Power plants – Internal combustion engine with treatment or handling of... – Methods
Reexamination Certificate
1999-05-19
2001-02-20
Chapman, Jeanette (Department: 3711)
Power plants
Internal combustion engine with treatment or handling of...
Methods
C060S285000, C123S443000
Reexamination Certificate
active
06189316
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a system and method for controlling the temperature of 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 gasses 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. When the desired lean and rich air/fuel ratios are adjusted to control trap temperature, poor control is achieved. In particular, when the trap is at a low temperature, a large difference between the rich and lean air/fuel ratios is desired to rapidly increase temperature. However, when the trap is at a low temperature and the air/fuel ratio difference is increased, an initial drop in temperature is experienced because it takes a certain amount of time for the exothermic reaction to begin. Thus, the trap temperature can drop below the light off temperature. At this point, the temperature continues to drop since the exothermic reaction is no longer sustainable due to the trap being below the light off temperature.
Another disadvantage encountered when using the above approach is that if the air/fuel ratio difference between the rich and lean cylinders is made too large, the trap temperature can drop even when well above the light off temperature. This is because the additional exothermic heat from the air/fuel ratio difference is not large enough to counteract the lower exhaust temperature caused by operating lean of, or rich of, stoichiometry.
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 controlling temperature of an 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 and a desired rich air/fuel ratio for the second cylinder based on the emission control device temperature, limiting said desired lean and rich air/fuel ratios based on said emission control device temperature, operating the first cylinder at said desired lean air/fuel ratio, and operating the second cylinder at said desired rich air/fuel ratio.
By limiting the desired lean and rich air/fuel ratios, it is possible to prevent inadvertent trap temperature changes in an undesirable direction. In other words, at low trap temperatures, the lean and rich air/fuel ratios are clipped at the point where maximum temperature increase is achieved. Allowing the lean and rich air/fuel ratios to be set beyond this value results in less than optimal temperature control and even temperature changes in an undesirable direction. By changing the limits with temperature, maximum control is always available, resulting in precise and rapid temperature control.
An advantage of the present invention is improved nitrous oxide trap temperature control.
Another advantage of the present invention is improved nitrous oxide conversion efficiency by improved desulfation.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.
REFERENCES:
patent: 5657625 (1997-08-01), Koga et al.
patent: 5758493 (1998-06-01), Asik et al.
patent: 5778666 (1998-07-01), Cullen et al.
patent: 5983627 (1999-11-01), Asik
patent: 6014859 (2000-01-01), Yoshizaki et al.
Farmer David George
Surnilla Gopichandra
Chapman Jeanette
Ford Global Technologies Inc.
Russell John D.
Varma Sneh
LandOfFree
Emission device temperature control system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Emission device temperature control system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Emission device temperature control system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2576273