A very interesting YouTube film on the workings of the BMW IIIa engine. It equipped the late Fokker DVII also known as the DVIIF.
https://www.youtube.com/watch?v=78nAd49BeVs
The above is part one, there is also a part two in the side bar.
https://www.youtube.com/watch?v=PSLE_4dUiWc
Greg appears to know his subject and explains the mysteries of how this revolutionary engine worked. He suggests, among other things, that the engine's horsepower may actually be under-rated as German figures were on the low side. Allied tests achieved up to 230 h.p. but this may have been with better quality petrol than the Germans possessed.
Wiki has the background below:
https://en.wikipedia.org/wiki/BMW_IIIa
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