Automatic transmission

Automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually. Similar but larger devices are also used for railroad locomotives.
However, some simple machines with limited speed ranges and/or fixed engine speeds only use a torque converter to provide a variable gearing of the engine to the wheels. Typical examples include forklift trucks and some modern lawn mowers.
Recently manufacturers have begun to make continuously variable transmissions commonly available. These designs can change the ratios over a range rather than between set gear ratios. Even though CVTs have been used for decades in two-wheeled scooters and in a few cars, the technology has recently gained greater acceptance among manufacturers and customers.