Abstract:
A vehicle is provided with a diesel engine, an exhaust treatment system including a LNT, and a controller configured to control the provision of a reductant to the exhaust for regenerating the LNT. The controller selectively provides the reductant to the exhaust based in part on a measure of whether the engine is undergoing a change in operating state. Preferably, the measure indicates if the engine is undergoing a speed or torque increase. During these types of transients, ideal conditions for LNT regeneration often occur. Ideal conditions include low exhaust oxygen concentrations combined with relatively low exhaust flow rates. Regenerating preferentially when these ideal conditions occur reduces the fuel penalty associated with LNT regeneration. These conditions also simplify regulating LNT and reformer temperatures. In a related concept, the transient is artificially created.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to pollution control devices for diesel engines. 
       BACKGROUND 
       [0002]    NO x  emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NO x  emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. 
         [0003]    In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NO x  emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective. 
         [0004]    Several solutions have been proposed for controlling NOx 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 NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NO x  catalysts, selective catalytic reduction (SCR), and lean NO x  traps (LNTs). 
         [0005]    Lean-burn NOx catalysts promote the reduction of NO x  under oxygen-rich conditions. Reduction of NOx 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. Lean-burn NO x  catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low. 
         [0006]    SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx 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. 
         [0007]    To clarify the state of a sometime ambiguous nomenclature, it should be noted that in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NOx catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NOx catalysis. Commonly when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NOx catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons. 
         [0008]    LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich condition. A LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO 3  and the catalyst is typically a precious metal, such as Pt or Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT. 
         [0009]    A LNT can produce ammonia during denitration. Accordingly, it has been proposed to combine a LNT and an ammonia-SCR catalyst into one system. Ammonia produced by the LNT during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone LNT with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes an exhaust treatment system with a LNT and a SCR catalyst in series. U.S. Pat. Pub. No. 2004/0076565 describes a combined LNT/SCR system wherein both components are contained within a single shell or disbursed over one substrate. 
         [0010]    Creating a reducing environment for LNT regeneration involves eliminating most of the oxygen from the exhaust and providing a reducing agent. Except where the engine can be run stoichiometric or rich, a portion of the reductant reacts within the exhaust to consume oxygen. The amount of oxygen to be removed by reaction with reductant can be reduced in various ways. If the engine is equipped with an intake air throttle, the throttle can be used. The transmission gear ratio can be changed to shift the engine to an operating point that produces equal power but contains less oxygen. However, at least in the case of a diesel engine, it is generally necessary to eliminate some of the oxygen in the exhaust by combustion or reforming reactions with reductant that is injected into the exhaust. 
         [0011]    The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream of the LNT, whereby the heat of reaction does not cause large temperature increases within the LNT at every regeneration. 
         [0012]    In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The temperature of the exhaust can be elevated by engine measures, particularly in the case of a lean-burn gasoline engine, however, at least in the case of a diesel engine, it is often necessary to provide additional heat. Typically, this heat is provided through the same types of reactions as used to remove excess oxygen from the exhaust. 
         [0013]    Reductant can be injected into the exhaust by the engine or a separate fuel injection device. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. Alternatively, or in addition, reductant can be injected into the exhaust downstream of the engine. 
         [0014]    U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the &#39;037 application”) describes an exhaust treatment system with a fuel reformer placed in the exhaust line upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate. For desulfations, heat produced by the reformer is used to raise the LNT to desulfations temperatures. 
         [0015]    WO 2004/090296 describes such a system wherein there is an inline reformer upstream of a LNT and a SCR catalyst. The reformer has a high thermal mass and is intended to operate at exhaust gas temperatures. 
         [0016]    The operation of an inline reformer can be modeled in terms of the following three equations: 
         [0000]      0.684CH 1.85 +O 2 →0.684CO 2 +0.632H 2 O  (1) 
         [0000]      0.316 CH 1.85 +0.316H 2 O→0.316 CO+0.608H 2   (2) 
         [0000]      0.316CO+0.316H 2 O→0.316CO 2 +0.316H 2   (3) 
         [0000]    wherein CH 1.85  represents an exemplary reductant, such as diesel fuel, with a 1.85 ratio between carbon and hydrogen. Equation (1) is exothermic complete combustion by which oxygen is consumed. Equation (2) is endothermic steam reforming. Equation (3) is the water gas shift reaction, which is comparatively thermal neutral and is not of great importance in the present disclosure, as both CO and H 2  are effective for regeneration. 
