JPH0577264B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an air-fuel ratio detection element for controlling or measuring the air-fuel ratio of a combustion device such as an internal combustion engine or a gas combustion device. [Prior Art] Traditionally, porous electrode layers (porous platinum electrodes) have been placed on both sides of an ion-conducting solid electrolyte (e.g. stabilized zirconia) with one side in contact with the measurement gas and the other side in contact with the atmosphere. An air-fuel ratio detection element formed by depositing is used. This air-fuel ratio detection element generates an electromotive force of about 1 volt between both porous electrode layers when the air-fuel ratio (λ) detected from the combustion gas is rich (λ<1). The voltage of the electromotive force generated between both porous electrode layers changes around the stoichiometric air-fuel ratio (λ=1), and almost no electromotive force is generated when the air-fuel ratio is lean (1<λ). When the electromotive force of the air-fuel ratio detection element changes around the stoichiometric air-fuel ratio, for example, when controlling the air-fuel ratio of an internal combustion engine of a car, for example, the electromotive force of the air-fuel ratio detection element changes. If it is lower than 500 millivolts (mV), it is determined that the air-fuel ratio of the measured gas is on the lean side, and the intake system, such as the fuel injection of a car, is controlled so that the air-fuel ratio becomes rich, and the air-fuel ratio detection element is activated. When the electric power was higher than 500 millivolts, it was determined that the air-fuel ratio of the measured gas was on the rich side, and the intake system was controlled so that the air-fuel ratio became lean. [Problems to be Solved by the Invention] However, when this type of air-fuel ratio detection element is used for a long time, the surface of the porous electrode layer that is exposed to the exhaust gas of the air-fuel ratio detection element for a long time has a small amount of enzyme. By being exposed to exhaust gas for a long time,
Oxygen adsorbed on the surface of the porous electrode layer is separated and reduced. This activates the surface of the porous electrode layer that is exposed to exhaust gas, which reduces the electromotive force near the stoichiometric air-fuel ratio and reduces the electromotive force when the air-fuel ratio shifts from the lean side to the rich side. Rise time becomes longer. When the air-fuel ratio detection element is used for a long time, the electromotive force near the stoichiometric air-fuel ratio decreases, and the air-fuel ratio control point moves from the lean side to the rich side. In addition, if the air-fuel ratio detection element is used for a long time, the rise time of the electromotive force when the air-fuel ratio changes from the lean side to the rich side becomes longer. Assuming that the fall time of the air-fuel ratio detection element does not change, the time when the electromotive force of the air-fuel ratio detection element generates a value lower than 500 mV is determined from the ratio of the time when the electromotive force generates a value lower than 500 mV and the time when the electromotive force generates a value higher than 500 mV. ratio becomes longer. In other words, if the air-fuel ratio detection element is used for a long time, the rise time of the electromotive force becomes longer, and the time during which the electromotive force of the air-fuel ratio detection element is less than 500 mV becomes relatively long, so the air-fuel ratio control point is shifted from the lean side. Move to Rich's side. For this reason, when installing the air-fuel ratio detection element, a delay circuit is installed in the oxygen concentration detection circuit that controls the air-fuel ratio so that the electromotive force shows an appropriate value after long-term use. Measures were taken such as shifting to the lean side. As a result, the air-fuel ratio is controlled on the lean side when installed. When the air-fuel ratio is controlled to the lean side, the amount of nitrogen oxide (NOx) emitted increases. When the air-fuel ratio detection element is used for a long time, the air-fuel ratio is controlled to the rich side and the amount of nitrogen oxide decreases. However, when the air-fuel ratio is controlled to the rich side, unburned gas (CO , HC) increases. As described above, the conventional air-fuel ratio detection element has a problem in that when used for a long time, the control point moves significantly, resulting in a large change in the amount of nitrogen oxide and unburned gas discharged. The present invention was made in view of the above circumstances, and
