JP6481702B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an exhaust emission control device for an engine.

  In the engine, the NOx catalyst disposed in the exhaust passage of the engine occludes NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and the exhaust gas There are some which reduce the stored NOx when the air-fuel ratio is near the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. In Patent Document 1, when the NOx occluded in the NOx catalyst becomes a predetermined value or more, the NOx reduction control is executed by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of NOx reduction. It is disclosed.

JP 2008-121596 A

  Incidentally, EGR is generally performed to recirculate exhaust gas to the intake passage. When performing NOx reduction control in an engine in which this EGR is performed, the amount of oxygen necessary for DeNOx control can be controlled by decreasing the EGR amount (exhaust gas recirculation amount) and increasing fresh air. I need it. However, since it is difficult to accurately adjust the ratio between the EGR amount and the fresh air amount, it is possible to accurately obtain the oxygen amount necessary for DeNOx control, that is, the air-fuel ratio of the exhaust gas is the target air-fuel ratio necessary for NOx reduction. Therefore, it is difficult to control with high accuracy, and some measures are desired in this respect.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an engine exhaust purification device that can accurately control the air-fuel ratio of exhaust gas to the target air-fuel ratio required for NOx reduction. There is to do.

In order to achieve the above object, the following solution is adopted in the present invention. That is, as described in claim 1,
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control is performed to reduce the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio that can reduce the NOx stored in the NOx catalyst. NOx reduction control means for
An EGR passage for returning exhaust gas to the intake passage;
An exhaust gas recirculation valve disposed in the EGR passage;
EGR control means for controlling the exhaust gas recirculation valve to control the amount of exhaust gas recirculated to the intake passage;
An electrically driven supercharger;
Supercharging control means for controlling the driving of the electric supercharger;
With
The EGR control means controls the opening degree of the exhaust gas recirculation valve to be maintained at a constant opening degree when the NOx reduction control means executes NOx reduction control,
When the NOx reduction control is executed by the NOx reduction control means, the supercharging control means drives the electric supercharger so that the air-fuel ratio of the exhaust gas is maintained at a target air-fuel ratio that can reduce NOx. control and,
The EGR control means controls the exhaust gas recirculation valve to be held at a constant opening when acceleration is detected during NOx reduction control,
When acceleration is detected when the NOx reduction control means is executing NOx reduction control, the supercharging control means increases the drive of the electric supercharger and the air-fuel ratio of the exhaust gas is increased. After reaching the target air-fuel ratio capable of NOx reduction, the electric supercharger is controlled so as to be driven at a constant rate.
It is like that.

  According to the above solution, by controlling the exhaust gas recirculation valve at a constant opening degree during DeNOx control, the fluctuation in the ratio between the exhaust gas amount and the fresh air amount due to the change in the opening degree of the exhaust gas recirculation valve is prevented. Alternatively, NOx reduction can be effectively performed by accurately maintaining the air-fuel ratio of the exhaust gas at the target air-fuel ratio required for NOx reduction by controlling the drive of the electric supercharger having excellent responsiveness while suppressing it. it can.

In addition to the above, the air-fuel ratio of the exhaust gas is also reduced during the DeNOx control when the amount of fresh air increases and it becomes difficult to accurately adjust the ratio between the EGR amount and the fresh air amount. The NOx reduction can be effectively performed while maintaining the target air-fuel ratio required for NOx reduction with high accuracy.

A preferred mode based on the above solution is as described in claim 2 and the following. That is,
An exhaust turbocharger controlled by the supercharging control means,
When the NOx reduction control is executed by the NOx reduction control means, the supercharging control means limits the supercharging of the exhaust turbocharger, and the air / fuel ratio of the exhaust gas can reduce NOx. The electric supercharger is driven and controlled so as to be maintained at
(Corresponding to claim 2 ). In this case, even when it becomes difficult to accurately adjust the ratio between the EGR amount and the fresh air amount due to a delay in the supercharging response by the exhaust turbocharger, the response while limiting the supercharging by the exhaust turbocharger. It is possible to effectively perform NOx reduction by accurately maintaining the air-fuel ratio of the exhaust gas at the target air-fuel ratio necessary for NOx reduction by using an electric supercharger having excellent performance.

