JPS62245148A - Oxygen concentration sensor - Google Patents

Abstract

PURPOSE:To averagely detect the concn. of oxygen, by mounting an oxygen concn. sensor element and a heat generator and covering the oxygen concn. sensor element with a substance occluding and releasing oxygen and a catalytic component. CONSTITUTION:A heater is provided in the zirconia element 4 of an oxygen concn. sensor element and a lead wire 6 is connected to said heater 5. an exhaust side electrode, a spacer and a surface cover layer are successively provided to the inside of the zirconia element 4 from the inside surface thereof and an atmospheric side electrode is provided to the outside thereof and an output lead wire 7 is connected to said electrode. The surface cover layer is constituted of a substance occluding and releasing oxygen and a substance containing a catalytic component. As the surface cover layer, one formed by coating cerium with platinum, palladium or rhodium being the catalytic component is used. Because the element 4 is covered with the substance occluding and releasing oxygen to occlude and release oxygen with respect to an exhaust gas atmosphere varying with the elapse of time and the component to be purified in exhaust gas is eliminated by the catalytic component, the average concn. of oxygen can be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oxygen concentration sensor, and more particularly to an oxygen concentration sensor used to control the exhaust gas of an internal combustion engine to a stoichiometric air-fuel ratio.

[Conventional method] Detects the oxygen concentration in the exhaust gas from the internal combustion engine of an automobile, etc., and controls the amount of air and fuel supplied to the internal combustion engine based on the detected value, thereby reducing fuel consumption and reducing the fuel consumption of the internal combustion engine. Oxygen concentration sensors are used to purify exhaust gas. Additionally, a three-way catalyst for exhaust gas purification is usually used together with the oxygen concentration sensor.

[Problem that the invention seeks to solve]

However, the oxygen concentration sensors conventionally used in three-way catalyst systems have different response speeds to changes in the exhaust gas atmosphere (from rich to rich or from rich to rich). Furthermore, this response speed is affected by the type of oxygen concentration sensor, product variations, operating temperature, and changes over time.

However, due to the influence of the response speed, the average air-fuel ratio of the exhaust gas flowing into the three-way catalyst deviates from the stoichiometric air-fuel ratio (A/F). Currently, as a countermeasure for this, for example, when switching from rich to rich, electrical delay circuits are used to compensate for this.
Although efforts are being made to equalize the two response speeds,
Control is complicated because a delay circuit is required, and reliability and accuracy are also insufficient.

As a result of various studies, the inventors of the present invention came to the conclusion that there is no prospect of a solution as long as the oxygen concentration sensor is made to respond at high speed and its output is directly used for A/F control. Ta.

Needless to say, the ideal oxygen concentration sensor is the theoretical A/F.
Since the output signal changes greatly after
In fact, if the oxygen concentration sensor can be made close to ideal, its output level and, by extension, the A/F control level should improve.

In other words, the conditions for using the oxygen concentration sensor in an ideal state are: (1) Among the exhaust gas of the internal combustion engine, at least the gas that comes into contact with the oxygen 6 degree sensor must reach chemical equilibrium; (2) The speed of atmospheric fluctuation is sufficiently slower than the response speed of the center and the center. Regarding (1), it can be solved by, for example, placing a catalyst on the surface of the oxygen concentration sensor (the part that comes into contact with exhaust gas). However, regarding (2), it cannot be solved by the conventional technology.

The present invention is intended to solve the above-mentioned problems in the prior art, and its purpose is to detect the average oxygen concentration by absorbing and releasing oxygen according to changes in the exhaust gas atmosphere. The purpose of this invention is to provide an oxygen concentration sensor that can

[Means for solving problems]

That is, the oxygen concentration sensor of the present invention includes an oxygen concentration sensor element and a heating element that heats the element, and a substance that absorbs and releases oxygen and a catalyst component on the surface of the element that comes into contact with the gas to be measured. It is characterized by being coated with.

As an oxygen concentration sensor element, an oxygen concentration battery type oxygen sensor using a solid electrolyte such as cubic zirconia can be used. An oxygen concentration sensor element, or an oxygen concentration sensor element that uses a semiconductor such as titania and whose resistance changes depending on the oxygen concentration, can be used. Among the components, components whose concentration changes greatly depending on A/F, that is, substances that can store and release oxygen, such as cerium oxide, are used as catalyst components, such as noble metals (
The structure includes a heater such as a ceramic heater having a predetermined shape such as a rod shape, and a heating element capable of heating the oxygen concentration sensor element to a predetermined temperature.

