Efficient use of Steam

Having had a recent conversation with a fellow enthusiast on Doncaster Station, who started chatting with good intention about cut off and use of the regulator , he unfortunately got a little muddled. After giving this some thought, my question is.....

Once a loco has achieved running speed and to maintain the same speed on a flat gradient, is it more efficient to have the reverser wound right in to say 15% and say 100 pounds on the steam chest gauge, or 40% cut off with only 50 pounds on the steam chest gauge? I realise this could be higher poundage but relative in difference to each cut off mentioned.

If these two examples are using approx the same amount of steam, is there one setting that is better? or was more often used by enginemen?

I appreciate this may be different for different locos too, but for the purpose of my enquiry, lets say this on a standard 4 loco with a 4 or 5 coach train.

 

Depends on the engine, the valve gear, lap and lead figures and a host of other things. Generally, the full regulator / short cut-off theory applied to the more modern locos, but there were other considerations than just the efficient use of steam (and hence coal): few men would run a Black Five with a fair mileage behind it below 30%, unless he was prepared to spend some time afterwards searching for his false teeth!

 

I think the point is that for the same load and speed they wouldn't use the same amount of steam.

 

This matter was discussed by S.O. Ell in his 1953 lecture. On starting the regulator is to be used, when running the reverser. He pointed at the investigation of Willans into this problem.
Kind regards
Jos Koopmans

 

As you were on Doncaster Station which is on the old LNER it might be best to consider the practice of this company.
However LMS2968 very rightly points out that there is more to consider than the thermodynamically optimum.
The long travel valve gear as designed and fitted to the Gresley Pacifics was optimised for 25% cut off. They also had good clearance volumes. That is the volume that exists between piston head and piston valve when the engine is at TDC, or at the commencement of stroke, it includes steam passage volume, compression space.
The ideal is for the steam chest pressure to be equal to the boiler pressure. It should remain constant during the admission period. But a glance at some indicator diagrams is most revealing.
Steam is not fully exhausted during the exhaust phase. A small amount remains in the cylinder to be compressed, acting as cushioning steam (to the benefit of the health of some mechanical components), but also to be compressed to the point where it is of the same pressure as the steam in the steam chest. Over compression and under compression are common.
Gresley Pacifics are well renowned for running with full regulator and 15% cut off. So are some of his other designs. They have been widely reported as working at even shorter cut offs as judged by the indicator on the footplate. The designs have good superheat. Very good for the time that they were built. However at a very short cut off, 10% for example, the expansion phase can reduce the energy in the steam to the point where condensation losses become an issue.
Some locomotives will not withstand the use of short cut off working. The high peak pressures are too much. Hence the Black Five with high mileage not being brought in shorter than 30%. The thermodynamic ideal is one thing but the steam locomotive is rather elastic. If the mechanical condition of the engine dictate or require it you can reduce the steam chest pressure and use a longer cut off to compensate, not the true ideal, but you can do it.
You could say that it is impossible to run at the thermodynamic optimum for high mileages without the locomotive design being the mechanical optimum.

 

Design and type of Valve gears can have a great deal of effect on running, both efficiently and otherwise, for example with a slide valve operated by stephensons valve gear the lead will increase as the cutoff is shortened, which is good for higher speeds, but depending on the valvegear geometry could increase to far, conversely the lead of a piston valve on the same valve gear will decrease with shortened cut off, which is going to put things in a right mess, hence piston valves would generally be rocker driven from Stephenson. Some valve gears do not even stand close scrutiny, visualise Joy gear for example, with the gear driven vertically from half way along the connecting rod, and a myriad of pins and links to wear, and the piece de resistance, a sprung driving axle moving up and down, is it any wonder that the original Wordsell designed F5's were known as 'Gobblers'? thankfully the F5 trust will be using the Holden/FV Russell designed Stephenson valve gear. Another one to think of is Hackworth, again a link vertically driven from a sprung axle forcing a die block up and down guided by a slotted link.........?

 

Actually, David Joy's gear worked very well and gave, for the time, quite decent valve events. There was some minor alterations in valve events during running due to the vertical movement of the driving axle on its springs, but it was the LNWR 'Super Ds' which showed the problem , and that was because their weight increased over the years in both new and rebuilt locos, with no corresponding modification to the springs. This, unlike the transient changes due to suspension movement over track irregularities, was a constant and gave these engine a very distinctive and uneven beat. But they pulled well and were economic on coal and water, as good as similar engines of that era.

The problem with Joy's gear was the pin in the connection rod. This required a hole to be machined in the rod, and was a point of weakness. This was not an issue in the early days, but as locos became more powerful with stronger piston thrusts, the rod could no longer withstand the stresses and failure started to become more common, sometime with disastrous consequence if the flailing end of a broken rod pierced the boiler or firebox.

 

The Joy valve gear seems to funcion well enough on Blodge new build L&B loco "Lyd". IIRC, it was thoroughly computer modelled before the first metal was cut.

