Steam engine timing

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?

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

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

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.

DC1 - There was no timing diagram or instructions at your link, only running and lubrication instructions.

JD

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

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

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

This might be helpful:

**LINK**

Joey

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

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?

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

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.

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