gregcwhat would it take to produce half/twice as much steam?
what does following table suggest?
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greg - Philadelphia & Reading / Reading
i believe the answer is to increase the strength of the fire, increase the amount of coal per unit of time, increasing its depth. the previous post discusses the depth of a fire, that it varies depending on load.
just as on a car, horsepower increases by allowing more fuel into the engine. fuel (gas, diesel, oil, coal, wood) are the source of energy. The answer may be more obvious on an oil burning steam locomotive where the fireman controls the fuel rate using a valve
Firing the Steam Locomotive simply says "maintain proper steam pressure". Is there any way to measure the strength of the fire except to maintain boiler pressure during the operation of the locomotive? so without a means to measure the strength of the fire, it depends on the experience of the fireman and engineer (i.e. art).
i had sincerely hoped some ex steam locomotive operator would have answered the question (and might still). i'm not surprised that most people thought the throttle/cutoff were the primary means of controlling the speed, I did as well until recently.
1188
gregcjust as on a car, horsepower increases by allowing more fuel into the engine.
you can't just pour gas into a car engine and expect it to run well, the proper amount of fuel and air are drawn by the engine depending on throttle position and rpm.
a steam engine turning twice as fast requires twice as much steam to maintain the same density of steam and corresponding pressure. But providing twice as much steam before it is needed doesn't mean it will enter the cylinders.
the steam flow into the cylinders depends on the difference in pressure between the cylinders and boiler. this results in a flow (lb/sec) that diminishes as the cylinder pressure increase, reducing the difference in pressure across the throttle. Cylinder pressure is unlikely to reach boiler pressure at speed. (A much different story when starting).
if the engine is turning too fast for the available amount of steam, the boiler pressure drops, reducing the difference in pressure across the throttle, resulting in less steam flowing into the cylinders, less density and pressure.
there is a balance between speed and boiler pressure. if the locomotive picks up speed, more steam is drawn, reducing boiler pressure, less pressure difference across the throttle, less steam flow (lb/sec), density and pressure in the cylinder and less tractive effort resulting in the train losing some speed.
there are just a few PSI difference across the throttle during sustained operation
if the fire is too strong for the speed of the engine and doesn't consume all the steam produced by the boiler, boiler pressure increases and the pop-off valves start venting, wasting steam.
so the strength of the fire should be increased gradually as speed increases to avoid wasting steam, water, fuel and fireman effort
if the engineer abruptly throttles down, limiting flow into the cylinders, the unused steam causes the boiler pressure to increase and the pop-off valve to start venting. the fire is allowed to diminish when anticpating the need to reduce speed.
the engineer uses the throttle and brakes to handle manage significant reductions in speed or tractive effort (e.g. downhill). The engineer uses cutoff to optimize the use steam, water and fuel.
so the fireman is essentially the gas pedal on a steam engine, determining the fuel rate (lbs coal / time) and the horsepower of the steam engine
1198
gregci believe the answer is to increase the strength of the fire, increase the amount of coal per unit of time, increasing its depth. the previous post discusses the depth of a fire, that it varies depending on load.
I think you are misunderstanding the jist of the source material. It is addressing fuel economy. If a train has a light load, the fireman doesn't need a heavy bed of coal, if the train has a heavy load the fireman needs a bigger bed of coal. It's not that the thickness of the bed makes the train go faster or haul more, its that a lighter bed can keep adequate steam to handle a light train without burning excess coal. A heavy train and are uses more steam, thus the fireman needs to provide more BTU's to the boiler to keep up with the steam demand.
Dave H. Painted side goes up. My website : wnbranch.com
A reciprocating steam locomotive is a 'pressure engine' as far as its producing power is concerned. The thing producing that pressure is heated steam, and while we discuss 'thermodynamics' in terms of heat, all the heat in the world won't propel a locomotive if it is not correctly applied to make pressure the engine can use.
