Patent Publication Number: US-2004052693-A1

Title: Apparatus and method for removing NOx from the exhaust gas of an internal combustion engine

Description:
FIELD OF THE DISCLOSURE  
       [0001] The present disclosure relates to generally to devices for removing NO X  from the exhaust gas of an internal combustion engine.  
       BACKGROUND OF THE DISCLOSURE  
       [0002] Traps for removing NO X  from an exhaust gas of an internal combustion engine typically include an absorber catalyst which removes NO X  from the exhaust gas flow by “trapping” the NO X  as the exhaust gas advances therethrough. Periodically, the NO X  trap must be regenerated to purge the absorber catalyst of the trapped NO X . In particular, the NO X  trap periodically undergoes a catalytic reaction in which the trapped NO X  is converted to less harmful gases that are subsequently exhausted from the trap. One way to regenerate a NO X  trap is by raising the temperature of the NO X  trap and thereafter injecting fuel, such as diesel fuel, into the trap.  
       SUMMARY OF THE DISCLOSURE  
       [0003] According to one illustrative embodiment, there is provided an emission abatement assembly having a NO X  trap, a heat exchanger, and a fuel reformer. The heat exchanger cools exhaust gases from an internal combustion engine prior to advancement thereof into the NO X  trap. The fuel reformer reforms hydrocarbon fuels so as to produce a reformate gas that is supplied to the NO X  trap thereby facilitating regeneration of the NO X  trap at relatively cool temperatures.  
       [0004] According to another illustrative embodiment, there is provided a method of operating an emission abatement device which includes the step of cooling exhaust gases from an internal combustion engine prior to advancement thereof into a NO X  trap. The method also includes the step of advancing reformate gas from a fuel reformer into the NO X  trap during regeneration of the trap.  
       [0005] The above and other features of the present disclosure will become apparent from the following description and the attached drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0006]FIG. 1 is a simplified block diagram of a fuel reforming assembly having a plasma fuel reformer under the control of an electronic control unit;  
     [0007]FIG. 2 is a diagrammatic cross sectional view of the plasma fuel reformer of FIG. 1;  
     [0008]FIG. 3 is a simplified block diagram of an emission abatement assembly for removing NO X  from the exhaust gases of an internal combustion engine;  
     [0009]FIG. 4 is a graph which shows the relationship between the NO X  conversion rate of a NO X  trap and the temperature of the NO X  trap; and  
     [0010]FIG. 5 is a flowchart of a control procedure executed by a control unit during operation of the emission abatement assembly of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0011] As will herein be described in more detail, a heat management scheme is utilized to maintain the temperature of a NO X  trap within a desired temperature range that corresponds to a desired, enhanced NO X  conversion rate. To prevent the temperature of the NO X  trap from having to be increased to a temperature outside of the desired range during regeneration of the trap, a fuel reformer is utilized to generate and supply a reformate gas to the NO X  trap to facilitate regeneration of the trap at cooler temperatures relative to the temperatures experienced in conventional approaches in which a hydrocarbon fuel, such as diesel fuel or gasoline, is utilized to regenerate the trap.  
     [0012] The fuel reformer described herein may be embodied as any type of fuel reformer such as, for example, a catalytic fuel reformer, a thermal fuel reformer, a steam fuel reformer, or any other type of partial oxidation fuel reformer. The fuel reformer of the present disclosure may also be embodied as a plasma fuel reformer. A plasma fuel reformer uses plasma to convert a mixture of air and hydrocarbon fuel into a reformate gas which is rich in, amongst other things, hydrogen gas and carbon monoxide. Systems including plasma fuel reformers are disclosed in U.S. Pat. No. 5,425,332 issued to Rabinovich et al.; U.S. Pat. No. 5,437,250 issued to Rabinovich et al.; U.S. Pat. No. 5,409,784 issued to Bromberg et al.; and U.S. Pat. No. 5,887,554 issued to Cohn, et al., the disclosures of each of which is hereby incorporated by reference.  
     [0013] For purposes of the following description, the concepts of the present disclosure will herein be described in regard to a plasma fuel reformer. However, as described above, the fuel reformer of the present disclosure may be embodied as any type of fuel reformer, and the claims attached hereto should not be interpreted to be limited to any particular type of fuel reformer unless expressly defined therein.  
