Abstract:
A method of regenerating a particulate filter of an exhaust system of an engine system is disclosed. The method may include determining a condition indicative of an accumulation of hydrocarbons in the particulate filter, and monitoring the particulate filter to determine when to initiate regeneration of the particulate filter. The method may also include selecting a regeneration profile for regeneration of the particulate filter based on the determined condition, and regenerating the particulate filter in accordance with the regeneration profile.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a system and a strategy for regeneration of exhaust after-treatment components, and more particularly, to a system and a strategy for the removal of accumulated hydrocarbons during regeneration. 
     BACKGROUND 
     Internal combustion engines may exhaust particulate matter and other pollutants into the atmosphere through its exhaust. Exhaust after-treatment devices, such as include Diesel Oxidation Catalysts (DOCs) and Diesel Particulate Filters (DPFs), may be placed in the path of the exhaust to separate or convert these pollutants to less noxious compounds. Since the volume of particulate matter collected in the DPF may eventually deteriorate engine performance, a method of regenerating these after-treatment devices is also provided. The present disclosure is directed to a system and a method for regeneration that may have advantages over existing regeneration technology. 
     SUMMARY OF THE INVENTION 
     In one aspect, a method of regenerating a particulate filter of an exhaust system of an engine system is disclosed. The method may include determining a condition indicative of an accumulation of hydrocarbons in the particulate filter, and monitoring the particulate filter to determine when to initiate regeneration of the particulate filter. The method may also include selecting a regeneration profile for regeneration of the particulate filter based on the determined condition, and regenerating the particulate filter in accordance with the regeneration profile. 
     In another aspect, an exhaust system of an engine system is disclosed. The exhaust system may include a particulate filter configured to remove particulate matter from an exhaust flow of the engine system, and a heating system configured to increase the temperature of the particulate filter for regeneration. The exhaust system may also include a controller that is configured to identify the occurrence of one or more of the following events: a burner of the exhaust system upstream of the particulate filter fails to ignite, a flame in the burner gets extinguished, a cumulative crank time of an engine of the engine system exceeds a preselected value, a servicing of the engine system, or a fault event of the engine system, and select a regeneration profile for regeneration of the particulate filter based on the identified events. 
     In yet another aspect, a method of regenerating a particulate filter positioned in an exhaust flow of an engine system is disclosed. The method may include monitoring the particulate filter to determine when to initiate regeneration of the particulate filter. The method may also include selecting a hold time at a first temperature based on an estimated accumulation of hydrocarbons in the particulate filter, and regenerating the particulate filter using a temperature profile that includes the hold time at the first temperature prior to an oxidation of particulate matter at a second temperature higher than the first temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary embodiment of the disclosed exhaust treatment system; 
         FIG. 2  is a flow chart illustrating an exemplary regeneration control process of the current disclosure; 
         FIG. 2A  is a flow chart illustrating an exemplary method of triggering regeneration in the regeneration control process of  FIG. 2 ; 
         FIG. 3  is a schematic illustration of an exemplary process to determine preheat time in the regeneration control process of  FIG. 2 ; 
         FIG. 4  illustrates an exemplary regeneration profile used to regenerate a diesel particulate filter (DPF) in the process of  FIG. 2 ; and 
         FIG. 5  is an exemplary modified regeneration profile used to regenerate a DPF having excess accumulated hydrocarbons therein, in the process of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of exhaust treatment system  16  of engine  14 . Engine  14  may include any type of internal combustion engine known in the art. Although engine  14  will be discussed hereinafter as being a diesel engine, engine  14  can, in general, be an internal combustion engine that burns any type of fuel. Exhaust from engine  14  may be directed through conduit  20  to exhaust treatment system  16 . Exhaust treatment system  16  may include one or more after-treatment devices  18  that are configured to remove at least some pollutants from the exhaust of engine  14 . For purposes of this discussion, DPF  19  and DOC  22  will be described as being the after-treatment devices  18 . However, it should be noted that, in general, exhaust treatment system  16  may include other after-treatment devices  18  (such as additional DPFs, DOCS, Selective Catalytic Reduction systems, Lean NOx Traps, etc.) in addition to DPF  19  and DOC  22 . 
