JP2009097469A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for increasing a NOx conversion rate by an exhaust emission control system for an internal combustion engine provided with an SCR. <P>SOLUTION: This exhaust emission control system includes the SCR 3 arranged in an exhaust passage 5 of the internal combustion engine 1 and selectively reducing NOx by using NH<SB>3</SB>as a reducing agent, generates NH<SB>3</SB>in exhaust gas by supplying a rich component into the exhaust gas by bank control or a rich spike, and supplies the generated NH<SB>3</SB>to the SCR 3. When a temperature of the exhaust gas made to flow into the SCR 3 exceeds a reference temperature which is controlled based on the upper limit of a temperature at which a NOx reduction reaction using the NH<SB>3</SB>as the reducing agent proceeds in the SCR 3, a channel selector valve 11 changes over an exhaust gas distribution channel to a passage passing through a U-shaped pipe 10. An air-fuel ratio difference between banks is reduced in the latter period of the bank control, and thereby the temperature of the SCR 3 immediately after the termination of the bank control does not exceed a reference temperature. The temperature of the SCR 3 does not exceed the reference temperature, and the NOx reduction reaction in the SCR 3 proceeds favorably. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

As a technique for reducing the amount of nitrogen oxides (NOx) contained in the exhaust gas from an internal combustion engine to the atmosphere, NOx is selectively reduced to N ₂ using ammonia (NH ₃ ) as a reducing agent in the exhaust passage. An exhaust gas purification system including a selective reduction type NOx catalyst (SCR) that is rendered harmless is known. A related technique is described in Patent Document 1.
Japanese Patent Laid-Open No. 10-47041 Japanese Patent Laid-Open No. 2000-18026 JP 2000-265828 A JP-A-8-189388

  An object of the present invention is to increase the NOx purification rate by an exhaust purification system equipped with an SCR.

In order to achieve the above object, an exhaust purification system for an internal combustion engine of the present invention comprises:
An SCR disposed in an exhaust passage of the internal combustion engine for selectively reducing NOx using NH ₃ as a reducing agent;
NH ₃ supply means for supplying NH ₃ to the SCR;
When the NOx reduction reaction takes place in the SCR and NH ₃ supplied by the NH ₃ supply means as a reducing agent, NOx reduction temperature of the SCR is to the NH ₃ supplied by the NH ₃ supply means and a reducing agent Temperature control means for controlling the temperature of the SCR so as to be equal to or lower than a predetermined reference temperature determined based on the upper limit value of the temperature at which the reaction proceeds,
It is characterized by providing.

NOx reduction reactions with a reducing agent to NH ₃ in the SCR is an exothermic reaction, it tends to progress as a low-temperature environment. Conversely, if the temperature of the SCR becomes too high, even if NH ₃ as a reducing agent is supplied to the SCR, the NOx reduction reaction by the NH ₃ is difficult to proceed, and a sufficient NOx purification rate cannot be obtained in the SCR. There is a fear. In this regard, in the exhaust purification system of the present invention, as described above, the temperature control means controls the SCR temperature to be equal to or lower than a predetermined reference temperature. The predetermined reference temperature is a temperature at which the NOx reduction reaction using NH ₃ as a reducing agent suitably proceeds if the SCR temperature is lower than the reference temperature, and the temperature at which the NOx reduction reaction using NH ₃ as a reducing agent advances. Based on the upper limit value, it is obtained in advance through experiments, simulations, or the like. According to the present invention, since the state in which the NOx reduction reaction proceeds suitably in the SCR is maintained, the NOx purification rate by the SCR can be increased.

The NH ₃ supply means may be any means as long as it can supply NH ₃ to the SCR. For example, in the case of a system in which an NOx storage reduction catalyst (NSR) or a three-way catalyst (TWC) is arranged in the exhaust passage upstream from the SCR, hydrocarbon (HC), hydrogen (H ₂ ), etc. by supplying the rich component, since the NOx desorbed from the NOx and NSR in exhaust those rich component is NH ₃ reacts generated, NH ₃ in the SCR that is disposed downstream of these catalysts Can be supplied. Also, by supplying urea water into the exhaust gas upstream from the SCR, a reaction in which the urea water is decomposed and NH ₃ is generated in the high-temperature exhaust gas proceeds, so NH ₃ can be supplied to the SCR. it can.

Here, the "when NOx reduction reaction is carried out in the SCR as a reducing agent and NH ₃ supplied by NH ₃ supply means", for example, and the NOx in the exhaust and NH ₃ supplied by the NH ₃ supply means , in the case of the reaction in the SCR, it means "when the NH ₃ is supplied by the NH ₃ supply means." The SCR has a zeolite structure and has a property of adsorbing NH ₃ . When the NH ₃ NH ₃ supplied by the supply means is adsorbed to the SCR, the NOx flowing into the NH ₃ and the SCR that the adsorbed reacts in SCR after the supply of NH ₃ is performed by the NH ₃ supply means means "even after the supply of NH ₃ is performed by the NH ₃ supply means, when the NOx flows into the SCR." For example, the NOx reduction reaction in SCR has a property that proceeds favorably in a lean environment. Therefore, if the NH ₃ supply means is a means for generating NH ₃ by supplying the rich component in the exhaust gas, when the ambient atmosphere SCR ended supply of the rich component becomes lean, the SCR A NOx reduction reaction is performed.

  In the present invention, the temperature control means may have means for acquiring the temperature of the SCR, and may control the temperature of the SCR so that the acquired temperature is lower than the reference temperature. In this case, the temperature of the SCR may be acquired by any means capable of estimating or measuring the temperature environment condition of the NOx reduction reaction in the SCR. For example, it can be a means for estimating the SCR catalyst bed temperature directly or based on the operating state of the internal combustion engine, or a means for estimating the SCR temperature based on the temperature of the exhaust gas flowing into the SCR.

For example, in the case of a configuration provided with a temperature acquisition means for acquiring the temperature of the exhaust gas flowing into the SCR, it further comprises an exhaust cooling means for cooling the exhaust gas flowing into the SCR,
Said temperature control means, the NH ₃ supplied by the NH ₃ supply means when the NOx reduction reaction takes place in the SCR as a reducing agent, if the temperature acquired by the temperature acquiring unit is higher than the reference temperature, the The exhaust gas flowing into the SCR may be cooled by an exhaust cooling means.

In this configuration, the exhaust cooling means may be any means as long as the exhaust flowing into the SCR can be cooled. For example,
A branch exhaust passage provided so as to branch from the exhaust passage upstream of the SCR and merge again with the exhaust passage upstream of the SCR;
A flow path switching means for switching a flow path of the exhaust flowing upstream from the SCR to a path passing through the branch exhaust passage or a path not passing through the branch exhaust passage;
Further comprising
The branch exhaust passage is such that the amount of heat lost when exhaust flows through a route passing through the branch exhaust passage is larger than the amount of heat lost when exhaust passes through a route not passing through the branch exhaust passage. If configured,
The exhaust cooling means cools the exhaust gas flowing into the SCR by switching the flow path of the exhaust gas flowing upstream from the SCR to a path passing through the branch exhaust passage by the flow path switching means. good.

