JP2008106717A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an NOx amount exhausted from an internal combustion engine from changing by a variation in an EGR ratio when deceasing a flow of an exhausted gas passing through an exhaust emission control device by means of decreasing an opening degree of an intake air throttling valve or an exhaust air throttling valve which are provided in an exhaust passage of the internal combustion engine. <P>SOLUTION: In a PM regeneration process for a filter, estimated is an EGR rate obtained when having changed an intake air throttling opening degree θin and an exhaust air throttling opening degree θex into target opening degrees θinT and θexT, respectively which are at a closing side (S105). Then, an EGR opening degree θr is changed into a target EGR opening degree θrT in order to keep the EGR rate R at a target EGR rate RT depending on a driving state. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine.

  An exhaust gas from an internal combustion engine, particularly a diesel engine, contains particulate matter (PM) such as soot. Therefore, a technique is known in which a particulate filter (hereinafter also referred to as a filter) that collects PM in exhaust gas is disposed in an exhaust system of an internal combustion engine. Here, when PM is excessively collected by the filter, the filter is clogged, and this clogging causes an increase in exhaust resistance and a decrease in the output of the internal combustion engine. Therefore, it is necessary to perform a so-called PM regeneration process in which the temperature of the filter is raised and the PM collected by the filter is oxidized and removed.

  Here, as an example of a method for raising the temperature of the filter, a technique for reducing the opening of an intake throttle valve or an exhaust throttle valve provided in an exhaust passage of an internal combustion engine is known. In other words, by reducing the opening of the intake throttle valve, reducing the amount of heat removed from the filter due to exhaust, and increasing the temperature of the filter by raising the exhaust temperature, and reducing the opening of the exhaust throttle valve A technique for increasing the engine load and raising the temperature of the filter by raising the exhaust gas temperature is known (see, for example, Patent Document 1 and Patent Document 2).

  Furthermore, the above-described prior art is provided with an exhaust gas recirculation device (hereinafter also referred to as “EGR device”) for returning a part of the exhaust gas passing through the exhaust passage of the internal combustion engine to the intake system of the internal combustion engine. ing. Normally, the EGR device recirculates EGR gas, which is part of exhaust gas, to the intake system of the internal combustion engine, and an EGR valve that adjusts the flow rate of EGR gas flowing through the EGR passage (hereinafter referred to as “EGR gas amount”). And. Here, Patent Document 1 discloses a technique for independently controlling the intake throttle valve and the EGR valve when the temperature of the filter is raised, and Patent Document 2 discloses EGR when the temperature of the filter is raised. A technique for reducing or reducing the amount of gas is disclosed.

However, if the intake throttle valve and the EGR valve are controlled independently as in the above prior art, the EGR rate [EGR gas amount / (EGR gas amount + intake fresh air amount)] may change. There is a concern that the amount of NOx discharged from the internal combustion engine may increase.
Japanese Patent Laid-Open No. 10-47112 JP 2003-343287 A JP 2001-303928 A

  The present invention has been made in view of the above prior art, and its object is to reduce the opening of an intake throttle valve or an exhaust throttle valve provided in an exhaust passage of an internal combustion engine and pass through an exhaust purification device. The present invention provides a technique capable of suppressing a change in the amount of NOx discharged from an internal combustion engine due to a change in the EGR rate when the flow rate of exhaust gas to be reduced is reduced.

In order to achieve the above object, the present invention changes an EGR passage that circulates a part of the exhaust gas that passes through the exhaust passage to the intake system of the internal combustion engine, and an EGR gas amount that is a flow rate of the exhaust gas flowing through the EGR passage. In an exhaust gas purification system for an internal combustion engine having a possible EGR valve, when the opening degree of the exhaust throttle valve and / or the intake throttle valve is changed, the EGR rate becomes a target EGR rate corresponding to the operating state of the internal combustion engine Thus, the greatest feature is to control the opening of the EGR valve.

More specifically, an exhaust passage that is connected at one end to the internal combustion engine and through which the exhaust from the internal combustion engine passes,
An exhaust gas purification device that is provided in the exhaust passage and purifies exhaust gas flowing in;
An exhaust throttle valve provided in the exhaust passage and capable of changing a flow rate of the exhaust gas and / or an intake throttle valve provided in an intake passage of the internal combustion engine and capable of changing a flow rate of intake air taken into the internal combustion engine;
Exhaust flow rate changing means for changing the flow rate of exhaust gas passing through the exhaust purification device by changing the opening of the exhaust throttle valve and / or the intake throttle valve;
An EGR passage for circulating a part of the exhaust gas passing through the exhaust passage to an intake system of the internal combustion engine;
An EGR valve capable of changing an EGR gas amount, which is a flow rate of exhaust gas flowing in the EGR passage,
EGR opening degree control for controlling the opening degree of the EGR valve so that an EGR rate corresponding to a ratio of EGR gas contained in intake air taken into the internal combustion engine becomes a target EGR rate corresponding to an operating state of the internal combustion engine. Means,
An exhaust purification system for an internal combustion engine comprising:
When the exhaust flow rate changing means changes the opening degree of the exhaust throttle valve and / or the intake throttle valve, the EGR opening degree control means sets the opening degree of the EGR valve so as to maintain the EGR rate at the target EGR rate. It is characterized by controlling.

  In the exhaust gas purification system for an internal combustion engine thus configured, when the temperature of the exhaust gas purification device is raised, the exhaust throttle valve and / or the intake throttle valve opening degree (hereinafter referred to as “exhaust gas” respectively). It is sometimes referred to as “throttle opening” or “intake throttle opening”. This is because the temperature of the exhaust gas purification device can be raised quickly by reducing the amount of heat removed by the exhaust gas passing through the exhaust gas purification device.

  Here, as a temperature increase request for the exhaust gas purification device as described above, when the exhaust gas purification device includes a filter capable of collecting particulate matter (hereinafter also simply referred to as “PM”) in the exhaust gas. A case where the temperature of the exhaust purification device is raised to a temperature at which PM can be oxidized (combusted) and the PM regeneration process is performed can be exemplified.

  Further, the exhaust gas purification device includes a NOx catalyst (for example, a storage reduction type NOx catalyst, a selective reduction type NOx catalyst, etc.) for purifying NOx in the exhaust gas, and when the temperature of the NOx catalyst is low, the exhaust gas purification For example, the temperature of the apparatus is raised to the activation temperature of the NOx catalyst.

