JPH061258B2 - Air-fuel ratio detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to an air-fuel ratio detection device, and more particularly to an air-fuel ratio detection device that detects an air-fuel ratio in a wide range with high accuracy using an oxygen sensor.

(Prior Art) Recently, in order to accurately control the air-fuel ratio of the engine intake air-fuel mixture to a target value, the oxygen concentration in the exhaust gas that has a correlation with the air-fuel ratio is detected, and the fuel supply amount is adjusted according to this oxygen concentration. A device for performing feedback control and detecting such an air-fuel ratio over a wide range has been developed.

A conventional air-fuel ratio detecting device is, for example, Japanese Patent Laid-Open No. 57-7.
There is one described in Japanese Patent No. 6450, which can be shown as in FIG. In FIG. 1, reference numeral 1 denotes an oxygen sensor, and the oxygen sensor 1 is an application of the principle of a kind of concentration battery that generates an electromotive force according to the oxygen concentration. Reference numeral 2 is an oxygen ion conductive solid electrolyte, and a reference electrode 3 containing platinum as a main component is provided on one surface of the solid electrolyte 2. An oxygen electrode 4 made of, for example, gold, platinum, and an alloy is provided at a position facing the reference electrode 3 with the solid electrolyte 2 interposed therebetween.
Are stacked. The solid electrolyte 2, the reference electrode 3, and the oxygen electrode 4 are covered with the porous protective layer 5.

Such an oxygen sensor 1 causes a reference oxygen partial pressure Pa to be generated in the reference electrode 3 when a flowing current Is is supplied to the reference electrode 3 in a gas to be measured (for example, exhaust gas).
On the other hand, the oxygen partial pressure Pb at the oxygen electrode 4 is the acid differential pressure of the gas to be measured, and based on these oxygen partial pressures Pa and Pb, E = RT / 4F · ln (Pa / Pb) ... However, R: gas constant, T: absolute temperature, F: Faraday constant, an electromotive force E represented by the Nernst equation is generated, and is taken out as the output Vs of the oxygen sensor 1 to the outside. When the electric power E is switched from the lean side to the rich side with a predetermined air-fuel ratio as a boundary,
The air-fuel ratio changes sharply to the plus side, and the air-fuel ratio changes depending on the value of the flow-in current Is. Further, this switching air-fuel ratio corresponds to the value of the inflow current Is, and as the inflow current Is increases, it shifts to the lean side from the theoretical air-fuel ratio. Therefore, the oxygen sensor output Vs
Is set as a target voltage Va (substantially an intermediate value between the upper limit and the lower limit of the oxygen sensor output Vs that changes abruptly in the switching air-fuel ratio), and the pouring current Is is supplied so that the oxygen sensor output Vs becomes the target voltage Va. The flow-in current Is has a value according to the current air-fuel ratio, and the air-fuel ratio can be continuously detected in a wide range by detecting the value.

However, in such a conventional air-fuel ratio detection device, the oxygen sensor output Vs is supplied with the inflow current Is so as to become the target voltage Va, and the value of this inflow current Is is detected. Since it is configured to judge the fuel ratio, it is leaner than the stoichiometric air-fuel ratio (λ = 1) (where λ is the equivalent ratio defined as CO + 1 / 2O ₂ → CO ₂ and λ = 1 / 2O ₂ / CO). Not only λ> 1),
There is a problem that it is not possible to accurately detect a wide range of air-fuel ratios including the rich side (λ <1).

That is, as shown in FIG. 2, the value of the inflow current Is corresponds to the switching air-fuel ratio of the oxygen sensor 1 (hereinafter, these relationships are referred to as Is-A / F characteristics), and is the minimum in the theoretical air-fuel ratio. However, it increases again as it moves to the rich side. This is because there is almost no oxygen in the exhaust gas in the rich region, so that oxygen ions in the solid electrolyte (the movement of ions, in other words, current) are not in an equilibrium state corresponding to the rich air-fuel ratio and become oxygen molecules. Therefore, as the exhaust gas moves to the rich side, the amount of movement of oxygen ions increases and the value of the inflow current Is increases. Therefore, in the vicinity of the stoichiometric air-fuel ratio, there are two values of the air-fuel ratio by switching to the same flow-in current Is value, and the present air-fuel ratio is uniquely determined only by determining the air-fuel ratio based on the value of the flow-in current Is. Can't decide on. In other words, the flow current I
A wide range of air-fuel ratio from rich to lean cannot be accurately detected only by the value of s.

