JP2007126991A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for suitably purifying NOx in exhaust gas in an exhaust emission control system for an internal combustion engine comprising a storage and reduction type NOx catalyst provided in an exhaust passage. <P>SOLUTION: When carrying out NOx reduction control by supplying a reducing agent to the storage and reduction type NOx catalyst while lowering the air-fuel ratio of exhaust gas exhausted from the internal combustion engine, a period of lowering the air-fuel ratio of exhaust gas exhausted from the internal combustion engine is set to a predetermined period shorter than a period when the temperature of the storage and reduction type NOx catalyst reaches a predetermined temperature. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine including an NOx storage reduction catalyst provided in an exhaust passage.

  Some exhaust purification systems for internal combustion engines include an NOx storage reduction catalyst (hereinafter simply referred to as a NOx catalyst) provided in an exhaust passage. The NOx catalyst is a catalyst that occludes NOx in the exhaust when the ambient atmosphere is an oxidizing atmosphere and reduces the NOx that is occluded when the ambient atmosphere is a reducing atmosphere.

In Patent Document 1, when the air-fuel ratio of exhaust discharged from an internal combustion engine (hereinafter referred to as engine exhaust air-fuel ratio) is switched from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio, an expansion stroke or an exhaust stroke is disclosed. A technique for supplying a reducing agent to a NOx catalyst by injecting fuel from a fuel injection valve is disclosed. Patent Document 1 also discloses a technique for supplying a reducing agent to the NOx catalyst by injecting fuel into the exhaust gas.
Japanese Patent No. 3424672 JP 2001-227325 A

  In an exhaust gas purification system for an internal combustion engine equipped with an NOx catalyst provided in an exhaust passage, when reducing NOx stored in the NOx catalyst, the engine exhaust air-fuel ratio is set so that the ambient atmosphere of the NOx catalyst is reduced. In some cases, the reducing agent is intermittently supplied to the NOx catalyst while lowering the NO.

  In this case, NOx is reduced when the reducing agent is supplied to the NOx catalyst. That is, when only the engine exhaust air-fuel ratio is lowered and no reducing agent is supplied to the NOx catalyst, NOx is stored without being reduced.

  However, when the engine exhaust air-fuel ratio is lowered, the temperature of the exhaust gas rises, and accordingly, the temperature of the NOx catalyst also rises. Further, when the temperature of the NOx catalyst increases, the NOx occlusion capacity may decrease. Therefore, if the temperature of the NOx catalyst rises excessively because the period during which the reducing agent is not supplied to the NOx catalyst and the engine exhaust air-fuel ratio is lowered becomes longer, the NOx purification rate at the NOx catalyst is excessively lowered. There is a fear.

  The present invention has been made in view of the above problems, and is a technology capable of more suitably purifying NOx in exhaust gas in an exhaust gas purification system for an internal combustion engine provided with a NOx catalyst provided in an exhaust passage. It is an issue to provide.

  The present invention relates to an exhaust gas purification system for an internal combustion engine having a NOx catalyst provided in an exhaust passage of the internal combustion engine, and when executing NOx reduction control to reduce NOx occluded in the NOx catalyst, The period during which the temperature is reduced is a period during which an excessive increase in temperature of the NOx catalyst can be suppressed.

More specifically, the exhaust gas purification system for an internal combustion engine according to the present invention is:
An NOx storage reduction catalyst provided in the exhaust passage of the internal combustion engine;
An air-fuel ratio control means for controlling an engine exhaust air-fuel ratio which is an air-fuel ratio of exhaust discharged from the internal combustion engine;
Reducing agent supply means for supplying a reducing agent to the NOx storage reduction catalyst;
NOx for reducing NOx stored in the NOx storage reduction catalyst by supplying a reducing agent to the NOx storage reduction catalyst by the reducing agent supply means while lowering the engine exhaust air fuel ratio by the air fuel ratio control means. NOx reduction control execution means for executing reduction control,
When executing the NOx reduction control, the NOx reduction control execution means sets a period during which the engine exhaust air-fuel ratio is lowered to a predetermined period shorter than a period during which the temperature of the NOx catalyst reaches a predetermined temperature.

