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The automotive industry principally uses automatic transmissions configured with multiple planetary gear sets that have integral clutches and bands to achieve discrete gear ratios. Current production automatic transmissions have four gears; however, several newly introduced transmissions have five gear ratios.
Typically, the additional fifth gear ratio is used to obtain and improve vehicle launch characteristics. If efficiency considerations were the only criteria for selecting the number of gears in a transmission, the ultimate number of gear ratios within this type of transmission would be six. With six gears, the fuel economy improvement benefits have effectively leveled off and any additional gears provide dramatically reduced incremental benefits.
A characteristic of these transmissions is that they have large efficiency variations throughout the wide dynamic range of torques and speed conditions that constitute a typical operating envelope. Typical efficiency ranges are as follows:
Southwest Research Institute (SwRI) presented an SAE technical paper at the 1996 International Congress that discussed the reasons why the best overall efficiency rating for an optimized automatic transmission is 86%, whereas most current production transmissions achieve values approaching 80%. This is in contrast to manual transmissions that can achieve a similar efficiency rating of approximately 95%.
A recent new initiative in alternative transmission configurations is automated manual gearboxes, such as are used in the Acura Legend and the Saturn. This type of transmission is devoid of planetary gear sets and uses clutch packs to engage spur gears.
A dramatic departure from these types of discrete ratio transmissions is continuously or infinitely variable transmissions. These types of transmissions may be grouped into three major types:
Typical efficiencies for these type of transmissions are as follows:
Belt-Type TransmissionsThe majority of the industry's efforts have been in applying belt types that either push or pull. Some examples are:
Belts work in conjunction with various sleeve mechanisms to achieve continuously changing input-to-output speed ratios. Examples of belt-type transmissions in current use are the Ford Festiva and the Subaru Justy.
A typical problem associated with belt-type transmissions is low torque-carrying capabilities, which limits their applications to vehicles with low power requirements. Much of this limitation results from the strength of the belt and the coefficient of friction between the belt and pulleys, allowing belt slippage to occur. To overcome this limitation, various steel-impregnated, improved geometry, and dry belt designs are being investigated.
Low-power vehicular applications include snowmobiles, go-karts, and all-terrain vehicles. Typically, the belts used in these applications are fabric-reinforced rubber. Improved power transmittal through Vee belt drives based on complex fabric-reinforced material has been
pursued by Gates, Dayco, Brando, and DAF. Another belt configuration uses a segmented belt concept to create a variable-pitch pulley continuously variable transmission (CVT) based on a flat belt developed by Kumm Industries.
Traction Drive TransmissionsOne basic type of traction drive consists of two rotating, conical steel rollers that transfer torque between themselves through the viscous shear of a thin film of special lubricant. Another basic type of traction drive is a pivotal traction roller interspaced between an input and output torroidal disc.
The limiting factor associated with either of these drives is its potential to withstand high throughput power and the torque impulses typical of automotive drivetrains. Durability is the basic problem associated with traction drives resulting from high surface fatigue stresses that occur in the traction elements. Advances in lubricant technology have provided improvements, but fluid issues are the limiting factor in these designs.
Many configuration variations are possible. However, most designs differ only in the speed in which power is transferred to the outer member or in the geometry of the traction roller and torroidal disk. Some examples are:
Traction-drive transmissions have long been used in the machine tool industry. No current production vehicles use traction-drive CVTs, although considerable development and investigations were conducted in the 1980s. Development activities were discontinued because of problems associated with excessive weight, excessive rotating inertia, high cost, and limited durability. The major problem is that much of the weight is in the rotating components, which decreases acceleration rates and increases vehicle emissions.
Variable Geometry TransmissionsEpicyclic devices create variable geometries in sine wave generating mechanisms that can continuously adjust the angle of the sine wave to create varying output speeds.
SummarySwRI is familiar with all types of continuously variable transmissions. SwRI routinely assists inventors and venture capitalists in the early stages of their designs, performing such tasks as initial design reviews, and kinematic and powerpath analyses. Testing a CVT is no different from testing a conventional transmission. Such testing consists of selectively controlling two of the following four conditions:
With regard to testing, a transmission may be thought of as a black box responding only to control parameters.
For more information about our continuously variable transmissions capabilities, or how you can contract with SwRI, please contact Douglas Fussner at dfussner@swri.org or (210) 522-3972.
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| Design and Development | Engine, Emissions & Vehicle Research Division | SwRI Home | |
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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions. |
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December 28, 2012 |
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