JP2560275B2 - Air-fuel ratio control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is intended to improve exhaust gas purification of an internal combustion engine.
The present invention relates to an air-fuel ratio control device that uses an exhaust gas purification catalyst and controls the air-fuel ratio of the internal combustion engine in a region where the purification rate of the exhaust gas purification catalyst is high.

[Prior Art] Conventionally, when an exhaust gas purifying catalyst, for example, a three-way catalyst, is used for purifying exhaust gas of an internal combustion engine, the air-fuel mixture ratio of the internal combustion engine is controlled in a region where the purifying rate of the catalyst is high. . It is the air-fuel ratio control device that executes the control of this air-fuel ratio, detects the residual oxygen concentration in the exhaust gas by the oxygen concentration sensor provided in the exhaust system of the internal combustion engine, the internal combustion engine based on the detection result. Feedback control of the air-fuel ratio is achieved by adjusting the amount of fuel supplied. The air-fuel ratio control device, in order to quickly detect the residual oxygen concentration in the exhaust gas of the internal combustion engine to ensure a quick response, an oxygen concentration sensor upstream of the exhaust gas purification catalyst near the exhaust manifold of the internal combustion engine. It is usually arranged.

Further, in order to improve the control characteristics of the air-fuel ratio control device, it has been proposed to install an oxygen concentration sensor separately not only on the upstream side of the exhaust gas purifying catalyst as described above but also on the downstream side thereof. (JP-A-52-102934).

This is because even if exhaust gas having the same theoretical air-fuel ratio as the output voltage of the sensor installed upstream of the exhaust gas purifying catalyst is detected due to variations in the output characteristics of the oxygen concentration sensors as shown in FIG. 2 (A). It is due to the difference. Even with the same oxygen concentration sensor having such variations in output characteristics, if the residual oxygen concentration is detected on the downstream side of the exhaust gas purifying catalyst, as shown in FIG. The output voltage of the sensor is almost the same. Therefore, according to the output of the oxygen concentration sensor installed on the downstream side, the comparison voltage of the output of the oxygen concentration sensor installed on the upstream side can be changed,
Alternatively, the integration constant of the air-fuel ratio control signal, the delay time of the oxygen concentration sensor output, or the like is changed to improve the accuracy of feedback control to the stoichiometric air-fuel ratio.

[Problems to be Solved by the Invention] Conventionally, even an air-fuel ratio control device using two oxygen concentration sensors aims at improving the accuracy of air-fuel ratio feedback control, and the air-fuel ratio control device and the air-fuel ratio control device The abnormality of the internal combustion engine system including the fuel ratio control device is not detected. For example, when an abnormality occurs in a fuel injection valve whose fuel injection time is strictly controlled to control the air-fuel ratio, it is no longer possible to feedback control the air-fuel ratio to a desired value. . But,
Conventionally, in such a case, the only countermeasure is to simply stop the air-fuel ratio feedback control and shift to open control, and the driver is not appropriately notified of the abnormal state. The internal combustion engine system may be operated in the abnormal state as described above.

In addition, if the variation in the output characteristics of the oxygen concentration sensor becomes excessive due to aging, or if the abnormality or variation not only in the oxygen concentration sensor but also in other mechanisms of the internal combustion engine becomes excessive, the exhaust gas purification Even if the comparison voltage for the oxygen concentration sensor on the upstream side, the integration constant, and the delay time are corrected by the output of the oxygen concentration sensor on the downstream side of the catalyst, the accuracy of the feedback control may not be maintained. Measures were also needed.

The present invention has been made in view of the above problems, and when some abnormality occurs in the internal combustion engine system including the air-fuel ratio control device, or the variation of the oxygen concentration sensor and the abnormality of other mechanisms of the internal combustion engine become excessive. With the correction by the output of the oxygen concentration sensor on the downstream side of the exhaust gas purification catalyst,
If the accuracy of feedback control cannot be maintained,
It is an object of the present invention to provide an excellent air-fuel ratio control device capable of promptly and accurately reporting the abnormality.

