JP2001141696A - Gas-detecting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly accurate and highly reliable gas-detecting apparatus by shortening a time required for stabilizing a sensor cell current value which detects an NOx concentration. SOLUTION: In the gas-detecting apparatus of a constitution in which, while a first inner space 31 is controlled to a low oxygen concentration by an oxygen pump cell 2 comprising a solid electrolyte A and a pair of electrodes 21 and 22, the NOx concentration in an exhaust gas introduced to a second inner space 32 is measured by an NOx sensor cell 6 comprised of a solid electrolyte B and a pair of electrodes 61 and 62. A first detecting electrode 61 of the NOx sensor cell 6 is constituted of an Rh or a Pt-Rh electrode or the like of a high NOx-dissolving efficiency. When the sensor starts operating, a second control voltage V2 higher than a normal first control voltage V1 is impressed for a predetermined time to the NOx sensor cell 6, thereby removing the oxygen adsorbing to the first detecting electrode 61. Thereafter, the voltage is switched to the normal first control voltage V1. A stabilization time is thus shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector for measuring a concentration of a specific gas component in a gas to be measured containing oxygen, and more particularly to a gas suitable for detecting nitrogen oxides in exhaust gas of an internal combustion engine. The present invention relates to a detection device.

[0002]

2. Description of the Related Art In order to prevent air pollution, regulations on exhaust gas from automobile engines and the like are becoming stricter year by year.
As means for reducing harmful components in exhaust gas, for example,
A system for suppressing generation of harmful components by controlling combustion of an engine and a system for purifying harmful components using a catalytic converter have been put into practical use. In recent years, in these systems, nitrogen oxides (NO
x) It has been studied to directly detect the concentration and use it for combustion control, determination of catalyst deterioration, and the like. For example, if the combustion state can be known from the NOx concentration in the exhaust gas and the fuel injection, the EGR rate, and the like can be feedback controlled, the combustion control can be performed more accurately to reduce harmful components. In addition, N on the downstream side of the catalytic converter for purifying exhaust gas is used.
By measuring the Ox concentration, it is possible to easily determine the deterioration of the catalyst supported on the catalytic converter.

[0003] Therefore, there is a demand for a gas detector capable of directly and accurately detecting the NOx concentration in exhaust gas for monitoring a combustion state and a catalyst state. Conventionally, as a gas detection device capable of directly detecting a gas containing oxygen in its composition such as NOx, a limiting current type gas detection device using an oxygen ion conductive solid electrolyte is known. FIG. 14 shows an example of the configuration. The gas detection device has an internal space 103 formed in a spacer disposed between a solid electrolyte body 101 and a solid electrolyte body 102. Exhaust gas as a gas to be measured is introduced through the hole 104. The solid electrolyte body 101 and the electrodes 101a and 101b on its upper and lower surfaces constitute an oxygen pump cell. When a voltage is applied in a direction as shown in the figure, oxygen in the internal space 103 becomes solid electrolyte 101.
The gas passes through the inside and is discharged to the outside, and the inside of the internal space 103 is controlled to a predetermined low oxygen concentration.

On the other hand, a sensor cell is constituted by the solid electrolyte member 102 and electrodes 102a and 102b on the upper and lower surfaces thereof.
When the electrode 102a facing the internal space 103 is formed of an electrode material that is active for reducing NOx, NOx is decomposed into nitrogen and oxygen on the electrode 102a to generate new oxygen. Since the inside of the internal space 103 is controlled to a predetermined low oxygen concentration by the oxygen pump cell, the increase or decrease of the current flowing when a predetermined control voltage is applied between the electrodes 102a and 102b is measured, so that the NOx concentration in the exhaust gas is measured. Can be detected.

[0005]

In the gas detection device having the above structure, when controlling the oxygen concentration in the internal space 103 by using the oxygen pump cell, it is necessary to prevent the decomposition of NOx from occurring.
Electrode 101b facing internal space 103 of oxygen pump cell
Are composed of a Pt-Au electrode that is inactive against NOx. The electrode 1 facing the internal space 103 of the sensor cell
02a is Pt which is conventionally active in reducing NOx.
Electrodes are used, but in recent years, use of Rh or Pt-Rh electrodes, which have higher reductive decomposition performance than Pt electrodes, has been studied.

