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Stirling Engine Project

Posted on October 24, 2008 by admin

Before I get into the details of my Stirling engine project I want to first talk abit about how a Stirling engine works, and the history behind it. A Stirling

Engine is a heat engine that works on the basis of an external appliedtemperature difference. By maintaining a hot and cold temperature differencethe engine is able to run and produce mechanical power. It is different fromthe Internal Combustion Engine (ICE) in that it is a closed cycle; that is, theworking gas is enclosed insidethe engine. This is in contrast to the InternalCombustion Engine (Otto cycle) in which the working gas (air) is drawn infrom the environment and expelled as exhaust. Where valves, air/fuel mixersand timing mechanisms are necessary in such engines, in a Stirling engine, nosuch components are required. Furthermore, the Stirling engine is notrestricted to the type of fuel used. It is indifferent to the source of heat,

which opens up many possibilities, including non-polluting solar energy, or theburning of biomass (wood, husks, ethanol, etc) which can be carbonneutral, meaning they absorb as much carbon (due to photosynthesis) asthey emit when burned. Since fossil fuels, which add to greenhouse gases,need not be used for such an engine we have a clear positive advantage.

Stirling Engine SoftwareUseful for designing Stirling engines and predicting performance. Simple touse. Only $89.95 CAD. Information manual included which explains in rich

detail how a Stirling engine works. Clickhere to find out more.
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The basic principle of the Stirling engine is this. The working gas inside theengine is heated, which increases its pressure and moves pistons as a result.The gas is then cooled, lowering its pressure. It is then heated again, and thecycle repeats. In a real engine this typically happens very fast, on the sameorder of speed as an ICE. The working gas is shuttled back and forth very

quickly inside the engine, between the hot and cold ends, continuouslygaining and losing heat and producing power as a result.

The Stirling Engine is by no means a new invention. It was invented nearly200 years ago by a Reverend, Robert Stirling, who was concerned about thenumber of injuries and deaths resulting from catastrophic explosions of high-pressure boilers of steam engines in the workplace. He proposed the StirlingEngine as a way to produce power without the same explosive risks.Unfortunately, due to the nature of the Stirling Engine, it requires a very hightemperature difference and high internal pressure to function. Although the

consequences of such a system failing were not as severe as in steamengines, it still proved to be restricted by the limitations of the materials atthe time (e.g. lack of high temperature resistant alloys). As a result, theStirling Engine never found its niche early on in high power applications. Andas Steam Engines became increasingly safer, the Stirling Engine was limitedto low and medium power applications, such as pumping water.

With the growth of the ICE in the 1900s, the Stirling Engine was pushedfurther into obscurity, namely because of cheap energy in the form of fossilfuels, but also because of the response characteristics of the ICE. The ICE,

unlike the Stirling Engine, doesnt need to warm up as much and is quick toaccelerate and change speed, which is ideal for automobiles, for example. Butthe Stirling cycle is inherently slower to respond to changes in powerrequirements. However, with proper design it can be made more efficient andconsume less fuel than a comparable spark ignition ICE (e.g. gasolineengine). For these reasons the Stirling Engine is ideal for steady poweroutput, and it is starting to catch on. Hobbyists, engineers, and inventorsfrom all over are working on the development of these engines. In fact, therehave been several promising engines with high power and high efficiencymade in the past, such as those produced by the Philips company, and NASA,which developed and tested the MOD I and MOD II engines. In a 1986 designreport for the MOD II automotive engine, it was stated in Appendix A thathighway gas mileage increased from 40 to 58 mpg, and in the city itincreased from 26 to 33 mpg with no change in gross weight of the vehicle(ref:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880002196_1988002196.pdf). However, due to the very high engine pressure required for highpower (15 MPa) which requires strong bulky components and thedifficulty in incorporating fast response and acceleration time into the engine,it never made its way into the market. It turns out that it only found realsuccess in niche applications such as submarines for military applications

where cost isnt prohibitive. And currently, high-power Stirling engines can bevery expensive, on the order or $10/Watt for high-power prototype systems.
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But the cost is much less for certain small-scale systems that arecommercially available.

But the Stirling Engine is currently gaining more attention as interest inalternative non-polluting sources of power is on the rise. And the technology

is catching up to the ICE, which has always been further along on thedevelopment curve. For an example see WhisperGen Heat and PowerSystems: http://www.whispergen.com.

Additional useful information can be found at the Wikipedia pagehttp://en.wikipedia.org/wiki/Stirling_engine. And if you have some knowledgeof thermodynamics you probably wont mindhttp://en.wikipedia.org/wiki/Stirling_cycle.

As it turns out there is a lot of information available online. I will touch on

some more of it as I get into the work I personally have done on the subject.

