Abstract:
A method and system for providing a dynamic torque band for hybrid electric vehicle (HEV) transient management includes determining a torque band indicative of an engine torque operation region representing efficient operation of the powertrain across a range of engine speeds. An engine torque command based on an actual speed of the engine is generated. The engine torque command is outputted to the engine if the engine torque command is within the torque band. The engine torque command is modified to be within the torque band if the engine torque command is out of the torque band and the modified engine torque command is outputted to the engine.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to managing the power from the two power sources of a hybrid electric vehicle (HEV) powertrain. 
       BACKGROUND 
       [0002]    A series-parallel hybrid electric vehicle (HEV) powertrain has two power sources for delivering power to the vehicle traction wheels. The first power source includes an engine and a generator mechanically coupled by a planetary gear arrangement. The second power source is an electric drive system including a battery, a motor, and the generator. 
         [0003]    When the powertrain is operating in a driving mode that includes the first power source, the planetary arrangement, together with the engine and the generator, cooperate to effect a power delivery characteristic that is analogous to the characteristic of a conventional continuously variable transmission. This is done by controlling generator speed, the generator being connected to the sun gear of the planetary arrangement and the engine being connected to a planetary carrier. The ring gear of the planetary arrangement is connected to the wheels through torque transfer gearing and a differential-and-axle-assembly. 
         [0004]    Because of the fixed ratio of the planetary arrangement and the variable generator speed, which achieve a decoupling of engine speed and vehicle speed, the planetary arrangement acts as a power divider that divides engine output power and distributes power to the torque transfer gearing and to the generator through separate power flow paths. The portion of the power delivered from the engine to the generator can be transmitted to the motor and then to the differential-and-axle assembly through the torque transfer gearing. Generator torque functions as a torque reaction as engine power is delivered through the planetary arrangement. 
         [0005]    When the powertrain is operating using the second power source, the motor draws power from the battery and provides driving torque to the wheels independently of the first power source. 
         [0006]    The two power sources can provide traction power either simultaneously or independently. However, the power sources must be integrated to work together seamlessly to meet a driver&#39;s demand for power within system power constraints while optimizing total powertrain system efficiency and performance. This requires a coordination of control of the power sources. 
         [0007]    To this end, the powertrain includes a vehicle system controller or the like configured to control the power sources. The controller determines an engine torque and engine speed operating region to meet a driver demand for power while maintaining optimal fuel economy and optimum emissions quality under various vehicle operating conditions. The powertrain can achieve better fuel economy by the controller operating the engine in its most efficient torque and speed operating region whenever possible. 
         [0008]    A problem is that real-world driving consists of many fast demand changes, which result in the powertrain experiencing rapid transients that adversely affect the fuel economy. In general, powertrain transient responses have more influence on ‘engine efficiency’ than ‘electrical efficiency’ in the powertrain. When the powertrain moves its operation point (torque and speed) from one point to another, there is a transient process that the engine can easily run off system-optimum settings thereby costing extra energy compared to steady-state optimum. On the other hand, it is difficult to confine the engine operation strictly along a steady-state optimal path. It not only requires more control efforts but it also causes more electrical re-circulation losses depending on driving conditions. Furthermore, it is infeasible to calculate the ‘true’ global-optimal engine torque command unless all future driving conditions are known a priori, and that underlying computation is extremely intensive. The challenge is due to the complex tradeoff between instant energy efficiency and long-term system losses. 
         [0009]    U.S. Pat. No. 7,398,147 describes an energy management strategy (EMS) based on static optimization. Such an energy management strategy utilizes offline computation to generate optimal EMS tables that are populated with steady-state values corresponding to minimum power losses. For example, the tables may contain steady-state ‘battery power’ and ‘engine speed’ targets as two degrees of control freedom. A drawback is that an offline static optimization process cannot incorporate transient dynamics into calculation. 
       SUMMARY 
