JP5093079B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an engine exhaust gas purification apparatus using a plurality of exhaust sensors.

Conventionally, a technology for purifying exhaust discharged from an engine by a catalytic converter has been developed. For example, in Patent Document 1 below, a three-way catalyst (30) or an HC trap catalyst (40) provided in multiple stages is developed. A technique for easily detecting deterioration is disclosed.
In the technique of Patent Document 1, as shown in FIG. 1 of the same document, a three-way catalyst (30) is provided as a first stage catalyst, and an HC trap catalyst (40) is provided as a second stage catalyst. Furthermore, a three-way catalyst (50) is provided as a third stage catalyst.

In the technique of Patent Document 1, according to the target intake air-fuel ratio (target A / F) set by feedback control based on the exhaust air-fuel ratio upstream of the first-stage three-way catalyst (30). The fuel injection amount from the fuel injection valve (6) is adjusted. The exhaust air-fuel ratio upstream of the three-way catalyst (30) is detected by the O ₂ sensor (22).

Furthermore, in the technique of Patent Document 1, an O ₂ sensor (24) is provided between the three-way catalyst (30) and the HC trap catalyst (40), and the HC trap catalyst (40) and the three-way catalyst ( 50) is also provided with an O ₂ sensor (26).
JP 2007-327394 A

If the deterioration of the catalytic converter is appropriately detected as in Patent Document 1 described above, the purification function of the catalyst can be recovered at an appropriate timing, and the performance of exhaust discharged into the atmosphere can be improved. .
However, in recent years when there is a strong need for protecting the global environment, further improvement in exhaust performance is desired.

  The present invention has been devised in view of such problems, and an object of the present invention is to provide an engine exhaust purification device that can improve exhaust performance by stabilizing exhaust components.

In order to achieve the above object, an exhaust purification device for an engine according to the present invention (Claim 1) includes a first catalytic converter disposed in an exhaust passage connected to the engine, and the downstream side of the first catalytic converter. A second catalytic converter disposed in the exhaust passage and the exhaust air / fuel ratio in the exhaust passage on the downstream side of the engine and upstream of the first catalytic converter are linearly detected as the first exhaust air / fuel ratio. First exhaust detection means and second exhaust detection means for detecting an exhaust air / fuel ratio in the exhaust passage downstream of the first catalytic converter and upstream of the second catalytic converter as a second exhaust air / fuel ratio A third exhaust detection means for detecting an exhaust air / fuel ratio in the exhaust passage downstream of the second catalytic converter as a third exhaust air / fuel ratio, and the first exhaust air detected by the first exhaust detection means. Main feedback control execution means for executing main feedback control for adjusting the target intake air-fuel ratio of the engine based on the ratio, and a control correction amount is defined according to the second exhaust air-fuel ratio and the third exhaust air-fuel ratio Applying the control correction amount map, the second exhaust air / fuel ratio detected by the second exhaust detection means, and the third exhaust air / fuel ratio detected by the third exhaust detection means to the control correction amount map. And sub-feedback control execution means for executing sub-feedback control for correcting the target intake air-fuel ratio set by the main feedback control execution means based on the control correction amount obtained in step (1).

  According to a second aspect of the present invention, there is provided the engine exhaust gas purification apparatus according to the first aspect, wherein both the second exhaust air-fuel ratio and the third exhaust air-fuel ratio are relatively included in the control correction amount map. A first leaning region that defines a first leaning correction amount for performing leaning correction of the target intake air-fuel ratio adjusted by the main feedback control when the control signal is rich, and the second exhaust air empty When the fuel ratio and the third exhaust air-fuel ratio are both relatively lean, a first enrichment correction amount that enriches and corrects the target intake air-fuel ratio that is adjusted by the main feedback control is defined as the control correction amount. When the first enrichment region, the second exhaust air-fuel ratio, and the third exhaust air-fuel ratio are both close to the theoretical air-fuel ratio, the target intake air space that is adjusted by the main feedback control A correction amount zero region defining the correction amount zero does not correct the ratio as control correction amount is characterized in that is provided.

