JP2664319B2 - Air-fuel ratio sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for an air-fuel ratio control device of an internal combustion engine, and particularly to a wide range of air-fuel ratios in three states of a rich region, a stoichiometric air-fuel ratio and a lean region. The present invention relates to an air-fuel ratio sensor for an automobile that can be detected at a time. 2. Description of the Related Art In an internal combustion engine, a region where the excess air ratio λ is λ <1 (rich region), λ = 1 (theoretical air-fuel ratio), and a region where λ> 1 (lean region) ), And it is required that a single air-fuel ratio sensor detects a wide range of air-fuel ratios from a rich region to a lean region. On the other hand, the relationship between the concentration of residual oxygen in exhaust gas and the concentration of carbon monoxide with respect to the air-fuel ratio or excess air ratio λ is shown in FIG.
In the (O ₂ ) concentration and rich region, the concentration of carbon monoxide (CO) as an unburned gas changes almost linearly in accordance with the air-fuel ratio. FIGS. 12A to 12C show the basic principle of a conventional air-fuel ratio sensor for individually detecting the air-fuel ratio in each region by using the residual oxygen concentration and the concentration of carbon monoxide. The air-fuel ratio sensor includes an electrode 1, a zirconia solid electrolyte 2, an electrode 3, a protective film 4, and an ammeter 5. FIG. 12 (A) shows an example in which an electrode 1 is used as a cathode and an electrode 3 is used as an anode, and an excitation voltage E of about 0.5 volt is applied between the two electrodes as known in Japanese Patent Application Laid-Open No. 53-66292. Thus, a rich region where λ <1 is detected. That is,
The protective film 4 functions as a gas diffusion resistor, and an oxygen gas that burns and reacts with unburned gas such as carbon monoxide diffused in the protective film 4 to the electrode 3 portion from the electrode 1 portion in contact with the atmosphere to the electrode 3 portion. , In the zirconia solid electrolyte 2 in the form of oxygen ions. Therefore, the pump current I _P measured by the ammeter 5 is the amount of oxygen ions transferred from the electrode 1 to the electrode 3 and corresponds to the amount of unburned gas diffusing in the protective film 4 to the electrode 3. it is intended to analog detect the air-fuel ratio of the rich region from the I _P value. As shown in FIG. 12B, when an electromotive force e _λ between the two electrodes is detected with reference to the electrode 3 in contact with the exhaust atmosphere via the protective film 4, the value of the e _λ becomes theoretically empty. ratio of about 1 volt, because the changes stepwise, are known for, for example, JP 47-37599 can be substantially digitally detect lambda = 1 from e _lambda value. [0007] As shown in FIG.
When applying an excitation voltage E of approximately 0.5 volts between the electrodes as a cathode, the oxygen ions are pumped from 3 electrodes to the electrode part, the pump current I _P by the ammeter 5 is measured.
The pumping current I _P via the protective film, thus corresponding to the amount of oxygen diffusing into 3 parts electrodes, it can be detected with this I from _P value lambda> 1 the lean region, for example, JP 52-69690
No. known. FIG. 13 shows an example of the characteristics of the conventional air-fuel ratio sensor shown in FIGS. 12 (A) to 12 (C). The characteristic in the lean region is indicated by a dashed line, the characteristic in the rich region is indicated by a dotted line, and the detection characteristic of the stoichiometric air-fuel ratio point is indicated by a solid line. As described above, it is known that each area is individually detected, but a configuration for smoothly detecting a wide range of air-fuel ratios by a consistent method has not yet been proposed. Since FIG. 12 (B) is not based on the principle based on diffusion control, the degree of gas diffusion resistance of the protective film 4 in FIG. 12 (B) is formed smaller than in FIGS. 12 (A) and 12 (C). In general, the thickness of the protective film 4 in FIG.
It is formed thin. Further, it is known that an air-fuel ratio can be detected in an analog manner from a terminal voltage generated between both electrodes by exciting a constant current between the electrodes, for example, in Japanese Patent Application Laid-Open Nos. 55-62349 and 55-1544.
Known as No. 50. It is shown that by switching the polarity of both electrodes, the air-fuel ratio in the rich and lean regions can be detected. However, it is not shown how and at what time the polarity is switched. A method for detecting the stoichiometric air-fuel ratio and the air-fuel ratio in the lean region by changing the connection between the electrodes and the electronic circuit and changing the measurement mode of the sensor is disclosed in Japanese Patent Laid-Open No.
