JP2018035771A - Exhaust purification system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust purification system capable of maintaining a high HC purification rate with a lean HC purification catalyst.SOLUTION: An exhaust purification system comprises: a three-way catalyst which is installed on an exhaust pipe of an engine capable of lean burn operation and purifies exhaust by oxidizing HC therein when an air-fuel ratio thereof is in a lean atmosphere; reducing means which reduces the three-way catalyst through a reducing treatment putting the exhaust flowing into the three-way catalyst in a rich atmosphere; and oxidation degree calculation means which calculates a value of an oxidation parameter TWCSTATE quantifying an oxidation degree of the three-way catalyst on the basis of the air-fuel ratio of the exhaust detected by an air-fuel ratio sensor and a three-way catalyst temperature obtained through a temperature sensor. The reducing means executes the reducing treatment on the basis of the oxidation parameter TWCSTATE.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system for an internal combustion engine including a lean HC purification catalyst that purifies by oxidizing HC in exhaust gas when the air-fuel ratio of the exhaust gas is a lean atmosphere.

  Exhaust gas purification systems that purify exhaust gas from internal combustion engines with a three-way catalyst have been widely put into practical use. The three-way catalyst purifies HC, CO, and NOx in the exhaust with high efficiency when the air-fuel ratio of the exhaust is controlled in the vicinity of the stoichiometric. When the air-fuel ratio of the exhaust gas becomes a lean atmosphere, the three-way catalyst can oxidize and purify HC and CO, but cannot sufficiently purify NOx. For this reason, in an exhaust gas purification system for an internal combustion engine capable of lean burn operation, in order to be able to purify NOx even during lean burn operation, in addition to a three-way catalyst, NOx in a lean atmosphere such as a lean NOx catalyst or an SCR catalyst. In many cases, a DeNOx catalyst that can purify gas is further provided (see, for example, Patent Document 1). As shown in Patent Document 1, in an exhaust purification system that combines a three-way catalyst and a DeNOx catalyst, NOx contained in the exhaust gas during lean burn operation is purified by the DeNOx catalyst, and HC and CO are oxidized by the three-way catalyst. It can be purified by doing.

International Publication No. 02/2916

  By the way, when the lean burn operation is continued in the exhaust purification system including the three-way catalyst, the oxidation of the catalyst noble metal contained in the three-way catalyst proceeds, and the HC purification rate by the three-way catalyst gradually decreases. However, in the conventional exhaust purification system, the HC purification rate of the three-way catalyst is lowered during the lean burn operation, and the recovery of the HC purification rate has not been sufficiently studied.

  An object of the present invention is to maintain a high HC purification rate by a lean HC purification catalyst in an exhaust purification system including a lean HC purification catalyst that can oxidize and purify HC in exhaust during lean burn operation.

  (1) An exhaust purification system (for example, an exhaust purification system 2 described later) of an internal combustion engine is provided in an exhaust passage (for example, an exhaust pipe 11 described later) of an internal combustion engine (for example, an engine 1 described later) capable of lean burn operation. A lean HC purification catalyst (for example, a three-way catalyst described later and an upstream catalytic converter 31 on which this is supported) that purifies by oxidizing HC in the exhaust gas when the air-fuel ratio of the exhaust gas is in a lean atmosphere; Exhaust air / fuel ratio detecting means (for example, an air / fuel ratio sensor 35 to be described later) for detecting the air / fuel ratio of the exhaust gas flowing through the exhaust passage, and temperature acquiring means (for example, to be described later for the temperature sensor 33 and ECU7 etc.) and reduction for reducing the lean HC purification catalyst by performing a reduction process for making the exhaust gas flowing into the lean HC purification catalyst a rich atmosphere A degree of oxidation (for example, ECU 7 to be described later) and an oxidation degree calculating means for calculating the degree of oxidation of the lean HC purification catalyst (for example, an oxidation parameter TWCSTATE described later) based on the detected air-fuel ratio of the exhaust gas and the acquired temperature. For example, the ECU 7), which will be described later, is characterized in that the reduction means performs the reduction process based on the calculated degree of oxidation.

