Abstract:
A hydraulic control system for distributing pressurized fluid to a multi-mode hybrid-type power transmission is provided, as well as a method for regulating the same. The hydraulic control system includes an engine-driven main pump in fluid communication with a main regulator valve, and an electrically-driven auxiliary pump in fluid communication with an auxiliary regulator valve. One pressure control solenoid provides feedback (boost) pressure to both regulator valves, and thereby modify output of the main and auxiliary pumps. A controller selectively modifies distribution of boost pressure to ensure a continuous and controllable feed of hydraulic pressure to the transmission during all vehicle operations. The method includes: determining if the vehicle is transitioning to engine auto-start or auto-stop; determining the line pressure requirements of the transmission under current operating conditions; starting the oncoming-pump; adjusting PCS command so started oncoming-pump pressure equals line pressure requirements; and shutting down off-going-pump.

Description:
CLAIM OF PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/039,904, filed on Mar. 27, 2008, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to motorized vehicle powertrains. More specifically, the present invention relates to hydraulic control systems for multi-mode hybrid-type power transmissions, and methods of operating the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    In general, motorized vehicles, such as the conventional automobile, include a powertrain that is comprised of an engine in power flow communication with a final drive system (e.g., rear differential and wheels) via a multi-speed power transmission. Hybrid type powertrains generally employ an internal combustion engine (ICE) and one or more motor/generator units that operate either individually or in concert to propel the vehicle—e.g., power output from the engine and motor/generators are transferred through planetary gearing in the multi-speed transmission to be transmitted to the vehicle&#39;s final drive. The primary function of the multi-speed power transmission is to regulate speed and torque to meet operator demands for vehicle speed and acceleration. 
         [0004]    To operate properly, the power transmission requires a supply of pressurized fluid, such as conventional transmission oil. The pressurized fluid may be used for such functions as cooling and lubrication. The lubricating and cooling capabilities of transmission oil systems greatly impact the reliability and durability of the transmission. Additionally, multi-speed power transmissions require pressurized fluid for controlled engagement and disengagement, on a desired schedule, of the various torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement. 
         [0005]    Transmissions are traditionally supplied with hydraulic fluid by a wet sump (i.e., internal reservoir) oil system, which is separate from the engine&#39;s oil system. The fluid is typically stored in a main reservoir or main sump volume where it is introduced to a pickup or inlet tube for communication to the hydraulic pump(s). The pump operates to pressurize the fluid for subsequent communication to the transmission. 
         [0006]    It is well known to utilize a fixed displacement (or “PF”, according to industry custom) pump in multi-speed transmissions. A PF pump can generate relatively instantaneous pressure and flow to a hydraulic circuit when the circuit is opened due to the positive displacement characteristic of PF type pumps. In addition to, or in lieu of a PF pump, it is also known to use a variable displacement (or “PV”, according to industry custom) pump to satisfy the hydraulic fluid needs of a multi-speed transmission. The PV pump produces a variable flow on demand. Thus, in standby conditions, PV pump systems do not circulate as much hydraulic fluid. 
         [0007]    One premise behind hybrid-type vehicles is that alternative power is available to propel the vehicle, minimizing reliance on the engine for power, thereby increasing fuel economy. Since hybrid-type vehicles can derive their power from sources other than the engine, engines in hybrid-type vehicles typically operate at lower speeds more often than their traditional counterparts, and can be turned off while the vehicle is propelled by the alternative power source(s). For example, electrically-variable transmissions alternatively rely on electric motors housed in the transmission to power the vehicle&#39;s driveline. 
         [0008]    Engines in hybrid-type vehicles are required to start and stop more often than engines in non-hybrid systems. When the engine in a hybrid-type vehicle is not operating (i.e., in a power-off state), hydraulic pumps which derive their power solely from the engine may become inoperable. As such, many hybrid powertrains include an electrically driven secondary or auxiliary pump that runs independent of the engine—e.g., powered by the vehicle drive lines or a battery, to provide hydraulic pressure during periods when the engine is shutdown. 
         [0009]    Packaging space in and around the powertrain in hybrid-type vehicles is normally scarce, often restricting use of a larger auxiliary pump motor. In addition to architectural limitations, installing a larger pump motor is not always possible due to mass, cost, and fuel economy constraints. As such, the motor of a transmission auxiliary pump may be so small that it may not be able to start reliably under certain conditions. However, a transmission auxiliary pump which fails during transition periods (e.g., transition to hybrid “engine-off driving mode”) can potentially result in slip in the transmission launching clutch, and may result in “engine-on” operation only. 
