JP2003307121A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a sulfur poisoning recovery control from continuing for a long time, and properly restrain deterioration of fuel economy and emission control regardless of a rich degree in the sulfur poisoning recovery control. <P>SOLUTION: An engine 10 is equipped with an NOx occlusion reducing catalyst 22 for occluding sulfur components in exhaust gas at an exhaust passage 18. An electronic control device 40 executes the sulfur poisoning recovery control for controlling an exhaust air-fuel ratio to a rich air-fuel ratio when the occlusion volume of the sulfur components occluded by the NOx occlusion reducing catalyst 22 exceeds a tolerance during lean combustion of the engine 10. At that time, the electronic control device 40 sets an allowable continuing time of rich combustion according to the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control, and completes the sulfur poisoning recovery control when the continuing time of rich combustion reaches to an allowable continuing time. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, which is applied to an internal combustion engine that performs lean combustion and purifies exhaust gas by a catalyst provided in an exhaust passage thereof.

[0002]

2. Description of the Related Art In recent years, an internal combustion engine has been proposed which is designed to reduce the fuel consumption rate by performing combustion at a lean air-fuel ratio in which the fuel concentration is lower than the stoichiometric air-fuel ratio (theoretical air-fuel ratio). There is. In such an internal combustion engine, a large amount of nitrogen oxides (NO
The NOx storage reduction catalyst device for efficiently purifying x) is provided. This catalyst device temporarily stores nitrogen oxides (NOx) contained in the exhaust gas during lean combustion. When the air-fuel ratio is burning at a stoichiometric air-fuel ratio or a rich air-fuel ratio having a higher fuel concentration than this, the stored NOx is contained in exhaust gas such as hydrocarbon (HC) or carbon monoxide. (CO) reduces and purifies and releases.

In this NOx storage reduction catalyst device,
The sulfur component oxide (SOx) contained in the fuel is occluded by a mechanism substantially similar to NOx. Such a phenomenon is generally referred to as SOx poisoning, and when the SOx storage amount (SOx poisoning amount) increases, the NOx storage amount decreases by that amount, and thus the NOx storage capacity of the catalyst device decreases.

Therefore, in this NOx occlusion reduction catalyst device, the exhaust air-fuel ratio is set in order to calculate the stored SOx amount and desorb SOx on the condition that the calculated SOx amount exceeds the allowable value. Sulfur poisoning recovery control is performed to control the rich air-fuel ratio.

The desorption amount of the sulfur component from the NOx storage reduction catalyst per unit time increases as the catalyst temperature increases. Therefore, in Japanese Unexamined Patent Publication No. 6-229231, in order to desorb a sulfur component while suppressing deterioration of fuel efficiency and deterioration of emission, the rich degree of the exhaust air-fuel ratio in the sulfur poisoning recovery control is changed according to the catalyst temperature. It is suggested to set. In addition, Japanese Patent Laid-Open No. 2000-16
In 1107, the degree of enrichment of the exhaust air-fuel ratio in the above sulfur poisoning recovery control is determined by the catalyst temperature and the SOx stored in the catalyst.
It is proposed to variably set according to the amount.

[0006]

By the way, in the above sulfur poisoning recovery control, the sulfur component stored in the catalyst is desorbed,
The process is terminated on the condition that the storage amount is below a predetermined value. However, when the catalyst temperature is low and the sulfur component is difficult to be desorbed, the sulfur poisoning recovery control is continued for a long period of time, which leads to deterioration of fuel consumption and emission. Fuel consumption deterioration and emission deterioration are occurring depending on the degree of enrichment in control.

The present invention has been made in view of such conventional circumstances, and an object thereof is to prevent the sulfur poisoning recovery control from being continued for a long period of time and to enrich the sulfur poisoning recovery control. An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that can appropriately suppress deterioration of fuel consumption and deterioration of emission regardless of the degree.

