JPH0826823B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control method for an internal combustion engine, and more particularly to a device for purging fuel vapor generated in a fuel tank into an intake passage downstream of a throttle valve. The present invention relates to an air-fuel ratio control method during air-fuel ratio feedback control operation of an internal combustion engine.

(Prior Art) Various correction factors and correction values (these are referred to as correction variables) are used to adjust the amount of fuel supplied to an internal combustion engine to a parameter representing the engine load, for example, a basic amount determined by the air flow rate and the engine speed. For example, a correction coefficient according to the concentration of a specific component in the exhaust gas (for example, O ₂ concentration) (hereinafter referred to as “feedback correction coefficient”), a correction coefficient according to the engine cooling water temperature, a correction value according to the battery voltage. Is corrected by multiplying and / or adding, etc., and the fuel amount thus corrected is injected / supplied to the engine to control the air-fuel ratio to a required value (for example, a value 14.6 giving the theoretical air-fuel ratio). Widely adopted.

On the other hand, in order to prevent the fuel evaporated from the fuel tank etc. from leaking to the atmosphere, the fuel tank is connected to the intake passage on the downstream side of the throttle valve via a communication passage (hereinafter referred to as “purge passage”). A canister is provided in the middle of this purge passage, and a solenoid valve (hereinafter referred to as "purge control valve") is provided in the purge passage between the canister and the intake passage.
Is installed to diffuse the evaporated fuel toward the canister filled with activated carbon when the engine is stopped, while the purge control valve is opened to take in the atmosphere from the outside of the canister during a predetermined operation, and the fuel separated from the activated carbon is discharged. Is discharged into the intake passage downstream of the throttle valve.

The air that contains evaporated fuel from the canister (hereinafter referred to as "purge air") does not have a constant ratio of air and fuel, and such purge air is discharged to the intake passage during a predetermined low load operation such as idling. Then, the operation of the engine becomes unstable. Therefore, during such a predetermined low load operation, the purge control valve is closed so that the purge air is not discharged to the intake passage.

When the purge air is discharged to the intake passage during the operation in which the air-fuel ratio is feedback-controlled to the required value by the feedback correction coefficient, the feedback correction coefficient value is lean as much as the purge air containing the fuel vapor is discharged to the intake passage. The value is corrected to the above value, and the feedback correction coefficient thus corrected is controlled to hold the air-fuel ratio at the required value. Fig. 4 shows the change in the output value of the O ₂ sensor that detects the O ₂ concentration in the exhaust gas, and the change in the output value of this O ₂ sensor, depending on the on / off state of the purge control valve (PVC) during air-fuel ratio feedback control operation. Shows the relationship between the time change of the feedback correction coefficient value IFB set according to and the time change of the air-fuel ratio of the air-fuel mixture supplied to the engine, at the time when the purge control valve is switched from on to off (open to closed) (At time t1 shown in FIG. 4)
The previous feedback correction coefficient value IFB is as described above.
It is set to a value for correcting the air-fuel ratio to the lean side (value shown by the solid line before time t1 in FIG. 4 (c). Usually, this value is smaller than 1.0).

(Problems to be Solved by the Invention) However, when the purge control valve is closed (OFF) during the air-fuel ratio feedback control operation (see FIG. 4).
At time t1, the fuel contained in the purge air is not suddenly supplied, and the air-fuel ratio suddenly changes to the fuel lean side (fourth point).
Therefore, in order to maintain the air-fuel ratio at the required value, it is necessary to suddenly change the feedback correction coefficient value IFB to a value for increasing the fuel amount commensurate with the fuel amount contained in the purge air. However, in order to ensure the stability of the feedback control system, it is not possible to set the rate of change of the feedback correction coefficient value IFB set according to the change in the output value of the O ₂ sensor to a large value. As shown by the solid line in (c), the feedback correction coefficient value IF
B will be gradually set to a large value, and it will take time to reach the value necessary to maintain the air-fuel ratio at the required value (for example, to increase the feedback correction coefficient value IFB to 1.0). , T2 from t1 as shown in FIG.
It will take some time). During this time, the air-fuel ratio supplied to the engine becomes a value on the over lean side, and there is a possibility that the engine may malfunction (stalling) in some cases. In such a case, it is desirable that the feedback correction coefficient value IFB be reset to the value 1.0 at the same time when the purge control valve is switched.

