Swing-piston engine
A swing-piston engine is a type of internal combustion engine in which the pistons move in a circular motion inside a ring-shaped "cylinder", moving closer and further from each other to provide compression and expansion. Generally two sets of pistons are used, geared to move in a fixed relationship as they rotate around the cylinder. In some versions the pistons oscillate around a fixed center, as opposed to rotating around the entire engine. The design has also been referred to as a oscillating piston engine, vibratory engine when the pistons oscillate instead of rotate, or toroidal engine based on the shape of the "cylinder".
Two successful swing-piston models have been made: The American-made MYT engine and a Russian ORE which was used in the Yo-Mobile hybrid car. These engines utilize fuel much more efficiently than other internal combustion engines and generate a high torque output with twelve compressions per oscillation. Due to the many flaws within the engine itself, it has not been very successful in the past, but with the technological advances and more practical construction methods, TRPE's (Twin-rotor piston engine) have come a long way in terms of design.