JP2016176428A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve exhaust emission control performance.SOLUTION: An exhaust passage 10 of an engine 2 includes: an NOx occlusion catalyst 11; and a selective reduction catalyst 13 provided downstream of the NOx occlusion catalyst 11 for reducing and eliminating NOx by using ammonia generated from urea water. An engine control unit 30 of the engine performs purge control for reducing and eliminating NOx occluded in the NOx occlusion catalyst 11 by increasing an exhaust temperature of the engine 2 and making an air-fuel ratio of exhaust gas stoichiometric or rich when NOx occlusion amount Qa of the NOx occlusion catalyst 11 exceeds a predetermined value A, and performs temperature increase control for supplying ammonia to the selective reduction catalyst 13 by making the air-fuel ratio of the exhaust gas lean and increasing the exhaust temperature to an activation temperature of the selective reduction catalyst 13 or higher when the NOx occlusion amount Qa becomes larger than a predetermined value B larger than the predetermined value A.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust emission control device for an engine provided with an exhaust purification catalyst that purifies nitrogen oxides in exhaust gas.

The exhaust passage of the engine is provided with an exhaust purification device for purifying the exhaust. For example, exhaust purification catalysts such as NOx storage catalysts and selective reduction catalysts have been developed to purify NOx (nitrogen oxides) in engine exhaust.
The NOx storage catalyst stores NOx under a lean atmosphere and reduces NOx to nitrogen under a rich atmosphere. Note that the NOx storage catalyst has a property that a large storage amount can be secured in a low temperature region and the storage amount decreases in a high temperature region. On the other hand, the selective reduction catalyst reduces NOx in the exhaust to nitrogen mainly using a reducing agent such as ammonia in a high temperature region.

  For example, in Patent Document 1, a diesel particulate filter that collects PM (particulate matter) in exhaust gas is provided in an exhaust passage of a diesel engine, and a NOx storage catalyst is provided in an exhaust passage upstream of the diesel particulate filter. At the same time, an exhaust emission control device including a selective reduction catalyst in an exhaust passage downstream of a diesel particulate filter is disclosed.

Japanese Patent No. 4730336

By the way, in an engine provided with a NOx storage catalyst in the exhaust passage, in order to remove NOx stored in the NOx storage catalyst, for example, NOx purge for supplying exhaust gas of rich air-fuel ratio to the NOx storage catalyst by engine fuel injection control is required. Is done according to.
However, for example, when the temperature of the NOx storage catalyst is lowered due to low load operation, NOx purge cannot be performed, and the NOx storage amount in the NOx storage catalyst further increases. If the NOx occlusion amount increases in this way, NOx will flow out from the NOx occlusion catalyst when the exhaust gas temperature then rises due to, for example, high load operation or the like and NOx purge is started. In the configuration in which the selective reduction catalyst is provided downstream of the NOx storage catalyst as in Patent Document 1, the NOx flowing out from the NOx storage catalyst can be reduced and removed by the selective reduction catalyst, but the low load operation has continued. Then, the temperature of the selective reduction catalyst is lowered, and there is a possibility that NOx cannot be sufficiently removed in the selective reduction catalyst and flows out downstream.

  The present invention has been made to solve such problems, and provides an exhaust purification device having excellent exhaust purification performance in an engine provided with a nitrogen oxide storage reduction catalyst and a selective reduction catalyst in an exhaust passage. It is in.