However, it has been found that when the Rh or Pt-Rh electrode is used, the time required for the sensor cell output to stabilize at startup is longer than that of the Pt electrode. In the NOx detection device having the above configuration, Rh or Pt-Rh
The output current value when the sensor is activated when the electrodes are used is shown in FIG.
(A), and changes to a predetermined control voltage (0.5
After application of V), it takes 2 to 3 minutes for the sensor cell current value to stabilize. It is considered that this is because the electrode containing Rh easily adsorbs O ₂ in the atmosphere, and the output current value increases due to the current based on the decomposition of O ₂ adsorbed on the electrode at the time of startup. FIG. 6 shows the relationship between the Pt / Rh ratio of the Pt-Rh electrode and the stabilization time. The stabilization time increases as the Rh content increases. For this reason, at the time of starting the engine, there is a problem that it is not possible to monitor the combustion control system, the catalytic converter system, and the like using the NOx concentration.

An object of the present invention is to provide a NOx decomposition electrode with Rh.
Alternatively, it is an object of the present invention to provide a highly accurate and highly reliable gas detection device in which the time required for stabilizing a sensor cell current value when a Pt-Rh electrode is used is reduced.

[0008]

A first aspect of the present invention is a gas detection apparatus for detecting the concentration of a specific gas component in a gas to be measured in which oxygen and a specific gas component containing oxygen in its composition exist. There is an internal space in which the gas to be measured is introduced under a predetermined diffusion resistance, and one of a pair of electrodes formed on the surface of an oxygen ion-conductive solid electrolyte is used to reduce one of the electrodes to reduce the specific gas component. An oxygen pump section that is disposed so as to be exposed to the internal space as an electrode that is inert to decomposition and controls the oxygen concentration in the internal space by energizing the pair of electrodes, and an oxygen ion-conductive solid electrolyte One of the pair of electrodes formed on the surface of the body is disposed so as to be exposed to the internal space as an electrode active for reductive decomposition of the specific gas component, and a predetermined control is performed on the pair of electrodes. Mark voltage A gas concentration detector for detecting the concentration of the specific gas component from the amount of oxygen generated by the reductive decomposition of the specific gas component when the device is activated. A means for removing oxygen is provided.

In the gas detecting device having the above structure, at the time of starting the device, the oxygen adsorbed on the one electrode of the gas concentration detecting section is removed by the means for removing oxygen.
When detecting the specific gas component, if oxygen contained in the gas to be measured is adsorbed to the one electrode, decomposition of oxygen occurs due to the application of voltage, the output current value fluctuates, and a long time is required for stabilization. . In the present invention, the stabilization time can be shortened by removing the adsorbed oxygen in advance, and the concentration of the specific gas component can be accurately detected immediately after the start-up.
Therefore, various systems can be monitored when the engine is started, and the reliability of the system is improved.

According to the second aspect of the present invention, as the means for removing the oxygen, a control means for controlling a voltage applied between the pair of electrodes of the gas concentration detector is provided. The control means controls the voltage applied between the pair of electrodes at the time of activation of the sensor to be higher than the predetermined control voltage, thereby adsorbing the one electrode of the gas concentration detector. Oxygen that has been decomposed can be quickly decomposed and removed.

In the invention according to claim 3, as the means for removing the oxygen, a flammable gas introducing means for introducing a flammable gas into the gas to be measured is provided. The flammable gas introducing means introduces a flammable gas into the gas to be measured at the time of starting the sensor, and the flammable gas and the oxygen adsorbed on the one electrode of the gas concentration detector are By reacting, oxygen can be quickly removed.

Specifically, as the specific gas component to be detected, nitrogen oxide can be mentioned. At this time, if the one electrode of the gas concentration detection unit is formed of an electrode material containing rhodium or platinum and rhodium as a main component metal, the reductive decomposition performance against nitrogen oxides is high, and the detection accuracy of the gas detection device is high. improves.
Further, as in claim 5, for the one electrode of the oxygen pump section, an electrode material containing platinum and gold as main components metals, which is inactive against reductive decomposition of nitrogen oxide, can be used.

In the configuration of claim 6, the internal space communicates with the measured gas existence space by the first diffusion resistance means, and the first internal space communicates with the first internal space by the second diffusion resistance means. (2) partitioning the inner space and exposing the one electrode of the oxygen pump section to the first inner space;
An oxygen concentration in the first internal space is controlled to a predetermined low concentration by flowing a current between the pair of electrodes so as to discharge oxygen in the first internal space. Further, when the one electrode of the gas concentration detection unit is exposed to the second internal space and a predetermined voltage is applied between the pair of electrodes,
The concentration of the specific gas component in the gas to be measured is detected from the value of an oxygen ion current flowing between the two electrodes based on oxygen generated by the reductive decomposition of the specific gas component.

