An automatic gearbox is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Most automatic transmissions have a defined set of gear ranges, often with a parking pawl feature that locks the output shaft of the transmission.
Similar
but larger devices are also used for heavy-duty commercial and
industrial vehicles and equipment. Some machines with limited speed
ranges or fixed engine speeds, such as some forklifts and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.
Besides automatics, there are also other types of automated transmissions such as continuous variable transmissions (CVTs) and semi-automatic transmissions,
that free the driver from having to shift gears manually, by using the
transmission's computer to change gear, if for example the driver were redlining
the engine. Despite superficial similarity to other automated
transmissions, automatic transmissions differ significantly in internal
operation and driver's feel from semi-automatics and CVTs. An automatic
uses a torque converter instead of clutch
to manage the connection between the transmission gearing and the
engine. In contrast, a CVT uses a belt or other torque transmission
schema to allow an "infinite" number of gear ratios instead of a fixed
number of gear ratios. A semi-automatic retains a clutch like a manual
transmission, but controls the clutch through electrohydraulic means.
A
conventional manual transmission is frequently the base equipment in a
car, with the option being an automated transmission such as a
conventional automatic, semi-automatic, or CVT. The ability to shift
gears manually, often via paddle shifters, can also be found on certain
automated transmissions (manumatics such as Tiptronic), semi-automatics (BMW SMG), and continuous variable transmissions (CVTs) (such as Lineartronic).
