gregcplease look at the plot from the tables above 10/14 5:09AM for throttle full (not plotted),
Continuing to use data extracted from the PRR Test Lab does not reflect 'real world' operating characteristics. I'm sure the test lab was using 'full throttle' simply to remove that variable from the calculations of the test results.
On another note, this explanation of the function of the steam locomotive valve, including a good explanation of lap and lead, is worth studying in light of the dialog Mr. Overmod provided above:
Regards, Ed
DoughlessIf a loco is going at only 20 mph, it would likely be at only partial throttle
please look at the plot from the tables above 10/14 5:09AM for throttle full (not plotted), 4 speeds: 10, 20, 30, 40 mph (green), relatively constant boiler pressure (white), various tractive efforts (orange) and cutoff (violet)
greg - Philadelphia & Reading / Reading
thanks
i believe you're just describing an indicator diagram. i'm posting these to make you aware that i believe i fully understand what happens inside the cylinder during a cycle. MEP is the area inside the red contour vs the area of the cylinder enclosing it. of course it's expanding inside the cylinder, ...
and more realistically (pg 116)
Overmod gregc what happens to the steam during the time that neither cylinder valve is open? It's expanding, Greg. Expanding as intended since about 1835. does it just "pile" up in the steam chest and piping, building pressure...
gregc what happens to the steam during the time that neither cylinder valve is open?
It's expanding, Greg. Expanding as intended since about 1835.
does it just "pile" up in the steam chest and piping, building pressure...
... i'm asking about the steam outside the cylinder when the valve is close
if the steam flow (lb steam/hr) is the same regardless of cutoff (!!) and all of the flow enters the cylinder when the valve is open (consumption equals production), the mass (lb) of steam in the cylinder each cycle is the same, but the initial pressure is higher at less cutoff (less volume, higher density) and results in a higher MEP with lower cutoff (can't be right)
so the steam flow must change when cutoff is changed. but i don't know how to calculate that value. i don't understand how the "piling" up affects the flow.
is it the opposite of expansion inside outside the cylinder where the increasing pressure impedes flow, resulting in a decreased average consumption?
Should the mean effective pressure in the steam chest (i.e. back pressure) be used to determine the pressure across the throttle and the flow it allows !?
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gregcto increase speed it makes sense that increasing cutoff, increasing the % of the cylinder cycle that the intake valves are open allows more steam into the cylinders, increasing tractive effort and possibly speed.
The boiler and the steam chests are essentially at equilibrium if the throttle is even slightly cracked, and sufficient time is left for things to warm up -- if there is no steam demand FROM THE VALVES. That is why all these discussions about the throttle being some kind of 'speed control' are fallacious for modern reciprocating locomotives.
There are two phases -- associated with two different physical piston thrusts -- during admission, and both are timed by the position of the valve corresponding to cutoff.
In the beginning of admission, the valve opens to steam. That steam comes from the chests, which are fed by the branch pipes, which are fed through the elements by the overcritical water 'steam liberation'. You can take this for purposes of argument as "boiler pressure" as it reaches the chests, or some nominally smaller percentage like the 80% pressure that we use in PLAN calculations. At 20mph, essentially the entire period of admission will involve steam 'mass flow' at that presure, passing into the cylinders. It exerts "that pressure' on the piston, as the piston moves and the swept volume in the cylinder increases. This pressure doesn't get any higher, and it only gets lower progressively if there is something holding up the free flow of steam from the boiler. It does not take much intuition to figure out when that might be something desirable to achieve. (Remember that the 'pressure on the piston' has to be translated through the mechanical linkage of the rods to get 'wheelrim torque' if you want to calculate it a la Wardale)
Now the valve passes the steam edge, and (with a modern piston valve) shrouding and effective cutoff happen very quickly, within a small fraction of an inch of physical valve motion. (The cutoff is quicker if the valve is moving more quickly, which is why the long-travel part of long-lap, long-travel valves is so utterly important for modern high-speed steam locomotives!) At this point, the mass flow that has entered the cylinders becomes orphaned completely from what the boiler does or provides. There is still substantial pressure being exerted on the piston, but as it continues further back in the stroke, the only thing developing that pressure is the heat contained in the mass of steam. That's considerable, so while the pressure starts falling it doesn't do so precipitously. All the way from here to the point in the stroke that the exhaust opens to steam, the piston thrust is determined by the characteristics of that expanding steam. Note that if you were to graph indicated piston thrust, you'd have a constant horizontal line during admission, and some kind of hyperbolic curve jiggered for thermal and phase-change losses during the expansion after cutoff.
