Abstract:
The exhaust from a diesel-fueled internal combustion engine is treated by a lean NO X  trap. The maximum temperature used for desulfating the lean NO X  trap is kept relatively lower during early life and progressively increased as the trap ages. Designing for adequate late life performance entails excess capacity during early life. The method utilizes the excess capacity available during early life to slow aging of the trap and thereby extend the trap lifetime. The method facilitates meeting durability requirements for diesel-powered vehicles with exhaust aftertreatment.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to systems having diesel-fueled internal combustion engines with exhaust aftertreatment and methods of operating those systems. 
       BACKGROUND 
       [0002]    Diesel-fueled internal combustion engines are used to power vehicles such as medium and heavy duty trucks. Diesel engines are also used in stationary power generation systems. While exhaust aftertreatment systems for gasoline engine have been widely used since the 1970s, diesel engine aftertreatment systems have only recently coming into widespread use. 
         [0003]    Whereas gasoline engines use spark ignition, diesel engines use compression ignition. As a consequence, the composition of diesel exhaust is much different from that of gasoline engines. The major pollutants in gasoline engine exhaust are carbon monoxide, unburned hydrocarbons, and some NO x . The major pollutants in diesel engine exhaust are NO X  and particulate matter (soot). 
         [0004]    A catalytic converter, which is an exhaust treatment device comprising a so-called three-way catalyst, can effectively control gasoline engine emissions by oxidizing carbon monoxide and unburned hydrocarbons while also reducing NO X . This approach is unsuitable for diesel engine exhaust because diesel exhaust contains from about 4 to 20% oxygen. The excess oxygen and dearth of oxygen accepting species (reductants) makes catalytic converters ineffective for reducing NO X  in diesel exhaust. 
         [0005]    Several solutions have been proposed for controlling NO X  emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NO X  emissions. Another set of approaches remove NO X  from the vehicle exhaust. These include the use of lean-burn NO X  catalysts, selective catalytic reduction (SCR) catalysts, and lean NO X  traps (LNTs). 
         [0006]    Lean-burn NO X  catalysts promote the reduction of NO x  under oxygen-rich conditions. Reduction of NO X  in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NO x  catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NO X  reduction, adding 3% or more to the engine&#39;s fuel requirement. Currently, the sustainable NO X  conversion efficiencies provided by lean-burn NO X  catalysts are unacceptably low. 
         [0007]    SCR generally refers to selective catalytic reduction of NO X  by ammonia. The reaction takes place even in an oxidizing environment. The NH 3  can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NO X  reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment. 
         [0008]    LNTs are devices that adsorb NO X  under lean conditions and reduce and release the adsorbed NO X  under rich conditions. An LNT generally includes a NO X  adsorbent and a catalyst. The adsorbent is typically an alkali or alkaline earth compound, such as BaCO 3  and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust (exhaust containing an excess of oxygen and other oxidizing species in comparison to reducing compounds), the catalyst speeds reactions that lead to NO X  adsorption. In a rich exhaust (containing reductants in excess of oxidizing compounds), the catalyst speeds reactions by which reductants are consumed and adsorbed NO X  is reduced and desorbed. In a typical operating protocol, a rich condition (reducing environment) is created within the exhaust from time-to-time to regenerate (denitrate) the LNT. 
         [0009]    In addition to accumulating NO X , LNTs accumulate SO X . SO X  is the product of combusting sulfur-containing fuels. Even with low sulfur diesel fuels, the amount of SO X  produced by combustion is significant. SO X  adsorbs more strongly than NO X  and necessitates a more stringent, though less frequent, regeneration (desulfation). Desulfation requires elevated temperatures, e.g., 700° C. 
         [0010]    A desulfation process requires much more time than a denitration process. Whereas denitration can be completed in a few seconds, desulfation takes several minutes, commonly on the order of 5-15 minutes. Desulfation could be carried out more rapidly if higher temperatures were used, but normal desulfating temperatures already approach the point at which the LNT will undergo rapid thermal degradation. For example, a temperature of 800° C. may cause a particular LNT to deteriorate and lose a substantial portion of its functionality after a single desulfation. 
         [0011]    U.S. Pat. No. 6,637,198 proposes a desulfation process in which several partial desulfations are performed between each full desulfation. The partial desulfations use lower temperatures and have shorter duration than the main desulfations. The patent asserts that this process facilitates making opportunistic use of higher than normal exhaust temperatures to reduce the amount of fuel expended heating the LNT for desulfations. 
         [0012]    In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel requirement), and reduces NO X  emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations that limit NO X  emissions from trucks and other diesel-powered vehicles. 
       SUMMARY 
       [0013]    The invention is a method of operating a diesel power generation system in which the exhaust from a diesel-fueled internal combustion engine is treated by a lean NO X  trap. The invention extends to encompass systems configured to implement that method. The systems and methods are particularly concerned with how the lean NO X  trap is desulfated. According to the invention, the maximum temperature used for desulfating the lean NO X  trap is kept relatively lower during early life and increased as the trap ages. 
         [0014]    The storage performance of a lean NO X  trap will invariably diminish over time. A lean NO X  trap designed to provide adequate late life performance must have excess capacity during early life. The method utilizes the excess capacity available during early life to slow aging of the trap and thereby extend the trap&#39;s lifetime. The method facilitates meeting durability requirements for diesel-powered vehicles with exhaust aftertreatment systems. 
         [0015]    In one embodiment, the invention is a method of operating a diesel power generation system in which the diesel exhaust is treated using a lean NO X  trap. The engine produces exhaust containing NO X  and SO X  and the exhaust is treated using the lean NO X  trap. From time-to-time, the lean NO X  trap is desulfated by heating it to a desulfating temperature, or equivalently, to within a desulfating temperature range. The heated trap is exposed to rich conditions, under which the lean NO X  trap desulfates. The lean NO X  trap is aged through many desulfations. As the lean NO X  trap ages, the highest temperature used for the desulfations is increased. The highest desulfating temperatures used are therefore lower during early lean NO X  trap life as compared to mid and late lean NO X  trap life. 
         [0016]    The primary purpose of this summary has been to present certain of the inventors&#39; concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventors&#39; concepts or every combination of the inventors&#39; concepts that can be considered “invention”. Other concepts of the inventors will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventors claim as their invention being reserved for the claims that follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a flow chart of an exemplary method of adaptive desulfation according to the present invention. 
           [0018]      FIG. 2  is a schematic illustration of an exemplary power generation system that can be configured to operate according to the present invention. 
           [0019]      FIG. 3  is a flow chart of an exemplary method for determining when desulfation is currently required. 
           [0020]      FIG. 4  is a flow chart of an exemplary method for executing a desulfation request. 
