JP2016176430A - 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 for occluding NOx in exhaust gas; and a selective reduction catalyst 13 provided downstream of the NOx occlusion catalyst 11 for reducing and eliminating NOx by using ammonia generated by hydrolyzing urea water. An engine control unit 30 of the engine estimates NOx occlusion amount Qa of the NOx occlusion catalyst 11, 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 the exhaust gas stoichiometric or rich when the NOx occlusion amount Qa exceeds a predetermined value A, and performs temperature increase control for 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 C that is set larger than the predetermined value A and larger than a slip determination value B of the NOx occlusion catalyst 11.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, in order to purify NOx (nitrogen oxide) in engine exhaust, an exhaust purification catalyst such as a NOx storage catalyst has been developed.
The NOx storage catalyst stores NOx under a lean atmosphere and reduces NOx to nitrogen under a stoichiometric or rich atmosphere. Furthermore, in order to improve the purification rate of NOx in the exhaust gas, there are some which arrange an NOx purification catalyst for purifying nitrogen oxides in series with the NOx storage catalyst in the exhaust passage of the engine.

  For example, Patent Document 1 discloses a diesel particulate filter that includes a NOx storage catalyst in an exhaust passage of a diesel engine and collects PM (particulate matter) in exhaust gas downstream thereof, and selective reduction that purifies NOx. An exhaust purification device including a catalyst (NOx purification catalyst) 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, the NOx purge cannot be performed. In the configuration in which the NOx purification catalyst such as 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 purified by the downstream NOx purification catalyst, but the load is low. In the state where the operation has continued, the temperature of the NOx purification catalyst is lowered along with the temperature decline of the NOx storage catalyst, and there is a risk that the NOx purification catalyst cannot sufficiently remove NOx and flows 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 nitrogen oxide purification catalyst in an exhaust passage. There is to do.

  In order to achieve the above object, an exhaust emission control device for an engine of the present invention includes a nitrogen oxide storage reduction catalyst that is provided in an exhaust passage of an engine and stores nitrogen oxides in the exhaust, and the nitrogen oxide storage reduction A nitrogen oxide purification catalyst that is provided in the exhaust passage on the downstream side of the catalyst and purifies nitrogen oxide; and a nitrogen oxide storage amount estimation unit that estimates a nitrogen oxide storage amount of the nitrogen oxide storage reduction catalyst; When the predetermined purge condition is satisfied in a state where the nitrogen oxide storage amount is larger than the predetermined first storage amount, the air-fuel ratio of the exhaust is made stoichiometric or rich so that the nitrogen oxide storage reduction catalyst A purge control unit that executes purge control for reducing the stored nitrogen oxides, and when the purge condition is not satisfied, the nitrogen oxide storage amount is set to be larger than the predetermined first storage amount. The purge control standby state in which the purge control is not performed is maintained until the predetermined second storage amount is reached, and the nitrogen oxide storage amount is not increased and the nitrogen oxide storage amount is greater than the first storage amount. A temperature increase control unit that executes a temperature increase control that makes the air-fuel ratio of the exhaust lean and raises the exhaust temperature of the engine to an activation temperature or higher when it exceeds a predetermined second storage amount that is set And.

Preferably, the predetermined second occlusion amount is the nitrogen oxide occlusion in which release of the nitrogen oxide is not reduced by the nitrogen oxide occlusion reduction catalyst when the purge control is performed. Preferably, the temperature increase control is performed by decreasing the intake air amount of the engine when the engine load is lower than a predetermined value.

Preferably, the temperature increase control is performed by increasing the fuel injection amount of the engine when the engine load is equal to or greater than the predetermined value.
Preferably, the nitrogen oxide purification catalyst is a selective reduction catalyst that purifies nitrogen oxide using ammonia, and the urea aqueous solution that supplies the urea aqueous solution to the exhaust passage upstream of the nitrogen oxide purification catalyst. It is preferable to provide a supply unit, and the temperature increase control may raise the exhaust gas temperature to a temperature at which ammonia is generated by hydrolyzing the urea aqueous solution supplied by the urea aqueous solution supply unit.

