Continuously Variable Transmission
The most common type of CVT employs a steel belt and two pulleys. Rather than step among four or five gears of different sizes, the CVT varies the effective diameter of the drive pulley and driven pulley to create a broad range of drive ratios free of the shift feel of conventional transmissions.
Imagine riding a bike on which the sprocket and rear gear magically change size without requiring the chain to jump from one gear to another, and you have the basic idea. By increasing or decreasing the space between the pulleys’ respective halves, the CVT varies the effective diameter of the conical inner surfaces on which the belt rides. It varies these parameters at any time, at any engine or vehicle speed. CVTs may use hydraulic pressure, centrifugal force and/or spring tension to adjust the pulley halves.
Smaller and more compact than a comparable automatic trans, the CVT is also somewhat more efficient, but its main advantage with regard to efficiency and fuel economy is that it can be designed to keep the engine running at its most efficient or powerful speed. By varying the drive ratio, a CVT can accelerate a vehicle when the engine rpm is constant or even decreasing.
Likewise, by limiting a gasoline engine’s speed range, engineers can better control emissions. CVTs will become even more efficient as they incorporate automated versions of mechanical clutches akin to those in manual transmissions, in place of inefficient fluidic torque converters. Theoretically, CVTs can be more efficient than manual transmissions as well.
CVTs do have shortcomings, not the least of which is the fact that they’re new technology compared to conventional automatics, though they are far more popular overseas. Belt-drive CVTs also are limited in the amount of torque they can handle, though refinements keep inching the capabilities upward. Audi is working on a CVT that uses a chain in place of a belt for a reported 200-plus pounds-feet of torque capacity.
Even greater capacities are likely to come from designs using complicated roller arrangements in lieu of pulleys and belts, such as Mazda’s Toroidal and Nissan’s Extroid CVTs. These transmissions are not expected on these shores anytime soon, but Honda has offered a belt-drive CVT in the Civic HX since 1996 and will offer it on the Insight later in the 2001 model year. A new Saturn model will debut with a belt-drive CVT in 2002, and Audi’s chain-drive CVT arrives in the 2002 A6.