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Continuously Variable Transmission(CVT)

    Continuously Variable Transmission(CVT)

    After more than a century of research and development, the internal combustionengine (ICE)is nearing both perfection and obsolescence:engineers continue to explore the outer limits of IC efficiency and performance,but advancements in fuel economy andemissions have effectively stalled. While many ICE vehicles meet Low Emissions Vehiclestandards,these will give way to new,stricter government regulations in the very nearfuture. With limited room for improvement, automobile manufacturers have begun full-scale development of alternative power vehicles. Still,manufacturers are loath to scrap acentury of development and billions or possibly even trillions of dollars in ICEinfrastructure, especially for technologies with no history of commercial success. Thus,the ideal interim solution is to further optimize the overall efficiency of IC vehicles.

    One potential solution to this fuel economy dilemma is the continuously variabletransmission (CVT),an old idea that has only recently become a bastion of hope toautomakers. CVTs could potentially allow IC vehicles to meet the first wave of new fuelregulations while development of hybrid electric and fuel cell vehicles continues. Ratherthan selecting one of four or five gears, a CVT constantly changes its gear ratio to optimizeengine efficiency with a perfectly smooth torque-speed curve. This improves both gasmileage and acceleration compared to traditional transmissions.

    The fundamental theory behind CVTs has undeniable potential,but lax fuelregulations and booming sales in recent years have given manufacturers a sense ofcomplacency:if consumers are buying millions of cars with conventional transmissions,why spend billions to develop and manufacture CVTs? Although CVTs have been used inautomobiles for decades, limited torque capabilities and questionable reliability haveinhibited their growth. Today, however,ongoing CVT research has led to ever-morerobust transmissions,and thus ever-more-diverse automotive applications.  As CVTdevelopment continues,manufacturing costs will be further reduced and performance willcontinue to increase, which will in turn increase the demand for further development. Thiscycle of improvement will ultimately give CVTs a solid foundation in the world'sautomotive infrastructure.

    Today's automobiles almost exclusively use either a conventional manual or automatictransmission with "multiple planetary gear sets that use integral clutches and bands toachieve discrete gear ratios.”A typical automatic uses four or five such gears, while amanual normally employs five or six. The continuously variable transmission replacesdiscrete gear ratios with infinitely adjustable gearing through one of several basic CVTdesigns.

    Push Belt

    This most common type of CVT uses segmented steel blocks stacked on a steelribbon, as shown in Fig. 9. 14 This belt transmits power between two conical pulleys, orsheaves,one fixed and one movable with a belt drive. In essence, a sensor reads theengine output and then electronically increases or decreases the distance between pulleys,and thus the tension of the drive belt. The continuously changing distance between thepulleys一their ratio to one another一is analogous to shifting gears.

    Push-belt CVTs were first developed decades ago, but new advances in belt designhave regently drawn the attention of automakers worldwide.

    Toroidal Traction-Drive

    These transmissions use the high shear strength of viscous fluids to transmit torquebetween an input torus and an output torus. As the movable torus slides linearly, theangle of a roller changes relative to shaft position, as seen in Fig. 9. 15 This results in achange in gear ratio.

    Variable Diameter Elastomer Belt

    This type of CVT uses a flat, flexible belt mounted on movable supports. Thesesupports can change radius and thus gear ratio. However, the supports separate at highgear ratios to form a discontinuous gear path. This can lead to the problems with creep andslip that have plagued CVTs for years. This inherent flaw has directed research anddevelopment toward push belt CVTs.

    Other CVT Varieties

    Several other types of CVTs have been developed over the course of automotivehistory, but these have become less prominent than push belt and toroidal CVTs. Anutating traction drive uses a pivoting, conical shaft to change "gears" in a CVT. As thecones change angle, the inlet radius decreases while the outlet radius increases,or viceversa,resulting in an infinitely variable gear ratio.  A variable geometry CVT usesadjustable planetary gearsets to change gear ratios,but this is more akin to a flexibletraditional transmission than a conventional CVT.

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