In evaluating claims of steam locomotive thermal efficiency under different conditions and circumstances, I choose to consider the efficiency in which coal is converted into superheated steam by the boilers separately from the efficiency in converting steam into indicated horsepower or “power in the cylinders.” I will get to power “at the wheel rim” and “at the drawbar” later on.
Alfred Bruce (1952) The Steam Locomotive in America, p 142 Chart 15 gives curves “of a 484 steam locomotive.” He is being coy: a boiler capable of evaporating 110,000 lbs steam/hr to produce a peak of 6600 indicated horse power ("in the cylinders) is no garden-variety Northern – it is probably Kieffer’s Niagara built by Bruce’s employer ALCo. This stated evaporation and horsepower is a steam rate of 16.7 lb/hp-hr – under some unstated conditions of speed and cutoff.
Bruce goes on for some pages talking about “Cole ratios” (an ALCo employee preceding Bruce by some decades) and grate area and how it really isn’t grate area that matters but firebox volume, with 7 cu ft of firebox volume typically provided for every square-foot of grate area leading to Chart 16 on p 176. Here, the evaporation in pounds of water/pounds of coal, my second efficiency factor, is charted to be about 7.5 at the low end of firing rate to 5 at the high end. The low end corresponds to about 50 lb coal fired/sq-ft of grate and the high end reaches 200 lb coal/sq-ft of grate.
Now 50 lb coal/sq-ft of grate is a rather low rate, but Tom Morrison (2018) The American Steam Locomotive in the 20th Century hints that towards the end of steam, some railroads were aiming to “derate” their steam locomotives to fire them at this low rate to get better efficiency. 200 lb coal/sq-ft, on the other hand, is what one would fire during test to have bragging rights as the the peak horsepower of a particular locomotive.
Keep in mind that all the factors you’re considering are not necessarily scaled correctly for modern power, and that a number of trends in late big-steam practice were counterproductive in actual practice. In my opinion it’s best to start with first principles to be sure you’ve modeled the subsystems reasonably ‘accurately’ (which is a big problem in Fry’s detail formulae, which quietly rely on empirical constants from early-'20s practice not good for later designs).
I presume you have downloaded and read Fry’s book now that it’s been released as a PDF. It is one of the must-read references if you actually want to design locomotives.
I have lost the patent reference to Snyder’s preheaters, but you should look them up (and, arguably, use them). These use relatively spent exhaust steam to preheat the primary air entering between the ashpan and the grate, with the option of further condensing or pumping the condensate into the tender to increase the effective water rate. Reportedly C&O on test got a better than 10% improvement (although I disremember exactly on what) – the point being that all this improvement was substantially free as far as cost or flexibility of operation are concerned.
Likewise, DNB in the water legs, although a serious problem, cannot be directly observed or quantified, but it can be addressed with better ‘waterwall’ circulation. Best is probably still a Lamont firebox, which runs about 6x the steam demand per hour through waterwall passages and then uses a cyclone arrangement to do mechanical steam separation with the circulation velocity. But you can get a long way toward this by using the Cunningham arrangement, which is a jet pump fed from downcoming areas in the convection boiler that feeds nozzles strategically placed in the water legs with fan diffusers above them. Cunningham did not (to my knowledge) subsequently optimize where the enhanced leg circulation goes once
The place to start is with the coal fuel, and that right there opens a sort of can-of-worms controversy. There are some who advise designing the locomotive to burn ‘run-of-mine’ coal, to minimize cost – perhaps the most glaring example of this was the ACE3000, which would have used sophisticated modular ‘coal pods’ designed to be taken off the tender and fueled remotely … likely on swap-body truck chassis … at mines offering the cheapest price. (Which I think would be suicide on a sophisticated heavy 4-8-4 with obligate condensing and at least the premise of semi-automated firing… but that’s just an opinion.) The AAR and other industry sources recommended (through the late 1940s, after which it increasingly ceased to matter) the opposite approach: coal mix of good ranks and ashing characteristics, properly sized to about 2" and then handled to minimize fines, and kept well-washed. I would note that some clean-coal approaches recommend co-firing with dolomite (to flux the glassing components in the ash and knock down some of the sulfur if the coal contains it, as many do) and this can be applied as a slurry to the lump coal at a convenient time during the sizing and washing.)
Thermodynamics often fails to consider the effects of having to deal with the post-combustion ash. Porta did some studies (regrettably mostly still untranslated from his somewhat idiomatic Spanish) including Detroit-style ‘ashaveyor’ systems to move ash to modular storage for longer run time and better ecological ‘optics’ – but there are issues to be dealt with. I believe Wardale’s concern with unburned fuel falling through the grates and ‘quenching’ in the ash is interesting, but it may be much less difficult on a North American engine with proper FireBar-style rocking grates and good secondary-air arrangements – the difficulty being that with proper ‘fuel design’ there is no more place for ‘recycling’ the remnants in
If you mean how is the figure calculated – no argument about that, is there? Convert the BTUs of the coal burned to foot-pounds, and compare to the foot-pounds in the cylinder, or at the drawbar.
