JP2006322344A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a three-way catalyst effectively function as early as possible after completion of a sulfur poisoning recovery control of a NOx catalyst. <P>SOLUTION: An exhaust emission control device for an internal combustion engine has a plurality of cylinders #1 to #4. The cylinders are divided into at least two cylinder groups. The cylinder groups are respectively connected with exhaust branch pipes 5, 6, and the exhaust branch pipes are merged downstream to be connected with a common exhaust pipe 7. The three-way catalysts 8, 9 are respectively disposed in the exhaust pipes, and the NOx catalyst 10 is disposed in the common exhaust pipe. The exhaust emission control device performs a sulfur poisoning recovery control of the NOx catalyst, by discharging exhaust gas of a rich air-fuel ratio from one cylinder group and discharging exhaust gas of a lean air-fuel radio from the other cylinder group. When the sulfur poisoning recovery control is completed, an exhaust air-fuel ratio reverse control is performed to discharge the exhaust gas of the lean air-fuel ratio from the cylinder group which has discharged the exhaust gas of the rich air-fuel ratio during the sulfur poisoning recovery control. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  As a catalyst for reducing and purifying nitrogen oxide (NOx) in exhaust gas discharged from an internal combustion engine, it absorbs NOx in exhaust gas when the air-fuel ratio of the exhaust gas flowing into it is leaner than the stoichiometric air-fuel ratio Or a catalyst of the type that reduces and purifies NOx retained when the air-fuel ratio of the exhaust gas flowing into it becomes richer than the stoichiometric air-fuel ratio or the stoichiometric air-fuel ratio (hereinafter referred to as “NOx catalyst”). ") Is known. Further, when the air-fuel ratio of the exhaust gas flowing into the exhaust gas is at or near the stoichiometric air-fuel ratio, NOx, carbon monoxide (CO), and hydrocarbon (HC) in the exhaust gas are simultaneously purified with a high purification rate. A three-way catalyst is known as a catalyst. Patent Document 1 discloses an internal combustion engine provided with these NOx catalyst and three-way catalyst.

  The internal combustion engine disclosed in Patent Document 1 includes six cylinders, and these cylinders are divided into two cylinder groups. An exhaust pipe (hereinafter referred to as “exhaust branch pipe”) is connected to each cylinder group, and these exhaust branch pipes merge downstream to form one common exhaust pipe (hereinafter referred to as “common exhaust pipe”). . A three-way catalyst is disposed in each exhaust branch pipe, and a NOx catalyst is disposed in the common exhaust pipe.

  By the way, the exhaust gas contains sulfur oxide (SOx) in addition to NOx. When NOx is held in the NOx catalyst, SOx is also held in the NOx catalyst. Thus, if SOx is retained in the NOx catalyst (that is, the NOx catalyst is poisoned by the sulfur component), the amount of NOx that can be retained by the NOx catalyst is reduced accordingly. For this reason, in order to maintain the NOx retention capacity of the NOx catalyst as high as possible, it is necessary to remove SOx from the NOx catalyst. In order to remove SOx from the NOx catalyst (that is, to recover the NOx catalyst from poisoning by the sulfur component), the temperature of the NOx catalyst is raised to a temperature at which SOx can be removed, and flows into the NOx catalyst. It is necessary to make the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or rich (weakly rich).

  Therefore, in Patent Document 1, in order to remove SOx from the NOx catalyst, the following sulfur poisoning recovery control is performed. That is, the air-fuel ratio of exhaust gas discharged from one cylinder group is made rich, and the air-fuel ratio of exhaust gas discharged from the other cylinder group is made lean, and these rich air-fuel ratio exhaust gases (hereinafter referred to as “rich exhaust gas”) And a lean air-fuel ratio exhaust gas (hereinafter referred to as “lean exhaust gas”) are combined upstream of the NOx catalyst and then flowed into the NOx catalyst. Here, in Patent Document 1, when the rich exhaust gas and the lean exhaust gas are merged, the rich degree of the rich exhaust gas and the lean exhaust gas are set such that the total air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio. The lean degree of has been adjusted.

  According to this, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is the stoichiometric air-fuel ratio. Further, when the rich exhaust gas and the lean exhaust gas merge, the HC in the rich exhaust gas is in the lean exhaust gas. It reacts with oxygen, and the heat of reaction raises the temperature of the exhaust gas. As a result, the temperature of the NOx catalyst is raised. Thus, in Patent Document 1, the temperature of the NOx catalyst is raised to a temperature at which SOx can be removed, and exhaust gas having a stoichiometric air-fuel ratio is supplied to the NOx catalyst to remove SOx from the NOx catalyst. .

JP 2004-68690 A Japanese Patent Laid-Open No. 11-343836 JP 2000-18025 A

  By the way, when performing the above-described sulfur poisoning recovery control, rich exhaust gas continues to flow into one of the three-way catalysts, and lean exhaust gas continues to flow into the other three-way catalyst. For this reason, when the sulfur poisoning recovery control is finished, one of the three-way catalysts is poisoned by CO or HC in the rich exhaust gas, or the other three-way catalyst is poisoned by oxygen in the lean exhaust gas. I'm going. For this reason, even if the conditions under which the three-way catalyst should function after the completion of the sulfur poisoning recovery control are satisfied, the three-way catalyst may not function effectively.

  Therefore, an object of the present invention is to provide a three-way catalyst as early as possible after the sulfur poisoning recovery control of the NOx catalyst is completed in the exhaust gas purification apparatus for an internal combustion engine provided with the three-way catalyst and the NOx catalyst as described above. Is to enable the three-way catalyst to function effectively when the sulfur poisoning recovery control of the NOx catalyst is completed.

