CN1014460B - The air-fuel ratio sensor that is used for controlling combustion engine - Google Patents

Abstract

An air-fuel ratio detector placed in the exhaust gas of an internal combustion engine. Used to detect mixed exhaust gas to determine the air-fuel ratio of an internal combustion engine, the detector consists of a solid zirconia electrolytic layer, a pair of electrodes formed on both sides of the electrolytic layer and a coating on the electrolytic layer enclosing one electrode . The detector has a diffusion area and a gas diffusion hole communicating the outside of the detector with the diffusion space. The electrode located in the diffusion area is used as the positive electrode and the other electrode is used as the negative electrode. There is a current conduction between the two electrodes to control the diffusion area and the oxygen concentration as a reference oxygen concentration. The air-fuel ratio can be known by detecting the voltage between the two electrodes.

Description

The air-fuel ratio detector involved in the present invention detects whether the air-fuel ratio of the automobile engine is equal to the ideal air-fuel ratio in the mixed exhaust gas discharged from the internal combustion engine. In particular, it can be used to control the air-fuel ratio detector of the automobile engine, by controlling the oxygen concentration in the diffusion area of the detector to make it a reference value for measurement, and by detecting the difference between the exhaust gas oxygen concentration and the oxygen concentration in the diffusion area The electromotive force generated by the difference can determine whether the air-fuel ratio is equal to the ideal air-fuel ratio.

In order to detect whether the air-fuel ratio of the automobile engine is equal to the ideal air-fuel ratio (the excess air factor λ=1 in the ideal ratio) from the exhaust gas mixture, people are generally accustomed to using an oxygen detector, which includes There are a pair of catalytic platinum electrodes and a solid zirconia electrolyte. This oxygen detector puts one platinum electrode in the exhaust gas and the other platinum electrode in the atmosphere, and uses the "atmospheric reference method" to detect the state of λ=1. The so-called "atmospheric reference method" is actually because the oxygen pressure in the exhaust gas and the oxygen pressure in the atmosphere are different, which can generate an electromotive force. However, oxygen detectors based on the "atmospheric reference method" are complex and bulky, and therefore expensive.

In order to solve these problems, there have been proposals about oxygen detectors, but in these proposals, the detectors are placed in the exhaust gas, and one electrode is made of non-catalytic gold, while the other The electrodes are made of platinum. However, electrodes made of gold are not durable. It cannot withstand drastic changes in the operating environment of an oxygen detector, and the realization of such a detector is impractical.

People then proposed another oxygen detector, which is also completely placed in the exhaust gas, And one of its platinum electrodes is also in the exhaust gas, and at the other platinum electrode, a reference oxygen concentration is generated by using the oxygen pumping phenomenon. The oxygen pumping phenomenon is to transfer oxygen from the positive electrode to the negative electrode, and the positive and negative electrodes are connected to the solid electrolyte. Through the conduction of current, oxygen ions move from the negative electrode to the positive electrode through the solid electrolyte.

Japanese Patent Application Publication No. 69690/77 patent document discloses an oxygen detector utilizing the oxygen pumping phenomenon, which includes a solid state zirconium dioxide electrolyte and a reference oxygen region generated in the electrolyte, and has a barrier for the diffusion of oxygen into the Refer to diffusion holes in the oxygen zone. The purpose of designing this oxygen detector is to evacuate the oxygen in the reference oxygen zone by means of the oxygen pumping phenomenon, and measure the oxygen in the detector from the amount of oxygen entering through the diffusion hole according to the reduced oxygen in the reference oxygen zone. oxygen concentration. Japanese Patent Application Publication No. 69690/77 patent document does not make any disclosure for the concept of detecting λ=1.