         [0017]    The inline reformer of the &#39;037 application is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby Reaction (1) takes place. After warm up, the fuel injection rate is increased to provide a rich exhaust. Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the reformer tends to either heat or cool as reformate is produced. 
         [0018]    In theory, the temperature of the reformer can be controlled through the fuel injection rate. For example if the reformer is heating, the fuel injection rate can be increased to increase the extent of endothermic Reaction (2) (endothermic steam reforming) while the extent of Reaction (1) (exothermic complete combustion), which is limited by the exhaust oxygen concentration, remains essentially constant. In practice, this approach often cannot be used. The size and catalyst loading of the reformer are limited for economic reasons, among others, and the efficiency of the fuel reformer is generally insufficient to accommodate high fuel injection rates. As a result, the reformer tends to heat as reformate is being produced, particularly when exhaust oxygen concentrations are in the 8-15% range. 
         [0019]    One approach suggested by the &#39;037 application for controlling this heating is to pulse the fuel injection. The reformer is allowed to cool between pulses. In this manner, the reformer can be kept at a desired temperature while a LNT regeneration completes. A desired temperature is, for example, 600° C.±50° C. 
         [0020]    Many publications propose reducing the fuel penalty by providing two or more LNTs in a parallel arrangement. During regeneration of an LNT, all or part of the exhaust flow can be diverted to the other LNTs. The implementation of this method requires the use of at least one exhaust valve that for a heavy duty truck must generally fit an exhaust pipe with an inner diameter of at least about 10 cm. U.S. Pat. No. 6,820,417 describes a four-way valve for this purpose. U.S. Patent Pub. No. 2004/0139730 describes a valve that divides reductant and exhaust between two LNTs. In a first position the valve directs reductant to one LNT and exhaust to the other and in a second position switches the flows. The durability and reliability of these valves is not known, although experience with smaller EGR valves suggest durability and reliability will present challenges for these valves. 
         [0021]    In certain applications that employ LNTs, as in lean-burn gasoline engines, stoichiometric air-fuel ratios occur during normal operation. It is known to preferentially carry out denitration when such favorable conditions occur during normal vehicle operation. For example, U.S. Patent Pub. No. 2003/0115858, teaches preferentially regenerating an LNT when engine power demand is high, and U.S. Patent Pub. No. 2003/0089103 teaches avoiding regeneration when an engine is at idle. 
         [0022]    It is also known that regeneration, especially desulfation, can be carried out more efficiently if initiated while an LNT is relatively hot. U.S. Pat. No. 6,128,899 teaches regenerating a LNT just before fuel cut-off events to avoid having to regenerate the LNT after it becomes cold. U.S. Pat. No. 6,637,198 teaches carrying out partial desulfation when an LNT is at a critical temperature as a result of a normal driving cycle. 
         [0023]    U.S. Pat. No. 6,742,328, suggests reducing the fuel penalty for regenerating a LNT in an exhaust treatment system of a compression ignition diesel engine by performing partial regenerations during deceleration to take advantage of low flow conditions. 
         [0024]    In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and can practically be used to reduce NOx emissions across the spectrum of diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations. 
       SUMMARY 
       [0025]    One of the inventor&#39;s concepts relates to a vehicle comprising a diesel engine, an exhaust treatment system including a LNT, and a controller configured to control the provision of a reductant to the exhaust for regenerating the LNT. The controller selectively provides the reductant to the exhaust based in part on a measure of whether the engine is undergoing a change in operating state. Preferably, the measure indicates if the engine is undergoing a speed or torque increase. During these types of transients, ideal conditions for LNT regeneration often occur. Ideal conditions include low exhaust oxygen concentrations combined with relatively low exhaust flow rates. Regenerating preferentially when these ideal conditions occur reduces the fuel penalty associated with LNT regeneration. These conditions also simplify regulating LNT and reformer temperatures. 
