https://images.app.goo.gl/dQXTCeSUQebiCo3V8
Several USA steamers had smoke lifter 'elephant ears', notably the New York Central S1b Niagara 4-8-4, some of the NYC's 4-8-2's and the several FEF 4-8-4's on the Union Pacific. There were others. I would not include Canada as a place where they were widely used.
Boston & Maine late model P4a and P4b Pacifics and R1a and R1b Mountains had small smoke lifters as delivered from Lima.
Western Pacific GS-64 Northerns also had them, as well as the Wabash P1 Hudsons.
I've never understood how attaching vertical plates to each side of the smokebox causes the smoke to be blown upward. Seems like wind would enter the slots in front, exit in back, and that would be that. Is it the steps that actually deflect the wind, and the "ears" just keep that force concentrated?
Lithonia OperatorI've never understood how attaching vertical plates to each side of the smokebox causes the smoke to be blown upward.
I'm sure there are videos on YouTube that show the shape and magnitude of the vortices and their action above the boiler.
Perhaps the best-designed version from a scientific perspective was the German Witte design and its design-approach clones; these used shaped plates relatively high on the smokebox explicitly as tailored-vortex generators with minimal cross-sectional visible area in the engineer's or fireman's view of important areas forward. I think the tight B&M deflectors were intended to have similar induced effect but carried further inboard in the wider loading gage to 'work' just as well but not block the view forward. While most of the mature deflector designs used on American road power had minimal 'visible metal obstruction fore-and-aft (the thickness of the stiffened ear and of the braces holding the top to the boiler, jacket or smokebox), I suspect that in poor weather conditions the vortices made forward vision a trying thing all too much of the time ...
I have been hoping many years now for one of the folks doing modeling on YouTube to put up a visualization of CFD results from various types of ear to assess the actual airflow patterns involved in what, back in the day, was largely a black art using the wrong basic aerodynamics...
Overmod... I have been hoping many years now for one of the folks doing modeling on YouTube to put up a visualization of CFD results from various types of ear to assess the actual airflow patterns involved in what, back in the day, was largely a black art using the wrong basic aerodynamics...
Just see if you can find a YT concerning the aerodynamics being used in auto racing - amongst all the various aero devices, wickers, flaps, airfoils and everything else - the black art gets explained to some degree.
Never too old to have a happy childhood!
On an ordinary no-wing locomotive at speed, the flat front of the engine shoves air aside, creating low-pressure areas along the side of the engine. Which suck exhaust smoke downward into the engineers eyes. The wings are supposed to extend forward, ahead of the smokebox front, to channel air alongside the boiler, eliminating the low pressure.
OM and timz: So now we have two pretty thorough technical answers about elephant ears, the second more cogent. Are both right, just different sides of the same coin?
In my opinion, laminar flow doesn't exist behind most North American ears, and I think the experimental results (including the failure to work even at massive size on UP articulated power) are largely explained better with turbulent flow and vortices than with an assumption of simplistic air displacement and pressurized laminar flow in the absence of aerodynamic flow shaping.
If you look at most of the Witte deflectors you can see the model of 'scooping high-pressure air' from the displaced bowshock or whatever is not particularly optimized, yet the devices are said to work well to keep smoke away from cabs.
I lean towards timz explanation.
If the plates worked by creating vortices, wouldn't they have a bank of small fins -- those of you old enough to have flown on the 707 may have seen such small fins on the wing. These vortex generators were meant to generate a small amount of roughness so the airflow would not separate from the top surface of the wing and cause a stall under slower flight and higher angle-of attack of the wing?
I think the "strakes" (fins) you see on the sides of the wide-fan jet engines on newer planes do a similar thing?
On the other hand, such fins or strakes generating vorticity could have the same effect of preventing the airflow from separating from the sides of the boiler, preventing suction drawing smoke downward towards the faces of the locomotive crew?
The use of a flat surface, however, to keep air flowing past a blunt surface such as a locomotive smokebox door, has a precedent in efforts to reduce drag of radial piston engines for aircraft.
