JPH0690179B2 - Air-fuel ratio measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an apparatus for measuring an air-fuel ratio of an air-fuel mixture from exhaust gas of an internal combustion engine, for example.

(Prior Art) Oxide zirconia is an electrical insulator at room temperature, but it is conventionally known that it becomes a conductor at high temperatures (about 500 ° C or higher), and zirconia at high temperatures is selective. Characteristic that allows only oxygen to pass through (hereinafter, "selective oxygen permeation characteristics"
It is also known to have.

It is also known that an electromotive voltage depending on the difference in oxygen concentration on both main surface sides of a zirconia plate can be obtained from the selective oxygen permeation characteristics. Therefore, if this electromotive voltage and the oxygen concentration on the one main surface side of the zirconia plate are known, the oxygen concentration on the other main surface side can be calculated from a constant equation (Nernst equation),
By utilizing this property, the zirconia plate can be used as an oxygen concentration detecting element.

Further, the zirconia plate has a function of moving oxygen in a direction opposite to the direction of the current when a current is passed between the two main surfaces so as to penetrate the plate. That is, the zirconia plate has a pumping action of pumping out oxygen on one main surface side to the other main surface side.

Therefore, conventionally, by combining the oxygen concentration detecting action and the pumping action of such zirconia, the combustion exhaust gas for both the combustion of a mixture leaner than the stoichiometric air-fuel ratio or the combustion of a rich mixture An air-fuel ratio detector has been developed in which the ratio can be suddenly measured from gas (see, for example, JP-A-59-64755).

FIG. 5 illustrates the principle of measuring the air-fuel ratio using such an air-fuel ratio detector.

In the figure, 1 is an air-fuel ratio detector (hereinafter, simply referred to as a detector). This detector 1 includes a first zirconia 2 for detecting oxygen concentration and a second zirconia for pumping operation.
Zirconia 3 and the first and second zirconia 2,
A thin chamber electrode 5 (5a, 5b, 5c, 5a, 5b, 5c, 5c, 5c, 5c, 5c, 5c, 5c, 5c, 5c, 5c) formed on both main surfaces of the hollow chamber 4 and the first and second zirconia 2, 3 5d) and.

Small holes 6 are provided in the second zirconia 3. The portion of the small holes 6 may be a porous coating film having characteristics equivalent to those of the small holes 6.

In addition, the first and second zirconia of the electrode 5
The electrodes 5b and 5c inside 2 and 3 are grounded, the electrode 5a outside the first zirconia 2 is the (−) input terminal of the differential amplifier 7, and the electrode 5d outside the second zirconia 3 is 5d. Are respectively connected to the output terminals of the differential amplifier 7.

The reference voltage _Er is applied to the (+) input terminal of the differential amplifier 7.

For example, when detecting the air-fuel ratio from the exhaust gas of an internal combustion engine, the outer main surface of the first zirconia 2 (electrode 5a side) is brought into contact with the atmosphere, and the outer main surface of the second zirconia 3 (electrode 5d side). ) Is brought into contact with the exhaust gas (gas to be measured) that has flowed due to concentration diffusion, and the gas to be measured is caused to flow into the hollow chamber 4 through the small holes 6.

Hereinafter, the operation will be described separately for the case of lean combustion with a large air-fuel ratio and the case of rich combustion with a small air-fuel ratio.

(1) Operation during lean burn During lean burn, a relatively large amount of residual oxygen is present in the gas to be measured. When such a gas to be measured flows into the hollow chamber 4, the oxygen concentration C _v in the hollow chamber 4 and the oxygen concentration C _r in the atmosphere are increased _.
The difference in the concentration between and is relatively small. Therefore, the electromotive voltage generated in the first zirconia 2, that is, the voltage E between the electrodes 5a and 5b (between the oxygen concentration detection electrodes) becomes smaller than the reference voltage _Er .

Therefore, during lean burn, the output of the differential amplifier 7 becomes positive, and the pumping current in the direction of arrow A (positive direction) corresponding to the difference between the reference voltage E _r and the electromotive voltage E is transmitted through the electrode 5d to the second direction. Supplied to zirconia 3.

As a result, the oxygen in the hollow chamber 4 penetrates the second zirconia 3 by the pumping action of zirconia and is pumped to the outer main surface side. When the amount of oxygen pumped out by the positive direction pumping current and the amount of oxygen flowing into the hollow chamber 4 become balanced, the pumping current stabilizes at a certain constant value.

