JPH08210164A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

PURPOSE: To control the air-fuel ratio of an engine to a target air-fuel ratio according to the output characteristic of an air-fuel ratio sensor with good responsiveness and with high accuracy by changing a lean side feedback control constant to be larger than the rich side feedback control constant the larger the recirculation quantity is. CONSTITUTION: The density of HC and CO of exhaust gas exposed to an air-fuel ratio sensor 13 becomes higher according to the quantity of exhaust recirculation by an exhaust recirculating means B, so that the output characteristic of the air-fuel ratio sensor 13 shifts. Even if the air-fuel ratio feedback control is continued while the output characteristic of the air-fuel ratio sensor 13 still shifts, the lean side feedback coefficient is changed to be larger than the rich side feedback coefficient the larger the recirculation quantity circulated by the exhaust recirculating means B is by an air-fuel ratio coefficient changing means D. Accordingly, the air-fuel ratio of an internal combustion engine which becomes rich due to the recirculation quantity of the exhaust recirculating means B is returned to the lean side with good responsiveness and with good accuracy and controlled to be a theoretical air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to an exhaust gas recirculation system (EGR) for reducing NOx and improving fuel consumption, which is adapted to the exhaust gas recirculation flow rate by EGR. Thus, the present invention relates to an air-fuel ratio control device for an internal combustion engine, which controls the air-fuel ratio of the air-fuel mixture detected from the exhaust gas of the internal combustion engine to a target air-fuel ratio.

[0002]

2. Description of the Related Art Generally, an air-fuel ratio control system for an internal combustion engine sets the air-fuel ratio of the engine to a target air-fuel ratio in accordance with the output of an air-fuel ratio sensor provided in the exhaust system of the internal combustion engine (hereinafter referred to as "engine"). Feedback control is performed so that the fuel injection amount required for each combustion cycle of the engine is corrected, and the fuel is injected from the fuel injection valve toward the intake port of the engine. This correction is performed by calculating the valve opening time of the fuel injection valve, that is, the fuel injection time τ from the following equation. τ = Tp · (FAF + α) + β where Tp is determined from the intake pipe pressure and engine speed in the speed density method, and is measured by a vane type, Karman vortex type, or thermal type air flow meter in the mass flow method. The basic fuel injection time determined from the intake air amount is shown, FAF is an air-fuel ratio correction coefficient, α is another correction coefficient, and β is another correction term.

The air-fuel ratio correction coefficient FAF is an air-fuel ratio of the engine according to the air-fuel ratio of the air-fuel ratio of the mixture supplied to the engine detected from the exhaust gas of the engine, for example, the oxygen concentration by an air-fuel ratio sensor provided in the exhaust system of the engine. Is rich, fuel injection time τ
Is a coefficient for correcting the fuel injection time τ so that the air-fuel ratio of the engine becomes the target air-fuel ratio by making τ short and making lean. The air-fuel ratio correction coefficient FAF is corrected by the feedback control constant, that is, the delay time TD, the integration constant KI or the skip amount RS.

The air-fuel ratio control device for an internal combustion engine equipped with EGR disclosed in Japanese Patent Laid-Open No. 2-55849 prevents fluctuations in the air-fuel ratio that are affected by uneven cylinder distribution of exhaust gas recirculation flow due to EGR. Therefore, the feedback control constant is changed according to whether or not the circulation flow rate exceeds a predetermined amount (EGR ON / OFF).

[0005]

Further, in the air-fuel ratio control system for an internal combustion engine, generally, the theoretical air-fuel ratio at which the exhaust gas of the engine is most purified is the center of the feedback control. On the other hand, the output voltage characteristic of the air-fuel ratio sensor changes under the influence of the exhaust gas recirculation flow rate due to EGR. That is, when EGR is on, the concentration of HC or CO in the exhaust gas becomes high, the output voltage characteristic of the air-fuel ratio sensor deviates, and the output voltage of the air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio, which is the control center of air-fuel ratio feedback, changes. When feedback control of the air-fuel ratio is performed based on the output voltage characteristic of the air-fuel ratio sensor that has deviated, the engine will generate emissions. The device disclosed in Japanese Patent Laid-Open No. 2-55849 corrects the fluctuation of the air-fuel ratio caused by the non-uniform distribution of the exhaust gas in the cylinders due to the EGR. The air-fuel ratio sensor is based on the exhaust gas recirculation flow rate due to the EGR. It is not disclosed that the output characteristics of the above shift. That is, the air-fuel ratio control device is
There is a problem in that emissions are not feedback-controlled so that the air-fuel ratio of the engine becomes the stoichiometric air-fuel ratio due to a shift in the output voltage characteristic of the air-fuel ratio sensor that changes according to the GR exhaust gas recirculation flow rate.

Further, in the air-fuel ratio control device of the above-mentioned Japanese Patent Laid-Open No. 2-55849, the air-fuel ratio sensor is HC or CO when EGR is on.
Therefore, the rich correction coefficient is set asymmetrically larger than the lean correction coefficient as the feedback control constant for correcting the air-fuel ratio of the engine according to the output of the air-fuel ratio sensor. When the EGR is switched from on to off, the exhaust gas exposed to the air-fuel ratio sensor decreases the concentration of HC and CO, so the output characteristic of the air-fuel ratio sensor is that there is no recirculation flow due to EGR from the output characteristic when there is recirculation flow due to EGR. When the output characteristics change. However, since the rich correction coefficient of the feedback control constant of the air-fuel ratio sensor is set to be larger than the lean correction coefficient, there is a problem that the air-fuel ratio of the engine is corrected to the rich side and the rich state continues to be maintained. Then, there arises a problem that emission occurs.

Therefore, in order to solve the above-mentioned problems, the present invention provides an air-fuel ratio control system for an internal combustion engine equipped with an EGR engine according to the output characteristic of an air-fuel ratio sensor which changes according to the exhaust gas recirculation flow rate by the EGR engine. It is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine, which controls the air-fuel ratio to be a target air-fuel ratio with good response and accuracy.

