gregcnot many of you seem to think that a thicker coal bed is needed to increase the rate of steam produced by the boiler
despite that test data table showing the throttle full at various speeds that presumably need optimal performance, i'm trying to understand why an experienced engineers would operate the engine differently (not optimally)
I'll guess the reason an engineer operates with the throttle partially closed is to be able to quickly increase the amount of steam (lb/sec) entering the cylinders.
a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would.
Steam is not 'drawn' anywhere -- it is PRESSURIZED and flows from higher pressure to lower. Angus Sinclair repeatedly laughed at various Gilderfluke types whose patent drawings neatly indicated how their steam flow would go... the problem being that no one told the steam that, let alone ordered it to. And even were you to ask politely... it goes where thermodynamics says it goes.
The entire steam path from the dome, through the dry pipe, to the superheater header and elements, through the front-end throttle and down the pipes to the steam chests are all part of the steam supply -- there may be some 'pressure drops' from saturation pressure along the way, but remember that we try to minimize them wherever economically justifiable. ONLY the steam volume and pressure in the chests has any real bearing on each admission event, with the enormous ¾ of each revolution time serving to allow steam mass to 'repressurize' the chest. (As noted we cheat and use unavoidable compression pressure to get rid of dead-space effects past the valves, but that isn't part of how steam locomotives go faster until you get really whacking fast...)
(my understanding of a throttle is that it doesn't give you some fraction of what is availble, but allows some flow (lb/hr) depending on its setting.
regardless of the throttle setting, the maximum amount of steam that can pass through it is limited by how much steam is produced by the boiler. in other words, the throttle will only allow so much steam (lb/hr), it can't pass more than is available)
Yes, if you slam the throttle closed, or quickly wind the reverse to mid, your boiler was generating a particular mass flow of steam that now has 'nowhere to go'. You very quickly quench the overpressure that results with a little judicious injection -- and it is a relatively LITTLE mass flow of liquid to produce a large nominal pressure reduction. This is simply the inverse of 'trading water for steam' for our purposes here. Naturally, as I keep saying, an alert fireman will 'plan ahead' and have his fire and water regulated accordingly for upcoming 'conditions'.
So while the throttle controls steam flow, it dictates the difference between boiler and cylinder pressure once equilibrium is established as speed stabilizes. If boiler pressure is maintained at some value by the fireman, then it determines the cylinder pressure. not seeing how the throttle can restrict the flow to anything less than the steam production by the boiler without the boiler pressure rising
not seeing how the throttle can restrict the flow to anything less than the steam production by the boiler without the boiler pressure rising
Yes, even though what happens with steam generation is not at all according to 'general gas laws' were you to start closing the throttle or centering the reverse, the engine is drawing less steam, expanding less steam, doing less work, and starting to have fewer strokes per minute. Were you dumb enough not to reduce the fire or limit saturated water temperature, you would indeed start generating more steam than being used, and if you were operating right up on pop pressure, one or perhaps more might actuate. This is neither inevitable nor desirable in a real-world operating locomotive run by the experienced, however. And believe me, it is but the work of several moments to ensure that the pops won't lift, unless you conduct a crash stop from 80mph upgrade without warning. That happens seldom, so you should not assume it always does.
a locomotive running without a train (no tonnage) requires little boiler pressure and/or a slightly open throttle
In practice, you'll crack the throttle slightly more than 'required' to give better low-speed response and eliminate torque peakiness. But your higher effective water rate won't matter that much for casual drifting or even light-engine moves. The problem for modern engines (see the discussions about air horns on steam locomotives) is akin to the issue with whistles or crude venting of open feedwater heaters. Blowing the whistle for one second at ~275psi loses about six pounds of steam -- which was six pounds of water first brought to saturation pressure and then converted with an enormous addition of latent heat of vaporization into actual steam. You lose all that mass, all that heat, and have to dose more water treatment for something that could have been done with a few diaphragms vibrated by brake air...
when the throttle is opened further, the steam flow (lb/s) increases into the cylinders and from the boiler resulting in the boiler pressure dropping. The boiler pressure is restored when the fireman adds coal.
i'm suggesting a higher rate of coal (lb/hr) is being added by the fireman as the speed increases.
