Patent Publication Number: US-9422873-B2

Title: Methods and systems for operating an engine

Description:
FIELD 
     The present description relates to a system and methods for improving engine efficiency and performance. The systems and methods may be particularly useful for engines that include cylinder deactivation and variable compression ratio. 
     BACKGROUND AND SUMMARY 
     An engine of a vehicle may include cylinder deactivation to improve engine efficiency. Engine cylinders may be selectively activated and deactivated to reduce engine pumping losses and adjust engine torque output. The engine cylinders may be activated and deactivated based on an engine torque threshold. For example, if the desired engine torque is greater than a threshold torque, all engine cylinders may be activated. If desired engine torque is less than the threshold torque, a fraction of engine cylinders maybe activated. Thus, the threshold engine torque is a condition for selecting between differing active cylinder displacements. However, it may not be desirable for the engine to switch between different active cylinder displacements at the same engine torque threshold at all times. 
     The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: varying an engine torque at which engine cylinders are activated in response to a compression ratio of the engine. 
     By adjusting an engine torque at which deactivated engine cylinders are reactivated in response to a compression ratio of an engine, it may be possible to provide the technical result of reducing the possibility of engine knock during cylinder reactivation. Further, it may be possible to increase engine efficiency since the method described herein provides a way to select different engine compression ratios in response to when the engine may operate more efficiently with the selected compression ratio. 
     In some examples, the torque threshold at which deactivated engine cylinders are reactivated may be adjusted in response to a rate of engine torque increase. For example, if engine torque is increased at a higher rate, the torque threshold at which deactivated engine cylinders are reactivated may be decreased as compared to the torque threshold at which deactivated engine cylinders are reactivated when engine torque is increased at a lesser rate. Further, adjustment of a compression ratio of the engine may be delayed until deactivated cylinders are reactivated when the rate of change in engine torque is greater than a threshold rate of change in engine torque. 
     The present description may provide several advantages. Specifically, the approach may reduce engine knock. Additionally, the approach may improve engine efficiency. Further, the approach may improve vehicle drivability by reducing the possibility of adjusting engine compression ratio at the same time cylinders are being activated or deactivated. 
     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; 
         FIGS. 2 and 3  show example simulated plots of engine efficiency versus engine torque; 
         FIG. 4  shows an example simulated engine operating sequence; and 
         FIGS. 5-8  show an example method for operating an engine. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to controlling operation of an engine that may selectively activate and deactivate cylinders to vary active cylinder displacement. The engine may also include capabilities for variable compression rates.  FIG. 1  shows an example engine system that includes mechanisms for varying both active cylinder displacement and compression ratio. The engine may operate as indicated in the engine efficiency versus engine torque plots as shown in  FIGS. 2 and 3 . The engine may also operate as shown in the sequence shown in  FIG. 4 .  FIGS. 5-8  are a flowchart of a method for operating an engine. The engine of  FIG. 1  may be operated according to the method of  FIGS. 5-8  to provide the sequence shown in  FIG. 4 . 
     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 . Variable compression adjusting device  31  may increase or decrease compression in cylinders by increasing or decreasing piston height. Alternatively, variable compression adjusting device  31  may adjust the effective connecting rod length, cranktrain geometry, crankshaft position, cylinder head position, or clearance volume to adjust the engine compression ratio. In still other examples, the effective engine compression ratio may be adjusted via advancing or retarding timing of intake valve  52  via valve adjusting mechanism  71 . 
     Flywheel  97  and ring gear  99  are coupled to crankshaft  40 . Starter  96  includes pinion shaft  98  and pinion gear  95 . Pinion shaft  98  may selectively advance pinion gear  95  to engage ring gear  99 . Starter  96  may be directly mounted to the front of the engine or the rear of the engine. In some examples, starter  96  may selectively supply torque to crankshaft  40  via a belt or chain. In one example, starter  96  is in a base state when not engaged to the engine crankshaft. 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 an intake cam  51  and an exhaust cam  53 . The position of intake cam  51  may be determined by intake cam sensor  55 . The position of exhaust cam  53  may be determined by exhaust cam sensor  57 . Intake cam  51  and exhaust cam  53  may be moved relative to crankshaft  40  via valve adjusting mechanisms  71  and  73 . Valve adjusting mechanisms  71  and  73  may also deactivate intake and/or exhaust valves in closed positions so that intake valve  52  and exhaust valve  54  remain closed during a cylinder cycle. 
     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 (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). In one example, a high pressure, dual stage, fuel system may be used to generate higher fuel pressures. In addition, intake manifold  44  is shown communicating with optional electronic throttle  62  which adjusts a position of throttle plate  64  to control air flow from air intake  42  to intake manifold  44 . 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 accelerator pedal  130  for sensing force applied by driver  132 ; a measurement of engine manifold 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 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 . 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. 2 , a plot of engine torque versus engine efficiency for an engine not having selective cylinder deactivation and activation is shown. The Y axis represents engine efficiency and engine efficiency increases in the direction of the Y axis arrow. The X axis represents engine torque and engine torque increases in the direction of the X axis arrow. 