As described above, the internal space may be divided into the first and second internal spaces communicating with each other by the second diffusion resistance means. At this time, the atmosphere in the first internal space controlled to a predetermined low oxygen concentration by the oxygen pump section diffuses into the second internal space via the second diffusion resistance means, and the atmosphere in the second internal space becomes stable. Therefore, more accurate detection is possible.

According to a seventh aspect of the present invention, one of a pair of electrodes formed on the surface of the oxygen ion-conductive solid electrolyte body is connected to a reference oxygen so that one of the electrodes is exposed to the first internal space. An oxygen concentration detector is disposed so as to be exposed to the concentration gas existing space, and detects an oxygen concentration in the first internal space from an electromotive force generated between the pair of electrodes.
By controlling the amount of electricity supplied to the oxygen pump unit based on the oxygen concentration detected by the oxygen concentration detection unit, the oxygen concentration in the first internal space can be controlled to a predetermined value.

In the configuration of claim 8, the exhaust gas of the internal combustion engine is used as the gas to be measured. The flammable gas introducing means is provided with a flammable gas introduction path in the exhaust passage or by adjusting the air-fuel ratio of the internal combustion engine to bring the exhaust gas into an excessively flammable gas state. When a specific gas component in the exhaust gas of the internal combustion engine is detected, the flammable gas is introduced into the exhaust passage, or the air-fuel ratio introduced into the internal combustion engine is set to be excessive in fuel, so that the Flammable gas can be easily introduced and reacted with oxygen.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Nitrogen oxides (N
One embodiment of a gas detection device for Ox) detection will be described with reference to the drawings. FIG. 3 is an overall cross-sectional view of the main body of the gas detection device. The gas detection unit 1 is housed in a cylindrical housing H with an outer periphery held by an insulating material. The gas detector 1
Is an elongated flat plate, and a tip end (lower end in the figure) protrudes from the housing H and extends downward in the figure, and is located in a container-shaped exhaust cover H1 fixed to a lower end of the housing H. The exhaust cover H1 has a double structure of an inner cover H11 and an outer cover H12 made of stainless steel. Exhaust gas as a gas to be measured is provided inside the exhaust cover H1 on the side walls and the bottom wall of the covers H11 and H12. Exhaust ports H13 and H14 for taking in the air are formed respectively.

At the upper end of the housing H, an air cover H2 composed of a cylindrical main cover H21 and a sub-cover H22 covering the rear end is fixed. The rear end (the upper end in the figure) and the lead wire H3 connected to the rear end are protected. The main cover H21 and the sub cover H
At 22, air ports H23 and H24 are respectively formed at positions facing the side walls, and the air as the reference oxygen concentration gas is taken into the air cover H2 from the air ports H23 and H24. In addition, the above air ports H23, H24
Is formed between the main cover H21 and the sub-cover H22, a water-repellent filter H25 for waterproofing is provided. The other end of the lead wire H3 extends outside from an upper end opening of the air cover H2.

FIGS. 1 and 2 are a schematic sectional view and a developed view, respectively, of the tip of the gas detector 1. As shown in FIGS. The gas detection unit 1 includes:
As shown in the figure, a first internal space 31, a spacer 3 forming a second internal space 31, and a solid electrolyte body are provided below an oxygen pump cell 2 serving as an oxygen pump unit including a solid electrolyte body A and a pair of electrodes 21 and 22. A NOx sensor cell 6 serving as a gas concentration detecting unit composed of B and a pair of electrodes 61 and 62, a spacer 4 forming an air duct 41, and a heater 7 for heating each cell are sequentially laminated.

1 and 2, the oxygen pump cell 2
Is for controlling the oxygen concentration in the first internal space 31 to a predetermined concentration, and a screen is provided at a position opposed to the upper and lower surfaces of an oxygen ion conductive solid electrolyte body A such as zirconia formed in a sheet shape. A pair of electrodes 2 formed by printing or the like
1, 22. Of the pair of electrodes 21 and 22,
The upper first pump electrode 21 is exposed to the space where the gas to be measured exists, that is, the space inside the exhaust cover H1 in FIG. As the first pump electrode 21, for example, platinum (P
A porous cermet electrode containing a noble metal such as t) is used.