Specifically, for example, electrodes made of porous platinum are provided on the outside (exhaust side) and inside (atmosphere side) of a cylindrical cubic zirconia element with one end closed, and then electrodes made of porous platinum are placed on the surface of the exhaust side electrode. Examples include those coated with a catalyst layer prepared by supporting palladium and cerium oxide on alumina powder, and equipped with a ζ-layer inside a cylinder made of zirconia. Further, the shape of the sensor element may be a plate shape such as a disk shape, and is not particularly limited.

Needless to say, other than Zilphnia sensor! The same effect can be obtained by performing the same processing as above on an I2 elemental pressure sensor, for example, a titania sensor.

When coating the sensor surface with a catalyst layer, it may be applied directly, but it is preferable to coat the sensor surface with a suitable separator interposed therebetween. The role of this separator is to prevent deterioration in performance due to reaction between catalyst components and the element or electrode of the oxygen concentration sensor during use. Therefore, the components of the separator include, for example, metal oxides such as spinel (MgAl Snow 04), zirconia (ZrOz
) or corundum (tt-klxOs), which are chemically stable against high temperatures and various exhaust gas compositions, are desirable, and also allow the gas that has passed through the catalyst layer to quickly reach the sensor element surface. Therefore, it needs to be porous. The thickness of the separator is 0 to 20
0 pm, preferably 20 to 50 μm. 2
At 0.00 pm or higher, gas permeation deteriorates, although it depends on the material used as the separator. In addition, the particle size of the separator component that determines the porosity of the separator is 1
105 to 1 μm is good. If the particle size is 105 pm or more, gas permeation will deteriorate, and if the particle size is 1 pm or more, the catalyst component will enter the pores, resulting in a loss of effectiveness as a separator.

Substances that store and release oxygen other than ceria (Ce02) include, for example, nonstoichiometric compounds such as vanadium oxide and cobalt oxide, rare earth oxides, and defective fluorite-type structures such as zirconia added with calcium, yttrium, etc. It may also be a compound. The physical shape of these substances allows exhaust gas to quickly pass through to the electrodes.
In addition, it is necessary that gas exchange be easy, and the particle size IIL is preferably 1 to 5 pm and the thickness of the coating layer is 20 to 200 pm, depending on the target engine control system. If this particle size cannot be achieved with the materials described above, the materials may be formed on a suitable chemically stable material, such as a powder of theta-alumina, zirconia, spinel, etc. In addition, since the gas exchange rate is slow with only substances that have the effect of occluding and desorbing, it is recommended to support IA (Pt%Pd, Ph, or a combination of two or more types) as a catalyst component. It is necessary. By catalyzing the gas in this manner, there is also the effect of bringing the gas reaching the sensor element into a chemically balanced state.

The amount of supported catalyst components is determined by
1.5wt% is good.

The above powder is coated on the electrodes of the sensor element or on the separator using a suitable sintering aid (aluminum nitrate, etc.) if necessary, and baked.

The type of heating element such as a heater attached to the sensor does not matter, but it must be capable of heating both the sensor element and the coated substance to 500°C or higher, preferably 600°C to 800°C. Preferably, a heating element such as a platinum wire that exhibits a positive resistance change with temperature is used. This has the advantage of being able to be set at a nearly constant temperature by using a constant voltage.

The method of using the oxygen concentration sensor of the present invention (hereinafter referred to as average oxygen concentration sensor) is as follows. There are two methods: using the oxygen concentration sensor alone and using it in combination with a conventional oxygen concentration sensor.

In the latter method, it is preferable to use the conventional oxygen concentration sensor for main control and correct the response deviation of the conventional oxygen concentration sensor based on the signal of the average oxygen concentration sensor of the present invention. The maximum correction delay time is preferably about 200m5. FIG. 10 shows a schematic configuration diagram of an apparatus for correcting response deviation of a conventional oxygen concentration sensor using an average oxygen concentration sensor.

In FIG. 10, rich/lean conditions are determined based on the signal from the conventional oxygen concentration sensor, and the results are used for various air-fuel ratio controls (for example, the three-way solenoid valve 13 in FIG. 8). As things stand, the drawbacks of conventional oxygen concentration sensors (
This is corrected by adding an appropriate delay to the above output using the average oxygen concentration sensor.

