US20090035194A1

US20090035194A1 - Exhaust treatment system with an oxidation device for NO2 control

Info

Publication number: US20090035194A1
Application number: US11/882,326
Authority: US
Inventors: Wade J. Robel; Praveen Shivshankar Chavannavar; William Lyle Hardy; Eric Lee Schroeder
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2007-07-31
Filing date: 2007-07-31
Publication date: 2009-02-05
Also published as: WO2009017639A1

Abstract

An exhaust treatment system includes an oxidation device having a plurality of channels through which a flow of exhaust flows. A first portion of the channels of the oxidation device are coated with catalytic material for converting NO to NO₂and a second portion of the channels are uncoated with the catalytic material. The exhaust treatment system also includes a selective catalytic reduction device disposed downstream from the oxidation device. The selective catalytic reduction device is configured to receive the flow of exhaust passing through the oxidation device.

Description

TECHNICAL FIELD

The present disclosure relates generally to an exhaust treatment system, and more particularly, to an exhaust treatment system with NO₂control.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of NOx exhausted to the environment has been to implement a strategy called selective catalytic reduction (SCR).
SCR is a process by which gaseous or liquid reductant (e.g., urea or ammonia) is added to the flow of exhaust from an engine. The combined flow is then absorbed onto a catalyst. The reductant reacts with NOx in the flow of exhaust to form H₂O and N₂. SCR may be more effective when a ratio of NO to NO₂in the flow of exhaust supplied to the SCR catalyst is about 50:50. Some engines, however, may produce a flow of exhaust having a NO to NO₂ratio of approximately 95:5. In order to increase the relative amount of NO₂to achieve a NO to NO₂ratio of closer 50:50, a diesel oxidation catalyst (DOC) may be located upstream of the SCR catalyst to convert NO to NO₂.
One system that includes a DOC to increase a relative amount of NO₂in a flow of exhaust is described in U.S. Pat. No. 6,846,464 (the '464 patent) issued to Montreuil et al. The '464 patent describes a catalytic device including two chambers. The first chamber includes tubes coated with a catalytic material such as platinum that oxidizes NO and hydrocarbons. The second chamber includes tubes coated with a catalytic material such as palladium that oxidizes NO and hydrocarbons. An SCR catalyst is provided downstream from the two chambers of the catalytic device.
Although the system of the '464 patent may provide an oxidation catalyst that increases the amount of NO₂in the flow of exhaust, all of the tubes of the oxidation catalyst are coated with an NO oxidizing material, such as platinum or palladium. Therefore, the entire flow of exhaust contacts either the platinum or the palladium coating on the oxidation catalyst. As a result, there is a risk of providing too much NO₂compared to NO. When there is too much NO₂, NOx reduction in the reduction catalyst is much slower, and therefore, a larger reduction catalyst is necessary to effectively reduce NOx in the flow of exhaust.
The disclosed system is directed to overcoming one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed to an exhaust treatment system. The exhaust treatment system includes an oxidation device having a plurality of channels through which a flow of exhaust flows. A first portion of the channels of the oxidation device are coated with catalytic material for converting NO to NO₂and a second portion of the channels are uncoated with the catalytic material. The exhaust treatment system also includes a selective catalytic reduction device disposed downstream from the oxidation device. The selective catalytic reduction device is configured to receive the flow of exhaust passing through the oxidation device.
In another aspect, the present disclosure is directed to a method for treating a flow of exhaust. The method includes generating the flow of exhaust and passing the flow of exhaust through a plurality of channels of an oxidation device. A first portion of the channels of the oxidation device is coated with catalytic material for converting NO to NO₂, and a second portion of the channels is uncoated with the catalytic material. The method also includes directing the combined flow of exhaust to a downstream selective catalytic reduction device disposed downstream from the oxidation device.
In yet another aspect, the present disclosure is directed to an exhaust treatment system. The exhaust treatment system includes an upstream injector configured to inject reductant into a flow of exhaust and an upstream selective catalytic reduction device disposed downstream from the upstream injector. The upstream selective catalytic reduction device is configured to receive the flow of exhaust. The exhaust treatment system also includes an oxidation device disposed downstream from the upstream selective catalytic reduction device. The oxidation device has a plurality of channels through which the flow of exhaust flows. A first portion of the channels is coated with catalytic material for converting NO to NO₂, and a second portion of the channels is uncoated with the catalytic material. The exhaust treatment system further includes a downstream selective catalytic reduction device disposed downstream from the oxidation device. The downstream selective catalytic reduction device is configured to receive the flow of exhaust.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed engine and exhaust treatment system;

FIG. 2 is a cross-sectional view of an exemplary disclosed oxidation device for the exhaust treatment system of FIG. 1;

FIG. 3 is a diagrammatic illustration of an exemplary disclosed engine and exhaust treatment system having two legs;

FIGS. 4A-4D are diagrammatic illustrations of exemplary disclosed first legs of the exhaust treatment system of FIG. 3;

FIG. 5 is a diagrammatic illustration of another exemplary disclosed engine and exhaust treatment system having two legs;

FIG. 6 is a diagrammatic illustration of another exemplary disclosed engine and exhaust treatment system;

FIG. 7 is a diagrammatic illustration of yet another exemplary disclosed engine and exhaust treatment system having two legs;

FIG. 8 is a diagrammatic illustration of a further exemplary disclosed engine and exhaust treatment system having two legs;

FIGS. 9A and 9B are diagrammatic illustrations of exemplary disclosed first legs of the exhaust treatment system of FIG. 8; and

FIG. 9C is a diagrammatic illustration of an exemplary disclosed second leg of the exhaust treatment system of FIG. 8.

