Abstract:
A method of correcting a soot mass estimate in a vehicle exhaust aftertreatment device includes monitoring an exhaust gas pressure drop across a particulate filter included with the vehicle exhaust aftertreatment device. Following the detection of a pressure drop, a controller may determine a soot mass estimate from the monitored pressure drop; determine an ash volume estimate representative of an amount of ash within the particulate filter; determine an ash correction factor from the soot mass estimate and the ash volume estimate; and calculate a corrected soot mass value by multiplying the ash correction factor with the soot mass estimate. If the corrected soot mass value exceeds a threshold, the controller may generate a corresponding particulate filter regeneration request.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of monitoring a particulate filter in an exhaust gas aftertreatment system using a differential pressure module. 
     BACKGROUND 
     Various exhaust aftertreatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. In the case of compression-ignition or diesel engines, a great deal of effort continues to be expended to develop practical and efficient devices and methods to reduce emissions of largely carbonaceous particulates otherwise present in the engine&#39;s exhaust gas. 
     An aftertreatment system for a modern diesel engine exhaust typically incorporates a diesel particulate filter (DPF) for collecting and disposing of the sooty particulate matter emitted by the diesel engine prior to the exhaust gas being discharged to the atmosphere. A typical DPF acts as a trap for removing the particulate matter from the exhaust stream. The DPF may contain precious metals, such as platinum and/or palladium, which serve as catalysts to further oxidize soot and hydrocarbons present in the exhaust stream. In many instances, the DPF may be regenerated or cleaned using superheated exhaust gas to burn off the collected particulate. 
     The particulate matter included in the engine exhaust gasses may include carbonaceous soot particulates that may be oxidized to produce gaseous carbon dioxide, as well as other non-combustible particulates (i.e., ash) that are not capable of being oxidized. The composition and morphology of exhaust gasses is largely a function of the fuel, engine type, engine design, engine operation and control methodology, environmental operating conditions, and other factors. For example, engine lubricating oil that passes into the combustion chamber and is partially burned produces the majority of ash. As a further example, combustion in gasoline engines may produce submicron organic matter (OM), as well as sulfates and elemental silicon, iron, zinc, or sulfur. The elemental silicon, iron, and zinc are non-combustible particulates and may comprise ash. As another example, combustion in diesel engines may also produce OM, sulfates and elemental silicon, iron, zinc or sulfur, as well as soot and ammonium. 
     SUMMARY 
     A vehicle may include an engine and an exhaust aftertreatment device in fluid communication with the engine. The exhaust aftertreatment device may include a particulate filter for separating soot from combustion gasses exhausted from the engine. 
     A system for monitoring the particulate filter of the exhaust aftertreatment device includes a first fluid tube, a second fluid tube, a differential pressure module and a controller. The first fluid tube may be disposed in fluid communication with the exhaust aftertreatment device between the particulate filter and the engine. The second fluid tube may be disposed in fluid communication with the exhaust aftertreatment device and on an opposite side of the particulate filter from the first fluid tube. As such, the first fluid tube may be “upstream” of the particulate filter, and the second fluid tube may be “downstream” of the particulate filter. 
     A differential pressure module may be in communication with a controller, and may be configured to monitor a pressure difference between the first fluid tube and the second fluid tube. The differential pressure module may be configured to generate a delta pressure signal corresponding to the monitored exhaust gas pressure drop. 
     The controller may be in communication with the differential pressure module and may be configured to receive the delta pressure signal from the differential pressure module and determine a soot mass estimate from the received delta pressure signal. Additionally, the controller may determine an ash volume estimate representative of an amount of ash within the particulate filter, determine an ash correction factor from the soot mass estimate and the ash volume estimate, and calculate a corrected soot mass value by multiplying the ash correction factor with the soot mass estimate. If the corrected soot mass value exceeds a threshold, the controller may generate a regeneration request to queue a regeneration of the particulate filter. 
     In one configuration, the ash correction factor may be a numeric value less than 1.0, and may scale the soot mass estimate to account for ash accumulation. The controller may include a two-dimensional look-up table expressing the ash correction factor as a function of both the soot mass estimate and the ash volume estimate. The controller may then be configured to determine an ash correction factor by selecting an ash correction factor from the two-dimensional look-up table. As such, the ash correction factor may decrease as the soot mass estimate increases, for a fixed ash volume estimate. 