[0008]

A structure for achieving the above-mentioned object and its function and effect will be described below. The invention according to claim 1 is an internal combustion engine in which a catalyst for storing a sulfur component in exhaust gas is provided in an exhaust passage when the exhaust air-fuel ratio is at least a lean air-fuel ratio, and the sulfur component stored in the catalyst is The storage amount of is calculated, while performing a sulfur poisoning recovery control for controlling the exhaust air-fuel ratio to a rich air-fuel ratio on condition that the calculated storage amount exceeds an allowable value, the calculated storage amount is In an exhaust purification device of an internal combustion engine, which terminates the sulfur poisoning recovery control when the value falls below a predetermined value smaller than an allowable value, the sulfur poisoning recovery is performed according to an exhaust air-fuel ratio controlled by the sulfur poisoning recovery control. A setting means for setting a continuation permission time of control, and an end means for forcibly ending the sulfur poisoning recovery control when the duration of the sulfur poisoning recovery control reaches the continuation permission time. And butterflies.

The sulfur poisoning recovery control is performed by controlling the exhaust air-fuel ratio to a rich air-fuel ratio. When the catalyst temperature is low and the sulfur component is difficult to be desorbed even if the sulfur poisoning recovery control is executed, the sulfur poisoning recovery control may be prolonged, which may cause deterioration of fuel consumption and emission. In this respect, according to the configuration of claim 1, the continuation permission time is set according to the exhaust air-fuel ratio during the sulfur poisoning recovery control, and the sulfur poisoning recovery control is performed when the duration reaches the continuation permission time. Since the poison recovery control is terminated, it is possible to prevent the sulfur poisoning recovery control from being continued for a long period of time, and it is possible to suppress deterioration of fuel consumption and deterioration of emission.

Moreover, since the above-described continuation permission time is set according to the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control, the fuel consumption is deteriorated regardless of the degree of enrichment of the exhaust air-fuel ratio in the sulfur poisoning recovery control. Emission deterioration can be appropriately suppressed.

According to a second aspect of the present invention, in the first aspect, the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control is varied so that the degree of enrichment becomes larger at least as the catalyst temperature of the catalyst becomes lower. It is characterized by being.

When performing the sulfur poisoning recovery control, the lower the catalyst temperature, the more difficult it is for the sulfur component to be desorbed, and the sulfur poisoning recovery control may be prolonged. Therefore, the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control is At least when the catalyst temperature is set to be variable so that the degree of richness increases as the temperature lowers, fuel consumption and emission tend to deteriorate significantly. However, according to the configuration of claim 2, even if the rich degree is increased as the catalyst temperature is lower, it is possible to preferably suppress deterioration of fuel consumption and deterioration of emission.

[0013]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an exhaust emission control device according to the present embodiment and a vehicle direct injection 4-cylinder gasoline engine (hereinafter simply referred to as “engine”) 10 to which the device is applied.

As shown in FIG. 1, the engine 10 directly injects fuel into the combustion chamber 12 of each cylinder # 1 to # 4 (in FIG. 1, only the combustion chamber of one cylinder is shown). The injector 14 and the spark plug 16 that ignites the injected fuel are provided.

Further, the exhaust passage 1 connected to the combustion chamber 12
In FIG. 8, a three-way catalyst device (hereinafter, simply referred to as “three-way catalyst”) 20 and a NOx storage reduction catalyst device (hereinafter, simply referred to as “NOx catalyst”) 22 are disposed downstream thereof. ing. The three-way catalyst 20 and the NOx storage reduction catalyst 22 purify HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) contained in the exhaust gas.

That is, in the three-way catalyst 20, HC, CO, and NOx contained in the exhaust gas are simultaneously purified by the redox reaction. On the other hand, in the NOx catalyst 22, the NOx contained in the exhaust gas in the lean combustion mode is temporarily stored, and the stored NOx is contained in the exhaust gas in the rich combustion mode (or stoichiometric combustion mode).
It is reduced by using C and CO as reducing agents and purified.