  In order to solve the above-mentioned problem, in the first invention, a plurality of cylinders are provided, these cylinders are divided into at least two cylinder groups, exhaust branch pipes are connected to the respective cylinder groups, and these exhaust branch pipes are connected to the downstream side. In which the three-way catalyst is disposed in each exhaust branch pipe, and the NOx catalyst is disposed in the one common exhaust pipe. In the exhaust purification apparatus that performs control for exhausting rich air-fuel ratio exhaust gas from one cylinder group and exhausting lean air-fuel ratio exhaust gas from the other cylinder group as sulfur poisoning recovery control of the NOx catalyst, When the sulfur poisoning recovery control is terminated, exhaust air / fuel ratio reverse control is performed to exhaust the lean air / fuel ratio exhaust gas from the cylinder group that exhausted the rich air / fuel ratio exhaust gas during the sulfur poisoning recovery control. .

In the second invention, in the first invention, in the exhaust air-fuel ratio reverse control, the lean air-fuel ratio exhaust gas is discharged from the cylinder group that has exhausted the rich air-fuel ratio exhaust gas during the sulfur poisoning recovery control. After that, the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are alternately discharged from the cylinder group.
In the third aspect of the invention, in the first or second aspect of the invention, in the exhaust air-fuel ratio reverse control, the rich air-fuel ratio exhaust gas is discharged from the cylinder group that has exhausted the lean air-fuel ratio during the sulfur poisoning recovery control. Let it drain.

In the fourth aspect of the invention, in the third aspect of the invention, in the exhaust air-fuel ratio reverse control, the rich air-fuel ratio exhaust gas is discharged from the cylinder group that has discharged the lean air-fuel ratio exhaust gas during the sulfur poisoning recovery control. Thereafter, the lean air-fuel ratio exhaust gas and the rich air-fuel ratio exhaust gas are alternately discharged from the cylinder group.
In the fifth aspect of the invention, in the first or second aspect of the invention, the above-described exhaust air-fuel ratio reverse control should be performed in the normal operation in the cylinder group that has exhausted the exhaust gas having the lean air-fuel ratio during the sulfur poisoning recovery control. Combustion takes place.
In a sixth aspect of the invention, in any one of the first to fifth aspects, the air-fuel ratio of the exhaust gas when exhaust gas having a lean air-fuel ratio is discharged from a specific cylinder group during the exhaust air-fuel ratio reverse control is as follows. The lean air-fuel ratio is close to the stoichiometric air-fuel ratio. When exhaust gas having a rich air-fuel ratio is discharged from a specific cylinder group during the exhaust air-fuel ratio reverse rotation control, the air-fuel ratio of the exhaust gas is rich near the stoichiometric air-fuel ratio.

  In order to solve the above-mentioned problems, in the seventh invention, a plurality of cylinders are provided, these cylinders are divided into two cylinder groups, exhaust branch pipes are connected to the respective cylinder groups, and these exhaust branch pipes are connected downstream. An exhaust purification device for an internal combustion engine that is joined and connected to a common exhaust pipe, wherein a three-way catalyst is disposed in each exhaust branch pipe, and a NOx catalyst is disposed in the common exhaust pipe, In the exhaust purification apparatus for performing control for exhausting the rich air-fuel ratio from one cylinder group and exhausting the lean air-fuel ratio exhaust from the other cylinder group as the sulfur poisoning recovery control of the NOx catalyst, During the sulfur poisoning recovery control, the cylinder group for exhausting the rich air-fuel ratio exhaust gas and the cylinder group for discharging the lean air-fuel ratio exhaust gas are repeatedly switched.

  According to an eighth aspect, in the seventh aspect, when the end of the sulfur poisoning recovery control is requested, the cylinders are made to be a cylinder group that exhausts exhaust gas having a rich air-fuel ratio first in the sulfur poisoning recovery control. When the exhaust gas having a lean air-fuel ratio is discharged from the group and the exhaust gas having a rich air-fuel ratio is discharged from a cylinder group that first discharges the exhaust gas having a lean air-fuel ratio, the sulfur poisoning recovery is performed. End control.

  According to a ninth aspect, in the seventh or eighth aspect, an exhaust branch pipe or an exhaust gas downstream of the three-way catalyst is used as air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture filled in each cylinder to the target air-fuel ratio. An air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture to a target air-fuel ratio is performed using an output of an air-fuel ratio sensor arranged in a pipe, and a learning control for learning a control amount for the control target of the air-fuel ratio control In the case of an internal combustion engine, execution of the learning control is prohibited for a predetermined period after the completion of the sulfur poisoning recovery control.

According to the first invention (the first to sixth inventions), the sulfur poisoning recovery control is completed for the three-way catalyst into which the exhaust gas having a rich air-fuel ratio has flowed during the sulfur poisoning recovery control. Thereafter, since the lean air-fuel ratio exhaust gas is supplied, the three-way catalyst can be effectively functioned earlier.
According to the second invention (the seventh to ninth inventions), the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas alternately flow into the three-way catalyst during the sulfur poisoning recovery control. Therefore, when the sulfur poisoning recovery control is finished, the three-way catalyst can be effectively functioned.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an internal combustion engine equipped with the exhaust emission control device of the present invention. In FIG. 1, 1 indicates a main body of an internal combustion engine, and # 1 to # 4 indicate a first cylinder, a second cylinder, a third cylinder, and a fourth cylinder, respectively. Each cylinder is provided with fuel injection valves 21, 22, 23, 24 correspondingly. Each cylinder is connected to an intake pipe 4 via a corresponding intake branch pipe 3. Further, a first exhaust branch pipe 5 is connected to the first cylinder and the fourth cylinder, and a second exhaust branch pipe 6 is connected to the second cylinder and the third cylinder. That is, when the first cylinder and the fourth cylinder are collectively referred to as a first cylinder group, and the second cylinder and the third cylinder are collectively referred to as a second cylinder group, the first cylinder group includes the first cylinder group. An exhaust branch pipe 5 is connected, and a second exhaust branch pipe 6 is connected to the second cylinder group. These exhaust branch pipes 5 and 6 merge on the downstream side and are connected to a common exhaust pipe 7.

