what is the CVT?

Answer 1

The continuously variable transmission (CVT) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios.

The continuously variable transmission should not be confused with the power split transmission (PST), as used in the Toyota Prius and other hybrid vehicles that use two or more inputs with one output, despite some similarities in their function.

A CVT need not be automatic, nor include zero or reverse output. Such features may be adapted to CVTs in certain specific applications.

Other mechanical transmissions only allow a few different discrete gear ratios to be selected, but the continuously variable transmission essentially has an infinite number of ratios available within a finite range, so it enables the relationship between the speed of a vehicle engine and the driven speed of the wheels to be selected within a continuous range. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient speeds within a narrow range.

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


Types

Infinitely Variable Transmission (IVT)
A specific type of CVT is the infinitely variable transmission (IVT), which has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined "higher" ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output/input which have high input/output ratios that are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit. Most continuously variable transmissions are not infinitely variable.


The IVT is a specific type of CVT that has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined "higher" ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. [Note: remember that so-called "low" gears are a reference to low ratios of output/input, which have high input/output ratios that are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit.]

Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system) that facilitates the subtraction of one speed from another speed within the set of input and planetary gear rotations. This subtraction only needs to result in an continuous range of values that includes a zero output; the maximum output/input ratio can be arbitrarily chosen from infinite practical possibilities through selection of extraneous input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. Importantly, the IVT is distinguished as being "infinite" in its ratio of high gear to low gear within its range; high gear is infinite times higher than low gear. The IVT is always engaged, even during its zero output adjustment.

The term "Infinitely Variable Transmission" does not imply reverse direction, disengagement, automatic operation, or any other quality except ratio selectabilty within a continuous range of input/output ratios from a defined minimum to an undefined, "infinite" maximum. This means continuous range from a defined output/input to zero output/input ratio.


Ratcheting CVT
The Ratcheting CVT is a transmission that relies on static friction and is based on a set of elements that successively become engaged and then disengaged between the driving system and the driven system, often using oscillating or indexing motion in conjunction with one-way clutches or ratchets that rectify and sum only "forward" motion. The transmission ratio is adjusted by changing linkage geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted, even when the average linkage speed remains constant. Power is transferred from input to output only when the clutch or ratchet is engaged, and therefore when its locked into a static friction mode where the driving & driven rotating surfaces momentarily rotate together without slippage.

These CVT transmissions can transfer substantial torque because their static friction actually increases relative to torque throughput, so slippage is impossible in properly designed systems. Efficiency is generally high because most of the dynamic friction is caused by very slight transitional clutch speed changes. The drawback to ratcheting CVT's is vibration caused by the successive transition in speed required to accelerate the element which must supplant the previously operating & decelerating, power transmitting element. An Infinitely Variable Transmission (IVT) that is based on a Ratcheting CVT and subtraction of one speed from another will greatly amplify the vibration as the IVT output/input ratio approaches zero.

Ratcheting CVT's are distinguished from Variable Diameter Pulleys (VDP's) and Roller-based CVT's by being static friction-based devices, as opposed to being dynamic friction-based devices that waste significant energy through slippage of twisting surfaces.


Variable-diameter pulley (VDP)
This type of CVT uses pulleys, typically connected by a rubber-covered metal or laminated steel 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 "Extroid"), and hence lose efficiency.

Diagrams:

Torotrak IVT

Hydrostatic CVT
Hydrostatic transmissions use a variable displacement pump and a hydraulic motor. All power is transmitted by hydraulic 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
Main article: Hydristor
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 but real-world ratios are constrained by physics.


Simkins' Ratcheting CVT
This transmission is an example of a Ratcheting CVT, prototyped as a bicycle transmission, protected under U.S. Patent #5516132. The input is the crank with a round hub integrated with it, and an array of twelve arms that are pivotally mounted to pins in the hub circle. Each arm has a pinion gear mounted on a one way clutch that allows only clockwise rotation of the pinion relative to the arm. All of these pinions are engaged with a large ring gear that is integrated with the chainwheel as the output, and the ring gear/chainwheel assembly is mounted to a mechanism that enables it to be adjusted from a position of concentricity with the crank hub to various amounts of eccentricity with the crank hub. Adjustment of this eccentricity variably changes the output/input ratio from 1:1 to 2.6:1 as the ring gear/sprocket assembly is moved from a position concentric with the crank hub to an eccentric position.

The eccentricity control mechanism is connected to a spring that pushes the transmission into its eccentric high gear position shown in the picture. The largest spread of the arms is indicative of the gear ratio because the spreading arms are the only arms whose pinions (and one-way clutches) are locked and driving the ring gear/chainwheel assembly. Strong pedaling torque causes this mechanism to react against the spring, moving the ring gear/chainwheel assembly toward a concentric, lower gear position. When the pedaling torque relaxes to lower levels, the transmission self-adjusts toward higher gears, accompanied by an increase in transmission vibration that produces a foot massage! This transmission behaves according to the definition of a Ratcheting CVT.


Anderson A+CVT
A+CVT Prototype (Dual Cone Version)Anderson A+CVT is a technology invented by Larry Anderson, under US patents 6,575,856 and 6,955,620. Two parallel cones have "floating sprocket bars" mounted in longitudinal grooves around the circumference of each cone.[1]

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 floating sprocket bars make the A+CVT positive-drive, non-friction-dependent. Another advantage of the A+CVT is the simplicity of its design, as it consists of far fewer components than other transmissions. The technology is also adaptable to a variable diameter pulley-type CVT, by mounting the floating sprocket bars on the inner face of the pulley sheaves. A few critics[Please name specific person or group] have speculated that noise could be a problem with the A+CVT. However, Anderson has said that he believes noise will be no more of an issue with the A+CVT than with other transmissions, as the A+CVT will be lubricated and encased in a housing.