An oxygen sensor 24 for detecting oxygen in exhaust components is provided in the exhaust passage 18 between the three-way catalyst 20 and the NOx catalyst 22. Further, the NOx catalyst 22 is provided with a temperature sensor 26 that detects the temperature of the catalyst.

On the vehicle C, a rotation speed sensor 31 for detecting the engine rotation speed, an accelerator sensor 32 for detecting the depression amount of an accelerator pedal (not shown), and a traveling speed (vehicle speed SPD) of the vehicle C are detected. A vehicle speed sensor 33 is provided. The detection signal of the oxygen sensor 24 and the detection signals of the sensors 31 to 33 are input to the electronic control unit 40 that executes various controls of the engine 10.

The electronic control unit 40 executes various controls such as fuel injection control based on the operating state of the engine 10 and the running state of the vehicle detected by the oxygen sensor 24, the temperature sensor 26, the sensors 31 to 33, and the like. To do. The electronic control unit 40 also includes a memory 41 that stores a program for executing such various controls, a calculation map, various data calculated when executing the control, and the like.

In the engine 10 of this embodiment, the engine combustion mode is changed to the stratified combustion (lean combustion) mode or stoichiometric combustion by changing the fuel injection mode by the injector 14 or the ignition timing by the spark plug 16. The mode (normal combustion mode), the rich combustion mode, and the temperature rising combustion mode are switched.

For example, in the stratified charge combustion mode, the fuel injection timing is set to the latter stage of the compression stroke. Therefore, at the time of ignition, only the air-fuel mixture in the vicinity of the spark plug 16 is in a combustible air-fuel mixture state where it can be partially ignited. Further, the average air-fuel ratio (A / F) of the air-fuel mixture in this case is uniformly leaner than the stoichiometric air-fuel ratio (A / F = 14.5) in each of the cylinders # 1 to # 4 (for example, A / F = 25 to 50).

Further, in the stoichiometric combustion mode, the fuel injection timing is set during the intake stroke. Therefore, the air-fuel ratio in the combustion chamber 12 at the time of ignition becomes substantially uniform, and the air-fuel ratio of the air-fuel mixture is uniformly set near the stoichiometric air-fuel ratio in each of the cylinders # 1 to # 4.

Switching between the engine combustion modes is basically performed by the engine 1 such as engine load and engine speed.
It is performed based on the driving state of 0. Normally, the engine combustion mode is set to the stratified charge combustion mode in the low load / low speed operation region, and the engine combustion mode is set to the stoichiometric combustion mode in the high load / high speed operation region.

Further, in the rich combustion mode, the fuel injection timing is set during the intake stroke as in the stoichiometric combustion mode, and the air-fuel ratio of the air-fuel mixture is set in each cylinder # 1.
All of # 4 to # 4 are uniformly set richer than the stoichiometric air-fuel ratio. This rich combustion mode is used, for example, for the NOx catalyst 2
When the NOx storage amount of No. 2 exceeds a predetermined amount, the exhaust air-fuel ratio is temporarily made rich to increase the amounts of HC and CO contained in the exhaust gas, so that the NOx stored in the NOx catalyst 22 is increased. Process for reducing and purifying (rich spike process)
Selected at the time.

On the other hand, the temperature rising combustion mode is NOx.
It is selected when there is a temperature increase request for SOx poisoning recovery in the catalyst 22, that is, when a predetermined condition is satisfied, such as the SOx poisoning amount of the NOx catalyst 22 exceeds a predetermined amount.

In this temperature rising combustion mode, each cylinder # 1 to #
Among the four, the air-fuel ratio of some cylinders is set richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the remaining cylinders are set leaner than the stoichiometric air-fuel ratio. The average air-fuel ratio of all the cylinders # 1 to # 4 is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio.

