Systems And Methods To Mitigate Nox And Hc Emissions At Low Exhaust Temperatures

Abstract

Systems and methods are provided for managing low temperature NO.sub.x and HC emissions, such as during a cold start of an internal combustion engine. The systems and methods include storing NO.sub.x and HC emissions at low temperatures and passively releasing these emissions as the temperature of the exhaust system increases.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of the filing date of Provisional Application No. 61/650,722 filed on May 23, 2012, which is incorporated herein by reference.

BACKGROUND

[0002] Control of selective catalytic reduction (SCR) catalysts is of increasing interest to meet modern internal combustion engine emissions standards. The effectiveness of a typical SCR catalyst in removing oxides of nitrogen (NO.sub.x) emissions is sensitive to the temperature of the exhaust gas at the inlet to the SCR catalyst. Copper exchanged zeolite based SCR catalysts are formulated to operate satisfactorily over a fairly wide temperature range. However, current state of the art Cu-Zeolite formulations operate at peak efficiency when subjected to exhaust gas temperatures of 200-400.degree. C. For certification of certain diesel engines, the emissions performance of the engine during the cold portion of the certification cycle is weighted almost equally with the emissions performance of the engine during the hot portion of the certification cycle. For this reason, improvements in preventing hydrocarbon (HC) and NO.sub.x emissions produced by the engine from slipping through the aftertreatment system at low temperature exhaust conditions, such as at temperatures less than 200.degree. C., are desired.

[0003] Typical diesel A/T systems include a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) in addition to the SCR. The DOC is responsible for oxidation of hydrocarbons (HC), carbon monoxide (CO), and nitric oxide (NO). Similar to the SCR catalyst, the DOC is not able to effectively and efficiently oxidize these molecules at cold exhaust temperatures. In order to meet NO.sub.x and HC emissions levels (for example, 0.02 and 0.01 g/mile, respectively for Tier 2 Bin 2 federal certification) at low temperature conditions, improvements in aftertreatment designs are required to mitigate the slip of these criteria pollutants through the exhaust flowpath during low temperature operation. Accordingly, further technological developments in this area are desirable.

SUMMARY

[0004] One embodiment is a unique method and system for managing low temperature NO.sub.x and HC emissions to improve the NO.sub.x conversion efficiency of diesel aftertreatment systems under low exhaust temperature conditions. In one embodiment, there is provided a multiple component aftertreatment system that includes passively operated HC and NO.sub.x storage devices for improved low temperature mitigation of NO.sub.x and HC emissions to achieve desired emissions levels for light duty vehicles, although applications with other vehicles are not precluded. The systems and methods reduce the need for external devices intended to artificially increase the exhaust gas temperature for cold start cycles, which is beneficial since such external devices tend to increase fuel consumption and greenhouse gas emissions, although the use of such external devices are not precluded. The systems and methods disclosed herein provide reductions in cost and fuel consumption over current thermal management strategies to mitigate low temperature NO.sub.x and HC emissions. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is an exemplary system for reducing emissions of HC and NO.sub.x of an internal combustion engine during low exhaust temperature operating conditions.

[0006] FIG. 2 is a schematic of a controller comprising a portion of the system of FIG. 1.

[0007] FIG. 3 is a graph showing an estimated impact of a hydrocarbon storage device on HC slip flow over time during low temperature exhaust operating conditions.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0008] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein.

[0009] There is disclosed systems and methods for reduction in HC and NO.sub.x emissions at low exhaust temperature operating conditions for an internal combustion engine. The systems and methods reduce criteria pollutants at least during low exhaust temperature conditions. The disclosed systems and methods are configured so that the vehicles equipped therewith are operable to meet emissions standards during low exhaust temperature operating conditions without the need for external aftertreatment heating systems, which increase fuel consumption and greenhouse gas emissions from the vehicle, although the use of such external systems is not precluded. In one embodiment, the systems and methods have application for light duty certified chassis vehicles, although applications with other vehicle types are not precluded.