The early radials had the cylinder sticking out into the airstream for engine cooling, which creates a lot of drag. One of the early drag reductions was to place a ring around the cylinders, essentially, a curved version of the flat-plate locomotive "smoke deflectors." That ring was refined into a cowling that was open in the front, streamlined along the sides, and had a way of expelling cooling air towards the back.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
The vortices as I saw them explained for Witte were large-scale, a bit similar to the tip vortices in the absence of winglets, arranged to provide organized high-pressure regions back along the upper sides of the tubular boiler where slipstream was causing the vacuum effect pulling the smoke down there. I suspect the effect from 'elephant ears' is a bit more like turbulent entrainment in the front end, with 'burbles' of pressure turbulence providing the envelope of high pressure held close to the boiler by slipstream but 'holding the smoke up'... if that picture makes sense.
There were attempts to provide ducting with multiple strakes. I got the impression these were an attempt at laminar flow shaping, like turning vanes in HVAC ducting, rather than small-scale turbulence for flow separation as in aircraft or the now-somewhat-discredited Airtab-style vortex generation to enhance Kamm effect behind van trailers or containers. (See the GTW version of the 'scientific' 6400-style steam streamlining for a possible evolutionary example). To my knowledge none of the straked systems worked as intended (or by accident either). I remember reading, although I don't remember where, about at least one experiment that used horizontal braces between 'outside' ears and the boiler shell, creating the effect of stacked rectangular ducts (but crippling the ability of crews to pass through behind the ears on the 'running board') and not, as an airfoil flow shaper would do, greatly compromise forward visibility.
Perhaps ironically, the specialized vacuum exhaust I designed for high-speed reciprocating locomotives used both 'feathers' active in the ejected gas plume behind the economizer train and vanes in the slipstream around the duct exits to raise the line of the exhaust above the boiler. So it's not as if defined laminar high pressure is either unworkable or wrong, just that I doubt the effect of airflow exiting behind a baffle of that kind would stay either laminar or follow the right theoretical 'streamlines' back along the barrel to accomplish the lifting rather than 'vacuum destruction' in the right areas far removed from just those keeping the forward cab-window-level view clear.
It would be easy to prove or disprove much of this with a simple arrangement of instrumented 'yaw strings' on a skeleton framework behind the ear on a locomotive like 844 equipped with a known-effective functional deflector setup. Even in the presence of quartering wind or other effects this would quickly show much of the nature of flow in the regions of peak interest... like an inside-out wind tunnel
timz Lithonia Operator I've never understood how attaching vertical plates to each side of the smokebox causes the smoke to be blown upward. That's a puzzle, all right. Answer is, there's no need to blow the smoke upward. It's high enough already -- just need to keep it up there, at stack level. On an ordinary no-wing locomotive at speed, the flat front of the engine shoves air aside, creating low-pressure areas along the side of the engine. Which suck exhaust smoke downward into the engineers eyes. The wings are supposed to extend forward, ahead of the smokebox front, to channel air alongside the boiler, eliminating the low pressure.
Lithonia Operator I've never understood how attaching vertical plates to each side of the smokebox causes the smoke to be blown upward.
That's a puzzle, all right. Answer is, there's no need to blow the smoke upward. It's high enough already -- just need to keep it up there, at stack level.
Thanks so much timz, for explaining it in a way the average person can understand.
Lots of speculation, here.
I would expect, back in the day when an answer would have been useful, that there were (or should have been) wind tunnel experiments to see what worked and what didn't.
Without that, you don't even know what worked, let alone HOW it worked.
Judging from the extent of these devices on US steam locomotives, the need wasn't pressing.
Ed
I have seen photos showing just that somewhere. I think that it was done in England.
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BigJim I have seen photos showing just that somewhere. I think that it was done in England.
This photo came up quickly:
Not elephant ears, but in an English wind tunnel. It says. Wonder who got to take the model home afterwards!
