JPH11183429A - Gas sensor for natural gas engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable highly accurate air-fuel ratio control by preventing sensor output from deviating due to H2 in exhaust gases peculiar to a natural gas engine and improving detection accuracy. SOLUTION: An outside electrode 23 to be exposed to exhaust gases is provided at the outer surface of an oxygen ion conductive solid electrolyte 21, and an inside electrode 22 to be exposed to gaseous oxygen of reference concentration is provided at the inner surface. A protective coating layer 24 to cover the surface of the outside electrode 23 is provided and constituted of a porous film with an average pore diameter of 1000 Å or more. As the difference in diffusing steed between H2 and O2 becomes sufficiently small with an average pore diameter of 1000 Å, and the outside electrode 23 does not become richer in H2 than reality it is possible to eliminate the effects of H2 to sensor output and to prevent output deviations.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the gas sensor for detecting a specific gas component contained in exhaust gas from a natural gas engine, for example, relates to a gas sensor which is used as the O ₂ sensor or the like for the air-fuel ratio control.

[0002]

2. Description of the Related Art In recent years, a natural gas vehicle has been attracting attention as an alternative fuel vehicle to oil. Natural gas vehicles have the advantages of being able to reduce CO ₂ emissions compared to gasoline engines, and being able to expect clean exhaust emission characteristics.
2. Description of the Related Art A system (gas mixer system) to which a carburetor system used in an LPG vehicle is applied is known. Also, in order to further improve the exhaust emission characteristics and make it an ultra-low-emission vehicle comparable to an electric vehicle, it is necessary to have a more accurate air-fuel ratio control than a gas mixer system,
For this reason, the development of a natural gas vehicle equipped with an injection system having a small air-fuel ratio fluctuation and excellent fuel control responsiveness has been promoted.

[0003]

Generally, the air-fuel ratio is feedback-controlled based on the output of an O ₂ sensor installed in an exhaust pipe. Therefore, the detection accuracy of the O ₂ sensor is an indispensable factor for improving the accuracy of the air-fuel ratio control. However, it has been found that if an O ₂ sensor for a gasoline engine is used as it is in order to detect the air-fuel ratio of a natural gas engine, the output characteristics of the O ₂ sensor will deviate. This is because natural gas contains methane (CH ₄ ) as a main component and therefore has a higher H / C ratio than gasoline and a larger amount of hydrogen (H ₂ ) contained in exhaust gas. It occurs remarkably in the system.

[0004] This is because the gas is directly mixed in the vicinity of the combustion chamber in order to ensure the fuel control response to the gas mixer system that uniformly mixes the gas upstream of the intake passage and then sucks the gas into the combustion chamber. In the injection system for injection, the mixture in the combustion chamber tends to be non-uniform. For this reason, in the injection system, a large amount of H ₂ is generated in a region where the air-fuel ratio is rich in the combustion chamber, and the H ₂ concentration in the exhaust gas becomes higher than that of the gas mixer system. Therefore, in the O ₂ sensor for a natural gas engine, in order to improve the air-fuel ratio control with high accuracy, it is a major problem to eliminate the influence of H ₂ on the sensor output and to eliminate the deviation of the sensor output. .

However, the present invention can prevent the output of the sensor from being shifted due to H ₂ in the exhaust gas and improve the detection accuracy. For example, as in the case of a natural gas engine of an injection system, a large amount of the exhaust gas is used. even if it contains the H _2, it is an object to realize a gas sensor for natural gas engines capable of high-precision air-fuel ratio control.

[0006]

According to a first aspect of the present invention, there is provided a gas sensor for a natural gas engine, comprising: a first electrode exposed to the exhaust gas on a surface of an oxygen ion conductive solid electrolyte; And a second electrode exposed to
A coating layer is provided to cover the surface of the first electrode. Then, the coating layer is formed so that the average pore diameter is 10
It is characterized by being constituted by a porous film of not less than 00 °.

