US20060021335A1

US20060021335A1 - Exhaust treatment system having particulate filters

Info

Publication number: US20060021335A1
Application number: US10/901,328
Authority: US
Inventors: Cornelius Opris
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2004-07-29
Filing date: 2004-07-29
Publication date: 2006-02-02
Also published as: DE102005025924A1; CN1727652A

Abstract

An exhaust treatment system for a power source has an air induction system and an exhaust system. The exhaust system has a first particulate filter and a second particulate filter disposed in series with the first particulate filter. The exhaust treatment system further has a recirculation system configured to draw at least a portion of an exhaust flow from between the first particulate filter and the second particulate filter and to direct the at least a portion of the exhaust flow to the air induction system.

Description

TECHNICAL FIELD

The present disclosure relates generally to an exhaust treatment system and, more particularly, to an exhaust treatment system having particulate filters.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous compounds, which may include nitrous oxides (NOx), and solid particulate matter, which may include unburned carbon particulates called soot.
Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of gaseous compounds emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions is exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas byproducts into the intake air supply of the internal combustion engine. The exhaust gas, which is directed to the engine cylinder, reduces the concentration of oxygen within the cylinder, which in turn lowers the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature can slow the chemical reaction of the combustion process and decrease the formation of NOx.
In many EGR applications, the exhaust gas is diverted directly from the exhaust manifold by an EGR valve. However, the particulate matter in the recirculated exhaust gas can adversely affect the performance and durability of the internal combustion engine and EGR system. As disclosed in U.S. Pat. No. 6,526,753 (the '753 patent), issued to Bailey on Mar. 3, 2003, a filter can be used to remove particulate matter from the exhaust gas that is being fed back to the intake air stream for recirculation. Specifically, the '753 patent discloses an exhaust gas regenerator/particulate capture system that includes a first particulate filter and a second particulate filter. A regenerator valve operates between a first position where an EGR inlet port fluidly connects a portion of an exhaust flow with the first particulate filter and a second position where the EGR inlet port fluidly connects the portion of the exhaust flow with the second particulate filter. The filtered EGR gases are then supplied for mixing with compressed air prior to or during entry into the intake manifold.
Although the exhaust gas regenerator/particulate capture system of the '753 patent may protect the engine from harmful particulate matter, it may be complex and difficult to package. For example, because the exhaust gas regenerator/particulate capture system of the '753 patent must alternate exhaust flow between the first and second particulate filters to avoid clogging, additional piping, valving, and control strategies may be required. These additional components coupled with the space required to mount and house the components within the engine compartment can increase the cost of the exhaust gas regenerator/particulate capture system and the difficulty of retrofitting the exhaust gas regenerator/particulate capture system to older vehicles. In addition, the portion of the exhaust gas not flowing through the exhaust gas regenerator/particulate capture system of the '753 patent may be completely unfiltered and untreated.
The disclosed exhaust treatment 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 for a power source that includes an air induction system and an exhaust system. The exhaust system includes a first particulate filter and a second particulate filter disposed in series with the first particulate filter. The exhaust treatment system also includes a recirculation system configured to draw at least a portion of an exhaust flow from between the first particulate filter and the second particulate filter and to direct the at least a portion of the exhaust flow to the air induction system.
In another aspect, the present disclosure is directed to an exhaust treatment system for a power source that includes an air induction system having at least one compressor. The exhaust treatment system also includes an exhaust system having a particulate filter and at least one turbine connected to the at least one compressor. The at least one turbine is disposed downstream of the particulate filter. The exhaust treatment system further includes a recirculation system configured to draw at least a portion of an exhaust flow from downstream of the at least one turbine and to direct the at least a portion of the exhaust flow to the air induction system upstream of the at least one compressor.
In yet another aspect, the present disclosure is directed to a method of operating an exhaust treatment system. The method includes directing air into the power source with an air induction system and directing exhaust away from the power source. The method further includes filtering out particulates entrained within the exhaust flow with a first particulate filter, directing a first portion of the exhaust flow from downstream of the first particulate filter into the power source, directing as second portion of the exhaust flow from downstream of the first particulate filter to a second particulate filter, and filtering particulates out of the exhaust flow with the second particulate filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an engine having an exhaust treatment system according to an exemplary disclosed embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a power source 10 having an exemplary exhaust treatment system 12. Power source 10 may include an engine such as, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other engine apparent to one skilled in the art. Power source 10 may, alternately, include another source of power such as a furnace or any other source of power known in the art. Exhaust treatment system 12 may include an air induction system 14, an exhaust system 16, and a recirculation system 18.