  In the present invention, the lean atmosphere refers to a state in which the oxygen concentration in the exhaust gas in the lean HC purification catalyst is relatively high with respect to the concentration of reducing components such as hydrocarbons and carbon monoxide. As described later with reference to FIG. 5, in the lean atmosphere, the oxidation rate in the three-way catalyst as the lean HC purification catalyst becomes positive. Specifically, this lean atmosphere is realized, for example, by making the air-fuel ratio in an internal combustion engine leaner than stoichiometric. In the present invention, the rich atmosphere means a state in which the oxygen concentration in the exhaust gas in the lean HC purification catalyst is relatively low with respect to the concentration of the reducing component. As will be described later with reference to FIG. 5, in a rich atmosphere, the oxidation rate in the three-way catalyst becomes negative and the reduction reaction proceeds. Specifically, the rich atmosphere is made, for example, by making after-injection or the like the air-fuel ratio of the internal combustion engine richer than stoichiometric, performing post-injection, or injecting fuel into the exhaust with a fuel injector provided in the exhaust pipe This is realized by supplying unburned fuel to the lean HC purification catalyst.

  (2) In this case, the oxidation degree calculation means increases the oxidation degree when the detected air-fuel ratio of the exhaust gas is lean, and the oxidation degree calculating means when the detected air-fuel ratio of the exhaust gas is rich. The reduction means starts the reduction process in response to the oxidation degree exceeding an upper limit threshold (for example, an upper limit threshold Th_H described later), and the oxidation degree is lower than the upper limit threshold. It is preferable that the reduction process is terminated in response to falling below (for example, a lower limit threshold Th_L described later).

  (3) In this case, it is preferable that the reduction means changes the air-fuel ratio of the exhaust gas to the rich side when the reduction process is performed as the acquired temperature increases.

  (1) The oxidation rate and reduction rate of the lean HC purification catalyst vary mainly depending on the air-fuel ratio of the exhaust gas and the temperature of the lean HC purification catalyst. Therefore, in the present invention, the degree of oxidation of the lean HC purification catalyst is calculated based on the air / fuel ratio of the exhaust gas detected using the exhaust air / fuel ratio detection means and the temperature of the lean HC purification catalyst obtained using the temperature acquisition means. Then, based on the calculated degree of oxidation, the exhaust gas flowing into the lean HC purification catalyst is made a rich atmosphere, the lean HC purification catalyst is reduced, and a reduction process is performed to recover the HC purification rate. As a result, the reduction process can be performed at an appropriate timing before the oxidation of the lean HC purification catalyst proceeds excessively, so that the HC purification rate in the lean HC purification catalyst during the lean burn operation can be maintained high. Further, since the reduction process requires extra fuel to make a rich atmosphere, if the reduction process is frequently performed, fuel consumption may be deteriorated. In contrast, in the present invention, the reduction process is performed based on the degree of oxidation calculated using the air-fuel ratio and temperature of the exhaust gas, so that the reduction process is not performed more frequently than necessary. Can be suppressed.

  (2) When the lean HC purification catalyst is exposed to the exhaust gas in the lean atmosphere, the oxidation proceeds, and when it is exposed to the exhaust gas in the rich atmosphere, the reduction proceeds. In the present invention, the degree of oxidation of the lean HC purification catalyst is increased by increasing the degree of oxidation when the air-fuel ratio of the exhaust gas is lean in accordance with this characteristic, and by decreasing the degree of oxidation when the exhaust is rich. Can be calculated appropriately according to In the present invention, the reduction process is started in response to the degree of oxidation calculated in this way exceeding the upper limit threshold. Thereby, since the reduction process can be started at an appropriate timing before the oxidation of the lean HC purification catalyst proceeds excessively, the HC purification rate can be maintained high. Further, the reduction process is terminated in response to the degree of oxidation falling below the lower limit threshold. Thereby, since the reduction process is not performed over a longer time than necessary, it is possible to suppress the deterioration of fuel consumption associated with the reduction process.

