JP2016176429A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve exhaust emission control performance.SOLUTION: An exhaust emission control device in an exhaust passage 10 of an engine 2, includes: a filter 12 for collecting PMs in exhaust gas; a selective reduction catalyst 13 provided downstream of the filter 12 for reducing and eliminating NOx by using ammonia generated by hydrolyzing urea water; and an NOx occlusion catalyst 11 provided upstream of the filter 12. An engine control unit 30 performs filter regeneration control for burning and removing the PMs stored in the filter 12 and purge control for removing a sulfur constituent in exhaust gas adsorbed in the NOx occlusion catalyst 11 by increasing exhaust temperature in accordance with request determination. When the request determination of the filter regeneration control or the purge control is made, the engine control unit performs reducing agent supply control for supplying ammonia to the selective reduction catalyst 13 by increasing a temperature of the selective reduction catalyst 13 to an activation temperature or higher and injecting an urea water solution before the filter regeneration control or the purge control is started.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust emission control device for an engine provided with an exhaust purification catalyst for purifying nitrogen oxides in exhaust gas and a filter for collecting particulate matter.

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, and particulate matter (PM) in diesel engine exhaust can be removed. Diesel particulate filters and the like to be collected have been developed.
The NOx storage catalyst stores NOx under a lean atmosphere and reduces NOx to nitrogen under a rich atmosphere. The selective reduction catalyst reduces NOx in the exhaust gas to nitrogen using a reducing agent such as ammonia.

  For example, in Patent Document 1, a diesel particulate filter is provided in an exhaust passage of a diesel engine, an NOx storage catalyst is provided in an exhaust passage upstream of the diesel particulate filter, and an exhaust passage downstream of the diesel particulate filter is selected. An exhaust emission control device including a reduction catalyst is disclosed.

Japanese Patent No. 4730336

  By the way, in an engine provided with a diesel particulate filter in the exhaust passage, filter regeneration is performed as necessary in order to remove PM collected in the diesel particulate filter. Further, in an engine provided with a NOx occlusion catalyst in the exhaust passage, NOx purge for reducing and removing NOx occluded in the NOx occlusion catalyst and S for removing sulfur components in the exhaust that are adsorbed by the NOx occlusion catalyst. Purge is performed as necessary. These filter regenerations and various purges are performed, for example, by raising the temperature of the exhaust gas or making the air-fuel ratio in the exhaust gas rich by controlling fuel injection of the engine. Further, there is also known an apparatus that includes an oxidation catalyst instead of the NOx storage catalyst, and can easily perform filter regeneration and various purges by oxidizing the fuel in the exhaust discharged by post injection and raising the exhaust temperature.

  However, among these, filter regeneration and S purge require the exhaust gas temperature to be high, and there is a risk that the amount of NOx generated will increase. In particular, in an engine equipped with an exhaust gas recirculation device, it is necessary to control the exhaust gas recirculation device so that unburned fuel flowing into the exhaust gas does not recirculate to the intake air during filter regeneration or S purge, which also generates NOx. The amount further increases.

  In the engine having a selective reduction catalyst downstream of the exhaust passage as in Patent Document 1, even if the NOx emission amount increases during filter regeneration or S purge, it can be purified by the selective reduction catalyst. For example, when the low-reduction operation continues and the temperature of the selective reduction catalyst is lowered, or when ammonia is not sufficiently stored in the selective reduction catalyst, NOx cannot be sufficiently processed in the selective reduction catalyst, There is a possibility that it will flow 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 having an exhaust purification filter and a selective reduction catalyst in an exhaust passage. is there.

