Patent Abstract:
A method of pressure control for transmissions having a hydraulic circuit selectively communicating with first and second pumps via first and second regulators includes commanding communication to the hydraulic circuit to transition from the first to the second pump and the PCS to set the regulators to a high pressure-regulation level. The hydraulic circuit line pressure is raised to an elevated value exceeding a target value. The line pressure is lowered from the elevated to the target value at or after completing the transition to the second pump. The method may include transitioning communication with the hydraulic circuit from the second to the first pump. The PCS maintains the regulators at the high level and line pressure is raised to the elevated value. PCS sets the regulators to a low level, and line pressure returns to the target only at or after completing the transition to the first pump.

Full Description:
TECHNICAL FIELD 
     The present invention relates generally to hybrid powertrains for motorized vehicles, and hydraulic control thereof. 
     BACKGROUND OF THE INVENTION 
     Motorized vehicles include a powertrain operable to propel the vehicle and power the onboard vehicle electronics. The powertrain, or drivetrain, generally includes an engine that powers the final drive system through a multi-speed power transmission. Many vehicles are powered by a reciprocating-piston type internal combustion engine (ICE) because of its ready availability, relatively-inexpensive cost, light weight, and relative efficiency. Such engines include four-stroke compression-ignited diesel engines and four-stroke spark-ignited gasoline engines. 
     Hybrid vehicles utilize alternative power sources to propel the vehicle, minimizing reliance on the engine for power, and increasing overall fuel economy. A hybrid electric vehicle (HEV), for example, incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and power the vehicle systems. The HEV generally employs one or more electric machines that operate individually or in concert with an internal combustion engine to propel the vehicle. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles may be turned off while the vehicle is stopped or is being propelled by the alternative power source(s). 
     Series hybrid architectures, sometimes referred to as Range-Extended Electric Vehicles (REEVs), are generally characterized by an internal combustion engine in driving communication with an electric generator. The electric generator provides power to one or more electric motors operable to rotate the final drive members. There may be no direct mechanical connection between the engine and the drive members in a series hybrid powertrain. The lack of a mechanical link between the engine and wheels allows the engine to run at a constant and efficient rate, even as vehicle speed changes. The electric generator may also operate to start the internal combustion engine. The system may also allow the electric motor(s) to recover energy by slowing the vehicle and storing it in the battery through regenerative braking. 
     Parallel hybrid architectures are generally characterized by an internal combustion engine and one or more electric motor/generator assemblies, all of which have a direct mechanical coupling to the transmission. Parallel hybrid designs utilize combined electric motor/generators, which provide traction and may replace both the conventional starter motor and alternator. The motor/generators are electrically connected to an energy storage device (ESD). The energy storage device may be a chemical battery. A control unit is employed for regulating the electrical power interchange between the energy storage device and motor/generators, as well as the electrical power interchange between the first and second motor/generators. 
     Electrically-variable transmissions (EVT) provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures, and also elements of traditional, non-hybrid transmissions. EVTs may be designed to operate in both fixed-gear (FG) modes and EVT modes. When operating in a fixed-gear mode, the rotational speed of the transmission output member is a fixed ratio of the rotational speed of the input member from the engine, depending upon the selected arrangement of the differential gearing subsets. EVTs are also configured for engine operation that is mechanically independent from the final drive, thereby enabling high-torque continuously-variable speed ratios, electrically dominated launches, regenerative braking, engine-off idling, and two-mode operation. 
     An EVT may combine the motor/generators with differential gearing to achieve continuously variable torque and speed ratios between the input and output. The EVT can utilize the differential gearing to send a fraction of its transmitted power through the electric motor/generator(s) and the remainder of its power through another, parallel path that is mechanical. One form of differential gearing used is the epicyclic planetary gear arrangement. However, it is possible to design a power split transmission without planetary gears, for example, as by using bevel gears or other differential gearing. 
     Hydraulically-actuated torque-transmitting mechanisms, such as clutches and brakes, are selectively engageable to selectively activate the gear elements for establishing different forward and reverse speed ratios and modes between the transmission input and output shafts. The term “clutch” is used hereinafter to refer generally to torque transmitting mechanisms, including, without limitation, devices commonly referred to as clutches and brakes. Shifting from one speed ratio or mode to another may be in response to vehicle conditions and operator (driver) demands. The “speed ratio” is generally defined as the transmission input speed divided by the transmission output speed. Thus, a low gear range has a high speed ratio, and a high gear range has a relatively lower speed ratio. Because EVTs are not limited to single-speed gear ratios, the different operating states may be referred to as ranges or modes. 
     The range or mode change may be controlled through a multi-clutch synchronization and release process. A first clutch associated with a currently-active range is carrying torque in an applied state, while a second clutch associated with a currently-inactive second range is carrying no torque in a released state. Shifting from a first range to a second range is accomplished by controlling the second, unapplied clutch to zero slip speed, and applying the second clutch (the oncoming clutch) thereby placing the EVT in a state with both clutches applied. The second range is then entered by the release of the first clutch (the offgoing clutch). 
     SUMMARY OF THE DISCLOSURE 
     A method of hydraulic pressure control for a hybrid transmission is provided. The hybrid transmission has a main pump deriving power from an internal combustion engine and in selective fluid flow communication with a hydraulic circuit of the hybrid transmission through a main regulator. The hybrid transmission also has an auxiliary pump not deriving power from the internal combustion engine and in selective fluid flow communication with the hydraulic circuit through an auxiliary regulator. 
     The method includes commanding the internal combustion engine to begin transition to an engine-off state, and commanding the main pump to begin a transition to a non-operating state. A pressure control solenoid (PCS) is moved from a low position to a high position. The PCS is operatively in communication with both the main regulator and the auxiliary regulator, such that the main regulator moves from a low pressure setting to a high pressure setting and the auxiliary regulator moves from a low pressure setting to a high pressure setting. 
     The method includes commanding the auxiliary pump to begin a transition to an auxiliary-on state. The method further includes operating the PCS in the high position during the transition of the main pump to the non-operating state. The transition of the auxiliary pump to the auxiliary-on state is finished before finishing the transition of the main pump to the non-operating state. 
     The high pressure setting of the main regulator may be greater than the high pressure setting of the auxiliary regulator. The method may further include placing the main regulator in fluid flow communication with the hydraulic circuit; transitioning fluid flow communication with the hydraulic circuit from the main regulator to the auxiliary regulator; and ending fluid flow communication from the main regulator to the hydraulic circuit before completing the transition of the main pump to the non-operating state. 