[Means for Solving Problems] Means adopted by the present invention for solving the above problems are exhaust gas purification provided in an exhaust system of an internal combustion engine EG, as illustrated in the basic configuration diagram of FIG. Catalyst CT
Of the first oxygen concentration detector S1 provided on the upstream side of the exhaust gas, the second oxygen concentration detector S2 provided on the downstream side of the exhaust gas purification catalyst CT, and the detection results of the first oxygen concentration detector S1 The air-fuel ratio feedback control means C1 for controlling the air-fuel ratio of the internal combustion engine EG to a desired value based on the above, and the first oxygen concentration detector which is an air-fuel ratio feedback constant used for control of the air-fuel ratio feedback control means C1. At least one of the delay time of the feedback correction coefficient based on the output of S1, the integration constant, and the fluctuation range of the output comparison voltage of the first oxygen concentration detector S1 is set to the second oxygen concentration detector S2. Feedback constant correction means C2 that corrects based on the detection result of, when the air-fuel ratio feedback constant is a set value or more,
The gist is an air-fuel ratio control device including a determination means C3 for determining an abnormality and an informing means C4 for informing the determination result of the determination means C3.

[Operation] In the air-fuel ratio control device of the present invention, the air-fuel ratio feedback control means C1 feedback-controls the air-fuel ratio of the internal combustion engine EG based on the detection result of the first oxygen concentration detector S1. In this air-fuel ratio feedback control, the feedback constant correction means C2 corrects the air-fuel ratio feedback constant used for the control of the air-fuel ratio feedback control means C1 based on the detection result of the second oxygen concentration detector S2. Therefore, even when only the first oxygen concentration detector S1 is used, the accuracy of the air-fuel ratio feedback control is further improved.

The air-fuel ratio feedback constant changed by the output of the second oxygen concentration detector S2 is the delay time of the feedback correction coefficient based on the output of the first oxygen concentration detector S1, the integration constant, and the first oxygen. Concentration detector S1
Is at least one of the fluctuation widths of the output comparison voltage, and is not the feedback correction coefficient itself. The control variation due to the abnormality of the first oxygen concentration detector S1 and the abnormality of the other mechanism of the internal combustion engine EG is difficult to appear in the feedback correction coefficient itself, but is well reflected in the value in the air-fuel ratio feedback constant. It

That is, if there is a deviation in the air-fuel ratio feedback control that is a degree of normal variation of the first oxygen concentration detector S1 and other mechanisms of the internal combustion engine EG, feedback based on the detection result of the second oxygen concentration detector S2 is performed. The constant correction means C2 corrects the deviation by correcting the air-fuel ratio feedback constant, and maintains high-precision air-fuel ratio feedback control.Therefore, the deviation of the air-fuel ratio feedback control due to various variations in the air-fuel ratio feedback constant Appears directly.

However, when the variation of the first oxygen concentration detector S1 becomes excessive, or the abnormality or variation of other mechanism of the internal combustion engine EG becomes excessive, based on the detection result of the second oxygen concentration detector S2. In some cases, the feedback constant correction means C2 can no longer maintain highly accurate air-fuel ratio feedback control by correcting the air-fuel ratio feedback constant. This state also directly appears in the air-fuel ratio feedback constant as the magnitude of the constant value.
However, in the above-mentioned feedback correction coefficient, such control deviation indirectly influences and does not appear promptly in the value of the feedback correction coefficient.

Therefore, in the present invention, by correcting the air-fuel ratio feedback constant based on the detection result of the second oxygen concentration detector S2, it is possible to perform highly accurate air-fuel ratio feedback control over a long period of time as much as possible, and by the determination of the determination means C3,
When it is determined that the highly accurate air-fuel ratio feedback control cannot be maintained by the correction of the air-fuel ratio feedback constant, the notification means C4 notifies the abnormal state.

This enables highly accurate air-fuel ratio feedback control over a long period of time, and of course not only excessive variations in the first oxygen concentration detector S1 but also abnormalities and excessive variations in other mechanisms of the internal combustion engine EG. Can be detected quickly and easily, and immediate countermeasures can be taken.

In the determination by the determination means, it is not determined that the air-fuel ratio feedback constant is abnormal immediately when the air-fuel ratio feedback constant becomes equal to or more than a set value, but is determined to be abnormal when the state where the air-fuel ratio feedback constant is equal to or more than the set value continues for a predetermined engine. You may do it. In this way, it is possible to avoid noise and accurately determine the abnormality.