Perhaps it would not be necessary to mention that it is MUCH easier to lose boiler pressure via injection than it is to increase it through firing. But I'll say it anyway to stave off any future misunderstanding of the importance of that point...
Something else that may be valuable to consider here is how experienced enginemen were observed to fun large American locomotives in practice. I thought by now we'd have seen at least one YouTube video showing, for example, the NYC Hudson engineer at work, with an experienced engineman explaining why he is working the controls as we see him doing, looking at what he's looking at, and what kind of power increase he is commanding (for a smooth start of what may be a very fast Pullman train).
But also keep in mind that, while the NYC engineer is a seasoned 'at the top of his game' he may have only months or even days actual experience on a Hudson -- before this, he ran Pacifics of various vintage, Atlantics before that, and 4-4-0s before that, all probably with dome throttles of traditional (non-Wagner) construction through expedient-at-best mechanical linkages. In some cases I'd expect his knowledge of 'cutoff control' to have started before the era of Stephenson riding cutoff, something that (very unfortunately!) disappeared as outside radial gear and piston valves became more common.
We might gainfully recall, in this context, why PRR engineers were so often slipping the T1s. I consider it a mistake that there weren't separate throttle manifolds for each 'engine' on a T1, but even so, trying to control 300psi steam with a lever over your shoulder is not exactly a candidate for precision haptics. You will note that one of the 'secret sauces' for successfully starting a T1 involved specifically using partially-closed throttle until the engine had successfully 'found its feet' (or stayed stalled) and only then starting to wind to the very short precise cutoff that the Franklin System could provide. But by no more than 30-35mph you'd have the throttle all the way out regardless of the road speed, and slip prevention/traction control would have had to be 'by other means' to be at all effective...
Douglas, thanks for one of the more thoughful posts
Doughlessyou said that throttle was wide open but boiler pressure was NEAR maximum. I would say that to accelerate, you have to increase BP to maximum and expect that the throttle is engineered to deliver that much steam to the cylinders.
thats what i was thinking, why i specifically said near, but ..
DoughlessMy question would be, can you get more steam to the cylinders when the throttle is wide open?
... in retrospect (i'm sincerely trying to figure things out) i don't believe a locomotive capable of 80 mph would operate at 20 mph with boiler pressure near max. it may be possible with the throttle set very low (but unnecessarily inefficent)``
if the throttle were full, the boiler pressure would need to be much lower, 35 psi. And i believe the fire would be relatively small to maintain conditions
my understanding of a steam engine throttle is that it is simply an orifice of some adjustable size. it doesn't restrict the flow as a percentage of its setting. it's not like the butterfly throttle on a car that restricts an unlimited amount of air into the engine
the size determines how much steam can flow (lb/sec) depending on the difference in pressure across itg. it can't pass more than what is available. so the flow may be maximum at 50% throttle and opening any further that 50% doesn't increase the flow.
the pressure difference means that it may pass 20 lb/s with a pressure difference of 50 psi when set to 80 %, but will pass 20 lb/s with a pressure differnce of 30 at 100%.
DoughlessI'm looking at steam as being the fuel....ultimately being the product of water heated by coal. The steam is what pushes the cylinders, and the throttle is what determines how much steam goes to the cylinders
i understand. i can see the analogy to a car (carburator). but steam is a transmitter of energy, like brake fluid or the siderods of the engine. not the fuel that generated that energy
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greg - Philadelphia & Reading / Reading
gregc a steam locomotive is traveling 20 mph on and approaching level grade throttle is open full cutoff is reduced to the minimum brakes are off boiler pressure is being maintained near maximum coal is being added to maintain the fire and boiler pressure what needs to happen to increase and maintain speed at 30 mph?
a steam locomotive is traveling 20 mph on and approaching level grade
what needs to happen to increase and maintain speed at 30 mph?
Getting back to this, I'm going to ask you or the forum for help. I don't know the answers to these related questions. Keep in mind I'm a layman with little knowledge of thermodynamic physics or steam loco terminology:
Can a steam loco, as typically designed and built, build to maximum boiler pressure when the throttle is wide open...in any reasonable period of time?
If a throttle is designed to relieve the maximum volume of steam that can be maintained in the boiler (via cubic pounds per second?), wouldn't you have to close the throttle a bit in order to fully build maximum pressure over some time period?