  In order to achieve the above object, an exhaust emission control device for an engine according to the present invention is provided in an exhaust passage of an engine, and includes a filter that collects particulate matter in exhaust, and an exhaust passage downstream of the filter. A selective reduction catalyst provided for reducing and removing nitrogen oxides using a reducing agent supplied from a reducing agent supply unit that supplies a reducing agent; and an exhaust purification catalyst provided in the exhaust passage on the upstream side of the filter; A filter regeneration control unit for performing filter regeneration control for increasing the exhaust gas temperature to burn and remove the particulate matter collected by the filter; and exhaust gas adsorbed by the exhaust gas purification catalyst by increasing the exhaust gas temperature A purge control unit that performs purge control to remove sulfur components therein, a determination unit that performs a request determination when a request for performing the filter regeneration control or the purge control is made, and the filter When the regeneration regeneration control or the purge control is determined to be required, before the start of the filter regeneration control or purge control, the temperature of the selective reduction catalyst is raised to a predetermined temperature or more and the reducing agent supply unit And a purification preparation control unit that supplies a reducing agent.

Preferably, a timer for measuring a time after the request determination of the filter regeneration control or the purge control is provided, and the purification preparation control unit has a predetermined time after the request determination by the timer. When the time has elapsed, the filter regeneration control or the purge control may be forcibly started.
Preferably, the predetermined time when the request for the filter regeneration control is determined is set to be shorter than the predetermined time when the request for the purge control is determined.

Preferably, when the filter regeneration control or the purge control is interrupted, the predetermined time at the start of the next filter regeneration control or the purge control may be set shorter than the first time.
Preferably, when both the filter regeneration control and the purge control are requested and determined, the purge control is performed after the filter regeneration control is performed.

  According to the engine exhaust gas purification apparatus of the present invention, when the filter regeneration control or the purge control is determined to be required, the temperature of the selective reduction catalyst rises above a predetermined temperature and the reducing agent is supplied before the start of the execution. Therefore, it becomes possible to reduce and remove nitrogen oxides in the selective reduction catalyst. When filter regeneration control or purge control is performed, even if nitrogen oxide in the exhaust increases and nitrogen oxide flows out downstream of the filter, nitrogen oxide is reduced and removed by the selective reduction catalyst downstream of the filter. And emission of nitrogen oxides into the atmosphere 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 filter regeneration control procedure in the engine control unit of this embodiment. It is a flowchart which shows the S purge control procedure 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, 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 (exhaust purification catalyst), a diesel particulate filter (hereinafter referred to as 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 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 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 engine control unit 30 (filter regeneration control unit, purge control unit, determination unit, purification preparation 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 exhaust gas recirculation valve 17 are calculated. The operation control of the engine 2 is performed by controlling the operation of the various devices.

Further, the engine control unit 30 can perform filter regeneration control that burns and removes the PM collected by the filter 12 by raising the exhaust gas temperature by intake throttle, post injection, or the like.
Further, the engine control unit 30 raises the exhaust temperature of the engine 2 by, for example, a so-called intake throttle that throttles the opening degree of the electronic control throttle valve 6 and makes the air-fuel ratio of the exhaust rich, and is adsorbed by the NOx storage catalyst 11. S purge control (purge control) for reducing and removing the sulfur oxide (sulfur component) is possible (purge control unit).

Next, filter regeneration control for burning and removing PM collected by the filter 12 will be described with reference to FIG.
FIG. 2 is a flowchart showing a filter regeneration control procedure in the engine control unit 30.
The filter regeneration control 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 there is a filter regeneration request. The filter regeneration request is determined when a predetermined amount or more of PM has accumulated on the filter 12. The accumulation of PM is estimated based on the difference in the exhaust gas pressure before and after the filter 12 detected in the engine control unit 30 based on the operation time and the operation state of the engine 2 since the end of the previous filter regeneration. It may be determined (determination unit). If there is a filter regeneration request, the process proceeds to step S10. If there is no filter regeneration request, this routine ends.

