https://www.nextbigfuture.com/2008/09/free-piston-engine-could-be-twice-as.html
Free-piston engine could be twice as fuel efficient as combustion engines | NextBigFuture.com
Apr 7, 2017 - 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 combustion enginesas fuelfreepistoncould