Model Steam Engine Design

[John]

I plan to build a small steam engine of the oscillating type to fit into Meccano models.

I have on hand some brass tubing with an 8.8mm (11/32") bore which would make an ideal cylinder.
Is there any rule-of -thumb or data tables which would tell me the following:

With a cylinder bore of 11/32" is there an optimum length of piston so that it runs easily in the bore?

What is the best piston material to use in a brass cylinder. Brass?, Aluminium?, Steel?. I intend to use a plain piston, no rings, just a couple of grooves to carry oil.

What would be the optimum stroke length with an 11/32" bore, assuming a steam pressure of around 20psi (seems to be the usual running pressure of toy steam engines)?

Any comments appreciated.

John

 

[Bogstandard]

John,
Because you will be using live steam, brass would be the best selection for piston, with a length of about 3/8", with 2 or 3 oil grooves about 0.050" apart and 0.015" deep and positioned at the top part of the piston. I would also suggest a centre pivot cylinder, rather than an end pivot. A stroke of 1/2" to 5/8" is about right, any shorter and the engine will be a fast runner, the longer the stroke the slower the engine will be.

Hopes this helps.

John

 

[tmuir]

John,
When you start building please post pictures as I hope to attempt to build my first engine early next year which will be a simple oscillator too.
I want to start simple to build my confidence up before I try something harder.

 

[deere_x475guy]

Bogstandard, can you explain why brass would be the best choice for a piston on a steam engine? Also since he is going to use brass for a cylinder is it a good idea to also run brass as a pison? Also how did you come up with 3/8" for length. Is there a formula for coming up with this number?

John please take pics when you start building. It will help my addiction along..))

 

[Bogstandard]

Hi Bob,
No formulas other than a lot of experience.
Brass wouldn't be the best choice for a piston but in the circumstances it is really the only viable one.
Because he is going to be using live steam, you would normally use materials because of their expansion and wear resistance properties.
If I was making a steam engine from scratch I would use either cast iron or maybe bronze for the cylinder, brass at a push. If a cast iron cylinder was used then a piston of cast iron, bronze or brass would be suitable materials. The list for all the combinations is endless. Because John is going to be using brass for his cylinder, then a piston of the same expansion and wear properties points to a piston of brass, cast iron or bronze would most probably create too much wear on the cylinder bore in the long run, being much harder materials. To my mind the piston should be the sacrificial part not the cylinder.
He could have a piston six inches long, but it is wasteful of materials plus the friction on the overall length would cause the engine to not be very efficient. So I would go for a piston length that is somewhere near the bore size or because john is most probably only be making a single acting engine then maybe slightly longer, this is because the piston and rod in an oscillator is being used to also swing the cylinder side to side, which if he went for a shorter piston, would tend to rock in the bore and cause undue wear. If however he goes for a double acting engine he can make the piston thickness only about half the cylinder bore width because the bottom sealing gland is taking most of the swing forces required to move the cylinder side to side.
I could carry on for hours about this but I think this is enough for now other than to say that if running on just air then almost any materials can be used for any part of the cylinder and piston makeup, as long as it is kept lubricated. Live steam is a totally different ball game.

John

 

[John]

Thanks Bogstandard.

You have confirmed pretty much what I thought.

Looking at a 1950's Fleischman (I think) engine I have, it has a bore to piston length ratio of about 1:1.6 (6.6mm:10.5mm).

Piston stroke is 20mm (crank throw 10mm) so that gives a bore:stroke ratio of about 1:3

Pivot point on the Fleischman is a bit above centre but a centre pivot would put the steam inlet and exhaust holes the maximum distance apart, which may be an a good idea.

Any advantage making these holes oval to increase the 'open' time?

I agree a brass piston is probably best from a wear and corrosion point of view. An aluminium piston would save weight but brass and Al are not compatable due to corrosion problems.

I will try to fit in the longest stroke possible given space constraints as that should extract the most power from the expanding steam. Fast running is not an issue.


Thanks for all the comments

John

 

[Bogstandard]

John,
You might find that on the one you have measured the pivot point is in fact central, it is just that they have maybe got a longer bit of the cylinder below the bottom of the piston compared to the bit above the top of the piston.
There is really no advantage to elongating the holes. What you do have to be careful of is the distance from the centre pivot point to the centre of the crank. The danger is the more the distance the closer the port holes are together, a bit too far and you run the risk, especially with a shorter stroke engine, of inlet and exhaust overlapping the port hole in the cylinder at the same time.
I usually work it all out on graph paper beforehand to get the port diameters and positioning. Then scale it down to the size I want to make.

John

 

[John]

Thanks John, got all that.

What about putting the cylinder pivot point a bit below centre? That would increase the swing at the top of the cylinder and allow the ports to be further apart.

Another question is where to place the port on the cylinder. The simple engine I have puts it in the side, so leaves a dead space between the top piston and the cylinder head, equal to the diameter of the inlet port.

