JP2005147106A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent emission of NOx which is not completely treated by rich spike, since a large amount of NOx is trapped in a NOx trap catalyst during engine cold operation. <P>SOLUTION: The NOx trap catalyst is disposed to an exhaust passage of a diesel engine, and a NOx sensor for detecting NOx concentration is disposed to an outlet side. After the cold start (S101), lean operation is started (S102). When the NOx concentration detected by the NOx sensor becomes lower than a first threshold value SNOx1, the start of the treatment of the NOx is determined and the operation is switched to stoichiometric operation (S105). When the NOx concentration becomes lower than a second threshold value SNOx2, complete activation of the catalyst is determined and the stoichiometric operation is completed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine using a NOx trap catalyst, and more particularly to an exhaust gas purification device suitable for an engine that performs basically lean combustion such as a diesel engine.

In an internal combustion engine that performs lean combustion, NOx (nitrogen oxide) contained in exhaust gas is trapped when the exhaust air-fuel ratio is lean, and the trapped NOx is released and purified by switching the exhaust air-fuel ratio to rich An exhaust emission control device using a trap catalyst is known. The NOx treatment function has progressed year by year, and the treatment performance has become very high with progress. In the improvement of the processing performance, the performance improvement particularly at a low temperature has made great progress. As disclosed in Patent Document 1, even when the exhaust gas temperature is as low as 200 ° C., it is exhausted from the lean combustion internal combustion engine. It is possible to process NOx.
Japanese Patent Laid-Open No. 2000-299832

  In an internal combustion engine including a diesel engine, purification of exhaust gas components at the cold start is very important in order to prevent harmful exhaust gas components from being released into the atmosphere.

  The NOx trap catalyst has a function of trapping NOx and also has a redox function as a three-way catalyst. Considering the early activation of this catalyst, it is necessary to promote the oxidation reaction of HC and CO. Considering this oxidation reaction, the exhaust air-fuel ratio of the exhaust gas given to the NOx trap catalyst in the cold state is leaner than rich. Is better. The temperature rise characteristic of the catalyst performance in lean is the same as the temperature rise characteristic for NOx as compared with the case of the stoichiometric air-fuel ratio, but the oxidation characteristics for HC and CO rise from a lower temperature.

  Thus, for early activation of the purification performance of HC and CO, it is preferable that the exhaust air-fuel ratio is lean in the cold state. However, as a treatment for NOx, under conditions where the exhaust air-fuel ratio is lean When the NOx trap (adsorption) function is used and the NOx trap catalyst is activated while the exhaust air-fuel ratio remains lean after the cold start, a large amount of NOx is stored in the NOx trap catalyst. Accordingly, when a rich spike (temporary exhaust air-fuel ratio enrichment) is given for the desorption purification of trapped NOx, the trapped NOx cannot be processed by the first rich spike, and it is returned to the atmosphere. There is a risk of release.

  In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, a NOx trap catalyst is disposed in the exhaust passage of the internal combustion engine. This NOx trap catalyst traps NOx (nitrogen oxide) in the exhaust gas when the exhaust air-fuel ratio of the inflowing exhaust gas is lean, and desorbs the trapped NOx when the exhaust air-fuel ratio is rich In addition to having a function of purifying, the exhaust air-fuel ratio also functions as a three-way catalyst under the stoichiometric air-fuel ratio.

  And in this invention, it has a means to determine the rising state of the NOx processing performance of the NOx trap catalyst when the engine is cold, and the function of processing the NOx of the NOx trap catalyst based on the determination of the rising state Are to be switched. Alternatively, the exhaust air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is changed based on the determination of the rising state.

  That is, the NOx trap catalyst has a function of trapping NOx and a redox function as a three-way catalyst. The NOx trap catalyst has a NOx trap performance under lean conditions and a reduction performance under a stoichiometric air-fuel ratio. Since the temperature rise characteristics are substantially the same, the NOx treatment function can be switched during the cold state according to the catalyst temperature rise, and release of NOx into the atmosphere can be prevented.

  In the present invention, it is desirable that the exhaust air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is controlled to be lean at a temperature lower than the temperature at which the catalytic performance of the NOx trap catalyst rises after the engine is cold started.

  As described above, the NOx trap performance of the NOx trap catalyst under lean conditions and the reduction performance under the stoichiometric air-fuel ratio are almost the same, but the oxidation performance of HC and CO is that the exhaust air-fuel ratio is lean. Therefore, the oxidation activity of HC and CO in the NOx trap catalyst can be raised at the same temperature as the rise of NOx performance.

