Abstract:
A catalyst system that may regenerate while removing pollutants from an exhaust gas of an engine. The system may have a converter with multiple segments of chambers. At least one of the chambers may be regenerated while the remaining chambers are removing pollutants from the exhaust. The chambers may be rotated in turn for one-at-a-time regeneration. More than one chamber may be regenerated at a time to remove collected pollutants. The system may have plumbing and valves, and possibly mechanical movement of the chambers, within the system to effect the changing of a chamber for regeneration. The chambers connected to the exhaust may be in series or parallel. A particulate matter filter may be connected to the system, and it also may be regenerated to remove collected matter.

Description:
BACKGROUND  
       [0001]     The present invention relates to engine exhaust systems and particularly to exhaust catalyst systems. More particularly the invention relates to catalyst units.  
         [0002]     Spark ignition engines often use catalytic converters and oxygen sensors to help control engine emissions. A gas pedal is typically connected to a throttle that meters air into engine. That is, stepping on the pedal directly opens the throttle to allow more air into the engine. Oxygen sensors are often used to measure the oxygen level of the engine exhaust, and provide feed back to a fuel injector control to maintain the desired air/fuel ratio (AFR), typically close to a stoichiometric air-fuel ratio to achieve stoichiometric combustion. Stoichiometric combustion can allow three-way catalysts to simultaneously remove hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx) in attempt to meet emission requirements for the spark ignition engines.  
         [0003]     Compression ignition engines (e.g., diesel engines) have been steadily growing in popularity. Once reserved for the commercial vehicle markets, diesel engines are now making real headway into the car and light truck markets. Partly because of this, federal regulations were passed requiring decreased emissions in diesel engines.  
         [0004]     Many diesel engines now employ turbochargers for increased efficiency. In such systems, and unlike most spark ignition engines, the pedal is not directly connected to a throttle that meters air into engine. Instead, a pedal position is used to control the fuel rate provided to the engine by adjusting a fuel “rack”, which allows more or less fuel per fuel pump shot. The air to the engine is typically controlled by the turbocharger, often a variable nozzle turbocharger (VNT) or waste-gate turbocharger.  
         [0005]     Traditional diesel engines can suffer from a mismatch between the air and fuel that is provided to the engine, particularly since there is often a time delay between when the operator moves the pedal, i.e., injecting more fuel, and when the turbocharger spins-up to provide the additional air required to produced the desired air-fuel ratio. To shorten this “turbo-lag”, a throttle position sensor (fuel rate sensor) is often added and fed back to the turbocharger controller to increase the natural turbo acceleration, and consequently the air flow to the engine.  
         [0006]     The pedal position is often used as an input to a static map, which is used in the fuel injector control loop. Stepping on the pedal increases the fuel flow in a manner dictated by the static map. In some cases, the diesel engine contains an air-fuel ratio (AFR) estimator, which is based on input parameters such as fuel injector flow and intake manifold air flow, to estimate when the AFR is low enough to expect smoke to appear in the exhaust, at which point the fuel flow is reduced. The airflow is often managed by the turbocharger, which provides an intake manifold pressure and an intake manifold flow rate for each driving condition.  
         [0007]     In diesel engines, there are typically no sensors in the exhaust stream analogous to that found in spark ignition engines. Thus, control over the combustion is often performed in an “open-loop” manner, which often relies on engine maps to generate set points for the intake manifold parameters that are favorable for acceptable exhaust emissions. As such, engine air-side control is often an important part of overall engine performance and in meeting exhaust emission requirements. In many cases, control of the turbocharger and EGR systems are the primary components in controlling the emission levels of a diesel engine.  
         [0008]     Most diesel engines do not have emissions component sensors. One reason for the lack of emissions component sensors in diesel engines is that combustion is about twice as lean as spark ignition engines. As such, the oxygen level in the exhaust is often at a level where standard emission sensors do not provide useful information. At the same time, diesel engines may burn too lean for conventional three-way catalysts.  
         [0009]     After-treatment is often needed to help clean up diesel engine exhaust. After-treatment often includes a “flow through oxidation” catalyst. Typically, such systems do not have any controls. Hydrocarbons, carbon monoxide and most significantly those hydrocarbons that are adsorbed on particulates can sometimes be cleaned up when the conditions are right. Other after-treatment systems include particulate filters. However, these filters must often be periodically cleaned, often by injecting a slug of catalytic material with the fuel. The control of this type of after-treatment may be based on a pressure sensor or on distance traveled, often in an open loop manner.  
         [0010]     Practical NOx reduction methods presently pose a technology challenge and particulate traps often require regeneration. As a consequence, air flow, species concentrations, and temperature should be managed in some way in order to minimize diesel emission levels.  
