DE102011120221B4 - Mixer for an exhaust system and exhaust system - Google Patents

Abstract

Mixer (18) for an exhaust system (12) with a tubular housing (20) with an inlet (22) and an outlet (24) for integrating the mixer (18) into the exhaust system (12) and with a plurality of static mixing elements (28) arranged within the housing (20) and arranged one behind the other on the inner wall of the housing (20) in the flow direction of the exhaust gas, characterized in that the mixing elements (28) are designed in the form of twisted mixing blades and/or are designed in the form of mixing walls which are concavely curved in the flow direction of the exhaust gas.

Description

The invention relates to a mixer for an exhaust system and to an exhaust system having such a mixer.

It is known to treat the exhaust gases of internal combustion engines, particularly those operating on the diesel principle, using so-called SCR processes, i.e., processes for selective catalytic reduction. This is intended, in particular, to reduce the proportion of nitrogen oxides in the exhaust gas. SCR processes are based on converting the nitrogen oxides contained in the exhaust gas into non-pollutant end products such as nitrogen, carbon dioxide, and water using a reducing agent, particularly ammonia, introduced into the exhaust gas.

In current applications, ammonia is not introduced into the exhaust gas in its pure form due to its toxic properties. Instead, it is extracted from the non-toxic carrier substance urea in the exhaust system itself. Because urea is very soluble in water, it is regularly used as an easily dosed aqueous urea solution.

The extraction of the ammonia required for the SCR process from the aqueous urea solution involves several chemical reactions, which can primarily be divided into hydrolysis and thermolysis. These reaction steps must be carried out to the greatest possible extent within the process from the addition of the aqueous urea solution to the first subsequent purification component. The remaining bound ammonia in the urea-water solution (HWL) is then converted into ammonia in the subsequent purification element or from a wall film formed (consisting of HWL deposits and chemical byproducts) within the described process.

To achieve a high conversion rate of nitrogen oxides into primarily nitrogen and water, a high conversion rate of the ammonia contained in the urea solution into pure chemically active ammonia is required, as well as the best possible even distribution of this ammonia over the downstream purification component. To achieve this, static mixing elements are used, among other things, to ensure the desired mixing by swirling the exhaust gas and the previously injected aqueous urea solution, thus ensuring effective conversion.

Such static mixers are known for example from DE 10 2007 012 790 B4 , the DE 10 2007 035 226 A1 and the DE 10 2008 028 626 B4 These have a circular housing with a cross-section and are integrated into a pipe section of the exhaust system, which is located between the metering device for the aqueous urea solution and the SCR catalyst. Within the housing of these static mixers, a multitude of mixing blades are arranged side by side, distributed across the entire cross-section of the housing, which ensure the desired swirling of the exhaust gas mixed with the urea solution.

US 2011/0174407 A1 describes a mixing and flow conditioning device for use within a conduit. The device comprises a disc arranged transversely to the flow direction in the conduit, with a plurality of segments cut from the disc and bent out in a predetermined direction.

US 2010/0186382 A1 describes an exhaust gas purification system comprising multiple exhaust aftertreatment devices. A cap connected to the outlet of one exhaust aftertreatment device and the inlet of another provides a mixing chamber and is intended to improve the mixing between exhaust gas and a reducing agent. Within the cap, several mixing elements are arranged transversely to the exhaust gas flow direction.

State-of-the-art static mixers for exhaust systems are characterized by good mixing. However, they also exhibit a relatively high flow resistance.

Due to the ever-increasing complexity of the technology used in today's motor vehicles, there is less and less space available in the engine compartment for individual components. This also applies to the part of the exhaust system that connects the metering device for the urea solution and the SCR catalyst downstream in the direction of exhaust gas flow. Until now, relatively long (for the longest possible mixing distance) and circular cross-section pipe sections were provided for this purpose, into which the static mixers known from the aforementioned publications could be easily integrated. This is no longer possible in all vehicles due to the increasing installation space constraints. In particular, it may be necessary to integrate the aforementioned static mixer for swirling the exhaust gas mixed with the aqueous urea solution into a section of the exhaust system in which the exhaust pipe does not have a (cost-effective) It has been shown that a simple adaptation of the static mixers known from the aforementioned publications to non-circular cross-sections often leads to a further increase in flow resistance.

