external combustion engine
as defined in Regulation (EU) No 168/2013 of the European Parliament and of the Council of…, Article 3, point (76) (Definitions)
a heat engine in which combustion and expansion chambers are physically separated and where an internal working fluid is heated by combustion in an external source; heat from the external combustion expands the internal working fluid which then, by expanding and acting on the mechanism of the engine, produces motion and usable work;