Patent Publication Number: US-11378028-B2

Title: System and method for diagnosing cylinder deactivation

Description:
FIELD 
     The present description relates to a system and methods for diagnosing operation of cylinder deactivation mechanisms. The system and methods may determine degradation of cylinder deactivating devices based on engine air charge estimates. 
     BACKGROUND AND SUMMARY 
     An engine may include one or more devices that deactivate intake valves and/or exhaust valves in a closed state so that one or more cylinders may be temporarily deactivated. By deactivating the one or more cylinders, the engine may operate in a variable displacement mode to reduce fuel consumption. For example, an eight cylinder engine may operate with four deactivated cylinders and four activated cylinders when driver demand torque is low. The engine may operate at a higher intake manifold pressure for a given engine speed and driver demand torque when four cylinders are deactivated as compared to if all eight cylinders were operated at the same given engine speed and driver demand torque. The higher intake manifold pressure allows the four active cylinders to generate the same torque as all eight cylinders at the given engine speed and driver demand torque. Increasing the intake manifold pressure reduces engine pumping losses, thereby increasing engine efficiency. However, it may be possible for a device that deactivates poppet valves of a cylinder to degrade such that intake and exhaust valves continue to operate while fuel flow to the cylinder is deactivated. Such a condition may cause excess air flow to catalysts in the engine&#39;s exhaust system, which may degrade emissions. Therefore, it may be desirable to provide a way of assessing whether or not degradation of a valve deactivation device has occurred. 
     The inventors herein have recognized the above-mentioned issues and have developed an engine control method, comprising: estimating an air charge of an engine according to data generated when one or more of an engine&#39;s cylinders are deactivated via a controller, where the air charge is adjusted according to an adaptive term determined from data generated when all of the engine&#39;s cylinders are activated; and adjusting engine operation according to the estimate. 
     By learning operating characteristics of an engine&#39;s cylinders while an engine is operating as is expected with all of its cylinders, it may be possible to provide the technical result of reducing false positive indications of degraded valve deactivators when one or more cylinders are commanded deactivated. In particular, it may be possible to reduce the influence of noise sources that may cause a control system to conclude that valve deactivators are degraded based on engine air charge or flow as determined from engine air-fuel ratio. In one example, the noise sources may include manifold absolute pressure (MAP) sensor signal offsets and errors in the determination of percentage ethanol included in gasoline. 
     The present description may provide several advantages. In particular, the approach may reduce false positive indications of valve deactivator degradation. Further, the approach may be provided without increasing system cost. In addition, the approach may be robust over a range of engine loads. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, where: 
         FIG. 1  is a schematic diagram of an engine; 
         FIG. 2A  is a schematic diagram of an eight cylinder engine with two cylinder banks; 
         FIG. 2B  is a schematic diagram of a four cylinder engine with a single cylinder bank; 
         FIG. 3  is plot that shows air-fuel ratio error as a function of engine load; 
         FIG. 4  shows a flow chart of an example method for operating an engine; and 
         FIG. 5  shows an example engine operating sequence according to the method of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to improving detection of valve deactivating devices. The valve deactivating devices may hold intake and exhaust valves in closed positions throughout an engine cycle so that air does not flow through the engine via deactivated cylinders. The engine may be of the type shown in  FIGS. 1-2B . The load that the engine is operating at may affect the engine&#39;s air-fuel ratio as shown in  FIG. 3 . The method of  FIG. 4  may reduce influence of signal noise sources so that a more reliable assessment of valve deactivation devices may be provided. The method of  FIG. 4  may provide an engine operating sequence as shown in  FIG. 5  to assess whether or not a valve deactivating device is degraded. 
     Referring to  FIG. 1 , internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in  FIG. 1 , is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . 
     Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Each intake and exhaust valve may be operated by a variable intake valve operator  51  and a variable exhaust valve operator  53 , which may be actuated mechanically, electrically, hydraulically, or by a combination of the same. For example, the valve actuators may be in a roller finger follower configuration or of the type described in U.S. Patent Publication 2014/0303873 and U.S. Pat. Nos. 6,321,704; 6,273,039; and 7,458,345, which are hereby fully incorporated for all intents and purposes. Intake valve operator  51  and an exhaust valve operator may open intake  52  and exhaust  54  valves synchronously or asynchronously with crankshaft  40 . The position of intake valve  52  may be determined by intake valve position sensor  55 . The position of exhaust valve  54  may be determined by exhaust valve position sensor  57 . 
     Fuel injector  66  is shown positioned to inject fuel directly into cylinder  30 , which is known to those skilled in the art as direct injection. Alternatively, fuel may be injected to an intake port, which is known to those skilled in the art as port injection. Fuel injector  66  delivers liquid fuel in proportion to the pulse width of signal from controller  12 . Fuel is delivered to fuel injector  66  by a fuel system  175 . In addition, intake manifold  44  is shown communicating with optional electronic throttle  62  (e.g., a butterfly valve) which adjusts a position of throttle plate  64  to control air flow from air filter  43  and air intake  42  to intake manifold  44 . Throttle  62  regulates air flow from air filter  43  in engine air intake  42  to intake manifold  44 . In one example, a high pressure, dual stage, fuel system may be used to generate higher fuel pressures. In some examples, throttle  62  and throttle plate  64  may be positioned between intake valve  52  and intake manifold  44  such that throttle  62  is a port throttle. 
