Continuously variable transmission

Simple rubber belt (non-stretching fixed circumference manufactured using various highly durable and flexible materials) CVTs are commonly used in small motorized vehicles, where their mechanical simplicity and ease of use outweigh their comparative inefficiency. Nearly all snowmobiles, utility vehicles, golf carts and motor scooters use CVTs, typically the rubber belt or variable pulley variety.

CVTs, along with several other electronic systems and driver aids were prohibited from Formula 1 in 1994 due to concerns over escalating research and development costs, and maintaining a specific level of driver involvement with the vehicles.[3]

More recently,, CVT systems have been developed for go-karts and have proven to increase performance and engine life expectancy.

Some vehicles that offer CVT are:

• The Chrysler Pacifica hybrid
• The Datsun Go CVT and Datsun GO+ CVT
• The Ford C-MAX hybrid
• The Toyota Corolla, Allion, Premio, Avalon, Mark X, Camry, and Prius
• The Honda Insight, Fit, CR-Z hybrid, CR-V, Capa, Civic, and Accord
• The Dodge Caliber
• The Mitsubishi Lancer, Mirage, Outlander, RVR/ASX/Outlander Sport
• The Scion iQ,
• The Nissan Tiida/Versa (SL, SV, and Note S Plus or higher models), Cube, Juke, Sentra, Altima, Maxima, 2013 1.2 Note, Rogue, X-Trail, Murano, Pathfinder, Sunny and the non-Mexican Micra
• The Jeep Patriot and Compass
• The Suzuki SX4, S-Cross, Baleno and Suzuki Kizashi
• The Subaru Forester, Impreza, Legacy, Outback and Crosstrek
• The Tomcar range of off-road vehicles

CVTs should be distinguished from power-sharing transmissions (PSTs), as used in newer hybrid cars, such as the Toyota Prius, Highlander and Camry, the Nissan Altima, and newer-model Ford Escape Hybrid SUVs. CVT technology uses only one input from a prime mover and delivers variable output speeds and torque, whereas PST technology uses two prime mover inputs and varies the ratio of their contributions to output speed and power. These transmissions are fundamentally different.

Farm equipment, namely harvester combines, used variable belt drives as early as the 1950s, as well. Many small tractors and self-propelled mowers for home and garden also use simple rubber belt CVT. Hydrostatic systems are more common on the larger units—the walk-behind self-propelled mowers are of the slipping belt variety.

Ratcheting CVT converting rotary motion to oscillating motion and back to rotary motion using roller clutches are well adapted to reciprocating engines when the oscillating movement is synchronized with that of the pistons. This solution could have a bright future because such ratcheting CVT are also IVT (providing the clutch function), and have a very high energy efficiency. They could help automakers comply with the future emission standards, while also improving the reciprocating engines' performance.

Instead of being dimensioned according to the maximum torque (e.g. that required for starting, or in case of momentary mechanical overload), the motors using this type of CVT may be dimensioned by matching the maximum power with the maximum desired speed (that of a vehicle by example). Such CVT is used only at startup or in case of mechanical overload, and may be disconnected most of the time, the engine transferring the torque directly to the output. New concepts adapting the transmission ratio to the resistant torque and centrifugal clutches may be used to make these changes automatic.

Bicycles with CVT gearing have had limited commercial success, and can give comparable shifting to an eight-speed cycle.[4] Automatic CVTs on cycles also exist and can make riding uphill more comfortable.[5]

Hydrostatic CVTs are common in small to medium-sized agricultural and earthmoving equipment. As the engines in these machines are typically run at constant power settings to provide hydraulic power or to power machinery, losses in mechanical efficiency are offset by enhanced operational efficiency, such as reduced forward-reverse shuttle times in earthmoving operations. Transmission output is varied to control both travel speed and direction. This is particularly beneficial in equipment designed to pivot or skid steer through differential power application as the required differential steering action can easily be supplied by independent CVTs, allowing steering to be accomplished without braking losses or loss of tractive effort and allowing the machine to pivot in place. In mowing or harvesting operations a CVT allows the forward speed of the tractor or combine harvester to be adjusted independently of the engine speed. This allows the operator to slow or accelerate as needed to accommodate variations in thickness of the crop.

CVTs have been used in aircraft electrical power generating systems since the 1950s and in Sports Car Club of America (SCCA) Formula 500 race cars since the early 1970s.

Some drill presses and milling machines contain a pulley-based CVT system where the output shaft has a pair of manually adjustable conical pulley halves through which a wide drive belt from the motor loops. 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 speed 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. In most cases, the speed must be changed with the motor running.

Doubly-fed induction generators are usually coupled with multi-stage gearboxes to increase the rotational speed. These gearboxes might be replaced by fully CVT in the future, but only fully geared one because they are the only ones providing a sufficient mechanical efficiency.[6]

A CVT and flywheel may be inserted between an energy source (e.g. a wind turbine) and the electricity generator. When the energy source is sufficient, the generator is connected directly to the CVT which serves to regulate its speed of rotation. When it is too low, the generator is disconnected and the energy stored in the flywheel. It is only when the speed of the flywheel is sufficient that the kinetic energy is converted into electricity, intermittently, but always at the optimal speed of the generator.

It's also an application of CVTs, especially for those adapting the transmission ratio to the resistant torque.

In this most common CVT system,[7] there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The drive ratio is changed by moving the two sheaves of one pulley closer together and the two sheaves of the other pulley farther apart. The V-shaped cross section of the belt causes it to ride higher on one pulley and lower on the other. This changes the effective diameters of both pulleys, which changes the overall drive ratio. 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 in order 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. The V-belt needs to be very stiff in the pulley's axial direction in order to make only short radial movements while sliding in and out of the pulleys. The Chinese gy6-type scooter uses this type of CVT drive system.

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 lots of very small elements. The shape of the elements is governed by the static of a column. The pulley-radial thickness of the belt is a compromise between maximum gear ratio and torque. For the same reason the axis between the pulleys is as thin as possible. In a chain-based CVT a film of lubricant is applied to the pulleys. It needs to be thick enough so that the pulley and the chain never touch and it must be thin in order not to waste power when each element dives into the lubrication film. Additionally, the chain elements stabilize about 12 steel bands. Each band is thin enough so that it bends easily. If bending, it has a perfect conical surface on its side. In the stack of bands each band corresponds to a slightly different drive ratio, and thus they slide over each other and need oil between them. Also the outer bands slide through the stabilizing chain, while the center band can be used as the chain linkage.[note 1]

While some CVTs transmit torque only through the tension of the belt or chain, a push-belt CVT transmits torque both through "pulling" belt ring tension and also "pushing" link element compression.[8][9][10]

Nissan Motors Extroid CVT

Toroidal CVTs are made up of discs and rollers that transmit power between the discs. The discs can be pictured as 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 disc is the input, and the other is the output. Between the discs are rollers which vary the ratio and which transfer power from one side to the other. When the roller's axis is perpendicular to the axis of the near-conical parts, it contacts the near-conical parts at same-diameter locations and thus gives a 1:1 drive ratio. The roller can be moved along the axis of the near-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the near-conical parts at varying and distinct diameters, giving a drive 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, and hence lose efficiency.

Some toroidal systems like the torotrak, are also infinitely variable, and the direction of thrust can be reversed within the CVT.[11]

Diagrams:

• Animated image of a toroidal CVT on HowStuffWorks

Examples:

• Nissan Extroid CVT (pdf on Nissan-Global site)

A magnetic continuous variable transmission system was developed at the University of Sheffield in 2006 and later commercialized.[12] mCVT is a variable magnetic transmission which gives an electrically controllable gear ratio. It can act as a power split device and can match a fixed input speed from a prime-mover to a variable load by importing/exporting electrical power through a variator path. The mCVT is of particular interest as a highly efficient power-split device for blended parallel hybrid vehicles, but also has potential applications in renewable energy, marine propulsion and industrial drive sectors.

A subset of CVT designs are called infinitely variable transmissions (IVT or IVTs), in which the range of ratios of output shaft speed to input shaft speed includes a zero ratio that can be continuously approached from a defined "higher" ratio. A zero output speed (low gear) with a finite input speed implies an infinite input-to-output speed ratio, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output speed to input speed. This low ratio is taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit, in which the output speed is zero. Unlike neutral in a normal automotive transmission, IVT output rotation may be prevented because the back-driving (reverse IVT operation) ratio may be infinite, resulting in impossibly high backdriving torque; in a ratcheting IVT, however, the output may freely rotate in the forward direction.

In the early decades of the 20th century, several tractors and small locomotives were built with friction-disk transmissions with an output disk rolling on the face of the input disk. For disks of identical diameter, the effective gear ratio could be varied from 1:1 when the point of contact was at the perimeter of the input disk, to infinity when the point of contact was at the center, to -1:1 when the point of contact was at the opposite extreme. The transmission on early Plymouth locomotives worked this way, while on tractors using friction disks, the range of reverse speeds was typically limited.[13]

Many IVTs result from the combination of a CVT with a planetary gear system which enforces an IVT output shaft rotation speed which is equal to the difference between two other speeds within the IVT. This IVT configuration uses its CVT as a continuously variable regulator (CVR) of the rotation speed of any one of the three rotators of the planetary gear system (PGS). If two of the PGS rotator speeds are the input and output of the CVR, there is a setting of the CVR that results in the IVT output speed of zero. The maximum output/input ratio can be chosen from infinite practical possibilities through selection of additional input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. The IVT is always engaged, even during its zero output adjustment.

IVTs can in some implementations offer better efficiency in the preferred range of operation when compared to other CVTs because most of the power flows through the planetary gear system and not the controlling CVR. Torque transmission capability can also be increased. Staging power splits is also possible for further increase in efficiency, torque transmission capability and better maintenance of efficiency over a wide gear ratio range.

