aer109-aircraft propulsion unit 1

AER109-AIRCRAFT PROPULSION

Unit-1Fundamentals of air Breathing

Engines

TEXT BOOKS:

• Hill, P.G. & Peterson, C.R. , Mechanics & Thermodynamics of Propulsion, Addison – Wesley Longman INC, 1999.

REFERENCE BOOKS:

• Gas Turbine Theory, Cohen, H. Rogers, G.F.C. and Saravanamuttoo, H.I.H. “Longman, 1989.

• Aero thermodynamics of Aircraft Engine Components, Oates, G.C., , AIAA Education Series, New York, 1985.

• Rolls Royce Jet Engine – Third Edition – 1983

• Gas Turbine, Jet and Rocket Propulsion, Mathur, M.L. and Sharma, R.P., , Standard Publishers & Distributors, Delhi, 1999.

• Gas Turbine, V. Ganesan, Tata McGraw Hill Pub. Co. Ltd., 1996.

Course Pre-requisites

• A course in Engineering Thermodynamics• Additionally : A course in Fluid Mechanics

would be helpful

• There are two general types of jet propulsion air-breathing and non air-breathing engines.

• Air-breathing engines use oxygen from the atmosphere in the combustion of fuel. They include the turbojet, turboprop, ramjet, and pulse-jet. The term jet is generally used only in reference to air-breathing engines.

• Non air-breathing engines carry an oxygen supply. They can be used both in the atmosphere and in outer space. They are commonly called rockets and are of two kinds liquid-propellant and solid-propellant.

• Air-breathing engines may be further divided into two groups, based on the way in which they compress air for combustion. The turbojet and turboprop each has a compressor, usually turbine-driven, to take in air. They are called gas-turbine engines. The ramjet and the pulse-jet do not have compressors.

Define propulsion

• Propulsion is a means of creating force leading to movement.

• A propulsion system has a source of mechanical power (some type of engine or motor, muscles), and some means of using this power to generate force, such as wheel and axles, propellers, a propulsive nozzle, wings, fins .

Review of Basic Thermodynamics

Definition for Thermodynamics

• Thermodynamics is the branch of science or physics that studies various forms of energies and their conversion from one form to the other like electrical energy to mechanical energy, heat to electrical, chemical to mechanical, wind to electrical etc.

• The study of thermodynamics is comprised of important laws of thermodynamics namely first law of thermodynamics, second law of thermodynamics, third law of thermodynamics and Zeroth law of thermodynamics.

Cont…..

• When any of the properties of the system such as temperature, pressure, volume etc change, the system is said to have undergone thermodynamic process.

• Various types of thermodynamic processes are: isothermal process, adiabatic process, isochoric process, isobaric process, and reversible process.

Macroscopic and microscopic view point

• There are two points of view from which the behavior of matter can be studied.

1.Macroscopic

2.Microscopic

In the macroscopic approach a certain quantity of matter is considered, without the events occurring at the molecular level being taken into account.

From the microscopic point of view matter composed of molecule. if it is a gas each molecule at a given instant has a certain position ,velocity and energy and for each molecule these change occurred very frequently as a result of collisions.

Thermodynamics = Therme + Dynamis (Heat) (Power)

Aspects related to Energy and Energy Transformation

- Power Generation

- Refrigeration

- Relationships among Properties of Matter

System & Surroundings

SYSTEM

SURROUNDINGS

BOUNDARY

SYSTEM :Quantity of matter or region in space, chosen for study.

SURROUNDINGS :Mass or region outside the SYSTEM.

BOUNDARY :Real / Imaginary surface that separates the SYSTEM from SURROUNDINGS.

BOUNDARY :

Fixed / Movable

Shared by both, SYSTEM and SURROUNDINGS

No ThicknessNo Mass / Volume

Close System

CLOSED System

m = const.

Mass NO

Energy YES

GAS2 kg1 m3

GAS2 kg3 m3

CLOSED System with Moving Boundary

Also known as CONTROL MASS

Isolated System

ISOLATED System

m = const.E = const.

Mass NO

Energy NO

Open System

OPEN System

Mass YES

Energy YES

Also known as CONTROL VOLUME

e.g. Water Heater, Car Radiator, Turbine, Compressor

BOUNDARY of OPEN System is known as

CONTROL SURFACE

In Out

Imaginary Boundary

Real Boundary

Properties of System

Any characteristic of a System is known as its PROPERTY.

e.g. Pressure (P), Volume (V), Temperature (T) and mass (m), etc. also Viscosity (μ), Electric Resistance (R), Thermal Conductivity (k), etc.

Intensive : Independent on mass of system. - e.g. Velocity (c), Elevation (h), etc.

Extensive : Dependent on mass of system. - e.g. Pressure (P), Density (ρ), etc.

Specific : Extensive properties per unit mass.- e.g. Sp. Vol (v=V/m), Sp. Enthalpy (h=H/m), etc.

