JP2002349316A - Exhaust emission purifying system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible, in an exhaust gas purifying system in which a three way catalyst is located upstream of an NOx catalyst, to supply, in just proportion, rich components to both of the catalysts, without being affected by the states of the three way catalyst and the NOx catalyst, operating conditions, or the like, at the time of an NOx purge control. SOLUTION: The NOx purge control, by which a target air-fuel ratio is temporarily switched to be rich during a lean operation and the adsorbed NOx of the NOx catalyst is purged, is performed. At the time of starting this NOx purge control, the target air-fuel ratio is set to be a first rich level at which the rich degree is large, thereby starting the NOx purge control. In the middle of the NOx purge control, the target air-fuel ratio is switched to a second rich level, at which the rich degree is smaller than that of the first rich level, when an output of an oxygen sensor in the downstream of the three way catalyst is changed from the vicinity of a stoichiometric amount to the rich side, thereby continuing the NOx purge control. Then, the NOx purge control is ended when an output of an oxygen sensor in the downstream of the NOx catalyst is changed from the vicinity of a stoichiometric amount to the rich side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust purification system for an internal combustion engine having at least a NOx storage-reduction type catalyst installed in an exhaust passage of the internal combustion engine.

[0002]

2. Description of the Related Art In recent years, so-called lean burn engines and in-cylinder injection engines, which control the air-fuel ratio to a side leaner than stoichiometric air (stoichiometric air-fuel ratio), have been put to practical use for the purpose of improving the fuel efficiency of automobiles. In these engines, the amount of generated NOx (nitrogen oxide) is larger than that of a normal engine. Therefore, a NOx storage-reduction type catalyst (hereinafter, referred to as “NOx catalyst”) is employed to reduce NOx emissions. There is something that I did. This NOx catalyst stores NOx when the air-fuel ratio of the exhaust gas is lean, and reduces and purifies and purges (releases) the stored NOx when the air-fuel ratio becomes rich (or stoichiometric). Therefore, when the lean operation continues for a long time, the target air-fuel ratio is intermittently switched to rich during the lean operation to purge the stored NOx of the NOx catalyst in order to prevent the NOx storage amount of the NOx catalyst from being saturated. The NOx purge control (NOx reduction purification control) is performed.

Further, recently, as disclosed in Japanese Patent Application Laid-Open No. 11-62666, in order to increase the exhaust gas purification rate,
In some cases, a three-way catalyst is provided upstream of the NOx catalyst.
In the system of this publication, NOx purge control is performed during NOx purge control.
x The air-fuel ratio on the downstream side of the catalyst is detected by an air-fuel ratio sensor, and NO
After the start of the x purge control, the time during which the air-fuel ratio on the downstream side of the NOx catalyst is maintained in the vicinity of stoichiometry and the time during which the air-fuel ratio is maintained in the rich state thereafter are measured.
The rich level of x purge control (the degree of richness of the target air-fuel ratio) and the duration thereof are learned and corrected.

[0004]

However, the three-way catalyst installed upstream of the NOx catalyst has the ability to occlude oxygen in the exhaust gas. (HC, CO, etc.) is consumed by reacting with the stored oxygen of the three-way catalyst, and the NO
x rich catalyst is not supplied with sufficient rich components,
Immediately after the start of the x purge control, the oxygen concentration (lean component concentration) in the exhaust gas flowing into the NOx catalyst becomes lower than that during the lean operation, so that NO from the NOx catalyst becomes lean component.
x begins to detach. However, as described above, after the start of the NOx purge control, for a while, the rich component serving as the reducing agent for the stored NOx is not sufficiently supplied to the NOx catalyst.
NOx released from the Ox catalyst is exhausted without being sufficiently reduced and purified. Therefore, after the start of the NOx purge control, the stored oxygen of the three-way catalyst is
In order to supply a sufficient rich component to the Ox catalyst at an early stage, it is necessary to increase the rich level of the NOx purge control to some extent.

