US20100199634A1

US20100199634A1 - Exhaust treatment system implementing selective doc bypass

Info

Publication number: US20100199634A1
Application number: US12/598,417
Authority: US
Inventors: David Mark Heaton
Original assignee: Perkins Engines Co Ltd
Current assignee: Perkins Engines Co Ltd
Priority date: 2007-05-02
Filing date: 2007-05-02
Publication date: 2010-08-12
Also published as: WO2008135805A1; EP2142771A1

Abstract

An exhaust treatment system for use with a power system is disclosed. The exhaust treatment system may have an SCR device (32), and an oxidation device (26) located upstream of the SCR device (32) to convert NO to NO2. The exhaust treatment system may also have an exhaust passageway (14) extending from an exhaust source (10) to the oxidation device (26), and a bypass passageway (24) extending from the exhaust passageway at a location upstream of the oxidation device to the exhaust passageway at a location downstream of the oxidation device (26). The exhaust treatment system may further have a valve element (20) configured to selectively direct exhaust from the exhaust source (10) through the oxidation device (26) and through the bypass passageway (24), at least one sensor configured to sense operating parameters of the exhaust source (10), and a controller (36) in communication with the valve element (20). The controller (36) may be configured to move the valve element (20) in response to an estimated ratio of NO to NO2 based on sensed operating parameters of the exhaust source (10).

Description

TECHNICAL FIELD

The present disclosure is directed to an exhaust treatment system and, more particularly, to an exhaust treatment system that implements selective Diesel Oxidation Catalyst (DOC) bypass.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H₂O and N₂. Although SCR can be effective, it is most effective when a concentration of NO to NO₂supplied to the SCR is about 1:1. In order to achieve this optimum ratio, a Diesel Oxidation Catalyst (DOC) is often located upstream of the SCR to convert NO to NO₂.
In addition to facilitating the reduction process of the SCR, the produced NO₂also facilitates the combustion of particulate matter. Specifically, a particulate trap is commonly used to collect unburned particulates also known as soot. Over time, the particulate matter builds up in the trap and, if left unchecked, the particulate trap could negatively affect performance of the engine. As such, the particulate matter collected by the trap must be periodically removed through a process called regeneration. To regenerate the particulate trap, a liquid catalyst (typically diesel fuel) is injected into the exhaust flow upstream of the trap. The fuel, in the presence of NO₂, ignites and burns away the particulate matter.
During operation of an associated engine, it may be desirable to selectively divert exhaust away from the DOC (i.e., bypass the DOC). For example, during some engine operating conditions, the ratio of NO to NO₂may naturally be about 1:1. In this situation, if all of the exhaust is passed through the DOC, the ratio of NO to NO₂could actually exceed the desired 1:1 ratio and reduce the effectiveness of the SCR process. Thus, under some conditions, the exhaust flow can be directed to bypass the DOC. In another example, the DOC is only necessary during trap regeneration events. In this example, in order to conserve the DOC, the exhaust flow can again be directed to bypass the DOC during non-regeneration events.
One system implementing DOC bypass is described in Japanese Laid-Open Patent Application JP 2005-2968 A (the '968 publication) by Mitsubishi Fuso, published Jan. 6, 2005. The '968 publication discloses an exhaust gas purifying system having an oxidation catalyst, a temperature sensor, a bypass path, a bypass switching device, and an SCR device. The system is designed to create a 1:1 ratio of NO:NO₂in an exhaust flow. The system estimates a ratio of NO:NO₂based on a sensed exhaust gas temperature. The oxidation catalyst converts NO to NO₂, and the SCR device converts NO and NO₂to N₂in the presence of ammonia. The SCR device operates most efficiently when the ratio of NO:NO₂is 1:1. At a certain temperature, above which the ratio of NO:NO₂in the exhaust is estimated to be 1:1, the bypass switching device diverts exhaust flow so that the exhaust gas flows through the bypass path, and not through the oxidation catalyst. In this way, the system aims to prevent excessive NO₂in the exhaust gas flow. The '968 publication also discloses the possibility of employing a NOx sensor to directly sense a NO:NO₂ratio.
Although the exhaust gas purifying apparatus of the '968 publication may disclose a method of operation that aims to achieve a 1:1 ratio of NO to NO₂in an exhaust flow, it may be limited. For example, there currently are no commercially available NOx sensors that satisfy desired performance requirements to measure NOx effectively and quickly enough to provide real-time control over NO:NO₂ratio in an exhaust flow. Additionally, estimating an NO:NO₂ratio based on a measurement of exhaust gas temperatures and other exhaust parameters may not accurately reflect the NO:NO₂ratio, because, for example, changes in engine operating parameters may cause changes in an exhaust gas NO:NO₂ratio, but not an exhaust gas temperature. Similarly, changes in engine operating parameters may cause changes in exhaust gas temperature but not in an exhaust gas NO:NO₂ratio.
The system of the present disclosure solves one or more of the problems set forth above.

