Unlike manual vehicles, automatic models aren't operated with a driver-controlled gear stick. For this reason, vehicles with automatic transmission don't have a clutch. Instead, the driver either pushes the acceleration or the brake pedal to control the car. The speed and torque of the vehicle are controlled automatically to imitate manual transmission cars - but how?
During automatic transmission, every component connecting the vehicle's engine and wheels is also linked to a torque converter. In effect, this is a substitution for the clutch pedal. Torque converters are mechanisms which direct the rotational power of a machine to its input shaft bearing via transmission fluid.
Automatic vehicles use sensors to figure out when it needs to change gears. When it's time to switch gears, the torque converter disengages the engine and the transmission. To achieve the same effect as manual transmission systems, the vehicle's torque converter moves the transmission fluid between the car's impeller and turbine to create varying forms of pressure.
Unlike manual transmissions, automatic transmission systems don't have a separate wheel for each gear. Instead, the vehicle uses a gear system which relies on three components: sun, planetary and ring gears.
When the torque converter identifies that the car needs to change gears, it engages the sun, planetary and ring gears with the pressure created by the transmission fluid. The gears then mesh together in different combinations to achieve the desired effect.