In step S20, the timer 31 starts counting. Then, the process proceeds to step S30.
In step S30, it is determined whether or not the ammonia adsorption amount Qb in the selective reduction catalyst 13 is smaller than a predetermined value A. 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 A may be set to an ammonia adsorption amount that can sufficiently reduce and remove NOx flowing into the selective reduction catalyst 13 when the filter regeneration control is performed. If the ammonia adsorption amount Qb is smaller than the predetermined value A, the process proceeds to step S40. If the ammonia adsorption amount Qb is greater than or equal to the predetermined value A, the process proceeds to step S50.

In step S40, it is determined whether or not the count value Ct of the timer 31 is equal to or greater than a predetermined value t1. The predetermined value t1 may be set to a time that can wait for the start of filter regeneration after filter regeneration is requested. If the count value Ct is greater than or equal to the predetermined value t1, the process proceeds to step S50. If the count value Ct is less than the predetermined value t1, the process proceeds to step S80.
In step S50, the timer 31 is turned off (reset to 0). Then, the process proceeds to step S60.

In step S60, it is determined whether or not a filter regeneration condition is satisfied. The filter regeneration condition may be determined, for example, that the engine 2 is a predetermined or higher high load operation capable of performing filter regeneration. If the filter regeneration condition is satisfied, the process proceeds to step S70. If the filter regeneration condition is not satisfied, this routine is terminated.
In step S70, filter regeneration control is executed. As described above, the filter regeneration control raises the exhaust gas temperature by intake throttling, post-injection, etc., and burns and removes the PM collected by the filter 12 (filter regeneration control unit). 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 T1 (predetermined temperature). The predetermined value T1 may be set to a temperature that is equal to or higher than the activation temperature of the selective reduction catalyst 13 and that ammonia water is hydrolyzed to generate ammonia. When the selective reduction catalyst temperature Tb is lower than the predetermined value T1, the process proceeds to step S90. When the selective reduction catalyst temperature Tb is equal to or higher than the predetermined value T1, the process proceeds to step S100.

In step S90, temperature rise control is executed. The temperature rise control is a 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. The intake air amount is reduced or the fuel injection amount is increased. By doing so, the exhaust temperature is raised. 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.

Next, S purge control for removing sulfur oxide adsorbed on the NOx storage catalyst 11 will be described with reference to FIG.
FIG. 3 is a flowchart showing the S purge control procedure in the engine control unit 30.
The S purge control of this embodiment shown in FIG. 3 is repeatedly executed at predetermined intervals in the engine control unit 30 during engine operation.

  First, in step S210, it is determined whether or not there is an S purge request. The S purge request is determined as a request when sulfur oxide is adsorbed on the NOx storage catalyst 11 and the performance of the NOx storage catalyst 11 approaches an allowable level. This S purge request may be determined in the engine control unit 30 based on the engine operating time and operating conditions since the end of the previous S purge (determination unit). If there is an S purge request, the process proceeds to step S220. If there is no S purge request, this routine ends.

In step S220, the timer 31 starts counting. Then, the process proceeds to step S230.
In step S230, it is determined whether or not the ammonia adsorption amount Qb in the selective reduction catalyst 13 is smaller than a predetermined value B. The predetermined value B may be set to an ammonia adsorption amount that can sufficiently reduce and remove NOx flowing into the selective reduction catalyst 13 when the S purge control is performed. If the ammonia adsorption amount Qb is smaller than the predetermined value B, the process proceeds to step S240. If the ammonia adsorption amount Qb is greater than or equal to the predetermined value B, the process proceeds to step S250.

  In step S240, it is determined whether or not the count value Ct of the timer 31 is equal to or greater than a predetermined value t2. The predetermined value t2 may be set to a time during which the start of the S purge control can be waited after the S purge control is requested. The predetermined value t2 may be set longer than the predetermined value t1 set in the filter regeneration control. If the count value Ct is greater than or equal to the predetermined value t2, the process proceeds to step S250. If the count value Ct is less than the predetermined value t2, the process proceeds to step S280.