         [0011]     Development of exhaust catalyst systems has been useful for meeting engine emissions requirements around the world. There has been a need for emission reduction efficiency and improved fuel economy in such developed catalyst systems.  
       SUMMARY  
       [0012]     The present invention addresses a reduction of the total amount of catalyst (i.e., precious metal) needed in exhaust gas catalyst system to provide a needed NOx/SOx removal efficiency. The invention involves a multi-element catalyst that may be integrated with regeneration relative to a catalyst element configuration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]      FIG. 1  shows a three member catalyst system connected an exhaust of an internal combustion engine;  
         [0014]      FIG. 2  is a graph of fuel injector events and the magnitudes reflecting some injection rate control for an engine;  
         [0015]      FIG. 3  is a graph combination showing engine performance relative to exhaust temperature management with several patterns of post injection events;  
         [0016]      FIG. 4  is a graph illustrating an example of a rate of depletion of adsorption sites on catalyst over time;  
         [0017]      FIG. 5  shows an illustrative example of a regenerative catalyst system with valves and a connected processor;  
         [0018]      FIGS. 6-9  show the example regenerative catalyst system, with series-connected chambers, showing the various flow circuits for the regeneration of each chamber;  
         [0019]      FIGS. 10   a  and  10   b  reveal a catalyst system having a rotatory structure to effect regeneration for each of the segments;  
         [0020]      FIG. 11  shows a multi-segment catalyst system having parallel-connected chambers;  
         [0021]      FIG. 12  reveals a particulate matter filter;  
         [0022]      FIG. 13  shows the multi-segment catalyst system having parallel chambers but with the flow diverted for regeneration of a chamber;  
         [0023]      FIGS. 14   a ,  15   a  and  16   a  show the availability of adsorption sites for each segment of a multi-segment catalyst system over time for various loads;  
         [0024]      FIGS. 14   b ,  15   b  and  16   b  show the relative amount of NOx versus time at the output of each segment of a multi-segment catalyst system for various loads;  
         [0025]      FIG. 17  is a graph showing filter time to regeneration as a function of the load for a catalyst system;  
         [0026]      FIGS. 18   a ,  19   a ,  20   a ,  21   a  and  22   a  are graphs showing the number of adsorption sites available for each of segments of a multi-segment system for certain regeneration periods, NOx inputs and amounts of metal of a catalyst system;  
         [0027]      FIGS. 18   b ,  19   b ,  20   b ,  21   b  and  22   b  are graphs showing the relative amount of NOx particles coming out of each of the segment stages of a multi-segment system relative to an input of particles over time for certain regeneration periods, NOx inputs and amounts of metal of a catalyst system;  
         [0028]      FIGS. 23, 24  and  25  illustrate the geometry of various catalyst batch-type operations;  
         [0029]      FIGS. 26   a  and  26   b  are graphs illustrating NOx concentration for a first geometry of catalyst operation;  
         [0030]      FIGS. 27   a  and  27   b  are graphs illustrating NOx concentration for a second geometry of catalyst operation;  
         [0031]      FIG. 28  is a graph showing NOx profiles for a multi-element catalyst system;  
         [0032]      FIGS. 29   a  and  29   b  are graphs showing a comparison of absorption sites depletion in time for the first and second geometries of the catalyst system;  
         [0033]      FIGS. 30   a  and  31   a  reveal relative amounts of NOx versus time for a catalyst system with precious metal reduction for the first and second geometries of the system, respectively;  
         [0034]      FIGS. 30   b  and  31   b  show adsorption sites depletion in space for a catalyst system with a catalyst reduction for the first and second geometries, respectively;  
         [0035]      FIGS. 32   a  and  32   b  are graphs showing absorption sites depletion in space for a multi-segment catalyst system without and with flow direction switching, respectively;  
         [0036]      FIGS. 33   a ,  33   b  and  33   c  are graphs showing the relative amount of NOx in time, the relative amount NOx in space, and absorption sites depletion in space for the second geometry of the catalyst system; and  
         [0037]      FIGS. 34   a ,  34   b ,  35   a ,  35   b ,  36   a  and  36   b  are graphs showing an impact of the segment regeneration order for regenerating the segment attached last, attached first and sequentially in view of available adsorption sites in time and the relative amount of NOx, respectively, with regard to an achievable catalyst reduction for a multi-segment catalyst system. 