However, when the air-fuel ratio on the downstream side of the NOx catalyst is switched to rich with the rich level increased, the NOx purge control is terminated. A large amount of rich component remains in the exhaust passage to the combustion chamber of the engine, and the large amount of rich component is discharged to the atmosphere without being consumed by the two catalysts. As a result, the exhaust emission immediately after the end of the NOx purge control deteriorates.

As a countermeasure against this, Japanese Patent Laid-Open Publication No.
In Japanese Patent No. 2666, the rich level is maximized at the time of starting the NOx purge control, and the rich level is reduced with time. However, as described above, N
In a configuration in which a three-way catalyst is installed upstream of the Ox catalyst, NO
Since the rich components (HC, CO, etc.) in the exhaust gas are consumed by reacting with the stored oxygen of the three-way catalyst for a while after the start of the x purge control, if the reduction rate of the rich level during the NOx purge control is increased, NO released from the NOx catalyst at the beginning
x may be discharged without being sufficiently reduced and purified. Conversely, if the rate of decrease of the rich level during the NOx purge control is reduced, an excess rich component may be discharged immediately after the end of the NOx purge control.

In view of the above, the above-mentioned Japanese Patent Application Laid-Open No. 11-62666 has been proposed.
In Japanese Patent Application Laid-Open Publication No. H11-260, the time during which the output of the air-fuel ratio sensor downstream of the NOx catalyst is maintained near the stoichiometric state after the start of the NOx purge control and the time during which the output is maintained in a rich state are measured. The maximum rich level at the start of the control and the rate of decrease of the rich level are learned and corrected.

However, the relationship between the oxygen storage amount of the three-way catalyst and the NOx storage amount of the NOx catalyst changes in a complicated manner. For example, even if the operating conditions at the start of the NOx purge control are the same, if the previous operating conditions are different, the relationship between the oxygen storage amount of the three-way catalyst and the NOx storage amount of the NOx catalyst becomes different. Further, the oxygen storage amount and the NOx storage amount of each catalyst also change depending on the degree of deterioration of each catalyst. Therefore, in order to accurately learn and correct the rich level during the NOx purge control,
Although it is necessary to accurately estimate the oxygen storage amount of the three-way catalyst and the NOx storage amount of the NOx catalyst, the oxygen storage amount of the three-way catalyst and NOx are determined based on the output of the air-fuel ratio sensor downstream of the NOx catalyst.
It is impossible to accurately learn and correct the rich level by estimating the NOx storage amount of the catalyst. Therefore, in the learning correction based on the output of the air-fuel ratio sensor on the downstream side of the NOx catalyst, NO
It is unavoidable that the rich component supply amount becomes excessive or deficient during the x purge control, and the exhaust emission temporarily deteriorates.

The present invention has been made in view of such circumstances, and accordingly, an object of the present invention is to provide both the three-way catalyst and the NOx catalyst without being affected by the conditions and operating conditions during the NOx purge control. An object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine that can supply rich components without excess and deficiency and reduce exhaust emissions.

[0010]

To achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention uses a three-way catalyst as an upstream catalyst and NO as a downstream catalyst.
x storage reduction type catalyst (NOx catalyst) is used, and a sensor for detecting the air-fuel ratio of exhaust gas or rich / lean or gas component concentration is installed downstream of the three-way catalyst and downstream of the NOx catalyst, respectively. The NOx purge control means sets the target air-fuel ratio to the first rich level having a large rich degree and starts the NOx purge control. During the NOx purge control, based on the output of the sensor on the downstream side of the three-way catalyst. The target air-fuel ratio is switched to a second rich level having a smaller degree of richness than the first rich level.

