I get a lot of questions about CVT transmissions, so I thought I would take the time to discuss it, how it works, and why you would want one if you do. They have become quite popular with Nissans, Subarus, and even some Audi products.
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. CVTs, 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.
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, CVTs 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?
CVTs 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. CVTs 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 CVTs, 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.
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.