That is, the direction and magnitude of the deviation in the average air-fuel ratio are determined using a rich/lean discriminator, an integrator, and a delay direction discriminator with respect to the output signal of the average oxygen concentration sensor, and the results are also used. In addition, an appropriate delay is applied to the output of the conventional sensor mentioned above.

Needless to say, this device can be applied to air-fuel ratio compensation circuits using other general oxygen sensors. Further, FIG. 11 shows the voltage waveforms of the signals A%B and C in FIG. @10, and the response deviation of the delay circuit of the conventional oxygen concentration sensor is corrected by the signal C. It is also possible to apply the signal of the average oxygen concentration sensor to a rich/lean discriminator of a conventional sensor. In other words, the rich/lean discriminator discriminates between rich and lean based on a certain voltage value in the case of a zirconia sensor and a certain resistance value in the case of a titania sensor. Even if the output is changed according to the output of the delay circuit, the same effect as when using a delay circuit can be obtained. Although this method has a slightly narrower control range, it has the advantage of increasing the overall control speed because it does not include a delay circuit. Of course, it may be used in combination with delay circuit control.

To use the average oxygen concentration sensor alone, for example, adjust the exhaust gas atmosphere from the internal combustion engine to be slightly rich using a capretor, etc., and introduce secondary air into the exhaust pipe, which corresponds to the theoretical A/F. There is a way to make it into exhaust gas. Specifically, air is intermittently injected from an exhaust pipe at a frequency of 1 to 10 Hz, and the time period of 08 hours is controlled by a sensor signal. FIG. 12 shows a schematic configuration diagram of the apparatus in this case. In FIG. 12, it is assumed that the exhaust gas discharged from the engine is slightly air-deficient (rich).

The solenoid valve is opened intermittently according to the signal from the oscillator (which generates a triangular wave), and air is introduced into the exhaust pipe in a pulsed manner. An average oxygen concentration sensor is installed downstream of this to detect the oxygen concentration of the exhaust gas. After shaping this output through a rich/lean discriminator and an integrator, a comparator 5 changes the time ratio of opening and closing of the solenoid valve. That is, if the average air-fuel ratio is rich, the time ratio for introducing air is increased, and if the average air-fuel ratio is leaf, the opposite effect is applied. Comparator 3 controls the triangular wave solenoid valve operation level.

In this way, the exhaust gas introduced into the catalyst can be brought to the stoichiometric air-fuel ratio.

Note that the method of using the average oxygen concentration sensor alone is not limited to this diagram. Further, the air inlet may be upstream of the sensor, for example, upstream of the engine. FIG. 15 shows the voltage waveforms of the signals B%D and E in FIG. 12. Note that the voltage waveform of the signal Ao is the same as that shown in FIG.

〔Example〕

The invention will be explained in further detail in the following examples. Note that the present invention is not limited to the following examples.

The inside (positive electrode: atmosphere side) and outside (negative +f&:
The exhaust side) is plated with platinum.

Sample 1 was prepared by plasma spraying spinel (MgA/sO+) on the outside of this element.

An aqueous solution of chloroplatinic acid and rhodium chloride was added to a powder of r-alumina ground to an average particle size of 1° pm or less using a ball mill, stirred to form a slurry, dried at 110°C for 10 hours, and then heated at 600°C for 5 hours. A catalyst powder was prepared by calcining in air for an hour. The amount of platinum supported is 2wt% based on the catalyst powder.
, the amount of rhodium supported was α2 wt%.

For 100 parts of the above catalyst powder, 70 parts of alumina sol (alumina content 10wt%), 3 parts of aluminum nitrate, 3 parts of water
0 part was added and stirred in a wet-type whole mill to prepare a slurry. □Immerse the outer part of sample 1 in this slurry,
Sample 2 was prepared by drying at 0''Q for 10 hours and then firing at 700 T! for a portion in a vacuum chamber.

