Patent Publication Number: US-9835101-B2

Title: System and method for selective cylinder deactivation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/602,395 entitled “SYSTEM AND METHOD FOR SELECTIVE CYLINDER DEACTIVATION,” filed on Jan. 22, 2015. U.S. patent application Ser. No. 14/602,395 claims priority to U.S. Provisional Patent Application No. 62/021,621 entitled “SYSTEM AND METHOD FOR SKIP FIRE,” filed on Jul. 7, 2014. The entire contents of each of the above-referenced applications are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates to skip fire operation in an internal combustion engine. 
     BACKGROUND AND SUMMARY 
     In order to improve fuel economy during low load conditions, some engines may be configured to operate in a selective cylinder deactivation mode where one or more cylinders of the engine are deactivated via disabling of intake and/or exhaust valve actuation, interruption of fuel injection, and/or disabling of spark ignition to the deactivated cylinders, for example. During operation in the selective cylinder deactivation mode, also referred to as “skip fire,” the total engine fuel amount may be redistributed to the fired cylinders, increasing per-cylinder load and reducing pumping work, thus increasing fuel economy and improving emissions. The cylinder(s) selected for deactivation may change with each engine cycle, such that a different cylinder or combination of cylinders is deactivated per engine cycle. Further, the number of cylinders deactivated per engine cycle may change as engine operating conditions change. 
     The inventors herein have recognized that during skip fire operation, valve deactivation/reactivation mechanisms may not be fully reliable. This may lead to unintended combustion events in cylinders scheduled to be skipped and/or unintended skipping of cylinders scheduled to be fired. Unintended firing or skipping of cylinders may cause undesired torque changes, NVH issues, degraded emissions, and/or other problems. 
     In light of the above issues, the inventors herein have devised an approach to maintain robustness of a skip fire strategy. One example method comprises: for a given engine cycle of an engine operating in a skip fire mode, selecting a number of cylinders of the engine to skip based on engine load and setting a commanded firing order of non-skipped cylinders of the engine, where the commanded firing order includes scheduling at least a first cylinder to be fired and at least a second cylinder to be skipped. The method further includes determining if combustion occurs as commanded in the first cylinder. If combustion does not occur, the commanded firing order is adjusted to fire the second cylinder of the engine. In one example, combustion may be detected based on feedback from an ionization sensor. 
     Similarly, combustion may sometimes occur in both the first cylinder and the second cylinder, although the second cylinder was intended to be skipped. In this case, the commanded firing order is adjusted to skip a later cylinder in the firing order which was originally planned to fire. 
     In this way, the commanded firing order of the engine may be dynamically updated in response to unintended combustion events, including combustion occurring in cylinders scheduled to be skipped and lack of combustion in cylinders scheduled to be fired. 
     The present disclosure may offer several advantages. For example, by updating the firing order to compensate for unintended cylinder events during skip fire, desired torque may be maintained, even if valve actuation does not occur as commanded. 
     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 
         FIG. 1  shows a schematic diagram of a single cylinder of a multi-cylinder engine. 
         FIG. 2  shows an example cylinder firing plot of an engine operating without skip fire according to an original engine firing order. 
         FIG. 3  shows an example cylinder firing plot of an engine operating with skip fire according to a commanded firing order. 
         FIG. 4  is a high level flow chart for an engine configured to operate with skip fire. 
         FIG. 5  is a flow chart illustrating a method for adjusting fuel injection during a skip fire mode. 
         FIG. 6  is an example engine operation plot of an engine operating according to the method of  FIG. 5 . 
         FIG. 7  is a flow chart illustrating a method for sensing combustion events during skip fire.  FIG. 8  is an example cylinder firing plot of an engine operating according to the method of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Operating an engine with skip fire, where at least one cylinder of the engine is skipped and not fired during each engine cycle, may improve fuel economy and emissions during certain operating conditions, such as low engine load. An engine configured to operate with skip fire is illustrated in  FIG. 1 , and  FIGS. 2-3  illustrate cylinder firing plots for the engine of  FIG. 1  in a non-skip fire mode ( FIG. 2 ) and in a skip fire mode ( FIG. 3 ). Additionally, the engine of  FIG. 1  may include a controller to execute one or more methods for carrying out skip fire operation, such as the method illustrated in  FIG. 4 . 
     During certain periods of skip fire operation, such as during transition into or out of skip fire, intake manifold dynamics may vary, making cylinder air-fuel ratio control difficult, particularly for port fuel injection systems. As described in more detail below, a split injection routine may be executed during skip fire, where some of the fuel is injected via port injection during an earlier portion of the cylinder cycle (when accurate estimation of cylinder air charge is more challenging) and a make-up pulse of fuel is injected via a direct injector during a later portion of the cylinder cycle (when the trapped cylinder air charge is more accurately measured).  FIG. 5  illustrates a method for carrying out the split injection routine, while  FIG. 6  illustrates example engine operation plots during the execution of  FIG. 5 . 
     Further, while some skip fire operation may include deactivation of intake/exhaust valve actuation, fuel injection, and spark ignition, other skip fire operation may maintain spark, even in deactivated cylinders. Additionally, valve deactivation mechanisms may not be fully reliable. During skip fire operation, if fuel vapors are present in the charge air (from a fuel vapor canister purge, for example, or from a positive crankcase ventilation system), and the intake and exhaust valves of a deactivated cylinder are inadvertently actuated, an unintended combustion event in the deactivated cylinder may occur, leading to torque disturbances. To minimize the consequences of unintended cylinder events during skip fire, combustion status may be monitored via ionization sensing, and if an unintended combustion event occurs in a cylinder scheduled to be skipped, the firing order of the engine may be dynamically updated to skip the next cylinder scheduled to be fired, thus maintaining requested torque.  FIG. 7  illustrates a method for monitoring combustion during skip fire.  FIG. 8  illustrates an example cylinder firing plot including a dynamically updated firing order. 