An example of a true IVT is the Hydristor because the front unit connected to the engine can displace from zero to 27 cubic inches (440 cm³) per revolution forward and zero to −10 cubic inches (−160 cm³) per revolution reverse. The rear unit is capable of zero to 75 cubic inches (1,230 cm³) per revolution. However, whether this design enters production remains to be seen. Another example of a true IVT that has been put into recent production[14] and which continues under commercial development[15] is that of Torotrak.

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.

One type of ratcheting CVT that is not dependent on friction uses a scotch yoke mechanism to convert rotation to linear oscillation. The magnitude of oscillation, sometimes called "stroke", depends on the distance of the crank pin in the scotch yoke mechanism from the axis of rotation. The stroke is altered by altering the distance of the crank pin from the axis of rotation. This linear oscillation is converted back to rocking motion using a rack and pinion. This rocking motion is rectified to rotation using either computer controlled clutch, sprag clutch or one-way bearing. The main advantage of this type of CVT is that it is not dependent on friction to transmit power. One drawback here is that the input to output ratio is sinusoidal and not constant. However, patented designs exist to overcome this drawback by altering the instantaneous rotational speed of the scotch yoke mechanism using non-circular gears. An example of a non-friction-dependent ratcheting CVT having a constant input to output ratio, is patent protected under U.S. Patent 9,970,520B2.

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 and decelerating, power transmitting element.

Ratcheting CVTs are distinguished from 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. An example of a ratcheting CVT is one prototyped as a bicycle transmission protected under U.S. Patent 5,516,132 in which strong pedalling 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.

The ratcheting IVT dates back to before the 1930s; the original design converts rotary motion to oscillating motion and back to rotary motion using roller clutches.[16] The stroke of the intermediate oscillations is adjustable, varying the output speed of the shaft. The fundamental limitation is that when the torque transfers between the separate oscillatory paths the change in deflection causes high vibration at higher torques. The original design is still manufactured today, and an example and animation of this IVT can be found here.[17] Paul B. Pires created a more compact (radially symmetric) variation that employs a ratchet mechanism instead of roller clutches, so it does not have to rely on friction to drive the output. An article and sketch of this variation can be found here [18]

Honda DN-01 motorcycle: Swashplate animation

The Japanese Type 10 tank uses hydraulic mechanical transmission (HMT).

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.

An integrated hydrostatic transaxle (IHT) uses a single housing for both hydraulic elements and gear-reducing elements. This type of transmission has been effectively applied to a variety of inexpensive and expensive versions of ridden lawn mowers and garden tractors.

One class of riding lawn mower is the zero-turn mower, which has traditionally been powered with wheel hub mounted hydraulic motors driven by continuously variable pumps. This design tends to be more expensive than alternatives.

Some heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural machinery including foragers, combines, and some tractors. A variety of heavy earth-moving equipment, e.g. compact and small wheel loaders, track type loaders and crawler tractors, skid-steered loaders and road rollers use hydrostatic transmission. Hydrostatic CVTs are usually not used for extended duration high torque applications because of the heat that is generated by the flowing oil, which may require a substantial oil cooler.

The Honda DN-01 motorcycle is the first road-going consumer vehicle with hydrostatic drive that employs a variable displacement axial piston pump with a variable-angle swashplate.

AGCO Corporation has employed a hydrostatic CVT in agricultural equipment. The transmission 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.[19]

The Evans friction cone, a type of cone CVT

A cone CVT varies the effective gear ratio using 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 the variation between the narrow and wide diameters of the cone.

The more-sophisticated twin cone mesh system is also a type of cone CVT.[20][21]

In a CVT with oscillating cones, the torque is transmitted via friction from a variable number of cones (according to the torque to be transmitted) to a central, barrel-shaped hub. The side surface of the hub is convex with a specific radius of curvature which is smaller than the concavity radius of the cones. In this way, there will be only one (theoretical) contact point between each cone and the hub at any time.

A new CVT using this technology, the Warko, was presented in Berlin during the 6th International CTI Symposium of Innovative Automotive Transmissions, on 3–7 December 2007.

A particular characteristic of the Warko is the absence of a clutch: the engine is always connected to the wheels, and the rear drive is obtained by means of an epicyclic system in output.[22] This system, named “power split”,[23] allows the engine to have a "neutral gear": when the engine turns (connected to the sun gear of the epicyclic system), the variator (i.e., the planetary gears) will compensate for the engine rotation, so the outer ring gear (which provides output) remains stationary.

The working principle of this CVT is similar to that of conventional oil pumps, but, instead of pumping oil, common steel rollers are compressed.[24]

The motion transmission between rollers and rotors is assisted by an adapted traction fluid, which ensures the proper friction between the surfaces and slows down wearing thereof. Unlike other systems, the radial rollers do not show a tangential speed variation (delta) along the contact lines on the rotors. From this, a greater mechanical efficiency and working life are claimed.[25]

Description: Infinitely Variable Transmission with rigid friction members (shaped as double cones) in planet configuration.[26]

In a planetary CVT, the gear ratio is shifted by tilting the axes of spheres in a continuous fashion, to provide different contact radii, which in turn drive input and output discs. The system can have multiple "planets" to transfer torque through multiple fluid patches. One commercial implementation is the NuVinci Continuously Variable Transmission.