[0008]

FIG. 1 is a basic configuration diagram of the present invention. An air-fuel ratio control system for an internal combustion engine according to the present invention which achieves the above object, comprises a fuel supply means A for supplying fuel to an intake system of the internal combustion engine, and an exhaust gas discharged from the internal combustion engine according to an operating state of the internal combustion engine. Exhaust gas recirculation means B for recirculating a part to the intake system, and an air-fuel ratio sensor 1 provided in the exhaust system.
In the air-fuel ratio control device for an internal combustion engine, the feedback control means C feedback-controls the fuel supply amount so that the air-fuel ratio of the internal combustion engine becomes a target value according to the output signal of the air-fuel ratio sensor 13. A rich side feedback control constant that corrects the air-fuel ratio correction coefficient that corrects the fuel supply amount toward the rich side and a lean side feedback control constant that corrects toward the lean side are returned to the exhaust gas recirculation means. An air-fuel ratio correction coefficient changing means D is provided for changing the lean side feedback control constant to the rich side feedback control constant as the flow rate increases.

Another air-fuel ratio control system for an internal combustion engine according to the present invention which achieves the above object, is a fuel supply means A for supplying fuel to an intake system of the internal combustion engine, and an exhaust from the internal combustion engine according to an operating state of the internal combustion engine. The exhaust gas recirculation means B for recirculating a part of the exhaust gas to the intake system, the air-fuel ratio sensor 13 provided in the exhaust system, and the air-fuel ratio of the internal combustion engine according to the output signal of the air-fuel ratio sensor 13 reach a target value. In the air-fuel ratio control device for the internal combustion engine, which comprises the feedback control means C for feedback controlling the fuel supply amount as described above, the exhaust gas recirculation means B
Is ON, the rich side feedback control constant for correcting the air-fuel ratio correction coefficient for correcting the fuel supply amount toward the rich side and the lean side feedback control constant for correcting for the lean side are set asymmetrically, Exhaust recirculation means B
Is off, the air-fuel ratio correction coefficient changing means D for symmetrically setting the rich side feedback control constant and the lean side feedback control constant is provided.

[0010]

In the air-fuel ratio control system for an internal combustion engine of the present invention,
The concentration of HC and CO in the exhaust gas exposed to the air-fuel ratio sensor becomes high according to the recirculation flow rate of the exhaust gas by the exhaust gas recirculation means, and the output characteristics of the air-fuel ratio sensor deviate. The output voltage of the air-fuel ratio sensor, which corresponds to, changes. Even if the air-fuel ratio feedback control continues with the output characteristics of the air-fuel ratio sensor shifted, the rich-side feedback control constant that corrects the air-fuel ratio correction coefficient toward the rich side and the lean-side feedback that corrects toward the lean side As the control constant and the larger the recirculation flow rate recirculated by the exhaust recirculation means, the lean-side feedback coefficient is largely changed with respect to the rich-side feedback coefficient by the air-fuel ratio correction coefficient changing means of the present invention.
The air-fuel ratio of the internal combustion engine, which has become rich due to the recirculation flow rate of the exhaust gas recirculation means, is controlled with high responsiveness and accuracy to the lean side to reach the theoretical air-fuel ratio.

Further, the air-fuel ratio correction coefficient changing means in the air-fuel ratio control device for an internal combustion engine of the present invention sets the feedback control constant asymmetrically when the exhaust gas recirculation means is on,
When the exhaust gas recirculation means is off, the feedback control constants are set symmetrically. Therefore, when the exhaust gas recirculation means is turned on, HC and C of the exhaust gas exposed to the air-fuel ratio sensor arranged upstream of the catalytic converter provided in the exhaust system of the engine.
The concentration of O becomes high, the output characteristic of the air-fuel ratio sensor shifts, and the output voltage of the air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio, which is the control center of air-fuel ratio feedback, changes. Even if the feedback control of the air-fuel ratio is continued while the output characteristic of the air-fuel ratio sensor is deviated, the feedback control constant is set asymmetrically when the exhaust gas recirculation means is turned on by the air-fuel ratio correction coefficient changing means of the present invention. The air-fuel ratio of the internal combustion engine, which has become rich due to the exhaust gas recirculation means being turned on, is responsively returned to the lean side and controlled to become the stoichiometric air-fuel ratio. Also,
When the exhaust gas recirculation means is switched from on to off,
Since the exhaust gas recirculation flow rate disappears, the concentration of HC and CO in the exhaust gas exposed to the air-fuel ratio sensor decreases, the output characteristics of the air-fuel ratio sensor change, and the exhaust gas recirculation means returns to the original state with no recirculation flow rate. Since the feedback control constant is set symmetrically by the air-fuel ratio correction coefficient changing means when the exhaust gas recirculation means is off, the air-fuel ratio of the internal combustion engine that becomes rich when the exhaust gas recirculation means is on is corrected to the lean side and the responsiveness is improved. It is well controlled to achieve the stoichiometric air-fuel ratio.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is an overall configuration diagram of an embodiment of the present invention.
In the figure, reference numeral 1 is an engine body, 2 is an intake passage, 3 is an air flow meter, 4 is a distributor, 5 is a crank angle reference sensor, 6 is a crank angle sensor, 7 is a fuel injection valve, 8 is a water jacket, and 9 is Water temperature sensor, 10 control circuit, 11 exhaust manifold, 12 catalytic converter, 13 upstream O ₂ sensor, 14 EGR valve, 1
5 is a TVV (Thermal Vaccum Valve), 16 is a valve opening sensor, 17 is a fuel tank, and 23 is a downstream O ₂ sensor. As shown, an air flow meter 3 is provided in the intake passage 2 of the engine body 1 to directly measure the intake air amount. The air flow meter 3 has a built-in potentiometer, for example, and generates an electric signal of an analog voltage proportional to the intake air amount and outputs it to the A / D converter 101 with a built-in multiplexer of the control circuit 10. The distributor 4 includes, for example, a crank angle reference sensor 5 that generates a pulse signal serving as a crank angle reference for each 720 ° CA (crank angle) of the crank shaft converted to a crank angle, and 30 ° CA converted to the crank angle. A crank angle sensor 6 is provided for generating a pulse signal for detecting the engine rotation in detail. One rotation of the camshaft corresponds to two rotations of the crankshaft. The pulse signals of the crank angle reference sensor 5 and the crank angle sensor 6 are supplied to the input / output interface 102 of the control circuit 10, of which the output of the crank angle sensor 6 is the I / O interface 10.
2 to the interrupt terminal of the CPU 103.

Further, in the intake passage 2, a fuel injection valve 7 for supplying the pressurized fuel from the fuel tank 17 to the intake port is individually provided for each cylinder. In addition, the engine body 1
The water jacket 8 of the cylinder block is provided with a water temperature sensor 9 for detecting the temperature of the cooling water. The water temperature sensor 9 generates an analog voltage electric signal corresponding to the cooling water temperature THW. This output is also supplied to the A / D converter 101.