(Object of the invention) Therefore, the present invention adjusts the direction and magnitude of the current flowing into the pump unit so that the sudden change point of the output voltage of the sensor unit coincides with the predetermined air-fuel ratio, and based on this current, the above-mentioned predetermined value is adjusted. An object of the present invention is to make it possible to detect a deviation from the air-fuel ratio quantitatively and to uniquely detect a wide range of air-fuel ratio from the rich region to the lean region.

(Structure of the Invention) An air-fuel ratio detection device according to the present invention has an oxygen electrode in contact with exhaust gas and a reference electrode in contact with the atmosphere with an oxygen ion conductive solid electrolyte sandwiched between the oxygen electrode and the reference electrode, which is in accordance with the oxygen concentration of the oxygen electrode atmosphere. A sensor unit, which generates a voltage and whose voltage suddenly changes when the oxygen concentration corresponds to a predetermined air-fuel ratio, is arranged facing the sensor unit at a predetermined interval,
An oxygen sensor including a pump unit that adjusts the oxygen concentration of the oxygen electrode atmosphere according to the direction and magnitude of the flowing current, and detects a voltage difference between the voltage generated by the sensor unit and a predetermined target voltage. A difference detection circuit, a current supply circuit that determines the direction and magnitude of the inflow current in response to the output of the difference detection circuit and supplies the current to the pump unit, and detects the value of the inflow current, and the detected current. And a current detection circuit that outputs a signal that correlates to the change in the oxygen electrode atmosphere as a signal that represents the oxygen concentration of the oxygen electrode atmosphere.

(Operation) In the present invention, the pouring current is adjusted so that the output voltage and the target voltage always match.

Here, when the output voltage> the target voltage and the output voltage <the target voltage, the direction of the flowing current is just opposite.

Therefore, if the magnitude of the target voltage is adjusted to the output voltage of the sensor section when the oxygen concentration in the oxygen electrode atmosphere is the predetermined air-fuel ratio, the flow-in current becomes the minimum value at this predetermined air-fuel ratio, and it is richer than that. One side has a value of one polarity (for example, negative polarity), and the lean side has a value of the other polarity (for example, positive polarity).

Then, the two values on the both polar sides have respective magnitudes corresponding to the degree of deviation from the predetermined air-fuel ratio,
After all, according to the present invention, it is possible to accurately detect a wide range of air-fuel ratios from the rich range to the learn range.

(Example) Hereinafter, the present invention will be described with reference to the drawings.

3 to 6 are views showing the first embodiment of the present invention. First, the structure will be described. FIG. 3 is a perspective view showing an assembling procedure of the oxygen sensor. In FIG. 3, reference numeral 11 denotes an alumina substrate, and an atmosphere introducing plate 13 is laminated on the upper surface (upper end surface in the drawing) of the alumina substrate 11 with a heater 12 interposed therebetween. The air introduction plate 13 contains oxygen ion conductive solid electrolyte as a main component, and an air introduction groove 13a is formed on the upper surface side thereof. Then, a flat plate-shaped first solid electrolyte 14, a support 15, and a second solid electrolyte 16 are sequentially laminated in parallel on the upper surface side of the atmosphere introducing plate 13. Also, on the first solid electrolyte 14,
An oxygen electrode 17 and a reference electrode 18 each containing platinum as a main component are provided on the lower surface (for example, they are laminated on the upper surface and the lower surface by a printing process).
Sensor lead wires 19 and 20 are connected to each. On the other hand, an anode electrode 21 and a cathode electrode 22 are provided on the upper and lower surfaces of the second solid electrolyte 16, respectively, and pump lead wires 23, 24 are connected to these electrodes 21, 22, respectively. The electrodes 17, 18, 21, 22 may be coated with a porous protective layer on the outside to increase the durability. The first solid electrolyte 14, the oxygen electrode 17 and the reference electrode 18 form a sensor section 25, and the second solid electrolyte 16, the anode electrode 21 and the cathode electrode 22 form a pump section 26. Further, the sensor unit 25, the pump unit 26, the atmosphere introduction plate 13,
Alumina substrate 11 and heater 12 as a whole are oxygen sensors
Makes up 27.