         [0010]    Due to size limitations, the auxiliary pump is generally limited in the pressure it can operate against. In most instances, the auxiliary pump operates at significantly less pressure than the engine-driven transmission pump. As such, the auxiliary pump may stall if it is forced to operate at excessive transmission pressures. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides an improved hydraulic control system for a multi-mode hybrid-type power transmission. In order to enhance the efficiency, reliability, and response time of the vehicle powertrain, the present invention also provides improved methods of operating the hydraulic control system. The methods of the present invention ensure continuous clutch pressure at specific controlled levels during all vehicle operations, including engine auto-start and auto-stop, and transitionary periods thereto. In doing so, the present invention protects against pressure drops during pump transitions and pressure handoffs that might otherwise cause a clutch slip or bump. This invention also protects the auxiliary pump from high transmission pressures by providing additional pressure exhaust paths and a boost accumulator valve. 
         [0012]    In accordance with a first embodiment of the present invention, a method of regulating a hydraulic control system operable to distribute pressurized fluid to a multi-mode hybrid-type power transmission is provided. The hybrid transmission is in power flow communication with (e.g., selectively drivingly connectable to) an engine and one or more motor assemblies. The hydraulic control system includes an engine-driven main pump in fluid communication with a main regulator valve, and an electrically-driven auxiliary pump in fluid communication with an auxiliary regulator valve. 
         [0013]    The method includes: determining if the engine is transitioning to an engine auto-stop; if so, determining the line pressure requirements of the transmission under current operating conditions; starting the auxiliary pump; modifying the boost pressure being delivered to both the auxiliary and main regulator valves via a pressure control solenoid that is in fluid communication with both the auxiliary and main regulator valves such that the current pressure output of the auxiliary pump is adjusted to equal the current line pressure requirements of the transmission; and stopping the main pump. 
         [0014]    According to one aspect of this embodiment, the method also includes determining the start-up pressure of the auxiliary pump prior to modifying the boost pressure being delivered to the regulator valves. The method may also include determining the boost pressure based, at least in part, upon the current line pressure requirement prior to starting the auxiliary pump. In this instance, determining the start-up pressure of the auxiliary pump is based, at least in part, upon the boost pressure prior to starting the aux pump. 
         [0015]    In accordance with another aspect, stopping the main pump when the engine is transitioning to engine auto-stop is in response to the current auxiliary pump pressure being equal to the current line pressure requirements of the transmission. 
         [0016]    According to yet another aspect of this embodiment, the method also includes modifying the auxiliary pump speed prior to stopping the main pump. 
         [0017]    It is further desired that the method includes: determining if the engine is transitioning to an engine auto-start; determining the current line pressure requirements of the transmission if the engine is transitioning to an engine auto-start; starting the main pump; modifying the current boost pressure being delivered to the auxiliary and main regulator valves via the pressure control solenoid such that the current main pump pressure is adjusted to equal the current line pressure requirements of the transmission; and stopping the auxiliary pump. 
         [0018]    In this instance, it is further preferred that the method also includes determining the start-up pressure of the main pump prior to modifying the feedback pressure. The method may also include determining the boost pressure based, at least in part, upon the current line pressure requirements prior to starting the main pump. In this instance, determining the start-up pressure of the main pump is based, at least in part, upon the boost pressure prior to starting the main pump. In addition, stopping the auxiliary pump when the engine is transitioning to engine auto-start is preferably in response to the current main pump pressure being equal to the current line pressure requirements of the transmission. Finally, the method may also include modifying the main pump speed prior to stopping the auxiliary pump. 
         [0019]    In a second embodiment of the present invention, a method of regulating a hydraulic control system for a multi-mode, hybrid-type power transmission is provided. The transmission is in power flow communication with an engine and at least one motor. The hydraulic control system includes an engine-driven main pump in fluid communication with a main regulator valve, and an electrically-driven auxiliary pump in fluid communication with an auxiliary regulator valve. 
