Patent Publication Number: US-6990799-B2

Title: Method of determining emission control system operability

Description:
This application is a divisional of Ser. No. 09/528,220 filed Mar. 17, 2000, now U.S. Pat. No. 6,810,659. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for determining degradation in an emission control system. 
     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, additional three-way catalyst, sometimes referred to 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 to provide emission control in a lean burn engine uses a sensor downstream of the NOx trap. The sensor is capable of measuring an amount of NOx in exhaust gas exiting the NOx trap. Engine air-fuel ratio is changed from lean of stoichiometry to rich of stoichiometry when measured NOx reaches a predetermined threshold. Such a method is described in U.S. Pat. No. 5,942,199. 
     In particular, the sensor described in U.S. Pat. No. 5,942,199 is capable of measuring both NOx and oxygen partial pressure in exhaust gas. The sensor has a first chamber where exhaust gas first enters and 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. Next, the exhaust gas enters a second chamber where NOx is decomposed and measured by a second pumping current, knowing the predetermined level. 
     The inventors herein have recognized a disadvantage with the above approach. In particular, if the output of the NOx sensor inadvertently indicates a NOx concentration greater than the predetermined threshold when NOx concentration is actually less that the threshold, lean operation will be ended prematurely. In other words, lean operation will be ended when it is actually possible to continue lean operation. 
     SUMMARY OF THE INVENTION 
     An object of the invention claimed herein is to provide a system and method for determining degradation in an emission control system. The above object is achieved, and disadvantages of prior approaches overcome, by a method for determining degradation in an emission control system coupled to an internal combustion engine comprising: providing first and second signals from a sensor coupled downstream of the engine, said first output signal and said second output signal respectively indicative of a first exhaust gas constituent flowing through the emission control system and a second exhaust gas constituent flowing through the emission control system; and determining degradation of said second output signal based on said first output signal. 
     Since a single sensor mutually supplies two signals, it is possible to determine when one signal has degraded based on the other signal. For example, in one aspect of the invention, the sensor supplies indications of both oxygen partial pressure and nitrogen oxide. In this example, exhaust gases must first be affected to provide an indication of oxygen partial pressure. Then, the affected exhaust gases are used to provide an indication of nitrogen oxide. By observing the first output, oxygen partial pressure, it is possible to determine whether the second output, nitrogen oxide, is degraded. In other words, if the first output is degraded, the second output will also be degraded. This is because the functioning of the second output relies on operability of the first output. 
     An advantage of the above aspect of the present invention is improved control since it is possible to determine operability of sensors. 
     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. 1–2  are block diagrams of an embodiment wherein the invention is used to advantage; and 
         FIGS. 3–15  are high level flow charts of various operations performed by a portion of the embodiment shown in  FIG. 1 . 
     
    
    
     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 (SIGNAL 1 ) and second output signal (SIGNAL 2 ) 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 a preferred embodiment, SIGNAL 1  indicates exhaust air-fuel ratio and SIGNAL 2  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. 
     In one aspect of the present invention, a determination of degradation of the nitrogen oxide concentration measurement can be made if it is determined that the exhaust air-fuel ratio measurement is degraded. This is because nitrogen oxide concentration is not accurately detected in the second chamber unless the first chamber controls oxygen partial pressure properly. In other words, if it is determined that operation of the first chamber (where partial pressure of oxygen is measured) is degraded, then it is possible to determine that the second signal (SIGNAL 2 ) indicating nitrogen oxide concentration is degraded as described later herein with particular reference to  FIG. 13 . 
     Referring now to  FIG. 2 , a port fuel injection engine  10  is shown where fuel is injected through injector  66  into intake manifold  44 . Engine  10  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, in view of this disclosure, that the methods of the present invention can be used to advantage with either port fuel injected or directly injected engines. 
     Referring now to  FIGS. 3–5 , routines for determining performance impacts of operating in various engine operating conditions are described. In a preferred embodiment, performance impact is a fuel economy percentage impact over stoichiometric operation. The impact can be a benefit, where fuel is saved over stoichiometric operation, or a fuel loss. In other words, the following routines determine fuel economy saved relative to stoichiometric operation or fuel economy lost relative to stoichiometric operation. However, those skilled in the art will recognize in view of this disclosure various other performance impacts that can be used to compare different operation modes such as, for example, fuel usage impact, fuel efficiency impact, fuel savings, fuel loss, engine efficiency impact, fuel savings per distance traveled by the vehicle, or a driveability impact. 
