DE19607151C1 - Regeneration of nitrogen oxide storage catalyst - Google Patents

Abstract

Nitrogen oxides storage catalyst (4) is regenerated in accordance with the operational state of the catalyst (4). During regeneration, the mixture supplied to the internal combustion engine corresponds to a stoichiometry ratio less than one (rich), ahead of the catalyst. The operational state corresponds to at least a limiting quantity of NOx compounds issuing from the catalyst. The quantity of NOx is evaluated from a characteristic diagram, which is a function of the loading and rotary speed of the engine.

Description

The invention relates to a method for the regeneration of a NOx storage catalyst according to the preamble of the claim 1.

NOx storage catalytic converters are used to with lean combustion the required exhaust gas limits to be able to comply. The NOx storage catalytic converters absor beers of the NOx connection generated during lean combustion fertilize. However, since the storage capacity of a NOx storage catalyst is limited, it is necessary a needs right regeneration of the storage catalytic converter. This is done by briefly operating the engine with egg a rich mixture, which saves the stored NOx connection in the catalytic converter.

A method for raining is already known from EP 0 597 106 A1 ration of a NOx storage catalyst, in which the Amount of NOx compounds absorbed by the storage catalytic converter depending on the intake air and the engine load is calculated. When a predetermined limit is exceeded amount of NOx Ver stored in the NOx storage catalytic converter bindings, the internal combustion engine becomes a rich mixture Regeneration of the storage catalyst supplied. To this However, reliable compliance with the exhaust gas limit is advisable values not guaranteed.

DE 195 11 548 A1 describes a method for regeneration a NOx storage catalyst, in which the regeneration phase se is started when the from the NOx storage catalyst given amount of NOx compounds over a given Limit is. The NOx connections are made with a sensor measured in the exhaust gas stream after the NOx storage catalytic converter. At During the regeneration phase, the internal combustion engine becomes a force substance mixture supplied that has an air ratio <1 before the NOx Storage catalytic converter corresponds.

The object of the invention is to provide a method for re generation of a NOx storage catalytic converter ensure safe compliance with the exhaust gas limit values guarantees and an improved, needs-based regeneration on the NOx storage catalyst.  

The object of the invention is characterized by the features of the spell 1 solved. A major advantage of the invention be rests in the regeneration of the NOx storage catalyst is started depending on the NOx emissions. To this In this way, safe compliance with the exhaust gas limit values is guaranteed accomplishes.

Advantageous developments and improvements of the invention are specified in the subclaims.

The invention is illustrated by the figures; it demonstrate:

Fig. 1 shows a schematic arrangement of an internal combustion engine having an NOx storage catalytic converter,

Fig. 2 is a schematic representation of the method according to the invention,

Fig. 3 shows a method for determining the NOx emissions and

Fig. 4 shows a method for determining the loading of the storage catalyst.

Fig. 1 shows an arrangement in which the inventive method is applied. An internal combustion engine 2 is connected to an intake tract 1 and an exhaust tract 3 . In the suction tract 1 , a temperature sensor 9 and a Lastmeßein device 11 , for example an air mass meter or a pressure meter, is arranged. The internal combustion engine 2 comprises an injection system with a valve arrangement and a cooling circuit. The exhaust tract 3 leads to a NOx storage catalyst 4 , to which a temperature sensor 13 is connected. The NOx storage catalytic converter 4 is referred to as storage catalytic converter 4 in the following. Furthermore, a Steuerge advises 5 with a memory 6 , the control device 5 via a load measuring line 12 with the load measuring device 11 , via a temperature measuring line 10 with the temperature sensor 9 , via a data and control line 8 with the internal combustion engine 2 and via a measuring line 7 is connected to the temperature sensor 13 . In addition, a lambda probe 14 is introduced into the gas tract 3 in front of the storage catalytic converter 4 and connected to the control unit 5 via a second measuring line 15 .

Fig. 2 schematically shows a method for determining the NOx raw emission NR. The control unit 5 preferably checks one or more starting conditions at program point 20 before further calculations are carried out. It is first checked via whether the internal combustion engine is in the “start” operating state. If this is the case, no further calculation is carried out, but is waited until the internal combustion engine 2 has left the “start” operating state. It is also checked whether there is a post-start control of the internal combustion engine 2 . If this is the case, further calculations are waited until the post-start control has ended. In addition, it is checked whether the catalytic converter temperature KT is greater than a predetermined minimum value. If this is the case, then it is checked whether the air ratio in the exhaust gas upstream of the catalytic converter has a value greater than 1. If the conditions mentioned are met, the program branches to program item 21 . In a simple embodiment, the conditions queried at program item 20 can also be dispensed with.

At program point 21 , the calculation of the raw NOx emission NR or the corrected raw NOx emission NRK is carried out. This is carried out using a subroutine, which is shown in FIG. 3.

