Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-11-01
2003-10-28
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, C060S277000, C060S286000
Reexamination Certificate
active
06637198
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for operating an emission control system having nitrogen oxide storage, for cleaning up a nitrogen oxide-containing, sulfur-contaminated exhaust gas from combustion equipment, especially a predominantly lean-combustion operated Otto or Diesel engine in a motor vehicle, according to the method, from time to time desulfating phases being performed for the releasing of sulfur intercalated in the nitrogen oxide storage.
BACKGROUND INFORMATION
As is conventional, for fuel consumption reasons, it is desirable to operate combustion equipment, such as an Otto or Diesel engine in a motor vehicle, predominantly in lean-combustion operation, i.e., having excess air in the combusted air/fuel mixture. However, in the case of Otto engines, lean-combustion operation can only be used in the range of low and medium engine load. At high engine load, it is necessary to change to at least stoichiometric operation, so as to be able to supply the desired torque. When a torque-based engine control is used, an appropriate threshold value for the so-called indicated engine torque can be used as changeover threshold, the latter being an operand formed in the engine control which is determined with the aid of a torque model involving air mass, fuel mass, and, depending on the particular application case, other variables. The indicated engine torque actually differs from engine torque available, for example, at the flywheel of an engine by friction losses not being considered.
During lean-combustion operation, a conventional three-way catalyst is not suitable for effective nitrogen oxide reduction. That is why, in emission control systems for predominantly lean-combustion operated internal combustion engines, nitrogen oxide storages, also known as nitrogen oxide storage catalysts, are installed, which intermediately store nitrogen oxides emitted during lean-combustion operation of the combustion equipment in the form of nitrate. From time to time, short-time desorption phases or regeneration phases are performed using rich mixture operation of the combustion equipment, so as to desorb the nitrogen oxides intermediately stored in nitrate form from the nitrogen oxide storage, and convert them using the available reduction media, such as uncombusted hydrocarbons and carbon monoxide. Diverse procedures suitable for this are conventional, as described, for example, in European Published Patent Application No. 0 585 900 and European Published Patent Application No. 0 598 916. The essential parameters in the nitrogen oxide regeneration of the nitrogen oxide storage are desorption duration and the exhaust gas/air ratio during the desorption. Special strategies for the selection of these desorption parameters are described, for example, in European Published Patent Application No. 0 636 770, European Published Patent Application No. 0 733 786 and German Published Patent Application No. 199 15 793.
The necessity for nitrogen oxide desorption can be detected, for example, by an NO
x
sensor downstream from the nitrogen oxide storage, or by a mathematical model of the nitrogen oxide storage which considers, among other things, the quantity of nitrogen oxide brought in since the last desorption, as described, for example, in European Published Patent Application No. 0 598 917, European Published Patent Application No. 0 867 604 and German Published Patent Application No. 196 35 977. Performing a nitrogen oxide desorption before changeover from lean-combustion operation to stoichiometric operation is also conventional, in order to avoid an otherwise threatening, uncontrolled release of stored nitrogen oxide, as described, for example, in German Published Patent Application No. 197 41 079.
A nitrogen oxide storage has effective NO
x
storability in a certain temperature window typically between 200° C. and approximately 500° C. If greater temperatures are reached because of increased exhaust gas temperatures, depending on the operational state of the combustion equipment, the change may be made to stoichiometric operation because of the storage effectiveness of the nitrogen oxide storage, which has become worse, but this leads to increased fuel consumption as compared to lean-combustion operation. Temperatures above approximately 800° C. may lead to irreversible damage of the nitrogen oxide storage, and should therefore be avoided by suitable measures.
Nitrogen oxide storages are damaged, in the sense of reduced nitrogen oxide storability, by sulfur contained in the exhaust gas, which mostly traces back to sulfur-containing fuel, by the fact that, during lean-combustion operation, beside the intercalation of nitrate intercalation of sulfur, especially in the form of sulfate can occur. The intercalated sulfates are not released or decomposed, as the case may be, under the conditions of the usual nitrogen oxide desorption states, so that they increasingly lower the nitrogen oxide storability of the nitrogen oxide storage.
From time to time, desulfating phases are performed as a remedy, in which the nitrogen oxide storage is subjected to suitable desulfating conditions, by which intercalated sulfur can be released again. These desulfating conditions typically include the setting of a rich exhaust gas composition and an increased nitrogen oxide storage temperature of over 600° C., e.g., above 650° C., for a sufficient desulfating time which is longer than typical nitrogen oxide desorption time, as described, for example, in European Published Patent Application No. 0 869 263, European Published Patent Application No. 0 899 430, German Published Patent Application No. 197 31 624 and German Published Patent Application No. 197 47 222. The desulfating process is accompanied by a corresponding increase in fuel consumption, because of the requisite rich air ratio and the possibly required measures for heating up the nitrogen oxide storage. In addition, during the setting of the rich air ratio, the effect of a three-way catalyst, frequently used in emission control systems, which in this case, if necessary, can simultaneously function as nitrogen oxide storage, is limited, since its point of optimal functioning occurs at the stoichiometric air ratio. Besides that, after desulfating by nitrogen oxide storage heating up, its cooling down during change to an operating state having lower engine load takes longer, so that one can change only at a later time to fuel-saving lean-combustion operation.
Since the nitrogen oxide storage temperature is an important parameter for desulfating processes, it should be determined as accurately as possible. This can be done using a sensor or with the aid of a mathematical model, the latter involving a model which can be adapted by a temperature sensor, as described in German Published Patent Application No. 197 52 271.
During a desulfating process, there is the danger of a noticeable emission of sulfur compounds, particularly SO
2
and H
2
S. A desulfating method, in which the formation and emission of undesired amounts of H
2
S is avoided, is described in German Published Patent Application No. 199 20 515. It is conventional that, in desulfating in the case of only a slightly rich mixture, e.g., with &lgr;=0.99, less hydrogen sulfide is formed than with setting a smaller air ratio of, for example, &lgr;=0.9. This basically makes desirable the setting of air ratios slightly below the stoichiometric value &lgr;=1 for the desulfating phases, the further advantage being achieved thereby that, using such air ratios at equal additional heating measures, higher nitrogen oxide storage temperatures are reached than with smaller air ratios, and the only very weak enrichment leads to a substantially negligible additional fuel consumption and an only slight deterioration of the effect of a possibly present three-way catalyst.
In full load operating conditions of the combustion equipment, particularly in the case of an internal combustion engine, usually a clearly rich air ratio of, e.g., &lgr;=0.9 is set, for the p
Daimler-Chrysler AG
Denion Thomas
Kenyon & Kenyon
Nguyen Tu M.
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