WO2001051779A1 - METHOD AND DEVICE FOR CONTROL OF DESULPHURISATION OF AN NOx STORAGE CATALYST ARRANGED IN AN EXHAUST SYSTEM OF AN INTERNAL COMBUSTION ENGINE - Google Patents

Abstract

The invention relates to a method and a device for the control of a desulphurisation of an NOX storage catalyst, arranged in an exhaust system of an internal combustion engine, with at least one NOx probe arranged downstream of the NOx storage catalyst, which produces a signal dependent upon the concentration of NOx present in the exhaust gas. An NOx activity for the NOx storage catalyst (18) is determined, from the value of the NOx concentration measured downstream from the NOx storage catalyst (18). Should the NOx activity (NOA) for the NOx storage catalyst (18) fall below a preset threshold value (SW), a desulphurisation with preset desulphurisation parameters is initiated. An irreversible deterioration of the NOx storage catalyst (18) is determined by means of the desulphurisation success and, if a preset threshold value for deterioration is exceeded (SWIR), a hard desulphurisation is initiated, whereby at least one desulphurisation parameter corresponding to an elevated desulphurisation efficiency is selected. The further operating parameters of the internal combustion engine (10) are made dependent upon the NOx initial activity (NOAMX) achieved after the hard desulphurisation.

Description

 Method and device for controlling desulfurization in an exhaust gas duct of an internal combustion engine

NO _x storage catalytic converter

The invention relates to a method and a device for controlling a desulfurization of a NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine with the features mentioned in independent claims 1 and 13.

It is known to aftertreat exhaust gases from internal combustion engines, which are operated at least temporarily in a lean operating mode, with the aid of NO _x storage catalytic converters. During a lean operating mode, the NO _x storage catalyst stores nitrogen oxides NO _x , which in this phase are present in excess in the exhaust gas compared to reducing exhaust gas components, such as carbon monoxide or unburned hydrocarbons and therefore cannot be fully converted, in the form of nitrate. The storage catalytic converter is subjected to NO _x regeneration at regular intervals or if the NO _x storage capacity decreases, for which purpose the storage catalytic converter is charged with a rich exhaust gas atmosphere and a minimum catalytic converter temperature is set.

In addition to the NO _x absorption, there is an undesirable incorporation of sulfur oxides in the NO _x storage catalytic converter. The sulfur storage is not reversible under the conditions of NO _x regeneration, which leads to increasing sulfurization of the storage catalyst, which reduces the NO _x storage capacity. In addition, an agglomeration of sulfur can lead to the formation of sulfate grains, which can irreversibly damage a catalytic activity of the NO _x storage catalyst. The removal of such sulfate grains becomes increasingly difficult with increasing grain size. It is therefore known to carry out a desulfurization of the NO _x storage catalyst at regular intervals, the stored sulfur being discharged mainly in the form of sulfur dioxide SO2 and hydrogen sulfide H2S at catalyst temperatures of over 650 ° C. and when exposed to a rich exhaust gas atmosphere. In addition to the irreversible damage caused by the formation of sulfate grains, other permanent types of damage are known which contribute to a reduction in the storage capacity of the catalyst. Thermal damage is of primary importance here.

The invention is based on the object of proposing a method for controlling desulfurization of a NO _x storage catalytic converter, with the aid of which irreversible damage to the NO _x storage catalytic converter can be identified and tracked.

According to the invention, this object is achieved by a method and a device for controlling desulfurization of a NO _x storage catalytic converter with the features mentioned in the independent claims. According to the inventive method, a NO _x of the NO _x -Speicheraktivität - determined from a storage catalyst downstream of the NO _x storage catalytic converter and the measured NO _x concentration falls below a predetermined threshold value for the NO _x activity of the NO _x -Speicherkataiysators desulfurization with predeterminable Desulfurization parameters initiated. Furthermore, irreversible damage to the NO _x storage catalytic converter is tracked on the basis of a desulfurization success and hard desulfurization is initiated if a predetermined damage limit value is exceeded, at least one desulfurization parameter being selected in accordance with a higher desulfurization efficiency, and further operation of the internal combustion engine from the level after hard desulfurization recovered NO _x activity is made dependent. Carrying out the hard desulfurization ensures a quantitative sulfur output, even if previous ones

Standard desulphurization should have been incomplete. Thus, the NO _x storage activity recovered after the hard desulfurization can be directly correlated with an existing irreversible damage to the NO _x storage catalytic converter. Consequently, permanent damage to the NO _x storage catalytic converter which can no longer be tolerated can be recognized.