On the other hand, the lower second pump electrode 22
It is exposed to the internal space 31 (FIG. 1). First internal space 3
1 is formed by a hole 3a (FIG. 2) provided in a spacer 3 made of alumina or the like laminated below the solid electrolyte body A. As the second pump electrode 22, an electrode whose electrode activity is adjusted so as to be active for reductive decomposition of oxygen and inactive for reductive decomposition of NOx is preferably used. Specifically, an electrode containing platinum (Pt) and gold (Au) as main metal components, preferably a porous cermet electrode made of a Pt-Au alloy powder and a ceramic such as zirconia or alumina is suitably used.

The first internal space 31 is formed in the exhaust cover H1 in which the gas to be measured exists by the solid electrolyte member A and the pinhole 51 as the first diffusion resistance means penetrating the pair of electrodes 21 and 22. (FIG. 3). The size of the pinhole 51 is appropriately set so that the diffusion speed of the exhaust gas passing through the pinhole 51 and introduced into the first internal space 31 becomes a predetermined speed. Further, the first pump electrode 21 and the pinhole 5 on the exhaust gas side are used.
1 to form a porous protective layer 53 made of porous alumina or the like.
Is prevented from being clogged with soot and the like contained in the exhaust gas.

The upper and lower surfaces of the solid electrolyte body A are shown in FIG.
The leads 21a and 22a connected to the pair of electrodes 21 and 22 are formed as shown in FIG. It is preferable that an insulating layer of alumina or the like be interposed between the solid electrolyte body 2 and the leads 21a and 22a except for the electrode forming part of the solid electrolyte body A, especially in the lead forming part.

The NOx sensor cell 6 detects the NOx concentration from the amount of oxygen generated by the reduction and decomposition of NOx, and is a sheet-like solid electrolyte B made of zirconia or the like.
And a pair of electrodes 61 and 62 formed by screen printing or the like at opposing positions on the upper and lower surfaces. Of the pair of electrodes 61 and 62, the upper first detection electrode 61 is exposed to the second internal space 32 that communicates with the first internal space 31 through the communication hole 52 that is the second diffusion resistance means. The communication hole 52 and the second internal space 32 are formed by holes 3 b and 3 c (FIG. 2) provided in the spacer 3. As the first detection electrode 61, an electrode having an activity for reductive decomposition of NOx is preferably used. Specifically, an electrode containing rhodium (Rh) or Pt and Rh as main metal components, preferably a porous cermet electrode made of Rh powder or Pt-Rh alloy powder and ceramics such as zirconia and alumina is used.

On the other hand, the lower second detection electrode 62 faces the air duct 41 into which the air is introduced. Atmospheric duct 41
Is a notch 4a provided in a spacer 4 made of alumina or the like.
(FIG. 2), the right end opening of the air duct 41 communicates with the space inside the air cover H2 (FIG. 3). This second
The detection electrode 62 is formed of a porous cermet electrode containing a noble metal such as Pt.

On the upper and lower surfaces of the solid electrolyte body B, leads 61a and 62a connected to a pair of electrodes 61 and 62 are formed. Also in this case, it is preferable that an insulating layer such as alumina is interposed between the solid electrolyte body B and the leads 61a and 62a, except for the electrode forming section of the solid electrolyte body B, especially at the lead forming section.

The heater 7 has a structure in which a heater electrode 71 is embedded between sheet-like insulating layers 72 and 73 made of alumina or the like. The heater electrode 71 is formed of, for example, a cermet of Pt and a ceramic such as alumina. A lead 71a (FIG. 2) is connected to the heater electrode 71, and heat is generated by external power supply to heat the cells 2 and 6 to the activation temperature.

Next, the principle of operation of the gas detecting device having the above-described configuration will be described. In FIG. 1, an exhaust gas as a gas to be measured passes through a porous layer 53 and a pinhole 51 and is introduced into the first internal space 31. In the exhaust gas, oxygen (O ₂ ), nitrogen oxide (NOx), carbon dioxide (C
Gas components such as O ₂ ) and water (H ₂ O) are included, and the amount of gas to be introduced is determined by the diffusion resistance of the porous layer 53 and the pinhole 51. A pair of electrodes 2 of the oxygen pump cell 2
When a voltage is applied to the first and second pump electrodes 22 so that the first pump electrode 21 on the exhaust gas side becomes a positive electrode, oxygen in the exhaust gas is reduced on the second pump electrode 22 on the first internal space 31 side. It becomes oxygen ions and is discharged to the first pump electrode 21 side by a pumping action. Here, the second pump electrode 22
Is an electrode (Pt-Au electrode) that is inert to NOx, so that NOx is not decomposed.

