It Still Works: The very first Otto Langen engine from 1867.
More than a century has passed and the Otto Langen engine designed by Germans Nikolaus Otto and Eugen Langen still works. This combustion four-stroke engine works on the Otto cycle, which the inventor cleverly designed himself.
View it being fired up and running here:
Details on the engine can be found here: https://oldmachinepress.com/2018/01/20/otto-langen-atmospheric-engine/