JP2010229916A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine in which a time required for sulfur poisoning regeneration and the control of a NOx catalyst is reduced and fuel consumption is improved. <P>SOLUTION: The exhaust emission control device of the internal combustion engine includes: an oxidation catalyst; a NOx catalyst disposed downstream of the oxidation catalyst; a first exhaust passage which feeds exhaust gas upstream of the oxidation catalyst; and a second exhaust passage which feeds exhaust gas between the oxidation catalyst and the NOx catalyst. When controlling a catalyst bed temperature for raising a temperature of the NOx catalyst so that a temperature of a sulfur component is raised to be desorbed, exhaust gas is made to flow into both the first exhaust passage and the second exhaust passage, thus raising the temperature of the NOx catalyst rapidly. This prevents a sulfur component desorbed from the oxidation catalyst from being absorbed by the NOx catalyst. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

  Japanese Patent Laid-Open No. 2000-282850 discloses an NOx occlusion reduction type provided downstream of an oxidation catalyst, a bypass passage that bypasses the oxidation catalyst, and a location where the bypass passage and the exhaust passage on the oxidation catalyst side merge. An internal combustion engine comprising: a catalyst; a fuel addition valve that adds fuel to the upstream side of the oxidation catalyst; and a control valve that controls a ratio of an exhaust gas flow rate passing through the oxidation catalyst and an exhaust gas flow rate passing through the bypass passage. An exhaust emission control device is disclosed. In this apparatus, when regenerating the NOx occlusion reduction type catalyst that has deteriorated due to the accumulation of sulfur, the fuel is added to the upstream side of the oxidation catalyst, the exhaust gas flow rate on the oxidation catalyst side is increased, and the exhaust gas on the bypass passage side is increased. By reducing the flow rate, the NOx occlusion reduction catalyst is set to a high temperature (see paragraph 0016 of the same publication).

JP 2000-282850 A JP 2007-23867 A JP 2008-261294 A JP 2008-25438 A JP 2005-330940 A

  When a basic carrier such as alumina is used as the oxidation catalyst, sulfur components also accumulate in the oxidation catalyst. However, the oxidation catalyst is weaker in holding the sulfur component than the NOx catalyst. For this reason, the sulfur component accumulated in the oxidation catalyst is relatively easily desorbed, but it takes a long time to desorb the sulfur component accumulated in the NOx catalyst.

  In the conventional apparatus described above, when performing sulfur poisoning regeneration control for desorbing the sulfur component from the NOx catalyst, the exhaust gas containing a large amount of unburned HC added from the fuel addition valve is first supplied to the oxidation catalyst. After flowing in and passing through the oxidation catalyst, it flows into the NOx catalyst. Therefore, when the added unburned HC is burned by the oxidation catalyst, the temperature of the oxidation catalyst rises before the temperature of the NOx catalyst.

  Since the sulfur component accumulated in the oxidation catalyst is easily desorbed, the sulfur component is rapidly desorbed from the oxidation catalyst when the temperature of the oxidation catalyst rises. At this time, the temperature of the NOx catalyst has not yet sufficiently increased. For this reason, the sulfur component desorbed from the oxidation catalyst is again absorbed by the NOx catalyst. As a result, the amount of sulfur that needs to be desorbed from the NOx catalyst increases, so that the time required to desorb the sulfur component from the NOx catalyst is further increased. That is, there is a problem that the sulfur poisoning regeneration control for a longer time is required and the fuel consumption is deteriorated.

  The present invention has been made to solve the above-described problems, and provides an exhaust gas purification apparatus for an internal combustion engine that can shorten the time required for sulfur poisoning regeneration control of a NOx catalyst and improve fuel efficiency. For the purpose.

In order to achieve the above object, a first invention is an exhaust purification device for an internal combustion engine,
An oxidation catalyst,
A NOx catalyst installed downstream of the oxidation catalyst;
A first exhaust path for introducing exhaust gas upstream of the oxidation catalyst;
A second exhaust path for introducing exhaust gas between the oxidation catalyst and the NOx catalyst;
A passage control means for controlling the passage of exhaust gas;
When it is required to recover sulfur poisoning of the NOx catalyst, the temperature of the NOx catalyst is raised by supplying exhaust gas containing unburned HC and oxygen that oxidizes the unburned HC. Catalyst temperature raising means for performing catalyst temperature raising control;
With
The passage path control means causes exhaust gas to flow into both the first exhaust path and the second exhaust path during the execution of the catalyst temperature increase control.

The second invention is the first invention, wherein
During the execution of the catalyst temperature increase control, the passage path control means increases the temperature of the NOx catalyst almost simultaneously with the temperature of the oxidation catalyst or increases the temperature of the NOx catalyst earlier than the temperature of the oxidation catalyst. Thus, the ratio of the exhaust gas flow rate in the first exhaust path and the exhaust gas flow rate in the second exhaust path is controlled.

The third invention is the first or second invention, wherein
After the temperature of the NOx catalyst rises to a temperature at which the sulfur component can be desorbed or higher, sulfur desorption is performed by desorbing the sulfur component from the NOx catalyst by supplying exhaust gas containing a reducing agent to the NOx catalyst. Comprising sulfur desorption means for performing the control,
The passage path control means allows exhaust gas to flow only into the first exhaust path during execution of the sulfur desorption control.

According to a fourth invention, in any one of the first to third inventions,
A first turbocharger having a turbine installed in the middle of the first exhaust path;
A second turbocharger having a turbine installed in the middle of the second exhaust path;
It is characterized by providing.

