Take it to the backshop for repairs? If the throttle is fully open, I believe the cylinders are getting all the steam they can get. Kinda like if you're driving your car with the accelorator to the floor and you're only going 20 MPH. When starting the engine, the engineer might put the throttle fully open to get it started, but then back off as the engine gained speed.
greg - Philadelphia & Reading / Reading
First, you answered your own question - How to make it go faster: open the throttle and brakes off. (ofcourse you have steam or it wouldnt move to begin with)
However, you posted throttle wide open - I'd say just sit back and watch her go at this point. You cant open the throttle any more than WOT (wide open throttle).
When shes 'over the shoulder' (meaning throttle wide open) and "lever's in the corner" (meaning full forward - no cutoff), you cant get more from a loco than that. If you cant move the train with all that..... get a helper loco youve exceeded the tonnage rating for this one loco.
With the 6 points you listed, all you need do is spot your pocket waltham and count posts as they go by ( how engineers used to gauge their speed).
A#1 North!
PMR
(PS: you just dont WOT a steam loco. They must be coaxed to speed. Do so, you will most certainly spin the drivers, yank a coupler [which is all kinds of bad news], pound the rails, even blow a few pipe or piston seals. One must remember, even the sportier versions of steam locos must be finessed to speed. They arent race cars. And the only traction control steamers had was sitting in the cab. Dont forget shock control - [slack action] people and products dont like being slammed around. Ive heard steamers break cadence into din of exhaust noise even at a good clip. impressive, but detrimental. PRR T-1s are known for slipping, so much so I think it was part of their design. Very touchy throttles requiring almost a master of the art. You cant just throw it over your shoulder if you want to gain speed. In all actuality, your rear end does more driving than your brain does, its a seat of the pants job - experience! The best job on the planet BAR NONE!)
To echo what is already said - it seems the only way to go faster there would be to go downhill! There may be some situational environmental conditions that could effect stuff - if it is cold out you might need to open the cylinder cocks to let out condensed water in the pistons, if it is cold you may also check for wheel slip - you may need some sanding action. This is why it tended to be important for engineers and firemen to be used to their locomotives, there's a lot of little things that they'd know or not know from experience about the situation to know why they aren't hitting 30.
wjstixIf the throttle is fully open, I believe the cylinders are getting all the steam they can get
so you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed?
what happens to the excess steam that isn't needed at lower speeds?
PM RailfanFirst, you answered your own question - How to make it go faster: open the throttle and brakes off.
those are the conditions when the loco is running at 20 mph.
allegedlynerdyit seems the only way to go faster there would be to go downhill!
trains needs to be able to run at different speeds. this could be a passenger train capable of running a 80 mph that is moving thru a yard
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gregc wjstix If the throttle is fully open, I believe the cylinders are getting all the steam they can get so you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed? what happens to the excess steam that isn't needed at lower speeds? PM Railfan First, you answered your own question - How to make it go faster: open the throttle and brakes off. those are the conditions when the loco is running at 20 mph. allegedlynerdy it seems the only way to go faster there would be to go downhill! trains needs to be able to run at different speeds. this could be a passenger train capable of running a 80 mph that is moving thru a yard 286
wjstix If the throttle is fully open, I believe the cylinders are getting all the steam they can get
PM Railfan First, you answered your own question - How to make it go faster: open the throttle and brakes off.
allegedlynerdy it seems the only way to go faster there would be to go downhill!
If the boiler is at max pressure, and the throttle is fully open, that means all of the steam the boiler is capable of producing (maximum pressure) is being used (let out through the throttle). If the boiler was under temperature, the pressure would be going down with open throttle, as there would be more steam exiting than being produced. If there was more steam being produced than there was being used, it would be gaining pressure. In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph.
Putting more coal/wood/oil on the boiler may give an increase in speed, as going over pressure may give an increase in throughput, but that would very much be a matter of judgement for an experienced engineer, I wouldn't want to increase pressure on the boiler past maximum if I wasn't certain of the result.
allegedlynerdyIn this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph.
so are you suggesting that speed depends on boiler pressure?
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here's test data showing that boiler pressure is roughly the same (near 200 psi) for various speeds: ~9, ~20, ~800, ~40 mp
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gregc here's test data showing that boiler pressure is roughly the same (near 200 psi) for various speeds: ~9, ~20, ~800, ~40 mp 313
While this is interesting, I would note my qualifier relating to the boiler being at maximum pressure - unless we know the "red line" for this locomotive was 200 psi, this would not fall under that criteria. Most consolidation boilers ran around 215 psi from what I've read.
It's like with your car - if you aren't redlining, you can rev up more to get to a higher speed then settle into a similar RPM at that new speed, but if you are already redlining and aren't increasing speed, you aren't going to get any faster.
gregc allegedlynerdy In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph. so are you suggesting that speed depends on boiler pressure? 302
allegedlynerdy In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph.
allegedlynerdythe issue in your scenario is that you are already at maximum pressure.
gregcboiler pressure is being maintained near maximum
i said "near" for a reason.
what would cause an increase in boiler pressure (other than closing the throttle)?
allegedlynerdybut it'll settle back down when you return to status quo
does status quo mean orignal speed or new (higher) speed?
allegedlynerdyIt's like with your car - if you aren't redlining, you can rev up more to get to a higher speed then settle into a similar RPM at that new speed
i don't believe the car analogies work here because motor rpm is directly related to speed.
the table shows that locomotive speed is not directly related to speed boiler pressure
362
Watch the video that Ed posted earlier on valve gear. There's actually 2 valves inline in the steam passages. One is the throttle/regulator. The next in line is the cylinder valve gear and valve. The throttle admits a given volume of steam to be used by the cylinders. The valve gear regulates the steam to the cylinders.
To 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. Closing the throttle valve is like taking your foot off the gas while in gear but no braking pressure from the engine. Putting the quadrant in the center with the throttle closed is essentially like neutral. Opening the throttle with the quadrant in center can blow steam into the smoke box because of leaking cylinder valves. Not a good idea.
Pop 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. Before a large use of steam he will add more heat and maybe shut down the injectors. When boiler pressure is almost to the valve setting he will open the injectors and add water instead of making steam.
Hopefully this explains it for you.
Pete
Sheesh, there is some wack stuff in this thread.
Standard operating procedure for 'best thermodynamic efficiency' has long been 'get the throttle fully open as fast as you can (to avoid throttling losses from the boiler/superheater to the steam chests) and then 'drive on the reverse'.
Under the stated initial conditions, developed power is just balancing running resistance at 20mph. To 'go faster' you will lengthen the cutoff to increase the mass flow per stroke. This will draw additional steam from the boiler, which will cause a (relatively slight) drop in boiler water level. (And if the pops were getting ready to lift, they won't then...)
Water level in the glass will drop and eventually require the injector or feedwater-heater hot-water pump to be actuated. Even heated feedwater is many degrees colder than the saturation temperature of the boiler water, so nominal pressure will tend to drop. On the other hand the additional mass flow of steam means additional mass flow in the exhaust... hence more draft, hence a brighter fire. (This is part of what the English call 'automatic action')
At some point, probably before balancing at 30mph, you will resume the 'firing early and often' that you should have been attending to at 20mph. A good fireman will estimate the additional fuel the engine will need at the new higher speed on the relevant part of the railroad... and fire and trim accordingly.
(Incidentally, the more correct way to start, if you are concerned with slipping/spinning -- as in a PRR T1 with a heavy train negotiating a bunch of station switches with low joints and worn frogs -- is to open the throttle only partway and 'drive on it'. A slip then produces wiredrawing loss that more or less promptly exhausts the steam in the chests, in the same sense that a slipping LP engine on a Mallet quickly exhausts the receiver. And the slip stops, without the overshoot and stall that would likely result if you tried arresting a momentarily overpowered slip by slamming the throttle closed and trying to horse it out again.)
There is an interesting quirk about reciprocating steam locomotives, which explains why big boilers and fireboxes/chambers are a key determinant of high speed capability. The 'balancing point' between steam generation and steam demand with the engine(s) in full gear, even with radical fixed cutoff like the 50% in a PRR I1s, would limit speed to a comparatively low number. So you shorten cutoff to increase how effectively the steam mass in the cylinder produces piston thrust -- you let it expand and do work, rather than blow a cylinder still full of pressurized steam wastefully to exhaust.
But eventually you'll reach a point where the decreasing steam mass going into the cylinder in a given admission no longer produces acceleration. To go faster, you need to start lengthening cutoff again, which of course requires more mass flow out of your boiler.
Now, plenty of 'fast' engines run into a wall of pain at this point because their steam tracts or valves are incapable of handling the increased necessary mass flow in the shorter and shorter interval that admission occurs per stroke. The ATSF 3460 class, with a ginormous boiler and 84" drivers, could easily run 100mph on the relatively flat east end. But it could barely reach 105mph under the best conditions... valve limited. The C&NW E-4b, another monster Hudson, couldn't even crack 100mph with the AAR test train in 1938.
Engines not so limited turn out to have an interesting statistic: the English A4 Pacific, the German BR 05, and the N&W J all made their fastest road speed with cutoff in the lower 40s
Greg) "...so are you suggesting that speed depends on boiler pressure?"
Yes, that and volume. Pressure AND volume.
Pressure = The force you can throw at the work load. (if you dont have psi, you cant turn the wheel over once)
Volume = The amount of time you can continue that work load. (if you dont have volume, you cant turn the wheel over a second, third, of fourth time)
".... the table shows that locomotive speed is not directly related to speed"
wait.... what? That doesnt make sense. In any lan'guage'.
I wanna go back to your very first post. You said,
"you are doing '20', throttle is 'open full', cut-off at 'minimum', brakes 'off', bpsi near 'max', fire and psi are 'maintained', .... "
"what needs to increase and maintain at 30?"
First off, if thats your top speed at WOT then (full open throttle and boiler is ok you said), your either on an 0-8-0 with 25" drivers (LOL) or the Best Friend of Charelston. Running light!
Second, if thats WOT, then thats all shes got. WOT is WOT! You cant really change things on the fly with a built loco in operation. Oh sure, you could tie down the safetys, drop off on the cut-off a bit and flog the cylinders, maybe gain a few miles that way..... until she blows or flies apart!
10mph increase to 30mph is 1/3 the rate of speed for this (????) loco. Thats a big jump for a loco. You might up the driver size and the boiler psi. that might do it. though for a bit as youd run out of steam (no volume). And you cant make those changes at 20mph. SO,
For a 1/3 gain in speed, Id do what the Railroads did.....
get a bigger, more modern loco.
Now,
if you mean your loco (????) is already capable of 30mph., but only doing 20mph at WOT and you wanna do the 30 its capable of, well.... then either your doing something wrong, your train is too heavy (too much tonnage for this loco), or you have something dragging (like brakes or a hot box).
Again a loco at WOT doesnt have any more notches left to go thru.
I think a bit of clarification would be required here.
PM RailfanAND volume.
yes and what determines the amount (lbs.) of steam produced?
PM Railfanif you mean your loco (????) is already capable of 30mph., but only doing 20mph at WOT and you wanna do the 30 its capable of, well.... then either your doing something wrong,
i'm suggesting it's capable of 80 mph
aren't steam locomitives designed to run at different speeds, efficiently?
(what determines, not controls, the horsepower produced by a car)?
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Overmodget the throttle fully open as fast as you can
as that table suggests, the engine is typically operated near max boiler. allowing full boiler pressure into the cylinders while the engine moving slowly and cylinder valve is opened for long periods of time is likely to cause slip.
while that table also shows the throttle open full, this is while running. my understanding is the throttle is just "cracked" to get the locomotive started to limit the amount of steam into the cylinders and the cylinder pressure. throttles are designed to have more finer control near the beginning of there range
the throttle is opened fully once the locomotive is running.
OvermodTo 'go faster' you will lengthen the cutoff to increase the mass flow per stroke. This will draw additional steam from the boiler, which will cause a (relatively slight) drop in boiler water level.
as the locomotive picks up speed, more steam is required to maintain the steam density/pressure in the cylinders (twice as much steam is needed at twice the speed).
while increasing cutoff may allow more steam into the cylinders and increase speed momentarily as more steam flows from the boiler, boiler pressure will drop ... until a new equilbrium is reached, more likely at a lower speed
my understanding is cutoff is reduced to improved the efficiency of the steam of the engine
at the least, reduced cutoff allows greater boiler pressure to be maintain if not a greater amount of steam to enter the cylinder resulting in higher density/pressure
630
Greg) What determines the psi produced in a boiler? Holy cow any number of a million things, at any different time, under any number of a million circumstances.
thats rather an unfair question as there are so many ways to make steam psi. in a steam loco.
Ok, your loco is capable of 80mph. that tells me you have to have atleast 70" drivers. Having that size driver, and the fact it can do 80 tells me its a modern steam loco (90% chance). Being a modern loco tells me its prolly Superpower design capable of producing all the steam it can use, and a good chance of being superheated aswell.
Knowing this, plus your givens about throttle, boiler status, cutoff, and speed of 20mph, its safe to say your prolly spinning the drivers right now.
After you correct that and open the throttle again - more gently this time, your train should pick up speed.
sit back and count the posts you pass lineside against your watch your on your way to 30mph. Thats if you are doing 20 and JUST opened the throttle to full. Shouldnt take long if your train is of proper tonnage and your loco is up to snuff after you stop the wheel slippage.
However, if you pulled the throttle open about 10 miles back and your still doing 20......
Your definately doing it wrong. Hit alt-Q and reset your game.
What determines hp in a car? Again, that is such a can of worms question it cant really be answered here. suffice it to say everything from cylinder size to octane of the gas your using, to the intelligence of the engine builder, is in the formula to figure that one out.
