DE102015214734A1 - Exhaust after-treatment system for a diesel engine - Google Patents

Abstract

The invention relates to an exhaust aftertreatment system (1) for a diesel engine (50), with
- A discharge path (4) for exhaust gases, which leads from a motor-side receiving point (2) to a delivery point (3),
- One of a branch point (6) on the discharge path (4) to a motor-side return point (8) leading return path (7) for exhaust gases, and
- A between the receiving point (2) and the branching point (6) arranged first catalyst (9) for the selective catalytic reduction of nitrogen oxide by a reducing agent.
In order to reduce the emission of pollutants, in particular nitrogen oxides, in the combined use of an exhaust gas recirculation and an SCR catalyst, the invention provides that between the first catalyst (9) and the return point (8) a second catalyst (11) for reducing the reducing agent is arranged.

Description

The invention relates to an exhaust aftertreatment system for a diesel engine having the features of the preamble of claim 1.

Exhaust gas recirculation is a common technique for reducing pollutant emissions in diesel engines. More specifically, this reduces the amount of nitrogen oxides (NO _x ) produced in the engine. The nitrogen oxides are formed by the endothermic reaction of nitrogen and oxygen contained in the intake air. Their formation depends on the one hand on the temperature prevailing in the engine, on the other hand on the oxygen concentration. The basic principle in exhaust gas recirculation is based on the fact that - in addition to fuel and air - also exhaust gases, which are predominantly chemically inert, are introduced into the engine, whereby the oxygen concentration and thus the combustion air ratio are lowered. On the one hand, and on the other hand by a resulting lower combustion temperature, the amount of nitrogen oxides produced decreases.

Normally in addition to this, the exhaust gases can also be catalytically treated in order to further reduce the amount of nitrogen oxides. Of importance here is u. a. Selective catalytic reduction (SCR), in which a reducing agent (eg ammonia or ammonia precursor such as a urea-water solution) is injected into the exhaust gas stream and, on suitable catalysts with nitrogen oxides, to less harmful products ( in the case of ammonia as a reducing agent: to nitrogen and water) are reacted.

In addition, for the treatment of the exhaust gas stream, further catalysts and particulate filters in particular can be used.

A problem with the use of SCR catalysts is the generally incomplete reaction of the introduced reducing agent. This, in contrast to the combustion gases often can not be considered as chemically inert, but when it is returned together with combustion gases in the engine, be oxidized, which can lead to the formation of other pollutants. In particular, ammonia is oxidized to form further nitrogen oxides. Thus, the measures for the removal of nitrogen oxides from the exhaust gas flow lead to increased formation of the same in the engine. A complete implementation of the reducing agent used in all operating ranges and under various conditions on the SCR catalyst, however, is hardly feasible. The problem described above arises in particular in the case of low-pressure exhaust gas recirculation (LP EGR), in which the reducing agent is added upstream of the recirculation.

In view of the above-mentioned prior art, a more effective reduction in the production or emission of pollutants, in particular nitrogen oxides, in the context of exhaust aftertreatment, which employs both an exhaust gas recirculation and an SCR catalyst, still offers room for improvement.

The invention has for its object to reduce the emission of pollutants, especially nitrogen oxides in the combined use of exhaust gas recirculation and an SCR catalyst.

According to the invention the object is achieved by an exhaust aftertreatment system with the features of claim 1, wherein the dependent claims relate to advantageous embodiments of the invention.

It should be noted that the features listed in the following description as well as measures in any technically meaningful way can be combined with each other and show other embodiments of the invention. The description additionally characterizes and specifies the invention, in particular in connection with the figure.

The invention provides an exhaust aftertreatment system for a diesel engine. The exhaust aftertreatment system can be used in particular for a diesel engine of a motor vehicle, in particular of a passenger car or truck. It comprises a discharge path for exhaust gases, which leads from a motor-side receiving point to a delivery point. The receiving point is connected in the installed state with the diesel engine and allows the reception or decrease of exhaust gases thereof. The delivery point serves to deliver the exhaust gases to the outside. It may, for example, open into a tailpipe or be formed by a tailpipe. The Abführweg is of course formed by components that are able to lead exhaust gases from the receiving point to the delivery point. The term "path" can be understood here and below also in the sense of "line" or "line system".

