JPS6034065B2 - Air fuel ratio detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an air-fuel ratio detection method particularly suitable for detecting an air-fuel ratio leaner than the stoichiometric air-fuel ratio (the ratio of air to fuel in a stoichiometric state).

Oxygen sensor elements are known that use an oxygen ion-conducting collective electrolyte and apply the principle of an oxygen concentration battery to detect oxygen concentration.

FIG. 1 is an explanatory cross-sectional view showing an example of this type of oxygen sensor element, in which reference marks are placed on the inner and outer surfaces of a solid electrolyte tube 1 made by shaping and sintering Zr02 solid electrolyte powder stabilized by adding Y203 into a tube shape. A polar electron conductive layer 2 and a measuring electrode electronic conductive layer 3 are provided, and the reference electrode electronic conductive layer 2 is exposed to the atmosphere A, and the measuring electrode electronic conductive layer 3 is exposed to a test gas (for example, from an internal combustion engine) through a protective layer 4. The structure is such that it is exposed to exhaust gas). At this time, the protective layer 4 is provided to prevent the high-temperature, high-speed exhaust gas of the internal combustion engine from directly contacting the measurement electrode electron conductive layer 3, thereby improving the durability of the oxygen sensor element. Therefore, according to the principle of an oxygen concentration battery, an electromotive force E is generated based on the following equation {1} in accordance with the difference in oxygen partial pressure on both sides of the solid electrolyte tube 1. E spicy 1 Nozo .・・・． ...[11 However, R: gas constant T: absolute temperature F: Faraday constant P.

2: Measured polar oxygen partial pressure By the way, as shown in FIG. 2, the oxygen concentration in the exhaust gas gradually increases from around the stoichiometric air-fuel ratio, but the measured polar oxygen partial pressure P.

2, as shown in FIG. 3, the air-fuel ratio is higher than the stoichiometric air-fuel ratio (A/F<14 in the case of a gasoline internal combustion engine) due to the catalytic action of platinum (Pt) forming the measurement electrode electron conductive layer 3. ．．
7), it becomes approximately tm, and when the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, the oxygen partial pressure increases sharply.

Therefore, the electromotive force (E) and air-fuel ratio (A
/F), as shown in Figure 4, shows the characteristic that a large electromotive force is generated on the fuel excess side, and the stoichiometric air-fuel ratio (input = 1
) can be detected. In addition, as this type of oxygen sensor element, a membrane structure type oxygen sensor element in which a reference electrode electron conductive layer, an oxygen ion conductive solid electrolyte layer, and a measurement electrode electron conductive layer are successively laminated on a substrate has also been attempted. In order to prevent the measurement electrode electron conduction layer from being directly exposed to high-temperature, high-speed exhaust gas flow, a protective layer was similarly provided on the entire surface of the measurement electrode electron conduction layer or the surface of the oxygen sensor element to improve durability. The same characteristics as in the case shown in FIG. 4 are obtained and the stoichiometric air-fuel ratio can be detected.

However, when detecting the air-fuel ratio using the conventional oxygen sensor element described above, a test gas such as exhaust gas is brought into direct contact with the electron conductive layer of the measurement electrode, and the oxygen concentration at the measurement electrode is caused by the catalytic action of the electron conduction layer of the measurement electrode. Since the stoichiometric air-fuel ratio is detected based on a sudden change in pressure around the stoichiometric air-fuel ratio (see FIG. 3), it has been impossible to detect anything other than the stoichiometric air-fuel ratio.

Therefore, although it is suitable for controlling the air-fuel ratio of an internal combustion engine at the stoichiometric air-fuel ratio, it has the disadvantage that it cannot be used for controlling the air-fuel ratio of an internal combustion engine when trying to improve fuel consumption efficiency by performing lean combustion. was. An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide an air-fuel ratio detection method capable of detecting an air-fuel ratio leaner than the stoichiometric air-fuel ratio. The purpose is to enable lean combustion in control and increase fuel consumption efficiency.

