CVTs differ from traditional automatic transmissions in that they don't have gears that provide "steps" between low- and high-speed operation. Hydrostatic CVTs are more common on the larger units. [26] It is claimed to produce a 3 to 5 percent reduction in fuel consumption, compared with a mechanical system.[26]. Some CVTs can also function as an infinitely variable transmission (IVT) which offer an infinite range of low gears (e.g. 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. Surprisingly, the CVT, in fact, isn't a very new idea at all. The Multitronic transmission was developed by Ford, Van Doorne, and Fiat, with work on the transmission starting in 1976. CVTs were once derided as the shiftless "rubber band" transmission that made cars seem as if they were revving up forever. Automotive Engineering International Online: Audi takes CVT from 15th century to 21st century, MSN Autos: Engineering New Transmissions, Audi takes CVT from 15th century to 21st century, Notes on Continuously Variable Transmissions. But CVTs are no longer a mere experiment by automakers looking for new ways to improve efficiency. [19][20] The Evans Friction Cone— produced in the 1920s— uses two rollers which are oriented so that a small constant-sized gap is present between the rollers. Another version of the CVT -- the toroidal CVT system -- replaces the belts and pulleys with, Both the pulley-and-V-belt CVT and the toroidal CVT are examples of frictional CVTs, which work by varying the radius of the contact point between two rotating objects. One side of the pulley is fixed, and the other side is movable, actuated by a hydraulic cylinder. Some CVTs transfer power to the output pulley via tension in the belt (a "pulling" force), while others use link element compression where the input pulley "pushes" the belt which in turn pushes the output pulley. Generally speaking, the more gears in a transmission, the better it can optimize engine speed in a variety of driving conditions. Also the outer bands slide through the stabilizing chain, while the center band can be used as the chain linkage. Between the discs are rollers which vary the ratio and which transfer power from one side to the other. But now eight, nine and even 10 speeds are becoming commonplace. [2] 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 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. Encyclopedia Britannica 2005, s.v. This video, produced by Nissan, is a bit self-promotional, but it provides a good explanation of how a CVT works (beginning at about the 30-second mark): For decades, three or four gears were considered sufficient for a car's transmission. In the latter case, the required speed differential to steer the equipment can be supplied by independent CVTs, allowing steering to be accomplished without several drawbacks associated with other skid steer methods (such as braking losses or loss of tractive effort). The transmission on early Plymouth locomotives worked this way, while on tractors using friction disks, the range of reverse speeds was typically limited. If you've been shopping for a new car recently, you've undoubtedly found that large numbers of late-model vehicles are equipped with a continuously variable automatic transmission (CVT). Uses of hydrostatic CVTs include forage harvesters, combine harvesters, small wheeled/tracked/skid-steer loaders, crawler tractors, and road rollers. 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. Where a common automatic transmission utilizes gears to achieve this driving behavior, the CVT does not have any gears and is instead supposed to seamlessly speed up and slow down without the gear-changing hesitation most people are used to. [33], The 1996 Honda Civic (sixth generation) introduced a pulley-based Multi Matic CVT which included a torque converter, for low-speed "creep" action).[37]. Constant lubrication is required and so the housing is usually partially filled with oil. In the modern market, there are various models of the CVT transmission. Winches and hoists are also an application of CVTs, especially for those adapting the transmission ratio to the resistant torque. 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. [citation needed] The design used a variable-displacement swash-plate pump and fixed-displacement gear-type hydraulic motor combined into a single compact package. Up to 40 km/h (25 mph), the transmission uses the launch gear, in order to increase acceleration and reduce the stress on the CVT. The pulley on the motor is usually fixed in diameter (or sometimes with discreet steps to allow a selection of speed ranges). [12] 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.[13]. Some motor scooters include a centrifugal clutch, to assist when idling or manually reversing the scooter. As with most hydraulic power transmissions, transmitting high torque for extended durations requires cooling of the hydraulic fluid. The CVT came back to the modern automobile as manufacturers began looking for ways to increase fuel efficiency. In an epicyclic CVT (also called 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. Bicycles with CVT gearing have had limited commercial success, with one example providing a range of gearing equivalent to an eight-speed shifter. The most common type of CVT uses a V-belt which runs between two variable diameter pulleys. As early as the 16th century, Leonardo da Vinci sketched a drawing that appears to describe the technology. The stepless nature of its design is the CVT's biggest advantage for automotive engineers. CD-ROM, 2005. Which CVT Transmission Brand Is The Most Reliable? [21] A leather strap sits in the gap between the rollers, pressed against both rollers in order to transfer motion from the input roller to the output roller. ", "Honda Worldwide - Technology Picture Book - CVT", "Nissan Technological Development Activities Overview: Xtronic Cvt", "Banned! 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. And some carmakers have been doing so for many years. The pulley-radial thickness of the belt is a compromise between maximum gear ratio and torque. We also share information about your use of our site with our social media, advertising and analytics partners who may combine it with other information that you’ve provided to them or that they’ve collected from your use of their services. 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. [clarification needed]. ", "What Is a Continuously Variable Transmission (CVT)? a wind turbine) and the electric generator. What kind of makes and models incorporate CVTs? 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. A belt-driven design offers approximately 88% efficiency,[3] 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. [4][5][6], 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. In 1879, Milton Reeves invented a CVT (then called a variable-speed transmission) for use in saw milling. [23], Friction-disk transmissions were used in several tractors and small locomotives built in the early decades of the 20th century. [38] Marketing terms for CVTs include "Lineartronic" (Subaru), "Xtronic" (Jatco, Nissan, Renault), INVECS-III (Mitsubishi), Multitronic (Volkswagen, Audi), "Autotronic" (Mercedes-Benz) and IVT (Hyundai, Kia). So what's with the trend toward more and more gears? A CVT transmission utilizes pulleys and a steel belt to adjust RPMs as you accelerate and decelerate. What about disadvantages? But it's taken a while for the technology to gain a foothold in the United States. Although there are several variations on the CVT theme, in most passenger cars the halves of each pulley are aligned with the pointed ends of the cones touching. In this way, there will be only one (theoretical) contact point between each cone and the hub at any time. Despite the similar name, these are fundamentally different devices from CVTs (which are powered by a single source). The side surface of the hub is convex with a specific radius of curvature which is smaller than the concavity radius of the cones. The most common type operates on a pulley system. There is a smooth transition through all forward and reverse speeds, which can be controlled using a single lever. Hydrostatic CVTs use a variable displacement pump and a hydraulic motor, therefore the transmission converts hydraulic pressure to rotation of the output shaft. A tensioner pulley is implemented in the belt transmission to take up or release the slack in the belt as the speed is altered. 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. Cars.com Glossary. When the engine is producing sufficient power, the generator is connected directly to the CVT which serves to regulate the engine's speed. Still, in the right applications and under most circumstances, a CVT's advantages outweigh its disadvantages, so it's easy to understand why manufacturers of high-efficiency vehicles often incorporate CVT technology into their drivetrains. The 2008-2010 Honda DN-01 cruiser motorcycle used a hydrostatic CVT, in the form of a variable displacement axial piston pump with a variable-angle swashplate. The most common type of CVT uses two pulleys connected by a belt or chain, however several other designs have also been used at times. 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. In a chain-based CVT a film of lubricant is applied to the pulleys.
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