US20160177801A1

US20160177801A1 - Exhaust system of an internal combustion engine with mixer for a liquid reductant

Info

Publication number: US20160177801A1
Application number: US14/965,247
Authority: US
Inventors: Danny Wohlfahrt
Original assignee: Audi AG
Current assignee: Audi AG
Priority date: 2014-12-17
Filing date: 2015-12-10
Publication date: 2016-06-23
Also published as: DE102014018852B4; DE102014018852A1; CN105715339A

Abstract

An exhaust system for an internal combustion engine includes an injection nozzle for injecting a liquid reductant into exhaust gas streaming through the exhaust system. A mixer is arranged in the exhaust-gas stream for swirling and dispersing the liquid reductant in the exhaust-gas stream. The mixer is provided with a heat-resistant, wear-resistant and corrosion-resistant non-stick coating which is made of a glass or ceramic material to promote rolling off of the reductant. A SCR catalyst is arranged downstream of the mixer for selective catalytic reduction of nitrogen oxides with the aid of the reductant or decomposition products of the reductant.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2014 018 852.3, filed Dec. 17, 2014, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference in its entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust system of an internal combustion engine with mixer for a liquid reductant.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Internal combustion engines, constructed as diesel internal combustion engine for example, nowadays use selective catalytic reduction (SCR) to decrease the nitrogen oxide fraction in exhaust systems of current motor vehicles. A liquid reductant, typically in the form of aqueous urea, also known as AdBlue, is added by an injection nozzle as spray jet or spray mist into a stream of exhaust gas flowing through the exhaust system during operation of the internal combustion engine.
The reductant or urea in the reductant decomposes in the exhaust system to ammonia which reduces nitrogen oxides contained in the exhaust gas in a downstream SCR catalyst under formation of nitrogen and water. As nitrogen oxides can almost entirely be removed from the exhaust gas in this way, the internal combustion engines of motor vehicles equipped with SCR catalysts can be operated with a relatively lean mixture. This in turn enables the motor vehicle to run fuel-efficiently and with low emission.
To realize a dispersion of the aqueous urea solution in the exhaust gas as homogenously as possible and thus a better conversion to ammonia, the use of a mixed is typically arranged in the exhaust-gas stream, for example in the spray jet or spray mist of the injection nozzle and/or downstream of the injection nozzle. The mixer may be constructed in the form of a stationary mixing element or a movable, especially rotatably supported, mixing element. The stationary mixing element may, for example, be constructed as baffle plate mixer with one or more baffle plates. It may, however, also be constructed in the form of an impeller, i.e. several blades extending from a central hub. The blades may hereby be profiled. When movably supported mixing elements are involved, the mixer may be constructed as turbine mixer with a turbine wheel that is propelled by exhaust gas which flows through the turbine wheel.
The mixer is designed to accelerate evaporation of droplets of the injected aqueous urea solution that are formed when the urea solution impacts the mixer surface heated by exhaust gas, and to swirl possibly remaining droplets so as to effect a more homogenous mixture with the exhaust gas. The mixer or the mixing element is predominantly made of steel sheet and has typically an untreated surface.
When the liquid reductant is injected at high rate into the exhaust-gas stream and/or the temperature of the exhaust system and thus also of the mixer is still relatively low, for example, shortly after starting the internal combustion engine, a liquid film of aqueous urea solution may form on the mixer and/or solid urea may deposit in the area of the mixer. Both, the liquid film and the urea deposits on the mixer, cause a constant lowering of the temperature of the mixer as the metallic mixer is no longer directly contacted by hot exhaust gas. As a result, the mixer is increasingly wetted by the urea solution. This interferes significantly in particular with the evaporation of the liquid reductant when impacting the mixer and its dispersion and mixture with the exhaust gas.
In addition, the liquid film or the urea deposits on the mixer can cause an increase of the low resistance or, when a turbine mixer is involved, a decrease in the rotation speed of the turbine wheel so that again evaporation, dispersion, and mixture of the liquid reductant with exhaust gas is impaired.
It would therefore be desirable and advantageous to address these prior art problems and to obviate other prior art shortcomings.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an exhaust system for an internal combustion engine includes an injection nozzle for injecting a liquid reductant into exhaust gas streaming through the exhaust system, a mixer arranged in the exhaust-gas stream for swirling and dispersing the liquid reductant in the exhaust-gas stream, the mixer being provided with a heat-resistant, wear-resistant and corrosion-resistant non-stick coating which is made of a glass or ceramic material to promote rolling off of the reductant, and a SCR catalyst, arranged downstream of the mixer, for selective catalytic reduction of nitrogen oxides with the aid of the reductant or decomposition products of the reductant.
It has been surprisingly found that the application of such a non-stick coating prevents or at least greatly reduces formation of a liquid film of aqueous urea solution upon the mixer and the formation of urea deposits in the area of the mixer. It has been found that droplets of the aqueous urea solution that impact the mixer bounce off and/or roll off better from the mixer surface, when the injection rate is high and even when the temperature of the exhaust system is low, so that evaporation and/or homogenous dispersion in the exhaust-gas stream is much improved. Moreover, as the coating is heat-resistant, wear-resistant, and corrosion-resistant, the mixer provided with such a coating and also the coating itself have a service life that corresponds to the service life of the exhaust system.
Advantageously, the non-stick coating can be applied in particular on those surfaces of the mixer that are directly exposed to a spray jet or spray mist. Of course, other surface areas of the mixer may also be provided with a non-stick coating, for example in dependence of the selected coating process. It is also conceivable to apply the non-stick coating across the entire surface area of the mixer or a component of the mixer, for example using a dip coating process.
For example, provision may be made for coating at least those mixer surfaces in opposition to the injection nozzle with the non-stick coating because these surface areas are directly exposed to the spray jet or spray mist. Moreover, surface areas of the mixer or mixing element adjacent to or distal to the injection nozzle may be provided with the non-stick coating. This is especially advantageous, when the mixer is constructed as baffle plate mixer, although other configurations of the mixer are conceivable in this regard as well.
When the mixer is constructed as turbine mixer having thus a turbine wheel through which exhaust gas flows, provision may be made to apply the non-stick coating at least on those surfaces that are oriented in opposition to the flow direction of the exhaust gas. Advantageously, the turbine wheel is provided with the non-stick coating across its entire surface area.
According to another advantageous feature of the present invention, the glass or ceramic material can be of a composition suitable for application as liquid sol or gel upon the mixer and for sintering through heating. Such coatings can have a dense glassy surface to form the non-stick effect and are made commercially available for example under the trade name NanoSeal® by the company EPG Engineered nanoProducts Germany AG, whereas other coatings that form nanostructured hydrophobic material surfaces with lotus effect are made available by the company CTC Nanotechnology GmbH, Merzig, Germany.
In both types of coatings, the glass or ceramic material can contain silicon or silicon oxide in order to improve heat resistance, wear resistance, and corrosion resistance. The non-stick coating is thus composed as sol-gel coating. Such a coating has a dense, i.e. closed or closed-pore surface, so that the reductant can effectively no longer become deposited on the surface.
According to another advantageous feature of the present invention, the non-stick coating can have a layer thickness of 2 to 8 μm. The surface may be made of metal, especially steel. Of course, other materials may also be conceivable.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 is a perspective external view of components of an exhaust system according to the present invention;

