JP3023614B2 - Air-fuel ratio control device 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 device for an internal combustion engine, and more particularly, to an air-fuel ratio sensor provided upstream and downstream of an exhaust purification catalyst. The present invention relates to a device for performing feedback control of an air-fuel ratio with high accuracy based on an output value of a sensor.

<Prior Art> A conventional general air-fuel ratio control device for an internal combustion engine is disclosed in, for example, Japanese Patent Application Laid-Open No. 60-240840.

To explain the outline of this, the intake air flow rate Q of the engine
And the basic fuel injection amount Tp corresponding to the amount of air taken into the cylinder by detecting the engine speed N (= K · Q / N; K is a constant)
The basic fuel injection amount Tp corrected by the engine cooling water temperature or the like is set by a signal from an air-fuel ratio sensor (oxygen sensor) that detects the air-fuel ratio of the intake air-fuel mixture by detecting the oxygen concentration in the exhaust gas. The feedback correction is performed using the air-fuel ratio feedback correction coefficient, and the correction based on the battery voltage is also performed to finally set the fuel injection amount Ti.

Then, a predetermined amount of fuel is injected and supplied to the engine by outputting a drive pulse signal having a pulse width corresponding to the fuel injection amount Ti thus set to the fuel injection valve at a predetermined timing.

The air-fuel ratio feedback correction based on the signal from the air-fuel ratio sensor is performed so that the air-fuel ratio is controlled near the stoichiometric air-fuel ratio which is the target air-fuel ratio. It is interposed in the exhaust system, CO in the exhaust gas, when the purification efficiency of the exhaust gas purifying catalyst for purifying by reducing NO _X (three-way catalyst) (conversion efficiency) the stoichiometric air-fuel ratio combustion with oxidizes HC This is because it is set to function effectively in the exhaust state.

The electromotive force (output voltage) generated by the air-fuel ratio sensor has a characteristic that changes abruptly in the vicinity of the stoichiometric air-fuel ratio. By comparing this output voltage with a slice level voltage corresponding to the stoichiometric air-fuel ratio, the stoichiometric air-fuel ratio is determined. It is determined whether the fuel ratio is rich or lean. For example, when the air-fuel ratio is lean (rich), the air-fuel ratio feedback correction coefficient α multiplied by the basic fuel injection amount Tp is increased (decreased) by a large proportional amount P in the first time when the air-fuel ratio is changed to lean (rich). The air-fuel ratio is controlled to be close to the stoichiometric air-fuel ratio by gradually increasing (decreasing) by a predetermined integral I, thereby correcting (increase) the fuel injection amount Ti.

By the way, in the ordinary air-fuel ratio control device as described above,
One air-fuel ratio sensor is provided in the collection part of the exhaust manifold as close as possible to the combustion chamber in order to increase the responsiveness. However, since this part has a high exhaust temperature, the characteristics of the air-fuel ratio sensor are affected by thermal effects and deterioration. It is difficult to detect the average air-fuel ratio of all the cylinders that are easily changed and the exhaust gas of each cylinder is insufficiently mixed, and the detection accuracy of the air-fuel ratio is difficult, and the air-fuel ratio control accuracy is poor.

In view of this point, there has been proposed an air-fuel ratio sensor provided downstream of the exhaust gas purification catalyst and performing feedback control of the air-fuel ratio using the output values of the two air-fuel ratio sensors (Japanese Patent Application Laid-Open No. 58-48756). Gazette).

That is, the air-fuel ratio sensor on the downstream side is difficult to respond because it is far from the combustion chamber, but because it is downstream of the exhaust gas purification catalyst, the characteristic fluctuation due to the variation of the exhaust components (CO, HC, NO _X ) And the amount of poisoning by the toxic components in the exhaust gas is small, so it is hard to be affected by characteristic changes due to the poisoning. In addition, since the exhaust gas mixture is good, the average air-fuel ratio of all cylinders can be detected. As compared with the fuel ratio sensor, a highly accurate and stable detection performance can be obtained.

Therefore, the two air-fuel ratio feedback correction coefficients set by the same calculation based on the output values of the two air-fuel ratio sensors are combined, or the air-fuel ratio feedback correction coefficient set by the upstream air-fuel ratio sensor. By correcting the control constants (proportional component and integral component), the slice level voltage of the output voltage of the upstream air-fuel ratio sensor and the delay time, etc., the variation in the output characteristics of the upstream air-fuel ratio sensor can be used to reduce the downstream air-fuel ratio. High-precision air-fuel ratio feedback control is performed by compensation using a sensor.

