how does a steam locomotive increase speed?

[gregc]

after resolving 2 confusing issues:

•  the Laboratory Test data and
•  Overmod's statement that general practice is to get the throttle open full and use cutoff to control speed/TE

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.

7420

[Overmod]

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).

[gregc]

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

6757

[gmpullman]

 

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

 

[gregc]

Overmod

Yes

wow

... and thanks

6611

[Overmod]

gregc

Seems there are different ways to operate a loco/train depending on situation...

And this is one of the beauties of operating a modern engine with a good power reverse (e.g. Franklin Precision either with lever or wheel) and a good proportional air throttle: you have reproduceable control over both the degree of throttle opening and the cutoff.  With a little care (I'll admit, more care than was usually given even in the Forties) this could be made precise enough, with travel enough, to make quick settings "haptic", so that quick and positive learnable movement could produce not only effective but economically optimized operation.  For example, it would not be difficult to provide the more-continuous version of a 'shift light', indicating the relative movement of controls and when the control adjustment is correct to produce the result, as an improvement on the Valve Pilot system (which matches needles for speed and cutoff based on a cam cut for the particular class of engine -- Ed can provide extensive documentation and pictures of how that worked). 

• 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.

[Overmod]

gregc

are 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?

Yes, with one particular qualification.  If boiler pressure is sufficiently high, and passage volume large, and load slight enough or downgrade sufficient, the developed MEP may be more than enough to accelerate the engine to 10mph.

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.

[gregc]

seems 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
• use of low boiler pressure and throttle to move a loco in a yard or possibly switching just a few cars

• lower boiler pressure reduces tractive effort for low or no tonnage trains
• lower boiler pressure provide more room for pressure to rise while throttle is closed

• opening throttle and using "early" cutoff to control speed of a high tonnage train over it's run

6568

[gregc]

Overmod

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.

are you saying that

• since MEP decreases (not proportionally) as cutoff decreases,
• that even when initial cylinder pressure is near boiler pressure,
• cutoff can reduce MEP sufficiently to maintain the TE required at low (~10) speed?

 

[Overmod]

gregc

what do you mean by "observed indication performance"?

Reading the indicator diagram or examining the readings from the various indicated quantities -- in other words, working entirely in terms of IHP rather than actual thermodynamic performance with CFD and all that.

i and i think others would appreciate an explanation of how early cutoff controls steam flow, allowing the throttle to be open full?

Let's look at this again.

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

 

[gregc]

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.

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?

6340

[Overmod]

gregc

whether 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

The point here is that the throttle and the valve action 'limit steam flow' in very different, non-trivial, non-semantic ways, and they do it to modulate effective pressure (not "MEP") so as to produce drawbar 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...

Note that the force isn't 'sinusoidal'; that's a colossal simplification of the actual resultant being applied at a driver rim over one revolution.  Best to actually plot it, as Wardale did and as I've said, so you get an idea of how the thrusts actually produce torqu

 

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 ...

You're mixing things up a bit.  The expansion has no formal relation to the recovery, only that it is occurring while the recovery has a 'chance' to be happening.  It's not the 'pressure increasing' that is important in the recovery, it's the transfer of mass into the chest sufficient to drive the next admission completely -- pressure AND steam flow driven by that pressure.  And obviously you don't need 'longer recovery time as speed increases' as the cutoff is being shortened to reach high speed -- the average mass flow per stroke is decreasing.  

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.

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

I don't know why it's unclear to you, because pretty obviously the cutoff can be moved that way, and it will be using steam more efficiently above the speed where the better-expanding steam can balance the train resistance.  One big problem is that earlier cutoff reduces the contribution of admission to thrust, so that a smaller overall percentage of stroke sees full effective pressure (and the MEP goes down more sharply than with longer admission).  See your prior comments about data that show very small MEP numbers to keep a train moving under certain conditions.

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...