In this configuration, after the start of the NOx purge control,
For a while, the target air-fuel ratio is maintained at the first rich level where the degree of richness is large, and a large amount of rich component is supplied to the three-way catalyst to quickly purge the stored oxygen of the three-way catalyst.
As a result, when the stored oxygen of the three-way catalyst is low,
The concentration of the rich components (HC, CO, etc.) supplied to the NOx catalyst after passing through the three-way catalyst starts to increase.
The output of the sensor downstream of the three-way catalyst changes from near stoichiometric to rich. From this characteristic, it is determined when the rich component supply amount to the three-way catalyst starts to increase based on the output of the sensor on the downstream side of the three-way catalyst. At that time, the target air-fuel ratio is set to be lower than the first rich level. By switching to the second rich level having a small rich degree, an appropriate amount of rich component is supplied to the NOx catalyst while preventing the supply amount of rich component to the NOx catalyst from becoming excessive.
x can be purged.

Further, it is preferable to determine the end time of the NOx purge control based on the output of a sensor on the downstream side of the NOx catalyst. That is, when the NOx storage amount of the NOx catalyst decreases, the concentration of the rich component passing through the NOx catalyst increases, and accordingly, the output of the sensor downstream of the NOx catalyst changes from near stoichiometric to rich. From this characteristic, it is determined, based on the output of the sensor on the downstream side of the NOx catalyst, when the stored NOx of the NOx catalyst is almost gone (the time when the state of the NOx catalyst recovers to near stoichiometry). At that time, the NOx purge control is ended. To return to normal lean operation.

As described above, the end timing of the NOx purge control is determined based on the output of the sensor on the downstream side of the NOx catalyst, and even if the NOx purge control is ended, the rich gas component present in the exhaust passage at the end of the NOx purge control is determined. If the amount is large, the rich gas components are discharged into the atmosphere, so that the exhaust emission deteriorates. However, in the present invention, during the NOx purge control, the target air-fuel ratio is switched to the second rich level having a smaller degree of richness than the first rich level. Existing rich gas components can be reduced, and emissions emitted to the atmosphere can be reduced.

In this case, it is preferable to terminate the NOx purge control when the output of the sensor downstream of the NOx catalyst changes from near stoichiometric to rich during the NOx purge control. In this case, NO
After confirming that the purging of the stored NOx of the x catalyst is almost completed, the NOx purge control can be ended at the most appropriate time.

Further, when the output of the sensor downstream of the three-way catalyst changes from near stoichiometric to rich during the NOx purge control, the target air-fuel ratio is changed from the first rich level to the second rich level. It is better to switch to the rich level. By doing so, it is possible to confirm that the purge of the stored oxygen of the three-way catalyst has been substantially completed, and to switch the target air-fuel ratio to the second rich level at the most appropriate time.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a lean burn engine will be described below with reference to the drawings.
First, a schematic configuration of the entire engine control system will be described with reference to FIG. An air cleaner 13 is provided at the most upstream portion of an intake pipe 12 of an engine 11 which is an internal combustion engine, and an air flow meter 14 for detecting an intake air amount is provided downstream of the air cleaner 13. Downstream of the air flow meter 14, a throttle valve 15 and a throttle opening sensor 16 for detecting a throttle opening are provided.

Further, a surge tank 17 is provided downstream of the throttle valve 15.
7, an intake pipe pressure sensor 18 for detecting an intake pipe pressure is provided. In addition, the surge tank 17 is provided with an intake manifold 19 for introducing air into each cylinder of the engine 11, and a fuel injection valve 20 for injecting fuel is mounted near an intake port of the intake manifold 19 of each cylinder. .

On the other hand, in the middle of an exhaust pipe 21 (exhaust passage) of the engine 11, a three-way catalyst 22 and a NOx catalyst 23 (NOx storage reduction type catalyst) for purifying CO, HC, NOx, etc. in exhaust gas are provided. They are installed in series. In this case, NO
The three-way catalyst 22 disposed on the upstream side of the x catalyst 23 is formed to have a relatively small capacity so that the warm-up is completed early at the time of starting and the exhaust emission at the time of starting is reduced. on the other hand,
The downstream NOx catalyst 23 stores NOx when the air-fuel ratio of the exhaust gas is lean, and reduces and purifies and releases the stored NOx when the air-fuel ratio becomes rich (or stoichiometric). The NOx catalyst 23 is formed to have a relatively large capacity so as to be able to occlude NOx sufficiently even in a high load region where the amount of NOx in the exhaust gas is large.

