Cutoff Vs Regulator

In journals / memoirs of crews, you often read about about certain loco's being better driven on the cutoff and others on the regulator. Are these issues based upon the design of the loco of simply the preference of the driver?

 

Bit of both really, plus the type of working.

Generally, later designs with long lap, long travel gears were more suited to the full regulator - short cut-off technique while older designs with short travel valves needed a longer cut-off and therefore less regulator. But other factors intervened, such as speed; a big pacific might be pulled up to 15% or less at 80 mph (not up hill!), but a Black Five on a mixed freight would probably never drop below 25% and rarely get into second regulator.

Short cut-offs aren't recommended with two cylinder layouts anyway; the lack of inherent reciprocating balance due to having no pistons moving simultaneously in opposite directions sets up heavy knocks in the rods and boxes.

Drivers also had personal preferences, although this could change if there was an inspector aboard; these 'encouraged' the short cut-off approach, since it used the steam most expansively, thus getting the maximum energy from it and reducing coal consumption. Firemen tended to concur!

 

Thanks for that answer, Another related issue from reading old memoir type things and related to this, what are people's opinions / preferences in starting a loco with full cutoff, small regulator and wind up as speed increases versus setting cutoff to the minimum necessary to get the train moving, and winding the cutoff towards full gear to accelerate?

Are there benefits / problems with each technique (Slipping!)?

Also, as I'm sure others don't understand completely either, can someone explain the (obvious!?) reasons why long travel valves are better then short travel?

 

Just following on from LMS 2968's answer above, there was also a finite degree to which a piston valve engine could be 'pulled up' before it started fighting its own efficiency. The main line drivers' habit of exploiting shorter cut-offs with more modern locomotives was all fine, but it did have one important downside. As the cut-off is shortened the amount of steam admitted to the cylinder decreases as the valve opening becomes more and more restricted, which as the locomotive accelerates is exactly what you want. BUT, with a piston valve engine the same restriction is also happening on the exhaust side, which you don't want. The steam on the admission side has lots of energy and will spit through the small valve opening into the cylinder with no trouble. However, the lazy steam on the exhuast side also has to get through the same small gap on its side, and has only a tiny fraction of a second to do it. Then, as the loco accelerates, more and more exhaust steam remains in the cylinder because it's unable to get out in time and ends up being compressed by the piston on its return stroke. This uses valuable energy for a pointless task. So, bring on the poppet valves. These have a large area which opens for the minutest fraction of a second, and can be set to open for longer on the exhuast side than on the admission side. So why were poppet valves not more widely used? Cost, less durable than conventional valves, and needed much more accurate setting.

 

Have a look at this:

http://uk.youtube.com/watch?v=jLL-WoG5kzc

Watch how Donald Beale gets that 9F moving - seems to be lots of use of the reverser, even light engine. This isn't something I've ever seen elsewhere so I'd be interested to hear other people's experience.

I've always used the full cut-off, careful use of the regulator approach to get trains moving. Didn't know there was another way...

 

The second method I think was described in a book I read where a driver discussed starting a Duchess out of Euston, so someone must have used it! I think I have seen it used on an A4 as well when I was stood alongside the cab when it departed, but I am not sure

 

Hence the knocks!

The long cut-off at starting gives a tremendous amount of torque, which helps get the train moving as long as it doesn't overcome the wheels' adhesion with the rails. A shorter cut-off reduces the torque and therefore chance of slipping. Apparently with the Stanier pacifics, the normal cut-off at starting was no more than 40%, despite 75% being available, although 55% might be used on a heavy train.

 

This is the very reasion why modern locos were designed with large lap and lead and long travel valves. There is more dead space that the valves move in when not over the ports. In other words the designers were trying to design into the cylinder casting a way the ports are open compleatly more frequently that the partial openings more common in short travel gears at short cut offs. The valve is open and the steam is passing all the time the valve trales along most of its parth at longer cut offs. I'm not sure if this is making sence - just think about it for a few minites.

 

A lot depends on the valve gear too, altering the cut off on a stevensons valve geared loco alters the ammount of leading steam you have, leading to the effect that certain classes of locos are hard pullers when going slowly, but loose it at higher speeds (When they are pulled well back).
Some locos like a long cut off and a little regulator, others like it the other way round

 

Tried to get my simple mind round this statement and am struggling a bit! 'Cut off' is the point when the valve closes to admit steam to the cylinder. At 50% cut off no steam can be admitted to the cylinder for half the stroke, therefore at the moment of starting there is half a wheel revolution when that cylinder cannot produce any torque. To start a train you need to get steam to the cylinders. Anything less than 50% and there are points on the revolution where steam cannot get to any cylinder on a 2/4 cylinder loco (30850 excepted!). Even when it is open to steam the angularity of the crank(s) means that it may produce very little torque and the loco/train won't move. When you open the regulator you expect (or hope!) the train will start to move. When it doesn't the steam chest pressure soon builds up. If you then wind the reverser further down until a valve admits steam you then have a situation where you have full pressure on the piston, possibly at the position of max torque (c50% but dependent on design and direction of stroke) and a slip is more likely. Full gear and crack the regulator for me, I'm afraid! With poor rail conditions, I pump the regulator, so that if, it does slip, it hasn't got a boiler full of steam to go at to keep it slipping.