  (3) In the present invention, as the temperature of the lean HC purification catalyst becomes higher, the air-fuel ratio of the exhaust during the reduction process is changed to the rich side. As described above, the reduction of the lean HC purification catalyst proceeds in a rich atmosphere. However, the reduction rate increases as the air-fuel ratio at this time is changed to the rich side, so that the HC purification rate is quickly recovered by that amount. be able to. Therefore, according to the present invention, the reduction process can be completed as quickly as possible according to the temperature of the lean HC purification catalyst. Further, the HC purification rate of the lean HC purification catalyst during the reduction process, that is, in the rich atmosphere, tends to be lower than that in the lean atmosphere. Therefore, according to the present invention, the period during which the HC purification rate decreases by executing the reduction process can be shortened by ending the reduction process promptly.

1 is a diagram illustrating a configuration of an internal combustion engine and an exhaust purification system thereof according to an embodiment of the present invention. It is a figure which shows the change of the HC purification | cleaning rate of a three-way catalyst when a rich spike is intermittently performed during lean burn operation. It is a flowchart which determines the timing which performs the reduction process of a three-way catalyst. It is a flowchart which shows the specific procedure of the state estimation process of a three-way catalyst. It is a figure which shows an example of an oxidation rate setting map. It is a figure which shows an example of an exhaust flow volume correction map. It is a time chart which shows an example at the time of performing the reduction process of a three way catalyst according to the flowchart of FIG.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification system 2 according to the present embodiment. The exhaust purification system 2 includes a catalyst purification device 3 provided in an exhaust pipe 11 extending from an exhaust port of the engine 1, and an electronic control unit (hereinafter referred to as "ECU") that controls the engine 1 and the catalyst purification device 3. 7).

  The engine 1 is based on a so-called lean burn operation in which the combustion air-fuel ratio is leaner than the stoichiometry, and intermittently performs a so-called stoichiometric operation in which the combustion air-fuel ratio is in the vicinity of the stoichiometry during a high load operation such as when the accelerator pedal is greatly depressed. More specifically, for example, a diesel engine or a lean burn gasoline engine is used, but the present invention is not limited to this. The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected in accordance with a control signal from the ECU 7. The ECU 7 determines the fuel injection amount and fuel injection timing from the fuel injection valve 17 under fuel injection control (not shown), and drives the fuel injection valve 17 so that this is realized.

  The catalyst purification device 3 includes an upstream catalytic converter 31, a downstream catalytic converter 32, a temperature sensor 33, an air-fuel ratio sensor 35, and an air flow meter 36 provided in the exhaust pipe 11.

  The upstream catalytic converter 31 has a flow-through type honeycomb structure as a base material and a three-way catalyst containing a noble metal such as Pd or Pt supported on the base material. The three-way catalyst purifies HC, CO and NOx contained in the exhaust gas by a three-way purification reaction when the air-fuel ratio of the exhaust gas is controlled in the vicinity of stoichiometric. In addition, when the air-fuel ratio of the exhaust is controlled to a lean atmosphere, the three-way catalyst purifies these by oxidizing HC and CO contained in the exhaust.

  The downstream catalytic converter 32 has a flow-through honeycomb structure as a base material, and the base material carries an LNT catalyst that purifies NOx in the exhaust gas. The LNT catalyst occludes NOx contained in the exhaust when the air-fuel ratio of the exhaust is in a lean atmosphere, and occludes in the lean atmosphere using HC and CO in the exhaust as a reducing agent when the air-fuel ratio of the exhaust is in a rich atmosphere. This is purified by reducing the NOx that has been added and the newly flowing NOx.

  The temperature sensor 33 is provided, for example, on the downstream side of the upstream catalytic converter 31 in the exhaust pipe 11. The temperature sensor 33 detects the temperature of the exhaust gas flowing out from the upstream catalytic converter 31, and transmits a signal corresponding to the detected value to the EC 7. The ECU 7 calculates the temperature of the three-way catalyst carried on the upstream catalytic converter 31 (hereinafter also simply referred to as “three-way catalyst temperature”) based on the output signal of the temperature sensor 33.

  For example, the air-fuel ratio sensor 35 is provided on the upstream side of the upstream catalytic converter 31 in the exhaust pipe 11. The air-fuel ratio sensor 35 transmits a detection signal that is substantially proportional to the oxygen concentration (air-fuel ratio) of the exhaust gas flowing into the upstream catalytic converter 31 to the ECU 7. For the air-fuel ratio sensor 35, a so-called LAF sensor whose output signal has a linear characteristic substantially proportional to the air-fuel ratio at the detection location is used.