According to the above configuration, when the temperature of the exhaust gas flowing into the SCR is higher than the reference temperature, the exhaust flow path is switched to the path flowing into the SCR via the branch exhaust passage. When exhaust flows through a route that passes through the branch exhaust passage, more heat is taken away than when the exhaust passes through a route that does not pass through the branch exhaust passage, so that the exhaust is cooled before flowing into the SCR. . Therefore, the SCR temperature can be lowered. By increasing the length of the path that passes through the branch exhaust passage, or by configuring the branch exhaust passage so that the surface area of the inner wall surface of the branch exhaust passage is increased, more heat is circulated through the branch exhaust passage. Can be taken away from the exhaust.

  As the exhaust cooling means, a cooler that cools the exhaust by engine cooling water or air cooling is disposed in the exhaust passage upstream of the SCR, and a bypass passage that bypasses the cooler is provided, and the temperature of the exhaust flowing into the SCR is a reference. When the temperature is higher than the temperature, a route passing through the cooler may be selected as the exhaust flow route, and in other cases, a route passing through the bypass passage may be selected.

By the way, in the exhaust, there is a combustion stroke, and water vapor generated by a reaction between rich components (hydrocarbon (HC), hydrogen (H ₂ ), etc.) and oxygen (O ₂ ) in the exhaust. When the steam is further heated above 100 ° C., it becomes superheated steam with very high reactivity. The NOx reduction reaction in the SCR is activated in the presence of superheated steam. In addition, a decomposition reaction in which NOx in the exhaust gas is decomposed into nitrogen (N ₂ ) and O ₂ in the presence of superheated steam is promoted. Further, the superheated steam generates H ₂ by a water gas shift reaction (CO + H ₂ O → CO ₂ + H ₂ ), and the NOx reduction reaction using the H ₂ as a reducing agent is also promoted.

In order to effectively utilize the NOx purification effect by such superheated steam, in the exhaust purification system of the present invention,
An exhaust temperature raising means for raising the temperature of the exhaust gas flowing into the SCR;
It said temperature control means, the NH ₃ supplied by the NH ₃ supply means when the reduction reaction of NOx is performed in the SCR as a reducing agent, the temperature of the SCR is present water vapor as superheated steam in the SCR If the temperature is lower than a predetermined second reference temperature determined based on the lower limit value of the temperature, the exhaust gas flowing into the SCR may be heated by the exhaust gas temperature raising means.

  By doing so, it is possible to effectively utilize the effect of promoting the NOx reduction reaction and the effect of promoting the NOx decomposition reaction by the superheated steam as described above, and further increase the NOx purification rate of the exhaust purification system. Here, the predetermined second reference temperature is a temperature determined based on the lower limit value of the temperature at which the water vapor exists as superheated water vapor in the SCR, and is obtained in advance by experiments, simulations, or the like.

The SCR has a property of adsorbing NH ₃ and H ₂ O, but the amount of adsorption tends to increase as the temperature of the SCR decreases. According to the present invention, since temperature control is performed so that the temperature of the SCR does not become higher than the reference temperature, the temperature of the SCR is suppressed from becoming excessively high. Therefore, it is possible to more SCR adsorption of _{H 2} O NH ₃ and in the exhaust gas supplied by the NH ₃ supply means, more NOx purification effect due to the various effects of NOx reduction reaction and _{H 2} O by NH ₃ It can be further enhanced.

  Here, the exhaust temperature raising means may be any means as long as the temperature of the exhaust flowing into the SCR can be raised. For example, the temperature of the exhaust gas can be increased by oscillating the air-fuel ratio of the exhaust gas around the target air-fuel ratio. Here, “vibrating the air-fuel ratio” means that the air-fuel ratio is repeatedly changed at a relatively short time interval with a relatively small fluctuation range above and below the air-fuel ratio with a certain air-fuel ratio as the center. . The fluctuation range may be symmetric or asymmetric with respect to the air-fuel ratio that is the center of fluctuation. In addition, the repetition of the fluctuation may be periodic or may have no periodicity. Such control for oscillating the air-fuel ratio is hereinafter referred to as perturbation control.

By performing the perturbation control, the air-fuel ratio on the lean side of the target air-fuel ratio and the exhaust gas on the rich-side air-fuel ratio are mixed. The lean air-fuel ratio exhaust gas contains a relatively large amount of O ₂ , and the rich air-fuel ratio exhaust gas contains a relatively large amount of rich components. Therefore, when these exhaust gases coexist, an oxidation-reduction reaction proceeds between these O ₂ and rich components, and the temperature of the exhaust increases due to the reaction heat. Here, the target air-fuel ratio is the air-fuel ratio of the exhaust gas set according to the operating state of the internal combustion engine, the state of the exhaust purification system, and the like.

  Further, in a configuration in which the exhaust passage is provided with a TWC, perturbation control for oscillating the air-fuel ratio around the air-fuel ratio in the vicinity of the stoichiometry may be performed. In particular, if the vibration range of the air-fuel ratio is set to a range including an air-fuel ratio at which the HC purification rate by the TWC is maximum and an air-fuel ratio at which the CO purification rate is maximum, the HC oxidation reaction in the TWC and the CO Since the reactivity with the oxidation reaction can be enhanced, the exhaust gas temperature raising effect can be further enhanced.

In the present invention, the NH ₃ supply means may be means for generating NH _{3 in} the exhaust gas upstream of the SCR by supplying a rich component in the exhaust gas upstream of the SCR.

When the rich components in the exhaust gas (HC, H ₂ or the like) is supplied, the NOx in the exhaust gas and these rich component is NH ₃ reacts to occur. Since the NOx reduction reaction by NH ₃ in the SCR has a property that proceeds favorably in a lean atmosphere, NH ₃ generated by supplying the rich component into the exhaust is mainly adsorbed on the zeolite structure of the SCR, and the rich component When the supply of exhaust gas to the exhaust gas ends and the SCR becomes a lean atmosphere, the reduction reaction of NOx flowing into the SCR proceeds by the adsorbed NH ₃ . Thus, in this configuration, the "NH ₃ when the NOx reduction reaction is carried out in the SCR as a reducing agent and NH ₃ supplied by the supply means", SCR lean atmosphere supply is terminated to exhaust gas of a rich component It is time to become.

The case where a rich component is supplied to the exhaust gas upstream of the SCR is a case where a rich spike that temporarily controls the air-fuel ratio of the exhaust gas to a rich state is performed during the lean operation of the internal combustion engine in the lean combustion internal combustion engine. Can be illustrated. When the rich spike is executed, the rich component supplied into the exhaust gas by the rich spike reacts with the NOx in the exhaust gas or the NOx released from the NSR to generate NH ₃ . The generated NH ₃ is adsorbed by the SCR, and when the rich spike ends and the normal lean operation is restored, the adsorbed NH ₃ reduces NOx in the exhaust gas. The rich spike is executed when it is necessary to temporarily change the air-fuel ratio of the exhaust to a stoichiometric or rich atmosphere. For example, in an internal combustion engine equipped with NSR, a rich spike is executed when a known NOx desorption reduction process or SOx poisoning recovery process is performed.

In the present invention, in the internal combustion engine having a plurality of cylinders, the NH ₃ supply means sets the air-fuel ratio of some of the plurality of cylinders to an air-fuel ratio leaner than the target air-fuel ratio, and the air-fuel ratios of the other cylinders Can be used as means for performing cylinder-by-cylinder air-fuel ratio control in which the air-fuel ratio richer than the target air-fuel ratio is controlled so that the total air-fuel ratio becomes the target air-fuel ratio. When the cylinder-by-cylinder air-fuel ratio control is executed, more NOx is contained in the exhaust from the cylinder controlled to the lean side air-fuel ratio, and the exhaust from the cylinder controlled to the rich side air-fuel ratio is included Contains more rich ingredients. By mixing these exhaust gases, NOx and rich components react to generate NH ₃ .

