Patent Publication Number: US-7722499-B2

Title: Launch control of a hybrid electric vehicle

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to a powertrain for a hybrid electric vehicle having an engine and one or more electric machines and, in particular, to controlling torque transmitted to the drive wheels when the vehicle is being accelerated from a stopped or nearly stopped condition, called vehicle launch. 
   2. Description of the Prior Art 
   A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts. 
   The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear. 
   During a vehicle launch condition in a conventional vehicle whose powertrain includes a powershift transmission, the engine and transmission are concurrently controlled in a coordinated manner to provide acceptable vehicle launch performance. In a powershift transmission vehicle application, providing consistent and acceptable vehicle launch performance can be a rather difficult control problem due to the lack of a torque converter. During a vehicle launch condition in this type of vehicle application, the torque capacity of the transmission clutch and slip across the clutch are carefully controlled in coordination with the engine torque to provide the desired vehicle response. Problems which can occur during these events include engine stall, excessive clutch slip, reduced clutch durability, dead pedal feel, and inconsistent response are a few examples. 
   A powershift transmission may be used in a hybrid electric vehicle (HEV), in which one or more electric machines, such as a motor or an integrated starter-generator (ISG), are arranged in series and parallel with the engine. Unlike a conventional vehicle with a powershift transmission, in a hybrid electric vehicle with a powershift transmission, there are multiple propulsion paths and multiple power sources, the engine and electric machines, which can be used during a vehicle launch condition. Therefore, a more sophisticated powershift vehicle launch control system is needed to deal with the complexities and added powertrain operating modes of an HEV in response to a vehicle launch request from the vehicle operator. 
   SUMMARY OF THE INVENTION 
   The system and method for controlling vehicle launch in a HEV takes advantage of additional propulsion paths and torque actuators to improve vehicle launch performance and to overcome problems and deficiencies presented by a conventional vehicle with a powershift transmission. 
   This control strategy supports torque blending when multiple propulsion paths are used for propulsion during vehicle launch due to enhanced powershift transmission control. Moreover, the control coordinates clutch torque capacity control when propulsion assistance is provided by the additional torque actuators, which improves clutch durability since clutch load is reduced accordingly. Furthermore, the control supports battery charging by the first electric machine during vehicle launch conditions by controlling the net crankshaft torque accordingly. The control supports multiple HEV powertrain operating modes and transitions, automatically operates the same as a conventional vehicle with a powershift if the additional torque actuators are not used, and is applicable to any HEV powertrain architecture that employs a powershift transmission whether the input clutches are wet or dry clutches. 
   A powertrain to which the control of a vehicle launch may be applied includes a transmission having an input, a current gear, an input clutch associated with a target gear and an output, an engine and a first electric machine for driving the input. A method for controlling a vehicle launch using a transmission having an input, a current gear, an input clutch associated with a target gear and an output, an engine and an electric machine for driving the input, includes the steps of using the engine and the first electric machine to drive the transmission input at a desired magnitude of torque, determining a desired magnitude of torque capacity of the input clutch, determining a crankshaft speed error, determining a magnitude of a change in torque capacity of the input clutch that will reduce the crankshaft speed error, and increasing the torque capacity of the input clutch to a desired torque capacity whose magnitude is determined by adding the current magnitude of torque capacity of the input clutch and said change in torque capacity of the input clutch. 
   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 of a vehicle powertrain system to which the control can be applied; 
       FIG. 2  is a schematic diagram showing additional details of the vehicle powertrain system of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of the vehicle launch control system; 
       FIG. 4  is a diagram illustrating the vehicle launch control method steps; and 
       FIG. 5A  is a graph showing the variation with time of desired wheel torques during vehicle launch operation produced by executing the control algorithm; 
       FIG. 5B  is a graph showing the variation of the desired torque capacity of the subject input clutch with time during vehicle launch operation; 
       FIG. 5C  is a graph showing the variation of vehicle speed with time during vehicle launch operation; 
       FIG. 5D  is a graph showing the variation of engine crankshaft speed, desired engine crankshaft speed, and clutch output speed with time of wheel torques during vehicle launch operation; 
       FIG. 5E  is a graph showing the variation of engine torque and CISG torque with time during vehicle launch operation; 
       FIG. 5F  is a graph showing the variation of transmission output torque and ERAD torque with time during vehicle launch operation; and 
       FIG. 6  is a schematic diagram showing details of a powershift transmission. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIGS. 1 and 2 , a vehicle powertrain  12  includes an engine  14 , such as a diesel or gasoline engine; a transmission  16 , such as a dual wet clutch powershift transmission or another multiple ratio transmission having no torque converter; an electric machine  18 , such as an ISG (integrated starter generator) driveably connected to the transmission input  20 ; and an additional electric machine  22 , such as an electric motor. Electric machine  18  provides starter/generator capability. 
