JP2022077329A - Exhaust emission control device - Google Patents

Abstract

To suppress a generation amount of NO2.SOLUTION: An exhaust emission control device 1 includes: a first catalyst device 31 including an oxidation catalyst; a filter device 32 for collecting particulate matter included in exhaust gas; a second catalyst device 33 including a selective reduction catalyst for selectively reducing NOx; a reducing agent supply device 35 for supplying a reducing agent to the second catalyst device; EGR piping 26 recirculating a part of exhaust gas to an intake side of an engine as EGR gas; an EGR valve 28 adjusting a flow rate of EGR gas G2; and a control device 40 for controlling an operation of the EGR valve. The control device acquires a generated NO2 concentration that is an NO2 concentration of exhaust gas flowing into the filter device, acquires a required NO2 concentration that is an NO2 concentration required for oxidation treatment of the particulate matter collected by the filter device, and controls the EGR valve so as to increase the flow rate of the EGR gas when the generated NO2 concentration is higher than the required NO2 concentration.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to an exhaust gas purification device.

A filter device that collects particulate matter (PM) contained in the exhaust gas from the engine is used. For example, Patent Document 1 describes a continuously regenerative DPF (Diesel) that collects PM contained in an exhaust gas and removes the collected PM by oxidizing it with NO ₂ (nitrogen dioxide) in the exhaust gas. An exhaust purification device equipped with a particulate filter) is described.

Further, Patent Document 2 describes a DOC that oxidizes NO (nitrogen monoxide) in exhaust gas to NO ₂ , a continuously regenerated DPF, a urea water injection device that adds urea water to exhaust gas, and urea. It is equipped with an SCR that reduces NOx (nitrogen oxides) in the exhaust gas using water-derived ammonia as a reducing agent, and the amount of NOx discharged from the engine, the amount of NO ₂ produced by the DOC, and the DPF. Exhaust gas purification that controls the combustion state of the engine so that the ratio of NO to NO ₂ (NO / NO ₂ ratio) in the exhaust gas flowing into the SCR is 1: 1 based on the consumption of NO ₂ in The device is described.

Japanese Unexamined Patent Publication No. 2010-096039 Japanese Unexamined Patent Publication No. 2014-163253

In the apparatus described in Reference 2, NOx contained in the exhaust gas is purified by using ammonia as a reducing agent, but when ammonia and NO ₂ react on SCR, N ₂ O (nitrous oxide) May be generated. N2O is a powerful greenhouse gas with a global warming potential about 300 times that of CO _{2 (} carbon dioxide), and its emission into the atmosphere is strictly regulated in some areas.

Therefore, it is an object of the present disclosure to suppress the amount of _N2O produced.

The exhaust gas recirculation device of one embodiment is disposed in the exhaust passage through which the exhaust gas from the engine flows, and is disposed in the exhaust passage on the downstream side of the first catalyst device including the oxidation catalyst and the first catalyst device for exhaust gas. A filter device that collects particulate matter contained in the gas, a second catalyst device that is disposed in an exhaust passage on the downstream side of the filter device and includes a selective reduction catalyst that selectively reduces NOx, and a second A reducing agent supply device that supplies a reducing agent to the catalyst device, an EGR pipe that recirculates a part of the exhaust gas to the intake side of the engine as EGR gas, an EGR valve that regulates the flow rate of EGR gas in the EGR pipe, and EGR. It includes a control device that controls the operation of the valve. The control device acquires the generated NO ₂ concentration, which is the NO ₂ concentration of the exhaust gas flowing into the filter device, and is the required NO ₂ concentration, which is the NO ₂ concentration required for the oxidation treatment of the particulate matter collected in the filter device. When the concentration is obtained and the generated NO ₂ concentration is higher than the required NO ₂ concentration, the EGR valve is controlled to increase the flow rate of EGR gas.

In the exhaust gas recirculation device of the above embodiment, the flow rate of EGR gas is increased when the generated NO ₂ concentration is higher than the required NO ₂ concentration. As the flow rate of EGR gas increases, the combustion temperature of the engine decreases and the amount of NOx discharged from the engine decreases. As the amount of NOx discharged from the engine decreases, the NO ₂ concentration of the exhaust gas flowing into the filter device decreases, and most of the NO ₂ in the exhaust gas is consumed by the oxidation of particulate matter. .. As a result, the NO ₂ concentration of the exhaust gas flowing into the second catalyst device is reduced, and it is suppressed that N ₂ O is generated when NO x is reduced by the second catalyst device.