  According to the exhaust purification device for an engine of the present invention, the nitrogen oxide storage reduction catalyst is provided in the exhaust passage, and the nitrogen oxide purification catalyst is provided downstream thereof, so that the nitrogen oxide in the exhaust is first introduced. It is stored in the nitrogen oxide storage reduction catalyst. When the nitrogen oxide storage amount of the nitrogen oxide storage reduction catalyst becomes larger than the first storage amount and the purge condition is satisfied, the nitrogen oxide is reduced and removed by purge control, and the storage performance of the nitrogen oxide storage reduction catalyst is restored. To do.

  Here, for example, when the nitrogen oxide storage / reduction catalyst is at a low temperature and the nitrogen oxide cannot be reduced and removed and the purge condition is not satisfied, the purge control is waited, and the nitrogen oxide storage amount becomes the second storage amount. When the air-fuel ratio becomes larger, the temperature of the exhaust gas is made lean and the temperature rise control is performed to raise the nitrogen oxide purification catalyst to the activation temperature or higher, thereby suppressing the fuel consumption and the nitrogen oxide purification catalyst. Purification can be made possible.

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 (hereinafter referred to as filter) 12, and a selective reduction catalyst 13 (nitrogen oxidation) are sequentially arranged downstream from the engine 2. Material purification catalyst).

The exhaust passage 10 between the filter 12 and the selective reduction catalyst 13 is provided with a urea water injector 14 (urea aqueous solution supply unit) for injecting and supplying 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 occlusion catalyst 11 occludes nitrogen oxide (hereinafter referred to as NOx) in the exhaust gas, and reduces and removes NOx in a high temperature rich atmosphere.
The 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.

Further, the engine 2 is provided with an exhaust gas recirculation device 15. The exhaust gas recirculation device 15 includes an exhaust gas recirculation passage 16 that connects the intake passage 5 and the exhaust passage 10, and an exhaust gas recirculation valve 17 that opens and closes the exhaust gas recirculation 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 NOx storage catalyst 11 is provided with a NOx storage catalyst temperature sensor 23 that detects the temperature of the NOx storage catalyst 11.

  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). The detection information of various sensors such as the rotational speed sensor 20, the air flow sensor 21, the selective reduction catalyst temperature sensor 22, the NOx occlusion catalyst temperature sensor 23, and information such as the accelerator operation amount of the vehicle are input. Based on the various information, the fuel injection amount from the fuel injection nozzle 3, the fuel injection timing, the opening degree of the electronic control throttle valve 6, the urea water injection amount from the urea water injector 14, the urea water injection timing, the exhaust gas recirculation valve 17 The operation of the engine 2 is controlled by controlling the operation 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 selective reduction catalyst 13 to a predetermined temperature or higher so that the NOx stored in the NOx storage catalyst 11 is stored. Has a purge control function for performing NOx purge for reducing and removing (purge control unit).
Next, NOx purge 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 control of the exhaust purification device 1 of the present 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 of the engine 2 from the end of the previous NOx purge and the operation state thereof (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.

Here, while proceeding from step S10 to step S20, a purge control standby state in which purge control is not performed is maintained.
In step S20, it is determined whether or not the NOx storage amount Qa is larger than a predetermined value C (second storage amount). The predetermined value C is larger than the predetermined value A, and may be set to a value smaller than a so-called slip determination value B (slip value) that causes NOx to flow out without being reduced and removed when NOx purge is performed. . When the NOx occlusion amount Qa is larger than the predetermined value C, the process proceeds to step S30. When the NOx occlusion amount Qa is equal to or less than the predetermined value C, the process proceeds to step S40.