Air induction system 14 may be configured to introduce charged air into a combustion chamber (not shown) of power source 10. Air induction system 14 may include a induction valve 20 and a compressor 22. It is contemplated that additional components may be included within air induction system 14 such as, for example, one or more air coolers, additional valving, one or more air cleaners, one or more waste gates, a control system, and other components known in the art.
Induction valve 20 may be fluidly connected to compressor 22 via a fluid passageway 24 and configured to regulate the flow of atmospheric air to power source 10. Induction valve 20 may be a spool valve, a shutter valve, a butterfly valve, a check valve, a diaphragm valve, a gate valve, a shuttle valve, a ball valve, a globe valve, or any other valve known in the art. Induction valve 20 may be solenoid actuated, hydraulically actuated, pneumatically actuated, or actuated in any other manner. Induction valve 20 may be in communication with a controller (not shown) and selectively actuated in response to one or more predetermined conditions.
Compressor 22 may be configured to compress the air flowing into power source 10 to a predetermined pressure. Compressor 22 may be fluidly connected to power source 10 via a fluid passageway 26. Compressor 22 may include a fixed geometry type compressor, a variable geometry type compressor, or any other type of compressor known in the art. It is contemplated that more than one compressor 22 may be included and disposed in parallel or in series relationship. It is further contemplated that compressor 22 may be omitted, when a non-pressurized air induction system is desired.
Exhaust system 16 may be configured to direct exhaust flow out of power source 10. Exhaust system 16 may include a first particulate filter 28, a turbine 30, and a second particulate filter 32. It is contemplated that additional emission controlling devices may be included within exhaust system 16.
First particulate filter 28 may be connected to power source 10 via a fluid passageway 34 and to turbine 30 via a fluid passageway 36. First particulate filter 30 may include electrically conductive coarse mesh elements that have been sintered together under pressure. The mesh elements may include an iron-based material such as, for example, Fecralloy®. It is contemplated that mesh elements may also be implemented that are formed from an electrically-conductive material other than Fecralloy® such as, for example, a nickel-based material such as Inconel® or Hastelloy®, or another material known in the art. It is further contemplated that first particulate filter 28 may, alternately, include electrically non-conductive coarse mesh elements such as, for example, porous elements formed from a ceramic material or a high-temperature polymer.
First particulate filter 28 may include coarse mesh elements to reduce back-flow restriction within power source 10 that may adversely affect performance of power source 10. The mesh size of first particulate filter 28 may be such that the particulate-trapping efficiency of first particulate filter 28 is about 40% or less. It is contemplated that first particulate filter 28 may alternately have a particulate-trapping efficiency greater than 40%.
First particulate filter 28 may include either a catalyst to catalyze the particulate matter trapped by first particulate filter 28 (which may reduce an ignition temperature of the particulate matter), a means for regenerating the particulate matter trapped by first particulate filter 28, or both a catalyst and a means for regenerating. Because the catalyst included within first particulate filter 28 is immediately fluidly connected to power source 10, the catalyst may experience high temperatures that support reduction of hydrocarbons (HC), carbon dioxide (CO), and/or particulate matter. The catalyst may include, for example, a base metal oxide, a molten salt, and/or a precious metal that catalytically reacts with HC, CO, and/or particulate matter. The means for regeneration may include, among other things, a fuel-powered burner, an electrically resistive heater, an engine control strategy, or any other means for regenerating known in the art.
Turbine 30 may be connected to compressor 22 and configured to drive compressor 22. In particular, as the hot exhaust gases exiting power source 10 expand against the blades (not shown) of turbine 30, turbine 30 may rotate and drive connected compressor 22. It is contemplated that more than one turbine 30 may be included within exhaust system 16 and disposed in parallel or in series relationship. It is also contemplated that turbine 30 may, alternately, be omitted and compressor 22 be driven by power source 10 mechanically, hydraulically, electrically, or in any other manner known in the art.
In contrast to first particulate filter 28, second particulate filter 32 may be disposed downstream of turbine 30. Specifically, second particulate filter 32 may be fluidly connected to turbine 30 via a fluid passageway 38. Similar to first particulate filter 28, second particulate filter 32 may include electrically conductive mesh elements that have been sintered together under pressure. The mesh elements may include an iron-based material such as, for example, Fecralloy®. It is contemplated that mesh elements may also be implemented that are formed from an electrically-conductive material other than Fecralloy® such as, for example, a nickel-based material such as Inconel® or Hastelloy®, or another material known in the art. It is further contemplated that second particulate filter 32 may, alternately, include electrically non-conductive mesh elements such as, for example, porous elements formed from a ceramic material or a high-temperature polymer.
