How does a jet engine work


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A brief intro to how modern jet engine works.

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How does a jet engine work

  1. 1. How does a jet engine work? By Kelvin Lam
  2. 2. Engine• “A machine that converts energy into motion, either linear or circular.”
  3. 3. Different types of engine:• Internal combustion engine• Piston Engine• Steam engine• Pulsejet/Ramjet• Jet Engine (Generic)• Pneumatic/Hydraulics• Ion thruster (future generation for spacecraft)
  4. 4. Different types of jet engine• Turbofan • Turbojet• Ramjet • Pulsejet
  5. 5. Compressible fluid• Fluids are compressible substance.• Examples includes:• Liquid• Air• The jet engine make use of this property to generate thrust.
  6. 6. Revision of forces• A resultant force is a sum of all forces acting on an object.• Force comes with action and reaction pair. This is Newton’s Third Law of Motion• There are mainly four basic force acting on an aircraft while it is cruising.
  7. 7. Revision of forces• According to Newton’s Second Law, for acceleration to take place a force has to be applied to overcome the friction (reacted force).• Thus, for increasing speed (relative to still air): Thrust > Drag• For increasing altitude/take-off: Lift > Weight
  8. 8. Revision of forces • The thrust lever allows the pilot to regulate the amount of thrust produced by the engine. • Thus, controls the thrust force of an aircraft. • Normally, we can classify thrust to three “modes”: TOGA, reverse thrust, normal thrust (manually or autopilot controlled, known as autothrottle) • Higher the TLA (Thrust Lever Angle), higher the thrust that an enginer is producing. • One lever represents one engine.Thrust lever of a Boeing767
  9. 9. Structure of a turbofan engine
  10. 10. Stages of propulsion• SUCKThe air is dragged into the central part of the engine by big fan blades.• SQEEZEAir is compressed in the compressor using specially aligned “mini-blades”.
  11. 11. Stages of propulsion• BANGFuel is injected into the combustion chamber. The air ignites it, causing an explosion in the chamber as it rapidly expands.• BLOWThe expansion of gas causes the air flow quickly from high pressure to low pressure.
  12. 12. Engine state• Steady state: Wheras the variable/conditions are maintained throughout in the system.• Transcient state: Wheras the variable are changed and adjusted.
  13. 13. Temperature of airflow• Low-pressure stage:Using Boyle’s Law, as air is compressed, temperature rises. The temperature reaches around 500K.• High-pressure stage:Again with the same principal, air is heated to around 1300-1500K
  14. 14. Temperature of airflow• Combustion stageFuel is mixed with oxygen contained in air. The exothermic reaction heats it to around 3200K.• ExhaustAir propelled from the engine can reach at least 3000K.
  15. 15. Components• Fan casing Rather than aesthetical purpose, it holds the core engine parts together and attach it to the pylon. It also gives the engine its aerodynamic properties.
  16. 16. Components• Gearbox and shaftsThe purpose for the shafts are to drive the fan blade at the intake using the propelled force in the exhaust.
  17. 17. “The two sides”• Dry-side • Wet-side• “Water and electricity • It holds the hydraulics don’t mix!” and the fuel supply of• It holds the electrical the engine. systems of the engine.
  18. 18. Dry side• Components include• FADEC (or EEC): Full authority Digital Engine Control• Electrical generator• Ignition (Motor that generates bleed air)• Wires carrying information and signals The “dry-side” of the Trent 900 fitted on Airbus 380. back to the flight deck
  19. 19. Wet side• Components include• Hydraulics pump• Fuel supply and valves• Mechanical (for cooling and lubrication) oil circulation
  20. 20. How pilot starts an engine• In smaller aircrafts, starter motor is used to force air into the combustion chamber.• In larger airliners, starter motor cannot generate enough air flow to move a big fan blade.
  21. 21. How pilot starts an engine• APU (Auxiliary Power Unit) • It is like a smaller jet engine. • It generates electricity when the aircraft is on the ground (certainly, main engines are not running!) • It also generates pressurized air which forces the blade to start spinning. • As blade spins, air is drawn into the chamber.
  22. 22. How pilot starts an engine• Fuel pumpAs air is drawn into the engine, fuel is pumped and injected into the chamber.
  23. 23. How pilot starts an engine• Cranking Alternatively, if autostarter fails, some aircrafts (Airbus, ATR) offers cranking. The ground crew manually feed air and fuel mix into the engine. It is hazardous.
  24. 24. • In cranking mode, air is bled into the engine bleed valve.• FADEC is isolated.• The bleed air will spin the HP turbine.
  25. 25. Difference• Turbofan • Turbojet High-bypass ratio  Low-bypass ratio Usually huge  Aero-dynamically Used for subsonic designed so it is small flights  Used for supersonic Very fuel efficient at flights high altitudes  Fuel-hungry
  26. 26. Difference • Bypass-ratio: • At low-bypass engines, most of the air comes from the core part. Thus, air contains more thrust.
  27. 27. Interesting applications• ThrustSSC• The world’s fastest land vehicle.• Achieved a speed of 1228km/h (Mach 1.02)• Powered by Rolls- Royce Spey engines
  28. 28. Interesting applications• Road deicing• Somewhere in Russia, governments make use of remaining MiG-15 fighter engines to de-ice Siberian roads
  29. 29. Interesting applications
  30. 30. Questions?