JP2739715B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an air-fuel ratio control method for an internal combustion engine, and more particularly, to a method of mixing air supplied to an engine using an exhaust gas concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration. The present invention relates to an air-fuel ratio control method for performing feedback control of air to a target air-fuel ratio.

(Prior Art) Using an exhaust gas concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration, the air-fuel ratio of the air-fuel mixture supplied to the engine (hereinafter referred to as “supplied air-fuel ratio”) is set according to the engine operating state. In the air-fuel ratio control method for performing feedback control to a target air-fuel ratio, when the vehicle speed is low and the accelerator pedal is slightly depressed (low-speed fluctuation state), the target air-fuel ratio is gradually changed, that is, the change speed of the target air-fuel ratio is changed. A device in which the size is reduced has been proposed (JP-A-62-223425 (hereinafter referred to as "prior art 1")).

There has also been conventionally proposed a method in which the target air-fuel ratio changing speed is switched between when the stoichiometric air-fuel ratio is changed from the stoichiometric air-fuel ratio to the lean direction and when it is changed from the stoichiometric air-fuel ratio to the lean direction.
No. -12850 (hereinafter referred to as “prior art 2”).

(Problems to be Solved by the Invention) However, the above-mentioned prior art 1 focuses only on a low-speed fluctuation state in which the accelerator pedal is slightly depressed in a low-speed state.
The target air-fuel ratio change speed is not set in consideration of other engine operating conditions. Therefore, in an engine operating state other than the above-described low-speed fluctuation state, the target air-fuel ratio is stepwise changed to a value set according to the typical operating state of the engine, and a sudden change in the target air-fuel ratio causes a torque shock. In some cases, the drivability is deteriorated.

Further, according to the above prior art 2, when the target air-fuel ratio is changed from a value leaner than the stoichiometric air-fuel ratio to the rich direction, a constant change speed is applied. If the change speed is set low to prevent the occurrence of engine rotation, the engine speed will be unstable when the engine speed decreases or idle, while if the change speed is set high to stabilize the engine speed, the torque will increase. Shock increases and driving performance deteriorates.

The present invention has been made in view of the above points, and particularly, in the case where the target air-fuel ratio is changed from a value leaner than the stoichiometric air-fuel ratio to a rich direction, the change speed of the target air-fuel ratio is appropriately set, and drivability is improved. It is an object of the present invention to provide an air-fuel ratio control method capable of preventing deterioration of the engine speed and stabilizing the engine speed.

(Means for Solving the Problems) In order to achieve the above object, the present invention supplies to an engine using an exhaust concentration sensor provided in an exhaust system of an internal combustion engine and having an output characteristic substantially proportional to the exhaust gas concentration. In the air-fuel ratio control method for an internal combustion engine in which the air-fuel mixture is feedback-controlled to a target air-fuel ratio corresponding to an operation state of the engine, when the target air-fuel ratio is changed from a value leaner than the stoichiometric air-fuel ratio to a richer direction, The change speed of the target air-fuel ratio is set to be higher as the rotation speed is lower, or the change speed of the target air-fuel ratio is set to be larger when the engine is in an idle state than in an operation state other than the idle state. It is something to do.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is an overall configuration diagram of a control device to which a control method of the present invention is applied. In FIG. 1, reference numeral 1 denotes a DOHC in which each cylinder is provided with a pair of an intake valve and an exhaust valve (not shown). It is an in-line four-cylinder engine. In the engine 1, the operating characteristics of the intake valve and the exhaust valve (specifically, the valve opening timing and the lift amount, hereinafter referred to as "valve timing") are changed to a high-speed valve timing suitable for a high-speed rotation region of the engine. It is configured to be switchable to a low-speed valve timing suitable for a low-speed rotation region.

In the middle of the intake pipe 2 of the engine 1, a throttle body 3
And a throttle valve 3 ′ is disposed therein. Throttle valve opening (θTH)
A sensor 4 is connected, and outputs an electric signal corresponding to the opening degree of the throttle valve 3 and supplies it to an electronic control unit (hereinafter referred to as “ECU”) 5.

The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). Is electrically connected to ECU5 together with the EC
The valve opening time of fuel injection is controlled by a signal from U5.

