I may have an answer o your problem...ive been thinking of the same thing recently and perhaps try togenerate electric power with a magnet near the axle of one wheel. If you dont understand try removing the cover of a lawnmower and youll see what i mean.
Perhaps try an air pocket in the heal to jumpstart it just to keep the idea or ATs in ur design. Ive thought of making it out of an air-based engine however the onyl possible way to keep that running is to get to and maintaina speed of at least about 40mph.
