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?
greg - Philadelphia & Reading / Reading
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
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?
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
Greg : Read "Set Up Running", the biography of a PRR engineer.
Dave H. Painted side goes up. My website : wnbranch.com
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
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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gmpullmanYes, 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?
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
gregcwhat 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)...
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.
that increasing cutoff allows more steam and increases TE...
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...
but since cutoff can be the same for various TE values, it is related, but does not dictate TE. why/how?
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!)
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
"Set Up Running"
The best narrative about real life railroading!
.
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)...
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
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.
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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BigJimThe best narrative about real life railroading!
I think both our memories will outshine it.
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
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.
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.
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.
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.
what happens to the steam during the time that neither cylinder valve is open?
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)?
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?
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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
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!".
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.
It's expanding, Greg. Expanding as intended since about 1835.
does it just "pile" up in the steam chest and piping, building pressure...
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)?
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...)
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?
... 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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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)
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
gregc Doughless If a loco is going at only 20 mph, it would likely be at only partial throttle please look at the plot from the tables above 10/14 5:09AM for throttle full (not plotted), 4 speeds: 10, 20, 30, 40 mph (green), relatively constant boiler pressure (white), various tractive efforts (orange) and cutoff (violet)
Doughless If a loco is going at only 20 mph, it would likely be at only partial throttle
I didn't mean to suggest that the throttle could not be at full when the loco is going only 20 mph. It certainly can. But if the top speed of the loco is significantly higher than 20 mph (you're suggesting at least 50% higher) under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well.
Maybe relying upon the fireman to stoke coal at a faster pace is how crews typically accelerated the train. But I think there were probably better ways to manage the steam as they traversed a familiar route.
DoughlessHowever, at that point, your Fireman is effectively acting like the accelerator pedal in a car. The faster he shovels, the faster the loco goes.
I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.
The fireman adds fuel to the firebox. It takes time to get an additional layer of coal into the firebox, even with a stoker. It takes time for the coals to ignite and begin burning (probably a very short time, but longer than the time it takes for fuel in a gasoline engine to be admitted and ignited). Then there is the time it takes for the hot gases to heat the water to increase the temperature of the water. Probably a relatively short time, but still not instananeous.
Once the fireman stops adding coal to fire, it's not like taking your foot off the accelerator because all that coal that was added continues to burn and heat the water in the boiler for many minutes after the fireman stops adding coal.
The fuel of a steam engine is steam. All the discussion about what happens in the pipes and cylinders is independent of what's going on in the firebox. It could be wood fired, coal fired, oil fired. Doesn't change a thing about what is happening on the front end of the engine. The fireman is in charge of making sure there is adequate steam. He can increase the temperature and pressure in the boiler by adding more fuel, but there is a lag in how long that takes. The fireman has to "step on the accelerator" well before the engineer wants to accelerate and has to take his "foot off the gas" well before the demand decreases. If the engineer wants to accelerate at mp 10, then the fireman had better be shoveling coal back at mp 8 or 9.
gmpullmanOn another note, this explanation of the function of the steam locomotive valve, including a good explanation of lap and lead, is worth studying in light of the dialog Mr. Overmod provided above
Ed, the video describes the "general" function of the valve gear and cylinder valves which i believe i sufficiently understand. The video doesn't address my latest question about how to calculate the pressure in the steam chest and piping during operation with reduced cutoff to determine steam flow (lb/hr) vs cutoff
DoughlessBut if the top speed of the loco is significantly higher than 20 mph ... under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well.
