1 University of Limerick Design, Construction & Analysis of
a Pulsejet Engine Prepared by: Thomas Naughton 0542717 Under the
Supervision of: Dr. Patrick Frawley Final Year Report Submitted to
the University of Limerick, March, 2010 Aeronautical Engineering I
declare that this is my own work and that all contributions from
other persons have been appropriately identified and acknowledged i
Abstract This engineering reports the design construction and
analysis of a pulsejet engine to achieve static thrust. An engine
was designed using available theory. Following a delay due to
ignition system problems, the completed engine was tested
extensively in an attempt to achieve static thrust. A detailed
analysis of petal valve vibration was carried out while attempting
to get the engine to resonate. This included the experimental
verification of a mathematical model. Several tests were carried
out using different petal valve natural frequencies. The tests
resulted in the engine being capable of achieving sustained
resonance without the external supply of air for up to two minutes.
Petal valve failure was determined to be the cause of the short
running times. The operation of the engine was analysed using
available thermodynamic models but these models were determined to
be inaccurate for pulsejet cycle prediction. The pressure cycles
within the pulsejet engine were obtained experimentally using a
high-temperature pressure transducer. The resulting pressure/time
plots were compared to other plots which were obtained from
published literature. The plots were found to correlate well
together with peak pressures measured in three very different sized
engines being within 0.07 Bar of each other. ii Acknowledgements I
would like to thank my supervisor, Dr. Patrick Frawley, for his
support and guidance throughout the project. Without his support
this project would not exist. I would like to thank the technical
staff of the M&AE Department workshop for their help with
building the project. Especially Mr. Patrick ODonnell, Mr. Ken
Harris and Mr. Jim Caulfield. I would also like to thank the
technicians of the Aeronautical Laboratory, Mr. Jim Ryan, Mr. John
Cunningham and Mr. Adrian McEvoy for their help throughout testing.
I would also like to thank the technical staff of the Electronic
Engineering Department, Mr. John Bird and Mr. John Clifford for
their help with ignition system. Finally, I would like to thank my
family for their valuable support throughout the year. iii Table Of
Contents 1. Introduction 1 1.1. Brief History 1 1.2. Operation 3 2.
Objectives 5 3. Literature Review 6 3.1. Jet Design 7 3.2. Reed
Valve Design 7 3.3. Thermodynamics 10 4. Conceptual Design 11 4.1.
Valve System 12 4.2. Choice of Fuel 13 4.3. Fuel Delivery 14 4.4.
Ignition System 16 4.5. Test Stand 17 5. Theory & Design 18
5.1. Jet Design 18 5.2. Petal Valve Vibration Frequency 23 5.3.
Thermodynamic Analysis 28 5.4. Material Selection 30 6.
Construction 36 6.1. Jet Body 36 6.2. Intake Diffuser 37 6.3. Valve
Plates 38 6.4. Valve Retainer Plates 39 6.5. Petal Valve 40 6.6.
Fuel Injection Nozzles 41 6.7. Test Stand 43 7. Testing &
Troubleshooting 45 7.1. Ignition System Problems 45 7.2. Fuel
Mixing 48 7.3. Valve Frequency Ratio Tuning 49 7.4. Valve Frequency
High-Speed Camera Test 54 iv 7.5. Data Collection 55 8. Results 57
9. Discussion 59 9.1. Jet design 59 9.2. Petal Valve Vibration
Theory 59 9.3. Valve Life 59 9.4. Valve Response to Engine Forcing
Frequency 60 9.5. Ignition System 61 9.6. Thermodynamic Analysis 61
9.7. Pressure Cycle Visualisation 61 9.8. Exhaust Velocity
Determination 62 10. Conclusion 63 References 64 Appendices
Appendix A Engineering Drawings Appendix B Ignition Circuit Diagram
Appendix C Electro-chemical Etching Process Appendix D Turn-it-in
Originality Report Summary Appendix E Kistler Pressure Tranducer
Data Sheets (CD) Appendix F Excel Spreadsheets (CD) v List of
Figures Figure 1.1 Marconnet pulsating combustor [Reynst, 1961]
.............................................. 1 Figure 1.2 V-1
flying bomb with Argus AS-014 pulsejet engine [museumofflight.org,
2010]
.................................................................................................................................
2 Figure 1.3 Ignition Stage
...................................................................................................
3 Figure 1.4 Combustion/Power Stage
................................................................................
3 Figure 1.5 Intake Stage
.....................................................................................................
4 Figure 1.6 Compression/Re-ignition Stage
.......................................................................
4 Figure 3.1 Pressure-Time plot example for 50cm valved pulsejet
engine [Ordon, 2006] 6 Figure 3.2 Tharratt's mechanical valve which
was claimed to withstand 25 hrs operation at full thrust [Tharratt,
1965] ... 8 Figure 3.3 Cross-section of Standard Valve (left) and
Low-loss Modified Valve (right) [Bressman, 1946] . 9 Figure 4.1
Argus AS-014 Grid Valve Layout [FZG-76 Gerte-Handbuch, 1944] ... 11
Figure 4.2 Aprilia RS125 Reed Valve Assembly . 12 Figure 4.3 Petal
Valve ... 12 Figure 4.4 Normally Aspirated (left) and Injected
(right) Fuel Delivery [aardvark.co.nz, 2009] .. 14 Figure 4.5
Sketch of Valve Head Design . 16 Figure 4.6 Piezoelectric Oven
Igniter 17 Figure 4.7 CAD Model of Test Stand Used With Previous
Project .. 17 Figure 5.1 Intake Orifice Design ... 22 Figure 5.2
Final Jet Body Dimensions .. 23 Figure 5.3 Petal Valve Geometry ..
24 Figure 5.4 Simplified Petal Valve Model .. 25 Figure 5.5 Petal
Valve Geometry Split for Centroid Determination . 26 Figure 5.6
Points of Interest Within the Pulsejet Engine .. 28 Figure 5.7
Ideal pulsejet cycle [El-Sayed, 2008] .. 28 Figure 5.8 Pressure
Cycle plot in AS-014 pulsejet engine [Bressman, 1946] ... 31 Figure
5.9 Mechanical Properties of Grade 43A Steel at Elevated
Temperatures [Bailey, 2009] .. 33 vi Figure 6.1 Combustion Chamber
and Flange 37 Figure 6.2 Intake Diffuser . 38 Figure 6.3 Anodised
Valve Plates; Original (left) & Modified (right) .. 39 Figure
6.4 Valve Retainer Plate . 40 Figure 6.5 Electro-chemical Etching
Apparatus 41 Figure 6.6 Fuel Injection Nozzles; Internal (top)
& External (bottom) . 43 Figure 6.7 Engine Mounted on Test
Stand 44 Figure 7.1 Final Ignition Circuit 47 Figure 7.2 Uneven
Burning in the Combustion Chamber (left) & Burning With New
Nozzle Fitted (right) .. 48 Figure 7.3 Valve Motion Sign Convention
(left) & Simplified Valve Motion Plot (right) .51 Figure 7.4
0.010" Deformed Shim Steel Valve Following Engine Run .52 Figure
7.5 Impact and Fatigue Damage on a 0.006" Spring Steel Petal Valve
. 53 Figure 7.6 High-Speed Camera Experiment Setup ... 55 Figure
7.7 Equipment Set Up For Pressure Data Collection . 56 Figure 8.1
55mm Position Pressure/Time Plot .. 57 Figure 8.2 1075mm Position
Pressure/Time Plot .. 58 vii Nomenclature Symbol Description Units
A Area m2 A Mean Cross Sectional Area m2 Ac Exhaust Area m2 Av
Valve Area m2 CNC Computer Numerically Controlled - Cpc Specific
Heat Capacity at Constant Pressure (cold) J/kg/K Cph Specific Heat
Capacity at Constant Pressure (hot) J/kg/K Ch Specific Heat
Capacity at Constant Volume (hot) J/kg/K Diameter m comb Combustion
Chamber Diameter m c Exhaust Diameter m E Youngs Modulus of
Elasticity GPa F Thrust N FEM Finite Element Method - I Second
Moment of Area m4 ID Internal Diameter m I Length m Icomb
Combustion Chamber Length m Icng Engine Length m H Mach Number -
NACA National Advisory Committee for Aeronautics - OD Outside
Diameter m P Static Pressure Pa P0 Stagnation Pressure Pa Energy
Density of Fuel J/kg R Universal Gas Constant J/kg/K I Static
Temperature K I0 Stagnation Temperature K u Strain Energy J uc
Exhaust Velocity m/s viii un Inlet Velocity m/s u] Jet Velocity m/s
I Volume m3 Icng Engine Volume m3 b Breadth of Beam m c0 Local
Speed of Sound in Air m/s Frequency Hz Heat Added Per Unit Mass
J/kg k Spring Stiffness N/m m Mass of Lumped Mass kg mbcum Mass of
Beam kg m u Mass Air Flow kg/s m] Mass Fuel Flow kg/s r Radius m r
Internal Radius m ro External Radius m t Thickness m y Vertical
Distance to Centroid m yc Specific Heat Ratio (cold) - yh Specific
Heat Ratio (hot) - pb Burner Efficiency - pd Diffuser Efficiency -
p Density kg/m3 o Normal Stress Pa oh Hoop Stress Pa 1. IntroA
pulsejeexhaust gsuggests, constant combustiocontinuouthese
engipollutantsA system from exitiduring theonly one mtogether in
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originatepulsating coalveless desuntil the 19tial of the propulsion
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on. Becautremely effn using hydrvalves are front of the ase. The
put but its ope y ed in the eaombustor wsigns thoughFigure 1.1 M30s
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pulsatingutilised iof having use of the ficient combrocarbon fuused
at the engine and ulsejet is exeration is coarly 1900s iwithout
valvh it never seMarconnet pulPaul Schmidgine, that thd. Schmidt
system at tust jet enginnd of the g combustin ramjets a higher
deflagratingbustors, prouels. (El-Safront of theto allow
frextremely simmplex and rin France wves, figureerved as a plsating
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wengine. Pulion rather and gas thermodyng nature ofoducing praayed,
2008) e engine to esh air charmple mecharelies on mawhen George1.1.
