DE102017207767B4 - Emission control method for nitrogen oxides and / or ammonia - Google Patents

Abstract

Method for treating an exhaust gas stream (4) generated by an internal combustion engine (2), comprising: - generating an exhaust gas stream (4) in an internal combustion engine (2), - introducing the exhaust gas stream (4) into an exhaust gas system, - treating the exhaust gas stream (4) in a nitrogen oxide reduction catalyst (9), - subsequent division of the exhaust gas stream (4) into an exhaust gas stream (4a) to be returned to the internal combustion engine and an exhaust gas stream (4b) to be released to the environment, and - treatment of the exhaust gas stream (4a) to be returned into an SCR catalyst (11 ) - treating the exhaust gas stream (4b) to be released into the environment in an SCR catalytic converter (11). - supplying a reducing agent to the exhaust gas stream (4b) to be given off to the environment before its treatment in the SCR catalytic converter (11) - determining i. Ammonia and / or nitrogen oxide concentrations of the exhaust gas stream (4, 4a, 4b) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) and / or downstream of the SCR catalytic converter (11) arranged and downstream of the exhaust gas recirculation line (10) or downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line (10) and / or upstream of the nitrogen oxide reduction catalytic converter (9) and / orii. Ammonia loads of the SCR catalysts (9, 11), as control variables, - controlling or regulating the control variables by adjusting the supply of the reducing agent to the exhaust gas stream (4b) to be released to the environment before its treatment in the SCR catalyst (11) and / or Supply of the reducing agent to the exhaust gas stream (4) before its treatment in the nitrogen oxide reduction catalytic converter (9), characterized by - determining the ammonia concentration of the exhaust gas stream (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10), - comparing the determined ammonia concentration of the Exhaust gas flow (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) with a predetermined limit value and, if the limit value is exceeded, reducing the ammonia concentration in the exhaust gas flow (4) downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line ( 10) and increasing the amount of that to be released to the environment Exhaust gas stream (4b) prior to its treatment in the reducing agent to be supplied in the SCR catalytic converter (11).

Description

The invention relates to methods for treating an exhaust gas flow generated by an internal combustion engine.

In the exhaust gas from internal combustion engines there are pollutants, such as. As hydrocarbons, carbon monoxide and nitrogen oxides (NO _x ), the emission limit values are regulated by law in many countries. In order to reduce pollutant emissions from exhaust gases, exhaust gas aftertreatment devices, e.g. B. catalysts are used with which the pollutants mentioned can be converted into non-toxic substances such as carbon dioxide, water and nitrogen by oxidation or reduction.

Using conventional, regulated 3-way catalysts, nitrogen oxides can only be reduced to nitrogen if there is a constant stoichiometric ratio of air to fuel (λ = 1). Even with slight deviations in the lean area (λ> 1), i.e. H. If there is an excess of oxygen, the nitrogen oxides are no longer reduced sufficiently. The cause is the incomplete combustion of products in too small quantities, such as. B. carbon monoxide and hydrocarbons, since these in turn are oxidized to carbon dioxide by the excess oxygen. Accordingly, such a catalytic converter can only be used in vehicles with a gasoline engine and lambda control, i. H. with a monitoring of the composition of the air-fuel mixture. In contrast, other processes for the treatment of nitrogen oxides are necessary for diesel and lean mix petrol engines.

To reduce nitrogen oxide emissions, nitrogen oxide reduction catalysts are used, for example catalysts for selective catalytic reduction (SCR catalyst) or NO _x storage catalysts, also referred to as lean NO _x trap (LNT catalyst).

The functioning of a NO _x storage catalyst is based on intermediate storage of the nitrogen oxides and subsequent periodic reduction, for. B with fuel, while in the case of SCR catalytic converters nitrogen oxides are continuously reduced by means of ammonia, which can be obtained, for example, from an aqueous urea solution which is fed to the exhaust gas stream upstream of the SCR catalytic converter. A special form of an SCR catalytic converter is an SDPF catalytic converter, which is equipped as a diesel particle filter (DPF) with a coating for selective catalytic reduction (SCR coating). The use of SCR catalysts for exhaust gas aftertreatment is described, for example, in DE 20 2015 104 462 U1 as well as the AT 507 865 A2 described.

In order to enable an SCR catalytic converter to work optimally, it is necessary to dose a quantity of ammonia or urea that is dependent on the nitrogen oxide emission of the engine to form ammonia. It should be noted that SCR catalysts can store ammonia.

If the dosage is too low, the efficiency of the nitrogen oxide reduction decreases and the nitrogen oxide emissions into the environment increase. If, however, too much ammonia or urea is dosed, excess ammonia can get into the environment. To prevent the release of ammonia (ammonia slip) into the environment, an oxidation catalytic converter (AMOX catalytic converter) can be provided downstream of the SCR catalytic converter. In the event of an ammonia overdose, this converts ammonia into nitrogen and water.

A further measure for reducing the emission of nitrogen oxides into the environment is exhaust gas recirculation, as a result of which the combustion temperature of the fuel in the combustion chamber of the engine can be reduced. By lowering the combustion temperature, fewer nitrogen oxides are formed, since the reaction rate of nitrogen oxide formation increases exponentially with the combustion temperature.

In addition to an internal exhaust gas recirculation, there is the possibility of an external exhaust gas recirculation, in which the exhaust gas is first discharged and returned to the intake manifold of the engine via an exhaust gas recirculation line, which can be provided with a cooler. For example, the exhaust gas recirculation line can be located downstream (low pressure exhaust gas recirculation, see e.g. DE 11 2009 000 229 T5 ) or upstream (high pressure exhaust gas recirculation) of the turbine of the turbocharger. In the case of low-pressure exhaust gas recirculation, exhaust gas aftertreatment devices can also be arranged between the turbine and the exhaust gas recirculation line. A combination of low pressure and high pressure exhaust gas recirculation is also possible, such as in the DE 10 2014 018 037 A1 described.