[0007] exhaust gas from natural gas engines, the low molecular weight hydrogen (H ₂₎ is included in a large amount, the H ₂
The difference between the rate at which the gas diffuses through the coating layer and the rate at which oxygen (O ₂ ) diffuses through the coating layer is considered to be the cause of the sensor output deviation. The present invention
By making the average pore diameter of the coating layer 1000 °, it has been found that diffusion of O ₂ having a higher molecular weight than that of H ₂ can be facilitated, and the difference between the two diffusion rates can be sufficiently reduced. Accordingly, since the influence of H _{2 on} the sensor output can be eliminated to prevent the output deviation, even when used for a natural gas engine of an injection system, highly accurate air-fuel ratio control is possible, and Has an excellent effect on improving emissions.

More preferably, the coating layer is
90% or more of all the pores are composed of a membrane having a pore diameter of 1000 ° or more (claim 2). By increasing the ratio of the pores having a pore diameter of 1000 ° or more, the difference between the diffusion rates of H ₂ and O ₂ can be reduced to almost zero.
And the above-described effect of preventing output deviation can be further enhanced.

Specifically, a configuration in which the first electrode is provided on one surface of the oxygen ion conductive solid electrolyte and the second electrode is provided on the other surface of the oxygen ion conductive solid electrolyte is considered. (Claim 3). For example, a configuration in which the oxygen ion conductive solid electrolyte is a test tube, the first electrode is provided on the inner peripheral surface, and the second electrode is provided on the outer peripheral surface, or the oxygen ion conductive solid electrolyte is formed in a plate shape It is possible to adopt a configuration in which the first electrode is provided on the front surface of the conductive solid electrolyte and the second electrode is provided on the back surface.

On the outer side of the coating layer, a porosity of 4
A porous layer of 0 to 50% by volume may be provided as a trap layer (claim 4) to prevent deposit components generated by combustion of engine oil or the like from entering the coating layer and causing clogging. can do.

Further, by providing a heater member for heating the sensor (claim 5), a deposit component generated by combustion of engine oil or the like is burned in the coating layer to prevent clogging of the coating layer. can do. Further, there is an effect that the sensor is activated early to prevent deterioration of exhaust emission.

A catalyst layer having an oxidizing effect can be formed on the surface of the coating layer. As a result, H ₂ in the exhaust gas is oxidized by the catalyst layer, the amount of H ₂ reaching the coating layer is reduced, and a shift in sensor output due to H ₂ can be suppressed more reliably.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example in which the present invention is applied to an O ₂ sensor used for controlling an air-fuel ratio of a natural gas engine will be described. Figure 2 shows the overall configuration of the O ₂ sensor, O ₂ sensor comprises a cylindrical housing 1 which is attached to the exhaust pipe wall of an unillustrated natural gas engines, gas detection element 2 to be inserted and held therein And The cylindrical housing 1 has a flange 11 protruding from the outer periphery of the central portion in contact with the exhaust pipe wall, and can be attached to the exhaust pipe wall of the natural gas engine with a thread formed below the flange. The gas detecting element 2 is a test tube whose lower end protrudes from the housing 1 and is located in the exhaust pipe, and is accommodated in a cover 3 fixed to the lower end of the housing 1. The cover body 3 has a double cylindrical shape and has a plurality of gas circulation holes 31 and 32 on the side surfaces of the inner and outer cylinders. The exhaust gas of the natural gas engine is introduced into the gas detection element 2 through the gas circulation holes 31 and 32. It is made to be done.

Above the housing 1, there is provided a tubular cover 12 which covers the upper outer periphery of the gas detecting element 2. The tubular cover 12 has a lower cover 121 fixed to the upper end of the housing 1 and an upper cover 122 fixed to the upper half of the lower cover 121. The outer periphery of the upper half of the upper cover 122 is a cylindrical member 123. It is covered with a double structure. The upper end opening of the upper cover 122 is the rubber bush 1
A lead wire 4 for extracting the output of the gas detection element 2 extends to the outside through the rubber bush 13. The upper cover 122 and the cylindrical member 123 have a plurality of air holes 124,
25 are formed, and these air holes 124, 1
The atmosphere introduced through the gas detecting element 25 is introduced into the hollow portion of the gas detecting element 2 as a reference oxygen concentration gas. A water-repellent filter 14 is provided between the upper cover 122 and the cylindrical member 123 at the position where the air holes 124 and 125 are formed.