  In the present invention, the engine exhaust air-fuel ratio is lowered when the NOx reduction control is executed. At this time, since the temperature of the exhaust gas rises, the temperature of the NOx catalyst gradually rises accordingly. Therefore, in the present invention, the period during which the engine exhaust air-fuel ratio is reduced is shorter than the predetermined period.

  Here, the predetermined period is a period shorter than the period in which the temperature of the NOx catalyst gradually increases to reach the predetermined temperature by lowering the engine exhaust air-fuel ratio. Further, the predetermined temperature here is a temperature equal to or lower than the lower limit value of the temperature at which it can be determined that the NOx storage capacity of the NOx catalyst is excessively reduced.

  In the present invention, when the NOx reduction control is executed, the control for reducing the engine exhaust air-fuel ratio is stopped before the temperature of the NOx catalyst reaches a predetermined temperature. Thereby, it is suppressed that the temperature of a NOx catalyst becomes more than predetermined temperature.

  Also in the present invention, the reducing agent is supplied to the NOx catalyst while the engine exhaust air-fuel ratio is being lowered. NOx occluded in the NOx catalyst is reduced when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is lowered due to a decrease in the engine exhaust air-fuel ratio and the reducing agent is supplied to the NOx catalyst. is there. That is, if the air-fuel ratio of the exhaust gas flowing into the NO catalyst is reduced only when the reducing agent is supplied, the NOx stored in the NOx catalyst can be reduced.

  Therefore, according to the present invention, NOx stored in the NOx catalyst can be reduced while suppressing a decrease in the NOx storage capacity of the NOx catalyst. That is, it becomes possible to more suitably purify NOx in the exhaust gas.

  When the NOx reduction control is performed, when the engine exhaust air-fuel ratio is lowered, it is only necessary to lower the air-fuel ratio so that NOx stored in the NOx catalyst can be reduced by supplying a reducing agent. .

  Further, when the operating state of the internal combustion engine is a low load, since the exhaust gas flow rate is relatively small, it is possible to reduce NOx only by supplying a reducing agent to the NOx catalyst without reducing the engine exhaust air-fuel ratio. There is a case. However, when the operating state of the internal combustion engine becomes a high load, the exhaust gas flow rate increases. Therefore, to make the ambient atmosphere of the NOx catalyst a reducing atmosphere, it is necessary to lower the engine exhaust air-fuel ratio. Further, when the operating state of the internal combustion engine becomes a high load, the exhaust temperature increases.

  Therefore, in the present invention, the NOx reduction control execution means reduces the NOx catalyst to the NOx catalyst by the reducing agent supply means without lowering the engine exhaust air-fuel ratio when the engine load of the internal combustion engine is equal to or less than a predetermined load when the NOx reduction control is executed. The NOx reduction control may be executed by supplying an agent. Here, the predetermined load is a value equal to or lower than the upper limit value of the engine load that can make the ambient atmosphere of the NOx catalyst a reducing atmosphere by supplying the reducing agent to the NOx catalyst without lowering the engine exhaust air-fuel ratio. And a predetermined value.

  In the above case, when the engine load of the internal combustion engine is higher than a predetermined load during the execution of the NOx reduction control, the reducing agent is supplied to the NOx catalyst by the reducing agent supply means while reducing the engine exhaust air-fuel ratio by the air-fuel ratio control means. Thus, the NOx reduction control is executed. At this time, the period during which the engine exhaust air-fuel ratio is reduced is set as a predetermined period.

  ADVANTAGE OF THE INVENTION According to this invention, in the exhaust gas purification system of the internal combustion engine provided with the NOx catalyst provided in the exhaust passage, NOx in exhaust gas can be more suitably purified.

  Hereinafter, specific embodiments of an exhaust gas purification system for an internal combustion engine according to the present invention will be described with reference to the drawings.

<Schematic configuration of intake and exhaust system of internal combustion engine>
Here, the case where the present invention is applied to a diesel engine for driving a vehicle will be described as an example. FIG. 1 is a diagram showing a schematic configuration of an intake / exhaust system of an internal combustion engine according to the present embodiment.

