Fundamentals of Turbine Engine Operation

A gas turbine engine is a heat engine that uses air as a working fluid to produce thrust. It operates on the Brayton Cycle.

Operation Principle (Brayton Cycle)

Like a piston engine (Otto Cycle), a gas turbine has four phases: Induction, Compression, Combustion, and Exhaust.

• Continuous Process: Unlike the intermittent piston engine, the gas turbine process is continuous.
• Constant Pressure: Combustion occurs at a constant pressure (volume and temperature increase), whereas in a piston engine, it occurs at constant volume.

Main Components

Air flows axially from front to back:

1. Inlet: Designed to reduce air velocity (dynamic pressure) and increase static pressure and temperature (divergent duct) before the compressor.
2. Compressor: Increases the pressure of the air. Can be axial or centrifugal.
3. Combustion Chamber: Mixes fuel with compressed air and burns it at constant pressure.
4. Turbine: Extracts energy from the hot expanding gases to drive the compressor (and fan/propeller).
5. Exhaust/Nozzle: Accelerates exhaust gases to produce thrust (convergent duct increases velocity, decreases pressure).

Engine Types

Turbojet / Turbofan

• Single Spool: One shaft connecting compressor and turbine.
• Multi-Spool (e.g., Twin Spool):
  • N1 (Low Pressure): Fan + LP Compressor driven by LP Turbine.
  • N2 (High Pressure): HP Compressor driven by HP Turbine.
  • shafts rotate mechanically independently.
• Bypass Ratio: Ratio of air bypassing the core to air going through the core (HP compressor).
  • Formula: Bypass Ratio = (Total Inlet Flow - HP Core Flow) / HP Core Flow
  • High bypass engines are more efficient (lower SFC) and quieter.

Turboprop / Turboshaft (Free Turbine)

• Gas Generator: Core engine (Compressor + Combustion + Compressor-Turbine) creates high-energy gas.
• Free Turbine: A separate turbine (mechanically independent from the core) extracts energy to drive a propeller or rotor via a reduction gearbox.
• Control:
  • Power Lever: Controls fuel flow (Gas Generator/N1 speed).
  • Propeller Lever / CSU: Adjusts blade angle to maintain constant propeller/free-turbine (N2/Np) RPM.

Performance Concepts

Thrust

• Static Thrust: When the aircraft is stationary.
  • Produces Force, but Zero Mechanical Power (Power = Force x Velocity; if Velocity is 0, Power is 0).
  • Equal to the product of mass flow and exhaust velocity (plus pressure term).

Specific Fuel Consumption (SFC)

• Turbojet/Turbofan: Fuel flow per unit of Thrust. Unit: kg/hour/Newton (or lb/hr/lbf).
• Turboprop/Piston: Fuel flow per unit of Power. Unit: kg/hour/Watt (or lb/hr/shp).
• Lower SFC indicates better efficiency.

Start Sequence

1. Start Selected: Starter rotates HP compressor (N2).
2. Ignition ON: Igniters activated.
3. Fuel ON: Fuel sprayed into combustion chamber.
4. Light-up: Mixture ignites.
5. Self-sustaining: Engine accelerates to idle; starter disengages.