In one embodiment, the controller may control the EGR valve to reduce the flow rate of EGR gas when the generated NO ₂ concentration is lower than the required NO ₂ concentration. As the flow rate of EGR gas decreases, the combustion temperature of the engine rises and the amount of NOx discharged from the engine increases. As the amount of NOx discharged from the engine increases, the NO ₂ concentration of the exhaust gas flowing into the filter device increases, so that PM can be effectively burned in the filter device.

In one embodiment, the second catalyst device may include a vanadium-based catalyst as the selective reduction catalyst. The vanadium-based catalyst can reduce NOx with high efficiency even when the ratio of NO ₂ to NOx is low. Therefore, by including the vanadium-based catalyst as the selective reduction catalyst, NOx can be effectively purified even when the NO ₂ concentration of the exhaust gas flowing into the second catalyst device decreases.

In one embodiment, the second catalyst device may further include a copper-based catalyst, and the vanadium-based catalyst may be arranged upstream of the copper-based catalyst in the flow direction of the exhaust gas. The copper _- based catalyst is a selective reduction catalyst capable of reducing NOx with _high efficiency, but has a property of easily producing N2O when reducing NO2. In this embodiment, NO ₂ is reduced by the vanadium-based catalyst arranged on the upstream side, and the exhaust gas containing NOx having a low proportion of NO ₂ is purified by the copper-based catalyst, so that the amount of N ₂ O produced is suppressed. While doing so, NOx can be effectively purified.

In one embodiment, the second catalyst device may include an iron-based catalyst as the selective reduction catalyst. The iron-based catalyst can purify NOx even when the ratio of NO ₂ to NOx is low. Therefore, by including the iron-based catalyst as the selective reduction catalyst, NOx can be effectively purified even when the NO ₂ concentration of the exhaust gas flowing into the second catalyst device decreases.

In one embodiment, the second catalyst device may further include a copper-based catalyst, and the iron-based catalyst may be arranged upstream of the copper-based catalyst in the flow direction of the exhaust gas. In this embodiment, NO ₂ is reduced by the iron-based catalyst arranged on the upstream side, and the exhaust gas containing NOx having a low proportion of NO ₂ is purified by the copper-based catalyst, so that the amount of N ₂ O produced is suppressed. While doing so, NOx can be effectively purified.

In one embodiment, the control device may increase the flow rate of the EGR gas as the difference between the generated NO ₂ concentration and the required NO ₂ concentration increases. By increasing the flow rate of the EGR gas, the NO ₂ concentration of the exhaust gas flowing into the filter device can be further lowered. As a result, the NO ₂ concentration of the exhaust gas flowing into the second catalyst device can be lowered, so that the amount of N ₂ O produced can be suppressed.

According to _one aspect of the present invention and various embodiments, the amount of N2O produced can be suppressed.

It is a figure which shows schematically the exhaust gas purification apparatus which concerns on one Embodiment. It is a figure which shows the modification of the 2nd catalyst apparatus. It is a flowchart which shows the flow of the process executed by a control device.

Hereinafter, various embodiments will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicate explanations for the same or corresponding parts will be omitted. In the following description, the terms "upstream" and "downstream" are used with reference to the flow direction of the exhaust gas G1. Further, in the present specification, nitric oxide (NO), nitrogen dioxide (NO ₂ ) and the like are collectively referred to as nitrogen oxides (NOx).

FIG. 1 is a diagram schematically showing an exhaust gas purification device 1 according to an embodiment. The exhaust gas purification device 1 is mounted on a vehicle such as a truck and purifies harmful substances contained in the exhaust gas G1 from the engine 10.

The engine 10 is, for example, a diesel engine and includes a plurality of cylinders 10a. A plurality of injectors 10b are installed at positions close to the plurality of cylinders 10a. The plurality of injectors 10b inject fuel supplied from the fuel addition device 20 described later into the plurality of cylinders 10a, respectively. An intake manifold 11 that supplies air into the plurality of cylinders 10a and an exhaust manifold 12 that discharges exhaust gas G1 from the plurality of cylinders 10a are connected to the plurality of cylinders 10a.

