JP3328565B2 - NOx gas 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 NOx gas concentration detector, and more particularly to a NOx gas concentration detector used for detecting the concentration of NOx gas which is a component of exhaust gas from a combustor or an internal combustion engine.

[0002]

2. Description of the Related Art In recent years, with the tightening of exhaust gas regulations for vehicles,
Research for directly measuring the concentration of NOx contained in exhaust gas from an internal combustion engine or the like and controlling the internal combustion engine or a catalyst has been conducted. In particular, using a solid electrolyte (oxygen ion conductor) such as zirconia and the like,
By sufficiently pumping out oxygen to such a degree that Ox does not substantially decompose, and further pumping out oxygen from the residual gas containing NOx by the second oxygen ion pump cell, a current Ip2 generated by oxygen ions decomposed and dissociated by NOx is measured. A NOx gas concentration detector of the type that detects NOx concentration is considered to be capable of measuring the NOx gas concentration without being affected by interfering gases such as HC and CO contained in exhaust gas. I have.

[0003]

In such a NOx gas concentration detector, the current Ip2 has a temperature dependency and an oxygen concentration dependency, which hinders accurate measurement of the NOx gas concentration.

[0004] In view of such circumstances, an object of the present invention is to provide a NOx gas concentration detector which has small temperature dependence and oxygen concentration dependence and enables accurate NOx gas concentration measurement.

[0005]

The inventor of the present invention has a first measuring chamber into which a gas to be measured is introduced, and pumps oxygen from the first measuring chamber to control the oxygen concentration in the first measuring chamber to be constant. A first oxygen ion pump cell, wherein the first oxygen ion pump cell has an electrode having a length substantially equal to a length of the first measurement chamber. A large oxygen concentration gradient occurs in the measurement chamber,
An electromotive force was generated at the electrode of the oxygen ion pump cell near the tip of the measurement chamber, and it was found that this electromotive force had an effect on the NOx gas concentration measurement. This has led to the invention.

A first aspect of the present invention has the following elements. The gas to be measured is introduced via the diffusion resistance. Along the flow direction of the gas to be measured in the measurement chamber in which oxygen in the introduced gas is selectively pumped to the outside, at least on the wall of the measurement chamber among the electrodes of the oxygen ion pump cell with respect to the entire length of the measurement chamber. The ratio of the length of the provided electrode is (electrode / total length) = 1/4 to 3/4.

A feature of the first aspect is that a first measurement chamber into which a gas to be measured is introduced via a first diffusion resistor,
Based on the oxygen partial pressure detection electrode for measuring the oxygen partial pressure in the gas to be measured in the measurement chamber, and the potential of the oxygen partial pressure detection electrode,
A first oxygen ion pump cell that pumps oxygen in the gas to be measured from the first measurement chamber to the outside of the measurement chamber sufficiently so that NOx is not decomposed, and a second diffusion resistor from the first measurement chamber A second measurement chamber into which gas is introduced, a second oxygen ion pump cell to which a voltage is applied to decompose NOx in the second measurement chamber, and a current corresponding to the NOx gas concentration flows by dissociated oxygen; It is suitably applied to a NOx gas concentration detector provided with

Based on the first point of view, a preferred NOx gas concentration detector has a first diffusion resistor in one of the measuring chambers and a second diffusion resistor in the other, spaced apart from each other. An electrode on the measurement chamber side of the oxygen ion pump cell is formed on one surface side of the opposing surfaces of the measurement chamber, and an entrance opening of the second diffusion resistor is formed on the other surface side. At least the electrode on the measurement chamber side of the oxygen ion pump cell is formed so as to extend from the vicinity of the diffusion resistance to at least the first diffusion resistance side immediately above the opening on the inlet side of the second diffusion resistance. Preferably, no electrode is formed at least on the measurement chamber side of the oxygen ion pump cell immediately above or near the opening on the inlet side of the second diffusion resistor.

In another preferred NOx gas concentration detector, an electrode provided outside the measurement chamber among a pair of electrodes provided in the oxygen ion pump cell is sealed or covered, and the gas is electrically connected to the electrode. It communicates with the atmosphere via a lead having diffusion resistance.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. First, the principle of measurement in a NOx gas concentration detector equipped with two sets of diffusion resistors, an oxygen ion pump cell, and a measurement chamber is as follows. (1) Exhaust gas flows into the first measurement chamber through the first diffusion hole having diffusion resistance. (2) The first oxygen ion pump cell pumps out oxygen in the first measurement chamber to the extent that NOx decomposition (2NO → N ₂ + O ₂ ) does not substantially occur (output from the oxygen partial pressure (concentration) detection electrode) The partial pressure of oxygen in the first measurement chamber is controlled by the signal to be supplied). (3) The gas in the first measurement chamber (concentration-controlled O ₂ gas + NOx gas) flows into the second measurement chamber through the second diffusion hole having diffusion resistance. (4) NOx in the second measurement room
The gas is decomposed into N ₂ and O ₂ by the second oxygen ion pump cell pumping out more oxygen than the gas in the second measurement chamber. (5) At this time, since the current Ip2 flowing through the second oxygen ion pump cell has a linear relationship with the NOx gas concentration, the NOx gas concentration can be measured by detecting Ip2.

Next, the features of the NOx gas concentration detector according to an embodiment of the present invention will be described with reference to FIG. 1 illustrating a detector according to an embodiment of the present invention and FIG. 1 illustrating a detector according to a comparative example. 10 will be described in comparison. Note that the present invention is not limited to the detector shown in FIG. The difference between the detectors shown in FIGS. 1 and 10 is that the second diffusion holes 3 of the electrodes 6a and 6b of the first oxygen ion pump cell 6 in the detector of FIG.
Side has been removed (A> B). Therefore,
The length of the electrodes 6a and 6b is shorter in the detector of FIG. 1 (the electrode area is also smaller). Thus, the first
Since the electrodes of the oxygen ion pump cell 6 are shortened, the oxygen concentration gradient in the first measurement chamber 2 particularly in the flow direction of the gas to be measured (the direction from the first diffusion hole 1 to the second diffusion hole 3) is reduced. (The oxygen concentration difference from one end to the other end of the electrode 6b is reduced, and the oxygen concentration difference in the portion where the electrode 6b is present is reduced.) The electromotive force generated at the tip portions of the electrodes 6a and 6b is suppressed. (See FIG. 1C). Further, the electrode 6
The first oxygen ion pump cell voltage Vp1 is reduced along with the suppression of the electromotive force generated in
The temperature dependency and the oxygen concentration dependency of the Ox gas concentration measurement are reduced.

