clipped from nextbigfuture.com

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 gas 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.

Piston power: In an unconventional engine design, a rod with a piston at either end shuttles between two combustion chambers. Magnets at the center of the rod move past metal coils (orange) to create an electrical current. Credit: Peter Van Blarigan, Sandia National Laboratory