An electromagnetic valve 21 for controlling the switching of the valve timing is connected to the output side of the ECU 5, and the opening and closing operation of the electromagnetic valve 21 is controlled by the ECU 5. Solenoid valve 21
Is for switching the hydraulic pressure of a switching mechanism (not shown) for switching the valve timing between high and low. The valve timing is switched between high-speed valve timing and low-speed valve timing in accordance with the high / low of the hydraulic pressure. . The oil pressure of the switching mechanism is detected by an oil pressure (POIL) sensor 20,
The detection signal is supplied to ECU5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 8 is provided immediately downstream of the throttle valve 3 via a pipe 7, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. Is done. Further, an intake air temperature (TA) sensor 9 is mounted downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies the electric signal to the ECU 5.

The engine water temperature (TW) sensor 10 mounted on the body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5. The engine speed (NE) sensor 11 and the cylinder discrimination (CYL) sensor 12 are mounted around a camshaft or a crankshaft (not shown) of the engine 1. The engine speed sensor 11 outputs a pulse (hereinafter referred to as “TDC signal pulse”) at a predetermined crank angle position every time the crankshaft of the engine 1 rotates 180 degrees, and the cylinder discriminating sensor 12 outputs a predetermined crank angle of a specific cylinder. A signal pulse is output at the position, and each of these signal pulses is supplied to the ECU 5.

The three-way catalyst 14 is arranged in an exhaust pipe 13 of the engine 1 is performed HC in the exhaust gas, CO, purification components such as NO _x. An oxygen concentration sensor (hereinafter, referred to as an “LAF sensor”) 15 as an exhaust concentration sensor is mounted on the exhaust pipe 13 on the upstream side of the three-way catalyst 14 and outputs an electric signal having a level substantially proportional to the oxygen concentration in the exhaust gas. Output and supply to ECU5.

The ECU 5 is further connected to an atmospheric pressure (PA) sensor 16, a vehicle speed (VSP) sensor 17, a clutch sensor 18 for detecting clutch engagement / disengagement, and a gear position sensor 19 for detecting a shift position of the transmission. Is supplied to the ECU 5.

The ECU 5 shapes input signal waveforms from various sensors, corrects a voltage level to a predetermined level, and converts an analog signal value to a digital signal value. The input circuit 5a has a function of a central processing unit (hereinafter referred to as a “CPU”). 5b), a storage means 5c for storing various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, the solenoid valve 21, and the like.

Based on the various engine parameter signals described above, the CPU 5b determines various engine operation states such as a feedback control operation area and an open loop control operation area corresponding to the oxygen concentration in the exhaust gas, and determines the next according to the engine operation state. Based on the equation (1), a fuel injection time _TOUT of the fuel injection valve 6 synchronized with the TDC signal pulse is calculated.

T _OUT = Ti × KCMDM × KLAF × K ₁ + K ₂ (1) where Ti is the basic fuel amount, specifically the engine speed
This is a basic fuel injection time determined according to NE and the intake pipe absolute pressure PBA, and a Ti map for determining this Ti value is stored in the storage means 5c.

KCMDM is a corrected target air-fuel ratio coefficient KC which is set according to the program of FIG.
It is calculated by multiplying MD by the fuel cooling correction coefficient KETV. The correction coefficient KETV is a coefficient for correcting the fuel injection amount in advance in consideration of the fact that the supply air-fuel ratio changes due to the cooling effect by actually injecting the fuel, and according to the value of the target air-fuel ratio coefficient KCMD. Is set. As is apparent from the above equation (1), if the target air-fuel ratio coefficient KCMD increases, the fuel injection time T _OUT increases.
The MDM value is a value proportional to the reciprocal of the so-called air-fuel ratio A / F.

KALF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 15 matches the target air-fuel ratio during the air-fuel ratio feedback control, and a predetermined value corresponding to the engine operating state during the open-loop control. Is set to

K ₁ and K ₂ are other correction coefficients and correction variable computed according to various engine parameter signals, so that the fuel consumption characteristic according to engine operating conditions, the optimization of various properties such as the engine acceleration characteristics can be achieved Is set to an appropriate value.

