Also look at the Hydrid version of a hybrid vehicle. Consider especially the electric Hydrid version.
I have tried many times before to explain that a hydraulic hybrid power train can use any energy source.
HyDrid
Also consider the "Chiron' free piston engine. There is an article today in the Green Car Congress website that shows the Chinese are working on an almost identical design.
DCB, they will get to 60% energy conversion efficiency with this design, especially when they use super critical heated fuel. Contrary to your assumption about my "knowing it is not possible" I know it IS possible. The fuel utilized may be any combustible liquid or gas, but Hydrogen has the benefit of virtually 0 emissions without after treatment.
Fuels can be custom engineered for the specific requirements of the engine. Look at Transonic combustion. Their ultra high pressure preheated fuel delivery can even switch between several different fuels while the engine is running.
The potential also exists (if it is really necessary) to change from 2 to 4 cycle for power and cruising performance, however in my opinion it is not necessary if you can vary the compression ratios from 10-50 to one and run mixture ratios as high as 75 to 1 without any after treatment of the emissions and pass any emission regulations on the planet.
Also DCB, since you made the statement about inefficiency in hydraulic pumps and motors, maybe you missed this information.
regards
Mech