Detailed Description

Reference will now be made in detail to exemplary embodiments, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
As shown in FIG. 1, a power source, such as an engine 10, of a machine is provided. The disclosed embodiment may be applicable to various types of machines such as, for example, a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, power generation, tree harvesting, forestry, or any other industry known in the art. The engine 10 may be an internal combustion engine, such as, for example, a diesel engine, a gasoline engine, a gaseous fuel-powered engine, or any other engine apparent to one skilled in the art. The engine 10 may alternatively be another source of power such as a furnace or any other suitable source of power for a powered system such as a factory or power plant. Operation of the engine 10 may produce power and a flow of exhaust. For example, each combustion chamber (not shown) of the engine 10 may mix fuel with air and combust the mixture therein to produce a flow of exhaust directed into an exhaust passageway. The flow of exhaust may contain carbon monoxide, NOx, carbon dioxide, aldehydes, soot, oxygen, nitrogen, water vapor, and/or hydrocarbons such as hydrogen and methane.
An exhaust treatment system 20 is provided with the engine 10. The flow of exhaust may be fluidly communicated from the engine 10 to the exhaust treatment system 20. Although not shown, other components such as, for example, one or more turbochargers, or any other component for treating or handling exhaust known in the art may be disposed between the exhaust passageway of the engine 10 and the inlet of the exhaust treatment system 20. In addition, other emission control devices, e.g., exhaust gas recirculation devices, may be disposed within or fluidly connected to the exhaust passageway of the engine 10.
The exhaust treatment system 20 shown in FIG. 1 may optionally include an upstream injector 21 disposed upstream from an upstream SCR device 23. The upstream injector 21 may be provided to inject reductant, such as urea, ammonia, and/or other elements or compounds capable of chemically reducing, e.g., NOx, contained within the flow of exhaust at predetermined timings, pressures, and flow rates.
An upstream SCR device 23, such as an SCR catalyst, may be disposed downstream of the upstream injector 21. The upstream SCR device 23 may chemically reduce the amount of NOx in the flow of exhaust. Reductant injected into the flow of exhaust upstream from the upstream SCR device 23 may be absorbed onto the upstream SCR device 23 so that the reductant may react with NOx in the flow of exhaust to form H₂O (water vapor) and N₂(nitrogen gas).
The exhaust treatment system 20 shown in FIG. 1 includes an oxidation device 22, such as a DOC, that receives the flow of exhaust directly from the engine 10 or from the upstream SCR device 23, if provided. If the upstream SCR device 23 is provided, it may be close-coupled to the oxidation device 22. The oxidation device 22 may be a device with a porous ceramic honeycomb-like or metal mesh structure. FIG. 2 shows a cross-sectional view of an exemplary honeycomb-like structure of the oxidation device 22. The oxidation device 22 includes holes 22 a, channels, passageways, or other openings through which the flow of exhaust may pass. According to an embodiment, the oxidation device 22 may be “partially-loaded,” i.e., a percentage less than 100% (e.g., approximately 50%) of the holes may be coated with platinum or another material for oxidizing NO, such as palladium, metal oxide, rhodium, or other precious metal. Alternatively, the percentage of holes 22 a coated with the NO oxidizing material may be, e.g., approximately 25%, 35%, 45%, 55%, 65%, 75%, etc., or any percentage therebetween. The remaining holes may be uncoated. The percentage of holes 22 a coated may be determined experimentally as described below based on the application to obtain a target amount of NO₂in the resulting flow of exhaust, e.g., 50% NO and 50% NO₂. Accordingly, only a percentage of the total flow of exhaust flowing through the oxidizing device 22 contacts the NO oxidizing material, and the percentage of the total flow of exhaust contacting NO oxidizing material depends on the percentage of holes 22 a coated.
As shown in FIG. 1, the exhaust treatment system 20 may include a particulate filter 24, which may be disposed downstream of the oxidation device 22. The particulate filter 24 may be a non-catalyzed filter and may include a wire mesh or ceramic honeycomb filtration media utilized to remove particulate matter from the flow of exhaust. Alternatively, the particulate filter 24 may be another type of device that physically captures particulates, ash, or other materials from the flow of exhaust. The particulate filter 24 may be a wall flow type filter, flow through type filter, or other type of filter known in the art. A “wall flow type” filter may refer to, e.g., a filter that includes a plurality of passages and opposite ends of adjacent passages may be blocked or plugged in order to force the flow of exhaust to travel radially through a plurality of relatively thin porous walls. A “flow through type” filter may refer to, e.g., a filter that can capture and store particulate matter while allowing open passages through which the flow of exhaust may flow. For example, a flow through type filter may include sheets or other blocking mechanisms that may divert particulate matter toward a mesh lining sides of the open passages. Alternatively, the particulate filter 24 may be omitted if the flow of exhaust from the engine 10 has lower amounts of particulate matter.
The particulate filter 24 may then be connected to a heat source 25. The heat source 25 may be any conventional heat source known in the art. Such heat sources may include, e.g., a furnace, an electric heater, a fuel burner, etc. The heat source 25 may direct heat to any particulate filter located in the exhaust treatment system 20, such as the particulate filter 24, so as to thermally age the particulate matter deposited in the particulate filter. Alternatively, the heat source 25 may be omitted, and the engine 10 may heat the flow of exhaust, which may heat the particulate matter in the particulate filter 24. The heat source 25 may be disposed upstream from the particulate filter 24 and downstream from the engine 10. In the exemplary embodiment shown in FIG. 1, the heat source 25 is disposed upstream from the upstream injector 21 and the upstream SCR device 23, and downstream from the engine 10.
An injector 26, such as an injector described above in connection with the upstream injector 21, may be provided to inject reductant, such as urea or ammonia, into the flow of exhaust downstream from the particulate filter 24. A controller 12 may receive input via communication lines 14 from a variety of sources including, for example, sensors configured to measure temperature, speed, fuel quantity consumed, and/or other operating characteristics of the engine 10. For example, the timing of the injections by one or more of the injectors 21, 26 may be synchronized with sensory input received from a sensor 40 (FIGS. 4C, 4D, 6, and 8), such as a temperature sensor as described below, a NOx sensor, a flow sensor, a pressure sensor, a timer, or any other similar sensory device. It is further contemplated that injections may occur on a set periodic basis, in addition to or regardless of pressure or temperature conditions, if desired. In order to accomplish these specific injection events, the controller 12 may control operation of one or more of the injectors 21, 26 in response to the one or more inputs.