SUMMARY OF THE DISCLOSURE

One aspect of the present disclosure is directed to an exhaust treatment system. The exhaust treatment system may include an SCR device, and an oxidation device located upstream of the SCR device to convert NO to NO₂. The exhaust treatment system may also include an exhaust passageway extending from an exhaust source to the oxidation device, and a bypass passageway extending from the exhaust passageway at a location upstream of the oxidation device to the exhaust passageway at a location downstream of the oxidation device. The exhaust treatment system may further include a valve element configured to selectively direct exhaust from the exhaust source through the oxidation device and through the bypass passageway, at least one sensor configured to sense operating parameters of the exhaust source, and a controller in communication with the valve element. The controller may be configured to move the valve element in response to an estimated ratio of NO to NO₂based on sensed operating parameters of the exhaust source.
Another aspect of the present disclosure is directed to a method of treating exhaust. The method may include generating a flow of exhaust, treating at least a portion of the flow of exhaust by a catalyst, and directing the flow of exhaust through an SCR device. The method may also include estimating a ratio of NO to NO₂in the flow of exhaust based on sensed operating parameters of a power source that generates the flow of exhaust, and changing an amount of the at least a portion in response to the estimation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and diagrammatic illustration of an exemplary disclosed power system; and

FIG. 2 is a flowchart depicting an exemplary disclosed operation of the power system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates a power source 10 having an exemplary embodiment of an exhaust treatment system 12. Power source 10 may include an 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. Power source 10 may also include any non-engine source of power, such as a furnace. Power source 10 may combust a mixture of air and fuel to produce a power output and an exhaust gas flow. The exhaust gas flow from power source 10 may be diverted through exhaust treatment system 12.
Exhaust treatment system 12 may include components that cooperate, by way of a main passageway 14, to treat the exhaust gas flow from power source 10. In particular, exhaust treatment system 12 may include a diesel oxidation catalyst (DOC) 26, an SCR device 32, and a urea injection unit 30. Exhaust treatment system 12 may also include a bypass circuit having a bypass valve 20 and bypass passageway 24.
DOC 26 may be located within main passageway 14 and include a porous ceramic honeycomb-like or metal mesh substrate. The substrate may be coated with a material such as, for example, a precious metal, that catalyzes a chemical reaction to alter the chemical composition of exhaust gas. For example, DOC 26 may include platinum or vanadium to facilitate the conversion of NO constituents into NO₂, which may be more susceptible to catalytic treatment in SCR device 32.
SCR device 32 may be disposed in main passageway 14 downstream of DOC 26. SCR device 32 may chemically reduce NOx into N₂in the presence of a catalyst such as ammonia or urea. Efficiency of NOx reduction by SCR device 32 may be at least partially dependent on the ratio of NO to NO₂in the exhaust. In particular, NOx reduction by SCR device 32 may be most efficient when the ratio of NO to NO₂in the exhaust is about 1:1. In a lean gas flow, a lean NOx SCR device 32 may need reductants for the chemical reaction and may utilize a reductant injector to introduce the reductant into the lean gas flow. Reductants employed may be diesel fuel, ethanol, blended fuels, or any other reductant known in the art. SCR device 32 may include a catalyst support material and a metal promoter dispersed within the catalyst support material. The catalyst support material may include at least one of alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. The catalyst support material may also include at least one of alumina and zeolite, and the metal promoter may include silver metal (Ag). Combinations of these materials may be used, and the catalyst material may be chosen based on the type of fuel used, the ethanol additive used, the air to fuel-vapor ratio desired, and/or for conformity with environmental standards. One of ordinary skill in the art will recognize that numerous other catalyst compositions may be used without departing from the scope of this disclosure. More than one SCR device 32 may be included in main passageway 14.
Urea injection unit 30 may be located adjacent to or upstream of SCR device 32 to inject urea directly into SCR device 32 and/or into main passageway 14. The injected urea may be broken down into ammonia, which may be retained within SCR device 32. The ammonia stored in SCR device 32 may be used to reduce the amount of NO_xin the exhaust gases passing through SCR device 32 by converting NO₂to N₂. Alternatively or additionally, other agents suitable for reducing NO_xmay be injected into main passageway 14 and/or SCR device 32.
