JP2008128115A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide technology enabling an NOx catalyst to quickly rise in temperature while reducing NOx formation quantity in an internal combustion engine when temperature of the NOx catalyst provided in an exhaust passage is low in the exhaust emission control system for the internal combustion engine provided with an EGR device re-circulating part of exhaust gas to an intake system of the internal combustion engine. <P>SOLUTION: A change position of a flow-in path change valve 54 is changed to a second route position to establish communication of a third EGR passage 51 and a fifth EGR passage 53, and shut off communication between the third EGR passage 51 and a fourth EGR passage 52 when the temperature Tn of the NOx catalyst is lower than an activation temperature Tn0. The change position of the flow-in path change valve 54 is changed to a first route position to establish communication of the third EGR passage 51 and the fourth EGR passage 52, and shut off communication between the third EGR passage 51 and the fifth EGR passage 53 when the temperature Tn of the NOx catalyst is equal to the activation temperature Tn0 or higher. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine equipped with an exhaust gas recirculation device.

  As a technique for reducing the amount of nitrogen oxide (hereinafter referred to as “NOx”) discharged from the internal combustion engine into the atmosphere, an exhaust gas recirculation device (hereinafter referred to as “EGR device”), a NOx catalyst, or a NOx catalyst. There is known an exhaust gas purification system provided with an exhaust gas purification device such as a particulate filter (hereinafter referred to as “filter”).

  The NOx catalyst purifies exhaust gas by storing NOx exhausted from the internal combustion engine into the NOx catalyst from the exhaust gas.

  The EGR device recirculates part of the exhaust gas to the intake system of the internal combustion engine, and lowers the combustion temperature of the air-fuel mixture in the combustion chamber, thereby reducing the amount of NOx generated in the internal combustion engine (that is, the amount of NOx emitted). Is.

  In recent years, in an exhaust purification system that includes a filter and a NOx catalyst provided downstream of the filter as an exhaust purification device, EGR that communicates a portion of the exhaust passage between the filter and the NOx catalyst and an intake passage of the internal combustion engine. There has been proposed an exhaust purification system having an EGR device that has a passage and recirculates part of the exhaust gas to the internal combustion engine via the EGR passage (see, for example, Patent Document 1). In the exhaust purification system having the above configuration, the EGR passage can be shortened rather than recirculating the exhaust gas in the exhaust passage downstream of the NOx catalyst to the intake system, and the exhaust purification system is made compact.

In the exhaust gas purification system provided with the EGR device as described above, the NOx purification rate of the NOx catalyst decreases when the temperature of the NOx catalyst is low. In some cases, the emission amount of NOx may be reduced by simply performing “EGR”.
JP 2001-115826 A

  However, in the exhaust purification system having the above configuration, it is considered that when EGR is performed, the flow rate of the exhaust gas passing through the NOx catalyst is reduced. In that case, since the heat energy of the exhaust gas is not efficiently transmitted to the NOx catalyst, it may be difficult to quickly raise the temperature of the NOx catalyst to the activation temperature, for example. On the other hand, when the temperature of the NOx catalyst is raised quickly without performing EGR, it may be difficult to reduce the amount of NOx generated in the internal combustion engine by EGR over that period.

  The present invention has been made in view of the above prior art, and an object of the present invention is an exhaust gas purification system for an internal combustion engine including an EGR device that recirculates a part of exhaust gas to an intake system of the internal combustion engine. It is an object of the present invention to provide a technique capable of quickly raising the temperature of a NOx catalyst while reducing the amount of NOx produced in an internal combustion engine when the temperature of the NOx catalyst provided in the exhaust passage is low.

In order to achieve the above object, the present invention guides a part of exhaust gas from the upstream side of the NOx catalyst provided in the exhaust passage to the intake system and / or from the downstream side of the NOx catalyst to the intake system. An EGR device that recirculates exhaust gas through the second path, and a second path for the sum of the flow rates of exhaust gas that is guided to the intake system by the first path and the second path when the temperature of the NOx catalyst is lower Thus, the greatest feature is that the ratio of the flow rate of the exhaust gas guided to the intake system is made equal to or higher than the ratio when the temperature of the NOx catalyst is higher.

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,
A NOx catalyst provided in the exhaust passage and purifying NOx contained in the exhaust;
A first path for leading a part of the exhaust gas from the upstream side of the NOx catalyst in the exhaust passage to the intake system of the internal combustion engine and / or a part of the exhaust gas downstream of the NOx catalyst in the exhaust passage An EGR device that recirculates part of the exhaust gas to the intake system through a second path leading from the engine to the intake system of the internal combustion engine;
EGR gas that changes the first EGR gas amount that is the flow rate of the exhaust gas guided to the intake system by the first path and / or the second EGR gas amount that is the flow rate of the exhaust gas guided to the intake system by the second path A quantity changing means;
With
The EGR gas amount changing means determines the ratio of the second EGR gas amount to the total EGR gas amount which is the sum of the first EGR gas amount and the second EGR gas amount when the temperature of the NOx catalyst is lower. It is characterized in that the ratio is not less than the above ratio when the temperature of is higher.

  The EGR device according to the present invention has a first path for leading a part of the exhaust gas from the upstream side of the NOx catalyst in the exhaust passage to the intake system of the internal combustion engine and a part of the exhaust gas from the downstream side of the NOx catalyst in the exhaust path. Exhaust gas is recirculated through at least one of the second path leading to the intake system of the engine. The exhaust gas recirculated to the intake system of the internal combustion engine is hereinafter referred to as “EGR gas”. The EGR gas guided by the first path is referred to as a first EGR gas, and the EGR gas guided by the second path is referred to as a second EGR gas.

  Here, the first path and the second path are different in that the exhaust gas before the EGR gas passes through the NOx catalyst is guided to the intake system or the exhaust gas after passing through the NOx catalyst is guided. In the present invention, the EGR gas amount changing means is configured so that the ratio of the second EGR gas amount to the total EGR gas amount when the NOx catalyst temperature is lower is equal to or higher than the ratio when the NOx catalyst temperature is higher. The 1EGR gas amount and / or the second EGR gas amount is changed.

  Thereby, when the temperature of the NOx catalyst is low, the flow rate of the exhaust gas passing through the NOx catalyst can be increased. That is, when raising the temperature of the NOx catalyst as described above, it is possible to quickly raise the temperature of the NOx catalyst by suitably flowing the heat energy of the exhaust gas into the NOx catalyst.

  In the present invention, the ratio of the second EGR gas amount to the total EGR gas amount is increased as the temperature of the NOx catalyst is lower, and the ratio of the second EGR gas amount to the total EGR gas amount is decreased as the temperature of the NOx catalyst is higher. Anyway. Alternatively, as the temperature of the NOx catalyst increases, the ratio of the second EGR gas amount to the total EGR gas amount may be decreased stepwise.

Further, the EGR gas amount changing means in the present invention is configured such that a ratio of the second EGR gas amount to the total EGR gas amount when the temperature of the NOx catalyst is lower than a predetermined temperature, the temperature of the NOx catalyst is equal to or higher than the predetermined temperature. It may be larger than the ratio of the second EGR gas amount to the total EGR gas amount at the time. As a result, the temperature of the NOx catalyst can be quickly raised to a predetermined temperature.

  The “predetermined temperature” means a target temperature required for the NOx catalyst. For example, the predetermined temperature in the present invention may be a temperature required for purifying NOx in the exhaust gas flowing into the NOx catalyst suitably with respect to the NOx catalyst, and a predetermined margin in the activation temperature of the NOx catalyst. It is good also as the temperature which anticipated.

  Here, the first path for guiding the exhaust gas from the upstream side of the NOx catalyst to the intake system is shorter than the second path for guiding the exhaust gas from the downstream side of the NOx catalyst to the intake system. Then, after the temperature of the NOx catalyst rises to the predetermined temperature, the first EGR gas ratio is relatively small so that the ratio of the second EGR gas amount to the total EGR gas amount is relatively smaller than when the NOx catalyst temperature is lower than the predetermined temperature. The gas amount and / or the second EGR gas amount is changed.