State & Equilibrium

Assume a System NOT undergoing any change.

Set of properties to completely describe the condition of the system is known as its

STATE

m = 2 kgT1 = 25 ºCV1 = 3 m3

m = 2 kgT1 = 25 ºCV1 = 1 m3

STATE 1 STATE 2

State & Equilibrium

EQUILIBRIUM : State of Balance

Thermal Equilibrium :- NO Temperature Gradient throughout the system.

Mechanical Equilibrium :- NO Pressure Gradient throughout the system.

Phase Equilibrium :- System having more than 1 phase. - Mass of each phase is in equilibrium.

Chemical Equilibrium :- Chemical composition is constant - NO reaction occurs.

Path & Process

Any change a system undergoes from one equilibrium state to another is known as

PROCESS.

Series of states through which system passes during the process is known as its PATH.

Property A

State 1

State 2

Pro

pert

y B

Path State 1State 2

Path & Process

t=0t=t1

t=0t=t2t

t2 < t1

Quasi-Static

Non-Quasi-Static

Process proceeds in such a manner that

system remains infinitesimally close to

equilibrium conditions at all times.

It is known as QUASI-STATIC or

QUASI-EQUILIBRIUM Process.

Path & Process

Pre

ssur

e (P

)

Volume (V)

V=ConstIsochoric

P=ConstIsobaric

Tem

pera

ture

(T

)

Enthalpy (h)/ Entropy (s)

T=ConstIsothermal

h=ConstIsenthalpic

s=ConstIsentropic

Cycle

CYCLE :

A system is said to have

undergone a cycle if it returns to its

ORIGINAL state at the end of the

process.

Hence, for a CYCLE, the

INITIAL and the FINAL states are

identical.Property A

State 1

State 2

Pro

pert

y B

Reversible / Irreversible Process

Reversible Process : Process that can be reversed without leaving any trace on the

Surroundings.

i.e. Both, System and Surroundings are returned to their initial

states at the end of the Process.

This is only possible when net Heat and net Work Exchange

between the system and the surroundings is ZERO for the Process.

t=0t=t1

Pendulum

Quasi-Static Compression and Expansion

Reversible / Irreversible Process

Most of the Processes in nature are IRREVERSIBLE.

i.e. Having taken place, they can not reverse themselves spontaneously and restore the

System to its original State.

e.g. Hot cup of coffee Cools down when exposed to

Surroundings.

But, Warm up by gaining heat from Surroundings.

i.e. w/o external Heat supply.

Temperature

TEMPERATURE :

- No EXACT Definition.

- Broad Definition : “Degree of Hotness / Cold”

- This definition is based on our physiological sensation.

- Hence, may be misleading.

- e.g. Metallic chair may feel cold than Wooden chair; even at SAME temperature.

- Properties of materials change with temperature.

- We can make use of this phenomenon to deduce EXACT level of temperature.

Temperature Scales

1. Celsius Scale ( ºC ) – SI System

2. Fahrenheit Scale ( ºF ) – English System

3. Kelvin Scale ( K ) – SI System

4. Rankine Scale ( R ) – English System

Celsius Scale and Fahrenheit Scale – Based on 2 easily reproducible fixed states,

viz. Freezing and Boiling points of water.

i.e. Ice Point and Steam Point

Thermodynamic Temperature Scale – Independent of properties of any substance.

- In conjunction with Second Law of Thermodynamics

Thermodynamic Temperature Scale – Kelvin Scale and Rankine Scale.

Hot End

Regenerator Pulse Tube

Temperature Scales

T ( K ) = T ( ºC ) + 273.15

T ( R ) = T ( ºF ) + 459.67

T ( ºF ) = 1.8 T ( ºC ) + 32

T ( R ) = 1.8 T ( K )-273.15 0

273.16 0.01

0 -459.67

491.69 32.02

ºC K ºF R

Conversion Factors :

Pressure

Local Atmospheric Pressure( 1.01325 bar @ Sea Level )

Absolute Zero Pressure

P (gauge)

P (abs) P (atm)P (vacuum)

P (gauge) = P (abs) – P (atm)

P (vacuum) = P (atm) – P (abs)

Ideal & Real GasAny equation that relates the Pressure, Temperature and Sp. Volume of the

substance is known as Equation of State.

In 1662, Robert Boyle, observed that Pressure of the gas is inversely proportional to

its Volume.

i.e. PV = C

In 1802, J. Charles and J. Gay-Lussac, observed that Volume of the gas is directly

proportional to its Temperature.

i.e. V /T= C

v

TRP OR Pv = RT

This equation is called Ideal Gas Equation of State.

The hypothetical gas that obeys this law, is known as Ideal Gas.

R is the Constant of Proportionality, given by the unit ( kJ / kg.K )

Ideal & Real Gas

Now, V (Total Volume) = m.v (Sp. Vol.)