  By the way, the exhaust gas purification system according to the present invention is provided with an exhaust gas recirculation device (hereinafter referred to as “EGR device”) composed of the EGR passage and the EGR valve, and exhaust gas passing through the exhaust passage. A part is recirculated as EGR gas to the intake system of the internal combustion engine. In this case, since the EGR gas has a relatively high specific heat and can absorb a large amount of heat, the amount of NOx produced in the internal combustion engine can be reduced by lowering the combustion temperature of the air-fuel mixture in the combustion chamber.

However, when the flow rate of the exhaust gas passing through the exhaust purification device is changed by the exhaust flow rate changing means, a differential pressure is generated before and after the exhaust throttle valve and / or the intake throttle valve. In such a case, even when the operating state of the internal combustion engine (for example, the fuel injection amount, the engine speed, etc.) or the opening of the EGR valve (hereinafter simply referred to as “EGR opening”) is not changed. The EGR rate [EGR rate = EGR gas amount / (EGR gas amount + intake fresh air amount)] may change due to the occurrence of the differential pressure. When the EGR rate changes, the combustion temperature of the air-fuel mixture in the combustion chamber of the internal combustion engine may change. In such a case, the NOx generation amount (NOx emission amount) changes and the emission deteriorates. There was a fear.

  On the other hand, in the present invention, when the exhaust flow rate changing means changes the exhaust throttle opening and / or the intake throttle opening, that is, there is a differential pressure before and after the exhaust throttle valve and / or the intake throttle valve. When generated, the EGR opening degree control means controls the EGR opening degree so as to maintain the EGR rate at a target EGR rate corresponding to the operating state of the internal combustion engine. Here, the target EGR rate is the optimum EGR rate for the operating state of the internal combustion engine, and may be obtained experimentally in advance. Further, the target EGR rate may be determined based on, for example, the fuel injection amount and the engine speed.

  When the EGR opening control controls the EGR opening as described above, the EGR rate is maintained at the target EGR rate even after the exhaust throttle opening and / or the intake throttle opening is changed. The amount of NOx discharged can be kept substantially constant. Therefore, it is possible to suppress the deterioration of the emission due to the increase in the discharge amount of the NOx catalyst.

  Here, the present invention for maintaining the EGR rate at the target EGR rate by the EGR opening degree control means includes, for example, an EGR rate and a fresh intake air amount that is a fresh air amount sucked into the internal combustion engine, A relationship between the exhaust throttle opening and / or the intake throttle opening and the EGR opening is experimentally obtained in advance, and the EGR opening when the EGR rate becomes the target EGR rate is derived based on these relationships. Anyway.

  Further, in the present invention, the amount of change in the EGR rate that changes due to the exhaust flow rate changing means changing the exhaust throttle opening and / or the intake throttle opening is estimated. You may make it change the said EGR opening degree so that it may become substantially zero.

  Here, if the oxygen concentration contained in the intake air taken into the internal combustion engine is different, the amount of NOx produced in the internal combustion engine may be different even when the EGR rate is the same. This is because, for example, when the required load of the internal combustion engine is different, the fuel injection amount is also different, so that the oxygen concentration in the EGR gas may be different. The intake air in the present invention means the sum of fresh air and EGR gas sucked into the combustion chamber of the internal combustion engine.

  The target EGR rate in the present invention for more reliably suppressing the increase in the amount of NOx generated in the internal combustion engine is further determined based on the oxygen concentration of the intake air sucked into the internal combustion engine. Also good. That is, when the exhaust flow rate changing means changes the exhaust throttle opening and / or the intake throttle opening, the target EGR rate is determined based on the fuel injection amount, the engine speed, and the oxygen concentration contained in the intake air. In addition, the EGR opening degree control means may control the EGR opening degree so as to maintain the EGR rate at the target EGR rate. Thereby, even when the exhaust throttle opening degree and the intake throttle opening degree are changed, it is possible to suppress an increase in the amount of NOx generated in the internal combustion engine with higher accuracy.

  Further, it is considered that the oxygen concentration in the intake air (hereinafter referred to as “target oxygen concentration”) optimum for the operating state changes when the operating state of the internal combustion engine (for example, fuel injection amount, engine speed, etc.) changes. Therefore, in the present invention, when the exhaust throttle opening and / or the intake throttle opening is changed, the EGR opening control means should maintain the oxygen concentration of the intake air at the target oxygen concentration according to the operating state. The EGR opening degree may be controlled.

Thereby, even after the exhaust throttle opening and / or the intake throttle opening is changed, it is possible to prevent the oxygen concentration contained in the intake air from deviating from the target oxygen concentration. That is, it becomes possible to maintain the NOx amount discharged from the internal combustion engine substantially constant, and to suppress the deterioration of the emission due to the increase in the NOx emission amount.

  Note that the target oxygen concentration of the intake air according to the operating state of the internal combustion engine may be obtained in advance based on the fuel injection amount, the engine speed, and the like.

  In the present invention, when the exhaust flow rate changing means changes the opening degree of the exhaust throttle valve and / or the intake throttle valve, the opening degree of the EGR valve controlled by the EGR opening degree control means is the intake air amount. Correction may be made based on the oxygen concentration of the gas.

  That is, the oxygen concentration contained in the intake when the EGR rate that changes with the change in the exhaust throttle opening and / or the intake throttle opening is maintained at the target EGR rate according to the operating state is acquired, and the oxygen concentration is You may make it correct | amend the said EGR opening degree when it deviates from the target oxygen concentration optimal for this driving | running state.

  For example, when the oxygen concentration of the intake air is larger than the target oxygen concentration, the EGR opening degree may be corrected to the open side. On the other hand, when the oxygen concentration of the intake air is smaller than the target oxygen concentration, the EGR opening degree may be corrected to the close side. As a result, the EGR opening degree control means can control the EGR opening degree with higher accuracy.

  Further, when the EGR opening is corrected based on the oxygen concentration contained in the intake air as described above, the EGR opening is corrected when the difference between the oxygen concentration of the intake air and the target oxygen concentration is larger than a predetermined threshold value. You may make it do. Thereby, the EGR opening degree control means can rationally control the EGR opening degree. Note that the above threshold value may be obtained experimentally in advance.