The fifth invention is the first invention, wherein
After the temperature of the NOx catalyst rises to a temperature at which the sulfur component can be desorbed or higher, sulfur desorption is performed by desorbing the sulfur component from the NOx catalyst by supplying exhaust gas containing a reducing agent to the NOx catalyst. A sulfur desorption means for performing the control;
Combustion control means for alternately and repeatedly switching between rich combustion and lean combustion in the internal combustion engine during execution of the sulfur desorption control;
A fuel addition valve provided in the middle of the first exhaust path;
Fuel addition means for adding fuel from the fuel addition valve to the exhaust gas of the lean combustion during the execution of the sulfur desorption control;
With
The passage path control means causes exhaust gas to flow into both the first exhaust path and the second exhaust path during execution of the sulfur desorption control.

  According to the first invention, during the execution of the catalyst temperature increase control for regenerating the sulfur poisoning of the NOx catalyst, not only the exhaust gas is supplied to the upstream side of the oxidation catalyst but also the NOx without passing through the oxidation catalyst. It is also possible to supply exhaust gas directly to the upstream side of the catalyst. Thereby, it is possible to prevent the temperature rise of the NOx catalyst from being delayed with respect to the temperature rise of the oxidation catalyst. Therefore, it is possible to reliably suppress the sulfur component desorbed from the oxidation catalyst from being absorbed again by the NOx catalyst. That is, it is possible to reliably suppress an increase in the sulfur accumulation amount of the NOx catalyst during the execution of the catalyst temperature increase control. For this reason, the execution period of sulfur desorption control can be shortened, and fuel consumption can be improved.

  According to the second aspect of the present invention, the temperature of the NOx catalyst is controlled by controlling the ratio of the exhaust gas flow rate in the first exhaust path and the exhaust gas flow rate in the second exhaust path during the catalyst temperature increase control. Or the NOx catalyst temperature can be raised faster than the oxidation catalyst temperature. Thereby, it can suppress more reliably that the sulfur component desorbed from the oxidation catalyst is absorbed by the NOx catalyst.

According to the third invention, the exhaust gas is caused to flow only to the upstream side of the oxidation catalyst during execution of the sulfur desorption control. As a result, all the O ₂ remaining in the exhaust gas reacts with unburned HC and the like by the oxidation catalyst and is consumed without remaining. Therefore, oxygen does not flow into the NOx catalyst during execution of sulfur desorption control. For this reason, the sulfur component accumulated in the NOx catalyst can be desorbed with higher efficiency.

  According to the fourth aspect, the above effect can be achieved by switching the operating states of the first turbocharger and the second turbocharger.

  According to the fifth invention, during the execution of sulfur desorption control, a large amount of CO is generated by performing rich combustion in the internal combustion engine, and a part of the exhaust gas containing this CO does not pass through the oxidation catalyst and passes through the NOx catalyst. Can be supplied directly upstream. Thereby, it is possible to reliably suppress the consumption of CO by the oxidation catalyst, which has a sulfur desorption effect superior to that of unburned HC, and to increase the amount of CO inflow into the NOx catalyst. For this reason, the sulfur component accumulated in the NOx catalyst can be desorbed with high efficiency. Further, by repeatedly switching between rich combustion and lean combustion during execution of sulfur desorption control, HC and CO can be purified during the lean combustion period, and emissions can be reduced. Furthermore, fuel can be added from the fuel addition valve to the exhaust gas for lean combustion, and the added fuel can be burned by the oxidation catalyst. For this reason, since it is possible to reliably prevent the temperature of the NOx catalyst from decreasing during the lean combustion period, the sulfur component can be desorbed from the NOx catalyst with high efficiency.

It is a figure for demonstrating the system configuration | structure of Embodiment 1 of this invention. It is a graph showing the time change of each temperature (bed temperature) and sulfur deposition amount of the oxidation catalyst and NSR catalyst in sulfur poisoning regeneration control. It is a flowchart of the routine performed in Embodiment 1 of the present invention. It is a flowchart of the routine performed at the time of catalyst warm-up. And the temperature of the oxidation catalyst is a diagram showing the relationship between the ratio of NO ₂ in NOx. It is a flowchart of the routine performed when an oxidation catalyst is low temperature.

Embodiment 1 FIG.
FIG. 1 is a diagram for explaining a system configuration according to the first embodiment of the present invention. As shown in FIG. 1, the system of the present embodiment includes an engine (internal combustion engine) 10. The engine 10 of this embodiment is a diesel engine. The number of cylinders and the cylinder arrangement of the engine 10 are not particularly limited. Each cylinder of the engine 10 is provided with a fuel injector (not shown) that injects fuel into the cylinder. The fuel injector can perform fuel injection a plurality of times during one cycle. That is, in addition to the main injection, pilot injection, after injection, post injection, and the like can be performed.

  The engine 10 includes a primary turbocharger (first turbocharger) 11 and a secondary turbocharger (second turbocharger) 12.

  Exhaust gases discharged from the cylinders of the engine 10 merge at an exhaust manifold (not shown) and flow into the exhaust passage 13. The exhaust passage 13 is branched into a first exhaust passage 14 and a second exhaust passage 15. A turbine 111 provided in the primary turbocharger 11 is disposed in the middle of the first exhaust passage 14. The first exhaust passage 14 on the downstream side of the turbine 111 is connected to the upstream side of the oxidation catalyst 16. That is, the exhaust gas that flows into the first exhaust passage 14 and passes through the turbine 111 flows into the oxidation catalyst 16.

  An NSR catalyst 17 is installed on the downstream side of the oxidation catalyst 16. That is, the exhaust gas that has passed through the oxidation catalyst 16 flows into the NSR catalyst 17. The NSR catalyst 17 is a NOx storage reduction type NOx catalyst. The NSR catalyst 17 carries an alkali metal or an alkaline earth metal as a NOx storage material. The NSR catalyst 17 stores NOx when the air-fuel ratio of the inflowing exhaust gas is leaner than the stoichiometric air-fuel ratio, and stores the NOx stored when the air-fuel ratio of the inflowing exhaust gas is rich below the stoichiometric air-fuel ratio. Can be removed and purified.