Furthermore, it is known that in the case of low-pressure exhaust gas recirculation, a certain proportion of nitrogen oxide (nitrogen oxide slip) is contained in the exhaust gas to be recirculated, which is fed back into the combustion chamber and subsequently reaches the exhaust gas aftertreatment device again.

In addition, a nitrogen oxide slip can be observed downstream in the direction of the tailpipe. Can treat these nitrogen oxides another exhaust gas aftertreatment device, e.g. B. an SCR catalyst may be arranged downstream of the low pressure exhaust gas recirculation line. The ammonia requirement of this SCR catalyst can e.g. B. can be covered by a corresponding desired ammonia slip of an SCR catalytic converter arranged upstream of the low-pressure exhaust gas recirculation line, which is accompanied by a rectified nitrogen oxide slip, the ammonia slip and the nitrogen oxide slip also occurring at different times. However, this means that the SCR catalytic converter arranged downstream of the low-pressure exhaust gas recirculation line can only be operated as a function of the SCR catalytic converter arranged upstream of the low-pressure exhaust gas recirculation line.

However, this means that not only the exhaust gas stream which is fed to the SCR catalytic converter arranged downstream of the exhaust gas recirculation line contains ammonia, but also that ammonia is also contained in the exhaust gas stream to be recirculated and is thus fed to the combustion chamber. In the combustion chamber there is at least partial oxidation of the ammonia, i. H. further nitrogen oxides are formed.

This process is undesirable because, on the one hand, further nitrogen oxides are formed which have to be post-treated using ammonia and, on the other hand, this part of the ammonia which is recirculated in the exhaust gas stream is no longer available for the reduction of the nitrogen oxides in the SCR catalytic converter. Accordingly, an increased consumption of ammonia can be determined.

In the event that an exhaust gas aftertreatment device for reducing the nitrogen oxide emission is designed as an LNT catalyst, ammonia can be formed during the regeneration of the LNT catalyst by reducing the stored nitrogen oxides with unused fuel. If this ammonia gets into the exhaust gas stream to be recycled, the fuel consumption increases, since in this case unused fuel must be used as an additional reducing agent for reducing the nitrogen oxides formed by oxidation in the combustion chamber from the ammonia in the exhaust gas stream to be recycled.

Ammonia slip can occur, for example, with increasing aging of an SCR catalytic converter, since its storage capacity for ammonia decreases and a correspondingly greater amount of ammonia slip can be observed with a constant ammonia supply. As already stated, a further ammonia source can be an LNT catalyst, since ammonia can be formed from the nitrogen oxides previously stored during its regeneration.

The object of the present invention is therefore to provide a possibility of controlling the emission of nitrogen oxides and / or ammonia contained in an exhaust gas stream of an internal combustion engine into the environment. In addition, an undesired supply of ammonia into the combustion chamber of the internal combustion engine is to be prevented by the exhaust gas stream to be recirculated.

This object is achieved by the subject matter of the independent claims. The dependent claims contain design variants of these solutions according to the invention.

The invention is based on the basic idea of arranging an additional SCR catalytic converter for the treatment of nitrogen oxides in the low-pressure exhaust gas recirculation line. As a result, ammonia contained in the exhaust gas stream to be recirculated can be sensibly used, namely to reduce nitrogen oxides also contained in the exhaust gas stream to be recirculated. Thus, on the one hand, ammonia can be prevented from entering the combustion chamber of the internal combustion engine and, on the other hand, the nitrogen oxide emission into the environment is reduced by using the additional exhaust gas aftertreatment device.

Furthermore, an additional feed device for ammonia or an ammonia-forming reagent, such as. B. an aqueous urea solution, arranged between the exhaust gas recirculation line and a downstream of the exhaust gas recirculation line further exhaust gas aftertreatment device, z. B. be provided as an SCR catalyst. This can enable very precise control or regulation of the ammonia and nitrogen oxide flows.

An arrangement comprises an internal combustion engine which generates an exhaust gas flow and an exhaust gas system connected to the internal combustion engine for receiving the exhaust gas flow. The exhaust system thus serves to discharge the exhaust gases from the combustion chamber of the internal combustion engine. To reduce harmful emissions to the environment, the exhaust system includes exhaust after-treatment devices.

The exhaust system has a nitrogen oxide reduction catalytic converter arranged in the exhaust gas stream, i. H. a catalyst which generally serves to reduce nitrogen oxides. For example, the nitrogen oxide reduction catalyst as an SCR catalyst, e.g. B. as SDPF catalyst, or LNT catalyst according to the introductory description. It is also possible to arrange several nitrogen oxide reduction catalysts in series or in parallel.

Optionally, the exhaust system can have a turbine arranged upstream of the nitrogen oxide reduction catalytic converter arranged in the exhaust gas flow.

Furthermore, the exhaust system has an exhaust gas recirculation line, which is arranged downstream of the nitrogen oxide reduction catalytic converter. It serves to connect the exhaust system downstream of the nitrogen oxide reduction catalytic converter to the intake side of the combustion chamber of the internal combustion engine. For example, the exhaust gas recirculation line can be designed as a low-pressure exhaust gas recirculation line in the case of an exhaust system with a turbine.

For example, by means of an exhaust gas recirculation valve, the exhaust gas flow can be divided into an exhaust gas flow to be returned via the exhaust gas recirculation line and an exhaust gas flow to be released to the environment. The distribution of the exhaust gas flow can be arbitrary, i. H. each split exhaust stream can have a volume fraction in the range of 0% to 100% of the total volume of the exhaust stream.

An SCR catalytic converter arranged in the exhaust gas recirculation line is also provided. This is used to treat the exhaust gas flow to be returned via the exhaust gas recirculation line.