In the exhaust system downstream of the intake manifold 11.
Are the three toxic components HC, CO, NOx in the exhaust gas
Catalytic converter 12 containing a three-way catalyst that purifies at the same time
Is provided. The exhaust manifold 11 has
The upstream side of the catalytic converter 12 is close to the combustion chamber of the engine.
Upstream O₂A sensor 13 is provided. Upstream O ₂
The sensor 13 is an electric signal corresponding to the concentration of oxygen components in the exhaust gas.
Issue. That is, upstream O₂The sensor 13 has an air-fuel ratio
Is lean or rich with respect to the theoretical air-fuel ratio,
Different analog voltage V₁Control circuit 1 for generating the electric signal of
0 to the A / D converter 101. Catalyst converter
The exhaust pipe 21 on the downstream side of ₂The sensor 23
It is provided. Downstream O₂The sensor 23 also exhaust gas
Analog voltage V according to the oxygen concentration₂Electrical signal
Generated and output to the A / D converter 101 of the control circuit 10
I do. Downstream O₂The output characteristics of the sensor are:₂Sensor
The response is slower than the output characteristics, but the variation is small.

Further, the configuration shown in the figure is such that exhaust gas is converted into EGR.
A negative pressure control system EGR that lowers the maximum temperature at the time of combustion and reduces the production of NOx by recirculating to the intake side through the valve 14 and mixing with the intake air-fuel mixture is provided. As shown in the figure, the TVV 15 is provided in the negative pressure passage,
The TVV 15 acts to close the EGR valve 14 when the engine body 1 is cold or at low speed, and when the engine body 1 goes from medium speed to high speed, the negative pressure in the intake pipe increases and the EGR valve 14 opens properly. Act on. EGR
The valve 14 is provided with a valve opening sensor 16 that outputs an analog voltage corresponding to the opening amount of the EGR valve 14. The valve opening sensor 16 is also called a lift sensor, and the opening amount of the EGR valve 14 is also called a lift amount. The output of the valve opening sensor 16 is input to the A / D converter 101 and converted into digital by the A / D converter 101. Although this figure shows a closed-loop EGR device, the internal combustion engine air-fuel ratio control device according to the present invention is provided with a step-type solenoid EGR valve (not shown) instead of the closed-loop EGR device. A pulse signal is sent from a valve excitation circuit (not shown) to excite the EGR valve, and the opening of the EGR valve is changed according to the number of pulse signals sent (also called the number of steps) to adjust the exhaust gas recirculation flow rate. An open loop EGR device may be provided.

The control circuit 10 is configured as, for example, a microcomputer, and has a ROM in addition to the A / D converter 101, the input / output interface 102, and the CPU 103.
104, RAM 105, backup RAM 106, clock generator 107, etc. are provided. Further, in the control circuit 10, the down counter 108, the flip-flop 109, and the drive circuit 110 are for controlling the fuel injection valve 7. That is, when the fuel injection time TAU, which is the valve opening time of the fuel injection valve 7 that adjusts the fuel injection amount, is calculated in the routine described later, T
The value of AU is set in the down counter 108 and the flip-flop 109 is also set. As a result, the drive circuit 110 opens the fuel injection valve 7 and sends fuel from the fuel tank 17 to the combustion chamber of the engine body 1. On the other hand, when the down counter 108 down counts the clock signal (not shown) output from the clock generator 107 to the set number, the flip-flop 109 is reset, and the drive circuit 110 closes the fuel injection valve 7 and the fuel tank 17 The fuel supply from the engine to the combustion chamber of the engine body 1 is stopped.

The CPU 103 interrupts the A / D
The error occurs when the input / output interface 102 receives the pulse signal of the crank angle sensor 6 after the A / D conversion of the converter 101 is completed. The intake air amount data Q and the cooling water temperature data THW of the air flow meter 3 are fetched by an A / D conversion routine executed at predetermined times or at predetermined crank angles and stored in a predetermined area of the RAM 105. That is, the intake air amount data Q and the cooling water temperature data THW in the RAM 105 are updated every predetermined time. Further, the rotation speed data Ne is calculated by interruption of the crank angle sensor 6 every 30 ° CA, and R
It is stored in a predetermined area of the AM 105.

FIG. 3 is a diagram showing the λ characteristic of the O ₂ sensor which changes depending on the exhaust gas recirculation flow rate by EGR. In this figure, the horizontal axis is the air-fuel ratio A / F of the air-fuel mixture supplied to the engine.
And the vertical axis represents the output voltage of the O ₂ sensor. Usually, the relationship between the air-fuel ratio A / F of the engine and the output voltage of the O ₂ sensor is represented by a λ curve, and this characteristic is called the λ characteristic of the O ₂ sensor. Further, the λ characteristic of the O ₂ sensor shown in the drawing is shown by plotting O ₂ when the recirculation flow rate of exhaust gas by EGR is 15% and plotting when the recirculation flow rate is 0% by cross mark based on the experimental data. It is a characteristic view of a sensor. As shown
When part of the exhaust gas is recirculated to the engine by EGR, O
It can be seen that the λ characteristic of the _two sensors shifts to the rich side of the air-fuel ratio A / F. That is, the λ characteristic of the O ₂ sensor is from the first λ characteristic whose center value is the air-fuel ratio 14.6 when the exhaust gas recirculation flow rate is 0% to the air-fuel ratio 14. Shift to a second λ characteristic with 5 being the median.
As a result, the air-fuel ratio is not the stoichiometric air-fuel ratio 14.6 of the first λ characteristic, but the air-fuel ratio of the second λ characteristic 14.
It is controlled to become 5. That is, the control center shifts from the stoichiometric air-fuel ratio 14.6 to the air-fuel ratio 14.5. Therefore, the air-fuel ratio control device of the present invention controls the air-fuel ratio to the stoichiometric air-fuel ratio of 14.6 by the air-fuel ratio correction coefficient changing means even when a part of the exhaust gas is recirculated to the engine by the EGR. . Hereinafter, an example in which the air-fuel ratio correction coefficient changing means in the air-fuel ratio control device according to the present invention is applied to a single O ₂ sensor system in which the O ₂ sensor 13 is provided upstream of the catalytic converter 12 arranged in the exhaust system of the engine A detailed description will be given with reference to the accompanying drawings.