 

The point of cut-off - say 50% - is the point at which the valve closes to admission so no further steam enters, BUT the steam that is already in the cylinder is still at, or near, steam chest pressure, so still exerts a force on the piston and torque at the wheels. As the piston moves this pressure reduces, so is using the expansive properties of the steam admitted. The longer the cut-off, the longer steam chest presure will be maintained in the cylinder; you are using the expansion of the steam from the boiler, not that in the cylinder, to apply the piston force.

It is possible at starting, especially with a two or four cylinder engine, that the engine is on a bad quarter so no steam will be admitted. Lengthening the cut-off might open the port so a start can be made, but occassionally it is necessary to reverse the engine a little so a port is opened when in forward gear.

Everyone had their own ways of driving, but in a pacific's steam circuit there is a lot of steam left after the regulator is closed. This is uncontrolled, and with an engine in full gear, the resulting torque will ensure that once a slip starts, it will develop. I can't remember where I read this, but I'll try to find the relevant text.

 

Quite so. In theory this problem should be more pronounced with two or four cylinder engines, so why in practice does it seem to be more common with three cylinder locomotives?

 

That's perfectly true once the loco is under way and the cycle of valve events has started but my interpretation of what you were saying previously is that locos were started at 40-50% cut off, not wound back as soon as they started to move. You can't get steam into a cylinder in the first place unless the valve is open to inlet, which at 40% cut off it is not for 60% of its stroke.

 

To create an aside to this, as it seems that many are advocating full gear to start, on a preserved line, what actually happens in terms of reg / cutoff at line speed? As a basis to cover a few variables let's say Black 5 with 8 on the level 25mph, ideal conditions, and how would you get there from the starting setup?

 

Having started away in full gear with the small valve open I'd very quickly wind it back to about 45% and slowly wind it back from there. No need for second valve, usually. It depends on how much of a rush you are to get up to line speed but with level track it won't take long. Once up to speed the reverser will be about 20% and the regulator just cracked to give (at a guess) 75psi in the steam chest. Much more and the speed will still be rising. However, you always play it by ear as each loco is different and it depends on how well the valves are set, whether there are any sharp curves and a host of other things. The hard bit is keeping the speed down to 25 in the circumstances described. We don't have much in the way of level track on the NYMR though! Leaving Grosmont will be a similar technique to start but the reverser is unlikely to get wound back below 40-45%. Once the train is moving, has got through the tunnel and the advance starter is seen to be clear I put the handle in the roof and leave it there till Goathland is in sight. With 7 coaches (the normal load) the reverser will be around 50%, wound out to 55% on the curves but that assumes that the fireman has the steam on the mark. Less pressure and you have to compensate with the reverser. All this will keep speed at 18-20mph on the 1 in 49, which is enough to maintain the schedule. With 8 coaches the reverser is even further down the rack to get the same performance. Not every run is like this, though. There are, as you say, so many variables.

 

I knew I'd read it somewhere! "The LMS Duchesses", edited by Douglas Dougherty, 1973, Model and Allied Press, Page 40. This was a chapter entitled "A Performance Evaluation" by John Powell, C.Eng., B.Sc (Eng).,M.I.Mech.E.

Driving a 'Duchess'
In your hands you had a big, imposing, powerful but sensitive machine; you had to show her who was in charge, or she would very soon take charge of you!
The first thing to remember was that she was very touchy on her feet. The adhesion factor was only 3-73, and even on straight track you could not play the fool with the regulator. On curves and point-work, she could be even less surefooted, because of weight transfer from the coupled wheels as a result of the action of the bogie and trailing truck.
In addition, the volume of steam in the main steam pipes, 40-element superheater, header and 4 valve chests was very large, so that even after the regulator was closed there was enough uncontrollable steam to feed the 4 cylinders for 1½ revolutions at full pressure in full gear. So, by the time you had realised she was starting to slip, reacted to the idea, closed the regulator and waiting for the steam already past it to be exhausted, she could be AWAY in a spectacular slip. That assumes you could close the regulator; if the water level was high and she started to entrain water with the steam, this could be difficult or impossible until you had wound her into mid-gear and minimised the steam flow, all of which took time.
So the prudent driver, anxious to make a clean start, would put the reverser to about 40% cutoff, open the regulator and close it again immediately, and see if she moved away; then repeat the trick, leaving the regulator open a shade longer this time; if she still did not move, he would drop her down to about 55 % and repeat the process. This time it would be most unusual for her not to move, and suggestive that the large ejector might be needed to get a full brake release at the rear of the train. In practice the whole process
as described rarely took longer than 7-8 seconds, and almost certainly saved time in the long run.
Apart from this, a 'Duchess' was a machine you could really become a part of. The riding was superb, free from rolling, vibration or axle knock, easy at high speeds and devoid of any tendency to vicious kicking on curves. Full regulator or first valve, she would do her work competently and economically. Most of her work could be done at 15-18% cutoff, and it was unusual to go above 40 % even on Shap or Beattock. For really fast running on a reasonable road, full regulator and 25% was a sure indication that the crew meant business and would not be put off!

I have to admit that I too looked a bit askance when I first read it!

 

Well, apart from the disagreement about first trying to get away with 40% cut off, that's pretty much the way I'd expect to start out. Pump the regulator so that you don't get a full head of steam and the reg is already closed if it does lose its feet. Never driven a Duchess. Perhaps one day.....
A 3.73:1 adhesion factor is very much on the low side for a steam loco. Usually more like 4 or 5:1. I wonder what an A4 comes in at? Suppose I could do the sums but does anybody out there know?

 

Isn't that experience which comes of practice with any driver?