PV = mRT→

Thus, for a fixed mass;

2

22

1

11

T

VP

T

VP

Behaviour of a Real Gas approaches to the that of an Ideal Gas, at low densities.

Thus, at low pressures and high temperatures, the density of the gas decreases

and the gas approaches to Ideal Gas.

Thermal Equilibrium

Thermal Equilibrium : NO change w.r.t. Temperature

NO Temperature Gradient.

HOT cup of tea / coffee cools off w.r.t. time.

COLD Drink warms up w.r.t. time.

When a body is brought in contact with another body at different temperature, heat

is transferred from the body at higher temperature to that with lower one; till both

attain a THERMAL EQUILIBRIUM.

Heat & Work

CLOSED System

Heat

Work

Energy can cross the Boundary of the System in 2 forms : 1. Heat

2.

Work Heat is a form of Energy transferred between 2 Systems

( or a System and the surroundings ) by virtue of

Temperature Difference (∆T).

i.e. Heat is Energy in TRANSITION.

Process involving no Heat Exchange is known as

ADIABATIC Process.

Atmosphere 25ºC

25 ºC

15 ºC

Heat, QQ=0

Adiabatic

Heat & Work

Possibilities of Adiabatic Process :

1. Perfect Insulation : Negligible Energy transfer through Boundary.

2. Both System and Surrounding at same temperature.

No Energy transfer due to absence of driving force (∆T).

NOTE : Adiabatic Process ≠ Isothermal Process

No Heat Transfer Energy content & temperature of the system can

be changed with help of Work.

Heat & Work

Energy Transfer in from of Heat by 3 ways :

CONDUCTION : Transfer of Energy from a more energetic particle of a substance

to the adjacent less energetic one, as a result of interaction

between them.

CONVECTION : Transfer of Energy between a solid surface and the adjacent fluid

that is in motion. It involved both, the combined effect of

conduction and fluid motion.

RADIATION : Transfer of Energy due to the emission of electromagnetic waves.

Heat & Work

WORK : Work is the Energy transfer associated with a Force acting through a distance.

Denoted by J or kJ.

∆X

Force

e.g. Raising Piston, Rotating Shaft, etc.

ME0223 SEM-IVApplied Thermodynamics & Heat

Engines

Heat & Work

Sp. Work = Work per unit Mass

w = W/m ( J/kg )

Power = Work per unit Time

P = W/time ( J/sec OR W )

Sign Convention :

Heat Transfer TO a System : + ve

Heat Transfer FROM a System : - ve

Work done BY a System : + ve

Work done ON a System : - ve

SYSTEM

SURROUNDINGS

Qin

Qout

Win

Win

Heat & Work

Similarities between HEAT & WORK :

1. Both are recognised at the Boundary of the System, as they cross the

Boundary. Hence both are Boundary Phenomena.

2. System possesses Energy, but neither Heat nor Work.

3. Both are associated with Process, not State. Heat and Work have NO meaning

at a State.

4. Both are Path Functions.

Path Function : Magnitude depends on the Path followed during the Process, as

well as the End States.

Point Function : Magnitude depends on State only, and not on how the System

approaches that State.

Specific Heat

Specific Heat at Constant Pressure (CP) :

The Energy required to raise the temperature of a unit mass of a substance by 1 degree, as

the Pressure is maintained CONSTANT.

Specific Heat at Constant Volume (CV) :

The Energy required to raise the temperature of a unit mass of a substance by 1 degree, as

the Volume is maintained CONSTANT.

m = 1 kg∆T = 1 ºC

Sp. Heat = 5 kJ/kg ºC

5 kJ

DEFINITION :

The Energy required to raise the temperature of a

unit mass of a substance by 1 degree.

Zeroth Law of Thermodynamics

STATEMENT :

If two bodies are in Thermal Equilibrium with the third body, then they are also in

Thermal Equilibrium with each other.

This statement seems to be very simple.

However, this can not be directly concluded from the other Laws of Thermodynamics.

It serves as the basis of validity of TEMPERATURE measurement.

A

25 ºC 25 ºC 25 ºC

BC


By replacing the Third Body with a Thermometer; the Zeroth Law can be stated as :

Two bodies are in Thermal Equilibrium, if both have same TEMPERATURE,

regarding even if they are not in contact with each other.

A

25 ºC 25 ºC

25 ºCB

i.e. Temp (A) measured by Thermometer and is known.

(A) is in Thermal Equilibrium with (B).

Then, Temp (B) is also known, even not in contact with Thermometer.


- Formulated and labeled by R.H. Fowler in 1931.

- However, its significance is realised after half a century after formation of First and

Second Laws of Thermodynamics.

- Hence named as Zeroth Law of Thermodynamics.

First Law of Thermodynamics

Also known as Law of Conservation of Energy

Important due to its ability to provide a sound basis to study between different

forms of Energy and their interactions.