  In order to achieve the above object, in the exhaust emission control device for an engine according to claim 1, a nitrogen oxide storage and reduction catalyst provided in an exhaust passage of the engine for storing nitrogen oxide in the exhaust, and the nitrogen oxide storage A selective reduction catalyst provided in the exhaust passage on the downstream side of the reduction catalyst, for reducing and removing nitrogen oxides using a reducing agent; a reducing agent supply unit for supplying the reducing agent to the selective reduction catalyst; and the nitrogen oxidation A nitrogen oxide storage amount estimation unit for estimating the nitrogen oxide storage amount of the material storage reduction catalyst, and a predetermined purge condition is established in a state where the nitrogen oxide storage amount is greater than a predetermined first storage amount. In this case, a purge control unit that performs a purge control for reducing the nitrogen oxide stored in the nitrogen oxide storage reduction catalyst by making the air-fuel ratio of the exhaust stoichiometric or rich, and the nitrogen oxide storage amount is the First If the purge condition is not satisfied in a state where the amount is greater than the storage amount, the reducing agent is supplied by the reducing agent supply unit while the exhaust air-fuel ratio is lean, and the reduction catalyst is supplied to the selective reduction catalyst. And a temperature increase control unit that executes temperature increase control for supplying the agent and increasing the temperature of the selective reduction catalyst to a predetermined temperature or higher.

Preferably, the purge control unit forcibly switches from the temperature increase control to the purge control when the purge condition is satisfied during the temperature increase control.
Preferably, the temperature increase control is performed by at least one of throttle of the intake air amount of the engine and increase of the fuel injection amount of the engine.
Preferably, the reducing agent supply unit supplies an aqueous urea solution to the exhaust passage upstream of the selective reduction catalyst, and supplies ammonia obtained by hydrolysis of the aqueous urea solution to the selective reduction catalyst. Good.

  Preferably, when the engine load is equal to or higher than a predetermined value, the temperature increase control is executed regardless of the nitrogen oxide storage amount.

  According to the exhaust purification device for an engine of the present invention, the exhaust passage is provided with the nitrogen oxide storage reduction catalyst having a high purification efficiency in the low temperature region and the selective reduction catalyst having a high purification efficiency in the high temperature region. Thus, the purification efficiency can be improved. Furthermore, since the selective reduction catalyst is provided on the downstream side of the nitrogen oxide, even if the nitrogen oxide flows out from the nitrogen oxide storage reduction catalyst, it can be reduced and removed by the selective reduction catalyst.

  In particular, in a state where the nitrogen oxide storage reduction catalyst cannot reduce and remove nitrogen oxides at low temperatures and the purge condition is not satisfied, and the storage amount of the nitrogen oxide storage amount increases, the air-fuel ratio of the exhaust gas is reduced. By executing the temperature raising control that leans and raises the exhaust temperature to a predetermined temperature or higher of the selective reduction catalyst, it is possible to prepare to immediately enable purification with the selective reduction catalyst while suppressing fuel consumption. Thereby, even when nitrogen oxide flows out of the nitrogen oxide storage reduction catalyst when the purge control is started, the selective reduction catalyst can sufficiently reduce and remove the nitrogen oxide, Nitrogen oxide emissions can be reduced.

1 is a schematic configuration diagram of an intake / exhaust system of an engine in an embodiment of the present invention. It is a flowchart which shows the control procedure of the exhaust gas purification apparatus in the engine control unit of this embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an intake / exhaust system of a diesel engine (hereinafter referred to as an engine 2) according to a first embodiment to which an exhaust emission control device 1 of the present invention is applied.
The engine 2 is mounted on a vehicle as a travel drive source, and is a multi-cylinder direct injection type engine. In FIG. 1, only one cylinder is illustrated in a simplified manner. The engine 2 is configured to be able to inject fuel into the combustion chamber 4 of each cylinder at an arbitrary injection timing and injection amount from a fuel injection nozzle 3 provided in each cylinder.

An electronic control throttle valve 6 for adjusting the flow rate of fresh air is provided in the intake passage 5 of the engine 2.
In the exhaust passage 10 of the engine 2, a NOx storage catalyst 11 (nitrogen oxide storage reduction catalyst), a diesel particulate filter 12, and a selective reduction catalyst 13 are provided in order from the engine 2 toward the downstream.

