Continuously Variable Transmissions: the Next Generation

Cars equipped with a Continuously Variable Transmission (CVT) offer better fuel efficiency, a smoother ride and better acceleration from smaller engines.

By Jim Henry

Posted April 15, 2008, at 1 p.m.

If squeezing out every last mile per gallon is a major issue, you may want to consider a growing number of cars — including many gasoline-electric hybrids — that are equipped with a Continuously Variable Transmission (CVT). Other benefits include a smoother ride and better acceleration from smaller engines.

A CVT "is more economical than an automatic transmission and quicker in acceleration than a manual," says Marc Trahan, director of product quality and technical service for Audi of America. "You can have your cake and eat it, too."

This type of transmission uses a flexible V-shaped metallic belt and adjustable pulleys in place of traditional gears. The effect is like a super-strong bicycle chain, connecting the front bicycle sprocket to the rear hub. However, instead of toothed gears like a bicycle, the CVT relies on friction between the belt and the pulleys to make the car go.

There's no clutch. The driver selects Drive, Neutral or Reverse, the same as with a traditional automatic. In fact, many modern CVTs are computer-programmed to mimic a traditional automatic, so drivers might not even notice the difference.

Catching On

All of the world's biggest automakers have CVTs, but they are more popular in Europe and Japan than in the United States, primarily due to decades of much higher gas prices in those markets. Other countries also have tax incentives for owning cars with smaller engines, and CVTs have traditionally been paired with smaller engines.

In the United States, CVTs were almost unheard-of in passenger cars until about five years ago. The Subaru Justy was a rare exception. It was sold in the U.S. market from 1988 to 1992, but was never very popular because it was small and underpowered, and gas mileage was less of a concern at that time than it is now.

Today, CVTs are becoming more common in this country, especially in hybrid models like the Toyota Prius, the Ford Escape Hybrid, the Nissan Altima Hybrid and others.

Audi and Nissan also offer CVTs in conventionally powered cars, including some variants of the Audi A4 and A6. Trahan says that the order rate for Audi models where a CVT is offered is about 20 percent. For some of Nissan's newest models, like the 2008 Rogue and the 2009 Murano, the automaker offers CVTs exclusively.

Older CVTs could only handle the torque from smaller engines, because the flexible belts weren't strong enough, explains Larry Dominique, vice president of product planning for Nissan North America. He says modern CVTs have much stronger belts, and together with other improvements, they can now handle the torque of bigger engines.

According to Dominique, EPA gas-mileage estimates for cars equipped with CVTs are about 2 percent to 3 percent better than the same car with an automatic transmission, but drivers with a lead foot may not see the benefit. "If you don't stomp on the throttle, there is some benefit," he adds.

In addition, a CVT has fewer parts and weighs less than the equivalent automatic transmission, and that also helps save gas, Audi's Trahan says. For instance, he notes that Audi's CVT saves 25 kilos, or about 55 pounds, compared with a five-speed automatic.

"You don't have to shift gears, you have continuous power flow and you don't have interruptions," Trahan adds.

How It Works

The CVT's function is the same as that of other transmissions': to vary the ratio of revolutions per minute the engine makes, versus how many rpms the car's wheels make.

Returning to the bicycle analogy, it's useful to think of a 21-speed mountain bike. When starting out, or climbing a steep hill, it takes less energy for your legs to pedal in low gear, with a relatively small gear in front and a relatively large gear in back. When your legs go around once, the rear wheel also goes around once — or even less than once.

When you get up to speed, you switch to high gear, which uses a large gear in front and a small gear in back. When your legs go around once, the rear wheel goes around, say, five times. That would have been impossible starting out, or climbing the hill, because your legs wouldn't have had the strength.

A CVT achieves the same effect by using its adjustable pulleys. Each one is split, like the halves of a yo-yo. The halves are slightly conical in shape, with the points facing the middle. When the two halves move together, the V-shaped belt in-between is forced toward the perimeter. When the two halves move apart, the belt rides closer to the hub.

In low gear, the halves of the engine-driven front pulley are apart, allowing the belt to ride closer to the axis. The rear pulley halves are closer together, creating a larger diameter for the belt. As the car picks up speed, they switch. The front pulley halves converge, and the rear ones move father apart, creating a high gear ratio.

Instead of changing abruptly — step by step — like a conventional transmission, a CVT's ratios change from low gear to high gear smoothly and continuously. They are continuously variable, hence the name.

An engine with a CVT could theoretically operate all the time at the same speed, without revving up and down, even as the car's speed changes. That produces a smooth ride and allows engineers to build a fuel-efficient drivetrain around an engine's most efficient speeds.

That's how the CVT in the old Justy worked, and it took some getting used to. The engine would rev up to a certain rpm and stay there. It created the impression that the car wasn't going any faster, even as it was accelerating to highway speed. Drivers are accustomed to associating the car's speed with the ups and downs in engine speed that occur with traditional gear changes.

Partly to give people what they're used to, new-generation CVTs are pre-programmed to employ certain pre-set gear ratios, an ability that CVTs are getting better at all the time, according to Nissan's Dominique. He adds that drivers with a CVT can also opt for a manual mode, which enables them to select pre-set gear ratios.

"The previous generation of CVTs had around 30 unique algorithms," Dominique explains. "The newest generation has more intelligence: more than a thousand algorithms — different scenarios that the transmission can recognize — and different combinations of throttle, acceleration and G-force, whereby the transmission knows to go to exactly the right ratio."

"Clearly it's high-tech," he says. "There are always going to be people who like to tell their friends, 'I've got this very smart transmission.'"

Bio: Jim Henry is a veteran freelance automotive writer. He was a one-man bureau in New York City for Detroit-based Automotive News for many years. He is also a past president and a board member of the International Motor Press Association.

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