JPS63205450A - Air-fuel ratio control method for internal combustion engine - Google Patents

Abstract

PURPOSE:To stabilize air-fuel ratio quickly by arranging a solenoid valve in a path communicating between a canister and an intake path in the downstream of a throttle valve, and setting correction variables to current values or ineffective values according to a plurality of specific conditions when the solenoid valve is switched. CONSTITUTION:A fuel tank 25 is communicated through a purge path 26 arranged with a canister 28 with an intake path 12 in the downstream of a throttle valve 18. Here, a purge control valve 30 is arranged in the purge path 26. An electronic control unit 16 controls a solenoid 30d in the valve 30 based on signals being fed from various sensors 14, 19-24 for detecting operational conditions of an engine 10 so as to open/close a valve body 30c. If open/close operation of the valve 30 is executed during feedback control operation of air-fuel ratio, specific correction variables are compared respectively with ineffective values. If none of a plurality of specific conditions is satisfied, the correction variables are reset to ineffective values.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an air-fuel ratio control method for an internal combustion engine, and in particular, the present invention relates to an air-fuel ratio control method for an internal combustion engine. The present invention relates to an air-fuel ratio control method during air-fuel ratio feedbank control operation of an internal combustion engine.

(Prior Art) The amount of fuel supplied to an internal combustion engine is determined by adding various correction coefficients and correction values (these are called correction variables) to a basic amount determined by parameters representing engine load, such as air flow rate and engine rotation speed. For example, a correction coefficient (hereinafter referred to as "
The corrected amount of fuel is injected and supplied to the engine, and the corrected amount of fuel is injected and supplied to the engine. Set the fuel ratio to the required value (e.g.
An air-fuel ratio control method that controls the air-fuel ratio to a value of 14.6) that gives the stoichiometric air-fuel ratio is widely used.

On the other hand, in order to prevent fuel that evaporates from the fuel tank, etc. from leaking into the atmosphere, the inside of the fuel tank is routed through a communication passage (hereinafter referred to as the "purge passage") into the intake passage downstream of the throttle valve. A canister is installed in the middle of this purge passage, and a solenoid valve (hereinafter referred to as a "purge control valve") is installed in the purge passage between the canister and the intake passage, and when the engine is stopped, the evaporated fuel is removed, for example. While the activated carbon is diffused toward the canister filled with activated carbon, the purge control valve is opened during specified operation to take in air from outside the canister, and the fuel that has separated from the activated carbon is discharged into the intake passage downstream of the throttle valve. .

Air containing evaporated fuel from the canister (hereinafter referred to as “
Purge air (referred to as "purge air") does not have a constant ratio of air to fuel, and if such purge air is discharged into the intake passage during specified low-load operation such as idling, engine operation will become unstable. During low-load operation, the purge control valve is closed to prevent purge air from being discharged into the intake passage.

When purge air is discharged into the intake passage during operation in which the air-fuel ratio is feedback-controlled to a desired value by the feedback correction coefficient, the feedback correction coefficient value is equal to the amount of purge air containing fuel vapor discharged into the intake passage. The air-fuel ratio is controlled to be maintained at the required value using the feedbank correction coefficient thus corrected. Figure 4 shows changes in the output value of the 02 sensor that detects the OTM degree in exhaust gas according to the on/off state of the purge control valve (PVC) during air-fuel ratio feedbank control operation, and the changes in the output value of the 02 sensor that detects the OTM degree in the exhaust gas. It shows the relationship between the time change of the feed bank correction coefficient value IFB set by As mentioned above, the feed bank correction coefficient value IFB before time t1 shown in FIG.
The value shown by the solid line before time t1 of C1. Typically this value is less than the value 1.0. ) is set.