     
    
     DESCRIPTION  
       [0038]     In the present description, please note that much of the material may be of a hypothetical or prophetic nature even though stated in apparent matter-of-fact language. The present catalyst system may include controlled regeneration resulting in a reduction of precious metal use and of fuel consumption of the engine incorporating the system. In a monolithic catalytic NOx removal system, the effectiveness of a catalyst may be reduced along a direction of the flow of exhaust gases. To achieve a required average NOx removal (e.g., 90 percent) with a periodic pattern of catalyst usage, (e.g., a 60 second NOx adsorption mode/5 second regeneration mode), some amount of precious metal may be needed. If the total volume of the catalyst is split into “n+1” elements, with “n” elements in the exhaust gas stream used in an NOx adsorption mode and one element regenerated, and the arrangement of the elements is periodically reshufffled, the total amount of the precious metal needed may be significantly reduced. By monitoring NOx emissions, switching times and regeneration parameters may be optimized to result in reduced fuel consumption of the engine. Reference may be made to “fluid” which may be either a gas or liquid.  
         [0039]     There may be several alternative mechanical configurations (based on switching the flow by valves or rotation of the catalyst elements), that may provide the above-noted operability. Exhaust gases may pass through “n” cleaning segments, and an “n+1” element may be regenerated. The manifold may be laid out to provide controlled flow distribution. A control system may monitor an average performance and provide control over the element configuration in the exhaust gas and regeneration streams.  
         [0040]     In one example, NOx sensors may be provided at an inlet and outlet of an after-treatment system. These sensors may be used to determine the degree of loading of the catalyst so that a regenerated segment may be brought into the exhaust gas flow and a loaded segment be brought into the regeneration flow. In another example, only one NOx sensor might be provided, for instance at the outlet, and its reading may be used to determine when to reconfigure the multi-element catalyst. Alternatively, a combination of sensors and numerical models may be used to determine the NOx loading (adsorption site depletion) of each catalyst element.  
         [0041]     In still another example, the state of regeneration of the element under regeneration may be monitored. Once a sufficient state is reached, then the regeneration may be halted. Since regeneration in many cases could require the burning of excess fuel, the fuel efficiency of the after-treatment may be improved.  
         [0042]     In yet another example, the “multi-element” catalyst may be a continuously rotating device, with a speed and/or phasing of rotation matched to the effectiveness of the catalyst, and controlled through the sensing of NOx and possibly other parameters with or without supplementary use of mathematical models, such as, for example, one or more models of the regeneration process.  
         [0043]     In the present system, the number elements may be as few as two. There is not necessarily an upper limit except as restricted by technological capabilities available at the time of application of the system.  
         [0044]     The engines dealt with relative to the present system may be the diesel engines (or lean-burn gasoline/natural gas or alternate fuel engines). For such engines, the most significant pollutants to control may be particulate matter (PM), oxides of nitrogen (NOx), and sulfur (SOx). An example catalyst system is shown in  FIG. 1 . A pre-catalyst  12  may primarily be an oxidation catalyst connected to the exhaust output of an engine  11 , which may for example be a 1.9 liter diesel engine. The pre-catalyst may be used to raise the temperature of the exhaust for a fast warm-up and to improve the effectiveness of the catalytic system downstream when the exhaust temperatures are too low. An underbody NOx adsorber catalyst (NAC)  13 , connected to the pre-catalyst  12  may be primarily for adsorbing and storing NOx in the form of nitrates. Diesel (or lean combustion) engine exhaust tends to have excess oxygen. Therefore, NOx might not be directly reducible to N2. The NOx may be stored for a short period of time (as an example, for about a 60 second capacity). A very short period (i.e., about 2 to 5 seconds) of near stoichiometric fuel air mixture operation may be conducted to get the exhaust stream down to a near-zero oxygen concentration. The temperature may also be raised to a desirable window. Under these conditions, NOx may react with CO and HC in the exhaust to yield N2, CO2 and H2O. A base and precious metal catalyst may be used. Sensors may be situated at various places in the catalytic exhaust system and be used to detect the capacity saturation point, the need to raise the exhaust temperature, the end of the clean up, and the restoration of normal operation.  
         [0045]     A catalytic diesel particulate filter (CDPF)  14  may be connected to the output of the NAC  13 . Filter  14  may provide physical filtration of the exhaust to trap particulates. Whenever the temperature window is appropriate, then oxidation of the trapped particulate matter (PM) may take place.  
         [0046]     In addition to the 60/2-5 second lean/rich swing for NOx adsorption/desorption reduction, there may be other “forced” events. They are desulfurization and PM burn-off. The NOx adsorption sites may get saturated with SOx. So periodically the SOx should be driven off which may require a much higher temperature than needed for NOx desorption. As to PM burn-off, there may be a “forced” burn-off if driving conditions (such as long periods of low speed or urban operation) result in excessive PM accumulation. The accumulation period may be several hours depending on the duty cycle of operation. The clean up may be several minutes (about 10). Higher temperatures and a reasonable oxygen level may be required.  
         [0047]     It can be seen that the above-noted catalytic system may involve a complex chemical reaction process. This process may utilize a control of flows and temperatures by a computer.  
