Dynamo test

[jamespetts]

On Saturday, I tested my Stuart Score with my new dynamo to see how much voltage and current that I could get out of it.

The video is here:



I found that, although I was able to produce a good voltage from the dynamo, the boiler was not able to sustain it for very long before running down the pressure - it went from 50 to 25psi in less than a minute of running, whether under heavy or light load.

I think that, in order to be able to power anything useful from this arrangement, I am eventually going to need another boiler (my current boiler (a gas fired Stuart 504), on which my father and I have done much work, can usefully be employed to power the Stuart S50 that I am building that I am hoping can be used to polish shoes).

Does anyone have any thoughts about the sort of boiler that might be suited to an engine and application such as I have? It should fit on the board that I have (1.5m x 1m - there is a fairly generous space around the existing boiler), and not require an electric blower (since that would defeat the point of being able to generate useful amounts of electricity in a power cut, or away from a mains supply, etc.). I think that I shall have to scour Station Road Steam for a few months to see what is around, and also save up, since larger boilers are terribly expensive!

Moving on from the boiler, looking back over the video footage of the engine, especially under load, it seems to have a very inconsistent power output, not consistent with a two cylinder engine at all - I am wondering whether there is a problem with the valve mechanism/timing on one of the cylinders: to anyone who has seen the video: does this seem to be borne out by the way that the engine performs? How best to diagnose such a problem?

I am also noticing that a number of the gaskets (cylinder back covers, steam chest covers, cylinder feed pipes) seem to be leaking: I am wondering whether the steam oil that I am using (from Forest Classics) is somehow perishing the paper.

Further, I am having some difficulty with operating the rear cylinder drain cocks - although I have now attached pipes (which greatly reduces the mess underneath the cylinders when the drain cocks are open), the rear drain cocks are very hard to access, even with pliers. Does anyone with a model like this have any tips?

Also, I had some problems with the chain: the large sprocket initially slipped on the shaft, causing the chain to become detached - I eventually fixed that by tightening the grub screws. The sprocket on the dynamo became dislodged, but I reattached it satisfactorily using Loctite (it had previously simply been a dry interference fit). The chain between the shaft and the dynamo is also too loose - I am looking into ways of moving the dynamo slightly further from the shaft (by packing the brass straps, which are a smidgen loose) to fix that.

Finally, I had improved the fitting of the water gauge greatly since the last steaming, using a number of home made copper washers, but the blowdown cock was loose. I have sine reduced the amount of PTFE tape used to pack it, and tightened it much more: it is now very tight, and should not come loose when pressure is applied to move the lever.

Any general tips or thoughts on the improvement or operation of a plant such as this? Any comments would be most welcome! Photographs follow:


Water gauge fitted with home made copper washers.




Dynamo and shaft set up. The dynamo is a brushed DC motor designed for a radio controlled boat.




Voltmeter attached - in the rain

Edit: Incidentally, this engine will be on display as a static exhibit as a work in progress model at the Ascot Exhibition this year, on the Ickenham & District Society of Model Engineers' club stand.

[Deleted]

Interesting. I am having the same problem. I'm running the Stuart 10H powering a small dynamo. In my case the desired output is 2 amps at 12 volts. With my drive ratio I need to run my steam engine at about 800 rpm. Assuming 50 psi operating pressure I would need about 0.56 cubic inches of water evaporated per minute. To get this I would need (in theory) a boiler with about 56 square inches of heating area. I expect you should need about double that. I'm not sure what the 504 has for area, but you could probably calculate it pretty quickly.

My boiler is the vertical fire tube type. I seem to have about enough area, so I'm thinking I need a better heat source. I'm currently using the esbit tablets, so I probably need to switch to a ceramic burner and LPG.

If you do calculate the area, please post the result.

[jamespetts]

Ztarum,

I should be interested to see your plant! Calculations about the amount of steam necessary for a given electrical power output are quite complicated, as well you can imagine. 2a at 12v is 24w - really quite a lot of electricity. Can your dynamo cope with that? What sort of dynamo do you have? What did you want to drive with it?

Are you sure that a 10V is powerful enough to produce 24w of electrical power given all of the inefficiencies inherent in the system? The Score is rated at 1/10th of a horsepower at 60psi, which equates to about 74w of mechanical power. A 10V is like the Score, but with only one cylinder, and vertical. One would expect its power output to be half, at 60psi, being 1/20th of a horsepower, or 37w of mechanical power. Half of that is 18.5w, so you would struggle to generate 24w with a 10V at 60psi, let alone 50psi. If you had a big enough boiler, you could probably be sure of no more than about 16-18w constant with a sufficiently efficient dynamo, peaking at 20 if you were lucky. Incidentally, there is a dynamo, rated at 18w/12v, sold especially for model steam engines, and stated to be particularly suitable for the 10h/10v. Given the above analysis of power outputs, it is easy to see why!