The invention permits various process variants for specifying the damage limit value. According to a preferred embodiment, the threshold value, the lower value of which triggers a desulfurization, is reset after each desulfurization in such a way that it is proportional to an NO _x starting activity of the NO _x after the desulfurization has ended. Storage catalytic converter is. In this case, the damage limit value represents a minimum limit that cannot be undercut for ^~ d Fe ^ O ^ storage efficiency td ar, the threshold value being greater than the damage limit value. According to an alternative embodiment, the threshold value, the lower value of which triggers desulfurization, is not varied. According to this variant, a desulfurization frequency increases with increasing irreversible damage to the catalyst, so that a predetermined maximum desulfurization frequency serves as the damage limit value. According to a further alternative embodiment, in which the threshold value is also constant, the damage limit value is specified as a lower limit of an initial NO _x activity recovered after desulfurization.

Desulfurization, in particular hard desulfurization, is associated with higher fuel consumption and may influence the operating behavior of a vehicle. The initiation of desulfurization is therefore not desirable in every operating situation. An advantageous embodiment of the method therefore provides that, if the damage limit value is exceeded, lean operation of the internal combustion engine is first prohibited and hard desulfurization is only initiated when suitable, predetermined boundary conditions exist. This can be, for example, a minimum temperature of the NO _x storage catalytic converter and / or a minimum vehicle speed that is maintained over a minimum period.

The method according to the present invention provides that the further operation of the internal combustion engine after hard desulfurization is dependent on the desulfurization success, that is to say on the level of an initial NO _x activity recovered after the desulfurization. A preferred embodiment provides that if after the end of hard desulfurization a recovered NO _x starting activity corresponds almost completely or at least to a predetermined extent to that of a sulfur-free, NO _x -free and undamaged NO _x storage catalytic converter, lean operation of the internal combustion engine continues is allowed. In this case, it can be assumed that the loss of NO _x storage activity observed before the hard desulfurization is due to sulfur storage and thus to incomplete, previous desulfurization and not to irreversible damage to the NO _x storage catalyst. After a successful hard desulfurization measure, it is therefore further preferred that Adapt desulfurization parameters so adaptively that subsequent ones

Desulphurization works more effectively.

If, on the other hand, hard desulfurization is largely unsuccessful and an initial NO _x activity recovered is not significantly higher than an initial NO _x activity present after the last desulfurization, a strong irreversible damage to the NO _x storage catalytic converter is concluded and lean operation of the internal combustion engine is restricted or locked to keep the NO _x emissions low. It is also conceivable to inform a driver of a need for maintenance of the catalytic converter, for example by means of a warning.

It is further preferably provided that the device comprises means, for example a control unit, in which a procedure for controlling the process steps of the desulfurization of a NO _x storage catalytic converter is stored in digital form. Such a control unit can advantageously be integrated in a mostly existing engine control unit.

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

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

Figure 1 is a schematic diagram with an exhaust gas cleaning system and

Means for controlling the desulfurization of a NO _x storage catalytic converter;

FIG. 2 shows a time course of a NO _x storage activity of a NO _x .

Storage catalytic converter according to a preferred embodiment of the invention;

FIG. 3 shows a time course of a NO _x storage activity of a NO _x .

Storage catalyst according to a further preferred embodiment of the present invention and FIG. 4 shows a flow chart of the method steps according to the invention in accordance with the exemplary embodiment shown in FIG. 2.

FIG. 1 schematically shows an internal combustion engine 10 with a downstream exhaust line 12. A pre-catalytic converter 16 and an NO _x storage catalytic converter 18 are arranged in an exhaust gas duct 14 of the exhaust line 12. The exhaust duct 14 also houses various instruments for recording selected operating parameters. For example, the gas probes 20, 22 detect a concentration of a gas component in the exhaust gas. In the present example, the gas probe 20 designed as a lambda probe serves to record an oxygen fraction after the internal combustion engine 10 and before the catalyst components 16, 18, while the NO _x probe 22 measures a NO _x concentration behind the NO _x storage catalyst 18. The temperature sensors 24, 26 arranged in front of and behind the NO _x storage catalytic converter 18 serve to determine a catalytic converter temperature. Alternatively, one or even both temperature sensors 24, 26 can be dispensed with and a temperature of the NO _x storage catalytic converter can be derived empirically. All signals detected by the gas probes 20, 22 and the temperature sensors 24, 26 find their way into an engine control unit 28, where they are first digitized and then further processed in order to control an operating mode of the internal combustion engine 10. For this purpose, the engine control unit 28 regulates, for example, an air-fuel mixture to be fed into the internal combustion engine 10 by influencing a position of a throttle valve 30 in an intake pipe 32 and / or an exhaust gas recirculation device 34. The actuators 30, 34 shown by way of example can, for example, lean or an Fat mode for the internal combustion engine 10 can be set.