  Further, when the operating state of the internal combustion engine in the present invention is a fuel cut state, it is considered that almost no NOx is discharged from the internal combustion engine. In addition, since the exhaust gas having a low temperature is discharged from the internal combustion engine when in the fuel cut state, the amount of heat taken away from the exhaust purification device when the exhaust passes through the exhaust purification device becomes excessive, and the exhaust purification device There is a possibility that the temperature of the liquid drops excessively.

  Therefore, in the present invention, the EGR opening degree control means may increase the opening degree of the EGR valve when the operating state of the internal combustion engine is in a fuel cut state.

  In other words, the EGR rate is increased by increasing the opening of the EGR valve, and the amount of heat that the exhaust purification device is carried away by low-temperature exhaust is reduced, thereby suppressing the temperature of the exhaust purification device from being lowered. it can.

  For example, when the exhaust purification device includes the filter and is performing PM regeneration processing on the filter, the temperature of the filter needs to be maintained at a temperature at which PM can be oxidized. Further, when the exhaust purification device includes the NOx catalyst, it is required to maintain the temperature of the NOx catalyst at an activation temperature or higher in order to maintain the activity of the NOx catalyst. On the other hand, in the present invention, it is possible to suppress the temperature of the exhaust emission control device from decreasing by increasing the opening of the EGR valve at the time of fuel cut when the temperature of the exhaust gas decreases.

In the present invention, the communication portion between the EGR passage and the exhaust passage may be upstream or downstream of the exhaust purification device. That is, a part of the exhaust before passing through the exhaust purification device may be recirculated as an EGR gas to the intake system of the internal combustion engine, or a part of the exhaust after passing through the exhaust purification device may be recirculated to the intake system. It may be recycled.

  For example, in the case of the former configuration, the EGR opening degree control means can increase the EGR opening degree to reduce the flow rate of the exhaust gas passing through the exhaust passage. The amount of leaving can be reduced. Even in the latter configuration, the temperature of the intake air sucked into the internal combustion engine is increased by reducing the amount of intake fresh air having a low temperature and increasing the amount of EGR gas having a temperature higher than that of fresh air. be able to. This makes it possible to keep the purification device warm.

  In the present invention, when the flow rate of the exhaust gas passing through the exhaust purification device is decreased by reducing the opening of the intake throttle valve or the exhaust throttle valve provided in the exhaust passage of the internal combustion engine, the exhaust gas is discharged from the internal combustion engine. It is possible to prevent the emission from deteriorating due to the increase in the amount of NOx.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. It should be noted that the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are intended to limit the technical scope of the invention only to those unless otherwise specified. is not.

  Here, a case where the present invention is applied to a diesel engine for driving a vehicle will be described as an example. FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 according to this embodiment and an intake / exhaust system thereof.

  The internal combustion engine 1 is a diesel engine for driving a vehicle. An intake pipe 2 through which intake air flowing into the internal combustion engine 1 flows is connected to the internal combustion engine 1, and an intake throttle valve 3 that adjusts the flow rate of intake air flowing through the intake pipe 2 is connected to the intake pipe 2. Is provided. An air flow meter 4 that outputs an electrical signal corresponding to the amount of intake air flowing through the intake pipe 2 is disposed upstream of the intake throttle valve 3 in the intake pipe 2.

  Further, an exhaust pipe 5 through which exhaust from the internal combustion engine 1 flows is connected to the internal combustion engine 1, and this exhaust pipe 5 is connected downstream to a muffler (not shown). A filter 10 that collects PM in the exhaust is disposed in the middle of the exhaust pipe 5. Hereinafter, the upstream side of the exhaust pipe 5 with respect to the filter 10 is referred to as a first exhaust pipe 5a, and the downstream side is referred to as a second exhaust pipe 5b. The second exhaust pipe 5b is provided with an exhaust throttle valve 6 for adjusting the flow rate of the exhaust gas flowing through the second exhaust pipe 5b. In the present embodiment, the first exhaust pipe 5a and the second exhaust pipe 5b correspond to the exhaust passage in the present invention. Further, in the present embodiment, the filter 10 corresponds to the exhaust purification device of the present invention. Further, the filter 10 according to the present invention only needs to be able to collect PM in the exhaust gas. For example, the filter 10 may be a particulate filter carrying a NOx catalyst.

  The first exhaust pipe 5a is provided with an electric heater 11 that generates heat when energized. The second exhaust pipe 5b is provided with an exhaust throttle valve 6 for adjusting the flow rate of the exhaust gas flowing through the second exhaust pipe 5b. Furthermore, the first exhaust pipe 5a and the second exhaust pipe 5b are provided with a differential pressure sensor 7 that detects the differential pressure before and after the filter 10 (upstream and downstream).

Further, the internal combustion engine 1 is provided with an exhaust gas recirculation device 20 that recirculates a part of the exhaust gas flowing through the exhaust system of the internal combustion engine 1 to the intake system. The exhaust gas recirculation device 20 includes an EGR passage 21 that communicates a portion of the second exhaust pipe 5b upstream of the exhaust throttle valve 6 and a portion of the intake pipe 2 downstream of the intake throttle valve, and an electromagnetic valve. The EGR valve 22 for adjusting the flow rate of exhaust gas (hereinafter referred to as “EGR gas”) flowing in the EGR passage 21 according to the magnitude of the applied voltage, and the EGR passage 21 upstream of the EGR valve 22 in the EGR passage 21. And an EGR cooler 23 that cools the EGR gas flowing through the.

  In the exhaust gas recirculation device 20 configured in this way, when the EGR valve 22 is opened, a part of the exhaust gas flowing through the second exhaust gas 5b passes through the EGR passage 21 and is cooled by the EGR cooler 42, It flows into the pipe 2. The EGR gas flowing into the intake pipe 2 is distributed to a combustion chamber of a cylinder (not shown) while being mixed with fresh air flowing from the upstream side in the intake pipe 2 and burned.

Here, the EGR gas does not burn itself and contains an endothermic inert gas component such as water (H ₂ O) and carbon dioxide (CO ₂ ). When the EGR gas is contained in the air-fuel mixture in the combustion chamber, the combustion temperature of the air-fuel mixture becomes low, and the amount of nitrogen oxide (NOx) generated is suppressed.

  The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 30 for controlling the internal combustion engine 1 and the intake / exhaust system. The ECU 30 is a unit that controls the EGR valve 22 in addition to controlling the operating state of the internal combustion engine 1 in accordance with the operating conditions of the internal combustion engine 1 and the driver's request.