any train needs to operate at various speeds and on various grades requiring different amounts of tractive effort as the lab chart shows examples for
the following shows the tractive effort and corresponding cylinder pressure vs mph and grade. Note that there is insufficient boiler pressure at higher speeds as grade increases
tePsi: 5000 T, max boiler PSI 220 --- 0.0 --- --- 0.5 --- --- 1.0 --- --- 1.5 --- mph TE psi TE psi TE psi TE psi 5 21261 25 71261 86 121261 147 171261 208 10 22841 27 72841 88 122841 149 172841 210 20 27298 33 77298 93 127298 154 177298 215 30 34287 41 84287 102 134287 163 184287 224 40 45248 55 95248 115 145248 176 195248 237 50 62438 75 112438 136 162438 197 212438 258 60 89398 108 139398 169 189398 230 239398 291
dehusmanI maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.
but there is also a reservoir of steam in the boiler to handle short term changes in steam consumption. Abrupt reductions in the need for steam (i.e. consumption) can result in increasing boiler pressure and waste of steam thru pop-off valves
again, the first chapter of Firing a Steam Locomotive is "Cooperation" and as Ed has noted, the fireman needs to be familiar with the route and anticpate the need for more/less steam
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gregcEd, the video describes the "general" function of the valve gear and cylinder valves which i believe i sufficiently understand. The video doesn't address my latest question about how to calculate the pressure in the steam chest and piping during operation with reduced cutoff to determine steam flow (lb/hr) vs cutoff
As Greg notes, valve-gear action is not instrumented to measure 'mass flow', and it has been a fun topic over the years to find ways to meter or approximate this. The most 'usual approach', back in the day, was to take the various indicated quantities and interpolate measured qualities of steam determined in the lab to get a number for admission, then use that to figure out what was going on during expansion (and then of course exhaust, but we're not concerned with that here).
Where you start with the steam chest is to 'assume' its starting pressure, and then how that decreases IN EQUILIBRIUM WITH THE ADMISSION VOLUME. There were attempts to indicate steam-chest pressure to get the initial pressure, net of any other effects, and then an indication of instantaneous pressure as admission proceeded, but as can be appreciated by an electronics engineer, this is no small problem and the data received from those sensors has to be integrated without losing the instantaneous pressure v. time information. You will appreciate Chapelon et al's concern for larger steam chests, and Wardale's concern for very well-insulated cylinders, when you look at the process through admission.
If the flow 'into' the steam chest (from the boiler) is less overall than the flow into the cylinder during admission, any equilibrium upset from flow restriction in the ports and passages may be COMPOUNDED by the fall in effective pressure through early expansion in the steam-chest volume (which causes pressure to fall there through the mechanism you keep trying to bring up). Naturally the mathematics to describe what happens going into the cylinder referred back up to the flow into the steam chest are extremely complicated, and have a nasty component of nondeterministic dependence on things you can't effectively measure even with the best analytic equipment. (See the origin of 'quaternions', for example, something that I refuse to discuss any further than this citation for any reason here).
Again, your take-home from this is that ANYTHING that impedes clear steam flow from the superheater elements down to the steam chest is going to impair how the steam gets from the chest into the cylinder during admission. That conclusively answers any concern about what the throttle in this particular discussion does... for our purposes, "less than nothing". (So we eliminate that source of loss quickly, positively, and as early as possible.)
Part of where cutoff becomes important is, again, that it varies both admission duration and expansive-working duration mechanically, by adjusting the points during the piston stroke where the valve starts to admit steam, and then stops admitting steam. There are some shrouding and unshrouding effects, and Julius Kirchoff went on at some length trying to demonstrate their significance at high cyclic, but the effects can be measured in fractions of an inch and their overall impact on effective pressure, between 20 and 30mph, is almost certainly very slight.
Doughless -- But if the top speed of the loco is significantly higher than 20 mph ... under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well.Any train needs to operate at various speeds and on various grades requiring different amounts of tractive effort as the lab chart shows...
Doughless -- But if the top speed of the loco is significantly higher than 20 mph ... under fully loaded tractive requirements and is already at wide open throttle, I don't think the crew managed their train very well.