An intake pipe 13 is connected to the intake manifold 11. The intake pipe 13 is provided with an air cleaner 14, a compressor 15, and an intercooler 16 in this order from the upstream side of the intake air.

An exhaust pipe 18 is connected to the exhaust manifold 12 via a turbine 17. The exhaust pipe 18 provides a part of the exhaust passage 25 through which the exhaust gas G1 from the engine 10 flows. The turbine 17 is connected to the compressor 15 via a connecting shaft. The turbine 17 is rotated by the flow of the exhaust gas G1 discharged from the exhaust manifold 12 of the engine 10. The compressor 15 rotates with the rotation of the turbine 17, takes in air (intake) from the intake pipe 13, and sends the compressed air to the intake manifold 11. That is, the compressor 15 and the turbine 17 form a turbocharger. The air introduced into the intake manifold 11 is introduced into the plurality of cylinders 10a. The air introduced into the plurality of cylinders 10a is compressed in each cylinder 10a.

As shown in FIG. 1, the exhaust gas purification device 1 includes a fuel addition device 20, an EGR unit 24, an aftertreatment device 30, and a control device 40. The fuel addition device 20 includes a fuel tank 21, a pump 22, and a fuel addition valve 23, and for example, fuel such as light oil stored in the fuel tank 21 is pumped to a plurality of injectors 10b via a fuel supply path 10c. The plurality of injectors 10b inject the fuel supplied from the fuel addition device 20 into the plurality of cylinders 10a, respectively.

The fuel injected by the plurality of injectors 10b is burned in each cylinder 10a, and the pistons arranged in each cylinder 10a are reciprocated in the cylinder 10a. The reciprocating motion of the piston causes the crankshaft connected to the engine 10 to rotate, and the propeller shaft of the vehicle to rotate via the clutch. The exhaust gas G1 discharged from the exhaust manifold 12 due to the combustion of fuel is discharged to the exhaust pipe 18.

The fuel addition valve 23 is provided in the exhaust pipe 18 on the upstream side of the aftertreatment device 30, and injects the fuel stored in the fuel tank 21 into the exhaust passage 25. The fuel injected into the exhaust passage 25 is added to the exhaust gas G1 and supplied to the aftertreatment device 30. The amount of fuel added to the exhaust gas G1 from the fuel addition valve 23 is controlled by the control device 40.

The EGR unit 24 is a device that recirculates a part of the exhaust gas G1 to the intake system of the engine 10. The EGR unit 24 includes an EGR pipe 26, an EGR cooler 27, and an EGR valve 28. The EGR pipe 26 connects the intake manifold 11 of the engine 10 and the exhaust manifold 12, and a part of the exhaust gas G1 is returned to the intake system of the engine 10 as the EGR gas G2. For example, the EGR gas G2 that has recirculated in the EGR pipe 26 is supplied into the cylinder 10a of the engine 10 together with the intake air flowing through the intake pipe 13.

The EGR cooler 27 is installed in the EGR pipe 26 and cools the EGR gas G2 flowing through the EGR pipe 26. The EGR valve 28 is arranged on the downstream side of the EGR cooler 27 in the flow direction of the EGR gas G2. The EGR valve 28 includes a valve body whose opening degree can be adjusted, and adjusts the flow rate of the EGR gas G2 flowing through the EGR pipe 26. The opening degree of the EGR valve 28 is controlled by the control device 40.

The aftertreatment device 30 is connected to the exhaust pipe 18. As shown in FIG. 1, the aftertreatment device 30 includes a first catalyst device 31, a filter device 32, a second catalyst device 33, and an ammonia reduction catalyst 34. The first catalyst device 31, the filter device 32, the second catalyst device 33, and the ammonia reduction catalyst 34 are housed in a case constituting the exhaust passage 25 together with the exhaust pipe 18 from the upstream side in the flow direction of the exhaust gas G1. They are arranged in this order.

