EP1183454B1 - METHOD OF CONTROLLING AN NOx STORAGE CATALYST - Google Patents

Abstract

The invention relates to a method of controlling the regeneration of at least one NOx catalyst that is mounted in the exhaust gas duct of an internal combustion engine. According to the inventive method, an NOx concentration of an exhaust gas is detected by an NOx sensor that is arranged downstream of the NOx storage catalyst. The inventive method is characterized by the following steps: (a) comparing the measured NOx concentration downstream of the storage catalyst with an NOx theoretical concentration and determining a comparative value that corresponds to a deviation of said NOx concentration from the NOx theoretical concentration; (b) taking a regenerative measure when the comparative value exceeds a predetermined threshold value, said regenerative measure influencing at least temporarily at least one operational parameter of the internal combustion engine (14); (c) selecting the regenerative measure depending on a regenerative measure that has optionally taken place before.

Description

The invention relates to a method for controlling a regeneration of at least one NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine with the features mentioned in the preamble of claim 1.

It is known to clean an exhaust gas of the internal combustion engine with the aid of suitable catalysts which are arranged in the exhaust gas duct of the internal combustion engine. During a combustion process, gaseous pollutants are generated which can be divided into reducing agents such as CO, HC or H ₂ or oxidizing agents such as NO _X or SO _X. The reducing agents are oxidized on the catalyst with oxygen, while the oxidizing agents are reacted with the reducing agents.

The internal combustion engine can be operated in different working modes. If there is an excess of oxygen over a fuel in the fuel-air mixture to be burned, then λ> 1 (lean working mode). Under stoichiometric conditions, λ = 1, while in an excess of fuel over oxygen the lambda value is less than 1 (rich working mode). In the rich working mode, incomplete combustion leads to an increase in reducing agents, whereas their share of exhaust gas in a lean atmosphere drops significantly. Nevertheless, if the oxygen concentration is sufficient, an at least partial conversion of the reducing agents on the catalyst is also possible under a rich atmosphere.

In contrast, the reduction of NO _X or SO _{X is} only possible to a sufficient extent if λ ≤ 1. In order to keep a NO _X emissions still low as possible, is known to arrange in the exhaust passage a NO _x storage, which can also be combined with the catalyst for a NO _x storage catalyst. At λ> 1, the NO _X storage catalytic converter absorbs the NO _X as nitrate, until it does so a NO _x desorption temperature and / or a NO _x storage capacity is exceeded.

It is therefore known to regenerate the NO _x storage catalytic converter in regular cycles (NO _x regeneration). During the NO _X regeneration, there is a brief change to a working mode with λ ≤ 1. Under such conditions, the stored NO _X is desorbed again and reduced on the catalyst with the reducing agents. A need for regeneration can be made dependent on individual operating parameters that can be measured directly by suitable measuring devices. For example, it is known from DE 197 29 676 A1 to initiate the regeneration as a function of certain limit temperatures. A more complex model for such a control can be found in DE 197 16 275 C1. Among other things, the current driving situation, a loading condition and a current lambda value can be taken into account. Furthermore, the conditions can be linked to certain parameters during the regeneration. It is known from DE 197 44 409 A1 to change the duration of a regeneration cycle as a function of an oxygen storage capacity of the catalyst and / or a storage capacity of the NO _x store. From DE 198 30 829 C1 it is also known to control the masses of reducing agent necessary for the regeneration of the NO _x storage catalytic converter as a function of a signal detected by a NO _x sensor.

In the course of operating the NO _x storage catalytic converter, however, various types of damage can impair its functionality with regard to the NO _x storage capacity and catalytic activity. Such damage can affect both the catalyst and the accumulator and is either reversible or irreversible in nature.

Reversible damage can occur, for example, by covering the catalyst with a condensate or soot. In addition, when sulfurous fuel is burned, SO _{x is} formed, which, like NO _x , is absorbed by the storage. Storage of the SO _x as sulfate leads, among other things, to a reduction in the NO _x storage capacity, but can also lead to damage to the catalyst in the long term, for example due to sulfite formation. In addition, sulfate grain formation during absorption can lead to tensions within the structure of a NO _x storage catalytic converter, which can result in irreversible damage to the NO _x storage catalytic converter.