By switching the engine combustion mode to the temperature rising combustion mode in this manner, the unburned fuel components in the exhaust gas discharged from some of the cylinders whose air-fuel ratio is set rich, and the air-fuel ratio Oxygen in the exhaust gas discharged from the remaining lean cylinders is the three-way catalyst 20 or NO.
The x catalyst 22 burns due to the catalytic function. As a result, the NOx catalyst 22 rises in temperature due to the combustion heat, and NOx
The SOx stored in the catalyst 22 comes to be removed from the catalyst 22.

Next, details of the SOx poisoning recovery process for the NOx catalyst 22 will be described with reference to FIGS. 2 and 3. 2 and 3 are flowcharts showing the processing procedure in the SOx poisoning recovery processing. The series of processes shown in this flowchart is executed by the electronic control unit 40 as an interrupt process of a predetermined crank angle.

When the processing of the electronic control unit 40 shifts to this series of processing, first, at step 100, NOx
The total SOx amount stored in the catalyst 22 is calculated. This total SOx amount is added when lean combustion is being performed at a lean air-fuel ratio, and is subtracted when rich combustion is being performed at a rich air-fuel ratio. The addition amount at this time is set to a larger value as the intake air amount increases and as the air-fuel ratio decreases (that is, the fuel concentration increases). This is because the amount of SOx that comes into contact with the NOx catalyst 22 per unit time increases as the intake air amount increases and the fuel concentration increases as the SOx amount increases. Further, the subtraction amount in the SOx amount calculation is set to a larger value as the air-fuel ratio is richer and the catalyst bed temperature is higher. This is because the higher the catalyst bed temperature and the richer the air-fuel ratio, the more the desorption of SOx in the NOx catalyst 22 is promoted. Note that total SOx
The calculation of the amount includes the element of the sulfur content of the fuel used. For example, high octane gasoline has a lower sulfur content than low octane gasoline, so the SOx amount stored per unit time at this time is small. On the contrary, since the low octane gasoline has a higher sulfur content than the high octane gasoline, the SOx amount stored per unit time at this time becomes large. Incidentally, in the case of high-octane gasoline, knocking is less likely to occur, so the ignition timing of the engine 10 is set to a more advanced timing through ignition advance control, whereas in the case of low-octane gasoline, comparison is made with this. Then, there is a tendency that the timing is set on the retard side. Therefore, such a fuel type determination can be performed based on such a tendency, for example. The relationship between the intake air amount, the air-fuel ratio and the SOx addition amount, and the relationship between the air-fuel ratio, the catalyst bed temperature and the SOx subtraction amount are obtained in advance through experiments and the like, and the electronic control unit
Stored in the memory 41.

After calculating the total SOx amount in this way,
It is determined whether the poisoning recovery request flag indicating that the SOx poisoning recovery process needs to be performed is on (step 102). When it is determined that the poisoning recovery request flag is not on (step 102: NO), the process proceeds to step 104. When it is determined that the poisoning recovery request flag is on (step 102: YES), the process proceeds to step 108.

When it is determined that the poisoning recovery request flag is not on, the total S calculated in the previous step 100 is calculated.
It is determined whether the Ox amount is equal to or more than the allowable value Ast (step 104). This allowable value ASt is a value for determining that the SOx poisoning amount has increased to such an extent that the decrease in NOx storage capacity cannot be ignored. If it is determined that the total SOx amount is equal to or greater than the allowable value Ast (step 104: YES), the poisoning recovery request flag is set to be on (SOx poisoning recovery process is necessary) (step). 106), and the post-processing proceeds to step 108,
The combustion mode is switched from the lean combustion mode to the temperature rising combustion mode. Further, when it is determined that the SOx amount is less than the allowable value ASt and there is still a margin for the SOx poisoning amount in the determination in step 104 (step 10).
If the answer is NO, the process proceeds to step 128.