The controller 12 may use these inputs to form a control signal based on a pre-set control algorithm. The control signal may be transmitted from the controller 12 via the communication lines 14 to various actuation devices, such as the injectors 21, 26. The controller 12 may embody a single microprocessor or multiple microprocessors that include a means for controlling an operation of the injectors 21, 26. Numerous commercially available microprocessors can be configured to perform the functions of the controller 12. The controller 12 may include components required to run an application such as, for example, a memory, a secondary storage device, and a processor, such as a central processing unit or any other means known in the art. It is contemplated that the controller may further be communicatively coupled with one or more components of the engine 10 to change the operation thereof. Thus, the engine 10 and the injectors 21, 26 may be connected to the controller 12, and optionally, the controller 12 may be integrated into the engine 10.
Alternatively, the sensor 40 may embody both a physical sensor and a virtual sensor, which is included in the controller 12, that generates a signal based on a map-driven estimate. The physical sensor may detect and communicate to the controller 12 parameters, e.g., one or more of engine fuel/air settings, engine operating speed, engine load, engine fuel injection profile, other engine operating parameters, exhaust temperature, exhaust flow rate, a temperature of any element within the exhaust treatment system 20, etc. The virtual sensor may evaluate the signals received from one or more physical sensors, and, using relationships contained within one or more maps stored in a memory of the controller 12, may estimate an operating parameter, e.g., the expected exhaust gas NO:NO₂ratio, based on the sensed parameters. Alternatively, the sensor 40 may be a physical sensor that is capable of sensing the NO:NO₂ratio, an amount of NOx, etc.
An SCR device 28, such as an SCR catalyst or other type of SCR device described above in connection with the upstream SCR device 23, may be disposed downstream of the particulate filter 24 and the injector 26. Urea injected by the injector 26 may decompose to ammonia, and the SCR device 28 may facilitate a reaction between the ammonia and NOx in the flow of exhaust to produce water and nitrogen gas, thereby removing NOx from the flow of exhaust. After exiting the SCR device 28, the flow of exhaust may be output from the exhaust treatment system 20, e.g., released into the surrounding atmosphere. Alternatively, a second SCR device 28 a may be disposed downstream from the SCR device 28 to receive the flow of exhaust before it is output from the exhaust treatment system 20. The second SCR device 28 a may further reduce the amount of NOx in the flow of exhaust, e.g., if the volume of exhaust flow is higher.
FIG. 3 illustrates another exemplary embodiment of an exhaust treatment system 30. As shown in FIG. 3, the oxidation device 22 of the exhaust treatment system 20 shown in FIG. 1 may be replaced with a dual-leg subsystem having a first leg 30 a and a second leg 30 b. The SCR device 28 described above may be located downstream from the two legs 30 a, 30 b. The first leg 30 a includes a catalyzed particulate filter 32, such as a catalyzed diesel particulate filter (CDPF). The catalyzed particulate filter 32 may be, e.g., a particulate filter, such as the particulate filters described above in connection with the particulate filter 24 (e.g., a wall flow type filter, a flow through type filter, or other type of filter) having a sponge-like or other type of porous or foam-like material that may be coated uniformly with platinum or another material for oxidizing NO, such as palladium, metal oxide, rhodium, or other precious metal. Accordingly, either a substantial portion of the exhaust or all of the exhaust flowing through the catalyzed particulate filter 32 contacts the NO oxidizing material. In the catalyzed particulate filter 32, the NO oxidizing material oxidizes NO to form NO₂. The NO₂may react with carbon (soot) in the particulate matter to form CO and NO. As a result, the amount of carbon (soot) in the catalyzed particulate filter 32 may be reduced, thereby regenerating the catalyzed particulate filter 32.
The second leg 30 b may include, in order from upstream to downstream, the oxidation device 22 and the particulate filter 24 described above in connection with the embodiment of FIG. 1. The oxidation device 22 and the particulate filter 24 in the second leg 30 b may be provided in a single can or housing. Optionally, a valve 34 may be provided downstream from the particulate filter 24 in the second leg 30 b. The valve 34 may control the amount of exhaust flowing through the second leg 30 b and therefore may control the allocation of flow between the two legs 30 a, 30 b. For example, one or more sensors 40 described above in connection with the embodiment of FIG. 4C may be provided for monitoring the NO:NO₂ratio of the flow of exhaust in the first leg 30 a and/or the second leg 30 b, e.g., downstream from the catalyzed particulate filter 32 and/or the particulate filter 24. Alternatively, the valve 34 may be omitted, and the catalyzed particulate filter 32 in the first leg 30 a may be sized with respect to the oxidation device 22 and the particulate filter 24 in the second leg 30 b to control the allocation of flow between the two legs 30 a, 30 b. For example, the catalyzed particulate filter 32 in the first leg 30 a may be sized so that approximately 50% of the total flow of exhaust is directed to the catalyzed particulate filter 32. Alternatively, the catalyzed particulate filter 32 may be sized so that approximately 45%, 55%, 60%, etc., or any percentage therebetween of the flow of exhaust is directed to the catalyzed particulate filter 32. The remaining exhaust flows to the second leg 30 b. The SCR device 28 and the second SCR device 28 a, described above in connection with the embodiment of FIG. 1, may be located downstream from the two legs 30 a, 30 b. Alternatively, the second SCR device 28 a may be omitted.