  The first exhaust branch pipe 5 is one exhaust branch pipe on the downstream side, but is branched into two on the upstream side, and the exhaust branch pipes branched into these two are the first cylinder and the first cylinder, respectively. It is connected to 4 cylinders. Similarly, the second exhaust branch pipe 6 is also one exhaust branch pipe on the downstream side, but is branched into two on the upstream side, and the two exhaust branch pipes branched into the second cylinder and the second branch branch pipe, respectively. Connected to the third cylinder. In the following description, when a portion divided into two on the upstream side of the exhaust branch pipes 5 and 6 is specified and expressed, this is expressed as “a branch portion of the exhaust branch pipe”, and the exhaust branch pipes 5 and 6 are expressed. In the case where one portion on the downstream side is specified and expressed, this is expressed as “a collection portion of exhaust branch pipes”.

  Three-way catalysts 8 and 9 are disposed in the gathering portions of the exhaust branch pipes 5 and 6, respectively, and a NOx catalyst 10 is disposed in the exhaust pipe 7. In addition, air-fuel ratio sensors 11 and 12 are arranged at the collection portions of the exhaust branch pipes 5 and 6 upstream of the three-way catalysts 5 and 6, respectively. Air-fuel ratio sensors 13 and 14 are also disposed in the exhaust pipe 7 upstream and downstream of the NOx catalyst 10, respectively.

  As shown in FIG. 2, the three-way catalysts 8 and 9 have a temperature equal to or higher than a certain temperature (so-called activation temperature), and the air-fuel ratio of the exhaust gas flowing into the three-way catalysts 8 and 9 is the stoichiometric air-fuel ratio. When in the vicinity (in the region X of FIG. 2), nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbon (HC) in the exhaust gas are simultaneously purified at a high purification rate. On the other hand, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is leaner than the stoichiometric air-fuel ratio, the three-way catalyst absorbs oxygen in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the exhaust gas is greater than the stoichiometric air-fuel ratio. When it is also rich, it has an oxygen absorption / release capability of releasing absorbed oxygen. As long as this oxygen absorption / release capability functions normally, the air / fuel ratio of the exhaust gas flowing in is leaner or richer than the stoichiometric air / fuel ratio. Therefore, NOx, CO, and HC in the exhaust gas are simultaneously purified with a high purification rate.

  The NOx catalyst 10 has an exhaust gas when its temperature is higher than a certain temperature (so-called activation temperature) and the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 10 is leaner (larger) than the stoichiometric air-fuel ratio. It is retained by absorbing or storing NOx therein, and when the air-fuel ratio of the exhaust gas flowing therein becomes richer than the stoichiometric air-fuel ratio or stoichiometric air-fuel ratio, the retained NOx is reduced and purified.

  By the way, if SOx is contained in the exhaust gas under the condition in which NOx is held in the NOx catalyst 10, this SOx is also held in the NOx catalyst. As described above, when SOx is held in the NOx catalyst, the amount of NOx that can be held by the NOx catalyst is reduced accordingly. For this reason, in order to maintain the NOx retention capacity of the NOx catalyst as high as possible, it is necessary to remove SOx from the NOx catalyst. Here, if the exhaust gas having a stoichiometric air-fuel ratio or rich (preferably, very close to the stoichiometric air-fuel ratio) is supplied to the NOx catalyst while the temperature of the NOx catalyst is set to a temperature at which SOx can be removed, the NOx catalyst SOx can be removed. In other words, it can be said that the NOx catalyst of the present embodiment releases SOx when the stoichiometric or rich air-fuel ratio exhaust gas is supplied to the NOx catalyst in a state where the temperature is set to a certain temperature.

  Therefore, when it is required to remove SOx from the NOx catalyst 10, in the present embodiment, the following sulfur poisoning recovery control is executed, so that the temperature of the NOx catalyst is set to a temperature at which SOx can be removed and NOx. An exhaust gas having a stoichiometric or rich air-fuel ratio is supplied to the catalyst. That is, in the sulfur poisoning recovery control of the present embodiment, rich air-fuel ratio exhaust gas (hereinafter referred to as “rich exhaust gas”) is discharged from the first cylinder and the fourth cylinder (that is, the first cylinder group) and the first. The air-fuel ratio of the air-fuel mixture filled in each cylinder is controlled so that the exhaust gas having a lean air-fuel ratio (hereinafter referred to as “lean exhaust gas”) is discharged from the second cylinder and the third cylinder (that is, the second cylinder group). To do.

  Here, the richness of the rich exhaust gas discharged from each cylinder and the leanness of the lean exhaust gas are determined when the rich exhaust gas and the lean exhaust gas are mixed upstream of the NOx catalyst 10 and flow into the NOx catalyst. The air-fuel ratio of the exhaust gas is adjusted so as to be the stoichiometric air-fuel ratio or a desired rich air-fuel ratio.