  The ECU 30 operates the internal combustion engine 1 such as an air flow meter 4 for detecting the amount of fresh air drawn into the internal combustion engine 1, a crank position sensor 8 for detecting the rotational speed, and an accelerator position sensor 9 for detecting the accelerator opening. In addition to sensors related to state control, a differential pressure sensor 7 is connected via electrical wiring, and an output signal thereof is input to the ECU 30. On the other hand, the ECU 30 is connected with a fuel injection valve 12, an intake throttle valve 3, an exhaust throttle valve 6, an EGR valve 22, an electric heater 11 and the like for injecting fuel into the cylinder of the internal combustion engine 1 through electric wiring. The ECU 30 is controlled.

  The ECU 30 includes a CPU, a ROM, a RAM, and the like. The ROM stores a program for performing various controls of the internal combustion engine 1 and a map storing data. The PM regeneration processing routine for the filter 10 is one of the programs stored in the ROM of the ECU 30.

  <PM regeneration process>

  Next, regarding the exhaust purification system of the internal combustion engine 1 in the present embodiment, the control related to the PM regeneration process for oxidizing and removing the PM collected by the filter 10 will be described.

  In this embodiment, control related to PM regeneration processing is performed when a predetermined PM regeneration request is issued. Here, the PM regeneration request may be issued when the PM accumulation amount (hereinafter simply referred to as “PM deposition amount”) Qs collected by the filter 10 becomes equal to or greater than the PM regeneration requirement accumulation amount. good. Specifically, for example, the PM accumulation amount is obtained by obtaining the filter differential pressure ΔPf based on the detection value of the differential pressure sensor 7 that detects the differential pressure before and after the filter 10 (hereinafter referred to as “filter differential pressure”). When the pressure ΔPf is equal to or higher than a predetermined reference differential pressure ΔPfb, it may be determined that the PM accumulation amount has become equal to or greater than the PM regeneration request accumulation amount, and a PM regeneration request may be issued. Alternatively, the PM regeneration request may be issued based on the integrated value of the travel distance from the end of the PM regeneration process performed last time.

Here, the PM regeneration request accumulation amount is the PM accumulation amount when a PM regeneration request is issued to the filter 10 and is experimentally determined in advance. The PM regeneration required accumulation amount is the filter 1
The PM accumulation amount that allows a certain margin to the lower limit PM accumulation amount in which the back pressure of the internal combustion engine 1 excessively increases due to the PM collected at 0 and causes problems such as a decrease in output may be used.

  <Temperature increase control of filter>

  Next, temperature increase control of the filter 10 in the present embodiment will be described. In this embodiment, when a PM regeneration request is issued to the filter 10, the electric heater 11 is energized by a command from the ECU 30. The temperature of the filter 10 (hereinafter referred to as “filter temperature”) is raised to a temperature at which PM can be oxidized (combusted) by the heat generated by the energization, whereby the PM is removed by oxidation.

  In the temperature increase control of the filter 10 in the present embodiment, the opening degree of the intake throttle valve 3 (hereinafter referred to as “intake throttle opening degree”) θin and the opening degree of the exhaust throttle valve 6 (hereinafter referred to as “intake throttle opening degree”) according to the operation state according to a command of the ECU 30 Hereinafter, it is referred to as “exhaust throttle opening”.) Θex is changed to a target intake throttle opening θinT and a target exhaust throttle opening θexT that are predetermined closing-side openings. As a result, the flow rate of exhaust gas flowing into the filter 10 (hereinafter simply referred to as “inflow exhaust gas flow rate”) Ve is reduced, and the amount of heat taken away by the exhaust gas is reduced, so that the filter 10 is efficiently made into the PM oxidation temperature (for example, , 500 ° C. to 700 ° C.).

  Here, the target intake throttle opening θinT and the target exhaust throttle opening θexT are predetermined closing-side openings including full closing, and are experimentally obtained in advance. ΘinT and θexT may be θin and θex that allow the inflow exhaust flow rate Ve to be equal to or less than a predetermined value, or may be determined according to the operating state. Therefore, in this embodiment, the ECU 30 that changes the intake throttle opening θin and the exhaust throttle opening θex to the target intake throttle opening θinT and the target exhaust throttle opening θexT, which are predetermined closing side openings, is the exhaust in the present invention. It corresponds to the flow rate changing means.

  <EGR opening control>

  Next, the control related to the opening degree of the EGR valve 22 (hereinafter simply referred to as “EGR opening degree”) θr in the present embodiment will be described. As described above, in the present embodiment, in the temperature increase control of the filter 10, the intake throttle opening θin and the exhaust throttle opening θex are changed to the target intake throttle opening θinT and the target exhaust throttle opening θexT, respectively. Then, a differential pressure is generated before and after the intake throttle valve 3 and the exhaust throttle valve 6 (upstream side and downstream side). As a result, even if the operating state of the internal combustion engine 1 (for example, the fuel injection amount Qa, the engine speed Ne, etc.) and the opening degree of the EGR valve (hereinafter simply referred to as “EGR opening degree”) θr are not changed. It is considered that the EGR rate [EGR rate = EGR gas amount / (EGR gas amount + intake fresh air amount)] R changes due to the occurrence of the differential pressure. As a result, when the combustion temperature of the air-fuel mixture in the combustion chamber of the internal combustion engine 1 changes, the NOx generation amount (NOx emission amount) in the internal combustion engine 1 changes and the emission may deteriorate.

  Therefore, in this embodiment, the EGR opening θr is controlled so as to cancel the influence on the EGR rate R when the intake throttle opening θin and the exhaust throttle opening θex are changed. Specifically, based on the fuel injection amount Qa and the engine speed Ne in the internal combustion engine 1, an optimal target EGR rate RT in the operating state is experimentally determined in advance, and the EGR rate R is set to the target EGR rate RT. Therefore, the EGR opening degree θr is changed to the target EGR opening degree θrT in accordance with a command from the ECU 30 in order to maintain the target EGR opening degree. Therefore, in this embodiment, the ECU 30 corresponds to the EGR opening degree control means in the present invention.

  <PM regeneration processing routine>

  Here, FIG. 2 is a flowchart showing a PM regeneration processing routine in the present embodiment. This routine is a program stored in the ROM in the ECU 30, and is executed every predetermined period.