           [0021]      FIG. 5  is a flow chart of another exemplary method for adapting desulfating conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  is a flow chart of an exemplary adaptive desulfation method  100  according to the present invention. The method  100  begins with a determination  101  whether desulfation is required. If desulfation is required the method  100  proceeds to step  102 , which analyzes lean NO X  trap performance over a preceding interval to determine the effectiveness of prior desulfations using a current set of desulfating conditions. The conditions include a desulfating temperature and optionally additional parameters, duration in this example. If one or more desulfations have been successfully completed using the current desulfating conditions but have failed to produce a satisfactory result, step  103  directs the method  100  to step  105 . Otherwise the method  100  proceeds with step  104  and carries out a desulfation using the extant conditions. 
         [0023]    Step  105  determines whether measures can be taken to improve desulfation without increasing the desulfating temperature. In this example, step  105  determine whether the desulfating time currently in use is less than a maximum. If not, the method  100  increases the desulfating time and proceeds with the desulfation. If the desulfating time is already at a maximum, the method proceeds to step  107 . 
         [0024]    Step  107  determines whether the desulfating temperature is already at a limit. The method  100  raises the desulfating temperature over the lifetime of a lean NO X  trap, but still uses a lifetime limit on how high the desulfating temperature can go. If the limit has been reached, a fault is indicated in step  110  and the desulfation is carried out without further increasing the severity of the desulfating conditions. If the limit has not yet been reached, the method  100  proceeds with step  108  in which the desulfating temperature is increased. The desulfating time is reduced in step  109  and the desulfation carried out in step  104 . 
         [0025]    The method  100  will increases the desulfating time again, later, as necessary, while retaining the increased desulfating temperature. Further increases to the desulfating temperature will be deferred until at least one successfully completed desulfation using the higher desulfating and the maximum desulfating time has proven inadequate. Optionally, decreases to desulfation time can be made between steps  103  and  104 , whereby the duration of desulfation is adapted either upwards or downwards according to desulfation performance as determined from the data analyzed in step  102 . Increases to the maximum temperature used for desulfation, however, are preferably monotonic. 
         [0026]      FIG. 2  provides a schematic illustration of an exemplary power generation system  200  that can be configured, adapted, and functional to implement methods of the invention. The exemplary power generation system  200  can be installed in a vehicle or in a stationary power application, such as a back-up generator for a hospital. While the invention may be described and claimed in terms of power generation systems, it should be appreciated that the full value of the invention may be realized in a larger system, such as vehicle. For example, the invention extends the period over which a vehicle can operate within the confines of emission control regulations without requiring replacement of any aftertreatment system components. This can be critical in meeting regulatory and customer requirements. 
         [0027]    The exemplary power generation system  200  comprises an engine  201 , a manifold  202 , and an exhaust aftertreatment system  203 . The exhaust aftertreatment system  203  comprises an exhaust line  204  configured to channel exhaust from the manifold  202  through, in order, a fuel reformer  205 , a thermal mass  206 , an LNT  207 , a DPF  208 , and an SCR catalyst  209 . A fuel injector  211  is configured to inject fuel into the exhaust line  204  upstream from the fuel reformer  205  at times and at rates determined by the controller  210 . Implementation of the present invention does not require either the fuel reformer  205 , the thermal mass  206 , the DPF  208 , the SCR catalyst  209 , and the fuel injector  211 . The major device that relate to the invention are the engine  201 , the LNT  207 , and the controller  210 . 
         [0028]    The controller  210  may be a control unit for the engine  201  or a separate control unit. If separate, the controller  210  preferably communicates with the engine control unit. Configuring the system  200  to practice a method of the invention generally involves providing the controller  210  with suitable programming. With suitable programming and any other necessary adaptations, the system  200  will be functional to carry out the method. 
         [0029]    The controller  210  receives data from various sensors, such as a temperature sensor  212 . The sensor  212  is configured to sense a characteristic temperature for the LNT  207 . Other sensors that may be provided include, without limitation, a temperature sensor for the fuel reformer  205  and one or more exhaust composition sensors that can be used to monitor performance of the LNT  207 . Usually there will be at least one composition sensor downstream from the LNT  207 , such as a NO X  sensor. Suitable locations include locations upstream and downstream from the SCR catalyst  209 . 
         [0030]    The engine  201  can be any engine that operates to produce a lean exhaust stream comprising NO X  and SO X . Generally the engine  201  is a diesel-fueled compression ignition internal combustion engine that produces an exhaust containing from 2 to 20% oxygen. The diesel exhaust is typically at temperatures in the range from about 200 to about 500° C., with temperatures in the range from 250 to 450° C. beginning common. The manifold  202  couples the exhaust aftertreatment system  203  to an exhaust stream from the engine  201 . Preferably the exhaust system  203  comprises a single exhaust line  204  that receives the entire exhaust from the engine  201 . 
         [0031]    The exhaust aftertreatment system  203  and the exhaust line  204  preferably have no valves or dampers that control the flow of exhaust. Exhaust system valves and dampers provide control over the distribution of exhaust between a plurality of flow paths. Such control is desirable in terms of limiting fuel usage. Reducing the flow of exhaust to the fuel reformer  205  and the LNT  207  during rich regeneration would reduce the amount of fuel expended eliminating oxygen from the exhaust in order to provide rich conditions. The reduced flow rate would also increase residence times, and thus the efficiency with which reductants are used. Nevertheless, it is preferred that the exhaust treatment system  203  operate without exhaust line valves or dampers in order to avoid failures resulting from reliance on such devices. 
         [0032]    The LNT  207  is a device that adsorbs NO X  under lean conditions and reduces NO X  releasing the reduction products (N 2  and NH 3 ) under rich conditions. Some alternate terms for a lean NO X  trap (LNT) are NO X  absorber-catalyst and NO X  trap-catalyst. An LNT generally comprises a NO X  absorbent and a precious metal catalyst in intimate contact on an inert support. Examples of NO X  adsorbent materials include certain oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. The adsorption can be physical or chemical, but is generally primarily chemical. The precious metal typically comprises one or more of Pt, Pd, and Rh. The support is typically a monolith, although other support structures can be used. The monolith support is typically ceramic, although other materials such as metal and SiC are also suitable for LNT supports. The LNT  207  may be provided as two or more separate bricks. 
         [0033]    The fuel reformer  205  and the fuel injector  211  are part of a system for producing the rich conditions and providing the reductant required for denitration and desulfation. A reductant is a compound that is reactive to accept oxygen and become oxidized. The reductant is generally diesel fuel or a substance derived from diesel fuel by partial combustion and or steam reforming reactions. A rich condition for the exhaust is one in which the concentration of reductants is more than stoichiometric for combustion with any oxygen and other oxidizing compounds present. In other words, a rich environment is one in which there is an excess of reductant and the overall composition is reducing rather than oxidizing. 
         [0034]    Optionally, the engine  201  is used to assist in producing rich conditions. If the engine  201  can be operated with rich combustion or with post-combustion fuel injection, than the engine  201  can provide a rich mixture and the exhaust line fuel injector  211  is optional. The engine  201  can also facilitate generating rich conditions by measures that reduce the exhaust oxygen flow rate. Such measures may include, for example, throttling an air intake for the engine  201 , increasing exhaust gas recirculation (EGR), modifying cylinder injection controls, and shifting gears to reduce the engine speed. 
