Abstract:
In a powertrain that includes wheels for driving a vehicle, a crankshaft, a machine driveably connected to the crankshaft and able to operate alternately as an electric motor and electric generator, a transmission including an input clutch driveably connected to the crankshaft and an output driveably connected to the wheels, a method for controlling idle speed including producing a desired magnitude of input clutch torque capacity, producing a desired wheel torque, using an error represented by a difference between a desired crankshaft idle speed and a current crankshaft speed to determine a desired change in torque produced by the machine, using the magnitude of input clutch torque capacity and the desired change in torque produced by the machine to determine a desired magnitude machine torque, and using the machine to produce said desired magnitude of machine torque.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to an apparatus and method for controlling crankshaft idle speed during a vehicle creep condition in a hybrid electric vehicle (HEV). 
   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 creep condition while the engine is idling in a conventional vehicle having an engine, powershift transmission, single propulsion path and single power source, the transmission clutch torque capacity is controlled by slipping the transmission input clutch as the driver releases the brake pedal. In a powershift transmission vehicle application, providing consistent, acceptable vehicle creep performance can be a difficult control problem due to the absence of a torque converter. 
   As the driver releases the brake pedal, the increase in clutch torque capacity loads the engine and disturbs the control of the engine idle speed. Therefore, engine idle speed control must be coordinated with any increase in the clutch torque capacity. 
   Unlike a conventional vehicle having a powershift transmission, a hybrid electric vehicle with a powershift transmission, multiple power sources can be used during a vehicle creep condition to provide robust, responsive engine idle speed control while accounting for the battery charging needs of the vehicle. 
   A need exists for responsive idle speed control that corrects for input clutch torque capacity disturbance, delayed engine torque response due to intake manifold filling as clutch torque capacity is increased, and potential engine stall if too much clutch torque capacity is provided while the engine torque has not increased sufficiently. The engine idle speed control must provide good coordination between transmission clutch torque capacity control and crankshaft speed control during a vehicle creep condition. 
   SUMMARY OF THE INVENTION 
   In a powertrain that includes wheels for driving a vehicle, an engine including a crankshaft, a machine driveably connected to the crankshaft and able to operate alternately as an electric motor and electric generator, a transmission including an input clutch driveably connected to the crankshaft and an output driveably connected to the wheels, and an electric storage battery having a variable state of charge and electrically connected to the machine, a method for controlling engine idle speed during a vehicle creep condition including producing a desired magnitude of input clutch torque capacity, producing a desired wheel torque, determining a desired battery charge torque, using the engine to produce the desired battery charge torque, using an error represented by a difference between a desired crankshaft idle speed and a current crankshaft speed to determine a desired change in torque produced by the machine, using the magnitude of input clutch torque capacity, magnitude of desired battery charge torque and the desired change in torque produced by the machine to determine a desired magnitude machine torque, and using the machine to produce said desired magnitude of machine torque. 
   The HEV idle speed control system provides a responsive idle speed control while accounting for battery charging needs and transmission clutch torque capacity actuation. Furthermore, by taking advantage of the crank-integrated electric machine to control the crankshaft idle speed, the control system accounts for engine manifold filling delays and transmission input clutch torque capacity actuation during vehicle creep conditions, is robust and responsive due to the short period required to produce electric machine torque, and is applicable to any HEV powertrain that includes a crankshaft-integrated electric machine and a transmission having no torque converter and either a wet or dry input clutch, i.e., a dual clutch powershift, automated manual transmission or any converterless automatic transmission. 
   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 an automotive vehicle powertrain of a hybrid electric vehicle utilizing a powershift transmission; 
       FIG. 2  is a schematic diagram showing propulsion and power flow of the vehicle powertrain of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of a crankshaft idle speed control system; 
       FIGS. 4A-4G  are graphs of various powertrain and vehicle parameters before, during and following an idle speed condition during vehicle creep in which a CISG provides torque to control engine idle speed; and 
       FIG. 5  is a schematic diagram showing details of a powershift transmission. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIGS. 1 and 2 , the powertrain  10  configuration includes a first power source such as an internal combustion engine  12 , a diesel engine or a gasoline engine; a power transmission  14  driveably for producing multiple forward and reverse gear ratios, such as a wet-clutch powershift transmission; an electric machine  16  driveably connected to the engine crankshaft and transmission input  18 , such as a crankshaft integrated starter/generator (CISG) for providing starter/generator capability; and an additional electric machine  20  driveably connected to the rear axles  22 , 23 , such as a electric rear axle drive (ERAD), for providing additional propulsion capability in either an electric drive or hybrid drive mode. The transmission output  24  is connected through a final drive unit and differential mechanism  26  to the front axles  28 ,  30 , which drive the front wheels  32 ,  33 , respectively. ERAD  20  drives the rear wheels  34 , through ERAD gearing  48 , a differential mechanism  36 , rear axles  22 ,  23  and wheels  34 ,  35 . 
