JPH1026043A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precisely control an air-fuel ratio to the theoretical fuel ratio even if the exhaust gas amount is changed, by suitably setting the speed of response of target air-fuel ratio feedback control according to the operating state of an internal combustion engine. SOLUTION: When the changes of the engine speed Ne and the intake air pressure PM are detected by an engine speed sensor 30 and an intake air pressure sensor 32, an air-fuel ratio controller controls the target air-fuel ratio correction amount so that the speed of response of target air-fuel ratio feedback control may be suitable. Therefore, the speed of response of target air-fuel ratio feedback control is suitably set, the high catalyst purifying factor is maintained, and the air-fuel ratio downstream a three-way catalyst 38 is approximately maintained to the theoretical air-fuel ratio. Therefore, the air-fuel ratio downstream the catalyst can be maintained to the theoretical air-fuel ratio even if the speed of response of the air-fuel ratio downstream the catalyst in relation to the air-fuel ratio upstream the catalyst is changed after the purifying capability of the three-way catalyst 38 is changed after the catalyst capacity per unit exhaust gas amount is changed, and exhaust of harmful components in exhaust gas can be restrained regardless of the operating state of an engine 10.

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 controlling an air-fuel ratio of an internal combustion engine. The present invention relates to a device for performing feedback control of an air-fuel ratio.

[0002]

2. Description of the Related Art Conventionally, as an apparatus for controlling an air-fuel ratio so that an air-fuel ratio downstream of a catalyst becomes a stoichiometric air-fuel ratio, there is an apparatus described in, for example, JP-A-3-185244.
In the device described in this publication, a target air-fuel ratio upstream of the catalyst is set such that the air-fuel ratio downstream of the catalyst becomes the stoichiometric air-fuel ratio in accordance with the operation state of the internal combustion engine. Feedback control is performed so that the fuel ratio is obtained.

Further, in the above publication, a fixed amount of correction is added to the target air-fuel ratio to the lean side when the air-fuel ratio downstream of the catalyst is rich, and to the rich side when the air-fuel ratio is lean, to control the air-fuel ratio more accurately. Note that, when the operating state of the internal combustion engine changes, the air-fuel ratio upstream of the catalyst that changes the air-fuel ratio downstream of the catalyst to the stoichiometric air-fuel ratio changes because the purification performance of the catalyst changes depending on the exhaust gas temperature and the amount of exhaust gas passing through the catalyst. That's why.

[0004]

In the apparatus described in the above-mentioned publication, the target air-fuel ratio is corrected by a fixed amount based on the air-fuel ratio downstream of the catalyst, thereby improving the accuracy of the air-fuel ratio control.
However, when the purification ability of the catalyst changes, the time required for the exhaust gas to pass through the catalyst changes accordingly.
That is, when the catalyst capacity (SV ratio) per unit exhaust gas amount is large, the purification ability of the catalyst increases, and the time required for the exhaust gas to pass through the catalyst increases. Therefore, the response of the air-fuel ratio downstream of the catalyst to the air-fuel ratio upstream of the catalyst becomes slow. Conversely, when the SV ratio is small, the purifying ability of the catalyst decreases, and the responsiveness of the air-fuel ratio downstream of the catalyst to the air-fuel ratio upstream of the catalyst increases.

[0005] As described in the above publication, if a fixed amount of correction is performed irrespective of the purifying ability of the catalyst, there is a problem that overcorrection occurs when the SV ratio is large and the response of the air-fuel ratio downstream of the catalyst is slow. Further, when the SV ratio is small and the response of the air-fuel ratio downstream of the catalyst is fast, there is a problem that the convergence to the optimum target air-fuel ratio is delayed. An object of the present invention is to provide an air-fuel ratio control device capable of controlling the air-fuel ratio with high accuracy even when the amount of exhaust gas changes.

[0006]

Means for Solving the Problems In order to achieve the above object, the present invention has the configuration described in claim 1. According to this configuration, since the response speed of the target air-fuel ratio feedback control is set according to the operating state of the internal combustion engine, the correction speed can be reduced when the SV ratio is large and the response of the air-fuel ratio downstream of the catalyst is slow. . Further, when the SV ratio is small and the response of the air-fuel ratio downstream of the catalyst is fast, the correction speed can be increased.

