The Parting of the Ways
In the episodes "Boom Town" and "Utopia", the TARDIS needed to 'refuel' by absorbing energy from a 'scar' left by a closed space/time rift in Cardiff, implying that it is not powered entirely by the Eye at this time. The former episode also revealed that the TARDIS console concealed the 'Heart of the TARDIS', a mysterious glowing energy source that the Doctor associated (in "The Parting of the Ways") with the energies of the time vortex.