This effect is due to the fact that the first oxygen ion pump cell voltage Vp1 necessary for pumping out excess oxygen is reduced, so that dissociation and decomposition of NO gas in the first measurement chamber other than pumping out of oxygen are eliminated. That is, the NO gas flowing into the second measurement chamber does not decrease, and the decrease of ΔIp2 (gain) can be suppressed.

Further, since the oxygen concentration gradient generated in the first measurement chamber 2 is reduced, the oxygen concentration in the vicinity of the second diffusion hole 3 becomes constant, and the oxygen concentration introduced into the second measurement chamber 4 for measuring the NOx concentration. Becomes constant. That is, the concentration of oxygen pumped in the second measurement chamber 4 becomes constant (stabilization of Ip2), and the value of Ip2 depends only on the amount of dissociated and decomposed NO.

In the detector of the comparative example shown in FIG. 10, since the electrodes 6a and 6b extend over the entire length of the first measurement chamber 2, the tip of the first measurement chamber 2 (the tip of the electrodes 6a and 6b). In the meantime, an electromotive force in a direction opposite to the first oxygen ion pump cell voltage Vp1 is generated depending on the oxygen concentration difference between the inside and outside of the first measurement chamber 2, and the inlet side of the first measurement chamber 2 (the first diffusion hole 1).
Oxygen is pumped out at the outlet side (second diffusion hole 3).
On the side), it is considered that a phenomenon occurs in which oxygen is pumped up. On the other hand, in the detector shown in FIG. 1, since the tip of the electrode 6a is omitted, the electromotive force on the tip side of the first measurement chamber 2 (the second diffusion hole 3 side, the first measurement chamber exit side). Is prevented (see FIG. 1C).

If at least one of the electrodes of the first oxygen ion pump cell provided on the wall surface of the first measurement chamber is too long, the voltage Vp1 of the first oxygen ion pump cell rises and NOx in the first measurement chamber. On the contrary, if it is too short, the pumping of oxygen from the first measurement chamber becomes insufficient and the gain of the second oxygen ion pump cell current decreases, so that the flow rate of the gas to be measured in the first measurement chamber is reduced. And the first
The ratio of the length of at least one of the electrodes of the first oxygen ion pump cell provided on the wall surface of the first measurement chamber to the total length of the measurement chamber is (electrode / total length) = 1/4 to 3 / 4, preferably 2/7 to 4/7, 5/7. In the present invention, the description of the numerical range includes not only the upper and lower limits but also any intermediate value.

As shown in FIG. 1, the preferred NOx gas concentration detector of the present invention comprises a first oxygen ion pump cell 6, an oxygen concentration measurement cell 7, and a second oxygen ion pump cell 8.
Are provided in solid electrolyte layers different from each other. Thereby, the leak current flowing between the electrodes of each cell is reduced, and the oxygen concentration in the first measurement chamber 2 can be controlled with high accuracy. More preferably, an insulating film such as alumina or an insulating layer is provided between the cells.

Preferably, a heater layer for heating the detector is laminated between the laminated solid electrolyte layers. By providing the heater layer, the oxygen partial pressure detection electrode can be maintained at an appropriate temperature.

As a solid electrolyte of each cell, a solid solution of zirconia and yttria, a solid solution of zirconia and calcia, and the like are used. As a porous electrode formed by a method such as screen printing and sintering on both surfaces of a thin solid electrolyte layer, it is preferable to use platinum, rhodium, or an alloy thereof having a catalytic action. As the first and second diffusion holes, it is preferable to use porous ceramics, such as porous alumina ceramics. It is preferable that the heat generating portion of the heater be formed of a composite material of ceramics and platinum or a platinum alloy, and the lead portion be formed of platinum or a platinum alloy.

The measuring method of the present invention can be applied to a CO gas sensor and an HC gas sensor. As in the case of a NOx gas concentration detector, the effect of H ₂ O is reduced and the target gas concentration can be accurately measured. Becomes

Other preferable features are as described in Japanese Patent Application No. 8-160812 filed by the present applicant. This application may be referred to and incorporated herein as necessary.