(2) Operation during rich combustion At the time of rich combustion, a relatively large amount of hydrogen and carbon monoxide exist in the measured gas. When such a gas to be measured flows into the hollow chamber 4 through the small holes 6, it is combined with oxygen in the hollow chamber 4 into water and carbon dioxide. As a result, the oxygen concentration C _v in the hollow chamber 4 is significantly reduced, and the concentration difference with the oxygen concentration C _r in the atmosphere is increased.

For this reason, the voltage E (electromotive voltage generated in the first zirconia 2) applied to the (−) input terminal of the differential amplifier 7 becomes larger than the reference voltage _Er . Therefore, during the rich combustion, the output of the differential amplifier 7 becomes negative, and the pumping current in the direction of arrow B (negative direction) according to the difference between the reference voltage _Er and the electromotive voltage E is supplied to the second zirconia 3. .

As a result, oxygen on the outer main surface side of the second zirconia 3 (oxygen in the measurement gas) is taken into the hollow chamber 4 by the pumping action of zirconia.

When the amount of oxygen taken in by the negative pumping current and the amount of oxygen combined with hydrogen or carbon monoxide are balanced, the pumping current settles to a certain value.

When positive and negative pumping currents are obtained by the circuit having the configuration and operation as described above, the air-fuel ratio is determined by the pumping current and each gas component (oxygen, hydrogen, carbon monoxide) in the measured gas. It can be obtained by a known calculation based on the sensitivity coefficient of the detector 1 with respect to the concentration and the water charcoal ratio.

The sensitivity coefficient for the concentration of each gas component of the detector 1 is a constant determined by the mechanical structure of the detector 1 (for example, the size of the small hole 6), and the sensitivity coefficient for a specific gas component is determined. Then, the sensitivity coefficient for the remaining gas components is also determined by a known method.

Further, the water-carbon ratio is a ratio of hydrogen atoms and carbon atoms contained in the fuel consumed by the internal combustion engine, and is a constant determined in accordance with the type of fuel.

(Problems to be Solved by the Invention) However, in actual combustion, the fuel is not completely burned and various hydrocarbons are discharged. The amounts of oxygen, hydrogen, and carbon monoxide change according to the amount of discharge of this hydrocarbon. Depending on its composition, this hydrocarbon may be methane (CH ₄ )
From there about 100 kinds until methylpropyl benzene _{(CH 3 C 3 H 7 C} 6 H 6), the rate at which they are diffused toward the electrode with oxygen varies greatly. Therefore, the air-fuel ratio detection accuracy of the sensor output depends on the diffusion rate (diffusion coefficient) of these hydrocarbons, and when a large amount of hydrocarbons with a slow diffusion rate and large molecular weight are contained, a correct air-fuel ratio cannot be measured with good response. The detection error of the air-fuel ratio also becomes large. Such hydrocarbons are previously catalytically oxidized to produce carbon dioxide and H ₂
If converted to O (water), the detection accuracy of the air-fuel ratio will be improved, but the air-fuel ratio detector will be large and costly due to the provision of the processing device having the catalyst. Further, this catalytic action can achieve its effect in the lean region of the air-fuel ratio in which oxygen exists, but cannot attain its effect in the rich region where oxygen does not exist.

With respect to such a problem, the present invention focuses on the point that the hydrocarbon emission characteristic changes according to the air-fuel ratio, and corrects the value of the air-fuel ratio according to this hydrocarbon emission characteristic to calculate the air-fuel ratio. It is an object of the present invention to provide an air-fuel ratio measuring device that improves measurement accuracy.

(Means for Solving the Problems) The present invention relates to an air-fuel ratio detector including an oxygen concentration detector formed by arranging electrodes on both main surfaces of two solid electrolytes and a pump portion, and an oxygen concentration detector. Means for supplying a positive and negative pumping current to the pump portion according to the voltage difference between the reference part and the reference electrode, a sensitivity coefficient setting means for setting a sensitivity coefficient corresponding to a specific component in the measurement gas, and the measurement gas Of the combustion fuel, the water-carbon ratio setting means for setting the water-carbon ratio, the hydrocarbon emission coefficient setting means set according to the hydrocarbon emission characteristics, and these respective set values and the pumping An arithmetic means for calculating the air-fuel ratio based on a predetermined arithmetic expression according to the current value is provided.

(Operation) In this way, the calculated value of the air-fuel ratio is corrected according to the emission characteristics of the hydrocarbon contained in the exhaust gas, so that it is not necessary to pretreat the hydrocarbon with a catalyst or the like, and therefore the hydrocarbon is The correct air-fuel ratio can be measured with good response, without being affected by the diffusion rate of.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention.

