Releasing the brakes is generally helpful.
OvermodThis is true for such obvious reasons I don't see why it keeps being brought up in this connection
yet you wrote a 403 word reply. i don't understandwhy you write so much and say so little. brevity would be appreciated
gregcit show an increase in speed (purple) from 10-30 mph the TE to maintain speed the increase in TE accelerate the somewhat exponential TE required to match the exponential increase in train resistance
Overmod3)With the same assumptions and initial conditions, figure out what is needed to sustain the train, on the original level grade, once 30mph has been reached. Likewise translate that into cutoff settings and steam circulation.
the plot simply shows what must happen to increase speed, not how to increase speed nor the details within the engine
Overmod4) ... This is going to involve both flow calculation (during admission) and then recuperation (during expansion when there is no admission)
if all steam produced by the boiler enters the cylinder, a decent estimate of cylinder pressure is due to the density when the valve closes at cutoff.
how does < 50% cutoff affect flow (lb steam/ hr)?
if flow varies with cutoff, the "piling" up must impede flow.
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greg - Philadelphia & Reading / Reading
gregci don't understand why you write so much and say so little. Brevity would be appreciated
Reminds me increasingly of that Far Side cartoon where he illustrates 'what we say to dogs' vs. 'what dogs hear'. If you are counting the number of blahs... well, there we are.
Oh look, now we have another plot. Let's see how this differs from the multicolored one with the excessively complex assumptions...
the plot is for level grade plot shows the TE for constant acceleration from 10-30 and TE for sustained speed at 10, 30 mph train resistance rises with speed (hence exponential curve) constant 2 mph/min acceleration (x-axis should be seconds) TE is 22814 lb, ~11T at 20 and 34287 at 30 mph 5000 T train this is simple physics, not a steam engine model the plot simply shows what must happen to increase speed, not how to increase speed nor the details within the engine
And while it isn't really 'simple' physics, any more than lolog heat uptake from combustion gas to boiler water is, we're actually getting to the original question.
Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank? And then that something magical makes all that steam go obediently into the cylinder?
how does <50% cutoff affect flow (lb steam/ hr)?
You don't know enough to get an answer until you know some of the details within the engine, so you're in deep doo-doo already. But in the interests of brevity I'll have to leave it at that until you self-educate a bit better, or get less simplistic physics that apply. In any case, if you haven't understood flow of steam into the cylinder depending on cutoff in over 790 words, I don't think any further amount of logorrhea will help you.
f flow varies with cutoff, the "piling" up must impede flow.
Overmod Oh look, now we have another plot.
Oh look, now we have another plot.
no, same plot as before, which you then asked for, which i posted to address your comments on "ACCELERATION", to focus on how cutoff is used to control speed
OvermodAnd while it isn't really 'simple' physics
simple application of Newton's Law (F=ma) knowing the tonnage, train resistance and force required to accelerate at 2 mph/min
Overmod if all steam produced by the boiler enters the cylinder, a decent estimate of cylinder pressure is due to the density when the valve closes at cutoff. Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank?
Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank?
a more serious response would address why the throttle can be full for various speeds and what is limiting steam flow between the boiler and cylinders
Overmod flow varies with cutoff, the "piling" up must impede flow. Succinctly, no. Can you please provide simple physics that illustrate what 'piling up' is supposed to mean in this context?
flow varies with cutoff, the "piling" up must impede flow.
Succinctly, no. Can you please provide simple physics that illustrate what 'piling up' is supposed to mean in this context?
how is steam flow (lb steam/hr) affected when both cylinder valves are closed?
analogous to
Water hammer arrestors have air-filled cylinders that absorb the jolt of a sudden water pressure increase when a valve shuts off.
why is there an optimal cutoff dependent on speed? why does steam consumption increase when cutoff is either +/-?
