Abstract:
A method for purging a catalyst containing oxidants operates the engine at different air-fuel ratios during different intervals. The intervals are adaptively adjusted based on a model that predicts an amount of fuel needed to perform the purging. The intervals are also responsive to an exhaust gas sensor located downstream of the catalyst.

Description:
This application is a continuation of U.S. patent application Ser. No. 09/544,318, filed Apr. 6, 2000 having the same assignee as the parent application, the entire contents of which is incorporated herein in its entirety by reference. As such, this application claims the benefit under 35 USC 120 of the filing date of the parent application indicated above. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device. 
     BACKGROUND OF THE INVENTION 
     Engine and vehicle fuel efficiency can be improved by lean burn internal combustion engines. To reduce emissions, these lean burn engines are coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, another three way catalyst known as a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter, where the NOx trap is optimized to further reduce NOx. The NOx trap typically stores NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry. 
     One method for controlling air-fuel ratio to release, or purge, stored NOx operates the engine rich until an air-fuel sensor downstream of the NOx trap indicates a rich air-fuel ratio. In other words, while the air-fuel ratio entering the NOx trap is rich, the output air-fuel ratio exiting the NOx trap will be near stoichiometry until a majority of the stored NOx is released. When the air-fuel ratio downstream becomes rich, there is little stored NOx and thus hydrocarbons are not used to reduce NOx and exit. Stated another way, the NOx trap is purged of stored NOx. Then, the engine air-fuel ratio can again become lean and the NOx trap can again store NOx. Such a system is described in EP 733786. 
     The inventors herein have recognized a disadvantage of the above approach. In particular, when the air-fuel sensor is place downstream of the NOx trap, there is always extra fuel used. In other words, since there is a delay from when fuel is injected until it reaches the air-fuel sensor, there will always be a certain amount of rich exhaust in the exhaust system when a purge is ended. All of the fuel in this bit of rich exhaust is excess and degrades fuel economy. 
     As an attempt to solve the above disadvantages, another approach is to place the air-fuel sensor somewhere in the NOx trap. In other words, the air-fuel sensor may be place at a location two-thirds from the from of the NOx trap. In this way, there is still some catalyst material after the air-fuel sensor to use the excess fuel in the rich exhaust. 
     The inventors herein have recognized a further disadvantage with the above approach. In particular, to obtain optimum performance, the sensor location is dependent on exhaust mass flow. Stated another way, at high exhaust mass flows, the sensor should be located closer to the front of the catalyst since a greater amount of fuel will be stored in the exhaust. Similarly, at low exhaust mass flows, the sensor should be located closer to the rear of the catalyst. Since only a single location is practical, performance is degraded. 
     SUMMARY OF THE INVENTION 
     An object of the invention claimed herein is to provide a method for controlling an engine during emission control device purging. 
     The above object is achieved, and disadvantages of prior approaches overcome, by claim 1. 
     By using a less rich value to complete purging of the emission control device, only a small amount of fuel is stored in the exhaust system when a purge completion signal is obtained. Thus, minimal emissions are produces during purging. Also, total purge time is minimized since most purge fuel is supplied at the richer air-fuel ratio. 
     An advantage of the above aspect of the present invention is that over purging is minimized 
     Another advantage of the above aspect of the present invention is that fuel economy is optimized while excess rich emissions are also minimized. 
     In another aspect of the present invention, the disadvantages of prior approaches are overcome by a method for controlling an internal combustion engine coupled to an emission control device with an exhaust sensor coupled downstream of the emission control device, the method comprising: operating the engine at a lean air-fuel ratio during a first interval, operating the engine at a first rich air-fuel ratio during a second interval following said first interval, and operating the engine at a second rich air-fuel ratio during a third interval following said second interval, wherein a duration of said second interval is based on a parameter indicative of a fuel quantity used during previously performed second and third intervals. 
     By adaptively adjusting the first rich interval, it is possible to account for catalyst again, while minimizing the rich operating time. 
     Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Description of Preferred Embodiment, with reference to the drawings, wherein: 
     FIGS. 1 and 2 are block diagrams of embodiments wherein the invention is used to advantage; 
     FIGS. 3-6 are high level flow charts of various operations performed by a portion of the embodiments shown in FIGS. 1 and 2; and 