In the figure, the current-voltage conversion circuit 21 indicates that the pumping current I _p
Is converted into a voltage signal with a predetermined conversion gain. The A / D conversion circuit 22 converts the analog voltage signal output from the current-voltage conversion circuit 21 into a digital signal.

The first digital switch 23 is a sensitivity coefficient setting digital switch for setting a predetermined sensitivity coefficient for oxygen concentration of the detector 1 with a digital value. The second digital switch 24 is a water / charcoal ratio setting digital switch for setting a water / water ratio previously determined according to the type of fuel consumed by the internal combustion engine.

It should be noted that in the present embodiment, since a three-digit digital switch is used as the second digital switch 24, it is 0.
Each water char ratio can be set in increments of 01.

The third digital switch 32 is an HC emission coefficient setting digital switch that sets the emission coefficient of hydrocarbons (hereinafter referred to as "HC").

The arithmetic circuit 25 includes an input / output interface 29, a central processing unit (CPU) 30 and a memory 31,
The arithmetic processing described later is executed. The display unit 26 is an arithmetic circuit 25.
The air-fuel ratio calculated in step 1 is displayed digitally.

By setting the first digital switch 23 to the known sensitivity coefficient for oxygen concentration of the detector 1, the sensitivity coefficient for oxygen concentration of the detector 1 is stored in the memory via the interface 29.
It is stored in RAM in 31.

When the oxygen concentration sensitivity coefficient of the detector 1 is stored in the RAM in this way, the CPU 30 causes the detector 1 to detect the hydrogen concentration sensitivity coefficient, the carbon monoxide concentration sensitivity coefficient, and the hydrocarbon resistance by a known method. Calculate the density sensitivity coefficient. Then, the calculated sensitivity coefficients are stored in the RAM in the memory 31.

Subsequently, the second digital switch 24 is set to the water-carbon ratio determined according to the type of fuel.

As a result, the water-charcoal ratio is stored in the RAM in the memory 31 via the interface 29.

Further, the third digital switch 32 is set to an HC emission coefficient obtained according to an HC emission characteristic described later. This allows
The HC emission factor is stored in the memory 31 via the interface 29.
Stored in RAM.

Thus, when the water-charcoal ratio, the oxygen concentration sensitivity coefficient of the detector 1 or the HC emission coefficient is newly set by the digital switch, the first, second, and third digital switches are stored in the RAM in the memory 31. All the sensitivity coefficients, the water-carbon ratio, and the HC emission coefficient of the detector 1 corresponding to the numerical values 23, 24, 32 are stored.

The arithmetic circuit 25 calculates the air-fuel ratio by a predetermined arithmetic expression based on the new water-charcoal ratio or the oxygen concentration sensitivity coefficient of the detector 1 and the HC emission coefficient stored in the RAM of the memory 31. However, since the calculation formulas are different between the rich combustion side and the lean combustion side with respect to the theoretical air-fuel ratio, as will be described later, the rich combustion side will be described first.

On the rich combustion side, the calculation formula of the air-fuel ratio becomes extremely complicated, and therefore it is practically difficult to calculate the air-fuel ratio in real time using a low-cost and small calculation circuit. Since it is easy to calculate the pumping current from the ratio and the HC emission coefficient, a numerical value correspondence table (map) showing the relationship between the air-fuel ratio and the pumping current is created using the water-carbon ratio and the HC emission coefficient as parameters. The air-fuel ratio is calculated based on

So, in CPU30, corresponding to the above-mentioned each set value,
Create a numerical value correspondence table for calculating the air-fuel ratio on the rich combustion side as follows.

That is, for example, a plurality of preset excess air ratios λ having different numerical values are read from the battery-backed ROM in the memory 31, and the water carbonization and HC emission coefficient set as described above are read from the RAM in the memory 31. D, the hydrogen concentration sensitivity coefficient, the carbon monoxide concentration sensitivity coefficient, and the hydrocarbon concentration sensitivity coefficient of the detector 1 are read out, and the following (1)
From equations (8), the pumping current I _p corresponding to each air-fuel ratio AFR is calculated.