Known as 48749. [0012] However, in the above-mentioned prior art, each region of the rich region or the lean region is individually detected, but consideration is given to the point of smoothly detecting a wide air-fuel ratio. Had not been. An object of the present invention is to provide an air-fuel ratio sensor capable of detecting the air-fuel ratio in three states of a rich region, a stoichiometric air-fuel ratio, and a lean region with a simple configuration and with high accuracy. [0014] The above objects are attained by forming a bag-shaped tubular zirconia solid electrolyte, a first electrode formed on the atmosphere side of the solid electrolyte, and formed on the exhaust atmosphere side of the solid electrolyte. A second electrode, a detection unit including a diffusion resistor formed on the second electrode, and a heater provided inside the solid electrolyte , for heating the solid electrolyte to 600 ° C. or higher , An air-fuel ratio sensor comprising a drive circuit for driving the detection unit, wherein the drive circuit has means for flowing current bidirectionally between the first electrode and the second electrode. This is achieved by setting the potential potential of the second electrode to a value higher than the ground level of the drive circuit. Since the potential potential of the exhaust atmosphere side electrode is set to a value higher than the ground level of the drive circuit for driving the detection unit, the potential is rich from the amount of oxygen ions flowing through the zirconia solid electrolyte. The air-fuel ratio in three states of the region, the stoichiometric air-fuel ratio and the lean region is continuously detected. FIG. 1 shows a mounting state of an air-fuel ratio sensor according to the present invention.
It is shown in FIG. The tubular detection unit 10 is disposed in a protective tube 12 having a hole 11, fixed in a plug 14 having a screw 13, and attached to an exhaust pipe 15 through which exhaust gas flows. 1
Reference numeral 6 denotes an electrode terminal, and 17 denotes a heater terminal. The detection unit 10 is connected to an electronic circuit (not shown) via these terminals. Note that a rod-shaped heater (such as a W heater formed on an alumina rod) is mounted inside the zirconia solid electrolyte 10, which is a bag-shaped detection unit, to heat it. Prior to the description of an embodiment of the present invention, the principle underlying the present invention will be described with reference to FIGS. Now, in FIG. 2 between the atmosphere atmosphere side electrode and the exhaust atmosphere side electrode, the characteristic of the curve a showing a stepwise change at the stoichiometric air-fuel ratio (λ = 1) is compared with the excitation voltage characteristic b in the figure. As shown by, a voltage V _E (for example, 0.45 volt) of a predetermined magnitude is excited regardless of the excess air ratio λ, and the excitation voltage reduces the electromotive force of the curve in the rich region of λ <1. In the lean region where λ> 1, it is configured to increase conversely. The voltage V _E may be applied to vary the predetermined inclination or stepwise as the characteristic c, d as described below. FIG. 1 is a diagram showing a principle configuration according to the present invention. In FIG. 1, it is composed of an oxygen concentration detecting section and a drive circuit for driving the detecting section. Reference numeral 20 denotes a bag-shaped tubular zirconia solid electrolyte, into which air is introduced. Reference numeral 21 denotes a rod-shaped heater which heats the zirconia solid electrolyte 20 to a high temperature of at least 600 ° C. or higher to improve the conductivity of oxygen ions. A first electrode 22 is formed on the zirconia solid electrolyte 20 on the air atmosphere side, and a second electrode 23 is formed on the exhaust gas atmosphere side. These electrodes are made of a platinum material having a thickness of several to several tens of μm, and are formed porous. A diffusion resistor 24 is formed on the surface of the second electrode 23, and the second electrode 2 is removed from the exhaust gas atmosphere.
It suppresses the inflow of oxygen and unburned gas such as carbon monoxide which flow into the third part by diffusion. The diffusion resistor 24 is obtained by plasma spraying spinel or the like, and is made porous.
