DE10156476A1 - Detection of progressive sulfur accumulation in nitrogen oxides storage catalyst of petrol engine, determines regenerant quantities under standard running conditions - Google Patents

Abstract

A nitrogen oxides (NOx) storage catalyst is subjected to sulfur oxides (SOx) regeneration. The state of aging of the sulfur-free NOx storage catalyst is determined from the NOx storage capacity which is reduced in operation after a given aging cycle. On the basis of the known aging state, a number of time and distance running cycles are carried out with NOx regeneration using the required regenerant quantity over a restricted operational range of the engine.

Description

The invention relates to a method for detecting the progressive sulfurization, in particular a NO _x storage catalytic converter arranged in the exhaust duct of a direct-injection gasoline engine, in which NO _{x is} stored during operation with a lean fuel / air mixture, which during a cyclical regeneration phase Operation of the NO _x storage catalytic converter with a rich fuel / air mixture with the supply of a regeneration agent is converted catalytically into less environmentally harmful reaction products, the exhaust gas component concentration being measured via a gas sensor arranged downstream of the NO _x storage catalytic converter.

For the exhaust gas purification of internal combustion engines, in particular of direct-injection gasoline engines, catalysts are arranged in the exhaust gas duct of these internal combustion engines. These are used to convert the gaseous pollutants that arise during the combustion process of the air-fuel mixture, such as nitrogen oxides NO _x , carbon monoxide CO and unburned hydrocarbons HC, into less environmentally harmful reaction products.

As is known, the working mode of the internal combustion engines can be characterized on the basis of a lambda value which reflects the stoichiometric ratios of oxygen to a fuel. If there is an excess of oxygen, the internal combustion engine is in the so-called lean working mode. If the fuel predominates, there is a rich working mode. The formation of reducing gas components such as CO, HC or H ₂ is generally favored under a rich atmosphere, reducing gas components can be oxidized with oxygen (conversion reaction). The catalyst supports the establishment of the equilibrium of such a reaction.

Modern internal combustion engines are for the purpose of optimizing fuel consumption preferably in the lean operating mode operated, wherein for reducing the NO _x emission, the NO _x in _x -Save NO is absorbed. The NO _x stores can also be combined directly with the catalysts supporting the conversion reaction of NO _x to the known NO _x storage catalysts. NO _x is stored in the NO _x storage catalytic converters by chemisorption until a storage capacity or a NO _x desorption temperature is exceeded. Depending on predefinable limit values, such as the content of NO _x , CO or HC downstream of the NO _x storage catalytic converter, the internal combustion engine is then operated again under a rich atmosphere. The stored NO _x is desorbed again and converted in the conversion reaction in the NO _x storage catalytic converter using the reducing gas components (NO _x regeneration). To detect the parameters that initiate a change in the working mode of the internal combustion engine, gas sensors are arranged in the exhaust gas duct, which provide a signal corresponding to a content of the gas components in the exhaust gas downstream or upstream of the NO _x storage catalytic converter. Such gas sensors can be, for example, lambda sensors or component-specific sensors, such as NO _x sensors.

In dynamic operation of the internal combustion engine, damage to the NO _x storage catalytic converter can occur, which can lead to a deterioration in NO _x storage capacity and / or deactivation of the catalytic converter. The damage can be reversible or irreversible. Irreversible damage is, for example, age-related, often thermal damage, such as sintering of a catalyst component, segregation of the catalyst and storage component and an increasingly inhomogeneous NO _x loading of the NO _x storage catalyst near the surface.

In addition, reversible damage occurs, as can occur, for example, through sulfurization of the NO _x storage catalytic converter.

Fuels contain sulfur in varying proportions. During the combustion process, especially in a lean atmosphere, sulfur oxides SO _{x are formed} , which are absorbed by the NO _x storage catalyst as sulfate. Sulphate grain formation within the NO _x storage catalytic converter can on the one hand lead to a reduction in the storage capacity, but can also form a point of attack for corrosive processes. If the sulfur content is above 10 ppm (up to 10 ppm is defined as sulfur-free), sulfur poisoning of the catalytic converters can be expected over the running distance, but this is reversible and can be driven out by greasing at high NO _x catalytic converter temperatures.

