A continuously variable transmission (CVT) is a type of automatic transmission that can change the "gear ratio" (gears are not generally involved) to any arbitrary setting within the limits.
The CVT is not constrained to a small number of gear ratios, such as the 4 to 6 forward ratios in typical automotive transmissions. CVT control computers often emulate the traditional abrupt gear changes, especially at low speeds, because most drivers expect the sudden jerks and will reject a perfectly smooth transmission as lacking in apparent power.
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
THE VARIABLE DIAMETER PULLEY CVT variable diameter pulley
A CVT transmission consists of a driving shaft and a driven shaft, each of which has a pulley splined onto it. Each pulley comprises two conical sheaves that face each other. A metallic pushbelt runs in the V-groove between the two conical sheaves of each pulley.
The pushbelt (See glossary) forms the link between the driving and the driven shaft, transmitting the drive power by means of shear force from the engine to the wheels. The transmitted shear force makes the pushbelt as strong as a steel rod, thereby enabling torques of over 400 Nm to be transmitted. Pushbelt is made of hundreds of specially pressed steel elements which are strung together between two rings made of layers of high-alloy steel. This design makes the pushbelt flexible and at the same time strong enough to transmit torque.
Another version of the CVT -- the toroidal CVT system -- replaces the belts and pulleys with discs and power rollers.
One disc connects to the engine. This is equivalent to the driving pulley.
Another disc connects to the drive shaft. This is equivalent to the driven pulley.
Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other.
The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.