Abstract:
A method and system for controlling the operation of “lean-burn” internal combustion engines, wherein initial values for measures representing current levels of NO x  and SO x  stored in an emissions control device, such as a lean NO x  trap, upon vehicle start-up are determined based at least in part upon values for the measures immediately preceding engine shut-off. Such initial values obviate the need for performing a purge event immediately upon engine start-up in appropriate circumstances, such as a brief engine shut-down, whereby the fuel economy penalty associated with such an initial purge event is avoided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to methods and systems for controlling the operation of “lean-burn” internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy. 
     2. Background Art 
     The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO 2 ) and oxygen (O 2 ). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio ( 8 ), engine speed and load, engine temperature, ambient humidity, ignition timing (“spark”), and percentage exhaust gas recirculation (“EGR”). The prior art often maps values for instantaneous engine-generated or “feedgas” constituents, such as NO x , based, for example, on detected values for instantaneous engine speed and engine load. 
     To limit the amount of feedgas constituents, such as HC, CO and NO x , that are exhausted through the vehicle&#39;s tailpipe to the atmosphere as “emissions,” motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NO x  “trap.”Both the upstream and downstream catalyst store NO x  when the exhaust gases are “lean” of stoichiometry and releases previously-stored NO x  for reduction to harmless gases when the exhaust gases are “rich” of stoichiometry. 
     Under one prior art approach, the duration of any given lean operating excursion (or its functional equivalent, the frequency or timing of each purge event) is controlled based upon an estimate of how much NO x  has accumulated in the trap since the excursion began. For example, in U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153, a controller seeks to estimate the amount of NO x  stored in the trap by accumulating estimates for feedgas NO x  which are themselves obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). The controller discontinues the lean operating excursion when the total feedgas NO x  measure exceeds a predetermined threshold representing the trap&#39;s nominal NO x -storage capacity. In this manner, the prior art seeks to discontinue lean operation, with its attendant increase in engine-generated NO x , before the trap is fully saturated with NO x , because engine-generated NO x  would thereafter pass through the trap and effect an increase in tailpipe NO x  emissions. 
     With the trap thus deemed to have been “filled” with NO x , the prior art teaches the immediate switching of the engine operating condition to a rich engine operating condition characterized by combustion of an air-fuel ratio that is substantially rich of the stoichiometric air-fuel ratio. The rich operating condition is continued, for example, for either a fixed time period sufficient to purge the trap of all stored NO x , or until a downstream oxygen sensor indicates the “break-through” of rich exhaust gas, thereby signaling the release from the trap of all stored NO x . 
     Because of the risk of emissions break-through if the trap is over-filled, the prior art teaches an initialization procedure at engine start-up characterized by the immediate purging of the trap of any stored NO x . Accordingly, immediately upon engine start-up, the controller selects the trap-purging rich engine operating condition and continues to so operate the engine until the trap is confirmed to be empty of stored NO x , either by running rich for a predetermined minimum time period, or until rich exhaust gas is detected downstream of the trap. As a result, each engine start-up incurs an immediate fuel economy penalty. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to control the operation of a lean-burn internal combustion engine at start-up so as to reduce any fuel economy penalty associated with trap initialization. 
     In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust purification system including a lean NO x  trap which stores an exhaust gas constituent when the exhaust gas is lean and releases previously-stored exhaust gas constituent when the exhaust gas is rich. Under the invention, the method includes determining a first measure representing an amount of the first exhaust gas constituent stored in the device at a time when the engine is shut off; and enabling lean engine operation upon an immediately-subsequent engine start-up based on the first measure. In an exemplary embodiment, the method also includes determining a second measure representing the amount of the first exhaust gas constituent stored in the device at the time of the subsequent engine start-up, wherein the second measure is based at least in part on the first measure and a temperature of the device; and downwardly adjusting the first measure based on the length of time. 
     In accordance with another feature of the invention, the enabling step preferably includes determining an amount of fuel, in excess of a stoichiometric amount of fuel, required to release substantially all of the previously-stored amount of the exhaust gas constituent based on the first measure; and prohibiting lean engine operation until the engine has been operated at a rich operating condition sufficient to add the excess fuel amount to the exhaust gas passing through the device. 
     Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The Drawing is a schematic of an exemplary system for practicing the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary system  10  for controlling the operation of a lean-burn internal combustion engine  12  in a motor vehicle (not shown) is illustrated in the Drawing. The exemplary system  10  includes an electronic engine controller  14  having a processor or “CPU”  14   a ; RAM  14   b ; and ROM  14   a , i.e., a processor-readable storage medium which is encoded with executable instructions for controlling the operation of the engine  12 . The engine  12  includes a set of fuel injectors  16  whose operation is controlled by the controller  14 . The fuel injectors  16 , which are of conventional design, are each positioned to inject fuel into a respective cylinder  18  of the engine  12  in precise quantities as determined by the controller  14 . The controller  14  similarly controls the individual operation, i.e., timing, of the current directed through each of a set of spark plugs  20  in a known manner. 
     The controller  14  also controls an electronic throttle  22  that regulates the mass flow of air into the engine  12 . An air mass flow sensor  24 , positioned at the air intake of engine&#39;s intake manifold  26 , provides a signal regarding the air mass flow resulting from positioning of the engine&#39;s throttle  22 . The air flow signal from the air mass flow sensor  24  is utilized by the controller  14  to calculate an air mass value AM which is indicative of a mass of air flowing per unit time into the engine&#39;s induction system. 
     A first oxygen sensor  28  coupled to the engine&#39;s exhaust manifold detects the oxygen content of the exhaust gas generated by the engine  12  and transmits a representative output signal to the controller  14 . The first oxygen sensor  28  provides feedback to the controller  14  for improved control of the air-fuel ratio of the air-fuel mixture supplied to the engine  12 , particularly during operation of the engine  12  at or near the stoichiometric air-fuel ratio (λ=1.00). A plurality of other sensors, including an engine speed sensor and an engine load sensor, indicated generally at  30 , also generate additional signals in a known manner for use by the controller  14 . It will be understood that the engine load sensor  30  can be of any suitable configuration, including, by way of example only, an intake manifold pressure sensor, an intake air mass sensor, or a throttle position/angle sensor. 
     An exhaust system  32  receives the exhaust gas generated upon combustion of the air-fuel mixture in each cylinder  18 . The exhaust system  32  includes an upstream three-way catalytic converter (“three-way catalyst  34 ”) and a downstream NO x  trap  36 . The three-way catalyst  34  contains a catalyst material that chemically alters the exhaust gas generated by combustion of the supplied air-fuel mixture within the cylinders  18  of the engine  12 . The resulting catalyzed exhaust gas is directed past a second oxygen sensor  38 , through the trap  36 , and past a third oxygen sensor  40 . The trap  36  functions in a known manner to reduce the amount of engine-generated NO x  exiting the vehicle tailpipe  42  during lean engine operation, based upon such factors as intake air-fuel ratio, trap temperature T (as determined by a trap temperature sensor, not shown), percentage exhaust gas recirculation, barometric pressure, humidity, instantaneous trap “fullness,” instantaneous sulfur poisoning, and trap aging effects (due, for example, to permanent thermal ag˜ing, or to the “deep” diffusion of sulfur into the core of the trap material which cannot subsequently be purged). While the invention contemplates the use of second and third oxygen sensors  38  and  40  of any suitable type or configuration, in the exemplary system  10 , the second and third oxygen sensors  38 , 40  are conveniently of the “switching” type. 
     Upon initialization, which typically occurs no later than the commencement of a trap purge event, except as described in greater detail below, the controller  14  resets a run timer used to track a first time period and adjusts the output of the fuel injectors  16  to thereby achieve a lean air-fuel mixture for combustion within each cylinder  18  having an air-fuel ratio greater than about 1.3 times the stoichiometric air-fuel ratio. In accordance with the invention, for each subsequent background loop of the controller  14  during lean engine operation, the controller  14  determines a value representing the instantaneous rate FG_NOX_RATE at which NO x  is being generated by the engine  12  as a function of instantaneous engine operating conditions, which may include, without limitation, engine speed, engine load, air-fuel ratio, EGR, and spark. 
     By way of example only, in a preferred embodiment, the controller  14  retrieves a stored estimate FG_NOX_RATE for the instantaneous NO x -generation rate from a lookup table stored in ROM based upon sensed values for engine speed N and engine load LOAD, wherein the stored estimates FG_NOX_RATE are originally obtained from engine mapping data. 
     During a first engine operating condition, characterized by combustion in the engine  12  of a lean air-fuel mixture (e.g., λ&gt;1.3), the controller  14  determines incremental or delta feedgas emissions from the engine, in grams/hr, generated since the last time through this loop, and preferably expressed by the following relationship: 
     