A haladium nitrate aqueous solution was added to commercially available cerium oxide powder, stirred, and dried at 110°C for 10 hours.
It was baked at ℃ for 3 hours. Sample 3 was prepared by coating Sample 1 with this palladium-cerium 0 powder in the same manner as Sample 2. Similarly, Samples 4 and 5 were prepared by attaching a platinum heating element inside Samples 2 and 3. Note that the heater can heat the oxygen concentration sensor to about 700°C. FIG. 1 shows a cross-sectional view of the sensor element portion of the average oxygen concentration sensor (sample 3) of the present invention. In the diagram, 4
5 is a zirconia element, 5 is a heater, 6 is a heater lead wire, and 7 is a sensor output lead wire. Also, Figure 2 shows the first
An enlarged cross-sectional view of the portion (3) surrounded by a solid line in the figure is shown. In the figure, 8
is an atmosphere side electrode, 9 is an exhaust side electrode, 10 is a separator, 1
1 is a surface coating layer containing a substance that stores and releases oxygen and a catalyst component.

FIG. 3 shows the relationship between the air-fuel ratio (A/F = air weight I)/fuel weight I) and the purification rate of each harmful component in the exhaust gas. As is clear from Figure 21-3, the air-fuel ratio is adjusted to the stoichiometric air-fuel ratio (14,6
), the purification rate of the exhaust gas purification catalyst is improved overall.

Experiment 1゜A model gas that resembles the exhaust gas of an automobile internal combustion engine (first
(see table) were prepared, and the static characteristics of each of the oxygen concentration sensors of Samples 1 to 5 were measured at gas temperatures of 400°C and 600"C.

If S = 1, that is, the signal of the oxygen a degree sensor changes at the current point, as shown in Figure 4, the gas temperature is 400.
In the case of ℃, only samples 4 and 5 were used. Also, gas temperature 600
In the case of ℃, as shown in Fig. 5, the signal changed for samples 2, 5.4, and 5. Alternately play Q for 5 seconds each,
We looked at the responsiveness of the oxygen concentration sensor. The gas temperature was 500°C. As is clear from FIGS. 6 and 7, sample 1 does not provide a sufficient change in output. In addition, in samples 2 and 4, the proportion of time at the output level of α4■ or more (rich region) is not equal to that of 14V or less (lean region). That is, it can be seen that in such an oxygen concentration sensor, there is a difference in responsiveness from lean to rich and from rich to lean.

Therefore, when such an oxygen concentration sensor is used for A/F control, the average A/F cannot be adjusted to the theoretical A/F. It can be seen that samples 3 and 5 do not respond to gas fluctuations in this period.

Experiment 3 A device as shown in FIG. 8 was attached to the exhaust pipe of a gasoline engine (displacement approximately 1,500 cc). In FIG. In other words, if the signal from the oxygen concentration sensor 14 indicates that the exhaust gas is deficient (rich) in oxygen, the air in the air control pulp 18 is activated via the three-way solenoid valve 13. Negative pressure is introduced into the chamber and the valve opens. Since a constant air pressure is applied to the valve by the pressure regulating valve, an amount of air is introduced into the exhaust gas according to the opening degree of the valve.

Conversely, if the oxygen concentration sensor indicates that the oxygen concentration in the exhaust gas is excessive, the three-way solenoid valve 13 switches, and the air chamber of the air control valve 1B becomes atmospheric pressure, closing the valve and preventing air from being introduced into the exhaust gas. will be stopped. With such control,
The exhaust gas introduced into the three-way catalyst 16 is brought to a chemical concentration of 14 g near the theoretical equivalence point air-fuel ratio. However, it is assumed that the engine exhaust is always controlled to be slightly rich. Carburetor, 9! ! M was set at 200 Orpm, and the intake manifold pressure was -350 Hf. To this, the conventional acid of sample 4 = -j (o temperature sensor was attached, three-way catalyst 16
The pressure regulating valve was adjusted so that the NO and Co%HC emissions after passing were the lowest, and the manifold negative pressure was changed to -450 mm, Hg, and -250 imHg, and the purification rate was measured. The results are shown in Table 2.

Table 2 6 component purification rate (%) Negative pressure (Hg) No Co He As is clear from Table 2, in the case of -a50msHg, the No purification rate is low, and it is controlled on the lean side, and -250 m
In the case of iHg, the He purification rate is low and it can be seen that it is controlled on the rich side. The reason for this is that the average air-fuel ratio cannot be measured with conventional oxygen concentration sensors.

Experiment 4 A similar experiment was carried out by modifying the mounting part of the oxygen concentration sensor of the equipment in Experiment 3 as shown in Fig. 9. In other words, in Fig. 9, the oxygen concentration sensor of Fig. 8 was 14 and the amplifier 15 are connected to the oxygen concentration sensor 21 (conventional sensor, sample 4) and the oxygen concentration sensor 22 of the present invention (sample 3).
and changed to computer. The contents of the computer are shown in FIG. The results are shown in Table 3.