If the poisoning recovery request flag is turned on in the previous step 102, or if the poisoning recovery request flag is set on in step 106, the temperature sensor 26 detects it in step 108. It is determined whether the catalyst bed temperature is equal to or higher than the predetermined temperature THc. The predetermined temperature THc is used to determine that the NOx catalyst 22 can desorb SOx based on its temperature state.

When it is determined that the catalyst bed temperature is lower than the predetermined temperature THc (step 108: NO), it is first necessary to raise the catalyst bed temperature, and the combustion mode is changed from the lean combustion mode to the stoichiometric air-fuel ratio. The temperature rising combustion mode is switched to. As described above, in the temperature rising combustion mode, the air-fuel ratio of some of the cylinders # 1 to # 4 is richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the remaining cylinders are leaner than the stoichiometric air-fuel ratio. Is set to. Also,
The average air-fuel ratio of all the cylinders # 1 to # 4 is set to the stoichiometric air-fuel ratio, and the exhaust gas reaching each of the catalysts 20 and 22 is stoichiometric when the average air-fuel ratio is controlled by the air-fuel ratio feedback control by the oxygen sensor 24. The fuel ratio is maintained. Due to this temperature rise combustion, unburned fuel components in the exhaust gas discharged from some cylinders whose air-fuel ratio is set to rich and oxygen in the exhaust gas discharged from the remaining cylinders whose air-fuel ratio is set to lean And are burned by the catalytic function of the three-way catalyst 20 or the NOx catalyst 22, and the heat of combustion raises the temperature of the NOx catalyst 22.

On the other hand, when the catalyst bed temperature is higher than the predetermined temperature THc (step 108: YES), SOx can be desorbed from the NOx catalyst 22. The rich combustion with the average air-fuel ratio of # 4 to # 4 as the rich air-fuel ratio is executed, and first, the rich combustion duration Te is measured (step 110).

Next, at step 112, the target air-fuel ratio is set to the rich side according to the catalyst bed temperature. As shown in FIG. 4, the SOx desorption amount from the NOx catalyst 22 tends to increase as the air-fuel ratio becomes a rich air-fuel ratio and increase as the catalyst bed temperature increases. When performing this rich combustion, as shown in FIG. 5, feedback is performed through the air-fuel ratio feedback control by the oxygen sensor 24 such that the control center is closer to the rich side than the stoichiometric air-fuel ratio.
B) The coefficient kaf is calculated. Therefore, step 112
In, the feedback (F / B) constant used for the air-fuel ratio feedback control is set. That is, referring to the map shown in FIG. 6A, the delay time TD from the output reversal timing of the oxygen sensor 24 to the addition of the rich side skip amount that decreases the F / B coefficient kaf according to the catalyst bed temperature.
R is set, and the delay time TDL until the lean side skip amount for increasing the F / B coefficient kaf is added is set. Also, referring to the map shown in FIG. 6 (b),
An integration constant CR that increases the F / B coefficient kaf and makes the air-fuel ratio rich according to the catalyst bed temperature, and an integration constant CL that decreases the F / B coefficient kaf and makes the air-fuel ratio lean are set. Therefore, the control center of the air-fuel ratio at this time is on the rich side of the control center of the stoichiometric air-fuel ratio shown by the broken line, as shown by the chain line in FIG. In this temperature rising combustion mode, the air-fuel ratio of the rich cylinder in the temperature rising combustion mode which is the stoichiometric air-fuel ratio in the previous step 126 is set to be richer, and the air-fuel ratios of the remaining cylinders are set to be closer to the stoichiometric air-fuel ratio. The average air-fuel ratio of all the cylinders # 1 to # 4 is set to the rich air-fuel ratio.

When the target air-fuel ratio is set to the rich side in this way, the rich combustion continuation permission time Tp is set in step 114. When the exhaust air-fuel ratio is set to the rich side and the rich combustion is actually performed, the fuel consumption and the emission are deteriorated. The deterioration of fuel consumption and the deterioration of emission become worse as the rich combustion duration Te becomes longer. Therefore, referring to the map shown in FIG. 7, the rich continuation permission time Tp is set based on the air-fuel ratio at the time of rich combustion.

