Abstract:
Short term power limits of an electrical energy storage device are progressively tightened toward long term power limits in accordance with net energy flow or magnitude or duration metrics.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/985,222 filed on Nov. 3, 2007 which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure pertains to control systems for electro-mechanical transmissions. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed. 
         [0005]    The electrical energy storage device includes an maximum discharge and charge power at which the electrical energy storage device can discharge or charge at over extended time periods without damaging the electrical energy storage device. However, powertrain operations including transmission shifts, engine starts, and regenerative braking may require discharge or charge rates for short time periods that exceed the maximum discharge and charge rates. Therefore, it would be advantageous to provide short term power limits for the electrical energy storage device. 
       SUMMARY 
       [0006]    A hybrid powertrain includes an electro-mechanical transmission coupled to an internal combustion engine and an electric machine to transmit mechanical power to an output member is described. A method for controlling the hybrid powertrain includes providing long term power limits of the electric energy storage device, determining short term power limits of the electric energy storage device based upon the long term power limits, and controlling power output of the hybrid powertrain system based upon the short term power limits of the electric energy storage device during an electrical energy storage device power excursion outside of the long term power limits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a schematic diagram of an exemplary powertrain, in accordance with the present disclosure; 
           [0009]      FIG. 2  is a schematic diagram of an exemplary architecture for a control system and powertrain, in accordance with the present disclosure; 
           [0010]      FIG. 3  is a parametric flow diagram, in accordance with the present disclosure; 
           [0011]      FIG. 4  graphically illustrates an exemplary short term and long term power limits over a temperature range, in accordance with the present disclosure; 
           [0012]      FIG. 5  is a flow chart illustrating the method for an energy-based approach to controlling the short term power limits, in accordance with the present disclosure; 
           [0013]      FIG. 6  graphically illustrates a method for calculating an energy area, in accordance with the present disclosure; 
           [0014]      FIG. 7  graphically illustrates a method for controlling the short term power limits using the energy-based approach, in accordance with the present disclosure; 
           [0015]      FIG. 8  is a flow chart illustrating the method for a time-based approach to controlling the short term power limits, in accordance with the present disclosure; and 
           [0016]      FIG. 9  graphically illustrates a method for controlling the short term power limits using the time-based approach, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,  FIGS. 1 and 2  depict an exemplary electro-mechanical hybrid powertrain. The exemplary electro-mechanical hybrid powertrain in accordance with the present disclosure is depicted in  FIG. 1 , comprising a two-mode, compound-split, electro-mechanical hybrid transmission  10  operatively connected to an engine  14  and first and second electric machines (‘MG-A’)  56  and (‘MG-B’)  72 . The engine  14  and first and second electric machines  56  and  72  each generate power which can be transferred to the transmission  10 . The power generated by the engine  14  and the first and second electric machines  56  and  72  and transferred to the transmission  10  is described in terms of input and motor torques, referred to herein as T I , T A , and T B  respectively, and speed, referred to herein as N I , N A , and N B , respectively. 
         [0018]    The exemplary engine  14  comprises a multi-cylinder internal combustion engine selectively operative in several states to transfer torque to the transmission  10  via an input shaft  12 , and can be either a spark-ignition or a compression-ignition engine. The engine  14  includes a crankshaft (not shown) operatively coupled to the input shaft  12  of the transmission  10 . A rotational speed sensor  11  monitors rotational speed of the input shaft  12 . Power output from the engine  14 , comprising rotational speed and engine torque, can differ from the input speed N I  and the input torque T I  to the transmission  10  due to placement of torque-consuming components on the input shaft  12  between the engine  14  and the transmission  10 , e.g., a hydraulic pump (not shown) and/or a torque management device (not shown). 
         [0019]    The exemplary transmission  10  comprises three planetary-gear sets  24 ,  26  and  28 , and four selectively engageable torque-transferring devices, i.e., clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75 . As used herein, clutches refer to any type of friction torque transfer device including single or compound plate clutches or packs, band clutches, and brakes, for example. A hydraulic control circuit  42 , preferably controlled by a transmission control module (hereafter ‘TCM’)  17 , is operative to control clutch states. Clutches C 2   62  and C 4   75  preferably comprise hydraulically-applied rotating friction clutches. Clutches C 1   70  and C 3   73  preferably comprise hydraulically-controlled stationary devices that can be selectively grounded to a transmission case  68 . Each of the clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75  is preferably hydraulically applied, selectively receiving pressurized hydraulic fluid via the hydraulic control circuit  42 . 
         [0020]    The first and second electric machines  56  and  72  preferably comprise three-phase AC machines, each including a stator (not shown) and a rotor (not shown), and respective resolvers  80  and  82 . The motor stator for each machine is grounded to an outer portion of the transmission case  68 , and includes a stator core with coiled electrical windings extending therefrom. The rotor for the first electric machine  56  is supported on a hub plate gear that is operatively attached to shaft  60  via the second planetary gear set  26 . The rotor for the second electric machine  72  is fixedly attached to a sleeve shaft hub  66 . 