In step S250, the timer 31 is turned off (reset to 0). Then, the process proceeds to step S260.
In step S260, it is determined whether the S purge condition is satisfied. The S purge condition may be determined, for example, that the engine 2 is operating at a high load that is higher than a predetermined level that allows the S purge control. If the S purge condition is satisfied, the process proceeds to step S270. If the S purge condition is not satisfied, this routine is terminated.

In step S270, S purge control is executed. In the S purge control, as described above, the exhaust gas temperature is increased by intake throttling, post injection, or the like, and the sulfur oxide adsorbed on the NOx storage catalyst 11 is reduced and removed. Then, this routine ends.
In step S280, 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. The predetermined value T1 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 T1, the process proceeds to step S290. When the selective reduction catalyst temperature Tb is equal to or higher than the predetermined value T1, the process proceeds to step S300.

In step S290, temperature increase control is executed. The temperature rise control is a 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. The intake air amount is reduced or the fuel injection amount is increased. By doing so, the exhaust temperature is raised. Then, the process proceeds to step S300.
In step S300, 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 S230.

  As described above, in the present embodiment, since the exhaust passage 10 includes the filter 12 and the selective reduction catalyst 13, PM and NOx in the exhaust are removed. When a predetermined amount or more of PM accumulates on the filter 12 or a sulfur oxide adheres to the NOx storage catalyst 11 or more, a filter regeneration control or S purge control is required. Is executed, the exhaust gas recirculation valve 17 is closed so that the unburned fuel in the exhaust gas recirculates to the intake air and the accuracy of the fuel injection control of the engine 2 does not decrease. Therefore, when filter regeneration control or S purge control is executed, NOx in the exhaust gas flowing out downstream of the filter 12 increases.

  On the other hand, in the present embodiment, when the filter regeneration control is requested, if the ammonia adsorption amount Qb in the selective reduction catalyst 13 is smaller than the predetermined value A, urea water injection is performed before the filter regeneration control is performed. Is done. Further, when the S purge control is requested, if the ammonia adsorption amount Qb in the selective reduction catalyst 13 is less than the predetermined value B, the urea water injection is executed before the execution of the S purge control. Further, when the urea water injection is performed, if the temperature of the selective reduction catalyst 13 is lower than the predetermined value T1, the temperature of the selective reduction catalyst 13 is raised by performing temperature increase control before the urea water injection. Let As a result, NOx can be reduced and removed in the selective reduction catalyst 13. Therefore, when NOx in the exhaust gas increases and NOx flows out downstream of the filter 12 when the filter regeneration control or NOx purge control is performed, NOx is reduced and removed by the selective reduction catalyst 13 downstream of the filter 12. And NOx emissions into the atmosphere can be reduced.

  In addition, the time from when the filter regeneration control or the S purge control is requested is measured, and when a predetermined value t1 has elapsed for the filter regeneration control request and when the predetermined time t2 has elapsed for the S purge control request, Regardless of the ammonia adsorption amount Qb, filter regeneration control or S purge control is started. Thus, for example, if the ammonia adsorption amount Qb in the selective reduction catalyst 13 does not reach the predetermined values A and B even if the temperature increase control and urea water injection are performed, excessive start standby of the filter regeneration control and S purge control is performed. It is possible to prevent PM and NOx storage catalyst 11 from accumulating on the filter 12 and to suppress the accumulation of sulfur oxides.

  Further, since the predetermined value t2 that is the S purge waiting time is set to be longer than the predetermined value t1 that is the filter regeneration waiting time, the filter regeneration control is immediately started rather than the S purge control. It will be. Thereby, excessive accumulation of PM in the filter 12 can be further prevented, an increase in exhaust resistance can be suppressed, and engine operation can be prevented from becoming unstable particularly in a low rotation range.