The first catalyst device 31 is provided in the upstream portion of the exhaust passage 25. The first catalyst device 31 includes an oxidation catalyst. This oxidation catalyst is supported on, for example, a ceramic carrier. Examples of the oxidation catalyst contained in the first catalyst device 31 include precious metal catalysts such as platinum, rhodium, and palladium. The first catalyst device 31 receives the exhaust gas G1 discharged from the engine 10 and oxidizes a part of NO contained in the exhaust gas G1 to NO ₂ by the oxidation catalyst. Further, the oxidation catalyst of the first catalyst device 31 purifies by oxidizing HC (hydrocarbon), CO (carbon monoxide) and the like in the exhaust gas G1.

The filter device 32 is arranged in the exhaust passage 25 on the downstream side of the first catalyst device 31. The filter device 32 is a so-called DPF (Diesel Particulate Filter), and collects particulate matter (PM: Particulate Matter) contained in the exhaust gas G1. Further, the filter device 32 receives the exhaust gas G1 containing NO ₂ generated by the first catalyst device 31 and oxidizes the PM collected in the filter device 32 by NO ₂ to remove the PM. As described above, the process of removing PM by NO ₂ in the exhaust gas is called a continuous regeneration process, and is continuously executed, for example, when the temperature of the exhaust gas G1 is 250 ° C. to 400 ° C.

The filter device 32 may contain an oxidation catalyst that oxidizes harmful components contained in the exhaust gas G1. Examples of the oxidation catalyst contained in the filter device 32 include precious metal catalysts such as platinum, rhodium, and palladium. These oxidation catalysts are coated, for example, on the filter member of the filter device 32. The filter device 32 may contain more oxidation catalysts than the amount of the oxidation catalyst contained in the first catalyst device 31. NO ₂ generated by the first catalyst device 31 may react with HC contained in the exhaust gas to be returned to NO before reaching the filter device 32. As described above, when a part of NO ₂ in the exhaust gas G1 is returned to NO, NO ₂ used in continuous regeneration may be insufficient. On the other hand, when the filter device 32 contains more oxidation catalyst than the amount of the oxidation catalyst contained in the first catalyst device 31, more NO ₂ is generated in the vicinity of PM, so that PM It can promote combustion. As a result, even when the temperature of the exhaust gas G1 is relatively low, the filter device 32 can effectively perform the continuous regeneration process.

The second catalyst device 33 is arranged in the exhaust passage 25 on the downstream side of the filter device 32. The second catalyst device 33 receives the exhaust gas G1 that has passed through the filter device 32, and reduces and purifies the NOx contained in the exhaust gas G1. The second catalyst device 33 includes a selective reduction catalyst. The selective reduction catalyst is supported on a ceramic carrier, for example. The selective reduction catalyst is a catalyst that selectively reduces NOx contained in the exhaust gas G1 by using a reducing agent, and as the selective reduction catalyst, for example, a vanadium-based catalyst or an iron-based catalyst is used. The vanadium-based catalyst is a catalyst containing a compound containing vanadium (V) as a main component, and the iron-based catalyst is a catalyst containing a compound containing iron (Fe) as a main component. The vanadium-based catalyst and the iron-based catalyst can purify NOx even when the ratio of NO ₂ to NOx is low. In particular, the vanadium-based catalyst can reduce NOx with high reduction efficiency when the ratio of NO ₂ to NOx is low.

The second catalyst device 33 may contain a plurality of types of selective reduction catalysts. For example, as shown in FIG. 2A, a vanadium-based catalyst 51 and a copper-based catalyst 52 are supported on the carrier of the second catalyst device 33 as selective reduction catalysts, and the vanadium-based catalyst 51 is more than the copper-based catalyst 52. May be located on the upstream side. The copper _- based catalyst 52 is a selective reduction catalyst capable of reducing NOx with _high efficiency, but has a property of easily producing N2O when reducing NO2. In the second catalyst device 33 shown in FIG. 2A, NO ₂ is reduced by the vanadium-based catalyst 51 arranged on the upstream side, so that the NO ₂ concentration of the exhaust gas G1 introduced into the copper-based catalyst 52 is determined. Can be lowered. Therefore, by using such a _second catalyst device 33, NOx can be effectively purified while suppressing the amount of N2O produced.

