Newbie question on oscillating engine

[Duncan.e]

Hi,

An old, but newbie here - first post - from the south east of England. I have only just found the forum and have searched through it and read lots - It looks a great place to learn!

I want to build a twin cylinder, double acting, oscillating steam engine ultimately to fit into a 1:13.7 scale coffee pot loco for the garden railway I'm building (slowly).

I have to say straight away that I am completely new to steam engines, but have some machining skills. I'm designing the engine myself, but paying a lot of attention to existing plans.... I want to make it out of 316 stainless steel, with bronze (or brass) inserts in the valve areas to prevent frictional and pick up problems.

I was discussing the piston with an engineering colleague and explained that a lot of people use PTFE tape twisted into string to pack the grooves in a brass piston. He looked at me and said that that seemed daft, why not just make the piston out of PTFE.... This has got me wondering if there is any reason why I shouldn't do just that.

I have looked at the coefficient of thermal expansion of 316 stainless and PTFE and the two are similar, with the 316 being slightly greater, so there shouldn't be a problem there. The working temperature is well in excess of a steam engine's heat, so again no problem.

Can anyone suggest any reason not to try this?

Also, I want the engine to be reasonably small - height max ~ 2.5" and be slow revving but have pulling power, so I presume I need a longer stroke with a smaller bore to give me the torque I require. Can anyone recommend what sort of bore and stroke I should go for? My thoughts are perhaps ⅜" bore and ⅝" stroke (10mm x 16mm), but I really have no idea....

Any insights will be gratefully received....

Thanks!

Duncan

 

[arnoldb]

Hi Duncan

Welcome to HMEM wEc1

I don't know if you've done it yet, but have a look at Dave Watkins' site; both Idris and Chaloner are coffee-pots.

PTFE for the pistons would work, but on the small sizes there's always the problem of connecting the piston rod to them. I don't think it would last too long with just a threaded section in an all-ptfe piston, especially in double-acting cylinders where the piston is inclined to be fairly thin.

As to piston size, that's a difficult one. A lot depends on what pressure you want to run at; oscillating engines work best at lower pressures to minimise friction on the port faces while running, but for lower pressures, you need bigger pistons to get a bit of power. Oscillators are not good at using the expansive capabilities of steam either, so running at high pressure would basically be a waste. Torque in steam engines works a bit differently from internal combustion engines; basically, while you can supply steam to the cylinder, you're getting a lot of torque - unlike the IC's that just get a big whoomf that have to be contained to be useful.

The stroke in oscillators are limited by geometry; too short or too long a long stroke makes for complications on the port faces and design. If you get a hold of Charlie Dockstader's Valve Gear program, there's a very nice simulation in it that you can use to custom-design an oscillating engine to your specifications.

Not much to help you then, but maybe a bit of food for thought.

Kind regards, Arnold

 

[Duncan.e]

Hi Arnold,

Thanks for the reply!

I appreciate the fact that PTFE won't be that strong on a threaded section, so my idea is to make it as a short piston with metal either side of smaller diameter - a stepped piston rod, with a washer on the crank side and a threaded cap on the other side, sandwiching the PTFE securely....

I have looked at both Idris and Chaloner and they are great locos - I shall build my loco based on their general layout, but to my own design.

I have a copy of Charlie Dockstader's valve gear program, but can't run it yet as I run Macs, not Windows. My wife has a Windows laptop, so I shall be grabbing that later

I must admit that my thoughts about torque are based on infernal combustion engines, but I understand what you are saying about steam - a re-tune of the brain is in order!

You have been a great help and I thank you!

Duncan

 

[GailInNM]

Duncan,
I think you missed a decimal point in comparing PTFE to Stainless.
PTFE has a coefficient of expansion about 7 times greater than 316 stainless.
Typical specifications for PTFE are expressed as x.x times 10^-5 power and staineless as x.x times 10^-6.
Gail in NM

 

[Deanofid]

Hi Duncan;
I don't know if Teflon will work in your steam engine. I think it expands more than you think. I once tried a PTFE piston in a low-ish temp stirling engine that used a cylinder made from aluminum. The engine would start and run a few minutes, then stop as the piston expanded in the cylinder bore. (The same engine worked well with a graphite piston. The engine ran at about 200 F.)
Also PTFE does absorb some water. I don't know if that would be a show stopper either, but you may have a couple of things here working against you from the get-go.