     Likewise, a method of correcting a soot mass estimate in a vehicle exhaust aftertreatment device includes: monitoring an exhaust gas pressure drop across a particulate filter included with the vehicle exhaust aftertreatment device; determining a soot mass estimate from a monitored exhaust gas pressure drop; determining an ash volume estimate representative of an amount of ash within the particulate filter; determining an ash correction factor from the soot mass estimate and the ash volume estimate; calculating a corrected soot mass value by multiplying the ash correction factor with the soot mass estimate; and generating a particulate filter regeneration request if the corrected soot mass value exceeds a threshold. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an engine and an exhaust gas aftertreatment system for treating exhaust gas from the engine. 
         FIG. 2  is a schematic diagram of a soot model including a soot estimator, an ash estimator, and an ash correction map. 
         FIG. 3  is a schematic graph of an ash correction factor as a function of a soot mass estimate, for a fixed ash volume estimate. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numerals are used to identify like or identical components in the various views,  FIG. 1  schematically illustrates a vehicle  10 , including an engine  12  and an exhaust gas aftertreatment system  14 . As may be appreciated, the engine  12  may combust a mixture of fuel and air to provide a motive force for the vehicle  10 . The exhaust gas aftertreatment system  14  may then direct and treat the byproducts of the combustion (i.e., exhaust gasses) as they flow from the engine  12  (indicated by flow arrows  16 ). In general, the exhaust gas aftertreatment system  14  may remove suspended particulate matter and NOx gasses from the exhaust flow  16  prior to the gas being expelled from the vehicle  10 . In one configuration, the engine  12  may be a compression-ignited diesel engine; however, other types of engine technology may similarly be used. 
     The exhaust gas aftertreatment system  14  may generally include a particulate filter  20  that may be configured to filter the particulate matter, i.e., soot, from the exhaust gas of the engine  12 . The particulate filter  20  may include one or more substrates  22  that define a plurality of apertures  24 , through which the exhaust gas must flow. As the exhaust gas passes through the particulate filter  20 , suspended airborne particulate matter may collect on the substrates  22 , where it may be separated from the flow  16 . 
     Over the life of the vehicle  10 , the particulate filter  20  may occasionally need to be regenerated to remove any collected particulate matter. In one configuration, regeneration of the particulate filter  20  may include heating the particulate filter  20  to a temperature sufficient to burn the particulate matter off of the substrate  22 . This high temperature may then be maintained for a period of time sufficient to burn off a majority of the particulate matter from the substrate  22 . In general, the process of “burning off” the particulate matter may involve converting the sooty trapped particulate matter into carbon dioxide, which may be more permissibly dissipated into the atmosphere. 
     To determine when a particulate filter  20  regeneration event is required, a controller  30  may monitor an exhaust flow impedance of the particulate filter  20  via a differential pressure sensor module  32  disposed across the particulate filter  20 . The differential pressure sensor module  32  may monitor a pressure drop across the substrate  22  and between a first fluid tube  34  in fluid communication with the aftertreatment system  14  at a location upstream of the filter  20  (i.e., between the filter  20  and the engine  12 ) and a second fluid tube  36  in fluid communication with the aftertreatment system  14  at a location downstream of the filter  20  (i.e., on an opposite side of the particulate filter  20  from the first fluid tube  34 ). In another configuration, one or more electronic pressure sensors may be used to determine the pressure drop across the particulate filter  20 . The electronic pressure sensor may include a piezoresistive sensor, a piezoelectric sensor, a MEMS sensor, and/or a capacitive sensor configured to convert a sensed pressure into an analog or digital signal representative of the sensed pressure. The differential pressure module  32  may detect a pressure drop between the respective first and second fluid tubes  34 ,  36 , and may provide a signal  38  (i.e., the delta pressure signal  38 ) to the controller  30  that is indicative of the magnitude of the difference. 