       [0010]    In an embodiment, a method for a hybrid electric vehicle (HEV) powertrain having an engine and a battery is provided. The method includes determining a torque band indicative of an engine torque operation region representing efficient operation of the powertrain across a range of engine speeds. An engine torque command based on an actual speed of the engine is generated. The engine torque command is outputted to the engine if the engine torque command is within the torque band. The engine torque command is modified to be within the torque band if the engine torque command is out of the torque band and the modified engine torque command is outputted to the engine. 
         [0011]    In an embodiment, a method for a HEV powertrain having an engine and a battery is provided. The method includes determining a desired engine torque target and generating an engine torque command. The engine torque command is outputted to the engine if the engine torque command is within a predetermined threshold of the engine torque target. The engine torque command is modified to either be the engine torque target or be within the threshold of the engine torque target if the engine torque command is out of the threshold of the engine torque target, and the modified engine torque command is outputted to the engine. 
         [0012]    In an embodiment, a system for managing transient operation of a HEV powertrain having an engine and a battery is provided. The system includes a torque band determination unit configured to determine a torque band indicative of an engine torque operation region representing efficient operation of the powertrain across a range of engine speeds. The system further includes an engine torque command generator configured to generate an engine torque command based on an actual speed of the engine. The system further includes an arbitrator configured to output the engine torque command to the engine if the engine torque command is within the torque band. The arbitrator is further configured to modify the engine torque command to be within the torque band if the engine torque command is out of the torque band and output the modified engine torque command to the engine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  illustrates a schematic representation of a series-parallel hybrid electric vehicle (HEV) powertrain capable of embodying the present invention; 
           [0014]      FIG. 2  illustrates a block diagram of power flow in the powertrain shown in  FIG. 1 ; 
           [0015]      FIG. 3  illustrates a block diagram of a system for implementing an energy management strategy based on static optimization for a HEV powertrain; 
           [0016]      FIG. 4  illustrates a block diagram of a system for implementing a transient operation energy management strategy in accordance with an embodiment of the present invention for a HEV powertrain; 
           [0017]      FIG. 5  illustrates a plot of a torque band determined by the torque band determination unit of the system shown in  FIG. 4 ; 
           [0018]      FIG. 6  illustrates a block diagram of the torque band determination unit in greater detail; 
           [0019]      FIG. 7  illustrates a block diagram of power flow in the powertrain shown in  FIG. 1  when the powertrain is operating pursuant to a drive and charge powersplit operating mode; 
           [0020]      FIG. 8  illustrates a block diagram of an alternate implementation for carrying out the steady-state reference signal determination function of the torque band determination unit; 
           [0021]      FIG. 9A  illustrates a plot of the upper delta torque between the steady-state reference signal and the torque band upper limit signal as a function of the engine power command; 
           [0022]      FIG. 9B  illustrates a plot of the lower delta torque between the steady-state reference signal and the torque band lower limit signal as a function of the engine power command; 
           [0023]      FIG. 10  illustrates a block diagram of the transient operation arbitrator of the system shown in  FIG. 4  in greater detail; and 
           [0024]      FIG. 11  illustrates a block diagram of the performance compensator of the system shown in  FIG. 4  in greater detail. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0026]    Referring now to  FIG. 1 , a schematic representation of a series-parallel hybrid electric vehicle (HEV) powertrain capable of embodying the present invention is shown. The powertrain includes two power sources that are connected to the driveline: 1) an engine  16  and a generator  50  connected together via a planetary gear arrangement  20  and 2) an electric drive system including a battery  12 , an electric motor  46 , and generator  50 . Battery  12  is an energy storage system for motor  46  and generator  50 . 
         [0027]    A vehicle system controller (VSC)  10  is configured to send control signals to and receive sensory feedback information from one or more of battery  12 , engine  16 , motor  46 , and generator  50  in order for power to be provided to vehicle traction wheels  40  for propelling the vehicle. 
         [0028]    Transmission  14  includes planetary arrangement  20 , which includes a ring gear  22 , a sun gear  24 , and a carrier assembly  26 . Ring gear  22  distributes torque to step ratio gears comprising meshing gear elements  28 ,  30 ,  32 ,  34 , and  36 . A torque output shaft  38  of transmission  14  is driveably connected to wheels  40  through a differential-and-axle mechanism  42 . 
