Internal-combustion engines – Charge forming device – Including exhaust gas condition responsive means
Reexamination Certificate
2000-03-17
2001-10-30
Kwon, John (Department: 3747)
Internal-combustion engines
Charge forming device
Including exhaust gas condition responsive means
C060S274000, C060S276000, C060S285000, C701S108000, C701S114000
Reexamination Certificate
active
06308697
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a system and method for controlling air-fuel ratio of an engine system having an emission control device.
BACKGROUND OF THE INVENTION
Engine and vehicle fuel efficiency can be improved by lean burn internal combustion engines. To reduce emissions, these lean burn engines are coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, an additional three-way catalyst, known as a NOx trap or catalyst, is typically coupled downstream of the three-way catalytic converter, where the NOx trap is optimized to further reduce NOx. Both the three way catalyst and to a greater extent the NOx trap store NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry.
Accurate air-fuel ratio control allows optimized performance of this three way catalytic converter and NOx trap emission control system. One method to provide accurate air-fuel control uses a so-called universal exhaust gas oxygen sensor in the engine exhaust. This universal exhaust gas oxygen sensor provides an output that represents exhaust air-fuel ratio over a range, including lean, stoichiometric, and rich operation. Engine fuel injection is adjusted so that measured exhaust air-fuel ratio converges to a desired stoichiometric air-fuel ratio. Such a method is described in U.S. Pat. No. 5,473,887.
The inventors herein have recognized a disadvantage with the above approach. In particular, when such a system is applied to operation where exhaust air-fuel ratio is varied from lean to stoichiometric or rich, or exhaust air-fuel ratio is varied from stoichiometric or rich to lean, air-fuel ratio control is degraded. Consider the universal exhaust gas oxygen positioned downstream of the NOx trap. In this configuration, the sensor does not indicate the air-fuel ratio entering the NOx trap during some operating conditions. For example, when the air-fuel ratio entering the NOx trap is changed from stoichiometric or rich to lean, the sensor indicates stoichiometric until the oxygen storage capacity of the NOx trap is saturated. If this measurement is used for control, the engine will be operated excessively lean. In other words, in this example, the desired engine air-fuel ratio will be lean and the sensor will indicate stoichiometric. Since the sensor is used for feedback control, less fuel will be injected and the engine operated excessively lean until the oxygen storage capacity of the NOx trap is saturated.
SUMMARY OF THE INVENTION
An object of the invention claimed herein is to provide a method for determining capacity of a NOx trap coupled to an internal combustion engine.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for an emission control device coupled to an internal combustion engine with a sensor coupled to an exhaust gas of the engine downstream of the emission control device, the method comprising: providing a first output signal of the sensor indicative of a first quantity of the exhaust gas and a second output of the sensor indicative of a second quantity of the exhaust gas; determining a desired engine air-fuel ratio based on the second output signal; and adjusting a fuel injection amount into the engine to provide said desired engine air-fuel ratio based on the first output signal when the first output signal indicates an exhaust air-fuel ratio away from stoichiometry.
By using the air-fuel ratio indicated by the sensor only when it indicates a value away from stoichiometry, it is possible to use the information to improve control accuracy. In other words, sensor information is used for closed loop air-fuel control when this information is representative of the engine air-fuel ratio.
An advantage of the above aspect of the present invention is improved air-fuel ratio control during lean and rich operation.
Another advantage of the above aspect of the present invention is reduced emissions and improved fuel economy.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.
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Bidner David Karl
Surnilla Gopichandra
Ford Global Technologies Inc.
Kwon John
Lippa Allan J.
Russell John D.
Vo Hieu T.
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