JP2002257778A - Combustible gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a combustible gas sensor capable of suppressing the output fluctuation in measurement when a gas to be measured contains a combustible gas of high concentration and performing a stable detection with high precision. SOLUTION: A combustible gas sensor element 1 is formed of an oxygen ion conductor 14 and a pair of electrodes 13a and 13b, and a diffusive resistance layer 12 is provided so as to cover the surface of the electrode 13a on the side of the gas to be measured. The diffusive resistance layer 12 is set to a minimum thickness necessary for completing the reaction of the combustible gas with oxygen before the gas to be measured reaches the surface of the electrode 13a, for example, about 1000 μm, so that the fluctuation of oxygen concentration on the surface of the electrode 13a to stabilize the output.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flammable gas sensor for detecting the concentration of flammable gas such as hydrocarbon. In particular, the present invention relates to a combustible gas sensor suitable for detecting a high-concentration combustible gas, such as when detecting fuel vapor flowing into an intake system of an engine.

[0002]

2. Description of the Related Art There is known a fuel vapor processing system in which fuel vapor generated by evaporation of fuel in a fuel tank is temporarily adsorbed and collected in a canister filled with an adsorbent. The collected fuel vapor is desorbed (purged) from the adsorbent by introducing the atmosphere into the canister using the negative pressure of the intake air at the time of engine operation, is sent out from the purge passage to the intake system, and is sent to the combustion chamber. Burn.

On the other hand, the fuel injection amount is controlled based on the detection result of an air-fuel ratio sensor installed in an exhaust system. In order to reduce exhaust emissions, it is necessary to control the fuel injection amount with high accuracy. It is important. However,
In an internal combustion engine equipped with a canister, the air-fuel ratio in the combustion chamber fluctuates as fuel vapor flows from the purge passage into the intake system. The air-fuel ratio sensor is installed in the exhaust system and indirectly measures the air-fuel ratio in the combustion chamber, and cannot determine the fuel vapor concentration flowing into the intake system. there were.

For this reason, it has been studied to install a sensor for directly detecting the concentration of the fuel vapor flowing into the intake system to correct the fuel injection amount. However, since the fuel vapor is a flammable gas and may become ignited when the concentration becomes high, the limiting current type oxygen sensor used for the air-fuel ratio sensor cannot be used as it is. Therefore,
The present inventors previously protected the outer periphery of a sensor element having a limiting current type oxygen sensor structure with a double cylindrical heat-resistant cover body, and devised the diameter and arrangement of ventilation holes provided in the outer cover. By doing so, a flammable gas sensor was proposed that prevented the flame from leaking to the outside (Japanese Patent Application No. 11-290113).
issue).

[0005]

However, as a result of actual measurement using the flammable gas sensor having the above structure, it has been found that even under a steady condition of a constant flow rate, a constant pressure, and a constant concentration, the measurement is performed in seconds. Output fluctuation was observed. For example, when measurement was performed under the condition that the concentration of butane gas as a flammable gas was 6% by weight, it was found that the range of output fluctuation reached 1% by weight at the maximum. Actually 0.1% by weight
Given that it is desirable to detect with a degree of accuracy,
This phenomenon is a serious obstacle in practical use.

As described above, unlike an air-fuel ratio sensor usually used in a positive pressure range and a low hydrocarbon concentration (less than the lower limit of the explosion concentration), in the negative pressure range and a high hydrocarbon concentration (about 10%), It was not easy to directly detect the fuel vapor concentration of the intake system under the conditions and to achieve both the flame-extinguishing property and the responsiveness.

[0007] Therefore, an object of the present invention is to suppress the fluctuation of output during measurement even when the gas to be measured contains a high-concentration flammable gas, and to provide a highly accurate flammable gas capable of stable detection. It is to provide a gas sensor.