[0010] The systems and methods include an aftertreatment system architecture configured to temporarily store HC emissions and NO.sub.x emissions during periods of low exhaust temperature operation, and then passively release the stored NO.sub.x and HC emissions for aftertreatment as the exhaust temperature increases. The systems and methods are configured so that at temperatures where HC and NOx emissions are released, the diesel oxidation catalyst (DOC) and SCR catalysts are effective at mitigating the released HC and NO.sub.x emissions from the storage devices before exiting the tailpipe.

[0011] In one embodiment, the aftertreatment system includes a close coupled HC storage device (HCSD) located directly downstream of the turbocharger, with a close coupled NO.sub.x storage device (NSD) located directly downstream of the HCSD. At low exhaust temperatures, the HCSD readily adsorbs and stores HC. In one specific embodiment, the HCSD includes a catalyst, such as a zeolite-based catalyst, for storing and adsorbing HC. As the exhaust temperature increases, the HCSD effectively oxidizes HC in the gas phase that are stored on the surface of the HCSD to form H.sub.2O and CO.sub.2. The NSD is any suitable component, such as a NO.sub.x adsorber, capable of passively storing NO.sub.x at low exhaust temperature, and then releasing the stored NO.sub.x as the exhaust temperature increases. The NSD may also have an oxidation function, primarily for NO oxidation to NO.sub.2, but under normal operating conditions is not capable of effectively reducing NO.sub.x to N.sub.2 and H.sub.2O. The NSD relies on a downstream selective catalytic reduction (SCR) catalyst or other NO.sub.x reduction catalyst to chemically reduce the NO.sub.x to N.sub.2 and H.sub.2O.

[0012] In a further embodiment, systems and methods for reducing the emission of HC and NO.sub.x from lean burn internal combustion engines for low exhaust temperature operating conditions are disclosed. As shown in FIG. 1, an exemplary aftertreatment system 10 includes a close coupled HCSD 12 and NSD 14 followed by a standard SCR or NAC-type NO.sub.x reduction catalyst 16 to receive exhaust gas produced by an internal combustion engine 20 into exhaust flowpath 18. The HCSD 12 and NSD 14 are passively operated devices, which require little or no active control strategies, although the use of active control strategies is not precluded.

[0013] At low exhaust gas temperatures which result in low catalyst temperatures, the HCSD 12 readily adsorbs and stores HC on the surface of a catalyst until the HCSD 12 reaches a surface temperature where it can effectively oxidize the stored HC to form CO.sub.2 and H.sub.2O. The NSD 14 readily adsorbs and stores NO.sub.x on the surface of its catalyst under low exhaust temperature conditions, and then begins to desorb this NO as the exhaust temperature and therefore the NSD catalyst temperature increases. The NSD 14 is configured to release the stored NO.sub.x at an exhaust temperature where the reduction catalyst 16 is highly effective for reducing NO.sub.x to N.sub.2 and H.sub.2O. Once the aftertreatment system 10 reaches operating temperature, either or both of the catalysts of the HCSD 12 and the NSD 14 may operate as DOC catalysts, and together are responsible for the oxidation of HC, CO and NO.

[0014] System 10 may include a controller 100 and other aftertreatment components in addition to those shown in FIG. 1. For example, system 10 typically includes a reductant doser 17 operationally coupled to the exhaust conduit at a position upstream of the reduction catalyst 16. The reductant injected into exhaust flowpath 18 is any type of reductant utilized in a NO.sub.x reduction system. For example, the reductant can include at least ammonia (gaseous or aqueous) and urea. System 10 may also include a diesel particulate filter (DPF) forming, with the HCSD 12 and NSD 14, a DOC/DPF system positioned upstream of reduction catalyst 16 during engine operation where exhaust temperatures are effective for NO.sub.x reduction with reduction catalyst 16. The system 10 may include an ammonia oxidation catalyst (AMOX) downstream of the reduction catalyst 16. In certain embodiments, the AMOX may not be present, or the AMOX may be commingled with the reduction catalyst 16 (or the last SCR catalyst, where multiple SCR catalysts are present), for example with a washcoat applied toward the rear portion of the reduction catalyst 16 that is responsive to at least partially oxidize ammonia. In other embodiments, any of these components may be present or missing, catalyzed or not catalyzed, and may be arranged in alternate order. Further, certain components or all components may be provided in the same or separate housings.

