JPS6157846A - Air fuel ratio sensor - Google Patents

Abstract

PURPOSE:To reduce deterioration in the lapse of time in characteristics and perform air fuel ratio detection without any influence of temperature variation of an atmosphere to be measured by communicating one of a couple of oxygen sensors with the atmosphere to be measured and bringing the other into contact with an atmosphere whose oxygen concentration is controlled, and detecting an air fuel ratio from the potential at the intermediate point between both sensors. CONSTITUTION:Oxygen sensors 26 and 36 consisting of transition metal oxides 23 and 33, and electrodes 24 and 25, and 34 and 35 are provided on both surfaces of a plate type electric insulator 22. A pump current Ip is flowed from a pump current source 63 to the oxygen pump 46 consisting of an oxygen ion conductive electrolyte 43 and a couple of electrodes 44 and 45 to control the oxygen concentration of a control atmosphere space 43. The small hole 46 of the oxygen pump 46 contacts the atmosphere to be measured together with the oxygen sensor 36. Both sensors 26 and 36 are connected to a power source 61 in series and the voltage Vs at the intermediate point is measured by a voltage measuring instrument 62. The resistance of the sensor decreases when Ip is flowed, so Ip when Vs is constant is measured to detect the air fuel ratio. Deterioration in the lapse of time in characteristics which is caused by clogging, etc., is reduced because of multilayer structure.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an air-fuel ratio sensor used to detect the ratio of air to fuel, that is, the air-fuel ratio, based on the oxygen concentration in combustion exhaust gas. be.

(Prior Art) Conventionally, as this type of air-fuel ratio sensor, there was one having the configuration shown in FIG.
Publication No. 946). This air-fuel ratio sensor 1 is made of alumina (A
A first electrode 3 and a second electrode 4 made of platinum or the like are provided side by side on the surface of a dense insulating substrate 2 made of a material such as U203), and on both electrodes 3 and 4, a material made of titania (T i O2) or the like is provided. A porous insulating layer 6 made of transition metal oxide 5, alumina (AJL203), etc., a third electrode 7 made of platinum, etc., a porous oxygen ion conductive solid electrolyte 8 made of partially stabilized zirconia (Zr02-Cao), etc. The fourth made of platinum etc.
It has a structure in which an electrode 2 and a porous protective layer 10 are sequentially laminated,
A resistance meter 11 is connected between the first electrode 3 and the second electrode 4, and a current source 12 is connected between the third electrode 7 and the fourth electrode 2. Electrode 2, solid electrolyte 8
The current source 12 constitutes an oxygen pump for use.

In the air-fuel ratio sensor 1 having such a configuration, the third electrode 7
When no current is passed between and the fourth electrode 2 (in the case of I-0 in FIG. 13), the porous protective layer 10 → the fourth electrode 9 → the porous solid electrolyte 8 → the third electrode 7 → Depending on the oxygen concentration in the gas to be measured that has passed through the porous insulating layer 6 and reached the transition metal oxide 5, the stoichiometric air-fuel ratio (A/F=14 .8), the electrical resistance of the transition metal moltenide 5 changes rapidly, and this resistance change is picked up by the resistance meter 11 via the first electrode 3 and the second electrode 4, and the air-fuel ratio change is detected. Ru.

Furthermore, when the oxygen pump is activated and a current flows from the fourth electrode 2 to the third electrode 7 (I>O in FIG. 13),
), oxygen ions move from the third electrode 7 to the fourth electrode 2 within the solid electrolyte 8 and oxygen is consumed at the third electrode 7. Since the oxygen partial pressure in the gas to be measured that reaches the transition metal oxide 5 decreases, the oxygen concentration in the gas to be measured reaches the stoichiometric air-fuel ratio, as shown in the region I>O in FIG. When the transition metal oxide 5 is larger than , the electrical resistance of the transition metal oxide 5 changes rapidly, and it can be used as a sensor for detecting a lean air-fuel ratio. On the other hand, when the current is passed in the direction from the third electrode 7 to the fourth electrode 2, contrary to the above (
In the case of I<O in FIG. 13), oxygen is generated at the third electrode 7, so the transition metal oxide 5 has an electrical resistance on the rich side as shown in the region of I<O in FIG. Characteristics that change rapidly.

In this way, the air-fuel ratio sensor 1 shown in FIG.
The stoichiometric air-fuel ratio (A/F
- approx. 14,8) and lean side (A/F > approx. 14.8)
), it has the advantage of being able to detect the air-fuel ratio on the rich side (A/F<approximately 14,8).