         [0020]    The method of this embodiment includes: determining if the engine is transitioning to either an engine auto-stop or an engine auto-start; if so, determining the current line pressure requirements of the transmission; starting either the auxiliary pump if the engine is transitioning to an engine auto-stop or the main pump if the engine is transitioning to an engine auto-start; modifying the current boost pressure being delivered to both the auxiliary and main regulator valves via a pressure control solenoid that is in fluid communication with both the auxiliary and main regulator valves such that the current pressure of the started pump is adjusted to equal the current line pressure requirements; and stopping the other of the pumps (e.g., the pump already in an on-state). 
         [0021]    In accordance with one aspect of this embodiment, the pressure control solenoid is in direct fluid communication with both the auxiliary regulator valve and the main regulator valve. 
         [0022]    In accordance with yet another embodiment of the present invention, a hydraulic control system for regulating the distribution of pressurized fluid to a hybrid transmission is provided. The transmission is in power flow communication with an engine and one or more motor assemblies. The transmission has a power source, such as a battery or motor/generator assembly, and one or more hydraulic fluid reservoirs. The transmission also has a current line pressure requirement which is dependent upon, for example, current vehicle operating conditions and operator demands. 
         [0023]    The hydraulic control system includes a main pump in fluid communication with one of the hydraulic fluid reservoirs, and in driving communication with the engine. The main pump is selectively operable to provide a first flow of pressurized hydraulic fluid to the transmission. The hydraulic control system also includes an auxiliary pump in fluid communication with one of the hydraulic fluid reservoirs, and in driving communication with the power source. The auxiliary pump is selectively operable to provide a second flow of pressurized hydraulic fluid to the transmission. A main regulator valve is in direct fluid communication with the main pump, and configured to regulate the flow of pressurized hydraulic fluid therefrom. Similarly, an auxiliary regulator valve is in direct fluid communication with the auxiliary pump, and configured to regulate the flow of pressurized hydraulic fluid therefrom. 
         [0024]    A single pressure control solenoid is in direct fluid communication with both the main regulator valve and the auxiliary regulator valve. The pressure control solenoid is configured to simultaneously provide a boost pressure to both regulator valves, and thereby boost output of the main pump and auxiliary pump. A controller is in operative communication with the pressure control solenoid, and operable to control the same. The controller is configured to selectively modify distribution of the boost pressure to the regulator valves such that at least one of the first and second flows of pressurized fluid is equal to the current line pressure requirements of the transmission during engine auto-start and auto-stop, and transitions thereto. 
         [0025]    According to one aspect of this embodiment, the pressure control solenoid is in direct fluid communication with both the main regulator valve and the auxiliary regulator valve. 
         [0026]    In accordance with another aspect, the hydraulic control system includes a boost accumulator valve that is in direct fluid communication with the auxiliary regulator valve. The boost accumulator valve operates to damp fluid pressure fluctuations generated by the pressure control solenoid and movement of the auxiliary regulator valve. 
         [0027]    According to yet another aspect, the auxiliary regulator valve includes an exhaust port. In this particular instance, the auxiliary regulator valve is configured to exhaust hydraulic fluid through the exhaust port if the flow of pressurized hydraulic fluid from the auxiliary pump exceeds a maximum regulated auxiliary pump pressure. 
         [0028]    In accordance with yet another aspect, the hydraulic control system also includes a transmission oil cooler system (TOC). The TOC is in fluid communication with both the main regulator valve and the auxiliary regulator valve. A cooler relief valve is placed in between the main regulator valve and TOC, and is operable to restrict the flow of pressurized fluid from the main regulator valve—i.e., the main pump, therethrough. In this instance, the auxiliary regulator valve is preferably in direct fluid communication with the cooler relief valve. A thermal bypass valve may also be placed intermediate the transmission oil cooler system and both the main and auxiliary regulator valves. The thermal bypass valve is operable to redirect fluid flow past the transmission oil cooler system when the fluid temperature is below a predetermined threshold value. 
