JPH0736278Y2 - Air-fuel ratio measuring device for multi-cylinder engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an air-fuel ratio measuring device for a multi-cylinder engine.

(Prior art) Zirconia made into ceramics is at high temperature (about 500 ℃ or more)
Is known to selectively permeate only oxygen, and from this characteristic, an electromotive voltage corresponding to the difference in the acidity concentration on both main surface sides of the zirconia plate can be obtained. That is, if the electromotive voltage and the acidity concentration on one principal surface side of the zirconia plate are known, the acidity concentration on the opposite principal surface side can be calculated from a constant formula (Nernst's formula). It can be used as an element.

Further, the zirconia plate has a pumping action of pumping out the acidity in the direction opposite to the direction of the current when a current is passed between the two main surfaces so as to penetrate the zirconia plate.

There is known an air-fuel ratio detector that combines the oxygen concentration detecting action and the pumping action of zirconia.

FIG. 3 is a circuit diagram for explaining the principle of detecting the air-fuel ratio using such an air-fuel ratio detector.

In the figure, the air-fuel ratio detector 1 is a first zirconia plate (also referred to as a sensing cell) that performs an oxygen concentration detection operation.
2, a second zirconia plate (also referred to as a pumping cell) 3 for performing a pumping operation, and thin-film electrodes (for example, platinum electrodes) 4 formed on both main surfaces of each zirconia plate 2, 3.
7 and a hollow chamber 8 provided between both zirconia plates 2 and 3,
The second zirconia plate 3 and the small holes 9 formed so as to penetrate therethrough, and the outer main surface (electrode 4 side) of the first zirconia plate 2
Is in contact with the atmosphere, and the outer main surface (electrode 7 side) of the second zirconia plate 3 is brought into contact with the exhaust gas (measurement gas) flowing due to temperature diffusion, and the measurement gas passes through the small holes 9. It flows into the hollow chamber 8 as a rate-controlling part. The small hole 9
The portion may be a porous coating layer having characteristics equivalent to those of small pores.

Reference numeral 11 is a circuit for driving the air-fuel ratio detector 1, in which the outer electrode 4 of the first zirconia plate 2 is differentially connected to the (-) input terminal of the differential amplifier 12 and the outer electrode 7 of the second zirconia plate 3. amplifier
Connected to each of the 12 output terminals. Further, the inner electrodes 5 and 6 of the zirconia plates 2 and 3 are grounded. Note that the differential amplifier
The reference voltage Er of the power supply 13 is applied to the (+) input terminal of 12.

The operation of FIG. 3 will be described below separately for the case of performing lean combustion with a large air-fuel ratio and the case of performing rich combustion with a small air-fuel ratio.

(1) Operation during lean combustion Since a relatively large amount of residual oxygen exists in the measurement gas during lean combustion, when such measurement gas flows into the hollow chamber 8, the oxygen in the hollow chamber 8 is reduced. Concentration Cv and oxygen concentration in the atmosphere
The concentration difference from Cr becomes relatively small, and the electromotive voltage generated in the first zirconia plate 2, that is, the voltage E between the electrodes 4 and 5 (between the oxygen concentration detection electrodes) becomes smaller than the reference voltage Er.

Therefore, the output of the differential amplifier 12 becomes positive, and the pumping current Ip in the direction of arrow A (positive direction) corresponding to the difference between Er and the electromotive voltage E is supplied to the second zirconia plate 3 via the electrode 7. It Oxygen in the hollow chamber 8 is pumped to the outer main surface side (electrode 7 side) of the second zirconia plate 3 by the pumping action of zirconia, and the positive direction pumping current is applied. The amount of oxygen pumped out and the hollow chamber 8
When the amount of oxygen flowing into the interior becomes balanced,
The pumping current Ip stabilizes at a certain value.

(2) Operation during rich combustion At the time of rich combustion, since a relatively large amount of hydrogen and carbon monoxide are present in the gas to be measured, such gas to be measured enters the small chamber 9 into the hollow chamber 8. When it flows in, it combines with oxygen in the hollow chamber 8 to form water and carbon dioxide, so the oxygen concentration Cv in the hollow chamber 8 is greatly reduced, and the difference in concentration with the oxygen concentration Cr in the atmosphere becomes large. The voltage E applied to the (-) input terminal of the dynamic amplifier 12 exceeds the reference voltage Er.