If anyone has watched the Walschaerts discussion from the C&O group (in another current thread) they will appreciate how piston thrust is not evenly applied to the driver rim proportional to rotation, and it will not be far from there to understand why there are torque peaks (four times per revolution) with fixed or short cutoff at high boiler pressure and low speed. Near starting, you easily trade throttled lower peak pressure and larger mass flow achieved through longer admission for spiky torque, but (as with Tuplin) you will have increasing mass flow to go with your lower pressure to make the same effective average torque, and at some rotational speed you will start running out of steam-generation capacity to supply that mass flow. You could, at that point, open the throttle full, but all that does is remove any pressure restriction between the boiler and the valves, and if the cutoff is set 'late' you are filling a larger and larger volume of cylinder with full-pressure steam, which will still have considerable pressure as the exhaust valves start to crack open. A great deal of the potential work, and heat content, of that steam then gets more or less wasted as it starts to thunder into the exhaust tract... and chokes it, still at relatively high pressure, so there are back-pressure and compression issues out the wazoo.
For all intents and purposes, on a well-designed engine the pressure ANYWHERE won't exceed what the pops are set for. (In fact on large engines there are multiple pops set 2psi apart to provide larger relief capacity for very large boilers that are abruptly throttled while on solid-fuel higher firing rate... but that is a story for a different question.) We can arrange compression (of the residual steam in the cylinder after the exhaust valve closes on a stroke) to be higher, and in some cases dramatically higher, than nominal MEP, but it can waste momentum and cause wear, tear, tribological and sealing issues as the piston goes through the subsequent dead center -- it has a use other than 'cushioning' the rod mass through center; it assures that the cylinder pressure and saturation temperature is close to admission pressure as the valves come open, so there are no flow problems in the 'dead space' that would decrement performance at high cyclic. This can be a bit technically involved to design, but very simple to operate in practice.
In order for speed in the example to increase from 20 to 30 mph:
1) The engine will have to have enough 'steam' to accelerate itself and its train against the running resistance, including that approximated by the Davis formula. That is done by increasing the thrust per stroke, which in turn is accomplished by admitting more steam THROUGH THE VALVES.
2) To run steady at the new speed, the mass flow of steam has to produce drawbar pull that 'just balances' train resistance at the new speed. That will be less than what was required to accelerate it that last mph from 29 to 30, and accordingly you can MOVE THE REVERSE slightly so the engine settles at the new speed economically.
3) To get the necessary mass flow, your boiler has to be capable of producing it. If you have even remotely competent firing and reasonable engine maintenance, the boiler will have been reasonably near popping off the whole time -- which may or may not imply that the fireman had to fire more intensively, or 'trade water for steam', or conversely use the injector a bit more aggressively to keep the engine from popping (and wasting steam mass and its heat content) when the demand for acceleration is relaxed. But that is done entirely relative to gauge pressure and level... no one but an idiot thinks you can pressurize the boiler to "104%" of its safety-valve capacity, like the engine rating on the Space Shuttle, for a little emergency wartime power or whatever. This is part of the reason why it Does Not Matter If The Throttle Is Fully Opened ASAP. On any modern engine with long-lap, long-travel valves ALL the effective admission of steam, and hence consumption of steam, is regulated via the valve gear cutoff once you're above relatively trivial speed.
There are places of course where a limited throttle opening is highly desirable -- one such just came up in a discussion on RyPN involving 614T, a couple of days ago. If the engine is drifting, or producing relatively limited power keeping a train stretched on a downgrade, you'll find benefit in opening the throttle only a relatively slight amount (some engines in fact have a designated 'drifting throttle' or setting that maintains a limited pressure in the steam chests to exclude air and keep cylinder oil warm). We have had a number of discussions over in the Trains forums about 'snifting valves' over the years, and while those aren't relevant directly to this discussion, they will aid understanding of precisely how these locomotives make power and run smoothly.
dehusmanI think you are misunderstanding the jist of the source material.
all i tried to point out is that the strength of the fire, the depth of the coal, can be adjusted.
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It might be useful for some of the would-be Chapelons in this thread to think about the concept and measurement of 'grate limit' with respect to extending the engine performance out to more extreme limits than the 30mph in the example. There is usually a peak efficiency for a given boiler's fuel consumption vs. steam mass produced, but that is not the 'highest' mass flow that the boiler can produce -- if it is more heavily fired and drafted, an increased proportion of the fuel will be 'wasted' BUT at least some additional mass flow at 'governed' pressure can be produced. And that means the engine can run somewhat faster under load than it would if cleanly fired.