(The leak in the boiler is too big to build pressure...make the hole smaller)
Is steam produced at a faster rate (build pressure) than what is dispensed through a wide-open throttle (reducing pressure)?
(despite the big hole in the boiler, it can still build pressure over time)
What I'm getting at is, that it seems likely that the crew will have to build maximum pressure in the boiler to reach maximum speed from a wide open throttle, BEFORE the crew calls for maximum throttle. The crew can maintain maximum pressure when the throttle is wide open, but it may not be able to increase to maximum boiler pressure with a wide open throttle...in any practical useable time period.
- Douglas
A USRA Mikado ran at 200 psi in the boiler. If you weren't at or near that psi, the engine couldn't pull it's train. So the fireman would want to stoke the fire so the boiler pressure would be that high (or at least close to it) while the engine is standing still, so the power is there for the engineer when he opens the throttle to start the train. From a cold start, it could take hours to get an engine fully up to steam and ready to go.
Having steam in the boiler isn't wasteful or anything, it's there waiting to be used. The fire in the firebox has to be kept up; if the heat in the firebox goes down, some of the steam will cool back into water. Boiler pressure will drop, and the train may come to a stop.
"Boiler pressure" can be anything from the minimum to run the auxiliaries effectively (50 to 75psi for the air pumps for example) up to the set safety-valve pressure. It was apparently not uncommon for crews to 'sliding-pressure fire' a larger locomotive to keep fuel use proportional -- a Niagara, for example, could be run at 180psi to do the work of a 2-8-0 on a 2-8-0's amount of coal and water.
Where you need 'full boiler pressure' is when starting a train with assigned resistance very close to what the engine can produce. Many times trains would be made up to take advantage of calculated engine power (at 80% rated pressure or whatever) but improperly use variables in the Davis formula or fail to recognize problems with maintenance, crappy coal, student firing (etc.) which make getting every ounce of achievable starting TE to 'start any train it can pull'
DoughlessKeep in mind I'm a layman with little knowledge of thermodynamic physics or steam loco terminology:
i'm no expert either, just trying to figure it out
DoughlessCan a steam loco, as typically designed and built, build to maximum boiler pressure when the throttle is wide open...in any reasonable period of time?
if production exceeds consumption, boiler pressure will rise. and my contention is that production depends on the strength of the fire, the # of BTUs heating the boiler.
i think consumption primarily depends on speed, the # of cylinder volumes that need to be filled.
as Overmod has said and the table below shows that normal operation can be with full throttle, but i don't think(?) the table values are near max boiler pressure
Doughless The crew can maintain maximum pressure when the throttle is wide open, but it may not be able to increase to maximum boiler pressure with a wide open throttle...in any practical useable time period.
not sure why you're asking specifically about max pressure. the table shows that various speeds can be maintained with full throttle.
i believe maintaining boiler pressure means consumption equals production. and that by increasing the strength of the fire, production increases raising pressure in both the boiler and cylinder, increasing tractive effort acceleration and speed, until the new consumption rate equals the new production rate.
i'm thinking, if the fireman increases the fire, boiler pressure goes up, and then as speed increases, drops back to what it was. All the while, the fireman has increased the rate of coal added to the fire (or opened the oil valve a little).
Increasing the fire more, results in further increase in speed, and again boiler pressure returns to what it was (see table)
boiler pressure remains constant when steam production equals consumption
the speed can be increased without adjusting throttle or cutoff by adjusting BTU production.
Douglas, is this making sense to you?
(ya gotta grok it)
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Overmod gregc a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would. Read that sentence over a couple of times. Do you see how dumb its assumption is?
gregc a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would.
Read that sentence over a couple of times. Do you see how dumb its assumption is?
my understanding is the flow thru the throttle depends on the pressure difference. So a more closed throttle requires a bigger pressure difference than a less closed throttle.
for example, for a boiler pressure of 220, a cylinder pressure of 200 has the same flow (lb/s) at 100% throttle as a cylinder pressure of 192 at 80% throttle
Evening
Don't Know, but probably should.
We could ask Mr Weatherby, ...but He might be just as Confused as You are?