Therefore, the output of the differential amplifier 12 becomes negative and the reference voltage Er
When a pumping current Ip in the direction of arrow B (negative direction) corresponding to the difference between the electromotive force E and the electromotive voltage E is supplied to the second zirconia plate 3, oxygen on the outer main surface side of the zirconia plate 3 (in the measured gas Oxygen) is generated by the pumping action of zirconia.
Taken in. Then, when the amount of oxygen taken in by the negative pumping current Ip and the amount of oxygen combined with hydrogen or carbon monoxide are balanced, the pumping current Ip settles to a certain value.

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

The sensitivity coefficient for each gas component concentration of the air-fuel ratio detector 1 is a constant determined by the mechanical structure of the air-fuel ratio detector 1 (for example, the size of the small hole 9), and by a known appropriate method, When the sensitivity coefficient for the specific gas component is determined, the sensitivity coefficient for the remaining gas components is also determined by a known method.

Further, the water-carbon ratio is the ratio of hydrogen atoms and carbon atoms contained in the fuel consumed by the engine, and is a constant that is determined according to the type of fuel.

Incidentally, the calculation of the air-fuel ratio is specifically performed while the pumping current is measured in real time in the arithmetic circuit. In this case, since the air-fuel ratio can be easily calculated on the lean burn side by simple arithmetic operations, real-time air-fuel ratio detection can be realized without great difficulty.

On the other hand, it is generally known that extremely complicated calculation is required on the rich combustion side. Therefore, it is virtually impossible to calculate the air-fuel ratio on the rich combustion side in real time using a small-sized and low-cost arithmetic circuit.

However, on the contrary, it was possible to easily calculate the pumping currents corresponding to these from the air-fuel ratio and the water charcoal ratio.
Conventionally, in order to calculate the air-fuel ratio on the rich combustion side using a small-sized and low-cost arithmetic circuit, the following method has been adopted.

That is, a numerical value correspondence table (hereinafter simply referred to as “numerical value correspondence table”) showing the relationship between the air-fuel ratio and the pumping current is created using the water-to-carbon ratio as a parameter. Then, this numerical value correspondence table is stored in the memory in the arithmetic circuit.

When actually calculating the air-fuel ratio on the rich combustion side,
The air-fuel ratio corresponding to the pumping current is calculated from the numerical value correspondence table by the interpolation method based on the pumping current detected as described with reference to FIG.

When the air-fuel ratio on the rich combustion side is calculated using the numerical correspondence table as described above, the air-fuel ratio on the lean combustion side is also calculated using the same numerical correspondence table. It is common.

(Problems to be solved by the invention) By the way, in the case of individually measuring the air-fuel ratio of each cylinder in a multi-cylinder engine, if the conventional device is simply attached to each cylinder, it is possible to measure the number of cylinders multiple times. A device is required, which causes an increase in size and cost of the entire device. Further, in order to measure the air-fuel ratio difference (air-fuel ratio distribution) between the cylinders, a new measuring device for that purpose is required.

On the other hand, if only one air-fuel ratio detector is provided and the air-fuel ratio detector is used to calculate the air-fuel ratio for each cylinder for the purpose of cost reduction, from the position where the air-fuel ratio detector is attached, The air-fuel ratio measurement accuracy of the cylinders separated from each other deteriorates.

Therefore, all cylinders are divided into two cylinder groups, one air-fuel ratio detector (oxygen concentration sensor) is provided for each cylinder group, and these are switched to detect the air-fuel ratio for each cylinder group. A method of doing so has been proposed (JP-A-62-255551). According to this one, the number of air-fuel ratio detectors smaller than the number of cylinders is sufficient, which is advantageous in terms of cost, but in terms of accuracy of air-fuel ratio measurement, compared to the case where an air-fuel ratio detector is provided for each cylinder. I have no choice but to lower it,
As for the drive circuit of the air-fuel ratio detector, the same number as the air-fuel ratio detector is required.

Therefore, while an air-fuel ratio detector is provided for each cylinder, only one drive circuit for the air-fuel ratio detector is prepared, and this and each air-fuel ratio detector are sequentially switched and connected.
Since only one drive circuit is required, it is possible to realize a significant cost reduction and downsizing of the device.

