Patent Publication Number: US-7908847-B2

Title: Method and apparatus for starting up a fuel-fired burner of an emission abatement assembly

Description:
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/536,327, filed on Jan. 13, 2004 and U.S. Provisional Patent Application Ser. No. 60/546,139 filed on Feb. 20, 2004, the entirety of both of which is hereby incorporated by reference. 
     CROSS REFERENCE 
     Cross reference is made to copending U.S. patent applications Ser. No. 10/931,028 entitled “Method and Apparatus for Cooling the Components of a Control Unit of an Emission Abatement Assembly” by Wilbur H. Crawley and Randall J. Johnson; Ser. No. 10/931,091 entitled “Method and Apparatus for Monitoring Engine Performance as a Function of Soot Accumulation in a Filter” by Randall J. Johnson and Wilbur H. Crawley; Ser. No. 10/931,008 entitled “Method and Apparatus for Shutting Down a Fuel-Fired Burner of an Emission Abatement Assembly” by Wilbur H. Crawley and Randall J. Johnson; Ser. No. 10/931,090 entitled “Method and Apparatus for Controlling the Temperature of a Fuel-Fired Burner of an Emission Abatement Assembly” by Wilbur H. Crawley, Randall J. Johnson, and Samuel N. Crane, Jr.; Ser. No. 10/931,092 entitled “Emission Abatement Assembly and Method of Operating the Same” by Wilbur H. Crawley and Randall J. Johnson; Ser. No. 10/931,020 entitled “Method and Apparatus for Cleaning the Electrodes of a Fuel-Fired Burner of an Emission Abatement Assembly” by Wilbur H. Crawley, Randall J. Johnson, Stephen P. Goldschmidt, and Edward C. Kinnaird; Serial No. 10/931,017 entitled “Method and Apparatus for Operating an Airless Fuel-Fired Burner of an Emission Abatement Assembly” by William Taylor, III, Yougen Kong, Mert E. Berkman, Jon J. Huckaby, and Samuel N. Crane, Jr.; Ser. No. 10/931,009 entitled “Method and Apparatus for Directing Exhaust Gas Through a Fuel-Fired Burner of an Emission Abatement Assembly” by Wilbur H. 
     Crawley, Randall J. Johnson, Yougen Kong, John Abel, Shoja Farr, Nicholas Birkby, and David Pearson; Ser. No. 10/931,027 entitled “Method and Apparatus for Controlling a Fuel-Fired Burner of an Emission Abatement Assembly” by William Taylor, III, Yougen Kong, Wilbur H. Crawley, and Randall J. Johnson; Ser. No. 10/931,026 entitled “Method and Apparatus for Determining Accumulation in a Particulate Filter of an Emission Abatement Assembly” by Wilbur H. Crawley and Randall J. Johnson; Ser. No. 10/931,010 entitled “Method and Apparatus for Monitoring Ash Accumulation in a Particulate Filter of an Emission Abatement Assembly” by Wilbur H. Crawley and Randall J. Johnson; and Ser. No. 10/931,088 entitled “Method and Apparatus for Monitoring the Components of a Control Unit of an Emission Abatement Assembly” by Wilbur H. Crawley, Randall J. Johnson, and Navin Khadiya, each of which is assigned to the same assignee as the present application, each of which is filed concurrently herewith, and each of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to diesel emission abatement devices. 
     BACKGROUND 
     Untreated internal combustion engine emissions (e.g., diesel emissions) include various effluents such as NO X , hydrocarbons, and carbon monoxide, for example. Moreover, the untreated emissions from certain types of internal combustion engines, such as diesel engines, also include particulate carbon-based matter or “soot”. Federal regulations relating to soot emission standards are becoming more and more rigid thereby furthering the need for devices and/or methods which remove soot from engine emissions. 
     The amount of soot released by an engine system can be reduced by the use of an emission abatement device such as a filter or trap. Such a filter or trap is periodically regenerated in order to remove the soot therefrom. The filter or trap may be regenerated by use of a burner or electric heater to burn the soot trapped in the filter. 
     SUMMARY 
     According to one aspect of the disclosure, an emission abatement assembly includes a pair of fuel-fired burners. Both of the fuel-fired burners are under the control of a single control unit. The fuel-fired burners may be selectively operated by the control unit to regenerate particulate filters. 
     According to another aspect of the disclosure, a method of monitoring a fuel-fired burner during filter regeneration includes determining the temperature of the heat being produced by the burner and adjusting the amount of fuel supplied to the burner based thereon. A predetermined temperature range may be used with the amount of fuel supplied to the burner being adjusted if the temperature is outside of the predetermined temperature range. An electronic controller configured to control the fuel-fired burner in such a manner is also disclosed. Temperature measurements may be obtained by use of a temperature sensor. 
     According to another aspect of the disclosure, a control unit for controlling operation of a fuel-fired burner is disclosed. The control unit includes a housing having an air inlet which is open to an interior chamber of the housing. An air pump is positioned in the interior chamber of the housing and has an air inlet which is open to the interior chamber of the control unit&#39;s housing. The air pump generates reduced air pressure in the interior chamber which draws air into the housing and into the pump&#39;s inlet. This flow of air cools an electronic controller along with other components position in the housing. In one exemplary embodiment, the air pump draws air from the interior chamber of the housing and supplies the air to a combustion chamber of the fuel-fired burner to facilitate operation of the burner. An associated method of advancing air to a fuel-fired burner is also disclosed. 
     According to another aspect of the disclosure, a method of operating a fuel-fired burner of an emission abatement assembly is disclosed. The method includes supplying a reduced amount of fuel to the fuel-fired burner in response to detection of a burner shutdown request. Such a reduced fuel supply continues for a predetermined time period after which fuel is no longer supplied to the burner. In the exemplary embodiment described herein, the supply of both combustion air and atomization air, along with spark generation, continues for a period of time after the fuel is shutoff. After a period of time, combustion air is no longer supplied to the burner, but atomization air continues to be supplied and spark generation is maintained. After a period of time, the supply of atomization air is shutoff and spark generation ceases. In the exemplary embodiment described herein, a supply of cleaning air is substantially continuously supplied to the fuel-fired burner to reduce, or even prevent, clogging of the burner&#39;s fuel inlet nozzle. An electronic controller configured to control the components of the emission abatement assembly in such a manner is also disclosed. 
     According to another aspect of the disclosure, a method of monitoring engine performance as a function of soot accumulation in a particulate filter includes determining characteristics of soot accumulation in the filter, analyzing the characteristics, and generating an error signal if the characteristics are indicative of predetermined engine performance conditions. In one exemplary embodiment, the rate in which soot accumulates in the filter may be monitored. An increase in the rate in which soot accumulates in the filter (beyond predetermined limits) may be indicative of an engine condition such as excess oil usage or a stuck/leaking fuel injector. An electronic controller configured to monitor soot accumulation in such a manner is also herein disclosed. 
     According to another aspect of the disclosure, a smoke detector is used to detect the presence of fuel particles and/or smoke in the interior chamber of the control unit. If the presence of fuel particles and/or smoke is detected, the control unit may be shutdown thereby potentially avoiding damage to the control unit. A method of monitoring output from such a smoke detector is also disclosed. 
     According to another aspect of the disclosure, a temperature sensor is used to monitor the temperature within the interior chamber of the control unit. If the temperature exceeds a predetermined upper temperature limit, the control unit may be shutdown thereby potentially avoiding damage to the control unit. A method of monitoring output from such a temperature sensor is also disclosed. 
     According to another aspect of the present disclosure, a fuel pressure sensor is used to monitor fuel pressure in a fuel return line associated with the control unit&#39;s fuel pump. If fuel pressure in the return line exceeds a predetermined upper pressure limit, the control unit may be shutdown thereby potentially avoiding damage to the control unit. A method of monitoring output from such a fuel pressure sensor is also disclosed. 
     According to another aspect of the disclosure, a method of monitoring ash buildup in a particulate filter includes determining particulate accumulation in the filters subsequent to filter regeneration and generating an error signal if particulate accumulation exceeds a predetermined threshold. The particulate matter remaining in the filter subsequent to filter regeneration may be attributable to ash. As such, by monitoring the amount of particulate matter in the filter relatively soon, if not immediately, after filter regeneration, a determination may be made as to when the filter is in need of servicing to remove ash therefrom. An electronic controller configured to monitor ash buildup in such a manner is also disclosed. 
     According to another aspect of the disclosure, the electronic controller of the emission abatement assembly is electrically coupled to an engine control unit of an internal combustion engine. The electronic controller may be coupled to the engine control unit via a communications interface such as a Controller Area Network or “CAN” interface. In such a way, information may be shared between the electronic controller of the emission abatement assembly and the engine control unit. 
     According to another aspect of the present disclosure, a method of operating a fuel-fired burner includes monitoring the temperature at the outlet of a particulate filter during a filter regeneration cycle and adjusting operation of the fuel-fired burner if the filter outlet temperature exceeds a predetermined limit. In one embodiment, the fuel-fired burner is shutdown if the filter outlet temperature exceeds the predetermined limit. Prior to, or in lieu of, shutdown of the burner, the amount of fuel supplied to the fuel-fired burner may be reduced if the filter outlet temperature exceeds the predetermined limit. 
     According to another aspect of the present disclosure, a method of starting up a fuel-fired burner of an emission abatement assembly includes lowering the fuel rate being supplied to the burner once flame ignition is detected. The fuel rate is maintained at this lower level as the assembly preheats. Once preheated, the fuel level is ramped up to a predetermined operational fuel level. 
     According to another aspect of the disclosure, the electrodes of a fuel-fired burner are energized for a predetermined period of time prior to the introduction of fuel into the burner thereby removing any soot or other debris deposited on the electrodes. 
     According to another aspect of the disclosure, the operating conditions of the engine are monitored to facilitate airless filter regeneration. In one specific implementation, filter regeneration occurs when engine operating conditions are within a predetermined range. 
     According to another aspect of the disclosure, the exhaust gas flow entering through the gas inlet port of the fuel-fired burner is separated into a combustion flow which is advanced through the combustion chamber, and a bypass flow which bypasses the combustion chamber. 
     According to another aspect of the disclosure, soot loading in a particulate filter is monitored as a function of exhaust mass flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a rear elevational view of an on-highway truck with an emission abatement assembly installed thereon; 
         FIG. 2  is a perspective view of one of the soot abatement assemblies of the emission abatement assembly of  FIG. 1 ; 
         FIG. 3  is an elevational view of the end of the soot abatement assembly as viewed in the direction of the arrows of line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cross sectional view of the soot abatement assembly of  FIG. 2  taken along the line  4 - 4  of  FIG. 3 , as viewed in the direction of the arrows; 
         FIG. 5  is an enlarged cross sectional view of the fuel-fired burner of the soot abatement assembly of  FIG. 4 ; 
         FIG. 6  is a perspective view of the control unit of the emission abatement assembly of  FIG. 1 , note that the cover has been removed for clarity of description; 
         FIG. 7  is a side elevational view of the control unit of  FIG. 6 ; 
         FIG. 8  is a diagrammatic view of the emission abatement assembly of  FIG. 1 ; 
         FIG. 9  is a flowchart of a control routine for monitoring operation of the fuel-fired burners of the emission abatement assembly during a filter regeneration cycle; 
         FIG. 10  is an exemplary temperature graph which demonstrates aspects of the control routine of  FIG. 9 ; 
         FIG. 11  is a flowchart of a control routine for monitoring the filter outlet temperature during a filter regeneration cycle; 
         FIG. 12  is a flowchart of a control routine for monitoring engine performance as a function of soot accumulation in the particulate filters of the emission abatement assembly of  FIG. 1 ; 
         FIG. 13  is an exemplary delta pressure versus time graph which demonstrates aspects of the control routine of  FIG. 12 ; 
         FIG. 14  is a flowchart of a control routine for monitoring ash buildup in the particulate filters of the emission abatement assembly of  FIG. 1 ; 
         FIG. 15  is a flowchart of a control routine for shutting down the fuel-fired burners of the emission abatement assembly of  FIG. 1 ; 
         FIG. 16  is an exemplary fuel level versus time graph which demonstrates aspects of the control routine of  FIG. 15 ; 
         FIG. 17  is a flowchart of a control routine for monitoring fuel pressure in the control unit&#39;s fuel return line; 
         FIG. 18  is a flowchart of a control routine for monitoring the output from the control unit&#39;s smoke detector; 
         FIG. 19  is a flowchart of a control routine for monitoring the output from the control unit&#39;s temperature sensor; 
         FIG. 20  is a diagrammatic view of another emission abatement assembly; 
         FIG. 21  is view similar to  FIG. 20 , but showing the emission abatement assembly configured with a diesel oxidation catalyst positioned upstream of the filter substrate; 
         FIGS. 22 and 23  are diagrammatic views showing the fuel-fired burner of the assemblies of  FIGS. 20 and 21  in greater detail; 
         FIG. 24  is a perspective view showing a portion of the combustion chamber of the assemblies of  FIGS. 20 and 21  in greater detail; 
         FIG. 25  is an elevation view of the portion of the combustion chamber of  FIG. 24  as viewed in the direction of arrow  25 - 25  of  FIG. 24 ; 
         FIG. 26  is an elevation view of a gas distributor; 
         FIG. 27  is a view similar to  FIGS. 22 and 23 , but showing a different embodiment of the combustion chamber; 
         FIG. 28  is an elevation view of a gas distributor; 
         FIG. 29  is a diagrammatic view showing both the engine and the emission abatement assembly under the control of the engine control unit of the engine; 
         FIG. 30  is a flowchart of a control routine for starting up the fuel-fired burners of the emission abatement assembly of  FIG. 1 ; 
         FIG. 31  is an exemplary fuel level versus time graph which demonstrates aspects of the control routine of  FIG. 30 ; 
         FIG. 32  is a flowchart of a control routine for cleaning the electrodes of the fuel-fired burner; 
         FIG. 33  is a flowchart of a control routine for regenerating an airless fuel-fired burner; 
         FIG. 34  is a flowchart of a control routine for triggering filter regeneration; 
         FIG. 35  is a diagrammatic view of another emission abatement assembly; 
         FIGS. 36-43  are views similar to  FIG. 5 , but showing the fuel-fired burner with modification thereto; 
         FIG. 44  is a development view of a plate which may be positioned around the combustion chamber; and 
         FIG. 45  is a fragmentary perspective view showing the plate of  FIG. 44  positioned around the combustion chamber. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     As will herein be described in more detail, an emission abatement assembly  10  for use with an internal combustion engine, such as the diesel engine of an on-highway truck  12 , includes a pair of soot abatement assemblies  14 ,  16  under the control of a control unit  18 . As shown in  FIG. 1 , each of the soot abatement assemblies  14 ,  16  has a fuel-fired burner  20 ,  22  and a particulate filter  24 ,  26 , respectively. The fuel-fired burners  20 ,  22  are positioned upstream (relative to exhaust gas flow from the engine) from the respective particulate filters  24 ,  26 . During operation of the engine, exhaust gas flows through the particulate filters  24 ,  26  thereby trapping soot in the filters. Treated exhaust gas is released into the atmosphere through exhaust pipes  28 ,  30 . From time to time during operation of the engine, the control unit  18  selectively operates the fuel-fired burner  20  to regenerate the particulate filter  24  and the fuel-fired burner  22  to regenerate the particulate filter  26 . 