Calculating the amount of steam required is especially complicated because: (1) at higher speeds, more steam volume at any given pressure is required, but; (2) at higher torques, more pressure is required for any given speed; and (3) for any given level of cutoff, peak efficiency is only reached at particular speeds, and for the fixed cutoff on simple slide valve engines such as the Stuart no. 10 series, the engine is more efficient at lower speed and higher torque.

So, you will probably find that, at 50psi, your engine cannot run fast enough when the dynamo is loaded at 2a to produce 12v. A Score (such as my engine), or the vertical version, the D10, should produce enough power at 60psi (and quite possibly enough at 50psi), but would require twice as much steam volume to run at that speed as the 10v.

Another thing to consider is the drivebelt - the higher the torque required, the more likely the drivebelt is to slip. The more current that you draw from your dynamo, the more torque required to turn it. The higher the speed that you require to produce any given voltage, the less torque that any given amount of current will require, but, at higher speeds, slide valve engines with fixed cutoffs are less efficient. At lower speeds, the engines are more efficient, but the higher torque is more likely to make the drivebelt slip. You could use, as I do, a geared chain drive that cannot slip, but that is heavier and less efficient than a flat drive belt.

The number of possible tradeoffs is, as will be appreciated, quite considerable: the simple answer, of course, is just to have a boiler that is more than large enough to power the engine at the requisite torque and speed even if the engine is of less than optimal efficiency and, use the chain drive, which, although less efficient, at least will not slip or come off so easily. Because of the larger boiler required, however, that is the more expensive route.

I should be intrigued as to how you progress with your project - you shall have to post pictures!

[Deleted]

My initial statement was a bit misleading. I don't plan to actually draw 2 amps at 12V. The dynamo is rated for that, but I will be drawing less. I simply wish to power some lamps, at least initially. I probably will only draw 0.5 amps or so.

You make some excellent points. My initial experimentation suggests that the engine has the power I need at 50 psi and will indeed run fast enough. It's my boiler that will not keep up. My rough calculations suggest that the heating area I have is marginal, but it should be enough. I'm hoping that if I can supply more heat it will be able to keep up, but that may not be the case.

Before I spend the money on the ceramic burner I may try to fire the boiler with my oxy/acetylene torch. Overkill for sure, but if it won't keep up with that then I know I will need a bigger boiler.

[jamespetts]

Ztarum,

ahh, if you only want to draw 0.5a at 12v, then that is 6w, which should be fine at about 50psi for a 10v, provided that everything is working properly. What kind of boiler do you have, exactly? Perhaps a picture would help.

If you are firing with esbit tablets, then you will definitely not be disappointed with a ceramic burner. I recommend Maccsteam for ceramic burners - their products are very high quality, and you can get a custom made one for the price of a mass produced one elsewhere. Get the largest burner that you can fit in your boiler, and power it using disposable butane/propane mix gas tanks such as these (excluding the propane only one at the top).

The Maccsteam people will explain to you how to use it, and what you will need (a burner, a jet, a gas cartridge and a regulator). You can certainly get more heat - and more controllable heat - from gas than from esbit tablets. If you are only a little under in terms of steam output, or even moderately, upgrading to a ceramic burner (which is not likely to be particularly expensive) is very worthwhile. Gas is by far the easiest of all the means to fire a model boiler.

[Deleted]

This is the boiler I have built.
www.pmresearchinc.com/store/product.php?productid=3111&cat=7&page=1

I'm hoping to find a US based source for the burner, but it doesn't look promising. There are Stuart Turner dealers that carry their burners, but that is about it. I'll have to drop the MacSteam folks a line and see what they can do.

[jamespetts]

Aug 15, 2008 22:00:13 GMT @ztarum said:

This is the boiler I have built.
www.pmresearchinc.com/store/product.php?productid=3111&cat=7&page=1

I'm hoping to find a US based source for the burner, but it doesn't look promising. There are Stuart Turner dealers that carry their burners, but that is about it. I'll have to drop the MacSteam folks a line and see what they can do.

Ahh, a nice looking boiler, but probably too small to run anything substantial with a load. A gas burner might help a little, but, really, you could do with a larger boiler.

Let us know how you get on!