The exhaust gas flow generated in the combustion chamber of the internal combustion engine therefore first reaches the nitrogen oxide reduction catalytic converter. The exhaust gas stream treated by means of the nitrogen oxide reduction catalytic converter then divides, as a result of which part of the exhaust gas stream is returned to the combustion chamber via the exhaust gas recirculation line after treatment by means of the SCR catalytic converter arranged in the exhaust gas recirculation line (exhaust gas stream to be recirculated). The other part of the exhaust gas stream (exhaust gas stream to be released to the environment) leaves the exhaust system and is released to the environment, possibly after a further exhaust gas treatment.

The SCR catalytic converter arranged in the exhaust gas recirculation line primarily serves to reduce the proportion of ammonia in the exhaust gas stream to be recirculated, in that the ammonia is at least partially stored in the SCR catalytic converter and is used to reduce nitrogen oxides also present in the exhaust gas stream to be recirculated. It is thereby achieved that less or no more ammonia is fed to the combustion chamber and correspondingly fewer or no additional nitrogen oxides can form from ammonia in the combustion chamber, which in turn would otherwise require post-treatment. The reducing agent necessary for such post-treatment, such as. B. ammonia or fuel can therefore be saved.

The reducing agent consumption can also be further reduced by using the ammonia contained in the exhaust gas stream to be recycled to reduce nitrogen oxides also contained in the exhaust gas stream to be recycled by means of the SCR catalytic converter arranged in the exhaust gas recirculation line.

This can also enable the use of a nitrogen oxide reduction catalytic converter upstream of the exhaust gas recirculation line with smaller dimensions, since the SCR catalytic converter arranged in the exhaust gas recirculation line is also available for the treatment of the nitrogen oxides. Problems with the arrangement of the nitrogen oxide reduction catalytic converter in the exhaust system due to lack of space in a vehicle equipped with the internal combustion engine and the exhaust system can thereby be minimized.

It was also found that the effect according to the invention of reducing the ammonia portion in the exhaust gas stream to be recirculated cannot be brought about with other exhaust gas aftertreatment devices arranged in the exhaust gas recirculation line, or only with other disadvantages.

The arrangement of an oxidation catalytic converter would at the same time cause an oxidation of the hydrocarbons and / or carbon monoxide contained in the exhaust gas stream to be recirculated, which, however, should only be burned (oxidized) in the combustion chamber in order to be able to use the contained calorific value for conversion into mechanical energy. In addition, such an oxidation in an oxidation catalyst would be an exothermic reaction that occurs in the exhaust gas recirculation line, and the like. a. for safety reasons or possible damage to components due to an increase in temperature caused by the exothermic reaction, is undesirable.

The arrangement of an LNT catalyst in the exhaust gas recirculation line would not contribute to reducing the ammonia content in the exhaust gas stream to be recirculated, as long as there is an excess of oxygen in the exhaust gas stream, i. H. X is> 1. A particle filter, e.g. B. diesel particulate filter, would have little or no impact on the ammonia content.

The SCR catalytic converter arranged in the exhaust gas recirculation line can, for example, have the same function as a conventional SCR catalytic converter, e.g. B. a nitrogen oxide reduction catalyst designed as an SCR catalyst upstream of the Exhaust gas recirculation line, be formed, but have significantly smaller dimensions, since only part of the exhaust gas flow, for. B. 10 vol .-%, 20 vol .-% or 30 vol .-%, is recycled.

Upstream of the exhaust gas recirculation line, additional exhaust gas aftertreatment devices, e.g. B. a particle filter to reduce the particles present in the exhaust gas, such as a diesel particle filter, may be arranged.

Optionally, the exhaust system can have a turbine as part of a turbocharger, which is arranged upstream of the nitrogen oxide reduction catalytic converter and possible further exhaust gas aftertreatment devices. The turbine serves to drive a compressor for the intake air of the internal combustion engine and draws its energy from the relaxation work of the expansion via the turbine.

According to various design variants, the exhaust system can also include an SCR catalytic converter and / or further exhaust gas aftertreatment devices, e.g. B. an AMOX catalyst, downstream of the exhaust gas recirculation line. This means that the non-recyclable portion of the exhaust gas stream, i.e. H. the exhaust gas stream to be released to the environment flows through this SCR catalytic converter and / or further exhaust gas aftertreatment devices and is subsequently released to the environment.

This SCR catalytic converter arranged downstream of the exhaust gas recirculation line also serves to reduce the ammonia content in the exhaust gas stream and to convert nitrogen oxides into non-toxic gases. It therefore contributes to effective exhaust gas aftertreatment and reduces nitrogen oxide emissions to the environment.

According to further embodiment variants, the exhaust system can furthermore have a reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line. This reducing agent supply device is used to supply a reducing agent, such as. B. ammonia or urea, to the exhaust gas stream before the exhaust gas stream reaches the SCR catalyst downstream of the exhaust gas recirculation line. For example, ammonia can be supplied in gaseous form or urea in the form of an aqueous solution.

It is thus possible to adjust the amount of the reducing agent in accordance with the composition of the exhaust gas flow, the needs of the SCR catalytic converter and / or specifications with regard to the exhaust gas flow to be released to the environment, e.g. B. Adjust limit values for mass emissions of nitrogen oxides or other components of the exhaust gas flow, d. H. to control or regulate.

In this respect, the SCR catalytic converter downstream of the exhaust gas recirculation line is no longer dependent on an ammonia slip from the upstream nitrogen oxide reduction catalytic converter and its function can be controlled or regulated independently of the upstream nitrogen oxide reduction catalytic converter.

According to further embodiment variants, nitrogen oxide reduction catalytic converter can be designed as an SCR catalytic converter or LNT catalytic converter. If the nitrogen oxide reduction catalytic converter is designed as an SCR catalytic converter, a reducing agent supply device is arranged upstream of the SCR catalytic converter.

