Abstract:
Engine systems and methods for accomplishing regeneration of a NOx adsorber ( 28 ) using in-cylinder post-injection in a way that creates a lean-rich transition (FIG.  3 ) for regenerating the NOx adsorber while attenuating engine torque output fluctuations during the transition without the necessity of using torque sensing to attenuate the fluctuations.

Description:
This application is a division of application Ser. No. 10/702,889, filed Nov. 6, 2003 now U.S. Pat. No. 7,191,591. 

   FIELD OF THE INVENTION 
   This invention relates to motor vehicles that are powered by internal combustion engines whose operation may for any of various reasons temporarily transition from running lean to running rich, one reason being to purge a NOx adsorber in the engine exhaust system of adsorbed NOx so that it can continue to be effective as the engine continues running. More particularly, the invention relates to systems and methods for attenuating fluctuations in engine output torque that contribute to harness in the operation of such vehicles during lean-rich transitions such as those for regenerating a NOx adsorber. 
   BACKGROUND OF THE INVENTION 
   An exhaust system of a diesel engine that comprises a NOx adsorber is capable of adsorbing significant amounts of NOx in exhaust gas passing through the exhaust system from the engine, thereby reducing the amount of NOx that otherwise would enter the atmosphere. From time to time, such a device must be regenerated in order to purge it of adsorbed NOx so that it can continue to be effective in adsorbing NOx as the engine continues to run. A known technique for regenerating a NOx adsorber comprises creating an excess of CO for reaction with adsorbed NOx to reduce the NOx to molecular nitrogen (N 2 ) while the CO oxidizes to CO 2  during the process. 
   One known method for creating excess CO comprises injecting fuel in proper amount into the exhaust leaving engine combustion chambers. Because that fuel does not contribute to the thermal energy of combustion that is converted by thermodynamic processes in the combustion chambers acting on the engine&#39;s kinematic mechanism to create engine torque, it has essentially no influence on engine torque. 
   For one or more reasons, post-injection of fuel that does contribute to the thermal energy of combustion that produces engine torque may be considered a more desirable alternative, although both methods require the injection of extra fuel to purge the NOx adsorber. However, the post-injection alternative has consequences on engine torque output that can lead to undesirable torque fluctuations that contribute to engine and vehicle harshness as the engine continues to run during NOx adsorber regeneration. 
   A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine. The amount and the timing of engine fueling are two functions that are controlled by an engine control system. A typical diesel engine that comprises fuel injectors for injecting fuel into the engine cylinders under control of an engine control system controls both the duration and the timing of each fuel injection to set both the amount and the timing of engine fueling. During an engine cycle, it is also capable of pre-injection of fuel (pilot-injection) in advance of a main injection and post-injection after the main injection, although the use of either typically depends on how the engine is being operated. 
   SUMMARY OF THE INVENTION 
   The present invention relates to engine systems and methods for accomplishing regeneration of a NOx adsorber using in-cylinder post-injection in a way that creates a lean-rich transition for regenerating the NOx adsorber while attenuating engine torque output fluctuations during the transition without the necessity of using torque sensing to attenuate the fluctuations. 
   Accordingly, one generic aspect of the present invention relates to a method for control of output torque developed by an internal combustion engine during lean-rich modulation of engine operation. With the engine running lean, data values of certain parameters are processed to develop a data value for desired engine fueling for causing the engine to develop a corresponding desired output torque at a given engine speed. 
   As the engine operation changes from running lean to ruing rich, engine output torque is maintained substantially at the corresponding desired output torque at the given engine speed by processing i) the data value for desired engine fueling resulting from the processing of certain parameters, ii) a data value for engine speed, and iii) a data value for actual air-fuel ratio at which the engine is operating, to thereby develop a data value for desired engine fueling for causing the engine to run rich while striving to maintain engine output torque at the corresponding desired output torque at the given engine speed when the engine was running lean. 
   Another generic aspect relates to an engine incorporating a control strategy for implementing the foregoing generic method. 
