JP4826398B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and in particular, in an internal combustion engine provided with a catalyst having oxygen storage capacity in an exhaust passage, the fuel injection amount is adjusted so that the air-fuel ratio of exhaust gas flowing into the catalyst becomes the stoichiometric air-fuel ratio. The present invention relates to an air-fuel ratio control device to be controlled.

  Conventionally, as disclosed in Patent Document 1, a full-range air-fuel ratio sensor is arranged upstream of the catalyst, an oxygen sensor is arranged downstream of the catalyst, and the air-fuel ratio is controlled based on the signals of these two sensors. An air-fuel ratio control device is known. The full-range air-fuel ratio sensor is a sensor that exhibits linear output characteristics with respect to the air-fuel ratio. The oxygen sensor is a sensor that outputs a signal corresponding to the oxygen concentration in the gas, and has an output characteristic that the output signal is inverted with respect to the air-fuel ratio with respect to the stoichiometric air-fuel ratio. In the air-fuel ratio control device, the fuel injection amount is feedback-controlled so that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes the stoichiometric air-fuel ratio based on a signal output from the global air-fuel ratio sensor.

In addition to feedback control based on the output signal of the entire air-fuel ratio sensor (this is called main feedback control), control for reflecting the signal output from the oxygen sensor in the fuel injection amount (this is called sub-feedback control) is also performed. ing. In the sub-feedback control, a correction value is calculated based on the deviation between the signal output from the oxygen sensor and the reference signal corresponding to the theoretical air-fuel ratio, and the correction value is reflected in the output signal of the global air-fuel ratio sensor. This correction value indicates which side of the air / fuel ratio of the exhaust gas flowing into the catalyst is lean / rich with respect to the stoichiometric air / fuel ratio. According to this, even when there is a deviation in the output signal of the entire area air-fuel ratio sensor, the deviation can be corrected and the fuel injection amount can be controlled so that the actual air-fuel ratio approaches the theoretical air-fuel ratio.
Japanese Patent Laid-Open No. 5-321721 Japanese Patent Laid-Open No. 10-246139

  By the way, the catalyst can purify the exhaust gas most efficiently when the surrounding atmosphere is in the vicinity of the theoretical air-fuel ratio. The catalyst has an oxygen storage capacity (OSC) in which oxygen is stored. When the air-fuel ratio of the exhaust gas flowing into the catalyst is lean, the catalyst takes in oxygen in the gas phase and stores it, and conversely, when the air-fuel ratio of the exhaust gas flowing into the catalyst is rich, the catalyst stores itself. Oxygen is released into the gas phase. The catalyst maintains its ambient atmosphere in the vicinity of the stoichiometric air-fuel ratio by storing or releasing oxygen in accordance with the air-fuel ratio of the inflowing exhaust gas.

  The oxygen storage capacity of the catalyst greatly affects the exhaust gas purification efficiency. That is, if the oxygen storage capacity is high, oxygen can be stored or released even when the air-fuel ratio of the exhaust gas is greatly deviated from the stoichiometric air-fuel ratio or when it vibrates with a large amplitude. The exhaust gas can be purified while maintaining the atmosphere in the vicinity of the theoretical air-fuel ratio. It is known that the oxygen storage capacity of the catalyst can be maintained high by activating the catalyst noble metal through repeated oxygen storage / release. According to the above-described feedback control of the fuel injection amount, the air-fuel ratio of the exhaust gas oscillates across the stoichiometric air-fuel ratio, so that the catalyst can repeatedly store / release oxygen.

  However, as a result of recent research, it has been found that the decrease in the oxygen storage capacity of the catalyst is influenced not only by the deterioration of the catalyst itself but also by the air-fuel ratio of the exhaust gas flowing into the catalyst. Specifically, even if the air-fuel ratio of the exhaust gas oscillates across the stoichiometric air-fuel ratio, the oxygen storage capacity of the catalyst is reduced when the amplitude is small.

