Hello

[jamespetts]

Having signed up to this forum recently, and having been lurking here reading for a while, I thought that I ought to introduce myself.

Having not had any involvement worthy of note with stationary steam engines since a boyhood interest, sparked by regular visits to the excellent Kew Bridge Steam Museum, I somehow got into my head recently a thoroughly hair-brained scheme to build a small computer entirely powered by a minature steam engine.

Last week-end, I dusted off and ran the two toy stationary engines that I had as a small child, a Wilesco D8 and a Wilesco D16, which reminded me both of the great pleasure and satisfaction that running minature steam engines brings, and also of the frustration brought about by the very basic Wilesco boilers that raise steam unevenly and have no feed pump, so cannot be run for more than a quarter of an hour or so before the water runs low and the fire has to be extinguished, and the engine left to cool before it can be run again.

I had always desired as a boy to own one of the beautiful engines made by Stuart Turner Ltd., but such things far were beyond the reach of a ten-year-old's budget, so, frustrated by the curtailed running times of the Wilesco engines, I turned my interests to other things, and very nearly forgot about the stationary engines - until a week or two ago, when my interest was suddenly and inexplicably rekindled.

I kept a watchful eye for bargains on eBay, and recently managed to snare a delightfully (and newly) built Stuart Score for the below-market price of £300. I do not yet have a boiler to go with it (unsure of whether it is wise, in terms of safety, to buy a second-hand boiler, and am in any case planning to wait before acquiring anything else until after I have attended this year's Model Steam Extravaganza, held in October, so that I can see how other people do it), but the engine is delightful to look at in itself, even without being run.

In matters of mechanical engineering, I am a rank novice: I do not have any engineering background (my line of work being of the paper-heavy variety), and do not know one end of a lathe from another, let alone own one, nor do I even have anything that could be termed a workshop, although, with some difficulty, I might be able to use the garage for simple tasks such as the cutting of wood. I am in some awe of those here about whom I have read who can fashion a fully-functioning miniature steam engine from little more than a pile of scrap metal and a few fixings.

Much as I admire those who are handy with a lathe, however, my interest (largely dictated by my time and available resources: it would, in theory, be wonderful to be able to build steam engines from relatively cheap castings or even scrap) is more on putting things together from a near-finished state, and then running them, than building them from scratch or sets of castings. I am, nonetheless, not entirely incapable of handiwork: by way of reference, when I owned a car, I bought a Haynes manual for it, and was able quite successfully to complete some of the maintenance tasks listed in it, but only those marked as one spanner (i.e., the lowest level of difficulty - tasks such as draining the radiator or changing the spark plugs). I would have been tempted to buy a machined kit from Stuart if it wasn't for the fact that finished engines are available on eBay more cheaply.

I do not know whether my Heath Robinson like project could ever work, but I do know that, if it is to have any chance of success, it will need greater expertise than mine, in the two quite separate fields of steam engines and small-scale electricity generating equipment (most likely an automotive alternator of some sort). As to the third required field, computing, that is a subject on which my own experience is somewhat greater than the others, having built, maintained and upgraded numerous computers over the last ten years or so; I know, for example, that it is possible to build a small desktop computer, including monitor, that consumes less than 40W of power at anything between 12 and 24v (which voltage does not have to be stable, providing that it always remains at least 12v, and not more than 24v, thus hopefully obviating the need for a governor).

Given what I have read so far from all the helpful people on this forum, it seems that this is a very good place to seek advice on things relating to the steam engine/mechanical engineering side of things; in return, I will try to keep people updated on its (inevitably very slow) progress - if people are interested, that is.

James E. Petts

[dickdastardly40]

Hi James,

Nice to see a new enthusiast to the hobby.

A steam powered computer is an interesting concept, to my (by no means expert) mind a couple of things you will have to get sorted or bear in mind before you boot up windows, in no particular order, may be:

Boiler output will have to be sufficient to continue to provide enough steam to run the engine on load for long periods. Also consider what you will use as a heat source and fumes

Engine power will have to be sufficient to turn the alternator on load at the required speed to obtain at least 12v output.

