Stirlingengineforum.com

Hello All, Im new hear and hoping for advice from the more experienced members on the design brief i have set myself.

I am going to design a small stove top stirling engine which will charge a 12v battery which can be left unattended and not overcharge the battery or cook the alternator
and be fail safe. I want to make the flywheel axial flux alternator from scratch and the engine from stainless steel using my tig welder.
I would also like to later design an attachable heat pipe solat trough so that the engine can take solar input in summer when detached from the small stove.
The stove is small but has strong heat output and i would like as much watt output as possible but i realise the output will likely be in the range of charging gadgets
or trickle charging the 12v battery very slowly. I would also like the option of charging a boats 12v battery which may or may noy be under load. I could consider a presurized
engine design if it is fail safe and will still have function on atmospheric pressure if pressure leeks. Reliability durability longevity high quality and low maintainance are
prioritys over output power but maybe presurized is essential to get any decent wattage. I have determination to do this at any cost to pocket and get a quality design i can
replicate and sell even if i have to sharpen my tig welding skills :)

Can anyone tell me in what order i must go about such a project?. I imagine i must first build any engine to fit the small stove size and calculate the engines
heat exchanger size based on max fuel temperature then measure the RPM of the engine on highest stove temperature output to know how to design the axial flux alternator to charge
a 12v battery?. Then how do i stop the battery from over charging if the stove is left unattended or takes ages to cool down?

a few other questions I have:
which type: alpha, beta, gamma is best for power output?
Which parts of a stirling engine make the most noise and how do you design for low noise
which matiarials are best for long life, durability andlow maintainance?

Advice much appreciated.

Regards

.Wellington.

I'll start where you finish, Stirling Engines are virtually silent runners in general, some types, Ringbom, and other free piston types can be noisy unless you get the design just right.
Find a published design, and go for it. There is only about two parts that I get TIG welded, the end cap on the hot cap, and the hot end on the displacer of a GAMMA, or BETA motor, or the Heylandt Crown on the hot piston on an ALPHA motor. One of the main things is careful work on the lathe boring the power cylinder, and making the piston, the other is a firm understanding of how the motor works, and perhaps build a simple motor , and get it going, before you start on a more complex one.
Ian S C

Thanks for the quick reply and pointers. There is so much info on the net on stirling engines its overwhelming to navigate through the tin can stuff and find useful info. Can you save me many hrs of surfing the net and provide any links to the crème de le crem of impresive diy projects you've seen and also which designs/projects i should avoid like the plague.

Thanks

.Wellington.

Wellington, not really, I use info I gathered before I got to the internet, look for books by James G. Rizzo, Dr Senft, magazines such as Model Engineer although there has not been any Stirling Engine articles lately.
They all work to the basic principal, with variations depending on whether it is a high or low temperature motor.
The thing is to study what a part is supposed to do, if it's the hot cap, and displacer, they must be able to stand the heat, and maintain strength, with out corroding to fast, also it should not conduct heat too fast between the hot and cold end of the motor, and as far as the displacer is concerned, it should be as light as possible, answer stainless steel.
When we look at the power piston you'll hear of graphite being used in a glass cylinder, that's good, and has been used quite a bit, but you wont find it in your junk pile, but a cylinder made of steel, or better still cast iron, with a cast iron piston, if well made has a coefficient of friction similar to the graphite one.
Don't discard the idea of tin can motors all together, they have their place in the learning about Stirling Engines.
Ian S C

Thanks for the book recommendations. shame they are quite expensive on amazon. I think I will go with stainless steel like you recommend since I have a tig welding machine. I was curious to know how displacer stroke length effects performance. I would like to aim for a long length to suit a low rpm alternator but am not sure how to design for a long displacer stroke? can you offer up any tips on this? Also how are pressure engines usually filled with gas? is there an el-cheapo method using a bike valve or something? I have an argon tig welding bottle and sooner or later I will move onto pressure designs. I don't want to take up too much of your time but you are obviously someone with a lot of experience to offer.
.Wellington..