       
         FG_NOX_RATE=FNXXX1(N,LOAD)*FNXXA(λ)*FNXXB(EGRACT) *FNXXC(SPK_DELTA)*FMXXD(ECT-200) 
       
     
     where: FNXXX1(N,LOAD) is a lookup table containing NO x  emission rate values in gram/hr for current engine speed N and engine load LOAD; 
     FNXXA(λ) is a lookup table for adjusting the FG_NOX_RATE value for air-fuel which inherently adjusts the FG_NOX_RATE value for barometric pressure; 
     FNXXB(EGRACT) is a lookup table for adjusting the FG_NOX_RATE value for actual exhaust gas recirculation percentage; 
     FNXXC(SPK_DELTA) is a lookup table for adjusting the FG_NOX_RATE value for the effect of knock sensor or hot open-loop induced spark retard, with NO x  production being reduced with greater spark retard; and 
     FMXXD(ECT-200) is a lookup table for adjusting the FG_NOX_RATE value for the effect of engine coolant temperature above 200° F. 
     Preferably, the determined feedgas NO x  rate FG_NOX_RATE is further modified to reflect any reduction in feedgas NO x  concentration upon passage of the exhaust gas through the three-way catalyst  34 , as through use of a ROM-based lookup table of three-way catalyst efficiency in reducing NO x  as a function of the current air-fuel ratio λ, to obtain an adjusted instantaneous feedgas NO x  rate FG_NOX_RATE_ADJ. The adjusted feedgas NO x  rate FG_NOX_RATE_ADJ is accumulated over the length of time t i,j  that the engine  12  is operated within a given engine speed/load cell for which the feedgas NO x  generation rate R i,j  applies, which is typically assumed to be the duration of the control process&#39;s nominal background loop, to obtain a value representing an instantaneous amount FG_NOX_ADJ of feedgas NO x  entering the trap during the background loop. 
     Also during the lean operating condition, the controller  14  calculates an instantaneous value NOX_INCR representing the incremental amount of NO x  stored in the trap  36  during each background loop executed by the controller  14  during a given lean operating condition, in accordance with the following formula: 
     