After the NOx reduction control by the NOx reduction control means ends, the supercharging control means cancels the supercharging limitation of the exhaust turbocharger and the supercharging by the exhaust turbocharger is a predetermined supercharge. Until the state is reached, the electric supercharger is continuously driven (corresponding to claim 3 ). In this case, the response delay of the exhaust turbocharger after the restriction on supercharging of the exhaust turbocharger is released can be compensated by the electric supercharger, so that a sufficient supercharging pressure can be secured.

The exhaust turbocharger is a movable vane type having a movable vane or a wastegate valve type having a wastegate valve capable of opening and closing a bypass passage bypassing the turbine,
The supercharging control means controls the movable vane or the wastegate valve to limit supercharging of the exhaust turbocharger;
(Corresponding to claim 4 ). In this case, while using a general movable vane type or waste gate valve type as the exhaust turbocharger, the supercharging limitation can be easily and greatly performed.

  According to the present invention, it is possible to effectively perform NOx reduction by accurately controlling the air-fuel ratio of the exhaust gas to the target air-fuel ratio necessary for NOx reduction.

The figure which shows an example of the engine to which this invention was applied. The figure which shows the example of a setting of the execution area | region of DeNOx control. The time chart which shows the control content of this invention. The flowchart which shows the example of control of this invention. The flowchart which shows the example of control of this invention.

  In FIG. 1, reference numeral 1 denotes an engine (engine body), which is an in-line four-cylinder automobile diesel engine in the embodiment. The engine 1 has a cylinder 2, a cylinder head 3, and a piston 4 as is known. An intake port 6 and an exhaust port 7 are opened with respect to the combustion chamber 5 formed above the piston 4. The intake port 6 is opened and closed by an intake valve 8, and the exhaust port 7 is opened and closed by an exhaust valve 9. A fuel injection valve 10 is attached to the cylinder head 3 so as to face the combustion chamber 5. In the embodiment, the fuel injection is of a common rail type, and extremely high pressure fuel is injected from the fuel injection valve 10.

  In the intake passage 20 connected to the intake port 6, an air cleaner 21, a compressor wheel 22 a of the exhaust turbocharger 22, a compressor wheel 23 a of the electric turbocharger 23, an intercooler are sequentially arranged from the upstream side to the downstream side. 24, a surge tank 26 is provided. The surge tank 26 and each cylinder (the intake port 6 thereof) are independently connected by a branched intake passage 27.

  The exhaust turbo supercharger 22 is a movable vane type having a movable vane 22c, and the movable vane 22c is driven and controlled by an actuator 22d. The supercharging capability of the exhaust turbocharger 22 is changed by the drive control of the movable vane 22c. Further, the compressor wheel 23a of the electric supercharger 23 is electrically driven by a motor 23b.

  A bypass passage 28 is provided in the intake passage 20. The bypass passage 28 has an upstream end opened to the intake passage 20 between the compressor wheels 22a and 23a. Further, the downstream end of the bypass passage 28 is opened to the intake passage 20 between the compressor wheel 23 a and the intercooler 24. A control valve 29 is disposed in the bypass passage 28. A control valve 30 is arranged in the intake passage 20 immediately downstream of the compressor wheel 23 a so as to be in parallel with the control valve 29.

  The exhaust passage 40 connected to the exhaust port 7 is connected to the turbine wheel 22b, NOx catalyst 41, DPF 42, urea catalyst 43, and ammonia processor 44 of the exhaust turbocharger 22 sequentially from the upstream side to the downstream side. Has been.

  The intake passage 20 and the exhaust passage 40 are connected via a first EGR passage 50 for High-Pressure. The upstream end of the first EGR passage 50 is opened to the exhaust passage 30 upstream of the turbine wheel 22b. Further, the downstream end of the EGR passage 50 is opened between the intercooler 24 and the surge tank 26 in the intake passage 20.

  An EGR cooler 51 is connected to the EGR passage 50, and an EGR valve 52 is disposed on the downstream side of the EGR cooler 51. The EGR passage 50 is provided with a bypass passage 53 that bypasses the EGR cooler 51. The bypass passage 53 has an upstream end opened to the EGR passage 50 upstream of the EGR cooler 51, and a downstream end opened to the EGR passage 50 downstream of the EGR valve 52. The bypass passage 54 is provided with a control valve 54 (also referred to as a second EGR valve).

