Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2002-12-18
2004-02-03
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, C060S286000
Reexamination Certificate
active
06684631
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 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 NO
x
, 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 release previously stored NO
x
, for reduction to harmless gases when the exhaust gases are “rich” of stoichiometry. Typically, such traps include ceria, which characteristically operates to store a quantity of available oxygen during the initial portion of lean engine operation.
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. For example, in U.S. Pat. Nos. 5,473,887 and 5,437,153, a controller seeks to estimate the amount of NO
x
stored in the trap by accumulating estimates for feedgas NO
x
which are themselves obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). The controller discontinues the lean operating excursion when the total feedgas 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.
However, the disclosed NO
x
-estimating means fails to account for any instantaneous reduction in trap efficiency, i.e., the trap's ability to store an additional amount of feedgas NO
x
. The disclosed NO
x
-estimating means further fails to account for the trap's initial storage of oxygen, which likewise reduces the trap's overall NO
x
-storing capacity.
The prior art has also recognized that the trap's actual or maximum NO
x
-storage capacity is a function of many variables, including trap temperature, trap history, sulfation level, and thermal damage, i.e., the extent of damage to the trap's NO
x
-absorbing materials due to excessive heat. See, e.g., U.S. Pat. No. 5,437,153, which further teaches that, as the trap approaches its maximum capacity, the incremental rate at which the trap absorbs NO
x
may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NO
x
capacity which is significantly less than the actual NO
x
capacity of the trap, to thereby theoretically provide the trap with a perfect instantaneous NO
x
-absorbing efficiency, i.e., the trap absorbs all engine-generated NO
x
, as long as stored NO
x
remains below the nominal capacity. A purge event is scheduled to rejuvenate the trap whenever accumulated estimates of engine-generated NO
x
reach the nominal trap capacity. Unfortunately, however, the use of such a fixed nominal NO
x
capacity necessarily requires a larger trap, because this prior art approach relies upon a partial, e.g., fifty-percent NO
x
fill in order to ensure absorption of engine-generated NO
x
.
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 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.
When the engine is operated using a fuel containing sulfur, SO
x
accumulates in the trap to cause a decrease in both the trap's absolute NO
x
capacity and the trap's instantaneous efficiency. When such trap sulfation exceeds a critical level, the accumulated SO
x
must be “burned off” or released during a desulfation event, during which trap temperatures are raised above perhaps about 650° C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a trap desulfation method which includes raising the trap temperature to at least 650° C. by introducing a source of secondary air into the exhaust upstream of the NO
x
trap when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the trap temperature to the desired level to purge the trap of stored SO
x
.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus for controlling the filling and purging of a NO
x
trap which can more accurately regulate overall tailpipe NO
x
emissions than prior art methods and apparatus.
In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust purification system including a lean NO
x
trap. Under the invention, during lean engine operation, the method includes determining a value representing an incremental amount, in grams per second, of feedgas NO
x
generated by the engine as a function of current values for engine speed, engine load or torque, and the lean operating condition's air-fuel ratio. The method also includes determining a value representing the incremental amount of NO
x
being instantaneously stored in the trap, preferably, as a function of trap temperature, an amount of NO
x
previously stored in the trap, an amount of sulfur which has accumulated within the trap, and a value representing trap aging (the latter being caused by a permanent thermal aging of the trap or the diffusion of sulfur into the core of the trap material which cannot be purged).
The method further includes calculating a value representing instantaneous tai
Cullen Michael John
Hepburn Jeffrey Scott
Robichaux Jerry D.
Surnilla Gopichandra
Denion Thomas
Kolisch Hartwell PC
Nguyen Tu M.
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