  Generally, the temperature at which SOx can be removed from the NOx catalyst 10 (hereinafter referred to as “the temperature at which SOx can be removed”) is higher than the temperature at which the NOx catalyst holds NOx or purifies NOx, so that SOx is removed from the NOx catalyst. In order to remove it, it is necessary to raise the temperature of the NOx catalyst. In this regard, according to the sulfur poisoning recovery control of the present embodiment, the rich exhaust gas and the lean exhaust gas are mixed and the HC in the rich exhaust gas reacts with the oxygen in the lean exhaust gas, so that the reaction heat This reaction heat can raise the temperature of the NOx catalyst to a temperature at which SOx can be removed.

  As described above, in order to remove SOx from the NOx catalyst 10, it is necessary to set the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to the stoichiometric air-fuel ratio or the rich air-fuel ratio. In this regard, according to the sulfur poisoning recovery control of the present embodiment, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is the stoichiometric air-fuel ratio or the rich air-fuel ratio. Thus, according to the sulfur poisoning recovery control of the present embodiment, SOx can be removed from the NOx catalyst 10.

  Note that the air-fuel ratio of the rich exhaust gas discharged from each cylinder in the sulfur poisoning recovery control is preferably a rich air-fuel ratio close to the stoichiometric air-fuel ratio. Therefore, the lean exhaust gas discharged from each cylinder in the sulfur poisoning recovery control. The air / fuel ratio of the gas is also preferably a lean air / fuel ratio close to the stoichiometric air / fuel ratio.

  By the way, during the sulfur poisoning recovery control, the rich exhaust gas always flows into one of the three-way catalysts 8. Thus, if only rich exhaust gas continues to flow into the three-way catalyst, HC and CO in the rich exhaust gas will adhere to the three-way catalyst. If such a three-way catalyst state continues even after the end of the sulfur poisoning recovery control, the three-way catalyst is effective even if conditions for the three-way catalyst to function effectively are prepared. May not work. Further, during the sulfur poisoning recovery control, lean exhaust gas always flows into the other three-way catalyst 9. As described above, when only the lean exhaust gas continues to flow into the three-way catalyst, oxygen in the lean exhaust gas adheres to the three-way catalyst. If such a three-way catalyst state continues even after the end of the sulfur poisoning recovery control, the three-way catalyst is effective even if conditions for the three-way catalyst to function effectively are prepared. May not work.

  Therefore, in the present embodiment, after completion of the sulfur poisoning recovery control, the following three-way catalyst poisoning recovery control is executed, so that HC, CO, or oxygen adhering to the three-way catalyst is removed from the three-way catalyst. Remove from. That is, in the three-way catalyst poisoning recovery control of the present embodiment, the mixture filled in each cylinder so that the lean exhaust gas is discharged from the first cylinder group and the rich exhaust gas is discharged from the second cylinder group. Control the air / fuel ratio of the air. That is, in other words, lean exhaust gas is discharged from the cylinder group that exhausted the rich exhaust gas during the sulfur poisoning recovery control, and from the cylinder group that discharged the lean exhaust gas during the sulfur poisoning recovery control. Exhausts rich exhaust gas.

  According to this, lean exhaust gas is supplied to the three-way catalyst 8 to which HC and CO are attached because only rich exhaust gas has flowed in during the sulfur poisoning recovery control. According to this, oxygen in the supplied lean exhaust gas reacts with HC and CO adhering to the three-way catalyst 8 to remove these HC and CO. On the other hand, rich exhaust gas is supplied to the three-way catalyst 9 to which oxygen is attached because only lean exhaust gas has flowed in during the sulfur poisoning recovery control. According to this, HC and CO in the supplied rich exhaust gas react with oxygen adhering to the three-way catalyst 9 to remove this oxygen.

  Next, a second embodiment of the present invention will be described. In the present embodiment, in the three-way catalyst poisoning recovery control, the lean exhaust gas is discharged from the cylinder group that has exhausted the rich exhaust gas during the sulfur poisoning recovery control, and the lean exhaust gas is discharged during the sulfur poisoning recovery control. After exhausting the rich exhaust gas from the exhausted cylinder group, the cylinder group that exhausts the lean exhaust gas and the cylinder group that exhausts the rich exhaust gas are alternately switched.

  Thus, performing the three-way catalyst poisoning recovery control has the following advantages. That is, according to the present embodiment, during the three-way catalyst poisoning recovery control, not only the lean exhaust gas is supplied to the three-way catalyst 8 poisoned by the adhesion of HC or CO, but the lean exhaust gas and the rich exhaust gas are not supplied. Gas is supplied alternately. For this reason, compared with the case where only lean exhaust gas is supplied, the reaction at the three-way catalyst 8 is more stimulated, so that HC and CO can be rapidly removed from the three-way catalyst 8. Further, according to the present embodiment, during the three-way catalyst poisoning recovery control, not only the rich exhaust gas is supplied to the three-way catalyst 9 poisoned by the adhesion of oxygen, but the rich exhaust gas, the lean exhaust gas, Since oxygen is supplied alternately, oxygen can be quickly removed from the three-way catalyst 9 for the same reason as above.

  Note that the timing of switching between the cylinder group that discharges lean exhaust gas and the cylinder group that discharges rich exhaust gas may be, for example, a timing at which a fixed time has elapsed or a timing calculated based on some other parameter. Good.