According to a third aspect of the present invention, there is provided the engine exhaust purification apparatus according to the first or second aspect of the present invention, wherein the second exhaust air-fuel ratio approximates the stoichiometric air-fuel ratio and the control correction amount map includes the control correction amount map. When the third exhaust air-fuel ratio is relatively rich, or when the second exhaust air-fuel ratio is relatively rich and the third exhaust air-fuel ratio approximates the stoichiometric air-fuel ratio, the main feedback control A second lean region defining the second lean correction amount as a control correction amount for lean correction of the target intake air-fuel ratio to be adjusted to a degree weaker than the first lean correction amount; and the second exhaust When the air-fuel ratio approximates the stoichiometric air-fuel ratio and the third exhaust air-fuel ratio is relatively lean, or when the second exhaust air-fuel ratio is relatively lean and the third exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio If approximate, the main feed A second enrichment region that defines, as the control correction amount, a second enrichment correction amount that performs the enrichment correction of the target intake air-fuel ratio adjusted by the engine control to a degree weaker than the first enrichment correction amount. It is characterized by being provided.
According to a fourth aspect of the present invention, there is provided the engine exhaust gas purification apparatus according to any one of the first to third aspects, wherein the sub-feedback control execution means is obtained using the control correction amount map. When the obtained control correction amount is equal to or greater than a predetermined value and stagnated for a predetermined period, a learning value obtained from the control correction amount is recorded, and the learning value is used during the next execution of the sub-feedback control.

According to the exhaust emission control device for an engine of the present invention, exhaust performance can be improved by stabilizing the exhaust components. (Claim 1)
Further, by specifically defining the control correction amount map used in the sub feedback control, it is possible to appropriately obtain the control correction amount of the main feedback control for adjusting the target intake air-fuel ratio. (Claim 2)
Further, by defining the control correction amount map in more detail, the control correction amount can be obtained in more detail. (Claim 3)

Hereinafter, an engine exhaust gas purification apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic block diagram showing the overall configuration, and FIG. 2 shows the concept of the main feedback control and sub feedback control. A schematic block diagram mainly showing the control correction amount map, and FIG. 3 is a schematic flowchart showing its operation.
As shown in FIG. 1, a spark plug 11 is provided on the cylinder head 2 of the engine 1 mounted on the vehicle 10 so as to face the cylinder 3.

Further, an ignition coil (not shown) for supplying high voltage power is connected to the spark plug 11.
An intake port 5 is formed in the cylinder head 2. The intake port 5 is provided with an intake valve 14.
The intake valve 14 opens and closes according to the operation of an intake cam (not shown) of an intake camshaft (not shown) that rotates according to the rotation of the crankshaft 7, and opens and closes the intake port 5 with respect to the combustion chamber 4. It has become.

The rotational speed of the crankshaft 7, that is, the engine rotational speed Ne is detected by an engine rotational speed sensor 23, and the detection result is read by an ECU (Electronic Control Unit) 60 described later.
A downstream end of the intake manifold 15 is connected to the intake port 5.
The intake manifold 15 is provided with a throttle valve 16 and a throttle position sensor 17 for detecting the opening (throttle opening) θth of the throttle valve 16.

Further, an air flow sensor (air amount measuring means) 20 is provided in an intake pipe (intake passage) 19 upstream of the intake manifold 15. The air flow sensor 20 detects an inflow air amount Qin flowing through the intake pipe 19 and flowing into the intake manifold 15, and a detection result is output to an ECU 60 described later.
An electromagnetic fuel injection valve 21 is attached to the intake manifold 15. The fuel injection valve 21 is supplied with fuel from a fuel tank (not shown) via a fuel pipe 22.

