Fabrication and Testing of JetEngineSandesh Parajuli1

1Capital College and Research Center, Koteshwor, Kathmandu, Nepal

March, 2019

A irways stand as the most important trans-portation medium in this current world fa-cilitating even the intercontinental travel

within a very short time frame. Human fascinationto air travel had begun long before Wright brothersinvented the world’s first successful airplane. Atthis current era of unprecedented technological ad-vancement, the world had already witnessed thou-sands of remarkable flying machines.They havemade such an enormous impact on current civiliza-tion that we cannot imagine a better world withoutaircrafts. Traditional aircrafts used to be the onesthat were powered by propellers. Today they havebeen largely replaced by the airplanes that use JetEngine. Being an aerospace enthuusiast there wasan urge to learn about this Jet Engine that has trulyrevolutionized the airway transportation. And, thebest way to learn is to build one. So our team in-cluding Mr.Ghanashym Sarowar,Mr.Tushar Bohoraand Ms. Dikshya Khadka worked on its fabricationfor around a month before it was finally ready fortest and presentation.

1 Introduction

Jet Engine is a major component of modern air-craftsthat keeps them moving forward being based onNewton’s Third Law. The reaction force that isproduced by the high speed exhaust at the tail of theengine makes the aircraft move forward. The higherthe speed of exhaust air, the greater the reaction force(thrust).

Before the World War II propeller powered air-crafts were approaching limits and their efficiency

declined as blades tips approached the speed of sound.This demanded a new form of propulsion, and whenthe German Airforce flew turbojet powered HeinkelHe 178 in August of 1939, the problem was solvedforever. Currently several variations of jet enginesare available like Airbreathing, Turbine Powered,Turbojet, Turbofan, Ram Compression,etc. This paperpresents the fabrication, testing, and theoretical andcomputational analysis of, specifically, a TurbojetEngine.

1.1 Block Diagram

Following is the block diagram of the jet engine thatwe created. Although it lacks several components ascompared to those available in the industry, it is func-tional and fulfills our purpose of understanding a realworking engine.

Figure 1: Block Diagram of model Jet Engine

Fabrication and Testing of Jet Engine

1.2 Theoretical Working of the Model

The fabricated ’Jet Engine’ works on the synchronizedcombination of three major components: compressor,combustion chamber and turbine. To achieve the highspeed exhaust, this combination of different compo-nents is done. The motive is to heat the incoming airto a high temperature so that it will tremendously ex-pand and create a high velocity exhaust. So we used acombustion chamber for this purpose. For the effectivecombustion we require the air entering the combustionchamber to be at moderately high temperature andpressure. So we used a series of compressor units thatconsist of rotor and stator blades. The starting processuses an electric motor to spin the main turbine shaft.When the blades connected to the shaft rotate, air isdriven into the engine because of the suction effect(i.e when the blades rotate the air molecules aroundgain velocity reducing the pressure in the immediateperiphery; air at comparatively high pressure region(outside) experiences a force to the low pressure region(inside) and gets sucked into the compressor unit). Thecompressor’s rotating blades add energy to the flowingair because of which the temperature and pressure ofthe air rises to a level suitable to sustain combustion.Stator blades are designed in such a way that theyprevent the back-flow of the air and facilitate in theforward flow.The compressor receives energy from the rotation ofthe turbine which is placed right after the combustionchamber. The compressor and turbine are attached tothe same shaft. The blades of turbine are designed intospecial air-foil shape. When the exploded air leavesthe combustion chamber and moves forward it exertsforce on the turbine and makes its blades rotate. Asthe blades of turbine receive energy from the air, thetemperature and pressure of the air drops. The out-let is made narrower, which makes the exhaust speedeven higher. The thrust is experienced in the directionopposite to the exhaust air.

2 Construction of Different Parts

Some components of the engine were constructedwhereas several household and laboratory equipmentfulfilled the remaining purpose.

2.1 Compressor and Turbine

They were made with the use of steel plates. Usinga round cutting machine, the plates were given thecircular shape. The blades of compressor and turbinewere cut into the shape with the help of angle grinderand bent manually into the airfoil shape.

2.2 Combustion Chamber

It was made from cylindrical iron box with properwelding. Holes were drilled on it for allowing the fuelsprayer and the heating coil to come in contact withthe moving air.

2.3 Power Supply

A converter was used at 12 V as the power supply tothe heating coil. It converted 230 V (RMS) AC frommainline to DC.

2.4 Heating Coil

A heating coil of Nichrome 80/20 was connected to the12 V power supply to support in the internal combus-tion. Initially, the plan was to use a spark plug but sinceit required an additional system to operate,heating coilwas economical.

2.5 Fuel Tank

Tank of a ’Household Kerosene Pressure Stove’ wasused as the fuel storage. Kerosene was used as fuel.

2.6 Spray Cap

A tube connected the fuel tank to the spray cap(nozzle).It sprayed the fuel into the combustion chamber which,when coming in contact with the compressed air (ofmoderate temperature and pressure) and the heatingcoil, produced the explosion inside the combustionchamber.