     Additionally, the method may include commanding the internal combustion engine to begin a transition to an engine-on state, and commanding the main pump to begin a transition to an operating state. The auxiliary pump is then commanded to begin a transition to an auxiliary-off state, and the PCS is operated in the high position during the transition of the main pump to the operating state. The transition of the main pump to the operating state is completed before finishing the transition of the auxiliary pump to the auxiliary-off state. The PCS is moved from the high position to the low position, such that the main regulator moves from the high pressure setting to the low pressure setting and the auxiliary regulator moves from the high pressure setting to the low pressure setting. The method may further include transitioning fluid flow communication with the hydraulic circuit from the auxiliary regulator to the main regulator, and ending fluid flow communication from the auxiliary regulator to the hydraulic circuit before completing the transition of the auxiliary pump to the auxiliary-off state. 
     The above features and advantages, and other features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiments and other modes for carrying out the present invention when taken in connection with the accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic lever diagram illustration of an exemplary vehicle powertrain with a multi-mode, electrically-variable hybrid transmission in accordance with the present invention; 
         FIG. 2  is a truth table listing the engaged torque-transmitting mechanisms for each of the operating modes of the transmission illustrated in  FIG. 1 ; 
         FIG. 3  is a graphical representation of various regions of operation with respect to input and output speeds of the transmission illustrated in  FIG. 1 ; 
         FIG. 4  is a schematic illustration of an exemplary pressure control system for feeding transmission fluid to the hydraulic circuit of the transmission illustrated in  FIG. 1 , shown while the auxiliary pump is supplying the hydraulic circuit; 
         FIG. 5  is an exemplary graphical representation of main and auxiliary regulator pressure as a function of pressure control solenoid (PCS) pressure; 
         FIG. 6A  is a graphical representation of an exemplary control process for a transition from the main to the auxiliary pump of  FIG. 4 ; and 
         FIG. 6B  is a graphical representation of an exemplary control process for a transition from the auxiliary to the main pump of  FIG. 4 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The claimed invention is described herein in the context of a hybrid-type vehicular powertrain having a multi-mode, multi-speed, electrically-variable, hybrid transmission, which is intended solely to offer a representative application by which the present invention may be incorporated and practiced. The claimed invention is not limited to the particular powertrain arrangement shown in the drawings. Furthermore, the hybrid powertrain illustrated herein has been greatly simplified, it being understood that further information regarding the standard operation of a hybrid powertrain, or a hybrid-type vehicle will be recognized by those having ordinary skill in the art. 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views, there is shown in  FIG. 1  a lever diagram depiction of an exemplary vehicle powertrain system, designated generally as  10 . The powertrain  10  includes a restartable engine  12  that is selectively drivingly connected to, or in power flow communication with, a final drive system  16  via a multi-mode, electrically-variable hybrid-type power transmission  14 . 
     A lever diagram is a schematic representation of the components of a mechanical device such as an automatic transmission. Each individual lever represents a planetary gearset, wherein the three basic mechanical components of the planetary gear are each represented by a node. Therefore, a single lever contains three nodes: one for the sun gear member, one for the planet gear carrier member, and one for the ring gear member. The relative length between the nodes of each lever may be used to represent the ring-to-sun ratio of each respective gearset. These lever ratios, in turn, are used to vary the gear ratios of the transmission in order to achieve appropriate ratios and ratio progression. Mechanical couplings or interconnections between the nodes of the various planetary gear sets and other components of the transmission (such as motor/generators) are illustrated by thin, horizontal lines. Torque transmitting devices such as clutches and brakes are presented as interleaved fingers. If the device is a brake, one set of the fingers is grounded. 
     The transmission  14  is designed to receive at least a portion of its driving power from the engine  12 , through an input member  18 , for example. The transmission input member  18 , which is in the nature of a shaft, may be the engine output shaft (also referred to as a “crankshaft”). Alternatively, a transient torque damper (not shown) may be implemented between the engine  12  and the input member  18  of the transmission  14 . The engine  12  transfers power to the transmission  14 , which distributes torque through a transmission output member or shaft  20  to drive the final drive system  16 , and thereby propel the vehicle (not shown). 
     In the embodiment depicted in  FIG. 1 , the engine  12  may be any of numerous forms of petroleum-fueled prime movers, such as the reciprocating-piston type internal combustion engines, which includes spark-ignited gasoline engines and compression-ignited diesel engines. The engine  12  is readily adaptable to provide its available power to the transmission  14  at a range of operating speeds, for example, from idle, at or near 600 revolutions per minute (RPM), to over 6,000 RPM. Irrespective of the means by which the engine  12  is connected to the transmission  14 , the input member  18  is connected to a differential gear set encased within the transmission  14 , as explained in more detail herein. 
     Referring still to  FIG. 1 , the hybrid transmission  14  utilizes one or more differential gear arrangements, preferably in the nature of three interconnected epicyclic planetary gear sets, designated generally at  24 ,  26  and  28 , respectively. Each gear set includes three gear members: a first, second and third member. In referring to the first, second and third gear sets in this description and in the claims, these sets may be counted “first” to “third” in any order in the drawings (e.g., left to right, right to left, etc.). Likewise, in referring to the first, second and third members of each gear set in this description and in the claims, these members may be counted or identified as “first” to “third” in any order in the drawings (e.g., top to bottom, bottom to top, etc.) for each gear set. 
     The first planetary gear set  24  has three gear members: a first, second and third member  30 ,  32  and  34 ; respectively. In a preferred embodiment, the first member  30  includes of an outer gear member (which may be referred to as a “ring gear”) that circumscribes the third member  34 , which may include of an inner gear member (which may be referred to as a “sun gear”). In this instance, the second member  32  acts as a planet carrier member. That is, a plurality of planetary gear members (which may be referred to as “pinion gears”) are rotatably mounted on the second member, planet carrier  32 . Each planetary gear member is meshingly engaged with both the first member, ring gear  30  and the third member, sun gear  34 . 
     The second planetary gear set  26  also has three gear members: a first, second and third member  40 ,  42  and  44 , respectively. In the preferred embodiment discussed above with respect to the first planetary gear set  24 , the first member  40  of the second planetary gear set  26  is an outer “ring” gear member that circumscribes the third member  44 , which is an inner “sun” gear member. The ring gear member  40  is coaxially aligned and rotatable with respect to the sun gear member  44 . A plurality of planetary gear members are rotatably mounted on the second member  42 , which acts as a planet carrier member, such that each planetary gear meshingly engages both the ring gear member  40  and the sun gear member  44 . 
     The third planetary gear set  28 , similar to the first and second gear sets  24 ,  26 , also has first, second and third members  50 ,  52  and  54 , respectively. In this arrangement, the first member  50  is preferably the outer “ring” gear, which circumscribes the third member  54  or inner “sun” gear. The second member  52  is the planet carrier in this particular gear set, and is coaxially aligned and rotatable with respect to the sun gear member  54 . As such, a plurality of planetary or pinion gear members are rotatably mounted on the carrier member  52 . Each of the pinion gear members is aligned to meshingly engage either the ring gear member  50  and an adjacent pinion gear member or the sun gear member  54  and an adjacent pinion gear member. 