The sulfur poisoning but will result in the near future by the legislature required emission test values in layered driving before a distance of 4000 km no longer be met, if not detected the Verschwefelungszustand the NO _x storage and SO _x regeneration of the NO _x storage in regular cycles. However, the running distance / cycle time of the internal combustion engine, which is dependent on the sulfur content of the fuel, between the desulfurizations is relatively long compared to the NO _x regeneration cycle. The precise detection and detection of the sulfurization of a NO _x storage catalytic converter is therefore of great importance for efficient pollutant reduction.

Since the thermal aging of the NO _x catalytic converter described at the outset, like the sulfurization, has a reducing effect on its storage capacity, it must be taken into account in the duration of the storage cycles and the frequency of the regeneration cycles. In order to comply with the required emission limit values and to achieve the lowest possible fuel consumption, it is important to determine the sulfur content and thermal aging as precisely as possible, e.g. B. with a NO _x sensor or a lambda sensor behind the NO _x catalyst.

A method is known from DE 199 51 544 C1 with which the NO _x storage capacity of a NO _x storage catalytic converter is ascertained and thus the aging condition of the storage device can be deduced. For this purpose, the exhaust gas component concentration is measured during the regeneration phase and the end of the regeneration phase is determined. The amount of regeneration agent supplied during the regeneration phase is recorded and serves as a measure of the total storage amount of the NO _x storage catalytic converter. After deduction of the known amount of oxygen also stored in the catalytic converter, the available NO _x storage capacity of the NO _x storage catalytic converter is obtained; the values obtained in this way are used for further operation of the NO _x storage catalytic converter.

As stated above, however, the increasing sulfurization of the NO _x storage catalytic converter may have significantly reduced the storage capacity. A separation of thermal aging and sulfurization is not easily possible with the known methods.

The object of the present invention is to provide a method which, in addition to the thermal aging of the NO _x catalytic converter, enables the degree of sulfurization of the catalyst system to be estimated with the greatest possible precision, in order to either block lean operation or to initiate a desulfurization cycle if a threshold value is exceeded.

• a) der NOx- Speicherkatalysator wird einer SO_x- Regeneration unterzogen,
• b) der Alterungszustand des schwefelfreien NO_x- Speicherkatalysators wird über das im Betrieb nachlassende NO_x-Speichervermögen nach einem vorgegebenem Alterungszyklus bei vorgegebener Alterungstemperatur messtechnisch ermittelt,
• c) unter Zugrundelegung des ermittelten Alterungszustandes wird während einer zeitlich und laufstreckenmäßig begrenzten Anzahl von Fahrzyklen mit NO_x-Regeneration die benötigte Regenerationsmittelmenge in einem eingeschränkten Betriebsbereich des Motors adaptiert,
• d) die adaptierte Regenerationsmittelmenge wird über eine Filterung als die speicherbare Standard-Regenerationsmittelmenge eines unverschwefelten NO_x-Speicherkatalysators abgespeichert,
• e) im weiteren Betrieb werden die Regenerationsmittelmengen mindestens einzelner Zyklen mit der abgespeicherten Standard-Regenerationsmittelmenge verglichen,
• f) bei einer Abnahme der Regenerationsmittelmenge wird auf eine fortschreitende Verschwefelung des NO_x-Speicherkatalysators erkannt und nach Überschreiten eines Schwellwertes eine Entschwefelung des Adsorbers angefordert bzw. eingeleitet.

To solve the problem, a method is proposed which is characterized by the following method steps:

• a) the NOx storage catalytic converter is subjected to SO _x regeneration,
• b) the aging state of the sulfur-free NO _x storage catalytic converter is measured using the NO _x storage capacity which decreases during operation after a predetermined aging cycle at a predetermined aging temperature,
• c) on the basis of the determined state of aging, the required amount of regeneration agent is adapted in a restricted operating range of the engine during a limited number of driving cycles with NO _x regeneration in terms of time and running distance,
• d) the adapted amount of regenerant is stored by filtering as the storable standard amount of regenerant of an unsulfurized NO _x storage catalytic converter,
• e) in further operation, the amounts of regenerant at least individual cycles are compared with the stored standard amount of regenerant,
• f) when the amount of regenerant decreases, a progressive sulfurization of the NO _x storage catalytic converter is detected and, after a threshold value has been exceeded, desulfurization of the adsorber is requested or initiated.

