JP2006348904A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, comprising a NOx storage-reduction catalyst device arranged on the downstream side of a combined part between a first exhaust passage for a first cylinder group and a second exhaust passage for a second cylinder group for carrying out lean-burn operation wherein the worsening of exhaust emission at the beginning of temperature rise control is suppressed during the temperature rise control of the NOx storage-reduction catalyst device. <P>SOLUTION: In the exhaust emission control device, the first exhaust passage 2a has a longer passage length than the second exhaust passage 2b. During the temperature rise control of the NOx storage-reduction catalyst device 6, exhaust gas of a set rich air-fuel ratio is exhausted from one of the first cylinder group 1a and the second cylinder group 1b and exhaust gas of a set lean air-fuel ratio is exhausted from the other. The exhaust gas of the set lean air-fuel ratio or the set rich air-fuel ratio is exhausted from the second cylinder group later than the exhaust gas of the set rich air-fuel ratio or the set lean air-fuel ratio is exhausted from the first cylinder group. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine that performs lean combustion.

An internal combustion engine that performs a lean combustion by spark ignition of a homogeneous air-fuel mixture that is leaner than the stoichiometric air-fuel ratio is known. Such dilute the exhaust gas of combustion includes a NO _X, in the exhaust passage of the internal combustion engine, arranged _the NO _X storage reduction catalyst device to satisfactorily absorb NO _X from the exhaust gas of a lean air-fuel ratio Has been.

The NO _X storage reduction catalyst device stores not only NO _X but also SO _X in the exhaust gas by the same mechanism as NO _X. When occlusion amount of SO _X increases, correspondingly, to storable amount of the NO _X is decreased, when the set amount of SO _X is occluded, the must be released SO _X from _the NO _X storage reduction catalyst device . SO _x is occluded as a stable substance, and cannot be released unless the NO _x occlusion reduction catalyst device is heated to a high temperature.

Thus it may increase the temperature of _the NO _X storage reduction catalyst device is needed, the temperature increase control for generally in _the NO _X storage reduction catalyst device, abundant in the exhaust gas of the rich air-fuel ratio HC and CO to be burned by oxygen contained in a large amount of lean air-fuel ratio exhaust gas.

As such temperature increase control, one of the first cylinder group and the second cylinder group corresponding to the first exhaust path and the second exhaust path that are independent from each other on the upstream side of the NO _x storage reduction catalyst device is operated with a lean air-fuel ratio. On the other hand, the rich air-fuel ratio operation is performed, and the lean air-fuel ratio exhaust gas and the rich air-fuel ratio exhaust gas are mixed at the junction of the first exhaust path and the second exhaust path to mix NO _X It has been proposed to flow into the storage reduction catalyst device (see, for example, Patent Document 1).

JP 2004-68690 A JP 2003-201892 A

In general, the path length of the first exhaust path from the first cylinder group to the merging portion is greatly different from the path length of the second exhaust path from the second cylinder group to the merging portion. In the temperature increase control, the lean air-fuel ratio of one cylinder group and the rich air-fuel ratio of the other cylinder group are selected so that the average of both becomes the stoichiometric air-fuel ratio. When the lean air-fuel ratio in the temperature raising control thus selected is leaner than the lean air-fuel ratio in normal lean combustion, when the temperature raising control is performed, the lean of one cylinder group is caused by the difference in path length. When the exhaust gas in the rich air-fuel ratio operation of the other cylinder group reaches the merging portion earlier than the exhaust gas in the air-fuel ratio operation, the exhaust gas flowing into the NO _x storage reduction catalyst device becomes rich in the initial stage of the temperature rise control, and the NO _{In the X} storage reduction catalyst device, HC and CO that could not be combusted due to lack of oxygen are released into the atmosphere, and exhaust emission is deteriorated.

Further, in normal lean combustion, a lean air-fuel ratio with a relatively small amount of NO _x generation is selected. In contrast, the lean air-fuel ratio in the Atsushi Nobori control, when the NO _X generation amount Unlike normal lean combustion is relatively large lean air-fuel ratio, in carrying out temperature increase control, the path length the difference, the exhaust gas of a lean air-fuel ratio operation of the one cylinder group from the exhaust gas of a rich air-fuel ratio operation of the other cylinder group reaches the early merging portion, the initial Atsushi Nobori control, NO _X occluding and reducing catalyst device The exhaust gas flowing into the engine becomes lean, and the NO _x storage reduction catalyst device cannot sufficiently store the NO _x produced in the lean air-fuel ratio operation of one of the cylinder groups, and the amount of NO _x released into the atmosphere Increases exhaust emissions.

