Pulsejet Project

Fabrication of a valveless Pulsejet Engine Project Report 2015

Department of Aeronautical Engineering 1 Mount Zion college of engineering,Kadammanitta

1. INTRODUCTION

A pulsejet is one of the simplest of engines from a design and manufacturing

aspect but this simplicity is offset by the complications involved in understanding its

working. It should be borne in mind that there is no conclusively established

comprehensive mathematical law governing the working of a pulsejet, hence all new

and innovative modifications to pulsejets are done on a trial and error basis. This greatly

hinders progress since the effect of a change in the design is 'unpredictable'. But this has

not deterred academicians and scientist from attempting to develop a theoretical model

of the working mechanism. A considerable number of analyses ranging from using an

acoustic analogy to solving the flow-field internal to the pulsejet have been performed

in the past and though each one sheds fresh insight into a specific process/processes

occurring in the pulsejet, no single theoretical model has been able to sufficiently

explain all the processes. The systemic nature of the processes involved in this jet

engine leaves a fragmented analysis of it wanting, hence requiring further understanding

of 'how it works' and 'what makes it work'. The pulsejet operation cycle, as has been

observed experimentally, can be summarized in four phases.

1. Combustion occurs in the combustion chamber and the ensuing heat release increases

the pressure and drives out the hot gases through the exhaust and produces thrust. The

hot gases expand down the exhaust and inlet tubes, but due to a difference in the cross-

sections of the inlet and exhaust pipes, a major portion of hot gases are expelled through

the exhaust pipe.

2. Once the combustion gases have expanded to atmospheric pressure, over-expansion

of the gases due to inertia (Kadenacy effect), inside the combustion chamber, causes the

chamber pressure to decrease to sub-atmospheric levels.



3. The sub-atmospheric pressure causes fresh reactants to enter the combustion chamber

through the inlet (the inlet air column has a lower inertia) and a small fraction of the

exhaust gases from the exhaust tube.

4. The residual gases and heat transfer from the walls raise the reactants temperature to

the auto-ignition temperature, initiating combustion. The entire cycle repeats itself at a

regular interval.

1.1 DESCRIPTION OF PULSE COMBUSTION

Pulsating combustion is a combustion process that occurs under oscillatory conditions.

That means that the state variables, such as pressure, temperature, velocity of

combustion gases, etc., that describe the condition in the combustion zone, vary

periodically with time. Pulse combustion is a very old technology. The phenomenon of

combustion-driven oscillations was first observed in the year 1777, subsequently

explained by Lord Rayleigh in the year 1878, and used in a variety of applications

around the turn of the Century.

Fig 1.1: General Pulse Combustion Process



One of the better known examples of a pulse combustor is the German V-1 "Buzz

Bomb " of World War II; Although the technology of pulse combustion has been

known for many years, devices using pulse combustion have not been implemented

widely despite their many attractive characteristics.

Fig1.2: Combustion chamber explosion

Compared to conventional combustion systems, their heat transfer rates are a factor of

two to five higher than normal turbulent values, their combustion intensities are up to

on order of magnitude higher, their emissions of oxides of nitrogen are a factor of three

lower, their thermal efficiencies are up to 40% higher, and they may be self-

aspirating, obviating the need for a blower. This combination of attributes can result in

favorable economic trade off with conventional combustors in many applications. Most

of the research on pulse combustors has been directed toward applied examinations of

the engineering aspects of pulse combustors: heat transfer, efficiency, frequency of

operation, pollutant formation, etc.

There is also uncertainty over the behavior of frequency as a function of geometry,

energy input, and mass input. Zinn states that the pulse combustor can be modeled as a

Helmholtz resonator, while Dec and Keller found that the frequency of operation is a

function of the magnitude of the energy input and of the magnitude of the mass flux.



These results indicate that a Helmholtz resonator model is insufficient to predict the

frequency of operation. These fundamental questions must be answered before the

prediction of an optimum resonant condition is possible.



2. LITERATURE SURVEY

2.1 STUDY OF EXISTING SYSTEM

A jet engine is a reaction engine discharging a fast moving jet that

generates thrust by jet propulsion in accordance with Newton's laws of motion. This

broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, and pulse

jets. In general, jet engines are combustion engines but non-combusting forms also

exist.

In common parlance, the term jet engine loosely refers to an internal combustion air

breathing jet engine (a duct engine). These typically consist of an engine with a rotary

(rotating) air compressor powered by a turbine ("Brayton cycle"), with the leftover

power providing thrust via a propelling nozzle. Jet aircraft use these types of engines for

long-distance travel. Early jet aircraft used turbojet engines which were relatively

inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass

turbofan engines. These engines offer high speed and greater fuel efficiency than piston

and propeller aeroengines over long distances. Jet engines power aircraft, cruise

missiles and unmanned aerial vehicles. In the form of rocket engines they

power fireworks,model rocketry, spaceflight, and military missiles.

Jet engines have propelled high speed cars, particularly drag racers, with the all-time

record held by a rocket car. A turbofan powered car, Thrust SSC, currently holds

the land speed record.

Jet engine designs are frequently modified for non-aircraft applications, as industrial gas

turbines. These are used in electrical power generation, for powering water, natural gas,

or oil pumps, and providing propulsion for ships and locomotives. Industrial gas

turbines can create up to 50,000 shaft horsepower. Many of these engines are derived

from older military turbojets such as the Pratt & Whitney J57 and J75 models. There is

also a derivative of the P&W JT8D low-bypass turbofan that creates up to 35,000 HP.

Propeller engines are useful for comparison. They accelerate a large mass of air but by a

relatively small maximum change in speed. This low speed limits the maximum thrust



of any propeller driven airplane. However, because they accelerate a large mass of air,

propeller engines, such as turboprops, can be very efficient.

On the other hand, turbojets accelerate a much smaller mass of intake air and burned

fuel, but they emit it at the much higher speeds which are made possible by using a de

Laval nozzle to accelerate the engine exhaust. This is why they are suitable for aircraft

traveling at supersonic and higher speeds.

Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion

product gas from the core engine. The amount of air that bypasses the core engine

compared to the amount flowing into the engine determines what is called a turbofans

bypass ratio (BPR).

While a turbojet engine uses all of the engine's output to produce thrust in the form of a

hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields

between 30 percent and 70 percent of the total thrust produced by a turbofan system.

The Diffuser : The Diffuser is a low pressure circular vent that is responsible for

converting the kinetic energy of the atmospheric air into a static pressure rise. The

pressure of the atmospheric air is high thus this air flows into the vent where it gets

reduced in volume thus increasing its pressure. This intake air is then fed to the

compressor.