An air-fuel ratio sensor 24 (A / F sensor) for outputting a linear air-fuel ratio signal according to the air-fuel ratio of the exhaust gas is provided upstream of the three-way catalyst 22.
An oxygen sensor 2 whose output voltage is inverted depending on whether the air-fuel ratio of the exhaust gas is rich or lean with respect to the stoichiometric (the stoichiometric air-fuel ratio) is provided on the downstream side of the NOx catalyst 23 and the downstream side of the NOx catalyst 23, respectively.
5, 26 are provided. Hereinafter, the oxygen sensor 25 on the downstream side of the three-way catalyst 22 is referred to as a first oxygen sensor 25,
The oxygen sensor 26 on the downstream side of the Ox catalyst 23 is referred to as a second oxygen sensor 26.

An oxygen sensor may be provided upstream of the three-way catalyst 22 instead of the air-fuel ratio sensor 24, and an oxygen sensor may be provided downstream of the three-way catalyst 22 and / or downstream of the NOx catalyst 23. Instead of 25, an air-fuel ratio sensor may be provided, or a gas sensor or the like for detecting the concentration of gas components such as HC and CO may be provided.

The cylinder block of the engine 11 is provided with a cooling water temperature sensor 27 for detecting a cooling water temperature and a crank angle sensor 28 for detecting an engine speed.

These various sensor outputs are input to an engine control circuit (hereinafter referred to as "ECU") 29.
The ECU 29 is mainly configured by a microcomputer and executes the NOx purge control program of FIG. 2 stored in a built-in ROM (storage medium),
It functions as a NOx purge control means referred to in the claims.

When the NOx purge control program shown in FIG. 2 is started, first, at step 101, it is determined whether or not NOx purge control execution conditions are satisfied. Where NO
x Purge control execution conditions are determined by NOx during lean operation.
For example, the lean operation time is set to NO so that the NOx purge control is performed before the NOx storage amount of the catalyst 23 is saturated.
The determination may be made based on whether or not a predetermined time set within a range where the NOx storage amount of the x catalyst 23 is not saturated.
At this time, NOx in the exhaust gas depends on the engine operating conditions.
Considering that the concentration changes and the rate of increase of the NOx storage amount of the NOx catalyst 23 changes, the lean operation time (predetermined time) until the start of the NOx purge control is changed to the engine operating conditions (for example, engine speed, intake air). (Pipe pressure, intake air amount, target air-fuel ratio, etc.).

When the NOx purge control execution condition is satisfied, the process proceeds from step 101 to step 102, where the target air-fuel ratio is switched to the first rich level having a large rich degree as shown in FIG. , The fuel injection amount TAU is calculated (step 103), and the NOx purge control is started. TAU = TP × LFAF × FALL × K1

Here, TP is a basic injection amount, which is calculated from a map or the like according to the current engine speed and intake pipe pressure. LFAF is an air-fuel ratio correction coefficient during lean operation, and FALL is another correction coefficient such as a water temperature correction coefficient.
K1 is a first fuel increase coefficient for enriching the air-fuel ratio of the exhaust gas to a first rich level, and is set to a value larger than a second fuel increase coefficient K2 described later. As a result, the fuel injection amount TAU at the start of the NOx purge control is greatly increased, and exhaust gas containing a large amount of rich components such as HC and CO is discharged from the engine 11.