     The after-treatment devices  18  may be configured to be thermally regenerated. Exhaust treatment system  16  may include a heating system  26 , which may be configured to increase the temperature of an after-treatment device  18  (such as, for example, DPF  19 ) for regeneration. There are a number of different ways that heating system  26  may be configured to increase the temperature of DPF  19 . For example, in some embodiments, heating system  26  may be configured to apply heat directly to DPF  19  via a heating device integral with, or positioned adjacent to, DPF  19 . An example of such a heating device may include an electric heating element (not shown). Alternatively or additionally, heating system  26  may be configured to increase the temperature of DPF  19  by transferring heat to DPF  19  from the exhaust gases flowing therethrough. In such embodiments, heating system  26  may be configured to apply heat to exhaust gases upstream of DPF  19 . Heat may be applied to the exhaust gases upstream of DPF  19  in multiple ways. For example, in some embodiments, the engine operating parameters may be altered to increase the temperature of the exhaust. In some of these embodiments, a rich air/fuel mixture may be temporarily directed to the combustion chambers of engine  14  to increase the exhaust gas temperature. In other embodiments, other operating parameters, such as engine speed, load, timing, and/or exhaust valve actuation may be manipulated to control the exhaust gas temperature. In some embodiments, exhaust gases may be heated by post injection techniques. For example, additional fuel may be injected into the combustion chambers of engine  14  after the combustion has taken place. This may result in the additional fuel being burned in the exhaust system, thereby increasing the exhaust gas temperature. In some embodiments, heating system  26  may include one or more heating devices (such as, for example, an electric heating element and/or flame producing burner) which may heat the exhaust gases flowing through conduit  20  upstream of an after-treatment device  18 . Any type of these or other types of heating systems  26  that are configured to provide heat for regeneration may be used to regenerate DPF  19 . 
     In one embodiment, heating system  26  may include a burner assembly  30  positioned upstream of DPF  19 . Burner assembly  30  may be positioned anywhere along exhaust conduit  20  between engine  14  and after-treatment device  18 . Burner assembly  30  may include a fuel injector  34  to supply fuel and an air supply  36  to supply air to burner assembly  30 . Burner assembly  30  may also include a spark plug  46  to provide a spark to ignite the air/fuel mixture delivered to the burner assembly  30 . Combustion of fuel in the burner assembly  30  may provide heat to the exhaust flowing therethrough to regenerate DPF  19 . Although exhaust treatment system  16  is shown with a single DPF  19 , a single DOC  22 , and a single burner assembly  30 , exhaust treatment system  16  may include any number of these devices. For example, in one embodiment, exhaust treatment system  16  may include a single burner assembly  30  configured to regenerate multiple DPFs. 
     Exhaust treatment system  16  may include multiple sensors configured to detect operating parameters of engine  14  and exhaust treatment system  16 . Exhaust treatment system  16  may also include a controller  32  configured to control the system based at least in part on this data. The sensors may include a first temperature sensor  50  and a first pressure sensor  52  upstream of DPF  19 , and a second temperature sensor  54  and a second pressure sensor  56  downstream of DPF  19 . These sensors may be configured to measure the temperature and pressure of exhaust entering and exiting DPF  19 . Controller  32  may maintain a record of the operating parameters measured by one or more of the sensors. Some embodiments of exhaust treatment system  16  may also include a flame sensor  60  configured to detect a flame in burner assembly  30 . It should be noted that the above description of sensors is exemplary only and, in general, exhaust treatment system  16  may include any number and type of sensors configured to detect operating parameters of the system. For example, any type of sensor capable of detecting a flame in burner assembly  30  may function as flame sensor  60 . In some embodiments, a temperature sensor  48  positioned downstream of burner assembly  30  (or within the burner assembly  30 ) may function as flame sensor  60 . In some other embodiments, a temperature sensor may be positioned upstream of burner assembly  30 , and the difference in temperature upstream and downstream of burner assembly  30  may indicate the presence of a flame in burner assembly  30 . 
