Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2000-11-13
2002-10-08
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S295000, C060S297000
Reexamination Certificate
active
06460328
ABSTRACT:
BACKGROUND AND SUMMARY OF INVENTION
This application claims the priority of German patent document DE 199 54 549.9, filed on Nov. 12, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method of operating an exhaust-emission control system with a nitrogen oxide adsorber and an assigned nitrogen-oxide loading sensor, the method providing that the nitrogen oxide adsorber is operated alternately in adsorption phases with an at least stoichiometric exhaust air ratio and in regeneration phases with an at most stoichiometric exhaust air ratio.
Operating methods for exhaust-emission control systems which have a nitrogen oxide adsorber are known in many forms, in particular for cleaning exhaust gas from motor-vehicle combustion engines operated predominantly on a lean mixture. In lean operating phases of the combustion device emitting the exhaust gas to be cleaned, such as a motor-vehicle spark-ignition engine operated predominantly on a lean mixture, the nitrogen oxide adsorber stores nitrogen oxides contained in the exhaust gas by adsorption. It is possible for the nitrogen oxides not to be reduced adequately to nitrogen, for example by a three-way catalyst, because of the oxygen excess and consequently the lack of reducing agents in the exhaust gas. The loading of the nitrogen oxide adsorber, also known as a nitrogen-oxide adsorber catalyst, with nitrogen oxides, mainly in nitrate form, increases continuously in the course of a lean operating phase. When its storage capacity is exhausted and it cannot adsorb any further nitrogen oxides, a changeover is made from the lean operation of the combustion device, which corresponds to an adsorption phase of the nitrogen oxide adsorber, briefly to a rich operating phase, in which the nitrogen oxide adsorber is fed an exhaust gas with an at most stoichiometric exhaust air ratio (generally with a substoichiometric air ratio, i.e. with a rich composition of the exhaust gas). This may take place, for example, by changing over the combustion device from the previous lean operation with an at least stoichiometric oxygen component in the fuel/air mixture to be burned to rich operation with a rich mixture; by injecting reducing agents directly into the exhaust gas upstream of the nitrogen oxide adsorber; and/or by other known methods. The rich operating phase corresponds to a regeneration phase of the nitrogen oxide adsorber, in which the nitrogen oxides temporarily stored in it are desorbed and then converted by reducing agents adequately present in the fed-in rich exhaust gas. The conversion may take place, for example, in the nitrogen-oxide adsorber body itself if a three-way catalyst is integrated therein; or in a downstream nitrogen-oxide reduction catalyst; or, for example, also by exhaust gas recirculation. The use of a three-way catalyst ensures effective nitrogen oxide conversion even in the stoichiometric range of the combustion device.
During the operation of such an exhaust-emission control system, it is desirable to change over between the alternating adsorption and regeneration phases of the nitrogen oxide adsorber at the most favourable possible time in each case. In general, the longest possible lean operation of the combustion device is desired for fuel consumption reasons, interrupted only from time to time by shortest possible rich operating phases for the regeneration of the nitrogen oxide adsorber fully loaded with nitrogen oxides. To find the most favourable switching times for the changes between the usually relatively long adsorption phases and the usually relatively short regeneration phases of the nitrogen oxide adsorber, the most accurate possible knowledge of the loading state of the nitrogen oxide adsorber at a given time is to be desired.
Conventionally, it is attempted in particular to determine the loading of the nitrogen oxide adsorber indirectly in the form of an estimate of the same on the basis of operating parameters of the exhaust-emission control system and the combustion device with the assistance of a mathematical modelling of the system, see for example the laid-open patent application EP 0 598 917 A1. A further indirect method of determining the loading uses the signal of a lambda probe arranged downstream of the nitrogen oxide adsorber, see for example the laid-open patent application EP 0 733 787 A2.
Alternatively, laid-open patent application DE 196 36 790 A1 proposes a direct determination of the loading of the nitrogen oxide adsorber by means of a corresponding loading sensor system, which comprises a nitrogen oxide sensor respectively upstream and downstream of the nitrogen oxide adsorber. With this loading sensor system, the increase in loading during a respective adsorption phase can be read off directly from the differential signal of the two nitrogen oxide sensors. At the end of a respective regeneration phase, an assigned loading counter is set to zero, and a changeover is then made from the following adsorption phase to the next regeneration phase if a predetermined maximum loading state is exceeded, i.e. the loading counter has exceeded a predetermined value.
A further directly measuring nitrogen-oxide loading sensor is described in German Patent Application No. 199 16 677.3, which is not a prior publication. This loading sensor makes use of the fact that the dielectric constant of the adsorber material depends in a one-to-one way on the degree of loading, so that the loading of the nitrogen oxide adsorber with adsorbed nitrogen oxides at a given time can be ascertained directly from a measurement of the dielectric constant of the adsorber material. It is of particular advantage in the case of this loading sensor that the decrease in loading of the nitrogen oxide adsorber during a respective regeneration phase can also be continuously sensed.
As is known, during the operation of the nitrogen oxide adsorber there is frequently a gradual decrease in its storage capacity, in particular due to chemical changes on account of excessive thermal effects and due to sulphur contained in the fuel, which is adsorbed in the adsorption phases in the form of sulphur compounds, in particular sulphates, in competition with the nitrogen oxides. By suitable special regeneration phases in the form of desulphating phases, this sulphur-dependent reduction in storage capacity can be at least partially reversed. The laid-open patent application EP 0 869 263 A1 discloses the carrying out of such desulphating phases. The incorporation of sulphur into the nitrogen oxide adsorber is modelled and a respective desulphating operation is introduced if a corresponding threshold value is exceeded. In addition, the incorporation of nitrogen oxides into the nitrogen oxide adsorber during a respective adsorption phase is also estimated on a model basis. The underlying exhaust-emission control system in this case comprises, inter alia, a lambda probe respectively upstream and downstream of the nitrogen oxide adsorber.
The technical problem on which the invention is based is that of providing an operating method of the type stated at the beginning for an exhaust-emission control system with a nitrogen oxide adsorber and an associated loading sensor which makes possible in particular a comparatively good control of the changes between the adsorption and regeneration phases of the nitrogen oxide adsorber in dependence on the loading state of the nitrogen oxide adsorber.
In the case of the method according to the present invention, it is specifically provided to sense the loading of the nitrogen oxide adsorber with nitrogen oxides continuously during a respective regeneration phase with a directly measuring loading sensor provided for this purpose, and to increase the air ratio of the exhaust gas fed to the nitrogen oxide adsorber in dependence on the measured loading as the measured loading decreases. As a result, the regenerating operation is adapted in an advantageous way to the current, decreasing loading of the nitrogen oxide adsorber (i.e., the proportion of reducing
Crowell & Moring LLP
Daimler-Chrysler AG
Denion Thomas
Tran Binh
LandOfFree
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