         [0026]    Another of the inventor&#39;s concepts relates to a vehicle on-board computer method of determining whether to initiate regeneration of a LNT disposed in an exhaust passage of a diesel engine. The method comprises obtaining an indication of whether the engine is undergoing a change in operating state and applying the indication in determining whether to regenerate the LNT, whereby the LNT is preferentially regenerated when the engine is undergoing a positive speed or torque gradient. 
         [0027]    A further concept of the inventor&#39;s is a method of operating a diesel power generation system in which a LNT is used to adsorb NOx from the engine exhaust under lean conditions. From time-to-time, a control signal to regenerate the LNT is generated by a controller. In response to the control signal, a transmission gear ratio is changed to create transient exhaust conditions. The gear ratio can take place without altering the systems power output. Also in response to the control signal, rich exhaust conditions are created to regenerate the LNT. In one embodiment, the transient exhaust conditions created by the gear change overlap the period of rich exhaust conditions. In another embodiment, the transient conditions overlap a period in which a reformer configured within the exhaust line is warmed. Transient conditions can facilitate or reduce the fuel penalty for either the reformer warm up or the LNT regeneration processes. 
         [0028]    The primary purpose of this summary has been to present certain of the inventor&#39;s 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 inventor&#39;s concepts or every combination of the inventor&#39;s concepts that can be considered “invention”. Other concepts of the inventor 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 inventor claims as his invention being reserved for the claims that follow. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a schematic illustration of a power generation system in which the inventor&#39;s concepts can be implemented. 
           [0030]      FIG. 2  includes plots of torque, speed, and pedal position for a truck undergoing a FTP cycle without motoring. 
           [0031]      FIG. 3  includes plots of time-averaged speed and pedal position corresponding to the cycle of  FIG. 2 . 
           [0032]      FIG. 4  includes plots of the gradients of the quantities charted in  FIG. 3 . 
           [0033]      FIG. 5  includes plots showing the occurrence of peaks in average pedal position gradient and average speed gradient and corresponding exhaust conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    One of the inventor&#39;s concepts is to initiate LNT regenerations during the unique exhaust conditions that occur during engine transients. An engine transient, as the term is used herein, is a period over which the engine operating state is changing, as opposed to merely a temporary state. An operating state relates primarily to the engine speed and torque. Accordingly, an engine transient involves a gradient in engine speed and or engine torque, preferably both. 
         [0035]    The engine operating state can be varied through fuel and air supply rates. When an accelerator pedal is depressed, the air supply rate changes more slowly than the fueling rate. For example, if the engine has a turbo-charger, the turbo-charger takes up to a few seconds to accelerate to a new steady state. During the transition period from one engine operating state to the next, the exhaust composition and flow rate reflect both the previous state and the emerging state. 
         [0036]    Ideal conditions for LNT regeneration often occur during engine speed or torque accelerations (increases). During this type of acceleration, the exhaust oxygen concentration drops to low levels. Oxygen concentrations from about 4 to about 8% may be considered low for diesel exhaust. The exhaust flow rate is also relatively low during engine speed or torque acceleration. The exhaust flow rate eventually increases, but it increases more slowly than the exhaust oxygen concentration drops. Low oxygen concentrations and low flow rates reduce the fuel penalty for LNT regenerations. 
         [0037]    Accordingly, one of the inventor&#39;s concepts is to initiate regenerations preferentially during engine transients, particularly transients involving engine speed or torque accelerations. A preferred transient creates a relatively low exhaust oxygen concentration and a relatively low exhaust flow rate, the later of which typically increases towards the end of the transient. An engine transient should not be confused with a vehicle speed transient. While engine speed and vehicle speed are correlated as long as a vehicle remains in one gear, by and large torque and engine speed can and do vary independently of vehicle speed. 
         [0038]    Any suitable measure can be used to detect engine transients. Examples include measures of engine speed gradient, engine torque gradient, engine load gradient, engine power gradient, and accelerator pedal position gradient. Engine speed refers to engine RPM. Engine load is essentially the same as engine torque, load being a measure of torque as a percentage of maximum torque. Accelerator pedal position can refer generically to any device that a vehicle operator uses to manually increase the engine fueling rate. 