  Thus, more EGR gas can be recirculated to the internal combustion engine by the first path having a short path through which the EGR gas is led to the intake system. As a result, for example, it is possible to improve control responsiveness when changing the EGR gas amount according to the operating state of the internal combustion engine.

  Further, the exhaust gas after passing through the NOx catalyst may have a lower specific heat than the exhaust gas before passing through the NOx catalyst due to a change in composition. On the other hand, in order to lower the combustion temperature of the air-fuel mixture in the combustion chamber with the EGR gas, it is desirable that the specific heat of the EGR gas is high.

  In contrast, the first path can guide part of the exhaust gas from the upstream side of the NOx catalyst in the exhaust passage to the intake system of the internal combustion engine. Therefore, after the temperature of the NOx catalyst becomes equal to or higher than a predetermined value, the ratio of the second EGR gas amount to the total EGR gas amount is reduced, and the first EGR gas amount is preferably made relatively larger than the second EGR gas amount. The amount of NOx generated in the internal combustion engine can be reduced.

  Here, when the temperature of the NOx catalyst is lower than the predetermined temperature, it is more preferable to increase the ratio of the second EGR gas amount to the total EGR gas amount as much as possible so that the NOx catalyst passes through more exhaust gas.

  Therefore, in the present invention, the EGR gas amount changing means may set the ratio of the second EGR gas amount to the total EGR gas amount to approximately 1 when the temperature of the NOx catalyst is lower than the predetermined temperature. That is, the exhaust gas recirculated to the internal combustion engine may be guided to the intake system only by the second path. Thereby, the temperature of the NOx catalyst can be raised to a predetermined temperature more quickly.

  On the other hand, when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature, the ratio of the second EGR gas amount to the total EGR gas amount is made as small as possible, and more exhaust gas is taken into the intake air of the internal combustion engine by the first route having a short route. It is more preferable to introduce it to the system. Therefore, in the present invention, the EGR gas amount changing means may set the ratio of the second EGR gas amount to the total EGR gas amount to be substantially zero when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature. That is, the exhaust gas recirculated to the internal combustion engine may be guided to the intake system only by the first path. As a result, it is possible to improve the responsiveness when changing the EGR gas amount and more efficiently reduce the amount of NOx generated in the internal combustion engine.

By the way, for example, when the internal combustion engine is started at a low temperature, the temperature of the NOx catalyst becomes lower than the activation temperature of the NOx catalyst, and the NOx purification rate in the NOx catalyst may be significantly reduced.

  Therefore, the predetermined temperature in the present invention may be the activation temperature of the NOx catalyst. That is, the EGR gas amount changing means in the present invention provides the ratio of the second EGR gas amount to the total EGR gas amount when the temperature of the NOx catalyst is lower than the activation temperature, and the total amount when the temperature of the NOx catalyst is equal to or higher than the activation temperature. The ratio may be larger than the ratio of the second EGR gas amount to the EGR gas amount.

  The EGR gas amount changing means sets the ratio of the second EGR gas amount to the total EGR gas amount to approximately 1 when the temperature of the NOx catalyst is lower than the activation temperature, and when the temperature of the NOx catalyst is equal to or higher than the activation temperature. In addition, the ratio of the total EGR gas second EGR gas amount may be substantially zero.

  Then, when the temperature of the NOx catalyst is lower than the activation temperature, the temperature of the NOx catalyst can be quickly raised to the activation temperature, and the period during which the NOx purification rate in the NOx catalyst is significantly reduced can be shortened. And before the temperature of the NOx catalyst reaches the activation temperature, a part of the exhaust is led to the intake system by at least the second path, so that the combustion temperature of the air-fuel mixture in the combustion chamber of the internal combustion engine can be lowered. . Therefore, even when the temperature of the NOx catalyst is lower than the activation temperature, it is possible to reduce the amount of NOx produced and suppress the emission from being excessively deteriorated.

  In the present invention, a temperature raising means for raising the temperature of the NOx catalyst by raising the exhaust temperature of the internal combustion engine may be further provided. Thereby, the temperature of the NOx catalyst can be raised to a predetermined temperature (for example, the activation temperature) more rapidly.

  The exhaust purification system for an internal combustion engine according to the present invention may further include a filter that is provided in the exhaust passage and collects particulate matter (hereinafter also simply referred to as “PM”) in the exhaust. The first route and the second route may guide a part of the exhaust gas after passing through the filter to the intake system of the internal combustion engine.

  Thereby, it can suppress that PM is suck | inhaled by an internal combustion engine. For example, when the exhaust purification system includes a turbocharger and an intercooler for cooling the intake air supercharged by the turbocharger, the inflowed PM accumulates in the turbocharger, the intercooler, and the like. This can be suppressed.

  In the present invention, the exhaust passage further includes a reducing agent adding means that is provided upstream of the NOx catalyst and adds a reducing agent to the exhaust gas in the exhaust passage, and the temperature of the NOx catalyst is the predetermined value. When the temperature is higher than the temperature, the reducing agent adding means may add a reducing agent.

  Here, the predetermined temperature may be set to a different temperature depending on the purpose of adding the fuel to the reducing agent adding means. For example, a NOx reduction process may be performed in which a reducing agent (for example, fuel) is allowed to flow into the NOx catalyst and NOx is reduced and purified in the NOx catalyst. In such a case, the predetermined temperature is preferably the activation temperature of the NOx catalyst. By doing so, since the reducing agent is added to the exhaust gas after the temperature of the NOx catalyst has surely exceeded the activation temperature, there is no possibility that the reducing agent added to the exhaust gas will slip through the NOx catalyst. Accordingly, it is possible to suppress the reducing agent from being released into the atmosphere without being purified. And it can suppress that the added reducing agent is wasted.

Further, when the NOx catalyst is poisoned by SOx contained in the exhaust gas, the temperature of the NOx catalyst is maintained at a high temperature (for example, 600 ° C. to 650 ° C.), and the SOx poisoning for supplying the reducing agent to the NOx catalyst. Recovery processing may be performed. In such a case, it is preferable that the predetermined temperature is a temperature at which SOx can be released from the NOx catalyst (for example, 600 ° C. to 650 ° C.). This is because the temperature of the NOx catalyst can be quickly raised to the temperature at which SOx is released.

  In the present invention, the first path may lead a part of the exhaust before the reducing agent is added by the reducing agent addition means to the intake system of the internal combustion engine. Thereby, it is possible to suppress the reducing agent added from the reducing agent adding means from being led to the intake system of the internal combustion engine through the second path, and the combustion of the air-fuel mixture in the combustion chamber from becoming unstable. Moreover, it can suppress that drivability deteriorates.

  This is because the reducing agent is added from the reducing agent addition means after the first EGR gas and / or the second EGR gas amount is changed so that the ratio of the second EGR gas amount to the total EGR gas amount becomes small. Because. Further, in the above, it is preferable that the EGR gas amount changing means changes the first EGR gas and / or the second EGR gas amount so that the ratio of the second EGR gas amount to the total EGR gas amount becomes substantially zero. This is because the reducing agent can be more reliably prevented from entering the intake system.

Moreover, the following structures can be illustrated as an exhaust gas purification system for an internal combustion engine according to the present invention. That is,
The EGR device
A first EGR passage that communicates a portion of the exhaust passage upstream of the NOx catalyst and the intake passage of the internal combustion engine;
A first EGR valve provided in the first EGR passage and capable of changing a flow rate of exhaust gas flowing through the first EGR passage;
A second EGR passage communicating the portion of the exhaust passage downstream of the NOx catalyst and the intake passage of the internal combustion engine;
And a second EGR valve provided in the second EGR passage and capable of changing a flow rate of exhaust gas flowing through the second EGR passage.