FIG. 4 is a flowchart of the air-fuel ratio correction coefficient calculation routine in the single O ₂ sensor system. The numbers shown in the flowcharts after this figure show the step numbers. The air-fuel ratio correction coefficient calculation routine shown in this figure uses, for example, 30 ° according to the output signal of the crank angle sensor 6.
It is executed every CA or every predetermined time, for example, every 4 msec.
First, the recirculation flow rate of the exhaust gas recirculated by EGR is
In the closed loop system, it is read from the lift amount of the EGR valve opening sensor 16, and in the open loop system, it is read from the step number of the step valve (step 401).

FIG. 5 is a map of the correction amount of the feedback control constant with respect to the exhaust gas recirculation flow rate by EGR,
(A) is a lean skip correction amount ΔRSL and lean integration correction amount ΔKIL, and (B) is a rich skip correction amount ΔRSR.
And a rich integration correction amount ΔKIR map. In this figure, the horizontal axis represents the lift amount or the number of steps corresponding to the exhaust gas recirculation flow rate, and the vertical axis represents the skip correction amount or the integral correction amount. These maps are created from experimental data. After step 401, the lean skip correction amount ΔRSL, the lean integration correction amount ΔKIL, and the rich skip correction amount ΔRS corresponding to the exhaust gas recirculation flow rate by EGR are performed.
R and the rich integral correction amount ΔKIR are calculated from the map stored in advance in the RAM 105 (step 402). Next, the corresponding ΔRSR and ΔKIR calculated in step 402 are added to the RSR and KIR written in the RAM 105 to obtain new RSR and KIR, and the RAM 10
Step 402 in RSL and KIL written in 5
The corresponding ΔRSL and ΔKIL calculated in step 3 are added to obtain new RSL and KIL, which are written and updated in the RAM 105 (step 403). The air-fuel ratio correction coefficient FAF is calculated based on the updated feedback control constants RSR, KIR and RSL, KIL (step 404), that is, the update routine of the air-fuel ratio correction coefficient FAF shown in FIG. 6 is executed and this routine is ended. . Next, the fuel injection amount calculation routine shown in FIG. 7 is executed to calculate the fuel injection amount.

FIG. 6 is a flowchart of a routine for updating the air-fuel ratio correction coefficient FAF. This routine is interrupted at every predetermined crank angle or every predetermined time. Step 6
In 01, it is determined whether or not the condition for feedback controlling the air-fuel ratio of the engine is satisfied. If the determination result is YES, the process proceeds to step 602, and if NO, step 6
Proceed to 26. The feedback control of the air-fuel ratio of the engine is carried out when the engine is in a starting state, during fuel increase control after starting, during warm-up increase control, during power increase control, during lean control, during fuel cut, and when the O ₂ sensor is not The condition is that it is in an active state and does not correspond to any of the above. In step 602, it is determined whether the air-fuel ratio of the engine is rich, that is, it is determined whether the output voltage V ₁ of the upstream O ₂ sensor 13 is higher than the voltage V _R1 corresponding to the theoretical air-fuel ratio, and V ₁ >.
When it is V _R1 , it is judged to be rich, and the routine proceeds to step 603,
When V ₁ ≧ V _R1 , it is judged to be lean and the routine proceeds to step 613. In step 603, it is determined whether or not the inversion flag CAF is 0. This reversal flag CAF is set to 0 in step 615 if the air-fuel ratio was lean (reversed from rich to lean) before that, and step 6 if the air-fuel ratio was rich (reversed from lean to rich) before that. It is set to 1 in 605. When step 603 is executed immediately after the air-fuel ratio is changed from lean to rich, the reversal flag CAF is still 0, so in step 604, the lean-skip amount RSL is subtracted from the previous air-fuel ratio correction coefficient FAF to obtain the air-fuel ratio correction coefficient. The FAF is corrected to the lean side, and the reversal flag CAF is set to 1 in step 605. Then, in step 606, the lean integration constant KI is further calculated from the air-fuel ratio correction coefficient FAF.
L is subtracted, and this routine ends. Thereafter, while the air-fuel ratio is rich, the reversal flag CAF is 0, so the routine proceeds from step 603 to 606, and the air-fuel ratio correction coefficient FAF
The integration constant KIL is subtracted from

On the other hand, when the air-fuel ratio is reversed from rich to lean, the routine proceeds from step 602 to step 613. Immediately after this reversal, the reversal flag CAF is 1 in step 613, so in step 614 the previous air-fuel ratio is lost. Rich skip amount RS in the fuel ratio correction factor FAF
R is added, and the reversal flag CAF is added in step 615.
Is cleared to 0. Then, in step 616, the rich integration constant KIR is further added to the air-fuel ratio correction coefficient FAF, and this routine is ended. Thereafter, while the air-fuel ratio is lean, the reversal flag CAF is 1, so step 6
From 13 to 616, the rich integration constant KIR is added to the air-fuel ratio correction coefficient FAF. In step 626, the air-fuel ratio correction coefficient FAF is set to 1 and this routine is ended.

FIG. 7 is a flowchart of the fuel injection amount calculation routine. This routine is executed every predetermined crank angle, for example, every 360 ° CA. In step 701, R
The intake air amount Q and the engine speed data Ne are read from the AM 105 to calculate the basic fuel injection amount TAUP. For example, TAUP = K · Q / Ne (K is a constant). In step 702, the final fuel injection amount TAU is calculated from the following equation. TAU = TAUP (FAF + α) + β where FAF is an air-fuel ratio correction coefficient, α is another correction coefficient, and β is another correction term. Then step 703
Then, the final fuel injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection, and this routine is finished. When the time corresponding to the final fuel injection amount TAU has elapsed, the flip-flop 1 is turned on by the carry-out signal from the down counter 108.
09 is reset and the fuel injection ends.

As described above, referring to FIGS. 4 to 7, single O ₂
The air-fuel ratio correction coefficient changing means applied to the sensor system has been described. For the sake of explanation, the skip amount and the integration constant are used as the feedback control constants for updating the air-fuel ratio correction coefficient FAF in proportion to the exhaust gas recirculation flow rate by the EGR, but the delay time is used instead of the skip amount and the integration constant. Good.
At least one of the delay time, the integration constant, and the skip amount may be selected as the feedback control constant for updating the air-fuel ratio correction coefficient FAF in proportion to the exhaust gas recirculation flow rate by EGR. Further, in any of the feedback control constants of the delay time, the integration constant, and the skip amount, one of the control constants on the rich side and the lean side is fixed and the other is adjusted to EG.
It may be set to a value proportional to the exhaust gas recirculation flow rate by R, which saves the storage capacity of the control device.

