Advanced Uniflow Steam Engine
The cylinder of a uniflow steam engine has large temperature gradient along its length, since - in order to maintain efficiency - it cannot be cooled. An "Athermalized" cylinder design can maintain good sealing of the piston at all cylinder temperatures. A constant diameter cylinder has an inner liner made of a material with a higher coefficient of thermal expansion and a lower Young's modulus than outer cylinder material. Depending on the materials selected for the cylinder liner and piston plating, Teflon piston rings may - or may not - be required. Superheated steam provides little lubrication for a piston moving in a cylinder. By carefully choosing compatible materials for the cylinder liner and the piston coating, an engine requiring no lubrication can be achieved. A scale model of the engine was built with a cylinder ID of 18 mm, stroke of 36 mm, working pressure of about 30 bar, and speed of up to 6000 RPM. The model was coupled to a small generator and was tested with an electrically heated boiler and atmospheric pressure condenser.