DE19626836A1 - Diesel engine with nitrous oxide storage catalyser - has exhaust gas diverted along by-pass line during regeneration of catalyser by fuel injected into preceding electrically-heated catalyser - Google Patents

Abstract

The diesel engine has a catalyser (7) inserted in the exhaust gas line for removal of nitrous oxides, with the exhaust gas selectively directed along a bypass line in parallel with the catalyser by operation of a pivoted control flap (203) during regeneration of the catalyser. The regeneration is effected by heating the catalyser to a temp. of 300 degrees C by injection of fuel in a preceding electrically-heated catalyser (214) so that the fuel is oxidised.

Description

The invention relates to a method for operating a diesel internal combustion engine with NOx Memory according to the preamble of claim 1 and a corresponding arrangement according to the preamble of claim 15.

NOx storage catalytic converters are known for lean lean-burn engines which operate in lean operation the nitrogen oxides initially up to the load limit dependent on the catalyst design store. This is followed by a short stoichiometric or slightly rich operation Regeneration of the NOx storage catalytic converter, with subsequent lean operation wise. The degree of retention of these NOx storage catalytic converters is very high, the whole NOx reduction with storage discharge and NOx conversion at λ 1 for lean Petrol engines in new condition <90%. In principle, such NOx storage catalysts are also can be used in diesel vehicles, with a certain larger dimension for Compensation of SOx inclusions is advantageous. In contrast to gasoline engines work Diesel engines, however, always with excess air, so that λ <during all operating conditions 1 is. Loading the NOx storage catalytic converter would thus be possible without any problems, regeneration by enriching the exhaust gas by means of, for example, injection of Fuel in the exhaust system would, on the one hand, become an intolerable ver consumption increase and on the other hand, due to the high oxygen content of the diesel exhaust a high heat of oxidation, because before the stored NOx is converted injected fuel is oxidized. This creates the risk of kata destruction lysators.

DE 43 42 062 A describes an exhaust gas purification device for diesel internal combustion engines is known in which the NOx storage is blocked for regeneration from the exhaust gas flow. This happens regularly when the memory has reached its capacity. Around during the regeneration still have no NOx emissions, the exhaust gas  current via a second NOx storage. Alternatively, the exhaust gas flow is throttled and initiated a complicated regeneration algorithm. Here, the function is reliable problematic. In addition, the doubling of the NOx storage requires a considerable amount Chen effort, although good exhaust gas purification is only partially achieved despite the effort becomes.

The object of the present invention is to provide a diesel internal combustion engine with a to equip a simple NOx storage system, with good emission control values being achieved that should. A corresponding procedure is also a task.

In the method described in the introduction, this object is achieved with the characteristics measures of claim 1 and / or 3; in the diesel engine solved the problem with the characterizing features of claim 15.

NOx storage catalysts contain, for example, oxides or carbonates of the alkali, Alkaline earth and / or rare earth metals that flow through with a lean exhaust gas Store nitrogen oxides in the form of nitrate or nitrite and flow through with stoichiometry chemical or rich exhaust gas release them again, in one with 3-way catalytic converters comparable procedure for the reduction of nitrogen oxides to nitrogen hydrocarbon substances are oxidized to water and carbon dioxide. Such a conventional NOx storage catalyst, as it can also be used in the present invention, is in the DE 43 42 062 A described.

According to the invention, when fuel is injected (or exhaust gases are passed through) high oxidizable portion) exploited the oxidation catalyst effect of the NOx storage or upstream of this an oxidation catalyst in which in the presence of oxygen the oxidizable constituents (fuel) are oxidized to generate heat Heating up NOx storage. This is particularly the case with efficiency-optimized diesel engines and especially useful for turbocharging, because here high exhaust gas mass flows exist at low temperatures. Such diesel engines can be weak during load phases the minimum temperature required for NOx storage (usually at least 200 ° C). Furthermore, even when cold starting (cold Catalyst) always a certain time until this minimum temperature is reached. Through the measure described, an additional NOx storage heater is available  which both shortens the heating-up time and if the exhaust gas temperatures are too low sufficient catalyst temperatures guaranteed during operation of the internal combustion engine. In a further embodiment, this can also be achieved with a burner. Usually, burners for diesel fuel have a slightly lean vote, so that for Regeneration of the NOx storage requires an additional fuel injection. If Operation at λ <1 is possible at least for a short time due to burner-side measures this injection can be omitted.