STATEMENT :

Energy can neither be created nor

destroyed during a process; but can be

only converted from one form to another.

m g Δz = ½ m ( v12 - v2

2 )

PE = 7 kJKE = 3 kJ

m = 2 kg PE = 10 kJKE = 0

Δz

First Law of Thermodynamics

This forms the basis for Heat Balance / Energy Balance.

Net change ( increase / decrease ) in the total Energy of the System during a Process

= Difference between Total Energy entering and Total Energy leaving the System

during that Process.

Total Energy entering the System

Total Energy leaving the System

= Change in Total Energy of the System

( EIN ) ( EOUT ) ( ΔE )

_

Second Law of Thermodynamics

Kelvin – Planck Statement :

It is impossible for any device that operates on a Cycle to receive Heat

from a single Reservoir and produce net amount of Work.

Alternatively;

No Heat Engine can have a thermal

efficiency of 100 per cent.

Thermal Energy Reservoir

Wnet = 100 kW

QH = 100 kW

QL = 0

Heat Engine

Second Law of Thermodynamics

Clausius Statement :

It is impossible to construct a device that

operates in a Cycle, and produces no effect

other than the transfer of Heat from a

Lower Temperature Body to a Higher

Temperature body.

Alternatively;

No Refrigerator can operate unless its

compressor is supplied with external

Power source.

Warm Environment

Wnet = 0

QH = 5 kJ

QL = 5 kJ

Refrigerator

Refrigerated Space

Various form of Energy

Examples for energy conversion

Application of energy conversion

Basic steam power plant

Internal combustion engine(spark ignition engine)

Simple gas turbine engine

Engine • An engine is a device which transforms one form of energy into

another form.

• Most of the engines convert thermal energy into mechanical work.

Heat engine

Heat engine is a device which transforms the chemical energy or thermal energy and utilizes this thermal energy to perform useful work. Thus thermal energy is converted to mechanical energy in a heat engine

Classification of heat engine 1.Internal combustion engines(IC ENGINE)2.External combustion engines(EC ENGINE)

Internal combustion enginesIn case of gasoline or diesel engine the products

of combustion generated by the combustion of fuel and air with in the cylinder.External combustion engines

External combustion engines are those in which combustion takes place outside the engine. For example in a steam engine or steam turbine the heat generated due to the combustion of fuel is employed to generate high pressure steam which is used as the working fluid in a turbine.

Heat engine

IC engines1.Rotary engine-Wankel engine ,open cycle gas

turbine engine.2.Reciprocating engines-Gasoline Engine, diesel

Engine.EC engine1.Rotary engine-steam engine ,stiriling engine2.Reciprocating engines-steam turbine, closed cycle

gas turbine

Reciprocating engine

55

Four Stroke Cycle

• Intake• Compression • Power• Exhaust

56

Intake Stroke

• Intake valve opens.• Piston moves down, ½

turn of crankshaft.• A vacuum is created in the

cylinder.• Atmospheric pressure

pushes the air/fuel mixture into the cylinder.

57

Compression Stroke

• Valves close.• Piston moves up, ½

turn of crankshaft.• Air/fuel mixture is

compressed.• Fuel starts to vaporize

and heat begins to build.

58

Power Stroke

• Valves remain closed.• Spark plug fires

igniting fuel mixture.• Piston moves down, ½

turn of crankshaft.• Heat is converted to

mechanical energy.

59

Exhaust Stroke

• Exhaust valve opens.• Piston move up,

crankshaft makes ½ turn.

• Exhaust gases are pushed out polluting the atmosphere.

Operation of two-stroke engine

The two stroke engine employs the crankcase aswell as the cylinder to achieve all the elements ofthe cycle in only two strokes of the piston.

360 degrees rotation of crankshaft completes the cycle.

• The two cycle engine completes its cycle of intake, compression, power, and exhaust with only two strokes of the piston.

• It takes only one revolution of the shaft to complete the 2-stroke cycle.

Air Standard Cycles assumptions

1. Cylinder contains constant amount of air and it is treated as ideal gas.

2. The specific heats and other physical and chemical properties remain unchanged during the cycle.

3. Instead of heat generation by combustion, heat is transformed from external heat source.

4. The process of heat removal in the exhaust gases is represented by heat transfer from the cycle to external heat sink.

5. There is neither friction nor turbulence; all processes are assumed to be reversible.

6. No heat loss from the working fluid to the surroundings.

Advantages of I.C engines over E.C engines

1. More mechanical simplicity and lower weight/power ratio.

2. They do not need auxiliary equipment, such as boiler & condenser.

3. They could be started and stopped in a short time.

4. Their thermal efficiency is higher than other heat engines.

5. Their initial cost is low.

These advantages make I.C.E. more suitable in the transport sector; motor cars, small ships, submarines, and small aircrafts.

PISTON-PROP ENGINES

• Nearly all present day small aircraft engines employing piston engine using gasoline as fuel, and air as oxidizer for combustion.

