JPH059623B2 - - Google Patents

Description

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

(Industrial application field) Conventionally, exhaust sensors such as O ₂ sensors are installed in the engine exhaust system to detect the air-fuel ratio of the intake system from the concentration of exhaust components, and supply air to the intake system accordingly. Engine air-fuel ratio control techniques that perform closed-loop (feedback) control of the air-fuel ratio are well known.

However, in the closed-loop control of the air-fuel ratio described above, there is a time delay between detecting the air-fuel ratio supplied to the intake system in the exhaust system, and in order to avoid sudden fluctuations in the air-fuel ratio, there is a time delay in detecting the air-fuel ratio supplied to the intake system. The air-fuel ratio adjusting means is controlled according to the integral signal.
Therefore, the actual air-fuel ratio fluctuations do not remain constant with respect to the set value, for example, the stoichiometric air-fuel ratio, but cause a hunting phenomenon in which they periodically fluctuate up and down around this set value, and during idling etc. This hunting fluctuation in the air-fuel ratio causes a problem of torque fluctuations, that is, rotation fluctuations, and in the case of a three-way catalyst installed in the exhaust system, there is a risk that the exhaust purification performance will deteriorate due to the three-way atmosphere. be.

Therefore, in order to improve the hunting phenomenon of the closed-loop control described above, a multi-cylinder engine is divided into a first cylinder group and a second cylinder group, and the first cylinder An exhaust sensor is disposed in the exhaust system of each cylinder group, and the first cylinder group is controlled in a closed loop by an integral signal based on the output of the exhaust sensor, while the second cylinder group is controlled by inverting the integral signal for the first cylinder group. A technique has been proposed in which air-fuel ratio control is performed in an opposite phase state. In this technique, the fluctuations in the air-fuel ratio of the first cylinder group and the second cylinder group are reversed so that they cancel each other out, thereby suppressing the hunting phenomenon.

However, in the above-mentioned prior art, the second cylinder group is not substantially subjected to closed-loop control based on the detection of the air-fuel ratio supplied to its intake system, but is simply controlled by the first cylinder group.
The air-fuel ratio is simply controlled in a phase opposite to the first cylinder group based on the assumption that the air-fuel ratio will fluctuate in the same way as the cylinder group. Due to system accuracy errors, intake system filling efficiency errors, etc., for example, even if the same fuel injection signal is output to multiple fuel injection valves, there will be differences in the amount of fuel actually injected. As a result, the air-fuel ratio control in the second cylinder group may be controlled to an air-fuel ratio that is far from the target value, resulting in a significant decrease in control accuracy, and the original purpose of closed-loop control of the air-fuel ratio may not be fully achieved. This results in deterioration in combustibility, fuel efficiency, and emission performance.

(Object of the Invention) In view of the above circumstances, the present invention provides an air-fuel ratio control system for a multi-cylinder engine that improves control accuracy and improves the hunting phenomenon associated with closed-loop control. The purpose of this invention is to provide a control device.

(Structure of the Invention) The air-fuel ratio control device of the present invention includes a first cylinder group and a second cylinder group.
Exhaust sensors are disposed in the exhaust systems of the cylinder groups, respectively, so that both cylinder groups perform closed-loop control using integral signals based on the respective exhaust sensor outputs, and a first integral signal for the first cylinder group and a first integral signal for the first cylinder group are provided. The air-fuel ratio is controlled by correcting and controlling the inversion timing and control gain of the first or second integral signal so that the fluctuation with the second integral signal for the second cylinder group is in opposite phase and the fluctuation period of both coincides. It is characterized by the fact that it is designed to be carried out.

FIG. 1 is an overall configuration diagram for clearly showing the configuration of the present invention.

The multi-cylinder engine E is divided into two cylinder groups, a first cylinder group 1 and a second cylinder group 2, and a first exhaust system extracts exhaust gas corresponding to the first cylinder group 1 and the second cylinder group 2. 3 and a second exhaust system 4 are connected to each other, and both exhaust systems 3 and 4 meet on the downstream side, and a catalyst device 5 is interposed therebetween.

The first exhaust system 3 and the second exhaust system 4 include a first exhaust sensor 6 and a second exhaust sensor 7 such as an O ₂ sensor that detects the component concentration of exhaust gas discharged from the respective cylinder groups 1 and 2. are arranged respectively.

