A brief intro to how modern jet engine works.

1. How does a jet engine
work?
By Kelvin Lam

2. Engine
• “A machine that converts energy into
motion, either linear or circular.”

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. Different types of jet engine
• Turbofan • Turbojet
• Ramjet • Pulsejet

5. Compressible fluid
• Fluids are compressible substance.
• Examples includes:
• Liquid
• Air
• The jet engine make use of this property to
generate thrust.

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. 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. 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 Boeing
767

9. Structure of a turbofan engine

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. 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. Engine state
• Steady state: Wheras the
variable/conditions are maintained
throughout in the system.
• Transcient state: Wheras the variable are
changed and adjusted.

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. 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. 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. 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. “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. 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. Wet side
• Components
include
• Hydraulics pump
• Fuel supply and
valves
• Mechanical (for
cooling and
lubrication) oil
circulation

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. 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. How pilot starts an engine
• Fuel pump
As air is drawn into
the engine, fuel is
pumped and
injected into the
chamber.

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. • In cranking mode, air is bled into the
engine bleed valve.
• FADEC is isolated.
• The bleed air will spin the HP turbine.

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. Difference
• Bypass-ratio:
• At low-bypass
engines, most of
the air comes from
the core part.
Thus, air contains
more thrust.

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. Interesting applications
• Road deicing
• Somewhere in
Russia,
governments make
use of remaining
MiG-15 fighter
engines to de-ice
Siberian roads

29. Interesting applications

30. Questions?