The Eye of Harmony is serviced on two sides by what looks like energy converters, and temporal field generators which probably feedback the energy to the eye of harmony's statis halo, which holds it outside of time, keeping it in a constant state. In my Compendium, and my thinking in regards to the TARDIS is that the energy used for the stability of the bubble universe comes directly from the eye of harmony, and is used for that, and the architectural reconfiguration systems exclusively. As for the movement of the ship, and secondary power source is needed, because stability of the bubble universe takes most of the infinite source of power. The Eye of Harmony is an infinite source of power.. NOT an infinite distributor of power. That is to say the energy needed to provide the artificial universe the Ship exists in is there, but the output is not necessarily high enough to spare to all of the ship's functions, which would make sense, which is why the doctor would need the dynamorphic generators and a 2nd power source to charge the engines up, like rift energy, once the direct link and power line from the original Eye of Harmony was lost after the Time War. that extra power was probably fed directly to the copy eye, and syphoned off to power the engines in addition to the additional power generators. Once that additional power source was lost, an alternative had to be found to open the engines to and soak up to be used to travel.