   An electronic engine control module (ECM)  24  controls operation of engine  12 . An electronic transmission control module (TCM)  26  controls operation of transmission  14  and the input clutches  38 ,  39 . An integrated starter controller (ISC)  40  controls operation of CISG  16 , ERAD  20  and the system for charging an electric storage battery  42 , which is electrically coupled to the electric machines  16 ,  20 . 
     FIG. 2  shows the power and energy flow paths from the power sources  12 ,  16 ,  20  to the load at the vehicle wheels  32 - 35 . Power produced by engine  12  and power produced by CISG  16  is combined at  44  and is transmitted to the transmission input  18 . Electric power produced by both electric machines  16 , is combinable at  46  for charging the battery  42 , or is transmitted from the battery to the electric machines. Mechanical power produced by ERAD  20  is transmitted through ERAD gearing  48  to the load at the rear wheels  34 ,  35  through the rear final drive  36 . 
   In the HEV powertrain  10 , power can be transmitted to the wheels  32 - 35  during a vehicle creep condition solely in an electric drive mode by the electric machine  20  independently of the engine  12  and transmission  14 , or in a parallel drive mode by a combination of engine  12 , transmission  14  and the electric machines  20 ,  16 . Two propulsion paths, mechanical and electrical, can be used to meet a given propulsion demand request. The engine  12  and CISG  16  can provide power to the wheels by transmitting torque through the transmission  14  in the mechanical propulsion path to the front axles  28 ,  30 , and the ERAD motor  20  can provide power directly in the electrical propulsion path to the rear axles  22 ,  23 . 
   Referring now to  FIG. 3 , a crankshaft idle speed control system during vehicle creep for an HEV includes a controller  70 , which includes an electronic microprocessor, accessible to electronic memory containing stored functions, variables, and control algorithms and electronic signals produced by various sensors representing operating parameters and variables of the vehicle, engine  12 , CISG  16 , ERAD  20 , transmission  14 , input clutches  38 ,  39 , ERAD gearing  48  and final drive  26 , front and rear differentials  26 , 36 , such as CISG and ERAD speed and temperature sensors, a vehicle speed sensor, brake pressure sensor. The microprocessor executes the algorithms and produces control commands to which the engine  12 , CISG  16  and ERAD  20  respond by producing torque, and the transmission  14  responds by engage and disengaging input clutches  38 ,  39  and alternately engaging a forward gear and reverse gear. 
   At  94 , the desired torque capacity T CL     —     CAP     —     CRP  of the input clutch  38 ,  39  that is associated with the current gear of transmission  14  during vehicle creep is determined by controller  70 . At  96 , a desired clutch torque capacity T CL     —     CAP     —     DES  command is sent by the controller  70  to TCM  26 . The torque capacity of the subject clutch is produced in response to the desired clutch torque command T CL     —     CAP     —     DES , and a signal representing clutch torque capacity during vehicle creep T CL     —     CAP     —     CRP  is transmitted to a summing junction  98 . The subject input clutch is always slipping when vehicle creep is being controlled by controller  70 . 
   If the SOC of battery  42  is less than a reference SOC, at  100 , controller  70  determines a desired battery charge torque T QBAT     —     CHG  and, at  102 , commands ECM  24  to produce the desired engine torque T ENG     —     DES , substantially equal to the engine torque required to charge the battery  42 . If the SOC is greater than the reference SOC, engine torque is controlled at  102  to zero brake torque since CISG  16  will control idle speed. The signal representing the battery charge torque T QBAT     —     CHG  is a first feed-forward signal transmitted to summing junction  98 . 
   A crankshaft idle speed closed-loop controller  104  is used to determine a desired change in CISG torque ΔT CISG     —     CL  based on a crankshaft speed feedback error  108  represented by the difference between the desired idle speed  110 , determined at  106 , and the actual crankshaft speed  112 , which is feedback to summing junction  107  from ECM  24 . Preferably a PID closed-loop controller  105  or a comparable controller determines the desired change in CISG torque ΔT CISG     —     CL  that is also transmitted to summing junction  98 . 