         [0048]     Fuel injection systems may be designed to provide injection events, such as the pre-event  35 , pilot event  36 , main event  37 , after event  38  and post event  39 , in that order of time, as shown in the graph of injection rate control in  FIG. 2 . After-injection and post-injection events  38  and  39  do not contribute to the power developed by the engine, and may be used judiciously to simply heat the exhaust and use up excess oxygen. The pre-catalyst may be a significant part of the present process because all of the combustion does not take place in the cylinder.  FIG. 3  is a graph showing management of exhaust temperature. Line  41  is a graphing of percent of total torque versus percent of engine speed. The upper right time line shows a main injection event  42  near top dead center (TDC) and a post injection event  43  somewhat between TDC and bottom dead center (BDC). This time line corresponds to a normal combustion plus the post injection area above line  41  in the graph of  FIG. 3 . The lower right time line shows the main injection event  42  and a first post injection event  44  just right after main event  42 , respectively, plus a second post injection event  43 . This time line corresponds to a normal combustion plus two times the post injection area below line  41  in the graph of  FIG. 3 .  
         [0049]     In some cases when the temperature during expansion is very low (as under light load conditions), the post injection fuel may go out as raw fuel and become difficult to manage using the pre-catalyst  12 . Under such conditions, two post injections  44  and  43  may be used—one to raise temperatures early in the expansion stroke and the second to raise it further for use in downstream catalyst processes. There could be an impact on the fuel economy of the engine.  
         [0050]     One aspect of the present system may be based on information from process control. Normally in a catalytic flow system, the effectiveness of a catalyst may be reduced exponentially along the direction of flow as shown in  FIG. 4 .  FIG. 4  is a graph showing an example of a deterioration rate of a catalyst. The graph shows a percent of absorptions sites depleted versus the percent of the total length of the catalyst device. Curves  45 ,  46 ,  47  and  48  are plots of sites depleted versus catalyst length for different time periods with increasing time as shown in the graph.  
         [0051]     Another aspect of the present system may be a segmented or sectioned NAC  13 . The NAC may be divided into “n” sections. As an illustrative example, a three section NAC with intelligent control valves  51  is shown in  FIG. 5 . Valves  51  with actuators may be connected (as shown by dashed lines) to a controller or processor  52  for control.  FIGS. 6-9  show various configurations of operation of the three-section NAC  13 . The valves  51  and processor  52 , not shown in  FIGS. 6-9 , may be used to provide the various flow paths for the exhaust gases and regeneration fluid. Under conditions when the catalyst is fresh, the flow may go through all three sections  15 ,  16  and  17 , in series, as shown in  FIG. 6 . When the first section  15  of the catalyst is depleted with adsorbed NOx, the exhaust flow  55  may be diverted to the second section  16  and third section  17 , as shown in  FIG. 7 , without a loss of effectiveness. The first section  15  may then be regenerated by a flow  54 . As shown in  FIG. 8 , the flow  55  may be diverted to the first section  15  and third section  17 , with the second section  16  being regenerated by flow  54 .  FIG. 9  shows the flow  55  being run through the first and second sections  15  and  16 , with the regeneration flow  54  in the third section  17 .  
         [0052]     System  13  may have sensors for detecting pressure, temperature, flow, NOx, SOx, and other parameters, situated in various locations of the system as desired and/or needed. The sensors may be connected to processor  52 . Exhaust gases  55  may enter an inlet  56 , go through several segments  15 ,  16  and or  17 , and then exit outlet  57 . A regeneration fluid  54  may come through an inlet  53  to be directed by valves  51  to the segment or chamber that is to be regenerated.  
         [0053]     Another illustrative example, shown in  FIGS. 10   a  and  10   b , reveals a configuration  18  of the NAC  13 . In configuration  18 , the exhaust gases  55  may pass through five cleaning segments  21 ,  22 ,  23 ,  24 , and  25 , with a sixth segment  26  being regenerated with a flow  54 . A distribution manifold  19  for the NAC may provide an input  61  and flow distribution of exhaust  55  through the segments in place for cleaning the exhaust. A collection manifold  58  may provide flow distribution, in conjunction with manifold  19 , of exhaust through the cleaning segments. Manifold  58  also may provide an outlet  62  for the exhaust  55  from device  18 .  
         [0054]     Intake  63  may convey a regeneration fluid  54  through a segment  26  for cleaning out the collected pollutants from the exhaust  55 . An outlet  64  may provide for an exit of the cleaning or oxidizing fluid  54  from segment  26 . The catalyst segments may be rotated to switch in another segment for regeneration. For instance, after the sixth segment  26  is regenerated, then the first segment  21  may be moved in and regenerated, and the exhaust may flow through the second to sixth segments  22 - 26 . This rotation may continue with the second segment  22  being regenerated and the exhaust flowing through the remaining segments, and so on. Structure  65  may mechanically support the rotation of the segments and be a support for manifolds  19  and  58 . Also, structure  65  may include a manifold and support of the input  63  and output  64  for the regeneration with fluid  54  of the segment in place for the regeneration. An analysis for the configuration  18  of the NAC  13  is noted below.  
