A 4 cylinder vacuum engine, built part of a drawing and the most off personal idea.I have a drawing of a single engine called the Luts, which I made as the Red Baron, and now a four-cylinder called The Baron.
source/image: Ed van den Berg
It was not easy to make, the most sizes are not there and I should find it while building but it has been successful.
How it works? This is traditionally known as an external combustion engine in contrast to an internal combustion engine where the heat input is by combustion of a fuel within the body of the working fluid.
A vacuum engine also called flame-licker enginederives its force from air pressure against one side of the piston, which has a partial vacuum on the other side of it.