Abstract:
A system and method for controlling automatic stop-start of a motor vehicle is provided. The system and method is configured to enable an automatic stop-start mode of operation based on vehicle conditions. In addition, the system and method is configured to selectively actuate an accumulator to prime the transmission for a smooth restart.

Description:
FIELD 
       [0001]    The present disclosure relates to a system and method for controlling an automatic engine stop-start, and more particularly to a system and method for controlling an automatic engine stop-start using measured vehicle conditions and an accumulator. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0003]    A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to the plurality of torque transmitting devices within the transmission. The pressurized hydraulic fluid delivered to the torque transmitting devices is used to engage or disengage the devices in order to obtain different gear ratios. 
         [0004]    In order to increase the fuel economy of motor vehicles, it is desirable to stop the engine during certain circumstances, such as when stopped at a red light or idling. However, during this automatic stop, the pump is no longer driven by the engine. Accordingly, hydraulic fluid pressure within the hydraulic control system drops. This leads to clutches and/or brakes within the transmission to be fully disengaged. As the engine restarts, these clutches and/or brakes may take time to reengage fully, thereby producing slippage and delay between engagement of the accelerator pedal or release of the brake and the movement of the motor vehicle. Additionally, there are conditions where automatically stopping the engine is not desirable, such as during brief stops or idling while still moving. 
         [0005]    Therefore, there is a need in the art for a system and method for controlling automatic engine stop-starts based on motor vehicle operating conditions as well as providing controllability of the motor vehicle during engine restart. 
       SUMMARY 
       [0006]    A system and method for controlling automatic stop-start of a motor vehicle is provided. The system and method is configured to enable an automatic stop-start mode of operation based on vehicle conditions. In addition, the system and method is configured to selectively actuate an accumulator to prime the transmission for a smooth restart. 
         [0007]    In one example, the system and method uses engine speed, vehicle speed, transmission temperature, and engine temperature to determine whether an automatic stop should be activated. 
         [0008]    In another example, the system and method uses the state of the transmission to determine whether an automatic stop should be inhibited. 
         [0009]    In yet another example, the system and method controls the accumulator using engine status indicators. 
         [0010]    In yet another example, the system and method controls the accumulator using brake pedal position. 
         [0011]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0012]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0013]      FIG. 1  is a schematic diagram of an exemplary powertrain in a motor vehicle; and 
           [0014]      FIG. 2  is a schematic diagram of a portion of an exemplary hydraulic control system. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0016]    With reference to  FIG. 1 , a motor vehicle is shown and generally indicated by reference number  5 . The motor vehicle  5  is illustrated as a passenger car, but it should be appreciated that the motor vehicle  5  may be any type of vehicle, such as a truck, van, etc. The motor vehicle  5  includes an exemplary powertrain  10 . It should be appreciated at the outset that while a rear-wheel drive powertrain has been illustrated, the motor vehicle  5  may have a front-wheel drive powertrain without departing from the scope of the present invention. The powertrain  10  generally includes an engine  12  interconnected with a transmission  14 . 
         [0017]    The engine  12  may be a conventional internal combustion engine or an electric engine, or any other type of prime mover, without departing from the scope of the present disclosure. The engine  12  supplies a driving torque to the transmission  14  through a flexplate  15  or other connecting device that is connected to a starting device  16 . The starter device  16  may be a hydrodynamic device, such as a fluid coupling or torque converter, a wet dual clutch, or an electric motor. It should be appreciated that any starting device between the engine  12  and the transmission  14  may be employed. 
         [0018]    The transmission  14  includes a typically cast, metal housing  18  which encloses and protects the various components of the transmission  14 . The housing  18  includes a variety of apertures, passageways, shoulders and flanges which position and support these components. Generally speaking, the transmission  14  includes a transmission input shaft  20  and a transmission output shaft  22 . Disposed between the transmission input shaft  20  and the transmission output shaft  22  is a gear and clutch arrangement  24 . The transmission input shaft  20  is functionally interconnected with the engine  12  via the starting device  16  and receives input torque or power from the engine  12 . Accordingly, the transmission input shaft  20  may be a turbine shaft in the case where the starting device  16  is a hydrodynamic device, dual input shafts where the starting device  16  is dual clutch, or a drive shaft where the starting device  16  is an electric motor. The transmission output shaft  22  is preferably connected with a final drive unit  26  which includes, for example, propshaft  28 , differential assembly  30 , and drive axles  32  connected to wheels  33 . The transmission input shaft  20  is coupled to and provides drive torque to the gear and clutch arrangement  24 . 