  Here, the configuration related to the exhaust purification system is referred to as a first configuration. In the exhaust purification system according to the first configuration, when the exhaust is guided to the intake system of the internal combustion engine by the first path, the first EGR passage is circulated through the exhaust and the exhaust is discharged by the second path. When leading to the intake system of the internal combustion engine, the second EGR passage may be circulated through the exhaust.

  The EGR gas amount changing means changes the opening degree of the first EGR valve and / or the opening degree of the second EGR valve when changing the ratio of the second EGR gas amount to the total EGR gas amount. Also good.

Further, the following configuration can be exemplified as the exhaust gas purification system for an internal combustion engine according to the present invention. That is,
The EGR device
A third EGR passage having one end connected to the intake passage of the internal combustion engine;
A fourth EGR passage having one end connected to a portion upstream of the NOx catalyst in the exhaust passage and the other end connected to the other end of the third EGR passage;
A fifth EGR passage having one end connected to a portion of the exhaust passage on the downstream side of the NOx catalyst and the other end connected to a connection portion between the third EGR passage and the fourth EGR passage;
The ratio of the flow rate of the exhaust gas flowing from the fifth EGR passage to the third EGR passage to the total flow rate of the exhaust gas flowing from the fourth EGR passage and the fifth EGR passage to the third EGR passage can be changed. An inflow ratio change valve,
And a third EGR valve provided in the third EGR passage and capable of changing a flow rate of the exhaust gas flowing through the third EGR passage.

  Here, the configuration related to the exhaust purification system is referred to as a second configuration. In the exhaust gas purification system according to the second configuration, when the exhaust gas is guided to the intake system of the internal combustion engine through the first path, the exhaust gas is caused to flow through the fourth EGR passage and the third EGR passage through the exhaust gas. When the exhaust is led to the intake system of the internal combustion engine by a route, the fifth EGR passage and the third EGR passage may be circulated through the exhaust.

  The EGR gas amount changing means changes the opening of the inflow ratio changing valve and / or the opening of the third EGR valve when changing the ratio of the second EGR gas amount to the total EGR gas amount. May be.

  That is, in the second configuration, the ratio of the second EGR gas amount to the total EGR gas amount can be changed by adjusting the opening of the inflow ratio changing valve. Then, by adjusting the opening degree of the third EGR valve, it is possible to adjust the EGR gas amount while maintaining the ratio of the second EGR gas amount to the total EGR gas amount determined by the opening degree of the inflow ratio changing valve.

  Further, the flow rate ratio changing valve may be an inflow path switching valve capable of electrically connecting one of the fourth EGR passage and the fifth EGR passage to the third EGR passage and blocking the other. For example, when the temperature of the NOx catalyst is lower than a predetermined temperature, the flow rate ratio changing valve conducts the third EGR passage and the fifth EGR passage so as to guide the exhaust to the intake system of the internal combustion engine through the second passage, and the third EGR passage. The passage and the fourth EGR passage may be blocked.

  On the other hand, when the temperature of the NOx catalyst is equal to or higher than a predetermined temperature, the flow rate ratio changing valve conducts the third EGR passage and the fourth EGR passage so as to guide the exhaust gas to the intake system of the internal combustion engine through the first passage, and the third EGR passage. And the fifth EGR passage may be blocked.

  Thereby, it is possible to switch whether the exhaust gas discharged from the internal combustion engine is guided to the intake system by either the first path or the second path with a simpler configuration.

  Further, in the exhaust gas purification system according to the second configuration, the first path and the second path merge at the connection portion from the third EGR passage to the fifth EGR passage. Accordingly, the EGR device can be made more compact than the first configuration, and the cost for installing the EGR device can be reduced.

  Further, in order to efficiently reduce the combustion temperature of the air-fuel mixture in the internal combustion engine, the EGR passage is often provided with an EGR cooler for cooling the EGR gas flowing through the EGR passage. Also in the present invention, the EGR device may include an EGR cooler that is provided in the third EGR passage and can cool the exhaust gas flowing through the third EGR passage.

  In this case, according to the present configuration (second configuration), the cost for constructing the exhaust purification system can be further reduced. That is, originally, it is necessary to provide the EGR cooler for each of the first route and the second route, but in this configuration, it is sufficient to provide a single EGR cooler. As described above, in this configuration, the number of EGR valves and EGR coolers arranged can be reduced by providing the flow rate ratio changing valve. As a result, the total cost required to construct an exhaust gas purification system for an internal combustion engine can be kept low.

In the present invention, when the temperature of the NOx catalyst provided in the exhaust passage is low, the NOx catalyst can be quickly heated while reducing the amount of NOx produced in the internal combustion engine.

  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.

  FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 and its intake / exhaust system and control system in this embodiment. An internal combustion engine 1 shown in FIG. 1 is a diesel engine having four cylinders 2. The internal combustion engine 1 is provided with a fuel injection valve 3 for injecting fuel directly into the combustion chamber of the cylinder 2 in each cylinder.

<Intake system>
An intake manifold 8 is connected to the internal combustion engine 1, and each branch pipe of the intake manifold 8 is communicated with a combustion chamber of each cylinder 2 through an intake port. A first intake throttle valve 11 capable of adjusting the flow rate of the intake air flowing through the intake passage 9 is provided in the vicinity of the connection portion between the intake manifold 8 and the intake passage 9. An intercooler 13 that cools the gas flowing through the intake passage 9 is provided upstream of the first intake throttle valve 11 in the intake passage 9.

  Further, a compressor housing 6 of a turbocharger 10 that operates using exhaust energy as a drive source is provided upstream of the intercooler 13 in the intake passage 9. An air flow meter 14 that outputs an electrical signal corresponding to the amount of intake air flowing through the intake passage 9 is disposed upstream of the compressor housing 6, and an air cleaner 4 is disposed upstream of the air flow meter 14. Is provided. Further, the flow rate of the intake air flowing through the intake passage 9 is adjusted downstream of the air flow meter 14 in the intake passage 9 and upstream of the connecting portion between the second EGR passage 41 and the intake passage 9 described later. A possible second intake throttle valve 12 is provided.

  In the intake system of the internal combustion engine 1 configured as described above, dust and dirt in the intake air are removed by the air cleaner 4 and then flow into the compressor housing 6 through the intake passage 9. The intake air flowing into the compressor housing 6 is compressed by the rotation of the compressor wheel built in the compressor housing 6. Then, the compressed intake air having a high temperature is cooled by the intercooler 13, and then the flow rate is adjusted by the first intake throttle valve 11 as necessary to flow into the intake manifold 8. The intake air flowing into the intake manifold 8 is distributed to each cylinder 2 through each branch pipe, and burned by using the fuel injected from the fuel injection valve 3 of each cylinder 2 as an ignition source.

<Exhaust system>
On the other hand, an exhaust manifold 18 is connected to the internal combustion engine 1, and each branch pipe of the exhaust manifold 18 is connected to a combustion chamber of each cylinder 2 via an exhaust port. A turbine housing 7 of the turbocharger 10 is connected to the exhaust manifold 18. An exhaust passage 19 is connected to the turbine housing 7, and the exhaust passage 19 is connected downstream to a muffler (not shown).

  A particulate filter (hereinafter simply referred to as “filter”) 20 that collects PM in the exhaust is provided in the middle of the exhaust passage 19. Further, an NOx storage reduction catalyst (hereinafter referred to as “NOx catalyst”) 21 is provided downstream of the filter 20 in the exhaust passage 19. Here, the NOx catalyst 21 stores NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and releases the stored NOx when the oxygen concentration of the inflowing exhaust gas is low. At that time, if a reducing component such as fuel is present in the surrounding exhaust gas, the released NOx and the fuel undergo a reduction reaction on the NOx catalyst 21 to reduce and purify NOx.

