Abstract:
A system comprises a particulate matter (PM) filter that comprises an upstream end for receiving exhaust gas, a downstream end and at least one portion. A control module initiates combustion of PM in the PM filter using a heater and selectively adjusts oxygen levels of the exhaust gas to adjust a temperature of combustion adjacent to the at least one portion of the PM filter. A method comprises providing a particulate matter (PM) filter that comprises an upstream end for receiving exhaust gas, a downstream end and at least one portion; initiating combustion of PM in the PM filter using a heater; selectively adjusting oxygen levels of the exhaust gas to adjust a temperature of combustion adjacent to the at least one portion of the PM filter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/972,517, filed on Sep. 14, 2007. 
         [0002]    This application is related to U.S. patent application Ser. Nos. 11/959,753 filed on Dec. 19, 2007, 11/956,722 filed on Dec. 14, 2007, 11/561,100 filed on Nov. 17, 2006, 11/561,108 filed on Nov. 17, 2006, and 11/557,715 filed on Nov. 8, 2006. The disclosures of the above applications are incorporated herein by reference in their entirety. 
     
    
     STATEMENT OF GOVERNMENT RIGHTS 
       [0003]    This disclosure was produced pursuant to U.S. Government Contract No. DE-FC-04-03 AL67635 with the Department of Energy (DoE). The U.S. Government has certain rights in this disclosure. 
     
    
     FIELD 
       [0004]    The present disclosure relates to particulate matter (PM) filters, and more particularly to reducing face cracks in electrically heated PM filters. 
       BACKGROUND 
       [0005]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0006]    Engines such as diesel engines produce particulate matter (PM) that is filtered from exhaust gas by a PM filter. The PM filter is disposed in an exhaust system of the engine. The PM filter reduces emission of PM that is generated during combustion. 
         [0007]    Over time, the PM filter becomes full. During regeneration, the PM may be burned within the PM filter. Regeneration may involve heating the PM filter to a combustion temperature of the PM. There are various ways to perform regeneration including modifying engine management, using a fuel burner, using a catalytic oxidizer to increase the exhaust temperature with after injection of fuel, using resistive heating coils, and/or using microwave energy. The resistive heating coils are typically arranged in contact with the PM filter to allow heating by both conduction and convection. 
         [0008]    Diesel PM combusts when temperatures above a combustion temperature such as 600° C. are attained. The start of combustion causes a further increase in temperature. While spark-ignited engines typically have low oxygen levels in the exhaust gas stream, diesel engines have significantly higher oxygen levels. While the increased oxygen levels make fast regeneration of the PM filter possible, it may also pose some problems. 
         [0009]    PM reduction systems that use fuel tend to decrease fuel economy. For example, many fuel-based PM reduction systems decrease fuel economy by 5%. Electrically heated PM reduction systems reduce fuel economy by a negligible amount. However, durability of the electrically heated PM reduction systems has been difficult to achieve. 
         [0010]    Face cracks have been an issue with the electrically heated PM filters. These cracks are created by large thermal expansion forces generated by the burning soot near a rear endplug region of the PM filter. 
       SUMMARY 
       [0011]    A system comprises a particulate matter (PM) filter that comprises an upstream end for receiving exhaust gas, a downstream end and at least one portion. A control module initiates combustion of PM in the PM filter using a heater and selectively adjusts oxygen levels of the exhaust gas to adjust a temperature of combustion adjacent to the at least one portion of the PM filter. 
         [0012]    A method comprises providing a particulate matter (PM) filter that comprises an upstream end for receiving exhaust gas, a downstream end and at least one portion; heating the PM filter with a heater; initiating combustion of PM in the PM filter; selectively adjusting oxygen levels of the exhaust gas to adjust a temperature of combustion adjacent to the at least one portion of the PM filter. 
