Turbo Props... Wish to teach me the basics???
Hi guys,
I have just started a university mechanical engineering module on turbo prop engines and have been researching the basics. I've tried to summarise in simple terms for my own benefit and reference going forward during the module.
Can you please take a look at below and tell me if I am on the right tracks?
Starting, Ground Idle & Taxi
Boost pump runs to circulate motive flow from aircraft ejectors to engine. Front stages of compressor have tendency to stall at starting conditions. Fuel flow rate circa 150pph.
Take Off
Pilot moves PLA to full throttle, increasing fuel supply to LP/HP turbine which now spins @100% and also spins (matched) LP/HP compressor @100% via concentric twin shaft. Boost pump turns off and geared positive displacement main fuel pump (MFP) takes over, driven by the accessory gear box (AGB) which is connected via a shaft to engine.
The MFP is sized in accordance with max fuel flow (take off) conditions and is located in the FMU. The MFP circulates high pressure fuel, at circa 550pph, to fuel nozzles in combustion chamber. Note: Air density and pressure are high at sea level (1000hPa) and decrease with altitude.
Cruise
Air density and pressure have decreased (200hPa), however the compressor still needs to spin at 100% to get a suitable amount of conditioned air into engine for combustion. Due to the less dense and lower pressure air, the compressor still needs to spin at 100% however the compressor outlet pressure is lower than the equivalent value at sea level.
To maintain the correct air/fuel ratio for combustion, a lower fuel flow rate is therefore required (350pph) but it should be noted the MFP continues to operate at 100% duty as its rotational speed is directly proportional to the shaft speed of the engine. A pressure drop regulator valve returns the excess pressurised fuel to before the MFP and ensures there is a fixed pressure drop across the Fuel Metering Valve (FMV).
The FMV measures fuel by mass flow (i.e. pph), as opposed to volume (i.e. m3/sec), as the density changes at altitude however weight does not. Note: For FADEC control, fuel flow (wf) must be linked to compressor outlet pressure (p3).