Fabrication and Testing of a Walking Beam Gamma Type Stirling Engine-Randall

7/28/2019 Fabrication and Testing of a Walking Beam Gamma Type Stirling Engine-Randall

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[Type text]

December 10, 2010

Nicholas Randall | Faculty Advisor Daniel Martinez, PhD

EAST

UNDERGRADUATE

FELLOWSHIP

STIRLING ENGINE


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Nicholas Randall December 10, 2010

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Purpose

The purpose of this project is to fabricate a walking beam Gamma type Stirling engine and test

the engine characteristics when operating using an external heat source. Additionally, the long-

term goals of the project are to understand the laws of thermodynamics as they apply to the

Stirling engine.

Introduction

One of the purposes of this project was to understand the laws and principles of a Stirling Cycle.

The Stirling engine works on the laws of thermodynamics mainly the first and second law. The

first law states that energy is neither created nor destroyed but it just changes form. The second

law states that heat would flow from hot to cold and can only flow from cold to hot if there is

work done upon it.

The other purpose of this project was to take the knowledge learned and apply it to building a

model of a Stirling engine. This project was based off an engine found on a website (Boyd2005). This engine used a walking-beam configuration in conjunction with a gamma

configuration of a Stirling engine and was adapted for use with this project.

In 1816, Robert Stirling was awarded a patent for an external combustion engine he designed and

built to help reduce the horrendous accidents that frequently occurred with steam engines boilers

that were in use at that time. Robert Stirling made an engine that did not require a boiler to run,

but still used external combustion. After Stirlings invention, the engines became widely used for

pumping water out of the ground (Boyle et al. 2003). Stirling engines are undergoing a new

phase of development using a number of energy sources, such as solar, geothermal, and biomass.

Stirling engines are used to produce electricity from the sun's energy by putting one part of the

Stirling engine in a device that will collect the sun's heat. Stirling engines have also been used ina geothermal application to drive heat pumps for pumping hot water out of the ground (Lian, et

al. 2005). In biomass power plants a Stirling engine is used to recover heat that would be lost in

the combustion process and they are able to generate power from Stirling engine output (Obara,

et al. 2008). Stirling engines are used in either very high scale or very low scale applications with

little utilization in between. The ability to access the medium scale could allow for a number of

opportunities in thermal generation, electricity generation, and transportation. The reason Stirling

engines have been used in high and low scale applications is because of the power to volume

ratio. Some research has been done on making a hybrid car with an internal combustion engine

and a Stirling engine by using the exhaust gases to power the Stirling engine. The Stirling engine

only added 30% efficiency (Cullen, et al. 2009).

The Stirling engine requires two sources of energy creating a difference in temperatures. One

source of energy is created by using a Sterno gel fuel. The other energy source is a cooling

source at the top of the engine, which is ice. The purpose of having the hot and the cold source is

to create a pressure difference between the displacement cylinder and the power cylinder, which

will cause the power cylinder to move back and forth depending on whether there is an increase

in pressure or a decrease in pressure. When the pressure increases (expansion), the power


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cylinder gets pushed outward and when the pressure decreases (compression), the power cylinder

will get sucked inward.

Methods

Once all the preliminary work was done, the next thing was to choose a design to build an

engine. The design was based off a tin can Stirling engine with a walking beam with some

modifications (Boyd 2005). We used the gamma configuration, which consists of a separate

power cylinder and displacement cylinder. The displacement cylinder will displace the air, which

will cause the power piston to move in a linear direction which causes the flywheel to turn. This

creates momentum which drives the engine.

Displacement cylinder assembly

For the main displacement cylinder assembly, instead of a can we used a 3 steel pipe for the

main cylinder. This made for some complications by using steel pipe instead of a can. One of the

complications was that with a can, you have one end already sealed. We needed to be able to sealone end but have the other end accessible so that we could try different displacer pistons. This

was done by welding a plate on the bottom end of the pipe but there was trouble getting an

airtight seal at first. There had to be a grove cut in the plate so the pipe would sit down in the

plate (figure 1). Also there had to be a bevel put on the outside edge of the pipe so the welding

bead would have someplace to sit (figure 2). This was done by a professional welder (figure 3).