After the rich combustion continuation permission time Tp is set, as shown in FIG. 5, the exhaust air-fuel ratio is controlled so as to reach the target air-fuel ratio on the rich side, and rich combustion is performed (step 116).

When rich combustion is performed in this way,
It is determined whether the rich combustion duration time Te is equal to or longer than the duration permission time Tp set in the previous step 114 (step 118). The continuation time Te is the continuation permission time Tp
If it is determined that the value is less than (step 118: N
O), the process proceeds to step 120. Also, the duration T
When it is determined that e is the continuing permission time Tp (step 118: YES), the process proceeds to step 122.

When it is determined that the continuation time Te is less than the continuation permission time Tp, it is determined whether the total SOx amount calculated in the previous step 100 is the predetermined value ASb or more (step 120). This predetermined value ASb reduces the SOx poisoning amount by executing the poisoning recovery process, and the NOx catalyst 2
This is a value for determining that the NOx storage capacity of 2 has recovered, and the predetermined value ASb is set so that Ast>ASb> 0.

When it is determined that the total SOx amount is the predetermined value ASb or more (step 120: YES),
It is necessary to continue the SOx poisoning recovery process, and this series of processes is once ended. In addition, the previous step 120
When it is determined that the total SOx amount is less than the predetermined value ASb and the NOx storage capacity of the NOx catalyst 22 has recovered (step 120: NO), the process proceeds to step 122.

If it is determined in the previous step 118 that the duration time Te is the duration permission time Tp, or if it is determined in step 120 that the total SOx amount is less than the predetermined value ASb, SOx poisoning recovery The poisoning recovery request flag is set to OFF to stop the processing (step 1).
22).

Then, in the next step 124, stratified charge combustion is carried out by controlling the exhaust air-fuel ratio to a lean air-fuel ratio. In addition, in the determination in the previous step 104, S
When it is determined that the Ox amount is less than the predetermined value Ast and the SOx poisoning amount still has a margin, it is determined whether the combustion mode at that time is the stoichiometric combustion mode (step 128). If it is determined that the stoichiometric combustion mode is not set (step 128: NO),
As it is not necessary to perform the SOx poisoning recovery process yet, the lean combustion with the lean air-fuel ratio is left as it is, and this series of processes is once ended. When it is determined that the stoichiometric combustion mode is set (step 128: YES), the process proceeds to step 130.

In step 130, it is determined whether the total SOx amount calculated in the previous step 100 is larger than the second predetermined value AS0 (step 130). This second
The predetermined value AS0 is a value for determining that the NOx storage capacity of the NOx catalyst 22 has recovered, and is set to the predetermined value AS0 = 0 in the present embodiment.

If it is determined that the total SOx amount is less than or equal to the predetermined value AS0 and the NOx storage capacity of the NOx catalyst 22 has recovered (step 130: NO), this series of processing is temporarily terminated.

In the determination at the previous step 130, the total SO
When it is determined that the x amount exceeds the predetermined value AS0 (step 130: YES), the process proceeds to step 132. In step 132, it is determined whether the catalyst bed temperature detected by the temperature sensor 26 is equal to or higher than a predetermined temperature THc at which SOx can be desorbed. When it is determined that the catalyst bed temperature is lower than the predetermined temperature THc (step 132: NO), this series of processes is temporarily terminated.

On the other hand, if the catalyst bed temperature exceeds the predetermined temperature THc (step 132: YES), NO.
Since SOx can be desorbed from the x catalyst 22, the combustion mode is switched to the rich combustion at the rich air-fuel ratio, and the rich combustion duration Te is measured (step 13).
4).