On the other hand, a fuel supply device 8 that supplies fuel to the first and second cylinder groups 1 and 2 is provided, and a first air-fuel ratio that adjusts the air-fuel ratio supplied to the first cylinder group 1 with respect to the fuel supply device 8. Adjustment means 9 and second cylinder group 2
A second air-fuel ratio adjusting means 10 for adjusting the air-fuel ratio supplied to the second air-fuel ratio is provided.

The first air-fuel ratio adjustment means 9 and the second air-fuel ratio adjustment means 10 are connected to the first exhaust sensor 6 and the second air-fuel ratio adjustment means.
It is controlled by a control means that receives the detection output of the exhaust sensor 7, and this control means includes a closed loop control means 11, a reversal detection means 12, a phase correction means 13, a period detection means 14, The period correction means 15 is also provided.

The closed loop control means 11 controls the first air-fuel ratio adjusting means 9 according to a first integral signal based on the first exhaust sensor 6, and controls the second air-fuel ratio adjusting means 9 according to the second integral signal based on the second exhaust sensor 7. It controls the fuel ratio adjusting means 10.

Further, the reversal detection means 12 detects a signal related to the reversal of the first integral signal and the second integral signal by the closed loop control means 11 from the first and second integral signals or from the first and second exhaust sensors 6, Detection is made from the detection output of No. 7, etc.

The phase correction means 13 corrects at least one of the first or second integral signal based on the signal of the reversal detection means 12 so that the increasing/decreasing directions of the first integral signal and the second integral signal are in opposite phases. This changes the reversal timing.

On the other hand, the period detection means 14 detects a signal related to the period of the first integral signal and the second integral signal from the reversal to the reversal of the first and second integral signals or the outputs of the first and second exhaust sensors 6 and 7. It is detected based on the time etc.

Further, the period correction means 15 includes the period detection means 1
Based on the signal No. 4, the control gain (integral constant) of at least one of the first or second integral signal is corrected and the cycle is changed so that the fluctuation cycles of the first integral signal and the second integral signal match. It is.

By using the above-mentioned control means, for example, when there is an output fluctuation (comparison output) of the first exhaust sensor 6 and the second exhaust sensor 7 as shown in A and B in FIG. 2, that is, an air-fuel ratio fluctuation, both of them are controlled. The reversal timing and period are different between rich detection and lean detection, and correspondingly, the first integral signal and the second air-fuel ratio for the first air-fuel ratio adjusting means 9 as shown in C and D in FIG. the second for the adjusting means 10;
An integral signal is created, but if left as is, a hunting phenomenon will occur, so for example, the second
As shown in E of FIG. 2, the second integral signal to the air-fuel ratio adjusting means 10 is the second integral signal from when the first integral signal is inverted to when the second integral signal is inverted in the opposite phase state. While performing phase correction to maintain the period of the first integral signal, the control gain is changed to perform period correction so that the period of the second integral signal matches the period of the first integral signal, and closed-loop control is performed in which the periods match with the opposite phase. It is something.

As a result, closed-loop control is performed in the first and second cylinder groups 1 and 2 according to the outputs of the first and second exhaust sensors 6 and 7, respectively, and increases and decreases in the first and second cylinder groups 1 and 2 are performed. The phase is adjusted so that the integrated signals are opposite in direction and have the same period, thereby improving control accuracy through closed loop control and suppressing hunting through opposite phase/periodic control.

The multi-cylinder engine E is a 6-cylinder or 8-cylinder V-type engine, with one bank serving as the first cylinder group 1 and the other bank serving as the second cylinder group.
The present invention is applicable to various multi-cylinder engines, such as those with a cylinder group 2, or those with an in-line 4 to 8 cylinder engine divided into two, a first cylinder group 1 and a second cylinder group 2.

The fuel supply device 8 and the first and second air-fuel ratio adjusting means 9, 10 are equipped with a fuel injection amount calculation section, and the fuel supply device 8 and the first and second air-fuel ratio adjusting means 9, 10 are equipped with a fuel injection amount calculation section and adjust the air-fuel ratio by adjusting the fuel injection amount from the fuel injection valve. injection type,
Alternatively, a carburetor system may be used in which the supplied air-fuel ratio is adjusted by adjusting the amount of bleed air in the carburetor.

