Many years ago (1976) Ford had a 'Philips-Stirling' engine fitted to one of their standard Torino chassis models (see GM link below re engine), was 20% more fuel efficient than a standard engine of similar weight. Had fewer parts etc:. Started almost straight away (20 secs), but there was not enough advantage to change cars also they still require fuel although the range and types of fuel can vary significantly.
[link|http://abcnews.go.com/sections/tech/Geek/geek000320.html|Stirling & Miller Engines]
This link below highlights the potential of HYBRID DRIVE vehicles where there is more than one type of engine. This may prove the direction the industry goes in. Here is a link based on research done by the big US vehicle mfgs. The article goes back to 1997 but shows how long the industry has been experimenting behind the scenes.
[link|http://www.uscar.org/techno/ng_power.htm|USCAR ORG - HYBRID CARS]
[link|http://www.uscar.org/techno/hyb_gm.htm|GM's Stirling Engine]
Cheers Doug