Need clarification of some thermodynamic concepts dealing with a Stirling engine. I've read descriptions that talk about the temperature of the heat source. Is this temperature referring to the surface temperature of the metal that contains the gas? Or is it referring to the temperature of the gas itself? Also, does this temperature remain constant at the heat source or is some kind of averaged temperature? Finally, if there is a constant influx of heat energy from an outside source, such as a flame heating the metal surface of the heat source, does the influx of energy remain constant during the whole cycle? Thanks for any insight you may have on this subject.