     Solid line  202  represents engine efficiency versus engine torque for a knock limited engine operating with a higher compression ratio (e.g., 11:1). Dash dot line  204  represents engine efficiency versus engine torque for the same engine operating with a lower compression ratio (e.g., 9:1). Dotted line  203  represents engine efficiency for the same engine when the engine is not knock limited while operating with a higher compression ratio. Dotted line  203  is not visible where solid line  202  overlaps dotted line  203 . 
     Thus, it may be observed that the engine operates more efficiently at lower to middle torque levels when the engine is operated with a higher compression ratio. However, at higher engine torques, when the engine is knock limited with the higher compression ratio, engine efficiency and torque are reduced at higher engine torques (e.g., due to spark retard) as compared to when the same engine is operated with a lower compression ratio or when the engine is not knock limited. Additionally, when the engine is operated with a higher compression ratio and is not knock limited, engine efficiency and torque are improved as compared to when the same engine is operated with a lower compression ratio. 
     Referring now to  FIG. 3 , a plot of an engine operating with knock limited higher compression ratio, lower compression ratio, not knock limited higher compression ratio, cylinder deactivation, and all cylinders active is shown. The Y axis represents engine efficiency and engine efficiency increases in the direction of the Y axis arrow. The X axis represents engine torque and engine torque increases in the direction of the X axis arrow. 
     Vertical line  301  represents a first engine torque threshold for adjusting engine compression ratio. Vertical line  305  represents a second engine torque threshold for activating deactivated cylinders when the engine is operating with a higher compression ratio and is knock limited. Vertical line  309  represents a third engine torque threshold for activating deactivated cylinders when the engine is operating with a lower compression ratio. Vertical line  311  represents a fourth engine torque threshold for activating deactivated cylinders when the engine is operating with a higher compression ratio and is not knock limited. Vertical line  323  represent a fifth engine torque threshold for adjusting engine compression ratio. Vertical line  325  represents a maximum engine torque when the engine is operating with all cylinders active at a lower compression ratio. Vertical line  327  represents a maximum engine torque when the engine is operating with all cylinders active at a higher compression ratio and is not knock limited. 
     Solid lines  302  and  306  represent engine efficiency versus engine torque for a knock limited engine operating with a higher compression ratio (e.g., 11:1). Dash dot lines  304  and  308  represent engine efficiency versus engine torque for the same engine operating with a lower compression ratio (e.g., 9:1). Dotted lines  303  and  307  represent engine efficiency for the same engine when the engine is not knock limited while operating with a higher compression ratio. Dotted lines  303  and  307  are not visible where solid lines  302  and  306  overlap dotted lines  303  and  307 . Lines  302 ,  304 , and  303  represent engine efficiency versus engine torque when a fraction of engine cylinders are deactivated (e.g., no spark or fuel while intake and exhaust valves are closed during a cylinder cycle). Lines  306 ,  308 , and  307  represent engine efficiency versus engine torque when all engine cylinders are operating. 
     A first engine torque region A is indicated by arrow  320 . Region A begins at a low engine torque and it ends at an intersection of lines  302  and  304 . The intersection represents an engine operating condition where engine efficiency versus engine torque is equivalent when the engine is operating with higher or lower compression. It may be observed that it may be more desirable to operate the engine with a higher compression ratio in region A since engine efficiency is improved by the higher compression ratio. 
     A second engine torque region B is the amount of engine torque between vertical line  301  and vertical line  305 . Region B ends at the maximum engine torque for operating the engine in a knock limited higher compression mode when a fraction of engine cylinders are deactivated. 
     A third engine torque region C is the amount of engine torque between vertical line  301  and vertical line  309 , which is indicated by arrow  322 . Region C ends at the maximum engine torque for operating the engine in a lower compression mode when a fraction of engine cylinders are deactivated. The engine operates with higher efficiency in region C as compared to when the engine is operated in region B. The increase of engine torque between operating the engine in region B and C is indicated by arrow  324 . Thus, it may be more desirable to operate the engine with a lower compression ratio in region C since engine efficiency is improved with a lower engine compression ratio. 
     A fourth engine torque region D is the amount of engine torque between vertical line  301  and vertical line  311 . Region D ends at the maximum engine torque for operating the engine in a higher compression mode where the engine is not knock limited. The engine may not be knock limited during selected conditions such as when the engine is not warm and when ambient temperature is less than a threshold value, or when high octane fuel is used. The engine operates with higher efficiency and torque in region D as compared to when the engine is operated in regions B and C. The increase of engine torque between operating the engine in region C and D is indicated by arrow  331 . 
