DE102017206425A1 - exhaust system - Google Patents

Abstract

The invention relates to an exhaust system (1) for the aftertreatment of exhaust gases of an internal combustion engine with successively arranged in the flow direction (2) of the exhaust, a catalyst (3) for the oxidation of the exhaust gas and / or a catalyst for storing nitrogen oxides, with an introduction point (4 ) for supplying a reducing agent, with an SCR catalyst (6) for the selective catalytic reduction of nitrogen oxides and with a particle filter, wherein the particle filter (7) in the flow direction (2) after the SCR catalyst (6) is arranged and in the flow direction ( 2) after the particulate filter (7), a second SCR catalyst (8) and / or an ammonia slip catalyst is arranged.

Description

Technical area

The invention relates to an exhaust system for the aftertreatment of exhaust gases of an internal combustion engine with viewed in the flow direction of the exhaust gas arranged sequentially, a catalyst for the oxidation of the exhaust gas and / or a catalyst for storing nitrogen oxides, with an introduction point for supplying a reducing agent, with an SCR catalyst for selective catalytic reduction of nitrogen oxides and with a particle filter.

State of the art

In exhaust aftertreatment systems different types of catalysts are used to ensure the most effective and comprehensive conversion and filtering of pollutants contained in the exhaust gas. Among others, catalysts are used, for example, which absorb and bind nitrogen oxides (NO _x ). In addition, catalysts are also known which stimulate a conversion of nitrogen oxides to less harmful reaction products. Furthermore, filters are known which filter out particles of a certain size from the exhaust gas.

A disadvantage of the known in the prior art devices for cleaning exhaust gases of an internal combustion engine is that the arrangement of the individual catalysts in the exhaust system is not optimal and thus the individual catalysts do not achieve their individual optimum effect. In particular, in devices known in the prior art, the so-called SCR catalysts for the selective catalytic reduction of nitrogen oxides are often arranged downstream of the particle filter. As a result, the SCR catalyst can not operate at its optimum operating point.

Presentation of the invention, object, solution, advantages

It is therefore an object of the present invention to provide an exhaust system for the after-treatment of exhaust gases of an internal combustion engine, which has an optimized arrangement and further an optimized design of the individual catalysts.

The object with regard to the exhaust system is achieved by an exhaust system having the features of claim 1.

An embodiment of the invention relates to an exhaust system for the aftertreatment of exhaust gases of an internal combustion engine with viewed in the flow direction of the exhaust gas arranged sequentially, a catalyst for the oxidation of the exhaust gas and / or a catalyst for storing nitrogen oxides, with an introduction point for supplying a reducing agent, with an SCR Catalyst for the selective catalytic reduction of nitrogen oxides and with a particulate filter, wherein the particulate filter is arranged in the flow direction downstream of the SCR catalyst and in the flow direction downstream of the particulate filter, a second SCR catalyst and / or an ammonia slip catalyst is arranged.

By arranging an SCR catalyst upstream of the particulate filter and an SCR catalyst downstream of the particulate filter, a more efficient unit can be created. In particular, the large volume that the particulate filters have on a regular basis, and the cell geometries and cell densities needed to allow sufficient exhaust gas filtering, degrade the operating conditions for SCR catalysts downstream of the particulate filter.

This is for example due to the fact that the temperature of the exhaust gas is greatly reduced after flowing through the particulate filter, whereby the effect of the SCR catalyst is adversely affected. Furthermore, a large pressure loss can be generated by the particulate filter, which means that the exhaust gas is no longer sufficiently high flow rate to the downstream SCR catalyst to ensure optimum conversion. The integration of the SCR catalyst into a particulate filter, for example, by partially coating the particulate filter, is not optimal because the commonly used cell densities and cell geometries of the honeycomb bodies of the particulate filters are not optimal for the selective catalytic reduction of the exhaust gas in the SCR catalysts.

By means of an upstream SCR catalytic converter, a first aftertreatment of the exhaust gas can be carried out, in particular a conversion of the nitrogen oxides to water and nitrogen can take place with the aid of the already introduced and vaporized reducing agent, which is typically formed by an aqueous urea solution.