<Problems to be Solved by the Invention> However, as described above, the first air-fuel ratio correction amount is calculated according to the output value of the air-fuel ratio sensor on the upstream side of the exhaust purification catalyst, and the air-fuel ratio on the downstream side is calculated. A second air-fuel ratio correction amount is calculated according to the output value of the sensor, and a final air-fuel ratio correction amount is calculated based on the first air-fuel ratio correction amount and the second air-fuel ratio correction amount. In the fuel-fuel ratio feedback control, the first air-fuel ratio correction amount and the second air-fuel ratio correction amount are fixed under the non-air-fuel ratio feedback control condition. If the calculation of the first air-fuel ratio correction amount and the second air-fuel ratio correction amount (especially, the second air-fuel ratio correction amount) is immediately restarted when the condition is shifted to the feedback control condition, the following problem occurs.

In the non-fuel ratio feedback control condition, the secondary air introduction or deceleration leaning clamping is performed, the exhaust gas of the catalyst inlet is lean, but O ₂ adsorption amount of the catalyst is increased,
Thereafter, when the process shifts to the air-fuel ratio feedback control, until the O ₂ adsorption amount of the catalyst is stabilized, the O ₂ discharge of the catalyst causes the air-fuel ratio sensor on the downstream side of the catalyst to perform a lean determination, thereby erroneously detecting. If the air-fuel ratio is corrected based on this, the air-fuel ratio will be erroneously controlled in the rich direction, rather deteriorating the exhaust emission characteristics and the operability.

To deal with this, after shifting to air-fuel ratio feedback control,
It is conceivable that the calculation of the second air-fuel ratio correction amount based on the air-fuel ratio sensor on the downstream side of the catalyst is prohibited for a certain period of time, and the second air-fuel ratio correction amount is fixed.

However, the time O ₂ adsorption amount of the catalyst is stabilized, the catalyst time and exposed to the lean gas, varies greatly by the deterioration degree or the like of the catalyst (for example, up to O ₂ adsorption amount is stabilized becomes small when degraded Therefore, if the calculation is prohibited for a certain period of time, the convergence of the control is poor, and the emission varies.

In addition, the secondary air is introduced when the air-fuel ratio is clamped to the rich side in order to enhance the stability during idling.
In response to the increase of the C component, the process is performed between the catalyst and the air-fuel ratio sensor on the upstream side of the catalyst to promote the CO and HC purification function of the catalyst.

The present invention has been made in view of the above-described conventional problems, and has been made in consideration of the air-fuel ratio lean state (2
From the secondary air introduction or the deceleration lean clamp) to the air-fuel ratio feedback control condition.

<Means for Solving the Problems> For this reason, as shown in FIG. 1, the present invention is provided on the upstream side and the downstream side of an exhaust purification catalyst provided in an exhaust passage of an engine, respectively. First and second air-fuel ratio sensors whose output values change in response to the concentration of the specific gas component in the exhaust gas, which changes according to the output value of the first air-fuel ratio sensor under air-fuel ratio feedback control conditions A first air-fuel ratio correction amount calculating means for calculating a first air-fuel ratio correction amount, and a second air-fuel ratio correction amount according to an output value of the second air-fuel ratio sensor under air-fuel ratio feedback control conditions. A second air-fuel ratio correction amount calculating unit that calculates the final air-fuel ratio correction amount based on the first air-fuel ratio correction amount and the second air-fuel ratio correction amount; The fuel supply amount to the engine is provided by the air-fuel ratio correction amount. In the air-fuel ratio control device for an internal combustion engine that controls the air-fuel ratio, when the air-fuel ratio lean state in the non-air-fuel ratio feedback control condition shifts to the air-fuel ratio feedback control condition, the second The operation of the second air-fuel ratio correction amount calculating means is continued until the output value of the air-fuel ratio sensor indicates a rich state or until a predetermined time has elapsed, whichever comes first. And a second air-fuel ratio correction amount fixing means for fixing the second air-fuel ratio correction amount is provided.

