JP2005351108A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain a catalytic smell by stably securing a high purification rate even when having a plurality of catalysts in series to an exhaust passage. <P>SOLUTION: This device has a front stage catalyst having oxygen storage capacity and a rear stage catalyst provided in the exhaust passage on the downstream side more than the front stage catalyst and having oxygen storage capacity, a bypass passage for making at least a part of exhaust gas exhausted from an internal combustion engine flow in the rest stage catalyst by bypassing the front stage catalyst, a flow control valve for adjusting a flow rate of the exhaust gas flowing in the bypass passage, and a processing performing means for performing leanness forming processing for making the air-fuel ratio lean in a cylinder of the internal combustion engine when both the front stage catalyst and the rear stage catalyst are a rich state of reducing an oxygen storage quantity and making the exhaust gas exhausted from the internal combustion engine flow in the rear stage catalyst via the bypass passage by opening the flow control valve. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a catalyst purification device for an internal combustion engine, and more particularly to an apparatus in which a plurality of catalysts are installed in series in an exhaust passage.

As an exhaust purification device for an internal combustion engine, a plurality of catalysts are installed in series in an exhaust passage to reduce the amount of HC, CO, and NOx discharged into the atmosphere. For example, the exhaust emission control device described in Patent Document 1 is provided with a three-way catalyst, an oxidation catalyst, and a NOx absorbent from the upstream side. Moreover, the exhaust purification apparatus described in Patent Document 2 is provided with a three-way catalyst and a NOx occlusion reduction type catalyst from the upstream side.
JP-A-6-346725 Japanese Patent Laid-Open No. 11-148340 JP-A-5-26034

  Both the three-way catalyst and the NOx occlusion reduction type catalyst have the ability to occlude an appropriate amount of oxygen inside, and when the exhaust gas contains NOx, the NOx is reduced by occlusion of oxygen, In addition, when exhaust gas contains HC or CO, it is a catalyst that can oxidize them by releasing the stored oxygen.

  Therefore, when these catalysts are provided in series in the exhaust passage as a pre-stage catalyst and a post-stage catalyst, if the operating state in which the air-fuel ratio of the exhaust gas exhausted from the internal combustion engine becomes rich continues, both the pre-stage and post-stage catalysts are A rich state is obtained, in which the amount of oxygen stored in the catalyst is small. On the other hand, when the operation state in which the air-fuel ratio of the exhaust gas discharged from the internal combustion engine becomes lean continues, both the front and rear catalysts enter a lean state in which the amount of oxygen stored in the catalyst is large.

  When both the front and rear catalyst are in a rich state or a lean state, the ability to purify the exhaust gas decreases. For example, if the exhaust gas passes through these catalysts when both the front stage catalyst and the rear stage catalyst are in a rich state, HC and CO in the exhaust gas cannot be sufficiently oxidized because the oxygen storage amount in the catalyst is small. Further, if the exhaust gas passes through these catalysts when both the front stage catalyst and the rear stage catalyst are in a lean state, the amount of oxygen stored in the catalyst is large, so that NOx in the exhaust gas cannot be reduced sufficiently.

  Further, as in the exhaust gas purification apparatus for an internal combustion engine described in Patent Document 2, the front stage catalyst is a three-way catalyst and the rear stage catalyst is a NOx occlusion reduction type catalyst, where the front stage catalyst is in a lean state and the rear stage catalyst is rich. In this state, NOx in the exhaust gas cannot be purified by the front catalyst, and therefore purification is performed only by the rear catalyst. On the other hand, when the front stage catalyst is rich and the rear stage catalyst is lean, NOx in the exhaust gas can be reduced by the front stage catalyst and can be stored by the NOx occlusion reduction type catalyst that is the rear stage catalyst. The rate is high. Further, if the latter stage catalyst close to the exhaust gas outlet is in a rich state for a long time, hydrogen sulfide is likely to be generated, and thus a catalyst odor may be generated. When the front catalyst is lean and the rear catalyst is rich, or when the front catalyst is rich and the rear catalyst is lean, one of the catalysts is rich, so the HC and CO purification rates are high.