A Deeper Investigation Into The Workings Of The Stirling Engine

In the last year I put significant effort into learning about and attempting tobuild a working DIY Stirling Engine. In other words, I wanted to build ahomemade Stirling engine. My goal was to make a homemade Stirling engineof decent power (100 W or so) out of everyday materials you can purchase atCanadian Tire, Home Depot, or your local hardware store. A true DIY project,which just happens to be low-budget as well Now, given this self-imposed

restriction, this was a huge challenge as I wanted to make something morethan just a cute low-power model like the kind you see on YouTube So Iwent at it, and to be honest I never got an actual working engine, although Idid get a lot of insight into how it works. And I did show that by powering theengine in reverse (as a heat pump), you can make a refrigerator out of it.More on that later.

First off, here is a picture of the homemade Stirling engine I made:
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Figure 1

Its obviously made out of wood in large part. I chose wood because I happento have a lot of it around and its basically free, easy to work with and is oftentimes strong enough to do the job. Now, I could have chosen to make the

engine out of metal, but I dont have a machine shop or the necessary metalworking tools. So I would have to get someone to machine the parts for me,and because its a learning process there would be some trial and errorinvolved. It can get pretty expensive if you know what I mean. So doing it theinexpensive way I set myself up for a basic understanding, so that when I dodecide to make an engine out of metal it will be further along towards beingfunctional. And believe me, these beasts are not as easy to make as onemight think.

The above engine is an alphaconfiguration. I chose the alpha configuration

because it is easiest for me to build given what I have to work with, and issimpler conceptually than the other types of configurations, which are knownas beta, and gammaengines. You can find out more about them at the sameWikipedia link as before http://en.wikipedia.org/wiki/Stirling_engine and atthis website, by Israel Urieli, which talks about the different mechanicalconfigurations of Stirling engines:http://www.ent.ohiou.edu/~urieli/stirling/engines/engines.html.

The above engine is actually my fourth attempt at building a DIY StirlingEngine. As my understanding progressed it became obvious what I did wrongeach time. Hindsight is 20/20 after all. And some things you cant know until
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you try and build it yourself. Hands on experience is the only way to get agood understanding of Stirling engine design.

And BTW I wont bother showing you any of the work I did on those previousfailed versions

Lets start off by dissecting what I did for this latest version.

The big thing was that I had to make the engine airtight. I learned early onthat a key aspect of having a functioning, higher-power Stirling engine wasbeing able to contain the working gas (which is air in my case) inside theengine with no (or very little) leakage. If you have an air leak you will losepower fast. The only way around that is to have an air compressorcontinuously pumping in air in order to maintain pressure. This was not anoption for me, so I had to find a way to seal in the air using everyday DIY

materials. So that means that using a piston with seals, plus bore (cut from ametal block), is out, as its not DIY. So I opted to use a diaphragm of somesort, which would expand and compress.

To make the diaphragms I used pieces cut from silicone baking mats(purchased from the cookware section of Canadian Tire) and RTV high-temperature red silicone (also purchased at Canadian Tire) to attach themtogether. I found that RTV silicone forms a powerful and airtight bond withthe baking mats. See Figure 2-5.

Figure 2
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Figure 3

Figure 4
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Figure 5: RTV high-temperature red silicone

From Figure 3 and 4 you can see that I coated the inside surfaces of the

diaphragms with the RTV silicone. Since it can resist temperatures of up to340 degrees Celsius (and the baking mat can resist up to 230 degrees) it actsas a kind of heat resistant barrier. I also coated the top surface of thediaphragm with silicone to help protect it from wear since this is where thepiston sits and pushes against.

A key requirement in using the diaphragms is that the pressure inside theengine is always above atmospheric. This is a key part of the operatingconditions. It means that at no point during the piston stroke can thediaphragms be pulled (since they cant be pulled). They can only be

pushed.

Before the diaphragms could be attached to the frame I needed to make abase. See Figure 6 and 7.
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Figure 6: Hot engine side

Figure 7: Cold engine side
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The hot engine side is the part of the engine that receives heat, and the coldengine side is the part of the engine that is cooled. The heating and coolingtakes place by way of heat exchangers which feed hot and cold air into therespective parts of the engine. See Figure 8.

Figure 8

The heat exchangers are wound up lengths of 0.25 inch OD copper tube,roughly 80 cm in length, long enough to permit adequate heat exchange withthe hot and cold sources which are in direct contact with them. I made themby bending the turns around a 1.5 inch dia. pipe, big enough to minimizeflattening of the tube cross-section and avoid kinking. On the previous designI tried using much shorter copper tubes, flattened in order to increase thewetted perimeter relative to the cross-sectional area in order to increase theheat transfer rate, in theory. This wasnt very effective despite the fact that

the heat transfer formulas I used predicted a high heat transfer rate. So whatI then did was experiment with different lengths of tube (not flattening them)by blowing through them while they lay over a bed of coals. I then measuredthe exiting air temperature with a digital kitchen thermometer. I found thatwith about 80 cm of tube length the exiting temperature was around 300degrees Celsius, which is close to the measured temperature of the coals. Itwas a crude approach, but it gave me the information I needed.