         [0035]    It is preferable for the aftertreatment  203  to be capable of providing the rich conditions for denitration and desulfation of the LNT  207  while making few or no changes to the operation of the engine  201  in order to avoid having regenerations (denitrations and desulfations) adversely affect drivability and also to provide greater independence between the designs and configurations of the aftertreatment system  203  and the engine  201 . It would not be unusual for the engine  201  to be manufactured by one company while the power generation system  200  comprising the engine  201  is assembled by another company. A third company may build a vehicle using the assembled power generation system  200 . 
         [0036]    The fuel reformer  205  is a device that is functional to reform diesel fuel into reformate, especially CO and H 2 . Reformate is a better reductant than diesel fuel for denitrating the LNT  207 . Reformate is more reactive than diesel fuel and results in less NO X  slip. NO X  slip is the release of unreduced NO X  from the LNT  207  during denitration. 
         [0037]    Preferably, the fuel reformer  205  has a low thermal mass and comprises both oxidation and steam reforming catalysts. A low thermal mass allows the fuel reformer  205  to be heated to steam reforming temperatures for each denitration without requiring an excessive amount of time or fuel. Steam reforming temperatures have a minimum in the range from about 500 to about 600° C., typically requiring at least 550° C. At steam reforming temperatures, energy from oxidation and partial oxidation, which are exothermic, can drive steam reforming, which is endothermic. This improves the efficiency with which reformate is produced and decreasing the amount of waste heat. A sufficiently low thermal mass can be achieved by constructing the fuel reformer  205  around a monolith substrate formed of thin metal foils, e.g., 130 microns or less. Preferably the foils are 100 microns or less, and more preferably 50 microns or less. The preferred structure can be heated from a typical diesel exhaust temperature in the range from 250 to 300° C. to steam reforming temperatures in 2 or 3 seconds or less. 
         [0038]    The exhaust from the engine  201  generally comprises at least 2% oxygen. When fuel is added to the exhaust to produce a rich condition for denitration or desulfation, this oxygen is eliminated by combustion. In the system  200 , this combustion takes place in the fuel reformer  205 . If the combustion does not take place upstream from the LNT  207 , it will generally take place within the LNT  207 . The precious metal catalysts typically used by the LNT  207  are functional as oxidation catalysts. If too much combustion takes place within the LNT  207 , it can cause undesirable temperature excursions which are particularly problematic if they take place during denitrations. Such temperature excursions can cause wear and result in the release of unreduced NO X . 
         [0039]    Optionally, a burner or any device that is functional to bring about combustion, such as an oxidation catalyst, a three-way catalyst, or a suitably catalyzed diesel particulate filter can be used instead of the fuel reformer  205 . Like the fuel reformer  205 , these device can cause combustion to take place upstream from the LNT  207 . They may also accomplish a certain amount of fuel reformate through partial oxidation reactions. 
         [0040]    When combustion takes place upstream from the LNT  207  in preparation for or during denitration, the heat is preferably held temporarily within devices upstream from the LNT  207  to be released only slowly over a prolonged period. According to the preferred design, the fuel reformer  205  has a low thermal mass (thermal inertia) and is not very effective for holding heat. In the system  200 , the thermal mass  206  provides the desired heat retention function. 
         [0041]    The thermal mass  206  is any device that is effective for exchanging heat with the exhaust and storing the heat produced by the fuel reformer  205  over the course of a denitration without heating excessively. A suitable device can be simply a catalyst substrate, with or without a catalyst. A suitable device is, for example, an inert monolith substrate, either metal or ceramic. Preferably, the thermal mass  206  has a thermal inertia that is greater than that of the fuel reformer  205 . The DPF  208  can be used as the thermal mass  206 , although in the exemplary system  200 , the DPF is downstream from the LNT  208  and instead serves to help protect the SCR catalyst  209  from high temperatures during desulfations. 
         [0042]    The DPF  208  and the SCR catalyst  209  contribute to meeting emission control limits and durability requirements. The DPF  208  removes particulate matter from the exhaust, which is the major pollutant in diesel exhaust other than NO X . The SCR catalyst  209  provides supplementary NO X  mitigation. It improves durability by allowing sufficient NO X  mitigation to be maintained with less frequent denitration and desulfation of the LNT  208 . When some NO X  is reduced downstream from the LNT  207 , the LNT  207  does not need to be maintained at as high a level of efficiency. 
         [0043]    A DPF is a device that traps particulates matter (soot), removing it from the exhaust flow. The DPF  208  can be a wall flow filter, which uses primarily cake filtration, or a flow-through filter, which uses primarily deep-bed filtration. The DPF  208  can have any suitable structure. Examples of suitable structures include monoliths. A monolith wall flow filter is typically made from a ceramic such as cordierite or SiC, with alternating passages blocked at each end to force the flow through the walls. A flow-through filter can be made from metal foil. 
         [0044]    Trapped soot can be removed from the DPF  208  continuously by catalyzing reactions between soot and NO X , but typically the DPF  208  must be heated from time-to-time to a temperature at which it regenerates by combustion of trapped soot. The temperature required for soot combustion can be reduced by a catalyst. Suitable catalysts include precious metals and oxides of Ce, Zr, La, Y, and Nd. Soot combustion is exothermic and can be self-sustaining once ignited. 
         [0045]    The SCR catalyst  209  is an ammonia-SCR catalyst functional to catalyze reactions between NOx and NH 3  to reduce NOx to N 2  in lean exhaust. Examples of SCR catalysts include oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites, such as ZSM-5 or ZSM-11, exchanged with metal ions such as cations of Cu, Co, Ag, Zn, or Pt. 
         [0046]    The engine  201  operates to produce a lean exhaust comprising NO X , particulate matter, and SO X . The expression NO R  designates the family of molecules consisting of nitrogen and oxygen atoms, primarily NO and NO 2 . The subscript X indicates the family includes multiple species with varying proportions between nitrogen and oxygen atoms. The notation SO X  is similar. 
         [0047]    Under lean conditions, the LNT  207  absorbs a portion of the NO X  and a portion of the SO X  in the exhaust. If the SCR catalyst  209  contains stored ammonia, an additional portion of the NO X  is reduced therein. The DPF  208  removes at least a portion of the particular matter from the exhaust. 
         [0048]    From time-to-time, the controller  210  determines to denitrate the LNT  207 . For denitration, the fuel reformer  205  is heated to steam reforming temperatures by injecting fuel into the exhaust line  204  through the fuel injector  211  under the control of the controller  210  at rates that leave the exhaust lean. Under lean condition, most of the injected fuel combusts in the fuel reformer  205 , heating it. After the fuel reformer  205  has reached steam reforming temperatures, as may be determined using a temperature sensor, the fuel injection rate is controlled to make the exhaust condition rich for a period of time (rich phase) over which the LNT  207  denitrates. 
