Continuously variable transmission
A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. The flexibility of a CVT with suitable control may allow the engine to operate at a constant RPM while the vehicle moves at varying speeds.
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The most common type of CVT uses a V-belt which runs between two variable-diameter pulleys. 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 sheaves of one pulley closer together and the two sheaves of the other pulley farther apart.
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 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, 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 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 bend. When part of the belt is wrapped around a pulley, the sides of the elements form a conical surface. 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.
Some CVTs transfer power to the output pulley via tension in the belt (a "pulling" force), while others use compression of the chain elements (where the input pulley "pushes" the belt, which in turn pushes the output pulley).
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, these plates may be quite thin, around a millimetre 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.
Toroidal CVTs, as used on the Nissan Cedric (Y34), 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.
An advantage of a toroidal CVT is the ability to withstand higher torque loads than a pulley-based CVT. 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.
A ratcheting CVT uses a series of one-way clutches or 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 rotary motion to oscillating motion and back to rotary motion using roller clutches. This design remains in production as of 2017, for use with low-speed electric motors. An example prototyped as a bicycle transmission was patented in 1994. The operating principle for a ratcheting CVT design, using a Scotch yoke mechanism to convert rotary motion to oscillating motion and non-circular gears to achieve uniform input to output ratio, was patented in 2014.
Hydrostatic CVTs use a variable displacement pump and a hydraulic motor; thus, the transmission converts hydraulic pressure to the rotation of the output shaft. The name, although misusing the term hydrostatic, differentiates these transmissions, which use positive-displacement pumps, from fluid couplings such as torque converters that use rotodynamic pumps to transmit torque.
The advantages of hydrostatic CVTs are:
- their scalability to any torque capacity that can be achieved by a hydraulic motor.
- the transmission of power to the wheel hub with flexible hoses, allowing for a more flexible suspension system and simplifying the design of all-wheel drive articulated vehicles.
- the smooth transition through all forward and reverse speeds, which can be controlled using a single lever.
- arbitrarily slow crawl speeds at full torque, allowing for precise vehicle movement.
- the possible provision of speed control for other hydraulic components, such as hydraulic cylinders.
Compared to gear-based transmissions, hydrostatic CVTs are generally more expensive, but on machinery that already utilizes hydraulic power transmission, the added complexity and cost is less significant. As with most hydraulically powered transmissions, transmitting high torque for extended durations requires cooling of the hydraulic fluid.
Uses of hydrostatic CVTs include forage harvesters, combine harvesters, small wheeled/tracked/skid-steer loaders, crawler tractors, and road rollers. 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 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.
Electric CVTs, used in series hybrid electric vehicles (SHEVs), have three or four main elements. These are a power source, a generator, an electric motor, and a battery pack. The basic principle of the electric CVT is analogous to the hydrostatic CVT in that the power source drives an electric generator, while a motor is connected to the output shaft—the generator and motor are connected by an electric circuit. Unlike with the hydrostatic CVT, the addition of a battery pack can store excess power that would normally be wasted during vehicle operation.
It could easily be argued that a generator powering a motor through a kind of electronic speed control would constitute a continuously variable transmission. Electrical transmissions have the advantage of great flexibility in layout, as the generator can be located at any distance or orientation relative to the motor. Furthermore, any excess power generated can be stored in batteries and drawn upon when high loads are experienced. However, they are heavy and inefficient; the efficiency of a typical generator or motor is only 75% to 80%, and compounding two results in an efficiency of only 56% to 64%. This limits their use to situations where other types of transmissions cannot be used.
This series arrangement is the standard for heavy vehicles. Diesel locomotives and some ships (and more recently, hybrid electric cars) use such drivetrains. Electric CVTs serve well in situations where the power source and the load locations prohibit a direct mechanical drive and where accurate transmission of high power is necessary.
In addition, the Electrical Variable Transmission (EVT), not to be confused with the electric variable transmissions in some hybrid cars, is a "fully electromagnetic continuously variable transmission which provides two additional electrical ports for hybrid functionality", developed by Electrical Variable Transmission BV. It consists of two concentric rotors housed by a conventional stator. This type of CVT is very rare, with one example being tested at Ghent University. The advantages of this EVT include its multitasking ability (serving as either a hybrid motor or CVT), higher projected efficiency, and low-maintenance design without lubrication, sealing or valves, requiring only maintenance of the slip rings. The lack of a mechanical connection between the engine and the wheels also results in overload protection and vibration damping.
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. 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. 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.
Another type of cone CVT exists as the "Warko", which uses a number of smaller input cones arranged around a larger output cone. Power is transmitted via friction between the cones, and the number of input cones is determined by the transmission's torque threshold. The cross section of the output cone is slightly convex, with a smaller curvature than those of the slightly concave input cones. The transmission ratio is varied by tilting the axes of the input cones such that they contact the output cone at a different point along its axis.
In an 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 Toyota e-CVT (which debuted on the 1997 Toyota Prius) and the NuVinci CVT.
Friction-disk transmissions were used in several tractors and small locomotives built in the early 20th century. Used today in snow blowers, 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 locomotives worked this way, while on tractors using friction disks, the range of reverse speeds was typically limited.
Still in development, the magnetic CVT transmits torque using a non-contact magnetic coupling. The design uses two rings of permanent magnets with a ring of steel pole pieces between them in order to create a planetary gearset using magnets. It is claimed to produce a 3 to 5 percent reduction in fuel consumption compared to a mechanical system.
Infinitely variable transmissionsEdit
Some CVTs can also function as an infinitely variable transmission (IVT) which offers an infinite range of low gears (e.g. moving a vehicle forward at an infinitely slow speed). Some IVTs prevent backdriving (where the output shaft can freely rotate, like an automotive transmission in neutral) due to providing high back-driving torque. Other IVTs, such as ratcheting types, allow the output shaft to freely rotate. The types of CVTs which are able to function as IVTs include epicyclic, friction-disk, and ratcheting CVTs.
In an epicyclic IVT, infinitely low drive ratios are produced when the output shaft rotation speed is equal to the difference between two other speeds within the CVT. In this situation, the CVT is functioning as a regulator of the rotation speed of any one of the three rotators of the planetary gear system. Since two of the rotators are the input and output of the regulator, the CVT can be configured to result in an output speed of zero for any given input speed. The CVT's input speed is always the same as the motor, even when the output speed is zero.
In 1879, Milton Reeves invented a CVT (then called a variable-speed transmission) for use in sawmilling. In 1879, Reeves began fitting this transmission to his cars, and the Reeves CVT was also used by several other manufacturers.
The 1911 Zenith Gradua 6HP motorcycle used a pulley-based Gradua CVT. A year later, the Rudge-Whitworth Multigear was released with a similar but improved CVT. Other early cars to use a CVT were the 1913-1923 David small three-wheeled cyclecars built in Spain, the 1923 Clyno built in the U.K., and the 1926 Constantinesco Saloon built in the U.K.
The 1987 second-generation Ford Fiesta and first-generation Fiat Uno became the first cars to be equipped with steel-belted CVTs (as opposed to the less robust rubber-belted DAF design). The Multitronic transmission was developed by Ford, Van Doorne, and Fiat, with work on the transmission starting in 1976.
Also in 1987, the ECVT was introduced as an optional transmission on the Subaru Justy, Production was limited to 500 units per month due to Van Doorne's limited production output. In June of that year, supplies increased to 3,000 per month, leading Subaru to make the CVT available in the Rex kei car. Subaru has also supplied its CVTs to other manufacturers (an example being the 1992 Nissan Micra).
Use of CVTs then spread in the following years to models including the 1998 Nissan Cube, 1999 Rover 25 and 1999 Audi A6. Marketing terms for CVTs include "Lineartronic" (Subaru), "Xtronic" (Jatco, Nissan, Renault), INVECS-III (Mitsubishi), Multitronic (Volkswagen, Audi), "Autotronic" (Mercedes-Benz) and "IVT" (Hyundai, Kia).
The 1999 Nissan Cedric (Y34) used a toroidal CVT—unlike the pulley-based designs used by other manufacturers—marketed as the Nissan Extroid, which incorporated a torque converter. Nissan then switched from toroidal to pulley-based CVTs in 2003. The version of the CVT used with the VQ35DE engine in the fourth-generation Nissan Altima is claimed to be capable of transmitting higher torque loads than other belt CVTs.
The 2019 Toyota Corolla (E210) is available with a CVT assisted by a physical "launch gear" alongside the CVT pulley. At speeds of up to 40 km/h (25 mph), the launch gear is used in order to increase acceleration and reduce stress on the CVT. Above this speed, the transmission switches over to the CVT.
Several hybrid electric vehicles—such as the Toyota Prius, Nissan Altima and Ford Escape Hybrid—use electric variable transmissions (EVTs) 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.
In the United States, Formula 500 open-wheel racing cars have used CVTs since the early 1970s. CVTs were prohibited from Formula One in 1994 (along with several other electronic systems and driving aids) due to concerns over escalating research and development costs and maintaining a specific level of driver involvement with the vehicles.
Many small vehicles—such as snowmobiles, golf carts, and motor scooters—use CVTs, typically of the pulley variety. CVTs in these vehicles often use a rubber belt with a non-stretching fixed circumference manufactured using various highly durable and flexible materials, due to the mechanical simplicity and ease of use outweighing their comparative inefficiency. Some motor scooters include a centrifugal clutch, to assist when idling or manually reversing the scooter.
Farm and earthmoving equipmentEdit
Combine harvesters used variable belt drives as early as the 1950s. Many small tractors and self-propelled mowers for home and garden use simple rubber belt CVTs. Hydrostatic CVTs are more common on the larger units.[example needed] In mowing or harvesting operations, the CVT allows the forward speed of the equipment to be adjusted independently of the engine speed; this allows the operator to slow or accelerate as needed to accommodate variations in the thickness of the crop.
Hydrostatic CVTs are used in small- to medium-sized agricultural and earthmoving equipment. Since the engines in these machines are typically run at constant power output (to provide hydraulic power or to power machinery), losses in mechanical efficiency are offset by enhanced operational efficiency. For example, in earthmoving equipment, the forward-reverse shuttle times are reduced. The speed and power output of the CVT is used to control the travel speed and sometimes steering of the equipment. In the latter case, the required speed differential to steer the equipment can be supplied by independent CVTs, allowing the steering to be accomplished without several drawbacks associated with other skid steer methods (such as braking losses or loss of tractive effort).
The 1965 Wheel Horse 875 and 1075 garden tractors were the first such vehicles to be fitted with a hydrostatic CVT. The design used a variable-displacement swash-plate pump and fixed-displacement gear-type hydraulic motor combined into a single compact package. Reverse ratios were achieved by reversing the flow of the pump through over-centering of the swashplate. Acceleration was limited and smoothed through the use of pressure accumulator and relief valves located between the pump and motor, to prevent the sudden changes in speed possible with direct hydraulic coupling. Subsequent versions included fixed swash plate motors and ball pumps.
The 1996 Fendt Vario 926 was the first heavy-duty tractor to be equipped with a IVT transmission. It is not the same thing as a hydostatic CVT. Over 100,000 tractors have been produced with this transmission.
Power generation systemsEdit
CVTs with flywheels are used as a speed governor between an engine (e.g. a wind turbine) and the electric generator. When the engine is producing sufficient power, the generator is connected directly to the CVT which serves to regulate the engine's speed. When the power output is too low, the generator is disconnected and the energy is stored in the flywheel. It is only when the speed of the flywheel is sufficient that the kinetic energy is converted into electricity, intermittently, at the speed required by the generator.
Some drill presses and milling machines contain a simple belt-drive CVT system to control the speed of the chuck, including the Jet models J-A5816 and J-A5818. In this system, the effective diameter of only the output shaft pulleys is continuously variable. The input pulley connected to the motor is usually fixed in diameter (or sometimes with discrete steps to allow a selection of speed ranges). The operator adjusts the speed of the drill by using a hand wheel that controls the width of the gap between the pulley halves. A tensioner pulley is implemented in the belt transmission to take up or release the slack in the belt as the speed is altered.
Bicycles with CVT gearing have had limited commercial success, with one example providing a range of gearing equivalent to an eight-speed shifter. The bicycle's short gearing assisted when cycling uphill, but the CVT was noted to significantly increase the weight of the bicycle.
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