         [0021]    Each of the resolvers  80  and  82  preferably comprises a variable reluctance device including a resolver stator (not shown) and a resolver rotor (not shown). The resolvers  80  and  82  are appropriately positioned and assembled on respective ones of the first and second electric machines  56  and  72 . Stators of respective ones of the resolvers  80  and  82  are operatively connected to one of the stators for the first and second electric machines  56  and  72 . The resolver rotors are operatively connected to the rotor for the corresponding first and second electric machines  56  and  72 . Each of the resolvers  80  and  82  is signally and operatively connected to a transmission power inverter control module (hereafter ‘TPIM’)  19 , and each senses and monitors rotational position of the resolver rotor relative to the resolver stator, thus monitoring rotational position of respective ones of first and second electric machines  56  and  72 . Additionally, the signals output from the resolvers  80  and  82  are interpreted to provide the rotational speeds for first and second electric machines  56  and  72 , i.e., N A  and N B , respectively. 
         [0022]    The transmission  10  includes an output member  64 , e.g. a shaft, which is operably connected to a driveline  90  for a vehicle (not shown), to provide output power to the driveline  90  that is transferred to vehicle wheels  93 , one of which is shown in  FIG. 1 . The output power at the output member  64  is characterized in terms of an output rotational speed N O  and an output torque T O . A transmission output speed sensor  84  monitors rotational speed and rotational direction of the output member  64 . Each of the vehicle wheels  93  is preferably equipped with a sensor  94  adapted to monitor wheel speed, V SS-WHL , the output of which is monitored by a control module of a distributed control module system described with respect to  FIG. 2 , to determine vehicle speed, and absolute and relative wheel speeds for braking control, traction control, and vehicle acceleration management. 
         [0023]    The input torque from the engine  14  and the motor torques from the first and second electric machines  56  and  72  (T I , T A , and T B  respectively) are generated as a result of energy conversion from fuel or electrical potential stored in an electrical energy storage device (hereafter ‘ESD’)  74 . The ESD  74  is high voltage DC-coupled to the TPIM  19  via DC transfer conductors  27 . The transfer conductors  27  include a contactor switch  38 . When the contactor switch  38  is closed, under normal operation, electric current can flow between the ESD  74  and the TPIM  19 . When the contactor switch  38  is opened electric current flow between the ESD  74  and the TPIM  19  is interrupted. The TPIM  19  transmits electrical power to and from the first electric machine  56  by transfer conductors  29 , and the TPIM  19  similarly transmits electrical power to and from the second electric machine  72  by transfer conductors  31  to meet the torque commands for the first and second electric machines  56  and  72  in response to the motor torques T A  and T B . Electrical current is transmitted to and from the ESD  74  in accordance with whether the ESD  74  is being charged or discharged. 
         [0024]    The TPIM  19  includes the pair of power inverters (not shown) and respective motor control modules (not shown) configured to receive the torque commands and control inverter states therefrom for providing motor drive or regeneration functionality to meet the commanded motor torques T A  and T B . The power inverters comprise known complementary three-phase power electronics devices, and each includes a plurality of insulated gate bipolar transistors (not shown) for converting DC power from the ESD  74  to AC power for powering respective ones of the first and second electric machines  56  and  72 , by switching at high frequencies. The insulated gate bipolar transistors form a switch mode power supply configured to receive control commands. There is typically one pair of insulated gate bipolar transistors for each phase of each of the three-phase electric machines. States of the insulated gate bipolar transistors are controlled to provide motor drive mechanical power generation or electric power regeneration functionality. The three-phase inverters receive or supply DC electric power via respective ones of the transfer conductors  29  and  31  and transform it to or from three-phase AC power, which is conducted to or from the first and second electric machines  56  and  72  for operation as motors or generators. 
         [0025]      FIG. 2  is a schematic block diagram of the distributed control module system. The elements described hereinafter comprise a subset of an overall vehicle control architecture, and provide coordinated system control of the exemplary hybrid powertrain described in  FIG. 1 . The distributed control module system synthesizes pertinent information and inputs, and executes algorithms to control various actuators to meet control objectives, including objectives related to fuel economy, emissions, performance, drivability, and protection of hardware, including batteries of ESD  74  and the first and second electric machines  56  and  72 . The distributed control module system includes an engine control module (hereafter ‘ECM’)  23 , the TCM  17 , a battery pack control module (hereafter ‘BPCM’)  21 , and the TPIM  19 . A hybrid control module (hereafter ‘HCP’)  5  provides supervisory control and coordination of the ECM  23 , the TCM  17 , the BPCM  21 , and the TPIM  19 . A user interface (‘UI’)  13  is operatively connected to a plurality of devices through which a vehicle operator controls or directs operation of the electro-mechanical hybrid powertrain. The devices include an accelerator pedal  113  (‘AP’), an operator brake pedal  112  (‘BP’), a transmission gear selector  114  (‘PRNDL’), and a vehicle speed cruise control (not shown). The transmission gear selector  114  may have a discrete number of operator-selectable positions, including the rotational direction of the output member  64  to enable one of a forward and a reverse direction. 