       FIG. 1  depicts an example embodiment of a combustion chamber or cylinder of internal combustion engine  10 . Engine  10  may be controlled at least partially by a control system including controller  12  and by input from a vehicle operator  130  via an input device  132 . In this example, input device  132  includes an accelerator pedal and a pedal position sensor  134  for generating a proportional pedal position signal PP. Cylinder (i.e. combustion chamber)  14  of engine  10  may include combustion chamber walls  136  with piston  138  positioned therein. Piston  138  may be coupled to crankshaft  140  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft  140  may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a starter motor may be coupled to crankshaft  140  via a flywheel to enable a starting operation of engine  10 . 
     Cylinder  14  can receive intake air via a series of intake air passages  142 ,  144 , and  146 . Intake air passage  146  (otherwise referred to as the intake manifold) can communicate with other cylinders of engine  10  in addition to cylinder  14 . In some embodiments, one or more of the intake passages may include a boosting device such as a turbocharger or a supercharger. For example,  FIG. 1  shows engine  10  configured with a turbocharger including a compressor  174  arranged between intake passages  142  and  144 , and an exhaust turbine  176  arranged along exhaust passage  148 . Compressor  174  may be at least partially powered by exhaust turbine  176  via a shaft  180  where the boosting device is configured as a turbocharger. However, in other examples, such as where engine  10  is provided with a supercharger, exhaust turbine  176  may be optionally omitted, where compressor  174  may be powered by mechanical input from a motor or the engine. A throttle  162  including a throttle plate  164  may be provided along an intake passage of the engine for varying the flow rate and/or pressure of intake air provided to the engine cylinders. For example, throttle  162  may be disposed downstream of compressor  174  as shown in  FIG. 1 , or may alternatively be provided upstream of compressor  174 . 
     Exhaust passage  148  can receive exhaust gases from other cylinders of engine  10  in addition to cylinder  14 . Exhaust gas sensor  128  is shown coupled to exhaust passage  148  upstream of emission control device  178 . Sensor  128  may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor. Emission control device  178  may be a three way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof. 
     Each cylinder of engine  10  may include one or more intake valves and one or more exhaust valves. For example, cylinder  14  is shown including at least one intake poppet valve  150  and at least one exhaust poppet valve  156  located at an upper region of cylinder  14 . In some embodiments, each cylinder of engine  10 , including cylinder  14 , may include at least two intake poppet valves and at least two exhaust poppet valves located at an upper region of the cylinder. 
     Intake valve  150  may be controlled by controller  12  via actuator  152 . Similarly, exhaust valve  156  may be controlled by controller  12  via actuator  154 . During some conditions, controller  12  may vary the signals provided to actuators  152  and  154  to control the opening and closing of the respective intake and exhaust valves. The position of intake valve  150  and exhaust valve  156  may be determined by respective valve position sensors (not shown). The valve actuators may be of the electric valve actuation type or cam actuation type, or a combination thereof. The intake and exhaust valve timing may be controlled concurrently or any of a possibility of variable intake cam timing, variable exhaust cam timing, dual independent variable cam timing or fixed cam timing may be used. Each cam actuation system may include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems that may be operated by controller  12  to vary valve operation. For example, cylinder  14  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT. In other embodiments, the intake and exhaust valves may be controlled by a common valve actuator or actuation system, or a variable valve timing actuator or actuation system. 
     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  156  closes and intake valve  150  opens. Air is introduced into combustion chamber  14  via intake manifold  146 , and piston  138  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  14 . The position at which piston  138  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  150  and exhaust valve  156  are closed. Piston  138  moves toward the cylinder head so as to compress the air within combustion chamber  14 . The point at which piston  138  is at the end of its stroke and closest to the cylinder head (e.g., when combustion chamber  14  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  192 , resulting in combustion. During the expansion stroke, the expanding gases push piston  138  back to BDC. Crankshaft  140  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  156  opens to release the combusted air-fuel mixture to exhaust passage  148  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. 
     Cylinder  14  can have a compression ratio, which is the ratio of volumes when piston  138  is at bottom center to top center. Conventionally, the compression ratio is in the range of 9:1 to 10:1. However, in some examples where different fuels are used, the compression ratio may be increased. This may happen for example when higher octane fuels or fuels with higher latent enthalpy of vaporization are used. The compression ratio may also be increased if direct injection is used due to its effect on engine knock. 
     In some embodiments, each cylinder of engine  10  may include a spark plug  192  for initiating combustion. Ignition system  190  can provide an ignition spark to combustion chamber  14  via spark plug  192  in response to spark advance signal SA from controller  12 , under select operating modes. However, in some embodiments, spark plug  192  may be omitted, such as where engine  10  may initiate combustion by auto-ignition or by injection of fuel as may be the case with some diesel engines. 
     In some embodiments, each cylinder of engine  10  may be configured with one or more fuel injectors for providing fuel thereto. As a non-limiting example, cylinder  14  is shown including two fuel injectors  166  and  170 . Fuel injector  166  is shown coupled directly to cylinder  14  for injecting fuel directly therein in proportion to the pulse width of signal FPW- 1  received from controller  12  via electronic driver  168 . In this manner, fuel injector  166  provides what is known as direct injection (hereafter referred to as “DI”) of fuel into combustion cylinder  14 . While  FIG. 1  shows injector  166  as a side injector, it may also be located overhead of the piston, such as near the position of spark plug  192 . Such a position may improve mixing and combustion when operating the engine with an alcohol-based fuel due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be located overhead and near the intake valve to improve mixing. Fuel may be delivered to fuel injector  166  from high pressure fuel system  172  including a fuel tank, fuel pumps, a fuel rail, and driver  168 . Alternatively, fuel may be delivered by a single stage fuel pump at lower pressure, in which case the timing of the direct fuel injection may be more limited during the compression stroke than if a high pressure fuel system is used. Further, while not shown, the fuel tank may have a pressure transducer providing a signal to controller  12 . 
     Fuel injector  170  is shown arranged in intake passage  146 , rather than in cylinder  14 , in a configuration that provides what is known as port injection of fuel (hereafter referred to as “PFI”) into the intake port upstream of cylinder  14 . Fuel injector  170  may inject fuel in proportion to the pulse width of signal FPW- 2  received from controller  12  via electronic driver  171 . Fuel may be delivered to fuel injector  170  by fuel system  172 . 