However, since the air-fuel ratio sensor 1 shown in FIG. 12 has a multilayer structure, it is difficult to control the pores in each layer, resulting in large variations as a whole, and the pores in each layer may become clogged. However, it has the disadvantage that its characteristics tend to deteriorate over time.

In addition, although this air-fuel ratio sensor 1 can obtain the characteristics shown in FIG. 13 when the ambient temperature is constant,
The resistance value of the transition metal oxide 5 not only changes depending on the oxygen partial pressure in the gas to be measured that reaches the transition metal oxide 5, but also as shown in FIG. 14 (however, the case where I=O is illustrated). As such, there is a problem in that the air-fuel ratio cannot be immediately detected from the resistance change of the transition metal oxide 5 because it changes greatly depending on the temperature.

(Objective of the Invention) The present invention was made by focusing on the above-mentioned conventional problems, and because it does not have a multilayer structure, the variation in characteristics is extremely small. It is an object of the present invention to provide an air-fuel ratio sensor that has almost no deterioration and can detect air-fuel ratios with high accuracy without being affected by temperature changes in the atmosphere to be measured.

(Structure of the Invention) The air-fuel ratio sensor according to the present invention includes a pair of oxygen sensors that utilize resistance changes caused by changes in oxygen concentration of transition metal oxides on the surface of an electrical insulator, and one of the pair of oxygen sensors is covered. Both oxygen sensors are connected in series and connected to a power source, with the other being in communication with the measurement atmosphere and the other being in contact with a controlled atmosphere in which the oxygen concentration is controlled by an oxygen pump that uses oxygen ion conduction in a solid electrolyte. The air-fuel ratio is characterized by being configured to detect the air-fuel ratio based on the midpoint potential of .

A pair of oxygen sensors constituting the air-fuel ratio sensor according to the present invention each utilizes a resistance change due to a change in oxygen concentration of a transition metal oxide. The material and forming method of the electrode forming the sensor are not particularly limited, and may be appropriately selected from various materials and forming methods that have been employed or attempted in the past. Also,
The pair of oxygen sensors may be provided on the front and back surfaces of the electrical insulator, or may be provided on the same surface.

Further, the oxygen pump constituting the air-fuel ratio sensor according to the present invention utilizes the oxygen ion conduction of a solid electrolyte, and the material of the oxygen ion conductive solid electrolyte and the pair forming the oxygen pump together with the solid electrolyte are The material of the electrode and the method of forming the same are not particularly limited, and may be appropriately selected from various materials and forming methods that have been conventionally employed or attempted.

In the air-fuel ratio sensor according to the present invention, one of the pair of oxygen sensors is in communication with the atmosphere to be measured, and the other is in contact with a controlled atmosphere in which the oxygen concentration is controlled by the oxygen pump. , this controlled atmosphere can be formed from a simple space or from a porous gas diffusion layer with a large number of microspaces.

Furthermore, in the air-fuel ratio sensor according to the present invention, it is also desirable to provide a heating element as necessary.

(Embodiment) FIG. 1 is a schematic cross-sectional view showing an air-fuel ratio sensor according to an embodiment of the present invention, and this air-fuel ratio sensor 21 includes:
A first oxygen sensor 26 is provided on both the front and back surfaces of the plate-shaped electrical insulator 22, and is configured to detect resistance changes due to oxygen concentration changes in the transition metal oxides 23, 33 using a pair of electrodes 24, 25 and 34, 35, respectively. and second oxygen sensor 3
6, electrically connecting one electrode 24 of the first oxygen sensor 26 and one electrode 35 of the second oxygen sensor 36 through a through hole 22a provided in the electrically insulating plate 22, An oxygen ion conductive solid electrolyte 4 is placed on the surface of the electric insulator 22 on the first oxygen sensor 26 side of one of the two solute sensors 26 and 36, with a frame 41 in between.
2 to form a controlled atmosphere space 43 between the electrical insulator 22 and the solid electrolyte 42;
A pair of electrodes 44 and 45 are provided on the front and back surfaces of the solid electrolyte 4.
The oxygen pump 46 utilizes the oxygen ion conduction of No. 2, and a small hole 47 is provided in the center of the oxygen pump 46 to communicate the controlled atmosphere space 43 with the atmosphere to be measured.

