JPH10282054A - Hydrocarbon sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correctly measure a total concentration (THC concentration) of a plurality of kinds of hydrocarbons in a gas to be measured. SOLUTION: The hydrocarbon sensor is provided with a transforming part 11 and a measuring part 12. The transforming part 11 has a cracking catalyst 23 which uniformly dissolves a hydrocarbon in a gas to be measured to hydrogen irrespective of a kind of the hydrocarbon, thereby obtaining a hydrocarbon of a small carbon number. At the measuring part, the transformed hydrocarbon is oxidized by an oxidation catalyst 51 and a THC concentration in the gas to be measured is measured on the basis of the amount of oxygen supplied for the oxidation of the hydrocarbon. The THC concentration is accordingly correctly measured regardless of the kind and amount of the hydrocarbon in the gas to be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrocarbon sensor for measuring the concentration of hydrocarbons in a gas to be measured, and more particularly to a hydrocarbon sensor for measuring the total concentration of hydrocarbons containing a plurality of hydrocarbons in the gas to be measured. It relates to a hydrogen sensor.

[0002]

2. Description of the Related Art A hydrocarbon sensor measures the concentration of hydrocarbons in a gas to be measured. For example, harmful components (hydrocarbons, nitrogen oxides, etc.) in exhaust gas discharged from an internal combustion engine.
Is provided downstream of the three-way catalyst, which is used to diagnose deterioration of the three-way catalyst.

In recent years, there has been a demand for hydrocarbon sensors that are less susceptible to miscellaneous gases having high selectivity to hydrocarbons, for example, regulations on emission of harmful components have been strengthened. For this reason, some hydrocarbon sensors expose a gas to be measured to an oxidation catalyst having an oxidation activity for hydrocarbons so that the concentration of hydrocarbons is known based on the amount of oxygen consumed in the oxidation catalyst.

[0004]

In applications such as the diagnosis of deterioration of the three-way catalyst, the total hydrocarbon concentration (hereinafter referred to as THC concentration) in the gas to be measured is important. However, hydrocarbons include alkanes with unspecified carbon numbers,
There are many types such as alkenes, alkynes and aromatic hydrocarbons, and it is difficult to make an oxidation catalyst having a uniform oxidation activity for all types of hydrocarbons. Therefore, THC depends on the type and amount of hydrocarbons in the gas to be measured.
The measured values vary, and accurate measurements cannot be made.

Accordingly, an object of the present invention is to provide a gas sensor capable of accurately measuring the concentration of THC in a gas to be measured.

[0006]

According to the first aspect of the present invention, there is provided a hydrocarbon sensor, comprising: a converter for converting a hydrocarbon in a gas to be measured into a predetermined hydrocarbon having a small number of carbon atoms; Is oxidized by an oxidation catalyst, and THC in the gas to be measured is determined based on the amount of oxygen supplied to the oxidation of hydrocarbons at that time
A measuring unit for measuring the concentration.

[0007] Since the hydrocarbons in the gas to be measured are converted into predetermined hydrocarbons in the conversion section, the oxidation catalyst in the measurement section must have sufficient oxidation activity for the predetermined hydrocarbons among the hydrocarbons. By simply adjusting the pH, it is possible to accurately measure the THC in the gas to be measured, regardless of the presence or absence or the degree of oxidation activity for hydrocarbons other than the above-mentioned predetermined hydrocarbon.

[0008] In the second aspect of the present invention, the conversion section of the hydrocarbon sensor is configured as follows. An introduction chamber into which the gas to be measured is introduced via a gas introduction passage having a diffusion resistance is provided. A part of the introduction chamber wall is composed of an oxygen conductive solid electrolyte material, and a pair of electrodes is formed on both surfaces thereof to form an oxygen pump, and a voltage is applied between the electrodes of the oxygen pump to move oxygen in the introduction chamber to the outside of the introduction chamber. The pumping configuration is adopted. A cracking catalyst is provided for decomposing hydrocarbons in the gas to be measured introduced into the introduction chamber into the predetermined hydrocarbons.