Hi Wellington. . I think this form of cogeneration is one of very few truly useful areas to invest Stirling development. Check out the electronic competition of Thermal Electric Generators first though. Cheap TEGs are becoming more available and may be getting to the point of making all of this much less relevant, like cheap photovoltaics have already done to solar Stirlings. I still think Stirling woodstove cogeneration is worth pursuing though. Some further thoughts: http://spragueriver.com/index.php/bumpkin-tech
Bumpkin

Thanks for the useful link bumpkin. Tegs are indeed an interesting technology but i prefer the charecture of a solidly built chunky engine that makes a soothing and subtle puf puf sound. I also prefer stirlings for the fact that you can make them from junk in an emergency and nit have to go looking for some unobtanium exotic metal.
Regards
Wellington.

Wellington, I will offer some very practical advice to add to Ian's excellent suggestions. Village Press publishes Rudy Kouhuoupt's volume of Stirling engines. He was a master machinist who took a very practical approach to model engine building. There are complete drawings for three gamma engines in this book. Although I did not build the first engine in the book which is very simple and air cooled, I did build the other two. There are plans for a horizontal engine with a radiator which is a really fun build and produces a worthwhile model. The vertical engine is also a very worthwhile build. I first built the vertical engine version as printed, as I recall it has a 1.125" bore. Recently I completed a version at a scale 266% larger or 2.66 times as large. I chose this size because it worked out conveniently in finding a commercially produced 3" cast iron cylinder sleeve which was used for the power cylinder and a 4" thin walled pipe for the displacer cylinder. This larger engine is suitable for battery charging.

Even a table top model can charge a battery if the battery and generator are small enough. As far as a charge controller, it is a little difficult for me to imagine a homebuilt Stirling engine overcharging a 12v battery but charge controllers are interesting and useful and worth considering on such a project. In my very limited experience, using something like a pulse width modulation controller may be very useful to limit the charge coming from the generator, thereby allowing the engine to work in its most complimentary rpm range to avoid bogging down the engine. I recently built a couple of controllers, the SCC3 solar charge controller kit from CirKits.com , a beautifully designed and really fun build ! and a circuit from 555-timer-circuits.com , a pwm controller using a power mosfet. I am still experimenting with these. For those not interested in building their own controllers, these sorts of controllers are comercially available and not expensive.

The discussion of transmission is perhaps a more critical topic. Finding the right ratios and a suitable, low friction method of getting the power to the generator seems essential.

For transmission I use V pulleys, and heat joinable 3 mm round plastic belt. These belts don't stretch, or slip. On the smallest motors, belts from cassette tape recorders or similar gear.

Alfista wrote:Wellington, I will offer some very practical advice to add to Ian's excellent suggestions.....

Thanks Alfista, all good tips I will be looking into. Ive seen a small Andy Ross engine which generates aprox 100W output. Have you any idea where I might buy such an engine? or have you seen any engines that can top this output? Regards
.Wellington.

Andy Ross used to sell some engines as kits and once in a while one of his engines will come up on ebay. Several of his books are available and he gives excellent advice on what works and what does not. On youtube, I would recommend baruman.
I am in awe of Jim Symanski's work : I am sure that there are some there over 100w.
http://www.starspin.com/stirlings/
In effect, once you master some of the building techniques, you can build an engine as large as your equipment and budget will allow and as you build bigger you will get more efficiencies due to scale, and more power, just as with steam. Speaking of steam, getting to 100w is not very difficult and I would say it that it is quite possible for someone to buy the components or make them if you have something like a 6 inch lathe and a torch.