(Problem to be Solved by the Invention) However, when the purge control valve is closed (off) during air-fuel ratio feedbank control operation (t in Fig. 4),
1), the fuel included in par and share is not suddenly supplied, and the air-fuel ratio suddenly changes to the fuel lean side (at 4th point).
Therefore, in order to maintain the air-fuel ratio at the required value, it is necessary to suddenly change the feedbank correction coefficient value IFB to a value that increases the amount of fuel to match the amount of fuel contained in the purge air. However, in order to ensure the stability of the feedbank control system, it is necessary to set the feedback correction coefficient value IFB according to the change in the output value of the 0° sensor.
Since the rate of change of cannot be set to a large value,
As shown by the solid line in FIG. 4 (C1), the feedbank correction coefficient value IFB is gradually set to a larger value,
It takes time to reach the value necessary to maintain the air-fuel ratio at the required value (for example, the feedbank correction coefficient value IFB
4 (as shown in C1, it takes time from time t1 to time t2) to increase the value to 1.0. During this period, the air-fuel ratio supplied to the engine was on the overly lean side, and in some cases the engine stalled. In such a case, the feedbank correction coefficient value I
It is desirable that FB be reset to the value 1.0 at the same time as the purge control valve is switched.

By the way, due to manufacturing errors, adjustment errors, performance deterioration due to use, etc. of the fuel supply system, when the purge control valve is open, the feed bank correction coefficient value may be 1 or 0 (fuel supply correction coefficient as a feedback correction coefficient). Even if the amount is corrected, the fuel supply amount may be substantially set to a value larger than the value that is not corrected (invalid value). Figure 5 is a graph for explaining such a phenomenon, and Figure 5 (
bl and (C1, the air-fuel ratio is set to the required value (14,6
), the feedback correction coefficient value IFB is
As mentioned above, when it is necessary to set the purge control valve to a value larger than 1.0, when the purge control valve is switched from on to off (at time tlo in Figure 5), the air-fuel ratio that changes to the lean side is corrected. In order to do this, it is necessary to set the feed bank correction coefficient value to a value even larger than the value before switching the purge control valve. In such a case, if the feedback correction coefficient value IFB is reset to the value 1.0 as in the case shown in FIG. 4, the feedback correction coefficient will be set to a value in the opposite direction from the appropriate value. Resetting the feedback correction coefficient value IFB will instead set the air-fuel ratio to a leaner side (see the change in the solid line in Figure 5 (see the change in the solid line immediately after the tlo time point in C1), which will prevent the air-fuel ratio from stabilizing to the required value. Delay (Figure 5 (
(See the change in the solid line after the tlo time of bl), problems such as engine malfunction may occur.

The present invention has been made to solve this problem, and when the purge control valve is opened or closed during air-fuel ratio feedback control operation, the air-fuel ratio is quickly stabilized to a desired value, thereby stabilizing engine operation. The present invention aims to provide an air-fuel ratio control method for an internal combustion engine that improves exhaust gas characteristics, improves fuel efficiency, etc.

(Means for Solving the Problems) In order to achieve the above-mentioned object, according to the present invention, the air-fuel ratio of an internal combustion engine in which the inside of the fuel tank is communicated with the intake passage on the downstream side of the throttle valve through the canister is adjusted. During feedbank control operation, a correction variable value is set according to the concentration of a specific component in the exhaust gas, and the amount of fuel supplied to the internal combustion engine is corrected according to the correction variable value, thereby adjusting the air-fuel ratio to the required value. In the air-fuel ratio control method, a solenoid valve for shutting off and opening the communication passage is disposed in the middle of a communication passage connecting the canister and the intake passage downstream of the throttle valve, and the electromagnetic valve is operated during a predetermined operation of the engine. When the solenoid valve is opened and closed during the feedback control operation, the correction variable value and the correction variable The correction variable value is compared with an invalid value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected, and the correction variable value is determined when the electromagnetic valve is switched from open to closed and the electromagnetic valve is switched from open to closed. A condition in which the correction variable value at the time of valve switching is a value that corrects the air-fuel ratio to the fuel lean side from the invalid value, and the solenoid valve is switched from closed to open, and the correction at the time of switching the solenoid valve. When either one of the conditions in which the variable value is a value that corrects the air-fuel ratio to the fuel lean side from the invalid value is satisfied, the current value is held, and when none of the conditions are satisfied, the current value is reset to the invalid value. A method for controlling an air-fuel ratio of an internal combustion engine is provided.

