Designing steam engines

[chucketn]

I've been playing with the simulator Donrecardo posted. I managed to scale up the Full Double Oscillator model to near where I want to build it. It brought up a question. The default model shows the piston covering half the ports at TDC and BDC. What effect would this have on a running engine? Should I modify my design to not cover the ports? To cover them less, more?
What affect would this have on a running engine? If this was covered in the posts previous to this, I missed it.
Bear in mind, I have no experience running my engines on steam. I might build a boiler some day, but for now I will run them on compressed air. So explanations should be primarily for air, with maybe a contrast if run on steam.

 

[Anatol]

Don, that's a wonderful little app. Thanks for sharing. It only simulates oscillator, correct?

 

[Donrecardo]

Hi Anatol,
yes single and double acting oscillators

 

[gbritnell]

Steam engines used for motive power, locomotives and traction engines generally had something called cut-off. Being as steam is an expansive gas cut-off was used to make the engine more efficient. When the engine wasn't required to work hard the valve could be adjusted through linkage to cut the steam supply to the cylinder anywhere from 30% to 60% of the piston travel. Most stationery engines used a simple valve connection because once started they ran at a governed speed to provide power for industry.
The reason I state this is because if you're going to design an engine it's better to create it with valve timing used for steam as opposed to using air. An engine that runs on steam will also run on air. I don't know the exact cut-off numbers of say a Stuart #10 but I would guess somewhere around 60% of piston travel. Using air will provide enough pressure on the piston for this period of time to allow the engine to go past center where it will then receive another shot of air to push the piston in the opposite direction (on a double acting engine)
gbritnell

 

[goldstar31]

I tend to be somewhat jaundiced about steam because of 'family experierinces' . The great uncle working the Royals out of Carlisle and the buckled back from ever open fireboxes and and a father under a dropped fire tube in a locomotive boiler. I could go on but suggest that you might want access my very good friend- Don Ashton's website on steam locomotion. Last time information that Don was building a full size steam locomotive.

Don's CV is quite impressive, he wasn't just a steam boffin.

Regards

Norman

 

[Charles Lamont]

Jens - I realise the steam pressure is higher than compression but the total pressure can only be as high as steam pressure. So by raising the cylinder pressure before admitting steam, the amount of steam admitted is reduced, and available power is thus reduced, for a given cylinder pressure. The energy of the compressed air is generated by the engine so costs more energy than it produces, which is why I was confused. It is thermodynamically impossible for the compression to create more power.

It is not about power, it is about efficiency. A sudden inrush of steam into a (relatively) cold cylinder would be wasteful - it is always best if things happen smoothly in terms of both flow velocity and temperature change. The fact that with compression less steam is admitted is a good thing. If you are concerned about there being less power, your cylinder was too small in the first place. Make the cylinder bigger to compensate. For the same steam consuption a larger cylinder with the right amount of compression will give more power than the smaller cylinder without, because the steam is used more efficiently.

In a steam engine running on steam, what is being compressed is steam, not air.

 

[fcheslop]

Thinner piston they only need to be a little thicker than the seal or look at the drawings in the link
http://jpduval.free.fr/Moteurs_vapeur_simples/MV deo 10x20.pdf

 

[TimTaylor]

A couple of other interesting references:

Model Stem Engine Design by R.M. de Vignier 1936

Steam Engines - Engine Governors I.C.S. staff 1926-1929 (published by International Textbook Company)

The first is accessible online from the university of Chicago Library, don't have a source for the second....

 

[chucketn]

I found the Vignier book at the U of C library, but it is not available for download by the general public.

I cannot find Steam Engines - Engine Governors I.C.S. staff 1926-1929 (published by International Textbook Company) at all.

 

[TimTaylor]

Yeah, U of C requires a membership to download also.

The following is a link to a list of books on steam engine theory and design that are in the public domain - some interesting reading -
http://www.vapeuretmodelesavapeur.com/telechargements/livres-vapeur-sans-copyright.pdf

This is a ink to the ICS Blue Book archive - the governor book is not on the restored list, but there are several other interesting titles there -
http://icsarchive.org/icsarchive-org/bb/

 

[chucketn]

Thinner piston they only need to be a little thicker than the seal or look at the drawings in the link
http://jpduval.free.fr/Moteurs_vapeur_simples/MV deo 10x20.pdf

Thank you.