[0007] The air-fuel ratio upstream of the catalyst at which the air-fuel ratio downstream of the catalyst becomes the stoichiometric air-fuel ratio changes depending on the operating state of the internal combustion engine. Therefore, it is preferable that the target air-fuel ratio is set based on the operating state of the internal combustion engine. At this time, it is preferable to set, as the target air-fuel ratio, a target air-fuel ratio that maximizes the purification rate of the catalyst in the operating state of each internal combustion engine. At this time, it is preferable to use, as the parameter of the operating state of the internal combustion engine, a parameter having a correlation with the purification ability of the catalyst. In particular, it is preferable to use, as parameters, the rotational speed of the internal combustion engine and the load of the internal combustion engine (the amount of air (intake air amount) and the intake pipe pressure) that are highly correlated with the purification performance of the catalyst.

Further, as the feedback control of the target air-fuel ratio, a method of performing feedback control of the target air-fuel ratio by a predetermined amount based on the rich / lean air-fuel ratio downstream of the catalyst as described in claim 3 is used. Is also good. When such target air-fuel ratio feedback control is adopted,
It is preferable that the response speed setting means sets the predetermined amount according to the operating state of the internal combustion engine. At this time, the predetermined amount may be set in consideration of a deviation amount of the air-fuel ratio downstream of the catalyst from the stoichiometric air-fuel ratio.

Further, it is preferable to use a parameter correlated with the purifying ability of the catalyst as a parameter of the internal combustion engine for determining the response speed of the target air-fuel ratio feedback control. In particular, as described in claim 4, it is preferable to use, as a parameter, the amount of exhaust gas having a strong correlation with the purification ability of the catalyst. At this time, the exhaust gas amount may be estimated based on the rotational speed of the internal combustion engine and the load (for example, the intake air amount or the intake pipe pressure), or these parameters may be used instead.

According to the fifth aspect of the present invention, the response speed of the target air-fuel ratio feedback control is set according to the operating state of the internal combustion engine. Therefore, when the SV ratio is large and the response of the air-fuel ratio downstream of the catalyst is slow, the correction speed is reduced. Can be slowed down. Further, when the SV ratio is small and the response of the air-fuel ratio downstream of the catalyst is fast, the correction speed can be increased. Also,
The air-fuel ratio upstream of the catalyst at which the air-fuel ratio downstream of the catalyst becomes the stoichiometric air-fuel ratio changes depending on the operating state of the internal combustion engine. Therefore, it is preferable that the target air-fuel ratio be set based on the operating state of the internal combustion engine. On this occasion,
It is preferable to set, as the target air-fuel ratio, a target air-fuel ratio that maximizes the purification rate of the catalyst in each operating state of the internal combustion engine. At this time, it is preferable to use, as the parameter of the operating state of the internal combustion engine, a parameter having a correlation with the purification ability of the catalyst. In particular, it is preferable to use, as parameters, the rotational speed of the internal combustion engine and the load of the internal combustion engine (for example, the intake air amount and the intake pipe pressure) that have a strong correlation with the purification performance of the catalyst.

Further, as the feedback control of the target air-fuel ratio, a method of performing feedback control of the target air-fuel ratio by a predetermined amount based on the rich / lean air-fuel ratio downstream of the catalyst as described in claim 7 is used. Is also good. When such target air-fuel ratio feedback control is adopted,
It is preferable that the response speed setting means sets the predetermined amount according to the operating state of the internal combustion engine. In this case, when a linear air-fuel ratio sensor is used as the catalyst downstream air-fuel ratio sensor, the predetermined amount may be set in consideration of a deviation amount of the air-fuel ratio downstream of the catalyst from the stoichiometric air-fuel ratio.

Further, it is preferable to use, as a parameter of the internal combustion engine for determining a response speed of the target air-fuel ratio feedback control, a parameter having a correlation with the purifying ability of the catalyst. In particular, as described in claim 8, it is preferable to use an exhaust gas amount having a strong correlation with the purification ability of the catalyst as a parameter. At this time, a sensor that detects the rotational speed of the internal combustion engine and a sensor that detects the load of the internal combustion engine (for example,
An intake air amount sensor or an intake pipe pressure sensor) may be provided, and the exhaust gas amount may be estimated based on the rotation speed and load of the internal combustion engine, or these parameters may be used instead.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As an embodiment of the present invention, an air-fuel ratio control device for an internal combustion engine (engine) employing the present invention will be described below. FIG. 1 is a schematic configuration diagram showing an engine 10 for which air-fuel ratio control is performed and its peripheral devices. As shown in FIG. 1, in this embodiment, the engine 10
The control of the fuel injection amount TAU is performed by an electronic control unit (ECU).
20.