     Referring now specifically to  FIG. 3 , a routine is described for determining a maximum fuel economy benefit that can be provided when operating lean, assuming that emission control device  72  has been decontaminated. More specifically in a preferred embodiment, that a sulfur decontamination has been completed. In other words, the routine determines the maximum potential fuel economy benefit that can be provided after performing a decontamination cycle. First, in step  308 , counter j is reset equal to zero. Next, in step  310 , a determination is made as to whether a decontamination cycle has just been completed. A decontamination cycle, as described herein, refers to any operating cycle where engine operating conditions are changed to remove a contaminant. For example, a sulfur decontamination cycle where exhaust gas temperature is raised and the engine is operated substantially at or rich of stoichiometry to remove sulfur contaminating emission control device  72  is one such decontamination cycle. When the answer to step  310  is YES, the routine continues to step  312  where parameter OLD — FE — MAX is set equal to parameter FILTERED — FE — MAX. Also, in step  312 , counter j is set equal to one. Counter j keeps track of the number of NOx fill/purge cycles after a decontamination cycle over which the maximum fuel economy benefit is average. Next, in step  314 , a determination is made as to whether a NOx fill/purge cycle has just been completed. When the answer to step  314  is YES, a determination is made as to whether counter j equals one. When the answer to step  316  is YES, the routine continues to step  318 . In step  318 , the routine calculates temporary fuel economy benefit (FE — TEMP j ) based on current fuel economy benefit (FE — CUR), where current fuel economy benefit is calculated as described below. This temporary fuel economy benefit represents the fuel economy benefit averaged over a NOx fill/purge cycle that is achieved compared to operating the engine substantially at stoichiometry. Next, in step  320 , maximum fuel economy benefit (FE — MAX) is calculated based on temporary fuel economy benefit. Next, in step  322 , counter j is incremented. Next, in step  324 , a determination is made as to whether counter j is greater than predetermined value J 1 . Predetermined value J 1  represents the number of NOx fill/purge cycles after a decontamination cycle over which maximum fuel economy benefit, provided by lean operation relative to stoichiometric operation, is calculated. In a preferred embodiment, predetermined value J 1  represents the number of NOx fill/purge cycles after a decontamination cycle over which maximum fuel economy benefit, is averaged. This averaging allows variations in vehicle operating conditions to be accounted in determining maximum fuel economy benefit so that a representative value is obtained. When the answer to step  324  is YES, the filtered maximum fuel economy benefit (FIL — FE — MAX) is set equal to maximum fuel economy benefit. 
     Continuing with  FIG. 3 , when the answer to step  316  is NO, temporary fuel economy benefit (FE — TEMP j ) is calculated in step  326  based on current fuel economy benefit (FE — CUR). Current fuel economy benefit (FE — CUR) represents the current fuel economy benefit relative to stoichiometric operation provided by lean operation and is calculated based on operating conditions. In particular, as described in U.S. patent application Ser. No. 09/528,217, titled “METHOD AND APPARATUS FOR CONTROLLING LEAN BURN ENGINE BASED UPON PREDICTED PERFORMANCE IMPACT”, filed concurrently with the present application on Mar. 17, 2000, assigned to the same assignee as the present application, which is hereby expressly incorporated by reference, a performance impact is set as a percentage fuel economy benefit/loss associated with engine operation at a selected lean or rich operating condition relative to a reference stoichiometric operating condition at MBT, the controller  12  first determines whether the lean-burn feature is enabled. If the lean-burn feature is enabled as, for example indicated by the lean-burn running flag LB — RUNNING — FLG being equal to logical one, the controller  12  determines a first value TQ — LB representing an indicated torque output for the engine when operating at the selected lean or rich operating condition, based on its selected air-fuel ratio LAMBSE and the degrees DELTA — SPARK of retard from MBT of its selected ignition timing, and further normalized for fuel flow. Then, controller  12  determines a second value TQ — STOICH representing an indicated torque output for the engine  10  when operating with a stoichiometric air-fuel ratio at MBT, likewise normalized for fuel flow. In particular, TQ — LB is determined as a function of desired engine torque, engine speed, desired air-fuel ratio, and DELTA — SPARK. Further, TQ — STOICH is determined as a function of desired engine torque and engine speed. Next, the controller  12  calculates the lean-burn torque ratio TR — LB by dividing the first normalized torque value TQ — LB with the second normalized torque value TQ — STOICH. 