  As described above, in the exhaust gas purification apparatus provided with two catalysts having oxygen storage capacity, it is preferable that the front catalyst is in a rich state and the rear catalyst is in a lean state.

  By the way, when the fuel cut operation state is continued, air passes through the exhaust passage, so that both the front stage catalyst and the rear stage catalyst are in a lean state. In order to shift the front stage catalyst, which is optimal for exhaust gas purification from this state, to the lean state and the rear stage catalyst to the lean state, the air-fuel ratio of the exhaust gas flowing into the front stage catalyst is made rich, that is, the mixture gas in the cylinder What is necessary is to make the air-fuel ratio rich.

  On the other hand, when the high load operation is continued, since the state where the air-fuel ratio of the exhaust gas discharged from the internal combustion engine is rich is continued, there is a possibility that both the front stage catalyst and the rear stage catalyst become rich. In order to shift from this state to the lean state while the front catalyst, which is optimal for exhaust gas purification, is shifted to the lean state, first, the air-fuel ratio of the air-fuel mixture in the cylinder is made lean to be discharged from the internal combustion engine. After making the air-fuel ratio of the exhaust gas lean and making both the front and rear catalysts lean, it is necessary to make the air-fuel ratio of the air-fuel mixture in the cylinder rich and make the front catalyst rich. Therefore, the exhaust emission may be deteriorated for the reason described above in the process of shifting from the state where both the front and rear catalysts are rich to the state where the front catalyst is rich and the rear catalyst is lean.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to stably secure a high purification rate even when a plurality of catalysts are provided in series in an exhaust passage and to prevent a catalyst odor. An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can suppress the above-mentioned problem.

  In order to achieve the above object, in an exhaust gas purification apparatus for an internal combustion engine according to the present invention, a pre-stage catalyst provided in an exhaust passage of the internal combustion engine and having an oxygen storage capacity, and an exhaust passage downstream of the pre-stage catalyst A rear stage catalyst having oxygen storage capacity, a bypass passage for bypassing at least a part of the exhaust gas discharged from the internal combustion engine and flowing into the rear stage catalyst by bypassing the front stage catalyst, and an exhaust gas flowing through the bypass passage When the flow rate control valve for adjusting the flow rate of gas and the front catalyst and the rear catalyst are both in a rich state with a small oxygen storage amount, the air-fuel ratio in the cylinder of the internal combustion engine is made lean and the flow control valve And a process executing means for executing a leaning process for causing the exhaust gas discharged from the internal combustion engine to flow into the rear catalyst through the bypass passage.

  In the exhaust gas purification apparatus for an internal combustion engine configured as described above, when the processing execution unit is in a rich state in which both the front stage catalyst and the rear stage catalyst have a small oxygen storage amount, the air-fuel ratio in the cylinder of the internal combustion engine is made lean. In addition, the lean control process is performed in which the exhaust gas discharged from the internal combustion engine flows into the rear catalyst through the bypass path by opening the flow control valve, so that the lean air-fuel ratio exhaust gas directly flows into the rear catalyst, and the rear catalyst Becomes lean. As a result, the upstream catalyst is in a rich state and the downstream catalyst is shifted to a lean state, so that a high purification rate can be secured stably. Moreover, since the latter stage catalyst close to the exhaust gas outlet to the atmosphere is in a lean state, generation of hydrogen sulfide is suppressed, and generation of catalyst odor can be suppressed.

  Further, it is preferable that the processing execution means executes a leaning process when a rich state in which both the front catalyst and the rear catalyst have a small oxygen storage amount continues for a predetermined time or longer. In the lean process, the exhaust gas discharged from the internal combustion engine is caused to flow into the rear catalyst via the bypass passage, so that the exhaust gas purification during the process is worse than when the exhaust gas is passed through the front catalyst and the rear catalyst. There is a risk. Therefore, the deterioration of the purification rate associated with the leaning process can be suppressed by executing only when the rich state in which the pre-stage catalyst and the post-stage catalyst both have a small oxygen storage amount continues for a predetermined time or longer. Moreover, since the latter stage catalyst is not made rich for a long time, generation of hydrogen sulfide is suppressed, and generation of catalyst odor can be suppressed.