         [0049]    During the rich phase, injected fuel mixed with lean exhaust enters the fuel reformer  205 . A portion of the fuel combusts, consuming most of the oxygen from the exhaust. Another portion of the fuel is converted to reformate (syn gas), which is primarily H 2  and CO. The reformate enters the LNT  207  where it reacts to reduce and release trapped NO X . Most of the NO X  released during the rich phase is reduced to N 2  or NH 3 , although it is typical for a small amount to be released (slip) without being reduced. The NH 3  is mostly trapped by the SCR  209 , where it is generally consumed reducing NO X  over the course of the following lean phase. 
         [0050]    NO X  slip occurs primarily at the beginning of the rich phase and may be lessened by varying the reductant concentration over the course of the denitration. The preferred reductant concentration profile has the reductant concentration relatively low at the start of the rich phase and gradually increasing over at least a first portion of the rich phase. 
         [0051]    When the fuel reformer  205  heats for denitration, the thermal mass  206  also heats, but to a lesser degree. After denitration is complete, fuel injection ceases and the fuel reformer  205  cools down to exhaust temperatures. The thermal mass  206  also cools. The LNT  207  will heat over the course of the denitration and a short period following, but only to a modest degree. 
         [0052]    From time-to-time, the LNT  207  must be heated substantially in order to carry out a desulfation. In the system  200 , the LNT  207  can be heated by injecting fuel into the exhaust. The injected fuel combusts, primarily in the fuel reformer  205 . Over the course of a few minutes, the thermal inertias of the thermal mass  206  and of the LNT  207  are overcome and the LNT  207  reaches desulfating temperatures. Alternative and supplemental means of heating the LNT  207  include, without limitation, engine measures, such as operating the engine to produce a hot exhaust, burners, and electrical heaters. 
         [0053]    In the system  200 , after the LNT  207  has reached desulfating temperatures, the fuel injection rate is controlled to make the exhaust rich. It might be considered ideal to maintain the rich condition until the LNT  207  has desulfated to a desired degree. In the system  200 , however, it proves difficult to continuously maintain rich conditions while also maintaining the fuel reformer  205  and the LNT  207  within desired temperature ranges. In general, it is necessary to pulse the fuel injection over the course of a desulfation. 
         [0054]    In the context of maintaining the desired conditions for desulfation, pulsing the fuel injection means creating alternating rich and lean phases (periods). During the rich phases, the reformer  205  heats and produces reformate. During the lean phase, fuel injection ceases and the fuel reformer  205  cools. Typically, the durations of the rich phases are in the range from about 4 to about 30 seconds, with periods in the range from about 5 to about 15 seconds being preferred. 
         [0055]    When lean and rich phases are alternated in this manner, some combustion takes place in the LNT  207 . The LNT  207  typically comprises oxygen storage materials, which are materials that are functional to store oxygen. These materials accumulate oxygen during the lean phases, typically to the point of saturation. During the rich phases, the reductants react with the stored oxygen, consuming the stored oxygen and producing heat. The amount of heat can be significant, e.g., enough to make the LNT  207  50 to 100° C. hotter than the fuel reformer  205 . The amount of this heating is generally proportional to the oxygen storage capacity of the LNT  207  and to the frequency of switches between lean and rich phases. 
         [0056]    In a preferred embodiment, the fuel injection rate is selected to provide a desired reformate production rate or concentration while the temperatures of the fuel reformer  205  and the LNT  207  are controlled by varying two parameters that dictate the lean-rich pulse pattern. The lean rich pulse pattern consists of the lean phase lengths and the rich phase lengths or two other parameters that define these, such as the pulse frequency and the ratio between lean time and rich time. 
         [0057]    In one exemplary control strategy, the rich phases are terminated when the fuel reformer  205  reaches a pre-defined upper limit temperature, such as 650° C. The lean phase durations are then adjusted in a closed loop control algorithm to maintain the temperature of the LNT  207  within the desired temperature range. In a variation of this method, the adjusted parameter is a temperature to which the fuel reformer  205  is required to cool before terminating. The LNT temperature target by control is preferably a maximum temperature that the LNT reaches over the course of a lean-rich cycle, but could alternatively be another temperature, such as an average temperature. Making the pulse periods shorter by reducing the durations of the lean phases raises temperatures within the LNT  207 . The temperatures rise because the fuel reformer  205  is on average hotter resulting in more heat convection to the LNT  207  and because the more frequent alternation between lean and rich phases results in more combustion within the LNT  207 . Conversely, lengthening the lean phases lowers temperatures within the LNT  207 . 
         [0058]    In another exemplary control strategy, the durations of the rich phases are predetermined and both the upper and lower temperatures of the fuel reformer  205  (or two equivalent parameters) are set in order to achieve the desired rich phase length while maintaining the LNT  207  at the desired temperature. In this example, the temperature to which the fuel reformer  205  falls during a lean phase is raised or lowered to raise or lower the temperature of the LNT  207 . During the rich phases, the reformer heats by an amount that depends on the predetermined rich phase duration. By selecting the rich phase duration within suitable limits, the fuel reformer  205  can be prevented from either overheating or cooling excessively, e.g., cooling below steam reforming temperatures. Preselecting the rich phase duration can be beneficial in managing hydrocarbon emissions during desulfations. In terms of limiting hydrocarbon emissions, a suitable length for the rich phases is on the order 10 to 20 seconds for a fresh catalyst, decreasing to about 50-70% as much as the catalyst ages. 
         [0059]    It is generally also necessary to regenerate the DPF  208  from time-to-time. Regenerating the DPF  208  comprises heating the DPF  208  to temperatures at which soot trapped with the DPF  208  combusts. The DPF  208  can be heated in the same way as the LNT  207  is heated for desulfation. Soot combustion is generally self-sustaining. Once the DPF  208  is heated to soot combustion temperatures, it is generally not necessary to supply any additional heat. The upstream devices, including the fuel reformer  205  and the LNT  207 , can be allowed to cool while soot combustion is proceeding to completion. 
         [0060]    The DPF  208  is typically of the wall flow filter variety and must be regenerated often enough to avoid excessive back pressure. Ideally, regenerating the DPF  208  each time the LNT  207  is regenerated provides sufficient frequency. The DPF  208  is heated to soot combustion temperatures each time the LNT  207  is heated for desulfation. If the DPF  208  has sufficient capacity to require regeneration no more often than the LNT  207  is regenerated, supplemental fuel expenditure and additional heating of the LNT  207  for the sole purpose of regenerating the DPF  208  can be avoided. 
         [0061]    One approach that can facilitate not having to regenerate the DPF  208  more often than the LNT  207  is desulfated is to provide a second DPF downstream from the fuel reformer  205  and upstream from the LNT  207 . This second DPF can be used as the thermal mass  206 , but is preferably a low thermal mass device upstream from the thermal mass  208 . Preferably, this second DPF is of the flow-through type. Preferably, its thermal mass is sufficiently low that it heats and regenerates each time the fuel reformer  205  is heated to supply reformate for denitration. Accordingly, in this embodiment, there is a second DPF that regenerates as often as the LNT  207  is denitrated. 