         [0029]    Gears  30 ,  32 , and  34  are mounted on a counter shaft  31  with gear  32  engaging a motor-driven gear  44 . Motor  46  drives gear  44 . Gear  44  acts as a torque input for counter shaft  31 . 
         [0030]    Engine  16  distributes torque through input shaft  18  to transmission  14 . Battery  12  delivers electric power to motor  46  through power flow path  48 . Generator  50  is connected electrically to battery  12  and to motor  46 , as shown at  52 . 
         [0031]    While battery  12  is acting as a sole power source with engine  16  off, input shaft  18  and carrier assembly  26  are braked by an overrunning coupling (i.e., one-way clutch (OWC))  53 . A mechanical brake  55  anchors the rotor of generator  50  and sun gear  24  when engine  16  is on and the powertrain is in a parallel drive mode, sun gear  24  acting as a reaction element. 
         [0032]    Controller  10  receives a signal PRND (park, reverse, neutral, drive) from a transmission range selector  63 , which is distributed to transmission control module (TCM)  67 , together with a desired wheel torque, a desired engine speed, and a generator brake command, as shown at  71 . A battery switch  73  is closed after vehicle “key-on” startup. Controller  10  issues a desired engine torque request to engine  16 , as shown at  69 , which is dependent on accelerator pedal position sensor (APPS) output  65 . 
         [0033]    A brake pedal position sensor (BPPS) distributes a wheel brake signal to controller  10 , as shown at  61 . A brake system control module (not shown) may issue to controller  10  a regenerative braking command based on information from the BPPS. TCM  67  issues a generator brake control signal to generator brake  55 . TCM  67  also distributes a generator control signal to generator  50 . 
         [0034]    Briefly, it is noted that the powertrain may be employed in a plug-in hybrid electric vehicle (PHEV). In this case, battery  12  is rechargeable from a power source residing external the vehicle (e.g., an external electric grid). Battery  12  periodically receives AC electrical energy from the grid via a charge port  76  connected to the grid. An on-board charger  78  receives the AC electrical energy from charge port  76 . Charger  78  is an AC/DC converter which converts the received AC electrical energy into DC electrical energy suitable for charging battery  12 . In turn, charger  78  supplies the DC electrical energy to battery  12  in order to charge battery  12  during the recharging operation. 
         [0035]    Referring now to  FIG. 2 , a block diagram of power flow paths between the various components of the powertrain of  FIG. 1  is shown. Fuel is delivered to engine  16  under the control of the driver in known fashion using an engine throttle. Engine power delivered from engine  16  to planetary arrangement  20  is the product τ e ω e , where τ e  is engine torque and ω e  is engine speed. Power delivered from planetary arrangement  20  to counter shaft  31  is the product τ r ω r , where τ r  is the ring gear torque and ω r  is the ring gear speed. Power out (P out ) of transmission  14  via output shaft  38  is the product τ s ω s , where τ s  and ω s  are the torque and speed of output shaft  38 , respectively. 
         [0036]    Generator  50  can act as a motor and deliver power to planetary arrangement  20 . Alternatively, generator  50  can be driven by planetary arrangement  20 . Similarly, power distribution between motor  46  and counter shaft  31  can be distributed in either direction. Driving power from battery  12  or charging power to battery  12  is represented by the bi-directional arrow  48 . 
         [0037]    The engine output power (τ e ω e ) can be split into two paths. This can be done by controlling the speed of generator  50 . The mechanical power flow path (τ r ω r ) of the engine output power is from planetary arrangement  20  to counter shaft  31 . The electrical power flow path (τ g ω g  to τ m ω m ) of the engine output power is from planetary arrangement  20 , to generator  50 , to motor  46 , and to counter shaft  31 , where τt g  is the generator torque, ω g  is the generator speed, τ m  is the motor torque, and ω m  is the motor speed. As described, the engine power is split, whereby the engine speed is disassociated from the vehicle speed. In this so-called positive split mode of operation, engine  16  delivers power to planetary arrangement  20 , which delivers power (τ r ω r ) to counter shaft  31 , which in turn drive wheels  40 . A portion of the planetary gearing power (τ g ω g ) is distributed to generator  50 , which delivers charging power to battery  12 . The speed of generator  50  is greater than zero or positive, and the generator torque is less than zero. Battery  12  drives motor  46 , which distributes power (τ m ω m ) to counter shaft  31 . 
         [0038]    If generator  50 , due to the mechanical properties of planetary arrangement  20 , acts as a power input to planetary arrangement  20  to drive the vehicle, the operating mode is referred to as the so-called negative split mode of operation. In this mode, both the generator speed and generator torque are negative. In particular, generator  50  delivers power to planetary arrangement  20  as motor  46  acts as a generator and battery  12  is charging. Under some conditions motor  46  may distribute power to counter shaft  31  if the resulting torque at wheels  40  from counter shaft  31  does not satisfy the driver demand. Then motor  46  makes up the difference. 