[0008]

According to a first aspect of the present invention, there is provided a combustible gas sensor comprising: a pair of electrodes formed on a surface of an oxygen ion conductor; One of them is disposed in a space where a gas to be measured containing a combustible gas and oxygen is present, and a sensor element is provided which has a diffusion resistance layer covering the surface of the one electrode. The sensor element has means for completing the reaction between the combustible gas in the gas to be measured and oxygen before the gas to be measured reaches the surface of the one electrode.

The inventors of the present invention have found that when a conventional flammable gas sensor is used, the sensor output fluctuates because the reaction between flammable gas and oxygen occurs throughout the diffusion resistance layer. It was found that the oxygen concentration on the electrode surface fluctuated. By providing a means for completing the reaction between the combustible gas in the gas to be measured and oxygen before the gas to be measured reaches the surface of the one electrode, the fluctuation of the oxygen concentration on the surface of the one electrode is provided. Therefore, a stable sensor output can be obtained, and a high concentration of combustible gas can be detected with high accuracy.

In the second aspect of the present invention, as the above means, the thickness of the diffusion resistance layer is adjusted, and the reaction between the combustible gas in the gas to be measured and oxygen is completed while passing through the diffusion resistance layer. Thickness or more. If the diffusion resistance layer has a predetermined minimum thickness determined according to the concentration range of the flammable gas to be detected, the unreacted flammable gas is prevented from reaching the one electrode surface. In addition, the fluctuation of the oxygen concentration can be suppressed.

According to the third aspect of the present invention, the thickness of the diffusion resistance layer is made larger than 500 μm. The thickness of the diffusion resistance layer in the conventional sensor is usually about 500 μm, and it is difficult to suppress output fluctuation with this thickness.
It is desirable to make the diffusion resistance layer thicker than this.

According to a fourth aspect of the present invention, a catalyst can be carried on a trap layer formed on the surface of the diffusion resistance layer instead of increasing the thickness of the diffusion resistance layer. Since the reaction between the combustible gas and oxygen is promoted by the catalyst, a similar effect of preventing output fluctuation can be obtained.

According to a fifth aspect of the present invention, as the above means, a catalyst layer can be formed on the surface of the diffusion resistance layer. It is not necessary to form a catalyst layer on the trap layer. For example, the same effect can be obtained by forming a catalyst layer by coating the surface of the diffusion resistance layer with a catalyst.

According to a sixth aspect of the present invention, the sensor element is held in a cylindrical housing, and protects the outer periphery of the sensor element which protrudes from one end of the housing and is located in the space where the gas to be measured exists. A heavy-duty heat-resistant cover is provided.

When used in the presence of a high concentration of flammable gas, it is desirable to provide the cover body for protecting the sensor element, and by appropriately setting the size of the ventilation holes of the inner and outer covers, it is possible to provide a flame. Propagation can be prevented.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. The present invention can be applied to, for example, measurement of a fuel vapor concentration in a fuel vapor processing system of an automobile engine. In the fuel vapor processing system, a canister filled with an adsorbent is provided to adsorb and collect the fuel vapor generated in the fuel tank. Is sent to the combustion chamber together with the intake air. At this time, the flammable gas sensor of the present invention is, for example, installed on a surge tank wall provided in the middle of the intake passage to measure the flammable gas in the gas to be measured, that is, the concentration of hydrocarbon components in the intake air. Used.

FIGS. 1A and 1B show the structure of a combustible gas sensor S according to the present invention. In FIG. 1A, the combustible gas sensor S has a tubular housing H having both ends opened and a combustible gas sensor element 1 inserted and held in the cylinder. The front end (the lower end in the figure) of the sensor element 1 protruding below the housing H is housed in a cover body 2 fixed to the lower end of the housing H, and the rear end (not shown) of the sensor element 1 is It is housed in an air cover 3 fixed to the upper end of the housing H. The housing H is fixed to a surge tank (not shown) wall by a thread formed on the outer periphery, and the distal end portions of the cover body 2 and the sensor element 1 protrude into a space inside the surge tank which is a measured gas existing space.

