problem that it becomes complicated and the cost of the detection device increases.
Hereinafter, this problem will be described.

First, the conventional NOx sensor pumps out oxygen from the first measurement chamber using the 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 will be substantially only NOx components, and this gas to be measured will be the second gas. It was developed in accordance with the technical idea that the NOx concentration can be detected from the second pump current flowing through the second oxygen pumping cell if it is decomposed into nitrogen and oxygen by the catalytic action of the oxygen pumping cell.

However, in practice, if 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 from the second pump current will be increased. 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 measuring chamber is controlled to be substantially zero is that the first pump current is controlled to measure the measured object in the first measuring chamber. It is considered that this is because the nitrogen monoxide component in the gas is decomposed and the measured gas corresponding to the NOx component does not flow into the second measurement chamber.

In this way, the first oxygen concentration in the first measuring chamber is set to a low oxygen concentration of about 1000 ppm.
When the NOx concentration is detected from the second pump current while controlling the oxygen pumping cell, the second measurement chamber has
Not only NOx (nitrogen oxide) in the measured gas, but also the first
Since oxygen in the measurement chamber also flows in, the second pump current changes depending on the NOx concentration in the measurement gas, but is also affected by the oxygen concentration in the measurement gas. As a result, conventionally, the detection result of the NOx concentration may be affected by the oxygen concentration in the measured gas around the sensor, and the detected NOx concentration may not correspond to the actual value.

Such a problem can be solved by utilizing the current flowing in the first oxygen pumping cell (hereinafter referred to as the first pump current). That is, by measuring the first pump current and the second pump current, respectively, and detecting the nitrogen oxide concentration from these two values, even if the oxygen concentration in the measured gas around the sensor fluctuates, the nitrogen is accurately The oxide concentration can be detected.

However, as described above, the first pump current and the second pump current are
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 these problems, and when detecting the NOx concentration using the NOx sensor, these two values and NOx are detected when detecting the NOx concentration from the first pump current and the second pump current.
The purpose of the present invention is to connect the x concentrations with a simple relationship so that the NOx concentration can be detected with high accuracy.

[0013]

The nitrogen oxide concentration detector according to the present invention (claim 1), which has been made to achieve the above object, has a solid electrolyte layer having oxygen ion conductivity as a porous electrode. A first measuring chamber having a first oxygen pumping cell and an oxygen concentration measuring cell sandwiched therebetween and communicating with the measured gas side through a first diffusion-controlling layer, and an oxygen ion conductive solid electrolyte layer are porous. A second measurement chamber having a second oxygen pumping cell sandwiched between the electrodes and a second measurement chamber communicating with the first measurement chamber via a second diffusion-controlling layer, and the output voltage of the oxygen concentration measurement cell is Oxygen is pumped out of the first measuring chamber by the first oxygen pumping cell so as to have a constant value, and a constant voltage is applied to the second oxygen pumping cell in a direction of pumping oxygen out of the second measuring chamber. By
In a nitrogen oxide concentration detector for detecting nitrogen oxides in a gas to be measured from a current flowing through the first oxygen pumping cell and a current flowing through the second oxygen pumping cell, the oxygen concentration measuring cell is provided with the second diffusion gas. Formed on the periphery of the rate-controlling layer
It is characterized by that.

[0014]

As described above, in the nitrogen oxide concentration detector (NOx sensor) of the present invention, the oxygen concentration measuring cell is formed at the periphery of the second diffusion controlling layer.

This is because the oxygen concentration measuring cell, which is arranged in the conventional NOx sensor to detect the oxygen concentration in the first measuring chamber, can detect the amount of oxygen flowing from the first measuring chamber to the second measuring chamber. by this <br/> and forming the periphery of a second diffusion-controlling layer, the oxygen concentration in the measurement gas flowing into the second measurement chamber accurately detected, so that the oxygen concentration reaches the predetermined value, the first This is so that the oxygen pumping cell can be controlled.