Further, the present invention is suitably applied to the following NOx gas concentration detector. That is, it has a ceramic body capable of electrically controlling the oxygen ion conduction amount, and a flow path into which a gas to be measured containing NOx is provided so as to face the ceramic body, and the flow path is Through this, the NOx concentration in the gas to be measured is changed in the course of the movement of the gas to be measured, and further, oxygen gas is led out of the gas to be measured through the ceramic body to the outside of the flow path. Means for forming a residual gas having a different NOx concentration in the flow path before introduction into the flow path; and applying a voltage within a range in which H ₂ O in the residual gas is not substantially dissociated. A plurality of electrodes formed on the surface of the ceramic body so as to dissociate NOx in the residual gas into nitrogen and oxygen, between the electrodes,
Means for measuring a current flowing through the ceramic body, wherein the current is generated by an electrochemical action of oxygen dissociated from NOx, and a NOx concentration in the gas to be measured is determined based on the measured current. And a detector provided with the detector.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIGS. 1A to 1C show N according to an embodiment of the present invention.
FIG. 1 is a diagram for explaining an _OX gas concentration detector, and FIG.
1A is a cross-sectional view cut in the longitudinal direction, and FIG.
FIG. 1C is a plan view of a measurement chamber portion, and FIG. 1C is an enlarged sectional view of a main part of the first measurement chamber. The detector of FIG. 1 includes a layer of a first oxygen ion pump cell 6 including a solid electrolyte layer and electrodes 6a (positive electrode) and 6b (negative electrode) provided between the layers, and a solid electrolyte layer and the layers. The second oxygen ion pump including the layer of the oxygen concentration measurement cell 7 provided with the oxygen partial pressure detection electrode provided in the above, the solid electrolyte layer, and the oxygen ion pump electrode provided with the solid electrolyte layer and the layer interposed therebetween The layers are stacked in the order of the cells 8. Between the layer of the first oxygen ion pump cell 6 and the layer of the oxygen concentration measurement cell 7, a first measurement chamber 2 is defined by left and right insulating layers and upper and lower solid electrolyte layers in the figure. The second measurement chamber 4 is defined above the layer of the two-oxygen ion pump cell 8. Further, the first measurement chamber 2
To introduce the gas to be measured through the diffusion resistance
The openings of the diffusion hole 1 and the second diffusion hole 3 are provided separately. The second diffusion hole 3 communicates with the first and second measurement chambers 2 and 4 through the layer of the oxygen concentration measurement cell 7 and the layer of the solid electrolyte, and performs the first measurement of a gas containing at least NOx and O _2. It is sent from the chamber 2 to the second measurement chamber 4 via a diffusion resistor.

Note that an insulating layer made of alumina is provided between the layers of the solid electrolytes, respectively.
The heater layer for heating the detector is bonded via a cement layer so as to sandwich the entire detector in the stacking direction.
Each electrode is connected to the outside of a detector such as a power supply via a lead wire formed between layers.

One of the features of this detector is that the first measurement chamber 2 and the second measurement chamber 4 are arranged so as to be substantially overlapped with each other in the vertical direction. Also, the first diffusion hole 1
Are located on both sides of the detector, not on the tip side of the detector. The second diffusion holes 3 are filled with a porous material, and an insulating film (layer) is disposed between all the solid electrolyte layers. Are insulated from each other. The second measurement chamber 4 forms a space, but may be filled with a porous material.

Here, as compared with the detector of FIG. 10 which is a comparative example, the feature of the detector of FIG. 1 is that the electrodes 6 a (positive electrode) and 6 b (negative electrode) provided in the first oxygen ion pump cell 6.
The length along the flow direction (longitudinal direction) of the gas to be measured flowing from the first diffusion hole to the second diffusion hole is shorter than the length of the first measurement chamber 2 (A> B). That is, the electrodes 6a and 6b are not formed immediately above the opening of the two diffusion holes 3.

The operation of the electrodes 6a and 6b formed as described above is as described above in the section of the embodiment of the present invention, and the description is omitted.

The NOx gas concentration was measured using the detectors of the embodiment shown in FIG. 1 and the comparative example shown in FIG. In the detector of the embodiment used, the outer dimensions of the detector are 50 mm in length in the longitudinal direction, 4 mm in width (transverse direction), and 1.3 mm in thickness (stacking direction). The thickness of the first oxygen ion pump cell is 0.3 mm, the length B of the electrodes 6a and 6b in the longitudinal direction is 4 mm, the length in the short direction is 2 mm, and the length A of the first measurement chamber in the longitudinal direction is 7 mm. , The length in the short direction is 2 mm,
Height 50 μm, the length of the first diffusion hole in the longitudinal direction is 2 mm,
The length in the short direction is 1 mm, the thickness is 50 μm, the size of the second diffusion hole is φ1, and the distance from the end (right end) of the first diffusion hole in the figure is 5.5 mm. The detector of the comparative example (see FIG. 10)
The length B in the longitudinal direction of the electrodes 6a and 6b is 7 mm (A = B)
The dimensions are the same as those of the detector of the embodiment except for the above.

Next, the manufacturing method and layout of the detector used for the measurement will be described. FIG. 2 is a diagram for explaining a manufacturing method and a layout of the detector used for the measurement. The layouts of the example and the comparative example are the same as shown in FIGS. 1 and 10 except that the electrodes of the first oxygen ion main pump cell are different in length.

Referring to FIG. 2, a ZrO ₂ sheet, an electrode paste, and the like are laminated in order from the upper left to the lower left and the upper right to the lower right in the figure to form an integrated detector. In the detector used for the measurement, although not shown in FIGS. 1 and 10, a protective coat paste (3) is provided. Paste materials such as insulating coats and electrodes are specified Zr
By being screen-printed on an O ₂ sheet, a laminate is formed. Next, an example of manufacturing each component such as the ZrO ₂ sheet shown in FIG. 2 will be described.

[0030] [ZrO ₂ sheet forming] a ZrO ₂ powder 600
Calcination was performed in an air furnace at a temperature of 2 ° C. for 2 hours. 30 kg of calcined ZrO ₂ powder, 150 g of dispersing material, 10 kg of organic solvent, and 60 kg of cobblestone were prepared by trommel, mixed and dispersed for about 50 hours, and a solution obtained by dissolving 4 kg of binder in 10 kg of organic solvent was added. For 20 hours to obtain a slurry having a viscosity of about 10 Pa · s (pascal second). A ZrO ₂ green sheet having a thickness of about 0.4 mm was prepared from this slurry by a doctor blade method, and dried at 100 ° C. for 1 hour.

[Printing Paste] (1) First oxygen ion pump electrode a, oxygen partial pressure reference electrode (oxygen reference electrode a), second oxygen ion pump electrodes a and b
For use: 20 g of platinum powder, 2.8 g of ZrO ₂ powder, and an appropriate amount of an organic solvent are mixed with a grinder (or pot mill), mixed for 4 hours and dispersed, and 2 g of a binder is added to the organic solvent 2.
A solution dissolved in 0 g was added, and 5 g of a viscosity modifier was further added, followed by mixing for 4 hours to prepare a paste having a viscosity of about 150 Pa · s.