  In step S30, 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 higher than a predetermined value T1. 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 higher than the predetermined value T1, the process proceeds to step S40. When the selective reduction catalyst temperature Tb is equal to or lower than the predetermined value T1, the process proceeds to step S60.

  In step S40, 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 the temperature at which NOx purging is possible, or by determining that the operation is a high load operation at which the exhaust temperature becomes NOx purging. If the NOx purge condition is satisfied, the process proceeds to step S50. If the NOx purge condition is not satisfied, this routine is terminated.

In step S50, NOx purge (purge control) is executed. As described above, the NOx purge is performed by making the air-fuel ratio stoichiometric or rich below 14.7 while maintaining the NOx storage catalyst 11 at a temperature that can be purged or higher by post injection or the like. Then, this routine ends.
In step S60, the NOx storage catalyst temperature Ta is input from the NOx storage catalyst temperature sensor 23, and it is determined whether or not the NOx storage catalyst temperature Ta is lower than a predetermined value T2. The predetermined value T2 may be set to a temperature at which the NOx storage catalyst 11 can be heated by post injection. If the NOx storage catalyst temperature Ta is lower than the predetermined value T2, the process proceeds to step S70. If the NOx storage catalyst temperature Ta is equal to or higher than the predetermined value T2, the process proceeds to step S80.

In step S70, an intake throttle that reduces the intake amount of the engine 1 is executed. More specifically, the opening degree of the electronic control throttle valve 6 is decreased to lower the air-fuel ratio, and the NOx storage catalyst temperature Ta is increased. Then, the process returns to step S60.
In step S80, post injection is executed. 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 NOx is stored in the upstream NOx storage catalyst 11 more than the predetermined value A, NOx purge is required, but for example, low load operation such as idling operation is continued and the temperature of the NOx storage catalyst 11 is increased. When the NOx purge is impossible, the NOx occlusion amount Qa in the NOx occlusion catalyst 11 further increases. If the NOx occlusion amount becomes larger than the slip determination value B, NOx may flow out from the NOx occlusion catalyst 11. In the present embodiment, the exhaust gas recirculation device 15 is provided as a device that suppresses the generation of NOx. However, in order to ensure combustion stability in an operating condition such as an idling operation, the recirculation amount of the exhaust gas is suppressed and the generation of NOx is suppressed. It is difficult to suppress sufficiently.

  Therefore, in the present embodiment, when the NOx occlusion amount Qa exceeds the predetermined value C set larger than the predetermined value A, the temperature rise control is performed by intake throttling or post injection, and the temperature of the selective reduction catalyst 13 is set to the activation temperature or higher. To do. Thus, even if NOx flows out from the NOx storage catalyst 11, it can be removed by the selective reduction catalyst 13 on the downstream side. Further, since the predetermined value C is set lower than the slip determination value B, the temperature of the selective reduction catalyst 13 can be raised before the NOx occlusion amount Qa reaches the slip determination value B.

When performing temperature increase control, if the NOx occlusion catalyst temperature Ta is lower than the predetermined value T2, the fuel does not burn even after post injection and the temperature does not increase. Raise. If the NOx occlusion catalyst temperature Ta is equal to or higher than the predetermined value T2, post injection can be performed to increase the fuel injection amount and efficiently raise the temperature.
Further, in this temperature increase control, by keeping the air-fuel ratio lean, it is possible to suppress the outflow of NOx from the NOx storage catalyst 11 and to suppress fuel consumption.

  Thus, in the present embodiment, the NOx occlusion amount Qa in the NOx occlusion catalyst 11 is smaller than the slip determination value B when the NOx occlusion amount Qa exceeds the predetermined value A and the NOx purge is requested. When it becomes larger, the temperature of the selective reduction catalyst 13 can be raised before the NOx purge reaches the slip determination value B by switching from the NOx purge to the temperature raising control. Therefore, for example, even when NOx purge is possible when high load operation is performed and the NOx occlusion amount Qa exceeds the slip determination value B, NOx flows out of the NOx occlusion catalyst 11 at the start of NOx purge. The selective reduction catalyst 13 can immediately and sufficiently purify, reduce NOx emission into the atmosphere, and improve the exhaust purification performance. Moreover, since the NOx storage catalyst 11 is also heated by this temperature increase control, the start of the NOx purge can be accelerated.