The oxygen pump electrolyzes water by the action of an applied voltage to generate hydrogen at an electrode on the side of the introduction chamber.
In the cracking catalyst, hydrocarbons in the gas to be measured are decomposed into the above-mentioned predetermined hydrocarbons by generated hydrogen. In addition, in the introduction chamber, the diffusion of the gas to be measured is restricted by the diffusion resistance, and oxygen is reduced by the oxygen pump. Thus, the combustion of the hydrocarbon in the gas to be measured is prevented, and the measurement accuracy of the THC concentration is improved.

[0010]

FIG. 1 shows a gas detection apparatus to which the hydrocarbon sensor of the present invention is applied. The gas detector is provided through the exhaust pipe wall downstream of the three-way catalyst for purifying exhaust gas discharged from the internal combustion engine. In the gas detection device, a hydrocarbon sensor 1 is accommodated in a tubular housing H with an outer periphery held by an insulating material. The hydrocarbon sensor 1 has an elongated flat plate shape, and its tip (lower end in the figure) protrudes from the housing H, extends downward in the figure, is fixed to the lower end of the housing H, and has a container-like exhaust that projects into the exhaust pipe. It is accommodated in the cover H1. The exhaust cover H1 has a double structure of an inner cover H11 and an outer cover H12 made of stainless steel, and the side walls of these covers H11 and H12 are for taking the exhaust gas to be measured into the exhaust cover H1. Exhaust ports H13 and H14 are formed respectively.

At the upper end of the housing H, an air cover H2 comprising a cylindrical main cover H21 and a sub cover H22 covering the rear end is fixed. The main cover H21 and the sub-cover H22 have air ports H23 and H24 at positions opposing the side walls thereof, and the air is taken into the air cover H2 from the air ports H23 and H24. A water-repellent filter H25 is provided between the main cover H21 and the sub-cover H22 at the position where the atmospheric ports H23 and H24 are formed for waterproofing.

An upper end of the air cover H2 is open, and a lead wire H3 connected to the rear end of the hydrocarbon sensor 1 extends outside from the upper end opening.

FIG. 2 shows a cross section of the hydrocarbon sensor 1, and FIG. 3 shows an exploded view of the hydrocarbon sensor 1.

The hydrocarbon sensor 1 has, at its tip, a converter 11 for pretreating exhaust gas flowing into the exhaust cover H1 (FIG. 1), and a THC based on the pretreated exhaust gas.
A measuring unit 12 for measuring the concentration and a heater 7 for heating these are provided. The conversion unit 11 includes an introduction chamber 1b, a first pump cell 3 serving as an oxygen pump, and a cracking catalyst 23. The measurement unit 12 includes an intermediate chamber 1c, a measurement chamber 1d,
A second pump cell 4, a sensor cell 5, a detection cell 6, and a duct 1e are provided. Elements constituting the conversion unit 11 and the measurement unit 12 are solid electrolyte materials 21 and 24, a spacer 2
2, 25, a heater insulating sheet 26, a heater sheet 27, and other sheet-like members are laminated.

A rectangular hole 22 is formed in an insulating spacer 22 for separating the solid electrolyte material 21 from the solid electrolyte material 24.
1, 222 are formed. A thin notch 223 connecting these holes is formed in the meat part separating the holes 221 and 222. A cracking catalyst 23 made of a porous material having the same width as that of the cracking catalyst 23 is disposed at the position of the hole 221 on the tip side, and the hole 221 of the spacer 22 is inserted into the half 221a on the tip side and the half 221b on the base side. Is divided into two. The gas introduction chamber 1b is provided with a half 221a of the hole 221.
Thereby, the spacer 22, the cracking catalyst 23, and the solid electrolyte materials 21 and 24 are formed as chamber walls. Intermediate room 1c
Is formed by the half portion 221b of the hole 221.
2. The cracking catalyst 23 and the solid electrolyte materials 21 and 24 are formed as chamber walls. The measurement chamber 1d is formed by a hole 222 with the spacer 22, the cracking catalyst 23, and the solid electrolyte materials 21 and 24 as chamber walls.