     Distributorless ignition system  88  provides an ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . 
     Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. 
     Controller  12  is shown in  FIG. 1  as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106  (e.g., non-transitory memory), 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: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  134  coupled to an propulsive effort pedal  130  for sensing force applied by human driver  132 ; a measurement of engine manifold absolute pressure (MAP) from pressure sensor  122  coupled to intake manifold  44 ; an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position; a measurement of air mass entering the engine from sensor  120 ; brake pedal position from brake pedal position sensor  154  when human driver  132  applies brake pedal  150 ; and a measurement of throttle position from sensor  58 . Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . Controller  12  may also receive input from and provide output to human/machine interface  155  (e.g., a touch display panel, pushbuttons, or other known human/machine interface). For example, human  132  may request that engine  10  be operated in an economy mode or a performance mode via human/machine interface  155 . Alternatively, or in addition, controller  12  may provide vehicle status information, such as diagnostic indications and codes, human  132  via human/machine interface  155 . In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
     In some examples, the engine may be coupled to an electric motor/battery system in a hybrid vehicle. Further, in some examples, other engine configurations may be employed, for example a diesel engine. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  54  closes and intake valve  52  opens. Air is introduced into combustion chamber  30  via intake manifold  44 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  52  and exhaust valve  54  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  54  opens to release the combusted air-fuel mixture to exhaust manifold  48  and the piston returns to TDC. Note that the above is shown merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
     Referring now to  FIG. 2A , an example multi-cylinder engine that includes two cylinder banks is shown. The engine includes cylinders and associated components as shown in  FIG. 1 . Engine  10  includes eight cylinders  210 . Each of the eight cylinders is numbered and the numbers of the cylinders are included within the cylinders. Fuel injectors  66  selectively supply fuel to each of the cylinders that are activated (e.g., combusting fuel during a cycle of the engine). Cylinders 1-8 may be selectively deactivated (e.g., not combusting fuel during a cycle of the engine) to improve engine fuel economy when less than the engine&#39;s full torque capacity is requested. For example, cylinders 2, 3, 5, and 8 (e.g., a fixed pattern of deactivated cylinders) may be deactivated during an engine cycle (e.g., two revolutions for a four stroke engine) and may be deactivated for a plurality of engine cycles while engine speed and load are constant or vary slightly. During a different engine cycle, a second fixed pattern of cylinders 1, 4, 6, and 7 may be deactivated for a plurality of engine cycles while engine speed and load are constant or vary slightly. Such cylinder deactivation modes may be referred to as static cylinder deactivation modes. 
     In addition, the engine cylinders may be operating such that other patterns of cylinders may be selectively deactivated based on vehicle operating conditions. Additionally, engine cylinders may be deactivated such that a fixed pattern of cylinders is not deactivated over a plurality of engine cycles. Rather, cylinders that are deactivated may change from one engine cycle to the next engine cycle. For example, cylinders 1, 3, 2, 6, 4, and 8 may fire and cylinders  5  and  7  may be deactivated in an engine cycle; cylinders 3, 7, 6, 5, and 8 may fire and cylinders 1, 2, and 6 may be deactivated in the next engine cycle; cylinders 1, 7, 2, 5, and 4 may fire and cylinders 2, 3 and 8 may be deactivated in a next engine cycle; then the activated cylinder and deactivated cylinder pattern may repeat. Such cylinder deactivation modes may be referred to as rolling cylinder deactivation modes. 
     Each cylinder includes variable intake valve operators  51  and variable exhaust valve operators  53 . An engine cylinder may be deactivated by its variable intake valve operators  51  and variable exhaust valve operators holding intake and exhaust valves of the cylinder closed during an entire cycle of the cylinder. An engine cylinder may be activated by its variable intake valve operators  51  and variable exhaust valve operators  53  opening and closing intake and exhaust valves of the cylinder during a cycle of the cylinder. Engine  10  includes a first cylinder bank  204 , which includes four cylinders 1, 2, 3, and 4. Engine  10  also includes a second cylinder bank  202 , which includes four cylinders 5, 6, 7, and 8. Cylinders of each bank may be active or deactivated during a cycle of the engine. 