     Controller  32  may trigger regeneration of DPF  19  based on the detected operating parameters of engine  14  and exhaust treatment system  16 . In some embodiments, controller  32  may activate burner assembly  30  to trigger regeneration of DPF  19  in response to a determination that more than a predetermined amount of particulate matter (or “soot”) may be trapped in DPF  19 . In general, controller  32  may be configured to trigger regeneration in response to any triggering condition known in the art. Non limiting examples of triggering conditions that may trigger regeneration of DPF  19  may include: operation of engine  14  for a predetermined amount of time; consumption of a predetermined amount of fuel by engine  14 ; detection of an elevated backpressure in exhaust treatment system  16 ; detection of a pressure differential across DPF  19  greater than a predetermined amount, etc. Alternatively or additionally, regeneration of DPF  19  may also be triggered by an operator of engine  14  by activating a switch or by other known means. 
     During some operating conditions of engine  14 , the hydrocarbon content in the exhaust entering DPF  19  may be high. These operating conditions may include cold start conditions of engine  14  when cold combustion chamber walls lead to poor vaporization of fuel, resulting in over-rich concentrations of fuel in the exhaust from engine  14 . Improper functioning of engine or exhaust system components may also increase hydrocarbon emissions in the exhaust. In some cases, repair or service of engine  14  (such as, repair of fuel system, replacement of fuel injectors, etc.) may also temporarily increase the amount of hydrocarbons emitted into the exhaust. The excess hydrocarbon in the exhaust may accumulate in DPF  19  and DOC  22  (and/or in any other after-treatment device  18 ) positioned downstream of burner assembly  30 . These accumulated hydrocarbons may combust during regeneration resulting in unstable regeneration. Unstable regeneration may include rapid incineration of accumulated matter in the DPF resulting in a sudden increase in temperature. As the filter temperature increases, soot oxidation rates increase, resulting in production of enough heat from the exothermic reaction to further increase the soot oxidation rate. This uncontrolled exothermic reaction during regeneration may result in temperatures that are high enough to melt, crack, or otherwise damage the DPF filter. 
     To reduce the likelihood of these uncontrolled exothermic reactions during regeneration, controller  32  may be configured to estimate the amount of hydrocarbons emitted in the exhaust as a result of these operating conditions, and configure the regeneration profile of the DPF  19  to evaporate at least a portion of these hydrocarbons before a soot oxidation step of the regeneration profile. Controller  32  may include control logic or another mechanism that is configured to identify a condition that may cause excess hydrocarbon emissions into the exhaust. In some embodiments, this logic may include the setting of a flag at the occurrence of an excess hydrocarbon emission event. For instance, a flag may be set at the occurrence of a loss of combustion event or a fail to ignite event in burner assembly  30 . Similarly, flags may be set to indicate the occurrence of other excess hydrocarbon emission events. When a regeneration is triggered, the controller  32  may estimate the amount of hydrocarbons that may be present in the exhaust treatment system  16  based on the flags, and may include a preheat step in the regeneration profile if the temperature of the exhaust is below that required for the hydrocarbons to evaporate. This preheat step may be configured to evaporate at least a portion of the accumulated hydrocarbons in DPF  19  before a soot oxidation step of the regeneration profile. Evaporation of at least a part of the accumulated hydrocarbons may prevent an uncontrolled exothermic reaction during the soot oxidation step. If the estimated amount of hydrocarbons in the exhaust is low, or if the temperature of the exhaust is such that any hydrocarbons that were emitted would have evaporated, then the controller  32  may execute any desired regeneration profile configured to oxidize the accumulated particulate matter. 