         [0026]    The aforementioned control modules communicate with other control modules, sensors, and actuators via a local area network (hereafter ‘LAN’) bus  6 . The LAN bus  6  allows for structured communication of states of operating parameters and actuator command signals between the various control modules. The specific communication protocol utilized is application-specific. The LAN bus  6  and appropriate protocols provide for robust messaging and multi-control module interfacing between the aforementioned control modules, and other control modules providing functionality including e.g., antilock braking, traction control, and vehicle stability. Multiple communications buses may be used to improve communications speed and provide some level of signal redundancy and integrity. Communication between individual control modules can also be effected using a direct link, e.g., a serial peripheral interface (‘SPI’) bus (not shown). 
         [0027]    The HCP  5  provides supervisory control of the hybrid powertrain, serving to coordinate operation of the ECM  23 , TCM  17 , TPIM  19 , and BPCM  21 . Based upon various input signals from the user interface  13  and the hybrid powertrain, including the ESD  74 , the HCP  5  determines an operator torque request, an output torque command, an engine input torque command, clutch torque(s) for the applied torque-transfer clutches C 1   70 , C 2   62 , C 3   73 , C 4   75  of the transmission  10 , and the motor torques T A  and T B  for the first and second electric machines  56  and  72 . The TCM  17  is operatively connected to the hydraulic control circuit  42  and provides various functions including monitoring various pressure sensing devices (not shown) and generating and communicating control signals to various solenoids (not shown) thereby controlling pressure switches and control valves contained within the hydraulic control circuit  42 . 
         [0028]    The ECM  23  is operatively connected to the engine  14 , and functions to acquire data from sensors and control actuators of the engine  14  over a plurality of discrete lines, shown for simplicity as an aggregate bi-directional interface cable  35 . The ECM  23  receives the engine input torque command from the HCP  5 . The ECM  23  determines the actual engine input torque, T I , provided to the transmission  10  at that point in time-based upon monitored engine speed and load, which is communicated to the HCP  5 . The ECM  23  monitors input from the rotational speed sensor  11  to determine the engine input speed to the input shaft  12 , which translates to the transmission input speed, N I . The ECM  23  monitors inputs from sensors (not shown) to determine states of other engine operating parameters including, e.g., a manifold pressure, engine coolant temperature, ambient air temperature, and ambient pressure. The engine load can be determined, for example, from the manifold pressure, or alternatively, from monitoring operator input to the accelerator pedal  113 . The ECM  23  generates and communicates command signals to control engine actuators, including, e.g., fuel injectors, ignition modules, and throttle control modules, none of which are shown. 
         [0029]    The TCM  17  is operatively connected to the transmission  10  and monitors inputs from sensors (not shown) to determine states of transmission operating parameters. The TCM  17  generates and communicates command signals to control the transmission  10 , including controlling the hydraulic circuit  42 . Inputs from the TCM  17  to the HCP  5  include estimated clutch torques for each of the clutches, i.e., C 1   70 , C 2   62 , C 3   73 , and C 4   75 , and rotational output speed, N O , of the output member  64 . Other actuators and sensors may be used to provide additional information from the TCM  17  to the HCP  5  for control purposes. The TCM  17  monitors inputs from pressure switches (not shown) and selectively actuates pressure control solenoids (not shown) and shift solenoids (not shown) of the hydraulic circuit  42  to selectively actuate the various clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75  to achieve various transmission operating range states, as described hereinbelow. 
         [0030]    The BPCM  21  is signally connected to sensors (not shown) to monitor the ESD  74 , including states of electrical current and voltage parameters, to provide information indicative of parametric states of the batteries of the ESD  74  to the HCP  5 . The parametric states of the batteries preferably include battery state-of-charge, battery voltage, battery temperature, and available battery power, referred to as a range P BAT     —     MIN  to P BAT     —     MAX . 
         [0031]    A brake control module (hereafter ‘BrCM’)  22  is operatively connected to friction brakes (not shown) on each of the vehicle wheels  93 . The BrCM  22  monitors the operator input to the brake pedal  112  and generates control signals to control the friction brakes and sends a control signal to the HCP  5  to operate the first and second electric machines  56  and  72  based thereon. 
         [0032]    Each of the control modules ECM  23 , TCM  17 , TPIM  19 , BPCM  21 , and BrCM  22  is preferably a general-purpose digital computer comprising a microprocessor or central processing unit, storage mediums comprising read only memory (‘ROM’), random access memory (‘RAM’), electrically programmable read only memory (‘EPROM’), a high speed clock, analog to digital (‘A/D’) and digital to analog (‘D/A’) circuitry, and input/output circuitry and devices (‘I/O’) and appropriate signal conditioning and buffer circuitry. Each of the control modules has a set of control algorithms, comprising resident program instructions and calibrations stored in one of the storage mediums and executed to provide the respective functions of each computer. Information transfer between the control modules is preferably accomplished using the LAN bus  6  and serial peripheral interface buses. The control algorithms are executed during preset loop cycles such that each algorithm is executed at least once each loop cycle. Algorithms stored in the non-volatile memory devices are executed by one of the central processing units to monitor inputs from the sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles are executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing operation of the hybrid powertrain. Alternatively, algorithms may be executed in response to the occurrence of an event. 
         [0033]    The exemplary hybrid powertrain selectively operates in one of several operating range states that can be described in terms of an engine state comprising one of an engine-on state (‘ON’) and an engine-off state (‘OFF’), and a transmission state comprising a plurality of fixed gears and continuously variable operating modes, described with reference to Table 1, below. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Engine 
                 Transmission Operating 
                 Applied 
               
