JP2905805B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control method for an internal combustion engine, and more particularly to a method for controlling an air-fuel ratio of an air-fuel mixture supplied to an engine using an exhaust 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 on a fuel ratio.

[0002]

2. Description of the Related Art An air-fuel ratio of an air-fuel mixture supplied to an engine (hereinafter referred to as "supply air-fuel ratio") is feedback-controlled to a target air-fuel ratio using an exhaust concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration. In the fuel ratio control method, a proportional term (P) corresponding to a deviation between an air-fuel ratio detected by an exhaust gas concentration sensor (hereinafter referred to as “detected air-fuel ratio”) and a target air-fuel ratio.
Term), an integral term (I term), and a derivative term (D term) are calculated, and the supply air-fuel ratio is feedback-controlled by these PID terms (Japanese Patent Application Laid-Open No. Sho 6).
JP-A-2-251443).

[0003]

When the feedback control is performed by setting the target air-fuel ratio in a wide range from the lean side to the rich side from the stoichiometric air-fuel ratio, the purification efficiency of the three-way catalyst disposed in the exhaust system is high. Other than stoichiometric air-fuel ratio (A / F = 14.7)
From the target air-fuel ratio to the target air-fuel ratio near the stoichiometric air-fuel ratio
Control is also frequently performed. Therefore, CO, HC,
In order to suppress the increase in the emission of three components of NOx,
The transition time of the air-fuel ratio such as
It is desired to be as short as possible.

However, the above-mentioned proposed method does not take this point into consideration, and the feedback gain used for calculating the PID term is set according to the engine speed and the deviation between the detected air-fuel ratio and the target air-fuel ratio. However, there is still room for improvement in exhaust gas characteristics.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and when performing feedback control by setting a target air-fuel ratio in a wide range, it is possible to appropriately set a feedback gain and the like to improve exhaust gas characteristics. It is an object of the present invention to provide an air-fuel ratio control method that can be performed.

In order to achieve the above object, the present invention calculates an air-fuel ratio control amount based on an output of 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, wherein the air-fuel ratio of the air-fuel mixture supplied to the engine is feedback-controlled to a target air-fuel ratio using the air-fuel ratio control amount, the calculation of the air-fuel ratio control amount is executed at a predetermined cycle, When the air-fuel ratio is set near the stoichiometric air-fuel ratio, the feedback gain used for calculating the air-fuel ratio control amount is set to a larger value than when the target air-fuel ratio is set to other than near the stoichiometric air-fuel ratio. Together with the air-fuel ratio control amount.
The updating cycle is set shorter than when the target air-fuel ratio is set to a value other than the vicinity of the stoichiometric air-fuel ratio.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is an overall configuration diagram of a control device to which the control method of the present invention is applied. A throttle body 3 is provided in the middle of an intake pipe 2 of an engine 1, and a throttle valve 3 'is provided therein. Is arranged. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 ′, and outputs an electric signal corresponding to the opening of the throttle valve 3 to be supplied to an electronic control unit (hereinafter referred to as “ECU”) 5. I do.

A fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of an intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). The ECU 5 is electrically connected to the ECU 5 and controls a valve opening time of fuel injection based on a signal from the ECU 5.

On the other hand, immediately downstream of the throttle valve 3, a pipe 7
An absolute pressure signal (PBA) sensor 8 is provided through the intake pipe, and an absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. Further, an intake air temperature (TA) sensor 9 is attached downstream thereof.
Detects intake air temperature TA and outputs the corresponding electrical signal for EC
Supply to U5.

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

The three-way catalyst 14 is disposed in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO and NOx in the exhaust gas. An oxygen concentration sensor (hereinafter, referred to as an “LAF sensor”) 15 as an exhaust concentration sensor is an exhaust pipe 13.
Is mounted 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.
Supply to U5.

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 CPU 5b includes a storage unit 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, and the like.

The CPU 5b determines various engine operating states such as a feedback control operating area and an open loop control operating area corresponding to the oxygen concentration in the exhaust gas based on the various engine parameter signals described above, and sets the engine operating state. Accordingly, based on the following equation (1), the TDC
Fuel injection time To of the fuel injection valve 6 synchronized with the signal pulse
ut is calculated.

