DE19731624A1 - Reversible rich regeneration of nitrogen@ oxide absorption catalyst - Google Patents

Abstract

This method desulphurises the NOx absorption catalyst (3) of an engine (1) exhaust system. When this becomes necessary, the exhaust temperature is raised, to a level adequate for desulphurisation, which lies below a maximum predetermined temperature limit. Desulphurisation occurs with the engine running rich. On completion, normal engine operation is resumed. Equipment to carry out the process is also claimed. There are temperature sensors (T1,T2), measuring the exhaust gas temperature, situated immediately upstream and downstream the storage catalyst.

Description

The invention relates to a method for monitoring and controlling the de-sulfation NOx storage catalytic converters, the internal combustion engines of the Otto or diesel engine type Exhaust gas cleaning are downstream.

NOx storage catalytic converters are activated during operation by the in the fuel contained sulfur and its storage as sulfate in the NOx storage catalyst poisoned so that the NOx uptake of the storage catalyst is hindered or overall is prevented. It is therefore periodic desulfurization or de- Carrying out sulfation of the NOx storage catalyst. It is known that a Sulfur poisoning of the NOx storage catalytic converter is largely or completely reversible is a minimum desulfation temperature in a reducing environment is exceeded.

In the case of lean-burn gasoline engines, one can be used in any operating state appropriate additional consumption with stoichiometric or rich air-fuel mixture be driven. Measures such as retarding the engine also Exhaust gas temperature can be varied within wide limits, so that usual de- Sulphation temperatures of approx. 650 ° C in almost every map area Otto engine can be achieved.

Currently known NOx storage catalytic converters can drop below approx. 550 ° C no longer store NOx in lean conditions and show strong aging effects when Exceeding approx. 800 ° C.  

In diesel engines, longer-term operation with λ <1 can occur because of this associated underperformance and the increasing particle emissions not carried out become.

The invention is therefore based on the object of developing a method for controlled de- Sulfation of NOx storage catalysts and a device for performing the To create process that controlled desulfation without destroying the NOx Enable storage catalytic converter in a simple manner.

This object is achieved by the method according to claim 1 and the device for Implementation of the method according to claim 13 solved. Preferred embodiments of the Invention are the subject of the dependent claims.

The present invention relates to a method for de-sulfating a NOx accumulator connected downstream of an internal combustion engine, which has the following steps:

• a) existence of the need to de-sulfate the NOx storage,
• b) initiation and implementation of an exhaust gas temperature-increasing measure with which the NOx storage catalytic converter to above a predetermined desulphation temperature is heated
• c) de-sulfation of the NOx storage by operating the engine with grease, and
• d) initiation of normal operation of the internal combustion engine after the end of the de- Sulfation step.

Furthermore, the exhaust gas temperature increasing measure is controlled so that the temperature of the NOx storage catalyst always below the aging temperature of the NOx storage lies. The maximum NOx storage temperature is preferably around a predetermined one Safety distance below the aging temperature.

In an internal combustion engine of the gasoline engine type, the engine is present (Recognize) the need for desulfurization operated stoichiometrically before one measure to increase exhaust gas temperature is initiated. The exhaust gas temperature increasing  Measure can be achieved by adjusting the ignition timing to a later time become.

Preferably, in an internal combustion engine of the gasoline engine type, immediately after the De-sulfation step performed an operation with λ = 1 before falling below a predetermined second safety distance from the aging threshold of the fat-lean Operation of the engine is resumed. The second is preferably Safety distance approx. 100 ° C.

Furthermore, in an internal combustion engine of the diesel engine type, after the detection of the Necessity of de-sulfation is carried out first before a NOx regeneration catalyst temperature-increasing measure is initiated. The NOx regeneration will performed by an operation with λ <1.

The measure to increase the catalyst temperature is preferably carried out in the case of diesel engines an adjustment of the injection start time, change of the EGR rate (exhaust gas recirculation rate), Lowering the boost pressure, partial throttling, combined fat-lean operation, Injecting fuel into the exhaust system in front of the catalytic converter or one Post-injection performed on CR engines (common rail). Furthermore, an enrichment for example by suction air throttling and / or EGR increase and / or Post-injection and / or boost pressure reduction can be achieved.

In the case of diesel engines, normal engine operation is started immediately after the Sulfation resumed.

The device for carrying out the method according to the invention comprises a Internal combustion engine with downstream NOx storage, immediately before and after a temperature sensor for measuring the exhaust gas temperature is arranged.

