EP1365131B1 - Method for controling a NOx storage catalyst - Google Patents

Abstract

Process for controlling a nitrogen oxides storage catalyst in an internal combustion engine comprises evaluating the actual catalyst behavior with respect to oxygen storage of the oxygen-containing component in the catalyst coating at the start of the regeneration of the storage catalyst, and/or regenerating the nitrogen oxides storage catalyst. Process for controlling a nitrogen oxides storage catalyst in an internal combustion engine comprises evaluating the actual catalyst behavior with respect to oxygen storage of the oxygen-containing component in the catalyst coating at the start of the regeneration of the storage catalyst, and/or regenerating the nitrogen oxides storage catalyst when a prescribed threshold value for the saturation state of the catalyst is reached or increasing the regeneration when the prescribed threshold value for the saturation state of the catalyst is reached neglecting introduction of a reducing agent.

Description

The invention relates to a method for controlling a NO _x storage catalytic converter with the features mentioned in the preamble of claim 1.

For the aftertreatment of exhaust gases from internal combustion engines, it is common practice to purify the exhaust gas catalytically. For this purpose, the exhaust gas is passed over at least one catalyst, which performs a conversion of one or more pollutant components of the exhaust gas. Different types of catalysts are known. Oxidation catalysts promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), while reduction catalysts promote the reduction of nitrogen oxides (NO _x ) of the exhaust gas. Furthermore, 3-way catalysts are used to simultaneously catalyze the conversion of the three aforementioned components (HC, CO, NO _X ). However, the use of a 3-way catalyst is only possible if a strictly stoichiometric air-fuel ratio at λ = 1 is present.

In order to optimize the consumption of motor vehicles, inter alia lean combustion engines are used. In a low-consumption lean operation, in which the internal combustion engine with oxygen excess, that is, with λ> 1, driven, a complete 3-way catalytic conversion of NO _{X is} not possible. In such internal combustion engines, therefore, NO _x storage catalysts are used which, in addition to a catalytic component, contain an NO _x reservoir which stores NO _x in the form of nitrate in the lean operating phases. In intermediate fat regeneration phases at λ <1, where HC and CO are formed, which act as reducing agents, the nitrates are reduced to nitrogen N ₂ . Often the NO _x storage catalytic converter is still a catalyst, for example, a 3-way catalyst upstream.

In simple methods for controlling the NO _x storage catalytic converter, a regeneration period is fixed by means of a rich exhaust gas atmosphere. In this case, the actual load state of the NO _x storage catalytic converter and a current regeneration rate of the same can disadvantageously not be taken into account. Such The procedure therefore involves the risk that the regeneration period is too short or too long, in the first case, an incomplete regeneration of the memory and in the second case unnecessary fuel consumption and emission of environmentally harmful reducing agents (HC and CO) is accepted.

Furthermore, methods are known in which the regeneration profile is monitored by means of a sensor system arranged downstream of the NO _x storage catalytic converter in the form of a NO _x sensor or a lambda probe which measures an oxygen content of the exhaust gas. In this case, a decreasing proportion of oxygen in the exhaust gas indicates a reduced reduction agent conversion at the NO _x reservoir and thus increasing proportions of the reducing agent in the exhaust gas. To avoid reducing agent breakthroughs, the NO _x regeneration is stopped, that is, the internal combustion engine is switched back to a lean operating mode as soon as the measured oxygen content falls below a predetermined limit value or a sensor voltage exceeds a corresponding limit voltage. This method has the disadvantage that the sensor can only react if a certain reductant breakthrough already occurs. In addition, in these methods, the pipe between engine and catalyst at the end of the regeneration, when the engine leaves the regeneration mode, still filled with rich exhaust gas. This contributes significantly to an increase in reductant breakdown at the regeneration end.

Various methods are known for initiating the regeneration of the NO _x storage catalytic converter, which are usually based on stored behavior models of the NO _x storage catalytic converter or on emission profiles measured, for example, by means of an NO _x sensor. Especially in the latter case, it may be that the initiation of a regeneration is carried out exclusively on demand with a corresponding NO _x breakthrough. As already stated above, the NO _x regenerations are performed such that the regeneration is terminated as soon as the signal of the downstream oxygen-sensitive sensor system reaches a certain threshold value or a behavioral model for the catalytic converter has determined the time of its complete emptying. Both methods are usually designed so that there is only a small breakthrough of reducing agent at the end of regeneration. In exceptional cases, a small excess of reducing agent can be tolerated for various reasons.