Pic taken by TF
Or myself
TF
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
If anyone is still following along perhaps a background in Firing 101 might be helpful to some.
[edit] Looks like Bear had the same thoughts just a few minutes before me
Here is an album in Flickr where I reproduced a New York Central booklet pertaining to the subject.
https://www.flickr.com/photos/gmpullman/albums/72177720301339959/with/52289280349/
If anyone has trouble seeing the pages I can reproduce them here if needed.
NYC_Fire by Edmund, on Flickr
Note the amount of instruction devoted to the variables in the coal as a fuel. I attempted to allude to this in an earlier post.
Regards, Ed
gmpullman[edit] Looks like Bear had the same thoughts just a few minutes before me
perhaps the following will reveal some of the mystery of steam engines that i'm trying to figure out
the table shows the required tractive effort and cylinder pressure for a 5000 T freight at various speeds. Note that the cylinder pressures are far below the max boiler pressure of 220 PSI even at 80 mph
mph TE psi 0 20000 24 10 22841 27 20 27298 33 30 34287 41 40 45248 55 50 62438 75 60 89398 108
cylinder pressure is related to tractive effort by
boiler pressure remains stable when steam production equals steam consumption. boiler pressure can be maintained at various levels (PSI) as long as it is greater+ than the required cylinder pressure.
boiler pressure can be much higher than cylinder pressure because of the balance between steam production and consumption, the creation of steam in the boiler and the outflow into the cylinders. steam production is maintained by the fireman by maintaining boiler pressure.
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just in case anyone is questioning why a locomotive would be designed to require such low pressuress, here are similar values for the same train going up a 1.5% grade. clearly not possible much above 10 mph
mph TE psi 0 170000 206 10 172841 210 20 177298 215 30 184287 224 40 195248 237 50 212438 258 60 239398 291
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gregcperhaps the following will reveal some of the mystery of steam engines that i'm trying to figure out the table shows the required tractive effort and cylinder pressure for a 5000 T freight at various speeds. Note that the cylinder pressures are far below the max boiler pressure of 220 PSI even at 80 mph.
On a 1.5% grade, you are doing lifting work rather than just accelerating mass with low frictional resistance. Accordingly your required MEP is much larger. Again, what the boiler produces is a bound on this, but otherwise irrelevant.
boiler pressure remains stable when steam production equals steam consumption.
boiler pressure can be maintained at various levels (PSI) as long as it is greater+ than the required cylinder pressure.
boiler pressure can be much higher than cylinder pressure because of the balance between steam production and consumption, the creation of steam in the boiler and the outflow into the cylinders.
The 'balance between steam production and consumption' only comes in when the mass flow required (by the chosen valve-gear setting) exceeds what the overcritical water in the boiler can 'source'. And you deal with it by firing the boiler to restore the gauge pressure. Or stopping to allow the boiler to 'recover' pressure before proceeding. Those things have nothing to do, by themselves, with what 'made the steam locomotive go faster'. Only the steam did that.
Steam production is maintained by the fireman by maintaining boiler pressure.
OvermodWell, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate,
i don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal.
and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder
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gregc Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate... I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal. and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder
Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate...
I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal.
With the intent of having sufficient steam that you can use the valve gear to admit the mass flow necessary.
(I am purposely avoiding the whole can of worms about what happens to the steam after it has gotten done expanding in a stroke, because God knows what misconceptions about that would result... but it does matter, and we could take it up in its own thread.)
gregcif production exceeds consumption, boiler pressure will rise.
Yes, it makes sense. Thank you.
What I quoted above seems to be the crux of the issue. But I don't think I can find the solution within your response. Apologies if I couldn't figure it out.
Try to answer these questions simply.
Does boiler pressure have to increase in order to accelerate from 20 to 30 mph when the throtte is wide open, or can something other than BP create acceleration in that situation?
Can boiler pressure even increase when a throttle is wide open? If so, how long does it take? Seconds, minutes, hours?
Overmod gregc Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate... I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal. and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder The point is that it doesn't matter to the engine's physical performance (with respect to the question you originally posed) whether 'steam consumption' balances 'steam generation' from fuel consumption or not. Mass flow through the cylinders has been the accepted 'measure' determining engine power for over a century now.
