DE10057936A1 - Regenerating nitrogen oxides storage catalyst in exhaust gas channel of lean-burn I.C. engine involves using extrapolated oxygen-dependent signal - Google Patents

Abstract

Process for regenerating a NOx storage catalyst (18) in the exhaust gas channel (14) of a lean-burn I.C. engine comprises extrapolating an oxygen-dependent signal from an oxygen-sensitive measuring device (20) during NOx regeneration; determining a time point based on the extrapolated signal which does not exceed a prescribed signal threshold value; and determining the end of the regeneration, in which a time spread corresponding to an exhaust gas running time between the engine and the catalyst is subtracted from the theoretical time point. An Independent claim is also included for a device for regenerating a NOx storage catalyst arranged in the exhaust gas channel of a lean-burn I.C. engine. Preferred Features: The oxygen-dependent signal is extrapolated using actual operating parameters of the engine and/or exhaust gas purification device (12).

Description

The invention relates to a method for performing a regeneration of a NO _x storage catalyst in an exhaust line of an internal combustion engine and a device for performing the regeneration with the features mentioned in the preamble of claims 1 and 11.

Internal combustion engines that are operated at least temporarily in a lean operating mode, that is to say with an oxygen-rich exhaust gas with λ <1, for reasons of optimizing consumption, produce nitrogen oxides NO _x in a stoichiometric excess. As a result, in a catalytic oxidative conversion of unburned hydrocarbons HC and carbon monoxide CO, nitrogen oxides NO _{x are} not completely converted to environmentally neutral nitrogen. To remedy this, it is known to arrange NO _x storage catalytic converters in the exhaust gas channels of internal combustion engines, which store the NO _x as nitrate in lean operating phases. In order to avoid NO _x breakthroughs due to a fully loaded NO _x storage catalytic converter, the NO _x storage catalytic converter must be regenerated at recurring intervals. For this purpose, the internal combustion engine is briefly switched to a rich or stoichiometric working mode (λ ≦ 1). As a result, a mass flow of reducing agent in the exhaust gas increases, the nitrogen oxides stored as nitrate are desorbed and converted catalytically on the NO _x storage catalytic converter with simultaneous oxidation of CO and HC.

In simple processes, a regeneration period during which the rich exhaust gas atmosphere is applied to the storage catalytic converter is predefined. The disadvantage of this is that an actual loading state of the NO _x storage catalytic converter and a current regeneration rate thereof are not taken into account. Such a procedure harbors the risk that the regeneration period is too short or too long, with 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).

Refined processes attempt to estimate an actual loading state of the NO _x storage catalytic converter on the basis of certain operating parameters during the last lean phase and derive a necessary regeneration period from this. However, this method also has considerable inaccuracies, so that unsuitable NO _x regeneration times with the mentioned consequences can also result here.

Furthermore, methods are known in which the regeneration process is monitored with the aid of a sensor system which is arranged downstream of the NO _x storage catalytic converter and measures an oxygen fraction 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. To avoid reductant breakthroughs, 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. At this point in time, the entire exhaust gas path between the internal combustion engine and the NO _x storage catalytic converter is still filled with rich, that is to say exhaust gas containing the reducing agent. These reducing agents (HC and CO) then reach the environment largely unconverted as pollutants.

The invention is therefore based on the object of providing a method for NO _x regeneration of a NO _x storage catalytic converter which is optimized with regard to the lowest possible reducing agent emission and at the same time ensures complete regeneration of the storage catalytic converter. A device for carrying out the method is also to be provided.

This object is achieved by a method and a device with the features mentioned in independent claims 1 and 11. The method according to the invention provides that

• a) during the NO _x regeneration, the oxygen-dependent signal of the measuring device arranged downstream of the NO _x storage catalytic converter is extrapolated and a theoretical point in time is determined on the basis of the extrapolated signal at which a predetermined signal threshold value is exceeded, and
• b) the end of regeneration is determined by subtracting a time period corresponding essentially to an exhaust gas runtime between the internal combustion engine and the NO _x storage catalytic converter from the theoretical point in time.

Thus, by extrapolating the signal from the measuring device, the predefined signal threshold value can be determined in advance with sufficient reliability. This enables the exhaust gas runtime between the internal combustion engine and the NO _x storage catalytic converter to be taken into account, so that the NO _x regeneration can be stopped in good time, that is to say before a reducing agent breakthrough occurs. Overall, the method thus enables a reduction in CO and HC emissions and a minimization of an additional fuel consumption necessary for NO _x regeneration.

