Continuously Variable Transmission CVT Seminar Report

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Continuously Variable Transmission 1 Chapter –I INTRODUCTION After more than a century of research and development, the internal combustion (IC) engine is nearing both perfection and obsolescence: engineers continue to explore the outer limits of IC efficiency and performance, but advancements in fuel economy and emissions have effectively stalled. While many IC vehicles meet Low Emissions Vehicle standards, these will give way to new, stricter government regulations in the very near future.
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  Continuously Variable Transmission Chapter –I INTRODUCTION After more than a century of research and development, theinternal combustion (IC) engine is nearing both perfection andobsolescence: engineers continue to explore the outer limits of ICefficiency and performance, but advancements in fuel economy andemissions have effectively stalled. While many IC vehicles meet LowEmissions Vehicle standards, these will give way to new, stricter government regulations in the very near future. With limited room for improvement, automobile manufacturers have begun full-scaledevelopment of alternative power vehicles. Still, manufacturers areloath to scrap a century of development and billions or possibly eventrillions of dollars in IC infrastructure, especially for technologies withno history of commercial success. Thus, the ideal interim solution is tofurther optimize the overall efficiency of IC vehicles.One potential solution to this fuel economy dilemma is thecontinuously variable transmission (CVT), an old idea that has onlyrecently become a bastion of hope to automakers. CVTs could potentially allow IC vehicles to meet the first wave of new fuelregulations while development of hybrid electric and fuel cell vehiclescontinues. Rather than selecting one of four or five gears, a CVTconstantly changes its gear ratio to optimize engine efficiency with a perfectly smooth torque-speed curve. This improves both gas mileageand acceleration compared to traditional transmissions.  Dept. of CAEVTU-Bangalore 1  Continuously Variable Transmission The fundamental theory behind CVTs has undeniable potential, but lax fuel regulations and booming sales in recent years have givenmanufacturers a sense of complacency: if consumers are buyingmillions of cars with conventional transmissions, why spend billions todevelop and manufacture CVTs?Although CVTs have been used in automobiles for decades,limited torque capabilities and questionable reliability have inhibitedtheir growth. Today, however, ongoing CVT research has led to ever-more robust transmissions, and thus ever-more-diverse automotiveapplications. As CVT development continues, manufacturing costs will be further reduced and performance will continue to increase, whichwill in turn increase the demand for further development. This cycle of improvement will ultimately give CVTs a solid foundation in theworld’s automotive infrastructure.  Dept. of CAEVTU-Bangalore 2  Continuously Variable Transmission Chapter –II CVT THEORY & DESIGN Today’s automobiles almost exclusively use either aconventional manual or automatic transmission with “multiple planetarygear sets that use integral clutches and bands to achieve discrete gear ratios” . A typical automatic uses four or five such gears, while amanual normally employs five or six. The continuously variabletransmission replaces discrete gear ratios with infinitely adjustablegearing through one of several basic CVT designs. Push Belt This most common type of CVT uses segmented steel blocksstacked on a steel ribbon, as shown in Figure (1). This belt transmits power between two conical pulleys, or sheaves, one fixed and onemovable . With a belt drive:In essence, a sensor reads the engine output and thenelectronically increases or decreases the distance between pulleys, and  Dept. of CAEVTU-Bangalore 3  Continuously Variable Transmission thus the tension of the drive belt. The continuously changing distance between the pulleys—their ratio to one another—is analogous toshifting gears. Push-belt CVTs were first developed decades ago, butnew advances in belt design have recently drawn the attention of automakers worldwide. Toroidal Traction-Drive These transmissions use the high shear strength of viscousfluids to transmit torque between an input torus and an output torus. Asthe movable torus slides linearly, the angle of a roller changes relativeto shaft position, as seen in Figure (2). This results in a change in gear ratio . Variable Diameter Elastomer Belt This type of CVT, as represented in Figure (2), uses a flat,flexible belt mounted on movable supports. These supports can changeradius and thus gear ratio. However, the supports separate at high gear ratios to form a discontinuous gear path, as seen in Figure (3). This can  Dept. of CAEVTU-Bangalore 4
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