Catalyst – solid sorbent – or support therefor: product or process – Regenerating or rehabilitating catalyst or sorbent
Reexamination Certificate
2001-01-16
2003-10-14
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Regenerating or rehabilitating catalyst or sorbent
Reexamination Certificate
active
06632764
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and an apparatus for controlling the regeneration of an NO
x
storage converter that is disposed in the exhaust-gas system of an internal-combustion engine and can be operated in an absorption mode and a regeneration mode, with the operating parameters of the internal-combustion engine being changed as a function of the operating state of the NO
x
storage converter.
NO
x
storage converters are primarily used in lean-mix engines. In this type of engine, the so-called lean-mix operation, with a so-called lambda value>1 of the air-fuel mixture, is preferred over a stoichiometric operation, with a lambda value=1, or a so-called rich-mix operation, with a lambda value<1, because a significantly lower fuel consumption can be attained with a surplus of air in the air-fuel mixture, i.e., with a lambda value>1. Stratified-charge engines represent a special kind of lean-mix motor. In the stratified-charge operation of a stratified-charge engine, a lean air-fuel mixture is supplied to the engine, and an ignitable, rich air-fuel mixture is made available in the combustion chamber in the vicinity of the spark plug, while a lean mixture is present in the remainder of the combustion chamber. The spark plug first ignites the rich air-fuel mixture, which then ignites the lean mixture. The 3-way converter conventionally used up to this point, however, cannot single-handedly reduce the exhaust gases that are formed in the operation of a lean-mix engine to harmless gas components, because it requires the supply of air and fuel in a precisely-defined stoichiometric ratio. An NO
x
converter, in contrast, can absorptively store nitrogen oxides for a limited time under certain marginal conditions, with a lambda value>1, and re-release them at a later time, with a lambda value<1 or =1, and reduce them to harmless gases. In a stratified-charge engine, the NO
x
converter is operated in a so-called absorption mode during stratified-charge operation. In the homogeneous operation of the engine, in contrast, a stoichiometric or rich air-fuel mixture is supplied, and the NO
x
storage converter is operated in a so-called regeneration mode. Usually, NO
x
storage converters operate in storage cycles, which encompass at least one relatively slow absorption mode and a faster regeneration mode.
The function and efficiency of an NO
x
storage converter depend on numerous influential factors, and can particularly be subjected to reversible and irreversible damage. Reversible damage can be caused by, for example, a thionation of the converter, which notably leads to a reduction in the NO
x
storage capacity or the creation of mechanical stresses in the converter. Thermal damage, such as the sintering of a converter component, the separation of converter and storage components, or an increasingly inhomogeneous, near-surface NO
x
charge, cause irreversible damage to the NO
x
converter. Thermal damage typically results in not only a reduction in the NO
x
storage capacity, but also a reduced oxygen storage capacity of the converter. Production-related variations in properties, along with these operation-related types of damage, can also influence the efficiency and function of the converters. Under certain marginal conditions, reversible damage to an NO
x
converter can be at least partially remedied through regeneration measures during driving operation. For example, desulfurization can be achieved through a temporary increase in the exhaust-gas temperature. With the occurrence of irreversible damage, however, the only possible regeneration measure is to adapt the operating parameters of the engine and/or the exhaust-gas system, thereby taking into account the altered efficiency of the exhaust-gas purification.
DE 196 07 151 C1 discloses a method for regenerating an NO
x
storage converter, in which a regeneration phase is initiated as a function of an operating state of the NO
x
storage converter. The operating state corresponds to at least one limit quantity of NO
x
compounds that are emitted by the NO
x
storage converter. The emitted quantity of NO
x
compounds is ascertained from the signal of a lambda sensor disposed upstream of the NO
x
storage converter. It is impossible, however, to reliably ascertain the operating state of the NO
x
storage converter, particularly the degree of damage, so the control of the regeneration is correspondingly imprecise.
EP 0936349 A2 discloses a system for diagnosing an NO
x
converter that is connected to an internal-combustion engine, and in which the signals of an NO
x
-sensitive sensor disposed behind the converter are evaluated for assessing the extent of the damage. This document does not, however, describe a control of the regeneration of the NO
x
storage converter. In this connection, EP 0936349 A2 further discloses a reduction in the NO
x
concentration after the switch to a rich air-fuel mixture. The NO
x
concentration reaches a minimum after a certain time in order to increase subsequently to higher values, and to finally attain a value again that it had attained prior to the switch to an oxygen deficiency. In the known system, the state of the NO
x
storage converter, or the damage thereto, is ascertained from the rate of change in the NO
x
concentration after the minimum has been reached. This requires the use of values of the NO
x
concentration within a relatively large time interval after the switch to an oxygen deficiency, which results in a correspondingly long diagnosis period. A further drawback is that the rate of change of the NO
x
concentration in the used time interval is a function of the operating parameters of the engine and the exhaust-gas system, and therefore requires complex corrective measures.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a method and an apparatus for controlling the regeneration of an NO
x
storage converter that is essentially based on the assessment of values of the NO
x
concentration within a relatively short time interval, and a relatively fast, simple determination of the operating state of the NO
x
storage converter for achieving an optimum regeneration.
This object is accomplished with the features of the independent claims.
The invention is based on the realization that, when the NO
x
storage converter switches from an absorption mode to a regeneration mode within a short time interval, only a portion of the released NO
x
is catalytically converted. The unconverted portion of the NO
x
causes a temporary increase in the NO
x
concentration in the exhaust gas, the so-called desorption peak. Characteristic properties of this peak, such as duration, height or the like, relate to the function of, or, if applicable, the damage to the NO
x
converter. In accordance with the invention, the NO
x
concentration in the exhaust gas is measured downstream of the NO
x
storage converter, and for determining the operating state of the NO
x
storage converter in a transition of the NO
x
storage converter from the absorption mode to the regeneration mode, the values of characteristic features of an NO
x
desorption peak are ascertained in the time curve of the NO
x
concentration, then compared to predetermined test patterns; in the process, a comparison result is formed, and a converter-state signal that characterizes the operating state of the NO
x
storage converter is derived from the comparison result. Depending on the converter-state signal, a change is made to the operating parameters, which includes implementing a regeneration measure for attaining an optimum regeneration of the NO
x
storage converter. Because the NO
x
desorption peak occurs within a relatively short time interval, for example after the transition from a lean to a rich or stoichiometric air-fuel mixture, it is possible to ascertain the operating state in a relatively short time interval. In an ideal case, the duration of a single NO
x
desorption peak is sufficient. The ascertainment of values of characteristic features of the
Drückhammer Jens
Lang Axel
Schulze Frank
Johnson Edward M.
Rabin & Berdo PC
Volkswagen AG
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