As shown in FIG. 1, air is sucked into each cylinder of the engine 10 through an air cleaner 11, an intake pipe 12, a throttle valve 13, a surge tank 14, and an intake branch pipe 15. On the other hand, the fuel is pressure-fed from a fuel tank (not shown) and the fuel injection valve 1 provided in each intake branch pipe 15 is provided.
6 to be injected and supplied. The engine 10 includes a rotation speed sensor 30 for detecting the rotation speed Ne of the engine 10, a throttle sensor 31 for detecting the opening TH of the throttle valve 13, and a throttle valve 13.
An intake pressure sensor 32 that detects a downstream intake pressure PM, a water temperature sensor 33 that detects a cooling water temperature Thw of the engine 10, and an intake temperature sensor 34 that detects an intake temperature Tam are provided.

Further, the exhaust pipe 35 of the engine 10 includes
Harmful components (C) in the exhaust gas discharged from the engine 10
A three-way catalyst 38 for reducing O, HC, NOx, etc.) is provided. On the upstream side of the three-way catalyst 38, a linear air-fuel ratio sensor 3 serving as an upstream air-fuel ratio sensor that outputs a linear detection signal according to the air-fuel ratio λ of the exhaust gas upstream of the catalyst
6 are provided. On the downstream side of the three-way catalyst 38, an O ₂ sensor 37 as a downstream air-fuel ratio sensor that outputs a detection signal according to whether the air-fuel ratio λ downstream of the catalyst is rich or lean with respect to the stoichiometric air-fuel ratio λ ₀ . Is provided.

The electronic control unit 20 includes a well-known CPU 21,
The ROM 22, RAM 23, backup RAM 24, and the like are configured as an arithmetic and logic operation circuit, and communicate with each other via a bus 27 via an input port 25 for inputting from each sensor and an output port 26 for outputting a control signal to each actuator. It is connected to the. The electronic control unit 20 calculates a fuel injection amount TAU based on a signal from each sensor input through the input port 25, and outputs a control signal to the fuel injection valve 16 through the output port 26.

Hereinafter, the air-fuel ratio control will be described with reference to the flowchart shown in FIG. FIG. 2 shows the fuel injection amount TA.
This is a process for setting U, which is executed in synchronization with the rotation (every 360 ° CA in this embodiment). First, at step 101, the basic fuel injection amount Tp is calculated according to the intake pressure PM and the rotation speed Ne. In the following step 102, it is detected whether or not the air-fuel ratio feedback condition is satisfied. Here, the cooling water temperature Thw is equal to or higher than a predetermined value,
The air-fuel ratio feedback condition is that the vehicle is not under high load and high speed operation. If the air-fuel ratio feedback condition is not satisfied in step 102, the air-fuel ratio correction coefficient FAF is set to 1 in step 103, and the routine proceeds to step 106.

If the air-fuel ratio feedback condition is satisfied in step 102, a target air-fuel ratio λ _TG is set in step 104 (details will be described later). Then, the air-fuel ratio correction coefficient FAF is set so that the air-fuel ratio lambda becomes equal to the target air-fuel ratio lambda _TG in step 105. More specifically, the air-fuel ratio correction coefficient FAF is calculated by the following equation according to the target air-fuel ratio λ _TG and the air-fuel ratio λ (k) detected by the air-fuel ratio sensor 36.

FAF (k) = K ₁ · λ (k) + K ₂ · FAF (k-3) + K ₃ · FAF (k-2) + K ₄ · FAF (k-1) + Z ₁ (k) 1) Here, K _{1 to} K ₄ are feedback gains. Also, FAF (k-1), FAF (k-2), FAF (k
-3) is the air-fuel ratio correction coefficient of the previous time, the air-fuel ratio correction coefficient of two times before, and the air-fuel ratio correction coefficient of three times before. In addition,
The integral term Z ₁ (k) is the target air-fuel ratio λ _TG and the actual air-fuel ratio λ
This is a value determined from the deviation from (k) and the integration constant Ka, and is obtained by the following equation.

Z ₁ (k) = Z ₁ (k−1) + Ka · (λ _TG −λ (k)) (2) Then, in step 106, the air-fuel ratio correction coefficient FAF for the basic fuel injection amount Tp And the fuel injection amount TAU is set according to the following equation according to the correction coefficient FALL and the other correction coefficient FALL.

