JP2010242658A - System for raising temperature of exhaust emission control catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a temperature raising system suppressing deterioration of exhaust emission when burning fuel added to exhaust gas flowing in an exhaust passage during a request for raising temperature of an exhaust emission control catalyst. <P>SOLUTION: When the temperature rise of a DPNR (Diesel Particulate NOx Reduction) catalyst 4 is requested, fuel combustion control is performed so that fuel added by a fuel adding valve 5 is burnt by ignition of a glow plug 6. A small-diameter catalyst 7 is so disposed that combustion gas generated in the fuel combustion control and fuel contained in the combustion gas are introduced. Fuel is added by the fuel adding valve 5 so that when the fuel combustion control is performed, an air-fuel ratio of exhaust gas passing through the small-diameter catalyst 7 is made to be rich, and an air-fuel ratio of the whole exhaust gas led to flow into the DPNR catalyst 4 is made to be lean. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a temperature raising system for an exhaust purification catalyst that purifies exhaust gas from an internal combustion engine.

  As a technique for raising the temperature of an exhaust purification catalyst for purifying exhaust gas of an internal combustion engine, a technique for raising the temperature of exhaust gas flowing in by a combustion burner device is known. Patent Document 1 discloses a configuration in which a burner device including an injector that injects fuel and an ignition device (glow plug) for the fuel is arranged on the upstream side of the exhaust purification device.

Japanese Patent Publication No. 3-10845

  Here, a fuel addition valve and an ignition device are arranged in the exhaust passage upstream of the exhaust purification catalyst, and fuel combustion control is performed so that the fuel added by the fuel addition valve is combusted by ignition by the ignition device when the temperature increase of the exhaust purification catalyst is requested. Consider a temperature raising system that executes and supplies the generated high-temperature combustion gas to an exhaust purification catalyst. When the fuel combustion control is executed, if rich combustion is performed with the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst as a rich air-fuel ratio (hereinafter referred to as “total air-fuel ratio”), it is included in the inflowing exhaust gas. There is a possibility that the HC / CO is not purified by the exhaust purification catalyst but slips downstream and the emission deteriorates. On the other hand, when performing lean combustion with the entire air-fuel ratio being the lean air-fuel ratio when executing the fuel combustion control, it is expected that a large amount of NOx is generated as a contradiction.

  The present invention has been made in view of the above circumstances, and its purpose is to prevent exhaust emission from deteriorating when the fuel added to the exhaust gas flowing through the exhaust passage is combusted when a temperature increase request for the exhaust purification catalyst is required. It is to provide a temperature rising system that can be suppressed.

  In order to solve the above-described problems, the temperature raising system for an exhaust purification catalyst according to the present invention employs the following means.

  That is, an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine, a fuel addition valve provided in an exhaust passage upstream of the exhaust purification catalyst, for adding fuel to exhaust gas flowing through the exhaust passage, An ignition device provided in an exhaust passage upstream of the exhaust purification catalyst, ignites the fuel added from the fuel addition valve, and downstream of the fuel addition valve and ignition device and upstream of the exhaust purification catalyst And a pre-stage shunt catalyst having NOx reduction ability that leads only part of the exhaust gas flowing through the exhaust passage, and when the temperature of the exhaust purification catalyst is raised by ignition by the ignition device Control means for performing fuel combustion control for combusting the fuel added by the fuel addition valve, and the pre-stage shunt catalyst includes the combustion gas generated in the fuel combustion control and The control means is arranged so that fuel contained in the combustion gas is introduced, and when the fuel combustion control is performed, the control means performs an air-fuel ratio of the exhaust gas that passes through the pre-stage shunt catalyst (hereinafter referred to as “divided gas air-fuel ratio”). Is a rich air-fuel ratio, and fuel is added to the fuel addition valve so that the entire air-fuel ratio (total air-fuel ratio) flowing into the exhaust purification catalyst becomes a lean air-fuel ratio. .

  When the fuel combustion control is performed, lean combustion is performed in which the overall air-fuel ratio becomes the lean air-fuel ratio, so that HC / CO contained in the exhaust gas is oxidized in the exhaust purification catalyst. Therefore, it is possible to prevent HC / CO contained in the exhaust gas from being discharged into the atmosphere without being purified.