<Operation> Under the air-fuel ratio feedback control condition, the first air-fuel ratio correction amount calculating means calculates the first air-fuel ratio correction amount based on the output value of the first air-fuel ratio sensor, and calculates the second air-fuel ratio correction amount. The fuel ratio correction amount calculating means calculates a second air-fuel ratio correction amount based on an output value from the second air-fuel ratio sensor. Then, the air-fuel ratio correction amount calculating means calculates a final air-fuel ratio correction amount based on the first air-fuel ratio correction amount and the second air-fuel ratio correction amount.

Under the non-air-fuel ratio feedback control condition, the first air-fuel ratio correction amount and the second air-fuel ratio correction amount are both fixed, and the air-fuel ratio feedback control is stopped.

Then, when the air-fuel ratio lean state under the non-air-fuel ratio feedback control condition (secondary air introduction or deceleration lean clamp, etc.) is shifted to the air-fuel ratio feedback control condition, the operation of the first air-fuel ratio correction amount calculating means is restarted. However, the operation of the second air-fuel ratio correction amount calculating means is prohibited by the second air-fuel ratio correction amount fixing means, and the second air-fuel ratio correction amount is fixed. As a result, the air-fuel ratio correction based on the second air-fuel ratio sensor on the downstream side of the catalyst is prohibited, so that the air-fuel ratio in the rich direction is detected by the air-fuel ratio lean detection by O ₂ stored when secondary air is introduced into the catalyst. Erroneous control can be prevented.

Thereafter, when the output value of the second air-fuel ratio sensor indicates a rich state, it is known that the discharge of O ₂ adsorbed on the catalyst has already been completed, so the second air-fuel ratio correction is performed until this time. The operation of the quantity calculation means is prohibited,
Thereafter, by restarting the operation of the second air-fuel ratio correction amount calculating means, good control becomes possible.

Further, when the air-fuel ratio feedback control condition is shifted from the air-fuel ratio lean state under the non-air-fuel ratio feedback control condition to the air-fuel ratio feedback control condition, when a predetermined time has elapsed since the shift, the output of the second air-fuel ratio sensor is output. Irrespective of, since the fixation of the second air-fuel ratio correction amount has been released,
For some reason, even after the catalyst O ₂ adsorption amount is stabilized, if the output of the second air-fuel ratio sensor is lean, the operation of the second air-fuel ratio correction amount calculating means is quickly restarted, Good control becomes possible.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings.

2, an intake passage 12 of an engine 11 is provided with an air flow meter 13 for detecting an intake air flow rate Q and a throttle valve 14 for controlling the intake air flow rate Q in conjunction with an accelerator pedal, and a downstream manifold portion. An electromagnetic fuel injection valve 15 is provided for each cylinder.

The fuel injection valve 15 is driven to open by a drive pulse signal from a control unit 16 containing a microcomputer, and injects fuel supplied from a fuel pump (not shown) under pressure and controlled to a predetermined pressure by a pressure regulator.

The exhaust passage 17 is provided with a first air-fuel ratio sensor 18 for detecting the air-fuel ratio of the intake air-fuel mixture by detecting the oxygen concentration in the exhaust gas at the manifold collecting portion. exhaust purifying catalyst performs the reduction of oxidized and NO _X (three-way catalyst) 19 is provided, the second air-fuel ratio further having the same function as the first air-fuel ratio sensor 18 on the downstream side of the catalyst 19 A sensor 20 is provided.

Further, when the air-fuel ratio is crapped to the rich side under predetermined idling operation conditions to ensure stability, secondary air is introduced into the exhaust between the first air-fuel ratio sensor 18 and the catalyst 19 in order to introduce secondary air. A secondary air introduction device (EAI) 21 is provided.

Further, a crank angle sensor 22 is provided, and by counting the crank unit angle signal output from the crank angle sensor 22 in synchronization with the engine rotation for a certain period of time, or by measuring the cycle of the crank reference angle signal. , The engine speed N can be detected.

Further, a water temperature sensor 23 is provided to detect a cooling water temperature Tw in the water jacket of the engine 11.

Next, control of the air-fuel ratio by the control unit 16 via the fuel injection valve 15 will be described with reference to the flowcharts of FIGS.

FIG. 3 shows a fuel injection amount setting routine, which is executed at predetermined time intervals or in synchronization with engine rotation.