Not immediately relevant, but critically important, is what happens to the contained steam as the exhaust opens. You want to get rid of most of it, but not all of it, and you want to do so with reasonably minimized back-pressure (for the other strokes and the engine momentum to overcome in part).
Again, this is why seeing the four stroke pressure thrusts plotted on one graph, as in Wardale's example, would be so valuable to you.
at speed, one cylinder intake valve or the other cylinder valve is open when cutoff is at 50% allowing all steam produced by the boiler to enter one cylinder or the other.
At 50% cutoff, steam flowing from the boiler, thru the throttle and piping reaches the "Y" and continues either right or left into a cylinder
Yes, some of the flow characteristics are important; Chapelon's use of 'internal streamlining' and larger pipes is a good proof. But the object is, again, to get highest potential pressure and mass flow to the chests regardless of steam consumption. A 'throttle' only impairs that. (And as I've said, there are times you want that impairment. Franklin type D poppet gear actually depends on it to get the effect of 'cutoff' in those 2-8-0s... but that isn't for anything even remotely related to efficient thermodynamics; it's to provide one-lever simple direction control to soldiers who don't give a crap about how steam locomotives actually work)
... but at less than 50% cutoff, the cylinder valves are closed part of the time and steam flow from the boiler is blocked.
...at 20% cutoff, no cylinder valve is open 60% of the time.
The situation would go to a far greater extreme if any engineman attempted to make use of the full precision of British Caprotti on a 2-cylinder DA engine. First you'll have some percentage of fixed cutoff -- probably about 83% on an engine not designed to Super-Power or PRR wack ideas about 50% fixed cutoff and slot/Weiss-port kludging (or Herdner valves) to let them start at all. Then you have an 'effective' admission cutoff of 2 to 5 percent, which is the only interval that steam at boiler or any other pressure is being allowed into the cylinder. If you were going at high speed, this is just a 'wisp of steam' flicking in during a correspondingly short time... but that's the only steam that's going to be expanding to push the piston for the entire 'rest' of that particular stroke. That ain't going to overcome very much resistance, and if you were to increase admission pressure to try to make more out of it, expect your torque peakiness... and your extreme sensitivity to high-speed slipping! -- to increase as well.
what happens to the steam during the time that neither cylinder valve is open?
We do have some fun with momentum effects in getting faster admission at very high cyclic, which corresponds in a rough way to tuned 'ram' charging of air. But it is more about getting higher mass of steam into the cylinder quickly, so it can expand and produce useful and controllable piston thrust.
...and what are the consequences, reduced steam flow as momentum is lost thru much of the piping?
(do i need to consider the momentum of steam)?
doesn't all the "piled" up steam eventually enter the cylinder when the intake valve does open because any excess steam produced by the boiler results in increasing boiler pressure?
in my modeling, the amount of steam (lbs) in each cylinder at the end of cutoff determines the density and the initial pressure used to determine MEP.
the amount of steam includes what may have "piled" up in the steam chest as well the normal flow thru the throttle. doesn't that amount of steam always have to be half the steam produced by the boiler?
does increasing cutoff when operating at a constant speed avoid minimize steam "piling" up...
... reducing the back pressure (?)...
and allows increased steam flow from the reservoir of steam in the boiler until the fireman increases the rate of coal (lb/hr) being added to accomodate the need for greater steam production, maintaining boiler pressure?
