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Notes From the Road

I Want My CVT (Or Do I?)
By Aaron Robinson

The Gearless Gearbox
A CVT takes this basic idea and adapts it to cars, but with even greater flexibility. Instead of gears, most CVT transmissions have two pulleys wrapped by a single belt with a V-shaped cross-section (there are other kinds, which we’ll get to in a minute). Each pulley looks like a yo-yo and is actually made up of two cones that face each other. Under the force of hydraulic pressure, the cones either come together or separate, causing the V-shaped belt to either ride higher in the pulley groove or lower down and closer to the center. Higher equates to a bigger diameter sprocket; lower means a smaller diameter. As you drive, the two pulleys variously squeeze in or separate to change the ratio between the input pulley (from the engine) and the output pulley (to the wheels). 

Here’s an example: when you start from a stop, the control computer de-clamps the input pulley so that the belt turns the smallest diameter while the output pulley (which goes to the wheels) clamps tighter to make the belt turn its largest diameter. This produces the lowest gear ratio (say, 3.0:1) for the quickest acceleration. As speed builds, the computer varies the pulley diameters, as conditions dictate, for the best balance of fuel economy and power. 

As a result, instead of five or six ratios, you get an infinite number of ratios between the lowest (smallest-diameter pulley setting) and highest (largest-diameter pulley setting). The engine can always run at the optimum speed for power or for fuel economy, regardless of the wheel speed. No revving up or down with each gear change, just the right RPM for the right speed all the time.

There’s another advantage: the lowest and highest ratios are also further apart than they would be in a conventional step-gear transmission, giving the transmission a greater “ratio spread,” which means that it is even more flexible. In the case of the 2002 Audi A4 CVT, the ratios varied from 2.40:1 to 0.40:1, or a factor of 6 from the highest to the lowest. In the conventional six-speed automatic also offered on the A4 that year, the ratio factor was only 4.79. A bigger factor means a transmission that can do more and do it more efficiently. 

All of this may sound complicated, but it isn’t. In theory, a CVT is far less complex than a normal automatic transmission. A planetary gear automatic transmission – sold in the tens of millions last year – has hundreds of finely machined moving parts. It has wearable friction bands and elaborate electronic and hydraulic controls. A CVT transmission like the one described above has three basic moving parts: the belt and the two pulleys. Considering the savings involved in building transmissions with only three moving parts, and you’ll understand why car companies have become very interested in CVTs lately.



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