Further, the air-fuel ratio correction coefficient changing means in the air-fuel ratio control device according to the present invention described above updates the rich and lean feedback control constants to symmetrical equal values in proportion to the exhaust gas recirculation flow rate by EGR, The air-fuel ratio correction coefficient FAF is corrected by the updated feedback control constant.
The rich side feedback control constant may be set to be larger than the lean side feedback control constant in proportion to the exhaust gas recirculation flow rate by R. Here, the rich side feedback control constant is a control constant for correcting the air-fuel ratio correction coefficient FAF to the rich side, for example, RSR, KIR, and the lean side feedback constant is a control constant for correcting the air-fuel ratio correction coefficient FAF to the lean side. For example, RSL and KIL are called. E
By setting the rich side feedback control constant to a large value in the lean side feedback control constant in proportion to the exhaust gas recirculation flow rate by GR, the engine air-fuel ratio can be promptly changed according to the change in exhaust gas recirculation flow rate by EGR. Can be controlled to

Further, in an air-fuel ratio control system for an engine which does not have means for measuring the exhaust gas recirculation flow rate by EGR, feedback is provided according to the exhaust gas recirculation flow rate by EGR using a map as shown in FIG. Instead of calculating the control constant, it detects whether the EGR is on or off, and when the EGR is on, the rich side feedback control constant is set asymmetrically larger than the lean side feedback control constant,
When the EGR is off, the rich-side feedback control constant and the lean-side feedback control constant may be set to be equal to each other, and the air-fuel ratio correction coefficient may be corrected by the feedback control constant set according to on / off of the EGR. For example, steps 401 to 4 in the flowchart of FIG.
Instead of 03, the feedback control constant RSR is set to RSR1 when EGR is on and to RSR2 (RSR1> RSR2) when EGR is off. As a result, it is possible to realize feedback control of the air-fuel ratio according to the λ characteristic of the O ₂ sensor that differs depending on whether EGR is on or off.
The reason is that when the EGR is set asymmetrically with the rich side feedback control constant being larger than the lean side feedback control constant both on and off, the λ characteristic of the O ₂ sensor changes after the EGR is switched from on to off, and the air-fuel ratio feedback control is performed. Since the center of is deviated and the feedback control constant is set asymmetrical as described above, even if the air-fuel ratio becomes rich when EGR is on, it can be sufficiently corrected to the lean side even if EGR switches from on to off. Continue to keep the rich state. However, if EGR is set to the above-mentioned asymmetry when it is on, and if EGR is set to the above-mentioned symmetry when it is off, correction to the lean side will be sufficient after the EGR is switched from on to off, and the air-fuel ratio of the engine will be sufficient. Can be promptly set to the stoichiometric air-fuel ratio.

Next, a double O ₂ sensor system having an O ₂ sensor 13 upstream and a O ₂ sensor 23 downstream of the catalytic converter 12 arranged in the exhaust system of the engine is used in the air-fuel ratio control device according to the present invention. An example in which the fuel ratio correction coefficient changing means is applied will be described in detail with reference to the accompanying drawings.

FIG. 8 is a flowchart of a feedback control constant calculation routine in the double O ₂ sensor system. The feedback control constant calculation routine shown in this figure is executed every predetermined time, for example, every 1 second. First E
The recirculation flow rate of exhaust gas recirculated by the GR is read from the lift amount of the valve opening sensor 16 of the EGR in the closed loop system, and is read from the step number of the step valve in the open loop system (step 80).
1).

FIG. 9 is a map of the skip sub correction amount by the downstream O ₂ sensor with respect to the exhaust gas recirculation amount by EGR, and (A) is the lean skip sub correction amount ΔΔRS.
L and (B) are maps of the rich skip sub-correction amount ΔΔRSR. In this figure, the horizontal axis represents the lift amount or the number of steps corresponding to the exhaust gas recirculation flow rate, and the vertical axis represents the lean skip sub-correction amount Δ from rich to lean of the air-fuel ratio.
ΔRSL or the rich skip sub-correction amount ΔΔRSR from lean to rich is shown. These maps are created from experimental data. Next to step 801, the rich skip sub-correction amount ΔΔRSR and the lean skip sub-correction amount ΔΔRSL corresponding to the exhaust gas recirculation flow rate by EGR.
Is stored in the RAM 105 in advance, and ΔΔRSR and ΔΔRSL are calculated from the map (step 80).
2). The rich skip correction amount ΔRSR calculated in step 802 is added to the rich skip correction amount ΔRSR written in the RAM 105 to obtain a new ΔRSR, and the lean skip correction amount ΔRSL similarly written in the RAM 105 is calculated in step S6. The new lean skip sub correction amount ΔΔRSL is added to obtain a new ΔRSL.
Are updated and stored in the RAM 105 (step 803). Updated skip correction amount ΔRSR and Δ
Based on the RSL, the flowchart of the second air-fuel ratio feedback loop control routine shown in FIG. 10 by the downstream O ₂ sensor is executed to update the skip amounts RSR and RSL (step 804), and this routine is ended. Then, based on the updated RSR and RSL, the first shown in FIG.
The air-fuel ratio feedback control routine is executed to calculate the air-fuel ratio correction coefficient FAF. Next, the fuel injection amount calculation routine described with reference to FIG. 7 is executed to calculate the fuel injection amount. The second air-fuel ratio feedback loop control routine shown in FIG. 10 will be described below.

FIG. 10 is a flow chart of the second air-fuel ratio feedback control routine. The second air-fuel ratio feedback control routine shown in this figure is executed every predetermined time, for example, every 1 _second because the response of the downstream O ₂ sensor is poor, and the skip amount RSR, based on the output of the downstream O ₂ sensor 23,
Calculate RSL. First, in step 1001, the downstream O
_{2 It} is determined whether or not the air-fuel ratio feedback closed loop condition by the sensor 23 is satisfied. Since this condition is the same as the condition of the air-fuel ratio feedback control of step 601 of the flowchart shown in FIG. 6, its explanation is omitted. When the condition of the air-fuel ratio feedback closed loop is satisfied in step 1001, the process proceeds to step 1003, and when it is not satisfied, the process proceeds to step 1024 and the rich skip amount R
SR is set to a constant value RSR ₀ of 5%, for example, and the process proceeds to step 1025, and the lean skip amount RSL is set to a constant value RSL ₀ of 5%, for example. This step 102
Although 4 and 1025 may be deleted, the skip amount in this case uses a value immediately before the end of the air-fuel ratio feedback control.