The sensor element 1 has a limiting current type oxygen sensor structure utilizing oxygen ion conductivity of a solid electrolyte.
Specifically, it has a test tubular oxygen ion conductor 14 made of zirconia or the like, and electrodes 13a and 13b made of platinum or the like formed at the front end of the conductor at positions facing the inner and outer peripheral surfaces. The hollow portion of the oxygen ion conductor 14 communicates with the space in the atmosphere cover 3 into which the atmosphere as the reference oxygen concentration gas is introduced. As a result, the outer electrode 13a of the oxygen ion conductor 14 is exposed to the gas to be measured, and the inner electrode 13b is exposed to the atmosphere. The heater 4 is accommodated in the hollow portion of the oxygen ion conductor 14, and the heat generating portion heats the electrodes 13 a and 13 b of the oxygen conductor 14.

The cover body 2 is provided for keeping the combustible gas sensor element 1 warm and protected, and has a double structure of a bottomed container-like inner cover 21 and an outer cover 22. The inner cover 21 and the outer cover 22 are made of a metal material having excellent heat conductivity and heat resistance, for example, stainless steel, and have a plurality of ventilation holes 23 serving as an inlet or outlet for a gas to be measured on a side wall or a bottom wall. 24.

Here, the vent hole 24 of the outer cover 22 functions as a flame extinguishing hole, and the flame generated in the inner cover 21 is extinguished by the heat taken by the wall surface when passing through the vent hole 24. And prevents the flame from propagating to the outside and igniting the fuel vapor flowing through the surge tank. To obtain this quenching effect, the diameter of the vent hole 24 is
The combustion energy of the flame, that is, the type of combustible gas,
Since the thickness differs depending on the thickness of the outer cover 22 and the surface temperature of the outer cover 22, it is preferable to set the hole diameter in consideration of these factors.

The arrangement and size of the ventilation holes 23 of the inner cover 21 can be appropriately set so that gas exchange between the inside and outside can be performed well and responsiveness can be ensured. Specifically, the size of the vent hole 23 of the inner cover 21 is usually set to 1.5 to
At this time, the ventilation hole 24 of the outer cover 22 is set smaller than this. For example, the thickness of the outer cover 22 is about 0.5 mm and the surface temperature is 200 ° C.
If it is about 1.1 mm or less for butane gas and about 0.9 mm or less for gasoline vapor, a flame-extinguishing effect can be obtained.

As shown in FIG. 1B, the oxygen ion conductor 1
A diffusion resistance layer 12 is formed so as to cover the surface of the electrode 13a on the outer peripheral side (measured gas side) of the electrode 4. After the gas to be measured passes through the diffusion resistance layer 12 by diffusion, the electrode 13a. Diffusion resistance layer 12
Consists MgO-Al ₂ O ₃ spinel, so as to have a predetermined diffusion resistance, for example, porosity 3-5%, and is controlled to about the average pore diameter 3 nm.

In the present embodiment, the thickness of the diffusion resistance layer 12 is set to be equal to or more than the minimum thickness necessary for completing the reaction between the combustible gas and oxygen while the gas to be measured passes through the diffusion resistance layer 12. Set to. The minimum thickness is such that the gas to be measured is the electrode 13
Before reaching a, the combustible gas of the gas to be measured, for example, a thickness capable of completely consuming the hydrocarbon component,
It changes according to the type of flammable gas to be detected and its concentration range, and generally, the higher the concentration of flammable gas, the thicker it becomes. Preferably, the thickness of the diffusion resistance layer 12 is 500 μm, which is the thickness of the diffusion resistance layer in a normal oxygen sensor.
It is desirable to set it thicker. Thus, stable measurement can be performed while suppressing output fluctuation.