The top plate was bolted on using eight hex head machine screws. This process was done by first

machining the end of the pipe on the lath so the end was perfectly flush (figure 4). Then there

were eight holes drilled in the top of the pipe set apart at 45 degrees from one another (figure 5).

This was done by putting the pipe on a turn table that was bolted to a milling machine so the pipe

was standing on end (figure 6). Holes were drilled in the top of the pipe. Once that was donethere were threads tapped into the holes. The plate had to have a grove in it so that pipe would

bolt down and seal properly on the plate. Once the plate was finished, it was bolted on to the top

of the pipe with a high temperature silicone to get a good seal (figure 7).

Power piston assembly

The next step was to make the power piston assembly which was made out of a bronze sleeve

bearing and the piston was made from a piece of aluminum stock (figure 8). There was a hole

drilled and tapped in the side of the displacement cylinder about half-way up from the bottom so

the power piston assembly could be attached by screwing it into the displacement cylinder and

sealing it with high temperature silicone (figures 7, 9).

Displacer piston

The displacer piston was made from an empty tin can and a 1/8 brass rod. By only drilling two

holes in either end of the can and attach the displacer piston rod with silver solder on either end

of the can. The top steel plate had a center hole in it which had a 3/8 bolt running through it

serving as a guide for the displacer piston rod (figure 7). The first displacer piston that was made

was too small, so the displacer piston was changed out. The first thing it was replaced with was a


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bigger tin can. The next thing was another tin can but instead of using a tomato paste can, it was

an imported mandarin orange can (figure 10) and the main differences were that this new can had

smooth sides and was a little taller.

After the mandarin orange can, there was a more direct approach at reaching the optimal size

displacer piston. Research was done to figure out the optimal ratio for the displacer piston to the

displacer cylinder. Then, the measurements that were taken from the engine were compared to

other engines that were researched. The optimal ratio was a diameter of 2 3/4, a radius of 1

3/8, and a length of 4 (table 1, figure 11).

The next can was an aluminum can. Because it was aluminum, it was light and smooth on the

sides. The problem with the aluminum can was that, as a displacer, aluminum cans dont work as

well because they have a dimple on the bottom of the can which allows air to get trapped under

the can. To fix that problem, we used aluminum tape to tape over the dimple so that it was flat.

Assembly of the flywheel, walking beam, connecting rods, and structure

There was a structure created to hold all of the parts into place. The structure was made out ofwood and consisted of two masts and a base supported by four legs. The two masts, one

supported the flywheel and the other supported the walking beam. (figure 12). This was the

initial set-up, but there was trouble with this set-up so the design was changed by running tests

and figuring out which parts were causing problems. The base was modified by cutting off half

of it because having an open flame under a wooden base was a safety hazard. By cutting off the

base, it allowed for the engine to be put on top of the flame so that the wood would not catch on

fire. But this also caused a problem with having insufficient support for the engine. To resolve

this problem, there was an aluminum base made that allowed us to put the flame underneath the

engine and still be sturdy.

The original flywheel was found to be too heavy, which was made from hardwood. The new

flywheel was made from fiberboard. This flywheel was much lighter but had to be mounted in adifferent way than the previous flywheel by using a collar and then putting a bolt through the

collar.

The original design for the mast for the flywheel was to have two supports lined up parallel so

that a shaft would run through them. But this caused problems with friction so with the new

flywheel just had one support. The mast supporting the walking beam had to have several holes

drilled in it so that the height of the walking beam could be adjusted (figure 13).

The old walking beam was a piece of wood that was thicker than it needed to be and therefore

added extra weight. The new walking beam was much lighter because it was much thinner than

the original. The walking beam was centered between the flywheel and the displacer piston rod.Otherwise, the timing of the engine would be off.

One of the problems with the connecting rods is that they were binding and to help this problem,

we first bent the rods so that they were able to flex, but they were still binding at certain points.

The rods were replaced with wire to get rid of the binding problem, but this allowed the displacer

piston to move from side to side and hit the displacer cylinder walls.


