Anyone know the correct English term for this? I have done some research and think it might be a (vertical) roller mill. However, I'm not sure. I found a good diagram and description of a Gutbett-Walzenmuehle in German at: if that's of any help. I did find ...
The W-31 motors used flattop pistons and got a 10.5:1 CR with the stock steel shim head gaskets. The lesser "high compression" engines used a small dish and got about 10.25:1 CR. The low compression engines used a large dish and got 9:1, which dropped to about 8.5:1 for 1971.
A first survey of grinding with high-compression roller mills. Int. J. Miner. Process., 22: 401-412. The special feature of high-compression roller mills (HC-roller mills) is that a bed of particles is compressed between two rollers to a high solid density more than 70% of volume. The size reduction occurs by interparticle crushing.
This, along with as high a compression ratio as the fuel will stand, means significantly higher pressures on the power stroke. Moving on to the compression stroke itself, we find that the higher the compression ratio is, the higher the resultant combustion pressure is.