AFR = 138λ (1 + n / 4) / (12 + n) (1) I _p =-(Bη + Cζ + Dθ) (2) A = 1 + n / 2 + 3.78λ (1 + n / 4) -αR (1 + n / 2−1 /
m)… (3) C '= 2 (1 + n / 4) (1-αR-λ) -B' (5) Q = {(1 + n / 4) (1-αR-λ) + (n / 4 + K / 2)
(1-αR) / (K-1)} ² −2K (1 + n / 4) (1-α
R-λ) (1-αR) / (K-1) (6) B = B '× 100 / A (7) C = C' × 100 / A (8) However, the above (1) to In equation (8), AFR is the air-fuel ratio, n is water carbonization, m is the average carbon number of unburned hydrocarbons, λ
Is a variable called excess air ratio, I _p is the pumping current, A,
B, C and Q are intermediate variables, η is the sensitivity coefficient of the detector 1 to carbon monoxide concentration, ζ is the sensitivity coefficient of the detector 1 to hydrogen concentration, θ is the sensitivity coefficient of the detector 1 to hydrocarbon concentration, and K is The water-gas reaction constant is shown.

Here, αR is the HC correction coefficient on the rich combustion side, and can be calculated by the following equation (9).

αR = {1 + n / 2 + 3.78λ (1 + n / 4)} D / {1 + D (1 + n / 2−1 / m) (9) Here, D is the HC set by the third digital switch 32.
Emission factor (HC emission concentration).

The arithmetic circuit 25 stores each pumping current calculated as described above in the RAM in the memory 31 in the rich combustion. That is, the RAM in the memory 31 stores a numerical value correspondence table corresponding to a certain specific value of the water charcoal ratio n.

Therefore, the measurement of the air-fuel ratio on the rich combustion side is performed as follows.

On the rich combustion side, as described above, the pumping current I _p supplied to the detector 1 becomes negative (I _p <0). The negative pumping current I _p is converted into a voltage signal by the current-voltage conversion circuit 21 with a predetermined conversion gain and supplied to the A / D conversion circuit 22.

The voltage signal supplied to the A / D conversion circuit 22 is converted into a digital signal here and supplied to the arithmetic circuit 25. The arithmetic circuit 25 takes in the digital voltage signal to the CPU 30 via the interface 29.

Then, based on this voltage signal, that is, the pumping current I _p , the air-fuel ratio corresponding to the pumping current I _p is calculated by the interpolation method from the numerical value correspondence table previously stored in the RAM in the memory 31 as described above. To calculate.

The air-fuel ratio calculated in this way is supplied to the display device 26 via the interface 29, so that it is digitally displayed.

On the other hand, the measurement operation of the air-fuel ratio on the lean burn side is performed as follows.

On the lean burn side as well as on the rich burn side, create a numerical correspondence table showing the relationship between the air-fuel ratio and the pumping current using the water charcoal ratio and the HC emission coefficient as parameters, and calculate the air-fuel ratio based on this. To

Therefore, in the CPU 30, a numerical value correspondence table for calculating the air-fuel ratio on the lean burn side corresponding to the above-mentioned set values is created as follows.

That is, for example, the excess air ratio λ with different numerical values is read from the battery-backed ROM in the memory 31, and the water / carbon ratio, the HC emission coefficient D, and the detector set as described above are read from the RAM in the memory 31. The oxygen concentration sensitivity coefficient γ of 1 is read out, and the pumping current I _p for each air-fuel ratio AFR is calculated from the equation (1) and the following equations (10) to (13).

I _p = Eγ + Dθ (10) E '= (1 + n / 4) (λ-1 + αL) (11) E = E' × 100 / A (12) A = n / 4 + 4.78λ (1 + n / 4) + αL (1 / m−n / 4) (13) αL = {n / 4 + (1 + n / 4) 4.78λ} D / {1+ (n / 4−1 / m) D} (14) where αL Is a HC correction coefficient on the lean burn side, and D is an HC emission coefficient (HC emission concentration) set by the third digital switch 32.

The air-fuel ratio calculated in this way is
It is supplied to the display device 26 via 29 and is digitally displayed.

The HC correction coefficients αR and αL on the rich combustion side and the lean combustion side are obtained from the HC emission coefficient D set by the third digital switch 32 by the equations (9) and (14), respectively. The emission coefficient D is, as shown in FIG. 2, the theoretical air-fuel ratio point (λ =
The set value is the value determined based on the HC emission concentration in 1). Fig. 2 shows the relationship between the air-fuel ratio and the HC emission concentration by experiments. HC is not theoretically emitted at λ = 1, but it is actually HC as shown in the figure due to the piston clevis and wall flow. Occurs. Therefore, when the target excess air ratio λ = 1, a predetermined HC emission coefficient is set based on the HC emission concentration (point P) at the illustrated theoretical air-fuel ratio point. However, the discharge characteristic of HC is not constant as shown by the line a, and changes according to the actual air-fuel ratio.