An exhaust port 6 is formed in the cylinder head 2. The exhaust port 6 is provided with an exhaust valve 24.
The exhaust valve 24 opens and closes according to the operation of an exhaust cam (not shown) of an exhaust camshaft (not shown) that rotates according to the rotation of the crankshaft 7, and opens and closes the exhaust port 6 with respect to the combustion chamber 4. It is like that. The valve opening period, opening / closing timing, and lift amount of the intake valve 14 and the exhaust valve 24 are continuously changed by the variable valve mechanism 30.

An upstream end of an exhaust manifold (exhaust passage) 25 is connected to the exhaust port 6.
An exhaust pipe (exhaust passage) 26 is connected to the downstream end of the exhaust manifold 25. The exhaust pipe 26 is provided with a front three-way catalyst 31 as a first stage catalytic converter (first catalytic converter), and an HC trap catalyst 32 as a second stage catalytic converter (second catalytic converter). Is provided. Further, the exhaust pipe 26 is provided with a rear three-way catalyst 33 as a third-stage catalytic converter (third catalytic converter).

In other words, the exhaust pipe 26 is provided with the front three-way catalyst 31 on the most upstream side with respect to the flow of exhaust, and the HC trap catalyst 32 is provided on the downstream side of the front three-way catalyst 31. A rear three-way catalyst 33 is provided downstream of the HC trap catalyst 32.
Both the front three-way catalyst 31 and the rear three-way catalyst 33 convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen compounds (NO _x ) contained in the exhaust discharged from the engine 1 into nitrogen (N ₂ ) Exhaust gas is purified by chemical change to carbon dioxide (CO ₂ ) and water (H ₂ O).

  The front three-way catalyst 31 has a volume smaller than that of the rear three-way catalyst 33 and a purification capacity lower than that of the rear three-way catalyst 33. However, since the front three-way catalyst 31 is closer to the engine 1 than the rear three-way catalyst 33, the front three-way catalyst 31 can receive a relatively high-temperature exhaust immediately after being discharged from the engine 1. Yes. As a result, the front three-way catalyst 31 can reach the activation temperature in a very short period even when the engine 1 is cold-started.

The HC trap catalyst 32 purifies the exhaust by occluding hydrocarbons (HC) contained in the exhaust at the time of cold start and then desorbing the stored HC when the temperature is raised to a predetermined temperature. It is. The HC desorbed from the HC trap catalyst 32 is purified by the rear three-way catalyst 33.
In the exhaust passage 26, a first exhaust sensor (first exhaust detection means) 41, a second exhaust sensor (second exhaust detection means) 42, and a third exhaust sensor (third exhaust detection means) 43 are provided. Yes.

Of these, the first exhaust sensor 41 detects the exhaust air-fuel ratio in the exhaust passage 26 on the downstream side of the engine 1 and upstream of the front three-way catalyst 31 as the first exhaust air-fuel ratio AFex1. (Linear Air Fuel ratio Sensor).
The second exhaust sensor 42 is an O ₂ sensor that detects the exhaust air-fuel ratio in the exhaust passage 26 downstream of the front three-way catalyst 31 and upstream of the HC trap catalyst 32 as the second air-fuel ratio AFex2. is there.

The third exhaust sensor 43 is an O ₂ sensor that detects the exhaust air-fuel ratio in the exhaust passage 26 on the downstream side of the HC trap catalyst 32 as the third air-fuel ratio AFex3.
The detection results by the first exhaust sensor 41, the second exhaust sensor 42, and the third exhaust sensor 43, that is, the first exhaust air-fuel ratio AFex1, the second exhaust air-fuel ratio AFex2, and the third exhaust air-fuel ratio AFex3, are described later. Is to be read by.

The ECU 60 is an electronic control unit having a memory and a CPU (Central Processing Unit), not shown.
In the memory of the ECU 60, a main feedback execution unit (main feedback execution unit) 61 and a sub feedback control execution unit (sub feedback control execution unit) 62 are recorded as software.