     In one embodiment on the transmission  14 , the first and second planetary gear sets  24 ,  26  each include simple planetary gear sets, whereas the third planetary gear set includes a compound planetary gear set. However, each of the carrier members described above can be either a single-pinion (simple) carrier assembly or a double-pinion (compound) carrier assembly. Embodiments with long pinions are also possible. 
     The first, second and third planetary gear sets  24 ,  26 ,  28  are compounded in that the second member  32  of the first planetary gear set  24  is conjoined with (i.e., continuously connected to) the second member  42  of the second planetary gear set  26  and the third member  54  of the third planetary gear set  28 , as by a central shaft  36 . As such, these three gear members  32 ,  42 ,  54  are rigidly attached for common rotation. 
     The engine  12  is continuously connected to the first member  30  of first planetary gear set  24  through an integral hub plate  38 , for example, for common rotation therewith. The third member  34  of the first planetary gear set  24  is continuously connected, for example, by a first sleeve shaft  46 , to a first motor/generator assembly  56 , interchangeably referred to herein as “motor A”. The third member  44  of the second planetary gear set  26  is continuously connected by a second sleeve shaft  48 , to a second motor/generator assembly  58 , also interchangeably referred to herein as “motor B”. The first member  50  of the third planetary gear set  28  is continuously connected to transmission output member  20  through an integral hub plate, for example. The first and second sleeve shafts  46 ,  48  may circumscribe the central shaft  36 . 
     A first torque transfer device  70 —which is herein interchangeably referred to as clutch “C 1 ”—selectively connects the second gear member  52  with a stationary member, represented in  FIG. 1  by transmission housing  60 . The second sleeve shaft  48 , and thus gear member  44  and motor/generator  58 , is selectively connectable to the second member  52  of the third planetary gear set  28  through the selective engagement of a second torque transfer device  72 —which is herein interchangeably referred to as clutch “C 2 ”. A third torque transfer device  74 —which is herein interchangeably referred to as clutch “C 3 ”—selectively connects the first gear member  40  of the second planetary gear set  26  to the transmission housing  60 . The first sleeve shaft  46 , and thus third gear member  34  and first motor/generator  56 , is also selectively connectable to the first member  40  of the second planetary gear set  26 , through the selective engagement of a fourth torque transfer device  76 —which is herein interchangeably referred to as clutch “C 4 ”. 
     A fifth torque transfer device  78 —which is herein interchangeably referred to as clutch “C 5 ”—selectively connects the input member  18  of engine  12  and the first gear member  30  of the first planetary gear set  24  to the transmission housing  60 . Clutch C 5  is an input brake clutch, which selectively locks the input member  18  when engine  12  is off. Locking input member  18  provides more reaction for regenerative braking energy. As shown below, in reference to  FIG. 2 , C 5  is not involved in the mode/gear/neutral shifting maneuvers of transmission  14 . 
     The first and second torque transfer devices  70 ,  72  (C 1  and C 2 ) may be referred to as “output clutches.” The third and fourth torque transfer devices  74 ,  76  (C 3  and C 4 ) may be referred to as “holding clutches”. 
     In the exemplary embodiment depicted in  FIG. 1 , the various torque transfer devices  70 ,  72 ,  74 ,  76 ,  78  (C 1 -C 5 ) are all friction clutches. However, other conventional clutch configurations may be employed, such as dog clutches, rocker clutches, and others recognizable to those having ordinary skill in the art. The clutches C 1 -C 5  may be hydraulically actuated, receiving pressurized hydraulic fluid from a pump (not shown). Hydraulic actuation of clutches C 1 -C 5  is accomplished, for example, by using a hydraulic fluid control circuit. 
     In the exemplary embodiment described herein, wherein the hybrid powertrain  10  is used as a land vehicle, the transmission output shaft  20  is operatively connected to the final drive system (or “driveline”). The driveline may include a front or rear differential, or other torque transfer device, which provides torque output to one or more wheels through respective vehicular axles or half-shafts (not shown). The wheels may be either front or rear wheels of the vehicle on which they are employed, or they may be a drive gear of a track vehicle. Those having ordinary skill in the art will recognize that the final drive system may include any known configuration, including front wheel drive (FWD), rear wheel drive (RWD), four-wheel drive (4 WD), or all-wheel drive (AWD), without altering the scope of the claimed invention. 
     All of the planetary gear sets  24 ,  26 ,  28 , as well as the first and second motor/generators  56 ,  58  (motor A and motor B,) are preferably coaxially oriented about the intermediate central shaft  36  or another axis. Motor A or motor B may take on an annular configuration, permitting one or both to generally circumscribe the three planetary gear sets  24 ,  26 ,  28 . Such a configuration may reduce the overall envelope, i.e., the diametrical and longitudinal dimensions, of the hybrid transmission  14  are minimized. 
     The hybrid transmission  14  receives input motive torque from a plurality of torque-generative devices. “Torque-generative devices” include the engine  12  and the motors/generators  56 ,  58  as a result of energy conversion from fuel stored in a fuel tank or electrical potential stored in an electrical energy storage device (neither of which is shown). 
     The engine  12 , motor A ( 56 ,) and motor B ( 58 ) may operate individually or in concert—in conjunction with the planetary gear sets and selectively-engageable torque-transmitting mechanisms—to rotate the transmission output shaft  20 . Moreover, motor A and motor B are preferably configured to selectively operate as both a motor and a generator. For example, motor A and motor B are capable of converting electrical energy to mechanical energy (e.g., during vehicle propulsion), and further capable of converting mechanical energy to electrical energy (e.g., during regenerative braking or during periods of excess power supply from engine  12 ). 
     With continuing reference to  FIG. 1 , an electronic control apparatus (or “controller”) having a distributed controller architecture is shown schematically in an exemplary embodiment as a microprocessor-based electronic control unit (ECU)  80 . The ECU  80  includes a storage medium with a suitable amount of programmable memory, collectively represented at  82 , that is programmed to include, without limitation, an algorithm or method  100  of regulating operation of a multi-mode hybrid transmission, as will be discussed in further detail below with respect to  FIG. 4 . 
     The control apparatus is operable, as described hereinafter, to provide coordinated system control of the powertrain  10  schematically depicted and described herein. The constituent elements of the control apparatus may be a subset of an overall vehicle control system. The control system is operable to synthesize pertinent information and inputs, and execute control methods and algorithms to control various actuators to achieve control targets. The control system monitors target and parameters including, without limitation: fuel economy, emissions, performance, driveability, and protection of drivetrain hardware—such as, but not limited to, the engine  12 , transmission  14 , motor A, motor B, and final drive  16 . 
