Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
In some types of transmissions, a gear ratio between the input shaft and the output shaft is established by engaging one or more friction clutches. During some operating conditions, a clutch may be partially engaged such that it transmits torque between two elements that are rotating at different speeds or between a rotating element and a stationary element such as the transmission housing. For example, during an upshift, the oncoming clutch may be partially engaged while the engine speed gradually declines to the speed associated with the final gear, enabling the transmission to provide output torque during the shift event. As another example, a clutch may be partially engaged while the vehicle accelerates from stationary to the speed associated with complete engagement of first gear. When a clutch is partially engaged, power is converted into heat which must be absorbed by the clutch and eventually dissipated.
A common type of clutch utilizes a clutch pack having separator plates splined to a rotating housing and interleaved with friction plates splined to a rotating shell. When the separator plates and the friction plates are forced together, torque may be transmitted between the housing and the shell. Typically, a separator plate on one end of the clutch pack, called a reaction plate, is axially held to the housing. A piston applies axial force to a separator plate on the opposite end of the clutch pack, called a pressure plate, compressing the clutch pack. The piston force is generated by supplying pressurized fluid to a chamber between the housing and the piston. Fluid typically flows between the separator plates and friction plates keeping the friction material on each friction plate damp. Heat is generated at the interface when the clutch is partially engaged. The heat is absorbed by the separator plates and transferred to the fluid by convection and is removed from the clutch with the fluid.