  The intake passage 20 and the exhaust passage 40 are connected via a second EGR passage 60 for Low-Pressure. The upstream end of the second EGR passage 60 is opened between the urea catalyst 43 and the ammonia processor 44 in the exhaust passage 40. Further, the downstream end of the second EGR passage 60 is opened in the intake passage 20 between the air cleaner 21 and the compressor wheel 22a. An intercooler 61 is connected to the second EGR passage 60, and an EGR valve 62 is connected downstream of the intercooler 61.

  Next, a region in which NOx reduction control (DeNOx control) is executed will be described with reference to FIG. In FIG. 2, a line indicated by AL is a maximum torque line. The NOx reduction control is performed in two areas with hatching in FIG. 2 using the engine speed and the engine load as parameters. The first region in which the NOx reduction control is performed is a region in which the engine speed is set in a medium rotation region between N1 and N2 and the engine load is set in a medium load region between L1 and L2. The NOx reduction control in the first region is referred to as active NOx reduction control (active DeNOx control). The second region where the NOx reduction control is performed is a region near the maximum torque line AL. This NOx reduction control in the second region is referred to as passive NOx reduction control (passive DeNOx control).

  The active DeNOx control is started when the NOx occlusion amount in the NOx catalyst 41 becomes equal to or higher than a predetermined upper limit value, assuming that the NOx occlusion amount in the NOx catalyst 41 is equal to or greater than the predetermined upper limit value. It is basically done until it reaches zero. However, the active DeNOx control may be stopped in a state where the NOx occlusion amount in the NOx catalyst 41 is reduced to a lower limit value greater than zero. The detection (or estimation) of the NOx occlusion amount in the NOx catalyst 41 can be performed by a known appropriate method. For example, a sensor that detects the occluded NOx amount is used, or data indicating the engine driver state is used. And NOx occlusion amount can be estimated.

  In the active NOx reduction control, post injection is performed in addition to main injection for obtaining necessary torque. This post injection is performed in the first half of the expansion stroke so as to be burned in the cylinder in order to suppress discharge of unburned fuel from the cylinder and oil dilution. In order to ensure the time until this post-injected fuel is combusted (in order to ignite in a state where air and fuel are properly mixed), the amount of EGR gas, which is reduced compared to the normal time, is introduced. Thus, the ignition of the post-injected fuel is delayed.

  In the passive NOx reduction control, post injection is performed in addition to main injection for obtaining necessary torque. This post injection is performed in the latter half of the expansion stroke so that combustion is not performed in the cylinder. When this passive NOx reduction control is executed, the control is based on a situation where the air-fuel ratio of the exhaust gas becomes high and the exhaust gas is rich, as compared with the case where the active NOx reduction control is performed. Thus, the ratio of the post injection amount to the main injection amount can be reduced.

  During passive NOx reduction control, because of the high load, if post-injected fuel is burned in the cylinder, HC causes a large amount of smoke (soot). Is not done. In addition, in passive DeNOx control, there is no need for a positive ignition delay due to the introduction of EGR, and from the viewpoint of accurately controlling the air-fuel ratio of the exhaust gas to the target air-fuel ratio, “soot” that is easily generated is EGR. From the viewpoint of preventing accumulation in the passage 50 or 60, EGR is not performed.

  The driving of the electric supercharger 23 is executed at the initial stage of acceleration to compensate for the response delay of the exhaust turbocharger 22 and also when the above-described NOx reduction control is executed. In the embodiment, the driving of the electric supercharger 23 is stopped in cases other than the above, and the supercharging is performed exclusively by the exhaust turbocharger 22. When supercharging is performed by the electric supercharger 23, both control valves 29 and 30 are opened. Further, when the drive of the electric supercharger 23 is stopped and the supercharging by only the exhaust turbocharger 22 is performed, the control valve 30 is closed and the control valve 29 is opened.

