JPH09317530A - Air-fuel ratio compensating control unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-fuel ratio compensating control in which emission is improved by providing compensated auxiliary air fuel ratio with initial value. SOLUTION: This control unit is provided with a catalyst (a) disposed in the exhaust path of an internal combustion engine, an upstream side oxigen sensor (b) disposed on the exhaust path so as to locate on the upstream side of the catalyst (a), a downstream side oxigen sensor (c) disposed on the exhaust path so as to locate on the downstream side of the catalyst (a), a main air fuel ratio compensating means (d) for computing compensated main air fuel ratio value according to the output V1 of the upstream side oxigen sensor (b), an auxiliary air fuel ratio compensating means (e) for computing compensated auxiliary air fuel ratio value according to the output V2 of the downstream side oxigen sensor (c), an air-fuel ratio control means (f) for controlling the air fuel ratio of the internal combustion engine according to the compemsated main air fuel ratio value and the compensated auxiliary air fuel ratio value, a catalyst deterioration detecting mains (g) for detecting the degree of deterioration of the catalyst, a memorizing means (h) for memorizing the detected degree of deterioration, an initial value computing means (i) for computing the initial value of the compensated auxiliary air fuel ratio value. Thereby, this initial value is utilized to control the air fuel ratio being based on the compensated auxiliary air fuel ratio value after the internal combustion engine is started.

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 device for an internal combustion engine, and more specifically, it is equipped with oxygen sensors on the upstream side and the downstream side of a catalyst, respectively, and in addition to main air-fuel ratio control based on the output of the upstream oxygen sensor The present invention relates to an air-fuel ratio control device for an internal combustion engine that performs auxiliary air-fuel ratio control based on the output of a downstream oxygen sensor.

[0002]

2. Description of the Related Art It is well known that the air-fuel ratio of an internal combustion engine is controlled based on the output of an oxygen sensor provided on the upstream side of a catalyst. There is a problem in improving the correction accuracy of the fuel ratio. In order to compensate for such variations in the output characteristics of the oxygen sensor and deterioration over time, a new oxygen sensor is installed on the downstream side of the catalyst, and in addition to the main air-fuel ratio control based on the upstream oxygen sensor output, the downstream oxygen It is already known to introduce auxiliary air-fuel ratio control based on sensor output.

Regarding such an air-fuel ratio control device for an internal combustion engine, for example, Japanese Patent Application Laid-Open No. 4-342849 discloses a device in which the control response of a downstream oxygen sensor is changed according to the degree of deterioration of the catalyst. The control responsiveness is reduced when the degree of deterioration of is small, and the control responsiveness is increased when the degree of deterioration of the catalyst is large.

[0004]

However, in such a conventional air-fuel ratio control device, the control responsiveness by the downstream oxygen sensor is changed according to the degree of deterioration of the catalyst. After starting, when starting the auxiliary air-fuel ratio control by the downstream oxygen sensor, it takes time for the auxiliary air-fuel ratio correction amount based on the auxiliary air-fuel ratio control to converge to a desired value, and during this time, there is a problem that emission deteriorates. there were.

That is, as shown in FIG. 3, for some time after the engine is started, the air-fuel ratio in the catalyst remains rich regardless of the degree of deterioration of the catalyst. It is considered that this is because CO is adsorbed in the catalyst due to the increase in the amount of fuel at the start, but the degree varies depending on the degree of deterioration of the catalyst. That is,
When the degree of deterioration is small (corresponding to a new product), the precious metal of the catalyst is less deteriorated and CO is adsorbed more, so that the rich degree is increased. On the other hand, when the catalyst is deteriorated, the precious metal in the catalyst is deteriorated and CO adsorption is smaller than that of a new product, so that the rich degree is small.

Here, in the auxiliary air-fuel ratio control based on the downstream side oxygen sensor, the auxiliary air-fuel ratio correction amount is shifted to the lean side in accordance with the degree of richness in the catalyst, so that the air-fuel ratio in the catalyst is stoichiometric. It is required to maintain the vicinity and contribute to the improvement of emission.