It is therefore known, depending on the need for desulfurization, to desulfurize the NO _x storage catalyst (SO _x regeneration). Due to the higher thermodynamic stability of the sulfate compared to the nitrate, higher temperatures are necessary for this. The need for desulfurization can be determined on the basis of parameters characterizing a current catalyst state, such as a degree of sulfurization, a current lambda value or a catalyst temperature. The degree of sulfurization can be determined, for example, as a function of a NO _x concentration downstream of the NO _x storage catalytic converter. Component-specific sensors such as NO _x sensors are suitable for detecting the NO _x concentration. A method can be found in DE 198 37 074 A1 with which a regulation of an air-fuel ratio is influenced during a desulfurization phase. An actual / target comparison of an oxygen concentration during the desulfurization is carried out and a fuel signal generated from this is used to redefine the fuel concentration supplied during the desulfurization.

In general, fuel consumption of the internal combustion engine in the rich operating mode is higher than that of the lean operating mode. As a result of this and, if appropriate, an additional heating of the exhaust gas, there is increased fuel consumption during NO _X or SO _X regeneration. In the course of progressive efforts to reduce the fuel consumption of internal combustion engines, the longest possible operation with λ> 1 is desirable.

A disadvantage of the known methods is poor coordination of a sequence of individual regeneration measures. As a result, increased fuel consumption can occur and it is not possible to select the regeneration measure in accordance with a current catalytic converter condition.

The object of the method according to the invention is to continuously monitor a functionality of the NO _x storage catalytic converter with regard to the NO _x storage capacity and the catalytic activity and to adapt the regeneration measure to the current catalytic converter condition.

• (a) die gemessene NO_X-Konzentration nach dem Speicherkatalysator mit einer NO_X-Sollkonzentration verglichen wird und entsprechend einer Abweichung dieser NO_X-Konzentration von der NO_X-Sollkonzentration ein Vergleichswert bestimmt wird,
• (b) beim Überschreiten des Vergleichswertes über einen vorgebbaren Schwellenwert eine Regenerationsmaßnahme ergriffen wird, bei der wenigstens ein Betriebsparameter der Verbrennungskraftmaschine zumindest temporär beeinflusst wird und
• (c) die Regenerationsmaßnahme in Abhängigkeit von einer gegebenenfalls zuvor durchgeführten Regenerationsmaßnahme ausgewählt wird,

ist es möglich, den Ablauf aufeinanderfolgender Regenerationsmaßnahmen zu koordinieren und dem aktuellen Katalysatorzustand anzupassen.According to the invention, this object is achieved by a method for controlling a regeneration of at least one NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine with the features of claim 1. As a result of that

• (a) comparing the measured NO _x concentration downstream of the storage catalyst with a NO _X -Sollkonzentration and is determined according to a deviation of the NO _x concentration of the NO _X -Sollkonzentration a comparison value,
• (b) a regeneration measure is taken when the comparison value exceeds a predefinable threshold value, in which at least one operating parameter of the internal combustion engine is influenced at least temporarily and
• (c) the regeneration measure is selected as a function of a regeneration measure that may have been carried out beforehand,

it is possible to coordinate the sequence of successive regeneration measures and to adapt them to the current state of the catalyst.

In a preferred embodiment of the method, the comparison value is determined again during or after the regeneration measure. If the comparison value is exceeded, at least one further regeneration measure is taken. In addition, it is conceivable to vary a content of reducing agents in the exhaust gas during the NO _x or SO _x regeneration depending on the comparison value. This allows the current catalytic converter state to be taken into account in a simple manner.

It is also advantageous that if the comparison value is exceeded above the threshold during NO _x regeneration, that is to say in a rich or stoichiometric atmosphere, a regeneration measure is subsequently taken which leads to an increase in the exhaust gas temperature without changing to the lean working mode. If the comparison value is still above the threshold value, then the catalyst component of the NO _x storage catalytic converter is damaged.

It is also advantageous to record the comparison value after a change in a lean atmosphere. On the basis of a first NO _x regeneration, after each regeneration measure it is checked again whether the comparison value is above the threshold value and, if necessary, a further regeneration measure is taken. It has proven to be advantageous here, the first NO _x regeneration first a renewed NO _x regeneration and then an increase in Exhaust temperature to follow. If the comparison value is still too high, the SO _X regeneration can be initiated, which may be repeated. If the comparison value remains above the threshold value even after renewed SO _X regeneration, the NO _X storage catalytic converter is highly likely to be irreversibly damaged.

Further preferred embodiments of the invention result from the other features mentioned in the subclaims.