This ractical soutor [Reynst, 19ed a large amential of puy patents
fo prevent eworks by eulsejets, as than the cturbines. Inamic
efficf pulsejet cactically no prevent exrges to enteranically, as
any processes Marconnwas the purce of prop961] mount of reulsating
comfor various exhaust gase1ejecting hottheir
namecontinuous,Intermittentciency thancombustion,o hazardoushaust
gasesr the engineit containsses workingnet patentedrecursor
ofulsion. esearch intombustion injets whiches escaping t e , t n ,
s s e s g d f o n h g 2 through the inlet. At the same time the
Argus engine company were developing valveless pulsejet engines.
Schmidt joined the Argus company and, in 1939, together they
successfully developed a valved pulsejet design which was later
used to power the unmanned V-1 flying bomb, (figure 1.2). Figure
1.2 V-1 flying bomb with Argus AS-014 pulsejet engine
[museumofflight.org, 2010] Following the end of World War II, many
of the Argus engines were captured by the US and Russia. The
captured engines were reverse engineered and analysed extensively
in an attempt to create a viable propulsive device for use on
aircraft. The majority of this testing has been documented and
published by NACA. The main concern regarding the pulsejet was the
operating life of the reed valves in the front of the engine.
Efforts were made to increase the operating life but these were
quickly overtaken by the development of the turbojet engine. The
turbo-jet engine offered increased reliability and fuel efficiency
and the pulsejet was largely forgotten as a source of aircraft
propulsion. Today, small pulsejet engines still find use as radio
control model powerplants. This is mainly due to their simplicity
and low cost when compared to a model turbojet of the same size.
Pulsejets have also paved the way for Pulse Detonation Engine (PDE)
technology which is a major research interest among aircraft
manufacturers as a high-tech, fuel efficient form of propulsion. 3
1.2. Operation The operation of a pulsejet engine is similar to
that of a modern reciprocating engine. It operates in defined
cycles which draw in air/fuel mixture, compress it, ignite it and
exhaust it in stages before the cycle repeats itself. The following
figures 1.3 1.6 are used to explain the cycles more clearly. The
first stage begins with the ignition of a fuel/air charge, figure
1.3. Ignition is provided by a spark plug during start-up and by
residual combustion during normal operation. Figure 1.3 Ignition
Stage The ignited fuel/air mix expands rapidly, increasing the
pressure within the engine to greater than atmospheric pressure.
This forces the spring valves shut which then forces the expanding
gases to exit rapidly through the tailpipe producing thrust,
(figure 1.4). Figure 1.4 Combustion/Power Stage Due to the Kadenacy
effect, a partial vacuum is formed behind the rapidly escaping
exhaust gases. The pressure at the front of the engine is now lower
than atmospheric pressure. The pressure differential across the
reed valves causes them to open and draws fresh fuel/air mixture
into the engine, (figure 1.5). At the same time, the pressure
difference within the engine slows down the momentum of the small
proportion of exhaust gases which have not yet exited the tailpipe
and draws them back up towards the front of the engine. 4 Figure
1.5 Intake Stage The momentum of the small piston of burning
exhaust gases which has been sucked back into the engine helps to
compress the fresh fuel/air charge and ignite it (figure 1.6) and
the cycle repeats itself. The cycle repeats itself 40-250 times per
second depending on the size and length of the engine. Figure 1.6
Compression/Re-ignition Stage The reed valves are the only moving
part of the engine. The operating life of these valves can vary
from 1-2 minutes up to several hours depending on factors such as
valve material, valve construction and valve natural frequency.
These factors are discussed in more detail in section 3.2. Partial
Vacuum5 2. Objectives The main aims of the project were as follows:
To design and build a working pulsejet engine to provide static
thrust To analyse the operation of the engine both theoretically
and experimentally so as to gain a better understanding of the
principles behind the operation of these engines. To investigate
the pressure cycles within the engine during operation. 3. Liter
The vast between 1pulsejets fperiod wer Much resepart of Pustill
ongoithis projecIn recent Universityconductedforms of ithese
theswhich wapressure cfrom a preobtained f rature Revmajority o1944
and 1for aircraft re studied aearch on puulse Detonaing, most ofct.
years, somy, USA, und tests on dinstrumentases were maas to be
buicycle plots wessure transfrom the 50cFigure 3.1 Prview of
available1970. This propulsionas part of theulsating comation
Enginf this informe work hasnder the sudifferent sization to
collainly valvelilt for this within the ensducer test ocm valved
pressure-Time pe researchis mainly n after this e research
fmbustion hane (PDE) demation is cls been carriupervision ozed
pulsejetlect data anless designsproject. Hongines whicon the
comppulsejet is splot example forregarding due to thetime. Manyfor
this projas been carevelopmentassified andied out by of Dr Willts
ranging fnd analyse ts and were owever, thech could be pleted
enginhown in figr 50cm valved ppulsejet ene lack of cy research
pect. rried out sint. However,d could not students ofliam L. Rofrom
8cm ttheir operatconsiderabey did provcompared wne. One of tgure
3.1. pulsejet enginengines was commercial papers fromnce the earl,
since the be accessedf North Caroberts. Theto 50cm ustion. The
jebly smaller vide good ewith the plothe pressure e [Ordon,
2006]6conductedinterest inm that timely 1980s asresearch isd for
use inrolina Statese studentsing variousets tested inthat the
jetxamples ofots obtainede-time plots] 6 d n e s s n e s s n t f d
s 7 An additional source of information for the project was a final
year report submitted to the University of Limerick by David Curran
in 2004. This report details the construction and testing of a
valved pulsejet of similar design to the engine constructed during
the course of this project. Although the engine which was
constructed in 2004 was not able to self sustain without an
external source of air, the recommendations for future work given
in the report were considered and referred to during the course of
the design work. 3.1. Jet design The most comprehensive paper found
on pulsejet design was C.E. Tharratts The Propulsive Duct. These
were published as a series of articles in a journal entitled
Aircraft Engineering and Aerospace Technology between 1965 and 1966
while the author was involved in research for the Chrysler Space
Division, New Orleans. In these articles, Tharratt attempted to
produce a comprehensible theoretical approach to pulsejet design
and thermodynamic analysis. The first article proposed three basic
equations which one could use to successfully determine the basic
dimensions of a pulsejet tube as well as a theoretical analysis of
the pulsejet thermodynamic cycle. Several reports were published by
Cornell Aeronautical Laboratory in the late 1940s as part of the
Project Squid experiment carried out for the United States Navy.
These reports were studied but much of their content was decided to
be unnecessarily detailed or irrelevant for the purposes of this
project. Some content regarding reed valve design was used and is
detailed in section 3.2. 3.2. Reed Valve Design Tharratts second
article in The Propulsive Duct provides a brief overview of valve
design. Mechanical spring valves such as those used in the Argus
engine and aerodynamic valves used in all valveless designs are
discussed in detail. Tharratt has claimed that he developed a
mechanical spring valve to withstand 25hr. at full thrust,
including several continuous runs of 7hr. duration. (Figure 3.2)
This operating life is much higher than those experienced by NACA
during their tests on captured Argus 8 engines. Tharratt provides
an image of this valve in the article but does not include further
details regarding its design or operating characteristics. Figure
3.2 Tharratt's mechanical valve which was claimed to withstand 25
hrs operation at full thrust [Tharratt, 1965] A report published by
Cornell Aeronautical Laboratory in 1947 entitled 4x6 Pulsejet
Engine Project contains a section in which reed valve material is
discussed. The report tested the performance of a pulsejet engine
using reed valves of varying thickness and materials and discussed
the results briefly. According to this report, annealed spring
steel or soft steel reeds are superior to tempered and polished
spring steel reeds for longevity. The tests also showed that
heavier reeds, in general, showed a longer life than thinner reeds
although operation of the jet was more difficult to start and
resonance of the jet was more easily upset when using heavy reeds
than with the lighter reeds. These observations were taken into
account when choosing the appropriate valve material for the jet.