When the enriched exhaust gas begins to pass through the three-way catalyst 22 after the start of the NOx purge control, as shown in FIG.
Changes to near stoichiometric. That is, after the start of the NOx purge control, while the stored oxygen of the three-way catalyst 22 remains to some extent, the rich components (HC, CO
) Reacts with the stored oxygen of the three-way catalyst 22 and is consumed, so that the air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 22 becomes close to stoichiometric. As a result, after the start of the NOx purge control,
Until the stored oxygen of the three-way catalyst 22 becomes low,
The output of the first oxygen sensor 25 downstream of the three-way catalyst 22 is maintained near the stoichiometric state.

During this period, it is determined in step 103 whether the output of the first oxygen sensor 25 has changed from near stoichiometric to rich, and if the output of the first oxygen sensor 25 is near stoichiometric, The target air-fuel ratio is maintained at the first rich level having a large rich degree, and a large amount of rich component is supplied to the three-way catalyst 22 to quickly purge the stored oxygen of the three-way catalyst 22.

As a result, when the amount of oxygen stored in the three-way catalyst 22 decreases, the NOx catalyst 2 passes through the three-way catalyst 22 and passes through the three-way catalyst 22.
3, the output of the first oxygen sensor 25 downstream of the three-way catalyst 22 changes from near stoichiometric to rich as shown in FIG. .

Then, when the output of the first oxygen sensor 25 changes from the vicinity of the stoichiometric state to the rich side, step 1
From step 03 to step 104, as shown in FIG. 3, the target air-fuel ratio is switched to the second rich level having a smaller rich degree than the first rich level, and the fuel injection amount TAU is calculated by the following equation (step 104). 104), NOx purge control is continued. TAU = TP × LFAF × FALL × K2

Here, K 2 is the air-fuel ratio of the exhaust gas,
This is a second fuel increase coefficient for enriching the fuel to a rich level, and is set to a value smaller than the first fuel increase coefficient K1 described above. Thereby, the fuel injection amount TAU
Is made smaller than when the NOx purge control is started, and the exhaust gas whose rich component amount is smaller than when the NOx purge control is started is discharged from the engine 11. Thus, while preventing the supply amount of the rich component to the NOx catalyst 23 from becoming excessive, an appropriate amount of the rich component is supplied to the NOx catalyst 23 to purge the NOx stored in the NOx catalyst 23.

During this period, at step 105, it is determined whether or not the output of the second oxygen sensor 26 has changed from near the stoichiometric to a rich state. If the output of the second oxygen sensor 26 is near the stoichiometric, The NOx stored in the NOx catalyst 23 is purged while the target air-fuel ratio is maintained at the second rich level having a degree of richness smaller than the first rich level. Accordingly, when the NOx storage amount of the NOx catalyst 23 decreases, the concentration of the rich component passing through the NOx catalyst 23 increases, and accordingly, the output of the second oxygen sensor 26 on the downstream side of the NOx catalyst 23 becomes close to stoichiometric. To rich side.

When the output of the second oxygen sensor 26 changes from the vicinity of the stoichiometric state to the rich side, the NOx purge control ends, and as shown in FIG.
Is switched from the rich level to the target air-fuel ratio at the time of the lean operation, and the above-described processing is repeated.

In the present embodiment described above, the NOx purge control is started by setting the target air-fuel ratio to the first rich level having a large rich degree at the start of the NOx purge control.
During the Ox purge control, the first downstream side of the three-way catalyst 22
When the output of the oxygen sensor 25 changes from the vicinity of the stoichiometric state to the rich side, the target air-fuel ratio is switched to the second rich level having a smaller rich degree than the first rich level, and N
Ox purge control is continued, and when the output of the second oxygen sensor 26 downstream of the NOx catalyst 23 changes from near stoichiometric to rich, the NOx purge control is terminated. The two catalysts 2 are not affected by the state of the catalyst 22 and the NOx catalyst 23 and the operating conditions.
It is possible to supply rich components to the components 2 and 23 without excess or deficiency, thereby reducing exhaust emissions.