  FIG. 1 is a graph showing the relationship between the air-fuel ratio (A / F) of exhaust gas flowing into the catalyst and the amount of oxygen stored or released from the catalyst. As shown in this figure, the occluded oxygen amount of the catalyst increases as the air-fuel ratio deviates to the rich side from the stoichiometric air-fuel ratio (stoichiometric), and the released oxygen amount of the catalyst increases as it deviates to the lean side. Conversely, the closer the air-fuel ratio is to the stoichiometric air-fuel ratio, the lower the amount of oxygen stored and the amount of released oxygen of the catalyst. For this reason, if the vibration amplitude across the theoretical air-fuel ratio of the air-fuel ratio continues to be small, oxygen storage / release is repeated only in a small amount, and the catalyst becomes stable with low oxygen storage capacity. .

  The above-described decrease in the oxygen storage capacity is temporary, and the oxygen storage capacity of the catalyst is recovered by increasing the amplitude of the air-fuel ratio again. However, it takes time for the oxygen storage capacity to fully recover, and until then, emissions exceeding the purification capacity of the catalyst are discharged into the atmosphere. According to the feedback control of the fuel injection amount using the whole area air-fuel ratio sensor and the oxygen sensor, the air-fuel ratio of the exhaust gas can be maintained in the vicinity of the stoichiometric air-fuel ratio. However, on the other hand, since the oxygen storage capacity of the catalyst is reduced, there is a possibility that emissions may be discharged from the catalyst even if there is a slight change in the air-fuel ratio.

  The present invention has been made to solve the above-described problems, and provides an air-fuel ratio control device for an internal combustion engine that can suppress emission of emissions in a situation where the oxygen storage capacity of the catalyst is reduced. With the goal.

In order to achieve the above object, a first invention is an air-fuel ratio control device for an internal combustion engine comprising a catalyst having an oxygen storage capacity in an exhaust passage,
Fuel injection amount control means for controlling the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the catalyst oscillates across the theoretical air-fuel ratio;
Oxygen storage capacity estimation means for estimating the degree of oxygen storage capacity of the catalyst;
Amplitude reducing means for reducing the amplitude of the air-fuel ratio by the fuel injection amount control means when it is estimated that the oxygen storage capacity of the catalyst is lower than a predetermined reference capacity;
It is characterized by having.

According to a second invention, in the first invention,
An oxygen sensor that outputs a signal corresponding to the oxygen concentration of the exhaust gas that has passed through the catalyst;
The oxygen storage capacity estimation means estimates that the oxygen storage capacity of the catalyst is lower than the reference capacity when the amplitude of the output signal of the oxygen sensor becomes smaller than a predetermined reference value.

According to a third invention, in the first invention,
An air-fuel ratio sensor that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst,
The oxygen storage capacity estimation means determines that the oxygen storage capacity of the catalyst is greater than the reference capacity when the state in which the amplitude of the output signal of the air-fuel ratio sensor is smaller than a predetermined reference value continues beyond a predetermined reference period. It is also characterized by the fact that it has also been reduced.

A fourth invention is any one of the first to third inventions,
An air-fuel ratio sensor that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst,
The fuel injection amount control means is configured to control the fuel injection amount by feedback control based on a deviation between the output signal of the air-fuel ratio sensor and the theoretical air-fuel ratio,
The amplitude reduction means reduces the amplitude of the air-fuel ratio by reducing the gain when the deviation between the output signal of the air-fuel ratio sensor and the theoretical air-fuel ratio is reflected in the fuel injection amount in the feedback control, or by limiting the reflection amount It is characterized by being comprised.

