Abstract:
In a powertrain for a vehicle that includes an engine, a transmission having an input, a current gear, an input clutch, a target gear and an output for driving a load, and an electric machine for driving the load, a method for performing a gearshift from the current gear to the target gear comprising the steps of opening the input clutch, releasing a drive connection between the current gear and the output, producing a drive connection between the target gear and the output, using the electric machine to produce torque at the load, closing the input clutch; and reducing the torque produced by the electric machine.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a powertrain for a hybrid electric vehicle having an engine, an electric machine and a multiple-speed, powershift transmission. In particular, the invention pertains to controlling double-step gear shifts in the transmission. 
         [0003]    2. Description of the Prior Art 
         [0004]    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. 
         [0005]    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. 
         [0006]    In a hybrid electric vehicle (HEV) whose powertrain includes a powershift transmission power is supplied from multiple power sources including an engine, and two electric machines: a crank integrated starter/generator (CISG) and an electric rear axle drive (ERAD). 
         [0007]    The CISG, which may be located in a drive path between the engine and the transmission input, function as a conventional alternator and starter, and provides torque assist. 
         [0008]    The ERAD interfaces with the transmission output and provides torque to the rear driveline for electric-only drive and power boost. The ERAD also provides regenerative recovery of vehicle kinetic energy through the rear driveline while braking the vehicle wheels. 
         [0009]    Gear shift between two even-numbered gears or between two odd-numbered gears are referred to a “double step shifts.” When performing double step shifts between gears on the same layshaft, a powershift transmission can produce brief, sharp reductions in power at the transmission input, sometimes referred to a “torque holes.” Torque holes are perceived by the vehicle&#39;s occupants as a transient period of harsh shifting. 
         [0010]    There is a need in the industry for a technique that reduces or eliminates torque holes when performing double step shifts with a powershift transmission. 
       SUMMARY OF THE INVENTION 
       [0011]    The control method uses the ERAD motor to provide torque to the wheels during a double step shift, thereby providing means to complete the double step shift involving only a single clutch. This method will significantly improve the double step shift event time. 
         [0012]    The control system and method eliminates torque holes, providing positive torque and providing the sensation of performing a gear shift in a conventional automatic transmission. 
         [0013]    The control significantly shortens the period for performing a downshift in heavy driver demand conditions, and improves responsiveness in change-of-mind shifts, i.e., when the vehicle operator changes between tip-in and tip-out of the accelerator pedal. 
         [0014]    A powertrain operating under the control will experience enhanced durability of the input clutches and synchronizers due to their being used less in heavy driver demand conditions. 
         [0015]    The control method can be used to perform the same gearshift if one input clutch or its layshaft is damaged, whereas in a conventional powertrain subject to a performance restriction of the transmission operation is limited to a single gear. 
         [0016]    If the vehicle operator has a “change of mind” and tips out of the throttle, the change of mind shift event occurs much faster. When the tip-out occurs, a change of mind gear shift control strategy will return to fifth gear, which will again be a same shaft shift event. In a conventional vehicle this would require additional steps, since the power flow would have been transferred to the even shaft and another transfer back to the odd shaft would be needed. 
         [0017]    In a powertrain for a vehicle that includes an engine, a transmission having an input, a current gear, an input clutch, a target gear and an output for driving a load, and an electric machine for driving the load, a method for performing a gearshift from the current gear to the target gear comprising the steps of opening the input clutch, releasing a drive connection between the current gear and the output, producing a drive connection between the target gear and the output, using the electric machine to produce torque at the load, closing the input clutch; and reducing the torque produced by the electric machine. 
         [0018]    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 
         [0019]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a schematic diagram of a vehicle powertrain; 
           [0021]      FIG. 2  is a schematic diagram showing additional details of a dual input clutch powershift transmission and the vehicle powertrain of  FIG. 1 ; 
           [0022]      FIG. 3  shows the steps for performing a double-step gearshift from fifth gear to third gear; 
           [0023]      FIG. 4A  illustrates the variation of input shaft speed and engine speed during a power-on, double-step gearshift from fifth gear to third gear; 
           [0024]      FIG. 4B  illustrates the variation of input clutch torque capacity, ERAD torque and engine torque during the gear shift of shown in  FIG. 4A ; and 
           [0025]      FIG. 4C  illustrates the variation of output torque during the gear shift of shown in  FIG. 3A ; 
           [0026]      FIG. 5  is an alternate schematic diagram of the vehicle powertrain of  FIG. 1 ; 
           [0027]      FIG. 6  is a schematic diagram of a second embodiment of a vehicle powertrain; and 
           [0028]      FIG. 7  is a schematic diagram of a third embodiment of a vehicle powertrain. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    As shown in  FIGS. 1 and 2 , a HEV powertrain  12  includes an engine  14 , such as a diesel or gasoline engine; a transmission  16 , such as dual input clutch powershift transmission or another multiple speed transmission having no torque converter; a first electric machine  18 , such as a CISG, driveably connected to the transmission input  20 ; and a second electric machine  22 , such as an electric motor or ERAD, driveably connectable to the transmission output  24  and a rear axle  25 . 
         [0030]    ERAD  22  is located near a vehicle axle  25  and provides propulsion capability to axle  25  in either an electric drive operating mode or a hybrid drive operating mode. Power output by ERAD  22  drives vehicle wheels  26 ,  27  through ERAD gearing and a final drive unit  30 , such as an inter-wheel differential mechanism. Similarly, the transmission output  24  may be driveably connectable to a second set of vehicle wheels  34 ,  35  through a drive unit  36 , which includes an inter-wheel differential mechanism. 
         [0031]    Alternatively, in a second embodiment, transmission output  24  is connected to the front wheels  34 ,  35  but not to the rear wheels  26 ,  27 , in which case ERAD  22  drives the rear wheels but not the front wheels.  FIG. 6  illustrates the second embodiment. 
         [0032]    In third embodiment, transmission output  24  is connected to the front wheels  34 ,  35  but not to the rear wheels  26 ,  27 , ERAD  22  is deleted, and a third electric machine XX, such as an electric front axle drive (EFAD) is driveably connected to the front wheels  34 ,  35 .  FIG. 7  illustrates the third embodiment. 
         [0033]    CISG  18  is used to crank the engine  14  during an engine starting procedure. CISG  18  and ERAD  22  are used to recharge a battery  28  when the CISG is driven by the engine or by the ERAD, or the CISG or ERAD or both the CISG and ERAD are driven by the wheels of the vehicle. 
         [0034]      FIG. 2  illustrates details of the transmission  16  including a first input clutch  40 , which selective connects the input  20  of transmission  16  alternately to the even-numbered gears  46  associated with a first layshaft  44 , and a second input clutch  45 , which selective connects the input  20  alternately to the odd-numbered gears  47  associated with a second layshaft  49 . 
         [0035]    An electronic transmission control module (TCM)  50  controls the input clutches  40 ,  45  through command signals sent to solenoid-actuated servos, which actuate the input clutches. An electronic engine control module (ECM)  52  controls operation of engine  14 . The TCM  50  and ECM  52  each includes a microprocessor accessible to electronic memory and containing control algorithms expressed in computer code, which are executed repeatedly at frequent intervals. 
         [0036]    Shaft  44  supports pinions  60 ,  62 ,  64 , which are each journalled on shaft  44 , and couplers  66 ,  68 , which are secured to shaft  44 . Pinions  60 ,  62 ,  64  are associated respectively with the second, fourth and sixth gears. Coupler  66  includes a sleeve  70 , which can be moved leftward to engage pinion  60  and driveably connect pinion  60  to shaft  44 . Coupler  68  includes a sleeve  72 , which can be moved leftward to engage pinion  62  and driveably connect pinion  62  to shaft  44  and can be moved rightward to engage pinion  64  and driveably connect pinion  64  to shaft  44 . 
         [0037]    Shaft  49  supports pinions  74 ,  76 ,  78 , which are each journalled on shaft  49 , and couplers  80 ,  82 , which are secured to shaft  49 . Pinions  74 ,  76 ,  78  are associated respectively with the first, third and fifth gears. Coupler  80  includes a sleeve  84 , which can be moved leftward to engage pinion  74  and driveably connect pinion  74  to shaft  49 . Coupler  82  includes a sleeve  86 , which can be moved leftward to engage pinion  76  and driveably connect pinion  76  to shaft  49  and can be moved rightward to engage pinion  78  and driveably connect pinion  78  to shaft  49 . 
         [0038]    Output  24  supports gears  88 ,  90 ,  92 , which are each secured to shaft  24 . Gear  88  meshes with pinions  60  and  74 . Gear  90  meshes with pinions  62  and  76 . Gear  92  meshes with pinions  64  and  78 . 
         [0039]    Couplers  66 ,  68 ,  80  and  82  may be synchronizers, or dog clutches or a combination of these. 
         [0040]      FIG. 3  shows the steps for performing and controlling a power-on, double-step gearshift from fifth gear to third gear in a powershift transmission  16  located in powertrain  12  using ERAD  22  to participate in the gearshift. Note that the engagement of fourth gear and activation/deactivation of two input clutches is not required. 
         [0041]    When transmission  16  is operating in fifth gear, at step  100 , the sleeve  86  of coupler  82 , is engaged with pinion  78 , input clutch  40  is fully disengaged or open, and input clutch  45  is fully engaged. With transmission  16  so disposed, at step  102 , coupler  86  connects pinion  78  to input  20  through clutch  45  and shaft  49 , and pinion  78  is driveably connected to wheels  26 ,  27 , which drive the vehicle or load, through gear  92 , output  24 , ERAD  22  and drive unit  30  in fifth gear, or to wheels  34 ,  35  or to wheels  26 ,  27 ,  34  and  35 . 
         [0042]    A double step downshift of transmission  16  from fifth gear to third gear begins at step  104  by opening clutch  45 .  FIG. 4B  shows that the torque capacity of input clutch  45  is reduced first at a step  108 , is then reduced further along a ramp  110  to a magnitude  112 , which is held constant for a period, and is then reduced to zero torque capacity along a second ramp  114 . 
         [0043]    At step  116 , sleeve  86  of coupler  82  is moved leftward to a disengaged position out of engagement with pinion  78  and discontinuing operation in fifth gear, and sleeve  84  of coupler  80  is moved leftward into engagement with pinion  74 , thereby disposing transmission  16  to operate in third gear. 
         [0044]    At step  118 , ERAD  22  begins to produce output torque.  FIG. 4B  shows that the torque output by ERAD  22  first increases along a ramp  120  during period  121  to a magnitude  122 , which is held constant for a period  123 , and is then reduced to zero torque capacity along a ramp  124  during period  125 . The torque capacity increase along ramp  120  during period  121  is concurrent with ramp  114 . The torque cross linking shown in  FIG. 4B  ensures a smooth transition in torque. 
         [0045]    At step  126 , a transfer of torque from ERAD  22  to input clutch  45  occurs as the ERAD torque decreases to zero torque and the torque capacity of input clutch increases along the rise  128 . 
         [0046]    At step  130 , input clutch  45  is closed such that its torque capacity increases rapidly along a rise  128  to a fully engaged torque capacity  132 , which is maintained and held constant. 
         [0047]      FIG. 4C  illustrates that the magnitude of torque transmitted by output  24  decreases only slightly at  134  when input clutch  45  has its torque capacity reduced. The magnitude of torque transmitted by output  24  is a constant magnitude  136  during the downshift transition from fifth gear to third gear operation. 
         [0048]    In  FIGS. 4A-4C , vertical line  138  represents the point in time when fifth gear operation ends and vertical line  140  represents the point in time when third gear operation begins. The output torque produced by ERAD  22  during the downshift is a maximum when the transition from fifth gear to third gear occurs. 
         [0049]      FIG. 4A  illustrates the speed change of input  49  due to the change from fifth gear  142  to third gear  144 , and the variation of engine speed  146 . 
         [0050]    Input clutches  40 ,  45  cannot be fully engaged at the same time or the resulting tie-up would damage the transmission  16 . When a gearshift occurs, the input clutches  40 ,  45  must smoothly, synchronously transfer torque between shafts  44 ,  49 . Without synchronous torque transfer from one shaft to the other the result would be loss of torque to the wheels during the gearshift. 
         [0051]    Commanding torque from ERAD  22  can be prepared ahead of the gear shift since the scheduled shift is already known. ERAD  22  is a fast torque device that is used to provide torque to the wheels during a transition from fifth gear to third gear. The ERAD torque is then ramped off with reengagement of clutch  45 . Using ERAD to fill in torque bypasses the intermediate shift into 4 th  gear resulting in a significantly faster double step shift. 
         [0052]    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. The input clutches  40 ,  45  of the powershift transmission  16  may be hydraulically actuated wet clutches, or they may be electrically or electromagnetically actuated dry clutches. 
         [0053]      FIG. 5  illustrates schematically the powertrain of  FIG. 1 , in which the transmission output  24  is driveably connected both to the front wheels  34 ,  35  through differential  36 , and to ERAD  22 , which is driveably connected through differential  30  to the rear wheels  26 ,  27 . Torque produced at the output  60  of engine  14  and at the output of CISG  18  is transmitted to  150  for delivery to the transmission input  20 . 
         [0054]      FIG. 6  illustrates schematically a second embodiment of the powertrain, in which the transmission output  24  is driveably connected through differential  36  to drive the front wheels  34 ,  35  only. Torque produced at the output  60  of engine  14  and at the output of CISG  18  is transmitted to  150  for delivery to the transmission input  20 . ERAD  22  drives the rear wheel  26 ,  27 . 
         [0055]      FIG. 7  illustrates schematically a third embodiment of the powertrain, in which the transmission output  24  is driveably connected to EFAD  152 , whose output  154  drives the front wheels  34 ,  35  through differential  36 . Torque produced at the output  60  of engine  14  and at the output of CISG  18  is transmitted to  150  for delivery to the transmission input  20 . 
         [0056]    Although the method has been described with reference to a 5-3 power-on downshift, the control method and system can be applied to any double step gearshift. 
         [0057]    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.