Bill,
the most efficient diesel engines (large, like container ship large) are only thirty-something % effecient. Noise, jacket and exhaust temperature increase make up the rest of the power otherwise put in. Small engines, like those even in your american supersize cars, are pitifully inefficient in comparison to a ship size diesel.
Low 20s % is pretty good for new technology.
But hey! Someone wants to re-power a self-launcher with hydrogen, LI batteries (more than 2 birds needed!) nuke - I dont care, bring it on!
The best place for a rotax is on a lawnmower


Bagger (diving, rolling for cover)
:-)