However, in an air-fuel ratio sensor of the type that pumps a pumping current, it takes time from the time when the drive circuit is connected to the air-fuel ratio detector until the pumping current stabilizes. Therefore, if the connection between each air-fuel ratio detector and the drive circuit is switched, the measurement accuracy of the air-fuel ratio will decrease.

This invention was made in view of such conventional problems. While an air-fuel ratio detector is provided for each cylinder, only one drive circuit for the air-fuel ratio detector is prepared. It is an object of the present invention to provide a device capable of switching connection with a container at a predetermined cycle.

(Means for Solving the Problem) This invention is connected to a plurality of air-fuel ratio detectors (1a to 1d) attached to each cylinder of an engine and to each air-fuel ratio detector (1a to 1d). If the air-fuel ratio detector (1a ~
1d) driving circuit (11) and each of the air-fuel ratio detectors (1a
~ 1d) means for outputting a select signal having a predetermined cycle that is predetermined in accordance with the time required for the pumping current flowing through the drive circuit connected to the drive circuit to be settled; Means (41, 4) for sequentially switching the connection between the air-fuel ratio detectors (1a-1d) and the drive circuit (11)
2, 43) and means for calculating the air-fuel ratio based on the pumping current flowing through the drive circuit (11) by this connection.

(Operation) According to the present invention, the cost can be reduced because only one drive circuit is required in spite of the multi-cylinder engine.
The entire device is downsized.

On the other hand, in the air-fuel ratio sensor of the type that pumps the pumping current, the connection between the air-fuel ratio detector of the cylinder that does not detect it and the drive circuit is cut off (the disconnected air-fuel ratio detector depends on the surrounding air-fuel ratio). Output is not generated at all)
Even if the drive circuit is connected during detection, an accurate output cannot be obtained until a predetermined settling time has elapsed. For this reason, if the switching cycle is too short, the air-fuel ratio measurement accuracy decreases. At this time, in the present invention, the switching cycle is determined in advance according to the time required for the pumping current flowing through the drive circuit to settle, and the drive circuit is switched at that cycle, so the measurement accuracy of the air-fuel ratio decreases. There is nothing to do.

Also, since the settling time of the air-fuel ratio detector is within 100 msec, for example, even in an 8-cylinder engine with a large number of cylinders, if there is 1 second, one air-fuel ratio measurement will be completed for all cylinders, so even in time division. This is sufficient for measuring the air-fuel ratio during steady operation.

(Embodiment) FIG. 1 is a block diagram of an embodiment in which the present invention is applied to a 4-cylinder engine. The same reference numerals as those in FIG. 3 represent the same or equivalent parts.

In FIG. 1, 1a is an empty space for the first cylinder, which is composed of a pair of zirconia plates (sensing cell and pumping cell) that perform an oxygen concentration detecting action and a pumping action, and electrodes provided on both main surfaces of each zirconia plate. With the fuel ratio detector,
4a is an outer electrode of the sensing cell, 7a is an outer electrode of the pumping cell, and 10a is a terminal in which the sensing cell and the inner electrode of the pumping cell are made common. Similarly, 1b-1d
Is an air-fuel ratio detector for the 2nd to 4th cylinders, 4b-4
The meanings of d, 7b to 7d and 10b to 10d are the same as those of the first cylinder.

41 to 43 are the number of output terminals of one air-fuel ratio detector (electrode terminal 2
And three common terminals for grounding (3 in total), the outer electrodes 4a to 4d of the sensing cell are the first multiplexer 41, and the outer electrodes 7a to 7d of the pumping cell are the second. Of the multiplexer 42, and the common terminals 10a-10d for grounding are connected to the third multiplexer 43 by leads, respectively.

The multiplexers 41 to 43 that receive a total of four signals from the air-fuel ratio detectors 1a to 1d are selected from among the four input signals in accordance with a command signal (select signal) from the CPU 33 described later.
Select only one signal.