     Fuel may be delivered by both injectors to the cylinder during a single cycle of the cylinder. For example, each injector may deliver a portion of a total fuel injection that is combusted in cylinder  14 . Further, the distribution and/or relative amount of fuel delivered from each injector may vary with operating conditions, such as engine load and/or knock, as described herein below. The relative distribution of the total injected fuel among injectors  166  and  170  may be referred to as an injection ratio. For example, injecting a larger amount of the fuel for a combustion event via (port) injector  170  may be an example of a higher injection ratio of port to direct injection, while injecting a larger amount of the fuel for a combustion event via (direct) injector  166  may be a lower injection ratio of port to direct injection. Note that these are merely examples of different injection ratios, and various other injection ratios may be used. Additionally, it should be appreciated that port injected fuel may be delivered during an open intake valve event, closed intake valve event (e.g., substantially before an intake stroke, such as during an exhaust stroke), as well as during both open and closed intake valve operation. 
     Similarly, directly injected fuel may be delivered during an intake stroke, as well as partly during a previous exhaust stroke, during the intake stroke, and partly during the compression stroke, for example. Further, the direct injected fuel may be delivered as a single injection or multiple injections. These may include multiple injections during the compression stroke, multiple injections during the intake stroke, or a combination of some direct injections during the compression stroke and some during the intake stroke. 
     As such, even for a single combustion event, injected fuel may be injected at different timings from a port and direct injector. Furthermore, for a single combustion event, multiple injections of the delivered fuel may be performed per cycle. The multiple injections may be performed during the compression stroke, intake stroke, or any appropriate combination thereof. 
     Fuel injectors  166  and  170  may have different characteristics. These include differences in size, for example, one injector may have a larger injection hole than the other. Other differences include, but are not limited to, different spray angles, different operating temperatures, different targeting, different injection timing, different spray characteristics, different locations etc. Moreover, depending on the distribution ratio of injected fuel among injectors  170  and  166 , different effects may be achieved. 
     Fuel tank in fuel system  172  may hold fuel with different fuel qualities, such as different fuel compositions. These differences may include different alcohol content, different octane, different heat of vaporizations, different fuel blends, and/or combinations thereof etc. In one example, fuels with different alcohol contents could include gasoline, ethanol, methanol, or alcohol blends such as E85 (which is approximately 85% ethanol and 15% gasoline) or M 85  (which is approximately 85% methanol and 15% gasoline). Other alcohol containing fuels could be a mixture of alcohol and water, a mixture of alcohol, water and gasoline etc. 
     Controller  12  is shown in  FIG. 1  as a microcomputer, including microprocessor unit  106 , input/output ports  108 , an electronic storage medium for executable programs and calibration values shown as read only memory chip  110  in this particular example, random access memory  112 , keep alive memory  114 , and a data bus. Controller  12  may receive 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  122 ; engine coolant temperature (ECT) from temperature sensor  116  coupled to cooling sleeve  118 ; a profile ignition pickup signal (PIP) from Hall effect sensor  120  (or other type) coupled to crankshaft  140 ; thrott 1 e position (TP) from a thrott 1 e position sensor; and absolute manifold pressure signal (MAP) from sensor  124 . Engine speed signal, RPM, may be generated by controller  12  from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. Further, in some examples, controller  12  may receive a signal from a combustion sensor  194  positioned in the combustion chamber. In one example, combustion sensor  194  may be an ionization sensor that detects the presence of smoke or another indicator of combustion. While a communication line is removed for clarity from  FIG. 1 , it is to be understood that combustion sensor  194  is operably coupled to and configured to send signals to the controller, similar to the other sensors depicted in  FIG. 1 . Storage medium read-only memory  110  can be programmed with computer readable data representing instructions executable by processor  106  for performing the methods described below as well as other variants that are anticipated but not specifically listed. An example routine that may be performed by the controller is described at  FIG. 4 . 
     As described above,  FIG. 1  shows only one cylinder of a multi-cylinder engine. As such each cylinder may similarly include its own set of intake/exhaust valves, fuel injector(s), spark plug, etc. In some examples, engine  10  may be an inline four-cylinder engine, a V-6 engine, V-8 engine, or other engine configuration. 
     During standard engine operation, engine  10  is typically operated to fire each cylinder per engine cycle. Thus, for every  720  CA (e.g., two revolutions of the crankshaft), each cylinder will be fired one time. To allow for combustion in each cylinder, each intake and exhaust valve is actuated (e.g., opened) at a specified time. Further, fuel is injected to each cylinder and the spark ignition system provides a spark to each cylinder at a specified time. Accordingly, for each cylinder, the spark ignites the fuel-air mixture to initiate combustion. 
       FIG. 2  illustrates an example plot of cylinder firing events for an example four cylinder engine (e.g., engine  10  of  FIG. 1 ) during standard, non-skip fire operation. The engine position of each cylinder of the four cylinder engine is described by the traces labeled CYL.  1 - 4 . The vertical markers along the length of traces CYL.  1 - 4  represent top-dead-center and bottom-dead-center piston positions for the respective cylinders. The respective cylinder strokes of each cylinder are indicated by INTAKE, COMP., EXPAN., and EXH. identifiers. 