The method according to the invention optimizes the age detection method as described in DE 199 51 544 C1 and makes it applicable for the detection of the sulfurization of a NO _x adsorber. A variant of the method does not determine the oxygen content in the capacity of the storage. Instead, the oxygen content is assumed to be constant between two sulfurization cycles. This increases the signal precision that can be achieved.

As the engine continues to operate normally, the amount of regeneration agent is continuously monitored and compared with the adapted value. The sulfurization of the NO _x adsorber can then be deduced from the reduction in the amount of regeneration agent. If, after exceeding a minimum running distance, which results from the determination of the unfavorable released fuel, the temperature falls below a lean value, the lean operation is blocked and desulfurization of the adsorber is requested or initiated.

The following process diagram illustrates the method according to the invention:

The adaptation according to the invention captures all relevant system tolerances, including the engine-related NO _x raw emission tolerances of the engine. Only environmental fluctuations cannot be recorded, but these are leveled by filtering within the adaptation.

In a further embodiment of the method according to the invention, it is provided that two separate sulfur models are used for modeling and estimating the entry of sulfur into the NO _x storage catalytic converter, in which the extreme values of the maximum permissible value and one that is considered to be sulfur-free are preferably used to over the running distance and time in lean operation to determine the sulfur input which reduces the storage capacity, the storage behavior after aging depending on the temperature and the NO _x concentration being stored as a map in the engine control system.

It will prefer the extreme values 150 ppm (legally maximum permissible sulfur content in ROZ95 fuel) and 10 ppm (sulfur free) used. It could be another small number. about Running distance and lean period then result in a sulfur input and thus a reduction in storage capacity.

A map is stored in the engine control which describes the storage behavior after aging as a function of temperature, NO _x concentration and space velocity.

• a) der NO_x-Speicherkatalysator wird einer SO_x-Regeneration unterzogen,
• b) der Alterungszustand des schwefelfreien NO_x- Speicherkatalysators wird über das im Betrieb nachlassende NO_x-Speichervermögen nach einem vorgegebenem Alterungszyklus bei vorgegebener Alterungstemperatur gegebenenfalls messtechnisch ermittelt,
• c) unter Zugrundelegung des erkannten Alterungszustandes wird nach der Entschwefelung während einer zeitlich und laufstreckenmäßig begrenzten Anzahl von Fahrzyklen oder durch eine kurze Einspeicherungsphase im Schichtbetrieb die Sauerstoffspeicherfähigkeit der Abgasanlage ermittelt,
• d) die ermittelte Sauerstoffspeicherfähigkeit wird adaptiert und über eine Filterung als die Sauerstoffspeicherfähigkeit des unverschwefelten NO_x-Speicherkatalysators abgespeichert,
• e) im Weiteren wird die Verbrennungsmaschine im Normalbetrieb mit Schichtbetrieb und zyklischer NO_x-Regeneration gefahren,
• f) zwecks Überwachung des Schwefelgehalts im NO_x- Absorber wird die Regenerationsmittelmenge in einem eingeschränkten Betriebsbereich ermittelt,
• g) zur Berechung des relevanten NO_x-Anteils an der Regenerationsmittelmenge wird der oben adaptierte Sauerstoffanteil der unverschwefelten Anlage vom Signalwert abgezogen,
• h) im Entscheidungspfad für die Schwefelerkennung wird der aktuell berechnete NO_x-Anteil an der Regenerationsmittelmenge mit einem Schwellwert verglichen,
• i) bei Unterschreitung des Schwellwertes wird nach Abfrage einer Fehlerkennungslogik auf verschwefelten Zustand erkannt und die Entschwefelung eingeleitet.

