Method and system for reducing vehicle tailpipe emissions...

Details Method and system for reducing vehicle tailpipe emissions... Method and system for reducing vehicle tailpipe emissions...

2001-06-19

2002-09-24

Denion, Thomas (Department: 3746)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S274000, C060S295000, C060S297000

Reexamination Certificate

active

06453666

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 constituents, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO
x
). The respective rates at which an engine generates these constituents 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 various of these “feedgas” constituents based, for example, on detected values for instantaneous engine speed and engine load.
In order to comply with modern restrictions regarding permissible levels of selected exhaust gas constituents, vehicle exhaust treatment systems often employ one or more three-way catalysts, referred to as an emission control device, disposed in an exhaust passage to store and release selected exhaust gas constituents, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NO
x
when the exhaust gas is lean, and releases previously-stored NO
x
when the exhaust gas is either stoichiometric or “rich” of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Significantly, a device's actual capacity to store a selected constituent gas, such as NO
x
, is often finite and, hence, in order to maintain low tailpipe NO
x
emissions, the device must be periodically cleansed or “purged” of stored NO
x
. The frequency or timing of each purge event must be controlled so that the device does not otherwise reach its actual NO
x
storage capacity, because engine-generated NO
x
would thereafter pass through the device and effect an increase in tailpipe NO
x
emissions. Further, the timing of each purge event is preferably controlled to avoid the purging of only partially-filled devices, due to the fuel penalty associated with the purge event's enriched air-fuel mixture and, particularly, the fuel penalty associated with the release of oxygen previously stored in any other upstream emission control device.
In response, U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153 teach use of NO
x
-estimating means which seeks to estimate the cumulative amount of NO
x
which has been generated by the engine and, presumptively, has been stored in the device during a given lean operating condition. The incremental amount of NO
x
believed to have been generated and stored in the device is 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). However, the disclosed NO
x
-estimating means fails to account for any instantaneous reduction in device efficiency, i.e., the device's ability to store an additional amount of feedgas NO
x
. The disclosed NO
x
-estimating means further fails to account for the device's initial storage of oxygen which likewise reduces the device's overall NO
x
-storing capacity.
The prior art has also recognized that the device's actual or maximum capacity to store selected exhaust gas constituents is often function of many variables, including device temperature, device history, sulfation level, and thermal damage, i.e., the extent of damage to the device's constituent-storing materials due to excessive heat. See, e.g., U.S. Pat. No. 5,437,153, which further teaches that, as the device approaches its maximum capacity, the incremental rate at which the device stores 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 device, to thereby theoretically provide the device with a perfect instantaneous NO
x
-storing efficiency, i.e., the device stores all engine-generated NO
x
, as long as stored NO
x
remains below the nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NO
x
reach the nominal device capacity. Unfortunately, however, the use of such a fixed nominal NO
x
capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NO
x
fill in order to ensure retention of engine-generated NO
x
.
When the engine is operated using a fuel containing sulfur, SO
x
accumulates in the device to cause a decrease in both the device's absolute capacity to store the selected exhaust gas constituent(s) and the device's instantaneous efficiency. When such device sulfation exceeds a critical level, the accumulated SO
x
must be “burned off” or released during a desulfation event, during which device 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 device desulfation method which includes raising the device temperature to at least 650° C. by introducing a source of secondary air into the exhaust upstream of the NO
x
device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of stored SO
x
.
Therefore, the inventors herein have recognized a need for a method and system for controlling the filling and purging of an emission control device with a selected exhaust gas constituent which can more accurately regulate overall tailpipe emissions of the exhaust gas constituent than prior art methods and systems.
SUMMARY OF THE INVENTION
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 treatment system including an emission control device that stores an exhaust gas constituent during lean engine operation and releases previously-stored exhaust gas constituent during engine operation at or rich of stoichiometry. Under the invention, during lean engine operation, the method includes determining a value representing an incremental amount, in grams per second, of a selected exhaust gas constituent, such as NO
x
, present in the engine feedgas 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 the exhaust gas constituent (e.g, NO
x
) being instantaneously stored in the device, preferably, as a function of device temperature, the amount of the constituent that is already stored in the device, an amount of sulfur which has accumulated within the device, and a value representing device aging (the latter being caused by a permanent thermal aging of the device or the diffusion of sulfur into the core of the device material which cannot be purged).
The method further includes calculating a value representing instantaneous tailpipe emissions of the exhaust gas constituent (e.g., NO
x
) based on the difference between the feedgas value and the incremental constituent-storage value; comparing the instantaneous tailpipe constituent emissions value to a predetermined threshold value; and discontinuing the lean engine operating condition when the instantaneous tailpipe constituent emissions value exceeds the predetermined threshold level, either instantaneously or as averaged over the course of a device purge-fill cycle, whose duration is determined by a timer which is nominally reset to zero upon commencement of an immediately prior rich engine operating condition.
In accordance with another feature of the invention, in a preferred e

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