     Referring now to  FIGS. 2-5 , the soot abatement assembly  14  is shown in greater detail. It should be appreciated that the soot abatement assembly  14  is substantially identical to the soot abatement assembly  16 . As such, the discussion relating to the soot abatement assembly  14  of  FIGS. 2-5  is relevant to the soot abatement assembly  16 . 
     As shown in  FIG. 5 , the fuel-fired burner  20  of the soot abatement assembly  14  includes a housing  32  having a combustion chamber  34  positioned therein. The housing  32  includes an exhaust gas inlet port  36 . As shown in  FIG. 1 , the exhaust gas inlet port  36  is secured to a T-shaped exhaust pipe  38  which conducts exhaust gas from the diesel engine of the truck  12  to both soot abatement assemblies  14 ,  16 . 
     The combustion chamber  34  has a number of gas inlet openings  40  defined therein. Engine exhaust gas is permitted to flow into the combustion chamber  34  through the inlet openings  40 . In such a way, an ignition flame present inside the combustion chamber  34  is protected from the full engine exhaust gas flow, while controlled amounts of engine exhaust gas are permitted to enter the combustion chamber  34  to provide oxygen to facilitate combustion of the fuel supplied to the burner  20 . Exhaust gas not entering the combustion chamber  34  is directed through a number of openings  42  defined in a shroud  44  and out an outlet  46  of the housing  32 . 
     The fuel-fired burner  20  includes an electrode assembly having a pair of electrodes  48 ,  50 . As will be discussed in greater detail herein, the electrodes  48 ,  50  are electrically coupled to igniters of the control unit  18 . When power is applied to the electrodes  48 ,  50 , a spark is generated in the gap  52  between the electrodes  48 ,  50 . Fuel enters the fuel-fired burner  20  through a fuel inlet nozzle  54  and is advanced through the gap  52  between the electrodes  48 ,  50  thereby causing the fuel to be ignited by the spark generated by the electrodes  48 ,  50 . It should be appreciated that the fuel entering the nozzle  54  is generally in the form of a controlled air/fuel mixture. 
     The fuel-fired burner  20  also includes a combustion air inlet  56 . As will be discussed in greater detail herein, an air pump associated with the control unit  18  generates a flow of pressurized air which is advanced to the combustion air inlet  56  via an air line  58  (see  FIG. 1 ). During regeneration of the particulate filter  24 , a flow of air is introduced into the fuel-fired burner  20  through the combustion air inlet  56  to provide oxygen (in addition to oxygen present in the exhaust gas) to sustain combustion of the fuel. 
     As shown in  FIGS. 2 and 4 , the particulate filter  24  is positioned downstream from the outlet  46  of the housing  32  of the fuel-fired burner  20  (relative to exhaust gas flow). The particulate filter  24  includes a filter substrate  60 . As shown in  FIG. 4 , the substrate  60  is positioned in a housing  62 . The filter housing  62  is secured to the burner housing  32 . As such, gas exiting the burner housing  32  is directed into the filter housing  62  and through the substrate  60 . The particulate filter  24  may be any type of commercially available particulate filter. For example, the particulate filter  24  may be embodied as any known exhaust particulate filter such as a “deep bed” or “wall flow” filter. Deep bed filters may be embodied as metallic mesh filters, metallic or ceramic foam filters, ceramic fiber mesh filters, and the like. Wall flow filters, on the other hand, may be embodied as a cordierite or silicon carbide ceramic filter with alternating channels plugged at the front and rear of the filter thereby forcing the gas advancing therethrough into one channel, through the walls, and out another channel. Moreover, the filter substrate  60  may be impregnated with a catalytic material such as, for example, a precious metal catalytic material. The catalytic material may be, for example, embodied as platinum, rhodium, palladium, including combinations thereof, along with any other similar catalytic materials. Use of a catalytic material lowers the temperature needed to ignite trapped soot particles. 
     The filter housing  62  is secured to a housing  64  of a collector  66 . Specifically, an outlet  88  of the filter housing  62  is secured to an inlet  68  of the collector housing  64 . As such, processed (i.e., filtered) exhaust gas exiting the filter substrate  60  (and hence the filter housing  62 ) is advanced into the collector  66 . The processed exhaust gas is then advanced into the exhaust pipe  28  and hence released to the atmosphere through a gas outlet  70 . It should be appreciated that the gas outlet  70  may be coupled to the inlet (or a pipe coupled to the inlet) of a subsequent emission abatement device (not shown) if the truck  12  is equipped with such a device. 
     Referring now to  FIGS. 6-8 , there is shown the control unit  18  in greater detail. The control unit  18  includes a housing  72  which defines an interior chamber  112 . Numerous components associated with the control unit  18  are positioned in the interior chamber  112  of the housing  72 . For ease of description, a sealed cover  74  (see  FIG. 1 ) has been removed from the housing in  FIGS. 6 and 7  to expose the components within the housing  72 . The control unit  18  includes an electronic control unit (ECU) or “electronic controller”  76 . The electronic controller  76  is positioned in the interior chamber  112  of the housing  72 . The electronic controller  76  is, in essence, the master computer responsible for interpreting electrical signals sent by sensors associated with the emission abatement assembly  10  (and in some cases, the engine  80 ) and for activating electronically-controlled components associated with the emission abatement assembly  10 . For example, the electronic controller  76  is operable to, amongst many other things, determine when one of the particulate filters  24 ,  26  of the soot abatement assemblies  14 ,  16  is in need of regeneration, calculate and control the amount and ratio of air and fuel to be introduced into the fuel-fired burners  20 ,  22 , determine the temperature in various locations within the soot abatement assemblies  14 ,  16 , operate numerous air and fuel valves, and communicate with an engine control unit  78  associated with the engine  80  of the truck  12 . 
     To do so, the electronic controller  76  includes a number of electronic components commonly associated with electronic units utilized in the control of electromechanical systems. For example, the electronic controller  76  may include, amongst other components customarily included in such devices, a processor such as a microprocessor  82  and a memory device  84  such as a programmable read-only memory device (“PROM”) including erasable PROM&#39;s (EPROM&#39;s or EEPROM&#39;s). The memory device  84  is provided to store, amongst other things, instructions in the form of, for example, a software routine (or routines) which, when executed by the processor  80 , allows the electronic controller  76  to control operation of the emission abatement assembly  10 . 
     The electronic controller  76  also includes an analog interface circuit  86 . The analog interface circuit  86  converts the output signals from the various sensors (e.g., temperature sensors) into a signal which is suitable for presentation to an input of the microprocessor  82 . In particular, the analog interface circuit  86 , 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  82 . 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  82 . It should also be appreciated that if any one or more of the sensors associated with the emission abatement assembly  10  generate a digital output signal, the analog interface circuit  86  may be bypassed. 
     Similarly, the analog interface circuit  86  converts signals from the microprocessor  82  into an output signal which is suitable for presentation to the electrically-controlled components associated with the emission abatement assembly  10  (e.g., the fuel injectors, air valves, igniters, pump motor, etcetera). In particular, the analog interface circuit  86 , by use of a digital-to-analog (D/A) converter (not shown) or the like, converts the digital signals generated by the microprocessor  82  into analog signals for use by the electronically-controlled components associated with the emission abatement assembly  10 . 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  82 . It should also be appreciated that if any one or more of the electronically-controlled components associated with the emission abatement assembly  10  operate on a digital input signal, the analog interface circuit  86  may be bypassed. 
     Hence, the electronic controller  76  may be operated to control operation of the fuel-fired burners  20 ,  22 . In particular, the electronic controller  76  executes a routine including, amongst other things, a closed-loop control scheme in which the electronic controller  76  monitors outputs of the sensors associated with the emission abatement assembly  10  to control the inputs to the electronically-controlled components associated therewith. To do so, the electronic controller  76  communicates with the sensors associated with the emission abatement assembly to determine, amongst numerous other things, the temperature at various locations within the soot abatement assemblies  14 ,  16  and the pressure drop across the filter substrate  60 . Armed with this data, the electronic controller  76  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 injectors are operated, controlling the power level input to the electrodes  48 ,  50 , controlling the air advanced through combustion air inlet  56 , etcetera. 
     The control unit  18  also includes an air pump  90 . The air pump  90  is driven by an electric motor  92  which is under the control of the electronic controller  76 . The motor  92  drives a pulley  94  which in turn drives the air pump  90 . A signal line  96  electrically couples the air pump  90  to the electronic controller  76 . The outlet  98  of the air pump  90  is coupled to an inlet  100  of an electronically-controlled air valve  102  via an air line  104 . A first outlet  106  of the air valve  102  is coupled to the combustion air inlet  56  of the fuel-fired burner  20  via one of the air lines  58 , whereas a second outlet  108  of the air valve  102  is combustion air inlet  56  of the fuel-fired burner  22  via the other air line  58 . 
     The air valve  102  is electrically coupled to the electronic controller  76  via a signal line  10 . As such, the electronic controller  76  may control position of the valve  102 . In particular, the electronic controller  76  may position the air valve  102  in either a first valve position in which combustion air from the air pump  90  is directed to the fuel-fired burner  20  or a second valve position in which combustion air from the air pump  90  is directed to the fuel-fired burner  22 . As will herein be described in greater detail, the controller  76  operates the air valve  102  to direct combustion air to the fuel-fired burner  20 ,  22  associated with the particulate filter  24 ,  26  undergoing regeneration. 
     As shown in  FIGS. 6 and 7 , the inlet  114  of the air pump  90  is open to the interior chamber  112  of the control housing  72 . As such, the air pump  90  draws air from the interior chamber  112  of the control housing  72 . The control housing  72  has an air inlet  116 . The air inlet  116  is open to the interior chamber  112 . An air filter  118  is secured to the housing  72  and is positioned to filter air being drawn into the interior chamber  112  through the air inlet  116 . When operated, the air pump  90  generates reduced air pressure in the interior chamber  112  thereby drawing air from the atmosphere through the filter  118 , the air inlet  116 , and into the interior chamber  112 . Air in the interior chamber  112  is then drawn into the pump inlet  114  and pumped to the air valve  102 . When the cover  74  is secured in place (see  FIG. 1 ), the housing  72  is substantially sealed such that substantially all of the air drawn into the interior chamber  112  by the air pump  90  is drawn through the filter  118  (and hence the air inlet  116 ). 
     Since both the pump inlet  114  and the housing inlet  116  are open to the interior chamber  112  (as opposed to being coupled to one another, for example, by an air hose or other type of conduit), a flow of air is generated in the interior chamber  112  as air advances from the housing inlet  116  to the pump inlet  114 . Such an arrangement facilitates cooling of the electronic controller  76  since the controller  76  is exposed to at least a portion of the air flow in the interior chamber  112 . In particular, the electronic controller  76  generates heat during operation thereof. Heat from the electronic controller  76  is transferred to the air advancing through the interior chamber  112  thereby cooling the electronic controller  76 . Such an arrangement facilitates the placement of the controller  76  in the housing  72  (as opposed to positioning the controller outside the housing  72  to be exposed to atmospheric temperatures). Moreover, in certain embodiments, cooling the electronic controller  76  in such a manner eliminates the need for heatsinks or other heat dissipating devices. 
     The control unit  18  also includes a fuel delivery assembly  120  configured to supply a desired mixture of air and fuel (“air/fuel mixture”) to the fuel-fired burners  20 ,  22 . In particular, the fuel-fired burners  20 ,  22  combust or otherwise process fuel in the form of a mixture of air and fuel. As is defined in this specification, the term “air/fuel mixture” is defined to mean a mixture of any amount of air and any amount of fuel including a “mixture” of only fuel. Moreover, the term “air-to-fuel ratio” is intended to mean the relationship between the air component and the fuel component of such air/fuel mixtures. 
     One illustrative embodiment of the fuel delivery assembly  120  will herein be described in greater detail. However, it should be appreciated that such a description is exemplary in nature and that the fuel delivery assembly  120  may be embodied in numerous different configurations. 
     In the illustrative embodiment described herein, the fuel delivery assembly  120  includes a fuel pump  122  which draws diesel fuel from a fuel tank  124  of the truck  12  via a fuel line  126 . A fuel filter  128  filters the fuel drawn from the tank  124 . As shown in  FIGS. 6 and 7 , the motor-driven pulley  94  drives an input shaft  130  of the fuel pump  122 . As such, the motor  92  drives both the air pump  90  and the fuel pump  122 . 
     The fuel pump  122  supplies a pressurized flow of fuel to a pair of electronically-controlled fuel injectors  132 ,  134 . As shown in  FIG. 8 , a signal line  136  electrically couples the fuel injector  132  to the electronic controller  76  thereby allowing the controller  76  to control operation of the injector  132 . Similarly, a signal line  138  electrically couples the fuel injector  134  to the electronic controller  76  thereby allowing the controller  76  to control operation of the injector  134 . 
     An electronically-controlled fuel enable valve  140  selectively allows fuel to be supplied to the fuel injectors  132 ,  134  from the fuel pump  122 . Specifically, when positioned in an open valve position, the fuel enable valve  140  allows fuel to be advanced to the fuel injectors  132 ,  134 . However, when the fuel enable valve  140  is positioned in a closed valve position, fuel is not supplied to the fuel injectors  132 ,  134 . Fuel pumped by the pump  122 , but not supplied to the injectors  132 ,  134 , is returned to the truck&#39;s fuel tank  124  via a fuel return line  142 . The fuel enable valve  140  is electrically coupled to the electronic controller  76  via a signal line  144 . The electronic controller  76  generates output signals on the signal line  144  to control operation (e.g., position) of the fuel enable valve  140 . 
     The fuel injectors  132 ,  134  are selectively operated by the electronic controller  76  to inject quantities of fuel into a mixing chamber  146  where the fuel is mixed with air to produce an air/fuel mixture having a desired air-to-fuel ratio which is then delivered to the fuel inlet nozzle  54  of the fuel-fired burners  20 ,  22  by a pair of fuel lines  148 ,  150 . Specifically, the electronic controller  76  generates output signals on the signal line  136  which cause the fuel injector  132  to inject a specific desired quantity of fuel into the mixing chamber  146  where the fuel mixes with air and is delivered to the fuel inlet nozzle  54  of the fuel-fired burner  20  via the fuel line  148 . Similarly, the electronic controller  76  generates output signals on the signal line  138  which cause the fuel injector  134  to inject a specific desired quantity of fuel into the mixing chamber  146  where the fuel mixes with air and is delivered to the fuel inlet nozzle  54  of the fuel-fired burner  22  via the fuel line  150 . 
     In the exemplary embodiment described herein, the air delivered to the mixing chamber  146  is supplied from a pressurized air source  150  associated with the truck  12 . For example, the pressurized air source  150  may be the truck&#39;s pneumatic brake pump(s). Pressurized air from the air source  150  is supplied to the control unit  18  via an air line  152 . A pair of electronically-controlled air valves  154 ,  156  control the amount of air supplied to the mixing chamber  146 . 