         [0013]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0014]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0015]      FIG. 1  is a functional block diagram of an exemplary engine including a particulate matter (PM) filter with a zoned inlet heater that is spaced from the PM filter; 
           [0016]      FIG. 2  illustrates exemplary zoning of the zoned inlet heater of the electrically heated particulate matter (PM) filter of  FIG. 1  in further detail; 
           [0017]      FIG. 3  illustrates exemplary zoning of the zoned inlet heater of the electrically heated PM filter of  FIG. 1  in further detail; 
           [0018]      FIG. 4  illustrates an exemplary resistive heater in one of the zones of the zoned inlet heater of  FIG. 3 ; 
           [0019]      FIG. 5  illustrates the electrically heated PM filter having a zoned electric heater that is spaced from the PM filter; 
           [0020]      FIG. 6  illustrates heating within the zoned electric heater; 
           [0021]      FIG. 7  is a flowchart illustrating steps performed by the control module to regenerate the PM filter; 
           [0022]      FIG. 8  is a graph illustrating soot thickness as a function of axial location for various oxygen levels; 
           [0023]      FIG. 9  is a graph illustration peak temperature as a function of axial location for various oxygen levels; 
           [0024]      FIG. 10  is a graph illustrating adjustment of oxygen levels for a given flow rate; and 
           [0025]      FIG. 11  is a flowchart illustrating steps of a method for controlling a temperature of a cascading flamefront to reduce combustion temperatures near components of the PM filter. 
       
    
    
     DETAILED DESCRIPTION  
       [0026]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0027]    As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0028]    The present disclosure relates to zoned and unzoned heaters for PM filters. The electrical heater may be in contact with or spaced from the PM filter. The heater selectively heats all or portions of the PM filter. The PM filter may be in contact with or mounted close enough to the front of the PM filter to control the heating pattern. The length of the heater may be set to optimize the exhaust gas temperature. 
         [0029]    Thermal energy is transmitted from the heater to the PM filter. The PM filter may be heated by convection and/or conduction. The electrical heater may be divided in zones to reduce electrical power required to heat the PM filter. The zones also heat selected downstream portions within the PM filter. By heating only the selected portions of the filter, the magnitude of forces in the substrate is reduced due to thermal expansion. As a result, higher localized soot temperatures may be used during regeneration without damaging the PM filter. 
         [0030]    The PM filter may be regenerated by selectively heating one or more of the zones in the front of the PM filter and igniting the soot using the heated exhaust gas. When a sufficient face temperature is reached, the heater may be turned off and the burning soot then cascades down the length of the PM filter channel, which is similar to a burning fuse on a firework. 
         [0031]    In other words, the heater may be activated only long enough to start the soot ignition and is then shut off. Other regeneration systems typically use both conduction and convection and maintain power to the heater (at lower temperatures such as 600 degrees Celsius) throughout the soot burning process. As a result, these systems tend to use more power than the system proposed in the present disclaimer. 
         [0032]    The burning soot is the fuel that continues the regeneration. This process is continued for each heating zone until the PM filter is completely regenerated. 
         [0033]    The present disclosure uses a control strategy that is designed to limit thermal stress by reducing the temperature of combustion as the cascading flamefront nears selected portions of the PM filter. The basic electrically heated PM filter strategy is to maximize a temperature of a burn zone for robustness and soot removal but limit the temperature in areas that are easily damaged. 
         [0034]    The rear endplugs on the PM filter are an area where temperature control may be needed. The present disclosure reduces the temperature at the rear end plugs of the PM filter by monitoring the position of the cascading flamefront and using exhaust oxygen control to reduce the temperature when the burn zone nears the rear endplugs or other selected components. 
         [0035]    Many techniques could be used to limit the exhaust oxygen concentration. For example only, temperature control can be achieved by injecting fuel into a hot catalyst (Post fuel), increasing EGR (Exhaust Gas Recirculation), and/or throttling the engine. 
         [0036]    The heater zones may be spaced in a manner such that thermal stress is mitigated between active heaters. Therefore, the overall stress forces due to heating are smaller and distributed over the volume of the entire electrically heated PM filter. This approach allows regeneration in larger segments of the electrically heated PM filter without creating thermal stresses that damage the electrically heated PM filter. 
         [0037]    A largest temperature gradient tends to occur at edges of the heaters. Therefore, activating one heater past the localized stress zone of another heater enables more actively heated regeneration volume without an increase in overall stress. This tends to improve the regeneration opportunity within a drive cycle and reduces cost and complexity since the system does not need to regenerate as many zones independently. 