This reducing agent supply device is used to supply a reducing agent, such as. B. ammonia or urea, to the exhaust gas stream before the exhaust gas stream reaches the SCR catalyst upstream of the exhaust gas recirculation line. For example, ammonia can be supplied in gaseous form or urea in the form of an aqueous solution.

The combination of the SCR catalytic converter arranged in the exhaust gas recirculation line with an SCR catalytic converter downstream of the exhaust gas recirculation line and a reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line enables efficient control or regulation and thus control of the exhaust gas aftertreatment, as described below by way of example .

In order to reduce the ammonia content in the exhaust gas stream to be returned via the exhaust gas recirculation line, i. H. The ammonia slip of the SCR catalytic converter arranged in the exhaust gas recirculation line can be kept as low as possible, whereby the case is first considered that the nitrogen oxide reduction catalytic converter is designed as an SCR catalytic converter and a reducing agent supply device is arranged upstream of the SCR catalytic converter.

If there is a risk of ammonia slipping on the SCR catalytic converter arranged in the exhaust gas recirculation line, i.e. that the exhaust gas stream to be recirculated continues to contain ammonia, since this cannot be completely removed by means of the SCR catalytic converter arranged in the exhaust gas recirculation line, the Amount of ammonia supplied by means of the reducing agent supply device arranged upstream of the SCR catalytic converter or the amount of ammonia supplied urea, from which ammonia is subsequently formed, are reduced.

This leads to a higher proportion of nitrogen oxides in the entire exhaust gas stream downstream of the nitrogen oxide reduction catalytic converter and thus also in the exhaust gas stream in the exhaust gas recirculation line, while the ammonia component drops. This reduces the amount of ammonia stored in the SCR catalytic converter arranged in the exhaust gas recirculation line and reduces the risk of ammonia slipping in the direction of the combustion chamber.

However, a larger nitrogen oxide / ammonia ratio downstream of the nitrogen oxide reduction catalytic converter leads to a higher nitrogen oxide load on the SCR catalytic converter downstream of the exhaust gas recirculation line.

The arrangement of the reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line now enables a further supply of a reducing agent, e.g. B. ammonia, so that the reduction in the ammonia content can be compensated for by reducing the amount of reducing agent that is supplied by means of the reducing agent supply device upstream of the nitrogen oxide reduction catalyst designed as an SCR catalyst.

In the event that the nitrogen oxide reduction catalyst is designed as an LNT catalyst, the amount of ammonia which is formed during regeneration of the LNT catalyst under rich conditions can be reduced by shortening the duration of the regeneration. This leads to a larger nitrogen oxide / ammonia ratio downstream of the nitrogen oxide reduction catalyst. The reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line can now be used to compensate for the reduced ammonia content in the exhaust gas stream, as previously described in the case of the formation of the nitrogen oxide reduction catalytic converter as an SCR catalytic converter.

The risk of ammonia slip in the direction of the combustion chamber can be described by means of a model for the SCR catalytic converter arranged in the exhaust gas recirculation line, which describes the different reactions, such as. B. conversion, adsorption, desorption or ammonia oxidation. The change in the amount of reducing agent supplied by means of the reducing agent supply device (s) can then, for. B. preventive, d. H. with a future ammonia slip predicted by the model, or as a direct reaction to an estimated or observed ammonia slip.

It is also possible to react to short-term changes in the nitrogen oxide / ammonia ratio downstream of the nitrogen oxide reduction catalytic converter, since the nitrogen oxide / ammonia ratio, for example in the case of a nitrogen oxide reduction catalytic converter designed as an SCR catalytic converter, is heavily dependent on the volume flow and the temperature and thus on the operating state of the internal combustion engine is.

Due to the available space for the arrangement of the exhaust gas system in a vehicle, the SCR catalytic converter downstream of the exhaust gas recirculation line is often designed to be larger than the SCR catalytic converter arranged in the exhaust gas recirculation line. Therefore, at a certain nitrogen oxide / ammonia ratio, an ammonia slip can be observed more quickly in the SCR catalytic converter arranged in the exhaust gas recirculation line than in the SCR catalytic converter downstream of the exhaust gas recirculation line.

While the SCR catalytic converter can often temporarily store an excess amount of ammonia downstream of the exhaust gas recirculation line, the ammonia saturation of the SCR catalytic converter arranged in the exhaust gas recirculation line is reached more quickly. Among others by arranging a reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line, it is possible to react to such circumstances.

In order to reduce the nitrogen oxide content in the exhaust gas stream to be returned via the exhaust gas recirculation line, i. H. The nitrogen oxide slip of the SCR catalytic converter arranged in the exhaust gas recirculation line can be kept as low as possible, taking into account the case where the nitrogen oxide reduction catalytic converter is designed as an SCR catalytic converter and a reducing agent supply device is arranged upstream of the SCR catalytic converter.

If the nitrogen oxide slip of the SCR catalytic converter arranged in the exhaust gas recirculation line is to be reduced so that the exhaust gas stream to be recirculated has a lower nitrogen oxide content, the amount of ammonia supplied by means of the reducing agent supply device arranged upstream of the SCR catalytic converter or the amount of urea supplied can be reduced which subsequently produces ammonia. This reduces the nitrogen oxide / ammonia ratio downstream of the nitrogen oxide reduction catalytic converter and that in the Exhaust gas recirculation line arranged SCR catalyst stored amount of ammonia increases. Correspondingly, nitrogen oxides can be reduced in the SCR catalytic converter arranged in the exhaust gas recirculation line and the nitrogen oxide slip in the direction of the combustion chamber decreases.

The decrease in the nitrogen oxide / ammonia ratio simultaneously leads to a higher ammonia content in the part of the exhaust gas flow which is fed to the SCR catalytic converter downstream of the exhaust gas recirculation line. Correspondingly, the amount of ammonia supplied by means of the reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line or the amount of urea supplied, from which ammonia subsequently forms, can be reduced.

