JP2003307143A - Air-fuel ratio control apparatus for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce interruption of lean operation, and efficiently desorb sulfur component form a catalyst, and thereby to suppress degradation of fuel economy. <P>SOLUTION: An engine 10 has a NOx occlusion and reduction catalyst 22 disposed in an exhaust passage 18 to occlude sulfur components in exhaust gas. In the engine 10, a lean operation mode in which an exhaust air-fuel ratio is to be a lean air-fuel ratio, and a stoichiometric operation mode in which the exhaust air-fuel ratio is to be a stoichiometric air-fuel ratio are provided as the engine combustion mode. An electric control unit 40 controls an air-fuel ratio of air-fuel mixture so that the exhaust air-fuel ratio becomes a rich air-fuel ratio when a occlusion amount of the sulfur components of the NOx occlusion and reduction catalyst 22 exceeds a predetermined value during the stoichiometric operation mode of the engine 10. <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 air-fuel ratio control system for an internal combustion engine having a catalyst for storing a sulfur component in exhaust gas when the exhaust air-fuel ratio is lean.

[0002]

2. Description of the Related Art In recent years, an internal combustion engine having a lean operation mode in which the fuel concentration is lower than the stoichiometric air-fuel ratio (theoretical air-fuel ratio) and a stoichiometric operation mode in which the exhaust air-fuel ratio is the stoichiometric air-fuel ratio Institutions are proposed. In such an internal combustion engine, a NOx storage reduction catalyst device is provided in the exhaust passage in order to efficiently purify nitrogen oxides (NOx) contained in the exhaust gas during lean operation. This catalyst device temporarily stores nitrogen oxides (NOx) contained in the exhaust gas during lean combustion. And
When combustion is performed at an air-fuel ratio of the stoichiometric air-fuel ratio or a rich air-fuel ratio having a higher fuel concentration than that, hydrocarbons (HC) containing NOx stored therein are contained in the exhaust gas.
It is reduced and purified by carbon monoxide (CO) and released.

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, as disclosed in Japanese Patent Laid-Open Nos. 2000-80914 and 2000-161107, the above NO
In order to recover the sulfur poisoning of the x storage reduction catalyst device, the SOx amount stored in the catalyst is calculated, and when the calculated SOx amount exceeds the allowable value, the exhaust air-fuel ratio is controlled to the rich air-fuel ratio. Sulfur poisoning recovery control is being performed.

[0005]

By the way, the above NOx
The storage reduction catalyst device stores the sulfur component (SOx) in the exhaust gas when the exhaust air-fuel ratio is lean. Therefore, when the sulfur poisoning recovery control described above is executed, the lean operation of the internal combustion engine is interrupted, and the fuel concentration of the air-fuel mixture supplied to the internal combustion engine, that is, the amount of fuel supplied to the internal combustion engine is increased. become. Therefore, fuel consumption is deteriorated by the sulfur poisoning recovery control.

The present invention has been made in view of such circumstances, and an object thereof is to prevent interruption of lean operation and to efficiently desorb a sulfur component from a catalyst. Therefore, it is an object of the present invention to provide an air-fuel ratio control device for an internal combustion engine that can suppress deterioration of fuel efficiency.

[0007]

[Means for Solving the Problems] Means for achieving the above-mentioned objects and their effects will be described below. The invention according to claim 1 is applied to an internal combustion engine in which a catalyst that stores 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 exhaust air-fuel ratio is a lean air-fuel ratio. Has a lean operation mode and a stoichiometric operation mode in which the exhaust air-fuel ratio is the theoretical air-fuel ratio, and selects an operation mode based on the engine operating state, and the exhaust air-fuel ratio is theoretical when the stoichiometric operation mode is selected. In an air-fuel ratio control device for an internal combustion engine that controls the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine so as to have an air-fuel ratio, a calculating means for calculating the storage amount of the sulfur component stored in the catalyst, and this calculation When the stored amount exceeds the allowable value, the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio becomes the rich air-fuel ratio, and the calculated stored amount exceeds the allowable value. When even exceeds a predetermined small value, characterized in that a control means for controlling the air-fuel ratio of the mixture to a rich air-fuel ratio of the exhaust air-fuel ratio when the stoichiometric operation mode has been selected.

