So basically it transmits mechanical power in form of angular motion from aircraft gearbox which is driven by a electric motor , and transfer it to engine gearbox where the power is further transmitted to Central axle to provide torque .
View attachment 133128
See the above image , the accessories gearbox visible gets the initial torque from this shaft .
This system can be eliminated if u introduce it axially inside the engine as electric motor are smaller nowadays but for that engine should be a clean sheet design .
Similar for electricity generation , in air these axles alongwith constant speed drive unit can help in electricity generation , can be removed and inserted in the axis of engine ,like in the RR next gen turbofan design
The PTO shaft is not present in the diagram u shared. The 2 vertical shafts are radial shafts.
That electric motor called starter in current engines need APU or GPU for power.
Why i was confused was bcoz in civil engines there is no PTO shaft. There are radial & longitudinal shafts connected to engine gearbox, part of engine set. So why would DRDO highlight it if part of GE-414 engine.
So civil engine start is by APU bleed air usually.
- APU bleed air turns air-starter which is like a small windmill
- air-starter through gearbox turns longitudinal shaft (not PTO shaft)
- longitudinal haft through bevel gears turn radial shaft
- radial shaft through bevel gear turn high pressure compressor & shaft N2
- air flow created by N2 rotates low pressure compressor & shaft N1
Beyond 20% rpm the fuel is injected & sprk plugs ignite it.
After searching before my 1st reply itself i found that the PTO shaft is in military jets & actually b/w AMAD (Airframe Mounted Accesory Drive) & EMAD (Engine Mounted Accesory Drive), or simply airframe gearbox & engine gearbox.
And the APU/JFS turns the shafts (PTO then radial) through the gearbox rather than by its bleed air.
But from above 2-D diagram it appears that the PTO shaft is directly connected to fan shaft which would obstruct air duct which is not true.
The PTO shaft is actually below the engine in most cases or in western engines.
GE-414 also follows the same arrangement as above
The following is sketch of F-15 engine & gearbox where P-12 below engine are PTO shafts.
The following is example from MiG-29 where the PTO shaft is on top of engine & towards spine rather than forward:
So ultimately the PTO shaft is not part of engine set from manufacturer but just another part in the airframe.
Now coming to the Rolls Royce plans for Tempest 6th gen jet to install electromagnetic motor + generator directly with engine shaft -
- the motor/gen needs to be shielded well from intense heat.
- Because the engine could be throttled up/down randomly & rapidly, now the generator EM field will also rapidly & randomly change, means for a uniform current/voltage control is required, the stator-rotor magnetic field control needs very fine electronic calibration. It is not simple like mixer/juicer motor.
- the motor needs initial electricity from GPU or APU.
- the gearbox operates other rotating things also like pumps for fuel, engine-oil, hydraulic fluid, etc. All these have to be made electronic.
Let's see how US engine makers will respond.