Abstract:
A method is disclosed for regenerating a diesel particulate filter without excessively increasing NO 2  emissions. The system includes a fuel delivery device, an oxidation catalyst, and a diesel particulate filter. During a first operational mode, the fuel injection device injects a relatively smaller amount of fuel into the exhaust stream to reduce the capacity of the oxidation catalyst to oxidize NO in the exhaust stream to NO 2 . At a determined time, a second operational mode is initiated where a relatively larger amount of fuel is injected into the exhaust stream and is oxidized within the oxidation catalyst, thereby raising the exhaust temperature sufficiently to combust substantially all of the soot trapped on the diesel particulate filter.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/674,943, filed Apr. 26, 2005, which application is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present disclosure relates generally to diesel engine exhaust systems. More particularly, the present disclosure relates to systems and methods for controlling diesel engine exhaust emissions.  
       BACKGROUND  
       [0003]     Vehicles equipped with diesel engines may include exhaust systems that have diesel particulate filters for removing particulate matter from the exhaust stream. With use of the diesel particulate filters, soot or other carbon-based particulate matter may accumulate on the filters. As particulate matter accumulates on the diesel particulate filters, the restriction of the filters increases, causing the buildup of undesirable back pressure in the exhaust systems. High back pressures decrease engine efficiency and reduce engine performance. Therefore, to prevent diesel particulate filters from becoming excessively loaded, diesel particulate filters should be regularly regenerated by burning off (i.e., oxidizing) the particulates that accumulate on the filters. Under most diesel engine operating conditions, however, the engine exhaust temperature is too low to cause the diesel particulate filter to completely self-regenerate. Thus, it is necessary to provide a means for initiating regeneration of the diesel particulate filter.  
         [0004]     There are a number of methods for regenerating diesel particulate filters known to those skilled in the art. One known method is to operate the engine fuel injection apparatus so as to inject a quantity of fuel late in the combustion stroke of the engine piston, causing the fuel to burn and raise the exhaust temperature sufficiently to initiate regeneration without substantially increasing the engine output torque. Alternatively, a diesel particulate filter may be heated by an electrical heating element to a temperature sufficient to initiate regeneration. Although these systems are generally effective for initiating regeneration of a diesel particulate filter, each has certain drawbacks in application.  
         [0005]     Another method for regenerating a diesel particulate filter involves positioning a fuel injector and an oxidation catalyst upstream of a diesel particulate filter. To initiate regeneration, the fuel injector injects hydrocarbon fuel into the exhaust stream, which is oxidized in the oxidation catalyst to raise the temperature of the exhaust stream sufficiently to initiate regeneration of the diesel particulate filter. An example of such a system is disclosed in U.S. patent application Ser. No. 11/016,345, filed Dec. 16, 2004 (Attorney Docket Number 758.1794USU1), which is herein incorporated by reference in its entirety.  
         [0006]     Diesel exhaust contains nitrogen oxides (NO x ), which consist primarily of nitric oxide (NO) and nitrogen dioxide (NO 2 ). Typically, the NO 2  in the exhaust stream is a relatively small percentage of total NO x , such as in the range of 5 to 20 percent but usually in the range of 5 to 10 percent. Although nitrogen oxides have been a regulated constituent of diesel exhaust for some time, recent developments have suggested that emissions of NO 2  should be regulated separately from overall NO x  emissions for environmental and health reasons. Therefore, it is desired that a diesel exhaust treatment system does not cause excessive increases in the amount of NO 2  within the exhaust stream. One regulation proposed in California requires that the ratio of NO 2  to NO x  in the exhaust gas downstream from an exhaust treatment system be no more than 20 percent greater than the ratio of NO 2  to NO x  in the exhaust gas upstream from the exhaust treatment system. In other words, if the engine-out NO x  mass flow rate is (NO x ) eng , the engine-out NO 2  mass flow rate is (NO 2 ) eng , and the exhaust-treatment-system-out NO 2  mass flow rate is (NO 2 ) sys , then the ratio  
             (     NO   ⁢           ⁢   2     )     ⁢   sys     -       (     NO   ⁢           ⁢   2     )     ⁢   eng           (   NOx   )     ⁢   eng         
 
 must be less than 0.20. 