Furthermore, the correction that makes the increasing/decreasing directions of the first integral signal and the second integral signal have opposite phases involves temporarily holding the integral signal, increasing/decreasing the control gain, and temporarily changing the inversion interval to shift the phase. etc. will be adopted as appropriate.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

Figure 3 shows the specific configuration. The multi-cylinder engine E, which is a V-type 6-cylinder engine, is connected to one bank B1.
is divided into a first cylinder group 1 and the other bank B2 is divided into a second cylinder group 2. A first exhaust system 3 is connected to the exhaust port 16 of each cylinder C1 of the first cylinder group ₁ , and a first exhaust system 3 is connected to the exhaust port 16 of each cylinder C1 of the first cylinder group 1. The second exhaust system 4 is connected to the exhaust port 16 of each cylinder C ₂ of the cylinder C 2 , and a catalyst device 5 (three-way catalyst) is provided downstream from the merging portion. Further, the first exhaust system 3 and the second exhaust system 4 are respectively provided with a first exhaust sensor 6 and a second exhaust sensor 7 such as an O ₂ sensor.

On the other hand, an intake system 18 that supplies intake air is connected to the intake port 17 of each cylinder C ₁ and C ₂ of the first and second cylinder groups 1 and 2. This intake system 18
is equipped with an air cleaner 19 at the upstream end, an air flow sensor 20 for measuring the amount of intake air is interposed downstream of the air cleaner 19, and a first intake passage for the first cylinder group 1 is installed downstream of the air flow sensor 20. 18a and the second intake passage 18 for the second cylinder group 2
It is branched into b. The first and second intake passages 18a, 18b are connected to throttle valves 21, 2, respectively.
The downstream portion of the cylinder 2 is configured as a surge tank, and this surge tank is communicated with each cylinder C ₁ and C ₂ through an independent intake passage. Note that the two throttle valves 21 and 22 are synchronously opened and closed to the same opening degree.

A first fuel injection valve 23 for supplying fuel to each cylinder _C1 of the first cylinder group 1 is disposed in the first intake passage 18a, and a first fuel injection valve 23 for supplying fuel to each cylinder C1 of the first cylinder group 1 is disposed in the second intake passage 18b. The second supplying fuel to cylinder C ₂
Fuel injection valves 24 are respectively provided. A control signal from a control unit 25 (microcomputer) is outputted to the first and second fuel injection valves 23 and 24 according to the operating state, and from this first fuel injection valve 23 to the first cylinder group 1, And the fuel injection amount, that is, the air-fuel ratio, respectively supplied from the second fuel injection valve 24 to the second cylinder group 2 is controlled.

This control unit 25 includes the first
Exhaust gas component concentration signals, that is, detected air-fuel ratio signals are input from the exhaust sensor 6 and the second exhaust sensor 7, and an intake air amount signal from the air flow sensor 20 and the rotation speed sensor 26 are input as signals for detecting the operating state of the engine. The engine rotation speed signal from the engine speed sensor 27 and the coolant temperature signal from the water temperature sensor 27 are respectively input.

The control unit 25 includes a fuel supply device 8 that supplies fuel to the first and second cylinder groups 1 and 2 in FIG. 2nd cylinder group 1,
the first and second adjusting the air-fuel ratio supplied to the second
The air-fuel ratio adjusting means 9 and 10 are configured, and
The control unit 25 has the functions of the closed loop control means 11, the reversal detection means 12, the phase correction means 13, the period detection means 14, and the period correction means 15 shown in FIG. At the same time, it performs closed-loop control such that both are in opposite phases and have the same period.

Next, the operation of the control unit 25 will be explained using the flowchart shown in FIG.

First, start (steps shown in Figure 4).
S1), and after initializing to reset each timer t and flag F, the intake air amount Q
and the engine speed N (S2, S3), and from these, the first and second
Basic injection pulse output to fuel injection valves 23 and 24
Calculate To (S4). In addition, in steps S5 and S6, a correction coefficient Co for temperature correction based on the detection of the water temperature sensor 27 or other starting, acceleration correction, etc. is determined.
Calculate the corrected injection pulse Ti.

Subsequently, the comparison outputs E ₁ and E ₂ of the first exhaust sensor 6 and the second exhaust sensor 7 are read (S7, S8). The comparison outputs E ₁ and E ₂ output a 1 signal when the detected air-fuel ratio is richer than the set air-fuel ratio, and output a 0 signal when the detected air-fuel ratio is leaner.

Steps S9 to S34 detect the cycle of variation between the comparison output _E1 of the first exhaust sensor 6 and the comparison output E2 of the second exhaust sensor 7 to the rich direction and lean change, respectively, and detect the cycle of variation between the comparison output E1 of the first exhaust sensor ₆ and the comparison output E2 of the second exhaust sensor 7. This is a cycle correction routine in which the integral constant Ibm of the second integral signal is corrected in the negative direction in step S30, and the integral constant Ibp of the second integral signal in the positive direction is corrected in step S34.The details will be described later. .