In a further embodiment, the NOx storage heater can also be equipped with an E-cat (electrically heated catalyst). Electrically heated pre-catalysts are included Gasoline engines have been known and tested for a long time. The use is with diesel engines a full-flow e-cat does not make sense, because on the one hand the exhaust gas mass flow is essential is higher than that of gasoline engines and, moreover, comparatively low CO and HC raw emissions ions are available for catalytic combustion. To achieve the same effectiveness as with comparable gasoline engines are due to the cooling due to the high exhaust gas mass flow and as a result of the low catalytic conversion, in particular at charged direct-injection diesel engines E-cat currents from 300 to 500 A at 12 V. required. In the present invention, however, the use of an E-cat heating of the NOx storage possible. In this case, the E-Kat can be used for regeneration the NOx storage are operated, d. H. the E-Kat will have a small part flow of the exhaust gas stream and / or oxygen-containing through an additional air injection Gases supplied, which he catalytically with additional fuel under heat development. The total heating output and any air exhaust gas that may be present Partial flow ratio is determined by the fuel injection. By this measure on the one hand the oxygen content is reduced during regeneration (the regeneration is only possible with a low oxygen content in the gas flow through the catalyst) and on the other hand, the temperature required for regeneration is ensured. Beyond that The E-Kat can also be used when carrying out the majority of the exhaust gas masses currents through the NOx storage (NOx storage acts as storage) can be used here also with fuel injection, with this procedure the E-cat only one must reach such temperature that the injected fuel is oxidized, whereby again the heat generated to secure the necessary NOx storage temperature serves. The current consumption of such an e-cat, especially in the partial flow  operation, can be set at approx. 30-60 A and thus compares Otto-E-Kats less demands on the on-board electrical system and the control.

In the method according to the invention for operating a diesel internal combustion engine, in which the exhaust gas flow from the diesel engine is sent via a NOx accumulator, The regeneration advantageously takes place under a partial exhaust gas stream or with complete shutdown termination of the diesel exhaust stream, d. H. at least the majority of the exhaust gas flow The diesel engine is in front of the regeneration of the NOx storage branched off.

A maximum of 25% of the diesel fuel during regeneration will be advantageous exhaust gases emitted by the machine and in particular a maximum of 10% of the exhaust gas during regeneration through the NOx storage.

According to the invention, the NOx storage device — unlike in the prior art — is advantageous not regenerated only when its NOx storage capacity threatens to exhaust, but instead already targeted at certain operating conditions of the diesel engine. This Operating conditions are those with an average low NOx content in the Exhaust gas flow, in particular overrun phases, but also idle phases are suitable. Likewise, the regeneration can also be temperature-determined, with two criteria here Play a role, namely the decreasing exhaust gas temperature and the other Falling below a predetermined temperature value. The predetermined temperature value can be an empirical value, in particular temperature values of 200 ° C. are suitable, however, it can also be a value that depends on the storage capacity of the NOx store determined depending on the temperature, whereby here the decreasing NOx storage ability is turned off with decreasing temperature. Regeneration is preferred ren at or below a temperature at which the NOx storage capacity to 90% and in particular 80% of the NOx storage capacity of the NOx storage at 300 ° C. is. The temperature is advantageously not below the temperature at which the NOx Store at least 30% and in particular at least 40% of its NOx Storage capacity (based on 300 ° C).