However, in the conventional air-fuel ratio control device, as shown by the alternate long and short dash line in FIG. 3, it takes time for the auxiliary air-fuel ratio correction amount to converge to a desired value, and the degree of catalyst deterioration is reduced. When it is small, much more time is required, and during this period, the air-fuel ratio in the catalyst is so rich that the auxiliary air-fuel ratio correction amount has not converged, and the improvement of emission was insufficient.

The present invention has been made by paying attention to such a conventional problem, and provides an air-fuel ratio correction control device in which the emission is improved by giving an initial value to the auxiliary air-fuel ratio correction amount. With the goal.

[0009]

In order to solve the above-mentioned problems, the present invention, as shown in FIG. 1, provides a catalyst a provided in an exhaust passage of an internal combustion engine and a catalyst. An upstream oxygen sensor b provided in the exhaust passage on the upstream side of
The downstream oxygen sensor c provided in the exhaust passage on the downstream side of the catalyst, the main air-fuel ratio correction means d for calculating the main air-fuel ratio correction amount according to the output V1 of the upstream oxygen sensor, and the output of the downstream oxygen sensor. Auxiliary air-fuel ratio correction means e for calculating an auxiliary air-fuel ratio correction amount according to V2, and air-fuel ratio adjustment means f for adjusting the air-fuel ratio of the internal combustion engine according to the main air-fuel ratio correction amount and the auxiliary air-fuel ratio correction amount, Catalyst deterioration detecting means g for detecting the degree of deterioration of the catalyst
And a storage means h for storing the detected degree of deterioration and an initial value calculation means i for calculating an initial value of the auxiliary air-fuel ratio correction amount according to the stored degree of deterioration, and after starting the internal combustion engine,
The adjustment of the air-fuel ratio by the auxiliary air-fuel ratio correction amount is started at the initial value.

Further, as shown in FIG. 2, the invention according to claim 2 is provided with a catalyst temperature detecting means for detecting the temperature of the catalyst, and the initial value calculating means is for the stored degree of deterioration and the detected catalyst. The initial value of the auxiliary air-fuel ratio correction amount is calculated according to the temperature.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 6 is a diagram showing an air-fuel ratio correction control apparatus according to the first embodiment of the present invention.

First, to explain the configuration of the first embodiment, upstream of the engine 1, an air cleaner 2 for purifying intake air, an air flow meter 3 for measuring the intake air amount, and a throttle chamber 4 for controlling the intake air amount. And are attached. An injector 6 is attached to an intake pipe 5 downstream of the throttle chamber 4, and a water temperature sensor 7 for detecting a warm-up state of the engine and a crank angle sensor 8 for detecting a rotational speed of the engine are attached to the engine 1. The control module 9 calculates the basic fuel injection amount from the intake air amount sent from the air flow meter 3 and the rotation speed sent from the crank angle sensor 8, and uses this basic fuel injection amount as the water temperature data sent from the water temperature sensor 7. Various corrections such as increase correction based on the above are added. Then, the pulse width corresponding to the fuel amount thus obtained is output to the injector 6 and the fuel is injected into the intake pipe 6.

An upstream oxygen sensor 12 and a downstream oxygen sensor 13 are attached to an exhaust pipe 10 downstream of the engine 1 so as to sandwich a catalyst 11, and the upstream oxygen sensor 1
Based on the signals of 2 and the oxygen sensor 13 on the downstream side, the control module 9 calculates a correction value to obtain the stoichiometric air-fuel ratio, and feedback-controls the air-fuel ratio. The upstream oxygen sensor 12 is mounted directly above the catalyst 11 in order to make the exhaust gas entering the catalyst have a stoichiometric air-fuel ratio, and it is mounted downstream of the catalyst to correct such variations and deterioration with time of the upstream oxygen sensor 12. The downstream oxygen sensor 13 is installed. That is, the upstream oxygen sensor 12 performs the main air-fuel ratio correction control, and the downstream oxygen sensor 13 performs the auxiliary air-fuel ratio correction control that further corrects the main air-fuel ratio correction. Further, the catalyst 11 has a temperature sensor 1 for detecting the temperature of the catalyst.
4 is attached.

