JP2000171433A - Combustible gas concentration measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device capable of measuring combustible gas even in gas to be inspected containing oxygen in high concentration. SOLUTION: An oxygen detecting electrode 2 inactive to HC, an oxygen detecting electrode 3 active to HC and an oxygen pump 6 are arranged to solid electrolyte substrates 1, 5 opposed to each other in a form exposed to gas to be inspected and a reference electrode 4 is arranged to the back surface of the substrate 1. The electrodes 2, 3 and the oxygen pump 6 are connected by electric circuits 9, 11 and the potential difference between the reference electrode 4 and one electrode 3 is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for measuring the concentration of a combustible gas.

[0002]

2. Description of the Related Art Combustible gases such as HC and CO are contained in exhaust gas of an internal combustion engine mounted on an automobile. When these flammable gases are emitted, harm to humans becomes a problem in addition to the problem of fuel efficiency of vehicles. One of them is the risk of triggering cancer. Furthermore, the emission of such gases into the atmosphere is one of the causes of environmental pollution. For this reason, detection of flammable gas such as HC and CO in exhaust gas is a prerequisite for exhaust gas control and is a very important issue. Therefore, there is a need for a detector that can reduce the size and cost of combustible gas measuring devices and that can respond to various use environments.

A conventional apparatus for measuring the concentration of combustible gas such as HC is an electrochemical method, a method of detecting a change in conductivity using a metal oxide semiconductor, or a method of detecting a calorific value due to catalytic combustion by a change in resistance. The detection method and the like have been studied and partially put into practical use as measuring instruments. However, the semiconductor method has a low element operating temperature, and is not suitable for high-temperature exhaust gas of an internal combustion engine or for in-vehicle use. Also,
In the catalytic combustion system, the temperature control is very strict, and cannot be used for high-temperature exhaust gas of an internal combustion engine or for in-vehicle use like the semiconductor type. Therefore, most of the possible detection methods for measuring the exhaust gas of the internal combustion engine use an electrochemical method.

Heretofore, in the electrochemical measurement of the concentration of combustible gas, a method of measuring a decrease in oxygen concentration during catalytic combustion of HC using an oxygen sensor, or a method of keeping the oxygen concentration constant. A method of controlling an electrochemical oxygen pump using the output of an oxygen sensor and measuring a feedback current proportional to the concentration of HC at this time, or electrochemically oxidizing HC and the like, and limiting current at this time. Has been proposed. For example, No. 26 of the Electrochemical Society
Annual Meeting of the Chemical Sensors Society, proceeding (vol.14, p69)
-72, 1998).
In this method, the amount of decrease in the oxygen concentration during catalytic combustion of HC is controlled by an electrochemical oxygen pump using the output of an oxygen sensor, and a feedback current proportional to the concentration of HC at this time is measured. .

[0005] However, as can be seen from the above-mentioned principle, any method can only be used for measurement in a gas having a low oxygen concentration. That is, when the oxygen concentration is high, the change in the oxygen concentration due to the oxidation of HC becomes very small, and the measurement accuracy is greatly reduced. Particularly, as the oxygen concentration increases, the HC concentration in the combustion gas decreases due to combustion, and it is very difficult to detect the low concentration HC at this time by the above-described method.

[0006]

The above-mentioned conventional HC sensor and measuring method can be applied only when the oxygen concentration is low, or when the oxygen concentration and the HC concentration are almost the same. The present invention, compared to conventional exhaust gas with a higher oxygen concentration, or compared to the oxygen concentration,
It is an object of the present invention to provide an HC sensor capable of measuring even exhaust gas when the HC concentration is low, and a method for measuring the HC sensor.

[0007]

The conventional methods for measuring the concentration of HC can be classified into two types: measurement of electromotive force and measurement of limit current. These two methods are a method in which the HC in the test gas is completely burned and then the oxygen concentration is measured without treatment, and a method in which the oxygen in the test gas is removed as much as possible before measuring the HC concentration. Therefore, any of the methods is greatly affected by oxygen. According to the present invention, the HC concentration is determined from the measurement result of the oxygen concentration in which the oxygen concentration in the test gas is changed in accordance with the HC concentration present in the test gas and the concentration ratio between the two (that is, P _O2 / P _O2-HC ). Since measurement is based on the detection principle, it is possible to measure HC in the presence of oxygen.