As shown in FIG. 1, the gas detecting element 2 includes an oxygen ion conductive solid electrolyte 21 such as zirconia molded into a test tube, and an inner electrode as a first electrode provided along the inner peripheral surface thereof. 22, an outer electrode 23 serving as a second electrode provided along the outer peripheral surface. These electrodes 22, 2
Reference numeral 3 is made of, for example, platinum (Pt) or the like, and is formed by ordinary means such as vapor deposition. As shown in FIG. 2, the electrodes 22 and 23 are connected to the lead wire 4 through output metal terminals 41 and 42 that press and contact the surface of the oxygen ion conductive solid electrolyte 21.

On the outer surface of the outer electrode 23, a protective film 24 as a coating layer covering the outer electrode 23 is formed. The protective film 24 protects the surface of the outer electrode 23 from harmful substances in the exhaust gas, for example, spinel (MgO.Al ₂
A porous film made of a ceramic such as O ₃ ) is preferably used. The feature of the present invention resides in that the protective film 24 is constituted by a porous film having an average pore diameter of 1000 ° or more, whereby the difference between the diffusion rates of H ₂ and O ₂ contained in the exhaust gas is sufficiently reduced. By making it small, the deviation of the sensor output is prevented. Preferably, the pores having an average pore diameter of 1000 °
By setting it to be 90% or more of all the pores, the difference between the diffusion rates of H ₂ and O ₂ can be made almost zero, and the influence of H ₂ can be eliminated.

The formation of the protective film 24 can be performed, for example, using a known plasma spraying apparatus. In the plasma spraying method, a heated and melted spray material is converted into a plasma jet, which is sprayed at a high speed onto the surface of the object to be sprayed.
As a material for thermal spraying, ceramic powder such as spinel (MgO.Al ₂ O ₃ ) constituting the protective film 24 may be used.
A mixed gas of argon (Ar) and nitrogen (N ₂ ) is used as a working gas. The plasma arc voltage is typically 50
~ 60V, plasma arc current is usually 500 ~
The range is about 700 A, and the distance from the plasma gun to the object to be sprayed is about 80 to 100 mm. The pore size of the protective coating 24 can be controlled by adjusting the particle size of the ceramic powder to be sprayed and the spraying power. Generally, when the particle size of the ceramic powder is increased or the spraying power is reduced, the pore size is reduced. growing.

In consideration of the stability of the sensor output, the mechanical strength, etc., the thickness of the protective film 24 is usually 10
It is desirable to set the range to about 0 to 200 μm.

Next, the operation of the protective film 24 in the O ₂ sensor having the above configuration will be described. In FIG. 1, the exhaust gas passing through the protective film 24 is introduced to the outer electrode 23 of the gas detection element 2, and the atmosphere is introduced to the inner electrode 22. At this time, an electromotive force is generated in the oxygen ion conductive solid electrolyte 21 according to the difference in oxygen concentration between the inner electrode 23 and the outer electrode 22. The sensor output should theoretically change abruptly when the excess air ratio (λ) = 1.0 (shown as the theoretical value (a) in FIG. 3).

However, in the exhaust gas of the natural gas engine, H ₂ is present _two to three times that of the gasoline engine, and when the pore size of the protective film 24 is small, the excess air ratio (λ) at which the sensor output suddenly changes. Shifts to the lean side (shown as (b) in FIG. 3). This is presumably because when the pore size of the protective film 24 is small, gas diffusion is governed by the Knudsen flow, and a difference occurs in the diffusion rates of H ₂ and O ₂ .

The relationship between the average pore diameter of the protective film 24 and the diffusion rate was examined, and the results are shown in FIG. Each sample used for the measurement was formed as follows. In addition,
The measurement of the pore diameter was performed as follows.

