DE102020215507A1 - Exhaust gas aftertreatment arrangement and method for aftertreatment of an exhaust gas of an internal combustion engine - Google Patents

Abstract

An exhaust gas aftertreatment arrangement (2) for an internal combustion engine, in particular for an Otto engine (1), is proposed, having a first catalytic converter (3), a second catalytic converter (5) arranged downstream of the first catalytic converter (3) in relation to the exhaust gas flow direction, and at least one additional one , the second catalytic converter based on the exhaust gas flow direction downstream third catalytic converter (7), wherein exhaust gas downstream of the first catalytic converter (3) and exhaust gas upstream of the second catalytic converter (5) an air supply line (11) opens into the exhaust gas line (4). Furthermore, a corresponding method for after-treatment of the exhaust gas of an internal combustion engine, in particular an Otto engine (1), is proposed.

Description

State of the art

The invention is based on an exhaust gas aftertreatment arrangement and a method for aftertreatment of an exhaust gas according to the species of the independent claims.

From the WO 2015/135983 exhaust gas aftertreatment arrangements are already known which contain a plurality of catalytic converters arranged one behind the other, such as, for example, a plurality of three-way catalytic converters in combination with an SCR catalytic converter.

With a view to future exhaust gas legislation, e.g. Euro 7, the exhaust gas aftertreatment should ensure a very good setting of the operating points of the exhaust gas cleaning system at all times, if possible, in order to achieve sufficient conversion rates of the exhaust gas components provided with specified limit values.

Disclosure of Invention

The exhaust gas aftertreatment arrangement according to the invention and the inventive method for aftertreatment of an exhaust gas with the characterizing features of the independent claims have the advantage of improved adjustability of the operating point of the exhaust gas purification at any time during operation of the internal combustion engine while adhering to limit values. In particular, the use of one or more three-way catalysts or one or more SCR catalysts, i.e. catalysts for the selective catalytic reduction of nitrogen oxides, is advantageous here in order to remove or prevent proportions of carbon monoxide, nitrogen oxides and unburned hydrocarbons contained in the exhaust gas Ammonia produced during after-treatment leaves the exhaust system and is released into the environment via the exhaust pipe.

Advantageous further developments and improvements of the exhaust gas aftertreatment arrangement specified in the independent claims or of the specified method for exhaust gas aftertreatment are possible as a result of the measures listed in the dependent claims.

It is particularly advantageous to design the third catalytic converter as a catalytic converter for the selective catalytic reduction of nitrogen oxides or as an SCR catalytic converter that can temporarily store ammonia, which can arise in particular in gasoline engines when a three-way catalytic converter is operated, especially in the range of an air ratio of less than 1 and lower to medium operating temperatures. Even slight shifts from an operating point with an air ratio of 1 can lead to significant ammonia emissions, which can be temporarily stored by means of the SCR catalytic converter and then eliminated via a suitably designed air supply into the exhaust tract.

Furthermore, it is particularly advantageous here to design the second catalytic converter as a three-way catalytic converter, the mode of operation of which can be set separately from the mode of operation of the first catalytic converter by means of the air supply. The second catalytic converter can thus be put into an operating mode by means of air supply in which it generates nitrogen oxides with the aim of using them for the regeneration of the subsequent third catalytic converter in which ammonia has accumulated. The ammonia temporarily stored in the third catalytic converter thus reacts with the nitrogen oxides produced in the exhaust tract, the ammonia stock in the third catalytic converter is broken down and nitrogen is produced in the process, which can be emitted into the environment. The third catalytic converter, which serves as an ammonia store, is therefore regenerated by the nitrogen oxides. In this way, the regeneration of the third catalytic converter or the SCR catalytic converter can advantageously take place independently of optimal operation of the first catalytic converter, in particular a three-way catalytic converter. The functionality of the first catalytic converter is therefore fully retained even if the third catalytic converter is regenerated. The operation of the exhaust aftertreatment arrangement can be set up in such a way that targeted regeneration of the third catalytic converter takes place, in particular that only as much nitrogen oxides are produced in the second catalytic converter as are required for regeneration of the third catalytic converter. Furthermore, it is advantageous that the regeneration of the third catalytic converter is not dependent on an accidental entry of nitrogen oxides into the third catalytic converter due to a controllable or regulatable air supply upstream of the second catalytic converter. A breakthrough of ammonia through the relatively small ammonia reservoir formed by the SCR catalytic converter and thus an emission of ammonia into the environment can thus be prevented in a reliable manner. Furthermore, a reliable ammonia storage effect can be achieved and at the same time the storage potential of the third catalytic converter can be used much better compared to an arrangement or a method in which there is no separate air supply to the exhaust tract.