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gregcSame plot as before, which you then asked for, which i posted to address your comments on "ACCELERATION", to focus on how cutoff is used to control speed
... simple application of Newton's Law (F=ma) knowing the tonnage, train resistance and force required to accelerate at 2 mph/min
...a more serious response would address why the throttle can be full for various speeds and what is limiting steam flow between the boiler and cylinders
There are some instances where reflected shock has been said to be observed in a steam line when flow is abruptly interrupted. Note that this is a pre-existing flow shock that is reversed, not a "piling up" or some other naive foolery.
analogous to Water hammer arrestors have air-filled cylinders that absorb the jolt of a sudden water pressure increase when a valve shuts off.
Can I ask why you use data for an ancient 2-8-0 when you'll get clearer indication from something like, say, a Hudson or Niagara, or something halfway modern if you want to stick with PRR?
why is there an optimal cutoff dependent on speed?
why does steam consumption increase when cutoff is either +/-?
[/quote]
and of course, the fireman (fuel pump) controls the evaporation rate to match whatever the cylinders require by maintaining pressure
OvermodSuccinctly: it answers the original question you asked, if it's simple physics. Why keep asking about steam and all that other stuff?
the plot illustrates the required forces, not the operation of a steam locomotive to control the steam generating those forces
OvermodYou had your serious response on this not just once in the thread already. In the interest of brevity I'm not going over it again.
you don't need to repeat yourself. you can refer to your post explaining it (i.e. date time)?
OvermodIt... doesn't flow. It equilibrates with boiler pressure in a very short time, and then starts up again when there is a pressure differential.
this sounds new. have you said this before?
so how do you determine the average flow (lb steam / hr) during intermittent flow? i think this is a key part of the answer
how might you determine the average steam chest pressure, the diffential pressure across the throttle (full) that determines the flow (lb steam / hr) thru the throttle
OvermodCan I ask why you use data for an ancient 2-8-0 when you'll get clearer indication from something like, say, a Hudson or Niagara
Laboratory Tests of a Consoliation Locomotive is from 1918. do you have a similar more modern reference?
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gregcit's never been clear how steam flow is limited with full throttle, as suggested on pg 1, despite seeing test data maintaining various speeds with full throttle
The only reason to keep the throttle closed is if, for some artificial reason, you want to restrict steam flow during admission. As your question is asking about removing limitations from flow, this would not apply.
Overmod explained how increasing cutoff increases steam (lb/hr) into the cylinders, tractive effort and speed (without the need for throttle).
so it seems cutoff can also control steam flow but differently than a restrictive orifice, a throttle...
The net effect of cutoff is to "restrict" flow, but only in the sense of maximizing the potential expansive working. The valves are best designed to admit steam freely as soon as possible when they open in admission, and then keep it flowing freely for as much of the duration of admission as possible. (There is less problem with gas-cutting if admission close isn't quick and precise than there is for admission opening without proper compression control -- see the general experience with Franklin type A poppet admission valves, and the early ones on the T1s in particular).
because I didn't see a clear explanation, the conclusion i draw is that < 50% cutoff results in intermittent, stop/start flow of steam when both cylinder valves are closed...
...affecting steam flow similar to a partially closed throttle.
[/quote]shorter cutoff is required as speed increases to increase the time that the valves are closed and the time steam chest pressure has to recover[/quote]That's not the point of short cutoff. The idea is to extend the portion of the stroke during which expansive working is making best use of the heat in the steam. Steam-chest pressure is incidental in that context unless there is some restriction of steam into the chest (like, for example, a partially-restricting throttle).
Now, one place you see 'output depending on cutoff' is when admission calls for more steam than the boiler can source. (Remember that the 'admission time' is 4x admission for 'one stroke', per driver revolution on a 2-cylinder DA, and therefore the number of events increases with speed, as does the aggregate mass flow required during the admissions. It does not take long for an engine in full gear to outrun its steam-generation capacity at a high rate of consumption, even with a relatively large boiler -- and since more and more of the admitted steam has little chance to expand before it has to be exhausted, much higher pressure as well as mass flow have to be blown out the exhaust. This will tend to lift the fire off the grates before it generates high effective heat transfer.