     Referring now to  FIG. 2B , an example multi-cylinder engine that includes one cylinder banks is shown. The engine includes cylinders and associated components as shown in  FIG. 1 . Engine  10  includes four cylinders  210 . Each of the four cylinders is numbered and the numbers of the cylinders are included within the cylinders. Fuel injectors  66  selectively supply fuel to each of the cylinders that are activated (e.g., combusting fuel during a cycle of the engine with intake and exhaust valves opening and closing during a cycle of the cylinder that is active). Cylinders 1-4 may be selectively deactivated (e.g., not combusting fuel during a cycle of the engine with intake and exhaust valves held closed over an entire cycle of the cylinder being deactivated) to improve engine fuel economy when less than the engine&#39;s full torque capacity is requested. For example, cylinders 2 and 3 (e.g., a fixed or static pattern of deactivated cylinders) may be deactivated during a plurality of engine cycles (e.g., two revolutions for a four stroke engine). During a different engine cycle, a second fixed pattern cylinders 1 and 4 may be deactivated over a plurality of engine cycles. Further, other patterns of cylinders may be selectively deactivated based on vehicle operating conditions. Additionally, engine cylinders may be deactivated such that a fixed pattern of cylinders is not deactivated over a plurality of engine cycles. Rather, cylinders that are deactivated may change from one engine cycle to the next engine cycle. In this way, the deactivated engine cylinders may rotate or change from one engine cycle to the next engine cycle. 
     Engine  10  includes a single cylinder bank  250 , which includes four cylinders 1-4. Cylinders of the single bank may be active or deactivated during a cycle of the engine. Each cylinder includes variable intake valve operators  51  and variable exhaust valve operators  53 . An engine cylinder may be deactivated by its variable intake valve operators  51  and variable exhaust valve operators holding intake and exhaust valves of the cylinder closed during a cycle of the cylinder. An engine cylinder may be activated by its variable intake valve operators  51  and variable exhaust valve operators  53  opening and closing intake and exhaust valves of the cylinder during a cycle of the cylinder. 
     Additionally, six cylinder engines may also be configured similarly to provide static and rolling variable displacement cylinder modes. The six cylinder engines may be of V or inline configurations. 
     The system of  FIGS. 1-2B  provides for an engine system, comprising: an engine including one or more poppet valve deactivating mechanisms; a controller including executable instructions stored in non-transitory memory that cause the controller to adjust operation of the engine when the one or more valve deactivating mechanisms is determined to be degraded according to a correction factor that is based on data generated while all cylinders of the engine were activated and an air charge estimate that is based on the correction factor. The engine system includes where degradation of the one or more valve deactivating mechanisms includes the one or more valve deactivating mechanisms not causing an intake or exhaust valve to cease opening during an engine cycle. The engine system includes where the engine air charge estimate is further based on an amount of fuel injected to the engine. The engine system further comprises an oxygen sensor, and where the engine air charge estimate is further based on output of the oxygen sensor. The engine system includes where adjusting operation of the engine includes activating all engine cylinders. The engine system further comprises additional instructions to generate a metric for determining degradation of the one or more valve deactivating mechanisms. The engine system includes where the metric is determined from a ratio. The engine system includes where the engine air charge estimate that is based on the correction factor is included in the ratio. 
     Referring now to  FIG. 3 , a plot of percent air-fuel ratio error versus engine load is shown. The plot is produced from data generated via a V8 engine operating with all of its cylinders (e.g., eight cylinders). The plot shows that engine air-fuel ratio error may be influenced by engine load. 
     The vertical axis represents percentage air-fuel ratio error and the magnitude of percentage air-fuel ratio error increases in the direction of the vertical axis arrow. The horizontal axis represents engine load and engine load increases in the direction of the horizontal axes arrow. The data points that are generated via a first bank of engine cylinders are indicated by + signs  302 . Data points that are generated via a second bake of engine cylinders are indicated by * signs  304 . 
     It may be observed that the percentage air-fuel ratio error increases at lighter engine loads and the percentage air-fuel ratio error decreases at higher engine loads. Consequently, it may be determined that engine load may influence engine air-fuel ratio values. Therefore, it may be desirable to correct air charge estimates as a function of, or based on, engine load. 
     Referring now to  FIG. 4 , a flow chart describing a method for operating an engine and diagnosing operation of cylinder deactivation devices is shown. The method of  FIG. 4  may be incorporated into and may cooperate with the system of  FIGS. 1-2B . Further, at least portions of the method of  FIG. 4  may be incorporated as executable instructions stored in non-transitory memory while other portions of the method may be performed via a controller transforming operating states of devices and actuators in the physical world. 
     At  402 , method  400  determines engine operating conditions. Engine operating conditions may include, but are not limited to engine speed, driver demand torque, engine temperature, barometric pressure, engine load, vehicle speed, ambient humidity, and ambient temperature. The engine operating conditions may be determined via the sensors and actuators described herein. Method  400  proceeds to  404 . 
     At  404 , method  400  judges if all of the engine&#39;s cylinders are activated. For example, if the engine is an eight cylinder engine and all eight of the engine&#39;s cylinders are activated, the answer is yes and method  400  proceeds to  406 . Otherwise, if fewer than all of the engine&#39;s cylinders are activated, the answer is no and method  400  proceeds to  420 . For example, if the engine is an eight cylinder engine and the engine is operating with four cylinders activated and four cylinders deactivated, then the answer is no and method  400  proceeds to  420 . Method  400  proceeds to  406 . 
     At  406 , method  400  determines engine air charge estimate from engine fuel flow and oxygen sensor output. In one example, the engine air charge is determined via the following equation:
 