Therefore, an object of the present invention is to perform lean combustion by disposing the NO _x storage reduction catalyst device downstream of the junction of the first exhaust path of the first cylinder group and the second exhaust path of the second cylinder group. In the exhaust gas purification device for an internal combustion engine, when the temperature increase control of the NO _x storage reduction catalyst device is performed, deterioration of exhaust emission at the initial stage of the temperature increase control is suppressed.

An exhaust emission control device for an internal combustion engine according to claim 1 of the present invention is an exhaust emission control device for an internal combustion engine that has at least a first cylinder group and a second cylinder group and performs lean combustion, wherein _the NO _X storage reduction catalyst device at the downstream side of the merging portion of the second exhaust path between the first exhaust path of the cylinder groups the second cylinder group is arranged, the first exhaust path is compared to the second exhaust path The exhaust gas having a set rich air-fuel ratio is discharged from one of the first cylinder group and the second cylinder group during the temperature increase control of the NO _x storage reduction catalyst device. Exhaust gas having a set lean air-fuel ratio is discharged from the other of the cylinder group and the second cylinder group, and exhausting the exhaust gas having the set lean air-fuel ratio or the set rich air-fuel ratio from the second cylinder group is the set rich Exhaust gas with air-fuel ratio or set lean air-fuel ratio This is characterized by delaying the exhaust from the first cylinder group.

  According to a second aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein the first cylinder group is operated at the set rich air-fuel ratio during the temperature increase control. And the operation of the set lean air-fuel ratio is performed in the second cylinder group.

  According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein a catalyst device having an oxidation function is disposed in the first exhaust path, One exhaust path is positioned in front of the second exhaust path, and when the temperature increase control is performed at a high load higher than a set load, the first lean group is operated at the set lean air-fuel ratio. In addition, the operation of the set rich air-fuel ratio is performed in the second cylinder group.

According to the exhaust gas purification apparatus for an internal combustion engine according to claim 1 of the present invention, when the NO _x storage reduction catalyst device is in the temperature rise control, the second exhaust path having a shorter path length than the first exhaust path is provided. The discharge of the set lean air-fuel ratio or the set rich air-fuel ratio from the corresponding second cylinder group is the same as the exhaust gas of the set rich air-fuel ratio or the set lean air-fuel ratio from the first cylinder group corresponding to the first exhaust path. The exhaust gas having the set rich air-fuel ratio or the set lean air-fuel ratio discharged from the first cylinder group and the set lean air-fuel ratio or the set rich air-fuel ratio discharged from the second cylinder group because it is delayed from the exhaust. And the exhaust gas of the exhaust gas reach the confluence portion and are mixed almost at the same time, and flow into the NO _x storage reduction catalyst device as a mixed exhaust gas having a substantially stoichiometric air-fuel ratio. Accordingly, it is possible to suppress the deterioration of exhaust emission due to the rich air-fuel ratio mixed exhaust gas or the lean air-fuel ratio mixed exhaust gas flowing into the NO _x storage reduction catalyst device in the early stage of temperature increase control.

According to the exhaust gas purification apparatus for an internal combustion engine according to claim 2 of the present invention, in the exhaust gas purification apparatus for the internal combustion engine according to claim 1, a path that is longer than the second exhaust path during temperature increase control. The set rich air-fuel ratio is operated in the first cylinder group corresponding to the first exhaust path having a long length, and the set lean air-fuel ratio is operated in the second cylinder group corresponding to the second exhaust path having a short path length. It is supposed to be implemented. Since the exhaust gas temperature is higher during operation at the set lean air-fuel ratio than during operation at the set rich air-fuel ratio, the exhaust gas having a higher temperature can be led to the junction with a short path length. Therefore, the NO _x storage reduction catalyst device is advantageous in raising the temperature of the NO _x storage reduction catalyst device particularly at low loads when the exhaust gas temperature is lower than the set load and lower.