     [0014] Referring now to FIGS. 1 and 2, there is shown an exemplary embodiment of a plasma fuel reforming assembly  10  of an emission abatement assembly  14 . The plasma fuel reforming assembly  10  includes a plasma fuel reformer  12  and a control unit  16 . The plasma fuel reformer  12  reforms (i.e., converts) hydrocarbon fuels into a reformate gas that includes, amongst other things, hydrogen and carbon monoxide. As such, the plasma fuel reformer  12 , as described further herein, may be used in the construction of an onboard fuel reforming system of a vehicle or a stationary power generator. In such a way, the reformate gas produced by the onboard plasma fuel reformer  12  may be utilized to regenerate or otherwise condition an emission abatement device associated with the internal combustion engine such as a NO X  trap.  
     [0015] As shown in FIG. 2, the plasma fuel reformer  12  includes a plasma-generating assembly  42  and a reactor  44 . The reactor  44  includes a reactor housing  48  having a reaction chamber  50  defined therein. The plasma-generating assembly  42  is secured to an upper portion of the reactor housing  48 . The plasma-generating assembly  42  includes an upper electrode  54  and a lower electrode  56 . The electrodes  54 ,  56  are spaced apart from one another so as to define an electrode gap  58  therebetween. An insulator  60  electrically insulates the electrodes from one another.  
     [0016] The electrodes  54 ,  56  are electrically coupled to an electrical power supply  36  (see FIG. 1) such that, when energized, an electrical current is supplied to one of the electrodes thereby generating a plasma arc  62  across the electrode gap  58  (i.e., between the electrodes  54 ,  56 ). A fuel input mechanism such as a fuel injector  38  injects a hydrocarbon fuel  64  into the plasma arc  62 . The fuel injector  38  may be any type of fuel injection mechanism which injects a desired amount of fuel into plasma-generating assembly  42 . In certain configurations, it may be desirable to atomize the fuel prior to, or during, injection of the fuel into the plasma-generating assembly  42 . Such fuel injector assemblies (i.e., injectors which atomize the fuel) are commercially available.  
     [0017] As shown in FIG. 2, the plasma-generating assembly  42  has an annular air chamber  72 . Pressurized air is advanced into the air chamber  72  through an air inlet  74  and is thereafter directed radially inwardly through the electrode gap  58  so as to “bend” the plasma arc  62  inwardly. Such bending of the plasma arc  62  ensures that the injected fuel  64  is directed through the plasma arc  62 . Such bending of the plasma arc  62  also reduces erosion of the electrodes  56 ,  58 . Moreover, advancement of air into the electrode gap  58  also produces a desired mixture of air and fuel (“air/fuel mixture”). In particular, the plasma reformer  12  reforms or otherwise processes the fuel in the form of a mixture of air and fuel. The air-to-fuel ratio of the air/fuel mixture being reformed by the fuel reformer is controlled via control of the fuel injector  38  and an air inlet valve  40 . The air inlet valve  40  may be embodied as any type of electronically-controlled air valve. The air inlet valve  40  may be embodied as a discrete device, as shown in FIG. 2, or may be integrated into the design of the plasma fuel reformer  12 . In either case, the air inlet valve  40  controls the amount of air that is introduced into the plasma-generating assembly  42  thereby controlling the air-to-fuel ratio of the air/fuel mixture being processed by the plasma fuel reformer  12 .  
     [0018] The lower electrode  56  extends downwardly into the reactor housing  48 . As such, gas (either reformed or partially reformed) exiting the plasma arc  62  is advanced into the reaction chamber  50 . A catalyst  78  may be positioned in the reaction chamber  50 . The catalyst  78  completes the fuel reforming process, or otherwise treats the gas, prior to exit of the reformate gas through a gas outlet  76 . In particular, some or all of the gas exiting the plasma-generating assembly  42  may only be partially reformed, and the catalyst  78  is configured to complete the reforming process (i.e., catalyze a reaction which completes the reforming process of the partially reformed gas exiting the plasma-generating assembly  42 ). The catalyst  78  may be embodied as any type of catalyst that is configured to catalyze such reactions. In one exemplary embodiment, the catalyst  78  is embodied as substrate having a precious metal or other type of catalytic material disposed thereon. Such a substrate may be constructed of ceramic, metal, or other suitable material. The catalytic material may be, for example, embodied as platinum, rhodium, palladium, including combinations thereof, along with any other similar catalytic materials. In certain configurations, the plasma fuel reformer  12  may be embodied without the catalyst  78 .  