Tout = Ti × KCMDM × KLAF × K ₁ + K ₂ (1) Here, Ti is a basic fuel amount, specifically, a basic fuel amount determined according to the engine speed NE and the intake pipe absolute pressure PBA. This is the fuel injection time, and T is used to determine this Ti value.
The i-map is stored in the storage unit 5c.

KCMDM is a corrected target air-fuel ratio coefficient, which is set in accordance with the operating state of the engine. The target air-fuel ratio coefficient KCMD representing the target air-fuel ratio is replaced with a fuel cooling correction coefficient KE.
It is calculated by multiplying TV. Correction coefficient KE
TV is a coefficient for correcting the fuel injection amount in advance in consideration of a change in the supplied air-fuel ratio due to a cooling effect by actually injecting the fuel, and is a target air-fuel ratio coefficient KCM.
It is set according to the value of D. As is apparent from the above equation (1), if the target air-fuel ratio coefficient KCMD increases, the fuel injection time Tout increases.
The CMDM value is a value proportional to the reciprocal of the so-called air-fuel ratio A / F.

KLAF is an air-fuel ratio correction coefficient calculated by a program shown in FIG. 2, which will be described later. During the air-fuel ratio feedback control, KLAF is set so that the air-fuel ratio detected by the LAF sensor 15 matches the target air-fuel ratio. During the open loop control, it is set to a predetermined value according to the engine operating state.

K ₁ and K ₂ are other correction coefficients and correction variables calculated in accordance with various engine parameter signals, respectively, for optimizing various characteristics such as fuel consumption characteristics and engine acceleration characteristics according to the engine operating state. It is set to the value as intended.

The CPU 5b outputs a signal for driving the fuel injection valve 6 via the output circuit 5d based on the result calculated as described above.

FIG. 2 is a flowchart of a program for calculating the air-fuel ratio correction coefficient KLAF. This program is executed in synchronism with the generation of each TDC signal.

In step S21, a delay TDC variable PTDC indicating the exhaust gas arrival delay by the number of occurrences of the TDC signal.
From the PTDC table set according to the intake pipe absolute pressure PBA. The PTDC table is set by paying attention to the fact that the delay until the air-fuel mixture supplied to the engine reaches the LAF sensor 15 changes according to the intake pipe pressure. The higher the intake pipe absolute pressure PBA, the smaller the delay. Is set to a value.

In step S22, the flag FLAFFB set to the value 1 during execution of the air-fuel ratio feedback control is set to T.
It is determined whether the value was 1 when the DC signal was generated last time (when this program was last executed), and the answer is affirmative (YES).
In the case of, the process immediately proceeds to step S24, and negative (NO)
In the case of, the previously calculated value DKAF (n− ₁ ) of the deviation between the equivalent ratio indicating the air-fuel ratio detected by the LAF sensor 15 (hereinafter simply referred to as “detected air-fuel ratio”) and the target air-fuel ratio KCMD.
To the value 0, and the target air-fuel ratio coefficient KCM before P times
D (np) is set to the current value KACT (n) of the detected air-fuel ratio (step S23), and the process proceeds to step S24. Here, "P" is equal to the PTDC value calculated in step S21.

In step S24, the present value KACT (n) of the detected air-fuel ratio is subtracted from the target air-fuel ratio coefficient KCMD (np) P times before, thereby obtaining the current value DKAF (n) of the deviation.
Is calculated. When KCMD (np) = KACT (n) when this step is reached via step S23,
DKAF (n) = 0.

In step S25, the thinning TDC variable N
It is determined whether or not ITDC has a value of 0. If the answer is negative (NO), NITDC is decremented by 1 (step S26), and the program ends.
The thinning-out TDC variable NITDC is a variable for updating the air-fuel ratio correction coefficient KLAF every time the TDC signal is generated by the thinning-out number NI set according to the engine operating state, and the answer to step S25 is affirmative (YES). Ie, NI
If TDC = 0, the process proceeds to step S27 and the
The LAF value is updated.