  According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein the first exhaust path and the second exhaust path each have a catalytic device having an oxidation function. The first exhaust path is located on the vehicle front side of the second exhaust path, and when the temperature rise control is performed at a high load higher than the set load, the first cylinder group is operated at the set lean air-fuel ratio. At the same time, the operation at the set rich air-fuel ratio is performed in the second cylinder group. When the exhaust gas having a high temperature lean air-fuel ratio passes through the catalytic device having an oxidation function, the deterioration of the catalytic device is promoted. In the first exhaust path, exhaust gas having a lean air-fuel ratio at a high load where the exhaust gas temperature is higher than the set load is high, and the second exhaust path is located on the rear side of the vehicle and is difficult to be cooled. In this case, the rich air-fuel ratio exhaust gas is allowed to pass through, and thereby, the high-temperature lean air-fuel ratio exhaust gas is prevented from passing through the catalytic device having an oxidation function and promoting the deterioration of the catalytic device. .

  FIG. 1 is a schematic view showing an exhaust gas purification apparatus for an internal combustion engine according to the present invention. In the figure, reference numeral 1 denotes, for example, a V-type 6-cylinder engine body, and three cylinders are arranged in a first bank 1a as a first cylinder group and a second bank 1b as a second cylinder group, respectively. . In the present embodiment, the first bank 1a is located on the vehicle front side, and the second bank 1b is located on the vehicle rear side (vehicle compartment side). 2a is a first exhaust path of the first bank 1a, and 2b is a second exhaust path of the second bank 1b. The first exhaust path 2 a and the second exhaust path 2 b merge at the junction 4 and communicate with a single exhaust passage 5.

The engine body 1 is an internal combustion engine capable of performing lean combustion in which a homogeneous air-fuel mixture leaner than a stoichiometric air-fuel ratio such as an air-fuel ratio 20 is ignited. By appropriately selecting the air-fuel ratio for lean combustion, the amount of NO _x produced can be suppressed. However, since NO _x is still produced, the NO _x storage reduction catalyst device 6 is disposed in the exhaust passage 5.

In addition, when the engine is started, the stoichiometric air-fuel ratio (or rich air-fuel ratio) is homogeneously burned in order to improve the ignitability of the homogeneous mixture and ensure good startability. HC contained in the exhaust gas during the stoichiometric air-fuel ratio operation, CO, and three-way catalytic converter 3a and 3b for purifying NO _X each exhaust manifold of the first exhaust path 2a and the second exhaust path 2b It is arranged on the downstream side. Since these three-way catalyst devices 3a and 3b have relatively small capacities and are located in the vicinity of the engine body 1, they are warmed up immediately after the engine is started and start purification of exhaust gas.

  By providing a supercharger such as a turbocharger in the engine main body 1, it becomes possible to supply a large amount of intake air into the cylinder, whereby the aforementioned lean combustion can be performed up to the high load side. When operation at a very high load is required, the stoichiometric air-fuel ratio operation may be performed. Linear output type air-fuel ratio sensors 7a and 7b are respectively arranged immediately upstream of the three-way catalyst device 3a in the first exhaust path 2a and immediately upstream of the three-way catalyst device 3b in the second exhaust path 2b. The air-fuel ratio of the exhaust gas discharged from the first bank 1a and the second bank 1b can be detected.

_The NO _X storage reduction catalyst device 6, occludes NO _X when the air-fuel ratio of the atmosphere is lean, carrying _the NO _X storage agent air-fuel ratio to release the NO _X occluding becomes the stoichiometric air-fuel ratio or rich, lean It absorbs NO _x well from the lean air-fuel ratio exhaust gas during combustion.

NO _X in _the NO _X storage capability of the storage reduction catalyst device 6 is limited, it is necessary regeneration process to release the NO _X before _the NO _X storage amount reaches the NO _X storable amount. In this regeneration process, the air-fuel ratio of the exhaust gas flowing into the NO storage reduction catalyst device 6 may be set to a rich air-fuel ratio (or theoretical air-fuel ratio). In the regeneration process, the released NO _x is reduced and purified by the reducing substances CO and HC in the rich air-fuel ratio exhaust gas. A linear output type air-fuel ratio sensor 8 is disposed immediately upstream of the NO _x storage reduction catalyst device 6 in the exhaust passage 5, and the air-fuel ratio of the exhaust gas in the regeneration process can be monitored. Further, a step output type oxygen sensor 9 is disposed in the exhaust passage 6 directly downstream of the NO _x storage reduction catalyst device 6. In the regeneration process, the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst device 6 is made rich, but the exhaust gas exhausted from the NO _X storage reduction catalyst device 6 by the reduction purification of the released NO _X is exhausted. The fuel ratio is almost the stoichiometric air-fuel ratio. Thereby, if the output of the oxygen sensor 9 becomes rich, it can be determined that the regeneration process is completed.