  The air flow meter 36 is provided in the intake pipe 12 of the engine 1 and transmits a signal corresponding to the flow rate of intake air flowing through the intake pipe 12 to the ECU 7. The ECU 7 calculates the amount of air newly supplied to the engine 1 (hereinafter also referred to as “intake air amount”) based on the output signal of the air flow meter 36.

  The ECU 7 has an I / O interface for A / D converting sensor detection signals, a CPU for executing processing in accordance with flowcharts shown in FIGS. 3 and 4 to be described later, and various devices in a mode determined under this processing. The microcomputer includes a driving circuit for driving, and a RAM, a ROM, and the like for storing various data such as maps shown in FIGS.

Next, the characteristics of the HC purification function of the three-way catalyst carried by the upstream catalytic converter 31 will be described in detail with reference to FIG.
FIG. 2 shows the HC purification of the three-way catalyst when the rich spike that makes the exhaust air-fuel ratio rich during the lean burn operation is intermittently performed between the times t1 to t2 and t3 to t4. It is a figure which shows the change of rate [%].

  First, the three-way catalyst has an HC purification function of purifying the exhaust gas by oxidizing HC in a lean atmosphere, but the HC purification rate in the three-way catalyst is not always maintained during the lean burn operation. More specifically, when the lean burn operation is continued, the catalytic noble metal (for example, Pd) contained in the three-way catalyst is oxidized by oxygen contained in the exhaust, and as a result, the HC purification rate is also increased as shown in FIG. There is a characteristic that gradually decreases.

  In contrast, if the rich spike is intermittently executed during the lean burn operation, the HC purification rate temporarily decreases, but the reduction of the three-way catalyst proceeds and the HC purification rate in the lean atmosphere is reduced. It can be recovered in time. As shown in FIG. 2, the HC purification rate during the rich spike (time t1 to t2 and t3 to t4) temporarily falls, but the purification rate immediately before the start of the rich spike (time t1 or t3). Comparing (a1 or a2 in FIG. 2) with the purification rate (b1 or b2 in FIG. 2) immediately after the rich spike is performed (time t2 or t4), the ratio immediately after the rich spike is higher. That is, the HC purification rate of the three-way catalyst, which is reduced by continuously performing the lean burn operation, can be recovered by performing a reduction process that temporarily makes the air-fuel ratio of the exhaust rich at an appropriate timing. .

  FIG. 3 is a flowchart for determining the timing for executing such a three-way catalyst reduction process. The process of FIG. 3 is executed at predetermined control cycles in the ECU in response to, for example, turning on the ignition switch and starting the engine.

  First, in S1, the ECU calculates the value of the oxidation parameter TWCSTATE by performing a state estimation process for estimating the current state of the three-way catalyst, and proceeds to S2. Here, the oxidation parameter TWCSTATE is a numerical value of the degree of oxidation of the three-way catalyst, and becomes 0 when the oxidation of the three-way catalyst is not progressing at all, and becomes a value larger than 0 as the oxidation progresses. And When the catalytic noble metal contained in the three-way catalyst is palladium, the oxidation parameter TWCSTATE corresponds to the ratio of palladium to palladium oxide (PdO / Pd) in the three-way catalyst.

FIG. 4 is a flowchart showing a specific procedure of the state estimation process.
First, in S21, the ECU calculates the value of the oxidation rate Vox of the three-way catalyst in the current control cycle, and proceeds to S22. The oxidation rate Vox is positive when the oxidation of the three-way catalyst proceeds, becomes negative when the reduction of the three-way catalyst proceeds, and becomes zero when neither oxidation nor reduction proceeds. And

  More specifically, the ECU searches the oxidation rate setting map using the three-way catalyst temperature obtained using the exhaust air-fuel ratio and the temperature sensor detected using the air-fuel ratio sensor, thereby oxidizing the three-way catalyst. The value of the speed Vox is calculated.