     [0019] As shown in FIG. 2, the plasma fuel reformer  12  has a temperature sensor  34  associated therewith. The temperature sensor  34  is used as a feedback mechanism to determine the temperature of a desired structure of the plasma fuel reformer  12  or the gas advancing therethrough. For example, the temperature sensor  34  may be used to measure the temperature of the reformate gas being produced by the plasma fuel reformer  12 , the ambient temperature within the reaction chamber  50 , the temperature of the catalyst  78 , etcetera. The temperature sensor  34  may be located in any number of locations. In particular, as shown in solid lines, the temperature sensor  34  may be positioned within the reaction chamber  50  at location in operative contact with the a structure (e.g., the catalyst  78  or the walls of the reaction chamber  50 ) or a substance (e.g., the gas in the reaction chamber  50 ). To do so, the temperature sensor  34  may be positioned in physical contact with the structure or substance, or may be positioned a predetermined distance away from the structure or out of the flow of the substance, depending on the type and configuration of the temperature sensor.  
     [0020] Alternatively, the temperature of the desired structure or substance may be determined indirectly. In particular, as shown in phantom, the temperature sensor  34  may be positioned so as to sense the temperature of the reformate gas advancing through the reaction chamber  50  or a gas conduit  80  subsequent to being exhausted through the outlet  76 . Such a temperature reading may be utilized to calculate the temperature of another structure such as, for example, the catalyst  78  or the reactor housing  48 . Conversely, the temperature sensor  34  may be positioned to sense the temperature of the reactor housing  48  with such a temperature reading then being correlated to the temperature of the reformate gas. In any such case, an indirect temperature sensed by the temperature sensor  34  may be correlated to a desired temperature.  
     [0021] As shown in FIG. 1, the plasma fuel reformer  12  and its associated components are under the control of the control unit  16 . In particular, the temperature sensor  34  is electrically coupled to the electronic control unit  16  via a signal line  18 , the fuel injector  38  is electrically coupled to the electronic control unit  16  via a signal line  20 , the air inlet valve  40  is electrically coupled to the electronic control unit  16  via a signal line  22 , and the power supply  36  is electrically coupled to the electronic control unit  16  via a signal line  24 . Moreover, as will herein be described in greater detail, a number of other components associated with an emission abatement assembly may also be under the control of the control unit  16 , and, as a result, electrically coupled thereto. For example, as shown in FIG. 3, a flow diverter valve  86  for selectively diverting a flow of exhaust gas from an internal combustion engine  82  may be under the control of the control unit  16 . Moreover, a temperature sensor  98  for sensing the temperature of exhaust gases entering a NO X  trap  84  may also be under the control of the control unit  16 .  
     [0022] Although the signal lines  18 ,  20 ,  22 ,  24  (and the signal lines used to couple the diverter valve  86  and the temperature sensor  98  to the control unit  16 ) are shown schematically as a single line, it should be appreciated that the signal lines may be configured as any type of signal carrying assembly which allows for the transmission of electrical signals in either one or both directions between the electronic control unit  16  and the corresponding component. For example, any one or more of the signal lines  18 ,  20 ,  22 ,  24  (along with the signal lines used to couple the diverter valve  86  and the temperature sensor  98  to the control unit  16 ) may be embodied as a wiring harness having a number of signal lines which transmit electrical signals between the electronic control unit  16  and the corresponding component. It should be appreciated that any number of other wiring configurations may also be used. For example, individual signal wires may be used, or a system utilizing a signal multiplexer may be used for the design of any one or more of the signal lines  18 ,  20 ,  22 ,  24  (along with the signal lines used to couple the diverter valve  86  and the temperature sensor  98  to the control unit  16 ). Moreover, the signal lines  18 ,  20 ,  22 ,  24  (along with the signal lines used to couple the diverter valve  86  and the temperature sensor  98  to the control unit  16 ) may be integrated such that a single harness or system is utilized to electrically couple some or all of the components associated with the plasma fuel reformer  12  to the electronic control unit  16 .  