(Function) The correction variable value at the time of switching the solenoid valve is used as an invalid value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected, that is, the correction variable value is set as the correction variable value. By resetting the air-fuel ratio, which increases or decreases due to valve switching, to a value closer to the correcting value, the air-fuel ratio is quickly stabilized to the desired value. The correction variable value at the time of switching the solenoid valve is compared with the invalid value, and the solenoid valve is switched from open to closed, and the correction variable value at the time of switching the solenoid valve is lower than the invalid value. The condition is a value that corrects to the fuel lynch side,
and the solenoid valve is switched from closed to open, and the correction variable value at the time of switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel lean side from the invalid value. By resetting the correction variable value to the invalid value, it is possible to avoid an inconvenient situation in which the resetting actually delays the settling of the air-fuel ratio to the desired value.

(Embodiment) An embodiment of the present invention will be described below based on the drawings. First, referring to FIG. A multi-cylinder internal combustion engine, for example a four-cylinder gasoline engine, is shown, and reference numeral 12 indicates an intake pipe connected to an intake port of each cylinder. An air cleaner 13 is attached to the open end of the intake pipe 12 on the atmosphere side, and a Karman vortex type air flow sensor 14 is also attached. This air flow sensor 1
4C is electrically connected to the input side of the electronic control unit (ECU) 16 and supplies the Karman vortex generation periodic signal r to the electronic control unit 16.

A throttle valve 18 is disposed in the middle of the intake pipe 12, and an electromagnetic fuel injection valve 20 is disposed near the intake boat of each cylinder, and each fuel injection valve 20 is connected to the output side of the electronic control device 16. The valve is driven to open by a drive signal from the electronic control unit 16.

Reference numeral 15 indicates an exhaust pipe connected to the exhaust boat of each cylinder, and a three-way catalyst 17 is disposed in the middle of the exhaust pipe 15 to purify harmful gas components such as unburned hydrocarbons and nitrogen oxides in the exhaust gas. An 02 sensor 21 is attached to the exhaust pipe 15 between the engine 10 and the three-way catalyst 17 to detect the 0° concentration in the exhaust gas. The 02 sensor 21 is electrically connected to the electronic control unit 16 and receives the 0□ concentration detection signal V.
O2 is supplied to the electronic control unit 16.

On the input side of the electronic control unit 16, there is an idle switch 19 that generates an on signal when the throttle valve 18 is closed to a predetermined idle opening position, and an idle switch 19 that generates an on signal when the throttle valve 18 is closed to a predetermined idle opening position. position)
a crank angle position sensor (N) 22 that detects engine water temperature TW;
and sensors 24 for detecting other engine operating parameter values such as battery voltage and atmospheric pressure are electrically connected to each other.

Reference numeral 25 denotes a fuel tank, and the inside of the fuel tank 25 is communicated with the intake pipe 12 downstream of the throttle valve 18 via a purge passage (communication passage) 26.

A canister 28 and a purge control valve (electromagnetic valve) 3 are connected to the purge passage 26 from the fuel tank 25 side.
0 are arranged in this order. The canister 28 is filled with activated carbon 28a, and the inside of the canister 28 communicates with the atmosphere through an opening 28b opening at the bottom of the activated carbon 28a layer. The purge control valve 30 is an on-off valve, and the boat 3 connected to the purge passage 26
0a.

30b, and a valve body 30c that opens and closes the boat 30a;
A spring (not shown) that presses the valve body 30c toward the boat 30a so as to always close the port 30a and a biasing (on)
and a solenoid 30d that moves the valve body 30C in the valve opening direction against the spring force of the spring. It is energized by the energizing signal of.

Next, the operation of the fuel supply control device configured as described above will be explained.

First, the solenoid 30d of the purge control valve 30 is activated by the energizing signal from the electronic control unit 16.
When the boat 30a is opened by energizing, the canister 2
Atmospheric air (purge air) is sucked into the intake pipe 12 from the opening 28b of the activated carbon 28.
The fuel adsorbed on b is released and guided to the intake pipe 12.

Operation control of this purge control valve 30, that is,
Bar shear control is executed by the electronic control unit 16, which controls the engine 10 to cut off (purge cut) the bar shear from the canister 28 based on the detection signals from the various engine operating parameter sensors described above, and to control the intake of purge air. It is determined whether or not a predetermined operating state exists in which discharge into the pipe 12 should be prevented. The predetermined operating state in which the purge should be cut is, for example, an operating state in which the engine water temperature 7W value detected by the engine water temperature sensor 23 is a predetermined temperature (for example, 60° C.) or higher, and a Karman vortex generation period detected by the air flow sensor 14. f is less than a value representing a predetermined air flow rate at a medium load or less, the throttle valve 18 is closed and the idle switch 19 is in the on state, and the electronic control unit 16 performs such a purge cut. When one of the desired operating conditions is detected, the solenoid 30d of the purge control valve 30 is deenergized, the port 30a is closed, and the purge air from the canister 28 is cut off.

