DE10221568A1 - Method for controlling a NO¶x¶ storage catalytic converter - 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 catalytically clean the exhaust gas. For this purpose, the exhaust gas is at least one catalyst directed the conversion of one or more pollutant components of the exhaust gas performs. Different types of catalysts are known.

Oxidation catalysts promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), while reduction catalysts help reduce nitrogen oxides (NO _x ) in the exhaust gas. Furthermore, 3-way catalysts are used to catalyze the conversion of the three aforementioned components (HC, CO, NO _X ) at the same time. However, the use of a 3-way catalytic converter is only possible if there is a strictly stoichiometric air-fuel ratio at λ = 1.

Among other things, lean-burn internal combustion engines are used to optimize the consumption of motor vehicles. In a fuel-efficient lean-burn operation in which the internal combustion engine is operated with an excess of oxygen, that is to say with λ> 1, a complete 3-way catalytic conversion of NO _{x is} not possible. In internal combustion engines of this type, therefore, NO _x storage catalysts are used which, in addition to a catalytic component, contain a NO _x storage which stores NO _x in the form of nitrate in the lean operating phases. In intermediate rich regeneration phases at λ <1, in which HC and CO are formed, which act as reducing agents, the nitrates are reduced to nitrogen N ₂ . A catalyst, for example a 3-way catalyst, is often connected upstream of the NO _x storage catalyst.

In simple processes for controlling the NO _x storage catalytic converter, a regeneration period is predefined by means of a rich exhaust gas atmosphere. The actual loading state of the NO _x storage catalytic converter and a current regeneration rate thereof cannot be taken into account disadvantageously. Such a procedure therefore harbors the risk that the regeneration period is too short or too long, in the first case an incomplete regeneration of the storage and in the second case an unnecessary increase in fuel consumption and an emission of environmentally harmful reducing agents (HC and CO) are accepted.

Methods are also known in which the regeneration process is monitored with the aid 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. A decreasing proportion of oxygen in the exhaust gas indicates a reduced reductant conversion in the NO _x store and thus an increasing proportion of the reductant in the exhaust gas. In order to avoid breakthroughs in reducing agents, the NO _x regeneration is terminated, that is, the internal combustion engine is switched back to a lean operating mode as soon as the measured oxygen fraction 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 when a certain reductant breakthrough has already occurred. In addition, with these methods, the pipe between the engine and catalytic converter is still filled with rich exhaust gas at the end of regeneration, when the engine leaves regeneration mode. This contributes significantly to an increase in reducing agent breakthrough at the end of regeneration.

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 by means of a NO _x sensor, for example. In the latter case in particular, it may be that the initiation of regeneration is carried out only as required with a corresponding NO _x breakthrough. As already explained above, the NO _X regenerations are carried out in such a way that the regeneration is ended as soon as the signal from the downstream oxygen-sensitive sensors has reached a certain threshold value or a behavioral model for the catalytic converter has determined the time of its complete emptying. Both processes 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 The degree of saturation cannot be completely emptied. A certain amount remains stored nitrates in the coating. The one following the regeneration phase The storage phase of nitrates then runs with reduced effectiveness, whereby the Fuel consumption and emissions of pollutants increased.

The invention is therefore based on the object of providing a method for controlling a NO _x storage catalytic converter which is optimized with respect to the lowest possible reducing agent emissions compared to the prior art and which is too saturated and thus poorer regenerability of the NO _x - Storage catalytic converter avoids.

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

• 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 a NO _x storage catalytic converter of an internal combustion engine with a 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 catalytic converter, the current catalytic behavior with regard to oxygen storage of the oxygen-containing components in the catalytic converter coating is evaluated on the basis of the catalytic converter temperature, the air / fuel mixture (combustion lambda) supplied for regeneration of the internal combustion engine depending on this evaluation is basically determined, and / or
• b) when a predetermined threshold value for the saturation state of the NO _x storage catalytic converter is reached, the regeneration of the NO _x storage catalytic converter is initiated or
• c) if the predetermined threshold value for the saturation state of the NO _x storage catalytic converter is exceeded, at least the subsequent regeneration is prolonged, neglecting a breakthrough of the reducing agent.