2.7 Concentric Shafts

Two concentric shafts were used. One was the mainshaft at which the rotors of the compressor unit andthe turbines were connected.This shaft could rotate.Another shaft was fixed at which the stators of thecompressor unit were connected. These shafts passedthrough the combustion chamber.

2.8 Starter Motor

Angle grinder connected to the main lines was used asthe starter motor. The cutting disc of the grinder wasremoved and the sprocket was connected at its axle.Another sprocket was connected to the main shaft ofthe engine. When these two sprockets were used inconjunction with a chain, then the one connected withthe axle of the grinder transmitted the torque to theone connected to the main shaft.This caused the mainshaft along with the connected blades to rotate andcause a suction effect.

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Fabrication and Testing of Jet Engine

2.9 Shield

Al the internal components were covered by doublelayered iron casing.

3 Practical Measurement ofThrust

The setup for practical measurement of thrust is asshown below: Mass of engine and its casing was mea-

Figure 2: Setup for thrust measurement

sured as 20kg.Coefficient of friction between engine casing andground was assumed to be 0.4.When the engine was powered, the reading on the dig-ital scale was 68.1 kg (≈ 680N)So,Practical Value of Thrust (Thpractical)

= Reading on scale+Frictional Force= (680+0.4*20*9.8) N= 760 N

4 Theoretical Analysis

The mathematics presented henceforth is destinedto help in the theoretical measurement of thrust.Theoretically determined value of the thrust can becompared with the practical value and the inferencecan be deduced.Assumptions:1. The exhaust velocity is significantly greater thanthe inlet velocity.2. The mass flow across the engine is consideredsteady.3. The fuel-air ratio is small and hence negligible.4. The pressure at inlet is equal to the pressure atoutlet.5. Frictional losses are neglected.6. Compression is isentropic through compressor andthe expansion is isentropic through both turbine andnozzle.

Different components of the jet engine like compressor,turbine and nozzle are analyzed as control volume.

These devices have single inlet and single outlet. Ifwe denote inlet by subscript i and outlet by subscripto then the steady state energy equation for controlvolume is given as

˙QCV − ˙WCV = m[(ho − hi) +1

2(v2o − v2i ) + g(zo − zi)]

(1)where˙QCV = rate of heat supplied to the control volume˙WCV = rate of work done by the control volume

m = mass flow rate of working substancehi and ho refer the specific enthalpy at inlet and outletrespectivelyvi and vo refer the velocity of working substance atinlet and outlet respectivelyzi and zo refer the height of inlet and outlet respectively

Figure 3: Schematic Diagram of Jet Engine

The general thrust equation for jet engine is givenas follows:

Th = ma[ue(1 + F )− u] +Ae(Pe − Pa) (2)

Here,Th= Thrustma= Mass of air entering the engine per secondue= Velocity of exhaust from nozzleF= Fuel-Air Ratiou= Velocity of air entering the engineAe= Cross Sectional Area of NozzlePe= Exit PressurePa= Inlet Pressure

As per the aforementioned assumptions,

Pe-Pa=01+F≈ 1ue-u=ue

So (1) is reduced to

Th = ma.ue (3)

We need to calculate the values of ma and ue to theo-retically determine the value of Thrust.

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Fabrication and Testing of Jet Engine

Working of Jet Engine is based on a Brayton Cyclewhich is shown below:

Figure 4: Temperature(T)-Entropy(S) diagram for BraytonCycle

Above figure suggests that pressure and tempera-ture increase from 1 to 2 but, under ideal conditions,entropy remains same. Then from 2 to 3, pressureremains same but both temperature and entropy in-crease. From 3 to 5, entropy remains same (underideal conditions), whereas temperature and pressuredecrease. Here, P2=P3 and P1=P5

∴P2

P1=P3

P5(4)

4.1 Thermodynamic Processes

4.1.1 Isentropic Compression (1-2)

Entropy at state 1 (S1) = Entropy at state 2 (S2)So,

T2 = T1(P2

P1)γ−1γ (5)

This shows that T2 > T1 as P2/P1 > 1 and γ=1.4 (forair)

4.1.2 Isobaric Combustion (2-3)

It is a constant pressure process. So the rate of heatentering the combustion chamber is given by

Qin = macp(T3 − T2) (6)

cp is the specific heat capacity at constant pressure.So,

T3 = T2 +Qin

macp(7)

Clearly T3 > T2

As fuel is the source of Qin, we must have

Qin = mF · Cvfuel (8)

mF is the fuel flow rate and Cvfuel is the calorificvalue of fuel.