     Although in description above, flags are used to identify a condition where excess hydrocarbons may be present in the exhaust treatment system  16 , this is not a requirement. In general, any method that identifies a condition where excess hydrocarbons may be present the exhaust treatment system  16  may be used to trigger the inclusion of a preheat step in the regeneration temperature profile of DPF  19 . In some embodiments, the controller  32  may estimate the amount of hydrocarbons accumulated in DPF  19  and configure the duration of the preheat step to evaporate at least a portion of the accumulated hydrocarbons in DPF  19  prior to the soot oxidation step. In these embodiments, if controller  32  determines that the amount of accumulated hydrocarbons in the DPF  19  is low, or below a threshold value, the preheat time may be set to zero. In these embodiments, the duration of the preheat time may be selectively increased as the amount of accumulated hydrocarbons in the DPF  19  increases. 
     It should be noted that although controller  32  is described as executing a regeneration profile following a preheat step, this is not a requirement. For instance, in some embodiments, controller  32  may not regenerate DPF  19  after a preheat step. In these embodiments, engine  14  may continue to operate normally after a preheat step and controller  32  may continue to monitor the system to detect another regeneration trigger. 
       FIG. 2  is a flow chart illustrating an exemplary regeneration control process  100  after a regeneration is triggered (step  110 ). As discussed earlier, regeneration may be triggered by any known method. For instance, in some embodiments, regeneration may be triggered when the soot load in DPF  19  exceeds a predetermined value. In some embodiments, the predetermined value that triggers regeneration may depend upon the operating environment of engine  14 . In one such embodiment, regeneration may be triggered based on normal triggers for regeneration in addition to the altitude of the system. For instance, in an embodiment where controller  32  triggers regeneration of DPF  19  when the soot load in DPF  19  reaches a threshold value, then in an altitude based version of the embodiment, the threshold soot load that triggers regeneration may depend on altitude of the system. A lower threshold value of soot load may trigger regeneration at a higher altitude and a higher threshold value of soot load may trigger regeneration at a lower altitude. For the same engine operating conditions, the exhaust flow at a higher altitude may be lower than the exhaust flow at a lower altitude. A lower exhaust flow may make temperature control of DPF  19  during regeneration difficult. Decreasing the soot load that triggers regeneration at a higher altitude may help maintain the temperature of the DPF  19  during regeneration below an acceptable limit. The altitude of the system may be measured from altitude sensor  58 , or determined based on the readings of any of other sensor of engine  14  or exhaust treatment system  16 . 
       FIG. 2A  is a flow chart illustrating an exemplary method of triggering regeneration (step  100 ) based on soot load in the DPF and altitude of the system. Controller  32  may continuously (or periodically) monitor the soot load in DPF  19  (step  112 ). The controller  32  may then determine the threshold soot load as a function of the measured soot load and altitude (step  114 ). This determination may be made by any means. For instance, in some embodiments, a table or a map may indicate the threshold soot load for different altitudes. In other embodiments, a model may provide the threshold soot load. Controller  32  may then trigger regeneration if the soot load in DPF  19  is greater than or equal to the threshold soot load (step  116 ). 
     With reference to  FIG. 2 , after a regeneration is triggered, controller  32  may determine (step  120 ) if one or more events that increase the amount of hydrocarbons in the exhaust (“excess hydrocarbon emission events”) have occurred after the last regeneration. In one embodiment, this step may include determining if any flags that indicate the occurrence of an excess hydrocarbon emission event are set (step  120 ). In some embodiments, these flags may include a fail to ignite flag  122 , a loss of combustion flag  124 , an extended cranking flag  126 , a service flag  128 , and an engine faults flag  32 . Additionally or alternatively, other embodiments may include other flags that indicate the occurrence of an excess hydrocarbon emission event. 