Further, a fuel addition valve 15 is provided between the filter 20 and the NOx catalyst 21 in the exhaust passage 19 to open a valve as a command signal from an ECU 22 described later and add fuel as a reducing agent into the exhaust. The fuel added from the fuel addition valve 15 to the exhaust gas in the exhaust passage 19 reduces the oxygen concentration of the exhaust gas. The exhaust gas having a low oxygen concentration thus formed flows into the NOx catalyst 21 and is reduced to nitrogen (N ₂ ) while releasing NOx stored in the NOx catalyst 21.

  A temperature sensor 25 that detects the temperature of the exhaust gas is provided immediately upstream of the NOx catalyst 21 in the exhaust passage 19. Further, on the upstream side of the fuel addition valve 15 in the exhaust passage 19 and on the downstream side of the connecting portion between the first EGR passage 31 and the exhaust passage 19 described later, the flow rate of the exhaust gas flowing in the exhaust passage 19 is set. An adjustable first exhaust throttle valve 16 is provided. A second exhaust throttle valve 17 capable of adjusting the flow rate of the exhaust gas flowing in the exhaust passage 19 is provided downstream of the connection portion of the exhaust passage 19 between the exhaust passage 19 and a second EGR passage 41 described later. ing.

  In the exhaust system of the internal combustion engine 1 configured as described above, the burned gas burned in each cylinder 2 of the internal combustion engine 1 is discharged to the exhaust manifold 18 through the exhaust port, and then from the exhaust manifold 18 to the turbocharger 10. It flows into the turbine housing 7. The exhaust gas flowing into the turbine housing 7 rotates a turbine wheel that is rotatably supported in the turbine housing 7 using thermal energy of the exhaust gas. At that time, the rotational torque of the turbine wheel is transmitted to the compressor wheel of the compressor housing 6.

  Then, the exhaust discharged from the compressor housing 6 flows into the filter 20 via the exhaust passage 19 and PM in the exhaust is collected. Then, it flows into the NOx catalyst 21 and NOx in the exhaust is occluded. The exhaust gas from which PM and NOx have been removed or purified as described above is discharged into the atmosphere through the muffler after the flow rate is adjusted by the second exhaust throttle valve 17 as necessary.

<Exhaust gas recirculation mechanism>
Further, in the internal combustion engine 1, a part of the exhaust gas that is downstream of the filter 20 in the exhaust passage 19 and upstream of the NOx catalyst 21 is recirculated upstream of the compressor housing 6 in the intake passage 9. A first EGR device 30 for circulation is provided. The first EGR device 30 is downstream of the filter 20 in the exhaust passage 19 and upstream of the first exhaust throttle valve 16 and upstream of the compressor housing 6 in the intake passage 9 and is first. 2 A first EGR passage 31 that connects a portion downstream of the intake throttle valve 12 and a flow rate (hereinafter referred to as “first EGR gas”) flowing through the first EGR passage 31 (hereinafter referred to as “first EGR gas amount”). The first EGR valve 32 is adjustable, and a first EGR cooler 33 that cools the EGR gas that flows upstream of the first EGR valve 32 in the first EGR passage 31 is provided.

  In the first EGR device 30 configured as described above, when the first EGR valve 32 is opened, the first EGR passage 31 becomes conductive, and a part of the exhaust before flowing into the NOx catalyst 21 passes through the first EGR passage 31. It flows into the intake passage 9 via. The first EGR gas flowing into the intake passage 9 is supercharged into the combustion chamber of the internal combustion engine 1 via the compressor housing 6 and the intake manifold 8. Here, in the present embodiment, the path through which the EGR gas is circulated through the first EGR passage 31 and recirculated to the intake system of the internal combustion engine 1 corresponds to the first path in the present invention.

  In addition, the internal combustion engine 1 is provided with a second EGR device 40 that recirculates a part of the exhaust gas that passes downstream of the NOx catalyst 21 in the exhaust passage 19 to the upstream side of the compressor housing 6 in the intake passage 9. Yes. The second EGR device 40 is downstream of the NOx catalyst 21 in the exhaust passage 19 and upstream of the second exhaust throttle valve 17 and upstream of the connection portion between the first EGR passage 31 in the intake passage 9. And a second EGR passage 41 that connects a portion downstream of the second intake throttle valve 12 and a flow rate (hereinafter referred to as “second EGR gas”) flowing through the second EGR passage 41 (hereinafter referred to as “second EGR gas”). , Referred to as “second EGR gas amount”), and a second EGR cooler 43 that cools the EGR gas flowing upstream of the second EGR valve 42 in the second EGR passage 41.

  In the second EGR device 40 configured as described above, when the second EGR valve 42 is opened, the second EGR passage 41 becomes conductive, and a part of the exhaust gas flowing out from the NOx catalyst 21 passes through the second EGR passage 41. Flows into the intake passage 9. The second EGR gas that has flowed into the intake passage 9 is supercharged into the combustion chamber of the internal combustion engine 1 via the compressor housing 6 and the intake manifold 8. Here, in the present embodiment, the path through which the EGR gas is circulated through the second EGR passage 41 and recirculated to the intake system of the internal combustion engine 1 corresponds to the second path in the present invention.

  Here, the first EGR gas recirculated by the first EGR device 30 and the second EGR gas recirculated by the second EGR device 40 are both recirculated after passing through the filter 20. That is, the exhaust gas after the PM is removed by the filter 20 is recirculated to the intake system. As a result, the intake of PM into the internal combustion engine 1 can be suppressed. Further, PM can be prevented from being deposited on the intercooler 13, the first EGR cooler 33, the second EGR cooler 43, and the like.

  Further, the first EGR gas amount and the second EGR gas amount are adjusted by adjusting the opening degree of the first exhaust throttle valve 16 and the opening degree of the second exhaust throttle valve 17 so that the exhaust gas at the connection portion of the exhaust passage 19 with the first EGR passage 31 is exhausted. Or the pressure of the exhaust gas at the connection portion of the exhaust passage 19 with the second EGR passage 41 can be adjusted. For example, if the opening of the first exhaust throttle valve 16 is adjusted to the valve closing side, the exhaust pressure at the connection portion of the exhaust passage 19 with the first EGR passage 31 rises, so the upstream side and the downstream side of the first EGR passage 31 And the first EGR gas amount increases. Further, when the opening of the second exhaust throttle valve 17 is adjusted to the valve closing side, the exhaust pressure at the connection portion of the exhaust passage 19 with the second EGR passage 41 increases, so the upstream side and the downstream side of the second EGR passage 41 And the second EGR gas amount increases.

  Similarly, the first EGR gas amount and the second EGR gas amount are adjusted by adjusting the opening of the second intake throttle valve 12 and the intake pressure at the connection portion of the intake passage 9 with the first EGR passage 31 or the first EGR gas amount in the intake passage 9. It can be adjusted by increasing or decreasing the pressure of the intake air at the connection with the 2EGR passage 41. For example, when the opening degree of the second intake throttle valve 12 is adjusted to the valve closing side, a negative pressure is generated at the connection portion, so that the first EGR gas amount and the second EGR gas amount increase.

In addition, the first EGR gas and the second EGR gas contain inert gas components that do not burn themselves and have a high heat capacity, such as water (H ₂ O) and carbon dioxide (CO ₂ ). Therefore, when the first EGR gas or the second EGR gas is contained in the air-fuel mixture, the combustion temperature of the air-fuel mixture is lowered, and thus the amount of NOx generated in the internal combustion engine 1 is suppressed.

  Further, when the first EGR gas is cooled in the first EGR cooler 33 and the second EGR gas is cooled in the second EGR cooler 43, the temperature of the first EGR gas and the second EGR gas itself is lowered and the volume of the gas is reduced. When the 1EGR gas or the second EGR gas is supplied into the combustion chamber, the atmospheric temperature in the combustion chamber is not increased unnecessarily, and the amount of fresh air (volume of fresh air) supplied into the combustion chamber is unnecessary. There is no risk of a decrease.

