Originally Posted by PratoN
Cold air is denser (as is air under higher pressure, ergo Ram-air) which means more molecules of air are in the same size volume (of the combustion chamber) - the mass air flow sensor (in conjunction with the oxygen sensors) are very accurate at remaining at (or near) stoichiometric fuel-to-air ratios. More oxygen, more fuel = more power for the same size engine.
Anyway, a heat engine is most efficient thermally when the difference in temperature is greatest, that is, cold air (100% oxygen?) and a super hot engine/flame temperature. But there are thermal/mechanical limits for high-temperature applications... Best case scenario for your engine is really cold / high pressure air and a warmed up engine.
People start seeing MPG go down in the winter due to the dense air raising the drag of the car itself (good for the engine, bad for moving things...), idling to warm-up, increased rolling resistance of tires, and a bunch of other things that I don't feel like getting into right now.
Thanks for the note. Interesting reading that as it just so happened that I need to brush up my thermodynamics (I took a general physics class back in my freshman year more than 40 years ago, which I rather enjoyed) for a project I am considering taking on.
I was just reading about Carnot heat engine and its (theoretical) efficiency and realized that I could not count on Wikipedia to teach me everything...