Abstract:
A powertrain assembly for an automotive vehicle having an internal combustion engine, transmission, electric induction motor, a clutch for connecting and releasing the engine and transmission, and a controller for controlling the state of engagement and disengagement of the clutch and the torque produced by the motor during the launch. The engine increases torque produced by the induction motor during a vehicle launch condition, permitting the engine to be turned off when the vehicle is at rest. The torque multiplication that would normally be available from a hydrokinetic torque converter is replaced with auxiliary launch torque supplied by the induction motor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to hybrid vehicle powertrains in which an electric motor and an internal combustion engine provide power to a transmission having no torque converter. 
   2. Description of the Prior Art 
   In a automotive vehicle powertrain that includes a transmission, a hydrokinetic torque converter is conventionally located between the engine shaft and transmission input, the torque converter increasing engine torque during launch to provides a powerful launch feel to the driver during acceleration from a stop. The impeller of the torque converter receives engine torque and the turbine of the torque converter transfers torque to the torque input element of multiple-ratio gearing of the transmission. 
   The presence of the torque converter in the torque flow path introduces hydrokinetic power losses, particularly during startup of the vehicle, as the torque converter fluid in the converter torus circuit is accelerated and decelerated. These losses are manifested in heat loss to the hydrokinetic fluid, which requires a heat exchanger to maintain an acceptable hydrokinetic fluid temperature. 
   Attempts have been made to eliminate the power losses inherent in a powertrain having an internal combustion engine and a torque converter automatic transmission by replacing the engine with an electric motor and by replacing the torque converter by a clutch. The power output of the motor is connected to the clutch, which is connected to the transmission input. However, replacing the torque converter in this way has met with limited success because of the lack of acceleration at launch, i.e. when accelerating the vehicle from a stop. 
   Attempts to combine the advantages of an internal combustion engine with an electric motor drive have been made in hybrid vehicle arrangements, but the engine is required in such known designs to operate throughout a large speed range including startup speeds and to operate at idle speed while the vehicle is at rest. 
   In a hybrid gas-electric vehicle with a pre-transmission motor, it is highly desirable to eliminate the torque converter to minimize transmission losses. An electric torque convertless transmission has been designed to address drivability concerns while minimizing losses. That transmission uses a base power-shifting transmission with standard gear ratios, removes the torque converter, and places a high voltage motor on the transmission input. Without the torque converter, the launch for the transmission is accomplished by actively controlling the existing planetary clutches and the electric motor. A control system for coordinating the clutch and motor to provide acceptable launch feel is required. 
   SUMMARY OF THE INVENTION 
   The transmission and control system of this invention are particularly adapted for use with a hybrid electric vehicle including an internal combustion engine and a multiple-ratio transmission wherein provision is made for significantly improving fuel economy and reducing exhaust gas emissions. 
   It is an objective to provide an improved hybrid electric vehicle transmission and control system that permits the internal combustion engine to be deactivated when the vehicle is at rest. The improved driveline includes an induction motor that is useful to provide added launch performance, and which permits the multiple-ratio transmission to operate throughout a desired ratio range without the need for using a hydrokinetic torque converter between the engine and the transmission input. 
   The absence of a hydrokinetic torque converter from the hybrid electric vehicle driveline of the invention does not result in undesirable torsional vibration since the induction motor situated between the engine and the transmission may function as a vibration damping structure. 
   A input clutch also disconnects the engine from the torque path when the engine is required to operate at low throttle and during operation of the vehicle at low speed when only the induction motor is used to power the vehicle. It is at this time that the internal combustion engine is most inefficient. Thus, by disconnecting the engine, the engine may be reserved for operation in the speed range at which it is most efficient as the induction motor supplies the driving torque. 
   The clutch may be used also to rapidly restart the engine when the vehicle is moving by using the vehicle momentum since the engine is connected mechanically through the clutch to the transmission input. 
