Patent Publication Number: US-2011061954-A1

Title: Strong hybrid system

Description:
TECHNICAL FIELD 
     The present invention generally relates to a hybrid system for a vehicle, and more specifically to a strong hybrid system including a single electric motor/generator. 
     BACKGROUND OF THE INVENTION 
     Many vehicles now include a hybrid powertrain system that includes an engine and a multi-speed transmission. The engine typically includes an internal combustion engine, and produces a torque, which is transferred through the transmission to wheels of the vehicle. The transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The hybrid powertrain systems include a coupling system disposed between the engine and the transmission. The coupling system augments, i.e., adds to, the torque provided by the engine. 
     The coupling system may improve vehicle fuel economy in a variety of ways. For instance, the engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Captured braking energy (via regenerative braking) or energy stored by an electric motor acting as a generator during periods when the engine is operating is utilized during these engine off periods. Transient demand for engine torque or power is supplemented by the electric motor during operation in engine-on, electric modes, allowing for downsizing the engine without reducing apparent vehicle performance. Additionally, the engine may be operated at or near the optimal efficiency point for a given power demand. The motor/generator is able to capture vehicle kinetic energy during braking, which is used to keep the engine off longer, supplement engine torque or power and/or operate at a lower engine speed, or supplement accessory power supplies. Additionally, the electric motor/generator is very efficient in accessory power generation and electric power from the battery serves as an available torque reserve allowing operation at a relatively low transmission numerical speed ratio. 
     SUMMARY OF THE INVENTION 
     A powertrain for a vehicle is provided. The powertrain includes an engine and a transmission. The engine is configured for supplying torque. A coupling system interconnects the engine and the transmission. The coupling system includes an input shaft coupled to the engine. The input shaft is configured for receiving torque from the engine. The coupling system further includes an engine disconnect clutch, which selectively interconnects the engine and the input shaft. The engine disconnect clutch is moveable between an engaged position and a disengaged position. The engaged position is configured for selectively connecting the input shaft with the engine to transmit torque from the engine to the input shaft. The disengaged position is configured for selectively disconnecting the input shaft from the engine to prevent transmission of torque between the engine and the input shaft. The coupling system further includes an electric motor/generator coupled to the input shaft. The electric motor is configured for supplying torque to the input shaft for transfer to at least one of the engine and the transmission, and is also configured for receiving torque from the input shaft for generating electricity. The coupling system further includes a fluid coupling interconnecting the input shaft and the transmission. The fluid coupling is configured for transmitting torque from the input shaft to the transmission. The coupling system further includes a transmission fluid pump. The transmission fluid pump is coupled to the input shaft, and is operable in response to torque from the input shaft. The transmission fluid pump supplies a fluid pressure to the engine disconnect clutch and the transmission at a pre-determined pressure. The powertrain operates in a first state of operation, a second state of operation and a third state of operation. The electric motor/generator supplies torque to the input shaft to operate the transmission fluid pump in the first state of operation. The engine supplies torque to the input shaft to operate the transmission fluid pump in the second state of operation. Both the engine and the electric motor/generator supply torque to the input shaft to operate the transmission fluid pump in the third state of operation. 
     In another aspect of the invention, a coupling system for a vehicle having an engine and a transmission is provided. The coupling system includes an input shaft. The input shaft is configured for receiving torque from the engine. The coupling system further includes an electric motor/generator coupled to the input shaft. The electric motor/generator is configured for alternatively supplying torque to the input shaft for transfer to at least one of the engine and the transmission, and is also configured for receiving torque from the input shaft for generating electricity. An engine disconnect clutch is attached to the input shaft. The engine disconnect clutch is moveable between an engaged position and a disengaged position. The engaged position is configured for selectively connecting the input shaft with the engine to transmit torque from the engine to the input shaft. The disengaged position is configured for selectively disconnecting the input shaft from the engine to prevent transmission of torque between the engine and the input shaft. A fluid coupling is attached to the input shaft. The fluid coupling is configured for transmitting torque from the input shaft to the transmission. A transmission fluid pump is coupled to the input shaft. The transmission fluid pump receives torque from the input shaft, and is configured for supplying a fluid pressure to the engine disconnect clutch and the transmission at a pre-determined pressure in response to torque transmitted through the input shaft. 