[jamespetts]

Update: I have been working on the engine to-day, and have fixed what was probably quite a significant problem with the non-flywheel side slide valve which meant that it was not pressed firmly against the cylinder ports (if one looks at the video, one will see that the engine seems to have full power only one one cylinder). The eccentric rod does not quite run true, so the pressure of the slide valve against the port varies depending on where the timing is set. I solved that by using a brass shim, and then retimed that side of the engine, which timing now seems accurate.

Also, I replaced one of the gaskets and tightened the other gaskets considerably more than they had previously been tightened (one will see them leaking in the video), found that the pipe into the non-flywheel side steam chest was partly blocked with copper shards, so drilled that out, added a small stub of 1/16" copper pipe to the end of the burner so that the air holes are always kept out fully in the air (previously, they had partly slipped inside the firebox, reducing the airflow, and therefore the burner heat), connected the pipe from the boiler blowdown cock to the sump tank, and put some brass and copper shims underneath the dynamo strap to tighten the chain to the dynamo somewhat. I have also finished insulating the one remaining section of steam pipe, and replaced the PTFE tape on the insulation of the pipes that feed the individual cylinders.

I am hoping to run it in steam again to-morrow to see whether any of that makes any difference, although I am not sure whether I shall have a chance to upload a video anytime soon.

[circlip]

The only way you're going to be 100% certain James is to check each cylinder separately. Disconnect one side and do whatever tests you can (Volts, RPM, Torque etc.) and then do the other cylinder likewise.It's interesting that you found debris in one of the pipes,but it has illustrated that the only way to ensure that everything is working correctly, especially when you haven't built the engine yourself is to assume nothing and do a full analysis piece by piece. Not a criticism, but a cold hard fact of life.You're slowly getting to the top of the mountain.
Regards Ian.

[Deleted]

I did some experimenting again tonight. I tried to rig up the oxy/acet torch with a rosebud tip into the firebox, but that didn't work at all. I could never get the mixture right and it kept popping. A few loud bangs and I decided enough was enough.
Next I tried the same thing, but this time with my MAPP gas torch. That worked much better. Using the torch I was able to make enough steam to keep the engine running with enough speed to power the lights. Based on this development I am going to move ahead with acquiring and installing a ceramic gas burner. Hopefully it will prove sufficient.

I still need to figure out a decent drive belt. I have been using rubber bands, but most are too narrow. I really need something about 1/2" wide.

[Deleted]

Our DYSON uses a half inch wide strong rubber band about 6" long. £3.50 for two!
JB

[jamespetts]

Stuart models sells a range of suitable bands that will do just what you want. Incidentally, well done on getting it working! I should love to see a video of it going.

[Deleted]

I will certainly post a video when I get it all set up. Right now it is a temporary arrangement and I am 1 or 2 hands short as it is. Once I get the gas burner set up (instead of my hand held torch) and get a proper belt (which will free up the other hand that I am currently using to keep the rubber band on the pulley, I will get some decent videos of the contraption in action.

I did get pricing for a burner from forrest classics, but their current delivery is 5 - 6 weeks. I'm going to call a Stuart dealer here in the US to see if they have anything immediately available. I'll drop macsteam a line as well.

[circlip]

For a positive drive system, you could also look at timing (toothed) belts.
Regards Ian

[Deleted]

I ended up with the perfect flat belt for my application. A co-worker suggested a vacuum cleaner drive belt might do the trick. As luck would have it there was a vacuum supply shop just down the road from me and they had exactly what I needed.

With the belt in place, the whole set-up works great and lights up two 14V bulbs as bright as can be. I still don't have enough steam to keep it going, but I have a burner on order from Forrest Classics so hopefully that will do the trick.

[Deleted]

Great to hear it works: might have saved some time if you hadn't missed this post on this thread.....

Our DYSON uses a half inch wide strong rubber band about 6" long. £3.50 for two!

Oh well, c'est la vie!

[Deleted]

Oops. I didn't make the connection that a Dyson was a vacuum cleaner.

[Deleted]

I thought most serious ME's lived in a vacuum?
JB

[jamespetts]

Ztarum - you shall simply *have* to post photographs!

[Deleted]

Well, I got the new gas burner installed. It works better, but still not enough heat for sustained operation at the required speed. At this point I am considering a step up belt system so I can run the engine more slowly but keep the dynamo at the same speed. I fear that at the current speed much of the steam blows straight through and is effectively waisted. I'm hoping that by slowing the engine down I can extract more of the power from the steam and operate more efficiently. If not, a larger boiler will be the only solution.