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Stirling engines have been making some headway lately, mostly being used in solar energy and for marine propulsion/secondary electricty. The main problems are the size to power ratio, potential temperature difference, pistion seals, and engine controls, specifically engine pressure control. The working solar commercial models that I have seen have used molten sodium-- but more practically for automotive use the working fluid would be helium. Plain air could also be used, like the original stirling, but there have been problems with the lubricants oxygenizing and causing explosions. Nasa acutally did some automotive testing with stirling engine for main propulsion in the late 70's and early 80's in a chevette.
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