     The distributed controller architecture (ECU  80 ) may include a Transmission Control Module (TCM), an Engine Control Module (ECM), a Transmission Power Inverter Module (TPIM), and a Battery Pack Control Module (BPCM). A hybrid control module (HCP) may be integrated to offer overall control and coordination of the aforementioned controllers. 
     A User Interface (UI) is operatively connected to a plurality of devices (not shown) through which a vehicle operator typically controls or directs operation of the powertrain. Exemplary vehicle operator inputs to the UI include an accelerator pedal, a brake pedal, transmission gear selector, vehicle speed cruise control, and other inputs recognizable to those having ordinary skill in the art. 
     Each of the aforementioned controllers communicates with other controllers, sensors, actuators, etc., via a local area network (LAN) bus or communication architecture. The LAN bus allows for structured communication of control parameters and commands between the various controllers. The communication protocol utilized is application-specific. For example, and without limitation, one useable communication protocol is the Society of Automotive Engineers standard J1939. The LAN bus and appropriate protocols provide for robust messaging and multi-controller interfacing between the aforementioned controllers, and other controllers providing functionality such as antilock brakes, traction control, and vehicle stability. 
     The ECM is operatively connected to, and in communication with, the engine  12 . The ECM is configured to acquire data from a variety of sensors and control a variety of actuators of the engine  12  over a plurality of discrete lines. The ECM receives an engine torque command from the HCP, generates a desired axle torque, and an indication of actual engine torque, which is communicated to the HCP. Various other parameters that may be sensed by the ECM include engine coolant temperature, engine input speed to the transmission, manifold pressure, and ambient air temperature and pressure. Various actuators that may be controlled by the ECM include, without limitation, fuel injectors, ignition modules, and throttle control modules. 
     The TCM is operatively connected to the transmission  14 , and functions to acquire data from a variety of sensors and provide command signals to the transmission  14 . Inputs from the TCM to the HCP may include estimated clutch torques for each of the clutches C 1 -C 5 , and rotational speed of the transmission output shaft  20 . Additional actuators and sensors may be used to provide additional information from the TCM to the HCP for control purposes. 
     Each of the aforementioned controllers may be a general-purpose digital computer, generally including a microprocessor or central processing unit, read only memory (ROM), random access memory (RAM), electrically programmable read only memory (EPROM), high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O) and appropriate signal conditioning and buffer circuitry. Each controller has a set of control algorithms, including resident program instructions and calibrations stored in ROM and executed to provide the respective functions of each computer. Information transfer between the various computers may be accomplished using the aforementioned LAN. 
     In response to operator input, as captured by the UI, the supervisory HCP controller and one or more of the other controllers described above with respect to  FIG. 1  determine required transmission output torque. Selectively operated components of the hybrid transmission  14  are appropriately controlled and manipulated to respond to the operator demand. For example, in the embodiment shown in  FIG. 1 , when the operator has selected a forward drive range and manipulates either the accelerator pedal or the brake pedal, the HCP determines an output torque for the transmission, which affects how and when the vehicle accelerates or decelerates. Final vehicle acceleration is affected by other variables, including such factors as road load, road grade, and vehicle mass. The HCP monitors the parametric states of the torque-generative devices, and determines the output of the transmission required to arrive at the desired torque output. Under the direction of the HCP, the transmission  14  operates over a range of output speeds from slow to fast in order to meet the operator demand. 
     The ECU  80  also receives frequency signals from sensors for processing into input member  18  speed, N i , and output member  20  speed, N o , for use in the control of transmission  14 . The system controller may also receive and process pressure signals from pressure switches (not shown) for monitoring clutch application chamber pressures. Alternatively, pressure transducers for wide range pressure monitoring may be employed. Pulse-width modulation (PWM) and/or binary control signals are transmitted by the controller  80  to transmission  14  for controlling fill and drain of clutches C 1 -C 5  for application and release thereof. 
     Additionally, the controller  80  may receive transmission fluid sump temperature data, such as from thermocouple inputs (not shown), to derive a sump temperature. Controller  80  may provide PWM signals derived from input speed, N i , and sump temperature for control of line pressure via one or more regulators. 
     Fill and drain of clutches C 1 -C 5  may be effectuated, for example, by solenoid controlled spool valves responsive to PWM and binary control signals. Trim valves may be employed using variable bleed solenoids to provide precise placement of the spool within the valve body and correspondingly precise control of clutch pressure during apply. Similarly, one or more line pressure regulators (not shown) may be utilized for establishing regulated line pressure in accordance with the PWM signal. Clutch slip speeds across clutches may be derived from, for example: transmission input speed, output speed, motor A speed, and/or motor B speed. 
     The multi-mode, electrically-variable, hybrid transmission  14  is configured for several transmission operating modes. The truth table provided in  FIG. 2  presents an exemplary engagement schedule (also referred to as a shifting schedule) of the torque-transmitting mechanisms C 1 -C 4  to achieve the array of operating states or modes. The various transmission operating modes described in the table indicate which of the specific clutches C 1 -C 4  are engaged (actuated), and which are released (deactivated) for each of the operating modes. 
     In general, ratio changes in transmission  14  may be performed such that torque disturbances are minimized, and the shifts are smooth and unobjectionable to the vehicle occupants. Additionally, release and application of clutches C 1 -C 4  should be performed in a manner which consumes the least amount of energy, and does not negatively impact durability of the clutches. One major factor affecting these considerations is the torque at the clutch being controlled, which may vary significantly in accordance with such performance demands as acceleration and vehicle loading. Improved shifts may be accomplished by a zero, or close to zero, torque condition at the clutches at the time of application or release, which condition follows substantially zero slip across the clutch. Clutches having zero slip across the clutch may be referred to as operating synchronously. 
     Electrically-variable operating modes may be separated into four general classes: input-split modes, output-split modes, compound-split modes, and series modes. In an input-split mode, one motor/generator (such as either motor A or motor B) is geared such that its speed varies in direct proportion to the transmission output, and another motor/generator (such as the other of motor A or motor B) is geared such that its speed is a linear combination of the input and output member speeds. In an output-split mode, one motor/generator is geared such that its speed varies in direct proportion to the transmission input member, and the other motor/generator is geared such that its speed is a linear combination of the input member and the output member speeds. A compound-split mode, however, has both motor/generators geared such that their speeds are linear combinations of the input and output member speeds, but neither is in direct proportion to either the speed of the input member or the speed of the output member. 
     Finally, when operating in a series mode, one motor/generator is geared such that its speed varies in direct proportion to the speed of the transmission input member, and another motor/generator is geared such that its speed varies in direct proportion to the speed of the transmission output member. When operating in series mode, there is no direct mechanical power transmission path between the input and output members and therefore all power must be transmitted electrically. 
