GB2500059A

GB2500059A - Mixer for an exhaust gas after-treatment system

Info

Publication number: GB2500059A
Application number: GB1204186.9A
Authority: GB
Inventors: Domenico Spatafora
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-03-09
Filing date: 2012-03-09
Publication date: 2013-09-11
Anticipated expiration: 2032-03-09
Also published as: GB2500059B; GB201204186D0

Abstract

A mixer (100), for integration into an exhaust conduit (14, fig.4) of an exhaust after-treatment system, comprises a body portion 101 and a plurality of blades 102 projecting from the body portion, the blades 102 being elastically deformable under the action of the pressure seen by the mixer at different flow rates in the exhaust conduit (14). The blades may deform such that flow resistance decreases as flow rate increases. The body portion 101 may comprise inner and outer cylindrical frames 151, 150 joined by radial rods 152, 153. The blades 102 may be mounted on the rods 152, 153 equally angularly spaced around the longitudinal axis and may be curved in the longitudinal and/or transverse directions. The blades may be made from stainless steel, eg AISI 441, with a thickness between 0.1-0.8mm. The mixer may be placed in the exhaust conduit upstream of the after-treatment device, eg SCR catalyst (15), and downstream of a reactant injector (26, fig.4) of urea or ammonia.

Description

5

A mixer for an exhaust after-treatment system

Technical field

Exemplary embodiments of the present invention are related to a mixer for exhaust after-treatment systems and to exhaust after-treatment systems employing these mixers to 10 enhance system performance.

Background

Exhaust after-treatment systems for internal combustion engines of the diesel type employ diesel oxidation catalyst (DOC) devices, selective catalytic reduction (SCR) devices, diesel 15 particulate filter (DPF) devices and other exhaust after-treatment devices. In these systems, the DOC devices frequently employ injectors that are located upstream to inject a reactant, generally fuel, urea and/or ammonia, into the exhaust gas flow for oxidation in the DOC devices to raise the temperature of the exhaust gas flow, such as when regeneration of the DPF is desired.

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The efficient use of the reactant in the DOC devices is of critical importance, since it directly affects the efficiency (e.g., fuel economy) of the engine, as well as the emission performance of the engine and exhaust after-treatment system, since the emission of unburned HC (HC slip) is regulated by law.

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In order to ensure an efficient reactant utilization, these systems frequently employ mixers downstream from the reactant injectors, also referred to as evaporators or vaporizers, to ensure that the reactant injected into the system is completely vaporized and dispersed into the exhaust gas flow so that it can be oxidized to the greatest extent possible in the 30 DOC. These mixers are designed to promote turbulence in the exhaust gas flow to provide mixing and dispersion of the reactant.

While effective for this purpose, mixers also create backpressure in the exhaust gas flow associated with the partial obstruction of the flow passage and the creation of the intended

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turbulence. Since the mixers are permanently installed in these systems, they create backpressure and affect flow even when reactant is not being injected and their use is not needed.

5 Other mixers are also employed in conjunction with the use of other exhaust after-treatment devices. For example, SCR devices frequently employ urea as reactant. Urea SCR (U-SCR) catalysts require upstream injection of urea, such as a urea-water solution, into the exhaust gas flow. The performance, durability and operating cost of the U-SCR catalyst devices and other downstream after-treatment devices strongly depend on the 10 mixing and dispersion (e.g., evaporation) of the injected reactant fluid into the exhaust gas flow. Mixers are also used upstream of these devices to increase the dispersion of the injected reactant fluid into the exhaust gas flow and the production of ammonia for catalysis. These mixers also produce undesirable system backpressure and affect flow even when urea is not being injected and their use is not needed.

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In order to minimize such undesirable effects, US2011/0258983 in the name of the Applicant discloses a mixer that includes a body portion and a plurality of airfoil portions that are disposed on the body portion and reversibly movable between a deployed position and a retracted position as a function of the temperature. The airfoil portions are made of 20 two-way shape memory alloy, thus being steadily positioned only in two different positions: in the retracted position and in the deployed position.