The gas introduction chamber 1b and the intermediate chamber 1c communicate with each other through a porous cracking catalyst 23 as a communication passage, and the intermediate chamber 1c and the measurement chamber 1d communicate with the notch 22 of the spacer 22.
3 through a passage 1f. In the cracking catalyst 23 and the passage 1f, diffusion of the gas limited by the diffusion resistance is performed.

When the exhaust gas diffuses from the introduction chamber 1b to the intermediate chamber 1c, the cracking catalyst 23 hydrocracks hydrocarbons in the exhaust gas to convert them into hydrocarbons having a small number of carbon atoms (such as methane). And platinum (Pt) are preferably used.

The solid electrolyte material 21 and the electrode 3 described later
Pinholes 1a are formed in the gas introduction chambers 1 and 32 at substantially the distal end positions of the gas introduction chambers 1b. The pinholes 1a are gas introduction paths having a predetermined diameter, and the exhaust gas is introduced into the introduction chambers 1b. It has become. In the pinhole 1a, the flow of gas is restricted by its diffusion resistance.

Solid electrolyte material 24 and heater insulating sheet 26
Drilled holes 251 and 252 are formed in the spacer 25 separating the intermediate chamber 1c and the measurement chamber 1d. A cutout 253 connecting these holes is formed in the meat portion separating the holes 251 and 252. Further, a long slit-like notch 254 extending from the hole 252 to the base end in the longitudinal direction of the hydrocarbon sensor 1 is formed, and is opened at the base end position. The duct 1e has these holes 251 and 25.
2. Solid electrolyte material 2 by notches 253 and 254
4. The spacer 25 and the heater insulating sheet 26 are formed as duct walls. The duct 1e is connected to the atmosphere ports H23 and H24.
The air taken in from FIG. 1 flows from the base end face of the hydrocarbon sensor 1 and has an atmosphere having a constant oxygen concentration (reference oxygen concentration).

An introduction chamber 1b is provided on the upper and lower surfaces of the solid electrolyte material 21.
A pair of electrodes 31, 32 are formed at positions. Electrode 3
Reference numerals 1 and 32 denote the tip side portions 22 of the hole 221 of the spacer 22.
This is a porous electrode having substantially the same size as 1a. The first pump cell 3 is composed of a solid electrolyte material 21 and electrodes 31 and 32, and a voltage is applied between the electrodes 31 and 32 with the electrode 31 side being positive to pump out oxygen in the introduction chamber 1b. I have.

On the upper and lower surfaces of the solid electrolyte material 21, a pair of electrodes 41 and 42 are formed at the position of the intermediate chamber 1c. The electrodes 41 and 42 are connected to the half 22 of the hole 221 of the spacer 22.
This is a porous electrode having substantially the same size as 1b. The second pump cell 4 is composed of the solid electrolyte material 21 and the electrodes 41 and 42, and a voltage is applied between the electrodes 41 and 42 so that oxygen moves between the intermediate chamber 1c and the outside according to the applied voltage. Has become. Note that the second pump cell 4 may have a configuration in which the electrode 41 is exposed to the atmosphere instead of the exhaust gas.

The upper and lower surfaces of the solid electrolyte material 24 are
c and the duct 1e overlap each other, and a pair of electrodes 51, 5
2 are formed. The electrodes 51 and 52 are porous electrodes having substantially the same size as the holes 251 of the spacer 25. The electrode 51 serving as an oxidation catalyst has an oxidation activity on the hydrocarbon converted in the cracking catalyst 23, and is preferably used such as Pt added with 1% of Au. Sensor cell 5
Is composed of the solid electrolyte material 24 and the electrodes 51 and 52, and the electrodes 5 and 52 are formed according to the oxygen concentration ratio on the surfaces of the electrodes 51 and 52.
The electromotive force generated between the first and the second 52 is output. This electromotive force output is used to control the applied voltage of the second pump cell 4,
The applied voltage of the second pump cell 4 is controlled so that the electromotive force output is constant. The set value of the electromotive force of the sensor cell 5 is a value at which a certain amount of oxygen exists in the intermediate chamber 1c. For example, when the voltage is 0.45 V, the surface of the electrode 51 on the side of the intermediate chamber 1c is in a stoichiometric state.