That is, as described above, in the conventional NOx sensor, the oxygen concentration in the first measurement chamber is substantially zero (theoretically, 1).
If the first oxygen pumping cell is controlled so that the oxygen concentration is about 0 ^-9 atm), the NOx concentration could not be detected from the second pump current. First, so that the concentration
It controlled the oxygen pumping cell. The reason for this is that when the first oxygen pumping cell is controlled so that the oxygen concentration in the first measurement chamber becomes substantially zero, the nitric oxide component in the gas to be measured in the first measurement chamber is decomposed, It is considered that this is because the measured gas corresponding to the NOx component does not flow into the two measurement chambers.

The inventors of the present invention 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 in 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, the external measurement gas flows into the first measurement chamber through the first diffusion-controlling layer, and oxygen is extracted from the flown measurement gas in the first oxygen pumping cell.
The oxygen distribution in the first measurement chamber is not uniform, and as a result, in the conventional NOx sensor, the oxygen concentration of the measured gas 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, and its detection was difficult.

Therefore, in the present invention, the oxygen concentration measuring cell is provided with a second diffusion-controlling layer capable of detecting not the oxygen concentration in the first measuring chamber but the amount of oxygen flowing from the first measuring chamber to the second measuring chamber.
By forming on the periphery of the, in the oxygen 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 passed through the first oxygen pumping cell to such an extent that the NOx component in the first measuring chamber is decomposed, and the first measuring chamber Therefore, 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 to secure the detection accuracy of the NOx concentration in the same manner as the above-mentioned conventional NOx sensor. It is desirable to provide a heater for heating the. When the heater for heating and controlling the temperature is provided as described above, the first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell are different from each other as described in claim 2. It is formed in a thin plate-like solid electrolyte layer, and each solid electrolyte layer is laminated with a predetermined gap, with the solid electrolyte layer on which the first and second oxygen pumping cells are formed being on the outside, to form a gap in the gap. The first measurement chamber and the second measurement chamber are configured, and thin plate-shaped heater substrates provided with heaters for heating are respectively arranged on both sides of each solid electrolyte layer in the stacking direction with a predetermined gap therebetween. It is desirable to form the first diffusion-controlling layer at a position where the solid electrolyte layer having the first oxygen pumping cell formed therein faces the central portion of the heater formed on the heater substrate.

That is, according to the nitrogen oxide concentration detector of the present invention, the solid electrolyte layer having the oxygen concentration measuring cell has the first oxygen pumping cell and the second oxygen pumping cell. Since it is sandwiched between the solid electrolyte layers and the heater substrates are arranged on both sides in the stacking direction, it becomes easy to control each cell to a predetermined temperature by controlling the energization of the heater, and moreover, from the first diffusion layer. First
The gas to be measured flowing into the measurement chamber, and further 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, it is possible to prevent the temperature variation of each cell in the sensor main body from occurring and to prevent each cell from being influenced by the temperature of the gas to be measured, thereby detecting the NOx concentration. Can be further improved.

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

Further , an oxygen concentration measuring cell as in the present invention is used.
When it is formed on the peripheral edge of the second diffusion-controlling layer, in particular, the method according to claim 4
As described above, the second diffusion-controlling layer is composed of a porous solid electrolyte layer capable of diffusion-controlling the gas to be measured, and the first measurement chamber side electrode of the oxygen concentration measuring cell is provided on the second diffusion-controlling layer. or formed, or, as described in claim 5, the first measurement chamber side electrode of the oxygen concentration measuring cell, formed between the solid electrolyte layer and the first diffusion controlling layer constituting the first measurement chamber Good to do.
That is, by doing so, it becomes possible to more accurately detect the oxygen amount of the measured gas flowing from the first measurement chamber to the second measurement chamber in the oxygen concentration measurement cell, and thus 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 the configuration of a nitrogen oxide concentration detector (NOx sensor) according to an embodiment of the present invention, FIG.
FIG. 3 is a configuration diagram showing the overall configuration of a nitrogen oxide concentration detection device that detects the NOx concentration using this NOx sensor.