FIGS. 4A-4D illustrate alternative exemplary embodiments of the first leg 30 a of the exhaust treatment system 30 of FIG. 3. In a first leg 30 a′ shown in FIG. 4A, the catalyzed particulate filter 32 of the first leg 30 a shown in FIG. 3 may be replaced by, in order from upstream to downstream, the oxidation device 22 and the particulate filter 24 described above in connection with the embodiment of FIG. 1. Alternatively, in a first leg 30 a″ shown in FIG. 4B, the oxidation device 22, described above in connection with the embodiment of FIG. 1, may be disposed upstream of the catalyzed particulate filter 32 of the first leg 30 a shown in FIG. 3. As another alternative, in a first leg 30 a′″ shown in FIG. 4C, the upstream injector 21, described above in connection with the embodiment of FIG. 1, may be disposed upstream of the catalyzed particulate filter 32 of the first leg 30 a shown in FIG. 3. Alternatively, the upstream injector 21 may be disposed upstream of the first and second legs 30 a′″, 30 b and downstream of the heat source 25. The sensor 40 may be provided for monitoring a temperature of the catalyzed particulate filter 32. Signals indicating the sensed temperature are transmitted from the sensor 40 to the controller 12 via the communication line 14. As a further alternative, in a first leg 30 a′″ shown in FIG. 4D, the upstream injector 21 and the upstream SCR device 23, described above in connection with the embodiment of FIG. 1, may be disposed upstream of the catalyzed particulate filter 32 of the first leg 30 a shown in FIG. 3. The upstream SCR device 23 may be close-coupled to the catalyzed particulate filter 32. The sensor 40 described above in connection with the embodiment of FIG. 4C may be provided for monitoring the temperature of the catalyzed particulate filter 32.
FIG. 5 illustrates a further exemplary embodiment of an exhaust treatment system 50 in which the dual-leg subsystem of the exhaust treatment system 30 shown in FIG. 3 may be replaced with a dual-leg subsystem having a first leg 50 a and a second leg 50 b. The first leg 50 a may include, in order from upstream to downstream, the catalyzed particulate filter 32 and the valve 34, described above in connection with the embodiment of FIG. 3. The second leg 50 b may include the particulate filter 24 and the valve 34 described above in connection with the embodiments of FIGS. 1 and 3. The SCR device 28 and the second SCR device 28 a, described above in connection with the embodiment of FIG. 1, may be located downstream from the two legs 50 a, 50 b. Alternatively, the second SCR device 28 a may be omitted. As another alternative, for example, when a lesser reduction of particulate matter from the flow of exhaust is necessary, the particulate filter 24 in the second leg 50 b may be omitted. As yet another alternative, for example, when the flow of exhaust from the engine 10 has a low amount of particulate matter, the catalyzed particulate filter 32 in the first leg 50 a may be replaced by an oxidization device, such as a DOC or other type of oxidation device described above in connection with the oxidation device 22 of FIG. 1, and the particulate filter 24 in the second leg 50 b may be omitted.
FIG. 6 illustrates yet another exemplary embodiment of an exhaust treatment system 60. As shown in FIG. 6, the exhaust treatment system 20 shown in FIG. 1 may further include an upstream oxidation device 62, such as a DOC or other type of oxidation device described above in connection with the oxidation device 22 of FIG. 1. Downstream from the upstream oxidation device 62, the exhaust treatment system 60 may further include, in order from upstream to downstream, the upstream injector 21, the upstream SCR device 23, the oxidation device 22, the particulate filter 24, the injector 26, and the SCR device 28 described above in connection with the embodiment of FIG. 1. Alternatively, the particulate filter 24 may be omitted. The upstream oxidation device 62 and the oxidation device 22 may be similar, but may have different percentages of the holes 22 a (FIG. 2) coated with the NO oxidizing material. For example, the upstream oxidation device 62 may have approximately 25% of the holes 22 a coated, and the upstream oxidation device 72 in the second leg 70 b may have approximately 65% to 100% of the holes 22 a coated. The sensor 40 described above in connection with the embodiment of FIG. 4C may be provided for monitoring the temperature and/or the NO:NO₂ratio of the flow of exhaust downstream from the particulate filter 24. Alternatively, the sensor 40 may be provided anywhere downstream from the engine 10, e.g., between the oxidation device 22 and the SCR device 28.
FIG. 7 illustrates a further exemplary embodiment of an exhaust treatment system 70. As shown in FIG. 7, the exhaust treatment system 70 may include a dual-leg subsystem having a first leg 70 a and a second leg 70 b. The first leg 70 a includes, in order from upstream to downstream, the upstream oxidation device 62, the upstream injector 21, the upstream SCR device 23, and the particulate filter 24 described above in connection with the embodiments of FIGS. 1 and 6. The second leg 70 b also includes, in order from upstream to downstream, the upstream injector 21, the upstream SCR device 23, and the particulate filter 24, which are identical or similar to the like elements in the first leg 70 a. An upstream oxidation device 72 is located upstream from the upstream injector 21 in the second leg 70 b. The upstream oxidation device 62 in the first leg 70 a and the upstream oxidation device 72 in the second leg 70 b may be similar, but may have different percentages of the holes 22 a (FIG. 2) coated with the NO oxidizing material. For example, the upstream oxidation device 62 in the first leg 70 a may have approximately 25% of the holes 22 a coated, and the upstream oxidation device 72 in the second leg 70 b may have approximately 65% to 100% of the holes 22 a coated. The SCR device 28, described above in connection with the embodiment of FIG. 1, may be located downstream from the two legs 70 a, 70 b.
FIG. 8 illustrates a further exemplary embodiment of an exhaust treatment system 80. As shown in FIG. 8, the exhaust treatment system 80 may include a dual-leg subsystem having a first leg 80 a and a second leg 80 b. The first leg 80 a may be similar to the first leg 30 a″″ of the exhaust treatment system 30 shown in FIGS. 3 and 4D, i.e., the first leg 80 a may include, from upstream to downstream, the upstream injector 21, the upstream SCR device 23, and the catalyzed particulate filter 32 connected to the sensor 40. The second leg 80 b may be similar to the second leg 70 b of the exhaust treatment system 70 shown in FIG. 7, i.e., the second leg 80 b may include, from upstream to downstream, the upstream oxidation device 72, the upstream injector 21, the upstream SCR device 23, and the particulate filter 24. The injector 26 and the SCR device 28 described above may be located downstream from the two legs 80 a, 80 b.
FIGS. 9A and 9B illustrate alternative exemplary embodiments of the first leg 80 a of the exhaust treatment system 80 of FIG. 8. As shown in FIG. 9A, a first leg 80 a′ may include, from upstream to downstream, the upstream injector 21, the upstream SCR device 23, the oxidation device 22, and the particulate filter 24 as provided in the embodiment of FIG. 1. As shown in FIG. 9B, a first leg 80 a″ may include, from upstream to downstream, the upstream oxidation device 62, the upstream injector 21, the upstream SCR device 23, and the particulate filter 24 as described above in connection with the embodiment of FIG. 7. FIG. 9C illustrates an alternative exemplary embodiment of the second leg 80 b of the exhaust treatment system 80 of FIG. 8. As shown in FIG. 9C, a second leg 80 b′ may include, from upstream to downstream, the upstream injector 21, the upstream SCR device 23, the oxidation device 22, and the particulate filter 24 as provided in the embodiment of FIG. 1.