   The torque output of the induction motor can be optimized by maintaining the engine speed at a lower level than that which would be the case with a conventional automatic transmission having a torque converter. The launch performance is improved compared to a driveline with a conventional transmission because the output torque increases more rapidly during launch of the vehicle when the electric motor is used for launch purposes. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
   In realizing these advantages, a method according to this invention for controlling during launch a vehicle powertrain is applicable to an assembly that includes a transmission having an input speed, an internal combustion engine having an output and a throttle position, and an input clutch having a variable torque capacity for driveably connecting the input and output. The method includes the steps of determining a current input speed, determining a value representing requested powertrain output, producing an indication that a vehicle launch condition has been initiated, determining a target input speed based on the value representing requested powertrain output, determining a variable pressure for actuating the input clutch during the launch condition based on a difference between the current input speed and the target input speed, and using the variable pressure to control the torque capacity of the input clutch during the launch condition. 
   Torque produced by the motor during launch further includes the steps of determining the current gear in which the transmission is operating, determining a target motor output torque based on the current gear and throttle position, and using the target motor output torque to control the torque produced by the motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a vehicle driveline including an engine, starter/generator or motor, input clutch, and automatic transmission; 
       FIG. 2  is a schematic diagram showing transmission gearing, gear control elements, input clutch, electric motor, a controller; 
       FIG. 3  is a chart showing the engaged and disengaged state of the clutches and brakes of the transmission of  FIG. 2 , each state corresponding to a gear ratio produced by the transmission; 
       FIG. 4  is a partial cross section of a vehicle driveline assembly showing the arrangement of an engine crankshaft, starter/generator, input clutch, torsion damper and transmission input; 
       FIG. 5  is a graph illustrating the variation of output torque vs. motor speed for a motor suitable for use with the control of this invention; 
       FIG. 6  is a graph showing the variation of boost, commanded clutch pressure, filtered input speed, and target engine speed during various progressive phases or modes of a vehicle launch controlled in accordance with this invention; 
       FIG. 7  is a schematic diagram of the controller for a converterless hybrid electric vehicle during launch and steps for controlling input clutch pressure and motor torque; 
       FIG. 8  is a graph showing the variation of boost, vehicle speed and engine speed before during and after a 1-2 and 2-3 upshifts during a vehicle launch controlled in accordance with this invention; 
       FIG. 9  shows the variation of various vehicle and power train parameters during a vehicle launch at wide open throttle; 
       FIG. 10  shows the operation of the multiplier to the motor torque command controller during a wide open throttle 1-2 gear shift; and 
       FIG. 11  illustrates the ability of the control strategy to effectively accommodate changes in driver demand. In the example, a change-of-mind maneuver by the operator is shown. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, there is illustrated in  FIG. 1  a gasoline-electric hybrid vehicle driveline that includes an internal combustion engine  10 , a multiple-ratio vehicle transmission  12 , an induction motor  14  located in a drive path between the engine and transmission  12 , and a friction clutch  16  located between the engine and the motor for driveably connecting and disconnect the engine and transmission. The rotor of the induction motor is connected directly to the torque input element of the multiple-ratio synchronous transmission. It is connected also to the engine crankshaft  10  through the friction clutch  16 . 
     FIG. 2  is a schematic representation of the gearing and control elements for the transmission of  FIG. 1 . The input shaft of the transmission is connected to the torque input side of the clutch  16 . The electric motor is arranged so that it transmits torque in parallel relationship with respect to the engine torque input. The direct clutch (DC) shown at  20  connects transmission input shaft  22  to the ring gear  24  of a first simple planetary gear unit. Sun gear  26  of the simple planetary gear unit is connected through a forward clutch (FC) shown at  28  to the shaft  22 . Ring gear  24  is connected to sun gear  30  of a second planetary gear unit. The ring gear  32  of the second planetary gear unit is connected to the planetary carrier  34  of the first planetary gear unit. The planetary carrier  36  for the second planetary gear unit is braked selectively by low-and-reverse brake (L/R)  38 . Transmission input shaft  22  is connected through reverse clutch (RC)  40  to the sun gear  30  and is engaged during the first ratio drive operation. The brake  38 , during reverse drive operation, anchors planetary carrier  36 . 