According to a fifth invention, in any one of the first to fourth inventions,
An air-fuel ratio sensor that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst;
An oxygen sensor that outputs a signal corresponding to the oxygen concentration of the exhaust gas that has passed through the catalyst,
The fuel injection amount control means controls the fuel injection amount by feedback control based on a deviation between the output signal of the air-fuel ratio sensor and the stoichiometric air-fuel ratio, and a reference value corresponding to the output signal of the oxygen sensor and the stoichiometric air-fuel ratio. To reflect the deviation from the fuel injection amount,
The amplitude reduction means reduces the gain when reflecting the deviation between the output signal of the oxygen sensor and the reference value corresponding to the theoretical air-fuel ratio in the fuel injection amount, or reduces the amplitude of the air-fuel ratio by limiting the reflection amount It is characterized by being comprised.

A sixth invention is any one of the first to fifth inventions,
The apparatus further includes means for canceling the reduction in the amplitude of the air-fuel ratio by the amplitude reduction means when the fuel cut or the fuel increase is executed.

A seventh invention is the sixth invention, wherein
The apparatus further comprises means for increasing the frequency of fuel cut or fuel increase when the amplitude reduction of the air-fuel ratio by the amplitude reduction means is continued beyond a predetermined reference period.

In an eighth aspect based on the sixth aspect,
And means for increasing the frequency of fuel cut or fuel increase when the integrated value of the intake air amount after the amplitude reduction of the air-fuel ratio is started by the amplitude reduction means exceeds a predetermined reference value. It is a feature.

  According to the first invention, when the oxygen storage capacity of the catalyst is reduced, the amplitude of the air-fuel ratio that oscillates across the theoretical air-fuel ratio is reduced, so that the air-fuel ratio of the gas flowing into the catalyst can be released by the catalyst. Can be prevented from shifting to the rich side beyond the range of oxygen amount, or to the lean side beyond the range of oxygen amount that can be stored in the catalyst, suppressing emission of emissions from the catalyst. be able to.

  According to the second invention, it is possible to accurately detect the decrease in the oxygen storage capacity of the catalyst by using the output signal of the oxygen sensor downstream of the catalyst to estimate the degree of the oxygen storage capacity of the catalyst. If the air-fuel ratio of the exhaust gas flowing into the catalyst approaches the stoichiometric air-fuel ratio and the state of the vibration amplitude is small, the oxygen storage capacity of the catalyst eventually decreases, and as a result, the oxygen sensor disposed downstream of the catalyst This is because the amplitude of the output signal also decreases.

  According to the third aspect of the invention, the air-fuel ratio of the exhaust gas flowing into the catalyst is measured by the air-fuel ratio sensor, and the degree of oxygen storage capacity of the catalyst is estimated based on the output signal. It is possible to accurately detect a decrease in capacity.

  According to the fourth aspect of the present invention, the gain at the time of reflecting the deviation between the output signal of the air-fuel ratio sensor upstream of the catalyst and the theoretical air-fuel ratio in the fuel injection amount is reduced, or the reflection amount is limited, thereby reducing the catalyst. The amplitude of the air-fuel ratio of the exhaust gas flowing into the engine can be efficiently reduced.

  According to the fifth aspect of the invention, the gain for reflecting the deviation between the output signal of the oxygen sensor downstream of the catalyst and the reference value corresponding to the theoretical air-fuel ratio in the fuel injection amount is reduced, or the reflection amount is limited. As a result, the air-fuel ratio amplitude of the exhaust gas flowing into the catalyst can be efficiently reduced.

  According to the sixth aspect of the present invention, when the oxygen storage capacity of the catalyst is recovered by executing the fuel cut or the fuel increase, the reduction in the amplitude of the air-fuel ratio is canceled to quickly return to the normal air-fuel ratio control and the catalyst. It will be possible to make maximum use of the oxygen storage capacity.

