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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. 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 (IVT), 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 with one 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.

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

Contents

Types

Variable-diameter pulley (VDP)

This type of CVT uses pulleys, typically connected by a rubber-covered metal 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 higher power types, for example that produced by ZF-Sachs of Germany, an oil-cooled laminated steel belt is used.

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.

Image:Cvt_list01.gif Variable-Diameter Pulley CVT - Mitsubishi Motors

Roller-based CVT

(marketed as the Traction CVT, Extroid CVT, or IVT)

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, or donut-shape. One part is the input disc (which connects the crankshaft), and the other part is the output disc (which connects the driveshaft). Power is transferred from one disc to the other by one or more rollers. When the roller's axis is perpendicular to the axis of the discs, it contacts the discs 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 discs at varying and distinct diameters, giving a gear ratio of something other than 1:1. By varying the angle of rollers, different transmission ratios can be obtained. For example, for "low" gearing ratio, the rollers contact the input disc near its inside diameter, but meet the output disc near its periphery - the output disc therefore turns much slower than the input disc.

The rollers are electro-hydraulically actuated, but do not directly contact with the input/output discs. A specially developed viscous oil provides the traction between the rollers and discs, and reduces friction and wear.

Systems may be partial or full toroidal. Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter (e.g. JATCO) and hence lose efficiency. Roller-based CVTs can with more engine torque than cariable-diameter pulley CVTs.

Image:Extroid_cvt_autozineorg.jpg Autozine.org

Image:Extroid_cvt2_autozineorg.jpg Autozine.org

Diagrams:

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.

Electronically-controlled CVT

The E-CVT saw first commercial automotive use in Toyota's Hybrid Synergy Drive system. This system is not a true CVT, having a fixed gear ratio, but behaves very similar to a true CVT. In this system, the transmission is an integral part of the hybrid powertrain and is actually a torque combiner. The gear train is a permanently-engaged, fixed-ratio, 3-way planetary gear. The engine is attached to one input, the driveshaft and the main electric motor to another, and then a smaller motor-generator controls the differential's third input to create a continuously-variable ratio between engine speed and wheel speed, with the variation taken by the electric motor and generator. At the extremes, the vehicle can move under electric power without the engine turning, or it can run the engine while stationary during engine warm-up or if needed to prevent discharge of the batteries.

The advantage of the system is its mechanical simplicity - no clutches, torque converters or shifting gears. A disadvantage is that continuous electrical power transmission between the two motor-generators is needed even during cruise, with resulting conversion losses, but the total effect is to increase the net efficiency through four methods:

  • The internal combustion engine may be completely shut down, rather than idle.
  • The electric motor operates during high torque demands required to put the vehicle in motion.
  • The internal combustion engine operates mostly at higher power demands, where it is more efficient.
  • Energy may be recovered through the generation function when the vehicle is slowing or coasting downhill, with the energy (stored in the battery) applied to the initial acceleration of the vehicle and when high power demands require a that both the internal combustion engine and the electric motor operate.

The design of the system may be optimized for efficiency or for performance, as appropriate for the marketing segment for which the vehicle is targeted.

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 as quiet as any conventional transmission, as it is enclosed in a housing and lubricated. In addition to the existing dual-cone A+CVT, Anderson is currently developing a variable diameter pulley version of the A+CVT. (www.andersoncvt.com)

Cone CVTs

This category comprises all CVTs made up of one or more conical bodies which function together along their respective generatrices in order to achieve the variation.

In the single cone type, there is a revolving body (a wheel) that moves on the generatrix of the cone, thereby creating the variation between the inferior and the superior diameter of the cone.

In those CVTs comprising two or more cones (see GIF website), the cones are placed in opposition, with the smaller diameter of one cone facing the larger diameter of the other cone. The contact between all the possible diameters of the two cones is made by a ring interposed between the cones. The variation is obtained by moving the ring along the generatrix of the two cones.

In a CVT with oscillating cones, the torque is transmitted via friction from a variable number of cones ( according to the torque to be transmitted ) to a central, barrel-shaped hub. The side surface of the hub is convex according to a determined radius of curvature, which is smaller than the concavity radius of the cones. In this way, there will be only one ( theoretical ) contact point between each cone and the hub.

A revolutionary new CVT, the Warko, using this technology, was presented in Berlin during the 6th International CTI Symposium of Innovative Automotive Transmissions, on 3-7 December 2007.

A particular characteristic of the Warko is absence of a clutch: the engine is always connected to the wheels, and the rear drive is obtained by means of an epicyclic system in output. This system, named “power split”, allows the condition of geared neutral or "zero Dynamic": when the engine turns( connected to the sun gear of the epicyclic system), the variator ( which rotates the ring of the epicyclic system in the opposite sense to the sun gear), in a particular position of its range, will compensate for the engine rotation, having zero turns in output ( planetary = the output of the system ). As a consequence, the satellite gears roll within an internal ring gear.