The exhaust passage 10 between the diesel particulate filter 12 and the selective reduction catalyst 13 is provided with a urea water injector 14 (reducing agent supply unit) that injects and supplies urea water (urea aqueous solution). Urea water is supplied to the urea water injector 14 from a urea water tank (not shown) mounted on the vehicle.
The injection position of the urea water injector 14 is set so that the urea water injected from the urea water injector 14 into the exhaust passage 10 is hydrolyzed by the heat of the exhaust to generate ammonia and reach the selective reduction catalyst 13. .

The NOx storage catalyst 11 stores nitrogen oxide (hereinafter referred to as NOx) in the exhaust, and reduces and removes NOx under a high temperature rich atmosphere.
The diesel particulate filter 12 collects particulate matter whose main component is graphite in the exhaust gas.
The selective reduction catalyst 13 reduces and purifies NOx in the exhaust gas using ammonia generated from urea water as a reducing agent.

The engine 2 is provided with an EGR device 15. The EGR device 15 includes an EGR passage 16 that connects the intake passage 5 and the exhaust passage 10, and an EGR valve 17 that opens and closes the EGR passage 16.
Further, the engine 2 is provided with a rotation speed sensor 20 that detects the rotation speed of the engine 2. An air flow sensor 21 that detects an intake air flow rate is provided in the intake passage 5 of the engine 2. The selective reduction catalyst 13 is provided with a selective reduction catalyst temperature sensor 22 that detects the temperature of the selective reduction catalyst 13.

  The engine control unit 30 (nitrogen oxide storage amount estimation unit, purge control unit, temperature rise control unit) includes an input / output device, a storage device (ROM, RAM, nonvolatile RAM, etc.), a timer, and a central processing unit (CPU). Etc., and input detection information of various sensors such as the rotational speed sensor 20, the airflow sensor 21, the selective reduction catalyst temperature sensor 22, and information such as the accelerator operation amount of the vehicle, and based on the various information, The fuel injection amount and fuel injection timing from the fuel injection nozzle 3, the opening degree of the electronic control throttle valve 6, the urea water injection amount and urea water injection timing from the urea water injector 14, and the opening degree of the EGR valve 17 are calculated, Operation control of the engine 2 is performed by performing operation control of the various devices.

Further, the engine control unit 30 makes the air-fuel ratio stoichiometric or rich at 14.7 or less by post injection or the like, and raises the NOx storage catalyst 11 to a predetermined temperature or higher, so that the NOx stored in the NOx storage catalyst 11 is increased. It has a function of performing NOx purge for reducing and removing (purge control unit).
Next, NOx purge control for reducing and removing NOx stored in the NOx storage catalyst 11 will be described with reference to FIG.

FIG. 2 is a flowchart showing a NOx purge control procedure in the engine control unit 30.
The operation control of the exhaust emission control device 1 of this embodiment shown in FIG. 2 is repeatedly executed at predetermined intervals in the engine control unit 30 during engine operation.
First, in step S10, it is determined whether or not the NOx storage amount Qa (nitrogen oxide storage amount) stored in the NOx storage catalyst 11 is greater than a predetermined value A (first storage amount). The NOx occlusion amount Qa may be estimated based on, for example, the operation time and operation state of the engine 2 from the end of the previous NOx purge to the present (nitrogen oxide occlusion amount estimation unit). The predetermined value A is a threshold value for determining whether or not to perform NOx purge, and may be set to a value smaller than the maximum allowable amount of the NOx storage amount Qa in the NOx storage catalyst 11. When the NOx occlusion amount Qa is larger than the predetermined value A, the process proceeds to step S20. When the NOx occlusion amount Qa is equal to or less than the predetermined value A, this routine ends.

  In step S20, it is determined whether or not the NOx storage amount Qa is greater than a predetermined value B (second storage amount). The predetermined value B is a so-called slip determination value, which is larger than the predetermined value A, and determines whether or not NOx is not completely reduced and removed from the NOx storage catalyst 11 when NOx purge is performed. What is necessary is just to set to the possible threshold value. Further, the second storage amount may be set to the same value as the first storage amount. When the NOx occlusion amount Qa is larger than the predetermined value B, the process proceeds to step S30. When the NOx occlusion amount Qa is equal to or less than the predetermined value B, the process proceeds to step S60.