     A fifth engine torque region E is the amount of engine torque between vertical line  309  and line  323 . However, in some examples region E may be expressed as the amount of engine torque between line  305  and line  323 , or between line  311  and line  323 . Line  323  is the amount of torque at the intersection of lines  306  and  308 . It may be observed that it may be more desirable to operate the engine with a higher compression ratio in region E since engine efficiency is improved by the higher compression ratio. 
     A sixth engine torque region F is the amount of engine torque between vertical line  323  and line  325 , which is indicated by arrow  328 . Region F may be extended from line  323  to line  327 , thereby increasing engine torque by an amount indicated by arrow  333  if the engine may be operated with a higher compression ratio without being knock limited. 
     The inventors herein have recognized that the engine may be operated most efficiently when the engine is knock limited by following line  302  to vertical line  301 , following line  304  from vertical line  301  to vertical line  309 , following line  306  from vertical line  309  to vertical line  323 , and following line  308  to higher engine torques. Thus, if the engine transitions from a lower torque to a higher torque, the engine may start with a group of cylinders deactivated while active cylinders operate at a higher compression ratio, the engine switches to a lower compression as engine torque increases while selected cylinders remain deactivated, engine cylinders are reactivated and compression ratio in all cylinders is increased to a higher compression ratio as engine torque increases further, and the engine switches to lower compression with all cylinders active at even higher engine torques. In this way, engine efficiency may be maintained at a higher level while engine torque transitions from a lower torque to a higher torque. 
     On the other hand, if engine torque is changing by more than a threshold rate of engine torque increase or decrease, it may be desirable to maintain the engine at a higher compression ratio before and after reactivating cylinders so that the engine may produce as much torque as possible in a short amount of time. Further, by maintaining the engine compression ratio during cylinder reactivation, it may be possible to provide a smoother torque transition between deactivating and reactivating cylinders. Thus, changes in engine compression ratio may be inhibited or stopped when the rate of engine torque change is greater than a threshold amount. 
       FIG. 3  illustrates an engine which can only deactivate a constant fraction of the cylinders, for example a 6-cylinder engine which can deactivate 3 cylinders. It is well known that other arrangements are possible, for example a 6-cylinder engine which can deactivate either 2 or 3 cylinders at various times. Such engines would have multiple torque regions with various combinations of compression ratio and number of deactivated cylinders, but the logic would be similar to  FIG. 3 . 
     Referring now to  FIG. 4 , an example engine operating sequence is shown. The operating sequence of  FIG. 4  may be provided by the engine system of  FIG. 1  executing the method of  FIGS. 5-8 . Times of interest in the sequence are indicated by vertical time markers T0-T8. 
     The first plot from the top of  FIG. 4  is a plot of engine compression ratio versus time. The Y axis represents compression ratio. A lower compression ratio is indicated when the compression ratio trace is closer to the X axis. A higher compression ratio is indicated when the compression ratio trace is closer to the Y axis arrow. The X axis represents time and time increases in the direction of the X axis arrow. 
     The second plot from the top of  FIG. 4  is a plot of engine cylinder deactivation status versus time. The Y axis represents cylinder deactivation status. Deactivated cylinders are indicated when the cylinder deactivation trace is closer to the X axis. All cylinders active is indicated when the engine cylinder deactivation trace is closer to the Y axis arrow. The X axis represents time and time increases in the direction of the X axis arrow. 
     The third plot from the top of  FIG. 4  is a plot of engine torque versus time. The Y axis represents engine torque, or alternatively desired engine torque, and engine torque increases in the direction of the Y axis arrow. The X axis represents time and time increases in the direction of the X axis arrow. Horizontal line  402  represents a first engine torque amount where an engine may be switched from a higher compression ratio to a lower compression ratio (e.g., torque at line  301  of  FIG. 3 ). Horizontal line  404  represents a second engine torque amount where an engine may be switched from operating with deactivated cylinders to being operated with all active cylinders when the engine is knock limited and operated at a higher compression ratio (e.g., torque at line  305  of  FIG. 3 ). Horizontal line  406  represents a third engine torque amount where an engine may be switched from operating with deactivated cylinders to being operated with all active cylinders when the engine is operated at a lower compression ratio (e.g., torque at line  309  of  FIG. 3 ). Horizontal line  408  represents a fourth engine torque amount where an engine may be switched from operating with deactivated cylinders to being operated with all active cylinders when the engine is not knock limited and is operated at a higher compression ratio (e.g., torque at line  311  of  FIG. 3 ). Horizontal line  410  represents a fifth engine torque amount where the engine may be switched from operating with a higher compression ratio to operating with a lower compression ratio (e.g., torque at line  323  of  FIG. 3 ). 
     At time T0, the engine compression ratio is at a higher level, cylinders are deactivated, and the engine torque amount is low. Such conditions may be representative of when a vehicle in which the engine operates is at very low vehicle speed. 