The purpose is to provide an air-fuel ratio detection element that does not move its control point and does not change the amount of nitrogen oxide and unburned gas discharged even after long-term use. [Means for Solving the Problems] In order to achieve the above object, the present invention includes an oxygen ion conductive solid electrolyte provided on a measurement gas contact surface that contacts a measurement gas and an atmosphere contact surface that contacts the atmosphere; A porous electrode on the measurement gas side provided on the measurement gas contact surface and an atmosphere side porous electrode provided on the atmosphere contact surface, between the measurement side porous electrode and the atmosphere side porous electrode. In the air-fuel ratio detection element that generates an electromotive force according to the air-fuel ratio, the value of the electromotive force when the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio becomes 500 millivolts or less, and when the air-fuel ratio increases from the lean side to the rich side. The rise time of the electromotive force when the air-fuel ratio shifts from the rich side to the lean side is determined, and the rise time is calculated by combining the rise time and the rise time. The technical means is that the surface of the porous electrode on the measurement gas side is subjected to a reduction treatment so that the value divided by the time obtained by adding the fall time is 0.32 or more. [Function] By reducing the surface of the porous electrode on the measurement gas side so that the electromotive force value when slightly richer than the stoichiometric air-fuel ratio is 500 mV or less, the stoichiometric air-fuel ratio can be maintained even after long periods of use. Since there is almost no drop in electromotive force in the vicinity, there will be no major fluctuations in the control point even if used for a long time. By reducing the surface of the porous electrode on the measurement gas side so that the value obtained by dividing the rise time by the sum of the rise time and the fall time is 0.32 or more, the rise time is maintained even after long periods of use. Since there is almost no variation in time and fall time, there is no large variation in control points even after long-term use. [Effects of the Invention] According to the present invention, even if the air-fuel ratio detection element is used for a long time, there is little variation in the control point, so the amount of nitrogen oxide and the amount of unburned gas emitted by a combustion device such as an internal combustion engine can be reduced. changes can be kept small over a long period of time. Therefore, the amount of nitrogen oxide and unburned gas discharged can be kept within an appropriate range over a long period of time from the initial stage when the air-fuel ratio detection element is installed in the combustion apparatus. [Example] Next, the air-fuel ratio detection element of the present invention will be described based on an example shown in the drawings. FIG. 1 shows an oxygen sensor equipped with an air-fuel ratio detection element according to the present invention. The oxygen sensor 100 includes an air-fuel ratio detection element 200
and a mounting bracket 300 for mounting the sensor part α of the air-fuel ratio detection element 200 into an exhaust pipe (not shown).
When this mounting bracket 300 is attached, the sensor part α
A hard hat 400 that protects the air-fuel ratio detection element 20
and an output extraction section 500 for extracting the electromotive force generated at zero. The air-fuel ratio detection element 200 includes an oxygen ion conductive solid electrolyte 210. In this embodiment, the oxygen ion conductive solid electrolyte 210 is formed by sintering zirconia stabilized with yttrium oxide, magnesia oxide, calcium oxide, etc., and one end on the sensor part α side is closed, and the middle part is closed. Flange 2 on the outer periphery
It has a cylindrical shape with a bottom. The sensor part α of this oxygen ion conductive solid electrolyte 210 has a measurement gas contact surface 2 whose outer surface is in contact with the measurement gas.
12. The inner surface is provided with an atmosphere contact surface 213 that is in contact with the atmosphere. This measuring gas contact surface 212 is coated with a thick film of approximately 1 to
A 2μ porous electrode 220 on the measuring gas side (for example, a porous platinum electrode) is provided. Further, the surface of the measurement gas side porous electrode 220 is coated with an electrode protection coat (not shown) that protects the measurement gas side porous electrode 220 from exhaust gas. In addition, the atmosphere contact surface 213 is also coated with an electroplating method.
A porous electrode (e.g. porous platinum electrode) on the atmosphere side of approximately 1 to 2 μm is formed using thick film technology such as chemical plating method.
0 is set. The mounting bracket 300 has a cylindrical shape, and has a mounting screw portion 311 on its outer periphery for screwing into a mounting hole provided in an exhaust pipe, and a step 312 on the mounting surface of the mounting screw portion 311 for the gasket 301. The bolt part 313 has a large diameter and has a hexagonal bolt shape on the outer periphery to lock a spanner, plug wrench, etc.
, a caulking part 314 that fixes one end of the output extraction part 500 inside, and a hard hat attachment part 315 to which the hard hat 400 is attached at the end of the mounting screw part 311 .
Equipped with. Furthermore, steps 321 and 322 that support the air-fuel ratio detection element 200 are provided on the inner periphery of the mounting bracket 300. The step 321 supports the flange 211 of the oxygen ion conductive solid electrolyte 210 via the metallic sealing material 302.