         [0039]    Any of these measures can be time averaged. With respect to engine speed, time averaging can be used to average out variations across gears such as the variations that occur as a vehicle accelerates under constant torque. Averaging can smooth out the torque variations that occur during gear shifts. With respect to pedal position, time averaging can provide a more definitive and prolonged signal when an engine transient is initiated by depressing an accelerator pedal. A time average can be over any suitable period, for example, a period in the range from about to about 0.5 to about 10 seconds, more preferably from about 1 to about 5 seconds, most preferably around 3 seconds. 
         [0040]    A preferred measure for detecting engine transients is to monitor a time-averaged pedal position gradient. The pedal position is generally available to the engine ECU. A minimum pedal position gradient upon which to initiate regeneration can be, for example, from about 5 to about 10% per second, wherein the percentages are with respect to the pedal&#39;s full range of positions. 
         [0041]    Another preferred measure for detecting engine transients is the engine speed, which is also typically available to the engine ECU. An engine speed gradient upon which to initiate regeneration can be, for example, at least about 100 RPM per second, more preferably at least about 200 RPM per second. 
         [0042]    A combination of measures can be used to provide a more reliable indication of an engine operating transient suitable for regeneration. For example, a criteria for regeneration can involve a combination of engine speed and pedal position gradients. An ideal condition for regeneration occurs when the engine speed gradient is at least about 200 RPM per second and the time-averaged pedal position gradient is at least about 5% per second. 
         [0043]    The decision whether to regenerate generally involves consideration of several factors in addition to whether engine conditions are conducive to regeneration. One consideration is whether, and to what degree, LNT regeneration is needed. Another consideration can be whether one or more of the exhaust temperature, the reformer temperature, or the LNT temperature is sufficiently high. 
         [0044]    The need to regenerate the LNT can be a yes/no determination. In such an embodiment, the LNT is regenerated when the need to regenerate has been identified and a suitable engine transient occurs. If a transient does not occur within a certain period, or if the need to regenerate the LNT becomes too urgent, the LNT can be regenerated without waiting for a transient. 
         [0045]    A more preferred system uses a weighted determination, wherein the magnitude of the engine operational parameter gradient that will trigger a regeneration has an inverse relationship with a measure of LNT loading. The measure of LNT loading may involve, for example, an estimate of the amount of NOx stored, an estimate of the remaining NOx storage, or a measure of the NOx removal efficiency. If the LNT is heavily loaded, little or no engine transient may be required to initiate regeneration. If the LNT is moderately loaded, LNT regeneration may only be initiated if the engine transient involves a sufficiently large gradient. If the LNT has little or no NOx loading, than LNT regeneration may not be triggered regardless of engine transients. 
         [0046]    In a preferred embodiment, a measure of LNT loading is used to determine a critical value for a gradient in a measure of an engine operating state. If the gradient exceeds the critical value, regeneration is initiated. It should be appreciated that exceeding can be understood as exceeding in absolute value. 
         [0047]    In one embodiment, regeneration is initiated if the sum of a measure of engine state and a measure of LNT loading exceeds a fixed value. The measure of LNT loading can vary from zero to 1.0 as the LNT trap goes from empty to full. The measure of engine state can go from 0 to 0.5 as the engine state goes from no gradient to an ideal gradient, such as a pedal position gradient of at least about 5% per second. When the sum of these two numbers exceeds a fixed value, such as 1.0, LNT regeneration can be initiated. 
         [0048]    A LNT regeneration, as the term is used herein, generally refers to a denitration. The inventor&#39;s concepts can be applied to desulfations, but the transients are not long enough to do much more than help at the very start of a desulfation. 
         [0049]    An additional benefit of regenerating upon an engine speed or torque acceleration is that high NOx production rates tend to occur in the steady state conditions that follow these gradients. By regenerating during the gradients, a refreshed NOx trap can be provided when the demands for exhaust treatment are at their highest. 
         [0050]    Another benefit is that engine efficiency tends to drop during these transients. Engine inefficiency is associated with increased hydrocarbon content in the exhaust. The inefficiency can be so great that the fuel injection rate must be limited during the transient to prevent the formation of smoke. Increased hydrocarbon content in the exhaust can facilitate light-off a fuel reformer and or regeneration of a LNT. 