         [0055]     An aspect of the present system is the NOx regeneration (i.e., removal) or cleansing. The NOx regeneration process may be one of desorption and catalytic reduction of NOx by CO and HC (unburnt hydrocarbons) under controlled temperature, controlled CO and HC concentration and near-zero free oxygen conditions. Generally, in ordinary systems, all of the exhaust may be heated and the oxygen used up for short periods of time (about 2 to 5 seconds) at frequent intervals (every 60 seconds or so). In the present system, the regeneration flow may be independent of the exhaust flow. Regeneration flow may consist of controlled 1) diverted exhaust, 2) diverted EGR flow from upstream of the turbine, 3) fresh air diverted from the intake, or 4) fresh air supplied from an independent source. A control system for catalyst flow processes may thus be linked to a control system for the air/EGR flow processes, controlled by a VNT (variable nozzle turbine) turbocharger. Only a small portion of flow may be needed. Therefore, the amount of fuel needed to increase the temperature and use up all of the oxygen may be likewise very small. Thus, the impact on the fuel economy may be reduced significantly. Fuel may be burnt in commercially available burners (e.g., such burners for use in diesel exhaust may have been developed both for passenger car and heavy duty truck applications), or with the use of a small “pre-catalyst”.  
         [0056]     Additionally, because regeneration flow rates are small, space velocity may be low and the efficiency of NOx reduction may be high. Space velocity is a measure of gas volume flow rate/catalyst volume. Higher space velocity for a given temperature and chemistry may usually mean lower catalyst efficiency. Diverted flow may be controlled to be a very low flow rate and may result in high efficiency for NOx desorption and reduction. One other benefit may deal with PM emissions. The state of the process of after-injection may result in very high PM emissions. These emissions may be trapped in the downstream CDPF  14 , but this frequent high dose of PM may represent high back pressure, more forced CDPF regenerations—both of which may impose a fuel economy penalty. Thus, there may be more fuel saving to be had with the use of a controlled regeneration process, independent of the main exhaust flow rate. Previously, parallel flow paths may have been considered. One path may be trapping/catalyzing while the other is regenerating. This approach may make the regeneration process independent of the exhaust flow rate but may double the size of the catalyst. However, the present system may reduce the size of the catalyst to a size of “1/n”. There may be asymmetric flow paths.  
         [0057]     Another aspect of the present system may be of the pre-catalyst  12 . During an emissions test cycle, the first about 100 seconds of operation may be responsible for about 85 percent of the emissions, because during this time the catalyst may be too cold to be effective. The pre-catalyst may serve several functions—a fast warm-up of the catalytic system, and exhaust temperature and composition control by oxidizing unburnt fuel of secondary or post injections. The parallel regeneration flow stream described in a noted aspect of the present system may also be used for fast warm-up. The exhaust may be controlled to flow through one section of the NAC  13  during startup, while the other two sections are being heated to a desired temperature using very low flow rates resulting in a low fuel penalty. The pre-catalyst  12  may be eliminated. If instead of a burner, a catalytic device is used in the regeneration stream, then the size of the catalyst may be greatly reduced because of the low flow rates.  
         [0058]     Still another aspect of the present system may involve SOx regeneration. Sulfur is present in diesel fuel. Oxides of sulfur may occupy the sites that the NOx would have occupied. Therefore, over a period of time, SOx poisoning may render the NAC  13  ineffective. SOx may be driven off by temperatures higher than those needed for NOx regeneration. With control of the regeneration temperature, independently of the exhaust temperature of the main flow rate, it may be possible to re-optimize the SOx/NOx regeneration process to occur in overlapping temperature windows.  