  A turbine 121 included in the secondary turbocharger 12 is disposed in the middle of the second exhaust passage 15. The second exhaust passage 15 on the downstream side of the turbine 121 is connected to the exhaust passage between the oxidation catalyst 16 and the NSR catalyst 17. That is, the exhaust gas flowing into the second exhaust passage 15 and passing through the turbine 121 flows into the NSR catalyst 17 without passing through the oxidation catalyst 16.

  A first exhaust switching valve (Exhaust Control Valve 1) 18 is provided in the first exhaust passage 14 on the upstream side of the turbine 111. Further, a second exhaust switching valve (Exhaust Control Valve 2) 19 is provided in the second exhaust passage 15 upstream of the turbine 121.

  The first exhaust passage 14 on the upstream side of the turbine 111 and the first exhaust passage 14 on the downstream side of the turbine 111 are connected via a bypass passage 20. An exhaust bypass valve 21 is provided in the bypass passage 20.

  An intake passage 22 is connected to an inlet of a compressor 112 provided in the primary turbocharger 11. An intake passage 23 is connected to the inlet of the compressor 122 included in the secondary turbocharger 12. The other ends of both intake passages 22 and 23 are joined together and connected to an air flow meter 24. The air flow meter 24 detects the intake air amount of the engine 10.

  An intake passage 25 is connected to the outlet of the compressor 112, and an intake passage 26 is connected to the outlet of the compressor 122. The other ends of the intake passages 25 and 26 are joined together and connected to the intake passage 27. The intake passage 27 is connected to each cylinder of the engine 10 via an intake manifold (not shown). In the middle of the intake passage 27, an intercooler 28 and an intake throttle valve 37 are installed.

  The intake passage 22 and the intake passage 26 are connected via a bypass passage 29. The bypass passage 29 is provided with an intake bypass valve 30. The intake passage 25 and the intake passage 26 are connected via a flow path 31. A reed valve 32 is provided in the flow path 31. The reed valve 32 opens when the pressure in the intake passage 26 is higher than the pressure in the intake passage 25. Further, an intake switching valve (Air Control Valve) 33 and a pressure sensor 34 are provided in the intake passage 26.

  A fuel addition valve 35 is installed in the first exhaust passage 14 between the turbine 111 and the oxidation catalyst 16. A fuel addition valve 36 is installed in the second exhaust passage 15 between the turbine 121 and the NSR catalyst 17.

  The system of this embodiment includes an ECU (Electronic Control Unit) 50. The ECU 50 includes an air flow meter 24, a pressure sensor 34, a first exhaust switching valve 18, a second exhaust switching valve 19, an exhaust bypass valve 21, an intake bypass valve 30, an intake switching valve 33, fuel addition valves 35 and 36, In addition to the intake throttle valve 37, although not shown, various sensors and actuators such as a fuel injection device and a crank angle sensor provided in the engine 10 are electrically connected.

  In the system of the present embodiment described above, when both the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19 are opened, the exhaust gas flows into both the first exhaust passage 14 and the second exhaust passage 15. Therefore, both the primary turbocharger 11 and the secondary turbocharger 12 operate. In this case, the exhaust gas that has passed through the turbine 111 of the primary turbocharger 11 flows into the oxidation catalyst 16, and the exhaust gas that has passed through the turbine 121 of the secondary turbocharger 12 does not pass through the oxidation catalyst 16 but is NSR. It flows into the catalyst 17.

  Further, when the first exhaust gas switching valve 18 is opened and the second exhaust gas switching valve 19 is closed, the exhaust gas flows only into the first exhaust passage 14, so that only the primary turbocharger 11 operates. In this case, all exhaust gas flows into the oxidation catalyst 16.

  During normal operation, the ECU 50 switches between operating only the primary turbocharger 11 or operating both the primary turbocharger 11 and the secondary turbocharger 12 according to the engine speed of the engine 10 and the engine load. That is, the ECU 50 operates only the primary turbocharger 11 in the low rotation and low load operation region where the exhaust gas flow rate is small, and in the high rotation and high load side operation region where the exhaust gas flow rate is large. Control is performed so that both turbochargers 12 are operated.

  By the way, the NOx storage material of the NSR catalyst 17 has a property of absorbing not only NOx but also a sulfur component in the exhaust gas. The NOx storage material accumulates sulfur components as sulfates or sulfites. As the sulfur component accumulates in the NSR catalyst 17, the NOx storage capability of the NSR catalyst 17 decreases. Further, since the sulfur component accumulated in the NOx storage material is less likely to be desorbed than NOx, the sulfur component is not desorbed from the NOx storage material even when NOx reduction control is performed.

  Therefore, in the present embodiment, when the amount of accumulated sulfur of the NSR catalyst 17 exceeds a predetermined value, the sulfur poisoning regeneration for desorbing the accumulated sulfur component is performed in order to recover the NOx storage capacity of the NSR catalyst 17. Control is going to be executed.

  In order to desorb the sulfur component from the NSR catalyst 17, a reducing agent is supplied to the NSR catalyst 17 while maintaining the temperature of the NSR catalyst 17 at, for example, about 650 to 700 ° C., and the atmosphere in the NSR catalyst 17 is theoretically determined. It is necessary to make it richer than the air-fuel ratio. Therefore, in the present embodiment, the catalyst temperature increase control for increasing the temperature of the NSR catalyst 17 to a necessary temperature (temperature at which the sulfur component can be desorbed), and the NSR catalyst 17 is rich after the temperature of the NSR catalyst 17 has increased. Sulfur poisoning regeneration control is configured by sulfur desorption control in which air-fuel ratio exhaust gas is supplied to desorb sulfur components.