The air-fuel ratio control method of the present invention includes horizontally layering a diaphragm layer, a reference pole fin conductive layer, an oxygen ion conductive solid electrolyte layer, and a measurement pole electron conductive layer, and among the diaphragm layer and the oxygen ion conductive solid electrolyte layer. At least one of them is gas permeable, and a DC power source is connected between the reference electrode electron conductive layer and the measurement electrode electron conductive layer. The coefficient is 6×10
-9 None/2 x lo -7 mole/a skin/sec using an oxygen sensor element that is provided with a gas diffusion control layer that can control the diffusion of oxygen molecules and that can come into contact with the test gas; By forcing the movement of oxygen ions from the measurement electrode electron conduction layer toward the reference electrode electron conduction layer by means of the current supplied by It is possible to detect an air-fuel ratio leaner than the stoichiometric air-fuel ratio by lowering it by a desired value and measuring the electromotive force generated in response to the difference between the reduced measured polar oxygen partial pressure and the reference polar oxygen partial pressure. It is characterized by the fact that

FIG. 5 is a schematic vertical cross-sectional wiring diagram of the oxygen sensor element 10 that can be applied to the implementation of the present invention, and FIG. 6 is a schematic exploded perspective view of the oxygen sensor element 10 of FIG. It is made of an insulating material such as alumina, mullite, spinel, etc., and has a heating element 12 in the shape shown in FIG.
It also maintains the strength as a structural base.

On the surface of this diaphragm layer 11, two reference electrode electron conductive layers 13 and 14 are laminated, and an oxygen ion conductive solid electrolyte layer 15 is further laminated thereon. As the material for this solid electrolyte layer 15, Y203, Cao-stabilized Z2, and other known materials can be used. A measurement electrode electron conduction layer 16 is provided on the surface of this solid electrolyte layer 15, and a voltage measuring means 17 is connected between the measurement electrode electron conduction layer 16 and the reference electrode electron conduction layer 13. In addition to making it possible to measure the power E, the measurement electrode electron conductive layer 1
6 and the other reference electrode electron conductive layer 14, a DC power source 18 is connected. At this time, the reference electrode electron conductive layer 13,
14 and the measurement electrode electron conductive layer 16 are preferably made of catalytically active platinum, and in addition, alloys of platinum and platinum group elements and other materials are preferably selected and used as appropriate. Further, when laminating the electron conductive layers 13, 14, 16 and the solid electrolyte layer 15, for example, a method may be used in which powders thereof are made into a paste, printed on a screen, and then baked, or other methods may be used. It is also possible to do so by means. Further, a heating power source 19 is connected to the heating element 12 so that the temperature of the oxygen sensor element can be controlled. Furthermore, a gas diffusion control layer 20 capable of controlling the diffusion of oxygen molecules flowing from the test gas is provided on the measurement electrode electron conduction layer 16, and this gas diffusion control layer 20 is made to be in contact with the test gas. There is. In this case, the gas diffusion control layer 20
As shown in the examples below, the gas diffusion coefficient is 6×10-9 and 2×10-7 mole/atm
・It is better to set it within the range of sec. The material for the gas diffusion control layer 20 can be, for example, mullite, spinel, forsterite, calcium zirconate, etc., and is laminated by a screen printing method using powder, a thermal spraying method, or the like. At this time, as will be described later, the diffusion coefficient of the gas diffusion control layer 20 for oxygen molecules can be selected based on the thermal spraying conditions and the like. Therefore, when using the oxygen sensor element 10 and connecting the positive electrode side of the DC power source 18 to the other reference electrode electron conductive layer 14 and supplying current, the current is directed from the measurement electrode electron conductive layer 16 to the reference electrode electron conductive layer 14. This forces the movement of oxygen ions. At this time, since the inflow and diffusion of oxygen molecules from the test gas is controlled by the presence of the gas diffusion control layer 20, the measured polar oxygen partial pressure is lower than the oxygen partial pressure in the test gas. On the other hand, the oxygen partial pressure at the reference electrode is increased by the flow of oxygen ions toward the reference electrode electron conductive layer 14, but the oxygen molecules present in the reference electrode electron conductive layer 14 are The oxygen ions are diffused within the diaphragm layer 11, or when the corneal membrane layer 11 is dense and the solid electrolyte layer 15 is porous, the oxygen ions are diffused within the solid electrolyte layer 15, or both. A reference polar oxygen partial pressure is maintained in which the inflow and the diffusion of the oxygen molecules are balanced. Therefore, if the voltage measuring means 17 measures the electromotive force E generated in response to the difference between the measured polar oxygen partial pressure, which is lower than the oxygen partial pressure in the test gas, and the reference polar oxygen partial pressure, While maintaining a relationship in which the measured polar oxygen partial pressure is lower than the oxygen partial pressure in the test gas, the measured polar oxygen partial pressure changes in response to changes in the oxygen partial pressure in the test gas, so the theoretical It becomes possible to detect an air-fuel ratio that is leaner than the fuel ratio. Below, this will be further explained in the 8th section.
This will be explained in detail based on the figures. FIG. 8 is an explanatory functional cross-sectional diagram of an oxygen sensor element used in carrying out the air-fuel ratio detection method of the present invention, and in this case, a porous diaphragm layer 11 whose diffusion coefficient for oxygen is k, a reference A state in which a polar electron conductive layer 14, a solid electrolyte layer 15, a measurement polar electron conductive layer 16, and a diffusion layer 20 having a diffusion coefficient for oxygen of k2 are laminated in this order is shown. Therefore, when the DC power supply 18 is connected and a DC current 1 is applied from the reference electrode electron conduction layer 14 to the measurement electrode electron conduction layer 16, as shown in FIG. Q (mol/
sec) and is expressed by the relationship Q=1/Cape. Furthermore, if the oxygen partial pressure in the test gas is P, then the oxygen partial pressure P2 in the measurement electrode electron conductive layer 16 is Q=k2(P−
P2) ......From the relational expression '21, P2=P-Q/k2...
'3}.