In this procedure, however, a storage catalyst with a very high degree of saturation can not be completely emptied. There remains some residual amount of stored nitrates in the coating. The regeneration phase following Nitrate storage phase then proceeds with reduced efficiency, increasing fuel consumption and emission of pollutants.

The invention is therefore based on the object to provide a method for controlling a NO _x storage catalyst available, which is optimized with respect to the lowest possible reducing agent emission compared to the prior art and which is too high saturation and thus a poorer regenerability of NO _X - Storage catalyst avoids.

This object is achieved by a method having the features mentioned in claim 1.

• (a) zu Beginn der Regeneration des NO_X-Speicherkatalysators das aktuelle Katalysatorverhalten hinsichtlich einer Sauerstoffausspeicherung der sauerstoffhaltigen Komponenten in der Katalysatorbeschichtung auf der Basis der Katalysatortemperatur bewertet wird, wobei das zur Regeneration der Verbrennungskraftmaschine zugeführte Luft-Kraftstoff-Gemisch (Verbrennungslambda) in Abhängigkeit dieser Bewertung grundsätzlich festgelegt wird, und/oder
• (b) bei Erreichen eines vorgegebenen Schwellwertes für den Sättigungszustand des NO_X-Speicherkatalysators die Regeneration des NO_X-Speicherkatalysators eingeleitet wird oder
• (c) bei Überschreiten des vorgegebenen Schwellwertes für den Sättigungszustand des NO_X-Speicherkatalysators mindestens die sich anschließende Regeneration unter Vernachlässigung eines Reduktionsmitteldurchbruches verlängert wird.

The method according to the invention for controlling an NO _x storage catalytic converter of an internal combustion engine with an NO _x -sensitive measuring device arranged downstream of the NO _x storage catalytic converter and with an engine control unit provides that

• (A) at the beginning of the regeneration of the NO _x storage catalyst, the current catalyst behavior is evaluated in terms of oxygen storage of the oxygen-containing components in the catalyst coating on the basis of the catalyst temperature, wherein the regeneration of the internal combustion engine supplied air-fuel mixture (combustion lambda) in response Assessment is determined in principle, and / or
• (B) upon reaching a predetermined threshold value for the saturation state of the NO _x storage catalytic converter, the regeneration of the NO _x storage catalyst is initiated or
• (C) is extended when the predetermined threshold for the saturation state of the NO _x storage catalytic converter is exceeded, at least the subsequent regeneration neglecting a reducing agent breakthrough.

The first process step according to the invention, which takes place at the beginning of a regeneration phase, ensures that the rich exhaust gases located at the end of a regeneration phase upstream of the NO _x storage catalytic converter are optimal due to the definition of a combustion lambda which takes into account the diffusion rate of the oxygen-containing components of the catalyst coating

Regeneration of the NO _x storage catalyst can be used, so that a reducing agent breakthrough is advantageously reduced. Furthermore, the engine operating point, the exhaust gas mass flow and / or the catalyst state, which can be determined by known methods, for example by means of a conversion factor, can be used to determine the combustion lambda. To determine the beginning of a regeneration phase, it is preferable to use the NO _x signal of the NO _x sensor and no time scheme.

According to a particularly preferred embodiment of the method, the air-fuel mixture (combustion lambda) supplied for regeneration is not only determined at the beginning of the regeneration, but also varies during the regeneration, since the conditions for determining the combustion lambda may be variable after the regeneration begins. This variation can be carried out by known methods.

The two further method steps of the main claim, which are carried out alternatively, a saturation of the NO _x storage catalyst is avoided or their effects are compensated by inventive measures. Therefore, as a rule, the NO _x storage catalyst can be completely discharged during the regeneration phase.

The predetermined threshold value for the saturation state of the NO _x storage catalytic converter is determined in advance by suitable tests. This gives information about up to which load of the NO _X storage catalytic converter the NO _x regeneration with normal execution still leads to a sufficient conversion of all stored nitrogen oxides.