Of these, the main feedback execution unit 61 executes main feedback control. As shown in FIG. 2, the main feedback control is performed by setting the target intake air-fuel ratio of the engine 1 based on the first exhaust air-fuel ratio AFex1 detected by the first exhaust sensor 41 and setting the target In this control, the fuel injection amount by the fuel injection valve 21 is set so that the intake air-fuel ratio is obtained.

  The sub feedback control execution unit 62 executes sub feedback control. As shown in FIG. 2, the sub-feedback control controls the second exhaust air / fuel ratio AFex2 detected by the second exhaust sensor 42 and the third exhaust air / fuel ratio AFex3 detected by the third exhaust sensor 43. By applying the correction amount map 66 to the correction amount map 66, the control correction amount A is obtained. The sub feedback control is also control for correcting the target intake air / fuel ratio set by the main feedback control execution unit 62 based on the obtained control correction amount A.

In addition, when the control correction amount A obtained using the control correction amount map 66 is greater than or equal to a predetermined value and stagnates for a predetermined period, the sub feedback control execution unit 62 updates the learning value AS and defines it in the memory area of the ECU 60. The control learning value storage area 67 is recorded. The learning value A _S recorded in this control learning value storage area 67 is adapted to be used during the execution of the next sub-feedback control.

In addition, as a preparation for obtaining the control correction amount A, the sub-feedback control execution unit 62 determines whether or not the following conditions 1 to 5 are satisfied.
Condition 1: Engine 1 is in normal operation Condition 2: Front three-way catalyst 31, HC trap catalyst 32 and rear three-way catalyst 33 are activated Condition 3: Second O ₂ sensor 42 and third O ₂ sensor 43 are activated Condition 4: The _second O ₂ sensor 42 and the third O ₂ sensor 43 are not in failure. Condition 5: The main feedback control is being executed. Of these, it is determined that the condition 1 is satisfied. That is, the sub-feedback control execution unit 62 determines that the engine 1 has not stalled and that the cranking of the engine 1 is completed and the engine speed Ne exceeds the start determination speed. This is the case.

The case where it is determined that the condition 2 is satisfied, that is, the time after the determination that the cranking of the engine 1 is completed and the engine speed Ne exceeds the start determination speed is a predetermined time. This is the case where the sub-feedback control execution unit 62 determines that t1 has been exceeded.
The case where it is determined that the condition 3 is satisfied, that is, the determination that the voltages output from the _second O ₂ sensor 42 and the third O ₂ sensor 43 are equal to or higher than a predetermined value is the sub feedback control execution unit 62. This is the case.

When it is determined that the condition 4 is satisfied, that is, it is not detected that the _second O ₂ sensor 42 and the third O ₂ sensor 43 are disconnected / short-circuited, stuck or deteriorated, and This is a case where the sub-feedback control execution unit 62 determines that the disconnection / short circuit is not detected by the heaters (not shown) of the 2O ₂ sensor 42 and the third O ₂ sensor 43.

The case where it is determined that the condition 5 is satisfied means that the main feedback control execution unit 61 determines that the main feedback control is executed without any problem by the sub feedback control execution unit 62. is there.
Further, a control correction amount map 66 is recorded in the memory of the ECU 60, and a control learning value storage area 67 is set.

Among these, in the control correction amount map 66, as shown in FIG. 2, the first lean region L1, the second lean region L2, the first rich region R1, the second rich region R2, and the uncorrected region. N is defined.
Among these, the first lean region L1 defines the control correction amount A when the second exhaust air-fuel ratio AFex2 and the third exhaust air-fuel ratio AFex3 are both relatively rich, that is, the first lean correction amount AL1. It is an area to do.

