A basic introduction to aircraft fossil fuel driven propulsion systems

1. Introduction to Aircraft
Propulsion

2. Aircraft Propulsion : Objectives
• Identify the two types of aircraft engines
• Identify the two types of thrust producers
• Understand the applications of both types of thrust producer
• Understand the Otto cycle
• Understand the Brayton cycle
• Identify the main components of a Piston Engine
• Identify the main components of a Gas Turbine (Jet) Engine

3. Aircraft Propulsion : Engine Types
• Identify the two types of aircraft engines
Piston Engine:
Uses ‘Avgas’ fuel
Gas Turbine Engine or ‘Jet’ Engine
Uses ‘Avtur’ fuel

4. Engine Operation
Regardless of whether an engine is Piston driven, or a Gas
Turbine, it operates on the same basic cycle of:
• Suck The engine sucks air into the intake
• Squeeze The engine compresses this air, or fuel/air mix
• Bang Fuel is added to the air and it is burned
• Blow The exhaust gases are vented to atmosphere
These operations are referred to as:
• Induction
• Compression
• Power
• Exhaust

5. Piston engines were the first
type of engine to be fitted to
an aircraft.
They operate on the ‘Otto’
cycle, where all the operations
in the cycle take place in the
same location, ( the cylinder)
at different times.
Most piston engines on aircraft
are a ‘Four Stroke’ design,
where the operations of
induction, compression, power
and exhaust are carried out in
four ‘strokes’ of the piston.
Aircraft Propulsion : Piston Engines

6. Four Stroke engines can
usually be identified by valve
mechanisms at the top of the
cylinder.
The valves allow air ( or air &
fuel) to enter the cylinder, and
the exhaust gases to exit.
Two Stroke engines do not
have this mechanism, and
carry out the cycle in just two
strokes of the piston.
Two Stroke engines are often
fitted to microlight aircraft.
Aircraft Propulsion : Piston Engines

7. Four Stroke Cycle
The Four Stroke cycle is:
• INDUCTION: The inlet valve is
open, allowing air ( or fuel/air)
into the cylinder. The piston is at
the top of the cylinder, called
Top Dead Centre( TDC), and is
moving downwards to Bottom
Dead Centre, (BDC).
• COMPRESSION: The inlet valve
closes as the piston once more
rises in the cylinder towards
TDC, compressing the fuel/air
mixture as it travels.

8. Four Stroke Cycle
• POWER: Just before the piston
arrives at TDC, the air/fuel
mixture is ignited with a Spark
Plug. This combustion drives
the piston downwards once
more to BDC. This downwards
movement powers the crankshaft.
• EXHAUST: The piston now moves
back upwards towards TDC, with
the exhaust valve open to vent the
exhaust gases.

9. Piston Engines
Piston engines have been produced
in a variety of configurations based
on their shape, number of cylinders
and method of cooling.
The two types of cooling used are
Air Cooling, and Liquid Cooling.
For example, the engine shown
here is an 18 cylinder, air cooled
radial engine. It produced around
2,500 horsepower.

10. Piston Engine Cooling
Two types of cooling are used on Piston
Engines; Air Cooling or Liquid Cooling.
Air Cooling relies on cooling fins to increase
the surface area of the cylinders. Cooling air
circulates through the fins and takes away the
heat. This type of cooling is usually only used
on rotary, radial or flat engines.
Liquid Cooling uses a cooling liquid to carry
heat away from the engine to a radiator,
where cooling air cools the liquid. This type
of cooling is used on most car engines.
It is essential for inline engines where the
inside cylinders would be difficult to cool
using air cooling.

11. An early four cylinder, in-line
Liquid Cooled engine
Piston Engine Configuration

12. EARLY ROTARY ENGINE: THE CRANK
REMAINS STATIC & CYLINDERS ROTATE
Piston Engine Configuration

13. EARLY RADIAL ENGINE: THE CYLINDER
REMAINS STATIC & CRANK ROTATES
Piston Engine Configuration

14. Rolls-Royce Merlin: 12 cylinder, V- configuration
( or V12), liquid cooled engine
Piston Engine Configuration

15. MODERN ‘LYCOMING’ ENGINE: Air Cooled,
‘flat four’, flat configuration, four cylinder
As fitted to most General Aviation aircraft
Piston Engine Configuration

16. Gas Turbine / Jet Engines

17. Gas Turbine Engines
Gas Turbine engines provide a
much greater power/weight ratio
than Piston Engines, and can
develop far greater power,
allowing the aircraft to fly faster.
Gas Turbine Engines are usually
described by their configuration,
the type of compressor used, and
how many shafts ( or ‘Spool’s) they
have.
They provide thrust by either the
jet exhaust or by use of a propeller.
Sometimes both……

18. Gas Turbine Engines
Gas Turbine engine compressors
are of two types:
• Centrifugal ( top image)
• Axial ( bottom image)
Some engines have a mix of both
types, to ensure a short engine.
This is because a Centrifugal
Compressor raises air pressure
quickly over a short distance.

19. Gas Turbine Engines
The process of Induction, Compression, Power & Exhaust take
place within a Gas Turbine Engine at the same time, but in different
locations. This is called the Brayton Cycle.
A Gas Turbine consists of a Compressor, a Combustor, a Turbine
and an Exhaust. Combustion gases drive the Turbine,
which in turn drive the Compressor.

20. Gas Turbine Engines: Operation Cycle
Gas Turbine
Brayton Cycle
Piston Engine
Otto Cycle
SUCK SQUEEZE BANG BLOW

21. Gas Turbine Engines
Gas Turbine engines are designed in
five different configurations,
depending on the role required.
• Turbojet
• Low by-pass ratio Turbofan
• High by-pass ratio Turbofan
• Turbo-shaft ( used on helicopters)
• Turbo-prop

22. Gas Turbine Engine Components: Compressor
Both ‘axial’ & ‘Centrifugal’ shown

23. Gas Turbine Engine Components: Combustor
Notice the holes for cooler airflow

24. Components: Turbine & Exhaust

25. Gas Turbine Engines: Centrifugal Type

26. Gas Turbine Engines: Axial Type

27. Engine Thrust
Aircraft engines ‘propel’ the
aircraft by converting their
power into THRUST.
Jet propulsion allows for faster flight
Propellers are efficient
at slower speeds

28. Engine Thrust
A Jet Engine creates THRUST by
accelerating a small amount or air
to a high velocity, which blows the
aircraft alongA propeller driven aircraft
creates THRUST by accelerating
a large amount or air to a lower
velocity, which blows the aircraft
along

29. Propeller Engines
Aircraft fitted with Propellers
can either be fitted with ‘Otto’
cycle Piston Engines or
‘Brayton’ cycle Gas Turbines.
All aircraft with piston engines
use propellers.
Gas Turbine Engines fitted with
propellers are called ‘Turbo-
Prop’ aircraft.
Around 75% or ¾ of the thrust
comes from their propeller,
with the rest from the jet
exhaust.

30. Aircraft Propulsion : Our Objectives
• Identify the two types of aircraft engines
• Identify the two types of thrust producers
• Understand the applications of both types of thrust producer
• Understand the Otto cycle
• Understand the Brayton cycle
• Identify the main components of a Piston Engine
• Identify the main components of a Gas Turbine (Jet) Engine

31. Questions?