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How does Hyundai’s Continuously Variable Valve Duration (CVVD) System Really Work?

Motor Trend logo Motor Trend 12/3/2019 Motor Trend Staff
a toaster oven: Hyundai Vision T HDC7 Concept 3© Motor Trend Staff Hyundai Vision T HDC7 Concept 3

Caution: Nerds-only zone! If you just want to know what Hyundai's CVVD system can do and how it drives, see our companion story comparing the tech to Nissan's VCT system. Keep reading for more on how this advanced tech works.

Altering the length of time that a valve stays open is way trickier than changing when it begins its open-and-close cycle or how much it lifts. To achieve variable duration, Hyundai has basically had to toss out the conventional one- or two-piece machined or hydroformed camshaft and replace it with a hella-complicated multi-piece gizmo that may make your head hurt to try to comprehend.

First, the shaft that the timing chain drives (via a familiar variable-timing phaser, by the way) is now a smooth cylinder with four holes drilled across it. Into those holes go four little pins that slot into and drive four round steel discs that also include a separate perpendicular pin that ultimately drives the cam lobes. These discs rotate in elaborate needle roller bearings on a device that allows them to move a few millimeters inboard and outboard across the top of the engine, perpendicular to the camshaft axis.

a group of people posing for the camera: Hyundai-CVVD-detail-48.jpg

When the movable discs and all their pins are positioned concentric with the camshaft, the cam lobes behave like a normal machined camshaft would, opening the valves for the length of time dictated by their machined profile. The magic happens when you move these discs inboard or outboard. Doing so changes the location of the little perpendicular pins that drive the cam lobes, which has the effect of either speeding up or slowing down the cam lobes as their noses swing around to open and close the valves.

Two separate carriers operate the valves for two cylinders each, and each carrier moves two eccentric discs from side to side using a mechanism of worm-drive gears. It takes 0.4 to 0.5 second to fully transition from one extreme to the other. Note that Hyundai only adds this system to the intake valves. It could also be added to the exhaust valves, but the only potential benefits of doing so would be emissions reduction, and this wasn't deemed a priority worthy of the expense at this time.

The parts count to add this feature seems staggering, as does the added manufacturing complexity. It also adds about 5 kg (11 pounds). Managing the friction and lubricating all these sliding parts also seems a herculean task, but the engineers swear on a stack of service manuals that the engine has cleared all of Hyundai's durability hurdles with flying colors running standard 0W20 oil.

We suspect that part of the argument in favor of green-lighting this complicated project included an element of Korean pride—a desire to lead the industry in a new technology. We sincerely hope it works and delivers on its performance and fuel economy promises in the real world—especially now that the EPA figures indicate the new Sonata with this engine gets slightly lower fuel economy than its 1.6T predecessor.

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