Next, the air-fuel ratio feed-hack control method by the electronic control unit 16 will be explained. First, the electronic control unit 16 executes feed-hack control of the air-fuel ratio of the engine 10 based on the detected values of the various engine operating parameters. When it is detected that the engine 1° is in an operating state that requires air-fuel ratio feed-hack control, the valve opening time T of the fuel injection valve 20 is calculated based on the following equation. .

T=KIX (A/N)XIFBXK2+TnHere,
(A/N) indicates the intake air amount taken in one intake stroke of the engine 1o, and is calculated from the air flow rate A detected by the air flow sensor 14 and the engine rotation speed N detected by the crank angle position sensor 22. . K1
is a constant for converting into a valve opening time corresponding to the (A/N) value, K2 is a correction coefficient set according to the engine water temperature Tw detected by the engine water temperature sensor 23, atmospheric pressure, etc., and T
D is a correction value determined according to battery voltage and the like.

Further, IFB is a feed bank correction coefficient, and this correction coefficient IFB value is set according to the voltage value Vo2 (value corresponding to the Oz concentration) detected by the O2 sensor 21. The method for setting the feedbank correction coefficient value IFB will be explained in more detail below with reference to the flowcharts shown in FIGS. 2 and 3.

The program flowchart shown in FIG. 2 is a timer routine program that is interrupted and executed by a timing pulse (for example, a pulse generated every 25 m5ec) generated by the electronic control unit 16 at a predetermined period. First, in step 4o, the voltage value detected by the 02° sensor 21 is read and stored as the VO2 value. Next, the VO2 value is compared with a predetermined determination value Vx, and it is determined whether the VO□ value is larger than the Vx value (step 41).

If this determination result is affirmative (Yes), that is, if it is determined that the vO□ value is larger than the Vx value and the air-fuel ratio supplied to the engine 10 is on the Lynch side than the required value, the process proceeds to step 42. , subtracts a predetermined minute value Δ■ from the stored feedback correction coefficient value IFB, stores this as a new feedbank correction coefficient value IFB, and ends the program.

If the determination result in step 41 is negative (No), that is,
If it is determined that the VO□ value is smaller than the Vx value and the air-fuel ratio supplied to the engine 10 is leaner than the required value, the process proceeds to step 43, where a predetermined small value ΔI is calculated from the feedback correction coefficient value IFB. This is stored as a new feedback correction coefficient value IFB, and the program ends.

FIG. 3 shows a main routine program that is executed by the electronic control unit 16 each time, for example, a predetermined crank angle position is detected by the crank angle position sensor 22.
In step 50, a purge control valve (P
Cv) 30 is opened (on). If the result of this determination is affirmative, it is determined whether the purge control valve 30 was open (turned on) during the previous execution of the program (step 51).

If the determination result in step 51 is negative, that is, it was off last time and it is on this time, it means that the purge control valve 30 was opened from off to on during the previous execution of the program and the current execution time. If so, step 52
Then, the feed parameter correction coefficient value IFB set and stored in the timer routine program shown in FIG. 2 is set to the value 1.
．． Determine whether it is smaller than 0. When the purge control valve 30 is opened from off to on, the engine 1
If it is predicted that the air-fuel ratio supplied to The bank correction coefficient value IFB is reset to the value 1.0 (step 55), and the program is ended. If it is predicted that the air-fuel ratio will suddenly change to the Lynch side, it is better to set the feedbank correction coefficient value IFB to a value of 1.0, which is smaller than the current value, to adjust the air-fuel ratio to the stoichiometric air-fuel ratio (required value) can be reached quickly.