  Next, a third embodiment of the present invention will be described. In the present embodiment, in the three-way catalyst poisoning recovery control, the air-fuel ratio of the air-fuel mixture charged in each cylinder of the first cylinder group is controlled so that lean exhaust gas is discharged from the first cylinder group. The air-fuel ratio control performed in the normal operation is performed without controlling the air-fuel ratio of the air-fuel mixture filled in each cylinder of the second cylinder group so that the rich exhaust gas is necessarily discharged from the second cylinder group. In other words, lean exhaust gas is discharged from the cylinder group that exhausted the rich exhaust gas during the sulfur poisoning recovery control, but from the cylinder group that exhausted the lean exhaust gas during the sulfur poisoning recovery control. Rich exhaust gas is not necessarily exhausted, and normal operation is performed in this cylinder group.

  The reason why the three-way catalyst poisoning recovery control is performed in this manner is that the poisoning of the three-way catalyst 9 due to the adhesion of oxygen is less poisonous than the poisoning of the three-way catalyst 8 due to the adhesion of HC or CO. In this sense, it can be said that it is sufficient to perform poisoning recovery control only on the three-way catalyst 8 that is poisoned by the adhesion of HC or CO. In this way, it is not necessary to perform an operation different from the normal operation in one cylinder group, so that an advantage that the engine operation control becomes simpler can be obtained.

  In the three-way catalyst poisoning recovery control of the third embodiment, after exhausting the lean exhaust gas from the cylinder group that has exhausted the rich exhaust gas during the sulfur poisoning recovery control, the rich exhaust gas and the lean exhaust gas are exhausted. May be discharged alternately. In this case, the timing for switching between the lean exhaust gas and the rich exhaust gas may be, for example, a timing at which a fixed time has elapsed or a timing calculated based on some other parameter.

  FIG. 3 shows an example of a routine for performing the above-described sulfur poisoning recovery control and three-way catalyst poisoning recovery control. In the routine of FIG. 3, first, at step 10, it is determined whether or not the SOx retention amount S is larger than the predetermined amount SR (S> SR). Here, the SOx retention amount is the amount of SOx currently retained in the NOx catalyst 10. If it is determined in step 10 that S ≦ SR, it is determined that it is not necessary to remove SOx from the NOx catalyst at the present time, and the routine ends. On the other hand, when it is determined in step 10 that S> SR, it is determined that SOx needs to be removed from the NOx catalyst 10, and in the next step 11, the above-described sulfur poisoning recovery control is started.

  Next, at step 12, it is judged if the SOx retention amount S has become zero (S = 0). If it is determined that S ≠ 0, it is determined that the sulfur poisoning recovery control needs to be continued, and the routine returns to step 12. Then, step 12 is repeated until it is determined at step 12 that S = 0, and during this time, the sulfur poisoning recovery control is continued. If it is determined in step 12 that S = 0, the sulfur poisoning recovery control is terminated in step 13, and then any of the above-described three-way catalyst poisoning recovery control is started in step 14. .

  Next, in step 15, it is determined whether or not the time T that has elapsed since the start of the three-way catalyst poisoning recovery control has exceeded a predetermined time TR (T> TR). If it is determined that T ≦ TR, it is determined that the three-way catalyst poisoning recovery control needs to be continued, and the routine returns to step 15. Then, step 15 is repeated until it is determined in step 15 that T> TR. During this time, the three-way catalyst poisoning recovery control is continued. When it is determined in step 15 that T> TR, the three-way catalyst poisoning recovery control is ended in step 16.

  Next, a fourth embodiment of the present invention will be described. In the present embodiment, in the sulfur poisoning recovery control, first, the mixture filled in each cylinder so that the rich exhaust gas is discharged from the first cylinder group and the lean exhaust gas is discharged from the second cylinder group. After controlling the air-fuel ratio of the air, the air-fuel ratio of the air-fuel mixture charged in each cylinder is controlled so that the cylinder group from which the rich exhaust gas is discharged and the cylinder group from which the lean exhaust gas is discharged are switched. Thereby, SOx can be removed from the NOx catalyst 10.

  Further, during the sulfur poisoning recovery control, the rich exhaust gas and the lean exhaust gas alternately flow into the three-way catalysts 8 and 9, so that at the end of the sulfur poisoning recovery control, the three-catalyst by the attachment of HC and CO There is almost no poisoning of the three-way catalyst due to poisoning of the original catalyst or adhesion of oxygen. For this reason, the three-way catalyst is in a state of functioning effectively immediately after the end of the sulfur poisoning recovery control.

  In the fourth embodiment, when the end of the sulfur poisoning recovery control is requested, the cylinder group that is the cylinder group that first exhausts the rich exhaust gas in the sulfur poisoning recovery control (in the above-described example, The lean exhaust gas is discharged from the one cylinder group), and the rich exhaust gas is discharged from the cylinder group (the second cylinder group in the above example) that is the cylinder group that first discharges the lean exhaust gas in the sulfur poisoning recovery control. When exhausted, it is preferable to terminate the sulfur poisoning recovery control. That is, according to this, since the air-fuel ratio of the exhaust gas flowing into each of the three-way catalysts 8, 9 during the sulfur poisoning recovery control is averaged, it becomes the stoichiometric air-fuel ratio. This is preferable from the viewpoint of suppressing poisoning of the catalyst and poisoning of the three-way catalyst due to adhesion of oxygen.

  In the fourth embodiment, the timing for switching between the cylinder group that discharges the lean exhaust gas and the cylinder group that discharges the rich exhaust gas may be, for example, a timing at which a fixed time has elapsed, or based on some other parameter. May be calculated at the timing.