Japanese Patent Application Publication No. 30681/80 discloses an oxygen detector having two laminated zirconia solid state electrolytic cells together with a reference oxygen zone, and an air extraction hole, which It is used to evacuate oxygen from the reference oxygen zone formed between the two cells. In this detector, the oxygen concentration in the reference oxygen zone is constantly controlled and the ambient oxygen concentration relative to the reference oxygen zone is measured. This detector is only used to measure the oxygen concentration in the atmosphere, moreover, the publication is silent on the concept of detecting λ=1.

Japanese Patent Application Publication No. 111441/82 patent literature has announced a kind of oxygen detector with two batteries, wherein one battery is used for pumping oxygen and another is used for detecting the oxygen concentration of electromotive force jump point in the atmosphere, design The purpose of this device lies in the change of the jump point of the electromotive force, rather than being directly used for detecting the state of λ=1. In addition, this detector has a complex stacked structure, as in the above-mentioned Japanese Patent Application Publication No. 30681/80.

No. 154450/80 patent literature published by Japanese patent application has announced a kind of Oxygen detector using two solid state zirconia electrolytic cells, where one cell is used for controlled oxygen extraction and the other for measurement. The purpose of designing this detector is to control the local oxygen pressure at the reference electrode in the center of the laminated battery by pumping oxygen by applying a voltage or current to the oxygen pumping battery, so that the λ can be measured from the electromotive force of the detection battery. value. Due to the laminated battery, this method cannot avoid the problem of complex structure, and it is conceivable that the sealed structure of the reference electrode will cause the electrode to peel off, or the component will be damaged due to the pressure increase when oxygen is absorbed. Moreover, the contact area of the porous electrode pressed between the two layers with oxygen is reduced, so that the catalytic action is weakened, so that the transient characteristics can hardly be obtained. In addition, a porous sintered material is required to make a solid zirconia electrolyte for an oxygen pumping battery, and its productivity is very low because the production process is difficult to control.

No. 16419/79 patent literature and 156856/80 published in Japanese Patent Application have announced a kind of oxygen detector that uses a battery, and this battery can be used as oxygen extraction and can be used as detection again. In this type of detector, a cell is placed on a substrate to which a current or voltage is applied to control the local oxygen pressure near the electrode on the side contacting the substrate. And the state of λ=1 is detected from the electromotive force of the battery. This kind of detector also has the problems of electrode peeling, component damage and weakening of catalysis due to the sealed structure at the electrode interface. Its situation is described in No. 154450/80 patent literature published as Japanese Patent Application Publication.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an air-fuel ratio detector for controlling an internal combustion engine which overcomes the above-mentioned problems of the prior art. Among them, the detector is completely placed in the exhaust gas and has a diffusion area. In the diffusion area, the oxygen pumping phenomenon is used to control the oxygen concentration, and from the generated electromotive force, it is detected whether the air-fuel ratio of the engine is equal to the ideal air. Fuel ratio.

According to the present invention, the air-fuel ratio detector comprises a solid zirconia electrolytic layer; a For electrodes formed on both sides of the electrolytic layer; a cover layer, which is above the electrolytic layer, and which together with the electrolytic layer forms a diffusion zone and encloses an electrode therein; and a gas diffusion hole, which enables the diffusion zone to It communicates with the outside of the detector. There is a conduction current between the positive electrode in the diffusion zone and the external negative electrode, which controls the oxygen concentration in the diffusion zone. According to the difference between the oxygen concentration of the electrode placed in the diffusion zone and the oxygen concentration of the other electrode placed in the exhaust gas, and the resulting voltage between the two electrodes, it can be detected whether the air-fuel ratio has reached the ideal air-fuel ratio. than state. The detection results are used to control the air-fuel ratio of the car engine.

In the present invention, the air-fuel ratio detector has a diffusion area surrounding the electrode, which is placed in the region where the reference oxygen concentration exists, has a large contact surface with oxygen, so that the catalytic effect can be prevented from weakening, and the gas Diffusion holes prevent electrode peeling or component damage due to increased pressure in the diffusion area. According to the present invention, the air-fuel ratio detector can be manufactured by a thick film process, thus realizing the miniaturization of the air-fuel ratio detector and reducing the cost.