  The cylinder-by-cylinder air-fuel ratio control is executed, for example, when the temperature of the exhaust gas is raised during NSR NOx desorption reduction processing, SOx poisoning recovery processing, filter regeneration processing for collecting particulate matter, and the like. The target air-fuel ratio is determined according to the operating state of the internal combustion engine, the state of the exhaust purification system, and the like. For example, in the NSR NOx desorption reduction process and the SOx poisoning recovery process, the target air-fuel ratio control is performed so that the air-fuel ratio (stoichiometric or rich) for desorbing NOx and SOx from the NSR is obtained. Set the fuel ratio.

In the present invention, in the internal combustion engine having a plurality of cylinder groups, the NH ₃ supply means sets the air-fuel ratio of a part of the plurality of cylinder groups to an air-fuel ratio leaner than the target air-fuel ratio, and other cylinders The group air-fuel ratio may be a means for performing air-fuel ratio control for each cylinder group that controls the air-fuel ratio richer than the target air-fuel ratio so that the total air-fuel ratio becomes the target air-fuel ratio. When the air-fuel ratio control for each cylinder group is executed, more NOx is contained in the exhaust from the cylinder group controlled to the lean side air-fuel ratio, and from the cylinder group controlled to the rich side air-fuel ratio. More rich components are contained in the exhaust. By these exhaust gas is mixed, NH ₃ is produced by the reaction of the NOx and the rich components.

  The cylinder group-specific air-fuel ratio control is executed when it is necessary to raise the temperature of the exhaust gas similarly to the cylinder-specific air-fuel ratio control. The target air-fuel ratio is determined according to the operating state of the internal combustion engine, the state of the exhaust purification system, and the like. For example, in bank control when NSR NOx desorption reduction processing or SOx poisoning recovery processing is performed, the target air-fuel ratio is set so that the air-fuel ratio (stoichiometric or rich) for desorbing NOx and SOx from NSR is set. To do.

As described above, the NOx reduction reaction using NH ₃ in the SCR as a reducing agent suitably proceeds in a low temperature environment. However, immediately after the exhaust gas temperature raising control by the cylinder group air-fuel ratio control is finished, the temperature of the exhaust gas is high, so that the NOx reduction reaction in the SCR is difficult to proceed. Therefore, even if NH ₃ as a reducing agent is generated by the air-fuel ratio control for each cylinder group and adsorbed to the SCR, there is a possibility that a sufficient NOx purification rate cannot be obtained in the SCR immediately after the air-fuel ratio control for each cylinder group is finished.

Therefore, in the case where NH ₃ is supplied to the SCR using the cylinder group air-fuel ratio control as the NH ₃ supply means, the temperature control means performs the cylinder group air-fuel ratio in the latter half of the period during which the cylinder group air-fuel ratio control is performed. In the control, the difference between the air-fuel ratio of the cylinder group that is the lean side air-fuel ratio and the air-fuel ratio of the cylinder group that is the rich-side air-fuel ratio (inter-cylinder group air-fuel ratio difference) is performed by the air-fuel ratio control for each cylinder group. It may be made smaller than the air-fuel ratio difference between the cylinder groups in the first half of the period.

  In the air-fuel ratio control for each cylinder group, the exhaust gas temperature increasing effect increases as the air-fuel ratio difference between the cylinder groups increases. Conversely, by reducing the difference, the exhaust gas temperature raising effect by the cylinder group air-fuel ratio control can be reduced. According to the above configuration, the difference in air-fuel ratio between the cylinder groups is reduced in the second half of the cylinder group air-fuel ratio control, so that the exhaust gas temperature raising effect in the second half of the cylinder group air-fuel ratio control is reduced. Therefore, the temperature of the exhaust gas flowing into the SCR after the completion of the cylinder group air-fuel ratio control can be quickly reduced to a temperature equal to or lower than the reference temperature. Accordingly, it is possible to obtain a high NOx purification rate in the SCR after the air-fuel ratio control for each cylinder group.

  Here, the inter-cylinder group air-fuel ratio difference in the latter half of the cylinder group air-fuel ratio control takes into account both the temperature of the SCR after the cylinder group air-fuel ratio control and the purpose of the cylinder group air-fuel ratio control. It is preferable to set. For example, in the air-fuel ratio control by cylinder group for the purpose of increasing the exhaust gas temperature for the SOx poisoning recovery process of NSR, the exhaust gas temperature is as low as possible within a range that does not fall below the lower limit of the temperature at which SOx can be desorbed from NSR. It is preferable to set the air-fuel ratio difference between the cylinder groups in the second half of the cylinder-group air-fuel ratio control so as to be low. By doing so, it is possible to quickly realize a temperature environment in which the NOx reduction reaction in the SCR can proceed favorably after the completion of the air-fuel ratio control for each cylinder group while properly continuing the SOx poisoning recovery process of the NSR. Become.

  Further, in the air-fuel ratio control for each cylinder group, the temperature control means is configured so that the temperature of the exhaust gas becomes lower than a predetermined third reference temperature determined based on the upper limit value of the temperature at which the water gas shift reaction proceeds in the exhaust gas. An inter-air-fuel ratio difference may be set.

The water gas shift reaction is an exothermic reaction in which carbon dioxide (CO ₂ ) and H ₂ are generated from carbon monoxide (CO) and water vapor (H ₂ O) in exhaust gas. This reaction has a property that it is difficult to proceed in an excessively high temperature environment. The third reference temperature is a temperature determined based on the upper limit value of the temperature at which the water gas shift reaction suitably proceeds, and is obtained in advance by experiments, simulations, or the like. According to the above configuration, the inter-cylinder group air-fuel ratio difference in the air-fuel ratio control for each cylinder group is set so that the exhaust gas temperature is lower than the third reference temperature. The water gas shift reaction preferably proceeds. As a result, more H ₂ can be generated in the exhaust gas. Since NH ₃ is generated from this H ₂ and NOx in the exhaust, according to the present invention, more NH ₃ can be generated and supplied to the SCR during the air-fuel ratio control for each cylinder group. Further, since the reaction for reducing NOx by the H ₂ itself as a reducing agent proceeds, the NOx purification rate of the exhaust purification system can be further increased.

In the configuration in which the cylinder group air-fuel ratio control is performed, when each cylinder group has a plurality of cylinders, the NH ₃ supply unit performs the cylinder group air-fuel ratio control for at least one cylinder group among the plurality of cylinder groups. The air-fuel ratio of some cylinders in the cylinder group differs from the air-fuel ratios of other cylinders in the cylinder group, and the total air-fuel ratio of the cylinder group is the cylinder group in the cylinder-group air-fuel ratio control. It is also possible to perform cylinder-by-cylinder air-fuel ratio control that controls the air-fuel ratio to be the target air-fuel ratio.