  When this routine is executed, first in step S101, it is determined whether a PM regeneration request has been issued to the filter 10. Specifically, the PM regeneration request is issued when the PM accumulation amount of the filter 10 becomes equal to or greater than the PM regeneration request accumulation amount. If it is determined that a PM regeneration request has been issued, the process proceeds to step S102. On the other hand, if it is determined that the PM regeneration request has not been issued, this routine is temporarily terminated.

  In step S102, the engine speed Ne is acquired by the ECU 30. Here, the engine speed Ne is acquired based on the detection value of the crank position sensor 8. Then, when step S102 is completed, the process proceeds to step S103.

  In step S103, the target EGR rate RT is acquired based on the engine speed Ne and the fuel injection amount Qa acquired in step S102. Specifically, for example, the target EGR rate RT may be derived by reading from a map in which the relationship between the engine speed Ne, the fuel injection amount Qa, and the target EGR rate RT is stored. Then, when step S103 is completed, the process proceeds to step S104.

  In step S104, the intake fresh air amount Ga is acquired based on the detection value of the air flow meter 4. The target intake throttle opening degree θinT and the target exhaust throttle opening degree θexT are acquired based on the intake fresh air amount Ga and the engine speed Ne. Specifically, for example, the target intake throttle opening degree θinT and the target intake throttle opening degree θinT are read by reading out from the stored map the relationship between the intake fresh air amount Ga, the engine speed Ne, the target intake throttle opening degree θinT, and the target exhaust throttle opening degree θexT. The exhaust throttle opening degree θexT may be derived. Then, when the process of step S104 ends, the process proceeds to step S105.

  In step S105, the EGR rate R obtained when the intake throttle opening θin and the exhaust throttle opening θex are changed to the target intake throttle opening θinT and the target exhaust throttle opening θexT, respectively, is estimated. Specifically, for example, the EGR is read out from a map in which the relationship between the intake fresh air amount Ga, the engine speed Ne, the intake throttle opening θin, the exhaust throttle opening θex, the EGR opening θr, and the EGR rate R is read. The rate R may be derived. When the process of step S105 ends, the process proceeds to step S106.

  In step S106, it is determined whether or not the absolute value (| R−RT |) of the difference between the EGR rate R and the target EGR rate RT is larger than the reference value ΔRb. Here, the reference value ΔRb means an absolute value of an allowable difference between the EGR rate R and the target EGR rate RT that is allowable from the viewpoint of the amount of NOx generated in the internal combustion engine 1, and is determined experimentally in advance. When it is determined that the absolute value of the difference between the EGR rate R and the target EGR rate RT is equal to or less than the reference value ΔRb, the intake throttle opening θin and the exhaust throttle opening θex are changed to θinT and θexT. In step S109, it is determined that there is no possibility that the amount of NOx generated in the internal combustion engine 1 will increase excessively.

  On the other hand, if it is determined that the absolute value of the difference between the EGR rate R and the target EGR rate RT is greater than the reference value ΔRb, it is determined that the amount of NOx generated in the internal combustion engine 1 may increase excessively. The process proceeds to step S107.

In step S107, the target opening for changing the EGR opening θr so that the EGR rate R is maintained at the target EGR rate RT, that is, the difference between the EGR rate R and the target EGR rate RT is zero. (Hereinafter, simply referred to as “target EGR opening degree”.) ΘrT is acquired. Here, the target EGR opening degree θrT may be calculated based on the difference between the EGR rate R and the target EGR rate RT and the current EGR opening degree θr. Then, when step S107 is completed, the process proceeds to step S108.

  In step S108, the EGR opening degree θr is changed to the target EGR opening degree θrT by a command from the ECU 30. Then, when the process of step S108 ends, the process proceeds to step S109.

  In step S109, the intake throttle opening θin is changed to the target intake throttle opening θinT and the exhaust throttle opening θex is changed to the target exhaust throttle opening θexT in accordance with a command from the ECU 30. Then, when the process of step S109 ends, the process proceeds to step S110.

  In step S110, the electric heater 11 is energized by a command from the ECU 30, and the temperature of the filter 10 is raised to a temperature at which PM can be oxidized, whereby PM is removed by oxidation. Then, when the process of step S110 ends, this routine is once ended.

  As described above, according to this routine, when the PM regeneration process is performed, the intake throttle opening θin and the exhaust throttle opening θex are set to the target intake throttle opening θinT, the target exhaust throttle opening, By changing to θexT, the inflow exhaust flow rate Ve can be reduced, and the temperature of the filter 10 can be efficiently raised to the PM oxidation temperature. Further, even if the differential pressure generated before and after the intake throttle valve 3 and the exhaust throttle valve 6 is generated by changing the intake throttle opening θin and the exhaust throttle opening θex, the EGR rate R may deviate from the target EGR rate RT. Can be suppressed. That is, it is possible to suppress an increase in the combustion temperature of the air-fuel mixture in the combustion chamber of the internal combustion engine 1 and an increase in the amount of NOx produced (NOx emission amount).

  In the processing from step S105 to step S107 in this routine, the EGR rate R resulting from changing the intake throttle opening θin and the exhaust throttle opening θex to the target intake throttle opening θinT and the target exhaust throttle opening θexT. The absolute value of the difference between the EGR rate RT and the target EGR rate RT is estimated, and when the absolute value is larger than the reference value ΔRb, the EGR opening θr is changed so that the difference between the EGR rate R and the target EGR rate RT is zero. Although the control has been described as an example, the present invention is not limited to this.

  For example, the relationship between the target EGR rate RT, the intake fresh air amount Ga, the target intake throttle opening θinT, the target exhaust throttle opening θexT, and the target EGR opening θrT is obtained in advance through experiments or the like, and the relationship is represented in the control map. You may store in ECU30 in the form. Then, the target EGR opening degree θrT is derived by accessing the control map using the target EGR rate RT, the intake fresh air amount Ga, the target intake throttle opening degree θinT, and the target exhaust throttle opening degree θexT as parameters. Also good. In this way, the EGR opening degree θr can be controlled so that the EGR rate R is always maintained at the target EGR rate RT.