Some railroads routinely abused, or allowed, their engines to be 'beat' this way, usually by allowing longer or heavier trains to be pulled except on 'ruling' resistance sections. That was usually explained to be an economic, not a technically-based, decision.
if you give it some thought, the cylinder pressure does not need to be near maximum under all conditions (e.g. no train, slow speed, max speed and/or max grade).
as mentioned repeatedly, cylinder pressure depends on the amount of steam in the cylinders, it's density/pressure. And the amount of steam increases as speed increases as the # of cylinder volumes that need to be filled increase -- more steam is needed to maintain cylinder pressure as speed increases
the following is a crude estimate for the fire strength (% max) for several speeds (mph) and grade (%) working backwards from the combined train resistance and force due to grade, the required tractive effort, cylinder pressure (or mean effective pressure), steam density and cylinder volume per sec, lbs steam at that speed and the % of max evaporation rate to achieve that amount of steam, lb/sec, given the grate area
the following equation is used to map tractive effort ito cylinder pressure
Max evaporation rate is based on burning 150 lb coal/hr per sq.ft of grate area and 1520 BTU to raise water fro 62F to 400F or 220 psi.
engine parameters are based on a Reading I-10 2-8-0
the values show that a relatvely small (~3%) fire can maintain a 5000 ton train at 10 mph no grade but increases to 33% on a 2% grade. maximum steam 28.6 lb/sec is reached at 30 mph on a ~2% grade, at 40 mph on ~1.5% and at 50 mph on ~1%.
grate area 95.0, max steam lb/sec 28.6, tonnage 5000 mph cps res gr resT PSI lb/s fire 10 1.84 11 0.0 11.4 22 1 3 % 10 1.84 11 0.5 36.4 71 3 11 % 10 1.84 11 1.0 61.4 120 5 18 % 10 1.84 11 1.5 86.4 169 7 26 % 10 1.84 11 2.0 111.4 218 9 33 % 20 3.67 14 0.0 13.6 27 2 8 % 20 3.67 14 0.5 38.6 76 7 23 % 20 3.67 14 1.0 63.6 125 11 38 % 20 3.67 14 1.5 88.6 174 15 52 % 20 3.67 14 2.0 113.6 223 19 67 % 30 5.51 17 0.0 17.1 34 4 15 % 30 5.51 17 0.5 42.1 83 11 37 % 30 5.51 17 1.0 67.1 131 17 60 % 30 5.51 17 1.5 92.1 180 23 82 % 30 5.51 17 2.0 117.1 229 30 104 % 40 7.35 23 0.0 22.6 44 8 27 % 40 7.35 23 0.5 47.6 93 16 56 % 40 7.35 23 1.0 72.6 142 25 86 % 40 7.35 23 1.5 97.6 191 33 116 % 40 7.35 23 2.0 122.6 240 42 145 % 50 9.18 31 0.0 31.2 61 13 46 % 50 9.18 31 0.5 56.2 110 24 83 % 50 9.18 31 1.0 81.2 159 34 120 % 50 9.18 31 1.5 106.2 208 45 157 % 50 9.18 31 2.0 131.2 257 56 194 %
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I did have a reply to this thread, but in two sentences I lost the cursor five times to the popup ads. I will not install an ad blocker just for this site. I have better stuff to do. So bye.
Sheldon
of course those value are closer to idea than practical but valid for comparision. my understanding is that the overall efficiency of steam engine is ~6%, because of
hopefully no one continues to believe that the throttle controls the tractive effort of a steam locomotive like a gas pedal on a car
1590
gregc hopefully no one continues to believe that the throttle controls the tractive effort of a steam locomotive like a gas pedal on a car 1590
Greg, not trying to get this thread locked, but I believe!
Are you suggesting that the speed of the engine is controlled by how thick a bed of coal there is in the firebox?
the coal bed thickness will affect the tractive effort which needs to increase to increase speed on level grade or possibly maintain speed going up a grade
1671
The thickness of the bed of coal will affect the amount of steam generation. Not how fast the locomotive will go.
wrench567Not how fast the locomotive will go.
what determines "how fast the locomotive will go"?
if you're going to say when the tractive force matches the train resistance, what determines the tractive force?
if you're going to say the cylinder pressure, what determines the cylinder pressure?