     Continuing, the controller  12  determines a value SAVINGS representative of the cumulative fuel savings to be achieved by operating at the selected lean operating condition relative to the reference stoichiometric operating condition, based upon the air mass value AM, the current (lean or rich) lean-burn air-fuel ratio (LAMBSE) and the determined lean-burn torque ratio TR — LB, wherein
 
SAVINGS=SAVINGS+( AM*LAMBSE* 14.65*(1 −TR )).
 
The controller  12  then determines a value DIST — ACT — CUR representative of the actual miles traveled by the vehicle since the start of the last trap purge or desulfurization event. While the “current” actual distance value DIST — ACT — CUR is determined in any suitable manner, in the exemplary system, the controller  14  determines the current actual distance value DIST — ACT — CUR by accumulating detected or determined instantaneous values VS for vehicle speed.
 
     Because the fuel economy benefit to be obtained using the lean-burn feature is reduced by the “fuel penalty” of any associated trap purge event, in the exemplary system, the controller  12  determines the “current” value FE — CUR for fuel economy benefit only once per NOx fill cycle. And, because the purge event&#39;s fuel penalty is directly related to the preceding trap “fill,” the current fuel economy benefit value FE — CUR is preferably determined at the moment that the purge event is deemed to have just been completed, as described below. 
     Continuing with  FIG. 3 , in step  328 , maximum fuel economy benefit is calculated as a function (f 1 ) of maximum fuel economy benefit and temporary fuel economy benefit. In this way, the fuel economy benefit provided by a decontaminated emission control device is filtered over several NOx fill/purge cycles. In a preferred embodiment, the filtering is performed by a rolling average function of the form in the following equation where (fk) is a filter coefficient between zero and one. Those skilled in the art will recognize, in view of this disclosure, this as a single pole low pass filter.
 
output=(1 −fk )output+( fk )input or,
 
output=(1 −fk )old — output+( fk )input, old — output=output
 
Thus, according to the present invention, it is possible to determine the fuel economy benefit provided by a decontaminated emission control device.
 
     Referring now to  FIG. 4 , a routine is described for determining the present, or current, fuel economy benefit that is being provided by operating lean of stoichiometry with the emission control device  72  in its present state, be it contaminated or decontaminated. First, in step  410 , a determination is made as to whether a NOx fill/purge cycle has just been completed. When the answer to step  410  is YES, the routine continues to step  412  where parameter OLD — FE — CUR is set equal to parameter FIL — FE — CUR. Next, in step  414 , the routine calculates the current fuel economy benefit (FE — CUR). Next, in step  416 , the routine calculates the filtered current fuel economy benefit (FIL — FE — CUR) based on a filtered value of the current fuel economy benefit, and parameter OLD — FE — CUR. In other words, the current fuel economy benefit (FIL — FE — CUR) represents the fuel economy benefit that will be realized if the system continues to operate as it currently does and no decontamination is performed. Accordingly, (FIL — FE — CUR) is the fuel economy benefit that will be achieved by not performing a decontamination cycle. 
     In a preferred embodiment, function (f 2 ) represents the rolling average function describe above herein. Thus, according to the present invention, a fuel economy benefit averaged over several NOx fill/purge cycle can be determined. This value can then be used to advantage in various ways since it indicates an on-line measure of the improved fuel economy performance provided by lean operation averaged to remove cycle to cycle variation. 
     Referring now to  FIG. 5 , a routine is described for determining a fuel economy penalty experienced by performing a decontamination cycle. More specifically, in a preferred embodiment, a decontamination cycle that removes SOx. First, in step  510 , a determination is made as to whether a decontamination cycle has just been completed. When the answer to step  510  is YES, the routine continues to step  512  where a fuel economy penalty is calculated. The current fuel economy penalty of the last decontamination cycle (CUR — FE — PENALTY) is calculated by dividing the excess fuel used to generate heat or the excess fuel used to operate in one condition compared to another condition by the distance between decontamination cycles. In other words, the penalty of performing a decontamination cycle is spread over the distance between two decontamination cycles. Next, in step  514 , a filtered fuel economy penalty is calculated by filtering the current fuel economy penalty according to function (f 3 ) which, in a preferred embodiment, represents the rolling average function describe above herein. Thus, according to the present invention, it is possible to determine the fuel economy penalty experienced by performing a decontamination cycle. In an alternative embodiment, the fuel economy penalty to perform a decontamination cycle can be set to a predetermined value. 