In the present embodiment, the electric heater 11 is disposed upstream of the filter 10 in order to raise the temperature of the filter 10, but the present invention is not limited to this. For example, as a conductive filter capable of generating heat by energizing the filter 10, the temperature of the filter 10 may be raised by energizing the filter itself. Further, the electric heater 11 may have a configuration in which a catalyst having an oxidation function is supported (for example, an electrically heated oxidation catalyst). Further, the temperature of the filter 10 may be raised by a combustion burner or the like instead of the electric heater 11. Alternatively, the temperature of the filter 10 may be raised by increasing the fuel injection amount Qa injected from the fuel injection valve 12 and increasing the temperature of the exhaust gas flowing into the filter 10.

  Further, in the present embodiment, the EGR opening degree control at the time of performing the PM regeneration process on the filter 10 has been described, but the present invention can be applied regardless of whether the PM regeneration process is being performed. That is, the EGR rate R can be maintained at the target EGR rate RT by performing the above-described EGR opening degree control when the intake throttle opening degree θin and the exhaust throttle opening degree θex are changed. For example, when the filter 10 carries a NOx catalyst, or when the exhaust system is equipped with an oxidation catalyst or the like, the above catalyst temperature is maintained above the activation temperature or becomes lower than the activation temperature. You may apply when control which heats up a catalyst is made | formed. Further, the present invention can also be applied when performing NOx reduction processing or SOx poisoning recovery processing on the NOx catalyst.

  Next, an embodiment different from the first embodiment of the exhaust gas purification system for the internal combustion engine 1 according to the present invention will be described. FIG. 3 is a diagram showing a schematic configuration of the internal combustion engine 1 and its intake and exhaust system in the present embodiment. Here, the same or equivalent components as those in the exhaust purification system of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  The present embodiment differs from the exhaust purification system according to Embodiment 1 in the following points. That is, the internal combustion engine 1 in this embodiment is provided with an oxygen concentration sensor 13 for detecting the oxygen concentration on the upstream side of the intake throttle valve in the intake pipe 2. The oxygen concentration sensor 13 is connected to the ECU 30 via electrical wiring, and an output signal of the oxygen concentration sensor 13 is input to the ECU 30.

  Further, the exhaust throttle valve 6 in this embodiment is provided in the first exhaust pipe 5a. Further, the EGR passage 21 communicates a portion of the first exhaust pipe 5a on the upstream side with respect to the exhaust throttle valve 6 and a portion of the intake pipe 2 on the downstream side of the intake throttle valve.

  In the present embodiment, when the PM regeneration process is performed on the filter 10, when the intake throttle opening θin and the exhaust throttle opening θex are changed to the target intake throttle opening θinT and the target exhaust throttle opening θexT, respectively. The EGR opening degree θr is controlled so that the oxygen concentration (hereinafter referred to as “intake oxygen concentration”) Odm of the intake air sucked into the internal combustion engine 1 is maintained at the target oxygen concentration OdmT corresponding to the operating state of the internal combustion engine 1. It was decided. This is because even if the EGR rate R is equal, the amount of NOx produced may change if the oxygen concentration Od of the intake air differs. Below, the control which concerns on a present Example for not making the amount of NOx produced | generated in the internal combustion engine 1 increase more reliably is demonstrated.

  <Second PM regeneration processing routine>

  FIG. 4 is a flowchart showing a second PM regeneration processing routine in the present embodiment. This routine is also a program stored in the ROM in the ECU 30 and is executed at predetermined intervals while the internal combustion engine 1 is in operation. In this routine, detailed description of the same terms, symbols, and the like as those of the above-described PM regeneration processing routine is omitted.

  Here, the processing of steps 101 and 102 in this routine corresponds to each processing of the PM regeneration processing routine described above, and detailed description thereof is omitted. Then, when the process of step 102 is completed, the process proceeds to step 203.

In step S203, the target oxygen concentration OdmT is acquired based on the engine speed Ne and the fuel injection amount Qa. Specifically, for example, the target oxygen concentration OdmT may be derived by reading from a map in which the relationship between the engine speed Ne, the fuel injection amount Qa, and the target oxygen concentration OdmT is stored. Then, when the process of step S203 ends, the process proceeds to step S104. Here, step S104 is the same as the process in the same step of the PM regeneration processing routine, and a description thereof will be omitted. Then, when the process of step 104 is completed, the process proceeds to step 205.

  In step S205, the oxygen concentration (hereinafter referred to as “fresh air oxygen concentration”) Odn contained in the fresh air sucked into the internal combustion engine 1 and the oxygen concentration contained in EGR gas (hereinafter referred to as “EGR oxygen concentration”). .) Odr is obtained. Specifically, for example, the fresh air oxygen concentration Odn may be estimated based on the detection value of the oxygen concentration sensor 13. Further, the EGR oxygen concentration Odr may be estimated based on the current fuel injection amount Qa. Then, when the process of step 205 is completed, the process proceeds to step 206.

  In step 206, the intake oxygen concentration Odm is acquired when the intake throttle opening θin and the exhaust throttle opening θex are changed to the target intake throttle opening θinT and the target exhaust throttle opening θexT, respectively. Specifically, for example, the changed EGR rate R is estimated from the intake fresh air amount Ga, the engine speed Ne, the intake throttle opening θin, the exhaust throttle opening θex, and the EGR opening θr. The intake oxygen concentration Odm may be estimated based on the air oxygen concentration Odn and the EGR oxygen concentration Odr. Then, when the process of step 206 is completed, the process proceeds to step 207.

  In step S207, the target EGR opening degree θrT for obtaining the intake oxygen concentration Odm as the target oxygen concentration OdmT acquired in step S203 is acquired. Specifically, the relationship among the target oxygen concentration OdmT, the intake oxygen concentration Odm, the intake fresh air amount Ga, the target intake throttle opening θinT, the target exhaust throttle opening θexT, and the target EGR opening θrT is obtained in advance through experiments or the like. Alternatively, the relationship may be stored in the ECU 30 in the form of a control map. Then, the target oxygen concentration OdmT, the intake oxygen concentration Odm, the intake fresh air amount Ga, the target intake throttle opening θinT, and the target exhaust throttle opening θexT are accessed as parameters to the control map, thereby setting the target EGR opening θrT. It may be derived. Then, when the process of step 207 is completed, the process proceeds to step 108. Here, the processing after step S108 is the same as the corresponding step in the PM regeneration routine described above, and a description thereof will be omitted.