  For example, among a plurality of cylinders belonging to a cylinder group controlled to a predetermined lean air-fuel ratio by the cylinder group air-fuel ratio control, the air-fuel ratio of some cylinders is set to be slightly leaner than the predetermined lean air-fuel ratio, The air-fuel ratio of the other cylinders in the cylinder group is set to be slightly richer than the predetermined lean air-fuel ratio, and the total air-fuel ratio of the cylinder group is controlled to be the predetermined lean air-fuel ratio. Further, between a plurality of cylinders belonging to the cylinder group, one or a cylinder having an air / fuel ratio slightly leaner than the predetermined lean air / fuel ratio and a cylinder having an air / fuel ratio slightly richer than the predetermined lean air / fuel ratio You may make it switch sequentially for every several cycles.

In this way, by performing cylinder-by-cylinder air-fuel ratio control on the cylinders belonging to each cylinder group in the cylinder group-by-cylinder air-fuel ratio control, a lot of rich components are included even between exhausts from a plurality of cylinders belonging to a certain cylinder group. Exhaust gas and exhaust gas containing a large amount of NOx are mixed, and more NH ₃ can be generated. Therefore, more NH ₃ can be supplied to the SCR during the cylinder group air-fuel ratio control, and the NOx purification rate in the SCR can be further increased after the cylinder group air-fuel ratio control.

  In the present invention, the SCR may be arranged in a position as far away from the internal combustion engine as possible in the exhaust passage. By doing so, the temperature of the exhaust gas flowing into the SCR can be lowered, so that it is possible to more reliably suppress the temperature of the exhaust gas flowing into the SCR from exceeding the reference temperature. Therefore, the NOx purification rate by SCR can be increased.

According to the present invention, it is possible to increase the NOx purification rate by the exhaust purification system provided with the SCR that selectively reduces NOx using NH ₃ as a reducing agent.

  A specific embodiment of an exhaust gas purification system for an internal combustion engine according to the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine and its exhaust system according to the present embodiment. The internal combustion engine 1 is a 6-cylinder V-type engine having a left bank 12 and a right bank 13. The combustion chamber of each cylinder (# 1, # 2, # 3) belonging to the left bank 12 communicates with the left bank exhaust passage 6. The combustion chamber of each cylinder (# 4, # 5, # 6) belonging to the right bank 13 communicates with the right bank exhaust passage 7. The left bank exhaust passage 6 and the right bank exhaust passage 7 join at the junction 14 and are connected to the exhaust passage 5. A three-way catalyst (TWC) 4, an occlusion reduction type NOx catalyst (NSR) 2, and a selective reduction type NOx catalyst (SCR) 3 are arranged in the exhaust passage 5 downstream from the junction 14. The NSR 2 has a function of storing NOx in the exhaust when the air-fuel ratio of the inflowing exhaust gas is lean, and reducing the NOx stored in the presence of a reducing agent when the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich. The SCR ₃ has a function of selectively reducing NOx in the exhaust gas using NH ₃ as a reducing agent. For example, nitric oxide (NO) is reduced to N ₂ by a reaction of 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O. Nitrogen dioxide (NO ₂ ) is reduced to N ₂ by a reaction of 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O. The TWC 4 has a function of reducing NOx in the exhaust and oxidizing HC and CO in the exhaust when the air-fuel ratio of the inflowing exhaust is within a predetermined air-fuel ratio region near the stoichiometric. In addition, an exhaust gas purification device such as a filter that collects particulate matter in the exhaust gas may be provided in the exhaust gas passage 5.

  The exhaust passage 5 upstream of the SCR 3 and downstream of the NSR 2 is provided with a branch portion 15 having two branch points. Connected to the branch portion 15 is a U-shaped tube 10 that connects one branch location of the branch portion 15 to the other branch location. Since the path length of the path that passes through the U-shaped tube 10 is longer than the path length of the path that does not pass through the U-shaped pipe 10, the amount of heat lost when the exhaust gas passes through the path that passes through the U-shaped tube 10 It is greater than the amount of heat lost when passing through a path that does not pass through the tube 10. Therefore, the U-shaped tube 10 of this embodiment corresponds to the branch exhaust passage of the present invention. The branching unit 15 is provided with a flow path switching valve 11. The flow path switching valve 11 is a valve that switches the flow path of the exhaust gas flowing through the exhaust passage 5 to a path that passes through the U-shaped tube 10 or a path that does not pass through the U-shaped tube 10. The flow path switching valve 11 of the present embodiment corresponds to the flow path switching means of the present invention. The exhaust passage 5 immediately before the SCR 3 is provided with a temperature sensor 9 for measuring the exhaust temperature. The temperature sensor 9 of the present embodiment corresponds to the temperature acquisition means of the present invention.

  The internal combustion engine 1 configured as described above is provided with an ECU 8 that is a computer for controlling the internal combustion engine 1. The ECU 8 has a function of controlling the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver. Various sensors other than the temperature sensor 9 are connected to the ECU 8, and output signals from the sensors are input to the ECU 8. Various devices other than the flow path switching valve 11 are connected to the ECU 8, and the operation of each device is controlled by a control signal output from the ECU 8.

When performing the process of releasing and reducing NOx occluded in NSR2 (referred to as NOx desorption reduction process) or the process of desorbing SOx occluded in NSR2 (referred to as SOx poisoning recovery process), NSR2 It is necessary to make the ambient atmosphere of the stoichiometric or rich and raise the temperature of NSR2. In the exhaust purification system of the present embodiment, when the condition for executing the NOx desorption reduction process or the SOx poisoning recovery process is satisfied, the air-fuel ratio of the cylinder belonging to the left bank 12 is set to lean (for example, A / F15). The air-fuel ratio of the cylinders belonging to the right bank 13 is made rich (for example, A / F12), and the air-fuel ratio control is performed so that the air-fuel ratio of the exhaust gas at the junction 14 becomes the air-fuel ratio in the vicinity of the stoichiometric (for example, A / F14.6). Do. Such air-fuel ratio control is hereinafter referred to as bank control. The bank control of the present embodiment corresponds to the cylinder group air-fuel ratio control in the present invention. When the bank control is executed, the exhaust gas containing a relatively large amount of O ₂ is discharged from the cylinders belonging to the left bank 12, and the exhaust gas containing a rich component such as CO or HC is discharged from the cylinders belonging to the right bank 13. . These exhaust gases are merged at the merge section 14, and an oxidation-reduction reaction proceeds between O ₂ and rich components in the exhaust gas in the exhaust passage 5 and the TWC 4, and the temperature of the exhaust gas is increased by the reaction heat. Thereby, the temperature of NSR2 rises. Further, since the total exhaust air-fuel ratio is made slightly richer than stoichiometric, NOx occluded in NSR2 is released, and NOx desorption reduction processing is performed. Further, if the difference between the air-fuel ratio of the left bank 12 and the air-fuel ratio of the right bank 13 (referred to as the inter-bank air-fuel ratio difference) in the bank control is made larger (for example, the air-fuel ratio of the left bank 12 = A / F18, 13), the effect of increasing the exhaust gas temperature by the bank control is increased, and the temperature of the NSR 2 can be increased to such an extent that the SOx poisoning recovery process can be performed.

When you run a bank control, in TWC4 and NSR2, and rich component of HC, _{H 2} or the like contained in the exhaust from the cylinders belonging to the right bank 13, and the NOx released from the NOx and NSR2 in the exhaust gas reacts , NH ₃ is generated. In addition, a reaction in which NH ₃ is synthesized from N ₂ and H ₂ in the exhaust gas also proceeds. That is, in the present embodiment, a device that executes bank control (a fuel injection system (not shown) and an ECU 8 that controls the device) corresponds to the NH ₃ supply means of the present invention. Further, H ₂ O is also generated with the bank control. NH ₃ and H ₂ O generated by the bank control are mainly adsorbed by the SCR ₃ . When the conditions for ending the NOx desorption reduction process and the SOx poisoning recovery process are satisfied and the bank control is completed and the normal operation mode (lean burn mode) is restored, the NOx in the exhaust gas flowing into the SCR3 is converted into the SCR3. It is reduced and purified using NH ₃ adsorbed on the reducing agent.