     In each of the four general types of electrically-variable operating modes indicated above, the speeds of the motors are linear combinations of the input and output speeds. Thus, these modes have two speed degrees of freedom (which may be abbreviated for simplicity as “DOF”). Mathematically, the torque (T) and speed (N) equations of this class of modes take the form: 
               [           T   a               T   b           ]     =           [           a     1   ,   1             a     1   ,   2                 a     2   ,   1             a     2   ,   2             ]     ⁡     [           T   i               T   o           ]       ⁢           ⁢     and   ⁢           [           N   a               N     b   ⁢                     ]       =       [           b     1   ,   1             b     1   ,   2                 b     2   ,   1             b     2   ,   2             ]     ⁡     [           N   i               N   o           ]               
where a and b are coefficients determined by the transmission gearing. The type of EVT mode can be determined from the structure of the matrix of b coefficients. That is, if b 2,1 =b 1,2 =0 or b 1,1 =b 2,2 =0, the mode is a series mode. If b 1,1 =0 or b 1,2 =0, the mode is an input split mode. If b 2,1 =0 or b 2,2 =0, the mode is an output split mode. If each of b 1,1 , b 1,2 , b 2,1 , and b 2,2  are nonzero, for example, the mode is a compound split mode.
 
     An electrically-variable transmission may also contain one or more fixed-gear (FG) modes. In general, FG modes result from closing (i.e., actuating) one additional clutch than the number required to select an electrically-variable mode. In FG modes, the speed of the input and each motor are proportional to the speed of the output. Thus, these modes have only one speed degree of freedom. Mathematically, the torque and speed equations of this class of modes take the form: 
               [     T   b     ]     =           [           a     1   ,   1             a     1   ,   2             a     1   ,   3             ]     ⁡     [           T   a               T   i               T   o           ]       ⁢           ⁢     and   ⁢           [           N   a               N   b               N   i           ]       ⁢           =       [           b     1   ,   1             b     1   ,   2             b     1   ,   3             ]     ⁡     [     N   o     ]               
where a and b are again coefficients determined by the transmission gearing. If b 1,1  is nonzero, motor A can contribute to output torque during operation in the fixed-gear mode. If b 1,2  is nonzero, motor B can contribute to output torque during operation in the fixed-gear mode. If b 1,3  is nonzero, the engine can contribute to output torque during operation in the fixed-gear mode. If b 1,3  is zero, the mode is an electric-only fixed-gear mode.
 
     An electrically-variable transmission may also be configured for one or more modes with three speed degrees of freedom. These modes may or may not include reaction torque sources such that the transmission is capable of producing output torque proportional to engine torque or motor torque. If a mode with three speed degrees of freedom is capable of producing output torque, the torques of the engine and any motor connected as a reaction to the engine torque will generally be proportional to the output torque. If a motor is not connected as a reaction to the engine torque, its torque can be commanded to control its speed independently of the transmission input and output speed. 
     In a mode with three speed degrees of freedom, it is generally not possible to easily control battery power independently of output torque. This type of mode produces an output torque which is proportional to each of the reacting torque sources in the system. The fraction of total output power provided by each of the three torque sources may be adjusted by varying the speeds of the motors and input. These modes are hereafter referred to as electric torque converter (ETC) modes in recognition of the fact that power flows to or from the energy storage device as a function of the output torque and the speed of the engine, output, and one of the motors. Mathematically, the torque and speed equations of this class of modes take the form: 
               [           T   a               T   b               T   i           ]     =           [           a     1   ,   1             a     1   ,   2             a     1   ,   3             ]     ⁡     [     T   o     ]       ⁢           ⁢     and   ⁢           [     N   b     ]       =       [           b     1   ,   1             b     1   ,   2             b     1   ,   3             ]     ⁡     [           N   a               N   i               N   o           ]               
where a and b are coefficients determined by the transmission gearing. If a 1,1  is nonzero, motor A serves as a reaction member and its torque is proportional to output torque when operating in the ETC mode. If a 1,1  is zero, motor A is disconnected and its torque is not determined by the output torque. If a 1,2  is nonzero, motor B serves as a reaction member and its torque is proportional to output torque when operating in the ETC mode. If a 1,2  is zero, motor B is disconnected and its torque is not determined by the output torque. If a 1,3  is nonzero, the engine can contribute to output torque during operation in the fixed-gear mode. If a 1,3  is zero, the input is disconnected and its torque is not determined by the output torque. If all of a 1,1 , a 1,2 , and a 1,3  are zero, the mode is a neutral mode that is not capable of producing output torque.
 
     There are four neutral modes presented in  FIG. 2 . In Neutral  1 , all clutches are released. Neutral  1  may be utilized when the entire vehicle is stopped and in an off-state, and thus there is no power distribution, electrical, mechanical, or otherwise, being actively distributed throughout the powertrain  10 . In Neutral  1 , a 12-volt starting-lighting-and-ignition (SLI) battery may be used for engine start. 
     In Neutral  2 , only clutch C 3  is engaged, and motor A and motor B may react engine  12  for start or to charge the energy storage device. Similar to Neutral  2 , when transmission  14  is in Neutral  3 , motor A and motor B may react engine  12  for start or to charge the energy storage device, and clutch C 4  as the only engaged torque-transmitting device. In Neutral  4 , the third and fourth clutches C 3 , C 4  are both in an activated state. In this instance, motor A is locked or “grounded”, and motor B is geared with the engine  12  for engine start. 
     The first and second planetary gear sets  24 ,  26  cooperate with the first and second motor/generators  56 ,  58 , along with the selective engagement of the first and second clutches C 1 , C 2 , to constitute an electric torque converter (ETC). For example, when the transmission  14  is operating in an ETC mode, the electric output of motor A and/or motor B, depending upon the active control schedule, can be adapted to control the transfer of torque from the engine  12  through the transmission differential gearing to the output member  20 . When the vehicle is started, ETC1 Mode is established by engaging the first clutch C 1 . In ETC1 Mode, motor A reacts engine  12  with the first and third planetary gear sets  24 ,  28 , and motor B freewheels. In ETC1 Mode, the stationary vehicle can be smoothly started with the engine  12  held at a suitable speed by gradually increasing the amount of electric power generated by motor A—i.e., the reaction force of motor A. 
     There are two other alternative ETC modes available utilizing the transmission configuration presented herein. ETC2 Mode, also known as “compound ETC”, can be initiated by engaging clutch C 2 , and disengaging the remaining clutches. In ETC2 Mode, motor A reacts engine  12  with the first and third planetary gear sets  24 ,  28 , while motor B reacts engine  12  and motor A to the output member  20 . The distribution of engine torque is manipulated through the cooperative management of the amount of electric power output generated by motor A and motor B. 