     In another aspect of the invention, a transmission for a vehicle is provided. The transmission includes a housing having a bell portion and a gear portion. A coupling system is disposed within the bell portion of the housing. The coupling system is configured for coupling to an engine of the vehicle. The coupling system includes an input shaft configured for receiving torque from the engine. The coupling system further includes an engine disconnect clutch configured for selectively interconnecting the input shaft and the engine. The engine disconnect clutch is moveable between an engaged position and a disengaged position. The engaged position is configured for selectively connecting the input shaft with the engine to transmit torque between the engine and the input shaft. The disengaged position is configured for selectively disconnecting the input shaft from the engine to prevent transmission of torque between the engine and the input shaft. The coupling system further includes an electric motor/generator coupled to the input shaft. The electric motor/generator is configured for supplying a torque to the input shaft for transfer to at least one of the engine and the transmission, and is also configured for receiving a torque from the input shaft for generating electricity. The coupling system further includes a fluid coupling configured for interconnecting the input shaft and the transmission to transmit torque from the input shaft to the transmission. The coupling system further includes a transmission fluid pump coupled to the input shaft. The transmission fluid pump is operable in response to torque from the input shaft to supply a fluid pressure to the engine disconnect clutch and the transmission at a pre-determined pressure. The engaged position of the engine disconnect clutch is a default position of the engine disconnect clutch. 
     Accordingly, the coupling system is selectively coupled to the engine to permit selective engagement/disengagement between the engine and the transmission. As such, in the event the coupling system fails, the vehicle is operable as a standard vehicle, i.e., a vehicle without a coupling system, by disengaging the coupling system. Additionally, both the engine and the electric motor of the coupling system are directly coupled to the transmission fluid pump, thereby eliminating the need for the coupling system to include an auxiliary electric motor and transmission fluid pump. Elimination of an auxiliary electric motor and transmission fluid pump in the coupling system reduces the possibility of the coupling system failing by reducing the number of operation components in the coupling system. 
     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 perspective view of a transmission. 
         FIG. 2  is a schematic diagram showing a first embodiment of a coupling system in a powertrain of a vehicle. 
         FIG. 3  is a schematic diagram showing a second embodiment of the coupling system in the powertrain of the vehicle. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a powertrain for a vehicle is shown generally at  20 . The powertrain  20  includes an engine  22 , a coupling system  24  and a transmission  26 . The powertrain  20  produces a torque and transfers the torque to one or more wheels of the vehicle. 
     The engine  22  is configured to supply a torque. Depending upon the operational state of the powertrain  20 , described in greater detail below, the engine  22  may supply all of the torque necessary to operate the vehicle, or may only supply a portion of the torque necessary to operate the vehicle. The engine  22  preferably includes an internal combustion engine. However, it should be appreciated that the engine  22  may include some other type of engine capable of providing a torque sufficient to power the vehicle. 
     The transmission  26  is configured to receive a torque and transmit the torque to at least one wheel of the vehicle. The transmission  26  preferably includes a multi-speed automatic transmission  26  including a plurality of planetary gear-sets, clutches, bands, etc., as is well known in the art. The transmission  26  increases the overall operating range of the vehicle by permitting the engine  22  to operate through its torque range a number of times. While described as a multi-speed automatic transmission, it should be appreciated that the transmission  26  may include some other type of transmission not shown or described herein, capable of transmitting torque to the wheels of the vehicle, and suitable for use with a coupling system  24 . 
     Referring to  FIG. 1 , the transmission  26  includes a housing  28 . The housing  28  includes a bell portion  30  and a gear portion  32 . The gear portion  32  houses the planetary gear-sets, clutches, bands, etc. of the transmission  26 . Preferably, the bell portion  30  houses the coupling system  24 , i.e., the coupling system  24  is part of the transmission  26 . However, it should be appreciated that the coupling system  24  may be separate from the transmission  26  and disposed outside of the bell portion  30  of the housing  28 , between the engine  22  and the transmission  26 . 
