CN105715339A - Exhaust device of internal combustion engine with mixer for liquid reductant - Google Patents

Abstract

The invention relates to an exhaust system (1) for an internal combustion engine, which has nozzles (7) for injecting a liquid reducing agent into the exhaust gas flowing through the exhaust system (1), arranged in the exhaust gas flow A mixer (10) for the swirl and distribution of the liquid reducing agent in the exhaust gas flow, and an SCR catalyst (4) arranged after the mixer (10) in the exhaust gas flow, the SCR catalyst is used It is used to selectively catalyze the reduction of nitrogen oxides by using a reducing agent or a decomposition product of the reducing agent. In order to prevent the formation of liquid films or deposits on the mixer (10), the mixer (10) is at least partially equipped with an anti-adhesion coating made of glass or ceramic material (19), which is heat-resistant, wear-resistant and corrosion-resistant Layer (18), the anti-adhesive coating facilitates the rolling off of the reducing agent.

Description

Exhaust system for internal combustion engines with liquid reductant mixer with anti-adhesion coating

technical field

The invention relates to an exhaust device for an internal combustion engine having: nozzles for injecting a liquid reducing agent into the exhaust gas flowing through the exhaust device; a mixer in which the liquid reductant is swirled and distributed in the exhaust flow; and an SCR catalyst arranged in the exhaust flow after the mixer for selectively utilizing the reductant or the decomposition products of the reductant Catalytic reduction of nitrogen oxides.

Background technique

In the exhaust system of modern motor vehicles, the exhaust gas of internal combustion engines, for example designed as diesel internal combustion engines, is often subjected to selective catalytic reduction (SCR) in order to reduce the nitrogen oxide content. For this purpose, a liquid reducing agent, for example in the form of an aqueous urea solution, also known as AdBlue (diesel exhaust fluid; automotive urea), is carried in the motor vehicle and sprayed into the stream via nozzles as a jet or mist when the internal combustion engine is running Exhaust from exhaust equipment.

The reducing agent or urea in the reducing agent is decomposed into ammonia in the exhaust system, and the ammonia reduces the nitrogen oxides contained in the exhaust gas in the downstream SCR catalyst with the formation of nitrogen and water. Since nitrogen oxides are almost completely removed from the exhaust gas in this way, internal combustion engines of motor vehicles equipped with SCR catalytic converters can be operated with relatively lean mixtures. This in turn enables fuel-efficient and low-emissions operation.

In order to distribute the aqueous urea solution as uniformly as possible in the exhaust gas and thus to convert it into ammonia better, mixers are often used which are arranged inside the exhaust gas flow, for example in the spray or spray of nozzles and/or or downstream of the nozzle. The mixer can be in the form of a stationary mixing element or a movable, in particular rotatable, mounted mixing element. Stationary mixing elements are, for example, impingement plate mixers with one or more impingement plates. However, it can also be designed in the form of an impeller, ie with a plurality of blades protruding from a central hub, which blades have, in particular, a certain profile. In the case of movably mounted mixing elements, the mixer can be designed, for example, as a turbo mixer having a turbine driven by the exhaust gas.

With the help of the mixer, on the one hand, the evaporation of the sprayed-in droplets of aqueous urea solution on impact on the mixer surface heated by the exhaust gas is accelerated and any remaining droplets are swirled and thus more uniformly distributed. Mix with exhaust. Mixers or mixing elements are predominantly made of sheet steel and may have untreated surfaces. Of course, an optional surface treatment of the mixing part is also possible.

If the liquid reductant is injected into the exhaust gas flow at a high dosing rate and/or the temperature of the exhaust system and thus the mixer is still relatively low (e.g. within a short time after starting the internal combustion engine), there may be A liquid film of the aqueous urea solution forms on the mixer and/or deposits of solid urea occur in the region of the mixer. Both the liquid film and the urea deposits on the mixer lead to a constant drop in the mixer temperature, since the metal mixer is no longer directly inflowed by the hot exhaust gas. As a result, the mixer is more and more wetted with the urea solution, which is primarily detrimental to the evaporation of the liquid reducing agent when it hits the mixer, and also to its distribution and mixing with the exhaust gas.

In addition, a liquid film or urea deposits on the mixer can also lead to increased flow resistance or, in the case of turbo mixers, to a reduction in the turbine speed, which likewise adversely affects the evaporation, distribution and communication with the exhaust gas of the liquid reducing agent. mix.

