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Continuously variable transmission

Continuously variable transmission wheels and axle from car gearbox Anti-lock brakes (ABS), electronic stability control (ESP), gasoline direct injection (GDI): When it comes to naming a new technology, the auto industry really loves its TLA (three-letter acronyms). Here’s one you may not have heard: CVT.

The goal of continuously variable transmission (CVT) is to reduce expense and complication in your car and give you better fuel economy.

Nissan, for example, figures its V6 powered Murano SUV gets 10 to 12 percent better fuel economy with its standard-equipment CVT than it would with a conventional automatic transmission.

So why doesn’t every car use a CVT today? Let’s find out.

CVT history
The CVT is a type of automatic transmission that was developed by General Motors (GM) in the 1930s. The technology went on the shelf because the fluids and control devices needed to make it reliable weren’t available yet.

Subaru takes credit for introducing the first continuously variable transmission to America with its 1989 Justy GL ECVT, a small hatchback. In the past decade, interest in the CVT has really picked up. GM launched its own production CVT in 2001 on its Saturn Vue SUV. Honda put CVTs on the option sheets of its Civic subcompact (1996) and Insight hybrid (2000). Since 2003, Nissan has installed one called the Extroid CVT on its Murano SUV and sells several models in Japan with CVTs. Audi started offering CVTs on its A4 sedan in 2002. Nowadays, many vehicles equipped with CVTs are in American garages.

What a transmission does
You may already know that a transmission transmits power from the engine to the wheels. Since an engine isn’t flexible enough to operate at every wheel speed needed for regular driving (otherwise they’d bolt the wheels directly to the crankshaft), some kind of ratio device is required to allow the engine to operate in its limited speed range but to generate a broader range of wheel speeds. That device is the transmission.

Most passenger cars use a step-gear transmission with anywhere from two to six speeds (though Mercedes-Benz recently introduced a seven-speed transmission). Each “speed” is actually a different gear ratio created by varying the sizes of the gears relative to each other.

A small gear turning a big gear has a lot of mechanical advantage but must turn faster than the big gear to maintain a speed. A big gear turning a small gear has less mechanical advantage, but it gets to turn slower to maintain the same speed. That’s a conventional transmission. The ratio of the transmission – its flexibility – is always limited by the number of speeds.

Picture a multispeed bicycle – it has a transmission more like a CVT. You have a chain sprocket at the pedals, and as you shift to higher gears, the chain moves from a smaller sprocket to a larger one (relative to the rear sprocket). Although you pedal slower, the bicycle goes faster – but it also gets harder to pedal. Likewise, as you downshift, say to go up a hill, the chain moves from a larger sprocket to a smaller one. The bicycle goes slower, you pedal faster and it becomes easier to pedal.

CVT: The gearless gearbox
A CVT takes this basic idea and adapts it to cars but with even greater flexibility.

Instead of gears, most CVTs have two pulleys wrapped by a single belt with a V-shaped cross-section (there are other kinds, which we’ll get to in a minute). Each pulley looks like a yo-yo and is actually made up of two cones that face each other. Under the force of hydraulic pressure, the cones either come together or separate, causing the V-shaped belt to either ride higher in the pulley groove or lower down and closer to the center.

Riding higher equates to a bigger diameter sprocket; riding lower means a smaller diameter. As you drive, the two pulleys variously squeeze in or separate to change the ratio between the input pulley (from the engine) and the output pulley (to the wheels).

Here’s an example: When you start from a stop, the control computer de-clamps the input pulley so the belt turns the smallest diameter while the output pulley (which goes to the wheels) clamps tighter to make the belt turn its largest diameter. This produces the lowest gear ratio (say, 3.0-to-1) for the quickest acceleration. As speed builds, the computer varies the pulley diameters, as conditions dictate, for the best balance of fuel economy and power.

As a result, instead of five or six ratios, you get an infinite number of ratios between the lowest (smallest-diameter pulley setting) and highest (largest-diameter pulley setting).

The engine can always run at the optimum speed for power or for fuel economy, regardless of the wheel speed, which means no revving up or down with each gear change, and just the right rpm for the right speed all the time.

There’s another advantage: The lowest and highest ratios are also further apart than they would be in a conventional step-gear transmission, giving the transmission a greater “ratio spread” This means it is even more flexible.

In the case of the 2002 Audi A4 CVT, for example, the ratios varied from 2.40-to-1 to 0.40-to-1, or a factor of 6 from the highest to the lowest. In the conventional six-speed automatic also offered on the A4 that year, the ratio factor was only 4.79-to-1. A bigger factor means a transmission that can do more and do it more efficiently.

