Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-05-19
2001-06-12
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S285000, C060S286000
Reexamination Certificate
active
06244043
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a system and method for controlling the air/fuel ratio of a mixture of exhaust gasses entering an emission control device during sulfur purging.
BACKGROUND OF THE INVENTION
Engine systems are known which operate the engine with lean combustion, or a lean air/fuel ratio, to improve fuel economy. To accommodate lean burn conditions, emission control devices, such as nitrous oxide (NOx) traps, are used to adsorb nitrous oxide emissions produced during lean operation. Adsorbed nitrous oxide is periodically purged by operating the engine with rich combustion, or a rich air/fuel ratio.
During normal lean and rich operation, sulfur contained in the fuel can become trapped in the emission control device. This gradually degrades the emission device capacity for storing nitrous oxide, as well as the device efficiency. To counteract the sulfur effect, various sulfur decontamination methods are available.
One method for sulfur decontamination requires elevating the emission control device temperature to a predetermined value. Then, additional fuel is injected while the catalyst is at this elevated temperature to reduce the sulfur stored in the device. The temperature of the device is raised by operating some of the cylinders lean and some of the cylinders rich. When the lean and rich exhaust gases meet in the device, exothermic reactions takes place, thereby releasing heat to increase the device temperature. The lean and rich exhaust gases are kept at certain desired lean and rich air/fuel ratios to maintain the average air/fuel ratio of the mixed exhaust gases at a desired air/fuel ratio. The desired lean and rich air/fuel ratios are determined in table look-up fashion with various correction factors. An exhaust gas air/fuel ratio sensor is relied upon to correct the desired lean and rich air/fuel ratios for control errors in the correction factors. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach. In particular, the method described for maintaining the average mixed exhaust air/fuel ratio is cumbersome and overly complex. In addition the above method requires an additional exhaust air/fuel ratio sensor because the open loop methods are not robust. Extensive testing and development, along with excessive computer memory storage and cost are necessary to use the above method. In other words, a simple, straightforward, and accurate method is not shown for determining a desired lean air/fuel ratio for the lean cylinders and a desired rich air/fuel ratio for the rich cylinders such that a desired exhaust gas mixture air/fuel ratio is achieved.
SUMMARY OF THE INVENTION
An object of the invention claimed herein is to provide a system and method for controlling cylinder air/fuel ratios for desulfating an emission control device, whereby the emission control device is heated by operating some cylinders of an engine lean and some cylinders of an engine rich.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for air/fuel ratio control of an exhaust gas mixture entering an emission control device, the emission control device located in an exhaust passage of an internal combustion engine having at least a first and second cylinder. The method comprising the steps of generating a desired lean air/fuel ratio for the first cylinder so that a desired emission control device temperature is achieved, operating the first cylinder at said desired lean air/fuel ratio, generating a desired rich air/fuel ratio for the second cylinder based on said desired lean air/fuel ratio and based on a desired air/fuel ratio of the exhaust gas mixture, wherein exhaust gases from the first and second cylinders form the exhaust gas mixture, and operating the second cylinder at said desired rich air/fuel ratio.
By calculating the desired lean air/fuel ratio for the lean cylinders to control trap temperature and then calculating a corresponding rich air/fuel ratio for the rich cylinders, the exhaust mixture from the lean and rich cylinder can be accurately controlled to form a desired mixture air/fuel ratio. In other words, the temperature control task is used for calculating the desired lean air/fuel ratio, since increasing or decreasing the lean air/fuel ratio has the effect of increasing or decreasing trap temperature. Then, since a certain air/fuel ratio is desired for the mixture of the lean and rich cylinders, this desired mixture air/fuel ratio, along with the just calculated lean air/fuel ratio is used to calculate the desired rich air/fuel ratio. In this way, the temperature is controlled to the desired level and the mixture from the lean and rich cylinders forms a desired mixture air/fuel ratio with no additional correction or sensor feedback.
In an alternative embodiment, the above object is achieved, and disadvantages of prior approaches overcome, by a method for air/fuel ratio control of an exhaust gas mixture entering an emission control device, the emission control device located in an exhaust passage of an internal combustion engine having at least a first and second cylinder, the method comprising the steps of generating a desired rich air/fuel ratio for the first cylinder so that a desired emission control device temperature is achieved, operating the first cylinder at said desired rich air/fuel ratio, generating a desired lean air/fuel ratio for the second cylinder based on said desired rich air/fuel ratio and based on a desired air/fuel ratio of the mixture of exhaust gasses, wherein exhaust gasses from the first and second cylinders form the exhaust gas mixture, and operating the second cylinder at said desired lean air/fuel ratio.
In this embodiment, the calculation order is reversed, and the rich air/fuel ratio is first calculated to control the trap temperature. Then, the lean air/fuel ratio is calculated based on the rich air/fuel ratio and the desired mixture air/fuel ratio. By calculating the desired rich air/fuel ratio for the rich cylinders to control temperature and then calculating a corresponding lean air/fuel ratio for the lean cylinders, the exhaust mixture from the lean and rich cylinder can be accurately controlled the desired mixture air/fuel ratio.
In an alternative embodiment, the above object is achieved, and disadvantages of prior approaches overcome, by a method for air/fuel ratio control of an exhaust gas mixture entering an emission control device, the emission control device located in an exhaust passage of an internal combustion engine having at least a first and second cylinder, the method comprising the steps of generating a desired air/fuel ratio difference between the first cylinder and the second cylinder so that a desired emission control device temperature is achieved, generating a desired lean air/fuel ratio for the first cylinder based on said desired air/fuel ratio difference and based on a desired air/fuel ratio of the exhaust gas mixture, wherein exhaust gasses from the first and second cylinders form the exhaust gas mixture, generating a desired rich air/fuel ratio for the second cylinder based on said desired air/fuel ratio difference and based on said desired air/fuel ratio of said exhaust gas mixture.
In this embodiment, the air/fuel ratio difference between the rich and lean cylinders is used to control trap temperature. Then, both the lean and rich cylinder air/fuel ratios are calculated from the air/fuel ratio difference and the desired exhaust mixture air/fuel ratio. Because the air/fuel ratio difference between the rich and lean cylinders is proportional to the heat addition to the trap, this parameter can be used to control trap temperature. Then, to provide the desired difference with a certain mixture air/fuel ratio, the desired rich and lean cylinder air/fuel ratio is accurately calculated.
An advantage of all embodiments of the present invention is improved nitrous oxide trap durability.
Another advantage of all embodiments of the present invention is improved nitrous oxide conversion effi
Castro Salomone
Farmer David George
Robichaux Jerry Dean
Surnilla Gopichandra
Denion Thomas
Ford Global Technologies Inc.
Russell John
Tran D.
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