As shown in FIG. 4, in a region where the pore diameter of the protective film 24 is smaller than 1000 °, the diffusion rate of H ₂ having a small molecular weight becomes larger than that of O ₂ . in this case,
H ₂ passes through the protective film 24 earlier than O ₂ and the outer electrode 2
3 and the outer electrode 23 is H ₂ richer than it actually is. In other words, O ₂ deficiency occurs on the surface of the outer electrode 23, and the output characteristic of the O ₂ sensor shifts its output sudden change point to the lean side with respect to the theoretical value (FIG. 3).
(B)). As a result, the λ value in the O ₂ feedback control system is shifted, and the exhaust emission may be deteriorated.

On the other hand, in the region where the average pore diameter of the protective film 24 is 1000 ° or more, as shown in FIG. 4, the difference between the diffusion rates of H ₂ and O ₂ , which causes the output shift, is almost zero. Therefore, O ₂ deficiency on the surface of the outer electrode 23 is eliminated, and a shift in output characteristics can be prevented. Then, the relationship between the average pore diameter and the excess air ratio (λ) at which the output changes suddenly was examined and shown in FIG. As is apparent from FIG. 5, when the average pore diameter is 1000 ° or less, the sensor output is shifted to the lean side. It was confirmed that it did not occur.

FIG. 6 shows a second embodiment of the present invention.
In the present embodiment, a trap layer 5 that covers the outside of the coating layer 24 is provided in addition to the O ₂ sensor configuration of the first embodiment. Other configurations are the same as those of the first embodiment. The trap layer 5 is a porous fired body,
For example, it is preferably made of ceramics such as Al ₂ O ₃ , and its porosity is usually preferably 40 to 50%. Further, the thickness of the trap layer 5 is usually preferably 100 μm or less. By providing the trap layer 5, it is possible to prevent a deposit component generated by combustion of engine oil or the like from entering the protective film 24, thereby preventing the protective film 24 from being clogged.

FIG. 7 shows a third embodiment of the present invention.
In the present embodiment, the heater 6 for heating the sensor is disposed in the hollow portion of the oxygen ion conductive solid electrolyte 21 in the O ₂ sensor having the configuration shown in FIG. Other configurations are the same as those of the first embodiment. The heater 6 is made of an insulator having a rod-like, plate-like or tubular shape, for example, A
A heating element such as tungsten (W) or molybdenum (Mo) is embedded inside l ₂ O ₃ or the like, and the heating element is connected to an external power supply via a lead wire. The power supplied to the heater 6 is such that the temperature of the protective film 24 is usually 500
The temperature is controlled to be not less than ° C. by a map prepared in advance according to the engine conditions, or by feedback control based on the resistance value of the heating element of the heater 6.

According to the above configuration, by providing the heater 6, the deposit component generated by the combustion of the engine oil and the like is burned in the protective film 24, whereby the clogging of the protective film 24 can be prevented. .
In addition, the sensor 6 is activated early by the heater 6, so that the engine that cannot perform feedback control at the time of cold start of the engine or the like can be shortened, and deterioration of emission can be prevented.

FIG. 8 shows a fourth embodiment of the present invention.
In the present embodiment, the outer surface of the protective film 24 is covered with the catalyst layer 7 having an oxidizing effect. The above catalyst layer 5
Are porous ceramics such as alumina (Al ₂ O)
₃ ) a catalytic metal (Pt, Pt-Rh, etc.) is supported on H _2, and H ₂ contained in the exhaust gas can be oxidized before reaching the protective film 24. Here, the loading amount of the catalyst metal is preferably about 0.5 to 5% by weight based on the total weight of the catalyst layer 7. The thickness of the catalyst layer 7 is usually desirably 50 μm or less. By providing the catalyst layer 7, H ₂ in the exhaust gas can be oxidized in the catalyst layer 7 and the amount of H ₂ reaching the protective film 24 can be reduced. Therefore, it is possible to more reliably prevent the deviation of the sensor output due to H _2.