         [0059]     Another aspect of the present system may involve CDPF regeneration. A particulate filter  67  at the tail end of the catalytic process may be a device to physically filter, trap and oxidize PM  66 . It may continuously trap and oxidize—depending on the duty cycle/temperatures. Under prolonged light load driving conditions, the CDPF  14  may continuously accumulate trapped PM  66  without regeneration. This may impose a high back pressure and fuel economy penalty on the engine. “Forced regeneration” may have to be used imposing its own fuel penalty. In the present system, the CDPF  14  may be designed with segments, sections or chambers  68  and  69  like those of NAC  13  in  FIGS. 5-9 . However, in the CDPF  14 , the sections  68  and  69  may be in parallel flow with an input  71  and an output  72  for exhaust gases  55 , as shown in  FIG. 11 . This sort of flow may be necessary because, unlike the NAC  13 , the CDPF  14  may have a “wall flow” device configuration  67  as shown in  FIG. 12 . With the latter approach, alternate flow channels may be blocked with a filter device  12 . Gas  55  with PM  66  may enter device  67 . Gas  55  may flow through a porous filter element  74  which catches the particulate matter particles  66 . The gas  55  may exit filter  67  free of particles  66 . The effective flow path is not necessarily along a catalytic channel but may be more so through the porous wall  74 . Thus, a series flow configuration from section to section, such as in the present NAC  13 , may result in a greatly reduced effective flow area and a very high pressure drop with a filter  67  in the only throughput path. Hence, the present CDPF may incorporate a parallel flow configuration of sections  69  and  69  in  FIG. 11 .  FIG. 12  shows the PM filter  67  having wall-flow/filtering with the filtered exhaust exiting filter channels  33  and  34 .  
         [0060]     Under normal conditions, within a range of CDPF  14  self-cleaning temperatures, flow conditions may be like those of the CDPF as in  FIG. 11 . However, under prolonged low temperature and low flow conditions, the exhaust may be diverted to only one of the sections  68  and  60 , as shown in  FIG. 13 , via valves  51  and processor  52 , as shown in  FIG. 5 . Gas  55  may enter inlet  71  and be diverted to chamber or segment  69  for cleaning. The gas  55  may exit system  14  via outlet  72 . Chamber  68  may be blocked from receiving any gases  55  by valves  51  (not shown). However, another valve  51  may let in a regenerating fluid  54  through input  73  and on to chamber  68  for its regeneration. Fluid  54  may exit chamber  68  and leave system  14  via outlet  72 . This approach should not result in an excessive pressure drop because the flow rates are low and the system  14  may handle a full load rate (i.e., a high rate). However, this configuration might not necessarily reduce the overall size of the trap/catalyst required.  
         [0061]      FIG. 13  shows the CDPF  14  flow diversion during low flow/low temperature conditions. During such time, high temperature gases may be already available from the NOx process. This high temperature stream may be in a range in which the CDPF  14  may effectively oxidize trapped PM. However, the oxygen concentration may be low. One of two approaches may be used. One may be a controlled combination of a high temperature stream with a high oxygen concentration, low temperature exhaust stream to achieve an oxidation of trapped PM. The other may be a preheating of a section with the high temperature stream and then exposing the section to a high oxygen concentration of the low temperature stream at a controlled flow rate so as to sustain oxidation of the PM. Filter  67  may have one or more sensors situated in or about the filter. The filter sensors may be connected to a controller. The controller may determine and initiate regeneration of the filter based on inputs from the filter sensors and possibly also on one or more mathematical models, such as for example, a model of a filter regeneration process.  
         [0062]     Applications of the present system may be with heavy duty diesel engines since they seem to be more sensitive to fuel economy than other kinds of engines. With ratios of catalyst/trap volumes to engine displacements being about 3 to 1, a 12 liter on-highway diesel engine may need 36 liters of catalyst. Other applications may include light trucks and passenger vehicles. The control box may communicate with the fuel controller on a similar level.  
         [0063]     A model of a six-segmented catalyst, e.g., configuration  18  of the NAC  13  mentioned above and shown in  FIGS. 10   a  and  10   b , may be evaluated relative to a precious metal demand and control strategies. The model may be based on the following assumptions. In each segment, a number of adsorption sites may be evaluated as n(i,t), where i=1, . . . , 5 is the number of the segment and t(s) is time. The number of adsorption sites may be normalized, i.e., n=1 corresponds to a fresh catalyst (fully regenerated) catalyst. The concentration of NOx may be evaluated as c(i,t), where i=0, . . . , 5. i=0 corresponds to the catalyst input, i=1, . . . , 5 corresponds to the output of individual segments and t(s) is time. The concentration of NOx may be normalized, i.e., c=1 corresponds to the maximum expected concentration. The performance of the catalyst may be specified in terms of fresh catalyst performance defined by output NOx [c(5,t)&lt;0.25 in the following example] and of catalyst performance degradation that triggers the regeneration [output NOx exceeds the threshold c(5,t)=0.1 in the following example] and degradation period at maximum load [td=60 seconds in the following example]. The results cover a basic analysis of the single-element catalyst and the multi-element catalyst.  