FIG. 4 is a sectional view of the oxygen sensor 27 assembled as described above. In FIG. 4, the atmosphere introducing plate 13 and the first solid electrolyte 14 define an atmosphere introducing section 28.
Atmosphere is guided to the air as indicated by the arrow AIR. On the other hand, the first and second solid electrolytes 14 and 16 and the support body 15 are exhaust gas introducing portions.
29, and this exhaust introduction part 29 and pump part 26
Exhaust gas is guided to the anode electrode 21 side of the as shown by the arrow GAS. In addition, the interval L of the exhaust introduction portion 29 is extremely narrow,
For example, it is set to about 0.1 mm. Therefore, the sensor unit 2
In reference numeral 5, the reference electrode 18 side is in contact with the atmosphere, and the oxygen electrode 17 side is in contact with the exhaust gas of the exhaust gas introducing portion 29, forming an oxygen concentration cell and generating an electromotive force E according to the oxygen concentration of the exhaust gas in the exhaust gas introducing portion 29. To do. The constant power E is the sensor unit
It is taken out as the output Vs of 25. A pouring current Is is supplied to the pump section 26 from a current supply circuit described later, and the pouring current Is has a lean air-fuel ratio (hereinafter, simply referred to as the current air-fuel ratio) corresponding to the oxygen concentration in the exhaust gas. Sometimes it is supplied to the anode electrode 21, and when it is rich, it is supplied to the cathode electrode 22. In this case, when the current air-fuel ratio is lean, the flow-in current Is flows in the second solid electrolyte 16 from the anode electrode 21 to the cathode electrode 22 as shown by the arrow I _L. At this time, the flowing current Is causes oxygen ions O ₂ ⁻ to move from the cathode electrode 22 toward the anode electrode 21 in the second solid electrolyte 16. That is, oxygen ions move in the direction opposite to the direction in which the pouring current Is flows. Therefore,
The pump unit 26 moves oxygen ions by the flow current Ic (hereinafter, referred to as a positive oxygen pump action), so that the oxygen concentration of the exhaust gas introducing unit 29 (in other words, the oxygen concentration of the oxygen electrode 17 atmosphere of the sensor unit 25). Is maintained at a predetermined value (2 × 10 ⁻³ % in this embodiment as described later). On the other hand, when the current air-fuel ratio is rich, the flow-in current Is
Flows in the opposite direction to the lean case as shown by the arrow I _R.
Therefore, the pump unit 26 moves the oxygen ions from the anode electrode 21 toward the cathode electrode 22 (hereinafter, referred to as a reverse oxygen pumping action), so that the oxygen concentration in the exhaust gas introducing unit 29 is adjusted to the above-described value as in the lean case. Maintain a predetermined value. Then, the sensor unit 25 suddenly changes the output voltage Vs with an air-fuel ratio corresponding to the oxygen concentration of the exhaust gas introducing unit 29. In addition,
Although the heater 12 is omitted in FIG. 4, the heater 12 heats the first and second solid electrolytes 14 and 16 to an appropriate temperature so as to keep them active.