According to the above configuration, when the calculated storage amount of the sulfur component exceeds the predetermined value, the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio when the stoichiometric operation mode is selected becomes the rich air-fuel ratio. To do. At this time, desorption of the sulfur component from the catalyst is promoted, and the frequency with which the stored amount of the sulfur component stored in the catalyst exceeds the allowable value decreases. Therefore,
It is possible to suppress interruption of the lean operation for desorbing the sulfur component from the catalyst, and to efficiently desorb the sulfur component from the catalyst. as a result,
It is possible to suppress deterioration of fuel efficiency.

According to a second aspect of the present invention, in the air-fuel ratio control device for an internal combustion engine according to the first aspect, the control means is calculated on the condition that the temperature of the catalyst is a predetermined value or more. When the stored amount exceeds the predetermined value, the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio when the stoichiometric operation mode is selected becomes a rich air-fuel ratio.

When the temperature of the catalyst is low, even if the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio becomes a rich air-fuel ratio, it is difficult for the sulfur component to be desorbed, and there is unnecessary fuel consumption for raising the catalyst temperature. . In this regard, according to the configuration of claim 2, the exhaust air-fuel ratio when the stoichiometric operation mode is selected is set to the rich air-fuel ratio when the temperature of the catalyst is equal to or higher than a predetermined value as in the above configuration. Therefore, wasteful fuel consumption can be suppressed and deterioration of fuel efficiency can be suppressed.

According to a third aspect of the present invention, in the air-fuel ratio control device for an internal combustion engine according to the first or second aspect, the control means controls the rich air-fuel ratio when the stoichiometric operation mode is selected. It is characterized in that the duration of the combustion is limited to within the duration permitted time according to the same rich air-fuel ratio.

Desorption of the sulfur component from the catalyst is performed by controlling the exhaust air-fuel ratio to a rich air-fuel ratio. However, when the catalyst temperature is low and the sulfur component is difficult to be desorbed, rich combustion may be prolonged. , Fuel efficiency may deteriorate. In this respect, according to the configuration of claim 3, since the duration of the combustion at the rich air-fuel ratio is limited to be within the continuation permission time according to the rich air-fuel ratio, the rich combustion is continued for a long period of time. Can be prevented, and deterioration of fuel efficiency can be suppressed.

The invention according to claim 4 is the air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein:
The rich air-fuel ratio when the stoichiometric operation mode is selected is variable so that the enrichment degree increases at least as the temperature of the catalyst decreases.

According to the above arrangement, the rich air-fuel ratio is variable so that the rich degree becomes larger at least as the catalyst temperature becomes lower. Therefore, the lower the catalyst temperature, the more difficult the sulfur component is desorbed, but the rich degree becomes higher. By increasing the amount, the desorption of the sulfur component from the catalyst can be promoted, and the sulfur poisoning recovery of the catalyst can be suitably performed.

[0015]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an air-fuel ratio control device according to this embodiment and a vehicle in-cylinder injection four-cylinder gasoline engine (hereinafter simply referred to as “engine”).
(Referred to as 10).

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.

In 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.

Further, in the engine 10 according to the present 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 ignition 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).

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 thus switching the engine combustion mode to the temperature rising combustion mode, the unburned fuel components in the exhaust gas discharged from some of the cylinders in which the 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 the temperature 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.

If 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 step 118 that the duration 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 is performed. 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 performed by controlling the exhaust air-fuel ratio to be 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 judgment of 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 become 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 processing 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 increased. 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 increases as the catalyst temperature becomes lower, there is a marked tendency to deteriorate fuel efficiency and emission. 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.

Further, when the total SOx amount of the NOx catalyst 22 exceeds a predetermined value AS0 (<allowable value ASt), the exhaust air-fuel ratio when the stoichiometric operation mode of the engine 10 is selected is controlled to be the rich air-fuel ratio. To be done. At this time, desorption of the sulfur component from the catalyst is promoted, and the frequency with which the stored amount of the sulfur component stored in the catalyst exceeds the allowable value decreases. Therefore, it is possible to suppress interruption of the lean operation for desorbing the sulfur component from the catalyst, and it is possible to efficiently desorb the sulfur component from the catalyst. As a result, deterioration of fuel efficiency can be suppressed.