  Further, when only a part of the exhaust gas flowing through the exhaust passage is guided to the upstream branching catalyst, it is arranged so that the combustion gas generated in the fuel combustion control and the fuel contained in the combustion gas are introduced. Therefore, the air-fuel ratio of the exhaust gas that passes through the front-stage shunt catalyst during fuel combustion control can be relatively lowered as compared with the exhaust gas that bypasses the front-stage shunt catalyst. As a result, it is possible to add the fuel from the fuel addition valve so that the split gas air-fuel ratio becomes a rich air-fuel ratio while performing lean combustion during fuel combustion control. As a result, during the fuel combustion control, the inside of the upstream branch catalyst can be made a reducing atmosphere. Therefore, the NOx generated by the lean combustion can be reduced and purified by the upstream branch catalyst.

  Note that the NOx to be purified in the front-stage branching catalyst includes NOx generated in the combustion chamber during engine combustion, in addition to NOx generated by lean combustion in fuel combustion control. According to the present invention, NOx generated in the combustion chamber of the internal combustion engine and discharged into the exhaust passage can be purified by the pre-stage shunt catalyst, similarly to NOx generated by lean combustion in the fuel combustion control.

  Here, the NOx purification performance of the upstream branch catalyst is affected by the flow rate of the exhaust gas passing through the upstream branch catalyst. If the other conditions are the same, the greater the amount of exhaust gas discharged from the internal combustion engine, the higher the flow rate of the exhaust gas that passes through the upstream branch catalyst, and thus the NOx purification performance tends to decrease. Therefore, in the present invention, the control means is configured so that the bed temperature of the upstream branch catalyst during fuel combustion control is higher when the amount of exhaust gas discharged from the internal combustion engine is large than when the amount is small. It is preferable to add fuel to the fuel addition valve. Further, the control means may add the fuel to the fuel addition valve such that the larger the amount of exhaust gas discharged from the internal combustion engine, the higher the bed temperature of the upstream branching catalyst at the time of fuel combustion control.

  If fuel addition is performed by the fuel addition valve so that the bed temperature of the upstream branch catalyst becomes higher, the passage resistance of the exhaust gas passing through the upstream branch catalyst is further increased. As a result, it becomes difficult for the exhaust gas to flow into the upstream branch catalyst, and the amount of exhaust gas that bypasses the upstream branch catalyst increases. As a result, the flow rate of the exhaust gas that passes through the upstream branch catalyst decreases. According to this, even when the amount of exhaust gas discharged from the internal combustion engine is large, it is possible to suppress the NOx purification performance from being lowered. That is, the NOx purification performance can always be maintained at a high level regardless of the increase or decrease in the amount of exhaust gas from the internal combustion engine.

  ADVANTAGE OF THE INVENTION According to this invention, when burning the fuel added to the exhaust gas which flows through an exhaust passage at the time of the temperature increase request | requirement of an exhaust gas purification catalyst, the temperature increase system which can suppress that exhaust emission deteriorates can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which a temperature raising system for an exhaust purification catalyst in Embodiment 1 is applied and its exhaust system. It is explanatory drawing explaining the structure of a fuel addition valve, a glow plug, and a small diameter catalyst. It is the flowchart figure which showed the fuel combustion control routine. It is the map which illustrated the relationship between the exhaust gas amount Ga at the time of fuel combustion control, and the small diameter catalyst bed temperature THc.

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

<Example 1>
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which a temperature raising system for an exhaust purification catalyst in the present embodiment is applied and its exhaust system. The internal combustion engine 1 is a diesel engine for driving a vehicle. An exhaust passage 3 through which exhaust gas from the combustion chamber of the internal combustion engine 1 is discharged is connected to the internal combustion engine 1.

  The exhaust passage 3 is connected to a muffler (not shown), and in the middle of the exhaust passage 3, a NOx storage reduction catalyst (hereinafter referred to as “hereinafter referred to as“ NOx catalyst ”) is attached to a filter that removes particulate matter (PM) such as soot contained in the exhaust gas. A DPNR (Diesel Particulate NOx Reduction) catalyst is provided, which is configured to carry a "NOx catalyst". The NOx catalyst carried on the DPNR catalyst 4 occludes NOx when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio. Further, the NOx catalyst releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas decreases to a stoichiometric or rich air-fuel ratio, and the NOx is reduced by the presence of reducing components (for example, fuel) around it. To do. In this embodiment, the NOx catalyst 4 corresponds to the exhaust purification catalyst in the present invention.

