[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 7-20, 7-21, 7-22]
(Last Update 2020 Apr 24 1400Z)
What’s Turbine Engine?
The turbine engine is a kind of powerplant pulls air into the engine. The air is compressed before mixing with fuel and burnt to produce thrust by increasing the velocity of the air flowing through the engine.
Indeed, the modern Turbine engine is also known as “high bypass ratio turbofan engine”.
HOW does a Turbofan engine work?
In general, a big fan at the front of the engine pulls air around the engine and sucks air into the core.
The air enters the core is the primary airflow and the air bypass the core is the secondary flow.
In the core, the air will be compressed (by compressors) before mixing with jet-fuel (in combustion chambers). The fuel-air mixture will be auto-ignited and the expanded gas has great energy to turn the turbines which connected to the relative compressors at the front via a concentric shaft. The hot exhaust is partly used for propulsion;
However, majority of thrust is produced because of the bypass air outside the core.
The whole self-sustain process is the basic working principle of Turbine engine.
General component of a Turbine engine:
- air inlet
- combustion chambers
- turbine section (connecting to the compressor via “shaft”)
- Compress have stages which consists sets of rotor blade and stator blade
- combustion chamber is used to provide morderate high temperature and pressure environment for fuel to be burnt
- Exhaust outlet narrower (converging) than the Inlet (divergent) may help to create grater jet velocity
- 2 spool = 2 sets of turbine and compressor connected via 2 shaft – forming the low- and high- pressure spool
- and the inner spool (the closer to the combustion chamber) turn faster than the outer spool – N2 > N1 – this design helps to reduce the centrifugal stress, which improve the blades’life
(3) Three types of Turbine Engines – based on compressor
- centrifugal flow
- axial flow
- centrifugal-axial flow
(4) four types of aircraft turbine engines – based on design
Compressor compresses air by a series of rotating and stationary airfoils that moving the air parallel to the longitudinal axis. Then the inlet air at a high rate of speed is passed to the combustion chamber. The combustion chamber contains the fuel inlet and igniter for combustion. The expanding air drives a turbine, which is connected by a shaft to the compressor, sustaining engine operation. The accelerated exhaust gases from the engine provide thrust.
Disadvantage of turbojet
- limited in range and endurance
- slow to respond to throttle applications at slow compressor speeds
exhaust gases drive a power turbine connected by a shaft that drives the reduction gear assembly. Reduction gearing is necessary in turboprop engines because optimum propeller performance is achieved at much slower speeds than the engine’s operating rpm.
Inlet air that passes through a turbofan engine is usually divided into two separate streams of air – One stream passes through the engine core, while a second stream bypasses the engine core.turbofan’s bypass ratio refers to the ratio of the mass airflow that passes through the fan divided by the mass airflow that passes through the engine core.the secondary airflow around the combustion chamber – bypasses the engine core:
- create additional thrust
- cools the engine
- aids in exhaust noise suppression
Most of the energy produced by the expanding gases is used to drive a turbine rather than produce
thrust – example: Helicopters / Auxiliary Power Units (APU) on large aircraft
Turbine Engine Instruments (#Read More)
some (3) instruments that are unique to turbine engines :