Method For Operating An Electrically Heatable Catalyst

Abstract

A method for operating an electrically heatable catalytic converter in an exhaust tract of an internal combustion engine having at least one honeycomb body through which an exhaust-gas stream can flow, and having at least one electrically heatable heating conductor positioned upstream of the honeycomb body in a throughflow direction of the exhaust gas includes: applying an electrical current to the heating conductor such that the heating conductor is electrically heated in a manner dependent on an ambient temperature around the heating conductor; and electrically heating the heating conductor such that a dwell time of a temperature of the heating conductor is bounded in a temperature range defined by a first lower limit temperature T.sub.G1U and an upper limit temperature T.sub.G1O.

Claims

1-11. (canceled)

12. A method for operating an electrically heatable catalytic converter in an exhaust tract of an internal combustion engine having at least one honeycomb body through which an exhaust-gas stream can flow, and having at least one electrically heatable heating conductor positioned upstream of the honeycomb body in a throughflow direction of the exhaust gas, the method comprising: applying an electrical current to the heating conductor such that the heating conductor is electrically heated in a manner dependent on an ambient temperature around the heating conductor; and electrically heating the heating conductor such that a dwell time of a temperature of the heating conductor is bounded in a temperature range defined by a first lower limit temperature T.sub.G1U and an upper limit temperature T.sub.G1O.

13. The method as claimed in claim 12, wherein a temperature transient of the temperature of the heating conductor when passing through the defined temperature range from a higher temperature to a lower temperature does not undershoot a predefined limit value.

14. The method as claimed in claim 12, wherein the temperature of the heating conductor is, by the electric heating, kept above the upper limit temperature T.sub.G1O if the ambient temperature around the heating conductor lies below the upper limit temperature T.sub.G1O but lies above a second lower limit temperature T.sub.G2U.

15. The method as claimed in claim 14, wherein the electric heating of the heating conductor is ended if the ambient temperature of the heating conductor has undershot the second lower limit temperature T.sub.G2U.

16. The method as claimed in claim 15, wherein the second lower limit temperature T.sub.G2U is lower than the first lower limit temperature T.sub.G1U.

17. The method as claimed in claim 14, further comprising maintaining, by the electric heating, the heating conductor above the upper limit temperature T.sub.G1O for a predefinable duration.

18. The method as claimed in claim 17, further comprising ending the electric heating if the ambient temperature around the heating conductor is below the second lower limit temperature T.sub.G2U for a predefined time.

19. The method as claimed in claim 12, further comprising ending the electric heating of the heating conductor if a temperature difference between surroundings of the heating conductor and a present temperature of the heating conductor exceeds a predefinable value.

20. The method as claimed in claim 14, further comprising outputting, by a prediction element, a statement regarding a predicted change in the ambient temperature around the heating conductor, and performing the electric heating of the heating conductor in a manner dependent on the predicted change of the ambient temperature around the heating conductor.

21. The method as claimed in claim 20, further comprising, ending the electric heating of the heating conductor in the event the ambient temperature around the heating conductor is predicted to remain below the second lower limit temperature T.sub.G2U.

22. The method as claimed in claim 12, further comprising defining, by the first lower limit temperature T.sub.G1U and the upper limit temperature T.sub.G1O, a temperature range in which, due to action of heat, material of the heating conductor undergoes a conversion of material structure that leads to a change in the specific resistance of the heating conductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will be explained in detail in the following text with reference to the drawing. In the drawing:

[0038] The FIGURE shows a diagram in which, in the upper region, the temperature of the heating conductor is illustrated versus the time, and in the lower region, the heating current conducted through the heating conductor is illustrated versus the time.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0039] The FIGURE shows a diagram divided into an upper region 1 and a lower region 2. In the upper region 1, the temperature 3 of a heating conductor is illustrated versus the time 4, which is plotted along the X axis. The exhaust-gas temperature 5 is also illustrated versus the time 4.

[0040] In the lower region 2 of the diagram, the heating current 6 applied to the heating conductor is illustrated versus the time 4. The heating current 6 is illustrated with the value 0% if no current is flowing and with the value 100% if current is flowing.

[0041] Proceeding from a starting temperature of approximately 550 degrees Celsius at the heating conductor and the flowing exhaust gas, the heating is deactivated, and no current flows through the heating conductor. Owing, for example, to a decreasing load on the internal combustion engine, the exhaust-gas temperature 5 falls over the course of the following time. If the heating conductor were now not heated, the temperature of the heating conductor would consequently fall correspondingly to the exhaust-gas temperature or with a slight offset with respect thereto. Owing to the relatively low temperature transient of the cooling, a structure conversion and formation of the alpha-prime phase could occur in the heating conductor.

[0042] To prevent this, the heating of the heating conductor is commenced at approximately 30 seconds to keep the temperature 3 of the heating conductor at a constant high level and prevent entry into the temperature range of the structure conversion.

[0043] During this time, the exhaust-gas temperature 5 continues to fall approximately linearly, whereby the temperature delta between the temperature 3 of the heating conductor and the exhaust gas 5 is increased. The heating of the heating conductor is maintained until the exhaust-gas temperature has fallen to the level of a lower limit temperature T.sub.G2U 7. Proceeding from this time, the temperature delta between the temperature 3 of the heating conductor and the temperature 5 of the exhaust gas is large enough to realize sufficiently fast cooling of the heating conductor in the case of which the formation of the alpha-prime phase is prevented.

[0044] Subsequently, the temperature 3 of the heating conductor likewise begins to fall. As shown in the FIGURE, after the ending of the heating, the temperature 3 of the heating conductor falls with a considerably higher transient than the exhaust-gas temperature 5. Owing to the higher transient of the temperature 3 of the heating conductor, it is achieved that the temperature window in which the formation of the alpha-prime phase preferably occurs is passed through in a significantly shorter time than in the case of the exhaust-gas temperature 5. The formation of the alpha-prime phase in the heating conductor is thus greatly reduced or even prevented entirely.

[0045] The diagram of the FIGURE shows a specific situation in which heating of the heating conductor is performed in order to maintain a certain minimum temperature level, so as not to enter the temperature range of the structure conversion. Only when the temperature delta between the temperature of the heating conductor and the surroundings is large enough to ensure that a sufficiently high temperature transient can be attained during the cooling is the heating stopped. The temperature of the heating conductor subsequently falls quickly enough to pass through the temperature range of the structure conversion without actually undergoing a structure conversion.

[0046] The usage situation described by the FIGURE is exemplary, and is not of a limiting nature.

[0047] Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.