JPH0348148A - Detection of air/fuel ratio - Google Patents

Abstract

PURPOSE:To easily detect the shift from a theoretical air/fuel ratio by simple and highly durable constitution by exposing a solid electrolyte substrate having an electrode and a gas diffusion limiting means mounted on the surface thereof to gas to be measured and detecting the internal resistance change of the substrate and comparing the detected value with the max. value of internal resistance showing a theoretical air/fuel ratio. CONSTITUTION:Porous electrodes 4, 6 composed of an yttria-zirconia solid solution and platinum, a platinum heater 8 and the porous gas rate limiting layer 12 for limiting gas diffusion covering the electrodes 4, 6 are provided to the single surface of a solid electrolyte substrate composed of a yttria- zirconia solid solution to form an air/fuel ratio sensor 1. When this sensor 1 is exposed to the exhaust gas of a vehicle passing through a catalyst, the internal resistance of the substrate 2 is changed corresponding to an air/fuel ratio and the corresponding output is taken out through the electrodes 4, 6 and compared with the output corresponding to the theoretical air/fuel ratio making internal resistance max. to detect the shift from the theoretical air/fuel ratio. By this method, the shift from the theoretical air/fuel ratio can be easily and certainly detected by simple and highly durable constitution.

Description

[Detailed Description of the Invention] [Industrial Application Field] A friend of the present invention: Burning bag! For example, the present invention relates to a method of detecting an air-fuel ratio based on the concentration of gas components in exhaust gas from an engine or the like.

[Prior Art] Conventionally, three-way catalysts such as platinum and palladium have been used for the purpose of purifying exhaust gas from engines and the like.

This three-way catalyst 1. By controlling the fuel mixture near the stoichiometric air-fuel ratio, Co, HC, and N O in the exhaust gas can be reduced.
It is possible to promote the reaction between x and suppress the amount of discharge of each component to a low level. Therefore, 1: Usually, an air-fuel ratio sensor is placed upstream of the three-way catalyst to detect the air-fuel ratio.
Control is performed to feed back the air-fuel mixture to the stoichiometric air-fuel ratio.

The performance of this three-way catalyst may deteriorate after long-term use, so in recent years, in order to detect the decline in performance, the downstream side of the three-way catalyst has also been A device equipped with an air-fuel ratio sensor similar to the sensor used for feedback control has been proposed. This device is a friend
When the three-way catalyst deteriorates, the reaction between the above components decreases, so the oxygen partial pressure downstream of the three-way catalyst (hereinafter referred to as the exhaust gas air-fuel ratio) changes to a value corresponding to the stoichiometric air-fuel ratio (hereinafter referred to as the exhaust gas theoretical air-fuel ratio). When the output of the air-fuel ratio sensor on the downstream side of the three-way catalyst exceeds a predetermined value, that is, when the air-fuel ratio of exhaust gas deviates significantly from the stoichiometric air-fuel ratio. It was determined that the three-way catalyst had deteriorated (
(Refer to Utility Model Application Publication No. 1983-83415).

[Problems to be Solved by the Invention] However, since the air-fuel ratio sensor used for the above-mentioned feedback control usually precisely detects the air-fuel ratio of exhaust gas over the entire range of notch and lean, oxygen concentration battery elements and It requires a complicated configuration such as an oxygen pump element, a gas introduction hole, and a gas diffusion chamber, so it cannot necessarily be said to have excellent durability.It also has a large number of parts and a complicated manufacturing method, which can lead to failures. According to the present invention 1, there was an economical problem of increased costs.
．． It is an object of the present invention to provide a detection method that can easily detect deviations of the air-fuel ratio from the stoichiometric air-fuel ratio with a simple configuration without using a conventional air-fuel ratio sensor.

[Means for Solving the Problems] In accordance with the first invention to solve the problems, the air-fuel ratio can be determined from the gas component concentration of the measurement gas by utilizing changes in the electrical characteristics of a solid electrolyte substrate equipped with a pair of electrodes. A method for detecting, comprising: providing a gas diffusion limiting means for limiting gas diffusion by covering both the electrodes and a solid electrolyte substrate between the two electrodes; and extract the change in internal resistance of the solid electrolyte substrate according to the concentration of the gas component as an output between the two electrodes,
The gist is to detect the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio by comparing the output with an output corresponding to the maximum value of internal resistance indicating the stoichiometric air-fuel ratio.

Here, the above air-fuel ratio (table) shows the ratio of fuel and air in the air-fuel mixture as is usually used, but it is thought that it also corresponds to the oxygen partial pressure after combustion, so especially in the present invention, Friend: The oxygen partial pressure after the mixture is combusted is also expressed as the air-fuel ratio of the measured gas.In addition, when the mixture at the stoichiometric air-fuel ratio is combusted, the oxygen partial pressure can also be expressed as the exhaust gas. Expressed as the stoichiometric air-fuel ratio.