By the way, sulfur S is contained in the fuel, and this sulfur S reacts with oxygen in the cylinder and becomes SO _x . Therefore, the exhaust gas also contains SO _X not only NO _X, this SO _X is also occluded in the NO _X occluding and reducing catalyst device 6 by NO _X similar mechanisms. NO _X is stored as nitrate, whereas SO _X is stored as sulfate that is more stable than nitrate. Therefore, it is not possible to release the air-fuel ratio of the exhaust gas by a regeneration process even if it is simply a rich air-fuel ratio. gradually occlusion amount increases, reducing _the NO _X storage amount capable of the NO _X occluding and reducing catalyst device 6.

Accordingly, for example, the SO _X storage amount of the NO _X storage reduction catalyst device 6 is estimated based on the accumulated fuel injection amount, and when the estimated amount reaches the set amount, the NO _X storage reduction catalyst device 6 is kept at a predetermined temperature. Then, the S poisoning recovery process is performed in which the air-fuel ratio of the exhaust gas is set to the rich air-fuel ratio (or the stoichiometric air-fuel ratio) and SO _X is released. In the S poison recovery process, as in the regeneration process, the air-fuel ratio of the exhaust gas flowing into the NO _x storage reduction catalyst device 6 by the air fuel ratio sensor 8 and the exhaust gas flowing out from the NO _x storage reduction catalyst device 6 by the oxygen sensor 9 are used. The air / fuel ratio of the gas can be detected. In the S poisoning recovery process, as described above, the temperature increase control for increasing the temperature of the NO _x storage reduction catalyst device 6 to a predetermined temperature is required. Further, the NO _X storage reduction catalyst device 6 has a temperature range in which NO _X can be stored well, and temperature increase control is required even when the temperature of the NO _X storage reduction catalyst device 6 falls below this temperature range. It is.

The NO _x storage reduction catalyst device 6 also carries a noble metal catalyst such as platinum Pt, and the temperature rise control flows a relatively large amount of unburned fuel and a relatively large amount of oxygen into the NO _x storage reduction catalyst device 6. The unburned fuel may be burned with a noble metal catalyst. For this purpose, either the first bank 1a or the second bank 1b is made rich in the combustion air-fuel ratio, and a relatively large amount of HC and CO in the exhaust gas is caused to flow into the NO _x storage reduction catalyst device 6, The other of the first bank 1a and the second bank 1b makes the combustion air-fuel ratio lean, and a relatively large amount of O ₂ and NO _x in the exhaust gas is caused to flow into the NO _x storage reduction catalyst device 6. Assuming that the combustion air-fuel ratio is rich and lean for each cylinder in one bank, HC and CO discharged from the rich combustion air-fuel ratio cylinder use O ₂ and NO _X discharged from the lean combustion air-fuel ratio cylinder. Te, to become burned in the three-way catalytic converter 3a or 3b, it is impossible to raise the temperature of the _the nO _X storage reduction catalyst device 6. Thus, as described above, the temperature increase control needs to make the combustion air-fuel ratio rich and lean for each bank.

Generally, the path length of the first exhaust path 2a from the first bank 1a to the merging portion 4 and the path length of the second exhaust path 2b from the second bank 1b to the merging section 4 are greatly different from each other. Since the first bank 1a is located on the front side of the vehicle, the first exhaust path 2a is longer than the second exhaust path 2b. Thereby, when the temperature increase control is necessary, the combustion air-fuel ratio is made rich in one of the first bank 1a and the second bank 1b, and at the same time, the combustion air-fuel ratio is made lean in the other of the first bank 1a and the second bank 1b. Then, the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas do not reach the joining portion at the same time, and the air-fuel ratio of the mixed exhaust gas flowing into the NO _x storage reduction catalyst device 6 at the initial stage of the temperature rise control is Rich or lean.

