Volkswagen created what they’re calling the Budack cycle, with a unique twist on the typical four-stroke engine (intake, compression, power, exhaust). During the intake stroke, the intake valves close early, so the piston keeps moving down, but no additional air is coming in. This reduces the effective compression ratio, because there’s less air and fuel to squeeze down. During the power stroke, the engine essentially has a longer distance for the piston to move down. That is to say, the expansion ratio is greater than the compression ratio, thus allowing the engine to get more useful work out of the air and fuel burned, improving efficiency. It’s very similar to the Atkinson cycle; both allow a limited amount of combustion gasses more room to grow.
VW Budack Cycle (Full Video) - https://youtu.be/JjM14JBPdBI
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