Well, you said a mouthful, because as I said earlier there isn't a good way to indicate mass flow, and it's usually extrapolated from other factors for analysis.  What I think most engineers do is to understand that shortening the cutoff uses less mass of steam per hour in the range where that shortening still accelerates or pulls the train, and have the fireman aware of what anticipated need for water (and fuel to heat it) might be 'upcoming'.

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.)

[gregc]

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.

• 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
• ...

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)

6142

[wrench567]

  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.

[Overmod]

gregc

it'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

As said before, 'full throttle' implies removing any 'limitation' from the steam flow upstream of the valves.  As the valve action inherently involves both fixed and variable cutoff, anything that reduces 'limitation' before that ought to be minimized.  Which is why the throttle is opened full, and all the regulation of steam flow is handled valve-dependent.

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).

Again, we need a different convention in language.  Is "increasing cutoff" supposed to be the same thing as lengthening it? if so, say so (it would follow from the context, but I think this is part of the ongoing confusion).  I would personally prefer that we use 'early cutoff' and 'late cutoff' as it describes the physical action more succinctly.

so it seems cutoff can also control steam flow but differently than a restrictive orifice, a throttle...

The throttle restricts flow.  The valve cutoff meters flow.

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...

So far, that would be correct...

...affecting steam flow similar to a partially closed throttle.

Look, the restriction from a throttle is continuous.  The restriction from valves is stop-start during admission, with essentially no flow at other times.  Don't equate them.

[/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).

and of course, the fireman (fuel pump) controls the evaporation rate to match whatever the cylinders require by maintaining pressure

Not in the sense you keep thinking.  "Whatever the cylinders require" is what is metered into them by the valve gear.  Nothing the fireman does is within several orders of magnitude of influencing that as it happens.  What the fireman does is watch his pressure, and anticipate what it is expected to be, and fire accordingly to keep the available pressure in the range we have discussed.  That is essentially asynchronous from what happens at the cylinders, except on the grossly averaged time scale that corresponds to the fireman's, and the boiler's, response.

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.

[gregc]

• it'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

• 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

• 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.

• 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

and of course, the fireman (fuel pump) controls the evaporation rate to match whatever the cylinders require by maintaining pressure

Overmod

Succinctly: 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

Overmod

You 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)?

Overmod

It... 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

Overmod

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

Laboratory Tests of a Consoliation Locomotive is from 1918.   do you have a similar more modern reference?

5934

[Overmod]

gregc

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

Succinctly: it answers the original question you asked, if it's simple physics.  Why keep asking about steam and all that other stuff?

 

... simple application of Newton's Law (F=ma) knowing the tonnage, train resistance and force required to accelerate at 2 mph/min

Guess so, if you have a string to the front of the consist to exert F in opposing vector, or perhaps a rheostat to turn up the voltage on the electric motor in your steam locomotive.  Again, why ask if it's so simple?

...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

You 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.  It's really simple once you try to understand it.

how is steam flow (lb steam/hr) affected when both cylinder valves are closed?

It... doesn't flow.  It equilibrates with boiler pressure in a very short time, and then starts up again when there is a pressure differential.

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.

Not.  Go back and study actual physics.  Liquids that suffer hammer are essentially incompressible and have clear momentum effects.  Gases... don't.  (You will see naive 19th-Century references acting as though steam does.  And, in fact, there is one piece of evidence in the Q2 test-plant documentation that seemed to suggest there were what they characterized as 'momentum' effects in the steamlines at the slip-prevention devices, although I don't remember actually seeing a magnitude for the effect.  

 

 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?

It's not just 'dependent on speed' -- and the physical valve arrangement and geometry usually plays an important part you'll never see from a test-plant plot.

why does steam consumption increase when cutoff is either +/-?

If you haven't figured that out from what I've said by now, I despair.  But here's a hint: look at what's happening at the exhaust.

[/quote]

[gregc]

Overmod

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

Overmod

And 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?

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?

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 +/-?

5765

[Overmod]

gregc

i don't understand why you write so much and say so little.   Brevity would be appreciated

I tried brevity in the first few answers.  It didn't work and then you started overcomplicating.