         [0051]    Engine transients of the type identified herein as creating good conditions for LNT regeneration occur frequently during ordinary truck driving cycles, particularly stop and go driving.  FIG. 2  provides plots of engine torque, engine speed, and accelerator pedal position for a Caterpillar C9 engine going through a US federal test protocol (FTP) cycle (also known as the transient emission cycle) specified in connection with truck emission regulations. The cycle is typical of ordinary truck driving cycles. Although actual driving cycles vary, the efficacy of a method in connection with this cycle is indicative of efficacy for many, if not all, actual driving cycles. The plots shows engine speeds and torques through the cycle, except for the motoring portion of the cycle, which is excluded. 
         [0052]      FIG. 3  shows time averages of engine speed and accelerator pedal position for the cycle of  FIG. 2 .  FIG. 4  shows the gradients in the time-averaged engine speed and time-averaged pedal position. The average pedal position gradient frequently exceeds 5% per second and the average engine speed gradient frequently exceeds 200 RPM per second. 
         [0053]      FIG. 5  shows the averages of  FIG. 4 , filtering to preserve only the average pedal position gradient peaks that exceed 5% per second and the average engine speed gradient peaks that exceed 200 RPM per second. Exhaust oxygen and exhaust flow rate are also plotted. These plots show that ideal conditions for LNT regeneration occur frequently in the FTP cycle and that these conditions can be identified by the average pedal position gradient exceeding 5% per second and by the average engine speed gradient exceeding 200 RPM per second. 
         [0054]      FIG. 1  provides a schematic illustration of an exemplary power generation system  5  in which various concepts of the inventor can be implemented. The system  5  comprises an engine  9 , a transmission  8 , and an exhaust aftertreatment system  7 . The exhaust aftertreatment system  7  includes a controller  10 , a fuel injector  11 , a diesel particulate filter (DPF)  16 , a reformer  12 , a lean NOx-trap (LNT)  13 , an ammonia-SCR catalyst  14 , and a clean-up catalyst  17 . The controller  10  receives data from several sources; include temperature sensors  20  and  21  and NOx sensors  22  and  23 . The controller  10  may be an engine control unit (ECU) that also controls the exhaust aftertreatment system  7  or may include several control units that collectively perform these functions. 
         [0055]    The DPF  16  removes particulates from the exhaust. During lean operation (a lean phase), the LNT  13  adsorbs a portion of the NOx from the exhaust. The ammonia-SCR catalyst  14  may have ammonia stored from a previous regeneration of the LNT  13  (a rich phase). If the ammonia-SCR catalyst  14  contains stored ammonia, it removes a second portion of the NOx from the lean exhaust. The clean-up catalyst  17  may serve to oxidize CO and unburned hydrocarbons remaining in the exhaust. 
         [0056]    From time-to-time, the LNT  13  must be regenerated to remove accumulated NOx (denitrated). Denitration may involve heating the reformer  12  to an operational temperature and then injecting fuel using the fuel injector  11 . The reformer  12  uses the injected fuel to consume most of the oxygen from the exhaust while producing reformate. The reformate thus produced reduces NOx adsorbed in the LNT  13 . Some of this NOx is reduced to NH 3 , most of which is captured by the ammonia-SCR catalyst  14  and used to reduce NOx during a subsequent lean phase. The clean-up catalyst  17  oxidizes unused reductants and unadsorbed NH 3  using stored oxygen or residual oxygen remaining in the exhaust during the rich phases. 
         [0057]    From time-to-time, the LNT  13  must also be regenerated to remove accumulated sulfur compounds (desulfated). Desulfation involves heating the reformer  12  to an operational temperature, heating the LNT  13  to a desulfating temperature, and providing the heated LNT  13  with a reducing atmosphere. Desulfating temperatures vary, but are typically in the range from about 500 to about 800° C., with optimal temperatures typically in the range from about 650 to about 750° C. Below a minimum temperature, desulfation is very slow. Above a maximum temperature, the LNT  13  may be damaged. 
         [0058]    While the engine  9  is preferably a compression ignition diesel engine, the various concepts of the inventor are applicable to power generation systems with lean-burn gasoline engines or any other type of engine that produces an oxygen rich, NOx-containing exhaust. For purposes of the present disclosure, NOx consists of NO and NO 2 . 