So, relatively quickly, you want to start reducing admission duration as the cyclic starts to come up in acceleration, so that more of the work is produced by expansion (of the steam in the cylinder; we don't care what happens in the chest after its pressure equilibrates, which likely happens long before the next admission starts).
If you're looking at something like cylinder thrust over a stroke, you can use cutoff to set a range of admission and expansion for a given speed. In general if you were running the engine for least cost, you'd want to reduce both the fuel and water rate, so you'd pick the shortest admission that, together with subsequent expansion, ultimately produces your wheelrim torque or drawbar TE. But you might, for reasons including those I've mentioned, choose to operate with less 'peaky' thrust for a small part of the stroke at peak pressure.
One of the things Neil Burnell came up with regarding the T1s was a method they used to start them, which involves a number of practices we've discussed here. At a station stop, the fireman would intentionally NOT bring up his fire to recuperate full 300psi pressure; I think I remember seeing something in the range of 190psi. The engineer would then start the train on the reduced pressure, which of course translated into lower admission pressure at the valves and hence longer cutoff required to get the desired acceleration. A T1 was both a bit slow and a bit slippery getting to about 30mph, and a little less power made for a less slippery girl -- but during this, the fireman would be handling his fire to build pressure back to make it available above 35mph where the T1 could put it to the rail. What was not mentioned in the article -- and which of course is of very great importance in practical locomotive management -- is that this pressure cycling and accompanying thermal cycling weren't doing very good things for the boiler structure. (But that's another issue for another thread).
Again, we come back to why all the modern steam designers thought in terms of mass flow: here, only average and relatively slow changes in mass flow can be commanded, even on a locomotive with oil firing. If you graph sequential admission at the rate corresponding to your 20 to 30mph, with your (unspecified) driver diameter, you can see the admission times (measurable in milliseconds) and recognize that this can't be a factor in what happens during each particular stroke.
As far as I know, there were more test plant results for the Q2 than any other PRR locomotive. Use those.
Overmod.
Earlier I linked an article to a 1920 Railway Age magazine article about the I1s testing of limited cutoff. The I1s was built first with 50% cutoff later increased to 65%.. The big hippos were probably tested as much as the Q2. What struck me about the tests was that they found no appreciate advantages in stoker firing over hand firing. Just ease on the fireman. In some tests, hand firing generated more usable steam with a higher super heater temperature. Most tests were done at 40 revolutions per minute at 7 and a half MPH. Drawbar tests were taken up to 20 mph. Something like 3500 HP at 7 mph.
The T1 was a magnificent locomotive. It's been later found out that most of the problems with them turned out to be sabotage by the crews who were afraid that a successful design would have seen a greater reduction in work hours. They really were afraid that they would be sent back to freight pool service. The T1 spent a lot of time on the test plant too.
The Q2 while successful still didn't last past the decades older 2 cylinder beasts it was supposed to replace. One of the last ones to dump it's ashes was nearly a half century old 2 cylinder simple locomotive.
Pete.
whether it's called "restricted" or "metered", < 50% cutoff (wiki cutoff (steam engine)) appears to be another mechanism that limits steam flow into the cylinders to control the MEP and tractive effort
like the sinusoidal force applied to the rails (see fig 7 in Fundamentals of Steam Locmotive Tractive Force), there is an average flow (lb steam /hr) or simply a mass (lb) of steam that enters the cylinder each cycle while the valve opens
it seems there are several concepts involved.
it's not at all clear why cutoff can't be made "early" (reduced toward ~20%) even at low speeds to use steam more efficiently
but while steam flow across a throttle is proportional to the difference in pressure across ...
... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff and speed
table shows how little MEP is required on level ground and the time (msec) that valves are open at speed and for early cutoff
trainPsi: tonnage 5000T, drv Dia 61.5 in mph tonage TE MEP cps msec adMsec cut 10 5000 22842 24.8 1.8 548.9 109.8 20% 20 5000 27298 29.6 3.6 274.4 54.9 20% 30 5000 34287 37.2 5.5 183.0 36.6 20% 40 5000 45248 49.0 7.3 137.2 27.4 20% 50 5000 62439 67.7 9.1 109.8 22.0 20% 60 5000 89399 96.9 10.9 91.5 18.3 20%
(busy with guests)
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gregcwhether it's called "restricted" or "metered", < 50% cutoff appears to be another mechanism that limits steam flow into the cylinders to control the MEP and tractive effort
... like the sinusoidal force applied to the rails (see fig 7 in Fundamentals of Steam Locomotive Tractive Force), there is an average flow [...] or simply a mass [flow] (they are both in lb/hr) of steam that enters the cylinder each cycle while the valve opens...
it seems there are several concepts involved. expansion to extract more energy from steam after cutoff; recovery to allow steam chest pressure to increase after cutoff; the need for longer recovery time as speed increases ...
What's going to happen after the 'highest speed' you can get by using expansion most effectively is that you have to start lengthening out on the cutoff, and therefore starting to use more steam per stroke even as the number of strokes per minute keeps increasing. This starts to draw more steam mass flow from the water in the boiler... which has a shorter and shorter time to actually move into the cylinder volume in the time allowed by the stroke for admission. Hence for very high speed, you most certainly don't have 'wisps of steam flicking in and out of the cylinders' but you certainly have steam moving in very fast if it is to be there productively at all to sustain acceleration at higher and higher cyclic.
Now there is a sort of lower-speed equivalent of this, which is if the engine is overloaded. Longer cutoff is required, the engine both uses more steam per stroke and makes less use of expansion to produce thrust. The additional steam mass that can't expand blows out the stack with additional heat and pressure energy, driving the boiler to higher combustion rate and (uneconomical) additional steam mass-flow generation. This hits the wall at the 'grate limit' which is the greatest amount of fuel per square foot that can be burned with forced draft through that particular boiler and front end before combustion starts to suffer or combustion-gas flow starts to choke.
but while steam flow across a throttle is proportional to the difference in pressure across ... ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff...
You probably don't have the port opening information for the valves in question, but you can clearly figure out "a" steam mass flow per admission from the indicated chest pressures for start and close of admission, and the time the port is open to steam. Since the degree of superheat is relatively easily (in theory) measured near the valve, you can make the appropriate correction for the actual degree of superheat at admission.
Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation. (Now might be a time to remember Angus Sinclair's comment that engine performance did not inexorably correlate with nice indicator diagrams -- find the passage in History of the Locomotive Engine that contains the line 'a small leg of mutton' -- but for now we can ignore some of the factors that make that happen.)
Overmod gregc ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff... Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation.
gregc ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff...
Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation.
i didn't say simple. what do you mean by "observed indication performance"?
i and i think others would appreciate an explanation of how early cutoff controls steam flow, allowing the throttle to be open full?
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gregcwhat do you mean by "observed indication performance"?
Just to be sure everyone has a compatible hymnal, I assume the test locomotive has long-lap long-travel piston valves. These open and close to steam transversely, so that the port opens remarkably quickly at the start of admission, over effectively 360 degrees of the valve circumference. And the valve is already moving with considerable speed when the steam edge crosses the near edge of the admission port (that's the point of 'long-travel', not that the valve physically moves a greater distance)
Remember that in admission two things happen: the cylinder volume (which is increasing as the piston in it moves) fills with steam, and that steam exerts pressure against the enclosing space as it fills it. So you get a particular mass, corresponding comparatively closely to the volume, and full 'effective pressure' on the piston very quickly after admission.