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

So in the interest of brevity: you wanted to know what makes a steam engine accelerate, without reference to 'the details within the engine', then you bring up all kinds of details about what the steam is doing within the engine without comprehending them, and now you plot something that answers your self-asked question with 'simple physics'.

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.  

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.

Briefly, this is drivel.

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)?

Is that "<50%" intended as longer cutoff, or shorter cutoff?  50% FIXED cutoff, as on a PRR I1s, is something different from setting the valve gear to produce 50% cutoff, in any case. 

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.

Succinctly, no.  Can you please provide simple physics that illustrate what 'piling up' is supposed to mean in this context?

[gregc]

Overmod

This 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

gregc

it 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

 

Overmod

3)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 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

Overmod

4) ... 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.

 5674

[maxman]

Releasing the brakes is generally helpful.

[Overmod]

But you specifically did not have 'grade' in the original question, and then when you added the graph, you quietly slipped in not only grade, but irregularly increasing train resistance that remains unspecified.

Since you appear to have a computer modeling program running:

1) Model the exact conditions in the original question.  Assume (as would be correct, although I don't think you fully realized it) that your initial state at 20mph had the cutoff set as short as it could possibly be.  Were the combination lever (see the C&O video again) not contributing something, the engine would in fact be drifting, with full pressure in the chests but no admission other than from slop.

2)Now plot the power needed to ACCELERATE the train from 20 to 30mph.  Don't try to cheat and not state the time over which the acceleration is supposed to occur -- put it on a labeled, proportional axis with scale.  Note that the appropriate level of 'new cutoff' will only apply until you are done accelerating the train to 30mph.  That is one segment of a graph of whaever variables you're tracking.  DO NOT make any further dicking around with the train resistance, whether from grade or 'rising air resistance' as though that scaled in that speed range for standard-gauge rail equipment.  Just get the power needed to accelerate given your trailing resistance, and then backtranslate to the resultant of the cutoff at nominal 'full-throttle' boiler pressure that would produce that acceleration.

3)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.

4)Once you have all that safely and correctly plotted, you can start introducing your double-salient changing running resistance, from increasing grade or whatever.  Note that the desired force now needed to produce your constant acceleration is going to be changing, in ways that I frankly wouldn't want to have to try calculating deterministically, but as you seem to find it significant, have at it.  This will give you a set of nifty curves at fixed ACCELERATION with differently-varying (but consistent-delta) train resistance.  And you can figure out the steam demand to produce that acceleration over that time or distance... with a little more work.

4) Note that only after you get all of this done can you start looking at how the steam is supplied to the ports in order to produce the required power over the required time.  This is going to involve both flow calculation (during admission) and then recuperation (during expansion when there is no admission) and yes, there may be some family of cutoff vs. pressure that produces the 'same' acceleration over at least part of the desired range.

But in any case, with the throttle full, you can now start figuring out how the steam actually flows into the chest during the admission time, and what the average mass-flow change for steady-state acceleration under each condition will be, and what mass flow is being required to achieve it.  You'll see what I've been telling you: that any actions to increase steam generation are required only as the steam comes to be needed as flow at the valves, and changing it may require careful anticipation but is NOT the typical sort of load-following you see with an 'accelerator' on an IC engined vehicle.

[wrench567]

  Greg.

 For a good read with lots of graphs, charts, and data.

https://books.google.com/books/about/Railway_Age.html?id=w0E_AQAAMAAJ#v=onepage&q=PRR%20test%20plant%20I1s&f=false

    Pete.

[gregc]

Overmod

I have to take a deep breath and sigh here, because the game is being changed again, perhaps without someone realizing they're doing it.  The original question involved accelerating a train ON LEVEL GRADE from 20 to 30mph, and understanding even how that is accomplished has been an ongoing source of consternation.  Now we have someone slip in not only the effect of grade, but increasing grade with distance, without even mentioning the function describing the delta resistance over each of the constant-speed runs.

don't understand why you think this it outside the original quesiton?   

the values show how little pressure is required when running on level grade.  grade was added to show how it requires significantly greater TE compared to level grade

did this earlier this morning, have guests and don't have time to read the response(s) thoroughly

it 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

[Overmod]

gregc

[cutting to the chase...]I'm asking about the steam outside the cylinder when the valve is closed if the steam flow (lb steam/hr) is the same regardless of cutoff (!!)...