After the condition of the air-fuel ratio feedback closed loop is satisfied in step 1001, the output voltage V ₂ of the downstream O ₂ sensor 23 is read in step 1003, and the read output voltage V ₂ is compared voltage V _{R2 in} step 1004.
Whether or not the air-fuel ratio is rich is determined by whether or not (for example, 0.55 V) or less. The comparison voltage V _R2 is the same as the output voltage V ₁ of the upstream O ₂ sensor 13 in consideration of the difference in the output characteristics of the O ₂ sensor due to the influence of raw gas and the deterioration speed between the upstream and the downstream of the catalytic converter 12. Is set higher than the comparison voltage V _R1 (for example, 0.45 V). On the other hand, if the air-fuel ratio is lean (V ₂ ≦ V _R2 ) in step 1004, the second delay counter CDLY2 is decremented by 1 in step 1005, and step 100
At 61007, the second delay counter CDLY2 is guarded with the minimum value TDR2. The minimum value TDR2 is a rich delay time for maintaining a lean state even when there is a change from lean to rich, and is defined as a negative value.
On the other hand, in step 1004, the air-fuel ratio is rich (V ₂
> V _R2 ), the second delay counter CDLY2 is incremented by 1 in step 1008, and steps 1009, 1
At 010, the second delay counter CDLY2 is guarded with the maximum value TDL2. The maximum value TDL2 is a lean delay time for maintaining the rich state even when there is a change from rich to lean, and is defined by a positive value. Here, with the reference of the second delay counter CDLY2 as 0, the air-fuel ratio after delay processing is considered rich when CDLY2> 0, and the air-fuel ratio after delay processing is considered lean when CDLY2 ≦ 0. To do.

In step 1011, it is judged whether or not the second delay counter CDLY2 is CDLY2≤0. If this judgment result CDLY2≤0, the air-fuel ratio is judged to be lean and the routine proceeds to steps 1012 to 1017, and the other CD
If LY2> 0, the air-fuel ratio is determined to be rich and the routine proceeds to steps 1018 to 1023. On the other hand, if the air-fuel ratio is lean, in step 1012 RSR ← RSR + ΔRSR
(ΔRSR is step 8 in the flowchart of FIG.
The rich skip correction amount) updated in 03 and stored in the RAM 105), that is, the rich skip amount RSR is increased to shift the air-fuel ratio to the rich side. Step 101
In 3, 1014, the maximum RSR value is MAX, for example, 6.2%.
To guard. Further, in step 1015, RSL ←
RSL-ΔRSL (ΔRSL is the lean skip correction amount updated in step 803 in the flowchart of FIG. 8 and stored in the RAM 105), that is, the lean skip amount RSL is decreased to shift the air-fuel ratio to the rich side. In steps 1016 and 1017, RSL is set to the minimum value M.
IN guards at 2.5%, for example. On the other hand, if the air-fuel ratio is rich, then in step 1018 RSR ← RSR-Δ
RSR, that is, the rich skip amount RSR is decreased to shift the air-fuel ratio to the lean side. Step 1019,
At 1020, RSR is guarded by the minimum value MIN. Further, in step 1021, RSL ← RSL + ΔRSL is set, that is, the lean skip amount RSL is increased to shift the air-fuel ratio to the lean side. Steps 1022, 1023
Then, RSL is guarded by the maximum value MAX.

The skip amount RSR calculated as described above,
The RSL ends this routine after it is stored in the RAM 105. Note that FAF, RAR, and RSL calculated during the air-fuel ratio feedback are once different values FAF ′ and RA.
It can be replaced with R'and RSL 'and stored in the backup RAM 106, which is also useful for improving drivability at the time of restart. The minimum value M in FIG.
IN is a value at which the transient followability is not impaired, and maximum value MAX is a value at which driveability does not deteriorate due to air-fuel ratio fluctuations. As described above, according to the routine of FIG. 10, if the output of the downstream O ₂ sensor 23 is lean, the rich skip amount RSR is gradually increased and the lean skip amount RSL is gradually decreased, whereby the air-fuel ratio is on the rich side. Will be moved to. Also downstream O ₂ sensor 2
If the output of 3 is rich, the rich skip amount RSR is gradually reduced and the lean skip amount RSL is gradually increased, whereby the air-fuel ratio is shifted to the lean side.

FIG. 11 is a flow chart of the first air-fuel ratio feedback control routine. The first air-fuel ratio feedback control routine shown in this figure is based on the upstream O ₂ sensor 1
Since the response of No. 3 is good, the air-fuel ratio correction coefficient FAF is calculated based on the output of the upstream O ₂ sensor 13 at a predetermined time, for example, every 4 msec. First, at step 1101, it is determined whether or not the air-fuel ratio feedback closed loop condition by the upstream O ₂ sensor 13 is satisfied. This condition is the same as the condition of the air-fuel ratio feedback control in step 601 of the flowchart shown in FIG. When the condition of the upstream O ₂ sensor feedback closed loop is satisfied, the routine proceeds to step 1102, and when it is not satisfied, the routine proceeds to step 1122, where the air-fuel ratio correction coefficient FAF is set to 1.0 and this routine is finished.

After the condition of the air-fuel ratio feedback closed loop is satisfied in step 1101, the output voltage V ₁ of the upstream O ₂ sensor 13 is A / D converted and read in step 1102, and the read output voltage V ₁ is compared in step 1103. Whether or not the air-fuel ratio is rich is determined by whether or not the voltage is V _R1 (for example, 0.45 V) or less. On the other hand, step 1
If the air-fuel ratio is lean (V ₁ ≦ V _R1 ) at 103, at step 1104 the first delay counter CDLY
1 is subtracted by 1, and in steps 1105 and 1106, the first delay counter CDLY1 is guarded by the minimum value TDR1. It should be noted that the minimum value TDR1 is a rich delay time for holding the determination that the output is the lean state even when the output of the upstream O ₂ sensor 13 changes from lean to rich, and is defined as a negative value. On the other hand, if the air-fuel ratio is rich (V ₁ > V _R1 ) in step 1103, step 1
At 107, the first delay counter CDLY1 is incremented by 1, and at steps 1108 and 1109, the first delay counter CDLY1 is guarded with the maximum value TDL1. The maximum value TDL1 is a lean delay time for holding the determination that the output of the upstream O ₂ sensor 13 is in the rich state even if there is a change from rich to lean, and is defined by a positive value. Again, the first delay counter C
When the reference of DLY1 is 0, the air-fuel ratio after delay processing is considered rich when CDLY1> 0, and the air-fuel ratio after delay processing is considered lean when CDLY1 ≦ 0.