(2) First oxygen ion pump electrode b, oxygen partial pressure detection electrode (oxygen reference electrode b): 19.8 g of platinum powder, Z
A mixture of 2.8 kg of rO ₂ powder, 0.2 g of gold powder and an appropriate amount of an organic solvent mixed with a grinder (or pot mill), mixed for 4 hours, dispersed, and dissolved 2 g of a binder in 20 g of an organic solvent. Was added, and 5 g of a viscosity modifier was further added, followed by mixing for 4 hours to prepare a paste having a viscosity of about 150 Pa · s.

(3) For insulating coat and protective coat: 50 g of alumina powder and an appropriate amount of an organic solvent are mixed with a grinder (or pot mill), mixed for 12 hours, dissolved, and further added with 20 g of a viscosity modifier. And mix for 3 hours, viscosity 100Pa
・ Created about s paste.

(4) For Pt-containing porous material (for lead wire): Alumina powder 10 g, platinum powder 1.5 g, binder 2.5 g, and organic solvent 20 g are mixed with a grinder (or pot mill) and mixed for 4 hours. And further add 10 g of a viscosity modifier,
After mixing for a time, a paste having a viscosity of about 100 Pa · s was prepared.

(5) For the first diffusion hole: 10 g of alumina powder having an average particle size of about 2 μm, 2 g of a binder, and 20 g of an organic solvent are prepared by a grinder (or a pot mill), mixed, dispersed, and further dispersed in viscosity. 10 g of the adjusting agent was added and mixed for 4 hours to prepare a paste having a viscosity of about 400 Pa · s.

(6) For carbon coating: carbon powder 4
g, 2 g of a binder, and 40 g of an organic solvent were mixed by a grinder (or a pot mill), mixed and dispersed, 5 g of a viscosity modifier was further added, and mixed for 4 hours to prepare a paste. In addition, by printing and forming a carbon coat,
For example, contact between the first oxygen pump electrode b and the oxygen reference electrode b is prevented. In addition, the carbon coat is the first
It is used to form a measurement chamber and a second measurement chamber. Since carbon is burned off during firing, the carbon coat layer does not exist in the fired body.

[Pellet] For second diffusion hole: 20 g of alumina powder having an average particle size of about 2 μm, 8 g of binder, organic solvent
Mix 20g with a grinder (or pot mill)
The mixture was granulated for 1 hour, granulated, and a pressure of about 2 t / cm ² was applied by a mold press to prepare a cylindrical pressed molded body (green state) of φ1.3 × 0.8 t. This green pressed body is
After being inserted into a predetermined portion of the zirconia green sheet of the second and third layers, pressed and integrated, and then fired,
A second diffusion hole is formed in the detector.

[ZrO ₂ laminating method]
A portion (φ1.3) through which the second diffusion hole penetrates is punched. After the punching, a green columnar molded body serving as the second diffusion hole is embedded, and 1 to 4 ZrO ₂ green sheets are pressed with a pressing force of 5 k.
g / cm ² , pressurization time: 1 minute.

[Defatting and firing]
Degreasing at 0 ° C. × 2 hours and baking at 1500 ° C. × 1 hour.

A test for measuring the concentration of NO gas in the gas to be measured was performed using the NOx gas concentration detector thus manufactured. The measurement results are shown in Tables 1 and 2, and the measurement results are shown in FIGS. The common measurement conditions in the test examples described below are as follows. Gas component to be measured: NO
(0-1500 ppm), O ₂ (1-16%), CO ₂ 1
0%, balance N ₂ , exhaust gas (gas to be measured) temperature: 300
° C, heater power 18-25W (detector temperature 80 at 20W)
0 ° C).

[0041]

[Table 1]

[0042]

[Table 2]

[Test Example 1] With NO set to 0%, the variation of the offset value of the second oxygen ion pump current was measured by changing the oxygen concentration. Note that the offset is a value of Ip2 when NO is not injected into the gas to be measured, and it is preferable that the value be smaller.
For example, it is desirable that it is more insensitive and hard to fluctuate with respect to the fluctuation of the oxygen concentration and the temperature in the atmosphere of the measured gas. This corresponds to the concentration of residual oxygen pumped in the first measurement chamber 2 (see FIG. 1). FIG. 3 shows the results of Test Example 1. FIG. 3 is a graph showing the oxygen concentration dependency of the second oxygen ion pump current Ip2 offset. Triangles represent measurement data obtained by the detectors of the example and diamonds represent measurement data obtained by the detector of the comparative example. Referring to FIG. 3, according to the detector of the embodiment, the change amount of the Ip2 offset is about 1 μA with respect to the change amount of the oxygen concentration of 1 to 16%. It can be seen that has been reduced.

Test Example 2 NO: 1500 ppm was introduced into the gas to be measured, and the gain of the second oxygen ion pump current was measured at an oxygen concentration of 1 to 16%. The gain is the amount of change in Ip2 when a constant concentration of NO is injected. FIG. 4 shows the results of Test Example 2. FIG. 4 is a graph showing the dependence of the gain ΔIp2 of the second oxygen ion pump current on the oxygen concentration, in which the triangle represents the measurement data obtained by the detector of the example and the diamond represents the data measured by the detector of the comparative example. Referring to FIG. 4, according to the detector of the embodiment, for a change amount of oxygen concentration of 1 to 16%, I
It can be seen that there was almost no change in the p2 gain, and the oxygen concentration dependency of the gain was reduced as compared with that of the comparative example.

[Test Example 3] NO was set to 0%, and the first oxygen ion pump cell voltage Vp1 between the first oxygen ion pump electrodes with respect to the change in oxygen concentration was measured. The oxygen ion pump cell voltage Vp1 is a voltage necessary for pumping out excess oxygen in the first measurement chamber to the outside. FIG. 5 shows the results of Test Example 3. FIG. 5 is a graph showing the oxygen concentration dependency of the first oxygen ion pump electrode voltage Vp1. In each of the graphs, a triangle represents measurement data by the detector of the example and a diamond represents measurement data by the detector of the comparative example.
Referring to FIG. 5, according to the detector of the embodiment, oxygen concentration 1
The oxygen ion pump cell voltage Vp1 is reduced as compared with the comparative example for a change of １６16%.