Except that it isn't; you're either not listening or not trying to understand.  ANYTHING that goes into the cylinder is a result of valve action, as driven by cutoff action and characteristics.  The only 'variable' that is controlled is the relative duration of admission (variable) and expansion (fixed by admission-timing cutoff and exhaust opening).  There is no magic way other than that to tinker with the 'steam flow'.

Now if you treat steam pressure as a variable (which in most practical engines it most certainly is) then you correct admission flow, etc. to be relative to pressure at the valve.  That pressure, moment to moment, affects how the steam goes into the cylinder (and comes up to full pressure thrust as it does).  That's the only place that steam-chest pressure influences what the engine is doing.

Do you understand what the little 'loops' in an indicator diagram are showing?

The flow 'into' the steam chest does not need to be continuous.  Certainly the flow into a given cylinder end at any practical physical cutoff setting isn't, and the overall flow into the chest isn't even if you assume admission from 'either end' of the chest volume depending on whether the piston is advancing or retreating in DA.  And yes, it starts very quickly as the valve opens to steam (even quicker in a Corliss or poppet-valve engine, but without very substantial benefits even if it's assumed to open both immediately and fully without restriction -- see the numbers for Bulleid's Leader, which probably set a record, in theory, for doing that with physical valves).  And yes, it is cut off very quickly as the valve head passes the other admission steam edge in cutoff -- that's part of why it's called 'cutoff'.  The steam-chest pressure and the mass of steam at that pressure almost certainly reaches an equibrium during the interval, and this is 'recuperation' to produce as-good-as-possible filling of the cylinder as needed for the 'next' stroke.  You could overthink this out the ying-yang with velocity and 'momentum' calculations (the latter a sure sign of someone who doesn't really understand how fluid mechanics in steam work) but it all comes down to the same thing: you want high effective pressure in the chest, and enough steam mass that it fills the cylinder effectively for the expansion you want.

"MEP" is an artifact, something that gives you a handle on calculating average power out of the engine over more than a full rotation.  For instantaneous steam-flow calculation, using it as if it were "a" fixed value will leave you just as clueless why different cutoffs produce different 'mean effective pressures' under load as you are now.

and all of the flow enters the cylinder when the valve is open (consumption equals production)...

More witlessness. Steam 'generation' from heat transfer in the boiler isn't even remotely proportional to what 'enters the cylinder when the valve is open' (unless you have a really tiny engine, designed and run by buffoons).  It is controlled as an average, nominally with respect to boiler pressure as a primary control signal and empirically-observed rate-of-change to delta conditions.  It is NOT controlled with any direct reference to events during a particular stroke, and while you could theoretically go down a rabbit hole doing precise instrumentation and supercomputer-level simulation of changing inputs... you get no further in practice than an experienced fireman watching his pressure and using stoking and injection appropriately to control it.

the mass (lb) of steam in the cylinder each cycle is the same, but the initial pressure is higher at less cutoff (less volume, higher density) and results in a higher MEP with lower cutoff (can't be right)...

Oh, you're so right that it can't be right; you need to be less wrong about what the mass of steam in the cylinder actually is when cutoff is changed, especially if the recuperating flow into the steam chest to supply mass flow is restricted.

so the steam flow must change when cutoff is changed.

Pleeeeeease don't make me say it again again.  We already have someone over on the Trains forum that says I sound like Jesse Jackson preaching.

... but i don't know how to calculate that value.   i don't understand how the "piling" up affects the flow.

That's because you don't seem to be really trying.  The 'piling up' is recuperation, not accumulation to some 'higher than boiler pressure' trading of momentum for pressure.  It only restores chest pressure to 'starting value' in between admission events.

...is it the opposite of expansion inside outside the cylinder where the increasing pressure impedes flow, resulting in a decreased average consumption?