Since this kind of air-fuel ratio detector has a diffusion area and uses the oxygen pumping phenomenon to generate a reference oxygen concentration in the diffusion area, oxygen is introduced into the detector from the outside of the detector, so the oxygen concentration of the external electrode is slightly lower than that of the detector. The oxygen concentration of the exhaust gas, which makes the detector respond to the oxygen concentration of the exhaust gas slightly higher than the ideal air-fuel ratio. In order to reduce this error, the current value that contributes to oxygen pumping should be reduced. The detector provides an electromotive force at its electrode terminals which increases with the difference in oxygen concentration outside the detector and inside the diffusion zone. The terminal voltage consists of a voltage drop which is the product of the impedance of the zirconia electrolyte and the oxygen extraction current, the impedance of the zirconia electrolyte being temperature dependent. In order to minimize the impact of impedance changes on the electrode terminal voltage, it is also necessary to reduce the oxygen pumping current. On the other hand, carbon monoxide in the exhaust gas enters the diffusion zone through the diffusion holes, causing carbon monoxide and oxygen to react to form carbon dioxide Carbon, this is a reversible reaction. When the exhaust gas is in the oil-rich area, the oxygen demand for the above reaction increases. If the amount of oxygen pumped into the diffusion area is less, the oxygen concentration in the diffusion area will decrease. Therefore, the difference with the oxygen concentration outside the detector becomes smaller. small, this eventually leads to a drop in the output of the detector. For this reason, it is necessary to determine the oxygen pumping current at an appropriate value. The current value depends on the size of the area of the diffusion hole, and its function is to prevent the device from being damaged due to the increase of the pressure in the diffusion area when oxygen is introduced. The inventors of the present invention confirmed through experiments that the area of the diffusion hole should be equal to or smaller than 1/100 of the electrode area. At this time, the above-mentioned errors in detecting the ideal air-fuel ratio can be reduced, and the influence of the voltage drop across the zirconium dioxide electrolytic layer can also be reduced to a minimum, and the voltage drop in the dense region can also be reduced. Therefore, satisfactory detector characteristics are actually provided. It can be seen that this diffusion area is sufficient to withstand the boost in it.

From following description and accompanying drawing, can understand advantage and several embodiments of the present invention clearly:

Figure 1 illustrates a detector for controlling the air-fuel ratio of an internal combustion engine according to the principles of the present invention.

Figure 2 shows the relationship between the output characteristics of the air-fuel ratio detector and the current.

Figure 3 shows the relationship between the output characteristics of the air-fuel ratio detector and the temperature.

The graph of Figure 4 shows the effect of constant current on the output of the air-fuel ratio detector.

Fig. 5 is a curve of the experimental results of the detector prototype.

Figures 6a and 6b illustrate the shape of the gas diffusion holes in the detector prototype.

Figure 7 is used to illustrate the manufacturing process of the air-fuel ratio detector.

Fig. 8 is a static characteristic curve diagram of the detector prototype.

Fig. 9 is a graph of dynamic characteristics of the detector prototype.

Figure 10 is a schematic diagram of an actual air fuel ratio detector with a heater.

Fig. 11 is a graph illustrating the influence of the area of the gas diffusion holes on the detected value of λ.

According to the present invention, the structural principle of the air-fuel ratio detector is as shown in Figure 1, the structure includes a solid zirconia electrolyte, a platinum electrode 2 working as the positive electrode of the detector and a platinum electrode 3 working as the negative electrode of the detector, the diffusion Zone 4, gas diffusion holes 5, constant current source 6 and terminals 7 and 8 connected to electrodes 1, 3, respectively.