Coupling the engine to the alternator may be awkward, belt drive may be easiest but some sort of engineering solution will be required to mate the two.

I have no idea what the output of an automotive alternator might be beyond 14 v nominal output at minimum revs (say 700 rpm engine speed) however the rotor speed of the alternator will be far higher being belt drive from a large to a small pulley. Some further research may be needed to see if 12V 40W at the speed you are likely to get from your preferred boiler & engine combination is possible.

Please treat the above as my thoughts only, It is not my intention to pour cold water on your idea, just to appraise you of what I believe are certain limiting factors and to ensure you purchase the right sort of boiler.

I look forward to seeing your progress.

Regards

Al

[ron]

Hi James
Welcome to the forum.
A couple of thoughts on your posting, having built and run several ST engines [but not tried generating!]
I wouldn't be too bothered about a second hand boiler as long as a bit common sense is applied when buying and for peace of mind an hydraulic test is done after purchase.
The cheapest alternative is probably one of the previous generation of Stuart Turner boilers ie the 500 series which are well made and work well, rather than a someone's home made effort which might be of dubious quality.
I'm using the 504 boiler which is the largest of the series, it was originally spirit fired, this worked remarkably well but was a bit uncontrollable so I converted it to gas firing using a proprietary ceramic burner. This boiler will run any of the ST engines up to the No9 although it's struggling a bit with that, they appear from time to time on Ebay and don't usually coat a fortune [usually a bit less than £200] so I would recommend going for it rather than one of the smaller ones which are not much cheaper and would struggle with anything other than the smallest ST engines.
As I said earlier I've never tried generating electricity but I get a feeling that anything other than the largest ST engines would struggle to run a car alternator, it might be a better bet to look at some of the early ST generators that appear on Ebay from time to time, another alternative might be a motor bike generator if they have such a thing.
Good luck with your project, sounds like good fun.
Ron

[jamespetts]

Thank you both very much for your input: I very much appreciate it. I evidently have a great deal to learn, and I am looking forward to learning it.

A few specific questions arising out of your comments, if I may. First of all, boilers. I am not presently very clear on the difference between the old Stuart boilers and the new ones, except that the new ones do not have different models for stationary/marine applications, are a different colour, and come with a gas, rather than a spirit, burner as standard. I also notice the largest boiler in the old 500 series, the 504, seems to be the equivalent to the medium boiler (this one) in the new range, which was the one that I was considering getting (since the Score is one of the engines for which it claims that it is suitable). (Although I have just noticed now that their larger boiler, the HB6 seems to have disappeared from their website within the last day or two: I only managed to get that page by a bit of backwards browsing). I know that the old 500 series were water-tube boilers; I was wondering whether the new boilers were actually monotube boilers, since the gas burners would allow more even steam production even with a monotube design, that would take advantage of the greater efficiency of that over a water-tube boiler. That being the case, would the SH4 be better than a 504 with a gas burner, do people think? The SH4 claims 60psi: what was the pressure rating of the old 504? As to buying secondhand boilers, is there anything in particular that one should look for as far as safety is concerned (other than that it was made by a reputable manufacturer)? And how exactly does one conduct an hydraulic test? If that is something that I would have to pay to have somebody else do, might it be just as worthwhile simply to get a new one? Finally, as to fumes, the two options that had occurred to me were either to run the whole thing outdoors (which might not be ideal in the winter or in the rain), or to run some sort of pipe from the chimney out through an open window. Are there any other possible options of which people can think?

