JP2000337132A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect the deterioration of a NOx controlling means independently of change of operating condition and degree of deterioration in the case where a three-dimensional catalyst is provided. SOLUTION: In an exhaust system of an internal combustion engine for combustion at a lean air-fuel ratio, a three-dimensional catalyst 1, a lean NOx catalyst 2, and a CO sensor 3 are arranged in this order from the upstream to the downstream of the exhaust system. The output from the CO sensor 3 is fed to an ECU. A deterioration detecting means for ECU determines the degree of deterioration of the lean NOx catalyst 2 on the basis of the CO concentration detected by the CO sensor 3. For example, in the case where the peak value of the CO concentration detected in the rich spike is higher than the predetermined critical CO concentration, since the degree of deterioration is large, the deterioration is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing NOx in exhaust gas when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio.
To reduce NOx absorbed when lean
The present invention relates to an exhaust gas purification device for an internal combustion engine provided with an Ox purification device, and more particularly to a technique effective for accurately detecting deterioration of a NOx purification device.

[0002]

2. Description of the Related Art In a lean-burn engine operated at an air-fuel ratio larger than the stoichiometric air-fuel ratio (about 14.7), NOx is absorbed when the air-fuel ratio of the exhaust gas is lean, and the exhaust gas becomes rich from the lean atmosphere. It is known to arrange a lean NOx catalyst (NOx purifying means) for releasing and reducing absorbed NOx in an exhaust system of an engine when the atmosphere is reached.

[0003] However, when the lean NOx catalyst deteriorates, the NOx absorption capacity is reduced, so that the NOx in the exhaust gas is reduced.
Is released into the atmosphere without purification. Therefore, the degree of deterioration of the lean NOx catalyst is determined, and when the deterioration is detected, the air-fuel ratio is temporarily made rich (hereinafter, referred to as "rich spike") during the lean operation, thereby absorbing the absorbed lean fuel. It becomes necessary to reduce the NOx catalyst.

For example, in Japanese Patent Application Laid-Open No. 7-208151,
The air-fuel ratio is switched from lean to rich by disposing a NOx sensor that detects the concentration of NOx in exhaust gas downstream of a lean NOx catalyst (described as “NOx absorbent” in the publication). After that, the time from when the output of the NOx sensor reaches a predetermined value is measured, and when this time is shorter than a predetermined time set in advance, it is determined that the NOx absorbent has deteriorated.

In Japanese Patent Application Laid-Open No. Hei 8-232644, an O2 sensor that generates a current proportional to the air-fuel ratio is provided downstream of a lean NOx catalyst (described as "NOx absorbent") in an exhaust passage. By arranging, the time from when the air-fuel ratio is switched from lean to rich to when the NOx releasing action is completed is measured from the generated current value of the O2 sensor, and from this time, the degree of deterioration of the NOx absorbent is obtained.

[0006]

SUMMARY OF THE INVENTION However, NOx
When the sensor is used, unless the operating state is completely constant, the NOx concentration in the exhaust gas fluctuates, so that the degree of deterioration of the lean NOx catalyst cannot be accurately diagnosed. In addition, the NOx sensor has poor detection accuracy on the low concentration side, is easily affected by NH3 (ammonia), and is expensive.

When an O2 sensor is used, the exhaust passage of the lean NOx catalyst depends on the degree of deterioration of the three-way catalyst disposed upstream of the lean NOx catalyst, that is, the degree of the O2 storage capacity of the three-way catalyst. Since a difference appears in the output tendency of the O2 sensor on the downstream side, lean N
In some cases, the degree of deterioration of the Ox catalyst cannot be accurately diagnosed.

[0008] The present invention has been made in view of such circumstances, and its object is to change the operating state and, if a three-way catalyst is provided, without affecting the degree of deterioration of NOx. An object of the present invention is to determine the degree of deterioration of a purifying means and accurately detect the deterioration.

[0009]

SUMMARY OF THE INVENTION To achieve the above object, the present invention relates to an exhaust purification device provided in an exhaust system of an internal combustion engine, wherein the exhaust gas is exhausted when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. NOx purifying means (lean NOx catalyst 2) for absorbing NOx in the gas and reducing the absorbed NOx when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio;
A CO concentration detecting means (CO sensor 3) for detecting the CO concentration in the exhaust gas on the downstream side of the NOx purifying means, and an output of the CO concentration detecting means when the air-fuel ratio is made rich after the continuation of the lean operation. And a deterioration detecting means (ECU) for detecting deterioration of the NOx purifying means.

The operation of the present invention will be described below with reference to FIGS. These figures show the CO emission behavior when the rich spike is temporarily performed during the lean operation. FIG. 4 shows the CO concentration on the upstream side of the NOx purifying means (the feed CO concentration before CAT is shown in FIG. 4). 5 is shown in FIG. 5, and the CO concentration downstream of the NOx purifying means (in the figure, CAT
Displayed after CO concentration. ). "Durable" refers to NOx at 80,000 km
Purification means, "sulfur degradation" refers to NOx whose NOx absorption power has decreased by absorbing a large amount of sulfur components in fuel.
Refers to purification means.

