Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2002-08-09
2004-03-16
Tran, Binh (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S276000, C060S295000, C060S297000
Reexamination Certificate
active
06705078
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and a device for controlling a NO
x
storage catalyst which is arranged in an exhaust gas channel of an internal combustion engine by means of a NO
x
-sensitive measuring device, a particular a NO
x
sensor, that is arranged downstream of the NO
x
storage catalyst.
It is known to place NO
x
storage catalysts in an exhaust channel of internal combustion engines for post-processing exhaust gases of temporarily lean-running internal combustion engines. These NO
x
storage catalysts store nitrogen oxides present in the exhaust gas during lean operating phases of the internal combustion engine. The NO
x
storage catalyst is periodically subjected to a NO
x
regeneration to restore the initial NO
x
storage capacity, for which purpose the catalyst is exposed to a rich exhaust gas atmosphere. Desorbed NO
x
is reduced by the reducing exhaust gas components carbon monoxide CO and non-burned hydrocarbons HC to nitrogen N
2
. It is also in necessary to perform after extended time intervals a desulphurization of the NO
x
storage catalyst to remove unwanted sulfate deposits. Desulphurization is typically also performed by exposing the catalyst to a rich exhaust gas while the temperature is adjusted to above 600° C.
It is also known to control NO
x
storage catalysts, in particular the NO
x
regeneration and desulphurization of these catalysts, with NO
x
sensors located downstream of the NO
x
storage catalyst. NO
x
breakthrough can be detected and a NO
x
regeneration can be initiated by measuring the NO
x
concentration after the catalyst.
EP 0 916 941 A describes a method for detecting a NO
x
storage activity of a NO
x
storage catalyst with a downstream NO
x
sensor. After the internal combustion engine is switched into a lean mode, a signal that depends on the NO
x
concentration is measured and its curve form dependence analyzed to determine a reference time for monitoring the storage activity. As a criterion for determining the reference time an absolute minimum of the signal form within a predetermined wait time after switch-over of the internal combustion engine is used. In particular, the height of the signal of the NO
x
sensor is used to correct the offset of the sensor. WO 00/76636 describes a method for calibrating a NO
x
sensor, wherein likewise a minimum in the signal form after switch-over of the internal combustion engine into a lean mode is used for a correction of the offset.
The NO
x
storage catalyst can be controlled even more precisely if the NO
x
regeneration and desulphurization time intervals are not matched to an actual NO
x
concentration after the NO
x
storage catalyst, but to a quantity of NO
x
that had passed through the NO
x
storage catalyst during a lean phase of the internal combustion engine, wherein the NO
x
quantity can be determined by integrating the measured NO
x
concentration. This concept has the disadvantage that the time at which NO
x
is actually stored again after a regeneration phase, is unknown. The time at which the internal combustion engine is switched into the lean mode typically forms the basis for starting the NO
x
storage phase. This point in time, however, represents only a coarse approximation of the actual start of the storage process. For example, a time delay is ignored which represents the distance traveled by the exhaust gas between the internal combustion engine and the NO
x
storage catalyst. At the start of the lean phase, the NO
x
storage catalyst can also be in a temperature range which prevents a reliable storage of nitrous oxides. Another disadvantage is the typical response time associated with modern NO
x
sensors, which can cause a delay in the signal change after the NO
x
concentration in the vicinity of the sensor has changed. As a result, integration of the NO
x
mass flow typically starts too early, which entails a premature start of the regeneration, which can then adversely affect the fuel consumption. Many control processes of the NO
x
storage catalyst suffer from the problem that the temporal relationship between an operating point of the internal combustion engine and the behavior of the catalyst is not known.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to propose a method and a device for controlling a NO
x
storage catalyst of the aforedescribed type, which takes into account a temporal correlation between an operating state of the internal combustion engine and the measurement from a NO
x
sensor located after the NO
x
storage catalyst.
The object is solved by the characterizing features of the method of claim
1
and the device of claim
17
. According to the method of the invention, it is proposed to perform an analysis of a signal form of the NO
x
sensitive measuring device after the internal combustion engine is switched into a lean operating mode, for example, after regeneration of the NO
x
storage catalyst, with respect to at least one preset criterion. The time at which at least one of the preset criteria is satisfied, is defined as a reference time for controlling NO
x
regeneration and/or desulphurization processes of the NO
x
storage catalyst.
According to the invention, at least one of the preset criteria is a transition of a decreasing signal form to a curve with a slope of approximately zero or a positive slope of the signal form and/or a drop of the signal below a predetermined threshold value. A threshold value can be predetermined for the slope. The first criterion corresponds essentially to the presence of a minimum in the signal form of the measuring device, which can, of course, also be determined by other methods for determining a minimum, for example, by a comparison of the magnitude of sequentially measured signal values. The criteria should be preset in such a way that under standard conditions of catalyst and measuring device a reference time can always be determined, so that the control of the NO
x
storage catalyst can be enabled.
According to an advantageous embodiment, the signal form of the measuring device for determining the reference time is analyzed immediately at the time when the internal combustion engine is switched into a lean operating mode or only after a short delay. Starting the analysis of the signal form can also require that the NO
x
storage catalyst is in a temperature range that is suitable for storing NO
x
.
According to a particularly advantageous embodiment, the criterion to be satisfied can include a simultaneous drop of the signal below a predetermined threshold value instead of or in addition to a slope of approximately zero or a positive slope of the signal form.
Advantageously, one of the preset criteria may be that the signal level remains below a predetermined threshold value for a predetermined minimum time interval before the signal form is analyzed. This criterion can take into account signal forms which, after a regeneration of the NO
x
storage catalyst, have already a relative constant or low level when the internal combustion engine is switched into a lean operating mode, with the level subsequently slowly increasing.
Another criterion can be preset, whereby the measured signal drops below a predetermined threshold value during the analysis. Optionally, this may be required for a certain minimum time duration.
Regardless which of the criteria is/are selected, it can be particularly advantageous with a low signal-to-noise ratio to smooth the signal form before the analysis. This can be done, for example, by averaging adjacent signal values or by other known methods.
According to another embodiment of the invention, a maximum time interval can be defined after the start of the analysis of signal form, wherein if a reference time could not be successfully established based on the aforedescribed criteria after the maximum time interval, then the end time of the time interval is used as the reference time. According to an alternative embodiment, if the time interval has passed unsuccessfully, a renewed regeneration of the NO
x
storage catal
Hahn Hermann
Hinze Sören
Norris & McLaughlin & Marcus
Tran Binh
Volkswagen Aktiengesellschaft
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