• Gas pressure inside the cylinder acting through the piston-connecting rod-crank linkage , applies torque to the engine shaft for running the propeller , often through a speed reducing gear box. rotation of propeller produces thrust in accordance with the aerodynamic concept of propeller theories.

• Power equivalent of thrust is simply thrust power TP=T.V , which may takes as the propeller output . The power input to the propeller from the engine shaft is simply the engine brake horsepower(after gear box).

• The propeller efficiency is

ɳp = (Propeller thrust power)/(Engine shaft brake horsepower)

The maximum efficiency achieved by a propeller by accelerating large mass of air rearward with high velocity(slightly greater than flight speed) is 85-90%.

• The lost power appears mainly as unrecoverable kinetic energy of air pushed rearward and friction that are mainly dissipates as heat.

• Mechanical efficiency defined as

ɳm = (BHP)/(IHP) (appx 85% in aircraft engines)

This type of engine has its theoretical basis on Otto cycle.

For a piston engine increase in mass flow achieved by

1.Increase in rpm

2.Increase in engine size

3.Increase both

Increase in rpm causes sliding friction (less ɳ)

Increase in engine size causes more drag, more weight and less combustion efficiency.

Its use is thus limited to short haul small low-subsonic aircraft which is operated at M < 0.5

Otto Cycle(SI engine or petrol engine)

• Intake stroke • Compression stroke • Power (expansion) stroke • Exhaust stroke

70

The air-standard Otto cycle is the ideal cycle that approximates the spark-ignition combustion engine.

Process Description 1-2 Isentropic compression 2-3 Constant volume heat addition 3-4 Isentropic expansion 4-1 Constant volume heat rejection

71

P-v and T-s diagrams

• At the start of the cycle, the cylinder contains a mass M of air at the pressure and volume indicated at point 1. The piston is at its lowest position.

• It moves upward and the gas is compressed isentropically

to point 2. At this point, heat is added at constant volume which raises the pressure to point 3.

• The high pressure charge now expands isentropically, pushing the piston down on its expansion stroke to point 4 where the charge rejects heat at constant volume to the initial state, point 1.

73

Thermal Efficiency of the Otto cycle:

th net

in

net

in

in out

in

out

in

W

Q

Q

Q

Q Q

Q

Q

Q

1

Now to find Qin and Qout. Apply first law closed system to process 2-3, V = constant.

Thus, for constant specific heats,

Q U

Q Q mC T T

net

net in v

,

, ( )

23 23

23 3 2

74

Apply first law closed system to process 4-1, V = constant.

Thus, for constant specific heats,

Q U

Q Q mC T T

Q mC T T mC T T

net

net out v

out v v

,

, ( )

( ) ( )

41 41

41 1 4

1 4 4 1

The thermal efficiency becomes

th Otto out

in

v

v

Q

Q

mC T T

mC T T

,

( )

( )

1

1 4 1

3 2

75

th OttoT T

T T

T T T

T T T

,

( )

( )

( / )

( / )

1

11

1

4 1

3 2

1 4 1

2 3 2

Recall processes 1-2 and 3-4 are isentropic, so

Since V3 = V2 and V4 = V1, we see that

T

T

T

T

or

T

T

T

T

2

1

3

4

4

1

3

2

76

The Otto cycle efficiency becomes

th OttoT

T, 1 1

2

Is this the same as the Carnot cycle efficiency? Since process 1-2 is isentropic,

where the compression ratio is r = V1/V2 and

th Otto kr, 11

1

77

The thermal efficiency of the theoretical Otto cycle increases with increase in compression ratio and specific heat ratio but is independent of the heat added (independent of load) and initial conditions of pressure, volume and temperature.

The more important performance factors are

1.Heat release per mass of air depends upon both fuel heating value and fuel-air ratio . the greater heat release of the beat fuel-air mixture (chemically correct or stoichiometric) results in max temperature and pressure rise of mixture trapped inside the cylinder.

2.Quantity of charge per stroke introduced into the cylinder directly controls the quantity of heat that can be released and then converted into work per cycle . if the supercharger is used , the air filling the cylinder is above ambient pressure and density , hence weight of air consumed per cycle is greater than unsupercharged case.

Factors affecting performance of piston engine

1.Design of induction/exhaust systems-pressure loss.

2.Design of cooling system-excess cooling causes heat loss.

3.detonation/knocking-caused by instantaneous combustion of a part of the fuel-air mixture.

Normal combustion requires abt 2.5 millisec for completion. Detonation occurs in less than a microsecond.

Topics covered in black board

1.Reciprocating engine performance2.Gear box

Turbo-prop Engines

• Aircraft with propellers have high efficiency and when the propeller is powered withy the help of gas turbine engines , they are known as turbo-prop engines.

• The prop-engines have reasonably good efficiency compared to piston engines and pure jet engines for speeds less than M<0.65.

• Engines with propellers have the disadvantage that, when flow at the tip of the propeller blade reaches local sonic velocity , it causes severe shocks and vibrations in blades.

Turbo-prop Engine

Turbo-shaft Engines

• The turbo-shaft variety was developed to carter to the needs of helicopters, which were developed after the WW II and have found use in various sectors, military and civilian all over the world.

• Since helicopters are relatively small flying crafts, the engines powering them also need to be compact.

• The engines are normally mounted vertically to supply power to the nearly horizontal rotor , driven through a speed reducing gear box , that provides lifting and flying thrust to the helicopter.

Turbo-shaft Engines

The Brayton Cycle

• The cycle consists of an isentropic compression process, a constant pressure heat addition process, an isentropic expansion process and a constant pressure heat rejection process. Expansion is carried out till the pressure drops to the initial (atmospheric) value.

P-v and T-S diagrams

Heat supplied in the cycle, Qs, is given by

Cp(T3 – T2)

Heat rejected in the cycle, Qs, is given by

Cp(T4 – T1) Hence the thermal efficiency of the cycle is given by

)62(

1

1

1

1

2

3

1

4

2

1

23

14

T

T

T

T

T

T

TT

TTth

Now 4

3

1

4

3

1

1

2

1

2

T

T

p

p

p

p

T

T

And since 2

3

1

4

4

3

1

2

T

T

T

Thavewe

T

T

T

T

Hence, substituting in Eq. 62, we get, assuming that rp is the pressure ratio p2/p1

)63(1

1

11

1

1

1

1

2

2

1

p

th

r

p

p

T

T

This is numerically equal to the efficiency of the Otto cycle if we put

11

1

2

2

1 1

rV

V

T

T

so that )63(1

11

Arth

where r is the volumetric compression ratio.

Jet engines

• A jet engine is a reaction engine discharging a fast moving jet that generates thrust by jet propulsion in accordance with Newton's laws of motion.

Basic Aircraft Jet Engine types

Aircraft Turbofan Engines

Main Propulsion Engines 1.Reciprocating piston engine plus propeller-used in small low subsonic aircrafts.

2.Gasturbine based power plants-used in various subsonic to supersonic aircrafts.

a. Tuboprop - propeller and single and two spool gas turbine engines.

b. Turboshaft - single and two spool gas turbine engines.

c. Turbojet- single and two spool gas turbine engines.

d. Turbojet with afterburner- single and two spool gas turbine engines.

e. Turbofan –low bypass and high bypass-mixed and unmixed flows- single, double and triple spool gas turbine based engines.

3.Ram and scramjet – used in hypersonic aircraft or missiles.

4.pulsejet- used by germans in missiles –presently not in use.

5.Ram rocket and hybrid engines – projected for use in orbiting hypersonic aircraft.

6.Rocket engines – used in missiles, earth-to-orbit and space vehicles.

Development of Aircraft Jet Engines

• Sir Isaac Newton in the 18th century was the first to theorize that a rearward-directed acceleration could propel a machine forward at a great speed. This theory was based on his own third law of motion.

• As the hot air blasts backwards through the jet nozzle the aeroplane moves forward.

• In 1920’s a high powered committee in USA, working under NACA, produced a report that stated that a jet engine was not a feasible proposition. So very little work was done in USA on jet engine development till world war II.

• Frank Whittle patented his jet engine in England 1930. He later developed it in USA.

• Dr Hans Von Ohain patented his jet engine in Germany in 1936. It flew in 1939. He also late worked in USA

How Jet Propulsion works

• The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the compressor.

• The gas turbine was not an idea developed in the 1930s: the patent for a stationary turbine was granted to John Barber in England in 1791.

• The first gas turbine to successfully run was built in 1903 by Norwegian engineer ÆgidiusElling. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture.

Whittle’s jet engine that flew

Heinkel Engine by Von Ohain that flew

A modern aircraft jet engine

How the thrust is created for flying

• An aircraft does not fly simply by setting it out on the runway and allow strong wind to blow over its wings.

• The aircraft is to be moved forward, forcing it to run through still air at a high speed. Only then necessary lift is created for it to fly. This is a continuous requirement.

• This forward thrust for the aircraft comes from one of two sources: i) a rotating propeller blade powered by an engine or, ii) a pure Jet engine.

Thrust requirement for aircraft

• Thrust is a mechanical force which is generated through the reaction of accelerating a mass of gas, as explained by Newton's III Law of motion.

• A gas or air, used as a working fluid is accelerated to the rear and the engine attached to the aircraft are accelerated in the forward direction.

• To accelerate the gas, we need some kind of propulsion system. We assume that a propulsion system is a machine which accelerates a gas/ air.

• But if we are dealing with a fluid (liquid or gas) and particularly if we are dealing with a moving fluid, keeping track of the mass gets tricky. For a moving fluid, the important parameter is the mass flow rate.