     Referring to  FIG. 2 , a first embodiment of the coupling system  24  is shown. The coupling system  24  interconnects the engine  22  and the transmission  26 . More specifically, the coupling system  24  interconnects the engine  22  and the gear portion  32  of the transmission  26 . The coupling system  24  may be utilized to provide torque to the transmission  26  independently from the engine  22 , or in combination with the engine  22  as is described in greater detail below. 
     The coupling system  24  includes an input shaft  34 . The input shaft  34  is coupled to the engine  22 , and is configured to receive torque from the engine  22 . The engine  22  may be coupled to the input shaft  34  in any suitable manner. The coupling system  24  may include a damper  36  configured for attenuating vibration from the engine  22  in the input shaft  34 . The damper  36  is disposed adjacent the engine  22  and may include any suitable damper  36  known to those skilled in the art. 
     The coupling system  24  includes an electric motor  38  coupled to a battery  40 . The electric motor  38  may be coupled to the battery  40  in any suitable manner. The electric motor  38  includes a generator  42 , and is hereinafter referred to as the electric motor/generator  38 . Accordingly, the electric motor/generator  38  is operable to generate torque, and is alternatively operable to generate electricity as a generator  42 , with the electricity being stored in the battery  40  as is well known. The electric motor/generator  38  is a single operational unit, i.e., the electric motor  38  and the generator  42  are combined and are not separate components. The electric motor/generator  38  utilizes electricity stored in the battery  40  to generate a torque to operate the vehicle. Additionally, the electric motor/generator  38  receives a torque to generate electricity for storage in the battery  40 . 
     The electric motor/generator  38  is coupled to the input shaft  34 , and is configured for supplying torque to the input shaft  34 . The electric motor/generator  38  may be coupled to the input shaft  34  in any suitable manner. Accordingly, the input shaft  34  may receive torque from only the engine  22 , only the electric motor/generator  38 , or both the engine  22  and the electric motor/generator  38 . The torque supplied to the input shaft  34  from the electric motor/generator  38  is transferred to at least one of the engine  22  and the transmission  26 . The generator  42  of the electric motor/generator  38  is also configured to receive torque from the input shaft  34 . It should be appreciated that the electric motor/generator  38  receives the torque from the input shaft  34  to generating electricity, and more specifically, the generator  42  of the electric motor/generator  38  receives the torque from the input shaft  34  to generate electricity. 
     The coupling system  24  includes an engine disconnect clutch  44 . The engine disconnect clutch  44  selectively interconnects the engine  22  and the input shaft  34 . The engine disconnect clutch  44  is moveable between an engaged position and a disengaged position. The engaged position is configured for selectively connecting the input shaft  34  with the engine  22  to transmit torque from the engine  22  to the input shaft  34 . The disengaged position is configured for selectively disconnecting the input shaft  34  from the engine  22  to prevent transmission of torque between the engine  22  and the input shaft  34 . Accordingly, the operating states of the powertrain  20  are changed by changing the engine disconnect clutch  44  between the engaged position and the disengaged position. The engine disconnect clutch  44  may include a spring loaded clutch, or some other type of clutch capable of selectively connecting and disconnecting the engine  22  and the input shaft  34  that is not shown or described herein. 
     The transmission  26  includes a transmission fluid pump  46 . The transmission fluid pump  46  is preferably disposed within the transmission housing  28 , but may alternatively be disposed outside the transmission housing  28 . The transmission fluid pump  46  is coupled to the input shaft  34 , and is operable in response to torque from the input shaft  34 . Accordingly, the transmission fluid pump  46  is operable in response to torque supplied by the engine  22 , torque supplied by the electric motor/generator  38  or torque supplied by both the engine  22  and the electric motor/generator  38 . The transmission fluid pump  46  supplies a fluid at a pre-determined fluid pressure to the transmission  26  to enable the transmission  26  to function properly. 