Second particulate filter 32 may include mesh elements having a smaller mesh size than the mesh elements of first particulate filter 28. The mesh size of second particulate filter 32 may be such that the particulate-trapping efficiency of second particulate filter 32 is about 80% or more. It is contemplated that the particulate-trapping efficiency of second particulate filter 32 may alternately be less than 80%.
Similar to first particulate filter 28, second particulate filter 32 may include either a catalyst, which may reduce an ignition temperature of the particulate matter trapped by second particulate filter 32, a means for regenerating the particulate matter trapped by second particulate filter 32, or both a catalyst and a means for regenerating. The catalyst may support reduction of HC, CO, and/or particulate matter. The catalyst may include, for example, a base metal oxide, a molten salt, and/or a precious metal. The means for regeneration may include, among other things, a fuel-powered burner, an electrically resistive heater, an engine control strategy, or any other means for regenerating known in the art.
Recirculation system 18 may be configured to redirect a portion of the exhaust flow of power source 10 from exhaust system 16 into air induction system 14. Recirculation system 18 may include an inlet port 40, a recirculation particulate filter 42, a cooler 44, a recirculation valve 46, and a discharge port 48.
Inlet port 40 may be connected to exhaust system 16 and configured to receive at least a portion of the exhaust flow from power source 10. Specifically, inlet port 40 may be disposed downstream from filter 28 and turbine 30 and upstream from second particulate filter 32. It is contemplated that inlet port 40 may be located elsewhere within exhaust system 16.
Recirculation particulate filter 42 may be connected to inlet port 40 via a fluid passageway 50 and configured to remove particulates from the portion of the exhaust flow directed through inlet port 40. Similar to first and second particulate filters 28, 32, recirculation particulate filter 42 may include electrically conductive coarse mesh elements that have been sintered together under pressure. The mesh elements may include an iron-based material such as, for example, Fecralloy®. It is contemplated that mesh elements may also be implemented that are formed from an electrically-conductive material other than Fecralloy® such as, for example, a nickel-based material such as Inconel® or Hastelloy®, or another material known in the art. It is further contemplated that recirculation particulate filter 42 may, alternately, include electrically non-conductive coarse mesh elements such as, for example, porous elements formed from a ceramic material or a high-temperature polymer.
Similar to first and second particulate filters 28, 32, recirculation particulate filter 42 may include either a catalyst, which may reduce an ignition temperature of the particulate matter trapped by recirculation particulate filter 42, a means for regenerating the particulate matter trapped by recirculation particulate filter 42, or both a catalyst and a means for regenerating. The catalyst may support reduction of HC, CO, and/or particulate matter. The catalyst may include, for example, a base metal oxide, a molten salt, and/or a precious metal. The means for regeneration may include, among other things, a fuel-powered burner, an electrically resistive heater, an engine control strategy, or any other means for regenerating known in the art. It is contemplated that recirculation particulate filter 42 may be omitted, if desired.
Cooler 44 may be fluidly connected to recirculation particulate filter 42 via a fluid passageway 52 and configured to cool the portion of the exhaust flowing through inlet port 40. Cooler 44 may include a liquid-to-air heat exchanger, an air-to-air heat exchanger, or any other type of heat exchanger known in the art for cooling an exhaust flow. It is contemplated that cooler 44 may be omitted, if desired.
Recirculation valve 46 may be fluidly connected to cooler 44 via fluid passageway 54 and configured to regulate the flow of exhaust through recirculation system 18. Recirculation valve 46 may be a spool valve, a shutter valve, a butterfly valve, a check valve, a diaphragm valve, a gate valve, a shuttle valve, a ball valve, a globe valve, or any other valve known in the art. Recirculation valve 46 may be solenoid actuated, hydraulically actuated, pneumatically actuated, or actuated in any other manner. Recirculation valve 46 may be in communication with a controller (not shown) and selectively actuated in response to one or more predetermined conditions.
A flow characteristic of recirculation valve 46 may be related to a flow characteristic of induction valve 20. Specifically, recirculation valve 46 and induction valve 20 may both be controlled such that an amount of exhaust flow entering air induction system 14 via recirculation valve 46 may be related to an amount of air flow entering air induction system 14 via induction valve 20. For example, as the flow of exhaust through recirculation valve 46 increases, the flow of air through induction valve 20 may proportionally decrease. Likewise, as the flow of exhaust through recirculation valve 46 decreases, the flow of air through induction valve 20 may proportionally increase.
Discharge port 48 may be fluidly connected to recirculation valve 46 via a fluid passageway 56 and configured to direct the exhaust flow regulated by recirculation valve 46 into air induction system 14. Specifically, discharge port 48 may be connected to air induction system 14 upstream of compressor 22, wherein compressor 22 draws the exhaust flow from discharge port 40.

