Stirling Engine

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A Project Report On

DESIGN & MANUFACTURE OF A GAMMA STIRLING ENGINE

(A Working ProtoType)

Submitted in partial fulfillment of the requirement for the award of degree of

BACHELOR OF TECHNOLOGY

In MECHANICAL ENGINEERING

By

Syed Khairul Mubeen &

Oggu Raghunandan Reddy

Under the esteeemed guidance of

Mr. D. S. Nagaraju Associate Professor

Mechanical Department G.R.I.E.T

Department of Mechanical Engineering GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY (Affiliated to jawaharlal Nehru Technological University)

Bachupally,Hyderabad- 500 072 2011

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ACKNOWLEDGEMENT I am greatly thankful to all those who have given me the oppurtunity of doi ng my proj ect t raining a t Gokaraju Rangaraju Institute of Engineering and Technology. I eloquent m y gra titude t o m y gu ide Mr.D.S.Nagaraju of G.R.I.E.T for hi s gu idance, motivation and prov iding t he fa cilities t hat had helped me a lot in completing this project. I w ould l ike t o t hank Prof. P.S. Raju, D irector, Dr. Jandhyala Narayana Murthy, Pri ncipal a nd Dr. K.G.K. Murthy,Professor a nd Head of t he D epartment of M echanical E ngineering i n Gokaraju Rangaraju Institute of E ngineering and Technology for providing all the required facilities and for their encouragement and assistance. I would like to thank Dr.P.S.V. Kurma Rao Project coordinator of Gokaraju R angaraju In stitute of E ngineering a nd T echnology for providing a ll t he r equired fa cilities a nd for hi s e ncouragement and assistance. I am t hankful t o t he entire St aff, D epartment of M echanical Engineering for their encouragement and who had invested their valuable time in providing their feedback with a lot of useful sugggestions. I am indebted to my parents who have always been a constant source of my encouragement and i t i s because of their confidence in me that I was able to do my Graduation.

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CERTIFICATE

This is t o certify th at the thesis entitled “DESIGN & MANUFACTURING

OF A GAMMA STIRLING ENGINE” is carried out by Mr. Syed Khairul

Mubeen (07241A0367) & Mr. Oggu Raghunandan Reddy (07241A0347) of

Final ye ar, M echanical E ngineering bonafide students of G okaraju

Rangaraju Ins titute of E ngineering a nd T echnology a ffiliated t o JN TU

Hyderabad, i n pa rtial ful fillment of Fi nal ye ar proj ect , i s a re cord of

bonafide work carried out by them under my guidance and supervision,

The re sults em bodied i n t his t hesis ha ve not be en subm itted by ot her

university or institute for the award of any degree or any diploma.

Head of the Department Internal Guide

(Dr.K.G.K. MURTHY) (Mr.D.S. NAGARAJU) Professor of Mechanical Dept Associate Professor Department of Mechanical Engineering Department of Mechanical Engineering G.R.I.E.T G.R.I.E.T Hyderabad-72. Hyderabad-72.

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CONTENTS PAGE NO

1. INTRODUCTION 1

1.1. MOTIVATION 1

1.2. OBJECTIVE OF THE PROJECT 3

1.3. LIMITATIONS OF THE PROJECT 3

1.4. ORGANIZATION OF DOCUMENTATION 3

2. LITERATURE SURVEY 4

2.1. INTRODUCTION TO THE STIRLING ENGINE 4

2.2. EXTERNAL COMBUSTION ENGINE 4

2.3. STIRLING CYCLE 4

2.4. HISTORY OF STIRLING ENGINES 5

2.5. KEY COMPONENTS 6

2.6. WORKING OF AN STIRLING ENGINE 8

2.7. TYPES OF STIRLING ENGINES 10

3. ANALYSIS OF STIRLING CYCLE 14

3.1. THEORY 14

3.2. THE STIRLING CYCLE 15

4. DESIGN CONFEDERATIONS OF THE EXPERIMENTAL UNIT 16

4.1. CHOICE OF CONFIGURATION 16

4.2. CHOICE OF MATERIAL 16

4.3. LIST OF PARTS REQUIRED 18

5. SECTIONAL VIEW OF OUR GAMMA STIRLING ENGINE 19

6. LIST OF RAW MATERIALS FOR MACHINING 20

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7. THE ASSEMBLY DRAWING 21

7.1. CYLINDER COVER 22

7.2. HEATER 24

7.3. FLY WHEEL 26

7.4. CRANK DISK 28

7.5. PISTON HOLDER 30

7.6. PISTON & CYLINDER 32

7.7. JOINT BOARD 35

7.8. FRAME 37

7.9. BASE 38

7.10. SHAFT 39

7.11. CONNECTING RODS 40

7.12. BUSH 41

7.13. GASKETS 42

7.14. ASSEMBLY DRAWINGS 44

7.15. SUGGESTIONS TO ASSEMBLE 45

8. EXPERIMENTAL RESULTS 46

8.1. CYLINDER COVER 46

8.2. HEATER 46

8.3. FLY WHEEL 47

8.4. CRANK DISK 47

8.5. PISTONS & PISTON HOLDERS 48

8.6. JOINT BOARD 48

8.7. BASE & FRAME WITH BEARING IN IT 49

8.8. CONNECTING RODS 49

8.9 SHAFT & BEARING 49

8.10. ASSEMBLED PICTURES 50

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9. RESULT SUMMARY 51

9.1. STEP BY STEP PROCEDURE OF PROJECT COMPLETION 51

9.2. RESULTS OBSERVED 51

10. FAILURES 52

11. CONCLUSION 52

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ABSTRACT

Definition of Stirling Engine:

A S tirling engine i s a he at e ngine ope rating by c yclic compression a nd expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work.