As to generating, I appreciate that there are a large number of variables of which to take account. What I am lacking at present, it seems, are numbers. I ultimately need to know: (1) how much power/torque that a Stuart Score (3/4"x3/4"x2 cylinder) when connected to a 60Psi boiler can produce; (2) how much power/torque that any given alternator (motorcycle or car) needs when turning at its desired RPM and various different amounts of electrical power are being drawn from it; (3) the speed at which the alternator would have to be turned to produce enough voltage; and (4) the speed at which the engine could turn when providing the requisite amount of tractive effort. I don't imagine that, unless I am very lucky at least, anybody here would immediately be able to help with nos. (2) or (3), (although, if I remember correctly, there is a relatively simple calculation to work out no. (1)), but any pointers as to where to find the information would be very much appreciated. (As to the Stuart Turner dynamos, I did look into that, but two problems emerge: (1) they seem to be rated for 10W at 10V, which is not enough: if one tried to draw 40W, they might very well burn out; and (2) they are of a brushed design, which is likely to wear out quickly. Thank you for the suggestion, though).

The approach that I am thinking of taking at present is first of all to see how much electricity that I can generate with a Score, and secondly to see what I can run from that amount of electricity, and build the computer system around that power limit. If the amount is so small that no computer at all could be run, then I might consider some other, less power-hungry, electrical equipment.

Again, thank you all very much for your help: as noted above, I have much to learn, and very much appreciate a chance to learn it, the limiting factors especially so that I can take account of them and, if possible, overcome them. Your replies are much appreciated.

[AndrewP]

James
I can't help with the electrickery but fume extraction I have got sussed! I installed a bathroom type extractor fan in the wall of my workshop and connected it to a length of tumble drier flexible hose which I can suspend over various locations on the benches - about 4" dia I think, might be 6" though.
This was originally to extract silver soldering fumes but it comes in really handy when running an engine on the rolling road and saves all those complaints from the domestic authorities

[steam4ian]

G'day James
I will work out a watts/HP formula for you if you are interested, I have to work back from the loco tractive effort x speed for, draw-bar HP formula.
Regarding generators, they only have to turn fast enough to get the required volts, volts are proportional to speed. An auto alternator will start charging the battery at 600 rpm of the engine (at idle), the pulley ratio is 3:1 so the alternator is rotating at say 2000rpm. At this point the terminal voltage will be at least 14volts. The current is simply dependent on the load; however too much load will cause the prime mover to slow down and the voltage to fall. Car alternators have a voltage regulator to hold the voltage at an upper limit irrespective of the engine speed but they must not be operated without a load.
The electrical power in watts is volts x amps, the mechanical power is torque x speed (rpm). A car alternator would be 80% efficient.

regards,
Ian

[dickdastardly40]

Hi All again,

One horsepower is equal to roughly 750 Watts (exactly 745.69) therefore 40 W = 0.0536 HP

The best site for just about any conversion that does all the maths for you can be found here: ;D

www.convert-me.com/en/convert/power

Regards

Al

[ron]

Hi James
I have no experience of the latest ST boilers but they are CONSIDERABLY dearer than the old 504s which will do the job you want, I presume the new ones are better as everything moves on. If you buy a 504 off Ebay and decide to upgrade at a later date you can always resell it and probably won't lose much if anything.
The 504 is a very simple Babcock type boiler, working pressure is 65psi.
If you are buying look for one that hasn't been abused, ie no dents and general signs of mis-use, to pressure test you need to plug the connections, fill it completely with water and using a pressure gauge known to be reasonably accurate pump up the water pressure with a small hand pump to 2 X working pressure and leave it for say 5 or 10 minutes to see if the pressure holds. You will need a small hand pump and tank anyway to keep the boiler topped up for continuous running.
Water is used rather than air as it's incompressable therefore isn't dangerous in event of a catastrophic failure. The Stuart Turner boilers are very well made, I would be surprised if you were to have any problem with a second hand one unless it has been badly mistreated.
I think initially you want to try getting a working steam set up and worry about the generating side later, getting a small ST engine to run for long periods requires a fair bit of skill even with a gas boiler unless it is considerably oversized.
You will also need a displacement lubricator for the engine if it didn't come with one fitted, you can only really run one of these engines for very short periods without one and even that isn't really a great idea.
Ron

[steam4ian]

G'day James, (Me again)

For your engine the output power in watts =
MEP x D x D x S x r x 2 x Pi / (33000 x 0.746).
MEP = mean effective pressure in psi (I'll comment later)
D = cylinder diameter in inches
S = stroke in inches
r = rpm revolutions per minute
Pi = Pi (3.142)
33000 constant for horsepower
0.746 HP to watts conversion factor.