Two wartime reports published by NACA bear particular relevance to
reed valve design. The first of these reports, written by
Manganiello, Valerino & Breisch and published in 1945, attempts
to solve the issue of poor reed valve operating life which had been
observed during earlier sea-level performance tests of a 22-inch
pulsejet. The average valve life was reported to be 30 minutes
before the valve tips were damaged by the repeated impact with the
valve grid and a severe loss of thrust was observed as a result.
The authors attempted to extend the life of the valves on the same
engine by coating the valve grid with a thin layer of neoprene. The
reasoning behind the neoprene coating was to cushionon the
valsignificantvalves wabroken ofvalves wegrid showgrid. The due to
the The seconlow-loss auniform 0attempt tothick sprinthe two
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was consnd Low-loss Mond thereby d that the nf operation nration,
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0-330mph. Tthrust at lotly lower thated in only 1946) The iderably
shodified Valve (rreduce impneoprene cono deterioravalve was exure,
and . The modo the unmodght reductiooating. stigates thehis report,
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[Bressm9pact stressesoating had aation of thecompletelythree
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1946] 9 s a e y r e e r a d n f s , e d n o e 10 3.3.
Thermodynamics The pulsating nature of combustion in a pulsejet
engine makes it very complicated to analyse as the processes are
time dependant. Due to the lack of interest in pulsejet engines as
source of aircraft propulsion, not much research has been done to
accurately predict the processes within them. Therefore, very
little thermodynamic analysis of a pulsejet engine can be found in
modern literature. The most modern analysis which was found was
published as a small section in Ahmed F. El-Sayeds Aircraft
Propulsion and Gas Turbine Engines. The analysis assumes that
combustion takes place at a constant volume process and that the
exhaust gases expand isentropically in the tailpipe. In reality,
combustion in a pulsejet engine is neither a constant-volume nor a
constant-pressure process and, since most pulsejets glow red-hot
during operation, the expansion of exhaust gases cannot be
accurately modelled as isentropic. However, the analysis could
provide an estimate of the pressure and temperature conditions
within the engine early on in the design process. Another source of
theoretical analysis was found in Jet Propulsion, a reference text
prepared by the Guggenheim Aeronautical Laboratory for the Air
Technical Service Command and published in 1946. This text also
makes similar assumptions about the behaviour of gases within the
jet as those made in El-Sayeds text. These assumptions are bound to
result in inaccuracies in calculations but were nevertheless used
early on in the design process to provide a rough estimate of
operating conditions in the engine. This text uses a different form
of equation that El-Sayed to model the heat addition during
combustion. Both analyses would be carried out and their accuracy
determined from experimental results. 11 4. Conceptual Design This
section outlines the process used to determine the final layout of
the project jet, the choice of fuel, ignition system and test
stand. 4.1. Valve System The two most common valve systems found on
existing pulsejet engines are grid valves and petal valves. Grid
valves are the most common type of valve used for larger engines
producing more than 100N thrust. This is because they provide the
least amount of intake flow resistance and they provide the most
flexible layout on these bigger engines. Bigger engines require a
larger intake area and it is much easier and more reliable to use
several grid type valve assemblies to make up the required intake
area than to design one extremely large petal valve. The reed
valves in a grid layout are usually single valves for each intake
orifice or sometimes grouped together so that one valve covers
three or four orifices. This method of assembly is much more
practical for engine maintenance. This way, if one valve fails, it
is only necessary to replace that one valve or, at the most, a
group of three or four. If one valve fails in a petal system, the
entire set must be replaced. The main disadvantage with grid valve
systems is their complexity. The grid valve systems used in large
engines such as the Argus AS-014 are incredibly complex. (Figure
4.1) Figure 4.1 Argus AS-014 Grid Valve Layout [FZG-76
Gerte-Handbuch, 1944] 12 However, the grid valve systems used on
smaller engines look more like the reed valve assemblies used at
the crankcase inlet in modern two stroke engines. An example of the
reed valve assembly used in an Aprilia RS125 engine is shown in
figure 4.2. Figure 4.2 Aprilia RS125 Reed Valve Assembly Petal
valve systems, on the other hand, are much simpler to design and
construct. They consist of a flat plate which covers the front of
the engine in which a radial pattern of holes is machined. A single
spring steel valve shaped like a flower (figure 4.3) covers all the
holes. A circular, curved, steel disk called the valve retainer is
bolted to the valve plate behind the petal valve to limit the
distance which the valves can flex when open. Figure 4.3 Petal
Valve 13 The biggest disadvantage of the petal valve system is its
inefficiency. Because the valve is placed perpendicular to the
incoming airflow, they produce a lot of resistance. This limits
their use to smaller pulsejets. Also, if one petal of the valve
fails, the entire valve must be replaced. Considering both options,
it was chosen to use a petal valve system for the project jet. This
type of system would allow for quicker manufacturing and would also
help incorporate a central fuel delivery point as outlined in
section 4.3. 4.2. Choice of Fuel One of the major advantages of
pulsejet engines is their ability to run on most commercially
available fuels. Many small pulsejets used for model aircraft
propulsion are run on liquid fuels such as methanol or
nitro-methane. These fuels may be attractive for this purpose as
they have a high energy density and good flammability range in air.
However, nitro-methane is too expensive to be used in a larger
scale engine as the fuel consumption would make it costly to run.
Methanol has a very high flammability range but also has the
disadvantage of burning with a clear flame. This could prove to be
a safety hazard in a laboratory environment. Diesel or kerosene
fuels are also a good alternative. These fuels are cheap compared
to other alternatives but they do cause problems during cold
starting. These fuels need to be vaporised prior to injection which
requires the use of a heat exchanger coil. Engines using these
fuels are usually started on a more flammable fuel and then
switched over to diesel or kerosene when the engine has reached
operating temperature. The use of these fuels has the added
complexity of having a secondary fuel system for the starting fuel.
Ordinary low-octane petrol has been used with some success in
pulsejet engines. Petrol is easily ignited using a spark plug which
eliminates the need for a secondary fuel system. It is also cheap
and readily available. However it does need to be vaporised before
combustion which makes it unsuitable for direct injection into the
combustion chamber unless a heat exchanger coil is used. 14 The
other disadvantage with using liquid fuels is the need for a fuel
pump to provide the correct fuel pressure. This adds complexity to
the project. The use of a fuel pump can be avoided if the engine is
designed to be naturally aspirated. This method has the drawback of
not being throttle-able as outlined in section 4.3. The problem
with fuel pressure can be overcome if a gaseous fuel such as
propane or butane is used. The gas would be fed from the cylinder
already under pressure and the fuel flow could be regulated using a
gas regulator at the cylinder exit. The use of a gaseous fuel would
also facilitate direct injection into the combustion chamber
without the need for a vaporiser. Although the gas cylinder is
bulkier and heavier than a similar liquid fuel tank, this was
considered to be unimportant as the engine was to be a purely
static engine. It was decided to use propane as a fuel due to its
high energy density (50 MJ/kg), ease of cold starting by spark
plug, the elimination of a fuel pump system and relatively low
cost. 4.3. Fuel Delivery Fuel delivery to the combustion chamber
can either be normally aspirated or injected. (Figure 4.4) Figure
4.4 Normally Aspirated (left) and Injected (right) Fuel Delivery
[aardvark.co.nz, 2009] A normally aspirated engine operates on much
the same principle as the carburettor in a car or motorcycle. The
atomiser is placed in a venturi in the intake and fuel is drawn 15
from it as high-speed air passes through the venturi. Aspirated
engines are simple in construction but aspiration does pose some
restrictions. Fuel flow is very dependent on the vertical placement
of the fuel tank in relation to the atomiser. Aspirated engines are
not throttle-able as there is no method for varying fuel flow. The
venturi must be properly designed to produce the required pressure
difference across the fuel system so fuel can flow. Fuel injection
solves the problems associated with aspiration. Although an
injection nozzle is more complex to machine, it was decided to
inject the fuel in the project engine. Injection would allow
throttling of the engine and would also allow higher flexibility of
fuel use. It was decided to inject the fuel directly into the
combustion chamber behind the valve retainer plate. A second
retainer plate would be placed on the combustion chamber side of
the injection nozzle. There were several reasons behind choosing
this setup. The different valve retainer plates could be machined
with different radii of curvature. The effect of different retainer
plates on valve life could then be investigated simply by swapping
them around. The channel created between the two retainer plates
would guide the fuel out towards the point where the incoming air
is moving over the tips of the retainer plates at a higher velocity
and this would aid fuel mixing. The second retainer plate would
create a secondary barrier between the valves and the hot
combustion gases and would help keep the valves cooler during
operation. The heat absorbed by the second retainer plate during
combustion would help to preheat the fuel as it passed between the
plates and this would help increase the efficiency of combustion.
The fuel injector nozzle would also double as the central bolt to
clamp the entire valve head assembly together. A sketch of the
assembly is shown in figure 4.5. 16 Figure 4.5 Sketch of Valve Head
Design 4.4. Ignition System The ignition system in a pulsejet
engine is only required for starting to ignite the first charge of
fuel/air mixture. After the jet has achieved successful ignition
and is running correctly, the ignition system is no longer needed.
The easiest method of ignition is through a spark plug situated in
the wall of the combustion chamber. Normally a spark is created
across the plug gap through the excitation of an induction coil and
then switching off the current to the main coil which causes the
magnetic field in the coil to collapse rapidly. The change in
magnetic field around a secondary coil induces a very high voltage
across the coil which then jumps the plug gap in the form of a
spark. However, instead of using an automotive coil to produce the
spark, it was decided to use a piezoelectric igniter from a gas
oven. (Figure 4.6) This was an extremely simple method. It required
a push-button to be pressed repeatedly until the engine fired but
it was lightweight, cheap and it did not require any additional
equipment like a battery and separate switch. Air Fuel Valve
Retainer Plate 17 Figure 4.6 Piezoelectric Oven Igniter 4.5. Test
Stand The test stand needed to fulfil the following requirements:
Securely support the jet during tests Allow for the measurement of
thrust Support all necessary ancillary equipment such as ignition
system and measuring devices. Be mobile enough to allow easy setup
for tests. Since the project engine was to be approximately the
same size as the jet which a previous student built, it was decided
to use the same test stand design for this project also. A CAD
drawing of the test stand is shown in figure 4.7. Figure 4.7 CAD
Model of Test Stand Used With Previous Project Push Button Earth
Connection Electrode 18 5. Theory & Design This section details
the theory used in the design, analysis and troubleshooting of the
project engine. Due to safety concerns, it was decided that the
engine in this project should produce no more than 90N thrust. This
also corresponded with the thrust limitations associated with a
petal valve design. 5.1. Jet Design The theory governing the design
of the pulsejet engine in this project was adapted from C.E.
Tharratts The Propulsive Duct. Tharratt developed his equations in
the 1960s with the imperial system of units in mind. The equations
were modified to work with SI units before being used to design the
project engine. 5.1.1. Tailpipe Tharratt proposed the following
basic equation governed the basic design of the pulsejet engine
duct: II, = u.uuS16F 5.1Where: V = Engine Volume (cu. ft.) L =
Effective acoustic length of engine (ft.) F = Thrust (lbf)
Manipulating this equation to take SI unit inputs produces equation
5.2: II, = u.uuuu66F 5.2 The simplest form of pulsejet is simply a
straight tube of constant cross-section. It was decided to use this
as a starting point. 19 Since for a straight pipe, I = AI If this
relationship is substituted into equation 5.2 and simplified, then
a direct relationship between thrust and cross-sectional area is
established. This cross-section area will be used as the tailpipe
area and will be referred to as Ae (exhaust area) from here on. Ac
= u.uuuu66F 5.3 It was decided to make use of standard seamless
pipe sizes available on the market to make the tailpipe. This would
reduce the complexity of having a long welded seam running the
entire length of the pipe. Inputting the maximum desired thrust of
90N into equation 5.3 returned a tailpipe diameter of 87mm. The
next smallest seamless mild steel pipe available on the market was
3 Sch40 pipe. This gave an internal diameter of 78mm. Using 3 Sch40
pipe as the basis for the design, the expected thrust was
calculated by substituting the area of the pipe back into equation
5.3. The expected thrust returned was 72.6N. The total length of a
pulsejet engine is what determines the frequency at which it
operates according to another of Tharratts basic equations, 5.4. =
c04I 5.4 There is still quite a bit of debate regarding the correct
operating frequency for a given size of jet. Therefore it was
decided to look at some existing designs, their operating
frequencies and their length to diameter (L/D) ratios to determine
a suitable length for this engine. The known properties of some
existing pulsejet designs are shown in table 5.1. 20 Table 5.1
Known Pulsejet Properties Engine Static Thrust Output (N) Frequency
(Hz) L/D ratio Argus AS-014 2,200 46 9.6 Dynajet 20 260 15 By
assessing the above data and considering the intended thrust output
of the project engine, it was determined that an L/D ratio of 14
would be a good starting point for the engine. This would allow
trimming of the tailpipe later on during testing if needed. Using
this ratio and the internal diameter of 3 Sch40 pipe, a total
engine length of 1.1m was calculated. This length was substituted
into equation 5.4. The operating temperature of the engine was
estimated to be 1000K approx. = yRI4I 1.S6 287 1uuu4 1.1 142Ez This
frequency falls within the expected range for a pulsejet of this
size. 5.1.2. Valve Plate Another important relationship which
Tharratt developed was that which related the intake valve area to
the exhaust area. He proposed that: A = u.2SAc 5.5This equation
does not take into account the inefficiencies associated with
different valve layouts. It is generally assumed that a petal valve
layout has an efficiency of 0.5. Therefore the equation must be
modified to allow for this. To make calculation simpler, equation
5.5 can be modified to allow for the efficiency factor and to take
an input of tailpipe diameter rather than area: A pctuI = u.11Snc2
5.6 A pctuI = 22uSmm2 21 A combustion chamber is not necessary in a
pulsejet engine. However, due to the layout of a petal valve
system, it is usually necessary to include a wider section which
resembles a combustion chamber at the front of the tailpipe. This
wider section will be referred to as a combustion chamber for
convenience. The valve plate layout must be designed before the
dimensions of this section can be determined. The valve plate was
designed in ProEngineer by observing the following criteria and
attempting meet the required valve area while keeping the outer
diameter of the orifices as low as possible. It was desired to keep
the number of petals in the valve as low as possible so that the
probability of failure due to fatigue could be kept low. It was
also observed by studying previous designs that the maximum
diameter intake hole was 12mm to reduce deformation of the valve
during the positive pressure cycle of the engine. To try and solve
this, the valve orifices had to have a minimum distance of 12mm in
one direction. The valve plate needed to allow for 2mm valve
overlap minimum around each orifice. Due to machining constraints,
the smallest radius included in the design could be no smaller than
3mm. The final design, shown in figure 5.1, had 10 intake orifices
and the outer diameter of the intake orifice ring was 90mm. 5.1 To
prevenfrom beinwhich wouand the codetermine During histhe intake
was decidof fresh fu 1.3. Combunt the flow ong overly reuld allow
foombustion cd for the cos research inphase, a puded to makeuel/air
mixtuFustion Chamof air throuestricted, thor an area ochamber.
Uombustion cnto pulsejetulsejet drawe the combuure. Figure 5.1
Intakmber ugh the intakhe combustiof twice the Using this
guchamber. t design in tws in 15%-2ustion chamIComb =ke Orifice
Desike holes andion chambevalve area tuideline, anthe 1930s, P20%
of its vmber big eno= u.2Icng ign d over the tr was givento exist
betwinternal diaPaul Schmidolume in frough to acc tips of the vn an
internween the vaameter of 1dt observed resh fuel/aircommodate
22valve petalsal diameteralve orifices117mm wasthat duringr
mixture. Itthis charge2 s r s s g t e Using equangle wasmachining
The final which was 5.2. Pe During finvalves to ta pulsejet is the
fvalves. Tliterature. In order tmodel neeis essentiauation 5.7, s
included g constraintdesign for ts used to boetal Valve Vnal
testing the pressureengine. Thefrequency oThis is a det o
determineeded to be fally a cantilea combustias a reduces. the jet
bodyolt on the vaFiguVibration Fof the engie oscillatione response
iof the enginail of desige the naturafound to simever beam oIcomb
=ion chambeer to tailpipy is shown ialve head asure 5.2 Final
JFrequencyine, it becans within this a functionne and ngn which
seal frequencmplify the cof varying cu.2c2Icngcomb2er length ope
diameterin figure 5.2ssembly andJet Body Dimename apparee engine is
n of the drivis the naturems to havy of vibraticomplex
shacross-sectiong f 97mm wr. This ang2. This drawd diffuser
tonsions nt that the critical to thving frequenral frequence been
largion of the pape of the vn. (Figure 5was determingle was
detewing includo the front oresponse ohe correct oency ratio, ncy
of vibragely overloopetal valvesvalves. The 5.3) 23 5.7ned. A
30ermined bydes a flangeof the jet. of the petaloperation of/n,
whereation of theoked in pasts a suitablepetal valve3 7 y e l f e e
t e e 24 Figure 5.3 Petal Valve Geometry According to Singiresu S.
Raos text, Mechanical Vibrations, kcq = SEII3 Where I is the second
moment of area and for a simple beam: I = bt312 Therefore: kcq =
Ebt34I3 5.8 If all variables in equation 5.8 are kept constant and
only b is allowed to change, then it can be shown that, as b
increases, so does k. Therefore k increases with distance from the
root of the petal valve. If the petal were to be deflected through
a small distance, then the majority of bending would occur at the
root where k has its smallest value. To simplify the problem of
varying k, the petal valve was modelled as a cantilever beam of
constant cross-section equal to that at the root of the petal, with
a lumped mass at the end which would represent the extra mass of
the side lobes of the valve. The value of that extra mass was found
by: Finding the mass of the side lobes Finding the centre of
gravity of that mass Calculating the moments produced by this mass
about the root 25 Then calculating an equivalent mass which would
produce the same moment about the root if it were placed at the tip
of the valve. The result is a constant-section beam with a lumped
mass at its end as shown in figure 5.4 for which the natural
frequency of vibration can be easily calculated. Figure 5.4
Simplified Petal Valve Model The equivalent mass of this model can
be found using Raos equation: mcq = m +u.2Smbcum 5.9 The natural
frequency of vibration can now easily be calculated using = _km
5.10Where k is obtained using equation 5.8 and m is calculated from
equation 5.9. In order to find the position of the centroid of the
side lobes, a simple 2D CAD program called QCad was used. The
geometry of the valve was drawn and then split into the main beam
and up to five other simple shapes as shown in figure 5.5. The
areas into an Exfollowing The equiOr: The mass Figand centroxcel
spreadsequation: ivalent massof the beamgure 5.5 Petal Vids of
thesesheet to calcs to be place m was calc Valve Geometrye shapes
weculate the py = ed at the enm = mm = ptAculated usinmbcumy Split
for Centere found usposition of tAyA nd of the beamIobcsyI AIobcsyI
ng: m = ptbI troid Determinasing QCad their combiam was then ation
and these vined centroin calculated26values inputid using thed
using: 5.115.126 t e 227 In order to tune the petal valves to the
required natural frequency of vibration, it was necessary to
investigate the relationship between natural frequency , spring
stiffness k, length L and material thickness t. By combining
equations 5.8 5.12 and simplifying, the following equation 5.13 was
obtained. n = _ Ebt24pI2(AIobcsy +u.2SbI2) 5.13By varying t and L
in equations 5.8 and 5.13, it can be shown that an increase in t
will increase the natural frequency of vibration but will also
increase the static stiffness by a larger factor. n t but k t3 If L
is reduced instead, there is a smaller increase in static stiffness
for the same increase in natural frequency. n _1I4 but k 1I3
Therefore it is more desirable to tune the valve frequency by
reducing the effective length of the valve than by increasing the
thickness. Keeping the static stiffness as low as possible is also
necessary to allow the engine to produce static thrust as the
valves do not have the benefit of ram-air pressure to help open
them. 5.3. Thermodynamic Analysis The thermodynamic cycle of the
pulsejet engine was analysed using theory from two different
sources. The first is Ahmed El-Sayeds text; Aircraft Propulsion and
Gas Turbine Engines and the second is the Guggenheim Aeronautical
Laboratorys reference text Jet Propulsion. Both methods are
examined separately. Both methods refer to conditions within the
engine at certain points. These points are illustrated in the
following diagram, figure 5.6. 28 Figure 5.6 Points of Interest
Within the Pulsejet Engine 5.3.1. El-Sayed (2008) Ahmed El-Sayed
idealises the process within the pulsejet engine as illustrated in
the T-S diagram in figure 5.7 below. Figure 5.7 Ideal pulsejet
cycle [El-Sayed, 2008] Due to ram effect in flight and
inefficiencies due to diffuser shape and valve system, both the
pressure and temperature at point 2 can be calculated as follows:
P02 = Pu _1 +pd yc 12 H2] ycyc-1 5.14 I02 = I01 = I0u= Iu_1 +yc 12
H2] 5.15a 1 2 3 4 29 Where d is the efficiency of the diffuser and
valve system and M is the flight Mach number. It is assumed that
combustion takes place at a nearly constant volume. Therefore: P03
= P02_I03I02] 5.16T03 is determined from the energy balance in the
combustion chamber, equation 5.17. (mu + m])CphI03 = m uCphI02 +
pbm]R 5.17Where Cp is the specific heat capacity at constant
pressure and b is the burner efficiency. The exhaust gases expand
out the tailpipe to ambient pressure. This process is assumed to be
isentropic and the temperature of the exhaust gases is calculated
from the following relationship, equation 5.18: _I03I4 ] = _P03Pu
]yh-1yh 5.18The exhaust velocity and thrust are then calculated
from equations 5.19 and 5.20 respectively. Ic = _2CphI03_1
_PuP03]yh-1yh_ 5.19 F = m u|(1 + )Ic I] 5.205.3.2. Guggenheim
Aeronautical Laboratory (1946) The reference text, Jet Propulsion,
makes precisely the same assumptions as El-Sayed up until the
combustion stage. Although the assumptions about constant volume
combustion and isentropic expansion remain the same, this text
proposes different relationships to model the process of heat
during combustion. This is described in equation 5.21 below. 30 = R
= Ch(I03 I02) = 1yhCphI03_1 I02I03] 5.21 5.4. Material Selection
5.4.1. Jet Body In order to choose a suitable material for the jet
body, it was necessary to first determine the maximum pressures
which could be expected within the engine during operation. These
pressures were obtained from the preliminary thermodynamic analysis
in section 5.3. Also considered were pressure cycle plots obtained
from existing pulsejet engines. The analysis in section 5.3
provided a reasonable estimate for the stagnation pressure in the
combustion chamber as P03 = 4.1 Bar Adding in a factor of safety of
approximately 2, the jet body should be capable of withstanding
maximum internal pressures of up to 8 Bar. Comparing to existing
pulsejet analysis as shown in figure 3.1 and fig 5.8, 8 Bar
pressure appears to provide a huge factor of safety. Figure 3.1
shows a peak pressure of 27 psi or 1.86 Bar and figure 5.8 shows a
peak pressure of 28 psi or 1.93 Bar. Considering that both jets are
at different ends of the thrust scale with the project jet lying
between them, it seemed reasonable to assume that the actual
pressures experienced in the jet would be very similar. Therefore,
by choosing the material to withstand 8 Bar pressure, it could be
ensured that the safety concerns of the university could be
comfortably met. Since the not a majosteel or staMild steel Lo Be
Be Suma45ThDue to thwelded-serupturing material oapplicationA
stress athick-wall78.1mm aFigure project jet wor concern. ainless
steell has more aower cost anetter machinetter weld-abuperior
heatade from st seconds (his problem he combinaeam structurunder
operaof choice ans. analysis of led cylinderand 5.49mm5.8 Pressure
Cywas to be aAmateur-bul. appeal as a pnd higher avne-ability
bility t radiation painless stee(Beck, 2005is reported ation of
preral steel tubating conditas this wasthe 3 Schr theory. Thm
respectivelrt, = S9.uS.49Cycle plot in ASa purely statuilt
pulsejetpulsejet matvailability properties. el should no5) as the
boto be eliminessure and bing was untions. A106s designed h40 pipe
chhe pipe hasly. uS9 = 7.1 tngine [Bressmane, the totalcommonly
stainless steto one pulted staticallerheat and bld steel is
usperature in as the weldeteel pipe wahigh-tempction 5.1.1 l
diameter ick wollcJ an, 1946] al weight of constructedeel for
severlsejet websily for morburn holes ised. the engineed seam wawas
decided perature, higwas conduand wall thJ 31f the jet wasd from
mildral reasons:ite, enginesre than 30 -in the steel.e, commonas at
risk ofupon as thegh-pressureucted usinghickness of s d s - . n f e
e g f 32 According to thick-walled cylinder theory: oh = r2Pro2
r2_1 +ro2r2_ 5.22Since the maximum stress is needed and maximum
stress occurs at the outer wall, then r = ro and equation 5.22 can
be simplified to: oh mux = 2r2Pro2 r2 5.23Using the dimensions of
the pipe and the expected pressures then the hoop stresses were
calculated as follows: Table 5.2 Max Hoop Stress in Jet Tube
Pressure (Bar) 8 1.93 Max Hoop Stress, h max (MPa) 5.36 1.29 While
no information was found for the yield strength of A106 steel at
high temperature, the stress values in table 5.2 were compared
against information found for several other weaker grades of carbon
steel. The comparison showed that the stresses in the jet tube were
significantly lower than the yield strengths of mild steel grades
which were not designed to operate under high temperature. An
example of the temperature dependant properties of common grade 43A
steel is shown in figure 5.9. From the plot, the yield stress of
43A steel at 800C can be approximated to 15Mpa. 33 Figure 5.9
Mechanical Properties of Grade 43A Steel at Elevated Temperatures
[Bailey, 2009] 5.4.2. Valve Plate The valve plate must be able to
withstand the repeated impact of the valve tips up to 150-200 times
per second. As well as being able to withstand this impacting, the
valve plate should also provide a certain amount of damping force
to the valve tips. This damping force should absorb some of the
impact energy from the valves and thereby reduce the amount of
stress the valve tip experiences during impact. This would help to
increase the operating life of the valves. The concept of energy
being absorbed by a material during an impact was related back to
strain energy theory. The energy stored within a material when work
has been done on it is termed the strain energy (Hearn, 1997).
Since the work being done on the valve plate is the impact from the
valve, the material which stores the most amount of strain energy
will be the best material choice for the valve plate. Some of the
kinetic energy of the valve tip will be converted to strain energy
in the valve plate material. 34 Strain energy can be expressed as:
u = o2AI2E Or: u = o2I2E 5.24 V will be constant for a given valve
plate design. will also be constant for a given valve impact. If
the only variable is the material of the valve plate, then: u 12E A
material with smaller E will result in a smaller value of U To
maximise U, aluminium with E = 70GPa was chosen for the valve plate
material over steel with E = 200GPa. The downside to using
aluminium is that aluminium has a very low surface hardness. This
would most likely result in the repeated impact of the valves
damaging the surface of the valve plate and affecting the seal
between the valve plate and the valves. To avoid this, it was
decided to hard-anodise the machined valve plate. The thin layer of
aluminium oxide would increase surface hardness significantly
without affecting the underlying material properties. A table
showing Vickers hardness values for different materials is included
below in table 5.3 as a comparison. Table 5.3 Material Hardness
Comparison Table [Hard Anodising Ltd, 2005] Material Vickers
Hardness Number Untreated Al 6082 100 120 Hard Anodised Al 6082 400
460 Mild Steel 200 220 Stainless Steel 300 350 35 Additionally, it
was determined that an aluminium valve plate would conduct heat
from the valves quicker than steel and help keep them from
overheating. 5754 aluminium alloy was chosen as the final valve
plate material due to its excellent anodising properties and local
availability. 36 6. Construction This section details the
manufacturing and construction of the project engine, the problems
encountered and how they were overcome. The majority of
manufacturing of the components was carried out in the universitys
engineering workshop. Detailed engineering drawings of all
components can be found in appendix A of this report. 6.1. Jet Body
The tailpipe section of the jet body was made from a 1m length of 3
Sch40 seamless carbon steel pipe. The nominal wall thickness of
this pipe is 5.5mm, therefore, the combustion chamber was designed
to have the same wall thickness. The tailpipe was left ~150mm too
long to allow tuning of the exhaust during testing. The combustion
chamber was machined from a solid carbon steel piece to the
dimensions shown in appendix A. The finished combustion chamber was
welded to one end of the tailpipe. To allow easy assembly and
disassembly of the engine, a flange was machined from 3mm mild
steel plate. The flange incorporated eight 6mm holes which were
designed to take M5 bolts to bolt the engine together. The flange
was welded to the front of the combustion chamber. A small fitting
was machined to allow the spark plug to be incorporated into the
jet body. This fitting was simply a 10mm piece of 25mm diameter
round bar. A 5mm step was machined in the piece so that the OD of
the step was 18mm. The fitting was then drilled and tapped with an
internal M14x1.25 thread to take the spark plug. A 19mm hole was
drilled in the combustion chamber wall, 60mm from the front of the
engine. The spark plug fitting was inserted into this hole and
welded in place. The flat surface of the spark plug fitting
provides a good surface for the plugs crush washer to seal against.
The completed combustion chamber end of the jet body is shown in
figure 6.1. 37 Figure 6.1 Combustion Chamber and Flange 6.2. Intake
Diffuser The intake diffuser for the jet was initially designed to
be a simple cone rolled from 1mm mild steel sheet. However, the
correct facilities to roll a cone of this size did not exist in the
university and, after an unsuccessful attempt to roll the cone in
an external workshop, the design was abandoned for that described
below. The final intake diffuser (figure 6.2) was machined from a
solid block of aluminium. The design was kept simple with the ID by
the valve plate being 110mm and an internal wall slope of no more
than 7. A 10mm thick flange was incorporated into the design. Eight
holes were drilled in the flange and tapped M5x0.8 to match up with
the holes in the combustion chamber flange. 38 Figure 6.2 Intake
Diffuser 6.3. Valve Plates Two different valve plates were
manufactured. The first was as per the design described in section
5.1.2 and the second was a modification of the same design. The
second valve plate simply extended the intake orifices towards the
centre of the plate to increase the total intake area available.
This was designed as a back-up in case problems were found
regarding the original valve plate design during testing of the
engine. The two valve plates were CNC machined from 10mm thick 5754
aluminium alloy. The OD of the valve plates were machined to the
same OD of the flanges on the diffuser and the combustion chamber.
Eight 6mm holes were machined in them to match the flanges. One
18mm hole was machined in the centre of the valve plates to allow
the fuel delivery nozzle to pass through. Both machined valve
plates then had to be polished before being sent to Marchant
Engineering, Tramore, Co. Waterford to be hard anodised. The
completed valve plates are shown in figure 6.3. 39 Figure 6.3
Anodised Valve Plates; Original (left) & Modified (right) 6.4.
Valve Retainer Plates Three different valve plates were
manufactured, each with a different radius of curvature. The
largest radius of curvature was chosen so that the valve would have
a maximum tip travel of about 8mm. This was determined to be the
smallest tip travel allowable to allow the incoming air to flow
unrestricted. A second retainer plate was chosen to have a much
smaller radius of curvature which would allow the valve to open
further during the intake phase. This would also increase bending
stresses in the valve petals. The third retainer plate was given
the same radius of curvature as the first but without any flat
contact area in the centre. This would allow the valve to have
total flexibility from the root. The effects of different retainer
plates on valve life could then be investigated. The three valve
retainer plates were CNC machined from mild steel bar stock and an
18mm hole drilled in the centre to allow fitting of the fuel jet.
One of the completed retainer plates is shown in figure 6.4. 40
Figure 6.4 Valve Retainer Plate 6.5. Petal Valve The petal valve
was cut from 0.006 blue spring steel sheet. The intricate shape of
the petal valve cannot be cut with a snips as the material will
just split. Therefore, an electro-chemical etching process detailed
in appendix C of the report was used. The process involved first
coating the material to be etched with an electrically insulating
coating. An automotive primer was used in this case. The shape of
the petal valve was then drawn on the painted surface and the lines
scribed with a sharp knife to expose the metal underneath. The
spring steel was placed in a saturated salt/water solution so that
all the lines to be scribed were submerged. A stainless steel plate
of approximately the same size was placed in the solution also with
a sponge between the two pieces of metal to avoid contact between
them. The spring steel was connected to the positive terminal of a
12V power supply and the stainless steel plate was connected to the
negative terminal. When current was switched on, bubbles were seen
to rise from the cathode. The entire apparatus was placed under an
extractor hood and left for 40 mins approx until the spring steel
appeared to have been eaten away at the scribed lines. The petal
valve could then be popped from the rest of the material and the
paint cleaned off by immersing the valve in cellulose thinners for
up to 30 mins. The valve was then ready to be used in the engine
with no further modifications. 41 The electro-chemical etching
process had some drawbacks. Care must be taken to ensure a good
even coat of masking paint is applied to the valve material. Any
pinholes in the cured paint will result in pinholes being etched in
the valve. These holes render the valve useless. The masking paint
must be allowed to cure properly for at least 48 hours. Otherwise
the etching process undercuts the paint very easily and a poor
surface on the finished valve results. The valve must not be
allowed to sit in the etching solution for too long or the process
eats through weak points in the masking paint and the finished
valve will have holes in it. Figure 4.3 shows an electro-chemically
etched valve and the etching apparatus is shown below in figure
6.5. Figure 6.5 Electro-chemical Etching Apparatus During testing,
shim steel was used to make valves of different thicknesses. This
material was cut using a dremel tool and the burrs ground off with
a grinding wheel on the dremel tool. 42 6.6. Fuel Injection Nozzles
6.6.1. Internal Injector The fuel injection nozzle was turned from
25mm mild steel bar stock. A 5.5mm hole was drilled in the centre
of the piece to a depth of 45mm then six radial 2.5mm were drilled
to intersect with it and form the fuel injection outlets. An
internal chamfer was cut in the inlet hole which would help produce
a tight seal with the nipple on the propane hose. Both sides of the
piece were turned down to 18mm, leaving a 5mm wide collar at the
injection holes. An M18x1.5 thread was machined on the turned down
sections. A 15mm section at the inlet end was turned down to 16.6mm
and an external 3/8 BSP thread was cut to allow the propane hose
end to be threaded on. The completed internal fuel injection nozzle
is shown in figure 6.6. 6.6.2. External Injector During testing, it
was decided to move the point of injection forward into the intake
diffuser. (see section 7.2) The quickest and simplest way to do
this was to make a fitting which would screw directly onto the
original nozzles 3/8 BSP thread. A 75mm piece of 20mm round bar was
turned down to 16.6mm diameter. A 5.5mm hole was drilled from one
end to a depth of 48mm. Six radial 2mm holes were drilled to
intersect with the larger axial hole similar to what was done with
the original nozzle. The internal chamfer was cut in the inlet
also. At the opposite end, a 5mm hole was drilled to a depth of
20mm and then tapped M6x1. A spare brass propane hose fitting was
then fixed to this end using an M6x15 wide-head screw bolt. The new
external fuel injector can be seen in figure 6.6. 43 Figure 6.6
Fuel Injection Nozzles; Internal (top) & External (bottom) 6.7.
