Patent Application
20210095588
An exhaust system includes catalyst components to reduce emissions. The exhaust system includes an injection system that injects a diesel exhaust fluid (DEF), or a reducing agent such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst which is used to reduce NOx emissions. The injection system includes a doser that sprays the fluid into the exhaust stream. The fluid spray should be transformed as much as possible into ammonia (NH3) before reaching the SCR catalyst. Providing for ultra-low NOx emissions requires dosing at low temperatures to address reducing emissions at cold start and low load cycles. Dosing DEF at low temperatures is a thermolysis and deposit formation problem as there is insufficient heat.
In one exemplary embodiment, vehicle exhaust system includes a conduit defining an exhaust gas flow path extending along a center axis, and wherein the conduit includes a doser opening. An exhaust gas aftertreatment component is positioned downstream of the conduit and at least one doser is configured to inject fluid into the conduit through the doser opening. A heating element pre-heats the fluid prior to mixing with exhaust gas. A perforated pipe is positioned within the exhaust gas flow path to surround the fluid injected by the doser.
In a further embodiment of the above, a mixer is positioned upstream of the exhaust gas aftertreatment component and downstream of the perforated pipe.
In a further embodiment of any of the above, the mixer comprises an outer band fixed to an inner surface of the conduit and a plurality of deflecting elements supported by the band.
In a further embodiment of any of the above, the plurality of deflecting elements comprise flat tabs having one end associated with the band and extending to an unsupported distal end at an angle relative to the center axis.
In a further embodiment of any of the above, the perforated pipe includes a plurality of openings spaced apart from each other about the axis.
In a further embodiment of any of the above, an upstream portion of the perforated pipe extends outwardly of the conduit through the doser opening and wherein a downstream portion of the perforated pipe includes the plurality of openings that are axially spaced apart from each other and extend to an outlet end of the perforated pipe.
In a further embodiment of any of the above, the mixer is positioned immediately adjacent to an outlet end of the perforated pipe.
In a further embodiment of any of the above, the doser injects along an injection axis that is parallel to the center axis.
In a further embodiment of any of the above, the doser injects along an injection axis that is non-parallel to the center axis.
In a further embodiment of any of the above, the perforated pipe is defined by an outer diameter that remains constant along a length of the perforated pipe.
In a further embodiment of any of the above, the perforated pipe is defined by an outer diameter that varies along a length of the perforated pipe.
In a further embodiment of any of the above, the at least one doser comprises a plurality of dosers.
In a further embodiment of any of the above, a control system controls heating of the fluid and/or injection of the fluid based on one or more of exhaust gas temperature, backpressure, time, and wear.
In another exemplary embodiment, a vehicle exhaust system includes a conduit defining an exhaust gas flow path extending along a first portion defining a first center axis and a second portion defining a second center axis. The conduit includes a doser opening. An exhaust gas aftertreatment component is connected to a downstream end of the conduit and a mixer is positioned upstream of the exhaust gas aftertreatment component. The mixer includes a plurality of deflecting elements. At least one doser injects DEF into the conduit through the doser opening and upstream of the mixer. A heating element pre-heats the DEF prior to mixing with exhaust gas and a perforated pipe is positioned within the exhaust gas flow path to surround the DEF injected by the doser.
In a further embodiment of any of the above, the perforated pipe includes a plurality of openings spaced apart from each other about the axis, and wherein the plurality of openings extend along a length of the perforated pipe to a downstream end of the perforated pipe, and wherein the mixer is positioned directly adjacent to the downstream end of the perforated pipe.
In a further embodiment of any of the above, the first center axis is non-parallel with the second center axis and wherein the doser defines an injection axis that is non-parallel to the first center axis and is parallel or non-parallel to the second center axis.
In a further embodiment of any of the above, the first center axis is parallel with the second center axis and wherein the doser defines an injection axis that is parallel or non-parallel to the second center axis.
In another exemplary embodiment, a method for injecting DEF into an exhaust component includes: providing a conduit that defines an exhaust gas flow path extending along a center axis, wherein the conduit includes a doser opening for a doser; positioning an exhaust gas aftertreatment component downstream of the conduit; injecting DEF into the conduit through the doser opening; pre-heating the DEF prior to mixing with exhaust gas; and positioning a perforated pipe within the exhaust gas flow path to surround the DEF injected by the doser.
In a further embodiment of the above, the method includes positioning a mixer immediately downstream of the perforated pipe and immediately upstream of the exhaust gas aftertreatment component.