  In the catalyst temperature increase control, exhaust gas containing a large amount of unburned HC is supplied, and the NSR catalyst 17 is heated by burning the HC. Although the method for supplying unburned HC into the exhaust gas is not particularly limited, in this embodiment, unburned HC can be supplied by adding fuel from the fuel addition valves 35 and 36 or by post injection in the engine 10. .

  This embodiment is characterized in that both the primary turbocharger 11 and the secondary turbocharger 12 are operated during the catalyst temperature increase control. That is, this embodiment not only supplies exhaust gas containing a large amount of unburned HC to the upstream side of the oxidation catalyst 16 through the first exhaust passage 14 during execution of the catalyst temperature increase control, The exhaust gas containing a large amount is also supplied between the oxidation catalyst 16 and the NSR catalyst 17 by the second exhaust passage 15.

  On the other hand, the conventional exhaust purification device generally does not include a passage for supplying exhaust gas directly between the oxidation catalyst 16 and the NSR catalyst 17, so that the catalyst temperature increase control is being performed. It is usual to always supply all the exhaust gas including the upstream side of the oxidation catalyst 16. In the following description, a case in which all exhaust gas is supplied to the upstream side of the oxidation catalyst 16 during the execution of the catalyst temperature increase control will be described as a comparative example. And in order to make it easy to understand the effect of the sulfur poisoning regeneration control of this embodiment, first, the sulfur poisoning regeneration control of the comparative example will be described with reference to FIG.

  FIG. 2 is a graph showing temporal changes in the temperature (bed temperature) and sulfur deposition amount of the oxidation catalyst 16 and the NSR catalyst 17 in the sulfur poisoning regeneration control, and the upper half shows a comparative example. The lower half shows the case of this embodiment. In the drawings, sulfur is represented by the element symbol “S”.

  The oxidation catalyst 16 uses a basic metal oxide such as alumina as a carrier. For this reason, the oxidation catalyst 16 also has a property of capturing sulfur components in the exhaust gas. That is, a sulfur component is deposited also on the oxidation catalyst 16. However, the oxidation catalyst 16 is weaker than the NSR catalyst 17 in holding the sulfur component.

  In the catalyst temperature increase control of the comparative example, all exhaust gas first flows into the oxidation catalyst 16, so that almost all unburned HC contained in the exhaust gas burns in the oxidation catalyst 16. For this reason, as shown in the upper half of FIG. 2, the temperature of the oxidation catalyst 16 rises first, and the temperature of the NSR catalyst 17 rises later. As described above, since the oxidation catalyst 16 is weak in holding the sulfur component, when the temperature of the oxidation catalyst 16 rises, the sulfur component is quickly desorbed from the oxidation catalyst 16. The sulfur component desorbed from the oxidation catalyst 16 flows into the NSR catalyst 17. At this time, the temperature of the NSR catalyst 17 has not yet sufficiently increased. For this reason, the sulfur component desorbed from the oxidation catalyst 16 and flowing into the NSR catalyst 17 is absorbed by the NSR catalyst 17 and accumulated. Therefore, the sulfur accumulation amount of the NSR catalyst 17 is significantly increased.

  The NSR catalyst 17 has a strong ability to hold a sulfur component. Therefore, the sulfur component is desorbed little by little from the NSR catalyst 17 even during execution of the sulfur desorption control. In the case of the comparative example, as described above, the sulfur deposition amount of the NSR catalyst 17 is greatly increased during the execution of the catalyst temperature increase control. For this reason, it takes a long time to desorb all the sulfur components accumulated in the NSR catalyst 17. That is, long-term sulfur desorption control is required. As a result, there is a problem that fuel consumption deteriorates.

  On the other hand, in the catalyst temperature increase control of this embodiment, exhaust gas containing a large amount of unburned HC is also supplied between the oxidation catalyst 16 and the NSR catalyst 17. For this reason, unburned HC flows directly into the NSR catalyst 17 without passing through the oxidation catalyst 16, so that HC also burns in the NSR catalyst 17. Therefore, as shown in the lower half of FIG. 2, the temperature of the NSR catalyst 17 rises simultaneously with the temperature of the oxidation catalyst 16. Therefore, when the temperature of the oxidation catalyst 16 rises and the sulfur component is desorbed from the oxidation catalyst 16, the temperature of the NSR catalyst 17 also rises sufficiently. For this reason, it is possible to reliably suppress the sulfur component desorbed from the oxidation catalyst 16 and flowing into the NSR catalyst 17 from being absorbed by the NSR catalyst 17. That is, it is possible to reliably suppress an increase in the amount of sulfur accumulated in the NSR catalyst 17 during the execution of the catalyst temperature increase control. As a result, the sulfur desorption control period required until all the sulfur components accumulated in the NSR catalyst 17 are desorbed can be shortened compared to the comparative example, and fuel consumption can be improved.

  FIG. 3 is a flowchart of a routine executed by the ECU 50 in the present embodiment in order to realize the above function. According to the routine shown in FIG. 3, first, it is determined whether or not the sulfur accumulation amount of the NSR catalyst 17 is equal to or greater than a predetermined value (step 100). The ECU 50 sequentially calculates the current sulfur accumulation amount based on the travel distance or the integrated fuel injection amount after executing the previous sulfur poisoning regeneration control. In step 100, if the current sulfur accumulation amount is less than the predetermined value, it can be determined that it is not necessary to execute the sulfur poisoning regeneration control yet. In this case, the execution of this routine is terminated here.