     The air valve  154  supplies a flow of cleaning air which, as described herein in greater detail, is generally constantly supplied to the mixing chamber  146  during operation of the engine  80  of the truck  12 . Such a flow of air prevents the accumulation of debris (e.g., soot) in the fuel inlet nozzles  54  of the fuel-fired burners  20 ,  22 . Such a flow of cleaning air may be pulsed at relatively high pressure for short interval of time to reduce clogging of the nozzles  54  with soot or other debris. For example, under software control, the cleaning air flow may be pulsed such that the air is supplied at, for example, 60 psi for 15 seconds, and then shutoff (or reduced in pressure) for 45 seconds, and then pulsed again, and so on. It has been found that such rapid increases in air pressure create a force or “shock” which facilitates soot removal. 
     As shown in  FIG. 8 , the air valve  156  is positioned in a parallel flow arrangement with the cleaning air valve  154 . The air valve  156  supplies a flow of air which is summed with the air flow from the cleaning air valve  154 . This combined flow of air is used for fuel atomization during operation of the fuel-fired burners  20 ,  22 . As such, during regeneration of one of the particulate filters  24 ,  26 , both the atomization air valve  156  and the cleaning air valve  154  are positioned in their respective open valve positions to supply air to the mixing chamber  146  to atomize the fuel injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . 
     The cleaning air valve  154  is electrically coupled to the electronic controller  76  via a signal line  158 . The electronic controller  76  generates output signals on the signal line  158  to control operation (e.g., position) of the cleaning air valve  154 . Similarly, the atomization air valve  156  is electrically coupled to the electronic controller  76  via a signal line  160 . The electronic controller  76  generates output signals on the signal line  160  to control operation (e.g., position) of the atomization air valve  156 . 
     As shown in  FIG. 8 , air exiting the air valves  154 ,  156  is supplied to the mixing chamber  146  via an air line  162 . A pressure transducer  164  senses the air pressure in the air line  162 . The output from the transducer  164  is communicated to the electronic controller  76  via a signal line  166 . The output from the transducer  164  may be used by the electronic controller  76  to verify that a desired air flow is being supplied to the mixing chamber  146 . For example, in the exemplary embodiment described herein, the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burners  20 ,  22  is varied by varying the amount of fuel injected into the mixing chamber  146  with the amount of air supplied to the mixing chamber  146  remaining substantially constant. As such, the output from the pressure transducer  164  may be monitored by the electronic controller  76  to confirm that the desired, substantially constant flow of air is being supplied to the mixing chamber  146 . 
     As described above, fueling of the fuel-fired burners  20 ,  22  is adjusted by altering the amount of fuel added to a substantially constant flow of atomization air. For example, to increase the amount of fuel being supplied to the fuel-fired burner  20  (i.e., to decrease the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ), the electronic controller  76  operates the fuel injector  132  to increase the amount of fuel being injected into the mixing chamber  146  with the amount of air being introduced into the mixing chamber  146  remaining substantially constant. Similarly, to increase the amount of fuel being supplied to the fuel-fired burner  22  (i.e., to decrease the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ), the electronic controller  76  operates the fuel injector  134  to increase the amount of fuel being injected into the mixing chamber  146  with the amount of air being introduced into the mixing chamber  146  remaining substantially constant. 
     Conversely, to decrease the amount of fuel being supplied to the fuel-fired burner  20  (i.e., to increase the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ), the electronic controller  76  operates the fuel injector  132  to decrease the amount of fuel being injected into the mixing chamber  146  with the amount of air being introduced into the mixing chamber  146  remaining substantially constant. To decrease the amount of fuel being supplied to the fuel-fired burner  22  (i.e., to increase the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ), the electronic controller  76  operates the fuel injector  134  to decrease the amount of fuel being injected into the mixing chamber  146  with the amount of air being introduced into the mixing chamber  146  remaining substantially constant. 
     As shown in  FIG. 8 , a pressure regulator  168  regulates the fluid pressure in the mixing chamber  146 . Specifically, the pressure regulator  168  ensures that a predetermined pressure is not exceeded in the mixing chamber  146 . For example, in many commercial systems, air from the truck&#39;s pressurized air source  150  is present at 90 psi. The pressure regulator  168  reduces the pressure of the air delivered to the mixing chamber  146  to a lower level such as, for example, 40 psi. 
     The control unit  18  also includes a pair of ignition devices or igniters  170 ,  172 . The igniters  170 ,  172  are electrically coupled to the electronic controller  76  via signal lines  174 ,  176 , respectively. As such, the controller  76  may selectively generate control signals on the signal lines  174 ,  176  to control operation of the igniters  170 ,  172 . The igniter  170  is electrically coupled to the electrodes  48 ,  50  of the fuel-fired burner  20  via a high voltage cable  178 , whereas igniter  172  is electrically coupled to the electrodes  48 ,  50  of the fuel-fired burner  22  via a high voltage cable  180 . Actuation of the igniter  170  causes a spark to be generated in the gap  52  between the electrodes  48 ,  50  of the fuel-fired burner  20  thereby igniting the air/fuel mixture entering the burner  20  through the fuel inlet nozzle  54 . Similarly, actuation of the igniter  172  causes a spark to be generated in the gap  52  between the electrodes  48 ,  50  of the fuel-fired burner  22  thereby igniting the air/fuel mixture entering the burner  22  through the fuel inlet nozzle  54 . 
     The igniters  170 ,  172  may be embodied as any type of device suitable to generate the spark across the electrode gap  52  of the electrodes  48 ,  50 . For example, the igniters  170 ,  172  may be embodied as one or more of the devices disclosed in U.S. Patent Application Ser. No. 10/737,333, entitled “Power Supply and Transformer” which was filed on Dec. 16, 2003 by Stephen P. Goldschmidt and Wilbur H. Crawley. The entirety of this patent application is hereby incorporated by reference. 
     As alluded to above, the electronic controller  76  monitors the output of a number of sensors associated with the soot abatement assemblies  14 ,  16 . For example, each of the soot abatement assemblies  14 ,  16  includes a flame temperature sensor  182 , a control temperature sensor  184 , and a outlet temperature sensor  186 . The temperature sensors  182 ,  184 ,  186  are electrically coupled to the electronic controller  76  via signal lines  188 ,  190 ,  192 , respectively. As shown in  FIGS. 2-5 , the temperature sensors  182 ,  184 ,  186  may be embodied as thermocouples which extend through the housings of the soot abatement assemblies  14 ,  16 , although other types of sensors may also be used. 
     The electronic controller  76  monitors output from the flame temperature sensor  182  to detect or otherwise determine presence of an ignition flame in the combustion chamber  34  of the fuel-fired burner  20 ,  22 . Specifically, when the electronic controller  76  initiates ignition of the fuel-fired burner  20 ,  22 , the controller  76  may monitor output from the flame temperature sensor  182  to ensure that the air/fuel mixture entering the burner  20 ,  22  is being ignited by the spark from the electrodes  48 ,  50 . An error signal is generated if the output of the flame temperature sensor does not meet a predetermined criteria. 
     The electronic controller  76  monitors output from the control temperature sensor to adjust the fueling of the fuel-fired burner  20 ,  22  to maintain the temperature of the heat exerted the particulate filter  24 ,  26  within a predetermined temperature range. For example, a temperature control range may be designed that allows for sufficient heat to adequately regenerate the particulate filter  24 ,  26 , while also preventing the filter  24 ,  26  from being exposed to excessive temperatures that may damage the filter  24 ,  26 . It should be appreciated that a temperature control range may be designed to meet many other objectives. 
     An exemplary temperature control routine  200  for controlling the fuel-fired burners  20 ,  22  during filter regeneration is shown in  FIGS. 9 and 10 . The control routine  200  begins with step  202  in which the electronic controller  76  determines the temperature of the heat generated by the burner. In particular, the electronic controller  76  scans or otherwise reads the signal line  190  to monitor output from the control temperature sensor  184 . Once the electronic controller  76  has determined the temperature of the heat being generated by the fuel-fired burner  20 ,  22 , the routine  200  advances to step  204 . 
     In step  204 , the electronic controller  76  determines if the sensed temperature of the heat generated by the fuel-fired burner  20 ,  22  is within a predetermined temperature control range. In particular, as described herein, a predetermined temperature control range may be established. In the exemplary embodiment described herein, a target temperature (e.g., 650° C. if the particulate filter  24 ,  26  is non-catalyzed or 350° C. if the filter  24 ,  26  is catalyzed) may be utilized in conjunction with a predetermined upper and lower control limit (see  FIG. 10 ). As such, in step  204 , the electronic controller  76  determines if the sensed temperature of heat generated by the fuel-fired burner  20 ,  22  is within the predetermined temperature control range (i.e., less than the upper limit and greater than the lower limit). If the temperature of the heat generated by the fuel-fired burner  20 ,  22  is within the predetermined temperature control range, the control routine  200  loops back to step  202  to continue monitoring the output from the control temperature sensor  184 . However, if the temperature of the heat generated by the fuel-fired burner  20 ,  22  is not within the predetermined temperature control range, a control signal is generated and the control routine  200  advances to step  206  if the temperature of the heat generated by the fuel-fired burner  20 ,  22  is above the upper control limit or step  208  if the temperature of the heat generated by the fuel-fired burner  20 ,  22  is below the lower control limit. 
     In step  206 , the electronic controller  76  decreases the fuel being supplied to the fuel-fired burner  20 ,  22 . To do so, the electronic controller  76  increases the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ,  22  by reducing the amount of fuel being injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . For example, to decrease the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  generates a control signal on the signal line  136  that reduces the amount of fuel being injected by the fuel injector  132  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  20  via the fuel line  148 . Similarly, to decrease the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  generates a control signal on the signal line  138  that reduces the amount of fuel being injected by the fuel injector  134  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  22  via the fuel line  150 . Once the fuel being supplied to the fuel-fired burner  20 ,  22  has been decreased, the control routine advances to step  210 . 
     In step  210 , the electronic controller  76  determines if the out-of-range condition in step  206  is a repeat occurrence. More specifically, the controller  76  determines if a predetermined number of temperature readings have been outside of the temperature control range. In particular, the electronic controller  76  monitors the results of previous fuel adjustments to determine if the fuel-fired burner  20 ,  22  has returned to operation within the predetermined temperature control range. If the controller  76  determines that a predetermined number of temperature readings have been outside of the temperature control range, the electronic controller  76  concludes that the fuel-fired burner  20 ,  22  cannot be brought back into control, an error signal is generated, and the control routine  200  advances to step  212 . Otherwise, the control routine  200  loops back to step  202  to continue monitoring operation of the fuel-fired burner  20 ,  22  during filter regeneration. 
     In step  212 , the electronic controller  76  shuts down the fuel-fired burner  20 ,  22 . In particular, since the electronic controller  76  concluded in step  210  that the fuel-fired burner  20 ,  22  cannot be brought back into control, the controller  76  ceases to supply fuel to the affected burner  20 ,  22 , ceases to generate a spark between the electrodes  48 ,  50 , or otherwise ceases operation of the affected burner  20 ,  22 . 
     Referring back to step  204 , if the temperature of the heat generated by the fuel-fired burner  20 ,  22  is below the lower control limit, the control routine advances to step  208 . In step  208 , the electronic controller  76  increases the fuel being supplied to the fuel-fired burner  20 ,  22 . To do so, the electronic controller  76  decreases the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ,  22  by increasing the amount of fuel being injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . For example, to increase the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  generates a control signal on the signal line  136  that increases the amount of fuel being injected by the fuel injector  132  into the mixing chamber  146  thereby decreasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  20  via the fuel line  148 . Similarly, to increase the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  generates a control signal on the signal line  138  that increases the amount of fuel being injected by the fuel injector  134  into the mixing chamber  146  thereby decreasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  22  via the fuel line  150 . Once the fuel being supplied to the fuel-fired burner  20 ,  22  has been increased, the control routine advances to step  210  to determine if control of the fuel-fired burner has been regained in the manner previously discussed. 
     Output from the outlet temperature sensor  186  may also be utilized by the electronic controller  76  to control operation of the fuel-fired burner  20 ,  22  during regeneration of the particulate filter  24 ,  26 . In particular, as shown in  FIG. 11 , a control routine  250  may be executed by the electronic controller  76  during filter regeneration. The control routine  250  begins with step  252  in which the electronic controller  76  determines the temperature at the outlet of the particulate filter  24 ,  26 . In particular, the electronic controller  76  scans or otherwise reads the signal line  192  to monitor output from the outlet temperature sensor  186 . Once the electronic controller  76  has determined the temperature at the outlet of the particulate filter  24 ,  26 , the routine  250  advances to step  254 . 
     In step  254 , the electronic controller  76  determines if the sensed filter outlet temperature is above a predetermined upper temperature limit. If the filter outlet temperature is below the upper temperature limit, the control routine  250  loops back to step  252  to continue monitoring output from the outlet temperature sensor  186 . However, if the filter outlet temperature is above the upper control limit, the control routine  250  advances to step  256 . 
     In step  256 , the electronic controller  76  shuts down the fuel-fired burner  20 ,  22 . In particular, since the electronic controller  76  concluded in step  254  that the filter outlet temperature was above the upper control limit, the controller  76  ceases to supply fuel to the affected burner  20 ,  22 , ceases to generate a spark between the electrodes  48 ,  50 , or otherwise ceases operation of the affected burner  20 ,  22 . The control routine  250  then advances to step  258 . 
     In steps  258  and  260 , the electronic controller  76  determines if the filter outlet temperature has cooled to a temperature below the upper control limit. In particular, in step  258  the electronic controller  76  scans or otherwise reads the signal line  192  to monitor output from the outlet temperature sensor  186  to determine the temperature at the outlet of the particulate filter  24 ,  26 . Once the electronic controller  76  has determined the temperature at the outlet of the particulate filter  24 ,  26 , the routine  250  advances to step  260 . 
     In step  260 , the electronic controller  76  determines if the sensed filter outlet temperature is still above the predetermined upper temperature limit. If the filter outlet temperature is still above the upper control limit, the control routine  250  loops back to step  258  to continue monitoring output from the outlet temperature sensor  186 . However, if the filter outlet temperature is now below the upper temperature limit, the control routine  250  advances to step  262 . 
     In step  262 , the electronic controller  76  restarts the fuel-fired burner  20 ,  22 . In particular, since the electronic controller  76  concluded in step  260  that the filter outlet temperature is now below the upper control limit, the controller  76  commences to supply fuel to the affected burner  20 ,  22 , generates the spark between the electrodes  48 ,  50 , and otherwise re-commences operation of the affected burner  20 ,  22 . The control routine  250  then loops back to step  252  to monitor operation of the burner  20 ,  22 . 
     The electronic controller  76  also monitors the output of a number of pressure sensors associated with the soot abatement assemblies  14 ,  16 . For example, each of the soot abatement assemblies  14 ,  16  includes a filter inlet pressure sensor  264  and a filter outlet pressure sensor  266  (see  FIG. 8 ). The pressure sensors  264  and  266  are electrically coupled to the electronic controller  76  via signal lines  268  and  270 , respectively. The pressure sensors  264 ,  266  may be embodied as any type of pressure sensing device such as, for example, commercially available pressure transducers. 
