Steam Loco Braking Effect

[Rockdoc2174]

I have been reading the superb Toton Engineman and one series of anecdotes is about the introduction of diesel power at Toton. Diesels with similar power characteristics and nominally the same braking power were substituted for steam locos on unbraked freights but it was quickly discovered that the braking was far less effective and trains were over-running signals and so on. This was 'cured' by the use of brake tenders. That much I knew but the book gives the explanation: when a steam loco is shut off, the cylinders pump against the gases trapped in the cylinders and this adds to the braking effect of the loco. A diesel free-wheels so its brakes had to do all the work instead of only part of it.

Is this effect practical in TS? It could, I suppose, be simulated by increasing the rolling-resistance of the loco but unless there is a compensating increase in the loco's performance all you'll do is degrade the work the loco can do. I suppose what I'm asking the floor is can the rolling-resistance be increased only when the regulator is shut off?

Keith

[cjbarnes5294]

In a word, no, as far as I'm aware anyway. You'd have to have some brakes drag on or have the regulator a crack open with the reverser in the opposite gear to direction, using some scripting for whenever the virtual throttle is shut (and the loco is moving - otherwise there'd be a problem with having the reg shut whilst stationary!), or even better, when the steam chest is depleted.

Chris

[Rockdoc2174]

I thought that might be the case, Chris. Thanks.
Keith

[VictoryWorks]

Interesting. Are there any figures on how much resistance this might provide?
The best (only?) way to do it would be to apply some brake (unknown to the player, unless they're using the F5 HUD) whilst the user has a fake brake that would be unaffected, the same way a few of us are simulating the steam chest (and realistic wheel slip) using a fake regulator.

[Rockdoc2174]

There's nothing definitive in the book. It must have been fairly gentle because it was used to close the buffers up before braking proper started and didn't create a bump on the engine. On the other hand, it must have been consistent, continuous and have had a significant effect overall because its loss made drivers reach places at a higher speed than they would have expected from previous experience, leading them to snatch at the brakes, sometimes throwing guards around their vans. Unsurprisingly, there was considerable antagonism generated towards this new and, apparently, dangerous form of traction. The brake tenders helped but it says the other reason they were introduced was because the brake shoes on the locos were wearing very quickly, possibly because drivers were having to brake harder for the reasons we're discussing?

Something else in that same section that isn't simulated but would be fun, if entirely impractical, to implement was the different way you had to operate a diesel's throttle. Drivers were used to closing the regulator all the way in one movement and some found it impossible to break the habit. On the early diesels at least, there was no system to dump any spare load so closing the throttle in similar fashion caused a flashover in the contactors and the loco was put out of action in one fell swoop. Apparently, you had to put the throttle into first position and allow the current to drop well down before taking it the last step. I don't drive TS diesels that way and suffer no such consequences. I wonder what players would make of a loud bang plus a message saying the loco had failed as a result of poor driving and the scenario was over the first time they slammed the throttle closed?

Keith

[cjbarnes5294]

I'm not very knowledgeable in diesels, but I think the deltics, 37s, and any others with that throttle setup can be damaged by the flashover caused by snapping the throttle closed. They have a single starting and ending notch that apparently you can feel, to try and remind you to wait for the current to die down before you release the contacts and cause a big bang. Otherwise, the throttles seem to be continuous, rather than having several notches.

I once saw a video of a class 37 driver for a fiver session on youtube, where what you describe happened purely by accident, but thankfully the loco was undamaged, it just made a bit of a crack! The instructor quickly moved the throttle back into the notch and let the ammeter drop before shutting it again.

If I understood how these types throttles work (I've searched the web countless times, just for my own interest), it wouldn't be that hard to replicate. It doesn't seem to have featured in any advanced diesels that I've tried, but I'm guessing that's because it wasn't appropriate/correct for the class modelled.