Another solution is shown in US2011/0214415 in the name of the Applicant that discloses a baffle located upstream the HC or ammonia reactant injector and working in cooperation 25 with a mixer that is located upstream the DOC devices. The baffle is located upstream of the HC or ammonia injector and is configured to define a turbulent low pressure velocity field downstream thereof and proximate to the HC or ammonia reactant injector, thus enhancing mixing of the injected fluid with the exhaust gas flow.

30 It is an object of an embodiment of the present invention to provide for a mixer that is able to improve efficiency of the exhaust gas treatment in the SCR at all exhaust gas flow rate conditions.

A further object is to provide for a mixer that allows to reduce the overall dimensions of the 35 after-treatment system.

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Still, another object is to provide for a mixer that is able to reduce backpressure problems on the exhaust after-treatment system even when HC, urea or the like is not injected and their use is not needed.

5 Summary

According to an embodiment of the invention, these objects are achieved by a mixer for integration into an exhaust conduit of an exhaust after-treatment system, the mixer comprising a body portion and a plurality of blades projecting from the body portion, the blades being elastically deformable under the action of the pressure seen by the mixer at 10 different flow rates in the exhaust conduit.

The blades reduce the turbulence energy in the exhaust gas flow as the mass flow rate of the exhaust gas increases between a first configuration at a minimum flow rate and a second configuration at a maximum flow rate. In fact, as the mass flow rate increases the 15 blades of the mixer deform in such a way to reduce the resistance to the exhaust mass flow rate, while as the exhaust mass flow rate decreases the blades returns elastically to their starting position so that the resistance to the exhaust mass flow rate is increased.

The blades have a curved shape in longitudinal direction. The deformation of the blades, 20 resulting in an elastic change of the shape of the blades, is driven by an aerodynamic action of the mass flow rate. In practice, the blades change from a more curved shape in longitudinal direction at low mass flow rates, to provide an obstructive geometric profile that induces high "mechanical" turbulence in the exhaust gas flow, to a less curved shape in longitudinal direction to provide a less obstructive profile that reduces the "mechanical" 25 turbulence induced in the exhaust gas flow at a higher mass flow rates.

In this way, since the turbulence induced by the blades reduces as the mass flow rate increases, the backpressure problems at high mass flow rates are considerably reduced. On the contrary, since the turbulence induced by the blades increases as the mass flow 30 rate decreases, the temperature energy losses at low mass flow rates of the exhaust gas flow are fully compensated by the energy provided by the turbulence induced in the exhaust gas, thus favouring a complete mixing of the reactant, such as urea, HCs or other injected fluid, with the exhaust gas.

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Due to the improved mixing action, a reduction of the mixing length in the exhaust conduit between the mixer and an after-treatment device is also obtained, thus reducing the overall dimension of the after-treatment system.

5 In an exemplary embodiment, the blades may have a substantially quadrangular shape in plan view and a curved shape in the transverse direction. The curved shape in the transverse direction generates a difference in speed of the gas flow on the opposed surfaces of the blades, thus inducing turbulence on the gas flow.

10 The body portion comprises a central longitudinal axis of symmetry and the blades are equally angularly spaced along the body portion around the central longitudinal axis of symmetry of the body portion. Such a configuration allows to obtain a swirling action on the exhaust gas flow and thus an enhanced mixing of the injected fluid downstream of the mixer.

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The body portion consists of an outer cylindrical frame, an inner cylindrical frame and radial rods extending between the two cylindrical frames to increase the robustness of the mixer and give a support for the blades that are disposed on the same rods.

20 The blades are made of a stainless steel having a modulus of elasticity between 190 and 230 kN/mm2 at 20 °C in order to resist to the thermo-chemical action of the exhaust gas flow passing through the exhaust conduit and have the desired elastic properties.

A suitable stainless steel can be for example that identified as AISI 441, having a modulus 25 of elasticity of 220 kN/mm2 at 20 °C and widely used in the automotive industry for the components of the exhaust gas systems.

The blades have a thickness between 0.1 and 0.8 mm, preferably between 0.2 and 0.6 mm, as a function of the material used, so that the shape of the blades can be elastically 30 deformed by the exhaust gas flow as desired at different mass flow rates.