         [0029]    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 best modes for carrying out the invention when taken in connection with the accompanying drawings and appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0030]      FIG. 1  is a schematic illustration of a vehicle powertrain having a multi-mode, multi-speed, hybrid-type power transmission with a hydraulic control system in accordance with the present invention; 
           [0031]      FIG. 2  is a graphical representation of the boosted pressure output of the main pump and auxiliary pump of  FIG. 1  utilizing a single, shared pressure control solenoid; and 
           [0032]      FIGS. 3A and 3B  provide a flow chart illustrating a method of regulating a hydraulic control system in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    Referring to the drawings, wherein like reference numbers represent the same or corresponding parts throughout the several views, there is shown schematically in  FIG. 1  a hydraulic control system, identified generally as  10 , for providing lubrication and cooling fluid to various components of a power transmission  12 , as well as pressurized fluid for controlled engagement and disengagement of the various torque transmitting mechanisms that operate to establish the forward and reverse speed ratios within the transmission  12 . The hydraulic control system  10 , although described herein for supplying hydraulic fluid to a multi-mode hybrid-type power transmission  12  of an automobile, may also be applied in other various applications, such as, by way of example, aeronautical vehicles (e.g., airplanes, helicopters, etc.), agricultural vehicles (e.g., combine, tractor, etc.), construction vehicles (e.g., forklift, backhoe, excavator, etc.), and stationary machines (e.g., hydraulic press, hydraulic drill, etc.). 
         [0034]    A restartable engine  14  is selectively drivingly connected to, or in power flow communication with, a final drive system  16  via the hybrid-type power transmission  12 . The engine  14  transfers power, preferably by way of torque, to the transmission  12  via an engine output shaft  18  (most commonly referred to as a “crankshaft”). The transmission  12  is adapted to manipulate and distribute power from the engine  14  to the final drive system  16 , which is represented herein by a rear differential  15  and wheels  17 . Specifically, the rear differential  15  is configured to distribute power and torque from a transmission output shaft  20  to drive the plurality of wheels  17  and propel the hybrid vehicle (not specifically identified herein). In the embodiment depicted in  FIG. 1 , the engine  14  may be any engine, such as, but not limited to, a two-stroke diesel engine or a four-stroke gasoline engine, which is readily adapted to provide its available power output typically at a number of revolutions per minute (RPM). Although not specifically illustrated in  FIG. 1 , it should be appreciated that the final drive system  16  may comprise any known configuration—e.g., front wheel drive (FWD), rear wheel drive (RWD), four-wheel drive (4WD), or all-wheel drive (AWD). 
         [0035]    First and second electric motor/generator assemblies A and B, respectively, are concentric with and connectable to a main shaft (not shown) of the transmission  12 , preferably through a series of planetary gear sets (not shown), which operate in concert with one or more selectively engageable torque transmitting mechanisms (e.g., clutches, brakes, etc.) to rotate the transmission output shaft  20 . The motor/generator assemblies A, B are preferably configured to selectively operate as a motor and a generator. That is, the motor/generator assemblies A, B are capable of converting electrical energy to mechanical energy (e.g., during vehicle propulsion), and converting mechanical energy to electrical energy (e.g., during regenerative braking). 
         [0036]    The hydraulic control system  10  includes a first, main pump  22  (which is also referred to herein as “engine pump” or “engine-driven pump”) and a second, auxiliary pump  24  (which is also referred to herein as “aux pump” or “electrically-driven pump”). Specifically, the vehicle engine  14  is operatively connected to the main pump  22  to communicate a driving force (i.e., power) thereto. In a similar respect, the hydraulic control system  10  also includes a power source  26 , operable to communicate a driving force (i.e., power) to the auxiliary pump  24 . The power source  26  may comprise any one of various devices operable to provide electrical energy storage capacity and distribution, such as, but not limited to, a battery, fuel cell, capacitor, fly wheel, and the like. It should also be recognized that  FIG. 1  is merely a schematic representation and, thus, alternate means of driving each pump are available. 
         [0037]    The main pump  22  is preferably of the variable displacement (PV) pump type. The main pump  22  is selectively operable to provide a first flow of pressurized hydraulic fluid (represented for explanatory purposes by arrow F 1 ) to the transmission  12  at various volumes and pressures. The auxiliary pump  24  is preferably a fixed displacement (PF) pump of the positive displacement type. The auxiliary pump  24  is selectively operable to provide a second flow of pressurized hydraulic fluid (represented for illustrative purposes by arrow F 2 ) to the transmission  12 . Although not required, the auxiliary pump  24  may be a high-voltage, electric-motor driven 10-tooth gerotor pump. It is also considered to be within the scope of the present invention that both pumps  22 ,  24  be PV pumps, PF pumps, or any combination thereof. 