As the gas diffusion limiting means, for example, a porous gas regulating layer that covers both electrodes and the solid electrolyte substrate in the vicinity of both electrodes can be used. Furthermore, in order to control the gas rate, a plate material or the like can be used which is placed near both electrodes to form a gap.

As materials for solid electrolyte substrates: 1. t. Itria-Zirconia Solid Solution Calcia-zirconia solid solutions are known, and solid solutions of cerium dioxide, thorium dioxide, and hafnium dioxide, perovskite solid solutions, and trivalent metal oxide solid solutions can also be used.

As a material for the porous electrode (white rhodium, etc.) can be used.

[Function ■ Air-fuel ratio detection method of the present invention 1. This method is not based on the electromotive force of a solid electrolyte as in the past, but is based on the knowledge that the internal resistance of a solid electrolyte exhibits peculiar behavior depending on the surrounding gas concentration. The air-fuel ratio is detected based on the That is,
Solid electrolyte (friend) When exposed to atmospheres with different oxygen partial pressures,
Oxygen ions move to equalize the oxygen partial pressure, but the presence of these oxygen ions changes the internal resistance of the solid electrolyte, especially in an atmosphere with an oxygen partial pressure corresponding to the stoichiometric air-fuel ratio. Take advantage of the property of maximum resistance.

Therefore, for example, if a constant voltage is applied to a pair of electrodes formed on a solid electrolyte substrate via a voltage dividing resistor and the output voltage V out generated across this voltage dividing resistor is measured, the theoretical At the air-fuel ratio point (excess air ratio λ=1), a peak indicating the minimum value of the output voltage v6°, that is, the maximum value of the internal resistance, appears. This is due to the following reasons.

On the lean side of the air-to-air ratio, it is easy to secure oxygen ions flowing to the solid electrolyte substrate due to the presence of excess air.
This is considered to be because the internal resistance of the solid electrolyte substrate is therefore reduced.

On the other hand, on the rich side, 11. , analogy +1CO+1/20
This is thought to be because the presence of a gas in a state where the reaction to the right side cannot be completed, such as 2-CO2 H2+1/202-H,0, allows oxygen ions to be taken into the solid electrolyte substrate, reducing the internal resistance. .

However, when λ = 1, only CO2 or H2O exists after complete reaction on the right side of the above equation, so in order to incorporate oxygen ions, l, 1.100% reaction completed gas must be dissociated. As a result, the number of oxygen ions in the solid electrolyte becomes small, which is thought to increase the internal resistance.

For these reasons, it is thought that the internal resistance is greatest at the stoichiometric air-fuel ratio point of λ=1.

Therefore, by considering this change in internal resistance as, for example, a change in the output voltage V ouj or current amount Q mentioned above, and comparing it with the output value corresponding to the maximum value of the internal resistance,
It becomes possible to detect the deviation of the measured air-fuel ratio from the stoichiometric air-fuel ratio.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 shows a schematic configuration diagram of a detection device (hereinafter simply referred to as an air-fuel ratio sensor) 1 used for detecting the air-fuel ratio of this embodiment, and FIG. 3 shows its III-II+ sectional view.

As shown in both figures, an air-fuel ratio sensor 1 (one side 1 of a solid electrolyte substrate 2 mainly made of an yttria-zirconia solid solution) is provided with a porous electrode 4.6 made of an yttria-zirconia solid solution and platinum; A square-shaped platinum heater 8 is arranged.One of the porous electrodes 4 and 6 is connected to the negative terminal of the constant voltage power supply El through the resistor R1, and the other is connected to the positive terminal through the heater 8. The window 10 in contact with the measurement gas is directly connected to the electrode.
, that is, the above-mentioned both pot porous electrodes 4.6 and both pot porous electrodes 4.6
A porous gas rate controlling layer 12 is formed on the surface of the fixed electrolyte substrate 2 between the two. Further, dense shielding layers 13 and 14 made of alumina are laminated on parts other than the window portion 10.

Next, the manufacturing procedure of the air-fuel ratio sensor 1 will be explained based on FIG. 4.

First, the sheet 20 that will become the solid electrolyte substrate 2 is manufactured by the doctor blade method using ytria-zirconia powder to which about 2.5% by weight of silica is added as a sintering aid, a PVB binder, and an organic solvent. do.

Then, a thin and dense alumina coat layer 22 that will become the shielding layer 13 is formed on the sheet 20 except for the window portion 10 .

