Abstract:
The present invention provides a mechanism and method of launching a vehicle using a conventional automatic shifting power transmission. By slipping a first gear reaction clutch during a launch, an efficient hydrodynamic fluid drive with a low stall speed may be used with negligible impact on launch performance. The reduced stall speed characteristic of the hydrodynamic drive may provide improved efficiency in other vehicle operating regions.

Description:
TECHNICAL FIELD 
     The present invention relates to a mechanism and method of controlling an automatic shifting power transmission. 
     BACKGROUND OF THE INVENTION 
     In automatic shifting power transmissions, gear ratio changes are effected by selectively connecting members of planetary gear sets. This is accomplished by selectively engaging torque-transmitting devices such as brakes or clutches. For each gear ratio change, there is a corresponding sequence of off-going clutch disengagement and on-coming clutch engagement. By providing a regulated fluid pressure gain to the clutches, the smooth engagement and disengagement of the clutches may be accomplished. 
     Additionally, a majority of automatic shifting power transmissions employ a hydrodynamic fluid drive, such as a torque converter or a fluid coupling, between the power source (engine) and a multi-speed gear configuration, such as the planetary gear arrangement discussed above. This hydrodynamic fluid drive will allow the vehicle to come to rest without stalling the engine and will provide a measure of isolation preventing the torsional vibrations, caused by the firing events of the engine, from being transmitted though the powertrain. 
     A torque converter is a specialized form of fluid coupling that provides a torque multiplier between the engine and gearing to improve the launch performance of the vehicle. The torque ratio of the torque converter is generally in the range of 1.6 to 3.3 depending on the particular application. 
     As is well known, the hydrodynamic fluid drive is a slipping drive that has a high efficiency loss at vehicle launch. This loss decreases, but is still present, as the hydrodynamic fluid drive approaches a 1 to 1 speed ratio at high speed and low torque. A lock-up clutch is typically added to most automatic shifting power transmissions to effectively remove the hydrodynamic fluid drive from the power path and thereby improve the overall efficiency of the transmission. 
     The stall speed of the hydrodynamic fluid drive is an important consideration for proper vehicle launch. The stall speed is the speed at which the hydrodynamic fluid drive will hold the engine speed and not allow further gain. The stall speed is usually chosen based on engine torque characteristic, vehicle weight, vehicle duty cycle, etc. A properly selected stall speed will allow the engine to spin to the peak torque range to affect a strong vehicle launch. The stall speed of the converter cannot be varied or tuned while in use, therefore, there may be certain trade-offs such as part throttle efficiency when selecting the stall speed. 
     Other considerations have been given to improving the overall efficiency of the transmission. For example, the use of a starting clutch in lieu of a torque converter has been suggested and in some instances utilized. The advent of electronic controls improves the operation of a starting clutch as a vehicle launch device. The clutch is, however, a rotating device with all of the complexities associated with such a device. The control needs considerable accuracy to insure consistent fill times and to compensate for variable fluid leaks at the rotating shaft seals. This requires accurate hydraulic flow volumes and pressure control over a wide range of operating requirements. The starting clutch fulfills the requirements of the hydrodynamic fluid drive. These include, for example, shift and launch quality, driveline isolation, mass, peak acceleration, and durability. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mechanism and method of launching a vehicle using a conventional automatic shifting power transmission. By slipping a first gear reaction clutch during a launch maneuver, an efficient hydrodynamic fluid drive with a low stall speed may be used with negligible impact on launch performance. The high efficiency characteristic of the hydrodynamic fluid drive is effective to provide improved efficiency in other vehicle operating regions. 
     The invention also provides a mechanism that incorporates the use of an efficient hydrodynamic fluid drive device in series with an integrated friction launch within a transmission by using a first gear reaction clutch with enhanced slip capability to allow system tuning. The hydrodynamic fluid drive device provides a “tight” or “stiff” characteristic, which is well suited to the engine&#39;s part throttle requirements. At higher throttle conditions, slipping the reaction clutch in first gear creates a “looser” characteristic. In the drive idle operating condition, the first gear reaction clutch is disengaged to minimize fuel consumption at idle. In the second forward range and higher, the hydrodynamic fluid drive device and lock-up clutch with a damper provide the necessary isolation and slip. 
     Accordingly, the present invention provides a method of controlling an automatic shifting power transmission having a hydrodynamic fluid drive device and at least one reaction clutch disposed in series with the hydrodynamic fluid drive device. The method includes slipping the at least one reaction clutch corresponding to a first gear engagement to effect a first gear launch maneuver when engine load is at or above a predetermined value, and fully engaging the at least one reaction clutch corresponding to the first gear engagement when engine load is below the predetermined value. 
     The method may further include disengaging the first of the at least one reaction clutch corresponding to the first gear engagement when the automatic shifting power transmission is in either a drive or a neutral gear state and the engine is idling. The hydrodynamic fluid drive device may be a fluid coupling or a torque converter and may include a lock-up clutch. The lock-up clutch of the present invention may be an electronically controlled capacity clutch. Additionally, the method may include fully engaging another of the at least one reaction clutch corresponding to the appropriate gear engagement when the automatic shifting power transmission is in second gear or higher, and engaging a lock-up clutch disposed in parallel with respect to the hydrodynamic fluid drive device. The predetermined value may be approximately 50 KPa of manifold air pressure. 
     The present invention also provides an automatic shifting power transmission having a hydrodynamic fluid drive device and a reaction clutch in series relationship with the hydrodynamic fluid drive device, the reaction clutch being operable to slip during a first gear launch maneuver. 
     The hydrodynamic fluid drive device may be a fluid coupling or a torque converter and may include a lock-up clutch. The lock-up clutch of the present invention may be an electronically controlled capacity clutch. 
     The present invention also provides a method of controlling an automatic shifting power transmission including providing an efficient hydrodynamic fluid drive device and a first gear reaction clutch operable to slip during a first gear launch. The method of the present invention also includes slipping the first gear reaction clutch to effect the first gear launch maneuver when engine load is at or above a predetermined value and fully engaging first gear reaction clutch corresponding to the first gear engagement when engine load is below the predetermined value. 
     The method may further include disengaging the first gear reaction clutch when the automatic shifting power transmission is in either a drive or a neutral gear state and the engine is idling. The hydrodynamic fluid drive device may be a fluid coupling or a torque converter. The predetermined value may be approximately 50 KPa of manifold air pressure. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a vehicular powertrain of this invention illustrating a neutral/drive gear state and engine idle mode of operation; 
         FIG. 2  is a schematic representation of the vehicular powertrain illustrating a first gear launch mode of operation with a slipping first gear reaction clutch; 
         FIG. 3  is a schematic representation of the vehicular powertrain illustrating a first gear part throttle mode of operation with a fully engaged first gear reaction clutch; and 
         FIG. 4  is a schematic representation of the vehicular powertrain illustrating the second gear or higher mode of operation with a fully engaged first gear reaction clutch and an engaged lock-up clutch. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like characters represent the same or corresponding parts throughout the several views, there is seen in  FIGS. 1 through 4  a powertrain  10 . The powertrain  10  has a power source, such as a conventional internal combustion engine  12 , a multi-speed automatic shifting power transmission  14 , and a conventional final drive mechanism  16 . 
     The engine  12  has an output shaft  18  with a pump section  20  of a hydrodynamic fluid drive device  22  rigidly mounted thereto. A turbine section  24  of the hydrodynamic fluid drive device  22  is mounted with respect to an input shaft  26  of the transmission  14 . The hydrodynamic fluid drive device  22  may be either a fluid coupling or a torque converter, the operation of which are well known to those skilled in the art. Additionally, the present invention may include a lock-up clutch  28  in parallel with the hydrodynamic fluid drive device  22 . The lock-up clutch  28  is operable to lock the input shaft  26  of the transmission  14  to the output shaft  18  of the engine  12  thereby allowing them to rotate in unison. By locking the input shaft  26  to the output shaft  18 , the efficiency of the powertrain  10  will increase by reducing the slip losses of the hydrodynamic fluid drive device  22 . In the preferred embodiment, the lock-up clutch  28  will be an electronically controlled capacity clutch. The electronically controlled capacity clutch will allow a slight amount of slip to occur between the input shaft  26  and the output shaft  18 . This slip will decouple the input shaft  26  and the output shaft  18  and help to attenuate the transmission of torsional vibrations created by the firing events of the engine  12  to the remainder of the powertrain  10 . 
     Additionally, the input shaft  26  may include a damper  30  in series with the lock-up clutch  28  and the hydrodynamic fluid drive device  22 . The damper  30  operates to further isolate the torsional vibrations created by the firing events of the engine  12 . The damper  30  may include a compliant member  32  such as a spring. The damper  30  may also include a lock-out clutch  34  operable to bypass the compliant member  32 . This is useful when starting or stopping the engine  12  as the engine  12  may produce a resonance at low engine speeds. The design and operation of the damper  30  is well known to those skilled in the art of automatic shifting power transmissions  14 . 
     A first gear reaction clutch  36 , shown in  FIGS. 1-3 , is selectively engageable and operable to selectively lock and unlock the input shaft  26  from a transmission output shaft  38 . Additionally, there is shown in  FIG. 4  a reaction clutch  36 ′ that may or may not be the same clutch as the first gear reaction clutch  36  depending on which gear range is desired. The reaction clutches  36  and  36 ′ are conventional fluid-operated multi-plate clutches. The reaction clutches  36  and  36 ′ are selectively controlled in engaged and disengaged states by conventional electro-hydraulic mechanisms, not shown, which include a hydraulic valve arrangement and an electronic control unit (ECU) that incorporates a conventional programmable digital computer. The reaction clutches  36  and  36 ′ are engaged and disengaged in accordance with performance and operating signals such as, for example, engine speed, vehicle speed, and engine torque to name a few. Those familiar with the art of transmission control will be familiar with the many features and functions that are available with electronic controls. 
     The reaction clutches  36  and  36 ′ have a plurality of reaction plates  40  interleaved with a plurality of friction plates  42 . Each of the friction plates  42  has a first friction-facing layer  44  and a second friction-facing layer  46  each disposed on opposite faces of the friction plate  42 . The friction-facing layers  44  and  46  frictionally engage the reaction plates  40 . 
     As is well known in the art of power transmissions, gear members of planetary gear sets, not shown, may be selectively engaged and disengaged to provide a speed ratio change between the input shaft  26  and the transmission output shaft  38 . The operation of the planetary gear sets is well known to those skilled in the art. A final drive  16  is connected with respect to the transmission output shaft  38  and is operable to provide a final gear reduction to the drive wheels of the vehicle. 
       FIG. 1  is a schematic representation of the powertrain  10  illustrating the neutral/drive gear state and engine idle mode of operation. In this mode, the first gear reaction clutch  36  will remain disengaged, thereby decoupling the transmission output shaft  38  from the input shaft  26  thereby minimizing the reaction force imparted on the turbine section  24  of the hydrodynamic fluid drive device  22 . As a result, the parasitic losses produced by slipping the hydrodynamic fluid drive device  22  will be minimized. 
       FIG. 2  is a schematic representation of the powertrain  10  illustrating the first gear launch mode of operation. In this mode, the first gear reaction clutch  36  is controlled in a slipping condition. By slipping the first gear reaction clutch  36 , the engine  12  can spin to its peak torque band to allow a strong vehicle launch. In effect, slipping the first gear reaction clutch  36  artificially tunes the stall speed of the hydrodynamic fluid drive device  22  thereby enabling the usage of a highly efficient or “tight” hydrodynamic fluid drive device  22 , which may provide efficiency gains in other areas of vehicle operation such as part throttle operation. Consideration should be paid to the selection of materials for the friction facing layers  44  and  46 , as they will now be subject to a more severe duty cycle. 
       FIG. 3  is a schematic representation of the powertrain  10  illustrating the first gear, part throttle mode of operation. In this mode, the first gear reaction clutch  36  is closed or fully engaged. This will maintain a zero slip condition between the input shaft  26  and the transmission output shaft  38 . Any slip will occur within the hydrodynamic fluid drive device  22 . However, since the hydrodynamic fluid drive device  22  is highly efficient, the powertrain  10  will have a greater efficiency. 
       FIG. 4  is a schematic representation of the powertrain  10  illustrating the second gear and higher mode of operation. In this mode of operation, reaction clutch  36 ′ is closed or fully engaged. This will maintain a zero slip condition between the input shaft  26  and the transmission output shaft  38 . Any slip will occur within the hydrodynamic fluid drive device  22 . However, the hydrodynamic fluid drive device  22  may include a lock-up clutch  28  that is operable to selectively lock the output shaft  18  with the input shaft  26  such that they rotate in sync. By employing a highly efficient hydrodynamic fluid drive device  22 , the torque split across the lock-up clutch  28  and the hydrodynamic drive device  22  provides improved controllability of the lock-up clutch  28 . In the preferred embodiment, the lock-up clutch  28  will be an electronically controlled capacity clutch. The electronically controlled capacity clutch will allow a slight amount of slip between the output shaft  18  and the input shaft  26  thereby decoupling the remainder of the powertrain  10  from the torsional vibrations produced by the firing events of the engine  12 . The control of the electronically controlled capacity clutch will also improve with the utilization of an efficient hydrodynamic fluid drive device  22 . 
     To affect a vehicle launch the first gear reaction clutch  36  should be placed in a slipping condition, as shown in  FIG. 2 , thereby allowing the engine  12  to rev into its torque band. The operating condition shown in  FIG. 2  is effective at high engine load, such as values over 50 KPa of manifold air pressure (MAP). For low MAP values, the operating condition shown in  FIG. 3  will be commanded. In this operating condition, the first gear reaction clutch  36  will be locked or fully engaged. 
     The benefits of the present invention may include enhanced fuel economy, enhanced driveability over a wide range of operating conditions, and improved lock-up clutch performance. Additionally the present invention may reduce cost, mass, and packaging requirements by eliminating the stator within the torque converter or enabling the use of a smaller torque converter in some applications. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.