So I read the thread, and it got me thinking. I did some research into the sterling engine, and an article by how stuff works (
http://auto.howstuffworks.com/stirling-engine.htm) said that they are primarily used by submarines and yachts as backup power systems. this means they are mostly used for electrical power production. The main issue with using them in the place of an internal combustion engine (ICE) is that sterlings are low speed, and react slowly to changes in the throttle.
However, and this is where my own thinking starts, there is a work around. With hybrid cars so common, the technology is here. I propose the use of a sterling as an electric generator, spinning an over-sized alternator at a near-constant rate. The alternator would charge a battery bank, which in turn could be used to power electric motors in the drive train. The system I have just described is largely similar to that used in the Nissan Leaf, the primary difference being that in the place of the ICE, we have a sterling.
This is more efficient than a standard motor for a lot of reasons, chiefly because the Stirling would not require gear shifting and large gearing changes, which are one of the least efficient parts in a modern car's drive train. Sterling engines are also innately more efficient than ICEs.
If anyone knows if a car has already been powered in this fashion could they please let me know, as I am very much curious about the implications of this idea.