From the DE 10 2007 002 981 B4 A static mixer for an exhaust system of an internal combustion engine is also known. There, a dividing wall running along a pipe section arranged between a metering device for the aqueous urea solution and the SCR catalyst is provided, dividing the pipe section into two sections. The dividing wall is perforated and provided with guide vanes designed to ensure that the exhaust gas flows back and forth between the two sections. It is obvious that this static mixer requires a particularly long pipe section between the metering device and the SCR catalyst, making its use unsuitable in situations where space is limited.

Based on this prior art, the object of the invention was to provide an improved mixer for an exhaust system of a motor vehicle's internal combustion engine. In particular, the mixer should be characterized by good exhaust gas mixing, low installation space requirements, and/or low flow resistance—particularly when integrated into a housing with a non-circular housing.

This object is achieved by the subject matter of independent patent claim 1. An exhaust system having a mixer according to the invention is the subject matter of independent patent claim 7. Advantageous embodiments of the mixer according to the invention or of the exhaust system according to the invention are the subject matter of the dependent patent claims and emerge from the following description of the invention.

The invention is based on the idea of improving the flow resistance of a generic mixer while maintaining good mixing performance by arranging the (multiple) static mixing elements no longer side by side, thus forming a (transverse) mixing plane, but rather by arranging them one behind the other at the edges in the direction of exhaust gas flow, thereby providing a relatively large, free flow cross-section over the entire length of the mixer. The intensive throttling effect of known static mixers can thus be avoided.

According to the invention, it is therefore provided to improve a generic mixer for an exhaust system, which has a tubular housing with an inlet and an outlet for integration into the exhaust system and a plurality of static mixing elements arranged within the housing, in that the mixing elements are arranged one behind the other on the inner wall of the housing in the (main) flow direction of the exhaust gas (between the inlet and outlet), wherein the mixing elements are designed in the form of twisted mixing blades and/or in the form of mixing walls that are concavely curved in the flow direction of the exhaust gas. Preferably, it can be provided that the mixing elements are attached to the inner wall of the housing at different circumferential positions.

The inventive design of a mixer for an exhaust system allows for the construction of particularly compact mixers with good mixing characteristics and relatively low flow resistance. This applies in particular to mixers that are to be integrated into an exhaust system in such a way that they connect two parallel pipe sections of the exhaust system. Accordingly, a preferred embodiment of the mixer according to the invention provides for a directional deflection of the exhaust gas by at least 60° and preferably by approximately 180° between the inlet and outlet of the housing.

In order for the mixer to take up as little installation space as possible, it can further be provided that the tubular housing has a non-circular cross-section and is designed in particular as a flat component with two opposing cover walls, the widths and heights of which are greater than the width of the side walls connecting the main walls.

An advantageous embodiment of the mixer according to the invention can provide that a metering inlet for a fluid to be introduced into the exhaust gas, and in particular an aqueous urea solution, is integrated into one of the side walls, wherein the arrangement is preferably provided in the vicinity of the inlet of the static mixer.

Furthermore, the inlet and outlet can be arranged in the same cover wall, i.e., their opening cross-sections can be arranged in the same or in mutually parallel planes. Preferably, the mixing elements are then attached to the cover wall that does not have the inlet and outlet.

According to the invention, the mixing elements are in the form of twisted mixing blades are formed. An alternative embodiment of the invention provides that the mixing elements are designed in the form of mixing walls that are concavely curved (in particular semicircular) in the direction of flow of the exhaust gas. Furthermore, the mixing elements can also have, for example, a cylindrical or drop-shaped cross-section. Of course, it is possible to combine different shapes of mixing elements in one mixer.

An exhaust system according to the invention for an internal combustion engine comprises, in addition to a metering device for introducing a fluid into an exhaust gas flowing in the exhaust system and an exhaust aftertreatment device arranged downstream of the metering device in the flow direction of the exhaust gas, a static mixer according to the invention arranged between the metering device and the exhaust aftertreatment device (within the respective connecting component). The statement that the mixer is arranged between the metering device and the exhaust aftertreatment device refers to the static mixing elements that effect the mixing of the exhaust gas and the fluid, and not necessarily to the entire mixer.