In this embodiment, immediately before the temperature increase control, for example, lean combustion with an air-fuel ratio of 20, for example, is performed. When this lean combustion is on the low load side, the exhaust gas temperature is low, and the temperature of the NO _x storage reduction catalyst device 6 is also low. Accordingly, when raising the temperature to the desired temperature, a considerable amount of HC and CO must be burned, and in one bank, the combustion air-fuel ratio needs to be a rich air-fuel ratio of 9 or less, for example. In this case, since the number of cylinders in each bank is the same, in the other bank, the lean air-fuel ratio (14.7) is set so that the average air-fuel ratio with the rich air-fuel ratio in one bank becomes the stoichiometric air-fuel ratio (14.7). Combustion at 20.4 or more is started.

In such temperature rise control, when the rich air-fuel ratio exhaust gas first reaches the merging portion 4 due to the difference in the path length between the first exhaust path 2a and the second exhaust path 2b, the lean air of normal lean combustion As a result of mixing with the exhaust gas of the fuel ratio, the air-fuel ratio of the exhaust gas at the junction 4 becomes rich, and the exhaust gas of the rich air-fuel ratio flows into the NO _x storage reduction catalyst device 6 at the beginning of the temperature raising control. . At this time, there are HC and CO that cannot be oxidized due to lack of oxygen in the NO _x storage reduction catalyst device 6, and these are released into the atmosphere, which deteriorates exhaust emission.

When the air-fuel ratio of one bank is set to a rich air-fuel ratio (9.4) whose average value is the stoichiometric air-fuel ratio with respect to the air-fuel ratio 20 of normal lean combustion, the combustion air-fuel ratio of the other bank The air-fuel ratio remains normal lean combustion, and there is no particular problem. However, when the combustion air-fuel ratio of one bank is set to the other rich air-fuel ratio, the combustion air-fuel ratio of the other bank is set to an air-fuel ratio different from the lean air-fuel ratio of normal lean combustion. Since a relatively large amount of NO _x is generated in the combustion at the fuel ratio, the lean air-fuel ratio exhaust gas is first supplied to the junction 4 due to the difference in the path length between the first exhaust path 2a and the second exhaust path 2b. occlusion is reached, the initial Atsushi Nobori control, a large amount of the NO _X flows into _the NO _X storage reduction catalyst device 6, the exhaust gas of a rich air-fuel ratio for reducing the NO _X even absent, by _the NO _X storage reduction catalyst device 6 not be nO _X is released into the atmosphere, again, worsening the exhaust emissions.

  As described above, in the temperature rise control, even if the rich air-fuel ratio exhaust gas reaches the joining portion first or the lean air-fuel ratio exhaust gas reaches the joining portion first, it is a factor that deteriorates the exhaust emission. Become. In order to solve this problem, in the present embodiment, the temperature increase control is performed according to the flowchart shown in FIG.

First, in step 101, it is determined whether there is a request for temperature increase control. When this judgment is denied, the process is terminated as it is. On the other hand, when there is a request for temperature increase control, in step 102, the current operating region is detected based on the amount of depression of the accelerator pedal, the engine speed, and the like. As described above, when the current operation region is on the low load side smaller than the set load, the exhaust gas temperature is low, and a considerable amount of HC and CO is required to raise the NO _x storage reduction catalyst device 6 to the desired temperature. In one bank, combustion with a relatively small rich air-fuel ratio is required. On the other hand, when the current operation region is on the high load side that is equal to or higher than the set load, the exhaust gas temperature is high, and it is necessary to burn so much HC and CO in order to raise the NO _x storage reduction catalyst device 6 to the desired temperature. However, in one bank, combustion may be performed with a relatively large rich air-fuel ratio. When determining the rich air-fuel ratio, the higher the engine speed, the higher the HC and CO emissions per unit time at the same rich air-fuel ratio. Therefore, the engine speed is also considered in determining the rich air-fuel ratio.

  Thus, the combustion rich air-fuel ratio in one bank that is set to the rich air-fuel ratio is determined, and the number of cylinders in each bank is the same, so the average value is the stoichiometric air-fuel ratio with respect to the determined rich air-fuel ratio. Is determined as the combustion lean air-fuel ratio of the other bank.