  The internal combustion engine 1 is a diesel engine for driving a vehicle. An intake passage 3 and an exhaust passage 2 are connected to the internal combustion engine 1. An air flow meter 7 and a throttle valve 8 are provided in the intake passage 3.

On the other hand, a NOx catalyst 5 is provided in the exhaust passage 2. The NOx catalyst 5 is a catalyst that stores NOx in the exhaust when the ambient atmosphere is an oxidizing atmosphere and reduces the NOx that is stored when the ambient atmosphere is a reducing atmosphere. Further, a fuel addition valve 6 is provided in the exhaust passage 2 upstream of the NOx catalyst 5 to add fuel as a reducing agent into the exhaust.

  An air-fuel ratio sensor 11 that detects the air-fuel ratio of the exhaust gas is provided downstream of the fuel addition valve 6 and upstream of the NOx catalyst 5 in the exhaust passage 2. A temperature sensor 12 for detecting the temperature of the exhaust gas is provided downstream of the NOx catalyst 5 in the exhaust passage 2.

  The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 10 for controlling the internal combustion engine 1. The ECU 10 is electrically connected to an air flow meter 7, an air-fuel ratio sensor 11, a temperature sensor 12, and an accelerator opening sensor 13 that detects the accelerator opening of a vehicle on which the internal combustion engine 1 is mounted. These output signals are input to the ECU 10.

  The ECU 10 calculates the load of the internal combustion engine 1 based on the detection value of the accelerator opening sensor 13. Further, the ECU 10 estimates the temperature of the NOx catalyst 5 based on the detection value of the temperature sensor 12.

  In addition, a throttle valve 8 and a fuel addition valve 6 are electrically connected to the ECU 10. These are controlled by the ECU 10.

<NOx reduction control>
In this embodiment, NOx reduction control is executed to reduce NOx stored in the NOx catalyst 5. In the NOx reduction control, the atmosphere around the NOx catalyst 5 needs to be a reducing atmosphere.

  When the operating state of the internal combustion engine 1 is a low load, the exhaust gas flow rate is relatively small. Therefore, when performing NOx reduction control, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 5 simply by adding fuel from the fuel addition valve 6 (Hereinafter simply referred to as inflow exhaust air-fuel ratio) can be sufficiently reduced. That is, the atmosphere around the NOx catalyst 5 can be made a reducing atmosphere only by adding fuel from the fuel addition valve 6.

  However, when the operating state of the internal combustion engine 1 becomes a high load, the exhaust flow rate becomes relatively large. For this reason, it is difficult to sufficiently reduce the inflow exhaust air-fuel ratio simply by adding fuel from the fuel addition valve 6. Therefore, when NOx reduction control is executed when the operating state of the internal combustion engine 1 is high, NOx is reduced by adding fuel from the fuel addition valve 6 while reducing the engine exhaust air-fuel ratio to the first predetermined air-fuel ratio. The atmosphere around the catalyst 5 is a reducing atmosphere. Here, the first predetermined air-fuel ratio is that fuel is added from the fuel addition valve 6 if the engine exhaust air-fuel ratio is lowered to the predetermined air-fuel ratio even when the operating state of the internal combustion engine 1 is high. Thus, it is a value at which it can be determined that the inflowing exhaust air-fuel ratio can be sufficiently reduced, that is, the ambient atmosphere of the NOx catalyst 5 can be made the reducing atmosphere.

  In this embodiment, the operation state of the internal combustion engine 1 is a low load when the engine load of the internal combustion engine 1 is a predetermined load or less, and the operation state of the internal combustion engine 1 is a high load. “Sometimes” means when the engine load of the internal combustion engine 1 is higher than a predetermined load. Further, the predetermined load here is an upper limit value of the engine load at which the ambient atmosphere of the NOx catalyst 5 can be made a reducing atmosphere by supplying fuel to the NOx catalyst 5 without reducing the engine exhaust air-fuel ratio. is there.