       
         NOX_INCR=FG_NOX_RATE_ADJ*t i,j *μ, 
       
     
     where: μ represents a set of adjustment factors for instantaneous trap temperature T, open-loop accumulation of SO x  in the trap  36  (which, in a preferred embodiment, is itself generated as a function of fuel flow and trap temperature T), desired trap utilization percentage, and a current estimate of the cumulative amount of NO x  which has already been stored in the trap  36  during the given lean operating condition. 
     The controller  14  thereafter calculates a value TP_NOX_INST based on the difference between the adjusted instantaneous feedgas NO x  value FG_NOX_ADJ and the instantaneous value NOX_INCR representing the incremental amount of NO x  stored in the trap  36 . The controller  14  then compares the value TP_NOX_INST to a predetermined threshold level TP_NOX_MAX. If the controller  14  determines that the instantaneous tailpipe emissions value TP_NOX_INST exceeds the predetermined threshold level TP_NOX_MAX, the controller  14  immediately discontinues the on-going lean engine operating condition in favor of either near-stoichiometric engine operating condition or a trap-purging rich engine operating condition. 
     In accordance with another feature of the invention, in a preferred embodiment, the method further includes generating a value TP_NOX_TOT representing the cumulative amount of NO x  emitted to the atmosphere during a given trap purge-fill cycle, i.e., since the commencement of an immediately-prior trap-purging rich operating condition; generating a value DISTANCE representing a cumulative number of miles that the vehicle has traveled during the given cycle, as by accumulating detected values VS for vehicle speed over time; and determining a modified value TP_NOX_TOT_MOD representing the average tailpipe NO x  emissions in grams per mile using the cumulative tailpipe emissions value TP_NOX_TOT and the accumulated mileage value DISTANCE. 
     More specifically, when the system  10  is initially operated with a lean engine operating condition, the efficiency of the trap  36  is very high, and the tailpipe NO x  emissions are correlatively very low. As the trap  36  fills, the trap efficiency begins to fall, the tailpipe NO x  emissions value TP_NOX_INST will slowly rise up towards the threshold value TP_NOX_MAX. However, since the initial portion of the lean engine operating condition was characterized by very low tailpipe NO x  emissions, the lean engine operating condition can be maintained for some time after the instantaneous value TP_NOX_INST exceeds the threshold value TP_NOX_MAX before average tailpipe NO x  emissions exceed the threshold value TP_NOX_MAX. Moreover, since a purge event is likewise characterized by very low instantaneous tailpipe NO x  emissions, average tailpipe NO x  emissions are preferably calculated using a time period which is reset at the beginning of the immediately prior purge event. 
     In accordance with yet another feature of the invention, when determining the value DISTANCE representing the cumulative number of miles traveled by the vehicle during the given cycle, the controller  14  assumes a minimum vehicle speed VS_MIN to thereby provide a single modified emissions control measure TP_NOX_TOT_MOD, expressed in terms of emissions per effective vehicle mile traveled, applicable to vehicle speeds above and below the minimum vehicle speed VS_MIN, including stopped vehicle conditions. 
     To the extent that the calculated tailpipe NO x  emissions do not exceed the predetermined threshold level, the controller  14  continues to track trap fill time, as follows: the controller  14  iteratively updates a stored value NOX_STORED representing the cumulative amount of NO x  which has been stored in the trap  44  during the given lean operating condition, in accordance with the following formula: 
     
       
         NOX_STORED=NOX_STORED+NOX_INCR 
       
     
     The controller  14  further determines a suitable value NOX_CAP representing the instantaneous NO x  -storage capacity estimate for the trap  36 . By way of example only, in a preferred embodiment, the value NOX_CAP varies as a function of trap temperature T, as further modified by an adaption factor K i  periodically updated during fill-time optimization to reflect the impact of both temporary and permanent sulfur poisoning, trap aging, and other trap-deterioration effects. 
     The controller  14  then compares the updated value NOX_STORED representing the cumulative amount of NO x  stored in the trap  36  with the determined value NOX_CAP representing the trap&#39;s instantaneous NO x  -storage capacity. The controller  14  discontinues the given lean operating condition and schedules a purge event when the updated value NOX_STORED exceeds the determined value NOX_CAP. It will be appreciated that, by discontinuing lean engine operation, it is meant that the controller  14  selects a suitable engine operating condition from either a near-stoichiometric operating region or a rich engine operating region, rather than from a lean engine operating region. 
     For example, in a preferred embodiment, if the controller  14  determines that the value TP_NOX_INST exceeds the predetermined threshold level TP_NOX_MAX, the controller  14  immediately schedules a purge event using an open-loop purge time based on the current value NOX_STORED representing the cumulative amount of NO x  which has been stored in the trap  44  during the preceding lean operating condition. In this regard, it is noted that the instantaneous trap temperature T, along with the air-fuel ratio and air mass flow rate employed during the purge event, are preferably taken into account in determining a suitable open-loop purge time, i.e., a purge time that is sufficient to release substantially all of the NO x  and oxygen previously stored in the trap  36 . 
     As noted above, a temperature sensor is used to directly measure the trap temperature T; however, it will be appreciated that trap temperature may be inferred, for example, in the manner disclosed in U.S. Pat. No. 5,894,725 and U.S. Pat. No. 5,414,994, which disclosures are incorporated herein by reference. 
     If, at the end of the purge event, the controller  14  determines that the value TP_NOX_INST continues to exceed the predetermined threshold level TP_NOX_MAX, the controller  14  either selects a near-stoichiometric engine operating condition, or schedules another open-loop purge event. 
     Preferably, in accordance with another feature of the invention, the controller  14  initializes certain variables in a manner to account for instances where an engine may be turned off for short periods of time during which the trap  36  may not have cooled to ambient temperature. More specifically, rather than assuming that a purge event, with its resulting fuel economy penalty, must be scheduled immediately upon engine start-up in order to assure that a measure representing NO x  stored in the trap  36  may properly be set to a known (zero) value, the controller  14  estimates values NOX_STORED_INIT and SOX_STORED_INIT for the amounts of NO x  and SO x , respectively, which remain stored in the trap  36  at the time of the subsequent engine start-up, preferably as a function of one or more operating parameters, such as the respective values for stored NO x  and stored SO x  immediately preceding engine shut-off, a value TEMP_INIT representative of the instantaneous trap temperature at the time of the subsequent engine start-up, and at least one respective calibratable time constant representing an amount of time for the variable to deteriorate to a value corresponding to the passage of a relatively large amount of time. 
     More specifically, the controller  14  determines the value NOX_STORED_INIT, representing the amount of NO x  remaining in the trap  36  at the time of the subsequent engine start-up as the lower value of either a time-based bleed-estimated value based on the intervening time interval SOAKTIME and the amount of NO x  believed to be stored in the trap  36  at engine shut-down; and a start-up-temperature-based capacity estimate. 
     Thus, the controller  14  determines a bleed-based trap initialization variable NOX_STORED_BLEED after a soak time SOAKTIME is expressed as follows: 
     
