JP3266039B2 - Oxygen sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an oxygen sensor used for controlling an air-fuel ratio of an internal combustion engine, for example.
The present invention relates to an oxygen sensor including a diffusion-controlling layer for controlling the diffusion of a gas whose oxygen concentration is detected.

[0002]

2. Description of the Related Art In air-fuel ratio control of an internal combustion engine, feedback control using an oxygen sensor has been conventionally performed. In recent years, there has been an internal combustion engine adopting a lean burn method for the purpose of improving fuel efficiency and the like, and such an internal combustion engine requires an oxygen sensor capable of measuring a lean air-fuel ratio.

[0003] This type of oxygen sensor comprises a sensor element made of a solid electrolyte material such as zirconia, a pair of platinum electrodes provided on the main body, and a diffusion element provided on the sensor element so as to cover one of the platinum electrodes. It has a rate-limiting layer. In addition, a plurality of pores are formed in the diffusion-controlling layer to control the diffusion of gas coming into contact with the sensor element.

In such an oxygen sensor, a gas (for example, an engine exhaust gas) that has passed through the pores of the diffusion-controlling layer comes into contact with the sensor element, and a predetermined voltage is applied between the electrodes so that both electrodes are turned on. A constant current (limit current) corresponding to the oxygen concentration of the gas flows between them. Therefore, the oxygen concentration of the gas can be known from the magnitude of the limit current value.

[0005] By the way, the above-mentioned limit current value has a characteristic that its magnitude changes in accordance with the temperature of the environment in which the oxygen sensor is used, that is, the temperature dependence, even if the oxygen concentration of the gas is constant. Further, such temperature dependency has different characteristics depending on the diameter of pores formed in the diffusion-controlling layer. That is, when the pore diameter is relatively small (for example, 100
(Å), the diffusion state of the gas becomes a Knudsen diffusion state, and the limiting current value I and the temperature T have a relationship represented by the following equation (1).

[0006]

(Equation 1) On the other hand, when the pore diameter is relatively large (for example, 10,000
Å), the gas diffusion state becomes a molecular diffusion state,
The limit current value I and the temperature T have a relationship represented by the following equation (2).

[0007]

(Equation 2) As described above, since the conventional oxygen sensor has temperature dependency, there has been a problem that a stable limit current value cannot be obtained when the conventional oxygen sensor is used in an environment where the operating temperature changes.

Incidentally, the relationship between the limiting current value I and the temperature T has opposite characteristics in the above two diffusion states. That is, as is apparent from the above equations (1) and (2), in the Knudsen diffusion state, the limiting current value I tends to decrease with the rise of the temperature T, whereas the molecular diffusion state , The limit current value I tends to increase as the temperature T increases.

Therefore, the average value of the pore diameter is set to a predetermined value (for example, 3000 °) as shown by a solid line in FIG. 7, and the diffusion state of the gas is set to an intermediate diffusion state between Knudsen diffusion and molecular diffusion. This may reduce the temperature dependency of the oxygen sensor.

[0010]

However, even when the average value of the pore diameters formed in the diffusion-controlling layer is set to the same value, the distribution state of the diameters can be various. For example, as shown by the two-dot chain line in FIG. 7, when the maximum diameter of the pores formed in the diffusion-controlling layer increases, most of the total amount of gas passing through the diffusion-controlling layer has a diameter Through large pores. For this reason, gas tends to be diffused in a molecular diffusion state, and the oxygen sensor exhibits temperature dependence. As described above, there has been a problem that the temperature dependency cannot be sufficiently reduced only by setting the average value of the pore diameters to a predetermined value as in the related art.

The present invention has been made in view of the above circumstances, and an object of the present invention is to surely reduce temperature dependency.
An object of the present invention is to provide an oxygen sensor capable of obtaining a stable limit current value.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted studies to achieve the above object, and have found that a diffusion-controlling layer has a layer thickness of 100 to 1000 μm, a porosity of 5 to 30%, and a maximum pore size of 600-1300 °, the mode of pores is 400-
By constituting by a porous material of 1000 °, the diffusion state of the gas passing through the diffusion-controlling layer is maintained at an intermediate state between the Knudsen diffusion state and the molecular diffusion state,
It has been found that the temperature dependence of the oxygen sensor can be reliably reduced.