The Compressor : The compressor found in Turbo Jet engines are usually rotary

compressors. The rotary compressors are compressors that generate high volume of air

at a low pressure thus having a lower pressure ratio compared to the reciprocating

compressors.

The air intake from the diffuser is fed to the inlet of the rotary axial or centrifugal

compressor where the air gets compressed in various stages and reaches a high pressure.

This high pressure is reached due to the various stages of the compressor adding to the

pressure at each stage.

The compressor outlet is the inlet to the air-fuel feed nozzle.



Fig: 2.1 Typical Jet Engine interior

The Air-Fuel feed nozzle : The air-fuel feed nozzle mixes the compressed air with the

jet fuel. The fuel is mixed with a specific air-fuel ratio. This injector nozzle injects fuel

at a constant rate in the combustion chamber where it is burned to form high pressure

exhaust.

The Combustion Chamber : This is a c

hamber where the air-fuel mixture is burned with the help of flame stabiizers. The flame

stabilizers keep a constant flame ignited in the combustion chamber to continuously

burn the fuel and also ensure that the flame does not go out. The chamber consists of

two fuel injector nozzles and the flame stabilizer.

The exhaust created in the combustion chamber is passed to the Mechanical Turbine.

The Mechanical Turbine : The mechanical turbine consists of a rotary element having

fan blades. The high pressure exhaust from the combustion chamber strikes the fans of

the turbine causing it to rotate. This striking causes the exhaust to expand and lose its

pressure.



The turbine is responsible for driving the axial compressor. the turbine rotation is

connected to the rotor of the compressor with reduction gear. This causes the

compressor to rotate.

The exhaust nozzle from the turbine blades is further passed to the exhaust nozzle.

2.2 LIMITATION OF EXISTING SYSTEM

They are heavier for the same power, because they need to be made from

stronger material due to the higher compression ratio and because they need to

have larger cylinder volume because of lower maximal rpm. On longer flights

the reduction in fuel weight often makes up for the heavier engine.

Compared to a reciprocating engine of the same size, they are expensive.

Because of high speeds and high operating temperatures, designing and

manufacturing gas turbines is a challenge from both the engineering and

materials point of view.

Gas turbines also tend to use more fuel when they are idling.

Very heavy, so cant be implemented in light aircrafts and drones.

Need constant maintenance because of moving parts.

Thrust production is often low at low subsonic speeds.



2.3 STUDY OF PROPOSED SYSTEM

Though a lot of research had been performed prior to Marconnet by researchers like

Holtzwarth and Karavodine in the field of pulsed combustion, the use of pulsed

combustion as a method of direct thrust generation was first carried out by Marconnet in

his "reacture-pulsateur", which is in every way the precursor to the modern day

valveless pulsejet. P. Schmidt applied the concepts of Marconnet's 'wave engine' to the

development of an intermittent pulsejet engine, called the Schmidtrohr, directed towards

use for vertical take-off and landing vehicles.

Once the potential of pulsejets as direct thrust producing engines had been

demonstrated, the development of the same grew rapidly. The German Air Ministry,

deciding to investigate all forms of jet engines, asked the Argus Motoren Gesellschaft of

Berlin to develop the pulsejet. This project, under the development of Dr. Fritz Gosslau,

led to the development of the famous Argus AS 109-014 powering the

Vergeltungswaffe 1 (V-1) Buzz Bomb of World War-II [ref xiii]. It has been

erroneously reported in numerous publications that the Argus work was in conjunction

with P. Schmidt. Post World War-II, research in pulsejets was undertaken by the US

Navy under Project Squid. French engineers at SNECMA did extensive research on

pulsejets. Lockwood of Hiller Aircraft, with the support of the French engineers,

investigated the working of pulsejets and this work is a landmark achievement as it is

the only completely documented, systematic study in existence. He utilized analytical

tools developed by J.V.Foa, which though not conclusive, were the most complete

analytical approach available at that time. E.Tharratt's heuristic approach by *analytical

evaluation of pulsejets. J.A.C. Kentfield and his associates from the University of

Calgary pioneered work on developing computer simulations of the cyclic operations of

the valveless pulsejet. Dudley Smith of the University of Texas, Arlington developed a

numerical model of a valveless pulsejet to include combustion, while accessing the

performance of a pulsejet with a synchronous injection ignition system.



Since 2004, a fair amount of research on pulsejets including experimental, analytical

and numerical studies has been undertaken by North Carolina State University and these

studies have demonstrated the feasibility of operating pulsejets of sizes as small as 8cm

in length.

A pulse jet engine is a type of jet engine in which combustion occurs in pulses. Pulsejet

engines can be made with few or no moving parts, and are capable of running statically.

Pulse jet engines are a lightweight form of jet propulsion, but usually have a poor

compression ratio, and hence give a low specific impulse.

Pulsejet is an unsteady propulsive device with its basic components being the inlet,

combustion chamber, valve and valve head assembly and a tailpipe.

Fig2.2 Schematic of pulse combustion operation



2.4 TYPES OF PULSE JET ENGINES

There are two types of pulse jet engines: those with valves and those without. The ones

with valves allow air to come in through the intake valve and exit through the exhaust

valve after combustion takes place. Pulse jet engines without valves, however, use their

own design as a valve system and often allow exhaust gases to exit from both the intake

and exhaust pipes, although the engine is usually designed so that most of the exhaust

gases exit through the exhaust pipe.

A. Valved Pulsejet Engine

Valved engines use a mechanical valve to control the flow of expanding exhaust,

forcing the hot gas to go out of the back of the engine through the tailpipe only, and

allow fresh air and more fuel to enter through the intake. The valved pulsejet comprises

of a intake with a one-way valve arrangement. The valves prevent the explosive gas of

the ignited fuel mixture in the combustion chamber from exiting and disrupting the

intake airflow, although with all practical valved pulsejets there is some 'blowback'

while running statically and at low speed as the valves cannot close fast enough to stop

all the gas from exiting the intake.

Fig 2.3: Valved Pulsejet Engine

The hot exhaust gases exit through an acoustically resonant exhaust pipe. The valve

arrangement is commonly a "daisy valve" also known as a reed valve. The daisy valve



is less effective than a rectangular valve grid, although it is easier to construct on a

small scale.

B. Valveless Pulsejet Engine

The valveless pulse jet engine operates on the same principle, but the 'valve' is the

engine's geometry. Fuel as a gas or liquid vapor is either mixed with the air in the intake

or directly injected into the combustion chamber. Starting the engine usually requires

forced air and an ignition method such as a spark plug for the fuel-air mix. With modern

manufactured engine designs, almost any design can be made to be 'self-starting' by

providing the engine with fuel and an ignition spark, starting the engine with no

compressed air. Once running, the engine only requires input of fuel to maintain a self-

sustaining combustion cycle.