And just so you know, since the 70's, the "controls" ARE part of the horsepower making formula. So you cant discount that.
PM RailfanPM Railfan wrote the following post 8 minutes ago: Greg) What determines the psi produced in a boiler?
gregcwhat determines the amount (lbs.) of steam produced?
not pressure (PSI, lb/sq.in), mass (lbs)
PM RailfanKnowing this, plus your givens about throttle, boiler status, cutoff, and speed of 20mph, its safe to say your prolly spinning the drivers right now.
so a steam engine can only run at one speed ???
PM RailfanWhat determines hp in a car?
does a car only generate a constant amount of hp? only runs at one speed?
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Wait LOL, I totally missed that one.....
"What 'determines' hp in a car?"
I feel so stupid! What determines hp.... lol.... its the same in railroading as in cars.....
A DYNAMOMETER!
For trains its a dynamometer car. For cars its just Dynomometer, or 'Dyno' for short. It can be in ground or in room.
dang cant believe i didnt catch that! Ya got me!
Lbs of mass? Now why in the world would you wanna know that? No one I know figures that into the weight of a loco.
At any rate, the mass would be the area of steam in the cavity between water level and roof of boiler. This would be forever changing due to changing water level itself and the rocking of the locomotive (though not much as the flues would act as baffles).
Take the cubic area of the boiler, minus the volume of water, now you have the void. Multiply that amount of cubic area times the density of steam and you have your answer.
Now, you know very well they can run from one mile an hour to their repected top speed.
What I am saying is you just opened the throttle to WOT on a slow moving train with a powerfull locomotive. Thats like stomping the gas from a slow roll.... you think that VW Bug aint gonna pop a wheely and tear outta there?
As for the car part - same answer. On a dyno-scope, you will see a graph - RPM vs 100's. The 100's is the range for HP and Torque (both are usually measured at same time - SOP).
As RPMs grow, so does HP and Torque until @ 5252 RPM they meet. At some point the HP and Torque tend to drop off. Who knows where in the grapgh but always after the 5252 point.
And you know as well as the rest of us, cars run at varying speeds. lol.
PM RailfanA DYNAMOMETER!
not measure. By "determine" i mean the thing (purposely vague) that when changed results in a change in HP and i don't mean what "controls" it.
the thing that needs to change to accelerate or run at 50 vs 20 mph
PM RailfanLbs of mass? Now why in the world would you wanna know that?
gregc what determines the amount (lbs.) of steam produced?
not the amount "in" the boiler.
doesn't more steam result in more power ???
When running you cant change the physical parameters of an engine. But yo can alter its fuel, air, timing, all of which can either enhance or deter hp/torque.
In the old days we called them carberatuers. Now, its fuel injection, electronic spark control, and a computer controls it all through the reading of a myriad of sensors. this is why you cant discount the control side. It is half of the part that make HP now.
And, it even adjusts speed! My 2012 tacoma is not fly by wire aymore. Meaning the cable that connected my foot to the "speed control" is gone. Its now done by sensor.
Since it comes down to what happens in the cylinder, all you need to do is alter fuel, air, compression (turbo), timing. thats it.
(But what type of fuel? what temp is the air? what boost you running? How much advance in the timing you have? Alot of stuff huh!)
PM RailfanBut you can alter its fuel
and on a steam engine?
gregcthe thing that needs to change to accelerate or run at 50 vs 20 mph
More steam to the cylinders. Not necessarily more pressure in the boiler, more steam.
The nuance your question misses is that that there are multiple things that can affect that, the pressue, the timing of when the steam is admitted, how long before it's exhausted, the physical ability of the pipes and valves to admit the steam, etc.
Dave H. Painted side goes up. My website : wnbranch.com
dehusmanMore steam to the cylinders.
gregc PM Railfan Lbs of mass? Now why in the world would you wanna know that? gregc what determines the amount (lbs.) of steam produced? not the amount "in" the boiler.
PM Railfan Lbs of mass? Now why in the world would you wanna know that?
dehusmanThe nuance your question misses is that that there are multiple things that can affect that, the pressue, the timing of when the steam is admitted, how long before it's exhausted
there are of course a myriad of environmental and design factors that affect performance. I believe i addressed the run-time timing aspect by specifying cutoff. The first chapter of Firing the Steam Locomotive is Cooperation, so it's not all mechanical/thermal/...
but no one has mentioned what i believe is fundamental, even on the earliest of steam locomotives
gregcso you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed?
The fire heats the water in the boiler creating steam, and the steam goes up into the steam dome. The throttle controls how much of the steam in the steam dome goes to the cylinders. If the throttle is closed, the cylinders are getting no steam, so the engine doesn't move (or if it is, it drifts to a stop).
In your question, you state the throttle is fully open, so 100% of the available steam in the steam dome is already going to the cylinders. You can't go above 100%, so 20 MPH is apparently as fast as the engine can go - at least pulling the train it has.
Unless the engine in your question is a geared logging locomotive with a very low top speed, or unless the engine is pulling a very heavy train that the engine can just barely pull, it's unlikely (impossible?) an engine going 20 MPH would be at full throttle. A 4-6-2 with a six car passenger train would probably, once it got up to 20 MPH, be maintaining that speed at let's say 30% throttle. To increase to 30 MPH, maybe you'd increase the throttle to say 40-45%. If you increased to 100% full throttle, on level track, you'd be going 70-80 MPH.
wjstixThe fire heats the water in the boiler creating steam,
how much steam (lbs)? (not a specific number).
what would it take to produce half/twice as much steam?
wjstixso 20 MPH is apparently as fast as the engine can g
what does it take to run a loco at different (higher) speeds
wjstixyou state the throttle is fully open, so 100% of the available steam
this table shows that the speed of the locomotive is determined by something other than the throttle
gregcwhat would it take to produce half/twice as much steam?
what does following table suggest?
943
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.
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.
1350
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 %
1479
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?
1744
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.
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.
2160
gregcit probably takes just as long for a fire to burn down by not adding coal as it does to build up when adding coal
Evidently you've never used a charcoal grill. If you put fuel on the fire it catches fire in a matter of minutes and burns for a long time.
gregcwhat do you think determines the tractive force?
As other people have stated, wheel diameter, cyclinder size, steam pressure, etc., etc.
If the question is how does a locomotive change pulling power while running? Adjusting the throttle and cut off. Very simple.
All changing the fire does is keep an adequate supply of steam based on it's usage, in economics it would be called a "trailing indicator".
You started off this discussion by articifially constraining the the controls an engineer uses to operate the engine (throttle wide open and cut off at max) and then asked how do you control the engine without using the controls.
Get a picture of a really basic locomotive and look at the controls the engineer uses that are connected to the running gear. The answer to you question is in THOSE controls and in the devices attached to those controls.
You could do exactly the same with a car. How does a driver change speed and power in a car? Look at what controls the driver has available. The gas pedal and the gear shift.
The fire and the steam pressure in the boiler may have an effect, but they are not a CONTROL. The fuel of a steam engine is steam, not coal. The boiler's job is to provide an adequate supply a fuel. It takes a relatively long time to change the pressure in the boiler and adjust the fire. That is not a CONTROL.
You appear to be trying to figure out how to run an engine by adjusting the pressure in the boiler rather that by adjusting the pressure in the cyclinders.
dehusmanThe fuel of a steam engine is steam, not coal.
that's simply not true
2206
May be helpful to remember that a steam engine is an external combustion engine (unlike an automobile motor, which is internal combustion). The coal, oil or wood burning in the firebox creates heated air that passes through the boiler tubes to the smokebox, where it is then vented out the stack. The fire isn't in the boiler.
When a steam locomotive completed it's run, the fire would be dumped in the ashpit on the way to the roundhouse. Even with no fire, there was sufficient steam to run onto the turntable and into the roundhouse stall.
Will you stop asking the same question over and over no matter how many people answer it for you, and bringing up more irrelevant details and numbers you don't understand the meaning of?
That isn't even wack enough to be wrong, as a moment's reflection on your part ought to make clear. Maximum speed is the speed at which train resistance (rising with speed) exactly balances developed drawbar tractive effort (falling with speed). It can also be thought of as the point that the locomotive no longer develops enough power to accelerate the train under the instantaneous running conditions.
There is such a thing as 'high-speed slipping', and under certain conditions it is indeed a limitation on achievable top speed. One canonical example was described by E.T.Harley (who knew his duplexes!) in his book on the Q2s -- he was on a train pulled by a T1 in bad weather and poor rail conditions, and there came a point where no further power could be transmitted through the driver contact patches on at least one of the engines (probably the forward one) and it would momentarily slip. This would of course throw the load onto the other one, the engine would slow down, and the slipping engine would 'catch its feet' -- the result being a repeated rhythmic surge, damped by the mass of the tender, not progressing to wild spinning drivers as in the general FRN conception of a T1 slip, but not permitting any further acceleration of the consist.
But notice that this isn't like a tractor pull, where the wheels start slipping when the load is too great. Even for the T1, the much more likely consequence of an overload, even at starting, would be as clearly described in the C&O road testing described by Dave Stephenson (feltonhill here) -- the engine simply stalls, refuses to accelerate, or bogs down.
And you fix that (to the extent you can) by adjusting the REVERSE, to allow more admission. To the extent you do anything with or to the boiler, you control fire and feedwater to keep the nominal boiler pressure as close to the desired pressure as careful experience and knowledge permit.
I will happily get into a discussion of how admitted steam flow translates into internal-combustion engine terms, since nobody else seems to understand that competently, either. But it doesn't apply in any real sense that would give you better understanding of the question you keep trying to ask.
At some point we might have to take up a critical difference between 'gas' engines and steam engines, which is the loss via physical phase change to liquid during the stroke duration (and then the losses involved in re-expansion to vapor when you don't want it, during release). This is not just the usual-suspects wall condensation; it involves what is called 'nucleate condensation' throughout the expanding steam mass as work is extracted via piston thrust, and nucleate condensation is THE principal reason superheating, and not steam jacketing or better external insulation, has such a tremendous effect on locomotive performance.
Actual maximum high speed depends on a number of things like admission time per stroke, shock in the intake tract or flow instabilities near valve unshrouding, and balancing effectiveness. I can assure you that the reason a British 9F can reach 90mph has nothing whatsoever to do with any Reading I-class locomotive, and comparatively little to do with Reading locomotives built using components from I-class locomotives.
Incidentally, here is a question for you: how do we 'conventionally' determine the P in the PLAN formula, and what is one of the common reasons why we do that?
OvermodWill you stop asking the same question over and over no matter how many people answer it for you, and bringing up more irrelevant details and numbers you don't understand the meaning of?
if you can provide a succinct answer?
a basic law of physics is conservation of energy, that the source of energy is the coal. a constant amount of energy is needed to overcome train resistance to maintain speed. a higher amount of energy is needed to accelerate and then maintain a higher speed because train resistance is greater. just as more gas is needed in a car (your gas mileage drop when going faster)
a common answer is that the fire is maintained to maintain boiler pressure. no one has suggested that the rate of coal consumption varies in order to maintain that pressure. Somehow adjusting throttle or cutoff increases the speed while the boiler pressure remains constant.
Overmod 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.
Yes, coal consumption varies, or ought to, with changes in boiler work. A seasoned stoker, who knew the route, would know when to start pouring the fuel to the firebox, and then when to desist. All the while, the boiler is being drawn down, sometimes harder, sometimes less, but the water injected/fed was always cooler than what was emitting from the throttle at the front end.
The HP curve of a steamer shows it rising as speed increases, but only up to a point. By the time the pistons, on a typical steamer, are reciprocating at about 3.5 - 4 times each second, the highest possible horsepower is being put to the piston. This also coincides with the shortest cutoff. Rapid, short, pulses of full-throttle steam are being puffed through the inlets and then expelled via the outlet ports, which keep the steamer at speed.
selectorYes, coal consumption varies, or ought to, with changes in boiler work.
thank you
OvermodIncidentally, here is a question for you: how do we 'conventionally' determine the P in the PLAN formula, and what is one of the common reasons why we do that?
i had never heard of PLAN and had to find a description which says the P is the mean effective pressure that results from early cutoff and expansion.
but the MEP is based of the initial pressure while the intake valve is open. it's not obvious (to me) what that pressure is
one answer is it depends on the amount of steam (lb) in the cylinder and the volume of the cylinder at cutoff which determines the density and pressure (see steam table)
i've been trying to figure out how to estimate amount of steam that enters the cylinder before cutoff.
if each cylinder were at 50% cutoff, all the steam produced by the boiler passing thru the throttle can enter one cylinder or the other because when one cylinder reaches cutoff the other cylinder is just starting its cycle
at 50% cutoff, my understanding is MEP is 87% of the initial pressure
whatever steam produced by the boiler that doesn't enter the cylinders will build up in the boiler, increasing its pressure and possibly vented thru the pop-off valve. If boiler pressure builds, the steam flow (lb/s) will increase, along with density/pressure and tractive effort. there will be some equilibrium point at some speed where all the steam produced by the boiled enters the cylinders
but at less than 50% cutoff, both intake valve will be partially closed some period of time and steam pressure is likely to build during that brief period.
I believe the result in that the same amount of steam that enters the cylinders at 50% enters when at 20% cutoff. half of what is produced by the boiler. since the volume is 40% of what it was at 50% cutoff, the density is 2.5 time higher along with pressure.
MEP for 20% cutoff is 68%. the net difference in cylinder pressure is 1.7 (2.5 * .68) which is why cutoff makes such a big difference.