In between the hot and cold exchangers is a regenerator. A regeneratorincreases the efficiency of a Stirling Engine. It is not necessary to have onefor the engine to run but it does reduce the energy input requirement fromthe heater and the energy removal requirement of the cooler, making the
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engine run more efficiently. The way the regenerator works is by storingsome of the heat energy of the working gas as it moves from the hotexchanger to the cold exchanger, thereby reducing the cooling requirement ofthe cold source. And on the return path, as the working gas moves from thecold exchanger to the hot exchanger, it gains back that heat energy,

thereby reducing the heating requirement of the hot source. The Wikipedialink explains the regenerator in detailhttp://en.wikipedia.org/wiki/Stirling_engine. The literature I found indicatesthat stacked wire mesh screens, stacked metal fins, or a bed of sphericalballs, serve as effective regenerator material. I personally used steel wool, asrecommended by numerous online sources. Figure 9-11 shows a picture ofthe regenerator. I inserted steel wool into a 2 inch length of copper tube,clipped the ends, and put compression fittings on the ends which allowed it tobe attached to the hot and cold exchangers (Figure 12). Unlike some of theother calculations I did, I wasnt rigorous about choosing the 2 inch length. It

just seemed like a good length to use. Besides, I wasnt sure how to modelsteel wool in my calculations.

Figure 9
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Figure 10

Figure 11


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Figure 12

The base, as shown in Figure 6 and 7, was made by first drilling holes intotwo lengths of 26, sized to provide a snug fit for inserting the copper tube. Ithen flared the ends of the copper tube with needle nose pliers to prevent

them from pushing out due to the engine pressure. I then drilled a largershallow hole (not all the way through) around the tube holes which provideda means to make a good airtight seal using the RTV silicone. On the cold sidea tireless air valve (Figure 13) was inserted into another drilled hole andsecured in place with washers and screws (Figure 14). On the hot side a heatresistant paint (Figure 15) was applied to the wood which increases itsresistance to high temperature (Figure 16 and 17).
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Figure 13

Figure 14


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Figure 15

Figure 16


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Figure 17: Bottom side

Finally two silicone baking mats were cut to size and placed on top of the twopieces of 26, aligned with the holes, with beads of RTV silicone appliedunderneath for adhesion to the wood. The gaps around the holes were

carefully sealed using RTV silicone and then the baking mat surfaces werecoated with the RTV. This was especially important for the hot side. Theresult is shown in Figure 6 and 7.

The two diaphragms were then centered and placed on the hot and cold sidebases and carefully sealed around the bottom with the RTV silicone (Figure18).
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Figure 18

The regenerator was then coated with a copious amount of RTV silicone(Figure 19) to provide thermal insulation on the exterior, which reduces theheat lost to the environment as the hot air passes through. This improves the

engine efficiency. Theres also a chance the thick layer of silicone eliminatedany small air leak at the compression seal junctions. But most likely thecompression seal is already airtight anyway.

Figure 19
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Figure 20 shows a picture of the piston over top of the diaphragm. The strokelength of the pistons is 1.5 inches, and at the lowest position of the piston theinside top surface of the diaphragm leaves a 1/8 inch gap with the bottom.This was done to eliminate as much dead (unswept) volume as possible.Unswept volume is the volume in the engine that remains constant as the

engine is running. Swept volume is the volume that is swept by the motionof the pistons. So if the stroke length of a piston is L, and the area is A, thenthe swept volume is AxL.

Figure 20

Dead volume tends to reduce engine power, so it must be minimized,especially in the expansion space and compression space (the volumesenclosed inside the diaphragms). Dead volume unavoidably exists in Stirlingengines so it is best to keep it in the location of the hot and cold exchangers,

and regenerator, since in these locations it can be of benefit and not just adetriment (e.g. longer heat exchangers with greater dead volume can boostheat transfer between the hot and cold source and the working gas. And alarger regenerator can boost its thermal storage capacity making the enginemore efficient). Indeed, the optimal design of the Stirling Engine often meansbalancing conflicting variables.

Metal containers (the kind that hold canned goods) were then placed aroundthe diaphragms in order to help the diaphragms resist the lateral pressureforces (Figure 21). The breaking strength of the baking mats is only 4 MPa(they are 1.5 mm thick) and that of the RTV silicone is 1.5 MPa, so its a goodidea to support the diaphragms somehow. Also, the piston itself helps the
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diaphragm resist the pressure pushing up, and the smaller the difference indiameter between the metal container and the piston, the less pressure forceacts on the diaphragm. In my case, the diameter of the piston is 3 inches andthat of the metal container is 4 inches. This gap (0.5 inch all around) isdeliberate so that the diaphragm comfortably rolls in and out at the sides,

much like a roll sock seal used in some Stirling Engine models. I worked outthat with this arrangement the diaphragms can resist over 100 psi enginepressure without breaking.