The large diameter portion 304 of the spacer 303 placed between the air-fuel ratio detection element 200 and the mounting bracket 300 is supported together with the caulked portion 314 . Furthermore, an insulating filler 305 is filled between the air-fuel ratio detection element 200, the mounting bracket 300, and the spacer 303. As a result, the sealing material 302 of the flange 211
Since the air-fuel ratio detection element 200 is supported by the insulating filler 305 and the insulating filler 305, the air-fuel ratio detection element 200 is fixed within the mounting bracket 300. The purpose of the protective cap 400 is to protect the sensor part α from collisions of high-temperature gas when the oxygen sensor 100 is installed in the exhaust pipe, and to protect the sensor part α before the oxygen sensor 100 is installed in the exhaust pipe. It has a cylindrical shape that covers the sensor section α. This protective cap 400 has a plurality of holes 401
The exhaust gas passing through the exhaust pipe is brought into contact with the surface of the measurement gas side porous electrode 220. The output extraction section 500 includes an inner case 510 whose one end is supported between the spacer 303 and the caulking section 314, an intermediate case 520, and an intermediate case 520.
and an outer case 530 that is fitted around the outer periphery of the outer case 530. A resin grommet 531 and a coil spring 532 are arranged on the inner periphery of the outer case 530. When the outer case 530 is fitted around the outer periphery of the intermediate case 520, one end of the coil spring 532 comes into contact with the end of the inner case 510 and biases the grommet 531 against the end of the outer case 530. Furthermore, an output lead wire 540 is arranged on the inner periphery of the grommet 531. A connecting fitting 541 that is fitted into the inner periphery of the oxygen ion conductive solid electrolyte 210 and electrically connects the output wire 540 and the atmospheric-side porous electrode 230 is attached to the end of the output wire 540. has been done.
Note that this connection fitting 541 is connected to the air-fuel ratio detection element 2.
It is provided so that the atmosphere is introduced into the interior of 00. The oxygen sensor 100 provided in this way is
For example, it is attached to the exhaust pipe of a car's internal combustion engine,
When the air-fuel ratio of this internal combustion engine is rich, approximately
It generates an electromotive force of 1000 millivolts (mV), and when the air-fuel ratio is lean, it generates a slight electromotive force of less than 100mV. An example of a method for controlling the air-fuel ratio of an internal combustion engine using this oxygen sensor 100 will be shown. oxygen sensor 10
When the electromotive force at zero is 500 mV or more, it is determined that the air-fuel ratio of the internal combustion engine is rich, and the amounts of fuel and combustion air are adjusted in the intake system so that the air-fuel ratio shifts to the lean side. Conversely, when the electromotive force of the oxygen sensor 100 is 500 mV or less, it is determined that the air-fuel ratio of the internal combustion engine is lean, and the amounts of fuel and combustion air are adjusted to the intake air so that the air-fuel ratio shifts to the rich side. Adjust by system. As a result, the air-fuel ratio of the internal combustion engine repeats lean and rich, and the stoichiometric air-fuel ratio (λ=
1) Controlled nearby. Next, a method for manufacturing the air-fuel ratio detection element 200 will be explained. Add an appropriate amount of yttrium oxide, a stabilizing agent, to zirconium oxide, wet mix for 40 hours, for example, in a ball mill using alumina cobblestones, and then
Temporary sintering is performed at 1300°C for 2 hours. Next, this is processed again using a ball mill using alumina cobblestones, for example, 40
The mixture is wet-pulverized for a period of time, an appropriate amount of an organic binder is added thereto, and powder is dried using a spray dryer or the like. Then, this is press-molded into a bottomed cylindrical shape and fired in the atmosphere to form an oxygen ion conductive solid electrolyte 210. Next, this oxygen ion conductive solid electrolyte 210
A porous platinum electrode is provided on the inner and outer surfaces of the measuring gas side porous electrode 220 by a plating method.
and an atmosphere-side porous electrode 230. Next, an electrode protective coat is formed on the surface of the porous electrode 220 on the measurement gas side by a plasma spraying method. Next, the air-fuel ratio detection element 200 thus formed is placed in a hydrogen furnace and subjected to a reduction treatment at 750° C. for 4 hours. By this reduction treatment, the porous electrode 2 on the measurement gas side
The enzyme adsorbed on the surface of 20 is separated. Next, the air-fuel ratio detection element 20 that has undergone the reduction process
The oxygen sensor 100 using the oxygen sensor 100 is attached to a propane injection type electromotive force measuring device 600 shown in FIG. 2, and the electromotive force of the oxygen sensor 100 is measured. The electromotive force measuring device 600 performs combustion in the main burner 610 at a slightly lean air-fuel ratio of λ=1.02, and causes propane gas to flow into the auxiliary burner 620 downstream thereof, so that the air-fuel ratio in the portion where the oxygen sensor 100 is arranged is slightly lean. It is set so that a rich λ=0.95. And oxygen sensor 1 when air fuel ratio λ = 0.95
Air-fuel ratio detection elements 200 whose electromotive force at 00 is 500 mV or less are selected. In addition, in this example, an example was shown in which the value of the air-fuel ratio at which the electromotive force of the oxygen sensor 100 is 500 mV or less was selected at λ=0.95, but in this example, λ=0.95 is selected.