In step 1110, it is determined whether the sign of the first delay counter CDLY1 has been inverted. That is, it is determined whether or not the air-fuel ratio after the delay process has been reversed. If the air-fuel ratio is reversed in step 1110, it is determined in step 1111 whether or not the execution condition of the feedback control of the air-fuel ratio by the downstream O ₂ sensor 23 is satisfied. The feedback control of the air-fuel ratio by the downstream O ₂ sensor 23 is executed under the condition that the cooling water temperature of the engine is equal to or lower than a predetermined temperature, the downstream O ₂ sensor is in the inactive state, the transient operation, or the like. . Next, at step 1112, it is determined whether the reversal from rich to lean or the reversal from lean to rich. If it is the reverse from rich to lean, in step 1113 FAF ← FAF + R
If it is reversed from lean to rich by increasing in a skip manner with SR, FAF ← FAF− in step 1114
A skip process is executed to reduce RSL and skip. RSR and step 11 of these steps 1113
The RSL 14 is data obtained by executing the flowchart of the feedback control constant calculation routine in the double O ₂ sensor system shown in FIGS. 8 to 10 every 1 second, and the flowchart of the air-fuel ratio correction coefficient calculation routine shown in FIG. 4. In step 1, data obtained from a feedback control constant calculation routine (not shown) that executes steps 401 to 404 every 4 msec is used. These data are stored in the RAM 1 every 1 sec on the one hand and every 4 msec on the other hand.
It is updated to 05 and stored.

If the air-fuel ratio is not reversed in step 1110, integration processing is executed in steps 1115 to 1117. That is, in step 1115, it is determined whether or not the first delay counter CDLY1 is CDLY1 ≦ 0. If one is CDLY1 ≦ 0 (lean), then in step 1116 FAF ← FAF + KI is set, and the other is CDLY1> 0 (rich. If so, step 1
At 117, FAF ← FAF-KI. Here, the integration constant KI is set to be sufficiently smaller than the skip amounts RSR and RSL, that is, KI << RSR (RSL). Therefore, step 1116 is in the lean state (CD
When LY1 ≦ 0), the fuel injection amount is gradually increased, and in step 1117, the fuel injection amount is gradually decreased in the rich state (CDLY1> 0). Note that steps 1116 and 111
As the integration constant KI of 7, the data obtained from the feedback control constant calculation routine that executes steps 401 to 404 every 4 msec in the flow chart of the air-fuel ratio correction coefficient calculation routine shown in FIG. 4 is used like the skip amounts RSR and RSL. May be. Integral constant KI in this case
(KIR, KIL) is updated every 4 msec, RAM10
5 is stored.

Steps 1113, 1114, 1116,
The air-fuel ratio correction coefficient FAF calculated at 1117 is guarded at steps 1118 and 1119 with a minimum value of 0.8, for example, and at steps 1120 and 1121 with a maximum value of 1.2, for example. As a result, when the air-fuel ratio correction coefficient FAF becomes too large or too small for some reason, the air-fuel ratio of the engine is controlled by that value to prevent overrich or over lean. The FAF calculated as described above is stored in the RAM 105, and this routine ends.

FIG. 12 is based on the flowchart shown in FIG.
Time chart explaining the operation of feedback control
Where (A) is the air-fuel ratio signal A / F1, and (B) is the delay
E Counter CDLY1 count value, (C) delay processing
The subsequent air-fuel ratio signal A / F1 ', (D) is the air-fuel ratio correction coefficient F.
It is a figure which shows AF. Upstream as shown in FIG.
O₂Output voltage V of sensor 13₁From the engine air-fuel ratio
The air-fuel ratio signal A / F1 that determines whether it is lean or lean is obtained.
It Then, the delay counter CD shown in FIG.
The count value of LY1 is counted up in the rich state.
And is counted down in the lean state. This result is shown in Figure 1.
Air-fuel ratio signal A / F delayed as shown in (C) of 2
1'is obtained. For example, time t₁At air-fuel ratio signal A /
Even if F1 changes from lean to rich, the sky is delayed
The fuel ratio signal A / F1 'is a rich delay time (-TDR1)
Time t after being held lean₂Changes to rich
It Time t₃The air-fuel ratio signal A / F1 changes from rich to
Even if the air-fuel ratio signal A / F1 'is delayed,
After being held rich for the lean delay time TDL1
At time t_FourChanges to lean. However, the air-fuel ratio signal
A / F1 is time t_Five, T ₆, T₇Rich delay time as
If it is inverted in a period shorter than (-TDR1), the first
It takes time for the ray counter CDLY1 to cross the reference value 0.
As a result, time t₈Air-fuel ratio signal after delay processing at
A / F1 'is inverted. That is, the air-fuel ratio signal after delay processing
No. A / F1 'is compared with the air-fuel ratio signal A / F1 before delay processing.
And stable. In this way, the stable air-fuel ratio after delay processing is
The air-fuel ratio shown in (D) of FIG. 12 based on the signal A / F1 '
The correction coefficient FAF1 is obtained. Also, the first delay
Time t when unta CDLY1 crosses reference value 0₂,
t_Four, T₈Each time the air-fuel ratio correction coefficient FAF exceeds the guard value
The fuel injection amount is compensated to absorb the fluctuation of the air-fuel ratio of the engine.
The corrective air-fuel ratio learning correction amount is updated.

