@tk Any purely electric vehicle will have a very limited range. The use of highly efficient internal combustion engines as generators makes it feasible. The efficiency would be high because it would be optimized to only run at the optimal output.
@tk A rotary engine can be used to save space and weight. They can be pretty efficient as long as the seals are in good condition.
@tk @AmpBenzScientist fuel cell?
@a_breakin_glass @tk Fuel cells are still not feasible for nearly any application on earth. The only fuel cell that I know of that worked well was a hydrogen fuel cell that NASA used in space.