  In the present embodiment, the intake air amount is decreased by controlling the throttle valve 8 in the valve closing direction, thereby lowering the engine exhaust air-fuel ratio to the first predetermined air-fuel ratio. In this embodiment, when a variable displacement turbocharger is provided, the amount of intake air is reduced by lowering the supercharging pressure by the turbocharger, whereby the engine exhaust air-fuel ratio is reduced to the first predetermined air-fuel ratio. It may be lowered to the fuel ratio. Further, in the present embodiment, when an EGR device that introduces a part of the exhaust gas into the intake passage 3 is provided, the intake air amount is decreased by increasing the flow rate of the exhaust gas introduced into the intake passage 3. Thus, the engine exhaust air-fuel ratio may be lowered to the first predetermined air-fuel ratio.

<Exhaust air / fuel ratio control at high load>
Here, the control of the inflow exhaust air-fuel ratio when the NOx reduction control is executed when the operating state of the internal combustion engine 1 is at a high load will be described based on FIG. FIG. 2 is a diagram showing the relationship between the inflow exhaust air-fuel ratio and the temperature of the NOx catalyst 5 in the NOx reduction control when the operating state of the internal combustion engine 1 is at a high load. 2A represents the inflow exhaust air-fuel ratio, and the vertical axis in FIG. 2B represents the temperature of the NOx catalyst 5. The horizontal axis of (a) and (b) of FIG. 2 represents time.

  As described above, in this embodiment, when NOx reduction control is performed when the operating state of the internal combustion engine 1 is high, the engine exhaust air-fuel ratio is reduced to the first predetermined air-fuel ratio R1. Then, as shown in FIG. 2, when the inflow exhaust air-fuel ratio becomes the first predetermined air-fuel ratio R1 by reducing the engine exhaust air-fuel ratio to the first predetermined air-fuel ratio R1, the fuel addition valve 6 causes the fuel to Is added to further reduce the inflow exhaust air-fuel ratio to the second predetermined air-fuel ratio R2. Here, the second predetermined air-fuel ratio R2 is a value at which the ambient atmosphere of the NOx catalyst 5 becomes a reducing atmosphere.

  In this case, NOx is reduced when the fuel is added from the fuel addition valve 6 and the inflowing exhaust air-fuel ratio is reduced to the second predetermined air-fuel ratio R2. In other words, when no fuel is added from the fuel addition valve 6 and the inflowing exhaust air-fuel ratio is the first predetermined air-fuel ratio R1, NOx is stored without being reduced.

  However, when the engine exhaust air-fuel ratio is lowered, the temperature of the exhaust gas rises. Accordingly, the temperature of the NOx catalyst 5 also rises as shown in FIG. Further, when the temperature of the NOx catalyst 5 rises, the NOx storage capacity may be reduced.

  Therefore, in this embodiment, as shown in FIG. 2, the period during which the engine exhaust air-fuel ratio is reduced is set to a predetermined period Δtd0. Here, the predetermined period Δtd0 is a period shorter than a period in which the temperature of the NOx catalyst 5 gradually increases to reach the predetermined temperature T0 by lowering the engine exhaust air-fuel ratio. Further, the predetermined temperature T0 here is a temperature equal to or lower than the lower limit value of the temperature at which it can be determined that the NOx storage capacity of the NOx catalyst 5 is excessively reduced.

  The predetermined period Δtd0 may be determined according to the operating state of the internal combustion engine 1 when the engine exhaust air-fuel ratio is lowered. Further, the predetermined period Δtd0 may be corrected based on the detection value of the temperature sensor 12.

  Then, fuel addition by the fuel addition valve 6 is executed during the predetermined period Δtd0, and thereby the inflow exhaust air-fuel ratio is lowered to the second predetermined air-fuel ratio R2.

  By controlling the inflow exhaust air-fuel ratio as described above, the NOx catalyst 5 can be prevented from flowing to a predetermined temperature T0 or higher while the operating state of the internal combustion engine 1 is at a high load. The exhaust air / fuel ratio can be lowered to the second predetermined air / fuel ratio R2.

  Therefore, according to the present embodiment, the NOx stored in the NOx catalyst 5 can be reduced while suppressing a decrease in the NOx storage capacity of the NOx catalyst 5. That is, it becomes possible to more suitably purify NOx in the exhaust gas.

<NOx reduction control control routine>
Next, a control routine for NOx reduction control according to this embodiment will be described based on the flowchart shown in FIG. This routine is stored in advance in the ECU 20 and is executed at predetermined intervals during the operation of the internal combustion engine 1.