Further, as shown in FIG. 2B, an iron-based catalyst 53 and a copper-based catalyst 52 are supported on the carrier of the second catalyst device 33 as selective reduction catalysts, and the iron-based catalyst 53 is more than the copper-based catalyst 52. May be located on the upstream side. In the second catalyst device 33 shown in FIG. 2B, NO ₂ is reduced by the iron-based catalyst 53 arranged on the upstream side, so that the NO ₂ concentration of the exhaust gas G1 introduced into the copper-based catalyst 52 is determined. Can be lowered. Therefore, by using such a _second catalyst device 33, NOx can be effectively purified while suppressing the amount of N2O produced.

See FIG. 1 again. The ammonia reduction catalyst 34 is arranged in the exhaust passage 25 on the downstream side of the second catalyst device 33. The ammonia reduction catalyst 34 receives the exhaust gas G1 that has passed through the second catalyst device 33, and oxidizes and purifies the excess ammonia contained in the exhaust gas G1, for example.

A reducing agent supply device 35 is provided on the downstream side of the filter device 32 and on the upstream side of the second catalyst device 33. The reducing agent supply device 35 supplies the reducing agent to the second catalyst device 33. For example, the reducing agent supply device 35 injects urea water into the exhaust passage 25 on the upstream side of the second catalyst device 33.

The exhaust gas purification device 1 further includes temperature sensors 36, 37, 38 and a NOx sensor 39. The temperature sensor 36 is attached to the first catalyst device 31 and measures the temperature of the first catalyst device 31. The temperature sensor 37 is attached to the filter device 32 and measures the temperature of the filter device 32. The temperature sensor 38 is attached to the second catalyst device 33 and measures the temperature of the second catalyst device 33. The temperature sensors 36, 37, and 38 measure the temperature of the exhaust gas G1 in the exhaust passage 25, and the temperature of the exhaust gas G1 is used to determine the temperature of the first catalyst device 31, the filter device 32, and the second catalyst device 33. The temperature may be obtained indirectly. The temperature sensors 36, 37, and 38 output the acquired information indicating the temperature of the first catalyst device 31, the filter device 32, and the second catalyst device 33 to the control device 40.

The NOx sensor 39 is provided on the upstream side of the first catalyst device 31 and measures the NOx concentration of the exhaust gas G1 flowing in the exhaust passage 25. The NOx sensor 39 outputs information indicating the NOx concentration of the exhaust gas G1 to the control device 40.

The control device 40 is an electronic control unit having a CPU [Central Processing Unit], ROM [Read Only Memory], RAM [Random Access Memory], CAN [Controller Area Network] communication circuit, etc., and operates the entire exhaust purification device 1. To control. The control device 40 realizes various functions described later by, for example, loading a program stored in the ROM into the RAM and executing the program loaded in the RAM in the CPU.

The control device 40 is communicably connected to the EGR valve 28, the reducing agent supply device 35, the temperature sensors 36, 37, 38 and the NOx sensor 39. The control device 40 controls the operation of the EGR valve 28 and the reducing agent supply device 35 based on the data measured by the temperature sensors 36, 37, 38 and the NOx sensor 39.

Hereinafter, the details of the control device 40 will be described. As shown in FIG. 1, the control device 40 includes a reducing agent addition unit 41, a generated NO ₂ concentration acquisition unit 42, a required NO ₂ concentration acquisition unit 43, and an EGR control unit 44 as functional components.

The reducing agent addition unit 41 controls the reducing agent supply device 35 to control the amount of the reducing agent supplied to the second catalyst device 33. For example, the reducing agent addition unit 41 acquires the temperature information of the second catalyst device 33 from the temperature sensor 38, and when the temperature of the second catalyst device 33 is equal to or higher than the activation temperature (for example, 180 ° C.), The reducing agent supply device 35 is controlled to inject urea water into the exhaust passage 25. The urea water injected into the exhaust passage 25 is decomposed by the heat of the exhaust gas G1 to generate ammonia (NH ₃ ). The generated ammonia is supplied to the second catalyst device 33 together with the exhaust gas G1 and reduces NOx contained in the exhaust gas G1 on the selective reduction catalyst of the second catalyst device 33.

