how does a steam locomotive increase speed?

[Overmod]

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

It is, but the specific thing the OP appears to have been asking was not the sustained heat release for different firebed conditions, but the rate of additional heat release after the moment new fuel has been stoked -- which is complex and, in my opinion, dificult to obtain consistent precise data for.

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.

It does not; in fact, the thicker bed can restrict primary air and, in some conditions, add to ash glassing or clinkering.  The reason for establishing a thicker bed is to provide additional fuel mass, where that is desirable for sustained conditiomns of 'high fire', and to preclude holing of a lighter bed during conditions of strong draft or if poor fuel quality warrants.

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.

There is very often a considerable amount of unburned 'whatever' that makes it through the tubes and flues with temperature sufficient to drive reignition if it encounters sufficient oxygen.  Some of the physical energy in the exhaust results from this 'afterheat' re-expanding the entrained steam in the exhaust plume, but you will note that flaming fallout is still depressingly common in locomotives that have bounced their combustion gas around screens and various plate surfaces before mixing with ejected steam.  The whole phenomenon of the Franco-Crosti system hinges on recovering the considerable 'untransferred heat' in a typical boiler exhaust; good package boilers in fact recuperate down below the condensation point of the combustion water (which yields up the considerable latent heat of condensation back into the Rankine cycle) -- this didn't get done on locomotives in the past because they used cheap-as-possible steel for the heat exchangers and cheap-as-possible high sulfur coal fuel, and condensing sulfur compounds caused rapid and catastrophic corrosion if the gas temps fell below somewhere around 244F.

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... 

[gregc]

gmpullman

I 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

gmpullman

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?

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 ... ?

3928

[gmpullman]

gregc

i'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.

Regards, Ed

[gregc]

Doughless

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

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)

• acceleration requires increasing tractive force to increase speed
• increased tractive effort/cylinder pressure requires increased steam in the cylinder
• but increasing speed also increases steam consumption (# of cylinder volumes / sec)
• but at the desired higher speed, tractive effort can be reduced to match the higher train resistance and steam production can be reduced, but is higher than at the lower speed.

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

3816

[gregc]

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.

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

3786

[Doughless]

 

gregc

if 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.  That there isn't a 'flow meter' in the main steamline or header or whatever doesn't affect that you're not deeply concerned with what flows out of the boiler or boils up in it EXCEPT to maintain some average, economically-determined pressure at the gauge, or work to stay within a few psi of nominal both by strategic firing and feedwater injection.

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.)

[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

3701

[Overmod]

gregc

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.

Ah, what you're reading is the MEP that is needed to produce the desired effect in the cylinders, and of course neither the maximum pressure that the boiler can theoretically develop nor the actual boiler pressure at the time of observation have anything to do with that.

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.

Yes, but that's a tautology.  How the steam is being used in the cylinders to produce thrust is the thing that matters.  And there is far more 'heat to generate steam' in the boiler water than is passing into it from combustion gas running through the tubes and flues.  When you increase mass flow through the cylinders, much of the volume 'made up' comes out of the overcritical water in the barrel and legs as steam.  In your original example (no grade, acceleration from 20 to 30mph) he pressure falls  -- relatively slightly -- and the water temperature drops -- relatively slightly, and you do have to remediate that 'eventually', but the short-term action is all 'fireless cooker' (with steam-drying action through the elements, of course).       

boiler pressure can be maintained at various levels (PSI) as long as it is greater+ than the required cylinder pressure.

Another tautology.  You can sliding-pressure fire right down to where your air pumps barely run, and still hostle the engine or even run it under restricted conditions.  Note that the "boiler pressure" being maintained here is proportional to the 'steam chest' pressure, so it doesn't matter where the mechanical throttle is set to influence equilibrium between boiler and chest pressure after a (relatively short!) interval.  But this is an artificial situation with little applicability to actual running and none whatsoever to clearer understanding of the original question.

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.

Try as I might, I can't make anything out of what this sentence is supposed to be indicating.  Boiler pressure is higher because it's used as a 'reservoir' of working fluid for use in the cylinders.  Since any working fluid has to be metered through the valve gear before being used in the cylinders, it doesn't matter how high the boiler pressure is -- and as related ad nauseam so far, that is fixed (by law since 1911 in this country) by safety valves that can't be monkeyed with 'to get a few extra pounds when you want to'.

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.

Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate, and they are not cause-and-effect related that way.  "Steam production" is what the boiler can produce under given conditions.  In normal circumstances, the fireman acts to maximize economical steam production by firing, and using water judiciously, to hold the available steam pressure as high as designed.  When the engine is overloaded or stressed, the fireman may 'force the boiler' with uneconomical levels of firing to sustain a higher steam-generation rate, but that is scarcely the 'best' way to operate an economic asset.

[gregc]

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.

3559

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

3587

["JaBear"]

gmpullman

[edit] Looks like Bear had the same thoughts just a few minutes before me 

“Great minds think alike; rogues’ band together.”

 

An interesting read, Ed, apart from the woeful G class Garretts, mechanical stokers were not used on the NZR locomotives, though I believe that the grate area on the “modern” K, Ka and Kb classes was near at the limit of achievable hand firing.

So, I hadn’t considered how the coal was distributed over the grate when a mechanical stoker was used.

 

However, regardless of the skills (art) required by either method, the fireman’s’ main task was to keep steam pressure as close as possible to maximum pressure, without wasting it by blowing off the safeties, regardless of locomotive speed or weight on the drawbar.

 

As an aside, one of the firemen who crewed on various of the restored locomotives running on mainline excursions, told me that because of ingrained habit, he resented the drivers excessive use of the whistle, though while he acknowledged that it delighted the trackside railfans, the waste of steam still irked him! 

 

Cheers, the Bear.

[gmpullman]

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

["JaBear"]

I really need to take my time to read all the posts to this thread but my initial reaction after a very quick read, is that there is a fair bit of overthinking going on!

 

Disclaimer: The only steam locomotives that I got to “work” on were in long term static storage, so my work limited was to trying to keep them clean, (scrubbing off the bird crap then wiping over with oil), and to lubricate what we could.

However, it did bring me into contact with former NZR drivers who had come up through the system, starting as locomotive cleaners and, if worthy, ending up as fully fledged drivers. A couple of the gentlemen were still employed as drivers on the railway and still had valid steam tickets so they could drive restored steam locomotives on the mainline. By keeping my mouth shut and ears open I did learn a lot, though some of the information (?) could be classified as “Good Yarns”!

Unfortunately, I will probably unable to achieve my ambition to be able to fire a steam locomotive, the challenge of being able to actually a get a shovel full of coal in through the firebox door while on a swaying locomotive a speed, would be interesting to say the least. And I’m not including the ability of depositing the coal in the “correct” position on the grate, and multitude of other tasks required to keep the steam pressure at the maximum without “lifting” the safety valves.

As far as firing goes, the words, “often and light” springs to mind.

Also, that individual locomotives, even of the same class had their own particular quirks, to have your “own locomotive”, especially if she was a good one, was a big deal to a crew.

Lastly, sometimes “best practise” had to be ignored. I was told and have read that on a certain sections when the grade was against them and there were a series of tunnels in quick succession, it paid to have the safeties “howling” before the first tunnel was entered because the driver and fireman would be crouched on the floor with their overcoats wrapped around their heads trying to avoid asphyxiation!!

 

The following, (Click on them to enlarge), is from the “New Zealand Government Railways, Hand book of Instructions for the Guidance of Locomotive Staff” published 1960, though the pages featured were reprinted in 1967, given to me along with other handbooks and his spare drivers cap, by a former NZR driver, since passed.

Note: In 1967 the NZR still had a reasonable amount of steam locomotives in service; though they were definitely on their way out, the last scheduled steam service running 25 October 1971.

 

Firing 67 1 by Bear, on Flickr

Firing 67 2 by Bear, on Flickr

Firing 67 3 by Bear, on Flickr

 

Cheers, the Bear.

 

NZR Drivers Cap by Bear, on Flickr

[Track fiddler]

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

 

 

[gregc]

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?

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

[gregc]

Doughless

Keep 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

Doughless

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 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)

3344

[Overmod]

"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'

[wjstix]

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.

[Doughless]

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?

 

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. 

[gregc]

Douglas,  thanks for one of the more thoughful posts

Doughless

you 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 ..

Doughless

My 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%.

Doughless

I'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

3222

[Overmod]

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...

[Overmod]

gregc

not many of you seem to think that a thicker coal bed is needed to increase the rate of steam produced by the boiler

That's because it isn't.  The thicker bed merely would provide more fuel for ignition and volatilization before more has be be dispensed -- which means being flung or jetted against the stream of combustion gas -- and a little more insurance against holing, bun-throughs, clinkering, and the other bugbears of burning cheap-as-possible fuel on a grate.

