Abstract:
An exhaust system ( 16 ) of a diesel engine ( 10 ) has a diesel particulate filter ( 18 ) for treating exhaust gas. When trapped soot has accumulated to an extent that may affect performance of the filter, an engine control system ( 12 ) forces combustion of trapped soot by increasing exhaust back-pressure using a control device ( 20 ) such as a back-pressure control valve or vanes of a variable geometry turbocharger.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to diesel engines that have diesel particulate filters for treating exhaust gases passing through their exhaust systems. More particularly, the invention relates to engine systems and methods employing exhaust back-pressure for burning soot trapped by such a filter. 
     BACKGROUND OF THE INVENTION 
     An exhaust system of a diesel engine that comprises a diesel particulate filter (DPF) is capable of physically trapping diesel particulate matter (DPM) in exhaust gas passing through the exhaust system from the engine. This prevents significant amounts of DPM from entering the atmosphere. Soot is one constituent of DPM. Other constituents include the soluble organic fraction (SOF) and ash (i.e. lube oil additives etc.). The trapping of soot by a DPF prevents what is sometimes seen as black smoke billowing from a vehicle&#39;s exhaust pipe. 
     One type of known DPF is marketed by Johnson Matthey Company under the trade name “Continuously Regenerating Trap” or (CRT™). Another type of known DPF is marketed by Englehard Corporation under the trade name DPX™. 
     DPF&#39;s have previously been used in warm climates without forced regeneration. One possible strategy for forced regeneration of a DPF involves using the engine control system to adjust engine fueling in a way that elevates the exhaust gas temperature to a sufficiently high temperature to combust material trapped by the DPF. If such a strategy is to be as transparent as possible to a driver of the vehicle, it generally requires that the vehicle is being operated in a manner that is both compatible with and substantially unaffected by the extra fueling needed to elevate exhaust gas temperature. It is believed fair to say that a successful strategy will introduce a certain amount of complexity into an engine control system. 
     SUMMARY OF THE INVENTION 
     Accordingly, a strategy that does not introduce as much complexity into an engine control system may be advantageous for certain engines in certain motor vehicles. 
     The present invention relates to engines and methods that employ exhaust back-pressure (EBP) to create suitable exhaust gas temperatures for accomplishing forced combustion of soot trapped by a DPF. The forced combustion process is itself conducted according to an algorithm that processes certain data to control exhaust back-pressure. 
     Accordingly, one generic aspect of the present invention relates to a method for selectively forcing combustion of soot that has been trapped in a diesel particulate filter that treats exhaust gas passing through an exhaust system of a diesel engine. 
     The method comprises, with the engine running, repeatedly processing data indicative of exhaust gas temperature, data indicative of pressure drop across the diesel particulate filter, data indicative of mass flow through the engine, and data correlating various combinations of pressure drop and mass flow with conditions distinguishing between mandating forced combustion of trapped soot, permitting forced combustion of trapped soot, and not forcing combustion of trapped soot. 
     When a result of the processing step discloses a condition mandating forced combustion of trapped soot, operating a device, regardless of exhaust gas temperature, that increases exhaust back-pressure on the engine sufficiently to cause elevation of the temperature of exhaust gas entering and passing through the diesel particulate filter to a temperature sufficient to initiate combustion of trapped soot. 
     When the result of the processing step discloses a condition permitting forced combustion of trapped soot, operating the device, provided that exhaust gas temperature exceeds a defined temperature threshold, to increase exhaust back-pressure on the engine sufficiently to cause elevation of the temperature of exhaust gas entering and passing through the diesel particulate filter to a temperature sufficient to initiate combustion of trapped soot. 
     Another generic aspect of the present invention relates to a diesel engine comprising an exhaust system comprising a diesel particulate filter that treats exhaust gas from the engine, an exhaust back-pressure control device for increasing exhaust back-pressure on the engine sufficiently to cause elevation of the temperature of exhaust gas entering and passing through the diesel particulate filter to a temperature sufficient to initiate combustion of soot trapped by the diesel particulate filter, and a control system for selectively forcing combustion of soot trapped in the diesel particulate filter. 
     The control system comprises a processor that with the engine running repeatedly processes data indicative of exhaust gas temperature, data indicative of pressure drop across the diesel particulate filter, data indicative of mass flow through the engine, and data correlating various combinations of pressure drop and mass flow with conditions distinguishing between mandating forced combustion of trapped soot, permitting forced combustion of trapped soot, and not forcing combustion of trapped soot. 
