Transmission (mechanical device)

Multiple gear ratios are also needed to provide sufficient acceleration and velocity for safe and reliable operation at modern highway speeds.

The simplest transmissions used a fixed ratio to provide either a gear reduction or increase in speed, sometimes in conjunction with a change in the orientation of the output shaft.

A semi-automatic transmission is where some of the operation is automated (often the actuation of the clutch), but the driver's input is required to move off from a standstill or to change gears.

Many early versions of these transmissions were semi-automatic in operation, such as Autostick, which automatically control only the clutch, but still require the driver's input to initiate gear changes.

[9][10] An automatic transmission does not require any input from the driver to change forward gears under normal driving conditions.

[14] The design is often similar to two separate manual transmissions with their respective clutches contained within one housing, and working as one unit.

[15][16] In car and truck applications, the DCT functions as an automatic transmission, requiring no driver input to change gears.

The flexibility of a CVT with suitable control may allow the engine to operate at a constant RPM while the vehicle moves at varying speeds.

CVTs are used in cars, tractors, side-by-sides, motor scooters, snowmobiles, bicycles, and earthmoving equipment.