MEP is the mean pressure in the cylinder through the stroke. Start at 85% of boiler pressure and then for a self starting engine with fixed cut off at 75% the MEP will be say 60% of 85% of boiler pressure.

By my calculations an engine with "2 bore and 2" stroke running at 300 rpm with a boiler pressure of 100 psi will develop about 30.6 watts.

Hope this helps
Regards,
Ian

[steam4ian]

OOOPS -------- CORRECTION
G'day James.

For your engine the output power in watts =
MEP x D x D x S x r X 2 x Pi X 746/33000

I'm still not confident this is the answer but it may prompt some else to have a go.

Regards,
Ian

[steam4ian]

FINAL CORRECTION
G'day James.

For your engine the output power in watts =
MEP x D x D x S x r x Pi X 746/(33000 x 12)
MEP = mean effective pressure in psi
D = cylinder diameter in inches
S = stroke in inches
r = rpm revolutions per minute
Pi = Pi (3.142)
33000 constant for horsepower
746 HP to watts conversion factor.

Your little 3/4" by 2" engine with 60 psi will develop about 80 watts at 400 rpm

This is my best shot at the answer.

Regards,
Ian

[alanstepney]

The formula for engine power is PLAN/33000
P= pressure
L= length of piston stroke (feet)
A= area of piston (sq In)
N=number of power strokes per minute

Using 60 psi (see later), and 500 rpm, that works out at around 1/10th HP, or say, 75 Watts.
(That assumes that I hit the right buttons on the calculator!)

Of course, with 60 psi boiler pressure you can never have 60 psi applied to the piston for the full stroke.

So, for simplicity, say, half that pressure which will give half the above power.

However, I do know that the Double 10 WILL give 1/10th HP, and even slightly above that, so it is reasonable to expect the Score to do the same.
The one thing I cant remember is the pressure for that power.

[jamespetts]

Thank you all very much for your help - that is most kind. 75w of mechanical power sounds as if it should be just enough if I use a sufficiently efficient generator and a sufficiently low-power computer.

As for learning to run an engine for long periods before trying to generate, that does indeed seem like a good idea: the plan at this stage, I think, is to try to see if I can set up a working steam plant and become familiar with that, then see how much electricity that I can generate with it, and finally build a computer that takes that amount or less.

I imagine that such a learning process involves much in the way of observation and trial and error (I imagine the learning might be quite interesting in itself), although any tips would, of course, be appreciated. I do indeed, as one of the posters suggested, plan to fit a displacement lubricator, and the person from whom I am buying the engine on eBay (he is the one who built it from castings: I suspect that he builds engines from castings to sell on eBay to finance the engines that he builds to keep, which is a jolly clever idea) has agreed, for a small additional charge, to drill and tap holes for drain cocks and to fit oiler cups to the top of the crosshead guides (similar to what is depicted here) to enable the crossheads to remain properly lubricated during long runs.

Given the above, the next stage would be to acquire a suitable boiler; as to which, I wrote to Stuart yesterday asking them about their boilers, and to-day received this reply:

Dear Sir,

The SH4 and SV4 are multi tube boilers with smoke boxes, the SH4 is powered by a blow lamp style burner and the SV4 is fitted with a ceramic burner.

The HB3/HB6 are both helix style fire tube boilers with Ceramic Burners.

Best Regards,

Bryn Jones,
Stuart Models... (etc.)