When regenerating under these operating conditions, the NOx Storage blocked exhaust gas flow on a NOx storage or throttling as a rule  be dispensed with, since this exhaust gas stream contains hardly any NOx. A waiver of a NOx Storage of a conventional type can even be advantageous here because the cold exhaust gas flow increases a cooling of the NOx storage leads, which in this way in its NOx storage capacity strongly decreases. For this reason, it can also be advantageous if the shut-off continues until at least one of the shut-off conditions described above is no longer available.

In addition to the selection criteria for the start of regeneration described above, - as usual so far - regeneration can also be carried out if the above criteria are not met are present, but the NOx storage is largely or completely exhausted. This can for example, with a sustained high load of the diesel engine. However, at least 50% and in particular at least 75% of the rain fall advantageously Rierzeit in the average operation of the engine under the Invention according to criteria.

With the particular regeneration conditions of the present invention, that the cold exhaust gases of the engine, which is warm to operation, have a very low level Contain NOx, do not cool the NOx storage, so that it stores its NOx Retains its storage capacity in order to avoid a subsequent increase in NOx in the exhaust gas possess high effectiveness.

It is useful to regenerate the NOx storage in front of the exhaust gas System injected fuel, which flows through the NOx storage as above described reacts. Advantageously, the exhaust gas flow is reduced so far that it as Carrier current is sufficient for the injected fuel. Instead of the exhaust gas flow or additionally can also air or, for example, burner gases injected as a carrier stream for the Fuel this and the NOx storage are supplied. Instead of fuel injection a NOx reducing agent can also be supplied in another way, for example in Form of rich exhaust gases from a burner, for example a burner. However, the fuel injection is preferably used for regeneration, since this is relatively inexpensive and easy to dose.

In an advantageous embodiment, the NOx storage is located in a first branch of the Exhaust system and during regeneration, the at least predominant part of the  exhaust gas flow via a second branch bypassing the NOx storage guided. In a further advantageous embodiment, the two exhaust branches can again be combined after the NOx storage, being advantageous in the reunited part the exhaust system further exhaust treatment with regard to noise and / or exhaust gas detox tion can be made. In particular, get involved in this reunited part Place the oxidation catalyst and / or a soot filter. In the second branch you can advantageously be housed a second NOx storage, which is the same as the first NOx storage cher can be formed, but preferably, especially if the regeneration of first NOx storage takes place with a low NOx content in the exhaust gas stream per unit of time the second NOx accumulator can be made smaller. This second NOx store can be regenerate analogously to the first NOx store, if through the first NOx store predominant parts (analogous to that described above) of the exhaust gas flow. In In a further embodiment, an oxidation catalyst can also be provided in the second branch be housed, which then starts faster than an in due to its proximity to the engine oxidation catalyst housed in the merged part of the exhaust system. There the NOx storage in the lean exhaust gas also has an effect as an oxidation catalyst oxidation of CO and HC during NOx storage and regeneration of the NOx storage always ensured. The use of an oxidation catalyst in the merged part of the exhaust system brings in particular in the regeneration and high engine load the advantage that two oxidation catalysts (NOx storage and Oxida tion catalyst in the merged part) are connected in series and thus a particularly effective CO and HC cleaning takes place.

The heating systems offer the advantage that in addition to heating the NOx storage device, that too Heating behavior of a downstream oxidation catalyst is improved. Advantageous Both heating systems can also use heat to regenerate possible sulfate deposits the NOx storage and for the regeneration of a downstream particle filter (soot filter) provide. For example, with the burner or e-cat Fuel injection a temperature of above 700 ° C in the gas flow through the NOx Storage can be achieved, whereby sulfate deposits at lambda <1 are removed. Such a high temperature is expediently only when necessary or, if only with difficulty can be determined after a specified number of NOx storage regenerations (NOx removal tion). As a result, the system is not overloaded. The sulfate  Regeneration can occur every tenth, twentieth or even fiftieth NOx regeneration take place.