By the way, the NOx reduction reaction using NH ₃ as a reducing agent in SCR ₃ is an exothermic reaction. Therefore, when the temperature of the SCR ₃ is excessively high, the NOx reduction reaction by NH ₃ hardly proceeds in the SCR ₃ , and there is a possibility that a sufficient NOx purification rate cannot be obtained. Therefore, the system of this embodiment, after the bank control end, the temperature of the exhaust gas flowing into the SCR3 by the temperature sensor 9 measures, if the measured temperature T is higher than the predetermined reference temperature T ₁ (T> T ₁₎ The flow path switching valve 11 switches the exhaust flow path to a path that passes through the U-shaped tube 10. As described above, the exhaust gas is cooled by passing through the U-shaped tube 10 before flowing into the SCR 3. Thus, by making the temperature of the exhaust gas flowing into the SCR ₃ equal to or lower than the reference temperature T ₁ , the NOx reduction reaction using NH ₃ in the SCR ₃ as a reducing agent can be suitably advanced. The purification rate can be increased. Here, the reference temperature T ₁ is a temperature determined based on the upper limit value of the temperature at which the NOx reduction reaction by NH ₃ proceeds favorably in SCR ₃ , and in this embodiment, T ₁ = 300 ° C.

Here, in the exhaust, there is a steam (H ₂ O) generated by a combustion process, a reaction between rich components (HC, H _2, etc.) in the exhaust and O ₂ or the like. When the steam is further heated above 100 ° C., it becomes superheated steam with very high reactivity. The NOx reduction reaction in SCR3 is activated in the presence of superheated steam. Further, the NOx decomposition reaction in which NOx in the exhaust gas is decomposed into N ₂ and O _{2 in} the presence of superheated steam is promoted. Moreover, since superheated steam produces H ₂ by a water gas shift reaction (CO + H ₂ O → CO ₂ + H ₂ ), a NOx reduction reaction using this H ₂ as a reducing agent is also promoted.

In order to effectively utilize such NOx purification effect by superheated steam, in the exhaust purification system of the present embodiment, the temperature of the exhaust gas flowing into the SCR 3 measured by the temperature sensor 9 is excessively low, and the second reference temperature T _2. If below, the temperature of the exhaust gas flowing into the SCR3, and so as to slightly Atsushi Nobori not exceeding the reference temperature T _1. The second reference temperature T ₂ is a temperature determined based on the lower limit value of the temperature at which H ₂ O in the exhaust gas can exist as superheated steam at least in the SCR ₂ , and in this embodiment, T ₂ = 200 ° C. By doing so, H ₂ O suitably functions as superheated steam in the SCR 3, so that it is possible to effectively utilize the various NOx purification effects by the superheated steam as described above.

In this embodiment, when the temperature measured by the temperature sensor 9 is below a second reference temperature T ₂ by executing the control to small vibrations in a relatively short period of air-fuel ratio, performs a Atsushi Nobori of the exhaust I did it. Such air-fuel ratio control is hereinafter referred to as perturbation control. The perturbation control is a control for causing the air-fuel ratio to oscillate between a slightly leaner air-fuel ratio and a slightly richer air-fuel ratio with respect to the target air-fuel ratio at that time. In this embodiment, the perturbation control is performed during the operation in the lean burn mode after the end of the bank control. Therefore, the air-fuel ratio is oscillated in the range of A / F 25 ± 0.3, for example. By executing the perturbation control, the exhaust gas having a slightly rich O ₂ concentration and the exhaust gas having a slightly rich rich component concentration coexist even in the lean exhaust gas. Therefore, the redox between these O ₂ and the rich component is mixed. The exhaust gas is heated by the reaction heat of the reaction. In the present embodiment, a device for performing air-fuel ratio perturbation control (a fuel injection system, an intake air control system, and an ECU 8 for controlling them) not shown corresponds to the exhaust gas temperature raising means in the present invention.

As described above, according to the exhaust gas purification system according to the present embodiment, the lean-burn mode after bank control, the temperature of the exhaust gas flowing into the SCR3 second reference temperature T ₂ higher than the reference temperature T ₁ of the following temperatures Temperature control is performed to enter. This realizes a low temperature environment in which the NOx reduction reaction using NH ₃ as the reducing agent can proceed suitably in SCR3, and a high temperature environment in which the water vapor in the exhaust can exist as superheated water vapor in SCR3. Is done. Therefore, it is possible to achieve an extremely high NOx purification rate. In the present embodiment, ECU 8 that performs control of the channel switching valve 11 and the air-fuel ratio perturbation control to enter the temperature range temperature of T ₂ or T ₁ or less of the exhaust gas flowing into the SCR3 is, the temperature control of the present invention Corresponds to means.

  Here, a specific execution order of the exhaust gas temperature control of the present embodiment will be described with reference to FIG. FIG. 2 is a flowchart showing an exhaust temperature control routine of the present embodiment. This routine is repeatedly executed at predetermined intervals during the operation of the internal combustion engine 1.

  First, in step S101, the ECU 8 determines whether or not a bank control execution condition is satisfied. The bank control execution condition is satisfied when, for example, a condition for executing the NOx desorption reduction process or a condition for executing the SOx poisoning recovery process is satisfied. Known conditions can be used as the execution condition of the NOx desorption reduction process and the execution condition of the SOx poisoning recovery process. If an affirmative determination is made in step S101, the ECU 8 proceeds to step S102. On the other hand, if a negative determination is made in step S101, the ECU 8 proceeds to step S105.

  In step S102, the ECU 8 starts bank control.

  In step S103, the ECU 8 determines whether or not a bank control end condition is satisfied. The bank control end condition is satisfied when, for example, a condition for ending the NOx desorption reduction process or a condition for ending the SOx poisoning recovery process is satisfied. Known conditions can be used as the termination condition of the NOx desorption reduction process and the termination condition of the SOx poisoning recovery process. Step S103 is repeated until an affirmative determination is made.

  In step S104, the ECU 8 ends the bank control and shifts to a normal lean burn mode.

  In step S105, the ECU 8 acquires the temperature T of the exhaust gas flowing into the SCR 3. Specifically, the detection signal of the temperature sensor 9 input to the ECU 8 is read.

In step S106, ECU 8 determines whether or not higher than the temperature T is a reference temperature _{T 1} of acquired in step S105. If an affirmative determination is made in step S106 (T> T ₁ ), the ECU 8 proceeds to step S107. On the other hand, if a negative determination is made in step S106 (T ≦ T ₁ ), the ECU 8 proceeds to step S108.

In step S <b> 107, the ECU 8 switches the exhaust flow path to a path that passes through the U-shaped tube 10 by the flow path switching valve 11.

In step S108, ECU 8, the temperature T acquired in step S105 determines whether lower or not than the second reference temperature _{T 2.} If an affirmative determination is made in step S108 (T <T ₂ ), the ECU 8 proceeds to step S109. On the other hand, when a negative determination is made in step S108 (T ≧ T ₂ ), the ECU 8 ends the execution of this routine.