Embodiments of the present inventions have an even number of blades, for example four blades, to equally distribute the turbulence on the section of the exhaust conduit where the mixer is installed without generating an undesired backpressure.

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In practice, each blade behaves like an airfoil surface that elastically deforms under the action of the pressure of the exhaust gas flow that impacts the surface of the blade, thus modifying its configuration for each mass flow rate. In other words, the blades assume automatically a new and different configuration to obstruct more/less the section of the 5 exhaust conduit, that is to say to induce more/less turbulence in the exhaust gas, as the mass flow rate of the exhaust gas flow decreases/increases.

According to another aspect, an after-treatment system is provided for an internal combustion engine comprising:

10 - at least one exhaust after-treatment device through an exhaust conduit;

- a mixer disposed in the exhaust conduit upstream of the exhaust after-treatment device, the mixer comprising a body portion and a plurality of blades projecting from said body portion, said blades being elastically deformable under the action of the pressure seen by the mixer at different flow rates in the exhaust conduit; and

15 - an injector that is located upstream of the mixer to inject a reactant into the exhaust gas flow.

Brief Description of the Drawings

Further advantages and features of an embodiment of the present invention will be more 20 apparent from the description below, provided with reference to the accompanying schematic drawings, purely by way of a non-limiting example, wherein:

- Figure 1 is front view of an exemplary embodiment of a mixer with blades in the two limit configuration;

- Figure 2 is a lateral projection of one blade of the mixer of Figure 1 with a partial 25 section of the body portion;

- Figure 3 shows energy vs mass flow rate graph of the exhaust gas; and

- Figure 4 is a schematic exemplary embodiment of an internal combustion engine and exhaust gas treatment system including the mixer of Figures 1 and 2.

30 Detailed Description

Referring to Figures 1 and 2, a mixer 100 for an exhaust after-treatment system 10 comprises a substantially cylindrical body portion 101 configured to be disposed in an exhaust cylindrical conduit 14 (see Figure 4) of an exhaust after-treatment system upstream of an exhaust after-treatment device 15.

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Projecting from the body portion 101, the mixer 100 has four blades 102 which are elastically deformable under the action of the pressure of the exhaust gas flow 16 passing through the exhaust conduit 14. In particular, the body portion 101 consists of an outer cylindrical frame 150, an inner cylindrical frame 151, and radial rods 152, 153 extending 5 between the cylindrical frames 150 and 151. Four blades 102 are disposed on the radial rods 152, 153. The outer cylindrical frame is coupled with the cylindrical conduit 14 and the central longitudinal axis of symmetry C of the body portion 101 coincides with the longitudinal axis of the cylindrical conduit 14 when the mixer 100 is mounted into the exhaust conduit 14.

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Each blade 102 is elastically deformable under the action of the pressure of the exhaust gas flow 16 passing through the exhaust conduit 14 for assuming different configurations P1, P2 for each mass flow rate Q1, Q2 of the exhaust gas flow 16. In Figures 1 and 2, for the sake of clarity, only two different configurations P1, P2 are shown (configuration P2 is 15 shown in dashed line); in any case, other configurations - albeit not shown - can be comprised between the two limit configurations P1 and P2 shown in Figures 1 and 2.

Blades 102 are equally angularly spaced along the body portion 101 with respect to the central longitudinal axis of symmetry C (in Figure 1 perpendicular to the sheet) of the body 20 portion 101. In the embodiment shown, four blades 102 are located, each angularly spaced of 90° with respect to an adjacent blade 102. Such a disposition of the blades 102 is advantageously employed to obtain a swirling action on the exhaust gas flow 16 and thus an enhanced mixing of the injected fluid (HC or ammonia) downstream of the mixer 100.