  Incidentally, FIG. 2 shows the relationship between the combustion air-fuel ratio and the exhaust gas temperature. In FIG. 2, the solid line is when the load is low, and the dotted line is when the load is high. As shown in the figure, over the entire combustion air-fuel ratio, the exhaust gas temperature at low load is lower than the exhaust gas temperature at high load, and at any load, the exhaust gas temperature at rich air-fuel ratio is lean. It is lower than the air-fuel ratio exhaust gas temperature. In FIG. 2, R1 is the combustion rich air / fuel ratio A / F determined at low load, L1 is the combustion lean air / fuel ratio A / F determined at low load, and R2 is determined at high load. The combustion rich air-fuel ratio A / F, and L2 is the combustion lean air-fuel ratio A / F determined at the time of high load.

Thus, the lean air-fuel ratio exhaust gas temperature is higher than the rich air-fuel ratio, so the lean air-fuel ratio exhaust gas passes through a short path length, and the rich air-fuel ratio exhaust gas passes through a long path length. so as to, who when the exhaust gas was allowed to flow into _the NO _X storage reduction catalyst device 6, as compared to the opposite case, the entire exhaust gas to the exhaust gas flows into _the NO _X storage reduction catalyst device 6 This is advantageous for increasing the temperature of the NO _x storage reduction catalyst device 6. Thereby, particularly when the exhaust gas temperature is low and the load is low, lean air-fuel ratio combustion is performed in the second bank 1b corresponding to the second exhaust path 2b having a short path length, and the first exhaust path having a long path length is achieved. Rich air-fuel ratio combustion is performed in the first bank 1a corresponding to 2a.

  By the way, it has been found that passing a high-temperature lean air-fuel ratio exhaust gas through the three-way catalyst devices 3a and 3b promotes the deterioration of the noble metal catalyst supported on the three-way catalyst device, thereby providing the exhaust gas. During a high load when the temperature becomes high, a second exhaust path that is located on the front side of the vehicle and allows the exhaust gas having a lean air-fuel ratio to pass through the first exhaust path 2a that is easy to be cooled by the traveling wind and is not easily cooled on the rear side of the vehicle. A rich air-fuel ratio exhaust gas is passed through 2b. That is, at the time of high load, lean air-fuel ratio combustion is performed in the first bank 1a, and rich air-fuel ratio combustion is performed in the second bank 1b.

In this manner, in step 103, the combustion air-fuel ratios A / F of the first bank 1a and the second bank 1b in the temperature increase control are determined according to the current operation region. Next, in step 104, the delay time T1 and the temperature rise time T2 are determined. The delay time T1 is exhausted from the first bank 1a and passes through the first exhaust path 2a having a long path length, and exhausted from the second bank 1b and passing through the second exhaust path 2b having a short path length. The time difference between the gas and the first exhaust path 2a and the second exhaust path 2b, the amount of exhaust gas in the current operating state (at the start of temperature rise control), etc. To be determined. On the other hand, heating time T2 is the time from the change of the combustion air-fuel ratio of the first bank 1a to the combustion air-fuel ratio A / F of the Atsushi Nobori control until _the NO _X storage reduction catalyst device 6 is heated to the desired temperature It is determined based on the current exhaust gas amount, the temperature of the NO _x storage reduction catalyst estimated based on the current operating state, the combustion rich air-fuel ratio in the temperature rise control, and the like.

  Next, at step 105, the combustion air-fuel ratio of the first bank 1a is changed to the combustion air-fuel ratio A / F of the temperature increase control determined at step 103. In step 106, an elapsed time t after the combustion air-fuel ratio of the first bank 1a is changed is calculated. Next, at step 107, it is determined whether or not the elapsed time t has reached the delay time T1, and when this determination is negative, the combustion air-fuel ratio of the second bank 1b is not changed to the combustion air-fuel ratio of the temperature increase control. .

If the elapsed time t after the change of the combustion air-fuel ratio in the first bank 1a reaches the delay time T1, the determination in step 107 is affirmed. In step 108, the combustion air-fuel ratio in the second bank 1b is determined in step 103. It is changed to the combustion air-fuel ratio A / F of the raised temperature control. As a result, the exhaust gas from the first bank 1a and the second bank 1b, both of which have been changed to the combustion air-fuel ratio for temperature increase control, simultaneously reaches the merging section 4 and is mixed to become the exhaust gas of the stoichiometric air-fuel ratio. _It flows into the _X storage reduction catalyst device 6. Thus, HC and CO in the rich air-fuel ratio exhaust gas are oxidized without excess and deficiency using O ₂ and NO _x in the lean air-fuel ratio exhaust gas, and the NO _x storage reduction catalyst device without deteriorating exhaust emissions. 6 is started.