  In step S109, the ECU 8 performs air-fuel ratio perturbation control.

By executing the above routine, since SCR3 temperature T is feedback controlled to enter the second reference temperature T ₂ higher than the reference temperature T ₁ of less temperature range can be improved NOx purification rate due to SCR3 and Become.

  Next, a second embodiment of the present invention will be described. FIG. 3 is a diagram showing a schematic configuration of the internal combustion engine and its exhaust system according to the present embodiment. Components that are substantially the same as those of the internal combustion engine and its exhaust system of the first embodiment already described with reference to FIG. 1 are denoted by the same names and symbols, and detailed description thereof is omitted. In this embodiment, TWC4, NSR2, and SCR3 are arranged in this order from the upstream side of the exhaust passage 5. A temperature sensor 16 for measuring the catalyst temperature of the TWC 4, a temperature sensor 17 for measuring the catalyst temperature of the NSR 2, and a temperature sensor 18 for measuring the catalyst temperature of the SCR 3 are provided. Each of these temperature sensors may be a sensor that measures the temperature of the exhaust gas flowing into each catalyst, similarly to the temperature sensor 9 of Example 1, or a sensor that measures the catalyst bed temperature of each catalyst. There may be. In the present embodiment, the SCR 3 is arranged at a position farther from the internal combustion engine 1 than the TWC 4 and the NSR 2.

In the present embodiment, as in the first embodiment, bank control is performed when it is necessary to raise the temperature of the exhaust gas during the NOx desorption reduction process or the SOx poisoning recovery process of NSR2. Hereinafter, a case where the temperature of exhaust gas is raised during the SOx poisoning recovery process will be described. In the SOx poisoning recovery process, it is necessary to raise the temperature of NSR2 to a very high temperature of 600 to 700 ° C. Therefore, immediately after the bank control is finished, the temperature of the exhaust gas is very high, and the temperature of the SCR 3 tends to exceed the reference temperature T ₁ described in the first embodiment. Therefore, immediately after the end of bank control, the NOx reduction reaction in the SCR 3 does not proceed favorably, and the NOx purification rate by the SCR 3 may decrease.

  Therefore, in this embodiment, the period during which bank control is performed is divided into three periods, the first period, the middle period, and the second period, and the control mode of bank control in each period is made different. Specifically, in the first half of bank control, the inter-bank air-fuel ratio difference is further increased in order to raise the temperature of NSR 2 more reliably. For example, the temperature target value of TWC4 is 600 ° C., the temperature target value of NSR2 is 700 ° C., the air / fuel ratio of the left bank 12 controlled to a lean air / fuel ratio is A / F18, and the right bank 13 is controlled to a rich air / fuel ratio. Bank control is performed by setting the air-fuel ratio to A / F10. Thus, by increasing the inter-bank air-fuel ratio difference, the effect of exhaust gas temperature increase by bank control is increased, and the temperature of NSR2 is increased to a temperature at which SOx poisoning recovery processing can be performed at an early stage, and the SOx from NSR2 is increased. The elimination reaction can be activated.

Next, in the middle period of bank control, control is performed to vary the air-fuel ratio between cylinders in each bank in order to increase the amount of NH ₃ generated by bank control. For example, the temperature target value of TWC4 is 580 ° C., the temperature target value of NSR2 is 670 ° C., the air / fuel ratio of the left bank 12 controlled to a lean air / fuel ratio is A / F18, and the right bank 13 is controlled to a rich air / fuel ratio. The air-fuel ratio is set to A / F10.

Further, the air-fuel ratio of any one of the cylinders # 1, # 2, and # 3 in the left bank 12 is set to A.
The air / fuel ratio A / F 17.6 is slightly richer than / F18, the air / fuel ratio of the other two cylinders is A / F18.2 slightly leaner than A / F18, and the left bank 12 total is the air / fuel ratio A / F18. The lean side air-fuel ratio and the rich side air-fuel ratio are mixed between the cylinders constituting the left bank 12. Then, the cylinders to be set to be slightly rich in the air-fuel ratio among the cylinders in the left bank 12 are selected in order from # 1, # 2, and # 3 every one or a plurality of cycles.

  In addition, the air-fuel ratio of one of the cylinders # 4, # 5, and # 6 in the right bank 13 is set to an air-fuel ratio A / F10.4 that is slightly leaner than A / F10, and the air-fuel ratios of the other two cylinders The A / F 9.8 is slightly richer than the A / F 10, and the right bank 13 total is the air-fuel ratio A / F 10, but the rich air-fuel ratio and the lean air-fuel ratio are different between the cylinders constituting the right bank 13. Try to mix. Then, among the cylinders in the right bank 13, a cylinder to be a slightly lean air-fuel ratio is selected in order from # 4, # 5, and # 6 every one or a plurality of cycles.

By performing such control, even if the exhaust gas is from the cylinders belonging to the same bank, the exhaust gas from the cylinder having a slightly rich air-fuel ratio contains a larger amount of H ₂ , and the lean air-fuel ratio is slightly higher. The exhaust from the cylinder having the fuel ratio contains more NOx. These exhaust gases are mixed in the exhaust manifolds (not shown) and the bank exhaust passages 6 and 7 of each bank, whereby H ₂ and NOx react to generate more NH ₃ .

Further, in this case, the effect of raising the temperature of the exhaust gas is suppressed to some extent, so that the condition that the water gas shift reaction (CO + H ₂ O → CO ₂ + H ₂ ) proceeds favorably in the exhaust gas is satisfied, and H ₂ is also generated. The generation of NH ₃ in the exhaust is promoted by H ₂ generated by the water gas shift reaction. The temperature at which the water gas shift reaction suitably proceeds is generally about 200-400 ° C. However, according to the study of the applicant, the water gas shift reaction suitably proceeds to a temperature around 600 ° C. in the configuration of this example. It was found to be. Therefore, the temperature target value of the TWC 4 in the middle of the bank control is set to 580 ° C. The generated NH ₃ is adsorbed on the SCR _{3 and} functions as a reducing agent that reduces NOx flowing into the SCR 3 during operation in the lean burn mode after the end of the bank control. Therefore, the increase in the amount of NH ₃ generated increases the NOx purification by the SCR 3 The effect can be enhanced.

Next, in the second half of the bank control, the inter-bank air-fuel ratio is set so that the temperature environment of the SCR ₃ after the bank control is finished satisfies the conditions that allow the oxidation-reduction reaction between NH ₃ and NOx to proceed appropriately. Reduce the difference. Here, the inter-bank air-fuel ratio difference is determined such that the temperature of NSR2 does not fall below the lower limit value of the temperature at which the SOx desorption reaction from NSR2 can proceed. For example, the temperature target value of NSR2 is 650 ° C., the temperature target value of SCR 3 is 300 ° C. of the reference temperature, the air / fuel ratio of the left bank 12 controlled to a lean air / fuel ratio is A / F15, and the right air / fuel ratio is controlled to a rich air / fuel ratio. Bank control is performed by setting the air-fuel ratio of the bank 13 to A / F12. In this way, by reducing the inter-bank air-fuel ratio difference, the exhaust gas temperature raising effect by the bank control can be suppressed. Therefore, immediately after the bank control is completed, the temperature of the SCR ₃ can be lowered to a reference temperature or lower at which the NOx reduction reaction by NH ₃ proceeds favorably. Further, in the configuration of the present embodiment, since the SCR 3 is disposed at a position farther from the internal combustion engine 1 than the NSR 2 and the TWC 4, the temperature of the exhaust gas is easily cooled in the process of flowing through the exhaust passage 5 upstream from the SCR 3. The SCR 3 can be more reliably lowered to a reference temperature or lower.