In step S27, a proportional term (P term) coefficient KP, an integral term (I term) coefficient KI, and a differential term corresponding to the feedback gain are obtained by the programs shown in FIGS. 3A to 3C.
(D term) The coefficient KD and the thinning number NI are calculated.

In step S41 of FIG. 3A, it is determined whether or not the engine is in an idle state, and the answer is affirmative (YE
In the case of S), the thinning number NI and the coefficients KI, KP, K
Each value of D is a predetermined value NIIDL, K for idle.
IIDL, KPIDL, and KDIDL (for example, 4,0.063,0,0, respectively) are set (step S42), and the process proceeds to step S75 (FIG. 3C). When the answer to step S41 is negative (NO), that is, when the engine is not in the idle state, it is determined whether or not it is immediately after the fuel cut (step S43). Here, "immediately after the fuel cut" means a period from the end of the fuel cut to the lapse of a predetermined number of TDCs. If the answer to step S43 is affirmative (YES), that is, immediately after the fuel cut, the NI, KI, KP, and KD values are set. Are the predetermined values NIAFC, KI immediately after fuel cut, respectively.
AFC, KPAFC, KDAFC (for example, 2,
0.6, 1.2, 0.8) (step S44), and the flow proceeds to step S75.

Here, the decimation number N immediately after the fuel cut is performed.
IAFC is set to a value smaller than the value after the lapse of the predetermined period, and the PID term coefficients KPAFC, KIAFC, KDA
FC is set to a larger value. This takes into account that the feedback control is stopped with the air-fuel ratio correction coefficient KLAF being a constant value during the fuel cut, and with the above setting, the detected air-fuel ratio and the target air-fuel ratio immediately after the end of the fuel cut are set. The speed of correction of the supply air-fuel ratio according to the deviation DKAF is increased, and the supply air-fuel ratio can quickly follow the target air-fuel ratio. As a result, it is possible to prevent deterioration of exhaust gas characteristics and operability immediately after the end of the fuel cut.

If the answer to step S43 is negative (NO),
That is, if it is not immediately after the fuel cut, it is determined whether or not the acceleration is increasing (the fuel is increasing when the engine is accelerating) (step S45). When the answer is affirmative (YES), a counter NCACC for measuring a predetermined period after the end of the acceleration increase is set to a predetermined value NCDLY (for example, 10).
Is set (step S46), and it is determined whether the detected air-fuel ratio KACT (current value) is smaller than the target air-fuel ratio coefficient KCMD (current value) (lean side) (step S48). If the answer is affirmative (YES), that is, if KACT <KCMD, the process immediately proceeds to step S50, while negating (N
O), that is, when KACT ≧ KCMD, it is determined whether or not the KACT value is smaller than a value obtained by adding a predetermined value DKCMACC to the KCMD value (step S49). If the answer to step S49 is negative (NO), that is, KACT ≧ KCM
In the case of D + DKCMACC, the process proceeds to step S50,
Yes (YES), that is, KACT <KCMD + DKCMA
In the case of CC, the process proceeds to step S51.

In step S50, NI, KI, KP,
Each value of KD is a first predetermined value NI for increasing the acceleration.
ACCH, KIACCH, KPACCH, KDACCH
(E.g., 3, 0.1, 0.2, 0.1, respectively). In step S51, NI, KI, KP, and KD are respectively set to second predetermined values NIACCL and KIAC for increasing the acceleration.
CL, KPACCL, and KDACCL (for example, 4, 0.05, 0.1, and 0, respectively). After execution of step S50 or S51, the process proceeds to step S75. Here, the first and second predetermined values are respectively NIACCH <NIA
CCL, KIACCH> KIACCL, KPACCH>
KPACCL, KDACCH> KDACCL, and KACT <KCMD or KACT ≧ KCMD + D
When KCMACC is established and the first predetermined value is applied, the correction speed of the supply air-fuel ratio by the feedback control is set to be larger.