  Further, the second lean region L2 is such that the second exhaust air-fuel ratio AFex2 approximates to the theoretical air-fuel ratio and the third exhaust air-fuel ratio AFex3 is relatively rich, or the second exhaust air-fuel ratio AFex2 is relatively rich. And the control correction amount A when the third exhaust air-fuel ratio AFex3 is close to the stoichiometric air-fuel ratio, that is, the second leaning correction amount AL2. The second lean correction amount AL2 is set so that the correction degree is smaller than when the lean correction of the target intake air-fuel ratio is performed using the first lean correction amount AL1.

The first enrichment region R1 defines the control correction amount A when the second exhaust air-fuel ratio AFex2 and the third exhaust air-fuel ratio AFex3 are both relatively lean, that is, the first enrichment correction amount AR1. It is.
The second enrichment region R2 is such that the second exhaust air-fuel ratio AFex2 approximates to the stoichiometric air-fuel ratio and the third exhaust air-fuel ratio AFex3 is relatively lean, or the second exhaust air-fuel ratio AFex2 is relatively lean. And the control correction amount A when the third exhaust air-fuel ratio AFex3 is close to the theoretical air-fuel ratio, that is, the second enrichment correction amount AR2. The second enrichment correction amount AR2 is set to have a smaller degree of correction than when the target intake air-fuel ratio enrichment correction is performed using the first enrichment correction amount AR1.

  The zero correction amount region N is a region that defines the control correction amount A when the second exhaust air-fuel ratio AFex2 and the third exhaust air-fuel ratio AFex3 are both close to the theoretical air-fuel ratio, that is, the correction amount zero AN. is there. That is, when the correction amount zero AN is selected as the control correction amount A by the sub feedback control execution unit 62, the target intake air-fuel ratio adjusted by the main feedback control is not corrected.

In the control learning value storage area 67, the sub feedback control execution unit 62 is configured to record the learning value AS obtained from the control correction amount A.
Since the exhaust emission control device for an engine according to an embodiment of the present invention is configured as described above, the following operations and effects are achieved.
As shown in the flowchart of FIG. 3, the sub-feedback control execution unit 62 determines that the engine 1 is operating normally, that is, if it is determined that the above condition 1 is satisfied (step 1). Further, it is determined whether or not all the catalysts 31, 32, 33 are activated, that is, whether or not the above condition 2 is satisfied (step S12).

Here, the sub-feedback control execution unit 62 determines that the time after the determination that the cranking of the engine 1 is completed and the engine speed Ne exceeds the start determination rotation speed has exceeded the predetermined time t1. In other words, when it is determined that the above condition 2 is satisfied (Yes route of step S12), the sub feedback control execution unit 62 further determines that the second O ₂ sensor 42 and the third O ₂ sensor 43 are It is determined whether or not it has already been activated, that is, whether or not the above condition 3 is satisfied (step S13).

Here, when the sub feedback control execution unit 62 determines that the voltages output from the _second O ₂ sensor 42 and the third O ₂ sensor 43 are equal to or higher than a predetermined value, that is, when the above condition 3 is satisfied. When the determination is made (Yes route in step S13), the sub feedback control execution unit 62 further determines whether or not the _second O ₂ sensor 42 and the third O ₂ sensor 43 are in failure, that is, the above condition 4 is satisfied. It is determined whether or not it is present (step S14).

Here, when the sub-feedback control execution unit 62 determines that no failure is detected by the _second O ₂ sensor 42 and the third O ₂ sensor 43 (Yes route of Step S14), the sub-feedback control execution unit 62 further Then, it is determined whether or not the main feedback control is being executed, that is, whether or not the condition 5 is satisfied (step S15).

  Here, when the sub feedback control execution unit 62 determines that the main feedback control execution unit 61 executes the main feedback control without any problem, that is, when it is determined that the above condition 5 is satisfied. (Step S15 Yes route) Further, the sub-feedback control execution unit 62 calculates the control correction amount A (Step S16). That is, in this step S16, the sub-feedback control execution unit 62 calculates the second exhaust air / fuel ratio AFex2 detected by the second exhaust sensor 42 and the third exhaust air / fuel ratio AFex3 detected by the third exhaust sensor 43. The control correction amount A is obtained by applying to the control correction amount map 66.