Overmod gives a synopsis of late-steam era efforts and technology to improve the notably poor thermal efficiency of the Stephensonian-pattern steam locomotive along with what may have worked and what may have not been worth the trouble.
As to why consider thermal efficiency at all, especially in a time and a place when coal was cheap and plentiful (although the post-WW-II labor actions to give miners their due changed the equation, hastening Dieselization it is said), Wardale explains that the sheer quantity of coal placed a capacity burden on the railroads consuming in. If you could use coal more efficiently, that would free up a lot of track capacity, a lesser-noted advantage to the thriftier diesels. Even today, the cost of transporting coal from mine to the power plants and other remaining users is said to exceed to the cost of digging it out of the ground, especially for the bulkier, low-BTU Powder River Basin coal that is extracted from open-pit mines with special-purpose jumbo-sized excavating equipment.
Apart from the discussed improvements to steam, I am posing the question as to what it even means if someone says “this locomotive is 8% thermally efficient” or “that locomotive required 3 lb coal/hp-hr.”
Consider the efficiency of producing hp from superheated steam as claimed by annotations on cylinder indicator diagrams on pp 258-259 of Wardale’s “Red Devil.” This locomotive is claimed to have a particularly low flow resistance steam circuit although its boiler pressure is quite moderate by late-steam standards (1464 kPa translates to 212 psi in more conventional, human-centric units). At 42 MPH, 52% cutoff, it used 15.8 lb steam/hp-hr. This is data Wardale reduced from an electronic indicator diagram that he admits isn’t an exact match to data from a mechanical indicator diagram no showing such rounded corners of the trace, but still, this is not that far off from the 16.7 lb/hp-hr for putatively a Niagara at
Yes, what you say is well-known, but what I am asking, where does a figure of 3 lb coal/hp-hr come from? The case of Alfred Bruce’s “mystery 4-8-4” (surprise, an ALCo product and the legendary Niagara), I speculate it could come from 16.7 lb steam/hp-hr, a figure I suggest comes from achieving peak hp by operating at high speed that lowers condensation loss but at a generous cutoff using a lot of steam, and from raising 5.5 lb steam/lb coal, which occurs at a high, inefficient firing rate. The coal consumption may have been considerably lower if the Niagara was not operated at peak horsepower, which author Tom Morrison hints may have been railroad practice.
I am just speculating here, but could we consider the Niagara to be operated at 7 lbs water/lb coal, 13 lbs water/hp-hr, 4000 hp, less than half peak evaporation at 52,000 lb water/hr, 7400 lb coal/hr.
This firing rate is 74 lb/sq ft, almost a third of the firing rate for max hp, consistent with the higher specific evaporation rate I assume above. By not “forcing” this locomotive, 4000 indicated HP is still quite a bit, and your coal rate is now 1.85 lb/hp-hr or 9.7%, putting in the range of figures quoted for the more efficient British steam locomotives?
This isn’t from changing a thing about this excellent locomotive apart from how you choose to brag about its performance?
The Niagara has been observed to do even better than that.
Bill Tuplin ("The Steam Locomotive, Its Form and Function) was something of a low-pressure enthusiast, and seems to seldom have passed up an opportunity to note that increased maintenance of high-pressure boilers may have outweighed the thermodynamic advantages of higher throttle pressure (if adequate superheat is provided). It was therefore probably delightful to him to observe a Niagara, on what I recall being characterized as a way freight (probably break-in after service at Harmon) doing the job of a 2-8-0 on a 2-8-0’s budget of fuel and water. The fireman was reportedly using sliding-pressure firing, probably to about 180psi, and of course there was lavish superheat available from shortly after opening the throttle. So things scaled still lower with that boiler.
There was a concern, though, with the steam separation in the ‘domeless’ boiler, particularly at the expected steam mass flow associated with higher firing rate. The Timken rods were extremely intolerant of priming carryover, and probably excessive peak compression, and one set of repairs of the resulting damage would pay for many, many tons of coal and pans worth of water.
NYC was also one of the roads to replace whistle use with ‘pneuphonic’ horns, recognizing the heat and mass loss involved (much greater than that expended in the air compressors for a horn blown on main-reservoir pressure). I’ll grant you that the whistle had it all esthetically … but particularly when aggressive water treatment has to be in use for alloy boilers, the mass loss and water-rate complication involved in even a little crossing-signal blowing adds up fairly dramatically.
There is, as far as I know, no good test result on use of the Hancock Turbo-Inspirator by extending its shaft to drive a dynamo, and then using that power to run some of the auxliiaries that did not ‘scale’ well to
Not quite. Diesels are rated at horspower into the main generator to be used for traction (at AAR standard conditions - 60 degree F air and fuel, 28.86" barometer). Power for auxiliaries is already taken out. (fans, aux gen, TM blower, etc.). Builders will usually provide RRs with info stating BHP, THP, and NTHP (BHP is Engine brake HP, THP is HP for traction into main generator, and NTHP is net HP out of main generator head to TMs).
I think that is pretty much what I am saying. The 3000 HP rating of an SD40-2 is “before the traction generator” but “after the auxiliary loads.”