TAU = FAF × Tp × FALL (3) An operation signal corresponding to the fuel injection amount TAU set as described above is output to the fuel injection valve 16. Next, the setting of the target air-fuel ratio λ _TG (step 104 in FIG. 2)
This will be described based on the flowchart shown in FIG.

First, in step 201, the engine speed Ne and the intake pressure PM are read as parameters representing the operating state of the internal combustion engine. Next, at step 202, a target air-fuel ratio λ _TG is set. In the present embodiment, the target air-fuel ratio λ _TG is read from a map of the engine speed Ne and the intake pressure PM as shown in FIG. In this map, the target air-fuel ratio λ _{TG at} which the purification rate of the three-way catalyst 38 is maximized in each operation state is stored. In this embodiment, the target air-fuel ratio is set to be richer as the engine speed Ne and the intake pressure PM are higher.

Next, at step 203, a target air-fuel ratio correction amount λ _M is set. In the present embodiment, the target air-fuel ratio correction amount λ _M is determined based on the engine speed Ne and the intake pressure P shown in FIG.
Read from the map with M. This map stores the target air-fuel ratio correction amount λ _{M at} which the response speed of the target air-fuel ratio feedback control by the O ₂ sensor 37 becomes optimum in each operating state. In addition, step 20 described later
The response speed of the target air-fuel ratio feedback control performed at 4 to step 210 is determined by the target air-fuel ratio correction quantity lambda _M.

[0024] In the following step 204 to step 210, the target air-fuel ratio feedback process based on the output of the O ₂ sensor 37 is executed. First, in step 204, it is determined whether the air-fuel ratio downstream of the catalyst is rich this time. In the present embodiment, it is detected whether or not the air-fuel ratio detected by the O ₂ sensor 37 this time is rich. Here, when the air-fuel ratio detected by the O ₂ sensor 37 is rich, an affirmative determination is made, and the routine proceeds to step 205. Next, at step 205, it is determined whether or not the air-fuel ratio downstream of the catalyst was rich at the time of the previous processing. Here, as in step 204, it is determined whether the air-fuel ratio detected by the O ₂ sensor 37 was rich last time. Here also, when an affirmative determination is made, the process proceeds to step 206. And
In step 206, the target air-fuel ratio λ _TG set in step 202 is set to be larger by the correction amount λ _M set in step 203. That is, when the air-fuel ratio downstream of the catalyst is rich, the target air-fuel ratio is set to the lean side by a predetermined value (correction amount λ _M ), and this is finally set as the target air-fuel ratio λ _TG (λ
_TG ← λ _TG + λ _M ) This processing ends.

If a negative determination is made in step 205, skip processing is executed. In other words, added large skip amount lambda _SKP than the correction amount to the target air-fuel ratio lambda _TG is set as the final target air-fuel ratio _{λ TG (λ TG ← λ TG} + λ
_SKP ), and terminates the process. In this embodiment, the skip amount λ _SKP is a fixed value given in advance. On the other hand, the air-fuel ratio detected by the O ₂ sensor 37 in the case of lean air-fuel ratio detected during the previous processing in step 208 it is determined whether a rich at step 204. If the determination is affirmative, the process proceeds to step 209.
In step 209, skip processing is executed as in step 207. However, here it subtracts a large skip amount lambda _SKP than the correction amount to the target air-fuel ratio lambda _TG, was set as the final target air-fuel ratio _{λ TG (λ TG ← λ TG} -λ SKP),
This processing ends.

If a negative determination is made in step 208, the process proceeds to step 210. And step 21
At 0, similarly to step 206, the target air-fuel ratio λ _TG is reduced by the correction amount λ _M. That is, when the air-fuel ratio downstream of the catalyst is lean, the target air-fuel ratio is set to a predetermined value (correction amount λ _M ).
Only on the rich side (λ _TG ← λ _TG −λ _M ), and this processing ends.

FIG. 6 is a time chart showing the above processing. Hereinafter, the operation and effect of this embodiment of the present invention will be described with reference to FIG. The solid line in the figure represents the movement of each parameter of the apparatus of the present invention, and the broken line represents the movement of each parameter of the conventional apparatus. In FIG. 6, since the operation state of the internal combustion engine is in a low rotation and low load state until time t1,
The target air-fuel ratio upstream of the catalyst is substantially the stoichiometric air-fuel ratio, the target air-fuel ratio correction amount is small, and has substantially the same waveform as the conventional air-fuel ratio control.