The CPU 5b further outputs a valve timing switching instruction signal in accordance with the engine operating state to control the opening and closing of the solenoid valve 21.

The CPU 5b outputs a signal for driving the fuel injection valve 6 and the solenoid valve 21 based on the result calculated and determined as described above.
Output through the output circuit 5d.

FIG. 2 shows the target air-fuel ratio coefficient KCMD and the correction target when the engine is in a normal engine operation state, not in a predetermined high load operation state in which the fuel is to be increased or a low load operation state in which the fuel supply is to be cut off. Air-fuel ratio coefficient KC
5 is a flowchart of a program for calculating MDM. This program is executed in synchronization with each generation of the TDC signal.

In step S11, a reference value KBSM of a target air-fuel ratio coefficient is calculated, and this calculated value is set as a target air-fuel ratio coefficient KCMD (step S12). The calculation of the reference value KBSM in step S11 is based on the detected NE value and PBA value from a KBSM map set in accordance with the engine speed NE and the intake pipe absolute pressure PBA (or according to Japanese Patent Application Laid-Open No. 60-90948). This is performed by reading a value corresponding to a known predicted PBA value. Step S13
Then, the KCMD value limit processing is performed by the program shown in FIG. This limit processing is performed so that the difference between the previous value and the current value of KCMD does not exceed an upper limit set according to the engine operating state, so that the KCMD value is not suddenly changed. However, when the KCMD value is leaner than the stoichiometric air-fuel ratio and the accelerator pedal is suddenly depressed, the KCMD value is immediately increased to a value corresponding to the stoichiometric air-fuel ratio.

After the KCMD limit process, in step S14, a corrected target air-fuel ratio coefficient KCMDM is calculated by reading the fuel cooling correction coefficient KETV from a table set in accordance with the KCMD value and multiplying the KCMD value by the KCMD value (step S15). Then KCMD
Perform a limit check on the M value and terminate this program. In this limit check, it is determined whether or not the KCMDM value is within a predetermined range of upper and lower limits. If the value is outside the range, the KCMDM value is set to the upper limit or the lower limit.

After the execution of this program, in the engine operating state where the air-fuel ratio feedback control is possible, the calculated target air-fuel ratio coefficient KCMD and the equivalent ratio KACT, which is calculated based on the output of the LAF sensor 15 and represents the detected air-fuel ratio, are used. Are calculated such that the air-fuel ratio correction coefficient KLAF is equal.

FIG. 3 is a flowchart of a program for performing a KCMD value limit process in step S13 of FIG.

In step S21, the change amount DKCMD of the KCMD value is calculated as the current calculated value KC.
Difference between MD _(N) and previous calculated value KCMD _(N-1) (KCMD _(N) −KCMD
_(N-1) ), and it is determined whether or not the previously calculated value KCMD _(N-1) is smaller than a predetermined value KCMD0 corresponding to the stoichiometric air-fuel ratio (step S22). The answer is affirmative (YES), that is, KCMD _(N-1) <K
When CMD0 and the KCMD value is leaner than the stoichiometric air-fuel ratio, the value is obtained when the engine is in a predetermined high load state, that is, when the throttle valve is substantially fully opened or the intake pipe absolute pressure PBA is in a high load state equal to or higher than a predetermined value. WOT flag FWOT set to 1
Is determined to be 1 (step S23). If this answer is affirmative (Yes), that is, if FWOT = 1, the KCMD value is immediately set to the value KCMD0 corresponding to the stoichiometric air-fuel ratio, and step S48 is performed.
Proceed to.

Thus, when the KCMD value is leaner than the value KCMD0 corresponding to the stoichiometric air-fuel ratio and FWOT = 1, for example, when the accelerator pedal is suddenly depressed, the value immediately increases to the value corresponding to the stoichiometric air-fuel ratio. Let it.