air_chg_est=(Σmfi)×lam×afr_sto×(num_banks)/(Num_act_cyl)
 
where air_chg_est is the estimate of air flowing through the engine, mfi is mass of fuel injected to the cylinders of the cylinder bank being evaluated, lam is a lambda value (e.g., engine air-fuel ratio divided by the stoichiometric air-fuel ratio) based on output of the oxygen sensor, afr_sto is the stoichiometric air-fuel ratio for the fuel that is being combusted in the engine, num_banks is the number of cylinder banks in the engine, and num_act_cyl is the actual total number of cylinders of the engine that are presently activated. The variables mfi, lam, afr_sto, num_banks, num_act_cyl are data determined via sensor outputs that are Method  400  proceeds to  408 .
 
     At  408 , method  400  determines an engine air charge estimate based on output of a mass air flow (MAF) sensor or a MAP sensor. If the engine includes a MAF sensor, method  400  may determine the engine air charge for a cylinder bank as described in U.S. Pat. No. 5,331,936, which is fully incorporated by reference for all intents and purposes. On the other hand, if the engine air flow is determined via a MAP sensor, method  400  may determine the engine air flow via the following speed/density equation: 
                 air   -     ⁢     cyl   -     ⁢     air   -     ⁢     chg   -     ⁢   total     =       η   v     ·       n   e     2     ·     V   d     ·     p   RT             
where air_cyl_air_chg_total is the mass of air flowing through the engine, n y  is the engine volumetric efficiency, n e  is engine speed, V d  is volume of all engine cylinders, p is intake manifold pressure, R is gas constant, and T is intake manifold temperature. The variables air_cyl_air_total, V d , n e , p, R, T, and η v  are data determined from sensor outputs and values stored in controller non-volatile memory. Method  400  proceeds to  410 .
 
     At  410 , method  400  determines an adaptive term for adjusting engine air flow when less than all of the engine&#39;s cylinders are activated. The adaptive term may be determined via the following equation: 
                 adaptive   -     ⁢     term   -     ⁢   tmp     =         air   -     ⁢     chg   -     ⁢   est         air   -     ⁢     cyl   -     ⁢     air   -     ⁢     chg   -     ⁢   total             
where adaptive_term_tmp is the adaptive data term, air_chg_est is the previously described estimate of air flowing through the engine, and air_cyl_air_chg_total is the previously described air flow through the engine. The adaptive term is stored in volatile memory in an array of n data points, where n is an integer number, according to the following equation:
 
adaptive_term(load_idx)=rolav(adaptive_term(load),adaptive_term_tmp,rolave_tc)
 
where adaptive_term is an array of adaptive data terms, load_idx is an engine load index value that is used to index or reference the array of adaptive data terms, rolav is an function that averages arguments adaptive_term(load) and adaptive_term_tmp via applying the time constant rolave_tc, and load is engine load. Method  400  proceeds to exit.
 