The generated NO ₂ concentration acquisition unit 42 acquires the generated NO ₂ concentration indicating the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32. That is, the concentration of NO ₂ produced is the concentration of NO ₂ produced by the first catalyst device 31. The generated NO ₂ concentration is, for example, the NOx concentration in the exhaust gas G1 flowing into the first catalyst device 31, the temperature of the first catalyst device 31, the amount of the oxidation catalyst carried by the first catalyst device 31, and the exhaust gas G1. Calculated based on the flow rate. The NOx concentration in the exhaust gas G1 flowing into the first catalyst device 31 may be measured by the NOx sensor 39, or may be calculated based on the load and the rotation speed of the engine 10. The flow rate of the exhaust gas G1 may be measured by a flow rate sensor installed in the exhaust passage 25, or may be calculated based on the intake flow rate, the rotation speed, or the like of the engine 10.

The generated NO ₂ concentration acquisition unit 42 relates to the NOx concentration of the exhaust gas G1, the temperature of the first catalyst device 31, the amount of the oxidation catalyst carried, the flow rate of the exhaust gas G1, and the generated NO ₂ concentration. The generated NO ₂ concentration may be obtained by referring to the map showing. When the filter device 32 contains an oxidation catalyst, the generated NO ₂ concentration may be calculated including the amount of NO ₂ produced by the oxidation catalyst of the filter device 32. In this case, it can be said that the concentration of NO ₂ produced is the concentration of NO ₂ produced by the first catalyst device 31 and the filter device 32.

The required NO ₂ concentration acquisition unit 43 acquires the minimum required NO ₂ concentration required for the oxidation treatment of PM collected in the filter device 32. That is, it can be said that the required NO ₂ concentration is the NO ₂ concentration consumed by the oxidation treatment of PM collected in the filter device 32. The required NO ₂ concentration is obtained based on the temperature of the filter device 32 and the PM deposition amount of the filter device 32. For example, the storage unit of the control device 40 stores a map showing the relationship between the temperature of the filter device 32, the amount of PM deposited, and the required NO ₂ concentration, and the required NO ₂ concentration acquisition unit 43 refers to the map. Obtain the required NO ₂ concentration at. The PM accumulation amount of the filter device 32 may be measured based on the differential pressure between the upstream side and the downstream side of the filter device 32, or the PM amount discharged from the engine 10 and the PM removed by continuous regeneration. It may be calculated by accumulating the difference from the amount.

The EGR control unit 44 controls the EGR valve 28 so that the flow rate of the EGR gas G2 increases when the generated NO ₂ concentration is higher than the required NO ₂ concentration. When the generated NO ₂ concentration is higher than the required NO ₂ concentration, all the NO ₂ flowing into the filter device 32 is not consumed in the continuous regeneration process, and the exhaust gas G1 having a high NO ₂ concentration is downstream of the filter device 32. There is a risk of inflow into the second catalyst device 33 arranged in. NO ₂ flowing into the second catalyst device 33 is reduced by the second catalyst device 33, and N ₂ O is generated at this time.

On the other hand, when the flow rate of the EGR gas G2 is increased, the combustion temperature of the engine 10 is lowered and the amount of NOx of the exhaust gas G1 is reduced. Along with this, the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 decreases, and most of the NO ₂ in the exhaust gas G1 is consumed for the combustion of PM. As a result, the NO ₂ concentration of the exhaust gas G1 flowing into the second catalyst device 33 decreases, and the amount of N ₂ O produced during the reduction of NOx is suppressed.

In one embodiment, the EGR control unit 44 may increase the amount of change in the flow rate of the EGR gas G2 as the difference between the generated NO ₂ concentration and the required NO ₂ concentration increases. When the generated NO ₂ concentration is larger than the required NO ₂ concentration, it can be said that the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 is excessive. At this time, by increasing the flow rate of the EGR gas G2, the difference between the generated NO ₂ concentration and the required NO ₂ concentration can be reduced, and the excessive inflow of NO ₂ into the filter device 32 can be suppressed. As a result, the exhaust gas G1 having a low NO ₂ concentration flows into the second catalyst device 33, and it becomes possible to suppress the amount of N ₂ O produced.