  In step S30, it is determined whether or not the ammonia adsorption amount Qb in the selective reduction catalyst 13 is larger than a predetermined value C. The ammonia adsorption amount Qb may be obtained, for example, by subtracting the consumption amount from the increment of the ammonia adsorption amount Qb in the selective reduction catalyst 13. The increase in the ammonia adsorption amount Qb can be calculated from the integrated value of the ammonia production rate and the ammonia adsorption rate depending on the urea water injection amount and the exhaust temperature. Note that the ammonia adsorption rate decreases when the amount of ammonia already adsorbed is large, and also depends on the temperature of the selective reduction catalyst 13. The consumption amount of the ammonia adsorption amount Qb is obtained by adding the NOx amount at the inlet of the selective reduction catalyst 13 to the integrated value of the NOx purification rate depending on the temperature of the selective reduction catalyst 13 and the ammonia preadsorption amount. It is obtained as a value obtained by adding the amount of ammonia desorbed or oxidized from the selective reduction catalyst 13 depending on the amount of adsorption. The predetermined value C may be set to an ammonia adsorption amount that can sufficiently reduce and remove NOx that flows out from the NOx storage catalyst 11 and flows into the selective reduction catalyst 13 when the NOx purge is performed. When the ammonia adsorption amount Qb is larger than the predetermined value C, the process proceeds to step S40. If the ammonia adsorption amount Qb is less than or equal to the predetermined value C, the process proceeds to step S80.

  In step S40, the selective reduction catalyst temperature Tb is input from the selective reduction catalyst temperature sensor 22, and it is determined whether or not the selective reduction catalyst temperature Tb is lower than a predetermined value T1 (predetermined temperature). The predetermined value T1 may be set to an activation temperature at which NOx can be sufficiently reduced and removed in the selective reduction catalyst 13. When the selective reduction catalyst temperature Tb is lower than the predetermined value T1, the process proceeds to step S50. When the selective reduction catalyst temperature Tb is equal to or higher than the predetermined value T1, the process proceeds to step S60.

  In step S50, the selective reduction catalyst activation temperature increase control (temperature increase control) is executed (temperature increase control unit). The temperature increase control for selective reduction catalyst activation raises the exhaust temperature by reducing the intake air amount by the electronically controlled throttle valve 6 or increasing the fuel injection amount from the fuel injection nozzle 3. Here, the air-fuel ratio of the exhaust gas is controlled to be leaner than the NOx purge so that NOx does not flow out from the NOx storage catalyst 11. For example, when the engine 2 of the present embodiment is a lean burn engine having an air-fuel ratio of about 20 during normal combustion, stoichiometric or rich combustion with an air-fuel ratio of 14.7 or less is performed during NOx purge, and the selective reduction catalyst activity for this step is used. In the temperature increase control and the ammonia generation temperature increase control in step S90, which will be described later, the air-fuel ratio may be set to about 17-18. That is, although the air-fuel ratio is changed to a richer side than normal lean combustion, lean combustion with an air-fuel ratio of 14.7 or more is performed. Thereby, it is possible to raise the temperature of the selective reduction catalyst 13 while suppressing an increase in the outflow of NOx. In addition, the NOx outflow can be suppressed even in such a light lean manner because the NOx stored in the NOx storage catalyst is accumulated as nitrates (such as barium nitrate), so the nitrates are decomposed and NOx is released. This is because NOx is not released in light lean because it is necessary to have a reducing atmosphere. Then, the process proceeds to step S60.

  In step S60, it is determined whether or not a NOx purge condition is satisfied. The NOx purge condition may be determined, for example, by detecting that the temperature of the NOx occlusion catalyst 11 is equal to or higher than a predetermined temperature (first predetermined temperature) at which NOx purging is possible, or determining that the operation is a high load operation. If the NOx purge condition is satisfied, the process proceeds to step S70. If the NOx purge condition is not satisfied, this routine is terminated.