(Edit: I see Gail has already put some good expansion info here for you. Mine is anecdotal. His runs the numbers for you.)

Good luck!

 

[Tin Falcon]

I was discussing the piston with an engineering colleague and explained that a lot of people use PTFE tape twisted into string to pack the grooves in a brass piston. He looked at me and said that that seemed daft, why not just make the piston out of PTFE.... This has got me wondering if there is any reason why I shouldn't do just that.

The PM research engines use ptfe rings. IMHO that is a good way to go. IIRC viton has also been used for rings and seals on steam engines. As far as PTFE tape I think you need to limit that to gland packing. There has been discussion in the past on using PTFE on steam engines it can get weird at high temps so be careful . I personally would stay away from entire pistons made of PTFE for steam.

Also, I want the engine to be reasonably small - height max ~ 2.5" and be slow revving but have pulling power, so I presume I need a longer stroke with a smaller bore to give me the torque I require. Can anyone recommend what sort of bore and stroke I should go for? My thoughts are perhaps ⅜" bore and ⅝" stroke (10mm x 16mm), but I really have no idea....

If you want the engine short in height but high torque you want a large bore . a piston 3/8 diameter @ 30 PSI will have 3.31 lbs of force exerted on it a 3/4 bore will have 13.31 lbs of force exerted on it. IMHO you need to be generous on the bore and conservative on the stoke for what you want.
I would think 1/2 x 1/2 would be about right or 5/8 things get awkward with an oscillator if the stoke is too long.
Tin

 

[Blogwitch]

Duncan,

I spent a lot of my formative years making engines for model boats, many of them oscillators.

Long stroke engines are the way to go, they are more economical with the steam, slower revving and more powerful. Square or over square engines are fine to look at, spinning over much faster, but much less efficient when it comes to pushing or pulling power.

With regards to PTFE as a piston, what you have to bear in mind is that with an oscillator, the piston and piston rod are the main load bearing parts of the engine, physically swinging the cylinders side to side, so have to be fairly strong and 'beefy', whereas with say a piston or slide valve engine, they basically only have to go up and down and the power is transferred to the cross-heads and conrods. So not really a very good idea in an oscillator, you might be better with a solid metal piston, say brass or bronze, with a viton O-ring. My engines used to last about two or three years before the rings needed changing. They wear very well in this sort of configuration.

With long stroke oscillators, you are limited to about a ratio of 2 to 1, say 10mm bore by 20mm stroke, then things start to get rather tight at the bottom end, especially around the stuffing gland if you want it to be a double acting one.

There is a very nice 10 x 20 double acting one on a French website that you can download the plans for free, and modify to your hearts content. But it will give you an idea of what it should look like.

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

Click on the bottom left hand one and you should then be able to download a pdf file of the plans.

There have been a few built on here, the last only a month or so ago by Chuck Fellows.

If you want to keep your max height to 2.5" and still have a long stroke engine, you are going to have to look at a bore of around 6mm and say 10mm stroke (1/4" x 3/8"). What you need to do to get your required stroke is look at the overall height required and divide it by 5 (for a tightish fit for top and bottom bits) or by 6 to give you some room to play with. The reason for this is to allow for the overly large crank swing, support bearings and bedplate.

I hope this has helped a little.

John

 

[Duncan.e]

Thanks to all of your replies, it really is a great help!

Gail, you are absolutely correct, I didn't register that PTFE is 10-5, I just read the numbers off.... :-[

Deanofid, I like the idea of a graphite piston as it seems that I can't use PTFE, I'll look into that....

Tin, I originally thought of using a viton 'O' ring for the piston seal, but am concerned about it's life expectancy - would one last in a hot reciprocating environment? Thanks for the suggestions (and warning) on bore and stroke.

It seems that I have a lot to learn - thanks to you all, I hopefully will learn and not make too many mistakes along the way!

