Patent Publication Number: US-8112988-B2

Title: System and method for desulfating a NOx trap

Description:
BACKGROUND AND SUMMARY 
     Various mechanisms have been developed to reduce NO x  emissions from lean-burning engines. One mechanism is a catalyst known as a NO x  trap. The NO x  trap is a catalytic device typically positioned downstream of the catalytic converter in an emissions system, and is configured to retain NO x  when the engine is running a lean air/fuel mixture for eventual reduction when the engine runs a more rich air/fuel mixture. A typical NO x  trap includes an alkali or alkaline metal, such as barium or calcium, to which NO x  adsorbs when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture to produce carbon monoxide, hydrogen gas and various hydrocarbons to reduce the NO x  in the trap, thus decreasing NO x  emissions and regenerating the trap. 
     The use of a NO x  trap can substantially reduce NO x  emissions from a lean-burning engine. However, NO x  traps are also susceptible to poisoning from sulfur in fuels, which may adsorb to the NO x  adsorption sites in the form of sulfate (SO 4   2− ) or other oxidized sulfur compounds. These materials may be generally referred to as “SO x ”, and may prevent NO x  from adsorbing to trap surfaces, thereby impeding proper trap performance. 
     Various methods of desulfating (“deSO x ”) NO x  traps may be used. In general, these methods involve heating the NO x  trap to a temperature sufficient to allow the reduction of SO x , and then producing a rich exhaust to reduce the SO x . However, it may be difficult to determine when trap performance has degraded sufficiently due to sulfur poisoning to perform a deSO x  process. Furthermore, as the trap is aged thermally and/or chemically, the interval at which deSO x  processes are needed may change over time, thereby contributing to the difficulty in determining when to perform a deSO x  process. 
     The inventors herein have realized that desulfation may be more efficiently performed by following a method of controlling the engine, wherein the method comprises initiating a desulfation process in the NO x  trap; measuring a delay between initiating the desulfation process and detecting a temperature increase in the NO x  trap; and adjusting an engine operating parameter based upon the delay between initiating the desulfation process and detecting a temperature increase in the NO x  trap. The engine operating parameter may be related to the timing of performing a subsequent deSO x  process, and/or may be related to an engine operating condition used during a deSO x  process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic depiction of an exemplary embodiment of an internal combustion engine. 
         FIG. 2  shows a graph representing the radial temperature profile of a NO x  trap as a function of time during a deSO x  process, after the NO x  trap has undergone one prior deSO x  process. 
         FIG. 3  shows a graph representing the radial temperature profile of a NO x  trap as a function of time during a deSO x  process, after the NO x  trap has undergone twenty two prior deSO x  processes. 
         FIG. 4  shows a graph representing the radial temperature profile of a NO x  trap as a function of time during a deSO x  process, after the NO x  trap has undergone one hundred five prior deSO x  processes. 
         FIG. 5  shows a flow diagram of an embodiment of a method of controlling an engine according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS 
       FIG. 1  shows a schematic depiction of an exemplary embodiment of an internal combustion engine  10 . Engine  10  typically includes a plurality of cylinders, one of which is shown in  FIG. 1 , and is controlled by an electronic engine controller  12 . Engine  10  includes a combustion chamber  14  and cylinder walls  16  with a piston  18  positioned therein and connected to a crankshaft  20 . Combustion chamber  14  communicates with an intake manifold  22  and an exhaust manifold  24  via a respective intake valve  26  and exhaust valve  28 . An exhaust gas oxygen sensor  30  is coupled to exhaust manifold  24  of engine  10 . A three-way catalyst  32  is connected to and receives feedgas from exhaust manifold  24 , and a NO x  trap  34  is connected to and receives emissions from three-way catalyst  32 . Furthermore, a first temperature sensor  36  is positioned between three-way catalyst  32  and NO x  trap  34 , and a second temperature sensor  38  is positioned at NO x  trap  34 . Engine  10  is depicted as a port-injection spark-ignition gasoline engine. However, it will be appreciated that the systems and methods disclosed herein may be used with any other suitable engine, including direct-injection engines, and compression ignition engines including but not limited to diesel engines. 
