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The Stirling was much improved, but rendered more mysterious, in the free-piston type. Now, the latest development is the thermoacoustic Stirling, in which the only moving part is a magnet, and efficiencies are up to 30% - close to the Carnot limit using normal burners. It would not do that well on exhaust, already cooled by expansion, but it looks better than many other techniques. The motors consist of heavy tubing with heat exchangers at each end, with careful shaping and a magnet and pickup coil in the middle.
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