     Regeneration of the particulate filters  24 ,  26  may be commenced as a function of output from the pressure sensors  264 ,  266 . For example, the pressure sensors  264 ,  266  may be utilized to sense the pressure difference across the particulate filter  24 ,  26  (i.e., the “pressure drop” across the filter) to determine when the filter  24 ,  26  requires regeneration. Specifically, when the pressure drop across one of the particulate filters  24 ,  26  increases to a predetermined value, the filter regeneration process may be commenced for that particular filter  24 ,  26 . It should be appreciated that the pressure sensors  264 ,  266  may be embodied as a single sensor. In particular, a single sensor which measures a differential pressure may be used. Such sensors have two input ports, one of which measures pressure upstream of the filter, the other of which measures pressure downstream of the filter. In operation, such a sensor measures the pressure difference between its ports and generates an output relating to the same. Moreover, it should also be appreciated that in certain embodiments, a single pressure sensor on either side of particulate filter  24 ,  26  may be utilized. In such a configuration, output from the single pressure sensor is monitored to determine when pressure exceeds a predetermined upper threshold or is below a predetermined lower threshold (as opposed to monitoring the pressure drop across the filter). 
     It should be appreciated that the control scheme utilized to initiate filter regeneration may be designed in a number of different manners. For example, a timing-based control scheme may be utilized in which the regeneration of the particulate filters  24 ,  26  is commenced as a function of time. For instance, regeneration of particulate filters  24 ,  26  may be performed at predetermined timed intervals. 
     The output from the pressure sensors  264 ,  266  may also be used in conjunction with other information to trigger regeneration of the particulate filters  24 ,  26 . For example, the pressure drop across the filter  24 ,  26 , as a function of the exhaust mass flow from the engine  80 , may be used to trigger filter regeneration. To do so, a data table (e.g., a map) of the particulate filter  24 ,  26  is first experimentally generated. To generate such a map, the pressure drop across the filter  24 ,  26  as a function of exhaust mass flow at various particulate (soot) loadings is mapped. Specifically, the filter  24 ,  26  is first impregnated with a given amount of soot. Such an amount of soot may be indicative of a desired loading that would necessitate regeneration. For instance, if it is desirable to regenerate a particular type of particulate filter  24 ,  26  when it is loaded with, for example, 5.0 grams/liter, the filter being utilized to experimentally generate the map is first pre-loaded with such an amount of soot (i.e., 5.0 grams/liter). Once pre-loaded, the pressure drop across the filter is experimentally measured at a plurality different exhaust mass flows. A lookup table (e.g., a map) can then be generated which includes a plurality of the experimentally derived pressure drop values each of which corresponds to one of the plurality of different exhaust mass flow values. Such a map may be programmed into the controller  76 . 
     The map of such experimentally derived pressure drop values may then be used to determine when to trigger regeneration. In particular, during operation of the engine  80 , the controller  76  may determine the current pressure drop across the filter  24 ,  26  and exhaust mass flow from the engine  80 . As described herein, the pressure drop may be determined by monitoring output from the pressure sensors  264 ,  266 . As described in greater detail below, the controller  76  may determine exhaust mass flow by monitoring the output from a mass flow sensor  892  (see  FIG. 8 ), such as a hot wire mass flow sensor. It should be appreciated that the controller  76  may communicate with the mass flow sensor  892  directly, or may obtain the output from the sensor  892  from the engine control unit  78  via a CAN interface  314  (the CAN interface  314  is described in greater detail below). Alternatively, exhaust mass flow may be calculated by the controller  76  in a conventional manner by use of engine operation parameters such as engine RPM, turbo boost pressure, and intake manifold temperature (along with other known parameters such as engine displacement). It should be appreciated that the controller  76  may itself calculate the mass flow, or may obtain the calculated mass flow from the engine control unit  78  via the CAN interface  314 . 
     Once the controller  76  has determined both the pressure drop across the particulate filter  24 ,  26  and the exhaust mass flow from the engine  80 , the controller  76  queries the lookup table (i.e., the map) to retrieve the experimentally created limit value which corresponds to the sensed (or calculated) exhaust mass flow of the engine  80 . The controller  76  then compares the sensed pressure drop across the particulate filter  24 ,  26  to the retrieved limit value. If the sensed pressure drop across the filter  24 ,  26  exceeds the retrieved limit value, the controller  76  determines that the filter  24 ,  26  is in need of regeneration and commences a regeneration cycle. 
     An exemplary control routine  860  for triggering filter regeneration based on the pressure drop across the filter as a function of exhaust mass flow is shown in  FIG. 34 . The routine  860  commences with step  862  in which the electronic controller  76  determines the pressure drop (ΔP) across the particulate filter  24 ,  26 . Specifically, the controller  76  monitors the output from the pressure sensors  264 ,  266  and thereafter calculates the pressure drop (ΔP) across the filter. The control routine  860  then advances to step  864 . 
     In step  864 , the controller  76  determines the exhaust mass flow from the engine  80 . As described above, the controller  76  may determine the exhaust mass flow by monitoring the output from the mass flow sensor  892 , or by calculating it with the use of engine operation parameters such as engine RPM, turbo boost pressure, and intake manifold temperature (along with other known parameters such as engine displacement). In either case, once the controller determines the exhaust mass flow, the control routine advances to step  866 . 
     In step  866 , the controller  76  queries the lookup table (i.e., the filter map) to retrieve the experimentally created limit value which corresponds to the sensed (or calculated) exhaust mass flow (as determined in step  864 ). Once the controller  76  has retrieved the limit value from the lookup table, the control routine  860  advances to step  868 . 
     In step  868 , the controller  76  compares the sensed pressure drop across the particulate filter  24 ,  26  (as determined in step  862 ) to the retrieved limit value. If the sensed pressure drop across the filter  24 ,  26  exceeds the retrieved limit value, the controller  76  concludes that the filter  24 ,  26  is in need of regeneration, and the control routine  860  advances to step  870 . If the sensed pressure drop across the filter  24 ,  26  does not exceed the retrieved limit value, the control routine  860  loops back to step  860  to continue monitoring accumulation in the filter  24 ,  26 . 
     In step  870 , the controller  76  commences filter regeneration. Specifically, the electronic controller  76  operates the fuel-fired burner  20 ,  22  to regenerate the particulate filter  24 ,  26  in any of the numerous manners described herein. Once filter regeneration is complete, the control routine  870  ends. 
     The output from the pressure sensors  264 ,  266  may also be utilized to monitor performance of the engine  80 . In particular, characteristics of soot accumulation within the particulate filters  24 ,  26  may be indicative of certain engine performance characteristics. For example, excessive or otherwise irregular soot accumulation in the particulate filters  24 ,  26  may be indicative of excessive oil usage by the engine  80 . Excessive or otherwise irregular soot accumulation in the particulate filters  24 ,  26  may also be indicative of a stuck or leaky engine fuel injector. The electronic controller  76  may be configured to monitor and analyze the output from the pressure sensors  264 ,  266  to determine if any such engine conditions exist. 
     It should be appreciated that if a given design utilizes methods or devices other than pressure sensors to determine soot accumulation within the particulate filters  24 ,  26 , the output from such methods or devices may be monitored and analyzed to determine if any such engine conditions exist. As such, although an exemplary embodiment of a control scheme for monitoring engine performance as a function of soot accumulation in the filters  24 ,  26  based on output from the pressure sensors  264 ,  266  will now be described in greater detail, it should be appreciated that such a description is not intended to be limited to only pressure sensor-based systems. 
     Referring now to  FIG. 12 , there is shown an exemplary embodiment of a control routine  300  for monitoring engine performance as a function of soot accumulation within the particulate filters  24 ,  26 . The routine commences with step  302  in which the electronic controller  76  determines the rate of soot accumulation within the particulate filters  24 ,  26 . In particular, during operation of the engine  80 , the pressure drop across the particulate filters  24 ,  26  (ΔP) is continuously monitored by the controller  76 . Specifically, at a predetermined frequency, the output from pressure sensors  264 ,  266  is read so that the pressure drop (ΔP) may be calculated and thereafter stored in a table in a memory device (e.g., RAM or other memory device associated with the electronic controller  82 ). Over time, the pressure drop (ΔP) may be tracked. For example, a graphical representation which tracks the pressure drop (ΔP) across one of the filters  24 ,  26  as a function of time is shown in  FIG. 13 . In the exemplary embodiment described herein, the rate of soot accumulation may be determined by tracking the pressure drop (ΔP) over time as indicated with the line  312  in the graphical representation of  FIG. 13 . Once the electronic controller  76  has determined the rate of soot accumulation within the soot particulate filter  24 ,  26 , the routine  300  advances to step  304 . 
     In step  304 , the electronic controller  76  analyzes the rate of soot accumulation within the particulate filter  24 ,  26 . In the exemplary embodiment described herein, the controller  76  analyzes the rate of soot accumulation within the particulate filter  24 ,  26  by analyzing the slope of the line  312  generated by tracking the pressure drop (ΔP) over time. For example, if the slope of the line  312  remains relatively constant (i.e., within predetermined limits deemed to be indicative of a constant slope), such as indicated with a dashed line in  FIG. 13 , the electronic controller  76  concludes that there is no change in the rate in which soot is accumulating within particulate filter  24 ,  26 . However, if the slope of the line  312  increases beyond predetermined limits (as shown in the solid line in  FIG. 13 ), the electronic controller  76  concludes that there is a change in the rate in which soot is accumulating within the particulate filter  24 ,  26 . It should be appreciated that other methods may be utilized to analyze the rate of soot accumulation within the filter  24 ,  26  with the method described herein being merely exemplary in nature. Once the electronic controller  76  has analyzed the soot accumulation within the particulate filter  24 ,  26 , the control routine  300  advances to step  306 . 
     In step  306 , the electronic controller  76  determines if the rate of soot accumulation within particulate filter  24 ,  26  is indicative of a predetermined engine condition. Specifically, a lookup table stored in the in the memory device  84  (or other memory device associated with the electronic controller  82 ) may be queried to determine if the rate of soot accumulation, as analyzed in step  304 , matches predetermined criteria. For example, the contents of the lookup table are used to determine if the analysis of step  304  is indicative of no change in the rate of soot accumulation or change that is within predetermined acceptable limits. If so, the controller  76  concludes that the rate of soot accumulation is not indicative of an engine condition, and the control routine loops back to step  302  to continue monitoring soot accumulation within the filters  24 ,  26 . The contents of the lookup table may also be used to determine if the analysis performed in step  304  is indicative of change in the rate of soot accumulation that is outside of predetermined limits. If so, the controller  76  concludes that the rate of soot accumulation may be indicative of an engine condition, and the control routine  300  advances to step  308 . 
     In step  308 , the electronic controller  76  generates an error signal. For example, the electronic controller  76  may generate an output signal which causes a visual, audible, or other type of alarm to be generated for presentation to the operator (e.g., the driver of the truck  12 ). The error signal may simply cause an electronic log or the like to be updated with information associated with the filter analysis of steps  302 - 306 . 
     As indicated in step  310 , the error signal may be communicated to the engine control unit (ECU)  78  associated with the engine  80 . The details of doing so will now be described in greater detail. However, it should be appreciated that such a description is not limited to communication of the error signal generated in step  308  of the control routine  300 , but rather any error signal herein described (along with any other error signal generated by the controller  76 ) may be communicated to the engine control unit  78 . Moreover, as will be discussed herein in greater detail, the engine control unit  78  may communicate information, such as engine operation information, to the controller  76 . 
     In a conventional manner, engine systems, such as the engine  80  of the truck  12 , include an engine control unit which is, in essence, the master computer responsible for interpreting electrical signals sent by engine sensors and for activating electronically-controlled engine components to control the engine. For example, an engine control unit is operable to, amongst many other things, determine the beginning and end of each injection cycle of each engine cylinder, or determine both fuel metering and injection timing in response to sensed parameters such as engine crankshaft position and RPM, engine coolant and intake air temperature, and absolute intake air boost pressure. 
     Error signals generated by the controller  76  (or subsequent signals generated in response the error signal) may be communicated to the engine control unit  78 . Specifically, the electronic controller  76  of the emission abatement assembly  10  may be configured to communicate with the engine control unit  78  via an interface  314 . The interface  314  may be any type of communication interface which enables electronic communication between the electronic controller  76  and the engine control unit  78 . One type of interface which is suitable for use as the interface  314  is a Controller Area Network or “CAN” interface. A CAN interface is a serial bus network of microcontrollers that connects devices, sensors and actuators in a system or sub-system for real-time control applications. Details of a CAN interface, which was first developed by Robert Bosch GmbH in 1986, are documented in ISO 11898 (for applications up to 1 Mbps) and ISO 11519 (for applications up to 125 Kbps), both of which are hereby incorporated by reference. 
     By use of the CAN interface  314 , information such as engine RPM and turbo boost pressure may be obtained from the engine control unit  78  for use by the electronic controller  76 . Such information may be used by the controller  76  in the execution of certain control routines. By using information from the engine control unit  78 , a redundant sensor array to determine such information solely for use by the electronic controller is eliminated. 
     Moreover, the CAN interface  314  allows for the transfer of error signals (e.g., error flags) or the like to the engine control unit  78  for use by the engine control unit  78  during its operation. For example, an error signal indicative of an engine problem (as described in regard to the control routine  300 ) may be communicated to the engine control unit  78 . Armed with this information, the engine control unit  78  may be programmed to perform additional engine analysis, generate an error signal to the truck operator (e.g., an indicator light on the truck&#39;s instrument cluster), or store the error message in an error log which can be accessed by a service technician. The CAN interface  314  also allows an engine manufacturer to assume some degree of control over the operation of the emission abatement assembly  10 , if desired. 
     As such, it should be appreciated that the controller  76  of the control unit  18  monitors operation of the fuel-fired burners  20 ,  22  (and other components of the emission abatement assembly  10 ) to determine if any of the predetermined conditions described herein (or other conditions) are met. The controller  76  may then generate a signal, such as an error signal, indicative of such conditions and communicate such a signal to the engine control unit  78  via the CAN interface  314 . Moreover, the CAN interface  314  may be used by the engine control unit  78  to communicate information, such as information relating to engine operation, to the controller  76 . For example, information relating to engine RPM or turbo boost pressure may be communicated to the controller  76  via the CAN interface  314 . In addition to engine operation information, if so configured, the engine control unit  78  may also generate and communicate control signals for controlling operation of the fuel-fired burners  20 ,  22  to the controller  76 . For example, the engine control unit  78  may be programmed to initiate regeneration cycles of the particulate filters  24 ,  26 . In such a case, the engine control unit  78  may generate and communicate a control signal to the controller  76  which causes the controller  76  to commence regeneration of one of the particulate filters  24 ,  26 . 
     As shown in  FIG. 29 , the electronic controller  76  of the control unit  18  may be integrated with the engine control unit  78 . As such, in addition to controlling operation of the engine  80 , the engine control unit  78  also controls operation of the emission abatement assembly  10 . In such a way, the engine control unit  78  is also, in essence, the master computer responsible for interpreting electrical signals sent by sensors associated with the emission abatement assembly  10  and for activating electronically-controlled components associated with the emission abatement assembly  10 . For example, the engine control unit  78  is operable to, amongst many other things, determine the beginning and end of each filter regeneration cycle, determine the amount and ratio of fuel and air to be introduced into the fuel-fired burners  20 ,  22 , along with the other functions herein described as being performed by the controller  76  of the emission abatement assembly  10 . 