     In some instances, conditions (such as air-fuel ratio, exhaust flow, etc.) within burner assembly  30  may cause the fuel air mixture in burner assembly  30  to not ignite when desired. Such a ‘fail to ignite’ event within burner assembly  30  may cause the fuel delivered through fuel injector  34  to pass un-burnt to DPF  19 . The fail to ignite flag  122  may indicate occurrences of such events since the last regeneration. In some instances, the flame in the burner assembly  30  may get extinguished while fuel injector  34  continues to direct fuel to the burner assembly  30 . This un-burnt fuel may pass along with the exhaust flowing therethrough to DPF  19  and DOC  22 . The loss of combustion flag  124  may identify such occurrences of flame extinguishment in burner assembly  30  since the last regeneration. During starting of engine  14 , if all the fuel in the combustion chambers of engine  14  do not ignite, this fuel may pass along with the exhaust to DPF  19  and DOC  22 . Extended cranking of the engine may increase the amount of un-burnt fuel that may end up in the exhaust. Controller  32  may include a counter that keeps track of the cumulative cranking time of engine  14 . The extended cranking flag  126  may be set when the cumulative cranking time exceeds a predetermined value. Some defects of the engine  14  (such as, for example, defects in the common rail fuel system, fuel injectors, etc.) may also increase the amount of hydrocarbons in exhaust. The engine fault flag  132  may be set to identify the presence of such a defect. In some cases, replacement or the service of a part (such as fuel injector) may temporarily increase the amount of hydrocarbons in the exhaust. The service flag  128  may be set after the replacement or service of a such a part. 
     If a flag indicating an excess hydrocarbon emission event is set (step  140 ), the controller  132  may check to see if the temperature of the exhaust exiting DPF  19  has been above a predetermined value for a predetermined period of time (step  150 ). If the exhaust temperature has been higher than the predetermined value for a predetermined period of time, then any hydrocarbons accumulated in DPF  19  may have already evaporated due to the hot exhaust flowing through DPF  19 . Therefore, the controller  132  may perform a normal regeneration (step  200 ) of the controller  132 . The normal regeneration may be performed using any desired regeneration profile. Similarly, if a flag indicating an excess hydrocarbon emission event is not set (step  140 ), then there may not be sufficient accumulated hydrocarbons in DPF  19  to cause an uncontrolled exothermic reaction during regeneration, and the controller  132  may perform a normal regeneration (step  200 ) of DPF  19 . 
     If a flag is set (step  140 ) and the temperature of the exhaust exiting DPF  19  is not above a predetermined value for a predetermined period of time (step  150 ), then the controller  132  may estimate the amount of hydrocarbons emitted as a result of the excess hydrocarbon emission events, and determine a preheat time t p-heat  (step  160 ) to evaporate at least a portion of these emitted hydrocarbons. The controller  32  may then regenerate DPF  19  by using a modified regeneration profile. The modified regeneration profile may be a regeneration profile configured to evaporate at least a portion of the hydrocarbons emitted prior to the burning of soot in a soot oxidation step. In some embodiments, the modified regeneration profile may be a modification of the normal regeneration profile to include a preheat step at the beginning of the normal regeneration profile (step  300 ). The preheat step may include a step where DPF  19  (or the exhaust flowing through DPF  19 ) is heated and maintained at a preheat temperature for the calculated preheat time t p-heat . The preheat temperature may be any temperature below the soot oxidation temperature which can evaporate the hydrocarbons. In some embodiments, the soot oxidation temperature of DOC  22  may be around 550° C. and the soot oxidation temperature of DPF  19  may be around 650° C. In these embodiments, the preheat temperature may be any temperature (such as, for example, about 500° C.) below 550° C. 