Figur 1

eine Anordnung eines Katalysatorsystems in einem Abgaskanal einer Verbrennungskraftmaschine;

Figur 2

ein Blockschaltbild für eine Steuerung einer Regeneration eines NO_X-Speicherkatalysators in einem fetten Arbeitsmodus und

Figur 3

ein Blockschaltbild für eine Steuerung einer Regeneration eines NO_X-Speicherkatalysators in einem mageren Arbeitsmodus.

The invention is explained in more detail below in exemplary embodiments with reference to the associated drawings. Show it:

Figure 1

an arrangement of a catalyst system in an exhaust duct of an internal combustion engine;

Figure 2

a block diagram for controlling a regeneration of a NO _x storage catalyst in a rich working mode and

Figure 3

a block diagram for controlling a regeneration of a NO _x storage catalyst in a lean working mode.

An arrangement of a catalyst system 10 in an exhaust gas duct 12 of an internal combustion engine 14 is shown schematically in FIG. The catalyst system 10 comprises a NO _x storage catalytic converter 16 and a pre-catalytic converter 18 as well as the temperature sensors 22.

Furthermore, gas sensors 19, 20, 21 are assigned to the catalytic converter system, which can detect a gas content of a gas component in the exhaust gas. Lambda sensors or component-specific sensors are suitable, for example. In this case, the gas sensor 21 enables at least detection of a NO _x concentration and is therefore a NO _x sensor.

A working mode of the internal combustion engine 14 can be regulated by means of an engine control unit 24. If, for example, a working mode with λ <1 (rich atmosphere) is desired, an oxygen concentration in an intake manifold 26 must be reduced before a fuel-air mixture is combusted. This simultaneously increases the content of reducing agents such as CO, HC and H ₂ in the exhaust gas compared to an oxygen content. For example, such a working mode can be set by reducing a volume flow of intake air by means of a throttle valve 28 and by simultaneously supplying low-oxygen exhaust gas via an exhaust gas reflux valve 30.

Gaseous pollutants such as CO, HC, NO _X or SO _{X are} formed in varying proportions during a combustion process. The reducing agents can be oxidized by oxygen on the pre-catalyst 18. With sufficient oxygen concentration, a complete or extensive implementation is possible under almost all working modes.

In a working mode with λ> 1 (lean atmosphere) in addition to NO _X and SO _X absorbed in the NO _X storing catalyst 16, while the low levels of reducing agents are reacted almost completely, at least at low space velocity in the primary catalytic converter 18th Depending on a NO _x storage capacity and a NO _x desorption temperature of the NO _x storage catalytic converter 16, the internal combustion engine 14 must be operated with λ 1 1 for NO _x regeneration. In such a working mode, the previously absorbed NO _{x is} reduced on a catalytically active surface of the NO _x storage catalytic converter 16.

Likewise absorbed SO _X is stored in the form of sulfate in the NO _X storage catalytic converter 16, although reversibility of this storage process, in contrast to the storage of NO _X, requires significantly higher temperatures. A minimum desulfurization temperature and a lambda value ≤ 1 must therefore be available for desulfurization (SO _X regeneration parameter). By at least temporarily influencing at least one operating parameter of the internal combustion engine 14, an exhaust gas temperature can be increased and the NO _x storage catalytic converter 16 can be heated to the minimum temperature.

With the help of the gas sensor 21, a NO _x concentration in the exhaust gas behind the NO _x storage catalytic converter 16 is continuously detected. This concentration is compared with a target NO _x concentration and a comparison value is formed in accordance with a deviation of the concentration from the target concentration. If the comparison value exceeds a predefinable threshold value, a regeneration measure is taken. The regeneration measure includes a SO _x regeneration (desulfurization), NO _x regeneration or an increase in the exhaust gas temperature in order to eliminate an occupation of the storage catalytic converter with condensate or soot. Important when making a selection the regeneration measure is to match this to a regeneration measure that may have been carried out beforehand. In this way, individual regeneration measures can be coordinated and control of the regeneration of the storage catalytic converter 16 can be carried out much more effectively. If the comparison value continues to exceed the threshold value after the regeneration measure has been carried out, a further regeneration measure is selected and carried out.