     Between time T0 and time T1, the engine torque increases at a rate greater than a threshold rate of torque increase. The compression ratio is maintained at a higher level and the cylinders remain deactivated. 
     At time T1, cylinders are reactivated without the engine compression ratio having changed from a higher level to a lower level. The engine cylinders are reactivated at an engine torque level indicated by line  404  and in response to the rate of engine torque increasing by more than a threshold amount. 
     At time T2, the engine torque has continued to increase to a level indicated by line  410 . The engine compression ratio is reduced from a higher level to a lower level to improve engine efficiency and increase the amount of available engine torque since the engine is knock limited at higher engine torque in this example. In this way, a transition from operating the engine at a higher compression ratio to lower compression ratio may be avoided during engine torque changes that are greater than a threshold rate of torque change. However, in some examples, it may be desirable to switch engine compression ratio and activate cylinders at the same engine torque. Additionally, if the engine had been operating at a lower compression ratio before the engine cylinders were reactivated, the engine would have continued to operate in the lower compression mode until after all cylinders were reactivated. 
     Between time T2 and time T3, the engine torque increases and then begins to decrease. The engine compression ratio and number of active cylinders remains constant during this time. 
     At time T3, the engine torque is below a threshold and the rate of engine torque change is less than a threshold amount. Therefore, the engine compression ratio is increased from a lower compression ratio to a higher compression ratio to increase engine efficiency. The number of active cylinders remains the same and engine torque continues to decrease. 
     At time T4, the engine torque is reduced to a predetermined torque less than the engine torque level indicated by line  406 . Therefore, a portion of engine cylinders are deactivated and engine compression ratio is reduced in response to the engine torque being less than the torque indicated by line  406 . The engine torque levels out to a value between torques indicated by lines  402  and  406 . 
     At time T5, the engine torque has increased to a level of torque indicated by line  406 . Consequently, all engine cylinders are reactivated and the engine compression ratio is increased to a higher compression ratio to improve engine torque output and efficiency. The engine torque at and before time T5 is changing at a rate less than a threshold rate of torque change. As a result, the engine compression ratio and number of active cylinders is not changed in response to the rate of torque change, but rather in response to the engine torque amount. 
     At time T6, the engine torque amount has increased to a level indicated by line  410 . The engine compression ratio is reduced and all cylinders remain activated in response to the engine reaching the torque level of line  410  so that engine efficiency and maximum torque may be increased. The engine torque continues to increase after time T6. 
     Engine torque decreases at a rate greater than a threshold amount before time T7. The engine is also operating with all cylinders active and a lower compression ratio before time T7. Engine cylinders are deactivated at time T7 in response to the engine torque being lower than the level indicated by line  406  and the rate of engine torque changing by more than a threshold rate of torque change. In this way, engine cylinders may be deactivated without the engine first having to change back and forth between lower and higher compression modes before engine cylinders are deactivated. 
     At time T8, the engine torque is reduced to a level indicated by line  402 . The engine compression ratio is increased at time T8 in response to engine torque being less than the level indicated by line  402 . The engine compression ratio is increased at time T8 to improve engine efficiency at lower engine torques. 
     Thus, engine compression ratio and cylinder activation/deactivation may be adjusted or varied in response to engine torque and rate of engine torque change. By changing engine compression ratio responsive to engine torque, engine efficiency may be improved. However, frequency of engine compression ratio changes may be reduced in response to a higher rate of engine torque change so that busyness of compression ratio changes may be reduced, thereby reducing the possibility of inducing torque disturbances to the vehicle driveline. 
     Referring now to  FIGS. 5-8 , a method for operating an engine is described. The method of  FIGS. 5-8  may be included as executable instructions stored in non-transitory memory of a controller as shown in  FIG. 1 . The method of  FIGS. 5-8  may provide the operating sequence shown in  FIG. 4 . 
     At  502 , method  500  determines engine operating conditions, which may include engine torque, engine speed, engine temperature, ambient temperature, fuel octane, etc. In one example, engine torque may be estimated based on engine speed and an amount of air entering the engine. The engine air amount and speed are used to index a table or function that describes engine torque as a function of engine air amount and engine speed. Engine speed is determined from engine crankshaft position. Method  500  proceeds to  504  after engine operating conditions are determined. 
     At  504 , method  500  determines a present engine compression ratio. In one example, the present engine compression ratio may be determined via a position of an engine compression ratio adjusting device. Alternatively, method  500  may determine engine compression ratio from a value of a variable stored in memory. Method  500  proceeds to  506  after the engine compression ratio is determined. 
     At  506 , method  500  judges whether or not variable compression ratio (VCR) for engine cylinders is available. In some examples, all engine cylinders may include compression ratio adjusting devices. In other examples, only cylinders that are active at all times may include compression ratio adjusting devices. Method  500  may judge whether or not the engine is a variable compression ratio engine based on a value of a variable stored in controller memory. If method  500  judges that the engine is a variable compression ratio engine, the answer is yes and method  500  proceeds to  520 . Otherwise, the answer is no and method  500  proceeds to  508 . 