 

[chucketn]

https://books.google.com/books?id=gDwxAQAAMAAJ&pg=PR1&lpg=PR1&dq=r.m.+de+vignier&source=bl&ots=OTZfG6sis8&sig=5_aFBwlKb6yWsfBCBjlMYlmqN4c&hl=en&sa=X&ved=0ahUKEwja4rvFkerbAhUJjq0KHaweAhkQ6AEIOjAF#v=onepage&q=r.m. de vignier&f=false

Found the R. M. de Vignier book at the above address, link there to .pdf download. Found by searching on Google for R.M. deVignier.
I have read the copy I downloaded, and my head is swimming from all the info I found. Thanks Tim for posting the link that got me looking for that reference.
I will re-read this book several times and refer to it often.
I found a comment on the Google Books page for this book that the copy ended at page 81 while the index says there is another chapter starting on page 87, and indeed it does end at page 81!
Tim, or anyone else that may have a copy of this book, does your copy have the missing pages? May I get a copy of them from you?
The information presented in the book is fantastic, but brings further questions. Are the timing, power, and other calculations described for vertical and horizontal slide valve and piston valve engines pertain equally to oscillating engines? The missing chapter has info pertaining to ports, patterns and such. I would very much like to get the missing pages.
I will invariably be asking more questions, as I think this book has given me enough information to at least know what questions to ask.

 

[chucketn]

I have made the drawing above based on what I've learned from this forum and others, and the various books I have been referred to. I'm still trying to define relationships that could be added to this drawing, not as a steadfast rule but as a guideline. One question that has not yet been answered to my understanding is : should the piston cover the ports at the top/bottom of the stroke, half cover, or not cover them at all? What is the benefit of yes, no or half covered?
Bogstandard, are you looking?
Where I have the lable "Total length of piston and piston rod", there are supposed to be dimension leaders pointing to the center of the left most circle and the 9:00 o'clock point on the middle circle on the right.

 

[chucketn]

I tried to improve the drawing...

 

[bouch]

View attachment 102401
One question that has not yet been answered to my understanding is : should the piston cover the ports at the top/bottom of the stroke, half cover, or not cover them at all? What is the benefit of yes, no or half covered?

If you cover the ports at the top/bottom of the stroke, you're preventing any steam from getting into the cylinder. I wouldn't want that for any steam engine. The only thing that should prevent steam/air from getting into the cylinder is the valving, not the piston blocking the port. If the piston blocks the port while the valve is admitting steam, you've pretty much made the valve timing useless. (and for the purpose of an oscillator, the valving is the rocking of the cylinder)

It even more important with an oscillator. That'll cause the steam/air pressure in the port to push the piston away from the body, which will cause leakage.

 

[Anatol]

... If the piston blocks the port while the valve is admitting steam, you've pretty much made the valve timing useless. (and for the purpose of an oscillator, the valving is the rocking of the cylinder)
.

Good point. But with an oscillator the ports cannot be in the top of the cylinder, unless you make a manifold internal to the cylinder head. You can squeeze the ports into the top corner. So what size gap between piston and end of cylinder is deemed to be too much? Is some gap desirable for any (other) reason? (Some argue for it in unaflow engines.)

 

[Charles Lamont]

In a simple oscillating engine:

1) the clearance between piston and cover can be minimal - a few thou to prevent mechanical contact due to inaccuracies and wear;

2) the piston should not completely block the port, but remember that at dead centre the piston is not actually moving in the bore;

3) putting the ports at as large a radius from the pivot allows them to be as big as possible for free steam flow;

4) minimising the distance between the crankshaft and pivot bearing maximises the angle of swing, again allowing large ports;
assuming the engine is double-acting, this can involve clever design like hiding part of the gland in a recess in the rod side of the piston.

5) A trunk guide for the crosshead takes the lateral thrust off the gland, piston rod, and piston (a very bad thing for them anyway) allowing the bearing area
of the gland to be minimised, helping with (4).

6) In small engines the gas flow velocities and dimensions are small (and therefore so is the Reynolds number) meaning that none of this matters much unless
high speed and power output are required: mostly they will breathe just fine.

 

[Anatol]

"1) the clearance between piston and cover can be minimal

2) the piston should not completely block the port,

3) putting the ports at as large a radius from the pivot allows them to be as big as possible for free steam flow;"

these points seem mutually antagonistic. Presumably, a compromise is best - ports are as large as possible and as close to cover as possible, and cylinder only covers half the port at TDC?

 

[TimTaylor]

Chuck,

I got a copy of the de Vignier book from a friend that is broken into three pdf files - only issue is that the page orientation is rotated 90 deg so I wound up just printing it out - the one you found is much easier to read.

I checked and the pdf copy I have does have the missing information from pages 82 through 94. Since it is now in the public domain (original copyright 1907 and books copyrighted in the US before 1923 are expired and legal to copy), there should be no problem letting you have it.

Tim

 

[TimTaylor]

Here's a link to another good reference. While it's about full size steam engines and not model engines per se, the same principles apply.....

http://books.google.com/books?id=ms...r&dq=Steam+engine+principles+and+practice&lr=

Tim