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In practice, as visible in Figures 1 and 2, each blade 102 assumes a shape starting from a more obstructive geometric profile P1 (more curved shape in longitudinal direction - see Figure 2), at the lowest mass flow rate Q1, for increasing the turbulence induced in the exhaust gas flow, to a less obstructive geometric profile P2 (less curved shape in 30 longitudinal direction - see Figure 2), at the highest mass flow rate Q2, for decreasing the turbulence induced in the exhaust gas flow rate. For example, the mass flow rate of the exhaust gas 16 can vary between a minimum of 60 Kg/h at a maximum of 500 Kg/h. From the more obstructive configuration, i.e. P1, to the less-obstructive configuration, i.e. P2, each blade 102 assumes a plurality of intermediate configurations P (not shown) that 35 gradually become more a more less obstructive as the mass flow rate 16 changes.

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Figure 3 shows schematically some energy profiles as a function of the different flow rates between a low flow rate Q1 and a high flow rate Q2. The curve 160 is the heat energy due to the temperature of the exhaust gas which starts from a minimum value at the low flow rate Q1 and increases with a substantially continuous trend up to the high flow rate Q2.

5 Curve 161 is the "mechanical" energy induced by the mixer 100 which starts from a maximum value at the low flow rate Q1 and decreases as the flow increases up to a minimum value at the high flow rate Q2. Curve 162 represents the sum of the "heat" and "mechanical" energies compared with the same curve 160 of the heat energy alone. From an energetic point of view, the mixer 100 allows to compensate the temperature energy 10 losses at lower mass flow rates, by inducing more turbulence into the exhaust gas, so enhancing mixing of the injected fluid in the exhaust gas flow and reducing backpressure problems at high mass flow rates.

It should be underlined that other exemplary embodiments of a mixer 100 can be provided 15 with a different number of blades 102, such as two or more blades.

According to the exemplary embodiment of the invention herein disclosed, each blade 102 has a convex shape subjected to the action of the exhaust gas 16. In particular, each blade 102 has a substantially quadrangular shape disposed along the exhaust conduit 14 20 in the direction of the exhaust gas flow 16. Each blade 102 has a free edge 121 and an opposite edge 122 connected to one of the radial rods 152, 153 of the body portion 101. Each blade 102 can be made of stainless steel to resist to the thermo-chemical action of the exhaust gas flow passing through the exhaust conduit and could have a thickness between 0.1 and 0.8 mm, or between 0.2 and 0.8 mm, as a function of the elastic 25 properties of the elastic properties of the stainless steel used.

An exemplary embodiment of an exhaust gas after-treatment system including a mixer 100 as previously disclosed is shown in Figure 4, referred to generally as 10, for the reduction of regulated exhaust gas constituents emitted by an internal combustion engine 12. Engine 30 12 may include, but is not limited to, an internal combustion engine fuelled by gasoline, diesel, biodiesel or other hydrocarbon fuels. Such engine may include, but is not limited to, gasoline direct injection systems and homogeneous charge compression ignition engine systems.

35 The exhaust after-treatment system 10 includes an exhaust gas conduit 14, which may comprise several segments, that functions to transport exhaust gas flow 16 from the

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engine 12 to the various exhaust after-treatment devices of the exhaust after-treatment system 10. References herein to disposition of mixer 100 in exhaust gas conduit 14 includes disposition in exhaust gas conduit 14 as well as disposition within any of exhaust after-treatment devices that are in fluid communication with exhaust gas flow 16.

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The exhaust after-treatment device may be for example an SCR catalyst device 15 disposed downstream of the engine 12 along the exhaust conduit 14. The SCR catalyst device may be constructed for example with a flow-through ceramic or metal monolith substrate that is wrapped in an intumescent mat (not shown) that expands when heated to 10 secure and insulate the substrate which is packaged in a rigid shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit 14.

The substrate has a NOx reducing catalyst composition applied thereto. The SCR catalyst composition may contain for example a zeolite and one or more base metal components 15 such as iron (Fe), cobalt (Co), copper (Cu) or vanadium (V) that can operate efficiently to convert NOx constituents in the exhaust gas flow 16 in the presence of a reactant such as ammonia (NH3) that may be produced by thermal decomposition of urea within exhaust after-treatment system 10 or, in alternative, in presence of hydrocarbons (HCs). The SCR catalyst compound is preferably resistant to HC adsorption and poisoning as has been 20 shown with certain copper-based catalyst compounds.