On the other hand, from step S35 onwards, a phase correction routine is performed in which the reversal timing is detected, the first or second integrated signal is temporarily held, and the inversion timings of both are matched and the phases are opposite to each other.

In this phase correction routine, the comparison output E ₁ of the first exhaust sensor 6 and the comparison output E ₂ of the second exhaust sensor 7 are compared in step S35, and whether or not they are in the same phase with the same value of 1 or 0 is determined. If the phase is opposite (NO), flag F is set in step S36.
After resetting to 0, it is determined in step S37 whether or not the air-fuel ratio of the first cylinder group 1 is rich from the comparison output E1 of the first exhaust sensor _6. If it is rich (YES), the supplied air-fuel ratio is Calculate the correction coefficient C ₁ by subtracting the integral constant Iam in the negative direction so as to shift to the lean direction (S38), and select lean (NO).
In this case, the correction coefficient _C1 is calculated by adding the integral constant Iap in the positive direction so that the supplied air-fuel ratio shifts to the rich direction (S39). Based on this, a first integral signal for closed loop control of the first cylinder group 1 is obtained. The integral constants Iam and Iap in the negative and positive directions are set to predetermined values in advance.

Proceeding to step S40, the second comparison output _E2 to the second
It is determined whether the air-fuel ratio of cylinder group 2 is rich or not, and if it is rich (YES), the integral constant Ibm in the negative direction set in step S30 is subtracted so that the supplied air-fuel ratio shifts to the lean direction. A correction coefficient _C2 is calculated (S41), and in the case of lean (NO), step S34 is performed so that the supplied air-fuel ratio shifts to the rich direction.
A correction coefficient _C2 is calculated by adding the positive integration constant Ibp set in (S42). Based on this, a second integral signal for closed loop control of the second cylinder group 2 is obtained.

Next, in steps S52 and S53, the memories of the first comparison output _E1 and the second comparison output _E2 are rewritten, respectively, and the correction coefficient _C1 (first integral signal) obtained in the step S38 or S39 and the step
Based on the correction coefficient C ₂ (second integral signal) obtained in S41 or S42, the final injection pulse T ₁ for the first cylinder group 1 and the final injection pulse T ₂ for the second cylinder group 2 are determined (S54, S55) In response, in step S56, a fuel injection pulse is output to the first fuel injection valve 23 of the first cylinder group and the second fuel injection valve 24 of the second cylinder group 2, and a predetermined amount of fuel is injected. Closed-loop control of air-fuel ratio.

In the above-mentioned anti-phase control, if either the first exhaust sensor 6 output or the second exhaust sensor 7 output is reversed first and becomes the same phase state, the determination in step S35 is YES, and the first exhaust sensor 7 output is inverted in step S43. Determine whether flag F is set. When this flag F is set to 1, the integral signal is being held, and when it is reset to 0, the integral signal is not being held. If F=0, which is not in the holding state yet, the determination in step S43 is
If NO, the flag F is set to a holding state of 1 in step S44, and then it is determined in step S45 whether the comparative output _E2 of the second exhaust sensor 7 is the same as the previous comparative output _E2m registered in the memory. In other words, depending on whether the detected air-fuel ratio has reversed from the lean side to the rich side or from the rich side to the lean side with respect to the set air-fuel ratio, this same phase state is determined by whether the first exhaust sensor 6 output is It is determined whether the output of the second exhaust sensor 7 has been reversed or not.

If the judgment in step S45 is YES, the second
The exhaust sensor 7 output is not reversed and the first exhaust sensor 6
Since the output was reversed first, the first exhaust sensor 6
In order to delay the reversal timing of the first integral signal based on the output to match the reversal timing of the second exhaust sensor 7 output,
In step S46, the second flag P is set to 1 to hold the first integral signal. on the other hand,
When the determination in step S45 is NO, since the output of the second exhaust sensor 7 was inverted first, the timing of inversion of the second integral signal based on the output of the second exhaust sensor 7 is delayed, and the output of the first exhaust sensor 6 is In order to match the reversal timing, the flag P is reset to 0 in step S47 and set to hold the second integrated signal, and the setting of the flag F is maintained until the next phase changes to the opposite phase.

After determining the integral signal to be held by setting the second flag P, the process proceeds to step S48, where it is determined whether the second flag P set in step S46 or S47 is set to 1.