The 'ringer' is that no one has clearly stated what that graph is measuring, which is not in fact plotted and is highly, even suspiciously artificial. What is happening is that trailing resistance is being increased at constant speed, probably explained away as the effect of 'increasing grade', but there is no indication of the rate of load increase, although there is certainly a 'piecewise' effort during the runs to tinker with the cutoff to increase engine output. (In one case the 'engineer' actually gets the cutoff too far, and has to jerk it back, with somewhat predictable results on the performance).
What would normally be seen on a plot, of course, would be trailing resistance measured by a dynamometer car. If the car were equipped with variable electric dynamic braking, as at least one English car was, you can simulate grade effects just as desired in the graph, by cranking up the excitation and watching the dynamometer pen swing over. (Assuming you have proper surge damping, etc., but that's another discussion for another audience...) The more usual method of simulating constantly augmenting gradient was to use brake locomotives, and there are so many jokers in that pack that I won't start listing them.
What I understand to be the original practice for this sort of thing originated with Lomonossov, who did the experiments for constant gradient utilizing certain parts of Russian trackage that were inclined at a very consistent gradient for the number of miles needed to extract meaningful indication data. But that is not the same thing as having an engine sitting on the PRR test plant and having someone turn the Prony brake up and up and up while the measured 100lb bags go into the firebox. That is NOT to disparage what PRR was doing, in their context, and it would be somewhat valuable once the necessary understanding of the variables is clearly understood to examine some of those data and some of their plots.
examples for the following shows the tractive effort and corresponding cylinder pressure vs mph and grade. Note that there is insufficient boiler pressure at higher speeds as grade increases...
To give a little analogy for, perhaps, understanding: As you increase grade, a common occurrence in ordinary running as stated, you increase the trailing resistance, all of which still has to be overcome by... piston thrust as expressed through the rod geometry. This clearly implies higher piston thrust per stroke, and since piston thrust is generated via steam pressure, there should be no surprise that more steam is needed at a given starting pressure, or less expansion will result for a given expansion volume, with what amounts to higher back pressure on the piston. Not incidentally, this is going to involve both longer admission under constant pressure and a higher mass per stroke during expansion, but it may not be immediately clear that the higher mass during expansion is also going to have its pressure drop (assuming no thermal loss for a moment, for an attempt at theoretical clarity) reduced, and this means that your steam is not doing 'all the work its heat is capable of' AND that when it blows to exhaust there will be not only a higher mass to be exhausted, but it starts at a higher pressure... and both the mass and the heat you blow away each stroke has to be made up from somewhere. Can you now trace where that 'somewhere' is going to be, back up the steam circuit, and appreciate what has to be done to the water to support it?
dehusman -- I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.... but there is also a reservoir of steam in the boiler to handle short term changes in steam consumption.
dehusman -- I maintain that is a poor analogy because there is a lag between what the fireman is doing and what the steam production is.
Abrupt reductions in the need for steam (i.e. consumption) can result in increasing boiler pressure and waste of steam thru pop-off valves again, the first chapter of Firing a Steam Locomotive is "Cooperation" and as Ed has noted, the fireman needs to be familiar with the route and anticipate the need for more/less steam
Now it is important to realize that ONLY this afterheat is the issue here. When you closed the throttle there was not some enormous reflected pulse of pressure back up into the boiler. All that happened was that steam was no longer following its desire to expand down to the chests and into the cylinders, and any resonant effects that occur when the throttle is closed will die out within a fraction of a stroke. Were the fire to be cut off completely (for axample via an oil burner with continuous flameholding) the boiler would be happy holding at the grade-climbing pressure without further difficulty. (There will be awful problems with differential thermal expansion if you have an idiot who actually tries this, but that's a different discussion entirely.)
I think until there's a better understanding of the principles of external-combustion engines using two-phase working fluid, we're just going to keep going round and round jiggering with the variables.
gregc[cutting to the chase...]I'm asking about the steam outside the cylinder when the valve is closed if the steam flow (lb steam/hr) is the same regardless of cutoff (!!)...