  FIG. 5 is a diagram showing an example of an oxidation rate setting map that takes the exhaust air-fuel ratio and the three-way catalyst temperature as inputs and calculates the value of the oxidation rate Vox according to these inputs. In FIG. 5, the vertical axis represents the oxidation rate Vox, and the horizontal axis represents the exhaust air / fuel ratio. Further, the map of FIG. 5 illustrates only cases where the three-way catalyst temperature is 300 ° C. (solid line), 400 ° C. (dashed line), and 500 ° C. (one-dot chain line).

  As shown in FIG. 5, when the exhaust air-fuel ratio is stoichiometric, the value of the oxidation rate Vox becomes 0, and neither oxidation nor reduction proceeds with the three-way catalyst. When the exhaust air-fuel ratio is a lean atmosphere, the value of the oxidation rate Vox becomes positive, so the oxidation parameter TWCSTATE changes to the increasing side. In a lean atmosphere, the oxidation rate Vox increases as the exhaust air-fuel ratio increases, and the increase rate of the oxidation parameter TWCSTATE increases. When the exhaust air-fuel ratio is a rich atmosphere, the value of the oxidation rate Vox becomes negative, so the oxidation parameter TWCSTATE changes to the decreasing side. In a rich atmosphere, the oxidation rate Vox decreases as the exhaust air-fuel ratio decreases, and the decrease rate of the oxidation parameter TWCSTATE increases.

  Further, as shown in FIG. 5, the higher the three-way catalyst temperature, the larger the absolute value of the oxidation rate Vox. That is, in the lean atmosphere, the higher the three-way catalyst temperature, the higher the oxidation rate Vox becomes on the positive side, and the increase rate of the oxidation parameter TWCSTATE. In a rich atmosphere, the higher the three-way catalyst temperature, the smaller the oxidation rate Vox on the negative side, and the greater the rate of decrease of the oxidation parameter TWCSTATE. In S21, the ECU calculates the value of the oxidation rate Vox by searching the above oxidation rate setting map using the three-way catalyst temperature and the exhaust air / fuel ratio at that time.

  In S22, the ECU calculates the value of the correction coefficient K_QAIR by searching the exhaust flow rate correction map using the intake air amount detected using the air flow meter, and proceeds to S23. The correction coefficient K_QAIR is a parameter used for correcting the oxidation rate Vox according to the flow rate of the exhaust gas flowing through the three-way catalyst, and is used by multiplying the oxidation rate Vox in S23 later. The correction coefficient K_QAIR is set larger than 0 and in the vicinity of 1.

  FIG. 6 is a diagram showing an example of an exhaust flow rate correction map that takes an intake air amount as an input and calculates a value of a correction coefficient K_QAIR according to the input. In FIG. 6, the vertical axis represents the correction coefficient K_QAIR, and the horizontal axis represents the intake air amount. As shown in FIG. 6, when the intake air amount is larger than a predetermined reference amount, that is, when an amount of exhaust gas larger than the predetermined amount flows into the three-way catalyst, oxidation or reduction in the three-way catalyst is promoted. Therefore, the correction coefficient K_QAIR is set to a value larger than 1 so that the absolute value of the oxidation rate Vox is increased. In addition, when the intake air amount is smaller than the reference amount, that is, when an amount of exhaust gas smaller than the predetermined amount flows into the three-way catalyst, oxidation or reduction in the three-way catalyst is suppressed, so the absolute value of the oxidation rate Vox The correction coefficient K_QAIR is set to a value smaller than 1 so that the correction is performed so as to reduce the value. In S22, the ECU calculates the value of the correction coefficient K_QAIR by searching the exhaust flow rate correction map as described above using the intake air amount at that time.

  Next, in S23, the ECU adds the value obtained by multiplying the oxidation rate Vox and the correction coefficient K_QAIR to the value of the oxidation parameter TWCSTATE during the previous control cycle, thereby obtaining the latest value of the oxidation parameter TWCSTATE. Then, the process proceeds to S2 in FIG.

  Returning to FIG. 3, in S <b> 2, the ECU determines whether or not the rich flag is “1”. This rich flag is a flag indicating that the reduction process of the three-way catalyst is currently requested, and is set to 1 in S4 described later, and then reset to 0 in S6.