In step 1 (denoted as S1 in the figure, the same applies hereinafter), the intake air flow rate Q detected by the signal from the air flow meter 13 and the engine speed N calculated by the signal from the crank angle sensor 22 are used. The basic fuel injection amount Tp corresponding to the intake air amount per unit rotation is calculated by the following equation.

Tp = K · Q / N (K is a constant) In step 2, various correction coefficients COEF are calculated based on the cooling water temperature Tw detected by the signal from the water temperature sensor 23, and the like.
Set.

In step 3, an air-fuel ratio feedback correction coefficient α which is an air-fuel ratio correction amount (first air-fuel ratio correction amount) set by an air-fuel ratio feedback correction coefficient setting routine of FIG.

In step 4, the voltage correction is performed based on the battery voltage.
Set Ts. This is for correcting a change in the injection flow rate of the fuel injection valve 15 due to the battery voltage fluctuation.

In step 5, the final fuel injection amount (fuel supply amount)
Calculate Ti according to the following equation.

Ti = Tp · COEF · α + Ts In step 6, the calculated fuel injection amount Ti is set in the output register.

Accordingly, at a predetermined fuel injection timing synchronized with the engine rotation, a drive pulse signal having a pulse width of the fuel injection amount Ti is given to the fuel injection valve 15 to perform fuel injection.

FIG. 4 shows an air-fuel ratio feedback correction coefficient setting routine, which is executed at predetermined time intervals.

In step 11, it is determined whether or not the condition is the air-fuel ratio feedback control.

If NO (non-air-fuel ratio feedback control condition), this routine is terminated. As a result, the air-fuel ratio feedback correction coefficient α is fixed to the final value under the air-fuel ratio feedback control condition.

Therefore, under the rich clamp condition and the lean clamp condition, the air-fuel ratio feedback correction coefficient α is fixed, the air-fuel ratio feedback control is stopped, and a desired air-fuel ratio is obtained by setting the various correction coefficients COEF. Since the secondary air introduction by the secondary air introduction device 21 is performed under the idle-rich clamp condition,
The air-fuel ratio feedback correction coefficient α is also fixed during the introduction of the secondary air.

In step 12, the output voltage V _{1 of} the first air-fuel ratio sensor 18
And the output voltage V2 of the _second air-fuel ratio sensor 20 is read.

In step 13, the output voltage V _{1 of} the first air-fuel ratio sensor 18
Is compared with the slice level voltage SL. If V ₁ <SL, it is determined to be lean, and if V ₂ > SL, it is determined to be rich. If lean, the flag F1 is set to 0 in step 14; To set the flag F1 to 1.

In step 16, the output voltage V _{2 of} the second air-fuel ratio sensor 20
Is compared with the slice level voltage SL, and if V ₂ <SL, it is determined to be lean; if V ₂ > SL, rich is determined. If lean, the flag F2 is set to 0 in step 17; To set the flag F2 to 1.

In step 19, it is determined whether the flag F1 has been inverted.

If YES (at the time of inversion from rich to lean or from lean to rich), the process proceeds to step 20, where it is determined whether or not the prohibition flag is set.

If the prohibition flag is not set, step 21
23, the process proceeds to step 24, and a proportional correction amount PHOS, which is a second air-fuel ratio correction amount, is calculated.

That is, in step 24, the value of the flag F2 is determined,
When F2 = 0 (when the air-fuel ratio detected from the exhaust component downstream of the catalyst 19 is lean), the process proceeds to step 25, where the current proportional component correction amount PHOS (initial value is 0) is a predetermined amount ΔPHOS
(> 0) is added to update the proportional correction amount PHOS to the increasing side. If F2 = 1 (if the air-fuel ratio detected from the exhaust component downstream of the catalyst 19 is rich), step 26 is executed. Then, the predetermined amount ΔPHOS is subtracted from the current proportional correction amount PHOS, and the proportional correction amount PHOS is updated to a decreasing side.