This is because 0.95 was most suitable for the internal combustion engine used to measure nitrogen oxide (NOx), and the setting of the value of the air-fuel ratio λ at which the electromotive force of the oxygen sensor 100 is 500 mV or less is suitable for the internal combustion engine in which the oxygen sensor 100 is used. It is necessary to set it optimally according to the control method. In other words, if the air-fuel ratio requires richer control, set the value of the air-fuel ratio λ that is 500 mV or less to a low value, and if you do not require so rich control, set the value of the air-fuel ratio λ that makes the air-fuel ratio 500 mV or less. Set to a large value. It is desirable to use an oxygen sensor 100 suitable for the internal combustion engine used. In addition, the air-fuel ratio λ is 500mV or less.
The variable range of the value is 0.91 to 0.99. Next, oxygen sensor 1 selected as above
00 is attached to a device (not shown) that changes the air-fuel ratio stepwise between a rich side (λ=0.9) and a lean side (λ=1.1) using propane gas as fuel.
Then, when the value of the air-fuel ratio is changed from 0.9 to 1.1, the rise time TLR required for the electromotive force to change from 300mV to 600mV is calculated, and when the value of the air-fuel ratio is changed from 1.1 to 0.9, the rise time TLR is calculated. Find the fall time TRL required for the power value to change from 600mV to 300mV. Then, let the rise time TLR be the rise time
The air-fuel ratio detection element 200 whose value obtained by adding TLR and fall time TRL divided by the time is 0.32 or more is selected. FIG. 3 shows the relationship between the usage time of the oxygen sensor 100 and the electromotive force of the oxygen sensor 100 when the air-fuel ratio λ=0.95. The solid line A in FIG. 3 shows an example in which the electromotive force after reduction treatment, which is a condition of the present invention, is 500 mV or less, and the broken line in FIG. 3 shows an example in which the electromotive force after reduction treatment is 500 mV or more. As shown in the dashed line, the electromotive force after reduction treatment is 500
If the voltage is higher than mV, the reduction treatment has not been sufficiently performed, and the electromotive force will fluctuate greatly after long-term use. Therefore, the amount of nitrogen oxide and unburned gas discharged varies greatly between the initial stage when the oxygen sensor 100 is attached to the piping of the internal combustion engine and the time after it has been used for a long time. On the other hand, in the case of the solid line (A) that satisfies the present invention, the electromotive force does not change much between the initial stage and after long-term use, so there is no large change in the amount of nitrogen oxide and unburned gas discharged. Also, the oxygen sensor 10 when the air-fuel ratio λ = 0.95
By setting the zero electromotive force to 500 mV or less, the air-fuel ratio control point can be controlled slightly to the rich side, so the amount of oxygen and nitrogen discharged can be kept low. FIG. 4 shows the relationship between the usage time of the oxygen sensor 100 and the value obtained by dividing the rise time TLR by the sum of the rise time TLR and the fall time TRL. The solid line C in Figure 4 shows the rise time TLR, which is a condition of the present invention, and the rise time TLR and fall time.
An example is shown in which the value divided by the time added to TRL is 0.32 or more, and the broken line D in FIG. An example where the divided value is 0.32 or less is shown. As shown by the broken line B, if the value after the reduction treatment is 0.32 or less, the reduction treatment has not been sufficiently performed, and this value will fluctuate greatly after long-term use. For this reason, the amount of oxygen, nitrogen, and unburned gas discharged varies greatly between the initial stage when the oxygen sensor 100 is attached to the piping of the internal combustion engine and the time after long-term use. On the other hand, in the case of the solid line (c) that satisfies the present invention, this value does not change much between the initial stage and after long-term use, so there is no large change in the amount of nitrogen oxide and unburned gas discharged. Table 1 shows the long-term use of air-fuel ratio detection elements 200 (A in Table 1) that satisfy the above conditions and air-fuel ratio detection elements 200 (B in Table 1) that do not satisfy one or both of the above conditions. A comparison between before and after use is shown.

