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The Artful Bodger
Regular Member


64 Posts
Posted -  01/07/2009  :  22:52
Hi, this is my first post to this forum which I found while searching for information relating to the timing of steam engine valving.

After about 45 years in the aviation industry (ground communications etc)  I have bought myself a lathe for evening relaxation and of course the first project has to be a simple engine, right?

I am trying to make one that is at least partially my own design and you can read about my progress at http://madmodder.net/index.php?topic=1389.0

I am sorry if the design is a bit obscure but the general idea is that the sleeve and piston rotate slightly allowing me to choose inlet and exhaust port timing by drilling in the appropriate places.

My assumption is that for expansion working there should be a short duration inlet opening following TDC and the exhaust should be open for almost all of the return stroke.  Is that correct?

This little engine will only ever get to run on compressed air, does that change things at all?

P.S.  I recognise there will probably be a problem with sealing around the port openings but that is someing I can address if it proves to be an issue.

John


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 02/07/2009 : 08:19
Hee Hee! My God John, you’ve opened up a can of worms with this one!  I wish you all the luck in the world in your search for a more efficient valve gear. It’s people like you who make the discoveries that alter the world. However, and here comes the bucket of cold water.... You’re doing the steam engine equivalent of re-inventing the wheel. What the top engineers found was that in valve gear design you reach a point where the return you get for the improvement doesn’t justify the investment and the more complicated the gear, the less reliable it was. This is why the vast majority of stationary engines in the latter days of the industry were transverse cylindrical valves controlled by various versions of the Corliss gear which had one vital characteristic, it enabled steam valves to be shut instantaneously at any desired position on the expansion stroke and thus made governing and control of steam use more accurate. Many continental makers went down a different route using drop valves with a very short working stroke and multiple seats to allow them to pass large amounts of steam in a very short time.  Locomotive practice was different because of the greater demand for steam and coal economy. Like stationary engines they started off with the simple slide valve. One of the advantages the stationary engine builders had was size so they could spend some time on making different versions of slide valves with complications like secondary sliding ports controlled by the governor to give some control of expansion. Look up Meyer gear and equivalents. These all gave way to the cylindrical transverse valve which was in effect a circular slide valve controlled by Corliss gear. The loco men took a different route and went on to another version of the cylindrical slide valve but one which depended for its action not on rotation but on longitudinal movement of the valve bobbin in the bore. What we now call piston valves. These became standard and all the effort went into different configurations of the gear which controlled them. Walschaert gear was the most common but there were many variations, Joy, Baker, Stephenson etc. Look them up. The only major departure from this was in the last days of steam haulage when BR tried out the Caprotti rotary valve gear. (At the same time they tried a turbine as well). The Caprotti gear was good but complicated and expensive and even if steam had survived it is doubtful whether it would have succeeded. As it was diesel took over and all the valve gears went. Running on compressed air instead of steam gains no appreciable advantage from expansive working because it lacks the thermo-dynamical element of high temperature steam. The engine will run but totally different than HP steam and in practical terms it’s a dead end. As for the timing of the exhaust.... A huge basket of worms and one in which the theory may look sensible but in practice is useless. The theory is that the energy from the expanding steam is maximised by having the least back pressure on the piston. In other words, ideally the exhaust should open as soon as the steam valve admits steam on the other side of the piston. However.... No practical engine was built like this because of various factors. Exhaust valves are never fitted with Corliss trips and their speed of action is governed by the eccentric, they open at the same speed as the steam valves but close more slowly. If you think about it this means that you have to make sure the exhaust valve is fully closed before you admit steam on the same side of the piston for obvious reasons. So, for this reason alone, exhaust valves always have lead to make sure they are shut before steam enters the cylinder, There is another and more important matter. It was soon realised in practice that the engines ran quieter and more efficiently if the exhaust valve was closed soon enough to create compression in the exhaust side to cushion the end of the stroke.  The amount varied with the speed, size and load on the engine and getting this setting right was one of the chief skills in valve setting. You said ‘a short duration inlet opening following TDC’ for the steam admission. In practice you’ll find that the engine needs steam admission before TDC in order to ensure that full admission is available for the return stroke and also to increase the cushioning at the end of the stroke caused by the closed exhaust valve. Think back to your valve-setting days on IC petrol engines, always set the spark to fire before TDC. Exactly the same principle. (In fact I once had the problem of timing a Ferrari Dino engine with no instruction book and in the end set it like a steam engine. Everyone laughed but it fired first time and ran perfectly. When an expert with all the technical gear checked it afterwards he said it was as near perfect as it could be and only needed a fractional adjustment) So, arrange your admission point 5% or 10% before TDC and your exhaust closing to about 90% of the stroke and you won’t be far out. You can always tweak afterwards once you have it running. Beware because what will be OK on compressed air won’t do for steam. Sorry if all that sounds a bit negative John, it’s not meant to be. Newton and I used to have long discussions about how we would design a steam engine for modern practice and even though we hated to admit it we never came up with anything better than the Bellis and Morcom design for a high speed enclosed engine running compound with a central piston valve. Don’t let this put you off, you must follow you’re own star and who knows, you might come up with something better than us Old Farts. One thing is certain you’ll have a lot of entertainment trying and in the end you’ll be a better man. One final story that I have often quoted, I think it was Churchward at his retirement dinner. he said that it was a pity he was having to retire because he thought he had just reached the point where he fully understood the simple slide valve. It was a sort of joke but if you really look at it he was quite right, ‘simple’ is completely the wrong word, the further reaches of slide valve theory are fiendishly complicated and I for one will admit that I have never got my head round it. I can make them work but don’t fully understand them. Sorry about the length of that John but as I said, it’s a biggy!