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wrench567To start a train. The engineer first moves the quadrant to either forward or reverse, then opens the throttle. Like a manual shift car. Select first then gas. As your going down the track and you want to go faster, move the quadrant a little further. Once you are at desired speed, move the throttle valve to almost closed. Your still admitting steam but not the volume to go faster.
what happens to the steam being generated that is now blocked by throttle from reaching the cylinders?
wrench567Pop valves are safety valves to keep the boiler pressure up to a certain level. A good fireman will try to keep them from popping off. That means he is wasting steam.
The steam will stay in the steam dome until either the safeties lift or the throttle is opened again.
I've seen live steam locomotives continue for quite awhile after the fire was dumped. They run on what was left in the dome.
Pete.
wrench567The steam will stay in the steam dome until either the safeties lift or the throttle is opened again.
what about the steam that continues to be generated by the fire immediately after the throttle is "almost closed"?
and what determines "how fast the locomotive will go"?
in order to maintain speed, i believe the fireman just maintains boiler pressire.
brakes can always be applied to slow a train more quickly. presumably the throttle is also closed. with the cylinders no longer consuming steam, the boiler pressure goes up and the pop-off values start releasing steam
but if the crew knows they need to slow, the fire and boiler pressure can be maintained at a lower level. with reduced pressure, less steam enters the cylinders resulting in less tractive effort and the train slowing.
As the speed reduces to what the crew wants, the fireman may need to increase the fire to maintain the boiler pressure at the "normal" level. reducing speed without applying brakes wastes no steam, water or coal.
to increase speed, the fireman can build up the fire, maintaining boiler pressure close to max boiler pressure, anything great just wastes steam out the pop-off valves. the engineer can increase cutoff allowing more steam into the cylinders resulting in a higher pressure.
in order to maintain the higher boiler pressure as the train picks up speed, the fireman needs to correspondingly increase the strength of the fire.
higher tractive effort is needed to accelerate the train than to maintain speed. but exeeding max boiler pressure just wastes steam, water and coal
when the train gets close to the higher speed, the fireman and engineer allow the boiler pressure to drop to the "normal" level by not adding as much coal to the fire and the engineer reducing cutoff until the train settles at the desired speed ... of course this is an art, the result of experience
1818
gregcthe coal bed thickness will affect the tractive effort which needs to increase to increase speed on level grade or possibly maintain speed going up a grade
Indirectly maybe, but that isn't a "control" used on a locomotive. It's a one way thing, a fireman can increase the bed thickness, but there isn't a way to decrease the bed thickness (other than to let the coal burn down). You can only control increasing it, it can't be decreased in a controlled manner. If I have an 8" bed cresting the hill and I only need a 3" bed downhill, guess what, can't make that happen.
I could carry an 8" bed all the time, the downside is I would just be wasting coal and blowing steam out the release valves.
Yes how much fire there is can affect how much steam can be created but it's not really controlling the tractive effort, it's just affecting how fast and how much steam is produced. Its an axe, not a scalpel.
Ya gotta know adding coal to the fire and changing the bed thickness isnt instant steam production.
And yes you can drop the thickness of the coal bed via the action of "shaking the grates". Sometimes you have to do this to drop ash into the pan, or get unburnt coal out of the way. Either way the end result is to promote draft which directly results in better efficiency and get the right bed thickness for what you need ahead.
Shaking the grates lowers the coal bed, Stokers add to the coal bed (if stoker equipped, if not..... shovel and a stiff backside).
However, neither give an instantaneous steam generation effect. Try as you might, your not going to get away from the fact the 'throttle' is your source to make the loco go faster.
It takes all these things your talking about to make the throttle work. It takes a throttle to make the loco work. In that order.
PMR
dehusmanIt's a one way thing, a fireman can increase the bed thickness, but there isn't a way to decrease the bed thickness (other than to let the coal burn down).
how quickly do you think the fire needs to burn down and boiler pressure to start dropping for the crew to anticpate a change in speed for a 5000T train?
it probably takes just as long for a fire to burn down by not adding coal as it does to build up when adding coal
dehusmanI could carry an 8" bed all the time, the downside is I would just be wasting coal and blowing steam out the release valves.
right
i'm not discussing running a engine inefficently, such as with the throttle partially closed and the pop-ooff venting steam
dehusmanYes how much fire there is can affect how much steam can be created but it's not really controlling the tractive effort, it's just affecting how fast and how much steam is produced.
this is yet another statement saying "no, that's not how it works", without describing how it does?
what do you think determines the tractive force?
are you suggesting you can run twice as fast without increasing the amount of steam produced/consumed by the engine?