         [0062]    The times at which the LNT  207  is denitrated are determined by the controller  210  and can be determined in any suitable manner. Typically, certain threshold must be met before allowing a denitration to begin. Threshold criteria can be, for example, one or both the LNT  207  and the fuel reformer  205  being at minimum temperatures, the oxygen concentration being below a maximum (e.g., less than 15%), the flow rate being above a minimum (e.g., significantly greater than at idle), the engine speed variance, as determined by a moving average, being below a maximum, at a minimum time elapsed since the last denitration, and a gear shift not currently imminent or in progress. If the threshold criteria are met, regeneration will begin if an additional condition (or conditions) are met. An additional condition generally relates to a measure of how urgently denitration is needed and is optionally weighed against the suitability of current conditions for beginning a denitration. 
         [0063]    The urgency of the need to denitrate generally relates to one or more of NO X  loading of the LNT  207 , remaining NO X  storage capacity, NO X  trapping efficiency (optionally normalized for such factors as the LNT temperature and exhaust flow rate), NO X  concentration in the exhaust at a point downstream from the LNT  207 , and cumulative NO X  emissions since the last denitration (optionally normalized by the engine&#39;s toque production). A measure of suitability can relate to one or more of such factors as the exhaust oxygen concentration (low is preferred), the engine speed variance (low is preferred), and the exhaust flow rate. One procedure for weighing the urgency of the need to denitrate against the suitability of current conditions to denitration is to assign numerical values to the urgency and the conduciveness, multiplying the two together, and denitrating based on whether the result exceeds a predetermined critical value. 
         [0064]    Likewise, the times at which the LNT  207  is desulfated are determined by the controller  210  in any suitable manner. Threshold criteria may be employed similar to those used for denitration and a measure of suitability of current conditions to desulfation can be weighed against a measure of the urgency of the need for desulfation. The urgency of the need to desulfated can be based on, for example, one or more of an estimate of the amount sulfur trapped in the LNT, the frequency with which denitration is being required, an estimate of the post-denitration NO X  storage capacity of the LNT  207 , the amount of time since the last desulfation, the number of denitrations since the last desulfation, and an estimate of the average LNT efficiency following the last desulfation, or over the last several desulfations. The LNT efficiency can be normalized to separate changes intrinsic to the LNT  207  from changes in the operating regime of the engine  201 . Alternatively, normalization can be limited, whereby less sulfur loading is tolerated when the engine is in an operating regime that requires peak LNT efficiency, e.g., when the engine  201  is in a high speed-high load condition. Preferably, the determinations of when to desulfate the LNT  207  include a dynamic measure of LNT performance, whereby adjustments to the desulfation timing and the desulfating conditions can be adapted to measurable indications of aging. 
         [0065]      FIG. 3  provides a flow chart of an exemplary method  300  that determines whether there is a need to desulfate the LNT  207 . After initialization  301 , the method  300  begins with a threshold determination  302 . In this example, the threshold determination is whether a minimum number of denitrations, A, have taken place since the last desulfation. The use of a threshold determination provides a failsafe for preventing overly frequent desulfations. Alternate or additional criteria can be used to make this threshold determination, such as whether a minimum time of operation has elapsed since the last desulfation. 
         [0066]    If the threshold criteria are satisfied, the method  300  sets a desulfation request flag in step  306  if any of several criteria are met. These criteria are tested through a series of steps,  303 - 305 . The first criteria examines the median of a normalized LNT efficiency over the last several lean-rich cycles of NO X  trapping followed by denitration. The efficiency is normalized for the LNT temperature and the exhaust flow rate to provide a value that is largely independent of the engine  201 &#39;s speed and load. If the efficiency is below a threshold value B, the desulfation request flag is set. Step  304  checks the elapsed operating time (engine running hours) since the last desulfation. An alternative criteria could be based on the number of denitrations since the last desulfation. Step  305  checks whether the SO X  loading is greater than a critical value D. 
         [0067]    The SO X  loading is estimated. The estimate generally takes into account at least the amount of fuel used since the last desulfation and the estimated sulfur content of that fuel. The accumulation rate is optionally modified by an accumulation efficiency that depends on the temperature of the LNT  207  and or the exhaust flow rate. Optionally, the estimate includes an amount remaining after the last desulfation, which would be particularly relevant if the last desulfation was aborted prematurely. 
         [0068]    If the method  300  determines there is a need for desulfation, the desulfation request is set in step  306 . The actual start of the desulfation may be postponed until conditions are suitable, but a desulfation will begin in response to the determination. A separate algorithm checks the suitability of current conditions, The method  300  is executed periodically, e.g., after each denitration, within the course of a more broadly functioning control algorithm. If the desulfation request flag is set in step  306  before the routine exists through the return step  308 , the parameters for desulfation are set in step  307 . 
         [0069]    The parameters for desulfation include a desulfating temperature. Another parameter is typically set to determine a duration for the desulfation. This could be, for example, a total time at rich conditions, a total amount of reductant to be provided to the LNT  207  at desulfating temperature, or a total amount of sulfur to be removed. The later envisages a dynamic determination of the sulfur removal rate as a function of measured values such as the LNT temperature. Another parameter that may be set is a duration for individual rich phases to be used over the course desulfation. 
         [0070]    The example shows the determination of parameters for desulfation being made immediately after setting the desulfation request flag. Optionally the selection of parameters is postponed until it has been determined that conditions are suitable for beginning the desulfation. Postponement in this manner allows a desulfation duration parameter to be adjusted to account for additional sulfur accumulation that may occur between setting the desulfation request flag and finding that conditions are suitable to begin desulfating. 
         [0071]    Step  307 , selecting the parameters to use for the desulfation, invokes another method. This method is exemplified by the process  500 , which is illustrated with a flow chart in  FIG. 5 . The method  500  selectively modifies the desulfation parameters from previously used values based on how efficacious the previous desulfations have been in improving the performance of the LNT  207 . 
         [0072]    After initialization  501 , the first step in the method  500  is to determine  502  whether the last desulfation was completed successfully. This determination of whether a desulfation was successful may consider factors in addition to whether the desulfation was completed. Examples of additional factors are the number of rich pulses required to complete the desulfation, the median rich pulse duration, and the mean LNT temperature during the rich pulses. The desulfation may be considered unsuccessful if the number of rich pulses required to make up the rich time was excessive, if the median rich pulse duration was too short, or if the mean LNT temperature was too far below the intended temperature range. Only desulfation qualified as successful are considered in deciding whether to adapt the desulfation conditions. Step  502  prevents any adjustment to the current desulfating parameters before they have proven inadequate to restore the LNT  207  to effectiveness. If one or more desulfations using the current desulfating conditions have been completed successfully, the method  500  proceeds with step  503 , in which the efficacy of the last desulfation, or the last several desulfation, in improving LNT performance is evaluated. 