Based on this finding, the present invention provides a sensor element made of a solid electrolyte material, a pair of electrodes provided on the sensor element, and outputting a signal corresponding to the oxygen concentration of gas in contact with the sensor element. An oxygen sensor comprising a diffusion-controlling layer provided on the sensor element so as to cover one of the electrodes and having a plurality of pores for controlling the diffusion of gas, wherein the diffusion-controlling layer has a layer thickness of 100 to 1000 μm; Rate is 5-30%, maximum pore diameter is 600-1300mm,
It is intended to be made of a porous material having a mode of pores of 400 to 1000 °.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an oxygen sensor provided in a vehicle diesel engine will be described below.

FIG. 1 shows a schematic configuration of a diesel engine (hereinafter, simply referred to as “engine”) 1 in the present embodiment. As shown in FIG. 1, an intake pipe 2 and an exhaust pipe 3 are connected to a combustion chamber (not shown) of the engine 1. The intake air taken into the intake pipe 2 through an air cleaner (not shown) or the like passes through the pipe 2 and is introduced into a combustion chamber (not shown) of the engine 1. Further, a throttle valve 4 provided in the intake pipe 2 adjusts the amount of intake air introduced into the combustion chamber of the engine 1 according to its opening degree. Then, the fuel injected from the injector (not shown) and the intake air are mixed in the combustion chamber, and the air-fuel mixture explodes and burns, so that a driving force is obtained in the engine 1. After the combustion, the exhaust gas is introduced into the exhaust pipe 3 from the combustion chamber, and then is discharged outside through a catalyst or the like (not shown).

The engine 1 has an exhaust gas recirculation (EGR:
Exhaust Gas Recirculation) device 5 is provided. The EGR device 5 includes an EGR passage 6 that communicates the exhaust pipe 3 with a portion of the intake pipe 2 downstream of the throttle valve 4, and a flow control valve 7 provided in the passage 6. Part of the exhaust gas (EGR gas) in the exhaust pipe 3 passes through the EGR passage 6 and is returned to the intake pipe 2.
EGR gas amount passing through the EGR passage 6 (EGR amount)
Is adjusted by controlling the flow control valve 7 by an electronic control unit (not shown) of the engine 1.

In the EGR device 5, the engine 1
By mixing EGR gas, that is, an inert gas that is not used for combustion, into a part of the intake air introduced into the combustion chamber, the maximum temperature of the combustion gas in the combustion chamber is lowered, and NOx is reduced. ing.

An oxygen sensor 11 is attached to the intake pipe 2 at a position downstream of the opening 6a of the EGR passage 6 so that its tip projects into the intake pipe 2.

FIG. 2 shows a tip of the sensor element 12 in the oxygen sensor 11, and FIG. 3 shows a cross section taken along line 3-3 in FIG. The oxygen sensor 11 includes the sensor element 12, a perforated cover (not shown) for covering the outer periphery of the sensor element 12, and a housing (not shown) for supporting the sensor element 12 and the perforated cover.

The sensor element 12 has a laminated structure and has oxygen ion conductive zirconia (ZrO 2) having a predetermined particle size.
It is formed by laminating a plurality of plate members 21 to 26 made of a mixture (hereinafter, simply referred to as “raw material”) obtained by adding a binder and the like to yttria (Y 2 O 3) and firing them. The sensor element 12 includes a heater 13 and an element body 14 provided on the heater 13.
And a diffusion control plate 26 provided on the element body 14.

The heater 13 includes a pair of heater plates 21 and 22.
And a heating element 40 provided between the plates 21 and 22. The upper and lower surfaces of the heating element 40 are covered with an insulating layer (Al2 O3) (not shown) made of alumina, and the heating element 40 and the heater plates 21 and 22 are electrically insulated. . The heating element 40 is connected to a drive circuit (not shown) provided in the electronic control unit.