Next, at step 136, the target air-fuel ratio is set to the rich side according to the catalyst bed temperature. In the rich combustion in this case as well, as shown in FIG. 5, the feedback (F / B) coefficient kaf is calculated through the air-fuel ratio feedback control by the oxygen sensor 24 such that the control center is closer to the rich side than the stoichiometric air-fuel ratio. Therefore, in step 136, the feedback (F / B) constant used for the air-fuel ratio feedback control is set in the same manner as in step 112 above. In this rich combustion mode, the air-fuel ratios of all cylinders are set from the stoichiometric air-fuel ratio to the rich air-fuel ratio, and the average air-fuel ratios of all the cylinders # 1 to # 4 are also set richer than the stoichiometric air-fuel ratio. .

When the target air-fuel ratio is set to the rich side in this way, the rich combustion continuation permission time Tp1 is set in step 138. When the exhaust air-fuel ratio is set to the rich side and the rich combustion is actually performed, the fuel consumption and the emission are deteriorated. The deterioration of fuel consumption and the deterioration of emission become worse as the rich combustion duration Te becomes longer. Therefore, referring to the map shown in FIG. 7, the rich continuation permission time Tp1 is set based on the air-fuel ratio at the time of rich combustion.

After the rich combustion continuation permission time Tp1 is set, as shown in FIG. 5, the exhaust air-fuel ratio is controlled so as to reach the target air-fuel ratio on the rich side, and rich combustion is performed in all cylinders (step. 140).

When rich combustion is performed in this way,
It is determined whether the rich combustion duration time Te is equal to or longer than the duration permission time Tp1 set in the previous step 138 (step 142). The continuation time Te is the continuation permission time Tp
When it is determined that it is less than 1 (step 142:
NO), this series of processing is once ended. If it is determined that the continuation time Te is equal to or longer than the continuation permission time Tp1 (step 142: YES), the process proceeds to step 144.

Then, in the next step 144, the exhaust air-fuel ratio is controlled so as to be the stoichiometric air-fuel ratio, stoichiometric combustion is performed, and the SOx poisoning recovery process is ended. According to this embodiment described above, the following operational effects are exhibited.

When performing the sulfur poisoning recovery control of the NOx catalyst 22, the rich continuation permission time Tp is set according to the exhaust air-fuel ratio during the sulfur poisoning recovery control, and the rich combustion continuation time in the sulfur poisoning recovery control is set. Te is the continuous permission time T
When it reaches p, the sulfur poisoning recovery control is forcibly ended. Therefore, it is possible to prevent the sulfur poisoning recovery control from being continued for a long period of time, and it is possible to suppress deterioration of fuel efficiency and emission.

Moreover, since the rich continuation permission time Tp is set according to the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control, regardless of the degree of enrichment of the exhaust air-fuel ratio in the sulfur poisoning recovery control, the fuel consumption is reduced. It is possible to appropriately suppress the deterioration and the emission deterioration.

Further, since the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control is variable so that the degree of enrichment becomes larger as the catalyst temperature becomes lower, the tendency of deterioration of fuel consumption and deterioration of emission occurs remarkably. Even if the degree of enrichment is changed in this way, it is possible to preferably suppress deterioration in fuel consumption and deterioration in emissions.

The embodiment can be implemented by changing the configuration as follows. In the above embodiment, the oxygen sensor 24 provided between the three-way catalyst 20 and the NOx catalyst 22 in the exhaust passage 18
Instead of this, an air-fuel ratio sensor that linearly detects the exhaust air-fuel ratio may be provided. In this case, the rich combustion target air-fuel ratio in the poisoning recovery control may be directly set.

In the above embodiment, when the total SOx amount is the allowable value Ast or more and the catalyst bed temperature is THc or more, the rich cylinder in the temperature rising combustion is made richer and the air-fuel ratio of the lean cylinder is made richer. Although the rich combustion is performed by the above, the rich combustion may be performed by setting the air-fuel ratios of all the cylinders to be rich.