By the way, the relationship between the oxygen partial pressure P in the test gas, for example, the exhaust gas of a gasoline internal combustion engine, and the air-fuel ratio (A/F) is as shown in FIG. Partial pressure P increases. Also, from the relationship of Q=1/small, the above formula (3'', P2=P-1/(4F・k2)
...It is expressed by the relationship [4], and by selecting appropriate values for the current 1 and the diffusion coefficient k2,
For example, as shown in Fig. 10, the relationship between the measured polar oxygen partial pressure P2 and the air-fuel ratio (A/F) is such that the measured polar oxygen partial pressure P2 becomes almost 0 until the air-fuel ratio (A/F) = 16. can do.

In other words, the measured polar oxygen partial pressure P2 can be made lower than the oxygen partial pressure P in the exhaust gas. On the other hand, the reference electrode oxygen partial pressure P, in the reference electrode electron conductive layer 14 is Q=k, (P
, 1P) ...[From the relational expression 5', P
, =Q/k, 10P......(6}).

Therefore, by decreasing the diffusion coefficient k, of the diaphragm layer 11, P,
》》P, and the above formula ■ is P, = Q/k,
...{7}. Further, the reference electrode oxygen partial pressure P. of the other reference electrode electron conductive layer 13 is ' is approximately the oxygen partial pressure P
, so the above formula {7feP,'=Q/k.
......(8}.

From the above relationship, the electromotive force E of the oxygen sensor element shown in FIG.
．．・Represented by the side, E:-band nP fan Q voice K2 .・It is represented by the side or E=-obi ln no go itoki tengaku side (...).

Therefore, as described based on the above formula {4}, by selecting appropriate values for the current 1 and the diffusion coefficient k2, the measurement electrode can be adjusted up to the air-fuel ratio (A/F) = 16 as shown in Figure 10. Adjust the oxygen partial pressure P2 to approximately 0, and adjust the diffusion coefficient k so that P,' is approximately 10-latm in the relationship P,' = Q/k, shown in equation '8) above. Then, from the relationship of Equation '9} above, the relationship between the electromotive force E of the oxygen sensor element and the air-fuel ratio (A/F) will be determined as shown in FIG. Therefore, the stoichiometric air-fuel ratio (A/F=14.7
), it becomes possible to detect an air-fuel ratio (A/F=16) that is leaner than that of A/F=16.