The first method 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 in front of the NO _x storage catalytic converter, due to the determination of a combustion lambda, which takes into account the diffusion rate of the oxygen-containing components of the catalyst coating, optimally 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 start of a regeneration phase, the NO _x signal of the NO _x sensor and preferably no timing diagram is preferably used.

According to a particularly preferred embodiment of the method, this becomes Regeneration of air-fuel mixture (combustion lambda) not only at the beginning set during regeneration, but also varies during regeneration as the Conditions for determining the combustion lambda after the start of regeneration can be changeable. This variation can be carried out using known methods become.

The two further method steps of the main claim, which are carried out alternatively, avoid saturation of the NO _x storage catalytic converter, or their effects are compensated for by inventive measures. Therefore, the NO _x storage catalytic converter can usually be completely discharged during the regeneration phase.

The predefined threshold value for the saturation state of the NO _x storage catalytic converter is determined in advance by suitable tests. This provides information about the loading of the NO _x storage catalytic converter with which the NO _x regeneration with normal implementation still leads to an adequate conversion of all stored nitrogen oxides.

The current value for the loading of the NO _x storage catalytic converter is determined by balancing the amounts of NO _x before and after the NO _x storage catalytic converter. 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 can additionally be dependent on further factors, for example catalyst / exhaust gas temperature, exhaust gas mass flow, NO _x raw mass flow, HC content of the lean exhaust gas and the like, which may have to be taken into account as correction values for the threshold value.

If a value related to this threshold value is now reached in driving operation, regeneration can be initiated as a preventive measure, although this would not yet be necessary in terms of the conversion capacity. In this way, saturation of the NO _x storage catalytic converter is prevented.

Alternatively, at least one next NO _X regeneration can be extended explicitly. An increased breakthrough of reducing agents due to the more intensive regeneration is accepted.

Furthermore, reaching the threshold value can be used for diagnosis of the catalyst or other evaluating the current storage capacity of the catalyst Deactivate functionalities in the following period. The withdrawal of this Deactivation can, for example, be made dependent on a certain cumulative Amount of reducing agent or a predetermined minimum number of Regenerative processes.

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

Further preferred refinements of the invention result from the others in the Characteristics mentioned subclaims.

The invention is described below in exemplary embodiments on the basis of the associated Drawings explained in more detail. Show it:

Figure 1 is a schematic diagram of an internal combustion engine with an exhaust system.

FIG. 2 shows time profiles of various exhaust gas parameters during a conventional NO _x regeneration;

3 shows time profiles of various exhaust gas parameters during a NO _X regeneration following an initiation of the regeneration according to the invention to prevent the catalytic converter from being saturated, and FIG

FIG. 4 shows time profiles of various parameters during a exhaust gas NO _X - Regeneration of the present invention by establishing a combustion lambdas at the beginning of the NO _x regeneration.

The internal combustion engine 10 shown in FIG. 1 is followed by an exhaust system 12 . The exhaust system 12 has an exhaust duct 14 , in which a pre-catalytic converter 16 arranged near the engine and a large-volume NO _x storage catalytic converter 18 are located. In addition to the pre-catalytic converter 16 and the NO _x storage catalytic converter 18 , the exhaust gas duct 14 usually has various gas and / or temperature sensors for regulating the internal combustion engine 10 , but not shown here. Is shown in Fig. 1 only a _NOx sensor 20, the downstream of the NO _X storage catalyst 18 is arranged and which delivers a signal U for the _NOX content of NO _X in the exhaust gas. The NO _X sensor 20 is equipped with a lambda measurement function, so that a signal U _{λ which} is dependent on an oxygen component of the exhaust gas is additionally 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 information relating to the operating state of the internal combustion engine 10 is also input into the engine control unit 22 . A control unit 24 is also integrated in the engine control unit 22 . The engine control unit 22 and the control unit 24 influence at least one operating parameter of the internal combustion engine 10 , in particular an air / fuel mixture to be supplied (combustion lambda), as a function of the signals U _NOX and U _{λ of} the NO _X sensor 20 .