INDUSTRIAL APPLICABILITY

The disclosed exhaust treatment system may be applicable to any combustion-type device such as, for example, an engine, a furnace, or any other device known in the art where the recirculation of reduced-particulate gas into an air induction system is desired. Exhaust treatment system 12 may be a simple, inexpensive, and compact solution to reducing the amount of exhaust emissions discharged to the environment while protecting the combustion-type device from harmful particulate matter and/or poor performance caused by the particulate matter. The operation of exhaust treatment system 12 will now be explained.
Atmospheric air may be drawn into air induction system 14 via induction valve 20 to compressor 22 where it may be pressurized to a predetermined level before entering the combustion chamber of power source 10. Fuel may be mixed with the pressurized air before or after entering the combustion chamber. This fuel-air mixture may then be combusted by power source 10 to produce mechanical work and an exhaust flow containing gaseous compounds and solid particulate matter. The exhaust flow may be directed via fluid passageway 34 from power source 10 through first particulate filter 28, where a portion of the particulate matter entrained with the exhaust may be filtered out of the exhaust flow. Because first particulate filter 28 includes coarse mesh elements that may remove about 40% or less of the total particulate matter produced by power source 10, the increased back pressure due to first particulate filter 28 may be minimal.
The particulate matter, when deposited on the coarse mesh elements of first particulate filter 28 may be passively and/or actively regenerated. When passively regenerated, the particulate matter deposited on the coarse mesh elements may chemically react with a catalyst included within first particulate filter 28 to lower the ignition temperature of the particulate matter. Because first particulate filter 28 is located immediately downstream of the exhaust flow from power source 10, the temperatures of the exhaust flow entering first particulate filter 28 may be high enough, in combination with the catalyst, to facilitate passive regeneration. When actively regenerated, heat may be applied to the particulate matter deposited on the coarse mesh elements to elevate the temperature of the particulate matter to the ignition temperature of the trapped particulate matter. A combination of passive and active regeneration may include both catalytically lowering the ignition temperature of the particulate matter and applying heat to the mesh elements.
In addition to the particulate matter within the exhaust flow, HC and CO may also be partially catalyzed within first particulate filter 28. The high temperature exhaust being immediately directed to the catalyst of first particulate filter 38 may provide for sufficient catalytic conditions.
The flow of partially filtered exhaust from first particulate filter 28 coupled together with expansion of the hot exhaust gasses may cause turbine 30 to rotate, thereby rotating compressor 22 and compressing the inlet air. After exiting turbine 30, the exhaust gas flow may be divided into two flows, a first flow redirected to air induction system 14 and a second flow directed to second particulate filter 32.
As the exhaust flows through inlet port 40 of recirculation system 18, it may be filtered by recirculation filter 42 to remove additional particulate matter prior to communication with cooler 44. The particulate matter, when deposited on the mesh elements of recirculation particulate filter 42, may be passively and/or actively regenerated.
The flow of the reduced-particulate exhaust flow from recirculation particulate filter 42 may be cooled by cooler 44 to a predetermined temperature and then directed through recirculation valve 46 to be drawn back into air induction system 14 by compressor 22. The recirculated exhaust flow may then be mixed with the air entering the combustion chamber. As described above, the exhaust gas, which is directed to the combustion chamber, reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature slows the chemical reaction of the combustion process, thereby decreasing the formation of nitrous oxides. In this manner, the gaseous pollution produced by power source 10 may be reduced without experiencing the harmful effects and poor performance caused by excessive particulate matter being directed into power source 10.
The ratio of cooled and reduced-particulate exhaust from recirculation system 18 relative to inlet air may be regulated by recirculation valve 46 and induction valve 20. As described above, the flow position of recirculation valve 46 and induction valve 20 may be related. As the flow of inlet air into power source 10 via induction valve 20 increases, the flow of cooled reduced-particulate exhaust into power source 10 decreases. Similarly, as the flow of inlet air into power source 10 via induction valve 20 decreases, the flow of cooled reduced-particulate exhaust into power source 10 increases.
As the second flow of exhaust leaves turbine 30, it may be filtered by second particulate filter 32 to remove additional particulate matter. Similar to first particulate filter 28 and recirculation filter 42, second particulate filter 32 may also be passively and/or actively regenerated to reduce the amount of HC, CO, and/or particulate matter exhausted to the atmosphere.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed 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 for a power source, comprising:

an air induction system;

an exhaust system, including:

a first particulate filter; and

a second particulate filter disposed in series with the first particulate filter; and

a recirculation system configured to draw at least a portion of an exhaust flow from between the first particulate filter and the second particulate filter and to direct the at least a portion of the exhaust flow to the air induction system.

2. The exhaust treatment system of claim 1, wherein the recirculation system further includes a recirculation valve configured to regulate the flow of the at least a portion of the exhaust flow.

3. The exhaust treatment system of claim 2, wherein the air induction system includes an induction valve configured to regulate the flow of inlet air into the air induction system.

4. The exhaust treatment system of claim 3, wherein a flow characteristic of the recirculation valve is related to a flow characteristic of the induction valve.

5. The exhaust treatment system of claims 1, wherein the recirculation system further includes a recirculation particulate filter configured to remove particulates from the at least a portion of the exhaust flow.

6. The exhaust treatment system of claim 5, wherein the recirculation particulate filter includes a catalyst.

7. The exhaust treatment system of claim 5, wherein the recirculation particulate filter includes a means for active regeneration.

8. The exhaust treatment system of claim 1, wherein the first particulate filter includes a coarse mesh.

9. The exhaust treatment system of claim 1, wherein the second particulate filter has a greater particulate trapping efficiency than the first particulate filter.

10. The exhaust treatment system of claim 1, wherein at least one of the first and second particulate filters includes a catalyst.

11. The exhaust treatment system of claim 1, wherein at least one of the first and second particulate filters includes a means for active regeneration.

12. The exhaust treatment system of claim 1, wherein:

the air induction system further includes at least one compressor;

the exhaust system further includes at least one turbine connected to the at least one compressor and disposed downstream of the first particulate filter; and

the recirculation system draws the at least a portion of an exhaust flow from downstream of the at least one turbine and directs at least a portion of the exhaust flow to the air induction system upstream of the at least one compressor.