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)

It has been 'settled science' for over a century that the throttle be opened as full as possible, as rapidly as possible, and the engine operated thereafter using the cutoff, implemented through the reverse, for all control.  This is even true for those situations, as Tuplin described, where the engine is 'sliding-pressure fired' so that peak boiler pressure is constrained -- you operate with minimal throttling of the steam flow OTHER than what the valve characteristics admit, and can exhaust.

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.

Anyone who has actually manipulated a non-air-assisted throttle -- even a multiple poppet front-end type on a modern engine -- will know EXACTLY why you adjust the power reverse for any sort of precise change of required power or anticipated speed.  You might have the throttle closed for drifting, or to reduce torque peakiness under uncertain adhesion conditions, but in general you'll want the greatest 'thermodynamic efficiency' out of an already hideously inefficient machine.  That is not obtained by using a higher water rate... which is almost an inevitable consequence of attempting to extract a certain amount of steam work at a lower initial pressure.

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?

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.

The throttle does just what it indicates: restricts the opening so that less mass flow of steam at a given pressure goes through.  And yes, the steam starts to expand after the throttling restriction, and yes, this wastes some of the available expansion energy in the steam, and yes, you don't want to do it unless there is some relatively unusual reason to do so.

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)

That is really obvious.  Where you start to fall off the trolley is assuming that the reverse is true: not allowing 'as much as is available' quickly results in enormous blowing off and waste at the pops.

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

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

What it 'needs' is full boiler pressure, very short cutoff, and to the extent cutoff can't be shortened beyond a certain amount (which is, of course, not likely on any modern locomotive) a throttle cracked to give finer control.

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.

When the throttle is opened 'further', the steam flow only increases into the cylinders if the valve gear is set for it to do so.  You will notice that there is a fixed tract volume between the front-end throttle poppets and the steam chests, and this fills to equilibrium remarkably fast even with the throttle 'partially closed'.  This is extremely poor engineering thinking.

i'm suggesting a higher rate of coal (lb/hr) is being added by the fireman as the speed increases.

In the long run, yes, you need more heat input to match the heat being 'dispensed' through the valves and out the exhaust.  But for most short-term acceleration that comes out of the overcritical water in the boiler, just as it would for a fireless cooker, and there's plenty of heat on the elements to 'dry' the steam to superheated condition for expansion.  The actual heat transfer from the fire, both in the radiant and convection sections, takes much longer physically (and there is considerable heat transfer continually available both from the radiant plume and from the gas in the tubes and flues before you start the involved process of getting fuel in position, volatilizing it, combusting it, and moving the combustion gas accordingly).

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...

[Doughless]

Greg,  When I quote I get the 403 Forbidden.

No.  I'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.  In a car engine, the throttle adds more fuel (mixture) directly to the cylinder, and more fuel means more combustion and faster go. 

Its not exactly the same as an internal combustion engine because the "combustion" in this case happens outside of the cylinder (steam production) and is stored in the dome for use when needed.  But fuel (steam...water and air mixture) to the cylinder is still what makes it go.  Make sense?

This may not be the way the steam engineers think of it, but it helps me understand it.

To go faster, you need more steam.

I don't disagree with your last quotes. 

More coal (more fire) and more water (I would think is needed) creates more steam (fuel, as I see it).

My question would be, can you get more steam to the cylinders when the throttle is wide open?  It can only process has much as designed.  Like trying to drain a bucket with a straw.  Produce more steam and max out the boiler as much as you can, but if the throttle is wide open (maxed), I don't know that more steam can get to the cylinders.  Dumping more water into the bucket won't produce more water into the straw,  if pressure already maxed out.  

In your OP, you 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.  If not...if the throttle is maxed out (not wide open but maxed out as to volume of steam it can handle)....then increasing boiler pressure won't deliver more steam to the cylinders and it will popoff.

[gregc]

not many of you seems to think the 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.

 

(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)

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

a locomotive running without a train (no tonnage) requires little boiler pressure and/or a slightly open throttle

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.

2898

[gmpullman]

gregc

so you're saying the amount of steam produced by the boiler doesn't matter on how much coal is being burned?

How quickly the volatiles distill and 'burn off' will be a major determining factor in how much heat you derive out of each scoop of coal. Some coal varieties require a longer time on the bed to fully ignite and give off its heat through the burning of the volitale gases. Ash in the coal can vary from around 2% up to15%. A high-ash coal is going to build up much faster and this build up of ash isn't going to add anything to the heating value of the coal and will indeed impede draft through the fire.

The temperature of the fire at the time of stoking will make a difference in how quickly the coal gives off its heat, too. Obviously a fire at 950°F will take longer to ignite fresh coal than if it is at 1400°F.