         [0039]    If generator brake  55  is activated, a parallel operating mode is established. In the parallel operating configuration, engine  16  is on and generator  50  is braked. Battery  12  powers motor  46 , which powers counter shaft  31  simultaneously with delivery of power from engine  16  to planetary arrangement  20  to counter shaft  31 . 
         [0040]    In the powertrain of  FIG. 1 , engine  16  requires either the generator torque resulting from engine speed control or the generator brake torque to transmit its output power through both the electrical and mechanical paths (split modes) or through the all-mechanical path (parallel mode) to the drivetrain for forward motion. 
         [0041]    During operation with the second power source (previously described as including battery  12 , motor  46 , and generator  50 ), motor  46  draws power from battery  12  and provides propulsion independently from engine  16  to the vehicle for forward and reverse motions. In addition, generator  50  can draw power from battery  12  and drive against one-way clutch  53  coupling on the engine output shaft to propel the vehicle forward. 
         [0042]    As described, the powertrain has two power sources for delivering driving power to wheels  40 . The first power source generally includes engine  16  and the second power source includes battery  12 , motor  46 , and generator  50 . 
         [0043]    As further described, the operation of the powertrain integrates the two power sources to work together seamlessly to meet the driver&#39;s demand without exceeding the system limits (such as battery limits) while optimizing the total powertrain system efficiency and performance. Coordination control between the power sources is needed. As shown in  FIG. 1 , the powertrain includes controller  10  which performs the coordination control. 
         [0044]    Under normal powertrain conditions, controller  10  interprets the driver demands (e.g., acceleration and deceleration demand), and then determines the wheel torque command based on the driver demand and powertrain limits. In addition, controller  10  determines when and how much torque each power source needs to provide in order to meet the driver&#39;s torque demand and determines the operating point (torque and speed) of the engine. 
         [0045]    Referring now to  FIG. 3 , a block diagram of a system  80  for implementing an energy management strategy (EMS) based on static optimization for a HEV powertrain such as the powertrain shown in  FIG. 1  is shown. System  80  is implemented by, for instance, controller  10 . In general, this EMS utilizes offline computation to generate tables that are populated with steady-state target values corresponding to minimum power losses. For example, the tables contain steady-state ‘battery power’ and ‘engine speed’ targets as two degrees of control freedom. 
         [0046]    In system  80 , a driver power demand (P driver)  82  and a battery charge and discharge power request (P battery )  84  are combined to produce a total power command (P total     —     cmd )  86 . A system optimal engine operating management strategy (EOMS) controller  88  receives an actual vehicle speed (V vehicle )  90  and total power command  86 . EOMS controller  88  develops a target engine speed (ωeng   —     targ )  92  based on vehicle speed  90  and total power command  86 . EOMS controller  88  develops target engine speed  86  such that the total powertrain loss is a minimum. A first signal filter  94  filters target engine speed  92  to produce an engine speed command  96 . 
         [0047]    A divisor (math map)  100  computes an engine torque command (τ eng     —     cmd )  102  from total power command  86  and an actual engine speed (ω eng     —     act )  98  (i.e., τ eng     —     cmd =P total     —     cmd /ω eng     —     act ). A second signal filter  104  filters engine torque command  102  to produce a filtered engine torque command  106 . 
         [0048]    The filtering by second signal filter  104  is done to avoid an instantaneous, uncontrolled spike or pulse in battery power command when a sudden change in total power command is made. Controller  10 , at the instant an increased total power command is made, provides an increased engine torque during a transition from one total power command to another. This gives a quick response to a driver&#39;s demand for a new power level (new torque at the wheels). Second signal filter  104  filters the engine torque command, however, to introduce a filter time lag in the engine torque command change, which avoids a battery power command spike during the total power command transition. As the engine speed then increases, the engine torque command decreases over time to a steady-state value. 
         [0049]    EOMS controller  88  maximizes the total system efficiency for a given vehicle speed and total power command by adjusting the engine speed command, followed by an adjustment of the engine torque command. EOMS controller  88  maximizes total system efficiency by minimizing total losses in the system. 
         [0050]    Referring now to  FIG. 4 , a block diagram of a system  200  for implementing a transient operation energy management strategy in accordance with an embodiment of the present invention for a HEV powertrain such as the powertrain shown in  FIG. 1  is shown. Again, system  200  is implemented by, for example, controller  10 . 
