Abstract: A heat exchanging medium is provided which is constructed out of sintered diamond. The medium can be formed into various desired shapes such as tubes, mesh, screens, granules or the like. The sintered diamond forms the heat transfer medium of a heat exchanger or regenerator.

Abstract: An externally heated engine is provided which has at least two pistons. The first piston has a first side (working side) and a second side opposite the first side. The first side of the first piston and the first cylinder define a first working chamber containing working fluid. The second side of the first piston and the first cylinder define a first opposite chamber containing an opposing fluid. A heater heats the working fluid in the first cylinder. Preferably, the cylinder is heated by a heat source so that the working fluid has a temperature of no more than 500° Fahrenheit with a temperature difference between the heat source and the working fluid of less than 5° Fahrenheit. The second piston reciprocates within a second cylinder, and has a first side (working side) and a second side opposite the first side. The first side and the cylinder define a working chamber containing working fluid. The second side of the piston and the cylinder define a second opposite chamber containing an opposing fluid.