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 (e.g., Jatco "Extroid") also employ a torque converter. Fuel efficiency advantages as high as 20% over four-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
Leonardo da Vinci, in 1490, conceptualized a stepless continuously variable transmission.[2] [3] The first patent for a toroidal CVT was filed in 1886. [4]

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

A 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 Extroid, 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 Power Split Transmission (PST), allowing either the 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 ZF Sachs and is currently produced in Batavia, Ohio. The CVT is computer-controlled and combines fuel efficiency and smooth riding. Ford also sold Escort (European version) and Orion models in Europe with CVT transmissions in the 1980s and 1990s. A disadvantage of this transmission is its inability to transmit torque in reverse, meaning that the engine cannot be used to brake the vehicle on a downgrade or when the accelerator pedal is relaxed, thus causing accelerated brake wear and possible brake overheating during mountain driving scenarios.

The 2007 Dodge Caliber[5] and the related Jeep Compass employ a CVT using a variable pulley system as their optional automatic transmission.

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

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


Examples
Many small tractors for home and garden use have simple hydrostatic or rubber belt CVTs. For example, the John Deere Gator line of small utility vehicles (used by many parks, stadiums, colleges, and other places where miscellaneous items must be displaced by laborers) use a belt with a conical pulley system. They can deliver a lot of power but can also build up speed to 10-15 MPH, all without need for a clutch or gearshift. Most snowmobiles use CVTs. 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 go-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
Honda Civic HX 1.7 L 4 cyl
Honda Civic Hybrid 1.3 L 4 cyl
Honda City 1.5 L
Honda HR-V 1.6 L
Honda Insight 1.0 L 3 cyl
Honda Jazz 1.4L / Honda Fit 1.3 L/1.5 L
Jeep Compass 2.4 L
Jeep Patriot 2.4 L
Lexus GS450h 3.5 L 6 cyl
Lexus RX400h 3.3 L 6 cyl
Mercedes-Benz A-Class
Mercedes-Benz B-Class
Mercury Montego 3.0 L 6 cyl
Microcar MC1/MC2 505cc 2 cyl diesel or petrol
Microcar Virgo 505cc 2 cyl diesel or petrol
Mitsubishi Colt 1.5 L MIVEC 4 cyl with INVECS-III CVT (Asian-Oceanian version only, 72 kW)
Mitsubishi Lancer 1.6 L/1.8 L MIVEC 4 cyl with INVECS-III CVT (Asian version only)
MG F/MG TF 1.8L
BMW MINI One and Cooper.
Nissan Altima (from 2007)
Nissan Cube
Nissan Maxima (from 2007)
Nissan Micra 1.0 L/1.3 L
Nissan Murano 3.5 L
Nissan Primera 2.0 L
Nissan Qashqai 2.0 L
Nissan Sentra (from 2007)
Nissan Serena 2.0 L
Nissan Skyline 350GT-8
Nissan Tiida / Versa
Opel Vectra 1.8 L
Rover 25
Rover 45
Rover Streetwise
Saturn ION Quad Coupe (2003-2004)
Saturn VUE 2.2 L AWD (2002-2005), 2.2 FWD (2002-2004)
Subaru R1
Subaru R2
Subaru Stella
Toyota Highlander Hybrid 3.3 L 6 cyl
Toyota Camry Hybrid 2.4L 4 cyl
Toyota Prius 1.5 L 4 cyl

Old automobiles equipped with CVT
DAF 600
DAF 750
DAF Daffodil (types 30, 31 and 32)
DAF 33
DAF 44
DAF 46
DAF 55
DAF 66
Fiat Uno
Ford Fiesta
Honda Civic ESi
Honda Civic HX
Nissan Micra
Subaru Justy
Volvo 66
Volvo 300 series
Volvo 440/460
Daewoo Matiz II with E3CVT (Currently GM Daewoo)

Answer 2

Hi,

In Electrical Constant Voltage Transformer is called CVT.

Answer 3

CVT is a belt and pulley syetem with an infinite number of gear ratios. It doesn't have mechanical gears like a conventional automatic gearbox. When you step on the pedal CVT can feel like a car with a slipping clutch as it gains speed. Amazingly enough, many of these get better gas mileage and acceleration times than a conventional automatic. They are also simple in design as opposed to other gearboxes. Also, in response to drifting an Altima, Altimas are front wheel drive. You need rear wheel drive to drift. Then again, why would you want to drift it?

Answer 4

CVT= Continuously Variable Transmission...

"I think"... it's the transmission the has no feel of shifts, it is a continuous surge of acceleration as you press the accelerator

"/JOe

Answer 5

CVT is constant variable transmission.There are 3 main types the most common is the variable expanding pulley system.This is used very successfully by Audi it provides better acceleration and is more economic on fuel.If you go to howstuffworks.com it is explained in detail with animated drawings.Having worked on and driven CVT transmissions i have to disagree with others who say they are no good.Mick UK.

Answer 6

Joe is correct. It is a form of automatic transmission used by companies such as Ford. Great idea in theory, unreliable in practice.

Edit: Ethol, why copy and paste all that crap from another site when you could just add the link? Even more questionable is why do some fuckwits rate it as good!

My god the place is a joke. I'll leave the blind to lead the blind.

Answer 7

It is a continuously variable transmission. The transmission has no gears so it doesnt shift. Im not exactly sure how it works but it does not shift ever so it is very smooth. It also eliminates the problem of an automatic transmission shifting back and forth between gears on hills when there isno ideal gear. If you go to http://auto.howstuffworks.com/cvt1.htm all your questions will be ansered.

Answer 8

A CVT means CONVERTIBLE,,,,NMN