As shown in FIG. 1, the NOx sensor 2 has a first
Oxygen pumping cell (hereinafter referred to as the first pump cell)
4, oxygen concentration measuring cell (hereinafter referred to as Vs cell) 6, 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 parts 4bl, 4cl are formed, and further,
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,
The diffusion-controlling layer 4d is formed.

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

Then, the porous electrode 6 of this Vs cell 6 is used.
b, 6c and the porous electrodes 4b, 4c of the first pump cell 4 have substantially the same center position 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 controlling layers 6d and 4d are arranged to face each other. The circular porous electrodes 6b and 6c formed in the Vs cell 6 are arranged around the diffusion control layer 6d, and
Rectangular porous electrodes 4b, 4 formed in the pump cell 4
It is smaller than c. In particular, the pair of porous electrodes 6
b, 6c, porous electrode 6b on the first pump cell 4 side
Is formed only within an extremely narrow range on the periphery of 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 control layer 6d can be accurately detected by the porous electrodes 6b and 6c. In order to do so, the lead portions 6bl, 6
Insulating film 6 made of zirconia or the like so as to cover cl from the outside
e is formed, and between the lead portions 6bl and 6cl, a leakage resistance portion 6f for leaking a part of oxygen pumped into the porous electrode 6c side to the porous electrode 6b side by the energization control described later. Are formed.

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

Also on the porous electrode 6c side of the Vs cell 6,
A solid electrolyte layer 22 having the same shape as each of the solid electrolyte layers 4a and 6a is laminated. Then, in this solid electrolyte layer 22, a circular hole having the same size is formed at the same position as the diffusion controlling layer 6d of the Vs cell 6, and the circular hole is filled with a porous filler such as alumina. Thus, the diffusion rate controlling layer 22d is formed.

On the other hand, like the first pump cell 4, the second pump cell 8 has rectangular porous electrodes 8b and 8c and their lead portions 8bl and 8cl on both sides of the plate-shaped solid electrolyte layer 8a. Is formed. Then, the second pump cell 8 is laminated on the solid electrolyte layer 22 laminated on the Vs cell 6 via the 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 laminated body of the first pump cell 4, the Vs cell 6, and the second pump cell 8 thus laminated, that is, on the outer sides of the first pump cell 4 and the second pump cell 8, spacers 28, The heaters 12 and 14 are laminated at a predetermined interval by 29. The heaters 12 and 14 are 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 wires 12b and 14b, which are respectively sandwiched between 4a and 14c and embedded in each heater substrate,
It is composed of the lead portions 12bl and 14bl. In the spacers 28 and 29, the heaters 12 and 14 are the porous electrodes 4b of the first pump cell 4 and the second pump cell 8, respectively.
And 8c, and the heaters 12 and 14 and the first pump cell 4 and the second pump cell 8 are arranged so as to face each other with a gap therebetween.

Here, the solid electrolyte layers 4a, 6a,
Typical solid electrolytes that compose ... are solid solutions of zirconia and yttria and zirconia and calcia, but other solid solutions of hafnia, perovskite type oxide solid solutions, trivalent metal oxide solid solutions, etc. are also used. it can. Further, it is preferable to use platinum or rhodium or its alloy having a catalytic function for the porous electrode provided on the surface of each solid electrolyte layer 4a, 6a, 8a. Then, as the forming method, for example, a thick film forming method in which platinum powder is mixed with powder of the same material as the solid electrolyte layer to form a paste, screen-printed on the solid electrolyte layer, and then sintered, or thermal spraying A method for forming a coating film is known. Further, it is preferable that the diffusion-controlling layers 4d, 6d, 22d be made of ceramics having thin through holes 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 thereof are made of platinum or a platinum alloy in order to reduce the resistance value and reduce the electrical loss in the lead portion. preferable. Also, the heater substrate 1
2a, 12b, 14a, 14c and spacers 28, 29
Alumina, spinel, forsterite, steatite, zirconia or the like can be used as the material.

In particular, when zirconia is used as the material of the heater substrate and the spacer, the heater and each pump cell can be integrated and sintered at the same time.
This is suitable for manufacturing 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. (Alumina or the like) is provided.