Figure 21: Metal containers placed around the diaphragms andattached to the base with RTV silicone, as its a good adhesive

A closer look at the final assembled engine is shown in Figure 22-26. Notethat the hot side piston motion is identical to the cold side piston motion(sinusoidal). The only difference is that the hot side piston is 90 degreesfarther ahead of the cold side piston, in terms of rotational position. Forexample, when the hot side piston is at the top most position, the engine hasto rotate another 90 degrees in order for the cold side piston to reach thesame top most position. The Wikipedia link also explains this, for alphaengines: http://en.wikipedia.org/wiki/Stirling_engine.
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Figure 22: Heater side using a bed of coals. The measuredtemperature of the coals was about 300 degrees Celsius, which is

close to the temperature limit of the RTV silicone


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Figure 23: Cooler side using cold supply water from a garden hose,adjusted to a low flow rate, and collecting in a plastic container.

Sealing was done with waterproof GE Silicone II. The temperatureof the water was roughly 15-20 degrees Celsius

Figure 24: Water draining from an overflow tube. This keeps thewater level constant inside the plastic container
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Figure 25: Underside view of the cold side, showing the air valve,and the vinyl tube used to drain the water out of the plastic

container when finished. It is manually dropped down so that thewater simply drains out


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Figure 26: View of linkages and flywheel (taken from an oldwheelbarrow). The main parts consist of threaded rods and steel

strips cut to size, with holes drilled. The connection piece, throughwhich the vertical threaded rod goes, at the bottom of the picture,

was taken from an old workout bench I no longer use. There was nowelding. Only bolts were used to fasten.

Now, its best to use something cleaner burning than coal if you can, likeeco-flame warming gel (used for camping) which is derived from sugar cane.Or you can use another carbon-neutral bio-fuel.
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To test the engine it was pressurized with a manual air pump (Figure 27) to 0psig and 3 psig (at maximum engine volume Vmax the rest position duringpressurization). The engine flywheel was then given a quick spin. But afternumerous attempts the engine did not start running. The engine did not evenappear to be on the brink of running. However, the compression was good

and the engine showed no signs of leaking air. The video below shows theattempt.

Figure 27

http://www.youtube.com/watch?v=fDwBrTp0Ois

Afterwards, I tried running the engine as a Stirling refrigerator. All Stirlingengines will produce a hot and cold temperature difference when rotated byan external power source. So if you attached a motor to the flywheel, oneend of the engine would increase in temperature and the other end would
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decrease in temperature. This is basically a heat pump, and is not toodifferent from how an air conditioner works.

What I did was pressurize the engine to around 1 psig (at Vmax)and cranked the flywheel by hand for a few minutes in the same

direction as before, at a speed of about 1 rev/sec. And atemperature difference developed. The original hot exchanger gotcolder and the original cold exchanger got warmer. The temperaturedifference obtained was around 10 degrees Celsius. The original hotexchanger reached 16 degrees Celsius and the original coldexchanger reached 26 degrees Celsius. This might seem a bitcounter-intuitive, however.

Note that the initial temperature of the heat exchangers was 17degrees Celsius same as the ambient air temperature at the time.

See Figure 28 and 29.

Figure 28: Temperature of 26 degrees Celsius
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Figure 29: Temperature of 16 degrees Celsius

This result is interesting because it clearly shows that you can run such anengine in reverse and operate it as a refrigerator, or a heater, if you wish.Essentially, it draws heat from the expansion space (it gets colder) and

pumps it out of the compression space (it gets warmer).

I had the option of attaching a motor to the flywheel which would haveresulted in much faster and more uniform spinning. And no doubt this wouldhave increased the temperature difference, but the linkages where notholding up that well so I decided against it.

In summary, the engine did not run as a heat engine. I pressurized theengine (at Vmax) at 0 psig and 3 psig, and it did not start. It did not evenshow any signs of almost starting. There are clearly mechanical losses, in

the linkage mechanism with the jerky motion, and probably with the flexingand deforming of the diaphragms as well, just to name a few.

It should be mentioned that at start up (1-2 rev/sec) the power of the engineis low, maybe 5-10 W, with the 3 psig initial pressure (according to myStirling engine simulator). The simulator I created shows that a much higherengine pressure than what I used is necessary to overcome thefriction/losses, and obtain a good running speed. But this pressure, which ison the order of 8-10 bar (120-150 psi) would create too much stress on thelinkages (over 800 pounds of upward force!). And the diaphragms may nothold up either.
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With higher pressure the engine has more kick during the expansion stage.This is the stage of the engine which moves from minimum engine (internal)volume to maximum engine volume. It is this kick which allows it to pushthrough the compression stage, which is the stage of the engine which movesfrom maximum engine volume to minimum engine volume. With enough

kick, it does this with energy to spare, resulting in a net power output. Theflywheel is necessary because it receives the energy of the kick, and due tomomentum, pushes the engine though the compression stage. If this kick isnot strong enough it simply wont start. And it means that you either have toolittle net power output or your engine losses are too high. This kick can beincreased in two ways: increase internal pressure and/or increase hotside/cold side temperature difference (usually by increasing heatertemperature).