A control unit 36 is also integrated in the engine control unit 28, in which a procedure for controlling the desulfurization of the NO _x storage catalytic converter 18, which is explained in more detail below, is stored. Alternatively, the control unit 36 can also be implemented independently of the engine control unit 28.

FIG. 2 shows the time course of a relative NO _x activity NOA _{re |} of a NO _x storage catalytic converter 18. The relative NO _x activity denotes NOA _{re |} the ratio of the NO _x activity NOA of the present NO _x storage catalytic converter 18 to the NO _x activity of a NO _x and sulfur-free and undamaged NO _x storage catalytic converter. The NO _x activity NOA itself here is the ratio of the NO _x concentration measured behind the NO _x storage catalytic converter 18 to the NO _x probe 22 of the NO _x concentration present in front of the NO _x storage catalytic converter 18. The NO _x concentration upstream of the NO _x storage catalytic converter 18, that is to say the raw NO _x emission, is preferably calculated by the engine control unit 28 on the basis of current operating parameters of the internal combustion engine 10. Alternatively, it can also be measured with a NO _x probe arranged in front of the NO _x storage catalytic converter 18 in the exhaust line 14. The calculation of the NO _x activity NOA or the relative NO _x activity NOA _re | takes place in the engine control unit 28, in which the NO _x activity of the NO _x and sulfur-free undamaged NO _x storage catalytic converter is stored. At the start of vehicle operation, the NO _x storage catalytic converter 18 has the NO _x activity NOA similar to that of a fresh catalytic converter, so that the relative NO _x activity NOA _{re |} initially assumes a value close to "1". In the following course, there is an increasing sulfurization of the catalyst 18, so that the relative NO _x activity NOA _re | increasingly falling. Falling below a first threshold value SW for the relative NO _x activity triggers a first desulfurization 40 of the NO _x storage catalytic converter 18 at the time t- _| out. In accordance with the predetermined desulfurization parameters, a minimum temperature of the catalyst required for the desulfurization is set and the internal combustion engine 10 is operated in a rich operating mode in accordance with a lambda fat specification for a predetermined or regulated desulfurization period. In order to suppress H2S emission, it is known to carry out the desulfurization in fat-lean intervals. In this case, the rich-lean lambda specifications of the intervals, a switching frequency or a position of the rich-lean switching thresholds after the NO _x storage catalytic converter 18 can be specified as additional desulfurization parameters. After a desulfurization 40 has ended, the position of the threshold value SW is redefined by the control unit 36 in accordance with a recovered relative NO _x starting activity NOAMX, the threshold value SW preferably being proportional to the recovered NO _x starting activity NOAMX. The present after completion of a desulfurization NO _x -Anfangsaktivität NOAMX corresponds with progressive operating time t less and less of a fresh NO _x - storage catalyst and decreases with increasing aging of the catalyst. Reasons for this are, for example, incomplete desulfurization and / or irreversible thermal damage to the NO _x storage catalytic converter 18. As a result of the decreasing NO _x starting activity NOAMX, the threshold value SW, the undershoot of which triggers desulfurization, is increasingly reduced. The cycles of the sulphurisation 38 and desulphurization 40 are repeated until the NO _x activity NOA a not to be lower border threshold value, the Damage limit value SW _| ^, reached at time t5. Falling below the damage limit value SWJR initially leads to the internal combustion engine 10 being switched from a lean operating mode to a stoichiometric or rich operating mode in order to keep the NO _x emission low. The hard desulfurization is advantageously only initiated when predetermined boundary conditions, for example a minimum temperature of the NO _x storage catalytic converter 18 and / or a minimum vehicle speed maintained over a minimum period, are present. In this way, the fuel consumption for the energetically extremely demanding hard desulfurization can be kept relatively low. The hard desulfurization 42 differs from the previous desulfurization 40 in that at least one of the desulfurization parameters mentioned (for example catalyst temperature, lambda, time specification) is selected in accordance with a higher desulfurization efficiency. For example, an extended desulfurization period and / or a lower lambda fat specification can be provided for the hard desulfurization 42.