There is a technical term 'wiredrawing' for what happens when there is inadequate mass flow to accomplish this completely, for example if some fool has the throttle partway closed so that the effective pressure falls as the piston is moved. This creates the effect of partial expansion during the admission phase, and heat is lost without producing useful work on the piston, and you trade pressure for velocity you can't really use for a variety of reasons. (If you had a turbine with a Curtis first stage, like the one on PRR 6200, you could make use of some of the velocity, but that's not true for any reciprocating engine I have looked at, even those with Bulleid's sleeve valves.)
Early cutoff, whether fixed or variable, results in earlier stopping -- again, at a very high rate of speed -- of steam mass admission. The pressure now falls in expansion, so if you were to measure effective pressure on the piston you will see it falling as the piston moves further, and the volume increases.
Note that these effects are entirely unrelated to throttle modulation. All the throttle does is restrict the steam supply that the valves can admit to the cylinder.
Now, as already noted, a good modern valve gear can cut off reasonably early with sufficient timing precision that the 'beats' are regular and square. This produces a resultant of admission and expansion thrust (take this as "MEP" through to exhaust opening) which -- as already noted several times -- can be remarkably small. This implies that no further reduction of the chest steam supply (via a throttle) is needed if you want a low MEP to run your locomotive, and therefore at the speed that the valves can provide the appropriate MEP in the cylinder, there is no reason to have a throttle restriction upstream of the valves at all.
Now the plot thickens. Suppose I'm starting a train with relatively low 'MEP requirement' to accelerate. If I have good valves and high boiler pressure, my admission at early cutoff shortens to a small fraction of each revolution -- at high effective pressure -- but the small mass now has to expand through a greater volume, with EP trailing off fast especially toward the end. This produces a relatively short torque peak 4x per revolution, which makes the engine mechanically slippery. Most valve gears did not have a precise 'shorten-and-return' that would arrest the slip with a minimal effect on EP, and the typical throttle was a miserably imprecise thing, like trying to play a slide whistle with a bag of flour tied to the slide. So the normal time for response and recovery of a slip without losing power was comparably long, and the control much more like 'shut off and reopen' than, say, easing up on your car accelerator a moment.
You could get around that rather nicely by restricting the admission steam to a lower pressure, and lengthening the cutoff so the admitted mass, starting to expand a little later, will produce the desired MEP at the lower initial pressure. You're going to throw away more heat and pressure at exhaust, but it's often worth it to get a clean start on poor rail. And that can be done easily, if a little indirectly, by partially closing the throttle to 'force' longer admission
OvermodThis implies that no further reduction of the chest steam supply (via a throttle) is needed if you want a low MEP to run your locomotive, and therefore at the speed that the valves can provide the appropriate MEP in the cylinder, there is no reason to have a throttle restriction upstream of the valves at all.
are you saying that
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gregcare you saying that since MEP decreases (not proportionally) as cutoff decreases, that even when initial cylinder pressure is boiler pressure, cutoff can reduce MEP sufficiently to maintain the TE required at low (~10) speed?
Remember that the 'M' in MEP signifies an average.
If the 'resultant' of the valve gear (cutoff by the reach rod moving in the link plus contribution by the combination lever) may not be precise enough to produce a limiting balancing speed down below 10mph, in which case you'd restrict the chest pressure by closing the throttle down appropriately. In my opinion, you'd adjust the throttle after starting, wind the valve gear to where operation is 'smooth', and then trim the throttle as desired, but that's just my opinion on how to proceed.
gregcSeems there are different ways to operate a loco/train depending on situation...
use of a "cracked" throttle and full cutoff to get just loco or high tonnage train moving
Note that there are two approaches. The more 'usual' one on large or front-end-throttle-equipped locomotives has been to crack the throttle at full (or very late) cutoff with the cylinder cocks open to start the train, but then as quickly as possible pull the throttle open and wind up the reverser toward mid. As noted in Australia and elsewhere in this thread, in many cases you'll still pull the throttle open relatively quickly, just stop opening it at some level (corresponding as noted to a particular peak or 'recovery' chest pressure) and trim the acceleration or final speed via cutoff.
