Abstract:
A method for controlling regenerative braking in a hybrid electric powertrain includes using an e-pump to supply fluid to a transmission provided an indication that a torque converter clutch disengagement will occur under current powertrain operating conditions is present, and discontinuing use of the e-pump and using a mechanical pump to supply fluid to the transmission provided the indication that a torque converter clutch disengagement will occur under current powertrain operating conditions is absent.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for increasing regenerative braking in a hybrid electric vehicle. 
     2. Description of the Prior Art 
     The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In a hybrid electric powertrain an electric machine is coupled to the wheels through a torque converter and transmission gearing. 
     In the vehicle equipped with a modular hybrid transmission (MHT) and using a torque converter it is necessary to keep the torque converter bypass clutch locked during brake regeneration events. During such events an onboard electric machine is used as generator to recapture vehicle kinetic energy during braking. 
     Failure to keep the torque converter bypass clutch locked can result in the torque converter&#39;s impeller speed to rapidly decelerate to speeds where the transmission&#39;s mechanical pump can not operate efficiently. Such low speeds can result in loss of line pressure in the transmission&#39;s hydraulic system and loss of transmission clutch control. 
     Abandoning regenerative braking at higher speeds in anticipation of opening the torque converter bypass clutch results in kinetic energy that is not captured and thus in a reduced fuel economy benefit from regenerative braking. 
     In the previous solutions, regenerative braking is limited to the vehicle speeds and gears where the torque converter bypass clutch is sure to stay locked. During downshifts the amount of regenerative braking is limited to the level that would assure that torque converter bypass clutch slip is minimal. This also required a significant amount of blending, wherein torque smoothly transitions from reduced torque magnitudes to low negative regenerative torque magnitudes. 
     SUMMARY OF THE INVENTION 
     A method for controlling regenerative braking in a hybrid electric powertrain includes using an e-pump to supply fluid to a transmission provided an indication that a torque converter clutch disengagement will occur under current powertrain operating conditions is present, and discontinuing use of the e-pump and using a mechanical pump to supply fluid to the transmission provided the indication that a torque converter clutch disengagement will occur under current powertrain operating conditions is absent. 
     The method recovers more vehicle kinetic energy during regenerative braking events due to the torque converter bypass clutch being maintained locked by hydraulic pressure produced by the e-pump, which provides line pressure required by the transmission to maintain the clutch locked. 
     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 a modular hybrid electric powertrain for a motor vehicle; 
         FIGS. 2A and 2B  are a flow diagram representing an algorithm for coordinating transmission pressure with regenerative braking in the powertrain of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , a hybrid electric powertrain  10  includes an internal combustion engine  12 ; engine disconnect clutch  14 ; electric machine or motor/generator  16 ; transmission hydraulic pump  18 , i.e. a mechanical pump driven by the engine or electric machine or both of these; torque converter  20 ; torque converter lock-up clutch  22 ; transmission gearing  24 ; final drive gearing  26 ; shafts  28 ,  29 ; and driven wheels  30 . A low voltage starter  32 , powered by a low voltage battery  34 , cranks the engine while starting the engine  12  and producing sustained combustion. A high voltage battery  36  powers the electric motor/generator  16 . 
     The torque converter  20  is a hydraulic coupling that produces a hydrokinetic drive connection between an impeller, which is driveably connected to the engine  12  when clutch  14  is closed, and a turbine, which is driveably connected to the driven wheels  30 . 
     The torque converter lock-up clutch  22  alternately opens and closes a drive connection between the torque converter&#39;s turbine and the shaft  38 . 
     A vehicle equipped with this powertrain  10  can produce electric drive and hybrid electric drive and can charge the battery  36  either by regenerative braking, i.e., recovering and converting kinetic energy of the vehicle during a braking event to electric energy that can be stored in battery  36 , or by using the engine to charge battery  36 . 
     An electric pump (also referred to in the specification and claims as an e-pump)  40 , i.e. a hydraulic pump driven by an electric motor  42  has its input connected to a source of hydraulic fluid at low pressure, such as the transmission&#39;s sump, and its output connected to the transmission hydraulic system, by which the torque converter&#39;s bypass clutch  22  is actuated between locked and unlocked states. 
     In powertrain  10  motor  16  is coupled to the wheels through the torque converter  20 , transmission gearing  24  and final drive  26 . The torque converter  20  transmits torque through the combination of the hydraulic path and the mechanic path, provided the torque converter clutch  22  is slipping. If the torque converter clutch  22  is fully open, torque can only be transmitted through the hydraulic path. If the clutch  22  is fully locked, the torque can only be transmitted through the mechanical path. 