The invention is explained below with reference to the drawings, in which:  

Fig. 1 schematically shows the device for monitoring the de-sulfation both a gasoline and a diesel engine,

Fig. 2 shows the schematic sequence of a de-sulfation in the lean Otto engine, and

Fig. 3 shows schematically the desulfation process in the diesel engine.

Fig. 1, the device according to the invention shows for de-sulfation of an internal combustion engine comprising an engine 1 and an exhaust system 2, which includes a NOx storage catalyst 3, and temperature sensors T1 and T2, which are arranged immediately before and after the NOx storage catalyst 2. From the figure it can be seen that the temperature sensor T1 measures the exhaust gas temperature immediately upstream of the NOx storage 3, while the temperature sensor T2 measures the exhaust gas temperature immediately downstream of the NOx storage 3 .

Fig. 2 shows schematically the sequence of a de-sulfation at the lean Otto-engine 1. The course of time in relation to the exhaust gas temperature and the current λ value is shown. Curve I represents the temperature curve at the first temperature sensor T1, curve II the temperature curve at the second temperature sensor T2 and curve III the curve of λ.

Out of the usual alternating fat-lean operation is when the The need to desulfurize the engine first stoichiometrically operated and an exhaust gas temperature increasing measure (point A of curve III) - z. B. Late ignition - initiated until the temperature at the temperature sensor T1 before the Storage catalytic converter by a safety distance (e.g. 50 °) below the aging threshold lies, which is approximately 800 ° C with today's conventional NOx storage catalysts.

As a result of the rising exhaust gas temperature and the possibly increasing amount of pollutants in the exhaust gas, the temperature at the temperature sensor T2 behind the NOx storage catalytic converter also increases. If it can be seen that the aging threshold is exceeded by the catalytic reaction in the NOx storage catalytic converter, for example due to an excessively rapid temperature rise in curve point B of curve I, the effect of the exhaust gas temperature-increasing measure is reduced (point C of curve I). The risk of internal overheating of the NOx store 3 is likely if the temperature rise speed in the vicinity of the aging threshold is so high that the aging threshold must be exceeded without changing the exhaust gas temperature-increasing measure.

If the desulphation temperature threshold is exceeded before and after the NOx storage catalytic converter 3 (point D of curve II), the engine 1 is operated richly (from point E to point F of curve III) until the sulphate load has been safely reduced is. The duration and extent of the enrichment depend on the calculated or estimated sulfur load. Immediately following the de-sulfation (point F of curve III), lean operation of engine 1 must be avoided, since no NOx is stored anyway due to the high catalyst temperature and the addition of oxygen on the HC and CO-saturated carrier will lead to a safe exceeding of the age threshold at least in part of the storage catalytic converter 3 . Another short-term operation with λ = 1 is more favorable, but without measures to increase the exhaust gas temperature, in order to bring about a gradual decrease in the catalyst temperature.

Only after falling below a second safety distance (approx. 100 °) towards the aging threshold (point G of curve II) is engine 1 lean operation again permitted. The NOx thermal desorption threshold may be exceeded briefly in memory 3 ; at the relatively low exhaust gas temperatures of lean Otto engine vehicles, however, a rapid drop below the NOx thermal desorption threshold is to be expected.

Fig. 3 shows the schematic timing of a de-sulfation of a diesel engine 1. The designation of the curves corresponds to that of FIG. 2. In diesel engines, desulfurization cannot be carried out in the same way as in lean gasoline engines, since long-term operation with λ <1 is not possible because of the underperformance and the increasing particle emissions. Therefore, with a view to the NOx thermal desorption threshold, a NOx regeneration is first carried out when the need for desulfurization is recognized, so that the catalyst is heated when the NOx store 3 is empty. If the exhaust gas is lean, a measure to increase the catalyst temperature (point A of curve III) is taken, for example by adjusting the start of injection, changing the EGR rate (exhaust gas recirculation rate), lowering the boost pressure, partial throttling, combined rich-lean operation, injecting fuel into the exhaust system before the catalytic converter, post-injection (only for common rail engines). The exhaust gas temperature of the temperature sensor T1 upstream of the catalytic converter 3 is set analogously to lean-gasoline engines with a 50 ° safety margin from the aging threshold.