By the way, due to manufacturing error, adjustment error of fuel supply system, performance deterioration due to use, etc., when the purge control valve is open, the feedback correction coefficient value is 1.0 (corrects the fuel supply amount as the feedback correction coefficient). The fuel supply amount may be set to a value larger than the uncorrected value (invalid value) even if the above is performed. FIG. 5 is a graph for explaining such a phenomenon. As shown in FIGS. 5 (b) and 5 (c), the air-fuel ratio is a required value (14.6).
If it is necessary to set the feedback correction coefficient value IFB to a value larger than 1.0 as described above in order to keep the value at 1.0, the purge control valve is switched from on to off (at time t10 in FIG. 5), and the lean In order to correct the air-fuel ratio which changes to the side, it is necessary to set the feedback correction coefficient value to a value larger than the value before the switching of the purge control valve. In such a case, as in the case shown in FIG. 4, the feedback correction coefficient value IFB
If is reset to the value 1.0, the feedback correction coefficient is set to a value in the opposite direction rather than the proper value.
Resetting the feedback correction coefficient value IFB would rather set the air-fuel ratio to a leaner side (see the change in the solid line immediately after time t10 in Fig. 5 (c)), and settle the air-fuel ratio to the required value. (Refer to the change in the solid line after the time point t10 in FIG. 5 (b)), which rather causes an engine malfunction.

The present invention has been made to solve such a problem, and when the purge control valve is opened / closed during the air-fuel ratio feedback control operation, the air-fuel ratio is settled to a required value as quickly as possible to stabilize the engine operation. It is an object of the present invention to provide an air-fuel ratio control method for an internal combustion engine, which improves exhaust gas characteristics, improves fuel economy, and the like.

(Means for Solving the Problems) According to the present invention in order to achieve the above-mentioned object, according to the present invention, the air-fuel ratio of the internal combustion engine in which the inside of the fuel tank is connected to the intake passage on the downstream side of the throttle valve via the canister. During feedback 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 to control the air-fuel ratio to a required value. In the fuel ratio control method, a solenoid valve for shutting off / opening the communication passage is provided in the middle of the communication passage connecting the canister and the intake passage downstream of the throttle valve, and the solenoid valve is opened when the engine is in a predetermined operating state. Then, when the opening / closing switching operation of the solenoid valve is executed during the feedback control operation while the evaporated fuel is discharged to the intake passage, the correction variable value at the time of switching the solenoid valve and 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 changed from the open state to the closed state to the solenoid valve. The condition that the correction variable value at the time of switching is a value for correcting the air-fuel ratio from the invalid value to the fuel rich side, and the solenoid valve is switched from closed to open, and the correction variable at the time of switching the solenoid valve. The present value is held when any one of the conditions that the value is a value for correcting the air-fuel ratio to the fuel lean side from the invalid value is satisfied, and is reset to the invalid value when neither of the conditions is satisfied. An air-fuel ratio control method for an internal combustion engine is provided.

(Operation) An ineffective value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected by using the correction variable value when switching the solenoid valve as the correction variable value, that is, the correction variable value By resetting the air-fuel ratio, which increases or decreases by switching the valve, to a value closer to the correction value, the settling of the air-fuel ratio to the required value is accelerated. Further, the correction variable value when switching the solenoid valve and the invalid value are compared, the solenoid valve is switched from open to closed, and the correction variable value when switching the solenoid valve is lower than the invalid value by the air-fuel ratio. Is the value to correct the fuel rich side,
And that the solenoid valve is switched from closed to open and that the correction variable value when switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel lean side from the above invalid value is not satisfied. By resetting the correction variable value to the invalid value by the reset, it is possible to avoid an inconvenient situation in which the resetting rather delays the settling of the air-fuel ratio to the required value.