     The engine disconnect clutch  44  may include a hydraulically actuated clutch. If so, the engine disconnect clutch  44  may be in fluid communication with the transmission fluid pump  46 , wherein the engine disconnect clutch  44  is moveable into the disengaged position in response to a fluid pressure supplied by the transmission fluid pump  46 . As such, the transmission fluid pump  46  supplies the fluid at the pre-determined fluid pressure to the engine disconnect clutch  44  to actuate the engine disconnect clutch  44  between the disengaged position and the engaged position. Accordingly, the engine disconnect clutch  44  is operable to move into the disengaged position only when one of the engine  22  and/or the electric motor/generator  38  is supplying a torque to the input shaft  34  to actuate the transmission fluid pump  46 . 
     The coupling system  24  further includes a fluid coupling  48  interconnecting the input shaft  34  and the transmission  26 . The fluid coupling  48  is configured to transmit torque from the input shaft  34  to the transmission  26 . More specifically, the coupling system  24  includes an output shaft  50  interconnecting the fluid coupling  48  and the transmission  26 , with the fluid coupling  48  interconnecting the input shaft  34  and the output shaft  50 . The output shaft  50  is coupled to the gear portion  32  of the transmission  26  to supply the torque to the gear portion  32 . The fluid coupling  48  permits relative movement, i.e., slippage, between the input shaft  34  and the output shaft  50  until the output shaft  50  is brought up to a comparable rotational speed as the input shaft  34 . 
     The coupling system  24  further includes a locking clutch  52  interconnecting the input shaft  34  and the output shaft  50 . The locking clutch  52  is moveable between a locked position and an unlocked position. The fluid coupling  48  is locked when the locking clutch  52  is in the locked position to prevent relative movement between the input shaft  34  and the output shaft  50 . The fluid coupling  48  is released when the locking clutch  52  is in the unlocked position to permit relative movement between the input shaft  34  and the output shaft  50 . Once the output shaft  50  is brought up to a comparable rotational speed relative to the input shaft  34 , the locking clutch  52  locks the fluid coupling  48  to prevent slippage between the input shaft  34  and the output shaft  50  to improve fuel efficiency. 
     The coupling system  24  may further include a one way clutch  54  interconnecting the engine  22  and the input shaft  34 . The one way clutch  54  is configured to permit torque transfer from the engine  22  to the transmission  26  and to not transmit torque transfer from the electric motor/generator  38  to the engine  22 . As such, the one way clutch  54  is disposed between the engine  22  and the electric motor/generator  38 . In other words, the one way clutch  54  only transmits torque from the engine  22  into the powertrain  20 , and does not transmit torque back into the engine  22 . 
     The engine disconnect clutch  44  and the one way clutch  54  are arranged in parallel with each other. As such, when the engine disconnect clutch  44  is in the disengaged position, both the one way clutch  54  and the engine disconnect clutch  44  are configured to not transmit torque to the engine  22 , thereby disengaging the electric motor/generator  38  of the coupling system  24  from the engine  22 . However, the engine  22  is still coupled to the input shaft  34  to supply torque to the input shaft  34  through the one way clutch  54 . When the engine disconnect clutch  44  is in the engaged position, the input shaft  34  may transmit torque to the engine  22  through the engine disconnect clutch  44 , for example, to start the engine  22  as described below. 
     Preferably, but not necessarily, in the first embodiment of the coupling system  24 , the disengaged position of the engine disconnect clutch  44  is a default position of the engine disconnect clutch  44 , i.e., the disengaged position is the position the engine disconnect clutch  44  defaults to in the absence of a signal to change to the engaged position. Accordingly, the default position of the engine disconnect clutch  44  ensures that torque is not transferable to the engine  22 , while still allowing torque transfer from the engine  22  to the input shaft  34  through the one way clutch  54 . As such, the engine  22  is disconnected from the coupling system  24  as a default to ensure that the engine  22  may be utilized to power the vehicle in the event the coupling system  24  fails. 
