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Steam engine timing

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Wolfie24/06/2012 08:53:38
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OK well I'm almost at the point where I need to set the timing of my first engine.


So where does the valve need to be at what point if that makes sense. When the piston is at one end, should the valve be also at one end of its movement or already starting to move?

Martin W24/06/2012 09:25:21
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Wolfie

A quick start is to set the valve gear at 90 deg to piston i.e. when the piston is at one end of the cylinder, top or bottom dead centre, the valve gear is mid travel. Depending on the which is used will determine the direction of rotation for your engine. Fine tuning of the valve timing will then be required to get the best out of the engine, others on this site are far better qualified than me to advise you on this.

The quick set above should get you going.

Cheers

Martin

JasonB24/06/2012 10:15:15
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Stuarts used to have a good download for setting the timing of their engines but is gone from their site, hopefully Bridport will put it back up.

Martins 90deg is a good start but you will not get the smoothest of running engines, as a rough guide the valve should be 90degrees ahead of the crank plus another 7-10deg. Basically you are letting a bit of steam into the cylinder before the piston reaches the end of its stroke which has a cussioning effect.

So if its your S50, looking from the flywheel side have the crank back towards the cylinder in the 9 o'clock position and the thickest part of the eccentric at 12.30, with this setup the flywheel will rotate clockwise as viewed.

J

Stub Mandrel24/06/2012 20:40:49
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Hi Wolfie,

90 degrees is rarely ideal not just beacuse you want some 'advance' or 'lead', but also because the valve may be designed with 'lap' - this means that it's not at 90-degrees that the valve starts to open and shut, i.e. the valve opens for less than half a rotation each way. This means you may need to rotate the eccentric further forwards by much more than 10 degrees to get some lead.

Why have lap? So that the valve 'cuts off' and only lets steam in for the first part of the stroke, and after the valve closes it works expansively. This makes the engine slightly less powerful, but much more efficient. the idea was worked out by Jonathan Hornblower, and Watt and his followers worked hard for many years trying to bury the fact that Watt didn't think of it first.

For example, a vlave might only let in steam over 150 degrees of its rotation at each end, so to let steam in at TDC the eccentric needs to be set 15 degrees ahead of the crank - (180-150)/2. But you need some lead, say 10 degrees for the reasons Jason explains, so the net advance of the eccentric might be 15+10=25 degrees (Pay attention at the back!)

That means the valve cuts off at 25+15=40 degrees before bottom dead centre, allowing teh steam to expand and make full use of its energy.

Compressed air doesn't do this as it has little energy to give up in this way, which is why steam engines run better on air.

I noted that Morgen's compound engine uses eccentrics at 90-degrees - and also that he runs his enginbes in air, so this makes sense. However a compound on low pressure air is possibly pumping the air through the LP cylinder rather than extracting energy from it and may run better on the HP cylinder only

Hmm,. Sorry if that goes on a bit, I've probably made an error in there!

Neil

Due ackowledgement to Tom Walshaw for cultivating my shaky understanding of this topic.

Jeff Dayman24/06/2012 21:06:41
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DC1 - There was no timing diagram or instructions at your link, only running and lubrication instructions.

JD

David Clark 124/06/2012 21:31:30
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Hi There

Must have removed it then.

That is where I downloaded it a couple of months ago.

I only downloaded it for a reader, I must have deleted it afterwards.

It might be on a back up somewhere.

regards David

martin ranson25/06/2012 09:21:21
4 forum posts

wolfie

not sure from your question what sort of engine you have .. the answer from Jason B reads as a stuart S 50 .. if so then there are 2 things to set up .. FIRST make the valve travel equal at each end .. this can be done with the valve eccentric set anywhere in relation to the crank throw .. set it by adjusting the linkage so that the slide valve uncovers each end of the ports by an equal amount as the crankshaft is rotated by hand .. when this is done the position of the eccentric can then be set in relation to the crank .. turn the engine so it is at top or bottom centre .. slacken the grub screw on the eccentric and position the valve so it is just cracking open the port .. the eccentric wil be in advance of the crank by 100 degrees or so ( right angle plus a bit ) .. gently lock the eccentric in this position .. turn the engine by hand .. watch the slide valve move .. if it looks good at each end, put it all back together .. tighten the eccentric and try it on steam .. my engines run very sweetly like this .. hope I have got the right type of engine .. hope this is useful

martin

Terryd25/06/2012 10:20:02
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Hi Wolfie,

I don't know if this is of any help but these are the valve setting instructions for the Stuiart Beam engine from the instructions by Andrew Smith, They appear to be pretty generic so may give you a start for initial setting, Quote:

"...............Assembly complete, but leave the steamchest cover off until we set the valve, To do this, turn over the flywheel in the direction in which it is intended that the engine should run and watch the valve slide up and down across the port face of the cylinder. Adjust the valve by taking out the spindle (31) and screwing the valve rod up or down, until the valve opens the steam ports at top and bottom of the cylinder by, as near as possible, the same amount.