Test Example 4 NO: 0%, O ₂ 7% was introduced into the gas to be measured, and the heater power was changed to measure the variation of the offset value of the second oxygen ion pump current Ip2. FIG. 6 shows the results of Test Example 4. FIG. 6 is a graph showing the temperature (heater power) dependence of the offset of the second oxygen ion pump current Ip2, in which the triangle represents measurement data obtained by the detector of the example and the diamond represents data measured by the detector of the comparative example. Referring to FIG. 6, according to the detector of the embodiment, it can be seen that there is almost no change in the Ip2 offset with respect to the change amount of the heater power of 18 to 25 W, that is, the temperature dependency is improved.

[Test Example 5] NO: 1500 ppm,
O ₂ : 7% was introduced into the gas to be measured, and the gain of the second oxygen ion pump current was measured by changing the heater power. FIG. 7 shows the results of Test Example 5. FIG. 7 is a graph showing the temperature (heater power) dependency of the second oxygen ion pump current ΔIp2, in which the triangles represent measurement data obtained by the detector of the example and the diamonds represent measurement data obtained by the detector of the comparative example. Referring to FIG. 7, according to the detector of the embodiment, I
It can be seen that the change in the p2 gain became almost constant, and the temperature dependency was improved as compared with that of the comparative example.

[Test Example 6] NO: 0%, O ₂ : 7%
Into the gas to be measured, and the first oxygen ion pump cell voltage Vp1 at the first oxygen ion pump electrode with respect to the heater power.
Was measured. FIG. 8 shows the results of Test Example 6. FIG.
Is a graph showing the temperature (heater power) dependence of the first oxygen ion pump electrode voltage Vp1, where the triangles are the data measured by the detector of the example and the diamonds are the data measured by the detector of the comparative example. According to the detector of the embodiment, Vp1 is greatly reduced as compared with the comparative example for a change in the heater power of 18 to 25 W.

From the results of the above Test Examples 1 to 6 (FIGS. 3 to 8), the electrode of the first oxygen ion pump cell (at least the electrode on the side of the first measurement chamber) was longer than the length of the first measurement chamber. When the length is long, the voltage of the first oxygen ion pump cell is reduced due to the effect of the electromotive force generated at the tip of the first oxygen ion pump cell (the end of the electrode, opposite to the first diffusion hole) (effective voltage is reduced). Vp1 tends to increase (in order to compensate for a decrease in the effective voltage, to pump out the oxygen in the first measurement chamber, and to maintain the oxygen concentration in the first measurement chamber at a predetermined concentration). In such a case, the pump current is insufficient (a large amount of oxygen diffuses into the second measurement chamber, and the detection of a small amount of NOx gas becomes inaccurate).
It was found that the gain of p2 tended to decrease. FIG.
7 shows the relationship between the length of the first oxygen ion pump electrode, the first oxygen ion pump electrode voltage Vp1 and the second oxygen ion pump current ΔIp2 (gain). 9, the length of the first measurement chamber is 7 mm, while the length of the first oxygen ion pump electrode is 4 mm, that is, “electrode length” / “first electrode”.
It is understood that it is particularly preferable to set the measurement chamber to “4/7. In addition, the length of the first oxygen ion pump electrode is 2 to 5.4.
mm, more preferably 3.5 to 5 mm
It is about. Table 1 and Table 2 show that the length of this electrode is 4 mm.
Table 3 shows data in the case of a length of 2 mm.

[0050]

[Table 3]

As shown in FIG. 9, when the electrode of the first oxygen ion pump cell is lengthened and covered with the second diffusion hole, the first oxygen ion pump electrode voltage Vp1 tends to increase due to the effect of electromotive force. That is, it can be seen that ΔIp2 becomes small. If the electrode of the first oxygen ion pump cell is too short, it can be seen that the output of Ip2 decreases due to insufficient pumping ability.

[Built-in electrode for Vp2] FIG.
FIG. 11D is a view for explaining a NOx gas concentration detector according to another embodiment of the present invention. FIG. 11A is a cross-sectional view cut in a longitudinal direction, and FIG. FIG. 11C is a plan view of a measurement chamber portion, and is an enlarged sectional view of a main part of the first measurement chamber. FIG. 11D is a plan projection view of the second measurement chamber.
The detectors of FIGS. 11A to 11D include a layer of a first oxygen ion pump cell 66 including a solid electrolyte layer and electrodes 66a and 66b provided with the layer interposed therebetween, the solid electrolyte layer and the layer. A layer of an oxygen concentration measuring cell 67 having an oxygen partial pressure (concentration) detecting electrode 67a and an oxygen partial pressure (concentration) reference electrode 67b, a solid electrolyte layer, and a solid electrolyte layer and one surface of the layer An oxygen ion pump electrode 68 a provided in the second measurement chamber 64, and an insulating layer 71 outside the second measurement chamber 64.
-3 oxygen oxygen pump electrode 68b
Are stacked in the order of the layers of the second oxygen ion pump cell 68 provided with. In the embodiment shown in FIG. 1, the oxygen ion pump electrodes 68a and 68b are formed one on each side of the solid electrolyte layer. However, in this embodiment, both oxygen ion pump electrodes 68a and 68b are formed on both sides of the solid electrolyte layer. Ion pump electrode 6
8a and 68b are formed together, the electrode 68a is located in the second measurement chamber 64, and the electrode 68b is located outside the second measurement chamber 64 by being covered with the insulating layer 71-3 (furthermore, the electrode 68b).
“b” communicates with the atmosphere through the electrode leads and the leads with diffusion resistance (see FIGS. 11 and 16). Between the layer of the first oxygen ion pump cell 66 and the layer of the oxygen concentration measurement cell 67, a first measurement chamber (first flow path) 62 is defined by left and right insulating layers and upper and lower solid electrolyte layers in the figure. And a second oxygen ion pump cell 68
A measurement chamber (second flow path) 64 is defined. Further, the first measurement chamber 62 is provided with openings of a first diffusion hole 61 and a second diffusion hole 63 for introducing a gas to be measured via a diffusion resistor, separated from each other. The second diffusion hole 63 penetrates the layer of the oxygen concentration measurement cell 67 and the layer of the solid electrolyte, and
A gas containing at least NOx and O ₂ is communicated from the first measurement chamber 62 to the second measurement chamber 62 via a diffusion resistor.
Send to the measurement chamber 64.