   Still another generic aspect relates to a method for regenerating a NOx adsorber in an exhaust system of an internal combustion engine that is fueled in accordance with a data value for desired engine fueling. The method comprises processing data values of certain parameters to develop a data value for desired engine fueling for causing the engine to develop a corresponding desired output torque without conditioning engine exhaust passing into the exhaust system for NOx adsorber regeneration. 
   Data values of various parameters indicative of conditions relevant to initiation of NOx adsorber regeneration are processed and after that processing has disclosed that NOx adsorber regeneration can be initiated, NOx adsorber regeneration is initiated by changing engine fueling so as to condition engine exhaust passing into the exhaust system for regenerating the NOx adsorber. 
   The method develops a data value for desired engine fueling that is effective to condition the exhaust gas for NOx adsorber regeneration at a given engine speed while striving to maintain the output torque at the corresponding output torque that desired engine fueling would develop at the given engine speed without conditioning engine exhaust passing into the exhaust system for NOx adsorber regeneration. This is accomplished by processing data values for i) the desired engine fueling that would develop that corresponding output torque without conditioning engine exhaust passing into the exhaust system for NOx adsorber regeneration, ii) engine speed, and iii) actual air-fuel ratio at which the engine is operating. 
   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 portions of a diesel engine relevant to the present invention. 
       FIG. 2  is a schematic diagram of a portion of control strategy for the engine. 
       FIG. 3  is a graph plot showing time traces of several parameters relevant to engine operation. 
       FIG. 4  is a first graph plot useful in understanding principles of the invention. 
       FIG. 5  is a second graph plot useful in understanding principles of the invention. 
       FIG. 6  is a schematic diagram of another portion of the engine control strategy. 
       FIG. 7  is a schematic diagram of another portion of the engine control strategy. 
       FIG. 8  is a third graph plot useful in understanding principles of the invention. 
       FIG. 9  is a fourth graph plot useful in understanding principles of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a schematic diagram of an exemplary diesel engine  20  for powering a motor vehicle. Engine  20  has a processor-based engine control system  22  that processes data from various sources to develop various control data for controlling various aspects of engine operation. The data processed by control system  22  may originate at external sources, such as sensors, and/or be generated internally. 
   Control system  22  controls the operation of electric-actuated fuel injectors that inject fuel into engine combustion chambers. A processor of control system  22  can process data sufficiently fast to calculate, in real time, the timing and duration of injector actuation to set both the timing and the amount of fueling. The injection process comprises a main injection, and under certain conditions, a pilot injection and/or a post-injection. Control system  22  calculates a data value VF_des_m that represents the amount of fuel that is to be injected into a combustion chamber during an engine cycle. 
   Engine  20  flier comprises an intake system  24  through which charge air enters the combustion chambers, and an exhaust system  26  through which exhaust gases resulting from combustion leave the engine. Exhaust system  26  includes a NOx adsorber  28  that adsorbs significant amounts of NOx in exhaust gas passing from engine  26 , thereby reducing the amount of NOx that otherwise would enter the atmosphere. 
   From time to time, NOx adsorber  28  must be regenerated in order to purge it of adsorbed NOx so that it can remain effective as the engine continues to run. A known technique for regenerating a NOx adsorber comprises creating an excess of CO for reaction with adsorbed NOx to reduce the NOx to molecular nitrogen (N 2 ) while the CO oxidizes to CO 2  during the process. 
     FIG. 2  discloses a strategy  30  that is executed by control system  22  to determine when regeneration can be performed. The strategy is premised on the general factors: exhaust gas temperature; elapse of time since the previous regeneration; and driveability of the vehicle. 
   Temperature of exhaust gas proximate the inlet of NOx adsorber  28 , obtained by either estimation or measurement, is represented by the data value for a parameter ADS_INLET_T. Temperature of exhaust gas proximate the outlet of NOx adsorber  28 , obtained by either estimation or measurement, is represented by the data value for a parameter ADS_OUTLET_T. The data value for ADS_INLET_T is compared by a comparison function  32  with a data value INLET_T_LL representing a lower temperature limit at or above which it would be appropriate to regenerate NOx adsorber  28 . The data value for ADS_OUTLET_T is compared by a comparison function  34  with a data value OUTLET_T_LL representing a lower temperature limit at or above which it would be appropriate to regenerate NOx adsorber  28 . 
