How automatic car works??

How automatic car works??

Most automatic transmissions consist of stacks of differently ratioed planetary gearsets which are engaged or disengaged via hydraulically actuated clutches.  The hydraulic fluid functions as both a pressure medium and a cooling medium and is generally pressurized by a mechanically driven pump in the transmission assembly.  Changes in RPM's and engine load cause different valves to open or close, directing fluid pressure to the appropriate clutch plate actuators and automatically selecting the most ideal gear level.  Traditional automatic transmissions are purely hydro-mechanical devices, while newer designs incorporate electro-valves controlled by the vehicle computer which is fed pertinent data by various sensors throughout the vehicle.  Vehicles with automatic transmissions generally include a torque converter which allows the engine to freewheel or slip without stalling, applying rotational energy gradually and giving the transmission and wheels time to overcome inertia.  
Gasoline combustion engines are capable of relatively high RPM's, so automatic transmission gear ratios are tuned to maximize efficiency at ranges higher than those produced by Diesel engines which generate maximum power and lower RPM's.  Purely electric vehicles (not hybrids) are driven by electric motors which cannot stall, so gearboxes can be simpler and do not require energy-wasting torque converters

Audi a63 automatic car