Power plants – Internal combustion engine with treatment or handling of... – Material from exhaust structure fed to engine intake
Reexamination Certificate
2000-11-15
2002-07-23
Denion, Thomas E. (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Material from exhaust structure fed to engine intake
C060S274000, C060S276000, C060S285000, C060S297000, C060S301000
Reexamination Certificate
active
06422003
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to internal combustion engine control systems and, specifically, to an internal combustion engine control system capable of controlling NO
x
emissions.
2. Description of the Art
It is known in the art relating to internal combustion engines that operating an engine with a lean mixture of fuel and air improves efficiency of the engine. This means that for a given amount of work performed by the engine, less fuel will be consumed, resulting in improved fuel efficiency. It is also well known that reduction of NO
x
emissions when the fuel rate is lean is difficult to achieve, resulting in an almost universal use of stoichiometric operation for exhaust control of automotive engines. By operating an engine with a stoichiometric mixture of fuel and air, fuel efficiency is good and NO
x
emission levels are reduced by over 90% once the vehicle catalyst reaches operating temperatures.
In addition to air-to-fuel ratio, other parameters involved in the combustion process affect the level of NO
x
emitted from the engine. Exhaust gas recirculation, for example, is a process by which a portion of the exhaust gas produced from the engine is conducted back to the combustion chamber to reduce peak combustion temperatures. Because high combustion temperatures result in high NO
x
emissions, increasing the percentage of exhaust gas recirculated in the combustion process reduces NO
x
emissions. However, the level of exhaust gas recirculated into the combustion process is limited because excessive amounts would halt combustion.
Another parameter that affects NO
x
emissions from the engine is spark ignition timing in spark-ignition engines or, in the case of compression-ignition engines (diesels), fuel injection timing. The timing of spark ignition or fuel injection, whichever is applicable, in the combustion sequence affects both emissions and fuel consumption. To reduce NO
x
emissions, the engine's timing is retarded from its baseline—the point of optimum fuel economy—lowering peak combustion temperature, increasing exhaust gas temperature, and increasing the amount of fuel burned. Since this decrease in NO
x
emissions occurs at the expense of increased fuel consumption, retard of an engine's ignition or injection timing, whichever is appropriate, is subject to a maximum limit.
Because lean operation of the engine improves fuel efficiency, maximizing lean operating time is desirable. Catalysts and engine control technologies operate to minimize NO
x
emissions during lean operation of the engine. Two types of NO
x
catalyst devices are standard in the art. Most common is the NO
x
adsorber, which stores NO
x
emissions during fuel lean operations and allows release of the stored emissions during fuel rich conditions with conventional three-way catalysis to nitrogen and water. The other is the NO
x
converter, which continuously converts NO
x
emissions as the exhaust feedstream flows through the converter.
These NO
x
catalyst devices, however, are not 100% effective in reducing NO
x
emissions from vehicles under all operating conditions. Attempts to compensate for various operating conditions relied on controlling the amount of emissions in the feedstream through an indirect measure of catalyst efficiency. A signal from an oxygen sensor mounted after the catalyst controlled the air-to-fuel ratio in the cylinders. The presence of oxygen, as measured by the oxygen sensor, indicated the condition where NO
x
emissions were not being efficiently removed by the catalyst.
The catalyst was thus regenerated, or stoichiometric fueling was forced, both of which can lead to a loss in fuel efficiency.
Thus, it would be desirable to provide a means of directly controlling the NO
x
emissions in the feedstream to a level the catalyst can process efficiently under varying operating conditions to provide optimum emission control and improve fuel efficiency.
SUMMARY OF THE INVENTION
The present invention is a method for controlling NO
x
emissions in an internal combustion engine by monitoring the exhaust gases flowing from the engine to the NO
x
catalyst, detecting when this feedstream NO
x
is above the level of efficient processing by the NO
x
catalyst, and adjusting certain engine parameters to reduce the NO
x
level in the feedstream to a level the NO
x
catalyst can efficiently process. In this way, emissions of NO
x
are minimized while fuel efficiency is maximized.
Once the engine is operating, the engine-out NO
x
rate is calculated. Engine-out NO
x
rate is the level of NO
x
in the feedstream being processed by the NO
x
catalyst. The engine-out NO
x
rate can be modeled using known engine operating conditions or calculated from measurements obtained using existing sensor technology.
The control proceeds to calculate a maximum permissible engine-out NO
x
rate. The maximum permissible engine-out NO
x
rate is a function of NO
x
catalyst efficiency. NO
x
catalyst efficiency is a measure of the amount of NO
x
the catalyst can process, which is a function of certain characteristics of the catalyst feedstream, namely the temperature of the catalyst and the NO
x
level seen by the catalyst.
If the engine-out NO
x
rate is less than or equal to the maximum engine-out NO
x
rate, then the catalyst is not receiving more NO
x
than it can process. Since the engine is operating in a fuel efficient region, and NO
x
emissions are minimized, adjustments to engine parameters are not required. The control sequence then begins again with the determination of the engine-out NO
x
rate. When the engine-out NO
x
rate is greater than the maximum engine-out NO
x
rate, the catalyst is receiving more NO
x
than it can efficiently process. The control sequence then performs steps to increase at least one of the following engine parameters to reduce the engine-out NO
x
rate: exhaust gas recirculation; air-to-fuel ratio; and ignition or injection retard. Which order the adjustments are made in depends upon what type of NO
x
catalyst is in use, whether the engine is a spark-ignition or a compression-ignition engine, and the totality of engine operating conditions.
In one aspect of the present invention, under normal operating conditions in a spark-ignition engine where the NO
x
catalyst is in an adsorber, the exhaust gas recirculation (EGR) percentage is increased first, if possible, to reduce the engine-out NO
x
rate. The exhaust gas recirculation (EGR) percentage is compared to a pre-determined maximum EGR rate. If the percentage of EGR is less than the maximum EGR rate, the percentage of EGR is increased by a pre-determined increase factor according to conventional methods of increasing the amount of exhaust gas recirculated. The control sequence then begins again with the determination of the engine-out NO
x
rate at the engine's new operating point.
If, however, the percentage of EGR is at its maximum EGR rate, then no changes in exhaust gas recirculated are made. The air-to-fuel ratio (A/F ratio) is compared to a pre-determined maximum A/F ratio. If the A/F ratio is less than a maximum A/F ratio, then the A/F ratio is increased by a pre-determined increase factor according to conventional methods of increasing cylinder fuel level. The control sequence then begins again with the determination of the engine-out NO
x
rate at the engine's new operating point.
If, however, the A/F ratio is at its maximum A/F ratio, then the engine timing is compared to a pre-determined maximum value for engine timing. This maximum value for engine timing corresponds to the maximum allowable retard of an engine's ignition or injection timing. In a spark-ignition engine, engine timing is spark ignition timing. In a compression-ignition engine, engine timing is fuel injection timing. If the engine timing is at the value indicating maximum retard, then the control sequence begins again with the determination of the engine-out NO
x
rate. If the engine timing is less than the value indicating maximum retard
Ament Frank
Brown David B.
Frank David A.
Denion Thomas E.
General Motors Corporation
Simon Anthony Luke
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