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

First, referring to FIG. 1, a schematic structure of a fuel supply control apparatus for carrying out the method of the present invention will be described. Reference numeral 10 indicates a multi-cylinder internal combustion engine, for example, a 4-cylinder gasoline engine, and reference numeral 12 indicates intake air of each cylinder. Shows the intake pipe connected to the port.
An air cleaner 13 and a Karman vortex type air flow sensor 14 are attached to the open end of the intake pipe 12 on the atmosphere side. The air flow sensor 14 is electrically connected to the input side of an electronic control unit (ECU) 16 and supplies the Karman vortex generation period signal f to the electronic control unit 16. Throttle valve 18 in the middle of the intake pipe 12
Electromagnetic fuel injection valves 20 are provided in the vicinity of the intake ports of the respective cylinders.
Is connected to the output side of and the valve is opened by a drive signal from the electronic control unit 16.

Reference numeral 15 denotes an exhaust pipe connected to the exhaust port of each cylinder, and a three-way catalyst 17 for purifying harmful gas components such as unburned hydrocarbons and nitrogen oxides in the exhaust gas is provided in the exhaust pipe 15 midway. An O ₂ sensor 21 for detecting the O ₂ concentration in the exhaust gas is attached to the exhaust pipe 15 between the engine 10 and the three-way catalyst 17. The O ₂ sensor 21 is electrically connected to the electronic control unit 16 and supplies the O ₂ concentration detection signal VO ₂ to the electronic control unit 16.

On the input side of the electronic control unit 16, an idle switch 19 for generating an ON signal when the throttle valve 18 is closed at a predetermined idle opening position, a predetermined crank angle position of each cylinder (for example, the top dead center of the intake stroke). Position) (crank angle position sensor (N) 22), an engine water temperature (Tw) sensor 23 attached to the cylinder block of the engine 10 to detect the engine cooling water temperature Tw, and other engine operations such as battery voltage and atmospheric pressure. Sensors 24 for detecting parameter values are electrically connected to each other.

Reference numeral 25 is 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. In the purge passage 26, a canister 28 and a purge control valve (electromagnetic valve) 30 are arranged in this order from the fuel tank 25 side. The canister 28 is filled with activated carbon 28a.
The inside of the canister 28 communicates with the atmosphere through an opening 28b that opens at the bottom of the 8a layer. Purge control valve 30
Is an on / off valve and is connected to the purge passage 26
A valve body 30c having 30a and 30b for opening and closing the port 30a, and a valve body 3
A spring (not shown) that constantly presses 0c toward the port 30a so as to close the port 30a, and a solenoid 30d that moves the valve element 30c in the valve opening direction against the spring force of the spring when energized (on). The solenoid 30d is electrically connected to the electronic control unit 16 and is energized by an energizing signal from the electronic control unit 16.

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

First, when the solenoid 30d of the purge control valve 30 is energized by the energizing signal from the electronic control unit 16 to open the port 30a, the opening 2 of the canister 28 is opened.
Atmosphere (purge air) is sucked into the intake pipe 12 from 8b, and the sucked air removes the fuel adsorbed on the activated carbon 28b and guides it to the intake pipe 12.

Operation control of this purge control valve 30, that is,
The purge air control is executed by the electronic control unit 16, and the electronic control unit 16 cuts off the purge air from the canister 28 (purge cut) and inhales the purge air based on the detection signals from the various engine motion parameter sensors described above. It is determined whether or not it is in a predetermined operating state in which discharge to the pipe 12 should be prevented. The predetermined operating state to be purge cut is, for example, an operating state in which the engine water temperature Tw value detected by the engine water temperature sensor 23 is a predetermined temperature (for example, 60 ° C.) or higher, and the Karman vortex generation cycle detected by the air flow sensor 14. f is
It is less than or equal to a value representing a predetermined air flow rate of medium load or less, and
When the electronic control unit 16 detects one of the operating states in which the purge cut should be performed, such as the operating state in which the throttle valve 18 is closed and the idle switch 19 is in the ON state, the solenoid 30d of the purge control valve 30 is deactivated. Then, the port 30a is closed and the purge air from the canister 28 is shut off.