  According to the seventh and eighth inventions, by increasing the execution frequency of fuel cut or fuel increase, it is possible to quickly return to normal air-fuel ratio control in which the oxygen storage capacity of the catalyst can be utilized to the maximum.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a diagram for explaining the overall configuration of the internal combustion engine system in which the air-fuel ratio control apparatus according to the embodiment of the present invention is incorporated. As shown in the figure, an exhaust passage 4 is connected to the internal combustion engine 2. In the exhaust passage 4, catalysts 6 and 8 for purifying harmful components (NOx, CO, HC) in the exhaust gas are arranged in two stages. At least the upstream catalyst 6 is a catalyst having an oxygen storage capacity. The upstream catalyst 6 is disposed close to the exhaust manifold, and the downstream catalyst 8 is disposed under the floor of the vehicle. A whole area air-fuel ratio sensor 12 is attached upstream of the catalyst 6, and an oxygen sensor 14 is attached downstream of the catalyst 6. The global air-fuel ratio sensor 12 is a sensor that exhibits output characteristics linear with respect to the air-fuel ratio. The oxygen sensor 14 is a sensor that outputs a signal corresponding to the oxygen concentration in the gas, and has an output characteristic that the output signal is inverted with respect to the air-fuel ratio with respect to the stoichiometric air-fuel ratio.

  In the internal combustion engine system, an ECU (Electronic Control Unit) 10 is provided as a control device that comprehensively controls the operation of the entire system. The entire area air-fuel ratio sensor 12 and the oxygen sensor 14 are connected to the ECU 10. The ECU 10 feedback-controls the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the catalyst 6 becomes the stoichiometric air-fuel ratio based on the output signals of the entire area air-fuel ratio sensor 12 and the oxygen sensor 14. Hereinafter, this feedback control is referred to as air-fuel ratio feedback control.

  The air-fuel ratio feedback control executed by the ECU 10 includes main feedback control and sub feedback control. In the main feedback control, the deviation between the output signal of the global air-fuel ratio sensor 12 and the theoretical air-fuel ratio is reflected in the fuel injection amount. In the sub-feedback control, the deviation between the output signal of the oxygen sensor 14 and the reference value corresponding to the theoretical air-fuel ratio is reflected in the fuel injection amount. Since the air-fuel ratio feedback control using the global air-fuel ratio sensor 12 and the oxygen sensor 14 is a known technique, a detailed description thereof will be omitted in this specification.

  According to the air-fuel ratio feedback control, the air-fuel ratio of the exhaust gas can be maintained near the stoichiometric air-fuel ratio. On the other hand, however, there is a problem that the oxygen storage capacity of the catalyst 6 decreases due to a decrease in the amount of oxygen stored / released, and emissions are discharged from the catalyst 6 even if there is a slight change in the air-fuel ratio. Therefore, the ECU 10 executes control for actively reducing the amplitude of the air-fuel ratio (hereinafter referred to as amplitude reduction control) under a predetermined condition during execution of the air-fuel ratio feedback control. By actively reducing the amplitude of the air-fuel ratio by the amplitude reduction control, it is possible to prevent the air-fuel ratio from changing beyond the range of the amount of oxygen that can be stored / released by the catalyst 6.

  Hereinafter, the content of the amplitude reduction control executed by the ECU 10 in the present embodiment will be described. The amplitude reduction control is performed in the air-fuel ratio control routine shown in the flowchart of FIG. In the first step S2 of the routine shown in FIG. 3, it is determined whether air-fuel ratio feedback (F / B) control is being executed. If the air-fuel ratio feedback control is not being executed, this routine ends without executing the amplitude reduction control.

  If air-fuel ratio feedback control is being executed, the determination in step S4 is performed. In step S4, it is determined whether the amplitude of the output signal of the oxygen sensor 14 is equal to or less than a predetermined reference value. When the air-fuel ratio of the exhaust gas flowing into the catalyst 6 approaches the stoichiometric air-fuel ratio by air-fuel ratio feedback control and the vibration amplitude continues to be small, the amount of oxygen that can be released and the amount of oxygen that can be stored by the catalyst 6 decreases. To go. As a result, the change in the oxygen concentration in the exhaust gas that has passed through the catalyst 6 becomes small, and the amplitude of the output signal of the oxygen sensor 14 disposed downstream of the catalyst 6 decreases. Therefore, the degree of the oxygen storage capacity of the catalyst 6 can be estimated from the amplitude of the output signal of the oxygen sensor 14, and the decrease in the oxygen storage capacity of the catalyst 6 can be accurately determined by comparing the amplitude with the reference value. Can be detected.