         [0019]    The gear and clutch arrangement  24  includes a plurality of gear sets, a plurality of clutches and/or brakes, and a plurality of shafts. The plurality of gear sets may include individual intermeshing gears, such as planetary gear sets, that are connected to or selectively connectable to the plurality of shafts through the selective actuation of the plurality of clutches/brakes. The plurality of shafts may include layshafts or countershafts, sleeve and center shafts, reverse or idle shafts, or combinations thereof. The clutches/brakes, indicated schematically by reference number  34 , are selectively engageable to initiate at least one of a plurality of gear or speed ratios by selectively coupling individual gears within the plurality of gear sets to the plurality of shafts. It should be appreciated that the specific arrangement and number of the gear sets, clutches/brakes  34 , and shafts within the transmission  14  may vary without departing from the scope of the present disclosure. 
         [0020]    The motor vehicle  5  includes a control system  36 . The control system  36  may include a transmission control module, an engine control module, or a hybrid control module, or any other type of controller. The control system  36  may include one or more an electronic control devices having a preprogrammed digital computer or processor, control logic, memory used to store data, and at least one I/O peripheral. The control logic includes a plurality of logic routines for monitoring, manipulating, and generating data. The control module  36  controls the actuation of the clutches/brakes  34  via a hydraulic control system  38 . The hydraulic control system  38  is operable to selectively engage the clutches/brakes  34  by selectively communicating a hydraulic fluid to the clutches/brakes  34  that engages the clutches/brakes  34 . The control module  36  is also in communication with a plurality of sensors located throughout the motor vehicle  5 . For example, the control module  36  communicates with engine speed and temperature sensors  37 A and  37 B, a brake pedal position sensor  37 C, an ignition key sensor  37 D, a vehicle speed sensor  37 E, to name but a few. 
         [0021]    Turning to  FIG. 2 , a portion of the hydraulic control system  38  is illustrated. At the outset it should be appreciated that the portion of the hydraulic control system  38  shown in  FIG. 2  is exemplary and that other configurations may be employed. The hydraulic control system  38  is operable to selectively engage the clutches/brakes  34  by selectively communicating a hydraulic fluid  44  from a sump  46  to a clutch actuation circuit  48 . The clutch actuation circuit  48  includes clutch control solenoids, valves, and actuators operable to engage the plurality of clutches/brakes  34 . The hydraulic fluid  44  is communicated to the clutch actuation circuit  48  under pressure from either an engine driven pump  50  or an accumulator  52 . 
         [0022]    The sump  46  is a tank or reservoir to which the hydraulic fluid  44  returns and collects from various components and regions of the automatic transmission  14 . The hydraulic fluid  44  is forced from the sump  46  and communicated throughout the hydraulic control system  38  via the pump  50 . The pump  50  may be, for example, a gear pump, a vane pump, a gerotor pump, or any other positive displacement pump. The pump  50  includes an inlet port  54  and an outlet port  56 . The inlet port  54  communicates with the sump  46  via a suction line  58 . The outlet port  56  communicates pressurized hydraulic fluid  44  to a main line pressure circuit  60 . The main line pressure circuit  60  may include various optional features including, for example, a spring biased blow-off safety valve, a pressure side filter, or a spring biased check valve. 
         [0023]    The main line pressure circuit  60  communicates with the clutch actuation circuit  48  and a solenoid  76 . The solenoid  76  is in fluid communication with an accumulator supply line  77 . The solenoid  76  is electrically controlled by the control module  36  and is operable to control the charge state of the accumulator  52 . The solenoid  76  is preferably an on/off solenoid having a solenoid valve  76 A moveable between a first position and a second position. In the first position, the main line pressure circuit  60  is in fluid communication with a flow restricting orifice  76 B that limits the amount of hydraulic fluid  44  that can be bled off the main line pressure circuit  60  in order to prevent the clutch actuation circuit  48  from being starved of hydraulic fluid  44 . The flow restricting orifice  76 B communicates with a one way check ball or poppet valve  76 C. The check ball valve  76 C is configured to maintain pressure within the accumulator  52 . When the solenoid valve  76 A is energized and moved to the second position, the restriction orifice  76 B is positioned in parallel with a one way check or poppet valve  76 D. The check ball valve  76 D prevents fluid backflow into the accumulator  52 . 
         [0024]    The solenoid  76  communicates with the accumulator  52  and a pressure sensor  78 . The accumulator  52  is an energy storage device in which the non-compressible hydraulic fluid  44  is held under pressure by an external source. The accumulator  52  includes a piston that has a seal that slides along a bore of the accumulator housing. On one side of the piston there is hydraulic fluid  44  and on the other side of the piston there is one or more springs and air. The accumulator  52  uses a combination of spring(s) and air to generate the force on one side of the piston that reacts against the hydraulic fluid pressure on the opposite side of the piston. An example of an accumulator for use with the present invention is disclosed in commonly assigned U.S. patent application Ser. No. 12/635,587 filed Dec. 10, 2009, hereby incorporated by reference as if fully disclosed herein. The accumulator  52 , when charged, effectively replaces the pump  50  as the source of pressurized hydraulic fluid  44 , thereby eliminating the need for the pump  50  to run continuously. The pressure sensor  78  reads the pressure of the hydraulic fluid  44  within the supply line  77  in real time and provides this data to the control module  36 . Other types of sensors, such as volume or position sensors, may also be included. 
