DE19923498A1 - Controlling the regeneration of a nitrogen oxides storage catalyst in the exhaust gas channel of an IC engine comprises comparing the measured nitrogen oxides concentration with a set concentration after the storage catalyst - Google Patents

Abstract

Controlling regeneration of a NOx storage catalyst in exhaust gas channel of an IC engine comprises comparing the measured NOx concentration with a NOx set concentration after the storage catalyst and determining a comparison value corresponding to a deviation of this NOx concentration from the set concentration. Process for controlling the regeneration of a NOx storage catalyst in the exhaust gas channel of an IC engine comprises comparing the measured NOx concentration with a NOx set concentration after the storage catalyst and determining a comparison value corresponding to a deviation of this NOx concentration from the NOx set concentration. On exceeding the comparison value over a prescribed threshold value, a regeneration measurement is determined during which an operating parameter of the IC engine is temporarily influenced. The regeneration measurement is selected depending on a regeneration measure optionally carried out before. During and after the regeneration measurement the comparison value is determined again and a further regeneration measurement is determined on exceeding the comparison value via the threshold value.

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 formed 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 become. 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 is λ = 1 while in an excess of fuel compared to oxygen the lambda value is less than 1 (rich working mode). In the fat one Working mode increasingly arise due to incomplete combustion Reducing agents, whereas their share of the exhaust gas in a lean atmosphere is clear sinks. Nevertheless, if the oxygen concentration is sufficient, it is also under fat At least a partial implementation of the reducing agent in the atmosphere Catalyst possible.

In contrast, the reduction of NO _x or SO _{x is} only possible to a sufficient extent at λ ≦ 1. In order to maintain a _NOx emission while a minimum, 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 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.

In the course of operating the NO _x storage catalytic converter, however, various types of damage can impair its functionality with regard to 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, SO _{x is} formed during combustion of sulfur-containing fuel, which, like NO _x , is absorbed by the accumulator. 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 tension 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 a desulfurization requirement, to desulfurize the NO _x storage catalyst (SO _x regeneration). However, 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.

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, 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 individual regeneration measures. As a result, it can increase Fuel consumption come and it is not possible to take the regeneration measure to select according to a current catalyst condition.

Object of the method according to the invention is to provide a functionality of the _NOx - monitor storage catalyst with respect to the NO _x storage capacity and catalytic activity and to adapt the regeneration measure the current state of the catalyst.

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 _x) is the measured NO _x concentration downstream of the storage catalyst with a NO - is compared target concentration, and a comparison value is determined according to a deviation of this NO _x concentration of the NO _x -Sollkonzentration,
• b) when the comparison value is exceeded above a predeterminable threshold value a regeneration measure is taken in which at least one Operating parameters of the internal combustion engine at least temporarily is influenced and
• c) the regeneration measure depending on one, if necessary beforehand regeneration measure is selected,

it is possible to sequence successive regeneration measures coordinate and adapt to the current catalyst condition.

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, depending on the comparison value, to vary the content of reducing agents in the exhaust gas during the NO _x or SO _x regeneration. 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 value 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 to have the first NO _x regeneration followed by another NO _x regeneration and then an increase in the exhaust gas temperature. 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 refinements of the invention result from the remaining ones in the features mentioned in the subclaims.

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

Fig. 1 shows an arrangement of a catalyst system in an exhaust passage of an internal combustion engine;

Fig. 2 is a block diagram for controlling a regeneration of an NO _x storage catalyst in a rich operating mode, and Fig. 3 is a block diagram for controlling a regeneration of an NO _x storage catalyst in a lean operating mode.

In Fig. 1 in schematic fashion an arrangement of a catalyst system 10 is shown an internal combustion engine 14 in an exhaust gas passage 12. The catalytic converter system 10 comprises a NO _x storage catalytic converter 16 and a pre-catalytic converter 18 as well as the temperature sensors 22 .

Further, the catalyst system 10, gas sensors 19, 20, 21 assigned, which can detect a gas content of a gas component in the exhaust gas. Lambda sensors or component-specific sensors, for example, are suitable. The gas sensor 21 enables at least one detection of a NO _x concentration and is thus 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. At the same time, a content of reducing agents such as CO, HC and H ₂ in the exhaust gas increases compared to a content of oxygen. 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 reductants 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 one mode of operation with λ <1 (lean atmosphere) _x and SO _x absorbed in the NO _x storage catalyst in addition to NO 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 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). An exhaust gas temperature can be increased and the NO _x storage catalytic converter 16 can be heated to the minimum temperature by at least temporarily influencing at least one operating parameter of the internal combustion engine 14 .

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 SO _x regeneration (desulfurization), NO _x regeneration or an increase in the exhaust gas temperature in order to remedy an occupation of the storage catalytic converter with condensate or soot. When selecting the regeneration measure, it is important to match it to a previously carried out regeneration measure. 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 circuit 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 NO _x regeneration can be carried out anew as a function of the previously detected comparison value, for example in order 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 ). If this regeneration measure also does not lead to a reduction in the comparison value, a first SO _x regeneration 54 is initiated and, if appropriate, 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 a reducing agent content during 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, then 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. 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, a NO _x concentration of an exhaust gas being detected by a NO _x sensor arranged downstream of the NO _x storage catalytic converter.
characterized in that

• a is compared) is the measured NO _x concentration downstream of the storage catalyst with NO _x -Sollkonzentration and a comparison value is determined according to a deviation of this NO _x concentration of the NO _x -Sollkonzentration,
• b) when the comparison value is exceeded above a predefinable threshold value, a regeneration measure is taken, in which at least one operating parameter of the internal combustion engine ( 14 ) 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.

2. The method according to claim 1, characterized in that during or after of the regeneration measure, the comparison value is determined again and at Exceeding the comparison value above the threshold value at least one further regeneration measure is taken. 3. The method according to claim 1 and claim 2, characterized in that when the comparison value is exceeded above the threshold in a lean working mode of the internal combustion engine ( 14 ), a NO _x regeneration is initiated. 4. The method according to claim 1 and 2, characterized in that when the comparison value is exceeded above the threshold in a rich working mode of the internal combustion engine ( 14 ), an exhaust gas temperature is increased as a regeneration measure. 5. The method according to claim 3, characterized in that when the comparison value is exceeded again above the threshold value as a regeneration measure, the NO _x regeneration is repeated. 6. The method according to claim 5, characterized in that when renewed Exceeding the comparison value above the threshold as Regeneration measure the exhaust gas temperature is increased. 7. The method according to claim 6, characterized in that a So _x regeneration is initiated when the comparison value is exceeded again above the threshold value as a regeneration measure. 8. The method according to claim 7, characterized in that the SO _x regeneration is repeated when the comparison value is again exceeded above the threshold value as a regeneration measure. 9. The method according to claim 4 and 8, characterized in that irreversible damage to the NO _x storage catalytic converter can be determined when the comparison value is exceeded again above the threshold value. 10. The method according to any one of the preceding claims, characterized in that a content of the reducing agent in the exhaust gas is varied during the NO _x or SO _x regeneration as a function of the comparison value.