In the present embodiment, during the NOx purge control, the first fuel increase coefficient K1 for enriching the air-fuel ratio of the exhaust gas to the first rich level and the air-fuel ratio of the exhaust gas are set to the second value. The fuel injection amount TAU is calculated using the second fuel increase coefficient K2 for enrichment to a rich level. However, during the NOx purge control, the air-fuel ratio sensor 24 on the upstream side of the three-way catalyst 22 is used. An air-fuel ratio correction coefficient FAF is calculated based on a deviation between the detected actual air-fuel ratio and a target air-fuel ratio (first rich level or second rich level), and the air-fuel ratio correction coefficient FAF and other correction coefficients FALL are calculated. The fuel injection amount TAU may be obtained by multiplying the basic injection amount TP. TAU = TP × FAF × FALL

The present invention is not limited to the above-described embodiment, and three or more catalysts may be provided in the exhaust pipe 21. In short, at least one catalyst is a NOx catalyst, The catalyst on the side may be a three-way catalyst.

In addition, the present invention can be applied to an engine that controls the air-fuel ratio lean, such as a direct injection engine, in addition to a lean burn engine.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an entire engine control system showing an embodiment of the present invention.

FIG. 2 is a flowchart showing a flow of processing of a NOx purge control program;

FIG. 3 is a time chart showing behaviors of a target air-fuel ratio, a first oxygen sensor output, and a second oxygen sensor output during NOx purge control.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 ... Engine (internal combustion engine), 12 ... Intake pipe, 20 ... Fuel injection valve, 21 ... Exhaust pipe (exhaust passage), 22 ... Three-way catalyst, 23 ... NOx catalyst, 24 ... Air-fuel ratio sensor, 25 ... First Oxygen sensor, 26 ... second oxygen sensor, 29 ... EC
U (NOx purge control means).

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) F01N 3/28 301 B01D 53/36 103B 101B F term (reference) 3G091 AA12 AA17 AB03 AB05 AB06 BA11 BA14 BA27 BA31 BA32 BA33 CA18 CB02 DA02 DA03 DA10 DC01 EA01 EA05 EA06 EA07 EA16 EA34 FB10 FB11 FB12 FC01 HA10 HA36 HA37 HA42 3G301 HA01 HA15 JA21 JA25 LB02 LC01 MA01 ND01 NE02 NE07 PA01Z PA07Z PA11Z PD02Z PD09Z PE01Z PE03A PE08A02 A04 DA048

Claims (4)

[Claims] 1. An exhaust gas purification apparatus for an internal combustion engine having at least two catalysts arranged in series in an exhaust passage of the internal combustion engine, wherein a three-way catalyst is used as an upstream catalyst, and N is used as a downstream catalyst.
Sensors using an Ox storage reduction type catalyst (hereinafter referred to as "NOx catalyst") to detect the air-fuel ratio or rich / lean or gas component concentration of exhaust gas on the downstream side of the three-way catalyst and the downstream side of the NOx catalyst, respectively. And NOx purge control means for temporarily switching the target air-fuel ratio to rich during lean operation to perform NOx purge control for purging NOx stored in the NOx catalyst, wherein the NOx purge control means The NOx purge control is started by setting the target air-fuel ratio to the first rich level having a large rich degree, and during the NOx purge control, the target air-fuel ratio is set based on the output of the sensor on the downstream side of the three-way catalyst. An exhaust gas purifying apparatus for an internal combustion engine, which switches to a second rich level having a degree of richness smaller than the first rich level. 2. The NOx purge control means includes:
The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the end timing of the NOx purge control is determined based on an output of a sensor downstream of the x catalyst. 3. The NOx purge control means includes:
3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the NOx purge control is terminated when the output of the sensor downstream of the x catalyst changes from near stoichiometric to rich. 4. The NOx purge control means changes the target air-fuel ratio from the first rich level when the output of a sensor downstream of the three-way catalyst changes from near stoichiometric to rich during NOx purge control. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the switching is performed to the second rich level.