 
         [0007]     An exhaust treatment system that includes a diesel oxidation catalyst will typically oxidize some of the NO present in the exhaust to form NO 2 . Moreover, because the exhaust typically flows through the oxidation catalyst at all times, and not only when the diesel particulate filter is being regenerated, the oxidation catalyst will typically cause a significant overall increase in the amount of NO 2  emissions. Although total NO x  emissions will generally remain the same, this increase in NO 2  may be problematic under proposed diesel exhaust emissions regulations. Therefore, it is desired to create a diesel exhaust treatment system that provides for the regeneration of a diesel particulate filter without excessively increasing NO 2  emissions.  
       SUMMARY  
       [0008]     The present disclosure relates to a method for regenerating a diesel emissions control device without excessively increasing NO 2  emissions. The system includes a fuel delivery device, an oxidation catalyst, and a diesel particulate filter. During a first operational mode, the fuel injection device injects fuel at a relatively smaller rate into the exhaust stream. The injected fuel enters the oxidation catalyst and favorably occupies catalytic reaction sites therein to reduce NO occupancy of the same sites and minimize the amount of NO that is oxidized to NO 2 .  
         [0009]     At a determined time, such as when the exhaust backpressure becomes excessive or at predetermined time intervals, a second regeneration mode is initiated where fuel is injected at a relatively larger rate into the exhaust stream, where it oxidizes within the diesel oxidation catalyst and raises the exhaust temperature sufficiently to combust substantially all of the soot trapped on the diesel particulate filter. The system therefore enables regeneration of the diesel particulate filter without substantially increasing NO 2  emissions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  schematically illustrates an exhaust system having features that are examples of inventive aspects in accordance with the principles of the present disclosure; and  
         [0011]      FIG. 2  graphically illustrates the relationship between time and the engine power output, the exhaust temperature, the exhaust backpressure, and the fuel injection rate, in accordance with the principles of the present disclosure.  
     
    
     DETAILED DESCRIPTION  
       [0012]     The present disclosure relates to a method for regenerating a diesel emissions control device, such as a diesel particulate filter.  FIG. 1  illustrates an exhaust system  20  that is in accordance with the inventive aspects of the present disclosure. The system includes an engine  22  (e.g., a diesel engine) and an exhaust conduit  24  for conveying exhaust gas away from the engine  22 . A fuel injection device  26  is positioned within exhaust conduit  24  and is adapted to inject fuel into the exhaust stream. An oxidation catalyst  28  is positioned downstream in the direction of exhaust flow from the fuel injection device  26 . Downstream from the oxidation catalyst  28  is a diesel particulate filter  30 . It will be appreciated that the oxidation catalyst  28  and the diesel particulate filter  30  function to treat the exhaust gas that passes through the conduit  24 .  
         [0013]     The system further includes controller  32  that functions to control the rate that fuel is dispensed by the fuel supply device  26  into the exhaust conduit  24 . The controller  32  interfaces with a number of sensing devices or other data inputs that provide data representative of the exhaust gas traveling through the conduit  24 . This data may include the temperature, pressure, and mass flow of the exhaust gas. The controller  32  can use this data to determine the rate that fuel should be dispensed into the exhaust gas stream. Controller  32  provides output control signals to fuel injection device  26  via control line  34 .  
         [0014]     The oxidation catalyst  28  can have a variety of known configurations. Exemplary configurations include substrates defining channels that extend completely therethrough. Exemplary oxidation catalyst configurations having both corrugated metal and ceramic substrates are described in U.S. Pat. No. 5,355,973, that is hereby incorporated by reference in its entirety. The substrates preferably include a catalyst. For example, the substrate can be made of a catalyst, impregnated with a catalyst or coated with a catalyst. Exemplary catalysts include precious metals such as platinum, palladium and rhodium, and other types of components such as base metals or zeolites.  
         [0015]     In one non-limiting embodiment, the oxidation catalyst  28  can have a cell density of at least 200 cells per square inch. A preferred catalyst for the oxidation catalyst  28  is platinum with a loading level greater than 30 grams/cubic foot of substrate. In other embodiments the precious metal loading level is in the range of 30-100 grams/cubic foot of substrate. In certain embodiments, the oxidation catalyst  28  can be sized such that in use, the oxidation catalyst  28  has a space velocity (volumetric flow rate through the oxidation catalyst/volume of the oxidation catalyst) less than 450,000/hour or in the range of 10,000-450,000/hour.  