   Electric machine  22 , sometimes referred to as an electric rear axle drive unit (ERAD), is connected to the final drive of a rear axle  24  through gearing  28  and provides additional propulsion capability in either an electric drive or hybrid (series/parallel) drive mode. In full FWD applications, electric machine  22  could also connected to the final drive of a front axle at the output of the transmission, and would be referred to as an electric front axle drive (EFAD) unit. Power output by the electric machine  22  drives vehicle wheels  26 ,  27  through ERAD gearing  28  and a final drive unit  30 , which is in the form of an inter-wheel differential mechanism. Similarly, the transmission output  32  is driveably (mechanically) connected to vehicle wheels  34 ,  35  through a final drive unit  36 , which includes an inter-wheel differential mechanism. 
   Powertrain  12  can operate in major modes including: (1) series hybrid drive, in which engine  14  is running and producing combustion, CISG  18  is generating electric power, and ERAD  22  is alternately motoring and generating electric power; (2) engine drive, in which CISG  18  and ERAD  22  are both nonoperative and engine  14  is running, as in a conventional powertrain; (3) parallel hybrid drive, in which engine  14  is running, CISG  18  and/or ERAD  22  are operative to provide additional vehicle propulsion; (4) engine starting, in which CISG  18  is motoring to start the engine by driving the engine flywheel; and (5) engine stop, in which engine  14  is shut down. While operating in parallel hybrid drive mode, the powertrain can operate in several sub-modes including: (3.1) parallel hybrid drive  1 , in which CISG  18  is shutdown, ERAD  22  is motoring and generating; (3.2) parallel hybrid drive  2 , in which CISG  18  is motoring and ERAD  22  is shutdown; (3.3) parallel hybrid drive  3 , in which (CISG  18  and ERAD  22  are motoring; and (3.4) parallel hybrid drive  4 , in which CISG  18  is generating and ERAD  22  is alternatively shutdown, motoring and generating. 
     FIG. 2  illustrates the input clutches  40 ,  41 , which selectively connect the input shaft  20  of transmission  16  alternately to the even-numbered gears  42  and odd-numbered gears  43 ; an electronic transmission control module (TCM)  44 , which controls the input clutches and gearbox state through command signals to servos that actuate the input clutches and gearbox shift forks/synchronizers; an electronic engine control module (ECM)  46 , which controls operation of engine  14 ; an ISC  48 , which controls the CISG and ERAD operations. A vehicle control system (VCS), which is not shown, issues control commands to the TCM and ECM. Each of the VCS, TCM and ECM includes a microprocessor accessible to electronic memory and containing control algorithms expressed in computer code, which are executed repeatedly at frequent intervals. 
   There are two propulsion paths, a mechanical path and an electrical path, which are used to meet the propulsion demand produced by the vehicle operator. The engine  14  and CISG  18  can provide vehicle propulsion by transmitting torque through transmission  16  in the mechanical propulsion path to wheels  34 ,  35 , and the ERAD machine  28  can provide vehicle propulsion directly in the electrical propulsion path to wheels  26 ,  27 . 
   The vehicle launch control uses a torque-based control scheme to control the torque capacity of the transmission input clutches  40 ,  41  and engine crankshaft torque in response to an effective front axle propulsion demand produced by the vehicle operator during a launch condition. 
   The steps of an algorithm for controlling vehicle launch using the powertrain illustrated of  FIGS. 1 and 2  are shown in the control system diagram of  FIG. 3  and the method steps diagram of  FIG. 4 . The vehicle operator&#39;s demand for wheel torque is represented by the degree to which the engine accelerator pedal  50  is depressed, which depression is usually referred to as accelerator pedal position, pps. An electronic signal representing the accelerator pedal position produced by a pps sensor and an electronic signal representing the current vehicle speed  52  produced by a shaft speed sensor, are received as input by a driver demand determination function  54 , which accesses in electronic memory a function indexed by the two input variables to produce the magnitude of the current desired wheel torque demand T W     —     DES . 
