Video: The World’s First Production Variable-Duration Valvetrain

These days, with advanced electronic controls in automotive engines the norm, rather than some whiz-bang new gadgetry, it’s just assumed that OEMs can control everything about the engine. While variable valve lift and variable valve timing is almost old hat at this point, there is one valvetrain parameter which the OEMs haven’t been able to easily vary — duration. Until now, that is.

Hyundai recently released the first mass-produced engine utilizing variable camshaft duration. Dubbed CVVD — Continuously Variable Valve Duration — the technology debuted on the 1.6-liter turbocharged four-cylinder powering the 2020 Hyundai Sonata. Please, before you scoff at the new technology being used on such a mundane vehicle, read through to understand just how interesting this new engine technology is.

Being a rather complex subject, it only makes sense that Jason Fenske from Engineering Explained made a video breaking it all down and explaining everything from the concepts to the actual execution of the new system. Fenske’s knack for making utterly complicated things easy to understand is a huge help in understanding this new technology.

Understanding What Needs to be Controlled

As Fenske points out early on in the video, before you can understand how the control system works, you have to make sure you understand the system being controlled first. While this might seem like “Camshaft 101” stuff for some of you reading, a quick review never hurt anyone.

In the video, Fenske mentions that there are three dimensions that can be adjusted on a camshaft, and in reality, there are four (or three and a half, since one might fall under the umbrella of another). The first is lift, or how much the valve is raised off of the seat. The second is timing, or when the valve lift occurs in relation to crank position.

Hyundai’s new Continuously Variable Valve Duration system is the first mass-produced system allowing for continuously variable — both longer and shorter than what’s ground into the lobe — valve duration.

Third — and feel free to argue that this is part of the second category — is lobe-separation angle, or the timing between the intake and exhaust cam lobe peaks. By adjusting the cams in dual-overhead-camshaft applications independently of one another, LSA can be continuously altered for any given situation.

The fourth is a parameter which is dictated when the camshaft lobe is ground — duration. Duration is how long the valve is off of the seat. “Up until now, no mass-produced engine has been able to continuously vary how long a valve stays open, relative to engine speed,” says Fenske. That’s where this interesting new system comes in.

Like a traditional overhead camshaft valvetrain, the CVVD camshaft rotates and its lobes actuate a rocker arm, which follows the cam lobe and translates the lobe’s rotation and shape into vertical movement of the valve. Here’s where Fenske’s passion for breaking down really complex subjects comes in handy because this system is complex.

“What is special about the Hyundai setup is that the camshaft and the cam lobe are not directly attached to each other,” Fenske explains. “By adjusting the mechanical relationship of the cam lobe control wheel and the camshaft, it alters the duration of the cam lobe.”

No, this isn’t a cutaway or demo camshaft. The CVVD camshaft is actually made up of multiple pieces, allowing the lobes to move independently of the cam core.

How Do They Do It?

We’re going to warn you right here, if you’re more of a visual learner, you need to watch Fenske’s video (probably a couple of times) to truly grasp the mechanics going on in this setup. Right off the bat, we just said that the camshaft moves independently of the lobe, and that is probably paradigm-shattering for most readers.

“Both the camshaft and the cam lobe always have the same absolute RPM,” Fenske explains. “The thing that is changing is through that one revolution; their relative speeds will change. The [cam lobe] positioner changes the two items’ relative speeds.”

Here you can see the actuator in both of its extreme positions. When centered, the cam lobe equals the camshaft RPM and the duration is what is ground into the lobe. Moved in one direction, the lobe swings past the valve faster than the rotation of the camshaft, reducing duration. Moved in the opposite direction, the lobe slows down in relation to the camshaft as it passes across the valve, effectively increasing duration.

Physically, that is accomplished through the use of an electro-mechanical actuator, which alters the position of the wheel in relation to the lobe (left or right of the cam lobe’s center of rotation) via a worm gear. That movement left or right takes advantage of a geometric linkage which allows the lobe to speed up or slow down through specific parts of its rotation.

“When the axis of rotation is equal for both the cam actuator and cam lobe, cam lobe speed always equals camshaft speed,” explains Fenske. “If you were to move that alignment in one direction, the geometry of the linkage causes the lobe to accelerate (relative to the camshaft) through the valve actuation area, reducing duration.”

“Moving it in the other direction, causes cam lobe to slow down in relation to the camshaft’s rotation as it passes through the valve actuation area, increasing the length of time the valve is open.”

The system of actuation is completely mechanical (save for the electric actuator motor) in order to keep reliability high. The electronic actuator drives the worm-gear shaft, which then moves the cam lobe offset disc in one direction or the other. A failure in the drive system would simply mean a fixed duration.

What it Accomplishes

Given this is currently only being fielded in an economy sedan, obviously, the main goal isn’t performance. But, the technology is still in its infancy (production-wise), so the potential for the system is still largely untapped. However, Hyundai is claiming significant increases in performance, efficiency, and reduced emissions.

“This system allows you to optimize the camshaft duration for every single RPM point, not just one or two, which is what most engines currently do. The beauty of this system is its flexibility, allowing you to optimize duration at any RPM,” Fenske says.

“That also means that by running really long duration on the intake side, you can run a modified Atkinson cycle, allowing the engine to vent some of its intake charge, lowering its compression ratio while maintaining its expansion ratio, allowing for more efficient operation.”

All this is starting to sound similar to the articles we published on Nissan’s Variable Compression turbo system, which brings up another point… Before you start condemning the idea as too complex to work — much like a lot of you did in the comments of the VC Turbo articles — realize the engineers have been working on this project for nine years, before releasing it.

“This is a purely mechanical system by design. For something to fail, it must do so mechanically. That means a part has to physically break for things to go wrong,” says Fenske. Add in that it’s covered by a 10-year, 100,000-mile warranty, and it’s safe to say that the company believes in the reliability of the new system. Now, we’re interested to see this technology implemented in a performance application.

Here you can see the extremes in available durations for a given camshaft. We’re not sure if these numbers are the actual numbers of the Hyundai cam lobe, but that is one hell of a duration range.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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