         [0051]    As shown in  FIG. 4 , system  200  includes a system optimal EOMS controller  201  for producing a target engine speed  92  based on a total power command  86  and an actual vehicle speed  90 . System  200  further includes a first signal filter  202  for filtering target engine speed  92  to produce an engine speed command  96 . In these manners, system  200  is similar to system  80  shown in  FIG. 3 . 
         [0052]    System  200  further includes a torque band determination (TBD) unit  203 , an arbitrator for transient operation  204 , a performance compensator  206 , and a second signal filter  205 . As explained in greater detail below, system  200  generally differs from system  80  in that system  200  includes: (i) TBD unit  203  and arbitrator  204  and (ii) compensator  206 . As further explained below, such additional components  203 ,  204 ,  206  effectively convert system  200  into an add-on EMS feature to system  80  for the purpose of managing HEV engine torque during transients (i.e., the transient operation EMS). To this end, system  200  captures additional fuel economy benefit by managing powertrain transient operation. 
         [0053]    TBD unit  203  and arbitrator  204  carry out a mitigation function pursuant to the transient operation EMS. The mitigation function utilizes an adaptive band which represents the ‘system sweet spot’ for the engine operation so as to achieve total system optimum. A goal of the transient operation EMS is not to change the steady-state targets, but rather to regulate engine transients only if necessary. By doing so, the transient operation EMS effectively minimizes unnecessary transient deviations of the engine torque from the steady-state optimum. 
         [0054]    In the meanwhile, compensator  206  checks the battery power limits and adjusts the engine torque command to guarantee performance. In case the driver command exceeds the sum of the engine power command and the maximum electrical power, the engine torque will be adjusted accordingly. 
         [0055]    Steps of the transient operation EMS implemented by system  200  will now be described in greater detail below. 
         [0056]    Referring now to  FIG. 5 , with continual reference to  FIG. 4 , a plot  220  of a torque band determined by TBD unit  203  is shown. In order to determine the torque band, TBD unit  203  receives target input speed  92 , vehicle speed  90 , battery power request  84 , total power command  86 , and driver power demand  82 . 
         [0057]    A purpose of plot  220  is to conceptually illustrate the torque band determined by TBD unit  203 . The torque band serves as a real-time guideline to mitigate unnecessary transients by regulating the engine torque inside a high-efficient operation region. 
         [0058]    The following three signals are plotted to define the torque band as graphically shown in  FIG. 5 : a steady-state torque reference signal  222 ; a torque band upper limit signal  224 ; and a torque band lower limit signal  226 . Signals  222 ,  224 , and  226  are plotted as a function of engine speed in plot  220  only for simplicity of illustration. The actual functions of signals  222 ,  224 , and  226  contain more inputs in addition to ‘engine speed’. The location and width of the torque band dynamically change as a function of driver demands and vehicle operation states. 
         [0059]    Referring now to  FIG. 6 , with continual reference to  FIGS. 4 and 5 , a block diagram of TBD unit  203  in greater detail is shown. TBD unit  203  includes: a steady-state torque target determination sub-unit  302  for determining steady-state reference signal  222  of the torque band; an upper delta determination sub-unit  301  for determining torque band upper limit signal  224  of the torque band; and a lower delta determination sub-unit  303  for determining torque band lower limit signal  226  of the torque band. 
         [0060]    In determining steady-state reference signal  222 , steady-state torque target determination sub-unit  302  receives battery power request  84 , vehicle speed  90 , driver power demand  82 , and target engine speed  92 . In order to describe how sub-unit  302  determines steady-state torque reference signal  222 , an analysis of the energy flow and the efficiency for a given operating condition will be described with reference to  FIG. 7 . 
         [0061]      FIG. 7  illustrates a block diagram of power flow in the powertrain shown in  FIG. 1  when the powertrain is operating pursuant to a ‘drive and charge’ powersplit operating mode. In this operating mode, engine  16  provides power to meet a driver demand with power flowing into battery  12 . As indicated in  FIG. 7 , engine power output is delivered to the wheels to satisfy the driver demand through a mechanical power flow path and an electrical power flow path and engine power output is delivered to battery  12  through a portion of the electrical power flow path. The steady-state optimal torque target can be developed so that the total powertrain loss is a minimum as explained below. 
         [0062]    The system power output is calculated as follows: 
         [0000]        P   out   =F*V=τ   r ω r +τ m ω m   +P   bat tη charge η discharge η m     —     c2m  
 
