CVT

A continuously variable transmission, or CVT, is an automatic gearbox that has no fixed set of gears and can instead provide an effectively infinite range of ratios between its highest and lowest limits. Where a conventional automatic or manual steps through a handful of discrete gears, a CVT moves smoothly and steplessly through every ratio in between, so there are no shift points and no interruption to the flow of power. Its purpose is to keep the engine operating at the most efficient or appropriate speed for the situation, regardless of how fast the car is travelling.

The most widespread CVT design uses a steel or composite belt, or a segmented push-belt or chain, running between two pulleys, each made of a pair of cone-shaped halves that can move closer together or further apart. As the halves of one pulley squeeze together, the belt is forced to ride at a larger effective diameter, while the other pulley opens up and the belt sits at a smaller diameter; varying the two diameters in opposition changes the ratio continuously. Hydraulic pressure controls the pulley clamping and movement, and a torque converter or a clutch is generally used to couple the engine to the system from rest.

The principal advantage of this arrangement is efficiency and smoothness. Because the transmission can select any ratio, it can hold the engine at the speed where it makes best fuel economy during gentle cruising, or at the speed of peak power during hard acceleration, rather than being tied to whatever fixed gear is nearest. The absence of gearchanges also removes the small jolts and pauses of a stepped transmission, giving notably seamless progress that suits relaxed driving and stop-start traffic, and contributing to the strong economy figures of many small and hybrid cars.

The characteristic drawback is the driving sensation under hard acceleration. Since the CVT pins the engine at high revolutions while the car gradually gathers speed, the engine note rises and then stays high and constant, decoupled from the sense of acceleration in a way many drivers find disconcerting; this is the so-called rubber-band effect or droning. Manufacturers counter it by programming artificial stepped ratios that mimic gearchanges, by adding paddle-operated virtual gears, and by improving sound insulation. Belt-type CVTs have also historically been limited in the torque they can handle and can be costly to repair, which restricts their use in high-performance and heavy-duty applications.

Several variations exist. Toroidal CVTs replace the belt and pulleys with rollers running between rotating discs, and the electronic CVT, or eCVT, used in many hybrids dispenses with a belt entirely, using a planetary gear set and electric motors to blend power sources and achieve a stepless effect. Within the wider family of automatic transmissions, the CVT sits alongside the torque-converter automatic and the dual-clutch transmission, each offering a different balance of smoothness, efficiency, responsiveness and cost.