[0015] For system 10 including controller 100, controller 100 can include a number of modules structured to functionally execute operations for controlling the SCR system. In certain embodiments, the controller forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller 100 may be a single device or a distributed device, and the functions of the controller may be performed by hardware or software. The controller 100 may be in communication with any sensor, actuator, datalink, and/or network in the system.

[0016] In certain embodiments, such as shown in FIG. 2, the controller 100 includes a NO.sub.x determination module 102, a temperature determination module 104, and a dosing control module 106. The description herein including modules emphasizes the structural independence of the aspects of the controller 100, and illustrates one grouping of operations and responsibilities of the controller 100. Other groupings that execute similar overall operations are understood within the scope of the present application. Modules may be implemented in hardware and/or software on computer readable medium, and modules may be distributed across various hardware or software components. More specific descriptions of certain embodiments of controller operations are included in the section referencing FIG. 2.

[0017] The exemplary system 10 further includes various sensors. The illustrated sensors in FIG. 1 include a first NO.sub.x sensor 22 positioned upstream of the HCSD 12 and a second NO.sub.x sensor 24 positioned downstream of the reduction catalyst 16. Alternatively or additionally, a NO.sub.x sensor 38 can be provided at the outlet of NSD 14, or between the inlet to reduction catalyst 16 and the outlet of NSD 14. System 10 also includes a first temperature sensor 26 at the inlet of HCSD 12, a second temperature 28 between HCSD 12 and NSD 14, a third temperature sensor 30 at the outlet of NSD 14, and a fourth temperature sensor 32 at the outlet of reduction catalyst 16. Other sensors can be provided to measure or determine the mass flow through the exhaust system, the temperature of any component of the aftertreatment system, the amount of ammonia stored in reduction catalyst 16 or outlet therefrom, etc.

[0018] The illustrated sensors are exemplary only, and may be re-positioned, removed, substituted, and other sensors may be present that are not illustrated in FIG. 1. Further, certain sensors may instead be virtual sensors that are calculated from other parameters available to the system 10, or values that would be indicated by sensors may instead be supplied to a computer readable memory location, via a datalink or network communication, or otherwise be made available to the system 10 where the sensor providing the sensed parameter is not a part of the defined system 10.

[0019] The exemplary controller 100 in FIGS. 1-2 is configured for executing operations to provide a reductant doser command for the effective removal of NO.sub.x emissions with reduction catalyst 16. The controller operations of the controller 100 in FIG. 2 include operations that adjust nominal control operations for a NO.sub.x aftertreatment system utilizing a reductant. Nominal control operations for a NO.sub.x aftertreatment system, including an SCR aftertreatment system, are understood in the art and are not described further herein. Any nominal NO.sub.x aftertreatment control operations may be utilized by system 10 disclosed herein.

[0020] The controller 100 includes a NO.sub.x determination module 102 that receives NO.sub.x parameters 108 from NO.sub.x sensors 22, 24, 38 and determines an amount of NO.sub.x emitted from engine 20 and from reduction catalyst 16, respectively. Controller 100 can also be configured to determine or calculate and amount of NO.sub.x at the outlet of NSD 14. Controller 100 also includes a temperature determination module 104 that receives temperature signals from temperature sensors 26, 28, 30, 32 to determine a temperature of the exhaust gas in flowpath 18 and/or of the various catalysts of the aftertreatment components in flowpath 18. Controller 100 further includes a dosing control module 106 that determines an appropriate dosing command for reductant to be injected in flowpath 18 to provide a desired emissions level for NO.sub.x at the outlet of reduction catalyst 16 and ultimately to the tailpipe.