FIG. 9 shows a fifth embodiment of the present invention.
In the above embodiments, examples in which the first electrode and the second electrode are formed on the inner and outer peripheral surfaces of a test tubular oxygen ion conductive solid electrolyte as the gas detection element 2 have been described. The shape is not limited to this,
As shown in FIG. 9, a stacked structure in which a first electrode 82 and a second electrode 83 are formed at opposing positions on the upper and lower surfaces of a plate-shaped oxygen ion conductive solid electrolyte 81 may of course be used. A protective film 84 is formed on the first electrode 82 on the exhaust side,
On the lower surface of the second electrode 83 on the atmosphere side, a flat support 85 for forming an atmosphere chamber 85a and an atmosphere passage 85b communicating therewith is laminated. On the lower surface of the support 85, a heater 86 and an insulating sheet 87 are further laminated.

In each of the above embodiments, an example has been described in which the present invention is applied to an O ₂ sensor for controlling the air-fuel ratio of a natural gas engine. However, the present invention is not limited to these uses, and the present invention is not limited to these applications. The present invention can be applied to any gas sensor for a natural gas engine such as an air-fuel ratio sensor for detecting a fuel ratio.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a main part of a natural gas engine O ₂ sensor according to a first embodiment of the present invention.

FIG. 2 is an overall cross-sectional view of the O ₂ sensor according to the first embodiment.

FIG. 3 is a diagram showing sensor output characteristics of a natural gas engine.

FIG. 4 is a diagram showing a relationship between a pore diameter and a diffusion rate.

FIG. 5 is a diagram illustrating a relationship between a pore diameter and a sensor output.

FIG. 6 is an enlarged sectional view of a main part of an O ₂ sensor according to a second embodiment of the present invention.

FIG. 7 is an enlarged sectional view of a main part of an O ₂ sensor according to a third embodiment of the present invention.

FIG. 8 is an enlarged sectional view of a main part of an O ₂ sensor according to a fourth embodiment of the present invention.

FIG. 9 is a diagram showing a fifth embodiment of the present invention, in which (a)
Is a developed view showing the structure of the gas detection element, and (b) is A in (a).
FIG. 4 is a cross-sectional view taken along a line A.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Housing 11 Flange 12 Cylindrical member 2 Gas detection element 21 Oxygen ion conductive solid electrolyte 22 Inner electrode (first electrode) 23 Outer electrode (second electrode) 24 Protective film (coating layer) 3 Cover body 4 Lead wire 5 trap layer 6 heater 7 catalyst layer

Continued on the front page (72) Inventor Motomasa Iizuka 14 Iwatani, Shimowasakucho, Nishio City, Aichi Prefecture Inside the Japan Auto Parts Research Institute (72) Inventor Kenji Kanehara 14 Iwatani, Shimowasukamachi, Nishio, Aichi Prefecture Japan, Ltd. Within the Automotive Parts Research Laboratory (72) Inventor Isao Watanabe 1-1-1, Showa-cho, Kariya-shi, Aichi, Japan Denso Corporation

Claims (6)

[Claims] 1. A gas sensor for detecting a specific gas component contained in an exhaust gas from a natural gas engine, comprising: a first electrode exposed to the exhaust gas on a surface of an oxygen ion conductive solid electrolyte; A second electrode exposed to a reference oxygen concentration gas; and a gas sensor for a natural gas engine provided with a coating layer covering the surface of the first electrode. A gas sensor for a natural gas engine, comprising a porous membrane. 2. The gas sensor for a natural gas engine according to claim 1, wherein 90% or more of all the pores in the coating layer have a pore diameter of 1000 ° or more. 3. The oxygen ion conductive solid electrolyte according to claim 1, wherein the first electrode is provided on one surface of the oxygen ion conductive solid electrolyte, and the second electrode is provided on the other surface of the oxygen ion conductive solid electrolyte.
Or the gas sensor for a natural gas engine according to 2. 4. A porosity of 4 outside the coating layer.
The gas sensor for a natural gas engine according to any one of claims 1 to 3, wherein a trap layer made of a porous film of 0 to 50% is provided. 5. A gas sensor for a natural gas engine according to claim 1, further comprising a heater member for heating the sensor. 6. The gas sensor for a natural gas engine according to claim 1, wherein a catalyst layer having an oxidizing action is provided outside the coating layer.