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CATEGORIES (articles) > Transmission > Automatic Gearboxes > Continuously variable transmission explained

Continuously variable transmission explained


Transmission types
Manual
Automatic
  • Tiptronic
Semi-automatic
  • Twin-clutch Gearbox
Continuously-variable
Derailleur gears
Hub gears

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 gears 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.


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 Van Doorne's Transmissie (part of the Bosch group), 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. The chain driven transmission designed by LuK and VAG/Audi uses a special lubricant which undergoes a phase change under extreme pressure to form a glassy solid, enabling the chain to transmit considerable torque through small contact surfaces.


Roller-based CVT

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

Consider two almost-conical parts, point to point, with the sides dished 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. 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.

Diagrams:

  • JATCO Extroid CVT
  • Torotrak IVT

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. Some designs are also 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.


Hydristor IVT

The Hydristor torque converter is a true IVT in that the front unit connected to the engine can displace from zero to 27 cubic inches per revolution forward and zero to -10 cubic inches per revolution reverse. The rear unit is capable of zero to 75 cubic inches per revolution. The common 'kidney port' plate between the two sections communicates the hydraulic fluid under pressure and suction return in a 'serpentine-torodial' flow path between the two Hydristor internal units. The IVT ratio is determined by the ratio of input displacement to output displacement. Therefore, the theoretical range of Hydristor IVT ratios is 1/infinity to +-infinity/1.

Not all of that ratio range can be realized in the real world. For example, the input set at 27 divided into the rear set at zero results in the infinity/1 ratio and wont actually work. The front unit set at zero into the back unit at 75 is 1/infinity; ie: the engine turns forever for one turn of the output. This is actually achieved but does no work. Once a cruising speed has been achieved with front and rear Hydristors at some appropriate relative displacements, hydraulic braking is achieved by first simultaneously reducing both front and rear to zero displacement, then leaving the front Hydristor at zero (thus hydro mechanically disconnecting the engine from the torque converter hydraulic circuit and finally beginning to increase rear displacement as a braking function with the braking pressure and flow being directed to a hydraulic accumulator pressure tank. The decaying vehicle speed (kinetic energy), the rising tank pressure and the desired rate of deceleration determined by the driver all are variables which are easily managed by the Hydristor system. The stored braking energy can then be re-used for subsequent re-acceleration.

One result is that the city stop and go fuel economy nearly equals the highway mileage. The highway mileage is also doubled or more because of lowering the engine speed to idle in most cases and the resulting huge reduction in engine losses substantially raises the highway fuel economy. An important result is that the creation of CO2 greenhouse gas is quartered in existing vehicles in addition to substantially raising the average fuel economy. Last, the combination of hydraulic stored energy, the Hydristor's IVT nature, and the infinite ranges of control result in a vehicle which can zoom up to highway speed in a few seconds and literally spin the tires at highway speed even though the engine is idling due to the hydraulic storage. Most important, the Hydristor eliminates the transmission gears completely and is completely retrofittable into every vehicle already on the highways thus saving all the existing highway fleet.


Electronically-controlled CVT

The E-CVT saw first commercial automotive use in Toyota's THS (Toyota Hybrid System), first seen in the 1997 Toyota Prius, and subsequently in Toyota's Hybrid Synergy Drive system. This system is not a true CVT, having a fixed gear ratio, but behaves very similarly to a true CVT. In this system, the transmission is an integral part of the hybrid power train 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 (planetary carrier), the drive shaft and the main electric motor to another (ring gear), and then a smaller motor-generator controls the differential third input (sun gear) 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 ICE (Internal Combustion Engine) may completely shut down instead of idling when the vehicle is stopped or driven at slower speeds.
  • The electric motor operates during high torque demands required to put the vehicle in motion.
  • The ICE operates mostly at higher power demands, where it is more efficient.
  • Energy may be recovered through the generation function (regenerative braking) when the vehicle is slowing or coasting downhill, with the energy (stored in the battery) applied to the initial acceleration of the vehicle and/or when high power demands require that both the ICE and the electric motor operate simultaneously.
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 however, is flawed due to noise and smoothness. It is unlikely this technology will ever be implemented in a real vehicle. ([1] www.andersoncvt.com)


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 automatics 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.
  • Most 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

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 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 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 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.

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 eitherthe electric motor or the ICE (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 ICE 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 as an option on some of its larger-engine models, for example the A4 3.0 L V6.

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

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 Lexus GS 450h, however the Hybrid Synergy Drive is not a true CVT.

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

  • Audi A4 2.0/1.8T/2.4/3.0/2.5 TDI
  • Audi A6 2.0/1.8T/2.4/3.0/2.5 TDI
  • Dodge Caliber
  • Fiat Punto 1.2 L
  • Ford Escape Hybrid 2.3 L 4 cyl
  • Ford Five Hundred 3.0 L 6 cyl
  • Ford Focus C-MAX 1.6 L TDCi 110 PS
  • Ford Freestyle 3.0 L 6 cyl
  • [http://www.internetautoguide.co




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