From people here's knowledge of boilers, does that suggest that the SH4/SV4 are an improvement over the old 504? Is there any particular advantage of a blow lamp over a ceramic burner or vice versa? Is there any technical advantage either way in a horizontal versus a vertical boiler, or is the only thing to choose between them a matter of shape convenience? The reference to a smokebox - does that make the SV/H4 distinct from a 504, or did the 504 also have a smokebox? Sorry if any of those are silly questions - I am afraid that my knowledge of boilers is, as will probably be apparent, quite limited.

In any event, thank you again very much for your help; I shall look forward to keeping those here up to date with the progress of my project.

[jamespetts]

I know that an important thing as far as quantity and constancy of steam production is concerned is the heating area of the boiler: the proportion of the surface area of the boiler exposed to direct heat from the firebox. The higher the proportion, the better.

I know that everybody's recommended form of firing a boiler seems to be gas, which is far more controllable than either solid or liquid fuel. The old Stuart 504s did not come with gas burners, so one would have to use an aftermarket ceramic burner and retrofit it. The Stuart SV4 boiler also uses a similar ceramic burner (see the letter above).

I note that ceramic burners come in a range of sizes, although the largest that now appears to be on sale is 1 7/8 in. x 2 1/2 in., and that costs £48 excluding gas jets or piping. It strikes me that that would be too small to make use of the full potential heating area of a no. 504 boiler's firebox.

By contrast, the Stuart SH4 uses what Stuart describes as a "blowlamp style" burner, which seems to require being removed from the boiler to light it. From the diagrams, it seems that the SH4's burner runs the entire length of the boiler, with, presumably, a number of individual gas jets throughout its length. That would seem, therefore, at least to my untrained eye, to suggest that steam production in an SH4 boiler would tend to be better than steam production in either a 504 or SV4 boiler fitted with a ceramic burner, since the blowlamp style burner on the SH4 runs the entire length of the boiler, whereas the ceramic burner would not fill the 504's smokebox, and the SV4 has a smaller heating area to begin with.

Would anybody here disagree with that, or does that make sense?

[AndrewP]

I'm pretty sure the SH4 burner is just like a blowlamp, ie one gas jet just inside the casing and a long horizontal (noisy) flame. Ceramic burners are lot quieter, more like a gas fire.

[jamespetts]

agatehouse said:

I'm pretty sure the SH4 burner is just like a blowlamp, ie one gas jet just inside the casing and a long horizontal (noisy) flame. Ceramic burners are lot quieter, more like a gas fire.

Thank you for that information :-) Apart from the noise, are there any functional differences between them; does one burn hotter or more reliably, for instance?

[AndrewP]

I've never played with a ceramic burner so I'm on thin ice here but I suspect they are less adjustable - flames heat the ceramic which gives off radiant heat. We know how adjustable a gas blowlamp is and can even 'hear' when it's running right.
The shape of the heating envelope is obviously completely different so that probably has the greatest effect - a firetube boiler is going to need a blowlamp style burner whereas a simple cylinder (think Mamod) could best utilise a ceramic burner.
It probably boils (ouch) down to how much gas you can usefully burn and sums involving calorific value etc could be useful but now I'm really out of my depth.

[robert]

I had thought a steam powered computer would have by nature to be mechanical rather than powered from steam generated electricity. A Babage machine perhaps? The need for rotary power may not be necessary either. See these for ideas?
www.mechanicalcomputer.com/images/album_1/thumbnails.html

Robert

[jamespetts]

Robert,

while a the Babbage Differential Engine is what first springs to mind when one thinks of a steam powered computer, I don't think that my model engineering skills could stretch to that ;-) Also, my particular interest in relation to this hobby is miniature steam put to practical good use, and a modern computer is a bit more useful than a Babbage Differential Engine. Thank you for the thought, though, and the link :-)

[havoc]

I can't help much with the steam side but you already have some input there. As far as the electrical side goes, you need to take a hard look at "axial flux generators". These are the kind of alternators made for wind turbines by the alternative energy people. These are rugged and simple constructions made for low rpm operation without brushes to wear out. Just a few links to start you:
www3.telus.net/faheydumas/Wind_Turbine/Forum/AXIAL_FLUX_HowItWorks.pdf
www.radiolocman.com/shem/schematics.html?di=45613

This is where I started to build the alternator for my engine (see the Gauge 1 section): www.scoraigwind.com/pmgbooklet/index.htm Mine is smaller than what you will need but it can be scaled.