The main / auxiliary air-fuel ratio correction control will be described with reference to FIG. First, in step S1 (hereinafter, the wording of step is omitted and described), the upstream oxygen sensor output FVO2 of the catalyst is detected. In S2, it is determined whether or not the flag FLGRICH = 1. FLGRI
If CH = 1, it is determined to be rich last time, and the process proceeds to S3. If FLGRICH = 0, it is determined to be lean last time, and the process proceeds to S6. In S3, it is determined whether the output FVO2 of the upstream oxygen sensor 12 has become equal to or lower than the slice level SL. If FVO2 <SL, it is determined that the rich state is reversed to lean, and the process proceeds to S4, where FVO2 <S
If it is not L, it is determined that the rich state continues, and the process proceeds to S5. In S4, in response to the transition from rich to lean, the control P amount PL is made rich so as to make rich this time, and the flag FLGRICH is set to 0 in S9. Further, in S5, since the rich state continues, the control I is made lean.

On the other hand, if it is determined in S2 that FLGRICH = 0, that is, the previous lean was detected, it is determined in S6 whether the upstream oxygen sensor output FVO2 is equal to or higher than the slice level. If FVO2> SL, it is determined that the lean state has been inverted to rich, and the process proceeds to S7, where FVO2>
If it is not SL, it is determined that the lean state continues, and the process proceeds to S8. In S7, after the shift from lean to rich, this time control P to increase lean
To make it lean, and set the flag FLGRICH to 1 at S10
Set to. Further, in S8, since the lean state continues, the control I is enriched. As described above, the main air-fuel ratio correction is a correction that always keeps the air-fuel ratio upstream of the catalyst substantially stoichiometric by making the fuel lean in the rich state and making the fuel rich in the lean state. Shows the time chart). However, the upstream oxygen sensor 12 may control the air-fuel ratio upstream of the catalyst away from stoichiometry due to its variation and deterioration over time, and the auxiliary air-fuel ratio correction described below corrects this.

At S11, the output RVO2 of the downstream oxygen sensor 13 is detected, and if RVO2 is above the slice level RSL, the flag RFLGRICH is set to 1, and if it is below the slice level RSL, the flag RFLGRIC is set.
Set H to 0. In S12, the flag RFLGRI
It is determined whether CH = 1. If RFLGRICH = 1, it is determined to be rich, and the process proceeds to S13, where RFLGRICH =
If 0, it is judged to be lean and the process proceeds to S14. In S13,
Due to being rich, the main air-fuel ratio correction, for example, the control P amount is corrected so as to make the engine lean. In this case, the control P amount PR when the upstream oxygen sensor 12 shifts from the lean output to the rich output is increased, or the control P when the upstream oxygen sensor 12 shifts from the rich output to the lean output.
It is made lean by reducing the amount PL. In addition, leaning can also be achieved by increasing the decrease of the control I, decreasing the increase of the control I, or setting the SL to be small. On the other hand, in S14, the lean air-fuel ratio is corrected, so that the main air-fuel ratio is corrected, for example, by the control P so as to be rich. In this case, the upstream oxygen sensor 1
2 increases the control P amount PR when the lean output changes to the rich output or increases the control P amount PL when the upstream oxygen sensor 12 changes from the rich output to the lean output. In addition, decrease the amount of control I, increase the amount of control I, increase SL
Similarly, it can be made rich by setting a large value. This is the auxiliary air-fuel ratio correction control, which corrects variations and deterioration with time of the upstream oxygen sensor 12 in the main air-fuel ratio correction control. For example, the above-mentioned control P PL, PR is corrected so as to be increased or decreased. As a result, the air-fuel ratio in the catalyst can always be kept substantially stoichiometric.

Next, the operation of the first embodiment will be described.