  As described above, according to this routine, the EGR opening degree θr can be controlled to maintain the intake oxygen concentration Odm at the target oxygen concentration OdmT even when the intake throttle opening degree θin and the exhaust throttle opening degree θex are changed. it can. As a result, it is possible to suppress an increase in the amount of NOx generated (NOx emission amount) in the internal combustion engine 1.

  In step S205 in this routine, the fresh oxygen concentration Odn is estimated based on the detection value of the oxygen concentration sensor 13, but the present invention is not limited to this. For example, the oxygen concentration contained in the fresh air is considered to hardly change, and may be set to a predetermined value in advance. Then, since the new oxygen concentration Odn can be acquired without providing the oxygen concentration sensor 13, it is advantageous in terms of cost.

  In steps S205 to S206 in this routine, the intake oxygen concentration Odm is acquired based on the fresh air oxygen concentration Odn, the EGR oxygen concentration Odr, etc., but the intake oxygen concentration Odm may be directly acquired. . Specifically, for example, if the oxygen concentration sensor 13 is provided downstream of the communication portion between the intake pipe 2 and the EGR passage 21 instead of being provided upstream of the intake throttle valve in the intake pipe 2, the detection by the oxygen concentration sensor 13 is performed. It is possible to acquire the intake oxygen concentration Odm directly based on the value. Thus, by simplifying the control content, the control according to this routine can be executed in a shorter time.

  In this routine, the control for always maintaining the intake oxygen concentration Odm at the target oxygen concentration OdmT has been described as an example. However, the present invention is not limited to this. For example, the following control may be executed.

  That is, it is determined whether or not the absolute value (| Odm−OdmT |) of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT is larger than the reference value ΔOdb, and when it is larger than the reference value ΔOdb, the NOx of the internal combustion engine 1 is determined. It may be determined that the generation amount may increase excessively. That is, when it is determined as such, control may be performed to change the EGR opening θr to the target EGR opening θrT in order to maintain the intake oxygen concentration Odm at the target oxygen concentration OdmT. The above reference value ΔOdb means the absolute value of the allowable difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT that is allowable from the viewpoint of the amount of NOx generated in the internal combustion engine 1, and is experimentally determined in advance. .

  <Third PM regeneration processing routine>

  Next, control different from the above-described second PM regeneration processing routine will be described with reference to FIG. FIG. 5 is a flowchart showing a third PM regeneration processing routine in this embodiment. This routine is also a program stored in the ROM in the ECU 30 and is executed at predetermined intervals while the internal combustion engine 1 is in operation. In this routine, detailed description of the same terms, symbols, and the like as those of the above-described PM regeneration processing routine is omitted.

  Here, the processing of steps 101 to 107 in this routine corresponds to each processing of the above-described PM regeneration processing routine, and detailed description thereof is omitted. Then, when the process of step 107 is completed, the process proceeds to step 308.

  In step 308, the target oxygen concentration OdmT is acquired based on the engine speed Ne and the fuel injection amount Qa. In the subsequent step S309, the fresh oxygen concentration Odn and the EGR oxygen concentration Odr are acquired. Here, steps S308 and S309 correspond to steps S203 and S205 in the second PM regeneration routine, and detailed description thereof will be omitted. Then, when step 309 is completed, the process proceeds to step 310.

  In step 310, the intake oxygen concentration Odm is estimated based on the EGR rate R, the fresh air oxygen concentration Odn, and the EGR oxygen concentration Odr estimated in step S105. Then, when the process of step 310 is completed, the process proceeds to step 311.

  In step 311, it is determined whether or not the absolute value (| Odm−OdmT |) of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT is larger than the reference value ΔOdb. Here, the reference value ΔOdb means an absolute value of an allowable difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT that is allowable from the viewpoint of the amount of NOx generated in the internal combustion engine 1, and is determined experimentally in advance.

  When it is determined that the absolute value of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT is equal to or less than the reference value ΔOdb, it is not necessary to correct the target EGR opening θrT acquired in step S107. The determination is made and the process proceeds to step S314. On the other hand, when it is determined that the absolute value is larger than the reference value ΔOdb, it is determined that the target EGR opening degree θrT needs to be corrected, and the process proceeds to step S312.

In step 312, the correction coefficient K is acquired based on the difference (Odm−OdmT) between the intake oxygen concentration Odm and the target oxygen concentration OdmT. The correction coefficient K is a coefficient for correcting the above-described target EGR opening degree θrT. When the intake throttle opening degree θin and the exhaust throttle opening degree θex are changed to the target intake throttle opening degree θinT and the target exhaust throttle opening degree θexT, respectively. The correction coefficient is determined by paying attention to the obtained intake oxygen concentration Odm. Specifically, for example, the correction coefficient K may be derived by reading from a map in which the relationship between the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT and the correction coefficient K is stored.

  The correction coefficient K in the present embodiment is determined so that the target EGR opening degree θrT is increased according to the magnitude of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT (Odm> OdmT). On the other hand, when the intake oxygen concentration Odm is smaller than the target oxygen concentration OdmT (Odm <OdmT), the target EGR opening degree θrT is determined to be small according to the difference between the two. Then, when the process of step 312 ends, the process proceeds to step 313.

  In step 313, the corrected EGR opening degree θrTR is acquired based on the target EGR opening degree θrT and the correction coefficient K. Specifically, the corrected EGR opening degree θrTR may be calculated by multiplying the target EGR opening degree θrT by a correction coefficient K. Then, when the process of step 313 is completed, the process proceeds to step 314.

  In step 314, the EGR opening degree θr is changed to the corrected EGR opening degree θrTR in accordance with a command from the ECU 30. In the above-described process of step S311, when it is determined that the absolute value of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT is equal to or less than the reference value ΔOdb, it is necessary to correct the target EGR opening θrT. Therefore, in this step, the EGR opening degree θr is changed to the target EGR opening degree θrT. Then, when the process of step 314 is completed, the process proceeds to step 109. Here, the processing after step S109 is the same as the processing in the corresponding step in the PM regeneration routine described above, and the description thereof is omitted.