     At  508 , method  500  judges whether or not engine torque is greater than (G.T.) a threshold torque where engine cylinders are to be reactivated. If method  500  judges that engine torque is greater than the threshold torque, the answer is yes and method  500  proceeds to  510 . Otherwise, the answer is no and method  500  proceeds to  512 . 
     It should be noted that hysteresis may also be included at  508  so that busyness of cylinder activation and deactivation may be reduced. For example, if engine torque is decreasing, method  500  may not move to  512  until engine torque is less than the threshold torque by a predetermined amount. 
     At  510 , method  500  activates all or a subset of engine cylinders to increase the engine&#39;s output torque capacity. Engine cylinders may be reactivated by allowing intake and exhaust valves to open and close during a cylinder cycle. Further, spark and fuel may be supplied to engine cylinders to reactivate the cylinders. Method  500  proceeds to exit after engine cylinders are reactivated. 
     At  512 , method  500  deactivates a portion of engine cylinders to reduce the engine torque capacity and engine pumping losses. Engine cylinders may be deactivated by closing and holding closed intake and exhaust valves during an engine cycle. Fuel and spark may also stop being provided to cylinders that are deactivated. Method  500  proceeds to exit after engine cylinders are deactivated. 
     At  520 , method  500  determines engine spark retard from minimum spark advance for best torque (MBT spark timing). In one example, MBT spark timing for the present engine speed and torque is stored in controller memory. Spark retard from MBT spark timing is determined by subtracting the present spark timing from MBT spark timing. The present spark timing may be adjusted to a more retarded value in response to engine knock. The amount of spark retard from MBT spark timing may be a basis for switching from a higher compression ratio to a lower compression ratio. Method  500  proceeds to  522  after spark retard from MBT spark timing is determined. 
     At  522 , method  500  determines a rate of change in engine torque. In one example, rate of engine torque change is determined by subtracting a last past value of engine torque from a present value of engine torque. The rate of change of engine torque may be positive if engine torque is increasing and negative if engine torque is decreasing. Method  500  proceeds to  524  after the rate of engine torque change is determined. 
     At  524 , method  500  judges whether or not the engine is in a higher compression mode. Method  500  may judge whether or not the engine is in a higher compression ratio based on a position of a variable compression adjusting device or a value of a variable stored in controller memory. If method  500  judges that the engine is operating at a higher compression ratio, the answer is yes and method  500  proceeds to  593 . Otherwise, the answer is no and method  500  proceeds to  528 . 
     At  528 , method  500  judges whether or not the rate of engine torque change determined at  522  is greater than a threshold rate. The threshold rate of engine torque change may vary with engine speed, engine temperature, and other operating conditions. If method  500  judges that the rate of engine torque change is greater than a threshold rate, the answer is yes and method  500  proceeds to  530 . Otherwise, the answer is no and method  500  proceeds to  540 . 
     At  528 , method judges whether or not all engine cylinders are active. In one example, method  500  judges whether or not all cylinders are active based on engine valve states or a value of a variable stored in controller memory. If method  500  judges that all cylinders are active, the answer is yes and method  500  proceeds to  570 . Otherwise, the answer is no and method  500  proceeds to  532 . 
     At  530 , method  500  activates all engine cylinders when engine torque is greater than a third threshold torque level (e.g., line  309  of  FIG. 3 ), which represents engine torque for reactivating deactivated engine cylinders when the engine is operating at a lower compression ratio before cylinders are reactivated. If engine torque is less than the third threshold torque level and engine torque is increasing, engine cylinders are not reactivated. On the other hand, if engine torque is decreasing, a portion of engine cylinders are deactivated when engine torque is a predetermined amount of torque less than the third threshold engine torque. Method  500  returns to  528  after engine cylinders are conditionally reactivated. 
     At  540 , method  500  judges whether or not engine torque is less than (L.T.) a first threshold torque (e.g., line  301  of  FIG. 3 ). The first torque represents an engine torque for adjusting from a lower compression ratio to a higher compression ratio. The first torque is at an intersection of engine efficiency versus engine torque curves for higher and lower engine compression ratios. If method  500  judges that engine torque is less than the first torque, the answer is yes and method  500  proceeds to  542 . Otherwise, the answer is no and method  500  proceeds to  550 . 
     At  542 , method  500  deactivates a portion of engine cylinders. Engine cylinders are deactivated by closing and holding closed cylinder intake and exhaust valves during a cycle of the engine. Additionally, spark and fuel flow to the cylinders is also stopped. Method  500  proceeds to  544  after a portion of engine cylinders are deactivated. 