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The other problem with the brass rod was that it was causing too much friction in the

displacement rod guide sleeve and to get rid of the friction a bigger hole drilled in the guide

sleeve. If the hole was too big, there would be a loss of power due to the fact that the pressure

was escaping through the bigger guide sleeve hole instead of going to the power piston to do

work. The final conclusion was to go with a thinner rod for the displacer piston so that it would

allow for some flexibility, but still have some rigidness to keep the displacer piston going up anddown in a straight line so it wouldnt hit the sides of the displacer cylinder.

Assembly of the engine

Assembling the engine consisted of putting together the power piston assembly which screwed

into the main displacer piston cylinder and the displacer cylinder was screwed down to the base

for stability. The next step was to connect the displacer piston rod to the walking beam which

was done by connecting to a joint that had two pivot points to prevent the rod from flexing too

much and getting bound on the displacer cylinder guide sleeve. The walking beam was

connected to the flywheel by a double-hooked brass rod, which allowed the brass rod to pivotaround the flywheel. The assembly was tested without the power piston hooked up. The test was

to spin the flywheel gently to see how many times the flywheel would spin around without

stopping or getting hung up on something. After this it was time to hook up the power piston to

the flywheel then the engine assembly was complete and was in the test and debugging stage.

Results

Thermodynamics.

The second law is in effect by allowing for heat to transfer from the heat source to the displacercylinder. The first law is used to explain what happens in the Stirling Cycle. The first law states

that you can use heat energy to transfer into mechanical work. The Carnot cycle, being the ideal

cycle, does not work in reality because youll never use the full amount of heat that is put into an

engine. The energy input that you put in will never equal the energy that you get out due to heat

loss within the cycle.

Stirling Cycle

Understanding the Stirling Cycle was one of the key principles behind this project. There were

several weeks of research before the project was started. This was to understand the Stirling

cycle and terminology that goes along with it. The Stirling Cycle is made up of four different

stages. The process between stage one and two is isothermic. Volume is increasing, temperature

stays constant, and pressure is decreasing (compression). The displacer piston moves up. In the

stage between two and three, the volume stays the same, temperature drops, and this causes a

decrease in pressure (compression) which pulls in the power piston. Between the processes of

three and four, the volume decreases, the temperature is isothermic, and the pressure increases

(expansion) and pushes out the power piston. The temperature increases, the pressure is


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increasing (expansion), and the volume is isovolumetric. The power piston is at the maximum

outward push and the displacer starts to move up. The cycle repeats (table 2)

The objective:

During the process of building this engine, there were several problems that had to be overcome.

The engine would work when the displacer piston was moved up and down by hand. This caused

the power piston to move in and out accordingly. What didnt work is the mechanical

configuration that was designed, due to the force that the power piston was putting out, was not

enough force to turn the flywheel a full turn.

The first thing that was done to improve the power output was to change out the displacer piston

in order to try to get more power by displacing more air. This works until the piston is too big

and it doesnt have any room to move up and down, so there is less air getting displaced. To

figure out the optimal displacer piston size, there was a table produced by calculating different

ratios that were found on other Stirling engines of the size of the displacer cylinder to the

displacer piston (table 1). The result of changing the first displacer piston provided more powerbut was still too heavy and had a tendency to cause drag.

The changes that were made to the structure and components did not solve the problem of the

engine not running. However, they improved the performance and made it easier to see what

other problems there were. A result of changing the flywheel was that the flywheel was lighter.

This way was better in the sense that the flywheel moved a lot easier, but it didnt allow us to

hook up anything to it. The next plan is to use a similar set-up but attach a pulley to the flywheel

so that it would be possible to hook something else to it. By changing the walking beam, it

helped to make it easier to line up the engine for timing. But, it didnt make the difference

between the engine running and not running.

Future Plans

One of the future plans is to redesign the base for optimal efficiency. To have the engine placed

so that there would be no strains on different parts of the engine and to have everything perfectly

lined up to minimize any drag. One of the things that needs to be done is to raise the mast that

holds the walking beam so the connecting rod can be run from the power piston underneath the

raised walking beam because there was trouble with the piston connecting rod running into the

mast of the walking beam. This involved changing the setup for the flywheel so that the

connecting rod from the power piston to the flywheel would be straight.