Next, when the engine speed Ne and the load PM increase from time t1 to time t2, the air-fuel ratio control device of the present invention corrects the target air-fuel ratio so that the response speed of the target air-fuel ratio feedback control becomes optimal. Increase the quantity λ _M. Further, in the present embodiment, the target air-fuel ratio is also set to the rich side so that the purification rate of the catalyst becomes the maximum, so that the target air-fuel ratio largely shifts to the rich side. Therefore, the response speed of the target air-fuel ratio feedback control is optimally set, and a high catalyst purification rate is maintained, so that the air-fuel ratio downstream of the catalyst is maintained at substantially the stoichiometric air-fuel ratio. As a result, emission of harmful components HC and NOx in the exhaust gas can be suppressed.

However, the conventional air-fuel ratio control device is
Although the target air-fuel ratio is set to the target air-fuel ratio at which the purification rate of the catalyst is maximized, the air-fuel ratio upstream of the catalyst is disturbed for a while because the response of the target air-fuel ratio feedback control is not good, and thus the air-fuel ratio downstream of the catalyst. Is disturbed. That is, some of the harmful components in the exhaust gas are discharged without being purified by the catalyst.

Further, the same effect can be obtained when the intake pressure PM and the engine speed Ne increase between time t3 and time t4. Although the case where the engine speed Ne and the intake pipe pressure PM are increased has been described here, the same applies to the opposite case, that is, when the engine speed Ne and the intake pipe pressure PM shift from a high state to a low state. Needless to say, the effect is obtained.

As described above, in the above embodiment of the present invention, the response speed of the target air-fuel ratio feedback is optimally set according to the engine speed Ne and the intake pressure PM.
In other words, the response speed of the target air-fuel ratio feedback is set using the engine speed Ne and the intake pressure PM as parameters as substitutes for the amount of exhaust gas strongly related to the purification performance of the catalyst.

For this reason, even if the catalyst capacity per unit exhaust gas amount (SV ratio) changes and the purifying ability of the catalyst changes, the response speed of the air-fuel ratio downstream of the catalyst to the air-fuel ratio upstream of the catalyst changes. The air-fuel ratio downstream of the catalyst can be kept substantially at the stoichiometric air-fuel ratio. Therefore, regardless of the operating state of the engine, the emission of harmful components in the exhaust gas can be suppressed.

Further, in this embodiment, the engine speed Ne
The target air-fuel ratio is set in accordance with and the intake pressure PM. In other words, parameters having a strong correlation with the purification performance of the catalyst are used as parameters representing the operating state of the engine, and the target air-fuel ratio is set based on these parameters. Can be controlled.

In the above embodiment, step 105 in FIG. 2 corresponds to the air-fuel ratio feedback control means, step 202 in FIG. 3 corresponds to the target air-fuel ratio setting means, and steps 204 to 210 in FIG. Step 203 of FIG. 3 corresponds to the response speed setting means, and functions. Further, in the above embodiment, the O2 sensor which outputs only rich or lean than the stoichiometric air-fuel ratio is used as the downstream air-fuel ratio sensor. However, the present invention is not limited to this. A fuel ratio sensor may be used. At this time, since the amount by which the air-fuel ratio downstream of the catalyst deviates from the stoichiometric air-fuel ratio can be detected, the response speed of the target air-fuel ratio feedback may be set according to this deviation amount.

In the above embodiment, the engine speed and the intake pressure are used as parameters representing the operating state of the internal combustion engine. However, the present invention is not limited to this. For example, the engine speed and the intake air amount are used as parameters. Alternatively, the output may be used in a system provided with an air-fuel ratio sensor output upstream of the catalyst or / and downstream of the catalyst or an exhaust gas temperature sensor.

Further, in the above embodiment, the response speed of the target air-fuel ratio feedback is set to an optimum response speed by changing the target air-fuel ratio correction amount. However, the present invention is not limited to this. The middle skip amount may be changed, or both the skip amount and the target air-fuel ratio correction amount may be changed. In addition, it is needless to say that the scope of application of the present invention is not limited to the system of the above embodiment, but can be applied to, for example, a system in which skip control is not performed.