2329
OvermodSheesh To 'go faster' you will lengthen the cutoff to increase the mass flow per stroke. This will draw additional steam from the boiler On the other hand the additional mass flow of steam means additional mass flow in the exhaust... hence more draft, hence a brighter fire. At some point, probably before balancing at 30mph, you will resume the 'firing early and often' that you should have been attending to at 20mph. A good fireman will estimate the additional fuel the engine will need at the new higher speed on the relevant part of the railroad... and fire and trim accordingly.
To 'go faster' you will lengthen the cutoff to increase the mass flow per stroke. This will draw additional steam from the boiler
On the other hand the additional mass flow of steam means additional mass flow in the exhaust... hence more draft, hence a brighter fire.
I apologize for not recognizing this earlier (from page 1)
i hadn't considered the increased draft on the fire and temporary increase in BTUs
but "additional fuel the engine will need at the new higher speed" implies that the fireman increases the rate of coal being added to the fire
2388
I apologize, but I can no longer post from a phone at all, thanks to Kalmbach incompetence in advertising forcing page resets and very, very, very slow typing throughput (or clever shadowbanning, perhaps).
The 'succinct' answer is that changes in steam admission are the only thing that makes the locomotive go 'faster'.
The situation is of course much more complex than that, but you'd need to study about the physics of steam, understand why Mollier charts are valuable, etc. to see just how much power can be developed (as pressure) from a comparatively small charge of initially high-pressure, high-temperature steam vs. admitting a 'whole cylinder full' and then letting it blow to exhaust, as in an engine with low or no fixed cutoff operated in full gear. You have already noted that a very small actual mass flow can have very long expansion and still produce smooth 'enough' torque above a certain (and relatively low) cyclic rpm to give effective TE at speed. For very high speed, even with large diameter drivers, the effective duration of admission is measured in milliseconds, and it becomes important to "push" an adequate mass of steam into the cylinder in that time, but there is then more time for 'long expansion' and hence both lower pressure and lower mass to be exhausted when the cylinder opens to exhaust (the thing that most people looking at higher pressure as the 'answer' to Moar Power often don't think about...)
The 'automatic action' is something that an extraordinary amount of (relatively unsung) design, proportioning, and construction goes into facilitating. Note that to go from 20 to 30mph requires more admission steam mass flow. But it is NOT proportional in lb. of steam as "33% more" than at 20mph, unless the engine is well over the peak of the torque curve or has lousy port and passage flow, etc.
It's sort of a honeytrap for armchair thermodynamicists to concentrate on front-end arrangements that provide the greatest possible road speed -- for bragging-right speed records, for example, with engines that will remain nameless. Something more important, though, is arranging the front end for best overall fuel efficiency (at least in a world where fuel was an expensive cost and water was perceived as relatively cheap and fungible) and fuel once stoked would burn 'as it wanted' without the ability to "decrease the fire" as on an oil-burning engine, or wind a chain-grate backward as on the N&W M2 Automatics or the TE-1. In the engine's normal operating range, and 20 to 30mph is certainly in that range for most locomotive designs, the 'idea' is to have the draft change by just the amount that produces the additional heat release, then heat uptake, then steam generation to sustain the higher mass flow.
Naturally, at that firing rate, you might have to stoke more frequently or using a different pattern. But that is done, as mentioned, with reference directly to boiler pressure and water level, which makes it somewhat easier for a fireman.
A 'correct' fire doesn't look at all like something in a charcoal grill, and anyone using that as an analogy has almost certainly never seen an actual locomotive fire in an engine running under load. There is usually more fuel in there than required for instantaneous power change, so that you might not need "extra" firing right away when the added draft brightens the fire still further. Fred Westing interestingly observed that some PRR engines used in commuter service, if started with a properly-built fire, would run for as much as 37 miles without any stoking (nothing was said about trimming).
Now, there's no guarantee that 'automatic action' applies to an engine in poor condition, or burning strange fuel. It is quite possible for an engine to lose the ability to 'keep up' with steam demand, and not infrequently this results in what the English call a 'blow up', where the engine has to be stopped, the blower put on, and the fire brought to effective brightness and mass flow of combustion gas to restore steam pressure for a boilerful of water.
One of the potential complications that you shouldn't let snare you when trying to understand this is that the steam needed to accelerate the train from 20 to 30mph might be easily sourced from the boiler water and existing fire, and when the reverse is adjusted to hold at steady speed the steam demand drops down to a level that existing conditions could maintain. This is less weird if you think about how a fireless cooker could accelerate a train or cut of cars, with no "fire" at all...
Can somebody like Ed find and post the graph from Wardale's Red Devil book that shows actual wheelrim torque from a 2-cylinder DA over one full driver revolution, at 15-degree increments? I think that alone would give him tremendous insight into what is happening in each of the four strokes occurring in one revolution, and why they overlap without crippling torque peaks much of the time.
The 'succinct' answer is that changes in steam admission are the only thing that makes the locomotive go 'faster'. All the fire does... ALL the fire does... is to put heat energy into the steam so it can exert more pressure over a long expansion (and incidentally give up some of the heat/pressure as work as it does). The steam does the work, and that is what dhusman meant when he called it the 'fuel' of the actual engine of the locomotive.
i don't understand why Mollar Charts are needed to understand that coal/oil provides the energy that propels a train.
as the train speed increases, the train resistance increases, requiring the (MEP) cylinder pressure to increase, requiring more steam. (assuming optimal operation)
while various controls allow more steam into the cylinder, the amount of steam can only be what the boiler produces. Any excess steam produced by the boiler is wasted thru the pop-off valves. The fire is only as strong as needed to maintain boiler pressure and not waste steam/water/fuel.
100 lb of coal can burn per hour per sq.ft of grate area. more/less depending on depth. A 100 sq.ft grate can burn 10,000 lb/hr or 166 lb/min. The density of coal is 52 lb/cu.ft. At this rate, the fireman needs to shovel 3+ cu.ft/min. (it's not like a charcoal file the lasts for hours)
~1520 BTU are needed to produce 1 lb of steam. There are ~11000 BTU per lb of coal. Burning 10,000 lb/hr of coal can produce ~72000 lb/hr steam
these are ballpark numbers
working from train resistance, tractive effort, cylinder pressure, steam density and mass (lb), a 5000T train traveling 20 mph requires ~7000 BTU/sec and 38 lb/min coal. A 30 mph train requires ~13000 BTU/sec and ~72 lb/min coal. A 50 mph train requires ~40000 BTU/sec and ~219 lb/min coal.
conservation of energy
While cutoff can be adjusted to change the admission of steam, as well as use is optimally, steam production needs to increase. I don't see how that happens without increasing the rate of coal being added to the fire.
gregc PM Railfan "..... what determines "how fast the locomtive will go"? my mistake. should be how does a steam locomotive increase speed?
PM Railfan "..... what determines "how fast the locomtive will go"?
Gotcha. In that case it goes back to what ive already been saying.....
Ya turn the knob on the transformer, the light gets brighter on the loco, and the train goes faster. And like that.
I forgot we were modellers!
I don't understand what you're stuck on? The boiler was designed to produce more steam than it could use in a given time. When you admit more steam into the cylinders, the boiler is still making steam. The volume of the steam dome and super heater will have enough reserved for quite a distance. All the while the boiler is still evaporating water. The fire is still hot. The crown sheet doesn't instantly cool.
wrench567I don't understand what you're stuck on?
that you and others seem to believe the boiler is producing the maximum amount of steam at all times or only as much steam as needed.
of course you can have the boiler producing more steam than is required with the pop-off values wasting the unused steam and you can use the throttle to control speed.
The boiler pressure is not a measure of how much steam the boiler is producing. The boiler pressure will remain constant if the boiler is producing the same amount of steam (lbs) as being consumed by the cylinders.
why not build the fire only as strong as needed, some bed thickness, to maintain the boiler pressure maybe 5,10 PSI below max boiler pressure when operating at the desired speed. add more coal when it goes below that, stop adding coal when it starts to rise above it
to increase speed, the fire needs to be strengthened to accelerate. After increasing speed,
OvermodA good fireman will estimate the additional fuel the engine will need at the new higher speed
the stength of the fire, bed thickness, can vary to change the amount of steam produced by the boiler as needed.
the fireman is the "gas pedal". on an oil fired steam engine, it's a valve that can be opened more/less to control the strength of the fire and the amount of steam produced
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gregcthe fireman is the "gas pedal".
The fireman is the fuel pump. He make sure there is an adequate supply of steam to run the steam engine. If he supplies an inadequate amount of steam the engine will not be able to make speed. If he supplies too much steam it will be wasted.
The fireman can have the engine producing enough steam to lift the pop valves and the train can be standing still. The engine isn't going anyplace until the engineer opens the throttle.
If the throttle is fully open, all the available steam is going to the cylinders, so the engine is going as fast as it can. Theoretically, if you add more fuel to increase the boiler pressure up to the point that the safeties blow, then yes perhaps the engine will go a little bit faster.
But I think your question may point out that you're not understanding what the throttle is. The throttles regulates the amount of power - steam - that goes to the cylinders to turn the drive wheels. The more steam passed to the cylinders, the faster the engine goes. If, as in your original question, the throttle is fully open and the engine is going 20 MPH, increasing the boiler pressure by adding all the fuel you can will only increase the speed a few miles per hour. But again, that's unrealistic. A steam engine going 20 MPH is only going to have the throttle partially open, half or less.
dehusman gregc the fireman is the "gas pedal". The fireman is the fuel pump. He make sure there is an adequate supply of steam to run the steam engine. If he supplies an inadequate amount of steam the engine will not be able to make speed. If he supplies too much steam it will be wasted.
gregc the fireman is the "gas pedal".
i agree -- that's a better description
but if he builds up the fire, when the train is running at a steady speed and the boiler pressure is not maxed out, the boiler pressure will rise and more steam will increase the speed of the train.
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wjstixIf the throttle is fully open, all the available steam is going to the cylinders
all the available steam for that amount of fire. the fire can have different depths, producing different # of BTUs, generating different amounts of steam (lb/sec)
wjstixTheoretically, if you add more fuel to increase the boiler pressure
boiler pressure does not indicate how much steam is produce by the boiler. it indicates when more or less steam is being consumed by the cylinder than produced by the boiler. it rises when less is consumed and falls when more is consumed
adding more fuel increases steam production (lb/sec). even a slight increase in boiler pressure (~2 psi) results in more steam flow (lb/sec) into the cylinders, higher cylinder density/pressure, tractive effort, acceleration and speed.
more steam is consumed by the cylinders as speed increases, if speed doubles, there are twice as many cylinder cycles and twice as many cylinder volumes to fill.
a higher speed will consume steam at the new rate and boiler pressure will again remain constant
same boiler pressure as before, but higher steam production, consumption and speed
wjstixThe throttles regulates the amount of power - steam - that goes to the cylinders to turn the drive wheels.
it could be used that way; probably is on a switch engine in a yard running at much less than max boiler pressure. but the table below shows full throttle at 9, 20, 30 and 40 mph.
running with anything less than full throttle prevents all the steam produced by the boiler from entering the cylinders. the excess steam in the boiler builds pressure and is wasted thru the pop-off valves.
a throttle is absolutely necessary to start a locomotive, to limit the flow of steam into the cylinders when the wheels are barely turning. the throttle is just "cracked" to start the loco
In your thinking. If you dump the fire the locomotive will not move. Wrong. As long as there is steam in the boiler, you can start and move the locomotive. I read an article years ago where they put a K4s on the test plant. They tested different firing methods. One was a banked fire the other a flat bed. Lines West fireman liked running banked fires. Coal was piled up on the back of the firebox. Then raked forward as needed to cover the grates. Altoona men used level firing and liked thin beds. There was no discernable difference in steam generation. Banked firing had unburned coal when dumped while level fire consumed more coal during the testing.
While steam generation is a factor in locomotive performance. Without an ample boiler the locomotive will probably never leave the shop.
The fire is one small cog in the whole machine.
I'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!
gregcthe fire can have different depths, producing different # of BTUs, generating different amounts of steam (lb/sec)
Probably no where in the industrial world is the marriage of 'man and machine' more pronounced than in the operation of a steam locomotive.
The variables are broad. Various coals give up their volatiles at different rates. The ash, sulpher, fixed carbon and moisture content will be a big determining factor in the ability to carry a thin fire or not. Other impurities in the coal will be another factor in weather or not the fireman will be able to perform his duties efficiently. It can be the difference between having a run that is a 'stroll in the park' or a constant battle with clinkers, grate shaking, plugges flues, plugged netting in the smokebox and finally, how much of the volitale gases can be given up in the firebox and combustion chamber before being carried out the stack.
The variety of coal will be an indication to the fireman what type of fire he will want to carry. Most will bank a fire toward the back corners and along the sides of the firebox. Some coals burn best with a level fire and yet others like a flat bed and maybe a little thin in the center.
Locomotive condition is another variable. Leaking flues and cracked staybolts were something to contend with along with the aforementioned bad coal. A regular fireman on a regular run pretty much knew what kind of coal his road provided but there were still variables.
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. This borders on ridiculous. Likewise, I want to go faster, shovel more coal.
In the days when I was firing I kept the needle within a few pounds of 200 psig. while on the road. The engineer deserved to have the full capability of the locomotive at his disposal and it was the fireman's duty to deliver that.
I worked with about five different former B&O engineers and each one had 'his' style. One I recall had about the easiest-going demeanor and his running of the engine followed suit. Very easy going and 'low-impact' hardly working the engine at all. Another was a bit of a 'throttle jockey' and I had to work a little harder. His style was to nearly constantly work the throttle, automatic and reverser always seemingly trying to find the 'sweet spot' but never really be satisfied.