     At  544 , method  500  judges whether or not the engine is spark is limited to low compression efficiency for torques less than the first threshold torque (e.g., region A of  FIG. 3 ). In one example, the engine is spark limited to lower compression ratio engine efficiency when spark retard from MBT determined at  520  is greater than a threshold amount of spark retard. Further, the spark retard from MBT spark timing for particular engine operating conditions may be stored in memory and used to adjust a curve (e.g.,  302  of  FIG. 3 ) of engine efficiency versus engine torque. If method  500  judges that the engine is spark limited to lower compression ratio engine efficiency, the answer is yes and method  500  proceeds to  548 . Otherwise, the answer is no and method  500  proceeds to  546 . 
     At  546 , method  500  switches active cylinders to a higher compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a higher compression ratio. In one example, engine cylinders are adjusted to a higher compression ratio via commanding a variable compression adjusting device to a higher compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  548 , method  500  switches active cylinders to a lower compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a lower compression ratio. In one example, engine cylinders are adjusted to a lower compression ratio via commanding a variable compression adjusting device to a lower compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  550 , method  500  judges whether or not a portion of engine cylinders is deactivated. In one example, method  500  judges whether or not a portion of engine cylinders are deactivated based on a position of valve adjusting mechanisms or a value of a variable in controller memory. If method  500  judges that a portion of engine cylinders is deactivated, the answer is yes and method  500  proceeds to  552 . Otherwise, the answer is no and method  500  proceeds to  570 . 
     At  552 , method  500  judges whether or not the engine is spark is limited to low compression efficiency for torques less than the forth threshold torque (e.g., region D of  FIG. 3 ) and greater than the first threshold torque. In one example, the engine is spark limited to lower compression ratio engine efficiency when spark retard from MBT determined at  520  is greater than a threshold amount of spark retard. Further, the spark retard from MBT spark timing for particular engine operating conditions may be stored in memory and used to adjust a curve (e.g.,  302  of  FIG. 3 ) of engine efficiency versus engine torque. If method  500  judges that the engine is spark limited to lower compression ratio engine efficiency, the answer is yes and method  500  proceeds to  554 . Otherwise, the answer is no and method  500  proceeds to  560 . 
     At  560 , method  500  switches active cylinders to a higher compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a higher compression ratio. In one example, engine cylinders are adjusted to a higher compression ratio via commanding a variable compression adjusting device to a higher compression mode. Method  500  proceeds to  562  after the engine compression ratio is adjusted. 
     At  562 , method  500  judges whether or not engine torque is greater than or equal to a fourth threshold torque (e.g., torque at line  311  of  FIG. 3 ). The fourth threshold torque represents an engine torque for activating deactivated cylinder when the engine is operating with a higher compression ratio. If the engine is operating with a higher compression ratio at  562  the engine is not knock limited at the present engine operating conditions. If method  500  judges that engine torque is greater than or equal to the fourth torque, the answer is yes and method  500  proceeds to  564 . Otherwise, the answer is no and method  500  proceeds to exit. 
     At  564 , all engine cylinders are activated. All engine cylinders may be activated by allowing intake and exhaust valves to open and close during a cylinder cycle. Fuel and spark may also be supplied to the engine to reactivate cylinders. Method  500  proceeds to exit after all engine cylinders are activated. 
     At  554 , method  500  method  500  switches active cylinders to a lower compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a lower compression ratio. In one example, engine cylinders are adjusted to a lower compression ratio via commanding a variable compression adjusting device to a lower compression mode. Method  500  proceeds to  556  after the engine compression ratio is adjusted. 
     At  556 , method  500  judges whether or not engine torque is greater than or equal to a third threshold torque (e.g., torque at line  309  of  FIG. 3 ). The third threshold torque represents an engine torque for activating deactivated cylinder when the engine is operating with a lower compression ratio. If method  500  judges that engine torque is greater than or equal to the third torque, the answer is yes and method  500  proceeds to  558 . Otherwise, the answer is no and method  500  proceeds to exit. 
     At  558 , all engine cylinders are activated. All engine cylinders may be activated by allowing intake and exhaust valves to open and close during a cylinder cycle. Fuel and spark may also be supplied to the engine to reactivate cylinders. Method  500  proceeds to exit after all engine cylinders are activated. 
     At  570 , method  500  judges whether or not engine torque is less than a second torque threshold (e.g., torque at line  305  of  FIG. 3 ) torque by more than a predetermined amount. The predetermined amount provides hysteresis in the cylinder activation and deactivation torques to reduce undesirable cylinder deactivation. If method  500  judges that engine torque is less than the second threshold torque, the answer is yes and method  500  proceeds to  572 . Otherwise, the answer is no and method  500  proceeds to  580 . 
