Abstract: A thermal lag engine has a cooled cylinder and piston connected to a heated chamber. A working fluid in the cooled cylinder and heated chamber is alternately compressed and expanded by the piston. Since there is a thermal lag or time interval before the working fluid entering the hot chamber is heated and the working fluid expanding back into the cooled cylinder is cooled, the working fluid is at a lower average temperature and pressure during compression and a higher average temperature and pressure during expansion to provide net work.

Abstract: A Stirling engine has cylinders with closed upper ends and open lower ends. The cylinders are plunged out of phase into tanks of fluids at different temperatures. A connecting tube extends through the tanks and upward through the fluids into the cylinders to allow a working gas to be displaced between cylinders through the connecting tube. The connecting tube transfixes and mounts seals disposed within the cylinders and in the fluids. The seals may have a slight clearance between themselves and the cylinders as the fluids have a higher viscosity and inertia than the working gas to provide an effective seal while the engine is operating to displace, compress, and expand the working gas in a Stirling cycle. The fluids transfer and remove heat from the working gas through the connecting tubes, the cylinders, and direct fluid contact. If the seals are disks, have portions of their edges rounded, or are sections of a sphere, no crossheads are required for some configurations of this engine.