Difference between revisions of "Continuously variable transmission"

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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.  
 
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.
 
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.

Revision as of 07:50, 26 March 2007

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.

An extension to CVT design, sometimes known as the Infinitely Variable Transmission, allows the transmission to drive a vehicle backwards as well as forwards. Transmission input is connected to the engine, then it is split into 2 shafts, one connected to a regular CVT and the other connected to an epicyclic gear set. The output from the CVT shaft is connected to another shaft that connects to a different set of gear in the epicyclic. The gear that does not draw power from engine or CVT transfers torque to the transmission output. The gear set acts as a mechanical adding machine to subtract one speed from the other, allowing the car to go forwards, backwards, or neutral.

Types

Variable Diameter Pulley (VDP)

This type of CVT uses pulleys, typically connected by a metal-covered rubber belt. A chain may also be used. 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 pulleys formed as pairs of opposing cones. Moving the cones in and out has the effect of changing the pulley diameter, since the belt or chain must take a large-diameter path when the conical pulley halves are close together. This motion of the cones can be computer-controlled and driven, for example by a servo motor. However, in the light-weight VDP transmissions used in automatic motorscooters and light motorcycles, the change in pulley diameter is accomplished by a variator, an all-mechanical system that uses weights and springs to change the pulley diameters as a function of belt speed.

In the case of a chain the links bear on the pulleys via tapered sides on the links. Some such transmissions have been designed to transmit the forces between pulleys using compressive (pushing) rather than traction (pulling) forces. Some chain driven transmissions have used a special lubricant which under extreme pressure undergoes a phase change to a glassy solid.

Anderson A+CVT

It is a technology invented by Larry Anderson, under US patent 6,575,856. Two parallel cones have "floating sprocket bars" mounted in longitudinal grooves around each cone. A specially-designed chain meshes with the floating sprocket bars, and is free to slide along the length of cones, changing the gear ratio at each point. The technology is also adaptable to a variable diameter pulley-type CVT, by mounting the floating sprocket bars on the insides of two pulley sheaves. A chain engages the floating sprocket bars, and slides up and down as the pulley sheaves move apart or together. The principle is similar to a conventional variable diameter pulley-CVT, except that the A+CVT is positive-drive, not friction-dependent. Additional details are available at www.andersoncvt.com.

Roller-based CVT

(marketed as the Traction CVT or Extroid CVT)

Consider two almost-conical parts, point to point, with the sides dished in such that the two parts could fill the central hole of a torus. One part is the input, and the other part is the output (they do not quite touch). Power is transferred from one side to the other by one or more rollers. When the roller's axis is perpendicular to the axis of the almost-conical parts, it contacts the almost-conical parts at same-diameter locations and thus gives a 1:1 gear ratio. The roller can be moved along the axis of the almost-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the almost-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1:1.

Hydrostatic CVT

Hydrostatic transmissions use a variable displacement pump and a hydraulic motor. All power is transmitted by fluid. These types can generally transmit more torque, but are very sensitive to contamination. And some designs are very expensive. However, they have the advantage that the hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system and eliminating efficiency losses from friction in the drive shaft and differential components. This type of transmission has been effectively applied to expensive versions of light duty ridden lawn mowers, garden tractors and some heavy equipment.

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. Their torque handling capability is limited by the strength of the belt or chain, and the ability to withstand friction wear between torque source and transmission medium for friction-driven CVTs. Therefore 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 drivers are bothered by the resulting lack of feedback about vehicle speed provided by engine sound. One magazine described it as sounding like driving a powerboat when accelerating. The satisfying jerk of a non-CVT transmission can be emulated by CVT control software though, eliminating this marketing problem.

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 Van Doorne's Automobiel Fabriek (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.

In the 1980s and 1990s, the Subaru Justy was offered with a CVT. While the Justy saw only limited success, Subaru continues to use CVT in its keicars to this day, while also supplying it to other manufacturers.

Nissan first introduced CVT in the 1992 Nissan March with a unit sourced from Subaru. In the late 1990s, Nissan designed its own CVT that allowed for higher torque, and includes a torque converter. This gearbox was used in a number of Japanese market models. Nissan is also the only car maker to bring roller-based CVT to the market in recent years. Their toroidal CVT, named the X-troid, was available in the Japanese market Y34 Nissan Gloria and V35 Skyline GT-8. However, the gearbox was not carried over when the Cedric/Gloria was replaced by the Nissan Fuga in 2004.

In 1993, the Williams-Renault VDT FW15C - Experimental CVT set-up by Van Doorne Transmissie (VDT) on the championship-winning car bridged the gap to the late-sixties F3 and FJ efforts by DAF. The CVT paired to a Renault V10 constantly revving at its peak led to spectacular test times by David Coulthard, after which CVT was quickly and silently banned from Grand Prix racing by the FIA in 1994, after the worryingly fast test times by Williams.

After studying pulley-based CVT for years, Honda also introduced their own version on the 1995 Honda Civic VTi. Dubbed Honda Multi Matic, this CVT gearbox accepted higher torque than traditional pulley CVTs, and also includes a torque converter for "creep" action.

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.

Audi has, since 2000, offered a chain-type CVT as an option on some of its larger-engined models, for example the A4 3.0L V6.

The 2005 Ford Freestyle and Five-hundred use a new chain-driven CVT allowing engine torque to go up to 300 Nm. The transmission was designed in cooperation with the German Company ZF and is currently produced in Batavia, Ohio. The CVT is computer controlled and combines fuel efficiency and smooth riding.

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 3.5L V6 engine. The CVT is also available for Audi, Fiat, Honda, Mercedes-Benz and Mini Cooper cars.

Some combine harvesters have CVT. The machinery of a combine is adjusted to operate best at a particular engine speed. The CVT allows the forward speed of the combine to be adjusted independently of the machine speed. This allows the operator to slow down and speed up as needed to accommodate variations in thickness of the crop.

New automobiles equipped with CVT

Old automobiles equipped with CVT

External links

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