       FIG. 3  is a schematic illustration of an exemplary process used by controller  32  to estimate the amount of emitted hydrocarbons and determine the preheat time t p-heat  (that is, step  160  of  FIG. 2 ) needed to evaporate at least a portion of these hydrocarbons. The preheat time t p-heat  may be determined based on the excess hydrocarbon emission events that have occurred since the last regeneration. Controller  32  may include logic that assigns a time value for each individual excess hydrocarbon emission event that has occurred. A time value associated with any particular excess hydrocarbon emission event may be an estimate of the amount of time DPF  19  has to be maintained at the preheat temperature to evaporate a desired portion of the hydrocarbons emitted by that particular excess hydrocarbon emission event. In some embodiments, these time values may be assigned using maps or tables. For example, if a total of three fail to ignite events have occurred since the last regeneration, controller  32  may assign a time value of t 1  for this excess hydrocarbon emission event based on a map that indicates an estimated amount of time needed to evaporate a portion of the hydrocarbons that may be emitted by the three fail to ignite events, at a particular temperature (such as the preheat temperature). Similarly, based on the number of loss of combustion events that occurred, a map may assign a time value of t 2 , and based on the cumulative crank time, the number of service tests, and the number or type of engine faults, maps may assign time values of t 3 , t 4 , and t 5  respectively. In some embodiments, the maps may assign a time value based on the excess hydrocarbon emission event and other operating conditions of the system (such as, for example, the soot load in DPF  19 ). It is also contemplated that, in some embodiments, these time values may be computed using models or estimated by other means. 
     Controller  32  may include logic  180  or another mechanism that determines the preheat time t p-heat  based on the individual time values of t 1 , t 2 , t 3 , t 4 , and t 5 . In some embodiments, preheat time t p-heat  may correspond to the maximum time of the individual time values of t 1 , t 2 , t 3 , t 4 , and t 5 . However, in general, preheat time t p-heat  may be determined as any function of some or all of the individual time values t 1 , t 2 , t 3 , t 4 , and t 5 . For example, in some embodiments, preheat time t p-heat  may be the sum of some or all of the individual time values. In some embodiments, a map or table may determine preheat time t p-heat  based on at least some of the individual time values. Preheat time t p-heat  may represent an estimate of the amount of time DPF  19  has to be maintained at the preheat temperature to evaporate a desired portion of the estimated accumulated hydrocarbons in the after-treatment components  18  of the exhaust treatment system  16 . 
     As explained earlier, any regeneration profile may be used to regenerate DPF  19  when controller  32  determines that accumulated hydrocarbons (if any) in DPF  19  will not cause an uncontrolled exothermic reaction during the regeneration process.  FIG. 4  illustrates an exemplary regeneration profile that may be used for regeneration (step  200 ) when there is no concern of an uncontrolled exothermic reaction due to accumulated hydrocarbons. The regeneration profile may include a series of temperature increases and temperature holds. Controller  32  may effectuate the regeneration profile of  FIG. 4  by controlling burner assembly  30 . For example, controller  32  may effectuate a temperature increase of DPF  19  at a predetermined rate up to a second predetermined temperature T 2 . Controller  32  may then effectuate a temperature hold by maintaining the temperature of DPF  19  at T 2  for a predetermined amount of time (t hold1 ). Second predetermined temperature T 2  may represent a temperature at which soot oxidizes in DOC  22  (or another after-treatment device  18 ) of the exhaust treatment system  16 . Further, controller  32  may then effectuate another temperature increase of DPF  19  at a predetermined rate to a higher third predetermined temperature of T 3 . Third predetermined temperature T 3  may represent the temperature at which soot oxidizes in DPF  19 . At T 3 , the soot accumulated in DPF  19  may start to burn. As the soot burns, the temperature in DPF  19  may increase as shown by the curve marked “A” in  FIG. 4 . 
     In some embodiments, the third predetermined temperature T 3  may depend on the soot load in DPF  19 . In embodiments where the threshold soot load that triggers regeneration depends on the altitude, third predetermined temperature T 3  may be lower at higher altitudes. As the soot load in DPF  19  increases, third predetermined temperature T 3  may increase. For example, at a higher altitude, after the temperature hold at T 2 , controller  32  may effectuate a temperature increase of DPF  19  to a third predetermined temperature T 3 ′ which is lower than T 3 . From T 3 ′, the temperature in DPF  19  may increase as the soot in the DPF burns. Because the exhaust flow rate through DPF  19  is lower at higher altitudes, the rate of temperature increase in DPF  19  as the soot burns (shown by curve marked “B”) may be higher at a higher altitude. Since it is desirable to maintain a final DPF temperature at or below an acceptable temperature T 4 , the third predetermined temperature T 3 ′ at a particular altitude may be selected so that the final DPF temperature may be at or below the acceptable temperature T 4 . 
