Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
2000-03-17
2002-12-03
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S277000, C060S285000, C060S286000
Reexamination Certificate
active
06487849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods and apparatus for controlling the operation of “lean-burn” internal combustion engines used in motor vehicles to obtain improved engine and/or vehicle performance, such as improved vehicle fuel economy or reduced overall vehicle emissions.
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 (&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.
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.
Significantly, 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. Accordingly, 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.
Unfortunately, as a further impact of trap sulfurization, empirical data suggests that a trap's instantaneous NO
x
-storage efficiency, i.e., its instantaneous ability to incrementally store NO
x
, is increasingly affected by trap sulfurization as the trap begins to fill with NO
x
. Specifically, while a trap's instantaneous efficiency immediately after a trap purge event is believed to remain generally unaffected by trap sulfurization, the instantaneous efficiency begins to fall more quickly, and earlier in the fill event, with increasing trap sulfurization. Such reduced trap efficiency leads to increased instantaneous NO
x
emissions, even when the trap is not yet “filled” with NO
x
.
Accordingly, it is possible for the condition of a lean NO
x
trap deteriorate such that continued lean-burn operation either reduces overall vehicle fuel economy or increased overall vehicle emissions. What is needed, then, is a method and apparatus for controlling a lean-burn engine that prohibits lean engine operation when lean-burn operation is likely to have such a negative performance impact.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus for controlling a lean-burn internal combustion engine of a motor vehicle to prohibit lean engine operation when such lean engine operation is likely to generate a negative performance impact, such as a reduced overall vehicle fuel economy or increase overall vehicle emissions.
In accordance with the invention, a method and apparatus is provided for controlling a lean-burn engine which prohibits lean-burn operation when a measure representing a performance impact, such as a determined measure of fuel economy benefit relative to stoichiometric engine operation, and a measure of trap NO
x
-storage efficiency, sampled once per trap fill/purge cycle at end of fill cycle, fall below respective calibratable threshold values. Preferably, the determination of the performance impact includes determining a relative cost due to periodically purging the trap of stored NO
x
, as well as the determination of the performance improvement likely to be obtained upon initiating a trap decontamination event, such as desulfurization of the trap.
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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Bidner David Karl
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Lippa Allan J.
Nguyen Tu M.
Russell John D.
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