If the answer to step S23 is negative (No), that is, if FWOT = 0, the previous calculated value KCMD _(N-1) is set to the lean side predetermined value KCMDX.
It is determined whether it is larger than (for example, A / F = 17) (step S25). This answer is affirmative (Yes), that is, KCMD
_{When (N-1)} > KCMDX, the decrease variable DKC2 corresponding to the change speed of the target air-fuel ratio in the lean direction is set to a first lean-side decrease predetermined value DKC2L (for example, a value equivalent to A / F = 0.3) ( Step S26), and proceed to step S28. The decrease variable DKC2 is applied to a formula for calculating the present value KCMD _(N) of the KCMD value in step S47 described later, and decreases the KCMD value. If the answer to step S25 is negative (No), that is, if KCMD _(N-1) ≦ KCMDX, the decreasing variable DKC2 is set to the first lean decreasing predetermined value DK.
The second lean side predetermined decrease value DKC2M smaller than C2L (for example,
(A / F = value equivalent to 0.1) (step S27), and the process proceeds to step S28.

In step S28, it is determined whether or not the engine is in an idling state. If the answer is affirmative (Yes), the increasing variable DKC1 corresponding to the changing speed of the target air-fuel ratio in the rich direction is determined.
Is set to the predetermined increase value DKC1IDL for idle (for example, a value corresponding to A / F = 2.0) (step S32), and the process proceeds to step S43.
The increase variable DKC1 is applied to a formula for calculating the current value KCMD _(N) of the KCMD value in step S45 described later, and increases the KCMD value. When the answer to step S28 is negative (No), that is, when the engine is not in the idle state, it is determined whether or not the engine speed NE is lower than a predetermined number of times NKCMD (for example, 1800 rpm) (step S29). When the answer is affirmative (Yes), the increase variable DKC1 is set to the predetermined increase value DK for idle.
Predetermined increase value DKC1M1H for low rotation smaller than C1IDL (for example, A /
F = a value corresponding to 1.0) (step S30). on the other hand,
If the answer is negative (No), the increase variable DKC1 is set to the high rotation increase predetermined value DKC1M1L (for example, a value equivalent to A / F = 0.05) smaller than the low rotation increase predetermined value DKC1M1H (step S31). Proceed to step S43.

In step S43, it is determined whether or not the change amount DKCMD of the KCMD value is a negative value, and when the answer is affirmative (Yes), that is, the KCM
When the D value changes in the decreasing direction, it is determined whether or not the absolute value of the deviation DKCMD is smaller than the decreasing variable DKC2 (step S46). The answer to step S46 is negative (No), that is, | DKCMD
| ≧ DKC2, the current value KCMD _{(N) is changed} to (KCMD _(N-1) −DKC
Change to 2) (step S47) On the other hand, if the answer to step S46 is affirmative (Yes), the process immediately proceeds to step S48.

If the answer to step S43 is negative (No), that is, if DKCMD ≧ 0 and the KCMD value changes in the increasing direction, the change amount DK
It is determined whether the absolute value of CMD is smaller than the increase variable DKC1 (step S44). If the answer in step S44 is negative (N
o), that is, when | DKCMD | ≧ DKC1, the current value KCMD _(N) is changed to (KCMD _(N-1) + DKC1) (step S45).
When the answer to step S46 is affirmative (Yes), the process immediately proceeds to step S48.

According to steps S43 to S47, when the absolute value of the change amount DKCMD of the KCMD value is larger than the increase variable DKC1 or the decrease variable DKC2, the current value KCMD _(N) is set to the DKC1 value or DKC2 value and the previous value KCMD.
_(N-1)
This prevents a sudden change in the D value and a deterioration in drivability.

In steps S48 to S51, a limit check of the KCMD value is performed. That is, the KCMD value is compared with predetermined upper and lower limit values KCMLMH and KCMLML (steps S48 and S49). If the KCMD value is larger than the upper limit value KCMLMH, the KCMD value is set to the upper limit value (step S51). If lower than the lower limit KCMLML, the KCMD value is set to the lower limit (step S50),
Exit this program.

On the other hand, if the answer in step S22 is negative (No), that is, KCMD
_{When (N-1)} ≧ KCMD0 and the KCMD value is a value corresponding to the stoichiometric air-fuel ratio or on the rich side, in steps S33 to S42, the decreasing variable DKC2 or the increasing variable DKC1 is set, and the step S43 is performed. Proceed to.