In essence, the kick comes from the expansion of the working gas as it heats

up when flowing through the heater and into the hot piston space (which inmy case is the diaphragm on the hot side).

Summary

Clearly, I was not able to achieve my goal of building a DIY Stirling engine ofdecent power using common materials. But I was able to demonstrate itscapacity as a heat pump, to an extent, which serves as a way to evaluatehow well the thermodynamic aspects of the engine function. Hopefully inreading all of this you have acquired a better understanding of Stirling engine

design.

This project has paved the way for me to build another Stirling enginesometime in the future. But it will be made out of metal, especially thelinkage and flywheel assembly where smooth mechanical action, and strengthis required. The diaphragm/RTV silicone idea may still be used though, butunfortunately it may not have very long life due to the constant flexinginvolved. And also it will not be able to resist very high pressures, andtemperatures much greater than 300 degrees Celsius. And high heatertemperature is essential for higher efficiency and power. So a metalpiston/bore configuration is probably the best solution, especially in the case

of high heater temperatures and internal pressure. Good seals wouldobviously be needed though, such as PTFE polytetrafluoroethylene (Teflon).But since they cannot resist excess temperature then perhaps a betaengineconfiguration would have to be used instead of an alphaconfiguration. And itmay be that an air compressor would have to be connected to such an enginein order to maintain and control the engine pressure, with the compressoroutput regulated to increase or decrease the engine power (increase ordecrease the engine speed, respectively). To increase the engine power, extraair would be pumped in, and to decrease the engine power, air would bepumped out. Ideally any air leak would be small which would minimize the

steady-state compressor power required to maintain engine pressure.


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Also, its better to use a hot side heat exchanger made of stainless steel. Thecopper tube I used showed clear signs of oxidation which may be fine fortemperatures of 300 degrees Celsius, but it wont hold up for very hightemperatures in excess of that (on the order of 700-800 degrees Celsius).Stainless steel is much more resistant to the effects of oxidation at high

temperature.

One other point to make is that the regenerator was not that essential for myengine and wouldnt have made much difference to the outcome. This isbecause the heating and cooling power of my set up were quite a bit inexcess of what I needed to power the engine. Getting high thermal efficiencywas not a priority at this stage in my design because I was only interested inhaving lots of heating and cooling power to get it running. And once it getsrunning, you can then confidently make the heater and cooler more efficientand focus on minimizing the energy required for each for example, putting

an insulated enclosure around the heat source to help trap the heat so thatit only gets absorbed by the heat exchanger and doesnt bleed away to thesurrounding air. Once you do this, thats when a regenerator becomes veryimportant. It helps keep the minimum energy input low.

If youre looking for Stirling engine software to aid you in yourdesign efforts, I created a program. To learn more about it go here.

Here are some more good Stirling Engine links:

Stirling Engine Design Manual, William R. Martini, University ofWashington, April 1978. Link:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780016056_1978016056.pdf

Stirling Engine Design Manual, Second Edition, William R. Martini,Martini Engineering, January 1983. Link:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830022057_1983022057.pdf

Dish Stirling Systems. Link:

http://www.nrel.gov/csp/troughnet/wkshp_power_2007.html#dish

This one was key to me being able to create my Stirling engine simulator:

Non-linear Analysis of Stirling Engine Thermodynamics, Oak RidgeNational Laboratory, R. D. Banduric and N. C. J. Chen, June 1984.Link:

http://www.ornl.gov/sci/ees/etsd/btric/eere_research_reports/thermally_activ
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ated_technologies/engine_driven/stirling_rankine/modeling_and_simulation/ornl_con_154/ornl_con_154.html

This next one gives you a good overview of Stirling Engine terminology:

Efficiency Terms for Stirling Engine Systems, Oak Ridge NationalLaboratory, J. L. Crowley, June 1983. Link:

http://www.ornl.gov/info/reports/1983/3445605890836.pdf

Check this one out. This man built a 2.5 hp gamma Stirling out of hisworkshop. He calls it The Jim Dandy # 6. Link:

http://www.starspin.com/stirlings/jimd6.html

Lastly, here is a list of points I put together which you may find helpful inStirling Engine design:

The optimal compression ratio (maximum engine volume/minimum enginevolume) that results in the greatest power, is less than 2. Note that theengine volume is the whole of the volume contained inside the engine,which includes the volume in the heater space, the cooler space, theregenerator, and the hot and cold side piston spaces. While the volume in theheater, cooler and regenerator remains constant, the volume in the hot andcold piston spaces change over time. So there is a point at which the enginevolume is maximum and minimum, hence the ratio. In my design, thecompression ratio was around 1.9.