     The third ETC mode, ETC12 Mode, can be initiated by engaging both clutch C 1  and clutch C 2 . Similar to ETC1 Mode, motor A reacts the engine  12  with the first and third planetary gear sets  24 ,  28 . However, in this instance, motor B is grounded to the transmission housing  60 . In ETC12 Mode, the vehicle can be smoothly accelerated with the engine  12  held at a suitable speed by gradually increasing the reaction force generated by motor A; which may be proportional to the electric power generated by motor A. 
     When the engine  12  is in an off-state, the transmission  14  can utilize the ETC mode clutch control schedule to vary the amount of electric energy generated by motor A so as to gradually increase the drive torque of motor A and/or motor B. For example, if the transmission  14  is shifted into ETC1 Mode when the engine  12  is in an off-state, the engine  12  will create a reaction force, by way of input member  18 . The motive output of the motor A can then be controlled, and a continuous and uninterrupted transmission output torque maintained, without having to turn the engine  12  on. 
     The exemplary powertrain  10  described herein has three fixed-gear (FG), or “direct,” modes of operation. In all fixed-gear modes of this embodiment of transmission  14 , the vehicle is driven in the forward direction by operation of the engine  12 . The selective engagement of clutches C 1 , C 3  and C 4  shifts the transmission  14  into FG1 Mode. In FG1, motor A is grounded, and the engine drives the first planetary gear set  24  to the third planetary gear set  28  and, thus, the output member  20 . FG2 Mode is achieved by the selective engagement of clutches C 1 , C 2  and C 4 . In FG2, motor B is grounded, and the engine drives the first and second planetary gear sets  24 ,  26  to the third planetary gear set  28  and, thus, the output member  20 . Likewise, FG3 Mode is achieved by the selective engagement of clutches C 2 , C 3  and C 4 . In FG3, motor A is locked, and the engine drives the first planetary gear set  24  to the second and third planetary gear sets  26 ,  28  and the output member  20 . When operating in a fixed-gear mode of operation, the output member speed N o  is directly proportional to input member speed N i  and the selected gear ratio. N i =N o ×GR. 
     With continued reference to  FIG. 2 , the transmission  14  may also operate in four electrically-variable transmission (EVT) modes. In EVT1 and EVT4, the transmission  14  is operating in an input-split mode of operation, wherein the output speed N o  of the transmission  14  is proportional to the speed of one motor/generator  56 ,  58  (motor A or motor B). Specifically, EVT1 Mode is achieved through the selective engagement of the first and third clutches C 1  and C 3 . When in EVT1, motor A functions to react the engine  12  with the first planetary gear set  24 , to the third planetary gear set  28 , and the output member  20 ; while motor B drives the second and third planetary gear sets  26 ,  28 . Motor A propels the vehicle in EVT1. Alternatively, the transmission  14  may be selectively shifted into EVT4 Mode by actuating clutch C 2  and clutch C 3 . In EVT4, motor A functions to react the engine  12  with the first planetary gear set  24 , to the second and third planetary gear sets  26 ,  28 , and the output member  20 , while motor B drives the second and third planetary gear sets  26 ,  28 . Motor B propels the vehicle in EVT4. 
     In EVT2 and EVT3, the transmission  14  is operating in a compound-split mode, wherein the output speed N o  of the transmission  14  is not proportional to the speed of a single motor/generator, but is rather an algebraic linear combination of the speeds of both motor/generators. More particularly, EVT2 is achieved through the selective engagement of the first and fourth clutches C 1 , C 4 . In this mode, motor A and motor B operate to react the engine  12  with the first and second planetary gears sets. Alternatively, the transmission  14  may be selectively shifted into EVT3 Mode by actuating clutch C 2  and clutch C 4 . When operating in EVT3 Mode, the two motor/generator assemblies  56 ,  58  react the engine  12  with all three planetary gear sets  24 ,  26 ,  28 . 
     With reference to  FIG. 3 , a plot of transmission output speed, N o , along the horizontal axis versus input speed, N i , across the vertical axis is illustrated.  FIG. 3  is only a graphical representation of exemplary regions of operation for each operating mode with respect to input and output speeds of this embodiment of transmission  14 . 
     Synchronous operation in FG1—the input speed and output speed relationships where clutches C 1 , C 3  and C 4  are operating with substantially zero slip speed thereacross—is represented by line  91 . As such, line  91  represents an input and output speed relationship at which substantially synchronous shifting between EVT modes can occur. FG1 is also a range at which direct mechanical coupling from input to output can be effected by simultaneous application of clutches C 1 , C 3  and C 4 —i.e., fixed- or direct-ratio. 
     Synchronous operation in FG2—the input speed and output speed relationships where clutches C 1 , C 2  and C 4  are operating with substantially zero slip speed thereacross—is represented by line  93 . Similarly, the relationships between input and output speed during operation in FG3, whereat clutches C 2 , C 3  and C 4  are operating simultaneously with substantially zero slip speed there across, is represented by line  95 . 
     To the left of the shift ratio line  91  is an exemplary region of operation for the first EVT mode, EVT1, wherein both C 1  and C 3  are applied, and C 2  and C 4  are released. To the right of the shift ratio line  91  and left of shift ratio line  93  is an exemplary region of operation for the second EVT mode, EVT2, wherein C 1  and C 4  are applied, and C 2  and C 3  are released. 
     To the right of shift line  93  and left of shift ratio line  95  is an exemplary region of operation for the third EVT mode, EVT3, wherein both C 2  and C 4  are applied, and C 1  and C 3  are released. To the right of the shift ratio line  95  is an exemplary region of operation for the fourth EVT mode, EVT4, wherein C 2  and C 3  are applied, and C 1  and C 4  are released. As used herein with respect to clutches C 1 -C 5 , the terms “applied” or “actuated” indicate substantial torque transfer capacity across the respective clutch. Antithetically, the terms “released” or “deactivated” indicate insubstantial or no torque transfer capacity across the respective clutch. 
     While the regions of operation specified above may be generally favored for operation of the hybrid transmission  14 , it is not meant to imply that the various EVT regions of operation depicted in  FIG. 3  cannot or do not overlap. Generally, however, it may be preferred to operate in the specified regions because each particular mode of operation preferably employs gear sets and motor hardware particularly well suited in various aspects (e.g., mass, size, cost, inertial capabilities, etc.) for that region. Similarly, while the individual regions of operation specified above are generally preferred for the particular modes of operation indicated, it is not meant to imply that the regions of operation for the individual EVT modes cannot be switched. 
     Generally, a shift into Mode  1  may be considered a downshift and is associated with a higher gear ratio in accordance with the relationship of N i /N o . In contrast, a shift into Mode  4  is considered an upshift, and is associated with a lower gear ratio in accordance with the relationship of N i /N o . As discussed herein, other mode-to-mode shift sequences are feasible. For example, a shift from EVT1 to EVT3 is also an upshift, while a shift from EVT4 to EVT2 is considered a downshift. 