Next, the air-fuel ratio feedback control method by the electronic control unit 16 will be described. First, the electronic control unit 16 operates based on the various engine operation parameter detection values, and the operation in which the engine 10 should perform the air-fuel ratio feedback control. If it is detected that the engine 10 is in the operating state in which the air-fuel ratio feedback control should be performed, the valve opening time T of the fuel injection valve 20 is calculated based on the following equation.

T = K1 × (A / N) × IFB × K2 + T _D Here, (A / N) represents the amount of intake air taken in one intake stroke of the engine 10, and the air flow rate A detected by the air flow sensor 14 and Crank angle position sensor 22
Is calculated from the engine speed N detected by. K1
Is a constant for converting to the 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 the battery It is a correction value determined according to the voltage and the like. And
IFB is a feedback correction coefficient, and this correction coefficient IFB
The value is set according to the voltage value VO ₂ (value corresponding to the O ₂ concentration) detected by the O ₂ sensor 21. The method of setting the feedback correction coefficient value IFB will be described below with reference to FIGS.
A more specific description will be given with reference to the flowchart shown in the figure.

The program flow chart shown in FIG. 2 is a timer routine program executed by interruption by a timing pulse (for example, a pulse generated every 25 msec) generated in a predetermined cycle by the electronic control unit 16. , Step 40
At, the voltage value detected by the O ₂ sensor 21 is read and stored as the VO ₂ value. Next, the VO ₂ value is compared with the predetermined determination value Vx to determine whether the VO ₂ value is larger than the Vx value (step 41).

If this determination result is affirmative (Yes), that is, if the VO ₂ value is Vx
If it is determined that the air-fuel ratio supplied to the engine 10 is greater than the required value and is richer than the required value, step 42
And the stored feedback correction coefficient value IFB
Then, a predetermined small value ΔI is subtracted from this, this is stored as a new feedback correction coefficient value IFB, and the program ends.

If the determination result of step 41 is negative (NO), that is, VO
_{When the binary} value is smaller than the Vx value and it is determined that the air-fuel ratio supplied to the engine 10 is a leaner value than the required value, the process proceeds to step 43, and a predetermined minute value ΔI is added from the feedback correction coefficient value IFB. Then, this is stored as a new feedback correction coefficient value IFB, and the program ends.

FIG. 3 is a main routine program executed by the electronic control unit 16 each time a predetermined crank angle position is detected by the crank angle position sensor 22, for example. Control valve (PCV) 30
It is determined whether or not is opened (ON). If the determination result is affirmative, it is determined whether or not the purge control valve 30 was opened (ON) even when the program was executed last time (step 51). When the determination result of step 51 is negative, that is, when the previous time is off and the current time is on, it means that the purge control valve 30 was opened from off to on at the time of the previous execution and the current execution time of the program, If so, the routine proceeds to step 52, where it is judged if the feedback correction coefficient value IFB set and stored by the timer routine program shown in FIG. 2 is smaller than 1.0.
When the purge control valve 30 is opened from off to on, it is predicted that the air-fuel ratio supplied to the engine 10 will suddenly change to the rich side, and if the feedback correction coefficient value IFB is a value greater than 1.0, that is, , If the determination result of step 52 is negative, the feedback correction coefficient value IF
B is reset to the value 1.0 (step 55), and the program ends. If it is predicted that the air-fuel ratio will suddenly change to the rich side, it is better to set the feedback correction coefficient value IFB to 1.0, which is a value smaller than the current value, so that the air-fuel ratio becomes the theoretical air-fuel ratio (required value). It can be reached quickly.