     [0023] The electronic control unit  16  is, in essence, the master computer responsible for interpreting electrical signals sent by sensors associated with the emission abatement assembly  14  and for activating electronically-controlled components associated with the emission abatement assembly  14  in order to control the plasma fuel reformer  12 , the flow of reformate gas exiting therefrom, and the exhaust gas flow from an internal combustion engine. For example, the electronic control unit  16  of the present disclosure is operable to, amongst many other things, determine the beginning and end of each injection cycle of fuel into the plasma-generating assembly  42 , calculate and control the amount and ratio of air and fuel to be introduced into the plasma-generating assembly  42 , determine the temperature of the reformer  12  or the reformate gas, determine the power level to supply to the plasma fuel reformer  12 , determine whether to raise or lower the temperature of the exhaust gas entering the NO X  trap  84  to maintain the trap within a desired temperature range.  
     [0024] To do so, the electronic control unit  16  includes a number of electronic components commonly associated with electronic units which are utilized in the control of electromechanical systems. For example, the electronic control unit  16  may include, amongst other components customarily included in such devices, a processor such as a microprocessor  28  and a memory device  30  such as a programmable read-only memory device (“PROM”) including erasable PROM&#39;s (EPROM&#39;s or EEPROM&#39;s). The memory device  30  is configured to store, amongst other things, instructions in the form of, for example, a software routine (or routines) which, when executed by the processor  28 , allows the electronic control unit  16  to control operation of the plasma fuel reformer  12 .  
     [0025] The electronic control unit  16  also includes an analog interface circuit  32 . The analog interface circuit  32  converts the output signals from the various sensors associated with the emission abatement assembly  14  into a signal which is suitable for presentation to an input of the microprocessor  28 . In particular, the analog interface circuit  32 , by use of an analog-to-digital (A/D) converter (not shown) or the like, converts the analog signals generated by the sensors into a digital signal for use by the microprocessor  28 . It should be appreciated that the A/D converter may be embodied as a discrete device or number of devices, or may be integrated into the microprocessor  28 . It should also be appreciated that if any one or more of the sensors associated with the emission abatement assembly  14  generate a digital output signal, the analog interface circuit  32  may be bypassed.  
     [0026] Similarly, the analog interface circuit  32  converts signals from the microprocessor  28  into an output signal which is suitable for presentation to the electrically-controlled components associated with the emission abatement assembly  14  (e.g., the fuel injector  38 , the air inlet valve  40 , the power supply  36 , the exhaust gas flow diverter valve  86 , etcetera). In particular, the analog interface circuit  32 , by use of a digital-to-analog (D/A) converter (not shown) or the like, converts the digital signals generated by the microprocessor  28  into analog signals for use by the electronically-controlled components associated with the emission abatement assembly  14  such as the fuel injector  38 , the air inlet valve  40 , the power supply  36 , or the diverter valve  86 . It should be appreciated that, similar to the A/D converter described above, the D/A converter may be embodied as a discrete device or number of devices, or may be integrated into the microprocessor  28 . It should also be appreciated that if any one or more of the electronically-controlled components associated with the emission abatement assembly  14  operate on a digital input signal, the analog interface circuit  32  may be bypassed.  
     [0027] Hence, the electronic control unit  16  may be operated to control operation of the emission abatement assembly  14 . In particular, the electronic control unit  16  executes a routine including, amongst other things, a closed-loop control scheme in which the electronic control unit  16  monitors the outputs from a number of sensors in order to control the inputs to the electronically-controlled components associated with the emission abatement assembly  14 . To do so, the electronic control unit  16  communicates with the sensors associated with the emission abatement assembly  14  in order to determine, amongst numerous other things, the temperature of the exhaust gas entering the NO X  trap  84 , the saturation level of the NO X  trap  84 , the amount, temperature, and/or pressure of air and/or fuel being supplied to the plasma fuel reformer  12 , the amount of hydrogen and/or oxygen in the reformate gas, etcetera. Armed with this data, the electronic control unit  16  performs numerous calculations each second, including looking up values in preprogrammed tables, in order to execute algorithms to perform such functions as determining when or how long the fuel reformer&#39;s fuel injector is opened, controlling the power level input to the fuel reformer, controlling the amount of air advanced through the reformer&#39;s air inlet valve, controlling the position of the flow diverter valve responsible for directing the flow of exhaust gas (i.e., the diverter valve  86 ), etcetera.  