Then we get into the whole 'fire-soot-a pipe-scale-water' issues with how the combustion heat is getting to the overcritical water to make up its heat drop, and then how efficiently the fuel is vaporized and carbureted into the combustion plume that would start doing this better, and... well, let me reiterate that WHAT THE FIREMAN DOES IS VERY, VERY DISTANT IN TIME FROM WHAT THE STEAM IS DOING TO ACCELERATE THE LOCOMOTIVE.
The points about building the fire for better performance at starting are related more to lavish 'waste' of steam since the engine has to develop high starting TE on essentially zero rpm until the train has rolled an appreciable percentage of a driver diameter.. and that brings us to part of your answer about lengthening and shortening cutoff. Which is that the engine makes more strokes, and hence more torque-producing events per minute, when it is physically rotating at a higher cyclic (which of course corresponds to a road speed if the engine isn't slipping). It would stall pathetically if it were 'internal-combustion' at any normal starting rate (which is part of why a Kitson-Still didn't switch to combustion until the engine had gathered considerable speed). It does not, in part because at longer cutoff the admission keeps 'full steam pressure' on the piston, and this pressure is elastic (back through the open valve to the chest, pipes, and eventually boiler) so that if you have the independent on for traction control, you're not losing "horsepower" the way you would if you revved a drag engine with the wheels braked. (Railfans can have a hard time realizing this...)
Steam engines are designed to be able to hold a very high amount of steam pressure. It's how the engine works. One cubic inch of water creates 1700 cubic inches of steam. That's what creates the power to pull a train.
If a locomotive is designed to run at say 250 psi pressure, the fireman normally needs to have that much pressure available before the engine starts to pull it's train. If the engine reaches a point where too high a pressure has been created in the boiler, the safeties will lift and let off enough steam to go back down to the maximum safe level. If the pressure falls well below the 250 psi, the train grinds to a halt. As engineers used to tell their firemen, "I need steam, I can't run the train on water!".
gregcmy question is not about efficiency. it is what needs to happen to increase the speed of a train.
From my layman's understanding. If the throttle is wide open and more steam is not being fed to the cylinders by a high but decreasing BP, I would say you need a hotter fire (and more water?) to increase speed.
However, at that point, your Fireman is effectively acting like the accelerator pedal in a car. The faster he shovels, the faster the loco goes.
(You could make a funny Wile E Coyote cartoon as him driving a steam locomotive and shoveling like a blur trying to catch the Roadrunner).
So, while your answer of a hotter fire is the correct scientific answer, I think many experienced railroad crews would prefer to not have placed their Fireman in that situation (may be human physically impossible if in your scenario you want to go from 20 to 80 mph instead of just 30).
IMO, sufficent BP has to be built slowly in order to achieve the steam needed to get to, say, 80 mph with an (eventual) wide open throttle. The crew has to understand where along the route and when they need the extra steam...and think ahead so that the throttle is in position to do the work when needed.
If a loco is going at only 20 mph, it would likely be at only partial throttle (not unlike a car....you need some throttle in reserve in order to accelerate). And, if its at wide open throttle while APPROACHING level grade (meaning it just struggled to go 20 mph up a 3% grade), then ...like a car that has its throttled floored to go up a hill...it will accelerate on its own as the grade levels out, as long as the amount of steam is maintained. (The "load" decreases)
So methinks your scenario might have a bit of an unrealistic combination of circumstances.....but maybe not?
- Douglas
to increase speed it makes sense that increasing cutoff, increasing the % of the cylinder cycle that the intake valves are open allows more steam into the cylinders, increasing tractive effort and possibly speed.
at speed, one cylinder intake valve or the other cylinder valve is open when cutoff is at 50% allowing all steam produced by the boiler to enter one cylinder or the other. At 50% cutoff, steam flowing from the boiler, thru the throttle and piping reaches the "Y" and continues either right or left into a cylinder
but at less than 50% cutoff, the cylinder valves are closed part of the time and steam flow from the boiler is blocked. at 20% cutoff, no cylinder valve is open 60% of the time.
does it just "pile" up in the steam chest and piping, building pressure and what are the consequences, reduced steam flow as momentum is lost thru much of the piping? (do i need to consider the momentum of steam)?
in my modeling, the amount of steam (lbs) in each cylinder at the end of cutoff determines the density and the initial pressure used to determine MEP. the amount of steam includes what may have "piled" up in the steam chest as well the normal flow thru the throttle. doesn't that amount of steam always have to be half the steam produced by the boiler?
does increasing cutoff when operating at a constant speed avoid minimize steam "piling" up, reducing the back pressure (?) and allows increased steam flow from the reservoir of steam in the boiler until the fireman increases the rate of coal (lb/hr) being added to accomodate the need for greater steam production, maintaining boiler pressure?
