i'm trying to better understand, in detail, how a steam locomotive functions
i'm trying to determine the tractive effort at various speeds and cutoff. I believe this depends on the rate of steam consumption by the cylinders vs the steam production in the boiler.
when steam consumption is relatively small, at speed on level grade, heat from the fire can heat steam to a higher pressure. when steam consumption is nearer the production rate, steam pressure is lower. i believe it takes less heat to raise steam to a higher temperature and pressure
of course less steam pressure may result in less speed resulting in less consumption allowing steam prodcution, at a higher pressure, to increase.
i'm trying to work out numerical values for this not just a conceptual understanding and only accurate to the first order.
looking for help. threw out my book on thermodynamics long ago.
greg - Philadelphia & Reading / Reading
It's a whole different thing. What you are describing refers more to achievable speed (a function of swept volume x fixed rotational mechanical advantage) rather than achievable TE, as cutoff largely controls steam mass flow at any reasonable working speed, and things like wall condensation and droplet nucleation also become significant to consider.
The 'joker' in the model is the presence of adequate superheat. This ensures (in a good design, anyway) that whatever mass flow of steam is used in the cylinders does not suffer from premature nucleate condensation (either in aggregate on the walls, or at the tail end of expansion as the steam continues to do work on the piston). You aren't really concerned with the steam quality at the entrance to the dry pipe except insofar as it reflects priming or carryover that causes problems in the superheated-steam quality as delivered at the valves.
Meanwhile, the timeless topic of 'automatic action' comes into this discussion, in ways that do NOT scale to any particular design without a large empirically-determined percent of the relevant variables. This "assumes" that the effect of the mass flow of exhaust steam, acting through the geometry and configuration of the locomotive 'front end', produces a proportional draft and movement of combustion gas that 'just' assures the correct amount of steam generation (without popping off and wasting heat and water mass). There are a number of historical details that factor into this, including 'jumper caps' and other variable gasdynamic devices in the induced draft, dampers and secondary-air openings at the firebox, correct camming and use of Valve Pilot, etc.; along the way was the very interesting experimentation in the early post-WWI period doing automatic cutoff for best economy based on measured back pressure in the steam exhaust.
An unfortunate thing to remember is that pressure turns out to be much less important in power production than many people, including some very bright ones who perhaps 'should have known better', thought in the '20s and '30s. Tuplin in England was a stubborn proponent of comparatively low pressures 'to the end'; while there are indeed some mass-flow advantages to higher pressure these are largely reflected in lower water rate instead of greater achieved TE at speed.
When you get into the field of locomotives expected to run revenue service at peak speeds above 100mph some other considerations start to come in -- Union Pacific, for example, was notorious in building a locomotive that used steam so efficiently in the cylinders that it produced insufficient draft above about 85mph to keep steam production up at that efficient mass flow. That is not quite the same thing as you're describing, and I trust you see some of the differences involved.
gregci'm trying to determine the tractive effort at various speeds and cutoff.
gregcwhen steam consumption is nearer the production rate, steam pressure is lower.
timz gregc... when steam consumption is nearer the production rate, steam pressure is lower. The pressure isn't supposed to be lower. When they test engines at something like maximum continuous output, boiler pressure should be near maximum.
gregc... when steam consumption is nearer the production rate, steam pressure is lower.
The pressure isn't supposed to be lower. When they test engines at something like maximum continuous output, boiler pressure should be near maximum.
This raises a little additional consideration. A typical 'Stephenson' boiler operates at considerably higher 'forcing' in railroad service than in most other steam-generation applications, often approaching a grate limit at 'near maximum' anticipated output. Here the physical heat transfer from the combustion gas to actual steam generation starts to go 'upside down', including observed increasing departure from nucleate boiling (DNB) on many of the surfaces where highest physical steam generation are observed. These aspects can often be noted in tests, particularly those conducted by PRR on the stationary plant.
As noted, a given test run (whether on a test plant/brake dyno or with dynamometer car and brake vehicles) will note trends in steam pressure. Some tests (as, for example, many conducted with Bulleid's Leader) will be run to the point steam generation markedly falls off (or, indeed, drops so low as to require a "blow-up" or period where the steam blower is used to artificially overdraft the boiler to raise steam pressure in the absence of steam demand from the engine).