  In the present invention, the exhaust air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is within a temperature range from when the engine performance of the NOx trap catalyst rises until it reaches a sufficiently active state after cold start of the engine. It is desirable that the theoretical air-fuel ratio be controlled.

  When NOx performance rises when the NOx trap catalyst is heated under lean conditions, the NOx trap catalyst exhibits a function of trapping NOx. However, if the NOx performance of the NOx trap catalyst rises and is operated lean until it is fully activated, NOx continues to be trapped and reaches a saturated state. As a result, the amount of NOx to be processed in the first rich spike (for example, by supplying a reducing agent) after the cold escape of NOx trapped in the cold state becomes very large, and unpurified NOx is released into the atmosphere. There is a risk that. Therefore, in order to maintain the NOx performance at the time of cold and after cold escape with a high performance, it is necessary to reduce the amount of NOx trap at the time of cold. It is preferable to operate the exhaust air-fuel ratio as the stoichiometric air-fuel ratio and use the purification by the three-way catalyst function of the NOx trap catalyst without trapping NOx.

  In one aspect of the present invention, a means for detecting the NOx concentration contained in the exhaust gas that has passed through the NOx trap catalyst is provided at the outlet of the NOx trap catalyst, and the change in the detected NOx concentration when the engine is cold. Thus, the rise of the catalyst performance can be determined.

  Alternatively, a means for detecting the exhaust air / fuel ratio of the exhaust gas that has passed through the NOx trap catalyst is provided at the outlet of the NOx trap catalyst, and when the engine is cold, the rise in catalyst performance is determined from the detected change in the exhaust air / fuel ratio. can do.

  Alternatively, a means for detecting the exhaust gas temperature is provided on each of the inlet side and the outlet side of the NOx trap catalyst, and when the engine is cold, the rising of the catalyst performance can be determined from the temperature difference before and after the NOx trap catalyst. .

  Also, in one aspect of the present invention, a temperature detection means for detecting the exhaust gas temperature is provided on the inlet side of the NOx trap catalyst, and the catalyst performance predicted from the oxidation reaction rates of HC and CO in the NOx trap catalyst. Based on the rising temperature and the temperature detected by the temperature detecting means, the rising of the catalyst performance can be determined.

  That is, the oxidation reaction rate of HC and CO is a function of temperature, and the temperature at which the oxidation reaction rate changes can be estimated from the rate equation, and the catalyst performance has risen when the estimated temperature is reached. The exhaust air / fuel ratio of the exhaust gas flowing into the catalyst can be switched.

  According to this invention, after the cold start, the oxidation reaction of HC and CO can be started early, the NOx trap catalyst can be activated early, and NOx after cold escape due to trapping a large amount of NOx can be achieved. Emission into the atmosphere can be prevented.

  FIG. 1 is a structural explanatory view showing a first embodiment of an exhaust emission control device according to the present invention, in which 1 is a diesel engine and 2 is a control device for controlling the operation of the diesel engine 1. Although the intake system of the diesel engine 1 is not shown, the intake system includes means for controlling the amount of air flowing into each cylinder, for example, an intake throttle valve, and means for detecting the flow rate, for example, A hot-wire air flow meter is provided (not shown).

  A NOx trap catalyst 3 is disposed in the exhaust passage 7 of the diesel engine 1. In addition to the NOx trap catalyst 3, another three-way catalyst, an HC trap catalyst, and a diesel particulate filter may be disposed in the exhaust passage 7. The NOx trap catalyst 3 traps NOx in the exhaust gas when the exhaust air-fuel ratio of the exhaust gas discharged from the diesel engine 1 is lean in a predetermined temperature range, and trapped NOx when it is rich. It has a function to release and purify. The NOx trap catalyst 3 also functions as a three-way catalyst that performs redox under the stoichiometric air-fuel ratio condition. In this example, the NOx trap catalyst 3 includes an independent casing 6.

  An air-fuel ratio sensor 4 that detects the exhaust air-fuel ratio of the exhaust gas flowing into the exhaust passage 12 is disposed at a position upstream of the NOx trap catalyst 3 in the exhaust passage 12. Based on the actual exhaust air / fuel ratio detected by the air / fuel ratio sensor 4 and the amount of air flowing into the diesel engine 1 detected by the air flow meter, the control device 2 has a desired air / fuel ratio. The fuel injection amount and the injection timing are determined, and a fuel injection drive circuit (not shown) is controlled to inject a desired fuel amount from the fuel injection valve 8.