Furthermore, it can be stated that the nitrogen oxide / ammonia ratio without additional reducing agent supply by means of the reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line during a steady state for both SCR catalysts, i.e. H. the SCR catalytic converter arranged in the exhaust gas recirculation line and the SCR catalytic converter downstream of the exhaust gas recirculation line is the same in the non-stationary state, e.g. B. in the event of a change in the proportion of the exhaust gas flow to be recycled and / or in the event of a change in the nitrogen oxide / ammonia ratio in the exhaust gas flow, the ratio of the accumulated nitrogen oxides (proportion of the nitrogen oxides multiplied by the quantity of exhaust gas in the exhaust gas flow) to the accumulated ammonia (proportion of the ammonia multiplied by the amount of exhaust gas in the exhaust gas flow) is different for the two SCR catalysts mentioned.

By means of the additional reducing agent supply device downstream of the exhaust gas recirculation line and upstream of the SCR catalytic converter downstream of the exhaust gas recirculation line, the SCR catalytic converter arranged in the exhaust gas recirculation line and an SCR catalytic converter arranged downstream of the exhaust gas recirculation line can be operated or decoupled independently of one another. In other words, the function of the SCR catalytic converter arranged in the exhaust gas recirculation line can be independent of an SCR catalytic converter arranged downstream of the exhaust gas recirculation line by means of the ammonia source arranged upstream of the exhaust gas recirculation line, e.g. B. an upstream of the exhaust gas recirculation line SCR or LNT catalyst can be influenced.

In general, the nitrogen oxide or ammonia portion or the nitrogen oxide / ammonia ratio in the exhaust gas stream can be by means of one or more nitrogen oxide or ammonia sensors, e.g. B. in the form of gas sensors.

For example, in order to carry out the control or regulation described above, the exhaust system can have devices in further embodiment variants which are designed to determine the ammonia and / or nitrogen oxide concentrations downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line, downstream of the SCR catalytic converter arranged downstream of the exhaust gas recirculation line. downstream of the nitrogen oxide reduction catalyst upstream of the exhaust gas recirculation line and upstream of the nitrogen oxide reduction catalyst and for determining the ammonia loads of the SCR catalysts. These specific variables are also referred to below as controlled variables.

Furthermore, the exhaust system can have control devices that are designed to control or regulate the controlled variables by adjusting the supply of reducing agent by means of the reducing agent supply devices. According to further embodiment variants, the devices for determining the ammonia loadings of the SCR catalysts can be designed for determining the ammonia loadings by means of an ammonia load model. Such an ammonia loading model can depict a relationship between the nitrogen oxide and ammonia concentration upstream of the respective SCR catalytic converter, the temperature of the respective SCR catalytic converter, the volume flow in the respective SCR catalytic converter and the ammonia loading of the respective SCR catalytic converter.

According to further embodiment variants, the exhaust system can have a cooler arranged on the exhaust gas recirculation line downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line. This serves to cool the exhaust gas flow to be returned, which is thereby fed to the combustion chamber of the internal combustion engine at a lower temperature, which reduces the formation of pollutants, especially nitrogen oxides, during the combustion process.

According to further embodiment variants, the exhaust system can furthermore have a further exhaust gas recirculation line upstream of the nitrogen oxide reduction catalytic converter. In the case of an exhaust gas system with a turbine, this additional exhaust gas recirculation line can be designed as a high-pressure exhaust gas recirculation line, which is arranged upstream of the turbine.

The further exhaust gas recirculation line is used to connect the exhaust system upstream of the nitrogen oxide reduction catalytic converter with an inlet into the combustion chamber of the internal combustion engine, ie part of the exhaust gas flow is discharged before an exhaust gas aftertreatment and fed back to the combustion chamber.

By means of the further exhaust gas recirculation line, it is e.g. B. possible to switch between a low pressure exhaust gas recirculation and a high pressure exhaust gas recirculation or to provide a combined low and high pressure exhaust gas recirculation.

The additional exhaust gas recirculation line also opens up the possibility of influencing the amount of ammonia stored in the SCR catalytic converters. If the amount of ammonia stored in the SCR catalytic converter downstream of the exhaust gas recirculation line is too high compared to the amount of ammonia stored in the SCR catalytic converter arranged in the exhaust gas recirculation line, the exhaust system can be connected to the further exhaust gas recirculation line, e.g. B. a high pressure exhaust gas recirculation line, instead of with the exhaust gas recirculation line downstream of the nitrogen oxide reduction catalyst, for. B. a low pressure exhaust gas recirculation line, or at least operated with a reduced proportion of the exhaust gas flow in the exhaust gas recirculation line downstream of the nitrogen oxide reduction catalyst. This leads to a reduction in the amount of ammonia stored in the SCR catalytic converter downstream of the exhaust gas recirculation line downstream of the nitrogen oxide reduction catalytic converter, without the ammonia amount stored in the SCR catalytic converter arranged downstream of the nitrogen oxide reduction catalytic converter being influenced.

According to further embodiment variants, the exhaust system of the arrangement can also have devices for regulating the volume flows in the exhaust gas recirculation line and / or the further exhaust gas recirculation line upstream of the nitrogen oxide reduction catalytic converter. These can be an adjustment of the volume flows in the SCR catalysts, for. B. the SCR catalytic converter arranged in the exhaust gas recirculation line.

A vehicle has one of the arrangements described above.

A method for treating an exhaust gas stream generated by an internal combustion engine comprises: generating an exhaust gas stream in an internal combustion engine, introducing the exhaust gas stream into an exhaust gas system, treating the exhaust gas stream in a nitrogen oxide reduction catalytic converter, then dividing the exhaust gas stream into an exhaust gas stream to be returned to the internal combustion engine and one to be released into the environment Exhaust gas flow and treatment of the exhaust gas flow to be recirculated in an SCR catalytic converter.

