Abstract:
Systems and methods for operating a driveline of a hybrid vehicle are described. The systems and methods may improve hybrid driveline performance so that a driver may experience less turbocharger lag and/or less cam phasing lag. The methods and systems may hold engine torque at an elevated level in the presence of a decrease in driver demand torque.

Description:
FIELD 
     The present description relates to methods and a system for operating a driveline of a hybrid vehicle. The methods and systems may be particularly useful for hybrid vehicles that may be operated in a performance mode. 
     BACKGROUND AND SUMMARY 
     A hybrid vehicle may include an internal combustion engine and an electric machine. The internal combustion engine and the electric machine may be selectively operated to propel the vehicle and recover the vehicle&#39;s kinetic energy during deceleration and vehicle braking conditions. The torque demands for the engine and the electric machine may be based on a base strategy that seeks to increase driveline efficiency so that energy consumed by the vehicle is reduced. However, the hybrid vehicle may not perform as is desired under all operating conditions when engine torque and electric machine torque are determined with a primary focus on driveline efficiency. 
     The inventors herein have recognized the above-mentioned issues and have developed an operating method for a hybrid vehicle, comprising: receiving input to a controller; and maintaining an engine torque and adjusting an electric machine torque via the controller in response to a driving maneuver based on the input and in further response to a decrease in a driver demand torque, the driving maneuver being expected to last less than a threshold duration based on the input. 
     By maintaining engine torque or adjusting engine torque to a value closest to engine torque immediately before a decrease in driver demand torque, it may be possible to provide the technical result of improving hybrid vehicle performance. For example, an engine that includes a turbocharger and variable valve timing may not respond as quickly as is desired to an increase in driver demand torque following a decrease in driver demand torque because it may take seconds for the valve timing to change and the turbocharger to reach a speed where a desired engine air flow is provided. However, engine torque may be maintained at or near engine torque before the decrease in driver demand torque while still providing the driver demand torque in combination with the electric machine. Specifically, engine torque may be maintained or adjusted to a torque near the engine torque before the decrease in driver demand torque by increasing magnitude of negative electric machine torque while the engine operates. The magnitude of electric machine torque may be decreased after a driving maneuver to make a large amount of engine torque available almost immediately. In this way, hybrid vehicle performance may be improved so that a driveline torque production is not delayed after performing a maneuver, such as negotiating a road turn in a vehicle. 
     The present description may provide several advantages. Specifically, the approach may reduce driveline torque production hesitation. In addition, the approach may be selectively applied so that energy consumption remains low during less aggressive driving. Further, the approach may be applied to both parallel and serial hybrid drivelines. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, where: 
         FIG. 1  is a schematic diagram of an engine; 
         FIG. 2  is a schematic diagram of a hybrid vehicle driveline; 
         FIG. 3  shows an example hybrid vehicle operating sequence; and 
         FIG. 4  shows an example method for operating a vehicle driveline. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to improving performance of a driveline of a hybrid vehicle during regeneration. The hybrid vehicle may include an engine as is shown in  FIG. 1 . The engine of  FIG. 1  may be included in a driveline as is shown in  FIG. 2 . The driveline may be operated as is shown in  FIG. 3 . The driveline operates according to the method 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  is comprised of cylinder head  35  and block  33 , which include combustion chamber  30  and cylinder walls  32 . Piston  36  is positioned therein and reciprocates via a connection to crankshaft  40 . Flywheel  97  and ring gear  99  are coupled to crankshaft  40 . Starter  96  (e.g., low voltage (operated with less than 30 volts) electric machine) 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 valve  52  may be selectively activated and deactivated by valve activation device  59 . Exhaust valve  54  may be selectively activated and deactivated by valve activation device  58 . Valve activation devices  58  and  59  may be electro-mechanical devices. 