  As described above, according to the present invention, even when a plurality of catalysts are provided in series in the exhaust passage, a high purification rate can be secured stably and a catalyst odor can be suppressed.

  The best mode for carrying out the present invention will be exemplarily described in detail below based on the following embodiments with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in the embodiments are not intended to limit the scope of the present invention only to those unless otherwise specified.

  Based on FIG. 1, a schematic configuration of an exhaust emission control device for an internal combustion engine according to a first embodiment of the present invention will be described. The internal combustion engine 1 is a spark ignition type internal combustion engine (gasoline engine) using gasoline as fuel, and is an engine mounted on an automobile or the like.

  In the exhaust passage 2 of the internal combustion engine 1, two catalysts, that is, a front-stage catalyst 3 provided on the upstream side of the exhaust passage 2 and a rear-stage catalyst 4 provided on the downstream side are installed in series. The catalysts 3 and 4 are three-way catalysts or NOx occlusion reduction type catalysts, etc., which have the ability to occlude an appropriate amount of oxygen, and occlude oxygen when the exhaust gas contains NOx. Examples of the catalyst that can reduce the NOx and, when the exhaust gas contains HC and CO, can oxidize them by releasing the stored oxygen.

  A bypass passage 5 is connected to the exhaust passage 2 so as to branch upstream of the upstream catalyst 3, bypass the upstream catalyst 3, and rejoin the upstream of the downstream catalyst 4. The bypass passage 5 is provided with a flow rate adjusting valve 6 that adjusts the flow rate of the exhaust gas flowing through the bypass passage 5. The flow rate adjusting valve 6 is driven by an actuator (not shown) controlled by a command from an ECU (to be described later).

  When the flow rate control valve 6 is closed, exhaust does not flow through the bypass passage 5 but exhaust flows only through the exhaust passage 2, and all exhaust gas discharged from the internal combustion engine passes through the front catalyst 3. On the other hand, when the flow rate control valve 6 is opened, an amount of exhaust gas corresponding to the opening degree of the flow rate control valve 6 flows through the bypass passage 5 and exhaust gas also passes through the upstream catalyst 3.

A main A / F sensor 7 that detects the air-fuel ratio of the exhaust gas after being discharged from the internal combustion engine 1 is disposed downstream of the front catalyst 3 and in the exhaust passage 2 upstream of the front catalyst 3. The exhaust passage 2 upstream of the confluence of the bypass passage 5 and the bypass passage 5 is provided with a first sub O ₂ sensor 8 that detects the oxygen concentration in the exhaust gas after flowing out of the front catalyst 3. A downstream sub-O ₂ sensor 9 for detecting the oxygen concentration in the exhaust gas after flowing out of the rear catalyst 4 is provided in the exhaust passage 2 downstream of the exhaust passage 2.

  An electronic control unit (ECU: Electronic Control Unit) 10 is also provided in the internal combustion engine provided with the exhaust purification device configured as described above.

The ECU 10 includes a main A / F sensor 7, a first sub O ₂ sensor 8, a second sub O ₂ sensor 9, an air flow meter (not shown) provided in the intake passage, a crank position sensor (not shown), Various sensors such as an accelerator position sensor (not shown) attached to a vehicle on which the internal combustion engine 1 is mounted are connected via electric wiring, and output signals of the various sensors described above are input to the ECU 10.

For example, the ECU 10 executes input of output signals of various sensors, calculation of engine speed, calculation of fuel injection amount, calculation of fuel injection timing, and the like in a basic routine to be executed at regular intervals. Various signals input by the ECU 10 and various control values obtained by the ECU 10 in the basic routine are temporarily stored in the RAM of the ECU 10.

  Further, the ECU 10 reads various control values from the RAM in the interrupt process triggered by the input of signals from various sensors, the passage of a fixed time, or the input of a pulse signal from the crank position sensor. Control the fuel injection valve.

  As described above, both the pre-stage catalyst 3 and the post-stage catalyst 4 have an ability to occlude an appropriate amount of oxygen inside, and when the exhaust gas contains NOx, the NOx is reduced by occlusion of oxygen. In addition, when HC or CO is contained in the exhaust gas, it is a catalyst that can oxidize them by releasing the stored oxygen. Therefore, when the operation state in which the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 becomes rich continues, both the front catalyst 3 and the rear catalyst 4 enter a rich state in which the oxygen storage amount in the catalyst is small. On the other hand, when the operation state in which the air-fuel ratio of the exhaust gas discharged from the internal combustion engine becomes lean continues, the front catalyst 3 and the rear catalyst 4 both enter a lean state in which the amount of oxygen stored in the catalyst is large.