  On the other hand, when the current sulfur accumulation amount is equal to or greater than the predetermined value in step 100, it can be determined that execution of sulfur poisoning regeneration control is necessary. In this case, first, control for causing the secondary turbocharger 12 to run is executed (step 102). As described above, in the present embodiment, both the primary turbocharger 11 and the secondary turbocharger 12 are operated during the catalyst temperature increase control. For this reason, when executing the sulfur poisoning regeneration control, if the secondary turbocharger 12 is stopped, it is necessary to start the secondary turbocharger 12 in advance. In this step 102, the second exhaust gas switching valve 19 is opened to a predetermined opening so that the exhaust gas flows into the second exhaust passage 14 and the secondary turbocharger 12 is allowed to run.

  Next, the required opening of the second exhaust gas switching valve 19 is calculated (step 104). In the catalyst temperature increase control, in order to reliably prevent the sulfur component desorbed from the oxidation catalyst 16 from being absorbed by the NSR catalyst 17, is the temperature of the NSR catalyst 17 increased almost simultaneously with the temperature of the oxidation catalyst 16? Alternatively, it is ideal that the temperature of the NSR catalyst 17 rises faster than the temperature of the oxidation catalyst 16 (see the lower half of FIG. 2). Such an ideal can be realized by adjusting the ratio of the exhaust gas flow rate in the first exhaust passage 14 and the exhaust gas flow rate in the second exhaust passage 15. That is, as the ratio of the exhaust gas flow rate in the second exhaust passage 15 is increased, the temperature of the NSR catalyst 17 can be increased more quickly. The ratio between the exhaust gas flow rate in the first exhaust passage 14 and the exhaust gas flow rate in the second exhaust passage 15 can be controlled by the opening of the second exhaust switching valve 19. The ECU 50 stores in advance a map for calculating the opening degree of the second exhaust gas switching valve 19 that can realize the above ideal according to the operating region of the engine 10. In step 104, the required opening of the second exhaust gas switching valve 19 is calculated according to the map.

  When adjusting the ratio between the exhaust gas flow rate in the first exhaust passage 14 and the exhaust gas flow rate in the second exhaust passage 15, the opening degree of the first exhaust switching valve 18 in addition to the second exhaust switching valve 19. May be further controlled.

  Subsequent to the process of step 104, a process of opening the intake switching valve 33, a process of opening the second exhaust switching valve 19 to the opening calculated in step 104, and a process of starting the catalyst temperature increase control, respectively. It is executed (step 106). As a process for starting the catalyst temperature increase control, a process for adding fuel into the exhaust gas from each of the fuel addition valves 35 and 36 is performed. Thereby, unburned HC is supplied to the oxidation catalyst 16 and the NSR catalyst 17, and the temperature of the oxidation catalyst 16 and the NSR catalyst 17 can be raised. In this embodiment, the unburned HC supply amount to the oxidation catalyst 16 (that is, the fuel addition amount from the fuel addition valve 35) and the unburned HC supply amount to the NSR catalyst 17 (that is, the fuel addition amount from the fuel addition valve 36). ) Can be controlled independently, and the relationship between the temperature increase rate of the oxidation catalyst 16 and the temperature increase rate of the NSR catalyst 17 can also be controlled by adjusting these ratios.

  During the execution of the catalyst temperature increase control, it is determined whether or not the temperature of the NSR catalyst 17 has become equal to or higher than a predetermined value (for example, 650 to 700 ° C.) at which the sulfur component can be desorbed (step 108). The temperature of the NSR catalyst 17 may be detected directly by a catalyst bed temperature sensor, or may be estimated based on the exhaust gas temperature at the catalyst outlet, a history of various parameters, and the like.

  If it is determined in step 108 that the temperature of the NSR catalyst 17 is lower than the predetermined value, the process returns to step 106 and the catalyst temperature increase control is continued. On the other hand, if it is determined in step 108 that the temperature of the NSR catalyst 17 is equal to or higher than the predetermined value, the catalyst temperature increase control is ended (step 110).

  In the present embodiment, during the execution of the sulfur desorption control, the exhaust gas is caused to flow only into the first exhaust passage 14 and only the primary turbocharger 11 is operated. For this reason, in step 110 described above, processing for closing the second exhaust gas switching valve 19 and the intake air switching valve 33 is also executed.

  Subsequently, sulfur desorption control is executed (step 112). In the sulfur desorption control, fuel is added from the fuel addition valve 35 so that the air-fuel ratio of the exhaust gas flowing into the oxidation catalyst 16 becomes richer than the stoichiometric air-fuel ratio.

  When the sulfur desorption control is executed, the exhaust gas containing unburned HC as a reducing agent flows into the NSR catalyst 17 and the inside of the NSR catalyst 17 becomes a rich atmosphere, so it has accumulated in the NSR catalyst 17. Sulfur components can be eliminated. While the sulfur desorption control is being executed, it is determined whether or not the sulfur accumulation amount of the NSR catalyst 17 has become a predetermined value or less (step 114). The ECU 50 can estimate the sulfur accumulation amount of the NSR catalyst 17 based on the elapsed time from the start of the sulfur desorption control, the reducing agent supply amount, and the like. If it is determined in step 114 that the sulfur accumulation amount of the NSR catalyst 17 has become equal to or less than the predetermined value, the sulfur desorption control is terminated.