Here's a short but interesting article on smoke deflectors. It mentions wind tunnel testing:
https://en.wikipedia.org/wiki/Smoke_deflectors
and here is an undoubtedly interesting article on the subject, should you wish to invest $40:
https://journals.sagepub.com/doi/pdf/10.1243/JILE_PROC_1941_031_047_02
That looks like a photo of the "Hush-Hush" British 4-6-4 locomotive with the marine-type water-tube boiler?
Paul MilenkovicThat looks like a photo of the "Hush-Hush" British 4-6-4 locomotive with the marine-type water-tube boiler.
Keep in mind that the Hush-Hush had some very weird ducted preheating arrangements inside the boiler 'clothing' which did not show up in the model testing but most certainly did on the road, where reported turbulence in the ducts produced a visible 'breathing' ripple at speed.
No, neither of these approaches to duct the slipstream worked particularly well at 'smoke lifting' even though extremely well crafted to channel air directionally. None of the amazing variety of vanes, strakes, ports, little airfoils before or behind the stack, etc. particularly worked either.
On the other hand it is dramatically easy to visualize the flow from 'ears' with little more than an array of thin nozzled pipes connected to a heavy fluid, arranged at the trailing edge of, say, an ear on UP 844. In theory flow could also be followed by laser illumination or fast camera tracking of sequentially-released small or even 'smart' particles, as in looking at front-end action analytically.
Alfred W Bruce design engineer in the 1940's for Alco Locomotive Company discusses the "locomotive smoke ears" as really the problem of - Smoke Trailing.
To quote his book "The Steam Locomotive In America" 1952, gives the following Post War engineering perspective"
"A minor but nonetheless aggravating problem on steam locomotives has always been (and still is) smoke trailing. This nuisance was accentuated with the use of short smokestacks and became even worse with streamlining, especially at high speed with light exhaust. Many different schemes have been tried, all based on creating air steams that would prevent the smoke from dropping to the cab level, but these have met with only partial success. Apparently the smoother the contour of the locomotive jacketing, the more difficult the problem becomes, and fully streamlined designs are the hardest of all to correct.
Attempts to remedy the condition were made abroad long before serious efforts were undertaken in this country. In general, the answer was the application of a wing on the edge of the running board, right and left extending from the back of the smokebox to a suitable distance ahead of the smokebox front. This wing served to guide the head-on air currents upward, and these currents in turn were supposed to prevent the dropping of the smoke itself.
Introduced in the United States about 1940, this arrangement has been frequently applied. Though it is perhaps the most effective construction yet devised, it is still of rather doubtful value. Some reports are favorable and some are indifferent.
The ideal location for an operating cab is, of course, at the front end of the engine, but this location is quite difficult to achieve on conventional designs that burn coal fuel. Smokestack trailing is primarily a combustion problem, and as such it therefore resolves itself to a problem of either boiler design or boiler operation - or as is more likely, both. The heavy billowing black smoke of imperfect combustion, while undoubtedly spectacular and photogenic, has long been one of the black marks against the steam locomotive and is likely to remain so during its remaining life, for anthracite (coal), which would correct the condition, will never again be available as a fuel for steam locomotives."
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The only operational steam locomotives I have seen using conventional smoke lifters were the New York Central 4-8-4 and 4-8-2 designs. Of which I thought the NYC Niagara the most beautiful with its tapered rear edge. Paul Kiefer seemed very committed to their use, along with overfire jets, enough so that thes wings likely represented a truely viable effort at solving the smoke traililng problem.
Union Pacific was also so committed to their use and also has currently retained the "smoke lifting wings" for their practical value or for historic reasons.
I have also noticed the extremely clean exhaust stack discharge from the Union Pacific steamers these days, particularly the "Big Boy" 4-8-8-4 which was a famous locomotive given to its historic outrageous discharge of smoke owing to the cheap lignite coal it burned.