  If the exhaust gas passes through these catalysts when both the catalysts 3 and 4 are in a rich state, the amount of oxygen stored in the catalyst is small, so that HC and CO in the exhaust gas cannot be sufficiently oxidized. Further, if the exhaust gas passes through these catalysts when both the catalysts 3 and 4 are in a lean state, the amount of oxygen stored in the catalyst is large, so that NOx in the exhaust gas cannot be reduced sufficiently.

  Further, when the front stage catalyst 3 is a three-way catalyst and the rear stage catalyst 4 is a NOx occlusion reduction type catalyst, the front stage catalyst 3 is in a lean state and the rear stage catalyst 4 is in a rich state, the NOx in the exhaust gas is reduced to the front stage. Since it cannot be reduced by the catalyst 3, purification is performed only by the post-stage catalyst 4. On the other hand, when the front catalyst 3 is in the rich state and the rear catalyst 4 is in the lean state, NOx in the exhaust gas can be reduced by the front catalyst 3 and stored by the NOx storage reduction catalyst that is the rear catalyst 4. As a result, the NOx purification rate is increased. In addition, if the rear catalyst 4 close to the exhaust gas exhaust port is in a rich state for a long time, hydrogen sulfide is likely to be generated, and thus a catalyst odor may be generated. When the front catalyst 3 is in the lean state and the rear catalyst 4 is in the rich state, or when the front catalyst 3 is in the rich state and the rear catalyst 4 is in the lean state, either catalyst is in the rich state, so HC, CO The purification rate is high.

  For the above reasons, in the exhaust purification system of the present embodiment, it is not preferable that both the front catalyst 3 and the rear catalyst 4 are in a rich state or a lean state. Further, when the front catalyst 3 is a three-way catalyst and the rear catalyst 4 is a NOx occlusion reduction type catalyst, the front catalyst 3 is in a rich state and the rear catalyst 4 is in a rich state rather than in the lean state and the rear catalyst 4 is in a rich state. It is preferable that the catalyst 4 is in a lean state. Therefore, it can be said that the state where the front catalyst 3 is rich and the rear catalyst 4 is lean is the optimum state for exhaust gas purification.

  By the way, when the operation state in which the air-fuel ratio in the cylinder is lean continues, the operation state in which the air-fuel ratio of the exhaust gas discharged from the internal combustion engine becomes lean continues, and the fuel cut operation state is continued. In some cases, since only air passes through the exhaust passage 2, both the catalysts 3 and 4 are in a lean state. In order to shift the upstream catalyst 3 that is optimal for exhaust gas purification from this state to the rich state and the downstream catalyst 4 to the lean state, the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 3 is made rich, that is, in the cylinder The air-fuel ratio of the air-fuel mixture may be made rich, and the ECU 10 can easily shift to the optimum state by controlling the fuel injection valve.

On the other hand, when the high-load operation is continued, the state where the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 is rich continues, so there is a possibility that both the catalysts 3 and 4 become rich. . In order to shift from this state to an optimal state for exhaust gas purification in which the front catalyst 3 is rich and the rear catalyst 4 is in a lean state, first, the air-fuel ratio of the air-fuel mixture in the cylinder is made lean to be discharged from the internal combustion engine 1. The exhaust gas air-fuel ratio is made lean, and both the catalysts 3 and 4 are made lean. After that, it is necessary to make the air-fuel ratio of the air-fuel mixture in the cylinder rich to make the pre-catalyst 3 rich. Therefore, when both the catalysts 3 and 4 are shifted from the rich state to the state where the front catalyst 3 is rich and the rear catalyst 4 is lean, both the catalysts 3 and 4 are once in the lean state. Therefore, the purification rate cannot be increased for the reasons described above.

