Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-03-17
2001-10-30
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S277000, C060S285000, C060S301000
Reexamination Certificate
active
06308515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. 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 (8), 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 and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NO
x
“trap”. Both the upstream and downstream catalyst store 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.
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
. U.S. Pat. No. 5,473,887 teaches the purging of a NO
x
trap by subjecting the trap to an air-fuel mixture whose air-fuel ratio is rich of stoichiometric, for example, an air-fuel ratio of less than about 13. 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.
Because a finite amount of excess fuel is required during the trap purge to release the stored oxygen before stored NO
x
is released, the prior art has sought to estimate the amount of oxygen stored in the trap by inferring oxygen storage from the time period between combustion, immediately after a rich purge event, of a lean air-fuel mixture within the engine and the switching of the downstream air-fuel ratio from a near-stoichiometric air-fuel ratio to a lean air-fuel ratio, as measured by a downstream oxygen sensor. However, because a significant portion of the time period between the commencement of lean engine operation and the switching of the downstream sensor is caused by movement of the resulting lean exhaust gas through the vehicle's exhaust system, both upstream and downstream of the trap, the prior art teaches an involved process for separating out the resulting time lags from the overall period, in an attempt to obtain an accurate measure of oxygen storage. Changes in the engine operating condition, i.e., the engine's speed and load, during this period renders the determination of the trap's oxygen storage capacity increasingly problematic.
Therefore, a need exists for a method and apparatus for accessing the ability of an emissions control device, such as a lean NO
x
trap, to releasably store an exhaust gas constituent.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus for accessing the ability of a emissions control device for a lean-burn engine to releasably store an exhaust gas constituent.
In accordance with the invention, a method is provided for accessing the ability of an emissions control device to releasably store a quantity of a constituent of exhaust gas generated by lean-burn operation of an internal combustion engine. The method includes determining that the engine is operating at a first lean operating condition immediately following a first rich engine operating condition, the first rich engine operating condition having been sufficient to release substantially all of the previously-stored amount of the exhaust gas constituent from the device. The method further includes determining, during the first lean operating condition, a first measure representative of the instantaneous efficiency of the device to remove the exhaust gas constituent from the exhaust gas; and initiating a second rich operating condition when the third measure falls below a reference value, the subsequent rich operating condition being characterized by the combustion of a quantity of fuel in excess of a stoichiometric quantity of fuel sufficient to release substantially all previously-stored exhaust gas constituent from the device. The method further includes determining an ability of the device based on the quantity of excess fuel used in the second rich operating condition to release substantially all previously-stored exhaust gas constituent from the device.
In an exemplary method, determination of the first measure representative of the instantaneous efficiency of the device includes determining a second measure representative of the content of the exhaust gas constituent in exhaust gas entering the device, for example, using a look-up table based upon at least one of the group consisting of engine speed and engine load; and determining a third value representative of the content of the exhaust gas constituent in exhaust gas exiting the device, for example, based upon a signal, generated by a suitable sensor, representative of a detected concentration of the exhaust gas constituent in the exhaust gas exiting the device. The exemplary method further includes calculating the first measure based on the second and third measure, for example, by dividing the third measure by the second measure.
In accordance with another feature of the invention, wherein the reference value is preferably significantly greater than a minimum value necessary to maintain the content of the exhaust gas constituent in the exhaust gas exiting the trap below a predetermined threshold level. In this manner, the invention advantageously avoids “saturation” of the device when determining the device's ability to store the exhaust gas constituent, thereby avoiding unnecessary tailpipe constituent emissions. In an exemplary method, the reference value represents an efficiency of no less than about sixty-five percent.
Under the invention, a controller is also provided for controlling an engine operating in combination with an emissions control device that releases a previously-stored first exhaust gas constituent when the engine is operated at a rich operating condition for a predet
Bidner David Karl
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Lippa Alan J.
Tran Diem
LandOfFree
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