JPS62182458A - Air-fuel ratio control of internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent the overlean state due to the deterioration of sensors and deflection of the base air-fuel ratio by setting the initial value of the correction value to the rich side, in the learning of the correction value in the open loop control in the air-fuel ratio feedback control. CONSTITUTION:Each detection value of a suction pipe absolute pressure sensor 10, crank angle sensor 11, O2 sensor 14, car speed sensor 14, etc. is input into a control circuit 20, and the standard value corresponding to the operation region is read out on the basis of the absolute pressure in the suction pipe and the number of revolution, and in the feedback control operation region, the feedback control to the aimed air-fuel ratio on the basis of the detection value of the O2 sensor 14 is carried out, and the correction value at this time, is memorized into each operation region. Though, when the air-fuel ratio feedback control is suspended such as in deceleration, the correction value corresponding to the operation region is read out, and the standard value is corrected, the initial value of the correction value on engine start is set to the rich side, and the miss-fire due to overlean state is prevented.

Description

[Detailed description of the invention] 1 hill 11 The present invention relates to an air-fuel ratio control method for an internal combustion engine.

For the purpose of purifying the exhaust gas of internal combustion engines and improving penalties, the oxygen concentration in the exhaust gas is detected by an oxygen concentration sensor, and the air-fuel mixture supplied to the engine is adjusted according to the output level of the oxygen concentration sensor. An air-fuel ratio control device that performs feedback control of the fuel ratio to a target air-fuel ratio is known (Japanese Patent Publication No. 55-353
Publication No. 3).

In such an air-fuel ratio control device, a reference value for air-fuel ratio adjustment is set according to a plurality of engine operating parameters related to engine load, and the reference value is adjusted at predetermined intervals to
The output value is set by correcting it according to the output level of the 11 degree sensor, and the opening degree of the air-fuel ratio adjusting solenoid valve is controlled according to the output value.

Oxygen 181M, the air-fuel ratio feedback control according to the output level of the sensor is stopped when the air-fuel ratio should be made lean, such as during light load operation just before starting a fuel cut, and the air-fuel ratio is kept lean to the set reference value. The opening degree of the air-fuel ratio adjustment solenoid valve is controlled according to the value multiplied by the coefficient.

By the way, the set reference value may not correspond to the target air-fuel ratio due to changes over time in the detection characteristics of the sensor that detects engine operating parameters, deterioration of the sensor, or deviation of the base air-fuel ratio of the carburetor from a predetermined value of 1. This will cause an error. Therefore, a correction value for the U* value is calculated and stored as memory data in correspondence with the operating condition, and during operation where the air-fuel ratio should be made lean, such as during light load, the correction value corresponding to the operating condition at that time is stored. It is conceivable that the correction value is read from the data and multiplied by the N semi-scale set together with the lean coefficient. However, in order to control the air-fuel ratio to the stoichiometric air-fuel ratio according to the output value, each correction value in the stored data is initially set equal to 1.0 before starting the engine.
Since it is normal to convert the Il'I value into an Il'I value, the base air-fuel ratio of the carburetor is 14. If the air-fuel ratio is leaner than O, if the opening of the solenoid valve for adjusting the air-fuel ratio is controlled according to the output value obtained from the east line, the air-fuel ratio of the supplied air-fuel mixture will become over-lean, leading to a misfire. there is a possibility.

112 and 1 Transition Therefore, an object of the present invention is to prevent overlean operation during lean operation during the period from when the correction value of the stored data for correcting the reference value is calculated to when the initial value is rewritten to the actual correction value. Air-fuel ratio i, II tl that can be prevented
t1 method.

The air-fuel ratio control method of the present invention adjusts the air-fuel ratio to l! according to the output value.
I! It is characterized in that the initial value of the correction value in the stored data is set so as to be controlled to a value smaller than the stoichiometric air-fuel ratio.

Tomo-Ki-1 Hereinafter, actual droplet examples of the present invention will be explained with reference to the drawings.

The air-fuel ratio tilJ of the intake secondary air supply system of an on-vehicle internal combustion engine to which the air-fuel ratio control method of the present invention shown in FIG. 1 is applied
In the control device, intake air is supplied from an atmospheric air intake 1 to an engine 5 via an air cleaner 2, a carburetor 3, and an intake manifold 4.

The carburetor 3 is provided with a throttle valve 6, and a venturi 7 is formed upstream of the throttle valve 6.