However, the air-fuel ratio detectors 1a to 1d are sequentially arranged (for example, 1-2
The select signals to the multiplexers 41 to 43 are determined so that they are driven in the order of (3-4) cylinders. For example, when the select signal selects the first cylinder, the air-fuel ratio detector 1a for the first cylinder and the drive circuit 11 have three multiplexers 41-
A pumping current is supplied from the drive circuit 11 to the detector 1a through the connection 43. Select signal from No. 2 cylinder to 4
The same is true when selecting one of the numbered cylinders.

Here, the air-fuel ratio is calculated based on the pumping current, the sensitivity coefficient of the air-fuel ratio detector for each gas component concentration, which will be described later, and the water charcoal ratio. It is assumed that the circuit 21 handles the voltage converted value.

When the output waveform from the current-voltage conversion circuit 21 is shown in the lower part of FIG. 2, the air-fuel ratio detector outputs from the first cylinder to the fourth cylinder are time-divided as specified by the select signal shown in the middle part of the figure. It is obtained in. For comparison, the upper part of FIG. 2 shows the output when a drive circuit is connected to each air-fuel ratio detector. Note that # 1 to # 4 mean the first cylinder to the fourth cylinder, respectively.

Here, the cycle T of the select signal is determined in advance in accordance with the response speed (or settling time) of the air-fuel ratio detector, but if the air-fuel ratio detector is at a sufficiently high temperature, it will take a relatively short time (10
It is confirmed to respond within 0 msec). For this reason, even with an engine having a large number of cylinders, for example, an 8-cylinder engine, one air-fuel ratio measurement is completed for all the cylinders within 1 second, and thus time-division is sufficient for air-fuel ratio measurement during steady operation.

By using time division, even if the same number of air-fuel ratio detectors as the number of cylinders are installed, the drive circuit 11 (also for the circuit for calculating the air-fuel ratio and the circuit for outputting the result, which will be described later) is 1
Only one is enough. In other words, since the air-fuel ratio detectors 1a to 1d are installed corresponding to each cylinder, high measurement accuracy can be obtained, and moreover, since the drive circuit 11 to the air-fuel ratio calculation circuit and the output circuit are all required to be one, it is drastic. The cost can be reduced and the entire device can be compactly assembled.

When the exhaust temperature is low such as when the engine is under a low load, the air-fuel ratio detector becomes inactive and the response speed becomes slow. Therefore, the air-fuel ratio detectors 1a to 1d are provided with a heater, and the heater is operated only when the exhaust temperature is low (or may be always) to keep the air-fuel ratio detector at the active temperature. V _{H in} FIG. 1 is the heater operating voltage.

Returning to FIG. 1, the signal obtained by converting the time-division air-fuel ratio detector output shown in the lower part of FIG. 2 into a digital value by the A / D conversion circuit 22 and the signals from the digital switches 23a to 23d, 24 are input. Whenever the numerical value of the water-charcoal ratio changes, the arithmetic circuit 31 creates one numerical value correspondence table for each cylinder and stores it in the memory in the arithmetic circuit, and the stored cylinder The air-fuel ratio for each cylinder on the rich combustion side is calculated using another numerical value correspondence table. On the other hand, for the lean-burn side air-fuel ratio, the air-fuel ratio for each cylinder is calculated.

First, a set of digital switches 23a to 2 for setting the sensitivity coefficient.
In 3d, the oxygen concentration sensitivity coefficient (sensitivity coefficient for oxygen concentration) of the predetermined air-fuel ratio detectors 1a to 1d is set, and the other digital switch 24 for setting the water-carbon ratio is set to the fuel consumed by the engine. The water-charcoal ratios that are determined in advance according to the type are set as digital values. In addition, digital switch 24
If a 3-digit one is used, the water-charcoal ratio can be set in 0.01 steps, so an accurate air-fuel ratio can be calculated according to the type of fuel consumed by the engine.

The arithmetic circuit 31 includes an input / output interface 32, a central processing unit (CPU) 33 and a memory 34,
The arithmetic processing described later is executed. The display 35 has an arithmetic circuit 31.
The air-fuel ratio calculated in step 1 is displayed digitally. 36 is a display changeover switch.

Explaining the operation performed by the CPU 33, first, a set of digital switches 23a-23d, the corresponding air-fuel ratio detector 1a-
By setting the oxygen concentration sensitivity coefficient of 1d, all the oxygen concentration sensitivity coefficients of the air-fuel ratio detectors 1a to 1d are stored in the RAM in the memory 34 via the interface 32.