Valveless pulsejets, have no moving parts and use only their geometry to control

the flow of exhaust out of the engine. Valveless engines expel exhaust gases out of both

the intake and the exhaust, most try to have the majority of exhaust go out the longer tail

pipe, for more efficient propulsion.

Fig2.4:Valveless pulsejet engine

Fuel is drawn into the combustion chamber through the intake valve in either as an air-

gas mixture or in liquid form. The intake valve then closes and a spark plug is used to

ignite the fuel in the combustion chamber. The fuel then expands rapidly and tries to fill

the entire chamber in order to escape. The closed intake valve forces the fuel to the rear

of the combustion chamber and allows the exhaust gases to exit through the exhaust

valve.



3. TYPES OF VALVELESS PULSEJETS

The idea of pulsed combustion was conceived even before the use of steady state

combustion employed in gas turbine engines. Over the past hundred years various

number of valveless pulsejet designs have been invented and tested. These are classified

into three main systems

Inline systems

U-shaped systems

Linear systems

3.1 INLINE SYSTEMS

The systems, which have an intake pipe, combustion chamber and exhaust pipe, all on

the same axis with intake and exhaust held in opposite directions are called inline

systems. The advantage of this system is that when the engine has positive forward air

velocity the intake has air rushing into it creating a ram-air effect, similar to ram jet

engines.

Moreover the fabrication and fitting of inline systems is much easier than any other

systems. The disadvantage is that these engines have lower thrust than other systems

because the hot air exiting the intake after combustion does not to contribute to net

thrust and actually creates negative trust that has to be overcome.

To overcome this many complicated and mostly infeasible aerodynamic valves have

been created to allow the ram air effect to work without allowing the air to move back

through so as to increase thrust. However none have been proven effective.

3.1.1 Marconnet Design

In 1909 Georges Marconnet developed the first pulsating combustor without valves. It

was the father of all valveless pulse jets. Marconnet found that a blast inside a chamber

would prefer to go through a bigger exhaust opening rather than squeezing through a

relatively narrow intake. In addition a long diffuser between the intake and the

combustion chamber would direct the charge strongly towards exhaust, the way a

trumpet directs sound.



Fig 3.1 Marconnets Valveless Pulsejet Engine

3.1.2 Schubert design

The principle of the valveless pulsating combustor was discovered by Lt.William

Schubert of the US NAVY in the early 1940s. Schuberts design was called a Resojet

on the account on its dependence on resonance.

The taper less attachments of the inlet tube to the combustion chamber in Schuberts

design creates strong turbulence for better mixing of fuel and air so that high intensity

combustion takes place. Schubert carefully calculated the geometry of the intake so that

the exhaust gas could not exit by the time the pressure inside fell below atmospheric.

The resistance of a tube to the passage of gas depends steeply on the gas

temperature. Thus, the same tube will offer a much greater resistance to outgoing hot

gas than to the incoming cold air. The impedance is inversely proportional to the square

root of the gas temperature. This degree of irreversibility seems to offer the possibility

for the cool air necessary for combustion to get in during the intake part of the cycle, but

for the hot gas to encounter too much resistance to get out of the intake during

the expansion part.

Fig 3.2: Schuberts valveless pulsejet engine



3.2 U-SHAPED SYSTEMS

The U-shape design overcomes the shortfall of the inline design by bending the exhaust

pipe by 180 degrees, so that the exhaust and intake are aligned in the same direction.

The advantage of this design is that the thrust generated by the inlet contributes to the

net thrust of the engine as it flows in the same direction as the exhaust. The

disadvantage is that the ram-air affect is lost. Moreover fabrication is quite complex.

3.2.1 Lockwood-Hiller

The U-shaped Lockwood-Hiller engine was invented by Raymond Lockwood. It is said

that the Lockwood was the most effective pulse jet engine ever developed.

The air fuel mixture is generated by mixing fuel which is injected through a jet built into

the side of the combustion chamber or on a strut projecting into the chamber or on two

crossed struts spanning the front part of the chamber. The chamber is the drum like

broad part of the engine. The short straight tube attached to the combustion chamber is

the inlet. And the long U tube attached to the combustion chamber is the tail pipe. The

tailpipe is fitted with a flare at the end.

Fig3.3: U-shaped Lockwood Hiller engine

The Lockwood-Hiller design is the most successful example of U-shaped designs in

both performance and efficiency. Conversely it is difficult to construct because of

numerous cone sections are to be fabricated for it.



3.3 LINEAR SYSTEMS

There are many designs of valveless pulsejet engines that cannot be categorized by

either U-shape or inline designs. These engines are generally variations of inline designs

with the intake moved to the side of the combustion chamber. The typical feature of the

linear engine is that the intake emanates from the side of the combustion chamber. The

advantage of this type of engine is that the physical size is smaller than an equivalent U-

shaped engine making integration into airframe more practical.

These engines are also simpler to manufacture than U-shape design. The disadvantage

of this design is the tuning difficulty for optimized performance as the intake length is

directly proportional to exhaust length. Net thrust outputs are considerably greater than

inline while performance is less than the equivalent U-shape design as the efficiency is

limited by intake position.

3.3.1 Argus design

The capped tube design was first invented by the Argus Company (manufacturer of

German V-1 bombs). It consisted of combustion chamber (plenum chamber), which

formed a bottle shape design capped over with a hemispherical top. Fuel was injected

through a nozzle located on the tip of the cap and protected from the chamber with

metal grid. The grid functioned as a heat sink and prevented gas from burning at the

nozzle.

Pressurized air was forced into the plenum chamber continuously using a compressor,

the combustion took place and the hot gases expanded. The continuous supply of the

compressed air into the plenum chamber prevented hot gas from getting out of the

plenum chamber and almost all of it were thrust into the exhaust. The engine did not

self-sustain or resonate due to the reasons of smaller plenum chamber and exhaust

length.



Fig 3.4: Capped tube-Argus



4. COMPONENTS AND SYSTEM DESCRIPTION

The main components are used in this project are

SPARK PLUG

COMBUSTION CHAMBER

FUEL INLET

FRAME

4.1 SPARK PLUG

A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug)

is a device for delivering electric current from anignition system to the combustion

chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by

an electric spark, while containing combustion pressure within the engine. A spark plug

has a metal threaded shell, electrically isolated from a central electrode by

aporcelain insulator. The central electrode, which may contain a resistor, is connected

by a heavily insulated wire to the output terminal of anignition coil or magneto. The

spark plug's metal shell is screwed into the engine's cylinder head and thus

electrically grounded. The central electrode protrudes through the porcelain insulator

into the combustion chamber, forming one or more spark gaps between the inner end of

the central electrode and usually one or more protuberances or structures attached to the

inner end of the threaded shell and designated the side, earth, or ground electrode(s).

Spark plugs may also be used for other purposes; in Saab Direct Ignition when they are

not firing, spark plugs are used to measure ionization in the cylinders - this ionic current

measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire

measurement function. Spark plugs may also be used in other applications such

as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are

sometimes referred to as flame igniters.



The plug is connected to the high voltage generated by an ignition coil or magneto. As

the electrons flow from the coil, a voltage develops between the central and side

electrodes. No current can flow because the fuel and air in the gap is an insulator, but as

the voltage rises further, it begins to change the structure of the gases between the

electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases

become ionized. The ionized gas becomes a conductor and allows electrons to flow

across the gap. Spark plugs usually require voltage of 12,00025,000 volts or more to

"fire" properly, although it can go up to 45,000 volts. They supply higher current during

the discharge process, resulting in a hotter and longer-duration spark.

As the current of electrons surges across the gap, it raises the temperature of the spark

channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to

expand very quickly, like a small explosion. This is the "click" heard when observing a

spark, similar to lightning and thunder.

The heat and pressure force the gases to react with each other, and at the end of the

spark event there should be a small ball of fire in the spark gap as the gases burn on

their own. The size of this fireball, or kernel, depends on the exact composition of the

mixture between the electrodes and the level of combustion chamber turbulence at the

time of the spark. A small kernel will make the engine run as though the ignition

timing was retarded, and a large one as though the timing was advanced.



4.2 COMBUSTION CHAMBER

A combustion chamber is that part of an internal combustion engine (ICE) in

which the fuel/air mix is burned.

ICEs typically comprise reciprocating piston engines, rotary engines, gas turbine

and jet turbines.The combustion process increases the internal energy of a gas, which

translates into an increase in temperature, pressure, or volume depending on the

configuration. In an enclosure, for example the cylinder of a reciprocating engine, the

volume is controlled and the combustion creates an increase in pressure. In a continuous

flow system, for example a jet engine combustor, the pressure is controlled and the

combustion creates an increase in volume. This increase in pressure or volume can be

used to do work, for example, to move a piston on a crankshaft or a turbine discin a gas

turbine. If the gas velocity changes, thrust is produced, such as in the nozzle of a rocket

engine.

Head types

Various shapes of combustion chamber have been used, such as: L-head

(or flathead) for side-valve engines; "bathtub", "hemispherical", and "wedge" for

overhead valve engines; and "pent-roof" for engines having 3, 4 or 5 valves per

cylinder. The shape of the chamber has a marked effect on power output, efficiency and

emissions; the designer's objectives are to burn all of the mixture as completely as

possible while avoiding excessive temperatures (which create NOx). This is best

achieved with a compact rather than elongated chamber.

Swirl & Squish

The intake valve/port is usually placed to give the mixture a pronounced "swirl"

(the term is preferable to "turbulence", which implies movement without overall

pattern) above the rising piston, improving mixing and combustion. The shape of the

piston top also affects the amount of swirl. Another design feature to promote

turbulence for good fuel/air mixing is "squish", where the fuel/air mix is "squished" at

high pressure by the rising piston.



Flame front

Finally, the spark plug must be situated in a position from which the flame front

can reach all parts of the chamber at the desired point, usually around 15 degrees

after top dead centre. It is strongly desirable to avoid narrow crevices where stagnant

"end gas" can become trapped, as this tends to detonate violently after the main charge,

adding little useful work and potentially damaging the engine. Also, the residual gases

displace room for fresh air/fuel mixture and will thus reduce the power potential of each

firing stroke.

Why Valveless Pulsejets

A valveless pulse jet engine is a simple and ordinary engine. It is just a piece of

metal tube cut to the required dimensions. In a valveless pulsejet engine there are no

mechanical valves but they do have aerodynamic valves which for the most part resist

the flow in a single direction. They have no mechanically moving parts and sothey are

more reliable. All valveless engines have low thrust output, high fuel consumption and

overall poor performance.

Fig4.1: A 4-Pound Valveless Pulse Jet



Pulsejets can be used on a large scale as industrial drying systems, and there has been a

new surge to study and apply these engines to applications such as high output heating,

biomass conversion, and alternative energy systems,as pulsejets can run on almost

anything that burns including particulate fuels such as sawdust or coal powder.



5. PRINCIPLE OF OPERATION

5.1 RIJKE TUBE

Rijke's tube turns heat into sound, by creating a self-amplifying standing wave.

It is an entertaining phenomenon in acoustics and is an excellent example of resonance.

Fig 5.1: Rijke Tube

The Rijke tube is simply a cylindrical tube with both ends open and a heat

source placed inside it. The heat source may be a flame or an electrical heating element.

It has a wire gauze inside about one quarter the way from the bottom. Traditionally, the

tube is positioned vertically on a stand or even held in a hand and the heat source is

introduced from below into the tube. For certain ranges of position of the heat source

within the tube, the Rijke tube emits a loud sound. This phenomenon was discovered by

Rijke around 1850, and is therefore called the Rijke phenomenon. Sound production in

the Rijke tube is a classic example of a thermo-acoustic phenomenon.

In the case of the Rijke tube air can move in and out of both ends. A heated

metal mesh placed a quarter of the way up from the bottom heats the air flowing past it.

This flow of air is a combination of the convection current caused by the transfer of heat

from the metal mesh and the sound wave that is set up for the condition of two open

ends.



For half of the oscillation cycle of the sound wave air moves in from both ends

as it flows towards the center generating a pressure antinode (displacement node) there.

Even though some of the air moving past the hot metal mesh has already been heated

during the cycle prior to this, some additional cool air flows in, passing through it and

acquiring thermal energy and further increasing the pressure, thus reinforcing the

oscillation. For the remaining half cycle air passing by the metal mesh while flowing

outward from the center of the tube is already heated and therefore energy transfer is

minimal.

The sound comes from a standing wave whose wavelength is about twice the

length of the tube, giving the fundamental frequency. Lord Rayleigh, in his book, gave

the correct explanation of how the sound is stimulated. The flow of air past the gauze is

a combination of two motions. There is a uniform upwards motion of the air due to

a convection current resulting from the gauze heating up the air. Superimposed on this

is the motion due to the sound wave. For half the vibration cycle, the air flows into the

tube from both ends until the pressure reaches a maximum. During the other half cycle,

the flow of air is outwards until the minimum pressure is reached. All air flowing past

the gauze is heated to the temperature of the gauze and any transfer of heat to the air

will increase its pressure according to the gas law.

As the air flows upwards past the gauze most of it will already be hot because it

has just come downwards past the gauze during the previous half cycle. However, just

before the pressure maximum, a small quantity of cool air comes into contact with the

gauze and its pressure is suddenly increased. This increases the pressure maximum, so

reinforcing the vibration. During the other half cycle, when the pressure is decreasing,

the air above the gauze is forced downwards past the gauze again. Since it is already

hot, no pressure change due to the gauze takes place, since there is no transfer of heat.

The sound wave is therefore reinforced once every vibration cycle and it quickly builds

up to very large amplitude.This explains why there is no sound when the flame is

heating the gauze. All air flowing through the tube is heated by the flame, so when it

reaches the gauze, it is already hot and no pressure increase takes place.



Fig 5.2: Working of a Rijke Tube

When the gauze is in the upper half of the tube, there is no sound. In this case,

the cool air brought in from the bottom by the convection current reaches the gauze

towards the end of the outward vibration movement. This is immediately before the

pressure minimum, so a sudden increase in pressure due to the heat transfer tends to

cancel out the sound wave instead of reinforcing it.

The position of the gauze in the tube is not critical as long as it is in the lower

half. To work out its best position, there are two things to consider. Most heat will be

transferred to the air where the displacement of the wave is a maximum, i.e. at the end

of the tube. However, the effect of increasing the pressure is greatest where there is the

greatest pressure variation, i.e. in the middle of the tube. Placing the gauze midway

between these two positions (one quarter of the way in from the bottom end) is a simple

way to come close to the optimal placement.

The Rijke tube is considered to be a standing wave form of thermo

acoustic devices known as "heat engines" or "prime movers".



5.2 THE HELMHOLTZ RESONATOR

Helmholtz resonance is the phenomenon of air resonance in a cavity, such as

when one blows across the top of an empty bottle. The name comes from a device

created in the 1850s by Hermann von Helmholtz. The "Helmholtz resonator", which he,

the author of the classic study of acoustic science, is used to identify the

various frequencies or musical pitches present in music and other complex sounds. The

Helmholtz resonator can best be demonstrated by taking a normal soft drink bottle and

blowing over the mouth of the bottle. When air is forced into a cavity,

the pressure inside it increases. When the external force pushing the air into the cavity is

removed, the higher-pressure air inside will flow out. The cavity will be left at a

pressure slightly lower than the outside, causing air to be drawn back in. This process

repeats with the magnitude of the pressure changes decreasing each time.

The air in the port (the neck of the chamber) has mass. Since it is in motion, it

possesses some momentum. A longer port would make for a larger mass, and vice-

versa. The diameter of the port is related to the mass of air and the volume of the

chamber. A port that is too small in area for the chamber volume will "choke" the flow

while one that is too large in area for the chamber volume tends to reduce the

momentum of the air in the port.



Fig 5.3: Helmholtz Resonator

An important type of resonator with very different acoustic characteristics is the

Helmholtz resonator. Essentially a hollow sphere with a short, small-diameter neck, a

Helmholtz resonator has a single isolated resonant frequency and no other resonances

below about 10 times that frequency.

The resonant frequency (f) of a classical Helmholtz resonator, shown in Figure,

is determined by its volume (V) and by the length (L) and area (A) of its neck:

Here, f =

2



Figure 5.4: A Classic Helmholtz Resonator

where S is the speed of sound in air. As with the tubes discussed above, the value of the

length of the neck should be given as the effective length, which depends on its radius.

The isolated resonance of a Helmholtz resonator made it useful for the study of

musical tones in the mid-19th century, before electronic analyzers had been invented.

When a resonator is held near the source of a sound, the air in it will begin to resonate if

the tone being analyzed has a spectral component at the frequency of the resonator. By

listening carefully to the tone of a musical instrument with such a resonator, it is

possible to identify the spectral components of a complex sound wave such as those

generated by musical instruments.

Helmholtz Resonator Analogy in Pulse Jet Engines

The simplest analytical model of the valveless pulsejet is that of a Helmholtz resonator

in a combination with a quarter wave oscillator. While their analogy is one of the

simplest forms, it allows for a wealth of understanding of the fundamental operation of

a valveless pulsejet. The model assumes that the combustion chamber and inlet can be

modeled as a Helmholtz resonator and the exhaust as a matched, or tuned, quarter wave

oscillator (the familiar pipe organ)

It is a classic element in the study of acoustics. The pressure of the gas within the

cavity of the resonator changes as it is alternately compressed and expanded by the



influx and efflux of the gas through the opening and thus provide the stiffness element.

At the opening, there is a radiation of sound into the surrounding medium, which leads

to the dissipation of acoustic energy and thus provides a resistance element.



6. OPERATION OF A VALVELESS PULSE JET ENGINE

Fig 6.1: Valveless Pulse Jet during operation

The operation of valveless pulsejet requires a fundamental knowledge about

mixing ignition, combustion and wave initiation, wave propagation and wave reflection.

Any disturbance in the fluid medium creates a wave pattern. If the propagation of the

wave is parallel to the motion of the fluid, then it is termed as longitudinal waves e.g.

sound waves. This is the mode of wave propagation that occurs in a valveless pulsejet.

When the deflagration begins, a zone of significantly elevated pressure travels outward

through both air masses as a "compression wave". This wave moves at the speed of

sound through both the intake and tailpipe air masses.

(Because these air masses are significantly elevated in temperature as a result of

earlier cycles, the speed of sound in them is much higher than it would be in normal

outdoor air.) When a compression wave reaches the open end of either tube, a low

pressure rarefaction wave starts back in the opposite direction, as if "reflected" by the

open end. This low pressure region returning to the combustion zone is, in fact, the



internal mechanism of the Kadenacy effect. There will be no "breathing" of fresh air

into the combustion zone until the arrival of the rarefaction wave.

Mixing of air and fuel in a Valveless Pulsejet

In the combustion chamber fuel is injected into the flow of fresh air entering the

engine. At the beginning of the charging cycle the mixture is very rich, then it gets

leaned and at the end of the cycle it gets richer again but this mixing of fuel and air in a

flow stream are affected by the parameters of molecular size, concentration,

temperature, flow velocity in the vicinity of the injector and evaporation rate, vary

within wide bounds, the mixture is very non-homogeneous. The combustion chamber

consists of two distinct layers: a highly enriched layer with fuel and combustion

products from the previous cycle and a cold layer arising at the end of the suction cycle.

This mixture in-homogeneously causes a noticeable drop in its combustible properties.

The proper engine operation could be achieved with a mixture composition of air/fuel

ratio 1.1 - 1.4.

Ignition in a Valveless Pulsejet

Initially the fuel-air ignition is done manually with the help of blower and a

spark plug. Since the pressure inside the combustion chamber is above atmospheric

pressure, the combustion products along with the air flow towards the exhaust and

continue so long as the pressure in the chamber falls below atmospheric pressure. Now

the gases will retrace its path back into the combustion chamber since the atmospheric

pressure is greater than the combustion pressure. Because of the momentum or the

turbulence of the hot gas rushing back in, the pressure and temperature inside the

combustion chamber will increase drastically. Once the chamber temperature is above

the ignition temperature of the fuel the next ignition takes place and this cycle

continues.

Combustion process in a Valveless Pulsejet

The combustion process likely exists in two phases: an initial ignition which

gradually takes over the entire combustion chamber and this increases pressure and

temperature in the chamber and thereby facilitating the evaporation of the remaining

unburned mixture, and a main combustion process occurring almost instantaneously in



the entire chamber and lasting about 25% of the entire cycle. The combustion chamber

can reach up to a maximum approximate temperature of 2000K.

Since the pressure difference between the combustion chamber and exhaust is

oscillating, there will only be intermittent flow of air to the chamber to support

combustion. A pulse jet engine is an ideal example for an unsteady combustion process.

Here the combustion process is pulsating. The potential coupling between the unsteady

components of pressure and heat release can lead to sustained, large amplitude acoustic

oscillations which being driven by heat release is referred to as a thermo-acoustic

instability. Rayleigh was the first to hypothesize the onset of the instability and define a

criterion for positive coupling.

According to Rayleighcriteria if heat be periodically communicated to and

abstracted from a mass of air vibrating in a cylinder, the effect produced will depend on

the phase of vibration at which heat transfer takes place. If the heat be given to air at

moment of greatest compression or taken at the moment of greatest rarefaction the

vibration is encouraged. On the other hand heat is given moment of greatest rarefaction

or abstracted at the moment of greatest condensation, the vibration is discouraged.

Expansion of gases

Due to pressure being setup only at a certain region of engine, the gases at high

pressure migrate to low pressure regions in the engine and eventually out of the engine

(atmosphere). This happens at a very high velocity since the potential difference in

static pressure between atmosphere and the combustion chamber is very high.

This phenomenon occurs at the cost of losing the achieved high static pressure in

combustion chamber, a very high migration velocity implies a very high volume flow

rate of the engine, hence a very quick and drastic drop in static pressure

Suction of gases

Owing to the exit of the exhaust gases at very high velocities, the static pressure

in the combustion chamber drops drastically, the drop is to such an extent that a

negative gauge pressure (partial vacuum) is setup in the combustion chamber, which

forces to cease any further exit to the combustion gases, instead the combusted products

still dwelling in the engine is sucked back into the combustion chamber along with the



fresh atmospheric air. This leads to the fresh mixing of air and fuel inside the

combustion chamber for subsequent combustions.



7. WORKING OF A VALVELESS PULSE JET ENGINE

Fig 7.1: Working of Valveless Pulsejet Engine

The figure below shows a layout of a valveless pulsejet engine. It has a chamber

with two tubular ports of unequal length and diameter. The port on the right,

curved backwards, is the intake pipe. The bigger, flared one on the left is the exhaust,

or tailpipe. In some other engines, it is the exhaust pipe that is bent into the U-shape,

but the important thing is that the ends of both ports point in the same direction. When

the fuel-air mixture combusts in the chamber, the process generates a great amount of

hot gas very quickly. This happens so fast that it resembles, an explosion.



Fig 7.2: Layout of a Valveless Pulse Jet Engine

The immediate, explosive rise in internal pressure first compresses the gas inside

and then pushes it forcefully out of the chamber, two powerful spurts of hot expanding

gas are created a big one that blows through the tailpipe and a smaller one blowing

through the intake. Leaving the engine, the two jets exert a pulse of thrust they push

the engine in the opposite direction. As the gas expands and the combustion chamber

empties, the pressure inside the engine drops. Due to inertia of the moving gas, this

drop continues for some time even after the pressure falls back to atmospheric. The

expansion stops only when the momentum of the gas pulse is completely spent. At that

point, there is a partial vacuum inside the engine. The process now reverses itself. The

outside (atmospheric) pressure is now higher than the pressure inside the engine and

fresh air starts rushing into the ends of the two ports. At the intake side, it quickly

passes through the short tube, enters the chamber and mixes with the fuel. The tailpipe,

however, is rather longer, so that the incoming air does not even get as far as the

chamber before the engine is refilled and the pressure peaks.

One of the prime reasons for the extra length of the tailpipe is to retain enough

of the hot exhaust gas within the engine at the moment the suction starts. This gas is

greatly rarified by the expansion, but the outside pressure will push it back and increase

its density again. Back in the chamber, the gases of the previous combustion mix

vigorously with the fresh fuel/air mixture that enters from the other side. The heat of the

chamber and the free radicals in the retained gas will cause ignition and the process

repeats.



The spark plug shown on the picture is needed only at start-up. The retained hot

gas provides self-ignition and the spark plug becomes unnecessary. Indeed, if spark

ignition is left on, it can interfere with the normal functioning of the engine.In the J-

shaped and U-shaped valveless engines, gas spews out of two ports. Some valveless

pulsejet designers have developed engines that are not bent backwards, but

employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air

to come in but prevent the hot gas from getting out through the intake. A gentler, more

gradual entry would not generate the necessary swirling of gases. In addition,

turbulence increases the intensity of combustion and the rate of the heat release.

7.1 THERMODYNAMIC CYCLE

The thermodynamic working principle of a pulsejet engine does not have an

exact explanation; hence a popular and commonly accepted thermodynamic model is a

Lenoir cycle.

The Lenoir Cycle is an idealized thermodynamic cycle, where the ideal gas

undergoes basically 3 processes to produce work. The most interesting part of this cycle

is that the output work is obtained with no energy spent on compressing the working

fluid. The cyclic process are as follows,

(1) Constant volume (isochoric) heat addition and then

(2) Adiabatic expansion and

(3) Constant pressure (isobaric) heat rejection.

As Pulsejets typically have a very small compression ratio that reaches a

maximum at around (1.7). The Lenoir three cycle process can be seen below in.



Fig 7.3: Lenoir Cycle

As the expansion process is isentropic and hence involves no heat interaction.

Energy is absorbed as heat during the constant volume process and rejected as heat

during the constant pressure process. Hence the (P-V) diagram from fig (3.5-1)

represents the thermodynamic process of the Lenoir cycle.

