The Cyclone external combustion engine looks pretty interesting; and it would seem to have a high efficiency, given the relatively cool exhaust and very low emissions exhaust and continuous burning.
(click on image for link)
Video: A Steam Engine for Your Car
Could Waste Oil Power Your Range-Extended Electric Car One Day? - Green Car Reports
They are claiming over 30% efficiency -- not bad, but not as good as I hoped. This seems to solve many of the issues we've discussed in the internal combustion engines threads; like burn time, and mechanical torque, while it has simple mechanicals and no transmission required. (Sounds a lot like EV's?)
What do you think?