     At  420 , method  400  determines an engine air charge correction factor from an adaptive term that was previously determined when the engine operated with all of its cylinders active at  410 . The engine air charge correction factor may be determined via the following equation:
 
correction_factor=interp(adaptive_term(load_idx),adaptive_term(load_idx+1),cur_load)
 
where correction_factor is correction factor data, interp is a function that interpolates between adaptive terms load_idx and load_idx+1, and cur_load is the present engine load. Thus, the correction factor is interpolated data that is based on two nearest adaptive term values. The correction factor may compensate for MAP and percent ethanol adjustment factor offsets that may influence determination of engine air flow amounts. Method  400  proceeds to  422 .
 
     At  422 , method  400  determines an engine air charge estimate applying the correction factor. The engine air charge estimate may be determined via the following equation: 
                 air   -     ⁢     charge   -     ⁢     est   -     ⁢   new     =         (     ∑   mfi     )     ×   lam   ×     afr   -     ⁢   sto   ×       (       num   -     ⁢   banks     )       (       Num   -     ⁢     act   -     ⁢   cyl     )             correction   -     ⁢   factor             
where air_charge_est_new is the air flow though the engine data and where the other parameters are as previously described. Method  400  proceeds to  424 .
 
     At  424 , method  400  determines the engine air flow metric. The engine air flow metric is a percentage error value for the engine air flow. The engine air flow metric may be determined via the following equation:
 
metric=(air_charge_est_new−air_cyl_air_chg_total)/air_cyl_air_chg_total
 
where metric is the engine air flow metric, which is a percentage of engine air flow as determined via an engine air flow sensor. Method  400  proceeds to  426 .
 