Inhalation of PM into the internal combustion engine can be suppressed. In addition, for example, when the exhaust purification system includes a turbocharger or an intercooler for cooling the intake air supercharged by the turbocharger, it is possible to prevent the inflow PM from accumulating on the turbocharger or the like. it can.

  The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 22 for controlling the internal combustion engine 1 and the intake / exhaust system. The ECU 22 controls 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, and also the opening degree of the first EGR valve 32 (hereinafter referred to as “first EGR opening degree”) Df. The opening degree (hereinafter referred to as “second EGR opening degree”) Ds of the second EGR valve 42 is controlled.

  Further, the ECU 22 includes sensors and a temperature sensor 25 for controlling the operating state of the internal combustion engine 1 such as an air flow meter 14, a crank position sensor 23 for detecting the engine speed, and an accelerator position sensor 24 for detecting the accelerator opening. Are connected via electrical wiring, and their output signals are input to the ECU 22. On the other hand, the ECU 22 includes a fuel injection valve 3, a fuel addition valve 15, a first intake throttle valve 11, a second intake throttle valve 12, a first exhaust throttle valve 16, a second exhaust throttle valve 17, a first EGR valve 32, The 2EGR valve 42 and the like are connected via electric wiring and are controlled by the ECU 22.

  The ECU 22 includes a CPU, a ROM, a RAM, and the like, and the ROM stores a program for performing various controls of the internal combustion engine 1 and a map storing data. Various routines to be described later are one of programs stored in the ROM of the ECU 22.

  Next, EGR control in the present embodiment will be described. In the EGR control according to the present embodiment, the EGR rate ER is controlled so as to become the target EGR rate ERtg corresponding to the operating state of the internal combustion engine 1. The target EGR rate ERtg is a predetermined target value. For example, the target NOx reduction rate is determined based on a target NOx reduction rate determined in consideration of an exhaust gas regulation value and a conformance margin for the regulation value. May be defined as the EGR rate required to achieve the above.

  The target EGR rate ERtg is specifically derived by reading the target EGR rate ERtg from a map in which the relationship among the engine speed NE, the engine load Tq, and the target EGR rate ERtg of the internal combustion engine 1 is stored. At this time, the engine speed NE is obtained from the detected value of the crank angle by the crank position sensor 23, and the engine load Tq is obtained from the detected value of the accelerator opening by the accelerator position sensor 24. Then, the ECU 22 calculates the target EGR gas amount (the sum of the first EGR gas amount + the second EGR gas amount) based on the intake fresh air amount Ga obtained from the detection value by the air flow meter 14 and the target EGR rate ERtg.

  Here, the exhaust gas purification system according to the present embodiment includes a first EGR device 30 and a second EGR device 40. In this embodiment, the ratio of the first EGR gas amount to the EGR gas amount (hereinafter, “the recirculation ratio of the first EGR gas amount”) Rf and the second EGR gas amount relative to the EGR gas amount according to the temperature Tn of the NOx catalyst. The ratio (hereinafter, “the second EGR gas amount recirculation ratio”) Rs is changed. Hereinafter, this will be described in detail with reference to FIG.

  FIG. 2 is a diagram illustrating the relationship between the temperature Tn of the NOx catalyst, the NOx purification rate Rp, the recirculation ratio Rf of the first EGR gas amount, and the recirculation ratio Rs of the second EGR gas amount in the present embodiment. (A) is the figure which illustrated correlation with temperature Tn of NOx catalyst, and NOx purification rate Rp. (B) is a diagram illustrating the relationship between the temperature Tn of the NOx catalyst and the recirculation ratio Rf of the first EGR gas amount. (C) is a diagram illustrating the relationship between the temperature Tn of the NOx catalyst and the recirculation ratio Rs of the second EGR gas amount.

  Here, if the “NOx purification rate Rp” shown in FIG. 2A is equal to or greater than “Rp0”, it means that the NOx catalyst 21 can suitably purify NOx in the exhaust gas. The temperature “Tn0” shown on the horizontal axis in the figure is the temperature of the NOx catalyst 21 when the “NOx purification rate Rp” is “Rp0”. That is, “Tn0” means the activation temperature of the NOx catalyst 21, and means the lower limit temperature at which the NOx catalyst 21 can efficiently purify NOx in the exhaust gas.

  Further, when “the recirculation ratio Rf of the first EGR gas amount” shown in FIG. 2B is “1”, it means that the EGR is performed only by the first EGR device 30. Further, when “the recirculation ratio Rf of the first EGR gas amount” is “0”, it means that the EGR is performed only by the second EGR device 40.

  Moreover, when the “recirculation ratio Rs of the second EGR gas amount” shown in FIG. 2C is “1”, it means that the EGR is performed only by the second EGR device 40. Further, when the “recirculation ratio Rs of the second EGR gas amount” is “0”, it means that EGR is performed only by the first EGR device 30.

  When EGR is performed using both the first EGR device 30 and the second EGR device 40, for example, when EGR is performed so that the ratio of the first EGR gas amount to the second EGR gas amount is 2: 8, the first EGR gas is used. The recirculation ratio Rf of the amount is “0.2”, and the recirculation ratio Rs of the second EGR gas amount is “0.8”.

  As shown in FIG. 2A, when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the NOx purification rate Rp is lower than Rp0, and it becomes difficult for the NOx catalyst 21 to purify NOx well. Therefore, in this embodiment, when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the exhaust temperature is raised to raise the temperature of the NOx catalyst 21 to the activation temperature Tn0 or higher. For example, the exhaust temperature may be raised by sub-injecting fuel from the fuel injection valve 3 during the expansion stroke or exhaust stroke after the main injection in the combustion cycle of the internal combustion engine 1.

  As shown in FIGS. 2B and 2C, in this embodiment, when the temperature raising control is performed on the NOx catalyst 21 as described above, that is, when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0. In addition, the recirculation ratio Rf of the first EGR gas amount is controlled to 0, and the recirculation ratio Rs of the second EGR gas amount is controlled to 1. That is, the first EGR opening degree Df is controlled to be fully closed so that the first EGR gas amount becomes zero. On the other hand, the second EGR opening degree Ds is controlled to the target opening degree Dstg necessary for setting the second EGR gas amount to the above-described target EGR gas amount.

  As described above, EGR is performed only through the second EGR device 40 when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0. Then, since all the exhaust gas whose temperature has been raised by the temperature increase control passes through the NOx catalyst 21, the thermal energy of the exhaust gas can be efficiently transmitted to the NOx catalyst 21. That is, the NOx catalyst 21 can be quickly raised to the activation temperature Tn0.

  Even during the temperature rise of the NOx catalyst 21, a part of the exhaust gas is recirculated by the second EGR device 40, so that the combustion temperature of the air-fuel mixture in the combustion chamber of the internal combustion engine 1 is lowered and the NOx in the internal combustion engine 1 is reduced. The production amount is reduced.

  On the other hand, after the temperature Tn of the NOx catalyst rises to the activation temperature Tn0 or higher, the recirculation ratio Rf of the first EGR gas amount is changed to 1, and the recirculation ratio Rs of the second EGR gas amount is changed to 0. That is, the second EGR opening degree Ds is controlled to be fully closed so that the second EGR gas amount becomes zero. On the other hand, the first EGR opening degree Df is controlled to the target opening degree Dftg necessary for setting the first EGR gas amount to the above-mentioned target EGR gas amount.

  As described above, in this embodiment, EGR is performed only through the first EGR device 30 after the temperature Tn of the NOx catalyst becomes equal to or higher than the activation temperature Tn0. Here, the path of the first EGR passage 31 is shorter than that of the second EGR passage 41. Thereby, when the target EGR rate ERtg of the internal combustion engine 1 changes, the responsiveness when changing the first EGR gas amount can be improved.