Chris

[Rockdoc2174]

I suppose how big a flash-over you'd get would depend on the load being applied before you closed the throttle so it would almost certainly not cause major damage on a preserved line, where speeds and loads are relatively low. Something like a Class 44 on a heavy freight and cresting a bank might well be the kind of situation John Woolley was describing. No resistor-bank to absorb the spare power and lots of Amps with nowhere to go. Using that wonderful, mythical sense of hindsight, it's probably a shame the diesels didn't get a throttle akin to the tap-changer on the AC electrics, which would have forced a step-by-step change.

Although I don't remember this being written in Toton Engineman, I'm sure I've read that the usual practice with steam was to put the reverser into mid gear when coasting so the valves barely moved and any gases in the cylinder were trapped, giving the cushioning effect. You don't need any steam in the valve chests so closing the regulator fully would be the norm. When you drive a new car it can take time to get yourself adjusted to new foibles so it isn't hard to imagine drivers with years of experience with steam finding the new diesels a trial. It wouldn't be any easier while they were swapping from steam to diesel and vice versa on a daily basis.

Keith

[cjbarnes5294]

Although I don't remember this being written in Toton Engineman, I'm sure I've read that the usual practice with steam was to put the reverser into mid gear when coasting so the valves barely moved and any gases in the cylinder were trapped, giving the cushioning effect. You don't need any steam in the valve chests so closing the regulator fully would be the norm.

I think that very much depends on the loco in question, and a number of different factors, such as the lubrication setup, valve gear, valve type etc. For slide-valve locos, you should apparently always coast in full-forward (or reverse if going backwards of course) gear or risk breaking the loco (I don't know why this is, to be honest). Piston valve locos tend to have designated drifting cutoffs, such as 45% for a black 5, or 25% for an A4 (at any speed) to prevent the conjugated valve gear from shearing itself to pieces from the slackness caused by lack of steam in the chest. Locos that made use of Malesco multiple valve superheater regulators, like Britannias and Clans, needed the regulator to be always slightly open, because the first valve directs saturated steam past the superheater and straight to the lubricators that they operate, located in the main steam pipes above the cylinder. GWR locos seem to have a jigger/jockey valve that allows you to completely shut off the steam and close the regulator, but keep the lubricator open (hence the unusual notched guide above the regulator handle pivot point), so therefore leaving the handle open before the notch (I think, I'm not that good with GWR stuff ).

Then I've seen that 71000 has a notice on the regulator handle that says keep the regulator fully closed when drifting, so that's another completely different case for some reason that I don't know of. I think overall, both back in steam days and today, it comes down to what the different railways (possibly sheds?) taught the staff as they progressed, and they probably all had their own techniques, or adhered to special cases, such as the A4s. I know that alot of the excellent drivers on preserved lines prefer to always have the regulator a crack open, presumably for cylinder temperature reasons (and hence reducing the risk of hydraulicing whilst running at relatively pedestrian speeds) if not for lubrication, even whilst braking.

When you drive a new car it can take time to get yourself adjusted to new foibles so it isn't hard to imagine drivers with years of experience with steam finding the new diesels a trial. It wouldn't be any easier while they were swapping from steam to diesel and vice versa on a daily basis.

Indeed. I bought a book in Australia that was a series of accounts from drivers in the old Victorian Railways, most of which started in the steam era. However, one of them I think started out driving by the time the diesels had replaced steam, and his discussions on how this affected the older drivers was most interesting, almost a bit saddening. After being the veterans for donkeys years, it apparently knocked alot of their self esteem to be the completely back to square one and being taught by the younger generations how to drive these new diesel locomotives and suburban EMUs.

Thanks
Chris

[DominusEdwardius]

Let me try and back this up with theory

I apologise in advance if I make a mistake and this will be very rough

Say we are traveling along in a Jinty at 20mph ( speed is Irrelevant ) and we shutoff steam. We then place the reverser in notch 1. The valve events in that notch are as follows approximately ( forward stroke ) Cutoff 22%, release 65% ( Backward stroke ) Compression 65% and lead steam at approx 3%. Now the following will not happen if steam is not reapplied, this is because without steam pressure slide valves will float off their seats so any a vacuum cannot occur, nor can compression since the cylinder valves would be constantly open so negating pretty much all the effect except at high speeds.