               
                 Description 
                 State 
                 Range State 
                 Clutches 
               
               
                   
               
             
             
               
                 M1_Eng_Off 
                 OFF 
                 EVT Mode 1 
                 C1 70 
               
               
                 M1_Eng_On 
                 ON 
                 EVT Mode 1 
                 C1 70 
               
               
                 G1 
                 ON 
                 Fixed Gear Ratio 1 
                 C1 70 C4 75 
               
               
                 G2 
                 ON 
                 Fixed Gear Ratio 2 
                 C1 70 C2 62 
               
               
                 M2_Eng_Off 
                 OFF 
                 EVT Mode 2 
                 C2 62 
               
               
                 M2_Eng_On 
                 ON 
                 EVT Mode 2 
                 C2 62 
               
               
                 G3 
                 ON 
                 Fixed Gear Ratio 3 
                 C2 62 C4 75 
               
               
                 G4 
                 ON 
                 Fixed Gear Ratio 4 
                 C2 62 C3 73 
               
               
                   
               
             
          
         
       
     
         [0034]    Each of the transmission operating range states is described in the table and indicates which of the specific clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75  are applied for each of the operating range states. A first continuously variable mode, i.e., EVT Mode  1 , or M 1 , is selected by applying clutch C 1   70  only in order to “ground” the outer gear member of the third planetary gear set  28 . The engine state can be one of ON (‘M 1 _Eng_On’) or OFF (‘M 1 _Eng_Off’). A second continuously variable mode, i.e., EVT Mode  2 , or M 2 , is selected by applying clutch C 2   62  only to connect the shaft  60  to the carrier of the third planetary gear set  28 . The engine state can be one of ON (‘M 2 _Eng_On’) or OFF (‘M 2 _Eng_Off’). For purposes of this description, when the engine state is OFF, the engine input speed is equal to zero revolutions per minute (‘RPM’), i.e., the engine crankshaft is not rotating. A fixed gear operation provides a fixed ratio operation of input-to-output speed of the transmission  10 , i.e., N I /N O . A first fixed gear operation (‘G 1 ’) is selected by applying clutches C 1   70  and C 4   75 . A second fixed gear operation (‘G 2 ’) is selected by applying clutches C 1   70  and C 2   62 . A third fixed gear operation (‘G 3 ’) is selected by applying clutches C 2   62  and C 4   75 . A fourth fixed gear operation (‘G 4 ’) is selected by applying clutches C 2   62  and C 3   73 . The fixed ratio operation of input-to-output speed increases with increased fixed gear operation due to decreased gear ratios in the planetary gears  24 ,  26 , and  28 . The rotational speeds of the first and second electric machines  56  and  72 , N A  and N B  respectively, are dependent on internal rotation of the mechanism as defined by the clutching and are proportional to the input speed measured at the input shaft  12 . 
         [0035]    In response to operator input via the accelerator pedal  113  and brake pedal  112  as captured by the user interface  13 , the HCP  5  and one or more of the other control modules determine torque commands to control the torque generative devices comprising the engine  14  and first and second electric machines  56  and  72  to meet the operator torque request at the output member  64  and transferred to the driveline  90 . Based upon input signals from the user interface  13  and the hybrid powertrain including the ESD  74 , the HCP  5  determines the operator torque request, a commanded output torque from the transmission  10  to the driveline  90 , an input torque from the engine  14 , clutch torques for the torque-transfer clutches C 1   70 , C 2   62 , C 3   73 , C 4   75  of the transmission  10 ; and the motor torques for the first and second electric machines  56  and  72 , respectively, as is described hereinbelow. 
         [0036]    Final vehicle acceleration can be affected by other factors including, e.g., road load, road grade, and vehicle mass. The operating range state is determined for the transmission  10  based upon a variety of operating characteristics of the hybrid powertrain. This includes the operator torque request communicated through the accelerator pedal  113  and brake pedal  112  to the user interface  13  as previously described. The operating range state may be predicated on a hybrid powertrain torque demand caused by a command to operate the first and second electric machines  56  and  72  in an electrical energy generating mode or in a torque generating mode. The operating range state can be determined by an optimization algorithm or routine which determines optimum system efficiency based upon operator demand for power, battery state of charge, and energy efficiencies of the engine  14  and the first and second electric machines  56  and  72 . The control system manages torque inputs from the engine  14  and the first and second electric machines  56  and  72  based upon an outcome of the executed optimization routine, and system efficiencies are optimized thereby, to manage fuel economy and battery charging. Furthermore, operation can be determined based upon a fault in a component or system. The HCP  5  monitors the torque-generative devices, and determines the power output from the transmission  10  required in response to the desired output torque at output member  64  to meet the operator torque request. As should be apparent from the description above, the ESD  74  and the first and second electric machines  56  and  72  are electrically-operatively coupled for power flow therebetween. Furthermore, the engine  14 , the first and second electric machines  56  and  72 , and the electro-mechanical transmission  10  are mechanically-operatively coupled to transfer power therebetween to generate a power flow to the output member  64 . 