  As described above, in this routine, the optimum target EGR opening degree θrT for maintaining the EGR rate R at the target EGR rate RT determined according to the operating state of the internal combustion engine 1 is set to the intake oxygen concentration Odm and the target oxygen concentration OdmT. The EGR opening degree θr can be more suitably controlled by correcting with a correction coefficient K that is determined according to the difference between the EGR opening degree θr and the EGR opening degree θr. As a result, by changing the intake throttle opening θin and the exhaust throttle opening θex, the NOx emission amount in the internal combustion engine 1 increases even if the differential pressure generated before and after the intake throttle valve 3 and the exhaust throttle valve 6 occurs. It is possible to reliably suppress this.

  In this routine, whether or not the target EGR opening degree θrT is corrected is determined according to the magnitude of the absolute value of the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT. However, the present invention is not limited to this. It is not something. For example, the process of step S312 may always be executed. That is, the control for obtaining the correction coefficient K such that the difference between the intake oxygen concentration Odm and the target oxygen concentration OdmT is as small as possible and correcting the target EGR opening θrT by the correction coefficient K may be executed.

  Next, in the PM regeneration process for the filter 10 in the present embodiment, the control that is performed when the internal combustion engine 1 is fuel cut will be described. Since almost no NOx is discharged during the fuel cut of the internal combustion engine 1, in this embodiment, by increasing the EGR opening θr, the inflowing exhaust gas flow Ve to the filter 10 is decreased, and the temperature of the filter 10 is lowered by the low temperature exhaust gas. This may be suppressed.

Here, the contents of control when the above control is applied to the third PM regeneration routine will be described. First, when it is determined in step S101 of the third PM regeneration routine that a PM regeneration request has been issued, it is determined whether or not a fuel cut state has occurred. Here, when it is determined that the fuel cut state is not established, the processing after step S102 described above is executed.

  On the other hand, when it is determined that the fuel cut state is established, the target intake throttle opening θinT and the target exhaust throttle opening θexT are acquired based on the current intake fresh air amount Ga and the engine speed Ne. Here, θinT and θexT are opening amounts on the closing side of the current intake throttle opening θin and exhaust throttle opening θex, and are experimentally obtained in advance so that the inflow exhaust gas flow rate Ve to the filter 10 is sufficiently reduced. The target opening. The ECU 30 changes the intake throttle opening θin and the exhaust throttle opening θex to θinT and θexT, and changes the EGR opening θr to fully open. Thereafter, this routine may be temporarily terminated.

  Here, since the EGR passage 21 in the present embodiment communicates a portion of the first exhaust pipe 5a upstream of the exhaust throttle valve 6 and a portion of the intake pipe 2 downstream of the intake throttle valve, As described above, the flow rate of the exhaust gas passing through the filter 10 can be reduced by controlling each opening degree. That is, it is possible to suppress the temperature of the filter 10 from being lowered by reducing the amount of heat removed by the low temperature exhaust.

  The above-described EGR opening degree control at the time of fuel cut can also be applied to an exhaust purification system having a configuration in which the EGR passage 21 communicates the second exhaust pipe 5b and the intake pipe 2. For example, the present invention can be applied to the exhaust gas purification system according to the configuration of the first embodiment (FIG. 1). Then, by increasing the EGR opening θr by this control, the intake fresh air amount Ga having a low temperature is reduced, and the EGR gas amount having a higher temperature than fresh air is increased, thereby being sucked into the internal combustion engine 1. The temperature of the intake air can be raised. Thereby, the filter 10 can be kept warm.

1 is a diagram illustrating a schematic configuration of an internal combustion engine according to a first embodiment and an intake / exhaust system and a control system thereof. 3 is a flowchart showing a PM regeneration processing routine in Embodiment 1. It is a figure which shows schematic structure of the internal combustion engine which concerns on Example 2, its intake / exhaust system, and a control system. 6 is a flowchart showing a second PM regeneration processing routine in Embodiment 2. 10 is a flowchart showing a third PM regeneration processing routine in Embodiment 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Intake pipe 3 ... Intake throttle valve 4 ... Air flow meter 5 ... Exhaust pipe 5a ... First exhaust pipe 5b ... Second exhaust pipe 6 ... Exhaust Throttle valve 7 ... Differential pressure sensor 8 ... Crank position sensor 9 ... Accelerator position sensor 10 ... Filter 11 ... Electric heater 12 ... Fuel injection valve 13 ... Oxygen concentration sensor 20 ... Exhaust gas recirculation Device 21 ·· EGR passage 22 · · EGR valve 23 · · EGR cooler 30 · · ECU

Claims (4)

An exhaust passage, one end of which is connected to the internal combustion engine and through which the exhaust from the internal combustion engine passes;
An exhaust gas purification device that is provided in the exhaust passage and purifies exhaust gas flowing in;
An exhaust throttle valve provided in the exhaust passage and capable of changing a flow rate of the exhaust gas and / or an intake throttle valve provided in an intake passage of the internal combustion engine and capable of changing a flow rate of intake air taken into the internal combustion engine;
Exhaust flow rate changing means for changing the flow rate of exhaust gas passing through the exhaust purification device by changing the opening of the exhaust throttle valve and / or the intake throttle valve;
An EGR passage for circulating a part of the exhaust gas passing through the exhaust passage to an intake system of the internal combustion engine;
An EGR valve capable of changing an EGR gas amount, which is a flow rate of exhaust gas flowing in the EGR passage,
EGR opening degree control for controlling the opening degree of the EGR valve so that an EGR rate corresponding to a ratio of EGR gas contained in intake air taken into the internal combustion engine becomes a target EGR rate corresponding to an operating state of the internal combustion engine. Means,
An exhaust purification system for an internal combustion engine comprising:
When the exhaust flow rate changing means changes the opening degree of the exhaust throttle valve and / or the intake throttle valve, the EGR opening degree control means sets the opening degree of the EGR valve so as to maintain the EGR rate at the target EGR rate. An exhaust gas purification system for an internal combustion engine characterized by controlling the engine.   The exhaust purification system for an internal combustion engine according to claim 1, wherein the target EGR rate is further determined based on an oxygen concentration of intake air taken into the internal combustion engine.   When the exhaust flow rate changing means changes the opening degree of the exhaust throttle valve and / or the intake throttle valve, the opening degree of the EGR valve controlled by the EGR opening degree control means is based on the oxygen concentration of the intake air. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the exhaust gas purification system is corrected.   The exhaust of the internal combustion engine according to any one of claims 1 to 3, wherein the EGR opening degree control means increases the opening degree of the EGR valve when the operating state of the internal combustion engine is a fuel cut state. Purification system.

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