         [0038]    First and second sump volumes  26 A and  26 B, respectively (which may, in reality, consist solely of a single oil pan), are configured to stow or store hydraulic fluid, such as transmission oil  28 , for distribution to the transmission  12  and its various components. The main pump  22  is fluidly connected to the first (or main) sump volume  26 A to draw transmission oil  28  therefrom. The auxiliary pump  24  is fluidly connected to the second (or auxiliary) sump volume  26 B to draw transmission oil  28  therefrom. 
         [0039]    A distributed control system, which may include, but is not limited to, an engine control module (ECM), a transmission control module (TCM), and an energy storage control module (ESCM), is depicted collectively in  FIG. 1  in an exemplary embodiment as a single micro-processor based electronic control unit (ECU)  30 . The ECU  30  (also referred to herein as “controller”) has a suitable amount of programmable memory that is programmed to include, among other things, an algorithm or method  100  of regulating a hydraulic control system, as will be discussed in further detail below with respect to  FIGS. 3A and 3B . The ECU  30  is in operative communication with the main pump  22 , the auxiliary pump  24 , and a pressure control solenoid  42 . The ECU  30  is preferably programmed and configured, in part, to control the individual and cooperative operation of the hydraulic control system  10 , transmission  12 , and engine  14 . Those skilled in the art will recognize and understand that the means of communication utilized by the controller  30  is not restricted to the use of electric cables (“by wire”), but may be, for example, by radio frequency and other wireless technology, fiber optic cabling, etc. 
         [0040]    A main regulator valve, indicated generally at  32  in  FIG. 1 , is in direct fluid communication with the main pump  22  via first hydraulic conduit or circuit  34 . The main regulator valve  32 , which is in the nature of a spring-biased, multi-port spool valve assembly, is configured to regulate the flow of pressurized hydraulic fluid from the main pump  22  (e.g., the first flow of pressurized hydraulic fluid F 1 ). A decrease circuit  40  also fluidly connects the main regulator valve  32  directly to the main pump  22 . The decrease circuit  40  acts as a “feedback circuit” for the main pump  22 , essentially redirecting hydraulic fluid  28  from the main regulator valve  32  back to the main pump  22  to reduce output from the main pump  22  under predetermined circumstances (e.g., when pressure output from the main pump  22  exceeds a threshold value). 
         [0041]    An auxiliary regulator valve, indicated generally at  36 , is in direct fluid communication with the auxiliary pump  24  via second hydraulic conduit  38 . The auxiliary regulator valve  36 , which is also preferably in the nature of a spring-biased, multi-port spool valve assembly, is configured to regulate the flow of pressurized hydraulic fluid from the auxiliary pump  24  (i.e., the second flow of pressurized hydraulic fluid F 2 ). The auxiliary regulator valve  36  preferably includes an exhaust port  37 . As will be described in further detail hereinbelow, the auxiliary regulator valve  36  is configured to exhaust (i.e., evacuate) hydraulic fluid through the exhaust port  37  if the flow of pressurized hydraulic fluid from the auxiliary pump  24  exceeds a maximum regulated auxiliary pump pressure, thereby preventing the aux pump  24  from stalling. 
         [0042]    A pressure control solenoid  42  (referred to hereinafter as “PCS”) is in direct fluid communication with both the main regulator valve  32  and the auxiliary regulator valve  36  via third hydraulic conduit  44  (which also places the auxiliary regulator valve  36  in direct fluid communication with the main regulator valve  32 ). The PCS  42  is designed or adapted to provide a boost pressure to both the main regulator valve  32  and auxiliary regulator valve  36 , and thereby boost output of each pump assembly. In addition, the PCS  42  is manipulated in accordance with the present invention to selectively modify (i.e., control modulation of) the main regulator valve  32  and the auxiliary regulator valve  36  such that at least one of the flows of pressurized fluid output from the main pump  22  and auxiliary pump  24 , and distributed to the transmission  12 , is equal to the current line pressure requirements of the transmission  12  during all vehicle operations, including when the engine  14  is transitioning to auto-start and auto-stop, as well as when the engine  14  is in auto-start and auto-stop (e.g., when the vehicle  10  is operating in either engine-on or engine-off mode). Specifically, the PCS  42 , which may be an open- or closed-type solenoid, is in electric signal communication with the controller  30 , and is actuated upon receipt of a control signal therefrom. When commanded, the PCS  42  delivers a boost pressure (illustrated in  FIG. 1  by arrow B for illustrative purposes) to both the main and auxiliary regulator valves  32 ,  36 . The boost pressure B acts to bias both regulator valves  32 ,  36 , in a substantially simultaneously manner, increasing the pressure of fluid output therefrom and distributed to the transmission  12 . 