Further, when the stoichiometric operation mode is selected, the exhaust air-fuel ratio when the stoichiometric operation mode is selected is set to a rich air-fuel ratio on condition that the catalyst bed temperature is equal to or higher than the predetermined temperature THc. Controlled. Therefore, it is possible to suppress unnecessary fuel consumption for increasing the catalyst bed temperature and suppress deterioration of fuel efficiency.

Further, the rich air-fuel ratio when the stoichiometric operation mode is selected is variable so that the rich degree becomes larger at least as the catalyst temperature becomes lower. Therefore, as the catalyst temperature becomes lower, the sulfur component is less likely to be desorbed from the NOx catalyst 22, but by increasing the rich degree, the desorption of the sulfur component from the NOx catalyst 22 can be promoted, and the NOx catalyst 22 is suitable. Can recover sulfur poisoning.

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 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 combustion) in which the fuel injection timing is set to the latter stage of the compression stroke as the stratified 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 temperature sensor 26 directly detects the catalyst temperature of the NOx catalyst 22, but the catalyst temperature is indirectly determined 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 air-fuel ratio 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.

[Description of Reference Signs] 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 calculation means and control means, C ...
vehicle.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 3G091 AA02 AA12 AA17 AA23 AA24                       AA28 AB03 AB06 BA11 BA14                       BA15 BA19 BA33 CB02 CB03                       DA01 DA02 DA04 DB06 DB10                       DC01 EA01 EA07 EA18 EA30                       EA34 EA39 FB10 FB11 FB12                       FC02 HA08 HA36 HA37 HA39                 3G301 HA01 HA04 HA06 HA16 JA02                       JA21 KA06 KA23 LB04 MA01                       NA08 NB03 NB13 NC02 ND01                       NE13 NE15 NE23 PD02A                       PD02Z PD12Z PE01Z PF01Z                       PF03Z

Claims (4)

[Claims] 1. A lean system in which a catalyst for storing a sulfur component in exhaust gas when an exhaust air-fuel ratio is at least a lean air-fuel ratio is applied to an internal combustion engine provided in an exhaust passage, and the exhaust air-fuel ratio is a lean air-fuel ratio. While having an operation mode and a stoichiometric operation mode in which the exhaust air-fuel ratio is the stoichiometric air-fuel ratio, the operation mode is selected based on the engine operating state, and when the stoichiometric operation mode is selected, the exhaust air-fuel ratio becomes the theoretical air-fuel ratio. In the air-fuel ratio control device of the internal combustion engine for controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine, the calculation means for calculating the storage amount of the sulfur component stored in the catalyst, and the calculated storage amount When exceeding the allowable value, the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio becomes a rich air-fuel ratio, and the calculated storage amount is smaller than the allowable value. An air-fuel ratio of the internal combustion engine, comprising: a control means for controlling the air-fuel ratio of the air-fuel mixture so that the exhaust air-fuel ratio when the stoichiometric operation mode is selected becomes a rich air-fuel ratio when the stoichiometric operation mode is exceeded. Control device. 2. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the control means sets the calculated storage amount to the predetermined value on condition that the temperature of the catalyst is equal to or higher than a predetermined value. An air-fuel ratio control device for an internal combustion engine, wherein the air-fuel ratio of the air-fuel mixture is controlled so that the exhaust air-fuel ratio when the stoichiometric operation mode is selected is set to a rich air-fuel ratio when it exceeds. 3. The air-fuel ratio control device for an internal combustion engine according to claim 1 or 2, wherein the control means controls the combustion duration at the rich air-fuel ratio when the stoichiometric operation mode is selected. An air-fuel ratio control device for an internal combustion engine, wherein the air-fuel ratio control is restricted so that the duration is within a permission period according to the rich air-fuel ratio. 4. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the rich air-fuel ratio when the stoichiometric operation mode is selected is richer at least as the temperature of the catalyst becomes lower. An air-fuel ratio control device for an internal combustion engine, wherein the air-fuel ratio control device is variable so that the degree of conversion is increased.