Abstract:
A vehicle having an engine that drives a transmission includes a pump that provides fluid to the transmission and a temperature sensor that monitors a temperature of the fluid. The temperature sensor generates a temperature signal. A control module receives the temperature signal and operates the pump to provide the fluid at a first pressure when the temperature signal is below an upper temperature limit. The control module operates the pump to provide the fluid at a second pressure when the temperature signal is above the upper temperature limit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to automatic transmissions, and more particularly to reducing burst rattle of an automatic transmission during cold start.  
       BACKGROUND OF THE INVENTION  
       [0002]     Vehicles commonly include an automatic transmission that is driven by a powerplant. Powerplants produce drive torque that is transferred to the automatic transmission and can include, but are not limited to, internal combustion engines, electric machines or combinations thereof (e.g., hybrid powerplant). The automatic transmission transfers the drive torque to a driveline at varying gear ratios to propel the vehicle.  
         [0003]     The automatic transmission includes a plurality of gear sets and clutches. The clutches are hydraulically actuated to selectively engage the gear sets to provide a desired gear ratio. Pressurized hydraulic fluid is provided by a pump. Typically, the pump is driven by the powerplant and compresses the hydraulic fluid drawn in from a sump.  
         [0004]     When a vehicle sits for an extended period of time (i.e., soak), the hydraulic fluid completely drains into the sump. In colder ambient temperatures (e.g., 30° F. and lower), the hydraulic fluid is cooled and becomes more viscous. When the powerplant is initially cranked after an extended soak in a colder ambient environment, the pump can cavitate as a result of the highly viscous hydraulic fluid. The cavitation generates a rattle or noise that can be sensed by the vehicle operator, which is undesirable.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention provides a vehicle having an engine that drives a transmission. The vehicle includes a pump that provides fluid to the transmission and a temperature sensor that monitors a temperature of the fluid. The temperature sensor generates a temperature signal. A control module receives the temperature signal and operates the pump to provide the fluid at a first pressure when the temperature signal is below an upper temperature limit. The control module operates the pump to provide the fluid at a second pressure when the temperature signal is above said upper temperature limit.  
         [0006]     In one feature, the control module operates the pump to provide the fluid at the first pressure when the temperature signal is within a range defined between the upper temperature limit and a lower temperature limit.  
         [0007]     In one feature, the first pressure is less than the second pressure.  
         [0008]     In another feature, an engine diagnostic system selectively sets error flags. The control module operates the pump to provide the fluid at the second pressure when an error flag is set.  
         [0009]     In another feature, the control module operates the pump to provide the fluid at the first pressure when the transmission is in one of a neutral range and a park range.  
         [0010]     In another feature, the control module operates the pump to provide said fluid at the first pressure for a threshold time and operates the pump to provide the fluid at the second pressure when the threshold time expires.  
         [0011]     In still another feature, the first pressure is constant.  
         [0012]     In yet another feature, the second pressure varies based on vehicle operating parameters.  
         [0013]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a schematic illustration of a vehicle implementing the transmission cold start control system according to the present invention; and  
         [0016]      FIG. 2  is a flowchart illustrating steps performed by the transmission cold start control system of present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality.  
         [0018]     Referring now to  FIG. 1 , an exemplary vehicle  10  includes an engine  12  that drives an automatic transmission  14  through a torque converter  16 . The torque converter  16  enables start-off, provides torque multiplication and absorbs harmonic vibrations within the vehicle drivetrain. Air is drawn into an intake manifold  18  of the engine through a throttle  20 . The air is combusted with fuel in cylinders of the engine  12  to produce drive torque. The engine  12  drives a pump  22  that draws low pressure hydraulic fluid from a transmission sump and compresses the hydraulic fluid. The hydraulic fluid is provided to the transmission  14  to enable shifting of the transmission components, as described in further detail below.  
         [0019]     The transmission  14  includes gear sets (not shown) that selectively interconnect input and output shafts (not shown). Typically, the gear sets include planetary gear sets selectively manipulated by clutches (not shown) to control the transmission gear ratio. Clutch actuation and gear shifting are enabled using the pressurized hydraulic fluid provided by the pump  22 . The input shaft is coupled to the engine  12  via the torque converter  16 . The output shaft is coupled to a drive shaft (not shown).  
         [0020]     A control module  30  defines gear selection and shift points and regulates demand-response shifting of the transmission  14 . Demand-response shifting is based on a shift program that is selected by the driver using a selector  32 , a position of an accelerator  34 , engine operating conditions, vehicle speed and other vehicle operating conditions. A position sensor  35  generates an accelerator position signal that is communicated to the control module  30 . The drive selector  32  enables a vehicle operator to select a particular range including, but not limited to, park (P), drive (D), neutral (N) and reverse (R).  
         [0021]     The control module  30  commands hydraulic actuation of the clutches based on the shift program. The control module  30  actuates gear selection and modulates the clutch pressure electronically in accordance with the torque flowing through the transmission  14 . The control module  30  communicates with an engine speed sensor  36  that generates an engine speed signal and a position sensor  38  that generates a gear selector position signal. The control module  30  also communicates with a sump temperature sensor  40  that generates a temperature signal based on the sump temperature of the hydraulic fluid (T FLUID ).  
         [0022]     Under normal operating conditions, the control module  30  operates the pump  22  to provide the hydraulic fluid at a normal pressure (P NORMAL ). P NORMAL  is a variable pressure that varies based on vehicle operating conditions including, but not limited to, engine speed, transmission range and present gear ratio. Under cold start conditions, the control module  30  executes the transmission cold start control of the present invention to alleviate burst rattle generated by the pump  22 . More particularly, the control module  30  operates the pump  22  to provide the hydraulic fluid to the transmission  14  at a reduced pressure (P RED ). P RED  is a fixed value that is generally less than pressure range within which P NORMAL  varies. Cold start is generally defined as T FLUID  being within a specified temperature range defined by lower and upper temperature limits, T UPPER  and T LOWER , respectively (e.g., −40° C. to 20° C., respectively), at engine crank. The control module  30  executes the cold start transmission control when there are no overriding priorities, the transmission  14  is in either the P or N ranges and T FLUID  is within the specified temperature range. Overriding priorities include diagnostics errors indicating pressure control problems, such that the transmission  14  is operating in a non-normal mode at engine crank.  
         [0023]     Referring now to  FIG. 2 , steps executed by the transmission cold start control system will be described in detail. In step  100 , control determines whether the engine  12  is running. If the engine  12  is not running, control ends. If the engine  12  is running, control starts a timer in step  102 . In step  104 , control determines whether there are any diagnostic errors that would effect pressure control (e.g., force motor failure diagnostics, range position diagnostics and the like). If there are diagnostic errors, control continues in step  106 . If there are no diagnostic errors, control continues in step  108 .  
         [0024]     In step  108 , control determines whether the transmission  14  is in either the park range (P) or the neutral range (N) based on the gear selector position signal. If the transmission  14  is in neither the P range nor the N range, control continues in step  106 . If the transmission is in either the P range or the N range, control continues in step  110 . In step  110 , control determines whether T FLUID  is within a specified range (i.e., between T LOWER  and T UPPER ). If T FLUID  is not within the specified range, control continues in step  106 . If T FLUID  is within the specified range, control continues in step  112 .  
         [0025]     Control operates the pump to provide the hydraulic fluid at P LOW  in step  112 . In step  114 , control determines whether the timer has expired. If the timer has not expired, control loops back to step  104 . If the timer has expired, control continues in step  106 . In step  106 , control operates the pump  22  to provide the hydraulic fluid at P NORMAL .  
         [0026]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.