     Those skilled in the art will recognize, in view of this disclosure, various alterations of the present invention that achieve a similar result. For example, the average excess fuel used during several decontamination cycles can be divided by the total distance between all of the decontamination cycles, thereby providing an averaged fuel economy penalty for performing a decontamination cycle. 
     In an alternate embodiment, fuel economy penalty to perform a decontamination cycle can be stored as a function of vehicle and/or engine operating parameters. For example, fuel economy penalty can be stored versus vehicle speed and exhaust gas temperature experienced before performing said decontamination cycle. Those skilled in the art will recognize, in view of this disclosure, various other factors that affect a fuel economy penalty to perform a decontamination cycle such as, for example, engine speed, mass air flow, manifold pressure, ignition timing, air-fuel ratio, exhaust gas recirculation amount, and engine torque. 
     In yet another embodiment, fuel economy penalty can be determined as now described. First, controller  12  updates a stored value DIST — ACT — DSX representing the actual distance that the vehicle has traveled since the termination or “end” of the immediately-preceding desulfurization, or decontamination, event. Then, the controller  12  determines whether a desulfurization event is currently in progress. While any suitable method is used for desulfurizing the trap, an exemplary desulfurization event is characterized by operation of some of the engine&#39;s cylinders with a lean air-fuel mixture and other of the engine&#39;s cylinders with a rich air-fuel mixture, thereby generating exhaust gas with a slightly-rich bias. Next, the controller  12  determines the corresponding fuel-normalized torque values TQ — DSX — LEAN and TQ — DSX — RICH, as a function of current operating conditions. In particular, TQ — DSX — LEAN and TQ — DSX — RICH are determined as functions of desired engine torque, engine speed, desired air-fuel ratio, and DELTA — SPARK. Then, the controller  12  further determines the corresponding fuel-normalized stoichiometric torque value TQ — STOICH as a function of desired engine torque and engine speed. The controller  12  then calculates a cumulative fuel economy penalty value, as follows:
 
PENALTY=PENALTY+( AM/ 2 *LAMBSE* 14.65*(1−TR — DSX — LEAN))+( AM/ 2 *LAMBSE* 14.65*(1−TR — DSX — RICH))
 
Then, the controller  12  sets a fuel economy penalty calculation flag to thereby ensure that the current desulfurization fuel economy penalty measure FE — PENALTY — CUR is determined immediately upon termination of the on-going desulfurization event.
 
     If the controller  12  determines that a desulfurization event has just been terminated, the controller  12  then determines the current value FE — PENALTY — CUR for the fuel economy penalty associated with the terminated desulfurization event, calculated as the cumulative fuel economy penalty value PENALTY divided by the actual distance value DIST — ACT — DSX. In this way, the fuel economy penalty associated with a desulfurization event is spread over the actual distance that the vehicle has traveled since the immediately-prior desulfurization event. Next, the controller  12  calculates a rolling average value FE — PENALTY of the last m current fuel economy penalty values FE — PENALTY — CUR to thereby provide a relatively-noise-insensitive measure of the fuel economy performance impact of such desulfurization events. The value FE — PENALTY can be used in place of value FIL — FE — PENALTY. By way of example only, the average negative performance impact or “penalty” of desulfurization typically ranges between about 0.3 percent to about 0.5 percent of the performance gain achieved through lean-burn operation. Finally, the controller  12  resets the fuel economy penalty calculation flag FE — PNLTY — CALC — FLG, along with the previously determined (and summed) actual distance value DIST — ACT — DSX and the current fuel economy penalty value PENALTY, in anticipation for the next desulfurization event. 