When this is done, undo the grubscrew locking the eccentric sheave to the crankshaft. Now turn over the crankshaft in the direction of rotation until the piston is exactly at the bottom dead centre, i.e. at the bottom of its stroke; now, with the crankshaft stationary, turn the eccentric sheave, still in the direction of rotation, until the valve reaches the bottom of its stroke and begins to rise again. Watch it carefully and as soon as it starts to uncover the lower port, stop turning the sheave and lock its grub screw.

Now revolve the flywheel and see what the conditions are like at the top of the piston travel. The valve should have risen to its highest position and started to descend uncovering the top port by the same amount as the bottom one when the piston had reached top dead centre. If it is not so. slacken the eccentric sheave grub screw and slightly rotate the sheave to split the difference between the top and bottom ports."

Hope that is of some help,

Regards

Terry

Joseph Ramon25/06/2012 13:21:25
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107 forum posts

This might be helpful:

**LINK**

Joey

Joseph Ramon25/06/2012 13:25:04
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107 forum posts

And if you really want to get into it, check out this:

**LINK**

I agree the the point made earlier that 90 degrees is not a good setting for eccentrics, and most need a lot more than 10 degrees or so of advance.

Joey

Wolfie25/06/2012 19:34:03
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Brilliant thanks chaps, and to the man who sent me the Stuart Turner info. Going to have a go at that this weekend.

One more question. At least to begin with this engine is likely to be run on compressed air, does that change anything?

Stub Mandrel26/06/2012 22:01:13
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It will run sweeter and with more power on steam, but you will suddenly discover it has leaks all over the place that never showed with air

Steam is also more fun, especially when hot, oily gunge shoots out of the exhaust all over the workshop and you.

Neil

David Paterson 427/06/2012 04:58:32
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Posted by Stub Mandrel on 26/06/2012 22:01:13:

It will run sweeter and with more power on steam, but you will suddenly discover it has leaks all over the place that never showed with air

Steam is also more fun, especially when hot, oily gunge shoots out of the exhaust all over the workshop and you.

Neil

I have been having my first serious go with steam aver the last week or so. I thought the spray from exhaust was just me. (built the engine a bit over two years ago and have been fiddling with the boiler forever! **LINK**

I did take a short video of the run, but have been experimenting with the firing system. Started with 3 wicks, but last night think i got a first 'successful' effort going with vapour.

Now that I have a burner, will sort out a fuel tank so everything doesnt stop after 15 minutes. prototype tank is a small boot polish tin with a spigot silver soldered in. If that works (was very cheap) I will graduate to a bird bath.

Dave

David Paterson 427/06/2012 05:02:41
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What I really meant to say (so as not to hijack this thread) was that looks like I now need to look at the timing for the engine if I intend to run on Steam regularly.

Stub Mandrel29/06/2012 21:56:50
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I've looked up some simple diagrams I put together for four steam engine models.

Below I give the name of the engine, the advance required to allow for lap, and the advance required to have 'tram ticket' or 1/64" lead - all calculated from the drawings.

Stuart 10 (Stuart Turner) - 100degrees, 110 degrees

Trojan (Westbury) - 103 degrees, 116 degrees

Lady Stephanie (Tubal Cain/Tom Walshaw) - 106.5 degrees, 120-125 degrees

Norden (Me) - 99.5 degrees, ~110 degrees

As you can see, in all cases a lot more than 90 degree advance is needed.

The number 10 and Trojan have exhaust ports open for 180 degrees of the cycle, so the open well ahead (20-25 degrees) of bottom dead centre.

I designed Norden with noticeable exhaust lap, so the exhaust opens only about 10 degrees before BDC with 110 degrees advance.

Lady Stephanie has a small amount of exhaust lap, also the as-designed drilled round ports mean the open gradually, so this engine might benefit from even more advance! On the other hand, this is an engine expected to run quite slowly so perhaps the advance could be kept to more modest amount.

