Exhaust emission control device of internal combustion engine

Details Exhaust emission control device of internal combustion engine Exhaust emission control device of internal combustion engine

2000-12-06

2002-12-03

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S276000, C060S301000

Reexamination Certificate

active

06487851

ABSTRACT:

TECHNICAL FIELD
This invention relates to an exhaust emission control device of an internal combustion engine, which has an occlusion type NO
x
catalyst in an exhaust passage.
BACKGROUND ART
In recent years, a lean burn internal combustion engine designed to be operated at a lean air-fuel ratio for improved fuel economy has come into practical use. When this lean burn internal combustion engine is operated at a lean air-fuel ratio, a three-way catalyst cannot fully purify NO
x
(nitrogen oxides) in an exhaust gas because of the purification characteristics of the catalyst. Recently, for example, use has been made of an exhaust emission purification catalyst device with an occlusion type NO
x
catalyst which occludes or adsorbs (hereinafter referred to simply as occludes) NO
x
in an exhaust gas during operation at a lean air-fuel ratio, and which releases and reduces the occluded NO
x
during operation at a stoichiometric air-fuel ratio or a rich air-fuel ratio.
The occlusion type NO
x
catalyst has the properties of attracting and occluding NO
x
in an exhaust gas as nitrates (X—NO
3
) in the presence of excess oxygen in the internal combustion engine, and releasing the occluded NO
x
mainly in the presence of excess carbon monoxide (CO) to reduce it to nitrogen (N
2
) (and simultaneously form carbonates (X—CO
3
)). In actuality, therefore, when the operation at a lean air-fuel ratio continues for a predetermined period of time, the air-fuel ratio in a combustion chamber is changed. Alternatively, the operation at the lean air-fuel ratio is periodically switched to operation at a rich air-fuel ratio by such means as supply of a reducing agent to an exhaust pipe to control the air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio (this method is called rich spike). As a result, a CO-abundant reducing atmosphere is produced in an oxygen concentration-poor atmosphere to release the occluded NO
x
for purifying reduction (NO
x
purge). In this manner, the occlusion type NO
x
catalyst can be regenerated. This technology is disclosed, for example, in Japanese Patent No. 2,586,738.
Such an occlusion type NO
x
catalyst is limited in the amount of NO
x
that can be occluded onto the catalyst. When the amount of NO
x
, which the occlusion type NO
x
catalyst has occluded, reaches the limit, rich spike is performed as stated earlier, so that a rich air-fuel ratio operation is carried out at a predetermined rich air-fuel ratio for a predetermined period of time.
The time point, at which the occluded amount of NO
x
in the occlusion type NO
x
catalyst reaches the limit and rich spike is required, changes according to the degree of deterioration in the NO
x
purification efficiency of the occlusion type NO
x
catalyst, or according to the operating conditions for the internal combustion engine that affect the flow rates of NO
x
and CO. A technology for detecting that the amount of NO
x
occluded by such an occlusion type NO
x
catalyst has reached the limit, and regenerating this catalyst is disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 7-166851.
According to an “exhaust emission control device” as disclosed in this publication, an NO
x
absorber is disposed in an exhaust passage of an internal combustion engine, and an NO
x
sensor is disposed downstream from the NO
x
absorber. With this arrangement, the exhaust emission control device performs regenerating control (NO
x
purge) in the following manner: When a detection value detected by the NO
x
sensor (i.e., the concentration of the NO
x
component) has exceeded a judgment value, the exhaust air-fuel ratio is switched to a rich air-fuel ratio to release NO
x
from an upper region of the catalyst.
The regulatory value according to the NO
x
emission regulations in different countries of the world is, for example, total NO
x
discharge over a predetermined distance traveled by a vehicle. The aforementioned conventional “exhaust emission control device” executes regenerating control by detecting the concentration of the NO
x
component during each lean operation zone which is a certain short period. Thus, if the set judgment value allows only a narrow margin because of a driver's way of driving, it is likely that the desired NO
x
discharge, e.g., the regulatory value, for each predetermined distance traveled cannot necessarily be cleared reliably.
That is, in the above-mentioned “exhaust emission control device,” whether the total NO
x
discharge over the predetermined distance traveled has not exceeded a desired value, e.g., the regulatory value, is not grasped during driving. To keep the total NO
x
discharge over the predetermined distance traveled at a predetermined value or less under various driving conditions, therefore, there is need to set the aforementioned judgment value for initiation of regenerating control (NO
x
purge) at a low value with a sufficient margin. By setting the judgment value in this manner, the frequency of regenerating control (NO
x
purge) for making the air-fuel ratio a rich or stoichiometric value increases because of an ample margin, thereby increasing fuel consumption. That is, the problem of increasing CO
2
discharge arises.
A technology for controlling NO
x
discharge over a predetermined distance traveled to be within a predetermined value is disclosed, for example, in Japanese Patent No. 2,503,387. An “electronic internal combustion engine control device” as disclosed in this publication controls the ignition timing and the EGR amount to control NO
x
discharge, only in a stoichiometric operation region. If this control device is applied to a lean burn internal combustion engine operating at a lean air-fuel ratio as described earlier, the air-fuel ratio has to be constantly set in the stoichiometric operation region, and fuel economy cannot be improved.
The present invention has been accomplished to solve the above problems. Its object is to provide an exhaust emission control device of an internal combustion engine, which can directly manage NO
x
discharge, released into the atmosphere, under any driving conditions to keep the NO
x
discharge at a desired value reliably and achieve a decrease in NO
x
discharge and a decrease in CO
2
discharge at the same time, without deteriorating fuel economy.
DISCLOSURE OF THE INVENTION
An exhaust emission control device of an internal combustion engine according to the present invention includes an exhaust emission purification catalyst device provided in an exhaust passage of the internal combustion engine, and having a NO
x
decreasing function of purifying or occluding NO
x
in an exhaust gas when an exhaust air-fuel ratio is a lean air-fuel ratio, and the reducing function of reducing hazardous substances in the exhaust gas when the exhaust air-fuel ratio is a stoichiometric air-fuel ratio or a rich air-fuel ratio; NO
x
detection means for detecting or estimating the concentration of NO
x
released into the atmosphere; and control means for computing an NO
x
discharge released into the atmosphere based on an output from the NO
x
detection means, and stopping or suppressing an operation at a lean air-fuel ratio when detecting or expecting that a total NO
x
discharge exceeds a predetermined value before a vehicle reaches a predetermined driving period.
Thus, when the exhaust air-fuel ratio is a lean air-fuel ratio, NO
x
in the exhaust gas is purified or occluded. A total NO
x
discharge is computed from the concentration of NO
x
released into the atmosphere, with a predetermined driving period as a cycle. If it is detected or expected that the total NO
x
discharge exceeds the predetermined value before the predetermined driving period is reached, operation at the lean air-fuel ratio is stopped or suppressed, and the reducing function of the exhaust emission purification catalyst device is performed. Under any driving conditions, therefore, poor fuel economy is not induced. Under any driving conditions, moreover, the NO
x
discharge can be restricted to the desired value. Thus, a decrease

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