The main driver of successful firing is anticipation and cooperation. The fireman has to know the territory in order to have both the fire trimmed and the water level where he needs it (you'll carry a full glass going uphill but you'd better know when you are about ready to crest and all your water runs to the front of the boiler, leaving a pretty thin margin for keeping the crown sheet covered!) also water level can be a convenient way to bring pressure down to avoid 'popping off'.
In the steam days most roads were very concerned with heavy firing and unnecessary smoke. Charts were kept on fireman's performance and it wasn't unusual for the road foreman to give a little 'pep talk' to the under performers.
That's enough for now...
Ed
wrench567As long as there is steam in the boiler, you can start and move the locomotive.
On the engine I'm familiar with we would dump the fire on one track, move to another track for sand then head for the turntable, finally going into a stall of the roundhouse. Sometimes thirty or fourty minutes elapsing. Steam pressure would only drop about twenty pounds in this time frame.
I was more concerned with the loss of air pressure. Nosing toward the brick wall of the roundhouse isn't quite the time to find out your air is rapidly depleting!
Regards, Ed
gregcwhy not build the fire only as strong as needed, some bed thickness, to maintain the boiler pressure maybe 5,10 PSI below max boiler pressure when operating at the desired speed. add more coal when it goes below that, stop adding coal when it starts to rise above it to increase speed, the fire needs to be strengthened to accelerate.
I think you mean to say that more steam to the cylinders is needed to increase speed....not strengthening the fire?
But at some point prior to the acceleration, yes, the amount of steam produced must be in sufficient supply for the throttle to allow more steam into the cylinders for actual acceleration, so the fire must be increased some time before the need to accelerate.
The throttle puts steam (fuel combustion/mix of sorts) to the cylinders. To accelerate, its the throttle, not the fire intensity, that accelerates the locomotive, (provided that the fire already made enough steam and there is enough steam in the dome).
Just my take from following this discussion.
- Douglas
wrench567 In your thinking. If you dump the fire the locomotive will not move.
of course not. what did i say that suggests that?
there's steam in the boiler but no additional steam is being produced because there's not BTU being added to replenish what is drawn by the cylinders
boiler pressure will drop as the cylinders consume steam but will draw less and less
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gmpullmanI see this concept of a 'thick' fire = more heat as opposed to a thin fire = less heat. Hogwash!
so you're saying the amount of steam produced by the boiler doesn't matter on how much coal is being burned?
gmpullmanThe main driver of successful firing is anticipation and cooperation. The fireman has to know the territory in order to have both the fire trimmed
what do you mean by "trimmed"?
gmpullman I kept the needle within a few pounds of 200 psig.
what was max boiler pressure?
DoughlessI think you mean to say that more steam to the cylinders is needed to increase speed....not strengthening the fire?
yes, more steam needs to enter the cylinders. but the boiler also needs to produce more steam (see below)
DoughlessThe throttle puts steam (fuel combustion/mix of sorts) to the cylinders.
is this suggesting that the throttle increases the amount of fuel, coal to the engine ??
DoughlessTo accelerate, its the throttle, not the fire intensity, that accelerates the locomotive, (provided that the fire already made enough steam and there is enough steam in the dome).
there's probably enough steam in the boiler to accelerate the locomotive. but if the throttle is full, as that table above says it would be during normal operation at various speeds, it can't be opened further to allow more steam into the cylinders.
presumably cutoff can be increased, as Overmod described, but i'm not sure i understand how that works.
regardless of how more steam is allowed into the cylinders, additional steam needs to be generated by the boiler by increasing the BTUs added to the boiler by burning more coal.
gregcso you're saying the amount of steam produced by the boiler doesn't matter on how much coal is being burned?
How quickly the volatiles distill and 'burn off' will be a major determining factor in how much heat you derive out of each scoop of coal. Some coal varieties require a longer time on the bed to fully ignite and give off its heat through the burning of the volitale gases. Ash in the coal can vary from around 2% up to15%. A high-ash coal is going to build up much faster and this build up of ash isn't going to add anything to the heating value of the coal and will indeed impede draft through the fire.
The temperature of the fire at the time of stoking will make a difference in how quickly the coal gives off its heat, too. Obviously a fire at 950°F will take longer to ignite fresh coal than if it is at 1400°F.
How finely the coal is crushed is a factor, too. I've seen some loads of coal that are nearly dust. These fines get pulled right through the combustion chamber, some flashing off right away and others getting pulled through the tubes without combusting. I would wet the fine coal down with the squirt-hose which helped it from getting blown out the stack. Large lumps of coal will likewise take longer to ignite and give off heat. The stoker has a crusher which reduces most of the larger lumps but when hand firing you have to either spend some time with a pick and break up the lumps or risk firing heavy and the large lumps not burning very quickly.
gregcwhat do you mean by "trimmed"?
The fireman opens the firedoor occasionally and uses his scoop to direct the flow of air across the grates. Bright spots are thin and will need a few scoops directed there to avoid holes in the fire. The back inside corners always need a few scoops as the stoker has a hard time directing coal to these areas. Dark spots usually indicate a clinker forming or at least a build up of impurities in that spot.
The firebed is constantly changing, thus it needs to be 'trimmed' by hand. Constant monitoring of the stoker jets is required, too. A Simplex stoker had five steam jets that adjust the flow of the coal. A Duplex had a dividing plate plus two jets on each elevator in order to balance the distribution of the coal. An Elvin stoker has a pair of 'paddles' that slings the coal across the fire. I've never seen one of these in operation but it must have been interesting.
I never had to shake the grates while in motion, only while stopped. I imagine a road crew that spent a lot of time running would have to occasionally shake the grates 'on the run' especially with bad coal.
not many of you seems to think the a thicker coal bed is needed to increase the rate of steam produced by the boiler
despite that test data table showing the throttle full at various speeds that presumably need optimal performance, i'm trying to understand why an experienced engineers would operate the engine differently (not optimally)I'll guess the reason an engineer operates with the throttle partially closed is to be able to quickly increase the amount of steam (lb/sec) entering the cylinders. a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would.
(my understanding of a throttle is that it doesn't give you some fraction of what is availble, but allows some flow (lb/hr) depending on its setting
regardless of the throttle setting, the maximum amount of steam that can pass through it is limited by how much steam is produced by the boiler. in other words, the throttle will only allow so much steam (lb/hr), it can't pass more than is available)
So while the throttle controls steam flow, it dictates the difference between boiler and cylinder pressure once equilibrium is established as speed stabilizes. If boiler pressure is maintained at some value by the fireman, then it determines the cylinder pressure.
not seeing how the throttle can restrict the flow to anything less than the steam production by the boiler without the boiler pressure rising
a locomotive running without a train (no tonnage) requires little boiler pressure and/or a slightly open throttlewhen the throttle is opened further, the steam flow (lb/s) increases into the cylinders and from the boiler resulting in the boiler pressure dropping. The boiler pressure is restored when the fireman adds coal.i'm suggesting a higher rate of coal (lb/hr) is being added by the fireman as the speed increases.
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Greg, When I quote I get the 403 Forbidden.
No. I'm looking at steam as being the fuel....ultimately being the product of water heated by coal. The steam is what pushes the cylinders, and the throttle is what determines how much steam goes to the cylinders. In a car engine, the throttle adds more fuel (mixture) directly to the cylinder, and more fuel means more combustion and faster go.
Its not exactly the same as an internal combustion engine because the "combustion" in this case happens outside of the cylinder (steam production) and is stored in the dome for use when needed. But fuel (steam...water and air mixture) to the cylinder is still what makes it go. Make sense?
This may not be the way the steam engineers think of it, but it helps me understand it.
To go faster, you need more steam.
I don't disagree with your last quotes.
More coal (more fire) and more water (I would think is needed) creates more steam (fuel, as I see it).
My question would be, can you get more steam to the cylinders when the throttle is wide open? It can only process has much as designed. Like trying to drain a bucket with a straw. Produce more steam and max out the boiler as much as you can, but if the throttle is wide open (maxed), I don't know that more steam can get to the cylinders. Dumping more water into the bucket won't produce more water into the straw, if pressure already maxed out.
In your OP, you said that throttle was wide open but boiler pressure was NEAR maximum. I would say that to accelerate, you have to increase BP to maximum and expect that the throttle is engineered to deliver that much steam to the cylinders. If not...if the throttle is maxed out (not wide open but maxed out as to volume of steam it can handle)....then increasing boiler pressure won't deliver more steam to the cylinders and it will popoff.
gregcnot many of you seem to think that a thicker coal bed is needed to increase the rate of steam produced by the boiler
despite that test data table showing the throttle full at various speeds that presumably need optimal performance, i'm trying to understand why an experienced engineers would operate the engine differently (not optimally)
I'll guess the reason an engineer operates with the throttle partially closed is to be able to quickly increase the amount of steam (lb/sec) entering the cylinders.
a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would.
Steam is not 'drawn' anywhere -- it is PRESSURIZED and flows from higher pressure to lower. Angus Sinclair repeatedly laughed at various Gilderfluke types whose patent drawings neatly indicated how their steam flow would go... the problem being that no one told the steam that, let alone ordered it to. And even were you to ask politely... it goes where thermodynamics says it goes.
The entire steam path from the dome, through the dry pipe, to the superheater header and elements, through the front-end throttle and down the pipes to the steam chests are all part of the steam supply -- there may be some 'pressure drops' from saturation pressure along the way, but remember that we try to minimize them wherever economically justifiable. ONLY the steam volume and pressure in the chests has any real bearing on each admission event, with the enormous ¾ of each revolution time serving to allow steam mass to 'repressurize' the chest. (As noted we cheat and use unavoidable compression pressure to get rid of dead-space effects past the valves, but that isn't part of how steam locomotives go faster until you get really whacking fast...)
(my understanding of a throttle is that it doesn't give you some fraction of what is availble, but allows some flow (lb/hr) depending on its setting.
Yes, if you slam the throttle closed, or quickly wind the reverse to mid, your boiler was generating a particular mass flow of steam that now has 'nowhere to go'. You very quickly quench the overpressure that results with a little judicious injection -- and it is a relatively LITTLE mass flow of liquid to produce a large nominal pressure reduction. This is simply the inverse of 'trading water for steam' for our purposes here. Naturally, as I keep saying, an alert fireman will 'plan ahead' and have his fire and water regulated accordingly for upcoming 'conditions'.
So while the throttle controls steam flow, it dictates the difference between boiler and cylinder pressure once equilibrium is established as speed stabilizes. If boiler pressure is maintained at some value by the fireman, then it determines the cylinder pressure. not seeing how the throttle can restrict the flow to anything less than the steam production by the boiler without the boiler pressure rising
Yes, even though what happens with steam generation is not at all according to 'general gas laws' were you to start closing the throttle or centering the reverse, the engine is drawing less steam, expanding less steam, doing less work, and starting to have fewer strokes per minute. Were you dumb enough not to reduce the fire or limit saturated water temperature, you would indeed start generating more steam than being used, and if you were operating right up on pop pressure, one or perhaps more might actuate. This is neither inevitable nor desirable in a real-world operating locomotive run by the experienced, however. And believe me, it is but the work of several moments to ensure that the pops won't lift, unless you conduct a crash stop from 80mph upgrade without warning. That happens seldom, so you should not assume it always does.
a locomotive running without a train (no tonnage) requires little boiler pressure and/or a slightly open throttle
In practice, you'll crack the throttle slightly more than 'required' to give better low-speed response and eliminate torque peakiness. But your higher effective water rate won't matter that much for casual drifting or even light-engine moves. The problem for modern engines (see the discussions about air horns on steam locomotives) is akin to the issue with whistles or crude venting of open feedwater heaters. Blowing the whistle for one second at ~275psi loses about six pounds of steam -- which was six pounds of water first brought to saturation pressure and then converted with an enormous addition of latent heat of vaporization into actual steam. You lose all that mass, all that heat, and have to dose more water treatment for something that could have been done with a few diaphragms vibrated by brake air...
when the throttle is opened further, the steam flow (lb/s) increases into the cylinders and from the boiler resulting in the boiler pressure dropping. The boiler pressure is restored when the fireman adds coal.
i'm suggesting a higher rate of coal (lb/hr) is being added by the fireman as the speed increases.
Perhaps it would not be necessary to mention that it is MUCH easier to lose boiler pressure via injection than it is to increase it through firing. But I'll say it anyway to stave off any future misunderstanding of the importance of that point...
Something else that may be valuable to consider here is how experienced enginemen were observed to fun large American locomotives in practice. I thought by now we'd have seen at least one YouTube video showing, for example, the NYC Hudson engineer at work, with an experienced engineman explaining why he is working the controls as we see him doing, looking at what he's looking at, and what kind of power increase he is commanding (for a smooth start of what may be a very fast Pullman train).
But also keep in mind that, while the NYC engineer is a seasoned 'at the top of his game' he may have only months or even days actual experience on a Hudson -- before this, he ran Pacifics of various vintage, Atlantics before that, and 4-4-0s before that, all probably with dome throttles of traditional (non-Wagner) construction through expedient-at-best mechanical linkages. In some cases I'd expect his knowledge of 'cutoff control' to have started before the era of Stephenson riding cutoff, something that (very unfortunately!) disappeared as outside radial gear and piston valves became more common.
We might gainfully recall, in this context, why PRR engineers were so often slipping the T1s. I consider it a mistake that there weren't separate throttle manifolds for each 'engine' on a T1, but even so, trying to control 300psi steam with a lever over your shoulder is not exactly a candidate for precision haptics. You will note that one of the 'secret sauces' for successfully starting a T1 involved specifically using partially-closed throttle until the engine had successfully 'found its feet' (or stayed stalled) and only then starting to wind to the very short precise cutoff that the Franklin System could provide. But by no more than 30-35mph you'd have the throttle all the way out regardless of the road speed, and slip prevention/traction control would have had to be 'by other means' to be at all effective...