The current load value for the NO _X storage catalyst is determined by balancing the NO _X M levels before and after the NO _X storage catalyst. For this purpose, in addition to the signal of the NO _x -sensitive measuring device, for example, the signal of a second NO _X -sensitive measuring device located in front of the NO _x * storage catalytic converter or a corresponding modeling in the engine control unit can be used. The threshold value may additionally be dependent on further factors, for example catalyst / exhaust gas temperature, exhaust gas mass flow, NO _X mass flow, HC content of the lean exhaust gas and the like, which may have to be taken into account as correction values of the threshold value.

If a value in relation to this threshold value is reached while driving, a regeneration can be initiated as a preventive measure, even though this would not be necessary from the conversion performance. In this way, saturation of the NO _x storage catalyst is prevented.

Alternatively, at least one next NO _x regeneration can be explicitly extended. An increased reductant breakthrough due to the more intensive regeneration is accepted.

Furthermore, the achievement of the threshold value can be used to deactivate the diagnosis of the catalytic converter or other functionalities evaluating the current storage capacity of the catalytic converter in the following period. The withdrawal of this deactivation may, for example, be made dependent on a certain cumulative amount of reducing agent or a predetermined minimum number of regeneration processes.

Advantageously, the NO _X -sensitive measuring device is a _NOx sensor, which also provides an oxygen-dependent signal which can be used to monitor the regeration of the NO _X storage catalytic converter. Otherwise, however, an additional oxygen-sensitive measuring device such as a lambda broadband or jump probe can be used to monitor the regeneration phase.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

Figur 1

eine Prinzipdarstellung einer Verbrennungskraftmaschine mit einer Abgasanlage;

Figur 2

zeitliche Verläufe verschiedener Abgasparameter während einer herkömmlichen NO_X-Regeneration;

Figur 3

zeitliche Verläufe verschiedener Abgasparameter während einer NO_X-Regeneration im Anschluss an eine erfindungsgemäße Einleitung der Regeneration zur Verhinderung der Sättigung des Katalysators und

Figur 4

zeitliche Verläufe verschiedener Abgasparameter während einer NO_X-Regeneration gemäß der vorliegenden Erfindung nach Festlegung eines Verbrennungslambdas zu Beginn der NO_X-Regeneration .

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

a schematic diagram of an internal combustion engine with an exhaust system;

FIG. 2

time histories of various exhaust gas parameters during a conventional NO _x regeneration;

FIG. 3

temporal courses of various exhaust gas parameters during a NO _x regeneration following an inventive initiation of the regeneration to prevent the saturation of the catalyst and

FIG. 4

Timing of various exhaust gas parameters during a NO _x regeneration according to the present invention after establishing a combustion lambda at the beginning of the NO _x regeneration.

The internal combustion engine 10 shown in Figure 1 is downstream of an exhaust system 12. The exhaust system 12 has an exhaust gas channel 14, in which a pre-catalyst arranged close to the engine 16 and a large-volume NO _x storage 18 are located. In addition to the precatalyst 16 and the NO _x storage catalytic converter 18, the exhaust gas channel 14 usually has various gas and / or temperature sensors (not shown here) for regulating the internal combustion engine 10. In FIG. 1, only an NO _x sensor 20 is shown, which is arranged downstream of the NO _x storage catalytic converter 18 and which supplies a signal U _NOX for the proportion of NO _x in the exhaust gas. The NO _x sensor 20 is equipped with a Lambdamessfunktion, so that in addition a dependent of an oxygen content of the exhaust gas signal U _{λ is} provided. The signals U _NOX and U _λ are transmitted to an engine control unit 22 in which they are digitized and further processed. Further, the operating state of the internal combustion engine 10 information related also found in the engine control unit 22. In the engine control unit 22, a control unit 24 is also integrated. By means of the engine control unit 22 and the control unit 24, at least one operating parameter of the internal combustion engine 10, in particular an air-fuel mixture (combustion lambda) to be supplied, is influenced as a function of the signals U _NOX and U _{λ of} the NO _x sensor 20.

