While several people have mentioned the Napier Deltic, there was also the Junkers Jumo 205 opposed-piston diesel aero engine of the early 1930s.<p><a href="http:&#x2F;&#x2F;www.enginehistory.org&#x2F;Diesels&#x2F;CH4.pdf" rel="nofollow">http:&#x2F;&#x2F;www.enginehistory.org&#x2F;Diesels&#x2F;CH4.pdf</a><p>Claims of innovative internal combustion engine designs that will be significantly more efficient than current ones show up every few months. They never live up to the hype.