According to the first to third aspects, an electrode 2 which is inert to at least a combustible gas and active to oxygen is provided on one surface of an ion-conductive (for example, stabilized zirconia) solid electrolyte substrate 1.
Oxidation reaction of flammable gas with oxygen is caused and formed by an electrode 3 active on oxygen and a reference electrode active on oxygen on the opposite surface of the solid electrolyte substrate, or an oxide or chloride. Forming a potential-stable reference electrode or a potential-stable electrode 4 isolated from the test gas, and forming an oxygen pump comprising an oxygen-active electrode 6 and an electrode 7 on both surfaces of another solid electrolyte substrate 5, This is arranged at a position facing the surfaces of the electrodes 2 and 3 of the solid electrolyte substrate 1 so as to form a space,
Construct an HC sensor.

Further, the electrodes 2 and 3 are formed by an electronic circuit.
The oxygen concentration is controlled using an oxygen pump located opposite the detection electrode so that the potential difference becomes a predetermined value, and the electromotive force between the electrodes 2 and 4 at this time is detected. A HC sensor capable of measuring a flammable gas concentration characterized by a method of measuring an oxygen concentration in a measurement space and calculating a flammable gas concentration from a predetermined value of an electromotive force difference between the electrodes 2 and 3. I do.

The point of the present invention is that the HC of the present invention is used.
This is the principle of operation of the sensor. According to the third aspect, the electrode 2 that is inert to the HC gas and active to oxygen and the electrode 3 that is active to both HC and oxygen generate an electromotive force according to the Nernst equation according to the oxygen concentration on the electrode surface. . Here, the potential generated at the electrode 2 is a value corresponding to the oxygen concentration in the test gas,
The potential generated at the electrode 3 is a potential corresponding to the oxygen concentration after the combustible gas such as HC has decreased in oxygen concentration by combustion. Therefore, the potential difference can be expressed by the equation shown in Equation 1.

[0011]

(Equation 1)

Where N = P _O2 / P _O2-HC = constant, ΔE
Is the potential difference between electrode 2 and electrode 3, and R is the gas constant (8.314
J. mol ⁻¹ , K ⁻¹ ) and F are Faraday constants (964)
84 C.mol ^-1 ), P _O2 is the oxygen partial pressure on the surface of the electrode 2,
_O2-HC indicates the oxygen partial pressure on the surface of the electrode 3 (the oxygen partial pressure after the combustible gas is burned).

As described in claim 2, when this potential difference is held at a predetermined value by an electronic circuit, the concentration of combustible gas such as HC can be determined by measuring the oxygen concentration on the surface of the electrode 2 at this time. Further, by changing the predetermined value, the sensitivity of the HC sensor of the present invention can be arbitrarily set.

According to a fourth aspect of the present invention, in the HC sensor having the structure and principle described in the first to third aspects, the electrode 2 which is active only in oxygen is made of Au, an alloy of Pt and Au, or a metal in the electrode. It can be manufactured using an alloy material to which an oxide is mixed and added. Regarding an alloy electrode composed of Pt and Au, from the viewpoint of sintering resistance in particular, 1 to 20% of A
It is desirable to add u.

According to a fifth aspect of the present invention, in the HC sensor having the structure and principle described in the first to third aspects, the oxidation reaction of the combustible gas with oxygen is caused, and the electrode 3 active on oxygen is provided. 4. The flammable gas concentration measuring device having the configuration described in any one of (1) to (4) above,
The electrode can be manufactured using an electrode made of at least one of e and Ir, or a material in which a metal oxide is mixed and added into the electrode. For example, the electrode 3 may be made of only Pt, but in order to sufficiently oxidize HC, an alloy of Pt and Ru, an alloy of Pt and Pd, or Pt is used.
It is desirable to use an alloy of Re and Re. Furthermore, if a metal oxide is added from the viewpoint of the catalytic action of the metal oxide and the sintering resistance of the electrode, the performance of the electrode 3 can be further improved.