The subsequent particle filter, a purification of the exhaust gas flow is achieved. An after the particulate filter downstream SCR catalyst can then convert nitrogen oxides contained in the exhaust gas. For this purpose, the second SCR catalyst can particularly advantageously have a cell geometry and / or cell density deviating from the first SCR catalyst and also a different chemically active one Have coating. Thus, for example, a conversion of the nitrogen oxides can be achieved even at lower temperatures and flow rates.

It is particularly advantageous if the first SCR catalytic converter arranged in the flow direction upstream of the particle filter is electrically heatable. The electrical heating of the SCR catalyst is advantageous in order to heat the exhaust gas as quickly as possible to the required operating temperature in order to be able to carry out the selective catalytic reduction.

It is also advantageous if the first SCR catalytic converter in the flow direction has an extension of not more than 80 mm along the flow direction, wherein it preferably has an extension of less than 50 mm, with particular preference having an extension of less than 40 mm. A short extension of the first SCR catalytic converter relative to the particle filter or the second SCR catalytic converter is advantageous in order to only slightly influence the flow for the subsequent particle filter and thus to maintain a good filter result in the particle filter.

A preferred embodiment is characterized in that the first SCR catalyst in the flow direction is arranged at a distance along the flow direction of not more than 20 mm, preferably less than 10 mm and more preferably less than 5 mm to the particle filter. The smallest possible distance is advantageous in order to be able to summarize the pressure loss arising from the first SCR catalytic converter and the particle filter. This facilitates the design of the exhaust system.

It is also preferable if the first SCR catalytic converter in the flow direction and the second SCR catalytic converter in the flow direction with the particle filter are designed as a modular unit. This is advantageous to facilitate the assembly.

Moreover, it is advantageous if the SCR catalysts and the particle filter are formed by a common honeycomb body. This is advantageous to keep the number of installed elements as low as possible, whereby the assembly is simplified.

Furthermore, it is advantageous for the honeycomb body of the particle filter to have a cell geometry and / or cell density deviating from the honeycomb body of the first SCR catalytic converter in the flow direction and the honeycomb body of the second SCR catalytic converter in the flow direction. This is particularly advantageous in order to optimally adapt the individual components to their respective intended use and, overall, to achieve the lowest possible pressure loss across the individual components. In addition, the cell geometries can influence, for example, the filter properties and the velocity distribution.

It is also expedient if the cell geometry and / or the cell density and / or the chemically active coating of the honeycomb body and / or the coating amount of the honeycomb body of the first SCR catalyst in the flow direction of the cell geometry and / or the cell density and / or the chemically active Coating of the honeycomb body deviates in the flow direction second SCR catalyst.

This is particularly advantageous in order to be able to adapt the honeycomb body to the changed flow velocity, for example in the region of the second SCR catalytic converter. Also, in the area of the second SCR catalytic converter, a changed exhaust gas composition is to be expected, since this is downstream of the particle filter. As a result, for example, a higher cell density or a reduced cell cross-section can be used without risking blocking of the flow channels formed in the honeycomb body. The coating can also be advantageously adapted to the regularly lower concentration of ammonia and nitrogen oxides, in order nevertheless to produce as complete a conversion of the nitrogen oxides as possible.

Preferably, the coating amount of the first SCR catalyst in the flow direction is higher than the coating amount of the second SCR catalyst in the flow direction.

Advantageous developments of the present invention are described in the subclaims and in the following description of the figures.

list of figures

• 1 eine Schnittansicht durch einen beispielhaften Abgasstrang mit einer Mehrzahl von Katalysatoren.

In the following the invention will be explained in detail by means of an embodiment with reference to the drawing. In the drawing shows:

• 1 a sectional view through an exemplary exhaust system with a plurality of catalysts.

Preferred embodiment of the invention

The 1 shows a sectional view through an exhaust system of an exhaust system 1 , The exhaust system 1 comprises a plurality of catalysts, which are arranged in housings and are connected to each other by pipelines such that exhaust gas can flow through the catalysts.

In the embodiment of 1 is in the flow direction 2 the exhaust gas first a catalyst 3 arranged for the oxidation of the exhaust gas. The following is an introductory point 4 provided, through which a reducing agent, such as an aqueous urea solution can be introduced into the exhaust system. Immediately adjacent to this discharge point 4 is an evaporation element 5 arranged, on which the reducing agent can be evaporated, to then react as ammonia with the nitrogen oxides in the exhaust gas.