Subsequently, in order to form the porous electrodes 4 and 6 on the alumina coat layer 22 and the window portion 10, the co-base material 16″IL amount%
and a specific surface area of 10 m2/g or less (for example, 4 to 6 m'/g)
g) Platinum powder is made into a paste using a cellulose-based or PVB-based binder and a solvent such as butylcarpitol, and this paste is printed by a screen. At the same time, a heater 8 made of platinum is also printed.

Next 1: To cover the surface of each of the laminated members, except for the window portion 10 and the connection portion 24, with a dense alumina coat layer 26 that will become the shielding layer 14, and to form the porous gas rate controlling layer 12. Next, the window portion 10 is filled with alumina paste 28 containing a binder.

Then, in this laminated state, normal firing is performed at about 1500° C. for 1 hour. Thereafter, lead wires (not shown) and the like are connected to complete the air-fuel ratio sensor 1.

Next 1: Implementation using this air-fuel ratio sensor 1 However, the method for determining the deterioration of the three-way catalyst by detecting fluctuations in the oxygen partial pressure (air-fuel ratio of exhaust gas) on the downstream side of the three-way catalyst will be described below. 5
This will be explained based on the diagram and FIG.

As shown in FIG. A sensor 34 is arranged.The full-range air-fuel ratio sensor 34 detects the air-fuel ratio of exhaust gas immediately after being discharged from the internal combustion engine 30.
Based on the output of No. 4, the air-fuel ratio is feedback-controlled.

This feedback control causes the air-fuel ratio of the exhaust gas sent to the three-way catalytic converter 32 to pulsate around the stoichiometric air-fuel ratio of 1 to 1, as shown in FIG. 6(A).

Inside the exhaust gas 1 sent to the three-way catalytic converter 32, CO, HC, NOX, and 02 in the exhaust gas (7) react with each other. As a result, as shown in FIG. 6(B), the air-fuel ratio on the downstream side of the three-way catalytic converter 32 becomes approximately equal to the stoichiometric air-fuel ratio.

However, if the three-way catalyst 36 has deteriorated, the purification of the exhaust gas by the three-way catalyst 36 does not progress, and the problem shown in Fig. 6 (C) occurs.
As shown in 1:, the air-fuel ratio on the downstream side of the three-way catalyst 36 (upper
It pulsates greatly around the stoichiometric air-fuel ratio.

As a result, the internal resistance of the solid electrolyte substrate 2 of the air-fuel ratio sensor 1 disposed downstream of the three-way catalytic converter 32 also changes, so that its output voltage v0° (Fig. 6)
As shown in D), 1: pulsates greatly in response to pulsations in the air-fuel ratio of the exhaust gas discharged from the three-way catalytic converter 32. In other words, when the air-fuel ratio of exhaust gas is the stoichiometric air-fuel ratio,
The output voltage V 6 u t of the air-fuel ratio sensor is the minimum, and if it deviates from that value, the output voltage V ouL increases.

Therefore, the output voltage V of the air-fuel ratio sensor 1 pulsates. u
By using t, it becomes possible to detect deterioration of the three-way catalyst 36. That is, even if the air-fuel ratio of exhaust gas deviates from the stoichiometric air-fuel ratio to rich or lean, the output voltage V out will increase, so it is determined whether the output voltage v0□ exceeds the appropriate threshold value V. By this,
Deterioration of the three-way catalyst 36 can be detected.

Like this 1: The air-fuel ratio sensor 1 of the present embodiment (above) detects the deviation of the air-fuel ratio of exhaust gas from the stoichiometric air-fuel ratio and can very easily degrade the three-way catalyst 36 without adopting a complicated structure. T
h+, and has the advantage of being excellent in durability. Part 5: Because the number of parts is small, failures are less likely to occur, and because the number of manufacturing steps is reduced, manufacturing costs can be reduced.Next, regarding the air-fuel ratio sensor 40 of other embodiments, FIGS. This will be explained based on the diagram.

As shown in FIG. 7 CB), a detection element 4 consisting of a solid electrolyte substrate 42, etc., is placed on the air-fuel ratio sensor 401 of this embodiment.
4 and the heater 46 are manufactured separately and placed close to each other to form a gap 48 for restricting gas diffusion.

One step in manufacturing this air-fuel ratio sensor 40 is as shown in FIG. 8. First, one side of a sheet 50 that will become the solid electrolyte substrate 42 is laminated with an alumina sheet 52, and then a porous electrode 54 is layered on the surface. 56 is printed, and then an alumina sheet 58 is laminated again to form the detection element 44. Further, the heater 46 is manufactured separately and is spaced apart from the detection element 44 by a predetermined distance (
For example, by arranging the gap 48 while maintaining a distance of 0°05 mm, a gap 48 is formed that controls the rate of gas diffusion.