Another change that needs to be made is to run thermalcouples to the engine so the efficiency of

the engine could be measured and it would also help to troubleshoot the engine so that the engine

would run. Another thing would be to change the displacer piston to a steel-wool scrubber pad

which allows the shape and size of the displacer piston to be changed. Also, the steel-wool pad

is a good thermal mass because it allows for air to rush through it when it is going up and down

as a displacer piston which allows a rapid heating and cooling. This is done by the fact that steel-


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wool is porous. The porous material allows heat to be trapped so that it stores the heat to be used

in a later process. This is called the regenerative process.

Publishing

This research will be presented at Thinking Matters in spring, 2011 and I have created a website

at sites.google.com/site/usmstirlingengine/ that describes the process that I went through to build

this engine and make this project.

References

Boyd, D. 2005. Stirling engine and plans. http://boydhouse.com/stirling/. Accessed October 12,

2010.

Boyle, G., B. Everett, and J. Ramage. The Stirling engine. P. 322-327.In

Energy Systems andSustainability. Oxford University Press, New York, NY.

Cullen, B., J. McGovern. 2010 Energy system feasibility study of an Otto cycle/Stirling cycle

hybrid automotive engine. Energy 35: 1017-1023.

Hargreaves, C. M. 1991. The Philips Stirling engine. Elsevier, New York.

Lian, Z., S. Park, W. Huang, Y. Baik, and Y. Yao. 2005. Conception if combination of gas-

engine-driven heat pump and water-loop heat pump system. International Journal of

Refrigeration 28: 810-819.

Obara, S., I. Tanno, S. Kito, A. Hoshi and S. Sasaki. 2008. Eexergy analysis of the woodybiomass Stirling engine and PEM-FC combined system with exhaust heat reforming.

International Journal of Hydrogen Energy 33: 2289-2299.

Urieli, I., D. M. Berchowitz. 1984. Stirling cycle engine analysis. Adam Hilger, Bristol.


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Appendix: Tables and Figures

d r r2

l

3 1.5 2.25 3.318345

2.9 1.45 2.1025 3.551142

2.8 1.4 1.96 3.8093252.7 1.35 1.8225 4.096722

2.6 1.3 1.69 4.417915

2.5 1.25 1.5625 4.778417

2.8 1.4 1.96 3.809325

2.7 1.35 1.8225 4.096722

Final Dimensions 2.75 1.38 1.89 3.95

Table 1. Calculations for optimal displacement piston size.

Green is the optimal zone of the displacer piston and then the orange is the average of the

two green rows. These calculations allowed me to determine the optimal size of the

displacer piston by working with the pressure/volume equation. d= diameter; r= radius; l=

length(height) of piston; V= volume; Vdc= volume of displacement cylinder; Vpp=

volume of power piston; Vpc= volume of power piston cylinder.

Vdc Vpc - Vpp

24.74004 1.276743

Vpp Vdc-(Vpc-Vpp)

0.608212 23.4633

Vpc

1.884956

Pressure/volume equation p1V1-p2V2

Cycle 1 to 2

volume increases

temperature stays constant (isothermic)pressure decreases

Cycle 2 to 3

volume stays constant (isovolumetric)

temperature decreases

pressure decreases

Cycle 3 to 4

volume decreases

temperature stays constant (isothermic)

pressure increases

Cycle 4 to 1volume stays constant (isovolumetric)

temperature increases

pressure increases

Table 2. Pressure, volume and temperature table.

This is the process of the Stirling Cycle when looking at

it from a pressure, volume, and temperature standpoint.


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Figure 1. Grooved plates. Groves were made by using a milling machineand a turn table.

Figure 2. Beveled edge of displacement cylinder. Put the beveled edge

on displacement cylinder for welding on a steel plate.


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Figure 4. Cleaning ends. The pipe is on the lath, and using a cutting tool to

clean the end of the pipe so that it is flush.

Figure 3. Professionally welded the plate onto a pipe to make a displacercylinder.