[Table] C shown in Table 1 is the electromotive force (mV) of the air-fuel ratio detection element 200 measured by the electromotive force measuring device 600,
D is the value obtained by dividing the rise time TLR by the sum of the rise time TLR and the fall time TRL, and E
is the initial nitrogen oxide (NOx) emission amount when the air-fuel ratio detection element 200 is attached to the exhaust pipe of an internal combustion engine, and F is the oxidation amount after 1000 hours of operation when the air-fuel ratio detection element 200 is attached to the exhaust pipe of the internal combustion engine. The nitrogen emission amount, G, indicates the difference between E and F (the amount of change in nitrogen oxide emission after long-term use). As shown in Table 1 above, after the reduction treatment, the air-fuel ratio λ
= 0.95, the electromotive force of the oxygen sensor 100 (air-fuel ratio detection element 200) is 500 mV or less, and the value obtained by dividing the rise time TLR by the sum of the rise time TLR and the fall time TRL is 0.32 The air-fuel ratio detection element 200 that satisfies the above requirements can suppress changes in the amount of nitrogen oxide emissions even after long-term use. According to the present invention, changes in nitrogen oxide emissions can be kept low even when used for a long time, so
Changes in the amount of unburned gas emissions can also be kept low even after long periods of use. Therefore, the internal combustion engine can be operated at an appropriate air-fuel ratio for a long period of time from the initial stage when the oxygen sensor 100 equipped with the air-fuel ratio detection element 200 to which the present invention is applied is attached to the exhaust pipe of the internal combustion engine. Nitrogen oxides and unburned gases emitted by the engine can be kept within appropriate limits. In the above embodiment, a hydrogen furnace is used as the reduction processing means for the air-fuel ratio detection element 100, but if a long aging time is acceptable, the reduction processing may be performed using a gas burner. In addition, when performing reduction treatment in a hydrogen furnace, for example, aluminum, titanium, chromium, copper, iron, nickel, etc.
The reduction effect can be increased by adding a substance that adsorbs oxygen. Further, although an example was shown in which reduction treatment was performed in a hydrogen furnace at 750°C, similar reduction treatment can be performed at 700°C or higher if a substance that adsorbs oxygen as described above is added. Note that if the treatment temperature is too high, the quality of the oxygen ion conductive solid electrolyte 210 will change.
Preferably below 850℃.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the oxygen sensor, Figure 2 is a schematic diagram of a propane injection type electromotive force measurement device, and Figure 3
The figure shows the usage time of the oxygen sensor and the air-fuel ratio λ = 0.95
A graph showing the relationship between the electromotive force of the oxygen sensor and the electromotive force of the oxygen sensor when It is a graph. In the figure, 100...Oxygen sensor, 200...Air-fuel ratio detection element, 210...Oxygen ion conductive solid electrolyte, 212...Measurement gas contact surface, 213...
Atmospheric contact surface, 220...Measurement gas side porous electrode,
230...Atmospheric side porous electrode.

Claims (1)

[Scope of Claims] 1. An oxygen ion conductive solid electrolyte comprising a measurement gas contact surface with which the measurement gas contacts and an atmosphere contact surface with which the atmosphere contacts, and a measurement gas side porosity provided on the measurement gas contact surface. an air-fuel ratio detection device comprising: an electrode; and an atmosphere-side porous electrode provided on the atmosphere contact surface, and generating an electromotive force according to the air-fuel ratio between the measurement-side porous electrode and the atmosphere-side porous electrode. In the element, the value of the electromotive force when the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio is 500 millivolts or less, and the rise time of the electromotive force when the air-fuel ratio shifts from the lean side to the rich side; The falling time of the electromotive force when the air-fuel ratio shifts from the rich side to the lean side is determined, and the value obtained by dividing the rising time by the sum of the rising time and the falling time is: 0.32
As described above, the air-fuel ratio detection element is characterized in that the surface of the porous electrode on the measurement gas side is subjected to a reduction treatment. 2. The air-fuel ratio detection element according to claim 1, wherein the air-fuel ratio is on the slightly rich side when the air-fuel ratio is 0.95. 3 The above rise time is determined by adjusting the air-fuel ratio value from 0.9.
1.1, the time required for the value of the electromotive force to change from 300 millivolts to 600 millivolts, and the falling time is the time required for the value of the air-fuel ratio to change from 300 to 600 millivolts.
When changing from 1.1 to 0.9, the value of the electromotive force is
The air-fuel ratio detection element according to claim 1 or 2, characterized in that it is the time required to change from 600 millivolts to 300 millivolts.