A variant of the proposed method is characterized by the following method steps:

• a) the NO _x storage catalytic converter is subjected to SO _x regeneration,
• b) the aging state of the sulfur-free NO _x storage catalytic converter is determined by measurement, if necessary, using the NO _x storage capacity which decreases during operation after a given aging cycle at a given aging temperature,
• c) on the basis of the recognized state of aging, after desulfurization, the oxygen storage capacity of the exhaust system is determined during a limited number of driving cycles in terms of time and running distance or through a short storage phase in shift operation,
• d) the determined oxygen storage capacity is adapted and stored by filtering as the oxygen storage capacity of the unsulfurized NO _x storage catalytic converter,
• e) furthermore, the internal combustion engine is operated in normal operation with shift operation and cyclical NO _x regeneration,
• f) in order to monitor the sulfur content in the NO _x absorber, the amount of regenerant is determined in a restricted operating range,
• g) in order to calculate the relevant NO _x portion of the amount of regenerant, the oxygen portion of the unsulfurized system adapted above is subtracted from the signal value,
• h) in the decision path for sulfur detection, the currently calculated NO _x portion of the amount of regeneration agent is compared with a threshold value,
• i) if the threshold value is undershot, an error detection logic for a sulfurized state is recognized and the desulfurization is initiated.

After desulphurization of the NO _x adsorber has also been carried out, a method of aging detection is used, as described in patent claims 2 to 7 of DE 1999 51 544 C1. The aging condition of the sulfur-free adsorber can be determined with acceptable accuracy using this method. On the basis of the determined aging factor for thermal aging of the NO _x adsorber, the oxygen storage capacity of the exhaust gas system is then determined in a limited number of driving cycle times and running distances after the desulfurization, by means of a short storage phase in shift operation. There should be no overrun phase between the last NO _x regeneration and this brief storage, so that there is no signal increase due to the increased oxygen concentration. The NO _x content can be neglected due to the short injection period.

The advantage here is that the Oxygen adaptation takes place under identical conditions as that Measurement of the amount of regenerant. This will physical cross effects avoided.

The oxygen storage capacity is adapted and saved via filtering.

The adapted value represents the oxygen storage capacity for the unsulfurized exhaust system. The engine then runs in normal operation with stratified operation and NO _x regeneration, as described in claim 1, feature a) -f) of DE 1999 51 544 C1.

In order to monitor the sulfur content in the NO _x absorber, the amount of regenerant is determined in a restricted operating range. For this monitoring mode, the storage period of the NO _x cycle and the engine parameters u. U. be changed.

To calculate the relevant NO _x portion of the amount of regeneration agent, the above adapted oxygen portion of the unsulfurized system is subtracted from the signal value (not a currently calculated oxygen portion). Oxygen offset compensation of the signal value is thereby achieved. In the decision path for sulfur detection, the currently calculated NO _x portion of the amount of regeneration agent is compared with a threshold value or model value; if this is undershot, after querying an error detection logic (minimum running distance reached, etc), the following formula for a sulphurized state is recognized:

RAI_NOX_O2 = RAI_AKT - O2_SPK_OS_AD

IF RAI_NOX_O2 ⇒ RAI_NOX_O2_MIN THEN B_NOXKAT_S = TRUE

mean:
RAI_AKT: Current amount of regenerant
O2_SPK_OS_AD: adapted oxygen storage capacity without the influence of sulfur
RAI_NOX_O2: Oxygen offset compensated amount of regenerant
RAI_NOX_O2_MIN: Threshold value for oxygen offset-compensated amount of regenerant
B_NOXKAT_S: Status detection NO _x adsorber is contaminated

Various advantages are achieved through the above-mentioned offset compensation:
The reduction in oxygen storage capacity due to sulfur input can be used to increase the selectivity for sulfur detection.

Due to the sulfurization of the exhaust system the oxygen storage capacity is reduced. There in one Sulfurization cycle of up to 4000 km the vehicle due to thermal aging the oxygen storage capacity can be neglected can, this portion can directly influence the sulfur be attributed. This influence leads to reduction the current amount of regeneration agent and is according to the above. Offset compensation for the detection selectivity used.