The concept of a S tirling engine i s fairly s imple. The engine consist o f heat source, i n our cas e an al cohol flame, and a heat sink, ambient air, an enclosed cylinder, a ”heat” p iston, a ” power” pi ston, a nd a flywheel connected to the two pistons by a set of connecting rods. The concept is that the heat flowing through the air in the enclosed cylinder is modulate by the position of the ”heat” piston. When the ”heat” p iston is lo cated d irectly o ver th e flame the heat flow into the engine is minimized w hile t he h eat flow out of the cylinder to the heat sink is maximized. Similarly, w hen h eat flow in i s m aximize, h eat flow out is minimized. While the ”heat” pi ston i s moving, t he ” power” pi ston i s a lso moving t hus c onverting t he thermal energy being captured by t he air into mechanical motion. The flywheel then stores this mechanical energy, thus allowing the mechanical power to flow both in and out of the engine. The geometry of the linkages determines the relationship between the motion of the ”power” piston and the ”heat” piston.

Type of Stirling Engines:

1. Alpha type Stirling Engine 2. Beta type Stirling Engine 3. Gama type Stirling Engine 4. Free Piston type Stirling Engine

Gama Type Stirling Engine:

In ga mma Stirling t he pow er pi ston i s m ounted i n a s eparate c ylinder alongside t he di splacer piston c ylinder, but is s till c onnected t o t he s ame flywheel. The gas in the two cylinders can flow freely between them and remains a single body. This c onfiguration pr oduces a l ower c ompression r atio bu t i s m echanically s impler and often used in multi-cylinder Stirling engines.

Made by Mubeen Azad & Raghunandan Reddy

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1. INTRODUCTION

1.1. MOTIVATION

Why try and build a Stirling engine? Isn’t it an invention that has fallen by the way side for good reasons? It has been almost 200 years since Robert Stirling patented the device – if the engine had any merit it surely would have been researched, developed and adopted by now!?

To address this question lets conduct a simple thought experiment. Let’s compare the Stirling engine to today’s most commonly used prime mover; the internal combustion engine. The Stirling engine is less complex than an internal combustion engine because the only thing exchanged between the environment and the engine is heat. There is no need for valves, injectors, timing belts, and the list goes on. Simplicity implies two things for the Stirling engine; lower cost and higher reliability. Flexibility is another advantage of the Stirling engine has going for it. Again, because heat is the only thing transferred between the outside environment and the engine it can be adapted to any heat source. Let’s look at efficiency; Stirling engines routinely approach 30% efficiency. internal combustion engines rarely approach 20%. In the engineering world, that’s an enormous difference. Another notable difference between Stirling engines and internal combustion engines is the manner in which they burn carbon fuels (if one decided to use the Stirling in such a configuration). internal combustion engines have a very short period of time to mix, compress, ignite burn and expel fuel mixtures before the next cycle must occur – that is the nature of internal combustion engines. Combusting fuel to supply heat to a Stirling engine does not require a particular combustion process – you are free to choose the most efficient and clean burning option. To be fair, internal combustion engines do have a clear advantage in one area; performance, or more specifically, power to weight ratio. An internal combustion engine can produce a lot of power in a small package. This advantage is essential in only one field; transportation. So Stirling engines might not be the best option for your car, unless you value efficiency more than performance.

So to summarize, the Stirling engine is simple, reliable, efficient and flexible. This seems like a good set of attributes – where could we apply this engine effectively?

Electricity & Alternative Energy Option:

Electricity is so commonplace in our lives that we take it for granted. If we take a minute to think about how often we use electricity every day, and how difficult it would be to substitute that with an alternative, we begin to understand how versatile and powerful electricity is. For some people producing their own electricity is a very appealing idea, for a variety of reasons.


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Independence: for some people electricity is so important they want to be in charge of its production. Also, some people live so far away from it all and they have no choice but to produce their own electricity.

Sustainability: Some people choose to produce their own electricity so that they can control and negate environmental impacts.

Financial: These days producing electricity can be a profitable enterprise, and this is some people’s motivation.

People who want or need to produce their own electricity have four options: Solar panels, wind turbines, hydro, and gas generators. Let’s look at these options. We’ll begin with possibly the least appealing.

Gas generators can produce a lot of power in a small package. The acquisition cost is low but the operating cost is high. People resort to gas generators when they have no other options; as a backup or temporary measure. One advantage of a gas generator is you can choose when to operate it – it is power on demand, and thus does not require a means to store energy, unlike most alternative energy options.

Hydro is a great source of electricity, but is very location dependent. For most people this is not a possibility.

Wind turbines are cost effective and very environmentally friendly – an appealing option. Wind is however site and space dependent and is intermittent. Some places just don’t receive much wind, and some properties are too small to accommodate a turbine.

Finally we have solar panels. Solar panels are fairly expensive, even with government subsidies. That said they do pay for themselves within 10 years or so and don’t take up much space – they can usually be installed on roofs. Of course, solar panels are intermittent, and at night time you’re guaranteed to not produce any electricity.

Through its adaptability the Stirling engines overcomes some of the drawbacks of other alternative energy options. For example, in a solar thermal configuration the Stirling engine could be as effective as solar panels and with less upfront cost. People with a wood lot, or access to other carbon fuels could use a stove to run a Stirling engine and simultaneously heat their home or shop. This is a sustainable, power on demand solution which only hydro can rival! What we’re getting at is that there is a serious place for Stirling engines in the alternative energy field.

That motivation drives this goal: To produce a simple but reliable Stirling engine as a stepping stone towards larger and/or more specialized engines. So, we started with a small working prototype


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1.2. OBJECTIVE OF THE PROJECT

This paper presents a method to design & construct a small Gamma Stirling Engine by using a lathe, grinding wheel, drilling machine, taps.

As it is a small machine, Torque will be neglected, and will focus on RPM of the engine.