  When the catalyst downstream of the NOx storage catalyst 11 is the selective reduction catalyst 13 as in this embodiment, the temperature rise control is performed to increase the exhaust temperature and immediately after the urea water is injected. Ammonia can be generated, and ammonia can be secured in the selective reduction catalyst 13. Thereby, the NOx flowing out from the NOx storage catalyst 11 can be quickly purified in the selective reduction catalyst 13.

  The present invention is not limited to the above embodiment. In this embodiment, the selective reduction catalyst 13 is provided downstream of the NOx storage catalyst 11, but other catalysts may be used as long as they are catalysts for purifying NOx. For example, instead of the selective reduction catalyst 13, an NOx storage catalyst similar to the upstream NOx storage catalyst 11 may be provided. Even if two catalysts of the same type are provided in this way, the temperature of the two catalysts is different because the filter 12 is provided between them, so the upstream catalyst and the downstream catalyst have a wide range of exhaust temperatures. As in the present embodiment, the purification efficiency can be increased, and when the downstream catalyst is heated up and NOx flows out from the upstream side, the downstream catalyst can be immediately purified.

  The present invention can be widely applied to an engine having an NOx storage catalyst and a NOx exhaust purification 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 (nitrogen oxide purification catalyst)
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 nitrogen oxide purification catalyst that is provided in the exhaust passage downstream of the nitrogen oxide storage reduction catalyst and purifies nitrogen oxide;
A nitrogen oxide storage amount estimation unit for estimating the nitrogen oxide storage amount of the nitrogen oxide storage reduction catalyst;
When the predetermined purge condition is satisfied in a state where the nitrogen oxide storage amount is larger than the predetermined first storage amount, the air-fuel ratio of the exhaust is made stoichiometric or rich so that the nitrogen oxide storage reduction catalyst A purge control unit that performs purge control to reduce the stored nitrogen oxides;
When the purge condition is not satisfied, the purge control standby state in which the purge control is not performed until the nitrogen oxide storage amount reaches a predetermined second storage amount set larger than the predetermined first storage amount. When the nitrogen oxide storage amount becomes larger than the predetermined second storage amount set larger than the first storage amount without the nitrogen oxide storage amount, An engine exhaust gas purification apparatus comprising: a temperature increase control unit that executes a temperature increase control that makes the air-fuel ratio lean and raises the exhaust temperature of the engine to an activation temperature or higher.   The predetermined second occlusion amount is the occlusion amount of the nitrogen oxide storage reduction catalyst that starts to be released without reduction of the nitrogen oxide by the nitrogen oxide storage reduction catalyst when the purge control is executed. 2. The engine exhaust gas purification apparatus according to claim 1, wherein the engine exhaust gas purification apparatus is set smaller.   3. The engine exhaust purification system according to claim 1, wherein the temperature increase control is performed by reducing an intake amount of the engine when the load of the engine is lower than a predetermined value.   4. The engine exhaust gas purification apparatus according to claim 3, wherein the temperature increase control is performed by increasing a fuel injection amount of the engine when a load of the engine is equal to or greater than the predetermined value. The nitrogen oxide purification catalyst is a selective reduction catalyst that purifies nitrogen oxide using ammonia,
A urea aqueous solution supply unit for supplying urea aqueous solution to the exhaust passage upstream of the nitrogen oxide purification catalyst;
5. The temperature raising control increases the exhaust temperature to a temperature at which ammonia is generated by hydrolyzing the urea aqueous solution supplied by the urea aqueous solution supply unit. 6. Engine exhaust purification system.

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