Power plants – Internal combustion engine with treatment or handling of... – Having sensor or indicator of malfunction – unsafeness – or...
Reexamination Certificate
2000-08-29
2002-07-16
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Having sensor or indicator of malfunction, unsafeness, or...
C060S274000, C060S276000, C060S285000
Reexamination Certificate
active
06418711
ABSTRACT:
TECHNICAL FIELD
The invention relates to methods and apparatus for accessing the ability of a vehicle emissions control device, such as a lean NO
x
trap, to releasably store an exhaust gas constituent and, more particularly, to a method and apparatus for estimating the capacity of a lean NO
x
trap to store NO
x
.
BACKGROUND ART
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO
x
) and oxygen (O
2
). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (&lgr;), engine speed and load, engine temperature, ambient humidity, ignition timing (“spark”), and percentage exhaust gas recirculation (“EGR”). The prior art often maps values for instantaneous engine-generated or “feedgas” constituents, such as HC, CO and NO
x
, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
To limit the amount of feedgas constituents that are exhausted through the vehicle's tailpipe to the atmosphere as “emissions,” motor vehicles typically include an exhaust purification system having an upstream three-way catalyst and a downstream NO
x
absorbent or “trap.” The three-way catalyst is particularly effective at reducing tailpipe NO
x
emissions when the engine is operated using an air-fuel mixture that is at or near a stoichiometric air-fuel ratio. The trap, in turn, stores NO
x
when the exhaust gases are “lean” of stoichiometry and releases previously-stored NO
x
for reduction to harmless gases when the exhaust gases are “rich” of stoichiometry. In this manner, the trap permits intermittent lean engine operation, with a view toward maximizing overall fuel economy, while concomitantly serving to control vehicle tailpipe emissions.
More specifically, in a typical embodiment, the trap chemically stores NO
x
during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat. The NO
x
(NO and NO
2
) are stored in the trap in the form of barium nitrate, for example. The washcoat also includes precious metals, such as platinum and palladium, which operate to convert NO to NO
2
for storage in the trap as a nitrate. The trap's washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as increased trap sulfurization (sulfur accumulation) and trap aging.
The trap's actual capacity to store NO
x
is finite and, hence, in order to maintain low tailpipe NO
x
emissions when running “lean,” the trap must be periodically cleansed or “purged” of stored NO
x
. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately “break through” the three-way catalyst and enter the trap, whereupon the trap's barium nitrate decomposes into NO
2
for subsequent conversion by the trap's precious metals into harmless N
2
and O
2
. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break through the three-way catalyst.
Each purge event is characterized by a fuel “penalty” consisting generally of an amount of fuel required to release both the oxygen stored in the three-way catalyst, and the oxygen and NO
x
stored in the trap. Moreover, the trap's NO
x
-storage capacity is known to decline in a generally reversible manner over time due to sulfur poisoning or “sulfurization,” and in a generally irreversible manner over time due, for example, to component “aging” from thermal effects and “deep-diffusion”/“permanent” sulfurization. As the trap's capacity drops, the trap is “filled” more quickly, and trap purge events are scheduled with ever-increasing frequency. This, in turn, increases the overall fuel penalty associated with lean engine operation, thereby further reducing the overall fuel economy benefit of “running lean.”
In order to restore trap capacity, a trap desulfurization event is ultimately scheduled, during which additional fuel is used to heat the trap to a relatively elevated temperature, whereupon a slightly rich air-fuel mixture is provided for a relatively extended period of time to release much of the stored sulfur and rejuvenate the trap. As with each purge event, each desulfurization event typically includes the further “fuel penalty” associated with the initial release of oxygen previously stored in the three-way catalyst and the trap. The prior art teaches scheduling a desulfurization event only when the trap's NO
x
-storage capacity falls below a critical level, thereby minimizing the frequency at which such further fuel economy “penalties” are incurred.
Accordingly, there is a need for a method and apparatus for accurately determining the NO
x
-storage capacity or efficiency of a lean NO
x
trap in order to accurately schedule the desulfurization event as well as the purge event.
DISCLOSURE OF INVENTION
In accordance with the method of the present invention, the NO
x
absorption capacity is determined based on an estimate of the change in the oxygen storage capacity of the lean NO
x
trap. More particularly, after a desulfurization event is performed, to put the lean NO
x
trap in a known state, a number of estimates of the current value of oxygen storage capacity are determined in order to calculate a filtered or mean value of the oxygen storage capacity of the lean NO
x
trap when it is fresh. This initial capacity value is then stored in computer memory as a value P
1
and also as a value P
2
representing the current oxygen storage capacity of the trap. Subsequently, and at periodic time intervals the value of the current oxygen capacity of the trap is estimated and filtered and the value P
2
is updated. The current trap capacity to absorb NO
x
is then determined as a function of the value of P
2
/P
1
. When the trap capacity to absorb NO
x
falls below a predetermined minimum capacity value, a desulfurization event is performed and the forgoing steps are repeated.
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Bidner David Karl
Ingram Grant Alan
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Hanze Carlos
Nguyen Tu M.
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