The notification means C4 is a determination means C3 that makes the above determination.
Notify the determination result of. The information to be notified is the determination result and is binary information indicating whether or not the result is suitable. Therefore, the notification means C4 can be configured by simple means such as lighting of a lamp and a buzzer sound. In addition, other technologies such as voice notification by synthesized voice and information by characters on the CRT screen may be used by using the current information processing device.

Hereinafter, the present invention will be described in more detail with reference to Examples in order to more specifically describe the present invention.

[Embodiment] FIG. 3 is a schematic diagram of an internal combustion engine system equipped with the air-fuel ratio control device of the embodiment. In the figure, reference numeral 1 denotes a four-cylinder engine, and a fuel injection valve 3 for injecting and supplying fuel is provided in each intake pipe 2 of the engine. The intake air passing through the intake pipe 2 is measured by the air flow meter 5 after passing through the air filter 4, and the temperature thereof is measured by the intake air temperature sensor 6. Further, the intake air amount is adjusted by a throttle valve 7 linked with an accelerator pedal (not shown), and the throttle valve opening is detected by a throttle sensor 8. Exhaust pipe 9 of engine 1
Is provided with a three-way catalyst 10 for purifying exhaust gas, and maintains good emission. A first oxygen sensor 11 for detecting the exhaust gas residual oxygen concentration in the exhaust pipe is provided on the upstream side of the three-way catalyst 10, and a similar second oxygen sensor 12 is provided on the downstream side.
Is installed. The output of the igniter 14 that generates a high voltage applied to each spark plug of the engine 1 is input to a distributor 16 that interlocks with a crankshaft 15 and is appropriately distributed and supplied to each spark plug. Inside this distributor 16, each rotation sensor that outputs 24 pulse signals (hereinafter referred to as Ne signal) every one rotation of the distributor 16, that is, every two rotations of the crankshaft 15, and one pulse signal per one rotation of the distributor 16. (Hereinafter referred to as G signal)
A cylinder discrimination sensor that outputs is output. In addition, 17
Is a water temperature sensor that detects the cooling water temperature of the engine 1.

Driving of the fuel injection valve 3, which determines the operating state of the engine 1 as described above, ignition timing, etc. is controlled by the output from the electronic control unit 20, and the air flow meter 5 for detecting the operating state of the engine 1 Various sensor outputs such as the temperature sensor 6 and the first and second oxygen sensors 11 and 12 are input to the electronic control unit 20. What is the electronic control unit 20?
It is composed of a logical operation circuit centered on a normal microcomputer, an input / output port 21 that serves as a data transfer port with the external device and various sensors described above, a CPU 22 that executes a logical operation according to a predetermined program, RO that stores the program, various control constants, maps, etc.
An M23 and a RAM 24 for temporarily storing data are provided.
30 is a battery that serves as a power source for electronic control devices, and 40
Is a warning light for notifying when the slip ratio control provided in the indicator panel in front of the driver's seat becomes abnormal.

The internal combustion engine system configured as described above executes the air-fuel ratio feedback control of the engine 1 as follows.

The flowchart shown in FIG. 4 is the main control routine. This routine is started when the engine 1 is started, and first performs initialization such as clearing the internal register of the CPU 22 (step 100), and then sets the initial value of data used for controlling the engine 1, for example, fuel. Processing such as setting a flag indicating that cutting is being performed to 0 is performed (step 105). Subsequently, a process of reading the operating state of the engine 1, for example, signals from the air flow meter 5, the rotation angle sensor, and the water temperature sensor 17 is performed (step 110), and the intake air amount Q and the rotation speed of the engine 1 are determined from the data thus read. A process for calculating various quantities such as N or load Q / N that are the basis of control of the internal combustion engine 1 is performed (step 120). Thereafter, the known ignition timing control (step 130) is performed based on the various amounts obtained in step 120, and then the process for calculating the amount of fuel to be injected and supplied to the engine 1 is performed. In order to calculate the fuel amount, it is first determined whether or not the condition for feedback control of the fuel amount is satisfied (step 14
0), when the condition is not satisfied, the fuel amount by the control most suitable for the operating state of the engine 1 at that time is calculated by the open loop. For example, the fuel increase control at the time of starting the engine 1 and the power increase control at the time of high load operation, which have been conventionally performed, are such. When it is determined in step 140 that the feedback condition is satisfied, that is, when the internal combustion engine 1 is performing stable operation in a normal steady state, normal feedback control is executed (step 17
0). In this way, after the optimal control for the operating state of the internal combustion engine 1 is selected and the amount of fuel to be injected and supplied is calculated,
The fuel injection control of step 190 is executed, and the fuel is actually supplied to the internal combustion engine 1. After this process, the process returns to step 110 again and the above processes are repeatedly executed.