     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. Fuel injector  66  delivers liquid fuel in proportion to the pulse width 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 turbocharger compressor  162  and engine air intake  42 . In other examples, compressor  162  may be a supercharger compressor. Shaft  161  mechanically couples turbocharger turbine  164  to turbocharger compressor  162 . Optional electronic throttle  62  adjusts a position of throttle plate  64  to control air flow from compressor  162  to intake manifold  44 . Pressure in boost chamber  45  may be referred to a throttle inlet pressure since the inlet of throttle  62  is within boost chamber  45 . The throttle outlet is in 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. Compressor recirculation valve  47  may be selectively adjusted to a plurality of positions between fully open and fully closed. Waste gate  163  may be adjusted via controller  12  to allow exhaust gases to selectively bypass turbine  164  to control the speed of compressor  162 . Air filter  43  cleans air entering engine air intake  42 . 
     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 foot  132 ; a position sensor  154  coupled to brake pedal  150  for sensing force applied by foot  152 , 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 ; and a measurement of throttle position from sensor  68 . 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. 
     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. 
       FIG. 2  is a block diagram of a vehicle  225  including a driveline  200 . However, the method described herein is applicable to other configurations. For example, the method of  FIG. 4  may be applied to a system where an engine is selectively coupled to an electric machine via a disconnect clutch, and the electric machine is mechanically coupled to a step ratio transmission. 
     The driveline of  FIG. 2  includes engine  10  shown in  FIG. 1 . Driveline  200  is shown including vehicle system controller  255 , engine controller  12 , electric machine controller  252 , transmission controller  254 , and brake controller  250 . The controllers may communicate over controller area network (CAN)  299 . Each of the controllers may provide information to other controllers such as torque output limits (e.g., torque output of the device or component being controlled not to be exceeded), torque input limits (e.g., torque input of the device or component being controlled not to be exceeded), sensor and actuator data, diagnostic information (e.g., information regarding a degraded transmission, information regarding a degraded engine, information regarding a degraded electric machine, information regarding degraded brakes). Further, the vehicle system controller  255  may provide commands to engine controller  12 , electric machine controller  252 , transmission controller  254 , and brake controller  250  to achieve driver input requests and other requests that are based on vehicle operating conditions. 
     For example, in response to a driver releasing an accelerator pedal and vehicle speed, vehicle system controller  255  may request a desired wheel torque to provide a desired rate of vehicle deceleration. The desired wheel torque may be provided by vehicle system controller requesting a first braking torque from electric machine controller  252  and a second braking torque from brake controller  250 , the first and second torques providing the desired braking torque at vehicle wheels  216 . 
     In other examples, the partitioning of controlling driveline devices may be partitioned differently than is shown in  FIG. 2 . For example, a single controller may take the place of vehicle system controller  255 , engine controller  12 , electric machine controller  252 , transmission controller  254 , and brake controller  250 . 
     In this example, driveline  200  may be selectively powered by engine  10  and electric machine  244 . In other examples, engine  10  may be omitted. 
     Electric machine  244  is in electrical communication with DC/DC inverter  246  and in mechanical communication with two speed planetary transmission  242 . DC/DC inverter supplies electrical power to and receives electrical power from electric energy storage device  275 . Electrical energy storage device  275  may include a controller to regulate battery state of charge and output battery control parameters such as battery state of charge, battery voltage, etc. Alternatively, controller  252  may perform these functions. Planetary transmission  242  may include two or more gears  248  that may be activated or deactivated via clutches  249 . Planetary transmission  242  is mechanically coupled to axle  240 . Planetary transmission  242  may be shifted via controller  254 . The electric machine  244  and planetary transmission  242  may be described as an electric torque path. 
     Engine output torque may be transmitted to torque converter impeller  285  of torque converter  206 . Torque converter  206  includes a turbine  286  to output torque to input shaft  270 . Input shaft  270  mechanically couples torque converter  206  to automatic transmission  208 . Torque converter  206  also includes a torque converter bypass lock-up clutch  212  (TCC). Torque is directly transferred from impeller  285  to turbine  286  when TCC is locked. TCC is electrically operated by controller  12 . Alternatively, TCC may be hydraulically locked. In one example, the torque converter may be referred to as a component of the transmission. 