     Intake manifold  22  communicates with a throttle body  42  via a throttle plate  44 . Intake manifold  22  is also shown having a fuel injector  46  coupled thereto for delivering fuel in proportion to the pulse width of signal (fpw) from controller  12 . Fuel is delivered to fuel injector  46  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). Engine  10  further includes a conventional distributorless ignition system  48  to provide an ignition spark to combustion chamber  14  via a spark plug  50  in response to controller  12 . In the embodiment described herein, controller  12  is a conventional microcomputer including: a microprocessor unit  52 , input/output ports  54 , an electronic memory chip  56 , which may be electronically programmable memory, a random access memory  58 , and a conventional data bus. 
     Controller  12  receives various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from a mass air flow sensor  60  coupled to throttle body  42 ; engine coolant temperature (ECT) from a temperature sensor  62  coupled to cooling jacket  64 ; a measurement of manifold pressure (MAP) from a manifold absolute pressure sensor  66  coupled to intake manifold  22 ; a measurement of throttle position (TP) from a throttle position sensor  68  coupled to throttle plate  44 ; and a profile ignition pickup signal (PIP) from a Hall effect sensor  70  coupled to crankshaft  40  indicating an engine speed (N). 
     Exhaust gas is delivered to intake manifold  22  by a conventional EGR tube  72  communicating with exhaust manifold  24 , EGR valve assembly  74 , and EGR orifice  76 . Alternatively, tube  72  could be an internally routed passage in the engine that communicates between exhaust manifold  24  and intake manifold  22 . 
     As described earlier, NO x  trap  34  may become poisoned by SOX over time. This occurs when SOX molecules bind to the NO x  absorption sites, thereby preventing the absorption of NO x  and harming trap performance. Therefore, NO x  trap  34  may periodically undergo a deSO x  process to remove SO x  from NO x  adsorption sites. Typical deSO x  processes involve first heating the NO x  trap, for example, by oscillating the air/fuel ratio to cause exothermic catalytic reactions in the trap, and then providing a rich exhaust to the trap for the reduction of adsorbed SO x . A rich/lean oscillation during SO x  reduction may be used to help reduce hydrogen sulfide production. 
     Over time, the elevated temperatures used in operating and desulfating NO x  trap  34 , as well as the chemical processes that occur in the trap, may cause a coarsening of the active materials within NO x  trap  34 , which may thereby reduce the number of NO x  adsorption sites within NO x  trap  34 . This process may be referred to as aging. As the number of NO x  adsorption sites within NO x  trap  34  decreases with aging, NO x  trap  34  may require the use of more frequent deSO x  processes to ensure proper trap operation. However, the aging of a NO x  trap may dependent upon specific trap operating conditions. Therefore, difficulties may arise in determining when and how often to perform deSO x  processes as NO x  trap  34  ages. 
     One possible method of determining when deSO x  is required may be to utilize NO x  sensors positioned upstream and downstream of NO x  trap  34  to estimate the NO x  storage capacity of the trap during engine operation. When the NO x  storage capacity is determined to have dropped below a predetermined level, deSO x  may be performed. While such a method would allow the interval at which deSO x  is performed to be adapted over time to account for aging of the NO x  trap, it may also have drawbacks. For example, currently available NO x  sensors may be expensive. Furthermore, the output of current NO x  sensors may drift over time, making it difficult to determine whether the NO x  storage capacity estimate is correct. 
     As an alternative to NO x  sensors, a diagnostic process utilizing a temperature sensor associated with NO x  trap  34  (for example, temperature sensor  38 ) may be used to determine an aging condition of NO x  trap  34 . For example, at the initiation of a deSO x  process, the air/fuel ratio is oscillated in such a manner as to raise the temperature of the NO x  trap via exothermic catalytic reactions in the trap. Over time, as the surface area of the trap coarsens due to thermal and/or chemical aging, the delay between the initiation of deSO x  air/fuel oscillations and the initial temperature rise in the catalyst may increase due to the aging-related loss of catalytic sites in the NO x  trap. This delay may be measured and used to determine an interval at which to perform a subsequent deSO x  process. 
