Many questions and observations here have highlighted the idea of using some kind of electric towing system for towing airplane to and from the runway, instead of using fuel to taxi. One of the impediments to this has been the need to preheat the engines during taxi, prior to take-off at full thrust.
Why use the inner engines of a four-engine airliner for taxi instead of the outer engines? Are the outer engines designed to not require preheating prior to full thrust?
If the outer engines do require preheating and are no different from the inner engines, how much fuel do they use to preheat the engines and would it not be cheaper to use electric taxiing then to preheat all engines?