         [0038]    Referring now to  FIG. 1 , an exemplary diesel engine system  10  is schematically illustrated in accordance with the present disclosure. It is appreciated that the diesel engine system  10  is merely exemplary in nature and that the zone heated particulate filter regeneration system described herein can be implemented in various engine systems implementing a particulate filter. Such engine systems may include, but are not limited to, gasoline direct injection engine systems and homogeneous charge compression ignition engine systems. For ease of the discussion, the disclosure will be discussed in the context of a diesel engine system. 
         [0039]    A turbocharged diesel engine system  10  includes an engine  12  that combusts an air and fuel mixture to produce drive torque. Air enters the system by passing through an air filter  14 . Air passes through the air filter  14  and is drawn into a turbocharger  18 . The turbocharger  18  compresses the fresh air entering the system  10 . The greater the compression of the air generally, the greater the output of the engine  12 . Compressed air then passes through an air cooler  20  before entering into an intake manifold  22 . 
         [0040]    Air within the intake manifold  22  is distributed into cylinders  26 . Although four cylinders  26  are illustrated, the systems and methods of the present disclosure can be implemented in engines having a plurality of cylinders including, but not limited to, 2, 3, 4, 5, 6, 8, 10 and 12 cylinders. It is also appreciated that the systems and methods of the present disclosure can be implemented in a V-type cylinder configuration. Fuel is injected into the cylinders  26  by fuel injectors  28 . Heat from the compressed air ignites the air/fuel mixture. Combustion of the air/fuel mixture creates exhaust. Exhaust exits the cylinders  26  into the exhaust system. 
         [0041]    The exhaust system includes an exhaust manifold  30 , a diesel oxidation catalyst (DOC)  32 , and a particulate filter (PM filter) assembly  34  with an inlet heater  35 . The heater  35  may be zoned. Optionally, an EGR valve (not shown) re-circulates a portion of the exhaust back into the intake manifold  22 . The remainder of the exhaust is directed into the turbocharger  18  to drive a turbine. The turbine facilitates the compression of the fresh air received from the air filter  14 . Exhaust flows from the turbocharger  18  through the DOC  32 , through the heater  35  and into the PM filter assembly  34 . The DOC  32  oxidizes the exhaust based on the post combustion air/fuel ratio. The amount of oxidation increases the temperature of the exhaust. The PM filter assembly  34  receives exhaust from the DOC  32  and filters any soot particulates present in the exhaust. The inlet heater  35  is in contact with or spaced from the PM filter assembly  34  and heats the exhaust to a regeneration temperature as will be described below. 
         [0042]    A control module  44  controls the engine and PM filter regeneration based on various sensed information. More specifically, the control module  44  estimates loading of the PM filter assembly  34 . When the estimated loading is at a predetermined level and the exhaust flow rate is within a desired range, current may be controlled to the PM filter assembly  34  via a power source  46  to initiate the regeneration process. The duration of the regeneration process may be varied based upon the estimated amount of particulate matter within the PM filter assembly  34 . 
         [0043]    Current is applied to the heater  35  during the regeneration process. More specifically, the energy may heat selected zones of the heater  35  of the PM filter assembly  34  for predetermined periods, respectively. Exhaust gas passing through the heater  35  is heated by the activated zones. The heated exhaust gas travels to the downstream filter of PM filter assembly  34  and heats the filter by convection and/or conduction. The remainder of the regeneration process is achieved using the heat generated by the heated exhaust passing through the PM filter. 
         [0044]    Referring now to  FIG. 2 , an exemplary zoned inlet heater  35  for the PM filter assembly  34  is shown in further detail. The electrically heated PM filter assembly  34  is arranged spaced from or in contact with the PM filter assembly  34 . The PM filter assembly  34  includes multiple spaced heater zones including zone  1  (with sub-zones  1 A,  1 B and  1 C), zone  2  (with sub-zones  2 A,  2 B and  2 C) and zone  3  (with sub-zones  3 A,  3 B and  3 C). The zones  1 ,  2  and  3  may be activated during different respective periods. 