Negative pressure (mHg) NOCo HC From the results in the table above, both No and H
It can be seen that both C purification rates are high, and that they are operating under conditions extremely close to the theoretical A/F.

Experiment 5 Oxygen concentration sensor samples 4 and 5 were installed in the apparatus shown in Figure 12.
was installed, and an experiment similar to Experiment 5 was conducted. The results are shown in Table 4.

4th. & Purification rate of each component (H) Table 4 shows that when using the average oxygen concentration sensor of the present invention, compared to when using a conventional oxygen concentration sensor, N
Both O and HC purification rates are improved, and extremely accurate air-fuel ratio control can be performed.

〔Effect of the invention〕

As mentioned above, the concentration sensor of the present invention is an oxygen concentration sensor? It is equipped with an element and a heating element that heats the element, and the surface of the element that comes into contact with the gas to be measured is coated with a substance that absorbs and releases oxygen and a catalyst component. It stores and releases oxygen in the fluctuating exhaust gas atmosphere,
In addition, by eliminating the components to be purified in the exhaust gas by the action of the catalyst component, the average oxygen concentration can be measured without being affected by atmospheric fluctuations.

Therefore, the gas components that reach the part of the sensor element that detects the oxygen concentration have a chemically equilibrium composition, and the rate of fluctuation of the components is slow enough for measurement. Operates ideally and provides accurate A/
It can be used as a reference signal for F control.

As a result, compared to the conventional method, it is possible to perform A/F control with less change over time and higher accuracy, and harmful components in exhaust gas can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a sensor element of an embodiment of the oxygen concentration sensor of the present invention, FIG. 2 is an enlarged sectional view of a portion H surrounded by a solid line in FIG. 1, and a graph showing the relationship with the conversion rate. Figure 4 is a graph showing the relationship between the amount of oxygen used/required oxygen by various oxygen concentration sensors and the sensor output when the exhaust gas salt is 400'Q, and Figure 5 is the graph showing the relationship between the amount of oxygen used/the amount of oxygen required by various oxygen concentration sensors and the sensor output when the exhaust gas salt is 400'Q. Figures 6 and 7 are graphs showing the relationship between required oxygen amount and sensor output. Figures 6 and 7 show the output of various oxygen concentration sensors when exhaust gas with an oxygen concentration of 18% and 1.0- Graph showing changes; Figure 8 is a schematic configuration diagram of an example of an oxygen concentration sensor performance evaluation device; Figure 9 is a schematic configuration diagram of the oxygen concentration sensor mounting part of a device that is an improved version of the device in Figure 8; The figure is a schematic configuration diagram of a device for correcting the response deviation of a conventional oxygen concentration sensor using the oxygen concentration sensor of the present invention. Figure 11 is a voltage waveform diagram of signals A, B, and C in Figure 10. , 912 is a schematic configuration diagram of an apparatus for controlling the properties of exhaust gas using the oxygen concentration sensor of the present invention, and FIG. 13 is a voltage waveform diagram of signals B%D and E in FIG. 12. In the figure, 4... Zirconia element 5... Heater 8... Atmospheric side electrode? ...Exhaust side electrode 10...Separator 11...Surface coating layer (1 other person) Sai1 Izu Fang 2 Figure 5 - Sweater Tatami / Katosan ↑l Fang 5 diagram (t! for @ elemental quantity / Atsaiyu l Fang 6 diagram Haru Ma (scale) Spear 8 diagram

Claims (4)

[Claims] (1) It comprises an oxygen concentration sensor element and a heating element that heats the element, and is characterized in that the surface of the element that comes into contact with the gas to be measured is coated with a substance that absorbs and releases oxygen and a catalyst component. Oxygen concentration sensor. (2) The oxygen concentration according to claim 1, wherein the substance that absorbs and releases oxygen is composed of at least one selected from cerium oxide, vanadium oxide, cobalt oxide, or defective fluorite-type compounds. sensor. (3) The oxygen concentration sensor according to claim 1, characterized in that a substance that stores and releases oxygen and a catalyst component are coated with an inorganic porous separator interposed therebetween. (4) The oxygen concentration sensor according to claim 1, wherein the catalyst component is made of at least one of platinum, palladium, and rhodium.