In practice you'll want to get that throttle restriction out of the way as soon as the engine can manage the train on cutoff alone -- that this is possible is part of the adage that 'a good steam locomotive can pull any train it can start'.
use of low boiler pressure and [partially-closed]throttle to move a loco in a yard or possibly switching just a few cars
Yes -- but a caveat. A number of very expensively built or funded switch-engine development projects have foundered, sometimes utterly, against the rock of not understanding how flat switching works. What you want is maximal acceleration with even a heavy cut of cars -- but without overpowering to slip with a light one -- and then reasonably quick deceleration once the cut is positioned and then acceleration to go get the next one. This isn't a place where low pressure moseying around spotting a few boxcars in the equivalent of a model-railroad switching problem would apply. It is, in fact, perhaps the most demanding engine service of all, and on top of what it requires, the engine may, with little advance mechanical warning, be required to idle with minimal fuel consumption between bouts of full accelerationdeceleration. (It is not difficult to see why even primitive DC-motored diesel-electric switch engines were so successful so early... but had to be so enormous in some applications that don't appear to require the horsepower)
lower boiler pressure reduces tractive effort for low or no tonnage trains
Yes, again both by having lower continuous thrust during admission and lower MEP during expansion.
lower boiler pressure provide more room for pressure to rise while throttle is closed
That is a little less certain. All that the throttle does when closed is retard the potential flow to the steam chests. If there is no demand at the chests, no steam flows. What to look at if there is some steam demand, if considering what happens in the boiler, is the pressure fluctuation immediately upstream of the partly-closed throttle: it is this zone that is the other end of equilibrium with the overcritical boiler water at this point, and what will govern 'pressure rise' due to additional heat uptake from combustion gas to water.
"Pressure recovery" will be greater in proportion to lower mass flow. But again it is the valves that are the key determinant of mass flow. If they are closed, the restriction at the throttle doesn't matter as much (or indeed, perhaps at all) whereas if conditions at admission lower the EP during that part of the stroke, the boiler can only make up the mass flow that passes the restriction, and can only do it via pressure relaxation (e.g. 'starved' pressure) into the lower pressure measured at the chests.
And, as you've already said, all the heat going into the water has to go "somewhere" -- and that 'somewhere' is going to wind up as increased saturation pressure, and if that gets up to the popping-off point, the fireman will have to use feedwater to keep pressure limited. (As another issue, he can't get too much additional feedwater in there, so he may have to 'live with' safeties lifting while he reduces the fire... the effect of which may take several minutes or more to 'work'. This, too, is part of his job to anticipate...)
opening throttle and using "early" cutoff to control speed of a high tonnage train over its run
But be careful. You'll start a high-tonnage train with long cutoff and perhaps WOT; even there, you won't be able to start a 'train' more than a few cars long... which is why slack action was so important in steam days, and why continued acceleration of the part of the head end that was moving was sometimes a "necessary" thing... to the immense detriment of the crew in the caboose of a long train which might be abruptly snatched at shockingly high resultant acceleration the length of the caboose's draft gear and then even more abruptly banged to full achieved train speed, which might be 20mph or more at that moment. (This was I suspect worse than the stories in Railroad Magazine make it out to be, which is saying something.)
The more interesting part of a run with heavy tonnage is how you control the engine as a heavy consist runs over an uneven profile, even to the extent that parts of the train are going upgrade while other parts are in a sag, and you need to adjust power as overall train resistance as felt at the drawbar changes. Unsurprisingly this is both complicated and extremely reliant on experience; the key thing here being the need to do something with the throttle when the engine has to be 'drifting', but then open it up again but with the reverse 'preset' when power is again required.
Otherwise -- in my opinion -- you would 'drive on the reverse' to adjust train speed, and certainly if you were for some reason using power braking. As soon as the engine is at a speed that permits control with cutoff alone, adjusting cutoff 'early' as the means of power control is the most economical in terms of water rate and hence required fuel firing.