Furthermore, the exhaust aftertreatment system comprises a recirculation path for exhaust gases leading from a branching point on the discharge path to an engine-side recirculation point and a first catalyst arranged between the receiving point and the branching point for the selective catalytic reduction of nitrogen oxide by a reducing agent. The reducing agent may in particular be ammonia. The return path branches at the Abzweigstelle from the discharge and off to return exhaust gases in the diesel engine. This corresponds to the known in the prior art principle of exhaust gas recirculation, which serves to reduce the development of nitrogen oxide in the engine. It is understood that the branching point is located between the receiving point and the delivery point. The term "nitrogen oxide" is used here and below as synonymous with "nitrogen oxides" and serves as a collective term for a single nitrogen oxide or a mixture of several nitrogen oxides, for example a mixture of NO and NO _2. The first catalyst is designed as an SCR catalyst and sets in a known manner the reducing agent (eg ammonia) with nitrogen oxides to harmless or less harmful products (eg to nitrogen and water) upstream of the first catalyst is at least when installed a supply device for the reducing agent (eg Ammonia, or an aqueous urea solution from which ammonia and carbon dioxide are formed by hydrolysis) or the like, via which the supply of the reducing agent is prepared, The first catalyst may be applied to an ordinary substrate, for example, a support body Ceramic coated with a washcoat for surface enlargement but it can also be applied to a particulate filter. In the latter case, the particle filter forms a so-called SDPF (SCR-coated diesel particle filter). Also for catalysts discussed below there is the possibility that they are applied to a particulate filter.

The arrangement of the first catalyst relative to the branch point corresponds to the configuration of a low-pressure exhaust gas recirculation. The invention thus relates in particular to LP-EGR systems.

In order at least to reduce the above-described problem of undesired recycling of reducing agent into the diesel engine, it is provided according to the invention that a second catalyst for reducing the reducing agent is arranged between the first catalyst and the return point. The second catalyst thus favors at least one reaction in which an educt is the reducing agent used. It is located downstream of the first catalyst, but upstream of the recirculation point, in each case based on the exhaust gas flow in the operation of the exhaust aftertreatment system. It thus captures exhaust gases that have already passed through the first catalytic converter and are subsequently returned to the diesel engine. Thus, the risk is reduced that is oxidized after the return of the exhaust gases into the diesel engine reducing agent to form pollutants such as nitrogen oxides.

The second catalyst can be arranged on the one hand in the part of the discharge path, which extends downstream of the first catalyst to the branching point. In this case, all the exhaust gases that have passed through the first catalyst would be treated by the second catalyst, including those exhaust gases that are fed from the branch point to the discharge point. However, the second catalyst is preferably arranged in the return path. This is expedient insofar as the task of the second catalyst is primarily to prevent the return of reducing agent in the diesel engine.

According to one embodiment, the second catalyst is configured as an SCR catalyst for the selective catalytic reduction of nitrogen oxide by means of the reducing agent (i.e., the same reducing agent used on the first catalyst). In this case, the second catalyst can to some extent be regarded as the second stage of the first catalyst, since in both cases the same reactions take place in principle. The aim here is to complete the generally incomplete reaction on the first catalyst as possible on the second catalyst. This is based on the generally correct assumption that the reaction on the first catalyst has not removed all the nitrogen oxides from the exhaust gas and thus they are available as reactants for remaining reducing agent available.

According to another embodiment, the second catalyst is adapted for selective catalytic oxidation of the reducing agent by oxygen. If the reducing agent is ammonia, the second catalyst may also be referred to as ammonia oxidation catalyst (AMOX). The corresponding oxidation produces nitrogen and water.

In principle, the second catalyst can be arranged at various points in the return path. One possible choice is that of the warmest region of the return path in the operating state, since this promotes the corresponding reactions. The second catalyst may be disposed within a tube that is part of the return path. However, it can also be applied to a cooler or a particle filter of the return path. It is known in the prior art, in the context of an exhaust gas recirculation to carry out a cooling of the exhaust gases and / or a removal of particles from the exhaust gas stream. The second catalyst may, for example, be applied as a thin layer to the surface of a corresponding radiator or particle filter.