FIG. 5 is a circuit diagram of an air-fuel ratio detecting device using the oxygen sensor 27. In FIG. 5, the pumping portion 26 of the oxygen sensor 27 is supplied with the inflow current Is from the current supply circuit 30, and the value of this inflow current Is is the current value detection circuit.
Detected by 31. The current value detection circuit 31 includes operational amplifiers OP1 and OP2 and resistors R1, R2, R3, R4 and R5.
And a capacitor C1, which detects the value of the inflow current Is as a voltage drop across the resistor R1 and outputs a voltage signal Vi. The voltage signal Vi has a positive value when the flow-in current Is is supplied in the direction of the arrow I _{L in the} drawing, and has a negative value when supplied in the direction of the arrow I _R. The current supply circuit 30 includes an operational amplifier OP3, a transistor Q1,
It is composed of Q2, diodes D1 and D2, and a resistor R6, and controls the magnitude and direction of the inflow current Is according to the value of the output ΔV of the difference detection circuit 32.
That is, the difference detection circuit 32 includes operational amplifiers OP4, OP5,
It is composed of resistors R7, R8, R9, R10 and R11, and subtracts the target voltage Va from the output voltage Vs of the sensor unit 25 to obtain a difference value ΔV (ΔV = K (Vs−Va), where K is a constant). It is outputting to the current supply circuit 30. This target voltage V
a is an intermediate value between the upper limit and the lower limit of the abruptly changing voltage value of the sensor unit output Vs when the oxygen concentration of the exhaust gas introducing unit 29 is maintained at a predetermined value. The power source voltage 15V is divided by the resistors R7 and R8. , A value of 0.2 V, for example. The sensor unit output Vs corresponds to the oxygen concentration of the exhaust gas introducing unit 29, and the target voltage Va corresponds to the above-mentioned predetermined value. Therefore, the difference value ΔV
Represents the magnitude of the deviation of the current oxygen concentration in the exhaust gas introducing portion 29 from the predetermined value. Therefore, the current supply circuit 30 uses the transistor Q so that the difference value ΔV becomes zero, that is, the sensor output Vs matches the target voltage Va.
The complementary phase inversion circuit composed of 1, Q2 and the diodes D1, D2 controls the magnitude and direction of the inflow current Is. The current supply circuit 30, the current value detection circuit 31, and the difference detection circuit 32 constitute an air-fuel ratio detection means 33, and in the present embodiment, this air-fuel ratio detection means 33 is a sensor section.
While supplying the pouring current Is to the pump unit 26 so that the output voltage Vs of 25 becomes a predetermined value (target voltage Va),
The air-fuel ratio is detected by detecting the value of this pouring current Is.

Next, the operation will be described.

Generally, as shown by the Is-A / F characteristic, the switching air-fuel ratio of the oxygen sensor corresponds to the flow current, and λ = 1.
Although it becomes the minimum in point, it increases again as it shifts to the rich side. Therefore, the oxygen sensor that detects the air-fuel ratio using such characteristics cannot accurately determine the wide-range air-fuel ratio from the rich region to the lean region.

Therefore, in this embodiment, the oxygen sensor 27 is divided into a sensor section 25 and a pump section 26, and the output voltage Vs of the sensor section 25 is Vs.
Is supplied to the pump section 26 so that the value of the flow rate is always suddenly changed at the stoichiometric air-fuel ratio (λ = 1), the value and direction of the flow-in current Is can be varied over a wide range from the rich region to the lean region. The air-fuel ratio judgment is made accurate by making a unique correspondence.

That is, in the sensor section 25, the oxygen electrode 17 side is the exhaust introduction section.
29 is in contact with the exhaust gas and the reference electrode 18 side is in contact with the atmosphere.
Change suddenly. In this case, to supply the I _L direction of pouring current Is for example the pump unit 26, this flow current I
A positive oxygen pumping action is performed by s, and oxygen molecules in the exhaust gas introducing portion 29 move to the anode electrode 21 side and are released. Further, at this time, the gap L of the exhaust gas introducing portion 29 is set to be small (L≈0.1 mm). Therefore, when oxygen molecules are released to the other as described above, the oxygen concentration of the exhaust gas introducing portion 29 decreases.
On the other hand, at this time, oxygen molecules are flowing from the exhaust gas into the exhaust gas introducing unit 29, and the oxygen concentration in the exhaust gas introducing unit 29 is determined at the time when the amount of release and the amount of inflowing oxygen molecules are balanced.

