Abstract:
A method for operating a powertrain includes determining maximum and minimum series-drive power limits of powertrain electric components; operating in parallel-drive if vehicle speed exceeds a reference, demanded wheel power is between said limits, or demanded engine power exceeds a reference demanded engine power; and operating in series-drive if vehicle speed is less than a reference, demanded wheel power is between said limits, and demanded engine power is less than a reference engine power.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a powertrain for a hybrid electric vehicle (HEV). More particularly, it pertains to the control of transitions between series drive and parallel drive operation of the powertrain. 
     2. Description of the Prior Art 
     The powertrain for hybrid electric vehicle may include two electric machines in combination with an engine and transmission to operate in at least two operating modes, series and parallel drive, sometimes called a dual-drive hybrid-electric powertrain configuration. The first electric machine is mechanically coupled between the engine and transmission on the front axle in order to provide starter/generator capability. The second electric machine is connected to the rear axle in order to provide additional propulsion capability in either an electric or hybrid drive mode, resulting in two independently driven axles. The electric machines are powered by a high-voltage battery using inverters. 
     This powertrain configuration provides great flexibility for operating the powertrain in various modes, such as electric mode, series mode, and parallel or split mode to satisfy the driver&#39;s demand and achieve better fuel efficiency without compromising other vehicle performance attributes. 
     Given the architectural complexity and the operational flexibility of this powertrain, it is essential to have a highly coordinated vehicle control system to perform the blending of torque, speed, and power from multiple power sources in addition to managing transmission, engine and electric machine subsystem control. 
     A need exists in the industry for a control method that produces transition between series drive mode and parallel or split drive mode that takes into account various sources of information about the driveline and state of the electrical drive components. 
     SUMMARY OF THE INVENTION 
     A method for operating a powertrain includes determining maximum and minimum series-drive power limits of powertrain electric components; operating in parallel-drive if vehicle speed exceeds a reference, demanded wheel power is between said limits, or demanded engine power exceeds a reference demanded engine power; and operating in series-drive if vehicle speed is less than a reference, demanded wheel power is between said limits, and demanded engine power is less than a reference engine power. 
     The control method employs a calculation based on vehicle speed, engine power demand, and driver demanded wheel power. Dynamic signals for maximum power and minimum power levels of the electrical components of the driveline are calculated dynamically and are used to determine whether the vehicle should be operating in a series drive mode or a parallel drive mode. 
     The control method calculates the maximum and minimum power of the electric drive components dynamically, and bases the decision of whether to operate the vehicle in series mode or parallel mode upon these calculations. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing components of a dual-drive hybrid-electric powertrain; 
         FIG. 2  is schematic diagram showing the electric drive mode of operation of the powertrain of  FIG. 1 ; 
         FIG. 3  is schematic diagram showing the series drive mode of operation of the powertrain of  FIG. 1 ; 
         FIG. 4  is schematic diagram showing the parallel or split drive mode of operation of the powertrain of  FIG. 1 ; 
         FIG. 5  is a diagram that shows the steps of a PTOM algorithm that causes the powertrain to transition from series drive to parallel drive; 
         FIG. 6  is a diagram that shows the steps of a PTOM algorithm that cause the powertrain to transition from parallel drive to series drive; and 
         FIG. 7  is a signal diagram showing the variation over time of certain powertrain variable while transitions between series drive and parallel drive occur. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a dual-drive hybrid-electric powertrain  10  operates alternately in series drive and parallel drive. The powertrain  10  includes two electric machines  12 ,  14 ; an internal combustion engine  16 , such as a diesel engine, a multiple-speed transmission  18  that can produce a range of torque ratios, such as a wet-clutch powershift transmission; a first set of wheels  20 ,  21 ; a second set of wheels  22 ,  23 ; and a differential mechanism  24 . A clutch  36  alternately connects and disconnects the engine crankshaft and the transmission input shaft. 
     The first electric machine  12 , called Crankshaft Integrated Starter Generator (CISG), is mechanically coupled between the engine  16  and transmission  18  on the first (front) axle  28  in order to provide starter/generator capability. The second electric machine  14 , called the Electric Rear Axle Drive (ERAD), is connected to the second (rear) axle  30  in order to provide additional propulsion capability in either an electric or hybrid drive mode, resulting in two independently driven axles. The CISG  12  and ERAD  14  are powered by a high-voltage (HV) battery  32  using inverters. 
     Although this description refers to the electric machine being an ERAD  14 , implying that front axle  28  and front wheels  20 ,  21  are driven by the engine  16  and transmission  18 , the electric machine could instead be an Electric Front Axle Drive (EFAD)  14 , in which case the front axle  30  and the front wheels  22 ,  23  are driven by the EFAD  14  and the rear axle  28  and rear wheels  20 ,  21  are driven by the engine  16  and transmission  18 . 
     This powertrain  10  configuration provides great flexibility for operating the powertrain in various modes, such as electric mode, series mode, and parallel or split mode to satisfy the driver&#39;s demand and achieve better fuel efficiency without compromising other vehicle performance attributes. Given the architectural complexity and the operational flexibility of the powertrain, it is essential to have a highly coordinated vehicle control system to perform the blending of torque, speed, and power from multiple power sources in addition to managing transmission, engine and electric machine subsystem control. The decision of whether to operate the powertrain  10  in series mode or parallel (split) mode requires a calculation that takes into account various sources of information about the driveline and state of the electrical drive components. 
     In order to coordinate the actions of the subsystems (engine  16 , transmission  18 , CISG  12  and ERAD  14 ), a Vehicle System Controller (VSC) contains a function called Powertrain Operating Mode (PTOM) control. PTOM control coordinates operation of the CISG-ERAD subsystems in order to request electric drive, series drive, parallel drive, engine start, and engine stop. A control algorithm accessible to the PTOM control decides whether to request speed control or torque control from the subsystems based upon various vehicle inputs. 
     The CISG-ERAD powertrain  10  enables the vehicle to operate in one of three main operational modes. The first mode of operation, shown in  FIG. 2 , is electric-drive, wherein the battery  32  supplies power to the ERAD  14  in order to propel the vehicle by delivering torque to the wheels  22 ,  23 . Clutch  36  is open when the electric drive mode is operative. 
     The second mode of operation, shown in  FIG. 3 , is series-drive, wherein the engine  16  drives the CISG  12  in order to charge the battery  32 , which is supplying power to the ERAD  14  to propel the vehicle by delivering torque to the wheels  22 ,  23 . Clutch  36  is open when the series drive mode is operative. 
     The third mode of operation, shown in  FIG. 4 , is split or parallel-drive, wherein the engine  16  and transmission  18  provide torque to the wheels  20 ,  21  while the battery  32  and ERAD  14  provide torque to the wheels  22 ,  23  in order to propel the vehicle. Clutch  36  is closed when the parallel or split drive mode is operative. These primary modes, as well as supplementary and transitional modes are arbitrated and coordinated by the PTOM control algorithm. 
     The conditions that cause transitions between series mode and parallel mode are expressed in equations (1) and (2) and are illustrated in  FIGS. 5 and 6 . 
     The PTOM control issues control signals, to which the components of the powertrain  10  respond, causing a transition from series drive to parallel drive if:
 
