An external combustion
An external combustion Stirling engine is ideal for this charging only application where instant HP and torgue is not essential form the charging engine. Stirlings are more efficient than IC engines but physically larger for the same HP, and slow to start up (a plus is that no starter motor is needed) but this is not a drawback in this application. Stirlings are silent, much cleaner being just an efficient burner and much simpler. As only a small HP engine is needed only to charge the accumulator, the Stirling is the unit to use. The Segwy (sp?) man has spent millions on Stirling development and isusing one in small petro-electric hybrid under test. He proposes a small scooter using a petro-electric hybrid setup.
Stirlings are used in Swedish and Japanese submarines. One small Stirling Swedish sub "sunk" the USS Ronald Reagan in war games. The USN could not detect the quiet sub. The crew love the silence of the Stirling operation reducing stress on-board. It can stay submerged for weeks using deisel and liquid oxygen. Stirlings are not things of yesterday, despite predating the IC enhine by 100 years. They are used in Compined Heat & Power applications in domestic homes.
The one drawback with petro-hydraulic setups is public acceptance of all that fluid under high pressure. They will think of what happens when there is an impact. Another is when a flexible hose pipe bursts, and fluid is lost and over the road or driveway. The cost of the replacement fluid is not exactly cheap. Which make me think the petro-electric hybrid will win as battery and supercapacitor technology is improving at a decent rate. But right now the simple, efficient, petro-hydraulic setup using a Stirling engine is by far the best.