DE102017207449A1 - Exhaust system with evaporation element - Google Patents

Abstract

The invention relates to an exhaust system (9) for the treatment of exhaust gases of an internal combustion engine, with a flow path limited by walls, with a plurality of catalysts arranged in the flow path, with an injection point (1) for an aqueous urea solution and with an evaporation element (3) Evaporation of the injected aqueous urea solution, wherein the evaporation element (3) is formed by a honeycomb body having an inflow side and an outflow side (8), wherein the honeycomb body from the inflow side to the outflow side (8) along a plurality of flow channels (4) can be flowed through , wherein the honeycomb body is set at an angle (α) to the main flow direction (12) of the flow path.

Description

Technical area

The invention relates to an exhaust system for the treatment of exhaust gases of an internal combustion engine, with a flow-limited by walls, with a plurality of arranged in the flow path catalysts, with an injection point for an aqueous urea solution and with an evaporation element for evaporation of the injected aqueous urea solution.

State of the art

To improve the exhaust aftertreatment in exhaust gas strands of motor vehicles and in particular for reducing the amount of nitrogen oxides contained in the exhaust gas, an aqueous urea solution is added to the exhaust gas which is converted into ammonia in the exhaust gas line by thermolysis and hydrolysis and finally reacts with nitrogen oxides to nitrogen and water.

For this purpose, the aqueous urea solution is sprayed onto an evaporation element and converted by the heat supply to ammonia. The ammonia then reacts with the nitrogen oxides bound in the exhaust gas.

For mixing and evaporation of the injected aqueous urea solution with the flowing exhaust gas evaporation elements are used in the devices known from the prior art. Known here among other sheet metal structures or cast structures, which are known as so-called flap mixers or paddle mixers. These have a plurality of openings through which the evaporation elements can be flowed through in the flow direction of the exhaust gas.

A disadvantage of the evaporation elements in the prior art is in particular that the flow channels are configured opaque in the flow direction of the exhaust gas, so that no straight-through flow is possible. The opacity is regularly achieved by blades or flaps, which protrude into the flow. Due to the unfavorable design, there may be direct contact of the aqueous urea solution with the walls delimiting the flow path, as a result of which the drops may accumulate on the colder walls of the flow path. This can lead to contamination or damage. In addition, the pressure loss caused by such an evaporation element is very high, whereby the exhaust gas flow is adversely affected.

Presentation of the invention, object, solution, advantages

Therefore, it is the object of the present invention to provide an exhaust system which achieves an advantageous mixing of the injected aqueous urea solution with the exhaust gas, wherein the counterpressure generated by the evaporation element is kept as low as possible and the admission of the walls of the flow path is reduced as much as possible.

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 treating exhaust gases of an internal combustion engine having a wall-limited flow path, with a plurality of arranged in the flow path catalysts, with an injection point for an aqueous urea solution and with an evaporation element for evaporation of the injected aqueous urea solution the evaporation element is formed by a honeycomb body, which has an inflow side and an outflow side, wherein the honeycomb body can be flowed through from the inflow side to the outflow side along a plurality of flow channels, wherein the honeycomb body is employed at an angle to the main flow direction of the flow path.

The evaporation element serves to distribute the injected aqueous urea solution with the flowing exhaust gas and to reduce the droplet diameter. By a very uniform and fine distribution of the aqueous urea solution in the exhaust gas, the reaction with the nitrogen oxides bound in the exhaust gas can be favored.

The evaporation element according to the invention is constructed like a honeycomb body for a known catalyst and is arranged at an angle to the main flow direction of the exhaust system. The flow channels which preferably extend in a straight line through the evaporation element are thus at an angle to the direction of propagation of the aqueous urea solution, which is determined on the one hand by the injection direction and on the other hand by the main flow direction of the exhaust gas. This ensures that the drops of the urea aqueous solution in each case collide with one of the inner walls of the flow channels where they are further atomized and deflected. Due to the length of the extension of the flow channels and the angle of attack of the evaporation element is further ensured that the rebounding drops again be directed to the inner walls of the flow channels. An impact of the not yet vaporized drops on the flow path limiting walls is structurally avoided by the flow channels of the evaporation element are designed long enough that a complete evaporation takes place within the evaporation element.

It is also expedient if the angle is chosen such that, as a function of the diameter of the individual flow channels, a view-tightness along the main flow direction is generated.

