The Stirling cycle engine was invented by Robert Stirling in 1816, so it has been around for a while. It is a heat engine, and is based upon a cycle of heating, then cooling, of a gas (usually air) contained within the engine. Since a Stirling engine is air tight, during the heating phase the air pressure inside increases, and during the cooling phase the pressure decreases. A displacer connected to the crankshaft moves the internal air from hot side to cold side of a cylinder. The change in pressure drives a power piston, which is also connected to the crankshaft. Since there are two stages, hot and cold, it is a two cycle engine
This diagram shows the concept of a traditional LTD Stirling cycle engine. The components of the engine described in this Instructable are arranged differently, but the concept is the same.