On the other hand, the feedbank correction coefficient value IFB is already 1.0.
If the value is smaller, that is, if the determination result in step 52 is affirmative, the feedbank correction coefficient value I
The program is terminated by leaving the FB at its current value (the value set in the time routine shown in FIG. 2). When it is predicted that the air-fuel ratio will suddenly change to the Lynch side, it is preferable to set the feedback correction coefficient value IFB to a value of 1.0, which is a value larger than the current value, since the settling of the air-fuel ratio to the stoichiometric air-fuel ratio will be rather slow. do not have. Therefore, in such a case, the feedback correction coefficient value IFB is left at its current value.

If the determination result in step 51 is affirmative, that is, the purge control valve 30 is on both last time and this time, and there is no change in the operating state of the purge control valve 30, the program is ended without doing anything. .

On the other hand, if the determination result in step 50 is negative, that is, if the purge control pulp 30 is closed (off) this time, the purge control pulp 30 was also closed (off) when the program was executed last time. It is determined whether or not it has been completed (step 53). If the determination result in step 53 is negative, that is, it was on last time and off this time, it means that the purge control valve 30 was closed from on to off during the previous execution time and the current execution time of the program. In this case, the process proceeds to step 54, where it is determined whether the feed bank correction coefficient value IFB set by the timer routine program shown in FIG. 2 is greater than 1.0. When the purge control valve 30 is closed from on to off, it is predicted that the air-fuel ratio supplied to the engine 10 will suddenly change to the lean side, and if the feedback correction coefficient value TFB is smaller than the value 1.0. If yes, that is, if the determination result in step 54 is negative, the feedback correction coefficient value IFB is reset to the value 1.0 (step 54).
2 step 55), terminate the program. When it is predicted that the air-fuel ratio will suddenly change to the lean side, the feed bank correction coefficient value IFB is set to a value of 1.0, which is larger than the current value.
Setting it to 0 allows the air-fuel ratio to reach the stoichiometric air-fuel ratio more quickly (see Figure 4 [t1 of C1 and +dl]).
Feedbank correction coefficient value IF indicated by the broken line at the time point
(See changes in B and changes in air-fuel ratio).

On the other hand, the feedback correction coefficient value IFB is already 1.0.
If the value is larger, that is, if the determination result in step 54 is affirmative, the feedback correction coefficient value I
Leave FB at its current value ((i) set in the time routine shown in Figure 2) and end the program. If it is predicted that the air-fuel ratio will suddenly change to the lean side, the feedback correction coefficient value IFB Setting IFB to a value of 1.0, which is smaller than the current value, will actually slow down the air-fuel ratio to the stoichiometric air-fuel ratio, which is undesirable.Therefore, in such a case, leave the feed bank correction coefficient value IFB at the current value. (See FIG. 5 Fb and the change in the feed bank correction coefficient value IFB and the change in the air-fuel ratio shown by the broken line at the time of tlo of tel).

If the determination result in step 53 is affirmative, that is, the purge control valve 30 is off both last time and this time, and there is no change in the operating state of the purge control valve 30, the program is ended without doing anything. .

The feed bank correction coefficient value IFB set as described above is applied to the above-mentioned calculation formula to calculate the valve opening time T of the fuel injection valve 20. Electronic control unit 1
6 supplies a drive signal corresponding to the valve opening time T thus calculated to the fuel injection valve 20 to open the fuel injection valve 20 and inject and supply the required amount of fuel to each cylinder of the engine 10.

In addition, in the above-mentioned embodiment, the feedback correction coefficient value IFB
is the VO2 value detected by the 0□ sensor 21 as the predetermined judgment value V
The explanation has been given using an example of setting by so-called integral term control, which adds or subtracts a small value ΔI to the IFB value depending on whether or not the ■0□ value is larger than the Vx value. The method for setting the numerical value IFB is not limited to this, and may be set using, for example, a combination of the above-mentioned integral term control and known proportional term control.

Furthermore, when the purge control valve 30 is opened from OFF to ON, the feed bank correction coefficient value IFB immediately before opening is the normal value 1.0 if the fuel supply system is well adjusted. Since it is often set to a value close to 0, when the ON state of the purge control valve 30 is detected in step 50 of FIG. 3, steps 51 and 52 are omitted and nothing is done, that is, The program may be terminated while the feedback correction coefficient value IFB is held at the current value (the value set in the time routine program of FIG. 2).