The point is that it doesn't matter to the engine's physical performance (with respect to the question you originally posed) whether 'steam consumption' balances 'steam generation' from fuel consumption or not. Mass flow through the cylinders has been the accepted 'measure' determining engine power for over a century now.
what do you think i mean by flow (lb/s)? it is the "mass flow" you're referring to
and how can it not matter? if more steam is produced than consumed, pressure builds up and is wasted. if less steam is produced that needed, pressure drops until there is insufficient pressure in the cylinders and the train slows
Overmod EXCEPT to maintain some average, economically-determined pressure at the gauge,
and what might the "economic" pressure value be?
if the flow (lb/s) into the cylinders is the same at regardless of boiler pressure, why does it matter as long as it is sufficient to maintain the flow into the cylinder
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DoughlessDoes boiler pressure have to increase in order to accelerate from 20 to 30 mph when the throtte is wide open, or can something other than BP create acceleration in that situation
this is my understaning ...
if the engine is already being run optimally, i don't see how boiler pressure doesn't need to rise. (how much: 1, 2, 5, 10 psi)
on pg 1, Overmod suggested that cutoff can be increased, allowing more steam into the cylinder and out the exhaust stack, increasing draft and "brightening" the fire, increasing the BTU output of the fire increasing steam production.
but burning the coal at a faster rate requires replenishing it a faster rate (i.e. coal consumption increases)
once again, this table shows various speeds 10, 20, 30, 40 with full throttle, a variety of cutoff and boiler pressures in the range of 190-199 at all speeds. (it would be great if someone could explain how the conditions in the table were achieved)
i'm fairly confident that increasing boiler pressure means steam production is greater than consumption and should result in an increasing consumption rate until it equals production.
maybe (???) the amount consumption changes depends on how long boiler pressure is above or below some "maintained" pressure (???).
i'm guessing it would take a few minutes after increasing the rate of fuel for BTU/hr to increase, but have no data.
Douglas, thanks. it's taken me an hour+ plus to write this as i thought about the issues
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gregci'm guessing it would take a few minutes after increasing the rate of fuel for BTU/hr to increase, but have no data.
Yet I offered the OP data. Maybe I'm not quite conversant with what his particular goals are but I thought coal consumption vs. boiler efficiency was rather well covered in this ASME discussion:
https://asmedigitalcollection.asme.org/fluidsengineering/article-pdf/62/5/410/6988950/399_1.pdf
The OP didn't seem to agree with me.
I invite anyone following this thread to read the 21 page discussion of the report and please comment on the pertinance of the findings related to our discussion here.
My position is that fire bed thickness does not equate to the relation of volitale gases distilling and transfering heat to boiler evaporation surfaces. In fact during some tests it was shown that a greater temperature was recorded in the smokebox showing (to me, anyway) that unburned gases were escaping without communicating the heat generated to the evaporative surfaces.
gmpullmanI thought coal consumption vs boiler efficiency was rather well covered in this ASME discussion:
my question is not about efficiency. it is what needs to happen to increase the speed of a train.
I am convinced the # of BTU/hr needs to increase when operated optimally and this happens with an increase in fuel whether it be coal (lb/hr) or oil (gal/hr). But it's not clear exactly what happens (i'm curious about increasing cutoff)
I'm looking for ballpark numbers and have found that the max burn rate of coal is around 150 lb/hr per sq.ft of grate area and that there's around 11000 BTU/lb of coal. i know this should be different for anthracite vs bituminous.
these are design values that I'm interested in. They could be off by 50%. And of course not every chunk of coal is the same. My question isn't concerned with the mix of coal, bituminous/anthracite, or coal quality, whether the last shovel full has more/less BTU than the previous or whether it ignites faster or takes longer to burn
gmpullmanI have never once witnessed a situation where the engineer tells the fireman to lower his boiler pressure because he wants to slow the train down.
i'm curious about what happens in a cab when a change in tractive force is required.
does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
does the fireman need to build up the fire when leaving a station? how and when does he know to do this?
does the fireman need to build up the fire when approaching a grade? how and when does he know to do this?
can the fireman allow the fire to die down when cresting a grade and possibly going down hill? how and when ... ?
does the fireman need to build up the fire when reaching the bottom of a downhill grade? how and when ... ?
