This type of engine is known by various names; most commonly flame licker, flame gulper and vacuum engine. It works by sucking in hot gasses directly from an external flame which are then cooled rapidly on the power stroke.
It has a small meths burner to provide a small flame, This is positioned close to a port in the end of the cylinder. The port is opened for the inlet stroke, sucking in hot gasses from the flame. The port is then closed for the return stroke, during which the gasses cool down and create a partial vacuum. The gasses are exhausted at the very end of the stroke. The valve over the port is a thin strip of metal that is loosely suspended over the port. During the power stroke, it is held against the port face by the partial vacuum. At the end of the stroke, when the vacuum disappears, the valve is pushed away from the port face, allowing the remaining gasses to escape.
The valve is operated by linkage from a return crank on the crank shaft. The single cylinder is 12mm bore x 9mm stroke. The cylinder is steel and the piston is brass. The main frame appears to be aluminium, mounted on a wooden base.
The engine was purchased new by me. It is rather temperamental in operation, needing constant adjustment of the flame position relative to the port, and stops frequently. Considering the size of the cylinder, compared with most other flame licker engines I've seen, I don't think I can expect much better performance. The reliability might improve with practice.