     The engine has an original engine firing order of 1-3-4-2, such that CYL.  1  is fired first, followed by CYL.  3 , CYL.  4 , and CYL.  2 , each engine cycle. Thus, as shown, combustion in CYL.  1  occurs at or near TDC between the compression and expansion strokes, illustrated by star  200 . To achieve combustion, fuel is injected to CYL.  1 , the intake valve is actuated to drawn in charge air (and is subsequently closed to trap the charge in the cylinder), and combustion is initiated by a spark ignition event. Combustion in CYL.  3  is initiated by a spark, as illustrated by star  202 . While CYL.  3  is on a compression stroke, CYL.  1  is on an expansion stroke. Combustion is initiated in CYL.  4  by a spark, as illustrated by star  204 . While CYL.  4  is on a compression stroke, CYL.  1  is on an exhaust stroke, and CYL.  3  is on an expansion stroke. Combustion is initiated in CYL.  2  by a spark, as illustrated by star  206 . While CYL.  2  is on a compression stroke, CYL.  1  is on an intake stroke, CYL.  3  is on an exhaust stroke, and CYL.  4  is on an expansion stroke. Upon completion of combustion in CYL.  2 , a new engine cycle starts and combustion again occurs in CYL.  1 , as illustrated by star  208 . Combustion then continues according to the engine firing order, as illustrated. 
     During certain operating conditions, engine  10  may operate in a skip fire mode, where less than all cylinders of the engine are fired each engine cycle. Skip fire mode may be carried out during low load conditions, for example, or other conditions where the per-cylinder fuel quantity to be injected to each cylinder is relatively small (e.g., so small that accurate fuel delivery may be difficult). During skip fire, one or more cylinders of the engine is skipped (e.g., not fired) during each engine cycle. To maintain desired torque, the fuel is redistributed to the fired cylinders, increasing the per-cylinder fuel quantity, thus reducing fueling errors. Skip fire may also reduce pumping losses, increasing engine efficiency. 
     In order to skip a designated cylinder, the intake and exhaust valves of the designated cylinder are deactivated (via control of the actuators  152  and  154 , for example), e.g., the intake and exhaust valves are maintained closed throughout each stroke of the cylinder cycle. In this way, fresh charge is not admitted to the cylinder. Further, fuel injection, via port injector  170  and/or direct injector  166 , for example, is disabled. In some examples, spark (from spark plug  192 , for example) may be disabled as well. In other examples, spark may be provided to the designated cylinder. However, without charge air and fuel, even with spark, combustion will not occur in the designated cylinder. 
       FIG. 3  illustrates an example plot of cylinder firing events for an example four cylinder engine (e.g., engine  10  of  FIG. 1 ) during skip fire operation. Similar to  FIG. 2 , the engine position of each cylinder of the four cylinder engine is described by the traces labeled CYL.  1 - 4 . The vertical markers along the length of traces CYL.  1 - 4  represent top-dead-center and bottom-dead-center piston positions for the respective cylinders. The respective cylinder strokes of each cylinder are indicated by INTAKE, COMP., EXPAN., and EXH. identifiers. 
     As explained above, the engine has an original engine firing order of 1-3-4-2. During skip fire, one or more cylinders of the engine are skipped each engine cycle. The number of skipped cylinders may be selected based on operating conditions, such as engine load, as will be explained in more detail below with respect to  FIG. 4 . Further, a different cylinder may be skipped each engine cycle, such that over a plurality of engine cycles, each cylinder is fired at least once and each cylinder is skipped at least once. 
     During skip fire, the original engine firing order may be adjusted to achieve a commanded firing order where one or more cylinders are skipped. The commanded firing order may maintain the same basic firing order of the engine, with one or more cylinders skipped each engine cycle, and may alternate skipped cylinders from engine cycle to engine cycle. As shown in  FIG. 3 , the commanded firing order of the engine during skip fire may fire two cylinders, skip one cylinder, fire two cylinders, skip one cylinder, etc., resulting in a firing order of 1-3-X-2-1-X-4-2-X-3-4-X. In this way, a different cylinder is skipped each time a cylinder is skipped until the pattern repeats. 
     Thus, as shown, combustion in CYL.  1  occurs at or near TDC between the compression and expansion strokes, illustrated by star  300 . Next, combustion in CYL.  3  is initiated by a spark, as illustrated by star  302 . CYL.  4 , which is scheduled to be fired after CYL.  3  in the original firing order, is skipped. Thus, while a spark may still occur in CYL.  4  during the compression stroke, no combustion is initiated due to the lack of valve actuation and fuel injection, as illustrated by dashed star  304 . Combustion in CYL.  2  is initiated by a spark as illustrated by star  306 . 
     During the next engine cycle, combustion occurs in CYL.  1 , CYL.  4 , and CYL.  2  (as illustrated by star  308 , star  312 , and star  314 , respectively). Combustion does not occur in CYL.  3 , as illustrated by dashed star  310 . During the following engine cycle, CYLS.  1  and  2  are skipped, as illustrated by dashed stars  316  and  322 , respectively, while CYLS.  3  and  4  are fired, as illustrated by stars  318  and  320 , respectively. In this way, during some engine cycles, only one cylinder is skipped, while in other engine cycles, more than one cylinder is skipped. However, the commanded firing order as illustrated maintains an even combustion pattern (one cylinder skipped for every two cylinders fired), reducing NVH issues. However, it should be noted that the order and sequence illustrated by  FIGS. 2 and 3  are only exemplary in nature and not intended to limit the scope of the description. For example, in some embodiments three cylinders may combust an air-fuel mixture before combustion is skipped in a cylinder. In other embodiments, four cylinders may combust an air-fuel mixture before combustion is skipped in a cylinder. In other embodiments, combustion may be skipped in two cylinders in a row rather than one as depicted by  FIG. 3 . 
     Turning now to  FIG. 4 , a method  400  for operating an engine with skip fire is illustrated. Method  400  may be carried by a controller, such as controller  12  of  FIG. 1 , according to non-transitory instructions stored thereon, in order to operate engine  10  in a skip fire or non-skip fire mode, as described below. 