How finely the coal is crushed is a factor, too. I've seen some loads of coal that are nearly dust. These fines get pulled right through the combustion chamber, some flashing off right away and others getting pulled through the tubes without combusting. I would wet the fine coal down with the squirt-hose which helped it from getting blown out the stack. Large lumps of coal will likewise take longer to ignite and give off heat. The stoker has a crusher which reduces most of the larger lumps but when hand firing you have to either spend some time with a pick and break up the lumps or risk firing heavy and the large lumps not burning very quickly.

gregc

what do you mean by "trimmed"?

The fireman opens the firedoor occasionally and uses his scoop to direct the flow of air across the grates. Bright spots are thin and will need a few scoops directed there to avoid holes in the fire. The back inside corners always need a few scoops as the stoker has a hard time directing coal to these areas. Dark spots usually indicate a clinker forming or at least a build up of impurities in that spot.

The firebed is constantly changing, thus it needs to be 'trimmed' by hand. Constant monitoring of the stoker jets is required, too. A Simplex stoker had five steam jets that adjust the flow of the coal. A Duplex had a dividing plate plus two jets on each elevator in order to balance the distribution of the coal. An Elvin stoker has a pair of 'paddles' that slings the coal across the fire. I've never seen one of these in operation but it must have been interesting.

I never had to shake the grates while in motion, only while stopped. I imagine a road crew that  spent a lot of time running would have to occasionally shake the grates 'on the run' especially with bad coal.

 

Ed

[gregc]

Doughless

I think you mean to say that more steam to the cylinders is needed to increase speed....not strengthening the fire?  

yes, more steam needs to enter the cylinders.   but the boiler also needs to produce more steam (see below)

Doughless

The throttle puts steam (fuel combustion/mix of sorts) to the cylinders.

is this suggesting that the throttle increases the amount of fuel, coal to the engine ??

Doughless

To accelerate, its the throttle, not the fire intensity, that accelerates the locomotive, (provided that the fire already made enough steam and there is enough steam in the dome).

there's probably enough steam in the boiler to accelerate the locomotive.  but if the throttle is full, as that table above says it would be during normal operation at various speeds, it can't be opened further to allow more steam into the cylinders.

presumably cutoff can be increased, as Overmod described, but i'm not sure i understand how that works.

regardless of how more steam is allowed into the cylinders, additional steam needs to be generated by the boiler by increasing the BTUs added to the boiler by burning more coal.

[gregc]

gmpullman

I see this concept of a 'thick' fire = more heat as opposed to a thin fire = less heat. Hogwash!

so you're saying the amount of steam produced by the boiler doesn't matter on how much coal is being burned?

gmpullman

The main driver of successful firing is anticipation and cooperation. The fireman has to know the territory in order to have both the fire trimmed

what do you mean by "trimmed"?

gmpullman

I kept the needle within a few pounds of 200 psig.

what was max boiler pressure?

[gregc]

wrench567

In your thinking. If you dump the fire the locomotive will not move.

of course not.   what did i say that suggests that?

there's steam in the boiler but no additional steam is being produced because there's not BTU being added to replenish what is drawn by the cylinders

boiler pressure will drop as the cylinders consume steam but will draw less and less

2900

[Doughless]

gregc

why not build the fire only as strong as needed, some bed thickness, to maintain the boiler pressure maybe 5,10 PSI below max boiler pressure when operating at the desired speed.   add more coal when it goes below that, stop adding coal when it starts to rise above it to increase speed, the fire needs to be strengthened to accelerate.

I think you mean to say that more steam to the cylinders is needed to increase speed....not strengthening the fire?  

But at some point prior to the acceleration, yes, the amount of steam produced must be in sufficient supply for the throttle to allow more steam into the cylinders for actual acceleration, so the fire must be increased some time before the need to accelerate. 

The throttle puts steam (fuel combustion/mix of sorts) to the cylinders.  To accelerate, its the throttle, not the fire intensity, that accelerates the locomotive, (provided that the fire already made enough steam and there is enough steam in the dome).

Just my take from following this discussion. 

[gmpullman]

wrench567

As long as there is steam in the boiler, you can start and move the locomotive.

On the engine I'm familiar with we would dump the fire on one track, move to another track for sand then head for the turntable, finally going into a stall of the roundhouse. Sometimes thirty or fourty minutes elapsing. Steam pressure would only drop about twenty pounds in this time frame.

I was more concerned with the loss of air pressure. Nosing toward the brick wall of the roundhouse isn't quite the time to find out your air is rapidly depleting!

Regards, Ed