  Next, at step 109, it is determined whether or not the elapsed time t after the change of the combustion air-fuel ratio of the first bank 1a has reached the temperature raising time T2, and when this determination is negative, the first bank 1a And the combustion air fuel ratio of the 2nd bank 1b is maintained at the combustion air fuel ratio of temperature rising control. On the other hand, if the elapsed time t reaches the temperature rise time T2, the determination in step 109 is affirmed, the combustion air-fuel ratio in the first bank 1a is made the stoichiometric air fuel ratio (stoichiometric) in step 110, and the temperature rise time T2 in step 111. A further time lapse of the elapsed time t reached is calculated.

  Next, at step 112, it is determined whether or not the elapsed time t has reached the sum of the delay time T1 and the temperature rise time T2. When this determination is negative, the combustion air-fuel ratio of the first bank 1a is maintained at the stoichiometric air-fuel ratio, and the second bank 1b is maintained at the combustion air-fuel ratio of temperature increase control. On the other hand, if the elapsed time t reaches the above-mentioned sum (T1 + T2), the determination in step 112 is affirmed, and in step 113, the combustion air-fuel ratio in the second bank 1b is made the stoichiometric air-fuel ratio (stoichiometric).

Although it has been described with respect to the three-way catalyst devices 3a and 3b that the exhaust gas having a high temperature lean air-fuel ratio promotes the deterioration of the noble metal catalyst, of course, if the noble metal catalyst is at a high temperature, the lean air-fuel ratio is not so high. Even if the exhaust gas comes into contact with the noble metal catalyst, the deterioration is similarly promoted. As a result, if the combustion air-fuel ratio of both banks 1a and 1b is set to a normal lean air-fuel ratio immediately after the temperature raising control, the noble metal catalyst supported on the NO _x storage reduction catalyst device 6 is deteriorated.

In order to prevent this, it is preferable that the combustion air-fuel ratio of both banks 1a and 1b is the stoichiometric air-fuel ratio immediately after the temperature raising control, and the temperature of the NO _x storage reduction catalyst device 6 is lowered. In this case, if the combustion air-fuel ratios of both banks 1a and 1b are simultaneously set to the stoichiometric air-fuel ratio, the stoichiometric air-fuel ratio exhaust gas discharged from the second bank 1b first reaches the junction 4 due to the difference in path length. After all, since the air-fuel ratio of the mixed exhaust gas flowing into the NO _x storage reduction catalyst device 6 becomes rich or lean and the same problem as in the initial stage of temperature rise control occurs, in this flowchart, the first bank 1a The combustion air-fuel ratio is changed from the combustion air-fuel ratio of the temperature raising control to the stoichiometric air-fuel ratio first, and when the delay time T1 has elapsed from this time, the combustion air-fuel ratio of the second bank 1b is changed from the combustion air-fuel ratio of the temperature raising control to the stoichiometric air-fuel ratio. It is supposed to be. Thereby, the exhaust gas of the stoichiometric air-fuel ratio which is discharged from the first and second bank 1a, 1b reaches the merging portion 4 simultaneously, the mixed exhaust gas rich or lean flows into _the NO _X storage reduction catalyst device 6 Is suppressed.

If the temperature of the NO _x storage reduction catalyst device 6 decreases, the combustion air-fuel ratios of the first bank 1a and the second bank 1b are changed to normal lean air-fuel ratios. At this time, even if the exhaust gas of a lean air-fuel ratio which is discharged from the second bank 1b (e.g. 20) reaches the merging portion 4 above, the amount of the NO _X is suppressed, there is no particular problem.

When the temperature increase control is performed for the S poison recovery process, the combustion air-fuel ratios of the first bank 1a and the second bank 1b are made rich immediately after the temperature increase control is completed. When the output of the oxygen sensor 9 becomes rich and the S poisoning recovery process is completed, if the temperature of the NO _x storage reduction catalyst device 6 is higher than the temperature range in which the NO _x is stored well, the delay will occur as described above. It is preferable to set the combustion air-fuel ratio of the first bank 1a and the second bank 1b to the stoichiometric air-fuel ratio by providing time. Further, when the temperature increase control when the temperature of the NO _X storage reduction catalyst device 6 falls below the temperature range where NO _X is stored well exceeds the temperature range, the temperature of the NO _X storage catalyst device is increased. In addition, as described above, it is preferable to provide a delay time so that the combustion air-fuel ratio of the first bank 1a and the second bank 1b is the stoichiometric air-fuel ratio.