     The coupling system  24  may further include a torque limiting device  56  interconnecting the engine  22  and the input shaft  34 . The torque limiting device  56  is configured to limit torque transfer between the engine  22  and the input shaft  34  to below a pre-determined level. Accordingly, if the torque from the engine  22  exceeds the pre-determined level, the torque limiting device  56  begins to slip, i.e., limit, the torque transferred to the input shaft  34 . The torque limiting device  56  may include a spring loaded clutch, or some other device capable of limiting torque transfer to below a pre-determined level, that is not shown or described herein. 
     The powertrain  20  operates in a first state of operation, a second state of operation and a third state of operation. In the first state of operation, only the electric motor/generator  38  supplies torque to the input shaft  34  to operate the transmission fluid pump  46 . In the second state of operation, only the engine  22  supplies torque to the input shaft  34  to operate the transmission fluid pump  46 . In the third state of operation, both of the engine  22  and the electric motor/generator  38  supply torque to the input shaft  34  to operate the transmission fluid pump  46 . When the powertrain  20  is operating in the second state of operation, the engine  22  may also supply torque to the input shaft  34  to operate the generator  42  of the electric motor/generator  38  to generate electricity. 
     The first state of operation is generally associated with normal, initial startup and operation of the vehicle below a pre-determined speed or torque requirement. In the first state of operation, the electric motor/generator  38  generates the torque and supplies the torque to the input shaft  34 , and the engine disconnect clutch  44  is in the disengaged position. The input shaft  34  supplies the torque to the transmission fluid pump  46 , which pressurizes the transmission fluid. The torque from the electric motor/generator  38  flows from the electric motor/generator  38 , through the input shaft  34 , through the fluid coupling  48  into the output shaft  50 , and then into the transmission  26 . Once the output shaft  50  and the input shaft  34  are operating at a comparable speed, the locking clutch  52  may be engaged to lock the fluid coupling  48  to prevent relative slippage between the input shaft  34  and the output shaft  50 . The disengaged position of the engine disconnect clutch  44  and the one way clutch  54  do not transfer torque to the engine  22 . 
     The second state of operation is generally associated with a failure in the coupling system  24 , in which case the engine  22  provides all of the power to the vehicle and bypasses the coupling system  24 . In the second state of operation, the engine  22  generates the torque and supplies the torque to the input shaft  34 . The input shaft  34  transmits the torque through the torque limiting device  56 , the damper  36 , and the one way clutch  54  to supply the transmission fluid pump  46  with torque to actuate the transmission fluid pump  46 . The transmission fluid pump  46  pressurizes the transmission fluid to a sufficient pressure to operate the transmission  26 . The torque from the engine  22  flows through the input shaft  34 , through the fluid coupling  48  into the output shaft  50 , and then into the transmission  26 . Once the output shaft  50  and the input shaft  34  are operating at a comparable rotational speed, the locking clutch  52  may be engaged to lock the fluid coupling  48  to prevent relative slippage between the input shaft  34  and the output shaft  50 . If the engine  22  includes an internal combustion engine  22 , then the engine  22  must include a standard 12 volt starter or the like to start the engine  22  to operate the powertrain  20  in the second state of operation. 
     When in the second state of operation, it should be appreciated that the electric motor/generator  38  may receive torque from the engine  22  to operate the generator  42  of the electric motor/generator  38 , to thereby generate electricity and charge the battery  40 . 
     The third state of operation is generally associated with operation of the vehicle above the pre-determined speed or torque requirement. In the third state of operation, the engine  22  is the primary supply of torque, with the electric motor/generator  38  adding torque to supplement the engine  22  to meet the various high speed and/or high torque driving conditions. The powertrain  20  enters the third state of operation upon reaching the upper limits of the first state of operation, i.e., the vehicle begins startup and normal operation in the first state of operation and then moves into the third state of operation when the coupling system  24  is no longer capable of supplying sufficient torque to the powertrain  20  by itself. 