     Referring now to  FIG. 6 , a routine is described for determining whether to commence, or begin, a decontamination cycle. First, in step  610 , a determination is made as to whether the maximum potential fuel economy benefit provided a decontaminated emission control device minus the current fuel economy benefit being provided by the decontamination cycle in its present condition is greater than the fuel economy penalty experienced by performing a decontamination cycle. In particular, the difference between parameter FIL — FE — MAX and parameter FIL — FE — CUR is compared to parameter FIL — FE — PENALTY. When the answer to step  610  is YES, the routine has determined that greater fuel economy can be provided by performing a decontamination cycle rather than continuing with operating the engine lean of stoichiometry and performing NOx fill/purge cycles. When the answer to step  610  is NO, the routine has determined that greater fuel economy can be provided by continuing operation in the present condition. In other words, operating with the emission control device in its present condition provides better fuel economy than attempting to improve operation of the emission control device by performing a decontamination cycle. Next, in step  612 , a determination is made as to whether normalized NOx storage ability (FIL — NOX — STORED) of the emission control device is less than limit value Cl. Normalized stored NOx (FIL — NOX — STORED) is calculated as described later herein with particular reference to  FIGS. 9 and 10 . When the answer to step  612  is YES, the routine continues to step  613  where a determination is made as to whether vehicle distance traveled since the last decontamination cycle is greater than limit distance (DISTANCE — LIMIT). When the answer to step  613  is YES, the routine continues to step  614  where a determination is made as to whether parameter A 1  is equal to one. Parameter A 1  is determined based on vehicle activity as described later herein with particular reference to  FIG. 7 . When the answer to step  614  is YES a decontamination cycle is begun in step  616 . The embodiment shown in  FIG. 6  is that for the example of a port fuel injected engine. In an alternate embodiment which can be used for direct injection engines, step  614  is eliminated. This is because in port fuel injected engines, it is challenging to provide well controlled decontamination temperatures under all operating conditions. However, in a direct injection engine, since fuel can be injected during the exhaust stroke to heat the exhaust system, decontamination can be performed at almost any time. 
     Referring now to  FIG. 7 , a routine is described for determining vehicle activity. First, in step  710 , a the routine calculates engine power (Pe). In a preferred embodiment, this is the actual engine power, however, in a preferred embodiment, desired engine power can be used. Also, various other parameters can be used in place of engine power, such as, for example: vehicle speed, engine speed, engine torque, wheel torque, or wheel power. Next, in step  712 , engine power (Pe) is filtered with a high pass filter G 1 (s), where s is the Laplace operator known to those skilled in the art, to produce high pass filtered engine power (HPe). Next, in step  714 , the absolute value (AHPe) of the pass filtered engine power (HPe) is calculated. In step  716 , the absolute value (AHPe) is low pass filtered with filter G 1 (s) to produce signal (LAHPe). In step  718 , adjustment factor K 1  calculated as a predetermined function g of signal (LAHPe). Then, in step  720 , a determination is made as to whether signal (LAHPe) is less than the calibration parameter (DESOX — VS — ACT — ENABLE — CAL). When the answer to step  720  is YES, parameter A 1  is set to one in step  722 . Otherwise, value A 1  is set to zero in step  724 . 
     Referring now to  FIG. 8 , a graph of function g shows how adjustment factor K 1  varies as a function of signal (LAHPe) in a preferred embodiment. As shown in the preferred embodiment, as vehicle activity increases, adjustment factor K 1  is reduced. As vehicle activity decreases, adjustment factor K 1  is increased to a maximum value of 0.7. 
     Referring now to  FIGS. 9 and 10 , a routine for determining NOx stored in an emission control device is described. In particular, the routine describes a method for determining a consistent measure of NOx stored that can be averaged over several NOx purge/fill cycles. First, in step  910 , a determination is made as to whether a NOx purge has just been completed. In an alternate embodiment, an additional check as to whether lean operation has commenced can also be used. When the answer to step  910  is YES, NOx stored estimated (NOX — STORED) is reset to zero in step  912 . In particular, the routine assumes that a complete NOx purge was completed and all stored NOx was removed. However, in an alternate embodiment, if only part of the NOx was purged, NOx stored in step  912  would be set to this partial value rather than zero. Next, in step  913 , flag Z is set to zero to indicate that the stored NOx value is not fully estimated. Next, in step  914 , a determination is made as to whether the engine is operating lean of stoichiometry. When the answer to step  914  is YES, the routine continues to step  916 . In step  916 , a calculation of feedgas NOx (NOX — FG) based on operating conditions is generated. In particular, feedgas NOx generated by the engine is calculated based on function (h 1 ) using operating conditions such as, for example, SIGNAL 1  (or desired air-fuel ratio of the engine), mass air flow ({dot over (m)}air), engine temperature (TENG), and engine speed (RPM). This feedgas NOx can then be used to represent the NOx entering Nox trap  72 . Those skilled in the art will recognize, in view of this disclosure, that various additional factors can be used such as factors that account for an NOx storage or reduction due to activity of three way catalyst  70 . 