     FIG. 7 is a graph illustrating operation according to the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Direct injection spark ignited internal combustion engine  10 , comprising a plurality of combustion chambers, is controlled by electronic engine controller  12  as shown in FIG.  1 . Combustion chamber  30  of engine  10  includes combustion chamber walls  32  with piston  36  positioned therein and connected to crankshaft  40 . In this particular example, piston  36  includes a recess or bowl (not shown) to help in forming stratified charges of air and fuel. Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valves  52   a  and  52   b  (not shown), and exhaust valves  54   a  and  54   b  (not shown). Fuel injector  66  is shown directly coupled to combustion chamber  30  for delivering liquid fuel directly therein in proportion to the pulse width of signal fpw received from controller  12  via conventional electronic driver  68 . Fuel is delivered to fuel injector  66  by a conventional high pressure fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail. 
     Intake manifold  44  is shown communicating with throttle body  58  via throttle plate  62 . In this particular example, throttle plate  62  is coupled to electric motor  94  so that the position of throttle plate  62  is controlled by controller  12  via electric motor  94 . This configuration is commonly referred to as electronic throttle control (ETC) which is also utilized during idle speed control. In an alternative embodiment (not shown), which is well known to those skilled in the art, a bypass air passageway is arranged in parallel with throttle plate  62  to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway. 
     Exhaust gas oxygen sensor  76  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . In this particular example, sensor  76  provides signal UEGO to controller  12  which converts signal UEGO into a relative air-fuel ratio λ. Signal UEGO is used to advantage during feedback air-fuel ratio control in a manner to maintain average air-fuel ratio at a desired air-fuel ratio as described later herein. In an alternative embodiment, sensor  76  can provide signal EGO (not show) which indicates whether exhaust air-fuel ratio is either lean of stoichiometry or rich of stoichiometry. 
     Conventional distributorless ignition system  88  provides ignition spark to combustion chamber  30  via spark plug  92  in response to spark advance signal SA from controller  12 . 
     Controller  12  causes combustion chamber  30  to operate in either a homogeneous air-fuel ratio mode or a stratified air-fuel ratio mode by controlling injection timing. In the stratified mode, controller  12  activates fuel injector  66  during the engine compression stroke so that fuel is sprayed directly into the bowl of piston  36 . Stratified air-fuel ratio layers are thereby formed. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. During the homogeneous mode, controller  12  activates fuel injector  66  during the intake stroke so that a substantially homogeneous air-fuel ratio mixture is formed when ignition power is supplied to spark plug  92  by ignition system  88 . Controller  12  controls the amount of fuel delivered by fuel injector  66  so that the homogeneous air-fuel ratio mixture in chamber  30  can be selected to be substantially at (or near) stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. Operation substantially at (or near) stoichiometry refers to conventional closed loop oscillatory control about stoichiometry. The stratified air-fuel ratio mixture will always be at a value lean of stoichiometry, the exact air-fuel ratio being a function of the amount of fuel delivered to combustion chamber  30 . An additional split mode of operation wherein additional fuel is injected during the exhaust stroke while operating in the stratified mode is available. An additional split mode of operation wherein additional fuel is injected during the intake stroke while operating in the stratified mode is also available, where a combined homogeneous and split mode is available. 
     Nitrogen oxide (NOx) absorbent or trap  72  is shown positioned downstream of catalytic converter  70 . NOx trap  72  absorbs NOx when engine  10  is operating lean of stoichiometry. The absorbed NOx is subsequently reacted with HC and catalyzed during a NOx purge cycle when controller  12  causes engine  10  to operate in either a rich mode or a near stoichiometric mode. 
     Controller  12  is shown in FIG. 1 as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values, shown as read-only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a conventional data bus. 
     Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurement of inducted mass air flow (MAF) from mass air flow sensor  100  coupled to throttle body  58 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  40  giving an indication of engine speed (RPM); throttle position TP from throttle position sensor  120 ; and absolute Manifold Pressure Signal MAP from sensor  122 . Engine speed signal RPM is generated by controller  12  from signal PIP in a conventional manner and manifold pressure signal MAP provides an indication of engine load. 
     In this particular example, temperature Tcat of catalytic converter  70  and temperature Ttrp of NOx trap  72  are inferred from engine operation as disclosed in U.S. Pat. No. 5,414,994, the specification of which is incorporated herein by reference. In an alternate embodiment, temperature Tcat is provided by temperature sensor  124  and temperature Ttrp is provided by temperature sensor  126 . 