   If either comparison function  32 ,  34  is satisfied by the corresponding actual temperature being equal to or greater than the respective lower limit, then a logical OR function  36  enables regeneration to proceed. Additional conditions must also be satisfied however before regeneration actually proceeds. 
   Time elapsed since the last regeneration is measured by a timer function  38 . The data value for elapsed time is compared by a comparison function  40  with the data value for a minimum interval between regenerations. Once elapsed time equals or exceeds the minimum, regeneration is enabled. An AND logic function  42  assures that both a temperature minimum and a time minimum have been satisfied before regeneration is enabled. 
   Once strategy  30  has been enabled, actual regeneration becomes a function of driveability. Driveability refers to acceptable vehicle vibration and harshness during lean/rich transition. A torque lower limit and upper limit have been set to minimize vibration and harshness during the transition. The lower limit requires the engine to be running with some load, while the upper limit keeps the engine away from severe acceleration conditions. 
   The data value for a parameter TORQUE represents the torque which engine  20  is producing. TORQUE is the estimated torque based on fueling and engine speed, in other words TORQUE=f(VFDES, N). The data value for TORQUE is compared by a comparison unction  44  with a data value TORQUE_UL representing an upper torque limit above which regeneration would be inappropriate. The data value for TORQUE is compared by a comparison function  46  with a data value TORQUE_LL representing a lower torque limit below which regeneration would be inappropriate. An AND logic function  48  processes the results of both comparisons to assure that torque is within the allowable range for NOx adsorber regeneration. 
   A further AND function  50  processes outputs from both AND functions  42 ,  48  to allow regeneration when the three general factors of exhaust gas temperature, elapse of time since the previous regeneration, and driveability of the vehicle are satisfied. 
   In general, a diesel engine runs cooler, slower, and leaner than a spark-ignition engine. During lean running, engine  20  generates NOx that is adsorbed by NOx adsorber  28 . When the adsorber is to be regenerated, engine operation transitions from running lean to running rich in order to condition the exhaust for purging NOx adsorber  28  by generating the needed excess CO. A trace  60  in  FIG. 3  represents air-fuel ratio. Before time t 1  engine  20  is running lean, the NOx loading of NOx adsorber  28 , represented by a trace  62 , is increasing, and CO concentration, represented by a trace  64 , is relatively low. 
   At time t 1  post injection and air management decrease the air-fuel ratio, creating a surge in CO concentration in the process. At time t 2  lean running resumes. Trace  62  shows that the surge is effective to purge NOx adsorber  28  of a significant amount of its NOx load. 
   The conditions portrayed by  FIG. 3  assume that certain inputs to control system  22 , namely engine speed and accelerator pedal position are commanding engine  20  to develop a substantially constant torque. Because the regeneration process alters engine fueling from that which is otherwise being called for by engine speed and accelerator pedal position, engine torque may fluctuate during NOx adsorber regeneration, as represented by a perturbation  70  in a trace of engine torque  72 . A significant perturbation can contribute to harshness in engine operation that is consequently introduced into the vehicle drivetrain. It is toward attenuating such harshness that the present invention is directed. 
     FIG. 4  shows a trace  80  of engine torque versus air-fuel ratio where, for a given engine speed, the torque remains substantially constant. A trace  82  shows the corresponding fueling that engine  20  needs in order to develop the torque represented by trace  80 . 
   Principles of the invention resulted from the recognition that data closely approximating trace  82  can be developed by suitable data programming of, and data processing by, control system  22 , and the resulting data processed with other data to create desired engine fueling data that, for a given engine speed and desired engine output torque, can fuel the engine during a lean-to-rich transition that causes the engine to run rich while striving to maintain engine output torque at the corresponding desired output torque at the given engine speed when the engine was running lean, thereby attenuating undesired fluctuations in engine torque that would be experienced in the absence of the invention. 
     FIG. 5  shows a piecewise linear approximation  90  of trace  82  to comprise a first linear segment  92  extending between data points marked AFR_r and AFR_c and a second linear segment  94  extending between data points marked AFR_c and AFR_l. AFR symbolizes air-fuel ratio. 
   Segment  92  can be defined by the function
 