In the structure of Figure 1, the voltage e between the terminals 7 and 8 is the result of the joint action of the electromotive force E, the impedance r of the solid zirconia electrolyte, and the current Ip; where the electromotive force E is the difference between the oxygen pressure at the two electrodes While generated between the electrodes, the current Ip is the value flowing from the current source through the detector. The relationship between them is as follows:

e=E+r Ip (1)

From the local oxygen pressure P ₁ at the three-phase interface of the negative electrode (for example, the electrode, exhaust gas and zirconia is a three-phase interface surface in contact with each other), the local oxygen pressure P ₂ in the diffusion zone 4, the electromotive force E It is expressed by the Nernst formula as follows:

E＝ (RT)/(4F) ln (P ₂ )/(P ₁ ) (2)

Here, F is Faraday's constant, T is absolute temperature, and R is an atmospheric constant.

According to the invention, except when oxygen is being pumped, the air fuel detectors placed entirely in the exhaust provide equal values for _P1 and _P2 , therefore the EMF is zero. Just because of this, the oxygen concentration in the diffusion zone 4 is controlled with the constant current source 6, when the constant current Ip is turned on from the positive platinum electrode 2 to the negative platinum electrode 3, the three-phase junction of the negative platinum electrode 3 and the carbon dioxide The oxygen in the zirconium electrolyte 1 is ionized and absorbed by the positive electrode, and the ionized oxygen ions are oxidized by the positive platinum electrode and reduced to oxygen molecules. In other words, the oxygen molecules at the interface of the negative electrode are guided to the diffusion region by the excitation current. The number of oxygen molecules introduced is determined by the value of the constant current Ip, but the value of the current Ip is not always required to be constant.

Owing to exist oxygen, and carbon monoxide has entered diffusion region 4 from gas diffusion hole 5, so in diffusion region 4, the same situation with the reaction that takes place on the negative electrode has just taken place, and the chemical equation of this reaction is as follows:

In the exhaust gas from the automobile engine, in the dense region where the excess air factor λ is less than 1, carbon monoxide is more than oxygen, and in the so-called lean region where the excess air factor λ is greater than or equal to 1, carbon monoxide is less than oxygen.

Therefore, in the dense region where λ is less than 1, oxygen is consumed by the chemical reaction shown in (3). In the diffusion region 4, more oxygen needs to be obtained from the negative electrode through the conduction of a constant current. As a result, the positive electrode has a high oxygen concentration, and at the junction of the negative electrode, due to the catalysis of the platinum electrode, an oxygen-deficient state is generated, which in turn leads to a high electromotive force E. On the contrary, in the rarefied region, oxygen is excessive and There is very little carbon monoxide at the negative electrode. Therefore, the oxygen concentration difference at the junction of the diffusion zone 4 and the negative electrode is very small, and the electromotive force given by (1) Potential is actually equal to zero. Therefore, from the oxygen entering the diffusion region 4 from the negative electrode, the state of λ=1 can be detected.

Fig. 2 shows the output characteristics of the air-fuel ratio detector according to the present invention. The amount of oxygen entering the diffusion zone 4 is proportional to the value of the constant current Ip as described above. Therefore, increasing Ip leads to a decrease in oxygen at the three-phase interface of the negative electrode, which occurs in the lean region. The jump point of the electromotive force does not appear exactly at the excess air factor λ=1, but shifts toward the lean region. In order to prevent the offset of the transition point, an optimal Ip value should be selected.

Fig. 3 shows an example of the air-fuel ratio detection temperature characteristic of the present invention. In this graph, the temperature Temp ₁ is lower than Temp ₂ , but the output voltage is higher at the lower temperature Temp ₁ , the reason is that the impedance of the zirconia electrolyte increases as the temperature decreases. Therefore, the voltage drop also increases due to the influence of the current Ip and the excess voltage of the resistive loss. To avoid increasing the voltage drop, the current Ip must be small.