  Further, a NOx sensor 5 for detecting the NOx trap catalyst outlet NOx concentration is provided at the outlet portion of the casing 6 of the exhaust passage 7. The detection signal of the NOx sensor 5 is input to the control device 2. Based on the detected concentration, the control device 2 determines whether or not the NOx trap catalyst 3 starts to process NOx in the exhaust gas when cold, and is discharged from the diesel engine 1 as will be described later. The exhaust air / fuel ratio of the exhaust gas is controlled to reduce HC, CO, and NOx discharged in the cold state.

  FIG. 2 is a flowchart showing a flow of control of the exhaust air-fuel ratio during cold in the first embodiment, and based on this, the operation during cold of the exhaust purification apparatus of the above embodiment will be described below. .

  After the diesel engine 1 is started, first, at step 101, it is determined whether or not it is a cold start based on the coolant temperature or the like. If it is determined in step 101 that the engine is cold-started, the process proceeds to step 102, and an operation is performed in which the exhaust air-fuel ratio of the exhaust gas of the diesel engine 1 is lean. When it is determined that it is not a cold start, the routine is terminated.

  In step 103, it is determined whether or not the detected value RNOx of the NOx sensor 5 at the outlet of the casing 6 is smaller than a predetermined first threshold value SNOx1. That is, it is determined whether the NOx concentration is lower than the reference level. Under the lean air-fuel ratio, the oxidation performance of HC and CO starts early, that is, at a relatively low temperature. As the temperature of the NOx trap catalyst 3 increases, the NOx trap performance rises. If the detected value RNOx is smaller than the predetermined value SNOx1 in step 103, it means that the NOx trap catalyst 3 has started to trap the inflowing NOx, so the routine proceeds to step 104, where the stoichiometric operation flag is turned ON. On the other hand, if it is equal to or greater than the predetermined value SNOx, the NOx trap catalyst 3 is in an inactive state, and therefore the lean operation is continued.

  After the stoichiometric operation flag is turned ON in step 104, in step 105, stoichiometric operation with the exhaust air / fuel ratio as the stoichiometric air / fuel ratio is started. During the stoichiometric operation, the NOx trap catalyst 3 reduces NOx by its three-way catalyst function and does not trap NOx. Therefore, it is possible to prevent an increase in the amount of NOx trap during cold.

  In step 106, it is determined whether or not the detected value RNOx of the NOx sensor 5 at the outlet of the casing 6 is smaller than a predetermined second threshold value SNOx2. If the detected value RNOx is smaller than the second threshold value SNOx2, the NOx trap catalyst 3 indicates that the function of trapping NOx has been sufficiently developed. Therefore, the process proceeds to step 107, and the stoichiometric operation flag is set to OFF. finish. On the contrary, if it is larger than the second threshold value SNOx2, the NOx trap catalyst 3 is in the process of being activated, so the stoichiometric operation is continued.

  Next, FIGS. 3 and 4 show a second embodiment of the present invention. In this embodiment, as shown in FIG. 3, an air-fuel ratio sensor 5A is provided on the outlet side of the casing 6 of the exhaust passage 7 in place of the NOx sensor 5 described above. That is, in this embodiment, the air-fuel ratio sensors 4 and 5A are provided on both the inlet side and the outlet side of the NOx trap catalyst 3.

  FIG. 4 shows the control flow of the second embodiment. After the diesel engine 1 is started, it is first determined in step 201 whether or not it is a cold start based on the coolant temperature or the like. The If it is determined that the engine is cold-started, the routine proceeds to step 202, where the operation is performed with the exhaust air-fuel ratio of the exhaust gas of the diesel engine 1 being lean. When it is determined that it is not a cold start, the routine is terminated.

  In step 203, the air-fuel ratio detected value FA / F of the inlet-side air-fuel ratio sensor 4 and the air-fuel ratio detected value RA / F of the outlet-side air-fuel ratio sensor 5A are compared, and the catalyst performance of the NOx trap catalyst 3 is compared. Determine the rising state. If RA / F <FA / F, it indicates that the NOx trap catalyst 3 has started to be activated, so the routine proceeds to step 204, where the stoichiometric operation flag is turned ON. If RA / F ≧ FA / F, since the NOx trap catalyst 3 is in an inactive state, the lean operation is continued.

  After the stoichiometric operation flag is turned ON in step 204, in step 205, stoichiometric operation with the exhaust air / fuel ratio as the stoichiometric air / fuel ratio is started. During the stoichiometric operation, the NOx trap catalyst 3 reduces NOx by its three-way catalyst function and does not trap NOx. Therefore, it is possible to prevent an increase in the amount of NOx trap during cold.