Test Stand The test stand was constructed to the same basic design
as was used for a previous pulsejet. However, to simplify the build
for preliminary testing, it was decided to omit the bearings from
the support straps and the bearing tracks on the frame. Instead,
the jet supports would be bolted directly to the uprights in the
frame. This would not allow for thrust measurement but was a secure
and simple method of securing the jet until self-sustaining could
be achieved. The individual components of the frame were cut from
30mm box-section steel and welded together. 9mm holes were drilled
25mm from the top of each upright before welding. To make the
supporting straps for the jet, two 25mm wide straps of 2mm thick
mild steel were bent around a section of the tailpipe. The ends
were then bent up so that there was a gap of about 25mm between
them. A 9mm hole was drilled through the tabs to allow the straps
to be tightened with M8 bolts. Two 110mm lengths of 16mm round bar
were welded to the outside of each strap so that the bars were in
line with each other and normal to the curve of the strap. Four 9mm
holes were drilled in the bars to match with the holes in the frame
uprights. The straps were secured to the frame using four M8x50
bolts. Figure 6.7 shows the engine mounted in the completed test
stand. 44 Figure 6.7 Engine Mounted on Test Stand 45 7. Testing
& Troubleshooting The following section details the testing of
the engine, the problems encountered and how they were overcome.
This section also details the measurement techniques which were
used at the end of the project, after successful running of the
engine had been achieved. 7.1. Ignition system problems During
initial testing, it was found that the spark generated by the
piezoelectric igniter was inadequate to ignite the propane/air
mixture. No form of ignition could be achieved using this method so
it was decided to upgrade the system to use a motorcycle ignition
coil and an old motorcycle battery as a power source. A coil was
purchased and wired to the battery via a push-to-make switch. The
circuit was then tested by connecting the coil lead to the spark
plug. A spark was observed but it appeared weak and unreliable.
When the circuit was connected to the engine, this spark also
proved unable to ignite the fuel/air mixture in the engine. The
spark plug which was being used up to this point was an NGK BM6A
plug. This plug has a standard thread reach of 9.5mm. It was
decided to replace this plug with one with a longer reach thread.
This would place the electrode further into the engine and increase
the chances of ignition. An NGK BR9EH plug was purchased as a
replacement. The replacement plug had a 19mm thread reach and also
had a higher heat rating which would allow the engine to withstand
higher engine temperatures and therefore last longer. A 5kV power
supply was connected to the spark plug as a temporary solution to
the ignition problem. This system produced a continuous spark
across the plug gap. A continuous spark is undesirable in a
pulsejet engine as it can disrupt the pulsating combustion and
cause the engine to stop. It was decided to use this method anyway
to see if the engine would at least ignite with the current spark
plug position. Ignition with the continuous spark was achieved but
the jet did not pulsate at all. The result of this test is
discussed in more detail in section 7.3. It was decided that the
intermittent spark 46 which could be produced using an induction
coil was much more desirable for pulsejet ignition. The motorcycle
coil in the old circuit was replaced with an old-type distributor
coil from a car and a short length of silicone HT lead was
purchased to provide the connection to the plug. However, on
testing, the spark was again weak and very unreliable. The circuit
was checked over with a multimeter and the impedance of the coil
and spark plug were found to be 4 k each. The total resistance of 8
k between the coil and plug electrode was much too high and the HT
lead and plug were replaced. The spark plug was replaced with a
non-resistor type B9ES NGK plug and the silicone HT lead was
replaced with a length of standard copper-cored spark plug wire.
The performance of the new system was found to be very satisfactory
with a strong reliable spark being produced across the plug gap
each time the switch was pushed. Although this system was adequate
for ignition, it proved awkward to have to push a button each time
a spark was needed. This system meant that more people were needed
to run a test; one person to provide spark and a second to vary the
fuel flow until ignition was achieved. An automatic system would
solve this problem by allowing the operator to simply switch on the
ignition circuit, vary the fuel flow until ignition was achieved
and then switch off the ignition circuit. An ideal automatic system
would be switched on using a toggle switch and then discharge the
coil at preset regular intervals to send a steady stream of sparks
across the plug gap until the system was switched off again. After
contacting the Electronic Engineering department in the university
to help with automating the circuit, two possible solutions were
determined: 1. Use a 555 timer circuit with a large transistor
which would act as a switch to cut the current to the induction
coil at regular intervals which would be controlled by the 555
circuit. This had the advantage of being completely portable with
all power to the circuit being provided by the motorcycle battery.
The downside 47 was that the timing of the spark was dependant on
the 555 circuit and could not be easily changed. 2. Use almost the
same circuit as above but instead of using a 555 timer to control
the transistor, a signal generator would provide a square wave
signal to do the same thing. This system had the advantage that the
timing of the spark was easily adjustable by varying the frequency
of the output square wave on the signal generator. The disadvantage
was that the signal generator needed an A/C power source and so the
portability was reduced. The second solution was chosen over the
first as it was simpler to set up and the ease of adjustment was
attractive. The Electrical Engineering department also had such a
circuit already made up for demonstration purposes which was made
available to this project and could easily be integrated into the
existing circuit. The final circuit provided a reliable and
adjustable ignition source for the pulsejet during testing. It also
allowed tests to be conducted more easily and with minimal
personnel. The final ignition setup is shown in figure 7.1. A
circuit diagram can also be seen in appendix B. Figure 7.1 Final
Ignition Circuit Signal Generator Battery Ignition Coil On/Off
Switch & Transistor Circuit 48 7.2. Fuel Mixing During the
initial tests when the engine was igniting but was acting almost
like a simple propane burner (section 7.3), a video clip taken
looking up the tailpipe showed that the burning in the combustion
chamber appeared over-rich and uneven. This can be seen in a still
image from the video clip in figure 7.2 below. It was thought that
the fuel may have been introduced too far into the combustion
chamber for adequate mixing of fuel/air to take place before
combustion. To attempt to solve this, a new fuel injector nozzle
was machined as detailed in section 6.6.2. The new fuel injector
would be threaded in place between the original injector nozzle and
the propane hose and would move the point of injection forward into
the intake diffuser. This would give the fuel a much longer time to
mix with the air as it passed through the intake orifices and over
the valve tips. The new injector nozzle proved to be very
effective. The engine was never tested with the old injector nozzle
after the valve frequency tuning had allowed the engine to operate
correctly as the performance of the engine with the new fuel nozzle
was significantly improved. There was no evidence of inadequate
mixing with the new nozzle. Figure 7.2 Uneven Burning in the
Combustion Chamber (left) & Burning With New Nozzle Fitted
(right) 49 7.3. Valve Frequency Ratio Tuning The engine test using
the 5kV power supply to provide spark resulted in ignition of the
fuel/air mixture at a certain fuel pressure setting. The engine
would not resonate and the sound of burning was very low. If gas
flow was decreased, the burning would stop and if gas flow was
increased, yellow flames would appear from the tailpipe. This led
to the conclusion that the continuous spark had set up a standing
flame front inside the engine which would only sustain at a certain
fuel/air setting. The engine was acting as a simple propane burner.
It was this conclusion that led to the desire to create the
intermittent spark ignition system detailed in section 7.1.
However, the new improved ignition system did not improve the
quality of burning in the engine. Even with the sparking frequency
turned down to under 0.5Hz, the engine would still ignite the
fuel/air mixture in the same manner as before. With the sparking
frequency that low, it ruled out that the problem was a standing
flame front being set up in the engine. Further visual comparison
of the movement of the petal valves before and after ignition
concluded that the engine was taking in air by itself and was
therefore attempting to resonate. Since the tailpipe had been left
oversized, the excess length was trimmed back to the designed
length of 1.1m and the test was run again. The change in length did
not affect the quality of burning in the engine at all. When
comparing the project engine to the previous engine which had been
built in the university, it was noticed that the basic jet body
dimensions were almost identical. The previous engine had achieved
resonant combustion, albeit with an external supply of air. The
only major difference in design was the valve plate and petal
valves. This detail, coupled with the failed test following the
length reduction, prompted an investigation into the vibration of
the petal valves. 50 In Part II of The Propulsive Duct, C.E.
Tharratt explains how a mechanical reed valve made up of two
identical metal reeds sandwiched together provides added stiffness
whilst retaining, as closely as possible, the response
characteristics of a single metal reed. (Tharratt, 1965) In order
to narrow down the problem, the engine was tested using a double
set of 0.006 petal valves in place of one. This would increase
static stiffness of the valves but keep the natural frequency of
vibration as close as possible to that of a single petal valve.
Using the double valve setup, the engine ran almost exactly the
same way as it had in previous tests with a single 0.006 valve. The
burning characteristics were very similar but the engine required a
much higher air supply to be started and sustain burning. These
results suggested that successful resonant combustion was reliant
on the natural frequency of vibration of the petal valves. The
theory necessary to calculate the natural frequency of the petal
valves is detailed in section 5.2. An attempt was made to
theoretically plot the response of a petal valve to the forcing
frequency of the jet with varying frequency ratios. However, the
analysis was regarded inconclusive due to the following reasons:
The motion of the valve cannot be modelled as a simple spring/mass
system without damping. This is due to the effect of the valve
plate damping out one-half of the valves motion. This means that
the momentum of the valve does not carry through from one cycle to
the next and therefore renders conventional modelling inaccurate.