   At  56 , the desired front axle torque T W     —     FA  to be provided to front wheels  34 ,  35  by the engine  14  and CISG  18  of the mechanical propulsion path and the desired rear axle torque T W     —     RA  to be provided to rear wheels  26 ,  27  by the ERAD  28  of the electrical propulsion path are determined upon reference to the desired magnitude of front axle torque and rear axle torque, such that the sum of the distributed propulsion torques equals the total driver demanded wheel torque T W     —     DES . The strategy for propulsion distribution may take into account vehicle stability and dynamics constraints, energy management and efficiency criteria, torque capabilities of the various power sources, etc. 
   At  58 , the desired ERAD torque is determined, on reference to the distributed propulsion and the rear axle propulsion torque request T W     —     RA , and a command representing desired ERAD torque T ERAD     —     DES  is sent on communication bus  60  to the ISC  48  control interface, which command causes the ERAD  28  to produce the desired ERAD torque. 
   Similarly, at  59 , the desired transmission output torque T O     —     FA  is determined and a command representing desired transmission output torque, determined with reference to the distributed propulsion and the front axle propulsion torque request T W     —     FA , is sent to  62 , where input clutch torque capacity is determined, and to  64 , where engine crankshaft torque is determined. Details of the techniques employed at  62  and  64  are described below. 
   Control then passes to a powershift mode handling controller  66 , which receives input signals representing the position of the transmission gear selector PRNDL, actual crankshaft speed ω CRK  of engine  14 , current clutch output speed ω CL  at the gearbox input  21 , vehicle speed VS, and the HEV powertrain operating mode. Controller  66  activates a vehicle launch mode controller  68 , provided the desired output torque T O     —     FA  is equal to or greater than a predetermined magnitude and vehicle speed is low, thereby indicating that the vehicle is operating in vehicle launch mode and that the propulsion path that includes transmission  16  will be used during vehicle launch. 
   After controller  66  issues command  67 , which activates the launch mode control  68 , control passes to  62  where an open-loop control determines the magnitude of a desired open-loop input clutch torque capacity T CL     —     OL     —     LCH  on reference to the current transmission gear, its gear ratio, and the desired transmission output torque T O     —     FA . 
   The vehicle launch controller  68  determines at  70  the desired slip across the input clutch  40 ,  41  CL SLIP DES  from a function stored in memory and indexed by the current vehicle speed VS and accelerator pedal position. The subject input clutch is associated with the target gear, i.e., the current transmission gear during launch. 
   At  72 , the desired engine crankshaft speed ω CRK     —     DES  at the transmission input  20  is determined at summing junction  74  with reference to the desired clutch slip CL SLIP     —     DES  and current clutch output speed, i.e., ω CL  at the gearbox input  21 . The desired engine crankshaft speed ω CRK DES  is supplied as input to summing junction  78 . A signal representing the current clutch output speed ω CL  is carried on communication bus  60  from the TCM  44  to summing junction  74 . 
   At summing junction  78 , the magnitude of engine crankshaft speed error ω CRK     —     ERR , the difference between the desired engine crankshaft speed ω CRK     —     DES  at the transmission input  20  and the current crankshaft speed ω CRK , is determined and supplied as input to a PID controller  80  or a similar closed loop controller. A signal representing the current crankshaft speed ω CRK  is carried on communication bus  60  from the ECM  46  to summing junction  78 . 
   In order to control slip across the subject input clutch  40 ,  41  during vehicle launch, controller  80  determines a desired delta torque capacity of the subject input clutch ΔT CL CAP  that minimizes the current engine crankshaft speed error ω CRK ERR . 
   At summing junction  69  the desired torque capacity of the subject input clutch during launch T CL     —     CAP     —     LCH , the sum of the desired delta torque capacity ΔT CL CAP  determined at  80  that minimizes the current engine crankshaft speed error ω CRK ERR  and the open loop torque capacity determined at  62  of the subject input clutch T CL     —     OL     —     LCH , is determined and carried on bus  60  as the final input clutch torque capacity command  92  T CL     —     CAP     —     DES  and sent to TCM  44 , which produces a command signal that actuated the servo of the subject input clutch to produce the desired torque capacity. 