  In step 206, it is determined whether or not the air-fuel ratio detection value RA / F of the air-fuel ratio sensor 5A on the outlet side of the casing 6 is smaller than a predetermined threshold value SA / F. If the detected value RA / F is smaller than the threshold value SA / F, it indicates that the NOx trap catalyst 3 has sufficiently developed the function of trapping NOx. Therefore, the routine proceeds to step 207 and the stoichiometric operation flag is set to OFF. The process ends. On the contrary, if it is larger than the threshold value SA / F, the NOx trap catalyst 3 is in the process of being activated, so the stoichiometric operation is continued.

  Next, FIGS. 5 and 6 show a third embodiment of the present invention. In this embodiment, as shown in FIG. 5, instead of the air-fuel ratio sensors 4 and 5A and the NOx sensor 5 described above, a temperature sensor 4B for detecting the exhaust temperature on both the inlet side and the outlet side of the NOx trap catalyst 3 is used. , 5B are provided.

  FIG. 6 shows the flow of control of the third embodiment. After the diesel engine 1 is started, it is first determined in step 301 whether or not it is a cold start based on the cooling water temperature or the like. The When it is determined that the engine is cold-started, the routine proceeds to step 302, and an operation is performed with the exhaust air-fuel ratio of the exhaust gas of the diesel engine 1 being lean. When it is determined that it is not a cold start, the routine is terminated.

  In step 303, the detected value Ftemp of the inlet temperature sensor 4B is compared with the detected value Rtemp of the outlet temperature sensor 5B to determine the rising state of the catalyst performance of the NOx trap catalyst 3. If Ftemp <Rtemp, this means that the NOx trap catalyst 3 has started to be activated, so the routine proceeds to step 304, where the stoichiometric operation flag is turned ON. If Ftemp ≧ Rtemp, since the NOx trap catalyst 3 is in an inactive state, the lean operation is continued.

  After the stoichiometric operation flag is turned ON in step 304, in step 305, stoichiometric operation with the exhaust air / fuel ratio as the stoichiometric air / fuel ratio is started. During the stoichiometric operation, the NOx trap catalyst 3 reduces NOx by its three-way catalyst function and does not trap NOx. Therefore, it is possible to prevent an increase in the amount of NOx trap during cold.

  In step 306, it is determined whether the detected value Ftemp of the temperature sensor 4B on the inlet side of the casing 6 is higher than a predetermined threshold value Stemp. If the detected value Ftemp is higher than the threshold value Stemp, the NOx trap catalyst 3 indicates that the function of trapping NOx has been sufficiently developed. Therefore, the process proceeds to step 307, the stoichiometric operation flag is turned OFF, and the process is terminated. . On the contrary, if it is equal to or less than the threshold Stemp, the NOx trap catalyst 3 is in the process of activation, so the stoichiometric operation is continued.

  Next, FIGS. 7 and 8 show a fourth embodiment of the present invention. As shown in FIG. 7, this embodiment includes a temperature sensor 4B that detects the exhaust temperature only on the inlet side of the NOx trap catalyst 3, and no temperature sensor or the like is provided on the outlet side.

  FIG. 8 shows the flow of control of the fourth embodiment. After starting the diesel engine 1, it is first determined in step 401 whether or not it is cold start based on the coolant temperature or the like. The If it is determined that the engine is cold-started, the routine proceeds to step 402, where an operation is performed with the exhaust air-fuel ratio of the exhaust gas of the diesel engine 1 being lean. When it is determined that it is not a cold start, the routine is terminated.

  In step 403, the detected value Ftemp of the temperature sensor 4B on the inlet side is compared with a predetermined first threshold value Ftemp1, and the rising state of the catalyst performance of the NOx trap catalyst 3 is determined. If Ftemp> Ftemp1, it means that the NOx trap catalyst 3 has started to be activated, so the routine proceeds to step 404 and the stoichiometric operation flag is turned ON. If Ftemp ≦ Ftemp1, since the NOx trap catalyst 3 is in an inactive state, the lean operation is continued.

  After the stoichiometric operation flag is set to ON in step 404, in step 405, stoichiometric operation with the exhaust air / fuel ratio as the stoichiometric air / fuel ratio is started. During the stoichiometric operation, the NOx trap catalyst 3 reduces NOx by its three-way catalyst function and does not trap NOx. Therefore, it is possible to prevent an increase in the amount of NOx trap during cold.