     [0028] Referring now to FIG. 3, there is shown the emission abatement assembly  14  in greater detail. The emission abatement assembly  14  includes a NO X  trap  84  for removing and treating NO X  present in an exhaust from an internal combustion engine  82  such as a diesel engine, a gasoline direct injection (GDI) engine, or natural gas engine. The NO X  trap  84  may be any type of commercially available NO X  trap including a lean NO X  trap so as to facilitate the trapping and removal of NO X  in the lean conditions associated with exhaust gases from diesel engines, GDI engines, or natural gas engines. Specific examples of NO X  traps which may be used as the NO X  trap  84  of the present disclosure include, but are not limited to, NO X  traps commercially available from, or NO X  traps constructed with materials commercially available from, EmeraChem, LLC of Knoxville, Tenn. (formerly known as Goal Line Environmental Technologies, LLC of Knoxville, Tenn.).  
     [0029] As shown in FIG. 3, the flow diverter valve  86  is operable to divert the flow of exhaust gases from an internal combustion engine  82  between a pair of different flow paths. In particular, the diverter valve  86  may be operated to divert a flow of exhaust gas from the engine  82  between a cooling flow path  104  and a bypass flow path  106 . As shown in FIG. 3, a heat exchanger  90  is positioned in the cooling flow path  104  such that exhaust gases advancing through the cooling flow path  104  are cooled by the heat exchanger  90 . The heat exchanger  90  may be embodied as any type of heat exchanger for cooling hot gases such as an air-cooled or liquid-cooled heat exchanger.  
     [0030] As also shown in FIG. 3, the cooling flow path  104  and the bypass flow path  106  are recombined by a flow coupler  118 . The flow coupler  118  is positioned upstream of an inlet  116  of the NO X  trap  84 . As a result, cooled exhaust gas exiting the cooling flow path  104  is reintroduced into uncooled exhaust gas from the bypass flow path  106 .  
     [0031] In the exemplary embodiment described herein, a number of fluid lines such as tubes, pipes, or the like are utilized to create the various flow paths. In particular, an exhaust gas inlet  102  of the diverter valve  86  is fluidly coupled to an exhaust manifold  124  of the engine  82  via a fluid line  120 . A first exhaust gas outlet  108  of the diverter valve  86  is fluidly coupled to an inlet  112  of a heat exchanger  90  via a fluid line  92 , whereas a second exhaust gas outlet  110  of the diverter valve  86  is fluidly coupled to the flow coupler  118  via a fluid line  96 . An outlet  114  of the heat exchanger  90  is fluidly coupled to the flow coupler  118  via a fluid line  94 . A fluid line  122  fluidly couples the flow coupler  118  to the inlet  116  of the NO X  trap  84 .  
     [0032] In such a way, cooled exhaust gas from the exhaust manifold  124  of the internal combustion engine  82  may be advanced into the NO X  trap  84  by directing the exhaust gas along a fluid path which includes the fluid line  120 , the diverter valve  86 , the fluid line  92 , the heat exchanger  90 , the fluid line  94 , the flow coupler  118 , and the fluid line  122 . Conversely, uncooled exhaust gas from the exhaust manifold  124  of the internal combustion engine  82  may be advanced into the NO X  trap by directing the exhaust gas along a fluid path which includes the fluid line  120 , the diverter valve  86 , the fluid line  96 , the flow coupler  118 , and the fluid line  122 .  