As shown in the flow chart of FIG. 9, first, in order to determine whether the engine air-fuel ratio feedback control has been entered, it is determined in S20 whether the water temperature TW is equal to or higher than the air-fuel ratio feedback control start water temperature TWCLMP, and TW> TWCLM.
If P, the process proceeds to S21. If TW> TWCLMP, the process returns. At S21, the target air-fuel ratio TFBYA = 1
Is determined. TFBYA is a coefficient related to fuel injection, and it is TFBY when increasing at startup, increasing low water temperature, increasing high oil temperature, rapidly increasing acceleration, increasing at full opening, and increasing during knock control.
A> 1, and TFBYA <1 at the time of lean clamp during deceleration or fuel cut. If TFBYA = 1, the process proceeds to S22, and if TFBYA = 1, the process returns. In S22, the flag FLGCL is referred to in order to determine whether or not the upstream oxygen sensor 12 that performs the air-fuel ratio correction control is activated and can start the air-fuel ratio correction. FLG
CL indicates that the upstream oxygen sensor output has reached a predetermined number of times (for example, 1
If it is reversed, FLGCL = 1 is set, and if the upstream oxygen sensor is activated and the air-fuel ratio correction can be started, the process proceeds to S23. On the other hand FLGCL =
If it is not 1, it is determined that the upstream oxygen sensor 12 is not activated and the air-fuel ratio correction cannot be started, and the routine returns.

In step S23, the stored deterioration degree TRD of the catalyst is referred to. The detection of the deterioration degree TRD will be described later. Next, in S24, the initial value PHOSL of the P correction value according to the degree of deterioration is referred to from the map shown in FIG. S
25, the control P component is, for example, PL = PL-PHOSL
(If PR, PR = PR + PHOSL). Here, in detail about S23 to 25,
After the engine is started, the air-fuel ratio of the catalyst becomes rich as shown in FIG. It is considered that this is because CO in the exhaust gas is adsorbed on the precious metal of the catalyst. At this time, when the air-fuel ratio correction control is started, it is necessary to correct such enrichment due to CO adsorption, and the correction at this time is made lean by PHOSL. This tendency tends to become richer as the catalyst is not deteriorated as shown in FIG. If the catalyst is not deteriorated, the noble metal in the catalyst is not deteriorated and CO is easily adsorbed, so that the rich tendency is strong. On the contrary, when the catalyst is deteriorated, since the precious metal of the catalyst is deteriorated and CO is hardly adsorbed, the rich tendency is weak. Therefore, as the correction immediately after the start of the air-fuel ratio correction control after the start, it is necessary to correct according to the degree of deterioration of the catalyst. In addition, this correction is effective even during the activation of the catalyst, and after the catalyst is completely activated, the deterioration is diagnosed, and the correction is performed by changing the update speed of the correction. This is effective for emission because it is possible to make a proper correction immediately without taking time until it becomes.

Now, in S26, the downstream oxygen sensor RVO
The average value AVRVO2 of 2 is calculated, and DLTA is executed in S27.
Calculate V = AVRVO2-RSL. As shown in FIG. 12, if DLTAV is positive, it means that the output of the downstream oxygen sensor 13 is rich, and if it is negative, it means that it is lean. In S28, PHOS corresponding to DLTAV is referred to. As shown in FIG. 13, PHOS is rich if DLTAV is positive, so it has a positive value so that it becomes lean, and PHOS is lean if DLTAV is negative.
It has a negative value to make it rich. PL-P in S29
The main air-fuel ratio correction is further corrected as HOS (PR + PHOS). Next, in S30, it is determined whether DLTAV = 0. If DLTAV = 0, it means that the air-fuel ratio correction is determined to be appropriate, and the process proceeds to S31, where DLTAV = 0.
If not, the process returns to S26 and continues until DLTAV = 0, that is, an appropriate air-fuel ratio.

Next, in S31, the upstream oxygen sensor output F
The frequency ratio HZRATE of the frequency of VO2 and the downstream oxygen sensor output RVO2 is calculated, and the degree of catalyst deterioration obtained from HZRATE is detected and stored in S32. This means that if RVO2 is sufficiently larger than FVO2 as shown in FIGS. 14 and 15, HZRATE (= RVO2 frequency / FVO2 frequency) approaches 0, and the degree of catalyst deterioration is small. . Also, for FVO2, RV
HZRATE approaches 1 as the O2 waveform gets closer,
This means that the degree of deterioration of the catalyst is large. Therefore,
The degree of deterioration of the catalyst can be detected by HZRATE.

Next, a second embodiment of the present invention will be described.