       FIGS. 2-4  show the effect of NO x  trap aging on the heating of an exemplary NO x  trap during a deSO x  process as a function of the number of prior deSO x  processes performed on the trap. In each graph, each line represents the temperature at a different radial location in the NO x  trap. This data is taken from SAE 2002-01-2871, the disclosure of which is hereby incorporated by reference. Referring first to  FIG. 2 , line  202  of graph  200  shows the temperature of the NO x  trap at the center of the trap approximately 4″ from the rear of the trap as a function of time during a deSO x  cycle. As shown in  FIG. 2 , the temperature in this location of the trap begins to rise about 50 seconds after beginning the deSO x  process. The other lines show the temperature at other locations in the trap. Specifically, line  204  shows the temperature at the center of the trap approximately 2″ from the rear of the trap. Line  206  shows the temperature of the approximately 6″ from the rear of the trap and approximately 5.25″ radially from the centerline of the trap and line  208  shows the temperature approximately 1″ from the rear of the trap and approximately 5.25″ radially from the centerline of the trap. 
     Next,  FIG. 3  shows the radial temperature profile of the same NO x  trap as  FIG. 2  after performing 22 deSO x  cycles. Line  302  of graph  300 , which shows the temperature at the same radial and axial location as line  202  of  FIG. 2 , shows the increase in trap temperature beginning approximately 100 seconds after initiating the deSO x  process. Lines  304 ,  306  and  308  of  FIG. 3  correspond to temperatures taken at the same radial and axial locations of lines  204 ,  206  and  208  of  FIG. 2 , respectively. Likewise,  FIG. 4  shows the radial temperature profile of the NO x  trap after performing 105 deSO x  cycles. Line  402  of graph  400 , which shows the temperature at the same radial and axial location as lines  302  and  202  of  FIGS. 3 and 2 , shows the increase in trap temperature beginning approximately 250 seconds after initiating the deSO x  process. Lines  404 ,  406  and  408  of  FIG. 3  correspond to temperatures taken at the same radial and axial locations of lines  204 ,  206  and  208  of  FIG. 2 , respectively. From  FIGS. 2-4 , it can be seen that the trap heating delay may vary with aging, and therefore may be used to determine an estimate of the aging condition of the trap. The aging condition of the trap likewise may be correlated with a specific interval at which to perform a subsequent deSO x  process. This correlation may be performed in any suitable manner, for example, via an experimentally determined lookup table stored in memory on controller  12 , or via a mathematical model. 
       FIG. 5  shows, generally at  500 , a flow diagram of an exemplary embodiment of a method for operating an engine that uses a measurement of a NO x  trap heating temperature delay during a deSO x  process to operate an engine. Method  500  first includes initiating a NO x  trap deSO x  process at  502 . Next, method  500  includes measuring a delay between initiating the deSO x  process and detecting an increase in the temperature of the NO x  trap at  504 . Method  500  then includes adjusting an engine operating parameter based upon the delay measured. 
     The temperature of the NO x  trap may be measured in any suitable manner. For example, a suitable temperature sensor (for example, temperature sensor  34  shown in  FIG. 1 ) may be provided on or within the NO x  trap. Such a sensor may be located in any suitable position on the NO x  trap, and/or may be configured to determine the temperature of the NO x  trap at any desired radial depth within the NO x  trap. Suitable positions and/or radial depths include those positions and/or depths at which the production of heat by the deSO x  reaction can be detected. 
     Likewise, the delay between initiating the deSO x  process and detecting an increase in the NO x  trap temperature may also be measured in any suitable manner. For example, the delay may be measured as a function of time, engine cycles, or any other suitable quantity. 