     Continuing with  FIG. 9 , in step  918 , a determination is made as to whether the ratio of NOx exiting trap  72  to NOx entering trap  72  is greater than threshold C 2 . For example, threshold C 2  can be set to 0.65. When the answer to step  918  is NO, a NOx difference (NOX — DELTA) is calculated between NOx entering (NOX — FG) and NOx exiting (SIGNAL 2 ) in step  920 . Next, in step  922 , an accumulated NOx storage (NOX — STORED) is determined by numerically summing NOX difference (NOX — DELTA). When the answer to step  918  is YES, flag Z is set to one to indicate that a consistent measure of NOx stored has been completed and fully estimated. 
     Referring now to  FIG. 10 , in step  1012 , a determination is made as to whether a NOx purge has just been completed. When the answer to step  1012  is YES, the routine continues to step  1014 . In step  1014 , filtered normalized NOx stored (FIL — NOX — STORED) is calculated by filtering NOx stored (NOX — STORED) according to function (f 4 ) which, in a preferred embodiment, represents the rolling average function describe above herein. 
     Thus, according to the present invention, it is possible to calculate a value representing a consistant and normalized NOx storage value that can be used in determining degradation and determining whether to perform a decontamination cycle. 
     Referring now to  FIG. 11 , a routine is described for using first output signal (SIGNAL 1 ) of sensor  140  for performing closed loop air-fuel ratio control. First, in step  1110 , a determination is made as to whether the absolute value of the difference between SIGNAL 1  and stoichiometric air-fuel ratio (air — fuel — stoich) is greater than a predetermined difference (D 1 ). In other words, a determination is made as to whether the first output signal of exhaust sensor  140  is indicating an exhaust air-fuel ratio away from stoichiometry. When the answer to step  1110  is YES, the routine continues to step  1112 . In step  1112 , the routine determines an air-fuel error (afe) based on the difference between desired air-fuel ratio (air — fuel — desired) and the first output signal (SIGNAL 1 ). Next, in step  1114 , the routine generates fuel injection signal (fpw) based on the determined error (afe) and the cylinder charge (m — cyl — air) and desired air-fuel ratio (air — fuel — desired). In addition, function g 2  is used to modify the air-fuel error (afe) and can represent various control functions such as, for example, a proportional, integral and derivative controller. Also, function g 1  is used to convert the desired mass of fuel entering the cylinder into a signal that can be sent to fuel injector  66 . Also, those skilled in the art will recognize, in view of this disclosure, that various other corrections involving information from other exhaust gas sensors can be used. For example, additional corrections from sensor  76  can be used. 
     When the step  1110  is NO, the routine continues to step  1116  and calculates fuel injection signal (fpw) based on the cylinder charge amount and the desired air-fuel ratio using function g 1 . Thus, according to the present invention, it is possible to improve open-loop fueling control using the first output of sensor  140 , which is located downstream of NOx trap  72 , whenever the first output signal indicates a value away from stoichiometry. In this way, NOx storage and oxygen storage, as well as NOx reduction, do not adversely closed-loop feedback air-fuel control using a sensor located downstream of a NOx trap. 
     Referring now to  FIG. 12 , an alternate routine to that described in  FIG. 11  is shown. In this alternate routine, various timers are used to gate out the first output of exhaust sensor  140  for use in feedback air-fuel ratio control whenever it is determined that one of the following conditions is present: oxygen is being stored in NOx trap  72 , and/or nitrogen oxide is being released and reduced by a reducing constituent in the exhaust gas in NOx trap  72 . Also, this alternate embodiment can be used to advantage to determine when to enable monitoring of exhaust sensor  140  as described later herein with particular reference to  FIGS. 13 and 14 . 