When alumina is used for the heater substrate, a porous material is used as the spacer in order to prevent cracks and the like from occurring due to the difference in shrinkage ratio and the difference in thermal expansion coefficient during sintering with each pump cell. Good to use. It is also possible to sinter the heater and each pump cell separately, and later bond them by using an inorganic material such as cement as a bonding material that also serves 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 configured as described above has the first pump cell 4 and Vs constituting the NOx sensor 2. A drive circuit 40 for energizing the cell 6 and switching the energizing path, a detection circuit 45 for detecting a current (first pump current) IP1 flowing in the first pump cell 4 via a resistor R4, and a NOx sensor 2 And a pair of heaters 1 provided in the NOx sensor 2 and a detection circuit 42 for applying a constant voltage to the second pump cell 8 constituting the NOx sensor 2 to detect a current (second pump current) IP2 flowing at that time.
The heater energizing circuit 44 for energizing the cells 2 and 14 to heat the cells 4, 6 and 8, the driving circuit 40 and the heater energizing circuit 4
4, an electronic control circuit (hereinafter referred to as an ECU) composed of a microcomputer for driving and controlling No. 4 and calculating 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
It is composed of 0 and 0.

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

This control section 40a has an open / close switch SW1.
When is on, it operates as follows. First, oxygen in the first measurement chamber 20 is pumped to the porous electrode 6c side of the Vs cell 6 by causing a constant minute current iCP to flow through the Vs cell 6 via the resistor R2. This porous electrode 6c is
Since it is closed by the solid electrolyte layer 22 and communicates with the porous electrode 6b side through the leakage resistance portion 6f, the closed space in the porous electrode 6c has a constant oxygen concentration due to the application of the minute current iCP. , Acts as an internal oxygen reference source.

Further, as described above, the porous electrode 6 of the Vs cell 6 is
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 between the oxygen concentration in the first measurement chamber 20 and the oxygen concentration on the internal oxygen reference source side, and the porous electrode 6
The c-side voltage Vs becomes a voltage according to the oxygen concentration in the measured gas flowing into the second measurement chamber 26 from the first measurement chamber 20.
Since this voltage is input to the differential amplifier AMP, the differential amplifier AMP outputs a voltage corresponding to the deviation (VCO-input voltage) between the reference voltage VCO and its input voltage. Is the porous electrode 4 of the first pump cell 4.
applied to b.

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

In this embodiment, the amount of oxygen flowing from the first measuring chamber 20 into 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 made to flow a current to the extent that the NOx component in the first measurement chamber 20 is decomposed. For example, the reference voltage VCO that determines the amount of oxygen flowing from the first measuring chamber 20 into the second measuring chamber 26 is set to a value of about 450 mV. However, it is not always necessary to set this reference voltage VCO to 450 mV, and 350
It may be in the range of mV to 450 mV.

Next, in the drive circuit 40, the controller 40
In addition to a, a constant current circuit 40b that is connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW2 and flows 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 through the open / close switch SW3 and causes a constant current to flow between the porous electrodes 6b-6c in the same direction as the minute current iCP.
c and are provided.

Each of the constant current circuits 40b and 40c has V
This is for detecting the internal resistance RVS of the s cell 6.
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 on the ECU 50 side by applying the constant current. The constant currents passed by the constant current circuits 40b and 40c are set to the same current value except that the current directions are different. This current value is larger than the minute current iCP supplied to the Vs cell 6 via the resistor R2.

Further, 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 are the ECU 50.
Only when the control unit 40a is activated by turning on / off the control signal from the NOx concentration to perform the NOx concentration detection operation, 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 which constitutes the detection circuit 42. The application direction of this constant voltage VP2 is
In the second pump cell 8, a current flows from the porous electrode 8c to the 8b side, and oxygen in the second measurement chamber 26 is pumped out to the outside, so that the porous electrode 8c side is the positive electrode and the porous electrode 8
It is set such that the side b is a negative electrode. The constant voltage VP2 is applied to the diffusion rate controlling layers 6d and 22 from the first measuring chamber 20.
The voltage is set to, for example, 450 mV, which is capable of decomposing the NOx component in the gas to be measured in the second measurement chamber 26 that flows in via d and pumping out the oxygen component.

