Hi,
Sure, heat can be used to produce electricity, so you could replace the alternator.
But it would be much better to convert more of the fuel's energy to mechanical motion in the first place -- and produce less waste heat. The cam driven Revetec engine is ~2X as efficient (39.5%) as the typical piston engine, since the mechanical leverage of the cams can be designed -- crankshaft and connecting rod engines are locked into sinusoidal motion, which wastes a lot of energy.
Also, the weight of the vehicle is a huge drain -- check out Amory Lovins on the specifics:
Amory Lovins on winning the oil endgame | Video on TED.com