The distribution of the exhaust gas flow can be arbitrary, i. H. the partial exhaust gas flows can have a volume fraction between 0% and 100% of the total volume of the exhaust gas flow.

The method can be carried out, for example, by means of the inventive arrangement explained above. In this respect, the above statements serve to explain the arrangement according to the invention also to describe the method according to the invention.

According to the invention it is provided that the exhaust gas flow to be released into the environment in an SCR catalytic converter and / or further exhaust gas aftertreatment devices, for. B. an AMOX catalyst to treat.

According to the invention, a reducing agent, e.g. B. ammonia or an ammonia-forming agent such as urea, for example in the form of an aqueous urea solution, to the exhaust gas stream to be released to the environment before its treatment in the SCR catalyst.

According to various embodiments, a reducing agent can be supplied to the exhaust gas stream before its treatment in the nitrogen oxide reduction catalyst. Ammonia or an ammonia-forming agent such as urea, for example in the form of an aqueous urea solution, can serve as the reducing agent, e.g. B. if the nitrogen oxide reduction catalyst is an SCR catalyst. For example, in the case of an LNT catalyst as a nitrogen oxide reduction catalyst, fuel can be supplied as the reducing agent.

According to further design variants, the exhaust gas stream to be recycled can be cooled.

According to the invention, the method further comprises determining ammonia and / or nitrogen oxide concentrations downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line and / or downstream of the SCR catalytic converter arranged downstream of the exhaust gas recirculation line and / or downstream of the nitrogen oxide reduction catalytic converter upstream of the exhaust gas recirculation line and / or upstream of the nitrogen oxide reduction catalytic converter and / or have a determination of the ammonia loads of the SCR catalysts as control variables.

These controlled variables can be adjusted by means of an adjustment of the supply of the reducing agent to the Exhaust gas flow to be discharged from the environment before its treatment in the SCR catalytic converter and / or the supply of the reducing agent to the exhaust gas flow before its treatment in the nitrogen oxide reduction catalytic converter are controlled or regulated.

According to further embodiment variants, the ammonia loading of the SCR catalysts can be determined by means of an ammonia loading model, which has a relationship between the nitrogen oxide and ammonia concentration upstream of the respective SCR catalyst, the temperature of the respective SCR catalyst, the volume flow in the respective SCR catalyst and the ammonia loading of the respective SCR catalytic converter.

A first method according to the invention comprises: determining the ammonia concentration of the exhaust gas flow downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line, comparing the determined ammonia concentration of the exhaust gas flow downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line with a predetermined limit value and, if the limit value is exceeded, reducing it the ammonia concentration in the exhaust gas stream downstream of the nitrogen oxide reduction catalyst upstream of the exhaust gas recirculation line and increasing the amount of the reducing agent to be supplied to the exhaust gas stream to the environment before its treatment in the SCR catalyst.

Another method according to the invention comprises: determining the nitrogen oxide concentration of the exhaust gas flow downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line, comparing the determined nitrogen oxide concentration of the exhaust gas stream downstream of the SCR catalytic converter arranged in the exhaust gas recirculation line with a predetermined limit value and, if the limit value is exceeded, increasing the ammonia concentration in the exhaust gas stream downstream of the nitrogen oxide reduction catalyst upstream of the exhaust gas recirculation line and reducing the amount of the reducing agent to be supplied to the exhaust gas stream to be released to the environment before its treatment in the SCR catalytic converter.

In both methods according to the invention, the ammonia concentration in the exhaust gas stream downstream of the nitrogen oxide reduction catalytic converter upstream of the exhaust gas recirculation line can be reduced or reduced by designing the nitrogen oxide reduction catalytic converter as an SCR catalytic converter to reduce or increase the amount of ammonia or an ammonia-forming reducing agent that is upstream of the SCR catalytic converter is increased or by shortening or lengthening the duration of regeneration of the LNT catalyst in which ammonia is formed in the LNT catalyst when the nitrogen oxide reduction catalyst is designed as an LNT catalyst. According to further embodiment variants, the exhaust gas flow can be divided into an exhaust gas flow to be returned to the internal combustion engine and the exhaust gas flow to be treated in the nitrogen oxide reduction catalytic converter. This exhaust gas stream to be recirculated can, for example, by means of a further exhaust gas recirculation line, e.g. B. a high pressure exhaust gas recirculation line, the combustion chamber of the internal combustion engine are fed back.

According to further embodiment variants, the volume flows in the exhaust gas recirculation line and / or the further exhaust gas recirculation line upstream of the nitrogen oxide reduction catalytic converter can be controlled or regulated.

The advantages of the method correspond to those of the arrangement according to the invention and its corresponding design variants.

• 1 eine schematische Darstellung einer beispielhaften Anordnung,
• 2 eine schematische Darstellung einer weiteren beispielhaften Anordnung.

The invention will be explained in more detail below using an exemplary embodiment. The associated drawings show:

• 1 1 shows a schematic illustration of an exemplary arrangement,
• 2nd is a schematic representation of another exemplary arrangement.

In the examples set forth below, reference is made to the accompanying drawings which form a part of the examples and in which specific embodiments are shown by way of illustration in which the invention may be practiced. In this regard, the directional terminology such as “top”, “bottom”, “front”, “back”, “front”, “back” etc. is used with reference to the orientation of the figure (s) described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is illustrative and is in no way limiting.

It is understood that other embodiments may be used and structural or logical changes may be made without departing from the scope of the present invention. It is understood that the features of the various exemplary embodiments described herein may be combined with one another unless specifically stated otherwise. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

Identical or similar elements are provided with identical reference symbols in the figures, insofar as this is expedient.

An arrangement 1 in the embodiment shown in FIG 1 an internal combustion engine 2nd on. The internal combustion engine 2nd can be a self-igniting or spark-ignited internal combustion engine. The internal combustion engine 2nd has three cylinders 3rd on, but can also have a different number, e.g. B. two, four, five, six or eight cylinders.