When holding the first integral signal, step
Since the determination in S48 is YES (P=1), the process proceeds to step S40, where closed-loop control of the second integral signal corresponding to the output of the second exhaust sensor 7 is performed, and closed-loop control of the first integral signal is cut. its correction coefficient
Fix C ₁ and hold it at a constant value.

In addition, when holding the second integral signal, the determination in step S48 is NO (P=0), and step S48 is NO (P=0).
Proceed to S49. Steps S49 to S51 are the same as steps S37 to S39, and are for performing closed loop control of the first integral signal, cutting off closed loop control of the second integral signal and fixing the correction coefficient _C2 to a constant value. Hold.

The holding state of the first or second integral signal is
When the second or first integral signal that is not held is inverted, an anti-phase state is created, and the determination in step S35 becomes NO, and the process proceeds to step S36, where F is reset to 0, thereby canceling the state. , returns to the closed loop control of the first and second integral signals in opposite phases by steps S37-42 and S52-56.

On the other hand, the period correction routine is performed in step S9.
Whether the comparison output E ₁ of the first exhaust sensor 6 is the same as the previous comparison output E ₁ m registered in the memory or not,
That is, it is determined whether or not it has been reversed. When it is not inverted (YES), it is determined whether the first flag F is set (S10) and the second flag P
is reset (S11), and in a state where the first integral signal is not held, it is determined whether the first comparison output _E1 is rich (S12). If it is rich (YES), the timer tam is counted in the negative direction in step S13, and if it is lean (NO), the timer tam is counted in the negative direction.
In S14, count the timer tap in the positive direction.
Counting by both timers tam and tap continues until the comparison output _E1 is inverted and the determination at step S9 becomes NO. That is, when the output is inverted, the process proceeds to step S15, where it is determined whether the first comparison output _E1 is rich or not, and if it is rich (YES), the memory of the timer tam in the negative direction is rewritten (S16).
At the same time, this timer tam is reset to 0 (S17), and in the case of lean, the memory of the timer tap in the positive direction is rewritten (S18),
This timer tap is reset to 0 (S19).

After the timer has counted in step S13 or 14, or when the judgment in step S11 is NO (P=1) and the first integral signal is held, and when the timer is reset at the time of inversion, the process advances to step S20 and the first integrated signal is reset. 2 The inversion interval of comparison output _E2 is counted.

In step S20, the comparison output of the second exhaust sensor 7 is
Previous comparison output E ₂ m where E ₂ is registered in memory
It is determined whether or not it is the same as, that is, whether it is reversed. When it is not reversed (YES), the step
In S21 and S22, it is determined whether the first flag F and the second flag P are set, respectively, and if the second integral signal is not held, the second comparison output _E2 is set in step S23. Determine whether it is rich or not. If rich (YES), timer tbm is counted in the negative direction in step S24, and if lean (NO), step S25 is performed.
The timer tbp counts in the positive direction. Counting by both timers tbm and tbp continues until the comparison output _E2 is inverted and the determination at step S20 becomes NO.

That is, when the output is inverted, the process proceeds to step S26, where it is determined whether the second comparison output _E2 is rich or not.
If rich (YES), the timer in the positive direction
The memory of tbp is rewritten (S27) and the timer tbp is reset to 0 (S28). and,
This second comparison output E ₂ is inverted to the rich side, and the second comparison output E 2 is inverted to the rich side.
Regarding shifting the integral signal in the lean direction, since the first comparison output E1 was a rich output and the _second comparison output _E2 was a lean output until this second comparison output _E2 was inverted, step S29 Find the difference in the reversal interval or period between the two, tcm=tbm−tap,
In step S30, the integral constant Ibm for shifting the correction coefficient _C2 in the negative direction is corrected based on this count difference tcm. The correction value obtained by multiplying the above count difference tcm by the coefficient Cm is added to the integral constant Ibm to correct the integral constant Ibm. When the count difference tcm is a positive value, the reversal interval on the first cylinder group 1 side Therefore, in order to shorten the reversal interval on the second cylinder group 2 side, the value of the integration constant Ibm is increased.

Further, if the judgment in step S26 is NO and the second comparison output _E2 is lean, the memory of the timer tbm in the negative direction is rewritten (S31), and
This timer tbm is reset to 0 (S32). Then, this second comparison output _E2 is reversed to the lean side,
Regarding the shift of the second integrated signal in the rich direction, until this second comparison output E ₂ is inverted, the first
Since the comparison output E ₁ is a lean output and the second comparison output E ₂ is a rich output, in step S33, the difference in the inversion interval or period between the two, tcp = tbp - tam, is calculated, and in step S34, this count difference tcp is calculated. Based on this, the integral constant Ibp for shifting the correction coefficient _C2 in the positive direction is similarly corrected.