After program point 21 , the program point 22 is followed by a query as to whether the NOx emission NA that leaves the storage catalytic converter 4 is greater than a predetermined limit value NE. If this is the case, the program branches to program item 23 . The NOx emission NA is calculated using the following formula:

NA (n) = NRK (n) TA (1-KEK (n)) (1-NO),
where NRK is the corrected raw emission, TA is the predetermined time interval between the times n and n + 1, KEK is the corrected storage efficiency, and NO is a correction factor that takes into account the proportion of NOx emissions that is chemically reduced by the storage catalytic converter 4 . In a simple embodiment of the invention, the NOx raw emission NR will be used instead of the corrected raw NOx emission NRK.

After calculating the NOx emission NA (n), the question is asked whether the NOx emission NA (n) exceeds the limit value NG. If this is not the case, the program branches back to program item 20 . However, if the NOx emission NA (n) exceeds the limit value NG, the regeneration of the storage catalytic converter 4 is initiated at program point 23 , in which the internal combustion engine 2 is supplied with a fuel / air mixture which, in the exhaust tract 3 in front of the storage catalytic converter 4, results in an air ratio leads less than 1. The program then branches back to point 20 .

Fig. 3 shows the individual steps of the program point 21 for calculating the NOx raw emission NR. At program point 30 the query follows whether the air ratio λ measured in the exhaust tract 3 in front of the storage catalyst 4 is greater than a predetermined starting value LS, for example 1.0. If this is not the case, the program branches back to program item 20 . If, however, the query at program point 30 shows that the air ratio λ is greater than the predetermined starting value LS, then the NOx raw emission mass NR is read out from a load and speed-dependent first map at program point 31 . The first map is stored in memory 6 . In a simple implementation of the invention can be branched back to program point 22 after processing the program point 31 . However, an improvement of the method according to the invention is achieved by carrying out at least one of program steps 32 , 33 , 34 or 35 .

At program point 32 , an ignition angle correction factor KZ is calculated for a correction of the raw NOx emission mass NR, taking into account the parameter ignition angle. For this purpose, the predetermined target ignition angle is read out first from the memory 6 from a second map, which contains a target ignition angle ZS as a function of the load and the speed, and the predetermined target ignition angle is read out in accordance with the load and the speed of the internal combustion engine 2, and the current ignition angle ZG is measured. In addition, a correction factor KF is read out of the memory 6 from a third map as a function of the load and the speed of the internal combustion engine 2 . The ignition angle correction factor KZ is then calculated using the following formula:

KZ = 1 + KF · (ZG - ZS)

The program then branches to program point 36 or to program point 33 .

At program point 33 , an air ratio correction factor KL is determined for a correction of the raw NOx emission NR, in which the air ratio λ is taken into account. For this purpose, a target air number LS predefined in accordance with the load and the speed of the internal combustion engine 2 is read out from a fourth map as a function of the load and the speed. In addition, the actual air ratio LG is measured. A differential air ratio LD is then calculated using the following formula:

LD = LS - LG

On the basis of the differential air ratio LD and the engine load ML, an air ratio correction factor KL is read from a fifth map in the memory 6 .

The program then branches to either item 36 or item 34 .

At program point 34 , a temperature correction factor FT is calculated, taking into account the cooling water temperature TL and the intake air temperature TA. Based on the cooling water temperature TL and the intake air temperature TA, a temperature correction factor FT is read from a sixth map, which is stored in the memory 6 . The program then branches to program point 36 or to program point 35 .

At program point 35 , a correction factor for the valve overlap is calculated for a correction of the raw NOx emission NR taking into account the valve overlap during the injection. For this purpose, a setpoint VS depending on the load and the speed for the valve overlap is read from a seventh map, which is stored in the memory 6 , and the difference to a measured value VG for the valve overlap is calculated. From the difference VD = VS - VG, a correction factor KV for the valve overlap is read out from an eighth map depending on the engine load ML and the difference VD of the valve overlap. Then branched ver after program item 36 .

At program point 36 , the correction of the raw NOx emission NR is carried out. Depending on the program points 32-35 carried out, the correction factors calculated therein are taken into account.

If all of the program points shown in FIG. 3 are carried out, the following value results for the corrected raw NOx emission NRK:

NRK = NR · KZ · KL · FT · KV.

A person skilled in the art will calculate the corrected NOx Crude NRK emission the number of corrections to be considered Select door factors according to the circumstances, so that in simple process the NOx raw emission z. B. only with tempera correction factor KT is corrected so that for the  corrected raw NOx emission NRK results in the following calculation:

NRK = NR · KT.

After calculating the corrected raw NOx emission NRK, the program branches back to item 22 .