     When torque converter lock-up clutch  212  is fully disengaged, torque converter  206  transmits engine torque to automatic transmission  208  via fluid transfer between the torque converter turbine  286  and torque converter impeller  285 , thereby enabling torque multiplication. In contrast, when torque converter lock-up clutch  212  is fully engaged, the engine output torque is directly transferred via the torque converter clutch to an input shaft (not shown) of transmission  208 . Alternatively, the torque converter lock-up clutch  212  may be partially engaged, thereby enabling the amount of torque directly relayed to the transmission to be adjusted. The controller  12  may be configured to adjust the amount of torque transmitted by torque converter  206  by adjusting the torque converter lock-up clutch  212  in response to various engine operating conditions, or based on a driver-based engine operation request. 
     Automatic transmission  208  includes gear clutches (e.g., gears  1 - 10 )  211  and forward clutch  210 . Automatic transmission  208  is a fixed ratio transmission. The gear clutches  211  and the forward clutch  210  may be selectively engaged to change a ratio of an actual total number of turns of input shaft  270  to an actual total number of turns of wheels  216 . Gear clutches  211  may be engaged or disengaged via adjusting fluid supplied to the clutches via shift control solenoid valves  209 . Torque output from the automatic transmission  208  may be relayed to axle  240  via output shaft  260 . Specifically, automatic transmission  208  may transfer an input driving torque at the input shaft  270  responsive to a vehicle traveling condition before transmitting an output driving torque to axle  240 . The mechanical torque path includes engine  10 , torque converter  206  and automatic transmission  208 . Transmission controller  254  selectively activates or engages TCC  212 , gear clutches  211 , and forward clutch  210 . Transmission controller also selectively deactivates or disengages TCC  212 , gear clutches  211 , and forward clutch  210 . Transmission controller also selectively activates and deactivates clutches  249  to activate and deactivate gears  248 . 
     Axle  240  combines torque from the mechanical path with torque from the electrical path to rotate wheels  216 . A frictional force may be applied to wheels  216  by engaging friction wheel brakes  218 . In one example, friction wheel brakes  218  may be engaged in response to the driver pressing his foot on a brake pedal (not shown) and/or in response to instructions within brake controller  250 . Further, brake controller  250  may apply brakes  218  in response to information and/or requests made by vehicle system controller  255 . In the same way, a frictional force may be reduced to wheels  216  by disengaging wheel brakes  218  in response to the driver releasing his foot from a brake pedal, brake controller instructions, and/or vehicle system controller instructions and/or information. For example, vehicle brakes may apply a frictional force to wheels  216  via controller  250  as part of an automated engine stopping procedure. 
     In response to a request to accelerate vehicle  225 , vehicle system controller may obtain a driver demand torque from an accelerator pedal or other device. Vehicle system controller  255  then allocates a fraction of the requested driver demand torque to the engine  10  and the remaining fraction to the electric machine  244 . Vehicle system controller  255  requests the engine torque from engine controller  12  and the electric machine torque from electric machine controller  252 . 
     Transmission controller  254  selectively locks torque converter clutch  212  and engages gears via gear clutches  211  in response to shift schedules and TCC lockup schedules that may be based on input shaft torque and vehicle speed. In some conditions when it may be desired to charge electric energy storage device  275 , a charging torque (e.g., a negative electric machine torque) may be requested while a non-zero driver demand torque is present. Vehicle system controller  255  may request increased engine torque to overcome the charging torque to meet the driver demand torque. 
     In response to a request to decelerate vehicle  225  and provide regenerative braking, vehicle system controller may provide a negative desired wheel torque based on vehicle speed and brake pedal position. Vehicle system controller  255  then allocates a fraction of the negative desired wheel torque to the electric machine  244  (e.g., desired driveline wheel torque) and the remaining fraction to friction brakes  218  (e.g., desired friction brake wheel torque). Further, vehicle system controller may notify transmission controller  254  that the vehicle is in regenerative braking mode so that transmission controller  254  shifts gears  248  based on a unique shifting schedule to increase regeneration efficiency. Electric machine  244  supplies a negative torque to axle  240 , but negative torque provided by electric machine  244  may be limited by transmission controller  254  which outputs a planetary transmission input shaft negative torque limit (e.g., not to be exceeded threshold value). Further, negative torque of electric machine  244  may be limited (e.g., constrained to less than a threshold negative threshold torque) based on operating conditions of electric energy storage device  275 , by vehicle system controller  255 , or electric machine controller  252 . Any portion of desired negative wheel torque that may not be provided by electric machine  244  because of transmission or electric machine limits may be allocated to friction brakes  218  so that the desired wheel torque is provided by a combination of negative wheel torque from friction brakes  218  and electric machine  244 . 