Lets look at the left hand front side of the cylinder first.
Backward stroke
Initial at the start of the backward stroke the steam inlet would be open, steam will enter into the cylinders, but given the speed were traveling at a breath of steam means the steam chest pressure is still roughly zero ( atmospheric). At 22% of its backward stroke the port to the steam chest will be closed at this point pressure will be approximately atmospheric or 15psi ( ish ). Since there is no air entering from the steam chest as the it expands thereby maintaining the pressure as the piston continues to move back this causes the pressure on that side of the cylinder to reduce ( a vacuum ). At 66% the exhaust port will open but since the there is a partial vacuum vacuum air will actually flow into the cylinder through the blast pipe! that destroys the vacuum and negates the force on that side of the cylinder. The Cylinder will then remain at atmospheric for the remainder of the stroke.

Calculations:
Volume at 22% = 1450 cu inches, pressure = roughly 15psia
Volume at 66% = 4360 cu inches, pressure = ?
P1V1 = P2V2
so 1450 * 15 = 4360 * P2
P2 = 5psia

So at the moment before the exhaust port opens the pressure on the front side of the cylinder will be 5 psi absolute in other words a vacuum. Although this is rather negligible force in the grand scheme of things so we will effectively call this negligible.

Forward Stroke

On the return stroke the exhaust port remains open until 24% travel( relative to backward stroke ) when compression begins. At this point no ports are opened so any air on that side of the cylinder will begin to be compressed. At 5% the lead steam occurs, at this point no doubt the compressed air pressure will be greater than that of the steam chest so the air will flow out into the steam chest until pressure is equalised on both sides and the process then repeats.

Calculations
Volume at 24% = 1580 cu inches
Volume at 3% = 200 cu inchs
Pressure at P1 = 15psi

P1V1 = P2V2
1580*15 = 330*P2
P2 = 118Psia

Now that is a much bigger force! This one will actually have some effect since it is equivalent force to 103 psi of steam acting on that side of the cylinder.

At this point I should remind you there are two sides to the cylinders and 2 cylinders one being offset at 90 degrees. These events occur every 90 degrees, so the vacuum during some part of the travel will negate that of the compression so I'd estimate that you'd have about 80psi of compression at the end of each stroke. Granted at the end of each stroke the crank is now straight forward so there would be no force, but just before with that amount of pressure you'd definitely have some resistive force.

The graph of pressure against revolution I think would look something akin to this
http://www.atomic-album.co.uk/showPic.p ... ession.jpg

But at higher cutoff say full gear the compression point is 10% and the release at 3% ( with respect to backward stroke ), as such the pressure on the cylinder caused by compression is pretty much negligible hence why it is usual to coast slide valve engines at full cutoff.

It's probably pretty wrong but it's my best stab at it and someone may be able to improve upon it

anyway enough of me!
regards
Edward

[AndiS]

I am not as daring as Edward, so I just offer the bits and pieces that float around my memories in a much looser fashion.

Slide valves needed the lubrication effect of moving to and fro which is why they had some minimum settings. I vaguely remember that you could not close the regulator completely, too, or it was at least discouraged. You see "engine X running under grease steam" in image captions when they push about a preserved engine at the top of an excursion train using a Diesel or electric for environmental considerations.

I would assume that applying counter-steam by setting the reverser for the opposite direction and applying a bit of steam was standard practice when braking because it reduces brake shoe wear.

I read that steam engines had quite some internal friction at lowest speed and gently buffering up would not have been such an affair with them as it is with MSTS & RW. The background of that is that friction decreases with speed in a non-linear fashion or - put the other was - is very high below walking speed.

Engines for mountainous terrain has special counter-pressure brakes. Usually associated with the name of Riggenbach, they were a dedicated contraption to do the things Edwards described. The important bit is that air should not be sucked in from the exhaust as that was a dirty affair. In addition to closing the connection to the exhaust and opening to fresh outside air, they admitted water to the cylinders for cooling purposes and exhaust air was lead through a muffler. However, this brake was only fitted for dedicated engines. Wikipedia has a short summary in English and a longer description in German.