On the other hand, if the feedback correction coefficient value IFB is already smaller than 1.0, that is, if the determination result of step 52 is affirmative, the feedback correction coefficient value IFB is set to the current value (the time routine shown in FIG. 2). (The value set in step 4) is left and the program is terminated. When it is predicted that the air-fuel ratio will suddenly change to the rich side, setting the feedback correction coefficient value IFB to a value 1.0, which is a value larger than the current value, is not preferable because the stabilization of the air-fuel ratio to the stoichiometric air-fuel ratio is rather delayed. Therefore, in such a case, the feedback correction coefficient value IFB is left as it is.

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

On the other hand, if the determination result of step 50 is negative, that is, if the purge control valve 30 is closed (OFF) this time, the purge control valve 30 was closed (OFF) even when the program was executed last time. It is determined whether or not (step 53). The determination result of step 53 is negative,
That is, when the previous time is on and this time is off, it means that the purge control valve 30 was closed from on to off at the previous execution time and the current execution time of the program, in which case the process proceeds to step 54, Feedback correction coefficient value set by the timer routine program shown in FIG.
Determine if IFB is greater than the value 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 when the feedback correction coefficient value IFB is a value smaller than 1.0, 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 55), and the program ends.
If it is predicted that the air-fuel ratio will suddenly change to the lean side, setting the feedback correction coefficient value IFB to a value of 1.0, which is a value larger than the current value, will cause the air-fuel ratio to reach the stoichiometric air-fuel ratio quickly. (See the change in the feedback correction coefficient value IFB and the change in the air-fuel ratio indicated by the broken line at time t1 in FIGS. 4C and 4D).

On the other hand, if the feedback correction coefficient value IFB is already larger than 1.0, that is, if the determination result of step 54 is affirmative, the feedback correction coefficient value IFB is set to the current value (the time routine shown in FIG. 2). (The value set in step 4) is left and the program is terminated. When it is predicted that the air-fuel ratio will suddenly change to the lean side, setting the feedback correction coefficient value IFB to a value of 1.0, which is a value smaller than the current value, is not preferable because the stabilization of the air-fuel ratio to the stoichiometric air-fuel ratio is rather delayed. Therefore, in such a case, the feedback correction coefficient value IFB is left at the current value (the change in the feedback correction coefficient value IFB and the change in the air-fuel ratio indicated by the broken line at time t10 in FIGS. 5B and 5C). reference).

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

The feedback correction coefficient value IFB set as described above is applied to the above-described calculation formula to calculate the valve opening time T of the fuel injection valve 20. The electronic control unit 16 sends a drive signal corresponding to the calculated valve opening time T to the fuel injection valve.
20 to supply the required amount of fuel to the engine
Injection is supplied to each of the 10 cylinders.

In the above-mentioned embodiment, the feedback correction coefficient value IFB
Is a so-called integral term control that compares the VO ₂ value detected by the O ₂ sensor 21 with a predetermined determination value Vx and adds or subtracts a small value ΔI to the IFB value depending on whether the VO ₂ value is greater than the Vx value. Although the example of setting is explained, the feedback correction coefficient value IFB
The setting method is not limited to this, and may be set, for example, by adding the well-known proportional term control to the above integral term control.

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

Further, in the above-described embodiment, the valve opening time T of the fuel injection valve 20 is
Is calculated by multiplying the feedback correction coefficient IFB by the K1 × (A / N) value to correct it. In the case of such a multiplication type correction coefficient, an invalid value, that is, K1 × as the correction coefficient value is calculated.
The value that does not substantially change the K1 × (A / N) value when multiplied by the (A / N) value was 1.0, but as the feedback correction variable of the present invention, the multiplication as described above is performed. The correction variable is not limited to the type and may be a correction variable of the type to be added to the K1 × (A / N) value. In this case, the invalid value is 0.