Thus, the ability of the actual air-fuel ratio to follow the target air-fuel ratio during acceleration is improved, and the acceleration performance can be improved. That is, in the present embodiment, the detected air-fuel ratio (= actual air-fuel ratio) becomes a value richer than the target air-fuel ratio due to the acceleration fuel increase at the beginning of acceleration, but thereafter, the actual air-fuel ratio decreases as the acceleration increase amount decreases. Rapidly changes in the lean direction to become leaner than the target air-fuel ratio, and this change in the actual air-fuel ratio in the lean direction is caused by the fact that the actual air-fuel ratio greatly deviates from the target air-fuel ratio to the rich side at the beginning of acceleration. When KACT <KCMD or KACT ≧ KCMD + DKCMACC is satisfied, the correction speed by the feedback control is set to be larger than when KCMD <KACT ≦ KCMD + DKCMACC is satisfied. The engine speed does not increase or decreases against the intention of acceleration (Hesitation, It is possible to avoid a tumble).

If the answer to step S45 is negative (NO),
That is, when the acceleration increase is not being performed, the counter NCAC is used.
The count value of C (the counter NCACC is decremented by 1 in step S77 to be described later) is 0
Is determined (step S47), and if the answer is negative (NO) and no TDC signal pulse is generated by the predetermined number NCDLY after the end of the acceleration increase, the process proceeds to step S48, and it is determined that the acceleration is being increased. Performs the same processing. This is in consideration of the fact that the engine speed usually rises immediately after the end of the acceleration increase and the hesitation or stumbling may occur, and is taken into account for a predetermined period (the predetermined number of TDC signals) after the end of the acceleration increase. In the period during which a pulse is generated), such a problem can be avoided by performing the same processing as during the acceleration increase.

If the answer in step S47 is affirmative (YE
S), that is, when NCACC = 0, in steps S52 to S74 in FIG. 3B, according to the engine speed NE and the intake pipe absolute pressure PBA, as shown in FIG. 4, the thinning number NI and the PID term coefficients KP, KI , KD are set. That is, the detected engine speed NE and the predetermined engine speeds NENI1 to NENI3 (for example, 1000, 2500,
4000rpm) and the detected absolute pressure PB in the intake pipe
A and predetermined pressures PBNI1 and PBNI2 (for example, 36
0,560 mmHg) is set as follows in accordance with the magnitude relationship with (0,560 mmHg). In the present embodiment, these magnitude relationships are determined (steps S52, S53, S55, S58, S5 in FIG. 3).
9, S61, S64, S65, S67, S70, S7
Hysteresis is added to 2).

(1) When NE ≦ NENI1 is satisfied (1-1) If PBA <PBNI1, NI = NI11
(For example, 4), KI = KI11 (for example, 0.25), KP =
KP11 (for example, 0) and KD = KD11 (for example, 0).

(1-2) PBNI1 ≦ PBA <PBNI2
Then, NI = NI12 (for example, 4), KI = KI1
2 (eg, 0.6), KP = KP12 (eg, 1.2), KD
= KD12 (for example, 0.8).

(1-3) If PBNI2 ≦ PBA, N
I = NI13 (for example, 2), KI = KI13 (for example, 0.
6), KP = KP13 (for example, 0.95), KD = KD13
(For example, 0.25).

(2) When NENI1 <NE ≦ NENI2 is satisfied (2-1) If PBA <PBNI1, NI = NI21
(For example, 4), KI = KI21 (for example, 0.3), KP =
KP21 (for example, 1.15), KD = KD21 (for example, 0.4)
And

(2-2) PBNI1 ≦ PBA <PBNI2
Then, NI = NI22 (for example, 2), KI = KI2
2 (eg, 0.3), KP = KP22 (eg, 1.05), KD
= KD22 (for example, 0.4).

(2-3) If PBNI2 ≦ PBA, N
I = NI23 (for example, 2), KI = KI23 (for example, 0.
35), KP = KP23 (for example, 0.95), KD = KD23
(For example, 0.25).