  Next, the outline of the active NOx reduction control will be described in association with supercharging while referring to the time chart of FIG. First, until the time point t1, the vehicle is in a steady operation in a region other than the active NOx reduction control execution region shown in FIG. Prior to time t1, the flag for executing the passive NOx reduction control is set to 0, and the passive NOx reduction control is not executed. Further, supercharging is normally performed by the exhaust turbocharger 22 (the electric supercharger 23 is in a drive stopped state).

  At the time t1, the active NOx reduction control region shown in FIG. 2 is entered, the flag is set to 1, and the supercharging of the exhaust turbocharger 22 is restricted accordingly (the movable vane 22c is fully opened). ). The reason why the supercharging of the exhaust turbo supercharger 22 is limited is that it is difficult to control the air-fuel ratio of the exhaust gas if the supercharging by the exhaust turbo supercharger 22 is performed as usual. In addition, a certain amount of supercharging can be performed by the exhaust turbocharger 22 within a limited range without fully opening the movable vane 22c.

  At time t2, the accelerator is depressed, the accelerator opening is increased, the main fuel injection amount is increased accordingly, and the in-cylinder required oxygen amount is also increased. From this time t2, the driving of the electric supercharger 23 is started, and the driving force of the electric supercharger 23 is increased as the main injection amount increases.

  The time point t3 is the time point when the air-fuel ratio of the exhaust gas reaches the target air-fuel ratio necessary for NOx reduction control. After the time t3, the driving force of the electric supercharger 23 is kept constant and the target air-fuel ratio is maintained.

  From time t4 when the execution flag of the active NOx reduction control is reset to 0, the supercharging limitation of the exhaust turbocharger 22 is released, and the supercharging pressure by the exhaust turbocharger 22 is increased. However, since a response delay occurs until the supercharging pressure actually increases after the supercharging limit of the exhaust turbocharger 22 is released, the electric supercharger 23 is continuously driven for a while after the time t4. After that, the driving of the electric supercharger 23 is gradually stopped. The time from the time t4 to the time t5 is a time during which the electric supercharger 23 is constantly driven, and after the time t5, the drive reduction of the electric supercharger 23 is gradually performed, and the electric supercharger 23 is timed at the time t6. The driving of the feeder 23 is stopped.

  Next, among the control contents of the controller U (see FIG. 1) that performs the above-described active NOx reduction control, particularly the control for the electric supercharger 23 will be described with reference to the flowchart of FIG. The input / output relationship with respect to the controller U is not shown. The controller U includes, for example, various parameters such as an engine speed detected by a sensor or the like, an engine load (accelerator opening), a supercharging pressure, an intake pressure, an intake air amount, or a sensor detected or estimated. The NOx occlusion amount in the NOx catalyst 41 is input. Further, the controller U controls the fuel injection valve 10, the EGR valve 52, the control valve 54, the EGR valve 62, the actuator 22d for the exhaust turbocharger 22, the motor 23b for the electric supercharger 22, and the like.

  First, after data is input in Q1 of FIG. 4, it is determined in Q2 whether or not it is an area for executing active NOx reduction control (active DeNOx control) according to the map shown in FIG. When the determination in Q2 is YES, in Q3, the supercharging capability of the exhaust turbo supercharger 22 is minimized by fully opening the movable vane 22c. Thereafter, in Q4, the electric supercharger 23 is driven. The drive mode of the electric supercharger 23 is performed as shown in FIG.

  When the determination in Q2 is NO, in Q5, supercharging by the exhaust turbocharger 22 is performed according to the engine state of the engine (normal supercharging control). Thereafter, in Q6, it is determined whether or not the previous active NOx reduction control has been executed (determination as to whether or not it is immediately after time t4 in FIG. 3). If the determination in Q6 is YES, it is determined in Q7 whether or not the supercharging pressure has been sufficiently increased by releasing the supercharging restriction by the exhaust turbo supercharger 22. If the determination in Q7 is NO, the process proceeds to Q4 (in a state where the time t5 in FIG. 3 has not been reached, at this time, the drive of the electric supercharger 23 is stopped in a state in which the pressure has increased to a predetermined supercharging pressure). .

  If YES in Q7, the driving of the electric supercharger 23 is stopped in Q8. The driving stop of the electric supercharger 23 is gradually stopped from the time t5 to the time t6 in FIG.