  As a result of the determination in step S4, when the amplitude of the output signal of the oxygen sensor 14 is below the reference value, it can be determined that the oxygen storage capacity of the catalyst 6 has decreased. In that case, amplitude reduction control is executed in step S6. When the amplitude of the output signal of the oxygen sensor 14 is larger than the reference value, the determinations in steps S2 and S4 are repeatedly performed until the condition in step S2 is not satisfied or until the condition in step S4 is satisfied. .

  In the amplitude reduction control in step S6, a gain (hereinafter referred to as a main F / B correction gain) for reflecting the deviation between the output signal of the global air-fuel ratio sensor 12 and the theoretical air-fuel ratio in the fuel injection amount in the main feedback control is reduced. Is done. The main F / B correction gain is a fixed value. In the amplitude reduction control, the main F / B correction gain is multiplied by a correction coefficient smaller than 1. By reducing the main F / B correction gain, the amplitude of the air-fuel ratio of the exhaust gas flowing into the catalyst 6 can be efficiently reduced.

  In step S8, it is determined whether the air-fuel ratio feedback control is continuing. When the air-fuel ratio feedback control is interrupted, the processes of steps S10, S12, and S14 are skipped and the process of step S16 is executed. In step S16, the amplitude reduction control is stopped, and the main F / B correction gain reduced in step S6 is returned to the normal value.

  When the air-fuel ratio feedback control is continuing, the process of step S16 is performed on condition that fuel cut (F / C) is executed. By executing the fuel cut, the lean gas containing a large amount of oxygen flows into the catalyst 6, and the oxygen storage capacity of the catalyst 6 is restored. If the oxygen storage capacity of the catalyst 6 is restored, a slight change in the air-fuel ratio can be absorbed by the oxygen storage capacity of the catalyst 6. Therefore, in this case, the amplitude reduction control is stopped and the main F / B correction gain is returned to the normal value so that the oxygen storage capacity of the catalyst 6 can be utilized to the maximum. It is determined in step S14 whether or not fuel cut has been executed. More specifically, in step S14, it is determined whether or not the fuel cut has been executed continuously for a predetermined time. The predetermined time is sufficient to restore the oxygen storage capacity of the catalyst 6 by the inflow of lean gas.

  When the air-fuel ratio feedback control is continuing, the determination at step S10 is performed prior to the determination at step S14. In step S10, an elapsed time after the main F / B correction gain is reduced by the amplitude reduction control is measured. Then, it is determined whether or not the elapsed time has reached a predetermined reference time.

  If the elapsed time since the main F / B correction gain has been reduced has reached the reference time, the process of step S12 is executed. In step S12, the fuel cut execution condition is relaxed, and the condition for returning from the fuel cut is tightened, so that the frequency and execution period of the fuel cut are increased. By increasing the execution frequency and execution period of the fuel cut, the condition of step S14 can be established early. If the condition of step S14 is established at an early stage, it is possible to return to the normal air-fuel ratio feedback control that can make maximum use of the oxygen storage capacity of the catalyst 6 accordingly. If the elapsed time has not reached the reference time, the process of step S12 is skipped and the determination of step S14 is performed.

  In the present embodiment, the air-fuel ratio control by the above routine is executed together with the air-fuel ratio feedback control. According to the air-fuel ratio control by the above routine, when the oxygen storage capacity of the catalyst 6 is reduced, the amplitude of the air-fuel ratio that vibrates across the theoretical air-fuel ratio can be actively reduced. According to this, the air-fuel ratio of the gas flowing into the catalyst 6 shifts to the rich side exceeding the range of the oxygen amount that can be released by the catalyst, or exceeds the range of the oxygen amount that can be stored in the catalyst 6 to the lean side. And emission of emissions from the catalyst 6 at the time of air-fuel ratio feedback control can be suppressed.