Douglas, thanks for one of the more thoughful posts
Doughlessyou said that throttle was wide open but boiler pressure was NEAR maximum. I would say that to accelerate, you have to increase BP to maximum and expect that the throttle is engineered to deliver that much steam to the cylinders.
thats what i was thinking, why i specifically said near, but ..
DoughlessMy question would be, can you get more steam to the cylinders when the throttle is wide open?
... in retrospect (i'm sincerely trying to figure things out) i don't believe a locomotive capable of 80 mph would operate at 20 mph with boiler pressure near max. it may be possible with the throttle set very low (but unnecessarily inefficent)``
if the throttle were full, the boiler pressure would need to be much lower, 35 psi. And i believe the fire would be relatively small to maintain conditions
my understanding of a steam engine throttle is that it is simply an orifice of some adjustable size. it doesn't restrict the flow as a percentage of its setting. it's not like the butterfly throttle on a car that restricts an unlimited amount of air into the engine
the size determines how much steam can flow (lb/sec) depending on the difference in pressure across itg. it can't pass more than what is available. so the flow may be maximum at 50% throttle and opening any further that 50% doesn't increase the flow.
the pressure difference means that it may pass 20 lb/s with a pressure difference of 50 psi when set to 80 %, but will pass 20 lb/s with a pressure differnce of 30 at 100%.
DoughlessI'm looking at steam as being the fuel....ultimately being the product of water heated by coal. The steam is what pushes the cylinders, and the throttle is what determines how much steam goes to the cylinders
i understand. i can see the analogy to a car (carburator). but steam is a transmitter of energy, like brake fluid or the siderods of the engine. not the fuel that generated that energy
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gregc a steam locomotive is traveling 20 mph on and approaching level grade throttle is open full cutoff is reduced to the minimum brakes are off boiler pressure is being maintained near maximum coal is being added to maintain the fire and boiler pressure what needs to happen to increase and maintain speed at 30 mph?
a steam locomotive is traveling 20 mph on and approaching level grade
what needs to happen to increase and maintain speed at 30 mph?
Getting back to this, I'm going to ask you or the forum for help. I don't know the answers to these related questions. Keep in mind I'm a layman with little knowledge of thermodynamic physics or steam loco terminology:
Can a steam loco, as typically designed and built, build to maximum boiler pressure when the throttle is wide open...in any reasonable period of time?
If a throttle is designed to relieve the maximum volume of steam that can be maintained in the boiler (via cubic pounds per second?), wouldn't you have to close the throttle a bit in order to fully build maximum pressure over some time period?
(The leak in the boiler is too big to build pressure...make the hole smaller)
Is steam produced at a faster rate (build pressure) than what is dispensed through a wide-open throttle (reducing pressure)?
(despite the big hole in the boiler, it can still build pressure over time)
What I'm getting at is, that it seems likely that the crew will have to build maximum pressure in the boiler to reach maximum speed from a wide open throttle, BEFORE the crew calls for maximum throttle. The crew can maintain maximum pressure when the throttle is wide open, but it may not be able to increase to maximum boiler pressure with a wide open throttle...in any practical useable time period.
A USRA Mikado ran at 200 psi in the boiler. If you weren't at or near that psi, the engine couldn't pull it's train. So the fireman would want to stoke the fire so the boiler pressure would be that high (or at least close to it) while the engine is standing still, so the power is there for the engineer when he opens the throttle to start the train. From a cold start, it could take hours to get an engine fully up to steam and ready to go.
Having steam in the boiler isn't wasteful or anything, it's there waiting to be used. The fire in the firebox has to be kept up; if the heat in the firebox goes down, some of the steam will cool back into water. Boiler pressure will drop, and the train may come to a stop.
"Boiler pressure" can be anything from the minimum to run the auxiliaries effectively (50 to 75psi for the air pumps for example) up to the set safety-valve pressure. It was apparently not uncommon for crews to 'sliding-pressure fire' a larger locomotive to keep fuel use proportional -- a Niagara, for example, could be run at 180psi to do the work of a 2-8-0 on a 2-8-0's amount of coal and water.
Where you need 'full boiler pressure' is when starting a train with assigned resistance very close to what the engine can produce. Many times trains would be made up to take advantage of calculated engine power (at 80% rated pressure or whatever) but improperly use variables in the Davis formula or fail to recognize problems with maintenance, crappy coal, student firing (etc.) which make getting every ounce of achievable starting TE to 'start any train it can pull'
DoughlessKeep in mind I'm a layman with little knowledge of thermodynamic physics or steam loco terminology:
i'm no expert either, just trying to figure it out
DoughlessCan a steam loco, as typically designed and built, build to maximum boiler pressure when the throttle is wide open...in any reasonable period of time?
if production exceeds consumption, boiler pressure will rise. and my contention is that production depends on the strength of the fire, the # of BTUs heating the boiler.
i think consumption primarily depends on speed, the # of cylinder volumes that need to be filled.
as Overmod has said and the table below shows that normal operation can be with full throttle, but i don't think(?) the table values are near max boiler pressure
Doughless The crew can maintain maximum pressure when the throttle is wide open, but it may not be able to increase to maximum boiler pressure with a wide open throttle...in any practical useable time period.
not sure why you're asking specifically about max pressure. the table shows that various speeds can be maintained with full throttle.
i believe maintaining boiler pressure means consumption equals production. and that by increasing the strength of the fire, production increases raising pressure in both the boiler and cylinder, increasing tractive effort acceleration and speed, until the new consumption rate equals the new production rate.
i'm thinking, if the fireman increases the fire, boiler pressure goes up, and then as speed increases, drops back to what it was. All the while, the fireman has increased the rate of coal added to the fire (or opened the oil valve a little).
Increasing the fire more, results in further increase in speed, and again boiler pressure returns to what it was (see table)
boiler pressure remains constant when steam production equals consumption
the speed can be increased without adjusting throttle or cutoff by adjusting BTU production.
Douglas, is this making sense to you?
(ya gotta grok it)
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Overmod gregc a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would. Read that sentence over a couple of times. Do you see how dumb its assumption is?
gregc a partially closed throttle requires a greater difference in pressure between the boiler and cylinders to draw the same amount of steam (lb/sec) that a more open throttle would.
Read that sentence over a couple of times. Do you see how dumb its assumption is?
my understanding is the flow thru the throttle depends on the pressure difference. So a more closed throttle requires a bigger pressure difference than a less closed throttle.
for example, for a boiler pressure of 220, a cylinder pressure of 200 has the same flow (lb/s) at 100% throttle as a cylinder pressure of 192 at 80% throttle
Evening
Don't Know, but probably should.
We could ask Mr Weatherby, ...but He might be just as Confused as You are?
Pic taken by TF
Or myself
TF
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
If anyone is still following along perhaps a background in Firing 101 might be helpful to some.
[edit] Looks like Bear had the same thoughts just a few minutes before me
Here is an album in Flickr where I reproduced a New York Central booklet pertaining to the subject.
https://www.flickr.com/photos/gmpullman/albums/72177720301339959/with/52289280349/
If anyone has trouble seeing the pages I can reproduce them here if needed.
NYC_Fire by Edmund, on Flickr
Note the amount of instruction devoted to the variables in the coal as a fuel. I attempted to allude to this in an earlier post.
gmpullman[edit] Looks like Bear had the same thoughts just a few minutes before me
perhaps the following will reveal some of the mystery of steam engines that i'm trying to figure out
the table shows the required tractive effort and cylinder pressure for a 5000 T freight at various speeds. Note that the cylinder pressures are far below the max boiler pressure of 220 PSI even at 80 mph
mph TE psi 0 20000 24 10 22841 27 20 27298 33 30 34287 41 40 45248 55 50 62438 75 60 89398 108
cylinder pressure is related to tractive effort by
boiler pressure remains stable when steam production equals steam consumption. boiler pressure can be maintained at various levels (PSI) as long as it is greater+ than the required cylinder pressure.
boiler pressure can be much higher than cylinder pressure because of the balance between steam production and consumption, the creation of steam in the boiler and the outflow into the cylinders. steam production is maintained by the fireman by maintaining boiler pressure.
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just in case anyone is questioning why a locomotive would be designed to require such low pressuress, here are similar values for the same train going up a 1.5% grade. clearly not possible much above 10 mph
mph TE psi 0 170000 206 10 172841 210 20 177298 215 30 184287 224 40 195248 237 50 212438 258 60 239398 291
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gregcperhaps the following will reveal some of the mystery of steam engines that i'm trying to figure out the table shows the required tractive effort and cylinder pressure for a 5000 T freight at various speeds. Note that the cylinder pressures are far below the max boiler pressure of 220 PSI even at 80 mph.
On a 1.5% grade, you are doing lifting work rather than just accelerating mass with low frictional resistance. Accordingly your required MEP is much larger. Again, what the boiler produces is a bound on this, but otherwise irrelevant.
boiler pressure remains stable when steam production equals steam consumption.
boiler pressure can be maintained at various levels (PSI) as long as it is greater+ than the required cylinder pressure.
boiler pressure can be much higher than cylinder pressure because of the balance between steam production and consumption, the creation of steam in the boiler and the outflow into the cylinders.
The 'balance between steam production and consumption' only comes in when the mass flow required (by the chosen valve-gear setting) exceeds what the overcritical water in the boiler can 'source'. And you deal with it by firing the boiler to restore the gauge pressure. Or stopping to allow the boiler to 'recover' pressure before proceeding. Those things have nothing to do, by themselves, with what 'made the steam locomotive go faster'. Only the steam did that.
Steam production is maintained by the fireman by maintaining boiler pressure.
OvermodWell, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate,
i don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal.
and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder
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gregc Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate... I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal. and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder
Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate...
I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal.
With the intent of having sufficient steam that you can use the valve gear to admit the mass flow necessary.
(I am purposely avoiding the whole can of worms about what happens to the steam after it has gotten done expanding in a stroke, because God knows what misconceptions about that would result... but it does matter, and we could take it up in its own thread.)
gregcif production exceeds consumption, boiler pressure will rise.
Yes, it makes sense. Thank you.
What I quoted above seems to be the crux of the issue. But I don't think I can find the solution within your response. Apologies if I couldn't figure it out.
Try to answer these questions simply.
Does boiler pressure have to increase in order to accelerate from 20 to 30 mph when the throtte is wide open, or can something other than BP create acceleration in that situation?
Can boiler pressure even increase when a throttle is wide open? If so, how long does it take? Seconds, minutes, hours?
Overmod gregc Overmod Well, yes, but you are confusing 'maintaining the boiler pressure' with steam generation rate... I don't believe i am. since there's no gauge indicating flow (lb/s), a stable boiler pressure indicates that production and consumption are equal. and i've said this can be true at various boiler pressures as long it exceeds what is required in the cylinder The point is that it doesn't matter to the engine's physical performance (with respect to the question you originally posed) whether 'steam consumption' balances 'steam generation' from fuel consumption or not. Mass flow through the cylinders has been the accepted 'measure' determining engine power for over a century now.
The point is that it doesn't matter to the engine's physical performance (with respect to the question you originally posed) whether 'steam consumption' balances 'steam generation' from fuel consumption or not. Mass flow through the cylinders has been the accepted 'measure' determining engine power for over a century now.
what do you think i mean by flow (lb/s)? it is the "mass flow" you're referring to
and how can it not matter? if more steam is produced than consumed, pressure builds up and is wasted. if less steam is produced that needed, pressure drops until there is insufficient pressure in the cylinders and the train slows
Overmod EXCEPT to maintain some average, economically-determined pressure at the gauge,
and what might the "economic" pressure value be?
if the flow (lb/s) into the cylinders is the same at regardless of boiler pressure, why does it matter as long as it is sufficient to maintain the flow into the cylinder
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DoughlessDoes boiler pressure have to increase in order to accelerate from 20 to 30 mph when the throtte is wide open, or can something other than BP create acceleration in that situation
this is my understaning ...
if the engine is already being run optimally, i don't see how boiler pressure doesn't need to rise. (how much: 1, 2, 5, 10 psi)
on pg 1, Overmod suggested that cutoff can be increased, allowing more steam into the cylinder and out the exhaust stack, increasing draft and "brightening" the fire, increasing the BTU output of the fire increasing steam production.
but burning the coal at a faster rate requires replenishing it a faster rate (i.e. coal consumption increases)
once again, this table shows various speeds 10, 20, 30, 40 with full throttle, a variety of cutoff and boiler pressures in the range of 190-199 at all speeds. (it would be great if someone could explain how the conditions in the table were achieved)
i'm fairly confident that increasing boiler pressure means steam production is greater than consumption and should result in an increasing consumption rate until it equals production.
maybe (???) the amount consumption changes depends on how long boiler pressure is above or below some "maintained" pressure (???).
i'm guessing it would take a few minutes after increasing the rate of fuel for BTU/hr to increase, but have no data.
Douglas, thanks. it's taken me an hour+ plus to write this as i thought about the issues
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gregci'm guessing it would take a few minutes after increasing the rate of fuel for BTU/hr to increase, but have no data.
Yet I offered the OP data. Maybe I'm not quite conversant with what his particular goals are but I thought coal consumption vs. boiler efficiency was rather well covered in this ASME discussion:
https://asmedigitalcollection.asme.org/fluidsengineering/article-pdf/62/5/410/6988950/399_1.pdf
The OP didn't seem to agree with me.
I invite anyone following this thread to read the 21 page discussion of the report and please comment on the pertinance of the findings related to our discussion here.