     When a result of the processing discloses a condition mandating forced combustion of trapped soot, the control system operates the exhaust back-pressure control device, regardless of exhaust gas temperature, to increase exhaust back-pressure on the engine sufficiently to cause elevation of the temperature of exhaust gas entering and passing through the diesel particulate filter to a temperature sufficient to initiate combustion of trapped soot. 
     When a result of the processing discloses a condition permitting forced combustion of trapped soot, the control system operates the exhaust back-pressure control device, provided that exhaust gas temperature exceeds a defined temperature threshold, to increase exhaust back-pressure on the engine sufficiently to cause elevation of the temperature of exhaust gas entering and passing through the diesel particulate filter to a temperature sufficient to initiate combustion of trapped soot. 
     The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, now briefly described as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general schematic diagram of an exemplary diesel engine having a control system for forcing combustion of soot trapped by a DPF in the exhaust system in accordance with principles of the present invention. 
         FIG. 2  is a graph plot useful in explaining certain principles of the invention. 
         FIG. 3  is a flow diagram of an algorithm performed by the control system of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a schematic diagram of an exemplary diesel engine  10  for powering a motor vehicle. Engine  10  has a processor-based engine control system  12  that processes data from various sources to develop various control data for controlling various aspects of engine operation. The data processed by control system  12  may originate at external sources, such as sensors, and/or be generated internally. 
     Engine  10  comprises an intake system  14  through which charge air enters the engine. Fuel is injected into cylinders of engine  10  under the control of control system  12  to form a mixture that is combusted to power the engine. Exhaust gases resulting from combustion within the engine cylinders exit through an exhaust system  16  that includes a DPF  18  for treating the exhaust gases before they pass through an exhaust pipe  19  into the atmosphere. Before reaching DPF  18 , the exhaust gases must pass through an EPB control device  20 , an example of which is an electric-controlled butterfly valve, that is under the control of control system  12 . Another example of EPB control device is the vanes of a variable geometry, or variable nozzle, turbocharger. 
     When EBP control device  20  is maximally open, it imposes minimal restriction to exhaust gas flow. When EBP control device  20  is maximally closed, it imposes maximal restriction to exhaust gas flow. Within a range between maximally open and maximally closed, device  20  imposes restriction that corresponds to the extent to which it is closed in accordance with a control signal applied to it as a result of certain processing performed by control system  12 . 
       FIG. 2  shows several plots correlating pressure drop across DPF  18  (ΔP) with mass flow rate through engine  10 . Mass airflow rate through the engine, either measured or calculated, is a suitable approximation for mass flow rate. When EBP control device  20  is maximally open and DPF  18  is relatively free of trapped soot, large mass flow rates create relatively small ΔP across DPF  18 . This is exemplified by a zone of operation  30  that lies below a plot  32  representing values for a Lower Limit of ΔP versus mass flow rate. An example of a “clean” DPF is shown by a plot  33 . 
     A further plot  34  representing values for an Upper Limit of ΔP versus mass flow rate forms an upper boundary for a further zone of operation  36  whose lower boundary is plot  32 . Values of mass flow rate that result in values for ΔP lying within zone  36  are indicative of more substantial accumulations of trapped soot in DPF  18 . Values of mass flow rate that result in values for ΔP lying within a zone  38  above plot  34  are indicative of even more substantial accumulations of trapped soot in DPF  18 . 
     Briefly, the zone in which DPF  18  is operating determines whether forced burning of trapped soot is called for. In particular: 1) when data, either measured or estimated, indicates that DPF  18  is operating within zone  30 , control system  12  recognizes that the amount of soot accumulation in DPF  18  is below an amount that calls for forced burning; 2) when data, either measured or estimated, indicates that DPF  18  is operating within zone  36 , control system  12  recognizes that the amount of soot accumulation in DPF  18  is appropriate for forced burning, provided that exhaust gas temperature also exceeds some threshold; and 3) when data, either measured or estimated, indicates that DPF  18  is operating within zone  38 , control system  12  recognizes that the amount of soot accumulation in DPF  18  is appropriate for forced burning, regardless of exhaust gas temperature. When burning is not being forced, EBP control device  20  will typically be maximally open, unless being closed to some extent by a strategy that is unrelated to that of the present invention. 