FIG. 4A is a diagram showing the voltage-current characteristics of the oxygen pump cell 2, wherein the horizontal axis represents the voltage Vp applied to the oxygen pump cell.
The vertical axis is the oxygen pump cell current Ip. As shown in the figure, the oxygen pump cell 2 shows a limit current characteristic according to the oxygen concentration, and the limit current detection region (the straight line portion parallel to the Vp axis) shifts to the positive voltage side as the oxygen concentration increases. Therefore, to make an accurate oxygen pump using the above limit current detection area,
It is desirable that the voltage applied to the oxygen pump cell 2 is not fixed, and the voltage is changed according to the oxygen concentration. That is,
From the relationship between the oxygen pump cell applied voltage Vp and the oxygen pump cell current Ip in FIG. 4A, the first internal space 31 is applied by applying a voltage V0 according to the oxygen concentration so that the oxygen pump cell current Ip becomes a limiting current. The oxygen concentration in the inside can be controlled to a predetermined low concentration.

Here, as shown in FIG.
It is also possible to provide an oxygen sensor cell 8 as an oxygen concentration detecting unit for measuring the oxygen concentration in the inside, and control the voltage applied to the oxygen pump cell 2 based on the measurement result. The oxygen sensor cell 8 has a pair of electrodes 81 and 82 provided on the upper and lower surfaces of the solid electrolyte body B, and the upper electrode 82 is connected to the first internal space 3.
1, the lower electrode 81 is exposed to the atmosphere duct 41 in which the atmosphere as the reference oxygen concentration gas exists. The upper electrode 82 is made of a NOx inert electrode, for example, a porous Pt-Au alloy electrode, and the lower electrode 81 is made of a porous Pt electrode. In the oxygen sensor cell 8, an electromotive force is generated between the pair of electrodes 81 and 82 based on the oxygen concentration difference between the first internal space 31 and the air duct 41 that is the reference oxygen concentration gas, and by measuring this magnitude, The oxygen concentration in the first internal space 31 can be known. Therefore, if the amount of electricity supplied to the oxygen pump cell 2 is feedback-controlled so that the electromotive force becomes a predetermined constant value, the oxygen concentration in the first internal space 31 can be controlled to be constant.

As described above, the exhaust gas whose oxygen concentration is controlled by the oxygen pump cell 2 to a low oxygen concentration that does not affect the detection of the NOx concentration is introduced into the second internal space 32 through the communication hole 52. In the NOx sensor cell 6, the first detection electrode 61 exposed to the second internal space 32 is NO.
x-reducing active electrode (Rh electrode or Pt-Rh
Therefore, when a predetermined control voltage is applied between the pair of electrodes 61 and 62, NOx is decomposed on the first detection electrode 61, and an oxygen ion current corresponding to the amount of generated oxygen is generated by the pair of electrodes 61 and 62. It flows between 62. By measuring this current value, the NOx concentration in the exhaust gas can be detected. At this time, the second detection electrode 6 exposed to the air duct 41
2 is set to be a positive pole.

FIG. 4 shows the characteristics of an ideal NOx sensor cell 6.
This will be described with reference to FIG. As shown in the figure, the voltage-current characteristics of the NOx sensor cell 6 show a limit current characteristic with respect to the NOx concentration. In the figure, the horizontal axis is the NOx sensor cell applied voltage Vs,
The vertical axis is the NOx sensor cell current Is, and is a predetermined constant voltage V in the limit current detection region (a straight line portion parallel to the Vs axis).
When 1 (for example, about 0.5 V) is applied, a current flows according to the NOx concentration, and the NOx concentration can be detected from this value. The broken lines in FIGS. 4 (A) and 4 (B) are the currents due to the decomposition of H ₂ O. In order to perform accurate detection, the control voltage V0 of the oxygen pump cell 2 and the NOx sensor cell 6,
The V1, it is desirable that the voltage lower than the decomposition zone of H ₂ O.