  In this routine, first, in S101, the ECU 10 determines whether or not an execution condition for NOx reduction control is satisfied. Here, the execution condition of the NOx reduction control can be exemplified by the case where the integrated value of the fuel injection amount in the internal combustion engine 1 is equal to or greater than a predetermined injection amount. If an affirmative determination is made in S101, the ECU 10 proceeds to S102, and if a negative determination is made, the ECU 10 once ends the execution of this routine.

  In S102, the ECU 10 determines whether or not the load Qe of the internal combustion engine 1 is equal to or greater than a predetermined load Qe0. Here, the predetermined load Qe0 is a threshold value that makes it difficult to reduce the inflow exhaust air-fuel ratio to the second predetermined air-fuel ratio R2 only by adding fuel by the fuel addition valve 6 without reducing the engine exhaust air-fuel ratio. It is a load. If an affirmative determination is made in S102, the ECU 10 determines that the operating state of the internal combustion engine 1 is a high load and proceeds to S103. If a negative determination is made, the ECU 10 determines that the operating state of the internal combustion engine 1 is a low load. The process proceeds to S108.

  The ECU 10 having proceeded to S108 executes the fuel addition by the fuel addition valve 6 to reduce the inflow exhaust air-fuel ratio Rin to the second predetermined air-fuel ratio R2. As a result, the NOx catalyst occluded in the NOx catalyst 5 is reduced using the ambient atmosphere of the NOx catalyst 5 as a reducing atmosphere. Thereafter, the ECU 10 ends the execution of this routine.

On the other hand, the ECU 10 having advanced to S103 controls the throttle valve 8 in the valve closing direction,
The engine exhaust air-fuel ratio is reduced to the first predetermined air-fuel ratio R1. Note that the closing amount of the throttle valve 8 at this time is determined according to the intake air amount detected by the air flow meter 7 and the operating state of the internal combustion engine 1.

  Next, the ECU 10 proceeds to S104 and determines whether or not the inflow exhaust air-fuel ratio Rin detected by the air-fuel ratio sensor 11 has decreased to the first predetermined air-fuel ratio R1. In S103, after the engine exhaust air-fuel ratio is reduced to the first predetermined air-fuel ratio R1 by controlling the throttle valve 8 in the valve closing direction, until the inflow exhaust air-fuel ratio Rin becomes the first predetermined air-fuel ratio R1. It takes some time. Therefore, in this embodiment, after the inflow exhaust air-fuel ratio Rin detected by the air-fuel ratio sensor 11 becomes the first predetermined air-fuel ratio R1, fuel addition by the fuel addition valve 6 described later is executed. If an affirmative determination is made in S104, the ECU 10 proceeds to S105, and if a negative determination is made, the ECU 10 returns to S103.

  In S105, the ECU 10 executes fuel addition by the fuel addition valve 6 to reduce the inflow exhaust air-fuel ratio Rin to the second predetermined air-fuel ratio R2. As a result, the NOx catalyst occluded in the NOx catalyst 5 is reduced using the ambient atmosphere of the NOx catalyst 5 as a reducing atmosphere.

  Next, the ECU 10 proceeds to S106, and whether the elapsed time Δtd after controlling the throttle valve 8 in the valve closing direction in S103, that is, the engine exhaust air-fuel ratio has decreased, has reached the predetermined period Δtd0 described above. Determine whether or not. If an affirmative determination is made in S106, the ECU 10 proceeds to S107, and if a negative determination is made, the ECU 10 repeats S106.

  In step S107, the ECU 10 controls the throttle valve 8 in the valve opening direction. That is, the control for reducing the engine exhaust air-fuel ratio to the first predetermined air-fuel ratio R1 is stopped. Thereafter, the ECU 10 ends the execution of this routine.

  According to the control routine described above, even when the NOx reduction control is executed when the operating state of the internal combustion engine 1 is a high load, the period during which the engine exhaust air-fuel ratio is reduced is suppressed to the predetermined period Δtd0. Thereby, it can suppress that the temperature of NOx catalyst 5 becomes more than predetermined temperature T0.