         [0064]      FIGS. 14   a  and  14   b  are graphs of performance of a single segment catalyst system for a maximum load performance of c_input=1.  FIG. 14   a  shows the availability of adsorption sites for each of the five segments over time.  FIG. 14   b  shows the relative amount of NOx particles versus time for each of the five segments. One may note the catalyst tuning relative to the initial performance c_out=0.05 and the performance deterioration c_out=0.1 at time t=60 seconds.  FIGS. 15   a  and  15   b  are graphs for the same parameter of the system but for a reduced load performance of c input=0.8. Likewise,  FIGS. 16   a  and  16   b  are graphs of the parameters for a system with a reduced load performance of c input=0.6.  
         [0065]      FIG. 17  is a graph showing filter time to regeneration as a function of the catalyst load (c input). That is, the time of the filter&#39;s life prior to needed regeneration is a nonlinear relationship relative to the amount of NOx at the input.  
         [0066]     The performance of a multi-segment rotating catalyst is shown in  FIGS. 18   a ,  18   b ,  19   a ,  19   b ,  20   a ,  20   b ,  21   a ,  21   b ,  22   a  and  22   b .  FIG. 18   a  is a graph showing the number of adsorption sites available for each of segments  1 - 5  versus time for a six segment filter having a regeneration period of 60/5=12 seconds.  FIG. 18   b  is a graph shows the relative amount of NOx particles coming out of each of the segment stages relative to an input of NOx over time along with the 12 second regeneration times for the segments of the six segment filter. One may note that with an equivalent filter area, the regeneration threshold c out=0.01 appears never to be reached.  
         [0067]     For the six-segment filter as noted above, the filter area of the catalyst is reduced to 0.9 and performance checked as shown by  FIGS. 19   a  and  19   b .  FIG. 19   a  is a graph that shows the number of adsorption sites available for each of segments  15  versus time.  FIG. 19   b  is a graph that shows the relative amount of NOx coming out of each of the segment stages relative to an input over time.  
         [0068]      FIGS. 20   a  and  20   b  are graphs showing the impact of a reduced NOx input of 0.8 into the catalyst system with a reduced regeneration rate. The time axis is to 400 seconds versus 120 second in the immediate previous four graphs.  FIG. 20   a  shows the number of adsorption sites available for each of segments  15  versus time.  FIG. 20   b  shows the relative amount of NOx coming out of each of the segment stages relative to an input of particles over time.  
         [0069]      FIGS. 21   a  and  21   b  are graphs showing the impact of the reduced NOx input (0.8) along with a reduced amount of precious metal in the catalyst segments. The time axis is at 120 seconds.  FIG. 21   a  shows the number of adsorption sites available for each of segments  1 - 5  versus time.  FIG. 21   b  shows the relative amount of NOx particles coming out of each of the segment stages relative to an input of NOx over time.  
         [0070]      FIGS. 22   a  and  22   b  are graphs showing the impact of a further reduced NOx input of 0.6 along with also a reduced amount of catalyst.  FIG. 22   a  shows the amount of adsorption sites available for each of segments  1 - 5  versus time.  FIG. 22   b  shows the relative amount of NOx particles coming out of each of the segment stages relative to an input of particles over time.  
         [0071]     An NOx removal model may be established. c i  may be the concentration of NOx (normalized to 1=maximum input); n i  may be the number of adsorption sites (normalized to 1=fresh after regeneration); the catalyst may be divided into 5+1 elements/10 slices in each element; the residence time in each slice dx may be dt; diffusion and desorption may be neglected; the regeneration time may be 5 seconds; and a simple 1st order model may be used. The formulae may include: 
 
 n   i ( t+dt )= n   i ( t )− k   n   n   i ( t ) c   i ( t ) d   t ; and 
 
 c   i+1 ( t=dt )= c   i ( t )− k   c   n   i ( t ) c   i ( t ) dt.  
 
         [0072]     There may be an impact of geometry of the catalyst model. For a geometry 1 or first geometry, the “thick” aspect ratio, k n , k c  may be calibrated given an initial output (NOx=0.01) for a fully regenerated catalyst, and an average output NOx to trigger a regeneration (NOx=0.1) after a 60 second period. For a geometry 2 or second geometry, the “thin” aspect ratio, k n , k c  may be calibrated given an initial output (NOx=0.001) for a fully regenerated catalyst, and an average output (NOx_avg=0.1) to trigger a regeneration after a 60 second period. The geometry 1 versus geometry 2 may be a different ratio between k n , k c , relative to depletion of the catalyst per unit NOx removed.  
         [0073]     One may note the reference and rotatory geometries illustrated in  FIGS. 23, 24  and  25 .  FIG. 23  shows a single element catalyst  75  batch operation (a basis for comparison), where all of the segments are operated for time Δt 1 =60 s and all segments are regenerated for Δt 2 =5 s.  FIG. 24  shows a multi-element catalyst  76  batch operation (geometry 1, 2), where n+1 segments are used and n=5, n segments are operated for time Δt=6 s, the 1st segment is regenerated for the same time, a fresh segment  77  is swapped to the end of the catalyst pack  76 , and there is a correspondence to rotating design with a triggered rotation.  