In the case of a particularly large diameter of the flow channels, the angle of attack must be chosen correspondingly greater in order to ensure that there is opacity along the main flow direction, which is substantially identical to the central axis of the flow path, and the aqueous urea solution sprayed onto the evaporation element at least once within the Flow channels at the flow channels bounding inner walls rebounds and is deflected. The more finely atomized droplets of the aqueous urea solution come into contact with the walls of the evaporation element, the faster the heat can be transferred to the aqueous urea solution, which promotes the evaporation.

With correspondingly smaller diameters, even a smaller angle of attack may already be sufficient in order to create a visual density along the main flow direction. The angle of attack is thus essentially dependent on the dimensioning of the flow channels in the evaporation element.

It is particularly advantageous if an injector is arranged at the injection point, which injects the aqueous urea solution in a conical cloud into the flow path, wherein the evaporation element is employed at an angle to the central axis of the injected conical cloud. The injection in a cone-shaped cloud is advantageous in order to achieve the finest possible atomization of the aqueous urea solution and thus to keep the absolute surface of the drops of urea aqueous solution as large as possible in order to achieve rapid heat transfer from the exhaust gas to the aqueous urea solution on the one hand and thus on the other hand to achieve a rapid evaporation of the aqueous urea solution.

Alternatively, the injected cloud of the aqueous urea solution may also have a cross section in the form of a flat oval. The shape of the cloud is in particular to be adapted to the cross section of the affected flow path or to the cross section of the evaporation element to be acted upon in order to minimize or completely avoid an impact of the aqueous urea solution on the walls delimiting the flow path. Ideally, the aqueous urea solution is sprayed completely homogeneously distributed on the inflow side of the evaporation element, without any passing of aqueous urea solution past the evaporation element or a precipitate on the walls of the flow path, in which the evaporation element is arranged.

It is also advantageous if the evaporation element is set at an angle such that the evaporation element is opaque in the propagation direction of the injected urea solution. Opaque means in this case that a flow through the flow channels for the majority of the aqueous urea solution is not possible without colliding at least once with one of the inner walls. In the case of an orthogonal projection along the center axis of the flow path, there must be no rectilinear flow through the evaporation element without wall contact.

Viewed from the perspective of a single drop of aqueous urea solution, the evaporation element is opaque along the direction of propagation. Only through the deflection on one of the inner walls of the flow channels, the aqueous urea solution can flow through the evaporation element. As a result, the desired deflection and further atomization is achieved without generating an excessive pressure loss in the exhaust gas flow.

A preferred embodiment is characterized in that the evaporation element comprises a hydrolysis coating. A hydrolysis coating is advantageous to accelerate the hydrolysis of the aqueous urea solution.

As a result, the ammonia required for the reaction with the nitrogen oxides can be generated faster.

It is also preferable if the evaporation element has a cross-section which is adapted to the diameter of the conical cloud, such that the entire inflow side of the evaporation element is acted upon by a cloud adapted to the shape of the evaporation element.

Ideally, the entire inflow side of the evaporation element is uniformly charged with the aqueous urea solution. Depending on the angle by the injection relative to Evaporating element happens and the influence of the exhaust gas flow on the spread of the aqueous urea solution in the exhaust system, the evaporation element may have different shapes and be employed at different angles.

Moreover, it is advantageous if the evaporation element is a honeycomb body formed from a plurality of stacked metal layers, wherein the metal layers are wound up and accommodated in a jacket tube. Preferably, the evaporation element is constructed like other known honeycomb body. This has the advantage that the evaporation element can be built in a particularly simple and in a large number of variants, whereby an optimum for the respective exhaust system evaporation element can be produced.

Furthermore, it is advantageous if the vaporization element forming metal layers are at least partially structured. This is advantageous in order to generate the flow channels in a simple manner, since it is possible to produce, through the structured regions, spacings between the smooth regions of the metal layers which form the flow channels in the wound-up state.

It is also expedient if the flow channels in the evaporation element have a structured surface, wherein the structured surface is formed in particular by fins, webs, recesses, microstructures, bulges and / or openings. Structured surfaces can promote turbulence of the aqueous urea solution in the evaporation element, whereby improved mixing with the flowing exhaust gas can be achieved.

Moreover, it is advantageous if the extent of the evaporation element from the inflow side to the outflow side is at least as long that the components of the aqueous urea solution impinging on the walls of the flow channels and deflecting therefrom impinge at least one more time on one of the walls of the flow channels. This is particularly advantageous in order to prevent the drops of the aqueous urea solution from hitting the walls delimiting the flow path of the exhaust system and precipitating there unintentionally. By deflecting from an inner wall of the evaporation element to a further inner wall mixing of the aqueous urea solution is achieved, and further better atomization of the aqueous urea solution, whereby the subsequent reactions, in particular the evaporation, are favored.