         [0063]    where: 
         [0064]    P out =powertrain output power; 
         [0065]    τ r =ring gear torque (NM); 
         [0066]    ω r =ring gear speed (radians/second); 
         [0067]    τ m =motor torque (NM); 
         [0068]    ω m =motor speed (radians/second); 
         [0069]    P batt =battery power; 
         [0070]    η charge =battery charging efficiency; 
         [0071]    η discharge =battery discharging efficiency; and 
         [0072]    η m     —     e2m =the assumed electrical efficiency during the conversion of electrical power to mechanical power. 
         [0073]    The total system efficiency then is η total,  as defined by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
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                             η 
                             discharge 
                           
                         
                       
                       ) 
                     
                      
                     
                       η 
                       
                         m_e 
                          
                         
                             
                         
                          
                         2 
                          
                         
                             
                         
                          
                         m 
                       
                     
                      
                     
                       
                         P 
                         batt 
                       
                       / 
                       
                         ( 
                         
                           
                             τ 
                             e 
                           
                            
                           
                             ω 
                             e 
                           
                         
                         ) 
                       
                     
                   
                   } 
                 
               
             
           
         
       
     
         [0074]    where: 
         [0075]    τ e =engine output torque (NM); 
         [0076]    τ g =generator torque (NM); 
         [0077]    ω g =generator speed (radians/second); 
         [0078]    T e2r =torque ratio from engine to ring gear, 1/(1+ρ); 
         [0079]    T e2g =torque ratio from engine to generator, ρ/(1+ρ); 
         [0080]    ρ=the sun gear&#39;s teeth/the ring gear&#39;s teeth; 
         [0081]    η g     —     m2e =the assumed generator efficiency during the conversion of mechanical power to electrical power; and 
         [0082]    η e =the engine efficiency. 
         [0083]    It can be noted from “η total ” equations that the engine efficiency has the most influence on the total system efficiency. 
         [0084]    The steady-state engine torque to minimize the total system loss can then be solved from the following equation: 
         [0000]    
       
         
           
             
               [ 
               
                 
                   Min 
                    
                   
                     ( 
                     
                       P 
                       total_loss 
                     
                     ) 
                   
                 
                 
                   ω 
                   eng 
                 
               
               ] 
             
             = 
             
               
                 f 
                 1 
               
                
               
                 ( 
                 
                   
                     P 
                     total_cmd 
                   
                   , 
                   
                     V 
                     vech 
                   
                   , 
                   
                     P 
                     eng_loss 
                   
                   , 
                   
                     P 
                     mot_loss 
                   
                   , 
                   
                     P 
                     batt_loss 
                   
                   , 
                   
                     P 
                     mech_loss 
                   
                 
                 ) 
               
             
           
         
       
