Abstract: In an engine driven by liquefied gas, such as liquefied nitrogen or air, inlet means (31, 33) admits drive fluid to an expansion chamber defined by the volume between two relatively movable members (1, 28) guided for movement in a repetitive cycle by guide means (38, 40). The inlet means also allows a heat exchange liquid to be admitted to the chamber in which the liquid is in intimate contact with the drive fluid and gives up heat energy to the expanding drive fluid. The cooled heat-exchange liquid is withdrawn from the chamber, heated and re-circulated to the chamber. Part of said cycle includes a time period during which there is substantially no relative movement between the members (1, 28) and during which the chamber volume is maintained at or close to its minimum volume, the heat-exchange liquid and drive fluid being admitted to the chamber during said time period.