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wjstix The inherent problem with the original question makes it impossible to answer it given the situation the question describes - a steam engine with the throttle fully open, going only 20 MPH. When starting a steam engine with a long train, the throttle may be opened full or close to it to get as much power as possible to the pistons / cylinders to start the engine and bring it up to speed. Once the train has accelerated to the track speed, the throttle will be closed back down a considerable degree. Once the train is up to speed, the engine doesn't need all that much steam going to the cylinders to maintain the speed. It's roughly the same as driving your car. If you are going up an on-ramp to the freeway, you may put the accelorator down almost to the floor, to go from near zero to say 60 MPH so you can merge with the highway traffic. Once you get up to 60 MPH, if you keep the pedal all the way down, you'll continue to accelorate - so you back off the pedal, using just enough gas to maintain the 60 mph.
The inherent problem with the original question makes it impossible to answer it given the situation the question describes - a steam engine with the throttle fully open, going only 20 MPH.
When starting a steam engine with a long train, the throttle may be opened full or close to it to get as much power as possible to the pistons / cylinders to start the engine and bring it up to speed. Once the train has accelerated to the track speed, the throttle will be closed back down a considerable degree. Once the train is up to speed, the engine doesn't need all that much steam going to the cylinders to maintain the speed.
It's roughly the same as driving your car. If you are going up an on-ramp to the freeway, you may put the accelorator down almost to the floor, to go from near zero to say 60 MPH so you can merge with the highway traffic. Once you get up to 60 MPH, if you keep the pedal all the way down, you'll continue to accelorate - so you back off the pedal, using just enough gas to maintain the 60 mph.
Ha. Not around here. People just come off the on ramps putting along at 25. Leaving the left blinker on for at least 5 miles when they eventually get to 55.
Pete.
gregcbut others ("Hogwash", ...) seem to dispute this
Please cite where I said the addition of coal to the fire was not necessary to maintain boiler pressure.
gmpullmanHow 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.
gmpullmanI've avoided responding to this thread for as long as I could hold out. I'll reluctantly jump in here as I see this concept of a 'thick' fire = more heat as opposed to a thin fire = less heat. Hogwash! gregc the fire can have different depths, producing different # of BTUs, generating different amounts of steam (lb/sec)
Mr. Hogwash here.
You're right, hogwash may have been a bit extreme. I should have had more patience with your understanding of this this subject.
SO I've reduced my descriptor to poppycock.
You stated earlier:
"Fire can have varying thicknesses (different depths) providing different amounts of BTUs generating different amounts of steam."
Perhaps I didn't explain my answer thoroughly.
My contention is that the firebed thickness is a result of varying grades and qualities of coal.
If you would have taken the time to read my reply on this you may have noted that coal can vary widely in makeup and ash percentages. Any of the firing instructional textx I provided will explain this further.
The fireman determines, based on the burning qualities of the coal supply he has at hand, how will carry his fire and if it will be allowed to build to a thicker bed or if the grade of coal and operating conditions will allow him to carry a (prefered) thinner fire.
One point I was attempting to get through to you was the fact that the 'BTUs' are not entirely derived AT the grates of the firebox but rather in the combustion chamber and flues. I've previously explained that some coals do not give up (distill) their volitales right away but will need some time and exposure to surrounding heat in order to make the chemical transformation to gas.
There are times when the combustibles are carried right out the stack without giving up its distilled gases and converting them to heat. Poor efficiency and poor firing. But if the fireman is not 'on his game' and does not react to changes in operating conditions far enough in advance of these changes he may find himself having to play 'catch up' and may have to resort to overfeeding the fire (producing less heat and more smoke) or dangerously allowing the water level to get too low.
