Exhaust gas purification system for low emission vehicle

Details Exhaust gas purification system for low emission vehicle Exhaust gas purification system for low emission vehicle

1999-06-04

2001-01-02

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

Methods

C060S285000, C060S295000, C060S300000, C060S301000, C060S311000

Utility Patent

active

06167696

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to exhaust systems for vehicles and, more particularly, to an exhaust after-treatment system for a low emission, lean-burn vehicle.
BACKGROUND OF THE INVENTION
Three-way catalysts are generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. These three-way catalysts remove CO, HC, and NO
x
simultaneously from engine exhaust gases when the air-fuel ratio of the exhaust gas is at a stoichiometric air-fuel ratio. The ability of a three-way catalyst to remove NO
x
in the exhaust gas falls rapidly, however, when the air-fuel ratio of the exhaust gas becomes leaner. Therefore, in engines operating in a lean air-fuel ratio environment, such as stratified charge lean-burn engine, it is difficult to remove NO
x
from the exhaust gas using a conventional three-way catalyst.
Accordingly, to achieve NO
x
control, exhaust after-treatment systems have included a NO
x
trap. Presently, however, the performance of NO
x
trap technology is limited in several respects. NO
x
trap performance is affected by the operating temperature, and requires a relatively narrow temperature operating window. In addition, sulfur poisoning degrades the performance efficiency of NO
x
traps, and typical gasoline can contain high sulfur levels.
Governmental regulations of emissions are also becoming more stringent. Because the efficiency of three-way catalysts and NO
x
traps are temperature dependent, a significant portion of harmful emissions are generated during the cold start portion of the engine cycle, i.e., the first 100-200 seconds after cold start. Thus, there exists a need for an exhaust after-treatment system which provides an effective means of reducing cold start, HC, and NO
x
emissions.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an improved exhaust gas purification system.
The foregoing and other objects and advantages are achieved through the provision of an exhaust gas purification system for a lean burn internal combustion engine including a three-way catalyst, a hydrocarbon trap downstream of the three-way catalyst, a NO
x
trap downstream of the hydrocarbon trap, and an electrically heated catalyst located between the hydrocarbon trap and NO
x
trap. In one embodiment, the NO
x
trap includes an integral particulate filter having alternating blocked channels extending longitudinally. The inlet channels filter particulate matter and the outlet channels have a NO
x
absorbent wash coat for absorbing exhaust gas NO
x
.
A method of controlling the exhaust system is also provided. The disclosed method of purifying exhaust gas further includes a procedure for activating the electrically heated catalyst for NO
x
trap desulfation and particulate matter removal.
One advantage of the present invention is that it reduces the problems associated with NO
x
trap sulfur poisoning. Another advantage is that it reduces the limitations caused by the temperature operating window of the NO
x
trap. A further feature of the invention is improved reductions in tail pipe, HC, and NO
x
generated during engine cold start.
Other features and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.


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