         [0037]      FIG. 3  shows a control system architecture for controlling and managing torque and power flow in a powertrain system having multiple torque generative devices, described hereinbelow with reference to the hybrid powertrain system shown in  FIGS. 1 and 2 , and residing in the aforementioned control modules in the form of executable algorithms and calibrations. The control system architecture can be applied to any powertrain system having multiple torque generative devices, including, e.g., a hybrid powertrain system having a single electric machine, a hybrid powertrain system having multiple electric machines, and non-hybrid powertrain systems. 
         [0038]    The control system architecture of  FIG. 3  depicts a flow of pertinent signals through the control modules. In operation, the operator inputs to the accelerator pedal  113  and the brake pedal  112  are monitored to determine the operator torque request (‘To_req’). Operation of the engine  14  and the transmission  10  are monitored to determine the input speed (‘Ni’) and the output speed (‘No’). A strategic optimization control scheme (‘Strategic Control’)  310  determines a preferred input speed (‘Ni_Des’) and a preferred engine state and transmission operating range state (‘Hybrid Range State Des’) based upon the output speed and the operator torque request, and optimized based upon other operating parameters of the hybrid powertrain, including battery power limits and response limits of the engine  14 , the transmission  10 , and the first and second electric machines  56  and  72 . The strategic optimization control scheme  310  is preferably executed by the HCP  5  during each 100 ms loop cycle and each 25 ms loop cycle. 
         [0039]    The outputs of the strategic optimization control scheme  310  are used in a shift execution and engine start/stop control scheme (‘Shift Execution and Engine Start/Stop’)  320  to command changes in the transmission operation (‘Transmission Commands’) including changing the operating range state. This includes commanding execution of a change in the operating range state if the preferred operating range state is different from the present operating range state by commanding changes in application of one or more of the clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75  and other transmission commands. The present operating range state (‘Hybrid Range State Actual’) and an input speed profile (‘Ni_Prof’) can be determined. The input speed profile is an estimate of an upcoming input speed and preferably comprises a scalar parametric value that is a targeted input speed for the forthcoming loop cycle. The engine operating commands and the operator torque request are based upon the input speed profile during a transition in the operating range state of the transmission. 
         [0040]    A tactical control scheme (‘Tactical Control and Operation’)  330  is repeatedly executed during one of the control loop cycles to determine engine commands (‘Engine Commands’) for operating the engine, including a preferred input torque from the engine  14  to the transmission  10  based upon the output speed, the input speed, and the operator torque request and the present operating range state for the transmission. The engine commands also include engine states including one of an all-cylinder operating state and a cylinder deactivation operating state wherein a portion of the engine cylinders are deactivated and unfueled, and engine states including one of a fueled state and a fuel cutoff state. 
         [0041]    A clutch torque (‘Tcl’) for each clutch is estimated in the TCM  17 , including the presently applied clutches and the non-applied clutches, and a present engine input torque (‘Ti’) reacting with the input member  12  is determined in the ECM  23 . A motor torque control scheme (‘Output and Motor Torque Determination’)  340  is executed to determine the preferred output torque from the powertrain (‘To_cmd’), which includes motor torque commands (‘T A ’, ‘T B ’) for controlling the first and second electric machines  56  and  72  in this embodiment. The preferred output torque is based upon the estimated clutch torque(s) for each of the clutches, the present input torque from the engine  14 , the present operating range state, the input speed, the operator torque request, and the input speed profile. The first and second electric machines  56  and  72  are controlled through the TPIM  19  to meet the preferred motor torque commands based upon the preferred output torque. The motor torque control scheme  340  includes algorithmic code which is regularly executed during the 6.25 ms and 12.5 ms loop cycles to determine the preferred motor torque commands. 
         [0042]    During ongoing powertrain operations, the BPCM  21  monitors operating parameters of the ESD  74  including electrical current, open circuit voltage, and temperature. The operating parameters are preferably monitored during one of the loop cycles or multiples of the loop cycles in the BPCM  21 . 