Due to the finite time of combustion and incomplete filling of the chamber with

the fresh charge, the pressure at the end of the heat supply process depends on both the

fuel-air composition and on the relative volume of the fresh mixture entering through

the inlet valve. In this case the heat supply process is not isochoric. This deviation from

the ideal process demands for implementation of modifications to the existing ideal

process.

Designing of a valveless pulse jet engine

Valveless Pulse jets are much simpler in design than the valved engines, but

with simplicity you have to sacrifice kgs of thrust and loose the ram air effect. The

following section breaks a valveless pulsejet engine into major components and

investigates design approaches used in other designs for each component. The most

important components are the combustion chamber, the exhaust and intake pipes, the

fuel injection system, the spark ignition system and the air assist starting system. For



each of the components, various solutions are considered to guide in designing a

suitable pulsejet engine.

Combustion Chamber

The combustion chamber is arguably the most important component of a

valveless pulsejet design. For a valveless pulsejet engine, the combustion chamber

geometry is critical as any flow inconsistency can disrupt the pulsating combustion

cycle, as pressure waves may be reflected at sudden area changes. The most suitable

solution depends heavily on the selected configuration but there are several design

parameters that apply to all cases. The most significant attribute of a combustion

chamber is the circular cross section. This is because the pressure inside the combustion

chamber, positive or negative depending on the cycle, causes stress within the wall. This

stress is more evenly distributed by a circular cross-section design.

Fig 7.4: Comparison of conical sections

Combustion chambers also have conical sections leading into the intake and

exhaust pipes. These sections maintain smooth gas flow throughout the engine.The

above figure depicts the gas flow after combustion in both a conical section and a

stepped transition. The example on the left has a higher pressure increase because the

post ignition confinement is improved, but produces lower thrust because the gas suffers

choking due to entrance effects upon entering the exhaust, limiting the exiting velocity.

Conversely, a tapered cone that is too shallow has poor levels of post ignition

confinement, meaning thrust is also low. A good compromise is required in order to

have a practical engine.



Fig 7.5: Lockwood Hiller Combustion Chamber

Fig 7.6: Logan Combustion Chamber Section

The Logan combustion chamber section shows the implementation of the cone

sections on two different design solutions. Notably, the Lockwood-Hiller design has

steep cones while the Logan design features shallower tapers. This is because the

Lockwood- Hiller design has much larger intake and exhaust openings that allow the

flow to move relatively smoothly so post ignition confinement is the most critical

component of that design. Conversely, the Logan design has smaller openings and

requires unimpeded air flow exiting and entering the engine thus the conical section is

much shallower. From Simpson (2005), the optimum cone angle for an inline or linear

valveless configuration is approximately 30 degrees, depending on the size of the

engine. The cone section is a critical compromise between the flow of the gases and



post ignition confinement and as such, is a relatively critical consideration for our

design.

Exhaust and Intake

The exhaust and intake pipes of a valveless pulsejet engine are generally

straight, circular cross-section tubes with a critical length. The length is critical as it

must promote the acoustic resonance necessary to sustain engine operation. The

diameter of the pipe is also an important consideration as it needs to allow sufficient

flow to produce the required thrust; however, some degree of pressure must be retained

to aid in combustion chamber pressure increase.

Fig 7.7: Standard Exhaust Runner Design

The fig shows an arbitrary exhaust pipe section. The length to diameter ratio is

not as critical as the length is the critical dimension. Generally, however, the length to

diameter ratio is 7 to 10 percent of the length to give sufficient volume for gas flow.

This is similar for intake pipes to allow a sufficient fresh air charge into the combustion

chamber. Standard exhaust runner design also depicts the diffuser on the end of the

pipe. This is the same for both intake and exhaust and is necessary to control the flow of

gas exiting and entering the engine.



Fig 7.8: Sudden Expansion Exit Conditions

For the exit condition, this fig shows that when a sharp expansion occurs, the

flow creates turbulent eddies as it separates from these edges. This separation causes the

flow to lose energy, thus reducing the overall thrust developed by the engine. By

making this transition conical or bell-shaped these effects are negated keeping the flow

smooth and directing more of the energy of the flow into generating thrust from the

engine. Conversely, for the intake condition, the fig shows that the flow separates from

the surface at the sharp corner creating a vena contractor that effectively limits thecross-

sectional area through which the air can flow.

This limits the effectiveness of the intake to draw in the fresh air charge and the

exhaust to ingest the cool dense air required to confine the combustion event. Conical or

bell-shaped diffusers limit flow separation allowing smooth transition of the air into the

engine.

Fig 7.9: Entrance Flow Conditions



8. OBJECT AND APPROACH

Although not all waves have a speed that is independent of the shape of the

wave, and this property therefore is an evidence that sound is a wave phenomenon,

sound does nevertheless have this property .For instance, the music in a large concert

hall or stadium may take on the order of a second to reach someone seated in the

nosebleed section, but we do not notice or care, because the delay is the same for every

sound. Bass, drums, and vocals all head outward from the stage at 340 m/s, regardless

of their differing wave shapes. The speed of sound in a gas is related to the gas's

physical properties. It is a series of compressions and expansions of the air.

Fig 8.1: Propagation of Sound during the Operation of Pulse Jet

Even for a very loud sound, the increase or decrease compared to normal

atmospheric pressure is no more than a part per million, so our ears are apparently very

sensitive instruments. In a vacuum, there is no medium for the sound waves, and so they

cannot exist. The roars and whooshes of space ships in Hollywood movies are fun, but

scientifically wrong.



8.1 KADENACY EFFECT

Fig 8.2: Kadenacy Effect

In the explanation of the working cycle, inertia keeps driving the expanding gas

out of the engine all the way until the pressure in the chamber falls below

atmospheric. The opposite thing happens in the next part of the cycle, when the outside

air pushes its way in to fill the vacuum. The combined momentum of the gases rushing

in through the two opposed ports causes the chamber briefly to be pressurized above

atmospheric before ignition. There is thus an oscillation of pressure in the engine caused

by inertia.

The gases involved in the process (air and gaseous products of combustion) are

stretched and compressed between the inside and outside pressures. In effect, those

fluids behave like an elastic medium, like a piece of rubber. This is called the

Kadenacy Effect.

The elastic character of gas is used to store some of the energy created in one

combustion cycle and use it in the next.

The energy stored in the pressure differential (partial vacuum) makes the

aspiration (replacement of the burned gas with fresh fuel-air mixture) possible. Without

it, pulsejets would not work.