       
         NOX_STORED_BLEED=NOX_STORED*FNEXP(−SOAKTIME/NOX_TIME_CONST) 
       
     
     where: FNEXP is a lookup table value that approximates an exponential function; 
     SOAKTIME is the time elapsed since the engine was shut down, in seconds; and 
     NOX_TIME_CONST is an empirically derived time constant associated with the release from the trap  36  of stored NO x , in seconds. 
     Because the storage capacity of the trap is typically limited as a function of trap temperature, the controller  14  also determines a temperature-based capacity value NOX_CAP_TEMP as a function of the trap temperature value TEMP_INIT at the time of the subsequent engine start-up, as follows: 
     
       
         NOX_CAP_TEMP=FNXXXX(TEMP_INIT)) 
       
     
     where: FNXXXX is a lookup table value of mapped values for trap capacity versus trap temperature T; and 
     TEMP_INIT is a value representing the instantaneous trap temperature T at the time of the subsequent engine start-up. 
     The controller  14  then estimates the amount NOX_STORED_INIT of NO x  stored in the trap  36  upon engine start-up as follows: 
     
       
         NOX_STORED_INIT=MIN(NOX_STORED_BLEED, NOX_CAP_TEMP) 
       
     
     where: MIN(x,y) is a function which selects the lower of the two values, x and y. 
     While, in the exemplary system  10 , the controller  14  preferably samples the output T generated by a temperature sensor to thereby obtain a detected value TEMP_INIT for use in determining the above trap initialization values for remaining NO x  and SO x , the controller  14  alternatively estimates the trap&#39;s temperature at the subsequent engine start-up, i.e., after a soak time SOAKTIME using an appropriate algorithm. By way of example only, an exemplary initialization routine for the trap temperature variable TEMP-INIT is preferably expressed as follows: 
     
       
         TEMP_INIT=((TEMP-PREVIOUS−AMBIENT)*FNEXP(−SOAKTIME/TEMP_TIME_CONST) 
       
     
     where: TEMP_PREVIOUS is a value for trap temperature T during the immediately preceding engine operating condition; 
     AMBIENT is a measured or inferred value representing current ambient temperature; 
     FNEXP is a lookup table value that approximates an exponential function; 
     SOAKTIME is the time elapsed since the engine was shut down, in seconds; and 
     TEMP_TIME_CONST is an empirically derived time constant associated with the cooling-off of the exhaust gas at an identified location on the trap  36 , in seconds. 
     While the best mode for carrying out the invention has 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.