     [0033] The exhaust gas diverter valve  86  is embodied as a variable flow hot valve. In such a configuration, the diverter valve  86  is operable to direct a desired flow amount of exhaust gas through the cooling flow path  104 , while also directing a desired flow amount of exhaust gas through the bypass flow path  106 . It should be appreciated that the magnitude of the two flows determines the temperature of the gas entering the NO X  trap  84 . In particular, by operating the diverter valve  86  to increase the magnitude of the flow through the cooling flow path  104  (and hence decrease the magnitude of the flow through the bypass flow path  106 ), the temperature of the resultant gas (i.e., the recombined gas flow entering the NO X  trap  84 ) is decreased. Conversely, by operating the diverter valve  86  to decrease the magnitude of the flow through the cooling flow path  104  (and hence increase the magnitude of the flow through the bypass flow path  106 ), the temperature of the resultant gas (i.e., the recombined gas flow entering the NO X  trap  84 ) is increased. Hence, by adjusting the position of the diverter valve  86 , the temperature of the exhaust gas being introduced into the NO X  trap  84  can be varied.  
     [0034] The diverter valve  86  is electrically coupled to the electronic control unit  16  via a signal line  88 . As such, the position of the diverter valve  86  is under the control of the electronic control unit  16 . Hence, the electronic control unit  16 , amongst its other functions, selectively diverts the flow of exhaust gas from the engine  82  between the cooling flow path  104  and the bypass flow path  106 .  
     [0035] A temperature sensor  98  is positioned so as to sense the temperature of the recombined exhaust gas flow prior to advancement thereof into the NO X  trap  84 . To do so, the temperature sensor  98  is fluidly interposed between the flow coupler  118  and the inlet  116  of the NO X  trap  84 . As such, the temperature sensor  84  may be used to determine the temperature of the exhaust gas in the combined fluid line  122 . The temperature sensor  98  is electrically coupled to the control unit  16  via a signal line  100 . As such, the temperature sensor  98  generates output signals on the signal line  100  indicative of the temperature of the exhaust gas entering the NO X  trap  84 .  
     [0036] The temperature sensor  98  is used as a feedback mechanism to maintain the NO X  trap in a desired temperature range by determining the temperature of the exhaust gas advancing therethrough. In particular, the output from the temperature sensor  98  is indicative of the temperature of the NO X  trap  84 . As such, the temperature of the exhaust gas may be used to determine and monitor temperature of the NO X  trap  84 . The temperature sensor  98  may be located in any number of locations. In particular, the temperature sensor  98  may be positioned within the fluid line  122  to sense the temperature of the exhaust gas advancing therethrough. Alternatively, the temperature of the exhaust gas may be determined indirectly. In particular, the temperature of either the inner surface or the outer surface of the fluid line  122  may be sensed. Moreover, the temperature of other structures such as, for example, the inlet  116  of the NO X  trap  84  may be sensed. In any such a case, the indirect temperature sensed by the temperature sensor  98  is indicative of, or otherwise may be correlated to, the temperature of the exhaust gas introduced into the NO X  trap  84 . As such, the calculations performed by the herein described methods and systems may be adjusted to account for the use of such an indirect temperature measurements. Alternatively, the output from such an indirect gas temperature measurement may be extrapolated to a corresponding direct gas temperature or otherwise adjusted prior to input into the calculations performed by the herein described methods and systems.  
     [0037] Hence, it should be appreciated that the herein described concepts are not intended to be limited to any particular method or device for determining the temperature of the exhaust gas introduced into the NO X  trap  84 . In particular, the exhaust gas temperature may be determined by use any type of temperature sensor, located in any sensor location, and utilizing any methodology (e.g., either direct or indirect) for obtaining temperature values associated with the exhaust gas.  
     [0038] As alluded to above, the temperature of the exhaust gas may be utilized to determine temperature of the NO X  trap  84 . As such, the temperature of the NO X  trap  84  may be maintained within a desirable temperature range. In particular, as shown in FIG. 4, the NO X  conversion rate of a given design of a NO X  trap varies as a function of trap temperature. The graph shown in FIG. 4 is indicative of an exemplary barium carbonate (BaCO 3 ) NO X  trap. However, it should be appreciated that a similar graph may be created for any type of NO X  trap to fit the needs of a given system design and, as a result, the concepts of the present disclosure are not intended to be limited to any particular type of NO X  trap or temperature range.  
