Compound engine cylinder proportion

[kno3]

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

Thank you. Interesting that the ratios are quite different. Any idea why?

I have also found the Stuart triple expansion model engine data:

HP----------MP----------LP
19.05------31.75-------44.45 bore mm
25.40------25.40-------25.40 stroke mm

Wich gives the following ratios:

HP/MP of 2.78 (just as the model "Compound Launch")

MP/LP of 1.96

[Lopez Mike]

I suppose the most thoughtful answer I can come up with is that it really does't matter that much. Very few small engines (less than maybe 20 h.p.) are engineered very much. No corporation is going the suffer from the adverse effect of low efficiency in one of them.

Big marine engines were carefully adjusted for minimum fuel consumption because it really mattered. Railroad engines got away with murder in this area as issues of power and speed were dominant. When you can only make the damned thing so high and so wide and it won't go around corners if it's too long, then you force the issue which does not lead to very high efficiency. A decent marine engineer would fall down in a fit on his first view of a high performance locomotive. They had a maintenance to operating time ratio about equal to a jet fighter. Labor was cheap. Timetables usually were more important than fuel consumption.

I think you will doing very well to get your small engine to run smoothly. The friction and thermal losses will very high. And I think it might be hard to measure the final effect of design changes over such a narrow range.

Almost certainly more than you wanted to hear! Sorry.

[kno3]

I know you can get away with many things in model size. But even if it doesn't matter much in the end, I'd like to know, why I'm doing what I'm doing
So, all insights much appreciated!

[TahoeSteam]

Here are the dimensions of some compounds I'm intimately familiar with:

Grosejean compound 2" & 4" x 3.25"
William Charles Grosejean was a mechanical engineer and designed his engine with maximum efficiency in mind, keeping within the constraints of Stephenson link valve gear, and 150psi steam (500° superheat). Piston valve HP and balanced slide valve LP. Built in the 1970's

Scripps Compound 3.5" & 5" x 4"
Bob Scripps just loved steam and had the bankroll to make a bunch of patterns and different engines available. Nothing special, just brute force and generous castings. Slide or piston valves on HP, slide on LP. Built in the 1970's

(Small) Doty Compound 4.5" & 10" x 6"
Built 1909 by a large Canadian manufacturer, this engine went into a small logging tug near Sudbury Ontario. Slide valves HP and LP.

Claparede Compound 6" & 10" x 7"
Not much known about this engine or manufacturer. Built 1924 in Argentuil, France. It was "liberated" from Southeast Asia by a GI in the '60's. Slide valves HP and LP.

Here are a couple links to a number of other new and antique engines giving their dimensions as well:

https://prestonservices.co.uk/category/ ... e-engines/

http://steamlaunch.com/engines/index.html

[kno3]

Thank you Wesley, now I have some more to study and compare!

[fredrosse]

I agree with Mike's statements about the lack of efficiency for very small model steam engines, perhaps with the exception of the very high speed hydroplane steamers that turn in the vicinity of 10,000 RPM, and have driven racing boats at over 100 MPH!. Also the difficulty of getting them to run reasonably well unless fairly high precision in valve events is obtained. However a model compound can be fun to build and run, as illustrated by the Stuart offering of a small model compound for several decades.

As to efficiency, larger steam engines of the 19th century consumed about 30 pounds of steam per brake horsepower-hour, (One boiler horsepower =33,475 BTU per Hour). This was defined by the ASME in 1876, and is still the standard by which commercial boilers are rated in the USA. Those numbers show a plant cycle efficiency of 7.6%, reduced to about 5% if the boiler efficiency is considered.

Small model steam engines typically consume in the vicinity of 10X the steam flow required for the larger 19th century engines, but I still enjoy my coaster steamer model, at 56 inches length and 66 pounds displacement cruising on the lake.

[fredrosse]

From Wesley:
"Grosejean compound 2" & 4" x 3.25"
William Charles Grosejean was a mechanical engineer and designed his engine with maximum efficiency in mind, keeping within the constraints of Stephenson link valve gear, and 150psi steam (500° superheat). Piston valve HP and balanced slide valve LP. Built in the 1970's"

Is that "500° superheat" or 500° total steam temperature? Superheat is defined as the temperature rise above the steam saturation temperature. In this case would be 358F + 500F = 858F, which is indeed a very high temperature for a reciprocating steam engine, although it has been done.

My domestic heat-power uniflow is designed for 500F total temperature, with 150 PSIG steam pressure.

[fredrosse]

From Mike:
"Very few small engines (less than maybe 20 h.p.) are engineered very much."

You are probably very correct here, as the great majority of steam engines built today are "designed" by armchair engineers who are just copying general paths that were established more than a hundred years ago, when fuel cost as minimal, and IC engines were in their infancy. As a matter of fact, my slow turning walking beam steamer engine follows design of almost 200 years ago, and while its only rock and roll, I like it.

There have been small modern reciprocating engine designs developed that show extraordinary efficiency, as well as practical long life in continuous service. The best example I have run across is the Whitecliffs Solar Project, which is a uniflow engine running at 1500 RPM on 600 PSI steam. There is much information about this engine on the Web.

The steam car projects of the 1970s also developed modern steam engines, at the expense of USA taxpayers who financed millions of dollars for the auto manufacturers (and LEAR, and ...Bessler) to develop these power plants. The design results were published, but my copies have disapeared, and I cannot find how to get new copies of this interesting work.