I've just dug out the 'Trojan' drawings- this is a slip-eccentric engine and the advance is specified as 1/32" i.e. the face of teh cam is 1/32" past its centre line, allowing for the size of the crank pin this (measuring from ETWs drawing) gives eccentric advance of about 125 degrees (opening 35 degrees before TDC).Tthis will see the exhaust opening about 45 degrees before BDC - of teh same ordwer as would be seen in a uniflow engine! Note that Trojan is a small, high-speed engine designed as a practical power plant for small boats - mine whizzes around on air or low pressure steam.

All in all for an apparently simple mechanism, there is a lot of subtlety, and a lot can be tweaked to match an engine to its working fluid, its duty and its speed.

I'd be interested in folk's comments on these observations.

Neil

Byron Nelson15/07/2022 21:02:38
1 forum posts

I’ve read through all the osts here . Of all the good information I’ve not been able to fit the piece to my situation .

so I’ll give a go here. My little engine is actually two identical engines coupled together . They are double acting piston valve porting . There is the issue. Reading through the post I see note of grub screws. Set screws here in the USA I had a very bad experience in my early engineering career with set screws grub screws . From that point on I really find them disagreeable at best . My little steamer has a number of them and I had a bad experience right atvthe start that resulted in frustration with scared up crankshaft. I had to make a miniature puller to get the flywheel off. What happened was the pistons are attached to the connecting rod by screwing them on then using a hex nut as lock nut. Two cylinders managed to drop the hex nuts then eventually the engine locked up causing everything with grub screws to slip . I had a royal mess. I know many use these without issues but I run my engines a lot. Not fast but often they are shelf modeled. Fortunately the cylinders serviced and I was able to dress off the cylinder heads .

so here is the question the instruction s show adjusting the crankshaft to some position only in one picture. Then to set the piston valve you are supposed to position the eccentric as shown. I can’t post the usual saying. So I started looking for alternat ways to secure the flywheels and eccentrics . I came up with modified shaft collars as well as similar couplers a gentleman did some machine work for me by mailing parts and sketches . Excellent work too. Then I discovered the sample I sent him had 3 mm socket head screws in the cap while the new parts came with 4 mm screws. Now the screw threads interfered as well as the bottom of the counter bore for the screws broke into the relief bore I managed to rework enough screws to clear things but a mess I only have a drill press in my home hobby shop and it’s no a milking machine or lathe I managed to slice sever collars apart ans install heli coils but it’s really make shift at worst. The crank shafts are only 6mm so not real big. The collars take up a lot of shaft length that is un necessary. Anyway I’m just getting things back together and I’m back to the original problem of timing I can find and mark TDC for the piston the steam chest piston port block has an opening in the end so it’s possible to measure the position ofvthe piston valve pretty accurately relative to the power piston I’ll be running on compressed air actually I was only running abou 20 psi when the crash occurred less than 1000 rpm I can get 135 psi if needed but I hope not . Sonibghinknibunder stand how the steam needs to be let in a little early to allow it to expand an do it’s work . Also that steam timing could be radically different than just air timing . My boiler project is on hold for the time being untill I have reliable operation to start with . My plan is some value based on in the instruction picture relative to TDC THEN ADJUSTVTHE ECCENTRIC TONGIVE A LITTLE LEAD TO THE there will be two eccentrics per pair of cylinders pthe ower PISTON. ILL BE ABLE TO MEASUREVTHIS EASILY WITH MY “very near” calipers

the engine crankshafts are made so that once the piston valve is set up the engine will run I have a reverse valve not installed yet but I think this just reversed the intake and exhaust ports according to its instructions . I have air line oiling as well as syringe injection as needed air tool oil works well and 10 W motor oil works to although it’s much stiffer Any way I’d like comments on my timing method . There are sbout 30 YouTube videos but but none address exact measurement like this. I call it “ about measurement “

if I could adjust micro amounts or even greater but measurable it would be fine. I’ll take some pictures of what I have shortly . I’m goingvto assemble the steam chest too there is another issue to look at here too but I’ll get to it later . I hope spell check doesn’t mess this up I just hate it when I come back and have to undo things

byron

Nigel Graham 216/07/2022 00:54:46
2257 forum posts
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It may help to step back a little to the basics. Much of the above is all good advice but sort of jumps straight in. Let's look at the machine itself, first.

- A conventional steam-engine with slide or piston valve is symmetrical in shape and operation (allowing for slight geometrical subleties).

- A slide valve and a piston valve work in the same way except that the slide valve is of outside admission (the live steam surrounds the outside of the valve), the piston-valve is inside admission (holds the live steam between its two pistons). So different port and valve-gear layout but identical action.

'

- Either type of valve is given an extension called Lap. This is the width of metal overlapping the port when the valve is in its own mid-travel (not the piston stroke). Normally the same on both ends of the valve.

