# Thread: Valve timing on model Steam/air engines

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## Valve timing on model Steam/air engines

Although I have been playing about with model steam/air engines for 2 years now, I still do not have a clearly defined understanding of valve design for a double acting air engine. If we leave "lag" and "lead" out of the equation, I want to know the following information. For a "given" stroke and piston length, how does one go about calculating A--The "throw" on the valve eccentric, B--the length of the "big ends" on the spool valve, the distance between the inlet and outlet ports, C--the distance between the "big ends" on the spool valve. I am sure that this information can all be reduced to a set of formulae that will cover any double acting air engine, but I haven't been able to find it, and I don't seem to be able to figure it out by myself. This all shows up in the attached .gif which was so kindly provided by Maryak. If anyone can help withthis, it would be an immense help to myself and to all other model engineers.---Brian

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Brian, being such a highly experienced engine builder that I am now on my second engine! I have concluded that there is no fixed relationship between the dimensions of the valve cylinder and the main cylinder.

The length of throw on the eccentric depends on how long you make the valve cylinder and the length of the valve cylinder depends on how you can conveniently arrange the flow of air/steam from there to the main cylinder.

I think one would designe for bigger port openings and with that would go longer valve spool travel if you wanted good performance on low pressure/high volume air.

Just my thoughts.

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The stroke of the valve is determined by the dimensions of the steam ports. It must be long enough to shift the valve from one admission port to the other.

This is a cylinder from an engine I am currently building. The two positions are shown by the magic marker lines, the stroke is the difference between either of the two pairs. In this case it is approximately 3/16"+-.

Last edited by JCHannum; 12-06-2009 at 08:55 PM.

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## Theres a lot to learn, and so little time.

Brian, The subject of valve timing is possibly the most discussed, least understood and most forgiving part of the story of the steam engine. From the very earliest " Rocking beam " pumping engines which ran so slowly the valves were opened and closed manually to the high speed marine triple expansion engines made just before WW1 and on to the poppet valved railway factory engines and steam trucks there has been much investigation of, and little agreement about " What is best". For models just to run for demonstration, especially mainly on air, small valves, small passages, long cutoffs and little advance will give smooth running, easy starting from most crank positions( Assuming a single cylinder only) and, if everything fits as it should, a minimum consumption of air or steam. If you really want them to GO, and believe me mine usually do, and if they dont well they get modified,You need big valves, big passages, big port openinge, small clearance volumes arranged in conjunction with valve timing that is well advanced both for inlet and free exhaust. Almost anything can be made to " Run" but after you have built a few you should be able to feel if you have a good one once you begin making the valves, sleeves and passages. Scale effect often jumps into the picture, especially with smaller models, and models can waste a lot of steam if theres too much dead space in oversize passages. Incidentally one of the most successful users and improvers of his own Model Railway engines in recent years was the fellow who designed Lotus racing car engines( E.C. Martin) regards David Powell.

5. dp
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There was a thread on this BBs a couple years ago where a model locomotive was being built and the topic was the timing leverage that controlled the valves. Because the timing for best performance is different at different speeds, the engineer fine tunes the valve timing as appropriate. What I can't recall is what the name of that controller is, but it was a fascinating bit of engineering.

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## valve gears.

There were two basic types of valve gear for railway engines. Some provided varying lead( read advance for the gas engine fellows) of the valve timing deliberately. So when the engine was running slowly and probably pulling hard steam was admitted at or slightly after dead centre and let in for most of the stroke then exhausted at the end of the stroke, once up to speed the valve gear was adjusted ( Notched up) , the valves opened earlier so the pressure could build up on the piston a little before dead centre, then the steam inlet was cut off at whatever desired % of the stroke and the steam expanded for the rest of the stroke, the exhaust was also opened early so the exhaust was free and the used expanded steam released. The other type of valve gears provided virtually constant valve timing, as the designer decided but still allowed the actual point where the steam stopped being put into the cylinder to be adjusted as the driver needed, and so could get economy from expansion of the steam.In later years the latter was more common, Baker and Walschearts valve gears were the most often seen.regards David Powell.

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"Steam Engine Design" explains all.

8. x39
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Some years back one of the members of this board referred me to this site:
http://www.tcsn.net/charlied it might be of help to you.

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I have the textbook my father used for a course (ME 1695) in steam engines which he took at the University of Washington in 1935. It is now scanned and available here, on Google Books: http://books.google.com/books?id=KYJ...age&q=&f=false. The chapter on "the slide valve" (pp 60-89) has some formulas which you might find useful.

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Brian,

From 1917, a public domain book, "Modern Locomotives Valves and Valve Gears", by Chas. L. McShane, 1917, might help you.

4.66 MB PDF DL. I'd send it to you but don't know if you can handle a large file with your Provider.

I enjoyed your project in the magazine. Easier than viewing on line and printing to take to the basement shop.

Cheers,

George

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