Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2000-03-17
2002-08-20
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S277000, C060S301000
Reexamination Certificate
active
06434930
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods and systems for controlling the operation of “lean-burn” internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy.
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 NO
x
, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of engine-generated constituent gases, such as HC, CO and NOx, 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.
Under one prior art approach, the duration of any given lean operating excursion (or its functional equivalent, the frequency or timing of each purge event) is controlled based upon an estimate of how much NO
x
has accumulated in the trap since the excursion began. Specifically, a controller accumulates estimates of feedgas NO
x
over time to obtain a measure representing total generated NO
x
. The controller discontinues the lean operating excursion when the total generated NO
x
measure exceeds a predetermined threshold representing the trap's nominal NO
x
-storage capacity. In this manner, the prior art seeks to discontinue lean operation, with its attendant increase in engine-generated NO
x
, before the trap is fully saturated with NO
x
, because engine-generated NO
x
would thereafter pass through the trap and effect an increase in tailpipe NO
x
emissions.
Unfortunately, empirical evidence suggests that the instantaneous storage efficiency of the trap, i.e., the trap's instantaneous ability to absorb all of the NO
x
being generated by the engine, rarely approaches 100 percent. Indeed, as the trap begins to fill, the instantaneous storage efficiency of the trap appears to decline significantly, with an attendant increase in the amount of NO
x
being exhausted to the atmosphere through the vehicle's tailpipe. 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 the three-way catalyst during lean engine operation, would rapidly negate the fuel savings associated with lean engine operation.
Moreover, under certain engine operating conditions, for example, under high engine speed and high engine load, the NO
x
generation rate and correlative exhaust flow rate through the trap are both so high that the trap does not have an opportunity to store all of the NO
x
in the exhaust, even assuming a 100 percent trap storage efficiency. As a result, such operating conditions are themselves typically characterized by a significant increase in tailpipe NO
x
emissions, notwithstanding the use of the NO
x
trap.
For a majority of motor vehicles, the effectiveness of a given method and system for controlling tailpipe NO
x
emissions is generally measured by evaluating the vehicle's performance in a standardized test under the Federal Test Procedure (FTP), in which the vehicle is operated in a prescribed manner to simulate a variety of engine operating conditions, at a variety of different engine-speed and engine-load combinations. A graphical illustration of the various engine speed/load combinations achieved during the FTP City Driving Cycle is depicted as Region I in
FIG. 5
, while the various engine-speed and engine-load combinations achieved during the FTP Highway Driving Cycle are depicted in FIG.
5
.
During either FTP test, vehicle NO
x
emissions, as measured by a NO
x
sensor, are accumulated over the course of a thirty-minute test period. The vehicle is deemed to have passed the test if the accumulated value of tailpipe NO
x
, in grams, does not exceed a prescribed threshold amount. Often, the prescribed threshold amount of permissible NO
x
emissions under the Highway Driving Cycle is characterized as a multiple of the prescribed threshold amount for the City Driving Cycle.
The NO
x
emissions of certain other motor vehicles, such as heavy trucks, are measured using another approach, wherein the vehicle's engine is independently certified on a dynamometer, with the engine's instantaneous NOx emissions thereafter being normalized by the engine's peak horsepower, in grams per horsepower-hour. In either event, such emissions standards are said to be “scalar,” i.e., fixed or static, rather than dynamic.
Significantly, it has been observed that, while the FTP City and Highway Driving Cycles include the vast majority of operating conditions over which a given motor vehicle is likely to be operated, the Cycles themselves are not necessarily representative of the manner in which most vehicles are operated. For example, it is generally true that an engine generates increased NO
x
emissions under operating conditions characterized by increased engine speeds and increased engine loads. Thus, each FTP cycle necessarily permits its relatively lower NO
x
-generating operating conditions to offset its relatively higher NO
x
-generating operating conditions, with a vehicle “passing the test” so long as the average generated NO
x
does not rise to the level at which the total generated NO
x
exceeds the prescribed threshold after thirty minutes.
In contrast, in “real world” operation, a given engine operating condition, such as a “highway cruise” operating condition characterized by substantially-higher instantaneous rates of NOx generation, may continue unabated for substantial periods of time. Such continued operation of the engine, even at an engine speed/load falling within Region I or Region II of
FIG. 5
, is properly characterized as being “off-cycle.” Similarly, certain circumstances, such as the towing of a large trailer, or operation of the vehicle at relatively higher altitudes, may push the operating point of the engine fully outside of Regions I and II. Engine operation under these circumstances (with engine speed/loads falling in the area generally depicted as Region III in
FIG. 5
) are likewise properly characterized as being “off-cycle.” And, because off-cycle operation may constitute a substantial portion of any given driving session, the FTP cycles do not necessarily predict the likely real-world emissions of a given vehicle.
Therefore, a need exists for a method and system for controlling the operation of a “lean-burn” internal combustion engine which seeks to regulate all vehicle NO
x
emissions, including “off-cycle” NO
x
emissions.
SUMMARY OF THE INVENTION
In accordance with the invention, a method is provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NO
x
, and wherein exhaust gas is directed through an exhaust gas purification system including a NO
x
trap before being exhausted to the atmosphere. Under the invention, the method includes determining a current rate at which NO
x
is being exhausted to the atmosph
Bidner David Karl
Cullen Michael John
Hepburn Jeffrey Scott
Robichaux Jerry D.
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Lippa Allan J.
Russell John D.
Tran Diem
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