Editing Stirling engine (section)

=== Gas choice ===
[[File:Stirling Engine 1min NCTU.webm|thumb|Video of a bench top stirling engine demonstrating the speed and power.]]

The gas used should have a low [[heat capacity]], so that a given amount of transferred heat leads to a large increase in pressure. Considering this issue, helium would be the best gas because of its very low heat capacity. Air is a viable working fluid,<ref name="Organ-2008b" /> but the oxygen in a highly pressurized air engine can cause fatal accidents caused by lubricating oil explosions.<ref name=Hargreaves /> Following one such accident Philips pioneered the use of other gases to avoid such risk of explosions.
* [[Hydrogen]]'s low [[viscosity]] and high [[thermal conductivity]] make it the most powerful working gas, primarily because the engine can run faster than with other gases. However, because of hydrogen absorption, and given the high diffusion rate associated with this low [[molecular weight]] gas, particularly at high temperatures, H<sub>2</sub> leaks through the solid metal of the heater. Diffusion through [[carbon steel]] is too high to be practical, but may be acceptably low for metals such as [[aluminum]], or even [[stainless steel]]. Certain ceramics also greatly reduce diffusion. [[Hermetic seal|Hermetic]] pressure vessel seals are necessary to maintain pressure inside the engine without replacement of lost gas. For high-temperature-differential (HTD) engines, auxiliary systems may be required to maintain high-pressure working fluid. These systems can be a gas storage bottle or a gas generator. Hydrogen can be generated by [[electrolysis]] of water, the action of steam on red hot carbon-based fuel, by gasification of hydrocarbon fuel, or by the reaction of [[acid]] on metal. Hydrogen can also cause the [[hydrogen embrittlement|embrittlement]] of metals. Hydrogen is a flammable gas, which is a safety concern if released from the engine.
* Most technically advanced Stirling engines, like those developed for United States government labs, use [[helium]] as the working gas, because it functions close to the efficiency and power density of hydrogen with fewer of the material containment issues. Helium is [[inert gas|inert]], and hence not flammable. Helium is relatively expensive, and must be supplied as bottled gas. One test showed hydrogen to be 5% (absolute) more efficient than helium (24% relatively) in the GPU-3 Stirling engine.<ref name="Thieme-1981" /> The researcher Allan Organ demonstrated that a well-designed air engine is theoretically just as ''efficient'' as a helium or hydrogen engine, but helium and hydrogen engines are several times more ''powerful per unit volume''.
* Some engines use [[air]] or [[nitrogen]] as the working fluid. These gases have much lower power density (which increases engine costs), but they are more convenient to use and they minimize the problems of gas containment and supply (which decreases costs). The use of [[compressed air]] in contact with flammable materials or substances such as lubricating oil introduces an explosion hazard, because compressed air contains a high [[partial pressure]] of [[oxygen]]. However, oxygen can be removed from air through an oxidation reaction or bottled nitrogen can be used, which is nearly inert and very safe.
* Other possible lighter-than-air gases include [[methane]] and [[ammonia]].