         [0059]    The transmission  8  can be any suitable type of transmission. The transmission  8  can be a conventional transmission such as a counter-shaft type mechanical transmission, but is preferably a CVT. A CVT can provide a much larger selection of operating points than a conventional transmission and generally also provides a broader range of torque multipliers. In general, a CVT will also avoid or minimize interruptions in power transmission during shifting. Examples of CVT systems include hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multispeed gear boxes with slipping clutches; and V-belt traction drives. A CVT may involve power splitting and may also include a multi-step transmission. 
         [0060]    A preferred CVT provides a wide range of torque multiplication ratios, reduces the need for shifting in comparison to a conventional transmission, and subjects the CVT to only a fraction of the peak torque levels produced by the engine. This can be achieved using a step-down gear set to reduce the torque passing through the CVT. Torque from the CVT passes through a step-up gear set that restores the torque. The CVT is further protected by splitting the torque from the engine, and recombining the torque in a planetary gear set. The planetary gear set mixes or combines a direct torque element transmitted from the engine through a stepped automatic transmission with a torque element from a CVT, such as a band-type CVT. The combination provides an overall CVT in which only a portion of the torque passes through the band-type CVT. 
         [0061]    A wide range of torque ratios can be used to facilitate start-up of the reformer  12  and or regeneration of the LNT  13 . When regeneration is required, a torque ratio can be selected that meets current power demands while providing favorable exhaust conditions. The torque ratio selection may involve a compromise between engine fuel economy and ease or cost of the reformer warm-up and or LNT regeneration processes. 
         [0062]    A further concept of the inventor is to expand the range of available exhaust conditions for LNT regeneration by artificially creating an engine transient when LNT regeneration is required. An engine transient is artificially created by changing the transmission gear ratio independent of power demands. Typically, the power demand will be maintained at an operator determined level, which may be essentially constant throughout the engine transient. The transient is typically generated in response to a control signal to regenerate the LNT. 
         [0063]    The transient may involve either an increase or decrease in engine speed, the actual selection depending on the tuning of the engine  9 , the current engine operating state, and the state of the exhaust aftertreatment system  7 . A selection can be made by doing a search among all the available engine transients and comparing their effects using a model of the exhaust aftertreatment system  7 . The transient conditions themselves are generally reproducible but difficult to predict and may best be determined experimentally in a calibration process. 
         [0064]    In one embodiment, the engine transient is initiated just before or during a warm-up operation for the fuel reformer  12  and provides a more rapid or lower fuel penalty warm-up then would occur if the fuel reformer were started at the steady state condition following the transient. Fuel reformer warm-up is favored by higher exhaust temperatures and lower exhaust flow rates. 
         [0065]    In another embodiment, the gear change is timed whereby the engine transient overlaps the rich phase in which the LNT  13  is regenerated. Preferably, the overlap covers at least a large portion of the rich phase, for example, at least about 25%, more preferably at least about 50%. The period of the transient can be defined by the period from when engine exhaust flow and composition conditions, such as total flow rate and oxygen concentration, begin to change to when they have all completed 90% of their transition. Generally, the transient will be selected to stabilize the operation of the reformer  12  during the rich phase or to reduce the fuel penalty for the LNT regeneration. 
         [0066]    The fuel injector  11  can be of any suitable type. Preferably, it provides the fuel in an atomized or vaporized spray. The fuel may be injected at the pressure provided by a fuel pump for the engine  9 . Preferably, however, the fuel passes through a pressure intensifier operating on hydraulic principles to at least double the fuel pressure from that provided by the fuel pump to provide the fuel at a pressure of at least about 4 bar. 
         [0067]    A fuel reformer is a device that converts heavier fuels into lighter compounds without fully combusting the fuel. A fuel reformer can be a catalytic reformer or a plasma reformer. Preferably, the reformer  12  is a partial oxidation catalytic reformer comprising a steam reforming catalyst. Examples of reformer catalysts include precious metals, such as Pt, Pd, or Ru, and oxides of Al, Mg, and Ni, the later group being typically combined with one or more of CaO, K 2 O, and a rare earth metal such as Ce to increase activity. A reformer is preferably small in size as compared to an oxidation catalyst or a three-way catalyst designed to perform its primary functions at temperatures below 450° C. The reformer is generally operative at temperatures from about 450 to about 1100° C. 