Next, in step 27, the value of the flag F1 is determined, and F1 =
If the value is 0 (if the air-fuel ratio detected from the exhaust gas component downstream of the catalyst 19 is lean), the process proceeds to step 28, where the current air-fuel ratio feedback correction coefficient α is corrected by a predetermined proportional amount P and a proportional amount. PHOS is added to update the air-fuel ratio feedback correction coefficient α to the increasing side. If F1 = 1 (if the air-fuel ratio detected from the exhaust component upstream of the catalyst 19 is rich), go to step 29. Then, a predetermined proportional amount P is subtracted from the current air-fuel ratio feedback correction coefficient α, and the proportional amount correction amount PHOS is added to update the air-fuel ratio feedback correction coefficient α to a decreasing side. This α is used in the routine of FIG.

On the other hand, if it is determined in step 19 that the flag F1 is not being inverted, the process proceeds to step 30, where the value of the flag F1 is determined. Here, when F1 = 0 (when the air-fuel ratio detected from the exhaust component on the upstream side of the catalyst 19 is lean),
Proceed to 30, and the current air-fuel ratio feedback correction coefficient α
A predetermined integral I is added to update the air-fuel ratio feedback correction coefficient α to an increasing side, and when F1 = 1 (when the air-fuel ratio detected from the exhaust component upstream of the catalyst 19 is rich)
Goes to step 33, and subtracts a predetermined integral I from the current air-fuel ratio feedback correction coefficient α to update the air-fuel ratio feedback correction coefficient α to a decreasing side.

FIG. 6 shows the output voltages of the first and second air-fuel ratio sensors.
V ₁ , V ₂ , air-fuel ratio feedback correction coefficient α, proportional correction amount
An example of the behavior of PHOS is shown.

FIG. 5 is a prohibition flag setting routine of FIG. 5, which is executed at predetermined time intervals.

In step 41, it is determined whether or not the secondary air introduction device (EAI) 21 is operating (ON).

If it is ON, the EAI flag is set in step 42, the prohibition flag is set in step 43, and the timer (TIM) is reset in step 44.

Note that the secondary air introduction device 21 operates only when the idle
Since the conditions are the rich clamp condition and the non-air-fuel ratio feedback control condition, the routine of FIG. 4 has been completed at step 11 and the calculation of the air-fuel ratio feedback correction coefficient α (and the proportional correction amount PHOS) is performed at this time. Not done, fixed.

When the introduction of the secondary air by the secondary air introduction device 21 is stopped, the process proceeds from step 41 to step 45, and it is determined whether or not the EAI flag is set.

Here, the fact that the EAI flag is set means that the introduction of the secondary air has ended, and at the same time, that the air-fuel ratio feedback control condition has been satisfied.

Therefore, at this time, the process proceeds to step 46 to reset the EAI flag, and then the timer (TIM) is started in step 47.

When the air-fuel ratio lean control state (secondary air introduction state) is shifted to the air-fuel ratio feedback control condition under the non-air-fuel ratio feedback control condition in this way, the execution of the routine of FIG. Since the flag remains set, processing different from that described above is performed.

That is, when it is determined in step 19 that the flag F1 has been inverted and the process proceeds to step 20, the prohibition flag is set, and the process proceeds to step 21.

In step 21, the value of the timer (TIM) is compared with a predetermined value, and if it is less than the predetermined value (within a predetermined time), the process proceeds to step 22.

In step 22, the flag F2 = 1 (the second air-fuel ratio sensor
It is determined whether or not the output value of 20 is a rich state). If NO, the process proceeds to step 27 without executing steps 24 to 26.

That is, when the air-fuel ratio lean state (secondary air introduction state) shifts to the air-fuel ratio feedback control condition under the non-air-fuel ratio feedback control condition, the output of the second air-fuel ratio sensor 20 downstream of the catalyst 19 from that shift time. Until the value indicates a rich state, the calculation of the proportional correction amount PHOS, which is the second air-fuel ratio correction amount, in steps 24 to 26 is prohibited, and the proportional correction amount PHOS is fixed.

If the value of the timer (TIM) is equal to or greater than a predetermined value (elapse of a predetermined time) in step 21, or if the flag F2 = 1 in step 22,
If (the output value of the second air-fuel ratio sensor 20 becomes rich), the process proceeds to step 23, where the prohibition flag is reset, and then the process proceeds to steps 24-26. Therefore, the second
Is calculated for the proportional correction amount PHOS, which is the air-fuel ratio correction amount.

In this way, after the formation of the air-fuel ratio feedback control conditions as in Figure 7, the correction based on the output voltage V ₂ of the second air-fuel ratio sensor 20 of the catalyst 19 downstream from point of an A is resumed.