I stand by my Poppycock reaction to your statement of 'thick fire = high BTU = high steam generation vs. thin fire = low BTU = less heat transfer.
Comments from others welcomed.
Cheers, Mr. Hogwash
BigJimThe best narrative about real life railroading!
I think both our memories will outshine it.
It's been fun. But it isn't much fun anymore. Signing off for now.
The opinions expressed here represent my own and not those of my employer, any other railroad, company, or person.t fun any
Overmod gregc what i draw from it: coal consumption is proportional to TE and speed (which partially answers my original question)... This says coal consumption is proportional to horsepower, which is not exactly a surprise.
gregc what i draw from it: coal consumption is proportional to TE and speed (which partially answers my original question)...
coal consumption is proportional to TE and speed (which partially answers my original question)...
This says coal consumption is proportional to horsepower, which is not exactly a surprise.
but others ("Hogwash", ...) seem to dispute this
it hasn't been made clear that "maintaining" boiler pressure may require changing the rate of coal (lb/hr) added to the fire
Overmod but since cutoff can be the same for various TE values, it is related, but does not dictate TE. why/how? You have it backward. ... look at the indicated MEP
but since cutoff can be the same for various TE values, it is related, but does not dictate TE. why/how?
You have it backward. ... look at the indicated MEP
"backwards"? don't understand what you're trying to say
Overmod that increasing cutoff allows more steam and increases TE... yes, but precisely how the 'more steam' admitted increases TE is an important part of the equation.
that increasing cutoff allows more steam and increases TE...
yes, but precisely how the 'more steam' admitted increases TE is an important part of the equation.
"how" ... not clear
with the throttle full, it seems steam production just matches flow (lb steam / hr) into the cylinders. there is a balance and achieving a stable operation is the trick to operating a steam locomotive
makes sense that increasing ("lenghthening") cutoff allows more steam into the cylinder, increasing TE and changing the steady state conditions. cutoff is reduced (how much) after reaching a higher speed when acceleration is no longer needed.
it looks like i need a better understanding of the relationship between cutoff and HP instead of cutoff and TE/MEP. the ratio between HP and cutoff increases with speed, cutoff is less at higher speeds, and my understanding is that reduced cutoff is more effective as speed increases !
after accelerating to a higher speed, can cutoff be less than it was a the lower initial speed ?? (need the math that results in this)
+ HP (cyan)
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zugmann gmpullman Yes, I agree this is a good, first hand account of the ins and outs of a 'green' fireman. I read that many moons ago, but I would classify it more as a "historical fiction" than anything. Some of the stories seemed too made up and too-well recited to be believable at times. Unless the engineer kept really, really, good notes. Like multiple diary entries a day good?
gmpullman
I read that many moons ago, but I would classify it more as a "historical fiction" than anything. Some of the stories seemed too made up and too-well recited to be believable at times. Unless the engineer kept really, really, good notes. Like multiple diary entries a day good?
"Set Up Running"
The best narrative about real life railroading!
.
gregcwhat i draw from it: coal consumption is proportional to TE and speed (which partially answers my original question)...
The 'catch' is that it's average, with a great deal of fuel used to heat up the boiler and water before TE is exerted.
We can start looking at the situation in a Mallet or other locomotive that 'uses steam twice' -- it actually does no such thing, it uses the steam once but with longer overall expansion. Even up to recently, people have a hard time figuring out the 'optimal' HP exhaust pressure to get comparable expansion out of the LP engine... without losing too much of the heat in the steam. Chapelon and N&W found it better to 'cheat' -- to throttle in enough high-pressure and superheated steam to get both the piston thrust and change in thrust over the functional length of the LP stroke to match what the HP produces. If you do that 'the hard way', you get into de Glehn-du Bousquet country... where one of the great 'roads not taken' in locomotive compounding prior to the innovation of the Schmidt practical superheater can be observed. The engineer of a de Glehn engine not only had throttle and cutoff to play with, he had a separate set of valve gear optimized to the LP engine, and he had to play the HP and LP together like an organ, constantly adjusting both sets of gear up or down to keep the thrusts (a) balanced, and (b) the LP MEP high enough that that side actually pulled its own weight. It could be surprising just how high the nominal exhaust pressure had to be in order to produce (b)... much more often it was common to see LP pressure as low as 55psi (with the mass flow fixed as coming through the receiver from the conjugated HP exhaust!) and this was just abjectly pathetic EVEN BEFORE the question of reheat comes up...