  In the present embodiment, the “fuel injection amount control means” according to the first aspect of the present invention is realized by the ECU 10 executing the air-fuel ratio feedback control. Further, the “oxygen storage capacity estimating means” according to the first and second inventions is realized by the ECU 10 executing the process of step S4 of the routine shown in FIG. Further, the “amplitude reducing means” according to the first and fourth aspects of the present invention is realized by the ECU 10 executing the process of step S6.

  Further, in the present embodiment, the “means for canceling the reduction in the air-fuel ratio amplitude” according to the sixth aspect of the present invention is realized by the ECU 10 executing the processes of steps S14 and S16. Further, the “means for increasing the execution frequency of fuel cut or fuel increase” according to the seventh aspect of the present invention is realized by the ECU 10 executing the processes of steps S10 and S12.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the following modifications may be made.

  In the amplitude reduction control, the gain (hereinafter referred to as sub F / B correction gain) for reflecting the deviation between the output signal of the oxygen sensor 14 and the reference value corresponding to the theoretical air / fuel ratio in the fuel injection amount in the sub feedback control is reduced. May be. According to this, the “amplitude reducing means” according to the first and fifth inventions is realized. Alternatively, both the main F / B correction gain and the sub F / B correction gain may be reduced.

  In the amplitude reduction control, a limit may be provided for the amount of reflection when the deviation between the output signal of the entire area air-fuel ratio sensor 12 and the theoretical air-fuel ratio is reflected in the fuel injection amount in the main feedback control. Or you may make it provide a restriction | limiting in the reflection amount at the time of reflecting the deviation of the output signal of the oxygen sensor 14, and the reference value corresponding to a theoretical air fuel ratio in fuel injection amount in sub feedback control.

  As a condition for stopping the amplitude reduction control, a time when the fuel increase is executed at the time of acceleration or the like may be added as an OR condition in addition to when the fuel cut is executed. When the fuel increase is executed, the air-fuel ratio of the exhaust gas flowing into the catalyst 6 becomes rich. This is because the release of oxygen from the catalyst 6 is promoted by the flow of rich exhaust gas, and thereby the recovery of the oxygen storage capacity of the catalyst 6 is expected. When time elapses from the start of the amplitude reduction control, the fuel increase execution frequency may be increased together with the fuel cut execution frequency.

  Whether to increase the execution frequency of fuel cut or fuel increase may be determined from the integrated value of the intake air amount after the amplitude reduction control is started. Specifically, when the integrated value of the intake air amount exceeds a predetermined reference value, the execution frequency of fuel cut or fuel increase may be increased. According to this, the “means for increasing the execution frequency of fuel cut or fuel increase” according to the eighth invention is realized.

  Further, the degree of the oxygen storage capacity of the catalyst 6 can be estimated from the output signal of the global air-fuel ratio sensor 12. The output signal of the entire area air-fuel ratio sensor 12 indicates the air-fuel ratio of the exhaust gas flowing into the catalyst 6. Therefore, when the state in which the amplitude of the output signal becomes small continues, oxygen storage / release is repeated in the catalyst 6 only in a small amount, and the oxygen storage capacity of the catalyst 6 decreases. Can be expected. In this case, if the state in which the amplitude of the output signal of the global air-fuel ratio sensor 12 is smaller than the predetermined reference value continues beyond the predetermined reference period, the amplitude reduction control is executed to positively increase the air-fuel ratio amplitude. What is necessary is just to reduce. According to this, the “oxygen storage capacity estimating means” according to the first and third inventions is realized.

  The sensor disposed upstream of the catalyst may be any sensor (air-fuel ratio sensor) that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst, and is not limited to a global air-fuel ratio sensor. An oxygen sensor arranged downstream of the catalyst in the above embodiment can also be used as the air-fuel ratio sensor.