The intake manifold 4 and the vicinity of the air discharge port of the air cleaner 2 are communicated through an intake secondary air supply passage 8.

A linear solenoid valve 9 is provided in the intake secondary air supply passage 8 . The opening degree of the solenoid valve 9 changes in proportion to the current value supplied to the solenoid 9a.

On the other hand, 10 is an absolute pressure sensor that is installed in the intake manifold 4 and generates a pressure at a level corresponding to the absolute pressure inside the intake manifold 4, and 11 is a crankshaft of the engine 5 (
Crank angle sensors 1 and 12 generate pulses in accordance with the rotation of the engine 5 (not shown), a cooling water temperature sensor that generates an output at a level corresponding to the cooling water temperature of the engine 5, and 13 an intake air temperature sensor that detects the intake air temperature. Reference numeral 14 denotes an oxygen concentration sensor that is provided in the exhaust manifold 15 of the engine 5 and generates an output according to the oxygen concentration in the exhaust gas. A catalytic converter 33 is provided in the exhaust manifold 15 downstream of the oxygen concentration sensor 14 in order to promote reduction of harmful components in the exhaust gas. A control circuit 20 includes a linear solenoid valve 9, an absolute pressure sensor 10, a crank angle sensor 11, a water temperature sensor 12, an intake air temperature sensor 13, and an oxygen concentration sensor 14.
It is connected to the. The control circuit 20 further includes a vehicle speed sensor 16 that generates an output at a level corresponding to the speed of the vehicle, an atmospheric pressure sensor 17, a clutch switch 18 that turns on when it detects the release of the vehicle's clutch, and a gear position that is set to neutral. A neutral switch 19 is connected to the neutral switch 19, which is turned on when it is in the position. The clutch switch 18 and the neutral switch 19 generate a low level output when turned off, and generate a high level output when turned on.

The control circuit 20 has an absolute control circuit 10 as shown in FIG.
, a level conversion circuit 21 that converts each output level of the water temperature sensor 12, intake temperature sensor 13, acid level sensor 14, vehicle speed sensor 16, and atmospheric pressure sensor 17, and one of the outputs of each sensor that has passed through the level conversion circuit 21. a multiplexer 22 that selectively outputs the multiplexer 22, and an A/D converter 23 that converts the signal output from the multiplexer 22 into a digital signal.
, a waveform shaping circuit 24 that shapes the output signal of the crank angle sensor 11, a counter 25 that measures the generation interval of the TDC signal output as a pulse from the waveform shaping circuit 24, and a clutch switch 18 and neutral switch 19. A level conversion circuit 26 that converts each output level, a digital input camosulator 27 that converts each switch output via the level conversion circuit 26 into digital data, a drive circuit 28 that drives the solenoid valve 9, and a CPU that performs digital calculations according to a program. (Central performance circuit) 29
It consists of a ROM 30 in which various processing programs and data are written in advance, and a RAM 31. The solenoid 98 of the electromagnetic valve 9 is connected in series with a drive transistor and a current detection resistor (both not shown) of the drive circuit 28, and a power supply voltage is supplied across the series circuit. Multi-brancher 22, A/D converter 23, counter 25, digital input camosulator 27, drive circuit 28, CPU
29, ROM 30 and RAM 31 are connected to each other by an input/output bus 32.

In such a configuration, information on the absolute pressure in the intake manifold 4, cooling water temperature, intake temperature, oxygen concentration in exhaust gas, vehicle speed, and atmospheric pressure is alternatively transmitted from the A/D converter 23, and information on the engine rotation is transmitted from the counter 25. The information representing the number and the on/off information of the clutch switch 18 and neutral switch 19 from the digital input camosulator 27 are sent to the CPU 2.
9 via the input/output bus 32. CPU2
9 is a predetermined period T+ (for example, 5m5ec
), and in response to the interrupt signal, the output JaToLJT representing the current value supplied to the solenoid 9a of the solenoid valve 9 is calculated as data. The output value TOLJT is supplied to the drive circuit 28. The drive circuit 28 controls the solenoid 9 so that the current value flowing through the solenoid 9a corresponds to the output value TOUT.
The current value flowing through a is controlled in a closed loop.

Next, the procedure of the air-fuel ratio control method of the present invention will be explained in detail according to the operation flowcharts of the CPU 29 shown in FIGS. 3 to 5.

