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".
Many swing-piston engines have been proposed, but none have been successful. Two attempts in about 2010 are the prototype American-made MYT engine[1] and prototype Russian ORE for use in the Yo-Mobile hybrid car. Both claimed high fuel efficiency and high power-to-weight ratio, but there have been no successful demonstrations of claimed efficiency or that the engines are durable enough for practical use.