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I'd be interested in reading this paper if you can find it.
On gasoline cars, warm air does a couple things, some are positive and others are negative. Mainly, it reduces the density of the air coming into the engine. This essentially doesn't allow the engine to make as much power. The lower output of the engine therefore requires increased engine load to do the same work. Increased engine load causes a reduction in pumping losses which increases efficiency. This is the same reason smaller engines get better fuel economy than larger ones (all else being equal). Warmer air also speeds up the combustion process. This is a good thing to a point, but if you get too much heat in there, you start having to alter ignition timing which can hurt efficiency.
Thats kind of the short answer. Others can fill in with what I haven't remembered off hand.
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