As already mentioned, the mixer according to the invention is provided in particular for the mixing of exhaust gas and aqueous urea solution for the purpose of a selective catalytic reduction of nitrogen oxides in the exhaust gas, so that in a preferred embodiment of the exhaust system according to the invention it can be provided that the fluid which is introduced by means of the metering device is an aqueous urea solution and the exhaust gas aftertreatment device comprises a catalyst for converting nitrogen oxides into (at least) nitrogen, carbon dioxide and water (i.e. an SCR catalyst).

• 1: eine Brennkraftmaschine mit einer erfindungsgemäßen Abgasanlage;
• 2: eine erste Ausführungsform eines erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1;
• 3: eine zweite Ausführungsform eines erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1.
• 4: eine dritte Ausführungsform eines erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1;
• 5: eine vierte Ausführungsform eines erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1.
• 6: eine Ausführungsform eines nicht erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1;
• 7: eine Ausführungsform eines nicht erfindungsgemäßen statischen Mischers zur Integration in beispielsweise die Abgasanlage gemäß der 1.

The invention will be explained in more detail below with reference to exemplary embodiments shown in the drawings.

• 1 : an internal combustion engine with an exhaust system according to the invention;
• 2 : a first embodiment of a static mixer according to the invention for integration into, for example, the exhaust system according to 1 ;
• 3 : a second embodiment of a static mixer according to the invention for integration into, for example, the exhaust system according to 1 .
• 4 : a third embodiment of a static mixer according to the invention for integration into, for example, the exhaust system according to 1 ;
• 5 : a fourth embodiment of a static mixer according to the invention for integration into, for example, the exhaust system according to 1 .
• 6 : an embodiment of a static mixer not according to the invention for integration into, for example, the exhaust system according to 1 ;
• 7 : an embodiment of a static mixer not according to the invention for integration into, for example, the exhaust system according to 1 .

The 1 shows an internal combustion engine 10 with an exhaust system according to the invention. This is a diesel internal combustion engine.

The structure and functioning of such an internal combustion engine 10 are sufficiently known, so that further detailed explanations can be dispensed with.

The exhaust gas generated in the combustion chambers of the internal combustion engine 10 by the combustion of a fuel-air mixture is introduced into an exhaust system 12, where it is post-treated before being discharged into the environment. The exhaust system comprises an oxidation catalyst 14 and a combined particulate filter/SCR catalyst 16. Both the oxidation catalyst 14 and the combined particulate filter/SCR catalyst 16 have a substantially cylindrical housing and are arranged directly adjacent to one another (substantially parallel with respect to the longitudinal axes of their housings) and to a cylinder crankcase of the internal combustion engine 10. The exhaust gas first flows through the oxidation catalyst 14 and then through the combined particulate filter/SCR catalyst 16.

A mixer 18 according to the invention connects an outlet of the oxidation catalyst 14 with an inlet of the combined particulate filter/SCR catalyst 16 and thus represents a part of the exhaust system 12 according to the invention. The mixer 18 comprises a housing 20, into which an inlet 22 and an outlet 24 are integrated. The inlet 22 of the mixer 18 overlaps with the outlet of the oxidation catalyst 14, while the outlet 24 of the static mixer overlaps with the inlet of the combined particulate filter/SCR catalyst 16. The housing 20 is flat, with an upper cover wall and a lower cover wall forming the inlet 22 and the outlet 24 of the mixer 18 being longer and wider than the side walls connecting the two cover walls. The special design of the Housing 20 of the mixer 18 enables a particularly space-saving connection of the oxidation catalyst 14 and the combined particulate filter/SCR catalyst 16, whereby their parallel arrangement and the resulting integration of the inlet 22 and the outlet 24 into the same cover wall of the housing 20 of the mixer 18 also require a deflection of the exhaust gas flow within the mixer by 180°.