  A fuel addition valve 5 is provided in the exhaust passage 3 upstream of the DPNR catalyst 4 to add fuel to the exhaust gas by injecting liquid fuel (light oil). The fuel addition to the exhaust gas by the fuel addition valve 5 can be performed by performing NOx reduction processing for reducing NOx occluded in the DPNR catalyst 4 or SOx poisoning recovery processing for recovering SOx poisoning, as well as increasing the DPNR catalyst 4. It is also performed when controlling the temperature.

  Between the fuel addition valve 5 and the DPNR catalyst 4 in the exhaust passage 3, the heat generating portion 60 generates heat due to power supply from a battery (not shown), and the glow plug 6 ignites the fuel added from the fuel addition valve 5. Is provided. In this embodiment, the glow plug 6 corresponds to the ignition device according to the present invention. A small-diameter catalyst 7 having a diameter smaller than the inner diameter of the exhaust passage 3 is disposed between the glow plug 6 and the DPNR catalyst 4 in the exhaust passage 3. The small-diameter catalyst 7 is a catalyst having NOx reducing ability when the ambient atmosphere is a reducing atmosphere. The reducing atmosphere here means an atmosphere in which the reducing component exists in the surroundings when the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich.

  FIG. 2 is an explanatory diagram illustrating the configuration of the fuel addition valve, the glow plug, and the small-diameter catalyst. In the exhaust passage 3, a portion where the small diameter catalyst 7 is disposed has a so-called double pipe structure, and the small diameter catalyst 7 is disposed substantially at the center of the cross section of the exhaust passage 3. On the outer periphery of the small-diameter catalyst 7, a bypass passage 30 is formed for guiding the DPNR catalyst 4 by bypassing the small-diameter catalyst 7 to the exhaust gas. That is, it can be said that the small-diameter catalyst 7 is a catalyst formed so that only a part of the exhaust gas flowing through the exhaust passage 3 passes through a part thereof. In this embodiment, the small-diameter catalyst 7 corresponds to the upstream branching catalyst in the present invention.

The internal combustion engine 1 is provided with an ECU (Electronic Control Unit) 10 which is an electronic control unit for controlling the operation state according to the operation conditions of the internal combustion engine 1 and the driver's request. The ECU 10 includes a CPU that executes various arithmetic processes related to the control of the internal combustion engine 1, a ROM that stores programs and data necessary for the control, a RAM that temporarily stores arithmetic results of the CPU, and the like. An input / output port for inputting / outputting signals is provided. The fuel addition valve 5 and the glow plug 6 are connected to the ECU 10 via electric wiring and are controlled by the ECU 10.

  Further, a temperature sensor 8 that outputs an electrical signal corresponding to the temperature of the exhaust gas is disposed in a portion of the exhaust passage 3 between the small diameter catalyst 7 and the DPNR catalyst 4. The temperature sensor 8 is connected to the ECU 10 through electric wiring. The ECU 10 can estimate the temperature of the DPNR catalyst 4 based on the output signal of the temperature sensor 8.

  Hereinafter, fuel combustion control executed by the ECU 10 when the temperature of the DPNR catalyst 4 is raised will be described. This fuel combustion control is performed when a temperature increase request to the DPNR catalyst 4 is made. Specifically, the fuel added from the fuel addition valve 5 to the exhaust gas is combusted by ignition by the glow plug 6, and together with the fuel. The DPNR catalyst 4 is heated by supplying high-temperature combustion gas to the DPNR catalyst 4. The temperature increase request to the DPNR catalyst 4 is issued, for example, when the temperature of the DPNR catalyst 4 is not more than a specified temperature. This specified temperature is a temperature as a threshold at which it can be determined that the activity of the DPNR catalyst 4 is low, and can be set in advance based on empirical rules such as experiments.

  When executing the fuel combustion control, attention is paid to the entire air-fuel ratio of exhaust gas flowing into the DPNR catalyst 4 (hereinafter referred to as “total air-fuel ratio”). This total air-fuel ratio is substantially equal to the air-fuel ratio at the time of combustion of the exhaust gas to which fuel is added by the fuel addition valve 5. When combustion is performed with a rich air / fuel ratio lower than the stoichiometric air / fuel ratio (hereinafter referred to as “rich combustion”), the amount of oxygen in the exhaust gas flowing into the DPNR catalyst 4 becomes insufficient and is included in the exhaust gas. HC and CO cannot be oxidized sufficiently. As a result, HC and CO contained in the exhaust gas pass through to the downstream side of the DPNR catalyst 4, which may cause deterioration of the exhaust mission.

