One of the coolest motors I never heard of again was an engine built by Smokey Yanuck, the engine guy for Popular Mechanics way back when. Somewhere around 1960 or so he cut a four cylinder engine down to two...then had pure carbon cylinder sleeves and pistons made for it (Super $$$). Since the carbon could handle many times the heat and does not expand, he pumped it up to something like 250+ horses on regular gas and it got fantastic MPG. After all...an internal combustion motor is simply a heat engine, and the more BTU's...the more ponies. Until it all melts.
You'd think with today's technology, someone could be tapping into that heat.