     Some portion of the increase in temperature of NO x  trap  34  during a deSO x  process may occur due to increased exhaust temperatures, rather than due to catalytic reactions occurring in the NO x  trap. Therefore, when measuring the delay between starting the deSO x  process and detecting a temperature increase of the NO x  trap, distinguishing the portion of the temperature increase arising from the increased exhaust temperatures from the portion of the temperature increase arising from reaction exotherms may allow a more accurate measurement of the delay to be made, as the slower temperature increase due to aging arises primarily from the latter. Distinguishing the two components of the heating may therefore allow a more accurate determination to be made of the delay in temperature increase caused by exothermic reactions within the NO x  trap. 
     The temperature increase due to increased exhaust temperature may be distinguished from the temperature increase from the catalytic reactions in the trap in any manner. For example, the derivative of the signal from temperature sensor  38  may used to determine the interval at which the deSO x  reactions begin instead of the raw signal. This may allow the temperature increase due to the catalytic reactions in the NO x  trap to be distinguished from the temperature increase due to the exhaust temperatures by highlighting any changes in the rate of increase of the NO x  trap temperature caused by the catalytic reactions. Such heating rate changes can be seen in  FIGS. 2-4 , where the slope of each of lines  202 ,  302  and  402  increases upon the initiation of the catalytic reactions. 
     Alternatively, the temperature increase from exhaust heating may be distinguished from the temperature increase from the catalytic reactions through the use of two temperature sensors, such as temperature sensors  36  and  38  from  FIG. 1 . Temperature sensor  36 , positioned upstream of NO x  trap  34 , may be used to monitor the temperature of the exhaust provided to NO x  trap  34 . Temperature sensor  38 , associated with NO x  trap  34 , may be used to monitor the temperature of the NO x  trap, as described above. From the outputs of temperature sensor  36  and temperature sensor  38 , it may be determined what portion of the temperature rise of NO x  trap  34  is attributable to the exhaust gas temperature increase and what portion is attributable to heat produced by catalytic reactions within the NO x  trap. For example, variables such as the exhaust temperature, flow rate, and/or the thermal mass of NO x  trap  34  may be used to model the heating of NO x  trap  34  as a function of the exhaust conditions, and any heating additional to that predicted by the model may be attributed to the catalytic reactions. Alternatively, a map of the heating profile of NO x  trap  34  as a function of variable such as the exhaust flow rate and exhaust temperature may be experimentally determined and stored in memory on controller  12 . In this manner, the measured temperature of NO x  trap  34  may be compared to the map to distinguish the heating caused by the exhaust from the heating caused by the catalytic reactions within NO x  trap  34 . 
     Referring again to  FIG. 5 , any suitable engine operating parameter may be adjusted in response to determining the delay between initiating a deSO x  process and detecting a production of heat by a catalytic reaction in the NO x  trap at  504 . For example, engine parameters associated with starting a subsequent deSO x  process may be adjusted based upon the determined delay. This may involve adjusting an interval between a current deSO x  process and a subsequent deSO x  process (for example, a time interval, an engine cycle interval, etc.), and/or may involve adjusting a parameter related to exhaust conditions (for example, parameters related to valve timing, injection timing, exhaust gas recirculation, etc.). Furthermore, the delay determined between initiating a deSO x  process and detecting a production of heat by a catalytic reaction in the NO x  trap may also be used to estimate a NO x  storage capacity of the NO x  trap, and thereby to determine an interval at which to purge the NO x  trap of stored NO x . 
     The adjustment made to the engine operating parameter may be determined in any suitable manner. For example, as described above, a look up table correlating specific measured delays with specific operating parameter adjustments may be stored in memory on controller  12 . Alternatively, a mathematical model may be used that calculates the operating parameter adjustments from the delay and (potentially) other inputs such as exhaust flow rate, EGR rate, fuel injection timing, fuel injection volume, etc. It will be appreciated that these methods of adjusting the engine operating parameter are merely exemplary, and that any other suitable method may be used. 
     The embodiments of systems and methods disclosed herein for desulfating a NO x  trap are exemplary in nature, and these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. The subject matter of the present disclosure includes all novel and non-obvious combinations and subcombinations of the various systems and methods for monitoring a temperature rise in the NO x  trap, and other features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the various features, functions, elements, and/or properties disclosed herein may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.