 VF   —   des   —   m=αVF   —   des   —   m   —   c+ (1−α) VF   —   des   —   m   —   r 
 
and
 
   segment  94  by the function
 
 VF   —   des   —   m=βVF   —   des   —   m   —   l +(1−β) VF   —   des   —   m   —   c 
 
where
 
α=( AFR−AFR   —   r )/( AFR   —   c−AFR   —   r )
 
and
 
β=( AFR−AFR   —   c )/( AFR   — 1 −AFR   —   c ).
 
   These functional relationships define a control algorithm for desired engine fueling over a range of air-fuel ratios that will cause engine  20  to develop substantially constant torque, although it is to be appreciated that the engine may not necessarily operate all such ratios. In order to generate the excess CO needed for NOx adsorber regeneration, engine  20  needs to run at an AFR below stoichiometic (an AFR of approximately 13). Hence for a given torque, a fueling transition from lean to rich that strives to maintain that torque will take place along segment  92 . As can be appreciated, the specific parameters for a transition will be governed by a specific regeneration strategy for a particular engine. 
   Implementation of the control algorithm in control system  22  is accomplished by entering data values for AFR_r, AFR_c, and AFR_l for each pair of data values for engine torque and engine speed. A sufficient number of pairs of such torque and speed data values are programmed into control system  22  to adequately cover the range of engine operation with sufficient resolution within the range. 
   For given data values of torque and speed representing current engine torque and current engine speed, control system  22  operates to select from the closest pair of torque and speed data values that have been programmed into it, the corresponding data values for AFR_r, AFR_c, and AFR_l for use in calculating a data value for VF_des_m. A data value for the variable AFR is obtained in any suitably appropriate way and processed according to the algorithm to develop the data value for VF_des_m. The processing occurs sufficiently fast m real time to allow variables like AFR to be updated fast enough to follow changing engine operation. 
   The algorithm develops desired fueling data values in the manner represented by  FIG. 7 . For the selected pair of torque and speed data values, control system  22  determines whether actual AFR is above or below the AFR represented by the corresponding break point AFR_c (step  100  in  FIG. 7 ). 
   If AFR is greater than AFR_c, then desired fueling is controlled by
 
 VF   —   des   —   m=βVF   —   des   —   m   —   l +(1−β) VF   —   des   —   m   —   c 
 
   corresponding to step  102  in  FIG. 7 . 
   If AFR is equal to or less than AFR_c, then desired fueling is controlled by
 
 VF   —   des   —   m=αVF   —   des   —   m   —   c +(1−α) VF   —   des   —   m   —   r 
 
   corresponding to step  104  in  FIG. 7 . 
   When the actual AFR is other than AFR_r, AFR_c, and AFR_l, the algorithm calculates the data value for desired engine fueling by what amounts to interpolation, as graphically portrayed by the function  110  in  FIG. 8  for α, and the function  120  in  FIG. 9  for β. The implementation in control system  22  is represented by  FIG. 6 . 
   Data values for engine speed (parameter N) and accelerator pedal position (parameter APS) determine, via a map or look-up table  130 , a data value for engine output torque (parameter Torque_des). Control system  22  processes Torque_des according to an operating strategy for causing engine  20  to develop that torque at that speed. When NOx adsorber  28  is not being regenerated, it is Torque_des that controls desired engine fueling by a different portion of the strategy that is not shown here. When regeneration is occurring, Torque_des is still a factor in controlling desired engine fueling, but not the sole factor because the control algorithm that is used during regeneration takes AFR into account. 
   Another look-up table  132  contains data values for VF_des_c correlated with the pair of the data value for engine speed (N) and the data value for Torque_des that would be essentially exclusively controlling desired engine fueling if regeneration were not occurring. Still another look-up table  134  contains data values for VF_des_r correlated with the pair of the data value for engine speed (N) and the data value for Torque_des that would be essentially exclusively con trolling desired engine fueling if regeneration were not occurring. 
   Data values for N and Torque_des determine, via maps  132  and  134 , data values for VF_des_m_c (representing desired fueling when AFR=AFR_c) and VF_des_m_r (representing desired fueling when AFR=AFR_r). The control algorithm then utilizes those data values for its calculation. Because data values for N and Torque_des can change during regeneration the execution rate of the control algorithm is sufficiently fast to follow whose changes so that data values for VF_des_m_c and VF_des_m_r can be changed accordingly as called for by the maps. 
   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.