In order to increase the resistivity of diffusion, the thickness is several hundred μm, which is several times the thickness of the theoretical air-fuel ratio sensor. The detection unit of the air-fuel ratio sensor is configured as described above. Reference numeral 25 denotes a differential amplifier, and V _B is its power supply voltage. The second electrode 23 is connected to a potential ground 27 at a higher level than the real ground 26 by a fixed potential. The first electrode 22 is connected to the (−) side input terminal of the amplifier 25. Between (+) side input terminal and the potential ground of the amplifier 25, to connect the excitation voltage V voltage source 28 for _R configuration. The amount of the fixed resistor 29 of resistance R of oxygen ions flowing through the zirconia solid electrolyte 20 medium, that is, for converting the oxygen pump current I _P to the output voltage E _0. A drive circuit is configured by the above configuration. The operation will be described below. [0022] In the lean region, the potential of the second electrode 23 is lower by V _R than the potential of the first electrode 22, the oxygen of the second electrode 23 part by the excitation voltage V _R is oxygen in the electrode portion It is converted into ions (O ⁻ ) and is transferred to the first electrode 22 by the oxygen pump action in the zirconia solid electrolyte 20. Then, it is oxidized again at this electrode portion and released into the atmosphere. At this time, a positive pump current I _P (O ^- opposite to the) flows in the circuit, varying the output voltage E _0. Since the pump current value I _P satisfying I _P > 0 corresponds to the amount of oxygen flowing into the second electrode 23 from the exhaust gas atmosphere via the diffusion resistor 24 by diffusion, the following equation is established. That is, the excess air ratio lambda, when the K a proportionality constant _{I P = K (λ-1} ) ... (1) Consequently, the output voltage E ₀ of the air-fuel ratio sensor when the potential of the potential ground and V ₀ E ₀ = _{V R + V 0 + I P} R ... because a (2), (1) and _{(2), E 0 = V R + V} 0 + K (λ-1) R … (3) At the stoichiometric air-fuel ratio (λ = 1), the amount of residual oxygen and residual unburned gas such as carbon monoxide in the exhaust gas that diffuses into the second electrode 23 via the diffusion resistor 24. Is a chemical equivalent, and both are completely burned by the catalytic action of the second electrode. Since oxygen is depleted in the second electrode 23, even if a voltage is excited between the first electrode 22 and the second electrode 23, no oxygen ions are transferred in the zirconia solid electrolyte 20. Therefore, the pump current flowing through the electronic circuit becomes I _P = 0. The output voltage E _{0 at} this time is given by E ₀ = V _R + V ₀ ... (4) from the equation (3), and becomes a constant value determined by the circuit constant. Equation (4) is I _P
Because it does not change in value, the output voltage E _{0 at} λ = 1 has a very reliable characteristic. In the rich region, since the electromotive force between both electrodes is reduced to the excitation voltage level as described with reference to FIG. 2, oxygen ions are transferred from the first electrode 22 to the second electrode 23. It flows through the zirconia solid electrolyte 20 in the opposite direction as in the lean region. This oxygen ion flow acts to increase the oxygen concentration of the second electrode 23. The oxygen ions are oxidized at the second electrode 23 and become oxygen gas again, and unburned carbon monoxide or the like flowing into the second electrode 23 from the exhaust gas atmosphere through the diffusion resistor 24 by diffusion. Combustion with gas. Therefore, the amount of oxygen ions transferred from the first electrode 22 to the second electrode 23 in the zirconia solid electrolyte 20 depends on the amount of unburned gas flowing into the second electrode 23 by diffusion. The value corresponds to the quantity. In this case, the value of the pump current flowing through the electronic circuit is I _P <0. The relationship between the concentration of unburned gas such as carbon monoxide and the excess air ratio λ is shown in FIG.
Equations (1) to (3) also hold in the rich region. However, in the lean region, λ> 1 so that I _P > 0, and in the rich region, λ <1 so that I _P <0. Next, an embodiment of a driving circuit for an air-fuel ratio sensor according to the present invention will be described with reference to FIG. In the figure, the same parts as those in FIG. 1 are indicated by the same numbers as those in FIG. The second electrode 23 is a potential ground 2
7 (point Y in the figure), and is controlled to a constant potential V ₀ by the amplifier 30. The potential of the first electrode 22 is controlled by the amplifier 25 to (V ₀ + V _R ). Accordingly, the first electrode 22 differential voltage between the second electrode 23, i.e. the excitation voltage V _E is _{V E = (V 0 + V} R) -V 0 = V R ... (5) next, the air excess ratio λ It is controlled to be constant regardless. In the lean region (λ> 1), the pumping current _IP is changed from the point X → the resistance 29 → the zirconia solid electrolyte 2
0 → potential ground Y point → flow to real ground 26 via amplifier 30. In the rich region (λ <1), the current flows to the real ground 26 via the potential ground Y point → the zirconia solid electrolyte 20 → the resistor 29 → the X point → the amplifier 25. At the stoichiometric air-fuel ratio (λ = 1), since the sensor of the present invention has I _p = 0 in principle, the output voltage E ₀ is different from that of the atmosphere.
(4) as shown by equation becomes (V _R + V _0). As described above, according to the embodiment of the air-fuel ratio sensor according to the present invention, the three states of λ <1, λ = 1 and λ> 1 can be obtained without switching the polarity between the electrodes and using a single power supply circuit. Can be continuously detected. FIG. 4 shows an example of a result measured by the configuration of one embodiment of the present invention shown in FIG. FIG. 4 shows V ₀ = 4.5.