  In the above control routine, the fuel addition by the fuel addition valve 6 is executed after the inflow exhaust air-fuel ratio Rin detected by the air-fuel ratio sensor 11 becomes the first predetermined air-fuel ratio R1, but the engine exhaust air-fuel ratio is changed to the first. After the throttle valve 8 is controlled in the valve closing direction so as to decrease to the predetermined air-fuel ratio R1, fuel addition by the fuel addition valve 6 may be executed when a predetermined elapsed time has elapsed.

  Here, the predetermined elapsed time is a time during which it can be determined that the inflow exhaust air-fuel ratio Rin has decreased to the first predetermined air-fuel ratio R1. In this case, the predetermined elapsed time may be determined based on the capacity of the exhaust passage 2 and the operating state of the internal combustion engine 1.

  Further, after the throttle valve 8 is controlled in the valve closing direction to lower the engine exhaust air-fuel ratio to the first predetermined air-fuel ratio R1, fuel addition by the fuel addition valve 6 is executed when a predetermined number of combustion cycles has elapsed. May be.

  After the throttle valve 8 is controlled in the valve closing direction, the engine exhaust air-fuel ratio is reduced to the first predetermined air-fuel ratio R1, and then the exhaust of the first predetermined air-fuel ratio R1 approaches the NOx catalyst 5 every combustion cycle. It will be. In the above case, the predetermined number of combustion cycles is the number of combustion cycles in which it can be determined that the inflowing exhaust air-fuel ratio Rin has decreased to the first predetermined air-fuel ratio R1.

  In this embodiment, when the NOx reduction control is executed when the operating state of the internal combustion engine 1 is a high load, the inflow exhaust air-fuel ratio becomes the first predetermined air-fuel ratio R1 only when fuel is added from the fuel addition valve 6. If so, the NOx stored in the NOx catalyst 5 can be reduced. Therefore, the predetermined period Δtd0 may be set as short as possible within a range where the inflowing exhaust air-fuel ratio can be reduced to the second predetermined air-fuel ratio R2 by adding fuel by the fuel addition valve 6.

The figure which shows schematic structure of the intake / exhaust system of the internal combustion engine which concerns on the Example of this invention. The figure which shows the relationship between the inflow exhaust air-fuel ratio in the NOx reduction | restoration control when the driving | running state of the internal combustion engine 1 is a high load, and the temperature of a NOx catalyst. The flowchart which shows the control routine of NOx reduction control which concerns on the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Exhaust passage 3 ... Intake passage 5 ... Occlusion reduction type NOx catalyst 6 ... Fuel addition valve 8 ... Throttle valve 10 ... ECU
11. Air-fuel ratio sensor 12. Temperature sensor

Claims (2)

An NOx storage reduction catalyst provided in the exhaust passage of the internal combustion engine;
An air-fuel ratio control means for controlling an engine exhaust air-fuel ratio which is an air-fuel ratio of exhaust discharged from the internal combustion engine;
Reducing agent supply means for supplying a reducing agent to the NOx storage reduction catalyst;
NOx for reducing NOx stored in the NOx storage reduction catalyst by supplying a reducing agent to the NOx storage reduction catalyst by the reducing agent supply means while lowering the engine exhaust air fuel ratio by the air fuel ratio control means. NOx reduction control execution means for executing reduction control,
The NOx reduction control execution means, when executing the NOx reduction control, sets a period during which the engine exhaust air-fuel ratio is lowered to a predetermined period shorter than a period when the temperature of the NOx catalyst reaches a predetermined temperature. An exhaust purification system for an internal combustion engine. If the engine load of the internal combustion engine is equal to or lower than a predetermined load when the NOx reduction control is executed, the NOx reduction control execution means performs the NOx storage reduction type NOx catalyst by the reducing agent supply means without reducing the engine exhaust air-fuel ratio. The NOx reduction control is executed by supplying a reducing agent to the engine. If the engine load of the internal combustion engine is higher than the predetermined load when the NOx reduction control is executed, the air-fuel ratio control means lowers the engine exhaust air-fuel ratio. 2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the NOx reduction control is executed by supplying a reducing agent to the NOx storage reduction catalyst by the reducing agent supply means.

Images