     
         [0085]    where: 
         [0086]    P eng     —     loss =f 2 (ω eng , τ eng ) 
         [0087]    P eng     —     loss =f 3 (ω gen , τ gen ) 
         [0088]    P mot     —     loss =f 4 (ω mot , τ mot ) 
         [0089]    P batt     —     loss =f 5 (V batt , I batt ) 
         [0090]    P mech     —     loss =f 6 (P total     —     cmd , V veh ) 
         [0091]    τ eng =f 7 (P total     —     cmd , ω eng ) 
         [0092]    ω gen =f 8 (ω eng , V veh ) 
         [0093]    τ gen =f 9 (τ eng ) 
         [0094]    ω mot =f 10 (V veh ) 
         [0095]    τ mot =f 11 (τ eng , P total     —     cmd , V veh ) 
         [0096]    I bat t =f 12 (ω gen , τ gen , ω mot , τ mot ) 
         [0097]    V batt =f 13  (I batt ) 
         [0098]    The functions ƒ 2  through ƒ 6  are loss functions for each of the sub-systems and components of the powertrain. These loss functions are located in tables or maps that are pre-calibrated. Each map corresponds to one of the loss functions. The determinations of the loss functions are mapped and entered into an EMS table of steady-state torque target determination sub-unit  302 . The stored values are based on experimental data. 
         [0099]    The loss functions ƒ 7  to ƒ 11  are determined by the physical configuration of the powertrain, including the gearing ratio and the battery characteristics. Each mathematical formulation of a power loss indicates that for a given vehicle speed and a total power demand, there is a unique solution in the determination of target engine speed such that the total loss of the system is minimized. 
         [0100]    After the power loss calculations for the several sub-systems or components are carried out, the values are compared, pursuant to the function f 1 . 
         [0101]    For any given engine speed command, there will be a computation of the power losses as indicated above. The minimum value for those computations of power loss at that engine speed command then is determined. The engine speed that corresponds to the minimum total power loss will not be the same as the engine speed that would correspond to maximum engine efficiency, but it is a speed that corresponds to maximum total system efficiency. 
         [0102]    In an alternate control routine, it is possible to achieve minimization of total system losses by developing off-line, in a pre-calibration procedure, storeable lookup EMS tables which contain, for every total power command and for each corresponding vehicle speed, a predetermined engine speed that will achieve minimum total powertrain losses, which results in maximum powertrain efficiency. 
         [0103]    In sum, the proceeding optimization described with respect to  FIG. 7  is developed off-line in a pre-calibration procedure. Again, for each driver power command, for each battery power command, and for each corresponding vehicle speed, there is a predetermined engine torque that maximizes the total system efficiency as expressed by the above equations. 
         [0104]    Referring now to  FIG. 8 , a block diagram of an alternate implementation for carrying out the steady-state reference signal determination function of steady-state torque target determination sub-unit  302  of TBD unit  203  is shown. This alternate implementation provides an alternate method for determining steady-state reference signal  222  for easier, but equivalent, implementation in a HEV powertrain such as the powertrain shown in  FIG. 1 . 
         [0105]    As shown in  FIG. 8 , the alternate implementation employs a reverse lookup unit  401  (“reverse lookup of the EMS tables”). Reverse lookup unit  401  re-uses the lookup EMS tables generated using the alternate control routine described above. As described, these tables contain the predetermined steady-state engine speeds. Reverse lookup unit  401  re-uses these tables by doing a subtle ‘reverse table lookup’ to solve for the steady-state engine torque target. Such an equivalency is self-explanatory due to only one degree of freedom in the offline optimization, in which the ‘engine speed’ and ‘engine torque’ are interchangeable control variables. In other words, the reverse lookup is an equal process to replace ‘engine speed’ by ‘engine torque’ as the choice of the degree of freedom. 
         [0106]    Although the tables used in the reverse lookup by reverse lookup unit  401  can be converted from tables generated by the EOMS controller, the tables used by reverse lookup unit  401  are designed and stored as separately calibrated to retain freedom of in-vehicle calibration to regulate the transient operation and balance other attributes as needed. 
         [0107]    Referring now to  FIGS. 9A and 9B , with reference to  FIGS. 5 and 6 , the torque band upper and lower limits respectively carried out by upper delta determination sub-unit  301  and lower delta determination sub-unit  303  will be described.  FIG. 9A  illustrates a plot  420  of the upper delta torque  225  between steady-state reference signal  222  and torque band upper limit signal  224  as a function of the engine power command.  FIG. 9B  illustrates a plot  430  of the lower delta torque  227  between torque band lower limit signal  226  and steady-state reference signal  222  as a function of the engine power command. 
