Editing Continuously variable transmission (section)

== Types ==
{{more citations needed section|date=July 2020}}

=== Pulley-based ===
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| caption1 = Belt-driven CVT for a [[Scooter (motorcycle)|motor scooter]]
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| caption2 = A PIV chain drive
| image3 = Claas Mercator 12 August 2020 JM (7).webm
| caption3 = CVT in a [[Claas Mercator]] combine harvester. The pulley's effective diameter is changed by pushing the two conical discs either towards or away from each other.
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The most common type of CVT uses a [[V-belt]] which runs between two variable-diameter pulleys.<ref>{{cite journal|last=Fischetti|first=Mark|title=No More Gears|journal=Scientific American|volume=294|issue=1|pages=92–3|date=January 2006 |doi=10.1038/scientificamerican0106-92|pmid=16468439|bibcode=2006SciAm.294a..92F}}</ref> The pulleys consist of two cone-shaped halves that move together and apart. The V-belt runs between these two halves, so the effective diameter of the pulley is dependent on the distance between the two halves of the pulley. The V-shaped cross-section of the belt causes it to ride higher on one pulley and lower on the other; therefore, the gear ratio is adjusted by moving the two [[Sheave#Adjustable pulley|sheaves]] of one pulley closer together and the two sheaves of the other pulley farther apart.<ref name="howstuffworks" />

As the distance between the pulleys and the length of the belt does not change, both pulleys must be adjusted (one bigger, the other smaller) simultaneously to maintain the proper amount of tension on the belt. Simple CVTs combining a centrifugal drive pulley with a spring-loaded driven pulley often use belt tension to effect the conforming adjustments in the driven pulley.<ref name="howstuffworks">{{cite web|url=https://auto.howstuffworks.com/cvt2.htm|title=How CVTs Work |website=howsuffworks.com|access-date=26 August 2020|date=27 April 2005}}</ref> The V-belt needs to be very stiff in the pulley's axial direction to make only short radial movements while sliding in and out of the pulleys.

The radial thickness of the belt is a compromise between the maximum gear ratio and torque. Steel-reinforced V-belts are sufficient for low-mass, low-torque applications like utility vehicles and snowmobiles, but higher-mass and -torque applications such as automobiles require a chain. Each element of the chain must have conical sides that fit the pulley when the belt is running on the outermost radius. As the chain moves into the pulleys the contact area gets smaller. As the contact area is proportional to the number of elements, chain belts require many very small elements.

A belt-driven design offers approximately 88% efficiency,<ref>{{cite web |url=http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf |title=CVT Efficiency |website=zeroshift.com |archive-url=https://web.archive.org/web/20140714215029/http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf |access-date=22 April 2014 |archive-date=14 July 2014 |url-status=dead}}</ref> which, while lower than that of a [[manual transmission]], can be offset by enabling the engine to run at its most efficient RPM regardless of the vehicle's speed. When power is more important than economy, the ratio of the CVT can be changed to allow the engine to turn at the [[Revolutions per minute|RPM]] at which it produces the greatest power.

In a chain-based CVT, numerous chain elements are arranged along multiple steel bands layered over one another, each of which is thin enough to easily [[Bending|bend]]. When part of the belt is wrapped around a pulley, the sides of the elements form a conical surface.<ref>{{Cite web|title=XTRONIC CVT {{!}} Nissan {{!}} Technology|url=https://www.nissan-global.com/EN/TECHNOLOGY/OVERVIEW/cvt.html|url-status=live|access-date=2021-09-20|website=Nissan Motor Corporation Global Website|publisher=[[Nissan|Nissan Motor Co. Ltd.]]|archive-url=https://web.archive.org/web/20110120043856/http://www.nissan-global.com:80/EN/TECHNOLOGY/OVERVIEW/cvt.html |archive-date=20 January 2011 }}</ref><ref>{{Cite web|title=Pushbelt|url=https://www.bosch-mobility-solutions.com/en/solutions/transmission-technology/pushbelt/|url-status=live|access-date=2021-09-20|website=Bosch Mobility Solutions|publisher=[[Robert Bosch GmbH]]|language=en|archive-url=https://web.archive.org/web/20210508172104/https://www.bosch-mobility-solutions.com/en/solutions/transmission-technology/pushbelt/ |archive-date=8 May 2021 }}</ref> In the stack of bands, each band corresponds to a slightly different drive ratio, and thus the bands slide over each other and need sufficient [[lubrication]]. An additional film of lubricant is applied to the pulleys. The film needs to be thick enough to prevent direct contact between the pulley and the chain, but thin enough to not waste power as each chain element enters it.{{Citation needed|date=September 2021}}