The resistor R3 has a second pump current I flowing through the second pump cell 8 when the constant voltage VP2 is applied.
It is for converting P2 into voltage VIP2 and inputting it to the ECU 50 as a detection signal of the second pump current IP2. In the nitrogen oxide concentration detecting apparatus of the present 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 that has flowed in via the diffusion-controlling layer (first diffusion-controlling layer) 4d flows into the second measurement chamber 26 via the diffusion-controlling layers (second diffusion-controlling layer) 6d and 22d. Since the amount of oxygen at the time of performing can be suppressed sufficiently (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 calculated. It is possible to measure NOx concentration. That is, the ECU 50 side detects the detection signal VIP1 of the first pump current IP1 and the second pump current I1.
By reading the detection signal VIP2 of P2 and executing a predetermined arithmetic process, the detection signal VIP1 (in other words, the first detection signal VIP1) is read.
Pump current IP1) and detection signal VIP2 (in other words, the second
The NOx concentration in the measured gas can be detected from the pump current IP2).

By the way, in order to secure 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 measuring 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 reaches the target temperature. Therefore, the ECU 50 normally operates the open / close switch SW1.
Is turned on and the open / close switches SW2 and SW3 are turned off to read the detection signal VIP1 of the first pump current IP1 and the detection signal VIP2 of the second pump current IP2 to detect the NOx concentration in the measured gas. Although the NOx concentration detection processing is executed, the sensor 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 every predetermined time T0. However, that temperature is the target temperature (for example, 850
C)) from the heater energizing circuit 44 to the heater 1
The heater energization control process for controlling the energization amount to 2 and 14 is also executed.

That is, as shown in FIG. 3, when the heater energization control process is started (time point 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 is turned on to cause a constant current to flow in the Vs cell 6 in the direction opposite to the minute current iCP, and then at time t2 when a certain time T1 (for example, 60 μsec.) Has elapsed, the porous electrode of the Vs cell 6 is again provided. The 6c side voltage Vs is read. Further, at the time t3 when a predetermined time T2 (for example, 100 μsec.) Has elapsed after the start of control, the opening / closing switch SW2 is turned off and the opening / closing switch SW3 is turned on to make the Vs cell 6 in the same direction as the minute current iCP (that is, the first measurement). A constant current is passed in the direction in which oxygen in the chamber 20 is pumped to the closed space side. Then, at the time t4 when a predetermined time T3 (for example, 200 μsec.) Elapses after the control is started, the open / close switch SW3 is turned off, and after the control is started
When a predetermined time T4 (for example, 500 μsec.) Has elapsed, the open / close switch SW1 is turned on and the process returns to the NOx concentration detection process.

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 voltage Vs on the porous electrode 6c side detected at the time points t1 and t2, respectively, and the heater energizing circuit 44 is set so that the internal resistance RVS becomes a target value. To control the amount of current supplied to the heaters 12 and 14. As a result, the heaters 12, 14
The amount of current supplied to the Vs cell 6 is controlled so that the internal resistance RVS (and thus 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 detector of this embodiment, the first Vs cell 6 is formed.
By forming the porous electrode 6b on the side of the pump cell 4 only within an extremely narrow range of the periphery of the diffusion-controlling layer 6d, the amount of oxygen flowing from the first measuring chamber 20 to the second measuring chamber 26 in the Vs cell 6 can be accurately measured. When the NOx concentration is detected, the output voltage from the Vs sensor 6 becomes the reference voltage VCO, and the inflow amount of oxygen from the first measurement chamber 20 to the second measurement chamber 26 becomes a predetermined value. Then, the first pump current IP1 is controlled.