FIG. 2 is an enlarged detailed perspective view of the exhaust system of FIG. 1, with parts being broken away to show the interior of a section of the exhaust system;

FIG. 3 is a detailed perspective view, on a still further enlarged scale, of parts of a turbine mixer in the exhaust system;

FIG. 4 is a still further enlarged sectional view of a coated part of an impeller of the turbine mixer; and

FIG. 5 is a sectional view, similar to FIG. 4, with modified coating for application on an impeller of the turbine mixer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments may be illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
Turning now to the drawing, and in particular to FIG. 1, there is shown a perspective external view of components of an exhaust system according to the present invention, generally designated by reference numeral 1, for a motor vehicle having an internal combustion engine, e.g. a diesel internal combustion engine. The exhaust system 1 includes an exhaust pipe 2 sized to extend from the internal combustion engine to the exhaust. Exhaust gas streaming through the exhaust pipe 2 flows through a primary catalyst 3 and a SCR catalyst 4 arranged in flow direction of the exhaust gas, as indicated by arrow A, downstream of the primary catalyst 3. In the SCR catalyst, nitrogen oxides are converted by selective catalytic reduction with the aid of a liquid reductant added into the exhaust pipe 2 beforehand.
The liquid reductant involves, for example, an aqueous urea solution, known under the designation AdBlue, which is received in a reduction tank (not shown) installed in the motor vehicle. With the aid of a metering unit 5, the liquid reductant is added in measured quantities into a pipe portion 6 of the exhaust pipe 2 between the primary catalyst 3 and the SCR catalyst 4. The pipe portion 6 of the exhaust pipe 2 may be bent in a U shaped manner for example.
As is readily apparent from FIG. 1, the metering unit 5 is arranged in the leading half of the pipe portion 6, as viewed in flow direction A. As best seen in FIG. 1, which is an enlarged detailed perspective view of the exhaust system 1, with parts being broken away to show the interior of a section of the exhaust system 1, the metering unit 5 has an injection nozzle 7 porting into the interior of the pipe portion 6 so as to inject the aqueous urea solution in the form of a divergent spray jet 8 into the exhaust gas streaming through the pipe portion 6. FIG. 1 clearly show the attachment of the metering unit 5 at the end of a small pipe socket 9 which extends substantially tangentially into the pipe portion 6.
To mix the aqueous urea solution, injected into the pipe portion 6, with the exhaust gas, a mixer 10 is arranged inside the exhaust pipe 2 in the middle of the U-shaped bent pipe portion 6 so as to realize a dispersion of the aqueous urea solution in the exhaust gas or exhaust gas stream as homogenously as possible and to realize a rapid evaporation of water contained in the urea solution. For example, the mixer may involve a static mixer or a dynamic mixer.
Urea contained in the urea solution should be converted in the exhaust gas in this way completely to ammonia and carbon dioxide. The ammonia is then used to reduce the nitrogen dioxides contained in the exhaust gas with the aid of the SCR catalyst 4 into nitrogen and water. The mixer 10 involved here is a static mixer with a mixing element 11 which is inserted in the pipe portion 6 and secured there and which defines a center axis 12 (FIG. 3) in alignment with a center axis of the spray jet 8 ejected from the injection nozzle 7. This is readily apparent from FIG. 2. The mixing element 11 is constructed for example in the form of a turbine wheel or impeller and has several blades 14 which are secured to one another, in particular in the center. Thus, the blades 14 extend from a central hub outwards. Advantageously, the blades 14 are connected to one another by a material joint, e.g. by soldering.
As is readily apparent from FIG. 3, which shows the mixing element 11 on an enlarged scale, the mixing element 11 is advantageously made in one piece and includes the plurality of blades 14. Each blade 14 can include a radially oriented inner planar part 15 and a curved outer part 16. The mixing element 11 is further stiffened by a cylindrical sleeve 17 disposed in coaxial relationship to the center axis 12 and rigidly connecting the blades 14 between their inner and outer parts 15, 16. Both, the blades 14 and the cylindrical sleeve 17, are made of steel sheet.