4.1.3 Isentropic Expansion through Turbine (3-4)

Turbine is a steady state work application, so we musthave a net work done. There is a negligible difference inkinetic energy and potential energy between its inletand outlet. Therefore, for an adiabatic turbine, (1)reduces to

Wtu = ma(h3 − h4) = macp(T3 − T4) (9)

Turbine powers the compressor. In jet engine, workoutput of the turbine is equal to the work input to thecompressor and hence, the net work output is zero, i.e

Wtu = ˙Wcom

or, macp(T3 − T4) = macp(T2 − T1)

∴ T4 = T3 − (T2 − T1) (10)

4.1.4 Isentropic Expansion through Turbine andNozzle (3-5)

State 3 is the inlet of turbine, state 4 is the outlet ofturbine or inlet of nozzle and step 5 is the outlet ofnozzle.Entropy at state 3(S3)= Entropy at state 5(S5)So,

T5 = T3(P5

P3)γ−1γ (11)

From (4) we can say that

T5 = T3(1P2

P1

)γ−1γ (12)

Nozzle is a steady state flow application, so thenet work done must be zero. There is a negligibledifference in potential energy between inlet and outlet.Therefore, for an adiabatic nozzle, (1) reduces to

m[(h5 − h4) +1

2(v25 − v24)] = 0 (13)

Since v5�v4 and v5 is same as ue above equation re-duces to

ue =√2(h4 − h5) =

√2cp(T4 − T5) (14)

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Fabrication and Testing of Jet Engine

4.2 Experimental Data

Temperature of the surrounding(T1)= 13◦C=286KTemperature of exhaust air as measured bypyrometer(T5)= 377◦C=650KRate of fuel supply (mF ) was measured by trackingthe time at which the weighed amount of fuel kept inthe tank was exhausted.Since 0.5 litres(0.4kg) of kerosene in the tank gotnearly exhausted in 50 seconds

mf =0.4

0.5= 0.0008kg/s

4.3 Data Assumptions

cp=1000 J/kgK

γ=cp/cv=1.4

Calorific Value of Kerosene (Cvfuel)=43,000 KJ/kg

Pressure Ratio(P2/P1)=1.25

4.4 Calculations

From(5)T2 = T1(

P2

P1)γ−1γ

T2 = 286(1.25)1.4−11.4

∴T2 = 304.83K

From(12)T5 = T3(

1P2P1

)γ−1γ

650 = T3(1

1.25 )1.4−11.4

∴T3 = 692.79K

From(8)

Qin = mF · Cvfuel

Qin=0.008*43,000*1,000 J/s

∴ Qin=344,000 J/s

From(7)T3 = T2 +

˙Qinmacp

692.79 = 304.83 + 344,000ma∗1,000

∴ ma=0.89 kg/s

From (10)T4 = T3 − (T2 − T1)

T4 = 692.79− (304.83− 286)

∴ T4=673.96 K

From (14)

ue =√

2cp(T4 − T5)

ue =√2 ∗ 1, 000(673.96− 650)

∴ ue=218.9 m/s

Finally from (3)Th = maue

Th=0.89*218.9

∴ Th = 1948 N

5 Result and Discussion

Hence, the theoretical value of thrust was determinedas 1948 Newtons compared to 760 Newtons ofexperimental value. Though it is a huge difference,given the odds encountered during the project, thevalue is still acceptable.

Several factors were responsible for this huge differ-ence in theoretical and practical values.1. Working of compressor,turbine and nozzle werebased on isentropic analysis, and heat lost from thecombustion chamber was neglected. In fact, the systemwas continuously losing heat to the surrounding duringoperation.2. Hydraulic system wasn’t used. Therefore energy lossdue to friction could not be minimized.3. The assumption on pressure ratio was made merelyon the basis of personal judgement and cannot be jus-tified as such.4. Value of mass flow rate wasn’t exact.5. All the thermodynamic working were based on airstandard analysis.6. Though gross errors, measurement errors and blun-ders were carefully intended to minimize, they maystill be existential.

6 Conclusion

The development of a model Jet Engine was in factan impeccable way of learning about its components,working mechanism and engineering aspects. In ourmodel, there are still several rooms to further develop

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Fabrication and Testing of Jet Engine

and make it more effective and efficient. Use of propertechnology to correctly give airfoil shape to the blades,use of fuel of high octane number, addition of hydraulicsystem to reduce friction, among others, can give themodel a better working ability.The curiosity driven project was truly a satisfying intel-lectual pursuit. With larger funding, proper availabilityof resources and some workshop training, a completeworking scale jet engine can be made which can beused to power a self-constructed automobile. Makingan engine which could power an aeroplane demandshigher higher level of study and training.

7 References

M.C.Luitel. Fundamentals of Thermodynamics andHeat Transfer.Athrai Publication (P) Limited.R.Gurung, A. Kunwar, T.R Bajracharya.Fundamentalsof Engineering Thermodynamics and Heat Transfer. As-mita Books Publishers and Distributers (P) Limited.J.R. Howell, R.O. Buckius. Fundamentals of Engineer-ing Thermodynamics.McGraw Hill Publishers.Nicholas Cumpsty. Jet Propulsion (1997). CambridgeUniversity Press.Sir Frank Whittle, Hans Von Ohain. The History of theJet Engine.Klaus Hunecke. Jet Engines:Fundamentals of Theory,Design and Operation

8 Gallery

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Fabrication and Testing of Jet Engine

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