   Control then advances to  64 , where the base torque T CRK     —     OL  carried by the engine crankshaft and transmission input  20  is determined open-loop upon reference to the desired transmission output torque T O     —     FA , current transmission gear, the current gear ratio, and expected inertial torque losses associated with the rate of change of engine crankshaft speed and combined inertias of the engine and CISG (i.e., torque lost due to engine and CISG acceleration during vehicle launch). 
   At  76 , a command T ENG     —     DES  representing desired engine torque and a command T CISG     —     DES  representing desired CISG torque issue and are carried on communication bus  60  to an ECM  46  and to ISC  48  control interfaces. These commands adjust the engine torque and CISG torque to achieve the base crankshaft torque, such that the sum of the commanded engine torque and commanded CISG torque equals the base crankshaft torque T CRK     —     OL  from  64 . The desired CISG torque can be commanded to charge the battery is the state of charge is low, and the desired engine torque can be increased accordingly such that the sum of both commands equal the base crankshaft torque. 
   Control then returns to  66  to determine whether the powershift vehicle launch mode control  68  should be deactivated based on the current conditions. If the current clutch slip CL SLIP  is minimal, crankshaft speed ω CRK  is above the clutch output speed ω CL , and vehicle speed VS is above a threshold vehicle speed, then the vehicle launch mode control  68  is exited upon controller  66  issuing command signals  67  and  86 . 
   Upon exiting vehicle launch mode control, controller  66  activates an end of launch mode at  88 , where a command signal T CL CAP RAMP  causes a gradual, smooth increase in torque capacity of the subject input clutch  40 ,  41  until the input clutch is engaged. While controller  66  activates  88  for smoothly engaging the input clutch at the end of launch, the command signal T CL     —     CAP     —     RAMP  is sent to TCM  44  as the final input clutch torque capacity command  92 . 
   After the subject input clutch  40 , 41  is smoothly engaged with zero clutch slip at  88 , controller  66  activates a locked mode at  90  and a command T CL     —     CAP     —     LOCKED  is produced and carried on the communication bus  60  to the TCM  44  as the final input clutch torque capacity command  92  T CL     —     CAP     —     DES . After the subject input clutch  40 ,  41  is fully engaged, the command T CL     —     CAP     —     LOCKED  issued by  90  causes the subject input clutch to become fully engaged or locked at a clutch torque capacity well above the current crankshaft torque magnitude, thereby ensuring that the transmission will not slip. 
   If any of the conditions required to exit the control vehicle launch control is absent, control returns to  59 , where the subsequent steps of the control strategy are repeated. 
     FIG. 4  lists the steps of the vehicle launch control using the same reference numbers as are used in the sequence of steps described with reference to  FIG. 3 . 
   The graphs of  FIG. 5A-5F  show the variation with time of variables and parameters of the powertrain that are used or produced by executing the control algorithm. For example,  FIG. 5A  relates to wheel torques. At  100 , the vehicle operator tips into the accelerator pedal during the neutral mode when neither input clutch  40 ,  41  is engaged, thereby increasing the desired wheel torque T W DES    104 . The desired front axle torque  106  T W FA  increases as vehicle propulsion is distributed, thereby initiating a vehicle launch condition beginning at  102 . Desired wheel torque  104  T W DES  and desired front axle torque  106  T W     —     FA  differ in magnitude by the magnitude of the desired rear axle torque  108  T W     —     RA . At  110 , rear axle torque is blended out leaving only front axle torque to drive the vehicle during the vehicle launch mode. 
   In  FIG. 5B , at  102 , the open-loop clutch torque capacity  112  T CL     —     OL     —     LCH  is commanded during the vehicle launch mode based on desired transmission output torque T O     —     FA    FIG. 5B  shows the variation of the final desired torque capacity  113  of the subject input clutch T CL CAP DES  and the closed loop delta input clutch torque capacity ΔT CL CAP  located between graphs  112  and  113 . At the end of the launch mode, the clutch torque capacity is ramped at  114  to smoothly engage the transmission. Once the clutch is engaged at the end of the launch mode, the torque capacity of the subject input clutch is stepped up and held at  116  to fully lock the clutch and ensure the input clutch does not slip. 