The reactant R, supplied from a tank (not shown) through a conduit 17, is injected into the exhaust gas conduit 14 at a location upstream of the mixer 100 and the SCR catalyst device 15 using a reactant injector 26 (e.g., for injection of urea or NH3 or hydrocarbons) 25 that is in fluid communication with exhaust gas conduit 14, or other suitable method of delivery of the reactant R to the exhaust gas flow 16. The reactant R may be in the form of a gas, a liquid or an aqueous urea solution and may be mixed with air in the reactant injector 26 to aid in the dispersion of the injected spray.

30 Mixing of the injected reactant R with the exhaust gas flow 16 is obtained along the gas conduit 14 between the mixer 100 and the SCR catalyst device 15. In view of the efficiency of the new mixer 100 disclosed above, the mixing length can be reduced thus providing a more compact and packagable after-treatment system with respect to the prior art.

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Even if not shown for sake of clarity, other after-treatment devices may be provided along the exhaust conduit 14 upstream or downstream of the SCR device 15, such as other diesel oxidation catalyst (DOC) devices and a diesel particulate filter (DPF) device.

5 While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled 10 in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

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List of references in the drawings

5

10

exhaust after-treatment system

12

internal combustion engine

14

exhaust gas conduit

15

exhaust after-treatment device

16

exhaust gas flow

10

17

conduit

26

reactant injector

100

mixer

101

mixer body portion

102

blade(s)

15

121

free edge

122

opposite edge

150

outer cylindrical frame

151

inner cylindrical frame

152

radial rod

20

153

radial rod

160

temperature energy loss function at different mass flow rates

161

turbulence energy loss function at different mass flow rates

162

sum of the turbulence energy loss function and the temperature energy loss function at different mass flow rates 25 C central longitudinal symmetry axis P1, P2 blade geometric profile configurations Q1, Q2 mass flow rates R reactant

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Claims

5 1. A mixer (100) for integration into an exhaust conduit (14) of an exhaust after-treatment system, the mixer comprising a body portion (101) and a plurality of blades (102) projecting from said body portion, said blades being elastically deformable under the action of the pressure seen by the mixer at different flow rates in the exhaust conduit (14).

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2. The mixer according to claim 1, wherein said blades (102) have a curved shape in longitudinal direction.
3. The mixer according to claim 1, wherein said blades (102) have a curved shape in transverse direction.

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4. The mixer according to claim 1, wherein said body portion (101) comprises a central longitudinal axis of symmetry (C), said blades (102) being equally angularly spaced along said body portion (101) around said central longitudinal axis of symmetry of said body portion.

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5. The mixer according to claim 1, wherein said body portion (101) consists of an outer cylindrical frame (150), an inner cylindrical frame (151) and radial rods (152, 153) extending between said cylindrical frames (150, 151).

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6. The mixer according to claim 1, wherein at least said blades (102) are made of stainless steel having a modulus of elasticity between 190 and 230 kN/mm2 at 20 °C.
7. The mixer according to claim 1, wherein at least said blades (102) are made of stainless steel having a modulus of elasticity of 220 kN/mm2 at 20 °C.

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8. The mixer according to claim 1, wherein said blades (102) have a thickness between 0.1 and 0.8 mm.
9. The mixer according to claim 1, wherein said blades (102) have a thickness 35 between 0.2 and 0.6 mm.
10. The mixer according to claim 1 having an even number of blades.

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11. The mixer according to claim 1 having four blades.
12. An after-treatment system (10) for an internal combustion engine (12) comprising: - at least one exhaust after-treatment device (15) within an exhaust conduit (14);

5 - a mixer (100) disposed in said exhaust conduit (14) upstream of said exhaust after-treatment device (15), the mixer (100) comprising a body portion (101) and a plurality of blades (102) projecting from said body portion, said blades being elastically deformable under the action of the pressure seen by the mixer at different flow rates in the exhaust conduit (14); and

10 - an injector (26) that is located upstream of said mixer (100) to inject a reactant (R)

into the exhaust gas flow.

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