     During regenerative braking, torque is transmitted from the wheels  30  to the electric machine  16 . If clutch  22  is open, the torque converter&#39;s ability to transmit torque in the reverse direction is very limited. Any excessive regenerative torque can reduce the electric machine&#39;s speed. As a result, to recoup most of the kinetic energy using regenerative braking, the torque converter clutch  22  should be kept locked while the vehicle is slowing down. 
     The electric pump (e-pump)  40  provides an alternative source of hydraulic line pressure to the hydraulic pressure that is provided by mechanical pump  18 . Line pressure produced by e-pump  40  is used to supplement or limit line pressure drop if mechanical oil pump  18  fails to supply adequate line pressure. The e-pump motor  42  is supplied with electric power from one of the batteries  34 ,  36 . 
     An algorithm  44  shown in  FIGS. 2A and 2B  controls and coordinates torque converter clutch operation and regenerative braking. At step  46  a test is performed to determine whether regenerative vehicle wheel braking is currently occurring. If the result of test  46  is logically false, control advances to step  48 , where e-pump  40  is stopped and the control is terminated at step  50 . 
     If the result of test  46  is logically true, at step  52  vehicle speed and brake pedal pressure are monitored, and at step  54  scheduled transmission gearshifts and vehicle deceleration rate are monitored. 
     At step  56  a test is performed to determine whether a transmission downshift is pending, i.e., commanded by not yet completed. Similarly at step  56  a test is performed to determine whether vehicle speed is less than a reference speed, which is a calibratable vehicle speed. If the result of test  56  indicates that a downshift is pending or that vehicle speed is less than the reference vehicle speed, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     But if the result of test  56  indicates that a downshift is not pending or that vehicle speed is greater than the reference vehicle speed, at step  60  a test is performed to determine whether brake pressure is greater than a reference brake pressure, which is a calibratable pressure magnitude. If the result of test  60  is true, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     If the result of test  60  is logically false, at step  62  a test is performed to determine whether vehicle deceleration is greater than a reference vehicle deceleration, which is a calibratable magnitude. If the result of test  62  is true, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     At step  64  the transmission speed ratio, torque ratio or gear ratio is monitored to detect slipping of a control element, i.e., a clutch or brake whose engagement is required to produce the current speed ratio, or a transmission tie-up, i.e., a condition wherein the engaged transmission control elements produce no transmission speed ratio. 
     At step  66  a test is performed to determine whether the current transmission speed ratio, torque ratio or gear ratio is outside of a calibratable range of speed ratios, or torque ratios or gear ratios, respectively. If the result of test  62  is true, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     If the result of test  66  is false, at step  68  a test is performed to determine whether the torque converter bypass clutch (TCC)  22  is scheduled to unlock. If the result of test  68  is true, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     If the result of test  68  is false, at step  70  slip across the torque converter bypass clutch (TCC)  22  is monitored. 
     At step  72  a test is performed to determine whether slip across the torque converter bypass clutch  22  is greater than a reference slip, which is a calibratable magnitude. If the result of test  72  is true, control advances to step  58  where e-pump  40  is turned on to supply pressurized fluid to the transmission&#39;s hydraulic system. 
     If the result of test  72  is false, at step  48  e-pump  40  is stopped and the control is terminated at step  50 . 
     The control  44  monitors vehicle speed during a vehicle braking event. If regenerative braking is taking place during the braking event, the vehicle speed is compared to predicted vehicle speed to determine if the torque converter clutch  22  is likely to unlock. There are numerous reasons why torque converter could unlock including vehicle speed being too low, vehicle operator&#39;s depressing the brake pedal and producing high brake pressure resulting in rapid vehicle deceleration, slip across clutch  22  during regenerative braking downshifts, etc. 
     According to the control strategy, a transmission controller turns on e-pump  40  when there is a probability that impeller speed might not be sufficient to generate adequate line pressure. This could be done by monitoring brake pressure signal, torque converter slip, vehicle speed and other parameters affecting torque converter lock up state. To minimize wasting recoverable vehicle kinetic energy, the e-pump  40  should be activated only when a locked condition of the clutch  22  can not be ensured. 
     Alternatively transmission oil temperature, e-pump diagnostic signals, e-pump duty cycle, system voltage should be monitored, step  76 , to determine if the e-pump is capable of providing a sufficient magnitude of transmission hydraulic pressure, step  78 . If one of these parameters is restricting e-pump operation to the point where hydraulic pressure is not sufficient to maintain desired gear state, a signal is sent to a vehicle controller as an indication of this state. The vehicle controller then commands a reduced magnitude of regenerative braking and disables regenerative braking sooner to avoid loss of mechanical pump line pressure, step  80 . 
     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.