Continuously variable transmission

A continuously variable transmission (CVT) is a type of automatic transmission that varies the distance between a pair of belt or chain-linked pulleys, wheels or cones to smoothly alter its gear ratio without requiring any predefined steps or sudden changes in engine speed.

Mechanical overview

Continuously variable transmissions operate on the theory of pulleys; a large pulley connected to a smaller pulley with a belt or chain will operate in the same manner as a large gear meshing with a smaller gear. Typical CVTs have two opposing, cone shaped drums in them (one on the input, one on the output side) which can be moved in and out, controlled by the engine vacuum in the inlet manifold and engine RPM and centrifugal force acting on weights inside the conical drums. Moving the cones in and out has the effect of changing the driving and driven pulleys' diameter, thus changing the gear ratio. As there are no predefined steps in the system, it's referred to as continuously variable.

Advantages and drawbacks

CVTs have much smoother operation than hydraulic automatic transmissions, are simpler to build and repair and provide better fuel economy by avoiding torque converter losses; however, their torque handling capability is limited by the strength of the belt or chain, and so CVTs have typically been limited to low powered cars and other light duty applications.

CVTs can smoothly compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. This improves both fuel economy and exhaust emissions, but some car drivers are bothered by the resulting lack of feedback about vehicle speed provided by engine sound.

History

According to TOROTRAK, the first patent for a toroidal CVT was filed at the end of the 19th century .

The first workable CVT, called VARIOMATIC, was designed and built by dutch Hub van Doorne, co-founder of DAF, in the late 1950s, specifically to produce an automatic transmission for a small, affordable car. The first DAF car using van Doorne's CVT was produced in 1958. Van Doorne's patents were later sold to Volvo along with DAF's car business.

General Motors designed a CVT for use in small cars, which was first offered in 2002. After just three years, however, this transmission will be phased out in favor of conventional planetary automatic transmissions.

CVT transmissions have been refined over the years and are much improved from their origins.

Examples

Many small tractors for home and garden use have simple CVTs, as do most snowmobiles. Almost all motor scooters today are equipped with CVT.

Possibly the largest vehicle currently sold with a CVT is the Nissan Murano, a mid-size sport utility vehicle with a V6 engine sold in the North American market. The CVT is also available in i.e. Audi, Honda and Mini Cooper cars.