     At  572 , method  500  judges whether or not the engine is spark is limited to low compression efficiency for torques less than the second threshold torque (e.g., region B of  FIG. 3 ) and greater than the first threshold torque. In one example, the engine is spark limited to lower compression ratio engine efficiency when spark retard from MBT determined at  520  is greater than a threshold amount of spark retard. Further, the spark retard from MBT spark timing for particular engine operating conditions may be stored in memory and used to adjust a curve (e.g.,  302  of  FIG. 3 ) of engine efficiency versus engine torque. If method  500  judges that the engine is spark limited to lower compression ratio engine efficiency, the answer is yes and method  500  proceeds to  574 . Otherwise, the answer is no and method  500  proceeds to  578 . 
     At  574 , method  500  switches active cylinders to a lower compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a lower compression ratio. In one example, engine cylinders are adjusted to a lower compression ratio via commanding a variable compression adjusting device to a lower compression mode. Method  500  proceeds to  576  after the engine compression ratio is adjusted. 
     At  578 , method  500  switches active cylinders to a higher compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a higher compression ratio. In one example, engine cylinders are adjusted to a higher compression ratio via commanding a variable compression adjusting device to a higher compression mode. Method  500  proceeds to  576  after the engine compression ratio is adjusted. 
     At  576 , method  500  deactivates a portion of engine cylinders. Engine cylinders are deactivated by closing and holding closed cylinder intake and exhaust valves during a cycle of the engine. Additionally, spark and fuel flow to the cylinders is also stopped. Method  500  proceeds to exit after a portion of engine cylinders are deactivated. 
     At  580 , method  500  judges whether or not engine torque is less than a fifth torque threshold (e.g., torque at line  323  of  FIG. 3  or region E) torque. If method  500  judges that engine torque is less than the fifth threshold torque, the answer is yes and method  500  proceeds to  582 . Otherwise, the answer is no and method  500  proceeds to  588 . 
     At  582 , method  500  judges whether or not the engine is spark is limited to low compression efficiency for torques less than the fifth threshold torque (e.g., region E of  FIG. 3 ) and greater than the fourth threshold torque. In one example, the engine is spark limited to lower compression ratio engine efficiency when spark retard from MBT determined at  520  is greater than a threshold amount of spark retard. Further, the spark retard from MBT spark timing for particular engine operating conditions may be stored in memory and used to adjust a curve (e.g.,  302  of  FIG. 3 ) of engine efficiency versus engine torque. If method  500  judges that the engine is spark limited to lower compression ratio engine efficiency, the answer is yes and method  500  proceeds to  586 . Otherwise, the answer is no and method  500  proceeds to  584 . 
     At  586 , method  500  switches active cylinders to a lower compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a lower compression ratio. In one example, engine cylinders are adjusted to a lower compression ratio via commanding a variable compression adjusting device to a lower compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  584 , method  500  switches active cylinders to a higher compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a higher compression ratio. In one example, engine cylinders are adjusted to a higher compression ratio via commanding a variable compression adjusting device to a higher compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  588 , method  500  judges whether or not the engine is spark is limited when operated at a higher compression ratios (e.g., in region F of  FIG. 3 ). If method  500  judges that the engine is spark limited at higher compression ratios, the answer is yes and method  500  proceeds to  590 . Otherwise, the answer is no and method  500  proceeds to  592 . 
     At  590 , method  500  switches active cylinders to a lower compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a lower compression ratio. In one example, engine cylinders are adjusted to a lower compression ratio via commanding a variable compression adjusting device to a lower compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  592 , method  500  switches active cylinders to a higher compression ratio. In some examples where all engine cylinders are variable compression ratio, all engine cylinders may be adjusted to a higher compression ratio. In one example, engine cylinders are adjusted to a higher compression ratio via commanding a variable compression adjusting device to a higher compression mode. Method  500  proceeds to exit after the engine compression ratio is adjusted. 
     At  593 , method  500  method  500  judges whether or not the rate of engine torque change determined at  522  is greater than a threshold rate. The threshold rate of engine torque change may vary with engine speed, engine temperature, and other operating conditions. Alternatively, method  500  may judge if engine torque is greater than a threshold torque that is requested torque plus a gain factor multiplied by a rate of change of requested engine torque. If method  500  judges that the rate of engine torque change is greater than a threshold rate or if the alternative condition is true, the answer is yes and method  500  proceeds to  594 . Otherwise, the answer is no and method  500  proceeds to  540 . 
     At  594 , method  500  method judges whether or not all engine cylinders are active. In one example, method  500  judges whether or not all cylinders are active based on engine valve states or a value of a variable stored in controller memory. If method  500  judges that all cylinders are active, the answer is yes and method  500  proceeds to  570 . Otherwise, the answer is no and method  500  proceeds to  596 . 
     At  596 , method  500  judges whether or not engine spark is limited to less than low compression engine efficiency (e.g., efficiency of the engine when the engine is operated with a low compression ratio) at the present operating conditions. In one example, method  500  compares engine efficiency at the present operating conditions based on if the engine is operated at a high compression ratio and with spark retard from MBT as compared with operating the engine at the same conditions at a lower compression ratio and knock limited spark timing. If method  500  judges that the engine is spark limited to less than low compression ratio engine efficiency, the answer is yes and method  500  proceeds to  598 . Otherwise, the answer is no and method  500  proceeds to  599 . 
