There are a few cases where heating the air actually helps after you compress it, such as a Brayton cycle with thermal regeneration .

A diesel is different however cycle however. Diesels get more efficient with the more excess air they have inducted into the cylinder as more energy goes into heating the working fluid instead of the cooling system. This results in a higher BMEP or in other words more power to the wheels.

For the same reason, a cooler intake charge of the same mass will also result less loss of energy to the cooling system and result in a higher power output.

__________________
green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png