         [0043]    The ESD  74  exhibits physical power charge and discharge operating limits that are characteristic to the design and materials of the ESD  74 . The operating limits include maximum charge power and discharge power that the ESD  74  can support without sustaining internal physical damage that can affect service life or performance of the ESD  74 . The operating limits are described as long term power limits, also referred to as a steady-state limit. A maximum long term power limit describes the maximum discharge power of the ESD  74  over an extended time period, and a minimum long term power limit describes the maximum charge power of the ESD  74  over an extended time period. An extended time period with respect to long term power limits includes time periods in excess of short time periods with respect to short term power limits a described in further detail herein below. The maximum charge power and the maximum discharge power are referred to herein as the long term power limits. The long term power limits may be predetermined and can be based upon the state-of-charge and/or temperature of the ESD  74 . 
         [0044]    Powertrain operations, controlled through the HCP  5  in this embodiment, may require an electric power flow (discharge power or charge power) that exceeds one of the long term power limits for a period of time, referred to herein as a power excursion. Such powertrain operations include adjusting rotational speeds of the first and second electric machines  56  and  72  to synchronize clutch plates of one of clutches C 1   70 , C 2   62 , C 3   73 , and C 4   75  during transmission shifts, rotating the first electric machine  56  to effect an engine start, and generating reactive power in the first and second electric machines  56  and  72  to generate electric power during a regenerative braking event. For operations that require a charge or discharge power greater than the long term power limits, the powertrain operations can control operation of the first and second electric machines  56  and  72  through the motor control modules and power inverters of the TPIM  19  to charge or discharge the ESD  74  limited by the short term power limit for short time periods, e.g., one to ten second time periods. A maximum short term power limit can be defined based upon the maximum long term power limit of the ESD  74 , and a minimum short term power limit can be defined based upon the minimum long term power limit of the ESD  74 . The maximum short term power limit can exceed the maximum long term power limit by a power magnitude or percentage of the long term maximum power; likewise the minimum short term power limit can exceed the minimum long term limit by a magnitude or percentage of the long term minimum power. 
         [0045]      FIG. 4  illustrates long term power limits and associated short term power limits based upon temperature. A short term power limit is said to exceed the corresponding long term power limit when the magnitude of the short term power limit exceeds the magnitude of the long term power limit. The exemplary short term power limits are depicted as a predetermined magnitude greater than the corresponding long term power limit magnitudes. Alternatively, the short term power limits can comprise a percentage increase over the long term power limits. The exemplary power limits vary over the operating temperature of the ESD  74 , with the power limits at a maximum at operating temperatures around 20° C., and decrease with increasing or decreasing operating temperatures. 
         [0046]    During vehicle operation, the short term power limits can fluctuate based upon an actual charging and discharging power that is in excess of the long term power limits by the ESD  74 . When the actual charging or discharging power of the ESD  74  exceeds the long term power limits, the short term power limits can converge to the long term power limits. When the short term power limits converge to the long term power limits, the short-term power excursions of the ESD are limited. The short term power limits preferably converge to the long term power limits through a ramping function. The ramping function may be based on loop cycles on the BPCM  21  or HCP  5 , and decrease the short term power limits incrementally for each loop cycle. There can be different methods to determine when to ramp the short term power limits to the long term power limits. A first approach is energy-based and a second approach is time-based. 
         [0047]    Both the energy-based approach and the time-based approach control the maximum and minimum short term power limits. Both the maximum and minimum short term power limits are controlled substantially the same within the respective approaches. Hereafter, the discussion will describe the approach to controlling the maximum short term power limits, i.e. the maximum short term discharge power. One skilled in the art will recognize that the approach given for controlling the maximum short term power limit is applicable to the minimum short term power limit, i.e., the maximum short term charge power. 
         [0048]      FIG. 5  depicts a flow chart illustrating the energy-based method for controlling the short term power limits. The energy-based approach controls the short term limits based upon an energy area metric. The energy area metric comprises a measure of charging or discharging power flow from the ESD  74  that exceeds the long term power limits over an elapsed period of time. 
         [0049]    During vehicle operations, the BPCM  21  monitors operating parameters of the ESD  74  including electrical current, open circuit voltage, and temperature ( 403 ). Long term power limits, described with reference to  FIG. 4 , are input into the system ( 402 ). Predetermined short term power limits are determined based upon the long term power limits. Short term power limits may be a predetermined percentage increase over the long term limits, or a predetermined power magnitude over the long term limits. The HCP  5  monitors the short term power limits, including the energy area metric. 
         [0050]    The energy area metric may be determined using one of a plurality of methods. A first method for determining the energy area metric comprises integrating, as a function of time, the actual power output from the ESD  74 . This is shown with reference to Eq. 1: 
         [0000]    
       