I rebuilt a Stuart Turner S9 the other year, this engine is a double acting horizontal steam engine with a 1 1/2" bore and stroke, I only ran it on air, and with a pressure between 30 and 60psi it could run an alternator to light an 80 W 230 volt light, but my compressor could only just keep up with it's 1/2hp motor going continually. I was pleased to find that my endeavours resulted in the governor holding the revs to the design speed of 400rpm, governors on small engines rarely work. To make this engine a practical power producer it would require a quite substantial boiler with a feed pump, and a gas supply, unless you want to stock the fire with wood or coal. The hot, high pressure boiler was the reason Stirling built his Air motor.
Ian S C

Ian, I cannot find any fault in your thoughts on steam engines and boilers. You are quite right about the effort involved. I would like to talk about the boiler. It is generally accepted that one needs about 10 ft2 of heating surface area per boiler horsepower, but since not all boiler designs are equal, some may be able to reach 1hp with less. I have one boiler, 6" diameter, 14" tall, with 44 flues (fire tubes) of .5" o.d. The calculated surface area is about 1133 in2 or about 7.86 ft2. I would not want to depend on 375w constant output from it but something around 250w would seem reasonable.  In your case, the 1.5" x 1.5" steam engine at 50 psi mep at 250 rpm should yield about 125 watts.
You are also right about the history and the niche of the Stirling engine. I am going out on a limb here but I believe that it was Rider who said something like "With all of the modern improvements in boiler safety, the modern boiler is nearly as safe as the hot air engine". I paraphrase. 
I am not trying to promote a cavalier attitude towards steam power. I just wanted to make some comparisons in the effort required to build a 100w prime mover that will run on "free fuel". As you know, many people, such as Andy Ross, begin the hobby of Stirling engine building and within the first few months realize the very substantial technical challenges in producing useable power. He wanted to make a Stirling engine powered bicycle. Mr. Ross is a first class engineer and he has generously exposed the challenges of building small, useful Stirling engines. To put it differently, he shows us just how hard it is to make a 100w engine and it seems that most of his excellent designs run on propane fuel. 
In my opinion, building a 6" x 14" boiler is a much easier proposition than building a 50w Stirling engine and it will run quite happily on wood, charcoal, biomass, etc.
So far, I have not met any serious Stirling engine builders who do it because it is easy. Stirling engines are a fascination and in particular because of the difference between the potential or theoretical efficiency versus the highly inefficient engine that is standard for a novice builder. .

I think you calculations on the output of the steam engine are about right, the alternator I was using was probably less than 50% efficient, which would mean the motor had a BHP of 160 W or more. There was a photo in "Old Glory" the traction engine/steam magazine of a motor bike with a boiler about the size of your's.
Ian S C

Alfista wrote: I just wanted to make some comparisons in the effort required to build a 100w prime mover that will run on "free fuel". As you know, many people, such as Andy Ross, begin the hobby of Stirling engine building and within the first few months realize the very substantial technical challenges in producing useable power. He wanted to make a Stirling engine powered bicycle. Mr. Ross is a first class engineer and he has generously exposed the challenges of building small, useful Stirling engines. To put it differently, he shows us just how hard it is to make a 100w engine and it seems that most of his excellent designs run on propane fuel. 
In my opinion, building a 6" x 14" boiler is a much easier proposition than building a 50w Stirling engine and it will run quite happily on wood, charcoal, biomass, etc.
So far, I have not met any serious Stirling engine builders who do it because it is easy. Stirling engines are a fascination and in particular because of the difference between the potential or theoretical efficiency versus the highly inefficient engine that is standard for a novice builder. .

An Interesting points you make there Alfista comparing ease of boiler building vs stirling building for power generation. I have no experience with stirling engines but came this way because I was put off with the risk of boiler explosion and the need to be in attendance at all times with steam. One of the big issues for me is noise and from what I can gather a stirlings seems to be the quiter option in like for like steam/stiring engine size?. like Ian said the stirling was invented because of boiler risks. What I am curious to know is how much risk does a stirling charged to 10 bar pose if any? A bike tyre is only 3 bar so I don't imagine 10bar could do anyone much harm if it went pop. please correct me if I'm wrong but I assumed stirlings @ aprox 10 bar can be left unattended and do not require any kind of "pressure vessel manufacturing" licence or sales license like steam boilers do. The reason I ask is because I am attempting to design a small stirling to fit an existing small stove and to make it available for sale.
Wellington.