         [0073]    The assessment of LNT performance improvement can be made in any suitable manner. Any suitable measure of LNT performance can be used. In one example, LNT performance is determined according to a normalized LNT efficiency averaged over a period following the last desulfation. The efficiency is preferably measured with input from a NO X  sensor within the exhaust line  204  at a position downstream from the LNT  207 . The average is preferably taken over at least several cycles of NO X  trapping followed by denitration. As another example, the LNT performance can be determined based on how frequently it has been necessary to denitrate the LNT in order to meet pollution control targets. If a target degree of pollution control can be maintained with less frequent denitration, the LNT is performing better. 
         [0074]    The determination of efficacy can be made by evaluating the LNT performance in either relative or absolute terms. An example of a relative determination is one comparing the average LNT efficiency over a period before the last desulfation to an average LNT efficiency over a period following the last desulfation. In that case, the efficacy determination is based on the degree of improvement in performance. An example of an absolute determination is one comparing the LNT efficiency following the last desulfation against a fixed reference. A desulfation is effective if it improves LNT performance to a satisfactory degree. 
         [0075]    If the previous desulfation was not sufficiently effective, step  504  determines whether a desulfation duration parameter is already raised to its upper limit. When desulfation with current parameters is not producing a satisfactory result (is not proving sufficiently effective), the method  500  responds by increasing desulfating time before increasing the desulfating temperature. The method  500  uses a predetermined maximum, which can be a fixed value or a function of a parameter that correlates to the LNT&#39;s aging, such as the desulfation temperature. If the maximum desulfation time is variable, then it preferably diminishes as the LNT ages. An aged LNT has less functional storage capacity that is amendable to restoration by desulfation. Also, desulfation proceeds more quickly at higher temperatures. 
         [0076]    As an alternative to using a predetermined maximum duration, step  504  can instead analyze LNT performance data to determine whether the last increase in desulfation time produced a satisfactory degree of improvement in desulfation efficacy. In this alternative, the desulfation time is increased as required until the data shows these increases have reached a point of diminishing returns. In any event, if further increases to the desulfation time are tenable, because an upper limit or point of diminishing returns has not yet been met, then the step  504  directs an increase in desulfation time, step  511 . Otherwise the method proceeds with step  505 . 
         [0077]    Step  505  avoids premature increases to the desulfating temperature by preventing any increase unless desulfation using the current desulfating temperature and the maximum desulfating time has proven unsatisfactory through several attempts. Increases to the maximum temperature used for desulfation are preferably made monotonically. Also, the increases are preferably made only when it is clear that the LNT  207  cannot be operated satisfactorily without employing at least some desulfations at an increased temperature. These preferences provide the greatest deferment of LNT aging. If step  505  has been reached several times in a row due to several desulfations using the current desulfating temperature and maximum time failing to provide a satisfactory result (as determined by steps  503  and  504 ), then the method  500  proceeds with step  506 , from which the desulfating temperature can be raised. Otherwise, the method  500  proceeds with step  513 , which directs that the parameters for the last desulfation be used again. Steps  503  and  513  are no different. They are illustrated separately to make the process flow easier to display. 
         [0078]    Step  506  makes a final check before raising the desulfating temperature. This step ensures that the desulfating temperature is never raised above a predetermined upper limit. The upper limit temperature is set at the point where the deterioration experienced by the LNT is likely to outweigh the benefits of deeper desulfation regardless of how much the LNT  207  has already aged. 
         [0079]    A typical upper limit is the one specified by the LNT manufacturer, or a slightly higher temperature. While an aged LNT may be less vulnerable than a fresh LNT to the effects of high desulfating temperatures, the primary mechanism of the invention is to take advantage of overdesign. A fresh LNT is overdesigned in anticipation of diminishing performance over time. Overdesign for early life is necessary to ensure satisfactory late life performance. The invention takes advantage of this overdesign to use lower desulfating temperatures during early life and thereby slow the process of aging. The diminished effectiveness of low temperature desulfations is tolerated as long as possible in order to maintain functionality over a longer period. 
         [0080]    If the upper limit temperature has not yet been reached, then the method  500  proceeds with step  511  in which the desulfating temperature is raised. The magnitudes of the increases made in step  511  can be chosen in any suitable manner. Generally the magnitudes of the increases in desulfating temperature are predetermined. Examples of predetermined increases include, for example 5° C. or 10° C. each time the desulfating temperature is raised. Any suitable series of preselected temperatures can be used. Another example provides a series of temperatures that rise linearly on a logarithmic scale from the lowest desulfation temperature, used for a fresh catalyst, to the upper limit temperature. The upper limit temperature is not used until the LNT  207  has aged into mid or late life. 
         [0081]    If step  506  is reached with the upper limit desulfating temperature already in use, then the method proceeds with step  512 , Step  512  signals a fault and the need for service. When this fault condition is reached, operation may continue as shown. While the desulfations may be unsatisfactory, the aftertreatment system  203  may remain functional albeit outside specification in terms of either unsatisfactory pollution control or excessive fuel usage. The system  200  may begin denitrating very frequently in attempts to maintain the overly aged LNT at a satisfactory level of NO X  trapping efficiency. To limit this type of behavior, step  512  may implement other fault procedures. One option is to terminate the LNT regeneration cycle entirely until the unit has been serviced. 
         [0082]      FIG. 4  provides a flow chart of an exemplary method  400  in which a desulfation is carried out using the selected desulfating parameters. The method  400  is executed periodically whenever the desulfation request flag is set. For example, it might be executed after each denitration, or every few seconds. Following initialization, the first step in the method  400  is to determine whether conditions are suitable for starting a desulfation. Similar to the threshold determination for beginning a denitration, step  402  can require one or more of a minimum temperature for the LNT  207  or the fuel reformer  205 , a maximum exhaust oxygen concentration, a minimum exhaust flow rate, a maximum engine speed variance as determined by a moving average, and a check that a gear shift is not currently in progress. Optionally, the conduciveness of conditions to beginning a desulfation is weighed against the time since the desulfation request flag was set, or a similar criteria relating to the urgency of the need to desulfate, whereby more flexible criteria such as the engine speed variance can be given greater latitude over time in order to avoid unnecessary and excessive waiting for preferred conditions to arrive. 
         [0083]    Once step  402  determines that conditions are satisfactory, the method  400  proceeds with step  403  which checks the soot loading level of the DPF  208 . The amount of soot can be determined in any suitable manner. One option is to calculate the amount, based for example on the engine&#39;s speed load history since the last DPF regeneration (DeSoot operation) in combination with knowledge of the engine  201 &#39;s particulate matter production rate as a function of speed and load. Another option is to determine the amount of soot from the pressure differential across the DPF, the temperature of the DPF, and the engine exhaust flow rate. If it is determined that too much soot has accumulated, then step  404 , a special DeSoot operation  404  is performed before proceeding to heat the LNT  207  to desulfating temperatures in step  405 . 