     Assuming the powertrain  20  is operating in the first state of operation, the electric motor/generator  38  supplies the torque to the input shaft  34 , which actuates the transmission fluid pump  46 . The transmission fluid pump  46  supplies the pressurized fluid to the engine disconnect clutch  44  to move the engine disconnect clutch  44  into the engaged position when signaled to permit torque transfer to the engine  22 . The torque entering the engine  22  through the engine disconnect clutch  44  rotates a crankshaft of the engine  22 . After the crankshaft of the engine  22  reaches a sufficient rotating speed, the engine  22  fires and begins to operate. Once the engine  22  is operating, the torque flows from the engine  22  to the input shaft  34  through the one way clutch  54 . After the engine  22  has fired and is in operation, the engine disconnect clutch  44  is preferably, but not necessarily, moved back into the disengaged position. In the third state of operation, the electric motor/generator  38  is selectively utilized to supplement the engine  22 . When the electric motor/generator  38  is not required to supply additional torque to the powertrain  20 , the generator  42  of the electric motor/generator  38  may be engaged to generate electricity to charge the battery  40 . 
     Referring to  FIG. 3 , a second embodiment of the coupling system is shown generally at  124 . Throughout the description of the second embodiment of the coupling system  124  and  FIG. 3 , reference numerals utilized to describe features of the second embodiment of the coupling system  124  that are similar to the features of the first embodiment of the coupling system  24  include the same reference numeral increased by one hundred. For example, the electric motor, identified by the reference numeral  38  in the first embodiment of the coupling system  24 , is identified by the reference numeral  138  in the second embodiment of the coupling system  124 . 
     The second embodiment of the coupling system  124  includes an input shaft  134 . The input shaft  134  is coupled to the engine  122 , and is configured to receive torque from the engine  122 . The engine  122  may be coupled to the input shaft  134  in any suitable manner. The second embodiment of the coupling system  124  may include a damper  136  configured for attenuating vibration in the input shaft  134  from the engine  122 . The damper  136  is disposed adjacent the engine  122  and may include any suitable damper  136  known to those skilled in the art. 
     The second embodiment of the coupling system  124  includes an electric motor  138  coupled to a battery  140 . The electric motor  138  may be coupled to the battery  140  in any suitable manner. The electric motor  138  includes a generator  142 , and is hereinafter referred to as the electric motor/generator  138 . Accordingly, the electric motor/generator  138  is operable to generate torque as a motor, and is alternatively operable to generate electricity as a generator, which is stored in the battery  140  as is well known. The electric motor/generator  138  is a single operational unit, i.e., the electric motor  138  and the generator  142  are combined and are not separate components. The electric motor/generator  138  utilizes electricity stored in the battery  140  to generate a torque to operate the vehicle. Additionally, the electric motor/generator  138  receives a torque to generate electricity for storage in the battery  140 . 
     The electric motor/generator  138  is coupled to the input shaft  134 , and is configured for supplying torque to the input shaft  134 . The electric motor/generator  138  may be coupled to the input shaft  134  in any suitable manner. Accordingly, the input shaft  134  may receive torque from only the engine  122 , only the electric motor/generator  138 , or both the engine  122  and the electric motor/generator  138 . The torque supplied to the input shaft  134  from the electric motor/generator  138  is transferred to at least one of the engine  122  and the transmission  126 . The generator  142  of the electric motor/generator  138  is also configured to receive torque from the input shaft  134 . It should be appreciated that the electric motor/generator  138  receives the torque from the input shaft  134  to generate electricity, and more specifically, the generator  142  of the electric motor/generator  138  receives the torque from the input shaft  134  to generate electricity. 
     The second embodiment of the coupling system  124  includes an engine disconnect clutch  144 . The engine disconnect clutch  144  selectively interconnects the engine  122  and the input shaft  134 . The engine disconnect clutch  144  is moveable between an engaged position and a disengaged position. The engaged position is configured for selectively connecting the input shaft  134  with the engine  122  to transmit torque from the engine  122  to the input shaft  134 . The disengaged position is configured for selectively disconnecting the input shaft  134  from the engine  122  to prevent transmission of torque between the engine  122  and the input shaft  134 . Accordingly, the operating states of the powertrain  120  are changed by changing the engine disconnect clutch  144  between the engaged position and the disengaged position. The engine disconnect clutch  144  may include a spring loaded clutch, or some other type of clutch capable of selectively connecting and disconnecting the engine  122  and the input shaft  134  that is not shown or described herein. 