• Since the mass flow rate already contains the time dependence (mass/time), we can express the change in momentum across the propulsion device as the change in the mass flow rate times the velocity.

The general thrust equation is then given by:

F = [(m .V)e – (m.V)a] + [(Pe - Pa).Ae]

• Normally, the magnitude of the pressure-area term is small relative to the m-dot x Vterms.

A Jet Engine Schematic

Jet Engine fundamentally is a Heat Engine

Combustion is the energy input in to the engine and is key to the operation of a jet engine

Fundamental varieties of jet engine

Thrust needs to be created for all flight regimes of the aircraft:

• Take-off – normally maximum thrust• Climb – reducing from maximum thrust• Cruise –normally minimum thrust• Manoevres –variable thrust• Acceleration & Deceleration -variable• Descend –Low thrust• Landing –Less than maximum thrust

A modern very-low bypass (almost pure turbojet) engine

Jet Propulsion

Definition of a Jet Engine

• An engine that burns fuel and uses the expanding exhaust gases to turn a turbine and/or produce thrust

• The concept of thrust is based on the principle of Newton’s Third Law

Newton’s Third Law

• For every action there is an equal and opposite reaction

• An example of this is a spray nozzle on a garden hose

Newton’s Second LawF=M x A

• Newton’s second law states - The force of an object is equal to its mass times its acceleration

• The force of the spray nozzle is equal to the mass of the water multiplied by the acceleration of the water when it comes through the nozzle

• This is the same principle used in rocket and jet engines

Newton in Practice

Schematic of a rocket engine

Drawing Courtesy of Understanding Flight

Where are jet engines used?

Commercial Airliners – Boeing 757


Business and personal jets - Learjet


Military Bombers

B-52 “Stratofortress”

B-2 “Spirit”


Military FightersF-15 “EAGLE”

F-22 “Raptor”


Helicopters - Apache


M-1 Abrams Tank


Tractor Pulling


Speed boats

History of Jet Engines

• Invented in the 1930’s• Co-invented by Dr. Hans von Ohain (German)

and Sir Frank Whittle (British)• Developed their ideas separately and at the

time knew nothing of the other’s work


• Germans were the first to utilize the jet engine as a military tool

• The jet powered ME-262 was the first jet powered airplane to see combat– It had a top speed of 540 mph

Photo Courtesy of Stormbirds.com


• The SR-71 “Blackbird” set the current speed and altitude record for a jet powered aircraft in 1961– Its top speed is still classified but is in excess of 2,200

mph

Photo Courtesy of NASA

Advantages of Jet Engines

• High power to weight ratio• No reciprocating parts

– Less parasitic power loss – no need to constantly accelerate and decelerate pistons

– Less required maintenance

Disadvantages of Jet Engines

• The high speeds and high operating temperatures make designing and manufacturing gas turbines complex from both the engineering and materials standpoint

• These complexities lead to a higher price• Jet engines do not produce high torque levels, which

is why they aren’t used in automobiles

Six different types of jet engines

• Turbojet• Turbofan• Turboshaft• Turboprop• Pulsejet• Ramjet

X-15 with ramjet engine

Photo Courtesy of NASA

Turbojet Engine• Thrust produced by gasses expelled from the exhaust

nozzle• Very noisy• Used on high speed aircraft due to its small size


Turbofan• Some of the thrust is produced by gasses expelled

from the exhaust nozzle just like a turbojet engine• Most of the thrust is produced from the large inlet fan • The Bypass ratio of a turbofan is typically 8:1 (eight

times more air is bypassed than passes through the compressor and combustion chamber)


Turbofan Cont’• If one wanted to increase thrust you would either have to

increase the speed of the air being moved or increase the mass of the air being moved (Thrust = Mass x Acceleration) ... However…

• It is more efficient to accelerate a larger mass of air to a lower velocity

• Due to this principle the turbofan is more efficient than the turbojet

• Due to the lower velocity the turbofan is also significantly quieter than a turbojet

• Almost all modern commercial aircraft use turbofan engines (excluding the Concord)

Turbofan Cont’

Turboshaft• Exhaust gas is used to turn turbine shaft

which is then used to propel the vehicle• Exhausted gas produces little thrust

because most of the energy is used up by the turbine

Drawing Courtesy of www.aircraftenginedesign.com

Turboshaft Cont’• Because of the high speed (RPM) of a turboshaft

engine gear reduction must be used to obtain a usable shaft speed – much like the transmission in your car

• This gear reduction also produces torque multiplication


Turboprop• A turboprop is essentially a turboshaft

engine that is attached to a propeller• A propeller is more efficient at low speeds

than a turbofan or turbojet


Pulsejet

• Doesn’t Use a compressor or turbine• Doesn’t have the ability to produce thrust at

low speed (<100 mph)• Germans used this design during WWII in their

V-1 “Flying Bomb”

