Originally Posted by Eggplant Jeff
Actually that's an exhaggeration, cyccommute. A LOT of waste heat is generated, true, but some of that energy (something like 30% I think) is converted into work (I.E. moving the piston in the engine, which is what actually gets you going on your trip to the local donut shop). And then some of that energy is reconverted to heat again when you brake.
You also have to consider that some fraction of that gallon isn't burned anyway, and is just wasted. That's the HC in your emissions, and why you have to change your oil so often.
Still a good point, but it isn't 100% waste heat.
I had this discussion with the motorhead engineers at the heavy duty vehicle test facility that I work at. They were making the same argument about the heat being converted to work but we all finally came to the conclusion that work ultimately still gets converted to heat anyway.
As for the unburned hydrocarbons, the amount coming out of the tailpipe of modern vehicles is so small as to be insignificant. The unburned HC gets converted to heat in the catalytic converter. That very small fraction that doesn't can be oxidized further in the atmosphere which is a heat process also.