This has to mean something -- I desperately think it has to mean something -- but it just doesn't.

Look, you have steam in a volume (the steam chest) that is admitted to a volume (the ports/passages and cylinder end).  The steam then continues to flow into the cylinder, through the ports and passages, as the volume in the cylinder increases (because the piston is moving... and it ain't moving at a constant speed, as a moment's thought would show, too).  Now, in the absence of anything coming into the chest, that is a pure equilibrium thing (in the absence of weird stuff like wall losses or nucleate condensation, which we ignore at this level) so the pressure in the chest FALLS in proportion to how the pressure in the cylinder RISES.  If you have to do some sort of complecticated mathemagics, calculate the pressure and volume in the chest at the moment admission starts, and calculate the volume of the chest, ports, passages, and cylinder volume at the moment of cutoff, and do your little assumed-gas-law calculation.

Now it follows from this that if you DON'T want the steam-chest pressure to fall, and hence the cylinder pressure at the moment expansion begins to be limited, you will be feeding replacement steam mass, at something like initial pressure, into the chest.  If you have been following along, the 'average' amount you feed only has to be that which fills the sequential admission volumes to the desired pressure, with some pressure recovery during the cutoff expansion between admissions.  If you are controlling the engine with a throttle, you are cutting down the amount of steam that goes into the chests -- again, grossly on average -- and hence the achieved final admission pressure will be limited to what's there.

More steam admitted to the chest, more steam that can go into the cylinder.  With the amount at whatever final pressure determined by the point of VALVE stroke that produces cutoff.  (Where the piston is at that point is conjugated by the valve gear combination lever, so relatively determined by design, but only incidentally related to the engineer's cutoff setting -- something you should have gotten from that C&O Walschaerts video.

Should the mean effective pressure in the steam chest (i.e. back pressure) be used to determine the pressure across the throttle and the flow it allows !?

Ye gods and little fishes, you're changing the game again.

MEP in the chest has nothing to do with cylinder 'back pressure' and even less than nothing to do with the 'back pressure' we use the term to describe, which is in the exhaust tract, far far away from this stuff and in a completely different space in the cylinder porting.

MEP in the chest cycles between what happens during the admission event and what the boiler sources during admission and then 'expansion time' before admission starts again the other way.  In a proper engine it's as close to boiler pressure as good steam streamlining can produce, and it's maintained so that at the moment of admission it's as high as practical.  (Or restricted so that cylinder peak and average effective pressure are 'regulated' -- why the English called the thing a 'regulator' in the era before good link gear was introduced.)

Your only concern is maximizing that.  If anything restricts that, redesign it so it won't.  And only look at it if it's grossly inadequate to reach a particular power at speed.  (Probably not 20mph or 30mph on the level.)

[Overmod]

gregc

Ed, the video describes the "general" function of the valve gear and cylinder valves which i believe i sufficiently understand.  The video doesn't address my latest question about how to calculate the pressure in the steam chest and piping during operation with reduced cutoff to determine steam flow (lb/hr) vs cutoff

Answering the last day or so's traffic in reverse order:

As Greg notes, valve-gear action is not instrumented to measure 'mass flow', and it has been a fun topic over the years to find ways to meter or approximate this.  The most 'usual approach', back in the day, was to take the various indicated quantities and interpolate measured qualities of steam determined in the lab to get a number for admission, then use that to figure out what was going on during expansion (and then of course exhaust, but we're not concerned with that here).

Where you start with the steam chest is to 'assume' its starting pressure, and then how that decreases IN EQUILIBRIUM WITH THE ADMISSION VOLUME.  There were attempts to indicate steam-chest pressure to get the initial pressure, net of any other effects, and then an indication of instantaneous pressure as admission proceeded, but as can be appreciated by an electronics engineer, this is no small problem and the data received from those sensors has to be integrated without losing the instantaneous pressure v. time information.  You will appreciate Chapelon et al's concern for larger steam chests, and Wardale's concern for very well-insulated cylinders, when you look at the process through admission.