Therefore, according to this embodiment, the NOx concentration can be accurately detected from the first pump current IP1 and the second pump current IP2. Here, in the above-mentioned embodiment, by forming the diffusion-controlling layer (first diffusion-controlling layer) 4d in the center of the electrodes 4b and 4c forming the first pump cell 4,
The heater 12 (more specifically, the center of the heater wire 12b) is opposed to the heater 12. 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 rate controlling layer. By forming the rate controlling layers 20a and 20b, the gas to be measured may be taken in from the side surface of the NOx sensor 2.

Further, in the above embodiment, the first Vs cell 6 is constructed so that the Vs cell 6 can accurately detect the amount of oxygen flowing into the second measurement chamber 20 from the first measurement chamber 20. Although the porous electrode 6b on the side of the pump cell 4 is formed on the periphery of the diffusion-controlling layer 6d, the porous electrode 6b may be formed on the diffusion-controlling layer 6d as shown in FIG. 6, or 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 (upper surface on the first measurement chamber 20 side), it corresponds to the amount of oxygen flowing into the second measurement chamber 26 between the electrodes 6b-6c. In order to generate the above voltage, the diffusion-controlling layer 6d needs to be composed of a porous solid electrolyte (such as zirconia) capable of diffusion-controlling the gas to be measured.

The NOx sensor shown in FIGS. 4 to 7 has the same structure as that of the NOx sensor 2 shown in FIGS. 1 and 2, except for the above-mentioned description, and therefore detailed description thereof will be omitted. Next, it is measured how much the NOx concentration detection accuracy 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 result of the experiment will be described.

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 control layer is opposed to the heater 12) and FIGS. 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) and the NOx sensor of the second example shown in FIG. Further, in the experiment, for each of these types of NOx sensors, the size (diameter) of the porous electrode 6b was set to 2 mm and 1 mm, and further, the porous electrode 6b was used as shown in FIG. A sensor formed on the diffusion-controlling layer is produced, and each of these NOx sensors (total 6 types of NOx sensors) is attached to an exhaust pipe of the internal combustion engine, and when the internal combustion engine is accelerated and decelerated, Detection result of NOx concentration (that is, second pump current IP
It 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, the more 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 until the Ox concentration stabilizes becomes short, and the time when the NOx concentration stabilizes when the porous electrode 6b is formed directly on the diffusion control layer 6d rather than on the periphery of the diffusion control layer 6d. It was found that ΔT can be shortened. this is,
The closer the porous electrode 6d is to the diffusion control layer 6d, the first
This is because the amount of oxygen flowing from the measurement chamber 20 into the second measurement chamber 26 can be accurately detected, the relationship between the NOx concentration between the two values of the first pump current and the second pump current is simplified, and the accuracy is improved.

The first diffusion control layer is made of N of the second embodiment.
Rather than installing on the side of the sensor like the 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,
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 NOx concentration detection result can be reduced.

[0060]

[Table 1]

The N of 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 measurement chamber 20, the diffusion rate controlling layer 22d, and the solid electrolyte layers 4a, 6a, 8a, 18, 22, which constitute the second measurement chamber 26,
24 and heaters 12 and 14 have a thickness A of 0.25 m each
m, and the thickness B of the spacers 28 and 29 that determine the gap between the heaters 12 and 14 and the first and second pump cells 4 and 8.
Is 0.1 mm. Therefore, the thickness D of the NOx sensor main body portion excluding the heaters 12 and 14 is 1.5 mm, and the thickness C of the second diffusion controlling layer composed of the diffusion controlling layers 6d and 22d is 0.5.
mm.

The diameter of each of the diffusion rate controlling layers 4d, 6d, 22d forming the first and second diffusion rate 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 widths I of the porous electrodes 4b and 4c formed around the diffusion-controlling layer (first diffusion-controlling layer) 4d are 2.4 mm, which are measured 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 side surfaces. Than J (J
= 0.5 mm) and the porous electrode 6b is formed on the periphery of the diffusion controlling layer 6d, the diameter L is 2
It was formed to have a size of 1 mm and 1 mm. When forming the porous electrode 6b on the diffusion-controlling layer 6d, the diameter of the porous electrode 6b was 0.4 mm.