Regenerator volume is often large compared to the volume of the heaterand cooler. It can be as much as twice the volume of either the heater orcooler. It is required in order to achieve high thermal efficiency. In fact, a 1%improvement in regenerator efficiency can improve the overall thermalefficiency by several percent. The more inefficient the regenerator, thegreater the required heat flux from the heater and the greater the requiredsize of the cooler, to reach the same power level.

There is a paradox involving the effectiveness of the heat exchangers andthe regenerator. The more effective they are, the more pumping power ittakes to push the working gas through them. Effectiveness of heatexchangers can primarily be achieved by narrow flow passages, whichconstricts the flow and raises the convective heat transfer rate between thewall of the flow passages and the working gas. But this also increases thepumping power requirement this translates into an engine loss. Similarly,the effectiveness of the regenerator can be achieved by using a dense matrixmaterial which readily absorbs and imparts heat from and to the working gas,respectively. But a dense matrix material also requires a higher pumping

power to push the working gas through it. Nevertheless, despite the extrapumping requirements, a Stirling engine runs more efficiently and with
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greater power when using narrow flow passages and a dense regeneratormatrix. One must be careful, however, in not making the passages too narrowor the matrix material too dense, otherwise the pumping loss will exceed anyincrease in power. This is where optimization becomes critical.

To reduce regenerator losses it is necessary to limit the level of axial heatconduction (in the flow direction) withinthe regenerator. Axial heatconduction has the effect of short-circuiting the heat from the hot side tothe cold side and as a result, reducing the effective temperature difference inthe engine. The lower the effective temperature difference, the less efficientthe engine. To limit this type of loss one can stack wire mesh screens (theregenerator material) perpendicular to the flow direction, which forces axialheat transfer to occur betweenthe screens, thereby slowing it down.

A good way to start a Stirling Engine is by having lower internal pressure,

allowing easier effort to turn over the engine past the compression stage.And once it starts running increase the engine pressure to increase speed andpower. In some cases you need an electric starter.

The typical heater and cooler (heat exchangers) are made up of manyparallel small diameter tubes (1-5 mm inside dia.) with the (working) gasflowing inside them.

Very high working gas pressures are used because power density isproportional to average gas pressure. Engine losses increase only slowly with

gas pressure. A higher temperature difference between the hot and cold sidealso increases power density and adds to the efficiency.

Piston seals cannot be oil lubricated as this will foul the heat exchangersquickly. Filled teflon piston rings are sometimes used. Specially designedmechanical seals or oil backed roll sock seals can be used to almost eliminateleakage. In some low power systems, leakage, sliding friction and mechanicalwear have been eliminated by using diaphragms or bellows instead of pistons.

Hydrogen and helium as the working gas, function much better than anyother gas (including air) by producing the greatest power and efficiency.

Hydrogen is best because it has the highest thermal conductivity, the lowestdensity and viscosity (resulting in lower pumping losses through the narrowpassages of heat exchangers and regenerator), and a low heat capacity on avolume basis. So only a relatively small amount of heat is needed to changeits temperature. However, hydrogen permeates through metals and nocontainer is completely impermeable to it. And hydrogen is flammable. Somemetals are embrittled by hydrogen. Helium, however, is inert and can bepermanently contained in metal. It has an even lower volumetric heatcapacity than hydrogen but the viscosity is twice that of hydrogen.

To predict the rotational speed of a Stirling engine at zero load you have todetemine the engine losses due to mechanical losses in the linkages, such as


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friction, and thermodynamic losses insidethe engine, such as pumping andhysteresis losses. The rotational speed of the engine dictates the flowrate/losses of the working gas as it flows through the engine. The engine willaccelerate until equilibrium is reached (constant rotational speed). This isthe point at which the raw engine power is balanced by the losses. In other

words, the following equality applies in the case of constant engine speed:mechanical and thermodynamic losses = raw engine power. In thecase where you have a load applied to the engine (such as with a generator)the engine will reach a speed lowerthan the zero-load speed. Thiscorresponds to the same basic equality, but with an extra term added:

mechanical and thermodynamic losses + generator load = rawengine power. As mentioned, the engine speed at which this equality holdsis the finalrotational speed of the engine. Note that the variables on the leftside, including the generator load, are likely alla function of the enginespeed, making this a non linear relationship, mathematically speaking. Ideally,

you want this equality to correspond to the point of maximum generatorpower output, because that means you get more bang for your buck.

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18 Responses to Stirling Engine Project

1. Dawnsays:

January 6, 2009 at 5:27 pm

I am a huge fan of the Stirling Engine. Nice Job. Also, thanks for theresources.