In the CPU 29, as shown in FIG. 3, first, a reference value D[3ASE representing the I2i reference current value supplied to the solenoid valve 9 is set every time an interrupt signal is generated (step 51).
. As shown in FIG. 6, the reference value DBASE determined from the intake manifold absolute pressure PEA and the engine speed Ne is stored in the ROM 30 as a DBASE data map in advance, so the CPU 29 uses the absolute pressure PBA and the engine speed Ne. and searches the fi1 quasi-value DoASE corresponding to the read 8 values from the Ds8SE data map. After setting the reference value DBASE, it is determined whether the vehicle operating condition (engine operating condition) satisfies the air-fuel ratio feedback (F/B) control conditions (step 52). The determination is made from the intake manifold absolute pressure Pa A N cooling water temperature TWSII speed ■ and engine rotation speed N13, and for example, it is determined that the air-fuel ratio feedback control condition is not satisfied at low vehicle speeds and low cooling water temperatures.

Here, if it is determined that the air-fuel ratio feedback control conditions are not satisfied, a light load lean routine is executed (step 53).

On the other hand, if it is determined that the air-fuel ratio feedback control conditions are satisfied, it is determined whether the opening time of an internal timer counter A (not shown) of the CPU 29 has elapsed by a predetermined time Δt1 (step 56). . This corresponds to the response delay time from supplying the intake secondary air for a predetermined period of time Δj+G until the result is detected by the oxygen concentration sensor 14 as a change of 1t1 degree of oxygen in the exhaust gas. A predetermined period of time Δ1. from the time when the time counter A is reset and starts counting a1. When the time counter A has elapsed, the time counter A is reset and counting starts from the initial value (step 57). That is, after the time counter A starts counting +i'l from the initial value by executing step 57, it is determined in step 56 whether or not the predetermined time Δ[1 has elapsed. In this way time counter A
When the old religion starts for a predetermined period of time Δt1, oxygen! !
The output level LO2 of the oxygen concentration sensor 14 is determined from one-time information.
It is determined whether or not is greater than a reference level Lref corresponding to the target air-fuel ratio (step 58). In other words, it is determined whether the air-fuel ratio of the air-fuel mixture supplied to the engine 5 is leaner than the target air-fuel ratio.

If Loz>Lref, the air-fuel ratio is leaner than the target air-fuel ratio, so it is determined whether the air-fuel ratio flag FAF representing the determination result of the previous step 58 is 1'' (
Step 59). If FA F = 1, the air-fuel ratio was determined to be lean last time as well, so the subtraction value IL is calculated (step 60).

The subtraction value IL is obtained by multiplying the constant +, engine speed Ne and absolute pressure PBA by 1B (K+ ・Ne −Pa A
), and depends on the intake air amount of the engine 5. After calculating the subtraction value IL, by executing this A/F routine, the correction value 1 o u T that has already been output is read out from the storage location a1 of the RAM 31, and the read correction! iB I.

The reduced impediment IL is subtracted from UT, and its H,1 value is set as a new correction value l0LIT, and is stored in the memory location a of the RAM 31.
1 (step 61). If FAF=0, the previous air-fuel ratio was determined to be rich and reversed from ridge to lean, so 1'' is set to flag Fρ representing the reversal of the air-fuel ratio control direction (step 62), and subtraction (
A direct PL is issued (step 63). The subtraction value PL is obtained by multiplying the constant rXIK 3 (>1) and the subtraction value IL by each other (K
3・IL). After the subtraction value PL is output, the correction value 10LJT, which has already been calculated by executing this A/F routine, is read from the memory location @a1 of the RAM 31, and the subtraction value PL is calculated from the read correction value 1°LIT. The n output value is subtracted and the new correction [Iou
v and written to storage location Aa+ of the RAM 31 (step 64). After calculating the correction value l0LJT in step 61 or 64, the flag FAF is set to 1" to indicate that the air-fuel ratio is lean (step 65). On the other hand, in step 58, LO2≦Lre
If r, the air-fuel ratio is higher than the target air-fuel ratio, so it is determined in 711 whether the air-fuel ratio flag FAF is "0" (step 66). If FAF=O, the air-fuel ratio was determined to be rich last time as well, so an additional value IR is calculated (step 67). Add 'ti I R to constant 2
It is obtained by multiplying the engine speed Ne and the absolute pressure PEA by each other (K2 · Ne · Pa 8), and depends on the intake air amount of the engine 5. After calculating the additional value IR, the correction value 10LIT, which has already been calculated by executing the A/F routine, is stored in the RAM 31.
The correction value 1 is read from memory location Ua+ of
The force value IR is added to ouv, and the ejection value thereof is set as a new correction value fouT and written to the storage location a1 of the RAM 31 (step 68). In step 66 the FA
If F = 1, the previous air-fuel ratio was determined to be lean and was reversed from lean to rich, so 7 lags Fρ.
1'' is set (step 69), and the added value PR is outputted as 0 (step 70).The added value PR is set to 4 (>
1) and the addition value IR are multiplied together (Ka.IR) to obtain 1q. After calculating the addition 1 direct PR, the correction value l0UT that has already been calculated by executing this △/F routine is read from the memory location a1 of the RAM 31, and the read correction value 10LIT and the interference value PR are added. The calculated value is used as the new correction value IoUT, and R
It is read into the storage location a1 of AM31 (step 71
). n of the correction value 1ouv in step 68 or 71
After this, the flag FAF is set to "0" to indicate that the air-fuel ratio is rich (step 72). In this way, the correction value Iouv is changed to step 61, 64, 68 or 7.
1, the correction value l0ur is added to the reference value D3ASE set in step 51, and the result of the addition is set as the output value TOLIT (step 73). After outputting the output fitITouT, the drive circuit 2
The output value Tou is output for 8, and (step 7
4) Then, the Krer ejection subroutine is executed (step 75).