Now if you treat steam pressure as a variable (which in most practical engines it most certainly is) then you correct admission flow, etc. to be relative to pressure at the valve. That pressure, moment to moment, affects how the steam goes into the cylinder (and comes up to full pressure thrust as it does). That's the only place that steam-chest pressure influences what the engine is doing.
Do you understand what the little 'loops' in an indicator diagram are showing?
The flow 'into' the steam chest does not need to be continuous. Certainly the flow into a given cylinder end at any practical physical cutoff setting isn't, and the overall flow into the chest isn't even if you assume admission from 'either end' of the chest volume depending on whether the piston is advancing or retreating in DA. And yes, it starts very quickly as the valve opens to steam (even quicker in a Corliss or poppet-valve engine, but without very substantial benefits even if it's assumed to open both immediately and fully without restriction -- see the numbers for Bulleid's Leader, which probably set a record, in theory, for doing that with physical valves). And yes, it is cut off very quickly as the valve head passes the other admission steam edge in cutoff -- that's part of why it's called 'cutoff'. The steam-chest pressure and the mass of steam at that pressure almost certainly reaches an equibrium during the interval, and this is 'recuperation' to produce as-good-as-possible filling of the cylinder as needed for the 'next' stroke. You could overthink this out the ying-yang with velocity and 'momentum' calculations (the latter a sure sign of someone who doesn't really understand how fluid mechanics in steam work) but it all comes down to the same thing: you want high effective pressure in the chest, and enough steam mass that it fills the cylinder effectively for the expansion you want.
"MEP" is an artifact, something that gives you a handle on calculating average power out of the engine over more than a full rotation. For instantaneous steam-flow calculation, using it as if it were "a" fixed value will leave you just as clueless why different cutoffs produce different 'mean effective pressures' under load as you are now.
and all of the flow enters the cylinder when the valve is open (consumption equals production)...
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?
Look, you have steam in a volume (the steam chest) that is admitted to a volume (the ports/passages and cylinder end). The steam then continues to flow into the cylinder, through the ports and passages, as the volume in the cylinder increases (because the piston is moving... and it ain't moving at a constant speed, as a moment's thought would show, too). Now, in the absence of anything coming into the chest, that is a pure equilibrium thing (in the absence of weird stuff like wall losses or nucleate condensation, which we ignore at this level) so the pressure in the chest FALLS in proportion to how the pressure in the cylinder RISES. If you have to do some sort of complecticated mathemagics, calculate the pressure and volume in the chest at the moment admission starts, and calculate the volume of the chest, ports, passages, and cylinder volume at the moment of cutoff, and do your little assumed-gas-law calculation.
Now it follows from this that if you DON'T want the steam-chest pressure to fall, and hence the cylinder pressure at the moment expansion begins to be limited, you will be feeding replacement steam mass, at something like initial pressure, into the chest. If you have been following along, the 'average' amount you feed only has to be that which fills the sequential admission volumes to the desired pressure, with some pressure recovery during the cutoff expansion between admissions. If you are controlling the engine with a throttle, you are cutting down the amount of steam that goes into the chests -- again, grossly on average -- and hence the achieved final admission pressure will be limited to what's there.
More steam admitted to the chest, more steam that can go into the cylinder. With the amount at whatever final pressure determined by the point of VALVE stroke that produces cutoff. (Where the piston is at that point is conjugated by the valve gear combination lever, so relatively determined by design, but only incidentally related to the engineer's cutoff setting -- something you should have gotten from that C&O Walschaerts video.
MEP in the chest has nothing to do with cylinder 'back pressure' and even less than nothing to do with the 'back pressure' we use the term to describe, which is in the exhaust tract, far far away from this stuff and in a completely different space in the cylinder porting.