OvermodYes
wow
... and thanks
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I hesitate to provide additional documentation as it seems Mr. C only gives credence to texts that he believes support his theories. I thought the preceeding discussions might lead to a mention of the Loco-Valve-Pilot.
I do have several articles about how some of these units were tested on the New York Central but these are contained in copyrighted periodicals. For now here is some baseline information from the 'Cyc":
Valve-Pilot by Edmund, on Flickr
Interesting to note that the cutoff was only changed four times in the distance of seven + miles and a speed range of seventy MPH to a stop.
Valve-Pilot_0001 by Edmund, on Flickr
Valve_Pilot_fix by Edmund, on Flickr
In the development of the Valve Pilot I read that scores of trips were made by the designers who plotted the operating habits and characteristics of a sampling of the locomotive engineers in order to acquire the data for calculating the 'sweet spot' which would be used in determinating the shape of the cam.
Valve_Pilot_crop_fix by Edmund, on Flickr
Valve_Pilot by Edmund, on Flickr
Regards, Ed
my current understanding is that while cutoff may only be needed to control speed with full throttle at higher speeds and tonnage ...
... throttle is needed at low speed or low tonnage. (perhaps some of you are saying i told you so) of course what low speed or low tonnage is depends on the size of the locomotive
i also believe lower boiler pressure is more suited for operation with little tonnage
a 246 T RDG I-10 loco doesn't require full boiler pressure to move between coal, water and sanding stations. it requires an MEP of ~20 and an additional 25 psi to accelerate it 5mph/ 15 sec. so i'll guess boiler pressure of less than 100 psi is probably more than sufficient
i assume there's no one right way to operate a locomotive pulling a medium tonnage train with a combination of throttle and cutoff and that it depends on the engineers preferences and experience
it bothered me that one of the rebuilders of the western maryland scenic RR 1309 said he just opens the throttle to increase speed. But pulling a half dozen passenger cars behind a 2-6-6-2 capable of 45T of tractive force is a relatively light load, even up the 1.75% grade to Frostburg
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The guy on 1309 is likely simplifying things when he says that.
1309 is a true Mallet, a compound locomotive. This is started on straight steam using an intercepting valve (look it up for the varieties used and the technical details) and then switched over to compound working at comparatively low speed -- well below 10mph in almost any event. On one of these, the LP steam is governed by whatever the HP engine has 'just used' and one of the things this means is that cutoff on the HP engine may have to be kept at a longer setting that an equivalent simple engine that size would be. That does imply a situation where at low speed you might find it better to lower and raise the HP admission pressure, and work the cutoff to provide the right 'balance' between the output of the HP and LP engines. (There are discussions on the Web for both the Baldwin and Alco types of early Mallet if you are interested in this somewhat esoteric stuff).
after resolving 2 confusing issues:
and experimenting with a software simulation, i understand that the primary and obvious means to control locomotive speed/TE is the throttle (which many of you said).
at startup, cutoff needs to be maximized to open the valves to both cylinders. Of course throttle needs to be severally limited. But once the locomotive has some speed, cutoff should be reduced (shortened) to 50% to take advantage of expansion.
my understanding is there is minimal benefit to reducing cutoff further until steam production begins to be limited. When limited, reducing cutoff can maximize the initial cylinder pressure, MEP and speed/TE. But the range of this benefit is small. Of course, cutoff can be reduced at lower speeds to take advantage of expansion to use less steam/water/fuel. But the effect on speed/TE is small.
Overmod's caveat (since edited) that throttle may need to be reduced to use cutoff to control speed/TE effectively says throttle controls speed/TE.
as for the Laboratory Data, I finally realized it's also evaluating performance when steam production is limited. it does this with throttle full by controlling the rate (lb/hr) coal is added.
Under these conditions, insufficient cutoff has a detrimental effect if the valve is open too long and expansion includes the volume of the steam chest. So reducing cutoff has a benefit when it closes the valve just as initial pressure is maximized and expansion begins.
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