  When both filter regeneration control and S purge control are requested, filter regeneration control may be performed with priority. Usually, since the S purge control needs to raise the temperature more than the filter regeneration control, if the S purge control is performed first in a state where PM is deposited on the filter 12 and sulfur oxide is deposited on the NOx storage catalyst 11, the S purge control is performed. During the purge control, PM may burn abnormally in the filter 12 and the filter 12 may burn out. By performing the S regeneration control after the filter regeneration control is performed first and the PM of the filter 12 is removed, such abnormal combustion of PM can be prevented.

  Further, when the filter regeneration control or the S purge control is being executed, if these conditions are not satisfied and the process is interrupted, it is set when the next filter regeneration control or the S purge control is started. The predetermined value t1 or the predetermined value t2 may be set shorter than the predetermined value t1 or the predetermined value t2 when not interrupted. As described above, when the filter regeneration control or the S purge control is interrupted, it is highly possible that the temperature of the selective reduction catalyst 13 has increased and the amount of adsorption of ammonia has increased. Even if the S purge control is started, the NOx purification rate in the selective reduction catalyst 13 can be secured.

In addition, this invention is not limited to the said embodiment. For example, only one of the filter regeneration control and the S purge control may be controlled so that ammonia is sufficiently adsorbed on the selective reduction catalyst 13 before the start of the control as in the present embodiment.
Further, the NOx storage catalyst 11 in the present embodiment may be another exhaust purification catalyst such as an oxidation catalyst. Therefore, even in a configuration provided with an exhaust purification catalyst other than such an NOx storage catalyst, in the S purge control for removing the sulfur oxide adsorbed on the exhaust purification catalyst, the outflowing NOx is separated in the selective reduction catalyst 13. Can be removed.

  The present invention can be widely applied to an engine having a selective reduction catalyst in an exhaust passage and a filter for collecting PM and an exhaust purification catalyst upstream thereof as an exhaust purification device.

2 Engine 10 Exhaust passage 11 NOx storage catalyst (Exhaust gas purification catalyst)
12 Filter 13 Selective reduction catalyst 14 Urea water injector (reducing agent supply unit)
30 Engine control unit (filter regeneration control unit, purge control unit, determination unit, temperature rise control unit)
31 timer

Claims (5)

A filter provided in the exhaust passage of the engine for collecting particulate matter in the exhaust;
A selective reduction catalyst that is provided in the exhaust passage on the downstream side of the filter and that reduces and removes nitrogen oxides using a reducing agent supplied from a reducing agent supply unit that supplies a reducing agent;
An exhaust purification catalyst provided in the exhaust passage upstream of the filter;
A filter regeneration control unit for performing filter regeneration control for increasing the exhaust gas temperature and burning and removing the particulate matter collected by the filter;
A purge control unit that performs a purge control that raises the exhaust gas temperature and removes sulfur components in the exhaust gas adsorbed by the exhaust gas purification catalyst;
A determination unit that performs a request determination when an execution request for the filter regeneration control or the purge control is made;
When the request for the filter regeneration control or the purge control is determined, the temperature of the selective reduction catalyst is raised to a predetermined temperature or more and the reducing agent supply unit before the start of the filter regeneration control or the purge control. A purification preparation control unit for supplying the reducing agent;
An exhaust emission control device for an engine comprising: A timer for measuring a time from when the request is determined for the filter regeneration control or the purge control,
2. The purification preparation control unit forcibly starts the filter regeneration control or the purge control when a predetermined time has elapsed since the request determination by the timer. The engine exhaust gas purification apparatus as described.   The engine exhaust according to claim 2, wherein the predetermined time when the request for the filter regeneration control is determined is set shorter than the predetermined time when the request for the purge control is determined. Purification equipment.   When the filter regeneration control or the purge control is interrupted, the predetermined time at the start of the next filter regeneration control or the purge control is set shorter than the predetermined time when the filter regeneration control or the purge control is not interrupted. The exhaust emission control device for an engine according to claim 2 or 3.   5. The engine according to claim 1, wherein when both of the filter regeneration control and the purge control are determined to be required, the purge control is performed after the filter regeneration control is performed. 6. Exhaust purification device.

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