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quote:This requires electricity or fuel cells.
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A heat differential engine does not require a fuel as you describe. Any method of introducing a large enough temperature difference to the engine will act as "fuel", including concentrated sunlight, geothermal effects and excess heat in such things as the smokestack of a factory.
A Stirling only requires a "heat differential" not a physical fuel.
With the help of catalysts automobile engines do indeed emit cleaner air under some circumstances than they take in.
I am not debating fuel efficiency. The subject is cleanliness, ie pollution.