INDUSTRIAL APPLICABILITY

The disclosed exhaust treatment system may be provided in any machine or powered system that includes a power source producing a flow of exhaust, such as an engine. The disclosed exhaust treatment system may increase the amount of NO₂relative to NO in the flow of exhaust upstream of the SCR device so that the SCR device can more quickly and efficiently reduce the amount of NOx. The operation of the exhaust treatment system will now be explained.
According to the embodiment of the exhaust treatment system 20 shown in FIG. 1, the flow of exhaust from the engine 10 may be heated by the heat source 25 before being directed to the oxidation device 22. Optionally, e.g., during cold start conditions, reductant may be injected into the flow of exhaust by the upstream injector 21 and then the flow of reductant and exhaust may be directed to the upstream SCR device 23 to reduce the amount of NOx in the flow of exhaust before being directed to the oxidation device 22. Then, the flow of exhaust may be directed to the particulate filter 24 where particulate matter may be removed. After exiting the particulate filter 24, reductant is injected into the flow of exhaust by the injector 26, and the flow of exhaust is directed to the SCR device 28 and optionally the second SCR device 28 a, which reduce the amount of NOx in the flow of exhaust.
Efficiency of the NOx reduction by the SCR device 28 may be at least partially dependent on the ratio of NO₂to NOx in the flow of exhaust. In particular, NOx reduction by the SCR device 28 may be faster and more efficient when the ratio of NO₂to NOx in the flow of exhaust is approximately 50:50. According to an exemplary embodiment, the oxidation device 22 may convert some of the NO in the flow of exhaust to NO₂so that the NO:NO₂ratio is closer to 50:50. For example, approximately 50% to 75% of the holes 22 a of the oxidation device 22 may be coated with platinum. In one embodiment, approximately 75% of the holes 22 a of the oxidation device 22 may be coated with platinum. As a result, the exhaust treatment system 20 shown in FIG. 1 may reduce particulate matter and may provide a greater reduction in NOx before releasing the flow of exhaust to the surrounding atmosphere.
The oxidation device 22 also allows the regeneration of the particulate filter 24. The oxidation device 22 increases the amount of NO₂such that NO₂reacts with carbon (soot) in the particulate matter to form CO and NO. As a result, the amount of carbon (soot) in the particulate filter 24 is reduced, thereby regenerating the particulate filter 24 and reducing the risk of having the particulate matter build up and clog the particulate filter 24.
According to the embodiment of the exhaust treatment system 30 shown in FIG. 3, the flow of exhaust from the engine 10 may be heated by the heat source 25 before being directed to the first and second legs 30 a, 30 b. A first portion of the exhaust flows through the first leg 30 a, where all or a substantial amount of the first portion of the exhaust contacts the platinum coating of the catalyzed particulate filter 32 to convert some of the NO to NO₂. The increase in NO₂may allow the catalyzed particulate filter 32 to regenerate as described above and may provide an increased amount of NO₂from the first leg 30 a to the SCR device 28. A second portion of the exhaust flows through the second leg 30 b, where the second portion of the exhaust is directed to the partially-loaded oxidation device 22. In an exemplary embodiment, the oxidation device 22 has a percentage less than 50%, e.g., approximately 25%, of the holes 22 a coated with platinum. Accordingly, the oxidation device 22 may convert some of the NO to NO₂in the second portion of the exhaust. The increase in NO₂may allow the regeneration of the particulate filter 24 in the second leg 30 b, as described above in connection with the oxidation device 22 and the particulate filter 24 of the embodiment shown in FIG. 1. The increase in NO₂may also allow the SCR device 28 to perform NOx reduction faster and more efficiently as described above. After exiting the oxidation device 22, the second portion of the exhaust may be directed to the particulate filter 24 where particulate matter may be removed.
The respective portions of exhaust from the first and second legs 30 a, 30 b are combined, and reductant is injected by the injector 26 into the combined flow. Then, the combined flow is directed to the SCR device 28, which reduces the amount of NOx in the combined flow. According to the embodiments shown in FIGS. 3 and 4A-4D, the components in the first leg 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ may be sized relative to the oxidation device 22 and the particulate filter 24 in the second leg 30 b such that the combined flow from the first leg 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and the second leg 30 b has a NO:NO₂ratio that is closer to 50:50. Alternatively or in addition, the sensor 40 and/or the valve 34 may be provided in the second leg 30 b to also control the respective amounts of flow in the two legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and 30 b. As a result, the reduction of NOx in the SCR device 28 may be more efficient and faster. The exhaust treatment system 30 may reduce particulate matter (with the components in the first leg 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and the particulate filter 24 in the second leg 30 b), and may provide a greater reduction in NOx before releasing the flow of exhaust to the surrounding atmosphere.
Alternatively, one or more of the sensors 40 may be provided, e.g., in one or both of the two legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and 30 b or between the two legs and the SCR device 28. Accordingly, the controller 12 may adjust the allocation of flow between the two legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″ and 30 b, e.g., by controlling the valve 34, based on a sensed condition (e.g., the NO:NO₂ratio) to provide more accurate control of the amount of NO₂supplied to the SCR device 28. Closed loop control of the ratio of NO:NO₂in the exhaust may be achieved.
According to the embodiment shown in FIG. 4A, the first portion of the exhaust may flow through the first leg 30 a′, where the first portion of the exhaust may be directed to the partially-loaded oxidation device 22 and then the particulate filter 24. In the exemplary embodiment, the oxidation device 22 may have approximately 50% or a lower percentage of the holes 22 a coated with platinum. Accordingly, the oxidation device 22 may convert some of the NO to NO₂in the first portion of the exhaust. The increase in NO₂may allow the regeneration of the particulate filter 24 in the first leg 30 a′, as described above in connection with the oxidation device 22 and the particulate filter 24 of the embodiment shown in FIG. 1. The increase in NO₂may also allow the SCR device 28 to perform NOx reduction faster and more efficiently as described above. After exiting the oxidation device 22, the first portion of the exhaust may be directed to the particulate filter 24 where particulate matter may be removed.
According to the embodiment shown in FIG. 4B, the first portion of the exhaust may flow through the first leg 30 a″, where the first portion of the exhaust may be directed to the partially-loaded oxidation device 22 and then the catalyzed particulate filter 32. The partially-loaded oxidation device 22 may have approximately 50% or a lower percentage of the holes 22 a coated with platinum. Accordingly, the oxidation device 22 may convert some of the NO to NO₂in the first portion of the exhaust. The increase in NO₂may allow the regeneration of the catalyzed particulate filter 32 and may allow NOx reduction by the SCR device 28 to be performed faster and more efficiently as described above. In addition, the catalyzed particulate filter 32 may allow a substantial amount or all of the first portion of the exhaust to contact the platinum coating of the filter 32, thereby converting some of the NO to NO₂. The increase in NO₂may allow the catalyzed particulate filter 32 to regenerate as described above and may allow NOx reduction by the SCR device 28 to be performed faster and more efficiently as described above.
According to the embodiment shown in FIG. 4C, the first portion of the exhaust may flow through the first leg 30 a′″, where reductant may be injected by the upstream injector 21. Then, the flow of reductant and the first portion of the exhaust may be directed to the catalyzed particulate filter 32. The catalyzed particulate filter 32 may allow a substantial amount or all of the first portion of the exhaust to contact the platinum coating of the filter 32, thereby converting some of the NO to NO₂. The increase in NO₂may allow the catalyzed particulate filter 32 to regenerate as described above and may allow NOx reduction by the SCR device 28 to be performed faster and more efficiently as described above.
According to an exemplary embodiment, the controller 12 may send a signal to the upstream injector 21 inject reductant when a predetermined condition is satisfied, e.g., during cold start conditions and/or when a temperature measured by the sensor 40 monitoring the temperature of the catalyzed particulate filter 32 is within a predetermined range (e.g., approximately 200° C. to 300° C., approximately 200° C. to 350° C., or other temperature range in which NOx reduction in the SCR device 28 is dependent on the ratio of NO:NO₂). When the predetermined condition is satisfied, the controller 12 may send a signal to the upstream injector 21 to inject reductant into the first portion of the exhaust upstream from the catalyzed particulate filter 32. The addition of reductant may reduce the amount of NOx in the first portion of the exhaust that contacts the platinum coating in the catalyzed particulate filter 32.