The regeneration of the NOx store advantageously takes place - as explained above - in one low NOx content in the exhaust gas stream per unit of time, d. H. especially in overrun phases if necessary (depending on the diesel engine) also / or in idle phases. These phases contribute the usual operation of a motor vehicle from a high proportion of time, the NOx Emissions are only around 10%. The table below shows the percentage of time and the NOx emissions from a VW Golf III with a 66 kW turbo diesel direct injection engine in the MVEG cycle.

From this it can be seen that if the NOx accumulator only regenerates in overrun mode is practically no NOx emissions (possibly also with certain proportions in idle mode) ions or at least a reduction in NOx emissions of around 90% are.

The diesel internal combustion engine belonging to the invention has an exhaust system, in which is arranged a NOx storage. In the direction of flow in front of the NOx accumulator Branch is provided, through which at least part of the exhaust gas flow can optionally be guided is. The (partial) current led through the branch does not flow through the NOx storage. Means are also provided which divide the exhaust gas flow at the junction. This Means contain a controller that, in cooperation with the means, the exhaust gas flow with NOx storage at least predominantly, preferably at least 75% and leads in particular to at least 90% via the NOx storage and at least temporarily the regeneration of the NOx storage, the major part of the exhaust gas flow, advantageous to at least 75% and in particular at least 90% of the exhaust gas flow  the branch past the NOx storage. According to the NOx storage an oxidation catalyst upstream, or the NOx storage has the properties of Oxidation catalyst, the oxidation catalyst in particular electrically heated is and serves to heat the NOx storage. The criteria for branching the Exhaust gas stream are advantageous here that of the method described above.

The diesel internal combustion engine advantageously also contains the above in the process described elements. The device and the method are preferably used in a motor vehicle.

The invention is explained in more detail below with the aid of exemplary embodiments and drawings described.

Show it

Fig. 1 with an exhaust gas purifying NOx trap and the oxidation catalyst;

FIG. 2 shows the exhaust gas purification according to FIG. 1 with a burner;

Fig. 3, the exhaust gas purification according to Figure 1 with an E-Cat.;

FIG. 4 shows an exhaust gas purifying NOx with two memories;

Fig. 5, the exhaust gas purification according to Fig 4 with a burner.

Fig. 6 is the exhaust gas purification according to Figure 4, E-Cat.; and

Fig. 7 is a NOx exhaust gas temperature diagram.

In the embodiment shown in FIG. 1, the exhaust system 1, the exhaust gases (not shown) guided by a diesel engine (compression-ignition engine) via a turbocharger 2 to an exhaust filter 3. The exhaust switch 3 has two flaps that are alternately open or closed. This releases a first branch 4 or a second branch 5 of the exhaust system 1 for the exhaust gas flow. Shown is a release of the first branch 4 , in the seen in the direction of flow (arrow) after the exhaust switch 3 a fuel injection 6 opens. Further in the flow direction downward from the fuel injector 6 , a NOx storage catalytic converter is used in the first branch 4 , which is permeable to the exhaust gas flow in the flow direction and essentially flows through the entire exhaust gas flow which is passed through the first branch 4 . Further downstream, a lambda and / or temperature probe is guided into the first branch 4 after the NOx storage catalytic converter 7 . Downstream from the probe 8 , the two branches 4 and 5 are reunited 9 (the exhaust gas separator could also be provided here), so that the entire exhaust gas flow is led through an end pipe 10 of the exhaust system. After the tail pipe 10 nen muffler or other parts of exhaust systems can still be connected.

Oxidation catalysts 11 and 12 can be provided in the second branch 5 and / or in the tail pipe 10 .