[0021] During low temperature operating conditions for engine 20 and for exhaust gas and/or aftertreatment components in flowpath 18, reduction catalyst 16 is ineffective in treating emissions of NO.sub.x to meet desired emissions level targets. Furthermore, traditional oxidation catalysts upstream of reduction catalyst 16 are ineffective in removing HC to meet criteria emissions levels at low temperature operating conditions. Therefore, HCSD 12 is configured to store HC, and NSD 14 is configured to store NO.sub.x during low exhaust temperature operating conditions until the aftertreatment components of system 10 are raised to a temperature effective to remove the criteria pollutants from the emissions of engine 20.

[0022] The amount of accumulation during low exhaust temperature operating conditions of NO.sub.x in NSD 14 can be determined by the difference between the amount of NO.sub.x detected by upstream NO.sub.x sensor 22 and the corresponding amount detected by downstream NO.sub.x sensor 24 and converted by oxidation catalyst 16 during low temperature operation. Dosing control module 106 is configured to determine a dosing command that delays reductant dosing during low temperature operating conditions since NO.sub.x sensor 24 senses NO.sub.x levels that are reduced or lower than the levels determined by NO.sub.x sensor 22 due to the NO.sub.x storage at NSD 14. As the exhaust gas temperature increases to and above an effective temperature, dosing control module 106 is configured to increase reductant dosing to treat the NO.sub.x emissions released from NSD 14 as determined by one or both of NO.sub.x sensor 38 and NO.sub.x sensor 24, or by a calculated or sensed NO.sub.x amount at the outlet of NSD 14. Furthermore, in certain embodiments, dosing control module 106 can be configured to anticipate a future NO.sub.x emissions load and timing of the release of the stored NO.sub.x emissions by monitoring temperature parameters 110. The amount of reductant dosing can be increased prior to release of NO.sub.x emissions to provide a sufficient amount of reductant on reduction catalyst 16 to meet the expected increased in NO.sub.x emissions.

[0023] The dosing control module 106 provides a reductant doser command 114 to reductant doser 17 in response to a threshold deviation value 116. The threshold deviation value 116 includes a determination that NO.sub.x emissions at NO.sub.x sensor 24 is approaching or meeting a threshold deviation from an emissions level target value which requires reductant to be supplied to reduction catalyst 16. The reductant doser command 114 provided by the dosing control module 106 may include a reductant amount to be supplied to reduction catalyst 16. The dosing control module 106 provides the reductant doser command 114 in response to the threshold deviation value 116 indicating that the present NO.sub.x emissions level deviates more than a threshold amount from a NO.sub.x emissions level target. The reductant doser command 114 may be provided under any control scheme understood in the art, and/or under specific control schemes described herein. The reductant doser command 114 may include an actuator command value, a voltage or other electrical signal, and/or a datalink or network command. In certain embodiments, a reductant doser in a system including the controller 100 is responsive to the reductant doser command 114 to provide reductant to an exhaust stream at the position of the reductant doser 17 upstream of the reduction catalyst 16.

[0024] Dosing control module 106 can further be configured to provide a reductant doser command 114 that can be delayed to account for the amount of NO.sub.x stored by NSD 14 during low temperature exhaust conditions. As used herein, a delayed reductant doser command includes decreasing the rate at which reductant is injected and/or decreasing the range of engine operating conditions in which reduction catalyst 16 is utilized for treatment of NO.sub.x emissions, including those conditions which otherwise would have resulted in treatment of emissions of NOx from engine 20 by supplying reductant to reduction catalyst 16 without NO.sub.x storage by NSD 14 upstream of reduction catalyst 16. However, injection of reductant for storage on reduction catalyst 16 during low temperature operation is not precluded.

[0025] The descriptions here provide illustrative embodiments of performing procedures for controlling an aftertreatment system for low temperature operating conditions. Operations illustrated are understood to be exemplary only, and operations may be combined or divided, and added or removed, as well as re-ordered in whole or part, unless stated explicitly to the contrary herein. Certain operations illustrated may be implemented by a computer executing a computer program product on a computer readable medium, where the computer program product comprises instructions causing the computer to execute one or more of the operations, or to issue commands to other devices to execute one or more of the operations.