  If the determination in S2 is NO, that is, if execution of the reduction process is not requested, the ECU proceeds to S3 and determines whether or not the value of the oxidation parameter TWCSTATE is greater than a predetermined upper limit threshold Th_H. Thus, it is determined whether or not it is time to start the reduction process of the three-way catalyst. If the determination in S3 is YES, the ECU sets the value of the rich flag to 1 (see S4), and further proceeds to S7 to start the reduction process. If the determination in S3 is NO, the ECU determines that it is not time to start the reduction process, and ends the process of FIG. 3 without performing the reduction process.

  When the determination of S2 is YES, that is, when the execution of the reduction process is currently requested, the ECU proceeds to S5 and sets the value of the oxidation parameter TWCSTATE to a value smaller than the above-described upper limit threshold Th_H. By determining whether or not it is smaller than the lower limit threshold Th_L, it is determined whether or not it is time to end the reduction process of the three-way catalyst. If the determination in S5 is YES, the ECU resets the value of the rich flag to 0 (see S6), and ends the process of FIG. 3 without performing the reduction process. If the determination in S5 is NO, the ECU determines that it is not time to finish the reduction process, and proceeds to S7 to continue the reduction process.

  In S7, the ECU determines whether or not the current engine operating state is a state suitable for executing a reduction process. More specifically, the ECU performs a fuel cut to temporarily stop fuel injection when a sulfur purge process is currently being performed to remove sulfur oxides adhering to the three-way catalyst or LNT. Is determined to be not suitable for executing the reduction process, and if none of these processes are executed, it is determined to be suitable for executing the reduction process. .

  If the determination in S7 is NO, the ECU ends the process of FIG. 3 without performing a reduction process. When the determination in S7 is YES, the ECU performs a reduction process to make the air-fuel ratio of the exhaust gas flowing into the three-way catalyst rich by performing after injection, post injection, or the like (see S8). The process of FIG. 3 is terminated. Note that the exhaust air-fuel ratio during the execution of the reduction process of S8 is preferably changed to the rich side as the three-way catalyst temperature increases within a richer range than the stoichiometric range.

  FIG. 7 is a time chart showing an example when the reduction process of the three-way catalyst is performed according to the flowchart of FIG. FIG. 7 shows the exhaust air-fuel ratio [A / F], the HC purification rate [%] by the three-way catalyst, the oxidation parameter TWCSTATE, and the rich flag in order from the upper stage to the lower stage. Further, in FIG. 7, a change in the HC purification rate or the like in a conventional exhaust purification system that does not perform the reduction process of the three-way catalyst is shown by a broken line for comparison.

  First, lean burn operation is performed between time t0 and time t1. During this time, since the exhaust gas in a lean atmosphere flows into the three-way catalyst, oxidation of the three-way catalyst proceeds, and the HC purification rate gradually decreases. According to the process of the flowchart of FIG. 3, such a decrease in the HC purification rate of the three-way catalyst is detected as an increase in the oxidation parameter TWCSTATE of the three-way catalyst.

  The stoichiometric operation is performed between the times t1 and t2. During this time, the stoichiometric air-fuel ratio exhaust gas flows into the three-way catalyst, so that HC, CO, NOx in the exhaust gas is removed with high efficiency by the three-way purification reaction in the three-way catalyst. For this reason, the HC purification rate in the three-way catalyst is temporarily higher than that between time t0 and time t1. Further, as described with reference to FIG. 5, at the stoichiometric air-fuel ratio, neither oxidation nor reduction of the three-way catalyst proceeds, so the value of the oxidation parameter TWCSTATE is constant from time t1 to time t2. For this reason, the HC purification rate in the three-way catalyst under a lean atmosphere does not increase significantly due to the stoichiometric operation from time t1 to time t2.

  The lean burn operation is performed again from time t2 to time t3. As described above, since the value of the oxidation parameter TWCSTATE is constant during the stoichiometric operation, when the lean burn operation is started at time t2, the HC purification rate is approximately the same as the HC purification rate immediately before the stoichiometric operation is started. Depressed. Thereafter, by continuing the lean burn operation, the HC purification rate gradually decreases, and the oxidation parameter TWCSTATE gradually increases.