When the oxygen concentration sensitivity coefficients of the air-fuel ratio detectors 1a to 1d are stored in the RAM in this manner, the CPU 33 uses a known method to detect the hydrogen concentration sensitivity coefficient of the air-fuel ratio detectors 1a to 1d and carbon monoxide. The density sensitivity coefficient is calculated respectively. Then, these calculated sensitivity coefficients are also stored in the RAM in the memory 34.

Subsequently, the other digital switch 24 is set to the water-charcoal ratio determined according to the type of fuel. As a result, the water-charcoal ratio is stored in the RAM in the memory 34 via the interface 32.

As described above, when the water-charcoal ratio or the air-fuel ratio detectors 1a to 1d are newly set with respect to the oxygen concentration sensitivity coefficient by the digital switch, or when the air-fuel ratio measuring device is powered on, the RAM in the memory 34 is , Digital switches 23a-23
All the sensitivity coefficients of the air-fuel ratio detectors 1a to 1d corresponding to the numerical values of d and 24 and one water charcoal ratio are stored.

Then, in the arithmetic circuit 31, when a new water-charcoal ratio or oxygen-concentration sensitivity coefficient of the air-fuel ratio detector is stored in the RAM in the memory 34, as will be described later, in the CPU 33,
A numerical value correspondence table for calculating the air-fuel ratio on the rich combustion side corresponding to these values is created.

In this case, a numerical value correspondence table for each cylinder is created, but the creation method is the same for all cylinders, so the following description will be made using the first cylinder as a representative.

For example, a plurality of preset air-fuel ratios having different numerical values are read from the battery-backed up ROM in the memory 34, and the water-fuel ratio and the air-fuel ratio detector 1a set as described above are read out from the RAM in the memory 34. The sensitivity coefficient for hydrogen concentration and the sensitivity coefficient for carbon monoxide concentration are read out, and the pumping current corresponding to each air-fuel ratio is calculated from the following equations (1) to (6).

AFR ₁ = 138λ ₁ (1 + n / 4) / (12 + n) (1) Ip ₁ =-(B ₁ / η ₁ + C ₁ / ζ ₁ ) / A ₁ (2) A ₁ = 1 + n / 2 + 3.73λ ₁ (1 + n / 4) (3) C ₁ = 2 (1 + n / 4) (1-λ ₁ ) -B ₁ (5) Q ₁ = {(1 + n / 4) (1-λ ₁ ) + (n / 4 + K / 2) / (K-1 ^{)} 2 -2K (1 + n} / 4) (1-λ 1) / (K-1) ... (6) However, in the above (1) to formula (6), AFR ₁ is air, n represents Mizusumi Ratio, λ ₁ is a variable called equivalence ratio, Ip
₁ is the pumping current, A ₁ , B ₁ , C ₁ , Q ₁ are intermediate variables, η ₁ is the sensitivity coefficient of the air-fuel ratio detector 1a to carbon monoxide concentration, and ζ ₁ is the sensitivity of the air-fuel ratio detector 1a to hydrogen concentration. The coefficient K indicates a water gas reaction constant. It should be noted that "1" attached to AFR, λ, Ip, η, ζ, etc. represents the value for the first cylinder.

The arithmetic circuit 31 stores each pumping current calculated as described above in the RAM in the memory 34. That is, the RAM in the memory 34 stores a numerical value correspondence table corresponding to a certain specific value of the water charcoal ratio n.

Here, the operation of measuring the air-fuel ratio on the rich-fuel combustion side will be briefly described. On the rich-fuel combustion side, as described above, the pumping current Ip ₁ supplied to the air-fuel ratio detector 1a is negative (Ip ₁
<0), which is the current-voltage conversion circuit 21, A / D conversion circuit
It is input to the arithmetic circuit 31 via 22. In CPU33, based on the input pumping current Ip ₁ , by the interpolation method,
As described above, the air-fuel ratio corresponding to the pumping current Ip ₁ is calculated from the numerical value correspondence table stored in the RAM of the memory 31. The air-fuel ratio calculated in this way is output to the display 35 via the interface 32 and digitally displayed.