My position is that fire bed thickness does not equate to the relation of volitale gases distilling and transfering heat to boiler evaporation surfaces. In fact during some tests it was shown that a greater temperature was recorded in the smokebox showing (to me, anyway) that unburned gases were escaping without communicating the heat generated to the evaporative surfaces.
gmpullmanI thought coal consumption vs boiler efficiency was rather well covered in this ASME discussion:
my question is not about efficiency. it is what needs to happen to increase the speed of a train.
I am convinced the # of BTU/hr needs to increase when operated optimally and this happens with an increase in fuel whether it be coal (lb/hr) or oil (gal/hr). But it's not clear exactly what happens (i'm curious about increasing cutoff)
I'm looking for ballpark numbers and have found that the max burn rate of coal is around 150 lb/hr per sq.ft of grate area and that there's around 11000 BTU/lb of coal. i know this should be different for anthracite vs bituminous.
these are design values that I'm interested in. They could be off by 50%. And of course not every chunk of coal is the same. My question isn't concerned with the mix of coal, bituminous/anthracite, or coal quality, whether the last shovel full has more/less BTU than the previous or whether it ignites faster or takes longer to burn
gmpullmanI have never once witnessed a situation where the engineer tells the fireman to lower his boiler pressure because he wants to slow the train down.
i'm curious about what happens in a cab when a change in tractive force is required.
does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
does the fireman need to build up the fire when leaving a station? how and when does he know to do this?
does the fireman need to build up the fire when approaching a grade? how and when does he know to do this?
can the fireman allow the fire to die down when cresting a grade and possibly going down hill? how and when ... ?
does the fireman need to build up the fire when reaching the bottom of a downhill grade? how and when ... ?
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gmpullman... Maybe I'm not quite conversant with what his particular goals are but I thought coal consumption vs boiler efficiency was rather well covered in this ASME discussion: https://asmedigitalcollection.asme.org/fluidsengineering/article-pdf/62/5/410/6988950/399_1.pdf
There are discussions, including in handbooks for engine crews, discussing the sequence of events and approximate times involved in what happens after the coal arrives at the distributor plate or is hand-bombed to various positions. The coal is heated from the outside surface by a combination of radiation and convection; hydrocarbons and carbon vapor are expelled from the surface; the lump can start swelling and breaking, increasing the surface area for vaporization; eventually the luminous carbon under the action of the draft produces the combustion plume, in which is both glowing carbon and levitated bits of still-burning fuel.
In a large modern locomotive, the heating of the surface can occur almost immediately, much of it while the fuel is still falling through the evolving gas plume. As the fuel reaches the bed, there is additional heat transfer through conduction from the burning fuel already there. However, it is important to remember that admitted ixygen proportion is intentionally less than 'stoich' for the fuel -- the conditions in the firebox are both partial vacuum and reducing atmosphere. So drawing analogies from internal-combustion lambda and whatnot are specious even before taking up the radically different nature of internal-combustion gas engines vs. external-combustion engines.
My position is that firebed thickness does not equate to the relation of volatile gases distilling and transfering heat to boiler evaporation surfaces.
In GPCS, the bed is intentionally thick because combustion there is 'retorted' -- air plus steam is used to react with the carbon and hydrocarbons in the fuel to produce clean volatile gases, which are then burned nearly entirely above the firebed -- no sparks, no ash carryover, no sooting, and no front-end mass losses or exhaust ignition.etc. But that is a chemical, and not a thermodynamic, concern.
In fact during some tests it was shown that a greater temperature was recorded in the smokebox showing (to me, anyway) that unburned gases were escaping without communicating the heat generated to the evaporative surfaces.
As a perhaps interesting aside: Chapelon (and then Porta) proposed designing a 'sectional boiler', in which the forward end of the tubes and flues, where heat transfer becomes lower, is used as part of the feedwater-heater system rather than for evolving saturated steam. As part of the ESC work on the feasibility plan, the actual 'optimal' length of this section for the 300psi T1 boiler was calculated -- it would be only about 3'4" long...
gregcwhat do you think I mean by flow (lb/s)?
it is the "mass flow" you're referring to and how can it not matter?
The boiler heat release is adjusted to compensate for the steam mass flow that accomplishes the actual work. You seem to think that this is like heat release in IC engines, where the fuel burn is directly proportional within milliseconds to the MEP in the cylinder. THAT IS NOT TRUE ON A RECIPROCATING STEAM LOCOMOTIVE.
If more steam is produced than consumed, pressure builds up and is wasted.
if less steam is produced that needed, pressure drops until there is insufficient pressure in the cylinders and the train slows...
[quote][quote user= "Overmod"] ...EXCEPT to maintain some average, economically-determined pressure at the gauge...[/quote] and what might the "economic" pressure value be?[/quote]Determined by the operating railroad, in part through experience and empirical measurement. For example, going up a severe grade the engine might have to be heavily fired, but as it tops the grade the water level shifts forward in the barrel and the crown depth decreases. That indicates that reduction of the firing rate as the engine approaches the crest be conducted (to avoid waste) and this in turn might cause the gauge pressure to fall considerably from safety-valve pressure. You will trade a little cutoff for expansion economy by using higher mass flow at the lower pressure for your drawbar TE. You use more water, which again is an economic concern for the operating entity to assess.
The "flow into the cylinders" is NOT THE SAME REGARDLESS OF BOILER PRESSURE. Obviously (at least to me) if you want more power out of the engine, you need more mass flow into the cylinders at lower pressure. THAT IS DONE BY ADJUSTING THE CUTOFF.
Meanwhile, "sufficient flow into the cylinders" is again a function solely of cutoff, and it implicitly is completed "to produce desired drawbar pull at speed, or for a desired acceleration rate".
The boiler is fired to keep it at or near a desired pressure, which need not be up there where the safeties pop all the damn time. You fire the boiler to make steam available for use, that steam being metered into the cylinders by the valve gear to make best use of its expansion (and then as quickly and expediently removed from the cylinder as possible, which as I said is a completely 'other' discussion in relation to the question).
You can overcomplicate this with meaningless discussions of coal rank and heat release, which were usually known only in empirical terms through firing experience anyway. You could effectively fire a locomotive with Egyptian mummies -- in fact, locomotives supposedly have been. What matters is whether the boiler has been fired in the most economical (or easiest, for those who don't really track efficiency correctly) way that provides steam at your experience-determined pressure. This has little to do with the somewhat artificial test train's acceleration in this question, but that's more because you used arbitrary quantities in framing that question in the first place.
don't have time to read and decypher you post
OvermodThen why do you keep bibbling about BTU release and sliding boiler pressure being factors directly influencing short-term acceleration?
sounds like you're iterpreting my "flow (lb/s)" as lb coal /s when i thought it was clear i meant lb steam / sec
while you can't ignore short term acceleration, i'm asking about the conditions at the higher speed
acceleration (F=ma) and force equal to train resistance depend on tractive effort
tractive effort depends cylinder pressure
cylinder pressure depends on the steam density in the cylinder
steam flow (lb steam/hr) must equal steam production otherwise boiler pressure changes
steam production depends on # BTU/hr heating water
# BTU / hr depends on lb coal /hr
Stop changing the game if you want an answer to what your question asked.
ACCELERATION of the train from 20 to 30mph -- under the conditions as you stated them -- is done by lengthening the cutoff. You already have the throttle fully open, and of course nothing would be gained by trying to open it further, so there's nothing else 'steam-related' but lengthening cutoff. Most of the additional steam mass that is required to accelerate the train comes out of the enormous reservoir of heat that is the overcritical water -- it does NOT immediately require proportional additional fuel firing, let alone precise and equal additional fuel firing to just balance uptake from combustion gas with saturated-steam release.
Once you get to 30mph, you no longer need to accelerate, unless by steady-state you mean continued acceleration to the maximum speed the engine's construction will permit. Which wasn't the question, although I can discuss that situation some in a different thread. So at 30mph you SHORTEN the cutoff back to the mass flow that just keeps the train rolling against resistance at 30mph. Note that if you were firing to match the instantaneous flow for acceleration, you'd now have 'too much fire'... which, as noted ad nauseam by this point in the thread, you can't immediately relieve.
Now, most of what seems to be troubling you concerns what happens if the train now continues at 30mph, throttle still full, still on level ground. The additional resistance can be calculated for this (and as noted from some of the earlier misapplied data, it corresponds to surprisingly little MEP at the cylinders) but TECHNICALLY it does imply a larger long-term heat input from fuel consumption, and therefore more fuel would have to be fired.
But the fuel firing is 'on average', and it might be no more than the rough changes produced by adjusting the feed and distribution valves on the stoker. As I keep noting, while a good crew will anticipate what steam demand will be (including under the all-too-common conditions where the engine is in far from good maintenance, cf. for example the PRR fireman's story about running a M1 4-8-2 with a repeatedly recalcitrant feedwater-heater pump) what they do to address it is simply keep the boiler pressure in some range where cutoff can control cylinder power (either with lower physical mass at higher gauge pressure, or larger mass at a lower pressure). Even at surprisingly little pressure, the engine may make its 'rated' power... this being determined more by the physical flow arrangement of the valves than anything in the boiler. What suffers most is the water rate under those conditions.
You are nowhere near the kind of conditions that would involve 'forcing the boiler', keeping a larger heel, having to run the stoker continuously while jiggering the valves to change distribution, etc. In all probability, to run at 30mph you'd adjust a couple of stoker valves a hair (not quite a radio RCH, but with the same sort of meaning!) by experience, and along you'd go. Actually overthinking what you have to do to get the trick to work isn't something necessary on any well-designed locomotive.
Unless, of course, you have the misfortune to be on a railcar or Sentinel or Besler/Doble with some kind of once-through tapered monotube. These of course were almost never fired on solid fuel (and the ones that were... weren't around for very long) and with those you do have to juggle fuel feed and combustion-plume conditions very carefully, within no more than a second or two after steam-demand changes. Doble in particular worked out an ingenious set of firing and injection controls that could actually do this, and scaled the approach up to about 800-900hp (perhaps more in Germany, but it's difficult to find full technical details of that). As a design exercise, you could scale one of these larger, but it would NOT be a locomotive that most seniority-based crews could even begin to run effectively -- let alone manage if any of the automatics malfunctioned or broke.
Overmod a larger long-term heat input from fuel consumption, and therefore more fuel would have to be fired.
seems significant, but burried in 678 words of text
OvermodStop changing the game if you want an answer to what your question asked.
Mike
gregcgmpullman I have never once witnessed a situation where the engineer tells the fireman to lower his boiler pressure because he wants to slow the train down. i'm curious about what happens in a cab when a change in tractive force is required. does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
i'm curious about what happens in a cab when a change in tractive force is required. does the the fireman at least allow the fire to "die down" when stopping at a passenger station? how does he know when to do this? how did he learn this?
In a previouis email reply to you I answered all of these questions.
How does the fireman know when to 'do this' comes from months of learning the route and learning the 'running style' of each of the engineers PLUS the operating variables inherent in different locomotives, even within the same class, along with variables in the grade of coal or water (yes, some water treatment fails at removing oxygen and impurities) in order to get the engine and train over the road.
gregcdoes the fireman need to build up the fire when leaving a station? how and when does he know to do this?
For the most part he's been firing this route for years. He knows where every station, junction, signal, curve, grade and tunnel is on his route. IF he is running on territory he's not familiar with, a situation I have encountered, there would sometimes be a pilot in the cab who would inform the fireman of upcoming conditions or events. The engineer would communicate to the fireman (That 'cooperation' chapter in the firing manuals) what he intends to do. Most engineers will allow the fireman to occupy the right-hand seat for a while during the trip in order to get the feel for running. Firemen are engineer trainees.
gregcdoes the fireman need to build up the fire when approaching a grade? how and when does he know to do this?
He KNOWS the route, he knows the locomotive, he knows the weight of the train, he knows his firing style and he knows how the engineer is going to attack the grade.
The word I used in my email reply to you was ANTICIPATION. The fireman has to anticipate conditions by at least fifteen minutes or more.
gregccan the fireman allow the fire to die down when cresting a grade and possibly going down hill? how and when ... ?
Short answer, yes. But before cresting the grade he has to be sure to have enough water in the glass so that the crown sheet doesn't get overheated when the water sloshes to the front of the boiler. Any braking effect down the grade will also contribute to your water disappearing from the glass. In spite of good feedwater heaters you still need to maintain enough fire to overcome the chilling effect of the introduction of feedwater.
gregcdoes the fireman need to build up the fire when reaching the bottom of a downhill grade? how and when ... ?
How depends on the present condition of the fire. With the engine drifting it is a good opportunity for the fireman to inspect his fire and fill thin spots or build up banks if needed. The when depends on present boiler pressure and train speed. Once the engine begins 'working' the fireman has to have the fire prepared (anticipation) as once he is into the grade precious time is lost to make any adjustments to the fire.
I had provided you with a scan of the entire chapter from the book Perfecting The American Steam Locomotive by J. Parker Lamb 'The Physics of Steam Power, which detailed boiler thermodynamics and your reply was 'I don't need any more operators manuals'.
I also suggested securing a copy of William L. Withuhn's excellent book American Steam Locomotives, Design and Development, 1880 — 1960.
gregcdon't have time to read and decypher you post
I'll stop here for fear of writing too much that can be read without unnecessary anxiety.
Cheers, Ed
For those still following I'd like to submit an example of one of the sources for Greg's often cited charts in prior posts. These were gleaned from laboratory tests of locomotives at the Pennsylvania Railroad's Locomotive Testing Plant.