According to an advantageous embodiment of the method, the oxygen-dependent signal of the measuring device is extrapolated on the basis of current operating parameters of the internal combustion engine and / or the exhaust system. This can be, for example, a current air / fuel mixture (combustion lambda) fed to the internal combustion engine and / or an exhaust gas mass flow and / or an exhaust gas temperature and / or a catalyst temperature. The accuracy of the extrapolation can be increased further by taking into account a behavior model of the NO _x storage catalytic converter. Such a behavior model can include, for example, the course of a regeneration rate depending on the current reducing agent mass flow and / or the catalyst temperature. The behavior model can also take into account the signal curve measured during the current regeneration. An advantageous further development of the method can also be achieved by extrapolating the signal taking into account a behavior model of the oxygen-sensitive measuring device. In particular, an inertia, that is to say a time delay with which the measuring device displays changed exhaust gas conditions, but also a current temperature of the measuring device measured, for example, via an internal resistance, can be taken into account.

Although it is possible in principle to specify the exhaust gas runtime as a fixed value preferably provided, this on the basis of current operating parameters of the Calculate internal combustion engine. It can be based on known operating parameters such as engine load, speed or exhaust gas temperature or other suitable data be resorted to.

Since the reliability of the extrapolation of the signal curve can be reduced under certain extreme boundary conditions, a preferred embodiment of the method provides for limit values for different operating conditions of the internal combustion engine and / or the exhaust gas system to be specified and for the extrapolation to be suppressed if these limit values are not observed. Limit values for the exhaust gas mass flow and / or for the temperature of the NO _x storage catalytic converter are particularly useful, since the regeneration rates are too inconsistent if the exhaust gas mass flows are too high or the catalyst temperatures are too low to be able to be extrapolated with sufficient reliability. The signal projection can advantageously also be suppressed if there are interfering influences dependent on the operating point and which influence irregular NOX regeneration. This is the case, for example, in the case of a fuel cut-off of the internal combustion engine.

According to a further advantageous embodiment of the method, the Extrapolation not during the entire regeneration period of the Storage catalytic converter, but only after a specified time has elapsed Start of regeneration and / or after throughput of a predetermined exhaust gas mass and / or after a predetermined minimum threshold of the signal of the Measuring device. These measures ensure that the signal curve is already known for a certain minimum duration and is therefore extrapolated more reliably can be. After the projection has started, the signal curve should continue to be followed so that the projection can be updated continuously.

The device according to the invention comprises means by which the described Process steps are executable. The means comprise a control unit in which a Algorithm for controlling the process steps is stored in digital form. This Control unit can advantageously also be integrated into an engine control unit of the vehicle his.  

The oxygen-sensitive measuring device can be a lambda probe arranged downstream of the NO _x storage catalytic converter, in particular a broadband or a step response lambda probe, or a NO _x sensor that has a lambda output signal.

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:

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

Fig. 2 temporal profiles of various exhaust gas parameters during a conventional NO _x regeneration and

FIG. 3 is time profiles of various parameters during a exhaust NO _x - regeneration according to the present invention.

The internal combustion engine 10 shown in FIG. 1 is assigned an exhaust system, designated overall by 12 . The exhaust system 12 comprises an exhaust duct 14 , in which a pre-catalytic converter 16 arranged in a position close to the engine and a large-volume NO _x storage catalytic converter 18 are arranged. In addition to the catalyst system 16 , 18 , the exhaust gas duct 14 usually houses various gas and / or temperature sensors (not shown) for regulating the internal combustion engine 10 . Only an oxygen-sensitive measuring device 20 is shown here, which is arranged downstream of the NO _x storage catalytic converter 18 . The measuring device 20 can be, for example, a lambda probe or a NO _x sensor which is equipped with a lambda measuring function. In any case, the measuring device 20 provides a signal U _λ that is dependent on an oxygen component of the exhaust gas. This signal U _λ is transmitted to an engine control unit 22 , in which it is 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 integrated in the engine control unit 22 , in which an algorithm for carrying out the method for NO _x regeneration of the NO _x storage catalytic converter 18 is stored. The engine control unit 22 and the control unit 24 are capable of influencing 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 signal U _{λ of} the measuring device to be explained.