With a bit of careful drilling, the steam entry/exhaust port could be from the side, but then turn 90 degrees in the cylinder head and enter the cylinder from the top. This would allow the piston to come up to the very top of the cylinder at TDC, so eliminating the dead space and make the engine more efficient.(I hope this makes sense without a diagram)


John

 

[Bogstandard]

John,
I think you just might be trying to make it a little bit too complicated.
The oscillator is really one of the most basic steam engines we build, and is grossly inefficient. When in fact you look at the later developments of the steam engine they start to use the steam a lot more efficiently.
The oscillator in fact puts steam into the cylinder for usually over 90% of its stroke, so it doesn't use the steam in the best way possible. Valved engines on the other hand may only allow steam into the top of the cylinder for 10% of the stroke and use the properties of its expansion to do its work. That is how the triple expansion engine works, getting the most out of the expanding steam.
So for this little project I would just keep it simple to get to know your equipment and materials, then progress onto something a bit more ambitious later.

John

 

[Tin Falcon]

John:
Bogs is right stick to the K.I.S.S. (Keep It Simple Sailor) method of model engineering. There are dozens of drawings prints designs out there and available. So pick one that looks easy to you and build it or take a couple thee that are similar or variations of the same design and select the parts from each that suit your liking. No need to re- invent the fly wheel.
Tin

 

[John]

Thats fine, I will keep it simple. So many options present themselves when you start looking hard at designs.

Now I will get it down on paper to see how it fits together, and also see what extra I need in raw materials. A visit to the scrap metal dealer soon.


Thanks for the help and advice.

John

 

[m_kilde]

Hi John

Well I'm unsure if you allready have desided on a design, but if not this little engine that I have build is really not that complicated, and you can download the plan for free - don't worry I can't understand french neigther

The plan you find via this link :

http://jpduval.free.fr/Plans_moteurs_vapeur_p1.htm

 

[Bogstandard]

Hi Mogens,
I took this design and modified it to make it easier to make and corrected a few mistakes on the drawings.
It is a little over the top for a beginner but it would be ideal for maybe a third or fourth engine.
Here are my renditions of the same engine but modified to suit myself and customers, it does run very well, when good material selection and tolerances are kept tight.

John

 

[John]

Wow, that looks a nice engine. Probably a bit out of my class at the moment, but as Bogs says maybe a fourth...............?

John

 

[Bogstandard]

John,
Don't be afraid of taking a plan and modifying it to suit your needs, I do it all the time. The way I build my composite crankshafts was an idea taken from this design.
The people have taken the time to calculate all the port spacings and throws of these engines, so just use the dimensions to make your little engine from. I wouldn't try to scale up or down on the port or throw dimensions because you just might find that they won't work.
If you do 'borrow' an idea from a plan it is always nice to recognise the original designer. If you see any of my work, especially my vids, if it isn't all my own work, I do try and mention the original designer.

John

 

[Steam4ian]

G'day John et al.

Sorry about the delayed reply.
For a simple single acting oscillator the piston length should be more than 3 times the diameter. I got this value from discussions with some very experienced mechanical engineers who at the time were analysing the design of some extendable lighting towers. The towers were to telescope and these engineers were highly critical of the design on the grounds of insufficient overlap between sections. Their observations were not headed and the towers were built. They failed, often jambing, in fact one almost fell and killed two men. They were then replaced by the fixed towers you see today at Adelaide Oval (cricket ground).

Obviously this rule need not apply to double acting cylinders with extended bushes or crossheads. From checking Greenly, LBSC and other designs a thickness of about 1/2 diameter seems OK.

Regards,
Ian

 

[Loose nut]

You shouldn't use brass for steam usage, use bronze, gunmetal to the British, brass fittings on small boilers have caused failures, pistons and cylinders won't fair much better. The zinc Leach's out of brass and it becomes porous and the wearing properties of bronze is very much better even if only used under air pressure. Cast iron is a very good choice, good wearing properties, just keep it well oiled.

 

[compound driver 2]

Hi
Gun metal and bronze are not the same and even looking at bronze you have a good few compositions to choose from.fittings on boilers are fine in either but the choice for me is always bronze.
Some brasses are ok for steam fittings but not the first choice. if its marked as a CZ grade it will in general be ok BUT and this is a big but theres no gaurantee it wont fail. Always go with bronze or gunmetal for any fittings that are going on a boiler or a steam line.

Point of fact the faces on my slide valves are always hard brass and to date I have not had one fail. The wear is superb against cast iron and a shade better than bronze. The Piston on my 2 inch traction engine is 1.5 inch diameter and just a shade over 5/8 inch long.

Cheers kevin

 

[John]

Thanks for the comments.

The Flieshmann toy engine I have here has a 2:1 piston length to diameter but I think 3:1 would be better to make the piston run smoother in the bore, considering the sideways forces on it in an oscillating engine.


Thanks for the comments regarding brass, bronze, gun metal etc. As a novice who sources his supplies from the local scrap metal yard I have trouble sorting out brass from bronze, let alone the different types of each!

What is an easy way to tell brass from bronze when you are looking through a bin of scrap?

John

 

[Philjoe5]

John,
I don't know if this is completely reliable, but I take a steel needle point tool to the "yellow metal scrap pile" and do the scratch test. Brass will produce a deep scratch with a little pressure while bronze will barely scratch. Try it on a few pieces of brass and bronze and you'll see the difference.

Cheers,
Phil