  In addition, the exhaust gas after passing through the NOx catalyst 21 may have a lower specific heat than before passing through. The first EGR device 30 recirculates the exhaust gas before passing through the NOx catalyst 21. The combustion temperature of the air-fuel mixture can be efficiently reduced without reducing the specific heat of the exhaust. Thereby, the production amount of NOx in the internal combustion engine 1 can be more suitably reduced. In the present embodiment, the activation temperature Tn0 of the NOx catalyst corresponds to the predetermined temperature in the present invention.

<EGR control routine>
Hereinafter, the EGR control according to the first EGR device 30 and the second EGR device 40 when the temperature increase control for the NOx catalyst 21 in the present embodiment is performed will be described with reference to the flowchart of FIG. 3. FIG. 3 is a flowchart showing an EGR control routine in the present embodiment. This routine is a program stored in the ROM in the ECU 22, and is executed every predetermined period.

  First, in step S101, the ECU 22 detects the engine speed NE, the engine load Tq, and the intake fresh air amount Ga of the internal combustion engine 1. In step S102, a target EGR rate ERtg corresponding to the engine speed NE and the engine load Tq is calculated. In the subsequent step S103, the target EGR gas amount Gtg is calculated based on the target EGR rate ERtg and the intake fresh air amount Ga. Then, when step S103 is completed, the process proceeds to step S104.

  In step S104, the ECU 22 estimates the temperature Tn of the NOx catalyst. Specifically, the temperature Tn of the NOx catalyst is estimated based on the detection value of the temperature sensor 25. Further, for example, the relationship between the engine speed NE and the engine load Tq of the internal combustion engine 1 and the NOx catalyst temperature Tn is obtained in advance through experiments or the like, and the relationship is stored in the ECU 22 in the form of a control map. Also good. Then, the temperature Tn of the NOx catalyst may be derived by accessing the control map using the engine speed NE and the engine load Tq as parameters.

  In subsequent step S105, the ECU 22 determines whether or not the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0. That is, in this step, it is determined whether it is necessary to raise the temperature of the NOx catalyst 21 early. If it is determined that the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the process proceeds to step S103. On the other hand, when it is determined that the temperature Tn of the NOx catalyst is equal to or higher than the activation temperature Tn0, the process proceeds to step S111 described later.

  In step S106, a command is issued from the ECU 22 to the first EGR valve 32 and the second EGR valve 42, the first EGR opening Df is fully closed, the second EGR opening Ds is the second EGR gas amount calculated in step S103, and the target EGR is calculated. It is changed to the target opening degree Dstg necessary for making the gas amount. In this embodiment, the ECU 22 that changes the first EGR opening degree Df and the second EGR opening degree Ds corresponds to the EGR gas amount changing means in the present invention. Then, when the process of step S106 ends, the process proceeds to step S107.

In step S107, the temperature increase control of the NOx catalyst 21 is started by a command from the ECU 22. Specifically, a command is issued to the fuel injection valve 3 to perform sub-injection of fuel. Here, the amount of sub-injected fuel is experimentally determined in advance. Then, when step S107 is completed, the process proceeds to step S108.

  In step S108, the ECU 22 estimates the temperature Tn of the NOx catalyst. In subsequent step S109, it is determined whether or not the temperature Tn of the NOx catalyst is equal to or higher than the activation temperature Tn0. That is, in this step, it is determined whether or not the temperature raising control for the NOx catalyst 21 may be stopped.

  When it is determined that the temperature Tn of the NOx catalyst is equal to or higher than the activation temperature Tn0, the process proceeds to step S110. On the other hand, when it is determined that the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the state returns to the state after the process of step S106. That is, the temperature rise control for the NOx catalyst 21 is continued.

  In step S110, the temperature increase control for the NOx catalyst 21 is stopped. Specifically, the ECU 22 issues a command to the fuel injection valve 3 to stop the fuel sub-injection. Then, when the process of step S110 ends, the process proceeds to step S111.

  In step S111, a command is issued from the ECU 22 to the first EGR valve 32 and the second EGR valve 42, and the first EGR opening degree Df is a target required for making the first EGR gas amount the target EGR gas amount Gtg calculated in step S103. The second EGR opening Ds is changed to fully closed at the opening Dftg. Then, when the process of step S111 is finished, this routine is once ended.

  As described above, in this routine, when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the NOx catalyst 21 can be quickly raised to the activation temperature Tn0. Further, EGR can be performed by the second EGR device 40 even during the temperature rise of the NOx catalyst, and the amount of NOx generated in the internal combustion engine 1 can be reduced.

<Modification of EGR control>
In this embodiment, the recirculation ratio Rf of the first EGR gas amount and the recirculation ratio Rs of the second EGR gas amount are set to 1 or 2 depending on whether or not the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0. Although the case of changing to any one of 0 has been exemplarily described, it is not limited to this.

  For example, when the NOx catalyst temperature Tn is lower than the activation temperature Tn0, the recirculation ratio Rf of the first EGR gas amount may be larger than the recirculation ratio Rs of the second EGR gas amount. Is greater than the recirculation ratio Rf of the first EGR gas amount when the recirculation ratio Rs of the second EGR gas amount is higher than the activation temperature Tn0. Hereinafter, the recirculation ratio Rf of the first EGR gas amount when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0 is 0.8, and the recirculation ratio Rf of the first EGR gas amount when the temperature Tn0 is the activation temperature Tn0 or more is 0. In the case of 2, the application of the routine will be described.

  That is, in this EGR control, the target value of the first EGR gas amount and the target value of the second EGR gas amount may be calculated in step S106 of the above routine. Here, the target value is obtained by adding the recirculation ratio Rf (0.8) of the first EGR gas amount and the recirculation ratio Rs (0.2) of the second EGR gas amount to the target EGR gas amount calculated in step S103. Calculated by multiplication.

And what is necessary is just to change the 1st EGR opening degree Df and the 2nd EGR opening degree Ds to the opening degree required in order to change the 1st EGR gas quantity and the 2nd EGR gas quantity to each target value.

  In step S111 of the above routine, the target value for the first EGR gas amount and the target value for the second EGR gas amount are calculated again. That is, the respective target values are calculated with the recirculation ratio Rf of the first EGR gas amount being 0.2 and the recirculation ratio Rs of the second EGR gas amount being 0.8. Then, the first EGR opening degree Df and the second EGR opening degree Ds may be changed to change the first EGR gas amount and the second EGR gas amount to the respective target values.

  In this embodiment, after the temperature Tn of the NOx catalyst becomes equal to or higher than the activation temperature Tn0, the NOx catalyst 21 is subjected to NOx reduction processing and SOx poisoning recovery processing. May be added to the exhaust. Thereby, fuel can be supplied to the NOx catalyst 21 which is in an active state reliably. Further, when the fuel is added, the EGR is performed by the first EGR device, so that there is no possibility that the fuel is recirculated to the internal combustion engine 1 via the second EGR passage 41. Therefore, the combustion of the air-fuel mixture in the combustion chamber of the internal combustion engine 1 can be suppressed from becoming unstable, and the drivability can be prevented from deteriorating.

  In addition, the filter 20 in this embodiment may carry an oxidation catalyst or a NOx catalyst, and the NOx catalyst 21 may be a selective reduction type NOx catalyst or the like instead of a storage reduction type NOx catalyst. In that case, the reducing agent supplied when performing the NOx reduction process on the selective reduction type NOx catalyst may be an aqueous urea solution or an aqueous ammonia solution.

  In addition, the exhaust purification system in the present embodiment includes a turbocharger 10 that supercharges intake air, and a part of the exhaust is upstream of the compressor housing 6 in the intake passage 9 and more than the second intake throttle valve 12. Although it is recirculated to the downstream part, this is an example when it is recirculated to the intake system of the internal combustion engine 1. The present invention can naturally be applied to an exhaust purification system that does not include a turbocharger.

  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. 4 is a diagram showing a schematic configuration of the internal combustion engine 1 and its intake / exhaust system and control 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 exhaust purification system according to the present embodiment includes a third EGR device 50 instead of the first EGR device 30 and the second EGR device 40 as an exhaust gas recirculation mechanism that recirculates part of the exhaust gas to the intake system. Hereinafter, the configuration of the third EGR device 50 will be described.

