Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-09-14
2001-03-13
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S285000, C060S276000
Reexamination Certificate
active
06199373
ABSTRACT:
TECHNICAL FIELD
This invention relates to exhaust after-treatment and, more particularly, to a method and apparatus for removing contaminants accumulated in a NO
x
trap.
BACKGROUND ART
Lean burn engines usually operate at an air/fuel ratio (A/F) ≧ 18 to obtain improved fuel economy. However, the usual three-way catalyst (TWC) is most efficient at approximately stoichiometry, (A/F=14.65). Accordingly, it has been proposed to locate a NO
x
trap downstream of the TWC to store NO
x
during lean A/F operation and subsequently converting the NO
x
to N
2
and O
2
by operating the engine at a rich A/F. This NO
x
conversion occurs within an optimum temperature window of approximately 300° C. to 400° C. The trap is preferably located underbody so that during hard, wide-open throttle (WOT) driving, the trap temperature does not exceed 800° C. The temperature of the three-way catalyst should not exceed approximately 1000° C.
When operating with fuel containing contaminants such as sulfur, an accumulation of the contaminants in the trap causes a decrease in the amount of NO
x
the trap can absorb. The contaminants must be “burned” off or desorbed at temperatures ≧ 675° C. and with an A/F ≦ 14.65 (rich). A previous method of raising the temperature of the trap from its usual operating temperature of approximately 300° to 400° C. to a temperature of at least 675° C., employs secondary air injection. This approach to SO
x
purging or desulfation of contaminants is expensive. Another approach to the problem employs a split exhaust manifold, separated TWC, or separated exhaust pipes, as proposed in copending application Ser. No. 764,185 filed Dec. 13, 1996 and assigned to the assignee of the present invention.
SUMMARY OF THE INVENTION
In accordance with the present invention, contaminant purging is accomplished by modulating the amplitude of the A/F at a properly chosen frequency to create a substantial temperature increase in a lean NO
x
trap, positioned downstream of a TWC. The A/F ratio modulation period and amplitude are such that O
2
, HC and CO breakthrough occurs in the TWC (i.e., incomplete catalytic conversion), and relatively complete catalytic reaction of O
2
, HC, and CO occurs in the trap, generating the required exotherm.
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Asik Joseph R.
Hepburn Jeffrey S.
Meyer Garth M.
Denion Thomas
Ford Global Technologies Inc.
Lippa Allan J.
Tran Binh
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