         [0025]    The control of flow in out of the accumulator  52  is performed through two different processes using the same solenoid  76 . When the pump  50  is on, hydraulic fluid  44  flows from the main line pressure circuit  60  into the de-energized solenoid  76 . Once the hydraulic fluid  44  passes through the orifice  76 B, the hydraulic fluid  44  unseats the check valve  76 C and flows into the accumulator  52 . Therefore, in order to charge the accumulator  52 , the pressure in the main line pressure circuit  60  must be higher than the pressure in the accumulator  52  in order to unseat the check valve  76 C. The hydraulic fluid  44  pressure acts on the piston, pushing it against the air and spring(s) on the other side. If the force of the air and spring(s) is less than the force generated by hydraulic fluid  44  pressure, then the piston will move allowing more oil to flow into the accumulator  52 . If the force generated by air and spring(s) is equal to the force generated by hydraulic fluid  44  pressure, then there will be no movement of the piston. If the force generated by the air and spring(s) is greater than the force generated by hydraulic fluid  44 , the piston will move causing the accumulator  52  to exhaust. The accumulator  52  pressure is monitored by the pressure sensor  78  to determine if the accumulator  52  is fully filled. The accumulator  52  can be filled quickly by opening or energizing the solenoid  76 , however this places a large flow demand on the main line pressure circuit  60 . 
         [0026]    Hydraulic fluid  44  is stored in the accumulator  52  at a set volume and pressure while the engine  12  is off. While the solenoid  76  is off, hydraulic fluid  44  will remain in the accumulator  52  as there is no path for any hydraulic fluid  44  to bypass the solenoid  76 , excluding the minute amount of leakage that weeps past the clearances in the parts of the solenoid valve  76 A. When the solenoid  76  is energized electrically, it opens. The decision to energize the solenoid  76  is determined based on an engine start command in order to have the clutches/brakes  34  ready for vehicle launch. Energizing the solenoid  76  allows hydraulic fluid  44  to leave the accumulator  52 , enter the solenoid  76 , and flow into the main line pressure circuit  60  that feeds the clutch actuation circuit  48 . The clutch actuation circuit  48  controls the pressure and flow rate to the clutches/brakes  34  to control clutch capacity during the engine start up event to eliminate torque bumps and increase the isolation of engine start up vibrations. Once pressure within the main line pressure circuit rises due to the activation of the pump  50 , the solenoid  76  is closed electrically by turning off power to the solenoid  76 . The accumulator  52  charge process can start over again to allow for another engine off event. 
         [0027]    When the motor vehicle  5  stops (i.e., at a red light for example), it may be desirable to shut off the engine  12  in order to improve fuel economy. However, during an automatic engine stop event, the engine  12  is shut down which cause a loss of hydraulic fluid  44  pressure in the transmission hydraulic circuit and clutches. In order to properly control the transmission  14  upon engine re-start and vehicle launch, transmission oil circuits must be filled and clutches pre-staged before vehicle launch by discharging the accumulator  52 . For example, when an auto start signal is commanded the controller  36  energizes the solenoid  76  thereby discharging the accumulator  52  for a period of calibrated time. In addition, application of a brake pedal for a predefined period of time may also be used to initiate accumulator  52  discharge. An example of a method for determining when to discharge the accumulator  52  is disclosed in commonly assigned U.S. patent application Ser. No. 13/228,275 filed on Sep. 8, 2011, hereby incorporated by reference as if fully disclosed herein. The solenoid commands in the clutch actuation circuit  48  electrically set up the transmission  14  to engage a minimum number of clutches/brakes so that only the minimum number of clutches/brakes needs to be filled. An example of selecting the minimum number of clutches/brakes is disclosed in commonly assigned U.S. patent application Ser. No. 13/228,664 filed Sep. 9, 2011, hereby incorporated by reference as if fully disclosed herein. 
         [0028]    In the following transmission  14  conditions the automatic stop will be prohibited: the pressure in the accumulator  52  is not high enough or the hydraulic fluid  44  temperature is low. When the pressure in the accumulator  52  is not high enough, the pressure in the main line pressure circuit  60  is increased to charge the accumulator  52  at proper conditions. When the hydraulic fluid  44  temperature increases beyond a threshold for allowing automatic stop, the accumulator  52  is discharged to discharge the cold fluid in the accumulator  52  in order to exchange the cold fluid in the accumulator  52  with the warmer fluid before allowing automatic stop. An example of the transmission conditions overriding or inhibiting the automatic stop is described in commonly assigned U.S. patent application Ser. No. 13/228,658 filed on Sep. 9, 2011, hereby incorporated by reference as if fully disclosed herein. 
         [0029]    During an automatic stop, if the accumulator  52  pressure is detected as being low (for example due to an accumulator leak), the controller  36  will restart the engine  12  and finish the automatic stop start. After key off and vehicle stop, the accumulator  52  is discharged. 
         [0030]    The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.