Here, in the present embodiment, steps 19 and 27 in FIG.
The portion excluding the correction by the proportional component correction amount PHOS among the portions 29 to 30 and 32 corresponds to the first air-fuel ratio correction amount calculating means, and the portions of steps 24 to 26 correspond to the second air-fuel ratio correction amount calculating means. 5 corresponds to the air-fuel ratio correction amount calculating means, and the steps of steps 20 to 23 and the routine of FIG. 2 corresponds to an air-fuel ratio correction amount fixing means.

In the above, the first air-fuel ratio sensor 18 upstream of the catalyst 19
Based on the air-fuel ratio feedback control based on the output value of
Is applied to the one that is substantially corrected based on the output value of the second air-fuel ratio sensor 20, but the air-fuel ratio feedback correction coefficient is set by each air-fuel ratio sensor, and both values are set. Using the air-fuel ratio feedback correction coefficient obtained by combining the air-fuel ratio and the air-fuel ratio feedback control by the first air-fuel ratio sensor, the slice level voltage SL and the output delay time of the rich / lean determination are changed to the second air-fuel ratio. The present invention can be applied to a device that corrects with an output value of a sensor.

<Effects of the Invention> As described above, according to the present invention, when the air-fuel ratio lean state under the non-air-fuel ratio feedback control condition (such as secondary air introduction or deceleration lean clamp) is shifted to the air-fuel ratio feedback control condition. time commensurate with the O ₂ adsorption state of the catalyst, and prohibits the air-fuel ratio correction based on the air-fuel ratio sensor on the downstream side of the catalyst, that the erroneous control can be prevented, yet which can be carried out at the optimal time The effect is obtained.

Further, when a predetermined time determined in advance has elapsed, regardless of the output of the air-fuel ratio sensor on the downstream side of the catalyst, because it cancels the prohibition, for some reason, even after the O ₂ adsorption amount of the catalyst is stabilized, When the output of the air-fuel ratio sensor on the downstream side of the catalyst is lean, good control can be achieved by promptly restarting the air-fuel ratio correction based on the air-fuel ratio sensor on the downstream side of the catalyst.

[Brief description of the drawings]

1 is a functional block diagram showing the configuration of the present invention, FIG. 2 is a system diagram showing one embodiment of the present invention, FIG. 3 is a flowchart of a fuel injection amount setting routine, and FIG. 4 is an air-fuel ratio feedback correction coefficient. FIG. 5 is a flowchart of a setting routine, FIG.
FIG. 7 and FIG. 7 are diagrams showing the control characteristics. 11 ... engine, 15 ... fuel injection valve, 16 ... control unit, 18 ... first air-fuel ratio sensor, 19 ... catalyst, 20 ...
... Second air-fuel ratio sensor, 21 ... Secondary air introduction device

Claims (1)

(57) [Claims] An output value is provided in response to a concentration of a specific gas component in exhaust gas which is provided on an upstream side and a downstream side of an exhaust gas purifying catalyst provided in an exhaust passage of an engine, and changes depending on an air-heat ratio. A first air-fuel ratio correction amount calculation for calculating a first air-fuel ratio correction amount according to an output value of the first air-fuel ratio sensor under the air-fuel ratio feedback control condition; Means, a second air-fuel ratio correction amount calculating means for calculating a second air-fuel ratio correction amount according to an output value of the second air-fuel ratio sensor under an air-fuel ratio feedback control condition; Air-fuel ratio correction amount calculating means for calculating a final air-fuel ratio correction amount based on the fuel ratio correction amount and the second air-fuel ratio correction amount, and correcting the fuel supply amount to the engine with the air-fuel ratio correction amount. Air-fuel ratio control of an internal combustion engine that controls the air-fuel ratio In the device, when shifting from the air-fuel ratio lean state in the non-air-fuel ratio feedback control condition to the air-fuel ratio feedback control condition, from the time of the shift, until the output value of the second air-fuel ratio sensor indicates a rich state, or The operation of the second air-fuel ratio correction amount calculating means is prohibited and the second air-fuel ratio correction amount is fixed during the earlier one of the predetermined time periods. An air-fuel ratio control device for an internal combustion engine, comprising a second air-fuel ratio correction amount fixing means.