Near the inlet 22 of the mixer 18, a dosing inlet 26 is integrated into one of the side walls, through which an aqueous urea solution can be introduced into the mixer 18 by means of a dosing device. The urea solution is to be mixed there with the exhaust gas flowing through it in order to convert the nitrogen oxides initially still present in the exhaust gas into harmless end products as part of an SCR process.

The mixing effect of the mixer 18 is achieved by a plurality of static mixing elements 28 which are arranged in the interior space formed by the housing 20 of the mixer 18.

The 2 to 5 show various possibilities for the design and arrangement of these mixing elements 28. The concrete design of the mixer 18 according to the invention which is optimally suited for the respective intended use can be determined by tests and in particular by a fluid dynamic simulation.

The embodiments according to the 2 to 5 have in common that the static mixing elements 28 do not all protrude next to one another (and thus in a plane transverse to the flow direction of the exhaust gas), but at least some of them (in the flow direction of the exhaust gas) one behind the other into the interior of the housing 20 of the mixer 18. Furthermore, the mixing elements 28 are attached to both inner walls of the housing 20 and protrude from there into its interior.

For the mixers 18 according to the 2 to 4 Both mixing elements 28 are used which are designed as straight or twisted mixing blades, and those which have the shape of a curved wall, the orientation of the wall being such that a concave curvature results in the flow direction of the exhaust gas.

For mixers 18 according to 5 In contrast, twisted mixing blades attached to an inner wall are used as static mixing elements 28 in different configurations.

A large part of the mixing elements 28 of the mixer 18 according to the 2 to 5 are arranged in a region of the interior of the housing 20 which lies between the inlet 22 and the outlet 24 of the mixer 18. In all embodiments, however, individual mixing elements 28 are also arranged in the region of the outlet 24 of the mixer 18 and - in the non-inventive embodiment according to the 7 - arranged in the area of the dosing inlet 26.

The mixers 18 according to the invention are characterized by good mixing behavior with simultaneously low flow resistance (back pressure behavior), low installation space requirements and low chemical deposits on their walls.

List of reference symbols

10

internal combustion engine

12

exhaust system

14

Oxidation catalyst

16

combined particulate filter/SCR catalyst

18

mixer

20

Housing

22

inlet

24

Outlet

26

Dosing inlet

28

Mixing element

Claims (8)

Mixer (18) for an exhaust system (12) with a tubular housing (20) with an inlet (22) and an outlet (24) for integrating the mixer (18) into the exhaust system (12) and with a plurality of static mixing elements (28) arranged within the housing (20) and arranged one behind the other on the inner wall of the housing (20) in the flow direction of the exhaust gas, characterized in that the mixing elements (28) are designed in the form of twisted mixing blades and/or are designed in the form of mixing walls which are concavely curved in the flow direction of the exhaust gas. Mixer (18) according to Claim 1 , characterized in that between the inlet (22) and the outlet (24) the exhaust gas is deflected by at least 60° and preferably by approximately 180°. Mixer (18) according to Claim 1 or 2 , characterized in that the housing (20) is designed as a flat component with two opposite the cover walls, the widths and heights of which are greater than the widths of the side walls connecting the cover walls. Mixer (18) according to Claim 3 , characterized in that a metering inlet (26) for a fluid to be introduced into the exhaust gas is integrated into one of the side walls. Mixer (18) according to Claim 3 or 4 , characterized in that the inlet (22) and the outlet (24) are arranged in one of the cover walls. Mixer (18) according to Claim 5 , characterized in that the mixing elements (28) are attached to the cover wall which does not have the inlet (22) and the outlet (24). Exhaust system (12) for an internal combustion engine (10) with a metering device for introducing a fluid into an exhaust gas flowing in the exhaust system (12), an exhaust gas aftertreatment device arranged downstream of the metering device in the flow direction of the exhaust gas, and a mixer (18) according to one of the preceding claims arranged between the metering device and the exhaust gas aftertreatment device. Exhaust system according to Claim 7 , characterized in that the fluid is an aqueous urea solution and the exhaust gas aftertreatment device comprises a catalyst for converting nitrogen oxides into nitrogen, carbon dioxide and water.