  Next, at time t3, the rich flag is set to 1 in response to the value of the oxidation parameter TWCSTATE becoming larger than the upper limit threshold Th_H, thereby starting a reduction process that makes the exhaust air-fuel ratio rich (FIG. 3). S3, S4, S7, and S8). As a result, the reduction of the three-way catalyst proceeds, and the value of the oxidation parameter TWCSTATE begins to decrease.

  At time t4, the rich flag is set to 0 in response to the value of the oxidation parameter TWCSTATE becoming smaller than the lower limit threshold Th_L, whereby the reduction process ends (see S5 and S6 in FIG. 3) and the lean burn operation. Return to.

  Here, during the period from time t3 to t4, the reduction process is performed and the exhaust air-fuel ratio is temporarily made rich, whereby the HC purification rate in the three-way catalyst is temporarily lowered, but as shown in FIG. The HC purification rate of the three-way catalyst in the lean atmosphere can be recovered in a short time to a value larger than the HC purification rate of the conventional exhaust purification system that does not perform the reduction treatment and the HC purification rate immediately before starting the reduction treatment. it can.

Although one embodiment of the present invention has been described above, the present invention is not limited to this.
For example, in the above embodiment, after the reduction process is started, the value of the rich flag is reset from 1 to 0 in response to the value of the oxidation parameter TWCSTATE becoming smaller than the lower limit threshold Th_L, and the reduction process is ended. You may determine the timing which complete | finishes a reduction process, without using the oxidation parameter TWCSTATE. More specifically, when the O ₂ sensor for detecting the oxygen concentration in the exhaust gas downstream of the three-way catalyst is provided, the timing to end the reduction treatment by using the output signal of the O ₂ sensor You may decide. In other words, while the reduction of the three-way catalyst is in progress, the O ₂ sensor provided on the downstream side, exhaust higher oxygen concentration than a predetermined value is detected and the reduction of the three-way catalyst is completed, O ₂ The sensor detects exhaust gas having an oxygen concentration lower than a predetermined value. Therefore, when an O ₂ sensor is provided, the reduction process may be terminated when the oxygen concentration of the exhaust gas detected by the O ₂ sensor becomes equal to or lower than a predetermined value.

1. Engine (internal combustion engine)
2 ... Exhaust purification system 3 ... Catalyst purification device 31 ... Upstream catalytic converter (lean HC purification catalyst)
33 ... Temperature sensor (temperature acquisition means)
35 ... Air-fuel ratio sensor (exhaust air-fuel ratio detecting means)
7 ... ECU (reduction means, oxidation degree calculation means)

Claims (3)

A lean HC purification catalyst that is provided in an exhaust passage of an internal combustion engine capable of lean burn operation and purifies by oxidizing HC in the exhaust when the air-fuel ratio of the exhaust is in a lean atmosphere;
Exhaust air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage;
An exhaust purification system for an internal combustion engine, comprising: a temperature acquisition means for acquiring the temperature of the lean HC purification catalyst;
Reducing means for reducing the lean HC purification catalyst by performing a reduction treatment that makes the exhaust gas flowing into the lean HC purification catalyst a rich atmosphere;
An oxidation degree calculation means for calculating an oxidation degree of the lean HC purification catalyst based on the detected air-fuel ratio of the exhaust gas and the acquired temperature;
The exhaust gas purification system for an internal combustion engine, wherein the reduction means performs the reduction process based on the calculated degree of oxidation. The oxidation degree calculation means increases the oxidation degree when the detected air-fuel ratio of the exhaust gas is lean, and decreases the oxidation degree when the detected air-fuel ratio of the exhaust gas is rich,
The reduction means starts the reduction process in response to the degree of oxidation exceeding an upper limit threshold, and ends the reduction process in response to the oxidation degree falling below a lower limit threshold smaller than the upper limit threshold. The exhaust gas purification system for an internal combustion engine according to claim 1.   3. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the reduction means changes the air-fuel ratio of the exhaust gas when the reduction process is performed to a rich side as the acquired temperature becomes higher.

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