         [0043]    According to a preferred embodiment of the present invention, the main pump  22  has a non-boosted line pressure of approximately 300 kilopascals (kPa) and the main regulator valve  32  has a boost gain of approximately 2.05. Consequently, as can be seen in the graphical illustration of the boosted pressure output of the main pump  22  of  FIG. 2  (shown with solid line), the main pump  22  pressure regulation and boost function operates according to the following equation: MainLinePress=2.05*PCS+300. Accordingly, the flow of pressurized hydraulic fluid from the main pump  22  preferably has a maximum line pressure of approximately 1800-2000 kPa. In a similar regard, the auxiliary pump  24  preferably also has a non-boosted line pressure of approximately 300 kPa, whereas the auxiliary regulator valve  36  has a boost gain of approximately 1.24. Accordingly, the auxiliary pump  24  pressure regulation and boost function (shown with a dashed line in  FIG. 2 ) operates according to the following equation: AuxLinePress=1.24*PCS+300. Thus, the flow of pressurized hydraulic fluid from the auxiliary pump  24  preferably has a maximum line pressure of approximately 1200-1300 kPa. 
         [0044]    A boost accumulator valve  46  is in direct fluid communication with the auxiliary regulator valve  36  via fourth hydraulic conduit  48 . The boost accumulator valve  46  operates to damp fluid pressure fluctuations generated by the PCS  42  and movement of the auxiliary regulator valve  36  to protect the auxiliary pump, for example, from inadvertent pressure spikes which may cause the aux pump  24  to stall. Put another way, the boost accumulator valve  46  is positioned to accumulate control pressure fluid provided to the auxiliary regulator valve  36  from the PCS  42  through fourth hydraulic conduit  48 . 
         [0045]    With continuing reference to  FIG. 1 , the hydraulic control system  10  also includes a transmission oil cooler system (TOC) of known composition, which is represented schematically by ellipsoid  54 . The TOC  54  is in fluid communication with the auxiliary regulator valve  36  via fifth hydraulic conduit  56 . The main regulator valve  32  is fluidly communicated with the TOC  54  via fifth and sixth hydraulic conduits  56  and  58 , respectively. A cooler relief valve  60  is placed in between the TOC  54  and main regulator valve  32 . The auxiliary regulator valve  36  is also in direct fluid communication with the cooler relief valve  60  via fifth hydraulic conduit  56 . The cooler relief valve  60  is operable to restrict fluid flow from the main regulator valve  32  passing therethrough. In accordance with the embodiment of  FIG. 1 , the cooler relief valve  60  is adapted to exhaust fluid through an exhaust port  62  if the flow of pressurized hydraulic fluid from the main pump  22  exceeds a predetermined level. A thermal bypass valve  68  may be placed intermediate the TOC  54  and both the main and auxiliary regulator valves  32 ,  36 . The thermal bypass valve  68  is operable to redirect fluid flow past the TOC  54  under specified pressure and temperature conditions—e.g., when oil temperature is less than a predetermined level or cooler pressure drop is greater than a predetermined level. 
         [0046]    A shuttle-type ball check valve assembly, identified generally as  64  in  FIG. 1 , is shown in direct fluid communication with the main regulator valve  32  and aux regulator valve  36  via seventh and eighth hydraulic conduits  70  and  72 , respectively. The check valve  64  and regulator valves  32 ,  36  define, in part, a “shift valve system” that is configured to control engagement and disengagement of one or more torque transmitting devices, such as, but not limited to hydraulic clutches and brakes (not shown herein). The check valve assembly  64  determines whether pressurized fluid is supplied to the transmission  12  via ninth hydraulic conduit  74  from the seventh hydraulic conduit  70 , namely main pump  22 , or whether pressurized fluid is supplied to ninth hydraulic conduit  74  from the eighth hydraulic conduit  72 , namely aux pump  24 . 