  Incidentally, as the air-fuel ratio sensor, for example, there is a so-called linear air-fuel ratio sensor that outputs a current with the characteristics shown in FIG. This linear air-fuel ratio sensor outputs a current of 0 A when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, and outputs a current of 0 A or less that is larger as the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. As the air-fuel ratio of the exhaust gas becomes leaner than the stoichiometric air-fuel ratio, a larger current of 0 A or more is output. That is, the linear air-fuel ratio sensor outputs a current that changes linearly according to the air-fuel ratio of the exhaust gas.

As another air-fuel ratio sensor, for example, there is a so-called O ₂ sensor that outputs a voltage with the characteristics shown in FIG. This O ₂ sensor outputs a voltage of approximately 0 V when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and outputs a voltage of approximately 1 V when it is richer than the stoichiometric air-fuel ratio. The output voltage rapidly changes in a region where the air-fuel ratio of the exhaust gas is in the vicinity of the theoretical air-fuel ratio and crosses 0.5V. That is, the O ₂ sensor outputs a constant voltage that varies depending on whether the air-fuel ratio of the exhaust gas is lean or rich with respect to the stoichiometric air-fuel ratio.

In the embodiment of the present invention, a linear air-fuel ratio sensor is adopted as the air-fuel ratio sensor 11, 12 upstream of the three-way catalyst 8, 9 and the air-fuel ratio sensor 13 between the three-way catalyst and the NOx catalyst 10, and the NOx catalyst downstream. As the air-fuel ratio sensor 14, an O ₂ sensor is employed. In this embodiment, the air-fuel ratio of the air-fuel mixture filled in each cylinder is controlled to the target air-fuel ratio based on the outputs from these sensors. Next, as an example of such air-fuel ratio control, the control of the present embodiment for controlling the air-fuel ratio of the air-fuel mixture charged in each cylinder to the stoichiometric air-fuel ratio during normal operation (hereinafter referred to as “normal stoichiometric control”) will be described. .

  First, the outline of the normal stoichiometric control of this embodiment will be described. The air-fuel ratio sensors (hereinafter referred to as “linear air-fuel ratio sensors”) 11 and 12 upstream of the three-way catalysts 8 and 9 indicate that the air-fuel ratio of the exhaust gas (hereinafter referred to as “exhaust air-fuel ratio”) is leaner than the stoichiometric air-fuel ratio. In the illustrated case, since the air-fuel ratio (engine air-fuel ratio) of the air-fuel mixture filled in the corresponding cylinder is leaner than the stoichiometric air-fuel ratio, fuel injection is performed so that the engine air-fuel ratio in the corresponding cylinder approaches the stoichiometric air-fuel ratio. The amount of fuel injected from the valve (hereinafter referred to as “fuel injection amount”) is increased. Conversely, when the linear air-fuel ratio sensors 11 and 12 indicate that the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, the fuel injection amount is reduced so that the engine air-fuel ratio in the corresponding cylinder approaches the stoichiometric air-fuel ratio. Is done.

  By controlling the fuel injection amount in this way, basically, the engine air-fuel ratio should be controlled to the stoichiometric air-fuel ratio. However, if the linear air-fuel ratio sensors 11 and 12 have an output error, the engine air-fuel ratio is not controlled to the stoichiometric air-fuel ratio. For example, if the linear air-fuel ratio sensor tends to output a current value corresponding to the air-fuel ratio that is shifted to a richer side than the current value corresponding to the actual exhaust air-fuel ratio, the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio. Even if this is the case, the exhaust air-fuel ratio will be richer than the stoichiometric air-fuel ratio. For this reason, the fuel injection amount is reduced, and as a result, the engine air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio. On the other hand, if the linear air-fuel ratio sensor tends to output a current value corresponding to the air-fuel ratio that deviates to a leaner side than the current value corresponding to the actual exhaust air-fuel ratio, the engine air-fuel ratio is less than the stoichiometric air-fuel ratio. Is also richly controlled.

Therefore, in the present embodiment, such output errors of the linear air-fuel ratio sensors 11 and 12 are compensated using the output value of the O ₂ sensor 14 downstream of the NOx catalyst 10. That is, if the linear air-fuel ratio sensor has no output error and the engine air-fuel ratio is controlled to the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas flowing out from the NOx catalyst should be the stoichiometric air-fuel ratio. The O ₂ sensor outputs 0.5 V (hereinafter referred to as “reference voltage value”) corresponding to the theoretical air-fuel ratio.

However, if there is an output error in the linear air-fuel ratio sensors 11 and 12 and the engine air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio, for example, the air-fuel ratio of the exhaust gas flowing out from the NOx catalyst 10 is the stoichiometric air-fuel ratio. It is richer than. At this time, the O ₂ sensor 14 outputs a voltage value corresponding to an air-fuel ratio richer than the theoretical air-fuel ratio. Here, the difference between the voltage value output from the O ₂ sensor at this time and the reference voltage value indicates an output error of the linear air-fuel ratio sensor. Therefore, in this embodiment, the output of the linear air-fuel ratio sensor is compensated so that the output error of the linear air-fuel ratio sensor is compensated based on the difference between the voltage value actually output from the O ₂ sensor and the reference voltage value. Correct the current value.

Conversely, when the linear air-fuel ratio sensors 11 and 12 have an output error and the engine air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, the voltage value output from the O ₂ sensor 14 and the reference voltage value Based on the difference, the output current value of the linear air-fuel ratio sensor is corrected so that the output error of the lean air-fuel ratio sensor is compensated.

