Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-07-20
2002-06-04
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S286000, C060S295000, C060S297000, C060S301000, C060S303000, C422S171000, C422S177000
Reexamination Certificate
active
06397582
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the priority of Japanese patent documents No. 8-146981, filed Jun. 10, 1996; No. 8-153718, filed Jun. 14, 1996; and No. 8-209587, filed Aug. 8, 1996, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a purification apparatus for an exhaust gas which is discharged or emitted from an internal combustion engine such as an automobile, and particularly to an apparatus which includes a catalyst for purifying an exhaust gas from an internal combustion engine that is operated under a lean air-fuel ratio (a lean burn), and from an automobile which has such a lean burn internal combustion engine.
Exhaust gas discharged from an internal combustion engine such an automobile includes carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NO
x
) etc. which pollute the environment, adversely affect the human body, and disturb the growth and the development of plants.
Accordingly, up to now a great deal of effort has gone into reducing the amount of such pollutants by improving a combustion in the internal combustion engine, and developing a method for purifying the discharged or the emitted exhaust gas using a catalyst to obtain a steady result.
Gasoline engine vehicles frequently utilize a three component catalyst in which platinum (Pt) and rhodium (Rh) are main active components. The oxidation of HC and CO and the reduction of NO
x
are carried out at the same time to convert the above air pollution materials to harmless materials.
It is characteristic of a three component catalyst, that it operates effectively only for exhaust gases which are generated within a range (“window”) in the vicinity of a stoichiometric air-fuel ratio.
In the conventional technique, the air-fuel ratio fluctuates in accordance with an operation condition of the automobile. A fluctuation region is principally controlled to the vicinity of the stoichiometric air-fuel ratio, which is a ratio between A (weight of air) and F (weight of fuel), being about 14.7 in case of the gasoline. Hereinafter, in the present specification, the stoichiometric air-fuel ratio is represented by A/F=14.7, but this value varies in accordance with kinds of the fuels.
However, when the engine is operated under a lean air-fuel ratio in comparison with the stoichiometric air-fuel ratio atmosphere, the fuel consumption can be improved. Therefore, the development of a lean burn combustion technique is promoted; and recently automobiles have been developed in which the engine is combusted under the lean area having the air-fuel ratio of more than 18.
However, when a conventional three component catalyst is adopted for purification of a lean burn exhaust gas, although the oxidation purification with respect to HC and CO is performed effectively, the reduction of NO
x
is not.
Accordingly, to promote the application of the lean burn system for large size vehicles and to enlarge the lean burn combustion time (that is, enlarge the operation area of the lean burn system), it is necessary to develop an exhaust gas purification technique which is suitable to the lean burn system. Thus, the development of a technique for purifying HC, CO and particularly NO
x
where a large quantity of oxygen (O
2
) is included in the exhaust gas, has been promoted vigorously.
Japanese patent laid-open publication No. 61,706/1988 discloses a technique in which HC is supplied upstream of a lean burn exhaust gas. The operation of a catalyst is facilitated by lowering the oxygen (O
2
) concentration in the exhaust gas to a concentration area for effective functioning of the catalyst.
Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, propose a technique in which N included in the exhaust gas (after the conversion of an easily absorbable N
2
, by oxidizing NO) is absorbed and removed by contact with a catalyst having NO
x
absorbing ability. When the absorption efficiency decreases, by stopping a passing-through of the exhaust gas accumulated NO
x
is reduction-removed using H
2
, HC included in a methane gas and a gasoline etc., and so that NO
x
absorbing ability of the catalyst is regenerated.
Further, WO 93/07363 and WO 93/08383 discloses an exhaust gas purification apparatus in which an NO
x
absorbent material arranged at an exhaust gas flow passage absorbs NO
x
from a lean exhaust gas, and when an oxygen concentration in the exhaust gas is lowered the NO
x
absorbent material discharges the absorbed NO
x
. The exhaust gas absorbs NO
x
during the lean atmosphere and the absorbed NO
x
is discharged by lowering O
2
, concentration in the exhaust gas which flows into a NO
x
absorbent.
However, in Japanese patent laid-open publication No. 61,708/1988, to attain a composition of the exhaust gas which corresponds to the air-fuel ratio of A/F=14.7 where the catalyst can function (O
2
concentration having about 0.5%), it needs a very large quantity of HC. A use of a blow-by gas in this patent document is effective, but the blow-by gas does not have an amount which is sufficient for efficient to treat an exhaust gas during an operation of an internal combustion engine. It is possible technically to throw the fuel but it eliminates the fuel consumption gains achieved by the lean burn system.
In Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, to regenerate a NO
x
absorbent material a flow of exhaust gas is stopped and the gaseous reducing agent of HC etc. is contacted to NO
x
absorbent. Further, two NO
x
absorbent materials are provided and the exhaust gas flows alternately to these two NO
x
absorbent materials. It is therefore necessary to provide an exhaust gas change-over mechanism, which complicates the structure of the exhaust gas treatment apparatus.
In WO 93/07363 and WO 93/08383, the exhaust gas is flowed continuously to an NO
x
absorbent material, and the NO
x
in the exhaust gas is absorbed during the lean atmosphere. By lowering O
2
concentration in the exhaust gas, the absorbed NO
x
is discharged and the NO
x
absorbent material is regenerated. Accordingly, since the change-over of the exhaust gas flow is unnecessary, the problem in the above stated system can dissolved. However, these systems require a material which can absorb NO
x
during the lean condition and can discharge NO
x
when O
2
concentration in the exhaust gas is lowered. Since the repeated NO
x
absorption and discharge inevitably causes a periodic change of a crystal structure of the absorbent, it is necessary to take a careful consideration about the durability of the absorbent. Further, it is necessary to treat the discharged NO
x
; in the case of a large quantity of the discharged NO
x
it may be necessary to provide a post-treatment using a three component catalyst.
SUMMARY OF THE INVENTION
In the light of the problems in the above stated prior arts, the object of the present invention is to provide an internal combustion engine exhaust gas purification apparatus which has a simple structure, consumes a small amount of gaseous reducing agents, has superior endurance and which effectively removes harmful components such as NO
x
from a lean burn exhaust gas converting them to a harmless component.
Another object of the present invention is to provide a catalyst for use in an exhaust gas purification apparatus of an internal combustion engine.
According to the present invention, an exhaust gas purification apparatus for use in an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NO
x
gas passes, and a catalyst disposed in the exhaust gas duct so that it contacts the exhaust gas. The catalyst chemically adsorbs NO
x
when a stoichiometric amount of a gaseous oxidizing agent present in the exhaust gas is larger than the amount of a gaseous reducing agen
Azuhata Shigeru
Doi Ryouta
Hanaoka Hiroshi
Hiratsuka Toshifumi
Iizuka Hidehiro
Crowell & Moring LLP
Denion Thomas
Hitachi , Ltd.
Tran Binh
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