“Although not as amazing as Infiniti’s sophisticated variable compression ratio engine or Mazda’s Spark Controlled Compression Ignition Engine, VW’s Budack combustion cycle is yet another example of how automakers are approaching ever-increasing fuel economy and emissions regulations with internal combustion engines,” noted Jason Fenske from Engineering Explained. He then contrasted the Miller and Budack cycles.
“A Miller combustion cycle engine reduces an engine’s effective compression ratio by holding the intake valve open longer, which lets the piston push some intake gases back into the manifold, thereby expending less work compressing the charge. This means that, during the expansion stroke, energy from combustion can be more efficiently turned into work. But because of the reduced compression ratio, output is lower than in a conventional Otto cycle.”
“The Budack combustion cycle 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. 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.” (Video — Engineering Explained)