   Vehicle speed  118 , represented in  FIG. 5C , is zero in the neutral mode, and increases throughout the launch mode. 
   The variation of actual engine crankshaft speed  120 , desired engine crankshaft speed  122 , and clutch output speed  124  are represented in  FIG. 5D . The desired slip across the subject input clutch is represented by the difference  126  between desired crankshaft speed  122  and clutch output speed  124 . The control causes actual clutch slip to approach the desired clutch slip as actual crankshaft speed  120  is controlled to the desired crankshaft speed  122 . The locations where actual crankshaft speed passes through desired crankshaft speed are shown by dashed vertical lines as shown in  FIG. 5D . 
   Engine and CISG torques, are controlled such that the sum equals the desired open-loop crankshaft torque T CRK     —     OL    128 . The difference between the engine torque  80  and the open-loop crankshaft torque T CRK     —     OL    128  represents the torque needed to charge the battery which is the CISG torque  130  since the CISG is producing generating torque to charge the battery. Essentially, the engine torque is increased to accommodate battery charging while still achieving the desired open-loop crankshaft torque. At the point where the battery is no longer charged, the CISG torque is zero and the engine torque equals the desired open-loop crankshaft torque T CRK     —     OL    128 , 
     FIG. 5F  illustrates the variation of transmission output torque  132  T TRANS     —     OUT  and ERAD torque  134  T ERAD . The increase  136  in transmission output torque is due to increased clutch torque capacity  113 , shown in  FIG. 5B . 
   The effective propulsion request for the transmission propulsion path is the desired transmission output torque during a vehicle launch condition after propulsion distribution between both the mechanical and electrical paths has been determined. This approach compensates for any vehicle propulsion assistance provided by the ERAD during a vehicle launch condition since the overall vehicle propulsion request can be met by both the mechanical propulsion path and electrical propulsion path. In addition to supporting distributed propulsion, i.e. blending torque produced by the power sources, the clutch torque capacity is used to regulate clutch slip in coordination with CISG and engine torque control during the launch event. 
     FIG. 6  illustrates details of a powershift transmission  16  including a first input clutch  40 , which selective connects the input  20  of transmission  16  alternately to the even-numbered gears  42  associated with a first layshaft  244 , and a second input clutch  41 , which selective connects the input  20  alternately to the odd-numbered gears  43  associated with a second layshaft  249 . 
   Layshaft  244  supports pinions  260 ,  262 ,  264 , which are each journalled on shaft  244 , and couplers  266 ,  268 , which are secured to shaft  244 . Pinions  260 ,  262 ,  264  are associated respectively with the second, fourth and sixth gears. Coupler  266  includes a sleeve  270 , which can be moved leftward to engage pinion  260  and driveably connect pinion  260  to shaft  244 . Coupler  268  includes a sleeve  272 , which can be moved leftward to engage pinion  262  and driveably connect pinion  262  to shaft  244  and can be moved rightward to engage pinion  264  and driveably connect pinion  264  to shaft  244 . 
   Layshaft  249  supports pinions  274 ,  276 ,  278 , which are each journalled on shaft  249 , and couplers  280 ,  282 , which are secured to shaft  249 . Pinions  274 ,  276 ,  278  are associated respectively with the first, third and fifth gears. Coupler  280  includes a sleeve  284 , which can be moved leftward to engage pinion  274  and driveably connect pinion  274  to shaft  249 . Coupler  282  includes a sleeve  286 , which can be moved leftward to engage pinion  276  and driveably connect pinion  276  to shaft  249  and can be moved rightward to engage pinion  278  and driveably connect pinion  278  to shaft  249 . 
   Transmission output  32  supports gears  288 ,  290 ,  292 , which are each secured to shaft  32 . Gear  288  meshes with pinions  260  and  274 . Gear  290  meshes with pinions  262  and  276 . Gear  292  meshes with pinions  264  and  278 . 
   Couplers  266 ,  268 ,  280  and  282  may be synchronizers, or dog clutches or a combination of these. 
   Although the invention has been described with reference to a powershift transmission, the invention is applicable to any conventional manual transmission, automatic shift manual transmission, or automatic transmission that has no torque converter located in a power path between the engine and transmission input. 
   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.