     Referring now to  FIG. 4 , and with continued reference to  FIGS. 1-3 , there is shown a schematic illustration of an exemplary pressure control system  100  for feeding transmission fluid to the hydraulic circuit of the transmission  14  shown in  FIG. 1 . The pressure control system  100  supplies transmission fluid to a hydraulic circuit, partially shown at  102 , which supplies transmission fluid for clutches C 1 -C 5 , and transmission lubrication and cooling systems. 
     The pressure control system  100  includes first and second fluid pumps: a main pump  104  and an auxiliary pump  106 . The main and auxiliary pumps  104  and  106  draw transmission fluid from a sump  108 , which may also operate as an exhaust for other portions of the hydraulic circuit. 
     The main pump  104 , which may also be referred to as an engine pump, derives power from the engine  12  through belt, gearing, or other driving connection to the engine output shaft. Therefore, the main pump  104  is not operational when engine  12  is not running at sufficient speeds to operate the main pump, and main pump  104  is completely off when engine  12  is turned off—such as when transmission  14  is operating on electric-only motive power. 
     Auxiliary pump  106  operates independently from the engine  12 , and may, therefore, operate while the engine  12  is off or running at very low speeds. The word “auxiliary” is often shortened to “aux” and is used interchangeably herein. Aux pump  106  is either self-powered (having a motor incorporated therein) or is powered by an electric motor (not shown). The electric motor may be dedicated solely to operation of the aux pump  106 , and draws power from the energy storage device associated with powertrain  10  or a dedicated battery (not shown). 
     Fluid from the main pump  104  flows into a main channel  110 , and fluid from the aux pump  106  flows into an auxiliary channel  112 . The channels  110 ,  112  converge at a control valve  116  before entering hydraulic circuit  102 . 
     The pressure of the transmission fluid passing through the channels  110 ,  112  from the main and aux pumps  104 ,  106  is controlled by first and second regulators in order to regulate the pressure of fluid entering the hydraulic circuit  102 . Generally, regulators are units which, with a pressure variable inlet pressure, give substantially constant output pressure provided that the inlet pressure remains higher than the required outlet pressure. 
     A main regulator  120  maintains substantially constant fluid pressure in the main channel  110 , as long as the pressure generated by the main pump  104  exceeds the control pressure of the main regulator  120 . Similarly, an auxiliary regulator  122  maintains substantially constant fluid pressure in the aux channel  112 , as long as the pressure generated by the aux pump  106  exceeds the control pressure of the aux regulator  122 . Structure and processes for controlling the main and aux regulators  120 ,  122  are described in more detail herein. Depending upon the application-specific configuration chosen, both main and auxiliary regulators may be considered either the first regulator or the second regulator. 
     Fluid flow from the main and aux pumps  104 ,  106  enter hydraulic circuit  102  via the control valve  116 . In the embodiment shown in  FIG. 4 , control valve  116  selectively allows substantially exclusive fluid flow communication with hydraulic circuit  102  from only one of the channels  110 ,  112  at a time. 
     The control valve  116  may be an exclusive- or (‘XOR’) configuration. In an XOR embodiment, control valve  116  is mechanized to control flow of hydraulic fluid from the aux pump  106  and the main pump  104  by permitting flow of pressurized fluid into the hydraulic circuit  102  of the transmission  14  substantially exclusively from either the aux pump  106  or the main pump  104 , depending upon operating conditions of pressure and flow from each of the pumps. For short periods of time, control valve  116  may allow fluid flow to the hydraulic circuit  102  from both of channels  110 ,  112 ; or may allow small amounts of fluid from one channel while the other is selected or placed in control; but under most operating conditions, fluid flow will be exclusive from one or the other of the channels  110 ,  112 . 
       FIG. 4  schematically shows the control valve  116  in position to allow fluid flow communication between the hydraulic circuit  102  and the aux pump  106 , while substantially blocking flow from the main pump  104  through channel  110 . This indicates that  FIG. 4  represents an operating mode in which the engine  12  is not powering the main pump  104 . The conditions shown schematically in  FIG. 4  may be indicative of an EVT mode, or other mode in which the vehicle is being propelled solely by tractive power from motor A or B ( 56 ,  58 ). 
     Main and aux regulators  120 ,  122  control the pressure in channels  110  and  112 , respectively, in accordance with a control signal. The control signal is a hydraulic fluid pressure signal sent from a single pressure control solenoid (PCS)  124  through signal channels  126 . Each of the regulators  120 ,  122  varies its maximum pressure as a function of the pressure in signal channels  126 . PCS  124  is controlled by an electronic communication from a controller  128 , determined by ECU  80  or another part of the hybrid control system based upon operating conditions of the powertrain  10 . 
     Through this control structure, a single control device (PCS  124 ) receiving a single electronic signal can control two (or more) regulators. Furthermore, this one pressure signal can result in different pressure value outputs by changing the response functions of the individual regulators  120 ,  122 . 
     Referring now to  FIG. 5 , and with continued reference to  FIGS. 1-4 , there is shown an exemplary graph  200  of two possible control functions for the regulated output pressure of the main and aux regulators  120 ,  122  as a function of PCS  124  control pressure communicated through signal channels  126 . The exact functions shown in  FIG. 5  exemplary, and are meant only to demonstrate two possible control functions resulting in different values for the main and aux regulators  120 ,  122 . 
     On the x-axis (horizontal) of graph  200  is the value of the pressure signal being communicated by the signal channels  126  from PCS  124 , show in kilopascal (kPa). The y-axis (vertical) is the pressure of the main or aux regulators  120 ,  122 , shown in kPa. Line  202  traces the pressure of the main regulator  120  as a function of the pressure of PCS  124 . The function upon which line  202  is based is: Main(PCS)=2*PCS. Line  204  traces the pressure of aux regulator  122  as a function of the pressure of PCS  124 . The function upon which line  204  is based is: Aux(PCS)=PCS+150. 
     Note that across a majority of the operating region shown in graph  200 —specifically, any PCS ( 124 ) pressure greater than approximately 150 kPa—the main regulator  120  is set to a higher pressure than the aux regulator  122 . When PCS  124  is set to 350 kPa, aux regulator  122  has a resulting output of 500 kPa (shown as horizontal line  206 ); at the same PCS ( 124 ) pressure, however, main regulator  120  has a resulting output of 700 kPa (shown as horizontal line  208 ). 
     Referring now to  FIGS. 6A and 6B , and with continued reference to  FIGS. 1-5 , there are shown graphical representations of exemplary control processes for transitions between the main and auxiliary pumps  104 ,  106  shown in  FIG. 4 .  FIG. 6A  shows a transition from pressurizing the hydraulic circuit  102  exclusively with the main pump  104  to pressurizing exclusively with the aux pump  106 . Similarly,  FIG. 6B  shows a transition from pressurizing hydraulic circuit  102  exclusively with aux pump  106  to pressurizing exclusively with the main pump  104 . 