In this exhaust system 1 , only one branch of the exhaust gas lines 4 and 5 running in parallel is released via the two alternately coupled flaps 3 . During acceleration processes of the internal combustion engine and in constant driving phases, the first branch 4 is opened, so that the exhaust gas stream first flows through the NOx storage catalytic converter 7 , in which nitrogen oxides are stored. In addition, the NOx storage cat acts as an oxidation catalyst in the lean diesel exhaust gas, so that a CO and HC conversion takes place simultaneously. If this is not sufficient, the oxidation catalyst 12 is additionally provided. In a subsequent overrun and possibly also idle phase of the diesel engine, the exhaust switch 3 is switched so that the first branch 4 is closed and the second branch 5 is open. If there is too much CO and / or HC in the exhaust gas during these operating phases, the oxidation catalytic converter 11 can be used here in the second branch 5 , in particular if no oxidation catalytic converter 12 is provided.

The exhaust switch 3 closes at least the second branch 5 as completely as possible from the exhaust gas flow when the first branch 4 is open, the other way around, the first branch 4 is preferably only largely closed off from the exhaust gas flow, so that even with the second branch 5 open, a small partial flow of the exhaust gas flow through the first branch 4 flows. This basic flow serves as space velocity (minimum flow) in order to convey fuel injected via the fuel injection 6 through the NOx storage catalytic converter 7 . Alternatively or additionally, an air supply can also take place with the fuel injection, which in turn serves to achieve the minimum flow rate of the NOx storage catalytic converter 7 . The injected fuel converts in the NOx storage catalytic converter 7, partly by heating the same, with the oxygen still present in the base stream, the remaining fuel components (HC) react with the stored NOx to form nitrogen, water and carbon dioxide. The end of this NOx conversion (and corresponding fuel injection) is detected with the probe 8 , which registers a λ <1 as a lambda probe and a temperature drop as the temperature probe (the reaction is exothermic). Both probes are advantageously used and queried. Possibly not used in this implementation HC components mix again with the main exhaust gas stream which flows through the second branch 5 , and are converted with the oxygen contained therein in the Oxidationskataly sator 12 to water and CO₂.

After the regeneration of the NOx accumulator 7 , the exhaust gas switch 3 can be switched over again, so that the exhaust gas stream flows through the first branch 4 , even if the diesel engine is still in overrun mode or in an idling phase, so that the nitrogen oxides which are low here are also present be saved again. Since the regeneration process ends in approximately 0.2 to 20 seconds, in particular in 0.5 to 10 seconds, there is sufficient time for regeneration in overrun operation in normal motor vehicle operation. In order to ensure extensive elimination of the nitrogen oxides even during long acceleration and constant runs, a time and / or diesel internal combustion engine operating state control can be provided, which leads to regeneration cycles taking place in addition to the method described above (for example, also during constant runs and / or accelerations). Since the regeneration time is significantly shorter than the loading time of the NOx storage catalytic converter 7 , extensive cleaning of the nitrogen oxides is also achieved here.

The exhaust system 101 shown in FIG. 2, as can also be seen from the reference numerals, has in principle the same structure as described in FIG. 1. In addition, a burner 113 is provided here, the heated burner gases (double arrow), seen in the flow direction, are introduced into the first branch 4 after the exhaust gas switch 3 . With the Bren ner, the cold NOx storage catalytic converter 7 and the oxidation catalytic converter 12, if present, can be quickly brought up to operating temperature on the one hand when the vehicle is cold started, and on the other hand, even during low-load phases of the diesel internal combustion engine (here the exhaust gas temperature can be significantly below 200 °), the NOx Required minimum temperature can be ensured. Only a very low burner output is required for the latter purpose.

Since a slightly lean tuning is common when operating the burner 113 with diesel fuel, an additional fuel injection 106 - as described above - is required for the regeneration of the NOx storage catalytic converter 7 . If at least short-term operation at λ <1 is also possible due to burner-side measures, this injection can be omitted.