Furthermore, in the above embodiment, the valve opening time T of the fuel injection valve 20
The calculation calculates the feedback correction coefficient IFB as K 1 x
(A/N) value to correct this, and in the case of such a multiplication type correction coefficient, the K I X (A/N) value is multiplied as an invalid value, that is, a correction coefficient value. Also, the value that does not substantially change K I Zu, K
It may also be a type of correction variable that is added to the I x (A/N) value. The invalid value in this case is the value 0.

(Effects of the Invention) As detailed above, according to the air-fuel ratio control method for an internal combustion engine of the present invention, the communication path can be shut off or opened in the middle of the communication path connecting the canister and the intake path downstream of the throttle valve. The solenoid valve is opened during a specific operating state of the engine to discharge the evaporated fuel into the intake passage, while the internal combustion engine 7 When the opening/closing operation of the solenoid valve is performed during air-fuel ratio feedbank control operation in which the air-fuel ratio is controlled to a required value by correcting the amount of fuel supplied to the solenoid valve, the correction variable value at the time of switching of the solenoid valve is and an invalid value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected as a correction variable value, and the correction variable value is set when the solenoid valve is switched from open to closed; and the condition that the correction variable value at the time of switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel rich side from the invalid value, and the condition that the solenoid valve is switched from closed to open, and when the solenoid valve is switched When either one of the conditions in which the correction variable value is a value that corrects the air-fuel ratio to the fuel lean side than the invalid value is satisfied, the current value is held, and when neither condition is satisfied, the current value is reset to the invalid value. As a result, even if the purge control valve is opened or closed during operation and the air-fuel ratio supplied to the engine suddenly changes, this can be quickly stabilized to the desired value, allowing the engine to operate smoothly. It has the excellent effect of being able to stabilize and improve exhaust gas characteristics and fuel efficiency.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention; FIG. 1 is a schematic configuration diagram of a fuel supply control device that executes the method of the present invention, and FIG. 2 shows a feedback correction system executed by the electronic control unit shown in FIG. 1. FIG. 3 is a flowchart showing the procedure for setting the numerical value IFB, and FIG. Figure 4 is
This is an explanation of the inconvenience that occurs when the purge control valve is turned from on to off during air-fuel ratio feedbank control in the conventional air-fuel ratio control method. FIG. 5 is a timing chart showing the relationship between the bank correction coefficient value IFB and each change in the air-fuel ratio over time. It explains another aspect of the inconvenience that occurs,
Purge control valve (PCV) on/off status,
5 is a timing chart showing the relationship between the feed hack correction coefficient value IFB and each temporal change in the air-fuel ratio. DESCRIPTION OF SYMBOLS 10... Internal combustion engine, 12... Intake pipe (intake passage), 16... Electronic control unit, 18...
Throttle valve, 20...Fuel injection valve, 21...0□
Sensor, 25... Fuel tank, 26... Purge passage (communication passage), 28... Canister, 30... Purge control valve (electromagnetic valve). Applicant Mitsubishi Motors Corporation Agent Patent Attorney Kan Nagato Figure 3

Claims (1)

[Claims] During air-fuel ratio feedback control operation of an internal combustion engine in which the inside of the fuel tank is communicated with the intake passage on the downstream side of the throttle valve via the canister, a correction variable value is set according to the concentration of a specific component in the exhaust gas, and the correction variable is In the air-fuel ratio control method, the air-fuel ratio is controlled to a required value by correcting the amount of fuel supplied to the internal combustion engine according to the amount of fuel supplied to the internal combustion engine. A solenoid valve for blocking and opening a communication passage is provided, and the solenoid valve is opened during a predetermined operating state of the engine to discharge evaporated fuel into the intake passage, and the solenoid valve is switched to open and close during the feedback control operation. When the operation is executed, the correction variable value at the time of switching the solenoid valve is compared with an invalid value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected as the correction variable value. Then, the correction variable value is
The solenoid valve is switched from open to closed, and the correction variable value at the time of switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel rich side from the invalid value, and the solenoid valve is switched from closed to open. and the correction variable value at the time of switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel lean side from the invalid value, the current value is held, and the current value is held; An air-fuel ratio control method for an internal combustion engine, characterized in that the air-fuel ratio is reset to the invalid value when the above condition is not satisfied.