Abstract:
A method and system for controlling the operation of “lean-burn” internal combustion engines determines a current rate of vehicle NO x  emissions, and determines a threshold value for permissible vehicle NO x  emissions based on at least one current value for the intake air-fuel ratio, engine speed, engine load (e.g., brake torque, manifold air pressure, or throttle position), and/or vehicle speed. A differential NO x  emissions rate is calculated as the difference between the current rate and the threshold rate, and the differential rate is accumulated over time to obtain a differential measure representing the amount by which cumulative NO x  emissions have fallen below permissible levels therefor. Lean engine operation is disabled when the differential NO x  emissions measure exceeds a predetermined excess vehicle NO x  emission value. In this manner, vehicle NO x  emissions are favorably controlled even when the engine is operated “off-cycle,” i.e., under operating conditions falling outside of the FTP driving cycles.

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 x ) 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 engine-generated constituent gases, such as HC, CO and NOx, 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 NOx when the exhaust gases are “lean” of stoichiometry and release 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. Specifically, a controller accumulates estimates of feedgas NO x  over time to obtain a measure representing total generated NO x . The controller discontinues the lean operating excursion when the total generated 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. 
     Unfortunately, empirical evidence suggests that the instantaneous storage efficiency of the trap, i.e., the trap&#39;s instantaneous ability to absorb all of the NO x  being generated by the engine, rarely approaches 100 percent. Indeed, as the trap begins to fill, the instantaneous storage efficiency of the trap appears to decline significantly, with an attendant increase in the amount of NO x  being exhausted to the atmosphere through the vehicle&#39;s tailpipe. While increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event&#39;s enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen stored previously stored in the three-way catalyst during lean engine operation, would rapidly negate the fuel savings associated with lean engine operation. 
     Moreover, under certain engine operating conditions, for example, under high engine speed and high engine load, the NO x  generation rate and correlative exhaust flow rate through the trap are both so high that the trap does not have an opportunity to store all of the NO x  in the exhaust, even assuming a 100 percent trap storage efficiency. As a result, such operating conditions are themselves typically characterized by a significant increase in tailpipe NO x  emissions, notwithstanding the use of the NO x  trap. 
     For a majority of motor vehicles, the effectiveness of a given method and system for controlling tailpipe NO x  emissions is generally measured by evaluating the vehicle&#39;s performance in a standardized test under the Federal Test Procedure (FTP), in which the vehicle is operated in a prescribed manner to simulate a variety of engine operating conditions, at a variety of different engine-speed and engine-load combinations. A graphical illustration of the various engine speed/load combinations achieved during the FTP City Driving Cycle is depicted as Region I in FIG. 5, while the various engine-speed and engine-load combinations achieved during the FTP Highway Driving Cycle are depicted in FIG.  5 . 
     During either FTP test, vehicle NO x  emissions, as measured by a NO x  sensor, are accumulated over the course of a thirty-minute test period. The vehicle is deemed to have passed the test if the accumulated value of tailpipe NO x , in grams, does not exceed a prescribed threshold amount. Often, the prescribed threshold amount of permissible NO x  emissions under the Highway Driving Cycle is characterized as a multiple of the prescribed threshold amount for the City Driving Cycle. 
     The NO x  emissions of certain other motor vehicles, such as heavy trucks, are measured using another approach, wherein the vehicle&#39;s engine is independently certified on a dynamometer, with the engine&#39;s instantaneous NOx emissions thereafter being normalized by the engine&#39;s peak horsepower, in grams per horsepower-hour. In either event, such emissions standards are said to be “scalar,” i.e., fixed or static, rather than dynamic. 
     Significantly, it has been observed that, while the FTP City and Highway Driving Cycles include the vast majority of operating conditions over which a given motor vehicle is likely to be operated, the Cycles themselves are not necessarily representative of the manner in which most vehicles are operated. For example, it is generally true that an engine generates increased NO x  emissions under operating conditions characterized by increased engine speeds and increased engine loads. Thus, each FTP cycle necessarily permits its relatively lower NO x -generating operating conditions to offset its relatively higher NO x -generating operating conditions, with a vehicle “passing the test” so long as the average generated NO x  does not rise to the level at which the total generated NO x  exceeds the prescribed threshold after thirty minutes. 