  Therefore, in the fuel combustion control of the present embodiment, the above problem is avoided by performing combustion with a lean air-fuel ratio in which the overall air-fuel ratio is higher than the stoichiometric air-fuel ratio (hereinafter referred to as “lean combustion”). Here, when lean combustion is performed, NOx is easily generated as a contradiction. Therefore, in the temperature raising system according to the present embodiment, NOx produced by the lean combustion is purified by the small diameter catalyst 7.

  In order to purify the NOx generated during the fuel combustion control in the small diameter catalyst 7, the ambient atmosphere of the small diameter catalyst 7 needs to be a reducing atmosphere. Therefore, the ECU 10 performs fuel combustion control so that the air-fuel ratio of the exhaust gas passing through the small-diameter catalyst 7 (hereinafter referred to as “divided gas air-fuel ratio”) becomes a rich air-fuel ratio while setting the overall air-fuel ratio to be a lean air-fuel ratio. To do.

  Specifically, referring to FIG. 2, the injection hole 50 of the fuel addition valve 5, the heat generating part 60 of the glow plug 6, and the central part of the upstream end face 70 of the small diameter catalyst 7 are along the longitudinal direction of the exhaust passage 3. It is arranged on a substantially straight line. The ECU 10 detects the operating state of the internal combustion engine 1 when detecting a temperature increase request to the DPNR catalyst 4. Then, the target fuel addition amount related to the fuel combustion control is calculated based on the operating state of the internal combustion engine 1. This target fuel addition amount is a target value of the fuel addition amount so that the total air-fuel ratio becomes the lean air-fuel ratio. This target fuel addition amount is also a target value of the fuel addition amount so that the shunt gas air-fuel ratio at the time of fuel combustion control becomes a rich air-fuel ratio, which will be described later.

When a control command for fuel addition is issued from the ECU 10 to the fuel addition valve 5, the fuel is injected from the injection hole 50 of the fuel, and a spray spread in a substantially conical shape (represented by lattice hatching in FIG. 2) is formed. Is done. The heat generating portion 60 of the glow plug 6 is disposed in the area of fuel spray from the injection hole 50. The ECU 10 energizes the glow plug 6 in advance when performing the fuel ignition control, and heats the heat generating portion 60 to a temperature higher than the ignition temperature of the fuel (for example, 800 ° C.). As a result, when the fuel added from the fuel addition valve 5 at the time of fuel ignition control reaches the heat generating portion 60 of the glow plug 6, the fuel is ignited and burned using the heat generating portion 60 as an ignition source.

  Part of the fuel added from the fuel addition valve 5 flows uncombusted and flows downstream together with the combustion gas. Here, since the heat generating portion 60 of the glow plug 6 and the upstream end surface 70 of the small diameter catalyst 7 are opposed to each other, most of the combustion gas generated in the fuel combustion control and the fuel contained in the combustion gas is the small diameter catalyst 7. It is introduced from the upstream end face 70 (represented by an arrow in the figure). That is, rich exhaust gas having a low air-fuel ratio composed of combustion gas and fuel exclusively passes through the small-diameter catalyst 7, and in the bypass passage 30 formed on the outer peripheral side of the small-diameter catalyst 7, a lean exhaust gas having a high air-fuel ratio is formed. Is configured to pass through. Therefore, according to this configuration, it is possible to relatively reduce the shunt gas air-fuel ratio in comparison with the overall air-fuel ratio when performing the fuel combustion control.

  The ratio of the amount introduced into the small-diameter catalyst 7 with respect to the total amount (total amount) of the combustion gas (and fuel contained in the combustion gas) generated in the fuel combustion control is referred to as a combustion gas introduction ratio. The higher the combustion gas introduction ratio in the fuel combustion control, the greater the difference between the overall air-fuel ratio and the lean air-fuel ratio, and the shunt gas air-fuel ratio can be significantly reduced compared to the overall air-fuel ratio. The combustion gas introduction ratio is the relative positional relationship among various devices such as the fuel addition valve 5, the glow plug 6, the small diameter catalyst 7, the area of the upstream end surface 70 of the small diameter catalyst 7 (flow path cross-sectional area), and other conditions. It depends on. Therefore, when the fuel addition amount related to fuel combustion control is controlled within the range where the overall air-fuel ratio becomes the lean air-fuel ratio, the relative positions of the various devices are ensured so that the shunt gas air-fuel ratio becomes the rich air-fuel ratio. The hardware configuration such as the relationship and the cross-sectional area of the small-diameter catalyst 7 is determined.

