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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.

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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

External links

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