     Accordingly, torque control of the various driveline components may be supervised by vehicle system controller with local torque control for the engine  10 , transmission  208 , electric machine  244 , and brakes  218  provided via engine controller  12 , electric machine controller  252 , transmission controller  254 , and brake controller  250 . 
     As one example, an engine torque output may be controlled by adjusting a combination of spark timing, fuel pulse width, fuel pulse timing, and/or air charge, by controlling throttle opening and/or valve timing, valve lift and boost for turbo- or super-charged engines. In the case of a diesel engine, controller  12  may control the engine torque output by controlling a combination of fuel pulse width, fuel pulse timing, and air charge. In all cases, engine control may be performed on a cylinder-by-cylinder basis to control the engine torque output. 
     Electric machine controller  252  may control torque output and electrical energy production from electric machine  244  by adjusting current flowing to and from field and/or armature windings of the electric machine as is known in the art. 
     Transmission controller  254  receives transmission input shaft position via position sensor  271 . Transmission controller  254  may convert transmission input shaft position into input shaft speed via differentiating a signal from position sensor  271 . Transmission controller  254  may receive transmission output shaft torque from torque sensor  272 . Alternatively, sensor  272  may be a position sensor or torque and position sensors. If sensor  272  is a position sensor, controller  254  differentiates a position signal to determine transmission output shaft velocity. Transmission controller  254  may also differentiate transmission output shaft velocity to determine transmission output shaft acceleration. 
     Brake controller  250  receives wheel speed information via wheel speed sensor  221  and braking requests from vehicle system controller  255 . Brake controller  250  may also receive brake pedal position information from brake pedal sensor  154  shown in  FIG. 1  directly or over CAN  299 . Brake controller  250  may provide braking responsive to a wheel torque command from vehicle system controller  255 . Brake controller  250  may also provide anti-lock and vehicle stability braking to improve vehicle braking and stability. As such, brake controller  250  may provide a wheel torque limit (e.g., a threshold negative wheel torque not to be exceeded) to the vehicle system controller  255  so that negative ISG torque does not cause the wheel torque limit to be exceeded. For example, if controller  250  issues a negative wheel torque limit of 50 N-m, ISG torque is adjusted to provide less than 50 N-m (e.g., 49 N-m) of negative torque at the wheels, including accounting for transmission gearing. 
     Vehicle system controller  255  may also receive other vehicle information such as positioning information from global positioning system  280  and steering angle sensor  281  to determine if the vehicle is expected to perform a short duration maneuver such as negotiating a turn. 
     Referring now to  FIG. 3 , an example operating sequence is shown. The sequence of  FIG. 3  may be provided by the system of  FIGS. 1 and 2  according to the method of  FIG. 4 . The plots shown in  FIG. 3  are aligned in time. Vertical markers T 0 -T 4  represent times of particular interest in the sequence. 
     The first plot from the top of  FIG. 3  is a plot of accelerator pedal position versus time. The accelerator pedal may be operated via a driver. The vertical axis represents accelerator pedal position and accelerator pedal position increases (e.g., is applied or depressed further) in the direction of the vertical axis arrow. The horizontal axis represents time and time increases from the left side of the plot to the right side of the plot. 
     The second plot from the top of  FIG. 3  is a plot of driver demand torque versus time. In one example, the driver demand torque corresponds to a desired wheel torque. The vertical axis represents driver demand torque and driver demand torque increases in the direction of the vertical axis arrow. The horizontal axis represents time and time increases from the left side of the plot to the right side of the plot. 
