Download - Aer109-Aircraft Propulsion Unit 1
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AER109-AIRCRAFT PROPULSION
Unit-1Fundamentals of air Breathing
Engines
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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.
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Course Pre-requisites
• A course in Engineering Thermodynamics• Additionally : A course in Fluid Mechanics
would be helpful
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• 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.
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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 .
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Review of Basic Thermodynamics
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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.
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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.
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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.
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Thermodynamics = Therme + Dynamis (Heat) (Power)
Aspects related to Energy and Energy Transformation
- Power Generation
- Refrigeration
- Relationships among Properties of Matter
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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
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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
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Isolated System
ISOLATED System
m = const.E = const.
Mass NO
Energy NO
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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
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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.
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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
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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.
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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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 :
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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)
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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
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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.
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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.
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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
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Zeroth Law of Thermodynamics
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.
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Zeroth Law of Thermodynamics
- 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.
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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
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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 )
_
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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
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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
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Various form of Energy
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Examples for energy conversion
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Application of energy conversion
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Basic steam power plant
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Internal combustion engine(spark ignition engine)
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Simple gas turbine engine
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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)
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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.
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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
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Reciprocating engine
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Four Stroke Cycle
• Intake• Compression • Power• Exhaust
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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.
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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.
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Power Stroke
• Valves remain closed.• Spark plug fires
igniting fuel mixture.• Piston moves down, ½
turn of crankshaft.• Heat is converted to
mechanical energy.
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Exhaust Stroke
• Exhaust valve opens.• Piston move up,
crankshaft makes ½ turn.
• Exhaust gases are pushed out polluting the atmosphere.
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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.
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• 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.
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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.
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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.
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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%.
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• 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
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Otto Cycle(SI engine or petrol engine)
• Intake stroke • Compression stroke • Power (expansion) stroke • Exhaust stroke
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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
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P-v and T-s diagrams
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• 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.
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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
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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
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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
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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
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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.
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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.
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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.
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Topics covered in black board
1.Reciprocating engine performance2.Gear box
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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.
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Turbo-prop Engine
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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.
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Turbo-shaft Engines
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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.
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P-v and T-S diagrams
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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
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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.
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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.
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Basic Aircraft Jet Engine types
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Aircraft Turbofan Engines
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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.
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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.
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• 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
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How Jet Propulsion works
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• 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.
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Whittle’s jet engine that flew
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Heinkel Engine by Von Ohain that flew
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A modern aircraft jet engine
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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.
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Thrust requirement for aircraft
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• 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.
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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.
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A Jet Engine Schematic
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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
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Fundamental varieties of jet engine
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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
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A modern very-low bypass (almost pure turbojet) engine
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Jet Propulsion
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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
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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
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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
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Newton in Practice
Schematic of a rocket engine
Drawing Courtesy of Understanding Flight
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Where are jet engines used?
Commercial Airliners – Boeing 757
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Where are jet engines used?
Business and personal jets - Learjet
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Where are jet engines used?
Military Bombers
B-52 “Stratofortress”
B-2 “Spirit”
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Where are jet engines used?
Military FightersF-15 “EAGLE”
F-22 “Raptor”
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Where are jet engines used?
Helicopters - Apache
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Where are jet engines used?
M-1 Abrams Tank
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Where are jet engines used?
Tractor Pulling
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Where are jet engines used?
Speed boats
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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
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History of Jet Engines
• 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
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History of Jet Engines
• 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
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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
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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
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Six different types of jet engines
• Turbojet• Turbofan• Turboshaft• Turboprop• Pulsejet• Ramjet
X-15 with ramjet engine
Photo Courtesy of NASA
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Turbojet Engine• Thrust produced by gasses expelled from the exhaust
nozzle• Very noisy• Used on high speed aircraft due to its small size
Drawing Courtesy of Understanding Flight
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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)
Drawing Courtesy of Understanding Flight
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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)
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Turbofan Cont’
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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
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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
Drawing Courtesy of www.aircraftenginedesign.com
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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
Drawing Courtesy of www.aircraftenginedesign.com
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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”
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V-1 Flying Bomb
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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
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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
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Components of a Turbine Jet Engine
Turbine Blade
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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
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Basic Components of a Turbine Jet Engine
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Basic Components of a Turbine Jet Engine
• 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
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Basic Components of a Turbine Jet Engine
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Basic Components of a Turbine Jet Engine
• 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
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Basic Components of a Turbine Jet Engine
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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
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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
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Other Essential Systems
• Fuel System• Ignition System• Flame Holder• Lubrication System
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Other Auxiliary Components
• Turbofan – Inlet fan• Turboshaft – Gear reduction unit• Turboprop - Gear reduction unit• Pulsejet – reed valves• Afterburners• Thrust Vectoring Systems
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Turbofan Inlet Fan
The Bypass ratio of a turbofan is typically 8:1
Most of the thrust is produced from the large inlet fan
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Turboshaft Gear Reduction Unit
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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
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Turboprop
A turboprop is essentially a turboshaft engine that is attached to a propeller
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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
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Afterburners
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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
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Thrust Vectoring Maneuverability
Russian Su-37, which incorporates thrust vectoring
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Thrust Vectoring Systems
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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)
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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)
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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
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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
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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
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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
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Pulsejet
• Relatively inexpensive• Doesn’t have the ability to produce thrust at
low speeds• Simple construction
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Ramjet
• Only used in extremely high speed applications (mostly military / NASA)
• Only produces thrust at high speeds• No moving parts
X-15
SR-71