In the processing of the main routine described above, the control performed when it is determined in step 140 that the feedback condition is satisfied, which is a feature of the present embodiment, will be described.

Here, first, the basic valve opening time of the fuel injection valve 3 for injecting and supplying the fuel to the engine 1, that is, the basic fuel injection time TP is calculated based on the load Q / N calculated in step 120. Then, the feedback correction coefficient FAF for correcting the basic fuel injection time TP is calculated from the outputs of the first and second oxygen sensors 11, 12 in order to make the air-fuel ratio of the air-fuel mixture the stoichiometric air-fuel ratio. The fuel injection time TAU for performing the fuel injection is calculated.

TAU = TP · FAF · f (x) Here, f (x) represents a variable representing various correction coefficients and learning values determined according to outputs from other sensors that detect the operating state of the engine 1. .

The fifth figure is for explaining the feedback correction coefficient FAF.
FIG. As shown in the figure, it is compared whether the output of the first oxygen sensor 11 is larger or smaller than the comparison voltage, and the feedback correction coefficient FAF is integratedly changed according to the comparison result. As a result, the fuel injection time TAU is extended or shortened by a minute time as is clear from the above equation, and the engine 1 can be operated near the stoichiometric air-fuel ratio.

Further, as described above with reference to FIG. 2A, it is assumed that the output of the first oxygen sensor 11 changes as shown by the alternate long and short dash line in FIG. 5 due to variations and aging. At this time,
The fluctuation of the output is easily detected because the output of the second oxygen sensor 12 is stable near the stoichiometric air-fuel ratio as described above with reference to FIG. 2B. Therefore, the comparison voltage in FIG. 5 is changed by the amount of this fluctuation range (dashed line in the figure) to always ensure operation in the vicinity of the theoretical air-fuel ratio and improve the accuracy of air-fuel ratio control. The variation in the output of the first oxygen sensor 11 is controlled from the rich time like the delay times TDR and TDL in FIG. 5 by controlling the delay time of the feedback correction coefficient FAF regardless of the change of the comparison voltage as described above. The same effect can be obtained by giving a delay time commensurate with the output fluctuation range to the point where lean changes to lean to rich.

In the air-fuel ratio control apparatus of the present embodiment, the CPU 22 interrupts the following abnormality diagnosis routine every predetermined time in accordance with the above-described normal air-fuel ratio feedback control.

FIG. 6 is a flowchart of this abnormality diagnosis routine. When a predetermined time has elapsed and the processing of this routine is interrupted by the CPU 22, it is first determined whether or not the above-described air-fuel ratio feedback processing is being executed (step 20).
0). If it is determined that the feedback condition is not satisfied and the open control (step 150) described above with reference to FIG. 4 is being executed, the counter C is reset (step 210) and this routine ends. On the other hand, during the feedback control, it is determined whether the delay time is within the set value (step 220). This is because when the delay times TDL and TDR of the feedback correction coefficient FAF based on the output of the first oxygen sensor 11 shown in FIG. 5 are controlled by the output of the second oxygen sensor 12, a too large delay time is calculated. When it is performed, it is determined to be abnormal. If the delay time is within the set value,
With the delay time sufficiently corrected by the output of the second oxygen sensor 12, it is assumed that the air-fuel ratio feedback control is being performed with high accuracy, and step 210 is executed to end this routine, and the delay time exceeds the set value. If so, the correction by the output of the second oxygen sensor 12 is no longer possible to maintain the air-fuel ratio feedback control with high accuracy, and the processing from step 230 onward is performed. In step 230, the counter C is incremented by "1". Then, in step 240, it is judged whether or not the content of the counter C thus incremented is equal to or more than a predetermined value K. If C ≧ K, step 250 is executed, the output for instructing the warning light 40 to be lit is stored in the RAM 24, and this routine is terminated. If C <K, no other processing is executed Exit the routine.

