Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1999-05-20
2001-04-24
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...
C060S297000, C060S301000
Reexamination Certificate
active
06220018
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an exhaust emission control device for an engine.
BACKGROUND OF THE INVENTION
A technique is disclosed in Tokkai Hei 7-144119 published by the Japanese Patent Office in 1995 for the reduction of hydrocarbon (hereafter HC) emissions immediately after cold-starting an engine at which time a three-way catalyst disposed upstream has not reached an activation temperature. According to the technique, a composite catalyst, which is comprised of a three-way catalyst and an HC absorbent material and which allows for the absorption and release of HC depending on the temperature of the catalyst, is provided downstream of a three-way catalyst.
A composite catalyst enables the temporary absorption of HC exhausted immediately after cold-starting an engine. Thereafter when the upstream three-way catalyst reaches an activation temperature, the air-fuel ratio is varied to a lean air-fuel ratio resulting in a super-oxygenated air mixture which raises the catalyst temperature of the composite catalyst. As a result, absorbed HC is released and oxidized on the composite catalyst.
SUMMARY OF THE INVENTION
Metallic catalysts such as Pt, Ph, Pd and the like which are suitable for use in three-way catalysts have the function of binding oxygen. In particular, large amounts of oxygen can be bound in the catalyst by the use of an auxiliary catalyst with a large storage capacity such as cerium oxide in order to increase the oxygen binding capacity. The conversion efficiency of the catalyst can be increased by the operation of bound oxygen. The amount of bound oxygen is increased in the super-oxygenated atmosphere of a lean air-fuel ratio. Since oxygen is released from the catalyst in the oxygen-poor atmosphere of a rich air-fuel ratio, the amount of bound oxygen is reduced.
However, the composite catalyst discussed above may not function effectively as a result of the oxygen storage function of the three-way catalyst. This is a result of oxygen being released from the three-way upstream catalyst in an atmosphere of the rich air-fuel ratio immediately after cold-starting the engine. Therefore, oxygen is absorbed by the upstream catalyst even if the air-fuel ratio is subsequently switched to lean. So it is not possible to supply sufficient oxygen to the composite catalyst for a period of time. As a result, released HC released with rises in the catalyst temperature cannot be sufficiently oxidized and HC emissions rise.
It is therefore an object of this invention to improve the purification of HC immediately after cold-starting an engine by the provision of an exhaust emission control device provided with a catalyst which has an HC absorbing/releasing function and is placed downstream of a three-way catalyst.
In order to achieve the above object, this invention provides an exhaust emission control device for an engine provided with a fuel injection valve and an exhaust pipe. The exhaust emission control device comprises a three-way catalyst provided with a fixed oxygen storage capacity and which is disposed in the exhaust pipe, a composite catalyst which is disposed downstream of the three-way catalyst, which has a larger oxygen storage capacity than the three-way catalyst, which absorbs hydrocarbons (HC) below a fixed temperature and which releases the absorbed HC above the fixed temperature. A microcomputer is provided which is programmed to control the fuel injection valve so that an air-fuel ratio of the engine reaches a lean air-fuel ratio which is greater than a stoichiometric air-fuel ratio when HC absorbed by the composite catalyst is released and oxidized.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
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Nishizawa Kimiyoshi
Sato Ritsuo
Tsuzuki Mikio
Yamanashi Fuminori
Denion Thomas
Foley & Lardner
Nissan Motor Co,. Ltd.
Tran Diem
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