     At  402 , method  400  includes determining operating conditions. The operating conditions determined include, but are not limited to, engine load, engine speed, engine fuel demand, and engine temperature. The operating conditions may be determined based on output from one or more engine sensors described above with respect to  FIG. 1 . At  404 , method  400  determines if the engine is currently operating in skip fire, where one or more cylinders of the engine are skipped (e.g., not fired) per engine cycle. If the engine is not currently operating with skip fire, method  400  proceeds to  406  to determine if conditions indicate that skip fire should be initiated. The engine may transition into skip fire operation based on one or a combination of various engine operating parameters. These conditions may include engine speed, fuel demand, and engine load being below predetermined respective thresholds. For example, during idle engine operation, engine speed may be low, such as 500 RPMs, and the engine load may be low. Thus, fuel demand, which is based on speed, load, and operating conditions such as engine temperature, manifold pressure, etc., may be too low to accurately deliver the desired amount of fuel. Additionally, skip fire operation may mitigate problems with cold engine operation, and as such, skip fire operation conditions may be based on engine temperature. Skip fire operation conditions may further be based on the controller sensing the engine being in a steady state operating condition, as transient operating conditions may require a fluctuating fuel demand. Steady state operating conditions may be determined by an amount of time spent at current load, or any suitable method. 
     If conditions do not indicate that skip fire should be initiated (e.g., if engine load is high), method  400  proceeds to  407  to maintain current operating conditions. The current operating conditions include each cylinder of the engine being fired according to the original engine firing order, with all intake and exhaust valves actuated at appropriate times and fuel injection and spark activated for each cylinder. Method  400  then returns. 
     If at  406  it is determined that it is time to transition to skip fire operation, method  400  proceeds to  408  to determine the number of cylinders to skip per engine cycle, or per a plurality of engine cycles. That is, a cylinder pattern for selective cylinder deactivation may be determined. The cylinder pattern determined may specify the total number of deactivated cylinders relative to active cylinders, as well the identity of the cylinders to be deactivated. For example, the controller may determine that one cylinder should be skipped every engine cycle, or it may determine that four cylinders should be skipped every three engine cycles, or other appropriate cylinder skip pattern. The total number of cylinders to skip on each engine cycle may be based on operating conditions, such as engine load. 
     At  410 , a commanded firing order for the non-skipped cylinders is set. The commanded firing order may be based on the selected number of cylinders to be skipped per engine cycle, the original engine firing order, and which cylinders were skipped in a previous skip fire engine operation, such that the original firing order is maintained, with the exception of the selected skipped cylinders. The commanded firing order may also ensure that a different cylinder is skipped each time a cylinder is skipped. The commanded firing order described in  FIG. 3  is one non-limiting example of a commanded firing order that may be set by the controller for the engine. Therein, a firing order 1-3-4-2-1-3-4-2 of an in-line four cylinder engine is adjusted during skip fire to operate as 1-3-x-2-1-x-4-2. Alternatively, a first set of cylinders may be skipped for a first number of engine cycles while a second set of cylinders are fired, and thereafter the second set of cylinders may be skipped for a second number of engine cycles while the first set of cylinders are fired. This may result in a skip fire pattern of 1-x-4-x-1-x-4-x-x-3-x-2-x-3-x-2-x. 
     At  412 , the cylinders are fired according to the commanded firing order determined in the selected cylinder pattern. As described previously, the fired cylinders have activated valve actuation, fuel injection, and spark, to initiate combustion, while the non-fired cylinders have deactivated valve actuation and deactivated fuel injection (and in some examples, deactivated spark ignition). The fuel provided to the fired cylinders may be provided solely via a port injector, or solely via a direct injector, based on the engine configuration and operating conditions. However, in some examples as indicated at  414 , firing the cylinders may optionally include injecting fuel to the fired cylinders using a split PFI/DI injection protocol, which is described in more detail below with respect to  FIG. 5 . Briefly, during skip fire, the fuel to the fired cylinders may be split between the port injector and the direct injector, to leverage the benefits of port fuel injection with the increased air-fuel ratio control provided by direct injection. A first fuel quantity may be injected to a given cylinder by the port injector, based on a desired air-fuel ratio and an estimated air charge amount for that cylinder, at a first, earlier time in the cylinder cycle (e.g., while the intake valve is closed, prior to the intake stroke). Then, at a second, later time in the cylinder cycle (e.g., just before or after the intake valve closes, before the compression stroke), an updated air charge amount is determined for the cylinder, and a second fuel quantity is injected via the direct injector, based on the updated air charge amount, desired air-fuel ratio, and the first fuel quantity. In this way, overall desired air-fuel ratio may be maintained, even if a load change (which would cause the first estimated air charge amount to differ from the actual trapped air charge amount) occurs between the port injection and direct injection. 
     Additionally, method  400  may optionally include, at  416 , monitoring combustion events and dynamically updating the commanded firing order if indicated, as described in more detail below with respect to  FIG. 7 . Monitoring the combustion events includes determining if combustion occurs as commanded in cylinders scheduled to fire, as well determining if combustion did not occur as commanded in cylinders scheduled to be skipped, based on ioniziation sensing (e.g., based on feedback from combustion sensor  194 ). If an unintended combustion event occurs in a skipped cylinder, or if a planned combustion event does not occur in a cylinder scheduled to be fired, the commanded firing order may be updated to either skip a next cylinder scheduled to be fired or fire a next cylinder scheduled to be skipped. Method  400  then returns. 
     Returning to  404  of method  400 , where it is determined if the engine is currently operating with skip fire, if the answer is yes, method  400  proceeds to  418  to determine if conditions indicate if the controller is to transition out of skip fire. Skip fire may be terminated if engine load increases, for example, if the engine is undergoing a transient event, or other suitable change in operating conditions. If the controller determines it is time to transition out of skip fire, method  400  proceeds to  420  to continue to operate with the PFI/DI split injection protocol at least until the transition is complete, if the engine was being operated with the PFI/DI split injection protocol during skip fire. A completed transition out of skip fire may include, in one example, firing all cylinders for an entire engine cycle. Further, at  422 , combustion events may continue to be monitored until the transition out of skip fire is complete. Method  400  then returns. 
     However, if at  418  it is determined that skip fire operation is to be maintained, method  400  proceeds to  424  to fire the cylinders according to the commanded firing order. If applicable, the engine will continue to operate with the PFI/DI split injection protocol, as indicated at  426 , and continue to monitor combustion events and update the firing order, if indicated, as shown at  428 . Method  400  then returns. 
