The Stirling engine is, surprisingly, an old invention -- 1816. I read somewhere that a Swedish submarine manufacturer uses a Stirling engine for its main power largely because it's so quiet. Not sure how they get their heat while under water.

One wonders why this hasn't found wider application. Seems like one could use waste auto heat to power the alternator and other accessories. Would have to increase mpg a bit.

I've seen the tiny toy like versions but never anything even as big as 5 hp.

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1986 300SDL, 362K
1984 300D, 138K