What you'll find is that some rational percentage of the engine's working capability (sometimes as estimated from ihp calculations) will be used as a start for performance evaluation, and the subsequent testing will "home in" on the most efficient combination of firing and water rate for that particular design. Not that it will be operated there for much of its working life, or even in the general envelope of performance there! (See the Alleghenies as a somewhat dramatic demonstration of this.)
timz gregc when steam consumption is nearer the production rate, steam pressure is lower. The pressure isn't supposed to be lower. When they test engines at something like maximum continuous output, boiler pressure should be near maximum.
gregc when steam consumption is nearer the production rate, steam pressure is lower.
so what happens when steam consumption is far less than at the rate of production such as when cresting a hill?
gregcso what happens when steam consumption is far less than at the rate of production such as when cresting a hill?
This is why the pop capacity is so oversized compared to what might be considered 'normal' relief volume.
One of the skills a fireman learned was how to slow down the firing rate approaching the top of a grade, or a known location for stops or 'checks' in speed, or to get ready to use the injector to reduce the amount of popping off when the throttle is closed or the reverse wound toward mid. You can see some of the approach and methodology in the 'how to fire' videos and some of the guides (like the one Dr. Leonard provides, from 1944)
gregcwhat happens when steam consumption is far less than at the rate of production
In other countries, railfans studied steam locomotive working much more closely than Americans ever have, and they would note that on a climb from so-and-so to so-and-so in so-and-so minutes with so-and-so tons the boiler pressure dropped 10 or 20 or whatever pounds per sq inch during the climb -- showing that it was an exceptional effort. (And maybe the fireman was tired.)
Or they might note that pressure was maintained, implying that the engine could maintain that speed on that grade indefinitely.
Or if you do mean consumption is far less -- no reason it would be far less when cresting a hill, since the enginemen know they're approaching the top and will have cut steam production in anticipation.
i've read that cutoff needs to be reduced as speed increases when acceleration drops in order to partially restore acceleration to achieve the desired speed.
Sure-- at a given speed some less-than-maximum cutoff will give the maximum power. At 60 mph the engineer wants steam to enter the cylinder for maybe 40% of the stroke.
why?
gregcI've read that cutoff needs to be reduced as speed increases when acceleration drops in order to partially restore acceleration to achieve the desired speed.
Several reasons for this. One, of course, is that using the expansion of the steam effectively reduces the required mass flow, meaning that the boiler can supply adequate steam without being forced. Another is that the high piston force near the ends of the stroke that would be observed with long cutoff would tend to increase augment forces. and the time required to exhaust the relatively large mass of (still very energetic) steam would be longer than available even with ports larger than effectively feasible. (And then you get the effect of steam expansion in the exhaust tract, where you do NOT have any value from it, but that's a further part of the discussion...)
The 'kicker' for acceleration, though, is likely "compression." When the exhaust port closes to steam, there is still an unavoidable mass of steam 'trapped' in the cylinder, which is then reversibly compressed as the piston goes to dead center and away to the point of next stroke's admission. You want some of this, of course, to cushion the effective momentum of the piston so it doesn't slam as it reverses -- but you don't want too much, either, as it first causes substantial counterpressure braking over a certain number of degrees of rotation, then can rise far enough to cause blowing through the glands. One of the 'rules of thumb' I remember is that compression ought to be designed to get the cylinder pressure right around admission pressure at the moment the admission valve opens to steam -- this preventing "wiredrawing" or gas cutting of the steam edge close to the point of effective opening (and unshrouding, for poppets).
Note that the thermodynamic fiends always seem to advise winding up the reverse just as fast as you can after a start, pulling the throttle wide open and leaving it there. This is a nifty idea ... until you start to see slip induction at the now-peakier torque peaks in each revolution. Better to drive on the throttle at fairly long cutoff, where any propagating slip will quickly starve itself out perhaps in a fraction of a revolution without requiring that the steam flow be cut (either via winding the cutoff further toward mid or whacking the throttle closed) and perhaps lead to a stall.
OvermodOne, of course, is that using the expansion of the steam effectively reduces the required mass flow, meaning that the boiler can supply adequate steam without being forced.
what does adequate steam mean?
how is it possible that reducing cutoff, reducing the mass of steam drawn from the boiler and entering the cylinder results in greater power? is it the quality of the steam now coming from the boiler? which quality?
gregcwhat does adequate steam mean?
Not meant as a technical term here; I meant 'enough mass flow (at reasonable delivered steam quality, but that's implied) to do what you want with the engine".
Note that 'engine' here refers to the cylinders, valve gear, and drivers: the "expander" that actually uses the steam to make torque. Sometimes I will use the term 'locomotive' in discussions to make the distinction in the normal sense we use 'engine' (as in discussions of Mallets, which are single locomotives with multiple engines) and I apologize in advance if this seems excessively pedantic.