The drive circuit 28 detects the current value flowing through the solenoid 9a of the solenoid valve 9 using a current detection resistor, compares the detected current value with the output value ToutT, and turns on and off the drive transistor according to the comparison result to control the solenoid 9a. supply current to. Therefore, the solenoid 9a has an output value TO
The current represented by LJT flows, and the solenoid 9a of the solenoid valve 9
Input secondary air is supplied into the intake manifold 4 in proportion to the current value flowing through the intake manifold 4.

Note that after the time counter A is reset in step 57 and starts counting from the initial value, a predetermined time Δ
If it is determined in step 56 that tl has not elapsed, step 73 is immediately executed, and in this case, the correction &B r o UT which has been increased by 1q by the previous A/F routine execution is read out.

Next, as shown in the Krerri output subroutine, it is first determined from the contents of the flag Fs whether initialization of each correction value Kref of the 1 (rer data map) has been completed (step 78). As shown in FIG. 7, the correction value K ref h<K determined from the intake manifold internal absolute pressure PoA and the engine speed NO.
It is formed as a ref data map. Fs =
If O, the initialization of the correction value KrcJ has not been completed, so each correction value Krcr in the Kref data map is 0.
．． 7 (step 79) and set the flag Fs to 1''.
is set (step 80). Meanwhile, step 80
If Fs=1 by executing
11(+) is determined (step 81
), if PA > 73 Q m111g, the engine speed Ne is 90 Or.

It is determined whether p, m, larger than 1700 r, p, m, smaller (step 82.83>.17
00 r, p, n+, > N e > 90 Or,
p, m, then the absolute intake pressure PBA is 160++ mHg
It is determined whether it is larger than 560 mm and smaller than 11 g (steps 84 and 85), 16011
If 111<Pa A <56011ml, the engine is considered to be in a steady operating state, and this steady operating state is 2
It is determined whether the process has continued for more than Sec (Step 8
6). If the steady state of operation continues for 2 Sec or more, it is determined whether the flag Fp is equal to 1'' (step 87). If Fρ = 0, the flag FKO2P is 1''.
It is determined whether it is equal to (step 88). The flag FKO2P is a flag to indicate that step 88 is executed for the first time in this subroutine, and is initially set to O'' when the power is turned on.If FK02P=0, the flag is calculated by executing the current A/F routine. The output value TOUT thus obtained is held as the previous average value To UT + (step 89), and the flag FKO2P is set to 1'' (step 90). If FKO2p=1, then step 90 has been executed. The average value Tou of the output 1iciTouv is calculated by tightening the output 1UTouv released by the execution of the current A/F routine and the previous average value Tou r + and dividing it by 2.
v is calculated (step 91), and its average value TOU
T is held as the previous average value To u T I (step 92), and the output (average IUTou of UTo u
Add 1″ou to the flag F indicating that v has been calculated.
t is set (step 93).

On the other hand, if it is determined that Fp=1 in step 87, the control direction of the air-fuel ratio has been reversed, so °゛O'° is set to the flag Fp to the cell I (step 94), and the flag FTo
It is determined whether Ut is equal to 11111 (step 95). If FTOIJt=O, the average value Touv
has not been calculated, step 88 is executed.