         
           
             
               
                 
                   
                     ∫ 
                     0 
                     t 
                   
                    
                   
                     
                       ( 
                       
                         
                           Actual 
                            
                           
                               
                           
                            
                           Power 
                         
                         - 
                         
                           LT 
                            
                           
                               
                           
                            
                           Power 
                            
                           
                               
                           
                            
                           Limit 
                         
                       
                       ) 
                     
                      
                     
                       
                          
                         p 
                       
                       
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
         [0000]    where t is the elapsed time period, Actual Power is the measured electric power during the elapsed time period, and LT Power Limit is the long term power limit at the temperature of the ESD  74 . The energy area metric increases when the integral is positive and decreases when the integral is negative. The energy area metric may not decrease below zero. 
         [0051]    A second method for determining the energy area metric is illustrated in  FIG. 6  and comprises adding electric energy (‘E A ’) and subtracting reduction areas (‘R A ’). Electric energy is calculated by multiplying an output power in excess of the long term power limits with the elapsed time between power measurements. The reduction areas are calculated by multiplying the output power between the actual power and the long term power limits and the elapsed time between power measurements. Power measurements are based upon ESD current and ESD voltage measurements. As  FIG. 6  depicts, ESD current and ESD voltage are measured periodically during ongoing powertrain operation. 
         [0052]    When the energy area metric reaches a predetermined threshold value, the HCP  5  ramps down the short term power limits converging to the long term power limits. If actual power output is less than the long term power limits, the HCP  5  reduces the energy area metric. The energy area metric continues to increase if the actual power exceeds the long term power limits while the HCP  5  ramps the short term power limits to converge to the long term power limits. The energy area metric stops increasing when the actual power output is less than the long term power limits. After the HCP  5  ramps down the short term power limits to the long term power limits, the energy area metric is reduced when the actual power output is less than the long term power limits. When the energy area metric reduces to a second threshold, preferably zero, the HCP  5  ramps up the short term power limits to the predetermined short term power limits. 
         [0053]    The short term power limits may be controlled using one of several methods. A first method for controlling the short term power limits using the energy-based approach comprises controlling the short term power limits based upon the energy area metric. The first method comprises establishing the long term power limits and short term power limits ( 402 ) and monitoring the ESD  74 , including the actual power output ( 403 ). 
         [0054]    The HCP  5  determines whether the actual power output from the ESD  74  exceeds the long term power limits ( 404 ). If the actual power exceeds the long term power limits, the HCP  5  increases the energy area metric as described hereinabove ( 408 ). The HCP  5  determines whether the energy area metric exceeds a predetermined threshold. If the energy area metric is less than the predetermined threshold the HCP  5  continues the method at block  403 . If the energy area metric exceeds the predetermined threshold, the HCP  5  ramps the short term power limit to the long term power limit ( 416 ). The energy area metric is monitored again at block  403  wherein the energy area metric reduces to zero when the actual power output remains less than the long term power limits. The HCP  5  may ramp down the short term limits based upon a decaying function or may incrementally step the short term limits down based upon a step function. Once the energy area metric reduces to zero the HCP  5  ramps up the short term power limits to the predetermined short term power limits ( 402 ). 
         [0055]    If the actual power is not greater than the long term power limit the HCP  5  determines if the energy area metric is greater than zero ( 420 ). If the energy area metric is greater than zero the HCP  5  ramps the energy area metric down a predetermined amount ( 422 ). If the energy area metric is not greater than zero the HCP  5  sets the energy area metric to zero ( 424 ) and reestablishes the short term power limits at block  402 . 