In the heater 13, since the heating of the heating element 40 is controlled by the electronic control unit to generate heat, the front end portion of the sensor element 12 is heated to a predetermined activation temperature or higher. ing.

The element main body 14 includes a solid electrolyte plate 24, and an introduction chamber forming plate 25 and a discharge chamber forming plate 23 which are stacked on and under the solid electrolyte plate 24. On the upper and lower surfaces of the solid electrolyte plate 24, a first electrode (cathode) 31 and a second electrode (anode) 33 are provided. The first electrode 31 and the second electrode 33 are electrically connected to a drive circuit of an electronic control unit via a lead (not shown).

The discharge chamber forming plate 23 has a U-shape as a whole, and the discharge passage 34 is formed by a space defined by the inner wall surface of the plate 23, the upper surface of the heater plate 22, and the lower surface of the solid electrolyte plate 24. Are formed. The discharge passage 34 extends to the base end side of the sensor element 12 and is open to the atmosphere. Oxygen that has moved from an introduction chamber 36 described later through the inside of the solid electrolyte plate 24 is
4 to the atmosphere.

On the other hand, a rectangular hole 35 is formed at a position corresponding to the first electrode 31 on the distal end side of the introduction chamber forming plate 25. The diffusion-controlling plate 26 is stacked on the upper surface of the introduction chamber forming plate 25 so as to close the rectangular hole 35. An introduction chamber 36 is defined by a space defined by the lower surface of the diffusion control plate 26, the inner peripheral surface of the rectangular hole 35, and the upper surface of the solid electrolyte plate 24.

A plurality of pores (not shown) are formed in the diffusion control plate 26 so as to penetrate the plate 26 vertically. The pores are formed by gaps formed between the particles in the material of the diffusion-controlling plate 26 by firing the material. The pores have a function of introducing the intake air flowing through the intake pipe 2 into the introduction chamber 36 in a diffusion-controlled state.

The diffusion-controlling plate 26 in this embodiment is made of a porous material that satisfies the following conditions 1) to 4) in order to reduce the temperature dependence of the oxygen sensor 11. In the present embodiment, the conditions 1) to 4) are used.
The particle size of the material, the sintering temperature and the like are appropriately adjusted so as to satisfy the following.

1) Thickness: 100-1000 μm 2) Porosity: 5-30% 3) Maximum pore size: 600-1300Å 4) Mode of pore size distribution: 400-1000Å Here, condition 1) is The thickness of the diffusion control plate 26 is set to 10
The reason for setting the thickness in the range of 0 to 1000 μm is that
When the thickness is smaller than 00 μm, it becomes impossible to sufficiently control the diffusion of the intake air in the diffusion control plate 26. When the plate thickness is larger than 1000 μm, the limit current flowing between the electrodes 31 and 33 is limited. This is because as a result, the S / N ratio of the oxygen sensor 11 decreases. In addition, the plate thickness is desirably set in the range of 400 to 800 μm in order to secure a favorable diffusion-controlling effect of the diffusion control plate 26 and to improve the S / N ratio of the oxygen sensor 11. . From this point of view, in the present embodiment, the thickness is set to “600 μm”.

FIG. 4 is a graph showing the relationship between the porosity and the limit current value. As shown in FIG.
If it is larger than 0%, diffusion control of the intake air cannot be sufficiently performed in the diffusion control plate 26, and the limit current value tends to be excessively large. On the contrary,
If the porosity is smaller than 5%, the magnitude of the limit current tends to be small, and the S / N ratio of the oxygen sensor 11 tends to decrease.

Therefore, in the present embodiment, condition 2)
The porosity is set in the range of 5 to 30%, and more specifically, set to “20%”. The porosity is 5-3.
It is sufficient that the porosity is within the range of 0%. However, in order to secure a favorable diffusion-controlling effect by the diffusion-controlling plate 26 and to improve the S / N ratio of the oxygen sensor 11, the porosity is set to 15 to 25%.
It is desirable to set within the range.