5 shows the measurement results when V volts and V _R = 0.45 volts. As shown by the solid line in the figure, it is possible to continuously detect a wide air-fuel ratio from a rich region to a lean region. Further, it was also confirmed that the output voltage E _{0 at the} stoichiometric air-fuel ratio (λ = 1) was V ₀ + V _R = 5 volt, which is a value predicted in principle. According to this embodiment, the air-fuel ratio in the entire region can be linearly detected with high accuracy, and the smooth feedback control of the air-fuel ratio can be performed in accordance with the state of the engine. It is possible to provide a control system far superior to the conventional system. In particular, a great improvement in fuel efficiency can be expected by enabling the engine control in the lean region and the linear feedback control in the rich region. FIG. 5 shows the VI characteristics of the sensor detector. As shown in the figure, the pump current _IP shows a constant saturation current value at a certain excitation voltage. By measuring the saturation current value, the excess air ratio λ can be detected.
When the excitation voltage V _E is further increased, the pump current I _P indicates a higher saturation value value. This is because the zirconia solid electrolyte 20 moves from the ion conduction region to the electron conduction region. The smaller the excess air ratio λ, the lower the excitation voltage V
_R moves to the electron conduction region. In the region of λ> 1, I _P > 0, which corresponds to the amount of oxygen flowing into the second electrode 23 through the diffusion resistor 24 by diffusion. In the range of λ <1, I _P <0, which corresponds to the amount I _{P of} unburned gas such as carbon monoxide flowing into the second electrode 23 through the diffusion resistor 24 by diffusion. FIG. 6 shows that the temperature T _g of the zirconia solid electrolyte was 700 ° C.
This is the VI characteristic at the time of. If the saturation current value I _P for each excess air ratio λ can be detected, a wide air-fuel ratio from a rich to a lean region can be detected linearly. As can be understood from the VI characteristic in FIG. 5, by setting the excitation voltage characteristic with respect to the excess air ratio λ to the b characteristic, the c characteristic, or the d characteristic, these saturation current values can be measured. When the excitation voltage characteristic is the b characteristic, λ = 0.5
And the measurement of the saturation current value near λ = 1.5 becomes difficult.
This can be solved by changing the excitation voltage to a c-characteristic, preferably a d-characteristic. Since the internal resistance of the zirconia solid electrolyte increases as the temperature decreases, the area α of the VI characteristic decreases. Therefore, it becomes easier to measure the saturation current value at a lower temperature, and the b characteristic is the most remarkable. For this purpose, it is necessary to heat the zirconia solid electrolyte to a high temperature with a heater. The zirconia solid electrolyte is heated by a heater at about 750 ° C. when the excitation voltage characteristic is b characteristic and about 70 ° C. when the excitation voltage characteristic is c characteristic.
In the case of 0 ° C. and d characteristics, it is desirable to heat to about 670 ° C. or more. Considering the required power and durability of the heater,
The c characteristic is preferable to the b characteristic, and the d characteristic is preferable to the c characteristic. These excitation voltage characteristics correspond to the b, c, and d characteristics shown in FIG. FIG. 6 shows an embodiment for obtaining the excitation voltage characteristic c shown in FIG. That is, a resistor 33 and a resistor 34 are newly connected between the power supply 28 and the point X as shown in the drawing in FIG. As a result, the pump current value I _P
In response to the output voltage E ₀ that changes due to the potential change, a potential difference γI _P is generated in the resistor 34, and the first electrode 22,
The difference voltage between the second electrodes 23, that is, the excitation voltage V between the two electrodes
Change _E. When the resistance value γ of the resistor 34 is set to a value close to the internal resistance of the zirconia solid electrolyte 20, the output voltage E ₀ of the air-fuel ratio sensor is less affected by the exhaust gas temperature. Since the potential γI _P changes not only with the resistance value r but also with the pump current value I _P , it automatically changes with respect to the excess air ratio λ.