In FIG. 4, the calculation of the Beladungszu schematically level of the storage catalytic converter 4 is shown, which is preferential used as a start condition for a regeneration phase of the storage catalytic converter 4. At program point 40 , control unit 5 calculates the storage efficiency KE of storage catalytic converter 4 . The storage efficiency KE is read out from a ninth characteristic field in the storage 6 as a function of the air mass LM drawn in and the loading loading KB of the storage catalytic converter. The degree of loading KB of the storage catalytic converter is calculated from the current loading KA based on the storage capacity KS of the storage catalytic converter 4 using the following formula: KB = KA / KS.

The storage capacity KS is read from a tenth map in the memory 6 , which depends on the catalyst temperature KT and the number of regeneration phases SZ that have already taken place. The regeneration phases in which the storage catalyst 4 rich mixture is supplied to reduce the NOx storage, are counted by the control unit 5 and stored in the memory 6 as a regeneration number.

The storage efficiency KE is preferably dependent on the catalyst temperature KT and depending on the Corrected charging cycles SZ, whereby from one eleventh map, which is based on the charging cycles SZ and the catalyst temperature KT depends, a correction value KS read out and the storage efficiency KE multiply is decorated:

KEK = KE · KS,

where KEK represents the corrected storage efficiency.  

The current load KA of the storage catalytic converter 4 is then calculated at program point 41 using the following formula:

KA (n) = KA (n + NRK (n) · TA · KEK (n) · 1 (1-NO),

where with KA (n) the loading at time n, with KA (n-1) the loading at time n-1, with NRK the corrected raw NOx emission, with TA the time interval between two calculation times n and n-1, KEK is the corrected storage efficiency and NO is a correction factor that takes into account the proportion of NOx emissions that are chemically reduced by the storage catalyst 4 .

Subsequently, at program point 42, the query is made as to whether the current load KA is greater than a predetermined minimum load KAM. If this is the case, a regeneration phase for the NOx storage catalytic converter 4 is started at program point 43 . If this is not the case, the program branches back to program item 40 . After carrying out the regeneration phase, program point 43 branches back to program point 40 .

An advantageous development of the invention is based on carrying out a load determination of the storage catalytic converter 4 during a regeneration phase in order to terminate the regeneration phase in good time. During the regeneration phase, the loading of the storage catalytic converter 4 is decremented by a value KD and the regeneration phase is ended when the catalytic converter loading KA falls below a predetermined threshold value. The decrement is read from a twelfth characteristic diagram, which depends on the intake air mass LM and the air ratio LG measured in front of the storage catalytic converter 4 in the exhaust tract 3 . The current catalyst loading is calculated at specified time intervals as follows:

KA (n) = KA (n-1) - KD,

where KD is the decrement read from the map, KA (n) the loading at time n and KA (n-1) the loading at Represents time n-1.

A memory field is provided in the memory 6 , in which the number of regeneration phases that have elapsed so far are counted and stored as a non-volatile regeneration number. However, in order to take into account the replacement of a storage catalytic converter 4 , a bit is provided in the memory 6 , which can be assigned zero or one, with a zero setting the regeneration number being set to zero and the regeneration phases starting from zero being counted up again.

This enables a more precise count of the regeneration phases ensures that the regeneration phases are also counted, caused by a rich fuel mixture at unsteady colors drove, d. H. e.g. B. with acceleration.

The regeneration phases are detected, for example, with the lam probe 14 in the exhaust tract 3 in front of the storage catalytic converter 4 (λ <1) and counted by the control device 5 and stored in the storage 6 as a regeneration number.

Claims (5)

1. A method for the regeneration of a NOx storage catalytic converter ( 4 ), in which a regeneration phase is started depending on an operating state of the NOx storage catalytic converter ( 4 ), in which a fuel mixture is supplied to the internal combustion engine which has an air ratio less than 1 in front of the NOx - Storage catalytic converter ( 4 ), the operating state corresponding to at least a limit amount of NOx compounds, which is output by the NOx storage catalytic converter ( 4 ), characterized in that the amount of NOx compounds emitted by the NOx storage catalytic converter ( 4 ) consists of at least a map is determined, which depends on the load and / or the speed of the internal combustion engine ( 1 ). 2. The method according to claim 1, characterized in that the operating state corresponds to at least one limit storage of NOx compounds in the NOx storage catalyst ( 4 ). 3. The method according to claim 1, characterized, that the amount of NOx compounds released depends on from the ignition angle and or from the air ratio and / or from the Cooling water temperature and / or the intake air temperature and / or is corrected by a valve overlap. 4. The method according to claim 2, characterized in that is calculated in dependence on the SpeI cherwirkungsgrad of the NOx storage catalytic converter (4) as the value for the boundary storing the loading state of the NOx storage catalytic converter (4), wherein the memory efficiency of the already carried out depending on the number Regeneration phases is corrected. 5. The method according to claim 1, characterized in that the loading state of the NOx storage catalyst ( 4 ) is checked during the regeneration phase and the regeneration phase is interrupted when the loading state falls below a predetermined minimum load.