Mixers for mixing aqueous urea solutions with exhaust gases are known from US-A-7963104 and EP-A-0894523, the mixers having coated surfaces. The surface carries a catalytically active coating there, by means of which coating the urea can be hydrolyzed, ie decomposed into ammonia and carbon dioxide. Other documents also disclose mixers with catalytic coatings.

Contents of the invention

Proceeding from this, the object of the present invention is to provide a degassing device which, compared with other degassing devices, has the advantage that, in particular, the formation of a liquid film and/or deposits on the mixer is prevented.

This object is achieved according to the invention by an exhaust system having the features of claim 1 . It is provided here that the mixer is at least partially equipped with a heat-resistant, wear-resistant and corrosion-resistant anti-adhesion coating made of glass or ceramic material, which facilitates the rolling of the reducing agent. fall.

It has been found that such an anti-adhesion coating prevents or significantly reduces the formation of a liquid film of the aqueous urea solution on the mixer and at the same time prevents or significantly reduces the formation of urea deposits in the region of the mixer. It has been established not only at high dosing rates but also at low exhaust system temperatures that droplets of aqueous urea solution hitting the mixer bounce off and/or roll off the mixer surface better, thereby enabling Improved evaporation and/or uniform distribution in the exhaust stream. On the other hand, the heat resistance, wear resistance and corrosion resistance of the coating ensure that the mixer equipped with the coating and in particular the coating itself has a service life corresponding to the service life of the exhaust system.

Since, without an anti-adhesive coating, a liquid film forms primarily on those surfaces of the mixer which are directly exposed to the jet stream or spray, it is advantageous to provide at least these surfaces of the mixer with an anti-adhesive coating. In addition, however, it is also possible to equip other surface areas of the mixer with an anti-adhesive coating, for example depending on the coating process selected. For example, it is expedient to equip the entire surface of the mixer or a part of the mixer with an anti-adhesive coating, for example by means of a dip coating process.

For example, at least those surfaces of the mixer or of the mixing element which are opposite the nozzles are provided with an anti-adhesive coating, since these surfaces are directly exposed to the jet stream or spray. In addition, however, it is also possible to equip the adjoining surface of the mixer or the mixing element or the surface facing away from the nozzle with an anti-adhesive coating. This is particularly preferred when the mixer is an impingement plate mixer, but it can also be realized in all other mixer configurations.

If, for example, the mixer is a turbine mixer, ie comprises a turbine through which the exhaust gas flows, it is expedient to equip at least the surface of the turbine which is oriented opposite to the flow direction of the exhaust gas with an anti-adhesion coating. However, it is advantageous to equip the turbine wheel with an anti-stick coating over its entire surface.

The anti-adhesion coating is advantageously made of a sintered glass or ceramic material which is applied as a nanostructured sol or gel to the mixer and then sintered by heating. This coating that forms a dense, glass-like surface with an anti-adhesion effect is for example produced under the brand name EPGE Engineered Nano Products Germany AG (EPG Engineering Nano Products Germany AG). Provided, while the coating of other hydrophobic material surfaces forming nanostructures with a lotus effect is provided by CTC Nanotechnology (CTCNanotechnology) in Merzig.

Of these two types of cladding, a glass material or a ceramic material containing silicon or silicon oxide for improving heat resistance, wear resistance and corrosion resistance is preferably used. The anti-adhesion coating is therefore preferably a sol-gel coating. Such coatings have a dense, ie closed or closed and microporous surface, so that it is no longer possible for the reducing agent to easily deposit on the surface.

The layer thickness of the anti-adhesion coating, which is preferably applied directly to the mixer surface, is advantageously 2 μm to 8 μm. The surface can be made of metal, especially steel. However, other materials are also possible.

Description of drawings

The invention is explained in more detail below with the aid of an exemplary embodiment shown in the drawing.

Fig. 1 shows the three-dimensional exterior view of each part of exhaust equipment according to the present invention;

FIG. 2 shows an enlarged perspective internal view, partly in section, of the exhaust device section of FIG. 1 ;

Figure 3 shows a further enlarged perspective view of the components of the turbo mixer in the exhaust device;

Figure 4 shows a further enlarged cross-sectional view of the clad portion of the turbine blade of the turbo mixer;

FIG. 5 shows a sectional view corresponding to FIG. 4 , but with another coating.

detailed description

An exhaust system 1 of a motor vehicle having an internal combustion engine, preferably a diesel internal combustion engine, which is only partially shown in FIG. 1 , comprises an exhaust pipe 2 extending from the internal combustion engine to the exhaust system. The exhaust gas passing through the exhaust pipe 2 flows through the front catalytic converter 3 and the SCR catalytic converter 4 arranged downstream of the front catalytic converter in the exhaust gas flow direction (arrow A in FIG. 1 ), in which SCR catalytic converter is The liquid reducing agent previously fed into the exhaust pipe 2 selectively catalytically reduces nitrogen oxides.