  Therefore, in this embodiment, when the state where both the catalysts 3 and 4 are both rich continues for a predetermined time or longer, the air-fuel ratio of the air-fuel mixture in the cylinder of the internal combustion engine 1 is set to lean and the internal combustion engine 1 A post-catalyst leaning process is performed in which the air-fuel ratio of the exhaust gas discharged is made lean, and a part of the lean air-fuel ratio exhaust gas discharged from the internal combustion engine 1 directly flows into the post-catalyst 4 by opening the flow control valve 6. Make it run.

  As a result, the first catalyst 3 is in the rich state and the second catalyst 4 is in the lean state without going through the state in which both the catalysts 3 and 4 are both in the lean state. Since it can be in a certain state, a high purification rate of the catalyst can be secured stably.

  However, when the catalyst is not in an active state, the above-described post-catalyst leaning process is not executed because the purification ability is low regardless of whether the catalyst is in a rich state or a lean state. Also, the operation increases the amount of fuel, such as increasing the fuel because the load required for the internal combustion engine is high, or increasing the fuel for the purpose of lowering the temperature because the combustion temperature is high. When the exhaust gas is in the state, the exhaust gas has a rich air-fuel ratio, but the operation state is prioritized and the post-catalyst leaning process is not executed. Further, when the operating state is a large intake air amount, the oxygen storage amount changes quickly, and when the operating state is such that the load required for the internal combustion engine is high, the output is stable. In addition, the subsequent stage catalyst leaning process is not performed.

  Further, the valve opening amount of the flow rate adjusting valve 6 is controlled so that a predetermined amount of exhaust gas flows directly into the rear catalyst 4 through the bypass passage 5. If the amount of intake air is large, the amount of exhaust gas also increases. Therefore, even if the valve opening amount is small, a predetermined amount of exhaust gas can flow directly into the rear catalyst 4. In such a case, two catalysts can be used as much as possible. It is preferable to purify the exhaust gas. On the other hand, if the amount of intake air is small, the amount of exhaust gas also decreases. Therefore, if the valve opening amount is not increased, a predetermined amount of exhaust gas cannot directly flow into the rear catalyst 4. Therefore, the valve opening amount of the flow rate adjusting valve 6 is controlled to be inversely proportional to the intake air amount.

  Hereinafter, the catalyst state control in the present embodiment will be specifically described with reference to FIG. FIG. 2 is a flowchart showing a control routine for catalyst state control. This routine is stored in advance in the ECU 10, and is executed by the ECU 10 as an interrupt process triggered by elapse of a fixed time or input of a pulse signal from the crank position sensor.

In this routine, the ECU 10 first determines in step (hereinafter referred to as “S”) 101 whether or not the front catalyst 3 and the rear catalyst 4 are in an active state. This is because the current catalyst temperature is estimated from the history of the exhaust temperature that can be derived from the intake air amount and engine speed after the internal combustion engine 1 is started, and whether or not the estimated catalyst temperature is equal to or higher than the activation temperature of the catalyst. This is a judgment. Alternatively, each catalyst may be provided with a temperature sensor that directly detects the temperature of the catalyst, and it may be determined whether or not the temperature detected by the temperature sensor is equal to or higher than the activation temperature. If a positive determination is made, the process proceeds to S102, and if a negative determination is made, the process proceeds to S109.

  In S102, it is determined whether or not the above-described operation state in which the amount of fuel is increased (during fuel increase). If a negative determination is made, the process proceeds to S103, and if an affirmative determination is made, the process proceeds to S109. In S103, it is determined whether or not the intake air amount is equal to or less than a predetermined amount. This is to determine whether or not the intake air amount detected by the air flow meter is equal to or less than a predetermined amount. If a positive determination is made, the process proceeds to S104, and if a negative determination is made, the process proceeds to S109. Note that the predetermined amount is a predetermined amount for each internal combustion engine by experiments or the like in advance, and it is not possible to exhibit a desired output by performing the post-catalyst leaning process or limit the oxygen storage amount excessively. The amount is set to be smaller than the intake air amount.