     In contrast, in “real world” operation, a given engine operating condition, such as a “highway cruise” operating condition characterized by substantially-higher instantaneous rates of NOx generation, may continue unabated for substantial periods of time. Such continued operation of the engine, even at an engine speed/load falling within Region I or Region II of FIG. 5, is properly characterized as being “off-cycle.” Similarly, certain circumstances, such as the towing of a large trailer, or operation of the vehicle at relatively higher altitudes, may push the operating point of the engine fully outside of Regions I and II. Engine operation under these circumstances (with engine speed/loads falling in the area generally depicted as Region III in FIG. 5) are likewise properly characterized as being “off-cycle.” And, because off-cycle operation may constitute a substantial portion of any given driving session, the FTP cycles do not necessarily predict the likely real-world emissions of a given vehicle. 
     Therefore, a need exists for a method and system for controlling the operation of a “lean-burn” internal combustion engine which seeks to regulate all vehicle NO x  emissions, including “off-cycle” NO x  emissions. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a method is provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NO x , and wherein exhaust gas is directed through an exhaust gas purification system including a NO x  trap before being exhausted to the atmosphere. Under the invention, the method includes determining a current rate at which NO x  is being exhausted to the atmosphere; determining a threshold rate for maximum permissible NO x  emissions as a function of at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position; and determining a differential rate based on the current rate and the threshold rate. The method further includes selecting a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio. By way of example, in an exemplary embodiment, the restricted range of engine operating conditions is selected when an accumulated measure based on the differential rate exceeds a near-zero threshold value. 
     In accordance with a feature of the invention, determining the current rate is achieved either by sampling the output signal generated by a NO x  sensor positioned downstream of the NO x  trap or, alternatively, calculating the current rate by determining a generation rate representative of the NO x  content of the exhaust being instantaneously generated by the engine, determining a storage rate representative of an instantaneous rate at which NO x  is being stored by the trap, and subtracting the storage rate from the generation rate. 
     In accordance with another feature of the invention, an exemplary method for practicing the invention further includes calculating a cumulative amount of NO x  stored in the trap using the current rate; and selecting the first region of engine operating conditions when the cumulative amount exceeds a trap capacity value. Preferably, the method further includes determining the trap capacity value as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio. 
     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 
     FIG. 1 is a schematic of an exemplary system for practicing the invention; 
     FIG. 2 a flow diagram generally illustrating the broad method steps of an exemplary method for practicing the invention; 
     FIG. 3 is a plot generally illustrating an alternative tailpipe NO x  threshold, calculated using a series of ordered pairs as a function of vehicle speed; 
     FIGS. 4A-4D are a collection of related plots respectively illustrating vehicle speed VS versus time, intake air-fuel ratio λ versus time, current and threshold NO x  emissions rates versus time, and the differential measure versus time; and 
     FIG. 5 is a plot of engine load versus engine speed illustrating regions of engine operating conditions falling within and without certain FTP test cycles. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an exemplary control system  10  for a four-cylinder gasoline-powered engine  12  for a motor vehicle includes an electronic engine controller  14  having ROM, RAM and a processor (“CPU”) as indicated. The controller  14  controls the operation of each of a set of fuel injectors  16 . 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 (1=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 in a known manner. The trap  36  alternately stores and releases amounts of engine-generated NO x , based upon such factors as intake air-fuel ratio, trap temperature T (as determined by trap temperature sensor  38 ), percentage exhaust gas recirculation, barometric pressure, humidity, instantaneous trap “fullness,” instantaneous sulfur poisoning, and trap aging effects (due, for example, to permanent thermal aging, or to the “deep” diffusion of sulfur into the core of the trap material which cannot subsequently be purged). A NO x  sensor  40  of any suitable configuration is positioned downstream of the trap  36 . The NO x  sensor  40  generates a control signal representative of the instantaneous NO x  content of the exhaust gas exiting the tailpipe  42  and exhausted to the atmosphere during engine operation. 
