Usually the working gas is helium or hydrogen because the lightest molecules need the least heat energy for a given change in temperature. Pressurization is used to increase the specific power of the engine. But if the engine is pressurized, say, to 300 psi, then the same temperature difference raises the pressure to 600 psi, for a delta of 300 psi on the piston. For the same displacement, this amount of pressurization nets 20 times as much power. So it's important to operate the burner at as high a temperature as possible, and keep the cool end as cold as possible.
More temperature spread, more output.
Striling Engine is very clean and economical engine. This engine is very curious mecanism, because this engine needs only the temperature variation. When the air temperatue is forced to raise, the air pressure shall be raised. While the air is cooled, the air pressure shall be decompressed. In order to separate these high temperature portion and low temperature portion, this engine has the displacer.