Abstract:
A method for reverse drive mode operation of a hybrid electric vehicle includes determining an output power of the internal combustion engine, determining a power circulation loss between a generator and a motor, determining a benefit power based on the difference between the output power of the internal combustion engine and the power circulation loss, and stopping operation of the internal combustion engine during the reverse drive mode operation of the vehicle if the benefit power is less than a predetermined threshold power value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a strategy to control a split powertrain HEV while the vehicle travels in reverse and vehicle state-of-charge (SOC) is low. 
     2. Discussion of the Prior Art 
     The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky. 
     The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set. 
     Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another electric motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE. 
     A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a “powersplit” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed. 
     The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV&#39;s potential benefits. 
     One such area of HEV development is controlling a powersplit HEV while traveling in reverse. In the prior art, a reverse gear in a transmission is engaged when the vehicle operator moves a shift lever to the reverse, or “R”, position. In an HEV, a variety of powertrain configurations based on vehicle conditions can require new strategies to move the vehicle in reverse. 
     A strategy for moving an HEV in reverse is known in the prior art. See U.S. Pat. No. 5,847,469 to Tabata et al. Tabata et al. describes an HEV using a conventional transmission to power the vehicle&#39;s wheels. The patent describes a system for the electric traction motor alone to reverse the direction of the vehicle without reversing the rotation of the motor so long as there is enough battery charge. Otherwise, the engine is started to assist the motor. 
     A conventional transmission with a reverse gear could be considered an inefficient and unnecessary complication and expense in a split powertrain HEV. Alternatively, the electric traction motor alone is used to propel the vehicle in reverse direction. When moving in reverse, the ring gear torque, resulting from engine output, goes against the vehicle moving in reverse. Thus, using the engine while the vehicle is in reverse is undesirable. Nevertheless, if battery state-of-charge (SOC) is low, the engine may need to run to power a generator to charge the battery and allow the motor to operate. 
     Unfortunately, no strategy is known to control a split powertrain HEV while the vehicle travels in reverse with the engine running because the battery state-of-charge (SOC) is low and the electric traction motor requires electricity produced by the generator for reverse motive power. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a strategy to control a split powertrain hybrid electric vehicle (HEV) when the vehicle travels in reverse, vehicle state-of-charge (SOC) is low and the powertrain is configured to only use the motor while traveling in reverse. 
     The powersplit hybrid electric vehicle (HEV) powertrain of the present invention has an engine, a traction motor, a generator, an electric energy storage device for storing electric energy, the electric energy storage device connected to the traction motor to power the traction motor, and the electric energy storage device connected to the generator to receive energy generated by the generator. The powersplit HEV powertrain also has a power transmission device having at least one forward drive position to move the HEV in a forward direction and at least one reverse drive position to move the vehicle in a reverse direction. The power transmission device is connected to the engine, the traction motor, and the generator motor. In addition, the powersplit HEV powertrain has a driver operated drive position selector comprising a reverse drive mode, a vehicle system controller comprising a reverse drive mode controller activated when the drive position selector is in the reverse drive mode, wherein the reverse drive mode controller preventing the battery SOC from continuously falling while meeting driver demand. 
     The powertrain reverse drive mode controller can be configured to determine whether the engine and generator motor are running, calculate a benefit power from the engine if the engine and the generator motor are running, compare the benefit power with a first predetermined value, determine whether a driver torque request plus the generator torque is greater than a predetermined maximum motor torque if the benefit power is greater than or equal to the first predetermined value, calculate a new generator torque request if the determination of whether a driver torque request plus the generator torque is greater than a predetermined maximum motor torque, determine whether the new generator torque request is greater than or equal to a second predetermined value, calculate a new generator speed for the new generator torque request if the new generator torque request is greater than or equal to the second predetermined value, determine whether the new generator speed is less than or equal to a maximum generator speed, and determine a new motor torque request if the new generator speed is less than or equal to a maximum generator speed. 
