Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-03-17
2002-03-26
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S277000, C060S285000, C060S301000
Reexamination Certificate
active
06360530
ABSTRACT:
FIELD OF THE INVENTION
1. Field of the Invention
The invention relates to methods and apparatus for controlling a “lean burn” internal combustion engine of a motor vehicle to achieve enhanced vehicle fuel economy with continued emissions compliance.
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 NOx when the exhaust gases are “lean” of stoichiometry and release previously stored NO
x
for reduction to harmless gases when the exhaust gases are “rich” of stoichiometry.
Because the trap's actual capacity to store NO
x
is finite, the prior art teaches vehicle emissions compliance by discontinuing lean engine operation when the trap is deemed to have been “filled” to a predetermined level, thereby avoiding the condition wherein feedgas NO
x
would pass through a “filled” trap to effect an increase in tailpipe NO
x
emissions. Thus, in U.S. Pat. Nos. 5,473,887 and 5,437,153, a controller accumulates estimates of feedgas NO
x
, all of which is presumptively stored in the trap during a given lean excursion, and triggers a purge event when the accumulated feedgas NO
x
exceeds a predetermined threshold representing the trap's nominal NO
x
-storage capacity. And because a trap's instantaneous efficiency, i.e., the incremental rate at which the trap stores NO
x
present in the exhaust gas flowing through the trap, may begin to fall as the trap fills with NO
x
, U.S. Pat. No. 5,437,153 further teaches use of a nominal NO
x
-storage capacity which is significantly less than the actual NO
x
capacity of the trap, to thereby theoretically provide the trap with a near-perfect instantaneous NO
x
-absorbing efficiency as long as stored NO
x
remains below the nominal capacity.
Unfortunately, this prior art approach often discontinues lean engine operation while the trap still has meaningful available NO
x
-storage capacity, thereby degrading the fuel economy benefit to be achieved through lean-burn operation. Even further, while increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event's enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen stored previously stored in both the three-way catalyst and the trap during lean engine operation, can further degrade the fuel economy benefits associated with lean engine operation.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus for controlling the operation of a “lean-burn” internal combustion engine which seeks to maximize the fuel economy benefit from lean engine operation while providing overall emissions compliance.
In accordance with the invention, a method and apparatus are provided for controlling the operation of a lean-burn engine of a motor vehicle, wherein the engine generates exhaust gas including a first constituent gas, such as NO
x
, and wherein exhaust gas is directed through an exhaust gas purification system including an emissions control device that is operative to store an amount of the first constituent gas when the engine is operated in a first operating condition, and is further operative to release a previously-stored quantity of the first constituent gas when the engine is operating in a second operating condition. Under the invention, the method includes determining a first measure representative of a total amount of the first exhaust gas constituent exhausted to the atmosphere during a first engine operating period; determining a second measure representing a distance traveled by the vehicle during the first engine operating period; and calculating a third measure based on the first measure and the second measure. The method further preferably includes comparing the third measure to a predetermined threshold, and discontinuing lean engine operation when the third measure exceeds the predetermined threshold. In an exemplary embodiment, the predetermined threshold is periodically adjusted based upon an indication of vehicle activity determined, for example, based upon actual engine power.
In accordance with another feature of the invention, determining the first measure includes determining a first value representative of an instantaneous amount of the exhaust gas constituent being exhausted to the atmosphere, for example, based upon an output signal generated by a sensor positioned downstream of the device; and accumulating the first value during the first engine operating period. In an exemplary embodiment, the first value is determined as a function of instantaneous operating conditions and an instantaneous device storage efficiency.
In accordance with a further feature of the invention, determining the second measure includes detecting a value representative of instantaneous vehicle speed, with the detected instantaneous vehicle speed value being clipped at a predetermined, nonzero minimum value. The predetermined, nonzero minimum value preferably approximates a minimum vehicle speed characterized by a level of constituent emissions that is at least as great as the levels of constituent emissions generated by the engine when idling at a stoichiometric operating condition.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.
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pat
Cullen Michael John
Hepburn Jeffrey Scott
Robichaux Jerry D.
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Lippa Allan J.
Tran Diem
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