[0026] An exemplary HCSD 12 is operable to store HC over a range of operating temperatures below an effective operating temperature. For example, at temperatures below 200.degree. C., HCSD 12 is effective at storing HC emissions typically seen in operation of a diesel engine. As the exhaust temperatures approach and exceed 200.degree. C., the lightoff of the HC emissions stored in HCSD 12 is effective to oxidize the HC to form CO.sub.2 and H.sub.2O. In one embodiment, the catalyst of HCSD 12 is a zeolite catalyst with an oxidation catalyst thereon.

[0027] As shown in graph 300 of FIG. 3, line 302 indicates the cumulative HC received at the inlet to HCSD 12 over time at the start of a low temperature operating condition. Lines 304, 306 indicate the minimal outlet of HC from HCSD 12 over time during the low temperature operating condition. As the operating temperature increases over time, as indicated by line 308, the stored HC stored until a sufficiently high catalyst temperature for HCSD 12 is attained to oxidize HC.

[0028] As is evident from the figures and text presented above, a variety of embodiments according to the present invention are contemplated.

[0029] An exemplary set of embodiments is a method including storing in an exhaust flowpath, upstream of a NO.sub.x reduction catalyst, hydrocarbon and NO.sub.x emissions from an internal combustion engine during low exhaust temperature operation; releasing the stored hydrocarbons and NO.sub.x emissions into the exhaust flowpath as the exhaust temperature increases toward an effective operating temperature; and treating the released hydrocarbons and NO.sub.x emissions with the NO.sub.x reduction catalyst.

[0030] In certain embodiments of the method, the NO.sub.x reduction catalyst is a selective catalytic reduction catalyst. In other embodiments, the method includes storing hydrocarbon emissions on the surface of a hydrocarbon storage device catalyst. In other embodiments, the method includes storing NO.sub.x emissions includes storing NO.sub.x emissions on the surface of a NO.sub.x storage device catalyst. In other embodiments, the NO.sub.x storage device is a NO.sub.x adsorber. In certain embodiments, the effective operating temperature is around 200.degree. Celsius. In other embodiments, the method further includes delaying reductant dosing during the low exhaust temperature operation so that a reductant amount dosed into the exhaust flowpath during low exhaust temperature operation is insufficient for the NO.sub.x reduction catalyst to treat the NO.sub.x emissions from the internal combustion engine.

[0031] Another set of exemplary embodiments is a method comprising: operating an internal combustion engine to produce hydrocarbon and NO.sub.x emissions into an exhaust flowpath during low exhaust temperature operation; storing, upstream of a NO.sub.x reduction catalyst, HC and NO.sub.x emissions from the internal combustion engine during low exhaust temperature operation; and providing a reductant dosing command that treats the NO.sub.x emissions with the NO.sub.x reduction catalyst when the exhaust temperature reaches an effective operating temperature that releases the stored HC and NO.sub.x emissions.

[0032] In yet other embodiments, the method further comprises delaying reductant dosing during the low exhaust temperature operation so that a reductant amount dosed into the exhaust flowpath during low exhaust temperature operation is insufficient for a NO.sub.x reduction catalyst to treat the NO.sub.x emissions from the internal combustion engine. In another embodiment, storing hydrocarbon emissions includes storing hydrocarbon emissions on the surface of a hydrocarbon storage device catalyst and releasing the stored hydrocarbons includes oxidizing the hydrocarbons as the exhaust temperature increases toward the effective operating temperature. In one refinement of this embodiment, storing NO.sub.x emissions includes storing NO.sub.x emissions on the surface of a NO.sub.x storage device catalyst. In yet another refinement, the NO.sub.x storage device is a NO.sub.x adsorber. In additional embodiments, the effective operating temperature is around 200.degree. Celsius. In other embodiments, the NO.sub.x reduction catalyst is a selective catalytic reduction catalyst.

