DE102017200090A1 - Method for operating an exhaust system with NOx storage catalytic converter and SCR exhaust aftertreatment device - Google Patents

Abstract

The invention relates to a method for operating an exhaust system of an internal combustion engine (1), wherein the exhaust gas flows through at least one NOx storage catalytic converter (2) and subsequently at least one SCR exhaust gas aftertreatment device (3) whose aging state is determined. According to the invention, the NOx storage catalytic converter (2) is regenerated increasingly frequently in accordance with the determined aging state of the SCR exhaust gas aftertreatment device.

Description

The invention relates to a method for operating an exhaust system of an internal combustion engine, wherein the exhaust gas flows through at least one NOx storage catalytic converter and subsequently at least one SCR exhaust gas aftertreatment device, whose aging state is determined, and an exhaust system configured for carrying out the method according to the preambles of the independent claims.

Such a method and such an exhaust system are from the DE 10 2011 004 557 A1 known.

Internal combustion engines often generate significant amounts of nitrogen oxides (NOx) during operation. In particular, in diesel and Otto engines used in motor vehicles, the amounts of nitrogen oxide in the exhaust gas are generally above the permissible limits, so that an exhaust aftertreatment to reduce the NOx emissions is necessary. In many engines, the reduction of nitrogen oxides by the non-oxidized constituents contained in the exhaust gas, namely by carbon monoxide (CO) and unburned hydrocarbons (HC), using a three-way catalyst. In particular, in diesel and gasoline lean-burn engines, however, this method is not available due to the small amounts of non-oxidized exhaust gas constituents.

In the case of lean-burn engines, a NOx storage catalytic converter (also called LNT for the English Lean NOx Trap), which receives and stores the nitrogen oxides contained in the exhaust gas of the internal combustion engine, is therefore used in accordance with a widespread method. From time to time, there is a regeneration of the NOx storage catalytic converter, for which, for example, a fuel surplus is generated in the exhaust gas conducted through the NOx storage catalytic converter.

The functionality of the NOx storage catalyst decreases with increasing operating time, which is due, inter alia, to contamination of the NOx storage catalyst with the sulfur contained in the exhaust gas, as well as thermal aging as a result of high temperatures, such as occur in a regularly be performed desulfurization. It is therefore necessary to monitor the functionality of a planned in the exhaust system NOx storage catalyst. Due to the thermal aging, the desired conversion of the nitrogen oxides towards the end of the life of the NOx storage catalyst is adversely affected, in particular, the storage capacity decreases due to thermal aging. To determine the aging state or the nitrogen oxide storage capacity of NOx storage catalysts various methods are known in the art, for. B. from the DE 10 2012 218 728 A1 , of the DE 198 23 921 A1 and the DE 198 52 240 A1 ,

A disadvantage of NOx storage catalysts is the limited temperature window that can be achieved in achieving adequate NOx conversion efficiency. Above a maximum operating temperature of a NOx trap catalyst, NOx conversion efficiency becomes inefficient, or even the catalyst undergoes unintentional aging and deterioration of the catalyst properties.

For nitrogen oxide reduction at higher exhaust gas temperatures than the maximum operating temperature of NOx storage catalytic converters, SCR exhaust aftertreatment devices, i. H. with selective catalytic reduction (Selective Catalytic Reaction). SCR exhaust aftertreatment devices are z. B. SCR catalysts, hereinafter abbreviated to SCR, and SCR-coated diesel particulate filter, abbreviated to SDPF.

For selective catalytic reduction, ammonia (NH ₃ ) is admixed to the exhaust gas, in the case of motor vehicle internal combustion engines in the form of an aqueous urea solution called AdBlue, which is evaporated after the injection and decomposed into ammonia and other substances. The ammonia is stored in the SCR exhaust aftertreatment device and converts the nitrogen oxides contained in the exhaust gas, ideally in nitrogen.

If an upstream NOx storage catalyst no longer operates efficiently at high exhaust gas temperatures, excess NOx is converted to the SCR exhaust aftertreatment device or its input line by urea injection.

Also in SCR exhaust aftertreatment devices, it is known to monitor their ammonia storage capacity or aging state, for. B. from the aforementioned DE 10 2011 004 557 A1 or the US 2011/0153260 A1 ,

The invention is based on the object to extend the life of a combined LNT / SCR exhaust aftertreatment system.

This object is achieved by a method and an exhaust system having the features of the preambles of the independent claims.

Advantageous developments of the invention are specified in the dependent claims.

According to the invention, the NOx storage catalyst is determined according to the determined Aging condition of the SCR exhaust aftertreatment device increasingly regenerated with increasing operating time.