1916
PM RailfanYa gotta know adding coal to the fire and changing the bed thickness isnt instant steam production.
how quickly does a 5000 T train need to change speed. building speed or using brakes requires anticipation
and i'm not suggesting that the throttle isn't closed or brakes not used on a switching loco in a yard making frequent stops/reversals
i am suggesting what it takes to run a steam engine and run it efficiently
but consider how quickly the strength of a fire on a oil powered steam locomotive can change
gregc wrench567 The steam will stay in the steam dome until either the safeties lift or the throttle is opened again. what about the steam that continues to be generated by the fire immediately after the throttle is "almost closed"? and what determines "how fast the locomotive will go"?
wrench567 The steam will stay in the steam dome until either the safeties lift or the throttle is opened again.
Remember that steam is a gas. It will continue to build pressure until the safeties lift.
wrench567Remember that steam is a gas. It will continue to build pressure until the safeties lift.
of course. what do you think i'm missing?
what determines "how fast the locomtive will go"?
Wheel diameter, size and stroke of the cylinders, amount of steam getting through the ports, weight of train, gradient of track and courage of the crew.
The PRR T1 and K4s were known to go better than 100 mph at times. The N&W J was known to push 100 even with the smaller drivers.
By the 19 teens, locomotive development was to the point where they could design for specific jobs. There wasn't really a general purpose steam locomotive.
gregc how quickly does a 5000 T train need to change speed. building speed or using brakes requires anticipation
Ya know, thats a question that can again, can have a boxcar load of different answers. So, I am just gonna tell ya your gonna have to get some seat time and see for yourself..... just what it takes.
Best answer there is - hands on. But dont feel bad, everyone here needs some, including myself!
"..... what determines "how fast the locomtive will go"?
I dont know what rule people use today but, todays rule didnt exist back then. The rule of thumb for all time is this....
"1 mile an hour for every inch of driver size plus 10 miles an hour."
Loco with 53" drivers (would be a freight loco) could go 60mph. 70mph tops if your having a winning lotto ticket kinda day on the railroad. Anything faster......
You owe them a million dollars!
wrench567Wheel diameter, size and stroke of the cylinders, amount of steam getting through the ports, weight of train, gradient of track and courage of the crew.
i've considered all of the above, including grate area, to determine the flow (lb/sec) of steam entering the cylinder to determine the density of steam in the cylinder and its pressure
here are some value from the other thread on Bershire water consumption
25.0 mph 4.1 cps 30401.1 te 37.0 cylPsi 9.7 cylCuFt 78.4 cuFtPsec 6.4 lbPsec 23191.0 lbPhr 183480.0 lb tender 7.9 hr 197.8 mi
that ~150 lb coal per sq.ft of grate area is about the max. that there are ~11000 BTU/lb coal
but is any of that needed to understand that more energy, fuel (coal/oil) is needed to increase the power (tractive force) to overcome train resistance and gravity due to grade, to accelerate and maintain the speed of a train.
no one has a problem understanding the same for a car, that more gas is needed to go faster or up a hill
2110
PM Railfan"..... what determines "how fast the locomtive will go"?
my mistake. should be
how does a steam locomotive increase speed?
happen to look at some results that suggest a theoretical answer to this question.
of course max speed depends on various things. but i believe the simplest answer is the speed where the train resistance equals the max tractive effort above which results in slip.
i've been working with values for the Reading I-10 for which i have data for.
the following shows that max Tractive Effort (TE) is 71048 lb for an adhesive weight of 284190 lb. That max boiler pressure of 220 psi in a 27" cylinder is 224667 lb, easily exceeding the max TE. The train resistance roughly equals the max TE at 55 mph for a 5000T train
284190 adhesive wt (lb) 71048 max TE 220 Blr PSI 27 cylDia (in) 573 cylArea 224667 max cylPsi 5000 tonnage 55 mph 71794 res
of course, there are mechanical and efficiency limit that i don't understand that prevent such a locomotive from reaching much higher speed (e.g. 100), but i believe these values are in line with what the locomotive was designed for
why am i doing this?
because i'm trying to understand how a steam engine works and the nuances of it's operation. it would be great to get first hand experience from an operator.
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