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i can't speak for diesel in particular, but with a gasoline enigne, a hotter intake charge is the way to go for fuel economy.
hotter air is less dense, which provides a couple of benefits, mostly being the reduced pumping losses due to having to open the throttle more to achieve the same cylinder airmass and a less dense air charge providing less resistance to movement. hotter air, i would assume also helps gasoline vaporization.
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