Further advantages result from the features mentioned in the further dependent claims and in the description.

Brief description of the drawings

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. The only 1 shows an exhaust aftertreatment arrangement.

character list

• 1 shows an exhaust gas aftertreatment system 2 of an internal combustion engine 1, in particular a gasoline engine. As indicated by an arrow, an exhaust pipe 4 discharges the exhaust gas produced in the internal combustion engine and allows it to pass through a number of catalytic converters before it is released into the environment after it has been cleaned. A catalytic converter 3 is provided in the form of a three-way catalytic converter, downstream of which is flow-technically another catalytic converter 5, also in the form of a three-way catalytic converter. A further catalytic converter 7 in the form of an SCR catalytic converter, ie a catalytic converter for the selective catalytic reduction of nitrogen oxides, is downstream of the further catalytic converter 5 in terms of flow.

An air supply line 11 is provided which protrudes into the exhaust line 4 so that air can be introduced into the exhaust line by means of an air supply device 9, for example an air pump or a device which is fed from the fresh air path of the internal combustion engine. The air supply line 11 is positioned in such a way that the air can get into the catalytic converter 5, ie the second three-way catalytic converter, without affecting the catalytic converter 3, ie the first three-way catalytic converter. The air supply line 11 protrudes, for example, as in 1 shown, upstream of the second three-way catalytic converter into the exhaust pipe 4, so that the input side of the second three-way catalytic converter can be subjected to this air. The air supply line 11 and/or the air supply device 9 preferably have an air valve, not shown in detail, with which the air flow can be controlled or regulated.

The air supply device 9 or the air valve mentioned are controlled electrically by an electronic control unit 6, which can be a separate control unit, a control unit for exhaust gas aftertreatment with integrated air control or an engine control unit, which takes over the control or regulation of the exhaust gas aftertreatment and the air supply. In the illustrated embodiment, the control unit 6 receives measurement data from a measuring device 13 and a further measuring device 15. The first measuring device 13 protrudes between the further catalytic converter 5 and the further catalytic converter 7, i.e. here between the second three-way catalytic converter and the SCR catalytic converter, into the exhaust tract , so that measurement signals can be obtained directly from the exhaust gas flow which flows into the SCR catalytic converter, in particular without having previously flowed through any further catalytic device. The second measuring device 15 protrudes into the exhaust tract downstream of the further catalytic converter 7, i.e. here the SCR catalytic converter, so that further measuring signals can be obtained from the exhaust gas stream exiting the SCR catalytic converter without it first having to pass through any further catalytic device Has.

The measuring device 13 and the further measuring device 15 each comprise at least one sensor which detects the presence of nitrogen oxides or ammonia in the relevant exhaust gas stream. Sensors are suitable for this purpose, which are used in the motor vehicle sector for measuring the proportion of nitrogen oxides and which are sensitive both to nitrogen oxides and to ammonia.

In principle, the exhaust gas aftertreatment arrangement is operated in a configuration with a first three-way catalytic converter 3 and a second three-way catalytic converter 5 in such a way that the lowest possible carbon monoxide, hydrocarbon and nitrogen oxide emissions are produced, with the air ratio being basically equal to 1. Any ammonia that occurs is used in the SCR catalytic converter 7 cached. The dual sensitivity of the sensors described above to nitrogen oxide and ammonia can be used in combination with information about a deviation from the air ratio of 1 to determine an ammonia content (if the air ratio is less than 1, rich exhaust gas) or a nitrogen oxide content in the exhaust gas (if the air ratio is greater than 1, lean exhaust gas).