The predetermined value K corresponds to a sufficient time for surely determining that the delay time exceeds the set value in consideration of noise and the like.

Therefore, according to the air-fuel ratio control device of the present embodiment configured as described above, the first oxygen sensor is detected based on the output of the second oxygen sensor 12 that most accurately detects the air-fuel ratio state. With respect to the variation of 11 and the variation of other mechanisms of the engine 1, while the correction is possible, highly accurate air-fuel ratio feedback control can be performed, and the corrected air-fuel ratio feedback constant (delay time in this case) has a set value. When the exceeded state continues for a time corresponding to K, the limit of correction is determined, and the process proceeds to step 250. In step 250, when such an abnormality in the air-fuel ratio control is detected, a lighting command for the warning light 40 is stored in the RAM 24, and by the processing of another display routine (not shown), at the same time as the abnormality is detected or the driver or the like lights up. The warning will be executed when desired. As a result, even if some abnormality occurs in the entire system of the engine 1 under the air-fuel ratio control that normally has no confirmation method, the driver or the like can quickly and accurately grasp the abnormality. .

Moreover, the air-fuel ratio feedback control is allowed while the correction can maintain the air-fuel ratio feedback control with sufficiently high accuracy. Therefore, highly accurate air-fuel ratio feedback control can be performed for a long period of time. Further, in the state of the corrected air-fuel ratio feedback constant, the limit of correction is judged and the abnormality is notified. Therefore, the abnormality of the first oxygen sensor 11 and the engine 1 can be detected promptly and easily regardless of other processing, and immediate countermeasures can be taken.

Further, from another point of view, it can be said that highly accurate air-fuel ratio feedback control can no longer be maintained by correcting the air-fuel ratio feedback constant such as the delay time. That is, since the air-fuel ratio feedback constant is corrected based on the detection result of the second oxygen sensor 12 downstream of the three-way catalyst (exhaust gas purification catalyst) 10, if it becomes an excessive correction exceeding the set value, The air-fuel ratio feedback constant is the air-fuel ratio itself after passing through the three-way catalyst 10. This causes the deterioration of the emission, so by determining the excessive correction in step 220, the deterioration of the emission can be prevented.

Further, since it is sufficient to make effective determination by effectively using the air-fuel ratio feedback constant that has already been corrected by the highly accurate air-fuel ratio feedback control, it is possible to perform a simple operation as shown in FIG. 6 without adding special logic. Just adopt the means,
Abnormalities in the entire engine 1 can also be easily detected.

The second oxygen sensor 12 is not limited to the above two embodiments.
The fluctuation range of the comparison voltage for the first oxygen sensor 11 output, which is changed depending on the output of 1 above, and other integration constants may be targets for the abnormality determination.

Further, the respective abnormality determinations may be used in combination, and if used together with, for example, an abnormality detection device of a single oxygen sensor that has been conventionally proposed, the abnormality of the entire system of the engine 1 warned in this embodiment will be the engine 1 It is more effective because it is easy to determine in which part of the system.

[Advantages of the Invention] As described above in detail with reference to the embodiments, the air-fuel ratio control device of the present invention is the same as the air-fuel ratio control device of the present invention, in which the air-fuel ratio feedback control means C1 is the first oxygen concentration detector S1. The air-fuel ratio of the internal combustion engine EG is feedback-controlled based on the detection result of. In this air-fuel ratio feedback control, the feedback constant correction means C2 is an air-fuel ratio feedback constant used for the control of the air-fuel ratio feedback control means C1 based on the detection result of the second oxygen concentration detector S2.
At least one of the delay time of the feedback correction coefficient based on the output of the first oxygen concentration detector S1, the integration constant, and the fluctuation range of the output comparison voltage of the first oxygen concentration detector S1 is corrected. .

Therefore, the accuracy of the air-fuel ratio feedback control is further improved as compared with the case where only the first oxygen concentration detector S1 is used.

If the first oxygen concentration detector S1 or the air-fuel ratio feedback control is deviated to the extent of a normal variation of the internal combustion engine EG, the feedback constant correction means C2 determines, based on the detection result of the second oxygen concentration detector S2, By correcting the air-fuel ratio feedback constant, the deviation is corrected and high-precision air-fuel ratio feedback control is maintained.