     A flow chart generally illustrating an exemplary method of practicing the invention in connection with the exemplary system  10  is illustrated in FIG.  2 . Upon initialization, which typically occurs no later than the commencement of a trap purge event, 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 . The controller  14  then samples the output signal generated by the NO x  sensor  40  to obtain a current rate R c  for vehicle NO x  emissions. 
     The controller  14  also determines a threshold rate R t  for permissible vehicle NO x  emissions based on at least one of the group consisting of an engine speed N, a vehicle speed VS, an engine brake torque Tq, an engine manifold air pressure MAP, and a throttle position. In accordance with the invention, the threshold rate R t  preferably characterizes permissible vehicle NO x  emissions in terms of a gross vehicle output and/or another basic input to NO x  generation, for example, as may be defined as a function of engine speed N and engine brake torque Tq, or of engine speed in combination with another measure of engine load (e.g., engine manifold air pressure MAP or throttle position), such that the threshold rate R t  increases to permit greater NO x  emissions with increasing gross vehicle outputs. Characterization of the threshold rate R t  in terms of a gross vehicle output is preferred because such a determination inherently considers secondary inputs to NO x  generation, including but not limited to variations in grade and barometric pressure, use of vehicle accessories, and trailering. 
     The invention also contemplates use of any suitable approximation for gross vehicle output in the determination of the threshold rate R t . Thus, in the exemplary method illustrated in FIG. 2, the controller  14  calculates the threshold rate R t  as a function of instantaneous vehicle speed VS, with a minimum threshold rate R tMIN  established when the vehicle speed VS falls below a minimum value VS MIN . A calibratable factor is also preferably used, for example, to allow for an increase in the determined threshold rate R t  in the event that increased NO x  emissions are permitted due to a correlative reduction in other regulated emissions, e.g., in vehicle CO 2  emissions. 
     While the invention contemplates implementing the determination of the threshold rate R t  in any suitable manner, for example, through real-time estimation or through the use of modeled values stored in a ROM look-up table, in the exemplary system  10 , the controller  14  calculates the threshold rate R t  as a function of instantaneous vehicle speed using a piecewise linearized function of “n” ordered pairs, as illustrated in FIG.  3 . 
     In accordance with another feature of the invention, the controller  14  determines a differential rate ΔR for vehicle NO x  emissions by subtracting the threshold rate R t  from the current rate R c . The differential rate ΔR represents the rate at which current NO x  emissions exceed the permissible NO x  emissions, as determined from the current gross vehicle output. A negative value indicates relatively “clean” vehicle operation, with current NO x  emissions less than the determined permissible NO x  emissions. A positive value for the differential rate ΔR indicates that current NO x  emissions exceed the determined permissible NO x  emissions. 
     In accordance with another feature of the invention, the controller  14  accumulates the differential rate ΔR during the first time period to obtain a differential measure ΣΔR representing the amount by which current NO x  emissions have fallen below the determined permissible NO x  emissions. Thus, the differential measure ΣΔR provides a running total of the NO x -emissions “cushion” achieved during the first time period. And, because NO x  emissions cushions nearly always occur during a trap purge event (except under extremely high-speed/high-load engine operating conditions) and usually continue during the initial portion of a subsequent lean operating excursion (because of the trap&#39;s renewed storage efficiency), the invention permits the NO x  emissions cushions to be “banked” to later offset excessive NO x  emissions experienced during the latter portion of the first time period (when the trap operates with a reduced storage efficiency). 
     The controller  14  thereafter selects an engine operating condition based on the differential rate, for example, by comparing the differential measure ΣΔR to a near-zero threshold value, and discontinuing or disabling lean engine operation when the differential measure ΣΔR exceeds the near-zero threshold value. It will be appreciated that, by discontinuing or disabling 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. 