Bypass valve 20 may be fluidly connected to main passageway 14 and bypass passageway 24 at a point upstream of DOC 26. Bypass valve 20 may be any commonly known three-way valve capable of directing flow in variable proportion between two separate passageways (i.e. between main passageway 14 and bypass passageway 24). Alternatively, bypass valve 20 may be a two-way valve (not shown) located within bypass passageway 24. Bypass valve 20 may include a valve element 22 configured to control the amount of exhaust gas delivered to DOC 26. In particular, valve element 22 may be movable between a first, “open” position, at which substantially all of the exhaust gas flow from power source 10 is directed to flow through bypass passageway 24, toward a second, “closed” position, at which all of the exhaust gas flow from power source 10 is directed to flow through DOC 26. Valve element 22 may also be positioned at any intermediate position between the open and closed positions, to direct portions of the exhaust gas flow to both DOC 26 and bypass passageway 24. Valve element 22 may include a spool valve element, a ball valve element, a globe valve element, a butterfly valve element, or any other suitable type of valve element known in the art. Bypass valve 20 may include means for automatically moving valve element 22 in response to a control signal. Bypass passageway 24 may extend from main passageway 14 a point upstream of DOC 26 to main passageway 14 at a point downstream of DOC 26, and may provide an alternate path for exhaust flow from power source 10.
A control system 34 may regulate the operation of bypass valve 20 in response to one or more inputs. In particular, control system 34 may include a controller 36 that communicates with bypass valve 20 by way of a communication line 40, and with sensor 38 by way of a communication line 42. In response to an input from sensor 38, and/or from other sources such as power source 10 and/or DOC 26, controller 36 may adjust a setting of valve element 22.
Controller 36 is shown in FIG. 1 as a single controller, and it may include one or more microprocessors that include a means for controlling an operation of exhaust treatment system 12. Alternatively, controller 36 may be one or more controllers, each assigned to control a subsystem, and in communication with each other, for example a controller configured to control power source 10, and a separate controller configured to control exhaust treatment system 12. Numerous commercially available microprocessors may be configured to perform the functions of controller 36. It should be appreciated that controller 36 may alternatively embody a general engine control unit (ECU) capable of controlling numerous functions, including power source 10 and exhaust treatment system 12. Controller 36 may include all of the 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 for controlling bypass valve 20 and sensor 38. Various other known circuits may be associated with controller 36, including power supply circuitry, signal-conditioning circuitry, solenoid driver circuitry, communication circuitry, and other appropriate circuitry.
Controller 36 may receive and store in memory communication from various sensors and components commonly known in the art, such as, for example, sensor 38, including measurements of, for example, exhaust gas NOx composition and concentrations, power source fuel/air settings, power source operating speed, power source load, power source fuel injection profile, other power source operating parameters, and/or DOC 26 operating temperature. Controller 36 may analyze and compare received and stored data, and, based on instructions and data stored in memory or input by a user, determine whether action is required. For example, controller 36 may compare received values with target values stored in memory, and, based on the results of the comparison, controller 36 may transmit signals to adjust bypass valve 20.
Controller 36 may include memory means known in the art for storing data relating to engine operation. The data may be stored in the form of one or more maps that describe relationships between various power source 10 and/or DOC 26 operating parameters and resulting power source 10 exhaust gas compositions. Each of these maps may be in the form of tables, graphs, and/or equations, and include a compilation of data collected from lab and/or field operation of power source 10 and DOC 26. These maps may be generated by performing instrumented tests on the operation of power source 10 and DOC 26 under a variety of operating conditions, while varying parameters such as power source fuel/air settings, power source operating speed, power source load, power source fuel injection profile, other power source operating parameters, and while measuring DOC 26 operating temperature and exhaust gas NO:NO₂ratio. Data from the tests may be logged, and may show correlation, for example, among one or more power source and/or DOC operating parameters and exhaust gas NOx composition, including a NO:NO₂ratio. Additionally, controller 36 may be capable of updating the maps based on measured operating conditions of power source 10 and DOC 26, which may allow controller 36 to adjust the maps to match the particular operating characteristics and modes of an individual power source 10 and DOC 26. Controller 36 may reference these maps and control the position of bypass valve 20 to bring the operation of exhaust treatment system 12 in line with desired values.