  In the temperature increase control of the present embodiment, the combustion air-fuel ratio of one cylinder group is set to the set rich air-fuel ratio, and the combustion air-fuel ratio of the other cylinder group is set to the set rich air-fuel ratio. When the fuel injection valve for injecting fuel is provided, the combustion air-fuel ratio of both cylinder groups is set as the set lean air-fuel ratio for temperature rise control (the combustion air-fuel ratio of one cylinder group is also set as the normal lean air-fuel ratio). However, in each cylinder of one cylinder group, additional fuel may be injected into the cylinder during the expansion stroke or the exhaust stroke so that the inside of the cylinder has a set rich air-fuel ratio for temperature rise control. Even in this case, it is preferable that the lean air-fuel ratio exhaust gas exhausted from one cylinder group and the rich air-fuel ratio exhaust gas exhausted from the other cylinder group simultaneously reach the junction of each exhaust path.

  In the present embodiment, the internal combustion engine has only the first cylinder group and the second cylinder group, but this does not limit the present invention. For example, when three or more cylinder groups having different exhaust paths are provided, the rich air-fuel ratio or the lean air-fuel ratio exhaust gas discharged from these cylinder groups is merged in each exhaust path at the time of temperature increase control. In order to reach the engine at the same time, the exhaust gas having a rich air-fuel ratio or a lean air-fuel ratio may be discharged earlier in the cylinder group having a longer exhaust path length to the merging section. Further, if there is a cylinder group that exhausts the exhaust gas having the stoichiometric air-fuel ratio instead of the rich air-fuel ratio or the lean air-fuel ratio during the temperature rise control, the exhaust of the stoichiometric air-fuel ratio from this cylinder group is discharged to other cylinder groups. The exhaust gas of the rich air-fuel ratio and the lean air-fuel ratio exhausted from the exhaust gas can be freely set.

It is the schematic which shows the structure of the exhaust gas purification apparatus of the internal combustion engine by this invention. It is a graph which shows the relationship between a combustion air fuel ratio and exhaust gas temperature. It is a flowchart for implementing temperature rising control.

Explanation of symbols

1 engine body 1a first bank 1b second bank 2a first exhaust path 2b second exhaust path 3a, 3b three-way catalytic converter 4 merging portion 6 NO _X occluding and reducing catalyst device

Claims (3)

An exhaust emission control device for an internal combustion engine having at least a first cylinder group and a second cylinder group and performing lean combustion, wherein the first exhaust path of the first cylinder group and the second exhaust of the second cylinder group A NO _x storage reduction catalyst device is disposed downstream of the junction with the passage, the first exhaust path has a longer path length than the second exhaust path, and the NO _x storage reduction catalyst device is During temperature control, exhaust gas having a set rich air-fuel ratio is discharged from one of the first cylinder group and the second cylinder group, and exhaust gas having a set lean air-fuel ratio is discharged from the other of the first cylinder group and the second cylinder group. Exhausting the exhaust gas of the set lean air-fuel ratio or the set rich air-fuel ratio from the second cylinder group, the first cylinder group of the exhaust gas having the set rich air-fuel ratio or the set lean air-fuel ratio To be later than the discharge from An exhaust gas purification apparatus for an internal combustion engine characterized by the above.   2. The operation of the set rich air-fuel ratio is performed in the first cylinder group and the operation of the set lean air-fuel ratio is performed in the second cylinder group during the temperature increase control. Exhaust gas purification device for internal combustion engine.   When the catalyst device having an oxidation function is disposed in the first exhaust path, the first exhaust path is located on the vehicle front side with respect to the second exhaust path, and the temperature increase control is performed at a high load higher than a set load. 2. The internal combustion engine according to claim 1, wherein the operation of the set lean air-fuel ratio is performed in the first cylinder group and the operation of the set rich air-fuel ratio is performed in the second cylinder group. Exhaust purification equipment.

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