JPH05240829A - Air-fuel ratio sensor - Google Patents

Abstract

(57) [Summary] [Objective] To provide an air-fuel ratio sensor capable of highly accurately detecting an air-fuel ratio in three states of a rich region, a stoichiometric air-fuel ratio and a lean region with a simple configuration. [Structure] A plate-shaped 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 drive circuit unit for driving the detection unit, and setting the potential potential of the second electrode to a value higher than the ground level of the drive circuit unit.

Description

Detailed Description of the Invention

[0001]

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 more particularly to an air-fuel ratio sensor for an automobile capable of detecting a wide range of three-state air-fuel ratios of a rich region, a theoretical air-fuel ratio and a lean region. ..

[0002]

2. Description of the Related Art An internal combustion engine operates in 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), depending on the engine state. It is desirable that a single air-fuel ratio sensor be used to detect a wide range of air-fuel ratios from the rich region to the lean region.

On the other hand, the relationship between the residual oxygen concentration in the exhaust gas and the carbon monoxide concentration with respect to the air-fuel ratio or the excess air ratio λ is as shown in FIG.
In the (O ₂ ) concentration / rich region, the concentration of carbon monoxide (CO), which is an unburned gas, changes substantially linearly according to the air-fuel ratio.

FIGS. 12A to 12C show the basic principle of a conventional air-fuel ratio sensor which individually detects the air-fuel ratio in each region by utilizing the residual oxygen concentration and the carbon monoxide concentration. The air-fuel ratio sensor comprises an electrode 1, a zirconia solid electrolyte 2, an electrode 3, a protective film 4 and an ammeter 5.

In FIG. 12A, for example, as known from Japanese Patent Laid-Open No. 53-66292, an excitation voltage E of about 0.5 volt is applied between both electrodes with the electrode 1 as a cathode and the electrode 3 as an anode. Then, the rich region with λ <1 is detected. That is,
The protective film 4 functions as a gas diffusion resistor, and oxygen gas that burns and reacts with unburned gas such as carbon monoxide that diffuses in the protective film 4 to the electrode 3 part is transferred from the electrode 1 part in contact with the atmosphere to the electrode 3 part , In the form of oxygen ions in the zirconia solid electrolyte 2. 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 that diffuses into the electrode 3 portion in the protective film 4. The air-fuel ratio in the rich region is detected from this I _P value in an analog manner.

Further, as shown in FIG. 12 (B), when the electromotive force e _λ between both electrodes is detected with reference to the electrode 3 in contact with the exhaust atmosphere through the protective film 4, this e _λ value is theoretical It is known, for example, from Japanese Patent Laid-Open No. 47-37599 that λ = 1 can be detected almost digitally from the e _λ value because the fuel ratio changes stepwise by about 1 volt.

As shown in FIG. 12C, the electrode 3
When an excitation voltage E of about 0.5 V is applied between both electrodes with the cathode as the cathode, oxygen ions are pumped from the electrode 3 part to the electrode 1 part, and the pump current I _P is measured by the ammeter 5.
Since this pump current value I _P corresponds to the amount of oxygen diffused to the electrode 3 through the protective film, it is possible to detect a lean region of λ> 1 from this I _P value, for example, JP-A-52-69690.
Known by the issue.

An example of the characteristics of the conventional air-fuel ratio sensor shown in FIGS. 12A to 12C is shown in FIG. The characteristic of the lean region is shown by a dashed line, the characteristic of the rich region is shown by a dotted line, and the detection characteristic of the stoichiometric air-fuel ratio point is shown by a solid line. As described above, it is known to detect each region individually, but a configuration for smoothly detecting a wide air-fuel ratio by a consistent method has not been proposed yet.

Since the principle shown in FIG. 12B is not based on the diffusion control, the degree of gas diffusion resistance of the protective film 4 shown in FIG. 12 is smaller than that in the case of FIGS. 12A and 12C. Generally, the thickness of the protective film 4 in FIG.
It is thinly formed.