" ... how is it possible that reducing cutoff, reducing the mass of steam drawn from the boiler and entering the cylinder results in greater power?"
It is not 'greater power' that you want, it is more efficient operation or fewer deleterious effects.
Remember the Triplex, "too many legs and not enough steam"? Any locomotive will have 'not enough steam' if run at long cutoff at high rotational speed. More importantly, the 'machinery losses' go up, up, up as the thrusts on the pistons stay high through parts of the stroke that don't use available expansive force efficiently.
The automatic action is thrown off, perhaps dramatically, when 'needlessly' high steam mass flow is blown through the front end. This will be somewhat self-correcting if you are dumb enough to use long cutoff at high speed, as the additional steam will choke the exhaust tract and 'self-correct' by restricting the actual amount of steam that the engine can 'take' at WOT.
Of course, there is another power-limiting problem that comes up, perhaps very quickly, with very high steam demand...
is it the quality of the steam now coming from the boiler? which quality?
This is a sensible question, as there are at least two 'qualities' that are valuable to look at.
The first involves carryover at the dry pipe, the problem that started and finished the disaster with Blue Peter, and the potential condition that is the greatest nominal reason for the Elesco Steam Dryer and some of the other arrangements (as on the Niagara) for reducing the potential for carryover "normally" provided by a high vertical separation between boiler water level and dry-pipe entrance. As Porta liked to say, people often don't appreciate what treated water in a boiler 'looks like' -- he correctly likened it to boiling milk, and also noted that when pressure reduces, as it does when the throttle is opened, nucleate boiling at most of the heat transfer surfaces in the boiler also increases, causing water level to swell. The combination of the effects causes a preferential lift of foam under the area of the dry pipe, and what may be a substantial amount of droplets of water -- leading to large slugs if the effect is sustained, for example if the engine slips at high mass flow and isn't promptly corrected.
Now, in a saturated engine, this leads to predictable problems with water going through the cylinders and into the exhaust, destroying the lubrication, blowing off cylinder heads, etc. I am not concerned with water 'getting that far' on a modern engine with a superheater. But we need to look for a moment about the consequences of introducing shots of 11pH water, with what may be a high concentration of TDS (mostly dissolved salts), into the superheater elements. Thermal shock, uneven heat transfer effects, and corrosion are some of these.
The consequences are different between locomotives with a dome throttle and a front-end throttle. You may recall reading about discussions in the NABC ESC about the relative 'stupidity' of treating a superheater arrangement as if it were a closed pressure vessel (specifically requiring its own safety-valve arrangement and regulatory consequences). That is indeed stupid on a locomotive with a front-end throttle, as the pressure in the superheater is essentially equalized through a substantial pipe with the pressure in the boiler, and will not rise appreciably even if substantial water passes through to flash to steam there. The situation is quite different with a dome throttle, especially if the engine has long-lap/long-travel valves, as if the throttle is closed and the reverse at mid there is comparatively little potential passage for any steam contained in the superheater tract (e.g. only the admission contributed by the combination lever in Walschaerts valve gear) and potentially very high pressure may develop there.
Now, if the engine slips near starting, when the superheater elements are reasonably cold, some priming water can 'make it' through to the cylinders, and this can affect the delivered steam quality in ways your Foam-Meter will not discriminate. This is handled by the cylinder cocks, up to a point, but will be fouling up the tribology as well as throwing water away.
As Chapelon demonstrated in the late '40s, assuring good steam quality at the (HP) valves is Job 1 for good operation at any speed, with any degree of desired cutoff. There is a corollary for admission to subsequent compound stages (which is one reason the booster valve on N&W Y-class engines was a good idea, and is the reason for the reheat arrangements on 160 A1). In most cases, the steam 'quality' most cared about is this, but most locomotives do not have 'devices' that determine this directly. Fortunately, good driving and firing practices -- specifically including the use of as early a cutoff as you can manage in most periods of operation -- help to assure it.
Overmod is it the quality of the steam now coming from the boiler? which quality? This is a sensible question, as there are at least two 'qualities' that are valuable to look at.
can you be more specific what the two qualities are?
the amount of steam (mass) drawn from the boiler is less. How can that result in more power?
gregccan you be more specific what the two qualities are?
Not really two separate 'qualia'. What I mean is steam quality at the dome, and steam quality at the admission point of the valves. Steam quality having the normal definitions it has in the industry.
