A fully automatic transmission is designed to allow the gear ratio in the drivetrain to be selected as a function of accelerator pedal position, speed of travel and program switch position, and to let the driver influence automatic regulation.
The transmission drive shaft is connected via a torque converter (top left) to the crankshaft in such a way that it cannot be completely disengaged. Planetary gearing components (top right) are switched into the power transmission by multiplate clutches and are held fast at the same time by other multiplate brakes or band brakes. This permits a rapid change of gear ratio, even by more than one shift point, without interrupting power transmission. A pump on the drive shaft builds up the pressure required to operate the clutches and brakes in the automatic transmission. Until recently, oil flow was regulated hydraulically. Now, it’s done with electronics. In an automatic transmission, the “oil” used is a special ATF (automatic transmission fluid).
In a hydraulic regulation system (2nd photo from the top), the on-off valves, which are operated by oil pressure, and thus the selected transmission, are controlled by the suction tube pressure (accelerator pedal position) and the centrifugal governor (speed of travel). In an electronic regulation system, the on-off valves are operated electrically by the control unit via the appropriate sensors.
The various gear-selector positions permit reversing, idling, blocking while parking and barring one or more higher ratios. The driver can frequently influence a gear change by means of a second shifting gate. A selection may be made between switching programs with early (economical) or late (sporty) switching, or else the software itself finds the desired style of driving using fuzzy logic.