Furthermore, as is clear from the above formula, if the current 1 is increased,
The air-fuel ratio (A/F) at which the electromotive force E changes rapidly shifts to the leaner side, and even when the diffusion coefficient k2 of the gas diffusion control layer 20 is made smaller, the air-fuel ratio (A/F) at which the electromotive force E changes rapidly also shifts to the leaner side.
A/F) shifts to the leaner side.

In the oxygen sensor element 10 shown in FIG. 5, two reference electrode electron conductive layers 13 and 14 are provided, one of which is connected to a voltage measuring means 17 and the other to a DC power source 18.

This is to prevent the influence of the DC power supply 18 from affecting the output voltage of the voltage measuring means 17. That is, since the internal resistance of the solid electrolyte layer 15 increases when the temperature decreases, by connecting the DC power supply 18 and flowing a current, the voltage between the two electron conductive layers 14 and 16 increases, and the reference electrode electron This is because if the conductive layers 13 and 14 are common, the above voltage is added to the electromotive force and the output voltage increases, making it difficult to measure a normal electromotive force. In addition, a heating element 12 is provided to keep the temperature of the oxygen sensor element 10 constant (approximately 60°C).
The purpose of keeping the current 1 from the DC power source 18 at
．． This is also to keep the diffusion coefficient k2 of the gas diffusion control layer 20 constant.

Next, an example of implementing the present invention will be shown. Table 1 shows the specifications of the oxygen sensor element 10. Table 1 Table 2 also shows the diffusion coefficient for oxygen at a temperature of 60,000, and also shows the air-fuel ratio (A/F) at which the electromotive force E suddenly changes when the current 1 = 500 peaks A. .

Note that the diffusion coefficient k3 of the electron conductive layers 13, 14, and 16 is
In order to make the oxygen partial pressure in the electron conductive layer uniform, it is necessary that the diffusion coefficient be significantly larger than the diffusion coefficient of the gas diffusion control layer 20. Table 2 As shown in Table 2, current 1 = 500 A, diaphragm layer 1 1
The diffusion coefficient k. =6×10-9mole/atm・se
c, diffusion coefficient k2 of gas diffusion control layer 20=6×lo-8
The air-fuel ratio (A
/F) = 16, the electromotive force E exhibits a characteristic of rapid change, but when the current 1 further changes to 750 and 1000 A, the electromotive force E suddenly changes as shown in Figure 12. The air-fuel ratio (A/F) also changes.

Therefore, in air-fuel ratio control of an internal combustion engine using a single oxygen sensor element 10, when it is not desired to reduce the output of the internal combustion engine too much (for example, when climbing up a hill), the above-mentioned current is made relatively small to reduce the output. When it is desired to increase fuel consumption efficiency by performing lean combustion when the fuel is not needed so much (for example, when driving at a constant speed), it is convenient to perform control such that the current is relatively increased. In addition, in some conventional oxygen sensor elements, a protective layer is provided on the surface of the measurement electrode electron conductive layer. When manufactured with dimensions, the diffusion coefficient of the protective layer is k=1×
It was necessary to make it larger than about 10-6 mole/atm·sec.

In other words, the purpose of providing the protective layer is to protect the main body of the oxygen sensor element, and if a gradient of oxygen partial pressure occurs in the protective layer, the stoichiometric air-fuel ratio cannot be detected.
As mentioned above, the value of the diffusion coefficient of the protective layer needs to be larger than a certain level. On the other hand, in the oxygen sensor element 10 according to the present invention, when manufactured with the dimensions shown in Table 1, the diffusion coefficient k2 of the gas diffusion control layer 20 is 2×10-7 mol.
The value is set to a relatively small value of approximately e/atm·sec or less, and a gradient of oxygen partial pressure is generated in the gas diffusion control layer 201, thereby making it possible to detect a lean air-fuel ratio.