In this embodiment, during execution of sulfur desorption control, exhaust gas is allowed to flow only into the first exhaust passage 14 and not into the second exhaust passage 15. That is, all exhaust gas passes through the oxidation catalyst 16 during execution of sulfur desorption control. Therefore, all oxygen (O ₂ ) remaining in the exhaust gas reacts with unburned HC and the like in the oxidation catalyst 16 and is consumed without remaining. Therefore, oxygen does not flow into the NSR catalyst 17 during execution of sulfur desorption control. Thereby, the sulfur component accumulated in the NSR catalyst 17 can be desorbed with higher efficiency.

  In the first embodiment described above, the first exhaust passage 14 is the “first exhaust path” in the first invention, and the second exhaust passage 15 is the “second exhaust path” in the first invention. It corresponds. Further, when the ECU 50 executes the processing of the steps 104 and 106, the “passage path control means” in the first invention executes the processing of the step 106, and thus the “catalyst ascentment” in the first invention is executed. When the “temperature means” executes the processing of step 112, the “sulfur desorption means” in the first invention performs the processing of step 110, thereby “passing path control means in the third invention. "Is realized.

  In the system shown in FIG. 1, during the normal operation, as described above, only the primary turbocharger 11 is operated or both the primary turbocharger 11 and the secondary turbocharger 12 are operated according to the operation region of the engine 10. Switch what to do. When the catalyst is warmed up after the engine 10 is started, the load is not so high in many cases. Therefore, usually only the primary turbocharger 11 is operating.

  If only the primary turbocharger 11 is operating when the catalyst is warmed up, all the exhaust gas first passes through the oxidation catalyst 16 and then flows into the NSR catalyst 17. For this reason, since the temperature of the exhaust gas decreases when passing through the oxidation catalyst 16, it takes time to warm up the NSR catalyst 17, and the light-off (activation) of the NSR catalyst 17 tends to be delayed.

  On the other hand, in the system shown in FIG. 1, exhaust gas flows into only the second exhaust passage 15 by closing the first exhaust switching valve 18 and opening the second exhaust switching valve 19, so that only the secondary turbocharger 12 is connected. It can be activated. In this case, all exhaust gas flows directly into the NSR catalyst 17 without passing through the oxidation catalyst 16. In this way, since the high-temperature exhaust gas can be flowed into the NSR catalyst 17, warming up of the NSR catalyst 17 can be promoted.

  Therefore, when the catalyst is warmed up, only the secondary turbocharger 12 may be operated, or the secondary turbocharger 12 may be operated with priority over the primary turbocharger 11. Thereby, the light-off of the NSR catalyst 17 can be accelerated.

  FIG. 4 is a flowchart for realizing the above control. According to the routine shown in FIG. 4, it is first determined whether or not the NSR catalyst 17 is light-off (step 120). In this step 120, the temperature of the NSR catalyst 17 is detected or estimated. If the temperature is equal to or higher than a predetermined value, it is determined that the NSR catalyst 17 is lighted off, otherwise the NSR catalyst 17 is still lighted off. It is determined that it is not.

  If it is determined in step 120 that the NSR catalyst 17 is in the light-off state, it is not necessary to perform the following processing, so the execution of this routine is terminated here.

  On the other hand, if it is determined in step 120 that the NSR catalyst 17 has not yet been turned off, then the required opening of the first exhaust gas switching valve 18 and the required opening of the second exhaust gas switching valve 19 are next. Are calculated (step 122). In this step 122, the required opening degrees of the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19 are calculated so that the secondary turbocharger 12 is operated with priority in order to promote warming up of the NSR catalyst 17. . That is, when the engine load is relatively low and sufficient supercharging can be obtained only with the secondary turbocharger 12, the opening of the first exhaust gas switching valve 18 is set to zero. On the other hand, when the engine load is high and it is not sufficient to supercharge only the secondary turbocharger 12, the opening degree of the first exhaust gas switching valve 18 is calculated so that the primary turbocharger 11 operates by the amount of the shortage. .

  Next, a process of controlling the opening of the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19 to the opening calculated in step 124 and a process of opening the intake air switching valve 33 are executed (step 124). . Here, the second exhaust gas switching valve 19 is set to a large opening, and the first exhaust gas switching valve 18 is fully closed or small. That is, all or most of the exhaust gas passes through the turbine 121 of the secondary turbocharger 12 and flows into the NSR catalyst 17 without passing through the oxidation catalyst 16. For this reason, since the temperature of exhaust gas does not fall with the oxidation catalyst 16, the temperature of the NSR catalyst 17 can be raised early.

  Subsequently, it is determined again whether or not the NSR catalyst 17 is lighted off (step 126). If it is determined in step 126 that the NSR catalyst 17 has not yet been lighted off, the processing from step 122 onward is executed again. On the other hand, if it is determined in step 126 that the NSR catalyst 17 has been turned off, processing for returning the turbocharger switching control to the normal control is executed (step 128).

Next, control when the temperature of the oxidation catalyst 16 is low will be described. The oxidation catalyst 16 carries at least one noble metal including palladium. When the temperature of the oxidation catalyst 16 is sufficiently high, the oxidation catalyst 16 oxidizes NO in the exhaust gas and converts it into NO ₂ . However, when the temperature of the oxidation catalyst 16 is low, conversely, NO ₂ in the exhaust gas is selectively reduced to NO by the oxidation catalyst 16.

As described above, the NSR catalyst 17 carries an alkali metal or an alkaline earth metal as a NOx storage material. In this NOx occlusion material, NO ₂ is easier to occlude than NO. Therefore, towards the case the proportion of NO ₂ in NOx flowing into the NSR catalyst 17 is high, as compared with the case the ratio is low, higher NOx storage performance.