All of this may sound complicated, but it isn’t. In theory, a CVT is far less complex than a normal automatic transmission. A planetary gear automatic transmission – sold in the tens of millions last year – has hundreds of finely machined moving parts. It has wearable friction bands and elaborate electronic and hydraulic controls. A CVT like the one described above has three basic moving parts: the belt and the two pulleys.

Considering the savings involved in building transmissions with only three moving parts, you’ll understand why car companies have become very interested in CVTs lately.

Different types of CVTs
In addition to the belt-type CVT described above, here is a rundown of the basic types used in passenger cars:

Belt-type CVT: also called a “push-type” or “Van Doorne” CVT. Engineers call it a push-type because of the belt design. The belt, developed by a Dutch company called Van Doorne, which holds the patent, is actually a stack of 70 to 80 flat, trapezoid-shaped steel plates joined in a loop by steel bands.

The belt is clamped in the pulleys at pressures up to 4 tons and “pushed” though its circuit by the pulleys. The compression loads on the belt’s individual plates vary with engine size, but they’re high, starting at 1,200 pounds for a small four-cylinder engine. Because the clamping and load forces are so high even for small engines, the belt-type CVT is limited in how much torque it can take without flying apart. It’s currently in use on the Saturn Vue, Ford Freestyle and FiveHundred, Honda Civic and Insight, and Nissan Murano.

Chain-type CVT: Also called a “pull-type” or “Luk chain,” this CVT also uses clamping pulleys. But instead of a belt, a chain (developed by a German company called Luk) is pinched between the pulley halves.

The chain links are held together by dozens of pins, which also act as the contact surface with the pulley. The chain is pulled through its circuit by the engine pulley. The design’s two main advantages over the belt-type CVT: The chain can turn in a smaller diameter than the belt, allowing for a greater ratio spread in the same size transmission case, and the chain more easily handles higher horsepower engines. Chain-type CVTs are in use on the Audi A4.

Toroidal CVT: This one is tricky. A torus is a geometric shape similar to a cone but with concave sides. Imagine putting a cone on a potter’s wheel and shaping the sides as it spins until they are rounded in.

Now, put two tori facing each other with a disc-shaped roller in between, in contact with both. Next turn the disc so that as it rides higher on the curving side of one torus, it runs lower on the other. One will spin faster as the other spins slower. You have a toroidal CVT. Because it doesn’t rely on belts or chains or clamping pulleys, the toroidal CVT can transmit more torque. You won’t find a toroidal CVT in the United States – at least not now. It’s only found on some Nissan vehicles sold only in Japan.

To CVT or not CVT
To sum up, because CVTs are simple, with fewer moving parts (which cause friction), they offer better fuel economy and performance, and they have infinite ratios and a broader ratio spread, allowing the engine to always run at the optimal speed.

Even though they are cool and useful, there are a number of reasons we don’t see CVTs on every car: 

  • They’re a lot more complicated and expensive to produce than they look. Generating the high clamping forces that make them work is a challenge for hydraulic systems and lubricants. Parts must meet precise specifications during manufacturing and assembly. Various patent royalties also drive up the costs to the automaker.
  • Durability has been a big stumbling block. Belt slippage is an Achilles’ heel of the CVT. If the belt slips even the tiniest bit on the pulley, the transmission is ruined.
  • Some drivers don’t like them. Most people have no clue what kind of transmission they have, but they generally expect an engine to sound a certain way. A CVT-equipped car sounds a bit like a motorboat. When you floor it, the engine revs up to a high speed and stays there until you let up. There’s no gear shifting, so there’s no change in the engine’s tone that we’re all accustomed to with conventional transmissions. Owners often bring their cars back, thinking that the transmission is slipping or something is broken.
  • The fuel economy gains are now being matched by conventional transmissions. Improvements in step-gear transmissions have narrowed the gap. Some automakers feel they can get most of the fuel savings of a CVT by adopting six- or seven-speed automatics.

The future of CVTs
Still, the CVT has its loyal supporters. Nissan, for one, is sticking with its Extroid CVT on its 245-horsepower Murano and plans a fourfold expansion in its CVT usage in the next few years. Regarding the CVT’s customer acceptance, Nissan North America product specialist Tony Pearson acknowledges that manufacturers and dealers must do more.

“There’s still a lot of education on the operation and benefits of the CVT that has to be done with the public,” he says.

As for durability, “... this has been our lowest warranty (cost) transmission,” Pearson says. “All of our front-drive cars will be going in that direction.”

Does your future hold a CVT? Quite possibly.

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