On the other hand, when the generated NO ₂ concentration is lower than the required NO ₂ concentration, the EGR control unit 44 controls the EGR valve 28 so that the flow rate of the EGR gas G2 decreases. If the generated NO ₂ concentration is lower than the required NO ₂ concentration, the amount of NO ₂ to be used in continuous regeneration may be insufficient. On the other hand, by reducing the flow rate of the EGR gas G2, the amount of NOx discharged from the engine 10 increases, so that the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 increases. As a result, the shortage of NO ₂ is eliminated, and PM can be effectively burned in the filter device 32.

Hereinafter, the processing executed by the control device 40 will be described with reference to FIG. FIG. 3 is a flowchart showing a flow of processing executed by the control device 40.

As shown in FIG. 3, the control device 40 first acquires various parameters of the exhaust gas purification device 1 (step ST1). Specifically, the control device 40 includes the temperature of the first catalyst device 31, the temperature of the filter device 32, the temperature of the second catalyst device 33, the NO ₂ concentration in the exhaust gas G1, and the PM accumulation amount of the filter device 32. To get.

Next, the generated NO ₂ concentration acquisition unit 42 acquires the generated NO ₂ concentration, which is the NO ₂ concentration of the exhaust gas G1 supplied to the PM of the filter device 32 (step ST2). For example, the generated NO ₂ concentration acquisition unit 42 has a NOx concentration of the exhaust gas G1 flowing into the first catalyst device 31, a temperature of the first catalyst device 31, a flow rate of the exhaust gas G1, and a first catalyst device 31. The produced NO ₂ concentration is calculated based on the amount of the oxidation catalyst contained in.

Next, the required NO ₂ concentration acquisition unit 43 of the control device 40 acquires the required NO ₂ concentration required for the oxidation treatment of the PM collected in the filter device 32 (step ST3). The required NO ₂ concentration acquisition unit 43 acquires the required NO ₂ concentration by referring to a map showing the relationship between the temperature of the filter device 32, the PM accumulation amount of the filter device 32, and the required NO ₂ concentration, for example.

Next, the EGR control unit 44 determines whether or not the generated NO ₂ concentration is higher than the required NO ₂ concentration (step ST4). When the generated NO ₂ concentration is higher than the required NO ₂ concentration, NO ₂ in the exhaust gas G1 flowing into the filter device 32 is not sufficiently consumed in the continuous regeneration process, and a part of NO ₂ is second. It will be introduced into the catalyst device 33. Therefore, the EGR control unit 44 controls the EGR valve 28 to increase the flow rate of the EGR gas G2 (step ST5). As the flow rate of the EGR gas G2 increases, the amount of NOx discharged from the engine 10 decreases, so that the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 decreases. As a result, most of the NO ₂ introduced into the filter device 32 is consumed in the oxidation treatment of PM, and the exhaust gas G1 having a low NO ₂ concentration is introduced into the second catalyst device 33. Therefore, it is suppressed that N ₂ O is produced when NO ₂ is reduced by the second catalyst device 33. In step ST5, the flow rate of the EGR gas G2 may be increased as the generated NO ₂ concentration is larger than the required NO ₂ concentration.

On the other hand, when the generated NO ₂ concentration is lower than the required NO ₂ concentration, the EGR valve 28 is controlled to reduce the flow rate of the EGR gas G2 (step ST6). As the flow rate of the EGR gas G2 decreases, the amount of NOx discharged from the engine 10 increases, and the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 increases. As a result, sufficient NO ₂ is supplied to the filter device 32 and PM is effectively burned. If the generated NO ₂ concentration matches the required NO ₂ concentration, the flow rate of the EGR gas G2 may be maintained.

As described above, in the exhaust gas purification device 1, when the generated NO ₂ concentration is higher than the required NO ₂ concentration, the NO ₂ concentration of the exhaust gas G1 flowing into the filter device 32 by increasing the flow rate of the EGR gas G2 Is reduced. As a result, most of the NO ₂ in the exhaust gas G1 is consumed in the oxidation treatment of PM, and the NO ₂ concentration of the exhaust gas G1 flowing into the second catalyst device 33 decreases. As a result, it is possible to suppress the production of N ₂ O during the reduction of NO ₂ by the second catalyst device 33.