Duncan

 

[Tin Falcon]

The great thing about this board is varied perspectives. The engines I have build and the plans I have keep the bore and stroke about equal.The designers are folks I have met ,trust and are well known for there designs. . I also have to admit my engines are run on compressed air for display.
while doubling bore size does indeed more than double torque due to the area proportional to the square of the radius. it does also increase steam consumption. I do not see the power being double by doubling the stoke but probably close enough. . I do agree that it does use less steam and increase efficiency.I would also think increasing the stoke on an oscillator would increase side forces a bit. Bogs thanks for giving a practical limit on how long the stoke can be before it cases problems.
There are certainly practical limits to design but a lot of room creativity within the limits.
Learning a little math may help. scaling up or down has its place but understand making an engine smaller reduces power output increasing size increase power and also steam consumption power is proportional to the cube of linear dimensions. Double the size and power is increased by 8 times.
Tin

 

[Deanofid]

Deanofid, I like the idea of a graphite piston as it seems that I can't use PTFE, I'll look into that....
Duncan

Duncan, I didn't mean to recommend graphite for your pistons in a steam engine. I only used that example to show that PTFE expands too much with heat. Brass or bronze are well proven materials for steam pistons.

 

[Duncan.e]

Duncan, I didn't mean to recommend graphite for your pistons in a steam engine. I only used that example to show that PTFE expands too much with heat. Brass or bronze are well proven materials for steam pistons.

Don't worry, I understood that you were giving me a possible alternative, not a recommendation. I appreciate your time in responding to my questions.....

I'm still trying to work out the best materials to use - I don't want to use brass or aluminium as these are commonly used materials and I want to use something different.... I still like the idea of using 316 stainless steel for the the engine, with some brass, or preferably bronze, to create bearing surfaces for the ports, piston rod bearing and crank end cylinder gland. I'm considering making the piston out of 316, with a viton 'O' ring, or something similar to create the piston seal, although I am still pondering this....

There is one problem, my lathe is an old Myford and does not have provision for running coolant - 316 ideally needs coolant when machining, so it looks like I'll need to do some mods before I start, or rethink my material choices

I have so much to learn and am finding it 'interesting' researching the options. :-\

Duncan

 

[Tin Falcon]

I don't want to use brass or aluminium as these are commonly used materials and I want to use something different....

I hear you. I have built demonstration engines that I show, out of corian and lucte folks love them. I have been accused of not owning a box. So i can identify .
But you are talking a running engine. Brass, Bronze, aluminium, and cast iron have desirable properties for engines and are all quite machinable and are not prone to galling. They are commonly used for a reason . they work for the application . Your engine your shop your decision.
Tin

 

[Duncan.e]

I hear you. I have built demonstration engines that I show, out of corian and lucte folks love them. I have been accused of not owning a box. So i can identify .
But you are talking a running engine. Brass, Bronze, aluminium, and cast iron have desirable properties for engines and are all quite machinable and are not prone to galling. They are commonly used for a reason . they work for the application . Your engine your shop your decision.
Tin

I hear you too Tin and I understand (and agree) with what you say about the materials. It's just that I would really like to use something different, if only I could decide what..... :-\

With reality kicking in, I am leaning towards bronze and / or cast iron.

Question time:

Is there any reason why the whole engine could not be made of bronze?
If I can, what bronze would you recommend for this application?
I've looked at the specs and prices of Colphos (expensive!) and PB1, both of which look right to me, but is there an alternative grade that is a more realistic price, that won't gall and is easily machinable and silver solderable?

Duncan

 

[Tin Falcon]

Is there any reason why the whole engine could not be made of bronze?

In days of yore (Before a few years ago when the price of copper skyrocketed) pm research made many of there kits in choice of aluminum or bronze. I bought a couple bronze kits in IIRC 2000 or 2011. When the prices jumped PMR discontinued the 2B kit in favor of marketing the more economical 2A as a starter entry level engine. the 3B was replaced by the 3BI still many bronze parts but a cast iron base.
The down side of an all bronze engine is the cost. And of course you will need a few pieces of steel for some little mating parts.
I have seen some fine but expensive kits investment cast out of bronze .
some selection help here
http://www.mcmaster.com/#bronze-alloys/=dtanyu
Tin

 

[Duncan.e]

Tin,

Once again thanks for your input. I have made the decision to make the bulk of the engine out of cast iron. This decision has been made quite simply because I have a free source of more than sufficient cast iron to make several engines!

Is there any reason why I should not make the cylinders, pistons, frame and crank counter weights all out of cast iron? I know I will need to use some steel for the piston rod, crank pin and crankshaft.

I'm considering using a Viton 'O' ring for the piston seal and another for the piston rod seal - any potential problems I should be aware of?

Thanks for your help!

Duncan