         [0016]     The diesel particulate filter  30  can have a variety of known configurations. An exemplary configuration includes a monolith ceramic substrate having a “honey-comb” configuration of plugged passages as described in U.S. Pat. No. 4,851,015 that is hereby incorporated by reference in its entirety. Wire mesh configurations can also be used. In certain embodiments, the substrate can include a catalyst. Exemplary catalysts include precious metals such as platinum, palladium and rhodium, and other types of components such as base metals or rare earth metal oxides.  
         [0017]     The diesel particulate filter  30  preferably has a particulate mass reduction efficiency greater than 75%. More preferably, the diesel particulate filter  30  has a particulate mass reduction efficiency greater than 85%. Most preferably, the diesel particulate filter  30  has a particulate mass reduction efficiency equal to or greater than 90%. For purposes of this specification, the particulate mass reduction efficiency is determined by subtracting the particulate mass that enters the diesel particulate filter from the particulate mass that exits the diesel particulate filter, and by dividing the difference by the particulate mass that enters the diesel particulate filter.  
         [0018]     The controller  32  is used to determine when the diesel particulate filter  30  is in need of regeneration. Any number of strategies can be used for determining when the diesel particulate filter  30  should be regenerated. For example, the controller  32  can initiate regeneration of the diesel particulate filter  30  when a pressure sensor  36  indicates that the back pressure in the exhaust conduit  24  exceeds a predetermined level. The controller  32  can also initiate regeneration of the diesel particulate filter  30  at predetermined time intervals. The controller  32  can also be programmed to delay regeneration if conditions of the exhaust system are not suitable for regeneration (e.g., if the exhaust flow rate or exhaust temperature is not suitable for controlled regeneration). For such an embodiment, the controller  32  can be programmed to monitor the operating conditions of the exhaust system and to initiate regeneration only when predetermined conditions suitable for regeneration have been satisfied.  
         [0019]     An example of a control system is disclosed in PCT application PCT US04/18536, filed Jun. 10, 2004, entitled Method of Dispensing Fuel into Transient Flow of an Exhaust System, that is hereby incorporated by reference in its entirety.  
         [0020]     In operation, the controller  32  may determine the correct time for regeneration by receiving input on the exhaust conduit backpressure from pressure sensor  36  and temperature sensor  38 . Referring now to  FIG. 2 , the backpressure will generally increase steadily as the engine is operated and particulate matter is accumulated on the diesel particulate filter  30 . During the time period labeled T 1  on  FIG. 2 , the back pressure is less than the predetermined regeneration pressure labeled P 1 , indicating that the diesel particulate filter  30  is not in need of regeneration. A preferred value for P 1  is 20 kilopascals. Therefore, during time T 1 , at times when the exhaust temperature is above a predetermined value ET 1  the controller  32  operates the fuel injection device  26  so as to inject fuel at a relatively smaller rate, labeled as R 1  on  FIG. 2 . ET 1  is generally in the range of 180 degrees C. to 230 degrees C., and is preferably about 230 degrees C. Fuel injection is generally not required at temperatures below ET 1  because the NO x  content in the exhaust gas is sufficiently low that the amount of NO 2  produced in the oxidation catalyst is relatively small. The time during which fuel is injected at rate R 1  is denoted on  FIG. 2  as T A , T B , etc.  
         [0021]     The rate R 1  is calculated based on a predicted mass flow rate of engine output NO x  emissions (in milli-moles per second). This calculation is based on the engine power output, the air intake manifold pressure, the oxygen content in the exhaust stream, and the exhaust temperature at the outlet of the turbocharger. The mechanics of this calculation are disclosed in previously referenced and incorporated U.S. patent application Ser. No. 11/016,345, filed Dec. 16, 2004 (Attorney Docket Number 758.1794USU1). The flow rate R 1  is then calculated by multiplying the predicted NO x  mass flow rate by a constant F. The constant F is calculated by multiplying the C 1 -based average molecular weight of the hydrocarbon fuel (milli-grams/milli-mole) by a factor that is in the range of 1 to 5, and preferably is in the range of 1 to 3. For example, a typical C 1  value for diesel fuel is C 1 H 1.93 . The actual number within this range is determined based on the catalytic surface area within the oxidation catalyst, the catalyst composition, and the required NO 2  emission level. The rate R 1  is not a fixed value but varies continuously according to the engine operating conditions. It is desired that rate R 1  be as low as possible while maintaining the required NO 2  emission level in order to minimize fuel consumption.  