  In the internal combustion engine 1, a part of the exhaust gas that passes downstream of the filter 20 in the exhaust passage 19 and upstream of the NOx catalyst 21 and / or passes downstream of the NOx catalyst 21 in the exhaust passage 19. A third EGR device 50 that recirculates part of the exhaust gas upstream of the compressor housing 6 in the intake passage 9 is provided.

  The third EGR device 50 includes a third EGR passage 51 whose one end is connected to a portion upstream of the compressor housing 6 in the intake passage 9, and one end downstream of the filter 20 in the exhaust passage 19 and a NOx catalyst. A fourth EGR passage 52 connected to the portion upstream of 21 and the other end connected to the other end of the third EGR passage 51, and the other end connected to a portion downstream of the NOx catalyst 21 in the exhaust passage 19. Includes a fifth EGR passage 53 connected to a connection portion between the third EGR passage 51 and the fourth EGR passage 52.

  Further, the third EGR device 50 is provided at a connection portion to which the third EGR passage 51 to the fifth EGR passage 53 are connected, and exhaust gas flowing into the third EGR passage 51 is sent to any of the fourth EGR passage 52 and the fifth EGR passage 53. An inflow path change valve 54 that can switch whether to flow from the third EGR passage 51, a third EGR valve 55 that can adjust the exhaust gas flowing through the third EGR passage 51, and a third EGR cooler 56 that cools the EGR gas flowing through the third EGR passage 51. I have.

  Here, the inflow path changing valve 54 is connected to the ECU 22 via an electric wiring, and is controlled by the ECU 22. Then, the ECU 22 causes the inflow passage change valve 54 to connect the third EGR passage 51 and the fourth EGR passage 52 and shuts off the third EGR passage 51 and the fifth EGR passage 53 (hereinafter, the inflow passage change valve 54 at this time). The switching position is referred to as “first path position”), and a part of the exhaust before passing through the NOx catalyst 21 can be circulated in the order of the fourth EGR passage 52 and the third EGR passage 51 and can be recirculated to the internal combustion engine 1. .

  In the present embodiment, the path when the ECU 22 controls the inflow path changing valve 54 to the first path position side and causes the EGR gas to flow through the fourth EGR passage 52 and the third EGR passage 51 and recirculate to the internal combustion engine 1. This corresponds to the first route in the present invention. In this embodiment, the EGR gas recirculated through this route is referred to as “first EGR gas”. Here, in this embodiment, the flow rate of the first EGR gas corresponds to the first EGR gas amount in the present invention.

  Further, the ECU 22 causes the inflow path changing valve 54 to block the third EGR passage 51 and the fourth EGR passage 52, and connects the third EGR passage 51 and the fifth EGR passage 53 (hereinafter, the inflow path changing valve 54 at this time). The switching position is referred to as “second path position”), and part of the exhaust gas that has passed through the NOx catalyst 21 can be circulated in the order of the fifth EGR passage 53 and the third EGR passage 51 and recirculated to the internal combustion engine 1. .

  In the present embodiment, when the ECU 22 controls the switching position of the inflow path changing valve 54 to the second path position side and causes the EGR gas to flow through the fifth EGR passage 53 and the third EGR passage 51 and recirculates to the internal combustion engine 1. This path corresponds to the second path in the present invention. In this embodiment, the EGR gas recirculated through this route is referred to as “second EGR gas”. Here, in this embodiment, the flow rate of the second EGR gas corresponds to the second EGR gas amount in the present invention.

  Further, the third EGR valve 55 is connected to the ECU 22 through an electrical wiring, and the ECU 22 controls the opening degree of the third EGR valve 55 (hereinafter referred to as “third EGR opening degree”) Dt. In the present embodiment, the ECU 22 can change the first EGR gas amount and the second EGR gas amount by controlling the switching position of the inflow passage changing valve 54 and the third EGR opening degree Dt. In this embodiment, the ECU 22 that controls the switching position of the inflow path changing valve 54 and the third EGR opening degree Dt corresponds to the EGR gas amount changing means in the present invention.

  As described above, according to the exhaust gas purification system having the above-described configuration, the exhaust gas purification system according to the first embodiment is used because it passes through the third EGR passage 51 when either the first EGR gas or the second EGR gas is recirculated to the internal combustion engine 1. Compared with the system, the EGR device can be made compact, and the cost can be reduced.

  Further, since the number of EGR valves and EGR coolers is reduced instead of adding the inflow path change valve 54 as compared with the exhaust purification system of the first embodiment, the total cost required for constructing the exhaust purification system is kept low. Is possible.

Next, EGR control in the present embodiment will be described. In the present embodiment, the ECU 22 performs switching control of the switching position of the inflow passage changing valve 54 in accordance with the temperature Tn of the NOx catalyst. Specifically, when the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the inflow path change valve 54 is switched to the first path position, and when the temperature is equal to or higher than the activation temperature Tn0, the inflow path change valve 54 is switched to the second path position. It was supposed to be. Hereinafter, the EGR control of this embodiment performed by the ECU 22 will be described with reference to the flowchart of FIG.

<Second EGR control routine>
FIG. 5 is a flowchart showing a second EGR control routine in the present embodiment. This routine is a program stored in the ROM in the ECU 22, and is executed every predetermined period.

  Further, the same steps as those in the routine and the EGR control routine in the first embodiment are not described in detail by using the same numbers. Further, description will be made on the assumption that the switching position of the inflow path changing valve 54 is controlled to the first path position when executing this routine. Steps S101 to S105 are the same as the processing contents in the corresponding steps in the EGR transient control routine, and a description thereof will be omitted. Then, when the process of step S105 ends, the process proceeds to step S206.

  In step S206, a command is issued from the ECU 22 to the inflow path change valve 54 and the third EGR valve 55, and the switching position of the inflow path change valve 54 is switched to the second path position. Further, the target opening degree Dttg necessary for setting the flow rate of the exhaust gas flowing from the fifth EGR passage 53 and flowing through the third EGR passage 51 (in this step, the second EGR gas amount) to the target EGR gas amount calculated in step S103. The third EGR opening degree Dt is changed. Then, when the process of step S206 ends, the process proceeds to step S107.

  Here, steps S107 to S110 are the same as the processing contents in the corresponding steps in the EGR transient control routine, and detailed description thereof will be omitted. If a negative determination is made in step S109, that is, if it is determined that the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0, the state returns to the state after the process of step S206. In step S110, when the temperature increase control for the NOx catalyst 21 is stopped, the process proceeds to step S211.

  In step S211, a command is issued from the ECU 22 to the inflow path change valve 54 and the third EGR valve 55, and the switching position of the inflow path change valve 54 is switched to the first path position. As a result, the second EGR gas flows from the fourth EGR passage 52 into the third EGR passage 51 and is recirculated to the internal combustion engine 1. In this step, the fuel addition valve 15 is opened by a command from the ECU 22, and fuel is added from the fuel addition valve 15 into the exhaust gas. Thereby, NOx stored in the NOx catalyst 21 is reduced and purified. In this embodiment, the ECU 22 for adding fuel to the fuel addition valve 15 corresponds to the reducing agent adding means in the present invention. Then, when the process of step S211 is finished, this routine is once ended.

  As described above, according to this routine, when the temperature Tn of the NOx catalyst is low (in this routine, when it is lower than the activation temperature Tn0), the temperature increase control is executed to increase the exhaust gas temperature. Hot exhaust gas is introduced into the NOx catalyst 21. As a result, the NOx catalyst 21 can be quickly heated.