Here, as in the present embodiment, the first detection electrode 61 of the NOx sensor cell 6 is changed to Rh or Pt-Rh.
When the electrodes are used, the time from the activation of the gas detection device to the stabilization of the sensor cell output (stabilization time) is longer than that of the Pt electrode. As shown in FIG. 6, it is considered that the larger the Rh content, the longer the stabilization time, and O ₂ in the atmosphere is adsorbed on Rh, and the decomposition current flows. FIG. 7A shows a Pt-Rh electrode (Rh 50% by weight).
Shows the change in the sensor cell current after the application of the normal control voltage (0.5 V). It can be seen that the sensor cell current immediately after the application of the voltage is increased by the decomposition current of the adsorbed O ₂ . After that, the sensor cell current value sharply decreases, but it takes 200 seconds or more to become substantially constant.

Therefore, in the present invention, in order to remove the influence of the decomposition current of O ₂ , the first detection electrode 6 is activated at the time of starting.
Forcibly remove the O ₂ adsorbed on the 1. Means for removing O ₂ include a pair of electrodes 6 of the NOx sensor cell 6.
There is a method of providing a control means for controlling a voltage applied between the first and the second 62 to electrochemically remove O ₂ . That is, by applying a voltage higher than (the second control voltage V2) normal control voltage to the NOx sensor cell 6 (first control voltage V1), forcibly O ₂ decomposition, it can be removed. FIG.
(B) shows an example of a voltage application pattern. In this example, the first control voltage V1 is set to 0.5V and the second control voltage V1 is set to 0.5V.
2 is set to 1.0 V, the second control voltage V2 is applied for a predetermined time at the time of startup using the same Pt-Rh electrode as in FIG. 7A, and then switched to the first control voltage V1 for normal control. I'm back. As a result, the sensor cell current becomes the second control voltage V
7 is substantially constant at the time of switching to the first control voltage V1, indicating that the time during which the O ₂ decomposition current flows is shorter than that of FIG. 7A, that is, the stabilization time can be greatly reduced. I have.

FIG. 8 is a circuit diagram of the control means for controlling the voltage applied to the NOx sensor cell 6. When the switch SW1 is closed to the contact point P, the first control voltage V1 (for example, 0.5 V) is increased. When the contact is closed to the contact Q side, the second control voltage V2 (for example, 1.0 V) is applied to the NOx sensor cell 6. Therefore, when starting the sensor,
First, the switch SW1 is switched to the contact Q side to apply a second control voltage V higher than the first control voltage V1 to the NOx sensor cell 6.
2 is applied, and after a predetermined time elapses, the switch SW1 is switched to the contact P side to perform normal control. In this normal control, a current value ((Va−V) flowing when a predetermined constant voltage (first control voltage V1) is applied to the NOx sensor cell 6
b) Measure the NOx concentration from / R1).

FIG. 9 is a diagram showing the relationship between the time during which the second control voltage V2 is applied to the NOx sensor cell 6 and the stabilization time (exhaust gas condition: 5% O ₂ (lean)). The longer the is, the shorter the stabilization time, but thereafter, the stabilization time tends to be longer again. As described above, the reason why the stabilization time has a minimum value with respect to the application time is that if the application time is long, the atmosphere becomes rich and the combustible gas (H ₂ , HC, etc.) contained in the exhaust gas is applied to the first detection electrode 61 It is presumed to be due to adsorption to inhibit the decomposition of NOx. Further, as shown in FIG. 9, when the exhaust gas is lean, the application time is 10 seconds and the stabilization time is the shortest 20 seconds. On the other hand, when the exhaust gas is rich, the application time is 2 seconds and the stability time is the shortest.
5 seconds. The reason why the optimum value of the application time differs depending on the components of the exhaust gas (rich, lean) is that the combustible gas (H ₂ ,
HC etc.) binds to O ₂ adsorbed on the first detection electrode 61,
It is considered that removal is promoted. From these results, it is desirable to select an optimal value for the application time to the second control voltage V2 in accordance with the exhaust gas component and the like so that the stabilization time is the shortest.

FIG. 10 shows the relationship between the second control voltage value and the stabilization time. As shown in the figure, the higher the second control voltage V2, the shorter the stabilization time. However, as the voltage increases, the solid electrolyte B (zirconia or the like) constituting the NOx sensor cell 6 may be decomposed. According to the evaluations of the present inventors, when the voltage is higher than 1.2 V, the decomposition current of zirconia is detected,
Therefore, in order to improve the durability, the second control voltage V
2 is preferably 1.2 V or less.