As is readily apparent from FIG. 2, the spray jet 8 is so configured that its diameter is slightly smaller in the area of the mixing element 11 than the diameter of the cylindrical sleeve 17 of the mixing element 11 so that droplets of the spray jet 8 impact substantially only the inner planar part 15 of each blade 14.
To ensure optimum dispersion of the aqueous urea solution in the exhaust gas, the droplets of the spray jet 8 should bounce off or roll off when impacting the inner planar part 15 of the blades 14 and thereby evaporate and/or split into smaller droplets. In addition, the exhaust gas should be intensely swirled by the curved outer parts 16 of the blades 14, when passing through the mixing element 11, to reinforce turbulence of the exhaust-gas stream and thereby improve a thorough mixing with the bouncing-off or rolling-off droplets.
When injecting the aqueous urea solution at high rate and/or when the temperature of the mixing element 11 is too low, the formation of a liquid film of aqueous urea solution on the blades 14, in particular on the inner planar parts 15 of the blades 14, that causes a temperature drop there and thus an increasing wetting with urea solution that adversely affects evaporation and dispersion of the impacting droplets in the exhaust, is prevented by providing the midsection of the mixing element 11 with a heat-resistant, wear-resistant, and corrosion-resistant non-stick coating 18 which covers at least the surfaces of the inner planar parts 15 of the blades 14. Advantageously, the non-stick coating 18 covers the mixing element 11 in its entirety.
FIGS. 4 and 5 show sectional views of two variants of non-stick coatings 18 which are each made of glass or ceramic material 19 which contains silicon or silicon dioxide and which is applied onto the surfaces of the inner planar parts 15 of the blades 14 as sol or gel and thereafter sintered through heating. Application of the non-stick coatings 18 is implemented by spraying the gel or sol in liquid form at a layer thickness of 3 to 10 μm. During subsequent sintering for example, the layer thickness decreases as a result of shrinkage to 2 to 8 μm.
In the exemplary embodiment of FIG. 4, with the aid of a chemical nanostructure, generated in the liquid coating sol, and a subsequent sintering at temperatures between 300 and 500° C., a dense and smooth glassy surface is produced, which has high heat resistance, wear resistance and corrosion resistance and ensures during operation of the mixer 10 a rapid and complete rolling off or bouncing off of droplets of the aqueous urea solution from the non-stick coating 18. As the layer thickness of the non-Ostick coating 18 is minimal and does not appreciably impact a heat transfer to the surface of the blades 14, the droplets evaporate faster so that overall a very homogenous dispersion of urea in the exhaust gas is realized. The non-stick coating 18, shown in FIG. 4, is made commercially available under the trade name NanoSeal® by the company EPG Engineered nanoProducts Germany AG.
The exemplary embodiment of a non-stick coating 18, as shown in FIG. 5, also involves application of a liquid sol or gel, which has heat-resistant, wear-resistant and corrosion-resistant silicon particles 20, onto the surface of the blades 14 to be coated using a sol-gel process. The particles 20 have a grain size in the nanometer range. As shown in FIG. 5, after undergoing a heat treatment or sintering of the sol or gel, the particles 20 embedded in the glass or ceramic material 19 create a hydrophobic nano-coarse surface structure having a shape and dimension resembling the one of a lotus plant. Like in a lotus plant, rolling off of liquids is promoted. Still, the non-stick coating 18 is heat-resistant, wear-resistant, and corrosion-resistant so as to have sufficient service life when exposed to conditions prevailing in the exhaust pipe 2. Such nanostructured hydrophobic material surfaces with lotus effect are made available by the company CTC Nanotechnology, Merzig, Germany.
The described non-stick coatings 18 are also suitable for coating static mixers. Examples of static mixers are described, for example, in DE-A-102007048558, DE-A-102007012790, or DE-A-102006058715, to which reference is made herewith.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:

Claims

What is claimed is:

1. An exhaust system for an internal combustion engine, comprising:

an injection nozzle for injecting a liquid reductant into exhaust gas streaming through the exhaust system;

a mixer arranged in the exhaust-gas stream for swirling and dispersing the liquid reductant in the exhaust-gas stream, said mixer being provided with a heat-resistant, wear-resistant and corrosion-resistant non-stick coating which is made of a glass or ceramic material to promote rolling off of the reductant; and

a SCR catalyst, arranged downstream of the mixer, for selective catalytic reduction of nitrogen oxides with the aid of the reductant or decomposition products of the reductant.

2. The exhaust system of claim 1, wherein the glass or ceramic material is of a composition suitable for application as liquid sol or gel upon the mixer and for sintering through heating.

3. The exhaust system of claim 1, wherein the glass or ceramic material contains particles with a grain size in a nanometer range.

4. The exhaust system of claim 1, wherein the glass or ceramic material contains silicon or silicon oxide.

5. The exhaust system of claim 1, wherein the glass or ceramic material includes a dense glassy surface that promotes the rolling off of the reductant.

6. The exhaust system of claim 1, wherein the glass or ceramic material has a hydrophobic nano coarse surface structure that promotes the rolling off of the reductant.

7. The exhaust system of claim 1, wherein the non-stick coating has a layer thickness of 2 to 8 μm.

8. The exhaust system of claim 1, wherein the mixer has a turbine wheel which is coated at least in part by the non-stick coating.

9. The exhaust system of claim 1, wherein the mixer has a baffle plate which is coated at least in part by the non-stick coating.

10. The exhaust system of claim 1, wherein the mixer has at least one surface area which is exposed to a spray jet or spray mist and is coated by the non-stick coating.