In step S211, since the temperature Tn of the NOx catalyst is already equal to or higher than the activation temperature Tn0, the fuel added to the exhaust gas from the fuel addition valve 15 may pass through the NOx catalyst 21 and be released to the atmosphere in a large amount. Absent. Further, when fuel is added from the fuel addition valve 15, the switching position of the inflow path changing valve 54 is controlled to the first path position, so that the added fuel is not recirculated. Therefore, the combustion of the air-fuel mixture in the internal combustion engine 1 is suppressed from becoming unstable, and the drivability is prevented from deteriorating.

  In this routine, the case where the switching position of the inflow passage changing valve 54 is controlled based on whether or not the temperature Tn of the NOx catalyst is lower than the activation temperature Tn0 has been described, but the present invention is not limited to this. For example, when recovering the NOx catalyst 21 poisoned by SOx, it is necessary to maintain the temperature Tn of the NOx catalyst at a high temperature (for example, 600 ° C. to 650 ° C.). Therefore, it is preferable to apply the EGR control according to this embodiment when performing the SOx poisoning recovery process on the NOx catalyst 21. This is because the temperature Tn of the NOx catalyst can be rapidly raised to a temperature at which SOx is released from the NOx catalyst 21.

  Further, when the NOx catalyst 21 in the present embodiment is supported on a filter capable of collecting PM in exhaust gas, for example, the temperature of the filter is a temperature at which PM can be oxidized (combusted) (for example, 500 ° C. to 700 ° C. ) May be raised. Therefore, when the PM regeneration process is performed on the filter, the EGR control according to the present embodiment may be applied to quickly raise the temperature of the filter.

  Moreover, although the inflow path change valve 54 in a present Example is set as the structure which selectively flows in either the 1st EGR gas and the 2nd EGR gas to the 3rd EGR channel | path 51, it is not limited to this. The inflow path change valve 54 may be, for example, an inflow ratio change valve capable of changing the ratio of the second EGR gas amount to the sum of the first EGR gas amount and the second EGR gas amount (total EGR gas amount).

1 is a diagram illustrating a schematic configuration of an internal combustion engine and an intake / exhaust system and a control system thereof in Embodiment 1. FIG. It is the figure which illustrated temperature Tn of NOx catalyst in Example 1, NOx purification rate Rp, recirculation ratio Rf of the 1st EGR gas amount, and recirculation ratio Rs of the 2nd EGR gas amount. (A) is the figure which illustrated correlation with temperature Tn of NOx catalyst, and NOx purification rate Rp. (B) is a diagram illustrating the relationship between the temperature Tn of the NOx catalyst and the recirculation ratio Rf of the first EGR gas amount. (C) is a diagram illustrating the relationship between the temperature Tn of the NOx catalyst and the recirculation ratio Rs of the second EGR gas amount. 3 is a flowchart illustrating an EGR control routine according to the first embodiment. It is a figure which shows schematic structure of the internal combustion engine in Example 2, its intake-exhaust system, and a control system. 7 is a flowchart showing a second EGR control routine in Embodiment 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Cylinder 3 ... Fuel injection valve 4 ... Air cleaner 6 ... Compressor housing 7 ... Turbine housing 8 ... Intake manifold 9 ... Intake passage 10 ... Turbocharger 11 ··· First intake throttle valve 12 · Second intake throttle valve 13 · Intercooler 14 · Air flow meter 15 · Fuel addition valve 16 · · First exhaust throttle valve 17 · · Second exhaust throttle valve 18. Exhaust manifold 19 Exhaust passage 20 Particulate filter 21 NOx catalyst 22 ECU
23 .... Crank position sensor 24..Accelerator position sensor 30..First EGR device 31..First EGR passage 32..First EGR valve 33..First EGR cooler 40..Second EGR device 41..Second EGR passage 42. 2nd EGR valve 43 2nd EGR cooler 50 3rd EGR device 51 3rd EGR passage 52 4th EGR passage 53 5th EGR passage 54 Inflow passage change valve 55 3rd EGR valve

Claims (7)

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;
A NOx catalyst provided in the exhaust passage and purifying NOx contained in the exhaust;
A first path for leading a part of the exhaust gas from the upstream side of the NOx catalyst in the exhaust passage to the intake system of the internal combustion engine and / or a part of the exhaust gas downstream of the NOx catalyst in the exhaust passage An EGR device that recirculates part of the exhaust gas to the intake system through a second path that leads from the engine to the intake system of the internal combustion engine;
EGR gas that changes the first EGR gas amount that is the flow rate of the exhaust gas guided to the intake system by the first path and / or the second EGR gas amount that is the flow rate of the exhaust gas guided to the intake system by the second path A quantity changing means;
With
The EGR gas amount changing means determines the ratio of the second EGR gas amount to the total EGR gas amount which is the sum of the first EGR gas amount and the second EGR gas amount when the temperature of the NOx catalyst is lower. An exhaust purification system for an internal combustion engine, wherein the ratio is equal to or higher than the ratio when the temperature of the engine is higher.   The EGR gas amount changing means makes the ratio when the temperature of the NOx catalyst is lower than a predetermined temperature larger than the ratio when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature. Item 6. An exhaust purification system for an internal combustion engine according to Item 1.   The EGR gas amount changing means sets the ratio to approximately 1 when the temperature of the NOx catalyst is lower than the predetermined temperature, and sets the ratio to approximately zero when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature. 3. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the exhaust gas purification system is an internal combustion engine. And further comprising a reducing agent adding means provided upstream of the NOx catalyst in the exhaust passage and adding a reducing agent to the exhaust gas in the exhaust passage;
4. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature, the reducing agent is added to the reducing agent addition means.   5. The exhaust gas purification system for an internal combustion engine according to claim 4, wherein the first path guides a part of the exhaust before the reducing agent is added by the reducing agent addition means to the intake system of the internal combustion engine. . The EGR device
A first EGR passage that communicates a portion of the exhaust passage upstream of the NOx catalyst and the intake passage of the internal combustion engine;
A first EGR valve provided in the first EGR passage and capable of changing a flow rate of exhaust gas flowing through the first EGR passage;
A second EGR passage communicating the portion of the exhaust passage downstream of the NOx catalyst and the intake passage of the internal combustion engine;
A second EGR valve provided in the second EGR passage and capable of changing a flow rate of exhaust gas flowing through the second EGR passage;
Have
When the exhaust is guided to the intake system of the internal combustion engine by the first path, the first EGR passage is circulated through the exhaust, and when the exhaust is guided to the intake system of the internal combustion engine by the second path, the exhaust Circulating the second EGR passage to the exhaust,
The EGR gas amount changing means changes the opening degree of the first EGR valve and / or the opening degree of the second EGR valve when changing a ratio of the second EGR gas amount to the total EGR gas amount. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 5. The EGR device
A third EGR passage having one end connected to the intake passage of the internal combustion engine;
A fourth EGR passage having one end connected to a portion upstream of the NOx catalyst in the exhaust passage and the other end connected to the other end of the third EGR passage;
A fifth EGR passage having one end connected to a portion of the exhaust passage on the downstream side of the NOx catalyst and the other end connected to a connection portion between the third EGR passage and the fourth EGR passage;
The ratio of the flow rate of the exhaust gas flowing from the fifth EGR passage to the third EGR passage to the total flow rate of the exhaust gas flowing from the fourth EGR passage and the fifth EGR passage to the third EGR passage can be changed. An inflow ratio change valve,
A third EGR valve provided in the third EGR passage and capable of changing a flow rate of exhaust gas flowing through the third EGR passage;
Have
When the exhaust is guided to the intake system of the internal combustion engine by the first path, the exhaust gas is passed through the fourth EGR passage and the third EGR path through the exhaust, and the exhaust is sent to the intake system of the internal combustion engine by the second path. To the exhaust gas, the fifth EGR passage and the third EGR passage are circulated through the exhaust,
The EGR gas amount changing means changes the opening of the inflow ratio changing valve and / or the opening of the third EGR valve when changing the ratio of the second EGR gas amount to the total EGR gas amount. An exhaust purification system for an internal combustion engine according to any one of claims 1 to 5.

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