Why are all APUs turbines?
As far as I know, efficient turbine engines have never been developed for road vehicles. Around 45 years ago there was an experimental Rover turbine-powered car, but it never got beyond being an experiment experiment.
If turbine engines are so inefficient, especially at ground level, why are piston engines, whether spark or compression ignition, not employed as aircraft auxiliary power units?
I would have thought that paraffin was near enough Diesel to power a compression ignition engine, so fuel is probably not much of a problem.
So is the problem weight, installation complexity, engineering inertia, or something else?