Then, the sub-feedback control execution unit 62 updates the learning value AS from the control correction amount A obtained in step S16 and records it in the control learning value storage area 67 (step S17).
When the above condition 1 is not satisfied (No route at step S11), when the above condition 2 is not satisfied (No route at step S12), when the above condition 3 is not satisfied (No route at step S13), When the above condition 4 is not satisfied (No route of step S14) and when the above condition 5 is not satisfied (No route of step S15), the sub feedback control execution unit 62 performs the control correction in any case. The process returns without calculating the amount A (step S16) and learning the control correction amount A (step S17).

Thus, according to the engine exhaust gas purification apparatus according to one embodiment of the present invention, the second exhaust air-fuel ratio AFex2 detected by the second exhaust sensor 42 and the third exhaust gas detected by the third exhaust sensor 43 are detected. The sub-feedback control is executed based on the control correction amount A obtained by applying the air-fuel ratio AFex3 to the control correction amount map 66.
Thereby, it becomes possible to stabilize the component of the exhaust discharged from the engine 1, and the exhaust performance of the vehicle 10 can be improved.

  Further, by stabilizing the components of the exhaust discharged from the engine 1, even if the purification performance of the front three-way catalyst 31, the HC trap catalyst 32, and the rear three-way catalyst 33 is relatively low, it is released into the atmosphere. The exhaust can be sufficiently purified. In other words, the amount of components such as platinum (Pt), rhodium (Rh) or palladium (Pd) is relatively expensive active noble metal used for the front three-way catalyst 31, the HC trap catalyst 32, and the rear three-way catalyst 33. Even if the reduction is intended to reduce the cost, the exhaust performance required for the front three-way catalyst 31, the HC trap catalyst 32, and the rear three-way catalyst 33 can be sufficiently secured.

Further, since the first lean region L1, the first rich region R1, and the zero correction amount region N are defined in the control correction amount map 66 used in the sub feedback control, the main feedback for adjusting the target intake air / fuel ratio is adjusted. The control correction amount A for control can be obtained appropriately.
Further, in the control correction amount map 66, the second lean region L2 and the second rich region R2 are defined more finely, so that the control correction amount A can be obtained in more detail.

Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and various modifications can be made without departing from the spirit of the present invention. An example is shown below.
In the above-mentioned embodiment, although the case where the variable valve mechanism 30 was provided in the engine 1 was demonstrated, it is not limited to this. For example, a general engine that does not have the variable valve mechanism 30 may be applied instead of the engine 1.

In the above-described embodiment, the case where the HC trap catalyst 32 is used as the second catalytic converter has been described as an example. However, the present invention is not limited to this. For example, instead of the HC trap catalyst 32, a NOx trap catalyst or a three-way catalyst may be used as the second catalytic converter.
In the above-described embodiment, the case where the third catalytic converter (rear three-way catalyst 33) is provided in the exhaust passage (exhaust pipe 26) has been described. However, the present invention is not limited to this. A general exhaust system that does not have the third catalytic converter (rear three-way catalyst 33) may be applied.

1 is a schematic block diagram showing an overall configuration of an engine exhaust gas purification apparatus according to an embodiment of the present invention. FIG. 2 is a schematic block diagram mainly showing a concept of main feedback control and sub feedback control and a control correction amount map used in an engine exhaust gas purification apparatus according to an embodiment of the present invention. It is a typical flowchart which shows operation | movement of the engine exhaust gas purification apparatus which concerns on one Embodiment of this invention.