Parallel={( VS&gt;=VS   lim ) OR ( P   wheel   &gt;=P   series     —     max ) OR ( P   wheel   &lt;=P   series     —     min ) OR ( P   eng   &gt;=P   eng     —     threshold )}  (1)
 
wherein:
 
     VS is vehicle speed; 
     VS lim  is vehicle speed limit for series drive; 
     P wheel  is driver demanded wheel power; 
     P series     —     max  is Min[(P battery discharge limit −P CISG minimum ), P ERAD maximum ]; 
     P battery discharge limit  is maximum discharge power limit of battery; 
     P CISG minimum  is CISG minimum power limit, which is a negative number when the CISG  12  is charging the battery  32 ; 
     P ERAD maximum  is ERAD maximum power limit; 
     P series     —     min  is Max [(P battery charge limit −P CISG maximum ), P ERAD minimum ]; 
     P CISG maximum  is CISG maximum power limit, which is a negative number when the CISG  12  is charging the battery  32 ; 
     P ERAD minimum  is ERAD minimum power limit; 
     P eng  is power demanded from the engine; and 
     P eng     —     threshold  is engine power threshold for series driving. 
     The PTOM control algorithm whose execution indicates need to transition the powertrain  10  from series drive to parallel drive is explained with reference to  FIG. 5 . 
     At step  40  (P battery battery discharge limit −P CISG minimum ) is calculated. 
     At step  42  P series     —     max  is calculated from Min[ (P battery discharge limit −P CISG minimum ), P ERAD maximum ]. 
     At step  44  a test is made to determine whether (P wheel &gt;=P series     —     max ) is true. 
     At step  46  a test is made to determine whether (P eng &gt;=P eng     —     threshold ) is true. 
     At step  48  a test is made to determine whether (VS&gt;=VS lim ) is true. 
     At step  50  (P battery battery charge limit −P CISG maximum ) is calculated. 
     At step  52  P series     —     min  is calculated from Max [(P battery charge limit −P CISG maximum ), P ERAD minimum ]. 
     At step  54  a test is made to determine whether (P wheel &gt;=P series     —     max ) is true. 
     At step  56  a test is made of the results produced at steps  44 ,  46 ,  48  and  54  to determine whether equation (1) is satisfied. If the test at step  56  is logically true, the powertrain  10  transitions to parallel drive operation, as described with reference to  FIG. 4 . 
     The PTOM control issues control signals, to which the components of the powertrain  10  respond, causing a transition from parallel drive to series drive if:
 