As described above, in the air-fuel ratio control apparatus to which the present invention is applied, the response speed of the target air-fuel ratio feedback control is set according to the response speed of the air-fuel ratio downstream of the catalyst with respect to the air-fuel ratio upstream of the catalyst. Therefore, even if the amount of exhaust gas changes, the air-fuel ratio can be accurately controlled.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an air-fuel ratio control device according to an embodiment to which the present invention is applied.

FIG. 2 is a flowchart for calculating a fuel injection amount in one embodiment.

FIG. 3 is a flowchart for setting a target air-fuel ratio in one embodiment.

FIG. 4 is a map for setting a target air-fuel ratio according to an operation state of the internal combustion engine.

FIG. 5 is a map for setting a target air-fuel ratio correction amount according to an operation state of the internal combustion engine.

FIG. 6 is a time chart showing the movement of each parameter in one embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Internal combustion engine (engine) 20 Electronic control unit 30 Speed sensor 32 Intake pressure sensor 36 Linear air-fuel ratio sensor 37 O2 sensor 38 Three-way catalyst,

Claims (8)

[Claims] 1. An air-fuel ratio feedback control means for performing feedback control so that an air-fuel ratio upstream of a catalyst provided in an exhaust system of an internal combustion engine becomes a target air-fuel ratio, and a target air-fuel ratio setting for setting the target air-fuel ratio. Means, target air-fuel ratio feedback control means for feedback-controlling the target air-fuel ratio so that the air-fuel ratio downstream of the catalyst becomes the stoichiometric air-fuel ratio, and the response speed of the target air-fuel ratio feedback control based on the operating state of the internal combustion engine. An air-fuel ratio control device for an internal combustion engine, comprising: a response speed setting means for setting. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein said target air-fuel ratio setting means sets the target air-fuel ratio based on an operation state of the internal combustion engine. 3. The target air-fuel ratio feedback control means sets the target air-fuel ratio to a lean side by a predetermined value when the air-fuel ratio downstream of the catalyst is rich, and sets the target air-fuel ratio to a predetermined value when the air-fuel ratio downstream of the catalyst is lean. 3. The internal combustion engine according to claim 1, further comprising: means for performing feedback control on the rich side only by a value; and wherein the response speed setting means is means for setting the predetermined value in accordance with an operating state of the internal combustion engine. Engine air-fuel ratio control device. 4. The response speed setting means uses an exhaust gas amount as a parameter representing an operation state of the internal combustion engine,
4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the response speed is set faster as the amount of exhaust gas increases. 5. A catalyst provided in an exhaust system of an internal combustion engine, an upstream air-fuel ratio sensor provided upstream of the catalyst and linearly detecting an air-fuel ratio upstream of the catalyst, and provided downstream of the catalyst. A downstream air-fuel ratio sensor that detects an air-fuel ratio downstream of the catalyst, and an air-fuel ratio feedback control unit that performs feedback control based on the output of the upstream air-fuel ratio sensor so that the air-fuel ratio upstream of the catalyst becomes a target air-fuel ratio. Target air-fuel ratio setting means for setting the target air-fuel ratio; and target air-fuel ratio feedback for feedback-controlling the target air-fuel ratio based on the output of the downstream air-fuel ratio sensor so that the air-fuel ratio downstream of the catalyst becomes the stoichiometric air-fuel ratio. Control means; and response speed setting means for setting a response speed of the target air-fuel ratio feedback control based on an operation state of the internal combustion engine. Control device for an internal combustion engine. 6. The air-fuel ratio control device for an internal combustion engine according to claim 5, wherein the target air-fuel ratio setting means sets the target air-fuel ratio based on an operation state of the internal combustion engine. 7. The target air-fuel ratio feedback control means sets the target air-fuel ratio to a lean side by a predetermined value when the air-fuel ratio detected by the downstream air-fuel ratio sensor is rich, and sets the target air-fuel ratio to a predetermined value when the air-fuel ratio is lean. 3. The internal combustion engine according to claim 1, further comprising: means for performing feedback control on the rich side only by a value; and wherein the response speed setting means is means for setting the predetermined value in accordance with an operating state of the internal combustion engine. Engine air-fuel ratio control device. 8. The response speed setting means uses an exhaust gas amount as a parameter representing an operation state of the internal combustion engine,
The air-fuel ratio control device for an internal combustion engine according to any one of claims 5 to 7, wherein the response speed is set faster as the exhaust gas amount is larger.