In addition to the first and the second catalyst may further devices for After treatment of the exhaust gases may be provided. Thus, according to one embodiment, a first exhaust aftertreatment device is arranged between the receiving location and the first catalyst. Said first exhaust aftertreatment device is thus arranged upstream of the first catalyst in the discharge path. In principle, this can be any type of catalyst, filter or even a combination of both. Preferably, the first exhaust aftertreatment device is designed as a diesel particulate filter, diesel oxidation catalyst or nitrogen oxide storage catalyst. As is known, carbon monoxide (CO) and hydrocarbons are converted to carbon dioxide and water by oxidation with the residual oxygen present in the exhaust gas by a diesel oxidation catalyst (DOC). The corresponding catalysts normally contain precious metals such as platinum or palladium. In order to achieve the temperature necessary for the reaction, it is expedient to provide the corresponding catalyst near the diesel engine or in the vicinity of the receiving location. In storage catalysts, in addition to the actual noble metal catalyst, an NO _x storage component (eg, an alkaline earth metal) is present. It is provided that the engine control operates predominantly in a "lean" phase in which the exhaust gases are rich in oxygen. The nitrogen oxides are absorbed under the action of the catalyst. Meanwhile, in a "rich" phase, the engine controller adjusts a stoichiometric oxygen concentration, thereby releasing the NO _x molecules back into the exhaust stream and reducing them by incompletely combusted hydrocarbons and / or CO.

Alternatively or in addition to the first exhaust aftertreatment device, a second exhaust aftertreatment device may be arranged between the first catalyst and the branch point. This is thus located downstream of the first catalyst, but still in the region of the discharge path, which is upstream of the branch point. This exhaust aftertreatment device can basically be any type of catalyst, filter or even a combination of both. Preferably, the second exhaust aftertreatment device is designed as a diesel particulate filter, nitrogen oxide storage catalytic converter or SCR catalytic converter. The diesel particulate filter can also carry on its surface a catalyst, in particular an SCR catalyst.

Alternatively or in addition to at least one of the first and second exhaust aftertreatment device, a third exhaust aftertreatment device may be arranged between the branch point and the delivery point. This is thus located in the discharge path downstream of the branch point. Again, in principle all conceivable catalysts and / or filters can be used. Preferably, the third exhaust aftertreatment device is designed as a nitrogen oxide storage catalyst, SCR catalyst or as an oxidation catalyst for the reducing agent. The oxidation catalyst is used for the selective catalytic oxidation of the reducing agent by oxygen, it being preferred that at most nitrogen in the elemental form and possibly no nitrogen oxides are generated by the reaction. This may in particular be an ammonia oxidation catalyst.

Further advantageous details and effects of the invention are explained in more detail below with reference to an embodiment shown in the figure. It shows:

1 a block diagram of an exhaust aftertreatment system according to the invention.

1 illustrates in block diagram form an exhaust aftertreatment system according to the invention 1 , with a low-pressure exhaust gas recirculation, and a connected diesel engine 50 , The diesel engine 50 is via an air supply 51 with air and a fuel supply 52 supplied with diesel. The resulting in the combustion of the fuel-air mixture exhaust gases are primarily via one of a motor-side receiving point 2 to a delivery point 3 leading discharge path 4 discharged into the environment. The last section of the discharge path 4 can in this case by a tailpipe 5 be formed.

At a branch point 6 goes from the discharge path 4 a return route 7 in the present case at a recycling center 8th in the air supply 51 and thus further into the diesel engine 50 opens. There in the discharge path 4 and thus also in the return path from this 7 primarily combustion gases are conducted, reduced by this measure the oxygen content of the diesel engine 50 existing fuel-air mixture. As a result, fewer nitrogen oxides are produced during combustion.

To reduce the amount of nitrogen oxides passing through the delivery point 3 get into the environment, reduce further, is in the discharge path 4 a first catalyst designed as an SCR catalyst 9 intended. Upstream of the first catalyst 9 is via an injection device 10 an aqueous urea solution in the exhaust stream, from which forms by hydrolysis ammonia, which in turn on the first catalyst 9 reacted with nitrogen oxides to form nitrogen and water.

However, the reaction may be incomplete, leaving not only nitrogen oxides but also ammonia in the downstream portion of the discharge pathway 4 and the return route 7 reach. While the nitrogen oxides are the diesel engine 50 would undergo substantially unchanged, ammonia would be at least partially burned to form further nitrogen oxides, whereby the effectiveness of the exhaust aftertreatment would be impaired. Because of this, is in the return path 7 a second catalyst 11 provided, which is adapted for the degradation of ammonia. The second catalyst 11 may be applied as a thin layer on the surface of a return path 7 be present existing cooler or particulate filter. According to one embodiment, the second catalyst may be 11 to act another SCR catalyst, however, is not provided with a further injection direction, but works by means of remaining in the exhaust gas flow amounts of ammonia and nitrogen oxides. Alternatively, the second catalyst 11 for the catalytic oxidation of ammonia to nitrogen, wherein the ammonia is oxidized with oxygen present in the exhaust gas stream.