  In step 406, the detected value Ftemp of the temperature sensor 4B on the inlet side of the casing 6 is compared with a predetermined second threshold value Ftemp2. If Ftemp> Ftemp2, the NOx trap catalyst 3 indicates that the function of trapping NOx is sufficiently developed, so the process proceeds to step 407 and the stoichiometric operation flag is turned OFF and the process is terminated. If Ftemp ≦ Ftemp2, since the NOx trap catalyst 3 is in the process of activation, the stoichiometric operation is continued.

BRIEF DESCRIPTION OF THE DRAWINGS The structure explanatory drawing which shows 1st Example of the exhaust gas purification apparatus which concerns on this invention. The flowchart which shows the control at the time of the cold of 1st Example. Structure explanatory drawing which shows 2nd Example of the exhaust gas purification apparatus which concerns on this invention. The flowchart which shows the control at the time of the cold of 2nd Example. FIG. 6 is an explanatory diagram showing the configuration of a third embodiment of the exhaust emission control apparatus according to the present invention. The flowchart which shows the control at the time of the cold of 3rd Example. Structure explanatory drawing which shows 4th Example of the exhaust gas purification apparatus which concerns on this invention. The flowchart which shows the control at the time of the cold of 4th Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diesel engine 2 ... Control apparatus 3 ... NOx trap catalyst 4 ... Air fuel ratio sensor 4B ... Temperature sensor 5 ... NOx sensor 5A ... Air fuel ratio sensor 5B ... Temperature sensor

Claims (9)

It has a function of trapping NOx (nitrogen oxide) in exhaust gas when the exhaust air-fuel ratio of the inflowing exhaust gas is lean, and desorbing and purifying trapped NOx when the exhaust air-fuel ratio is rich In the exhaust gas purification apparatus for an internal combustion engine, in which the NOx trap catalyst that functions as a three-way catalyst under the stoichiometric air-fuel ratio is disposed in the exhaust passage of the internal combustion engine,
An internal combustion engine having means for determining a rising state of the NOx treatment performance of the NOx trap catalyst when the engine is cold, and switching a function of processing the NOx of the NOx trap catalyst based on the determination. Exhaust purification device. It has a function of trapping NOx (nitrogen oxide) in exhaust gas when the exhaust air-fuel ratio of the inflowing exhaust gas is lean, and desorbing and purifying trapped NOx when the exhaust air-fuel ratio is rich In the exhaust gas purification apparatus for an internal combustion engine, in which the NOx trap catalyst that functions as a three-way catalyst under the stoichiometric air-fuel ratio is disposed in the exhaust passage of the internal combustion engine,
It has means for judging the rising state of the NOx treatment performance of the NOx trap catalyst when the engine is cold, and the exhaust air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is changed based on the judgment. An exhaust purification device for an internal combustion engine.   3. The exhaust air / fuel ratio of the exhaust gas flowing into the NOx trap catalyst is controlled to be lean at a temperature lower than a temperature at which the catalytic performance of the NOx trap catalyst rises after the engine is cold started. An exhaust gas purification apparatus for an internal combustion engine as described.   The exhaust air / fuel ratio of the exhaust gas flowing into the NOx trap catalyst is controlled to the stoichiometric air / fuel ratio in the temperature range from when the engine performance of the NOx trap catalyst rises until it reaches a sufficiently active state after the engine is cold started. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3.   A means for detecting the NOx concentration contained in the exhaust gas that has passed through the NOx trap catalyst is provided at the outlet of the NOx trap catalyst, and when the engine is cold, the rising of the catalyst performance is determined from the change in the detected NOx concentration. An exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4.   Means for detecting the exhaust air / fuel ratio of the exhaust gas that has passed through the NOx trap catalyst is provided at the outlet of the NOx trap catalyst, and when the engine is cold, the rising of the catalyst performance is determined from the detected change in the exhaust air / fuel ratio. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4.   A means for detecting the exhaust gas temperature is provided on each of the inlet side and the outlet side of the NOx trap catalyst, and when the engine is cold, the rising of the catalyst performance is determined from the temperature difference before and after the NOx trap catalyst. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4.   Temperature detection means for detecting the exhaust gas temperature is provided on the inlet side of the NOx trap catalyst, the rising temperature of the catalyst performance predicted from the oxidation reaction rate of HC and CO in the NOx trap catalyst, and detection of the temperature detection means The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the rising of the catalyst performance is determined based on the temperature.   The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 8, which is used for a diesel engine.

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