Therefore, the HC emission concentration is generally set at the third digital switch 32 because the concentration is low from the theoretical air-fuel ratio point (point P) toward the rich side, slightly lower to the lean side, and higher than the lean side. HC emission coefficient D at the theoretical air-fuel ratio point
By compensating for the rich side and the lean side using the following equation (12), it is possible to measure the air-fuel ratio with a relatively small error in a wide range.

D ′ = D · KHC (12) Here, KHC is a correction coefficient according to the HC emission characteristic, and KHC at a predetermined air-fuel ratio is set to obtain the HC emission coefficient D ′ at a predetermined air-fuel ratio. To use. During that time, the value obtained by the interpolation calculation is used.

FIG. 3 shows a second embodiment.

In this embodiment, two third digital switches 32 are provided,
One is the rich-burn side digital switch 32a, and the other is the lean-burn side digital switch 32b.

This is because, for example, when the detector 1 is installed at the downstream position of the oxidation catalyst (not shown), residual oxygen may be generated on the lean combustion side.
HC reacts and the HC emission concentration becomes almost zero. Therefore,
In this case, it is necessary to set the HC emission coefficient D to 0 on the lean combustion side.

Therefore, the HC emission coefficient D can be set separately for the rich-burn side and the lean-burn side so that the detector 1 can be mounted at the mounting position (before and after the catalyst).

FIG. 4 shows a third embodiment. In this embodiment, in addition to the third digital switch 32 for setting the HC emission coefficient D, a fourth digital switch 33 and a memory reed switch 34 are provided. The fourth digital switch 33 sets the air-fuel ratio (or λ).

This operation will be described. 4th digital switch
A predetermined air-fuel ratio is set by 33, the HC emission coefficient at the air-fuel ratio is set by the third digital switch 32, and the HC emission coefficient at the predetermined air-fuel ratio is set by the memory mode switch 34 via the interface 29. It is stored in the RAM in the memory 31.

The fourth digital switch 33 uses a 2-digit switch, and the air-fuel ratio can be set in "1" increments. That is, when the relationship between the air-fuel ratio and the HC emission characteristic is clear, the HC emission coefficient can be set according to the characteristic, and the air-fuel ratio measurement accuracy can be further improved.

On the rich combustion side, the same effect can be obtained by providing a carbon monoxide emission correction coefficient, a hydrogen emission correction coefficient, and an oxygen emission correction coefficient instead of the HC emission coefficient.

Further, instead of the digital switch for setting the HC emission coefficient D, the HC emission coefficient D may be directly written in the memory (ROM).

As described above, according to the present invention, the sensitivity coefficient setting means for setting the sensitivity coefficient of the detector corresponding to the specific gas in the gas to be measured, and the charcoal ratio setting means determined according to the type of fuel, An HC emission coefficient setting means for setting according to the emission characteristics of HC is provided, and since the air-fuel ratio is calculated from these set values and the output of the detector based on a predetermined arithmetic expression, Even when a hydrocarbon with a slow diffusion rate is included, the correction calculation is performed in consideration of the hydrocarbon emission characteristics, so that the air-fuel ratio can be accurately measured with good response without the need for a catalyst for oxidizing the hydrocarbon. The effect of being able to measure is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention. FIG. 2 is an explanatory diagram showing the air-fuel ratio and the exhaust HC concentration characteristic. 3 and 4 are block diagrams showing the second and third embodiments, respectively. FIG. 5 is a circuit diagram for explaining the principle of detecting the air-fuel ratio. 1 ... Detector, 7 ... Differential amplifier, 21 ... Current-voltage conversion circuit, 22 ... A / D conversion circuit, 23 ... First digital switch, 24 ... Second digital switch, 25 ... … Operation circuit, 29 …… Interface, 30 …… CPU, 31 …… Memory, 32 …… Third digital switch.

Claims (1)

[Claims] 1. An air-fuel ratio detector comprising an oxygen concentration detector formed by arranging electrodes on each main surface of two solid electrolytes and a pump, and a voltage difference between the oxygen concentration detector and a reference electrode. According to the type of combustion fuel of the gas to be measured, the driving means for supplying positive and negative pumping current to the pump part, the sensitivity coefficient setting means for setting the sensitivity coefficient corresponding to the specific component in the gas to be measured, And a hydrocarbon discharge coefficient setting means for setting the discharge characteristics of hydrocarbons, and a predetermined arithmetic expression according to each of these set values and the pumping current value. An air-fuel ratio measuring device, comprising: