If you've been shopping for a new car recently, you've undoubtedly found that large numbers of late-model vehicles are equipped with a continuously variable automatic transmission (CVT). It's a type of automatic transmission that manufacturers increasingly favor as a way to improve fuel economy.
CVTs were once derided as the shiftless "rubber band" transmission that made cars seem as if they were revving up forever. But CVTs are no longer a mere experiment by automakers looking for new ways to improve efficiency. They're becoming so common that it's hard to find a mainstream manufacturer that doesn't offer them in at least some models. Audi, Chevrolet, Ford, Honda, Jeep, Nissan, Subaru and Toyota all provide CVTs as the standard transmission in at least one model. And some carmakers have been doing so for many years.
This video, produced by Nissan, is a bit self-promotional, but it provides a good explanation of how a CVT works (beginning at about the 30-second mark):
For decades, three or four gears were considered sufficient for a car's transmission. In fact, many early automatics had only two speeds. But now eight, nine and even 10 speeds are commonplace. So what's with the trend toward more and more gears?
Generally speaking, the more gears in a transmission, the better it can optimize engine speed in a variety of driving conditions. That means low gears can provide better acceleration while higher gears can maximize fuel economy. As a result, manufacturers can meet government efficiency standards and provide improved fuel economy for owners, even with existing engines.
CVTs differ from traditional automatic transmissions in that they don't have gears that provide "steps" between low- and high-speed operation. Instead, the majority of them work via a pair of variable-diameter, cone-shaped pulleys connected by a steel or composite belt.
Although there are several variations on the CVT theme, in most passenger cars the halves of each pulley are aligned with the pointed ends of the cones touching. These form a V-shaped groove in which the connecting belt rides. One side of the pulley is fixed, and the other side is movable, actuated by a hydraulic cylinder. The cylinder can increase or decrease the amount of space between the two sides of the pulley. This allows the belt to ride lower or higher along the walls of the pulley, depending on driving conditions, thereby changing the "gear" ratio.
The stepless nature of its design is the CVT's biggest advantage for automotive engineers. Because of it, a CVT can work to keep the engine in its optimum power range, increasing efficiency and fuel mileage by delivering an infinite number of smooth transitions from low to high. And it's this infinite variability that, according to the EPA, can boost a passenger car's fuel economy by about 6%.
Surprisingly, the CVT, in fact, isn't a very new idea at all. As early as the 16th century, Leonardo da Vinci sketched a drawing that appears to describe the technology. An early version of the CVT was a component on the first automobile, patented by Karl Benz in 1886. Although these transmissions fell out of favor for automobiles quite early, they've been widely used for more than a century in industrial applications (drill presses and lathes, for instance) and more recently in personal watercraft and snowmobiles.
The CVT came back to the modern automobile as manufacturers began looking for ways to increase fuel efficiency. In the 1989 model year, Subaru introduced the first modern automotive CVT in the U.S. on the subcompact Justy. Other automakers began developing their own CVTs throughout the 1990s.
Since then, the introduction of improved materials, such as high-strength belts, advanced hydraulics, and high-speed sensors and microprocessors, has been responsible for the CVT's growth in the automotive arena. New materials and other innovations have made it possible to design small, relatively inexpensive CVTs that reliably deliver valuable fuel efficiency improvements.
The CVT's value lies not only in its efficiency but also in its simplicity. It has very few components compared to a traditional automatic transmission, which can contain a mind-boggling array of hundreds of moving parts. Typical CVT components include a high-strength belt, a hydraulically operated driving pulley, a mechanical torque-sensing pulley, and an array of microprocessors and sensors.
Because of this design simplicity, CVTs offer a number of advantages over traditional automatic transmissions. In addition to improved fuel economy and reduced manufacturing costs, they offer steady acceleration, smooth operation, and the ability to adapt to varying road conditions and power demands to provide a smoother overall ride.
There are also drawbacks, however. Most significantly there is sometimes an unsettling disconnect between the pressure being applied to the accelerator and the engine's rpm. That's the so-called rubber band feeling that is caused by the CVT allowing the engine to operate in its most efficient powerband, even when that band is higher or lower than the accelerator pedal's position might indicate. In addition, belt-driven CVTs often provide less torque than a traditional automatic (although this is improving all the time), and transmitting power by friction can cause greater wear on belts and other components.
Still, in the right applications and under most circumstances, a CVT's advantages outweigh its disadvantages, so it's easy to understand why manufacturers of high-efficiency vehicles often incorporate CVT technology into their drivetrains. You can expect to see more CVTs in the coming years as technological advances further widen their functionality and the battle for improved fuel economy accelerates.
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