Alternatively, however, it is also possible to use sensors that specifically measure ammonia. Various different sensors can thus be used for direct or indirect determination of an ammonia content in the exhaust gas and/or the amount of regeneration air, for example lambda sensors, in particular jump sensors, nitrogen oxide sensors or even special ammonia sensors.

However, if the system behavior is known, the ammonia and nitrogen oxide quantities can also be determined on a model basis. A combined method is also possible. Depending on the configuration of the control unit and the corresponding control or regulation of the internal combustion engine or the exhaust gas aftertreatment or the air supply device 9, the measuring devices 13 and 15 or one of the two measuring devices 13 and 15 can be omitted.

The SCR catalytic converter 7 is able to intermediately store any ammonia that occurs, which can arise as a function of the air ratio and temperature during the operation of three-way catalytic converters. By introducing air as required before the further catalytic converter or the second three-way catalytic converter 5, nitrogen oxides are generated in the second three-way catalytic converter 5, which are used to regenerate the SCR catalytic converter 7, ie to remove temporarily stored ammonia. There is a need for such an introduction of air, for example, when a certain maximum fill level of ammonia in the SCR catalytic converter is reached or when ammonia begins to break through. After regeneration, the SCR catalytic converter can store ammonia again.

So if a need for regeneration has been identified, the second three-way catalytic converter 5 is no longer operated with an air ratio equal to 1, but lean, ie with an air ratio greater than 1, by controlling the air inlet. So-called secondary nitrogen oxides are formed in the second three-way catalytic converter 5 and can be used to regenerate the ammonia stored in the SCR catalytic converter. The amount of nitrogen oxides formed depends on the amount of additional air introduced, the composition of the remaining exhaust gas and the boundary conditions acting in the second three-way catalytic converter 5, here primarily on catalytic converter-specific parameters, the temperature and the space velocity. When enough nitrogen oxides have been formed to regenerate the SCR catalytic converter, the introduction of additional air via the air supply line 11 is terminated again.

• Zum Beispiel kann eine Bestimmung eines Ammoniak-Durchbruchs mittels einer Meßeinrichtung 15 abgasstromabwärts des SCR-Katalysators 7 erfolgen, woraufhin eine Ansteuerung der Luftzufuhreinrichtung 9 erfolgt, damit Sekundär-Stickoxide gebildet und schließlich solange eine Regeneration des SCR-Katalysators 7 erfolgen kann, bis die Messeinrichtung 15 Stickoxide detektiert.
• Ein Vorteil dieses Verfahrens ist die Einfachheit und der mögliche Mehrfachnutzen der Meßeinrichtung 15 an der letzten Position, zum Beispiel auch für Aufgaben des zukünftig gegebenenfalls geforderten sogenannten „On-Board-Measurements“.

Various implementation variants are possible:

• For example, an ammonia breakthrough can be determined by means of a measuring device 15 downstream of the SCR catalytic converter 7, whereupon the air supply device 9 is activated so that secondary nitrogen oxides are formed and finally the SCR catalytic converter 7 can be regenerated until the measuring device 15 nitrogen oxides detected.
• An advantage of this method is the simplicity and the possible multiple use of the measuring device 15 in the last position, for example also for tasks of the so-called "on-board measurements" that may be required in the future.

For example, the ammonia entry can also be determined by means of a measuring device 13 exhaust gas upstream of the SCR catalytic converter 7, with an integration model for the SCR catalytic converter being used in the control unit. The air supply device 9 is activated when a threshold value of the integration model for the entry of ammonia into the SCR catalytic converter 7 is reached. Secondary nitrogen oxides are then formed in the second three-way catalytic converter 5, and the SCR catalytic converter is regenerated. The integration model implemented in the control unit in this exemplary embodiment also calculates the quantity of nitrogen oxides formed. As soon as a threshold value of the integration model for the nitrogen oxides formed is reached, the SCR catalytic converter is considered to have been sufficiently regenerated and the introduction of air is stopped.