     [0039] As can be seen from the graph, a desirable, relatively high NO X  conversion rate can be maintained by maintaining the temperature of the trap within a predetermined temperature range. For example, in the exemplary plot of the BaCO 3  NO X  trap shown in the graph of FIG. 4, a ninety percent (90%) NO X  conversion rate may be maintained by maintaining the temperature of the NO X  trap  84  within a temperature range of 250° C. to  325 ° C.  
     [0040] To maintain the trap temperature in any such desirable temperature range, as shown in FIG. 5, the control unit  16  executes a temperature control routine  200 . The control routine  200  begins with step  202  in which the control unit  16  determines the temperature of the exhaust gas entering the NO X  trap  84 . In particular, the control unit  16  scans or otherwise reads the signal line  100  in order to monitor output from the temperature sensor  98 . As described above, the output signals produced by the temperature sensor  98  are indicative of the temperature of the exhaust gas entering the NO X  trap  84 . Once the control unit  16  has determined the temperature of the exhaust gas entering the NO X  trap  84 , the control routine  200  advances to step  204 .  
     [0041] In step  204 , the control unit  16  determines if the sensed temperature of the exhaust gas is within a predetermined temperature range. In particular, as described herein, a predetermined temperature range may be established which corresponds to a desired NO X  conversion rate of the NO X  trap  84 . In the exemplary embodiment described herein, a trap temperature range of 250° C. to  325 ° C. (which corresponds with a 90% NO X  conversion rate) is utilized. As such, in step  204 , the control unit  16  determines if the temperature of the exhaust gas is within the desired temperature range. If the temperature of the exhaust gas is within such a desired temperature range, the control routine  200  loops back to step  202  to continue monitoring the output from the temperature sensor  98 . However, if the temperature of the exhaust gas is outside the desired temperature range, the control routine  200  advances to step  206 .  
     [0042] In step  206 , the control unit  16  determines if the temperature of the exhaust gas is above the desired temperature range or below the desired temperature range. If the temperature of the exhaust gas is above the desired temperature range, the control routine  200  advances to step  208 . If the temperature of the exhaust gas is below the desired temperature range, the control routine  200  advances to step  210   
     [0043] In step  208 , the control unit  16  cools the exhaust gas entering the NO X  trap  84 . In particular, the control unit  16  generates a control signal on the signal line  88  thereby adjusting position of the diverter valve  86 . More specifically, the control unit  16  adjusts the position of the diverter valve  86  so as to increase the magnitude of the flow through the cooling flow path  104  (and hence decrease the magnitude of the flow through the bypass flow path  106 ), thereby causing the temperature of the resultant gas (i.e., the recombined gas flow entering the NO X  trap  84 ) to be decreased. Thereafter, the control routine  200  loops back to step  202  to continue monitoring the output from the temperature sensor  98 .  
     [0044] Referring back to step  206 , if the temperature of the exhaust gas is below the desired temperature range, the control routine  200  advances to step  210 . In step  210 , the control unit  16  heats the exhaust gas entering the NO X  trap  84 . In particular, the control unit  16  generates a control signal on the signal line  88  thereby adjusting position of the diverter valve  86 . More specifically, the control unit  16  adjusts the position of the diverter valve  86  so as to decrease the magnitude of the flow through the cooling flow path  104  (and hence increase the magnitude of the flow through the bypass flow path  106 ), thereby causing the temperature of the resultant gas (i.e., the recombined gas flow entering the NO X  trap  84 ) to be increased. Thereafter, the control routine  200  loops back to step  202  to continue monitoring the output from the temperature sensor  98 .  
     [0045] The above described control scheme may be utilized to maintain the NO X  trap in the desired temperature range during NO X  absorption. However, as will now be discussed in more detail, the NO X  trap  84  of the emission abatement assembly  14  may also be maintained in the desired temperature range during regeneration of the NO X  trap. This is desirable relative to regeneration schemes such as those which inject diesel fuel or gasoline into the NO X  trap. In particular, such regeneration schemes require the temperature of the trap to be significantly increased to elevated temperatures including temperatures exceeding typical exhaust gas temperatures (e.g., a temperature in excess of 600-650° C.). In such situations, the conversion rate of the NO X  trap is adversely affected during the transitional times associated with heating of the trap prior to regeneration and cooling of the trap subsequent to regeneration.  