The electrodes 51 and 52 are provided on the upper and lower surfaces of the solid electrolyte material 24 at positions where the measurement chamber 1d and the duct 1e overlap.
Another pair of electrodes 61 and 62 are formed. Electrode 6
Reference numerals 1 and 62 denote porous electrodes having substantially the same size as the holes 252 of the spacer 25. The electrode 61 on the measurement chamber 1d side is an electrode that is inert to the converted hydrocarbon. The detection cell 6 includes the solid electrolyte material 24 and the electrodes 61 and 62, and a constant voltage is applied between the electrodes 61 and 62 with the electrode 62 being positive. Oxygen moves between the measurement chamber 1d and the duct 1e by this applied voltage, a pump current flows through the solid electrolyte material 24, and the THC concentration is measured from the pump current.

The heater 7 is such that a heater wire 71 is formed on the upper surface of the heater sheet 27, and a normal Pt heater wire is used for the heater wire 71. By energizing the heater wire 71, the conversion section 11 and the entire measurement section 12 are heated to increase the operation sensitivity of each of the cells 3, 4, 5, and 6, and to increase the oxidation activity of the cracking catalyst 23 and the electrode 51. Has become.

Electrodes 31, 41, 32, 42, 51, 61
The leads 31a, 41a, 32a, 42a, 51a,
61a extends toward the base of the gas sensor 1, and the gas sensor 1
, That is, connected to each terminal of the terminal portion 81 formed on the upper surface of the solid electrolyte material 21 directly or through a through hole 224. Leads 52 a, 62 a, 71 a extend from the electrodes 52, 62 and the heater wire 71 toward the base of the hydrocarbon sensor 1, and are directly or directly connected to the terminals of the terminal 82 formed on the lower surface of the hydrocarbon sensor 1, that is, the lower surface of the heater sheet 27. They are connected via through holes 255, 261, 271.

The electrode 3 on the outside of the first pump cell 3
1 and a porous protective layer 28 made of alumina or the like which covers the pinhole 1a is formed to prevent the pinhole 1a from being clogged with particulates having a large particle size such as soot contained in exhaust gas. I have.

The spacers 22 and 25 and the heater sheet 2
7. As the heater insulating sheet 26, an alumina (Al ₂ O ₃ ) sheet is used, and is formed by a doctor blade method or the like together with the solid electrolyte material sheet. Of course, the manufacturing method is not limited to this, and an extrusion molding method, an injection molding method, or the like can be used. Also, the spacers 22 and 25, the heater insulating sheet 26
May be formed by screen printing. Solid electrolyte material 2
Nos. 1 and 24 describe yttria-doped zirconia (Y ₂ O ₃ -Z) widely used in a solid electrolyte material type gas sensor.
(rO ₂ ) based partially stabilized zirconia is preferred, but not limited thereto. The thickness of the solid electrolyte materials 21 and 24 is preferably in the range of 50 to 300 μm. However, considering the balance between electric resistance and sheet strength, 1
It is desirable to set it in the range of 00 to 300 μm. The heater wires 71 such as the electrodes 31 and 32 are formed by screen printing. The thickness of the electrodes 31 and 32 is usually 1 to 20 μm.
However, considering heat resistance and gas diffusivity, 5
It is desirable that the thickness be 10 to 10 μm.