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gmpullman... Maybe I'm not quite conversant with what his particular goals are but I thought coal consumption vs boiler efficiency was rather well covered in this ASME discussion: https://asmedigitalcollection.asme.org/fluidsengineering/article-pdf/62/5/410/6988950/399_1.pdf
There are discussions, including in handbooks for engine crews, discussing the sequence of events and approximate times involved in what happens after the coal arrives at the distributor plate or is hand-bombed to various positions. The coal is heated from the outside surface by a combination of radiation and convection; hydrocarbons and carbon vapor are expelled from the surface; the lump can start swelling and breaking, increasing the surface area for vaporization; eventually the luminous carbon under the action of the draft produces the combustion plume, in which is both glowing carbon and levitated bits of still-burning fuel.
In a large modern locomotive, the heating of the surface can occur almost immediately, much of it while the fuel is still falling through the evolving gas plume. As the fuel reaches the bed, there is additional heat transfer through conduction from the burning fuel already there. However, it is important to remember that admitted ixygen proportion is intentionally less than 'stoich' for the fuel -- the conditions in the firebox are both partial vacuum and reducing atmosphere. So drawing analogies from internal-combustion lambda and whatnot are specious even before taking up the radically different nature of internal-combustion gas engines vs. external-combustion engines.
My position is that firebed thickness does not equate to the relation of volatile gases distilling and transfering heat to boiler evaporation surfaces.
In GPCS, the bed is intentionally thick because combustion there is 'retorted' -- air plus steam is used to react with the carbon and hydrocarbons in the fuel to produce clean volatile gases, which are then burned nearly entirely above the firebed -- no sparks, no ash carryover, no sooting, and no front-end mass losses or exhaust ignition.etc. But that is a chemical, and not a thermodynamic, concern.
In fact during some tests it was shown that a greater temperature was recorded in the smokebox showing (to me, anyway) that unburned gases were escaping without communicating the heat generated to the evaporative surfaces.
As a perhaps interesting aside: Chapelon (and then Porta) proposed designing a 'sectional boiler', in which the forward end of the tubes and flues, where heat transfer becomes lower, is used as part of the feedwater-heater system rather than for evolving saturated steam. As part of the ESC work on the feasibility plan, the actual 'optimal' length of this section for the 300psi T1 boiler was calculated -- it would be only about 3'4" long...
gregcwhat do you think I mean by flow (lb/s)?
it is the "mass flow" you're referring to and how can it not matter?
The boiler heat release is adjusted to compensate for the steam mass flow that accomplishes the actual work. You seem to think that this is like heat release in IC engines, where the fuel burn is directly proportional within milliseconds to the MEP in the cylinder. THAT IS NOT TRUE ON A RECIPROCATING STEAM LOCOMOTIVE.
If more steam is produced than consumed, pressure builds up and is wasted.
if less steam is produced that needed, pressure drops until there is insufficient pressure in the cylinders and the train slows...
[quote][quote user= "Overmod"] ...EXCEPT to maintain some average, economically-determined pressure at the gauge...[/quote] and what might the "economic" pressure value be?[/quote]Determined by the operating railroad, in part through experience and empirical measurement. For example, going up a severe grade the engine might have to be heavily fired, but as it tops the grade the water level shifts forward in the barrel and the crown depth decreases. That indicates that reduction of the firing rate as the engine approaches the crest be conducted (to avoid waste) and this in turn might cause the gauge pressure to fall considerably from safety-valve pressure. You will trade a little cutoff for expansion economy by using higher mass flow at the lower pressure for your drawbar TE. You use more water, which again is an economic concern for the operating entity to assess.
The "flow into the cylinders" is NOT THE SAME REGARDLESS OF BOILER PRESSURE. Obviously (at least to me) if you want more power out of the engine, you need more mass flow into the cylinders at lower pressure. THAT IS DONE BY ADJUSTING THE CUTOFF.
Meanwhile, "sufficient flow into the cylinders" is again a function solely of cutoff, and it implicitly is completed "to produce desired drawbar pull at speed, or for a desired acceleration rate".