     The transmission  126  includes a transmission fluid pump  146 . The transmission fluid pump  146  is preferably disposed within the transmission housing, identified in  FIG. 1  by reference numeral  28 , but may alternatively be disposed outside the transmission housing  28 . The transmission fluid pump  146  is coupled to the input shaft  134 , and is operable in response to torque from the input shaft  134 . Accordingly, the transmission fluid pump  146  is operable in response to torque supplied by the engine  122 , torque supplied by the electric motor/generator  138  or torque supplied by both the engine  122  and the electric motor/generator  138 . The transmission fluid pump  146  supplies a fluid at a pre-determined fluid pressure to the transmission  126  to enable the transmission  126  to function properly. 
     The engine disconnect clutch  144  may include a hydraulically actuated clutch. If so, the engine disconnect clutch  144  may be in fluid communication with the transmission fluid pump  146 , wherein the engine disconnect clutch  144  is moveable into the disengaged position in response to a fluid pressure supplied by the transmission fluid pump  146 . As such, the transmission fluid pump  146  supplies the fluid at the pre-determined fluid pressure to the engine disconnect clutch  144  to actuate the engine disconnect clutch  144  between the disengaged position and the engaged position. Accordingly, the engine disconnect clutch  144  is operable to move into the disengaged position only when one of the engine  122  and/or the electric motor/generator  138  is supplying a torque to the input shaft  134  to actuate the transmission fluid pump  146 . 
     Preferably, the engaged position of the engine disconnect clutch  144  is a default position of the engine disconnect clutch  144 , i.e., the engaged position is the position the engine disconnect clutch  144  defaults to in the absence of a signal to change to the disengaged position. Accordingly, the default position of the engine disconnect clutch  144  ensures the engine  122  is coupled to the transmission  126  in the event the coupling system  124  fails. As such, the vehicle is operable as a standard internal combustion vehicle in the event the coupling system  124  fails. 
     The second embodiment of the coupling system  124  further includes a fluid coupling  148  interconnecting the input shaft  134  and the transmission  126 . The fluid coupling  148  is configured to transmit torque from the input shaft  134  to the transmission  126 . More specifically, the coupling system  124  includes an output shaft  150  interconnecting the fluid coupling  148  and the transmission  126 , with the fluid coupling  148  interconnecting the input shaft  134  and the output shaft  150 . The output shaft  150  is coupled to the gear portion  32  of the transmission  126  to supply the torque to the gear portion  32 . The fluid coupling  148  permits relative movement, i.e., slippage, between the input shaft  134  and the output shaft  150  until the output shaft  150  is brought up to the same rotational speed as the input shaft  134 . 
     The second embodiment of the coupling system  124  further includes a locking clutch  152  interconnecting the input shaft  134  and the output shaft  150 . The locking clutch  152  is moveable between a locked position and an unlocked position. The locked position locks the fluid coupling  148  to prevent relative movement between the input shaft  134  and the output shaft  150 . The unlocked position releases the fluid coupling  148  to permit relative movement between the input shaft  134  and the output shaft  150 . Once the output shaft  150  is brought up to a comparable rotational speed as the input shaft  134 , the locking clutch  152  locks the fluid coupling  148  to prevent slippage between the input shaft  134  and the output shaft  150  to improve fuel efficiency. 
     The powertrain  120  including the second embodiment of the coupling system  124  shown in  FIG. 3  operates in a first state of operation, a second state of operation and a third state of operation. In the first state of operation, only the electric motor/generator  138  supplies torque to the input shaft  134  to operate the transmission fluid pump  146 . In the second state of operation, only the engine  122  supplies torque to the input shaft  134  to operate the transmission fluid pump  146 . In the third state of operation, both of the engine  122  and the electric motor/generator  138  supply torque to the input shaft  134  to operate the transmission fluid pump  146 . When the powertrain  120  is operating in the second state of operation, the engine  122  may also supply torque to the input shaft  134  to operate the generator  142  of the electric motor/generator  138  to generate electricity. 