Here is one example of a test report:
https://hdl.handle.net/2027/pst.000003544839
and another comparing an L1 class to the H:
https://hdl.handle.net/2027/pst.000003544815
Interesting reading and the PRR Test Lab certainly earned its role in locomotive development but one must keep in mind when studying these results that these are 'laboratory test bed' conditions and not real-world over-the-road train handling tests. For those there are dynamometer tests available such as this record of road testing the New York central Niagara:
https://nycshs.files.wordpress.com/2014/07/roadtestingniagaras.pdf
Thank you, Ed
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?
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.
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.
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.
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?
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:
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.
Releasing the brakes is generally helpful.
OvermodThis is true for such obvious reasons I don't see why it keeps being brought up in this connection
yet you wrote a 403 word reply. i don't understandwhy you write so much and say so little. brevity would be appreciated
gregcit show an increase in speed (purple) from 10-30 mph the TE to maintain speed the increase in TE accelerate the somewhat exponential TE required to match the exponential increase in train resistance
Overmod3)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.
the plot simply shows what must happen to increase speed, not how to increase speed nor the details within the engine
Overmod4) ... This is going to involve both flow calculation (during admission) and then recuperation (during expansion when there is no admission)
if all steam produced by the boiler enters the cylinder, a decent estimate of cylinder pressure is due to the density when the valve closes at cutoff.
how does < 50% cutoff affect flow (lb steam/ hr)?
if flow varies with cutoff, the "piling" up must impede flow.
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gregci don't understand why you write so much and say so little. Brevity would be appreciated
Reminds me increasingly of that Far Side cartoon where he illustrates 'what we say to dogs' vs. 'what dogs hear'. If you are counting the number of blahs... well, there we are.
Oh look, now we have another plot. Let's see how this differs from the multicolored one with the excessively complex assumptions...
the plot is for level grade plot shows the TE for constant acceleration from 10-30 and TE for sustained speed at 10, 30 mph train resistance rises with speed (hence exponential curve) constant 2 mph/min acceleration (x-axis should be seconds) TE is 22814 lb, ~11T at 20 and 34287 at 30 mph 5000 T train this is simple physics, not a steam engine model the plot simply shows what must happen to increase speed, not how to increase speed nor the details within the engine
And while it isn't really 'simple' physics, any more than lolog heat uptake from combustion gas to boiler water is, we're actually getting to the original question.
Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank? And then that something magical makes all that steam go obediently into the cylinder?
how does <50% cutoff affect flow (lb steam/ hr)?
You don't know enough to get an answer until you know some of the details within the engine, so you're in deep doo-doo already. But in the interests of brevity I'll have to leave it at that until you self-educate a bit better, or get less simplistic physics that apply. In any case, if you haven't understood flow of steam into the cylinder depending on cutoff in over 790 words, I don't think any further amount of logorrhea will help you.
f flow varies with cutoff, the "piling" up must impede flow.
Overmod Oh look, now we have another plot.
Oh look, now we have another plot.
no, same plot as before, which you then asked for, which i posted to address your comments on "ACCELERATION", to focus on how cutoff is used to control speed
OvermodAnd while it isn't really 'simple' physics
simple application of Newton's Law (F=ma) knowing the tonnage, train resistance and force required to accelerate at 2 mph/min
Overmod if all steam produced by the boiler enters the cylinder, a decent estimate of cylinder pressure is due to the density when the valve closes at cutoff. Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank?
Did you think that 'steam produced by the boiler' is constrained to leave it, like someone taking a withdrawal out of a bank?
a more serious response would address why the throttle can be full for various speeds and what is limiting steam flow between the boiler and cylinders
Overmod flow varies with cutoff, the "piling" up must impede flow. Succinctly, no. Can you please provide simple physics that illustrate what 'piling up' is supposed to mean in this context?
flow varies with cutoff, the "piling" up must impede flow.
Succinctly, no. Can you please provide simple physics that illustrate what 'piling up' is supposed to mean in this context?
how is steam flow (lb steam/hr) affected when both cylinder valves are closed?
analogous to
Water hammer arrestors have air-filled cylinders that absorb the jolt of a sudden water pressure increase when a valve shuts off.
why is there an optimal cutoff dependent on speed? why does steam consumption increase when cutoff is either +/-?
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gregcSame plot as before, which you then asked for, which i posted to address your comments on "ACCELERATION", to focus on how cutoff is used to control speed
... simple application of Newton's Law (F=ma) knowing the tonnage, train resistance and force required to accelerate at 2 mph/min
...a more serious response would address why the throttle can be full for various speeds and what is limiting steam flow between the boiler and cylinders
There are some instances where reflected shock has been said to be observed in a steam line when flow is abruptly interrupted. Note that this is a pre-existing flow shock that is reversed, not a "piling up" or some other naive foolery.
analogous to Water hammer arrestors have air-filled cylinders that absorb the jolt of a sudden water pressure increase when a valve shuts off.
Can I ask why you use data for an ancient 2-8-0 when you'll get clearer indication from something like, say, a Hudson or Niagara, or something halfway modern if you want to stick with PRR?
why is there an optimal cutoff dependent on speed?
why does steam consumption increase when cutoff is either +/-?
[/quote]
and of course, the fireman (fuel pump) controls the evaporation rate to match whatever the cylinders require by maintaining pressure
OvermodSuccinctly: it answers the original question you asked, if it's simple physics. Why keep asking about steam and all that other stuff?
the plot illustrates the required forces, not the operation of a steam locomotive to control the steam generating those forces
OvermodYou had your serious response on this not just once in the thread already. In the interest of brevity I'm not going over it again.
you don't need to repeat yourself. you can refer to your post explaining it (i.e. date time)?
OvermodIt... doesn't flow. It equilibrates with boiler pressure in a very short time, and then starts up again when there is a pressure differential.
this sounds new. have you said this before?
so how do you determine the average flow (lb steam / hr) during intermittent flow? i think this is a key part of the answer
how might you determine the average steam chest pressure, the diffential pressure across the throttle (full) that determines the flow (lb steam / hr) thru the throttle
OvermodCan I ask why you use data for an ancient 2-8-0 when you'll get clearer indication from something like, say, a Hudson or Niagara
Laboratory Tests of a Consoliation Locomotive is from 1918. do you have a similar more modern reference?
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gregcit's never been clear how steam flow is limited with full throttle, as suggested on pg 1, despite seeing test data maintaining various speeds with full throttle
The only reason to keep the throttle closed is if, for some artificial reason, you want to restrict steam flow during admission. As your question is asking about removing limitations from flow, this would not apply.
Overmod explained how increasing cutoff increases steam (lb/hr) into the cylinders, tractive effort and speed (without the need for throttle).
so it seems cutoff can also control steam flow but differently than a restrictive orifice, a throttle...
The net effect of cutoff is to "restrict" flow, but only in the sense of maximizing the potential expansive working. The valves are best designed to admit steam freely as soon as possible when they open in admission, and then keep it flowing freely for as much of the duration of admission as possible. (There is less problem with gas-cutting if admission close isn't quick and precise than there is for admission opening without proper compression control -- see the general experience with Franklin type A poppet admission valves, and the early ones on the T1s in particular).
because I didn't see a clear explanation, the conclusion i draw is that < 50% cutoff results in intermittent, stop/start flow of steam when both cylinder valves are closed...
...affecting steam flow similar to a partially closed throttle.
[/quote]shorter cutoff is required as speed increases to increase the time that the valves are closed and the time steam chest pressure has to recover[/quote]That's not the point of short cutoff. The idea is to extend the portion of the stroke during which expansive working is making best use of the heat in the steam. Steam-chest pressure is incidental in that context unless there is some restriction of steam into the chest (like, for example, a partially-restricting throttle).
Now, one place you see 'output depending on cutoff' is when admission calls for more steam than the boiler can source. (Remember that the 'admission time' is 4x admission for 'one stroke', per driver revolution on a 2-cylinder DA, and therefore the number of events increases with speed, as does the aggregate mass flow required during the admissions. It does not take long for an engine in full gear to outrun its steam-generation capacity at a high rate of consumption, even with a relatively large boiler -- and since more and more of the admitted steam has little chance to expand before it has to be exhausted, much higher pressure as well as mass flow have to be blown out the exhaust. This will tend to lift the fire off the grates before it generates high effective heat transfer.
So, relatively quickly, you want to start reducing admission duration as the cyclic starts to come up in acceleration, so that more of the work is produced by expansion (of the steam in the cylinder; we don't care what happens in the chest after its pressure equilibrates, which likely happens long before the next admission starts).
If you're looking at something like cylinder thrust over a stroke, you can use cutoff to set a range of admission and expansion for a given speed. In general if you were running the engine for least cost, you'd want to reduce both the fuel and water rate, so you'd pick the shortest admission that, together with subsequent expansion, ultimately produces your wheelrim torque or drawbar TE. But you might, for reasons including those I've mentioned, choose to operate with less 'peaky' thrust for a small part of the stroke at peak pressure.
One of the things Neil Burnell came up with regarding the T1s was a method they used to start them, which involves a number of practices we've discussed here. At a station stop, the fireman would intentionally NOT bring up his fire to recuperate full 300psi pressure; I think I remember seeing something in the range of 190psi. The engineer would then start the train on the reduced pressure, which of course translated into lower admission pressure at the valves and hence longer cutoff required to get the desired acceleration. A T1 was both a bit slow and a bit slippery getting to about 30mph, and a little less power made for a less slippery girl -- but during this, the fireman would be handling his fire to build pressure back to make it available above 35mph where the T1 could put it to the rail. What was not mentioned in the article -- and which of course is of very great importance in practical locomotive management -- is that this pressure cycling and accompanying thermal cycling weren't doing very good things for the boiler structure. (But that's another issue for another thread).
Again, we come back to why all the modern steam designers thought in terms of mass flow: here, only average and relatively slow changes in mass flow can be commanded, even on a locomotive with oil firing. If you graph sequential admission at the rate corresponding to your 20 to 30mph, with your (unspecified) driver diameter, you can see the admission times (measurable in milliseconds) and recognize that this can't be a factor in what happens during each particular stroke.
As far as I know, there were more test plant results for the Q2 than any other PRR locomotive. Use those.
Overmod.
Earlier I linked an article to a 1920 Railway Age magazine article about the I1s testing of limited cutoff. The I1s was built first with 50% cutoff later increased to 65%.. The big hippos were probably tested as much as the Q2. What struck me about the tests was that they found no appreciate advantages in stoker firing over hand firing. Just ease on the fireman. In some tests, hand firing generated more usable steam with a higher super heater temperature. Most tests were done at 40 revolutions per minute at 7 and a half MPH. Drawbar tests were taken up to 20 mph. Something like 3500 HP at 7 mph.
The T1 was a magnificent locomotive. It's been later found out that most of the problems with them turned out to be sabotage by the crews who were afraid that a successful design would have seen a greater reduction in work hours. They really were afraid that they would be sent back to freight pool service. The T1 spent a lot of time on the test plant too.
The Q2 while successful still didn't last past the decades older 2 cylinder beasts it was supposed to replace. One of the last ones to dump it's ashes was nearly a half century old 2 cylinder simple locomotive.
whether it's called "restricted" or "metered", < 50% cutoff (wiki cutoff (steam engine)) appears to be another mechanism that limits steam flow into the cylinders to control the MEP and tractive effort
like the sinusoidal force applied to the rails (see fig 7 in Fundamentals of Steam Locmotive Tractive Force), there is an average flow (lb steam /hr) or simply a mass (lb) of steam that enters the cylinder each cycle while the valve opens
it seems there are several concepts involved.
it's not at all clear why cutoff can't be made "early" (reduced toward ~20%) even at low speeds to use steam more efficiently
but while steam flow across a throttle is proportional to the difference in pressure across ...
... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff and speed
table shows how little MEP is required on level ground and the time (msec) that valves are open at speed and for early cutoff
trainPsi: tonnage 5000T, drv Dia 61.5 in mph tonage TE MEP cps msec adMsec cut 10 5000 22842 24.8 1.8 548.9 109.8 20% 20 5000 27298 29.6 3.6 274.4 54.9 20% 30 5000 34287 37.2 5.5 183.0 36.6 20% 40 5000 45248 49.0 7.3 137.2 27.4 20% 50 5000 62439 67.7 9.1 109.8 22.0 20% 60 5000 89399 96.9 10.9 91.5 18.3 20%
(busy with guests)
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gregcwhether it's called "restricted" or "metered", < 50% cutoff appears to be another mechanism that limits steam flow into the cylinders to control the MEP and tractive effort
... like the sinusoidal force applied to the rails (see fig 7 in Fundamentals of Steam Locomotive Tractive Force), there is an average flow [...] or simply a mass [flow] (they are both in lb/hr) of steam that enters the cylinder each cycle while the valve opens...
it seems there are several concepts involved. expansion to extract more energy from steam after cutoff; recovery to allow steam chest pressure to increase after cutoff; the need for longer recovery time as speed increases ...
What's going to happen after the 'highest speed' you can get by using expansion most effectively is that you have to start lengthening out on the cutoff, and therefore starting to use more steam per stroke even as the number of strokes per minute keeps increasing. This starts to draw more steam mass flow from the water in the boiler... which has a shorter and shorter time to actually move into the cylinder volume in the time allowed by the stroke for admission. Hence for very high speed, you most certainly don't have 'wisps of steam flicking in and out of the cylinders' but you certainly have steam moving in very fast if it is to be there productively at all to sustain acceleration at higher and higher cyclic.
Now there is a sort of lower-speed equivalent of this, which is if the engine is overloaded. Longer cutoff is required, the engine both uses more steam per stroke and makes less use of expansion to produce thrust. The additional steam mass that can't expand blows out the stack with additional heat and pressure energy, driving the boiler to higher combustion rate and (uneconomical) additional steam mass-flow generation. This hits the wall at the 'grate limit' which is the greatest amount of fuel per square foot that can be burned with forced draft through that particular boiler and front end before combustion starts to suffer or combustion-gas flow starts to choke.
but while steam flow across a throttle is proportional to the difference in pressure across ... ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff...