     The PFI/DI split injection protocol described above will not be presented in more detail with respect to  FIG. 5 , which illustrates a method  500  for adjusting fuel injection during skip fire operation. As explained above, method  500  may be carried out by controller  12 , during the execution of method  400  of  FIG. 4 , to control injection via a port injector (e.g., injector  170 ) and a direct injector (e.g., injector  166 ). 
     At  502 , method  500  includes determining engine operating conditions. The determined operating conditions may include engine speed, engine load, MAP, MAF, commanded air-fuel ratio, exhaust air-fuel ratio (determined based on feedback from an exhaust oxygen sensor, such as sensor  128 ), and other conditions. At  504 , a first air charge amount is estimated for a first fired cylinder. The first air charge amount is estimated prior to the intake valve of the first cylinder opening, for example during the exhaust stroke of a previous engine cycle. The air charge amount may be estimated in a suitable manner, such as based on MAP and MAF, and/or other suitable parameters, including boost pressure (if the engine is turbocharged), exhaust gas recirculation rate (both external and internal), intake and exhaust variable cam timing phase angles, and/or engine temperature. 
     At  506 , a maximum possible change in air charge that may occur between when the first air charge amount is estimated and when combustion occurs in the first cylinder is determined based on operating conditions. The maximum possible change in air charge may reflect the possibility that the engine may enter into or exit out of skip fire operation or that the number of skipped cylinders may change, and thus may be based on a change in engine load. For example, the engine load may be decreasing, and thus the maximum possible change in air charge may predict that engine load will keep decreasing over the course of the cylinder cycle, causing a shift in the number of skipped cylinders (e.g., from none to one, or from one to two). Other parameters may also be considered when determining the maximum possible change in air charge amount. For example, an estimate of the maximum change in the air charge in a given cylinder, as a fraction of the current air charge, due to another cylinder being fired versus being skipped may be V_cyl/V_man, where V_cyl is cylinder displacement and V_man is the volume of the intake manifold. In a four-cylinder engine, for example, the maximum change may be ⅛ (12.5%). 
     At  508 , a desired air-fuel ratio is determined based on operating conditions (e.g., speed, load, output from one or more exhaust composition sensors, etc.). At  510 , a first fuel quantity is injected via the port injector at a first timing, such as prior to the intake valve opening. As indicated at  512 , the first fuel quantity is based on the desired air-fuel ratio and the estimated air charge amount. The first fuel quantity is an amount that is deliberately lean of a fuel quantity needed to reach the desired air-fuel ratio, as indicated at  514 . The first fuel quantity may be deliberately lean of the fuel quantity needed to reach the desired air-fuel ratio by an amount based on the maximum possible change in air charge determined at  506 . For example, if the maximum possible change in the air charge between the first, estimated air charge amount and the actual air charge trapped in the first cylinder at combustion is a negative value (e.g., indicates that the estimated air charge is likely to be greater than the actual air charge amount), the first fuel quantity may be lean of the fuel quantity needed to reach the desired air-fuel ratio by a first, larger amount. If the maximum possible change in air charge is a positive value (e.g., indicates that the estimated air charge is likely to be less than the actual air charge amount), the first fuel quantity may be lean of the fuel quantity needed to reach the desired air-fuel ratio by a second, smaller amount. In this way, if the controller predicts the air charge amount is likely to increase, the first fuel quantity may be larger than if the controller predicts the air charge amount is likely to decrease. Further, in some examples, the first fuel quantity may be decreased below the amount needed to reach the desired air-fuel ratio based on other parameters, such as knock, NVH issues, etc. 
     At  516 , a second, updated air charge amount is calculated and a final desired air-fuel ratio is determined based on operating conditions, at a later time in the cylinder cycle, such as near intake valve closing. Due to the relatively long amount of elapsed time between when the first air charge amount is calculated (before intake valve opening, prior to port injection) and when the updated air charge amount is calculated (at intake valve closing, prior to direct injection), engine operating conditions may change that affect intake manifold dynamics and ultimately change the amount of charge air that is trapped in the cylinder once the intake valve closes. Such operating conditions may include transition into or out of skip fire operation or adjustment to the number of skipped cylinders. To compensate for the changed air charge amount, a second, “make-up” pulse of fuel is injected via the direct injector. As indicated at  518 , a second fuel quantity is injected via a direct injector at a second, later timing, where the second fuel quantity is an amount based on the first fuel quantity, updated air charge amount, and final desired air-fuel ratio. 
     In one example, the first estimated air charge amount and second, updated air charge amount may be equal. In this case, the second fuel quantity injected by the direct injector is equal to the amount of fuel needed to bring the cylinder to the first desired air-fuel ratio, minus the first fuel quantity. In other words, the “deliberate leanness” of the first fuel quantity is simply made up by the second fuel quantity. In another example, the first estimated air charge amount may be less than the second, updated air charge amount. In this case, the second fuel quantity may be an amount that includes the “deliberate leanness” of the first fuel quantity (e.g., the amount added to the first fuel quantity in order to reach the desired air-fuel ratio), plus an additional amount of fuel to compensate for the increased amount of charge air. In a still further example, the first estimated air charge amount may be greater than the second, updated air charge amount. In this case, the second fuel quantity may be an amount that is less than “deliberate leanness” of the first fuel quantity to compensate for the decreased amount of charge air. In all the above examples, the final desired air-fuel ratio is reached at combustion. 
     At  520 , the PFI/DI split injection is repeated for all fired cylinders until the skip fire mode (and transition out of the skip fire mode) is complete. Method  500  then returns. 