FIG. 2 shows the time profile of various parameters of the internal combustion engine 10 and of the exhaust system 12 during a NO _x regeneration of the NO _x storage catalytic converter 18, which takes place according to the prior art. First, the internal combustion engine 10 is in a lean mode of operation in which it is supplied with an oxygen-rich air-fuel mixture with λ _M »1 (graph 100). In this phase, the exhaust gas contains an excess of nitrogen oxides NO _x , which can not be completely converted by the precatalyst 16. NO _x is therefore stored in the NO _x storage catalytic converter 18, whose NO _x charge increases continuously to saturation NO _XMAX (graph 102). Based on a suitable criterion, a NO _x regeneration need is recognized at a time t _A. This may be, for example, a NO _x breakthrough detected by the NO _x sensor 20. As a result, the internal combustion engine 10 is switched by influencing the engine control unit 22 in a rich operating mode with λ _F <1. Due to the now increased mass flow the reducing agent CO and HC in the exhaust gas is desorbed in the NO _x storage 18 stored NO _X and reduced to nitrogen. A decrease in the NO _x charge of the storage catalytic converter 18 (graph 102), however, is recorded only after a certain time delay after switching the internal combustion engine 10, since at time t _{A of} the exhaust duct 14 is still filled with lean exhaust gas, which initially still the storage catalytic converter 18 must happen before the reducing agents reach it. The course of the NO _x regeneration is in the meantime tracked with the aid of the signal U _λ provided by the NO _x sensor 20. The signal U _λ (graph 104) behaves inversely proportional to an oxygen concentration of the exhaust gas downstream of the storage catalytic converter 18. As the reducing agents are consumed to an increasing extent as regeneration progresses, the signal U _{λ of} the NO _X sensor 20 increases slowly. At a time t _E , the signal U _{λ reaches} a predetermined threshold value U _SE , whereupon the internal combustion engine 10 is usually switched back into a lean operating mode with λ _M »1. At the time of the regeneration end t _E , however, there is still exhaust gas with a high proportion of reducing agent in the exhaust gas duct 14 between the internal combustion engine 10 and the storage catalytic converter 18. This flows through the storage catalytic converter 18, which is emptied of some stored nitrates, and enters the converter without conversion Environment. The course of the measured downstream of the catalyst concentration of carbon monoxide CO and unburned hydrocarbons HC (graph 106) therefore shows after regeneration end t _E still an undesirably high increase. The graph 102 shows that after regeneration in the NO _x storage catalyst 18, a residue of nitrates remains, so that the value NO _XMIN for the complete discharge of the NO _x storage 18 is not achieved. This leads to a reduction of the effectiveness of the subsequent storage of NO _X , which in turn causes an increased fuel consumption or a higher emission of pollutants.

In order to prevent the saturation of the NO _x storage catalyst and the associated incomplete removal of the nitrates in the regeneration phase, the approach shown in Figure 3 is pursued according to the invention, wherein the time course of the same parameters as in Figure 2 and in addition the course of the signal U _NOX (graph 108) of the _NOx sensor 20 is shown for the NO _X emissions. The NO _x emission increases sharply with increasing loading of the NO _x storage catalytic converter 18, wherein at the time t _A upon reaching a predetermined threshold value NO _XSE , which is determined by balancing the NO _x quantities before and after the NO _x storage catalytic converter 18 and which is in relation to the saturation of the NO _x storage 18, the regeneration of the NO _x storage 18 is initiated. The threshold NO _XSE is determined experimentally in advance and gives the point at the NO _X storage loading, in which a complete emptying of the NO _X storage catalytic converter is still possible in a subsequent regeneration phase. After initiation of the regeneration, the NO _x emission drops steeply and remains at a constantly low level. The course of the NO _x charge of the NO _x storage catalytic converter 18-represented by graph 102-substantially corresponds to the course according to FIG. 2, since the loading and the discharge are subject to the same mechanisms. However, the graph 102 is at a lower level since the emptying begins at a lower loading condition and ends only at complete emptying.