You probably don't have the port opening information for the valves in question, but you can clearly figure out "a" steam mass flow per admission from the indicated chest pressures for start and close of admission, and the time the port is open to steam. Since the degree of superheat is relatively easily (in theory) measured near the valve, you can make the appropriate correction for the actual degree of superheat at admission.
Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation. (Now might be a time to remember Angus Sinclair's comment that engine performance did not inexorably correlate with nice indicator diagrams -- find the passage in History of the Locomotive Engine that contains the line 'a small leg of mutton' -- but for now we can ignore some of the factors that make that happen.)
Overmod gregc ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff... Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation.
gregc ... it's not obvious how to determine steam flow (lb steam/hr) based on cutoff...
Unfortunately, this is not the simple calculation you seem to think it would be, and in my opinion you're better off staying in terms of observed indication performance rather than mathematical approximation.
i didn't say simple. what do you mean by "observed indication performance"?
i and i think others would appreciate an explanation of how early cutoff controls steam flow, allowing the throttle to be open full?
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gregcwhat do you mean by "observed indication performance"?
Just to be sure everyone has a compatible hymnal, I assume the test locomotive has long-lap long-travel piston valves. These open and close to steam transversely, so that the port opens remarkably quickly at the start of admission, over effectively 360 degrees of the valve circumference. And the valve is already moving with considerable speed when the steam edge crosses the near edge of the admission port (that's the point of 'long-travel', not that the valve physically moves a greater distance)
Remember that in admission two things happen: the cylinder volume (which is increasing as the piston in it moves) fills with steam, and that steam exerts pressure against the enclosing space as it fills it. So you get a particular mass, corresponding comparatively closely to the volume, and full 'effective pressure' on the piston very quickly after admission.
There is a technical term 'wiredrawing' for what happens when there is inadequate mass flow to accomplish this completely, for example if some fool has the throttle partway closed so that the effective pressure falls as the piston is moved. This creates the effect of partial expansion during the admission phase, and heat is lost without producing useful work on the piston, and you trade pressure for velocity you can't really use for a variety of reasons. (If you had a turbine with a Curtis first stage, like the one on PRR 6200, you could make use of some of the velocity, but that's not true for any reciprocating engine I have looked at, even those with Bulleid's sleeve valves.)
Early cutoff, whether fixed or variable, results in earlier stopping -- again, at a very high rate of speed -- of steam mass admission. The pressure now falls in expansion, so if you were to measure effective pressure on the piston you will see it falling as the piston moves further, and the volume increases.
Note that these effects are entirely unrelated to throttle modulation. All the throttle does is restrict the steam supply that the valves can admit to the cylinder.
Now, as already noted, a good modern valve gear can cut off reasonably early with sufficient timing precision that the 'beats' are regular and square. This produces a resultant of admission and expansion thrust (take this as "MEP" through to exhaust opening) which -- as already noted several times -- can be remarkably small. This implies that no further reduction of the chest steam supply (via a throttle) is needed if you want a low MEP to run your locomotive, and therefore at the speed that the valves can provide the appropriate MEP in the cylinder, there is no reason to have a throttle restriction upstream of the valves at all.
Now the plot thickens. Suppose I'm starting a train with relatively low 'MEP requirement' to accelerate. If I have good valves and high boiler pressure, my admission at early cutoff shortens to a small fraction of each revolution -- at high effective pressure -- but the small mass now has to expand through a greater volume, with EP trailing off fast especially toward the end. This produces a relatively short torque peak 4x per revolution, which makes the engine mechanically slippery. Most valve gears did not have a precise 'shorten-and-return' that would arrest the slip with a minimal effect on EP, and the typical throttle was a miserably imprecise thing, like trying to play a slide whistle with a bag of flour tied to the slide. So the normal time for response and recovery of a slip without losing power was comparably long, and the control much more like 'shut off and reopen' than, say, easing up on your car accelerator a moment.
You could get around that rather nicely by restricting the admission steam to a lower pressure, and lengthening the cutoff so the admitted mass, starting to expand a little later, will produce the desired MEP at the lower initial pressure. You're going to throw away more heat and pressure at exhaust, but it's often worth it to get a clean start on poor rail. And that can be done easily, if a little indirectly, by partially closing the throttle to 'force' longer admission
OvermodThis implies that no further reduction of the chest steam supply (via a throttle) is needed if you want a low MEP to run your locomotive, and therefore at the speed that the valves can provide the appropriate MEP in the cylinder, there is no reason to have a throttle restriction upstream of the valves at all.
are you saying that
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gregcare you saying that since MEP decreases (not proportionally) as cutoff decreases, that even when initial cylinder pressure is boiler pressure, cutoff can reduce MEP sufficiently to maintain the TE required at low (~10) speed?
Remember that the 'M' in MEP signifies an average.
If the 'resultant' of the valve gear (cutoff by the reach rod moving in the link plus contribution by the combination lever) may not be precise enough to produce a limiting balancing speed down below 10mph, in which case you'd restrict the chest pressure by closing the throttle down appropriately. In my opinion, you'd adjust the throttle after starting, wind the valve gear to where operation is 'smooth', and then trim the throttle as desired, but that's just my opinion on how to proceed.
gregcSeems there are different ways to operate a loco/train depending on situation...
use of a "cracked" throttle and full cutoff to get just loco or high tonnage train moving
Note that there are two approaches. The more 'usual' one on large or front-end-throttle-equipped locomotives has been to crack the throttle at full (or very late) cutoff with the cylinder cocks open to start the train, but then as quickly as possible pull the throttle open and wind up the reverser toward mid. As noted in Australia and elsewhere in this thread, in many cases you'll still pull the throttle open relatively quickly, just stop opening it at some level (corresponding as noted to a particular peak or 'recovery' chest pressure) and trim the acceleration or final speed via cutoff.
In practice you'll want to get that throttle restriction out of the way as soon as the engine can manage the train on cutoff alone -- that this is possible is part of the adage that 'a good steam locomotive can pull any train it can start'.
use of low boiler pressure and [partially-closed]throttle to move a loco in a yard or possibly switching just a few cars
Yes -- but a caveat. A number of very expensively built or funded switch-engine development projects have foundered, sometimes utterly, against the rock of not understanding how flat switching works. What you want is maximal acceleration with even a heavy cut of cars -- but without overpowering to slip with a light one -- and then reasonably quick deceleration once the cut is positioned and then acceleration to go get the next one. This isn't a place where low pressure moseying around spotting a few boxcars in the equivalent of a model-railroad switching problem would apply. It is, in fact, perhaps the most demanding engine service of all, and on top of what it requires, the engine may, with little advance mechanical warning, be required to idle with minimal fuel consumption between bouts of full accelerationdeceleration. (It is not difficult to see why even primitive DC-motored diesel-electric switch engines were so successful so early... but had to be so enormous in some applications that don't appear to require the horsepower)
lower boiler pressure reduces tractive effort for low or no tonnage trains
Yes, again both by having lower continuous thrust during admission and lower MEP during expansion.
lower boiler pressure provide more room for pressure to rise while throttle is closed
That is a little less certain. All that the throttle does when closed is retard the potential flow to the steam chests. If there is no demand at the chests, no steam flows. What to look at if there is some steam demand, if considering what happens in the boiler, is the pressure fluctuation immediately upstream of the partly-closed throttle: it is this zone that is the other end of equilibrium with the overcritical boiler water at this point, and what will govern 'pressure rise' due to additional heat uptake from combustion gas to water.
"Pressure recovery" will be greater in proportion to lower mass flow. But again it is the valves that are the key determinant of mass flow. If they are closed, the restriction at the throttle doesn't matter as much (or indeed, perhaps at all) whereas if conditions at admission lower the EP during that part of the stroke, the boiler can only make up the mass flow that passes the restriction, and can only do it via pressure relaxation (e.g. 'starved' pressure) into the lower pressure measured at the chests.
And, as you've already said, all the heat going into the water has to go "somewhere" -- and that 'somewhere' is going to wind up as increased saturation pressure, and if that gets up to the popping-off point, the fireman will have to use feedwater to keep pressure limited. (As another issue, he can't get too much additional feedwater in there, so he may have to 'live with' safeties lifting while he reduces the fire... the effect of which may take several minutes or more to 'work'. This, too, is part of his job to anticipate...)
opening throttle and using "early" cutoff to control speed of a high tonnage train over its run
But be careful. You'll start a high-tonnage train with long cutoff and perhaps WOT; even there, you won't be able to start a 'train' more than a few cars long... which is why slack action was so important in steam days, and why continued acceleration of the part of the head end that was moving was sometimes a "necessary" thing... to the immense detriment of the crew in the caboose of a long train which might be abruptly snatched at shockingly high resultant acceleration the length of the caboose's draft gear and then even more abruptly banged to full achieved train speed, which might be 20mph or more at that moment. (This was I suspect worse than the stories in Railroad Magazine make it out to be, which is saying something.)
The more interesting part of a run with heavy tonnage is how you control the engine as a heavy consist runs over an uneven profile, even to the extent that parts of the train are going upgrade while other parts are in a sag, and you need to adjust power as overall train resistance as felt at the drawbar changes. Unsurprisingly this is both complicated and extremely reliant on experience; the key thing here being the need to do something with the throttle when the engine has to be 'drifting', but then open it up again but with the reverse 'preset' when power is again required.
Otherwise -- in my opinion -- you would 'drive on the reverse' to adjust train speed, and certainly if you were for some reason using power braking. As soon as the engine is at a speed that permits control with cutoff alone, adjusting cutoff 'early' as the means of power control is the most economical in terms of water rate and hence required fuel firing.
OvermodYes
wow
... and thanks
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I hesitate to provide additional documentation as it seems Mr. C only gives credence to texts that he believes support his theories. I thought the preceeding discussions might lead to a mention of the Loco-Valve-Pilot.
I do have several articles about how some of these units were tested on the New York Central but these are contained in copyrighted periodicals. For now here is some baseline information from the 'Cyc":
Valve-Pilot by Edmund, on Flickr
Interesting to note that the cutoff was only changed four times in the distance of seven + miles and a speed range of seventy MPH to a stop.
Valve-Pilot_0001 by Edmund, on Flickr
Valve_Pilot_fix by Edmund, on Flickr
In the development of the Valve Pilot I read that scores of trips were made by the designers who plotted the operating habits and characteristics of a sampling of the locomotive engineers in order to acquire the data for calculating the 'sweet spot' which would be used in determinating the shape of the cam.
Valve_Pilot_crop_fix by Edmund, on Flickr
Valve_Pilot by Edmund, on Flickr
my current understanding is that while cutoff may only be needed to control speed with full throttle at higher speeds and tonnage ...
... throttle is needed at low speed or low tonnage. (perhaps some of you are saying i told you so) of course what low speed or low tonnage is depends on the size of the locomotive
i also believe lower boiler pressure is more suited for operation with little tonnage
a 246 T RDG I-10 loco doesn't require full boiler pressure to move between coal, water and sanding stations. it requires an MEP of ~20 and an additional 25 psi to accelerate it 5mph/ 15 sec. so i'll guess boiler pressure of less than 100 psi is probably more than sufficient
i assume there's no one right way to operate a locomotive pulling a medium tonnage train with a combination of throttle and cutoff and that it depends on the engineers preferences and experience
it bothered me that one of the rebuilders of the western maryland scenic RR 1309 said he just opens the throttle to increase speed. But pulling a half dozen passenger cars behind a 2-6-6-2 capable of 45T of tractive force is a relatively light load, even up the 1.75% grade to Frostburg
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The guy on 1309 is likely simplifying things when he says that.
1309 is a true Mallet, a compound locomotive. This is started on straight steam using an intercepting valve (look it up for the varieties used and the technical details) and then switched over to compound working at comparatively low speed -- well below 10mph in almost any event. On one of these, the LP steam is governed by whatever the HP engine has 'just used' and one of the things this means is that cutoff on the HP engine may have to be kept at a longer setting that an equivalent simple engine that size would be. That does imply a situation where at low speed you might find it better to lower and raise the HP admission pressure, and work the cutoff to provide the right 'balance' between the output of the HP and LP engines. (There are discussions on the Web for both the Baldwin and Alco types of early Mallet if you are interested in this somewhat esoteric stuff).
after resolving 2 confusing issues:
and experimenting with a software simulation, i understand that the primary and obvious means to control locomotive speed/TE is the throttle (which many of you said).
at startup, cutoff needs to be maximized to open the valves to both cylinders. Of course throttle needs to be severally limited. But once the locomotive has some speed, cutoff should be reduced (shortened) to 50% to take advantage of expansion.
my understanding is there is minimal benefit to reducing cutoff further until steam production begins to be limited. When limited, reducing cutoff can maximize the initial cylinder pressure, MEP and speed/TE. But the range of this benefit is small. Of course, cutoff can be reduced at lower speeds to take advantage of expansion to use less steam/water/fuel. But the effect on speed/TE is small.
Overmod's caveat (since edited) that throttle may need to be reduced to use cutoff to control speed/TE effectively says throttle controls speed/TE.
as for the Laboratory Data, I finally realized it's also evaluating performance when steam production is limited. it does this with throttle full by controlling the rate (lb/hr) coal is added.
Under these conditions, insufficient cutoff has a detrimental effect if the valve is open too long and expansion includes the volume of the steam chest. So reducing cutoff has a benefit when it closes the valve just as initial pressure is maximized and expansion begins.
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