     Continuing with  FIG. 12 , in step  1210 , a determination is made as to whether the desired air-fuel ratio (air — fuel — desired) has been changed. In particular, a determination is made as to whether the desired air-fuel ratio has changed from rich or stoichiometric to lean, or whether the desired air-fuel ratio has changed-from lean to stoichiometric or rich. When the answer to step  1210  is YES, the counter C 3  is reset to zero. Otherwise, in step  1214 , counter C 3  is incremented. Next, in step  1216 , a determination is made as to whether the desired air-fuel ratio is stoichiometric or rich. When the answer to step  1216  is YES, a determination is made as to whether counter C 3  is greater than threshold value D 2  in step  1218 . Otherwise, when the answer to step  1216  is NO, a determination is made as to whether counter C 3  is greater than threshold value D 3  in step  1220 . When the answer to either step  1218  or step  1220  is YES, the routine enables monitoring in step  1222 . 
     In other words, duration D 2  and duration D 3  represent periods before which first output of exhaust sensor  140  cannot be used for feedback control because it will indicate stoichiometric even when the exhaust air-fuel ratio entering NOx trap  72  is not stoichiometric. Thus, when changing from stoichiometric or rich to lean, first output of exhaust sensor  140  is valid for monitoring or feedback control after duration D 3 . Similarly, when changing from lean operation to rich or stoichiometric operation, first output of exhaust sensor  140  is valid for monitoring or feedback control after duration D 2 . In a preferred embodiment, duration D 2  is based on oxygen storage of trap  72  and duration D 3  is based on both oxygen storage and NOx storage of trap  72 . Stated another way, once the oxygen storage is saturated when changing from rich to lean, SIGNAL 1  is indicative of the air-fuel ratio entering trap  72 . And once the oxygen stored and NOx stored is reduced when changing from lean to rich, SIGNAL 1  is indicative of the air-fuel ratio entering trap  72 . 
     Continuing with  FIG. 12 , in step  1224 , a determination of air-fuel error (afe) is made by subtracting desired air-fuel ratio (air — fuel — desired) and first output of exhaust sensor  140  (SIGNAL 1 ). Next, in step  1226 , fuel injection signal (fpw) is calculated in a manner similar to step  1114 . 
     When the answers to either step  1218  or step  1220  are NO, the routine continues to step  1228  to calculate fuel injection signal (fpw) as described herein in step  1116 . Thus, according to the present invention, it is possible to utilize the first output of exhaust sensor  140  for feedback air-fuel control. 
     Referring now to  FIG. 13 , a routine is described for determining degradation of the second output signal of exhaust sensor  140 . In particular, a routine is described for determining degradation of indicated NOx concentration based on the first output signal of exhaust gas sensor  140 , when the first output signal is indicative of an exhaust air-fuel ratio. First, in step  1310 , a determination is made as to whether monitoring is enabled as described in step  1222 , or whether the engine is operating in a near stoichiometric condition. Further, a determination is also made as to whether the first output signal of exhaust sensor  140  is degraded. In other words, when SIGNAL 1  is indicative of the air-fuel ratio entering trap  72 , it can be used to provide an estimate of NOx concentration exiting trap  72 . When the answer to step  1310  is YES, the routine continues to step  1312 . In step  1312 , the routine estimates the second output signal (est — signal  2 ) based on several conditions. In particular, function h 2  is used with the feed gas NOx (NOx — fg) and the first output signal of exhaust sensor  140  (SIGNAL 1 ). In other words, the routine attempts to estimate NOx exiting trap  72  based on NOx entering trap  72  and exhaust air-fuel ratio. In addition, various other dynamic effects of NOx trap  72  can be added to account for oxygen storage and nitrogen oxide and oxygen reduction. Further, efficiency of trap  72  can be included to estimate NOx exiting based on NOx entering trap  72 . However, if performed during stoichiometric operation, it can be assumed that net NOx stored is constant. Next, in step  1314 , the absolute value of the difference between the estimated NOx exiting trap  72  (EST — SIGNAL 2 ) and measured second output of exhaust sensor  140  (SIGNAL 2 ) is compared to threshold value C 4 . When the answer to step  1314  is YES, counter C 5  is incremented in step  1316 . Next, in step  1318 , a determination is made as to whether counter C 5  is greater than threshold C 6 . When the answer to step  1318  is YES, the routine indicates degradation of the second output of exhaust sensor  140  in step  1320 . 
