JPH11107807A - Emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To regenerate a NOx catalyzer simply and efficiently by controlling an exhaust air-fuel ratio to be rich when more than a specified amount of nitrogen oxides is adsorbed to the NOx catalyzer, and also installing an air-fuel ratio control means to terminate this rich control on the basis of output of a combustible sensor. SOLUTION: When an adsorbed quantity of nitrogen oxides in a NOx catalyzer 3 has exceeded the specified quantity, this exhaust emission control device is equipped with an air-fuel ratio control means 5a to discharge the nitrogen oxides adsorbed to the catalyzer 3. Also in this emission control device, an operating control part 5 is equipped with a reducing gas quantity detecting means 5b detecting a reducing gas quantity in the exhaust gas on the basis of output of a combustible sensor 4 at a time when it temporarily operates an engine body 1 to be rich under the control of the air-fuel ratio control means 5a and the exhaust air-fuel ratio is made into being rich and thereby the nitrogen oxides are discharged out of the NOx catalyzer 3. At the air-fuel ratio control means 5a, when more than a specified amount of the reducing gas is detected at the downstream side of the catalyzer 3, the activation of the NOx catalyzer 3 by the rich control of the exhaust air-fuel ratio is terminated by the detecting.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine capable of efficiently activating a NOx catalyst provided in an exhaust passage of the internal combustion engine.

[0002]

2. Related Background Art In a so-called lean burn engine which is operated at a lean air-fuel ratio in order to improve fuel efficiency, one of harmful substances is used as compared with an engine which is operated at a normal stoichiometric air-fuel ratio. A large amount of nitrogen oxides (NOx) are emitted. In order to remove NOx in such exhaust gas, a storage NOx is inserted into an exhaust passage of an internal combustion engine (engine).
It has been proposed to provide a catalyst. In addition, the storage type N
The Ox catalyst adsorbs NOx in the exhaust gas when the exhaust air-fuel ratio is lean. For example, when the exhaust air-fuel ratio is enriched and the oxygen concentration in the exhaust gas decreases, the Ox catalyst already adsorbs NOx.
It acts to release Ox.

[0003]

When the NOx catalyst adsorbs a predetermined amount or more of NOx, the internal combustion engine is temporarily operated in a rich manner as described above to reduce the oxygen concentration in the exhaust gas. The air-fuel ratio control is executed to release the NOx adsorbed on the NOx catalyst, that is, to restore the purification efficiency of the NOx catalyst. However, when the rich operation is insufficient, NOx is not completely released, so that a problem occurs that the purification ability of the NOx catalyst is not sufficiently restored. If the rich operation is excessive, the amounts of HC and CO discharged along with the rich operation increase, and the fuel efficiency is deteriorated. Therefore, it is desirable to make the exhaust air-fuel ratio rich only until the release of the NOx adsorbed on the NOx catalyst is completed.

[0004] However, it cannot be denied that the purifying ability (adsorbing ability) of the NOx catalyst gradually decreases as NOx is adsorbed and released repeatedly. In addition, the time required for releasing NOx from the NOx catalyst (restoring the NOx catalyst) varies depending on the enrichment conditions at that time, specifically, the oxygen concentration in the exhaust gas and the catalyst temperature. Therefore, in order to efficiently recover the NOx catalyst without deteriorating the exhaust gas characteristics, it is necessary to grasp the state of the recovery of the NOx catalyst due to, for example, enrichment of the exhaust air-fuel ratio.

The present invention has been made in view of such circumstances, and an object of the present invention is to simply and efficiently recover a NOx catalyst provided in an exhaust passage without causing a problem such as deterioration of fuel efficiency. An object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine that can be operated.

[0006]

In order to achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention adsorbs NOx in exhaust gas when the exhaust air-fuel ratio is lean, and removes NOx in the exhaust gas. N already adsorbed when oxygen concentration decreases
A NOx catalyst for releasing Ox is provided in an exhaust passage of the internal combustion engine, and a combustible sensor is provided downstream of the NOx catalyst. When a predetermined amount or more of NOx is adsorbed on the NOx catalyst, the exhaust air-fuel ratio As an air-fuel ratio control means for controlling the enrichment of the fuel cell, means for terminating the enrichment control based on the output of the combustible substance sensor is provided.