     To do so, the engine control unit  78  includes a number of electronic components commonly associated with electronic units which are utilized in the control of engine systems. For example, the engine control unit  78  may include, amongst other components customarily included in such devices, a processor such as a microprocessor  728  and a memory device  730  such as a programmable read-only memory device (“PROM”) including erasable PROM&#39;s (EPROM&#39;s or EEPROM&#39;s). 
     The memory device  730  is provided to store, amongst other things, instructions in the form of, for example, a software routine (or routines) which, when executed by the processing unit, allows the engine control unit  78  to control operation of both the engine  80  and the emission abatement assembly  10 . To do so, as shown in  FIG. 29 , the engine control unit  78  is electrically coupled to both the engine  80  and the emission abatement assembly  10 . In particular, the engine control unit  78  is electrically coupled to the engine  80  via the signal line  718 , whereas the engine control unit  78  is electrically coupled to the emission abatement assembly  10  via the signal line  720 . Although each is shown schematically as a single line, it should be appreciated that the signal lines  718 ,  720  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 engine control unit  78  and the engine  80  or the emission abatement assembly  10 , respectively. For example, either one or both of the signal lines  718 ,  720  may be embodied as a wiring harness having a number of signal lines which transmit electrical signals between the engine control unit  78  and the engine  80  or the emission abatement assembly  10 , respectively. In such an arrangement, signals generated by operation of a number of engine sensors  734  or the sensors  736  associated with the emission abatement assembly  10  are transmitted to the engine control unit  78  via the corresponding wiring harness, and signals generated by the engine control unit  78  are transmitted to the engine  80  or the emission abatement assembly  10  by the corresponding wiring harness. It should be appreciated that any number of other wiring configurations may be used. For example, individual signal wires may be used, or a system utilizing a signal multiplexer may be used for the design of either one or both of the signal lines  718 ,  720 . Moreover, the signal lines  718 ,  720  may be integrated such that a single harness or system is utilized to electrically couple both the engine  80  and the emission abatement assembly  10  to the engine control unit  78 . 
     The engine control unit  78  also includes an analog interface circuit  732 . The analog interface circuit  732  converts the output signals from the various analog engine sensors  734  and the emission abatement sensors  736  into a signal which is suitable for presentation to an input of the microprocessor  728 . In particular, the analog interface circuit  732 , by use of an analog-to-digital (A/D) converter (not shown) or the like, converts the analog signals generated by the sensors  734 ,  736  into a digital signal for use by the microprocessor  728 . 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  728 . It should also be appreciated that if any one or more of the sensors  734 ,  736  generate a digital output signal, the analog interface circuit  732  may be bypassed. 
     It should be appreciated that the emission abatement sensors  736  communicating with the engine control unit  78  may be any of the sensors herein described in relation to the emission abatement assembly  10 . For example, the pressure sensors  264 ,  266  and the temperature sensors  182 ,  184 ,  186  associated with the soot abatement assemblies  14 ,  16  may be coupled to the engine control unit  78 . Moreover, the sensors and detectors  164 ,  426 ,  460 ,  510  of the control unit  18  may be coupled to the engine control unit  78 . 
     The analog interface circuit  732  also converts signals from the microprocessor  728  into an output signal which is suitable for presentation to the electrically-controlled components  744  associated with the engine  80  and the electronically-controlled components  746  associated with the emission abatement assembly  10 . In particular, the analog interface circuit  732 , by use of a digital-to-analog (D/A) converter (not shown) or the like, converts the digital signals generated by the microprocessor  728  into analog signals for use by the electronically-controlled components  744  associated with the engine such as the fuel injector assembly, ignition assembly, fan assembly, etcetera, along with analog signals for use by electronically-controlled components  746  associated with the emission abatement assembly  10  such as the pump motor  92 , the air valve  102 , the fuel injectors  132 ,  134 , the valves  140 ,  154 ,  156 , the igniters  170 ,  172 , etcetera. 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  728 . It should also be appreciated that if any one or more of the electronically-controlled components  744  associated with the engine  80  or electronically-controlled components  746  associated with the emission abatement assembly  10  operate on a digital input signal, the analog interface circuit  732  may be bypassed. 
     Hence, the engine control unit  78  may be operated to control operation of both the engine  80  and the emission abatement assembly  10 . In particular, the engine control unit  78  operates in a closed-loop control scheme in which the engine control unit  78  monitors outputs of the sensors  734 ,  736  in order to control the inputs to the controlled components  744 ,  746  thereby managing the operation of both the engine  80  and the emission abatement assembly  10 . In particular, the engine control unit  78  communicates with the sensors  734  in order to determine, amongst numerous other things, the engine coolant temperature, manifold air pressure, crankshaft/flywheel position and speed, and the amount of oxygen in the exhaust gas. Armed with this data, the engine control unit  78  performs numerous calculations each second, including looking up values in preprogrammed tables, in order to execute routines to perform such functions as varying spark timing or determining how long the fuel injector is to be left open in a particular cylinder. 
     Contemporaneous with such control of the engine  80 , the engine control unit  78  also executes a routine for controlling operation of the emission abatement assembly  10 . In particular, the engine control unit  78  communicates with the sensors  736  in order to determine, amongst numerous other things, the soot accumulation level in the particulate filters, various temperature and pressure readings, etcetera. Armed with this data, the engine control unit  78  performs numerous calculations each second, including looking up values in preprogrammed tables, in order to execute algorithms to perform such functions as supplying fuel and air to the fuel-fired burners  20 ,  22 , energizing the electrodes  48 ,  50 , etcetera. 
     As such, the engine control unit  78  controls operation of both the engine  80  and the emission abatement assembly  10 . In particular, during operation of the engine  80 , the engine control unit  78  executes a fuel injector control routine which, amongst other things, generates a number of injection signals in the form of injection pulses which are communicated to the individual injectors of the engine&#39;s fuel injector assembly. In response to receipt of the injection pulse, a fuel injector is opened for a predetermined period of time, thereby injecting fuel into the corresponding cylinder of the engine  80 . Contemporaneous with execution of the fuel injection routine, the engine control unit  78  executes a burner control routine which, amongst other things, generates a number of control signals which are communicated to the various electronically-controlled components  746  associated with the emission abatement assembly  10 , thereby controlling operation of the fuel-fired burners  20 ,  22 . For example, signals are generated and communicated for, amongst other things, varying the amount of fuel being supplied to the fuel-fired burner  20 ,  22 , energizing the electrodes  48 ,  50 , etcetera. 
     Moreover, the engine control unit  78  also monitors input from the various sensors  736  associated with the emission abatement assembly  10  in order to utilize such input in the closed-loop control of the assembly  10 . For example, signals communicated to the engine control unit  78  are utilized to monitor the temperature of certain areas within the soot abatement assembly  14 ,  16 , the pressure drop across the particulate filter  24 ,  26 , along with the numerous other functions herein described. 
     It should be appreciated that such routines (i.e., the fuel injector control routine and the fuel reformer control routine) may be embodied as separate software routines, or may be combined as a single software routine. 
     Referring now to  FIG. 14 , there is shown a control routine  350  for monitoring ash buildup in the particulate filters  24 ,  26 . Over time as multiple filter regenerations occur, ash may accumulate in the particulate filters  24 ,  26 . By monitoring (e.g., measuring and data logging) the pressure drop (ΔP) across the particulate filter  24 ,  26  subsequent to each filter regeneration process, it can be determined when the filter requires the ash to be cleaned. Specifically, as will herein be described in greater detail, shortly after each filter regeneration cycle, the pressure drop (ΔP) across the particulate filter  24 ,  26  is obtained and stored in memory. Once the pressure drop (ΔP) across the particulate filter  24 ,  26  exceeds a predetermined upper limit, an error signal indicative of the need to service the filter by removing the ash from the filter is generated. 
     The control routine  350  commences with step  352  in which the electronic controller  76  regenerates one of the particulate filters  24 ,  26 . Specifically, as described in greater detail herein, the electronic controller  76  operates the fuel-fired burner  20 ,  22  to generate heat to regenerate the particulate filter  24 ,  26 . Once the regeneration cycle is complete, the control routine  350  advances to step  354 . 
     In step  354 , the electronic controller  76  measures the pressure drop (ΔP) across the recently regenerated particulate filter  24 ,  26 . Specifically, the output from pressure sensors  264 ,  266  of the recently regenerated filter is read so that the pressure drop (ΔP) may be calculated. 
     Thereafter, the control routine advances to step  356  where the value of the pressure drop (ΔP) across the recently regenerated particulate filter  24 ,  26  is stored in a table in a memory device (e.g., RAM or other memory device associated with the electronic controller  82 ). The control routine  350  then advances to step  358 . 
     In step  358 , the electronic controller  76  determines if the pressure drop (ΔP) across the recently regenerated particulate filter  24 ,  26  is above a predetermined upper limit. If the pressure drop (ΔP) across the recently regenerated particulate filter  24 ,  26  is below the upper limit, the control routine  350  ends until reinitiated subsequent to completion of the next filter regeneration cycle. However, if the pressure drop (ΔP) across the recently regenerated particulate filter  24 ,  26  is above the upper control limit, the control routine  350  advances to step  360 . 
     In step  360 , the electronic controller  76  generates an error signal. For example, the electronic controller  76  may generate an output signal which causes a visual, audible, or other type of alarm to be generated for presentation to the operator (e.g., the driver of the truck  12 ). Alternatively, the error signal may simply cause an electronic log or the like to be updated with information associated with the filter analysis of steps  352 - 358 . It should be appreciated that the error signal generated in step  360  may be configured for use with any type of alarming or error tracking arrangement to fit the needs of a given system design. 
     As indicated in step  362 , if the electronic controller  76  is so equipped, the error signal (or a subsequent signal generated in response the error signal) may be communicated to the engine control unit  78  via the CAN interface  314 . Armed with this information, the engine control unit  78  may be programmed to perform additional filter analysis, generate an error signal to the truck operator (e.g., an indicator light on the truck&#39;s instrument cluster) indicating that the affected filter(s)  24 ,  26  requires servicing (i.e., ash removal), or store the error message in an error log which can be accessed by a service technician. The control routine  350  then ends. 
     As described above, the electronic controller  76  may use a number of different control schemes to determine when one of the particulate filters  24 ,  26  is in need of regeneration. For example, a sensor-based scheme or a timing-based scheme may be utilized. In either case, when the controller  76  determines that one of the filters  24 ,  26  is in need of regeneration, a regeneration cycle is commenced in which the electronic controller  76  operates the fuel-fired burners  14 ,  16  to regenerate the filters  24 ,  26 , respectively. To do so, the air pump  90  and the air valve  102  are operated to supply combustion air to the appropriate burner  20 ,  22 . Contemporaneously, fuel is supplied to the appropriate burner  20 ,  22  via the fuel delivery assembly  120 . In particular, to supply fuel to the fuel-fired burner  20 , the fuel injector  132  is operated to inject fuel into the mixing chamber  146  where it is atomized in a flow of atomization air being supplied to the mixing chamber  146  by the air valves  154 ,  156 . The resultant air/fuel mixture is conducted to the fuel inlet nozzle  54  of the fuel-fired burner  20  via the fuel line  148 . On the other hand, to supply fuel to the fuel-fired burner  22 , the fuel injector  134  is operated to inject fuel into the mixing chamber  146  where it is atomized in the flow of atomization air being supplied to the mixing chamber  146  by the air valves  154 ,  156 . The resultant air/fuel mixture is conducted to the fuel inlet nozzle  54  of the fuel-fired burner  22  via the fuel line  150 . 
     The air/fuel mixture entering the burner  20 ,  22  via the fuel inlet nozzle  54  is ignited by the electrodes  48 ,  50 . In the case of operation of the fuel-fired burner  20 , the igniter  170  is actuated to generate a spark across the electrode gap  52  between the electrodes  48 ,  50  of the burner  20  thereby igniting the air/fuel mixture exiting the fuel inlet  54 . In the case of operation of the fuel-fired burner  22 , the igniter  172  is actuated to generate a spark across the electrode gap  52  between the electrodes  48 ,  50  of the burner  22  thereby igniting the air/fuel mixture exiting the fuel inlet  54 . 
     As described above, the electronic controller  76  monitors output from the flame temperature sensor  182  to detect or otherwise determine presence of an ignition flame in the combustion chamber  34  of the fuel-fired burner  20 ,  22  being activated. Specifically, when the electronic controller  76  initiates ignition of the fuel-fired burner  20 ,  22 , the controller  76  monitors output from the flame temperature sensor  182  to ensure that the air/fuel mixture entering the burner  20 ,  22  is being ignited by the spark from the electrodes  48 ,  50 . An error signal is generated if the output of the flame temperature sensor does not meet a predetermined criteria. 
     Once the fuel-fired burner  20 ,  22  is activated, it begins to produce heat. Such heat is directed downstream (relative to exhaust gas flow) and into contact with the upstream face of the particulate filter  24 ,  26 . The heat ignites and burns soot particles trapped in the filter substrate  60  thereby regenerating the particulate filter  24 ,  26 . Illustratively, heat in the range of 600-650 degrees Celsius may be sufficient to regenerate a non-catalyzed filter, whereas heat in the range of 300-350 degrees Celsius may be sufficient to regenerate a catalyzed filter. 
     In an illustrative embodiment, regeneration of the particulate filter  24 ,  26  may take only a few minutes. Moreover, it should be appreciated that regeneration of the particulate filter  24 ,  26  may be self-sustaining once initiated by heat from the fuel-fired burner  20 ,  22 , respectively. Specifically, once the filter  24 ,  26  is heated to a temperature at which the soot particles trapped therein begin to ignite, the ignition of an initial portion of soot particles trapped therein can cause the ignition of the remaining soot particles much in the same way a cigar slowly burns from one end to the other. In essence, as the soot particles “burn,” an amount of heat is released in the “burn zone.” Locally, the soot layer (in the burn zone) is now much hotter than the immediate surroundings. As such, heat is transferred to the as yet un-ignited soot layer downstream of the burn zone. The energy transferred may be sufficient to initiate oxidation reactions that raise the un-ignited soot to a temperature above its ignition temperature. As a result of this, heat from the fuel-fired burners  20 ,  22  may only be required to commence the regeneration process of the filter  24  (i.e., begin the ignition process of the soot particles trapped therein). 
     During the regeneration cycle, the fuel-fired burners  20 ,  22  may be controlled in the manner described herein in regard to  FIGS. 9-11 . Specifically, the control routines  200  and  250  may be utilized to monitor temperatures within soot abatement assemblies  14 ,  16  in the manner described herein. 
     Referring now to  FIGS. 30 and 31 , there is shown a control routine  750  for starting up the fuel-fired burners  20 ,  22  during commencement of a regeneration cycle. The routine begins with step  752  in which the routine determines if a request to startup the fuel-fired burner  20 ,  22  (i.e., a burner startup request) has been executed. It should be appreciated that a burner startup request may take many different forms including, for example, a startup request generated by a software control routine in response to sensed, timed, or otherwise determined indication that one of the particulate filters  24 ,  26  is in need of regeneration. For example, a sensor-based scheme, map-based scheme, or a timing-based scheme may be utilized to generate a startup request. As such, in step  752 , if the control routine  750  detects a burner startup request, a control signal is generated and the routine  750  advances to step  754 . If the control routine  750  does not detect a burner startup request, the routine  750  loops back to step  752  to continue monitoring for such a request. 
