What I picture in my head is a generator with a small engine built into the trunk that could put out just enough to run the electric motors and propel the car by it'self at say 55 mph on it's own like a "limp in mode" but could charge the batteries at anything less sort of like how a diesel electric train works but with batteries. It would probably work with about a 15hp honda generator in a tesla with it's superior aromatics. It could run on regular gasoline in a pinch but could also have the batteries charged via plug in when you are sleeping, eating or at work.