Series={( VS&lt;VS   lim ) AND ( P   wheel &lt;( P   series     —     max   −P   series     —     max     —     hyst )) AND ( P   wheel &gt;( P   series     —     min   +P   series     —     min     —     hyst )) AND ( P   eng   &lt;P   eng     —     threshold )}  (2)
 
wherein:
         VS is vehicle speed;   VS lim  is vehicle speed limit for series drive;   P wheel  is driver demanded wheel power;   P series     —     max  is Min[(P battery discharge limit −P CISG minimum ), P ERAD maximum ];   P battery discharge limit  is maximum discharge power limit of battery;   P CISG minimum  is CISG minimum power limit, which is a negative number when the CISG  12  is charging the battery  32 ;   P ERAD maximum  is ERAD maximum power limit;   P series     —     max     —     hyst  is hysteresis value for maximum driver demanded power in series drive;   P series     —     min  is Max[(P battery charge limit −P CISG maximum ), P ERAD minimum ];   P battery charge limit  is maximum charge power limit of battery;   P CISG maximum  is CISG maximum power limit, which is a negative number when the CISG  12  is charging the battery  32 ;   P ERAD minimum  is ERAD minimum power limit;   P series     —     min     —     hyst  is hysteresis value for minimum driver demanded power in series drive;   P eng  is power demanded from the engine; and   P eng     —     threshold  is engine power threshold for series driving.       

     The PTOM control algorithm whose execution indicates need to transition the powertrain  10  from series drive to parallel drive is explained with reference to  FIG. 6 . 
     At step  60  (P battery battery discharge limit −P CISG minimum ) is calculated. 
     At step  62  P series     —     max  is calculated from Min[ (P battery discharge limit −P CISG minimum ), P ERAD maximum ]. 
     At step  64  (P series     —     max −P series     —     max     —     hyst ) is calculated. 
     At step  66 , a test is made to determine whether (P wheel &lt;(P series     —     max −P series     —     max     —     hyst ) is true. 
     At step  68  a test is made to determine whether (P eng &lt;P eng     —     threshold ) is true. 
     At step  70  a test is made to determine whether (VS&lt;VS lim ) is true. 
     At step  72  (P battery battery charge limit −P CISG maximum ) is calculated. 
     At step  74  P series     —     min  is calculated from Max [(P battery charge limit −P CISG maximum ), P ERAD minimum ]. 
     At step  76  (P series     —     min +P series     —     min     —     hyst ) is calculated. 
     At step  78  a test is made to determine whether (P wheel &gt;(P series     —     min +P series     —     min     —     hyst ) is true. 
     At step  80  a test is made of the results produced at steps  66 ,  68 ,  70   30  and  78  to determine whether equation (2) is satisfied. If the test at step  80  is logically true, powertrain  10  transitions to series drive operation, as described with reference to  FIG. 3 . 
       FIG. 7  is a signal diagram showing the variation over time of certain powertrain parameters while transitions between series drive mode and parallel drive mode occur.  FIG. 7  shows the transitions from series mode to parallel mode due to the conditions in Equations  1  and  2 . 
     At time t 1 , the vehicle operator or driver steps into the accelerator pedal  82 , and the driver demanded wheel power  84  P wheel  increases. P series     —     max  driver demanded wheel power  84  and P series     —     min    88  increase as vehicle speed  90  increases. 
     P eng     —     threshold    96 , the engine power threshold for series driving, and the vehicle speed limit for series driving  98  VS lim , are constant. 
     At time t 2 , vehicle conditions are such that the engine  16  is turned on, series drive mode  100  is entered, and power demanded from the engine  102  P eng  increases. 
     At time t 3 , the driver releases the pedal  82 . Driver demanded wheel power  84  P wheel  and engine speed  102  P eng  decrease. P series     —     max    86  decreases and P series     —     min    88  increases as vehicle speed  90  decreases. 
     At time t 4 , the driver steps back into the pedal. 
     At time t 5 , the driver steps further into the pedal  82 , which action causes driver demanded wheel power  84  P wheel  to increase to a magnitude greater than P series     —     max    86 , and power demanded from the engine  102  P eng  to increase to a magnitude greater than the engine power threshold  96  for series driving P eng     —     threshold . At time t 5 , parallel drive mode is entered. 
     At time t 6 , the driver steps out of the pedal  82 , which action causes driver demanded wheel power  84  P wheel  and power demanded from the engine  102  P eng  to decrease. 
     At time t 7 , the vehicle speed  90  drops below the vehicle speed limit for series driving  98  VS lim , and series drive mode  100  is reentered. 
     In  FIG. 7 , the difference between P series     —     max    86  and (P series     —     max −P series     —     max     —     hyst )  92  is represented graphically by a space or gap P series     —     max     —     hyst . The difference between P series     —     min    88  and (P series     —     min +P series     —     min     —     hyst )  94  is represented graphically by a space or gap P series     —     min     —     hyst . The presence of hysteresis values P series     —     max     —     hyst  and P series     —     min     —     hyst  in equation (1) avoids undesired cycling from series drive to parallel drive and maintains the powertrain  10  in parallel-drive longer, than if the hysteresis values were absent from equation (1). 
     Although this description refers to the electric machine being an ERAD  14 , implying that front axle  28  and front wheels  20 ,  21  are driven by the engine  16  and transmission  18 , the electric machine could instead be an Electric Front Axle Drive (EFAD)  14 , in which case the front axle  30  and the front wheels  22 ,  23  are driven by the EFAD  14  and the rear axle  28  and rear wheels  20 ,  21  are driven by the engine  16  and transmission  18 . 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.