However, it is not necessarily the case that the diffusion coefficient k2 is small, and if it becomes smaller than about 6×10 −9 mole/atm·sec, it will adversely affect the response. Therefore, in the case of the dimensions shown in Table 1, it is preferable that the diffusion coefficient k2 of the gas diffusion control layer 20 is in the range of approximately 6×10-9 to 2×10-7 mole/atm·sec. It can be said. As described above, according to the present invention, a gas diffusion control layer having a diffusion coefficient for oxygen within a predetermined range and capable of controlling the diffusion of oxygen molecules is provided on the surface of the measurement electrode electron conductive layer to The oxygen partial pressure at the measurement electrode is applied by using an oxygen sensor element which can be brought into contact with the oxygen sensor element, and by causing the movement of oxygen ions from the measurement electrode electron conduction layer to the reference electrode electron conduction layer by means of a current supplied by a DC power source. lower than the oxygen partial pressure in the sample gas,
Since the electromotive force generated in response to the difference between the reduced measured polar oxygen partial pressure and the reference polar oxygen partial pressure is measured,
It is possible to detect an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, and brings about extremely excellent effects such as enabling lean combustion in internal combustion engines and the like, thereby increasing fuel consumption efficiency.

[Brief explanation of drawings]

Figure 1 is a cross-sectional explanatory diagram of an oxygen sensor element used for conventional air-fuel ratio detection, Figure 2 is a graph showing the relationship between air-fuel ratio and oxygen concentration in exhaust gas, and Figure 3 is a graph showing the relationship between air-fuel ratio and Figure 1. Graph showing the relationship between the oxygen sensor element and the measured polar oxygen partial pressure, 4th
The figure is a graph showing the relationship between the air-fuel ratio and the electromotive force of the oxygen sensor element shown in FIG. 7 is an explanatory diagram of the heating element portion of the oxygen sensor element in FIG. 5; FIG. 8 is an enlarged sectional explanatory diagram showing the action of the oxygen sensor element in FIG. 5; Figure 9 is a graph showing the relationship between the air-fuel ratio and the oxygen partial pressure in the residual gas; Figure 10 is a graph showing the relationship between the air-fuel ratio and the polar oxygen partial pressure measured by the oxygen sensor element in Figure 5; Figure 11 is a graph showing the relationship between the air-fuel ratio and the electromotive force of the oxygen sensor element shown in Figure 5, and Figure 12 is a graph showing the relationship between the air-fuel ratio and the electromotive force of the oxygen sensor element when the current is changed. be. 10... Oxygen sensor element, 11... Corneal membrane layer, 13, 14... Reference electrode electron conductive layer, 15...
... Oxygen ion conductive solid electrolyte layer 16 ... Measurement electrode electron conductive layer, 17 ... Voltage measuring means, 18
...DC power supply, 20... Gas diffusion control layer. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12

Claims (1)

[Scope of Claims] 1. A diaphragm layer, a reference electrode electron conductive layer, an oxygen ion conductive solid electrolyte layer, and a measurement electrode electron conductive layer are laminated, and at least one of the diaphragm layer and the oxygen ion conductive solid electrolyte layer is laminated. is gas permeable, and the surface of the measurement electrode electron conduction layer of the oxygen sensor element, in which a DC power source is connected between the reference electrode electron conduction layer and the measurement electrode electron conduction layer, has a diffusion coefficient of 6×10^ for oxygen. -＾9 or 2×10＾-＾7
An oxygen sensor element is provided with a gas diffusion control layer capable of controlling the diffusion of oxygen molecules in the range of mole/atm sec, and is capable of contacting the test gas, and is operated by the current supplied by the DC power supply. The measurement electrode oxygen partial pressure is lowered than the oxygen partial pressure in the test gas by causing the movement of oxygen ions from the measurement electron conduction layer toward the reference electrode electron conduction layer, and the reduced measurement electrode Air-fuel ratio detection characterized by making it possible to detect an air-fuel ratio leaner than the stoichiometric air-fuel ratio by measuring the electromotive force generated in response to the difference between the oxygen partial pressure and the reference polar oxygen partial pressure. Method. 2. Forming a reference electrode electron conductive layer from two electron conductive layers,
A patent in which an electromotive force measuring means is connected between one reference electrode electron conductive layer and a measurement electrode electron conductive layer, and a DC power source is connected between the other reference electrode electron conductive layer and the measurement electrode electron conductive layer. An air-fuel ratio detection method according to claim 1. 3. The air-fuel ratio detection method according to claim 1 or 2, wherein a porous material is used in the diaphragm layer to enable diffusion of oxygen molecules.