How Continuously Variable Transmissions Work

Unlimited possibilities for constant change

Jeff Karr / autoMedia.com

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.

The concept of the CVT has been around in one form or another as far back as the 1800s. 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.