FIG. 5 is a graph showing the relationship between the temperature of the oxidation catalyst 16 and the ratio of NO ₂ in NOx. The vertical axis of FIG. 5 shows the percentage of NO ₂ in NOx after passing through the oxidation catalyst 16, a value obtained by dividing a ratio of NO ₂ in NOx before passing through the oxidation catalyst 16. As shown in the figure, this value increases as the temperature of the oxidation catalyst 16 increases, and decreases as the temperature of the oxidation catalyst 16 decreases. Let T ₁ be the temperature of the oxidation catalyst 16 when this value is 1.0.

When the temperature of the oxidation catalyst 16 is higher than T ₁ , NO is oxidized to NO ₂ by passing through the oxidation catalyst 16, so the ratio of NO ₂ in NOx increases. In such a case, passing the exhaust gas through the oxidation catalyst 16 increases the ratio of NO ₂ in NOx, so that the NOx storage performance of the NSR catalyst 17 can be improved.

On the other hand, when the temperature of the oxidation catalyst 16 is lower than T ₁ , NO ₂ is selectively reduced to NO by passing through the oxidation catalyst 16, so the ratio of NO ₂ in NOx decreases. In such a case, passing the exhaust gas through the oxidation catalyst 16 reduces the ratio of NO ₂ in NOx, so that the NOx occlusion performance of the NSR catalyst 17 is lowered. Therefore, in such a case, higher NOx occlusion performance can be obtained by flowing the exhaust gas directly into the NSR catalyst 17 without passing through the oxidation catalyst 16.

Therefore, when the temperature of the oxidation catalyst 16 is lower than T _1, whether only the secondary turbocharger 12 is operated in order to allow the exhaust gas to flow directly into the NSR catalyst 17 without passing through the oxidation catalyst 16 as much as possible. Alternatively, the secondary turbocharger 12 may be operated in preference to the primary turbocharger 11. Thereby, the NOx occlusion performance of the NSR catalyst 17 can be improved.

FIG. 6 is a flowchart for realizing the above control. According to the routine shown in FIG. 6, it is first determined whether or not the temperature of the oxidation catalyst 16 is equal to or lower than T ₁ (step 130). The temperature of the oxidation catalyst 16 may be directly detected by a catalyst bed temperature sensor, or may be estimated based on the exhaust gas temperature at the catalyst outlet, the history of various parameters, and the like.

If it is determined in step 130 that the temperature of the oxidation catalyst 16 is higher than T ₁ , it is not necessary to perform the following processing, and thus the execution of this routine is terminated here.

On the other hand, if it is determined in step 130 that the temperature of the oxidation catalyst 16 is lower than T ₁ , the required opening of the first exhaust gas switching valve 18 and the required opening of the second exhaust gas switching valve 19 are next. Are calculated (step 132). In step 132, the required opening degrees of the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19 are calculated so that the secondary turbocharger 12 is operated with priority. That is, when the engine load is relatively low and sufficient supercharging can be obtained only with the secondary turbocharger 12, the opening of the first exhaust gas switching valve 18 is set to zero. On the other hand, when the engine load is high and it is not sufficient to supercharge only the secondary turbocharger 12, the opening degree of the first exhaust gas switching valve 18 is calculated so that the primary turbocharger 11 operates as much as the shortage. .

Next, a process of controlling the opening of the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19 to the opening calculated in step 132 and a process of opening the intake air switching valve 33 are executed (step 134). . Here, the second exhaust gas switching valve 19 is set to a large opening, and the first exhaust gas switching valve 18 is fully closed or small. That is, all or most of the exhaust gas passes through the turbine 121 of the secondary turbocharger 12 and flows into the NSR catalyst 17 without passing through the oxidation catalyst 16. For this reason, even if the temperature of the oxidation catalyst 16 is equal to or lower than T ₁ , it is possible to prevent the NO ₂ ratio in NOx from being reduced by the oxidation catalyst 16. Can be prevented.

Subsequently, it is determined whether or not the temperature of the oxidation catalyst 16 has become equal to or higher than T ₁ (step 136). In step 136, when the temperature of the oxidation catalyst 16 is determined to be still less than T _1, the above step 132 following processing is executed again. On the other hand, in step 136, when the temperature of the oxidation catalyst 16 is determined to be above T _1, the process of returning the switching control of the turbocharger to the normal control at the time is executed (step 138).

Embodiment 2. FIG.
Next, the second embodiment of the present invention will be described. The difference from the first embodiment will be mainly described, and the description of the same matters will be simplified or omitted. The present embodiment is the same as the first embodiment except that the content of sulfur desorption control is different.

  In the sulfur desorption control of the present embodiment, the reducing agent is supplied by performing rich combustion in the engine 10. The rich combustion is a control for reducing the intake air amount and making the combustion air-fuel ratio in the cylinder rich below the stoichiometric air-fuel ratio by performing after injection after main injection.

  In rich combustion, a large amount of CO is generated. When this CO flows into the NSR catalyst 17, it functions as an excellent reducing agent that desorbs the sulfur component. That is, when CO is used as the reducing agent, the sulfur component accumulated in the NSR catalyst 17 is desorbed with higher efficiency than when unburned HC supplied mainly from the fuel addition valve 35 is used as the reducing agent. Can be made.

However, in a diesel engine, even when rich combustion is performed, O ₂ (oxygen) is not completely consumed in the cylinder. For this reason, O ₂ remains in the exhaust gas of rich combustion. For this reason, when rich combustion exhaust gas passes through the oxidation catalyst 16, CO is oxidized by O ₂ remaining in the exhaust gas and converted to CO ₂ . For this reason, since the amount of CO flowing into the NSR catalyst 17 decreases, the desorption efficiency of the sulfur component decreases.