       FIG. 6  is a diagram  600  illustrating a plurality of example engine operational plots that may be produced during the execution of method  500 . Specifically, diagram  600  includes a load plot, a skip fire status plot, a PFI and DI split ratio plot (which also illustrates the fuel injected via PFI as a proportion of the fuel needed to reach the desired air-fuel ratio at the time of the first air charge estimate), and air-fuel ratio plot. For each plot, time is depicted along the horizontal axis, and each respective operating parameter is depicted along the vertical axis. For the skip fire status plot, a binary on/off status is depicted. For the PFI and DI split ratio plot, the relative proportion of fuel injected by each injector is depicted per injection event for a single cylinder (e.g., cylinder  1 , according to the firing order of  FIG. 3 ), not absolute amounts of fuel. As such, the PFI and DI split ratio plot depicts a range of relative ratios, from 0 to 1, where if all the fuel is injected via the port injector, the PFI split ratio is 1 and the DI split ratio is zero, and vice versa. As mentioned above, the fuel injection events for one cylinder are illustrated. These events correspond in time to the cylinder strokes for that cylinder, represented by the hatch marks along the horizontal axis, along with combustion events, represented by the stars also along the horizontal axis. For the PFI injected/commanded for AFR curve, the proportion of injected fuel vs. fuel needed to reach the desired air-fuel ratio is depicted as a proportion in a range from 0-1. 
     Prior to time t 1 , the engine is operating with mid-to-high engine load, as illustrated by curve  602 , and thus skip fire is off (as combustion in all cylinders is needed to deliver the requested torque), as illustrated by curve  604 . All the fuel is injected via the port injector, and as such the proportion of PFI fuel to reach the desired AFR actually injected via PFI is 1, as illustrated by curve  606 . Accordingly, the PFI split ratio is one (illustrated by injection event  608 ) and the DI split ratio is zero. Air-fuel ratio is maintained around a desired air-fuel ratio of stoichiometry, as illustrated by curve  610 . 
     Just prior to time t 1 , engine load starts to drop. As such, the controller beings to initiate a transition into skip fire operation at time t 1 . During the transition into skip fire, MAP, MAF, and other intake manifold and charge air parameters may change as the number of fired cylinders decreases. To compensate for a possible transition into skip fire mode, at time t 1 , the controller initiates the PFI/DI split injection protocol described above with respect to  FIG. 5 . As a result, the fuel quantity injected by the port injector is decreased, e.g., the air-fuel ratio is temperorarily made deliberately lean. For example, rather than delivering 100% of the fuel needed to reach the desired air-fuel ratio, 90% of the fuel needed to reach the desired air-fuel ratio may be delivered via port injection. Then, later in the cylinder cycle, the direct injector injects a make-up pulse to reach the desired air-fuel ratio. Accordingly, the PFI split ratio decreases while the DI split ratio increases. The decreased quantity of fuel injected by the port injector may be based on anticipated changes to the air charge, from the transition into skip fire, for example, and/or from the decreasing engine load. 
     Thus, as illustrated in  FIG. 6 , for the second firing event of cylinder  1 , a port injection event  612  occurs immediately after time t 1 . The port injection event  612  is less than the entire amount of fuel needed to reach the desired air-fuel ratio, due to an anticipated change in air charge between the port injection event and when the intake valve is closed (and thus the air charge amount in the cylinder is set). Then, at direct injection event  614 , the rest of the fuel needed to reach the desired air-fuel ratio, based on the updated air charge amount, is provided. 
     Skip fire operation begins between injection event  612  and injection event  614 . That is, during the first firing event following time t 1 , the engine starts to skip fire. As such, during the course of firing cylinder  1  (e.g., at a time between intake valve opening and closing), a cylinder originally scheduled to be fired is instead skipped (such as cylinder  4 , according to the firing order illustrated in  FIG. 3 ). The skipping of this cylinder results in an increase in the actual air charge as compared to the air charge estimated, and thus an additional amount of fuel is injected via the direct injection event to maintain air-fuel ratio, even as air charge changes over the course of the cylinder cycle for cylinder  1 . The next scheduled firing event for cylinder  1  is a skip fire event, where cylinder  1  is not fired, as illustrated by the dashed star. 
     Prior to time t 2 , the engine load decreases again. This decreasing engine load may cause a change to the maximum possible change in air flow, as the controller may anticipate a shift in the number of skipped cylinders (e.g., the number of skipped cylinders may increase). This increase in the number of skipped cylinders may cause a reduction in the amount of actual charge air trapped in the cylinder  1 , and so the relative proportion of fuel injected by the port injector decreases, as shown by injection event  616 , and the relative proportion of the fuel injected by the direct injector increases, as illustrated by injection event  618 . In some examples, the switch from skipping one to skipping two cylinders may cause a greater air flow disturbance than the switch from skipping no cylinders to skipping one cylinder, and thus the relative proportion of fuel injected by the port injector may be less around time t 2  than the proportion of fuel injected by the port injector around time t 1 . 
     Following time t 2 , engine load stabilizes and the PFI split ratio increases (and the DI split ratio decreases) slightly due to the stabilized engine conditions (for example, the maximum possible change in charge air may be smaller if the load remains steady). This is illustrated by injection event  618  and injection event  620 . 
     The engine load increases again prior to time t 3 , relatively rapidly. Due to the increasing engine load, the controller may predict a transition out of skip fire operation. During a transition out of skip fire, the difference between the estimated air charge and the actual air charge may be a negative value, as the air charge may decrease following the reactivation of all the cylinders. As such, the amount of fuel injected by PFI, as a proportion of the fuel needed to reach the desired air-fuel ratio, illustrated by curve  606 , may decrease. This is because the total amount of fuel needed to maintain the desired air-fuel ratio, after the transition out of skip fire, may be low, and thus to avoid an over-fueling event, the fuel quantity injected by the port injector may be made even lower than the previous injection events, as demonstrated by the injection event  622 . However, because the engine does not actually transition out of skip fire, the air charge amount does not change as anticipated, and thus a relatively large amount of fuel is injected via the direct injector, as illustrated by injection event  624 . After the cylinder firing event following time t 3 , skip operation is terminated. Once termination is complete, the PFI ratio returns to one, as shown by injection event  626 . 
     It is to be understood that the cylinder firing events illustrated in  FIG. 6 , including the combustion events and fuel injection events, are illustrative in nature, and not meant to be limiting. Other configurations are possible. For example, multiple firing events for cylinder  1  may occur between the illustrated firing events, including skipped firing events, in order to maintain an established firing order. In particular, additional firing events may occur between the firing event before time t 3  and the firing event after time t 3 , or the firing order of the engine may change, for example due to the additional number of skipped cylinders following the load drop at time t 2 . 