The controller 12 may be configured to determine a desired (or target) amount of reductant to inject with the upstream injector 21 based on one or more mappings correlating the sensed temperature of the catalyzed particulate filter 32 and the desired amount of reductant to inject. In addition, the mapping(s) may determine the desired amount of reductant to inject based on a desired amount of NO₂to supply to the SCR device 28, e.g., to reach a desired NO:NO₂ratio of 50:50. The controller 12 may also determine the amount of reductant based on other factors, e.g., characteristics of the catalyzed particulate filter 32, an estimated amount of NO₂in the second portion of the exhaust from the second leg 30 b, etc. Accordingly, by using the mapping(s), the controller 12 may adjust the amount of reductant to inject into the first leg 30 a′″ based on a sensed condition (e.g., the temperature of the catalyzed particulate filter 32) to provide more accurate control of the amount of NO₂supplied to the SCR device 28. Closed loop control of the ratio of NO:NO₂in the first portion of the exhaust may be achieved. The determination of whether and how much reductant to inject with the upstream injector 21 is adjusted based on the sensed temperature of the catalyzed particulate filter 32.
According to the embodiment shown in FIG. 4D, the first portion of the exhaust may flow through the first leg 30 a″″, where reductant may be injected by the upstream injector 21. Then, the flow of reductant and the first portion of the exhaust may be directed to the upstream SCR device 23 to reduce the amount of NOx in the flow of exhaust before being directed to the catalyzed particulate filter 32. The catalyzed particulate filter 32 may allow a substantial amount or all of the first portion of the exhaust to contact the platinum coating of the filter 32, thereby converting some of the NO to NO₂. The increase in NO₂may allow the catalyzed particulate filter 32 to regenerate as described above and may allow NOx reduction by the SCR device 28 to be performed faster and more efficiently as described above.
The controller 12 may send a signal to the upstream injector 21 to inject reductant into the first portion of the exhaust when a predetermined condition is satisfied, as described above in connection with the embodiment shown in FIG. 4C. The addition of reductant upstream from the upstream SCR device 23 may reduce the amount of NOx in the first portion of the exhaust. Since there is less NOx (which includes NO and NO₂), there is proportionally less NO and NO₂supplied to the catalyzed particulate filter 32. Therefore, the catalyzed particulate filter 32 outputs less NO₂than without the addition of reductant.
The controller 12 may also be configured to adjust the amount of reductant injected by the upstream injector 21, as described above in connection with the embodiment shown in FIG. 4C. Accordingly, by using the mapping(s), the controller 12 may adjust the amount of reductant to inject into the first leg 30 a″″ based on a sensed condition (e.g., the temperature of the catalyzed particulate filter 32) to provide more accurate control of the amount of NO₂supplied to the SCR device 28. Closed loop control of the ratio of NO:NO₂in the first portion of the exhaust may be achieved. The determination of whether and how much reductant to inject with the upstream injector 21 is adjusted based on the sensed temperature of the catalyzed particulate filter 32.
According to the embodiment of the exhaust treatment system 50 shown in FIG. 5, the flow of exhaust from the engine 10 may be heated by the heat source 25 before being directed to the first and second legs 50 a, 50 b. A first portion of the exhaust flows through the first leg 50 a, where all or a substantial amount of the first portion of the exhaust contacts the platinum coating of the catalyzed particulate filter 32 to convert some of the NO to NO₂. The increase in NO₂may allow the catalyzed particulate filter 32 to regenerate as described above and may provide an increased amount of NO₂from the first leg 50 a to the SCR device 28. A second portion of the exhaust flows through the second leg 50 b, where the second portion of the exhaust is directed to the particulate filter 24. The particulate filter 24 may remove particulate matter from the second portion of the exhaust. Then, the respective portions of the exhaust from the first and second legs 50 a, 50 b are combined, and reductant is injected by the injector 26 into the combined flow. The combined flow is directed to the SCR device 28, which reduces the amount of NOx in the combined flow.
The valves 34 in the first and second legs 50 a, 50 b may be controlled by the controller 12 to control the respective amounts of flow through the legs 50 a, 50 b. In an exemplary embodiment, the controller 12 may control the respective amounts of flow through the valves 34 to provide a NO:NO₂ratio that is approximately 50:50 in the flow of exhaust directed to the SCR device 28. As a result, the reduction of NOx in the SCR device 28 may be more efficient and faster. The exhaust treatment system 50 may reduce particulate matter (with the catalyzed particulate filter 32 in the first leg 50 a and the particulate filter 24 in the second leg 50 b), and may provide a greater reduction in NOx before releasing the flow of exhaust to the surrounding atmosphere.
In addition, the controller 12 may control the valves 34 such that the valves 34 in the first and second legs 50 a, 50 b may be closed simultaneously. When the valves 34 are closed simultaneously, back pressure is created in the engine 10, which raises the temperature of the flow of exhaust. The higher temperature exhaust may be used to regenerate the catalyzed particulate filter 32 in the first leg 50 a and the particulate filter 24 in the second leg 50 b. Accordingly, the heat source 25 may be omitted.
According to the embodiment of the exhaust treatment system 60 shown in FIG. 6, the flow of exhaust from the engine 10 may be heated by the heat source 25 before being directed to the partially-loaded upstream oxidation device 62. In an exemplary embodiment, the upstream oxidation device 62 has a percentage less than 50%, e.g., approximately 25%, of the holes 22 a coated with platinum. Alternatively, the upstream oxidation device 62 may have approximately 50% or another percentage less than 100% of the holes 22 a coated with platinum. Accordingly, the oxidation device 22 may convert some of the NO to NO₂in the second portion of the exhaust. Then, reductant may be injected into the flow of exhaust by the upstream injector 21 and the flow of reductant and exhaust may be directed to the upstream SCR device 23 to reduce the amount of NOx in the flow of exhaust. Since the upstream oxidation device 62 converts some of the NO to NO₂upstream from the upstream SCR device 23, greater NOx reduction efficiency may be achieved with the upstream SCR device 23.
The flow of exhaust is then directed to the partially-loaded oxidation device 22. In an exemplary embodiment, the oxidation device 22 has a percentage equal to or greater than 50%, e.g., approximately 65%, of the holes 22 a coated with platinum. Alternatively, the oxidation device 22 may have approximately 75% or another percentage less than 100% of the holes 22 a coated with platinum. Accordingly, the oxidation device 22 may convert some of the NO to NO₂in the flow of exhaust. The oxidation device 22 may convert more NO to NO₂than the upstream oxidation device 62 since the oxidation device 22 has a greater percentage of the holes 22 a coated with platinum. Then, optionally, the flow of exhaust may be directed to the particulate filter 24 where particulate matter may be removed. The increase in NO₂by the oxidation device 22 may allow the regeneration of the particulate filter 24, as described above in connection with the oxidation device 22 and the particulate filter 24 of the embodiment shown in FIG. 1.
After exiting the particulate filter 24, reductant is injected into the flow of exhaust by the injector 26, and the flow of exhaust is directed to the SCR device 28, which reduces the amount of NOx in the flow of exhaust. The remaining NO₂in the flow of exhaust may be used by the SCR device 28 to reduce the amount of NOx with greater efficiency. In an exemplary embodiment, the components of the exhaust treatment system 60, e.g., the upstream oxidation device 62, the upstream SCR device 23, the oxidation device 22, and the particulate filter 24, may be configured to provide a NO:NO₂ratio in the flow of exhaust to the SCR device 28 of approximately 50:50.