I was contrasting this with my figures for a steam locomotive, also “before the mechanical rod transmission of power” but without taking deductions for auxiliary loads.
I suggest separating the auxiliary loads needed to ‘run the boiler’ or steam generator from those used to operate the engine – with one special category (draft induction) kept separate from both. There is also a different set of ‘auxiliaries’ in play for starting (cylinder cocks open) and certain modes of drifting, when you get to operation.
I used the ‘idling’ consumption (in the absence of a Direct Steam type system, or electrical pressure maintaining as on rebuilt 8055) and this involves things like continuous blowdown that you might not know to account for (personally, I don’t like current continuous blowdown as an option and plan to use different approaches that recover at least some proportion of the water mass and heat)
Problem is that many of the auxiliaries were designed to run efficiently at or near peak cylinder efficiency mass flow, or to suit the anticipated demand of a forced boiler, and are inefficient at idle. Therefore some of the heat-balance data are nonlinear and change nonproportionally.
Whereas I am offering speculation, I am indeed suggesting that a Niagara could be run that efficiently at part-power, and I am invoking Tom Morrison (2018) The American Steam Locomotive in the 20th Century as a source that this could have been a practice in the late steam era.
There is a narrative that especially the French and to some extent the Germans and the British were building much more efficient steam locomotives than the ham-fisted Americans. Perhaps some of the shade thrown on late-era US designs is in the way figures are quoted?
That is more than a ‘narrative’; it is demonstrable in a great deal of practice and explained in a great many sources… both technical and ‘railfan-oriented’. French railroads, for example, had very poor access to locomotive coal, but comparatively good access to ‘mecaniciens’ to run complex machinery to wring the last erg and dyne out of steam. And a willingness to engage in weird thermodynamic contraptions that make the Yellott coal-burning turbine look simple by comparison, in furtherance of nominal economies made practical by management.
There are equally clear explanations on the American side why cheap, robust construction and cheap maintenance concerns ‘win the day’ over expensive and often fragile or illusory thermodynamic improvements.
I will not make exhaustive (and ever more boring) lists or citations of the various issues. But one fairly dramatic one might serve. You may remember the 614T testing, ‘supposedly’ providing data for modern boiler design. Not only were some of those tests conducted with leaking staybolts, NONE of them was conducted with a working feedwater heater.
Another approximation useful for calculating locomotive efficiency is:
TE (lbf) X Speed (mph) X 1.99 = power in Watts (“2” is a good approximation for 1.99)
Kratville’s book on the Big Boys had dynamometer data from eastbound runs from Ogden, recall coming up with a figure of 4 to 4.5% for efficiency converting energy in coal to DBHP.
7400 lb/hr of 14000 BTU/lb coal = 103600000 BTUs per hour
= 80600800000 ft-lb per hour
= 40707 horsepower
So 4000 hp is 9.8% thermal efficiency. Offhand guess: no Niagara did better than 7% on any test. NY Central didn’t care enough about efficiency to find out the 4-8-4 was actually that much better?
Other US engines could hope for 9% anyway? But tests never revealed their efficiency either?
William Withuhn (2019) American Steam Locomotives, p 253 gives the coal rate of the J3 Hudson as 2.03 lbs/indicated (cylinder) hp or 8.7 percent.
So performance under some operating condition of late-era American steam better than the well-quoted 3 lb/hp-hr or 6% thermal efficiency is not unheard of. The question is, under what operating condition are people talking about because boiler efficiency charts (lb water/lb coal) and Wardale’s indicator diagrams (lb water/hp-hr) show that these supporting elements of efficiency can vary widely.
Again, efficiency percentages above 6% are widely quoted for French, German and British late-era steam, but did they have some “special sauce” that US designers ignored, or is it a question of how these locomotives were operated?
I think you left a layer of mechanical conversion efficiency out of your calculation. I think the 6% rule of thumb applies to overall thermal efficiency of a typical late steam era locomotive, possibly operating under steady state conditions. There is a figure in Jeffries book, “Norfolk & Western: Giant of Steam” on page 62 that sheds some light on the assumptions of this 6% rule of thumb. Hopefully, you have this book and can reference, but it is a comparison of the operating cost between a diesel-electric locomotive and a steam locomotive on the basis of “work at the rail per dollar”. It shows 400,000 BTU of work being done at the rail for 6,666,666 BTU/$1 of coal at the then-current cost of 13,666 BTU/lb coal. This works out to 6% thermal efficiency (400,000/6,666,666). I tried to post a scan of this figure, but apparently the Trains forum requires an image to exist somewhere on the internet to link. It is important to note that this is work at the rail (tractive force), and not work in the cylinders.
If we adjust your 6% number (derived from figures roughly applicable to the maximum indicated horsepower output levels of fthe NYC Niagara) for the rather jaw-dropping mechanical and resistance efficiency loss factor (4600 DBHP/6600 IHP @86mph)= 69.6%, we get an overall estimate of thermal efficiency of only about 4.2%. Of course, that would be at the very inefficient operating condition of maximum IHP and presumably very high (and inefficient) firing rate. And it should also be po