Fig. 4 shows the detector output characteristic curves obtained by different constant current values. In this graph, the current Ip ₃ is larger than the current Ip ₄ , the constant current Ip ₃ exhibits good output characteristics in the whole range of the excess air factor λ, and the constant current Ip ₄ gives a lower The output voltage. The reason for this phenomenon is that the carbon monoxide in the exhaust gas in the dense area increases, as a result, more carbon monoxide enters the diffusion area, and due to the lack of oxygen intake required for the chemical reaction shown in (3), the positive electrode and the negative electrode are three The oxygen concentration at the phase junction is close, so the electromotive force drops.

As mentioned above, there is a problem with the air-fuel ratio detector, that is, the jump point of the voltage output will move to the side of the lean region relative to the point of λ=1. As carbon monoxide increases, the output voltage will drop. The above problems depend to a large extent on the value of the constant current and the area of the gas diffusion holes. Since the oxygen concentration is from The number of oxygen molecules entering the diffusion region from the negative electrode is controlled, so the size of the gas diffusion holes is closely related to the problem that the device may be damaged due to the increase in the pressure of the diffusion region.

In view of these circumstances, the inventors of the present invention have made experimental air-fuel ratio detectors _S1 and _S2 , the diameters of their gas diffusion holes are different, thereby further confirming the above facts, Figure 5 shows the experimental result. Due to the action of the excitation current, the drift of the detected lambda value gradually increases with the increase of the current. The fundamental reason is that the increase of current can cause more oxygen to enter the diffusion region, so that an oxygen-deficient state is produced at the three-phase interface of the negative electrode farther away from the λ=1 point. Therefore, the lambda value at which the electromotive force of the detector changes suddenly produces a drift phenomenon between the Ip ₁ and Ip ₂ curves, as shown in Figure 2. In order to minimize the deviation of λ at the output transition point, the value of the constant current Ip must be selected as small as possible. As shown in Figure 3, a small Ip value is also good for preventing the increase of voltage drop due to the increase of impedance in the low temperature region. If the Ip value can be made small, the gas diffusion hole can be allowed to have a smaller aperture, which is beneficial to the miniaturization of the air-fuel ratio detector. However, as shown in FIG. 4, if the excitation current Ip is too small, the oxygen entering the diffusion region 4 also decreases, and as a result, the output in the dense region decreases due to an increase in the diffusion amount of carbon monoxide. The dotted line in Figure 5 shows the characteristics of this area and the characteristics of different gas diffusion holes compared. For example, the aperture of detector S ₁ is 0.12mm, and the aperture of detector S ₂ is 0.03mm. It can be found from the figure that the gas The detector S ₂ with a smaller diffusion hole diameter can detect the point λ=1 more accurately.

As mentioned above, in the diffusion area, the pressure of the diffusion area increases due to the too small diameter of the gas diffusion hole, which causes the problem of device damage. After the inspection of the detector _S2 , the results confirm that the detector stack The maximum force v of is so small that there is no need to worry about the problem of device damage.

Finally, the experiments further confirmed that although the characteristics of the detector depend on the length of the diffusion pore, However, good detector characteristics can be obtained as long as the area of the gas diffusion holes is equal to or less than 1/100 of the area of the negative electrode.

Fig. 6a and Fig. 6b are drawings related to the gas diffusion hole shape of the detector prototype. Fig. 6 a is a transverse sectional view, Fig. 6 b is a longitudinal sectional view, Fig. 7 is used to illustrate the manufacturing process of the air-fuel ratio detector of the present invention, the diameter of the gas diffusion hole 5 is so small that it can be formed by drilling or perforation technology Thus, as shown in Figure 6b, it is a lateral groove made in the detector cell.

