Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1998-10-29
2001-01-09
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...
C060S288000, C060S295000, C060S297000, C060S301000
Reexamination Certificate
active
06170259
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the priority of German Application No. 197 47 670.8, filed on Oct. 29, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an emission control system for an internal-combustion engine having at least two nitrogen oxide adsorbers arranged in parallel for the alternating periodic adsorption and desorption of nitrogen oxides contained in the exhaust gas of the internal-combustion engine and devices for providing a desorption gas flow for the nitrogen oxide adsorbers running in each case in the desorption operation.
Nitrogen oxide adsorbers in emission control systems are known for adsorbing in the adsorption operation nitrogen oxides contained in the exhaust gas and thus removing them from the exhaust gas which can then be discharged to the outside largely without nitrogen oxides. From time to time, i.e., no later than when it reaches its nitrogen oxide storage capacity, the respective nitrogen oxide adsorber is run in the desorption operation in which the nitrogen oxides adsorbed therein are desorbed again. The exhaust gas flow which takes up the desorbed nitrogen oxides will then be treated appropriately, for example, and will be returned into an intake pipe of the internal-combustion engine in order to react the desorbed nitrogen oxides by the combustion in the internal-combustion engine to nitrogen and oxygen.
An emission control system is described in DE 43 19 294 C1. There, two parallel nitrogen oxide adsorbers are alternatingly run in the adsorption and in the desorption operation. For the desorption, fresh air is fed into the corresponding nitrogen oxide adsorber by way of an air pipe and this nitrogen oxide adsorber or the fed fresh air is heated externally while simultaneously the feeding of the exhaust gas flow remains blocked. In contrast, the feeding of air and the external heating of the respective adsorber do not take place in the adsorption operation in which the exhaust gas flow of the internal-combustion engine flows through the adsorber.
German Patent Application No. 196 28 796.0 describes an emission control system which has two adsorbers, which are arranged in parallel and are alternatingly run in the adsorption and in the desorption operation, and an oxidation catalyst arranged downstream of the adsorbers close to the engine, for the oxidation of NO contained in the exhaust gas to NO
2
. An exhaust pipe section situated upstream of the adsorbers is divided into a main flow pipe branch and into a partial flow pipe branch which is parallel to the main flow pipe branch and is shorter and/or more thermally insulated than the main flow pipe branch. The adsorber which, in each case, is operated in the adsorption mode is fed by the exhaust gas flow of the main flow pipe branch. and the respective other adsorber operated in the desorption mode is fed by the exhaust gas flow of the partial flow pipe branch. This measure takes into account the fact that a low temperature level promotes the adsorption operation and a higher temperature level promotes the desorption operation and nitrogen oxide adsorbers frequently adsorb NO
2
much more easily than NO.
Another emission control system with two nitrogen oxide adsorbers arranged in parallel is disclosed in DE 196 26 837 A1. There, the desorption gas flow providing devices consist, for example, of two electrically heatable catalysts which are connected directly in front of the respective nitrogen oxide adsorber and in front of which, in the respective branch of the exhaust pipe system, a respective fuel injecting unit is connected; or the desorption gas flow providing devices consist of a burner which is assigned jointly to the nitrogen oxide adsorbers and is operated by a slightly lean diesel fuel/air mixture, and of a fuel injecting unit which is connected behind this burner. In the latter, the hot burner exhaust gas acts as a carrier gas for the injected fuel which is fed to the respective nitrogen oxide adsorber by way of a corresponding pipe branch. One or two lambda probes, which are arranged in parallel downstream of the nitrogen oxide adsorbers recognize the end of a respective desorption phase.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an emission control system type which has an advantageous emission control functionality for nitrogen-oxide-containing exhaust gases.
This object has been achieved by an emission control system in which the desorption gas flow providing devices have a desorption gas pipe which the nitrogen oxide adsorbers have in common and which has an externally heatable catalyst, a lambda probe arranged between the externally heatable catalyst and the nitrogen oxide adsorbers, and devices for feeding a mixture of air and/or exhaust gas, on the one hand, as well as fuel at an at least stoichiometric, controllable proportion, on the other hand, to the externally heatable catalyst, in which case the externally heatable catalyst burns the fed mixture to form a desorbing gas flow which is fed to the nitrogen oxide adsorber which is each case is in the desorption operation, by way of a pertaining desorption pipe branch and in which case a predefinable lambda value of the desorbing gas flow is adjusted by way of the lambda probe.
In this system, a desorption gas pipe is provided which the nitrogen oxide adsorbers share and in which an externally heatable catalyst is arranged to which, on one hand, a mixture of air and/or exhaust gas can be fed and to which, on the other hand, fuel can be fed at an at least stoichiometric proportion. and which burns this mixture to form a desorbing gas flow which is fed to the respective nitrogen oxide adsorber situated in the desorption operation. By way of a lambda probe, the lambda value of the desorbing gas flow is adjusted to a desired value, for the purpose of which the lambda probe is inserted downstream of the externally heatable catalyst and upstream of the nitrogen oxide adsorbers into the desorption gas pipe.
As a result, the exhaust gas of a combustion operation of an air/fuel mixture with a controllable, at least stoichiometric fuel fraction (i.e., with a stoichiometric or rich air/fuel ratio) can be fed to the respective nitrogen adsorber to be desorbed, without limiting the operation of the internal-combustion engine. As required, the internal-combustion engine can also be operated with a lean air/fuel mixture during the desorption operation of an adsorber because the adsorber to be desorbed obtains the desorbing gas flow from the burning of the stoichiometric or rich air/fuel mixture in the externally heated catalyst.
It is known that the exhaust gas of such combustion contains unburnt hydrocarbons, carbon monoxide and hydrogen, while the oxygen contained in the air/exhaust gas flow fed to the catalyst burns up almost completely which promotes the desorption operation. The feeding of exhaust gas instead of fresh air for the combustion in the externally heated catalyst has the advantage that the oxygen content in the exhaust gas is already significantly reduced and therefore less fuel must be apportioned in order to obtain the desired, at least stoichiometric fuel fraction.
In addition, the exhaust gas has a higher temperature than fed fresh air. This reduces the energy requirement for the heating of the catalyst and thereby improves the overall energy balance of the system. The nitrogen oxides which are released from the respective adsorber by this desorption operation advantageously uncoupled from the operation of the internal-combustion engine can then be treated appropriately, for example, by the return into an intake pipe system of the internal-combustion engine. The emission control system is particularly suitable also for the emission control of internal-combustion engines of motor vehicles.
In an emission control system of the present invention, the externally heatable catalyst is formed by an electrically heatable catalyst with a comparatively low volume, a so-called electrically heatable minicatalyst (also
Boegner Walter
Haak Karl-Ernest
Krutzsch Bernd
Voigtlaender Dirk
Wenninger Guenter
Daimler-Chrysler AG
Denion Thomas
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Nguyen Tu M.
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