  When the determination in Q6 is NO, the time is irrelevant to the active NOx reduction control. At this time, in Q9, it is determined whether or not the acceleration is in the initial stage. If the determination in Q9 is NO, it is determined that driving of the electric supercharger 23 is not necessary, and the process proceeds to Q8. If YES in Q9, the electric supercharger 23 is driven in Q10 (drive of the electric supercharger 23 to compensate for the delay in supercharging by the exhaust turbocharger 22). The driving is stopped when the turbocharging by the exhaust turbocharger 22 is sufficiently performed).

  Next, the point of EGR control is demonstrated, referring the flowchart of FIG. First, after data is input in Q21, it is determined in Q22 whether or not the engine is running at low speed and low load. If YES in Q22, the second EGR passage 60 is cut in Q23 (EGR valve 62 is closed). Thereafter, in Q24, it is determined whether or not the passive DeNOx control is being executed. If YES in Q24, the first EGR passage 50 is cut (the EGR valve 52 and the control valve 54 are closed) because EGR is unnecessary. As described above, when the passive DeNOx control is executed, the EGR is not performed, so that the air-fuel ratio of the exhaust gas is accurately set to the target air-fuel ratio and the like, and for the DeNOx control. This is preferable in preventing a situation where “soot” that is likely to be generated along with the post injection is accumulated in the first EGR passage 50.

  If the determination in Q24 is NO, it is determined in Q26 whether or not active DeNOx control is being executed. If the determination in Q26 is YES, it is determined in Q27 whether or not the vehicle is accelerating. If YES in Q27, the EGR valve 52 (and the control valve 54) is held at a constant opening in Q28. As a result, the ratio between the exhaust gas amount and the fresh air amount due to the change in the opening degree of the EGR valve 52 is prevented from greatly fluctuating, and the air-fuel ratio of the exhaust gas is accurately controlled to the target air-fuel ratio even during acceleration. Thus, active DeNOx control can be performed effectively.

  When the determination in Q27 is NO, the EGR amount from the first EGR passage 50 is reduced in comparison with the normal time. Note that this reduction is a fixed amount, which is a fixed amount with respect to the normal amount set according to the operating state of the engine. When the determination in Q26 is NO, the EGR amount from the first EGR passage 50 is the normal amount.

  When the determination in Q22 is NO, the first EGR passage 50 is cut in Q31 (EGR valve 52 and control valve 54 are closed). Thereafter, in Q32, it is determined whether or not the passive DeNOx control is being executed. If YES in Q32, the second EGR passage 60 is cut (EGR valve 62 is closed) in E33, assuming that EGR is unnecessary. As described above, when the passive DeNOx control is executed, the EGR is not performed, so that the air-fuel ratio of the exhaust gas is accurately set to the target air-fuel ratio and the like, and for the DeNOx control. This is preferable in preventing “soot” that is likely to be generated along with the post injection from accumulating in the second EGR passage 60.

  If the determination in Q32 is NO, it is determined in Q34 whether or not active DeNOx control is being executed. If the determination in Q34 is YES, it is determined in Q35 whether the vehicle is accelerating. If YES in Q35, the EGR valve 62 is held at a constant opening in Q36. As a result, the ratio between the exhaust gas amount and the fresh air amount due to the change in the opening degree of the EGR valve 62 is prevented from greatly fluctuating, and the air-fuel ratio of the exhaust gas is accurately controlled to the target air-fuel ratio even during acceleration. Thus, active DeNOx control can be performed effectively.

  When the determination in Q35 is NO, the EGR amount from the second EGR passage 60 is reduced in comparison with the normal time. Note that this reduction is a fixed amount, which is a fixed amount with respect to the normal amount set according to the operating state of the engine. If the determination in Q34 is NO, the EGR amount from the second EGR passage 60 is made the normal amount in Q38.