         [0084]    The special DeSoot operation  404  is performed to avoid overheating the DPF  208 . When the DPF  208  is heavily loaded with soot, there is a danger that once heating initiates soot combustion, the combustion will further heat the DPF  208  to a point where damage takes place. To minimize this risk, step  404  ceases the provision of supplemental heating once the DPF  208  is hot enough for soot combustion to be underway. After the DPF  208  has had time to regenerate, the step  404  allows the method  404  to resume heating for desulfation with step  405 . 
         [0085]    Step  405  checks whether the LNT  207  is at a suitable temperature to begin the preferred cycles of alternating lean and rich phases to provide temperature control and desulfation conditions as described above. Step  405  may also check the temperature of the fuel reformer  205 . If the LNT  207  is not yet hot enough, the method  400  proceeds with step  406 , which heats the LNT  207 . Heating may be accomplished in any suitable manner. For example, fuel may be injected into the exhaust line  204  at a rate where the fuel combusts within the reformer  205  to raise it to its maximum operating temperature and then maintain the reformer  205  at that temperature until the LNT  207  is adequately heated. 
         [0086]    Once the LNT  207  is adequately heated, the method  400  proceeds with step  407  in which desulfation proceeds. In this example, desulfation proceeds through pulsed fuel injection as described above. In this context, pulsed fuel injection alternates periods of fuel injection that creates rich condition with period of no fuel injection that leave the exhaust lean. Pulsing in this context should not be confused with pulse width modulated flow control, which may be used by the fuel injector  211  to provide a desired fuel injection rate. Pulse width modulated flow control has a high frequency and results in an essentially constant fuel dosing rate as opposed to low frequency pulsing which creates alternating lean and rich conditions within the exhaust line  204 . 
         [0087]    After each rich phase or with other suitable timing, the method  400  proceeds to step  408  in which a progress variable for the desulfation is advanced. The progress variable can be of the types described previously. For example, the progress can be measured by the accumulated time at which the LNT  207  has been at rich conditions or the amount of reductant that has been provided to the LNT  207  at desulfating temperature. After advancing the progress variable, the method  400  proceeds with step  409  in which the value of the progress variable is tested to determine whether desulfation is complete. In this example, the total time at rich conditions is compared to the desulfation time set by the method  500 . 
         [0088]    If desulfation is complete, the desulfation request flag is reset (turned off) in step  411  and the process  400  completes. If, not the method  400  proceeds to step  410 , which determines whether conditions remain suitable for desulfation. This determination can be similar to step  402 , but generally with less stringent criteria. The criteria ensure that the exhaust conditions are amendable to controlling temperatures in the reformer  205  and the LNT  207 . 
         [0089]    The method  400  illustrates only one of several possible procedures that may be followed if conditions become unsuitable for continuing a desulfation that is in progress. The procedure shown has the desulfation aborting and resetting the desulfation request flag. Optionally, the desulfation flag can remain set, whereby desulfation resumes once conditions are again suitable. Optionally, step  410  waits for a period to determine if the problematic condition is fleeting before aborting. During this waiting period, fuel reformer and LNT temperatures can be maintained by fuel injection. The procedure taken can be made dependent on the condition that caused the interruption. For example, if the problem condition is excessive engine speed variance, it is likely to pass soon and waiting is preferred. If the problem condition is idle (assuming that desulfation at idle is problematic in the system  200 ), then aborting the desulfation may be the better option. 
         [0090]    Each complete desulfation takes from about 3 to about 30 minutes, more typically 5 to 10 minutes, e.g., 7.5 minutes. Shorter desulfations are generally avoided because it usually takes several minutes to heat the LNT  207  to desulfating temperatures. Longer desulfation is generally unnecessary. 
         [0091]    Desulfation comprises heating the LNT  207  to a desulfating temperature, or equivalently, to within a range of desulfating temperatures. A desulfating temperature refers to any characteristic temperature for the LNT  207 . A characteristic temperature can be, for example, a measured temperature, an estimated temperature, or an average of several measured or estimated temperatures for one or more points within the LNT  207  or the exhaust within or immediately downstream from the LNT  207 . A characteristic temperature can be an actual value, an estimated value, a target value, or a blend of the foregoing. A target value is an objective value (set point) used by a control system controlling the heating of the LNT  207 . 
         [0092]    As a practical matter, most measures of LNT temperature have a degree of variability. During desulfation, the LNT  207  will have a range of temperatures within its volume, particularly if it is being heated. Temperatures constantly vary over time due to perturbations in exhaust conditions, noise in temperature measurements, and lag in the system&#39;s response to temperature control measures. This is all in addition to the ranging of temperatures inherent if a control method in with lean-rich cycling is employed. Accordingly, even if a method purports to raise the LNT  207  to one particular desulfating temperature, it would be more accurate to say the LNT  207  is raised to within a desulfating temperature range. 
         [0093]    Descriptions of the present invention refer to a either a desulfating temperature, or a desulfating temperature range having an upper limit (peak or maximum). These descriptions are coextensive. A “desulfating temperature” can be understood as either a single characteristic temperature or the peak or average of a range for a characteristic temperature. A description referring to a range of desulfating temperatures is inclusive of cases that purport to hold the LNT  207  at a single desulfating temperature. The range can be considered either the single value (upper and lower limits the same), or the single value plus or minus a measure of uncertainty or variability, e.g. ±25° C. 
         [0094]    The invention can be employed regardless of how the LNT temperature is characterized. If the examination of any one characteristic temperature demonstrates the invention is being employed, then the invention is being employed regardless of whether that characteristic temperature is used by a controller. An increase in any one characteristic temperature or upper limit temperature will inherently result in increases to all other characteristic temperatures and upper limit temperatures. 
         [0095]    Aging of the LNT  207  refers to irreversible physical or chemical changes that occur over time with use. Aging causes a progressive deterioration in functionality. The affected functionality includes at least NO X  uptake efficiency. Aging is not relieved by desulfation. Aging can occur through a variety of mechanisms, which may or may not be elucidated. Typically, however, an important mechanism of aging is thermal aging or sintering. At elevated temperatures, small catalyst particles gradually coalesce into larger particles, the rate depending on temperature. This coalescence results in a reduction in surface area. As catalyst surface area goes down, so does catalyst activity. Aging can also occur through essentially irreversible poisoning. For example, SO X  or another compound can become bound in such a way that the poison cannot be removed by a standard desulfation process. The time over which LNT aging occurs is time in operation, especially time at desulfating temperatures. The higher the temperature, the more quickly aging is taking place. Aging will occur more quickly if high sulfur fuels, necessitating more frequent desulfations, are used. 
         [0096]    While aging can occur catastrophically, more typically aging is a process that occurs gradually over the lifetime of the LNT  207 , including many desulfations. Typically, the LNT  207  has a service life spanning several hundred thousand kilometers of highway driving or the equivalent. The service life is typically thousands of hours over which hundreds of desulfations take place. 