V-1 Flying Bomb

Pulsejet

• Uses one-way reed valves in the front of the engine to force exhaust gasses out the rear of the engine and allow fresh air in the front

Ramjet

• Used for extremely high speeds (minimum 400 mph)

• Doesn’t contain any moving parts (I.e.compressor, turbine, reed valves)

• Relies on the inertia of the incoming air for compression

• Used in the SR-71 Blackbird at supersonic speeds

Components of a Turbine Jet Engine

Turbine Blade

Basic Components of a Turbine Jet Engine

• Housing – The rigid frame that supports and contains the parts needed for operation as well as the combustion event

• Air inlet and diffuser – The area of the jet where fresh air comes in, the design of the diffuser straightens and alters the speed of the incoming airs


• Compressor – Compresses the incoming air at a ratio of approximately 30:1

• Burner or combustion chamber – The area of the engine where fuel is ignited


• Exhaust Nozzle – accelerates the engine exhaust to the most efficient and effective speed for producing thrust

• Turbine – Converts the energy from the heated and expanding exhaust gasses to a rotating shaft which is used to turn the compressors, or in the case of a turboshaft engine, power the vehicle

Radial vs. Axial Flow

• Axial flow compressors – the air travels along the axis of the engine

• Radial flow engines use a centrifugal compressor – they push the air out radially rather than along the axis of the engine

Radial vs. Axial Flow

• Axial flow compressors are more efficient • Radial flow compressors are less expensive• Most large and high-performance jet engines

use an axial flow configuration

Other Essential Systems

• Fuel System• Ignition System• Flame Holder• Lubrication System

Other Auxiliary Components

• Turbofan – Inlet fan• Turboshaft – Gear reduction unit• Turboprop - Gear reduction unit• Pulsejet – reed valves• Afterburners• Thrust Vectoring Systems

Turbofan Inlet Fan

The Bypass ratio of a turbofan is typically 8:1

Most of the thrust is produced from the large inlet fan

Turboshaft Gear Reduction Unit

Pulsejet Reed Valves

The reed valves force the expanding exhaust gasses out the rear of the engine and allow fresh air to enter the front

Turboprop

A turboprop is essentially a turboshaft engine that is attached to a propeller

Afterburners

• An afterburner injects fuel directly into the exhaust stream and burns it using the remaining oxygen.

• This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.

• The advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine

Afterburners

Thrust Vectoring Systems

• Thrust Vectoring redirects exhaust gasses to create thrust on a vector other than the centerline of the aircraft

• Thrust Vectoring is used in aircraft such as the Harrier, F-22 Raptor, and Joint Strike fighter

• Thrust Vectoring can be used to increase maneuverability or allow a plane to takeoff / land vertically

Thrust Vectoring Maneuverability

Russian Su-37, which incorporates thrust vectoring

Thrust Vectoring Systems

Naturally Aspirated Piston Engine

• Relatively inexpensive• Limited power at high

altitudes due to the lower air density

• Speed is limited due to propeller inefficiencies at high speeds (>500 mph)

Supercharged or Turbocharged Piston Engine

• Able to operate at higher altitudes than a naturally aspirated engine– Turbocharging or Supercharging increases the

density of the air entering the engine (the engine thinks it is at a lower altitude)

Still somewhat limited by altitudeSpeed is still limited due to propeller inefficiencies at high speeds (>500 mph)

Turbojet

• No reciprocating parts• Thrust is not greatly affected by altitude• Relatively small frontal area is desirable for

high speed (supersonic) use• Relatively high-speed, low-mass of exhaust

gasses make the turbojet somewhat inefficient

• High speed exhaust is extremely noisy

Turbofan• Because the large inlet fan moves a larger volume of air

at a lower velocity, the turbofan is more efficient that the turbojet

• Because of the lower exhaust speeds the noise level is greatly reduced

• The large inlet fan creates a large frontal area which negatively affects drag at high speeds (especially supersonic)

• Most effective at speeds below supersonic (Mach .5 – Mach .9)• However modern fighters are now using state of the art

turbofans for supersonic flight

Turboprop

• Propellers are most efficient at low speeds • Produce greater power than a comparable

piston engine with less weight, noise, and maintenance

• More expensive than a piston engine• Must use a gearbox to reduce the high

turboshaft rpm’s down to prop rpm’s

Turboshaft

• Used in turboprop, helicopter, and land based applications

• Must use a gearbox to reduce rpm’s• M-1 Abrams tank – 1500 hp turboshaft

engine

Pulsejet

• Relatively inexpensive• Doesn’t have the ability to produce thrust at

low speeds• Simple construction

Ramjet

• Only used in extremely high speed applications (mostly military / NASA)

• Only produces thrust at high speeds• No moving parts

X-15

SR-71

aer109-aircraft propulsion unit 1

Documents

thermodynamics thermodynamics

law of thermodynamics

study of thermodynamics

gasturbine engines

propulsion system

gas turbine theory

isothermal process

adiabatic process