FIG. 11 shows various examples of voltage application patterns when controlling the voltage applied to the NOx sensor cell 6. FIG. 11 (A) is the same as the pattern shown in FIG. 7 (B), and the most effective method is to apply a constant second control voltage V2 for a predetermined period of time to such an extent that the solid electrolyte B is not decomposed. It is. Also, as shown in FIG. 11B, the voltage is gradually increased to the second control voltage V2, and after maintaining for a predetermined time, the voltage is gradually decreased to the first control voltage V1. As shown in FIG. 11C, the voltage may be changed in a gentle curve from zero voltage to the first control voltage V2 via the second control voltage V2, and an effect almost equivalent to that of FIG. In addition, it is possible to prevent the solid electrolyte B from deteriorating due to a sudden change in the applied voltage. Alternatively, as shown in FIG. 11D, the second control voltage V2 and the first control voltage V1 may be alternately switched for a predetermined period, and any method of applying a voltage higher than the normal control voltage at the time of starting the sensor. The effect can also be obtained.

As means for removing O ₂ adsorbed on the first detection electrode 61, in addition to the above-described means for controlling the voltage applied to the NOx sensor cell 6, a flammable gas is introduced into the exhaust gas to be measured. Providing flammable gas introduction means to
There is a method of reacting the adsorbed O ₂ with a combustible gas. That is, when the sensor is activated, the measurement atmosphere is set in an excessively combustible gas state for a predetermined time, so that O ₂ adsorbed on the first detection electrode 61 can be removed by reacting with the combustible gas. FIG. 12 shows the relationship between the time (rich time) and the stabilization time when the measurement atmosphere is in the state where the combustible gas is excessive. In this example, the measurement atmosphere where the combustible gas is excessive is A / A.
F12 and the subsequent normal measurement atmosphere were 5% O ₂ .
As shown in the figure, the stabilization time becomes shorter as the state of excess combustible gas becomes longer, and according to the desired stabilization time,
The amount of the flammable gas to be introduced and the time for making the flammable gas excessive may be appropriately set.

As a specific example of the flammable gas introducing means, as shown in FIG. 13, when the gas detecting device of the present invention is installed in the exhaust passage 9 of the engine, the gas introducing passage connected to the upstream side of the main body is provided. 91 is provided to introduce a combustible gas such as a fuel gas, or to adjust the air-fuel ratio of the air-fuel mixture to be taken into the engine so that the exhaust gas introduced into the gas detection device becomes excessively combustible gas. In each case, the same effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a main part of a main body of a gas detection device according to a first embodiment.

FIG. 2 is a development view of FIG.

FIG. 3 is an overall sectional view of a gas detection device main body.

FIG. 4A is a voltage-current characteristic diagram of an oxygen pump cell,
(B) is a voltage-current characteristic diagram of the NOx sensor cell.

FIG. 5 is a sectional view of a main part of a gas detection device according to a second embodiment.

FIG. 6 is a diagram showing the relationship between Rh content and stabilization time.

7A and 7B show a relationship between a control voltage of a NOx sensor cell and a sensor cell current, FIG. 7A shows a change in sensor cell current by conventional control, and FIG. 7B shows a change in sensor cell current by control of the present invention; is there.

FIG. 8 is a control circuit diagram of a NOx sensor cell.

FIG. 9 is a diagram showing a relationship between a second control voltage application time and a stabilization time.

FIG. 10 is a diagram showing a relationship between a second control voltage and a stabilization time.

FIGS. 11A to 11D are diagrams showing voltage application patterns of a NOx sensor cell.

FIG. 12 is a diagram showing a relationship between a rich time and a stable time in which a state of excessive combustible gas occurs.

FIG. 13 is a diagram showing a specific configuration of a flammable gas introduction unit.

FIG. 14 is a sectional view of a main part of a conventional gas detection device.