MEP in the chest cycles between what happens during the admission event and what the boiler sources during admission and then 'expansion time' before admission starts again the other way. In a proper engine it's as close to boiler pressure as good steam streamlining can produce, and it's maintained so that at the moment of admission it's as high as practical. (Or restricted so that cylinder peak and average effective pressure are 'regulated' -- why the English called the thing a 'regulator' in the era before good link gear was introduced.)
Your only concern is maximizing that. If anything restricts that, redesign it so it won't. And only look at it if it's grossly inadequate to reach a particular power at speed. (Probably not 20mph or 30mph on the level.)
OvermodI have to take a deep breath and sigh here, because the game is being changed again, perhaps without someone realizing they're doing it. The original question involved accelerating a train ON LEVEL GRADE from 20 to 30mph, and understanding even how that is accomplished has been an ongoing source of consternation. Now we have someone slip in not only the effect of grade, but increasing grade with distance, without even mentioning the function describing the delta resistance over each of the constant-speed runs.
don't understand why you think this it outside the original quesiton?
the values show how little pressure is required when running on level grade. grade was added to show how it requires significantly greater TE compared to level grade
did this earlier this morning, have guests and don't have time to read the response(s) thoroughly
it show
Greg.
For a good read with lots of graphs, charts, and data.
https://books.google.com/books/about/Railway_Age.html?id=w0E_AQAAMAAJ#v=onepage&q=PRR%20test%20plant%20I1s&f=false
But you specifically did not have 'grade' in the original question, and then when you added the graph, you quietly slipped in not only grade, but irregularly increasing train resistance that remains unspecified.
Since you appear to have a computer modeling program running:
1) Model the exact conditions in the original question. Assume (as would be correct, although I don't think you fully realized it) that your initial state at 20mph had the cutoff set as short as it could possibly be. Were the combination lever (see the C&O video again) not contributing something, the engine would in fact be drifting, with full pressure in the chests but no admission other than from slop.
2)Now plot the power needed to ACCELERATE the train from 20 to 30mph. Don't try to cheat and not state the time over which the acceleration is supposed to occur -- put it on a labeled, proportional axis with scale. Note that the appropriate level of 'new cutoff' will only apply until you are done accelerating the train to 30mph. That is one segment of a graph of whaever variables you're tracking. DO NOT make any further dicking around with the train resistance, whether from grade or 'rising air resistance' as though that scaled in that speed range for standard-gauge rail equipment. Just get the power needed to accelerate given your trailing resistance, and then backtranslate to the resultant of the cutoff at nominal 'full-throttle' boiler pressure that would produce that acceleration.
3)With the same assumptions and initial conditions, figure out what is needed to sustain the train, on the original level grade, once 30mph has been reached. Likewise translate that into cutoff settings and steam circulation.
4)Once you have all that safely and correctly plotted, you can start introducing your double-salient changing running resistance, from increasing grade or whatever. Note that the desired force now needed to produce your constant acceleration is going to be changing, in ways that I frankly wouldn't want to have to try calculating deterministically, but as you seem to find it significant, have at it. This will give you a set of nifty curves at fixed ACCELERATION with differently-varying (but consistent-delta) train resistance. And you can figure out the steam demand to produce that acceleration over that time or distance... with a little more work.
4) Note that only after you get all of this done can you start looking at how the steam is supplied to the ports in order to produce the required power over the required time. This is going to involve both flow calculation (during admission) and then recuperation (during expansion when there is no admission) and yes, there may be some family of cutoff vs. pressure that produces the 'same' acceleration over at least part of the desired range.
But in any case, with the throttle full, you can now start figuring out how the steam actually flows into the chest during the admission time, and what the average mass-flow change for steady-state acceleration under each condition will be, and what mass flow is being required to achieve it. You'll see what I've been telling you: that any actions to increase steam generation are required only as the steam comes to be needed as flow at the valves, and changing it may require careful anticipation but is NOT the typical sort of load-following you see with an 'accelerator' on an IC engined vehicle.