 

There is a book that gives considerable detail of the valve gear for the new Lyd - both the background and the details of the care
that went into the Joy gear that was fitted. The locomotive seems to go decidedly well: it took 9 carriages up the Ffestiniog as
a test when converted for coal burning and with some more recent improvements has taken 6 over the Welsh Highland.
When there is a bit more length to run on the Lynton & Barnstable and some more stock to haul it would be very interesting
to see how it compares to the original engines - which I hope would inspire some additions and a reprint of the book:
"LYD a new Lynton and Barnstable locomotive" by James Evans, Paul Lewin, David Paling, & Jon Whalley published in 2011
ISBN 978 0 901848 09 3.

The risk of a connecting rod with the pin in it for Joys fracturing is a rational concern but does not seem to have actually given the sort
of considerable trouble you might think to expect. There were not repetitive major accidents from it, however much people
may have been convinced there should have been. There was a major review on the early LMS and some redesign of the connecting rods
which appears to have resolved things. In the end the Joys motion under the LNWR 0-8-0s soldiered on through the 1950s
until remarkably near the end of steam. And now we do actually have far better techniques for investigation, design & oversight.

 

The best way of describing the regulator and reversing setting is to to say.

Use the most regulator opening posible with the shortest posible cutoff for smoth motion and maintaining power.

Mostly it's down to valve events and the length of the valve travel.

Joys valve gear will always be a second choice valve gear as its eccentric rod is affected by the suspension and weight of the loco. In one instance I have found that the springing on the axel with the return crank is omitted so as to eliminate the effect.

 

There is a book that gives considerable detail of the valve gear for the new Lyd - both the background and the details of the care
that went into the Joy gear that was fitted. The locomotive seems to go decidedly well: it took 9 carriages up the Ffestiniog as
a test when converted for coal burning and with some more recent improvements has taken 6 over the Welsh Highland.
When there is a bit more length to run on the Lynton & Barnstable and some more stock to haul it would be very interesting
to see how it compares to the original engines - which I hope would inspire some additions and a reprint of the book:
"LYD a new Lynton and Barnstable locomotive" by James Evans, Paul Lewin, David Paling, & Jon Whalley published in 2011
ISBN 978 0 901848 09 3.


I have searched on line for a copy but without success..............

(Sorry, but I lost the quote bit)

 

I think that was more true with modern superheated locos, where the general driving style would be to leave the regulator largely alone and control power output with the reverser setting. Quite apart from any thermodynamic advantage, you get a quicker response between making an adjustment and that adjustment being felt if you control the valve gear setting, rather than move the regulator.

With older, saturated locos, most things I've read about, as well as practical experience, tend to suggest doing things the other way: leaving the cut-off largely alone (once up to speed: obviously you start in full gear and move it back when starting out) and controlling power with the regulator. There is a thermodynamic advantage of doing so, which is that as the saturated steam goes past a partially-closed regulator, there is a pressure drop but no significant temperature drop - the effect of that is in essence a small degree of superheat, i.e. taking the steam out of equilibrium with the saturated steam in the boiler into a regime where the temperature is above the temperature of saturated steam at the new pressure. That in turn helps avoid condensation in the cylinders, which apart from the obvious compression risk, significantly reduces non-productive heat transfer to the cylinders themselves.

Tom

 

Al - I can't help thinking that you are viewing the past from a modern perspective there. 50-60 years ago only a very small proportion of (steam)trains on the system went above 75mph (and of those that did it was often taking advantage of gravity while drifting downhill). The fact that many modern engines could run fast does not mean that there was a need for them to do so. Even expresses were for the most part not that tightly timed. Operational inefficiencies abounded back in steam days just the same, on what was a far more complex railway than we have today. And the other thing to remember is that poor coal is no new problem. I suspect that most of us 21st Century steam men have only the slightest inkling of the truely awful coal quality that some of our predecessors had to contend with, even sometimes on top link jobs.

Peter James

 

one of the pleasant aspects of our days is the possibility of simulation on a pc. I used Prof. Halls Perform program with the BR 5 data.
The steamchest pressure is 200 psi. With 70 mph, 15% cut-off the MEP = 33psi and steam consumption is 11200 lbs/hr.
I simulated with a systematic change of 20 psi steamchest pressure and adapting the cut-off to get the same 33 psi MEP.
The numbers are 180 psi,c.o 17,5% 11300lbs/hr. 160 psi, c.o 20.2%, 11200 lbs/hr
140 psi, 24.3%, 11400 lbs/hr. 120 psi 29.55% c.o. 12000 lbs/hr and 100 psi 35.9% c.o. with 12900lbs/hr.
So it appears that for some steps there is little difference but at 60% of the steamchest pressure the steam consumption starts to make
a fair difference.
Kind regards
Jos Koopmans

 

As mentioned by Jos, the Willans lines will give you the answer but the theory and reality for any loco will be quite different. You will only get a true answer by indicating a loco. As a general rule, though, long lap valves drive on the reverser and short lap, drive on the regulator if you can. If you're on a slide valve loco, you won't think twice: set the reverser and drive on the reg.