If F -1, since the average value Touv has been calculated by executing step 91out-, O'' is set to flag F (step 96), and Ko 2 P = 5 ・To U
02P representing the error of the reference value D[3ASE is calculated from the formula T/DsAS (step 97). Here, K5 is a constant. Then, Krer=・K6
・From the formula Ko2ρ+7-Krcfx, a correction value KrC to compensate for the error in the reference value DBASE is calculated, and this correction 11CiKrcr determines the current intake manifold internal pressure PGA and engine rotation speed. It is stored in the position of the K rcf data map of the RAM 31 corresponding to NO (step 98). Here, Ks and K7 are constants, and K refx is the correction 1iQKrcf obtained by the previous execution of step 98 (correction value). (ref
is gradually approached from Krerx (0, 7) to 02P. For example, K6 =
128/256, if K7=128/256, KO2
The width ratio between P and Krcfx is 1:1. After calculating the correction value Krcf, the correction value Krcf is the previous correction 11f.
i K refx (step 99). By repeating this routine, the correction value K r in the K rat data map is adjusted according to the aging and deterioration of the sensor.
at is rewritten to a new value.

Note that the flags Fs, Fp, and F are set when the power is turned on.
is initially set to O゛, but if you press step 87 and press F
If it is determined that p = O, that is, the air-fuel ratio al(I
At the time of the next real-time execution after step 94 is executed after the t11 direction is reversed, or if it is determined in step 95 that F=O, step 96 is executed after calculating out, that is, the average faTouv.
When this subroutine is executed next after execution of step 8
8 is executed.

Next, in the light load subroutine, as shown in FIG. 5, it is first determined whether the intake air temperature TA is higher than 25°C.
It is also determined whether the engine cooling water Q T w is higher than 70° C. (steps 101 and 102). T
If A≦25℃ or T W, 670℃, engine 4
The output value Tout is made equal to 0 in order to stop the lean air-fuel ratio since step 11 is low temperature (step 10).
3). If TA>25°C and TW>70°C, it is determined whether the clutch switch 18 is on and whether the neutral switch 19 is on (step 104.
105). If the clutch switch 18 is on, it means that the clutch of the vehicle is released and the power transmission system from the engine to the wheels is cut off, and if one neutral switch is on, it indicates the gear position of the vehicle at dawn. This indicates that the power transmission system from the engine to the wheels is cut off because it is in the neutral position. Therefore, when it is detected that each switch is turned on in this way, it is assumed that the operation is in a transient state such as deceleration, and in order to stop the lean air-fuel ratio, the output value TOUT is set to 0 by executing step 103.
is made equal to If the clutch switch 18 is off and the neutral switch 19 is off, check whether the engine speed Ne is greater than 1100 rpm and the vehicle speed V.
It is determined whether or not the speed is greater than 17 km/h (
Steps 106.107>. Ne≦1100r, p, m
, or if V≦17km/11, step 10 is performed to stop the lean air-fuel ratio when the intake air amount is low.
By executing step 3, the output value TOLJT is made equal to O.

If Ne > 110 Or, o, m or V > 17 km/h, it is determined whether the shift position of the shift fold is at the 11th speed (step 108). If the gear position is the first speed, the output 1/j T o uT is made equal to O by executing step 103 in order to stop making the air-fuel ratio lean when the vehicle starts. If the gear position is other than first speed, it is determined whether the gear position is fifth speed (step 10).
9). If the gear position is 2nd or 4th speed other than 5th speed, the absolute pressure PBA in the intake manifold 4 is 420.
It is determined whether or not the absolute pressure PEA in the intake manifold 4 is smaller than 480 mm11g (step 110). 111). When the gear position becomes the fifth speed, the engine speed Ne decreases, so the light load discrimination reference value of the absolute pressure PBA in the intake manifold 4 is increased from 420 mm11g to 480 mm11g. In step 110, P8A≧
420 mm11g, or P8 in step 111
If A≧480 mi+l1g, it is assumed that the load is not light and the output value TOLJT is made equal to O by executing step 103 in order to stop making the air-fuel ratio lean.