Although the valve motion cannot be regarded as being damped, it
cannot be modelled as a damped system either. Viscous damping and
coulomb damping both restrict the motion of a spring system
regardless of whether the amplitude is positive or negative. In a
reed valve system, the valve is not restricted at all when the
amplitude is positive but the valve plate does not allow the
amplitude to become negative at any time. (Figure 7.3) Essentially
the valve is returned to initial conditions [ x(u) = u ; x (u) = u]
before the beginning of each negative pressure cycle. 51 Figure 7.3
Valve Motion Sign Convention (left) & Simplified Valve Motion
Plot (right) These issues prevented an accurate theoretical
solution for the response of the valve to be obtained without
considerable further work. It was decided to carry out various
tests, varying the natural frequency of the valves each time and
observe the results. The next step in testing was to use a petal
valve with a higher natural frequency of vibration than the
original. Using the theory in section 5.2, the original 0.006 valve
was calculated to have a natural frequency of 66Hz. If the
thickness of the reed was increased to 0.010, the natural frequency
would rise to 110Hz. Due to the unavailability of additional sheet
spring steel in Ireland, it was decided to carry out testing using
valves cut from shim steel. The shim steel valve would have the
same vibrational characteristics as a spring steel valve of equal
thickness but would be more prone to deformation. The shim steel
valves would help determine whether or not the engine would
resonate with different frequency ratios. A test was carried out
using a 0.010 thick shim steel valve. The engine achieved resonance
immediately but would only sustain for 15-20 seconds. Additionally,
the engine would not sustain combustion without an external supply
of air. Several 52 subsequent attempts were made to start the
engine. Engine started each time but failed to sustain for more
than 10 seconds. When the engine was disassembled following the
test and the valves were examined, it was found that one petal had
been bent so much that it no longer seated against the valve plate.
(Figure 7.4) The sections of valves which covered the intake
orifices were also visually deformed from the pressure of
combustion. Figure 7.4 0.010" Deformed Shim Steel Valve Following
Engine Run The audio was extracted from a video clip of the test
and analysed using Audacity sound editor to determine the operating
frequency of the engine. An operating frequency of ~150Hz was
measured from the audio file. This is very close to the frequency
of 142Hz which was estimated in section 5.1.1. Although the engine
started with a valve frequency of 110Hz, the performance was not
satisfactory. It was decided to try and run the engine with a valve
frequency of roughly double that of the previous test. The simplest
method of doing this was to insert a steel washer of a certain size
behind the petal valve. This would shorten the effective length of
the valve and thereby increase the natural frequency of vibration.
It was calculated that a 47mm diameter washer would reduce the
length of the valves by 9mm and increase natural frequency to
~250Hz. 53 The engine did not fire at all with this valve in place.
It was determined that the static stiffness of the valve was too
high to allow the air from the external supply to open the valves
and create an air flow through the engine. It was calculated that
using one of the 0.006 petal valves with the same diameter washer
would result in a natural frequency of 150Hz but that the stiffness
would be much lower. The valve was changed immediately and another
test was run. The engine ran very erratically using this setup and
was not able to sustain at a constant setting. It was determined
that the frequency of the valves was too close to the forcing
frequency of the engine for normal operation to be achieved. To
solve this problem, it was decided to make a second washer which
would increase the natural frequency of a 0.006 petal valve to
250Hz. It was calculated that a 53mm diameter washer was needed for
this. Using this setup, the engine fired and sustained for over
1:30 minutes. The external air supply was shut off about five
seconds after starting with no noticeable difference in running.
When the engine cooled and the valves removed, visual inspection
showed that the tips of many petals were broken and one petal had
cracked along its line of flexure with the washer. (Figure 7.5)
Figure 7.5 Impact and Fatigue Damage on a 0.006" Spring Steel Petal
Valve 54 Due to time constraints with the project, it was decided
to discontinue further valve frequency testing and use the
remaining two valves to attempt to get pressure plots and inlet
velocity data from the jet. 7.4. Valve Frequency High-Speed Camera
Test To determine the accuracy of the theoretical valve vibration
model, an experiment was conducted using a high-speed camera to
measure the frequency of vibration. A 0.010 thick shim steel valve
was used for the test. The valve was set up in front of the camera
so that it was clamped between two pieces of steel at the root of
the petal. (Figure 7.6) The valve was deflected by hand and
released so that it vibrated naturally. The first recording of the
vibration was taken at 200 fps (frames per second) but this was not
high enough to accurately capture the vibration. A second recording
was then taken at 600 fps. On playback, the time for five complete
oscillations was noted. The period of one oscillation could then be
determined and, hence, the natural frequency. The test recorded a
frequency of 104Hz. The calculated frequency was 110Hz. The test
was run again with the valve length shortened by 9mm. This was
calculated to have a frequency of 250Hz. The vibration was first
recorded at 600 fps but it was not high enough to accurately
capture the motion. The recording speed was increased to 1500 fps
and the natural frequency determined the same way as the first
test. The recorded frequency was 227Hz. The calculated frequency
was 250Hz. The experimental values of natural frequency
corresponded relatively closely to the calculated values. This
verified the theoretical model. 55 Figure 7.6 High-Speed Camera
Experiment Setup 7.5. Pressure Cycle Data Collection To obtain
pressure cycle plots within the engine, a high-temperature pressure
transducer was fitted to the engine. The pressure transducer which
was used was a Kistler 4045A5 with a cooling adapter. Two BSP
sleeves were purchased to fit the transducer to the engine. These
sleeves were welded to the engine at positions 55mm and 1075mm from
the front of the engine. The transducer was connected to a laptop
computer via a Handyscope to collect and store the data. The water
for the cooling adapter was supplied by a 12V pump from a car
windscreen wiper system. The pump was powered directly from the
same battery which powered the ignition system. Since only one
transducer was available for testing, the pressure readings from
both points had to be taken from two separate tests. In order to
keep the data as consistent as possible, the first test was started
as normal but then the fuel was cut off via the main High-Speed
Camera Petal Valve 56 valve on the propane tank. By leaving the
setting on the regulator, a consistent fuel flow into the engine
could be ensured. The tests were carried out using one 0.006 petal
valve. The valve was one which had not been etched properly. When
the engine did not sustain without air, it was put down to the
defects in the valve. However, due to shortage of valves, the test
was carried out anyway with the external air supply. The data from
the transducer was recorded for the forward measurement position.
The engine was then stopped and the transducer was moved to the
rear measurement position. The engine was restarted and the
transducer data recorded. Shortly after recording the last set of
data, the valves failed and the engine ceased to run. Figure 7.7
shows the equipment set up for pressure data collection. Figure 7.7
Equipment Set Up For Pressure Data Collection Transducer Power
Supply Transducer Cooling Pump Cooling Water Reservoir 57 8.
Results This section displays the results of the experimental data
acquired during testing of the engine. The following plot (figure
8.1) of pressure vs. time was obtained for the front end of the
engine: Figure 8.1 55mm Position Pressure/Time Plot The plot
displays the characteristic oscillating pressure cycles which occur
at the front of a pulsejet engine. By analysing the plot, a burning
frequency of 160Hz was calculated. A pressure/time plot was also
obtained for a position just 25mm from the end of the tailpipe.
This plot is displayed in figure 8.2. 0.10.40.91.41.92.40 0.01 0.02
0.03 0.04 0.05AbsPressure(Bar)Time(s)58 Figure 8.2 1075mm Position
Pressure/Time Plot 00.20.40.60.811.21.40 0.005 0.01 0.015 0.02
0.025 0.03 0.035 0.04 0.045 0.05AbsPressure(Bar)Time(s)59 9.
Discussion 9.1. Jet Design This project has shown that a pulsejet
engine can be designed using a set of simple equations. The
equations derived by C.E. Tharratt do allow the basic dimensions of
a pulsejet engine to be determined on the back of an envelope
(Tharratt, 1965). However, very little emphasis is placed of the
design of the intake valve system. Little or no research has been
carried out to accurately determine the response of a mechanical
spring valve to a forcing frequency. This response was determined
to be a crucial aspect of the correct operation of a pulsejet
engine. For an engine to achieve static thrust, the correct
relationship between the frequency of operation of the engine and
the natural frequency of the reed valves must be determined. The
static stiffness of the valves must also be correct to allow the
valves to open under static conditions. 9.2. Petal Valve Vibration
Theory The theoretical model for determining the natural frequency
of vibration of the petal valves was verified by capturing the
vibration of the valve in front of a high-speed camera. However, as
the length of the valve was reduced, the error in calculations
appeared to increase. This may have been an error in the position
where the valve was clamped. Section 5.2 explains how natural
frequency is inversely proportional to L3. If the valve position in
the clamp was even slightly off, it would result in a relatively
large error in natural frequency reading. 9.3. Valve Life The
operating life of the petal valves used in the project engine was
extremely low. The longest continuous engine run lasted for only
two minutes. After the engine runs the valves were found to have
suffered severe impact damage and also fatigue damage at the valve
root. 60 Fatigue cracks at the valve root can easily be eliminated
by machining a new curved valve retainer plate with the correct
root diameter. However, using the steel washer is the simplest and
fastest way of varying natural frequency of vibration. The washers
should be used to determine if a frequency ratio exists where valve
impact damage is minimised. A new ret