The boiler is fired to keep it at or near a desired pressure, which need not be up there where the safeties pop all the damn time. You fire the boiler to make steam available for use, that steam being metered into the cylinders by the valve gear to make best use of its expansion (and then as quickly and expediently removed from the cylinder as possible, which as I said is a completely 'other' discussion in relation to the question).
You can overcomplicate this with meaningless discussions of coal rank and heat release, which were usually known only in empirical terms through firing experience anyway. You could effectively fire a locomotive with Egyptian mummies -- in fact, locomotives supposedly have been. What matters is whether the boiler has been fired in the most economical (or easiest, for those who don't really track efficiency correctly) way that provides steam at your experience-determined pressure. This has little to do with the somewhat artificial test train's acceleration in this question, but that's more because you used arbitrary quantities in framing that question in the first place.
don't have time to read and decypher you post
OvermodThen why do you keep bibbling about BTU release and sliding boiler pressure being factors directly influencing short-term acceleration?
sounds like you're iterpreting my "flow (lb/s)" as lb coal /s when i thought it was clear i meant lb steam / sec
while you can't ignore short term acceleration, i'm asking about the conditions at the higher speed
acceleration (F=ma) and force equal to train resistance depend on tractive effort
tractive effort depends cylinder pressure
cylinder pressure depends on the steam density in the cylinder
steam flow (lb steam/hr) must equal steam production otherwise boiler pressure changes
steam production depends on # BTU/hr heating water
# BTU / hr depends on lb coal /hr
Stop changing the game if you want an answer to what your question asked.
ACCELERATION of the train from 20 to 30mph -- under the conditions as you stated them -- is done by lengthening the cutoff. You already have the throttle fully open, and of course nothing would be gained by trying to open it further, so there's nothing else 'steam-related' but lengthening cutoff. Most of the additional steam mass that is required to accelerate the train comes out of the enormous reservoir of heat that is the overcritical water -- it does NOT immediately require proportional additional fuel firing, let alone precise and equal additional fuel firing to just balance uptake from combustion gas with saturated-steam release.
Once you get to 30mph, you no longer need to accelerate, unless by steady-state you mean continued acceleration to the maximum speed the engine's construction will permit. Which wasn't the question, although I can discuss that situation some in a different thread. So at 30mph you SHORTEN the cutoff back to the mass flow that just keeps the train rolling against resistance at 30mph. Note that if you were firing to match the instantaneous flow for acceleration, you'd now have 'too much fire'... which, as noted ad nauseam by this point in the thread, you can't immediately relieve.
Now, most of what seems to be troubling you concerns what happens if the train now continues at 30mph, throttle still full, still on level ground. The additional resistance can be calculated for this (and as noted from some of the earlier misapplied data, it corresponds to surprisingly little MEP at the cylinders) but TECHNICALLY it does imply a larger long-term heat input from fuel consumption, and therefore more fuel would have to be fired.
But the fuel firing is 'on average', and it might be no more than the rough changes produced by adjusting the feed and distribution valves on the stoker. As I keep noting, while a good crew will anticipate what steam demand will be (including under the all-too-common conditions where the engine is in far from good maintenance, cf. for example the PRR fireman's story about running a M1 4-8-2 with a repeatedly recalcitrant feedwater-heater pump) what they do to address it is simply keep the boiler pressure in some range where cutoff can control cylinder power (either with lower physical mass at higher gauge pressure, or larger mass at a lower pressure). Even at surprisingly little pressure, the engine may make its 'rated' power... this being determined more by the physical flow arrangement of the valves than anything in the boiler. What suffers most is the water rate under those conditions.
You are nowhere near the kind of conditions that would involve 'forcing the boiler', keeping a larger heel, having to run the stoker continuously while jiggering the valves to change distribution, etc. In all probability, to run at 30mph you'd adjust a couple of stoker valves a hair (not quite a radio RCH, but with the same sort of meaning!) by experience, and along you'd go. Actually overthinking what you have to do to get the trick to work isn't something necessary on any well-designed locomotive.