In the air-fuel ratio correction control apparatus according to the second embodiment, the air-fuel ratio in the catalyst after the start of the air-fuel ratio correction control is different after the start even if the deterioration of the catalyst is the same, depending on the catalyst temperature at that time. . The air-fuel ratio in the catalyst becomes less rich as the temperature of the catalyst rises. This is because, as the catalyst temperature rises, the conversion rate of the catalyst is improved, and the adsorbed CO is converted, so that the tendency of enrichment of the catalyst is reduced. As shown in FIG. 5, the degree of deterioration of the catalyst, the tendency of the catalyst temperature and the degree of richness are as follows:
The rich degree is large, and conversely, when the deterioration degree is large at high temperature, the rich degree becomes small. For example, in the case of restarting at high temperature, the temperature of the catalyst is high, so the degree of richness is smaller than that at normal temperature. Therefore, in order to cope with this, S24 of the first embodiment is replaced with S33 and S34 of FIG. 16 and control is performed.

First, after referring to the degree of deterioration TRD of the catalyst in S23, the catalyst temperature TC is detected in S33. this is,
The same applies to the prediction made by integrating the engine speed and the load value. Next, in S34, the PHOS map allocated by TRD and TC shown in FIG. 17 is referred to. The lower the degree of deterioration at low temperature, the greater the degree of richness of the catalyst.
PHOS becomes large, and conversely, when the degree of deterioration is large at high temperature, the degree of rich becomes small, so PHOS becomes small.

As described above, according to the invention shown in the first embodiment, the auxiliary air-fuel ratio correction amount is set to the initial value P.
Since HOSL is applied, the convergence time until the supplementary air-fuel ratio correction amount converges as shown by the alternate long and short dash line in FIG. 3, and the supplementary air-fuel ratio control by the downstream oxygen sensor 13 is started, and at the same time, the catalyst 11 It becomes possible to control the air-fuel ratio to a desired value (for example, stoichiometry), and emission can be improved during this period. Furthermore, since this initial value depends on the degree of deterioration TRD of the catalyst, such an improvement in emission can be obtained regardless of the degree of deterioration of the catalyst.

Further, according to the invention shown in the second embodiment, the initial value PHOSL of the auxiliary air-fuel ratio control is calculated in consideration of the temperature TC of the catalyst in addition to the degree of deterioration TRD of the catalyst 11. Improved emissions are obtained regardless of catalyst temperature at start-up. That is, the auxiliary air-fuel ratio correction amount desired by the catalyst depends not only on the degree of deterioration of the catalyst as described above, but also on the temperature of the catalyst as shown in FIG. The initial value is required to be small. This is because when the catalyst temperature rises, the conversion rate of the catalyst improves, so the conversion of adsorbed CO is promoted,
This is because the rich degree in the catalyst tends to be small. Therefore, the air-fuel ratio in the catalyst depends on the degree of deterioration of the catalyst TRD.
Since the relationship between the catalyst temperature TC and the catalyst temperature TC has a tendency as shown in FIG. 5, if the initial value PHOSL of the auxiliary air-fuel ratio correction amount is obtained from the relationship shown in FIG. 17, the convergence time of the auxiliary air-fuel ratio correction amount is further shortened. It becomes possible to improve the emission.

[0028]

As described above, according to the invention of claim 1, since the initial value is given to the auxiliary air-fuel ratio correction amount, the auxiliary air-fuel ratio correction amount is indicated by, for example, the one-dot chain line in FIG. It becomes possible to control the air-fuel ratio in the catalyst to the desired value (for example, stoichiometric) at the same time as starting the auxiliary air-fuel ratio control by the downstream oxygen sensor, and improving the emission during this time. can get. Furthermore, since this initial value depends on the degree of deterioration of the catalyst, such improvement in emission can be obtained regardless of the degree of deterioration of the catalyst.

Further, according to the second aspect of the present invention, the initial value of the auxiliary air-fuel ratio control is calculated in consideration of the catalyst temperature in addition to the degree of catalyst deterioration, so that the catalyst temperature at the time of starting is calculated. Emission improvement is obtained. That is, since the conversion rate of the catalyst increases as the catalyst temperature rises, the conversion of adsorbed CO is promoted, and the rich degree in the catalyst tends to decrease. Therefore, when the temperature of the catalyst is high, it is not necessary to shift the auxiliary air-fuel ratio correction amount to the lean side compared to when it is low, and by considering the catalyst temperature in the calculation of the initial value of the auxiliary air-fuel ratio control, the auxiliary air-fuel ratio The convergence time of the correction amount can be shortened, and the emission can be further improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram corresponding to the invention of claim 1.