     Thus, the description above with respect to  FIGS. 5 and 6  discloses “make-up” pulses of fuel that may be injected after the main fuel injection event, to compensate for air flow changes that may occur between when port injection occurs (before intake valve opening) and when direct injection occurs (after the intake valve opens and near intake valve closing). However, such an approach relies on a port injector and a direct injector, which may be costly to install and complicated to control. Thus, a more cost-effective mechanism for compensating for air flow changes during skip fire includes using only port injection and compensating for air charge changes during a subsequent firing event. For example, if there is a deviation between a first, predicted air charge, determined at the time of the port injection of a first cylinder, and an air charge calculated later during the cylinder cycle (such as at intake valve closing, when the actual air charge can be determined), additional fuel may be injected during the port injection of a second cylinder that follows the first cylinder in the engine firing order. 
     In this way, the proper amount of fuel for reaching a desired air-flow ratio, based on the first predicted air charge amount, can be injected to the first cylinder (e.g., the amount injected to the first cylinder will not be made purposely lean). Then, if the actual air charge admitted to the first cylinder is different than the predicted air charge amount, the amount of fuel injected to the second cylinder can be increased or decreased accordingly, so that overall engine air-fuel ratio remains steady. The first and second cylinders may be on the same cylinder bank and/or plumbed to the same catalyst to ensure that the exhaust air-fuel ratio and the catalyst remains at the desired air-fuel ratio. 
     Turning now to  FIG. 7 , a method  700  for sensing combustion events during skip fire is illustrated. Method  700  may be carried out as part of method  400 , as explained above, according to instructions stored on controller  12  in order to maintain a set number of skipped cylinders of engine  10 , even in the event of unintended combustion or skip events during skip fire operation. It is to be understood that method  700  is executed after skip fire operation has commenced, for example after setting a commanded firing order that includes firing at least a first cylinder and skipping at least a second cylinder. Method  700  includes, at  702 , activating fuel injection, valve actuation, and spark ignition to fire the first cylinder. At  704 , feedback from one or more ionization sensors is received to determine the combustion status of the first cylinder, following spark. For example, the first cylinder may include an ionization sensor (such as sensor  194 ) that detects the presence of smoke or other combustion products. As such, feedback from the ionization sensor may indicate if combustion did or did not occur in the cylinder follow spark. 
     At  706 , method  700  includes determining if combustion occurred in the first cylinder, based on the feedback from the ionization sensor. If combustion did not occur, method  700  proceeds to  708  to adjust the commanded firing order to fire a next cylinder scheduled to be skipped in the commanded firing order. At  710 , fuel injection, valve actuation, and spark are activated to fire the next cylinder. At  712 , after the next cylinder is fired (based on feedback from the ionization sensor, for example), the original commanded firing order is resumed, and then method  700  returns. 
     However, if combustion does occur as scheduled in the first cylinder at  706 , method  700  proceeds to  714  to deactivate fuel injection and valve actuation to skip the second cylinder (e.g., the cylinder scheduled to be skipped in the commanded firing order). While some engine configurations may also disable spark during skipping of a cylinder, other engine configurations may maintain spark even to skipped cylinders. At  716 , feedback is received from an ionization sensor (e.g., an ionization sensor of the second cylinder) to determine the combustion status of the second cylinder. 
     At  718 , method  700  includes determining if combustion occurred in the second cylinder. If combustion did not occur, and the second cylinder was skipped as scheduled, method  700  proceeds to  720  continue firing and skipping cylinders according to the commanded firing order and dynamically adjusting the commanded firing order if indicated, for example in response to an unintended combustion or skip event. Method  700  then returns. 
     If at  718  it is instead determined that combustion did occur in the second cylinder, method  700  proceeds to  722  to adjust the commanded firing order to skip the next cylinder scheduled to be fired. At  724 , fuel injection and valve actuation are deactivated to skip the next cylinder. At  726 , after the next cylinder has been skipped, the original commanded firing order is resumed, and method  700  returns. 
     Thus, method  700  provides for firing and skipping cylinders according to a commanded firing order of the engine during a skip fire operation. For each cylinder, whether the cylinder is scheduled to be fired or scheduled to be skipped, the combustion status of the cylinder is monitored via ionization sensing. For example, spark ignition, and hence combustion, typically occur at some time in the late compression stroke or early expansion stroke. Thus, the feedback from the one or more ionization sensors may be collected and monitored during the compression and expansion strokes for each cylinder, at each engine cycle. If combustion occurs in a cylinder scheduled to be skipped, the commanded firing order of the engine is updated to skip the next cylinder in the firing order scheduled to be fired, thus maintaining the correct number of skipped cylinders and maintaining torque. Similarly, if combustion does not occur in a cylinder scheduled to be fired, the next cylinder in the firing order scheduled to be skipped may instead be fired. While the above examples adjust the firing status of the next cylinder in the firing order if an unintended combustion event or skip event is detected, in some circumstances a later cylinder in the firing order may be adjusted, to balance the firing order of the engine and prevent NVH issues, for example 
       FIG. 8  illustrates example firing events for cylinders of an engine according to the method of  FIG. 7 . The cylinder firing plots of  FIG. 8  are similar to the firing plots of  FIGS. 2-3 . As such, the same original engine firing order (1-3-4-2) and commanded firing order during skip fire (skip one cylinder for every two cylinders fired) apply. Thus, a first combustion event occurs in CYL.  1 , illustrated by star  800 , and a second combustion event occurs in CYL.  3 , illustrated by star  802 . According to the commanded firing order of the engine, CYL.  4  is scheduled to be skipped. However, an unintended combustion event occurs in CYL.  4 , as illustrated by star  804 . To compensate, the next cylinder scheduled to be fired, CYL.  2 , is instead skipped, as shown by dashed star  806 . The commanded firing order then resumes with a combustion event in CYL.  1  (star  808 ) and so forth. 
     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. 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, 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 engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.