Modularity, wide ratio range (= 9), high efficiency (95%), high torque capability (up to 500 Nm) and compactness (less than 36 cm length for 31 cm diameter and 60 kg) are the most important characteristics of the Warko.

The same device, with the same identical cone, in different configurations, covers the 90% of the engines produced all over the world, with a power range that goes from 60 to 200 Hp, gasoline and diesel. As a consequence, being manufactured by the millions, its production costs will be comparable to mechanical transmission costs.

Diagrams:



Advantages and drawbacks

Compared to hydraulic automatic transmissions:

  • CVTs can smoothly compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. They may also avoid torque converter losses. This improves both fuel economy and exhaust emissions. However, some units (eg. Jetco Extroid) also employ a torque converter. Fuel efficiency advantages as high as 20% over 4 speed automatic transmissions can be obtained.
  • CVTs have much smoother operation. This can give a perception of low power, because many drivers expect a jerk when they begin to move the vehicle. The satisfying jerk of a non-CVT transmission can be emulated by CVT control software though, eliminating this marketing problem.
  • Since the CVT keeps the engine turning at constant RPMs over a wide range of vehicle speeds, pressing on the accelerator pedal will make the car move faster but doesn't change the sound coming from the engine as much as a conventional automatic transmission gear-shift. This confuses some drivers and again, leads to a mistaken impression of a lack of power.
  • CVTs are simpler to build and repair.
  • CVT torque handling capability is limited by the strength of their belt or chain, and by their ability to withstand friction wear between torque source and transmission medium for friction-driven CVTs. CVTs in production prior to 2005 are predominantly belt or chain driven and therefore typically limited to low powered cars and other light duty applications. More advanced IVT units using advanced lubricants, however, have been proven to support any amount of torque in production vehicles, including that used for buses, heavy trucks, and earth moving equipment.

History

Leonardo da Vinci sketches what is considered to be basis for the first continuously variable transmission in 1490.

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

From the 1950's, CVTs have been applied to aircraft electrical power generating systems.

The first workable CVT, called Variomatic, was designed and built by the Dutchman Huub van Doorne, co-founder of DAF Trucks (Van Doorne's Automobiel Fabriek), 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 and CVT was used in Volvo 340.

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 (Japanese minicars) 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 Cedric/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.

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.

Toyota introduced the E-CVT in the 1997 Prius, and all subsequent Toyota and Lexus hybrids sold internationally continue to use the system (marketed under the Hybrid Synergy Drive name). Although sold as a CVT, it is in fact not such a device as the gear ratios are fixed and the transmission is actually a torque blending device, allowing either the electric motor or the internal combustion engine, or both, to propel the vehicle. The response of the complete system (under computer control) is similar in feel to a CVT in that the internal combustion engine speed is relatively low and constant under low power or high and constant under high power.

BMW used a belt-drive CVT as an option for the low and middle range MINI in 2001, forsaking it only on the supercharged version of the car where the increased torque levels demanded a conventional automatic gearbox. The CVT could also be manually 'shifted' if desired with software simulated shift points.

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 (Multitronic) as an option on some of its larger-engine models, for example the A4 3.0L V6.

Launched in 2005, the Ford Freestyle, Five Hundred and Mecury Montego use a new chain-driven CVT that can handle engine torque up to 300 N•m. The transmission was designed in cooperation with the German company Sachs ZF and is currently produced in Batavia, Ohio.

Sachs ZF supplied its belt drive CVT unit to many car manufacturers including BMW and MG Rover.

Contract agreements were established in 2006 for the first full toroidal system to be manufactured for outdoor power equipment such as jetskis, ski-mobiles and ride on mowers.

Examples

Many small tractors for home and garden use have simple hydrostatic or rubber belt CVTs, as do most snowmobiles. Most new motorscooters today are equipped with CVT. Virtually all snowmobile and motor scooter CVTs are rubber belt/variable pulley CVTs.

Possibly the largest vehicle currently sold with a CVT is the Toyota Highlander Hybrid.

Some combine harvesters have CVTs. 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.

CVTs have been used in SCCA Formula 500 race cars since the early 1970s.

More recently CVT systems have been developed for karts, and have proved to increase performance, and engine life expectancy.

New automobiles equipped with CVT

Old automobiles equipped with CVT

  • DAF 600
  • DAF 750
  • DAF 30 (Daffodil)
  • DAF 31
  • DAF 32 (DAF 33)
  • DAF 44
  • DAF 46
  • DAF 66
  • Fiat Uno
  • Ford Fiesta
  • Honda Civic ESi
  • Nissan Micra
  • Subaru Justy 1.2 3 cyl with ECVT (49/55 kW)
  • Volvo 66
  • Volvo 340
  • Daewoo (currently GM Daewoo) Matiz II with E3CVT

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