The liquid reducing agent is, for example, an aqueous urea solution known as AdBlue, which is carried in a reducing agent tank (not shown) in the motor vehicle and is dosed into the exhaust pipe 2 by means of a dosing unit 5 is arranged in the for example U-shaped curved pipe section 6 between the front catalytic converter 3 and the SCR catalytic converter 4 .

As shown most clearly in FIG. 1 , the metering unit 5 is arranged in the front half of the for example U-shaped bent pipe section 6 in the direction of flow. As shown most clearly in FIG. 2 , the dosing unit 5 has a nozzle 7 opening into the interior of the pipe section 6 , from which the aqueous urea solution in the form of a diverging jet 8 is sprayed into the drain flowing through the pipe section 6 . in the air. As shown most clearly in FIG. 1 , the metering unit 5 is mounted on the end of a small connecting piece 9 , which opens substantially tangentially into the pipe section 6 .

In order to mix the aqueous urea solution sprayed into the pipe section 6 with the exhaust gas, a mixer 10 is arranged inside the exhaust pipe 2 in the center of the U-shaped curved pipe section 6, for making the aqueous urea solution in the exhaust gas or in the exhaust gas. distribution in the exhaust gas stream as evenly as possible and rapid evaporation of the water contained in the urea solution. For example, the mixer is designed as a static or dynamic mixer.

In this way it can be achieved that the urea contained in the urea solution is completely converted into ammonia and carbon dioxide in the exhaust gas. The nitrogen oxides contained in the exhaust gas are then reduced by ammonia in the SCR catalyst 4 to form nitrogen and water. The mixer 10 is a static mixer having a mixing element 11 mounted in the pipe section 6 and fixed there, the center point axis 12 of which is aligned with the central axis of the jet 8 from the nozzle 7, as most clearly seen in as shown in Figure 2. The mixing element 11 is designed, for example, in the form of a turbine or impeller and has a plurality of blades 14 which are fastened relative to one another, in particular centrally. For this it can be assumed that the blades 14 extend outwards from the central hub. The blades 14 are preferably fastened to one another in a materially bonded manner—for example by welding.

As shown most clearly in FIG. 3 , the preferably integrally formed mixing element 11 includes a plurality of vanes 14 which may consist of a radially oriented inner planar portion 15 and a curved outer portion 16 , respectively. The mixing element 11 is also reinforced by a cylindrical sleeve 17 coaxial with the center point axis 12, which rigidly connects the blades 14 to each other between the inner part 15 and the outer part 16 of the blades. Not only the vanes 14 but also the cylindrical sleeve 17 are made of sheet steel.

As most clearly shown in FIG. 2, the jet 8 is formed such that the diameter of the jet is slightly smaller than the diameter of the cylindrical sleeve 17 of the mixing element 11 in the region of the mixing element 11, whereby the jet 8 The droplets substantially only reach the inner planar portion 15 of each blade 14.

In order to ensure an optimal distribution of the aqueous urea solution in the exhaust gas, on the one hand the droplets of the jet 8 bounce or roll off when they hit the inner flat part 15 of the blade 14 and at the same time evaporate and/or decompose into Smaller droplets, while on the other hand the exhaust gas can be strongly swirled by the curved outer part 16 of the blade 14 when passing through the mixing element 11, so as to enhance the turbulence of the exhaust gas flow and thereby improve the connection with the Thorough mixing of droplets that bounce or tumble.

In order to avoid the formation of a liquid film of the aqueous urea solution on the blades 14, in particular on the inner planar parts 15 of the blades 14 due to an excessively high dosing rate of the aqueous urea solution and/or an excessively low temperature of the mixing element 11, and cause cooling (which leads to more and more wetting with urea solution and thus to the evaporation of the colliding droplets and the distribution in the exhaust gas), the middle part of the mixing element 11 is equipped with heat-resistant, wear-resistant and a corrosion-resistant anti-adhesion coating 18 which covers at least the surface of the inner planar portion 15 of the blade 14 , but preferably covers the entire mixing element 11 .