     The controller can also be configured to add a stop engine command if the benefit power is less than the first predetermined value, the new generator torque request is less than the second predetermined value, or the new generator speed is greater than a maximum generator speed. 
     Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which: 
     FIG. 1 illustrates a general hybrid electric vehicle (HEV) configuration. 
     FIG. 2 illustrates the power flow within the powertrain. 
     FIG. 3 illustrates the strategy of the present invention for an HEV while traveling in reverse. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to electric vehicles and, more particularly, to hybrid electric vehicles (HEVs). FIG. 1 demonstrates just one possible configuration, specifically a parallel/series hybrid electric vehicle (powersplit) configuration. 
     In a basic HEV, a planetary gear set  20  mechanically couples a carrier gear  22  to an engine  24  with a one-way clutch  26  to prevent the engine from rotating in a counter clock wise (CCW) direction. The planetary gear set  20  also mechanically couples a sun gear  28  to a generator motor  30  and a ring (output) gear  32 . The generator motor  30  also mechanically links to a generator brake  34  and is electrically linked to an electric energy storage device (battery)  36  to receive electric energy converted from mechanical energy by the generator motor  30 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to drive wheels  42  via an output shaft  44 . The mechanical coupling represents collectively a power transmission device, the power transmission devise being connected to the engine  24 , the traction motor  38  and the generator motor  30 . This power transmission device can be configured to have at least one forward drive position to move the HEV in a forward direction and at least one reverse drive position to move the HEV in a reverse direction. A driver operated drive position selector (gear selector) (not shown) determines whether the vehicle is to move in the reverse direction. 
     The planetary gear set  20 , splits the engine  24  output energy into a series path from the engine  24  to the generator motor  30  and a parallel path from the engine  24  to the drive wheels  42 . Engine  24  speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments the engine  24  power to the drive wheels  42  on the parallel path through the second gear set  40 . The traction motor  38  also provides the opportunity to use energy directly from the series path, essentially running off power created by the generator motor  30 . This reduces losses associated with converting energy into and out of chemical energy in the battery  36  and allows all engine  24  energy, minus conversion losses, to reach the drive wheels  42 . 
     A vehicle system controller (VSC)  46  controls many components in this HEV configuration by connecting to each component&#39;s controller. An engine control unit (ECU) (not shown)connects to the engine  24  via a hardwire interface. The ECU and VSC  46  can be based in the same unit, but are actually separate controllers. The VSC  46  communicates with the ECU, as well as a battery control unit (BCU)  50  and a transaxle management unit (TMU)  52  through a communication network such as a controller area network (CAN)  54 . The BCU  50  connects to the battery  36  via a hardwire interface. The TMU  52  controls the generator motor  30  and traction motor  38  via a hardwire interface. 
     All vehicles require movement in a reverse direction from time to time. Such movement usually begins with a driver manually shifting a gear selector to a reverse (or “R”) position. In the powertrain configuration of the present invention, the engine  24  does not provide primary drive to the vehicle while traveling in reverse. There is no true rear drive shifting means in that there is no discrete exchange of power flow elements that produces a reverse range as opposed to a forward range. In fact, torque from the engine  24  while in reverse would work against the traction motor  38  traveling in reverse. Nevertheless, to operate the traction motor  38  in a reverse rotation, the engine  24  may be needed to charge the battery  36  if a low state-of-charge (SOC) exists. During engine  24  operation to generate the electricity, the engine  24  would produce torque through the second gear set  40  that would attempt to drive the vehicle in a forward direction. It is only by the balance of the relative forward and reverse torques that the net vehicle rearward torque is augmented. 
     For example, if the vehicle is moving in reverse at a certain vehicle speed (equivalent to ω r ), the traction motor&#39;s  38  output power required for the vehicle speed is P V . Also assume the engine  24  is running at a constant power output (τ e ω e ), and both efficiencies of the planetary gear set  20  and the second gear set  40  are one. The resulted ring output gear  32  torque from the engine  24  torque (τ e ) is τ r =τ e /(1+ρ). ρ is the gear ratio between the sun gear  28  and the ring gear  32  (N s /N r ). η g  and η m  are the efficiencies for the generator motor and the traction motor respectively. The power flow within the powertrain under these assumptions is illustrated in FIG.  2 . The following symbols listed below will assist in understanding the present invention: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 ω r  = 
                 ring gear speed 
               