Furthermore, it is expedient for the flow channels of the evaporation element to extend in a straight line from the inflow side to the outflow side, whereby an impingement of the aqueous urea solution on the inner walls of the flow channels is ensured by the employment of the evaporation element at an angle to the central axis of the injected conical cloud. By a suitable combination of the angle of attack with the length of the flow channels, it can be achieved that flow through the aqueous urea solution through the evaporation element is avoided without hitting at least one of the inner walls of the flow channels for most of the injected aqueous urea solution.

It is also advantageous if the direction along which the flow passages flow is deviating from the main flow direction of the flow path.

This is advantageous, on the one hand, to prevent the exhaust gas from flowing through the evaporation element essentially without being deflected or, respectively, flowing the injected aqueous urea solution through the evaporation element in an undeflected manner. By the contact with the inner walls of the flow channels of the evaporation element, the heat transfer from the evaporation element is improved to the aqueous urea solution, whereby the evaporation is accelerated. By providing additional coatings of the evaporation element which act as a catalyst, the thermolysis and the hydrolysis of the aqueous urea solution can be further accelerated.

Furthermore, it is preferable if by the offset between the flow direction of the flow channels and the main flow direction of the flow path on the outflow side of the flow channels of the evaporation element Abrisskanten arise, whereby a turbulence of the fluid flowing through the flow channels is generated.

Due to the angle between the main flow direction of the flow path, which is substantially identical to the center axis of the flow path, and the flow direction of the flow channels is formed on the outflow side of the evaporation element one or more separation edges for the fluid flowing through the flow channels. These demolition edges are formed, for example, by the flow channels bounding edge on the outflow side. The overflow of the spoiler edge creates a turbulence of the gas volume flowing through the flow channels, whereby a mixing with the other exhaust gas flowing in the exhaust system is achieved.

In addition, it is helpful if at least one deflection of the main flow direction of the flow path is directly downstream of the evaporation element by at least 90 degrees. By a further deflection of the main flow direction of the flow path by at least 90 degrees, an improved mixing of the substantially evaporated after the evaporation element aqueous urea solution and the exhaust gas flowing through the exhaust gas is also generated.

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

list of figures

• 1 im linken Teil eine schematische Ansicht eines Verdampfungselementes und dem Injektor für die Einspritzung der wässrigen Harnstofflösung, und im rechten Teil eine Detailansicht des Verdampfungselementes, und
• 2 eine schematische Ansicht einer Strömungsstrecke, mit einem Verdampfungselement und zwei Umlenkungen der Hauptdurchströmungsrichtung.

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

• 1 in the left part of a schematic view of an evaporation element and the injector for the injection of the aqueous urea solution, and in the right part of a detailed view of the evaporation element, and
• 2 a schematic view of a flow path, with an evaporation element and two deflections of the main flow direction.

Preferred embodiment of the invention

The 1 shows an injector 1 through which the aqueous urea solution can be injected into the exhaust system. The aqueous urea solution is removed from the injector 1 injected cone-shaped, resulting in the exhaust system, the conical cloud 2 results.

The evaporation element 3 is formed by a metallic honeycomb body having a plurality of flow channels 4 having. The evaporation element is at an angle α to the central axis 5 the cone-shaped cloud 2 hired. This causes the aqueous urea solution to deviate in a direction to align the flow channels 4 emotional. This ensures that the aqueous urea solution on the inner walls of the evaporation element 3 meets and is diverted there. The more common the drops of the aqueous urea solution in the evaporation element 3 be redirected and meet the inner walls, the finer the generated mist in the exhaust system, which favors the evaporation and conversion to ammonia.

The evaporation element 3 is preferably constructed analogously to already known catalysts, which consist of a plurality of stacked and then wound metal layers.

With the reference number 6 is a detail of the evaporation element 3 marked in the right part of the 1 is shown enlarged.

The detail 6 shows the flow channels 4 in the evaporation element 3 , Furthermore, it is exemplary for one of the flow channels 4 shown how the aqueous urea solution on one of the inner walls 7 meets and is deflected by this and to the opposite inner wall 7 is steered. This further atomizes the aqueous urea solution.

The length of the flow channels is preferred 4 such that after the first impact of the aqueous urea solution on the inner wall 7 at least a second contact with an inner wall 7 is generated before the aqueous urea solution from the outflow 8th of the evaporation element 3 exit.