     The third plot from the top of  FIG. 3  is a plot of engine torque versus time. The vertical axis represents engine torque and engine torque increases in the direction of the vertical axis arrow. The horizontal axis represents time and time increases from the left side of the plot to the right side of the plot. 
     The fourth plot from the top of  FIG. 3  is a plot of electric machine torque versus time. The vertical axis represents electric machine torque. Positive electric machine torque is above the horizontal axis and negative electric machine torque is below the horizontal axis. The magnitude of positive torque increases in a direction of the vertical axis arrow above the horizontal axis. The magnitude of negative torque increases in a direction of the vertical axis arrow below the horizontal axis. 
     The fifth plot from the top of  FIG. 3  is a plot of battery state of charge (SOC) versus time. The vertical axis represents battery state of charge and battery state of charge increases in the direction of the vertical axis arrow. The horizontal axis represents time and time increases from the left side of the plot to the right side of the plot. Horizontal line  302  is a threshold battery state of charge above which battery charging is reduced as compared to battery charging below line  302 . Horizontal line  304  is a threshold level for a maximum battery state of charge that is not to be exceeded. 
     At time T 0 , the accelerator pedal position is applied to a middle level and the driver demand torque is also at a middle level. The engine torque is at a middle level and the motor torque is a small positive value. The battery state of charge is at a higher level, but below threshold  302 . 
     At time T 1 , the driver (not shown) releases the accelerator pedal to reduce the driver demand torque. However, because controller inputs indicate a short duration vehicle maneuver is expected, engine torque is held at its value prior to the decrease in accelerator pedal position and driver demand torque. Instead, the electric machine torque changes from a positive torque to a negative torque. The negative electric machine torque combined with the present engine torque provides the driver demand torque. The battery state of charge also increases since the electric machine supplies electrical energy to the battery via operating in a charging mode. Further, if the road grade is negative or vehicle deceleration is desired, the vehicle may operate in a regeneration mode where the vehicle&#39;s kinetic energy is converted into electrical energy via the electric machine. 
     Between time T 1  and time T 2 , the battery state of charge continues to increase as the electric machine outputs electrical energy to the battery by operating as a generator. The accelerator pedal position is at a lower level and the driver demand torque is also at a lower level. The engine torque continues at a level it was at prior to the decrease in driver demand torque. By holding engine torque at a level it assumed prior to the decrease in driver demand torque, turbocharger lag and cam indexing (moving) may be reduced so that engine torque is available immediately when driver demand torque increases. 
     At time T 2 , the driver (not shown) applies the accelerator pedal and increases driver demand torque. The vehicle exits regenerative braking mode as indicated by the electric machine torque transitioning from a negative value to a positive value. Consequently, the engine torque is made available to drive the vehicle&#39;s wheels and the battery ceases charging. The driver demand torque is met by reducing the magnitude of the negative electric machine torque. 
     Between time T 2  and time T 3 , the accelerator pedal position gradually increases and the driver demand torque increases with the accelerator pedal position. The engine torque also increases and the motor torque remains at a lower level. The battery state of charge is greater than threshold  302 . 
     At time T 3 , the driver (not shown) releases the accelerator pedal a second time while controller inputs indicate a short duration vehicle maneuver. But, because battery state of charge is greater than threshold  302 , electric machine torque is reduced but the electric machine does not operate in a generator mode charging the battery. Instead, the engine torque is reduced based on the driver demand torque and the reduction in electric machine torque. The engine torque plus the electric machine torque is equal to the driver demand torque. Thus, the engine torque is reduced by an amount that provides the driveline torque equivalent to driver demand torque when the electric machine is not available to provide negative torque. 
     Between time T 3  and time T 4 , the battery state of charge is maintained because the electric machine output is near zero so that the engine may provide as much of the driver demand torque without assistance from the electric machine. As such, the engine torque output is at a level of the driver demand torque. In this way, the engine torque may be adjusted to a highest engine torque near the driver demand torque without the driver demand torque being exceeded. By operating the engine at the torque near the driver demand torque, turbocharger lag and cam timing adjustment delays may be reduced. 