     In general, the controller  30  may use the sensed pressure drop, as measured by the differential pressure module  32 , along with any available estimates of exhaust gas flow rate, as inputs into a soot model  40  to estimate the status of the particulate filter  20 . As will be described in greater detail below, the soot model  40  may use the sensed pressure drop across the particulate filter to estimate the number of grams of soot collected within the particulate filter  20 . 
     When the soot model  40  estimates that the particulate filter  20  requires regeneration (i.e., the amount of estimated soot exceeds a soot threshold), the controller  30  may adjust the operation of the engine  12  to perform a regeneration. In one configuration, the controller  30  may initiate a filter regeneration event by increasing the amount of fuel provided to the engine until the fuel/air ratio is slightly rich of a stoichiometric balance. 
     The controller  30  may include a computer and/or processor, and include all software, hardware, memory, algorithms, connections, sensors, etc., necessary to monitor and control the exhaust gas aftertreatment system  14 , engine  12 , and/or the differential pressure module  32 . As such, a control method operative to evaluate the soot model  40  and/or to initiate a regeneration may be embodied as software or firmware associated with the controller  30 . It should be appreciated that the controller  30  may also include any device capable of analyzing data from various sensors, comparing data, making the necessary decisions required to control the exhaust gas aftertreatment system  14 , as well as monitoring the differential pressure sensor module  32 . 
       FIG. 2  schematically represents one configuration of a soot model  40 . The soot model  40  may include a soot estimator  50 , an ash estimator  52 , and an ash correction map  54 . The soot estimator  50  may receive various inputs, such as an exhaust temperature  56 , an exhaust flow rate  58 , and/or a differential pressure reading/signal  38 , and may generate an output estimate  60  of collected soot mass within the particulate filter  20  (i.e., the “soot mass estimate  60 ”) according to techniques known in the art. In one configuration, the soot estimator  50  may include a multi-dimensional look-up table  62  that may provide the soot mass estimate  60  as a function of the various sensory inputs  38 ,  56 ,  58 . The look-up table  62  may be populated using numeric data obtained either through empirical testing or through analytic formulation. In general the soot mass estimate  60  may be an increasing function of the differential pressure reading  38 , though may also be affected to a lesser degree by temperature  56  and/or flow rate  58 . In one configuration, the soot mass estimate  60  may be a value expressed in grams of soot. 
     The ash estimator  52  may estimate the total amount of ash within the particulate filter  20  based on engine operating parameters and/or other diagnostic algorithms. The ash estimator  52  may then add the real-time estimate to a running ash volume estimate  64 , which may be output to the ash correction map  54 . Because ash may not be burned off during a regeneration, the ash volume estimate  64  may be maintained for the life of the particulate filter  20  and/or may only be reset when the filter is replaced or manually cleaned. 
     The ash correction map  54  may include a two-dimensional look-up table  66  that may select an ash correction factor  68  using the soot mass estimate  60 , together with the ash volume estimate  64 . The ash correction factor  68  may be a value less than 1.0 that corresponds to the scaled efficiency of the particulate filter  20  for a given soot/ash level. This correction factor  68  may then be multiplied with the soot mass estimate  60  to form a corrected soot mass value  70 . The corrected soot mass  70  value may then be compared to a soot threshold (at  72 ) to determine if a regeneration is required. 
     In general, the ash correction factor  68  prevents over-regeneration of the particulate filter  20 . Said another way, as ash accumulates within the particulate filter  20 , it will contribute to an increased pressure drop across the filter. This increased pressure drop would ordinarily cause the aftertreatment device  14  to regenerate more frequently, since the pressure drop is the primary proxy for trapped particulate matter. If not corrected for ash accumulation, each successive regeneration would see a progressively larger decrease in efficiency (measured as soot reduction per regeneration), since ash is not burned off in the same way that soot is. By correcting for the amount of trapped ash, the efficiency of the regeneration may remain relatively constant (i.e., a roughly constant amount of soot may accumulate before a regeneration event is triggered). Furthermore, the ash correction factor  68  may be dependent on both the ash volume estimate  64  and the soot mass estimate  60  to account for the non-linearity of ash estimate as soot mass increases.  FIG. 3  illustrates this non-linear relationship through a schematic graph  74  of the required ash correction factor  68  as a function of the soot mass estimate  60 , at a fixed ash volume estimate  64 . 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not as limiting.