In step 110, Pa A < 420 mm11
g or in step 111 Pa A < 480m
If mt1g, it is determined whether the change width ΔV of the vehicle speed V per unit time is smaller than 0.5 km/h (step 112). If Δv≧0.5kII/h,
Since g7NR is not constant, the output value To U T /J< O is made equal to the output value by executing step 103, and ΔV<0.
If the speed is 5 km/h, it is assumed that the air-fuel ratio should be leaner during light load operation, and TOUT=D[3ASE −)(rer
The output 11iTour is generated by the formula 0KLs (step 113). In this formula, 1(ref
is a correction value for compensating for the error in the reference value DeAsE set in step 51, and KLS is a lean coefficient [
For example, 1.2). The CPU 29 sets a correction value Krer corresponding to the absolute pressure PBA and the engine speed Ne.
It is searched from the ref data map and used to calculate the output value Touv.

In addition, the gear position of the transmission depends on the engine rotation speed and vehicle speed.
It is determined by the size of the ratio.

In the embodiments of the present invention described above, the case where the present invention is applied to an air-fuel ratio control device using an intake secondary air supply method has been described, but it can also be applied to an air-fuel ratio control device for an internal combustion engine using a fuel injection method using an injector. Can be applied.

In this case as well, the correction value is set to the operating state in which air-fuel ratio feedback control is stopped (fuel injection base t% using ref).
The error in the reference value DBASE between 5 and 5 is compensated for by tCt. For example, when the engine is lightly loaded, the output value TOLJT as the fuel injection time is TOLJT=DBASE 0
It is calculated by the formula Kref 0KLS. This lean coefficient is, for example, 0.8.

Furthermore, in the embodiment of the present invention described above, in J3, the control circuit 2
Each correction value Kre' in the Krer data map is initialized to 0.7 each time power supply to 0 is started, but if the correction value Kre is initialized each time the engine is started, The present invention can also be applied to.

As described above, in the air-fuel ratio control method of the present invention, the correction value in the stored data such as the K rat data map described above is used so that the air-fuel ratio is controlled to a value smaller than the stoichiometric air-fuel ratio according to the output value. Since the initial value of is set, if the base air-fuel ratio of the engine due to the carburetor etc. deviates to the lean side from 14.0, the exhaust gas concentration will change by the time the correction value of the stored data is replaced with the actual value. Even in operations such as light load operation where the corresponding air-fuel ratio control should be stopped to make the air-fuel ratio lean, over-leaning of the air-fuel ratio of the supplied air-fuel mixture can be avoided. Therefore, misfires and the like can be prevented, and a good operating condition can be obtained until the correction value of the stored data is rewritten from the initial value to the actual value.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an air-fuel ratio control device to which the control method of the present invention is applied, FIG. 2 is a block diagram showing a specific configuration of a control circuit in the device in FIG. 1, and FIGS. 3 to 5 is CPU
6 is a diagram showing the DBASE data map written in the ROM, and FIG. 7 is a diagram showing the Kref data map written into r(AM. Explanation of the symbols of the main parts 2 ... Air cleaner 3 ... Carburetor 4 ... Intake manifold 6 ... Throttle valve 7 ... Venturi 8 ... Intake Secondary air supply passage 9...Linear type solenoid valve 10...Absolute pressure sensor 11...Crank angle sensor 12...Cooling water temperature sensor 13... ... Intake temperature sensor 14 ... Oxygen concentration sensor 15 ... Exhaust manifold 33 ... Catalytic converter Applicant Honda Motor Co., Ltd. Agent Patent attorney Motohiko Fujisaku Figure 5 Figure 6 Figure 7 r, p, m・

Claims (1)

[Claims] A reference value for air-fuel ratio adjustment is set according to multiple engine operating parameters related to the load of the internal combustion engine, and the set reference value is corrected at predetermined intervals according to the engine exhaust component concentration to determine the output value for the target air-fuel ratio. Then, the air-fuel ratio is adjusted according to the output value, and a correction value for correcting the error in the reference value is calculated each time the output value is determined, and is stored as memory data in correspondence with each value of the plurality of engine operating parameters. When the air-fuel ratio of the air-fuel mixture supplied to the engine is to be made leaner than the target air-fuel ratio, the correction according to the exhaust component concentration of the reference value is stopped, and the correction value according to the plurality of engine operating parameters at that time is adjusted. An air-fuel ratio control method in which an output value is determined by reading out the stored data and correcting a set reference value using the correction value, the air-fuel ratio being set to a value smaller than the stoichiometric air-fuel ratio according to the output value. An air-fuel ratio control method characterized by setting an initial value of a correction value in the stored data.