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Old 11-14-2007, 06:17 AM   #1
John
 
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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



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Old 11-14-2007, 07:35 AM   #2
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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



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Old 11-14-2007, 10:40 AM   #3
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Default Model Steam Engine Design

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.

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Old 11-14-2007, 04:10 PM   #4
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Default Model Steam Engine Design

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..))

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Old 11-14-2007, 04:55 PM   #5
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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

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Old 11-15-2007, 08:15 AM   #6
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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

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Old 11-15-2007, 09:20 AM   #7
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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

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Old 11-16-2007, 03:47 AM   #8
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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

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Old 11-16-2007, 06:28 AM   #9
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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

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Old 11-17-2007, 04:46 AM   #10
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Default Model Steam Engine Design

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



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