  Although the embodiment has been described above, the present invention is not limited to the embodiment, and can be appropriately changed within the scope described in the scope of claims. For example, the invention includes the following cases. . The engine 1 may be a gasoline engine. Passive DeNOx control can also be executed at times other than during acceleration (particularly, when the supercharging pressure by the exhaust turbocharger 22 is expected to increase significantly). The exhaust turbocharger 22 may be a wastegate valve type having a wastegate valve that can open and close a bypass passage that bypasses the turbine 22b. In this case, the supercharging restriction is performed by opening the wastegate valve. (It can be fully open.) During passive DeNOx control, supercharging control by the exhaust turbocharger 22 and the electric supercharger 23 can be performed in the same manner as in the case of active DeNOx control. That is, during the outer panel portion DeNOx control, the exhaust turbo supercharger 22 is supercharge-limited, while the drive of the electric supercharger 23 is gradually increased so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. After the electric supercharger 23 is driven at a certain point in time and the active DeNOx control is completed, it is possible to perform control through the drive modes from t4 to t5 and t6 in FIG.

  During active DeNOx control, the electric supercharger 23 may be driven only at the time of acceleration, while when not accelerated, the electric supercharger 23 may be stopped. The exhaust turbocharger 22 may not be provided but only the electric supercharger 23 may be provided. When only the electric supercharger 23 is provided, the active DeNOx control may be started from a state in which the electric supercharger 23 is driven. If acceleration is detected at that time, the electric supercharger 23 may be started. The feeder 23 may be driven to increase. Each step or step group shown in the flowchart indicates the function of the controller U, and the name indicating the function can be attached to the name of the means so as to be grasped as a constituent requirement of the controller U.

  The present invention is suitable for application to, for example, an automobile diesel engine.

U: Controller 1: Engine 2: Cylinder
10: Fuel injection valve 20: Intake passage 22: Exhaust turbocharger 23: Electric supercharger 40: Exhaust passage 41: NOx catalyst 50: First EGR passage 52: EGR valve (exhaust gas recirculation valve)
54: Control valve (exhaust gas recirculation valve)
60: Second EGR passage 62: EGR valve (exhaust gas recirculation valve)

Claims (4)

NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control is performed to reduce the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio that can reduce the NOx stored in the NOx catalyst. NOx reduction control means for
An EGR passage for returning exhaust gas to the intake passage;
An exhaust gas recirculation valve disposed in the EGR passage;
EGR control means for controlling the exhaust gas recirculation valve to control the amount of exhaust gas recirculated to the intake passage;
An electrically driven supercharger;
Supercharging control means for controlling the driving of the electric supercharger;
With
The EGR control means controls the opening degree of the exhaust gas recirculation valve to be maintained at a constant opening degree when the NOx reduction control means executes NOx reduction control,
When the NOx reduction control is executed by the NOx reduction control means, the supercharging control means drives the electric supercharger so that the air-fuel ratio of the exhaust gas is maintained at a target air-fuel ratio that can reduce NOx. control and,
The EGR control means controls the exhaust gas recirculation valve to be held at a constant opening when acceleration is detected during NOx reduction control,
When acceleration is detected when the NOx reduction control means is executing NOx reduction control, the supercharging control means increases the drive of the electric supercharger and the air-fuel ratio of the exhaust gas is increased. After reaching the target air-fuel ratio capable of NOx reduction, the electric supercharger is controlled so as to be driven at a constant rate.
An exhaust emission control device for an engine. Oite to claim 1,
An exhaust turbocharger controlled by the supercharging control means,
When the NOx reduction control is executed by the NOx reduction control means, the supercharging control means limits the supercharging of the exhaust turbocharger, and the air / fuel ratio of the exhaust gas can reduce NOx. The electric supercharger is driven and controlled so as to be maintained at
An exhaust emission control device for an engine. In claim 2 ,
After the NOx reduction control by the NOx reduction control means ends, the supercharging control means cancels the supercharging limitation of the exhaust turbocharger and the supercharging by the exhaust turbocharger is a predetermined supercharge. The engine exhaust gas purification apparatus is characterized in that the electric supercharger is continuously driven until the state is reached. In claim 2 or claim 3 ,
The exhaust turbocharger is a movable vane type having a movable vane or a wastegate valve type having a wastegate valve capable of opening and closing a bypass passage bypassing the turbine,
The supercharging control means controls the movable vane or the wastegate valve to limit supercharging of the exhaust turbocharger;
An exhaust emission control device for an engine.

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