Engine Operation

Air comes into the engine via an inlet under the propeller hub where it is regulated into the compressor section via variable inlet guide vanes/IGV. The IGV also operate in conjunction with the engine anti-ice/start bleed valve to restrict or dump engine bleed air as necessary. The compressor section has 5 axial stages and one centrifugal stage driven by a two-stage gas generator turbine. Compressed air enters the combustion chamber where fuel is added via the HMU (fuel pump or "brawn") . Of the total air that enters the engine, about 30% is used for the combustion process. The remainder is used for cooling and other purposes.

A hydromechanical unit/HMU functions in conjunction with the Digital Electronic Control Unit/DECU (computer control or "brain") to regulate the fuel flow to the engines by power lever/PL and condition lever/CL positions. Once the mixture is ignited the process is self sustaining and exhaust gases flowing out then drive the gas generator and power turbines. A 2 stage power turbine drives the prop through the propeller gear box/PGB via a concentric shaft within the gas generator turbine shaft. The gas generator and power turbines are not mechanically connected. Consequently, power turbine rpm (Np) is independent of gas generator rpm (Ng). This is commonly called a "free turbine". 

Power Levers 

Each power lever/PL is connected to its related Hyrdo Mechanical Unit and Prop Control Unit (governor and HP oil pump). The PLs set Ng by controlling fuel flow via the HMU and also controls prop blade angle via the prop control unit/PCU for taxi and reverse thrust thrust. The PLs also indirectly control the torque motor via the DECU. 

The Condition Levers/CLs are connected to their associated HMU shutoff valve for engine shutdown. Each CL is also connected to its related PCU for prop feathering and RPM control. The CL range is from FUEL OFF, to FEATHER (with a start position), UNF (unfeather), MIN-MAX range, to T/M (torque motor lockout).

Automatic Flight Idle Stop 

Movement of the PLs below flight idle into the beta range is prohibited by the AFIS. A mechanical stop in the control quadrant aft of FLT IDLE (ground beta range) prevents inadvertent movement of the PLs into this range while airborne. The AFIS is electrically controlled and will engage once airborne and disengages upon touchdown. Once on the ground, the AFIS opens to allow PL movement into the GND IDLE (min thrust) and REV or beta range. In case the AFIS fails to open on touchdown (indicated by the absence of the blue FI STOP light on the flight status panel and the inability to move PLs aft of FLT IDLE), there is a red manual override knob that will release the stop allowing PL movement. If the override was inadvertently pulled, when an attempt to start the left engine is made, a CONFIG warning alert will sound and can't be canceled until the left CL is returned to the FUEL OFF position.