Further, it is possible to detect an air-fuel ratio in an analog manner from a terminal voltage generated between both electrodes by exciting a constant current between the electrodes, for example, JP-A-55-62349 and JP-A-55-1544.
Known as No. 50. It is shown that the air-fuel ratios in the rich and lean regions can be detected by switching the polarities of both electrodes. However, it is not shown how and at which point the polarity is switched.

Further, there is a method of detecting the theoretical air-fuel ratio and the air-fuel ratio in the lean region by switching the connection between both electrodes and the electronic circuit and changing the measurement mode of the sensor.
It is known as 48749.

[0012]

However, in the above-mentioned prior art, although each of the rich region and the lean region is individually detected, no consideration is given to the smooth detection of a wide air-fuel ratio. It was

An object of the present invention is to provide an air-fuel ratio sensor capable of highly accurately detecting an air-fuel ratio in three states of a rich region, a stoichiometric air-fuel ratio and a lean region with a simple structure.

[0014]

The above-mentioned object is to provide a solid electrolyte having a plate-like structure, a first electrode formed on the atmosphere side of the solid electrolyte, and a second electrode formed on the exhaust atmosphere side of the solid electrolyte. In an air-fuel ratio sensor including an electrode and a detection unit including a diffusion resistor formed on the second electrode, and a drive circuit unit for driving the detection unit, the potential potential of the second electrode is This is achieved by setting the value higher than the ground level of the drive circuit section.

[0015]

Since the potential of the electrode on the exhaust atmosphere side is set to a value higher than the ground level of the drive circuit section for driving the detection section, the amount of oxygen ions flowing in the zirconia solid electrolyte can be changed from the rich area to the theoretical area. The air-fuel ratio and the three-state air-fuel ratio in the lean region are continuously detected.

[0016]

EXAMPLE FIG. 1 shows a mounting state of an air-fuel ratio sensor according to the present invention.
4 shows. The bag-shaped detection unit 10 is arranged in a protective pipe 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 is an electrode terminal, 17 is a heater terminal, and the detection unit 10 is connected to an electronic circuit (not shown) via these terminals. In addition, 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 the solid electrolyte 10.

Prior to the description of one embodiment of the present invention, the principle underlying the present invention will be described with reference to FIGS.

Now, in FIG. 2 between the electrode on the atmosphere side and the electrode on the exhaust side, in contrast to the characteristic of the curve a showing the stepwise change in the theoretical air-fuel ratio (λ = 1), for example, the excitation voltage characteristic b As shown in, the voltage V _E of a predetermined magnitude (for example, 0.45 V) is excited regardless of the excess air ratio λ, and this excitation voltage reduces the electromotive force of the curve in the rich region of λ <1. As described above, in the lean region of λ> 1, it is configured to increase conversely. The voltage V _E may be applied so as to change in a predetermined slope or step like the characteristics c and d as described later.

FIG. 1 shows a principle block diagram according to the present invention. In FIG. 1, it is composed of an oxygen concentration detecting section and a drive circuit for driving this detecting section. Reference numeral 20 is a bag-shaped zirconia solid electrolyte into which the air is introduced. Reference numeral 21 is a rod-shaped heater that 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 air atmosphere side of the zirconia solid electrolyte 20, 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 made porous. A diffusion resistor 24 is formed on the surface of the second electrode 23 so that the second electrode 2 can be removed from the exhaust gas atmosphere.
It suppresses the inflow of oxygen and unburned gas such as carbon monoxide that flow into the third part by diffusion. The diffusion resistor 24 is formed by plasma spraying spinel or the like, and is made porous.
In order to increase the diffusion resistivity, the thickness thereof is several hundred μm, which is several times as thick as the theoretical air-fuel ratio sensor. The above constitutes the detection unit of the air-fuel ratio sensor.