     At time T 4 , the driver (not shown) applies the accelerator pedal and increases driver demand torque. The electric machine is activated to provide a small positive torque. The electric machine torque added with the engine torque equals the driver demand torque. The electric machine is operated in a motoring mode to consume battery charge so that the motor may operate in a generator mode during a subsequent decrease in driver demand torque. The engine torque is also increased, but because engine torque was reduced at time T 3 , there may be a small delay in engine torque production at time T 4 . 
     In this way, hybrid vehicle performance may be improved by allowing an engine to operate at a torque where driveline torque may be increased almost instantaneously via reducing a magnitude of negative electric machine torque. Such driveline operation may be beneficial for operating a hybrid vehicle on a race track or while operating the hybrid vehicle in a sport mode where a driver wishes to drive more aggressively on a road having many turns. 
     Referring now to  FIG. 4 , a method for operating a vehicle driveline is shown. At least portions of method  400  may be implemented as executable controller instructions stored in non-transitory memory. Additionally, portions of method  400  may be actions taken in the physical world to transform an operating state of an actuator or device. 
     At  402 , method  400  determines vehicle operating conditions. Vehicle operating conditions may include but are not limited to vehicle speed, driver demand torque, steering angle, battery state of charge, vehicle location, accelerator pedal position, and brake pedal position. Vehicle operating conditions may be determined via a controller querying its inputs. Method  400  proceeds to  404  after operating conditions are determined. 
     At  404 , method  400  judges if there is a decrease in driver demand torque. A decrease in driver demand torque may be determined via monitoring accelerator pedal position. The decrease in driver demand torque is not required to be a complete release of the accelerator pedal. If method  400  judges that there is a decrease in driver demand torque, the answer is yes and method  400  proceeds to  406 . Otherwise, the answer is not and method  400  proceeds to  432 . 
     At  406 , method  400  judges if the decrease in driver demand torque is expected to be for a short time duration. In one example, method  400  may judge that the driver demand torque is expected to last for a short duration in response to vehicle inputs that indicate vehicle maneuvers. For example, if the steering angle indicates that the vehicle is beginning to turn, method  400  may indicate that the driver demand torque is expected to be reduced for a short time duration (e.g., less than 5 seconds). Similarly, method  400  may indicate that the driver demand torque is expected to be reduced for a short time duration based on the vehicle entering a turn based on global positioning satellite information. Determination that the vehicle is performing a specific maneuver may be indicative that it is likely that the driver will request a torque increase after at least a portion of the maneuver is completed. Therefore, it may be desirable to operate the engine at a torque level at a time immediately before the decrease in driver demand torque so that engine torque may be available when the driver applies the accelerator pedal. If method  400  judges that the decrease in driver demand torque is expected to be for a short duration, the answer is yes and method  400  proceeds to  408 . Otherwise, the answer is no and method  400  proceeds to  432 . 
     At  408 , method  400  determines an electric machine torque based on engine torque immediately before the decrease in driver demand torque. In one example, the electric machine torque is given by the equation:
 
 E   T   =DD   T   −EN   TBTO  
 
where E T  is electric machine torque after the decrease in driver demand torque, DD T  is driver demand torque after the decrease in driver demand torque, and EN TBTO  is engine torque immediately before the decrease in driver demand torque. For example, if engine torque immediately before the decrease in driver demand torque is 70 N-m and driver demand torque after the decrease in driver demand torque is 30 N-M, the electric machine torque is −40 N-m.
 
     Alternatively, method  400  may determine a desired turbocharger compressor speed and engine torque based on the desired turbocharger compressor speed. In one example the desired turbocharger compressor speed is above a threshold compressor speed where a desired compressor flow rate may be provided. For example, if the desired turbocharger compressor speed is 140,000 RPM and the turbocharger compressor volumetric flow rate is 0.06 m 3 /second, the engine may be operated at a load at its present speed such that air flow through the engine is within a threshold flow rate of the turbocharger compressor volumetric flow rate. The engine air flow rate may be converted to an estimated engine torque and electric machine torque may be determined based on the following equation:
 
 E   T   =DD   T   −EN   TBTS  
 
where E T  is electric machine torque after the decrease in driver demand torque, DD T  is driver demand torque after the decrease in driver demand torque, and EN TBTS  is engine torque based on a desired turbine speed. By operating the turbocharger at a compressor speed above a threshold speed, it may be possible to reduce turbocharger lag. Method  400  proceeds to  410  after motor torque is determined.