Next, as in the conditions 3) and 4), in the present embodiment, the maximum pore diameter is set within the range of 600 to 1300 °, and the mode of the pore diameter distribution is set to 400 to 1300 °.
It is set within the range of 1000 °. In this embodiment,
By setting the maximum pore diameter and the mode value of the pore diameter distribution within the above-mentioned predetermined ranges, the diffusion state of the intake air in the diffusion control plate 26 is maintained at an intermediate state between the molecular diffusion and the Knudsen diffusion. Confirmed by experiment.

FIG. 5 is a graph showing the relationship between the maximum pore diameter and the limit current value. In this case, the oxygen concentration of the intake air is 20.9%, and the temperature of the sensor element 12 is about 750.
° C, and the applied voltage between the electrodes 31 and 33 is set to 0.75V. As shown in FIG.
If it is set smaller, the S / N ratio of the oxygen sensor 11 tends to decrease as a result of the limit current value becoming smaller. On the other hand, when the maximum pore diameter is larger than 1200 °, the diffusion control of the intake air by the diffusion control plate 26 becomes difficult, and the size of the limit current value tends to increase. For this reason, the diffusion state of the intake air is maintained in an intermediate state between the molecular diffusion and the Knudsen diffusion, and a suitable diffusion-limiting action and S
In order to secure the / N ratio, the maximum pore diameter should be 800 to 12
It is desirable to set within the range of 00 °.

Also, the mode of the pore diameter distribution should be set within the range of 600 to 800 ° in order to ensure that the diffusion state of the intake air is in an intermediate state between the molecular diffusion and the Knudsen diffusion. Is desirable.

FIG. 6 is a graph showing the result of measurement of the distribution of pore diameters in the diffusion control plate 26 using a mercury porosimeter. As shown by the solid line in FIG.
The distribution state having the mode value at 00 ° is shown, and the maximum pore diameter is 1100 °.

In such an oxygen sensor 11, when a predetermined voltage is applied between the first electrode 31 and the second electrode 33 by the electronic control device, the movement of oxygen ions inside the solid electrolyte plate 24. Each electrode 31, 33
A current flows between them. At this time, the oxygen molecules introduced into the introduction chamber 36 are diffusion-controlled by the pores, so that the current flowing between the electrodes 31 and 33 takes a constant value (limit current value) according to the oxygen concentration of the exhaust gas. Become like Therefore, the electronic control unit can calculate the air-fuel ratio of the engine 1 based on the magnitude of the limit current value.

Here, in the present embodiment, the diffusion control plate 26
Is formed of a porous material that satisfies the conditions 1) to 4) described above. Therefore, the diffusion state of the intake air in the diffusion control plate 26 is reliably maintained at an intermediate state between the molecular diffusion and the Knudsen diffusion. As a result, the oxygen sensor 1
1 is reliably reduced, and a limit current value corresponding to the oxygen concentration of the intake air can be stably obtained.

Further, in the present embodiment, since the plate thickness, porosity, and maximum pore diameter are set to appropriate values in consideration of the magnitude of the limiting current value, a reliable diffusion-controlling action and a good S / S N
The ratio can be obtained, and the detection accuracy of the oxygen sensor 11 can be improved.

In this embodiment, the oxygen sensor 11 is attached to the intake pipe 2 of the engine 1 provided with the EGR device 5, and detects the oxygen concentration of the intake air flowing through the pipe 2. As described above, the oxygen sensor 11 attached to the intake pipe 2 is in a situation where the temperature change is larger than that of the oxygen sensor attached to the exhaust pipe 3, for example.

That is, relatively high-temperature EGR gas intermittently flows into the intake pipe 2 by the operation of the EGR device 5, and the temperature of the oxygen sensor 11 (sensor element 12) decreases with the inflow of the EGR gas. Because it changes. Further, the oxygen sensor 11 is deprived of heat by the contact of the intake air. However, the flow rate of the intake air changes according to the rotation speed of the engine 1, and the amount of heat deprived changes the temperature of the sensor 11. To do that.

In this regard, the oxygen sensor 1 of this embodiment
Since 1 has extremely low temperature dependency, it can output a stable limit current value even under the above-mentioned situation of temperature change, and can be suitably used as an oxygen sensor for an intake pipe.

