Continuously variable transmission (CVT)
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01-03-2009, 01:22 PM

Continuously variable transmission (CVT)

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. With limited room for improvement, automobile manufacturers have begun full-scale development of alternative power vehicles. Still, manufacturers are loath to scrap a century of development and billions or possibly even trillions of dollars in IC infrastructure, 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 variable transmission (CVT), an old idea that has only recently become a bastion of hope to automakers. CVTs could potentially allow IC vehicles to meet the first wave of new fuel regulations while development of hybrid electric and fuel cell vehicles continues. Rather than selecting one of four or five gears, a CVT constantly changes its gear ratio to optimize engine efficiency with a perfectly smooth torque-speed curve. This improves both gas mileage and acceleration compared to traditional transmissions.

The fundamental theory behind CVTs has undeniable potential, but lax fuel regulations and booming sales in recent years have given manufacturers a sense of complacency: if consumers are buying millions of cars with conventional transmissions, why spend billions to develop and manufacture CVTs?

Although CVTs have been used in automobiles for decades, limited torque capabilities and questionable reliability have inhibited their growth. Today, however, ongoing CVT research has led to ever-more robust transmissions, and thus ever-more-diverse automotive applications. As CVT development continues, manufacturing costs will be further reduced and performance will continue to increase, which will in turn increase the demand for further development. This cycle of improvement will ultimately give CVTs a solid foundation in the world's automotive infrastructure.

CVT Theory & Design

Today's automobiles almost exclusively use either a conventional manual or automatic transmission with "multiple planetary gear sets that use integral clutches and bands to achieve discrete gear ratios" . A typical automatic uses four or five such gears, while a manual normally employs five or six. The continuously variable transmission replaces discrete gear ratios with infinitely adjustable gearing through one of several basic CVT designs.
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A continuously variable transmission (CVT) is a transmission which can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that only allow a few different distinct gear ratios to be selected. The flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.

CVTs can compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. This improves fuel economy and by effect, exhaust emissions.
CVTs operate smoothly since there are no gear changes which can cause sudden jerks.

CVT torque-handling capability is limited by the strength of their transmission medium (usually a belt or chain), and by their ability to withstand friction wear between torque source and transmission medium (in friction-driven CVTs). CVTs in production prior to 2005 are predominantly belt- or chain-driven and therefore typically limited to low-powered cars and other light-duty applications. Units using advanced lubricants, however, have been proven to support a range of torques in production vehicles, including that used for buses, heavy trucks, and earth-moving equipment.
Some CVTs transmit torque in only one direction, rendering them useless for regenerative or engine-assisted vehicle braking; all braking would need to be provided by disc brakes, or similar dissipative systems.

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.pdf   cvt.pdf (Size: 157.96 KB / Downloads: 185)
Continuously variable transmission (CVT)

Kevin R. Lang
21W. 732
May 3, 2000

As the U.S. government enacts new regulations for automotive fuel economy and emissions, the
continuously variable transmission, or CVT, continues to emerge as a key technology for improving the
fuel efficiency of automobiles with internal combustion (IC) engines. CVTs use infinitely adjustable
drive ratios instead of discrete gears to attain optimal engine performance. Since the engine always runs
at the most efficient number of revolutions per minute for a given vehicle speed, CVT-equipped vehicles
attain better gas mileage and acceleration than cars with traditional transmissions.
CVTs are not new to the automotive world, but their torque capabilities and reliability have been
limited in the past. New developments in gear reduction and manufacturing have led to ever-more-robust
CVTs, which in turn allows them to be used in more diverse automotive applications. CVTs are also
being developed in conjunction with hybrid electric vehicles. As CVT development continues, costs will
be reduced further and performance will continue to increase, which in turn makes further development
and application of CVT technology desirable.
This paper evaluates the current state of CVTs and upcoming research and development, set in
the context of past development and problems traditionally associated with CVTs. The underlying
theories and mechanisms are also discussed.

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