Further, regarding the NOx sensor of the second embodiment, the length of the diffusion rate controlling layers (first diffusion rate controlling layers) 20a and 20b formed on the side surface of the sensor along the sensor longitudinal direction is set to 1 mm, and The porous electrodes 4b and 4c constituting one pump cell had a size of 2.4 mm × 3.0 mm and were formed on the entire surface without forming a hollow portion. The other dimensions are the same as those of the NOx sensor of the first embodiment.

[Brief description of drawings]

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

FIG. 2 is a schematic configuration diagram showing an overall configuration of a nitrogen oxide concentration detecting device of an embodiment.

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

FIG. 4 is a NO in which a first diffusion limiting layer is provided on a side surface of a sensor.
It is a disassembled perspective view showing the structure of an x sensor.

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

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

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

FIG. 8 is a time chart showing changes in the first pump current and the second pump current with respect to a transient change in the 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 ... First pump cell (first oxygen pumping cell) 6 ... Vs cell (oxygen concentration measuring cell) 8 ... Second 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 rate controlling layer (first diffusion rate controlling layer) ) 6d, 22d ... Diffusion rate controlling layer (second diffusion rate controlling layer) 6e
Insulating film 6f Leakage resistance portion 12a, 12c, 14a, 14c
... Heater substrates 12b, 14b ... Heater wiring 20 ... First measurement chamber
26 ... Second measuring room

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noboru Ishida 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Japan Special Ceramics Co., Ltd. (72) Inventor Takafumi Oshima 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi No. Japan Special Ceramics Co., Ltd. (56) Reference JP 10-232232 (JP, A) JP 8-271476 (JP, A) JP 10-19843 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01N 27/416 G01N 27/419 G01N 27/41

Claims (5)

(57) [Claims] 1. A first oxygen pumping cell having an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes and an oxygen concentration measuring cell, which communicates with a gas to be measured side through a first diffusion controlling layer. And a second oxygen pumping cell in which a solid electrolyte layer having oxygen ion conductivity is sandwiched between porous electrodes, and is communicated with the first measurement chamber via a second diffusion-controlling layer. And a second measurement chamber, wherein oxygen is pumped out of 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 chamber is also provided. By applying a constant voltage to the cell in the direction of pumping oxygen from the second measurement chamber, the nitrogen oxide in the gas to be measured from the current flowing through the first oxygen pumping cell and the current flowing through the second oxygen pumping cell. Detect In-containing oxide concentration sensor, the oxygen concentration measurement cell, the peripheral edge of the second diffusion-controlling layer
A nitrogen oxide concentration detector characterized by being formed . 2. The first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell are respectively formed on different thin plate-like solid electrolyte layers, and the solid electrolyte layers are formed on the first and second solid electrolyte layers, respectively. The solid electrolyte layer on which the second oxygen pumping cell is formed is placed outside, and the first measurement chamber and the second measurement chamber are formed in the gap, and the solid electrolyte layers are laminated with a predetermined gap therebetween. Thin plate-shaped heater substrates provided with heaters for heating are respectively arranged on both sides in the stacking direction of the layers with a predetermined gap, and further, the heater of the solid electrolyte layer in which the first oxygen pumping cell is formed. 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 the substrate. 3. The first diffusion-controlling layer, when the nitrogen oxide concentration detector is projected 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 control layer. The method according to claim 4, wherein said second diffusion controlling layer, constituted by a solid electrolyte layer of the measurement gas diffusion rate-determining possible porous, the oxygen concentration of the first measuring chamber side electrode of the measuring cell second diffusion claims 1-3 Izu, characterized in that formed on the control layer
Nitrogen oxide concentration sensor according to any Re. The method according to claim 5 wherein said first measuring chamber side electrode of said oxygen concentration measuring cell, characterized by being formed between the solid electrolyte layer and the second diffusion-controlling layer forming the first measurement chamber 請
The nitrogen oxide concentration detector according to any one of claims 1 to 3 .