FIG. 2 is a diagram showing the procedure for manufacturing the air-fuel ratio sensor 21 having the configuration shown in FIG. Prepare this electrical insulator 22
The electrodes 24 and 25 on the first oxygen sensor 26 side and the electrode lead part 27 are formed on the negative side of the electrical insulator 2 using a paste made of platinum, for example, and dried.
2, electrodes 34 and 35 on the second oxygen sensor 36 side are formed using a paste made of platinum and dried, and the platinum paste is also applied to the through holes 22a and 22b formed in the electrical insulator 22. is poured so that electrical continuity can be obtained between the electrodes 24, 35 and the electrode lead portion 27 after firing, and electrical connection to the electrode 34 is made possible via the through hole 22b. Make it. Then, a transition metal oxide 23 made of, for example, titania (T i 02 ) paste is layered and dried on the pair of electrodes 24 and 25 on the first oxygen sensor 26 side.
A transition metal oxide 33 also made of titania paste i is laminated on the pair of electrodes 34 and 35'' on the oxygen sensor 36 side and dried.

Next, the transition metal oxide 2 of the electrical insulator 22 is
A U-shaped frame 41 made of alumina, for example, is placed around the third side, and the through hole 22b, the electrode lead portion 27 and the tip of the electrode 25 are placed on the electrode lead portion 27 side of the electrical insulator 22. For example, platinum lead wire 54
．． An electrically insulating plate (spacer) 48 made of, for example, alumina is placed with the tips of 57 and 55 placed respectively.

Then, for example, 5 mol% Y2O3-95 mol% ZrO2
A green sheet made of solid electrolyte is cut to a predetermined size, and a through hole 47b for forming a small hole 47 and through holes 42a and 42b for obtaining electrical connection are formed in advance. Then, this green sheet-like solid electrolyte 4
Oxygen pump electrodes 44 and 45 are laminated on the lower surface of No. 2 using platinum paste, for example, and electrode lead portions 44a and 45a are formed thereon, as well as through holes 47a and 47c for obtaining the small hole 47, respectively. Form it.

Next, the solid electrolyte 42 is superimposed on the electrically insulating plate (spacer) 48 with the tips of the platinum lead wires 64 and 65 placed at intervals corresponding to the through holes 42 a and 42 b, Platinum paste is poured into the re-through holes 42a and 42b to ensure electrical connection between both electrodes 44.45 and lead wires 64.65.

Subsequently, the laminate thus obtained is heated to a temperature of, for example, 1400~
Fire at 1500°C.

When providing a heating element, a heating element 61 is laminated using platinum paste, for example, on an electrically insulating substrate 60 made of, for example, an alumina green sheet, dried, and lead wires 62 are connected to both ends of the heating element 62. After fixing .63, 1
The oxygen sensor side electric insulator 22 and the heating element side electric insulating substrate 60 are then fixed with a heat-resistant adhesive.

Figure 3 shows an air-fuel ratio sensor 21 having the structure shown in Figure 1 above.
FIG. 3 is a diagram showing an example of the wiring connection, in which a constant voltage source 61 is connected between one electrode 25 of the first oxygen sensor 26 and one electrode 34 of the second oxygen sensor 36 via lead wires 55 and 54. At the same time, a voltage measuring device 62 is connected between both electrodes 34 and 35 of the second stimulant sensor 36, and the oxygen pump 46
An oxygen pump current source 63 is connected between both electrodes 44, 45 via lead wires 64, 65.

FIG. 4 shows a similar circuit between the first oxygen sensor 26 and the second oxygen sensor 36 in FIG. 3, and shows the resistance of the transition metal oxide 23 between the electrodes 24 and 25 of the first oxygen sensor R1 and the resistance of the transition metal oxide 33 between the electrodes 34 and 35 of the second oxygen sensor 36 are shown as R2, the voltage of the constant voltage source 61 is shown as Vc, and the voltage measured by the voltage measuring device 62 is shown as Vs. In such a relationship, the measured voltage Vs is expressed by the following equation. Here, the resistance value R2 hardly changes on the spring side and can be handled in accordance with the reference resistance. Also,
When the resistance R1 is kept in a rich state, the resistance value changes depending on the air-fuel ratio. Therefore, it is preferable to keep the controlled atmosphere space 43 in a warm atmosphere using the oxygen pump 46.

By the way, since both oxygen sensors 26.36 are composed of the same transition metal moltenide 23.33, they have the same temperature dependence.Therefore, the ratio of R1/R2 in the -temperature type is constant, so the K111 constant voltage The oxygen sensor output, denoted Vs, does not change with temperature.

