Comment by dahart

> Jumo 204 held the record until nineties

And what has happened since then? Google is showing me several engines with breakthrough efficiency in the last 10 years.

When I was a kid in the 90s, SUVs commonly got 12 MPG. The new models are 25 sometimes 30 MPG. Emissions have gotten considerably better in the last 30 years.

I’m looking and can’t find any info to back up the claim that this 1920s engine was more efficient than engines designed in the 80s and 90s. I am curious about it, not just is it true, but specifically what kind of efficiency you mean and what design features made it efficient. Do you have any sources or reading? Wikipedia talks about how the arrangement of the valves increased the efficiency, but only says this made it approach four stroke efficiency (at the time), not that it exceeded other designs. The 204 was a two stroke, and it seems to be common knowledge that even today, four strokes are more efficient. https://en.wikipedia.org/wiki/Junkers_Jumo_204