[0033] Another exemplary set of embodiments is a system including an internal combustion engine; an exhaust conduit fluidly coupled to the internal combustion engine; a hydrocarbon storage device fluidly coupled to the exhaust conduit; a NO.sub.x adsorber fluidly coupled to the exhaust conduit; a NO.sub.x reduction catalyst downstream of the NO.sub.x adsorber; and a reductant doser operationally coupled to the exhaust conduit upstream of the NO.sub.x reduction catalyst and downstream of the NO.sub.x adsorber.

[0034] In certain embodiments, the system includes the NO.sub.x reduction catalyst downstream of the hydrocarbon storage device. In one refinement of this embodiment, the reductant doser is operationally coupled to the exhaust conduit downstream of the hydrocarbon storage device and the NO.sub.x adsorber. In another embodiment, the reductant doser is operationally coupled to the exhaust conduit downstream of the hydrocarbon storage device and the NO.sub.x adsorber.

[0035] In certain embodiments, the system includes a controller comprising a NO.sub.x ratio determination module structured to determine a NO.sub.x amount at an outlet of the NO.sub.x reduction catalyst; a temperature determination module structured to determine a present operating temperature of the exhaust gas in the exhaust flowpath; and a dosing control module structured to determine a reductant doser command in response to the NO.sub.x amount to achieve a desired NO.sub.x emissions from the NO.sub.x reduction catalyst.

[0036] In another exemplary embodiment of the system, the controller is configured to provide a delayed reductant doser command during low exhaust temperature operating conditions. In another embodiment, the hydrocarbon storage device includes a catalyst configured to store hydrocarbon emissions thereon during low exhaust temperature operating conditions and to oxidize the stored hydrocarbons when the exhaust temperature reaches an effective temperature. In a refinement of this embodiment, the NO.sub.x adsorber is structured to adsorb NO.sub.x emissions during low exhaust temperature operating conditions and release and oxidize NO.sub.x emissions when the exhaust temperature reached an effective temperature. In another embodiment, the system includes a first NO.sub.x sensor at an inlet of the hydrocarbon storage device and a second NO.sub.x sensor at an outlet of the nitrous oxide reduction catalyst. In another embodiment, the system includes a NO.sub.x sensor at an outlet of the NO.sub.x storage device.

[0037] While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the inventions are desired to be protected. In reading the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language "at least a portion" and/or "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

1. A method, comprising: storing in an exhaust flowpath, upstream of a NO.sub.x reduction catalyst, hydrocarbon (HC) and oxides of nitrogen (NO.sub.x) emissions from an internal combustion engine during low exhaust temperature operation; releasing the stored HC and NO.sub.x emissions into the exhaust flowpath as the exhaust temperature increases toward an effective operating temperature; and treating the released HC and NO.sub.x emissions with an oxidation catalyst and the NO.sub.x reduction catalyst.

2. The method of claim 1, wherein the NO.sub.x reduction catalyst is a selective catalytic reduction catalyst.

3. The method of claim 1, wherein the oxidation catalyst is a hydrocarbon storage device catalyst and storing HC emissions includes storing HC emissions on the surface of the hydrocarbon storage device catalyst.

4. The method of claim 3, further comprising oxidizing the stored HC with the hydrocarbon storage device catalyst.

5. The method of claim 1, wherein storing NO.sub.x emissions includes storing NO.sub.x emissions on the surface of a NO.sub.x storage device catalyst.

6. The method of claim 5, wherein the NO.sub.x storage device is a NO.sub.x adsorber.

7. The method of claim 1, wherein the effective operating temperature is around 200.degree. Celsius.

8. The method of claim 1, further comprising delaying reductant dosing during the low exhaust temperature operation so that a reductant amount dosed into the exhaust flowpath during low exhaust temperature operation is insufficient for the NO.sub.x reduction catalyst to treat the NO.sub.x emissions from the internal combustion engine.

