A jet engine works by compressing air from its intake with a fan, mixing this compressed air with fuel, combusting the air-fuel mixture, and expelling the hot combustion gases through a nozzle to generate thrust. The main stages of operation are intake, compression, combustion, and exhaust. Different types of jet engines include turbofans, turbojets, ramjets, and pulsejets. Jet engines make use of the compressibility of air to generate thrust by accelerating air and expelling it at high speeds from the rear of the engine.

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?