Fig. 2 shows the time course of various parameters of the internal combustion engine 10 and the exhaust system 12 during an NO _x - regeneration of the NO _x storing catalyst 18, which is carried out by a conventional method. Initially, the internal combustion engine 10 is in a lean operating mode, in which an oxygen-rich air-fuel mixture with λ _M »1 is fed to it (graph 100 ). In this phase, the exhaust gas contains an excess of nitrogen oxides NO _x , which cannot be completely converted by the pre-catalyst 16 . NO _x is therefore stored in the NO _x storage catalytic converter 18 , the NO _x load of which increases continuously in the process (graph 102 ). On the basis of a suitable criterion, a need for regeneration of NO _{x is} recognized at a point in time t _A. This can be, for example, a NO _x breakthrough detected by the measuring device 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. As a result of the now increased mass flow of reducing agents CO and HC in the exhaust gas which the NO _x is - embedded storage catalyst 18 NO reduction _x desorbed and nitrogen. A decrease in the NO _x loading (graph 102 ) of the storage catalytic converter 18 can, however, only be recorded after a certain time delay after switching over the internal combustion engine 10 , since at time t _A the exhaust gas duct 14 is still filled with lean exhaust gas, which initially still contains the storage catalytic converter 18 must happen before the reducing agents reach it. The course of the NO _x regeneration is meanwhile tracked with the aid of the signal U _λ provided by the measuring device 20 - usually a voltage. The probe voltage U _λ (graph 104 ) is inversely proportional to an oxygen concentration of the exhaust gas downstream of the storage catalytic converter 18 . Since the reducing agents are consumed to an ever lesser extent as regeneration progresses, the signal U _{λ of} the measuring device 20 rises slowly. At a time t _E , the signal U _{λ reaches} a predetermined threshold value U _S , whereupon the internal combustion engine 10 is generally switched back to 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 now practically NO _x -free storage catalytic converter 18 and reaches the environment unconverted. The course of the concentration of carbon monoxide CO and unburned hydrocarbons HC (graph 106 ) measured downstream of the catalyst therefore shows an undesirably high increase after the end of regeneration t _E.

In order to reduce this emission of pollutants, another approach is used according to the invention to determine the end of regeneration t _E. This is shown in FIG. 3, the time course of the same parameters as shown in FIG. 2. After the start of the regeneration at time t _A , the signal U _{λ of} the measuring device 20 (graph 104 ) is first measured and recorded in a known manner. After a predetermined period of time has elapsed, the control unit 24 begins at a time t _AH with an extrapolation of the signal U _λ . This is done on the basis of various operating parameters of the internal combustion engine 10 and the exhaust system 12 . In addition, behavioral models of the storage catalytic converter 18 and the measuring device 20 can be taken into account in the extrapolation. On the basis of the extrapolated signal curve, a point in time t _{S is} determined at which the predetermined signal threshold value U _{S is} theoretically reached. On the basis of selected operating parameters, the control unit 24 also calculates a time span Δt that corresponds to the current exhaust gas runtime that the exhaust gas requires until the storage catalytic converter is reached. The time of the end of regeneration t _E is then determined by subtracting the exhaust gas runtime Δt from the time t _S. During the period T _H , the extrapolation and thus the end of regeneration t _{E are} continuously updated on the basis of current operating parameters and on the basis of the actual signal profile U _λ . When the end of regeneration t _E determined in this way is reached, the internal combustion engine 10 is switched back to the lean operating mode with λ _M. At this point in time, there is still a small amount of stored NO _x in the storage catalytic converter 18 (graph 102 ), which is sufficient to convert the remaining reducing agents contained in the exhaust gas. Consequently, only very small amounts of pollutants are measured downstream of the storage catalytic converter 18 after the regeneration end t _E (graph 106 ).