     FIG. 3  is a chart, which illustrates the state of the clutches and brakes of the transmission  12  for each of the gear ratios. First gear is achieved by engaging the forward clutch and the low-and-reverse brake. Second forward drive ratio is achieved by engaging the forward clutch and the 2/4 band brake. Direct drive or third drive ratio is achieved by simultaneously engaging the forward clutch and the direct clutch, and fourth ratio or overdrive ratio is achieved by engaging the direct clutch and the 2/4 band brake. Reverse clutch  40  and low-and-reverse brake  38  are engaged during reverse drive operation. 
   The ring gear  32  acts as a torque output element for the gearing. It defines a sprocket wheel  42 , which drives a sprocket wheel  44  by means of a drive chain  46  engaged with both sprocket wheels. Sprocket wheel  44  drives the sun gear  48  of the final drive gear unit. The ring gear  50  of the final drive gear unit is anchored, and the planetary carrier  52  brings torque output to differential gearing  54 , which transfers driving torque to each of two axle half shafts  56  and  58 . 
     FIG. 4  is a detailed cross sectional view of the electric induction motor  14 , a starter/alternator, engine crankshaft  18 , and transmission input shaft  22 . The induction motor  14  includes a rotor  60  supported on a rotor hub  61 , which is supported by needle bearing  62 ,  64  on a sleeve shaft  66 , surrounding the transmission input shaft  22 . The rotor  60  and rotor hub  61  are continually driveably connected to input shaft  22  by a spline connection  68 . Surrounding rotor  60  is a motor stator  70 , which includes stator windings  72 , the stator and winding being fixed to a casing against rotation. 
   A second torque delivery path connects the engine shaft  18  and input shaft  22 , and is arranged in parallel with the path from the motor  14  to input shaft  22 . The second path includes a torsion damper  74 , connected by bolts  75 ,  76  to the engine shaft. The damper  74  is connected by a spline  78  to a disc  80 , on which the input or start-up clutch  16  is supported. When clutch  16  is disengaged, the engine  10  is disconnected from the input shaft  22 , and the induction motor is the only source of positive torque for the transmission  12 . The clutch  16  disengaged during vehicle launch at low-throttle operating conditions when the induction motor is capable of providing sufficient torque to propel the vehicle. The engine, which may be inoperative during low-speed operation, can be rapidly restarted merely by engaging the clutch  16  and actuating the engine ignition system. 
   The input clutch  16  includes a set of friction discs  82 , splined to the radially outer surface of disc  80 . Interleaved with discs  82  are spacer plates  84 , splined to the radially inner surface of rotor hub  61 , and a blocker ring, secured to hub  61  by a snap ring  84  to limit axial displacement of the blocker ring and of the friction discs and spacer plates. Clutch  16  is controlled by a servo  88 , which includes a hydraulic cylinder  90 , a sealed piston  92  located in the cylinder, a Belleville return spring  94 , and balance dam  96 . The cylinder is pressurized and vented through passage  98 . When the servo is pressurized, piston  92  moves rightward, forcing the discs  82  and plates  84  into mutual frictional contact, thereby driveably connecting the rotor hub  61  and the engine shaft  18 . Clutch  16  is disengaged by venting cylinder  90 , which allows spring  94  to push piston  92  away from the discs  82  and plates  84 , thereby disconnecting the engine shaft  16  and rotor hub  61 . 
   The control strategy of this invention can be applied to a powertrain in which clutch  16  is deleted, and the forward clutch  28  and low-and-reverse brake  38  are controlled instead according to this invention during vehicle launch conditions. In this case, vehicle launch is controlled through operation of the forward clutch  28  during vehicle forward launch and of the low-and-reverse brake  38  during vehicle reverse launch, in the same manner as will be described with reference to clutch  16 . As in the case of the configuration of  FIG. 1 , in a system in which the forward clutch  28  and low-and-reverse brake  38  operate under control of the present invention to produce vehicle launch, the torque converter of the transmission is eliminated, and the induction motor improves launch performance in the absence of the torque converter. 
   A controller  100  receives signals generated by sensors, processes, and uses the input signals to determine the magnitude of pressure to be applied to clutch  16  in accordance with a clutch control strategy. Based upon this determination, the controller generates a command signal that causes the torque capacity of the clutch  16  to change, whereby the clutch slips, fully engages or fully disengages. The magnitude of clutch pressure establishes the magnitude of torque transmitted by the clutch  16 . The controller also determines and command the magnitude of torque to be produced by the motor  12  by controlling the magnitude of current to by applied to the field windings of the motor. 
   In the preferred embodiment, the controller  100  is a powertrain controller that includes one or more digital microprocessors or digital computers, which cooperatively perform calculations, and execute subroutines and control algorithms. Controller  100  preferably generates a pulse width modulated (PWM) command or output signal, which controls the amount of slippage between the friction discs and spacer plates of clutch  16 , thereby controlling the relative magnitudes of torque and power transmitted through the clutch from the engine shaft  18  to the transmission input shaft  22 . The duty cycle of the PWM signal is the percentage of the cycle time for which the signal is activated or enabled. The output signal of the controller is communicated to a solenoid  122 , which operates a valve  124  that opens and closes a source of fluid pressure  126  to the servo  88  of clutch  16 . The clutch duty cycle is interchangeably referred to as a pressure command, or clutch command. 
   Controller  100  is preferably a microprocessor-based controller, which provides integrated control of engine  10  and transmission  12 . The present invention may be implemented with a separate engine or transmission controller depending upon the particular application. Controller  100  includes a microprocessor MPU in communication with input ports, output ports, and computer readable media via a data/control bus  102 . Computer readable media may include various types of volatile and nonvolatile memory such as random access memory (RAM), read-only memory (ROM), and keep-alive memory (KAM). These functional descriptions of the various types of volatile and nonvolatile storage may be implemented by any of a number of known physical devices including, but not limited to EPROMs, EEPROMs, PROMS, flash memory, and the like. Computer readable media include stored data representing instructions or algorithms executable by microprocessor MPU to implement the method for controlling input hydraulic pressure and motor torque according to the present invention. 
   Vehicle launch is commanded when the driver steps on the accelerator pedal and the vehicle is at rest or at a low speed. The pressure command to the input clutch  16  is the output of a closed loop controller  100 , which controls the transmission input speed through an incremental PID controller. The set point for the controller  100  is a function of actual throttle position (TP — raw) and vehicle speed (VS). The launch controller also controls the magnitude of torque produced by the electric motor  14 , a starter/alternator. The torque produced by motor  14  provides extra boost to the engine torque, and aids in vehicle launch feel and acceleration so that the launch is comparable to today&#39;s torque converter-equipped vehicles. 
   A torque converter&#39;s boost is limited to low speed, up to about 10 mph. However, the motor  14  can produce torque at relatively high motor speed and vehicle speed, as shown in  FIG. 6 . This torque producing ability improves launch performance time and feel by permitting boost to continue after the input clutch  16  is fully engaged and during operation in second gear. 
   There are two primary functions of the launch controller. One is to control the input clutch  16  such that it provides smooth torque delivery path to the wheels, while maintaining input speed. A second function of the controller is to control the output torque produced by the motor  14 . The control strategy of this invention applies to a mechanical throttle, i.e., an engine control system in which a fixed relationship exists between the extent to which the accelerator pedal is depressed and the degree to which the engine throttle is open. The control strategy of this invention applies also to an electronic throttle control system, in which a sensor produces as input to a controller a signal representing the position of the accelerator pedal. The controller processes the signal and its variations with time to interpret the vehicle operator&#39;s desire for changes in engine and powertrain operation. 
   The time based implementation of the control strategy that produces the pressure command (prs — oncoming) to the solenoid  122  that controls clutch  16  is illustrated in  FIG. 6 . The vehicle is at rest in the forward drive range or reverse drive range (sft — mode=3) when a transmission range selector (PRNDL) is in the D or R position. Vehicle launch control is initialized when the vehicle operator depresses the accelerator pedal, i.e. increases TP, and the control enters sft — mode=4. When the launch controller is first called, it enters a short open loop phase. This is needed when coming from an idle condition  130  because the closed loop controller can actually decrease clutch pressure if the engine stumbles during the throttle transition or another event causes the engine speed to be less than the desired engine speed, which may cause an undesirable bump in the output torque. During this time, the pressure command is increased gradually along ramp  132  in accordance with an open loop parameter, slo — rmp, which is a function of filtered throttle position (TP — flt), motor state (SA — state), and the state of the air conditioner (AC — state). The function slo — rmp is stored in electronic memory accessible to the controller  100 . 
   The desired engine speed, Ne — des — raw,  134  is set based on a function of throttle position. When the actual engine speed  136  exceeds the desired engine speed  134 , the closed loop pressure controller is enabled at  138 . If the engine speed (N 1 ) is coming up normally, Ne — des is equal Ne — des — raw at the entry into closed loop. In case an unusual event, such as a neutral slam, causes the engine speed (N 2 ) to be above Ne — des — raw, then Ne — des is filtered down to Ne — des — raw. 
   When input clutch  16  is nearly fully engaged or locked-up, sft — mode 5 is entered at  146 , which causes an open loop parabolic increase  148  of commanded clutch pressure used to complete an upshift from first gear to second gear. After the clutch pressure is increased to its maximum magnitude, the pressure control is complete at  150 , the sft — mode is set to 9 and the shift is completed. 
   Boost time  146  is a parameter that is used to control the length of time that the motor  14  adds torque. During a normal launch or drive-away, boost time is set to be sufficient to allow full boost during operation in first gear and second gear. In case the vehicle does not drive away as expected, the boost timer will expire. The motor stops adding boost to minimize thermal durability issues and to allow charging of the battery. The boost timer is also used to improve drivability. It is not desirable to boost after a downshift from third gear to second gear. This condition is avoided by setting the boost time. 
   The launch controller is illustrated schematically in  FIG. 7 . Two controllers are used: a closed loop pressure controller and an open loop motor torque controller. For the closed loop pressure controller, the input parameters are throttle position (TP — raw)  160 , vehicle speed (VS)  162  and transmission input speed (Ni — flt)  164 . The output  166  is the pressure command (prs — oncoming) to the input clutch  16 . 
   A target input speed is determined from a lookup table  168 , which is a function of current filtered throttle position TP — flt, the output from a first order filter  170 , whose input is TP — raw. The target input speed is target is added at a summing junction  172  to a factor of VS multiplied at  174  by the gain Knov to provide a target input speed that allows the engine speed to increase as the vehicle accelerates to provide a more natural experience for the driver. 
   Target input speed is then filtered through a low-pass first order filter  176  to determine Ne — desired. For drivability, the input speed should always be greater than the target input speed because the controller attempts to keep the input speed down by controlling the load on the engine/motor. A switch  178  sets the feedback to the filter  176  and an input to the summing junction  180  to Ni — flt instead of Ne — desired whenever the actual speed Ni — flt is lower than the target input speed Ne — desired. 
   After switch  178 , Ni — flt is subtracted from Ne — desired to obtain the error at summing junction  180 . This error is then multiplied at  182  by the gain Kc. Kc is a proportional function of TP — flt because it is necessary to have higher gains at higher speeds. The output of this gain  182  is multiplied by an incremental Proportional+Integral+Derivative controller  184 . The proportional term is dominant since steady state error is not critical to driveline smoothness, but maintaining very tight control on speed can cause transients in the pressure of clutch  16  pressure, which can be felt by the driver and passengers. Fast increases in pressure can cause drivability problems, so the output of the PID controller is rate limited at  186  in the up direction to limit the rate of increase in the pressure of clutch  16 . The controller issues a pressure command  166  to the solenoid  122 , and the state of engagement or torque transmitting capacity of clutch  16  changes in response to that command. 
   In the open loop control for the motor torque command, calibration tables  190  are used to determine the desired torque magnitude produced by motor or starter/alternator  14 . These tables are a function of throttle position, and there is a table for each of the forward gears. The controller determines the current gear from signals produced by sensors whose output represents the speed of the input shaft  22  and the transmission output  56 ,  58 . 
   When the throttle position is increasing, switch  192  sets TP — raw directly as an index to tables  190  to provide a fast response to the demand. When TP is decreasing, switch  192  sets TP — flt as the index to tables  190 . To provide a natural acceleration feel, different calibration tables are used in each gear. The commanded magnitude of motor torque, SA — torque — command,  194  is highest in first gear, and lower in second gear. It is calibrated so that boost is not increased in third gear and fourth gear, although it is possible to add boost in those gears also. 
   The output of the tables  190  is multiplied at  196  by TRQ — SA — Mult  198  to produce the motor commanded torque, SA — torque — command. The multiplier TRQ — SA — Mult, graphically shown in  FIG. 8 , is used to turn the motor off prior to a gear ratio change. TRQ — SA — MULT is equal to 1.0 until vehicle speed reaches VS — END1, which occurs in sufficient time before the 1-2 gear shift at launch to assure that the commanded motor torque is zero when the gear shift begins. The length of the period during which TRQ — SA — MULT ramps down from 1.0 to zero is in the range 0.5–1.0 seconds. After the 1-2 upshift is completed, TRQ — SA — MULT is ramped back to 1.0. The same technique is used before the 2-3 upshift from second gear to third gear. 
   Preferably the motor  14  is a multi-pole synchronous induction motor. The rotor is of the laminated squirrel cage type. The output torque is controlled by the magnitude of current applied to the field windings. The commanded output torque signal, SA — torque — command, produced by the controller changes the magnitude of current applied to the field windings in response to the command to produce the commanded output torque. 
   A vehicle launch with the accelerator pedal fully depressed causing the throttle position  200  to become wide open throttle. The launch begins at time equal to 4.5 seconds. The open loop control briefly increases clutch  16  pressure on a ramp  202  until the closed loop control begins at time=4.65 sec. Initially, the rate of clutch pressure increase is clipped by the rate limiter  186  until just over time=5 sec. Some engine flare is allowed during this time. This flare keeps the engine speed  204  above the desired speed  206  in order to provide extra inertia torque when the engine speed is pulled down. This feature of the control produces a wheel torque  208  having a shape similar to that produced by a torque converter. The PID controller is tuned to keep the engine speed smooth by allowing the offset error rather than forcing a sudden change that holds tight speed control. As vehicle speed  210  increases, the target speed  206  is increased proportionally. Boost torque  212  from the motor  14  is commanded to 70 ft-lbs. As the speed of the motor increases, the motor becomes power limited so that at WOT the commanded torque  214  is not achieved. Ideally, at WOT the torque produced by the motor is at its maximum torque capacity. At lower throttles, the torque command is lower and the motor is able to provide it. 
     FIG. 10  shows the operation of the multiplier to the motor torque command controller (TRQ — SA — Mult)  198  during a wide open throttle 1-2 gear shift. At time=8 sec, the vehicle speed reaches VS — CMD1 and TRQ — SA — Mult ramps down from 1.0 to during one second before the start of the 1-2 shift. With the multiplier at zero, the commanded motor torque is also zero. After the 1-2 shift is completed, the multiplier TRQ — SA — Mult is ramped up again over about one second to the value 1.0. This enables the torque boost produced by the motor  14  to continue in second gear. The one second ramps allow for a smooth transition in launch feel. 
     FIG. 11  illustrates the ability of the control strategy to effectively accommodate changes in driver demand. In the example, a change-of-mind maneuver by the operator is shown. The launch begins with the accelerator pedal at about 20 percent of its maximum range. The accelerator pedal is held at that position for about one second, then the pedal is fully depressed to the wide open throttle position. The control system smoothly and quickly controls the clutch pressure and SA boost to respond to this change to provide maximum vehicle acceleration. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.