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I've always wanted to build a reversable vacuum engine, but I could never shrink down a valve gear mechanism. I also thought it would be nice to have VVT for higher speeds. Eventually I thought of using a differential to do both jobs. The engine's crankshaft drives one of the differential's outputs to spin the case. The other differential output is held in place by a worm gear. Spinning the output to turn the differential case makes a 2:1 reduction, so I attached a chain from the 24t end of the differential to a 12t spur gear to get the ratio back to 1:1.

 This engine runs very smooth. I can advance or delay the valve timing in small increments by turning the worm gear, or reverse the engine by advancing the timing by 180 degrees. I hope someone puts this mechanism to use!

https://bricksafe.com/pages/LegoLord1880/vvt-vacuum-engine

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