In this embodiment, paying attention to such characteristics, the movement amount of oxygen molecules is controlled so that the oxygen electrode 17 atmosphere of the sensor unit 25 is always maintained at the oxygen concentration corresponding to the theoretical air-fuel ratio, and based on this movement amount, The oxygen concentration is 0.2% in the theoretical air-fuel ratio, and the oxygen concentration in the exhaust gas introducing portion 29 may be maintained at 0.2% itself, but in the case of the present embodiment, , Due to the catalytic action of platinum, which is the main component of the oxygen electrode 17, on the surface of the oxygen electrode 17, the exhaust gas temperature = 10 at the theoretical air-fuel ratio.
When the temperature is 00 ° K, the oxygen concentration is about 2 × 10 ^-3 % (derived from the Nernst equation). Therefore, this value (2 × 10 ⁻³ %) is determined as the predetermined oxygen concentration (predetermined value) at which the sensor unit output Vs constantly changes suddenly, and the target voltage Va corresponding to this is set to 0.2V. Therefore, in the lean region where the oxygen concentration in the exhaust gas is 0.2 to 20%, the flow-in current Is for supplying the oxygen concentration of the exhaust gas introduction portion 29 to 2 × 10 ⁻³ % is supplied to the pump portion 26. That is, when the current air-fuel ratio is in the lean range, Vs <V0, and the output ΔV (ΔV = K (Vs−Va)) of the difference value detection circuit 32 is Δ.
Since the V <O, the pouring current Is I _L direction is supplied to the pump unit 26 by the current supply circuit 30. As a result, a positive oxygen pumping action is performed and oxygen molecules in the exhaust gas introducing portion 29 are released to the anode electrode 21 side.
Oxygen concentration decreases to a predetermined value. The value of the inflow current Is at this time is proportional to the amount of movement of oxygen molecules, and is a value corresponding to the oxygen concentration in the exhaust gas.

On the other hand, in the rich region where the oxygen concentration in the exhaust gas is 0.1 to 0.2%, the oxygen concentration on the surface of the oxygen electrode 17 decreases to about 10 ^-20 % due to the catalytic action of platinum. In order to maintain this at 2 × 10 ⁻³ % and cause the sensor unit output Vs to suddenly change at the stoichiometric air-fuel ratio, it is necessary to move (inflow) oxygen molecules to the exhaust introduction unit 29 from the other. Therefore, in the rich region, the pouring current Is in the opposite direction to the lean region is supplied. That is, when the current air-fuel ratio is in the rich region, Vs> Va and ΔV>
0 from becoming, the pouring current Is I _R direction is supplied to the pump unit 26 by the current supply circuit 30. As a result, the reverse oxygen pumping action is performed, oxygen molecules flow from the anode electrode 21 side into the exhaust gas introducing portion 29, and the exhaust gas introducing portion 29 is discharged.
The oxygen concentration of increases to a predetermined value. The value of the inflow current Is at this time also has a magnitude proportional to the amount of movement of oxygen molecules, and has a value corresponding to the oxygen concentration in the exhaust gas.

In this way, the pouring current Is is supplied so that the oxygen concentration in the exhaust gas introducing portion 29 is always maintained at a predetermined value. Therefore, the output Vi of the current value detection circuit 31 that detects the value of the flow-in current Is is uniquely corresponding to the current air-fuel ratio as shown by the Vi-A / F characteristic in FIG. As a result, it is possible to accurately determine the air-fuel ratio and accurately detect a wide range of air-fuel ratio from the rich region to the lean region. Further, if the air-fuel ratio control of the engine is performed using this device, it is possible to accurately control the air-fuel ratio in a wide range including the rich region.