9. A method, comprising: operating an internal combustion engine to produce hydrocarbon (HC) and oxides of Nitrogen (NO.sub.x) emissions into an exhaust flowpath during low exhaust temperature operation; storing, upstream of a NO.sub.x reduction catalyst, HC and NO.sub.x emissions from the internal combustion engine during low exhaust temperature operation; and providing a reductant dosing command that treats the NO.sub.x emissions with the NO.sub.x reduction catalyst when the exhaust temperature reaches an effective operating temperature that releases the stored HC and NO.sub.x emissions.

10. The method of claim 9, further comprising delaying reductant dosing during the low exhaust temperature operation so that a reductant amount dosed into the exhaust flowpath during low exhaust temperature operation is insufficient for a NO.sub.x reduction catalyst to treat the NO.sub.x emissions from the internal combustion engine.

11. The method of claim 9, wherein storing HC emissions includes storing HC emissions on the surface of a hydrocarbon storage device catalyst and wherein releasing the stored HC emissions includes oxidizing the HC as the exhaust temperature increases toward the effective operating temperature.

12. The method of claim 11, wherein storing NO.sub.x emissions includes storing NO.sub.x emissions on the surface of a NO.sub.x storage device catalyst.

13. The method of claim 12, wherein the NO.sub.x storage device is a NO.sub.x adsorber.

14. The method of claim 9, wherein the effective operating temperature is around 200.degree. Celsius.

15. The method of claim 9, wherein the NO.sub.x reduction catalyst is a selective catalytic reduction catalyst.

16. A system, comprising: an internal combustion engine; an exhaust conduit fluidly coupled to the internal combustion engine; a hydrocarbon storage device fluidly coupled to the exhaust conduit; a NO.sub.x adsorber fluidly coupled to the exhaust conduit; a NO.sub.x reduction catalyst downstream of the NO.sub.x adsorber; and a reductant doser operationally coupled to the exhaust conduit upstream of the NO.sub.x reduction catalyst and downstream of the NO.sub.x adsorber.

17. The system of claim 16, wherein the NO.sub.x reduction catalyst is downstream of the hydrocarbon storage device.

18. The system of claim 17, wherein the reductant doser is operationally coupled to the exhaust conduit downstream of the hydrocarbon storage device and the NO.sub.x adsorber.

19. The system of claim 16, wherein the reductant doser is operationally coupled to the exhaust conduit downstream of the hydrocarbon storage device and the NO.sub.x adsorber.

20. The system of claim 16, further comprising a controller, comprising: a NO.sub.x ratio determination module structured to determine an NO.sub.x amount at an outlet of the NO.sub.x reduction catalyst; a temperature determination module structured to determine a present operating temperature of the exhaust gas in the exhaust flowpath; a dosing control module structured to determine a reductant doser command in response to the NO.sub.x amount to achieve a desired NO.sub.x emissions from the NO.sub.x reduction catalyst.

21. The system of claim 20, wherein the controller is configured to provide a delayed reductant doser command during low exhaust temperature operating conditions

22. The system of claim 16, wherein the hydrocarbon storage device includes a catalyst configured to store hydrocarbon (HC) emissions thereon during low exhaust temperature operating conditions and to oxidize the stored HC when the exhaust temperature reaches an effective temperature.

23. The system of claim 22, wherein the NO.sub.x adsorber is structured to adsorb NO.sub.x emissions during low exhaust temperature operating conditions and release and oxidize NO.sub.x emissions when the exhaust temperature reached an effective temperature for NO.sub.x conversion over the NO.sub.x reduction device.

24. The system of claim 16, further comprising a first NO.sub.x sensor at an inlet of the hydrocarbon storage device and a second NO.sub.x sensor at an outlet of the NO.sub.x reduction catalyst.

25. The system of claim 16, further comprising a NO.sub.x sensor between the NO.sub.x adsorber and the NO.sub.x reduction catalyst.

26. The system of claim 16, further comprising a NO.sub.x sensor upstream of the NO.sub.x reduction catalyst.