In todays highly charged green enviornmental era, the smoke control of the steam locomotive seems of particular political importance. However, I have enjoyed the glorious smoke plumes of the past as seen on the Cumbres and Toltec 2-8-2 engines still climbing Cumbres Pass and I have celebrated the moment to see this remaining historic witness of the smoke plumes in high mountain air.
Historically the steam locomotive is "less of a show" without it.
--------------------------
Dr. D
duplicate of above - deleated
One of the articles I read said the problem wasn't smoke, it was steam.
Smoke was simply a demonstration that the fireman wasn't in control of his fire, and this could be corrected by better practice.
The steam problem was made worse by modern locomotives having a lower exhaust pressure, and thus a lower stack exhaust velocity. This allowed the steam to linger at lower levels and possibly obstruct view from the cab.
Of course, if the exhaust velocity were raised, any smoke would also be lifted with the steam.
So the author said, I recall.
7j43kOne of the articles I read said the problem wasn't smoke, it was steam.
The issue in North American practice far more often involved actual smoke; perhaps nowhere was this more compellingly demonstrated day after day than on the PRR T1s, which were burning entirely the wrong kind of coal at necessarily higher forced firing level than they were designed for. Part of the objective importance is that smoke doesn't dissipate at the same rate steam vapor does, and it can be highly relevant what the seasonal climate or other weather factors are whether vapor or smoke in the exhaust (there isn't any 'steam' by the time the plume gets into the crew's line of sight from the cab, of course) is the major obscuring factor. Keeping the plume from vacuum displacement was, and remains, the key to keeping vision unobscured from exhaust. (In my opinion, but it is historically and physically extremely well grounded on large, and particularly laminarly well-streamlined, locomotives -- the latter point something that Holcroft noted enough to remark on, but not thoroughly explain.)
The steam problem was made worse by modern locomotives having a lower exhaust pressure, and thus a lower stack exhaust velocity.
This was certainly convenient in the days of restrictive front ends, where back pressures on the order of 22psi or greater could be accepted as 'best practice' -- you can gauge for yourself how much expansive work is lost in just the difference between that level and the ~5psi of a good modern front-end arrangement with equivalent draft -- but that certainly wasn't the case even by 1941. The issue then rightly shifted to the actual aerodynamics tending to pull the smoke and steam lower in the first place, and that is where any practical 'smoke lifting' device that actually operates to purpose on a large modern locomotive has its effect.
One critical thing that all these devices share is the very long effective 'fetch' along the sides of the boiler where the natural aerodynamics tend to pull air from the top. Anything purporting to 'lift' the exhaust plume, which remember is a process, not a physical 'thing' with structural integrity, would exhaust its propulsive effort within not more than a second or so, and without coherent maintenance of what amounts to 'clear air' in the entire subsequent path of the plume to behind the cab, the plume will be susceptible to any perturbation, specifically including a persistent one, that would tend to move it.
A potentially relevant example of another effect in the plume, which is its considerable heat content (which naively would tend to lift its components like the hot air in a balloon) can be seen in the British designs that used a Franco-Crosti economizer. Here the exhaust is subjected to a very long gas pass, almost the length of the boiler, in contact with heat-exchange surface at a relatively very cold temperature, and yet the exhaust plume characteristics were similar at the point of entrainment of exhaust steam in the exit nozzle and duct arrangement. This had the somewhat nominal advantage of exhausting high up just in front of the cab, with the entrained gas likely still being superheated relative to the exhaust through the acceleration zone (this is enhanced at low effective back pressure with respect to thermodynamics, and probably with difference in mass action) and therefore lifting to clear the cab before vacuum effects in the following train might pull the plume in some 'undesirable' direction or dissipate it preferentially in a direction that would block following view.
With respect to operating a coal-fired steam locomotive in the cab-forward direction to protect the crew from smoke, besides those Italian 4-6-0 locomotives (or were they 0-6-4's?) with the awkward hand-firing from side bunkers, were not the ill-fated C&O turbine electric's along with the solitary N&W Jawn Henry also cab-forward?