Incidentally, an insulating layer made of alumina is provided between the layers of the solid electrolytes.
The heater layer for heating the detector is bonded via a cement layer so as to sandwich the entire detector in the stacking direction.
Each electrode is connected to the outside of a detector such as a power supply via a lead wire formed between layers. For example, referring to FIG. 11 (D), electrodes 68a, 68 of second oxygen ion pump cell 68
b is electrically connected to the leads 68c and 68d.

One of the features of this detector is that the first measurement chamber 62 and the second measurement chamber 64 are arranged so as to substantially overlap each other in the vertical direction. The first diffusion holes 61 are not on the tip of the detector but on both sides of the detector. The second diffusion holes 63 are filled with a porous material, and an insulating layer (film) is provided between all the solid electrolyte layers. The arrangement is such that the electrodes of each cell are insulated from each other. The second measurement chamber 64 forms a space, but may be filled with a porous material. Another feature is that both oxygen ion pump electrodes are formed on one surface of the solid electrolyte layer, and one electrode 68a
Is located in the measurement chamber 64, and the other electrode 68b is the insulating layer 7
Measuring chamber 64 covered (sealed or built-in) by 1-3
It is located outside. This built-in electrode 68
b indicates the atmosphere of the gas to be measured (exhaust gas) because the insulating layer 71-3 serves as protection means for the electrode 68b and the lead 68d serves as diffusion resistance means and communicates with the atmosphere via diffusion resistance. The electrode 68b is shut off from the electrode 68b to prevent direct contact with the outside air, and oxygen pumped around the electrode 68b is pooled.
b The oxygen concentration around (near) is stabilized. As a result, the pair of electrodes 68a, 68 of the second oxygen ion pump cell 68
The electromotive force generated between b is stabilized. Further, since the generated electromotive force is stabilized, the effective pump voltage (Vp2-one electromotive force) of the pump voltage Vp2 applied to the second oxygen ion pump cell 68 is stabilized, and the oxygen concentration dependence of the NOx gas concentration measurement is measured. Decrease. In the manufacturing process of such a detector, there is an advantage that both electrodes 68a and 68b of the second oxygen ion pump cell 68 can be printed at one time. In addition, the oxygen concentration reference electrode is substantially sealed in a ceramic body (preferably a self-generated electrode), and is connected to the atmosphere with a predetermined diffusion resistance via a porous conductive lead. The reference potential of the concentration reference electrode is stabilized. The same applies to an electrode arranged outside the flow path for deriving oxygen.

Here, as compared with the detector of FIG. 12 which is a comparative example for the detectors of FIGS. 11A to 11D, FIG.
The features of the detectors 1 (A) to 1 (D) are that electrodes 66a (positive electrodes) and 66 provided in the first oxygen ion pump cell 66 are provided.
The length of the negative electrode b along the flow direction (longitudinal direction) of the gas to be measured flowing from the first diffusion hole 61 to the second diffusion hole 63 is shorter than the length of the first measurement chamber 62 (A > B) and the electrode 66b is not formed up to the position immediately above the opening of the second diffusion hole 63. In the comparative example,
Reference numeral 41 is a first diffusion hole, 42 is a first measurement chamber, 43 is a second diffusion hole, 44 is a second measurement chamber, 46 is a first oxygen ion pump cell, 46a and 46b are first oxygen ion pump electrodes, 47 is an oxygen concentration measurement cell, 47a is an oxygen concentration detection electrode, 47b is an oxygen concentration reference electrode, 48 is a second oxygen ion pump cell, 48a and 48b are second oxygen ion pump electrodes, and 50c is an insulating layer.

The NOx gas concentration was measured using the detectors of the embodiment shown in FIGS. 11A to 11D and the comparative example shown in FIG. In the detector of the embodiment used, the outer dimensions of the detector are 50 mm in length in the longitudinal direction, 4 mm in width (transverse direction), and 1.3 mm in thickness (stacking direction). The thickness of the first oxygen ion pump cell 66 is 0.3 mm,
a, 66b are 7 mm in the longitudinal direction and 4 mm in the length B.
mm, the length in the short direction is 2 mm, the length A in the long direction of the first measurement chamber 62 is 7 mm, the length in the short direction is 2 mm, the height is 50 μm, and the length in the long direction of the first diffusion hole 61. Is 2 mm,
1 mm in width direction, 50 μm thickness, second diffusion hole 63
Is φ1, and the distance from the end (right end) of the first diffusion hole 61 in the figure is 5.5 mm. In the detector of the comparative example (see FIG. 12), the length B in the longitudinal direction of the electrodes 46a and 46b is 7 m.
The dimensions are the same as those of the detector of the embodiment except that m (A = B).

Referring to the NOx gas concentration measuring system shown in FIG. 13, a description will be given of a measurement example using the detector according to the present embodiment and the detector according to the comparative example. That is, a dummy signal is input to the air flow sensor, the injector pulse width is changed, and the air-fuel ratio is controlled so that the following settings are obtained (the oxygen concentration in the exhaust gas changes). Was compared with the output of the detector according to the comparative example. Since this control is an open loop control, λ
The correction was performed by UEGO (universal full range air-fuel ratio sensor). Other conditions are shown below. The loading mode of the NOx gas concentration detector will be described later with reference to FIG.