In step S70, NOx purge (purge control) is executed. As described above, the NOx purge is performed by maintaining the NOx storage catalyst 11 at or above the first predetermined temperature at which purging is possible by post-injection or the like and making the air-fuel ratio stoichiometric or rich below 14.7. Then, this routine ends.
In step S80, the selective reduction catalyst temperature Tb is input from the selective reduction catalyst temperature sensor 22, and it is determined whether or not the selective reduction catalyst temperature Tb is lower than a predetermined value T2. The predetermined value T2 may be set to a temperature at which urea water is hydrolyzed and ammonia is generated. When the selective reduction catalyst temperature Tb is lower than the predetermined value T2, the process proceeds to step S90. When the selective reduction catalyst temperature Tb is equal to or higher than the predetermined value T2, the process proceeds to step S100.

  In step S90, the temperature increase control for ammonia generation is executed. The temperature increase control for ammonia generation is control for hydrolyzing the urea water injected from the urea water injector 14 to generate ammonia and adsorb it to the selective reduction catalyst 13, and the temperature increase for selective reduction catalyst activity in step S50. Similar to the control, the exhaust gas temperature is raised by reducing the intake air amount or increasing the fuel injection amount. Then, the process proceeds to step S100.

In step S <b> 100, urea water for reducing and removing NOx in the selective reduction catalyst 13 is injected from the urea water injector 14. Then, the process returns to step S30.
As described above, in the present embodiment, the exhaust passage 10 includes the NOx storage catalyst 11 having excellent purification performance in the low temperature region and the selective reduction catalyst 13 having excellent purification performance in the high temperature region. The NOx purification performance can be improved. When the NOx storage catalyst 11 stores a large amount of NOx exceeding the predetermined value A, a NOx purge is required. For example, a low load operation such as an idling operation is continued, and the temperature of the NOx storage catalyst 11 decreases. When NOx purge is impossible, the NOx occlusion amount Qa in the NOx occlusion catalyst 11 further increases. In the present embodiment, the EGR device 15 is provided as a device that suppresses the generation of NOx. However, in order to ensure combustion stability in an operating situation such as an idling operation, the exhaust gas recirculation amount is suppressed, and the generation of NOx is sufficient. It is difficult to suppress.

  Therefore, in the present embodiment, when the NOx occlusion amount Qa in the NOx occlusion catalyst 11 becomes larger than the predetermined value B which is the slip determination value, the exhaust gas temperature is raised by performing the temperature reduction control for selective reduction catalyst activation. Thus, the temperature of the selective reduction catalyst 13 is raised to a predetermined value T1, which is the activation temperature. Thereby, even if NOx flows out from the NOx storage catalyst 11, it can be purified by the selective reduction catalyst 13 downstream of the NOx storage catalyst 11, and the discharge of NOx into the atmosphere can be reduced. In this temperature increase control, fuel consumption can be suppressed by keeping the air-fuel ratio lean.

  Further, when the NOx occlusion amount Qa in the NOx occlusion catalyst 11 is larger than the predetermined value B and the ammonia adsorption amount Qb of the selective reduction catalyst 13 has not reached the predetermined value C, the temperature rise control for ammonia generation is performed to increase the urea. The selective reduction catalyst 13 is heated to a temperature at which it can be hydrolyzed, and urea water is injected, so that the selective reduction catalyst 13 is not only sufficiently activated but also the ammonia adsorption amount of the selective reduction catalyst 13. Qb can be secured sufficiently.

  As described above, in the present embodiment, when the NOx storage amount Qa exceeds the predetermined value A and the NOx purge is requested, the NOx storage amount Qa in the NOx storage catalyst 11 becomes larger than the slip determination value. Is switched from NOx purge to temperature increase control (temperature increase control, ammonia generation temperature increase control), so that when the NOx purge is impossible, the selective reduction catalyst 13 is sufficiently adsorbed and sufficiently activated. The selective reduction catalyst 13 can be prepared so as to be able to purify NOx. Therefore, for example, when NOx purge becomes possible due to high load operation, even if NOx flows out from the NOx storage catalyst 11 at the start of the NOx purge, it is immediately and sufficiently purified by the selective reduction catalyst 13. It is possible to reduce NOx emission into the atmosphere and improve exhaust purification performance. In addition, since the temperature of the NOx storage catalyst 11 is also raised by these temperature rise controls, the start of the NOx purge can be accelerated.