In addition, by using the offset-compensated adaptation value the fault tolerance caused by Scattering of measured values in oxygen storage determination arises, tempered. Since all individual values of the Amount of regeneration agent with the identical oxygen storage value are corrected, the detection selectivity is only systematically, but not stochastically influenced.

The following process diagram illustrates this variant of the method according to the invention:

The proposed method enables a sulfurization detection with a lambda sensor concept with two or more lambda sensors behind the NO _x adsorber without the need for NO _x sensors. It enables continuous certification and longer lean-burn operation using low sulfur fuel. Ultimately, this means a similarly good reduction in consumption as in a system with a NO _x sensor at significantly lower system costs.

Claims (3)

1. Method for detecting the progressive sulfurization, in particular a NO _x storage catalytic converter arranged in the exhaust duct of a direct-injection gasoline engine, in which NO _{x is} stored during operation with a lean fuel / air mixture, which in a cyclical regeneration phase during operation of the NO _x storage catalytic converter with a rich fuel / air mixture with the addition of a regeneration agent is converted catalytically into less environmentally harmful reaction products, the exhaust gas component concentration being measured via a gas sensor arranged downstream of the NO _x storage catalytic converter, characterized by the following process steps: a) the NO _x storage catalytic converter is subjected to SO _x regeneration, b) the aging state of the sulfur-free NO _x storage catalytic converter is determined by measurement, if necessary, using the NO _x storage capacity which decreases during operation after a given aging cycle at a given aging temperature, c) on the basis of the recognized aging condition, the required amount of regeneration agent is adapted in a restricted operating range of the engine during a limited number of driving cycles with NO _x regeneration in terms of time and running distance, d) the adapted amount of regenerant is stored by filtering as the storable standard amount of regenerant of an unsulfurized NO _x storage catalytic converter, e) in further operation, the amounts of regenerant at least individual cycles are compared with the stored standard amount of regenerant, f) when the amount of regenerant decreases, a progressive sulfurization of the NO _x storage catalytic converter is detected and, after a threshold value has been exceeded, desulfurization of the adsorber is requested or initiated. 2. The method according to claim 1, characterized in that two separate sulfur models are used for modeling and estimating the entry of sulfur into the NO _x storage catalytic converter, in which the extreme values of the maximum permissible and one considered to be sulfur-free values are preferably used to over the running distance and time in lean operation to determine the sulfur input which reduces the storage capacity, the storage behavior after aging depending on the temperature and the NO _x concentration being stored as a map in the engine control system. 3. A method for detecting the progressive sulfurization, in particular a NO _x storage catalytic converter arranged in the exhaust duct of a direct-injection gasoline engine, in which NO _{x is} stored during operation with a lean fuel / air mixture, which in a cyclical regeneration phase during operation of the NO _x storage catalytic converter with a rich fuel / air mixture with the addition of a regeneration agent is converted catalytically into less environmentally harmful reaction products, the exhaust gas component concentration being measured via a gas sensor arranged downstream of the NO _x storage catalytic converter, characterized by the following process steps: a) the NO _x storage catalytic converter is subjected to SO _x regeneration, b) the aging state of the sulfur-free NO _x storage catalytic converter is measured using the NO _x storage capacity which decreases during operation after a predetermined aging cycle at a predetermined aging temperature, c) on the basis of the determined aging condition, the oxygen storage capacity of the exhaust system is determined during a limited number of driving cycles after desulfurization or through a short storage phase in shift operation, d) the determined oxygen storage capacity is adapted and stored by filtering as the oxygen storage capacity of the unsulfurized NO _x storage catalytic converter, e) furthermore, the internal combustion engine is operated in normal operation with shift operation and cyclical NO _x regeneration, f) in order to monitor the sulfur content in the NO _x adsorber, the amount of regenerant is determined in a restricted operating range, g) in order to calculate the relevant NO _x portion of the amount of regenerant, the oxygen portion of the unsulfurized system adapted above is subtracted from the signal value, h) in the decision path for sulfur detection, the currently calculated NO _x portion of the amount of regeneration agent is compared with a threshold value, i) if the threshold value is undershot, an error detection logic for a sulfurized state is recognized and the desulfurization is initiated.