FIG. 2 shows the time course of various parameters of the Internal combustion engine10 and the exhaust system12 during a NO_X- Regeneration of the NO_Xstorage catalyst18, which takes place according to the prior art. The internal combustion engine is located first10 in a lean Operating mode in which you have an oxygen-rich air-fuel mixture with λ_M >> 1 fed will (graph100). In this phase, the exhaust gas contains an excess of nitrogen oxides NO_X. through the pre-catalyst16 cannot be fully converted. NO_X therefore in the NO_Xstorage catalyst18 stored, the NO_X- Loading continuously until saturation NO_XMAX increases (graph102). Using a suitable criterion, a time t_A a NO_X- Regeneration need recognized. For example one through which NO_X-Sensor20 detected NO_X-Breakthrough. As a result, the Internal combustion engine10 by influencing the engine control unit22 in a bold operating mode switched with λ_F <1. As a result of the now increased mass flow the reducing agents CO and HC in the exhaust gas will be in the NO_Xstorage catalyst18  stored NO_X desorbed and reduced to nitrogen. A decrease in the NO_X-Loading of the storage catalytic converter18 (Graph102) is however only after a certain time Delay after switching over the internal combustion engine10 to be recorded since Time t_A the exhaust duct14 is still filled with lean exhaust gas, which is initially still the storage catalytic converter18 must happen before the reducing agents reach it. The Course of the NO_XRegeneration is meanwhile carried out with the aid of the NO_X-Sensor20  provided signal U_λ tracked. The signal U_λ (Graph104) behaves the other way round proportional to an oxygen concentration of the exhaust gas downstream of the storage catalyst18, Because with progressive regeneration, the reducing agents in always are consumed to a lesser extent, the signal U rises_λ of the NO_Xsensor20 slowly on. At a time t_e reaches signal U_λ a predetermined threshold value U_SE. whereupon the internal combustion engine10 usually back into a lean one Operating mode with λ_M >> 1 is switched. At the time of 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 engine10 and the storage catalytic converter18, This flows through the storage catalytic converter18that, except for a certain remaining portion stored nitrate is emptied and enters the environment unconverted. The course of the downstream of the catalyst measured concentration of carbon monoxide CO and unburned hydrocarbons HC (graph106) therefore shows t after the end of regeneration_e  another undesirably high increase. The graph102 shows that after regeneration in the NO_Xstorage catalyst18 a residue of nitrates remains, so that the value NO_{X MIN} For the complete discharge of the NO_Xstorage catalyst18 is not reached. this leads to a reduction in the effectiveness of the subsequent storage of NO_X, what on the other hand an increased fuel consumption or a higher emission of Contaminants caused.

In order to prevent the saturation of the NO _x storage catalytic converter and the associated incomplete outsourcing of the nitrates in the regeneration phase, the approach shown in FIG. 3 is followed according to the invention, the course over time of the same parameters as in FIG. 2 and additionally the course of the Signal U _NOX (graph 108 ) of the NO _X sensor 20 is shown for the NO _X emission. The NO _x emission rises steeply with increasing loading of the NO _x storage catalytic converter 18, at the time t _A when a predetermined threshold value NO _{XSE is reached} , which is determined by balancing the _amounts of NO _x before and after the NO _x storage catalytic converter 18 and which is related to the saturation of the NO _x storage catalytic converter 18 , the regeneration of the NO _x storage catalytic converter 18 is initiated. The threshold value NO _XSE is determined experimentally in advance and indicates the point in the NO _X storage _loading at which a complete emptying of the NO _X storage catalytic converter is still possible in a subsequent regeneration _phase . After initiation of regeneration, the NO _X emission drops sharply and remains at a constantly low level. The course of the NO _x loading of the NO _x storage catalytic converter 18 - represented by graph 102 - essentially corresponds to the course according to FIG. 2, since the loading and the emptying are subject to the same mechanisms. However, the graph 102 is at a lower level since the emptying begins at a lower loading state and only ends when the emptying is complete.