This is what comes of letting mathematicians get involved with steam engines. We use "mass flow" as an index because we assume it means 'mass of good-quality steam'. If the steam is compromised in its 'ability to make power', mass flow is no longer a reasonable index.
A perhaps-comparable example is the old wives' tale that locomotives with higher boiler pressure develop more power. An immediate point is that, no matter how high the admission pressure, if the engine cannot clear its own exhaust steam (either in terms of back pressure or mass flow) it will not produce the expected power as a function of admitted mep.
Remember also that 'power' is a time-related quality, as opposed to TE (or 'drawbar pull' measured at a particular speed or during a particular acceleration). As such, in a reciprocating steam locomotive, it is related to factors such as achieved rotational speed, stroke frequency (and flow factors associated with the diminishing time per nominal valve event) and wiredrawing effects through the admission tract.
We call a steam engine a 'heat engine' and use thermodynamics to explain much of its workings. But heat doesn't make torque. Pressure created by heat does. And without understanding the conditions under which heat creates pressure (and sustains it as work is extracted) you may not understand why "more heat per second" doesn't result in more output power per second, let alone more economical power in terms of fuel, water, and machine expense.
I repeat that knowledge of 'how to make power' out of reciprocating internal-combustion engines does not always translate over to steam. There are a number of decided faux-amis in automobile or truck engine optimization that do NOT apply to reciprocating external-combustion engines.
CPR High Pressure 2-10-4 experimental. Flopped. Must have been hopeless in order to scrap it in 1940 as we were at war and everything was pressed into service.
T4a 8000 one-of-a-kind experimental three cylinder high pressure boiler 2-10-4. CPR 5/1931 Cyl. 15 1/2" x 28" and 24" x 30" Drv. 63" Press. 850 lbs. and 250 lbs. t.e 83% Oil 4, 100 gals. water 11,500 gals. Retired 12/23/1936 Scrapped 12/1940
OvermodWe use "mass flow" as an index because we assume it means 'mass of good-quality steam'. If the steam is compromised in its 'ability to make power', mass flow is no longer a reasonable index.
rather than explain what isn't, can you explain what is a reasonable index?
once again, reducing the amount of steam into the cylinder somehow increases performance. how?
reducing the consumption seems to result in the boiler able to prodcue higher quality steam. From your explanation, it's not clear what makes it higher quality.
OvermodPressure created by heat does.
seems key!
it's still not clear why reducing the amount of steam into the cyclinder ends up increasing the drawbar force resulting in greater acceleration -- increases the drawbar force.
ultimately, increased power (ft-lb/min) come down to an increase in force.
gregcit's still not clear why reducing the amount of steam into the cyclinder ends up increasing the drawbar force
The first reason is obvious. On a large 4-8-4, a full cylinder of 300 psi steam weighs something like 5 pounds. So in one turn of the drivers, four full cylinders is something like 20 pounds of steam used. In other words, you're using a pound of steam for each foot you travel -- at 20 mph you're using 100000 lb of steam per hour. The 4-8-4's boiler can't produce more steam than that. It can't come close to producing the 300000 lb/hr needed at 60 mph.
So if it were possible to run the engine at 100% cutoff at 60 mph, boiler pressure would drop to zero in ... there's a math problem for you: how long would it take?
So we need to use the steam more efficiently than that -- at 60 mph we'll admit steam to the cylinder for a third of the stroke and let it expand for the rest of the stroke. Note that it my simple-minded calculation, steam consumption will be 100000 lb/hr at 60 mph, and the 4-8-4 can't do better than that.
In reality the engine's maximum power at 60 mph will demand more than 33% cutoff, but steam consumption won't exceed 100000 lb/hr. Steam pressure in the cylinder won't be 300 psi when the valve closes, just because the steam doesn't have time to get in there.
So what's the maximum TE you can get at 60 mph for a short time, if we ignore the plummeting boiler pressure? None of us knows what sets the limit.
gregcrather than explain what isn't, can you explain what is a reasonable index?
Not unless you give up some unreasonable assumptions. Fry and others settled on mass flow (of reasonable quality, defined empirically and with some respect to contemporary lowest cost) in the 1920s, and I see little reason in finding something different. Better to improve the sources of potential loss. But you can't go against normal operating economic practice without being prepared to pay a price. There are time-honored reasons for the use of early cutoff going back nearly to the beginnings of steam power on railways.