 
     At  410 , method  400  judges if electric machine torque determined at  408  (the new electric machine torque), is within electric machine torque and battery power limits. For example, the electric machine negative torque limit (e.g., not to exceed value) may be X N-m and the electric machine positive torque limit (e.g., not to exceed value) may be Y N-m. If the electric machine torque determined at  408  is positive and greater than Y N-m, the answer is no and method  400  proceeds to  420 . If the electric machine torque determined at  408  is negative and has a greater magnitude than X N-m, the answer is no and method  400  proceeds to  420 . Otherwise, the answer may be yes, and method  400  may proceed to  412 . 
     Further, battery power limits may also be a basis for determining electric machine limits. For example, if the battery state of charge is high and battery power is limited to X Kw/hr, electric machine torque is limited (not to be exceeded) to a torque that provides less than or equal to X Kw/hr of charge to the battery. If the electric machine torque determined at  408  is greater in magnitude than the electric machine torque limit based on battery charge, the answer is no and method  400  proceeds to  420 . Otherwise, the answer is yes and method  400  proceeds to  412 . 
     At  412 , method  400  demands an electric machine torque equivalent to the electric machine torque determined at  408 . The electric machine torque may be demanded via commanding a DC/DC converter to output a specified current and/or voltage. Method  400  may also shift gears in transmission  242  to provide a desired driver demand torque at vehicle wheels. Method  400  proceeds to  414  after electric machine torque is demanded. 
     At  414 , method  400  demands engine torque equal to engine torque commanded immediately before the decrease in driver demand torque was determined. The engine torque may be commanded via opening a throttle, adjusting cam timing, spark timing, and boost pressure. Thus, the engine cam timing, boost, and spark timing may be maintained at same values as immediately before the decrease in driver demand torque. Method  400  may also shift gears in transmission  208  to provide a desired driver demand torque at vehicle wheels. Method  400  proceeds to  430  after the engine torque is demanded. 
     At  430 , method  400  judges if the short duration driving maneuver is complete or if the short duration driver demand torque response is still desired. Method  400  may judge that the short duration driving maneuver is complete when the accelerator pedal is applied or increased from a previous position. If the accelerator pedal has not been applied, method  400  may judge that the short duration maneuver response is still desired. Further, method  400  may judge that the short duration driving maneuver is complete based on an amount of time since the driver demand torque was reduced (e.g., 5 seconds). If method  400  determines that a threshold amount of time since driver demand torque was reduced has not expired, method  400  may judge that the short duration maneuver response is still desired. If method  400  judges that the response to the expected short duration driving maneuver is still desired, the answer is yes and method  400  returns to  408 . Otherwise, the answer is no and method  400  proceeds to  432 . 
     At  432 , method  400  demands engine torque and electric machine torque in response to driver demand torque and vehicle speed based on a base energy management strategy. The base energy management strategy may reduce engine torque and electric machine torque in response to a reduction in driver demand torque to conserve energy. The engine cam timing, boost, and spark timing may be adjusted responsive to engine speed and load. Method  400  proceeds to exit after engine and electric machine torque are adjusted. 
     At  420 , method  400  demands electric machine torque based on electric machine torque limits and a battery power limit. For example, an electric machine may have a positive torque limit of Y (e.g., 100) N-m and a negative torque limit of X (e.g., −100) N-m, the limits are not to exceed values. Thus, the electric machine may provide between X and Y N-m of torque. The magnitude of electric machine torque is not to exceed the negative and positive torque limits. Further, the electric machine torque may be limited to a value based on an amount of power vehicle batteries may accept during charging. For example, a battery may be capable of receiving 10 Kw/sec which corresponds to Z N-m of electric machine negative torque. Therefore, the electric machine negative torque magnitude is constrained to less than Z N-m. 