  FIG. 3 is a flowchart showing a fuel combustion control routine. Hereinafter, the specific processing content of the fuel combustion control executed by the ECU 10 will be described with reference to this flowchart. This routine is stored in advance in the ECU 10 and is repeated every predetermined time. In step S <b> 101, the ECU 10 estimates the temperature of the DPNR catalyst 4 based on the output signal of the temperature sensor 8.

  In step S102, the ECU 10 determines whether or not the temperature of the DPNR catalyst 4 is equal to or lower than a specified temperature. When it is determined that the temperature of the DPNR catalyst 4 is equal to or lower than the specified temperature, it is determined that a temperature increase request is issued to the DPNR catalyst 4, and the process proceeds to step S103. On the other hand, when it is determined that the temperature of the DPNR catalyst 4 is higher than the specified temperature, it is determined that a temperature increase request has not been issued to the DPNR catalyst 4, and this routine is temporarily exited.

  In step S103, the ECU 10 calculates a target fuel addition amount related to fuel combustion control based on the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 and the exhaust gas amount Ga. As described above, the target fuel addition amount is calculated as an amount such that the shunt gas air-fuel ratio becomes the rich air-fuel ratio and the total air-fuel ratio becomes the lean air-fuel ratio. In step S104, the ECU 10 performs fuel combustion control, and then ends this routine.

In step S104, the ECU 10 specifically adds a target fuel addition amount of fuel from the fuel addition valve 5 to the exhaust gas, and causes the glow plug 6 to ignite the fuel. As a result, lean combustion is performed, and most of the generated combustion gas and reducing components (added fuel by the fuel addition valve 5, CO, H ₂ generated by lean combustion, etc.) flow into the small-diameter catalyst 7. . As a result, the shunt gas air-fuel ratio decreases to the rich air-fuel ratio, and NOx flowing into the small diameter catalyst 7 is reduced. The small-diameter catalyst 7 has a smaller heat capacity than the DPNR catalyst 4, and can easily increase its activity by introducing high-temperature combustion gas.

The rich air-fuel ratio exhaust gas flowing out from the small-diameter catalyst 7 merges with the lean air-fuel ratio exhaust gas flowing through the bypass passage 30 and then flows into the DPNR catalyst 4 to heat the DPNR catalyst 4. Here, since the exhaust gas flowing into the DPNR catalyst 4 as a whole has a lean air-fuel ratio, an oxygen amount sufficient to oxidize HC and CO contained in the exhaust gas in the DPNR catalyst 4 is ensured. Therefore, HC and CO contained in the exhaust gas flowing into the DPNR catalyst 4 are surely oxidized, and deterioration of exhaust emission when the temperature of the DPNR catalyst 4 is raised is suppressed. Further, since the heat of reaction when HC and CO are oxidized contributes to the temperature increase of the DPNR catalyst 4, it is possible to realize an early temperature increase of the DPNR catalyst 4.

  Moreover, when the combustion gas produced | generated by lean combustion passes the small diameter catalyst 7, some oxygen contained in the gas is consumed. In the small-diameter catalyst 7, part of the fuel contained in the combustion gas is reformed or vaporization is promoted. As a result, the combustion reaction of the reducing component in the DPNR catalyst 4 becomes slow, and the generation of NOx in the DPNR catalyst 4 is suppressed.

  As described above, according to the temperature raising system for the exhaust purification catalyst in the present embodiment, when the fuel added to the exhaust gas flowing through the exhaust passage 3 when the temperature raising request for the DPNR catalyst 4 (exhaust purification catalyst) is combusted, Deterioration of exhaust emission is suppressed. In the present embodiment, the ECU 10 that performs the fuel combustion control when the temperature increase request of the DPNR catalyst 4 (exhaust gas purification catalyst) is required corresponds to the control means in the present invention.

<Example 2>
Next, a second example of the embodiment of the present invention will be described. The schematic configuration of the internal combustion engine to which the temperature raising system in the present embodiment is applied and its intake and exhaust system are the same as the configurations shown in FIGS. Hereinafter, the characteristic part in the temperature raising system of the present embodiment will be mainly described.

  In the fuel combustion control in this embodiment, the temperature THc of the small-diameter catalyst 7 during fuel combustion control (hereinafter referred to as “small-diameter catalyst bed temperature”) THc is controlled according to the exhaust gas amount Ga discharged from the internal combustion engine 1. It is characterized by doing. When the ECU 10 detects a temperature increase request to the DPNR catalyst 4 based on the output signal of the temperature sensor 8, the ECU 10 estimates the exhaust gas amount Ga. For example, the ECU 10 can estimate the exhaust gas amount Ga based on an output signal of an air flow meter (not shown) that measures the amount of air flowing through the intake passage.