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

NO _x

storage catalyst

20

oxygen sensitive measuring device

22

Engine control unit

24

control unit

100

combustion lambda

102

NO _x

Loading of the NO _x

storage catalyst

104

Waveform (U _λ

) of the measuring device

106

Reducing agent content in the exhaust gas
t _A

regeneration start
t _E

regeneration end
t _p

When the threshold was exceeded
t _AH

Extrapolation beginning
T _H

Extrapolation time
Δt exhaust gas runtime
U _λ

Signal from the measuring device
U _S

threshold
λ _M

Lambda lean value
λ _F

Lambda fat value

Claims (14)

1. Method for NO _x regeneration of a NO _x storage catalytic converter arranged in an exhaust gas duct of a lean-running internal combustion engine, wherein the NO _x storage catalytic converter is subjected to a rich to stoichiometric exhaust gas atmosphere with λ ≦ 1 until a regeneration end is reached, and a regeneration course based on a an oxygen-dependent signal provided downstream of the NO _x storage catalytic converter is provided, characterized in that

• a) during the NO _x regeneration, the oxygen-dependent signal (U _λ ) from the measuring device ( 20 ) is extrapolated and, based on the extrapolated signal, a theoretical point in time (t _S ) is determined at which a predetermined signal threshold value (U _S ) is exceeded, and
• b) the end of regeneration (t _E ) is determined by subtracting a time period (Δt) corresponding essentially to an exhaust gas runtime between the internal combustion engine ( 10 ) and NO _x storage catalytic converter ( 18 ) from the theoretical point in time (t _S ).

2. The method according to claim 1, characterized in that the oxygen-dependent signal (U _λ ) is extrapolated based on current operating parameters of the internal combustion engine ( 10 ) and / or the exhaust system ( 12 ). 3. The method according to claim 2, characterized in that the oxygen-dependent signal (U _λ ) is extrapolated depending on an air-fuel mixture supplied to the internal combustion engine ( 10 ) and / or an exhaust gas mass flow and / or an exhaust gas temperature and / or a catalyst temperature. 4. The method according to claim 2 or 3, characterized in that the oxygen-dependent signal (U _λ ) is extrapolated taking into account a behavior model of the NO _x storage catalyst ( 18 ). 5. The method according to any one of claims 2 to 4, characterized in that the oxygen-dependent signal (U _λ ) is extrapolated taking into account a behavior model of the oxygen-sensitive measuring device ( 20 ), in particular an inertia of the measuring device ( 20 ). 6. The method according to any one of the preceding claims, characterized in that the extrapolation begins after an elapse of a predetermined time after the start of regeneration and / or after throughput of a predetermined exhaust gas mass and / or after exceeding a predetermined threshold of the signal (U _λ ). 7. The method according to any one of the preceding claims, characterized in that the exhaust gas runtime (Δt) is calculated or specified based on current operating parameters of the internal combustion engine ( 10 ). 8. The method according to any one of the preceding claims, characterized in that the extrapolation of the signal (U _λ ) is omitted if predetermined limit values for operating conditions of the internal combustion engine ( 10 ) and / or the exhaust system ( 12 ) are not met. 9. The method according to claim 8, characterized in that limit values for the exhaust gas mass flow and / or for the temperature of the NO _x storage catalytic converter ( 18 ) are specified. 10. The method according to claim 8 or 9, characterized in that the extrapolation of the signal (U _λ ) is omitted in the event of a fuel cut-off during the NO _x regeneration. 11. Apparatus for carrying out regeneration of a arranged in an exhaust passage of a lean executable internal combustion engine _NOx - storage catalyst, wherein the NO _x storage catalytic converter until it reaches a regeneration end with a fat is subjected to a stoichiometric exhaust gas atmosphere with λ ≦ 1, and a regeneration process on the basis of a by oxygen-sensitive signal provided downstream of the NO _x storage catalytic converter is provided, characterized in that means are available with which the method steps

• a) extrapolation of the oxygen-dependent signal (U _λ ) from the measuring device ( 20 ) during the NO _x regeneration and determination of a theoretical point in time (t _S ) on the basis of the extrapolated signal at which a predetermined signal threshold value (U _S ) is exceeded, and
• b) Determination of the end of regeneration (t _E ) by subtracting a time period (.DELTA.t) substantially corresponding to an exhaust gas runtime between internal combustion engine ( 10 ) and NO _x storage catalytic converter ( 18 ) from the theoretical time (t _S ).

12. The device according to claim 11, characterized in that the means comprise a control unit ( 24 ) in which an algorithm for controlling the method steps is stored in digital form. 13. The apparatus according to claim 12, characterized in that the control unit ( 24 ) is integrated in an engine control unit ( 22 ). 14. Device according to one of claims 11 to 13, characterized in that the oxygen-sensitive measuring device ( 20 ) comprises a broadband or step response lambda probe or a NO _x sensor.