In any case, the second catalyst provides 11 for at least substantial removal of ammonia from the via the recycling site 8th in the diesel engine 50 returning exhaust gas flow. Oxidation of ammonia to nitrogen oxide is thus largely prevented.

It is conceivable, optional exhaust aftertreatment devices 12 . 13 . 14 provide, by means of which the quality of the exhaust gases emitted to the outside can be further improved. A first exhaust aftertreatment device 12 can in this case between the receiving point 2 and the first catalyst 9 be arranged. This may in particular be a diesel particulate filter, diesel oxidation catalyst or nitrogen oxide storage catalyst. Alternatively or additionally, a second exhaust aftertreatment device 13 downstream of the first catalyst 9 between this and the branch point 6 be provided. This can be in particular a diesel particulate filter, nitrogen oxide storage catalytic converter or SCR catalytic converter. Alternatively or in addition to each of the first and second exhaust aftertreatment devices 12 . 13 may be in the downstream of the branch point 6 located part of the discharge path 4 a third exhaust aftertreatment device 14 be provided. This can be designed in particular as a nitrogen oxide storage catalyst, SCR catalyst or ammonia oxidation catalyst. In principle, however, any of the exhaust aftertreatment devices 12 . 13 . 14 be formed by a known type of filter and / or catalyst, for example. By a catalyst coated with a filter.

LIST OF REFERENCE NUMBERS

1

 aftertreatment system

2

 receiving location

3

 delivery point

4

 discharge path

5

 tailpipe

6

 branching point

7

 return path

8th

Recycle point

9

first catalyst

10

 Injector

11

second catalyst

12, 13, 14

exhaust aftertreatment device

50

 diesel engine

51

 air supply

52

 fuel supply

Claims (10)

Exhaust aftertreatment system ( 1 ) for a diesel engine ( 50 ), with - a discharge path ( 4 ) for exhaust gases, which from a motor-side receiving point ( 2 ) to a delivery point ( 3 ), - one from a branch office ( 6 ) at the discharge path ( 4 ) to a motor-side return point ( 8th ) leading return path ( 7 ) for waste gases, and - one between the receiving point ( 2 ) and the branch office ( 6 ) arranged first catalyst ( 9 ), for the selective catalytic reduction of nitrogen oxide by a reducing agent, characterized in that between the first catalyst ( 9 ) and the return center ( 8th ) a second catalyst ( 11 ) is arranged to reduce the reducing agent. Exhaust gas after-treatment system according to claim 1, characterized in that the second catalyst ( 11 ) in the return path ( 7 ) is arranged. Exhaust gas aftertreatment system according to claim 1 or 2, characterized in that the second catalyst ( 11 ) is configured as an SCR catalyst for the selective catalytic reduction of nitrogen oxide by the reducing agent. Exhaust gas after-treatment system according to one of the preceding claims, characterized in that the second catalyst ( 11 ) is adapted for selective catalytic oxidation of the reducing agent by oxygen. Exhaust after-treatment system according to one of the preceding claims, characterized in that between the receiving point ( 2 ) and the first catalyst ( 9 ) a first exhaust aftertreatment device ( 12 ) is arranged. Exhaust after-treatment system according to claim 5, characterized in that the first exhaust aftertreatment device ( 12 ) is designed as a diesel particulate filter, diesel oxidation catalyst or nitrogen oxide storage catalyst. Exhaust gas after-treatment system according to one of the preceding claims, characterized in that between the first catalyst ( 9 ) and the branch office ( 6 ) a second exhaust aftertreatment device ( 13 ) is arranged. Exhaust after-treatment system according to claim 7, characterized in that the second exhaust aftertreatment device ( 13 ) is designed as a diesel particulate filter, nitrogen oxide storage catalyst or SCR catalyst. Exhaust after-treatment system according to one of the preceding claims, characterized in that between the branching point ( 6 ) and the delivery point ( 3 ) a third exhaust aftertreatment device ( 14 ) is arranged. An exhaust after-treatment system according to claim 9, characterized in that the third exhaust aftertreatment device ( 14 ) is designed as a nitrogen oxide storage catalyst, SCR catalyst or as an oxidation catalyst for the reducing agent.

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