  The NOx purification performance of the small-diameter catalyst 7 varies according to the flow velocity Vg of the exhaust gas passing through the small-diameter catalyst 7. As the exhaust gas amount Ga from the internal combustion engine 1 increases, the flow velocity Vg of the exhaust gas passing through the small-diameter catalyst 7 increases. As a result, the NOx reduction reaction in the small-diameter catalyst 7 hardly occurs, and the NOx purification performance is lowered. Therefore, the ECU 10 controls the small-diameter catalyst bed temperature THc at the time of fuel combustion control to be higher when the exhaust gas amount Ga discharged from the internal combustion engine 1 is large than when it is small.

  FIG. 4 is a map illustrating the relationship between the exhaust gas amount Ga and the small-diameter catalyst bed temperature THc during fuel combustion control. In this figure, the corresponding temperature of the small-diameter catalyst bed temperature THc during fuel combustion control increases as the exhaust gas amount Ga discharged from the internal combustion engine 1 increases. The ECU 10 substitutes the estimated value of the exhaust gas amount Ga into the map shown in FIG. 4 and calculates a target value of the small diameter catalyst bed temperature THc (hereinafter referred to as “target small diameter catalyst bed temperature THct”). Then, the ECU 10 adds the fuel in the fuel addition valve 5 so that the small diameter catalyst bed temperature THc becomes the target small diameter catalyst bed temperature THct. For example, a single fuel addition period when fuel is added intermittently from the fuel addition valve 5 is referred to as an addition period, and a period between the addition periods is referred to as an addition interval. In this case, the small-diameter catalyst bed temperature THc is changed to the target small-diameter catalyst bed temperature THct by changing the addition parameters such as the amount of fuel added from the fuel addition valve 5 per unit time, the addition interval, and the number of additions per predetermined time. Can be controlled.

Here, as the small-diameter catalyst bed temperature THc becomes higher, the passage resistance of the exhaust gas passing through the small-diameter catalyst 7 increases. Therefore, as the exhaust gas amount Ga increases, the flow rate Vg of the exhaust gas passing through the small diameter catalyst 7 decreases by controlling the small diameter catalyst bed temperature THc at the time of fuel combustion control higher. As a result, the reduction reaction of NOx in the small-diameter catalyst 7 can be promoted, and the reduction in the NOx purification performance can be suppressed. Therefore, the NOx purification performance of the small-diameter catalyst 7 during fuel combustion control can always be maintained at a high level regardless of the increase or decrease in the exhaust gas amount Ga from the internal combustion engine 1.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 3 ... Exhaust passage 4 ... DPNR catalyst 5 ... Fuel addition valve 6 ... Glow plug 7 ... Small diameter catalyst 10 ... ECU

Claims (2)

An exhaust purification catalyst provided in the exhaust passage of the internal combustion engine;
A fuel addition valve that is provided in an exhaust passage upstream of the exhaust purification catalyst and adds fuel to exhaust gas flowing through the exhaust passage;
An ignition device that is provided in an exhaust passage upstream of the exhaust purification catalyst and ignites the fuel added from the fuel addition valve;
A pre-stage shunt having a NOx reduction ability that is provided in an exhaust passage downstream of the fuel addition valve and the ignition device and upstream of the exhaust purification catalyst, and in which only a part of the exhaust gas flowing through the exhaust passage is guided. A catalyst,
Control means for performing fuel combustion control for burning the fuel added by the fuel addition valve by ignition by the ignition device at the time of a temperature increase request of the exhaust purification catalyst,
The pre-stage shunt catalyst is arranged so that the combustion gas generated in the fuel combustion control and the fuel contained in the combustion gas are introduced,
When the fuel combustion control is performed, the control means has a rich air-fuel ratio of the exhaust gas passing through the upstream branching catalyst, and a lean air-fuel ratio of the entire exhaust gas flowing into the exhaust purification catalyst. As described above, a temperature raising system for an exhaust purification catalyst, wherein fuel is added to the fuel addition valve.   The control means supplies fuel to the fuel addition valve so that the bed temperature of the pre-stage shunt catalyst during fuel combustion control is higher when the amount of exhaust gas discharged from the internal combustion engine is small than when the amount is small. The temperature raising system for an exhaust purification catalyst according to claim 1, wherein the temperature increasing system is added.

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