An example of a method for setting the minimum thickness will be described below. FIG. 2A shows the relationship between the output of the gas to be measured containing 6% by weight of butane gas and the thickness of the diffusion resistance layer. As shown in FIG. The relationship is such that as the layer 12 becomes thicker, it decreases. Here, assuming that the allowable fluctuation width is ± 1% of the output value, the allowable fluctuation width is, for example, ± 0.0% when the thickness of the diffusion resistance layer 12 is 1000 μm.
7 mA When the thickness of the diffusion resistance layer 12 is 500 μm, ± 0.0
Since it is 8 mA, the required line of the allowable fluctuation range is shown as in FIG. 2B. In the range where the fluctuation range is larger than the required line of ± 1% (the range of the diagonal line in the figure), a stable output is obtained. You will not get it. Therefore, the actual fluctuation width obtained by changing the thickness of the diffusion resistance layer 12 is measured in advance, and the thickness at the intersection of the line of the actual fluctuation width and the required line of ± 1% is calculated as follows.
The minimum required thickness may be used. In the example of FIG. 2B, the minimum required thickness is about 700 μm, and it can be seen that the effect can be obtained if the thickness of the diffusion resistance layer 12 is more than this.

The surface of the diffusion resistance layer 12 is covered with
A trap layer 11 is formed. The trap layer 11 is an example
For example, α-Al_TwoO_Three, Γ-Al_TwoO_Three, MgO-Al
_TwoO _ThreeSpinel, mullite, etc.
Deposits generated from fine carbon particles, oil mist, and oil
Formed to protect the sensor element 1 from jets and the like.
You. To satisfy this purpose, the thickness of the trap layer 11
Is usually about 20 to 300 μm, and the porosity is 6 to 30 μm.
%, Average pore diameter should be in the range of about 0.1 to 50 μm
Is preferred.

The detection principle of the flammable gas sensor element 1 having the above configuration will be described. In FIG. 1B, the gas to be measured is
The light passes through the trap layer 11 and enters the diffusion resistance layer 12, and diffuses toward the electrode 13a with a predetermined diffusion resistance. In the diffusion resistance layer 12, a reaction of oxygen and hydrocarbons in the gas to be measured occurs, and their concentrations gradually decrease. Here, in the present embodiment, the thickness of the diffusion resistance layer 12 is
Since the thickness of the combustible gas is set to be equal to or more than the minimum thickness required for completing the oxidation reaction of the flammable gas during the passage through 2, the hydrocarbon is completely consumed by the combustion reaction, and only oxygen remains.
The remaining oxygen diffuses in the diffusion resistance layer 12 as it is, reaches the electrode 13a, and is ionized on the electrode 13a. The ionized oxygen diffuses in the oxygen ion conductor 14 to generate a sensor output. By detecting this, the concentration of the combustible gas can be known.

As described above, according to the above configuration, since the combustible gas is completely consumed in the diffusion resistance layer 12, the electrode 1
The fluctuation of the oxygen concentration on the surface 3a can be prevented, and a stable output can be obtained.

Next, a test was conducted to confirm this effect. FIG. 3 (c) shows the configuration of the apparatus used for the test. The flammable gas sensor S having the above-described configuration is installed in the decompression vessel so that the tip protrudes, and the sample gas is supplied from the gas introduction path provided at one end. (Composition: 6% by weight of butane, 22% by weight of oxygen, and 72% by weight of nitrogen) were introduced. The gas flow rate is 55 L / min (flow rate 0.5 m /
s), a vacuum pump was connected to the other end of the vacuum container, and the pressure was maintained at 100 kPa. The thickness of the diffusion resistance layer 12 of the combustible gas sensor S is 500 μm, which is smaller than the minimum required thickness in FIG.
FIGS. 3 (a) and 3 (b) show the results obtained by measuring the change in the sensor output at a time of 00 μm.