       FIG. 6A  shows a graph  300  plotting input speed, N i , as shown on line  302 , PCS command on line  306 , and hydraulic circuit pressure on line  304 , against time on the x-axis (horizontal). The transition begins at a start point  310  as the internal combustion engine  12  is commanded to begin a transition to an engine-off state, as shown by the decrease in input speed N i  and line  302 . The transition generally takes place between the start point  310  and an end point  312 , both marked with dashed vertical lines and coinciding to changes in input speed N i  as the engine  12  transitions from on to off, or the reverse. 
     Prior to start point  310 , the PCS  124  is operating in a low state, which may be approximately 250 kPa. Referring again to  FIG. 5 , while PCS  124  is set to the low position or state, the main regulator  120  has a pressure of approximately 500 kPa, and aux regulator  122  has a pressure of approximately 400 kPa. For illustrative purposes only, it may be assumed that transmission  14  requires approximately 500 kPa to properly operate clutches C 1 -C 5  and to cool components of the transmission  14 . Therefore, in order to maintain operation of powertrain  10 , the hybrid control system will attempt to maintain constant pressure in the hydraulic circuit  102  of at least 500 kPa. This is the target pressure of the hybrid control system. 
     At start point  310 , control valve  116  will place (or have already placed) channel  110 , and flow from the main regulator  120 , in substantially exclusive fluid flow communication with hydraulic circuit  102 . Control valve  116  selects flow from the main regulator  120  because the auxiliary regulator  122  is set to allow lower pressure (400 kPa versus 500 kPa from main regulator  120 ). Furthermore, the aux pump  106  may be in an off state while the engine  12  is running and the main pump  104  is capable of fully-pressurizing hydraulic circuit  102 . 
     As input speed N i  decreases, the ability of main pump  104  to maintain 500 kPa of pressure will also decrease, and eventually main pump  104  will be in a non-operating state. Therefore, the hybrid control system will command the aux pump  106  to transition to an auxiliary-on state at or before start point  310 , in order to take over control of the hydraulic circuit  102  immediately as the main pump  104  is no longer able to maintain the target pressure of 500 kPa. Referring again to  FIG. 5 , in order for the aux regulator  122  to provide 500 kPa, the PCS  124  must be set to a high position. For the function shown on line  204  of  FIG. 5 , the high position of the PCS  124  is 350 kPa. 
     Moving PCS  124  to the high position causes the main regulator  120  to increase its maximum allowable pressure in channel  110  to 700 kPa. Note that, referring to  FIG. 4 , signal channels  126  communicate the PCS control signal to both the main and aux regulators  120 ,  122 . Therefore, while PCS  124  is set to the high position in order to increase the maximum pressure from the aux regulator  122 , because there is only one PCS, the main regulator  120  pressure also increases. Maintaining both regulators ( 120 ,  122 ) at the elevated level—caused by the high PCS setting—causes an increase in the line pressure of hydraulic circuit  102 , shown as portion  304   a  in  FIG. 6A . 
     Main regulator  120  will continue to output 700 kPa until the input speed N i  decreases to a level that renders the main pump  104  incapable of maintaining 700 kPa of pressure. The aux pump  106  reaches full capacity (auxiliary-on) prior to the point at which the main pump  104  is incapable of maintaining the target pressure. 
     The pressure in channel  110  will then decrease to zero as input speed N i  decreases to zero. As the pressure in channel  110  decreases to below 500 kPa, the control valve  116  will handoff fluid flow communication with the hydraulic circuit  102  to channel  112  and aux regulator  122 . Main pump  104  will lose the ability to generate 500 kPa prior to input speed N i  falling to zero and the main pump  104  moving to a completely-inoperable state. For example, this may occur at an input speed of 200 rpm, shown in  FIG. 6A  as horizontal line  314 . At this point, the transition is complete and the line pressure of hydraulic circuit  102  will be maintained at the target pressure substantially exclusively by the aux pump  106 . 
       FIG. 6B  shows the transition from pressurizing hydraulic circuit  102  exclusively with aux pump  106  to pressurizing exclusively with the main pump  104 . This transition may occur after the transition shown in  FIG. 6A , or may be an independent transition. 
     A graph  350  shows input speed, N i , on line  352 , PCS command on line  356 , and hydraulic circuit ( 102 ) pressure on line  354 , plotted against time on the x-axis. The transition begins at a start point  360  as the internal combustion engine  12  is commanded to begin a transition from the engine-off state to an engine-on state, as shown by the increase in input speed N i . The transition generally takes place between the start point  360  and an end point  362 , both marked with dashed vertical lines and coinciding with changes in input speed N i  as engine  12  transitions from off to on. 
     Prior to start point  360 , while the transmission  14  is operating on electric-only power, PCS  124  is operating in the high state (approximately 350 kPa), placing the aux regulator  122  and main regulator  120  in respective high pressure settings. At start point  360 , control valve  116  will place (or have already placed) channel  112 , and flow from the aux regulator  122 , in substantially exclusive fluid flow communication with hydraulic circuit  102 , because the pressure from main pump  104  will be negligible. 
     As input speed N i  increases, the ability of main pump  104  to maintain 500 kPa of pressure will also increase, and eventually main pump  104  will be able to take over pressurization hydraulic circuit  102 . Because PCS  124  is in the high position, the main regulator  120  allows maximum pressure in channel  110  of 700 kPa. As input speed N i  increases above a minimum speed (represented by line  364 , which may be approximately 200 rpm), main pump  104  will create enough pressure to exceed the 500 kPa maximum allowed by the aux regulator  122 . 
     Flow handoff to the main pump  104  will occur as the control valve  116  registers that the pressure from main pump  104  in channel  110  exceeds the pressure from aux pump  106  in channel  112 . However, in order to maintain the target pressure in hydraulic circuit  102 , PCS  124  remains in the high position until the main pump  104  is fully operational. Because PCS  124  is still set to the high position, the continued increase in input speed Ni will continue to cause the line pressure in hydraulic circuit  102  to increase up to the maximum of 700 kPa allowed by the main regulator  120 . This pressure increase is shown as portion  354   a  of line  354  in  FIG. 6B . 
     Following complete flow handoff to the main pump  104 , the auxiliary pump  106  may be commanded to an auxiliary-off state. Furthermore, the PCS  124  may then move from the high position to the low position. As the signal channel  126  causes the main regulator  120  to move back to its low pressure setting, the line pressure in hydraulic circuit  102  returns to 500 kPa, as shown by the portion of line  354  to the right of end point  362 . 
     While the best modes and other modes for carrying out the present invention have been described in detail, those familiar with the art to which this invention pertains will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.

Technology Classification (CPC): 5