Depending on the irreversible damage to the NO _x storage catalytic converter 18, different levels of NO _x starting activities NOAMX are recovered after the hard desulfurization 42. In a scenario denoted by 44, the recovered NO _x activity NOAMX corresponds approximately to that of a sulfur-free, undamaged catalyst. In this case it can be assumed that the catalytic converter 18 has practically no permanent damage and the previous loss of activity is due to incomplete, previous desulfurization 40. In scenario 46, the hard desulfurization 42 does not completely restore the original NO _x activity, but it does so to a considerable extent. This indicates the presence of irreversible damage to the catalyst but also incomplete desulfurization 40. In both scenarios 44, 46, lean operation of the internal combustion engine 10 is still permitted, it being possible for a lower threshold value to be specified for the NO _x starting activity NOAMX to be recovered. In both cases 44, 46, the desulfurization parameters for subsequent desulfurization are advantageously adaptively corrected such that improved subsequent desulfurization results can be expected. The correction of the desulfurization parameters is all the more important the less the irreversible damage to the NO _x storage catalytic converter 18 and the higher the recovered NO _x activity NOAMX after the hard desulfurization 42. In accordance with scenario 48, hard desulfurization 42 is practically unsuccessful run. Extensive irreversible damage to the storage catalytic converter 18 must therefore be concluded here. In order to prevent further NO _x emissions, the lean operation of the internal combustion engine 10 is finally blocked in this case. Optionally, a warning display can also be provided, which informs a vehicle driver of the condition of the catalytic converter or indicates maintenance that becomes necessary.

A course of the relative NO _x activity NOA _{re |} according to another preferred embodiment of the invention is shown in Figure 3. Here the threshold value SW for the NO _x activity NOA is kept constant during the entire vehicle operation. With increasing aging of the NO _x storage catalytic converter 18, the NO _x starting activity NOAMX restored after desulfurization 40 decreases. As a result, the time interval τ in which the NO _x storage catalytic converter 18 stores sulfur in the lean mode of the internal combustion engine 10 until the threshold SW is reached becomes shorter and shorter. In other words, a frequency rises, with which desulfurization 40 becomes necessary. A criterion for recognizing the need for hard desulfurization 42 can consist in a predetermined maximum desulfurization frequency or in a lower threshold of an NO _x starting activity NOAMX recovered after desulfurization. All other method features of this embodiment of the invention correspond to the features shown in FIG. 2 and are not to be explained again here.

FIG. 4 shows a flow chart to explain the embodiment of the method shown in FIG. 2. The process sequence begins with step S1, in which the internal combustion engine 10 is subjected to a lean atmosphere, that is to say with a lambda value> 1. In step S2, the calculation of the NO _x activity NOA based on the measured NO _x from the NO probe 22 takes place _x - concentration downstream of the NO _x storing catalyst 18. The NO _x -Speicheraktivität NOA is compared in step S3 with the threshold value SW , If the NO _x activity NOA is above the threshold value SW, the method goes to step S1 and the internal combustion engine 10 continues to be operated in the lean mode. If, on the other hand, it is determined in step S3 that the threshold value SW has been reached or fallen below, the NO _x activity NOA is compared with the damage limit value SW | R in step S4. If the damage limit value SW | R has not yet been reached or fallen below, a desulfurization with the predetermined desulfurization parameters is initiated in step S5. After completing the Desulphurization, the recovered NO _x activity NOAMX is determined in step S6 and the threshold value SW is recalculated as a function of the determined initial activity NOAMX. If, on the other hand, it is determined in step S4 that the NO _x activity NOA has reached or fallen below the damage limit value SW | R, hard desulfurization is initiated in step S7. After hard desulfurization has ended, a query is made in step S8 as to whether the recovered NO _x activity NOAMX is less than a predetermined threshold value SWMX. If this question is answered in the negative, a new threshold value SW is calculated in step S6 as a function of the NO _x starting activity NOAMX, whereupon the lean operation is again permitted in step S1. If, on the other hand, the desulfurization success cannot be determined in step S8, the lean operation is finally blocked in step S9.