     In step  754 , the electronic controller  76  supplies a relatively high amount of fuel to the fuel-fired burner  20 ,  22  to facilitate ignition of a flame in the combustion chamber  34 . Specifically, an air/fuel mixture is supplied to the burner  20 ,  22  where it is to be ignited by the spark between the electrodes  48 ,  50  in the presence of combustion air supplied by the control unit  18 . The supply of this initial fuel level is shown graphically with the arrow  764  of  FIG. 31 . The control routine  750  then advances to step  756 . 
     In step  756 , the controller  76  determines if ignition has occurred. The controller  76  may do so in any number of different manners. For example, the electronic controller  76  may monitor output from the flame temperature sensor  182  to detect or otherwise determine presence of an ignition flame in the combustion chamber  34  of the fuel-fired burner  20 ,  22 . Specifically, when the electronic controller  76  initiates ignition of the fuel-fired burner  20 ,  22 , the controller  76  may monitor output from the flame temperature sensor  182  to ensure that the air/fuel mixture entering the burner  20 ,  22  is being ignited by the spark from the electrodes  48 ,  50 . Once ignition has been detected, the control routine  750  advances to step  758 . Ignition detection is shown graphically at point  766  in  FIG. 31 . 
     In step  758 , the electronic controller  76  decreases the fuel being supplied to the fuel-fired burner  20 ,  22 . To do so, the electronic controller  76  increases the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ,  22  by reducing the amount of fuel being injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . For example, to decrease the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  generates a control signal on the signal line  136  that reduces the amount of fuel being injected by the fuel injector  132  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  20  via the fuel line  148 . Similarly, to decrease the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  generates a control signal on the signal line  138  that reduces the amount of fuel being injected by the fuel injector  134  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  22  via the fuel line  150 . 
     The electronic controller  76  operates the fuel-fired burner  20 ,  22  at this reduced fuel level for a period of time to preheat the components of the soot abatement assembly  14 ,  16 . Such a preheating period may be time-based (i.e., continue for a predetermined period of time) or may be sensor-based (i.e., continue until a predetermined temperature is sensed by one or more of the temperature sensors  182 ,  184 ,  186 ). The preheating period is shown graphically with the arrow  768  of  FIG. 31 . Once this period of time has elapsed (i.e., once the system has been preheated), the control routine  750  advances to step  760 . 
     In step  760 , the electronic controller  76  ramps up or otherwise increases the fuel being supplied to the fuel-fired burner  20 ,  22 . To do so, the electronic controller  76  decreases the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ,  22  by increasing the amount of fuel being injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . For example, to increase the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  generates a control signal on the signal line  136  that increases the amount of fuel being injected by the fuel injector  132  into the mixing chamber  146  thereby decreasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  20  via the fuel line  148 . Similarly, to increase the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  generates a control signal on the signal line  138  that increases the amount of fuel being injected by the fuel injector  134  into the mixing chamber  146  thereby decreasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  22  via the fuel line  150 . 
     In step  760 , the fuel supplied to the fuel-fired burner  20 ,  22  may be increased at a predetermined ramp rate. For example, as shown graphically with arrow  770  in  FIG. 31 , the fuel level may be gradually increased at a predetermined ramp rate up to a specific, predetermined fuel level, as indicated by point  772  in  FIG. 31 . Such a predetermined fuel level may correspond with a desired regeneration temperature. Once the fuel level has been ramped up, the control routine  750  advances to step  762 . 
     In step  762 , the controller  76  adjusts the fuel level being supplied to the fuel-fired burner  20 ,  22  to facilitate filter regeneration. Specifically, as described above in regard to  FIGS. 9 and 10 , during a filter regeneration cycle, fueling of the burner  20 ,  22  is adjusted by closed-loop control. Such closed-loop control of the fueling of the burner  20 ,  22  is shown generally in the area indicated by the arrow  418  of  FIG. 31 . Once under closed-loop control, the startup control routine  750  ends. 
     Referring now to  FIG. 32 , there is shown another startup control routine  780  for starting up the fuel-fired burners  20 ,  22  during commencement of a regeneration cycle. The routine begins with step  782  in which the routine  780  determines if a request to startup the fuel-fired burner  20 ,  22  (i.e., a burner startup request) has been executed. It should be appreciated that a burner startup request may take many different forms including, for example, a startup request generated by a software control routine in response to sensed, timed, or otherwise determined indication that one of the particulate filters  24 ,  26  is in need of regeneration. For example, a sensor-based scheme, map-based scheme, or a timing-based scheme may be utilized to generate a startup request. As such, in step  782 , if the control routine  780  detects a burner startup request, a control signal is generated and the routine  780  advances to step  784 . If the control routine  780  does not detect a burner startup request, the routine  780  loops back to step  782  to continue monitoring for such a request. 
     In step  784 , the controller  76  energizes the electrode assembly of the fuel-fired burner  20 ,  22  that is to be regenerated prior to any fuel being supplied to the burner. Specifically, during startup of the fuel-fired burner  20 , prior to fuel being supplied to the burner  20 , the controller  76  operates the igniter  170  to commence spark generation between the electrodes  48 ,  50  of the burner  20 . In the case of startup of the fuel-fired burner  22 , prior to fuel being supplied to the burner  22 , the electronic controller  76  operates the igniter  172  to commence spark generation between the electrodes  48 ,  50  of the burner  22 . 
     The controller  76  continues to energize the electrode assembly of the fuel-fired burner  20 ,  22  for a predetermined period of time prior to the introduction of fuel to the burner. The duration of such a period of time may be configured to fit the needs of a given system design. In particular, it has been found that energizing the electrode assembly for such a period of time prior to fuel introduction cleans any fouled surfaces on the electrodes  48 ,  50  (i.e., removes any soot or other matter accumulated thereon). As such, any matter accumulated on the electrodes  48 ,  50  (e.g., soot, diesel fuel, water, oil, etcetera) can be removed from the electrodes prior to the introduction of fuel thereby enhancing operation of the fuel-fired burner  20 ,  22 . Once the predetermined period of time has elapsed, the control routine  780  advances to step  786 . 
     In step  786 , the electronic controller  76  supplies fuel and air to the fuel-fired burner  20 ,  22  to regenerate the particulate filter  24 ,  26  in the manner described above. Specifically, an air/fuel mixture is supplied to the burner  20 ,  22  where it is ignited by the spark between the electrodes  48 ,  50  in the presence of combustion air supplied by the control unit  18 . Heat generated by the combustion of the fuel regenerates the particulate filter  24 ,  26 . 
     It should be appreciated that the control routines  750 ,  780  may be combined, if desired. For example, the electrode assembly may be energized for a period of time (as described in step  784  of the control routine  780 ) prior to the introduction of the fuel for ignition (as described in step  754  of the control routine  750 ). 
     Referring now to  FIGS. 15 and 16 , there is shown a control routine  400  for shutting down the fuel-fired burners  20 ,  22  during a regeneration cycle. The control routine begins with step  402  in which electronic controller  76  supplies fuel and air to the fuel-fired burner  20 ,  22  to regenerate the particulate filter  24 ,  26  in the manner described above. Specifically, an air/fuel mixture are supplied to the burner  20 ,  22  where it is ignited by the spark between the electrodes  48 ,  50  in the presence of combustion air supplied by the control unit  18 . As described in regard to  FIGS. 9 and 10 , during such a filter regeneration cycle, fueling of the burner  20 ,  22  is adjusted by closed-loop control. Such closed-loop control of the fueling of the burner  20 ,  22  is shown generally in the area indicated by the arrow  418  of  FIG. 16 . 
     During the filter regeneration cycle, the control routine  400 , at step  404 , determines if a request to shutdown the fuel-fired burner  20 ,  22  (i.e., a burner shutdown request) has been executed. It should be appreciated that a burner shutdown request may take many different forms including, for example, a shutdown request generated by a software control routine in response to sensed, timed, or otherwise determined indication that the particulate filter  20 ,  22  has been regenerated or that filter regeneration is self-sustaining (as described above), an automatic shutdown request generated by a software control routine or the like, a timed shutdown request, or any other manual, software, or hardware-driven shutdown request. In certain embodiments, a burner shutdown request may be generated in response to the turning of an ignition key associated with the engine  80  of the truck  12  from an on position to an off position. As such, in step  404 , if the control routine  400  detects a burner shutdown request, a control signal is generated and the routine  400  advances to step  406 . Detection of a shutdown request is shown graphically at point  420  in  FIG. 16 . If the control routine  400  does not detect a burner shutdown request, the routine  400  loops back to step  402  to continue the filter regeneration cycle. 
     In step  406 , the electronic controller  76  decreases the fuel being supplied to the fuel-fired burner  20 ,  22 . To do so, the electronic controller  76  increases the air-to-fuel ratio of the air/fuel mixture being supplied to the burner  20 ,  22  by reducing the amount of fuel being injected into the mixing chamber  146  by the fuel injectors  132 ,  134 . For example, to decrease the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  generates a control signal on the signal line  136  that reduces the amount of fuel being injected by the fuel injector  132  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  20  via the fuel line  148 . Similarly, to decrease the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  generates a control signal on the signal line  138  that reduces the amount of fuel being injected by the fuel injector  134  into the mixing chamber  146  thereby increasing the air-to-fuel ratio of the air/fuel mixture being supplied to the fuel-fired burner  22  via the fuel line  150 . 
     The electronic controller  76  operates the fuel-fired burner  20 ,  22  at this reduced fuel level for a predetermined period of time. Such a period of time is shown graphically with the arrow  422  of  FIG. 16 . Once this predetermined period of time has elapsed, the control routine advances to step  408 . 
     In step  408 , the fuel supply to the burner  20 ,  22  is shutoff. Specifically, the electronic controller  76  deactuates the fuel delivery assembly  120  thereby ceasing the supply of fuel to the burner  20 ,  22 . To shutoff the fuel being supplied to the fuel-fired burner  20 , the electronic controller  76  closes the fuel enable valve  140  and ceases to generate control signals on the signal line  136  thereby causing the fuel injector  132  to cease to inject fuel into the mixing chamber  146 . Once the fuel remaining in the fuel line  148  is consumed by the burner  20 , no additional fuel enters the fuel inlet nozzle  54  of the burner  20 . Similarly, to shutoff the fuel being supplied to the fuel-fired burner  22 , the electronic controller  76  closes the fuel enable valve  140  and ceases to generate control signals on the signal line  138  thereby causing the fuel injector  134  to cease to inject fuel into the mixing chamber  146 . Once the fuel remaining in the fuel line  150  is consumed by the burner  22 , no additional fuel enters the fuel inlet nozzle  54  of the burner  22 . 
     In step  408 , the electronic controller  76  maintains the supply of combustion air and atomization air to the burners  20 ,  22 , and also maintains operation of the igniters  170 ,  172 . Specifically, in the case of shutdown of the fuel-fired burner  20 , even though fuel is no longer being supplied to the burner  20 , the electronic controller  76  continues to supply combustion air to the burner  20  via the air line  58  and continues to supply atomization air via the fuel line  148 . The controller  76  continues to operate the igniter  170  to continue spark generation within the combustion chamber  34  of the burner  20 . In the case of shutdown of the fuel-fired burner  22 , even though fuel is no longer being supplied to the burner  22 , the electronic controller  76  continues to supply combustion air to the burner  22  via the air line  58  and continues to supply atomization air via the fuel line  150 . The controller  76  continues to operate the igniter  172  to continue spark generation within the combustion chamber  34  of the burner  22 . Such continued air supply and spark generation ensures that any remaining fuel in the system is combusted by the burner  20 ,  22  thereby reducing, if not eliminating, the emission of unburned hydrocarbons. 
     The electronic controller  76  continues to supply combustion air and atomization air and operate the igniters as described above for a predetermined period of time. Such a period of time is shown graphically with the arrow  424  in  FIG. 16 . Once this predetermined period of time has elapsed, the control routine advances to step  410 . 
     In step  410 , the electronic controller  76  shuts off the flow of combustion air to the fuel-fired burner  20 ,  22 . Specifically, the electronic controller  76  ceases operation of the motor  92  thereby ceasing operation of the air pump  90 . Subsequent to shutdown of the air pump  90 , the electronic controller  76  continues to supply atomization air and continues to operate the igniters as described above for a predetermined period of time. Once this predetermined period of time has elapsed, the control routine advances to step  412 . 
     In step  412 , the electronic controller  76  shuts off the flow of atomization air to the fuel-fired burner  20 ,  22 . Specifically, the electronic controller  76  closes the atomization air valve  156  thereby reducing the flow of air to the mixing chamber  146  and hence the burners  20 ,  22 . Note that the cleaning air valve  154  remains open, and, as a result, a reduced flow of cleaning air continues to be advanced into the mixing chamber  146  and, as a result, supplied to the fuel-fired burners  20 ,  22 . As described above, the flow of cleaning air from the cleaning air valve  154  is generally constantly supplied to the mixing chamber  146  during operation of the engine  80  of the truck  12  to prevent the accumulation of debris (e.g., soot) in the fuel inlet nozzles  54  of the fuel-fired burners  20 ,  22 . 
     In step  412 , the electronic controller  76  ceases spark generation within the combustion chamber  34  of the fuel-fired burner  20 ,  22 . Specifically, the electronic controller  76  ceases operation of the igniter  170  (in the case of the burner  20 ) or the igniter  172  (in the case of the burner  172 ) thereby causing the spark to cease to be generated across the electrode gap  52  of the electrodes  48 ,  50  of the burner  20 ,  22 . The control routine  400  then ends. 
     As described above, during execution of the shutdown control routine  400  (along with other times as well), there are occasions in which the electronic controller  76  supplies combustion air to one of the fuel-fired burners  20 ,  22 , but does not supply fuel to either burner  20 ,  22 . As also described above, the motor  92  drives both the fuel pump  122  and the air pump  90 . Hence, when the motor  92  is driving the air pump  90  to supply combustion air, the fuel pump  122  is also being driven. During the occasions in which combustion air is being supplied a burner  20 ,  22 , but fuel is not being supplied to either burner  20 ,  22 , fuel pumped by the fuel pump  122  is returned to the truck&#39;s fuel tank  124  via the fuel return line  142 . As shown in  FIG. 8 , a fuel pressure sensor  426  senses fuel pressure in the fuel return line  142 . Output from the fuel pressure sensor  426  is communicated to the electronic controller  76  via a signal line  428 . If the fuel return line  142  becomes restricted such that fuel cannot readily flow back to the tank  124 , pressure on the seals of the fuel pump  122  may increase thereby potentially necessitating repair or replacement of the pump  122 . 
     As shown in  FIG. 17 , the electronic controller  76  executes a control routine  450  to monitor the return fuel line  142 . The control routine  450  commences with step  452  in which the electronic controller  76  determines the fuel pressure in the fuel return line  142 . Specifically, the electronic controller  76  scans or reads the signal line  428  to obtain the output from the fuel pressure sensor  426 . The control routine  450  then advances to step  454 . 
     In step  454 , the electronic controller  76  determines if the sensed fuel pressure is above a predetermined upper pressure limit. If the fuel pressure is below the upper pressure limit, the control routine  450  loops back to step  452  to continue monitoring output from the fuel pressure sensor  426 . However, if the fuel pressure is above the upper control limit, the control routine  450  advances to step  456 . 