The exhaust system 201 shown in FIG. 3 in turn differs only slightly from the exhaust system 1 from FIG. 1. Here the oxidation catalytic converter 11 (which is basically optional) is omitted and a flap solution 203 is used as an exhaust switch. In addition, an E-cat 214 (electrically heated oxidation catalytic converter) is provided in front of the NOx storage catalytic converter 7 and takes over the function of the burner 113 described in FIG. 2. The E-Kat 214 manages with a low heating output (approx. 40 A at 12 V), since in operation it only has to reach the temperature that is necessary to oxidize sprayed fuel. This then causes self-heating, which is released with the exhaust gas stream to the subsequent catalysts. In this embodiment, as little oxygen as possible should be present in the basic flow during the regeneration, so that the E-cat does not overheat here, in principle the E-cat (the power supply) can be switched off during the regeneration.

In the heating systems described above and below (burner and e-cat) for the NOx storage catalytic converter 7 , so little burner gas and or fuel are usually supplied that the resulting temperature in the exhaust gas is 250 ° to 400 °, in particular 300 ° to 350 ° .

FIG. 4 shows an exhaust system 301 which has a storage catalytic converter 7 with fuel injection 6 and a downstream probe 8 in both branches 4 and 5 ; an oxidation catalytic converter ( 12 ) connected downstream in the end pipe 10 is not shown, but can be provided. Here, too, is the basic procedure as described in FIG. 1, with the difference that with an equivalent design of the NOx catalytic converters 7, the switching of the exhaust gas switch 3 is not dependent on the driving phases, but solely on the basis of time and / or the diesel engine operating state. With regard to the storage capacity, one of the NOx storage catalytic converters 7 can also be designed to be weaker, taking this lower capacity into account can also be carried out as described above, but also after the regeneration described in FIG. 1 in different operating phases of the internal combustion engine, the weaker designed NOx storage catalytic converter advantageously takes over the NOx storage in the overrun and / or idling phases of the diesel internal combustion engine, while the more strongly designed NOx storage catalytic converter is regenerated.

Such an exhaust system can, as shown in FIG. 5 with 401 , be equipped with a burner system 413 . For separate supply of the two branches 4 and 5 with the heated burner gases, these are fed separately via a flap system 415 depending on the operating state in the branches 4 and 5 . This makes it possible to use a fuel injection 406 . If the burner is not only used for the regeneration of the NOx storage catalytic converter 7 (if sulfate deposits are also to be removed here, the regeneration temperature must be raised to above 700 ° C.), the distribution flaps 415 are controlled accordingly so that each branch has the required for it gets cheap heat and fuel.

FIG. 6 shows an exhaust system 501 constructed in accordance with FIGS. 4 and 5, which is equipped with E-catalysts 414 instead of a burner system. In principle, the function here is as described in FIG. 5, here again individual fuel injections 6 are provided. Here too, only one lambda probe 8 is used, which fulfills the function described in FIG. 1. Here, in addition, an oxidation catalytic converter 12 is also provided in the end pipe 10 in order to ensure in particular oxidation of possibly too much injected fuel.

From Fig. 7 it can be seen that there is a clear connection between the exhaust gas temperature and the NOx emissions of the diesel engine. If the exhaust gas temperature drops to 200 ° C (this value can vary somewhat depending on the diesel engine, whereby the described effect is always present at low temperatures), the NOx emissions are particularly low (with the engine at operating temperature), so that the exhaust gases are further freed of nitrogen oxides is almost insignificant with regard to the total emission of nitrogen oxides (10%). If the exhaust gas temperature drops below 200 ° C, or to values at which there is only a small NOx emission, the NOx storage can be separated from the exhaust gas stream and regenerated. In normal driving, there are enough such phases with lower exhaust gas temperatures, especially in diesel engines with high efficiency, as is the case with turbo diesel direct injection engines. Accordingly, the present invention is particularly suitable for such diesel units.