That is, according to the present invention, the air-fuel ratio is enriched from the output of a combustible sensor, such as a CO / HC sensor or a reducing gas sensor, provided downstream of the NOx catalyst in the exhaust passage, and the NOx catalyst is restored. When the reducing gas is detected by the combustible substance sensor, it is detected that the reducing gas is no longer oxidized, that is, the NOx catalyst is recovered, At this detection timing, the enrichment of the exhaust air-fuel ratio by the air-fuel ratio control means is terminated.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An exhaust gas purifying apparatus for an internal combustion engine according to one embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment, and 1 is an engine (internal combustion engine) main body. The exhaust passage 2 of the engine body 1 adsorbs NOx in the exhaust gas when the exhaust air-fuel ratio is lean, and releases the already adsorbed NOx when the oxygen concentration in the exhaust gas decreases. Is provided. Further, on the downstream side of the NOx catalyst 3, a combustible substance sensor 4 including, for example, a CO / HC sensor and a reducing gas sensor is provided. The combustible substance sensor 4 is composed of a semiconductor sensor using SnO ₂ or ZnO as a base material, for example, and contains hydrocarbon (HC), carbon monoxide (CO), or other components contained in exhaust gas discharged from the engine body 1. It plays the role of detecting the reducing gas component of the gas.

A three-way catalyst (not shown) may be provided downstream of the NOx catalyst 3 in some cases. This three-way catalyst uses NO and HC existing near the catalyst to produce NO.
It acts to reduce x. This three-way catalyst reduces the Nx
The reduction of NOx not adsorbed by the Ox catalyst 3 and the N released from the NOx catalyst 3 when the NOx catalyst 3 is reduced
Ox reduction takes place.

The operation control unit 5 for controlling the operation of the engine body 1 is mainly composed of, for example, a microprocessor, and basically has an operation state such as an accelerator operation and an engine load at that time. To control the throttle opening, fuel injection amount, and the like. Especially the operation control unit 5
For example, when the engine body 1 is operating at a lean air-fuel ratio (in a lean operation) and the NOx adsorption amount in the NOx catalyst 3 exceeds a predetermined amount, the engine body 1 is temporarily stopped. A rich operation is performed to reduce the concentration of oxygen in the exhaust gas and to reduce H ₂ and H
Air-fuel ratio control means 5 for releasing NOx adsorbed on said NOx catalyst 3 by increasing reducing gas such as C, CO, etc.
a. Further, the operation control unit 5 performs the rich operation of the engine body 1 temporarily under the control of the air-fuel ratio control unit 5a as described above, that is, makes the exhaust air-fuel ratio rich and removes NOx from the NOx catalyst 3. A reducing gas amount detecting means 5b for detecting the amount of reducing gas in the exhaust gas based on the output of the combustible substance sensor 4 when discharging is provided. When the reducing gas amount detecting means 5b detects a predetermined amount or more of reducing gas downstream of the NOx catalyst 3 in the air-fuel ratio controlling means 5a, the air-fuel ratio controlling means 5a receives the exhaust gas air-fuel ratio enrichment control. NO by
x The activation of the catalyst 3 is terminated.

That is, the air-fuel ratio control means 5a monitors the amount of reducing gas on the downstream side of the NOx catalyst 3 from the output of the combustible substance sensor 4 when the NOx catalyst 3 is restored by controlling the enrichment of the air-fuel ratio. When the reducing gas amount detecting means 5b detects that the amount of reducing gas in the exhaust gas exceeds a predetermined amount, the reducing gas such as HC and CO discharged from the engine body 1 passes through the NOx catalyst 3 as it is. It is detected that the gas passes through, that is, the reducing gas is no longer oxidized, and that the NOx catalyst 3 is completely restored. The air-fuel ratio control means 5a terminates the exhaust air-fuel ratio enrichment control at the detection timing of the reducing gas and returns the exhaust air-fuel ratio to the original state (lean operation state).