     FIGS. 4A-4D are a collection of related plots respectively illustrating vehicle speed VS versus time, intake air-fuel ratio λ versus time, and the NO x  emissions rates versus time, for two respective cycles as a theoretical vehicle is accelerated from a stop, through several vehicle speed changes, through a first “highway cruise” speed and, ultimately, to a second highway cruise speed. The relatively mild, near-stoichiometric acceleration beginning at point A of FIG. 4C produces current rates R c  of tailpipe NO x  emissions which are significantly less than the minimum threshold rate R tMIN . As the rate of acceleration is increased, the controller  14  temporarily enriches the air-fuel mixture supplied to each cylinder  18  in a known manner, thereby resulting again in relatively low levels of tailpipe NO x . Because, in the exemplary embodiment, the threshold rate R t  is conveniently calculated as a function of vehicle speed VS, the instantaneous NO x  emissions cushion increases, with “banked” cushions being represented by an increasingly negative accumulated value for the differential measure ΣΔR, as seen in FIG.  4 D. 
     As the rate of acceleration begins to rapidly decline, the controller  14  transitions through a near-stoichiometric operating condition to a first lean operating condition (beginning at point B of FIG.  4 C). Because of the relatively high vehicle speed and, correlatively, the relatively high air mass flow through the engine  12 , and with the trap  36  slowly being filled with stored NO x , the current rates R c  for vehicle NO x  emissions during the first lean operating condition ultimately begin to exceed the corresponding threshold rates R t  and the banked NO x  emissions cushion represented by the differential measure ΣΔR begins to fall slightly. At point C, the vehicle undergoes brisk deceleration, and the controller  14  temporarily “breaks out” of the first lean operating condition in a known manner in order to prevent engine roughness. At point D, the vehicle begins to slowly accelerate up to a “highway cruise” speed, and the controller  14  alters the air-fuel mixture supplied to each cylinder  18  to continue the first lean operating condition. 
     The current rates R c  for vehicle NO x  emissions quickly rise in excess of the respective threshold rates R t , with the resulting excess tailpipe NO x  emissions ultimately reducing the differential measure ΣΔR to a near-zero value at point E of FIG.  4 C. 
     Because continued lean engine operation beyond point E of FIG. 4C would result in a cumulative excess of vehicle NO x  emissions, the controller  14  discontinues or disables lean engine operation in favor of a near-stoichiometric operating condition or, more preferably as seen in FIG. 4B, a rich operating condition suitable for purging the trap of stored NO x . The run timer is also preferably reset by the controller  14  upon commencement of the purge event. As seen in FIG. 4D, the extremely low vehicle NO x  emissions incident to the trap purge event quickly restores the accumulated NO x  emissions cushion (represented by a high negative value for the differential measure ΣΔR). When the trap is purged of stored NO x , the controller  14  enables lean engine operation, as indicated at point F on FIG.  4 C. The trap slowly fills with stored NO x , whereupon the current rates R c  for NO x  emissions again slowly climb until the current rates R c  exceed the respective threshold rates R t  at point G of FIG.  4 C. The resulting excess tailpipe NO x  emissions again ultimately reduce the differential measure ΣΔR to a near-zero value at point H of FIG. 4C, whereupon the controller  14  discontinues the second lean operating condition in favor of a second trap purge event. 
     In accordance with another feature of the invention, the controller  14  determines a generation rate R g  at which the engine  12  generates NO x , and calculates a trap storage rate R s  based on the difference between the generation rate and the current rate R c , preferably using a suitable delay to accommodate time lag introduced by the exhaust gas purification system  32 . The controller  14  discontinues lean engine operation when an accumulated measure ΣR s  based on the trap storage rate R s  exceeds a trap capacity value, which may itself be determined in real time by the controller  14  as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio. 
     Significantly, if the controller  14  discontinues lean engine operation based upon the trap storage measure ΣR s  while the differential measure ΣΔR continues to register a NO x  emissions cushion, the controller  14  may, where appropriate, temporarily delay the trap purge event in favor of near-stoichiometric engine operation, until such time as the differential measure ΣΔR is again reduced to the near-zero threshold value therefor. Conversely, if the controller  14  discontinues lean engine operation based on the differential rate ΔR when the trap is not otherwise filled with stored NO x  (as may occur when operating the engine at extremely high-engine-speed/high-engine-load combinations), the trap storage measure ΣR s  is preferably used in an open-loop calculation of the purge time to be used in the ensuing trap purge event. 
     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.