The exhaust gas is supplied to the introduction chamber 1b by the pinhole 1
Instead of being introduced by a, it may be introduced by a porous body. In addition, a slit-shaped notch is formed in the spacer 22 from the hole 221 forming the gas introduction chamber 1b to the tip, whereby the solid electrolyte materials 21 and 24 and the spacer 22 are used as the flow path wall for exhaust gas introduction. May be used.

The operation of the hydrocarbon sensor 1 together with the above gas detection device will be described with reference to FIGS. In the converter 11, the exhaust gas flows into the exhaust cover H1 and is introduced into the gas introduction chamber 1b through the pinhole 1a of the hydrocarbon sensor 1. In the first pump cell 3, when a voltage is applied between the electrodes 31 and 32, electrolysis of water (H ₂ O) occurs on the surface of the electrode 32, and hydrogen (H ₂ ) is generated. Oxygen is pumped from the introduction chamber 1b by the pumping operation of the first pump cell 3. In the introduction chamber 1b, since the flow of oxygen is restricted by the diffusion resistance of the pinhole 1a and the cracking catalyst 23, the oxygen concentration decreases.

The exhaust gas in the introduction chamber 1b diffuses from the cracking catalyst 23 into the intermediate chamber 1c. At this time, in the cracking catalyst 23, the hydrocarbons in the exhaust gas are uniformly hydrocracked, including alkanes, alkenes, alkynes, and aromatic hydrocarbons having an unspecified number of carbon atoms, so that the predetermined hydrocarbons having a small number of carbon atoms are removed. (Eg, methane). The hydrocracking water (H ₂ O) hydrogen generated by the decomposition of the (H ₂₎ is used in the first pump cell 3. Also, under low oxygen concentrations, hydrocarbon combustion does not decrease because hydrocarbon combustion is restricted. The exhaust gas diffuses to the intermediate chamber 1c in such a state.

In the measuring section 12, the voltage applied to the second pump cell 4 is controlled by the electromotive force output of the sensor cell 5, and the pumping amount of oxygen between the outside of the hydrocarbon sensor 1 and the intermediate chamber 1c is adjusted.

For example, a voltage equal to the set value of the electromotive force of the sensor cell 5 is applied between the electrodes 61 and 62 of the detection cell 6.

The electrode 51 on the side of the intermediate chamber 1c of the sensor cell 5
Is active on the above-mentioned converted hydrocarbon having a small number of carbon atoms, so that the converted hydrocarbon is oxidized on the surface of the electrode 51 and T
An amount of oxygen corresponding to the HC concentration is consumed. The applied voltage of the second pump cell 4 is set high so that the electromotive force of the sensor cell 5 is constant, that is, the oxygen concentration on the surface of the electrode 51 is constant, and compensates for the lack of oxygen in the electrode 51 according to the THC concentration. As a result, in the intermediate chamber 1c, the oxygen concentration becomes higher in portions other than the surface of the electrode 51 by an amount corresponding to the THC concentration.

The exhaust gas in the intermediate chamber 1c flows into the measuring chamber 1d through the passage 1f. Since the inflowing exhaust gas has an oxygen concentration higher in accordance with the THC concentration, the detection cell 6 pumps excess oxygen corresponding to the THC concentration into the duct 1e. This pump current is a limit current at which the flow of gas with the intermediate chamber 1c is restricted by the cracking catalyst 23. From this pump current, the THC concentration is known as shown in FIG. Since the same voltage as the set electromotive force of the sensor cell 5 is applied between the electrodes 61 and 62 of the detection cell 6, if the THC concentration is 0, the pump current does not flow and the offset is 0, which is proportional to the THC concentration. The obtained pump current is obtained. Of course, the voltage applied to the detection cell 6 may be different from the set electromotive force of the sensor cell 5. In this case, the offset of the pump current may be corrected.

The electrode 5 on the intermediate chamber 1c side of the sensor cell 5
In 1, miscellaneous gases such as H ₂ and CO that are easily combustible also consume oxygen, and to eliminate this effect, the electrode 61 on the measurement chamber 1 d side of the detection cell 6 is oxidized with respect to the miscellaneous gas. It is desirable to prepare as shown, and a material obtained by adding 10% of Au to Pt is preferably used.