  The present invention is also applicable to an internal combustion engine system that includes an air-fuel ratio sensor only upstream of the catalyst and no oxygen sensor downstream of the catalyst, that is, an internal combustion engine system that performs air-fuel ratio feedback control only by main feedback control. Can do. In that case, the degree of the oxygen storage capacity of the catalyst can be estimated based on the output signal of the air-fuel ratio sensor arranged upstream of the catalyst.

It is a graph which shows the relationship between the air fuel ratio of the exhaust gas which flows into a catalyst, and the amount of occluded oxygen of a catalyst, or the amount of released oxygen. It is a figure for demonstrating the structure of the internal combustion engine system to which the air fuel ratio control apparatus as embodiment of this invention was applied. It is a flowchart which shows the routine of the air fuel ratio control performed in embodiment of this invention.

Explanation of symbols

2 Internal combustion engine 4 Exhaust passage 6, 8 Catalyst 10 ECU
12 Global air-fuel ratio sensor 14 Oxygen sensor

Claims (6)

An air-fuel ratio control apparatus for an internal combustion engine comprising a catalyst having an oxygen storage capacity in an exhaust passage,
Fuel injection amount control means for controlling the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the catalyst oscillates across the theoretical air-fuel ratio;
Oxygen storage capacity estimation means for estimating the degree of oxygen storage capacity of the catalyst;
Amplitude reducing means for reducing the amplitude of the air-fuel ratio by the fuel injection amount control means when it is estimated that the oxygen storage capacity of the catalyst is lower than a predetermined reference capacity;
An air-fuel ratio sensor that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst,
The oxygen storage capacity estimation means determines that the oxygen storage capacity of the catalyst is greater than the reference capacity when the state in which the amplitude of the output signal of the air-fuel ratio sensor is smaller than a predetermined reference value continues beyond a predetermined reference period. air-fuel ratio control apparatus for an internal combustion engine characterized in that it also be estimated to have decreased. Before SL fuel injection amount control means is configured to control the amount of fuel injected by feedback control based on the deviation between the output signal and the stoichiometric air-fuel ratio of the air-fuel ratio sensor,
The amplitude reduction means reduces the amplitude of the air-fuel ratio by reducing the gain when the deviation between the output signal of the air-fuel ratio sensor and the theoretical air-fuel ratio is reflected in the fuel injection amount in the feedback control, or by limiting the reflection amount 2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein Further comprising an oxygen sensor which outputs a signal corresponding to the oxygen concentration of the exhaust gas passing through the pre-Symbol catalyst,
The fuel injection amount control means controls the fuel injection amount by feedback control based on a deviation between the output signal of the air-fuel ratio sensor and the stoichiometric air-fuel ratio, and a reference value corresponding to the output signal of the oxygen sensor and the stoichiometric air-fuel ratio. To reflect the deviation from the fuel injection amount,
The amplitude reduction means reduces the gain when reflecting the deviation between the output signal of the oxygen sensor and the reference value corresponding to the theoretical air-fuel ratio in the fuel injection amount, or reduces the amplitude of the air-fuel ratio by limiting the reflection amount The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2 , characterized in that The internal combustion engine according to any one of claims 1 to 3, further comprising means for canceling the reduction of the amplitude of the air-fuel ratio by the amplitude reduction means when a fuel cut or a fuel increase is performed. Air-fuel ratio control device. When the reduction of the amplitude of the air-fuel ratio by the amplitude reduction means is continued beyond a predetermined reference period, the frequency of fuel cut or fuel increase is increased compared to before the continuation period exceeds the reference period . The air-fuel ratio control apparatus for an internal combustion engine according to claim 4 , further comprising means. When the integrated value of the intake air amount after the reduction of the amplitude of the air-fuel ratio by the amplitude reducing means exceeds a predetermined reference value, the fuel is compared with the value before the integrated value exceeds the reference value. 5. An air-fuel ratio control apparatus for an internal combustion engine according to claim 4 , further comprising means for increasing the execution frequency of cut or fuel increase.

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