     At  598 , method  500  activates all engine cylinders when engine torque is equal to or greater than a second threshold engine torque (e.g., engine torque at line  305  of  FIG. 3 ) if engine torque is increasing. If engine torque does not reach the thresholds engine torque and engine torque is increasing, method  500  returns to  593  without deactivating a portion or engine cylinders or activating all engine cylinders. On the other hand, if engine torque is decreasing, a portion of engine cylinders are deactivated when engine torque is a predetermined amount of torque less than the second threshold engine torque. 
     At  599 , method  500  activates all engine cylinders when engine torque is equal to or greater than a fourth threshold engine torque (e.g., engine torque at line  311  of  FIG. 3 ). If engine torque does not reach the fourth threshold engine torque, method  500  returns to  593  without activating all engine cylinders. In this way, engine torque at which cylinders are activated may be increased if knock is not detected (e.g., the engine is not spark limited). On the other hand, if engine torque is decreasing, a portion of engine cylinders are deactivated when engine torque is a predetermined amount of torque less than the fourth threshold engine torque. 
     It should be noted that method  500  may also include hysteresis between torque thresholds to activate and deactivate cylinders so that mode switching busyness may be reduced. For example, an engine torque to deactivate cylinders may be lower than an engine torque to activate cylinders. 
     Thus, the method of  FIGS. 5-8  provides for operating an engine, comprising: varying an engine torque at which engine cylinders are activated in response to a compression ratio of the engine. The method includes where the engine cylinders are activated at a first engine torque when the compression ratio is a first compression ratio, and where the engine cylinders are activated at a second engine torque when the compression ratio is a second compression ratio, the first compression ratio being less than the second compression ratio. The method includes where the first engine torque is greater than the second engine torque. 
     The method further comprises adjusting the compression ratio of the engine from a higher value to a lower value in response to an indication of engine knock. The method further comprises increasing the engine torque at which engine cylinders are activated in response to an absence of the indication of engine knock when the engine is operating at a second compression ratio, the second compression ratio greater than a first compression ratio. The method includes where engine cylinders are activated via opening and closing intake and exhaust valves during a cycle of the engine where the intake and exhaust valves were not opening. The method further comprises where the compression ratio is varied based on engine efficiency. 
     In another example, the method includes varying an engine torque at which deactivated cylinders are reactivated in response to a rate of change in engine torque. The method further comprises not varying the engine torque at which deactivated cylinders are reactivated in response to the rate of change in engine torque being less than a threshold rate of change in engine torque. The method includes where the engine torque is a first engine torque when the engine is operating with a lower compression ratio, and where the engine torque is a second engine torque when the engine is operating at a higher compression ratio. The method includes where the first engine torque is greater than the second engine torque. 
     In another example, the method further comprises adjusting a compression ratio of the engine after deactivated cylinders are reactivated in response to the deactivated cylinders being reactivated. The method further comprises not adjusting a compression ratio of the engine after deactivated cylinders are reactivated in response to the deactivated cylinders being reactivated. The method further comprises varying an engine torque at which active cylinders are deactivated in response to a rate of change in engine torque. 
     In another example, the method provides for operating an engine, comprising: varying an engine torque at which deactivated cylinders are reactivated in response to a rate of change of engine torque exceeding a threshold and an engine compression ratio immediately before the rate of change of engine torque exceeds the threshold. The method further comprises varying an engine torque at which active cylinders are deactivated in response to a rate of change in engine torque decrease exceeding a threshold and engine compression ratio immediately before the rate of change in engine torque decrease exceeding the threshold. The method includes where the engine torque at which deactivated cylinders are reactivated is lower when the engine compression ratio is a higher compression ratio. The method includes where the engine torque at which deactivated cylinders are reactivated is higher when engine compression ratio is a lower compression ratio. The method further comprises adjusting a compression ratio of the engine in response to reactivating the deactivated cylinders. The method further comprises not varying the engine torque at which deactivated cylinders are reactivated in response to the rate of change in engine torque being less than a threshold rate of change in engine torque. 
     The method described in  FIGS. 5-8  are for an engine which can only deactivate a constant fraction of the cylinders, for example a 6-cylinder engine which can deactivate 3 cylinders. It is well known that other arrangements are possible, for example a 6-cylinder engine which can deactivate either 2 or 3 cylinders at various times. Such engines would have multiple torque regions with various combinations of compression ratio and number of deactivated cylinders, but fundamentally the method would be similar to  FIGS. 5-8 . 
     As will be appreciated by one of ordinary skill in the art, method described in  FIGS. 5-8  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 steps 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 objects, features, and advantages described herein, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations, methods, 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. 
     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.