         [0047]    With reference now to the flow charts in  FIGS. 3A and 3B , a method of regulating a hydraulic control system for a multi-mode, hybrid-type power transmission is shown generally as  100  in accordance with a preferred embodiment of the present invention. The method or algorithm  100  is described herein with respect to the structure illustrated in  FIG. 1 . However, the methods of the present invention may also be incorporated into other powertrain configurations, and applied to various other types of power transmissions. The method  100  preferably includes at least steps  101 - 131 . However, it is within the scope and spirit of the present invention to omit steps, include additional steps, and/or modify the order of steps presented in  FIGS. 3A and 3B . 
         [0048]    Looking to  FIG. 3A , the method  100  begins with determining if the engine is transitioning to an engine auto-stop, in step  101 , or whether the engine  14  is transitioning to an engine auto-start, in step  103 . Notably, steps  101  and  103 , and any corresponding subsequent steps, can be performed simultaneously, or in any order. If the engine  14  is transitioning to an auto-stop, step  105  then includes determining the line pressure requirements of the transmission  12  for the current operating conditions. The line pressure requirements are based, for example, on operator demands for torque and acceleration, as well as the range the transmission  12  is in at that particular time. The current line pressure requirements will be equal to the main pump  22  output pressure (i.e., MainLinePress) during engine-on operation. The current boost pressure CPCS from PCS  42  can thus be determined, for example in step  107 , from the above noted relationship between pump output and PCS output. CPCS=(MainLinePress−300)/2.05. 
         [0049]    In step  109 , the auxiliary pump  24  is started. The start-up pressure of the auxiliary pump (P_Aux_Resultant) can be determined contemporaneously therewith, for example in step  111 , by first establishing the current boost pressure CPCS (step  107 ), and then utilizing the pressure regulation and boost function equation for the auxiliary pump  24  highlighted hereinabove. P_Aux_Resultant=1.24*CPCS+300. Thereafter, the current auxiliary pump pressure is modified to equal the current line pressure requirements of the transmission  12 . Specifically, in step  115 , the boost pressure being distributed by PCS  42  is modified (e.g., increased) such that the second flow of hydraulic fluid F 2  from aux pump  24  is pressurized according to current system demands. Prior to, or contemporaneously therewith, the aux pump speed is adjusted accordingly, in step  113 . Once the output from aux pump  24  meets current system demands—i.e., when the current auxiliary pump pressure is equal to the current line pressure requirements, the main pump  22  is shut down or stopped in step  117 . If the current auxiliary pump pressure is not equal to the current line pressure requirements, the boost feedback pressure from PCS  42  is further modified (i.e., repeat step  115 ). 
         [0050]    If the engine  14  is transitioning to an auto-start, the method  100  then includes determining what the line pressure requirements of the transmission  12  are for the current operating conditions, as indicated in step  119  of  FIG. 3B . Unlike step  105  above, the current line pressure requirements in this instance will be equal to the aux pump  24  output pressure (i.e., AuxLinePress) during engine-off operation. The current boost pressure CPCS from pressure control solenoid  42  can thus be determined, in step  121 , from the above noted relationship between pump output and PCS output. CPCS=(AuxLinePress−300)/1.24. 
         [0051]    In step  123 , the engine pump  24  is started. The start-up pressure of the engine pump (P_Eng_Resultant) can be determined contemporaneously therewith, in step  125 , by first establishing the current boost pressure CPCS (step  121 ), and then utilizing the pressure regulation and boost function equation for the engine pump  22 . P_Eng_Resultant=2.05*CPCS+300. The active main pump pressure is thereafter modified to equal the current line pressure requirements. Specifically, in step  129 , the boost pressure being distributed by PCS  42  is modified (e.g., decreased) such that the first flow of hydraulic fluid F 1  output from main pump  22  is pressurized in accordance to current system demands. Once the output from main pump  22  meets current transmission needs—i.e., the current main pump pressure is equal to the current line pressure requirements, the aux pump  22  is shut down or stopped in step  131 . Prior to, or contemporaneously therewith, the main pump speed is adjusted accordingly, in step  117 . If the current main pump pressure is not equal to the current line pressure requirements, the boost feedback pressure from PCS  42  is further modified (i.e., repeat step  129 ). 
         [0052]    While the best modes for carrying out the present invention have been described in detail hereinabove, 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.