Some CVTs transfer power to the output pulley via [[Tension (physics)|tension]] in the belt (a "pulling" force), while others use [[Compression (physics)|compression]] of the chain elements (where the input pulley "pushes" the belt, which in turn pushes the output pulley).<ref>{{cite book |last1=Ambrósio |first1=Jorge A. C. |title=Advances in Computational Multibody Systems |url=https://books.google.com/books?id=Jv6e3oxkI6YC |publisher=Springer |access-date=8 July 2020 |page=271 |language=en |date=5 July 2005|isbn=9781402033926 }}</ref><ref>{{cite book |last1=Pfeiffer |first1=Friedrich |title=Mechanical System Dynamics |date=2008 |publisher=Springer |isbn=978-3-540-79436-3 |url=https://books.google.com/books?id=_bjlZaxm5UwC |page=320 |access-date=8 July 2020 |language=en}}</ref><ref>{{cite web |title=CVT Transaxle Steel Push Belt Construction | date=16 September 2016 |url=https://www.youtube.com/watch?v=PiwRUfFEc5k | archive-url=https://ghostarchive.org/varchive/youtube/20211107/PiwRUfFEc5k| archive-date=2021-11-07 | url-status=live|via=YouTube |publisher=Weber State University |access-date=8 July 2020}}{{cbignore}}</ref>

Positively Infinitely Variable (PIV) chain drives are distinct in that the chain positively interlocks with the conical pulleys.  This is achieved by having a stack of many small rectangular plates in each chain link that can slide independently from side-to-side.  The plates may be quite thin, around a millimeter thick.  The conical pulleys have radial grooves.  A groove on one side of the pulley is met with a ridge on the other side and so the sliding plates are pushed back and forth to conform to the pattern, effectively forming teeth of the correct pitch when squeezed between the pulleys.  Due to the interlocking surfaces, this type of drive can transmit significant torque and so has been widely used in industrial applications.  However, the maximum speed is significantly lower than other pulley-based CVTs.  The sliding plates will slowly wear over years of usage.  Therefore the plates are made longer than is needed, allowing for more wear before the chain must be refurbished or replaced.  Constant lubrication is required and so the housing is usually partially filled with oil.<ref>{{Cite web|title=PIV Vertical Drives – Gayatri Gear|url=http://gayatrigear.com/piv-vertical-drives/|access-date=2020-09-15|language=en-US}}</ref><ref>{{Cite web|title=Positively Infinitely Variable (PIV) Chain|url=https://www.usarollerchain.com/PIV-Chain-s/2117.htm|access-date=2020-09-15|website=usarollerchain.com}}</ref>

=== Toroidal ===
[[File:Extroid CVT.jpg|thumb|Toroidal CVT used in the [[Nissan Cedric#Tenth generation Y34|Nissan Cedric (Y34)]] ]]

Toroidal CVTs, as used on the [[Nissan Cedric#Tenth generation (Y34; 1999)|Nissan Cedric (Y34)]],<ref>{{cite web |title=Nissan's Weird Double CVT Is Perfect for High-Torque Applications |url=https://www.roadandtrack.com/new-cars/car-technology/a25412165/nissan-double-toroidal-cvt-explained/ |website=Road & Track |access-date=16 July 2020 |date=5 December 2018}}</ref><ref>{{cite web |title=Tech & Trends: Nissan Producing Amazing New CVT |url=https://www.wardsauto.com/news-analysis/tech-trends-nissan-producing-amazing-new-cvt |website=wardsauto.com |access-date=16 July 2020 |language=en |date=1 December 1999}}</ref> and those built by CVTCORP,<ref>{{cite web |title=CVTCORP Technology |url=https://www.cvtcorp.com/expertise/?lang=en}}</ref> consist of a series of discs and rollers. The discs can be pictured as two almost-conical parts arranged point-to-point, with the sides dished such that the two parts could fit into the central hole of a [[torus]]. One disc is the input, and the other is the output. Between the discs are rollers, which vary the ratio and transfer power from one side to the other. When the rollers' axes are [[perpendicular]] to the axis of the discs, the effective diameter is the same for the input discs and the output discs, resulting in a 1:1 drive ratio. For other ratios, the rollers are rotated along the surfaces of the discs so that they are in contact with the discs at points with different diameters, resulting in a drive ratio of something other than 1:1.<ref>{{cite web |title=How CVTs Work – Toroidal CVTs |url=https://auto.howstuffworks.com/cvt3.htm |website=howstuffworks.com |access-date=16 July 2020 |language=en |date=27 April 2005}}</ref>

An advantage of a toroidal CVT is the ability to withstand higher torque loads than a pulley-based CVT.<ref>{{cite web |title=Extroid CVTs – For Application to Rear-Wheel-Drive Cars Powered by Large Engine |url=https://www.nissan-global.com/PDF/tcvt_e.pdf |website=nissan-global.com |access-date=16 July 2020}}</ref> In some toroidal systems, the direction of thrust can be reversed within the CVT, removing the need for an external device to provide a reverse gear.<ref>{{cite web |url=http://www.torotrak.com/pdfs/tech_papers/2007/CTI_Michigan_2007.pdf |title=Developments in Full-Toroidal Traction Drive Infinitely & Continuously Variable Transmissions (CTI Innovative Automotive Transmissions Conference and Exhibition) |publisher=Torotrak |date=August 2007 |url-status=dead |archive-url=https://web.archive.org/web/20120917085727/http://www.torotrak.com/pdfs/tech_papers/2007/CTI_Michigan_2007.pdf |archive-date=17 September 2012}}</ref>

=== Ratcheting ===
A ratcheting CVT uses a series of one-way [[clutch]]es or [[Ratchet (device)|ratchets]] that rectify and sum only "forward" motion. The on-off characteristics of a typical ratchet means that many of these designs are not continuous in operation (i.e. technically not a CVT), but in practice, there are many similarities in operation, and a ratcheting CVT is able to produce a zero-output speed from any given input speed (as per an Infinitely Variable Transmission). The drive 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.

Ratcheting 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 the vibration caused by the successive transition in speed required to accelerate the element, which must supplant the previously operating and decelerating power-transmitting element.

The design principle dates back to before the 1930s, with the original design intended to convert [[Rotation around a fixed axis|rotary motion]] to [[Oscillation|oscillating motion]] and back to rotary motion using roller clutches.<ref>{{cite book |last1=Franklin |first1=D. |title=Ingenious mechanisms for designers and inventors ... |date=1930 |publisher=Industrial Press |isbn=0-8311-1084-8 |pages=343–345 |edition=1st}}</ref> This design remains in production as of 2017, for use with low-speed electric motors.<ref>{{cite web |url=http://www.zero-max.com/products/drives/drivesmain.asp |title=drives |website=zero-max.com |access-date=19 September 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090301191004/http://www.zero-max.com/products/drives/drivesmain.asp |archive-date=1 March 2009}}</ref> An example prototyped as a bicycle transmission was patented in 1994.<ref>{{cite web |title=US patent US5516132A: Variable-speed transmission |url=https://patents.google.com/patent/US5516132 |access-date=17 July 2020 |language=en |date=22 July 1994}}</ref> The operating principle for a ratcheting CVT design, using a [[Scotch yoke]] mechanism to convert rotary motion to oscillating motion and [[non-circular gear]]s to achieve uniform input to output ratio, was patented in 2014.<ref>{{cite web |title=US patent US9970520B2: Continuous variable transmission with uniform input-to-output ratio that is non-dependent on friction |url=https://patents.google.com/patent/US9970520B2 |access-date=17 July 2020 |language=en |date=18 March 2014}}</ref>

=== Hydrostatic/hydraulic ===


A hydrostatic CVT uses an engine-driven, [[positive-displacement pump#positive-displacement_pumps|positive-displacement pump]] to deliver oil under pressure to one or more [[hydraulic motor]]s, the latter creating the torque that is applied to the vehicle's driving wheel(s).  The name "hydrostatic CVT", which misuses the term "[[hydrostatics|hydrostatic]]", differentiates this type of transmission from one that incorporates a [[torque converter|hydrodynamic torque multiplier]] ("torque converter") into its design.

In a hydrostatic CVT, the effective "gear ratio" between the engine and the driving wheel(s) is the result of a difference between the pump's displacement—expressed as cubic inches or cubic centimeters per revolution—and the motor's displacement.  In a closed system, that is, a system in which all of the pump's output is delivered to the motor(s), this ratio is given by the equation '''{{mono|1=GR = Dm ÷ Dp}}''', where '''{{mono|Dp}}''' is the pump's effective displacement, '''{{mono|Dm}}''' is the motor's displacement, and '''{{mono|GR}}''' is the "gear ratio".

In a hydrostatic CVT, the effective "gear ratio" is varied by varying effective displacement of the pump, which will vary the volume of oil delivered to the motor(s) at a given engine speed (RPM).  There are several ways in which this may be accomplished, one being to divert some of the pump's output back to the reservoir through an adjustable valve.  With such an arrangement, as more oil is diverted by opening the valve, the effective displacement of the pump is reduced and less oil is delivered to the motor, causing it to turn more slowly.  Conversely, closing the valve will reduce the volume of oil being diverted, increasing the effective displacement of the pump and causing the motor to turn more rapidly.

Another method is to employ a [[variable displacement pump]].  When the pump is configured for low displacement, it produces a low volume of oil flow, causing the [[Hydraulics|hydraulic]] motor(s) to turn more slowly.  As the pump's displacement is increased, a greater volume of oil flow is produced for any given engine RPM, causing the motor(s) to turn faster.

Advantages of a hydrostatic CVT include:
* Capacity scalability.  A hydrostatic CVT's power-transmission capacity is readily adapted to the application by using a correctly-sized pump and matching hydraulic motor(s).
* Flexibility.  As power transfer from the engine-driven pump to the hydraulic motor(s) is through the medium of flowing oil, the motor(s) can be mounted in otherwise-inconvenient locations by using hoses to convey oil from the pump to the motor(s), thus simplifying the design of [[all-wheel drive]] [[articulated vehicle]]s.
* Smoothness.  As the effective "gear ratio" of a hydrostatic CVT is infinitely-variable, there are no distinct transitions in torque multiplication, such as produced with conventional, geared transmissions.
* Simplified control.  Operation through the full range of forward and reverse speeds can be controlled using a single lever or a foot pedal to actuate a diversion valve or variable-displacement pump.
* Arbitrarily slow crawl speeds.  The potential for high torque multiplication at very low speeds allows for precise vehicle movement while under load.

Disadvantages of a hydrostatic CVT include:
* Reduced efficiency.  Gears are one of the most efficient methods of mechanical power transmission, with efficiencies as high as 90 percent in many cases.  In contrast, few hydrostatic transmission systems achieve more than about 65 percent efficiency.  This is due to a combination of internal losses in the pump and motor(s), and losses in the piping and valves.
* Higher cost.  For a given level of power-transmitting capacity, a hydrostatic CVT will be more expensive to produce than an equivalent geared transmission.  In addition to the pump and motor(s), a hydrostatic system requires the use of an oil reservoir, piping and in many applications, an oil cooler, this last item being necessary to dissipate the waste heat that results from hydrostatic power transmission's relatively low efficiency.
* Greater weight.  Due to the high oil pressure at which a hydrostatic CVT operates, the pump and motor(s) are under considerable mechanical stress, especially when maximum power and loading is being applied.  Hence these items must be very robust in construction, typically resulting in heavy components.  Additional weight will be found in the oil reservoir and its oil load, as well as the piping and valving.

Uses of hydrostatic CVTs include [[forage harvester]]s, [[combine harvester]]s, small wheeled/tracked/skid-steer [[Loader (equipment)|loaders]], crawler [[tractor]]s, and [[road roller]]s.  One agricultural example, produced by [[AGCO]], splits power between hydrostatic and mechanical transfer to the output shaft via a planetary gear in the forward direction of travel (in reverse, the power transfer is fully hydrostatic).  This arrangement reduces the load on the hydrostatic portion of the transmission when in the forward direction by transmitting a significant portion of the torque through more efficient fixed gears.<ref>{{cite web |url=https://www.youtube.com/watch?v=dgtIKMAjvFI | archive-url=https://ghostarchive.org/varchive/youtube/20211107/dgtIKMAjvFI| archive-date=2021-11-07 | url-status=live|title=AGCO's Continuously Variable Transmission (CVT) Explained |website=YouTube | date=2 August 2010|access-date=26 October 2012}}{{cbignore}}</ref>

A variant called the ''Integrated Hydrostatic Transaxle'' (IHT) uses a single housing for both hydraulic elements and gear-reducing elements and is used in some [[mini-tractor]]s and ride-on [[lawn mower]]s.

The 2008–2010 [[Honda DN-01]] [[Cruiser (motorcycle)|cruiser motorcycle]] used a hydrostatic CVT in the form of a variable-displacement axial piston pump with a variable-angle [[swashplate]].

=== Cone ===
[[File:Evans friction cone - Hagley Aug 2009.jpg|thumb|Evans Variable Speed Countershaft]]

A cone CVT varies the drive ratio by moving a wheel or belt along the axis of one or more conical rollers. The simplest type of cone CVT, the single-cone version, uses a wheel that moves along the slope of the cone, creating variation between the narrow and wide diameters of the cone.

Some cone CVT designs use two rollers.<ref>{{cite web|url=https://www.youtube.com/watch?v=MhFK5gfAGpM| archive-url=https://ghostarchive.org/varchive/youtube/20211107/MhFK5gfAGpM| archive-date=2021-11-07 | url-status=live|title=CVT Explained | date=February 2008|via=YouTube |access-date=27 August 2011}}{{cbignore}}</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=pyYNvwKPAKA| archive-url=https://ghostarchive.org/varchive/youtube/20211107/pyYNvwKPAKA| archive-date=2021-11-07 | url-status=live|title=CVT transmission | date=23 November 2007|via=YouTube |access-date=27 August 2011}}{{cbignore}}</ref> In 1903, William Evans and Paul Knauf applied for a patent on a continuously variable transmission using two parallel conical rollers pointing in opposite directions and connected by belts that could be slid along the cones to vary the transmission ratio.<ref>William Evans and Paul Knauf, Variable-Speed-Transmission Device, {{US Patent|759872|U.S. Patent 759872}}, granted 17 May 1904.</ref><ref>William Evans and Paul Knauf, Power-Transmission Device, {{US Patent|759873|U.S. Patent 759873}}, granted 17 May 1904.</ref> The Evans Variable Speed Countershaft, produced in the 1920s, is simpler—the two rollers are arranged with a small constant-width gap between them, and the position of a leather cord that runs between the rollers determines the transmission ratio.<ref>{{cite journal |title=Evans Friction Cone Co. advertisement |journal=Machinery Magazine |date=19 January 1922 |url=http://antiquemachinery.com/yahoo_site_admin/assets/docs/000-Machinery-Magizine-January-19-1922-Gorton-Panagrph-600dpi--top-.361154958.jpg |access-date=18 July 2020}}</ref>