Controller 36 may also contain one or more virtual models of exhaust treatment system 12. A virtual model may contain information such as tables, graphs, and/or equations, and include a compilation of data collected from lab and/or field operation of exhaust treatment system 12. The virtual model may contain data correlating exhaust gas NO:NO₂ratio as reported by sensor 38, bypass valve 20 setting, operating parameters of power source 10 and/or DOC 26, and an expected exhaust gas NO:NO₂ratio downstream of DOC 26. A virtual model may enable controller 36 to determine, based on sensed exhaust gas NO:NO₂ratio, power source 10 operating parameters, and/or DOC 26 operating parameters, a setting of valve element 22 that will produce a desired exhaust gas NO:NO₂ratio downstream of DOC 26. Controller 36 may use a virtual model in an open loop mode of operation of exhaust treatment system 12, as described below.
Sensor 38 may be associated with main passageway 14. Sensor 38 is shown, for example, downstream of DOC 26. One skilled in the art will recognize, however, that sensor 38 may alternatively or additionally include sensing elements associated with, for example, power source 10, DOC 26, and SCR device 32. Sensor 38 may directly sense a concentration of NO and NO₂in an exhaust gas flow, and generate a signal in response thereto. Alternatively, sensor 38 may sense a NO:NO₂ratio of an exhaust gas flow and generate a ratio signal in response thereto. Sensor 38 may be any type of sensor commonly known in the art for sensing NO and NO₂composition.
It is contemplated that sensor 38 may alternatively embody both physical sensors and a virtual sensor, included in controller 36, that generates a signal based on a map-driven estimate. Such a virtual sensor may include one or more physical sensing elements associated with, for example, power source 10 and/or DOC 26. Physical sensing elements may detect and communicate to controller 36 parameters including, for example power source fuel/air settings, power source operating speed, power source load, power source fuel injection profile, other power source operating parameters, and/or DOC 26 operating temperature. Virtual sensor 38 may evaluate the signals received from various physical sensors, and, using relationships contained within one or more maps stored in a memory of controller 36, estimate the expected exhaust gas NO:NO₂ratio based on the sensed parameters.
Controller 36 may monitor and regulate valve element 22 of bypass valve 20 to control the amount of exhaust gas delivered to DOC 26. Controller 36 may monitor an actual exhaust gas NOx composition via sensor 38 to determine a NO:NO₂ratio, and then adjust valve element 22 to deliver an amount of exhaust gas to DOC 26 necessary to provide SCR device 32 with exhaust gas having a desired ratio of NO:NO₂. In one embodiment, the desired ratio may be 1:1. Alternatively, if using a virtual sensor 38, controller 36 may monitor operating parameters of power source 10 and/or DOC 26, use the maps stored in memory to estimate an exhaust gas NO:NO₂ratio based on the sensed operating parameters, and then adjust valve element 22 to deliver an amount of exhaust gas to DOC 26 necessary to provide SCR device 32 with exhaust gas having a desired NO:NO₂ratio. In one embodiment, the desired ratio may be 1:1.
Controller 36 may control a NO:NO₂ratio using either a closed or open loop scheme. In closed loop operation, controller 36 may measure a NO:NO₂ ratio using sensor 38, determine that the ratio is too low, adjust valve element 22 toward its open position to direct more exhaust gas flow to bypass passageway 24, and then measure a NO:NO₂ratio again. If controller 36 determines the ratio is still too low, controller 36 may open bypass valve 20 further and then measure a NO:NO₂ratio again, continuing until the desired NO:NO₂ratio is obtained. Alternatively, controller 36 may measure a NO:NO₂ ratio using sensor 38, determine that the ratio is too high, adjust valve element 22 toward its closed position to direct more exhaust gas flow to DOC 26, and then measure a NO:NO₂ratio again. If controller 36 determines the ratio is still too high, controller 36 may close bypass valve 20 further and then measure a NO:NO₂ratio again, continuing until the desired NO:NO₂ratio is obtained.
In open loop operation, controller 36 may measure a NO:NO₂ ratio using sensor 38, compare that ratio to a desired NO:NO₂ratio, and then, based on a virtual model of exhaust treatment system 12 stored in memory of controller 36, adjust valve element 22 to a specific setting corresponding to the desired NO:NO₂ratio. For example, when sensor 38 reports a NO:NO₂ratio of 2:1, then controller 36 may use a virtual model of exhaust treatment system 12 to determine that valve element 22 should be moved to, for example, 25% bypass, so as to obtain a desired exhaust gas NO:NO₂ratio for SCR device 32.
FIG. 2 shows a flowchart illustrating an exemplary method of operating control system 34. FIG. 2 will be described in detail below.