Engine: 2.0 L inline 6-cylinder turbocharger Engine rotation speed: 3000 rpm Intake pipe negative pressure: -350 mmHg Frequency: 0.5 Hz Amplitude (target): λ ± 3% Mounting position: See FIG.

FIG. 14 shows the results obtained by the detector of the embodiment shown in FIGS. 11A to 11D, and FIG. 15 shows the results obtained by the detector of the comparative example shown in FIG. From these figures, it can be seen that in the results of the comparative example, when the oxygen concentration curve rises (when the stoichiometric point is passed), a large spike noise is generated in the detector output. On the other hand, no spike noise is observed in the results of the example. Therefore, according to the detector of the embodiment having the structure shown in FIGS. 11A to 11D, the generation of spike noise in the NOx gas concentration detection output at the time of passing the stoichiometric point is suppressed, and the variation in the oxygen concentration is affected. It can be seen that accurate measurement of the NOx gas concentration, which is difficult to perform, can be performed.

[Example of loading NOx gas concentration detector] FIG.
Next, an example in which the NOx gas concentration detector according to the present invention is mounted on a mounting bracket is shown. The detector element is fixed such that the lower part, in which the gas inlet to be measured is formed, is located in a perforated protector. The detector element is provided with a heater along its longitudinal direction. The upper part in the figure of the detector element and the heater is covered with a sealing material, the lower sealing material is made of a porous material that allows gas to pass through, and the upper sealing material is made of an airtight material. Further, a holder is provided on the outer peripheral side of the sealing material, and a stainless steel material and a talc material (a lower end of the talc material is in contact with a flange portion of the holder) are sealed between the holder and the metal shell. Thus, via the stainless steel and talc, the holder is fixed to the metal shell by the caulking force acting in the radial direction and the axial caulking force of the metal shell, and the detector element is also stably formed. Will be retained. In the upper part of the mounting bracket, the first outer cylinder and the second outer cylinder are coaxially combined and locked with each other. The first outer cylinder extends into the metal shell and is locked to the metal shell. Waterproof rubber is sealed in the upper part of the second outer cylinder. Also, an electrode (not shown) formed on the detector element
Is electrically connected to one end of a lead wire having gas diffusion resistance via an electrode lead having gas diffusion resistance, and the other end of this lead wire is connected to the atmosphere.

Further, according to the present invention, the NO
In the method for measuring the x gas concentration, the following facts can be used. That is, at room temperature, NO ₂
Is a minute amount, for example, 0.1% (1000 ppm) of NO
_The atmosphere where ₂ exists with another gas is, for example, 800 ° C.-1
In a high pressure atmosphere, most of the NO ₂ becomes N ₂
It may be considered that 0.1% of NO dissociates into O and (1/2) O ₂ . Therefore, for example, a gas containing a small amount of nNOx (n: the number of NOx molecules) is converted into (1) high temperature (700 ° C. or more)
Under, the degrade nNOx to nNOx and _{(n / 2) O 1-} x, through the step of removing the generated _{(n / 2) O 1-} x, (2)
Next, the nNO generated in the step (1) is converted into (n / 2) N ₂ and (n /
2) dissociated into O _2, when measuring the (n / 2) O ₂ is passed through the oxygen ion conductor as ions current and can be determined as a value proportional to n / 2 on current. That is,
If n can be determined, the amount of nNOx can be determined.

The NOx gas concentration detector test NO
As the x gas, both NO ₂ gas and NO gas can be used in the same manner. However, NO has a smaller molecular weight than NO _{2 and} is less subject to diffusion restriction. Used as gas.

The ideal NOx gas concentration detector according to the present invention uses the NO generated in the above step (1) in this step.
It is introduced into step (2) without being decomposed in (1). However, preferred detectors (eg, FIG. 2)
1 (A) to (D)), the above steps
In (1), the dissociation of NO has begun. The NO dissociation and oxygen dissociation rates are determined by the influence of the applied voltage in the above step (1) and design factors such as the electrode material and shape. Therefore, compensation of NO dissociation in the step (1) should be taken into consideration in determining an accurate NOx amount. Specifically, compensation is performed by the reciprocal of the degree of dissociation (60 to 95%) indicating the degree of dissociation of NO into N and O in step (1).

The generation of the residual gas in the first flow path (first measurement chamber) is performed under such a condition that the dissociation of NO is substantially allowed but compensated. Under this condition, the dissociated NO amount is compensated by the NOx flowing from the atmosphere,
The residual gas is in an equilibrium state. In an equilibrium state roughly determined by the temperature, where the temperature is such that the NO ₂ abundance decreases with increasing temperature.
NOx is regarded as the sum of the substantially NO and NO _2. For example, if there is 50% NO ₂ at room temperature, 700 ° C.
At higher temperatures, it is 5% or less.

The NOx gas concentration detector according to the present invention
It is preferably used at about 700 ° C. or higher (900 ° C. or lower), more preferably 750-850 ° C. The role of NO in NOx is relatively small. That is,
Under carefully set conditions as described above,
That is, NO ₂ may be regarded as NO.

In the step of dissociating NOx in the residual gas into nitrogen and oxygen, the voltage applied between the plurality of electrodes formed on the surface of the ceramic body is such as H ₂ O or CO _{2 in} the residual gas. It is preferable to apply the gas within a range where the interfering gas component does not substantially dissociate. As a result, the influence of the interfering gas is reduced and accurate measurement of the NOx gas concentration can be performed.

[0067]

As described above, according to the NOx gas concentration detector of the present invention, the electromotive force generated in the first measurement chamber is suppressed, and the first oxygen ion pump cell voltage Vp1 is reduced. (1) Dissociation and decomposition of NO gas in the measurement chamber is suppressed. Thereby, the oxygen dependency and the temperature dependency of the NOx gas concentration detection are improved, and more accurate NOx gas concentration detection can be performed.