For example, in a further embodiment variant, both the ammonia input into the SCR catalytic converter and the ammonia discharge from the SCR catalytic converter can be determined by two measuring devices 13 and 15 upstream and downstream of the SCR catalytic converter. In this case, an integration model for the SCR catalytic converter in control unit 6 can also be used. In this case, an air supply is introduced via the air supply line 11 when a threshold value of the integration model for the ammonia entry into the catalytic converter is reached and/or an ammonia breakthrough is detected with the measuring device 15 downstream of the SCR catalytic converter. Secondary nitrogen oxides are then formed and the SCR catalytic converter 7 is regenerated. The amount of nitrogen oxides formed is also determined by means of an integration model or the sensor after the SCR catalytic converter. When sufficient regeneration is achieved - determined from a nitrogen oxide threshold value of the integration model or by detecting nitrogen oxides with the sensor behind the SCR catalytic converter - the air intake is ended.

For example, in a further embodiment, based solely on the use of models to determine the ammonia load of the SCR catalytic converter, i.e. its possible regeneration requirement, and to determine the amount of nitrogen oxide required for regeneration, the control of the additional air by the control unit 6 via the air supply device 9 or the air supply line 11 in order to regenerate the SCR catalytic converter with a suitable amount of nitrogen oxides if required.

For example, combined methods can also be implemented in the control device in further embodiments. Combined methods are also possible through the use of sensor information from the measuring devices 13 and/or 15 and modeling values. For example, a model-based control of the regeneration of the SCR catalytic converter with additional adjustment and learning or also adapting the system behavior with available information from the measuring signals of the measuring devices.

For example, the control unit can also simply switch on the air supply device 9 regularly after a predetermined time, after a predetermined exhaust gas mass flow has passed, after a predetermined intake air flow has passed through the internal combustion engine, after a predetermined fuel mass flow has been consumed and/or after the exhaust gas aftertreatment system has reached a state derived from these variables . In this way, the SCR catalytic converter is regenerated regularly. However, this may happen more often than is actually necessary. Another disadvantage would be possibly unnecessary nitrogen oxide emissions from the second three-way catalytic converter 5. However, it can make sense to carry out such regular regeneration of the SCR catalytic converter 7 (at least also) in order to reliably prevent ammonia emissions, since a possible permitted limit value for ammonia can be significantly below a possible permitted limit value for nitrogen oxides.

In order to create storage conditions that are as robust as possible for the ammonia in the SCR catalytic converter 7, the following conditions for a regeneration, i.e. for an air supply, can alternatively be implemented, which can optionally also be used in addition to the alternatives already mentioned for conditions for initiating a regeneration can come: initiating a regeneration at the end of a driving cycle and/or initiating a regeneration after starting the internal combustion engine, the latter being able to happen as soon as the second three-way catalytic converter 5 is able to do so. The latter is intended to catch the ammonia emissions that occur during the heating-up phase of the first three-way catalytic converter 3 . It can be particularly useful here to heat the second three-way catalytic converter 5 externally, for example in the form of an e-cat or using a burner.

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was 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.

Patent Literature Cited

• WO 2015/135983 [0002]

Claims (24)