As shown in FIG. 3A, when the thickness of the diffusion resistance layer 12 is 500 μm, which is equivalent to that of a conventional oxygen sensor,
A variation of about 0.7 mA was observed. This is because, as shown in FIG. 4, when the thickness of the diffusion resistance layer 12 is as thin as 500 μm,
This is because the combustion reaction of hydrocarbons is not completed in the diffusion resistance layer 12 and the reaction also occurs on the surface of the electrode 13a.
The oxygen concentration on a becomes unstable, and the sensor output also fluctuates greatly. On the other hand, FIG.
As described above, when the thickness of the diffusion resistance layer 12 is 1000 μm, it is reduced to about 0.1 mA, and the effect of suppressing the output fluctuation is confirmed.

FIG. 5 shows a second embodiment of the present invention.
The basic configuration of this embodiment is the same as that of the first embodiment, and the following description will focus on the differences. In the present embodiment, instead of making the thickness of the diffusion resistance layer 12 equal to or more than a predetermined minimum required thickness, a metal having a catalytic action is used as a means for reacting the entire amount of hydrocarbons with oxygen before reaching the electrode 13a. Is provided. This trap layer 11 'functions as a catalyst layer and promotes the oxidation reaction of hydrocarbons. As the catalytic metal species, for example, Pt, P
t-Rh or the like can be used. Usually, the amount of the catalyst is preferably in the range of 0.5 to 5% by weight based on the total weight of the catalyst layer.

The formation of the catalyst layer is performed, specifically, by using a ceramic material constituting the trap layer 11 ′, for example, γ-Al
₂ O ₃ , a catalyst metal such as Pt, Pt-Rh, a dispersant,
The body of the sensor element 1 having the diffusion resistance layer 12 formed on the surface of the electrode 13a is immersed in a solution in which a binder or the like is slurried to form a film serving as the trap layer 11 'on the surface.
Bake by heat treatment at a high temperature of ℃ or more. This makes it possible to easily form the trap layer 11 ′ also serving as the catalyst layer. However, as long as the catalyst may be supported on the surface layer of the diffusion resistance layer 12, the diffusion resistance layer 12 and the trap After the formation of 11 ', the sensor element 1 body may be immersed in an aqueous solution containing a catalytic metal and heat-treated.

When the sensor element 1 having the above configuration is used,
The oxygen and hydrocarbon concentration distributions are as shown in FIG.
Due to the catalytic metal contained in the trap layer 11 ′, the oxidation reaction of the hydrocarbon is completed in the trap layer 11 ′, and only the remaining oxygen passes through the diffusion resistance layer 12 to form the electrode 13 a
Reach the top. Therefore, an oxidation reaction does not occur near the electrodes, the sensor output is stabilized, and the same effect can be obtained. In addition, according to the above method, the catalyst layer can be formed at the same time as the formation of the trap layer 11 ', so that the production is easy.

FIG. 6 shows an example using the apparatus shown in FIG.
It is the result of performing the same test for the combustible gas sensor S in which the trap layer 11 not supporting the catalyst and the trap layer 11 'supporting the catalyst were formed. Diffusion resistance layer 12
Was set to 500 μm. As evident in the figure,
In the trap layer 11 not carrying the catalyst, a change in the sensor output was observed (FIG. 6A), whereas in the trap layer 11 'carrying the catalyst, a stable sensor output was obtained.

As described above, the effect of stabilizing the sensor output can also be obtained by forming the catalyst layer. Note that the second
In the embodiment described above, the trap layer 11 ′ is configured to also serve as the catalyst layer. However, in the present invention, it is sufficient that the oxidation reaction is completed before reaching the electrode 13 a, for example, the surface layer of the diffusion resistance layer 1. The catalyst layer can be formed by coating a portion with a catalyst.

In the above embodiment, a combustible gas sensor element using a test tubular oxygen ion conductor is used. However, a combustible gas sensor element having a laminated structure using a flat oxygen ion conductor is used. Can also. Further, the present invention can be applied to detection of various combustible gases other than butane gas and gasoline vapor as combustible gases.