By monitoring the exhaust gas temperature of the temperature sensor T2 behind the NOx accumulator 3 , it is ensured, as in the case of the gasoline engine, that there is no overheating due to oxidation reactions on the NOx accumulator surface (point B of curve II); if necessary, thermal damage is avoided by partially withdrawing the measure that increases the catalyst temperature (point C of curve I). As soon as the temperature after the catalyst at temperature sensor T2 has exceeded the de-sulfation limit (point D of curve II), de-sulfation is initiated by enriching the exhaust gas (point E of curve III). The duration of the enrichment and the value of λ during the fat phase in turn depend on the calculated or estimated sulfur loading. The enrichment can be carried out analogously to the NOx regeneration, for example by suction air throttling, EGR increase, post-injection or boost pressure transmission.

After de-sulfation has ended (point F of curve III), no longer operation can take place more can be maintained with λ <1; it immediately becomes normal again Operating mode set. Because of the usually very low exhaust gas temperatures Diesel engines and the resulting rapid and strong cooling is despite the after Desulfurization occurring oxidation reactions only a small internal NOx storage Temperature peak to be expected.  

Reference list

1

engine

2nd

Exhaust system

3rd

NOx storage catalytic converter
T1 temperature sensor 1
T2 temperature sensor 2
I temperature curve T1
I temperature curve T2
II course λ
A Initiate KTM
B Temperature rise too fast
C KTM withdrawal
D Exceeds the desulfation temperature
E Initiate de-sulfation
F End of de-sulfation
G Fat-lean operation
KTM catalyst temperature increasing measure

Claims (13)

1. A method for de-sulfating an NOx accumulator ( 3 ) connected downstream of an internal combustion engine ( 1 ), which has the following steps:

• a) the need for desulfurization,
• b) initiation and implementation of an exhaust gas temperature-increasing measure with which the NOx storage catalytic converter ( 3 ) is heated to at least a predetermined desulfurization temperature,
• c) De-sulfation of the NOx storage by operating the engine with grease ( 1 ), and
• d) initiating normal operation of the internal combustion engine ( 1 ) after the end of the de-sulfation step.

2. The method according to claim 1, characterized in that the exhaust gas temperature-increasing measure is controlled so that the temperature of the NOx storage catalyst ( 3 ) is always below the aging temperature of the NOx storage ( 3 ). 3. The method according to claim 2, characterized in that the maximum NOx storage temperature by a predetermined safety distance below the Aging temperature. 4. The method according to any one of the preceding claims, characterized in that in an internal combustion engine ( 1 ) of the gasoline engine type, the engine ( 1 ) is operated stoichiometrically after recognizing the need for desulfurization before an exhaust gas temperature-increasing measure is initiated. 5. The method according to claim 4, characterized in that the Measure increasing the exhaust gas temperature by adjusting the ignition timing a later date is achieved. 6. The method according to any one of the preceding claims, characterized in that in an internal combustion engine ( 1 ) of the gasoline engine type immediately after the de-sulfation step, an operation with λ = 1 is carried out before after falling below a predetermined second safety distance to the aging threshold of the fat-lean - Operation of the engine ( 1 ) is resumed. 7. The method according to claim 6, characterized in that the second Safety distance is approx. 100 ° C. 8. The method according to any one of claims 1 to 4, characterized in that in an internal combustion engine ( 1 ) of the diesel engine type after the detection of the need for de-sulfation, a NOx regeneration is first carried out before a catalyst temperature-increasing measure is initiated. 9. The method according to claim 8, characterized in that the NOx regenerations is carried out by operation with λ <1. 10. The method according to any one of claims 8 or 9, characterized in that the catalyst temperature-increasing measure by adjusting the start of injection, changing the EGR rate, lowering the boost pressure, partial throttling, combined rich-lean operation, injecting fuel into the exhaust system before the catalyst or a post-injection in CR engines ( 1 ) or a combination of one or more of these measures is carried out. 11. The method according to any one of claims 8 to 10, characterized in that a Enrichment, for example by suction air throttling, EGR increase,  Post-injection, boost pressure reduction or a combination of one or more of these measures is achieved. 12. The method according to any one of claims 8 to 11, characterized in that the Normal engine operation again immediately after de-sulfation has ended is recorded. 13. Device for performing the method according to one of claims 1 to 12, wherein the device consists of an internal combustion engine ( 1 ) with a downstream NOx storage ( 3 ) and immediately before and after the NOx storage each have a temperature sensor (T1, T2) Measurement of the exhaust gas temperature is arranged.