That Lap is crucial, as it governs the Cut-off that allows "expansive" working.

Cut-off is the valve closing the port at a predetermined percentage of the piston-stroke, so stopping any more steam entering that end of the cylinder. The steam already there is trapped, doing its work by expanding, until as the piston arrives at the end of that power stroke the valve has now moved to open the port to exhaust.

- At which point the valve is also starting to open the opposite end port to live-steam, and that is the duty of the eccentric setting.

- Now, what if no lap? If the valve is sized to just bridge the port edges in mid-travel, so has no lap, it would be set to move 90º ahead of the crank... and would give no cut-off. The engine would run very jerkily, and waste a lot of steam. Or more likely not run at all. The lap allows that spell of closed port.

- To achieve that initial opening then, the eccentric is both given more travel and is set to 90º in advance of the crank; plus a further angle, called the Angle of Advance, to account for the lap making the valve physically longer. (Some engines also have a nicety called 'lead', not detailed here.) So if the eccentric leads the crank by 100º, the Angle of Advance is (100 - 90) = 10º. Which seems remarkably low, probably giving a very late cut-off.

The actual angle of advance is specific to the engine. It, the eccentric throw, and the port and valve dimensions are all interdependent, established when the engine is being designed.

.

So to setting the valves:

Assume all parts to correct dimensions, the valve and port face properly symmetrical; and designed without lead (normally found only on locomotives and big marine engines).

The centre of the valve-travel and centre of port face should co-incide; the valve travelling an equal distance each side of that. This is given by the valve's location along its rod (or 'spindle' even though it does not rotate), so set that first by measuring the width by which both ports open to admission at each end of travel, to verify they are equal. Easiest to use slips and feeler-gauges. Measuring from the ends of the valve-chest will work only if that too is fully concentric with the ports: not necessarily so. Note: measure port openings, not port widths.

Now the valve events, given by the eccentric's angular setting. Place the engine on either dead-centre: not easy to do accurately but there are various geometrical methods. The port should be only just opening to admission (of live steam), shown by a tiny dark line appearing. Test on the opposite dead-centre.

The effect should be the same. If not, adjust the eccentric forwards or backwards, test again... repeat as necessary.

(If the valve is designed to give lead opening, i.e. very slightly ahead of dead-centre and very small, measure that with a feeler-gauge.)

Now test for port-opening. The port is normally fully open to exhaust steam, but only partially to admission, perhaps 3/4 width according to design. Also equal both ends, and at the same point in the respective piston stroke.

Really, the second test should confirm the first, but the start of admission is the critical point.

.

To summarise:

The engine and its actions should be symmetrical, aside from slight geometrical subleties we need not about here.

To explain the significance of all those angles, the valve has an extension called "lap" which necessitates the eccentric throw catering for that and being 90º plus a certain angle of advance further, to let the valve open to admission just as the piston is ready to start its travel. This gives a full valve travel = [ 2(port width + lap) ], if no lead. Width in that equation, not opening, this time!

The valve will not necessarily open the port fully to live steam (usually doesn't) but normally fully to exhaust.

- The port just starts to open as the piston moves over dead-centre.

- The valve opens both ports by the same distance.

duncan webster16/07/2022 23:24:35
4105 forum posts
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- The port just starts to open as the piston moves over dead-centre.

- The valve opens both ports by the same distance.

Angularity of the eccentric rod means you can't get equal lead and equal port opening. On most set ups the effect is very small but where would we be without a bit of pedantry. 

Edited By duncan webster on 16/07/2022 23:25:40

Nigel Graham 217/07/2022 12:00:30
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Indeed, Duncan, but I would be surprised if the engine in question was designed with any lead, and its scale the angularity errors will be tiny.

I was trying in my reply above to state those two criteria while mentioning that although there are these more subtle details, we need not become bogged down them here.

If we want to be really pedantic we'd also worry about such matters as the piston-rods' volumes, as was sometimes worried over in full-size practice, but the OP not building a "Britannia" with an eye to winning IMLEC!

JasonB17/07/2022 13:02:40
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Why would you think it has been designed with no lead? Just about all the designs for small slide and piston valve engines I have seen have lead often around the 20-30degree mark for steam. Less can be used on air say 10deg

As for the grub screw problem, if the hole is deep enough drop in a slug of brass or soft aluminium and then insert the screw, it will protect the shaft. It is also possible to use the grub screw for initial setting and then when you have the engine running mark the eccentric positions, take apart and file/mill flats for the screw to subsequently bear against which is more secure and no risk of scoring the round surface

Edited By JasonB on 17/07/2022 13:11:22

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