Alternatively or in addition, to provide a NO:NO₂ratio in the flow of exhaust to the SCR device 28 of approximately 50:50 or other optimum ratio, the sensor 40 may be provided, e.g., downstream of the oxidation device 22. The sensor 40 allows the controller 12 to determine the NO:NO₂ratio and to provide closed loop control of the dosing of the reductant to the upstream SCR device 23 by the upstream injector 21, thereby allowing the conversion efficiency of the upstream SCR device 23 to be controlled. The sensor 40 may transmit a signal to the controller 12 indicating the NO:NO₂ratio in the flow of exhaust. For example, the sensor 40 may include the virtual sensor described above and one or more physical sensors. Then, the controller 12 may determine the timing and amount of reductant injected by the upstream injector 21 to control the NO:NO₂ratio of the flow of exhaust supplied to the SCR device 28, e.g., by setting the NO:NO₂ratio closer to 50:50. For example, one or more mappings may stored in the memory of the controller 12. The mappings may be used to determine a sensed NO:NO₂ratio of the flow of exhaust based on the characteristics sensed by the physical sensors, e.g., the temperature of the flow of exhaust, space velocity, air flow to the engine 10, etc. Then, the mappings may be used to determine the timing and amount of reductant to inject based on the sensed NO:NO₂ratio in the flow of exhaust in order to maintain the NO:NO₂ratio near 50:50. Thus, closed loop feedback control of the ratio of NO:NO₂in the flow of exhaust may be achieved. As a result, the reduction of NOx in the SCR device 28 may be more efficient and faster over a wider range of operating conditions (e.g., exhaust temperatures). Furthermore, the exhaust treatment system 60 may reduce particulate matter in the flow of exhaust with the particulate filter 24 and may provide a greater reduction in NOx before releasing the flow of exhaust to the surrounding atmosphere. Also, as the components of the exhaust treatment system 60 age, the closed loop feedback control on the dosing of reductant to the upstream SCR device 23 can compensate for the change in conversion efficiency of the other components, such as the oxidation devices 62, 22, thereby maintaining an optimum NO:NO₂ratio to the SCR device 28. The closed loop feedback control also allows the NO:NO₂ratio to the SCR device 28 to be actively controlled.
Some ammonia may remain in the flow of exhaust after passing through the upstream SCR device 23, e.g., in situations when there is an overdose of reductant by the injectors 21, 26 or when the oxidation device 22 is unable to convert all of the ammonia. The SCR device 28 may remove the remaining ammonia (i.e., the ammonia slip) by reacting the ammonia with the NOx in the flow of exhaust to form N₂and water. Furthermore, the sensor 40 may also be used to determine whether the amount of reductant injected by the upstream injector 21 is not within a desired range, e.g., too high or too low, so that ammonia slip may be reduced.
According to the embodiment of the exhaust treatment system 70 shown in FIG. 7, the flow of exhaust from the engine 10 may be heated by the heat source 25 before being directed to the first and second legs 70 a, 70 b. A first portion of the exhaust flows through the first leg 70 a, where a percentage of the first portion of the exhaust contacts the platinum coating in the upstream oxidation device 62 in the first leg 70 a to convert some NO to NO₂The percentage is determined based on the number of the holes 22 a of the upstream oxidation device 62 that are coated with platinum. At the same time, a second portion of the exhaust flows through the second leg 70 b, where a percentage of the second portion of the exhaust contacts the platinum coating in the upstream oxidation device 72 in the second leg 70 b to convert NO to NO₂. The percentage is determined based on the number of the holes 22 a of the upstream oxidation device 72 that are coated with platinum. Accordingly, an increased amount of NO₂may be provided in the first and second legs 70 a, 70 b by the respective upstream oxidation devices 62, 72.
Downstream from the upstream oxidation devices 62, 72 in the respective first and second legs 70 a, 70 b, reductant may be injected into the flow of exhaust by the respective upstream injectors 21. The flow of reductant and exhaust may be directed to the respective upstream SCR devices 23 to reduce the amount of NOx in the flow of exhaust. The increase in NO₂by the upstream oxidation devices 62, 72 may allow the upstream SCR devices 23 to perform NOx reduction faster and more efficiently as described above. The respective flows of exhaust are then directed to the particulate filters 24 in the respective first and second legs 70 a, 70 b, and the particulate filters 24 may remove particulate matter from the respective flows of exhaust. In addition, some of the NO₂introduced by the upstream oxidation devices 62, 72 into the flows of exhaust in the respective first and second legs 70 a, 70 b may be used by the particulate filter 24 for regeneration, as described above in connection with the oxidation device 22 and the particulate filter 24 of the embodiment shown in FIG. 1. The respective portions of exhaust from the first and second legs 70 a, 70 b are combined and directed to the SCR device 28, which reduces the amount of NOx in the combined flow.
The upstream oxidation devices 62, 72 may include different percentages of holes 22 a coated with platinum. In an exemplary embodiment, a percentage less than 50%, e.g., 25%, of the holes 22 a of the upstream oxidation device 62 in the first leg 70 a may be coated with platinum, and a percentage greater than 50%, e.g., 65%, of the holes 22 a of the upstream oxidation device 72 in the second leg 70 b may be coated with platinum. Alternatively, the upstream oxidation device 72 may have approximately 75% or another percentage less than 100% of the holes 22 a coated with platinum. The controller 12 may send signals to the upstream injectors 21 to control the respective amounts of reductant injected into the respective first and second legs 70 a, 70 b, e.g., to provide a NO:NO₂ratio of approximately 50:50, thereby providing more efficient and faster NOx reduction in the SCR device 28. As a result, the exhaust treatment system 50 may reduce particulate matter (with the particulate filters 24 in the first and second legs 70 a, 70 b), and may provide a greater reduction in NOx before releasing the flow of exhaust to the surrounding atmosphere.
In a dual-leg exhaust treatment system, such as the exhaust treatment systems 30, 50, 70, 80 shown in FIGS. 3, 5, 7, and 8, the particulate filter 24 and/or the catalyzed particulate filter 32 in one or both of the legs may be a flow through type filter, as described above. Compared to wall flow type filters, flow through type filters have less back pressure. When back pressure builds up upstream from the flow through type filter, there may be a difference between the target or desired amount of flow through the respective leg and the actual amount of flow through the leg. The difference in target/desired and actual amounts of flow may cause a deviation from the target NO:NO₂ratio, e.g., 50:50, and the actual NO:NO₂ratio to the SCR device 28. Therefore, when the particulate filter 24 and/or the catalyzed particulate filter 32 in one or both of the legs is a flow through type filter, the risk of having excessive back pressure is lower, thereby providing a more efficient exhaust treatment system.
One or more characteristics of the exhaust treatment system 20, 30, 50, 60, 70, 80, e.g., the percentage of holes 22 a coated in the oxidizing device 22, 62, 72, an allocation of flow between two or more legs 30 a, 30 a′, 30 a″, 30 a′″, 30 a″″, 30 b, 50 a, 50 b, 70 a, 70 b, 80 a, 80 a′, 80 a″, 80 b, 80 b′, etc., may be determined experimentally based on the application. For example, the characteristic(s) may be determined when one or more components of the exhaust treatment system 20, 30, 50, 60, 70, 80, e.g., the components of the exhaust treatment system 20, 30, 50, 60, 70, 80 upstream from the SCR device 28, are operating at a predetermined operating condition and when the flow of exhaust directed to the SCR device 28 achieves the target NO:NO₂ratio of 50:50. The predetermined operating condition may include, e.g., a predetermined mass flow, a predetermined temperature, or other operating condition when NOx conversion is difficult and/or when there is less than 50% NO₂. The predetermined mass flow may be, e.g., approximately 60,000 volume hour space velocity (VHSV) (60,000 hr⁻¹) or less, the predetermined temperature of the flow of exhaust may be, e.g., approximately 250 to 350° C. For example, in one embodiment, the percentage of holes coated in the oxidizing device and/or the allocation of flow between two or more legs, may be determined when the following operating conditions are achieved: the components of the exhaust treatment system upstream from the SCR device 28 have a VHSV of approximately 60,000 hr⁻¹, the temperature of the flow of exhaust directed to the SCR device 28 is between 250 to 350° C., and the flow of exhaust directed to the SCR device 28 achieves a NO:NO₂ratio of 50:50. Then, the determined characteristic (e.g., percentage of holes coated and/or allocation of flow) may become the target characteristic for the exhaust treatment system 20, 30, 50, 60, 70, 80.
In view of the foregoing disclosure, one skilled in the art may readily conceive or identify additional configurations of the exhaust treatment system sufficient to realize the desired NO₂control functions. For example, the embodiments of the exhaust treatment systems 80 shown in FIGS. 8 and 9A-9C illustrate additional configurations of the components, e.g., the upstream injector 21, the oxidation device 22, the upstream SCR device 23, and the particulate filter 24, of the exhaust treatment systems shown in FIGS. 1, 3, 4A-4D, and 5-7. The exhaust treatment systems 80 shown in FIGS. 8 and 9A-9C operate using the same principles of operation as described above in connection with FIGS. 1, 3, 4A-4D, and 5-7.
It will be apparent to those skilled in the art that various modifications and variations can be made to the exhaust treatment system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed exhaust treatment system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.