               
                 ω e  = 
                 engine speed 
               
               
                 P v  = 
                 output power 
               
               
                 τ e ω e =   
                 engine power output 
               
               
                 τ e = 
                 engine torque 
               
               
                 τ r  = 
                 ring gear torque 
               
               
                 ρ = 
                 gear ratio between sun gear and ring gear 
               
               
                 Ns = 
                 number of teeth in sun gear 
               
               
                 Nr = 
                 number of teeth in ring gear 
               
               
                 η g  = 
                 overall efficiency for generator 
               
               
                 η m  = 
                 overall efficiency for motor 
               
               
                 P batt  = 
                 power charging the battery 
               
               
                 η g τ e ω e  = 
                 engine&#39;s electrical output power (through 
               
               
                   
                 generator) 
               
               
                 P v /η m  = 
                 motor&#39;s input power (electrical) 
               
               
                 (1/η m  − η g ) τ r ω r  = 
                 power circulation loss between motor and 
               
               
                   
                 generator 
               
               
                 τ d—req@m  = 
                 driver&#39;s torque request at the motor 
               
               
                 τ g  = 
                 generator torque 
               
               
                 ω g  = 
                 generator speed 
               
               
                 W benefit  = 
                 benefit power from the engine 
               
               
                 K w  = 
                 a predetermined W benefit  value 
               
               
                 τ m—max  = 
                 maximum motor torque 
               
               
                 τ g—req  = 
                 generator torque request 
               
               
                 τ g—min  = 
                 generator torque request minimum 
               
               
                 ω g—cal  = 
                 calculated generator speed 
               
               
                 ω g—max  = 
                 maximum generator speed. 
               
               
                 T = 
                 gear ratio from generator to motor 
               
               
                   
               
             
          
         
       
     
     The power flow illustrated in FIG. 2 demonstrates part of the traction motor  38  output (τ r ω r  required to overcome the engine  24  output at the ring gear  32 ) is also part of the generator motor  30  input, which generates electricity. Clearly, this is a power circulation between the traction motor  38  and the generator motor  30 , which results in power circulation loss in the powertrain system. 
     The power charging the battery  36  is P batt  and is shown by: 
     
       
           P   batt =η g (τ e ω e +τ r ω r )−( P   v +τ r ω r )/η m , or  
       
     
     
       
           P   batt =η g τ e ω e   −P   v /η m −(1/η m −η g )τ r ω r .  
       