         [0045]    As exhaust gas flows through the activated zones of the heater, regeneration occurs in the corresponding portions of the PM filter that initially received the heated exhaust gas (e.g. areas downstream from the activated zones) or downstream areas that are ignited by cascading burning soot. The corresponding portions of the PM filter that are not downstream from an activated zone act as stress mitigation zones. For example in  FIG. 2 , sub-zones  1 A,  1 B and  1 C are activated and sub-zones  2 A,  2 B,  2 C,  3 A,  3 B, and  3 C act as stress mitigation zones. 
         [0046]    The corresponding portions of the PM filter downstream from the active heater sub-zones  1 A,  1 B and  1 C thermally expand and contract during heating and cooling. The stress mitigation sub-zones  2 A and  3 A,  2 B and  3 B, and  2 C and  3 C mitigate stress caused by the expansion and contraction of the heater sub-zones  1 A,  1 B and  1 C. After zone  1  has completed regeneration, zone  2  can be activated and zones  1  and  3  act as stress mitigation zones. After zone  2  has completed regeneration, zone  3  can be activated and zones  1  and  2  act as stress mitigation zones. 
         [0047]    Referring now to  FIG. 3 , another exemplary zoned inlet heater arrangement is shown. A center portion may be surrounded by a middle zone including a first circumferential band of zones. The middle portion may be surrounded by an outer portion including a second circumferential band of zones. 
         [0048]    In this example, the center portion includes zone  1 . The first circumferential band of zones includes zones  2  and  3 . The second circumferential band of zones comprises zones  1 ,  4  and  5 . As with the embodiment described above, downstream portions from active zones are regenerated while downstream portions from inactive zones provide stress mitigation. As can be appreciated, one of the zones  1 ,  2 ,  3 ,  4  and  5  can be activated at a time. Others of the zones remain inactivated. 
         [0049]    Referring now to  FIG. 4 , an exemplary resistive heater  200  arranged adjacent to one of the zones (e.g. zone  3 ) from the first circumferential band of zones in  FIG. 3  is shown. The resistive heater  200  may comprise one or more coils that cover the respective zone to provide sufficient heating. 
         [0050]    Referring now to  FIG. 5 , the PM filter assembly  34  is shown in further detail. The PM filter assembly  34  includes a housing  200 , a filter  202 , and the zoned heater  35 . The filter  202  may include a rear endplug  208 . The heater  35  may be arranged between a laminar flow element  210  and a substrate of the filter  202 . An electrical connector  211  may provide current to the zones of the PM filter assembly  34  as described above. 
         [0051]    As can be appreciated, the heater  35  may be in contact with or spaced from the filter  202  such that the heating is convection and/or conduction heating. Insulation  212  may be arranged between the heater  35  and the housing  200 . Exhaust gas enters the PM filter assembly  34  from an upstream inlet  214  and is heated by one or more zones of the PM filter assembly  34 . The heated exhaust gas is received by the filter  202 . 
         [0052]    Referring now to  FIG. 6 , heating within the PM filter assembly  34  is shown in further detail. Exhaust gas  250  passes through the heater  35  and is heated by one or more zones of the heater  35 . If spaced from the filter  202 , the heated exhaust gas travels a distance “d” and is then received by the filter  202 . The distance “d” may be ½″ or less. The filter  202  may have a central inlet  240 , a channel  242 , filter material  244  and an outlet  246  located radially outside of the inlet. The filter may be catalyzed. The heated exhaust gas causes PM in the filter to burn, which regenerates the PM filter. The heater  35  transfers heat by convection and/or conduction to ignite a front portion of the filter  202 . When the soot in the front face portions reaches a sufficiently high temperature, the heater is turned off. Combustion of soot then cascades down a filter channel  254  without requiring power to be maintained to the heater. 
         [0053]    Referring now to  FIG. 7 , steps for regenerating the PM filter are shown. In step  300 , control begins and proceeds to step  304 . If control determines that regeneration is needed in step  304 , control selects one or more zones in step  308  and activates the heater for the selected zone in step  312 . In step  316 , control estimates a heating period sufficient to achieve a minimum filter face temperature based on at least one of current, voltage, exhaust flow and exhaust temperature. The minimum face temperature should be sufficient to start the soot burning and to create a cascade effect. For example only, the minimum face temperature may be set to 700 degrees Celsius or greater. In an alternate step  320  to step  316 , control estimates current and voltage needed to achieve minimum filter face temperature based on a predetermined heating period, exhaust flow and exhaust temperature. 