The configuration of the above embodiment can be changed as described below. Even if the configuration is changed in this way, it is possible to achieve the same operation and effect as the above embodiment. The distribution state of the pore diameters formed in the diffusion control plate 26 is not limited to the distribution state shown by the solid line in FIG. 6, and if the above conditions 2) to 4) are satisfied, for example, the distribution state shown in FIG. The distribution state as shown by a chain line may be used.

In the above embodiment, the diffusion-controlling plate 26 is formed by firing the material of the diffusion-controlling plate 26. On the other hand, for example, a diffusion-controlling layer may be provided on the sensor element 12 using a method such as thermal spraying.

In the above embodiment, the diffusion-controlling plate 26 is made of oxygen ion conductive zirconia (Zr
Although the plate 26 is formed of a mixture of O2) and yttria (Y203), the plate 26 may be formed of, for example, other porous materials (alumina, magnesia, spinel, etc.).

In the oxygen sensor 11 of the above embodiment, the sensor element 12 having a laminated structure in which a plurality of plate members are laminated is used.
As shown in JP-A-8248, a sensor element having a test tube shape may be used.

In the above embodiment, the oxygen sensor 11 is
It is designed to be attached to the intake pipe 2 of the engine 1 provided with the GR device 5. On the other hand, the oxygen sensor according to the present invention may be attached to an intake pipe of an engine equipped with a known blow-by gas reduction device.

In the above-described embodiment, the oxygen sensor according to the present invention is applied to the engine 1. However, the oxygen sensor may be applied to, for example, general combustion equipment, particularly, combustion equipment in which the use environment temperature changes.

The technical ideas that can be grasped from the above embodiments will be described below together with their effects. (A) In the oxygen sensor according to the first aspect, the oxygen sensor is attached to an intake pipe of an internal combustion engine having an exhaust gas recirculation device or a blow-by gas reduction device.

As described above, in the oxygen sensor attached to the intake pipe, as described above, there is a tendency that the temperature change is large and the limit current value becomes unstable. The oxygen sensor described in the above (a) has a very small temperature dependency, and thus is suitable as the oxygen sensor having the above tendency.

[0049]

According to the present invention, the diffusion-controlling layer has a thickness of 10
0 to 1000 μm, porosity of 5 to 30%, maximum pore diameter of 600 to 1300 °, mode of pores of 400 to 1000
Å is made of a porous material. Therefore, the diffusion state of the gas passing through the diffusion-controlling layer is maintained at an intermediate state between the Knudsen diffusion state and the molecular diffusion state. Therefore, the temperature dependence of the oxygen sensor can be reliably reduced, and a limit current value corresponding to the oxygen concentration of the gas can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an engine system using an oxygen sensor.

FIG. 2 is a perspective view showing a sensor element of the oxygen sensor.

FIG. 3 is a sectional view taken along line 3-3 in FIG. 2;

FIG. 4 is a graph showing a relationship between a porosity and a limit current value.

FIG. 5 is a graph showing a relationship between a maximum pore diameter and a limit current value.

FIG. 6 is a graph showing a distribution state of pore diameters.

FIG. 7 is a graph showing a conventional pore size distribution state.

[Explanation of symbols]

11: oxygen sensor, 12: sensor element, 26: diffusion-controlling plate, 31: first electrode, 33: second electrode.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01N 27/41

Claims (1)

(57) [Claims] A sensor element made of a solid electrolyte material;
A pair of electrodes provided on the sensor element and outputting a signal corresponding to the oxygen concentration of the gas contacting the sensor element, provided on the sensor element so as to cover one of the electrodes,
An oxygen sensor comprising: a diffusion-controlling layer having a plurality of pores for controlling the diffusion of a gas; wherein the diffusion-controlling layer has a layer thickness of 100 to 1000 μm, a porosity of 5 to 30%, and a maximum pore diameter of 600. An oxygen sensor comprising a porous material having a maximum mode of pores of 400 to 1000 °.