Then, as shown in FIG. 7(A), 1:, heater 46
and porous electrode 54.561; voltage is applied from constant voltage power supply E2 through resistor R9*R3, output voltage V
Take out oul.

Since the air-fuel ratio sensor 40 of this embodiment does not use the porous gas regulating layer 12 as a means for restricting gas diffusion, clogging does not occur, and since the heater 46 is manufactured separately, the heater 46 There is an advantage that a suitable heating state can be obtained by manufacturing a heating pattern in a desired shape.

Next, an experimental example in which it was confirmed how the internal resistance of the solid electrolyte substrate 52 actually changes depending on the air-fuel ratio will be described based on FIG. 9. In this experiment, a sensor having almost the same configuration as the air-fuel ratio sensor 40 of the above embodiment was used, but the circuit for applying voltage to the heater 46 and the circuit for both electrodes 44 and 46 were separated, and the sensor In nine experimental examples, we assumed that the temperature could be controlled independently.
For example, 100 μA) was applied, the voltage between both electrodes 44°46 was measured, and the internal resistance R1a was calculated. Temperature Extent of diffusion restriction Applied voltage The resistance of the circuit varies depending on the type of solid electrolyte, etc., so the results of experiments were conducted by changing the temperature, i.e., the applied voltage of the heater 46, V, 4 and 0 as shown in Figure 9. As is clear from the figure, 1: Internal resistance R+nl increases rapidly as the air-fuel ratio approaches the stoichiometric air-fuel ratio from rich, and rapidly decreases as the air-fuel ratio exceeds the stoichiometric air-fuel ratio and becomes lean. Therefore, it is clear that the deviation of the air-fuel ratio of the exhaust gas from the stoichiometric air-fuel ratio can be easily detected by utilizing such a change in the internal resistance R1n and extracting it as a change in voltage or current. Note that as the temperature increases, the internal resistance RIn has the property of decreasing, so by changing the voltage vH applied to the heater 46, the internal resistance R1n can be changed, and thereby the output voltage Vout can be adjusted to any desired magnitude. Can be set to

In addition, it can be applied to detect deterioration of the air-fuel ratio sensor 1, 40 + 1 three-way catalyst 36 as described above, but it can also be used to detect deviation from the stoichiometric air-fuel ratio point, for example, by detecting incomplete combustion and issuing a warning. It can also be applied when

[Effects of the Invention] As explained above, 1. The air-fuel ratio detection method of the present invention.
By restricting the gas diffusion of the measurement gas, exposing the pair of electrodes and the solid electrolyte substrate to the measurement gas, and extracting the change including the maximum value of the internal resistance of the solid electrolyte substrate as the output between the two electrodes, the air-fuel ratio can be determined. Deviations from the stoichiometric air-fuel ratio can be detected very easily. In addition, since the state of the component concentration of the gas to be measured can be easily known with such a simple configuration, the structure of the device used for detection can also be simplified, resulting in fewer failures and high durability. Furthermore, since the manufacturing process of the device can be reduced, it is also excellent in economical efficiency.

[Brief explanation of drawings]

Fig. 1 is a graph explaining the present invention in detail, Fig. 2 is a schematic configuration of the fuel-fuel ratio sensor of this embodiment, and Fig. 3 is its III.
-I11 cross section is shown in Fig. 4, an explanation showing its manufacturing process, Fig. 5, an explanation showing its use position, Fig. 6, a graph showing the air-fuel ratio of exhaust gas and the output voltage of the air-fuel ratio sensor, and Fig. 7.
The figure shows a schematic configuration of an air-fuel ratio sensor according to another embodiment. FIG. 8 is an explanatory diagram showing its manufacturing process. FIG. 9 is a graph showing the relationship between internal resistance and air-fuel ratio. 40...Air-fuel ratio sensor 42...Solid electrolyte substrate 6.54.56...Porous electrode 46...Heater 2...Gas regulating layer 8...Gap Fig. 1

Claims (1)

[Claims] 1. A method for detecting an air-fuel ratio from the gas component concentration of a measurement gas by utilizing changes in the electrical characteristics of a solid electrolyte substrate provided with a pair of electrodes, the method comprising: A gas diffusion limiting means is provided to cover the solid electrolyte substrate between them to limit gas diffusion, and together with the gas diffusion limiting means, both the electrodes and the solid electrolyte substrate are exposed to a measurement gas, and the solid electrolyte substrate is adjusted to the concentration of the gas component. Take out the change in internal resistance as the output between the two electrodes,
An air-fuel ratio detection method for detecting a deviation of the air-fuel ratio from the stoichiometric air-fuel ratio by comparing the output with an output corresponding to a maximum value of internal resistance indicating the stoichiometric air-fuel ratio.