         [0054]    In step  324 , control determines whether the heating period is up. If step  324  is true, control determines whether additional zones need to be regenerated in step  326 . If step  326  is true, control returns to step  308 . Otherwise control ends. 
         [0055]    In use, the control module determines when the PM filter requires regeneration. Alternately, regeneration can be performed periodically or on an event basis. The control module may estimate when the entire PM filter needs regeneration or when zones within the PM filter need regeneration. When the control module determines that the entire PM filter needs regeneration, the control module sequentially activates one or more of the zones at a time to initiate regeneration within the associated downstream portion of the PM filter. After the zone or zones are regenerated, one or more other zones are activated while the others are deactivated. This approach continues until all of the zones have been activated. When the control module determines that one of the zones needs regeneration, the control module activates the zone corresponding to the associated downstream portion of the PM filter needing regeneration. 
         [0056]    The present disclosure may substantially reduce the fuel economy penalty, decrease tailpipe temperatures, and improve system robustness due to the smaller regeneration time. 
         [0057]    Referring now to  FIG. 8 , soot thickness is shown as a function of axial location for various oxygen levels. As can be appreciated, there is a tradeoff between soot thickness and oxygen levels. As the oxygen level increases (from 7% to 9% to 11% to 14%), the soot thickness after regeneration decreases. However, soot levels are reduced greater than 50% more at 9% as compared to 7%. Likewise, soot levels are reduced greater than 50% more at 11% as compared to 9%. 
         [0058]    Referring now to  FIG. 9 , peak temperature is shown as a function of axial location for various oxygen levels (including 7% to 9% to 11% to 14%). The peak temperature of the different oxygen levels are initially similar. As the axial location increases, the temperatures diverge from each other. Lower oxygen levels tend to have lower peak temperatures. For the exemplary values of 7%, 9%,11% and 14%, the temperature range at 7 inches is from approximately 790 degrees Celsius to 870 degrees Celsius. Therefore, a tradeoff can be made between soot reduction and peak temperature. As can be appreciated, the oxygen levels may be controlled (e.g. reduced) as the cascading flamefront nears sensitive components. 
         [0059]    Referring now to  FIG. 10 , oxygen levels of the exhaust gas can be adjusted during regeneration to change the resultant temperature of the cascading flamefront as it reaches the sensitive components of the PM filter. For example, the oxygen levels can be adjusted to reduce the likelihood that the flamefront will damage the rear end plugs of the PM filter. To that end, the oxygen levels may be set high initially to initiate heating of an upstream surface. After a predetermined period sufficient to allow the flamefront to reach the sensitive components, the oxygen level can be reduced to reduce a temperature of the burning soot. 
         [0060]    Referring now to  FIG. 11 , control begins with step  400 . In step  404 , control determines whether regeneration has been enabled. If step  404  is true, control continues with step  412  and determines whether the burn zone location is in a face crack area or other sensitive area. If step  412  is false, control returns to step  412 . When step  412  is true, control continues with step  416  and determines whether the burn zone temperature will greater than a first predetermined temperature T TH1  at a current oxygen level. The burn zone temperature may be measured or estimated. For example, the burn zone temperature may be estimated based on flow rates, oxygen levels in the exhaust stream and elapsed time since regeneration was started. 
         [0061]    If step  416  is true, control decreases oxygen in the exhaust stream in step  420 . The oxygen level in the exhaust can be adjusted using any suitable approach. For example only, the oxygen level in the exhaust can be adjusted using post fuel, exhaust gas regeneration (EGR), throttle or other suitable adjustments. 
         [0062]    If step  416  is false, control continues with step  421  and determines whether the burn zone temperature will less than a second predetermined temperature T TH2  at a current oxygen level. If true, control increases oxygen in the exhaust gas in step  422 . Control continues from steps  420 ,  422  and  421  (if false) with step  424 . In step  424 , control determines whether the zone is done. If not, control returns to step  412 . If step  424  is false, control continues with step  426 , where control determines whether another zone needs to be regenerated. If step  426  is true, control returns to step  412 . Otherwise control ends.