     The first state of operation is generally associated with normal, initial startup and operation of the vehicle below a pre-determined speed or torque requirement. In the first state of operation, the electric motor/generator  138  generates the torque and supplies the torque to the input shaft  134 , and the engine disconnect clutch  144  is in the engaged position. The input shaft  134  supplies the torque to the transmission fluid pump  146 , which pressurizes the transmission fluid and supplies the pressurized fluid to the engine disconnect clutch  144 . The pressurized fluid moves the engine disconnect clutch  144  from the default engaged position into the disengaged position to disengage the engine  122  from the input shaft  134  and the electric motor/generator  138 . The torque from the electric motor/generator  138  flows from the electric motor/generator  138 , through the input shaft  134 , through the fluid coupling  148  into the output shaft  150 , and then into the transmission  126 . Once the output shaft  150  and the input shaft  134  are operating at a comparable rotational speed, the locking clutch  152  may be engaged to lock the fluid coupling  148  to prevent relative slippage between the input shaft  134  and the output shaft  150 . 
     The second state of operation is generally associated with a failure in the coupling system  124 , in which case the engine  122  provides all of the power to the vehicle and bypasses the coupling system  124 . In the second state of operation, the engine disconnect clutch  144  is in the default engaged position. The engine  122  generates the torque and supplies the torque to the input shaft  134  through the engine disconnect clutch  144 . The input shaft  134  transmits the torque through the damper  136 , to supply the transmission fluid pump  146  with torque to actuate the transmission fluid pump  146 . The transmission fluid pump  146  pressurizes the transmission fluid to a sufficient pressure to operate the transmission  126 . The torque from the engine  122  flows through the input shaft  134 , through the fluid coupling  148  into the output shaft  150 , and then into the transmission  126 . Once the output shaft  150  and the input shaft  134  are operating at a comparable rotational speed, the locking clutch  152  may be engaged to lock the fluid coupling  148  to prevent relative slippage between the input shaft  134  and the output shaft  150 . If the engine  122  includes an internal combustion engine  122 , then the engine  122  must include a standard 12 volt starter or the like to start the engine  122  to operate the powertrain  120  in the second state of operation. 
     When in the second state of operation, it should be appreciated that the electric motor/generator  138  may receive torque from the engine  122  to operate the generator  142  of the electric motor/generator  138 , to thereby generate electricity and charge the battery  140 . 
     The third state of operation is generally associated with operation of the vehicle above the pre-determined speed or torque requirement. In the third state of operation, the engine  122  is the primary supply of torque, with the electric motor/generator  138  adding torque to supplement the engine  122  to meet the various high speed or high torque driving conditions. The third state of operation normally, but not necessarily, begins upon the powertrain  120  reaching the upper limits of the first state of operation, i.e., the vehicle begins startup and normal operation in the first state of operation and then moves into the third state of operation when the coupling system  124  is no longer capable of supplying sufficient torque to the powertrain  120  by itself. 
     Assuming the powertrain  120  is operating in the first state of operation with the engine disconnect clutch  144  in the disengaged position, the electric motor/generator  138  supplies the torque to the input shaft  134 , which actuates the transmission fluid pump  146 . The transmission fluid pump  146  supplies the pressurized fluid to the engine disconnect clutch  144  to move the engine disconnect clutch  144  into the engaged position to permit torque transfer to the engine  122 . The torque entering the engine  122  through the engine disconnect clutch  144  rotates a crankshaft of the engine  122 . After the crankshaft of the engine  122  reaches a sufficient rotating speed, the engine  122  fires and begins to operate. Once the engine  122  is operating, the torque flows from the engine  122  to the input shaft  134  through engine disconnect clutch  144 , which remains in the default engaged position. In the third state of operation, the electric motor/generator  138  is selectively utilized to supplement the engine  122 . When the electric motor/generator  138  is not required to supply additional torque to the powertrain  120 , the generator  142  of the electric motor/generator  138  may be engaged to generate electricity to charge the battery  140 . 
     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.