What is a Continuously Variable Transmission?
Love or Hate Them, CVTs Are Here To Stay
The continuously variable transmission (CVT) was once derided as the shiftless "rubber band" transmission that made cars seem as if they were revving up forever. But CVTs are no longer an experiment by automakers looking for new ways to improve fuel efficiency.
The federal energy department figures a CVT can boost a passenger car's fuel economy by about 6 percent, and with the government asking automakers to double average fuel efficiency by 2025, every percentage point helps.
So CVTs are here to stay, and their presence in the market is growing.
Ford uses them, as does Mitsubishi. With the 2014 Corolla, Toyota Motor Corp. has introduced its first true CVT for Toyota brand cars. Yes, the transmissions in Toyota hybrids, despite marketing claims, aren't true continuously variable models. They use planetary gears, like a conventional automatic transmission.
Acura, Audi, Honda, Jeep, Lincoln, Nissan, Scion and Subaru all provide CVTs as the standard transmission in at least one model. Some carmakers have been doing so for years.
The da Vinci Connection
The CVT, in fact, isn't a very new technology at all. Leonardo da Vinci drew one in the 16th century. Daimler-Benz filed the first patent for a CVT in 1886. Although the 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 carmakers 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.
Since then, the introduction of improved materials such as high-strength metal belts, advanced hydraulics and high-speed sensors and microprocessors has been responsible for the CVT's growth in the automotive arena. The new materials have made it possible to design small, relatively inexpensive CVTs that reliably deliver valuable fuel-efficiency improvements.
Less Is More
The CVT's value lies not only in its efficiency but in its simplicity. It has very few components, typically including a high-power metal or rubber belt; a hydraulically operated driving pulley; a mechanical torque-sensing driving pulley; and an array of microprocessors and sensors. That's it.
Because of this simplicity in design, CVTs offer some advantages over traditional transmissions. There are also drawbacks, including a sometimes disconcerting disconnect between the pressure being applied to the accelerator and the engine's RPM. That's the so-called rubber band feeling: The CVT allows the engine to operate in its most efficient power band, even when that band is at a far greater or lesser pace than the accelerator pedal's position might indicate.
There also are limitations on the size and power of engines with which they can be teamed. There are no V8s with CVTs, so far.
The transmissions are often smaller and lighter than the conventional automatic transmissions they replace, but they don't always provide that advantage, either. Because of limited space between a car's front wheels, the automatic transmissions designed for front-wheel-drive vehicles are pretty compact these days.
For example, the CVT that Toyota's engineers designed for the 2014 Toyota Corolla is slightly heavier than the conventional automatic it replaces, says David Lee, a transmission specialist with the company.
Still, in the right situation, a CVT's advantages outweigh its disadvantages. Less complexity and fewer moving parts theoretically means fewer things to maintain or to go wrong. More importantly, by eliminating the various fixed gear ratios that regular automatic or manual transmissions must move through, the CVT keeps a vehicle's engine operating at its most efficient level all the time, improving fuel efficiency.
How the CVT Works
Although there are several variations on the CVT theme, most passenger cars use a similar setup that utilizes a pair of variable-diameter, cone-shaped pulleys. Essentially, a CVT operates by varying the working diameters of the two main pulleys in the transmission.
The cone-shaped halves of each pulley are aligned with the pointed ends of the cones touching. These form V-shaped grooves in which the connecting belt rides. One side of the pulley is fixed; the other side is movable, actuated by a hydraulic cylinder. When actuated, the cylinder can increase or reduce 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 action is similar to the way a mountain bike shifts gears, by "derailing" the chain from one sprocket to the next. The difference here is that the action is infinitely variable in a CVT, with no "steps" between gearchanges.
The "stepless" nature of its design is the CVT's biggest draw for automotive engineers. Because of it, a CVT can work to keep the engine in its optimum power range, increasing efficiency and gas mileage. This translates to a gain of about 1-2 mpg, but as with any car, your mileage will vary based on your driving habits.
With these advantages, it's easy to understand why manufacturers of high-efficiency vehicles often incorporate CVT technology into their drivetrains.
Look for more CVTs in the coming years as the battle for improved gas mileage accelerates and technological advances further widen their functionality.