Therefore, it is possible to enjoy the highly accurate air-fuel ratio feedback control as long as it can be corrected.

On the other hand, when the variation of the first oxygen concentration detector S1 becomes excessive, or the abnormality or variation of other mechanisms of the internal combustion engine EG becomes excessive, based on the detection result of the second oxygen concentration detector S2. When the determination means C3 determines that the feedback constant correction means C2 can no longer maintain highly accurate air-fuel ratio feedback control by correcting the air-fuel ratio feedback constant, the notification means C4 promptly detects the abnormal state. To inform.

As a result, highly accurate air-fuel ratio feedback control can be performed while correction is possible, and the limit of correction is determined in the state of the corrected air-fuel ratio feedback constant to promptly notify the abnormality.

Further, from another viewpoint, it can be said that highly accurate air-fuel ratio feedback control can no longer be maintained by the feedback constant correcting means C2 correcting the air-fuel ratio feedback constant. That is, since the air-fuel ratio feedback constant is corrected based on the detection result of the second oxygen concentration detector S2 downstream of the exhaust gas purifying catalyst CT, if it is excessively corrected, the air-fuel ratio feedback constant is used for exhaust gas purification. It becomes the air-fuel ratio itself after passing through the catalytic CT, which causes deterioration of emission. Therefore, it is possible to prevent the emission from deteriorating by the determination means C3 determining the excessive correction.

Further, since the air-fuel ratio feedback constant, which has already been corrected by the highly accurate air-fuel ratio feedback control, can be effectively used for the determination, it is possible to detect the abnormality of the entire internal combustion engine EG without adding special logic. Can be detected quickly and easily.

Therefore, highly accurate air-fuel ratio feedback control can be performed over a long period of time, and when any abnormality occurs in the entire internal combustion engine including the air-fuel ratio control device, the abnormality can be promptly and accurately notified. As a result, a system abnormality that may worsen the emission of the internal combustion engine can be easily confirmed, and the operation will not be continued without noticing the abnormality of the internal combustion engine. Also, in terms of service and maintenance, the workability can be greatly improved, and the secondary effect thereof is great.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of the present invention, FIG. 2 is an explanatory view of variations in output when oxygen concentration sensors are arranged upstream and downstream of an exhaust gas purifying catalyst, and FIG. 3 is an air-fuel ratio control device of an embodiment. FIG. 4 is a flow chart of the main routine of the embodiment, FIG. 5 is an explanatory view of the feedback correction of the air-fuel ratio, and FIG. 6 is a flow chart of the abnormality diagnosis routine of the first embodiment. Show. EG ...... Internal combustion engine CT ...... Exhaust gas purifying catalyst S1 ...... First oxygen concentration detector S2 ...... Second oxygen concentration detector C1 ...... Air-fuel ratio feedback control means C2 ...... Feedback constant correction means C3 ...... Judgment means C4 ... Notification means 1 ... Engine (internal combustion engine) 9 ... Exhaust pipe 10 ... Three-way catalyst (catalyst for purifying exhaust gas) 11 ... First oxygen sensor 12 ... Second oxygen sensor 20 ... … Electronic control unit 40… Warning light

Claims (2)

(57) [Claims] 1. A first oxygen concentration detector provided upstream of an exhaust gas purification catalyst provided in an exhaust system of an internal combustion engine, and a second oxygen concentration detector provided downstream of the exhaust gas purification catalyst. And an air-fuel ratio feedback control means for controlling the air-fuel ratio of the internal combustion engine to a desired value based on the detection result of the first oxygen concentration detector, and an air-fuel ratio feedback constant used for the control of the air-fuel ratio feedback control means. At least one of a delay time of a feedback correction coefficient based on the output of the first oxygen concentration detector, an integration constant, and a fluctuation range of the output comparison voltage of the first oxygen concentration detector is set to 2. Feedback constant correction means for correcting based on the detection result of the oxygen concentration detector, and when the air-fuel ratio feedback constant is equal to or greater than a set value, it is determined that the abnormality An air-fuel ratio control device comprising: a determination means; and an informing means for informing a determination result of the determination means. 2. The air-fuel ratio control device according to claim 1, wherein the determining means determines that the air-fuel ratio feedback constant is abnormal when the air-fuel ratio feedback constant is equal to or more than a set value for a predetermined period.