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Here's How Volkswagen's New 'Budack Cycle' Engine Works

gif: Engineering Explained/YouTube

The 2018 Volkswagen Tiguan crossover has a new 2.0-liter turbocharged EA888 engine using a “B-Cycle” combustion method, a variation of the Miller Cycle named after VW’s own powertrain engineer Ralf Budack. Here’s how it works.

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If you clicked on this, chances are you’re familiar with the Miller Cycle. If not, here’s the gist of it: a Miller Cycle engine reduces an engine’s effective compression ratio by holding the intake valve open longer, letting the piston push some intake gases back into the manifold, and thus expending less work compressing the charge.

This means that, during the expansion stroke, energy from combustion can be more efficiently turned into work. Because of the reduced compression ratio, output is lower than in a conventional otto cycle.

The Budack Cycle—as Jason Fenske from Engineering Explained mentions in his video—uses the same concept, except it closes the intake valve early. Thus, as the piston goes down with the valve closed, the pressure inside the combustion chamber drops, and then rises during the compression stroke. Ultimately—like in a Miller Cycle—this yields a lower effective compression ratio, which makes it easier for the power stroke to turn energy from combustion into downward motion of the piston. Like in the Miller Cycle, the increased efficiency comes at the detriment of power output.

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It’s not a particularly complex or groundbreaking setup, using essentially a sliding pin (likely hydraulically actuated like Honda’s VTEC) to change cam profiles from a high-lift one that opens and closes normally to a lower lift profile that closes early. The former is used when the driver drops the gas pedal to the floor, and the latter is used during low-load cruising conditions.

So it’s not quite as amazing as Infiniti’s variable compression ratio engine or Mazda’s Spark Controlled Compression Ignition Engine, but it’s definitely still cool to learn about how different automakers are approaching ever-increasing fuel economy and emissions regulations.

Sr. Technical Editor, Jalopnik. Always interested in hearing from auto engineers—email me. Cars: Willys CJ-2A ('48), Jeep J10 ('85), Jeep Cherokee ('79, '91, '92, '00), Jeep Grand Cherokee 5spd ('94).

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DISCUSSION

I’m pretty sure what you’re describing is the Atkinson cycle. The Miller cycle is when you combine the Atkinson cycle with a supercharger to drive more air into the cylinder while keeping the intake valve open on the upstroke of the compression stroke.