Abstract:
A system and method for controlling an accumulator in a transmission of a motor vehicle includes the steps of determining whether the motor vehicle has been turned off, sensing at least one operating condition of the motor vehicle, and comparing the at least one operating condition to a reference condition. If the at least one operating condition of the motor vehicle fulfills the reference condition and if the motor vehicle has been turned off then the accumulator is discharged.

Description:
FIELD 
     The present disclosure relates to a system and method for controlling an accumulator in a hydraulic control system of a motor vehicle, and more particularly to a system and method for controlling the discharge of an accumulator based on measured transmission conditions within a motor vehicle. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices and to provide lubrication and cooling to the components of the transmission. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as automatic transmission 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 components of the transmission. For example 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. 
     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. One solution is to include an accumulator to pressurize the transmission at the start of an auto-start event. However in certain configurations the accumulator and the fluid stored within the accumulator are exposed to ambient temperature conditions and therefore may not be at the same temperature as the transmission during an engine start event after prolonged exposure. Therefore, there is a need in the art for a system and method for controlling the discharge of an accumulator that preserves the ability to perform engine stop-start events without affecting transmission operating performance. 
     SUMMARY 
     A system and method for controlling an accumulator in a transmission of a motor vehicle is provided. The method includes the steps of determining whether the motor vehicle has been turned off, sensing at least one operating condition of the motor vehicle, comparing the at least one operating condition to a reference condition, and discharging the accumulator if the motor vehicle has been turned off and if the at least one operating condition of the motor vehicle fulfills the reference condition. 
     In one aspect of the present invention the at least one operating condition is a temperature of a hydraulic fluid within the transmission, and the reference condition is a temperature range defined by a lower threshold and an upper threshold. 
     In another aspect of the present invention the at least one operating condition is a temperature of ambient air, and the reference condition is a temperature range defined by a lower threshold and an upper threshold. 
     In another aspect of the present invention the at least one operating condition is a pressure of a hydraulic fluid within the transmission, and the reference condition is a pressure range defined by a lower threshold and an upper threshold. 
     In another aspect of the present invention the at least one operating condition is a time value measured from a key off event, and the reference condition is a time range defined by a lower threshold and an upper threshold. 
     In another aspect of the present invention the at least one operating condition is one or more sensed conditions of the motor vehicle, and the reference condition is one or more conditions indicative of service being performed on the motor vehicle. 
     In another aspect of the present invention determining whether the motor vehicle has been turned off includes sensing a key off event by an operator of the motor vehicle. 
     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 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic diagram of an exemplary powertrain in a motor vehicle; 
         FIG. 2  is a schematic diagram of a portion of an exemplary hydraulic control system; 
         FIG. 3  is a flow chart illustrating a method of operating the motor vehicle of  FIGS. 1-2  according to the principles of the present invention; and 
         FIG. 4  is a flow chart illustrating another method of operating the motor vehicle of  FIGS. 1-2  according to the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     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 . 
     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. 
     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 . 
     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. 
     The motor vehicle  5  includes a control module  36 . The control module  36  may be a transmission control module (TCM), an engine control module (ECM), or a hybrid control module, or any other type of controller. The control module  36  is preferably an electronic control device 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 an ambient temperature sensor  37 A, a transmission fluid temperature sensor  37 B, and an ignition key sensor  37 C, to name but a few. 
     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 plurality of shift actuating devices  48 . The hydraulic fluid  44  is communicated to the shift actuating devices  48  under pressure from either an engine driven pump  50  or an accumulator  52 . 
     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 supply line  60 . The supply line  60  is in communication with a spring biased blow-off safety valve  62 , an optional pressure side filter  64 , and an optional spring biased check valve  66 . The spring biased blow-off safety valve  62  communicates with the sump  46 . The spring biased blow-off safety valve  62  is set at a relatively high predetermined pressure and if the pressure of the hydraulic fluid  44  in the supply line  60  exceeds this pressure, the safety valve  62  opens momentarily to relieve and reduce the pressure of the hydraulic fluid  44 . The pressure side filter  64  is disposed in parallel with the spring biased check valve  66 . If the pressure side filter  64  becomes blocked or partially blocked, pressure within supply line  60  increases and opens the spring biased check valve  66  in order to allow the hydraulic fluid  44  to bypass the pressure side filter  64 . 
     The pressure side filter  64  and the spring biased check valve  66  each communicate with an outlet line  68 . The outlet line  68  is in communication with a second check valve  70 . The second check valve  70  is in communication with a main supply line  72  and is configured to maintain hydraulic pressure within the main supply line  72 . The main supply line  72  supplies pressurized hydraulic fluid to a control device  76 . The control device  76  is electrically controlled by the control module  36  and is operable to control whether the accumulator  52  is charged or discharged. When the control device  76  is open, the accumulator  52  may charge or discharge depending on whether the main supply line pressure is higher or lower than the accumulator pressure, respectively. When the control device  76  is closed, the accumulator  52  remains charged. The control device  76  may be an on/off solenoid or a pressure or flow control solenoid. Alternatively, an optional check ball valve (not shown) may be disposed parallel to the control device  76  and configured to maintain hydraulic pressure within the accumulator  52 . The optional check ball valve allows the accumulator  52  to charge even when the control device  76  is closed if the main supply line pressure  72  is higher than the accumulator pressure. When the control device  76  is open, the accumulator  52  may charge or discharge as previously mentioned. 