If the flow 'into' the steam chest (from the boiler) is less overall than the flow into the cylinder during admission, any equilibrium upset from flow restriction in the ports and passages may be COMPOUNDED by the fall in effective pressure through early expansion in the steam-chest volume (which causes pressure to fall there through the mechanism you keep trying to bring up).  Naturally the mathematics to describe what happens going into the cylinder referred back up to the flow into the steam chest are extremely complicated, and have a nasty component of nondeterministic dependence on things you can't effectively measure even with the best analytic equipment.  (See the origin of 'quaternions', for example, something that I refuse to discuss any further than this citation for any reason here).

Again, your take-home from this is that ANYTHING that impedes clear steam flow from the superheater elements down to the steam chest is going to impair how the steam gets from the chest into the cylinder during admission.  That conclusively answers any concern about what the throttle in this particular discussion does... for our purposes, "less than nothing".  (So we eliminate that source of loss quickly, positively, and as early as possible.)

Part of where cutoff becomes important is, again, that it varies both admission duration and expansive-working duration mechanically, by adjusting the points during the piston stroke where the valve starts to admit steam, and then stops admitting steam.  There are some shrouding and unshrouding effects, and Julius Kirchoff went on at some length trying to demonstrate their significance at high cyclic, but the effects can be measured in fractions of an inch and their overall impact on effective pressure, between 20 and 30mph, is almost certainly very slight.

Doughless -- But if the top speed of the loco is significantly higher than 20 mph ... under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well.

Any train needs to operate at various speeds and on various grades requiring different amounts of tractive effort as the lab chart shows...

First, it's wack to state that any crew trying to get appreciable TE out of any locomotive is going to have the throttle partway closed doing it, so that objection falls to the ground before we even need to consider much about it.  Second, before we spend any more time on that graph, someone has to go through and put proper scales for the different variables on it, and arrange them so they all increase or decrease in appropriate proportion.  If you know what you're looking for you can extract some information from the wretched thing, but if not... well, here we are so far.

The 'ringer' is that no one has clearly stated what that graph is measuring, which is not in fact plotted and is highly, even suspiciously artificial.  What is happening is that trailing resistance is being increased at constant speed, probably explained away as the effect of 'increasing grade', but there is no indication of the rate of load increase, although there is certainly a 'piecewise' effort during the runs to tinker with the cutoff to increase engine output.  (In one case the 'engineer' actually gets the cutoff too far, and has to jerk it back, with somewhat predictable results on the performance).

What would normally be seen on a plot, of course, would be trailing resistance measured by a dynamometer car.  If the car were equipped with variable electric dynamic braking, as at least one English car was, you can simulate grade effects just as desired in the graph, by cranking up the excitation and watching the dynamometer pen swing over.  (Assuming you have proper surge damping, etc., but that's another discussion for another audience...)  The more usual method of simulating constantly augmenting gradient was to use brake locomotives, and there are so many jokers in that pack that I won't start listing them.

What I understand to be the original practice for this sort of thing originated with Lomonossov, who did the experiments for constant gradient utilizing certain parts of Russian trackage that were inclined at a very consistent gradient for the number of miles needed to extract meaningful indication data.  But that is not the same thing as having an engine sitting on the PRR test plant and having someone turn the Prony brake up and up and up while the measured 100lb bags go into the firebox.  That is NOT to disparage what PRR was doing, in their context, and it would be somewhat valuable once the necessary understanding of the variables is clearly understood to examine some of those data and some of their plots.

examples for the following shows the tractive effort and corresponding cylinder pressure vs mph and grade.   Note that there is insufficient boiler pressure at higher speeds as grade increases...

I have to take a deep breath and sigh here, because the game is being changed again, perhaps without someone realizing they're doing it.  The original question involved accelerating a train ON LEVEL GRADE from 20 to 30mph, and understanding even how that is accomplished has been an ongoing source of consternation.  Now we have someone slip in not only the effect of grade, but increasing grade with distance, without even mentioning the function describing the delta resistance over each of the constant-speed runs.