     Although the temperature ramp rates in  FIG. 4  are depicted to be uniform, in reality, they may not be. That is, the predetermined rates of temperature increases to T 2  and T 3  (or T 3 ′) may include multiple ramp rates. In general, the temperature ramp rate may be chosen to reduce regeneration time and minimize temperature over-shoot. Similarly, any value may be selected for the second predetermined temperature T 2 , and the hold time t hold1  at this temperature. In some embodiments, these values may be selected to oxidize an estimated amount of trapped soot in DOC  22  (or another after-treatment device  18 ) while optimizing the regeneration process. 
       FIG. 5  illustrates an exemplary regeneration profile including a preheat step (step  300 ) to evaporate at least a part of the accumulated hydrocarbons in DPF  19 . As explained earlier, such a regeneration profile may be used for regeneration (step  300 ) when controller  32  determines that accumulated hydrocarbons in DPF  19  may cause an uncontrolled exothermic reaction in the DPF  19  during regeneration. In this case, controller  32  may first effectuate an increase in temperature of DPF  19  to a preheat temperature of T 1  and maintain this temperature for a preheat time of t p-heat . Any values may be chosen for preheat temperature T 1 . Preheat temperature may be any temperature below the soot oxidation temperature that may evaporate the emitted hydrocarbons. As described earlier, the preheat time t p-heat  may be determined based on the expected amount of accumulated hydrocarbons in DPF  19  (step  160  of  FIG. 2 , and  FIG. 3 ). After the hold time, the controller  32  may then regenerate DPF  19  following any regeneration profile. In some embodiments, after the expiry of the preheat time t p-heat , the controller  32  may follow the same regeneration profile of  FIG. 4 . That is, after holding the temperature of DPF  19  at T 1  for a time of t p-heat , the controller  32  may effectuate a temperature increase of DPF  19  at a predetermined rate to the second predetermined temperature T 2  and hold the DPF  19  at this temperature for the hold time of t hold1  to oxidize the soot in DOC  22 . After this step, the controller  32  may increase the temperature of DPF  19  to the third predetermined temperature of T 3  to oxidize (burn) the soot in DPF  19 . As explained earlier, in some embodiments, the controller  32  may increase the temperature of DPF  19  to a third predetermined temperature of T 3 ′ such that the final DPF temperature after soot oxidation is at or below the acceptable temperature T 4 . Maintaining DPF  19  at T 1  for t p-heat  amount of time may evaporate a sufficient amount of accumulated hydrocarbons from DPF  19  such that any remaining accumulated hydrocarbons in DPF  19  may not cause an uncontrolled exothermic reaction when the temperature of the DPF is raised to the third predetermined temperature of T 3  to oxidize the soot in DPF  19 . 
     Although the preheat step is described as being included ( FIG. 5 ) or not included ( FIG. 4 ) based on the estimated amount of hydrocarbons in DPF  19 , it should be noted that in some embodiments the regeneration profile, as illustrated in  FIG. 5 , with a variable preheat time t p-heat  may be used for regeneration in all cases. In these embodiments, the duration of the preheat time t p-heat  may be selected based on the estimated amount of accumulated hydrocarbons in DPF  19 . And, when the estimated amount of accumulated hydrocarbons in DPF  19  is below a threshold value, the duration of preheat may be set to zero (i.e., t p-heat =0), to result in the regeneration profile of  FIG. 4 . 
     INDUSTRIAL APPLICABILITY 
     The exhaust preheat strategy of the current disclosure may be beneficial for any internal combustion engine application to remove accumulated hydrocarbons from the exhaust so that an uncontrolled exothermic reaction may be prevented during regeneration. Such engine applications may include stationary applications, such as engines used to drive power generation sets, or mobile applications, such as engines used in mobile machines. 