FIG. 4 shows that the CO concentration upstream of the NOx purifying means tends to temporarily increase during the rich spike, and FIG. 5 shows that the NOx during the rich spike
It can be seen that the greater the degree of deterioration of the purifying means, the more the CO release amount tends to increase. The behavior shown in FIG. 5 is that when the degree of deterioration of the NOx purifying means is large, the amount of NOx absorbed by the NOx purifying means decreases, so that the CO amount used for NOx reduction also decreases. C in exhaust gas
This is because the amount of released O increases.

Therefore, the amount of CO released during the rich spike,
That is, the degree of deterioration of the NOx purifying means can be determined based on the output of the CO concentration detecting means. For example,
A predetermined critical C for judging that the NOx purification means has deteriorated.
The O concentration (criterion) is set in advance, and when the CO concentration measured by the CO concentration detecting means is higher than the criterion, it is possible to determine that the deterioration has occurred.

Further, when the CO concentration measured by the CO concentration detecting means after a predetermined time has elapsed from the start of the rich spike operation is higher than a predetermined critical CO concentration (criterion), it is possible to judge deterioration. . Furthermore, it is determined that the larger the rate of increase of the CO concentration measured by the CO concentration detecting means from the start of the rich spike, that is, the steeper the output rise, the higher the CO emission concentration is. When it is higher than a predetermined critical CO concentration increase rate (criterion), it is also possible to judge deterioration.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an exhaust system of an internal combustion engine that performs combustion at a lean air-fuel ratio. In FIG. 1, reference numeral 1 denotes a three-way catalyst, 2 denotes a lean NOx catalyst (NOx purifying means), and 3 denotes a CO sensor (CO sensor). (Density detecting means).

The three-way catalyst 1 is formed by supporting a noble metal such as platinum, rhodium or palladium on a carrier such as alumina, and reduces NOx in exhaust gas at a stoichiometric air-fuel ratio,
The oxidation of C and CO is performed simultaneously. Lean NOx catalyst 2
Is formed by supporting at least one of an alkali metal, an alkaline earth metal, and a rare earth metal and a noble metal such as platinum on a carrier such as alumina.
The exhaust gas is interposed downstream of the exhaust passage to absorb NOx when the exhaust gas is in a lean atmosphere and reduce the absorbed NOx when the exhaust gas is in a rich atmosphere.

The CO sensor 3 is an application of a zirconia sensor, and is disposed downstream of the lean NOx catalyst 2 in the exhaust passage. As shown in FIG. 2, the CO sensor 3 has platinum electrodes 12a and 12b on both sides of the zirconia 11.
And one platinum electrode 12a is covered with an alumina-supported platinum catalyst 13 (indicated as "catalyst" in FIG. 2) which is a CO oxidation catalyst.
Operate at 00 ° C.

On the side of the platinum electrode 12a covered with the catalyst 13, CO is almost oxidized and an oxygen atmosphere of the same level as air is produced, while on the side of the platinum electrode 12b not covered with the catalyst 13, the platinum electrode Since an oxidation reaction of CO occurs on 12b, a mixed potential E is observed. FIG. 3 shows the characteristics of the CO sensor 3. Thus, the voltage E corresponding to the concentration of CO contained in the exhaust gas is output from the CO sensor 3.

An output voltage E from the CO sensor 3 is supplied to an engine control electronic control unit (not shown) (not shown).
Abbreviated as "ECU". ) Is supplied to the detection of deterioration of the lean NOx catalyst 2. For this reason, the ECU includes a deterioration detecting unit.

This deterioration detecting means determines the degree of deterioration of the lean NOx catalyst 2 based on the output of the CO sensor 3 when a rich spike is performed after the lean operation is continued. This determination indicates that if the lean NOx catalyst 2 is deteriorating, the amount of NOx absorbed by the lean NOx catalyst 2 during a rich spike is reduced, so that the CO amount used for NOx reduction is also reduced. The fact that the amount of CO released from the exhaust gas downstream of the lean NOx catalyst 2 is increased is used.

Next, the flow of the process for detecting the deterioration of the lean NOx catalyst 2 by the deterioration detecting means will be described with reference to the flowchart of FIG. First, in step S1, the ECU
Determines whether or not the air-fuel ratio control is performing a rich spike. If the determination result is “No”, that is, if the rich spike is being performed, the process ends, and the determination result is “Ye
s ", that is, during the rich spike, the process proceeds to step S2, and the CO concentration at that time is grasped from the output from the CO sensor 3.

Next, the process proceeds to step S3, where EC
The deterioration detection means of U uses the C acquired in step S2.
The degree of deterioration of the lean NOx catalyst 2 is considered (determined) based on the O concentration. As a first determination method, as shown in FIG. 7, a critical CO concentration (criterion) for determining that the lean NOx catalyst 2 is degraded is set in advance, and the CO sensor 3 measures the critical CO concentration. If the CO concentration is high, it is determined to be deteriorated.