         [0108]    Plots  420  and  430  respectively represent calibratable tables. Upper and lower delta determination sub-units  301  and  303  respectively determine upper and lower torque limits  225  and  227  through the two calibratable tables. The design rationale is to set wider the transient operation range at higher the engine power load. This can be explained by the high-efficient island on the engine efficiency map that corresponds to higher engine power. Additional candidate inputs to each table include engine speed, vehicle speed, and battery power. 
         [0109]    Referring now to  FIG. 10 , with continual reference to  FIG. 4 , a block diagram of arbitrator  204  of system  200  in greater detail is shown. In general, arbitrator  204  arbitrates a final target engine torque command  240  based on raw engine torque command  102  and the determined (dynamic) torque band, which is represented by steady-state engine torque reference signal  222  and torque band upper and lower limit signals  224  and  226 . 
         [0110]    The design rationale of the transient operation EMS is to maintain the engine operation inside a dynamic operation band (i.e., the determined torque band) that represents the system-optimal sweet spot. If raw engine torque command  102  resides inside the high-efficient band as determined in decision block  502 , then it is desirable to allow certain fast engine transient so as to speed up the powertrain&#39;s movement towards peak efficient points. In this case, arbitrator  204  outputs raw engine torque command  102  as final target engine torque command  240  (i.e., final torque command  240  (τ e )=raw engine torque command (τ e     —     raw )). As such, the whole mitigation function remains inactive. 
         [0111]    On the other hand, if raw engine torque command  102  falls out of the high-efficient band due to rapid transient as determined in decision block  502 , arbitrator  204 , according to the transient operation EMS, modifies the engine torque command such that it does not deviate too far from the steady-state optimum. Arbitrator  204  is configured to use either of two modification algorithms in making such modification. The modification algorithms can be selected by a calibratable switch (‘Arbitration Method’=0 or 1) as indicated in decision block  504 . 
         [0112]    The first modification algorithm (i.e., Arbitration Method=1) performs simple clipping of raw engine torque command  102  as indicated at  505  when raw engine torque command  102  is out of the band. In this case, arbitrator  204  outputs the clipped raw engine torque command as final target engine torque command  240 . The second modification algorithm (i.e., Arbitration Method=0) resets raw engine torque command  102  to the value of steady-state engine torque reference  222  as indicated at  507  when raw engine torque command  102  is out of the band. In this case, arbitrator  204  outputs steady-state engine torque reference  222  as final target engine torque command  240 . In both modification algorithms, electrical assist during the transient guarantees no compromise to performance. Simulation results of both modification algorithms have shown promising fuel economy benefits. 
         [0113]    Referring now to  FIG. 11 , with continual reference to  FIG. 4 , a block diagram of performance compensator  206  of system  200  in greater detail is shown. The vehicle fuel economy, during the transient, is much more sensitive to the engine operation variation than to the battery power variation. Therefore, an objective of the transient operation EMS is to keep the engine torque always inside the system-optimal band (i.e., the determined torque band). The battery power is forced to make up the entire transient deviation without violating the electrical limits. This takes advantage of the relatively high electrical efficiency by allowing more transient battery power variation, which has less impact to the overall fuel efficiency compared to the engine torque variation. Ideally, if the remaining electrical power is sufficient enough, it is desired to maintain the engine operation inside the band all the time during any transient shifting. However, in case the actual engine power command and the maximum electrical power, together, cannot meet the driver command, the engine torque can be adjusted accordingly for compensation by compensator  206  as shown in  FIG. 11 . 
         [0114]    As shown in  FIG. 11 , compensator  206  calculates the total desired battery power over the electric limits and adds an adjustment term  242  on the engine torque so that the engine can provide the power shortage to maintain performance. 
         [0115]    Turning back to  FIG. 4 , second filter system  205  filters the combined final target engine torque command  240  and adjustment term  242  to produce a filtered engine torque command  106 . 
         [0116]    As described, embodiments of the present invention provide a transient operation energy management strategy (EMS). Features of the transient operation EMS generally include: dynamically determining the system-optimum sweet spot; enabling the engine operation inside the system best-efficient region; mitigating and avoiding rapid torque change—minimizing unnecessary engine transients if battery can absorb driver&#39;s power “perturbation”; no performance compromise because of the automatic engine torque compensation; applicable to powersplit HEV, parallel HEV, series HEV, and other types of HEVs; and applicable to various driving conditions especially real-world driving. 
         [0117]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.