         [0022]     At some time, the back pressure within the exhaust conduit  24  will reach a predetermined level P 1  or a certain time interval will be reached. At this time, the controller  32  will operate the fuel injection device  26  so as to inject fuel at a relatively larger rate, labeled as R 2  on  FIG. 2 . To promote a controlled and efficient regeneration of the diesel particulate filter  30 , R 2  is selected to cause the temperature of the exhaust gas exiting the oxidation catalyst  28  to have a target temperature in the range of 500 to 700 degrees C. Preferably, the exhaust temperature is in the range of 550 to 650 degrees C., and most preferably the temperature of the gas exiting the oxidation catalyst is about 600 degrees C. Thus, the rate R 2  that fuel is dispensed upstream of the oxidation catalyst  28  is preferably selected so that upon oxidation of the fuel within the oxidation catalyst  28 , the exhaust gas exiting the oxidation catalyst  28  is within the target temperature range. The value of R 2  required to achieve the target temperature range will depend on a number of variables, including the temperature of the exhaust exiting the engine and the mass flow rate of the exhaust.  
         [0023]     The controller  32  will continue to operate the fuel injection device  26  at rate R 2  for time T 2 , until the exhaust backpressure reaches the level labeled as P 2  in  FIG. 2 . At this point, the controller  32  will revert to the smaller fuel injection rate R 1 , so long as the exhaust temperature remains above temperature ET 1 . The total time that the system operates at fuel injection rate R 1  is labeled T 3 , defined as the sum of T A +T B +T C + . . . T n . A typical value for T 3  is 50 to 95 percent of engine operating time and a typical value for T 2  is 2 to 20 minutes. A typical value for T 3 +T 2  may range for 50 percent of the engine operating time to over 95 percent of the engine operating time. In accordance with the above-specified example percentage for T 3 , for a 24 hour engine operating period, T 3  may be 12 to 22.8 hours. The sum of T 2  plus T 3  does not necessarily equal 100 percent of engine operating time because there are periods of time where the exhaust temperature is below the target value so no injection is required.  
         [0024]     The fuel dispensed into the exhaust conduit  24  by the fuel supply device  26  is oxidized within oxidation catalyst  28 . During time T 3 , the injection of fuel at rate R 1  raises the temperature of the exhaust, but does not raise the exhaust temperature to the level required to initiate full regeneration of the diesel particulate filter  30 . For example, the injection of fuel at rate R 1  may raise the temperature of the exhaust by 100 degrees Centigrade. Instead of causing regeneration of the diesel particulate filter  30 , the fuel is dispensed into the exhaust stream and favorably occupies catalytic reaction sites within the oxidation catalyst  28  in order to reduce the oxidation of NO to NO 2 . Because these reaction sites are favorably occupied, in part, by the injected fuel molecules, fewer sites are available to oxidize NO to NO 2  and consequently less NO 2  is produced by the oxidation catalyst  28 .  
         [0025]     During time T 2 , fuel is injected at rate R 2  to raise the temperature of the exhaust gas exiting the oxidation catalyst  28  to a temperature above the combustion temperature of the particulate matter accumulated on the diesel particulate filter  30 . In this manner, by oxidizing fuel in the oxidation catalyst  28 , sufficient heat is generated to cause regeneration of the diesel particulate filter  30 . Preferably, the rate that fuel is dispensed into the exhaust stream is also controlled to prevent temperatures from exceeding levels which may be detrimental to the diesel particulate filter  30 . For example, temperatures above 800 degrees Centigrade may be detrimental. Preferably, exhaust temperature sensor  38  is positioned downstream of the oxidation catalyst  28  and provides input to controller  32 . If controller  32  senses that the exhaust temperature is excessive, it can reduce the amount of fuel injected by fuel injection device  26 .  
         [0026]     In one preferred embodiment, time T 3  constitutes a majority of the operating time and time T 2  constitutes a minority of operating time. More preferably, time T 3  constitutes approximately 50 to 95 percent of operating time and time T 2  constitutes approximately 0.001 to 5 percent of the operating time.  
         [0027]     It will be appreciated that the specific dimensions disclosed herein are examples applicable for certain embodiments in accordance with the principles of the disclosure, but that other embodiments in accordance with this disclosure may or may not include such dimensions.