How Hyundai's New, More Fuel-Efficient Engine Tech Called CVVD Works

  • Hyundai claims it has created the world's first Continuously Variable Valve Duration (CVVD) engine technology, which it says increases engine performance by 4 percent and efficiency by 5 percent.
  • The first application will be in the company’s Smartstream G1.6 T-GDi inline-four engine, which makes 180 horsepower and 195 lb-ft of torque and is expected later this year in the new Sonata Turbo in the home market of South Korea.
  • Continuously Variable Valve Duration (CVVD) stretches or shortens the time the intake valves are open, depending on engine speed and load.

Engine building is an exercise in balancing efficiency with power. Much of that compromise involves the camshafts, which control the air that flows in and out of an engine’s cylinders. To minimize the trade-offs between competing priorities automakers use systems that are by now well known-variable valve timing, variable valve lift-and now Hyundai is announcing something new: variable valve duration. Hyundai calls the technology Continuously Variable Valve Duration (CVVD) and claims an increase in performance of 4 percent with 5 percent better efficiency, along with a 12 percent improvement in tailpipe emissions. The system works as a complement to existing variable-valve-timing systems, not as a replacement. It's just one more invention to make engines instantly able to adjust to any operating condition, and probably the final step short of a camless engine.

So what's going on here? First, let's back up to the basics for the uninitiated (experts skip ahead two paragraphs). Each piston in a four-stroke engine goes down, up, down, up-four strokes-in every combustion cycle. That's intake, compression, combustion, and exhaust. In simple terms, the intake valve(s) need to be open at the right time to pull air in, and the exhaust valve(s) need to be open to push those hot, post-combustion gases out to start the cycle all over again. In actual practice, the valves open at different times throughout the four-stroke cycle and even overlap at times.

The valves are pushed open by the eccentric lobes (think egg shaped) of a camshaft at a specific point in a 360-degree rotation. Adjust the shape of the camshaft lobe, and you can change when, how far, and how long a valve opens. The problem is that valve timing that works for high-rpm performance running might not work very well for idling or low-rpm cruising. Enter variable valve timing, which helps get closer to the best of both worlds.

Here’s a more detailed explanation, but the general idea uses a form of rotational black magic (cam phasing) to advance or delay the valve timing. Variable valve lift gets even more complicated.

Variable valve duration is not necessarily less complicated than variable timing or lift, but VCCD is an elegant solution. It works, as best as we can tell from patent drawings and Hyundai’s promotional video, with a rotating adjuster with a slot in the middle. The variable duration adjuster moves up and down, and shifts the contact point of the cam lobe. Where the duration adjuster is moved determines how long a valve is open.

The benefit is that you can have a long valve duration during low engine speeds and loads, allowing plenty of time for the air to enter the cylinder. At higher speeds, a short duration maximizes compression and thus power. Speaking of compression, VCCD can also be used to change the effective compression ratio, similar to how variable-valve-timing systems can close the intake valve late and effectively switch between Atkinson-cycle and Otto-cycle operation.

While the 1.6-liter debut of VCCD only uses adjustable duration on the intake valves, Hyundai’s patent applications state the system is not limited to a single camshaft. As this is a new derivative of the Gamma engine, used across the Hyundai and Kia lineups, it’s likely to see widespread use. Hyundai has not announced when we'll see this technology in the United States, however, or which vehicles will get the new engine.

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