=== Epicyclic ===
In an [[Epicyclic gearing|epicyclic]] CVT (also called a planetary CVT), the gear ratio is shifted by tilting the axes of spherical rollers to provide different contact radii, which in turn drive input and output discs. This is similar in principle to toroidal CVTs. Production versions include the [[NuVinci continuously variable transmission|NuVinci CVT]].<ref>{{cite web |title=Continuously variable planetary transmission |url=https://www.oemoffhighway.com/drivetrains/article/10218934/continuously-variable-planetary-transmission |website=oemoffhighway.com |access-date=18 July 2020 |language=en-us |date=21 February 2011}}</ref>

=== Hybrid electric ===
Several [[hybrid electric vehicle]]s—such as the Toyota Prius, Nissan Altima, Mitsubishi Outlander PHEV, and [[Ford Escape|Ford Escape Hybrid]]—use electric variable transmissions (EVTs, sometimes eCVT) to control the contribution of power from the electric motor and the internal combustion engine. These differ from standard CVTs in that they are powered by an electric motor in addition to the engine, often using [[Planetary gear|planetary gears]] to combine their outputs instead of a belt used in traditional CVTs. A notable example is the Toyota [[Hybrid Synergy Drive]].

=== Other types ===
[[Friction drive|Friction-disk]] transmissions were used in several vehicles and small locomotives built in the early 20th century, including the [[Lambert (automobile)|Lambert]] and [[Metz Company|Metz]] automobiles. Used today in [[snow blower]]s, these transmissions consist of an output disk that is moved across the surface of the input disk upon which it rolls. When the output disk is adjusted to a position equal to its own radius, the resulting drive ratio is 1:1. The drive ratio can be set to infinity (i.e. a stationary output disk) by moving the output disk to the center of the input disk. The output direction can also be reversed by moving the output disk past the center of the input disk. The transmission on early [[Plymouth Locomotive Works|Plymouth locomotives]] worked this way, while on tractors using friction disks, the range of reverse speeds was typically limited.<ref>{{cite journal |last1=Engineers |first1=Society of Automotive |title=Tractor Friction Transmissions |journal=The Journal of the Society of Automotive Engineers |year=1918 |page=440 |url=https://books.google.com/books?id=QWnmAAAAMAAJ&pg=PP446 |language=en}}</ref>

Still in development, the magnetic CVT transmits torque using a non-contact magnetic coupling.<ref>{{cite news |url=http://www.engineerlive.com/Design-Engineer/Power_Transmission/Magnets_offer_advantages_as_an_alternative_to_mechanical_gears/22078/ |title=Magnets offer advantages as an alternative to mechanical gears |website=engineerlive.com|date=7 February 2012 |access-date=2012-02-07}}</ref> The design uses two rings of permanent magnets with a ring of steel pole pieces between them to create a planetary gearset using magnets.<ref name="magneticsmag.com">{{cite web |title=Magnetic Continuously Variable Transmission |url=https://magneticsmag.com/magnetic-continuously-variable-transmission/ |website=magneticsmag.com |access-date=16 July 2020 |date=4 November 2013}}</ref> It is claimed to produce a 3 to 5 percent reduction in fuel consumption compared to a mechanical system.<ref name="magneticsmag.com"/>