         [0068]    The LNT  13  can comprise any suitable NOx-adsorbing material. Examples of NOx adsorbing materials include oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. Further examples of NOx-adsorbing materials include molecular sieves, such as zeolites, alumina, silica, and activated carbon. Still further examples include metal phosphates, such as phosphates of titanium and zirconium. Generally, the NOx-adsorbing material is an alkaline earth oxide. The absorbent is typically combined with a binder and either formed into a self-supporting structure or applied as a coating over an inert substrate. 
         [0069]    The LNT  13  also comprises a catalyst for the reduction of NOx in a reducing environment. The catalyst can be, for example, one or more transition metals, such as Au, Ag, and Cu, group VIII metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Pt and Rh. Precious metal catalysts also facilitate the adsorbent function of alkaline earth oxide absorbers. 
         [0070]    Adsorbents and catalysts according to the present invention are generally adapted for use in vehicle exhaust systems. Vehicle exhaust systems create restriction on weight, dimensions, and durability. For example, a NOx adsorbent bed for a vehicle exhaust systems must be reasonably resistant to degradation under the vibrations encountered during vehicle operation. 
         [0071]    The ammonia-SCR catalyst  16  is a catalyst effective 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, substituted with metal ions such as cations of Cu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCR catalyst  16  is designed to tolerate temperatures required to desulfate the LNT  13 . 
         [0072]    The particulate filter  15  can have any suitable structure. Examples of suitable structures include monolithic wall flow filters, which are typically made from ceramics, especially cordierite or SiC, blocks of ceramic foams, monolith-like structures of porous sintered metals or metal-foams, and wound, knit, or braided structures of temperature resistant fibers, such as ceramic or metallic fibers. Typical pore sizes for the filter elements are about 10 μm or less. 
         [0073]    The inclusion of, and the location of, the DPF  15  is optional. Between the reformer  12  and the LNT  13 , the DPF  15  can serve to protect the LNT  13  from temperature excursions associated with the operation of the reformer  12 . Between the LNT  13  and the ammonia-SCR catalyst  16 , the DPF  15  can help protect the SCR catalyst  16  from desulfation temperatures. Optionally, one or more of the reformer  12 , the LNT  13 , the additional catalyst  14 , and the ammonia-SCR catalyst  16  is integrated as a coating or within the structure of the DPF  15 . 
         [0074]    The DPF  15  is regenerated to remove accumulated soot. The DPF  15  can be of the type that is regenerated continuously or intermittently. For intermittent regeneration, the DPF  15  is heated, using a reformer  12  for example. The DPF  15  is heated to a temperature at which accumulated soot combusts with O 2 . This temperature can be lowered by providing the DPF  15  with a suitable catalyst. After the DPF  15  is heated, soot is combusted in an oxygen rich environment. 
         [0075]    For continuous regeneration, the DPF  15  may be provided with a catalyst that promotes combustion of soot by both NO 2  and O 2 . Examples of catalysts that promote the oxidation of soot by both NO 2  and O 2  include oxides of Ce, Zr, La, Y, Nd, Pt, and Pd. To completely eliminate the need for intermittent regeneration, it may be necessary to provide an additional oxidation catalyst to promote the oxidation of NO to NO 2  and thereby provide sufficient NO 2  to combust soot as quickly as it accumulates. Where regeneration is continuous, the DPF  15  is suitably placed upstream of the reformer  12 . Where the DPF  15  is not continuously regenerated, it is generally positioned downstream of the reformer  12 . 
         [0076]    The clean-up catalyst  17  is preferably functional to oxidize unburned hydrocarbons from the engine  9 , unused reductants, and any H 2 S released from the NOx absorber-catalyst  13  and not oxidized by the ammonia-SCR catalyst  16  or the additional catalyst  14 . Any suitable oxidation catalyst can be used. To allow the clean-up catalyst  17  to function under rich conditions, the catalyst may include an oxygen-storing component, such as ceria. Removal of H 2 S, where required, may be facilitated by one or more additional components such as NiO, Fe 2 O 3 , MnO 2 , CoO, and CrO 2 . 
         [0077]    The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. While a particular component or feature may have been disclosed herein with respect to only one of several concepts or examples or in both broad and narrow terms, the components or features in their broad or narrow conceptions may be combined with one or more other components or features in their broad or narrow conceptions wherein such a combination would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein.