Steam Locomotive Valve Gear Questions

Can someone explain how decreasing cutoff percent works with regards to too much steam limiting top speed.

I understand mechanically what is going on, and the theory that as cutoff is reduced, steam is admitted for less of the piston stroke, which uses less steam.

I've been playing with train simulator and with full cut-off, on a large locomotive, top speed is around 15MPH with full regulator.

What is it that causes this effect, is it possible to calculate parameters in the design of a locomotive?

If we need to talk about a specific type of gear then if we could use walschaerts as example please.

 

Have a look at this site.

http://www.donashton.co.uk/index.html

 

Unfortunately the text there is full of garbage characters. I've tried three different browsers on two different operating systems, all with the same problem. Anyone able to contact Don Ashton to see about sorting it out?

 

Works fime for me on Windows 10 with Google Chrome. Never had a problem accessing it with Mozilla and other Windows platforms.

 

I'm using Windows 10 and Chrome and get the garbage, eg on the Introduction page...

frequent typical statements 箯ring the third term, which is very small....ᮤ 姬ecting the connecting rod angularity....Ქ unhelpful and clearly unacceptable. The profoundly knowledgeable Willie Pearce advised against a mathematical treatment with good reason. We can੧nore the mathematics

 

I have to admit that I've just gone on the site again and I'm now also getting similar. I originally clicked on Profile and then Walschaert's and couldn't see anything wrong but going on other pages, I see what you mean.

 

I'm having no problems with Mozella. Well, I say no problems, but none with this post, anyway.

 

I have contacted Don Ashton to see if he can help, works for me with Mozilla.

 

And with most locomotives you will probably be using steam faster than the boiler can supply it

 

I haven't played with train sim steam loco physics for a year or two, the following assumes nothing relevant has changed in the program code in the meantime.

I think what you're seeing is an example of Train Simulator not being able to replicate the exact characteristics of a real steam loco. One of the TS configuration files for steam locos contains a parameter called 'ExhaustLimit'. This sets a limit on the amount of steam that can pass through the cylinders. If you leave a loco in full gear and use full regulator, the steam consumption will rise in line with the speed until you reach the value set in the ExhaustLimit parameter, at which point acceleration will abruptly cease and the loco will continue at a constant speed, using a large amount of steam in the process.

I've never tried to drive a real loco in that manner, but I'm pretty sure what happens in TS is not what would happen in real life. As speed rises you get so called 'wire drawing' of the steam flow, you can only get so much steam into and out of the cylinder ports in a given time. In full gear and full regulator I expect the acceleration would tail off gradually, rather than the abrupt limit you get in TS.

 

However the amount of steam that does go into and out of the cylinders will still be a lot more than at the same speed with a normal shorter cutoff. Therefore you will, as Johnb said, be using steam faster than the boiler can supply it. With so much steam going up the blast pipe you will probably tear the fire apart too.

All that said, a simulator that behaves as described, where full regulator and full cut-off won't get you above 15 mph, is clearly unrealistic (unless that is with a heavy train on a hill).

 

Completely agree

 

He had been hacked. I think you'll find its resolved now.

 

I have some doubts whether Trainsim gives the proper physical results. Prof. Bill Hall's Perform
would do better. One of the results that Hall gives is whether the exhaust blast is sonic or not.
15 mph+full regulator+highest cut-off is bound to give sonic blast pressure, around 12 psi,
a standard blast nozzle cannot cope with that.
Kind regards
Jos Koopmans

 

I get the same gobbledegook when trying to read that website.
As far as the original question goes, I can only speak from experience in the traction engine world. Here its invariably a fairly rough and ready Stephenson link set-up, but I can say that I have never found a speed limiting factor on the rare occasions of needing full gear and full regulator. Because of the gearing down between crankshaft and driven axle, which I think is about 10 to 1, this would equate my normal 4 mph to 40 mph on a steam railway locomotive. Therefore, if there **IS** some sort of inbuilt limiting factor at around 15mph for a railway loco, I would certainly have noticed it by now.

That being said, the only time I need full forward/full regulator is when I am down on steam, and on a hill. I may well just experiment next time I have chance to pull a decent load (eg 3 or 4 other engines on push-poles) up a hill.

 

Thanks for the links, very interesting reading, not that I fully understand all of it.

The reason I ask is I am trying to understand a 7.25" gauge romulus. I'm trying to work out whether it just needs the valves resetting, or whether upgrading from Hackworth to Walschaerts might give better results.

It runs very smoothly in full gear, pulls away easily, requiring very little regulator to move, which gives the impression that the valves are well set. But notched up in any way it gets very uneven and bouncy. Running in full gear all the time results in it being very hungry, thirsty, and uses steam at an alarming rate. Which agrees with comments so far.

I've been reading the links and will continue to read, but if those with experience could give some advice, it would be appreciated.

 

Have a look at the downloads available on the site linked below. It allows you to vary many of the settings e.g. rod lengths, fixing points etc.

http://www.billp.org/Dockstader/ValveGear.html

 

Aha! Now I understand! Look at this:
http://www.michaelguy.ca/lempor update 2006/Lempor 2006 update page.htm
About two months of exhaust experiments with a Romulus in which I was involved.

Apart from too much consumption in full gear, did you Google "Romulus Hackworth gear" ?
There is quite some comment on its (dis)functioning including comment by Don Ashton.
Does it run properly backwards?
Kind regards
Jos Koopmans