First, in step S33, it is determined whether or not the WOT flag FWOT has a value of 1. If the answer is negative (No), the increasing variable DKC1 is set to a normal increasing predetermined value DKC1M2 (for example, A / F = 0.3). Value) (step S39), and proceeds to step. When the answer to step S33 is affirmative (Yes), that is, when FWOT = 1 and the engine is in a predetermined high load operation state,
It is determined whether or not the previous value KCMD _(N-1) is larger than the low water temperature target air-fuel ratio coefficient KTWLAF used when the engine water temperature TW is low (step S34). When the answer is negative (No), the process proceeds to step S39, and when the answer is affirmative (Yes), it is determined whether a failure of a system such as a sensor connected to the ECU 5 is detected (step S35). When the answer to step S35 is affirmative (Yes), that is, when some failure is detected, the increase variable DKC1 is set to the high water temperature increase predetermined value DKC1H larger than the normal increase predetermined value DKC1M2 (for example, a value equivalent to A / F = 0.8). ) (Step S40), and proceeds to step S42.

If the answer to the step S35 is negative (No), that is, if no failure is detected, the high water temperature rich flag FXWOT, which is set to the value 1 when the engine is in the predetermined high load operation state and the engine water temperature TW is high, has the value 1 Is determined (step S36). When the answer is affirmative (Yes), the process proceeds to step S40, and when the answer is negative (No), it is determined whether or not the high-speed valve timing is selected (step S37). If the answer to step S37 is negative (No), that is, if the low speed valve timing is selected, it is determined whether or not the throttle valve fully open flag FTHWOT which is set to the value 1 when the throttle valve is substantially fully open is the value 1. (Step S38). When the answer to step S37 or S38 is affirmative (Yes), that is, when the high-speed valve timing or the low-speed valve timing is selected and the throttle valve is almost fully open, the process proceeds to step S39. If both the answers of steps S37 and S38 are negative (No), that is, the low speed valve timing is selected and the throttle valve is not substantially fully opened, the increase variable DKC1 is set to the high load increase predetermined value smaller than the normal increase predetermined value DKC1M2. The value is set to a value DKC1L (for example, a value corresponding to A / F = 0.05) (step S41), and the process proceeds to step S42.

In step S42, the decrease variable DKC2 is set to the rich-side decrease predetermined value D.
Set to KC2H (for example, A / F = 0.4 equivalent value) (step S4
1), and proceed to step S43.

According to the program shown in FIG. 3, the increase variable DKC1 and the decrease variable DKC2 corresponding to the change speed of the target air-fuel ratio are set as follows according to the engine operating state.

(1) When changing from the value KCMD0 corresponding to the stoichiometric air-fuel ratio to the rich direction (on the rich side in FIG. 4A), the engine is in a predetermined high load operation state (FWOT =
1) When a system failure is detected or when the engine water temperature is high and the air-fuel ratio should be enriched (FXWOT = 1)
DKC1 = DKC1H, the engine is in a predetermined high load operation state (FWOT = 1), when KCMD _(N-1) ≤ KTWLAF is satisfied, when high-speed valve timing is selected, or when the throttle valve is almost fully open, Alternatively, when the engine is not in the predetermined high load operation state and KCMD _(N-1) ≧ KCMD0 is satisfied (for example, when the KCMD value is set to a value richer than KCMD0 because the engine water temperature is low), DKC1 = DKC1M2, DKC1 = DKC1L when the engine is in the predetermined high load operation state, the low-speed valve timing is selected, and the throttle valve is not substantially fully opened.

(2) When changing from a value leaner than KCMD0 corresponding to the stoichiometric air-fuel ratio to a richer direction (toward the lean side in FIG. 4 (a)) When the engine is idle, DKC1 = DKC1ID
L, DKC1 = DKC1M1H when the engine speed is low (NE <NKCMD) instead of idling, and DKC1 = DKC1 = DKC1M1H when the engine speed is high (NE ≥ NKCMD) instead of idle. DKC1M1L.