SCR exhaust after-treatment devices have good NO x conversion efficiency above their light-off temperature, but virtually none when cold. Combining an SPDF and / or SCR catalyst with an upstream NOx trap catalyst, the latter may store NOx at lower temperatures and later self-regenerate or release the stored NOx to be converted in the SCR exhaust aftertreatment device. For most on-the-fly driving cycles, provided they are not too short, this ensures a good overall conversion efficiency of combined LNT / SCR exhaust aftertreatment systems.

It has been found that with combined LNT / SCR exhaust aftertreatment systems and practical driving cycles, aging of the SCR exhaust aftertreatment device is more critical than aging of the NOx storage catalytic converter. Because if the conversion efficiency of the NOx storage catalyst significantly deteriorates, the SCR will absorb this. If, on the other hand, the conversion rate of the SCR deteriorates, the entire exhaust system must be regarded as defective, even if the conversion efficiency of the LNT has only slightly deteriorated.

If the NOx storage catalyst ages faster than the SCR exhaust aftertreatment device, this merely results in the control electronics of the SCR exhaust aftertreatment device detecting either more NOx at the inlet or an increase in NOx slippage and therefore increasing the urea dosage accordingly.

If, however, the NOx storage catalytic converter ages faster than the SCR exhaust gas aftertreatment device, this can not be detected by the control electronics of the NOx storage catalytic converter since the SCR exhaust gas aftertreatment device is located downstream of the NOx storage catalytic converter and does not see the sensor of the NOx storage catalytic converter, which results from the SCR Exhaust gas aftertreatment device comes out.

The resulting difficulty in maintaining the NOx conversion efficiency of a combined LNT / SCR exhaust aftertreatment system as long as possible is eliminated with the invention.

In a preferred embodiment, the regeneration schedule or purge schedule of the NOx trap catalyst is modified based on the determined aging condition of the SCR exhaust after-treatment device such that the power loss of the SCR exhaust after-treatment device is compensated for by correspondingly more frequent regeneration.

In a preferred embodiment, the increase in the frequency of the regeneration of the NOx storage catalyst associated with the aging of the SCR exhaust aftertreatment device is achieved by monitoring the amount of NOx stored in the NOx storage catalyst and a stored threshold amount of NOx whose exceeding the NOx storage catalyst is regenerated is reduced.

Alternatively or additionally, the increase in the frequency of regeneration of the NOx storage catalyst along with the aging of the SCR exhaust aftertreatment device may be achieved by changing one or more stored regenerative regeneration thresholds.

Reduced or modified threshold values for the NOx quantity and / or economy or adaptation factors for these threshold values are preferably stored as corresponding new threshold values or adaptation factors, in order subsequently, even after a restart of the internal combustion engine, as the threshold values or their adaptation factors for the NOx Quantity and / or convenience to be used.

• 1 eine Abgasanlage mit einem LNT und einem motornahen SCR-Katalysator;
• 2 eine Abgasanlage mit einem LNT, einem SDPF und einem motorfernen SCR-Katalysator; und
• 3 eine Abgasanlage mit einem LNT und einem motorfernen SCR-Katalysator.

The following is a description of embodiments with reference to the drawings. Show:

• 1 an exhaust system with an LNT and a close-coupled SCR catalyst;
• 2 an exhaust system including an LNT, a SDPF and a remote SCR catalyst; and
• 3 an exhaust system with an LNT and a SCR catalytic converter remote from the engine.

In 1 contains an exhaust system of an internal combustion engine 1 an active LNT (NOx storage catalyst) 2 and downstream of it an active SCR catalyst 3 , Downstream of the LNT 2 and upstream of the SCR catalyst 3 protrudes a urea injector 4 in the exhaust stream.

In 2 contains an exhaust system of an internal combustion engine 1 an active LNT 2 and downstream of it, an SDPF 5 , with the urea injector 4 between. The LNT 2 and the SDPF 5 are relatively close to the combustion engine 1 and much further downstream, a small active underbody SCR catalyst closes 3. ' at.

In 3 contains an exhaust system of an internal combustion engine 1 an active LNT 2 close to the combustion engine 1 and much further downstream, right behind a urea injector 4 , a small active underbody SCR catalyst 3. ' ,

On the basis of in 1 to 3 Exhaust systems shown as examples will now be described as how the life of a combined LNT / SCR exhaust aftertreatment system can be improved by increasing the life of the SCR catalyst 3 or 3. ' and / or the SDPF 5 is extended.

After the SCR exhaust aftertreatment device in the form of the SCR catalyst 3 or 3. ' and / or the SDPF 5 it can do the NOx conversion alone. However, the NOx conversion efficiency of an aged SCR exhaust aftertreatment device becomes worse, and to that extent, the NOx conversion performance of the LNT becomes 2 elevated.

For this purpose, information about the aging state of the SCR exhaust aftertreatment device can be used, which are supplied by the diagnostic functionality, as in the prior art such. B. the DE 10 2011 004 557 A1 and the US 2011/0153260 A1 known.

