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Old 02-28-2009, 11:16 AM   #4 (permalink)
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Join Date: Aug 2008
Location: Buffalo, NY, USA
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Prius C - '12 Toyota Prius C
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The most efficient ICEs today are very, very large engines, such as this 25000 liter two-stroke turbo diesel ship engine with ~50% thermal efficiency. I would guess that large, stationary diesel generators are also efficient, since weight isn't much of a concern and they can add waste heat recovery systems, etc.

Or if by "cleanest", you mean you'd survive the longest while locked in a garage with one running, I'd go for the Honda 1.0L 3cyl that came in the Insight with CVT. I don't know of a car with lower NOx, CO, and HC emissions.
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