[Brief description of the drawings]

FIG. 1A is a partial cross-sectional view showing a main part configuration of a combustible gas sensor according to a first embodiment of the present invention, and FIG.
Is an enlarged sectional view of a main part of the flammable gas sensor element of the present invention,
It is a figure which shows the hydrocarbon and oxygen concentration distribution in the to-be-measured gas which passes this.

2A is a diagram illustrating a relationship between the thickness of a diffusion resistance layer and a sensor output, and FIG. 2B is a diagram illustrating a relationship between the thickness of a diffusion resistance layer and a current variation width.

3A is a diagram showing a sensor output when a diffusion resistance layer has a thickness of 500 μm, and FIG. 3B is a view showing a diffusion resistance layer having a thickness of 100 μm.
FIG. 3C is a diagram showing a sensor output in the case of 0 μm, and FIG. 4C is a schematic configuration diagram of a measuring device used for a measurement test using butane gas.

FIG. 4 is an enlarged sectional view of a main part of a combustible gas sensor element having a diffusion resistance layer having a thickness of 500 μm, and a diagram showing a concentration distribution of hydrocarbons and oxygen in a gas to be measured passing therethrough.

FIG. 5 is an enlarged cross-sectional view of a main part of a combustible gas sensor element according to a second embodiment of the present invention, and a diagram showing a concentration distribution of hydrocarbons and oxygen in a gas to be measured passing therethrough.

6A is a diagram illustrating a sensor output when a catalyst is not supported on a trap layer, and FIG. 6B is a diagram illustrating a sensor output when a catalyst is supported on a trap layer.

[Explanation of symbols]

 H Housing 1 Combustible gas sensor element (sensor element) 11 Trap layer 11 'Trap layer 12 Diffusion resistance layer 13a, 13b Electrode 14 Oxygen ion conductor 2 Cover body 21 Inner cover 22 Outer cover 23 Vent 24 Vent 3 Air cover 4 heater

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Naohisa Oyama 14 Iwatani, Shimowasumi-cho, Nishio City, Aichi Prefecture Inside the Japan Automobile Parts Research Institute (72) Inventor Shigeki Omichi 14 Iwatani, Shimowasumi-cho, Nishio-shi, Aichi Prefecture Stock Company (72) Inventor Fumihiko Sato 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation

Claims (6)

[The claims] 1. A pair of electrodes are formed on the surface of an oxygen ion conductor, and one of the pair of electrodes is disposed in a space where a combustible gas and oxygen are present. A flammable gas sensor for detecting a flammable gas concentration from a change in an oxygen concentration in the gas to be measured due to an oxidation reaction of the flammable gas, comprising a sensor element provided with a diffusion resistance layer covering the surface of the gas to be measured. A means for completing the reaction between the combustible gas in the gas to be measured and oxygen before the gas reaches the surface of the one electrode of the sensor element. 2. The method according to claim 1, wherein the thickness of the diffusion resistance layer is equal to or greater than a minimum thickness at which a reaction between the combustible gas and oxygen in the gas to be measured is completed while passing through the diffusion resistance layer. 2. The combustible gas sensor according to 1. 3. The flammable gas sensor according to claim 1, wherein the thickness of the diffusion resistance layer is larger than 500 μm. 4. The flammable gas sensor according to claim 1, wherein a catalyst is supported on a trap layer formed on the surface of the diffusion resistance layer. 5. The flammable gas sensor according to claim 1, wherein a catalyst layer is formed on the surface of the diffusion resistance layer. 6. A heat resistance of an inner / outer double structure which holds the sensor element in a cylindrical housing and protrudes from one end side of the housing to protect an outer periphery of the sensor element located in a measurement gas existence space. 6. The flammable gas sensor according to any one of 1 to 5, further comprising a cover.