TARDIS engine
TARDISes were equipped with engines which were the source of the distinctive sound heard upon materialisation. By turning the engines "on silent", TARDIS pilots had the option of forgoing the sound, usually as a means of being discreet. The Doctor employed this function on rare occasions. (TV: The Impossible Astronaut, The Time of the Doctor) According to the Eleventh Doctor, the engine would shut down if no one was in a TARDIS. (TV: The Pandorica Opens)
On Gallifrey, the Quadriggers were assigned the task of dismantling the engines of old TARDISes before they would be vaporised. Quadrigger Stoyn was assigned the task of dismantling the engine of the ship that would become the Doctor's TARDIS. As he worked on it, the First Doctor and Susan Foreman's theft of the craft caused the engine to burn Stoyn's face. (AUDIO: The Beginning)
When attempting to diagnose the problem with the TARDIS following its dematerialisation from Skaro, the First Doctor and Ian Chesterton visited the TARDIS engine and power rooms. (PROSE: The Edge of Destruction) When the Doctor later attempted a temporal slingshot to return Ian and Barbara to Earth, the attempt blew the engine, the TARDIS needing days to repair it. (AUDIO: The Invention of Death) As the TARDIS was a space-time vessel, its engine was vulnerable to the energies of the Tick-Tock World, only managing to break free of the planet when the engine was augmented by the power of the TARDIS force field. (AUDIO: Tick-Tock World)
During the Last Great Time War, the War Doctor, after having it done to him many times, came to consider that it was quite easy to interfere with the engines of a TARDIS. (PROSE: The Day of the Doctor)
Shortly following the Ninth Doctor's regeneration, the Tenth Doctor "open[ed] up" the engines as he took the TARDIS through the Time Vortex on a course to the Powell Estate on 24 December, 2006, resulting in a crash landing. (TV: Born Again)
Materialising the TARDIS in the Oval Office, the Doctor turned the engines on silent to be discreet. (TV: The Impossible Astronaut)
Taking the TARDIS to the source of the mysterious message beamed from Trenzalore, the Doctor turned the engines on silent in order to avoid "mak[ing] a fuss." (TV: The Time of the Doctor)
In 5343, River Song tried to use the TARDIS to escape Mendorax Dellora with the detached head of King Hydroflax, except her initial attempts were stalled due to interference with the engines. Despite River claiming that interfering with the TARDIS engines was "technically speaking, not possible", the Doctor, under the guise of theorising, explained that the engines were interfering with themselves due to the ship's failsafes to prevent it from taking off when a lifeform registers simultaneously as inside and outside, as Hydroflax's body was still connected to the head. (TV: The Husbands of River Song)
When the First and Twelfth Doctors were abducted by the Testimony spaceship, the latter tried to take off but discovered that the spacecraft was emitting a signal that blocked the command pathways to the engines. (TV: Twice Upon a Time)
In an alternate timeline, the Eleventh Doctor had turned off his TARDIS's shield oscillators, leaving it vulnerable to the magno-grab of the Van Baalen Bros. salvage ship. The impact caused the TARDIS's engine to explode, only for the ship's safety protocols to freeze the explosion in a time stasis field. This timeline was later undone when the Doctor gave his past self the remote to shut down the magno-grab. (TV: Journey to the Centre of the TARDIS)
When the TARDIS crashed on Desolation, the Thirteenth Doctor suspected that the engines were phasing in and out of time and space, leading to its reputation as "the Ghost Monument". (TV: The Ghost Monument)