In S104, it is determined whether or not the detection value of the first sub O ₂ sensor 8 detects a rich air-fuel ratio. That the detection value of the first sub O ₂ sensor 8 detects the rich air-fuel ratio means that the front catalyst 3 is in a rich state, and whether or not the front catalyst 3 is in a rich state in this step. It can be said that it is judged whether or not. If a positive determination is made, the process proceeds to S105, and if a negative determination is made, the process proceeds to S109.

In S105, it is determined whether or not the detection value of the second sub O ₂ sensor 9 detects a rich air-fuel ratio. That the detection value of the second sub O ₂ sensor 9 detects the rich air-fuel ratio means that the rear catalyst 4 is in a rich state, and whether or not the rear catalyst 4 is in a rich state in this step. It can be said that it is judged whether or not. If a positive determination is made, the process proceeds to S106, and if a negative determination is made, the process proceeds to S109.

  In S106, the catalyst rich counter is incremented by one. Thereafter, the process proceeds to S107, and it is determined whether or not the value of the catalyst rich counter counted so far is equal to or larger than a reference value. Note that the reference value is a value determined in advance for each internal combustion engine through experiments or the like, and if both the catalysts 3 and 4 are in a rich state, the exhaust emission or the generation of catalyst odor is excessively deteriorated. It is a value set smaller than the value of. If the determination is affirmative, the process proceeds to S108, and the above-described post-catalyst leaning process is executed. On the other hand, if a negative determination is made, the execution of this routine is terminated.

  In S109, it is determined whether the post-catalyst leaning process is being executed. If an affirmative determination is made, the routine proceeds to S110, where the post-catalyst leaning process is terminated, and then the routine proceeds to S111 where the catalyst rich counter counted so far is cleared, that is, set to zero, and the execution of this routine is terminated. On the other hand, if a negative determination is made in S109, the process proceeds to S111, the catalyst rich counter is cleared, and the execution of this routine is terminated.

  By executing this catalyst state control, if the state in which both the catalysts 3 and 4 are both in a rich state continues for a predetermined time or longer except in the case of a specific operation state, the subsequent stage catalyst leaning process is started. Is done. The lean air-fuel ratio exhaust gas then flows directly into the rear catalyst 4 via the bypass passage 5. Thereafter, except for the specific operating state described above, the negative catalyst leaning process is continued until a negative determination is made in S105, that is, until it is determined that the downstream catalyst 4 is in the lean state. As a result, the pre-catalyst 3 is shifted to a rich state and the post-catalyst 4 is shifted to a lean state, so that a high purification rate of the catalyst can be secured stably. Moreover, since it can avoid that the back | latter stage catalyst 4 becomes a rich state for a long time, generation | occurrence | production of a catalyst odor can be suppressed.

In the present embodiment, when the post-catalyst leaning process is performed, depending on the amount of opening of the flow control valve 6, a part of the lean air-fuel ratio exhaust gas flows into the pre-catalyst 3. In such a case, a negative determination is made in S104, and the post-catalyst leaning process is terminated in S110. As a result, the front catalyst 3 is in a lean state and the rear catalyst 4 is in a rich state, but this state is more than the state in which the front catalyst 3 having a high purification rate is rich and the rear catalyst 4 is in a lean state. Although the NOx purification rate is inferior, the exhaust gas can be sufficiently purified because the purification rate of HC, CO, and NOx is higher than the state in which the front catalyst 3 and the rear catalyst 4 are rich.

In the first embodiment, the bypass passage 5 and the flow rate adjusting valve 6 are provided. However, in the present embodiment, as shown in FIG. 3, the upstream side of the rear catalyst 4 and the downstream side of the first sub O ₂ sensor 8 are provided. A secondary air supply device 20 for supplying secondary air to the exhaust passage 2 is provided. The secondary air supply device 20 includes an air pump 21 driven by the internal combustion engine 1 as shown in FIG. 3, a secondary air supply valve 22 that opens according to a command from the ECU 10, and the like. Is connected to an intake passage downstream of the air cleaner through a conduit, or an electric air pump that is driven by a battery as a drive source in response to a command signal from the ECU 10, and air discharged from the air pump is sent to the exhaust passage. It may be composed of a reed valve that permits and does not allow exhaust gas to flow from the exhaust passage to the pump side.

