"This engine is not a 'vacuum' engine, it is a 'beta' type stirling engine. Beta means that the power piston and the displacer are both working in the same cylinder. This arrangement gives greater power output than the typical two cylinder engine. The reason for this is that the compression ratio can be higher due to less "dead" space inside the engine.
The Duplex Vacuum Company of Chicago, Illinois made this type of engine about 100 years ago. They were widely used in popcorn wagons and to operate dental drills among other applications due to their silent operation and reliability.
A unique feature of the Duplex engines is the oscillating shaft and linkage which produces an interesting motion in operating the displacer." - Jerry