         [0056]      FIG. 7  graphically illustrates the first method for controlling the short term power limits based upon the energy area metric. Initially, the long term (LT) power limits are set, and the short term (ST) power limits are set based upon the long term power limits as shown in region  1  in  FIG. 7 . The energy area metric increases when the actual power output exceeds the long term power limits as shown in regions  2  and  3  in  FIG. 7 . When the energy area metric exceeds a threshold the short term power limits ramp down to the long term power limits as shown in region  3  in  FIG. 7 . When the actual power output is less than the long term power limits the energy area metric decreases until reaching zero as shown in region  4  in  FIG. 7 . When the energy area metric is zero the short term power limits are ramped up to the predetermined short term power limits as shown in region  5  in  FIG. 7 . 
         [0057]    The HCP  5  can change the long term power limits at any time during the energy-based approach. Preferably, the long term power limits are determined before the short term power limits at block  402  in  FIG. 5 . Both the maximum and minimum short term power limits are concurrently determined. Once either energy area metric for the maximum short term power limit or the minimum short term power limit exceeds the threshold, both maximum and minimum short term power limits can be set to their respective long term limits until the HCP  5  commands them to ramp up to the predetermined short term power limits. 
         [0058]      FIG. 8  depicts a flow chart illustrating the time-based method for controlling the short term power limits. During ongoing powertrain operations, the BPCM  21  monitors operating parameters of the ESD  74  including electrical current, open circuit voltage, and temperature ( 603 ). Long term power limits are input into the system. The long term power limits may be predetermined or may be determined based upon state-of-charge and/or temperature of the ESD  74 . After the long term limits are set, the short term power limits are determined based upon the long term power limits ( 602 ). Short term power limits may be predetermined percentage increase over the long term limits, or a predetermined magnitude over the long term limits. 
         [0059]    The time-based method controls the short term power limits based upon a time counter. The time counter monitors an elapsed time actual power exceeds the long term power limits. When the actual power exceeds the long term power limits, the time counter increases. When the actual power is less than the long term power limits, the time counter decreases. The time counter does not decrease below zero. The time counter is initialized at a predetermined amount, preferably zero, and is controlled by the HCP  5 . 
         [0060]    During vehicle operations, the HCP  5  determines whether the actual power output of the ESD  74  exceeds the long term power limits ( 604 ). When the HCP  5  determines that the power output exceeds the long term power limits, the HCP  5  increases the time counter ( 608 ). 
         [0061]    If the actual power is not greater than the long term power limits, the HCP  5  determines if the time counter exceeds zero ( 620 ). If the time counter is greater than zero the HCP  5  will reduce the time counter a predetermined amount and continue the method at block  603 . If the time counter is not greater than zero, the HCP  5  is set to zero ( 624 ) and continues the method at block  602 . 
         [0062]    After increasing the time counter, the HCP  5  determines whether the time counter exceeds a predetermined threshold ( 610 ). If the time counter exceeds the predetermined threshold, the HCP  5  ramps down the short term power limits to the long term power limits ( 616 ). Actual power output is monitored again at block  603  wherein the time counter will then reduce to zero when the actual power output remains less than the long term power limits. The HCP  5  may ramp down the short term limits and reduce the time counter based upon a decaying function or may incrementally step the short term limits down based upon a step function. The short term power limits may not be ramped down less than the long term power limits. After ramping down the short term power limits to the long term power limits, the HCP  5  continues the method at block  602 . 
         [0063]      FIG. 9  graphically illustrates the time-based method for controlling the short term power limits. Initially, the long term power limits are set and the short term power limits are set based upon the long term power limits as shown in region  1  in  FIG. 9 . The time counter increases when the actual power output exceeds the long term power limits as shown in regions  2  and  3  in  FIG. 9 . When the time counter exceeds the predetermined threshold the short term power limits ramp down to the long term power limits as shown in region  3  in  FIG. 9 . When the actual power output is less than the long term power limits, the time counter decreases until reaching zero as shown in region  4  in  FIG. 9 . When the time counter is zero the short term power limits are ramped up to the predetermined short term power limits as shown in region  5  in  FIG. 9 . 
         [0064]    It is understood that modifications are allowable within the scope of the disclosure. The disclosure has been described with specific reference to the preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the disclosure.