Therefore, a current is supplied from a pump current source 63 to an oxygen pump 46 composed of an oxygen ion conductive solid electrolyte 43 and a pair of electrodes 44 and 45, and a pump current Ip is applied in the direction from the electrode 45 to the electrode 44, for example. When flowing, oxygen ions flow from the electrode 44 toward the electrode 45, so that the oxygen in the controlled atmosphere space 43 is exhausted. Here, since the combustion exhaust gas flowing into the controlled atmosphere space 43 is restricted by the small holes 47, the oxygen concentration in the controlled atmosphere space 43 is lower than the oxygen concentration in the combustion exhaust gas. Therefore, by flowing the pump current Ip, the value of the resistance R1 of the first oxygen sensor 26 becomes smaller, and therefore the measured voltage Vs becomes larger.

Conversely, in the oxygen pump 46, the electrode 44 to the electrode 45
When the pump current Ip is passed in the direction of the controlled atmosphere space 43
Since the oxygen concentration in the combustion exhaust gas becomes higher than the oxygen concentration in the combustion exhaust gas, the value of the resistance R1 of the first oxygen sensor 26 becomes large, and the measured voltage Vs becomes small.

Therefore, in order to obtain the pump current Ip such that the output Vs of the air-fuel ratio sensor 21 is constant, the output of the air-fuel ratio sensor 21 is converted to the R-V conversion circuit 71 as shown in FIG.
(FIG. 4), the differential amplifier 72 compares the reference voltage with the reference voltage from the reference voltage generation circuit 73, and differentially amplifies it. Oxygen pump 46 by changing the pump current from 63
When the pump current is measured by the current detection circuit 75, an equivalence ratio-pump current characteristic as shown in FIG. 7 is obtained, and the equivalence ratio can be detected by detecting the pump current.

In this case, more specifically, a circuit as shown in FIG. 6 can be used. FIG. 6 is a diagram showing an example of a circuit configuration for detecting the air-fuel ratio by determining the pump current IP such that the output Vs of the air-fuel ratio sensor 21 shown in FIG. 1 is constant. In the circuit configuration shown in the figure, a reference voltage is input from a reference voltage generation circuit 73 to one side of the differential amplifier 72, and the transition metal oxide 23 of the first oxygen sensor 26 is input to the other side of the differential amplifier 72.
and the transition metal oxide 33 of the first oxygen sensor 36 are electrically connected in series and a voltage V is applied to convert the resistance value change due to the atmosphere of the transition metal oxide 23 into a voltage change. Input via R3. Further, the output of the differential amplifier 72 is connected to one electrode 45 of the oxygen pump 46 via a resistor R4, and the other electrode 42 of the pump 46 is input as a change in pump current IP to, for example, the air-fuel ratio control circuit of the engine. be done.

In such a circuit configuration, for example, when the oxygen concentration in the gas to be measured becomes high (i.e., an atmosphere with excess air) and the resistance R1 of the transition metal oxide 23 becomes large, the oxygen is input to one side of the differential amplifier 51. The voltage becomes lower and smaller than the reference voltage, and as a result, the output of the differential amplifier 51 increases to the positive side, and the pump current flowing from one electrode 45 to the other electrode 42 increases. On the other hand, when the oxygen concentration in the gas to be measured becomes low (that is, a rich atmosphere with excess fuel) and the resistance value R1 of the transition metal oxide 23 becomes small, the voltage input to one side of the differential amplifier 51 becomes high. becomes larger than the reference voltage, the output of the differential amplifier 51 is inverted, and the pump current flowing from the electrode 42 to the electrode 45 becomes larger.As a result, the characteristics shown in FIG. 7 are obtained, and this pump current The air-fuel ratio can be detected by Ip, and combustion control of the automobile engine can be performed.

FIG. 8 is a diagram showing an example of the results of comparing the characteristics of the conventional air-fuel ratio sensor 1 and the air-fuel ratio sensor 21 according to the present invention.
The sensor temperature was set at 700'O, and the characteristics of each 10 sensors were evaluated. As shown in FIG. 8, in the air-fuel ratio sensor 21 according to the present invention, the range is between the lines 21a and 21b, whereas in the conventional air-fuel ratio sensor 11, the range is between the lines 11a and 11b.
It was observed that there was considerable variation between lbs.

FIG. 9 is a diagram showing another embodiment of the present invention, in which the first
In the air-fuel ratio sensor 21 shown in the figure, a small hole 47 through which combustion exhaust gas flows into a controlled atmosphere 43 is provided in a pair of electrodes 44, 45 and an oxygen ion conductive solid electrolyte 42 that constitute an oxygen pump 46. , air-fuel ratio sensor 2 in FIG.
1 has a structure in which a small hole 47 having the same effect as the above is provided in a portion of the frame 41 that is remote from the oxygen pump 46.