   At summing junction  98 , the desired change in torque produced by CISG  16  ΔT CISG     —     CL , the commanded or estimated creep clutch torque capacity T CL     —     CAP     —     CRP , and the battery charge torque T QBAT     —     CHG  are added algebraically. The desired change in torque ΔT CISG     —     CL  represents a closed-loop CISG torque required to maintain idle speed control, and the sum of battery charge torque T QBAT     —     CHG  and creep clutch torque capacity T CL     —     CAP     —     CRP  represent an open-loop feed-forward CISG torque, with which to maintain idle speed control. The battery charge torque T QBAT     —     CHG  is a negative value and reduces the feed-forward CISG torque since an increase in battery charge torque would cause an increase in idle speed. The creep clutch torque capacity T CL     —     CAP     —     CRP  is a positive feed-forward CISG torque since an increase in clutch torque would cause a decrease in idle speed. At  114 , controller  70  issues a command to ISC  40  to produce the overall desired CISG torque T CISG     —     DES  according to the output of summing junction  98  which includes both the closed-loop and feed-forward CISG torque commands. 
     FIGS. 4A-4G  are graphs of various powertrain and vehicle parameters before, during and following a vehicle creep condition in which torque blending is not used.  FIG. 4A  shows that the gear selector  88  may be in the N or neutral position during period A, thereafter it is moved to the D or drive position at the beginning of period B before vehicle creep control begins. The brake pedal  62  is depressed during periods A and B, and is released at the beginning of period C, where vehicle creep control begins at  120 , and remains released until the beginning of period D. The accelerator pedal displacement  121  is zero throughout periods A-D and is depressed gradually during period E, which terminates the vehicle creep control. 
   In  FIG. 4B , the overall desired wheel torque T W     —     DES    123  and desired front axle wheel torque T W     —     FA  increase at the beginning of creep control at  120  as the brake pedal is released during period C and remain constant during period D until creep control terminates at  124 . The desired front axle wheel torque T W     —     FA  is equal to the overall desired wheel torque T W     —     DES  since no rear axle wheel torque T W     —     RA  is provided by the ERAD  20 . During a point during period D, the vehicle reaches a steady-state creep speed once the wheel torque is equal to the road load  122 . 
   In  FIG. 4C , the input clutch torque capacity is zero until it begins to ramp-up at the beginning of creep control  120  to the desired clutch torque capacity  126  since there is an increase in the desired front axle wheel torque T W     —     FA . During period D, the desired clutch torque capacity  126  remains constant since the desired front axle wheel torque T W     —     FA  is also constant until creep control terminates at  124 . 
   In  FIG. 4D , vehicle speed is zero until it ramps-up at the beginning of creep control  120  as input clutch torque transmits the current crankshaft torque to the wheels. Crankshaft torque includes engine torque, or CISG torque or both of these. Vehicle speed reaches a controlled steady vehicle creep speed  128  once the wheel torque equals the road load  122 , which remains constant until creep control terminates at  124 . 
   In  FIG. 4E , the speed  130  at the gear box, i.e., clutch output, side of the input clutch  38 ,  39  is zero until it ramps-up at the beginning of creep control  120  as the input clutch gains torque capacity. Clutch speed  130  is smaller than the crankshaft idle speed and remains constant until creep control terminates at  124 . The speed  132  of crankshaft  18  is controlled to a constant desired crankshaft idle speed  134  until creep control terminates at  124 . 
   In  FIG. 4F , engine brake torque  136  is positive and constant while battery  42  is being charged to the reference SOC required to supply electric energy to the CISG  16 . Engine brake torque  136  decreases following the battery charge and remains constant until creep control terminates at  124  unless the battery SOC falls below the reference SOC. CISG torque  138  is negative during the battery charging period A &amp; B, ramps-up to a positive torque when vehicle creep control begins at  120  due to the increase in clutch torque capacity. During period D, it remains constant and positive until creep control terminates at  124 . 
   In  FIG. 4G , torque  140  produced by ERAD  20  is zero since only front axle wheel torque T W     —     FA , is desired. The transmission output torque  142  is zero until it ramps-up at the beginning of creep control  120  as the input clutch  38 ,  39  gains torque capacity, and remains constant during period D until creep control terminates at  124 . 
   The HEV idle speed control system provides a responsive idle speed control during vehicle creep conditions while accounting for battery charging needs and transmission clutch torque capacity disturbances. By taking advantage of the responsiveness of a crank-integrated electric machine to control the crankshaft idle speed and by directly accounting for clutch torque loading during vehicle creep, engine manifold filling delays are avoided and robust idle speed control is provided. 
     FIG. 5  illustrates details of a powershift transmission  14  that includes the first input clutch  38 , which selective connects the input  18  of the transmission alternately to the even-numbered gears  42  associated with a first layshaft  244 , and a second input clutch  241 , which selective connects the input  20  alternately to the odd-numbered gears  243  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  24  supports gears  288 ,  290 ,  292 , which are each secured to output shaft  24 . 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. 
   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.