7 and 8 are views showing a second embodiment of the present invention. In this embodiment, the structure of the air-fuel ratio detecting means is different from that of the first embodiment,
The air-fuel ratio is detected by supplying a predetermined flow current Is to the pump unit 26 and detecting the output voltage Vs of the sensor unit 25.

That is, in FIG. 7, 41 is an air-fuel ratio detecting means, and the air-fuel ratio detecting means 41 is composed of a current supply circuit 42 and a buffer amplifier 43. The current supply circuit 42 supplies a predetermined flow-in current Is (direction is the same as in the first embodiment) to the pump unit 26, and the oxygen concentration in the exhaust gas introduction unit 29 is determined in accordance with the value of the flow-in current Is. It Therefore, the output voltage Vs of the sensor unit 25 suddenly changes at the switching air-fuel ratio corresponding to the value of the inflow current Is, and the air-fuel ratio can be determined by detecting the sudden change point of the output voltage Vs. The sudden change point of the output voltage Vs is unique to a wide range of air-fuel ratios from the rich region to the lean region by changing the value of the flow current Is and the direction thereof as shown by Vs-A / F characteristics in FIG. It will correspond to each other. As a result, it is possible to accurately detect a wide range of air-fuel ratios including the rich region.

In this embodiment, since the air-fuel ratio is judged by detecting the sudden change point of the sensor output Vs, it is judged whether the current air-fuel ratio is richer or leaner than the air-fuel ratio, that is, so-called 2 Although it is convenient for the value judgment, it is a little inconvenient when judging the air-fuel ratio continuously. However, for example, 1
As a detection device used when controlling to one or a plurality of predetermined air-fuel ratios, there is an advantage that the controllability is good because the sensor unit output Vs becomes ON / OFF.

(Effect) According to the present invention, the flow current value or the sudden change point output of the sensor unit can be uniquely associated with a wide range of air-fuel ratios from the rich region to the lean region, and the air-fuel ratio determination can be made accurate. A wide range of air-fuel ratios from the rich range to the learn range can be accurately detected.

[Brief description of drawings]

1 and 2 are views showing a conventional air-fuel ratio detecting device, FIG. 1 is a sectional view of the oxygen sensor, and FIG. 2 is its Is-A.
/ F characteristics, FIGS. 3 to 6 are views showing a first embodiment of the present invention, FIG. 3 is a perspective view showing an assembly procedure of the oxygen sensor, and FIG. 4 is a cross section of the oxygen sensor. 5 and 5 are circuit configuration diagrams thereof, FIG. 6 is a diagram showing the Vi-A / F characteristic thereof, FIGS. 7 and 8 are diagrams showing a second embodiment of the present invention, and FIG. The circuit configuration diagram, FIG. 8 shows the Vs-A /
It is a figure which shows F characteristic. 25 …… Sensor section, 26 …… Pump section, 30 …… Current supply circuit, 31 …… Current detection circuit, 32 …… Difference value detection circuit.

Claims (1)

[Claims] 1. An oxygen electrode, which is in contact with exhaust gas, and a reference electrode, which is in contact with the atmosphere, sandwiching an oxygen ion conductive solid electrolyte,
A sensor unit that generates a voltage according to the oxygen concentration of the oxygen electrode atmosphere and that suddenly changes the voltage when the oxygen concentration corresponds to a predetermined air-fuel ratio, and faces the sensor unit at a predetermined interval. An oxygen sensor comprising: a pump unit that adjusts the oxygen concentration of the oxygen electrode atmosphere according to the direction and magnitude of the flow current, and a voltage generated by the sensor unit and a predetermined target voltage. Difference detection circuit for detecting the difference voltage of the current detection circuit, a current supply circuit for determining the direction and magnitude of the flow-in current and supplying it to the pump unit in response to the output of the difference detection circuit, and detecting the value of the flow-in current. In addition, the air-fuel ratio detection device is provided with a current detection circuit that outputs a signal correlating to a change in the detected current as a signal representing the oxygen concentration of the oxygen electrode atmosphere.