         [0097]    Increases in maximum desulfating temperature are made over a substantial portion of the lifetime and many desulfations. While a small increase can be made with each desulfation, use of the present invention is better characterized by increases in the maximum desulfating temperature within successive periods, each period comprising many desulfations, such as 10, 25, 50, or 100 desulfations. 
         [0098]    The increases in maximum desulfation are spread out over a substantial portion of the lifetime of the product, e.g., from about 25% to about 75% of the lifetime. Cumulative increase over the lifetime are typically from about 30 to about 100° C., e.g., about 50° C. A typical rate of increase is on the order of about 5 to about 10° C. over periods of about 50 to 100 desulfation cycles. 
         [0099]    The intervals between desulfations are typically from about 10 to 100 operating hours, e.g. 30 hours. The intervals depend on the sulfur content of the fuel and generally decrease over time due to diminishing storage capacity. Increases of 5 or 10° C. in maximum desulfating temperature are typically made over periods of about 500 to 5,000 hours of operation. 
         [0100]    The operating lifetime can be divided into consecutive time intervals, I 1 , I 2 , . . . I N , with N≧2, each interval comprising at 1,000 operating hours and at least 10 desulfations. The intervals can be selected so that the highest of the desulfating temperatures used within each interval I n  is at least 5° C. greater than that of the preceding interval, I n-1 . The total increase from I 1  to I N  is at least 30° C., preferably at least 50° C. Preferable, N is at least 3 so that the increases are made through a series of stages. Still more preferably, N is at least 5 so that the increases are gradual, e.g., about 10° C. or less per interval. 
         [0101]    The increases in maximum desulfating temperature can be stepwise or continuous. They can result from the application of a continuous function to a quantified measure of the LNT&#39;s aging, or they can be made through a series of gated stages. Each desulfation need not use a higher maximum temperature than the proceeding one. The relevant increases are those being made between successive intervals each of which comprises many desulfation. The relevant increases concern the highest among the range of temperatures used within each interval. 
         [0102]    For example, several mild desulfations with reduced maximum temperature can be used between deeper desulfations at the current maximum temperature. The mild desulfation temperatures or the fraction of desulfations that are mild can be increased, like maximum desulfation time, as a first response to desulfation ineffectiveness before raising the maximum desulfation temperature. 
         [0103]    The timing of the increases to the maximum desulfation temperature can be predetermined or dynamically determined. Predetermined increases can be made, for example, according to the number of desulfations executed or the accumulated time spent at desulfating temperatures. If the aging of a particular LNT is predictable and well characterized, a predetermined schedule for increasing the maximum desulfating temperature over time has the advantage of simplicity and avoiding the possibility of premature increases resulting from measurement error. Optionally, a predetermined timing can be used to provide a minimum period to wait before increasing desulfating temperatures regardless of whether an increase is indicated by a dynamically assessment of whether a temperature increase is warranted. 
         [0104]    A dynamic method of increasing maximum desulfating temperatures comprises analyzing data to assess the current state or performance of the LNT  207 . Such an analysis seeks to determine whether the LNT  207  can be adequately desulfated without further increases to the maximum desulfation temperature. The function of desulfation is to restore the LNT  207  to adequate levels of performance. When current desulfation condition no longer restore the LNT  207  to adequate levels of performance, an increase in the maximum desulfating temperature may be indicated. Thus, a dynamic method analyzes performance, or a measure of state that relates to performance, in order to determine whether there is a need to raise the maximum desulfation temperature. 
         [0105]    The data to be examined relates to the performance of the LNT  207  in terms of trapping NO X  during lean phases and reducing NO X  during rich phases. Measures that can be used include, for example and without limitation, NO X  trapping efficiency, NO X  storage capacity, frequency with which denitration is required, reductant usage during denitration, and oxygen storage capacity of the LNT  207 . Any suitable data can be used. The data actually used may depend on what is available to the controller  210  as a result of the design choices made for scheduling and controlling denitration. Typically, the data considered will span at least a plurality of desulfations in order that the effects of LNT aging can be distinguished from other sources of variability in desulfation effectiveness and subsequent LNT performance. 
         [0106]    Some of the forgoing examples analyze LNT efficiency data to determine whether to desulfate the LNT  207  or whether to adapt the desulfating conditions. This efficiency data can be normalized to eliminate factors other than catalyst aging and sulfur loading that affect LNT performance. Such factors include LNT temperature and the exhaust flow rate. It can, however, be difficult to make data collected under disparate operating conditions comparable. 
         [0107]    A method for analyzing LNT performance, which is applicable to a variety of methods for determining whether to desulfate an LNT or adapt LNT desulfation conditions, sorts LNT performance data into separate groups according to the conditions under which the data was collected. Each group corresponds to a distinct range of conditions. For example, there may be 16 groups indexed according to two parameters. The two parameters can be, for example, engine out exhaust gas temperature and engine out oxygen concentration. The span of exhaust temperatures is divided into four ranges, the span of exhaust oxygen concentrations is divided into four ranges, whereby the possible combination create the 16 distinct groups. 
         [0108]    When a decision is to be made based on LNT performance, the decision criteria is analyzed separately for each group&#39;s data. If the criteria is met based on the data from any one group for which a statistically significant sample has been collected, the decision, such as a decision to desulfate or adapt desulfating conditions, can be made. Variations on the method include using only the group with the most recently obtained statistically significant data, only the group with the most statistically significant data, or voting among the groups having data that is both sufficiently recent and statistically significant. 
         [0109]    The number of parameters to use, the identity of the parameters, the number of ranges to divide each parameter into, are all choices that can be flexibly made. Suitable parameters include, without limitation those that characterize the engine&#39;s operating state, such as torque, speed, and load, and those that characterize the exhaust or LNT condition, such as air-to-fuel ratio, oxygen concentration, flow rate, exhaust temperature, and LNT temperature. 
         [0110]    The number or parameters and their ranges are preferably defined in such a way that the majority of operating conditions fall within a relatively small or moderate number of groups, such as 4 to 20. The number of groups is preferably limited to ensure that at least one group generally accumulates a statistically significant set of data in a timely fashion for making the relevant decision. 
         [0111]    Several sets of data can be retained. For example there may be one set corresponding to LNT performance within limited intervals that follow successfully completed desulfations. That data set is used for adapting desulfating conditions. There may be another data set that contains the most recent LNT performance data. That data, optionally in combination with the post desulfation data, can be used to decide whether it is time for desulfation. 
         [0112]    Within each group, only a limited number of data points corresponding to the most recently collected data may be retained. For example, each group may retain a number of date points in the range from 3 to 7, for example 5. As new data is collected, the oldest is discarded. Alternatively, the number of data points retained is not limited by number, but is limited by age. In using the data, an average or median value can be taken. In one example, 5 data points are retained within each set of each group. Analysis of lean NO X  trap performance uses the averages taken after discarding the highest and lowest values.