     
     In this example, η g τ e ω e  is the engine&#39;s  24  electrical output power (through the generator motor  30 ), P V /η m  is the traction motor&#39;s  38  input power (electrical) required to propel the vehicle, and (1/η m −η g )τ r ω r  is the power circulation loss between the traction motor  38  and generator motor  30 . If the power circulation loss is greater than or close to the engine&#39;s  24  electrical output power, there is no benefit to operate the engine  24  since the engine  24  output only generates heat in the traction motor  38  and generator motor  30 , and does not charge the battery  36 . 
     To ensure the benefit of operating the engine  24  when the vehicle travels in reverse and the battery  36  SOC is low, it is necessary for the VSC  46  to control the powertrain system properly to avoid the result illustrated above. 
     The present invention is a control strategy within the VSC  46  to efficiently control the illustrated powersplit HEV powertrain system when the vehicle travels in reverse and the battery  36  SOC is low. The reverse drive mode controller is activated when the drive position selector is in the reverse drive mode. The present invention operates the powertrain system efficiently and prevents the battery  36  SOC from continuously falling while meeting the driver&#39;s demand. The strategy is illustrated in FIG.  3 . 
     At Step  60 , the reverse drive mode controller strategy first reads the following vehicle inputs  58 : PRND position, driver&#39;s torque request at the motor (τ d     —     req@m ), generator torque (τ g ) and speed (ω g ), vehicle speed (to calculate ring gear speed ω r ), engine speed (ω e ), and engine and generator status. PRND position represents a driver operated drive position selector, or gear selector, (not shown) that is manually shifted by the vehicle driver. If the gear selector is in the “R” position, the driver has requested the vehicle to move in reverse. 
     At Step  62 , the strategy next determines if the gear selector is in the “R” position. If no, the strategy ends. 
     If “R” is selected at Step  62 , the strategy next determines if both the engine  24  and generator motor  30  are running at Step  64 . If no, the strategy ends. If both the engine  24  and generator motor  30  are running at Step  64 , the strategy calculates the benefit power from the engine  24  W benefit  at Step  66  using the equation W benefit =η g τ e ω e −(1/η m −η g )τ r ω r . 
     Next the strategy determines at Step  68  whether the W benefit  is greater than or equal to a first predetermined value K w . This value indicates it is desirable to run the engine  24  to charge the battery  36 . 
     If W benefit  is greater than or equal to K w  at Step  68 , the strategy next determines at Step  72  whether the sum of the driver&#39;s torque request at motor(τ d     —     req@m ) plus the generator motor  30  torque reflected at the motor shaft (τ g T, where T is the gear ratio from generator to motor and is well known in the prior art) is greater than the predetermined maximum motor torque (τ m     —     max ) If the W benefit  is less than K w , the strategy executes a stop engine process at Step  70  and ends the strategy. 
     If the sum is greater than the predetermined maximum motor torque at Step  72 , the strategy calculates a new generator motor  30  torque request (τ g     —     req ) at Step  74  so that the driver&#39;s torque request is not compromised and the vehicle reverse acceleration performance meets driver demand. The calculation is as follows: τ g     —     req =(τ m     —     max −τ d     —     req@m )/T. Otherwise, the strategy proceeds to calculate a new traction motor  38  torque request (τ m     —     req ) at Step  76  using: τ m     —     req =τ d     —     req@m +τ g     —     req  T and the strategy ends. 
     Next, the strategy determines at Step  78  whether the new generator motor  30  torque request (τ g     —     req ) is greater than or equal to a second predetermined value (τ g     —     min ). This implies the generator motor&#39;s  30  torque can be accurately controlled. If (τ g     —     req ) is greater than or equal to the second predetermined value (τ g     —     min ) at Step  78 , the strategy proceeds to determine the calculated generator motor  30  speed (ω g     —     cal ) for the given new generator motor  30  torque request at Step  80 . Otherwise, the strategy proceeds to execute the stop engine process at Step  70  and ends the strategy. With the new generator motor  30  torque request (τ g     —     req ), a new engine  24  speed (ω e ) can be determined and then the calculated generator motor  30  speed (ω g     —     cal ) of Step  80  can be derived based on the new engine  24  speed and ring gear  32  speed (equivalent to present vehicle speed). 
     Next, at Step  82 , the strategy determines whether the calculated generator motor  30  speed (ω g     —     cal ) is less than or equal to a predetermined maximum generator motor  30  speed (ω g     —     max ). If yes, the strategy proceeds to Step  76  (described above) to determine the new traction motor  38  torque request to meet the driver&#39;s demand. This motor torque request compensates the ring gear  32  torque resulted from the engine  24  output to meet the driver&#39;s demand. If the calculated generator motor  30  speed (ω g     —     cal ) is greater than the maximum generator motor  30  speed (ω g     —     max ) at Step  82 , the strategy proceeds to execute the stop engine process at Step  70  and the strategy ends. 
     The above-described embodiment of the invention is provided purely for purposes of example. Many other variations, modifications, and applications of the invention may be made.