The 2 shows a schematic view of an exhaust system 9 which is at least one evaporation element 3 as it already is in the 1 was shown. The exhaust gas can go along the arrow 11 in the exhaust system 9 flow. It undergoes a deflection by 90 degrees due to the pipe routing of the flow path and continues to flow along the main flow direction 12 on the evaporation element 3 to. The evaporation element 3 is according to the invention in an angle of attack to the main flow direction 12 arranged. The embodiment of 2 serves the basic structure of the exhaust system 9 to clarify. The inclined arrangement of the evaporation element 3 is already in 1 shown in detail and described.

After flowing through it evaporation element 3 finds another redirection 90 degrees in the range 10 instead of the exhaust gas along the arrow 13 against the direction of the arrow 11 from the exhaust system 9 flows. The exhaust system 9 may additionally comprise further catalysts which serve the exhaust aftertreatment.

In particular, by the outflow from the employed evaporation element 3 and the associated overflow of the demolition edges formed on the outflow side and the deflection in the area 10 is a mixing of the evaporation element 3 vaporized aqueous urea solution with the through the exhaust system 9 reached flowing exhaust gas. The exhaust gas mixed as homogeneously as possible with the vaporized aqueous urea solution can subsequently be re-circulated in the area 10 be post-treated in a so-called SCR catalyst, whereby the unwanted nitrogen oxides are filtered from the exhaust gas.

The embodiments of the 1 and 2 In particular, they have no restrictive character and serve to clarify the inventive concept.

Claims (14)

Exhaust system (9) for treating exhaust gases of an internal combustion engine, with a wall section limited by flow, with a plurality of arranged in the flow path catalysts with an injection point (1) for an aqueous urea solution and with an evaporation element (3) for evaporation of the injected aqueous Urea solution, characterized in that the evaporation element (3) is formed by a honeycomb body having an inflow side and an outflow side (8), wherein the honeycomb body from the inflow side to the outflow side (8) can be flowed through along a plurality of flow channels (4), wherein the honeycomb body is set at an angle (α) to the main flow direction (12) of the flow path. Exhaust system (9) according to Claim 1 , characterized in that the angle (α) is selected such that, depending on the diameter of the individual flow channels (4) is a visual tightness along the main flow direction (12) is generated. Exhaust system (9) according to one of the preceding claims, characterized in that at the injection point (1) an injector (1) is arranged, which injects the aqueous urea solution in a conical cloud (2) in the flow path, wherein the evaporation element (3) at an angle (α) to the central axis (5) of the injected conical cloud (2) is employed. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation element (3) is set at an angle such that the evaporation element (3) is opaque in the direction of propagation of the injected urea solution. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation element (3) has a hydrolysis coating. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation element (3) has a cross-section adapted to the diameter of the conical cloud (2), such that the entire inflow side of the evaporation element (3) through one of Shape of the evaporation element (3) adapted cloud (2) is acted upon. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation element (3) is a honeycomb formed from a plurality of stacked metal layers, wherein the metal layers are wound and accommodated in a jacket tube. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation elements (3) forming metal layers are at least partially structured. Exhaust system (9) according to one of the preceding claims, characterized in that the flow channels (4) in the evaporation element (3) have a structured surface, wherein the structured surface in particular by fins, webs, recesses, microstructures, bulges and / or openings formed is. Exhaust system (9) according to one of the preceding claims, characterized in that the extent of the evaporation element (3) from the inflow side to the outflow side (8) is at least as long that on the walls (7) of the flow channels (4) impinges and of There deflected components of the aqueous urea solution at least once more on one of the walls (7) of the flow channels (4) impinge. Exhaust system (9) according to one of the preceding claims, characterized in that the flow channels (4) of the evaporation element (3) extend rectilinearly from the inlet side to the outlet side (8), wherein the employment of the evaporation element (3) at an angle (α) to the central axis (5) of the injected cone-shaped cloud (2) an impact of the aqueous urea solution on the inner walls (7) of the flow channels (4) is ensured. Exhaust system (9) according to one of the preceding claims, characterized in that the direction along which the flow channels (4) are flowed is different from the main flow direction (12) of the flow path. Exhaust system (9) according to Claim 12 , characterized in that by the offset between the flow direction of the flow channels (4) and the main flow direction (12) the flow path on the outflow side of the flow channels (4) of the evaporation element (3) Abrisskanten arise, whereby a turbulence of the flowing through the flow channels (4) fluid is generated. Exhaust system (9) according to one of the preceding claims, characterized in that the evaporation element (3) at least one deflection of the main flow direction (12) of the flow path is directly downstream by at least 90 degrees.

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