Here, since the air-fuel ratio on the rich combustion side is calculated from the numerical value correspondence table based on the pumping current, a relatively small memory capacity for storing the numerical value correspondence table is sufficient. Cost reduction can be realized.

On the other hand, on the lean burn side, as described above, the air-fuel ratio detector
The pumping current Ip ₁ supplied to 1a becomes positive (Ip ₁ ≧ 0), and in the CPU 33 to which this is input, the pumping current Ip ₁
Based on p ₁ and the water-charcoal ratio n and the oxygen concentration sensitivity coefficient γ _{1 of the} air-fuel ratio detector 1a stored in the RAM of the memory 34 as described above, the equation (1) and the following equation (7)
Calculate the air-fuel ratio from the formula.

λ ₁ = 1 + Ip ₁ / γ ₁ (0.209-Ip ₁ / γ ₁ ) × {1 + 0.209n / (4 + n)} (7) The air-fuel ratio calculated in this way is also an interface.
It is output to the display unit 35 via 32 and is digitally displayed.

In addition, instead of the numerical value correspondence table used on the rich combustion side,
Using equations (2) to (6), the water-carbon ratio n
It may be a numerical value correspondence table showing the relationship between the equivalence ratio and the pumping current using the as a parameter. However, the calculation of the air-fuel ratio when using such a numerical value correspondence table is performed by the CPU 33.
It is necessary to perform the calculation of the above formula (1).

Further, in order to obtain the air-fuel ratio difference between the cylinders, the four air-fuel ratios calculated for each cylinder are stored in the RAM, and the stored values are used for subtraction. And the display changeover switch
The air-fuel ratio difference is displayed on the display 35 by means of 36.

Finally, in FIG. 1, the inner electrode of the sensing cell is replaced by the multiplexer instead of the outer electrode of the sensing cell.
It goes without saying that you can connect to the 41. However, in that case, the outer electrodes 4a to 4d of the sensing cell and the inner electrode of the pumping cell are made common, and the common terminal is input to the multiplexer 43.

(Effects of the Invention) The present invention relates to a plurality of air-fuel ratio detectors attached to each cylinder of an engine, a circuit for driving the air-fuel ratio detectors when connected to each air-fuel ratio detector, and Means for outputting a select signal having a predetermined cycle that is predetermined according to the time required for the pumping current flowing through the drive circuit connected to each air-fuel ratio detector to settle, and in accordance with this select signal Since only one drive circuit is provided, the means for sequentially switching the connection between each of the air-fuel ratio detectors and the drive circuit and the means for calculating the air-fuel ratio based on the pumping current flowing through the drive circuit by this connection are sufficient. Therefore, it is possible to realize a significant cost reduction and downsizing of the device, and to reduce the accuracy of air-fuel ratio measurement by switching the connection between the air-fuel ratio detector and the drive circuit at too short a cycle. You can avoid the bottom.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of this embodiment, and FIG. 3 is for explaining the principle of detecting an air-fuel ratio by an air-fuel ratio detector. It is a circuit diagram of. 1a ~ 1d ... air-fuel ratio detector, 4a ~ 4d ... outer electrode of sensing cell, 7a ~ 7d ... outer electrode of pumping cell, 10a
~ 10d …… Common terminal, 11 …… Drive circuit, 21 …… Current-voltage conversion circuit, 22 …… A / D conversion circuit, 23a-23d …… Sensitivity coefficient setting digital switch, 24 …… Coal ratio setting Digital switch, 31 ... Arithmetic circuit, 32 ... Interface, 33 ... CPU, 34 ... Memory, 35 ... Display, 41-43
...... Multiplexer.

Claims (1)

[Scope of utility model registration request] 1. A plurality of air-fuel ratio detectors mounted corresponding to each cylinder of an engine, a circuit for driving the air-fuel ratio detectors when connected to each air-fuel ratio detector, and each air-fuel ratio detector. Means for outputting a select signal having a predetermined cycle that is predetermined according to the time required for the pumping current flowing through the drive circuit connected to the air conditioner to settle; and the air-fuel ratios according to the select signal. An air-fuel ratio measuring device for a multi-cylinder engine, comprising: means for sequentially switching the connection between the detector and the drive circuit; and means for calculating an air-fuel ratio based on a pumping current flowing through the drive circuit by this connection.