   


Stanley Challenger Graham




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The Artful Bodger
Regular Member


64 Posts
Posted - 02/07/2009 : 10:38
Wow Stanley!  I think you are giving me far too much credit for the idea!

It was not intended as an improvement in the science of steam but rather a simple engine that a first time engine maker might be able to manage.  I think a lot of people in my position first build a little oscillator engine and full credit to those that achieve thier aim.  But for me I wanted something a little different without having to devise slide valves of any kind.

My little engine will have only three moving pieces, the crank, the connecting rod and the piston with its attached sleeve.


Thanks for the comments on timing.  As you rightly pointed out internal combustion engines usually open the inlet before TDC and I think all have valve overlap where inlet and exhaust are open at the same time.  I will probably end up with a little overlap as the port openings cannot open or close instantly.

My design having a sleeve which although quite thin still displaces air/steam   would have more or less TDC cushioning depending on how close it fits the 'displacer'.

The idea of rotating the piston and sleeve came from recalling that some sleeve valve engines used a single sleeve that added the extra dimension to sleeve valve operation by rotating the sleeve. If I recall correctly it was a Scottish engineer who devised that.   I dont know how he rotated the sleeve but I hope to be able to by canting the plane of the big end bearing such that the 'swash plate' action will twist the connecting rod and hence the piston.

The idea of having the sleeve as an extension of the piston came from a WWII Junkers two stroke diesel aero engine I saw cut away in a museum in Poland.

I dont expect my little project to alter the future of mankind any but I personally will be very pleased if I can learn enough about working in metal to at least have it run.

I remade the piston sleeve and the cylinder this evening as the first attempts did not provide confidence I had the fit good enough.  Good practice and the old water pipe did not cost anything!


Thanks for the informative post Stanley which I can confidently claim increased my knowledge of steam engines several fold!

John 


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 02/07/2009 : 13:17
It's a pleasure John. Don't be worried about slide valves, dead simple. Have you had a look at Shed Culture? I built a few engines last year and posted the process right through... You might enjoy losing yourself in there! Go to forum search in the menu at the top of the page, alter search to subject heading, pop 'shed' into the search box and it'll throw the link up for you.


Stanley Challenger Graham




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stanley at barnoldswick.freeserve.co.uk Go to Top of Page
The Artful Bodger
Regular Member


64 Posts
Posted - 02/07/2009 : 20:18
Slide valves, double action, multi cylinder, link motions, they are all in the future Stanley!  Right now I found it hard enough to make a cyclinder and sleeve that fitte together!

Cheers


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 03/07/2009 : 07:42
Ahhh John, it all depends on what you call a fit. My attitude is that I know I am not the best turner in the world, I just do the best I can without going overboard. The consolation is that this usually works! Don't be too critical about yourself, practice maketh perfect or so they say....


Stanley Challenger Graham




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stanley at barnoldswick.freeserve.co.uk Go to Top of Page
The Artful Bodger
Regular Member


64 Posts
Posted - 03/07/2009 : 09:52
I have probably already committed some cardinal sins on this very first project.  I have the cylinder, the cylinder head and integral displacer, the piston and its attached sleeve all sitting nice and shiney on my bench and I am yet to actually measure anything....whatchutalkingabout


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 03/07/2009 : 10:51
If you dig out LBSC's articles in the Model engineer he was a great advocate of 'rack of the eye' and I still use the method especially if I have to get something dead accurate like a chuck back. Calipers and frequent trial fits. Works every time!


Stanley Challenger Graham




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stanley at barnoldswick.freeserve.co.uk Go to Top of Page
Tizer
VIP Member


5150 Posts
Posted - 03/07/2009 : 11:12
John, we can all see how you got Stanley very excited with your initial post - the level of excitement is inversely proportional to his use of paragraph breaks in his writing!   Wink


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The Artful Bodger
Regular Member


64 Posts
Posted - 03/07/2009 : 21:48
Could be I am getting a bit over excited too Tizer, looking at all my typos in my earlier post!


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 04/07/2009 : 06:34
Everyone's a critic! I wrote the piece in Msoft Word and then formatted it before copying and posting. It gets rid of the double breaks between the paras but this time I was surprised to see it had demolished the pars formatting. Stop nagging and concentrate on the pearls of wisdom!


Stanley Challenger Graham




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stanley at barnoldswick.freeserve.co.uk Go to Top of Page
The Artful Bodger
Regular Member


64 Posts
Posted - 15/08/2009 : 10:34
The little engine is complete and it runs  (on compressed air!)

Nutating engine!

You can read a few posts relating to the building of it at
http://madmodder.net/index.php?topic=1389.0

This is my first real attempt at metal turning and my first engine build of any kind.


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 15/08/2009 : 17:02
Splendid! Looks like a nice finish as well. Did you chop the crank out of solid or cheat and build it up?


Stanley Challenger Graham




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stanley at barnoldswick.freeserve.co.uk Go to Top of Page
The Artful Bodger
Regular Member


64 Posts
Posted - 16/08/2009 : 05:56
I cheated wherever I could Stanley including the crank!lolol


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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 16/08/2009 : 06:56
No shame in that John. Newton once told me about his dad making a crankshaft for a double engine and he turned the crank journals on the same centre. When it was done he heated the middle of the shaft, put it in the vice and twisted it 90degrees before straightening it. When the bloke watching him expressed surprise Johnny said it would be OK, it wasn't driving a thousand looms!


Stanley Challenger Graham




Barlick View
stanley at barnoldswick.freeserve.co.uk Go to Top of Page


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