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This is the Hugo Junkers -developed WWII 2-stroke aerodiesel.  It had both supercharging and turbocharging, good for climbing power of 750hp from sea level to nearly 40,000ft.  It uses 12 pistons, sharing six very long cylenders with a pair of crankshafts at the top and bottom geared together to an upper propshaft.  It's one of the most unusual engines I've ever read of, and was successfully used in German cargo aircraft in WWII.

(engine seen at the Silver Hill / Paul E. Garber Facility that restores aicraft and aeroengines to display status for the Smithsonian and the National Air and Space Museum)