2. Divyanshsays:

February 23, 2009 at 3:53 am

thank you.

i m very fascinated about renewable energy and study regarding thesame.

if its possible, can you please send me the design procedure includingall the formulae for designing a descent stirling engine

3. whowhywhatsays:

March 20, 2009 at 12:27 am

very interesting project. i have been toying with using much largervolumes and lower temperatures for a low cost sterling engine. for
http://newenergydirection.com/blog/category/do-it-yourself/stirling-engine-diy/http://newenergydirection.com/blog/category/do-it-yourself/stirling-engine-diy/http://newenergydirection.com/blog/2008/10/stirling-engine-project/http://newenergydirection.com/blog/2008/10/stirling-engine-project/http://newenergydirection.com/blog/2008/10/cost-to-make-ice-for-homemade-air-conditioner/http://newenergydirection.com/blog/2008/10/cost-to-make-ice-for-homemade-air-conditioner/http://newenergydirection.com/blog/2008/10/cost-to-make-ice-for-homemade-air-conditioner/http://newenergydirection.com/blog/2008/10/cost-of-electric-vs-gas-powered-vehicles/http://newenergydirection.com/blog/2008/10/cost-of-electric-vs-gas-powered-vehicles/http://newenergydirection.com/blog/2008/10/cost-of-electric-vs-gas-powered-vehicles/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/cost-of-electric-vs-gas-powered-vehicles/http://newenergydirection.com/blog/2008/10/cost-to-make-ice-for-homemade-air-conditioner/http://newenergydirection.com/blog/2008/10/stirling-engine-project/http://newenergydirection.com/blog/category/do-it-yourself/stirling-engine-diy/


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example using a large hot box type solar collector with bellows ascylinders. there are a couple of sterling patents along those lines.

4. Merlijn Janssensays:

November 7, 2009 at 5:55 pm

Hi!Fascinating to see how you have made this engine with the samerestraints I have: availability of components, workshop limitations,financesIt also took me a couple of tries to get a working engine, not tomention a vast amount of designing and building time.I am very impressed with your first effort, especially with the fact youhave your engine air-tight.

Looking at your project, I have the following observations:1) The flywheel seems to have too low kinetic energy to push throughthe compression of an Alpha engine. You could think of filling the tirewith water or sand (heavier).2) The regenerator is too narrow: gases (and thus the engine) looseenergy trying to push through the pipe with metal wool. Try to makethe regenerator wider, say 2x2x2 inch. (Of course at the cost of deadvolume.)3) Friction losses. Mainly the reason why people start with smallerengines. Where you can, use bearings. Of course when your engne

really generates 100W, this is less of a problem, but in the beginning,this is an important design aspect.4) Dead volume in the cylinders. Though amazing you are able tomake cylinders of sheet silicone, you could think of filling them asmuch as possible. (Without any material touching the cylinders whenrunning to avoid friction.) Also consider using silicone bellows. (Alsosee comment above.) Bellows are available, and I found a companythat is willing to make a silicone bellow with their standard rubbermould for a reasonable price.5) Of course connecting your heater and cooler to the regeneratorbecomes tricky when it has a larger diameter, you could use aluminiumpipes and weld with Durafix Easyweld.6) You can then use more than 1 pipe for the heater and the cooler: Itreduces pressure losses in the pipes when you use a multiple. (Againat the cost of dead volume, but you can use shorter pipes. You do notneed to heat or cool the air 99%, 60% is good enough with your heatsource of coals.)The design of Stirling engines is an interesting challenge because of allrequirements for heat transfer and dead volume. But history hasproven energy efficient engines can be built, and with enough effort, itshould be possible to make a nice one at home that generates some

power.I have spent the past few months designing my next version, and
http://www.merlijnjanssen.nl/stirling/stirling.phphttp://www.merlijnjanssen.nl/stirling/stirling.phphttp://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://www.merlijnjanssen.nl/stirling/stirling.php


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getting all components. I am now builing the next version but tinkeringwill take me a few months.Good luck! It is a terrific feeling when the engine really runs. Everytime it does for the first time, I open a bottle of champagne and watchit.

Excellent that you desribed in such detail how you made your engine.Lots of pictures, very instructive.Regards,Merlijn

5. Merlijn Janssensays:

November 8, 2009 at 4:46 pm

NB. Please ignore my suggestion of filling the flywheel with water, bad

idea. The flywheel needs to push through compression, and water willjust flow through the tire and not help pushing.

6. adminsays:

November 9, 2009 at 1:48 am

Hi Merlijn

Your suggestions are well noted. Sometime in the future I plan onrevisiting the project, but with a whole new design in mind. Havinglearned from my mistakes I have a good feel for what the limitationsare. I will need a machine shop for sure. And no more wood! All metalthis time.

I will also use my design program which will help me determine thenumber of heater/cooler tubes and regenerator size, to help reducetrial and error. Unfortunately, I created the program after I built theengine, but it did tell me that it was only capable of 5-10 Watts intheory, and I was also able to more quantitatively identify the mainweaknesses in the engine, such as, that I dont have enough

heater/cooler tubes and the regenerator is too small (as youmentioned). This results in flow losses that are too great since thereare too few passages for the working gas to flow through.

You have a great looking engine. Im wondering if you can increase thetemperature difference from 80 deg C to something higher. I realizethis may not be possible if youre using a rubber material for thepiston/diaphragm. But silicone can tolerate a high temperature if thediaphragm is made out of that. I think your speed and power wouldincrease quite a bit.