In the above embodiment, the exhaust passage 18 is provided with the three-way catalyst 20 and the NOx storage reduction catalyst 22, but the exhaust passage 18 may be provided with only the NOx storage reduction catalyst 22. In this case, during stoichiometric combustion operation, deterioration of HC and CO emissions can be avoided by exhaust gas purification by the NOx storage reduction catalyst.

In the above embodiment, the exhaust gas purification device is applied to the so-called in-cylinder injection type engine 10 in which the fuel is directly injected from the injector 14 into the combustion chamber 12, but the fuel is injected into the intake port. It can also be applied to an intake port injection type engine.

In the above embodiment, the engine is assumed to perform only the combustion (strong stratified charge combustion) in which the fuel injection timing is set to the latter stage of the compression stroke as the stratified charge combustion. In addition to such strong stratified charge combustion, for example, the fuel is divided into an intake stroke and a compression stroke and injected, so that the exhaust purification system according to the present invention is applied to an engine that performs combustion with weaker stratified strength (weak stratified charge combustion). It can also be applied. In addition, as homogeneous combustion, in addition to combustion that sets the air-fuel ratio to the stoichiometric air-fuel ratio (homogeneous stoichiometric combustion), combustion that sets the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio (homogeneous lean combustion) The exhaust gas purification device according to the invention may be applied.

In the above embodiment, the catalyst temperature of the NOx catalyst 22 is directly detected by the temperature sensor 26, but the catalyst temperature is indirectly detected based on the operating state of the internal combustion engine, more preferably based on the operating history of the internal combustion engine. Detected (estimated)
You may do it.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an exhaust emission control device for an internal combustion engine according to an embodiment.

FIG. 2 is a flowchart showing a processing procedure of poisoning recovery processing.

FIG. 3 is a flowchart showing a processing procedure of poisoning recovery processing.

FIG. 4 is an explanatory diagram showing a relationship between an exhaust air-fuel ratio and a SOx desorption amount.

FIG. 5 is an explanatory diagram showing a relationship between an F / B constant and an air-fuel ratio during poisoning recovery processing.

FIG. 6 (a) is a map showing a delay time used to set a target air-fuel ratio during poisoning recovery control;
(B) A map showing integration constants used to set the target air-fuel ratio during poisoning recovery control.

FIG. 7 is a map showing a relationship between an exhaust air-fuel ratio and a rich continuation permission time.

[Explanation of symbols]

10 ... Engine, 14 ... Injector, 16 ... Spark plug, 18 ... Exhaust passage, 20 ... Three way catalyst, 22 ... NOx storage reduction catalyst, 24 ... Oxygen sensor, 26 ... Temperature sensor, 4
0 ... Electronic control device as setting means and ending means, C ...
vehicle.

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Claims (2)

[Claims] 1. An internal combustion engine provided in an exhaust passage with a catalyst that stores a sulfur component in exhaust gas when the exhaust air-fuel ratio is at least a lean air-fuel ratio, and stores the stored amount of the sulfur component stored in the catalyst. Calculated, while performing the sulfur poisoning recovery control to control the exhaust air-fuel ratio to the rich air-fuel ratio on the condition that the calculated storage amount exceeds the allowable value, the calculated storage amount is greater than the allowable value. In an exhaust gas purification apparatus for an internal combustion engine, which terminates the sulfur poisoning recovery control when the value falls below a small predetermined value, a continuation permission of the sulfur poisoning recovery control according to an exhaust air-fuel ratio controlled by the sulfur poisoning recovery control Setting means for setting a time, and an end means for forcibly ending the sulfur poisoning recovery control when the duration of the sulfur poisoning recovery control reaches the continuation permission time. Exhaust gas purification device of the combustion engine. 2. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the exhaust air-fuel ratio controlled by the sulfur poisoning recovery control has a greater degree of enrichment as the catalyst temperature of the catalyst becomes lower. An exhaust gas purification device for an internal combustion engine, characterized in that it is variable.