13. A method of operating an exhaust treatment system for a power source, the method comprising:

directing air into the power source;

directing an exhaust flow away from the power source;

filtering out particulates entrained within the exhaust flow with a first particulate filter;

directing a first portion of the exhaust flow from downstream of the first particulate filter into the power source;

directing a second portion of the exhaust flow from downstream of the first particulate filter to a second particulate filter; and

filtering particulates out of the exhaust flow with the second particulate filter.

14. The method of claim 13, further including regulating the flow of the first portion of the exhaust flow directed into the power source.

15. The method of claim 14, further including regulating the flow of air into the power source.

16. The method of claim 15, wherein a flow characteristic of the air regulated into the power source is related to a flow characteristic of the first portion of the exhaust flow regulated into the power source.

17. The method of claim 16, further including filtering the first portion of the exhaust flow with a recirculation particulate filter.

18. The method of claim 17, further including passively regenerating the recirculation particulate filter with a catalyst.

19. The method of claim 17, further including actively regenerating the recirculation particulate filter.

20. The method of claim 13, wherein the second particulate filter has a greater particulate trapping efficiency than the first particulate filter.

21. The method of claim 13, further including passively regenerating at least one of the first and second particulate filters with a catalyst.

22. The method of claim 13, further including actively regenerating at least one of the first and second particulate filters.

23. An exhaust treatment system for a power source, comprising:

an air induction system, including at least one compressor;

an exhaust system, including:

a particulate filter; and

at least one turbine connected to the at least one compressor and disposed downstream of the particulate filter; and

a recirculation system configured to draw at least a portion of an exhaust flow from downstream of the at least one turbine and to direct the at least a portion of an exhaust flow to the air induction system upstream of the at least one compressor.

24. The exhaust treatment system of claim 23, wherein the recirculation system further includes a recirculation valve configured to regulate the flow of the at least a portion of an exhaust flow into the power source.

25. The exhaust treatment system of claim 24, wherein the air induction system includes an induction valve configured to regulate the flow of inlet air into the power source.

26. The exhaust treatment system of claim 25, wherein a flow characteristic of the recirculation valve is related to a flow characteristic of the induction valve.

27. The exhaust treatment system of claims 23, wherein the recirculation system further includes a recirculation particulate filter configured to remove particulates from the at least a portion of an exhaust flow.

28. The exhaust treatment system of claim 27, wherein the recirculation particulate filter includes a catalyst.

29. The exhaust treatment system of claim 27, wherein the recirculation particulate filter includes a means for active regeneration.

30. The exhaust treatment system of claim 23, wherein the particulate filter includes a coarse mesh.

31. The exhaust treatment system of claim 23, wherein the particulate filter includes a catalyst.

32. The exhaust treatment system of claim 23, wherein the particulate filter includes a means for active regeneration.

33. A power system, comprising:

a power source operable to produce an exhaust flow;

an air induction system configured to direct an airflow into the power source, the air induction system including:

an induction valve configured to regulate the flow of inlet airflow into the power source; and

at least one compressor configured to pressurize the airflow;

an exhaust system, including:

a first particulate filter having a coarse mesh;

a second particulate filter disposed in series with the first particulate filter and having a greater particulate trapping efficiency than the first particulate filter, at least one of the first and second particulate filters including a catalyst; and

at least one turbine connected to the at least one compressor and disposed downstream of the first particulate filter; and

a recirculation system configured to draw at least a portion of the exhaust flow from between the first particulate filter and the second particulate filter downstream of the at least one turbine and to direct the at least a portion of the exhaust flow to the air induction system upstream of the at least one compressor, the recirculation system including a recirculation valve configured to regulate the flow of the at least a portion of the exhaust flow, wherein a flow characteristic of the recirculation valve is related to a flow characteristic of the induction valve.

34. The exhaust treatment system of claims 33, wherein the recirculation system further includes a recirculation particulate filter configured to remove particulates from the at least a portion of the exhaust flow.

35. The exhaust treatment system of claim 34, wherein the recirculation particulate filter includes a catalyst.

36. The exhaust treatment system of claim 34, wherein the recirculation particulate filter includes a means for active regeneration.

37. The exhaust treatment system of claim 33, wherein at least one of the first and second particulate filters include a means for active regeneration.