     Fuel system  130  is coupled to intake manifold  44  via tube  132 . Fuel vapors (not shown) generated in fuel system  130  pass through tube  132  and are controlled via purge valve  134 . Purge valve  134  receives control signal PRG from controller  12 . 
     Exhaust sensor  140  is a sensor that produces two output signals. First output signal (SIGNAL1) and second output signal (SIGNAL2) are both received by controller  12  Exhaust sensor  140  can be a sensor known to those skilled in the art that is capable of indicating both exhaust air-fuel ratio and nitrogen oxide concentration. 
     In one embodiment, SIGNAL1 indicates exhaust air-fuel ratio and SIGNAL2 indicates nitrogen oxide concentration. In this embodiment, sensor  140  has a first chamber (not shown) in which exhaust gas first enters where a measurement of oxygen partial pressure is generated from a first pumping current. Also, in the first chamber, oxygen partial pressure of the exhaust gas is controlled to a predetermined level. Exhaust air-fuel ratio can then be indicated based on this first pumping current. Next, the exhaust gas enters a second chamber (not shown) where NOx is decomposed and measured by a second pumping current using the predetermined level. Nitrogen oxide concentration can then be indicated based on this second pumping current. 
     Referring now to FIG. 2, a port fuel injection engine  11  is shown where fuel is injected through injector  66  into intake manifold  44 . Engine  11  is operated homogeneously substantially at stoichiometry, rich of stoichiometry, or lean of stoichiometry. Fuel is delivered to fuel injector  66  by a conventional fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail. 
     Those skilled in the art will recognize that the methods of the present invention can be used to advantage with either port fuel injected or directly injected engines. 
     Referring now to FIG. 3, a routine for controlling the engine is described. First, in step  300 , a determination is made as to whether the engine is operating lean. When the answer to step  300  is YES, the routine continues to step  310  where a determination is made as to whether a NOx purge cycle is required. Typically, a NOx purge cycle is required when an amount of NOx stored in trap  72  reaches a predetermined level, or when an amount of NOx discharged from trap  72  per distance reaches a predetermined value. When the answer to step  310  is YES, the routine continues to step  312  where engine  10  is operated at a first rich air-fuel ratio. In this way, NOx stored in trap  72  and catalyst  70  is reduced. Typically, first rich air-fuel ratio is about a relative air-fuel ratio of 0.7. Then, in step  314 , where a determination is made as to whether purge fuel used (pfu) is greater then upper fuel threshold hi_pg_fuel. Upper fuel threshold (hi_pg_fuel) is determined as described later herein with particular reference to FIG.  6 . In other words, when excess fuel delivered in the exhaust to trap  72  is greater than upper fuel threshold, engine operation is changed to operate at a second rich air-fuel ratio, usually about 0.9. However, second rich air-fuel ratio can range between 0.7 and 1. Determination of extra fuel (pfu) is described later herein with particular reference to FIG.  5 . 
     Continuing with FIG. 3, when the answer to step  314  is NO, the routine continues to step  316  to determine whether sensor  140  indicates rich. In other words, if purge fuel is overestimated and NOx is prematurely purged, the purge is ended in step  322 . Otherwise, in step  318 , the engine is then operated at the second rich air-fuel ratio. This operation is continued until sensor  140  indicates rich in step  320  and then the purge is ended in step  322 . Then, in step  324 , the NOx storage model is updated based on the total fuel used to purge trap  72  as described later herein with particular reference to FIG.  4 . 
     Thus, according to the present invention, during trap purging, the engine is first operated at a first rich air-fuel ratio until purge fuel used reaches threshold. Then, engine is first operated at a second rich air-fuel ratio until the trap is purged as indicated by a downstream air-fuel ratio sensor changing to rich. 
     Referring now to FIG. 4, in step  410 , a NOx estimation model is used to estimate NOx stored in trap  72  based on current operating conditions. These operating conditions include engine airflow, fuel injection amount, ignition timing, exhaust gas recirculation amount, engine speed, and temperatures. Then, in step  412 , an estimate of fuel required to purge to the stored NOx is determined at the start of the NOx purge. In general terms, a predetermined ratio as a function of trap  72  temperature is used to convert total stored NOx to a total required fuel amount estimate (efr). Then, the previously learned offset value (of) is subtracted to provide the adapted total required fuel amount estimate (lefr). This parameter is used as described later herein with particular reference to FIG. 6 to determine threshold (hi —pg   —fuel).    
     Continuing with FIG. 4, in step  414 , at the end of the trap purge, a new offset value is learned based on the total fuel used to complete the purge (pfu) (determine from the fuel injection pulse width, fpw) and the estimate of the total fuel required (efr) using the following equations: 
     
