Multiple stage aftertreatment system

Details Multiple stage aftertreatment system Multiple stage aftertreatment system

1999-06-23

2001-09-25

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...

C060S295000, C060S297000, C060S301000, C060S303000

Reexamination Certificate

active

06293096

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to an aftertreatment system for reducing the amount of NO
x
and particulate matter in engine exhaust gases, and more particularly to such a system that oxidizes NO to NO
2
in a first stage, reduces a portion of the NO
2
to N
2
in a second stage, and further reduces a remaining portion of the NO
2
to N
2
and oxidizes carbon to CO
2
in a third stage.
2. History of Related Art
Heretofore, the diesel engine industry has been successful in meeting exceedingly tight emission limits without heavy reliance on expensive exhaust aftertreatment technology. Recently, oxidation catalysts have been used in medium-heavy-duty diesel engine applications to oxidize the soluble organic fraction (SOF) in particulate. Also, even though great strides have been made in controlling the soot emitted from the diesel combustion process, carbonaceous matter in the exhaust stream, generally referred to as particulate matter (PM), continues to make it difficult to meet increasingly stringent proposed particulate limits. Control of soot emissions has been compounded by employing exhaust gas recirculation (EGR) to achieve proposed emission standards.
Using significant EGR rates to achieve very low NO
x
emission is forcing some manufacturers to reconsider the use of particulate traps or filters, for their potential in reducing insoluble particulate matter. Oxidation catalysts and traps, with certain limitations on fuel sulfur content, have been combined in a two-stage exhaust aftertreatment system. For example, U.S. Pat. No. 4,902,487 issued Feb. 20, 1990 to Barry J. Cooper, et al. for Treatment of Diesel Exhaust Gases describes an aftertreatment process in which carbonaceous matter is trapped in a filter and oxidized to carbon dioxide. The first stage of the treatment system oxidizes nitrogen oxide (NO) to NO
2
by means of a platinum catalyst. Nitrogen dioxide formed in this stage is then used to oxidize carbon that is trapped in the second stage filter. In this two-stage process, the oxidation of carbon is dependent upon the amount of nitrogen dioxide present in the exhaust stream. If an insufficient amount of NO
2
is present, complete carbon oxidation does not occur.
U.S. Pat. No. 5,746,989 issued May 5, 1998 to Mikio Murachi, et al. for a Method for Purifying Exhaust Gas of a Diesel Engine, describes a method by which NO (nitrogen oxide) in the exhaust gas stream of a diesel engine is oxidized to NO
2
(nitrogen dioxide) by an oxidizing catalyst. Carbon particles in the exhaust gas are trapped by a particulate filter. The exhaust gas containing NO
2
is fed to the particulate filter whereat the NO
2
reacts with carbon particles trapped in the particulate filter and the carbon particles are oxidized. NO
2
is reduced to nitrogen (N
2
), H
2
O (water in a gaseous state) and carbon dioxide (CO
2
) by enriched fuel mixture combustion in the engine. The oxidizing catalyst uses an electric heater to raise the temperature of the exhaust gas stream to ensure a chemical reaction between the NO
2
with the carbon particles in the particulate filter. Also in the Murachi et al. method, it is necessary to periodically purge the NO
x
absorbent. This is accomplished by changing the air-fuel ratio of the engine by injecting fuel into each cylinder of the engine twice during each complete cycle, thus creating a relatively low temperature exhaust gas having a rich air-fuel mixture. This practice generally disadvantageously affects the operating characteristics of the engine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an aftertreatment system for reducing the amount of nitrogen oxide and particulate matter in engine exhaust gases without the need for electric heating elements to increase the temperature of the exhaust gas to periodically regenerate the particulate filter. It is also desirable to have such an aftertreatment system that does not require the injection of additional fuel in each cylinder of the engine to provide additional necessary hydrocarbon to reduce the NO
x
to N
2
(nitrogen). It is desirable to also have an internal bypass for the NO
2
trap to control the rate of NO
2
directed to the carbon trap and, accordingly, the amount of NO
2
bypassed to the carbon particulate filter.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a multiple stage aftertreatment system for reducing the amount of NO
x
and carbonaceous particulate matter in engine exhaust gases includes a NO
x
oxidation catalyst, a lean NO
x
trap, a carbon trap, and a hydrocarbon fuel injector. The NO
x
oxidation catalyst is positioned to receive exhaust gases discharged from the internal combustion engine and oxidize at least a portion of the NO
x
compounds in the exhaust gas to nitrogen dioxide. The lean NO
x
trap is adapted to store nitrogen dioxide in the exhaust gas discharged from the catalyst and reduce the nitrogen dioxide to nitrogen and water (in a gaseous state) in the presence of at least a portion of the carbonaceous particulate matter in the engine exhaust gas in combination with supplemental hydrocarbon fuel. As a result of this reaction, at least portions of the carbonaceous particulate matter and the hydrocarbon fuel are oxidized to carbon dioxide. The carbon trap is adapted to oxidize other portions of the carbonaceous particulate matter to carbon dioxide. The hydrocarbon fuel injector is disposed adjacent the lean NO
x
trap at a position adapted to control the selected amounts of hydrocarbon fuel into the exhaust gas prior to entry into the lean NO
x
trap.
In accordance with another aspect of the present invention, a multiple stage aftertreatment system for reducing the amount of NO
x
and particulate matter in engine exhaust gases includes first, second and third stages and a hydrocarbon fuel injector. The first stage is positioned to receive exhaust gases discharged from an internal combustion engine which typically contain NO (nitrogen oxide), NO
2
(nitrogen dioxide), HC (hydrocarbons), CO (carbon monoxide), CO
2
(carbon dioxide), and PM (particulate matter) in the exhaust. The first stage has a catalyst adapted to oxidize the NO in the exhaust stream and discharge a gaseous stream containing NO
2
and PM from the first stage. The second stage of the aftertreatment system is positioned in fluid communication with the first stage described above and is adapted to receive the gaseous stream discharged from the first stage. The second stage includes a trap portion and a bypass portion. The trap portion is adapted to store NO
2
discharged from the first stage in a typical lean (oxygen-rich) environment and, in the presence of a supplemental hydrocarbon fuel, reduce the stored NO
2
to N
2
(nitrogen), O
2
(oxygen), H
2
O (water in a gaseous state), and CO
2
(carbon dioxide) and discharge the N
2
, O
2
, H
2
O, and CO
2
and residual unreduced NO
2
from the trap portion. The hydrocarbon fuel injector is interposed between the first and second stages at a position adapted to controllably inject selected amounts of hydrocarbon fuel into the gaseous stream discharged from the first stage. The third stage of the aftertreatment system is in fluid communication with the second stage and has a carbon trap adapted to receive NO
2
from the bypass portion of the second stage and residual unreduced NO
2
from the trap portion of the second stage, and reduce the NO
2
to nitrogen, and oxidize the PM and the carbon component to carbon dioxide.
In accordance with yet another aspect of the present invention, a multiple stage aftertreatment system for reducing the amount of NO
x
and carbonaceous particulate matter in engine exhaust gases includes a first stage positioned to receive exhaust gases discharged from an internal combustion engine which has NO
x
and carbonaceous particulate matter carried therein. The first stage includes a Pt catalyst adapted to oxidize components of the NO
x
to form nitrogen dioxide and discharge a gaseous stream fro

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