Techsplanations: Continuously Variable Transmission (CVT)

CVT-equipped vehicles popping up more and more to improve fuel economy

Before every car, SUV and truck sold in America gets turned into a gas-electric hybrid or all-electric vehicle to meet stiffer fuel economy standards imposed by the government, automakers are adding more and more technology that has been around for years to the internal combustion engine to reduce the amount of gas they burn.

One of those technologies is the continuously variable transmission, and every new car buyer should understand the basics of how it works before spending $20,000 and up on a new set of wheels that has the system under the hood.

What is it?

A continuously variable transmission, or CVT, is a type of transmission that doesn't have fixed gears. A conventional transmission, whether manual or automatic, has a defined set of gear ratios or "speeds," which is why we refer to it as a "5-speed manual transmission" or a "6-speed automatic." These types of transmissions move through their gears as the speed of the car increases, which results in the engine speed dropping every time a higher gear is selected. CVT's, however, allow an engine to operate at or near its optimal speed, with a varying gear ratio that can be adjusted in real time for maximum efficiency.

How does it work?

A CVT is actually quite simple, comprised of just two pulleys connected by a belt. One pulley is attached to the crankshaft of the engine – this one is called the drive pulley. The other pulley, the driven pulley, is attached to the output shaft of the transmission. This is connected to the drive-shaft, which in turn rotates the wheels. The belt that connects the pulleys is usually flexible but made of steel for strength. Both pulleys have a deep V groove that can have its width adjusted by the computer in the transmission. As the width in the pulley grooves change, the belt rides higher or lower in each, changing the gear ratio between the engine and the wheels.

When a car accelerates from a stop, the drive pulley has a wide groove and the driven pulley has a narrow groove, corresponding to a low gear. As the drive pulley's groove shrinks, the driven pulley's groove widens to keep the tension on the belt, changing the gear ratio as the belt slides up or down in the grooves. When the drive pulley has a narrow groove and the driven pulley's groove is wide, the CVT has reached its highest gear ratio.

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Why would I want it?

One reason: Gas mileage. An engine coupled to a CVT can operate in its most efficient range for more of the time, meaning it uses less fuel to drive the same distance.

According to the United States Department of Energy, CVT's have a potential efficiency improvement of six percent. But just like every other automotive technology, CVT's are improving beyond that. Nissan has said that its newest CVT has a 10 percent improvement in efficiency over its older designs, a result of reduced friction, shrinking the size and weight of the transmission, and improving some of the basic design to increase the maximum spread of gear ratios available. The all-new 2013 Nissan Altima has the improved system.

Is there any downside?

CVT's take some getting used to. Most drivers are familiar with the sounds and sensations of a vehicle with a conventional transmission. The engine "revs" or runs faster and the accompanying sound of its intake and exhaust grows until the transmission shifts into the next highest gear and the process repeats itself. CVT's are different, allowing the engine to rev higher and much faster, usually leading to increased noise. Since the CVT does not "shift" to a higher gear, it can result in the engine sounding a bit like a vacuum cleaner.

Some manufacturers have had problems with the reliability of their CVT's, most notably General Motors and MINI, neither of which currently offer them. The technology is mature enough today that durability concerns should be a thing of the past.

What vehicles offer it?

Nissan has offered the most vehicles with CVT's in the North American market, from its smallest subcompact Versa to its large Maxima sedan and Murano SUV. Subaru has embraced the technology in it latest generation of vehicles, and Audi also uses CVT's in some of its models. Look for more vehicles to come equipped with CVTs, though, as many are adding technology like this to improve the fuel economy of their vehicles across the board to meet the higher government requirements.

Bottom line

In the quest for maximum mileage, there are plenty of new technologies. Continuously variable transmissions may not be as sexy as hybrids, but they are a key piece of the puzzle as manufacturers try to meet stringent fuel economy standards.

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