7. Hydrogen Helium Leak Testingsays:
http://www.merlijnjanssen.nl/stirling/stirling.phphttp://www.merlijnjanssen.nl/stirling/stirling.phphttp://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://www.abhobley.co.uk/http://www.abhobley.co.uk/http://www.abhobley.co.uk/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://www.merlijnjanssen.nl/stirling/stirling.php


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November 19, 2009 at 1:19 am

Hi, Your project is fabulous and I think you worked really hard. Goodluck. Bye

8.Anggit Yuliartonosays:

December 28, 2009 at 12:05 am

i like your project, before designing an alpha engine, did you use somemathematical calculation?

thanks.

9. adminsays:

December 28, 2009 at 10:09 pm

I did. It helped a bit, but the model wasnt very refined at that point.After it was built I created a much better mathematical model topredict engine performance. Ill use that for my next engine.

10.Merlijn Janssensays:

April 26, 2010 at 5:35 am

Hi again!Because I left such an extensive reply in November, I wanted to sharewith you that I got my latest alpha engine running yesterday. I posteda video on my site. Good luck with your next project!Merlijn

11.panotsays:

July 25, 2010 at 12:31 am

I think the heater and cooler are in the wrong places.They should be at the cylinders, instead of in the tube between them.Heat input should stop (or slow down) when hot cylinder has low gasvolume.Heat output should stop (or slow down) when cold cylinder has lowgas volume.

12.john jsays:

December 30, 2010 at 3:17 pm
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Finally, after hours and I mean hours of search I finally foundyou..great stuff..the info, blog is worth the cost of the program.My project is alpha from a compressor and Im curious about the head(volume) is an asset or a problem. Im considering the head chamberas a preheater / temperature maintainer area of the hot side and Im

using glow plugs. The cylinder volume is 99cc and the head volume is80cc per side. Any constructive ideas are appreciated.

13.waqassays:

May 19, 2011 at 1:58 am

hello sir ur project is very nice and i wana help plz contact me at thisemail address plzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz sir i wana helpfor make striling engine ok sir my email is thisssssssssssssss

[email protected] ok bye sir and God blessuuuuuuuuuuuuuuuuuuuuu

14.Derek Robsonsays:


Hi,Just to let you know. I watched your vidio when you turned the engineby hand. It seemed to have some compresion like a petrol engine. Wellthere is something wrong because all the sterling engines I have seenor built do not have compression like yours. Look at the books again itis a smooth, none compressive cycle and I can not see how youpressurized your engine with 2 recycled food tins for cylinders? Maybeyou were cracking a joke?? I hope you get fun from your work.Seasons greetings.

15.adminsays:


No it wasnt a joke, and I think I explained my design as best I could.

16.Hemonsays:


So whats next? A beta configuration engine? Would that not reducethe dead space dramatically? (ie. having both pistons close together?) Ive seen some people on youtube make a piston out of steel wool,hence behaving both as a displacer and a regenerator looked like a

good idea, but not sure how effective it would have been. Im quiteinterested in beta confirations and linear generator stirlings, although
http://newenergydirection.com/blog/2008/10/stirling-engine-projecthttp://newenergydirection.com/blog/2008/10/stirling-engine-projecthttp://newenergydirection.com/blog/2008/10/stirling-engine-projecthttp://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/mailto:[email protected]:[email protected]:[email protected]://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/mailto:[email protected]://newenergydirection.com/blog/2008/10/stirling-engine-project/comment-page-1/http://newenergydirection.com/blog/2008/10/stirling-engine-project


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eventually when I get up enough skill looking towards a free pistondesign (super complex).

17.Doug Bullersays:

March 18, 2012 at 11:23 am

For years I was employed in manufacturing engineering in an industrywhich used many air operated diaphragm pumps. Which is basicallywhat you are attempting to do.I would suggest that you go to U-Tube and enter Wilden introduces fullflow PTFE diaphragms. You will note how increasing air pressure (ie.heated air volume) is converted to a linear action. It then becomesonly one step away to create rotation.Have fun.

18.Cliffsays:

May 8, 2012 at 6:04 am

A Very Well Done website! Never run across a stirling engine site thathad such, to the point information and suggestions on their design. Ialso was able to finally find a copy of NASAs Stirling Engine DesignManual #1.Again my thanks for the time, the effort, and the sharing.

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o The Stirling Engineo Solar Stirling Engine
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n.com/blog/2008/10/http://newenergydirection.com/blog/2008/11/http://newenergydirection.com/blog/2008/12/http://newenergydirection.com/blog/2009/03/http://newenergydirection.com/blog/2009/05/http://newenergydirection.com/blog/2009/06/http://newenergydirection.com/blog/2009/09/http://newenergydirection.com/blog/2009/10/http://newenergydirection.com/blog/2010/01/http://newenergydirection.com/blog/2010/06/

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