Exhaust gas aftertreatment arrangement (2) for an internal combustion engine, in particular for an Otto engine (1), with a first catalytic converter (3), a second catalytic converter (5) arranged downstream of the first catalytic converter (3) in relation to the exhaust gas flow direction and at least one further catalytic converter related to the second catalytic converter third catalytic converter (7) downstream of the exhaust gas flow direction, characterized in that an air supply line (11) opens into the exhaust gas line (4) downstream of the first catalytic converter (3) and upstream of the second catalytic converter (5). Exhaust aftertreatment arrangement (2) after claim 1 , characterized in that the air supply line (11) is connected to an air supply device (9). Exhaust gas after-treatment arrangement (2) according to one of the preceding claims, characterized in that the first catalytic converter (3) is set up to reduce proportions of carbon monoxide, nitrogen oxides and/or unburned hydrocarbons contained in the exhaust gas. Exhaust gas aftertreatment arrangement according to one of the preceding claims, characterized in that the first catalytic converter (3) is a three-way catalytic converter. Exhaust gas aftertreatment arrangement according to one of the preceding claims, characterized in that the second catalytic converter (5) is set up to reduce proportions of carbon monoxide, nitrogen oxides and/or unburned hydrocarbons contained in the exhaust gas. Exhaust gas aftertreatment arrangement according to one of the preceding claims, characterized in that the second catalytic converter (5) is a three-way catalytic converter. Exhaust gas aftertreatment arrangement according to one of the preceding claims, characterized in that the third catalytic converter (7) is set up for intermediate storage of ammonia. exhaust aftertreatment system claim 7 , characterized in that the third catalyst (7) is a catalyst for the selective catalytic reduction of nitrogen oxides. Exhaust aftertreatment arrangement according to one of the preceding claims, characterized in that a measuring device (15) is arranged in the exhaust pipe (4) downstream of the third catalytic converter (7) for determining the ammonia content in the exhaust gas escaping from the third catalytic converter. Exhaust gas aftertreatment arrangement according to one of the preceding claims, characterized in that a measuring device (13) is arranged in the exhaust pipe (4) between the second catalytic converter (5) and the third catalytic converter (7) for determining an entry of ammonia into the third catalytic converter (7 ). Exhaust aftertreatment arrangement according to one of the preceding claims, with an electronic control unit (6) which is set up to activate the air supply device (9). Method for after-treating the exhaust gas of an internal combustion engine, in particular a spark-ignition engine (1), in which the exhaust gas is passed through a first catalytic converter (3), through a second catalytic converter (5) arranged downstream of the first catalytic converter (3) in relation to the exhaust gas flow direction, and through at least one further , the third catalytic converter (7) downstream of the second catalytic converter in relation to the exhaust gas flow direction, characterized in that air is supplied downstream of the first catalytic converter (3) and upstream of the second catalytic converter (5) when the third catalytic converter (7) is regenerated target. procedure after claim 12 , characterized in that the regeneration of the third catalyst comprises a removal of intermediately stored ammonia. procedure after Claim 13 , characterized in that an ammonia content of the exhaust gas escaping from the third catalytic converter (7) is determined and in that the regeneration is carried out if there is an ammonia breakthrough downstream of the third catalytic converter (7). procedure after Claim 13 or 14 , characterized in that an entry of ammonia into the third catalytic converter (7) is determined and that when a threshold value of the ammonia entry is reached, the regeneration is carried out. procedure after Claim 13 , characterized in that the regeneration is carried out when a need for regeneration is recognized based on the exhaust gas model. procedure after Claim 16 and after one of Claims 14 or 15 , characterized in that the exhaust gas model-based detection of the need for regeneration with measurement signals from at least one measuring device (13, 15) for Determination of the ammonia content of the exhaust gas exiting from the third catalytic converter and/or is adjusted to determine the entry of ammonia into the third catalytic converter. procedure after Claim 16 and after one of Claims 14 or 15 , characterized in that the exhaust gas model-based detection of the need for regeneration learns from measurement signals from at least one measuring device (13, 15) for determining the ammonia content of the exhaust gas exiting the third catalytic converter and/or for determining the entry of ammonia into the third catalytic converter. adapted and/or improved. Procedure according to one of Claims 13 until 18 , characterized in that the regeneration is carried out periodically when a predetermined time has elapsed. Procedure according to one of Claims 13 until 19 , characterized in that the regeneration is carried out regularly when a predetermined exhaust gas mass flow has been generated by the internal combustion engine. Procedure according to one of Claims 13 until 20 , characterized in that the regeneration is carried out regularly when the internal combustion engine has sucked in a predetermined air mass. Procedure according to one of Claims 13 until 21 , characterized in that the regeneration is carried out regularly when the internal combustion engine has consumed a predetermined amount of fuel. Procedure according to one of Claims 13 until 22 , characterized in that regeneration is carried out at the end of a driving cycle. Procedure according to one of Claims 13 until 23 , characterized in that a regeneration is carried out after the start of the internal combustion engine as soon as the second catalytic converter is sufficiently warm for this purpose.

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