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Figure 5. Drilling holes in end of pipe. Holes were drilled in the end of the

pipe so I could tap in threads for bolting on the end plate.


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Figure 6. Set-up on milling machine. This is the set-up

that was used to drill the holes in the end of the pipe

for bolting on the plate.


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Figure 7. Displacement cylinder assembly. This is thedisplacement cylinder with the top plate bolted on and the

displacement piston and guide rod sleeve attached.


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Figure 8. Power piston assembly, disassembled. The power piston

cylinder is a brass sleeve bearing. The power piston is a 1" piece of

aluminum stock made into a piston.

Figure 9. Displacement cylinder assembly with power piston assembly

attached. There was high-temperature silicone, which has a red color, used to

seal all potential leaks.


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Figure 10. Displacer piston. Displacer piston with silver solder attaching the

brass rod.

Figure 11. Arrangement of displacer cylinders for testing. These are the

displacement cylinders that I cut after making the table of calculations

(table 1). The blue can was the optimal size. The object to the far left is a

jig made out of wood for cutting cans on the lath.


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Glossary

AAir- The surrounding gases near the earth surface and is made up of mostly nitrogen, oxygen,

and argon. Air is also one of the working fluids most Stirling engines use

Alpha Stirling engine- The alpha configuration uses two power cylinders, one hot, one cold,

with a regenerator in between them

Atmosphere- A unit of pressure (atm)

BBeta Stirling engine- The beta type engine is when the mechanical configuration has the power

piston and the displacer piston in the same cylinder

CClosed cycle engine- The engine working fluid never leaves the engine like in a Stirling engine

Compression- The stroke that decreases the volume of the cylinder and increases the pressure of

the cylinder

DDiaphragms- Is a flexible material that is used instead of a piston or in conjunction with a piston

Displacer- is a device that pushes matter from a side to another side or place

Displacer Piston- pushes the working fluid from one side to the other side and in most Stirlingengines is connected to the power piston by some mechanical means by a quarter turn.

EEnergy- The ability to do work for duration of time

Entropy- Measurement in thermodynamics for the amount of work that cannot be used in a

closed thermodynamic system

Ericsson Cycle- Works like the Stirling Cycle but the volume is what changes in the Ericsson

Cycle instead of the pressure

Expansion-To move apart from one another

External combustion- When the reaction that produces thermal energy happens outside the

engine.


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FFree piston- A piston that is not attached to anything mechanically. The piston moves up and

down by the increase and decrease of pressures in the cylinder.

Friction- The energy that is produced by when two contacting surfaces are rubbing together,

producing the result of thermal energy.

GGamma Stirling engine- A different mechanical configuration of the Stirling engine. In the

gamma configuration, the displacement and the power cylinders are separate.

HHeat engine- Needs heat to produce mechanical energy

Heat pumps- A mechanical device that moves thermal energy

Helium- A working fluid in some Stirling engines

Hydrogen- A working fluid in some Stirling engines

IIsocaloric process- Is when heat does not transfer from one place to another place. Also known

as adiabatic.

OOpen cycle engines-When the engines working fluid is used once and then pushed out of the

engine. An example is an internal combustion engine.

PPiston- A cylinder that moves in a linear direction inside another cylinder

Power- The amount of work done in a certain time period

Power Piston- The piston that is used to convert energy to mechanical motion

Pressure- Force that is uniformly spread across an area. Often measured as pressure per square

inch (psi).

RRegeneration-What happens when heat is moved by a working fluid thus cooling the area


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SSchmidt Theory- The way calculations are made on the efficiency of a Stirling engine

Sink- A piece of metal that will absorb thermal energy

Stirling Cycle- How a Stirling engine works in principle

Stirling engine- A heat engine

TThermal efficiency-The measure of the devices performance thermally

Thermal expansion- The amount that an object can expand by a certain amount of heat

Thermal Mass- The amount of heat that an object can store

VVacuum- Created by a change in pressures from a higher pressure to a lower pressure

WWorking fluid- The fluid or gas in an engine that carries heat so it can be reused in the

regenerator

Documents

Fabrication and Testing of a Walking Beam Gamma Type Stirling Engine-Randall