Size will be small, in order to make it more simple, easy to machine the parts.

More reliable materials will be used for better results.

Efforts will be made to reduce friction as far as possible.

Air leakage is to be minimized as far as possible.

1.3. LIMITATIONS OF THE PROJECT

Even a little air leakage, engine won’t work.

Friction is the biggest enemy of this engine.

For better elimination of air leakage, glass syringe is used, which may break if very high temperatures are used.

As it is a small engine, torque is nearly negligible.

1.4. ORGANIZATION OF DOCUMENTATION

In this project documentation we have initially put the definition and objective of the project as well as the design of the project which is followed by the implementation and testing phases. Finally the project has been concluded successfully and also the future enhancements of the project were given in this documentation.


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2. LITERATURE SURVEY

2.1. INTRODUCTION TO THE STIRLING ENGINE

A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work. The Stirling engine is traditionally classified as an external combustion engine, as all heat transfers to and from the working fluid take place through the engine wall. Unlike a steam engine's usage of a working fluid in both its liquid and gaseous phases, the Stirling engine encloses a fixed quantity of permanently gaseous fluid such as air. It works on Stirling cycle. The Stirling engine produces a higher efficiency rate than either the steam or internal combustion engines. However, it must run at very high temperatures to achieve maximum power output and efficiency.

2.2. EXTERNAL COMBUSTION ENGINE

An external combustion engine is a heat engine where an (internal) working fluid is heated by combustion of an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine produces motion and usable work. The fluid is then cooled, compressed and reused (closed cycle), or dumped, and cool fluid pulled in (open cycle air engine).

"Combustion" refers to burning fuel with an oxidizer, to supply the heat. Engines of similar (or even identical) configuration and operation may use a supply of heat from other sources such as nuclear, solar, geothermal or exothermic reactions not involving combustion; but are not then strictly classed as external combustion engines, but as external thermal engines.

The working fluid can be a gas as in a Stirling engine, or steam as in a steam engine. The fluid can be of any composition; gas is by far the most common, although even single-phase liquid is sometimes used. In the case of the steam engine, or the Organic Rankine Cycle the fluid changes phases between liquid and gas.

2.3. STIRLING CYCLE

The Stirling cycle is a thermodynamic cycle that describes the general class of Stirling devices. The cycle is reversible, meaning that if supplied with mechanical power, it can function as a heat pump for heating or refrigeration cooling, and even for cryogenic cooling. The cycle is defined as a closed-cycle regenerative cycle with a gaseous working fluid. "Closed-cycle" means the working fluid is permanently contained within the thermodynamic system. This also categorizes the engine device as an external heat engine. "Regenerative" refers to the use of an internal heat exchanger called a regenerator which increases the device's thermal efficiency.


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2.4. HISTORY OF STIRLING ENGINES

The original Stirling engine was designed and developed by Reverend Dr Robert Stirling, a fantastic engineer and He was a Scottish minister. At that time it was called a ‘hot air’ engine, no one knows when the term Sterling engine became widely accepted. Stirling received the original patent in 1816, and had his first engine built and working as a water pump in a quarry in 1818. Stirling was trying to come up with an alternative to the then current steam engine and later the internal combustion engine.

The main subject of Stirling's original patent was a heat exchanger which he called an "economizer" for its enhancement of fuel economy in a variety of applications. The patent also described in detail the employment of one form of the economizer in his unique closed-cycle air engine design in which application it is now generally known as a 'regenerator'. Subsequent development by Robert Stirling and his brother James, an engineer, resulted in patents for various improved configurations of the original engine including pressurization which had by 1843 sufficiently increased power output to drive all the machinery at a Dundee iron foundry.

The downside to the steam engine is the necessity to use boilers, which have the off chance to explode. Stirling sought to build an equivalent engine that would not have such a potentially deadly side effect. Although the Stirling engine eventually lost to the steam engine for popular support, it continues to be useful. It is not entirely lost, however, Philips, the large Dutch electric and electronic manufacturer began to design and produce a line of sterling engine based generators in the 1930’s. Development continued through WWII till the initial batch was produced in 1951. However by then the market was being taken over by the electric engine and the company lost out on the design.

Figure 1

Stirling Engine of Robert Stirling (1816)


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2.5. KEY COMPONENTS

As a consequence of closed cycle operation, the heat driving a Stirling engine must be transmitted from a heat source to the working fluid by heat exchangers and finally to a heat sink. A Stirling engine system has at least one heat source, one heat sink and up to five heat exchangers. Some types may combine or dispense with some of these.

2.5.1. Heat source

The heat source may be combustion of a fuel and, since the combustion products do not mix with the working fluid (that is, external combustion) and come into contact with the internal moving parts of the engine, a Stirling engine can run on fuels that would damage other internal combustion engines, such as landfill gas which contains siloxane. Other suitable heat sources are concentrated solar energy, geothermal energy, nuclear energy, waste heat, or even biological. If the heat source is solar power, regular solar mirrors and solar dishes may be used. Also, Fresnel lenses have been advocated to be used (for example, for planetary surface exploration). Solar powered Stirling engines are becoming increasingly popular, as they are a very environmentally sound option for producing power. Also, some designs are economically attractive in development projects.

2.5.2. Heater / hot side heat exchanger

In small, low power engines this may simply consist of the walls of the hot space(s) but where larger powers are required a greater surface area is needed in order to transfer sufficient heat. Typical implementations are internal and external fins or multiple small bore tubes.

Designing Stirling engine heat exchangers is a balance between high heat transfer with low viscous pumping losses and low dead space (unswept internal volume). With engines operating at high powers and pressures, the heat exchangers on the hot side must be made of alloys retaining considerable strength at temperature that also will not corrode or creep.

2.5.3. Regenerator

In a Stirling engine, the regenerator is an internal heat exchanger and temporary heat store placed between the hot and cold spaces such that the working fluid passes through it first in one direction then the other. Its function is to retain within the system that heat which would otherwise be exchanged with the environment at temperatures intermediate to the maximum and minimum cycle temperatures, thus enabling the thermal efficiency of the cycle to approach the limiting Carnot efficiency defined by those maxima and minima.


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The primary effect of regeneration in a Stirling engine is to increase the thermal efficiency greatly by 'recycling' internal heat which would otherwise pass through the engine irreversibly. As a secondary effect, increased thermal efficiency promises a higher power output from a given set of hot and cold end heat exchangers since it is these which usually limit the engine's heat throughput

The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer capacity without introducing too much additional internal volume ('dead space') or flow resistance, both of which tend to reduce power and efficiency. A typical design is a stack of fine metal wire meshes, with low porosity to reduce dead space, and with the wire axes perpendicular to the gas flow to reduce conduction in that direction and to maximize convective heat transfer.

Many small 'toy' Stirling engines, particularly low-temperature difference (LTD) types, do not have a distinct regenerator component and might be considered hot air engines, however a small amount of regeneration is provided by the surface of displacer itself and the nearby cylinder wall, or similarly the passage connecting the hot and cold cylinders of an alpha configuration engine.

2.5.4. Cooler / cold side heat exchanger

In small, low power engines this may simply consist of the walls of the cold space(s), but where larger powers are required a cooler using a liquid like water is needed in order to transfer sufficient heat.

2.5.5. Heat sink

The heat sink is typically the environment at ambient temperature. In the case of medium to high power engines, a radiator is required to transfer the heat from the engine to the ambient air. Marine engines can use the ambient water. In the case of combined heat and power systems, the engine's cooling water is used directly or indirectly for heating purposes.

Alternatively, heat may be supplied at ambient temperature and the heat sink maintained at a lower temperature by such means as cryogenic fluid or iced water.

2.5.6. Displacer

The displacer is a special-purpose piston, used in Beta and Gamma type Stirling engines, to move the working gas back and forth between the hot and cold heat exchangers. Depending on the type of engine design, the displacer may or may not be sealed to the cylinder, i.e. it is a loose fit within the cylinder and allows the working gas to pass around it as it moves to occupy the part of the cylinder beyond.


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2.6. WORKING OF AN STIRLING ENGINE

Stirling engines can be hard to understand. Here are the key points. Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (which is often air, helium, or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side it expands and pushes up on a piston. When it moves back to the cold side it contracts. Properly designed Stirling engines have two power pulses per revolution, which can make them very smooth running. Two of the more common types are two piston Stirling engines and displacer-type Stirling engines. The two piston type Stirling engine has two power pistons. The displacer type Stirling engine has one power piston and a displacer piston.

2.6.1. Displacer Type:

The displacer type Stirling engine is shown here. The space below the displacer piston is continuously heated by a heat source. The space above the displacer piston is continuously cooled. The displacer piston moves the air (displaces the air) from the hot side to the cold side.

Gas expands when heated, and contracts when cooled. Stirling engines move the gas from the hot side of the engine, where it expands, to the cold side, where it contracts.

DISPLACER PISTON

When there is a temperature difference between upper displacer space and lower displacer space, the engine pressure is changed by the movement of the displacer. The pressure increases when the displacer is located in the upper part of the cylinder (and most of the air is on the hot lower side). The pressure decreases when the displacer is moved to the lower part of the cylinder. The displacer only moves the air back and forth from the hot side to the cold side. It does not operate the crankshaft and the engine. In other words, the connecting rod to the displacer could be a string in this engine and it would still work.

Figure 2


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POWER PISTON

When the engine pressure reaches its maximum because of the motion of the displacer, a power piston is pushed by the expanding gas adding energy to the crankshaft. The power piston should ideally be 90 degrees out of phase with the displacer piston. The displacer type Stirling engine is operated by the power of the power piston.

2.6.2. Two Piston Type:

The two piston type Stirling engine is shown here. The space above the hot piston is continuously heated by a heat source. The space above the cold piston is continuously cooled.

HEATING

Let's start from top dead center of the hot piston. The hot piston moves to the upper part of the cylinder and the cold piston moves to the lower part of the cylinder during the first 90 degrees of revolution. The working air is moved from the cold space to the hot space. And the pressure in the engine is increased.

EXPANSION

During the next 90 degrees of revolution, the two pistons both move the lower part accepting the air pressure. The engine gets its power during this portion of its cycle.

COOLING

The crankshaft revolves by power stored in the flywheel for the next 90 degrees. The hot piston moves to the lower part and the cold piston moves to the upper part. The air is moved from the hot space to the cold space. And the pressure in the engine is decreased.

CONTRACTION

The two pistons are moved to upper part by the contraction of the air during the next 90 degrees. The engine also gets power during this portion of its cycle. The two piston type Stirling engine then repeats this cycle.

Figure 3


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2.7. TYPES OF STIRLING ENGINES

Stirling engines are classified to five types with viewpoint of working space.

1. The alpha-type has two power pistons. Also called Two Piston Type.

2. The beta-type has a displacer and a power piston with the same cylinder.

3. The gamma-type has a displacer and a power piston with independent cylinders. The beta- and gamma-type are also called the displacer type.

4. The double-acting type has for power pistons and uses both the top and bottom sides of the pistons.

5. The Free piston Stirling engine has a single piston and uses a spring.

2.7.1. Alpha Stirling

The alpha Stirling engine consists of two cylinders which have a 90 degree angle between their linear working paths. Both pistons are connected at the same point on the crankshaft and the two cylinders are connected by a pipe. As with all Stirling engines the gas called the working fluid never leaves these cylinders and pipes. There are no valves as in other engine types

therefore no other gas e.g. exhaust fumes moves in or out of the engine. This working fluid is usually air, hydrogen or helium. The two pistons are joined in such a way that they're linear motion can be translated into

rotational motion which can be used to power a mechanism like a power generator. On the outside of the cooling cylinder there could be cooling fins as in a normal air cooled combustion engine or a radiator system which would use liquid to cool the engine as in most modern cars. The heating cylinder would have an external heating source which could be a fuel burner e.g. gas or petroleum or it could be a renewable energy source like solar power. The alpha Stirling engine is similar to a two cylinder two stroke combustion engine in that each cylinder produces a power stroke in one rotation of the crankshaft.

Figure 4

H: hot region, R: regenerator, C: cold region

PP: Power Piston


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Low coefficient of friction, non-lubricated materials are used in order to prevent the use of lubricants which can clog the regenerator or the heat exchangers. These can be made from materials such as Rulon or Graphite. Rulon is a plastic and good choice of material because it does not require lubrication, is highly corrosion resistant and will work perfectly between -240°C and 288°C. Parts are designed to have low normal forces so that they don’t place too much pressure on any bearings which might lead to fatigue or fracture.

As with other heat engines the working fluid in the Stirling engine goes through four stages cooling, compression, heating and expansion. This is done in the alpha Stirling engine by moving the working fluid to the cooling cylinder and then through the regenerator to the heating cylinder. The change in temperature will cause the pressure of the working fluid to change and the movement of the pistons will also change the pressure.

This means that when the working fluid enters the heating cylinder it will be heated and expanded. As the working fluid expands it will firstly push the heating piston towards the bottom of its cylinder. This movement will push the cooling piston to the top of its cylinder. When the heating piston has reached half of its power stroke the cooling piston will begin its power stroke. Therefore at this point both pistons will be producing power. When the heating piston reaches the bottom of its cylinders it can go no further so the cooling piston will continue to provide power until it has reached the bottom of its cylinder. At this point the cooling piston is at the bottom of its cylinder and the heating piston is half way up its cylinder. The momentum created by the flywheel will spin the engine until the pistons return to they're original position and the heating piston can push again.

2.7.2. Beta Type Stirling Engines

A beta Stirling has a single power piston arranged within the same cylinder on the same shaft as a displacer piston. The displacer piston is a loose fit and does not extract any power from the expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold heat exchanger. When the working gas is pushed to the hot end of the cylinder it expands and pushes the power piston. When it is pushed to the cold end of the

Figure 5

H: hot region, R: regenerator C: cold region, PP: Power Piston

DP: Displacer Piston


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cylinder it contracts and the momentum of the machine, usually enhanced by a flywheel, pushes the power piston the other way to compress the gas. Unlike the alpha type, the beta type avoids the technical problems of hot moving seals.

Action of a beta type Stirling engine

1. Power piston has compressed the gas, the displacer piston has moved so that most of the gas is adjacent to the hot heat exchanger.

2. The heated gas increases in pressure & pushes the power piston to the farthest limit of the power stroke.

3. The displacer piston now moves, shunting the gas to the cold end of the cylinder.

4. The cooled gas is now compressed by the flywheel momentum. This takes less energy, since when it is cooled its pressure drops.

2.7.3. Gamma Stirling Engine

Gamma type engines have a displacer and power piston, similar to Beta machines, however in different cylinders. This allows a convenient complete separation between the heat exchangers associated with the displacer cylinder and the compression and expansion work space associated with the piston. Thus they tend to have somewhat larger dead (or un-swept) volumes than either the Alpha or Beta engines. Furthermore during the expansion process some of the expansion must take place in the compression space leading to a reduction of specific power. Gamma engines are therefore used when the

advantages of having separate cylinders outweigh the specific power disadvantage.

Because of the convenience of two cylinders in which only the piston has to be sealed, the gamma configuration is a favorite among modelers and hobbyists.

Figure 6

H: hot region, R: regenerator C: cold region, PP: Power Piston

DP: Displacer Piston


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2.7.4 The double-acting type

Double-acting type Stirling engine has four cylinders. Space is allotted at adjacent top and bottom of the piston, connecting it through a heat exchanger, the working space of double-acting type Stirling engine will be four times of a normal type Stirling engine. The engine can be downsized and the format is suitable for high-power engine. On the other hand, there are other problems that complicate the drive mechanism that moves a piston or four tight seal is needed to each piston and rod seals. Cams & valves are also used.

2.7.5. Free piston Stirling engines

Free piston Stirling engines include those with liquid pistons and those with diaphragms as pistons. In a "free piston" device, energy may be added or removed by an electrical linear alternator, pump or other coaxial device. This sidesteps the need for a linkage, and reduces the number of moving parts. In some designs friction and wear are nearly eliminated by the use of non-contact gas bearings or very precise suspension through planar springs.

Four basic steps in the cycle of a “Free piston” Stirling engine,

1. The power piston moves down by gravitation.

2. The volume in the engine decreases and the pressure of the working gas becomes higher. The higher pressure raises the displacer and piston and rod.

3. The hot space becomes bigger and the pressure in the engine becomes higher. The power piston then rises due to the higher pressure.

4. The volume decreases and the pressure become lower. Then the displacer piston moves down. The cold space becomes bigger and the pressure in the engine becomes lower.

Figure 7

H: hot region, R: regenerator, C: cold region

PP: Power Piston

Figure 8


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3. ANALYSIS OF STIRLING CYCLE

3.1. THEORY

The idealized Stirling cycle consists of four thermodynamic processes acting on the working fluid:

Isothermal Expansion: The expansion-space and associated heat exchanger are maintained at a constant high temperature, and the gas undergoes near-isothermal expansion absorbing heat from the hot source.

Constant-Volume (known as isovolumetric or isochoric) heat-removal: The gas is passed through the regenerator, where it cools transferring heat to the regenerator for use in the next cycle.

Isothermal Compression: The compression space and associated heat exchanger are maintained at a constant low temperature so the gas undergoes near-isothermal compression rejecting heat to the cold sink

Constant-Volume (known as isovolumetric or isochoric) heat-addition: The gas passes back through the regenerator where it recovers much of the heat transferred in 2, heating up on its way to the expansion space.

Theoretical thermal efficiency equals that of the hypothetical Carnot cycle - i.e. the highest efficiency attainable by any heat engine. However, though it is useful for illustrating general principles, the text book cycle it is a long way from representing what is actually going on inside a practical Stirling engine and should only be regarded as a starting point for analysis.

Other real-world issues reduce the efficiency of actual engines, due to limits of convective heat transfer, and viscous flow (friction). There are also practical mechanical considerations, for instance a simple kinematic linkage may be favored over a more complex mechanism needed to replicate the idealized cycle, and limitations imposed by available materials such as non-ideal properties of the working gas, thermal conductivity, tensile strength, creep, rupture strength, and melting point.

Figure 9

A pressure/volume graph of the idealized Stirling cycle


[15]

3.2. THE STIRLING CYCLE

Q1-2 → isothermal expansion

W1-2 → Q1-2 → RT, ln → W1-2

But,


[16]

4. DESIGN CONFEDERATIONS OF THE EXPERIMENTAL UNIT

4.1. CHOICE OF CONFIGURATION

It was stated earlier that there are five types of Stirling engines. And any Stirling engine requires components like heater, cylinder, piston, fly wheel, etc. we have made a choice of gamma type Stirling engine as it is more simple & reliable with better efficiency compared to alpha and beta engines.

This engine is a two piston type Stirling engine with a cap type heater made of stainless steel and it does not have a regenerator. Its cooling system is a natural convection cooling by air. The pistons and cylinders use medical syringes made of glass, so that the engine has few leakage of the working gas and small friction. To minimize the friction loss, at most of drive parts ball bearings are used. This engine has high speed performance. We hope that it can rotate about 1000 rpm.

4.2. CHOICE OF MATERIAL

4.2.1. Aluminum:

Lightweight - Aluminum is about one-third the weight of an equal volume of copper, steel or brass.

Strength - Aluminum can withstand heavy loads and pressure; when alloyed appropriately, its strength approaches that of steel.

High strength-to-weight ratio - The ratio of the tensile strength of aluminum, divided by density, is higher than any other metal.

Corrosion resistance - It has good resistence against the corrosive influences of water, salt and other influences.

Thermal conductivity - Aluminum spreads heat or cooling energy evenly and quickly.

Ductility - Aluminum is easy to cold work and fabricate.

Finishing - Aluminum can be finished with a variety of coatings and finishes such as paint, lacquer, porcelain or organic coatings, which can be anodized to bond to the metal.

Recyclability - Aluminum can be easily reclaimed and recycled into new, final aluminum products.

Cost-efficiency - Aluminum processing is inexpensive.


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4.2.2. Brass:

Density - Although not the most dense metal, brass alloys are a considerably dense substance. In Brass alloys, density=8 g/cm cubed, have a high density compared to many other substances. which is a important feature required in fly wheel.

Melting Point - The melting point for brass alloys is around 916 degrees Celsius, which makes brass alloys fairly resistant to heat.

Tensile Strength - Alpha brass alloys have a very high tensile strength in comparison to other metals and metal alloys. Alpha/beta brass alloys have a slightly lower tensile strength.

Elongation - In Alpha brass alloys, where Zinc is lower, the elongation is low, as opposed to alpha/beta alloys which have a considerably higher elongation. In fact, elongation increases with the amount of Zinc in the alloy.

Hardness - Brass alloys are generally considered to have a good score for hardness.

Modulus of Elasticity - For brass alloys, the modulus of elasticity is around 103 GPA or 15000000 psi (pressure per square inch). A 103 GPA implies that as a metal, brass alloys have a low modulus of elasticity as opposed to other types of metals.

4.2.3. Glass:

Minimum Friction – The smooth finishing and accurate shape makes it the most friction resistant material which could be used in this application

Minimum Leakage – a glass syringe provides an accurate leak proof seal between piston & cylinder with minimum friction

Non Expandable – glass is one of the materials which have lowest expansion coefficient. This makes them less subjected to stress.

Thermal conductivity – it has very less thermal conductivity which helps in little early cooling in cold region.

4.2.4. Stainless steel:

Hardness - Stainless steel is a hard and strong substance.

Ductility - Stainless steel possess high ductile strength. This means it can be easily shaped or machined into required dimensions.

Thermal Conductivity – it has the property to store heat for a long period of time, as we need a heater to store heat even after removing the heat source.


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Corrosion resistance - It is highly resistant to corrosion. And it does not get oxidized very easily.

Strength - Even at very high temperatures, stainless steel is capable of retaining its strength and resistivity towards oxidation and corrosion.

4.2.5. Silicon Rubber

Silicone rubber offers good resistance to extreme temperatures, being able to operate normally from -55°C to +300°C. At the extreme temperatures, the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers although still low relative to other materials.