Reference numeral 25 is a differential amplifier, and V _B is its power supply voltage. The second electrode 23 is connected to the high-level potential ground 27 at a constant potential from the real ground 26. The first electrode 22 is connected to the (−) side input terminal of the amplifier 25. A voltage source 28 for setting the excitation voltage V _R is connected between the (+) side input terminal of the amplifier 25 and the potential ground. The fixed resistor 29 having a resistance value R is for converting the amount of oxygen ions flowing in the zirconia solid electrolyte 20, that is, the oxygen pump current I _P into the output voltage E ₀ . The drive circuit is configured by the above configuration.

The operation will be described below.

In the lean region, the potential of the second electrode 23 is lower than the potential of the first electrode 22 by V _R, so that the excitation voltage V _R causes oxygen in the second electrode 23 to be oxygen at this electrode. It is converted into ions (O ⁻ ), and is transferred to the first electrode 22 part in the zirconia solid electrolyte 20 by the oxygen pump action. Then, it is oxidized again at this electrode portion and released into the atmosphere. At this time, a positive pump current I _P (opposite to O ⁻ ) flows in the circuit and changes the output voltage E ₀ .

Since the pump current value I _{P with} I _P > 0 corresponds to the amount of oxygen flowing into the second electrode 23 by diffusion from the exhaust gas atmosphere through the diffusion resistance 24, the following equation is established. That is, assuming that λ is the excess air ratio and K is a proportional constant, I _P = K (λ−1) (1) Therefore, the output voltage E ₀ of the air-fuel ratio sensor is E ₀ = when the potential of the potential ground is V _0. Since V _R + V ₀ + I _P R (2), E ₀ = V _R + V ₀ + K (λ-1) R _P (3) from the equations (1) and (2).

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 through the diffusion resistor 24. Is a chemical equivalence and both are completely burned by the catalytic action of the second electrode. Then, since the second electrode 23 part is depleted of oxygen, even if a voltage is excited between the first electrode 22 and the second electrode 23, there is no oxygen ion transferred in the zirconia solid electrolyte 20. Therefore, the pump current flowing in the electronic circuit becomes I _P = 0.

The output voltage E _{0 at} this time becomes E ₀ = V _R + V ₀ (4) according to the equation (3), which is a constant value determined by the circuit constant. Equation (4) is I _P
The output voltage E _{0 at} λ = 1 has an extremely reliable characteristic because it does not change in value.

In the rich region, the electromotive force between both electrodes is lowered to the excitation voltage level as described with reference to FIG. 2, so that oxygen ions move from the first electrode 22 part to the second electrode 23 part. It flows in the zirconia solid electrolyte 20 in the opposite direction to that in the lean region. This oxygen ion flow acts so as to increase the oxygen concentration in the second electrode 23 portion. The oxygen ions are oxidized in the second electrode 23 portion to become oxygen gas again, and unburned carbon monoxide or the like that diffuses into the second electrode 23 portion from the exhaust gas atmosphere through the diffusion resistor 24. Burns with gas.

Therefore, the amount of oxygen ions transferred from the first electrode 22 part to the second electrode 23 part in the zirconia solid electrolyte 20 is equal to that of the unburned gas flowing into the second electrode 23 part by diffusion. The value corresponds to the quantity. In this case, the pump current value flowing in the electronic circuit is I _P <0.

Since there is the relationship shown in FIG. 11 between the concentration of unburned gas such as carbon monoxide and the excess air ratio λ,
Equations (1) to (3) also hold in the rich region. However, since λ> 1 in the lean region, I _P > 0. In the rich region, λ <1 and I _P <0.

Next, an embodiment of the drive circuit of the 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 the 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 to (V ₀ + V _R ) by the amplifier 25. Therefore, the difference voltage between the first electrode 22 and the second electrode 23, that is, the excitation voltage V _E becomes V _E = (V ₀ + V _R ) −V ₀ = V _R (5), and the excess air ratio λ is obtained. It is controlled to be constant regardless.

In the lean region (λ> 1), the pumping current I _P is at point X → resistance 29 → zirconia solid electrolyte 2
0 → potential ground Y point → flows to real ground 26 via amplifier 30.