After the timer counts in step S24 or 25, or when the judgment in step S22 is NO and the second integral signal is held, and after the cycle of the integral constant of the second integral signal is corrected, the step
The process proceeds to S35 and phase control is performed.

Therefore, by the above control routine, when the phase changes from opposite to the same, the first or second
The integral signal is held, and when the other second or first integral signal reverses in the opposite direction and becomes antiphase again, the above holding is canceled and the air-fuel ratio change is held so that both are in an antiphase state. In order to maintain this anti-phase state, the period of the first integral signal and the second integral signal, that is, the inversion interval is detected, and the integral constants Ibm and Ibp of the second integral signal are determined according to the difference. increase/decrease, and match the reversal timings of the first integral signal and the second integral signal so that both cylinder groups 1 and 2 are activated in the same cycle.
It performs closed-loop control over the

Note that in the above embodiment, either the first integral signal or the second integral signal is held depending on the context of the reversal timing to shorten the holding time. Either one of them may be always held. Also,
In periodic control as well, in the above embodiment the control gain of only the second integral signal is corrected; however,
Either one of the first integral signals may be selected for correction.

Furthermore, in the flowchart, the first and second
Although the integral constant I used in calculating the integral signal is changed in the positive direction and the negative direction, that is, in the case where the air-fuel ratio is shifted to the rich direction and the lean direction, it may be the same value.

(Effects of the Invention) According to the present invention, both the first cylinder group and the second cylinder group are controlled in a closed loop using an integral signal based on the exhaust sensor output of each exhaust system, and the first cylinder group and the second cylinder group At the same time, it improves the control accuracy for the target value of the air-fuel ratio supplied to the group, and at the same time
Phase and period control is performed so that the cylinder group and the second cylinder group have opposite phases and the same reversal period to suppress the hunting phenomenon, stabilize the engine speed during idle operation, and improve emission performance. It is something that can be improved.

[Brief explanation of the drawing]

Fig. 1 is an overall configuration diagram to clarify the structure of the present invention, Fig. 2 is a characteristic diagram showing an example of correction of an integral signal together with an exhaust sensor output and an integral signal, and Fig. 3 is a specific structural example of the present invention. FIG. 4 is a flowchart illustrating the operation of the control unit in FIG. 3. E...Multi-cylinder engine, 1...First cylinder group, 2
...Second cylinder group, 3...First exhaust system, 4...Second
Exhaust system, 6...First exhaust sensor, 7...Second exhaust sensor, 8...Fuel supply device, 9...First air-fuel ratio adjustment means, 10...Second air-fuel ratio adjustment means, 11...
...Closed loop control means, 12...Reversal detection means, 1
3...Phase correction means, 14...Period detection means, 1
5... Period correction means, 23... First fuel injection valve,
24...Second fuel injection valve, 25...Control unit.

Claims (1)

[Claims] 1 The cylinders of the engine are divided into a first cylinder group and a second cylinder group, a first exhaust system is provided corresponding to the first cylinder group, and a second exhaust system is provided corresponding to the second cylinder group. In addition, in a multi-cylinder engine equipped with a fuel supply device that supplies fuel to the first and second cylinder groups, the component concentration of the exhaust gas disposed in the first exhaust system and discharged from the first cylinder group is determined. a first exhaust sensor that detects a component concentration of exhaust gas that is disposed in the second exhaust system and that detects a component concentration of exhaust gas discharged from a second cylinder group; a first air-fuel ratio adjustment means for adjusting the air-fuel ratio; a second air-fuel ratio adjustment means for adjusting the air-fuel ratio supplied to the second cylinder group; and a first air-fuel ratio adjustment according to a first integral signal based on the first exhaust sensor output. a second exhaust sensor based on the second exhaust sensor output;
closed-loop control means for controlling the second air-fuel ratio adjustment means in response to the integral signal; reversal detection means for detecting a signal related to reversal of the first integral signal and the second integral signal; and based on the signal of the reversal detection means. a phase correction means for correcting the inversion timing of at least one of the integral signals so that the increasing/decreasing directions of the first integral signal and the second integral signal are in opposite phases to each other; a period detection means for detecting the signal; and a period correction means for correcting the control gain of at least one of the integral signals so that the first integral signal and the second integral signal have the same period based on the signal of the period detection means. An air-fuel ratio control device for a multi-cylinder engine characterized by: