Abstract:
The present invention relates to a torsional damper for an electrically variable transmission. The torsional damper is equipped with a hydraulically actuable lock-out clutch to selectively directly couple the engine to the input shaft of the transmission. The electric motors provided with the electrically variable transmission can serve to effectively cancel out engine compression pulses when the springs of the torsional damper are locked out. During higher speeds the centrifugal loading placed on oil in the torsional damper increases, which may cause the lock-out clutch to inappropriately engage. The present invention hydraulically balances the hydraulic actuator (or piston) driving the lock-out clutch to appropriately regulate lock-out clutch engagement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application 60/555,141 filed Mar. 22, 2004, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to an electrically variable transmission with a torsional damper assembly having a hydraulically balanceable lock-out clutch assembly.  
       BACKGROUND OF THE INVENTION  
       [0003]     Automobile engines produce torsionals or vibrations that are undesirable to transmit through the vehicle transmission. To isolate such torsionals, torsional dampers can be implemented into the vehicle transmission. These dampers rest between the engine crankshaft and the input shaft or turbine shaft of the transmission to substantially counteract the unwanted torsionals generated by the engine. Dampers are configured with springs that have the capacity to carry maximum engine torque plus some margin above.  
         [0004]     One premise behind hybrid automobiles is that alternative power is available to propel the vehicle, thus reliance on the engine for power can be decreased, thereby increasing fuel economy. Since hybrid vehicles can derive their power from sources other than the engine, hybrid engines typically operate at lower speeds more often and can be turned off while the vehicle is propelled by the electric motors. For example, electrically variable transmissions alternatively rely on electric motors housed in the transmission to power the vehicle&#39;s driveline. Engines in hybrid vehicles are therefore required to start and stop more often than engines in non-hybrid systems. Compression pulses are generated by the engine during starts and stops that can produce undesirable vibration in hybrid vehicles such as those having an electrically variable transmission. Therefore, greater functionality is desirable in the damper assembly to aid the electrically variable transmission in canceling these compression pulses.  
         [0005]     Lastly, since the torsional damper assembly is securable to the engine crankshaft the torsional damper revolves at high annular speeds. Where hydraulic fluid is used to govern the torsional damper, the fluid is subjected to centrifugal loading as a result of these annular speeds.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a means of hydraulically balancing the actuator (or piston) driving a lock-out clutch for the torsional damper assembly of an electrically variable transmission (or EVT). The invention includes two separate hydraulic circuits transferring a hydraulic fluid to opposing sides of the piston when necessary to balance the piston. The need for this balancing depends upon the centrifugal loading placed on the hydraulic fluid resulting from the annular speed of the damper assembly.  
         [0007]     In one embodiment of the present invention, each circuit is in parallel with two pumps (one motor driven and the other engine driven) to assist in supplying the hydraulic fluid to the intended areas of the torsional damper assembly.  
         [0008]     More specifically, the present invention provides an electrically variable transmission with at least one electric motor and a rotatable torsional damper assembly. The torsional damper assembly includes a torsional spring operable to eliminate or reduce compression pulses and torsionals. A clutch assembly is further provided, having a hydraulically operable piston for selectively locking out the torsional spring; whereas at least one electric motor cancels out compression pulses when the torsional spring is locked out. Also included is a hydraulic fluid applicable to opposing sides of said piston to sufficiently hydraulically balance the piston so as to prevent the clutch assembly from locking out the torsional spring at least partially in response to centrifugal forces resulting from the rotational speed of the damper.  
         [0009]     Also provided is a method of operating a rotatable hydraulically actuated torsional damper of an electrically variable transmission in the start, stop and drive modes. The method includes hydraulically locking out the torsional damper during the start and stop modes with hydraulic fluid; and hydraulically counter balancing the lockout piston of the torsional damper to prevent the hydraulically locking out of the torsional damper during drive mode.  
         [0010]     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  
       [0011]      FIG. 1  is a schematic side view of an electrically variable transmission with parts broken away to show selected transmission components and an auxiliary pump mounted to the transmission;  
         [0012]      FIG. 2  is a fragmentary cross-sectional view of the torsional damper assembly taken along one side of the centerline of the front portion of the electrically variable transmission with two hydraulic circuits shown schematically;  
         [0013]      FIG. 3  is a graph indicating the piston charging pressure as a function of the damper assembly speed (line A) and the damper vessel volume required to balance the piston (line B).  
         [0014]      FIG. 4   a  is a schematic sectional view of the perforated thrust washer of  FIG. 2  isolated from the transmission; and  
         [0015]      FIG. 4   b  is a schematic front view of the perforated thrust washer of  FIG. 2  isolated from the transmission.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Referring to the drawings,  FIGS. 1 through 2 , wherein like characters represent the same or corresponding parts throughout the several views there is shown in  FIG. 1 a  side view of an electrically variable transmission  10 . Fundamentally, the present invention is implemented in an electrically variable transmission  10  with at least one electric motor (A or B) and a rotatable torsional damper assembly  26 , as shown in  FIG. 2 . The torsional damper assembly  26  includes a torsional spring  32  operable to eliminate or reduce compression pulses and torsionals. A clutch assembly (or lock-out clutch  33 ) is further provided, having a hydraulically operable piston  50  for selectively locking out the torsional spring  32 ; thereby enabling one or both of the electric motors (A or B of  FIG. 1 ) to cancel out engine compression pulses. Also included is a hydraulic fluid which is applicable to the piston cavity  58  and the damper vessel  34 , which are on opposing sides of the piston  50 , to sufficiently hydraulically balance the piston  50  so as to prevent the lock-out clutch  33  from locking out the torsional spring  32  as a result of centrifugal forces from the rotation of the torsional damper  26 .  
         [0017]     More specifically,  FIG. 1  displays selected components of an electrically variable transmission  10  including the input housing  12  and main housing  14  with dual electric motors (A and B), which are indirectly journaled onto the main shaft  19  of the transmission  10  through a series of planetary gear set (not shown). The motors (A, B) operate with selectively engaged clutches (not shown) to rotate the output shaft  20 . The oil pan  16  is located on the base of the main housing  14  and is configured to provide oil volume for the transmission  10  and its components. The main housing  14  covers the inner most components of the transmission such as the electric motors (A, B), planetary gear arrangements, the main shaft  19  and two clutches (all of which are mentioned for exemplary purposes and not all are shown). Finally, the input housing  12  is bolted directly to the engine block rear face of the engine  24  (schematically represented in  FIG. 2 ) and encases the transmission components that mechanically interface with the engine  24 . Namely, the input housing  12  covers the torsional damper assembly  26  (shown better in  FIG. 2 ). The input housing  12  also supports an auxiliary pump  27  (as shown in  FIG. 1 ), which is mounted to the base of the input housing  12  and secured nestably adjacent the oil pan  16 .  
         [0018]     The torsional damper assembly  26 , as shown in  FIG. 2 , generally functions to isolate the transmission  10  from unwanted torsionals generated by the engine  24  during operation and also to selectively aide the transmission electric motors (either A or B) in canceling engine compression pulses during starts and stops. The torsional damper assembly  26  consists of an engine side cover  28 , which is affixed to the engine crankshaft  29 . The engine side cover  28  is welded to the transmission side cover  30  at  31  and houses the damper springs  32 . The two covers ( 28  and  30 ) define a vessel  34 , which encloses the lock-out clutch  33  and a piston  50 . The torsional damper assembly  26  further houses a damper flange  38  with hub portion  40  that mates to the input shaft  18  at complementary splines  42 . The engine side cover  28  of the torsional damper assembly  26  is affixed to an engine flexplate  44 . The flexplate  44  functions to transmit to the transmission the torque produced by the engine  24  and also to absorb any thrust loads generated by the damper assembly  26 . The torsional damper assembly  26  consists of a series of damper springs  32  running annularly or circumferentially between the engine side cover  28  and transmission side cover  30 . The damper springs  32  absorb and dampen the unwanted torsionals produced by the engine  24  during normal or drive mode operation (i.e., speeds above 600 rpm). The torsional damper assembly  26  has a torque capacity equal to the maximum torque capacity of the engine plus some margin. The torsional damper assembly  26  may be configured, in part, similarly to the structure disclosed in commonly owned, U.S. Pat. No. 5,009,301, which is hereby incorporated by reference in its entirety.  
         [0019]     The electrically variable transmission  10  is equipped with two electric motors (A and B as shown in  FIG. 1 ). Electric motor A creates a torque during start and stop that effectively cancels out the engine compression pulses caused when the engine is operating at speeds below 600 rpm (or in start and/or stop mode). The damper springs  32  of the torsional damper assembly  26  can be locked out by applying the clutch plates  36  and  37  (of the lock-out clutch  33 ) when the engine  24  is operating within a predetermined speed range. In the preferred embodiment, the torsional damper assembly  26  is effectively locked out when the engine is operating at speeds less than or equal to 600 rpm. This mode of operation is desirable because in an electrically variable transmission either electric motor (A or B) can be used to actively cancel out engine compression pulses generated during start or stop. The lock-out clutch  33 , located inside the torsional damper assembly  26 , consists of two reaction plates  37  connected to the damper flange  38 , two friction plates  36  connected to the transmission side cover  30 , a backing plate  46  and a snap ring  48  that is attached to the damper flange  38  at arm  61 . The lock-out clutch  33  is adjacent a hydraulic piston  50  which moves against the reaction plates  37  forcing them to engage the friction plates  36 . The piston  50  moves in response to oil fed into cavity  58  from an oil circuit  57 . The load is reacted at the backing plate  46  and snap ring  48  and contained by the damper flange  38 . Adjacent the piston  50  and affixed to the damper flange  38  is the damper hub  40  of the torsional damper assembly  26 , which has a cross-drilled channel  56  to define a radially extending aperture  52  that allows oil from circuit  57  to pass through. The oil extends through a cross-drilled aperture  55  in the input shaft  18  through aperture  53 , into the channel  56  to the front side of the piston  50 . The piston  50  is restricted from engaging with the lock-out clutch  33  and held in the disengaged position by a return spring  54 . As oil is fed through channel  56  of the damper hub  40 , the pressure inside the piston cavity  58  increases creating a load sufficient to overcome the spring force and stroke the piston  50  thereby engaging the lock-out clutch  33 . The vessel  34  is also filled with oil from the hydraulic circuit  59 , through aperture  51 , into the inner diameter of tube  35 , which is fitted in the input shaft  18 , through a grooved thrust washer  41  (or bushing), into cavity or spacing  43  and to the interior of vessel  34 . The oil thus received in vessel  34  is on the right side of the piston  50 , as shown in  FIG. 2 , to counter balance the oil fed into cavity  58  on the other side of the piston  50 .  
         [0020]     The hydraulic circuits  57  and  59 , as shown in  FIG. 2 , supply oil to the piston cavity  58  and damper vessel  34  respectively; governing the lock-out clutch  33  and commanding it to engage and disengage under certain predetermined conditions. The first circuit  57  delivers hydraulic fluid to the piston cavity  58 . The second circuit  59  is regulated at a lower pressure and ultimately sends oil to the vessel  34  located on another side of the piston  50 . The piston  50  inside the torsional damper assembly  26  responds to the sufficiently higher pressure resulting from the oil fed through the first circuit  57  by stroking and engaging the lock-out clutch  33  to effectively lock out the damper springs  32 . When the lock-out clutch  33  is engaged the torsional damper springs  32  are deactivated or locked out so that the engine  24  is directly coupled to the input shaft  18  of the transmission  10 . This condition is only preferred for engine starts and stops (i.e., start and/or stop modes where engine speeds are within the predetermined speed range: between 0 and 600 rpm).  
         [0021]     The transmission  10  can operate in electric mode where the engine  24  is completely turned off. When the engine is off the main pump  62 , which derives its power from the engine, is inoperable. Since the damper vessel  34  is unsealed the oil inside drains from the damper vessel  34  to approximately half full when the main pump  62  and the auxiliary pump  27  are not in operation. As the engine is restarted, the remaining oil is forced to the perimeter of the torsional damper assembly  26  by the centrifugal loading resulting from the revolution of the input shaft  18  and torsional damper assembly  26 . Likewise, the oil remaining in the damper hub  40  is forced into the piston cavity  58  (i.e., its perimeter). Since the oil in the damper flange  38  is concentrated in the piston cavity  58  the oil in the piston cavity  58  weighs on the piston  50 . At high speeds the centrifugal loading on the oil (or hydraulic fluid) in the piston cavity  58  may overcome the force of the return spring  54  and stroke the piston  50 . To stroke the piston  50  the pressure difference between the piston cavity  58  and the damper vessel  34  must be greater than or equal to 4 psi to overcome the return spring and greater than or equal to 60 psi to acquire full capacity on the clutch  33 . Line A of  FIG. 3  illustrates the increased pressure differential of the oil in the piston cavity  58  as a function of the speed of the torsional damper assembly  26 . The x-axis represents the speed of the torsional damper assembly  26  and the y-axis represents the charging pressure on the piston  50 . As the torsional damper assembly speed approaches 4000 rpm the charging pressure resulting from the hydraulic fluid in the piston cavity  58  is approximately 60 psi—enough to have full torque design capacity on the clutch  33 . Inappropriate engagement of the lock-out clutch  33  and effectively locking out of the torsional damper assembly  26  can lead to additional wear on transmission components causing premature failure or reduced cycle life. However, as demonstrated by the intersection of lines A and B in  FIG. 3 , when using the provided pumps ( 27  and  62 ) to fill the damper vessel  34  the piston  50  can be hydraulically balanced prior to reaching a charging pressure of 60 psi. Though either pump is capable of supplying oil into the damper vessel  34 , the auxiliary pump  27  is responsible for sending oil to the vessel  34  or other side of the piston  50  when the transmission is operating in electric mode (or when the engine is off).  
         [0022]     One of the technical advantages of the present invention is that the hydraulic circuits,  57  and  59  as shown in  FIG. 2 , and pumps  27  and  62  are configured to balance the hydraulic piston  50  in preparation for engine operation. To balance the piston  50  at least 0.36 liters of oil must be inside the damper vessel  34  as shown by line B in  FIG. 3 . When the auxiliary pump  27  is operating it pulls oil from a sump and sends oil in parallel to the control module  64  and the priority regulator  70  (as shown in  FIG. 2 ). The priority regulator  70  regulates the pressure at which the auxiliary pump  27  operates (which is 60 psi in the preferred embodiment) and directs all excess oil to the thermal exchanger  68  which returns oil to the transmission  10 , through the lube regulator  72  and into hydraulic circuit  59 . The control module  64 , under certain predetermined conditions (or in start and/or stop modes of operation in the preferred embodiment), will supply oil to hydraulic circuit  57  to pressurize the oil in the piston cavity  58  up to 110 psi. When the engine is on and turning the main pump  62  pulls oil from the sump and sends it in parallel to the control module  64  and main regulator valve  66 . From the main regulator  66 , oil passes through the priority regulator  70 , flows through the thermal exchanger  68  to lube regulator  72  and into hydraulic circuit  59 .  
         [0023]     In the preferred embodiment, the lube regulator  72  ensures that the pressure of the oil in the damper vessel  34  does not exceed 30 psi. The control module  64  maintains oil pressure in the hydraulic circuit  57  to 2 psi. Therefore, the piston  50  cannot stroke to apply the clutch  33  with the oil in the piston cavity  58  at 2 psi, the oil in the damper vessel  34  at 30 psi, and the return spring  54  applying an adverse load. The piston  50  is thereby hydraulically balanced or prevented from engaging the lock-out clutch  33 . However, when desirable (or in start and/or stop modes of operation), the auxiliary pump  27  can apply the clutch  33  by supplying high-pressure oil to the piston cavity  58  overcome the 30 psi in the damper vessel  34  and the adverse force of the return spring  54 .  
         [0024]     The two circuits ( 57  and  59 ) are isolated by a set of rotating seal rings  74  and a steel tube  35  fitted within the input shaft  18  of the transmission  10 . A grooved thrust washer  41 , as better shown in  FIGS. 4   a  and  4   b , facilitates oil travel from the inner diameter of the tube  35  to the damper vessel  34 . The thrust washer  41  has grooves  76  in its perimeter to facilitate oil travel through the washer  41  and into the damper vessel  34 .  
         [0025]     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.