The actual manufacturing process of the detector structure shown in Fig. 6a and Fig. 6b can be briefly described with reference to Fig. 7 . First, brush platinum glue 20 and 30 on both sides of the unsintered zirconia sheet 10, thereby forming an electrode with a thickness of about 10 μm; secondly, carbon-containing organic adhesive 70 is printed on both sides of the platinum electrode 20. At the end, the ceramic sheets 60 and 60' that adhere well on the green sheet 10 are also printed on the outside of the adhesive 70, wherein the ceramic sheets 60 and 60' can be the same material as the green sheet 10 . The organic adhesive 70 is again printed on the ceramic sheet 60' and the platinum paste 20 to form gas diffusion holes and diffusion regions. They are then covered with a green covering 10' and heated and pressed to about 200°C. In the elevated temperature state during hot pressing, the organic binder 70 is burned off, thereby forming gas diffusion holes and diffusion regions. Finally, after a calcination process at 1500°C, the air-fuel ratio detector shown in Figure 6a and Figure 6b is obtained.

Fig. 8 and Fig. 9 show the output characteristic and response characteristic curves of the manufactured air-fuel ratio detector shown in Fig. 6a, Fig. 6b and Fig. 7, respectively. The device produces a somewhat delayed output transition at excess air factor λ=1. Compared with generally existing oxygen detectors, the dynamic characteristics of the device are sensitive.

A disadvantage of air-fuel ratio detectors utilizing the oxygen pumping phenomenon, including the present invention, is that they cannot operate in low temperature regions. For this reason, when using the detector, it should be heated to an operating temperature above 500°C.

As an example, Figure 10 shows an air-fuel ratio detector with a heater made by thick film technology, on both sides, an insulating layer (such as an insulating layer of aluminum oxide) is made to It is isolated from the zirconia electrolytic layer.

The inventors of the present invention have experimented with the influence of the area of the gas diffusion holes on the measurement accuracy of the ideal air-fuel ratio, and FIG. 11 is the experimental result. In this graph, the λ value detected by the test sample that does not destroy the output in the dense area is taken as the ordinate, and the ratio of the electrode area to the area of the gas diffusion hole is taken as the abscissa, thus obtaining the relationship between λ and the area ratio curve. For each parameter range, experiments were carried out using dozens of test prototypes. Among these parameters, the area of the electrode is from 3-200 mm, the width of the gas diffusion hole is 0.002-3 mm, and the length is from 0.1-7 mm. It is found through experiments that the lambda values detected by these samples (eg, the lambda value at the detector output jump point) are distributed in the shaded part of the graph. The trend of the characteristic curve can be seen from the figure, that is, the shorter the length of the gas diffusion hole, the larger the value of the detected λ value moves toward the lean area. The reason for this phenomenon is that a large amount of carbon monoxide enters through the short gas diffusion hole. the diffusion zone. As shown in the figure, in order to control the deviation of the λ value detected by the test prototype within a few percent range, it is required

Claims (4)

1. An air-fuel ratio detector placed in the exhaust gas discharged from the internal combustion engine can be used to detect the exhaust gas mixture to determine whether the air-fuel ratio in the internal combustion engine is equal to the ideal air-fuel ratio. The characteristics of the air-fuel ratio detector in: A solid zirconia electrolytic layer; A pair of electrodes respectively located on both sides of the electrolytic layer; a cover over said solid zirconia electrolytic layer and enclosing said one electrode, with a diffusion zone formed between said cover and electrolytic layer; a gas diffusion hole for communicating the diffusion region with the outside of the detector; Wherein, a constant excitation current provided by a constant current source device is conducted between an electrode in the diffusion zone as a positive electrode and another electrode as a negative electrode, so as to provide a constant amount of Oxygen molecules, so as to control the oxygen concentration in the diffusion region to make it a reference oxygen concentration, and can detect whether the air-fuel ratio is equal to the ideal air-fuel ratio from the voltage generated between the two electrodes. 2. The detector according to claim 1, characterized in that the area of the gas diffusion hole is 1/100-1/1000 of the area of the negative electrode. 3. The detector according to claim 2, characterized in that the gas diffusion hole and the diffusion area are made by firing an organic adhesive, and the diffusion hole is in the shape of a groove. 4. A detector according to claim 3, characterized in that said detector has a heater.

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