INDUSTRIAL APPLICABILITY

The exhaust treatment system of the present disclosure may be applicable to any power source, including, for example, an engine or a furnace that benefits from reduced NOx emissions. In particular, the disclosed system may improve reduction of NOx by providing an approximately 1:1 mix of NO and NO₂to an associated SCR device. The operation of exhaust treatment system 12 will now be explained.
Referring to FIG. 1, air and fuel may be drawn into power source 10 for subsequent combustion. Fuel may be injected into power source 10, mixed with the air therein, and combusted by power source 10 to produce a mechanical work output and an exhaust gas flow. The exhaust gas flow may contain a complex mixture of air pollutants composed of gaseous material, which can include oxides of nitrogen (NOx). As this NOx laden exhaust gas flow is directed from power source 10 through exhaust treatment system 12, DOC 26 may modify a NOx composition of exhaust gas by converting NO to NO₂, and SCR device 32 may remove NO₂from the exhaust gas flow by conversion to N₂(Step 100).
During operation of power source 10, controller 36 may determine a ratio of NO:NO₂based on a measured NO and NO₂concentration (i.e. based on a signal from sensor 38) (Step 102). Alternatively, controller 36 may determine an exhaust gas NO:NO₂ratio by sensing operating parameters of power source 10 and/or DOC 26, and then compare the parameters with relationships stored in one or maps in controller 36 memory. For example, controller 36 may use a map as a lookup table to determine the ratio of NO:NO₂based on sensed power source fuel/air settings, power source operating speed, power source load, power source fuel injection profile, other power source operating parameters, and/or DOC 26 operating temperature.
Controller 36 may evaluate the ratio of NO:NO₂to determine a further course of action by comparing the sensed or determined NO:NO₂ratio with an expected or desired NO:NO₂ratio ( Steps 104 a and 104 b). For example, a desired NO:NO₂ratio may be 1:1. When controller 36 determines the ratio of NO:NO₂equals 1:1, then controller 36 continues to determine a ratio of NO:NO₂based on a measured NO and NO₂concentration (Step 104 a). When controller 36 determines that the ratio of NO:NO₂is greater than 1:1 (Step 104 b), then controller 36 may adjust valve element 22 toward its closed position to increase the amount of exhaust gas flowing through DOC 26. Controller 36 may decrease the amount of exhaust gas flowing through bypass passageway 24 until the ratio of NO:NO₂in the exhaust gas reaches about 1:1 (Step 106).
However, when controller 36 determines that the exhaust gas ratio of NO:NO₂is less than 1:1 (Step 104 b), controller 36 may adjust valve element 22 toward its open position to decrease the amount of exhaust gas flowing through DOC 26. Controller 36 may increase the amount of exhaust gas flowing through bypass passageway 24 until the ratio of NO:NO₂in the exhaust gas reaches about 1:1 (Step 108).
The disclosure sets forth ways in which exhaust treatment system 12 may continuously control the conversion of NO to NO₂in preparation for the SCR process. This control of an exhaust gas flow NO:NO₂ratio may allow controller 36 to maintain a NO:NO₂ratio at about 1:1. This optimal ratio may allow SCR device 32 to operate at maximum efficiency when converting NO₂to N₂. Estimation of a NO:NO₂ratio based on power source operating parameters and a map may provide a more accurate estimate of a NO:NO₂ratio, as well as a more rapid estimation of a NO:NO₂ratio that enables real-time control of a NO:NO₂ratio. A rapid, more accurate estimate of a NO:NO₂ratio may provide emissions from power source 10 that are better able to meet stringent standards.
It will be apparent to those skilled in the art that various modifications and variations can be made to the system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.