To give a little analogy for, perhaps, understanding:  As you increase grade, a common occurrence in ordinary running as stated, you increase the trailing resistance, all of which still has to be overcome by... piston thrust as expressed through the rod geometry.  This clearly implies higher piston thrust per stroke, and since piston thrust is generated via steam pressure, there should be no surprise that more steam is needed at a given starting pressure, or less expansion will result for a given expansion volume, with what amounts to higher back pressure on the piston.  Not incidentally, this is going to involve both longer admission under constant pressure and a higher mass per stroke during expansion, but it may not be immediately clear that the higher mass during expansion is also going to have its pressure drop (assuming no thermal loss for a moment, for an attempt at theoretical clarity) reduced, and this means that your steam is not doing 'all the work its heat is capable of' AND that when it blows to exhaust there will be not only a higher mass to be exhausted, but it starts at a higher pressure... and both the mass and the heat you blow away each stroke has to be made up from somewhere.  Can you now trace where that 'somewhere' is going to be, back up the steam circuit, and appreciate what has to be done to the water to support it?

 

dehusman -- I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.

... but there is also a reservoir of steam in the boiler to handle short term changes in steam consumption.

. There is absolutely nothing the fireman can do to affect short-term steam generation, and very little he can do to affect short-term firing rate outside what draft permits.  All he can do, as has been said over and over and over by now, is learn where the load changes are likely to be, and start building or reducing appropriately so that steam neither 'runs shy' or 'blows out' during operation.  And as has been said over and over and over, this is a learned skill and requires both attentiveness and discipline.  It also happens to be very difficult to automate unless you have a much more complicated -- and nondeterministic -- control system and algorithms; even with vast changes in the cost of various components, and assuming zero capital cost in development and programming, even if you were given solid-fuel free you'll never earn back the cost for an engine in commodity service.  (Even the crude method of 'automatic' cutoff control built in 1922, wonderful though it was as a technical accomplishment, saw any further practical implementation...)

Abrupt reductions in the need for steam (i.e. consumption) can result in increasing boiler pressure and waste of steam thru pop-off valves again, the first chapter of Firing a Steam Locomotive is "Cooperation" and as Ed has noted, the fireman needs to be familiar with the route and anticipate the need for more/less steam

This is true for such obvious reasons I don't see why it keeps being brought up in this connection.  Look at the fire in a train ascending a steep grade.  The exhaust jet is dense and powerful, and it excites tremendous white luminous flame from the fuel bed (which is being replenished 'suitably' -- we won't go into the details of how, precisely, because the point is that the train IS climbing and not slowing down, and therefore IS having heat input balanced with steam extraction.  Now, suddenly and without warning, the throttle is slammed closed or the reverser brought toward center.  The draft reduces, so the fire isn't drawing as much primary and secondary air and the combustion plume may be slowing down dramatically, but the substantial bed on the grates is still very hot, and evolving hot incompletely-burned combustion products, and there will be some 'afterheat' in the radiant and convection sections that does not decrease immediately.  That will keep the boiler steam generation -- which, remember, was very high to produce the necessary steam mass flow at pressure to climb the grade -- high for a while.  If you were on the ragged edge of popping off, you should expect that the resulting pressure excursion might easily cause the safeties to lift... the logical response, as repeatedly stated, being to increase feedwater and perhaps start an injector, to get the boiler temperature down by the amount the 'afterheat' is putting in.

Now it is important to realize that ONLY this afterheat is the issue here.  When you closed the throttle there was not some enormous reflected pulse of pressure back up into the boiler.  All that happened was that steam was no longer following its desire to expand down to the chests and into the cylinders, and any resonant effects that occur when the throttle is closed will die out within a fraction of a stroke.  Were the fire to be cut off completely (for axample via an oil burner with continuous flameholding) the boiler would be happy holding at the grade-climbing pressure without further difficulty.  (There will be awful problems with differential thermal expansion if you have an idiot who actually tries this, but that's a different discussion entirely.)