  Therefore, in the present embodiment, both the primary turbocharger 11 and the secondary turbocharger 12 are operated during execution of the sulfur desorption control. Thereby, the exhaust gas passing through the turbine 121 of the secondary turbocharger 12 flows directly into the NSR catalyst 17 without passing through the oxidation catalyst 16, so that the amount of CO flowing into the NSR catalyst 17 can be increased. For this reason, the excellent sulfur desorption effect by CO can be fully utilized.

  In the present embodiment, the rich combustion and the lean combustion are alternately repeated in the engine 10 during the execution of the sulfur desorption control. If the air-fuel ratio of the exhaust gas is kept rich, there is no opportunity to purify HC and CO, so the amount of HC and CO released into the atmosphere increases. Therefore, by repeatedly performing rich combustion and lean combustion alternately, HC and CO can be purified during the lean combustion period, and release into the atmosphere can be suppressed.

  However, when lean combustion is performed during execution of sulfur desorption control, the temperature of the NSR catalyst 17 tends to decrease during the lean combustion period. Further, the rich combustion has a weak effect of maintaining the NSR catalyst 17 at a high temperature as compared with the addition of exhaust fuel. This is because, in rich combustion, the exhaust gas cools until the exhaust gas reaches the NSR catalyst 17, whereas in the addition of exhaust fuel, the added fuel is burned by the oxidation catalyst 16, so the NSR catalyst This is because the temperature of the exhaust gas flowing into 17 can be increased. For this reason, in the present embodiment, the temperature of the NSR catalyst 17 is particularly likely to decrease during the lean combustion period.

  Therefore, in the present embodiment, when lean combustion is performed, fuel is added from the fuel addition valve 35 on the upstream side of the oxidation catalyst 16. More specifically, when the lean combustion exhaust gas passes through the fuel addition valve 35, fuel is added to the exhaust gas from the fuel addition valve 35. Thereby, since the fuel added from the fuel addition valve 35 burns in the oxidation catalyst 16, the temperature of the exhaust gas flowing into the NSR catalyst 17 can be increased. For this reason, the temperature drop of the NSR catalyst 17 can be prevented, and can be reliably maintained at a temperature higher than the temperature at which sulfur can be desorbed.

  Since Embodiment 2 is the same as Embodiment 1 except for the points described above, further description is omitted. In the second embodiment, the ECU 50 controls the intake throttle valve 37 and the fuel injection device of the engine 10 during execution of the sulfur desorption control, and repeatedly performs rich combustion and lean combustion alternately. The “combustion control means” in the present invention adds the fuel from the fuel addition valve 35 to the exhaust gas of lean combustion, so that the “fuel addition means” in the fifth invention is The “passage path control means” in the fifth aspect of the present invention is realized by opening the first exhaust gas switching valve 18 and the second exhaust gas switching valve 19.

10 Engine 11 Primary turbocharger 111 Turbine 112 Compressor 12 Secondary turbocharger 121 Turbine 122 Compressor 13 Exhaust passage 14 First exhaust passage 15 Second exhaust passage 16 Oxidation catalyst 17 NSR catalyst 18 First exhaust gas switching valve 19 Second exhaust gas switching valve 24 Air flow meter 25, 26, 27 Intake passage 28 Intercooler 32 Reed valve 33 Intake switching valve 35, 36 Fuel addition valve 37 Intake throttle valve 50 ECU

Claims (5)

An oxidation catalyst,
A NOx catalyst installed downstream of the oxidation catalyst;
A first exhaust path for introducing exhaust gas upstream of the oxidation catalyst;
A second exhaust path for introducing exhaust gas between the oxidation catalyst and the NOx catalyst;
A passage control means for controlling the passage of exhaust gas;
When it is required to recover sulfur poisoning of the NOx catalyst, the temperature of the NOx catalyst is raised by supplying exhaust gas containing unburned HC and oxygen that oxidizes the unburned HC. Catalyst temperature raising means for performing catalyst temperature raising control;
With
The exhaust purification device for an internal combustion engine, wherein the passage path control means causes exhaust gas to flow into both the first exhaust path and the second exhaust path during the execution of the catalyst temperature increase control.   During the execution of the catalyst temperature increase control, the passage path control means increases the temperature of the NOx catalyst almost simultaneously with the temperature of the oxidation catalyst or increases the temperature of the NOx catalyst earlier than the temperature of the oxidation catalyst. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein a ratio between an exhaust gas flow rate in the first exhaust path and an exhaust gas flow rate in the second exhaust path is controlled. After the temperature of the NOx catalyst rises to a temperature at which the sulfur component can be desorbed or higher, sulfur desorption is performed by desorbing the sulfur component from the NOx catalyst by supplying exhaust gas containing a reducing agent to the NOx catalyst. Comprising sulfur desorption means for performing the control,
3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the passage path control means allows exhaust gas to flow only into the first exhaust path during execution of the sulfur desorption control. A first turbocharger having a turbine installed in the middle of the first exhaust path;
A second turbocharger having a turbine installed in the middle of the second exhaust path;
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3, further comprising: After the temperature of the NOx catalyst rises to a temperature at which the sulfur component can be desorbed or higher, sulfur desorption is performed by desorbing the sulfur component from the NOx catalyst by supplying exhaust gas containing a reducing agent to the NOx catalyst. A sulfur desorption means for performing the control;
Combustion control means for alternately and repeatedly switching between rich combustion and lean combustion in the internal combustion engine during execution of the sulfur desorption control;
A fuel addition valve provided in the middle of the first exhaust path;
Fuel addition means for adding fuel from the fuel addition valve to the exhaust gas of the lean combustion during the execution of the sulfur desorption control;
With
2. The exhaust of the internal combustion engine according to claim 1, wherein the passage path control means causes exhaust gas to flow into both the first exhaust path and the second exhaust path during execution of the sulfur desorption control. Purification equipment.

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