In the rich region (λ <1), the current flows to the real ground 26 through 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), this sensor theoretically has I _P = 0. Therefore, the output voltage E ₀ changes to the atmosphere.
(4) as shown by equation becomes (V _R + V _0).

As described above, according to one embodiment of the air-fuel ratio sensor of 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 with a single power supply circuit. Can be continuously detected.

FIG. 4 shows an example of the result measured by the configuration of the embodiment of the present invention shown in FIG. FIG. 4 shows V ₀ = 4.5
It shows the measurement results when 5 V and V _R = 0.45 V. As shown by the solid line in the figure, a wide range of air-fuel ratios from the rich region to the lean region can be continuously detected. It was also confirmed that the output voltage E _{0 at the} stoichiometric air-fuel ratio (λ = 1) was V ₀ + V _R = 5 V, which was a theoretically predicted value.

According to the present embodiment, the air-fuel ratio of the entire region can be detected linearly with high accuracy, and the smooth feedback control of the air-fuel ratio can be performed according to the state of the engine, and from the viewpoint of exhaust measures and fuel economy. It is possible to provide a control system far superior to the conventional system. In particular, a large improvement effect of fuel efficiency can be expected by enabling engine control in the lean range and enabling linear feedback control in the rich range.

FIG. 5 shows the VI characteristic of the sensor detecting portion.

As shown in the figure, the pump current I _P shows a constant saturation current value at a certain excitation voltage. The excess air ratio λ can be detected by measuring the saturation current value.
When the excitation voltage V _E becomes larger, the pump current I _P shows a value higher than the saturation value. This is because the zirconia solid electrolyte 20 shifts from the ion conduction region to the electron conduction region. The smaller the excess air ratio λ, the smaller the excitation voltage V
_R shifts to the electron conduction region.

In the region of λ> 1, I _P > 0, which corresponds to the amount of oxygen that diffuses into the second electrode 23 through the diffusion resistor 24. In the region of λ <1, I _P <0, which corresponds to the amount I _{P of} unburned gas such as carbon monoxide that diffuses into the second electrode 23 through the diffusion resistor 24. In addition, in FIG. 6, the temperature T _g of the zirconia solid electrolyte is 700 ° C.
It is a VI characteristic at the time of.

If the saturation current value I _P for each excess air ratio λ can be detected, it is possible to linearly detect a wide range of air-fuel ratios from the rich region to the lean region. As can be understood from the VI characteristic of FIG. 5, these saturation current values can be measured 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.

When the excitation voltage characteristic is the b characteristic, λ = 0.5
And, it becomes difficult to measure the saturation current value near λ = 1.5.
This is solved by changing the excitation voltage like the c-characteristic, preferably the d-characteristic.

Since the internal resistance of the zirconia solid electrolyte increases at lower temperatures, the region α of the VI characteristic becomes smaller. Therefore, the lower the temperature, the more difficult the measurement of the saturation current value becomes, and the b characteristic is most remarkable. Therefore, it is necessary to heat the zirconia solid electrolyte to a high temperature with a heater. Using a zirconia solid electrolyte with a heater, when the excitation voltage characteristic is b characteristic, about 750 ° C., and when it is c characteristic, about 70 ° C.
In the case of 0 ° C and d characteristics, it is desirable to heat to about 670 ° C or higher. 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. 2, respectively.

FIG. 6 shows an embodiment for obtaining the excitation voltage characteristic c shown in FIG. That is, the resistors 33 and 34 are newly connected to the configuration of FIG. 3 between the power source 28 and the point X as shown in the figure. As a result, the pump current value I _P
A potential difference γI _P is generated in the resistor 34 in accordance with the output voltage E ₀ which changes with the first electrode 22,
The difference voltage between the second electrodes 23, that is, the excitation voltage V between both 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 likely to be 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 also changes automatically with respect to the excess air ratio λ, and the difference voltage between both electrodes, that is, the excitation voltage V
_E becomes like the characteristic c in FIG. According to the configuration of this example, the temperature dependence of the oxygen ion conductivity of the zirconia solid electrolyte can be improved.