Downstream is followed by a first SCR catalyst 6 in which the nitrogen oxides in the exhaust gas with the ammonia and the chemically active coating of the SCR catalyst 6 react and thus form nitrogen and water.

The first SCR catalyst 6 is a particle filter 7 downstream, which is used for filtering the exhaust gas and filters in particular soot particles from the exhaust gas.

On the particle filter 7 followed by a second SCR catalyst 8th , In this chemically takes place the same reaction as in the first SCR catalyst 6 ,

The advantage of such a construction is that a reduction of the nitrogen oxides already takes place in the exhaust gas before the exhaust gas flows into the particle filter. The exhaust gas flowing into the first SCR catalytic converter therefore still has a particularly high temperature and the exhaust gas distribution over the cross section of the flow path is not adversely affected by the particle filter. As a result, a particularly effective conversion of the nitrogen oxides in the exhaust gas can take place. Due to the usually large-volume particulate filter, the temperature of the exhaust gas drops significantly, whereby the chemical reaction in the downstream SCR catalyst usually does not run optimally. The second SCR catalyst essentially serves to reduce the not yet reduced in the first SCR catalyst nitrogen oxide content.

Instead of or in addition to the second SCR catalyst, it is also possible to install an ammonia slip catalyst which can bind excess ammonia (NH ₃ ) which was not reacted in the reduction of the nitrogen oxides in the exhaust gas. In this way, for example, a breakthrough of ammonia can be avoided in the exhaust, which ammonia could escape into the environment and could lead to an odor nuisance or pollution of the environment.

The embodiment of 1 In particular, it has no restrictive character and serves to clarify the inventive idea.

Claims (8)

Exhaust system (1) for the aftertreatment of exhaust gases of an internal combustion engine with viewed in the flow direction (2) of the exhaust gas successively arranged, a catalyst (3) for the oxidation of the exhaust gas and / or a catalyst for storing nitrogen oxides, with an introduction point (4) for supplying a Reducing agent with an SCR catalyst (6) for the selective catalytic reduction of nitrogen oxides and with a particulate filter, characterized in that the particulate filter (7) in the flow direction (2) after the SCR catalyst (6) is arranged and in the flow direction (2 ) after the particulate filter (7), a second SCR catalyst (8) and / or an ammonia slip catalyst is arranged. Exhaust system (1) according to Claim 1 , characterized in that in the flow direction (2) upstream of the particulate filter (7) arranged first SCR catalyst (6) is electrically heated. Exhaust system (1) according to one of the preceding claims, characterized in that in the flow direction (2) first SCR catalyst (6) along the flow direction (2) has an extension of not more than 80 mm, wherein it preferably has an extension of less than 50 mm, wherein it particularly preferably has an extension of less than 40 mm. Exhaust system (1) according to one of the preceding claims, characterized in that in the flow direction (2) first SCR catalyst (6) with a distance along the flow direction (2) of a maximum of 20 mm, preferably of less than 10 mm, and particularly preferably is arranged smaller than 5 mm to the particle filter (7). Exhaust system (1) according to one of the preceding claims, characterized in that in the flow direction (2) first SCR catalyst (6) and in the flow direction (2) second SCR catalyst (8) with the particulate filter (7) as a modular unit are formed. Exhaust system (1) according to one of the preceding claims, characterized in that the SCR catalysts (6, 8) and the particulate filter (7) are formed by a common honeycomb body. Exhaust system (1) according to one of the preceding claims, characterized in that the honeycomb body of the particulate filter (7) one of the honeycomb body of the first in the flow direction (2) SCR catalyst (6) and the honeycomb body in the flow direction (2) second SCR catalyst (8) different cell geometry and / or cell density. Exhaust system (1) according to one of the preceding claims, characterized in that the cell geometry and / or the cell density and / or the chemically active coating of the honeycomb body and / or the coating amount of the honeycomb body of the flow direction (2) first SCR catalyst (6) deviates from the cell geometry and / or the cell density and / or the chemically active coating of the honeycomb body of the flow direction (2) second SCR catalyst (8).

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