The anti-adhesion coating 18 shown in FIGS. 4 and 5 consists of a silicon-containing or silicon-oxide-containing glass or ceramic material 19, which is applied as a sol or gel to the inner plane of the blade 14. The surface of part 15 is then sintered by heating. The anti-adhesion coating is applied by spraying the gel or sol in liquid form with a layer thickness of 3 μm to 10 μm, which is reduced to approximately 2 μm to 7 μm, for example by shrinkage during the subsequent sintering process.

In the example shown in Figure 4, a dense and smooth surface is produced by means of chemical nanostructures produced in a liquid coating sol and subsequent sintering at temperatures between 300°C and 500°C , the surface has a high degree of heat resistance, wear resistance and corrosion resistance, and ensures that the aqueous urea solution droplets roll off or bounce off the anti-adhesion coating 18 quickly and completely during the operation of the mixer 10 . Since the layer thickness of the anti-adhesion coating 18 is very thin, the heat transfer on the surface of the blade 14 is not perceptibly affected, and the droplets are evaporated more quickly, so that overall a very uniform distribution of urea in the exhaust gas can be achieved. distributed. The anti-adhesion coating 18 shown in Fig. 4 can be purchased from Germany EPG Engineering Nano Products Co., Ltd. branded anti-stick coating.

In the embodiment shown in FIG. 5 , a liquid sol or gel is applied to the surface of the blade 14 to be coated by means of a sol-gel process, the liquid sol or gel comprising heat-resistant and wear-resistant Silicon particles 20 having a particle size in the nanometer range. As shown in FIG. 5, after heat treatment or sintering of the sol or gel, a hydrophobic nano-roughness surface structure is produced by the particles embedded in the glass or ceramic material 19, which surface structure is in shape and size Close to the shape and size of a lotus plant. This facilitates the rolling off of the liquid, as in lotus plants. However, the coating 18 is heat-resistant, wear-resistant and corrosion-resistant, so that the coating has a sufficient service life under the conditions in the exhaust pipe 2 . Such nanostructured, hydrophobic material surfaces with a lotus flower effect are available from CTC Nanotechnology in Merzig.

The anti-adhesion coating 18 described is also suitable for coating static mixers, as described for example in documents DE-A-102007048558, DE-A-102007012790 and DE-A-102006058715, and also for For coating components of static mixers.

Claims (10)

1. An exhaust device (1) for an internal combustion engine, the exhaust device having: a nozzle (7) for injecting a liquid reducing agent into the exhaust gas flowing through the exhaust device (1); a mixer (10) arranged in the exhaust flow for swirling and distribution of the liquid reducing agent in the exhaust flow; and an SCR catalyst arranged downstream of the mixer (10) in the exhaust flow (4), the SCR catalyst is used to selectively catalyze the reduction of nitrogen oxides using a reducing agent or a decomposition product of a reducing agent, characterized in that the mixer (10) is at least partially equipped with a glass or ceramic material (19 ), heat-resistant, wear-resistant and corrosion-resistant anti-adhesion coating (18), the anti-adhesion coating is conducive to reducing agent rolling off. 2. The exhaust system according to claim 1, characterized in that the glass or ceramic material (19) from which the anti-adhesion coating (18) is made is applied as a liquid sol or gel to the mixer ( 10) and then sintered by heating. 3. The exhaust device according to claim 1 or 2, characterized in that the glass or ceramic material (19) contains particles with a particle size in the nanometer range. 4. The exhaust system as claimed in claim 1, characterized in that the glass or ceramic material (19) contains silicon or silicon oxide. 5 . The exhaust system as claimed in claim 1 , characterized in that the glass or ceramic material ( 19 ) has a glass-like compact surface, which facilitates the rolling off of the reducing agent. 6 . 6. The exhaust device according to any one of the preceding claims, characterized in that the glass or ceramic material (19) has a hydrophobic surface structure with nanoscale roughness, which favors the reductant tumble. 7. The exhaust gas system as claimed in claim 1, characterized in that the anti-adhesion coating (18) has a layer thickness of 2 μm to 8 μm. 8 . The exhaust system as claimed in claim 1 , characterized in that the mixer ( 10 ) comprises a turbine ( 11 ) which is at least partially equipped with an anti-adhesion coating ( 18 ). 9. The exhaust system as claimed in claim 1, characterized in that the mixer comprises at least one impingement plate which is at least partially equipped with an anti-adhesion coating (18). 10. The exhaust device according to any one of the preceding claims, characterized in that the mixer (10) is at least on those surfaces exposed to the jet (8) or spray from the nozzle (7) Equipped with anti-stick coating (18).