Claims

PATENT CLAIMS 1. Method for controlling a desulphurization of an arranged in an exhaust passage of an internal combustion engine NO _x storage catalytic converter with at least one downstream of the NO _x storage catalytic converter arranged NO _x - probe provides according to a present in the flue gas NO _x, a signal concentration, characterized in that (a) a NO _x activity (NOA) of the NO _x storage catalytic converter (18) is determined from the NO _x concentration measured downstream of the NO _x storage catalytic converter (18), (b) if a predeterminable threshold value (SW) for the NO _x activity (NOA) of the NO _x storage catalytic converter (18) is undershot, desulfurization is initiated with predeterminable desulfurization parameters, (c) an irreversible damage to the NO _x storage catalytic converter (18) is determined on the basis of a desulfurization success and hard desulfurization is initiated if a predetermined damage limit value (SW | R) is exceeded, at least one desulfurization parameter being selected in accordance with a higher desulfurization efficiency, and (d) further operation of the internal combustion engine (10) is made dependent on the level of an initial NO _x activity (NOAMX) recovered after the hard desulfurization. 2. The method according to claim 1, characterized in that the threshold value (SW) is specified after each desulfurization in such a way that it is proportional to a NO _x initial activity (NOAMX) of the NO _x storage catalytic converter (18) that is recovered after a desulfurization has ended, and the damage limit value (SW | R) represents a minimum limit value for the NO _x activity (NOA) which is not to be undercut, the threshold value (SW) being greater than the damage limit value (SW | R). 3. The method according to claim 1, characterized in that the damage limit value (SW | R) is a predetermined maximum desulfurization frequency with a non-variable threshold value (SW). 4. The method according to claim 1, characterized in that the damage limit value (SW | R) at a non-variable threshold value (SW) is a predetermined lower limit of a NO _x initial activity recovered after desulfurization (NOAMX). 5. The method according to any one of the preceding claims, characterized in that when the damage limit value (SW | R) is exceeded, lean operation of the internal combustion engine (10) is initially restricted or prohibited and hard desulfurization is only initiated when predetermined boundary conditions, such as a minimum temperature of the NO _x storage catalytic converter (18) and / or a minimum vehicle speed maintained over a minimum duration. 6. The method according to any one of the preceding claims, characterized in that if after the end of hard desulfurization a recovered NO _x starting activity (NOAMX) almost completely or at least to a predetermined extent that of a sulfur-free, NO _x -free and undamaged NO _x Storage catalyst (18) corresponds to a lean operation of the internal combustion engine (10) is still permitted. 7. The method according to any one of the preceding claims, characterized in that if after the end of hard desulfurization a recovered NO _x starting activity (NOAMX) almost completely or at least to a predetermined extent that of a sulfur-free, NO _x -free and undamaged NO _x Storage catalyst (18) corresponds to the desulfurization parameters are adjusted so that subsequent desulfurization run more effectively. 8. The method according to any one of the preceding claims, characterized in that if after the end of hard desulfurization a recovered NO _x starting activity (NOAMX) is not significantly higher than after a directly preceding desulfurization, to a strong irreversible Damage to the NO _x storage catalyst (18) is closed and lean operation of the internal combustion engine (10) is restricted or blocked. 9. The method according to any one of the preceding claims, characterized in that the NO _x activity (NOA) of the NO _x storage catalyst (18) is a ratio of the downstream of the NO _x storage catalyst (18) measured NO _x concentration to a measured or calculated on the basis of current operating parameters of the internal combustion engine (10) _NOx - concentration upstream of the NO _x storage catalytic converter (18). 10. The method according to any one of the preceding claims, characterized in that the NO _x activity (NOA) is a ratio of the measured NO _x concentration after the NO _x storage catalyst (18) and a modeled NO _x concentration after a sulfur-free and undamaged NO _x storage catalyst (18). 11. The method according to any one of the preceding claims, characterized in that the desulfurization parameters are a desulfurization temperature on the NO _x storage catalytic converter (18), a lambda fat specification and a desulfurization time. 12. The method according to any one of the preceding claims, characterized in that rich-lean lambda specifications, a switching frequency and a location of the rich-lean changeover thresholds after the NO _x - storage catalyst (18) additional desulfurization parameters in the case of a regulated rich-lean desulfurization are. 13. A device for controlling a desulphurization of an arranged in an exhaust passage of an internal combustion engine NO _x storage catalytic converter with at least one downstream of the NO _x storage catalytic converter arranged NO _x probe, which according to a present in the exhaust gas NO _x concentration provides a signal, characterized in that that means are provided with which the process steps (a) determining a NO _x activity (NOA) of the NO _x storage catalytic converter (18) from the NO _x concentration measured downstream of the NO _x storage catalytic converter (18), (b) initiation of a desulfurization with predeterminable desulfurization parameters when a predefinable threshold value (SW) for the NO _x activity (NOA) of the NO _x storage catalytic converter is undershot, (c) determining an irreversible damage to the NO _x storage catalytic converter (18) on the basis of a desulfurization success and, if a predetermined damage limit value (SW | R) is exceeded, initiation of hard desulfurization, at least one desulfurization parameter being selected in accordance with a higher desulfurization efficiency, and (d) Decision on further operation of the internal combustion engine (10) depending on the amount of NO _x activity (NOAMX) recovered after the hard desulfurization. are executable. 14. The apparatus according to claim 13, characterized in that the means comprise a control unit (36) in which a procedure for controlling the process steps for the desulfurization of a NO _x storage catalyst (18) is stored in digital form. 15. The apparatus according to claim 14, characterized in that the control unit (36) is integrated in an engine control unit (28).