Another embodiment of FIG. 6 is shown in FIG. 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 T _{g of the above} is 650 ° C., its output characteristic is the same as the value shown by the solid line in FIG. 4, and it is also effective for the measure against the temperature influence.

FIG. 8 shows an embodiment of a drive circuit for obtaining the excitation voltage characteristic d shown in FIG. Basically, the addition and subtraction amplifiers 281, 2 output comparators 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 signals V and V of the two-output comparator 41 which inverts when the pump current I _P = 0,
The voltage v is alternately applied to the + side input terminal and the − side input terminal of the amplifier 281 for addition and subtraction. + 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, the excitation voltage characteristic between both electrodes can be given as the characteristic d in FIG. Therefore, the fact that this excitation voltage characteristic d is suitable for detecting the saturated pump current value I _P corresponding to each excess air ratio λ can be easily understood from the VI characteristic shown in FIG. it can.

FIG. 9 shows an example of measurement results obtained by the circuit configuration of FIG. This figure shows the measurement results when v = 0.15 volts. 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 linearly in the entire region when λ ≦ 1 is added to the characteristic of FIG. become.

The structure of this embodiment is effective in improving accuracy deterioration due to electrode deterioration (increased interface resistance).

In the above description, the shape of the zirconia solid electrolyte of the detection portion of the air-fuel ratio sensor according to the present invention has been described as a bag-like shape, but the present invention is not limited to this. That is, any structure may be used as long as it can introduce the atmosphere into the first electrode portion, and for example, a flat plate structure as shown in FIG. 10 may be used.

FIG. 10 shows a case where the zirconia solid electrolyte is a flat plate and the diffusion resistor is, for example, one hole.

Items having the same numbers in FIGS. 1 and 10 have the same function. The atmosphere is introduced into the first electrode 22 section via the passage 32. Residual oxygen and unburned gas in the exhaust gas are passed through the diffusion resistor 24 in the shape of a hole to the diffusion chamber 3
It flows into the 2nd electrode 23 part in 1 by diffusion.
The heater 212 in the alumina insulating layer 211 fixed to the zirconia solid electrolyte 20 causes the zirconia solid electrolyte 20 to have a high oxygen ion conductivity at a high temperature (for example, 600).
The temperature is controlled to be higher than ℃.

[0054]

According to the present invention, it is possible to provide an air-fuel ratio sensor capable of highly accurately 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 structure.

[Brief description of 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 configuration diagram showing an embodiment of an air-fuel ratio sensor according to the present invention.

FIG. 4 is a diagram showing one characteristic of the air-fuel ratio sensor according to the present invention.

FIG. 5 shows an example of VI characteristics.

FIG. 6 is 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 is another characteristic example of the air-fuel ratio sensor of the present invention.

FIG. 10 is a circuit configuration diagram showing another embodiment of the air-fuel ratio sensor according to the present invention.

FIG. 11 shows the relationship between air-fuel ratio and exhaust gas concentration.

FIG. 12 is an explanatory view of the principle of a conventional air-fuel ratio sensor.

FIG. 13 is a characteristic explanatory view of a conventional air-fuel ratio sensor.

FIG. 14 is a mounting state diagram of the air-fuel ratio sensor according to the present invention.

[Explanation of symbols]

20 ... Zirconia solid electrolyte, 22 ... First electrode, 23
... second electrode, 24 ... diffusion resistor, 27 ... potential ground, 26 ... real ground.

Claims (1)

[Claims] 1. A solid electrolyte having a plate-like structure, 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 the second electrode. In an air-fuel ratio sensor including a detection unit having a diffusion resistor formed above and a drive circuit unit for driving the detection unit, the potential potential of the second electrode is set to be higher than the ground level of the drive circuit unit. An air-fuel ratio sensor characterized by being set to a high value.