     Thus, according to the present invention, it is possible to determine when the NOx sensor, which is the second output of exhaust sensor  140 , has degraded by comparing to an estimated value of exiting NOx trap  72 . 
     Referring now to  FIG. 14 , a routine is described for determining degradation of the second output signal of sensor  140  based on the first output signal of sensor  140 . First, a determination is made in step  1410  as to whether monitoring has been enabled or whether operating near stoichiometry. When the answer to step  1410  is YES, the routine continues to step  1412 . In step  1412 , the routine estimates air-fuel ratio that should be measured by the first output signal (SIGNAL 1 ) of exhaust sensor  140 . In other words, the routine estimates exhaust air-fuel ratio exiting NOx trap  72  based on various operating parameters. The estimated air-fuel ratio (AFTP — EST) is estimated based on air-fuel ratio measured by sensor  76  (UEGO), mass airflow measured by mass airflow sensor  100 , and fuel injection amount (fpw) in a preferred embodiment. Those skilled in the art will recognize, in view of this disclosure, various other signals and methods that can be used to estimate exhaust air-fuel ratio exiting a NOx trap. For example, dynamic effects of both catalyst  70  and  72  can be included that account for NOx storage, oxygen storage, temperature effects, and various other effects known to those skilled in the art. 
     Continuing with  FIG. 14 , in step  1414 , the absolute value of the difference between the estimated exhaust air-fuel ratio (AFTP — EST) in the first output signal of exhaust gas sensor  140  (SIGNAL 1 ) is compared to threshold C 7 . When the answer to step  1414  is YES, counter C 8  is incremented in step  1416 . Next, in step  1418 , counter C 8  is compared to threshold C 9  in step  1418 . When the answer to step  1418  is YES, an indication is provided in step  1420  that both the first output signal and a second output signal of exhaust sensor  140  have been degraded. Thus, according to the present invention, it is possible to determine that the NOx concentration measured by the second output signal of exhaust sensor  140  is degraded when it is determined that the oxygen partial pressure indicated in the first output signal of exhaust sensor  140  has been degraded. 
     Referring now to  FIGS. 15A–15C , these figures show an example of operation according to the present invention. In particular, the graphs show when first output signal (SIGNAL 1 ) of sensor  140  is valid for air-fuel control or for monitoring.  FIG. 15A  shows air-fuel ratio entering NOx trap  72  versus time.  FIG. 15B  shows air-fuel ratio exiting NOx trap  72  versus time.  FIG. 15C  indicates whether first output signal (SIGNAL 1 ) of sensor  140  is valid for air-fuel control or for monitoring. 
     Before time t 1 , the entering air-fuel ratio and exiting air-fuel ratio are both lean and first output signal (SIGNAL 1 ) is valid for control or monitoring. Then, at time t 1 , a determination is made to end lean operation and purge NOx stored in trap  72  due to tail-pipe grams of NOx/mile, or because a fuel economy benefit is no longer provided by operating lean, or for various other reasons as described above herein. At time t 1 , entering air-fuel ratio is changed from lean to rich. Similarly, at time t 1 , air-fuel ratio exiting changes to stoichiometric until all stored NOx and oxygen are reduced, which occurs at time t 2 . Thus, according to the present invention, the stoichiometric air-fuel ratio measured downstream of NOx trap  72  during the interval from time t 1  to time t 2 , is not equal to the air-fuel ratio upstream of NOx trap  72 . After time t 2 , a rich exhaust air-fuel ratio is measured downstream of NOx trap  72  and this measurement can be used for air-fuel control or monitoring. At time t 3 , entering air-fuel is changed back to a lean air-fuel ratio. Again, air-fuel ratio exiting changes to stoichiometric until all the oxygen storage capacity of NOx trap  72  is saturated at time t 4 . Thus, according to the present invention, the stoichiometric air-fuel ratio measured downstream of NOx trap  72  during the interval from time t 3  to time t 4  is not equal to the air-fuel ratio upstream of NOX trap  72 . After time t 4 , the entering air-fuel ratio can be measured by sensor  140  and thus can be used for control or monitoring. 
     Those skilled in the art will recognize in view of this disclosure that the above methods are applicable with any decontamination method. In a preferred embodiment, the decontamination method described in U.S. Pat. No. 5,758,493, which is hereby incorporated by reference, can be used. 
     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.