         [0074]      FIG. 25  shows a multi-element catalyst  78  semi batch operation (geometry 2), where two axial segments are used, the 1st segment is operated for time Δt=6 s, the 2nd element is regenerated for the same time, and a fresh segment is swapped to the NOx stream. A triggered or continuous operation is possible.  
         [0075]      FIGS. 26   a  and  26   b  are graphs revealing the NOx concentration for the first geometry of the catalyst.  FIG. 26   a  shows the relative amount of NOx in time for the multi-segment system. The initial NOx out is 0.01 at point  79 . At t=60 seconds at point  81 , the average NOx out=0.1.  FIG. 26   b  is a three-dimensional graph showing NOx concentration versus time and length. At point  82  is an NOx profile in space/time with an average NOx output=0.1.  
         [0076]      FIGS. 27   a  and  27   b  are graphs like those of  FIGS. 26   a  and  26   b  illustrating NOx concentration for a second geometry of catalyst operation. One may note that at point  83  the initial NOx out=0.001. At point  84  for t=60 seconds, the average NOx out=0.1.  FIG. 27   b  is a three-dimensional graph showing NOx concentration versus time and length. At point  85  is an NOx profile in space/time with an average NOx output=0.1.  
         [0077]      FIG. 28  is a graph showing NOx profiles where dt=2 seconds, such as at point  86 . The graph shows the relative amount of NOx particles versus length in space. Point  87  shows a first element output for n=2 where NOx_out&gt;0.1 at t=2.  
         [0078]      FIGS. 29   a  and  29   b  are graphs showing a comparison of absorption sites depletion in time for the first and second geometries, respectively, of the catalyst system. At point  88  for t=60 seconds, the first geometry appears to have a slower depletion. At point  89  for t=60 seconds, the second geometry appears to have a faster depletion. The relative depletion rate may be expressed as k n1 /k c1 &lt;k n2 /k c2 .  
         [0079]      FIGS. 30   a  and  31   a  reveal relative amounts of NOx versus time for a catalyst system with a catalyst reduction for the first and second geometries of the system, respectively. The regeneration period is 6 seconds. Point  91  in  FIGS. 30   a  and  31   a  appear to show a required average performance of NOx&lt;0.1.  
         [0080]      FIGS. 30   b  and  31   b  show adsorption sites depletion in space for a catalyst system with a catalyst reduction for the first and second geometries, respectively. Point  92  in  FIG. 30   b  appears to show a catalyst reduction of 0.67*6/5=0.8. Point  93  of  FIG. 31   b  appears to show a catalyst reduction of 0.56*6/5=0.67. The direct reduction from the respective graphs may be multiplied by the total number of segments of the system divided by the number of segments cleaning the exhaust.  
         [0081]      FIGS. 32   a  and  32   b  are graphs showing absorption sites depletion in space for a multi-segment catalyst system with without and with flow direction switching, respectively. The spatial profiles  94  may be at one second without flow direction switching. The spatial profiles  95  may be at one second with flow direction switching. The regeneration may be at 6 seconds. There appears to be a more uniform depletion in the segments. The impact on catalyst reduction appears to be minimal.  
         [0082]      FIGS. 33   a ,  33   b  and  33   c  are graphs showing the relative amount of NOx in time, the relative amount NOx in space and absorption sites depletion in space for the second geometry of a system with a catalyst load of 40 percent. Point  96  of the graph in  FIG. 33   a  shows a required average performance of NOx&lt;0.1. Point  97  in the graph of  FIG. 33   b  shows an output NOx sampled at one second. Point  98  show a catalyst depletion sampled at one second in the graph of  FIG. 33   c . The catalyst reduction may be noted at point  99  of the graph of  FIG. 33   c . The catalyst reduction achieved may be calculated as 0.4*2=0.8 for the second geometry.  
         [0083]      FIGS. 34   a ,  34   b ,  35   a ,  35   b ,  36   a  and  36   b  are graphs showing an impact of the segment regeneration order optimization for regenerating the segment attached last, attached first and sequentially in view of available adsorption sites in time and the relative amount of NOx particles, respectively, with regard to an achievable catalyst reduction for a multi-segment catalyst system. The system may be a six-segment catalyst having one of the segments being regenerated at a time while the remaining five segments are active. The saturation time of the segments may be 60 seconds while the regeneration time may be 12 seconds. Where the regeneration segment is attached last, the achievable catlayst reduction may be 0.9. Where the regeneration segment is attached first, the achievable catalyst reduction may be 0.96. In the case where the regeneration of the segments is done sequentially, the achievable catalyst reduction may be 0.96.  
         [0084]     Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.