[Brief description of the drawings]

1 (A) to 1 (C) relate to one embodiment of the present invention.
It is a diagram for explaining a NOx gas concentration detector,
(A) is a cross-sectional view cut in the longitudinal direction, (B) is a plan view of a first measurement chamber portion, and (C) is an enlarged cross-sectional view of a main part of the first measurement chamber.

FIG. 2 is a diagram for explaining a manufacturing method and a layout of a NOx gas concentration detector used for measurement.

FIG. 3 is a graph showing the oxygen concentration dependency of a second oxygen ion pump current Ip2 (offset) according to one example and a comparative example of the present invention.

FIG. 4 is a graph showing the oxygen concentration dependency of a second oxygen ion pump current ΔIp2 (gain) according to one example and a comparative example of the present invention.

FIG. 5 is a graph showing an oxygen concentration dependency of a first oxygen ion pump cell voltage Vp1 according to one example and a comparative example of the present invention.

FIG. 6 is a graph showing the temperature (heater power) dependence of a second oxygen ion pump current Ip2 (offset) according to one example and a comparative example of the present invention.

FIG. 7 is a graph showing temperature (heater power) dependence of a second oxygen ion pump current ΔIp2 (gain) according to one example and a comparative example of the present invention.

FIG. 8 is a graph showing a temperature (heater power) dependency of a first oxygen ion pump cell voltage Vp1 according to one example and a comparative example of the present invention.

FIG. 9 shows the relationship between the length of the first oxygen ion pump electrode, the voltage of the first oxygen ion pump electrode voltage Vp1, and the second oxygen ion pump current ΔIp2 (gain) according to one embodiment and a comparative example of the present invention. 6 is a graph showing a relationship with the graph.

FIG. 10 is a cross-sectional view illustrating a schematic configuration of a NO _X gas concentration detector according to a comparative example.

FIGS. 11A to 11D are diagrams for explaining a NOx gas concentration detector according to one embodiment of the present invention;
(A) is a cross-sectional view cut in the longitudinal direction, (B) is a plan view of the first measurement chamber portion, (C) is an enlarged cross-sectional view of a main part of the first measurement chamber, and (D) is a plane view of the second measurement chamber. It is a projection view.

FIG. 12 is a diagram for explaining a detector that is a comparative example with respect to the detector shown in FIG. 11;

FIG. 13 is a diagram showing an example of a system for measuring the concentration of NOx gas in exhaust gas.

FIG. 14 shows the system shown in FIG.
It is a figure which shows the measurement result at the time of applying the detector of the Example shown to (D).

15 is a diagram showing a measurement result when the detector of the comparative example shown in FIG. 12 is applied to the system shown in FIG. 13;

FIG. 16 is a view showing a state in which a NOx gas concentration detector according to one embodiment of the present invention is mounted on a mounting bracket.

[Explanation of symbols]

1, 61: first diffusion hole 2, 62: first measurement chamber 3, 63: second diffusion hole 4, 64: second measurement chamber 6, 66: first oxygen ion pump cell 6a, 6b, 66a, 66b: First oxygen ion pump electrode 7, 67: Oxygen concentration measurement cell 8, 68: Second oxygen ion pump cell 8a, 8b, 68a, 68b: Second oxygen ion pump electrode 71-3 Insulating layer A: In first measurement chamber Length (electrode length of first oxygen ion pump cell of comparative example) B: Electrode length of first oxygen ion pump cell

Continuation of the front page (72) Inventor Takafumi Oshima 14-18 Takatsuji-cho, Mizuho-ku, Nagoya Japan Special Ceramics Co., Ltd. (56) References JP-A-10-160703 (JP, A) JP-A-8-271476 (JP, A) JP-A-9-288085 (JP, A) JP-A-10-221298 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/416 G01N 27 / 419 G01N 27/41 G01N 27/406 G01N 27/409

Claims (3)

(57) [Claims] A first measuring chamber into which a gas to be measured is introduced via a first diffusion resistor; an oxygen partial pressure detecting electrode for measuring a partial pressure of oxygen in the gas to be measured in the first measuring chamber; A first oxygen ion pump cell for sufficiently pumping oxygen in the gas to be measured from the first measurement chamber to the outside of the measurement chamber based on the potential of the oxygen partial pressure detection electrode so that NOx is not substantially decomposed; A second measurement chamber into which gas is introduced from the first measurement chamber via a second diffusion resistor; and a voltage applied to decompose NOx in the second measurement chamber,
A second oxygen ion pump cell through which a current corresponding to the NOx gas concentration flows due to the dissociated oxygen, wherein the NOx gas concentration detector comprises: The ratio of the length of at least one of the electrodes of the first oxygen ion pump cell provided on the wall surface of the first measurement chamber to the total length of the first measurement chamber is (electrode / total length) = 1/4 A NOx gas concentration detector characterized in that: 2. The first diffused resistor and the second diffused resistor are spaced apart from each other on the first measurement chamber side, and the first diffused resistor and the first diffused resistor are disposed on one surface of the first measurement chamber facing each other. An electrode on the first measurement chamber side of the oxygen ion pump cell;
An inlet opening of the second diffusion resistor is formed on the other surface, and at least the electrode of the first oxygen ion pump cell on the first measurement chamber side is at least the second electrode from the vicinity of the first diffusion resistor. 2. The NOx gas concentration detector according to claim 1, wherein the NOx gas concentration detector is formed so as to extend on the first diffusion resistance side from immediately above the opening on the inlet side of the diffusion resistance. 3. A pair of electrodes provided in the second oxygen ion pump cell, one of which is provided in the second measurement chamber, and the other of which is provided outside the second measurement chamber is sealed or covered. 3. The NOx gas concentration detector according to claim 1, wherein the NOx gas concentration detector is electrically connected to the electrode and communicates with the atmosphere via a lead having gas diffusion resistance.