It is known that the reduction efficiency can be increased by bringing the ratio of NO to NO ₂ (NO / NO ₂ ratio) close to 1: 1 depending on the type of selective reduction catalyst. In the above-mentioned exhaust gas purification device 1, when the generated NO ₂ concentration is higher than the required NO ₂ concentration, the flow rate of the EGR gas G2 is increased, so that the second catalyst device 33 has an exhaust gas G1 having a low NO ₂ concentration. It will flow in. In particular, in the exhaust gas purification device 1 of one embodiment, the ratio of NO ₂ to NO x is brought close to zero. However, the second catalyst device 33 can suppress a decrease in NOx reduction efficiency by containing, for example, a vanadium-based catalyst that is less susceptible to the influence of the NO / NO ₂ ratio as a selective reduction catalyst as compared with a copper-based catalyst. It is possible. Even when the second catalyst device 33 includes an iron-based catalyst as the selective reduction catalyst, it is possible to reduce NOx.

Although the exhaust gas purification device according to the various embodiments has been described above, various modifications can be configured without being limited to the above-described embodiment without changing the gist of the invention.

For example, in the exhaust gas purification device 1 of the above embodiment, urea water is supplied as a reducing agent by the reducing agent supply device 35, but ammonia may be supplied, for example. Further, the aftertreatment device 30 does not necessarily have to include the ammonia reduction catalyst 34.

1 ... Exhaust purification device, 20 ... Fuel addition device, 25 ... Exhaust passage, 31 ... First catalyst device, 32 ... Filter device, 33 ... Second catalyst device, 40 ... Control device, 51 ... Vanadium-based catalyst, 52 ... Copper catalyst, 53 ... Iron catalyst, G1 ... Exhaust gas, G2 ... EGR gas.

Claims (7)

A first catalyst device, which is arranged in the exhaust passage through which the exhaust gas from the engine flows and contains an oxidation catalyst,
A filter device disposed in the exhaust passage on the downstream side of the first catalyst device and collecting particulate matter contained in the exhaust gas, and a filter device.
A second catalyst device, which is disposed in the exhaust passage on the downstream side of the filter device and includes a selective reduction catalyst that selectively reduces NOx, and a second catalyst device.
A reducing agent supply device that supplies a reducing agent to the second catalyst device, and a reducing agent supply device.
An EGR pipe that returns a part of the exhaust gas to the intake side of the engine as EGR gas,
An EGR valve that adjusts the flow rate of the EGR gas in the EGR pipe,
A control device that controls the operation of the EGR valve and
Equipped with
The control device is
Obtaining the generated NO ₂ concentration, which is the NO ₂ concentration of the exhaust gas flowing into the filter device,
Obtain the required NO ₂ concentration, which is the NO ₂ concentration required for the oxidation treatment of the particulate matter collected by the filter device.
An exhaust gas recirculation device that controls the EGR valve to increase the flow rate of the EGR gas when the generated NO ₂ concentration is higher than the required NO ₂ concentration. The exhaust gas recirculation device according to claim 1, wherein the control device controls the EGR valve to reduce the flow rate of the EGR gas when the generated NO ₂ concentration is lower than the required NO ₂ concentration. The exhaust gas purification device according to claim 1 or 2, wherein the second catalyst device includes a vanadium-based catalyst as the selective reduction catalyst. The exhaust gas purification device according to claim 3, wherein the second catalyst device further includes a copper-based catalyst, and the vanadium-based catalyst is arranged on the upstream side of the copper-based catalyst in the flow direction of the exhaust gas. .. The exhaust gas purification device according to claim 1 or 2, wherein the second catalyst device includes an iron-based catalyst as the selective reduction catalyst. The exhaust gas purification device according to claim 5, wherein the second catalyst device further includes a copper-based catalyst, and the iron-based catalyst is arranged on the upstream side of the copper-based catalyst in the flow direction of the exhaust gas. .. The exhaust gas purification according to any one of claims 1 to 6, wherein the control device increases the amount of change in the flow rate of the EGR gas as the difference between the generated NO ₂ concentration and the required NO ₂ concentration increases. Device.

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