As a second determination method, as shown in FIG. 8, the larger the rate of increase of the CO concentration measured by the CO sensor 3 from the start of the rich spike, that is, the steeper the rise of the output. , It is determined that the CO emission concentration is high, and if the CO concentration increase rate is higher than the critical CO concentration increase rate (criterion), it is determined that the fuel cell has deteriorated. As the CO concentration increase rate, for example, a rise coefficient C obtained by dividing the CO concentration increment Δ from the start of the rich spike to the vicinity of the peak value by the execution time t of the rich spike is employed.

As a third determination method, as shown in FIG. 9, after a lapse of a predetermined time from the start of execution of the rich spike,
If the CO concentration measured by the O sensor 3 is higher than a predetermined critical CO concentration (criterion), it is determined to be deteriorated. Here, the predetermined time can be set shorter than the time until the CO concentration reaches a peak, so that the time for determining the degree of deterioration can be shortened.

As described above, in the exhaust gas purifying apparatus for an internal combustion engine according to the present embodiment, the CO sensor 3 is disposed downstream of the lean NOx catalyst 2 interposed in the exhaust system.
The degree of deterioration of the lean / lean catalyst 2 is determined based on the amount of CO in the exhaust gas detected by the sensor, and the amount of CO is affected by a change in the operating state and the degree of deterioration of the three-way catalyst 1. Therefore, the degree of deterioration of the lean NOx catalyst 2 can be accurately determined, and accurate deterioration detection can be performed.

The "absorption" of NOx by the NOx purifying means (lean NOx catalyst 2 in the embodiment) according to the present invention also includes "adsorption" on the surface of the NOx purifying means. Further, in the present embodiment, the CO sensor 3 is provided only downstream of the lean NOx catalyst 2,
A CO sensor is also provided upstream of the lean NOx catalyst 2 so that each C
The accuracy of detection can be improved by comparing the output of the O sensor.

[0026]

As is apparent from the above description, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention uses the CO concentration detecting means disposed downstream of the NOx purifying means in the exhaust system to detect the amount of CO in the exhaust gas. Is detected, and the degree of deterioration is determined based on the CO amount. Therefore, NO is not affected by a change in the operating state or the degree of deterioration when a three-way catalyst is provided.
x It is possible to accurately detect the deterioration of the purifying means.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an exhaust gas purification apparatus according to the present invention.

FIG. 2 is a diagram illustrating a configuration example of a CO sensor.

FIG. 3 is a diagram showing characteristics of a CO sensor.

FIG. 4 is a diagram showing a CO emission behavior upstream of a lean NOx catalyst accompanying a change in an air-fuel ratio.

FIG. 5 is a diagram showing CO emission behavior downstream of a lean NOx catalyst with a change in air-fuel ratio.

FIG. 6 is a diagram showing an example of an ECU for detecting deterioration of a lean NOx catalyst.
3 is a flowchart showing the flow of the processing performed in step (a).

FIG. 7 is a diagram showing that the degree of deterioration can be determined from the peak value of the CO concentration detected during a rich spike.

FIG. 8 is a diagram showing that it is possible to determine the degree of deterioration from the CO concentration detected after a lapse of a predetermined time from the start of execution of the rich spike.

FIG. 9 is a diagram showing that the degree of deterioration can be determined from the rate of increase in the CO concentration detected during a rich spike.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Three-way catalyst 2 Lean NOx catalyst (NOx purification means) 3 CO sensor (CO concentration detection means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 45/00 314 F02D 45/00 314Z F-term (Reference) 3G084 AA04 BA13 BA14 BA33 DA10 DA27 FA28 3G091 AA12 AA13 AA17 AB03 AB05 AB06 AB09 BA14 BA15 BA19 BA33 BA34 CB02 DA01 DA02 DA03 DA10 DB05 DB09 DB10 EA30 EA33 FB10 FB11 FB12 FC02 GB01X GB02W GB03W GB04W GB05W GB06W GB10X GB16X HA08 HA18 HA36 HA37 HA42 3G301 HA01 HA15 JA01 LB01 JA01 LB01 MA11 NA08 NE06 NE13 NE14 NE15 PD01B PD01Z

Claims (1)

[Claims] 1. An exhaust purification device provided in an exhaust system of an internal combustion engine, wherein the exhaust gas absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, and converts the absorbed NOx into the exhaust gas. NO is reduced when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio
x purification means, CO concentration detection means for detecting the CO concentration in the exhaust gas downstream of the NOx purification means, and based on the output of the CO concentration detection means when the air-fuel ratio is made rich after the continuation of the lean operation. An exhaust purification device for an internal combustion engine, comprising: a deterioration detection unit that detects deterioration of the NOx purification unit.