Claims

1. An exhaust treatment system comprising:

an oxidation device having a plurality of channels through which a flow of exhaust flows, a first portion of the channels being coated with catalytic material for converting NO to NO₂, a second portion of the channels being uncoated with the catalytic material; and

a selective catalytic reduction device disposed downstream from the oxidation device, the selective catalytic reduction device being configured to receive the flow of exhaust passing through the oxidation device.

2. The exhaust treatment system of claim 1, wherein a percentage of the channels of the oxidation device that are coated with the catalytic material is based on a target NO:NO₂ratio in the flow of exhaust supplied to the selective catalytic reduction device.

3. The exhaust treatment system of claim 2, wherein the target NO:NO₂ratio in the flow of exhaust supplied to the selective catalytic reduction device is approximately 50:50.

4. The exhaust treatment system of claim 2, wherein the percentage of channels coated in the oxidation device is determined when operating at a predetermined operating condition when the target NO:NO₂ratio is achieved, the predetermined operating condition including at least one of a predetermined mass flow and a predetermined temperature.

5. The exhaust treatment system of claim 1, wherein the first portion of the channels of the oxidation device that are coated with the catalytic material constitute approximately 75% of the channels of oxidation device.

6. The exhaust treatment system of claim 1, further including a particulate filter downstream from the oxidation device.

7. The exhaust treatment system of claim 1, wherein the selective catalytic reduction device is a downstream selective catalytic reduction device, and the exhaust treatment system further includes:

an upstream selective catalytic reduction device disposed upstream from the oxidation device; and

an upstream injector disposed upstream from the upstream selective catalytic reduction device, the upstream injector configured to inject reductant into the flow of exhaust.

8. The exhaust treatment system of claim 7, wherein the oxidation device is a downstream oxidation device, and the exhaust treatment system further includes:

an upstream oxidation device disposed upstream from the upstream selective catalytic reduction device, the upstream oxidation device having a plurality of channels through which the flow of exhaust flows, and a percentage less than 100% of the channels of the upstream oxidation catalyst are coated with the catalytic material.

9. The exhaust treatment system of claim 8, further including:

a sensor for sensing a characteristic of the flow of exhaust upstream from the downstream selective catalytic reduction device and downstream from the upstream selective catalytic reduction device; and

a controller connected to the sensor, the controller being configured to receive the sensed characteristic and control an injection of reductant by the upstream injector based on the sensed characteristic.

10. The exhaust treatment system of claim 9, wherein the characteristic is a NO:NO₂ratio.

11. The exhaust treatment system of claim 8, wherein the percentage of coated channels in the downstream oxidation device is greater than 50%, and the percentage of coated channels in the upstream oxidation device is less than 50%.

12. The exhaust treatment system of claim 1, further including a first passageway configured to direct a portion of the flow of exhaust around the oxidation device, the oxidation device being disposed in a second passageway.

13. The exhaust treatment system of claim 12, further including:

a first particulate filter disposed in the first passageway; and

a second particulate filter disposed in the second passageway.

14. The exhaust treatment system of claim 13, wherein at least one of the particulate filters is coated with the catalytic material for converting NO to NO₂.

15. The exhaust treatment system of claim 13, wherein at least one of the particulate filters is a flow through type filter.

16. The exhaust treatment system of claim 12, wherein the oxidation device is a second oxidation device, and the exhaust treatment system further includes a first oxidation device in the first passageway, the first oxidation device having a plurality of channels through which the flow of exhaust flows, a percentage less than 100% of the channels of the first oxidation device being coated with the catalytic material.

17. The exhaust treatment system of claim 16, wherein:

the selective catalytic reduction device is a downstream selective catalytic reduction device disposed downstream from the first and second passageways; and

the exhaust treatment system further includes:

a first selective catalytic reduction device in the first passageway downstream from the first oxidation device; and

a second selective catalytic reduction device in the second passageway downstream from the second oxidation device.

18. The exhaust treatment system of claim 17, wherein the percentage of channels coated with the catalytic material in the first oxidation device is greater than 50%, and the percentage of channels coated with the catalytic material in the second oxidation device is less than 50%.

19. The exhaust treatment system of claim 17, further including:

a first particulate filter in the first passageway downstream from the first selective catalytic reduction device; and

a second particulate filter in the second passageway downstream from the second selective catalytic reduction device.

20. The exhaust treatment system of claim 1, wherein the catalytic material is platinum.

21. A method for treating a flow of exhaust comprising:

generating the flow of exhaust;

passing the flow of exhaust through a plurality of channels of a downstream oxidation device, a first portion of the channels being coated with catalytic material for converting NO to NO₂, a second portion of the channels being uncoated with the catalytic material; and

directing the combined flow of exhaust to a downstream selective catalytic reduction device disposed downstream from the downstream oxidation device.

22. The method of claim 21, further including:

passing the flow of exhaust through a plurality of channels of an upstream oxidation device disposed upstream from the downstream oxidation device, a first portion of the channels of the upstream oxidation device being coated with the catalytic material, a second portion of the channels of the channels of the upstream oxidation device being uncoated with the catalytic material;

injecting reductant with an upstream injector disposed downstream from the upstream oxidation device and upstream from the downstream oxidation device; and

passing the flow of exhaust through an upstream selective catalytic reduction device disposed upstream from the downstream oxidation device and downstream from the upstream injector.

23. The method of claim 22, further including:

sensing a characteristic of the flow of exhaust; and

controlling the injection of reductant by the upstream injector based on the sensed characteristic.

24. The method of claim 23, wherein the sensed characteristic is a sensed NO:NO₂ratio in the flow of exhaust.

25. The method of claim 24, wherein the injection of reductant by the upstream injector is controlled based on a target NO:NO₂ratio in the flow of exhaust supplied to the downstream selective catalytic reduction device.

26. The method of claim 25, wherein the target NO:NO₂ratio in the flow of exhaust supplied to the downstream selective catalytic reduction device is approximately 50:50.

27. An exhaust treatment system comprising:

an upstream oxidation device having a plurality of channels through which a flow of exhaust flows, a first portion of the channels being coated with catalytic material for converting NO to NO₂, a second portion of the channels being uncoated with the catalytic material;

an upstream injector configured to inject reductant into the flow of exhaust upstream from the upstream selective catalytic reduction device;

an upstream selective catalytic reduction device disposed downstream from the upstream injector, the upstream selective catalytic reduction device being configured to receive the flow of exhaust;

a downstream oxidation device disposed downstream from the upstream selective catalytic reduction device, the downstream oxidation device having a plurality of channels through which the flow of exhaust flows, a first portion of the channels of the downstream oxidation device being coated with the catalytic material, a second portion of the channels of the downstream oxidation device being uncoated with the catalytic material; and

a downstream selective catalytic reduction device disposed downstream from the downstream oxidation device, the downstream selective catalytic reduction device being configured to receive the flow of exhaust.

28. The exhaust treatment system of claim 27, further including:

a sensor configured to sense a characteristic of the flow of exhaust; and

29. The exhaust treatment system of claim 28, wherein the sensed characteristic is a sensed NO:NO₂ratio in the flow of exhaust.

30. The exhaust treatment system of claim 29, wherein the controller controls the injection of reductant based on a target NO:NO₂ratio in the flow of exhaust supplied to the downstream selective catalytic reduction device.

31. The exhaust treatment system of claim 30, wherein the target NO:NO₂ratio in the flow of exhaust supplied to the downstream selective catalytic reduction device is approximately 50:50.