[Explanation of symbols]

 A, B Solid electrolyte body 1 Gas detection unit 2 Oxygen pump cell (oxygen pump unit) 21, 22 A pair of electrodes 3 Spacer 31 First internal space 32 Second internal space 4 Spacer 41 Atmospheric duct 5 Solid electrolyte body 51 Pinhole (No. 1 diffusion resistance means) 52 communication hole (second diffusion resistance means) 53 porous protective layer 6 NOx sensor cell (gas concentration detection unit) 61, 62 pair of electrodes 7 heater unit 8 oxygen sensor cell (oxygen concentration detection unit) 81, 82 A pair of electrodes

Continued on the front page (72) Inventor Keigo Mizutani 14 Iwatani, Shimowasukamachi, Nishio City, Aichi Prefecture Inside the Japan Automobile Parts Research Institute (72) Inventor Tasuke Makino 14 Iwatani, Shimowasukamachi, Nishio City, Aichi Prefecture Japan Motor Corporation (72) Inventor Toshitaka Saito 1-1-1, Showa-cho, Kariya-shi, Aichi Pref. Denso Corporation

Claims (8)

[Claims] 1. A gas detecting device for detecting the concentration of a specific gas component in a gas to be measured in which oxygen and a specific gas component containing oxygen in the composition are present, wherein the gas to be measured has a predetermined diffusion property. An inner space introduced under the resistance, and a pair of electrodes formed on the surface of the oxygen ion conductive solid electrolyte body, wherein one of the electrodes is an electrode inert to the reductive decomposition of the specific gas component. An oxygen pump unit arranged to be exposed to the internal space and controlling the oxygen concentration in the internal space by energizing the pair of electrodes; and a pair of electrodes formed on the surface of the oxygen ion conductive solid electrolyte body. Among them, one electrode is disposed as an electrode active for reductive decomposition of the specific gas component, so as to be exposed to the internal space, and when a predetermined control voltage is applied to the pair of electrodes, the specific gas component is of A gas concentration detection unit for detecting the concentration of the specific gas component from the amount of oxygen generated by reductive decomposition, and a unit for removing oxygen adsorbed on the one electrode of the gas concentration detection unit when the apparatus is started. A gas detector characterized by the above-mentioned. 2. As means for removing the oxygen, a control means for controlling a voltage applied between the pair of electrodes of the gas concentration detecting section is provided, and the control means controls the voltage between the pair of electrodes at startup. The gas detection device according to claim 1, wherein control is performed such that the applied voltage is higher than the predetermined control voltage. 3. As the means for removing the oxygen, a flammable gas introducing means for introducing a flammable gas into the gas to be measured is provided. The gas detector according to claim 1, wherein a combustible gas is introduced. 4. The specific gas component is a nitrogen oxide,
2. The one electrode of the gas concentration detecting section contains rhodium or platinum and rhodium as main components metals.
4. The gas detection device according to any one of claims 3 to 3. 5. The one electrode of the oxygen pump section,
5. The gas detection device according to claim 4, wherein platinum and gold are contained as main components metals. 6. The internal space includes a first internal space that communicates with the measured gas existence space by first diffusion resistance means, and a second internal space that communicates with the first internal space and second diffusion resistance means. Exposing the one electrode of the oxygen pump unit to the first internal space and flowing a current between the pair of electrodes so as to discharge oxygen in the first internal space, thereby causing the first electrode to be discharged;
When controlling the oxygen concentration in the internal space to a predetermined low concentration, exposing the one electrode of the gas concentration detecting section to the second internal space, and applying a predetermined voltage between the pair of electrodes. The gas detection according to any one of claims 1 to 5, wherein the concentration of the specific gas component in the gas to be measured is detected from an oxygen ion current value flowing between the two electrodes based on oxygen generated by reductive decomposition of the specific gas component. apparatus. 7. A pair of electrodes formed on a surface of an oxygen ion conductive solid electrolyte body, and one of the electrodes is placed in a reference oxygen concentration gas existing space so that one of the electrodes is exposed to the first internal space. Arranged to be exposed, has an oxygen concentration detection unit that detects the oxygen concentration of the first internal space from the electromotive force generated between the pair of electrodes, by the oxygen concentration detected by the oxygen concentration detection unit, 7. The gas detector according to claim 6, wherein the amount of electricity supplied to the oxygen pump unit is controlled. 8. The gas to be measured is an exhaust gas of an internal combustion engine, and the flammable gas introducing means provides the flammable gas introduction path in an exhaust passage or adjusts the air-fuel ratio of the internal combustion engine to produce the exhaust gas. 4. The gas detecting device according to claim 3, wherein the gas is set in a state of excessive combustible gas.