(Effects of the Invention) As described above in detail, according to the air-fuel ratio control method for an internal combustion engine of the present invention, the communication passage is blocked / opened in the middle of the communication passage that connects the canister and the intake passage downstream of the throttle valve. A solenoid valve for opening the solenoid valve in a specific operating state of the engine to discharge the evaporated fuel to the intake passage, and to the internal combustion engine according to the correction variable value according to the concentration of the specific component in the exhaust gas. When the opening / closing switching operation of the solenoid valve is executed during the air-fuel ratio feedback control operation of correcting the fuel supply amount of the solenoid valve to control the air-fuel ratio to the required value, the correction variable value at the time of switching the solenoid valve and the correction variable As a value, the correction variable value is compared with an ineffective value that does not substantially change the fuel supply amount even if the fuel supply amount is corrected, and the correction variable value is set so that the solenoid valve is switched from open to closed and The correction variable value when switching Condition that is a value to correct the air-fuel ratio to the fuel rich side,
And when the solenoid valve is switched from closed to open, and the correction variable value when switching the solenoid valve is a value that corrects the air-fuel ratio to the fuel lean side from the invalid value, either one of the conditions is satisfied. Since the current value is held and the invalid value is reset when none of the conditions is satisfied, the purge control valve is opened / closed during the air-fuel ratio feedback control operation and the air-fuel ratio supplied to the engine suddenly changes. Even if it occurs, it can be quickly settled to the required value, the engine operation can be stabilized, and the excellent effects of being able to improve the exhaust gas characteristics and the fuel consumption are achieved.

[Brief description of drawings]

The drawings show an embodiment of the present invention. FIG. 1 is a schematic configuration diagram of a fuel supply control apparatus for executing the method of the present invention, and FIG. 2 is a feedback correction section executed by the electronic control unit shown in FIG. FIG. 3 is a flow chart showing a procedure for setting the numerical value IFB, FIG. 3 is a flow chart showing a procedure for resetting the feedback correction coefficient value IFB to a predetermined value when the purge control valve is turned on and off, which is executed by the electronic control unit shown in FIG. The figure illustrates the inconvenience that occurs when the purge control valve is switched from ON to OFF during air-fuel ratio feedback control in the conventional air-fuel ratio control method. The on-off state of the purge control valve (PCV), the O ₂ sensor Timing chart showing the relationship of output, feedback correction coefficient value IFB, and air-fuel ratio with time. FIG. 5 illustrates another aspect of the disadvantage that occurs when the purge control valve is switched from ON to OFF during the air-fuel ratio feedback control in the conventional air-fuel ratio control method. 3) is a timing chart showing the relationship between the on / off state of FIG. 4), the feedback correction coefficient value IFB, and the change over time of the air-fuel ratio. 10 ... Internal combustion engine, 12 ... Intake pipe (intake passage), 16 ...
… Electronic control unit, 18… Throttle valve, 20
…… Fuel injection valve, 21 …… O ₂ sensor, 25 …… Fuel tank,
26 ... Purge passage (communication passage), 28 ... Canister, 30 ...
… Purge control valve (solenoid valve).

Claims (1)

[Claims] 1. A correction variable value according to a concentration of a specific component in exhaust gas is set during an air-fuel ratio feedback control operation of an internal combustion engine in which a fuel tank communicates with an intake passage downstream of a throttle valve via a canister. In the air-fuel ratio control method for correcting the fuel supply amount to the internal combustion engine in accordance with the correction variable value and controlling the air-fuel ratio to a required value, the connection between the canister and the intake passage downstream of the throttle valve is connected. An electromagnetic valve that shuts off / opens the communication passage is disposed in the middle of the passage, and the electromagnetic valve is opened when the engine is in a predetermined operating state to discharge the evaporated fuel to the intake passage, while the feedback control operation is performed during the feedback control operation. When the opening / closing switching operation of the solenoid valve is executed, the correction variable value at the time of switching the solenoid valve and the fuel supply amount substantially change even if the fuel supply amount is corrected as the correction variable value. By comparing with an invalid value which is not given, the correction variable value, the solenoid valve is switched from open to closed, and the correction variable value at the time of switching the solenoid valve makes the air-fuel ratio from the invalid value to the fuel rich side. Of the condition that is a value to be corrected, and that 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 from the invalid value to the fuel lean side. An air-fuel ratio control method for an internal combustion engine, wherein the current value is held when one of the conditions is satisfied, and the current value is reset to the invalid value when none of the conditions is satisfied.