Next, the normal stoichiometric control of this embodiment will be described as a more specific example. In the present embodiment, the valve opening time (hereinafter referred to as “reference valve opening time”) of the fuel injection valve that is a reference for setting the engine air-fuel ratio to the stoichiometric air-fuel ratio is determined according to the following equation 1.
TAUB = α × Ga / Ne (1)
Here, α is a constant, Ga is the intake air amount (the amount of air taken into the cylinder), and Ne is the engine speed. That is, according to the present embodiment, the reference valve opening time is calculated based on the intake air amount per unit engine speed, and tends to become longer as the intake air amount per unit engine speed increases.

Then, the actual valve opening time (hereinafter referred to as “actual valve opening time”) TAU of the fuel injection valve is calculated according to the following equation 2.
TAU = TAUB × F1 × β × γ (2)
Here, F1 is a correction coefficient (hereinafter also referred to as “main correction coefficient”) obtained as described later, and β and γ are constants determined according to the engine operating state.

The main correction coefficient F1 is calculated according to the following equation 3.
F1 = Kp1 × (I−F2−I ₀ ) + Ki1 × ∫ (I−F2−I ₀ ) dt + Kd1 × d (I−F2−I ₀ ) / dt (3)
Here, I ₀ is a current value to be output from the linear air-fuel ratio sensors 11 and 12 when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, and I is actually output from the linear air-fuel ratio sensors 11 and 12. F2 is a correction coefficient (hereinafter also referred to as “sub-correction coefficient”) obtained as described later, Kp1 is a proportional gain, Ki1 is an integral gain, and Kd1 is a differential gain. . That is, according to this, the main correction coefficient F1 is PID-controlled.

On the other hand, the sub correction coefficient F2 is calculated according to the following equation 4.
_{F2 = Kp2 × (V 0 -V} ) + Ki2 × ∫ (V 0 -V) dt + Kd2 × d (V 0 -V) / dt ... (4)
Here, V ₀ is a voltage value that should be output from the O ₂ sensor 14 when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio, V is a voltage value that is actually output from the O ₂ sensor 14, Kp2 is a proportional gain, Ki2 is an integral gain, and Kd2 is a differential gain. That is, according to this, the sub correction coefficient F2 is also PID-controlled.

  Thus, according to this embodiment, the engine air-fuel ratio is maintained at the stoichiometric air-fuel ratio.

  By the way, in the embodiment of the present invention, the sub correction coefficient F2 is learned. Here, “learning” means storing a certain value and appropriately updating the stored value to the latest value calculated one after another. That is, the sub-correction coefficient F2 obtained every moment according to the above equation 4 compensates for the constant output error of the linear air-fuel ratio sensor. For example, normal stoichiometric control (engine air-fuel ratio is reduced). When the normal stoichiometric control is resumed after the control for controlling the stoichiometric air-fuel ratio is interrupted, the sub correction coefficient stored before the normal stoichiometric control is interrupted, rather than re-determining the sub correction coefficient F2 from scratch. The engine air-fuel ratio can be controlled to the stoichiometric air-fuel ratio earlier when F2 is used from the time when the normal stoichiometric control is resumed. In this embodiment, the sub correction coefficient F2 is stored for this reason.

  By the way, as described above, if the sub correction coefficient F2 is learned during the normal stoichiometric control, the sub correction coefficient F2 is also learned after the engine control shifts from the sulfur poisoning recovery control to the normal stoichiometric control. It will be. However, immediately after the engine control shifts from the sulfur poisoning recovery control to the normal stoichiometric control, there is a high possibility that the oxygen absorption / release capability of the three-way catalyst does not work effectively. That is, at this time, there is a high possibility that the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst is greatly deviated from the stoichiometric air-fuel ratio. For this reason, there is a high possibility that the sub correction coefficient F2 calculated immediately after the engine control shifts from the sulfur poisoning recovery control to the normal stoichiometric control is greatly deviated from the normally calculated value.

  Here, in the case where the learning of the sub correction coefficient F2 is performed, when the engine control is shifted from the normal stoichiometric control to another control, the sub correction coefficient F2 greatly deviating from the normally calculated value is stored as a learning value. Will be. In this case, when the engine control again shifts to the normal stoichiometric control, the sub correction coefficient F2 greatly deviated from the normally calculated value is used as the initial value of the sub correction coefficient. Here, if the oxygen absorption / release capability of the three-way catalyst is functioning effectively and the sub correction coefficient F2 is a value that is normally calculated, the engine air-fuel ratio is accurately controlled to the stoichiometric air-fuel ratio. If the sub correction coefficient F2 is greatly deviated from the normally calculated value, the engine air-fuel ratio is greatly deviated from the stoichiometric air-fuel ratio.

  Of course, when a certain period of time has elapsed, the sub correction coefficient F2 is a value that is normally calculated, but until this time, the engine air-fuel ratio has become an air-fuel ratio that deviates greatly from the stoichiometric air-fuel ratio. Will be.

  In order to avoid such an inconvenience, in the embodiment of the present invention, learning of the sub correction coefficient F2 is prohibited for a predetermined period after the completion of the sulfur poisoning recovery control even if the normal stoichiometric control is performed. .

  The technical idea of prohibiting learning in this way is to control the engine air-fuel ratio using the output from the air-fuel ratio sensor downstream of the three-way catalyst and learn the control coefficient calculated by the control. It is also applicable when

  In the above-described embodiment, during the sulfur poisoning recovery control, the outputs from the linear air-fuel ratio sensors 11 and 12 upstream of the three-way catalysts 8 and 9 are used in the control described with reference to FIGS. Instead of using the output from the linear air-fuel ratio sensor 13 between the three-way catalyst and the NOx catalyst 10, the engine air-fuel ratio is adjusted so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst becomes the stoichiometric air-fuel ratio. (That is, the fuel injection amount) is controlled.

