cut off - display engines

I'm approaching making the valve gear for my latest little engine. I've got drawings for 2 stationary engines, Tubal Cain's Williamson and the Stuart Turner #4. Doing the sums gives cut off in ST#4 as 81% (assuming 0.005" lead), which seems about right, but the Williamson is 95% (assuming 0.002" lead). This seems more than a lot.

I'm not going for efficiency, more for steady running, but the Williamson has only 1/64" lap. It runs quite nicely, but so do ST#4s that I've seen. What do others use?

I've tried pasting in the spreadsheet results, but it overlaps the right hand side

Edited By duncan webster on 20/02/2021 17:55:49

Edited By duncan webster on 20/02/2021 17:58:31

I have built a few Stuarts and been surprised at the relatively long cutoffs, given that on a small locomotive you can use around 75 percent and expect it to start under load, so that should be satisfactory and economical for display running.

I really don't give it any thought, just tend to start with the eccentric's max throw at 90+30deg ahead of the crank and see what it runs like from there, seldom needs further adjustment. This is for display on air not steam. Do have a couple with reversers so you can notch them up or down if you want.

But find it's easiest to set the compressor regulator to a low sub 5psi setting and use an inline valve to regulate air volume.

Cut-off is a function of port width and valve lap, so how much cut-off do you design for? What you are describing is valve setting, which is not quite the same thing.

Edited By Redsetter on 20/02/2021 19:18:54

30 degrees advance with little or no lead gives 75% cut off, in the same order as I'd expect. Regulator is on the shopping list

thanks

As I said I don't really give it ant thought, just set the valve and go!

I've never bothered to work out what the cut off is on any of the engines I have made to published designs. Those that I make to my own designs I'm more than likely to take the port and valve proportions from a known design that is a good runner. Quite a few members here a have built engines to my designs and they too find they run well with minimal pressure for display, those that I have seen on steam also run well.

Edited By JasonB on 21/02/2021 07:03:14

I I were to give Howards two example engines some thought then they really are at opposite ends of the "model" spectrum.

The Williamson is a detailed replica of an actual engine which even at full size was quite a slow runner at 130rpm. It therefore has a long stroke in relation to the bore 1 3/8" x 5/8" and is unlikely to be expected to do any significant work. I expect most builders of this would aim for 40rpm or less for display

The No4 on the other hand was never designed to represent a specific prototype and at the time it was first introduced was far more likely to have been expected to do real work. It's meant to run at far higher speeds and the 1.5" bore by 1 1/4" stroke are far more suited to this than the long stroke of the Williamson. A lot more of these models get built and used to drive something so would expect them to be run faster, even for display they may not run as slowly as the williamson,