Now just to give you some pointers as to what is involved in the points you asked earlier on without going into a complete physics/electricity course:

2: power = torque x angular speed and angular speed = 2 x pi x frequency = 120 x pi x rpm. This is so for all rotational engines. Now the total power an alternator needs is the amount you get in electricity divided by its efficiency. Or the other way round: your engine will have to deliver (needed power x efficiency of alternator). A start would be an efficiency of 0.8.

3: speed and voltage are related through the flux in the alternator and the number of windings in the coils. The voltage in a coil is equal to the rate of change of the flux through the coil times the number of windings (sign is inverse but this is not important in this case). The flux is the magnetic strenght divided by the surface. So more turns and more speed will give you more voltage. More flux will also give you more voltage but that means smaller coils or stronger magnets. Now a given amperage needs a certain size of wire...so that will give you how much windings you get into a certain surface.

So a given size will let you make a high voltage at low amps or a low voltage at high amps for the same rpm. This is a balancing act...

4: see above.

Everything depends on everything. There are only a few of the values you cannot change. You are going to use a certain engine and that will determine your rpm at max power. If you really want then you could rig a torque balance and measure a torque/rpm chart (for a given pressure) while braking the engine. That will let you work out the optimal rpm.

Then you need to determine at what voltage you want to work and the power needed. Don't forget that booting may need more power due to hard discs spinning up. But just let pretend 12V and 40W. (since the rpm of your engine can vary, you will need to design for a voltage range!) So that is 12V and 4A (already some spare).

Now you have the 4A you can check the wire size needed. Keep in mind this will be in a completely closed setup! So keep an eye on what the table says. I would not go under 1mm diameter. Also raise the voltage to 14V to count for losses in the diodes of the rectifier.

So now you have the rpm and wire size. Build a spreadsheet and arm yourself with some datasheets of magnet suppliers. This is going to be an iterative process.
- the flux: this will be determined by the magnets you use. It is the magnetic strenght divided by the surface of the magnet times 2 (because there is a magnet both sides of the rotor) and multiplied by the "filling factor" (how good will the field stay concentrated. A filling factor of 0.8 for this is a start.
- now depending on how much magnets you are going to put next to each other you are going to get a diameter for your rotor. You always need an even number of magnets around the circumference of your rotor!
- the rate of change of the flux at a point along the centerlines of the magnets is going to be (flux x 120 x pi x rpm x (number of magnets / 2))
- the voltage will be the number of windings times the rate of change of the flux. So now you have an idea of how many windings you need.
- see if you can get half that number under the surface of a magnet without making it too thick. If you can you are lucky. You need only half the numbers of calculated windings if like in the pdf you put 2 windings in series.

Remember that windings that are smaller than a magnet (in surface) do not capture the full flux and thus produce less voltage but those larger than the surface of a magnet do not produce more voltage (and often less if the magnetic field of near magnets start to overlap inside the coil). If possible you could see what a diameter smaller or larger of wire would do. Change to 0.9mm wire and this might give a few volt more or be more compact while keeping the same number of turns. Again balancing...

This is a quick calculation off the back of an envelope. If you want to do it properly you need to take into account:
- resistance of the wire
- surface of each winding properly
- self inductance of the windings
- losses in the rectifier

If you use rare earth magnets -they are much stronger than ordinary magnets- then the temperature of them is limited to 80°C! So another point to look at is cooling of your alternator....

I'll try to find back my calculations and see if any of the above needs correcting.

Edit: some typos
PS: everything in SI units.