The Steamboating Forum

Hello all,
I am looking for some information regarding the design of compound steam engines. More specifically, what the proportion between the high pressure and low pressure cylinders should be in terms of working volume. I couldn't find anything so far, but I assume this topic should have been a matter of thorough study during the steam age.
I would appreciate if you could explain if there's a rule, point me to some good sources or even to some data for the cylinder sizes of compound steam engines.
Calin

Ah, that it were that simple. But it's complicated and many of the variables are just not known very well. Rules that apply to large engines with great volume to surface area ratios may not work well with our little beasts. Condensation can be rather severe and there are lots of situations where compounding is hardly with the trouble.

There are any number of people on here who know better than I about what works. And I hope they will chime in. Left to myself I would look over the numbers on existing launch engines and be concerned if your design gets out the that range.

I agree with Lopez Mike's comments.
Compounding ratios probably change with scale, so it's not a "simple" issue. (pun intended.)

My launch engine is a Cliff Blackstaffe steeple compound design with 1.5" and 2.5" bores.
I suspect even those compound bore ratios might be different from those of an inline compound design, if carefully engineered.
Although I think Cliff also made successful inline compounds with the same bore diameters as my steeple engine.
The steeple compound designs have a thermal advantage over inline compounds, because there is less metal to absorb heat from the steam.

The largest compound engine I have extensively operated is 110-ton 2-6-6-2 railroad locomotive with cylinder bores of 18" and 28".
That's many more stories...

Interesting stuff.
RussN

Hello, thanks for the replies. Please don't just mention the cylinder bores for HP and LP, but also the stroke.

Specification correction:
Blackstaffe steeple compound marine engine Bore/stroke:
1.5" bore x 3" bore x 2.5" stroke.
RussN

As noted above, there are many other parameters to consider. Firstly is the steam pressure used? second is if you are condensing or exhausting to atmosphere? also important factors are the point of cut off of the cylinders particularly the H.P. Marine engines were usually designed to allow approximate equal powers on each cylinder. As a rule of thumb a condensing engine working at 120 P.S.I. or above would have a cylinder volume ratio of 4:1 although many have around 3:1. locomotives were of course non condensing and so use lower ratios.
Good reading for this is:- "steam-engine theory and practice" by William Ripper and "Marine engineering pocket book" by Seaton & Rounthwaite.
Regards
Jack

Thank you. What I have in mind is a small model engine. It would be non-condensing. I have here 2 cylinders which I could use, their volume ratio being 1.96, so roughly 1:2. The stroke is 11 mm and the bores are 8 mm and 11 mm. What I'd like to know if using these two cylinders at a pressure of say 3 bar (45 psi) would result in more or less equal power for both cylinders.

Given the very small size of the cylinders and the inescapable laws of thermodynamics I think that you are building a condenser with a lot of parts.

Put in other terms, mice need to eat a lot for their size compared to elephants. The volume, which generates the heat goes up as the cube of the animals size but the surface area only goes up as the square. Thus your very small engine will, inescapably, be running at such an incredibly low thermal efficiency that I'm finding it hard to imagine how one would measure the relative loads on the two cylinders.

I think building it with the parts you have and not stressing very much about theoretical considerations will be enough of a feat of model making.

I'm looking forward to progress reports!

A most popular model compound engine bt Stuart "Compound Launch Engine" HP=3/4 inch (19mm) LP=1-1/4 inch (31.75mm) diameters x 7/8 inch (22.25mm) stroke, gives a volume ratio of 2.78

A Stuart 6A, large enough to power a real launch: Stuart "Compound 6A Engine" HP=2-1/2 inch (63.5mm) LP=4 inch (101.6mm) diameters x 3 inch (76.2mm) stroke, gives a volume ratio of 2.56

Long existing Stuart dimensions

I've built two of those Stuart Compound Launch Engines. Not too surprisingly, I find that it's a lot harder to make a small engine of that size run smoothly than a much larger one. I've learned to cheat by making up a flywheel out of lead and such dodges.

It takes a lot of careful work fitting up to get such a small engine to run smoothy at low speeds. If you scale up the operating speeds as you scale down the size you find yourself operating at thousands of r.p.m.

However correct this might be, I like to see the motion without the aid of a stroboscope. I find buzzards more educational to watch than hummingbirds.