     During some operating conditions of engine, such as, for example, during the cold start phase of engine operation and during some engine faults (extended cranking, fuel injector faults, failed ignitions and flameouts of the burner, etc.), the hydrocarbons emitted in the exhaust may be high. These hydrocarbons may accumulate in an exhaust after-treatment device (such as, for example, a DPF or a DOC) along with particulate matter such as soot. The accumulated hydrocarbons may cause an uncontrolled exothermic reaction in the after-treatment device during regeneration. Such an uncontrolled exothermic reaction may increase the temperature of the after-treatment device to above an acceptable temperature and adversely affect the reliability of the device. Therefore, the after-treatment device may be preheated to evaporate at least a portion of the accumulated hydrocarbons prior to soot oxidation during regeneration. To illustrate the preheat strategy, an exemplary application will now be described. 
     In an exemplary application, engine  14  along with exhaust treatment system  16  of  FIG. 1  may be part of a mobile machine. During operation of the mobile machine, the machine may experience several excess hydrocarbon emission events, such as, for example, three fail to ignite events, two loss of combustion events, and a cumulative crank time of 10 seconds. At the occurrence of these events, controller  32  may set the fail to ignite flag  122 , the loss of combustion flag  124 , and the extended cranking flag  126 . The controller  32  may also assign time values (t 1 , t 2 , and t 3 ) to these excess hydrocarbon emission events from a map. For example, the controller  32  may assign a value of 30 seconds for t 1 , a value of 20 seconds for t 2  and a value of 0 seconds for t 3  based on maps that relate time to each of these events. These time values may represent the duration of time for which the DPF  19  must be held at a selected preheat temperature to evaporate a certain amount of the hydrocarbons emitted as a result of the respective excess hydrocarbon emission events. 
     During operation of the mobile machine, the controller  32  may continuously monitor the soot load of DPF  19 . The soot load may be monitored using the difference in pressure readings of the upstream pressure sensor  52  and the downstream pressure sensor  56 . When this monitored soot load reaches a threshold value, controller  32  may trigger regeneration of DPF  19  (step  110  of  FIG. 2 ). When the regeneration is triggered, controller  32  may check to see if a flag indicating the occurrence of an excess hydrocarbon emission event is set. Since a flag is set, the controller  32  may check a record of the temperature measured by the second temperature sensor  54  to determine if the temperature of the exhaust exiting the DPF  19  has been above a predetermined value for a predetermined amount of time. If it has not, the controller  32  may consider that the accumulated hydrocarbons in DPF  19  may cause an uncontrolled exothermic reaction during regeneration. To reduce the possibility of such a reaction, the controller  32  may determine the time t p-heat  (step  160  of  FIG. 2 ) for which the DPF  19  should be held at the selected preheat temperature so as to evaporate at least a portion of the hydrocarbons accumulated therein. The preheat time t p-heat  may be determined as the maximum of t 1 , t 2 , and t 3  (that is, 30 seconds in this case). The controller  32  may then preheat the DPF to the selected preheat temperature of T 1  (for example about 500° C.) and maintain this temperature for a hold time of 30 seconds. The controller  32  may heat DPF  19  and maintain the DPF at the preheat temperature by activating and controlling the burner assembly  30 . After the expiry of 30 seconds, the controller  32  may continue to heat the DPF  19  along any desired regeneration profile to regenerate DPF  19 . 
     By evaporating at least a portion of the accumulated hydrocarbons in the after-treatment device prior to soot oxidation, the presently disclosed system may reduce the likelihood of an uncontrolled exothermic reaction during regeneration. Also, since the preheating step is included in the regeneration profile only when excess hydrocarbons may be present in the after-treatment device, the presently disclosed system may reduce the total time of regeneration, and thereby optimize the regeneration process. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed regeneration strategy. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed regeneration strategy. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.