. In Fig 4 the inventive method are the same parameters taken into account as in the Figure illustrating 2. To initiate the regeneration of the NO _X -. Storage catalyst 18 at the time t _A, the temperature of the storage catalyst is determined 18 and transmitted to the engine control unit 22, which then switches the internal combustion engine 10 from a lean operating mode with λ _M >> 1 to a rich mode with λ _F <1, the determined catalyst temperature being used to determine 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 can be higher (λ _FT1 ), lower (λ _FT2 ) or equal to the combustion _lambda λ _FT0 , which can be carried out without evaluating the temperature of the NO _x - Storage catalyst 18 would have been set. The setting of a combustion lambda 4 , which takes into account at least the temperature of the NO _x storage catalytic converter 18 as a decisive factor, ensures that the rich exhaust gases at the time t _E at the end of a regeneration phase before the NO _x storage catalytic converter 18 still serve to regenerate the NO _x storage catalyst 18 can be used. A breakthrough of reducing agents can thus be significantly reduced. The comparison of the course of the graph 106 with that in FIG. 2, which represent the concentrations of carbon monoxide CO and unburned hydrocarbons HC downstream of the NO _x storage catalytic converter 18 , shows a sharp reduction in the regeneration-related pollutant emission. REFERENCE SIGN LIST 10 internal combustion engine
12 exhaust system
14 exhaust duct
16 pre-catalytic converter
18 NO _x storage catalytic converter
20 NO _X sensor
22 engine control unit
24 control unit
100 combustion lambda
102 NO _x loading of the NO _x storage catalytic converter
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 loading of the NO _X storage catalytic converter
NO _XMIN Value for complete discharge of the NO _X storage _{catalytic converter}
NO _XSE threshold to initiate NO _X regeneration
t _{A start of} regeneration
t _{E end of} regeneration
U _NOX signal from the NO _X sensor
U _λ signal of the Lambda measurement function of the NO _X sensor
U _{λ SE} threshold value for ending the NO _x regeneration
λ _M Lambda lean value
λ _F , λ _FT0 , λ _FT1 , λ _{FT2 Lambda fat value}

Claims (9)

1. A method for controlling a NO _x storage catalytic converter ( 18 ) of an internal combustion engine with a NO _x sensitive measuring device arranged downstream of the NO _x storage catalytic converter ( 18 ) and with an engine control unit, characterized in that a) at the beginning (t _A ) of the regeneration of the NO _x storage catalytic converter ( 18 ), the current catalytic behavior with regard to oxygen storage of the oxygen-containing components in the catalytic converter coating is evaluated on the basis of the catalytic converter temperature, the air supplied for the regeneration of the internal combustion engine ( 10 ) Fuel mixture (combustion lambda) is basically determined depending on this evaluation, and / or b) a predetermined threshold value (NO _XSE) for the state of saturation of the NO _X storage catalytic converter (18) the regeneration of the NO _X when it reaches - storage catalyst (introduced 18) or c) if the predetermined threshold value (NO _XSE ) for the saturation state of the NO _x storage _{catalytic converter} ( 18 ) is exceeded, at least the subsequent regeneration is prolonged while neglecting a _breakthrough of the reducing agent. 2. The method according to claim 1, characterized in that at the beginning (t _A ) of the regeneration of the NO _x storage catalytic converter ( 18 ) the exhaust gas mass flow, the engine operating point and / or the catalytic converter state are additionally assessed and that the combustion lambda is determined as a function of this assessment , 3. The method according to claim 1 or 2, characterized in that the air-fuel mixture supplied for regeneration (combustion lambda) is additionally varied during the regeneration of the NO _x storage catalytic converter ( 18 ). 4. The method according to any one of claims 1 to 3, characterized in that a NO _X sensor ( 20 ) is used as the NO _X - sensitive measuring device. 5. The method according to any one of claims 1 to 3, characterized in that in addition to the NO _X sensitive measuring device, an oxygen sensitive measuring device is used. 6. The method according to claim 5, characterized in that as oxygen-sensitive Measuring device a lambda broadband or jump probe is used. 7. The method according to any one of claims 1 to 6, characterized in that the catalyst / exhaust gas temperature, exhaust gas mass flow, NO _X raw mass flow, HC content of the lean exhaust gas are taken into account as correction variables of the threshold value (NO _XSE ). 8. The method according to any one of claims 1 to 7, characterized in that reaching the threshold value (NO _XSE ) leads to deactivation of the functionalities evaluating the current storage capacity of the NO _x storage catalytic converter ( 18 ). 9. The method according to claim 8, characterized in that the withdrawal of the Deactivation depends on a defined amount of reducing agent and / or one defined minimum number of regeneration processes.