(3) When NENI2 <NE ≦ NENI3 is satisfied (3-1) If PBA <PBNI1, NI = NI31
(For example, 4), KI = KI31 (for example, 0.3), KP =
KP31 (for example 1.1), KD = KD31 (for example 0.4)
And

(3-2) PBNI1 ≦ PBA <PBNI2
Then, NI = NI32 (for example, 2), KI = KI3
2 (for example, 0.35), KP = KP32 (for example, 0.95), K
It is assumed that D = KD32 (for example, 0.4).

(3-3) If PBNI2 ≦ PBA, N
I = NI33 (for example, 2), KI = KI33 (for example, 0.
4), KP = KP33 (for example, 0.85), KD = KD33
(For example, 0.3).

(4) When NE> NENI3 is satisfied (4-1) If PBA <PBNI1, then NI = NI41
(For example, 2), KI = KI41 (for example, 0.35), KP =
KP41 (for example, 1.05), KD = KD41 (for example, 0.4)
And

(4-2) PBNI1 ≦ PBA <PBNI2
Then, NI = NI42 (for example, 2), KI = KI4
2 (eg, 0.35), KP = KP42 (eg, 0.9), KD
= KD42 (for example, 0.4).

(4-3) If PBNI2 ≦ PBA, N
I = NI43 (for example, 2), KI = KI43 (for example, 0.
4), KP = KP43 (for example, 0.8), KD = KD43
(For example, 0.35).

In step S75, the target air-fuel ratio coefficient KC
MD is a lean predetermined value KCMDZML near the stoichiometric air-fuel ratio
(For example, A / F = 15.0) and the rich-side predetermined value KC
MDZMH (for example, a value corresponding to A / F = 14.3) is determined, and the answer is affirmative (YES), that is, KC
When MDZML <KCMD <KCMDZMH holds, the NI value is multiplied by α (α <1 and, for example, 0.5), and the respective values of KI, KP, and KD are β times (β> 1). , For example, 1.5), and then proceeds to step S77. If the answer to step S75 is negative (NO), that is, KCMD ≧ KCMDZMH or KCMD ≦ KC
When MDZML is established, step S77 is immediately performed.
Then, the count value of the counter NCACC is decremented by 1 and the program ends.

When the target air-fuel ratio is set to a value close to the stoichiometric air-fuel ratio by the processing in steps S75 and S76,
When the correction speed of the feedback control is set to a larger value than at other times, that is, when the proportional term coefficient KP, the integral term coefficient KI, and the derivative term coefficient KD corresponding to the feedback gain are set to larger values, the air-fuel ratio Since the decimation number NI corresponding to the update cycle of the correction coefficient KLAF is set to a smaller value, when the target air-fuel ratio is changed from a value other than near the stoichiometric air-fuel ratio to a value near the stoichiometric air-fuel ratio, the supply air-fuel ratio becomes smaller. Quickly converges to a value near the stoichiometric air-fuel ratio,
The duration of the transient state can be reduced. As a result, CO, HC or NO in such transient conditions
x emission amount can be reduced and exhaust gas characteristics can be improved. Further, in the present embodiment, since a so-called proportional output type oxygen sensor is used, the air-fuel ratio near the stoichiometric air-fuel ratio can be accurately detected, and the purification rate of the three-way catalyst can be further improved. .

Returning to FIG. 2, in step S28, it is determined whether or not the absolute value of the current value DKAF (n) of the deviation calculated in step S24 is larger than a predetermined value DKPID, and the answer is negative (NO), that is, When | DKAF (n) | ≦ DKPID, the process immediately proceeds to step S30. On the other hand, when the answer is affirmative (YES), that is, when | DKAF (n) |> DKPID, the previous value DKAF (n− ₁ ) of the deviation and the current Value DKA
F (n) is set to a value of 0 (step S29), and the process proceeds to step S30. In step S30, the following equations (2) to
According to (4), P-term KLAFP, I-term KLAFI and D
The term KLAFD is calculated.

KLAF = DKAF (n) × KP (2) KLAFI = KLAFI + DKAF (n) × KI (3) KLAFD = (DKAF (n) −DKAF (n− ₁ )) × KD (4) Therefore, when the answer to the step S28 is affirmative (YES) and | DKAF (n) |> DKPID is satisfied, both DKAF (n) and DKAF (n- ₁ ) are set to the value 0. KLAFP = KLAFD = 0, KLAFI =
KLAFI. That is, the feedback control by the P term and the D term is stopped, and the I term is maintained at the previous value.