[0016]

FIG. 1 is a sectional view showing an example of the most basic embodiment of the present invention. At least one electrode 2 (for example, an Au electrode) that is inert to flammable gas and active to oxygen and a combustion reaction of flammable gas (oxidation reaction with oxygen) are provided on one surface (test gas side) of the ion conductive solid electrolyte substrate 1. ), An active electrode 3 (for example, a Pt electrode) and a reference electrode (for example, a Pt electrode) that is active on oxygen exposed to the atmosphere on the opposite surface of the fixed electrolyte substrate, or an oxide or chloride formed on the atmosphere. A potential stable reference electrode or a potential stable electrode 4 isolated from the test gas is formed without exposure, and an oxygen active electrode 6 and an electrode 7 are formed on both surfaces of another solid electrolyte substrate 5. A pump (for example, a Pt-Pt electrode) is formed, and the pump is arranged at a position facing the surfaces of the electrodes 2 and 3 of the solid electrolyte substrate 1 so as to form a space.

Further, the electrodes 2 and 3 are formed by an electronic circuit.
The oxygen concentration is controlled using an oxygen pump so that the difference in electromotive force between the electrodes becomes a predetermined value. The electromotive force of the electrodes 2 and 4 at this time is detected, and the oxygen concentration in the measurement space is measured. Measure the gas concentration. Further, the flammable gas is oxidized on the surface of the electrode 3 between the surfaces of the electrodes 2 and 3 and the surface of the electrode 6, so that the potential difference between the electrodes 2 and 3 follows the above-mentioned equation. The ratio of the concentration of flammable gas to the concentration of flammable gas is controlled to a certain value, and the electrode potential based on the oxygen concentration at this time is measured with respect to the reference electrode. The concentration of the neutral gas can be detected.

The operation of the HC sensor and the electronic circuit having the structure shown in FIG. 1 is as follows. For example, a test gas is introduced from a gas inlet narrowed so as to be diffusion-controlled so that the oxygen concentration can be controlled in the measurement space. The introduced gas passes through the surface of the electrode 2 and reaches the surface of the electrode 3.
At this time, the electrode 2 generates an electromotive force corresponding to the oxygen concentration in the gas. On the other hand, HC is burned on the electrode 3 by the catalytic action of the electrode 3, and the oxygen concentration decreases. If the potential difference between the electrode 2 and the electrode 3 at this time is detected, the concentration of HC can be detected. However, when the oxygen concentration is higher than the HC concentration, the potential difference is too small to detect the HC concentration. Can not.

Here, in the present invention, if this potential difference is small, oxygen in the measurement space is discharged using an oxygen pump,
The oxygen concentration is reduced so that this potential difference becomes a predetermined value. That is, the value of the potential difference is set on the electric circuit, the oxygen pump is controlled by using the difference between the potential difference between the electrode 2 and the electrode 3 and the predetermined value as a signal, and the oxygen concentration in the measurement space follows the change in the HC concentration. N value. For example, when the set value of N is 2, if the concentration of HC is 200 ppm, the concentration of oxygen is controlled to 400 ppm. Therefore,
The concentration of HC can be obtained by measuring the electromotive force of the electrode 2 with respect to a known reference electrode, detecting the oxygen concentration, and converting it using N. With this measurement, the output of the sensor is proportional to the logarithm of the HC concentration, and has the advantage that it can be measured in a wide range and with high sensitivity. In FIG. 1, 9 denotes an operational amplifier, 10 denotes a high resistance voltage, and 11 denotes a voltage setting unit (setting of N).

[0020]

EXAMPLES Example 1 Au electrodes 2 and Pt electrodes 3, 4, 6, and 7 were formed on known zirconia substrates 1 and 5 by a printing method and baked. These were laminated to produce a sensor element having a structure as shown in FIG. Further, a Pt heater 8 was formed on the alumina substrate 13 by a known method, and was adhered to the substrate 1 to have a self-heating function. This element is 45
Heated to a temperature of 0 ° C. by the heater 8, and N in the equation 1 is changed to 2
The oxygen pump is controlled by the electronic circuit 11 to change the concentration of C ₃ H ₆ in 4% oxygen or 1% oxygen, and the electromotive force of the electrode 2 is measured with reference to the reference electrode 4 on the atmosphere side. did. FIG. 2 shows the dependency of the measured electromotive force on the concentration of C ₃ H ₆ . Although the electrode 2 is originally active only on oxygen, the oxygen pump is controlled by changing the electromotive force of the electrode 3 to change the oxygen concentration in the measurement space according to the HC concentration. Although the generated electromotive force is due to oxygen, it has become possible to detect the HC concentration in proportion to the logarithm of the HC concentration without being substantially affected by the oxygen concentration in the test gas.