FIG. 10 shows still another embodiment of the invention,
In the air-fuel ratio sensor 21 shown in FIG. 1O, the frame 41 configuring the air-fuel ratio sensor 21 in FIG. A heating element 61 is placed on the second oxygen sensor 36 side of the
The heating element 61 and the second oxygen sensor 36 are covered with a porous protective layer 67.

FIG. 11 is a diagram showing still another embodiment of the present invention, in which the air-fuel ratio sensor 21 is configured such that one end of an electrical insulator 22 provided with a first oxygen sensor 26 and a second oxygen sensor 36 is attached to a support 78. At the same time, one end of the solid electrolyte 42 constituting the oxygen pump 46 is also supported by the support 78, and the electric insulator 22 and the solid electrolyte 42 are brought close to each other via the controlled atmosphere space 43. , has a configuration in which the controlled atmosphere space 43 is communicated with the atmosphere to be measured via a slit 72. Even with such a configuration, the same effects as in the embodiment described above can be obtained.

(Effects of the Invention) As explained above, in the air-fuel ratio sensor according to the present invention, a pair of oxygen sensors are provided on the surface of an electrical insulator that utilize resistance changes due to changes in oxygen concentration of transition metal oxides, and One of the sensors is connected to the atmosphere to be measured, and the other is brought into contact with a controlled atmosphere in which the oxygen concentration is controlled by an oxygen pump that utilizes oxygen ion conduction in a solid electrolyte, and both oxygen sensors are connected in series to provide a power source. Since the sensor is connected to the sensor and the air-fuel ratio is detected by the midpoint potential of both oxygen sensors, the variation in the air-fuel ratio detection characteristics is extremely small because it does not have a multilayer structure, and the characteristics change due to clogging caused by the diffusion of combustion exhaust gas. The risk is very small, and the output characteristics do not change due to changes in ambient temperature, so it is possible to detect a stable air-fuel ratio, and it has the great effect of allowing highly accurate air-fuel ratio control.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of an air-fuel ratio sensor according to an embodiment of the present invention, FIG. 952 is an exploded perspective view showing the manufacturing procedure of the air-fuel ratio sensor of FIG. An explanatory diagram showing a wiring example of the air-fuel ratio sensor, Fig. 4 is an equivalent circuit diagram of the wiring shown in Fig. 3, Fig. 5 is a system diagram for controlling the output of the air-fuel ratio sensor shown in Fig. 1, and Fig. 6 is a diagram showing the air-fuel ratio sensor. FIG. 7 is an explanatory diagram showing an example of a circuit that controls the output of the fuel ratio sensor. FIG. 7 is an explanatory diagram showing the relationship between the equivalence ratio and pump life flow obtained by the output control in FIGS. 5 and 6. FIG. Graphs showing the evaluation results of the conventional air-fuel ratio sensor and the air-fuel ratio sensor according to one embodiment of the present invention, and FIGS. 9, 10, and 11 are all models of the air-fuel ratio sensor according to another embodiment of the present invention. FIG. 12 is a schematic cross-sectional diagram of a conventional air-fuel ratio sensor, FIG. 13 is an explanatory diagram showing the output characteristics of the air-fuel ratio sensor of FIG. 12, and FIG. 14 is a schematic cross-sectional diagram of a conventional air-fuel ratio sensor. FIG. 2 is an explanatory diagram showing temperature characteristics of a sensor. 21... Air-fuel ratio sensor, 22... Electric insulator, 23... Transition metal oxide, 24.25... Electrode, 26... First oxygen sensor, 33... Transition metal oxide , 34.35... Electrode, 36... Second oxygen sensor, 42... Oxygen ion conductive solid electrolyte, 43... Controlled atmosphere space, 44.45... Electrode, 46... Oxygen Pump, 47... Small hole (communicating part with the atmosphere to be measured), 77...
-Slit (communicating part with the atmosphere to be measured). Patent Applicant: Nissan Motor Co., Ltd. Representative Patent Attorney Yutaka Oshio (di) 3; 11. Fist! (℃t
U) '4:! death. Shisa V Fig. 12 1\ / /9y” JP-A-1-57846 (10) Fig. 14

Claims (1)

[Claims] (1) A pair of oxygen sensors that utilize resistance changes due to changes in oxygen concentration of transition metal oxides are provided on the surface of an electrical insulator, one of the pair of oxygen sensors is communicated with the atmosphere to be measured, and the other is connected to a solid electrolyte. At the same time, both oxygen sensors are connected in series and connected to a power source, and the air-fuel ratio is detected by the midpoint potential of both oxygen sensors. An air-fuel ratio sensor characterized by having the following configuration.