     The electric machine torque is commanded to a torque nearest the torque determined at  408  that does not exceed the electric machine positive and negative torque limits or the battery power limit. For example, if the negative battery torque limit is −50 N-m and method  400  determined a value of −60 N-m at  408 , method  400  commands −50 N-m. Method  400  may also shift gears in transmission  242  to provide a desired driver demand torque at vehicle wheels. Method  400  proceeds to  422  after the electric machine torque is commanded. 
     At  422 , method  400  demands engine torque based on the driver demand torque and the electric machine torque determined at  420 . In one example, method  400  determines engine torque based on the following equation:
 
 EN   T   =DD   T   −E   T  
 
where E T  is electric machine torque after the decrease in driver demand torque determined at  420 , DD T  is driver demand torque after the decrease in driver demand torque, and EN T  is engine torque after the decrease in driver demand torque. The engine is commanded to the determined engine torque via adjusting one or more torque actuators including a throttle, camshaft, and spark timing. Method  400  may also shift gears in transmission  208  to provide a desired driver demand torque at vehicle wheels Method  400  proceeds to  430  after engine torque is commanded.
 
     In this way, engine torque may be maintained during the duration of a short duration driving maneuver. Alternatively, engine torque may be held to a torque closest to the torque the engine output immediately before a reduction in driver demand torque based on electric machine torque limits and battery power limits. 
     Thus, the method of  FIG. 4  provides for an operating method for a hybrid vehicle, comprising: receiving input to a controller; and maintaining an engine torque and adjusting an electric machine torque via the controller in response to a driving maneuver based on the input and in further response to a decrease in a driver demand torque, the driving maneuver being expected to last less than a threshold duration based on the input. The method includes where the electric machine torque and the engine torque provide the driver demand torque. The method includes where the electric machine torque provides a negative torque to a vehicle driveline. 
     In some examples, the method includes where the engine torque is maintained at a torque an engine outputs before the decrease in driver demand torque. The method further comprises shifting a transmission downstream of the electric machine while maintaining the engine torque. The method further comprises shifting a transmission downstream of the engine while maintaining the engine torque. The method includes where engine speed is allowed to vary while maintaining the engine torque. 
     The method of  FIG. 4  also provides for an operating method for a hybrid vehicle, comprising: receiving input to a controller; and maintaining an engine turbocharger compressor speed above a threshold speed and adjusting an electric machine torque via the controller in response to a driving maneuver based on the input and in further response to a decrease in a driver demand torque, the driving maneuver being expected to last less than a threshold duration based on the input. The method includes where the driving maneuver is turning the hybrid vehicle. The method includes where the turbocharger compressor speed is maintained via maintaining a threshold air flow amount through an engine. The method includes where the electric machine torque and an engine torque provide the driver demand torque. The method further comprises maintaining engine cam timing at a timing that is based on an engine torque provided by an engine immediately before the decrease in driver demand torque, and adjusting cam timing in response to engine speed and load after an increase in driver demand torque. 
     The method of  FIG. 4  also provides for an operating method for a hybrid vehicle, comprising: receiving input to a controller; and adjusting an engine torque to a first torque that when combined with a limited electric machine torque provides a driver demand torque via the controller in response to a driving maneuver based on the input and in further response to a decrease in a driver demand torque, the driving maneuver being expected to last less than a threshold duration based on the input, a value of the first torque nearest to a value of engine torque immediately before the decrease in driver demand torque. The method includes where the limited electric machine torque is a negative torque. The method includes where the limited electric machine torque is a maximum negative electric machine torque. The method includes where the limited electric machine torque is based on a battery charging limit. The method includes where the driving maneuver is turning a vehicle. The method includes where the input is received via a steering angle sensor. The method includes where the input is a global positioning system signal. The method further comprises decreasing engine output in response to an amount of time after the decrease in driver demand torque exceeding the threshold duration. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, at least a portion of the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the control system. The control actions may also transform the operating state of one or more sensors or actuators in the physical world when the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with one or more controllers. 
     This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, hybrid electric vehicles including engines operating in natural gas, gasoline, diesel, or alternative fuel configurations could use the present description to advantage.