In a further embodiment of any of the above, the method includes forming the perforated pipe to include a plurality of openings that are spaced apart from each other about a pipe axis and are axially spaced apart from each other along a length of the perforated pipe.
These and other features of this application will be best understood from the following specification and drawings, the following of which is a brief description.
A mixer 30 is positioned downstream from the outlet 20 of the DOC 16 or DPF and upstream of the inlet 24 of the SCR catalyst 22. The upstream catalyst and downstream catalyst can be in-line or in parallel, for example. The mixer 30 is used to facilitate mixing of the exhaust gas.
An injection system 32 is used to inject a reducing agent, such as diesel exhaust fluid (DEF), for example, into the exhaust gas stream upstream from the SCR catalyst 22 such that the mixer 30 can mix the DEF and exhaust gas thoroughly together. The injection system 32 includes a fluid supply 34, a doser 36, and a controller 38 that controls injection of the fluid as known. The doser 36 injects the DEF upstream of the mixer 30. In one example, the mixer 30 comprises an outer band 40 having an upstream end 42, a downstream end 44, and a plurality of deflecting elements 46 (
Providing ultra-low NOx emissions requires dosing at low temperatures to address de-nox at cold start and low load cycles. Dosing DEF at low temperatures raises thermolysis and deposit issues as there is usually insufficient heat from the exhaust gas to manage deposits. To address these issues, the injection system 32 heats the DEF prior to entering the mixer 30, which provides for faster atomization and better mixing, and additionally includes a flow diverting device, such as a perforated pipe 48 for example, that is positioned around the injected DEF spray to minimize spray diversion and further facilitate mixing. Using this configuration for dosing and mixing reduces the overall required packaging space, provides for lower thermal inertia, and has a faster conversion to ammonia, while also providing a more uniform distribution on an upstream face of the SCR catalyst 22.
In one disclosed example shown in
The perforated pipe 48 defines a pipe center axis P as shown in
In one example, the mixer 30 is positioned directly adjacent to the downstream end 72 of the perforated pipe 48. In other examples, the mixer 30 may not be required; however, the mixer 30 is preferred because the mixing of the fluid and exhaust gas is more uniform across the section of the decomposition pipe 50. As discussed above, in one example configuration, the mixer 30 comprises an outer band 40 having an upstream end 42, a downstream end 44, and a plurality of deflecting elements 46 as shown in
As shown in
In one example, the perforated pipe 48 is defined by an outer diameter that remains constant along a length of the perforated pipe 48 as shown in
A control system includes the controller 38 that controls heating of the DEF and/or injection of the DEF based on one or more of exhaust gas temperature, backpressure, time, and wear. Additionally, there are a plurality of sensors 80 that can be used to determine temperature, flow rates, rate of deposit formation, and wear, for example.
In one example, the pipe can be made of a steel or other similar material and/or can have variable material properties. The pipe may also comprise a twin-wall pipe. Additional mixing elements such as baffles and/or perforated plates could also be used as needed. Also, a thermal or hydrolysis barrier could be added to the mixing elements if needed.
As discussed above, the SCR catalyst 22 is used to reduce NOx emissions by using NH3 as the catalytic reductant. The injection system 32 injects NH3 as heated DEF fluid. The catalytic reduction is based on the ammonia decomposition and SCR activation. It is difficult for these two actions to occur at lower temperatures. The first step in ammonia decomposition is to evaporate the water in the DEF fluid, which is a process called thermolysis. During the process of mixing, the DEF fluid takes this energy from the exhaust heat; however, at lower temperatures, as the exhaust gas does not have enough energy, the water does not evaporate completely and this results in an increase in deposit formation.
The subject injection system 32 is able to dose DEF fluid at lower temperatures as the pre-heating of the DEF will help atomize the DEF to smaller diameter particles. This will increase the heat transfer from the exhaust gas to the droplets resulting in faster decomposition. The deflecting device/perforated pipe 48 is added at the spray injection location to make sure the exhaust flow does not deflect the already fine particles of the DEF towards a wall of the decomposition pipe 50. This perforated pipe 48 also creates a finer mixing zone for the DEF with the exhaust. Optionally, an upstream mixing device can be used to further improve the mixing of the DEF with the exhaust gas. Further, the decomposition pipe heated dosing mixer can be used in different architectures where mixing is required in downpipes within a short distance. Examples of these architectures include a light off SCR in close coupled engine compartment position, a light off close coupled DOC in engine compartment position, or a dual dosing dual SCR configuration.
Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