FIG. 2 shows a block diagram for controlling the regeneration of the NO _x storage catalytic converter 16 in a rich atmosphere. A NO _x regeneration of the NO _x storage catalytic converter 16 can already be carried out in such an atmosphere. If the comparison value in a comparison 40 is below the threshold value, no further measure (42) is taken. However, if the comparison value reaches the threshold value, an operating parameter of the internal combustion engine 14 is influenced at least temporarily in such a way that the exhaust gas temperature is increased (exhaust gas temperature increase 44). A comparison 40 of the comparison value and the threshold value is then carried out again. If the comparison value continues to exceed the threshold value, irreversible damage to the NO _x storage catalytic converter 16 can be concluded.

In addition to detecting the NO _x concentration during the NO _x regeneration, this can be done in a lean atmosphere, as shown in a block diagram according to FIG. 3. First, as already explained, comparison 40 of the comparison value with the threshold value takes place and, if necessary, NO _x regeneration 48 is initiated as a regeneration measure. If, in a subsequent comparison 40, the comparison value is again greater than the threshold value, a second NO _X regeneration 50 is initiated. After the second NO _x regeneration 50 has ended, the comparison value and threshold value are again compared. If the regeneration measure was successful, no further measures 42 are taken. Optionally, an adaptation 52 of a content of reducing agents in the exhaust gas during the NO _x regeneration can be carried out as a function of the previously detected comparison value, for example to compensate for minor irreversible damage to the NO _x storage catalytic converter 16.

However, if the comparison value 40 continues to exceed the threshold value, the NO _x storage catalytic converter 16 is subjected to an exhaust gas temperature (exhaust gas temperature increase 44) that is elevated. Perform this too Regeneration measure not to reduce the comparison value, a first SO _X regeneration 54 is initiated and possibly also a second SO _X regeneration 56 subsequently. A comparison 40 of the comparison value and threshold value then takes place again. If the comparison value is again below the threshold value, an adjustment of the content of reducing agents during the SO _x regeneration can be carried out analogously to the method during the NO _x regeneration (adaptation 58).

If the entire regeneration measures taken do not lead to a reduction in the comparison value below the threshold value, irreversible damage to the NO _x storage catalytic converter 16 can be concluded. In addition, a possible extent of this damage can be determined from the position of the comparison value, on which any maintenance work can be made dependent.

Claims (10)

1. Method for controlling regeneration of at least one NO_X storage catalytic converter arranged in an exhaust pipe of an internal combustion engine, in which an NO_X concentration of an exhaust gas is recorded by an NO_X sensor arranged downstream of the NO_X storage catalytic converter, characterized in that

   (a) the measured NO_X concentration downstream of the storage catalytic converter is compared with a desired NO_X concentration, and a comparison value is determined as a function of a deviation in this NO_X concentration from the desired NO_X concentration,

   (b) if the comparison value exceeds a predeterminable threshold value, a regeneration measure is taken, which involves at least temporarily influencing at least one operating parameter of the internal combustion engine (14), and

   (c) the regeneration measure is selected as a function of a regeneration measure which has previously been carried out.
2. Method according to Claim 1, characterized in that the comparison value is determined again during or after the regeneration measure, and if the comparison value exceeds the threshold value at least one further regeneration measure is taken.
3. Method according to Claim 1 and Claim 2, characterized in that if the comparison value exceeds the threshold value in a lean operating mode of the internal combustion engine (14), an NO_X regeneration is initiated as a regeneration measure.
4. Method according to Claims 1 and 2, characterized in that if the comparison value exceeds the threshold value in a rich operating mode of the internal combustion engine (14), an exhaust-gas temperature is increased as a regeneration measure.
5. Method according to Claim 3, characterized in that if the comparison value again exceeds the threshold value, the NO_X regeneration is repeated as a regeneration measure.
6. Method according to Claim 5, characterized in that if the comparison value again exceeds the threshold value, the exhaust-gas temperature is increased as a regeneration measure.
7. Method according to Claim 6, characterized in that if the comparison value again exceeds the threshold value, an SO_X regeneration is initiated as a regeneration measure.
8. Method according to Claim 7, characterized in that if the comparison value again exceeds the threshold value, the SO_X regeneration is repeated as a regeneration measure.
9. Method according to Claims 4 and 8, characterized in that if the comparison value again exceeds the threshold value, irreversible damage to the NO_X storage catalytic converter can be determined.
10. Method according to one of the preceding claims, characterized in that a level of the reducing agent in the exhaust gas during the NO_X or SO_X regeneration is varied as a function of the comparison value.