Claims (15)

1. Method for operating a diesel internal combustion engine with a NOx store through which the exhaust gas flow flows for NOx storage, and regeneration of the NOx store in which the NOx store is at least predominantly shut off from the exhaust gas stream while the diesel internal combustion engine is in operation. characterized in that the NOx store is designed as an oxidation catalyst and / or has preceded it, and in that the fuel is oxidized in the presence of oxygen by introducing fuel in the presence of oxygen in order to heat up the NOx store. 2. The method according to claim 1, characterized in that the oxidation catalyst has electrical heating. 3. Method for operating a diesel internal combustion engine with a NOx store, the is flowed through by the exhaust gas stream for NOx storage, and regeneration of the NOx storage, in which the NOx storage during operation of the Diesel internal combustion engine at least predominantly shut off from the exhaust gas flow is characterized in that the NOx storage and / or this upstream oxidation catalyst is electrically heated. 4. The method according to claim 3, characterized in that the NOx storage as Oxidation catalyst is formed. 5. The method according to any one of the preceding claims, characterized in that the NOx storage is in a first branch of the exhaust system and that of this at least predominantly shut off exhaust gas flow via a second branch below Bypassing the NOx storage is performed, and that in the direction of flow after  NOx storage an oxidation catalyst is arranged, which is separated from that by the NOx Storage led exhaust gas flow and from that blocked by the NOx storage Exhaust gas flow is flowed through. 6. The method according to any one of the preceding claims, characterized in that the NOx storage is in a first branch of the exhaust system and that of this at least predominantly shut off exhaust gas flow via a second branch below Bypassing the NOx storage is performed, and that in the second branch no NOx Storage and / or an oxidation catalyst not functioning as a NOx storage. 7. The method according to claim 5, characterized in that of the NOx storage at least predominantly blocked off exhaust gas flow in the second branch via a second NOx storage is performed, which alternates with the (first) NOx storage is regenerated. 8. The method according to any one of claims 1 to 6, characterized in that the Shut off specifically with an average low NOx content in the exhaust gas flow Unit of time and / or in overrun phases and / or in idle phases of the Diesel engine is done. 9. The method according to claim 8, characterized in that the shut-off period At least 50%, especially at least 75%, occurs when an average low NOx content in the exhaust gas flow per unit of time and / or one overrun phase and / or there is an idling phase of the diesel engine. 10. The method according to any one of the preceding claims, characterized in that a fuel is introduced into the NOx storage device for regeneration. 11. The method according to any one of the preceding claims, characterized in that a burner for heating the NOX storage (s) and / or the Oxidation catalyst / s is provided.   12. The method according to any one of the preceding claims, characterized in that shutting off occurs when the temperature of the exhaust gas flow drops to one predetermined value, in particular 200 ° C, drops. 13. The method according to any one of the preceding claims, characterized in that the shut-off takes place specifically when the temperature of the exhaust gas flow to such a level low value decreases that the NOx storage capacity of the NOx storage due to temperature to 90% of the NOx storage capacity of the NOx storage at 300 ° C decreases. 14. The method according to any one of the preceding claims, characterized in that the shut-off continues until the overrun phase or the idle phase has ended and / or until the exhaust gas temperature again reaches a value above the predetermined one Value, in particular <200 ° C or <90% of the NOx storage capacity to the NOx storage capacity at 300 ° C. 15. Diesel engine with an exhaust system ( 1 , 101 to 501 ) in which a regenerable NOx storage ( 7 ), a branch in the flow direction (arrow) in front of the NOx storage ( 7 ) arranged for the exhaust gas flow and means ( 3 , 203rd ) are arranged, which at least during regeneration during operation of the diesel internal combustion engine send at least a predominant portion of the exhaust gas flow through the branch, so that the predominant portion does not flow through the NOx store, characterized by an electrically heatable oxidation catalytic converter which is located upstream of the NOx Storage and possibly arranged behind a fuel injection.