However, it is also conceivable to use diagnostic methods which have been described in the prior art for NOx storage catalysts in a similar form for the SCR exhaust aftertreatment device.

The information on the aging state of the SCR exhaust aftertreatment device can be obtained by comparing an actual measurement z. B. the NOx-slip of the SCR exhaust aftertreatment device with a modeled parameter. The corresponding model may be a kinetic SCR model that models the main reactions taking place in the SCR exhaust aftertreatment device. The model can run online in parallel with different aging state calibrations. Even better, the aging condition can be iterated to find an aging condition that corresponds to many measurements over a given interval. The output is a measure representing the state of aging.

The thus obtained information about the aging state of the SCR exhaust aftertreatment device in the form of the SCR catalyst 3 or 3. ' and / or the SDPF 5 may be used to generate an adaptation factor for the regeneration scheme of the NOx trap catalyst or LNT 2.

The adaptation factor may be used to adjust one or more thresholds of the LNT regeneration scheme during operation such that the NOx storage catalyst is gradually regenerated more frequently in accordance with the determined aging condition of the SCR exhaust aftertreatment device to increase the power loss of the SCR exhaust aftertreatment device compensate.

The adaptation factor or any adjusted thresholds can be stored in the control electronics of the exhaust system, so that they are available again after a restart of the internal combustion engine.

• - Ein Schwellenwert für die NOx-Menge, bei dessen Überschreiten der NOx-Speicherkatalysator regeneriert wird.
• - Ein oder mehrere Schwellenwerte für die Günstigkeit von Regeneration, d. h., wie günstig die gerade herrschenden Betriebsbedingungen des NOx-Speicherkatalysators, des Verbrennungsmotors usw. für eine Regeneration sind.

Examples of thresholds that are thus adjusted and, in particular, reduced are:

• - A threshold value for the amount of NOx, beyond which the NOx storage catalyst is regenerated.
• One or more regenerative regeneration thresholds, ie, how favorable the currently operating operating conditions of the NOx trap catalyst, internal combustion engine, etc., are for regeneration.

The more frequent regeneration of the NOx storage catalyst with increasing aging of the SCR exhaust aftertreatment device increases the NOx conversion performance provided to the NOx storage catalyst as a whole, thus maintaining the NOx conversion performance of the overall exhaust system over a longer lifetime. Also, the SCR exhaust aftertreatment device may age more strongly before the overall exhaust system needs to be considered defective. Therefore, this strategy substantially extends the life of a combined LNT / SCR exhaust aftertreatment system.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102011004557 A1 [0002, 0010, 0030]
• DE 102012218728 A1 [0005]
• DE 19823921 A1 [0005]
• DE 19852240 A1 [0005]
• US 2011/0153260 A1 [0010, 0030]

Claims (6)

Method for operating an exhaust system of an internal combustion engine (1), wherein the exhaust gas flows through at least one NOx storage catalytic converter (2) and subsequently at least one SCR exhaust aftertreatment device (3, 3 ', 5), wherein the aging state of the SCR exhaust aftertreatment device (3, 3 ', 5) is determined, characterized in that the NOx storage catalytic converter (2) according to the determined state of aging of the SCR exhaust gas treatment device (3, 3', 5) is regenerated with increasing frequency. Method according to Claim 1 characterized in that based on the determined aging state of the SCR exhaust aftertreatment device (3, 3 ', 5), a regeneration scheme of the NOx storage catalyst (2) is changed such that the power loss of the SCR exhaust aftertreatment device (3, 3', 5) is compensated by correspondingly more frequent regeneration. Method according to Claim 1 or 2 , characterized in that the increase in the frequency of the regeneration of the NOx storage catalytic converter (2) is achieved in conjunction with the aging of the SCR exhaust aftertreatment device (3, 3 ', 5) in that the NOx storage in the NOx storage catalytic converter (2) Amount is monitored and that a stored threshold for the amount of NOx, beyond which the NOx storage catalyst (2) is regenerated, is reduced. Method according to one of the preceding claims, characterized in that the increase in the frequency of regeneration of the NOx storage catalyst (2) associated with the aging of the SCR exhaust aftertreatment device (3, 3 ', 5) is achieved in that one or more stored threshold values be changed for the convenience of regeneration. Method according to one of the preceding claims, characterized in that reduced or altered threshold values for the amount of NOx and / or cheapness or adaptation factors for the threshold values are stored as corresponding new threshold values or adaptation factors. Exhaust system of an internal combustion engine (1), in particular in a motor vehicle, with at least one NOx storage catalyst (2) and downstream thereof at least one SCR exhaust aftertreatment device (3, 3 ', 5), characterized in that the exhaust system for performing the method according to a of the preceding claims.

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