Thus, when the detected air-fuel ratio KACT fluctuates greatly at the beginning of acceleration or at the time of misfire, | DKAF (n) |> DKPID, and the feedback control by the P and D terms While being suspended,
Since the I term is held at the previous value, it is possible to prevent the supply air-fuel ratio from fluctuating greatly and the drivability or the exhaust gas characteristics from deteriorating.

In steps S31 to S34, the I term KLAF
Perform I limit check. That is, the magnitude relationship between the KLAFI value and the predetermined upper and lower limit values LAFIH and LAFIL is compared (steps S31 and S32). If the KLAFI term exceeds the upper limit value LAFIH, the upper limit value is set (step S33). If it is smaller than LAFI, the lower limit is set (step S34).

In step S35, the PID term KLAF
The air-fuel ratio correction coefficient KLAF is calculated by adding P, KLAFI, and KLAFD, and the current calculated value DKAF _(N) of the deviation is set to the previous value DKAF _(N-1) (step S
36), and further reduce the thinning variable NITDC in the step S
It is set to the thinning number NI calculated in 27 (step S3
7) End this program.

[0052]

As described above in detail, according to the present invention, when the target air-fuel ratio is set near the stoichiometric air-fuel ratio,
The feedback gain used for calculating the air-fuel ratio control amount is set to a larger value than when the target air-fuel ratio is set to a value other than near the stoichiometric air-fuel ratio.
Since the update cycle is shorter than when the target air-fuel ratio is set to a value other than near the stoichiometric air-fuel ratio, the supply air is not changed when the target air-fuel ratio is changed from a value other than near the stoichiometric air-fuel ratio to a value near the stoichiometric air-fuel ratio. The fuel ratio quickly converges to a value near the stoichiometric air-fuel ratio, and the duration of the transient state can be reduced. As a result, CO, HC or N in such transient conditions
Ox emissions can be reduced and exhaust gas characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a fuel supply control device to which a control method of the present invention is applied.

FIG. 2A is a flowchart of a program for calculating an air-fuel ratio correction coefficient (KLAF).

FIG. 2B is a flowchart of a program for calculating an air-fuel ratio correction coefficient (KLAF).

FIG. 3A is a flowchart of a program for setting a thinning number (NI) and a gain (KI, KP, KD) for feedback control.

FIG. 3B is a flowchart of a program for setting a thinning number (NI) and a gain (KI, KP, KD) for feedback control.

FIG. 3C is a flowchart of a program for setting a thinning number (NI) and a feedback control gain (KI, KP, KD).

FIG. 4 is a diagram showing a setting result by the program of FIG. 3;

[Explanation of symbols]

 Reference Signs List 1 internal combustion engine 5 electronic control unit 6 fuel injection valve 15 exhaust concentration sensor (oxygen concentration sensor)

Continuation of the front page (56) References JP-A-61-106941 (JP, A) JP-A-2-211348 (JP, A) JP-A-61-197737 (JP, A) (58) Fields investigated (Int) .Cl. ⁶ , DB name) F02D 41/14 310

Claims (1)

(57) [Claims] An air-fuel ratio control amount is calculated based on an output of 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, and the air-fuel ratio control amount is used. In an air-fuel ratio control method for an internal combustion engine, wherein an air-fuel ratio of an air-fuel mixture supplied to an engine is feedback-controlled to a target air-fuel ratio, the calculation of the air-fuel ratio control amount is performed at a predetermined cycle, and the target air-fuel ratio is set near a stoichiometric air-fuel ratio. when setting the feedback gain used in the calculation of the air-fuel ratio control amount, as well as set to a value greater than when setting the target air-fuel ratio other than the stoichiometric air-fuel ratio near the air
An air-fuel ratio control method for an internal combustion engine , wherein an update cycle of the fuel-ratio control amount is shorter than when the target air-fuel ratio is set to a value other than near the stoichiometric air-fuel ratio.