[Comparative Example 1] An element similar to that of Example 1 was replaced with 45 elements.
Heated to a temperature of 0 ° C. by the heater 8, without changing the oxygen concentration, changing the concentration of C ₃ H ₆ in 4% oxygen or 1% oxygen and changing the electromotive force of the electrodes 2 and 3 to the reference electrode on the atmosphere side. 4 was used as a reference. Since the electrode 2 was inactive against HC and active only on oxygen, the electromotive force did not change. At the same time, the electromotive force of the electrode 3 active on both oxygen and HC was measured with respect to the reference electrode. FIG. 3 shows the dependence of the concentration on C ₃ H ₆ .
Shown in The electromotive force obtained at this time depends on the oxygen concentration in the measurement space. However, the oxygen concentration was reduced by the introduction of HC and the oxidation reaction, and the change in oxygen concentration at this time was detected. However, this change in oxygen concentration depends on the oxygen concentration in the test gas, and when the oxygen concentration is high, the output of the sensor is greatly reduced. FIG. 3 shows such a result.

[0022]

The present invention provides an HC sensor having a new measurement principle. This sensor can measure over a wide range of oxygen concentration with little effect from changes in oxygen concentration. Further, the output of this sensor is proportional to the logarithm of a multiple of the HC concentration, and higher sensitivity can be obtained as compared with the sensitivity of the conventional HC sensor.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of an element according to an example of the present invention.

FIG. 2 is a graph showing an output voltage C according to an embodiment of the present invention;
FIG. 4 is a graph showing the concentration dependency of ₃ H ₆ .

FIG. 3 is a graph showing the dependency of the sensor output voltage measured by a conventional method on the concentration of C ₃ H ₆ .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ion conductive solid electrolyte substrate 2 Oxygen detecting electrode 3 Electrode active for oxygen and HC 4 Reference electrode 5 Oxygen pump electrolyte substrate 6,7 Oxygen pump electrode 8 Self-heating platinum heater 9 Operational amplifier 10 High resistance voltmeter 11 Voltage setting Part (setting of N value)

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Yukio Nakanouchi 4-14-1, Suehiro, Kumagaya-shi, Saitama F-term in Riken Kumagaya Office (reference) 2G004 BB07 BD14 BE22 BE23 BE25 BJ02 BK04 BL09

Claims (5)

[Claims] 1. An oxidation reaction between an electrode (2) which is inert to at least a flammable gas and is active on oxygen and an oxygen of a flammable gas occurs on one surface of an ion-conductive solid electrolyte substrate (1). And an oxygen-active electrode (3) and an oxygen-active reference electrode exposed to the atmosphere on the opposite surface of the solid electrolyte substrate (1), or a potential stable reference formed of an oxide or chloride. An electrode or an electrode (4) having a stable electric potential separated from the test gas is formed, and an oxygen pump comprising an electrode (6) and an electrode (7) active on oxygen is provided on both surfaces of another solid electrolyte substrate (5). A flammable gas concentration characterized by being arranged so as to form a space for a test gas at a position facing the surfaces of the electrodes (2) and (3) of the solid electrolyte substrate (1). Measuring device. 2. A flammable gas concentration measuring device having a configuration according to claim 1, wherein an electromotive force difference between the electrode (2) and the electrode (3) is set to a predetermined value by an electronic circuit. The flammable gas is characterized by detecting the electromotive force between the electrode (2) and the electrode (4) at this time by using the oxygen pump, measuring the oxygen concentration in the measurement space, and measuring the flammable gas concentration. Gas concentration measurement device. 3. A flammable gas concentration measuring device according to claim 1, wherein a change in oxygen concentration caused by oxidization of the flammable gas on the surface of the electrode (3) is detected by the electrode (3). 2) and the potential difference between the electrode (3) is measured. At this time, the ratio of the concentration of oxygen to the concentration of flammable gas is controlled to a certain value. A device for measuring the concentration of flammable gas from the ratio of the oxygen concentration in the test gas to the oxygen concentration in the electrode (3). 4. The flammable gas concentration measuring device according to claim 1, wherein the electrode (2) is an Au electrode, an alloy electrode composed of Pt and Au, or the electrode. A combustible gas concentration measuring device characterized by mixing and adding a metal oxide. 5. A flammable gas concentration measuring device according to claim 1, wherein the electrode (3) is Pt,
An electrode comprising at least one of Pd, Rh, Ru, Re, and Ir, or a device for measuring a flammable gas temperature, wherein a metal oxide is mixed and added to the electrode.