The same parameters as in FIG. 2 are also taken into account in FIG. 4 in order to illustrate the method according to the invention. To initiate the regeneration of the NO _x storage catalytic converter 18, the temperature of the storage catalytic converter 18 is determined at time t _A and transmitted to the engine control unit 22, which subsequently the internal combustion engine 10 of a lean mode with λ _M »1 in a rich mode with λ _F <1 switched, the determined catalyst temperature is used to establish an optimized combustion lambda. Depending on the determined temperature of the NO _x storage catalytic converter 18, a combustion _lambda λ _{F is} set, which may be higher (λ _FT1 ), lower (λ _FT2 ) or equal to the combustion _lambda λ _FT0 , without the evaluation of the temperature of the NO _x Storage Catalyst 18 would have been adjusted. By setting a combustion lambda λ _F , which takes into account at least the temperature of the NO _x storage 18 as a relevant factor, it is ensured that at the time t _E at the end of a regeneration phase before the NO _x storage catalytic converter 18 fat exhaust gases are still for regeneration of the NO _x storage 18 can be used. A reducing agent breakthrough can thus be significantly reduced. Comparing the graph of graph 106 with that in FIG. 2, which illustrates the concentrations of carbon monoxide CO and unburned hydrocarbons HC downstream of NO _x storage catalyst 18, shows a large reduction in pollutant emission due to regeneration.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

NO _X storage catalyst

20

NO _x sensor

22

Engine control unit

24

control unit

100

combustion lambda

102

NO _x charge of the NO _x storage catalyst

104

Signal curve (U _λ ) of the lambda function of the NO _X sensor

106

Reducing agent content in the exhaust gas

108

Signal curve (U _NOX ) of the lambda function of the NO _X sensor

NO _XMAX

Saturation value of the NO _x charge of the NO _x storage catalyst

NO _XMIN

Value for complete discharge of the NO _X storage catalytic converter

NO _XSE

Threshold for the initiation of the NO _x regeneration

t _A

regeneration start

t _E

regeneration end

U _NOX

Signal from the NO _X sensor

U _λ

Signal of the lambda measurement function of the NO _X sensor

U _λSE

Threshold for ending the NO _X regeneration

λ _M

Lambda lean value

λ _F , λ _FT0, λ _FT1, λ _FT2

Lambda fat value

Claims (10)

1. Process for controlling an NO_X regeneration of an NO_X storage catalyst (18) downstream of an internal combustion engine (10), comprising an NO_X-sensitive measuring device (20) arranged downstream of the NO_X storage catalyst (18) and also comprising an engine control unit (24), the regeneration of the NO_X storage catalyst (18) on attainment of a predefined threshold value (NO_XSE) for the state of saturation of the NO_X storage catalyst (18) being initiated by establishment of a rich air-fuel mixture (λ_F) of the internal combustion engine (10), characterized in that, at the start (t_A) of the regeneration of the NO_X storage catalyst (18), a current oxygen release behaviour of the oxygen-containing components in a catalyst coating of the NO_X storage catalyst (18) on the basis of the catalyst temperature is evaluated and the rich air-fuel mixture (λ_F) supplied to the internal combustion engine (10) for regeneration is determined as a function at least of this evaluation.
2. Process according to Claim 1, characterized in that, at the start (t_A) of the regeneration of the NO_X storage catalyst (18), the exhaust gas flow rate, the engine operating point and/or the catalyst state is additionally evaluated, and in that the rich air-fuel mixture (λ_F) is determined as a function of this evaluation.
3. Process according to Claim 1 or 2, characterized in that the rich air-fuel mixture (λ_F) supplied for regeneration is additionally varied during the regeneration of the NO_X storage catalyst (18).
4. Process according to one of Claims 1 to 3, characterized in that the NO_X-sensitive measuring device (20) used is an NO_X sensor.
5. Process according to one of Claims 1 to 3, characterized in that an oxygen-sensitive measuring device is also used in addition to the NO_X-sensitive measuring device (20).
6. Process according to Claim 5, characterized in that the oxygen-sensitive measuring device used is a λ broadband or step-response sensor.
7. Process according to one of Claims 1 to 6, characterized in that catalyst/exhaust gas temperature, exhaust gas flow rate, untreated NO_X flow rate, HC content of the lean exhaust gas are taken into account as correction factors of the threshold value (NO_XSE).
8. Process according to one of Claims 1 to 7, characterized in that the attainment of the threshold value (NO_XSE) leads to the deactivation of the functionalities which assess the current storage capacity of the NO_X storage catalyst (18).
9. Process according to Claim 8, characterized in that the reversal of the deactivation is dependent upon a defined amount of reducing agent and/or a defined minimum number of regeneration operations.
10. Process according to one of Claims 1 to 9, characterized in that the predefined threshold value (NO_XSE) for the state of saturation of the NO_X storage catalyst (18) is such that the regeneration leads to a conversion of all stored nitrogen oxides.

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