That is, when the target air-fuel ratio coefficient KCMD is changed from a value leaner than a value corresponding to the stoichiometric air-fuel ratio to a rich direction,
When the engine is idle, the increase variable DKC1
Is set to a larger value than when the engine is not in the idle state, so that the engine speed can be stabilized immediately after shifting to the idle state. Further, the increase variable DKC1 is set to a larger value as the engine speed is lower, so that the rotation at the time of low engine speed can be stabilized.

(3) When changing from the value on the rich side of the stoichiometric air-fuel ratio KCMD0 to the lean direction (on the rich side in FIG. 4B), DKC2 = DKC2H.

(4) When changing from the value KCMD0 corresponding to the stoichiometric air-fuel ratio in the lean direction (on the lean side in FIG. 4 (b)) When KCMD _(N-1) > KCMDX holds, DKC2 = DKC2
L and KCMD _(N-1) ≤ KCMDX holds, DKC
2 = DKC2M.

That is, when the target air-fuel ratio coefficient KCMD is changed from the value corresponding to the stoichiometric air-fuel ratio in the lean direction, the decreasing variable DKC2 is set to the lean-side predetermined value KCM until the lean-side predetermined value KCMDX is reached.
Since it is set to a larger value when changing in the lean direction further than DX, the KCMD value passes through the value corresponding to A / F = about 16 in a short time, and after reaching the lean side predetermined value KCMDX, it is relatively high. Decreases slowly. As a result, it is possible to shorten the period during which the NO _x emission increases, prevent a sudden torque fluctuation thereafter, and reduce the NO _x emission without deteriorating the operating state.

(Effect of the Invention) As described above in detail, according to the present invention, when changing the target air-fuel ratio from a value leaner than the stoichiometric air-fuel ratio to the rich direction, the lower the engine speed, the richer the target air-fuel ratio. When the change speed to the target air-fuel ratio is set to a larger value and the engine is in an idle state, the change speed in the rich direction of the target air-fuel ratio is set to be higher than when the engine is in an operation state other than the idle state, so that the torque shock increases. , And stable engine rotation can be ensured.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a fuel supply control device to which the control method of the present invention is applied. FIG. 2 is a flowchart of a program for calculating a target air-fuel ratio coefficient (KCMD) and a corrected target air-fuel ratio coefficient (KCMDM). FIG. 4 is a program flowchart for performing a limit process of the target air-fuel ratio coefficient, and FIG. 4 is a diagram showing a mode of change of the target air-fuel ratio coefficient. 1: Internal combustion engine, 5: Electronic control unit (EC
U), 6: fuel injection valve, 15: exhaust gas concentration sensor (oxygen concentration sensor).

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hironao Fukuchi 1-4-1 Chuo, Wako-shi, Saitama Pref. Honda Technical Research Institute Co., Ltd. (56) References JP 1-2224427 (JP, A) JP JP-A-61-232347 (JP, A) JP-A-58-214649 (JP, A) JP-A-63-12850 (JP, A) JP-A-62-223425 (JP, A) JP 2-29851 (JP) , B2)

Claims (2)

(57) [Claims] An air-fuel mixture supplied to an engine using an exhaust concentration sensor provided in an exhaust system of an internal combustion engine and having an output characteristic substantially proportional to an exhaust gas concentration is adjusted to a target air-fuel ratio according to an operation state of the engine. In the air-fuel ratio control method for an internal combustion engine that performs feedback control, when the target air-fuel ratio is changed from a value leaner than the stoichiometric air-fuel ratio to a rich direction, the change speed of the target air-fuel ratio increases as the rotation speed of the engine decreases. An air-fuel ratio control method for an internal combustion engine, wherein the method is set. 2. An air-fuel mixture supplied to an engine using an exhaust concentration sensor provided in an exhaust system of an internal combustion engine and having an output characteristic substantially proportional to an exhaust gas concentration to a target air-fuel ratio corresponding to an operation state of the engine. In the air-fuel ratio control method for an internal combustion engine that performs feedback control, when the target air-fuel ratio is changed from a value leaner than the stoichiometric air-fuel ratio to a rich direction, when the engine is in an idle state, the operation state is changed to an operation state other than the idle state. An air-fuel ratio control method for an internal combustion engine, wherein the change speed of the target air-fuel ratio is set to be higher than at a certain time.