     The inventive strategy is implemented in control system  12  as an algorithm that is repeatedly executed as the engine operates.  FIG. 3  presents an example of such an algorithm  40 . One of the initial steps  42  comprises determining a data value for ΔP in any suitably appropriate way, such as by actual pressure sensing. Another of the initial steps  44  comprises determining, in any suitably appropriate way, a data value for mass flow rate through the engine. 
     Once those two data values have been obtained, a step  46  determines if the data value for ΔP exceeds the Lower Limit value corresponding to the mass flow rate data value. A data value for ΔP that does not exceed the Lower Limit value corresponding to the mass flow rate data value indicates operation in zone  30 , and hence no need for forced burning, in which event the algorithm loops back to the beginning to await its next execution. A data value for ΔP that does exceed the Lower Limit value corresponding to the mass flow rate data value indicates operation in zone  36  or zone  38 , and hence the possibility for initiating forced burning, in which event the algorithm continues to execute. 
     A step  48  next determines if the data value for ΔP is less than the Upper Limit value corresponding to the mass flow rate data value. A data value for ΔP that is less than the Upper Limit value corresponding to the mass flow rate data value indicates operation in zone  36 , and the possibility for initiating forced burning if exhaust gas temperature is greater than the aforementioned threshold. A data value for ΔP that does equal or exceed the Upper Limit value corresponding to the mass flow rate data value indicates operation in zone  38 , and this will result in initiating forced burning regardless of the exhaust gas temperature. 
     Hence, if the data value for ΔP is less than the Upper Limit value corresponding to the mass flow rate data value, a step  56  is performed to determine exhaust gas temperature. Data representing exhaust gas temperature may be obtained in any suitably appropriate way. If step  50  determines that exhaust gas temperature is not greater than the threshold, 200° C. for example, EBP control device  20  is left maximally open, and the algorithm loops back to the beginning to await its next execution. If step  50  determines that exhaust gas temperature is greater than the threshold, control system  12  operates EBP control device  20  to some degree of closure, as indicated by a step  52 . The degree of closure is determined via use of a look-up table that correlates various degrees; of closure with various combinations of values of engine speed and engine load. Data values for engine speed and engine load may be obtained in any suitably appropriate way. For example, engine speed data is typically published on a data bus while engine fueling data that is calculated by on-going processing in control system  12  is indicative of engine load. 
     The increased restriction created by increased closing of device  20  serves to elevate the temperature of the exhaust gases entering and passing through DPF  18  to temperatures sufficient to initiate combustion of soot trapped by the DPF. Repeated execution of the algorithm will continue to keep device  20  closed to some degree so that the combustion of trapped soot continues to be forced. The burned soot passes through exhaust pipe  19  to atmosphere as essentially carbon dioxide. 
     As the soot is burned off, the restriction that DPF  18  imposes on exhaust gas flow diminishes, and eventually DPF operation will return to zone  30 , at which time the forced burning of soot will cease and device  18  will be returned to maximally open condition because the conditions that initiated forced burning will no longer prevail. 
     Should the execution of step  48  have determined that the data value for ΔP equals or exceeds the Upper Limit value corresponding to the mass flow rate data value, control system  12  would have initiated forced burning regardless of the exhaust gas temperature by operating EBP control device  20  to some degree of closure, as indicated by a step  54 . The degree of closure is determined via use of the look-up table that contains correlations of degree of closure with various combinations of values of engine speed and engine load. 
     The increased restriction created by increased closing of device  20  serves to elevate the temperature of the exhaust gases passing through DPF  18  to temperatures sufficient to initiate combustion of trapped soot. Repeated execution of the algorithm will continue to keep device  20  closed to some degree so that the combustion of trapped soot continues to be forced. As the soot is burned off, the restriction that the DPF imposes on exhaust gas flow diminishes, and eventually DPF operation will return to zone  30 , at which time the forced burning of soot will cease and device  18  will be returned to maximally open condition because the conditions that initiated forced burning will no longer prevail. Although operation toward zone  30  from zone  38  will inherently pass through zone  36 , it is most likely that exhaust gas temperature will not diminish below the threshold level that would result in continued operation in zone  36  without reaching zone  30 . 
     It is believed that the inventive strategy avoids any significant interaction with existing engine control strategies so that the inventive strategy can be implemented in existing control systems without complicating such existing strategies. A benefit of this is the opportunity for retrofitting existing motor vehicles whose engines are already equipped with so-called “passive DPF&#39;s”. To the extent that an existing vehicle also has an EBP control device, such a device need not be added to the vehicle, a further advantage of the inventive strategy. 
     While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.