  In the above description, an example in which the present invention is applied when four cylinders are divided into two cylinder groups has been described. However, the present invention can also be applied when a plurality of cylinders are divided into two or more cylinder groups. It is.

It is the figure which showed an example of the internal combustion engine provided with the exhaust gas purification device of this invention. It is the figure which showed the purification characteristic of a three-way catalyst. It is the figure which showed an example of the routine which performs sulfur poisoning and the three-way catalyst poisoning recovery of this invention. It is the figure which showed the output characteristic of the linear air fuel ratio sensor. O ₂ is a graph showing the output characteristics of the sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine body 4 Intake pipe 5,6 Exhaust branch pipe 7 Exhaust pipe 8,9 Three way catalyst 10 NOx catalyst 11-14 Air fuel ratio sensor 21-24 Fuel injection valve

Claims (9)

  An internal combustion engine comprising a plurality of cylinders, divided into at least two cylinder groups, each connected to an exhaust branch pipe and connected to one common exhaust pipe by joining the exhaust branch pipes downstream. An exhaust emission control device for an engine, wherein a three-way catalyst is arranged in each exhaust branch pipe and a NOx catalyst is arranged in one common exhaust pipe, and one cylinder is used as sulfur poisoning recovery control of the NOx catalyst. When the sulfur poisoning recovery control is terminated in an exhaust purification system that controls exhaust of rich air-fuel ratio from the group and exhaust of lean air-fuel ratio from the other cylinder group, An exhaust gas purification apparatus that performs exhaust air-fuel ratio reverse control for exhausting a lean air-fuel ratio exhaust gas from a group of cylinders that exhausted a rich air-fuel ratio exhaust gas during poison recovery control.   In the exhaust air-fuel ratio reverse control, after exhausting the lean air-fuel ratio from the cylinder group that has exhausted the rich air-fuel ratio during the sulfur poisoning recovery control, exhaust the rich air-fuel ratio from the cylinder group. The exhaust emission control device according to claim 1, wherein the gas and the exhaust gas having a lean air-fuel ratio are alternately discharged.   3. The exhaust air / fuel ratio inversion control according to claim 1, wherein the exhaust gas having a rich air / fuel ratio is exhausted from a cylinder group that has exhausted the exhaust gas having a lean air / fuel ratio during the sulfur poisoning recovery control. Exhaust purification equipment.   In the exhaust air-fuel ratio reverse control, after exhausting the rich air-fuel ratio from the cylinder group that has exhausted the lean air-fuel ratio during the sulfur poisoning recovery control, the exhaust gas having the lean air-fuel ratio is exhausted from the cylinder group. The exhaust emission control device according to claim 3, wherein the gas and the rich air-fuel ratio exhaust gas are alternately discharged.   3. In the exhaust air-fuel ratio reverse control, combustion that should be performed in a normal operation is performed in a cylinder group that has discharged exhaust gas having a lean air-fuel ratio during sulfur poisoning recovery control. Exhaust gas purification device described in 1.   When exhaust gas having a lean air-fuel ratio is discharged from a specific cylinder group during the exhaust air-fuel ratio reverse control, the air-fuel ratio of the exhaust gas is lean near the stoichiometric air-fuel ratio. 6. The exhaust emission control device according to claim 1, wherein when exhaust gas having a rich air-fuel ratio is discharged from a cylinder group, the air-fuel ratio of the exhaust gas is rich close to the stoichiometric air-fuel ratio. .   An internal combustion engine having a plurality of cylinders, divided into two cylinder groups, each having an exhaust branch pipe connected to each cylinder group, and joining these exhaust branch pipes on the downstream side to one common exhaust pipe The exhaust purification apparatus of the present invention includes a three-way catalyst disposed in each exhaust branch pipe and a NOx catalyst disposed in the common one exhaust pipe, and as a sulfur poisoning recovery control of the NOx catalyst, one cylinder group Exhaust gas having a rich air-fuel ratio is discharged from the other cylinder group, and exhaust gas having a lean air-fuel ratio is discharged from the other cylinder group. An exhaust emission control device that repeatedly replaces a cylinder group to be discharged and a cylinder group to discharge exhaust gas having a lean air-fuel ratio.   When the end of the sulfur poisoning recovery control is requested, the lean air-fuel ratio exhaust gas is discharged from the cylinder group that is the first cylinder group that discharges the rich air-fuel ratio exhaust gas in the sulfur poisoning recovery control. The sulfur poisoning recovery control is terminated when rich air-fuel ratio exhaust gas is exhausted from the cylinder group that is the cylinder group that first discharges lean air-fuel ratio exhaust gas. Exhaust gas purification device described in 1.   As the air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture filled in each cylinder to the target air-fuel ratio, the internal combustion engine uses the output of the air-fuel ratio sensor disposed in the exhaust branch pipe or the exhaust pipe downstream of the three-way catalyst. In the case of an internal combustion engine that performs air-fuel ratio control for controlling the air-fuel ratio of the air-fuel mixture to a target air-fuel ratio and performs learning control for learning a control amount for the control target of the air-fuel ratio control, the sulfur poisoning recovery control is The exhaust emission control device according to claim 7 or 8, wherein execution of the learning control is prohibited for a predetermined period after the end.

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