Explanation of symbols

1 Engine 26 Exhaust pipe (exhaust passage)
31 Front three-way catalyst (first catalytic converter)
32 HC trap catalyst (second catalytic converter)
33 Rear three-way catalyst (third catalytic converter)
41 1st exhaust sensor 42 2nd exhaust sensor 43 3rd exhaust sensor 61 Main feedback control execution part (main feedback control execution means)
62 Sub-feedback control execution unit (sub-feedback control execution means)
66 Control correction amount map A Control correction amount AFex1 First exhaust air-fuel ratio AFex2 Second exhaust air-fuel ratio AFex3 Third exhaust air-fuel ratio

Claims (4)

A first catalytic converter disposed in an exhaust passage connected to the engine;
A second catalytic converter disposed in the exhaust passage downstream of the first catalytic converter;
First exhaust detection means for linearly detecting an exhaust air-fuel ratio in the exhaust passage downstream of the engine and upstream of the first catalytic converter as a first exhaust air-fuel ratio;
Second exhaust detection means for detecting an exhaust air-fuel ratio in the exhaust passage downstream of the first catalytic converter and upstream of the second catalytic converter as a second exhaust air-fuel ratio;
Third exhaust detection means for detecting an exhaust air-fuel ratio in the exhaust passage on the downstream side of the second catalytic converter as a third exhaust air-fuel ratio;
Main feedback control execution means for executing main feedback control for adjusting a target intake air-fuel ratio of the engine based on the first exhaust air-fuel ratio detected by the first exhaust detection means;
A control correction amount map for defining a control correction amount according to the second exhaust air-fuel ratio and the third exhaust air-fuel ratio;
The control correction obtained by applying the second exhaust air-fuel ratio detected by the second exhaust detection means and the third exhaust air-fuel ratio detected by the third exhaust detection means to the control correction amount map. An engine exhaust gas purification apparatus comprising: sub feedback control executing means for executing sub feedback control for correcting the target intake air-fuel ratio set by the main feedback control executing means based on a quantity. The control correction amount map includes
When the second exhaust air-fuel ratio and the third exhaust air-fuel ratio are both relatively rich, the first lean correction amount that corrects the target intake air-fuel ratio that is adjusted by the main feedback control is made lean. A first lean region defined as a correction amount;
When both of the second exhaust air-fuel ratio and the third exhaust air-fuel ratio are relatively lean, the control is performed with the first enrichment correction amount for enriching the target intake air-fuel ratio that is adjusted by the main feedback control. A first enrichment region defined as a correction amount;
When both the second exhaust air-fuel ratio and the third exhaust air-fuel ratio are close to the stoichiometric air-fuel ratio, the correction amount zero that does not correct the target intake air-fuel ratio adjusted by the main feedback control is set to the control correction amount. An exhaust emission control device for an engine according to claim 1, wherein a correction amount zero region defined as follows is provided. The control correction amount map includes
When the second exhaust air-fuel ratio approximates the stoichiometric air-fuel ratio and the third exhaust air-fuel ratio is relatively rich, or when the second exhaust air-fuel ratio is relatively rich and the third exhaust air-fuel ratio is When approximate to the stoichiometric air-fuel ratio, the second lean correction amount that corrects the target intake air-fuel ratio adjusted by the main feedback control to a lean degree to a degree weaker than the first lean correction amount is the control correction. A second lean region defined as a quantity;
When the second exhaust air-fuel ratio approximates the stoichiometric air-fuel ratio and the third exhaust air-fuel ratio is relatively lean, or when the second exhaust air-fuel ratio is relatively lean and the third exhaust air-fuel ratio is When approximate to the stoichiometric air-fuel ratio, the control value is corrected to a second enrichment correction amount that corrects the target intake air-fuel ratio that is adjusted by the main feedback control to a degree weaker than the first enrichment correction amount. The engine exhaust gas purification apparatus according to claim 1 or 2, further comprising a second enrichment region defined as a quantity. The sub-feedback control execution means records a learning value obtained from the control correction amount when the control correction amount obtained using the control correction amount map is equal to or greater than a predetermined value and stagnates for a predetermined period, and The engine exhaust gas purification apparatus according to any one of claims 1 to 3, wherein the engine is used during the next execution of the sub-feedback control.

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