     In step  456 , the electronic controller  76  shuts down components associated with the control unit  18 . In particular, since the electronic controller  76  concluded in step  454  that fuel pressure in the fuel return line  142  was above the upper control limit, the controller  76 , amongst other things, ceases operation of the drive motor  92  thereby ceasing operation of the fuel pump  122 . The control routine  450  then advances to step  458 . 
     In step  458 , the electronic controller  76  generates an error signal. For example, the electronic controller  76  may generate an output signal which causes a visual, audible, or other type of alarm to be generated for presentation to the operator (e.g., the driver of the truck  12 ). Alternatively, the error signal may simply cause an electronic log or the like to be updated with information associated with the fuel pressure analysis of steps  452 - 456 . It should be appreciated that the error signal generated in step  458  may be configured for use with any type of alarming or error tracking arrangement to fit the needs of a given system design. Moreover, if the electronic controller  76  is so equipped, the error signal (or a subsequent signal generated in response the error signal) may be communicated to the engine control unit  78  via the CAN interface  314 . Armed with this information, the engine control unit  78  may be programmed to perform additional analysis, generate an error signal to the truck operator (e.g., an indicator light on the truck&#39;s instrument cluster) indicating that the control unit  18  has shutdown, or store the error message in an error log which can be accessed by a service technician. The control routine  450  then ends. 
     Referring back to  FIG. 8 , the control unit  18  may be equipped with a one or more sensors for detecting the presence of predetermined environmental conditions within the interior chamber  112  of the control housing  72 . For example, the control unit  18  may be configured to include a smoke detector  460 . Output from the smoke detector  460  is communicated to the electronic controller  76  via a signal line  462 . As will herein be described in greater detail, the smoke detector  460  may be used to detect the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 . If the presence of fuel particles or smoke is detected, the system may be shutdown and an error signal generated. The smoke detector  460  may be embodied as any type of smoke detector. In the exemplary embodiment of the control unit  18  described herein, the smoke detector  460  is embodied as a non-ionizing smoke detector such as a commercially available IR-detector. 
     As shown in  FIG. 18 , the electronic controller  76  executes a control routine  500  to monitor for the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 . The control routine  500  commences with step  502  in which the electronic controller  76  scans or reads the signal line  462  to obtain the output from the smoke detector  460 . Once the controller  76  has obtained the output from the smoke detector  460 , the control routine  500  then advances to step  504 . 
     In step  504 , the electronic controller  76  determines if the output from the smoke detector  460  is indicative of the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 . If the output from the smoke detector  460  is not indicative of the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 , the control routine  500  loops back to step  502  to continue monitoring output from the detector  460 . However, if the output from the smoke detector  460  is indicative of the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 , a control signal is generated, and the control routine  500  advances to step  506 . 
     In step  506 , the electronic controller  76  shuts down components associated with the control unit  18 . In particular, since the electronic controller  76  concluded in step  454  that the output of the smoke detector is indicative of the presence of fuel particles or smoke in the interior chamber  112  of the control housing  72 , the controller  76 , amongst other things, ceases operation of the drive motor  92  thereby ceasing operation of the fuel pump  122  and the air pump  90 . The control routine  500  then advances to step  508 . 
     In step  508 , the electronic controller  76  generates an error signal. For example, the electronic controller  76  may generate an output signal which causes a visual, audible, or other type of alarm to be generated for presentation to the operator (e.g., the driver of the truck  12 ). Alternatively, the error signal may simply cause an electronic log or the like to be updated with information associated with the analysis of steps  502  and  504 . It should be appreciated that the error signal generated in step  508  may be configured for use with any type of alarming or error tracking arrangement to fit the needs of a given system design. Moreover, if the electronic controller  76  is so equipped, the error signal (or a subsequent signal generated in response the error signal) may be communicated to the engine control unit  78  via the CAN interface  314 . Armed with this information, the engine control unit  78  may be programmed to perform additional analysis, generate an error signal to the truck operator (e.g., an indicator light on the truck&#39;s instrument cluster) indicating that the control unit  18  has shutdown, or store the error message in an error log which can be accessed by a service technician. The control routine  500  then ends. 
     As shown in  FIG. 8 , the control unit  18  may be configured with other types of sensors for detecting the presence of predetermined environmental conditions within the interior chamber  112  of the control housing  72 . For example, the control unit  18  may be configured to include a temperature sensor  510 . Output from the temperature sensor  510  is communicated to the electronic controller  76  via a signal line  512 . As will herein be described in greater detail, the temperature sensor  510  may be used to monitor the temperature within the interior chamber  112  of the control housing  72 . If the temperature within the interior chamber  112  of the control housing  72  exceeds a predetermined upper temperature limit (e.g., 125° C.), the system may be shutdown and an error signal generated. The temperature sensor  510  may be embodied as any type of electronic temperature sensor. In the exemplary embodiment of the control unit  18  described herein, the temperature sensor  510  is embodied as a commercially available thermocouple. 
     As shown in  FIG. 19 , the electronic controller  76  executes a control routine  550  to monitor the temperature within the interior chamber  112  of the control housing  72 . The control routine  550  commences with step  552  in which the electronic controller  76  scans or reads the signal line  512  to obtain the output from the temperature sensor  510 . Once the controller  76  has obtained the output from the temperature sensor  510 , the control routine  550  then advances to step  554 . 
     In step  554 , the electronic controller  76  determines if the sensed temperature within the interior chamber  112  of the control housing  72  is above a predetermined upper temperature limit (e.g., 125° C.). If the temperature within the interior chamber  112  of the control housing  72  is below the upper temperature limit, the control routine  550  loops back to step  552  to continue monitoring output from the temperature sensor  510 . However, if the temperature within the interior chamber  112  of the control housing  72  is above the upper control limit, a control signal is generated, and the control routine  550  advances to step  556 . 
     In step  556 , the electronic controller  76  shuts down components associated with the control unit  18 . In particular, since the electronic controller  76  concluded in step  554  that the temperature within the interior chamber  112  of the control housing  72  is above the upper control limit, the controller  76 , amongst other things, ceases operation of the drive motor  92  thereby ceasing operation of the fuel pump  122  and the air pump  90 . The control routine  550  then advances to step  558 . 
     In step  558 , the electronic controller  76  generates an error signal. For example, the electronic controller  76  may generate an output signal which causes a visual, audible, or other type of alarm to be generated for presentation to the operator (e.g., the driver of the truck  12 ). Alternatively, the error signal may simply cause an electronic log or the like to be updated with information associated with the temperature analysis of steps  552  and  554 . It should be appreciated that the error signal generated in step  558  may be configured for use with any type of alarming or error tracking arrangement to fit the needs of a given system design. Moreover, if the electronic controller  76  is so equipped, the error signal (or a subsequent signal generated in response the error signal) may be communicated to the engine control unit  78  via the CAN interface  314 . Armed with this information, the engine control unit  78  may be programmed to perform additional analysis, generate an error signal to the truck operator (e.g., an indicator light on the truck&#39;s instrument cluster) indicating that the control unit  18  has shutdown, or store the error message in an error log which can be accessed by a service technician. The control routine  550  then ends. 
     Referring now to  FIG. 20 , there is shown an emission abatement assembly  600 . The emission abatement assembly  600  includes a number of common components with the emission abatement assembly  10 . Common reference numerals are utilized to designate common components between the two assemblies. 
     The emission abatement assembly  600  includes a controller  76 , a fuel supply unit such as a fuel pump  122  under the control of the controller  76 , and a fuel-fired burner  606 . The assembly  600  may be installed in the truck  12  either horizontally, vertically, or upside-down vertically. A diesel oxidation catalyst  608  may optionally be positioned upstream of the filter substrate  60 , as shown in  FIG. 20 . The diesel oxidation catalyst  608  (or any other type of oxidation catalyst) may be used to oxidize any unburned hydrocarbons and carbon monoxide (CO) thereby generating additional heat which is transferred downstream to the filter substrate  60 . Alternatively, as shown in  FIG. 21 , the emission abatement assembly  600  may be configured without the diesel oxidation catalyst  608 . 
     As described above, the filter substrate  60  may be impregnated with a catalytic material such as, for example, a precious metal catalytic material. The catalytic material may be, for example, embodied as platinum, rhodium, palladium, including combinations thereof, along with any other similar catalytic materials. Use of a catalytic material lowers the temperature needed to ignite trapped soot particles. 
     Unlike the assembly  10 , in the exemplary embodiment described herein, the emission abatement assembly  600  does not utilize supplemental air pumped from an air pump such as the air pump  90 . As such, the combustion process is supported by oxygen in the exhaust gas. 
     The fuel-fired burner  606  is shown in greater detail in  FIGS. 22 and 23 . Hot exhaust gas enters the housing  610  through an exhaust gas inlet  612 . Note that unlike the assembly  10  in which the exhaust gas enters through an inlet  36  which is perpendicular to the flow direction through the housing of the assembly, the exhaust gas inlet  612  is substantially co-axial with the flow direction of the housing  610 . As such, the gas inlet  612  and a gas outlet  614  of the housing  610  are arranged along the same general axis (see  FIGS. 20 and 21 ). 
     Exhaust gas entering the housing  610  is split into two streams. The inner stream  616  enters a chamber  618 , and then flows into a combustion chamber  620  through a number of holes  622 ,  624 . The hole pattern of the holes  622 ,  624  is shown in  FIGS. 24 and 25 . The hole pattern is configured such that exhaust gas flowing through the holes  622  swirls inside the combustion chamber  620 , thus facilitating the mixing of the injected fuel, the exhaust gas, and combustion gases. One or more rows of the holes  622  may be utilized to generate a desired flow/swirl. As shown in  FIGS. 24 and 25 , an upstream wall  628  of the combustion chamber  620  may also have a number of holes  626  defined therein to allow a portion of the exhaust gas flow to enter the chamber  620  without being first advanced through the chamber  618 . 
     The ends of the electrodes  48 ,  50  are placed downstream of the nozzle  54  to ignite the fuel in the presence of exhaust gas. The exhaust gas contains between 4%-20% oxygen which facilitates combustion of the fuel. The exhaust gas passing through holes  624  mixes with the hot combustion gas that may contain unburned fuel, hydrocarbons, CO, and other combustible gas. In the presence of the oxygen in the exhaust gas, these gases further combust. A flow of exhaust gas flows through a number of holes  630  thereby bypassing the fuel-fired burner  606 . This bypass flow of exhaust gas supplies additional oxygen for the combustion of the combustion gas exiting the combustion chamber  620 . 
     A flame holder  632  is placed downstream of the combustion zone to prevent the flame from reaching the diesel oxidation catalyst  608  (or the filter substrate  60  in configurations without a diesel oxidation catalyst such as shown in  FIG. 21 ). A gas distributor  634  may be positioned downstream of the combustion zone to facilitate the mixing of the hot combustion gas and the exhaust gas bypassing the fuel-fired burner  606 , thus enhancing the temperature distribution across the inlet of diesel oxidation catalyst  608  and/or filter substrate  60 . The distributor  634  may be positioned around a portion of the walls of the combustion chamber  620  as shown in  FIG. 22 . An exemplary design of a gas distributor  634  that may be positioned in such a manner is shown in  FIG. 26 . Alternatively, as shown in  FIG. 27 , the gas distributor  634  may be positioned downstream of the outlet of the combustion chamber  620 . An exemplary design of a gas distributor  634  that may be positioned in such a manner is shown in  FIG. 28 . 
     Referring now to  FIG. 27 , another exemplary design of the fuel-fired burner is shown in greater detail. In this embodiment, some exhaust gas flows through the holes  622  whose hole pattern is similar to the hole pattern shown in  FIG. 24 , thereby creating gas swirl inside the combustion chamber. The hot flame which contains unburned fuel, hydrocarbons, CO, and other combustible gas burns further downstream in the assembly of  FIG. 27  relative to the assembly of  FIG. 22 . 
     As shown in  FIG. 27 , an additional flame holder  636  may be positioned between the flame holder  632  and the fuel-fired burner  606 . As shown in solid lines, the flame holder  636  may be designed in a concave configuration or, as shown in phantom lines, a convex configuration. 
     Other variations of the exemplary designs of the emission abatement assemblies described herein are also contemplated. For example, as described above, the air pump  90  may be embodied as any type of air pump including a relatively high flow/high efficiency air pump. A variable air flow pump that increases output at high engine load conditions may also be used. Alternatively, a variable air flow pump that only operates at high engine load conditions may be used. The pump  90  may be embodied as a centrifugal compressor or a roots blower. 
     The size of the combustion chamber  34 ,  620  may also be varied to fit the needs of a given system design. For example, a relatively large (16″ diameter) combustion chamber  34 ,  620  may be used to slow exhaust gas velocity thereby enhancing combustion efficiency of the fuel-fired burners. Relatively smooth/efficient air flow configurations, such as the “axial” configurations shown in  FIGS. 20 and 21 , may also be used to enhance the flow characteristics of a given design. 
     The manner in which fuel is injected into the fuel-fired burners  20 ,  22 ,  606  may also be varied, if desired. For example, a staged fuel injection arrangement may be used in which a first amount of fuel is injected into the burner to create an initial flame. The initial flame is then used to ignite a second amount of injected fuel. 
     A modulated fuel flow arrangement could also be utilized to increase the surface area of the fuel spray. For example, a dithering fuel average may be used in which the amount of injected fuel is dithered around a desired average fuel amount. For instance, the injected fuel rate may be dithered between 25% and 75% to produce an average fuel rate of 50%. 
     Operation of the engine  80 , and its associated components, may also be controlled to facilitate operation of the emission abatement assemblies described herein. For example, in the case of operation of an emission abatement assembly that does not utilize supplemental air (e.g., the assemblies of  FIGS. 20 and 21 ), the position of the EGR valve of the engine  80  may be coordinated with regeneration of the particulate filter. For instance, to increase both the temperature and the oxygen content in the exhaust gas, the engine&#39;s EGR valve may be momentarily closed. It is estimated that filter regeneration may require about ten minutes of time. During such a brief period of time, the EGR valve may be closed. In such a case, filter regeneration may be coordinated with engine idle conditions. 
     In other embodiments, the engine  80  may be controlled such the EGR level is actually increased during filter regeneration. In such a case, a fuel or fuel additive such as hydrogen gas may be utilized to stabilize the flame of the fuel-fired burner. Hydrogen gas may be supplied by either a storage tank or an onboard fuel reformer. 
     Along a similar line, operation of the engine  80 , and its associated components, may be monitored to facilitate operation of the emission abatement assemblies described herein. For example, in the case of operation of an emission abatement assembly that does not utilize supplemental air (i.e., an airless burner such as the assemblies of  FIGS. 20 and 21 ), operation of the engine may be monitored so that, for example, filter regeneration occurs at desired, predetermined engine operating conditions. For example, in the case of an emission abatement assembly that does not utilize supplemental air (e.g., the assemblies of  FIGS. 20 and 21 ), it is desirable to perform filter regeneration in the presence of exhaust gas which contains a relatively high oxygen concentration. Such is generally the case when the engine  80  is under relatively low load conditions such as when the engine  80  is operating at idle or near idle conditions (e.g., 600-1,000 RPM depending on the engine). 