     The main supply line  72  communicates through a hydraulic circuit that may include other control devices, valves, etc., to the plurality of actuating devices  48 . The actuating devices  48  may be, for example, piston assemblies that when engaged in turn engage the clutches/brakes  34 . 
     The control device  76  communicates with the accumulator  52  and a pressure sensor  74 . The accumulator  52  is an energy storage device in which the non-compressible hydraulic fluid  44  is held under pressure by an external source. In the example provided, the accumulator  52  is a spring type or gas filled type accumulator having a spring or compressible gas or both that provides a compressive force on the hydraulic fluid  44  within the accumulator  52 . However, it should be appreciated that the accumulator  52  may be of other types, such as a gas-charged type, without departing from the scope of the present invention. Accordingly, the accumulator  52  is operable to supply pressurized hydraulic fluid  44  back to the main supply line  72 . However, upon discharge of the accumulator  52 , the second check valve  70  prevents the pressurized hydraulic fluid  44  from returning to the pump  50 . 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  74  reads the pressure of the hydraulic fluid  44  within the accumulator  52  or the main supply line  72  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. 
     With reference to  FIG. 3 , and with continued reference to  FIGS. 1 and 2 , a method  100  for operating the motor vehicle  5  will now be described. The method  100  is used to determine when to discharge the accumulator  52  based on the state of the motor vehicle  5 . For example, the method  100  begins at step  102  where the control module  36  determines whether a controller shutdown sequence has been activated. The controller shutdown sequence begins after a key-off event. A key-off event occurs when an operator of a motor vehicle turns off or otherwise commands the motor vehicle  5  to shut down. If the shutdown sequence has not been activated the method  100  starts over. If the shutdown sequence has been activated (i.e. the motor vehicle  5  has been keyed off), the method  100  proceeds to step  104 . 
     At step  104  the controller  36  performs one or more multiple processes (i.e. executes one or more control logic) to determine whether to discharge the accumulator  52 . In a first exemplary process the controller  36  compares the temperature of the automatic transmission fluid (ATF) sensed by the sensor  37 B to an ATF range defined by lower and upper ATF temperature thresholds. In a second exemplary process the controller  36  compares the temperature of the ambient air sensed by the sensor  37 A to an ambient temperature range defined by lower and upper ambient temperature thresholds. In a third exemplary process the controller  36  compares the pressure of the ATF within the accumulator  52  sensed by the pressure sensor  74  to a pressure range defined by lower and upper pressure thresholds. In a fourth exemplary process the controller  36  determines a time since the key-off event occurred. The key-off event is sensed by the key sensor  37 C. The controller  36  then compares the time since key-off event to a time threshold. These processes may be used individually or in combination to determine if the accumulator  52  should be discharged. For example, if the sensed ATF temperature is outside the ATF temperature range, and/or the sensed ambient temperature is outside the ambient temperature range, and/or the sensed ATF pressure is outside the pressure range, and/or the time since key off is less than the time threshold, then the method  100  ends and the accumulator  52  is not discharged. If, however, the sensed ATF temperature is within the ATF temperature range, and/or the sensed ambient temperature is within the ambient temperature range, and/or the sensed ATF pressure is within the pressure range, and/or the time since key off is greater than the time threshold, then the method  100  proceeds to step  106 . At step  106  the controller  36  commands the accumulator  52  to discharge and the method  100  ends. 
     Turning to  FIG. 4 , and with continued reference to  FIGS. 1 and 2 , another method  200  for operating the motor vehicle  5  will now be described. The method  200  may be used concurrently with the method  100  described above. The method  200  is used to determine when to discharge the accumulator  52  based on the state of the motor vehicle  5 . For example, the method  200  begins at step  202  where the control module  36  determines whether a controller shutdown sequence has been activated. The controller shutdown sequence begins after a key-off event. A key-off event occurs when an operator of a motor vehicle turns off or otherwise commands the motor vehicle  5  to shut down. If the shutdown sequence has not been activated the method  200  starts over. If the shutdown sequence has been activated (i.e. the motor vehicle  5  has been keyed off), the method  200  proceeds to step  204 . 
     At step  204  the controller  36  determines whether the accumulator  52  should be discharged based on whether the motor vehicle  5  is to be served. The controller  36  determines whether the motor vehicle  5  is being serviced based one or more inputs received by the controller  36 . For example, the controller  36  can receive a CAN message indicating that the motor vehicle will be serviced. This CAN message may be generated from a computer/device that a technician uses to read motor vehicle error codes. Alternatively, the CAN message may be generated from a change in various motor vehicle calibration values. If the controller  36  determines that the motor vehicle  5  is being serviced the method  200  proceeds to step  206 . At step  206  the controller  36  commands the accumulator  52  to discharge and the method  200  ends. If the controller  36  determines that the motor vehicle  5  is not being serviced the method proceeds to step  208 . 
     At step  208  the controller  36  determines whether one or more error codes have been generated by the controller  36 . One example of an error code includes a loss of communication with the controller  36  or body module or a combination of communication failures. Another example of an error code includes detecting a failed accumulator discharge which may indicate that the control device  76  may be malfunctioning. This would therefore indicate a need to relieve the pressure of the accumulator  52 . If the error codes are not generated the method  200  ends. If the error codes are generated then the method  200  proceeds to step  206  where the controller  36  commands the accumulator  52  to discharge and the method  200  ends. 
     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.