Next, the second air-fuel ratio feedback control by the downstream O ₂ sensor 23 will be described. As the second air-fuel ratio feedback control, the skip amount RSR as a constant involved in the first air-fuel ratio feedback control,
RSL, delay time TDR1, TDL1, integration constant KI
(In this case, the rich integration constant KI1R and the lean integration constant KI1L are set separately). Alternatively, there are a system that makes the comparison voltage V _R1 of the output voltage V ₁ of the upstream O ₂ sensor 13 variable and a system that introduces the second air-fuel ratio correction coefficient FAF2. For example, if the rich skip amount RSR is increased, the control air-fuel ratio can be shifted to the rich side, and even if the lean skip amount RSL is decreased, the control air-fuel ratio can be shifted to the rich side. On the other hand, if the lean skip amount RSL is increased, the control air-fuel ratio can be shifted to the lean side, and even if the rich skip amount RSR is decreased, the control air-fuel ratio can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the rich skip amount RSR and the lean skip amount RSL according to the output of the downstream O ₂ sensor 23. Also, rich delay time (-TDR1)> lean delay time (T
If DL1) is set, the control air-fuel ratio can shift to the rich side, and conversely, if the delay delay time (TDL1)> rich delay time (-TDR1) is set, the control air-fuel ratio can shift to the lean side. That is, the air-fuel ratio can be controlled by correcting the delay times TDR1 and TDL1 according to the output of the downstream O ₂ sensor 23.

Furthermore, if the rich integration constant KI1R is increased, the control air-fuel ratio can be shifted to the rich side, and even if the lean integration constant KI1L is decreased, the control air-fuel ratio can be shifted to the rich side. On the other hand, if the lean integration constant KI1L is increased, the control air-fuel ratio can be shifted to the lean side, and even if the rich integration constant KI1R is decreased, the control air-fuel ratio can be shifted to the lean side. Therefore, depending on the output of the downstream O sensor 23, the rich integration constant KI1R and the lean integration constant KI1
The air-fuel ratio can be controlled by correcting L. Furthermore, if the comparison voltage V _R1 is increased, the control air-fuel ratio can be shifted to the rich side, and if the comparison voltage V _R1 is decreased, the control air-fuel ratio can be shifted to the lean side. Therefore, the air-fuel ratio can be controlled by correcting the comparison voltage V _R1 according to the output of the downstream O ₂ sensor 23.

[0042]

As described above, according to the air-fuel ratio control system for an internal combustion engine of the present invention, the feedback control constant is changed according to the exhaust gas recirculation flow rate by the EGR, thereby correcting the air-fuel ratio correction coefficient. And fuel injection, so EG
The air-fuel ratio of the internal combustion engine is controlled to be the stoichiometric air-fuel ratio with good responsiveness and accuracy without being affected by the exhaust gas recirculation flow rate by R, and the exhaust gas purifying performance of the engine is improved.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of the present invention.

FIG. 2 is an overall configuration diagram of an embodiment of the present invention.

FIG. 3 is a diagram showing a λ characteristic of an O ₂ sensor which changes depending on a ring flow rate of exhaust gas due to EGR.

FIG. 4 is a flowchart of an air-fuel ratio correction coefficient calculation routine in the single O ₂ sensor system.

FIG. 5 is a map of a correction amount of a feedback control constant with respect to an exhaust gas recirculation amount by EGR, (A) shows a lean skip correction amount and a lean integration correction amount, and (B) shows a rich skip correction amount and a rich integration correction amount. It is a map.

FIG. 6 is a flowchart of a routine for updating an air-fuel ratio correction coefficient FAF.

FIG. 7 is a flowchart of a fuel injection amount calculation routine.

FIG. 8 is a flowchart of a feedback control constant calculation routine in the double O ₂ sensor system.

FIG. 9: Downstream O for exhaust gas recirculation flow rate by EGR
It is a map of skip sub correction amount by ₂ sensors,
(A) is the lean skip sub correction amount ΔΔRSL, (B)
Is a map of the rich skip sub-correction amount ΔΔRSR.

FIG. 10 is a flowchart of a second air-fuel ratio feedback control routine.

FIG. 11 is a flowchart of a first air-fuel ratio feedback control routine.

12 is a time chart for explaining the operation of feedback control according to the flowchart shown in FIG.
(A) shows the air-fuel ratio signal A / F1, (B) shows the count value of the delay counter CDLY1, (C) shows the delayed air-fuel ratio signal A / F1 ', and (D) shows the air-fuel ratio correction coefficient FAF. Is.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine main body 3 ... Air flow meter 4 ... Distributor 5 ... Crank angle reference sensor 6 ... Crank angle sensor 7 ... Fuel injection valve 9 ... Water temperature sensor 10 ... Control circuit 11 ... Exhaust manifold 12 ... Catalytic converter 13 ... Upstream O ₂ sensor ( First air-fuel ratio sensor) 14 ... EGR valve 15 ... TVV 16 ... Valve opening sensor 17 ... Fuel tank 23 ... Downstream O ₂ sensor (second air-fuel ratio sensor)

Claims (2)

[Claims] 1. A fuel supply means for supplying fuel to an intake system of an internal combustion engine, and an exhaust gas recirculation means for recirculating a part of exhaust gas discharged from the internal combustion engine to the intake system according to an operating state of the internal combustion engine. And an air-fuel ratio sensor provided in the exhaust system,
An air-fuel ratio control device for an internal combustion engine, comprising: feedback control means for feedback-controlling a fuel supply amount such that the air-fuel ratio of the internal combustion engine reaches a target value in accordance with an output signal of the air-fuel ratio sensor. The rich side feedback control constant that corrects the air-fuel ratio correction coefficient that corrects the amount toward the rich side and the lean side feedback control constant that corrects toward the lean side have a large amount of recirculation flow recirculated by the exhaust gas recirculation means. An air-fuel ratio control apparatus for an internal combustion engine, comprising an air-fuel ratio correction coefficient changing means for changing the lean-side feedback control constant largely with respect to the rich-side feedback control constant. 2. A fuel supply means for supplying fuel to an intake system of an internal combustion engine, and an exhaust recirculation means for recirculating a part of exhaust gas discharged from the internal combustion engine to the intake system according to an operating state of the internal combustion engine. And an air-fuel ratio sensor provided in the exhaust system,
In an air-fuel ratio control apparatus for an internal combustion engine, comprising: a feedback control unit that feedback-controls a fuel supply amount so that the air-fuel ratio of the internal combustion engine becomes a target value according to an output signal of the air-fuel ratio sensor. When the means is on, the rich side feedback control constant for correcting the air-fuel ratio correction coefficient for correcting the fuel supply amount toward the rich side and the lean side feedback control constant for correcting for the lean side are set asymmetrically. An air-fuel ratio control device for an internal combustion engine, comprising air-fuel ratio correction coefficient changing means for symmetrically setting the rich side feedback control constant and the lean side feedback control constant when the exhaust gas recirculation means is off. .