http://www.innas.com/CFPE.html
DOE papers 10 years ago stated 58%.
Add a transonic injection system.
Transonic Combustion | Our Technology | TSCi Technology - Fuel Injection Systems
Now an accumulator at 98%
And one of these in each wheel, with infinitely variable displacement
http://www.youtube.com/user/Ride122609
There is your complete power train, with a huge reduction in power train parts count.
That's technology available right now, not some predicted development in the next decade.
Do a parts count on the moving parts in that free piston engine.
Of course I like my rotary flywheel engine better, with variable displacement and compression and no parts moving rapidly in opposite directions.
regards
Mech