In the air-fuel ratio control means 5a, not only the control for stopping the enrichment operation based on the output of the combustible substance sensor 4 but also the operation for a certain period based on the duration of the enrichment operation is performed. Even when the enrichment operation is performed over a period of time, the stop control is executed. By using such stop control based on the duration of the rich operation, even if the detection characteristic of the combustible substance sensor 4 is deteriorated or a failure occurs in the detection system, it is undesirably long. The air-fuel ratio control is performed smoothly so that the enrichment operation is not performed over time.

FIG. 2 shows an example of a procedure of a control process in the air-fuel ratio control means 5a for executing the above-described air-fuel ratio control. This process is started by determining whether or not the NOx catalyst 3 has adsorbed a predetermined amount or more of NOx [Step S1]. Incidentally, the NOx adsorption amount in the NOx catalyst 3 is equivalently obtained by, for example, integrating the fuel injection pulse time width when the operation mode of the engine body 1 is the lean mode. Alternatively, the amount of NOx emission when the engine body 1 is operating lean is determined according to the engine speed Ne and the engine output Pe.
The emission amount is obtained by multiplying the product obtained by multiplying the emission amount by the adsorption coefficient depending on the NOx adsorption rate of the NOx catalyst 4 and the correction coefficient of the NOx adsorption rate depending on the catalyst temperature.

The NOx adsorption amount of the NOx catalyst 4 thus obtained is compared with a predetermined threshold (predetermined amount) to determine whether or not the adsorption amount exceeds the predetermined amount. It is determined whether activation (regeneration) of the NOx catalyst 3 is necessary. If the NOx adsorption amount is less than the predetermined value, it is determined that the NOx regeneration condition is not satisfied, and the routine returns without executing the following processing.

If it is determined in step S1 that the amount of NOx adsorbed on the NOx catalyst 3 has reached a predetermined value, the air-fuel ratio control means 5a starts regeneration control for the NOx catalyst 3. In this regeneration control, for example, the operation mode of the engine body 1 is temporarily set to the rich operation,
This is realized by enriching the exhaust air-fuel ratio to lower the oxygen concentration in the exhaust gas.

In this way, the exhaust air-fuel ratio is controlled to be rich to release the NOx adsorbed on the NOx catalyst 3,
After shifting to the operation mode in which the catalyst 3 is reactivated, the air-fuel ratio control means 5a next activates the reducing gas amount detection means 5b and outputs NOx from the output of the combustible substance sensor 4.
The amount of reducing gas on the downstream side of the catalyst 3 is detected. By comparing the detected amount of reducing gas with a predetermined value, NO
It is determined whether there is a reducing gas in the exhaust gas on the downstream side of the x catalyst 3 [Step S3]. Specifically, by detecting the amounts of HC and CO in the exhaust gas, it is determined whether the NOx catalyst 3 has been restored as described above.

Then, the air-fuel ratio control means 5a executes step S
When the reducing gas is detected from the output of the combustible substance sensor 4 in the process of 3, the determination is made that the reduction of the NOx catalyst 3 is completed, and the exhaust air-fuel ratio enrichment control described above is stopped at the detection timing [ Step S4]. That is, the temporarily executed air-fuel ratio enrichment is terminated, and the operation mode is returned to the original operation mode.

When the concentration of the reducing gas from the output of the combustible substance sensor 4 is equal to or lower than a predetermined value, the reducing gas introduced into the NOx catalyst 3 due to the enrichment of the exhaust air-fuel ratio is used to restore the NOx catalyst 3. It is determined that it is provided. Therefore, in this case, the exhaust air-fuel ratio enrichment control is continued as it is, and the processing from step S3 described above is repeatedly executed. However, at this time, the reduction processing time of the NOx catalyst 3 due to the enrichment of the exhaust air-fuel ratio, that is, the continuation time of the enrichment is determined [Step S5]. When the continuation time of the enrichment control reaches a predetermined time t, this is detected as the end point of the enrichment, and the above-described enrichment control of the exhaust air-fuel ratio is stopped [Step S4].