8.2 PROPAGATION OF SOUND IN PULSE JETS

The phenomenon of sound is easily found to have all the characteristics we

expect from a wave phenomenon Sound waves obey superposition. Sounds do not

knock other sounds out of the way when they collide, and we can hear more than one

sound at once if they both reach our ear simultaneously. The medium does not move

with the sound. Even standing in front of a titanic speaker playing earsplitting music, we

do not feel the slightest breeze. The velocity of sound depends on the medium. Sound

travels faster in helium than in air, andfaster in water than in helium. Putting more

energy into the wave makes it more intense, not faster.

Acoustic Theory

The pressure wave travels up and down the tube. When the wave front reaches

an end of the tube, part of it reflects back. Reflections from opposed ends meet and

form the so-called standing wave.

Fig 8.3: Standing Wave

A standing wave in a transmission line is a wave in which the distribution of

current, voltage, or field strength is formed by the superposition of two waves of the



same frequency propagating in opposite directions. The effect is a series of nodes (zero

displacement) and anti-nodes (maximum displacement) at fixed points along the

transmission line. Such a standing wave may be formed when a wave is transmitted into

one end of a transmission line and is reflected from the other end by an

impedancemismatch, i.e., discontinuity, such as an open circuit or a short. The failure of

the line to transfer power at the standing wave frequency will usually result in

attenuation distortion.

In practice, losses in the transmission line and other components mean that a

perfect reflection and a pure standing wave are never achieved. The result is a partial

standing wave, which is a superposition of a standing wave and a traveling wave. The

degree to which the wave resembles either a pure standing wave or a pure traveling

wave is measured by the standing wave ratio.

Fig 8.4: Wave Formation at the Exhaust

Another example is standing waves in the open ocean formed by waves with the

same wave period moving in opposite directions. These may form near storm centers, or

from reflection of a swell at the shore, and are the source of microbaroms and

microseisms. Graphically, the standing wave is best represented by a double sine

curve. The same is true for the pulsejet cycle. The undulations of a single sine curve

depict the changes of gas pressure and gas speed inside a pulsejet engine very well. The



doubling of the curve the addition of a mirror image, so to say shows that the places

where the pressure and speed are the highest in one part of the cycle will be the places

where they are the lowest in the opposite part.

The changes of pressure and the changes of gas speed do not coincide. They

follow the same curve but are offset from each other. One trails (or leads) the other by a

quarter of the cycle. If the whole cycle is depicted as a circle 360 degrees the speed

curve will be offset from the pressure curve by 90 degrees.The resonance establishes a

pattern of gas pressures and speeds in the engine duct that is peculiar to the pulsejet and

not found in the other jet engines.

In some ways it resembles a 2-stroke piston engine resonant exhaust system more

than in does a conventional jet engine. Understanding this pattern is very important, for

it helps determine the way the events in the engine unfold. When considering a pulsejet

design, it is always good to remember that those machines are governed by a complex

interaction of fluid thermodynamics and acoustics.

Elements of Resonance

In acoustic terms, the combustion chamber is the place of the greatest impedance,

meaning that the movement of gas is the most restricted. However, the pressure swings

are the greatest. The chamber is thus a speed node but a pressure antinode. The outer

ends of the intake and exhaust ports are the places of the lowest impedance. They are

the places where the gas movement is at the maximum and the speed changes are the

greatest in other words, they are speed antinodes. The pressure swings are minimal,

so that the port ends are pressure nodes. The pressure outside the engine is constant

(atmospheric).

The pressure in the combustion chamber seesaws regularly above and below

atmospheric. The pressure changes make the gases accelerate through the ports in one

direction or another, depending on whether the pressure in the chamber is above or

below atmospheric. The distance between a node and an antinode is a quarter of the

wavelength. This is the smallest section of a standing wave that a resonating vessel can

accommodate. In a valveless pulsejet, this is the distance between the combustion

chamber (pressure antinode) and the end of the tailpipe(pressure node). This length will

determine the fundamental wavelength of the standing wave that will govern the engine



operation.The distance between the chamber and the end of the intake is rather

shorter. It will accommodate a quarter of a wave of a shorter wavelength. This

secondary wavelength must be an odd harmonic of the fundamental.



9. DESIGNING AND MODELLING

9.1 CAD/CAE

Computer aided design or CAD has very broad meaning and can be defined

as the use of computers in creation, modification, analysis and optimization of a design.

CAE (Computer Aided Engineering) is referred to computers in Engineering analysis

like stress/strain, heat transfer, flow analysis. CAD/CAE is said to have more potential

to radically increase productivity than any development since electricity. CAD/CAE

builds quality form concept to final product. Instead of bringing in quality control

during the final inspection it helps to develop a process in which quality is there through

the life cycle of the product. CAD/CAE can eliminate the need for prototypes. But it

required prototypes can be used to confirm rather predict performance and other

characteristics. CAD/CAE is employed in numerous industries like manufacturing,

automotive, aerospace, casting, mould making, plastic, electronics and other general-

purpose industries. CAD/CAE systems can be broadly divided into low end, mid end

and high-end systems.

Low-end systems are those systems which do only 2D modeling and with only

little 3D modeling capabilities. According to industry statics 70-80% of all mechanical

designers still uses 2D CAD applications. This may be mainly due to the high cost of

high-end systems and a lack of expertise.Mid-end systems are actually similar high-end

systems with all their design capabilities with the difference that they are offered at

much lower prices. 3D sold modeling on the PC is burgeoning because of many reasons

like affordable and powerful hardware, strong sound software that offers windows case

of use shortened design and production cycles and smooth integration with downstream

application. More and more designers and engineers are shifting to mid end system.

High-end CAD/CAE softwares are for the complete modeling, analysis and

manufacturing of products. High-end systems can be visualized as the brain of



concurrent engineering. The design and development of products, which took years in

the passed to complete, is now made in days with the help of high-end CAD/CAE

systems and concurrent engineering.

9.2 MODELING

Model is a Representation of an object, a system, or an idea in some form other than

that of the entity itself. Modeling is the process of producing a model; a model is a

representation of the construction and working of some system of interest. A model is

similar to but simpler than the system it represents. One purpose of a model is to enable

the analyst to predict the effect of changes to the system. On the one hand, a model

should be a close approximation to the real system and incorporate most of its salient

features. On the other hand, it should not be so complex that it is impossible to

understand and experiment with it. A good model is a judicious trade off between

realism and simplicity. Simulation practitioners recommend increasing the complexity

of a model iteratively. An important issue in modeling is model validity. Model

validation techniques include simulating the model under known input conditions and

comparing model output with system output. Generally, a model intended for a

simulation study is a mathematical model developed with the help of simulation

software.

9.3 SOFTWARE FOR MODELING

Solid works

Creo

CATIA

Unigraphics, etc