It acts as air sealing between the joints to stop air leaks from the gaps

4.3. LIST OF PARTS REQUIRED

No. Name Material Number of parts

Notes

1 Cylinder Cover Aluminum 2 2 Heater Stainless 1 Steel of 316 grade 3 Fly Wheel Brass 1 4 Crank Disk Brass 1 5 Piston Holder Aluminum 2 6 Cylinder Glass 2

2ml Medical Syringe 7 Hot Piston Glass 1 8 Cold Piston Glass 1 9 Joint Board Aluminum 1 10 Frame Aluminum 1 11 Base Aluminum 1 12 Shaft Stainless 1 13 Connecting Rod Aluminum 2 14 Bush Steel Wire 2 15 Gasket Silicon Rubber Gel 3 16 Gasket Silicon Rubber Gel 1 17 Legs Rubber 4 18 Bearing Steel 2 Outer dia10mm,

inner 3mm 19 Bolt Steel 1 M4 x 12mm 20 Bolt Steel 4 M3 x 20mm 21 Bolt Steel 8 M3 x 12mm 22 Bolt Steel 4 M3 x 15mm 23 Bolt Steel 2 M2 x 20mm 24 Bolt Steel 2 M2 x 10mm 25 Washer Steel 4 M3 26 Nut Steel 4 M3 27 Nut Steel 4 M2 28 Cap Screw Steel 4 M3 x 5mm


[19]

5. SECTIONAL VIEW OF OUR GAMMA STIRLING ENGINE

This is the sectional view of the gamma Stirling engine which we have designed. It’s a small but friction less engine with less torque but good RPM.

It is a assembly of a heater, two cylinder covers, a cold piston, a hot piston, two cylinders, a fly wheel, a crank disk, a joint board carefully drilled, two connecting rods, and a frame & base.


[20]

6. LIST OF RAW MATERIALS FOR MACHINING

The above list of raw materials can be obtained from any metal shop. We got

them from FATHENAGAR in HYDERABAD.

2ml Medical glass syringe can be obtained at surgical shops. We got them from

NAMPALLY in HYDERABAD.

We got 3mm diameter stainless steel shaft from an old floppy drive door.


[21]

7. THE ASSEMBLY DRAWING

1: Cylinder Cover 2: Heater 3: Flywheel 4: Crank Disk

5: Piston Holder 6: Cylinder 7: Hot Piston 8: Cold Piston

9: Joint Board 10: Frame 11: Base 12: Shaft

13: Connecting Rod 14: Bush 15: Gaskets


[22]

7.1. CYLINDER COVER

All dimensions are in millimeters.

No. Material No. of parts notes 1 Aluminum 2 Inner Dia = outer

Dia of glass syringe


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Details:

• Cylinder cover is made up of aluminum metal.

• Lathe machine is required to turn the aluminum rod to required dimensions

• Inner diameter is taken as 11.5mm as external diameter of syringe we got was 11.45mm

• Inner Diameter of the cylinder cover must approximately equal to diameter of the 2ml syringe, such that glass cylinder fits inside the cylinder cover

• Fins are added for more exposure of metal surface for quick cooling

• A PCD marking drilling machine was used to mark exact location of tap holes. (this was used for more accuracy, it is not a compulsory)

• 2.5mm drills were used to make 4 holes in the cylinder cover

• M3 tap was used for tapping in those holes

• Filing is done to remove material peeled out after drilling & tapping

• Finally buffing is done to remove lines of turning


[24]

7.2. HEATER


No. Material No. of parts notes 2 Stainless steel 1 Steel of grade 316


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Details:

• Heater is made up of stainless steel of 316 grade.

• Lathe machine is required to turn the stainless steel rod to required dimensions

• thickness of walls will be less than 1mm, so internal bore must be carefully done to avoid crushing

• Internal surface must be as smooth as possible

• A PCD marking drilling machine was used to mark exact location of holes. (this was used for more accuracy, it is not a compulsory)

• 3mm drills were used to make 4 holes in the heater

• Filing is done to remove material peeled out after drilling



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7.3. FLY WHEEL


No. Material No. of parts 3 Brass 1


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Details:

• Flywheel is made up of brass.

• Lathe machine is required to turn the Brass rod to required dimensions

• Center hole is to be made on the lathe carefully, even a play in microns will fluctuate the rotation of the wheel

• 2.5mm & 1.5mm drills were used to make holes in the flywheel

• M3 & M2 holes are made till the depth required




[28]

7.4. CRANK DISK


No. Material No. of parts 4 Brass 1


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Details:

• Crank disk is made up of brass.

• Lathe machine is required to turn the Brass rod to required dimensions

• Center hole is to be made on the lathe carefully, even a play in microns will fluctuate the rotation of the wheel

• 2.5mm & 1.5mm drills were used to make holes in the crank disk





[30]

7.5. PISTON HOLDER


No. Material No. of parts 5 aluminum 2


[31]

Details:

• Piston holder is made up of aluminum.

• Lathe machine is required to turn the aluminum rod to required dimensions

• milling is done to make a slot in the holder as directed in the figure

• 1.5mm drills were used to make holes in the holder



• Finally buffing is done to remove lines of turning.


[32]

7.6. PISTON & CYLINDER:

7.6.1. Cylinders:


No. Material No. of parts notes 6 Glass 2 Medical syringe

7.6.2. Hot Piston




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7.6.3. Cold piston



Example of a Glass syringe:

How to cut a glass syringe:

• The syringe is cut using a grinder with a blue grindstone

• We found a glass cutting machine in an spectacle shop

• Cutting must be done slowly for avoiding cracks at cutting edges


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• Water must be used on the grinding wheel as a coolant while grinding

• Syringe must be cleaned & dried after cutting

• Cylinders must be glued to cylinder cover without any air gap between cylinder cover & cylinder.

Silicone gel for gluing:

• We used araldite to glue the glass cylinder to the cylinder cover

• It makes a tight seal between cylinder cover & cylinder by blocking the gaps

• Both the tubes in the pack are opened & mixed in equal ratio

• Mix them until gel converts to grayish color

• Keep hands safe by wearing gloves

• Apply the mixture on the both cylinder wall & inside cylinder cover

• Slowly insert cylinder into cylinder cover

• Keep it aside for drying for 2 to 3 hours

• Care is to be taken that gel doesn’t enter the cylinder.


