Drone On: How CVT Transmissions Improve Mileage

More gears means better fuel economy, and the potential for more performance. But more gears adds weight, complexity, and shift times. What if you could make a gearbox that wasn’t. One that had all the ratios instead of just a handful. Some sort of continuously variable transmission? That’s the CVT, and this is how it works.

The CVT is becoming more and more popular as fuel economy targets increase. Today they’re used by nearly every automaker. Fiat Crysler, GM, Ford, Toyota, Honda, Nissan, Subaru, and even Audi have given the non-gear gearboxes a shot. They were pioneered in power systems to make electricity on aircraft, as well as farm equipment. They’ve been used there since the 1950s.

But the idea is even older than that. The first to appear in a car was way back in 1896. DAF brought the idea closer to the mainstream in the 1950s, with a transmission was used in the DAF 600. After some more development time in snowmobiles and ATVs, Subaru launched a CVT-equipped Justy in 1987, developed by the same company that produced the DAF. Through the 90’s, Nissan, Ford, Fiat, and Honda also brought the idea onboard. Now, it’s taken over some automakers’ entire lineups, with Nissan and Honda moving to CVT over eight and nine-speed automatics in small cars.

Toyota CVT with first gear

Most CVTs work the same way. And despite their strange nature, they’re fairly simple. There are two pulleys and a belt. That’s it. Well, ok, that’s not it. But that’s most of it. There is a pulley on the input shaft and one on the output shaft.

The two pulleys are cone-shaped, widening toward the outside. The pulleys aren’t one piece, either. They’re two pieces. So they can squeeze together and pull apart. When they squeeze together, the belt moves to the outside of the pulley. A bigger circle for it to travel around. At the same time, the other pulley pulls apart. That lets the belt drop to the inside, sending it around a shorter path.

Nissan Xtronic Pulleys

Sending power from one pulley to the other is a belt. And no, it’s not some rubber belt. At least not anymore. It’s more like a chain. One or more flat steel bands make up the loop, then they have metal elements that are held by the bands. Those are what help transmit power. It’s basically a band made of metal. In Audi’s multitronic, the belt really is a chain. Like taking a bunch of bicycle chain links and putting them together, parallel.

The moving pulleys give a much wider ratio spread than fixed gears can without leaving big gaps in between gears.  Big gaps means that the revs fall too far after a shift. That leaves you sluggish and out of the power band. The ratio spread is the ratio between the shortest and tallest gears. The highest ratio divided by the lowest. A conventional four-speed auto has a ratio spread of around four. So if first is 4:1, fourth is 1:1. A six-speed auto has a spread of around 6. But a CVT can manage closer to 9 to 1. Something that it takes a 10-speed automatic to beat. The CVT can also hit every ratio in the middle, so it doesn’t drop out of the power band. It can also tick over at basically idle on a flat highway.

Audi multitronic with belt visible

The spread means lower RPM for highway cruising, and always in the power for acceleration. It’s why CVTs swing from low to high rpm so quickly. That, combined with fewer moving parts, means less power lost to poor efficiency. So in theory, they drive better, use power better, and sip gas.

So the power comes in one end, goes through the pulleys and the belt, then heads to the wheels. The complicated part, and where manufacturers vary, is how to get the power to the gearbox. Manuals use a clutch to separate the engine from the transmission, automatics use a torque converter. But the CVT can use either one.

JATCO CVT belt

Honda uses a pack of clutches soaked in fluid. They release when you stop and slip together when you pull away. Like a manual clutch, they can wear out over time. Audi’s multitronic system used one clutch for forward and one for reverse. Other manufacturers use a torque converter. Those ones can have a smoother pull away from a stop as the converter does the slipping instead of a real clutch. Toyota’s latest has taken it a step further and added a real first gear as well as a torque converter. The conventional first gear smooths out pulling away, removes the rubber band effect that CVTs have because of the converter or clutch and pully movement, and allows a wider ratio spread. Most CVTs are built by Jatco, the company that is responsible for Nissan’s design.

Audi multitronic belt

Problems with the belts, and slippage of the belt, limited maximum torque the CVT could handle. That’s why they are most commonly found on lower power, smaller engine vehicles. But as the new box matures, that problem goes away. Like in the new Subaru Ascent, which makes 277 lb-ft and can tow up to 5,000 lbs. Both respectable figures for a midsize crossover.

The CVT is best known for full throttle drone. Something automakers have tried to program out. But programming them to make fake shifts, or to rev more slowly makes them less efficient, and takes away some of the benefit. And those paddle shifters are still using the pulleys. They just pick a set gear ratio instead of letting the CVT do what it does best. So if you have a CVT, embrace the drone. It’s maximizing your economy and minimizing your fuel consumption. It just doesn’t always sound great.

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