Then, the post-catalyst leaning process in the first embodiment makes the air-fuel ratio of the air-fuel mixture in the cylinder of the internal combustion engine 1 lean, the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 is made lean, and the flow control valve 6, a part of the lean air-fuel ratio exhaust gas discharged from the internal combustion engine 1 directly flows into the rear catalyst 4. However, the rear catalyst leaning process in this embodiment flows into the rear catalyst 4. Secondary air is supplied from the secondary air supply device 20 so that the air-fuel ratio of the exhaust gas becomes lean. At this time, the air-fuel ratio of the exhaust gas flowing into the rear catalyst 4 is desired based on the exhaust gas amount estimated based on the intake air amount detected by the air flow meter and the oxygen concentration detected by the first sub O ₂ sensor 8. The secondary air is supplied by controlling the above-described secondary air supply valve or the electric air pump so as to obtain a lean air-fuel ratio. The other details are the same as those of the first embodiment, and thus detailed description thereof is omitted.

  In such a configuration, when the catalyst state control similar to that of the first embodiment is executed, the state where both the catalysts 3 and 4 are both in a rich state continues for a predetermined time or more except in a specific operation state. Then, the subsequent catalyst leaning process is started. Then, the exhaust gas supplied with the secondary air and having a lean air-fuel ratio flows into the rear catalyst 4. Thereafter, except for the specific operating state described above, the negative catalyst leaning process is continued until a negative determination is made in S105, that is, until it is determined that the downstream catalyst 4 is in the lean state. As a result, the pre-catalyst 3 is shifted to a rich state and the post-catalyst 4 is shifted to a lean state, so that a high purification rate of the catalyst can be secured stably. Moreover, since it can avoid that the back | latter stage catalyst 4 becomes a rich state for a long time, generation | occurrence | production of a catalyst odor can be suppressed.

  In the present embodiment, the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 becomes lean depending on the air-fuel ratio of the air-fuel mixture in the cylinder during the post-catalyst leaning process, and the exhaust gas having the lean air-fuel ratio becomes lean. Flows into the front catalyst 3, so that the front catalyst 3 may be in a lean state. In such a case, a negative determination is made in S104, and the post-catalyst leaning process is terminated in S110. As a result, the front catalyst 3 is in a lean state and the rear catalyst 4 is in a rich state. This state is higher than the state in which the front catalyst 3 having a high purification rate is rich and the rear catalyst 4 is in a lean state. However, since the purification rate of HC, CO, and NOx is higher than the state where the front catalyst 3 and the rear catalyst 4 are rich, the exhaust gas can be sufficiently purified.

1 is a diagram illustrating a schematic configuration of an exhaust emission control apparatus according to a first embodiment. It is a flowchart figure which shows the control routine of the catalyst state control which concerns on Example 1 and Example 2. FIG. It is a figure which shows schematic structure of the exhaust gas purification apparatus which concerns on Example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 3 Front stage catalyst 4 Rear stage catalyst 5 Detour passage 6 Flow control valve 7 Main A / F sensor 8 First sub O ₂ sensor 9 Second sub O ₂ sensor 10 ECU
20 Secondary air supply device 21 Air pump 22 Secondary air supply valve

Claims (2)

A pre-stage catalyst provided in an exhaust passage of the internal combustion engine and having an oxygen storage capacity;
A rear catalyst provided in the exhaust passage downstream of the front catalyst and having an oxygen storage capacity;
A detour passage for causing at least a part of exhaust gas discharged from the internal combustion engine to detour the front catalyst and flow into the rear catalyst;
A flow rate regulating valve for regulating the flow rate of the exhaust gas flowing through the bypass path;
When both the front catalyst and the rear catalyst are in a rich state with a small oxygen storage amount, the exhaust gas discharged from the internal combustion engine is made lean while the air-fuel ratio in the cylinder of the internal combustion engine is leaned and the flow control valve is opened. An exhaust purification device for an internal combustion engine, comprising: a process executing means for executing a leaning process for causing the exhaust gas to flow into the rear catalyst through the bypass passage.   2. The internal combustion engine according to claim 1, wherein the processing execution unit executes a leaning process when a rich state in which both the front stage catalyst and the rear stage catalyst have a small oxygen storage amount continues for a predetermined time or longer. Exhaust purification device.

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