The electrode 51 on the intermediate chamber 1c side of the sensor cell 5
Is activated to the converted hydrocarbon, and the electrode 62 on the measurement chamber 1d side of the detection cell 6 is made inert to the converted hydrocarbon. However, the electrode 51 of the sensor cell 5 is made inert to the converted hydrocarbon. Alternatively, the electrode 61 of the detection cell 6 may be activated to the converted hydrocarbon. In this case, the intermediate chamber 1 is placed in the measurement chamber 1d.
Exhaust gas with a constant oxygen concentration flows into the detection cell 6 and the detection cell 6
The surface of the electrode 61 becomes in a state of oxygen deficiency according to the THC concentration.
Oxygen is pumped up. Thus, also in this case, the THC concentration is known from the pumping current of the detection cell 6.

[0037] While the detection cell 6 using an oxygen pump for applying a voltage to the pair of electrodes formed on both surfaces of an oxygen conductive solid electrolyte material, an oxide semiconductor such as Ti O _2, Sn O ₂ semiconductor The sensor may be used to measure a change in oxygen concentration according to the THC concentration. The entire configuration of the measuring unit is also arbitrary as long as the configuration is such that hydrocarbons having a small number of carbon atoms are oxidized by the oxidation catalyst and the hydrocarbon concentration is measured based on the amount of oxygen used for oxidation.

The cracking catalyst is supplied to the introduction chamber 1b and the intermediate chamber 1
Although it is also used as a communication path for communicating with c, it may be arranged in the introduction chamber 1b, for example, on the surface of the solid electrolyte material 14.
Further, a configuration in which a cracking catalyst is supported on the ceramic protective layer 28 may be employed.

In the present embodiment, the present invention is applied to the measurement of the exhaust gas discharged from the internal combustion engine. However, the present invention is not limited to this. Further, if the gas to be measured does not contain oxygen and there is sufficient hydrogen used for hydrocracking, a configuration without the first pump cell of the conversion unit can be adopted.

[Brief description of the drawings]

FIG. 1 is an overall vertical sectional view of a gas detection device to which a hydrocarbon sensor of the present invention is applied.

FIG. 2 is a longitudinal sectional view of the hydrocarbon sensor of the present invention.

FIG. 3 is an exploded view of the hydrocarbon sensor of the present invention.

FIG. 4 is a graph illustrating the operation of the hydrocarbon sensor of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hydrocarbon sensor 11 Conversion part 12 Measurement part 1a Pinhole (gas introduction path to be measured) 1b Introducing chamber 23 Cracking catalyst 3 First pump cell (oxygen pump) 4 Second pump cell 5 Sensor cell 51 Electrode (catalyst) 6 Detection cell

Continued on the front page (72) Inventor Yuji Mori 1-1-1, Showa-cho, Kariya-shi, Aichi Pref.

Claims (2)

[Claims] 1. A conversion section for converting hydrocarbons in a gas to be measured into predetermined hydrocarbons having a small number of carbon atoms, and an oxidation catalyst for oxidizing the converted predetermined hydrocarbons. A measuring unit for measuring the total concentration of hydrocarbons in the gas to be measured based on the amount of oxygen supplied to the oxidation. 2. The gas sensor according to claim 1, wherein the conversion unit forms an introduction chamber into which the gas to be measured is introduced via a measurement gas introduction path having a diffusion resistance, and a part of the introduction chamber wall. An oxygen pump configured to apply a voltage to a pair of electrodes formed on both sides of the oxygen-conductive solid electrolyte material to pump oxygen inside the introduction chamber to the outside of the introduction chamber, and an oxygen pump in the gas to be measured introduced into the introduction chamber. A hydrocarbon sensor comprising a cracking catalyst for decomposing hydrocarbons into the predetermined hydrocarbons.