Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2003-03-18
2004-12-07
Tran, Binh Q. (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S276000, C060S285000, C060S297000
Reexamination Certificate
active
06826902
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to lean burn internal combustion engine exhaust gas aftertreatment systems and methods and more particularly to systems and method for estimating oxygen storage capacity and stored NOx in a lean NOx trap aftertreatment systems.
BACKGROUND
As is known in the 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 (NOx), carbon dioxide, nitrogen (N
2
), 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 NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
In order to limit the amount of certain ones of the 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 and downstream three-way catalyst. For engines capable of running lean of stoichiometry, a downstream three-way catalyst optimized to store and release NOx is used, referred to as a lean NOx trap. This catalyst stores NOx when the exhaust gases are “lean” of stoichiometry and releases previously-stored NOx for reduction to unregulated 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 one embodiment, the trap chemically stores NOx during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat, although other elements can also be used. The NOx (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 thermal degradation and trap aging.
The trap's actual capacity to store NOx is finite and, hence, in order to maintain low tailpipe NOx emissions when running “lean,” the trap must be periodically cleansed or “purged” of stored NOx. 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 and convert both the oxygen stored in the three-way catalyst, and the oxygen and NOx stored in the trap. Moreover, the trap's NOx-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.
Typically, an “optimal” operating policy which gives the best trade-off between fuel economy and emissions determines when the purge should be initiated or terminated based on operating conditions. The key variable in executing such an optimal solution is an estimate of the amounts of NOx and oxygen stored in LNT (the internal state of the LNT). The purge is initiated when the mass of stored NOx reaches a threshold, and terminated when the stored NOx is completely depleted. Any deviation from the optimal policy, resulting from estimation errors or control implementation errors may lead to adverse consequences on fuel economy and emissions.
Without on-line emission measurements to determine stored NOx or oxygen, one has to estimate the state of the LNT using models and other measured variables. Most commonly available sensors are the switching HEGO sensor located downstream of the trap, mass air flow rate sensor, etc. Since the switching HEGO sensor is positioned after the LNT, a significant time delay may occur between the HEGO switch signal and effective change of the engine feedgas air-to-fuel ratio. This will lead to HC and CO breaking through the exhaust system and cause emission concerns. To mitigate the effects of the time delay, a model-based strategy can be used to predict the end of purge cycle.
Thus, in the absence of on-line emission measurements, the control of lean burn engine aftertreatment system can be implemented by using models of feedgas emissions and the aftertreatment subsystems, together with measured variables such as mass air flow rate, feedgas and tailpipe relative air-to-fuel ratios, etc. When the ambient conditions (such as humidity, LNT brick temperature, and sulfur effects) change or the engine components age, oxygen storage capacity and NOx amount in the LNT vary significantly, and the performance of the control system that relies on these variables may deteriorate if the accuracy of the estimate deteriorate with the change in hardware.
The inventors have discovered that voltage time variations (trajectories) of a tailpipe HEGO sensor can provide sufficient information to predict oxygen capacity during storage phase and stored NOx during purge phase of a lean NOx trap (LNT). This result can be used to provide accurate prediction of the termination time of purge operation, leading to improved control strategies for LNT operation. Further, more expensive UEGO sensors can be avoided without tangible performance penalty on LNT control.
More particularly, the inventors have developed an estimation algorithm to determine the termination time of purge operation. This will reduce potential HC and CO emission problems, leading to improved control strategies for LNT operation. This finding also indicates that more expensive UEGO sensors can be avoided without tangible performance penalty on LNT control. Also, the need to determine the oxygen storage capacity of the LNT i
Kim Yong-Wha
Sun Jing
Wang Le Yi
Ford Global Technologies LLC
Ford Global Technologies LLC
Tran Binh Q.
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