I think until there's a better understanding of the principles of external-combustion engines using two-phase working fluid, we're just going to keep going round and round jiggering with the variables.

[gregc]

gmpullman

On another note, this explanation of the function of the steam locomotive valve, including a good explanation of lap and lead, is worth studying in light of the dialog Mr. Overmod provided above

Ed, the video describes the "general" function of the valve gear and cylinder valves which i believe i sufficiently understand.  The video doesn't address my latest question about how to calculate the pressure in the steam chest and piping during operation with reduced cutoff to determine steam flow (lb/hr) vs cutoff

Doughless

But if the top speed of the loco is significantly higher than 20 mph ... under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well. 

any train needs to operate at various speeds and on various grades requiring different amounts of tractive effort as the lab chart shows examples for

the following shows the tractive effort and corresponding cylinder pressure vs mph and grade.   Note that there is insufficient boiler pressure at higher speeds as grade increases

tePsi: 5000 T, max boiler PSI 220
     --- 0.0 --- --- 0.5 --- --- 1.0 --- --- 1.5 ---
 mph      TE psi      TE psi      TE psi      TE psi
   5   21261  25   71261  86  121261 147  171261 208
  10   22841  27   72841  88  122841 149  172841 210
  20   27298  33   77298  93  127298 154  177298 215
  30   34287  41   84287 102  134287 163  184287 224
  40   45248  55   95248 115  145248 176  195248 237
  50   62438  75  112438 136  162438 197  212438 258
  60   89398 108  139398 169  189398 230  239398 291

 

dehusman

I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.

but there is also a reservoir of steam in the boiler to handle short term changes in steam consumption.  Abrupt reductions in the need for steam (i.e. consumption) can result in increasing boiler pressure and waste of steam thru pop-off valves

again, the first chapter of Firing a Steam Locomotive is "Cooperation" and as Ed has noted, the fireman needs to be familiar with the route and anticpate the need for more/less steam

5414

[dehusman]

Doughless

However, at that point, your Fireman is effectively acting like the accelerator pedal in a car.  The faster he shovels, the faster the loco goes.

I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.

The fireman adds fuel to the firebox.  It takes time to get an additional layer of coal into the firebox, even with a stoker.  It takes time for the coals to ignite and begin burning (probably a very short time, but longer than the time it takes for fuel in a gasoline engine to be admitted and ignited).  Then there is the time it takes for the hot gases to heat the water to increase the temperature of the water.  Probably a relatively short time, but still not instananeous.  

Once the fireman stops adding coal to fire, it's not like taking your foot off the accelerator because all that coal that was added continues to burn and heat the water in the boiler for many minutes after the fireman stops adding coal.

The fuel of a steam engine is steam.  All the discussion about what happens in the pipes and cylinders is independent of what's going on in the firebox.  It could be wood fired, coal fired, oil fired.  Doesn't change a thing about what is happening on the front end of the engine.  The fireman is in charge of making sure there is adequate steam.  He can increase the temperature and pressure in the boiler by adding more fuel, but there is a lag in how long that takes.  The fireman has to "step on the accelerator" well before the engineer wants to accelerate and has to take his "foot off the gas" well before the demand decreases.  If the engineer wants to accelerate at mp 10, then the fireman had better be shoveling coal back at mp 8 or 9.

 

[Doughless]

gregc

 

 

Doughless

If a loco is going at only 20 mph, it would likely be at only partial throttle

 

 

please look at the plot from the tables above 10/14 5:09AM for throttle full (not plotted), 4 speeds: 10, 20, 30, 40 mph (green), relatively constant boiler pressure (white), various tractive efforts (orange) and cutoff (violet)

 

 

 

I didn't mean to suggest that the throttle could not be at full when the loco is going only 20 mph.  It certainly can.  But if the top speed of the loco is significantly higher than 20 mph (you're suggesting at least 50% higher) under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well. 

Maybe relying upon the fireman to stoke coal at a faster pace is how crews typically accelerated the train.  But I think there were probably better ways to manage the steam as they traversed a familiar route.