Mystery steam engine

[GreenTwin]

Here is the Stanley.
Looks like the interior of my valve layout needs to be a bit larger if it is going to adhere to a Stanley or the other references.

I don't make comparisons to model steam engine designs because they generally don't run under load, do not address efficiency, and are often just designed to make the flywheel go round and round.
Looks like your examples adhere to good design pretty well.
As the models get smaller, often there is no cutoff at all, and people use a typical simplified valve and valve travel.
The locomotive folks often pay more attention to getting the ports and valve gear correct, since otherwise they may not make it up the grade.


Looking at the Stanley, in general the interior of the valve pocket roughly spans between the inside steam ports, and the lap part of the valve extends beyond the port about the width of a steam port.
Generally I think there is a slight dead zone where the internal pocket is not open to either steam port.
I did my valve design in great haste, but it looks like we have some correlation.

The question still stands though; did the compound twins have a late or no cutoff on the LP cylinder?
There would be a reason perhaps for an early cutoff on the HP side, but once the steam exits the HP side, there is no efficiency to be gained by cutting it off early on the LP side ?
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Edit:
If we need a valve with no cutoff, then that would negate everything posted so far.
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[Jasonb]

Well I thought that was the whole idea to design the missing bits for Chris's engine not show what has been read in books about full size.

Your valve is designed around asymmetrical ports and bridges.
I am not aware of a steam engine that had assymetrical ports.
Asymmetrical valves were common on large vertical engines, but the ports where symmetrical.

The port dimensions should definitely not be asymmetrical, as far as anything I have ever seen in the old books.

As an example that you are likely to see when you get round to making an engine with a slide valve what the boox show and what you get are two different things

Here is a Stuart cylinder with cast ports, the inlet on the left is smaller than the one on the right. also teh "bridge" on th eleft is smaller than the one on the right. Not in a book or in th eplans but that is actually how it is.

In this case they are easily accessed so can be altered, I have enlarged the left port and reduced the right "bridge" to get things more symmetrical. The valve was altered to suit the new sizes

Although possible to adjust Chris's ports it won't be easy so make the valve to suit the sizes that we have and if better sizes can be obtained adjust if needed. Better than just a theoretical full size valve.

 

[GreenTwin]

Anything goes with model engine design, and so there is no way to compare apples to apples.
If one wants to make an accurate model engine, one has to know how the full sized engines were designed, and at least understand what one is trying to achieve.

No doubt the OP's engine is a rare bird, and it is hard to tell what is going on with it, or if it adheres to any design standard.

But what about my question in post #201 ?
Cutoff or no cutoff on the LP cylinder of a compound ?
ie: in full sized engines, how was it done ?

Edit:
Here is a discussion about compound locomotives.
Locomotives were designed to adjust the valve gear to give a very late cutoff, such as when starting the train with a heavy load, or pulling a heavy load up a steep hill. As the locomotive achieved speed on somewhat level ground, the valvegear position was changed to give an earlier cutoff, and higher efficiency.

According to this article, there was generally a ratio between HP and LP cutoff, and it states values of 50% HP and 73% LP (not sure how gear position figures into this, since gear position affects cutoff).
And a trick I had not heard of, they sometimes introduced a higher pressure into the LP steam chest, in order to pull steep grades.

I think there was an LP cutoff to allow sufficient time for the cylinder to exhaust, else you could have excessive end-of-piston-travel pressure remaining.

https://www.lner.info/article/tech/compound/compounds.php

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Don Ashton has a good article about equalizing the valve travel between full forward and full reverse of the gear, and also at positions in between, when using a Stephenson's link.
There is a bit of an art to getting it right.
For model engines run with no load, nobody will ever know the difference, but as Don Ashton points out in his article, getting the valve events (and ports) correct in a model locomotive can mean the difference between making it up the hill, or not.
Why not design it correctly if we know how to do that, or at least use that as a starting point of discussion ?
With model engines that will never see a load, I think it is just a matter of tidying things up a bit, but that is more of an art than a science.
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[Jasonb]

Answer to #201 As the OP is in Australia they have a limit of 100psi for model boiler so the one he wants to build can't go above that. So you are not really going to get any compounding worth worrying about with steam at those pressures and air does not compound either so all bets are off.

So best to design it correctly to what is needed than what theory and full size may do.

 

[GreenTwin]

I have seen a number of functional compound launch engines, but the model engine compounds at shows that run on air may be more for for looks.
Perhaps they are designed like the full sized units, and use something like a bypass valve in order to work.

I have to agree, for model compounds, design it to function somehow, especially if it is run on air.
I think compounds were designed to work well with higher pressure steam, such as 200-250 psi.
The locomotive article says that superheaters added to locomotives pretty much negated the advantages of compounding locomotives.

Place your bets on whether we will see this engine run.
I think there is a real potential to get it working, especially if the OP sticks with it and tries a few things to see what works.
I think JasonB is on the right path and has a handle on what is needed.
Its a nice engine, and it needs to run again.
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