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.
CVTs 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.
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 a 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 it is locked into a static friction mode where the driving & driven rotating surfaces momentarily rotate together without slippage.
These CVTs 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 CVTs 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 CVTs are distinguished from Variable Diameter Pulleys (VDPs) and Roller-based CVTs 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) or Reeves Drive
In this system, there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two sections of one pulley closer together and the two sections of the other pulley farther apart. Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which changes the overall gear ratio. The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) simultaneously to maintain the proper amount of tension on the belt.
Diagrams:
★
Pulley-based CVTs
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 'CVTs'
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 can be 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 is relatively easy to use because all forward and reverse speeds can be accessed using a single lever.
This type of transmission has been effectively applied to a variety of inexpensive and expensive versions of ridden
lawn mowers and
garden tractors. Many versions of riding lawn mowers and garden tractors propelled by a hydrostatic transmission are capable of pulling a reverse tine tiller and even a single bladed plow.
One class of riding lawn mower that has recently gained in popularity with consumers is zero
turning radius mowers. These mowers have traditionally been powered with wheel hub mounted hydraulic motors driven by continuously variable pumps, but this design is relatively expensive. A company called Hydro-Gear, a joint venture between
Sauer-Danfoss and Agri-Fab, Inc., of Sullivan, Illinois, created the first cost-effective integrated hydrostatic transaxle suitable for propelling consumer zero turning radius mowers. An integrated hydrostatic transaxle (IHT) uses a single housing for both hydraulic elements and gear-reducing elements. As of May 9, 2007, Hydro-Gear remains the only company producing integrated hydrostatic transaxles for consumer zero turning radius mowers in North America.
Some
heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural machinery including foragers and combines, but not anything that works the ground because the transmission cannot transmit enough torque.
Hydristor IVT
Main articles: 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. 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. This transmission behaves according to the definition of a Ratcheting CVT.
Variable toothed wheel CVT
Variable toothed wheel transmission relies on a toothed wheel positively engaged with a chain where the toothed wheel has the ability to add or subtract a tooth at a time in order to alter its ratio with relation to the chain it is driving. The "toothed wheel" can take on many configurations as is listed below in the patent specifications and include ladder chains, drive bars and sprocket teeth. This type of CVT is not a true CVT that can alter its ratio in infinite increments but rather approaches CVT capability by having a large number of ratios, typical 49, as is described in the VW owned German patent application DE10010741A1. The huge advantage of this type of CVT is that it is a positive mechanical drive and thus does not have the frictional losses and limitations of the Roller based or VDP CVT’s. The challenge in this type of CVT is to add or subtract a tooth from the toothed wheel in a very precise and controlled way in order to maintain synchronized engagement with the chain. This type of transmission has the potential to change ratios under load because of the large number of ratios resulting in the order of 3% ratio change differences between ratios, thus a clutch or torque convertor is only necessary for pull away. None of this type of CVT is in commercial use probably because of above mentioned development challenges. Other examples of this type of CVT can be found in the following patent specifications: US5406863, WO9404411, US2669885, and
WO2005036028.
Diagram and video clip
iCVT
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 might 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 expected jerk of a non-CVT 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.
[1] The first
patent for a toroidal CVT was filed in 1886.
[2]
From the
1950s, CVTs have been applied to aircraft electrical power generating systems.
A CVT, called
Variomatic, was designed and built by the
Dutchman 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, the
DAF 600,
[1] was produced in 1958. Van Doorne's patents were later transferred to a company called VDT (Van Doorne Transmissie B.V.) when the passenger car division was sold to
Volvo; it's CVT was used in
Volvo 340.
The
Ford Fiesta and
Fiat Uno were the first mainstream European cars to be equipped with steel-belted CVT (as opposed to the less robust rubber-belted DAF design), in
1987.
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. The CVT is also currently employed in the Honda City ZX that is manufactured in India.
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.
Ford introduced a chain-driven CVT known as the CFT30 — with a maximum torque capacity of — in their 2005
Ford Freestyle,
Ford Five Hundred and
Mercury Montego. The transmission was designed in cooperation with German automotive supplier
ZF Friedrichshafen and was produced in Batavia, Ohio at Batavia Transmissions LLC (a subsidiary of Ford Motor Company) until 3/22/07. The Batavia plant also produced the belt-driven CFT23 CVT which went in the Ford Focus C-MAX and
Escape Hybrid and still produces the 4-speed automatic (CD4E) for the Ford Escape and Mazda Tribute. Ford also sold Escort (European version) and Orion models in Europe with CVTs in the 1980s and 1990s.
ZF Friedrichshafen supplied its belt drive VT-1 CVT unit to BMW for use in some versions of the European Mini Cooper.
The 2007
Dodge Caliber[3] and the related
Jeep Compass employ a CVT using a variable pulley system as their optional automatic transmission.
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.
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 engine 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. The
Tomcar range of off-road vehicles also utilizes the CVT system.
Some older
drill presses contain a pulley-based CVT where the output shaft (which the chuck is connected to) has a pair of manually-adjustable conical pulley halves to which a wide drive belt from the motor loops through. The pulley on the motor, however, is usually fixed in diameter, or may have a series of given-diameter steps to allow a selection of speed ranges. A handwheel on the drill press, marked with a scale corresponding to the desired machine speed, is mounted to a reduction gearing system for the operator to precisely control the width of the gap between the pulley halves. This gap width thus adjusts the gearing ratio between the motor's fixed pulley and the output shaft's variable pulley, changing speed of the chuck; a tensioner pulley is implemented in the belt transmission to take up or release the slack in the belt as the speed is altered. However, the drill press' speed almost always cannot be changed without the motor running.
New automobiles equipped with CVT
★
Audi A4 2.0/1.8T/2.4/3.0/2.0 TDI/2.5 TDI
★
Audi A6 2.0/1.8T/2.4/3.0/2.5 TDI
★
Daihatsu Mira Custom 0.66l 3 cyl
★
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 1.3L/1.4L/1.5L /
Honda Fit 1.3 L/1.5 L
★
Honda Odyssey (JDM)
★
Honda Stream
★
Jeep Compass 2.4 L
★
Jeep Patriot 2.4 L
★
Lexus GS450h 3.5 L 6 cyl
★
Lexus RX400h 3.3 L 6 cyl
★
Lexus LS600h 5.0 L 8 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) the 2008 version also
★
Mitsubishi Lancer 2.0
MIVEC 4 cyl with
INVECS-III CVT 2008 (North America)
★
Mitsubishi Outlander 2.4
MIVEC 4 cyl with
INVECS-III CVT
★
MG F/MG TF 1.8L
★
BMW MINI One and Cooper.
★
Nissan Altima (from 2007)
★
Nissan Cube
★
Nissan Maxima (from 2007, Model SE)
★
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, Model SL)
★
Nissan Serena 2.0 L
★
Nissan Skyline 350GT-8
★
Nissan Teana 3.5 L
★
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)
Notes
1. 20th Century Cars, Hilton Holloway, Martin Buckley, , , Carlton, 2002, ISBN 1-84222-835-8
External links
★
Video-simulation of CVT belt in action
★
Video of a real CVT in operation on a racing kart
★
How CVTs Work on
HowStuffWorks
★
CVT - Continuously Variable Transmission homepage
★
Anderson A+CVT homepage
★
Torotrak IVT homepage
★
Fallbrook Technologies homepage
★
eCars.com.au page about CVT
★
AutoZine Technical School - CVT
★
Fixed Pitch Continuously Variable Transmission (FPCVT)
★
Gyroscopic gear
★
GyroTorque
★
InfiniTran Controlled Epicyclic Gear Train
★
Incremental CVT based on variable sprocket
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