     As will herein be described in more detail below, there are a number of ways to determine when desirable, predetermined engine conditions exist for filter regeneration of an emission abatement assembly that does not utilize supplemental air. For example, a predetermined engine speed range may be utilized in which case filter regeneration is only performed if the engine is operating within a predetermined range of engine speed. In such a case, the controller  76  may monitor output from an engine speed sensor  890  (see  FIG. 8 ) or the like to determine engine speed. It should be appreciated that the controller may communicate with the engine speed sensor  890  directly, or may obtain the output from the sensor  890  from the engine control unit  78  via the CAN interface  314 . 
     Moreover, a predetermined engine load range may be utilized to determine when desirable, predetermined engine conditions exist for filter regeneration of an emission abatement assembly that does not utilize supplemental air. In such a case, filter regeneration is only performed if the engine is operating within the predetermined range of engine load. To do so, the controller  76  may first sense or otherwise determine certain engine parameters (e.g., RPM, turbo boost, etcetera) and then query or otherwise access a preprogrammed engine load map to determine the load on the engine. It should be appreciated that the controller  76  may be preprogrammed with such an engine load map, or may obtain the engine load from an engine load map programmed in the engine control unit  78  via the CAN interface  314 . 
     In addition, exhaust mass flow from the engine  80  may be used to determine when desirable, predetermined engine conditions exist for filter regeneration of an emission abatement assembly that does not utilize supplemental air. For example, a predetermined exhaust mass flow range may be utilized in which case filter regeneration is only performed if the engine is operating within a predetermined range of exhaust mass flow. In such a case, the controller  76  may monitor output from a mass flow sensor  892  (see  FIG. 8 ), such as a hot wire mass flow sensor, to determine exhaust mass flow. It should be appreciated that the controller  76  may communicate with the mass flow sensor  892  directly, or may obtain the output from the sensor  892  from the engine control unit  78  via the CAN interface  314 . Alternatively, exhaust mass flow may be calculated by the controller  76  in a conventional manner by use of engine operation parameters such as engine RPM, turbo boost pressure, and intake manifold temperature (along with other known parameters such as engine displacement). It should be appreciated that the controller  76  itself may calculate the mass flow, or it may obtain the calculated mass flow from the engine control unit  78  via the CAN interface  314 . 
     Referring now to  FIG. 33 , there is shown a control routine  850  for controlling regeneration an emission abatement assembly that does not utilize supplemental air (i.e., an airless emission abatement assembly). The routine  850  begins with step  852  in which the routine determines if a request to startup the airless fuel-fired burner  20 ,  22  (i.e., a burner startup request) has been executed. It should be appreciated that a burner startup request may take many different forms including, for example, a startup request generated by a software control routine in response to sensed, timed, or otherwise determined indication that one of the particulate filters  24 ,  26  is in need of regeneration. For example, a sensor-based scheme, map-based scheme, or a timing-based scheme may be utilized to generate a startup request. As such, in step  852 , if the control routine  850  detects a burner startup request, a control signal is generated and the routine  850  advances to step  854 . If the control routine  850  does not detect a burner startup request, the routine  850  loops back to step  852  to continue monitoring for such a request. 
     In step  854 , the controller  76  determines if the engine  80  is operating within predetermined engine conditions. For example, if a predetermined engine speed range is being utilized, in which case filter regeneration is only performed if the engine is operating within a predetermined range of engine speed, the controller  76  monitors output from the engine speed sensor  890  or otherwise determines engine speed. Thereafter, the controller  76  determines if the speed of the engine is within the predetermined speed range. Alternatively, if a predetermined engine load range is being utilized, in which case filter regeneration is only performed if the engine is operating within the predetermined range of engine load, the controller  76  senses or otherwise determines certain engine parameters (e.g., RPM, turbo boost, etcetera) and thereafter queries or otherwise accesses a preprogrammed engine load map to determine the load on the engine. Thereafter, the controller  76  determines if the load of the engine is within the predetermined range of engine load. Moreover, if a predetermined exhaust mass flow range is being utilized, in which case filter regeneration is only performed if the engine is operating within a predetermined range of exhaust mass flow, the controller  76  senses, calculates, or otherwise determines exhaust mass flow from the engine. Thereafter, the controller  76  determines if the exhaust mass flow of the engine is within the predetermined range of exhaust mass flow. Hence, in step  854 , if the controller  76  determines that the engine  80  is operating within predetermined engine conditions, the control routine  850  advances to step  856 . However, if the engine is not operating within predetermined engine conditions, the control routine  850  loops back to step  854  to continue monitoring the engine to determine when it is operating within such conditions. 
     In step  856 , the controller  76  commences filter regeneration. Specifically, the electronic controller  76  operates the fuel-fired burner  20 ,  22  to regenerate the particulate filter  24 ,  26  in any of the numerous manners described herein. However, it should be appreciated that the fuel-fired burner  20 ,  22  is operated without the assistance of combustion air (i.e., without the use of supplemental air supply such as from the air pump  90 ). As such, oxygen present in the engine exhaust gas sustains combustion of the fuel delivered to the fuel-fired burner  20 ,  22 . Heat generated by the combustion of the fuel regenerates the particulate filter  24 ,  26 . Once filter regeneration is complete, the control routine  850  ends. 
     It should be appreciated that the control routine  850  may also be used to regenerate filters with the assistance supplemental air, if desired. It should also be appreciated that the control routine  850  may be modified in a manner in which filter regeneration occurs even in the absence of a startup request. For example, the controller  76  may be configured to regenerate one or both of the particulate filters  24 ,  26  when the engine  80  is operating within predetermined engine conditions irrespective of whether the filters  24 ,  26  are loaded to a predetermined limit. In such a way, the controller  76  can take advantage of any time oxygen rich conditions are present in the exhaust gas. 
     Referring now to  FIG. 35 , another exemplary embodiments of an emission abatement assembly  800  is shown. The assembly  800  includes a nozzle  802  which extends into an exhaust conduit to inject fuel into a flow of exhaust gas. The electrodes  48 ,  50  are positioned in a substantially vertical arrangement (as viewed in the orientation of the drawings). 
     A flame holder  636  may be positioned in a number of different positions relative to the electrodes  48 ,  50 . For example, as shown in  FIG. 35 , the flame holder  636  may be positioned downstream of the nozzle  802 , but upstream of the electrodes  48 ,  50 . Alternatively, the flame holder  636  may be positioned downstream of both the nozzle  802  and the electrodes  48 ,  50 . Moreover, the flame holder  632  may be designed in a concave configuration (as shown in  FIG. 35 ), or a convex configuration (not shown). 
     A flow diffuser  644  may be positioned upstream of the diesel oxidation catalyst  608  and/or the filter substrate  60  to facilitate the mixing of the hot combustion gas from combustion zone proximate to the nozzle  802  and the remaining exhaust gas, thus enhancing the temperature distribution across the inlet of diesel oxidation catalyst  608  and/or filter substrate  60 . The flow diffuser  644  may be embodied as any type of flow diffuser. In an exemplary embodiment, the flow diffuser  644  may be embodied as the any of the flow distributors  634  described above. 
     Referring now to  FIG. 36 , there is shown another exemplary embodiment of the fuel-fired burner  20 ,  22 . The embodiment shown in  FIG. 36  is similar to the embodiments previously described, with the same reference numerals being used to designate similar components. The fuel-fired burner  20 ,  22  has been modified to reduce the exhaust gas flow through the combustion chamber  34 . It has been found that such a modification reduces (perhaps significantly) hydrocarbon and CO slip, while also reducing other emissions. 
     In essence, the flow of exhaust gas entering through the exhaust gas inlet port  36  is separated into two flows, one of which is advanced through the combustion chamber  34  (i.e., a combustion flow), the other of which bypasses the combustion chamber  34  (i.e., a bypass flow). As such, exhaust gas flow through the combustion chamber  34  of the fuel-fired burner  20 ,  22  of  FIG. 36  is reduced relative to the burner of, for example,  FIG. 5 . As a result, the percentage of the exhaust gas flow bypassing the combustion chamber  34  (i.e., advancing through the openings  42  of the shroud  44 ) is increased relative to the design of  FIG. 5 . 
     As will herein be described in greater detail, the design of the combustion chamber  34  may be altered to provide control of the exhaust gas flowing therethrough (i.e., control the velocity and direction of exhaust gas flow through the combustion chamber). Moreover, components such as diverter plates may also be used to control the exhaust gas flow in such a manner. 
     One exemplary manner of controlling the exhaust gas flow through the fuel-fired burner  20 ,  22  in such a manner is shown in  FIG. 36 . In this case, the combustion chamber  34  includes a generally annular shaped outer wall  902  having two wall halves  904 ,  906 . The first wall half  904  faces away from the exhaust gas inlet port  36 , whereas the second wall half  906  faces toward the exhaust gas inlet port  36 . As shown in  FIG. 36 , the first wall half  904  has a number of the gas inlet openings  40  defined therein. The collective surface areas of the gas inlet openings  40  of the first wall half  904  define a first void area, whereas the collective surface areas of the gas inlet openings of the second wall half  906  define a second void area. The second void area of the second wall half  904  is less than the first void area of the first wall half. As such, a reduced portion of the exhaust gas entering the fuel-fired burner  20 ,  22  through the exhaust gas inlet  36  flows into the combustion chamber  34  relative to, for example, the design of the fuel-fired burner of  FIG. 5 . As a result, the magnitude of the combustion flow (i.e., the flow of exhaust gas entering the combustion chamber  34 ) is reduced relative to the design of  FIG. 5 . It should be appreciated that such a configuration not only reduces the magnitude of the exhaust gas entering the combustion chamber  34 , but also reduces the velocity of the exhaust gas entering the combustion chamber  34  (relative to, for example, the design of  FIG. 5 ). Moreover, such a configuration also reduces the flow of exhaust gas entering the fuel-fired burner  20 ,  22  through the exhaust gas inlet  36  that flows directly into the combustion chamber  34  (i.e., through the wall half  906 ), and, as a result, is impinged upon the flame generated therein. 
     Referring now to  FIG. 37 , there is shown another embodiment of the fuel-fired burner  20 ,  22  in which the second wall half  906  of the combustion chamber  34  is substantially devoid of the gas inlet openings  40 . For example, the collective surface areas of the gas inlet openings of the second wall half  906  define a void area of zero. As a result, exhaust gas entering the fuel-fired burner  20 ,  22  through the exhaust gas inlet port  36  does not flow directly into the combustion chamber  34 , and, as a result, is not impinged upon the flame generated therein. Rather, the combustion flow of exhaust gas enters the combustion chamber  34  through the gas inlet openings  40  formed in the first wall half  904  of the combustion chamber  34  (i.e., the surfaces that do not face the exhaust gas inlet  36 ). The balance of the flow of exhaust gas entering the exhaust gas inlet port  36  bypasses the combustion chamber  34 . 
     It should be appreciated that the size and location of the gas inlet openings  40  on either wall half  904 ,  906  may be configured to generate any desired flow characteristics within the combustion chamber  34  (e.g., velocity and direction). 
     Although the proportions of the separated flows (i.e., the combustion flow and the bypass flow) are described as being a function of the gas inlet openings  40  formed in the outer wall  902  of the combustion chamber  34 , the exhaust gas flow entering the exhaust gas inlet port  36  may be separated in other ways. For example, a plate or “patch” may be secured to the combustion chamber  34  to block any number of gas inlet openings  40  that may already exist in the chamber  34 . An example of such a plate  912  is shown in  FIG. 44 . The plate  912  may be positioned around the outer wall  902  of the combustion chamber  34  of the burner design shown in, for example,  FIG. 5 . The seam  918  created when the two ends  914  of the plate  912  are secured together faces the exhaust gas inlet port  36 . As shown in  FIG. 45 , when the plate  912  is installed in such a manner, the exhaust gas flow entering the exhaust gas inlet port  36  is impinged upon an area of the plate  912  (shown generally as the shaded area  916 ) which is devoid of holes thereby preventing the exhaust gas flow from being impinged directly on the flame within the combustion chamber  34 . 
     By controlling the flow of exhaust gas through the combustion chamber  34  stability of the flame generated by the fuel-fired burner  20 ,  22  may be enhanced. Indeed, it has been found that when the velocity of the flame is greater than the velocity of the exhaust gas moving through the chamber  34 , a stable flame may be more readily maintained. To the contrary, when the velocity of the exhaust gas moving through the chamber  34  is greater than the flame velocity, instability of the flame may occur. 
     As alluded to above, the size, number, and location of the gas inlet openings  40  may be predetermined to produce a desired flow through the combustion chamber. In an exemplary embodiment, the fuel-fired burner  20 ,  22  is configured such that about 70% of the exhaust gas entering through the inlet  36  is advanced through the combustion chamber  34  (with the balance of the exhaust gas bypassing the chamber  34 ). In another exemplary embodiment, the fuel-fired burner  20 ,  22  is configured such that about 50%-70% of the exhaust gas entering through the inlet  36  is advanced through the combustion chamber  34  (with the balance of the exhaust gas bypassing the chamber  34 ). In yet another exemplary embodiment, the fuel-fired burner  20 ,  22  is configured such that less than 50% of the exhaust gas entering through the inlet  36  is advanced through the combustion chamber  34  (with the balance of the exhaust gas bypassing the chamber  34 ). Flows other than these exemplary flow arrangements are contemplated. 
     As alluded to above, in lieu of, or in addition to, removal of the gas inlet openings  40  from the outer wall  902  of the combustion chamber  34 , the exhaust gas flow entering the gas inlet port  36  may be separated into a desired combustion flow and bypass flow in numerous different ways. For example, a number of diverter plates may be used to direct a desired amount of exhaust gas flow through the combustion chamber  34  while directing the balance of the flow to bypass the chamber. Examples of such plates  910  are shown in  FIGS. 38-43 , although other configurations are contemplated. It should be appreciated that such plates  910  may be configured to direct the desired portion of the flow through the combustion chamber  34  while also preventing an increase in backpressure within the exhaust system. 
     The size, shape, and/or location of the openings  42  defined in the bypass shroud  44  may also be altered to generate desired flow characteristics. For example, the size, shape, and/or location of the openings  42  may be configured to accommodate for “hot spots” or “cool spots” on the upstream face of the filter substrate  60 . Indeed, thermal analysis may be performed on the filter substrate  60  to determine where such hot spots or cool spots exist. The size, shape, and/or location of the openings  42  defined in the bypass shroud  44  may then be altered based on such an analysis. 
     For example, the size of the openings  42  upstream (relative to exhaust gas flow) of a cool spot may be reduced. This increases the temperature on the cool spot during filter regeneration by reducing the amount of exhaust gas flowing through the cool spot. 
     Conversely, the size of the openings  42  upstream (relative to exhaust gas flow) of a hot spot may be increased. This decreases the temperature on the hot spot during filter regeneration by increasing the amount of exhaust gas flowing through the hot spot. 
     As a result, it is contemplated to construct a bypass shroud  44  that includes a number of different sized openings  42  to accommodate varying surface temperatures on the upstream surface of the filter substrate  60 . 
     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. 
     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. 
     For example, it should be appreciated that the order of many of the steps of the control routines described herein may be altered. Moreover, many steps of the control routines may be performed in parallel with one another.