Part of the key is to assure adequate -- but not excessive -- superheat in the ranges of speed and load where you expect the engine to produce highest efficiency. In the 'olden days' of the '20s you'd frequently see an external linkage to a superheater damper, which had to be set a certain way to keep the elements from being overheated in certain firing conditions. These were considered 'obsolescent' into the '30s, as the Superheater Company found better alloys and computational methods to design particular installations (I have been trying to find documentation of their methodology for many years, without particular success).
The 'catch' is that for each degree of superheat only a small amount of additional enthalpy goes into the steam (and comes out again as it does a proportionally small amount of expansive work) while on the other hand the gauge pressure balloons (and with it, all the problems that go with higher-pressure boilers) and the temperature on the gas side of the elements starts climbing. Meanwhile, without dampers, operating a modern locomotive at high sustained speed results in considerable increase in temperature and effective mass flow of gas over the elements, often in particular the last few feet toward the firebox-end return bends. This produces 'crazy high' superheat, up toward the levels producing coking of the lube film, with relatively little contribution toward either useful TE or useful horsepower at speed.
There are a couple of effects that superheat has in assuring good steam quality 'at the moment the valve opens to exhaust'. One of them, the standard, is to provide enough energy to overcome the worst of the 'wall condensation' on piston and cylinder bore, which only involves thermal cycling of something like .007" metal adjacent to the bore, but can't be overcome with barrier coatings because it's a consequence of necessary expansion. The other effect (which I learned from an old-hand boiler designer) is nucleate condensation in the steam itself, as work is done by the expanding steam on the piston. While this is reversible, it results in phase change of the droplets and a resulting nonproportional drop in steam volume as soon as there is insufficient 'local' superheat to prevent nucleation. It is wise to keep enough superheat in the steam that, even on long expansion, there is 'enough' to assure what the old folks called 'dry steam' all the way to exhaust opening.
In between what I have said and what Tim Zukas has said, there's not much else to add to answer this. Perhaps someone else can try to formulate an explanation that gets the message through.
[quote]reducing the consumption seems to result in the boiler able to produce higher quality steam. From your explanation, it's not clear what makes it higher quality.[quote]
Less priming and better heat transfer with less DNB/voids translates into better saturated-steam generation per lb. of fuel burned. Lower speed through the elements allows more heat transfer in this poor-heat-transfer environment, and to an extent increases the tendency for any carryover to contact a hot wall (contact being much more positive in heating water than radiation from stainless is). Stuff like that. If that's not enough, think about what a grate limit is, and what happens to combustion and heat-transfer effectiveness as you approach it.
It's still not clear why reducing the amount of steam into the cylinder ends up increasing the drawbar force resulting in greater acceleration -- increases the drawbar force. ultimately, increased power (ft-lb/min) come(s) down to an increase in force.
Except that you forget that force on the piston head isn't the only force involved in a locomotive accelerating to higher speed. Aside from what Tim said, there are relatively quick limits to how much pounding from double-acting thrust a given chassis, or more importantly a given main pin or its seat, can take from higher admission or from quicker reversal of heavy force (as would be the case if we neglect exhaust considerations and look only at the effects of keeping timed admission relative to port location, which is fixed, but maintaining higher mep through sustained mass increase.
This is less of a problem on a modern locomotive with Franklin wedges and buffer, a good cast bed, disc main centers, etc. But something that remains a significant problem is the effect on the boiler of the rapid fall in pressure that occurs when steam demand intentionally outstrips supply -- perhaps the best example being starts with the PRR S2 turbine, which suffered relatively bad bypass through 'slip', which did not contribute to starting torque but most certainly did contribute to increased fire (through induced draft in those four big low-restriction tracts!) Pressure would fall and heat release would increase, a formula for causing thermal distortion in the firebox water spaces and an invitation to a staybolt popping fiesta. Doesn't take many popped staybolts to equal the "savings" from a little quicker acceleration...
Meanwhile, add something to Tim's numbers, if they haven't convinced you quite yet. Even assuming you have something like the steam-generation capability of a Q2 boiler ... think about where the high rate of mass flow came from before it was 'sourced' by the boiler (even assuming 12% or so recovery by an open FWH system). This was the thing that killed the V1 even at 8000 nominal turbine HP (let alone the increased promotional figure, which is part of how we know it was promo and not engineering) -- even with the largest PRR coast-to-coast cistern, you'd get something like 135 road miles full to empty, with necessary water treatment that wouldn't like trackpan-based filling so you'd need to stop each time. You could fix this with additional A-tanks, but each one weighs a considerable amount, deducts from train length, and itself must be kept maintained, staged, and filled appropriately...
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