     [0046] The plasma fuel reformer  12  is operated to facilitate regeneration of the NO X  trap  84  at the relatively cool temperatures within the desired temperature range. In particular, the reformate gas produced by the plasma fuel reformer  12  is rich in, amongst other things, hydrogen and carbon monoxide. Such reformate gas facilitates regeneration of the absorber catalyst of the NO X  trap  84  at relatively cool temperatures. Indeed, testing has shown that use of such reformate gas facilitates regeneration of the absorber catalyst of the NO X  trap  84  at a wide range of cool exhaust gas temperatures including transient temperatures such as the exhaust gas temperatures associated with engine idle. Hence, by use of reformate gas from the plasma fuel reformer  12  to facilitate regeneration of the NO X  trap  84 , the temperature of the NO X  trap  84  may be maintained within a desirable temperature range including during regeneration.  
     [0047] The control scheme executed by the control unit  16  includes a routine for regenerating the NO X  trap. Such a regeneration control scheme may be designed in a number of different manners. For example, a timing-based control scheme may be utilized in which the NO X  trap  84  is regeneration as a function of time. For instance, regeneration of the NO X  trap  84  may be performed at predetermined timed intervals.  
     [0048] Alternatively, a sensor-based control scheme may be utilized. In such a case, the NO X  trap  84  is regenerated as a function of output from one or more sensors associated with the trap. For instance, regeneration of one of the NO X  trap  84  may commence when the output from NO X  sensor(s) (not shown) associated with the trap is indicative of a predetermined saturation level.  
     [0049] However, in any such case, the control scheme executed by the control unit  16  may be configured to regenerate the NO X  trap  84  while contemporaneously maintaining the temperature of the NO X  trap within a temperature range corresponding to a desired, enhanced NO X  conversion rate. More specifically, the control unit  16  may maintain operation of the diverter valve  86  so as to maintain the trap temperature within the desired temperature range during regeneration of the trap. In other words, the NO X  trap  84  need not be subjected to elevated temperatures outside of the desired range during regeneration. In such a way, periods of a reduced NO X  conversion rate are not realized just prior to or subsequent to regeneration of the NO X  trap  84 .  
     [0050] While the disclosure is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and has herein be described in detail. It should be understood, however, that there is no intent to limit the disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure.  
     [0051] There are a plurality of advantages of the present disclosure arising from the various features of the apparatus, systems, and methods described herein. It will be noted that alternative embodiments of the apparatus, systems, and methods of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of apparatus, systems, and methods that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present disclosure.  
     [0052] For example, it should be appreciated that in lieu of diverting only a portion of the exhaust gas through a heat exchanger, the entire exhaust gas flow may be directed through a heat exchanger. In such a case, the heat exchanger itself may be operated to adjust the temperature of the exhaust gas exiting therefrom and into the NO X  trap  84 . For example, in the case of an air-to-air heat exchanger, the speed control of the air fan may be adjusted to adjust the temperature of the exhaust gas exiting the heat exchanger. Similarly, in the case of an air-to-liquid heat exchanger, the speed control of the pump may be adjusted to adjust the temperature of the exhaust gas exiting the heat exchanger.  
     [0053] In addition, the emission abatement assembly  14  may be configured to include one or more additional catalysts to function in conjunction with the NO X  trap  84 . For example, an oxidation catalyst may be positioned upstream of the NO X  trap  84  to remove oxygen from the exhaust gas thereby facilitating regeneration of the trap. Moreover, an oxidation catalyst may also be positioned downstream from the NO X  trap  84  remove (i.e., oxidize) any residual compounds such a H 2 S that may be present in the gases being exhausted from the NO X  trap  84 .  
     [0054] Moreover, as described above, the plasma fuel reformer  12  may be operated to produce a reformate gas that is rich in, amongst other things, hydrogen and carbon monoxide. However, a flow of reformate gas rich in other compounds may also be produced by the plasma fuel reformer  12 . For example, reformate gas rich in acetylene, methane, propanol, or ethanol may also be produced. Such a reformate gas may also be utilized to regenerate the NO X  trap  84  at relatively cool trap temperatures.