       
         
           of′=efr−pfu 
         
       
     
     
       
           of=fk*of+( 1− fk )* of′   
       
     
     where fk is a filter coefficient between zero and 1. 
     Then, in step  416 , total purge fuel used is reset to zero. 
     Referring now to FIG. 5, actual purge fuel used (pfu) is determined. First, in step  510 , a determination is made as to whether NOx purge has begun. When the answer to step  510  is YES, the routine continues to step  512 . In step  512 , purge fuel used is incremented based on the excess fuel supplied to the exhaust over the last sample interval as described in the equations below.          Δ                 f     =         m   air          (     1   -   λ     )         λ                   λ   s                                
     where Δf is the total fuel injected during the sample interval based on fuel pulse width (fpw), 
     m air  is the air charge for the current the sample interval, 
     λ is the engine relative air-fuel ratio, and 
     λ s  is the stoichiometric air-fuel ratio. 
     The integrated excess fuel is determined as: 
     
       
         
           pfu=pfu+Δf 
         
       
     
     This process is repeated until the purge cycle has ended as represented by step  514 . 
     Referring now to FIG. 6, in step  610  fuel threshold (hi_pg_fuel) is determined as a percentage (K 1 ) of the adapted total required fuel amount estimate (lefr). Typically, the percentage is greater than 50%. Thus, when the total excess fuel supplied to the exhaust (pfu) reaches a predetermined percentage of the adapted estimate of the total required to complete the purge, the engine air-fuel ratio is made less rich. Thus, when the air-fuel ratio downstream of trap  72  switches to rich, only a small amount of excess fuel is in the exhaust and over-purging is minimized. Stated another way, less extra fuel is used the the air-fuel ratio is only slightly rich at the end of the purge. However, purge time is still kept short since a majority of the purge is done at the first, richer, air-fuel ratio. 
     Referring now to FIG. 7, an example of operation according to the present invention is now described. In the upper graph, engine air-fuel ratio is shown versus time. At time t1, during the first interval, the engine is operating lean and NOx trap  72  is storing NOx. Similarly, sensor  140  is indicating a lean air-fuel ratio. At time t2, during the second interval, the engine is operated at the first rich air-fuel ratio until time t3. At time t3, during the third interval, purge fuel provided reaches a percentage of estimated total fuel required and the engine is operated at the second rich air-fuel ratio, which is closer to stoichiometry. Then, at time t4, a rich signal is provided by sensor  140  indicating purge completion and the engine is again operated lean. The cycle can then repeat. 
     Although several examples of embodiments which practice the invention have been described herein, there are numerous other examples which could also be described. The invention is therefore to be defined only in accordance with the following claims.