The bank control execution period is determined as a period during which SOx occluded in NSR2 can be completely desorbed from NSR2. For example, it can be determined based on the total amount of fuel injection since the previous SOx poisoning recovery process was executed. Further, the length of the bank control late period in which the inter-bank air-fuel ratio difference is reduced is that the temperature of the SCR 3 is reduced to a temperature equal to or lower than the reference temperature T ₁ between the start of the bank control late period and the end of the bank control. The period that can be reduced can be obtained by experiments or the like.

As described above, in the bank control of this embodiment, the exhaust gas temperature rising effect is suppressed and the temperature of the SCR ₃ is lowered in the latter half of the bank control. Therefore, immediately after the bank control is finished, the SCR ₃ is quickly converted into NOx by NH _3. reduction reaction can be suitably proceeds possible reference temperature T ₁ of the following low temperature. This makes it possible to increase the NOx purification rate by the SCR 3 even in a system that raises the exhaust gas temperature for SOx poisoning recovery processing and NOx desorption reduction processing by bank control.

  Here, a specific execution procedure of the bank control of the present embodiment will be described with reference to FIG. FIG. 4 is a flowchart showing the bank control routine of this embodiment. This routine is repeatedly executed at predetermined intervals during the operation of the internal combustion engine 1.

  First, in step S201, the ECU 8 determines whether or not a bank control execution condition is satisfied. In this embodiment, the bank control execution condition is satisfied when the condition for executing the SOx poisoning recovery process is satisfied. Known conditions can be used as conditions for executing the SOx poisoning recovery process. For example, the establishment of the execution condition for the SOx poisoning recovery process is determined based on the total amount of fuel injection since the previous SOx poisoning recovery process was executed. If an affirmative determination is made in step S201, the ECU 8 proceeds to step S202. On the other hand, if a negative determination is made in step S201, the ECU 8 once ends the execution of this routine.

  In step S202, the ECU 8 starts the previous bank control. The bank control is control performed in the first half of the bank control described above. That is, the target temperature of TWC4 is set to 600 ° C., the target temperature of NSR2 is set to 700 ° C., the air / fuel ratio of the left bank is set to A / F18, and the air / fuel ratio of the right bank is set to A / F10. Feedback control is performed so that the target temperature becomes the target temperature. In the temperature feedback control, in addition to the inter-bank air-fuel ratio difference in bank control, known exhaust temperature control means such as ignition timing control may be used in combination.

In step S203, ECU 8 is the elapsed time Delta] t ₁ from when the previous year bank control is started is determined whether reaches the first period Delta] t _10. The first period Δt ₁₀ includes each of the SOx occlusion amount in the NSR 2, the first-term bank control, the middle-term bank control (control performed during the above-described bank control middle period), and the second-stage bank control (control performed during the above-described latter bank control). This is determined based on the easiness of the SOx desorption reaction (SOx desorption rate) and the like. The ECU 8 continues the previous bank control until an affirmative determination is made in step S203. If an affirmative determination is made in step S203, the ECU 8 ends the previous bank control and proceeds to step S204.

  In step S204, the ECU 8 starts mid-term bank control. That is, the target temperature of TWC4 is set to 580 ° C., the target temperature of NSR2 is set to 670 ° C., the air / fuel ratio of the left bank is set to A / F18, the air / fuel ratio of the right bank is set to A / F10, Change the fuel ratio. Then, feedback control is performed so that the temperatures measured by the temperature sensors 16 and 17 become the target temperatures. In the temperature feedback control, in addition to the inter-bank air-fuel ratio difference in the bank control and the inter-cylinder air-fuel ratio difference in the bank, known exhaust temperature control means such as ignition timing control may be used in combination.

In step S205, ECU 8 is the elapsed time Delta] t ₂ from medium-term bank control is started is determined whether reaches the second period Delta] t _20. The second period Δt ₂₀ is determined in the same manner as the first period Δt ₁₀ . The ECU 8 continues the mid-term bank control until an affirmative determination is made in step S205. If an affirmative determination is made in step S205, the ECU 8 ends the medium-term bank control and proceeds to step S206.

  In step S206, the ECU 8 starts late bank control. That is, the target temperature of NSR2 is set to 650 ° C., the target temperature of SCR 3 is set to 300 ° C., the air / fuel ratio of the left bank is set to A / F15, and the air / fuel ratio of the right bank is set to A / F12. Feedback control is performed so that the target temperature becomes the target temperature. In the temperature feedback control, in addition to the inter-bank air-fuel ratio difference in the bank control, known exhaust temperature raising control means such as ignition timing control may be used in combination.

In step S207, ECU 8 determines whether the elapsed time Delta] t ₃ from late bank control is started has reached a third period Delta] t _30. In the third period Δt ₃₀ , in addition to the SOx occlusion amount in the NSR 2 and the SOx desorption rate in each bank control period as in the first period Δt ₁₀ and the second period Δt ₂₀ , the temperature of the SCR 3 becomes the reference temperature T ₁ . It is determined on the basis of the time required to decrease. The ECU 8 continues the late bank control until an affirmative determination is made in step S207. If an affirmative determination is made in step S207, the ECU 8 proceeds to step S208 and ends the bank control.

By executing the above routine, a low temperature environment in which the NOx reduction reaction by NH ₃ can proceed appropriately in SCR ₃ is realized early in SCR ₃ even immediately after the end of bank control, so that the NOx purification rate by SCR 3 is increased. Is possible.

  The above-described embodiment is an example for explaining the present invention, and various modifications can be made to the above-described embodiment without departing from the gist of the present invention.

For example, in the above embodiment, the embodiment in which the present invention is applied to a 6-cylinder V-type engine has been described. However, the present invention can be applied regardless of the number of cylinders and the arrangement form. For example, in the case of a straight engine, to generate NH ₃ in the exhaust gas by supplying the rich components in the exhaust by performing the air-fuel ratio control for varying the air-fuel ratio between the rich spike or cylinders in place of the bank control, The generated NH ₃ can be supplied to the SCR ₃ . For example, if the flow path switching valve 11 is switched according to the exhaust temperature measured by the temperature sensor 9 during the lean operation after the rich spike, the temperature of the exhaust gas flowing into the SCR 3 does not exceed the reference temperature. The temperature of the SCR 3 can be controlled.

  Further, the path length from the U-shaped tube 10 or the branching portion 15 to the SCR 3 is preferably extended or shortened so that the temperature of the SCR 3 does not exceed the reference temperature to be adapted to the target NOx purification rate.

  Further, in the above embodiment, when the temperature of the exhaust gas flowing into the SCR 3 exceeds the reference temperature, the exhaust gas is switched by switching the exhaust flow path to the path passing through the U-shaped tube 10 by the flow path switching valve 11. Although cooling is performed, the means for cooling the exhaust is not limited to this.