  In the above embodiment, the temperature rise control is performed when the NOx occlusion amount Qa in the NOx occlusion catalyst 11 becomes larger than the predetermined value B which is the slip determination value, but the NOx occlusion amount is less than the predetermined value B and is normal. Even in the case of combustion, when the load of the engine 2 is in a high-load operation state of a predetermined value or more, the operation control step S30 and subsequent steps shown in FIG. This predetermined value is a load in an operating state in which the NOx storage catalyst 11 becomes hot and NOx flows out.

In this way, by controlling the temperature increase of the selective reduction catalyst 13 even in a high load operation, the NOx purification rate of the NOx storage catalyst 11 is reduced, and even at high temperatures where NOx flows out of the NOx storage catalyst 11, NOx. NOx can be sufficiently removed in the selective reduction catalyst 13 downstream of the storage catalyst 11, and the emission of NOx into the atmosphere can be suppressed.
The present invention is not limited to the above embodiment. The present invention can be widely applied to an engine having an NOx storage catalyst and a selective reduction catalyst in an exhaust passage as an exhaust purification device.

2 Engine 10 Exhaust passage 11 NOx storage catalyst (nitrogen oxide storage reduction catalyst)
13 Selective reduction catalyst 14 Urea water injector (reducing agent supply unit)
30 Engine control unit (nitrogen oxide storage amount estimation unit, purge control unit, temperature rise control unit)

Claims (5)

A nitrogen oxide storage reduction catalyst that is provided in the exhaust passage of the engine and stores the nitrogen oxide in the exhaust;
A selective reduction catalyst that is provided in the exhaust passage on the downstream side of the nitrogen oxide storage reduction catalyst and that reduces and removes nitrogen oxide using a reducing agent;
A reducing agent supply unit for supplying the reducing agent to the selective reduction catalyst;
A nitrogen oxide storage amount estimation unit for estimating the nitrogen oxide storage amount of the nitrogen oxide storage reduction catalyst;
When a predetermined purge condition is satisfied in a state where the nitrogen oxide storage amount is larger than the predetermined first storage amount, the exhaust gas air-fuel ratio is stoichiometric or rich, and the nitrogen oxide storage reduction catalyst A purge control unit for performing purge control to reduce the nitrogen oxides occluded in
If the purge condition is not satisfied when the nitrogen oxide storage amount is larger than the first storage amount, the reducing agent supply unit causes the reducing agent supply unit to make the reducing agent in a lean state. And a temperature increase control unit for performing temperature increase control for supplying the reducing agent to the selective reduction catalyst and increasing the temperature of the selective reduction catalyst to a predetermined temperature or higher,
An exhaust emission control device for an engine comprising:   2. The engine according to claim 1, wherein the purge control unit forcibly switches from the temperature increase control to the purge control when the purge condition is satisfied during execution of the temperature increase control. Exhaust purification device.   3. The engine exhaust purification device according to claim 1, wherein the temperature increase control is performed by at least one of a restriction of an intake amount of the engine and an increase of a fuel injection amount of the engine.   The reducing agent supply unit supplies an aqueous urea solution to the exhaust passage on the upstream side of the selective reduction catalyst, and supplies ammonia obtained by hydrolysis of the aqueous urea solution to the selective reduction catalyst. The exhaust emission control device for an engine according to any one of claims 1 to 3.   The engine exhaust according to any one of claims 1 to 4, wherein when the engine load is equal to or greater than a predetermined value, the temperature increase control is executed regardless of the nitrogen oxide storage amount. Purification equipment.

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