One purpose of the exhaustive series of road tests comprising a PRR test-plant series was to determine engine performance against a constant wheelrim resistance (there, via a Prony brake system) and get some idea of how much coal was needed to produce different levels of sustained output. A problem with 'formulae' to calculate this is that there are empirical constants used in much of the work, just as there were in Fry's book on boiler design in the early 1920s, and this can make mathematical attempts something of a crapshoot "before you know the answers" (as the first law of engineering tells you!)
gmpullmanYes, I agree this is a good, first hand account of the ins and outs of a 'green' fireman.
what i draw from it
i also think Husman's analogy that the fireman is a fuel pump is insightful. the fuel pump adjusts its rate, lb coal/hr, as needed to maintain pressure.
gotta go, family
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dehusmanGreg : Read "Set Up Running", the biography of a PRR engineer.
Yes, I agree this is a good, first hand account of the ins and outs of a 'green' fireman.
Set Up Running tells the story of a Pennsylvania Railroad locomotive engineer, Oscar P. Orr, who operated steam-powered freight and passenger trains throughout central Pennsylvania and south-central New York. From 1904 to 1949, Orr sat at the controls of many famous steam locomotives; moved trains loaded with coal, perishables, and other freight; and encountered virtually every situation a locomotive engineer of that era could expect to see.
John W. (Jack) Orr, Oscar’s son, tells his father’s story, which begins at the Central Steam Heating Plant in Bellefonte, Pennsylvania. Oscar operated nearly every kind of steam locomotive the Pennsylvania Railroad owned, working from the bottom of the roster to the top position (number one in seniority). Orr has an ear for detail and a vivid memory. He tells about his father’s first encounter with an automobile along the right-of-way, about what it was like to operate a train in a blizzard, and about the difficulties railroadmen encountered in stopping a trainload of tank cars loaded with oil in order to take on water and coal―and many other stories.
This compelling railroad history will enthrall not only everyone in the railroad community but also the general reader interested in railroads and trains, past and present.
Also, if anyone is interested this 166 page booklet covers many aspects of locomotive operation. A good source of information.
https://archive.org/details/locomotivefiringcourse1944
An introductory page:
https://www.railarchive.net/firing/p005.htm
Contents: https://www.railarchive.net/firing/pv.htm
You can click on any entry in the contents page and be taken to that section. Use page 'next' or 'back' to navigate.
Cheers, Ed
Greg : Read "Set Up Running", the biography of a PRR engineer.
Dave H. Painted side goes up. My website : wnbranch.com
gregchow does he learn/know this? does he watch another firman? does the engineer tell him?
For about a week he is a 'student fireman' usually on at least five runs or so. After that he's on his own and a 'good' engineer would be helpful for him but not all were.
I've heard stories of engineers refusing to allow the fireman to use the stoker — "We didn't have any stokers when I was firing and you're not going to, either."
The New York Central booklet I offered in an earlier reply was issued to new firemen as well.
Good Luck, Ed
gmpullmanHow 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
how does he learn/know this?
does he watch another firman? does the engineer tell him?
gmpullman gregc don'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
gregc don'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
i have family visiting for the weekend
Water Level Route Overmod Stop changing the game if you want an answer to what your question asked.
Overmod Stop changing the game if you want an answer to what your question asked.
what game am i changing?
is description of flow (lb steam/ hr) what you mean by mass flow?
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
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
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.
OvermodStop changing the game if you want an answer to what your question asked.
Mike
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
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.
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
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.
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
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.