Constantly variable transmission

A continuously variable transmission (CVT) (also known as a single-speed transmission, stepless transmission, pulley transmission, or, in case of motorcycles, a twist-and-go) is an automatic transmission that can change seamlessly through a continuous range of effective gear ratios. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the input shaft to maintain a constant angular velocity.

A belt-driven design offers approximately 88% efficiency, which, while lower than that of a manual transmission, can be offset by lower production cost and by enabling the engine to run at its most efficient speed for a range of output speeds. When power is more important than economy, the ratio of the CVT can be changed to allow the engine to turn at the RPM at which it produces greatest power. This is typically higher than the RPM that achieves peak efficiency. In low-mass low-torque applications (such as motor scooters) a belt-driven CVT also offers ease of use and mechanical simplicity.

A CVT does not strictly require the presence of a clutch. Nevertheless, in some vehicles (e.g. motorcycles), a centrifugal clutch is added to facilitate a "neutral" stance, which is useful when idling or manually reversing into a parking space.

Simple rubber belt CVTs are commonly used in small motorized vehicles, where their mechanical simplicity and ease of use outweigh their comparative inefficiency. Nearly all snowmobiles, utility vehicles, golf carts and motor scooters use CVTs, typically the rubber belt or variable pulley variety. Many small tractors and self-propelled mowers for home and garden also use simple rubber belt CVT, though hydrostatic systems are more common.

Hydrostatic CVTs are common in small to medium-sized agricultural and earthmoving equipment. As the engines in these machines are typically run at constant power settings to provide hydraulic power or to power machinery, losses in mechanical efficiency are offset by enhanced operational efficiency, such as reduced forward-reverse shuttle times in earthmoving operations. Transmission output is varied to control both travel speed and direction. This is particularly beneficial in equipment designed to pivot or skid steer through differential power application as the required differential steering action can easily be supplied by independent CVTs, allowing steering to be accomplished without braking losses or loss of tractive effort and allowing the machine to pivot in place. In mowing or harvesting operations a CVT allows the forward speed of the tractor or combine harvester to be adjusted independently of the engine speed. This allows the operator to slow or accelerate as needed to accommodate variations in thickness of the crop.

...
Wikipedia