As described above, in the present apparatus, the NOx catalyst 3
As described above, the air-fuel ratio control means 5a
When the exhaust air-fuel ratio enrichment control is executed under the condition, the reducing gas component in the exhaust gas on the downstream side of the NOx catalyst 3 is detected from the output of the combustible substance sensor 4. By the way, when the engine body 1 is enriched, the oxygen concentration in the exhaust gas is reduced accordingly, and at the same time, H ₂ , HC, CO
And other reducing gas components are discharged. Then, these reducing gas components are oxidized by NOx taken into the NOx catalyst 3 and discharged downstream of the NOx catalyst 3. But N
When the release of NOx from the Ox catalyst 3 decreases, and the NOx in the NOx catalyst 3 decreases accordingly, the reducing gas becomes difficult to consume, and the reducing gas component flows out of the NOx catalyst 3 as it is downstream.

The combustible material sensor 4 detects such outflow of the reducing gas downstream of the NOx sensor, and when the reducing gas component is detected from the output of the combustible material sensor 4 by a predetermined amount or more, the amount of the reducing gas is detected. The detecting means 5b detects this as a state in which the reducing gas is no longer consumed in the NOx catalyst 3, that is, as a complete recovery of the NOx catalyst 3. The air-fuel ratio control means 5a terminates the above-described enrichment control at the detection timing of the reducing gas.

Therefore, according to the above-described control, the period of the rich operation of the engine body 1 can be minimized. That is, the completion of the reduction of the NOx catalyst 3 is promptly detected from the output of the combustible substance sensor 4, and the enrichment control for reducing the NOx catalyst 3 can be stopped at the detection timing. As a result, the enrichment control does not end in a state in which the NOx catalyst 3 cannot be completely restored, and the problem of insufficient restoration (regeneration) of the NOx catalyst 3 can be prevented. In addition, by continuing the rich operation despite the NOx catalyst 3 being restored, problems such as deterioration of fuel consumption and deterioration of exhaust gas characteristics due to emission of a large amount of HC and CO during the redundant period can be prevented. Can be prevented.

In particular, by using a simple method of detecting the reducing gas component in the exhaust gas by the combustible substance sensor 4 provided on the downstream side of the NOx catalyst 3, the period of the rich spike for restoring the NOx catalyst 3, especially Since the end point is reliably controlled in accordance with the degree of restoration of the NOx catalyst 3, the NOx catalyst 3 can be simply and effectively restored sufficiently without deteriorating the exhaust gas characteristics. . In addition, even when the enrichment conditions such as the enriched air-fuel ratio and the catalyst temperature are different, the enrichment control can be appropriately stopped in accordance with the reactivated state of the NOx catalyst 3.

The present invention is not limited to the above embodiment. For example, when a three-way catalyst is provided downstream of the NOx catalyst 3, the combustible sensor 4 may be provided downstream of the three-way catalyst. NOx catalyst 3
It is also possible to easily control the timing of the enrichment operation in order to restore the vehicle by determining the traveling distance by the lean operation. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0024]

As described above, according to the present invention, N
From the output of the combustible sensor provided downstream of the Ox catalyst, NOx
Since the reducing gas component at the time of catalyst recovery is detected, the degree of recovery of the NOx catalyst is determined based on the detected amount of reducing gas, and the exhaust air-fuel ratio enrichment control is stopped.
The period of enrichment required for the restoration of the Ox catalyst can be minimized. As a result, without deteriorating exhaust gas characteristics and without deteriorating fuel economy,
Practically great effects are obtained, such as the Ox catalyst can be surely restored (regenerated).

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a processing procedure of air-fuel ratio control according to the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 3 NOx catalyst 4 Combustible substance sensor 5 Operation control part 5a Air-fuel ratio control means 5b Reduction gas amount detection means

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 ZAB B01D 53/36 101A

Claims (1)

[Claims] 1. An exhaust passage provided in an internal combustion engine for adsorbing NOx in exhaust gas when an exhaust air-fuel ratio is lean,
A NOx catalyst that releases NOx that has already been adsorbed when the oxygen concentration in the exhaust gas is reduced; a combustible substance sensor provided downstream of the NOx catalyst in the exhaust passage; When NOx is adsorbed,
An exhaust gas purifying apparatus for an internal combustion engine, comprising: air-fuel ratio control means for controlling the exhaust air-fuel ratio to be rich and terminating the rich control based on the output of the combustible substance sensor.