To the internal combustion engine 2nd closes an exhaust system for receiving the from the internal combustion engine 2nd generated exhaust gas flow 4th on. In the exemplary embodiment, the exhaust system has a turbine 5 of a turbocharger. The turbine 5 is about a wave 6 with a compressor 7 connected in the intake tract 8th is arranged.

In the exhaust system is the turbine downstream 5 a nitrogen oxide reduction catalyst 9 arranged, which is an SCR catalyst in this embodiment. Upstream of the nitrogen oxide reduction catalyst 9 is a reducing agent supply device 12th present, by means of ammonia or an aqueous urea solution in the exhaust gas stream 4th upstream of the nitrogen oxide reduction catalyst 9 can be initiated.

Downstream of the nitrogen oxide reduction catalyst 9 is an exhaust gas recirculation line 10 arranged, which is designed in the example as a low-pressure exhaust gas recirculation line. The exhaust gas recirculation line 10 opens upstream of the compressor 7 in the intake tract 8th . The exhaust gas recirculation line 10 has an exhaust gas recirculation valve 13 on the exhaust gas recirculation line 10 part of the exhaust gas flow 4th , namely the exhaust gas flow to be recirculated 4a , can be fed. As an alternative to the exhaust gas recirculation valve 13 throttles or valves can also be used in the exhaust pipe and the intake side of the combustion chamber or a combination valve (combi valve). The rest of the exhaust gas flow 4th flows as an exhaust gas flow to be released to the environment 4b towards the exhaust pipe 15 .

In the exhaust gas recirculation line 10 is an SCR catalyst 11 to treat the exhaust gas flow to be recycled 4a arranged. Another SCR catalyst 11 is downstream of the exhaust gas recirculation line 10 to treat the exhaust gas flow to be released into the environment 4b intended.

Furthermore, the exhaust system of this exemplary embodiment has a further reducing agent supply device 12th downstream of the exhaust gas recirculation line 10 and upstream of the SCR catalyst 11 downstream of the exhaust gas recirculation line 10 on. By means of this further reducing agent supply device 12th can ammonia or urea, e.g. B. as an aqueous urea solution, downstream of the exhaust gas recirculation line 10 and upstream of the SCR catalyst 11 downstream of the exhaust gas recirculation line 10 in the exhaust stream 4th be initiated.

Optionally, on the exhaust gas recirculation line 10 downstream of the on the exhaust gas recirculation line 10 arranged SCR catalyst 11 a cooler (not shown) for cooling the exhaust gas stream to be recirculated 4a be arranged.

Further optionally, the exhaust system of the embodiment can have a further exhaust gas recirculation line, e.g. B. a high pressure exhaust gas recirculation line (not shown) upstream of the turbine 5 , exhibit.

By means of the 1 The arrangement shown can be an exhaust gas flow 4th in an internal combustion engine 2nd are generated, which is then introduced into an exhaust system. The exhaust gas flow 4th is initially in a nitrogen oxide reduction catalyst designed as an SCR catalyst 9 treated.

Below is the exhaust gas flow 4th into an exhaust gas flow to be returned to the internal combustion engine 4a and an exhaust gas stream to be released to the environment 4b divided up.

The exhaust gas flow to be recirculated 4a is in one in the exhaust gas recirculation line 10 arranged SCR catalyst 11 treated. The exhaust gas flow to be released to the environment 4b is in another SCR catalyst 11 treated.

In addition, ammonia or urea is used as a reducing agent in the exhaust gas flow 4th before its treatment in the nitrogen oxide reduction catalyst 9 and the exhaust gas flow to be released to the environment 4b before its treatment in the SCR catalyst 11 fed.

Another embodiment as shown in 2nd differs from the previously described embodiment in that the nitrogen oxide reduction catalyst 9 is designed as an LNT catalyst. Therefore, the reducing agent supply device 12th upstream of the nitrogen oxide reduction catalyst 9 omitted.

In addition, in this further exemplary embodiment there is an exhaust gas aftertreatment device 14 downstream of the nitrogen oxide reduction catalyst 9 and upstream of the exhaust gas recirculation line 10 arranged. With this exhaust gas aftertreatment device 14 in this exemplary embodiment it is a particle filter.

The exhaust systems of both exemplary embodiments can also have devices which are designed to determine the ammonia and / or nitrogen oxide concentrations downstream of that in the exhaust gas recirculation line 10 arranged SCR catalyst 11 , downstream of the exhaust gas recirculation line 10 arranged SCR catalyst 11 , downstream of the nitrogen oxide reduction catalyst 9 upstream of the exhaust gas recirculation line 10 and upstream of the nitrogen oxide reduction catalyst 9 and to determine the ammonia loading of the SCR catalysts 9 , 11 as controlled variables, as well as control devices that are designed to control or regulate the controlled variables by adjusting the supply of reducing agent by means of the reducing agent supply devices 12th .

Accordingly, it is possible to determine the controlled variables and the controlled variables by adjusting the supply of the reducing agent to the exhaust gas stream to be released to the environment 4b before its treatment in the SCR catalyst 11 and / or the supply of the reducing agent to the exhaust gas stream 4th before its treatment in the nitrogen oxide reduction catalyst 9 to control or regulate.