[35]

7.7. JOINT BOARD




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Details:

• Joint board is made up of aluminum block.

• milling machine is required to perform milling on the alluminum block to accurate step size

• Surface smoothness is important

• Edges are not to be champed

• Drilling is the main operation to be done on the board

• Holes of different diameters at accurate dimensions are to be drilled to accurate depths

• Drills bits of 3.2mm diameter, 3.5mm diameter, 12.5mm diameter are required in machining of this part

• M4 tap is used for tapping on one side


• Finally buffing is done to remove lines of milling.


[37]

7.8. FRAME



Details:

• Frame is made up of aluminum block.

• milling machine is required to perform milling on the alluminum block to accurate size


• 10mm drill bit & M3 tap is required




[38]

7.9. BASE



Details:

• Base is made up of aluminum block.

• milling machine is required to perform milling on the alluminum block to accurate size


• 3.2mm drill bit & M3 tap is required




[39]

7.10. SHAFT


No. Material No. of parts 12 Stainless steel 1

It is a stainless steel shaft available in disk drive or floppy drive doors.

It has good smoothness, light weight

It fits exactly in 3mm bearing

Can be cut to required length and can be used as fly wheel shaft

Shaft


[40]

7.11. CONNECTING RODS


No. Material No. of parts 13 Aluminum 2

Details:

• Connecting rods are made up of 2mm aluminum sheet.

• Sheet cutter machine is required to perform cutting on the alluminum sheet to accurate size

• 2mm drill bit is required


• Finally buffing is done.


[41]

7.12. BUSH


No. Material No. of parts 14 Steel wire 2

Details:

• This is a small bush made of steel wire.

• It needs a cutting player to mould the wire into circular shape.

• Steel wire is used as it is hard and smoothly finished to avoid friction


[42]

7.13. GASKETS


No. Material No. of parts 15 Silicon rubber sheet 2


No. Material No. of parts 16 Silicon rubber sheet 1


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Details:

• Silicon rubber sheet is used to make gaskets for this engine

• It acts as a air seal

• It even resists some heat transfer from heater to joint board

• It can with stand high temperatures

• Silicon rubber sheet is cut into required shape using a industrial scissors

• Holes are drilled using drilling machine

If silicon rubber is not available, gasket maker gel can be used.

Gel is to be applied carefully without letting it inside the cylinder or heater.

It takes more than 6 hours to get dry.


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7.14. ASSEMBLY DRAWINGS (follow the part No.)


[45]

7.15. SUGGESTIONS TO ASSEMBLE

A: Seal and fix between a cylinders cover (No.1) and a cylinder (No.6) with a silicone glue.

B: Fix between a piston holder (No.5) and a piston (No.7,8) with a quick drying glue.

C: Fix a connecting rod (No.13) to a piston holder (No.5) with a bolt (No.24) and a nut (No.26) to move light.

D: Cut a top of a bolt (No.23).

E: Fix the bolts (No.23) to a flywheel (No.3) and a crank disk (No.4) using double nut type.

F: Fix bolts (No.22) to a base (No.11) with double nut type.


[46]

8. EXPERIMENTAL RESULTS

8.1. CYLINDER COVER

8.2. HEATER


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8.3. FLY WHEEL

8.4. CRANK DISK


[48]

8.5. PISTONS & PISTON HOLDERS

8.6. JOINT BOARD


[49]

8.7. BASE & FRAME WITH BEARING IN IT

8.8. CONNECTING RODS

8.9 SHAFT & BEARING


[50]

8.10. ASSEMBLED PICTURES

Heater, cylinder, cylinder cover, joint board

Final assembly of Gamma Stirling Engine


[51]

9. RESULT SUMMARY

9.1. STEP BY STEP PROCEDURE OF PROJECT COMPLETION

First we have designed the parts of stirling engine by the use of thermodynamics & design of machine members

made the plan of the gamma stirling engine in AutoCAD

We have choose the material according to the requirement of the part

We crosschecked the material & dimensions of the part whether the part may withstand the forces produced in the engine

We designed the parts keeping in view that their weight must not reduce the RPM of the engine

Piston & cylinder were selected considering less friction with high air packing inside the engine

We bought 2ml syringes from surgical store in NAMPALLY, HYDERABAD

Material were bought from JAGADAMBA STEEL STORE at FATHENAGAR

Gasket gel, silicon glue have bought from a hardware shop

Most of the machining was done in our college, GOKARAJU RANGARAJU INTITUTE OF ENGINEERING & TECHNOLOGY

Materials like stainless steel and parts which required high accuracy were machined at machining shops, SHOBHANA COLONY, BALANAGAR.

Parts were assembled at college &Engine was tested at a welding shop using gas flame of 400°C to 450°C.

9.2. RESULTS OBSERVED

We got RPM of 300 to 400 at temperature of 400°C to 450°C in duration of 2mins

Because we used piston & cylinder of glass, friction was very less, air inside engine was locked

There is no air leak

Less weight

Fly wheel is efficient enough to push the air tight pistons to 4 revolutions without engine on


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10. FAILURES

→ We first tried piston with stainless steel, cylinder with brass. But, it failed because of weight of piston and small leakages from between piston & cylinder. A picture of metal piston and cylinder in cylinder cover is shown below

→ While testing, due to sudden exposure of heat, heater melted and a hole was created. We had to again machine a new heater as welded patch created extra heat at a single point.

→ Glass cylinder was broken due to high heat

→ There were many air leakages which we had to close using silicon gel

11. CONCLUSION

Gamma type Stirling engine was designed & manufactured successfully, which is running with an RPM of 300 to 400 which we would like to improve.

A better Stirling engine with high accuracy can be used for generation of electricity by using sun as heat source.

Documents

Stirling Engine