Claims

1. An exhaust treatment system, comprising:

an SCR device;

an oxidation device located upstream of the SCR device to convert NO to NO₂;

an exhaust passageway extending from an exhaust source to the oxidation device;

a bypass passageway extending from the exhaust passageway at a location upstream of the oxidation device to the exhaust passageway at a location downstream of the oxidation device;

a valve element configured to selectively direct exhaust from the exhaust source through the oxidation device and through the bypass passageway;

at least one sensor configured to sense operating parameters of the exhaust source; and

a controller in communication with the valve element, the controller being configured to move the valve element in response to an estimated ratio of NO to NO₂based on sensed operating parameters of the exhaust source.

2. The exhaust treatment system of claim 1, wherein the controller includes a map stored in a memory thereof relating the exhaust source operating parameters to an amount of NO and an amount of NO₂produced by the exhaust source.

3. The exhaust treatment system of claim 1, wherein the exhaust source operating parameters include at least one of fuel/air settings, operating speed, load, and fuel injection profile.

4. The exhaust treatment system of claim 1, wherein the valve element includes a three-way valve located at the junction of the bypass passageway and the exhaust passageway upstream from the oxidation device.

5. The exhaust treatment system of claim 1, wherein the valve element includes a two-way valve located within the bypass passageway.

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

the controller has stored in a memory thereof a virtual model of the exhaust treatment system; and

the valve element is moved based on the estimated ratio of NO to NO₂in the exhaust gas and the virtual model.

7. The exhaust treatment system of claim 1, wherein a greater amount of exhaust is directed through the DOC when the amount of NO₂in the exhaust is less than the amount of NO in the exhaust.

8. The exhaust treatment system of claim 7, wherein a greater amount of exhaust is directed through the bypass passageway when the amount of NO₂in the exhaust is greater than the amount of NO in the exhaust.

9. The exhaust treatment system of claim 1, wherein the oxidation device includes a substrate coated with a precious metal.

10. A method of treating exhaust, comprising:

generating a flow of exhaust;

treating at least a portion of the flow of exhaust by a catalyst;

directing the flow of exhaust through an SCR device;

estimating a ratio of NO to NO₂in the flow of exhaust based on sensed operating parameters of a power source that generates the flow of exhaust; and

changing an amount of the at least a portion in response to the estimation.

11. The method of claim 10, wherein estimating includes referencing a known relationship between power source operating parameters and the production of NO and NO₂.

12. The method of claim 11, wherein the power source operating parameters include at least one of fuel/air settings, operating speed, load, and fuel injection profile.

13. The method of claim 12, wherein changing an amount of the at least a portion includes estimating the amount of the at least a portion based on the ratio of NO to NO₂and the predicted behavior of the catalyst.

14. The method of claim 10, further including increasing the amount of the at least a portion when an amount of NO in the exhaust directed to the SCR device exceeds an amount of NO₂in the exhaust directed to the SCR device.

15. The method of claim 14, further including decreasing the amount of the at least a portion when the amount of NO₂in the exhaust directed to the SCR device exceeds the amount of NO in the exhaust directed to the SCR device.

16. A power system, comprising:

a power source configured to combust a fuel/air mixture and generate power and a flow of exhaust;

an SCR device;

an exhaust passageway fluidly communicating the power source with the SCR device;

an oxidation device to convert NO to NO₂located in the exhaust passageway between the power source and the SCR device;

a valve element configured to selectively direct exhaust from the power source through the oxidation device and through the bypass passageway;

a controller in communication with the valve element, the controller being configured to move the valve element in response to an estimated ratio of NO to NO₂based on sensed operating parameters of the power source.

17. The power system of claim 16, wherein the controller includes a map stored in a memory thereof relating power source operating parameters to an amount of NO and an amount of NO₂produced by the power source.

18. The power system of claim 17, wherein the power source operating parameters include at least one of fuel/air settings, operating speed, load, and fuel injection profile.

19. The power system of claim 16, wherein a greater amount of exhaust is directed through the DOC when an amount of NO₂in the exhaust is less than an amount of NO in the exhaust.

20. The power system of claim 19, wherein a greater amount of exhaust is directed through the bypass passageway when the amount of NO₂in the exhaust is greater than the amount of NO in the exhaust.