  For example, the U-shaped pipe 10 is a branch passage provided so as to branch at a location where the exhaust passage 5 is upstream from the SCR 3 and return to the location again, but branches at a location where the exhaust passage 5 is upstream from the SCR 3 and branches to the SCR 3. A branch passage provided so as to return to another place in the upstream exhaust passage 5, where the exhaust gas passes through a route where the amount of heat lost when the exhaust passes through the route passing through the branch passage does not pass through the branch passage. A branch passage larger than the amount of heat lost when passing through, a device capable of switching the flow path of the exhaust to either a path passing through the branch passage or a path not passing through the branch passage, and for the exhaust flowing into the SCR 3 When the temperature exceeds the reference temperature, the exhaust gas can be cooled by switching the flow path of the exhaust gas to a path passing through the branch passage by the device.

In addition, for example, an exhaust gas cooler that cools the exhaust gas by engine cooling water, air cooling, or the like is disposed in the exhaust passage 5 upstream from the SCR 3, and a bypass passage that bypasses the exhaust gas cooler is provided, and the exhaust passage is routed through the bypass passage. A switching valve that switches to a path that passes through or a path that passes through the exhaust gas cooler is provided, and when the temperature of the exhaust gas flowing into the SCR 3 exceeds a reference temperature, the switching valve switches the exhaust flow path to a path that passes through the exhaust gas cooler. Anyway. Any other means may be used as long as it has a function of cooling the exhaust.

  In the above embodiment, when the temperature of the exhaust gas flowing into the SCR 3 is lower than the second reference temperature, the exhaust gas is heated by performing the air-fuel ratio perturbation control, but the exhaust gas is heated. The means is not limited to this. For example, the exhaust gas may be heated using known exhaust gas temperature raising control such as ignition timing retard control, fuel injection control such as multi-injection, sub-injection, and injection timing, and EGR control.

  In the above embodiment, the temperature of the exhaust gas flowing into the SCR 3 is measured by the temperature sensor 9, and the temperature is controlled so that the measured temperature falls within a predetermined range (a temperature range between the second reference temperature and the reference temperature). Although the example has been described, the temperature of the exhaust gas flowing into the SCR 3 may be acquired by estimation based on, for example, the operating state of the internal combustion engine 1. Further, a sensor that directly measures the catalyst bed temperature of the SCR 3 may be provided to directly acquire the temperature environment of the NOx reduction reaction in the SCR 3.

1 is a diagram illustrating a schematic configuration of an internal combustion engine and an exhaust system of an exhaust purification system according to Embodiment 1. FIG. 3 is a flowchart illustrating a routine for exhaust gas temperature control in the first embodiment. It is a figure showing schematic structure of the internal combustion engine of the exhaust gas purification system which concerns on Example 2, and its exhaust system. 10 is a flowchart illustrating a bank control routine according to the second embodiment.

Explanation of symbols

1 Internal combustion engine 2 NOx storage reduction catalyst (NSR)
3 Selective reduction type NOx catalyst (SCR)
4 Three-way catalyst (TWC)
5 Exhaust passage 6 Left bank exhaust passage 7 Right bank exhaust passage 8 ECU
9 Temperature sensor 10 U-shaped tube 11 Flow path switching valve 12 Left bank 13 Right bank 14 Merge section 15 Branch section

Claims (7)

Disposed in the exhaust passage of the internal combustion engine, a NOx selective reduction catalyst that selectively reduces NOx and NH ₃ as a reducing agent,
And NH ₃ supply means for supplying NH ₃ to the selective reduction type NOx catalyst,
The NH ₃ supplied by the NH ₃ supply means when the NOx reduction reaction is performed in the NOx selective reduction catalyst as a reducing agent, the temperature of the selective reduction type NOx catalyst, supplied by the NH ₃ supply means NH Temperature control means for controlling the temperature of the selective reduction type NOx catalyst so as to be equal to or lower than a predetermined reference temperature determined based on an upper limit value of the temperature at which the NOx reduction reaction using ₃ as a reducing agent proceeds
An exhaust gas purification system for an internal combustion engine, comprising: In claim 1,
Temperature acquisition means for acquiring the temperature of the exhaust gas flowing into the selective reduction type NOx catalyst;
An exhaust cooling means for cooling the exhaust flowing into the selective reduction type NOx catalyst;
With
The temperature control means cools the exhaust gas flowing into the selective reduction type NOx catalyst by the exhaust cooling means when the temperature acquired by the temperature acquisition means is higher than the reference temperature. Purification system. In claim 2,
A branched exhaust passage provided so as to branch from an exhaust passage upstream from the selective reduction type NOx catalyst and rejoin the exhaust passage upstream from the selective reduction type NOx catalyst;
A flow path switching means for switching a flow path of exhaust flowing upstream from the selective reduction type NOx catalyst to a path passing through the branch exhaust passage or a path not passing through the branch exhaust passage;
Further comprising
The branch exhaust passage is such that the amount of heat lost when exhaust flows through a route passing through the branch exhaust passage is larger than the amount of heat lost when exhaust passes through a route not passing through the branch exhaust passage. Configured,
The exhaust cooling means flows into the selective reduction NOx catalyst by switching a flow path of exhaust flowing upstream from the selective reduction NOx catalyst to a path passing through the branch exhaust passage by the flow path switching means. An exhaust gas purification system for an internal combustion engine, wherein the exhaust gas is cooled. In claim 2 or 3,
Exhaust temperature raising means for raising the temperature of the exhaust gas flowing into the selective reduction type NOx catalyst,
Said temperature control means, the NH ₃ supplied by the NH ₃ supply means when the reduction reaction of NOx is performed in the NOx selective reduction catalyst as a reducing agent, the temperature of the selective reduction type NOx catalyst, the selective reduction When the temperature of the NOx catalyst is lower than a predetermined second reference temperature determined based on the lower limit value of the temperature at which the water vapor exists as superheated water vapor, the exhaust gas temperature riser raises the exhaust flowing into the selective reduction NOx catalyst. An exhaust purification system for an internal combustion engine characterized by heating. In claim 4,
The exhaust gas purifying system for an internal combustion engine, wherein the exhaust gas temperature raising means raises the temperature of the exhaust gas flowing into the selective reduction type NOx catalyst by oscillating the air fuel ratio of the exhaust gas around the target air fuel ratio. In any one of Claims 1 thru | or 5,
The internal combustion engine has a plurality of cylinder groups, and the air-fuel ratio of some cylinder groups among the plurality of cylinder groups is set to an air-fuel ratio leaner than the target air-fuel ratio, and the cylinders of other cylinder groups are empty. Means for performing air-fuel ratio control for each cylinder group for controlling the air-fuel ratio to be richer than the target air-fuel ratio and controlling the total air-fuel ratio to be the target air-fuel ratio;
In the second half of the period during which the cylinder group air-fuel ratio control is performed, the temperature control means calculates the difference between the lean side air-fuel ratio and the rich side air-fuel ratio in the cylinder group air-fuel ratio control. An exhaust gas purification system for an internal combustion engine, characterized in that it is made smaller than the difference between the lean side air-fuel ratio and the rich side air-fuel ratio in the first half of the period during which the separate air-fuel ratio control is performed. In claim 6,
The temperature control means is configured to control the lean side in the cylinder group air-fuel ratio control so that the temperature of the exhaust gas is lower than a predetermined third reference temperature determined based on an upper limit value of the temperature at which the water gas shift reaction proceeds in the exhaust gas. An exhaust gas purification system for an internal combustion engine, wherein a difference between the air-fuel ratio of the engine and the rich-side air-fuel ratio is set.

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