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A split-cycle engine includes a compression cylinder having a first volume for a first working fluid and a second volume for a second working fluid, the first volume and second volume being separated by the compression piston, an expansion cylinder having a first volume for the first working fluid and a second volume for the second working fluid, the first volume and second volume being separated by the expansion piston, and a fluid coupling between the second volume of the compression cylinder and the second volume of the expansion cylinder, wherein the two second volumes and the fluid coupling provide a closed volume for the second working fluid, wherein the fluid coupling includes a regenerator arranged such that the two second volumes are thermally decoupled.