_E becomes like the characteristic c in FIG. According to the configuration of the present embodiment, the temperature dependence of the oxygen ion conductivity of the zirconia solid electrolyte can be improved. FIG. 7 shows another embodiment of FIG. Note that the amplifier 280 has the same function as the voltage source 28 in FIG. According to this circuit configuration, the zirconia solid electrolyte 20
Even when the temperature _Tg is 650 ° C., its output characteristics become the same as the values shown by the solid lines in FIG. 4, which is also effective for countermeasures against temperature effects. FIG. 8 shows an embodiment of a drive circuit for obtaining the excitation voltage characteristic d shown in FIG. Basically, an addition / subtraction amplifier 281 and an output comparator 41 and switches 42 and 43 are newly added to the circuit configuration of FIG. The switches 42 and 43 are driven by the output signal V of the two-output comparator 41, which is inverted when the pump current I _P = 0,
The voltage v is alternately applied to the positive input terminal and the negative input terminal of the addition / subtraction amplifier 281. + Input terminal Z of amplifier 25
The potential at the point V *, the current value of the resistor 33 parts and _{i V * = V 0 + V} R + v + ri atλ> become _{1 V * = V 0 + V} R -v + ri atλ <1 ... (6). With such a circuit configuration, an excitation voltage characteristic between both electrodes can be given as a characteristic d in FIG. Therefore, to detect the saturation pumping current I _P corresponding to each excess air ratio lambda, that the excitation voltage characteristic d is suitable be readily understood from the V-I characteristic shown in FIG. 5 it can. FIG. 9 shows an example of the measurement results obtained by the circuit configuration of FIG. This figure shows the measurement result when v = 0.15 volt. In this case, as shown in the figure, the output voltage E ₀ changes stepwise by 2v at the stoichiometric air-fuel ratio λ = 1. This is not an essential problem in the present embodiment in which the output voltage E ₀ changes in a stepwise manner by 2 v. If the characteristic of FIG. 9 is added by 2 v in the range of λ ≦ 1, the characteristic of the output voltage E ₀ is linear in the entire range. become. According to the configuration of the present embodiment, there is an effect of improving accuracy reduction due to electrode deterioration (increase in interface resistance). FIG. 10 shows a case where the zirconia solid electrolyte is a flat plate and the diffusion resistor is a single hole, for example. 1 and 10 have the same function. The atmosphere is introduced into the first electrode 22 via the passage 32. The residual oxygen and unburned gas in the exhaust gas are diffused through the diffusion resistor 24 having a hole shape into the diffusion chamber 3.
1 flows into the second electrode 23 in the diffusion region 1 by diffusion.
By the heater 212 in the alumina insulating layer 211 fixed to the zirconia solid electrolyte 20, the zirconia solid electrolyte 20 is heated to a high temperature (for example, 600
(° C or higher). According to the present invention, it is possible to provide an air-fuel ratio sensor capable of detecting a wide range of air-fuel ratios in three states of a rich region, a stoichiometric air-fuel ratio, and a lean region with a simple configuration and high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a principle configuration diagram of an air-fuel ratio sensor according to the present invention. FIG. 2 is an electromotive force characteristic diagram for explaining the principle of the present invention. FIG. 3 is a circuit diagram showing an embodiment of an air-fuel ratio sensor according to the present invention. FIG. 4 is a diagram showing one characteristic of an air-fuel ratio sensor according to the present invention. FIG. 5 is an example of a VI characteristic. FIG. 6 shows another embodiment of the air-fuel ratio sensor according to the present invention. FIG. 7 shows another embodiment of the present invention. FIG. 8 shows another embodiment of the present invention. FIG. 9 shows another characteristic example of the air-fuel ratio sensor of the present invention. FIG. 10 is a circuit diagram showing another embodiment of the air-fuel ratio sensor according to the present invention. FIG. 11 shows the relationship between the air-fuel ratio and the exhaust gas concentration. FIG. 12 is a diagram illustrating the principle of a conventional air-fuel ratio sensor. FIG. 13 is a diagram illustrating characteristics of a conventional air-fuel ratio sensor. FIG. 14 is a mounting state diagram of an air-fuel ratio sensor according to the present invention. [Description of Signs] 20: zirconia solid electrolyte, 22: first electrode, 23
... Second electrode, 24. Diffusion resistor, 27. Potential ground, 26. Real ground.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Masayuki Miki               4026 Kuji-cho, Hitachi City, Ibaraki Prefecture Co., Ltd.                 Hitachi, Ltd.                (56) References JP-A-61-144563 (JP, A)                 JP-A-55-44999 (JP, A)                 JP-A-57-29941 (JP, A)                 JP-A-57-178152 (JP, A)                 JP-A-58-179351 (JP, A)                 JP-A-59-170758 (JP, A)

Claims (1)

(57) [Claims] A bag-shaped zirconia solid electrolyte, a first electrode formed on the atmosphere side of the solid electrolyte, a second electrode formed on the exhaust atmosphere side of the solid electrolyte, and a diffusion formed on the second electrode A detection unit including a resistor, and a detection unit provided inside the solid electrolyte,
In the air-fuel ratio sensor composed of a heater for heating to at least ℃ and a drive circuit unit for driving the detection unit, the drive circuit unit is located between the first electrode and the second electrode. An air-fuel ratio sensor, further comprising means for flowing a current in both directions, and setting a potential of the second electrode to a value higher than a ground level of the drive circuit.