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CVT | How Continuously Variable Transmissions Work

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CVT | How Continuously Variable Transmissions Work

First there were manual gearboxes, then automatic transmissions. Now here comes the latest—the CVT.

Conventional Transmissions.

It's a fact of engineering life: All vehicles need a transmission. That's because current internal-combustion engines, for all their benefits, have a number of requirements. First, almost all engines need a way to be disconnected from the drive wheels to allow for idling when the vehicle is momentarily stopped. Second, since engines have a relatively narrow range of rpm for best power production and/or efficient operation, there's got to be a way to change the gear ratio between the crankshaft and the wheels. That way, the engine can have a numerically high gear ratio for climbing hills at low speed with a heavy load, and at the other end of the spectrum, a numerically low gear ratio for very fast highway cruising at a relatively relaxed engine rpm.

Conventional transmissions do these jobs with a clutch or torque converter to disconnect the engine from the wheels for idling, and an arrangement of mechanical gears that deliver a finite number of possible gear ratios to cover the total speed and load range of the vehicle. The Continuously Variable Transmission does these jobs even better.

What's Old Is New.

Something of an elusive Holy Grail of the transmission world, the concept of the CVT has been around in one form or another as far back as the 1800s. Unlike conventional transmissions that have only a limited number of gear ratios, the CVT has an unlimited number of possible ratios available in between a fixed "low" ratio and a fixed "high" ratio. Given the way internal-combustion engines operate, this is a very good thing since it means that the engine never has to labor at the wrong (inefficient) rpm. And since the CVT doesn't allow slip when underway, no power is wasted as in conventional automatic transmissions. With infinite and constantly changeable ratios available, acceleration, pulling power, fuel economy—everything—improves.

If CVT is so great, the obvious question is why don't we have it on all our vehicles? Snowmobiles have used it for decades, and it's common on a wide range of low-powered utility vehicles. The challenge with CVT for automotive use is coming up with a design that is as cheap, durable and pleasant to use as a conventional transmission. Advancing materials technology and ever-smarter electronic controls are clearing these hurdles even as this is written.

Basic CVT.

The basic CVT system that has found the most favor over the years is a simple layout with two pulleys connected by a flexible belt. One pulley is connected to the engine, while the other is connected to the rest of the driveline and ultimately the wheels. Each pulley is split in the middle, and looks something like two saucers positioned bottom-to-bottom. When the saucers are pulled apart, the belt rides close to the center. When the saucers are pushed together, the belt moves out. So by changing the spacing of the saucers, the pulley's effective diameter changes too. To create a "neutral" that allows the engine to idle without moving the vehicle, one or both pulleys open up and the belt goes slack and slips harmlessly.

A number of low-powered cars have used CVT transmission over the years, but now strength and durability advancements mean that CVT transmission is headed into the realm of mid-powered vehicles. Flexible metal belt designs and advancing material technologies have made CVT transmissions tougher than ever. Audi now offers it on the 220-horsepower A6, and Saturn uses it on the 143-horsepower version of the Vue. Other applications are just around the corner.

Even the slightly odd driving feel of CVT transmissions is being dealt with. To drivers who've spent their lives hearing and feeling the distinct ratios of conventional transmissions, the "disconnected" feel of a CVT can be at first disconcerting. Audi addresses that subjective issue with a manual shift mode that makes the CVT mimic the operation of a six-speed transmission by creating six distinct ratios to choose from. True, a small amount of efficiency and smoothness is lost, but a bit of comforting familiarity is gained for old-school drivers who aren't quite ready to leave the feel of old conventional transmission behind. Inevitably they will, as Constantly Variable Transmissions move fully into the mainstream.

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