FIG. 2 is an explanatory diagram corresponding to the invention of claim 2.

FIG. 3 is an explanatory diagram showing a difference in air-fuel ratio between a conventional example and this control.

FIG. 4 is an explanatory diagram showing a relationship between a catalyst temperature and an air-fuel ratio in the catalyst.

FIG. 5 is an explanatory diagram showing a relationship between a catalyst temperature, a catalyst deterioration degree, and an in-catalyst air-fuel ratio.

FIG. 6 is an explanatory diagram showing the configuration of the first embodiment of the present invention.

FIG. 7 is a flowchart showing a conventional main / auxiliary air-fuel ratio correction control.

FIG. 8 is a time chart of main air-fuel ratio correction control.

FIG. 9 is a flowchart of air-fuel ratio correction control according to the first embodiment of this invention.

FIG. 10 is a degree of catalyst deterioration TRD and a correction value PHOSL for P component.
FIG.

FIG. 11 is an explanatory diagram showing a relationship between a catalyst deterioration degree TRD and an in-catalyst air-fuel ratio.

FIG. 12 is an explanatory diagram showing a relationship between a catalyst downstream side oxygen sensor output RVO2 and an air-fuel ratio.

FIG. 13 is an explanatory diagram showing the relationship between DLTAV, which is the difference between the catalyst downstream-side oxygen sensor output, and the slice level of the downstream-side oxygen sensor, and the P component correction value PHOS.

FIG. 14 is an explanatory diagram comparing frequencies of an upstream oxygen sensor output FVO2 and a downstream oxygen sensor output RVO2.

FIG. 15 is a frequency ratio of FVO2 and RVO2, HZR.
Explanatory drawing which shows the relationship between ATE and deterioration degree TRD.

FIG. 16 is a flowchart of the second embodiment of the present invention.

FIG. 17 is an explanatory diagram showing a relationship between a catalyst temperature TC, a catalyst deterioration degree TRD, and a P component correction value.

[Explanation of symbols]

 a catalyst b upstream oxygen sensor c downstream oxygen sensor d main air-fuel ratio correction means e auxiliary air-fuel ratio correction means f air-fuel ratio correction means g catalyst deterioration detection means h storage means i initial value calculation means j catalyst temperature detection means 1 engine 2 Air cleaner 3 Air flow meter 4 Throttle chamber 5 Intake pipe 6 Injector 7 Water temperature sensor 8 Crank angle sensor 9 Control module 10 Exhaust pipe 11 Catalyst 12 Upstream oxygen sensor 13 Downstream oxygen sensor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shunji Yamada 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd.

Claims (2)

[Claims] 1. A catalyst provided in an exhaust passage of an internal combustion engine, an upstream oxygen sensor provided in an exhaust passage upstream of the catalyst, and a downstream oxygen sensor provided in an exhaust passage downstream of the catalyst. A main air-fuel ratio correction unit that calculates a main air-fuel ratio correction amount according to the output of the upstream oxygen sensor; and an auxiliary air-fuel ratio correction unit that calculates an auxiliary air-fuel ratio correction amount according to the output of the downstream oxygen sensor. Air-fuel ratio adjusting means for adjusting the air-fuel ratio of the internal combustion engine according to the air-fuel ratio correction amount and the auxiliary air-fuel ratio correction amount,
A catalyst deterioration detecting means for detecting a deterioration degree of the catalyst, a storage means for storing the detected deterioration degree, and an initial value calculating means for calculating an initial value of the auxiliary air-fuel ratio correction amount according to the stored deterioration degree. An air-fuel ratio control apparatus for an internal combustion engine, comprising: starting the adjustment of the air-fuel ratio by the auxiliary air-fuel ratio correction amount at the initial value after the internal combustion engine is started. 2. A means for detecting the temperature of the catalyst, wherein the initial value calculation means calculates the initial value of the auxiliary air-fuel ratio correction amount according to the stored degree of deterioration and the detected temperature of the catalyst. An air-fuel ratio control system for an internal combustion engine according to claim 1.