Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-01-14
2001-02-13
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S277000, C060S276000, C060S285000
Reexamination Certificate
active
06185929
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to improvements in an exhaust emission control system for an internal combustion engine, particularly for an automotive internal combustion engine which is operated mainly on air-fuel mixtures having air-fuel ratios leaner than a stoichiometric level.
Hitherto, an exhaust emission control system including a NOx (nitrogen oxides) absorbing agent has been proposed to be installed to an internal combustion engine. The NOx absorbing agent is disposed in an exhaust gas passage of the engine. The NOx absorbing agent absorbs NOx when the air-fuel ratio of exhaust gas is leaner than a stoichiometric level, and releases the absorbed NOx when the air-fuel ratio becomes richer than the stoichiometric level. When judgment is made such that the amount of NOx absorbed in the NOx absorbing agent has reached a limit level, the air-fuel ratio can be controlled at a level richer than the stoichiometric level, thus accomplishing an enrichment treatment for the air-fuel ratio of exhaust gas to be introduced into the NOx absorbing agent. This causes the NOx absorbing agent to release the absorbed NOx. Such an exhaust emission control system is disclosed in Japanese Patent Provisional Publication No. 6-66185.
BRIEF SUMMARY OF THE INVENTION
Now, there has been a proposition that the NOx absorbing agent is provided with the function of a three-way catalyst. This is referred hereinafter as a “NOx storage type three-way catalyst” which is arranged not only to release NOx in a rich operation but also purify or reduce the thus released NOx with HC (hydrocarbons) and CO (carbon monoxide) which serve as a reducing agent and much emitted from the engine in the rich operation. In the rich operation, the engine is operated on an air-fuel mixture having an air-fuel ratio richer than the stoichiometric level.
It is to be noted that the NOx storage type three-way catalyst is apt to be subjected to a thermal deterioration and therefore is proposed to be disposed in the exhaust gas passage downstream of a three-way catalyst which is referred hereinafter to as simply the “three-way catalyst”. In this case, the three-way catalyst has an ability of absorbing oxygen, and therefore a part of HC and CO to be supplied to the NOx storage type catalyst is unavoidably oxidized or consumed with oxygen absorbed in the three-way catalyst. As a result, the amount of HC and CO to be supplied to the NOx storage type catalyst becomes deficient, so that NOx cannot be effectively reduced.
In order to cope with the above, a basic value of enrichment degree relative to the stoichiometric level in the rich operation is determined corresponding to an amount of HC and CO to be required for releasing and reducing NOx absorbed or stored in the NOx storage type three-way catalyst. Additionally, the enrichment degree basic value is increased to a side where the enrichment degree increases (i.e., a correction amount is added to the enrichment basic value), upon taking account of the fact that a certain amount of HC and CO is oxidized with oxygen absorbed in the three-way catalyst.
However, since the oxygen absorbing ability and the oxidizing ability of the three-way catalyst change owing to deterioration with age and different temperature conditions, the amount of HC and CO to be supplied to the NOx storage three-way catalyst becomes excessive or deficient in case that the abovementioned correction amount determined upon taking account of the oxygen absorbing ability of the three-way catalyst is constant. For example, assume that the correction amount is matched to be optimum for the three-way catalyst in a new condition. In this case, when the three-way catalyst has been deteriorated so that its oxygen absorbing ability lowers, the amount of HC and CO to be supplied to the NOx storage type three-way catalyst becomes excessive by an amount corresponding to a decreased amount of HC and CO oxidized by the three-way catalyst.
Therefore, an object of the present invention is to provide an improved exhaust emission control system for an internal combustion engine, including a three-way catalyst (having an oxygen absorbing ability) and a NOx storage type catalyst, by which the NOx storage type catalyst can be caused to be effectively function so as to reduce NOx at a high efficiency.
Another object of the present invention is to provide an improved exhaust emission control system for an internal combustion engine, including a three-way catalyst (having an oxygen absorbing ability) and a NOx storage type three-way catalyst, by which the amount of HC and CO can be effectively supplied to the NOx storage type three-way catalyst without excess and deficiency even in case that the three-way catalyst having the oxygen absorbing ability is deteriorated with age and/or is subjected to different temperature conditions.
A further object of the present invention is to provide an improved exhaust emission control system for an internal combustion engine, including a three-way catalyst (having an oxygen absorbing ability) and a NOx storage type three-way catalyst, by which an enrichment degree in an enrichment treatment for air-fuel ratio of air-fuel mixture to be supplied to the engine is altered in accordance with a changing oxygen absorbing ability of the three-way catalyst.
An aspect of the present invention resides in an exhaust emission control system for an internal combustion engines The exhaust emission control system comprises a first catalyst disposed in an exhaust gas passage of the engine The first catalyst functions to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level and reduce released NOx in presence of HC and CO. A second catalyst is disposed in the exhaust gas passage upstream of the first catalyst and having an oxygen trapping ability. A control unit is provided including a first section for accomplishing an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after the enriching the air-fuel ratio. A second section of the control unit is for estimating the oxygen trapping ability of the second catalyst. A third section of the control unit is for correcting the enrichment degree to decrease in the stepwise enriching as the estimated oxygen trapping ability decreases.
Another aspect of the present invention resides in an exhaust emission control system for an internal combustion engine. The exhaust emission control system comprises a first catalyst disposed in an exhaust gas passage of the engine. The first catalyst functions to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level and reduce released NOx in presence of HC and CO. A second catalyst is disposed in the exhaust gas passage upstream of the first catalyst and having an oxygen trapping ability. A control unit is provided including a first section for accomplishing an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after the enriching the air-fuel ratio. A second section of the control unit is for increasing the enrichment degree in the stepwise enriching by an amount corresponding to a quantity of HC and CO oxidized with oxygen trapped in the second catalyst, the quantity of HC and CO forming part of HC and CO to be supplied to the first catalyst A third section of the control unit is for restricting the enrichment de
Ishizuka Yasuji
Nishizawa Kimiyoshi
Denion Thomas
Foley & Lardner
Nissan Motor Co,. Ltd.
Trieu Thai-Ba
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