Reference list

1

arrangement

2nd

Internal combustion engine

3rd

cylinder

4th

Exhaust gas flow

4a

exhaust gas flow to be recirculated

4b

exhaust gas flow to be released into the environment

5

turbine

6

wave

7

compressor

8th

Intake tract

9

Nitrogen oxide reduction catalyst

10th

Exhaust gas recirculation line

11

SCR catalytic converter

12th

Reducing agent feeder

13

Exhaust gas recirculation valve

14

Exhaust aftertreatment device

15

Exhaust tailpipe

Claims (8)

Method for treating an exhaust gas stream (4) generated by an internal combustion engine (2), comprising: - generating an exhaust gas stream (4) in an internal combustion engine (2), - introducing the exhaust gas stream (4) into an exhaust gas system, - treating the exhaust gas stream (4) in a nitrogen oxide reduction catalyst (9), - subsequent division of the exhaust gas stream (4) into an exhaust gas stream (4a) to be returned to the internal combustion engine and an exhaust gas stream (4b) to be released to the environment, and - treatment of the exhaust gas stream (4a) to be returned into an SCR catalyst (11 ), - treating the exhaust gas stream (4b) to be released into the environment in an SCR catalytic converter (11). - supplying a reducing agent to the exhaust gas stream (4b) to be released to the environment before its treatment in the SCR catalytic converter (11) - determining i. Ammonia and / or nitrogen oxide concentrations of the exhaust gas stream (4, 4a, 4b) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) and / or downstream of the SCR catalytic converter (11) arranged and downstream of the exhaust gas recirculation line (10) or downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line (10) and / or upstream of the nitrogen oxide reduction catalytic converter (9) and / or ii. Ammonia loads of the SCR catalysts (9, 11), as control variables, - controlling or regulating the control variables by adjusting the supply of the reducing agent to the exhaust gas stream (4b) to be released to the environment before its treatment in the SCR catalyst (11) and / or Supply of the reducing agent to the exhaust gas stream (4) before its treatment in the nitrogen oxide reduction catalytic converter (9), characterized by - determining the ammonia concentration of the exhaust gas stream (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10), - comparing the determined ammonia concentration of the Exhaust gas flow (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) with a predetermined limit value and, if the limit value is exceeded, reducing the ammonia concentration in the exhaust gas flow (4) downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line ( 10) and increasing the amount of to be released to the environment The exhaust gas stream (4b) before its treatment in the SCR catalytic converter (11) to be supplied with reducing agent. Procedure according to Claim 1 , showing: - Determining the nitrogen oxide concentration of the exhaust gas stream (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10), - comparing the determined nitrogen oxide concentration of the exhaust gas stream (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) with a predetermined limit value, and - if the limit value is exceeded, increasing the ammonia concentration in the exhaust gas stream (4) downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line (10) and reducing the amount of the exhaust gas stream (4b) to be released to the environment before its treatment in reducing agent to be supplied to the SCR catalytic converter (11). Method for treating an exhaust gas stream (4) generated by an internal combustion engine (2), comprising: - generating an exhaust gas stream (4) in an internal combustion engine (2), - introducing the exhaust gas stream (4) into an exhaust gas system, - treating the exhaust gas stream (4) in a nitrogen oxide reduction catalyst (9), - subsequent division of the exhaust gas stream (4) into an exhaust gas stream (4a) to be returned to the internal combustion engine and an exhaust gas stream (4b) to be released to the environment, and - treatment of the exhaust gas stream (4a) to be returned into an SCR catalyst (11 ), - treating the exhaust gas stream (4b) to be released into the environment in an SCR catalytic converter (11). - supplying a reducing agent to the exhaust gas stream (4b) to be released to the environment before its treatment in the SCR catalytic converter (11) - determining i. Ammonia and / or nitrogen oxide concentrations of the exhaust gas stream (4, 4a, 4b) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) and / or downstream of the SCR catalytic converter (11) arranged and downstream of the exhaust gas recirculation line (10) or downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line (10) and / or upstream of the nitrogen oxide reduction catalytic converter (9) and / or ii. Ammonia loads of the SCR catalysts (9, 11), as control variables, - controlling or regulating the control variables by adjusting the supply of the reducing agent to the exhaust gas stream (4b) to be released to the environment before its treatment in the SCR catalyst (11) and / or Supply of the reducing agent to the exhaust gas stream (4) before its treatment in the nitrogen oxide reduction catalytic converter (9), characterized by - determining the nitrogen oxide concentration of the exhaust gas stream (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10), - comparing the determined nitrogen oxide concentration of the Exhaust gas flow (4a) downstream of the SCR catalytic converter (11) arranged in the exhaust gas recirculation line (10) with a predetermined limit value, and - if the limit value is exceeded, increasing the ammonia concentration in the exhaust gas flow (4) downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line ( 10) and reducing the amount to be discharged to the environment the exhaust gas stream (4b) before its treatment in the SCR catalyst (11) to be supplied reducing agent. Method according to one of the preceding claims, further comprising: - Feeding a reducing agent to the exhaust gas stream (4) before its treatment in the nitrogen oxide reduction catalyst (9). Method according to one of the preceding claims, further comprising: - Cooling the exhaust gas flow to be recirculated (4a). Method according to one of the preceding claims, wherein the ammonia loadings of the SCR catalysts (9, 11) are determined by means of an ammonia loading model which relates the nitrogen oxide and ammonia concentration upstream of the respective SCR catalyst (9, 11), the temperature of the respective one SCR catalyst (9, 11), the volume flow in the respective SCR catalyst (9, 11) and the ammonia loading of the respective SCR catalyst (9, 11). Method according to one of the preceding claims, wherein the ammonia concentration in the exhaust gas stream (4) downstream of the nitrogen oxide reduction catalytic converter (9) upstream of the exhaust gas recirculation line (10) is reduced or increased by the amount when the nitrogen oxide reduction catalytic converter (9) is designed as an SCR catalytic converter (11) is reduced or increased by upstream of the ammonia or an ammonia-forming reducing agent supplied as the nitrogen oxide catalyst (9) in the form of an SCR catalyst, or by a duration of regeneration of the LNT catalyst when the nitrogen oxide reduction catalyst (9) is designed as an LNT catalyst, in which ammonia is formed in the LNT catalyst, shortened or lengthened. Method according to one of the preceding claims, further comprising: dividing the exhaust gas stream (4) into an exhaust gas stream to be returned to the internal combustion engine and the exhaust gas flow to be treated in the nitrogen oxide reduction catalyst (9).

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