Unless, of course, you have the misfortune to be on a railcar or Sentinel or Besler/Doble with some kind of once-through tapered monotube. These of course were almost never fired on solid fuel (and the ones that were... weren't around for very long) and with those you do have to juggle fuel feed and combustion-plume conditions very carefully, within no more than a second or two after steam-demand changes. Doble in particular worked out an ingenious set of firing and injection controls that could actually do this, and scaled the approach up to about 800-900hp (perhaps more in Germany, but it's difficult to find full technical details of that). As a design exercise, you could scale one of these larger, but it would NOT be a locomotive that most seniority-based crews could even begin to run effectively -- let alone manage if any of the automatics malfunctioned or broke.
Overmod a larger long-term heat input from fuel consumption, and therefore more fuel would have to be fired.
seems significant, but burried in 678 words of text
OvermodStop changing the game if you want an answer to what your question asked.
Mike
gregcgmpullman I have never once witnessed a situation where the engineer tells the fireman to lower his boiler pressure because he wants to slow the train down. i'm curious about what happens in a cab when a change in tractive force is required. does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
i'm curious about what happens in a cab when a change in tractive force is required. does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
In a previouis email reply to you I answered all of these questions.
How does the fireman know when to 'do this' comes from months of learning the route and learning the 'running style' of each of the engineers PLUS the operating variables inherent in different locomotives, even within the same class, along with variables in the grade of coal or water (yes, some water treatment fails at removing oxygen and impurities) in order to get the engine and train over the road.
gregcdoes the fireman need to build up the fire when leaving a station? how and when does he know to do this?
For the most part he's been firing this route for years. He knows where every station, junction, signal, curve, grade and tunnel is on his route. IF he is running on territory he's not familiar with, a situation I have encountered, there would sometimes be a pilot in the cab who would inform the fireman of upcoming conditions or events. The engineer would communicate to the fireman (That 'cooperation' chapter in the firing manuals) what he intends to do. Most engineers will allow the fireman to occupy the right-hand seat for a while during the trip in order to get the feel for running. Firemen are engineer trainees.
gregcdoes the fireman need to build up the fire when approaching a grade? how and when does he know to do this?
He KNOWS the route, he knows the locomotive, he knows the weight of the train, he knows his firing style and he knows how the engineer is going to attack the grade.
The word I used in my email reply to you was ANTICIPATION. The fireman has to anticipate conditions by at least fifteen minutes or more.
gregccan the fireman allow the fire to die down when cresting a grade and possibly going down hill? how and when ... ?
Short answer, yes. But before cresting the grade he has to be sure to have enough water in the glass so that the crown sheet doesn't get overheated when the water sloshes to the front of the boiler. Any braking effect down the grade will also contribute to your water disappearing from the glass. In spite of good feedwater heaters you still need to maintain enough fire to overcome the chilling effect of the introduction of feedwater.
gregcdoes the fireman need to build up the fire when reaching the bottom of a downhill grade? how and when ... ?
How depends on the present condition of the fire. With the engine drifting it is a good opportunity for the fireman to inspect his fire and fill thin spots or build up banks if needed. The when depends on present boiler pressure and train speed. Once the engine begins 'working' the fireman has to have the fire prepared (anticipation) as once he is into the grade precious time is lost to make any adjustments to the fire.
I had provided you with a scan of the entire chapter from the book Perfecting The American Steam Locomotive by J. Parker Lamb 'The Physics of Steam Power, which detailed boiler thermodynamics and your reply was 'I don't need any more operators manuals'.
I also suggested securing a copy of William L. Withuhn's excellent book American Steam Locomotives, Design and Development, 1880 — 1960.
gregcdon't have time to read and decypher you post
I'll stop here for fear of writing too much that can be read without unnecessary anxiety.
Cheers, Ed
For those still following I'd like to submit an example of one of the sources for Greg's often cited charts in prior posts. These were gleaned from laboratory tests of locomotives at the Pennsylvania Railroad's Locomotive Testing Plant.
Here is one example of a test report:
https://hdl.handle.net/2027/pst.000003544839
and another comparing an L1 class to the H:
https://hdl.handle.net/2027/pst.000003544815
Interesting reading and the PRR Test Lab certainly earned its role in locomotive development but one must keep in mind when studying these results that these are 'laboratory test bed' conditions and not real-world over-the-road train handling tests. For those there are dynamometer tests available such as this record of road testing the New York central Niagara:
https://nycshs.files.wordpress.com/2014/07/roadtestingniagaras.pdf
Thank you, Ed