     At  426 , method  400  judges if the engine air flow metric is out of a predetermined range of values. In one example, method  400  may judge if the absolute value of metric is greater than a predetermined value (e.g., 0.25). If so, the answer is yes and method  400  proceeds to  428 . Otherwise, the answer is no and method  400  proceeds to exit. 
     At  428 , method  400  indicates that one or more poppet valve deactivators is degraded. In addition, method  400  takes mitigating action in response to the indication of poppet valve deactivator degradation (e.g., operation of a poppet valve deactivator that does not deactivate a poppet valve that is requested to be deactivated). In one example, the mitigating action may include preventing deactivation of one or more engine cylinders. For example, if intake valves of cylinder number three of an eight cylinder engine are determined to not be deactivated after being commanded deactivated, method  400  may operate the engine with all engine cylinders activated. Method  400  proceeds to exit. 
     In this way, adaptive terms based on operating an engine with all engine cylinders activated may be a basis for determining engine air flow when less than all engine cylinders are activated. In addition, the adaptive terms may allow a controller to determine whether or not cylinder deactivating devices are degraded or operating as expected. 
     Thus, method  400  provides for an engine control method, comprising: estimating an air charge of an engine according to data generated when one or more of an engine&#39;s cylinders are commanded deactivated via a controller, where the air charge is adjusted according to an adaptive term determined from data generated when all of the engine&#39;s cylinders are activated; and adjusting engine operation according to the estimate. The method includes where the adaptive term is determined from two air charge estimates. The method includes where the two air charge estimates include a first air charge that is based on an amount of fuel that is injected to the engine, and where the second of the two air charge estimates is a second air charge that is based on output of an air sensing device. The method includes where adjusting engine operation included preventing deactivation of one or more cylinders. The method includes where the one or more of the engine&#39;s cylinders are deactivated via ceasing to supply fuel to the one or more cylinders. The method includes where the data includes an amount of fuel injected to the engine. The method includes where the data includes a lambda value as determined from output of an oxygen sensor. 
     Method  400  also provides for an engine control method, comprising: adjusting engine operation according to a metric comprising a ratio of engine air charge values, where the ratio of engine air charge values includes a numerator that is based on a difference of two engine air charge values and a denominator that is one of the two engine air charge values. The method includes where one of the two engine air charge values is based on an amount of fuel injected to an engine and output of an oxygen sensor. The method includes where the other of the two engine air charge values is based on output of an air charge sensor. The method includes where the one of the two air charge values is adjusted via a correction factor. The method includes where the correction factor that is based on data generated when all cylinders of an engine are activated. 
     Referring now to  FIG. 5 , an engine operating sequence according to the method of  FIG. 4  is shown. The sequence of  FIG. 5  may be provided via the system of  FIGS. 1-2B . The present example may be performed at a constant engine speed and constant engine load. In this example, the engine includes a total of eight cylinders, four of which may be selectively deactivated based on vehicle operating conditions. 
     The first plot from the top of  FIG. 5  is a plot of the actual total number of active cylinders versus time. The vertical axis represents the actual total number of active cylinders and the actual total number of active cylinders is indicated along the vertical axis. The horizontal axis represents time and time increases from the left side of the figure to the right side of the figure. Trace  502  represents the actual total number of presently active cylinders. 
     The second plot from the top of  FIG. 5  is a plot of adaptive term determination state versus time. The vertical axis represents the adaptive term determination state and the adaptive term is being determined when trace  504  is at a higher level near the vertical axis arrow. The adaptive term is not being determined when trace  504  is near the horizontal axis. The horizontal axis represents time and time increases from the left side of the figure to the right side of the figure. Trace  504  represents the adaptive term determination state. 
     The third plot from the top of  FIG. 5  is a plot of an engine air charge metric versus time. The vertical axis represents the engine air charge metric and the engine air charge metric value increases in the direction of the vertical axis arrow. The engine air charge metric is zero at the level of the horizontal axis. The horizontal axis represents time and time increases from the left side of the figure to the right side of the figure. Trace  506  represents the air charge metric value. Line  550  represents a threshold metric value. Cylinder valve deactivator degradation may be indicated when the engine air charge metric is greater than threshold  550 . 
     The fourth plot from the top of  FIG. 5  is a plot of valve deactivation device degradation state versus time. The vertical axis represents the valve deactivation device degradation state and the valve deactivation device is indicated as degraded when trace  508  is at a higher level near the vertical axis arrow. The valve deactivation device is determined to not be degraded when trace  508  is near the horizontal axis. The horizontal axis represents time and time increases from the left side of the figure to the right side of the figure. Trace  508  represents the valve deactivation device degradation state. 
     At time t 0 , the engine is operating with all eight cylinders activated and the adaptive term is being determined. The engine air charge metric is not being determined and the variable displacement engine (VDE) degradation state is not indicated. 
     At time t 1 , the engine switches from operating with eight cylinders to operating with four active cylinders. The four engine cylinders may be deactivated when catalyst temperature (not shown) reaches a threshold temperature, the engine has operated at the present speed and load with all eight cylinders for a predetermined amount of time, or when another condition is satisfied. The adaptive term is not being determined and the air charge metric is determined and its value is equal to zero. Therefore, the estimated engine air charge as determined via an amount of fuel injected to the engine and output of an oxygen sensor is in agreement with the engine air charge as determined from the air sensor. Accordingly, valve deactivating device degradation is not indicated. 
     At time t 2 , the engine is switched from operating with four cylinders back to operating with eight cylinders. The engine may be switched back to operating with eight cylinders after operating with four cylinders for a predetermined amount of time, catalyst temperature, or other condition. The adaptive term begins to be determined again and the air charge metric is not being determined. Valve deactivating device degradation is not indicated. 
     At time t 3 , the engine switches from operating with eight cylinders to operating with four active cylinders again. The adaptive term is not being determined and the air charge metric is determined and its value is equal to zero. Therefore, the estimated engine air charge as determined via an amount of fuel injected to the engine and output of an oxygen sensor is in agreement with the engine air charge as determined from the air sensor. Accordingly, valve deactivating device degradation is not indicated. However, shortly after time t 3 , the air charge metric value increases above threshold  550  to indicate a difference between the engine air charge as determined via the air sensor and engine air charge as determined from fuel flow to the engine and oxygen sensor output data. 
     At time t 4 , valve deactivating device degradation is indicated and the engine is switched back to operating all eight cylinders in response to the valve deactivating device degradation indication. The adaptive term is determined once again and the engine air charge metric remains at an elevated level. 
     In this way, an engine air charge metric based on two different engine air charge estimation calculations may be a basis for determining valve deactivating device degradation. In addition, when valve deactivating device degradation is indicated, mitigating actions may be taken so that excess air flow is not delivered to a catalyst so that engine emissions may be at desired levels. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, at least a portion of the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the control system. The control actions may also transform the operating state of one or more sensors or actuators in the physical world when the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with one or more controllers. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage.