Free Piston Engine

http://www.technologyreview.com/Energy/21442/page2/

http://nextbigfuture.com/2008/09/free-piston-engine-could-be-twice-as.html

http://nextbigfuture.com/2008/09/free-piston-engine-could-be-twice-as.html

Sandia National Laboratory led by Sandia researcher Peter Van Blarigan that has been testing physical components of free-piston engines which could be 50% efficient. This is about twice as good as current combustion engines and about as efficient as fuel cells. Fuel cells are too expensive. He is assembling a complete free-piston engine prototype, a project that he expects to complete within a year.