I think Charles’s diagnosis of compression loops is correct, but it would be most unusual to have over-compression in full gear. Have you entered the clearance volume correctly? I can send you more understandable simulators, one written by Prof Hall in Borland C++, another by me using Python. Both of these produce more understandable oval diagrams. If the Hall one will work on modern Windows it would be easier than mine for which you need to install Python (not terribly difficult). There is also one by an American chap, whose name escapes me at the moment, no doubt someone will remember

In your 1st diagram, the exhaust compression loops may well be caused by incorrect entry of the exhaust back pressure.  Look at the exhaust line which is at 30 % of inlet pressure – very odd.

Charles has explained the 2nd diagram very well and in a perfect world it would show sine curves.  As Charles remarks, the kink at 100 % piston travel is odd.

I prefer Dockstaders offering but they all have a learning curve which takes time.  Time well spent, though – unless you enjoy lumpy running and re making valve gear parts.

Martin

On 22 May 2026 at 13:47 Martin Johnson 1 Said:

In your 1st diagram, the exhaust compression loops may well be caused by incorrect entry of the exhaust back pressure.  Look at the exhaust line which is at 30 % of inlet pressure – very odd.

Doh, I should have spotted that.

The only back pressure figure I can readily find for a full size engine is single chimney King working at well over normal evaporation rate. this was 10 psi. Double chimney conversion reduced this significantly, but I can’t find a figure. this doesn’t help with a model, where I would expect back pressure to be significantly lower. Jim Ewins did a series of tests with a loco on a stationary test set up. These are at last partially reported in Martin Evans book on model boilers, with his loco working hard he got a whole 0.9 psi back pressure.

Typical clearance volume on full size was 10-12%. GWR was less, but they suffered from compression loops at short cut off. Clearance is a necessary evil, how much you need depends how short a cut off you intend to have, and steam chest pressure. I’m not a big driver of model locos, but I doubt anyone runs at much less than 30% cut off, and steam chest pressure is unlikely to be above 100 psi. Most people seem to run with part closed regulator, so in reality a lot less.

If you are getting variable lead in an engine with Walschearts valve gear there is something sadly amiss with the design, probably either the angular position of the return crank or the length of the eccentric rod. Both my and Prof Hall’s simulations sort this out for you, but it is easy enough to do it on CAD. I did a few articles on valve gears in ME starting at issue 4718

I wouldn’t worry too much about the ‘indicator’ diagrams, what is important is that the cutoff each end is something like the same, the valves open far enough, and do not over run

Kevin

From your screen shots I agree with tcsmith, it looks like there is a problem with the radius rod length in your simulation. For comparison here are a couple of pictures from my Wallace simulation of the layout and indicator diagrams for a K28 Mikado I am working on. It’s a 3 1/2″ gauge loco, but is narrow gauge so the dimensions are practically the same as a 5″ gauge one (except for the wheel spacing).

John

On 22 May 2026 at 23:17 Kevan Shaw Said:

Right I put 30 psig in as the exhaust pressure as a guess. What should it be in a typical 5″ gauge logo to have a decent drafting? I also guessed at the likely space when the piston is at full stroke. Having remeasured it is going to be a lot more. There is also a problem with lead. The simulator shows virtually no lead or even negative lead at some valve setttings.

Hi Kevan, as Duncan says there is not the test data to be precise, but comfortably under 5 psi on a model I suggest.  The precise figure is not going to  matter greatly.  If your clearance volumes were wrong that will also help to eliminate the compression loops in your PV diagrams.

Sorry I can’t help with the other problems as I have never done a Walschaerts gear.

Good luck,

Martin

Kevan

The problem is that using the program (#1) for the case where the lifting link is behind the exp. link for dimensions of the case where the link is in front of the exp. link means that there are dimensions missing and the lifting link is not connected to the radius rod in the simulation. Which results in a non valid simulation. The #2 program requires the input of these extra dimensions so the lifting link is positioned correctly and is connected to the radius rod.

I have sent you a PM.

John

Kevan

Further to my last, I think making those two dimensions very long would simulate a solid valve rod and minimize or eliminate any vertical movement of the point “V”.

John

Kevan

Kevan
The length of the valve rod, VW, is defined from “V” to “W” so by increasing it’s length to say 100, “V” remains fixed and “W” moves way off to the right. Which is what we want.
But for the drop link, the fixed point is “S”, so when increasing the length of the drop link, “SV”, “S” remains fixed and “V” moves down off the page. Not what we want, we want “V” to remain fixed.
The way around this is to add exactly equal amounts, say 50 or 100 to the dimension AS suby “Axle to Hanger Top” and “SV”. This moves the top of the hanger “P” up while keeping the point “V” fixed on the combination lever.
Hope this makes sense.

John