The arrangement was coal bunker in front, cab next, firebox, boiler, smokebox, and in the case of the C&O turbine, the "turbine gallery" (from power-plant lingo)? The water, in both cases, taken from a trailing water-only tender? The in-front coal bunker having narrow sides so the crew could look out along each side -- no worse than the sightlines along a boiler in front?
Maybe that the turbines were mechanically fired helped from the fireman having to adopt an awkward posture to steal glances out front for signals while shoveling into the backhead in back (would that make it the boiler "fronthead"?). Perhaps if the bunker were overfilled, the crew could get coal lumps pelting the front windows.
But aside all of the warts the turbines suffered from, is placing the coal bunker in front of the cab instead of the boiler and smokebox such a bad arrangement? Wardale seemed to love the concept for his contribution to the many designs to come out of the ACE effort?
The significant detail was to keep the stoker between the coal and the back of the firebox. This necessarily puts the boiler 'facing backward' with the stack trailing, at the cost of the fireman's having to face the backhead or wherever the instrumentation is installed (there are good pictures of this for the turbines).
Ideally for a cab in 'normal' position to be leading, there would be nothing in front (as with the way German class 05 003 was set up). This pretty well means that fuel has to be brought past the firebox water legs and mud ring, grate, and ashpan arrangement if it is to be fired from the same cab used for driving. (The exception would be to use a front-end stoker, under active development in the late '30s and actually tried, apparently, in considerable numbers (over 70) on B&O, but the potential problems of this are enormous, so it isn't surprising no one goes on record as recommending it.)
On the original Loewy separable 'triplex' design patented in the mid-Thirties, the 'engine' (boiler system, cylinders, driver wheelbase) was on the 'center' unit, with fuel in a tender on one end and water in a tender on the other. Obviously one of those two needed to be 'leading', and since putting water at the rear simplifies 'having more than one' and improves the potential guiding -- nobody wants to push something sloshing at high speed! -- the bunker end leads.
Now it is not difficult if you have power reverse and air throttle to rig up a driving cab on the end of the coal tender, but this makes for a walk across the coal pile any time something needs attention in the autonomic firing. Which would likely be often, and perhaps without sufficient warning to know what a problem might be. The sensible thing was to put the cab at the backhead, put the fuel 'ahead' with the stoker mechanism right at hand under the cab, and configure things to allow forward vision, which as you note usually involved thinning the sides.
This made for a limited bunker space compared to, say, what could be provided in the NYC 64T pedestal design, side to side and capable of being piled relatively high if the collar extends up to the loading gage. Note that most of the turbines 'as built' had a retractable or folding cover over the coal pile; it might be arguable how long this would survive in service; at least on N&W this was replaced with a collar allowing more extended range... which, a bit like the first law of consulting, did not imply good range.
A point to remember here is that, even with the better thermal efficiency of the turbine at sustained design speed, there are still severe consequences for making 8000hp in a non-condensing design. The V1's practical range was slightly over 130 miles, which in the days where stops were required at every division point was not so bad, but for prospective high-speed service destroyed much of the point of having 8000hp in a single unit. (This would only be worse in the vaporware '9000hp' or larger designs PRR hinted at in the latter Forties...)
A design like the ACE3000 was meant to operate either way with equal facility (something that is arguable, but with care achievable, so I won't comment further) and was built with the somewhat naive assumption that automatic firing in the '80s would be sufficiently reliable ... on 'run-of-mine coal' in modular packs, no less! ... that the cabs could be placed on the ends of engine and 'support unit', with only a little sealed side door provided in case Something Needed Attention In There. This would have been swimmingly sensible if the engine had made the power of, say, two diesels of equivalent length cabs-out MUed, but the equivalent of 1500hp continuous per unit was not particularly up to 1960s standards even before noting it had to be run through just four axles out of the 14 present. A bit of somewhat double-edged good news was that the rather complicated draft-turbine arrangements would have, at least in theory, been addressable from the cab on that end (usually the 'leading' end with the exhaust adjacent, but behind) and a similar arrangement on one of the turbines could have been adopted had there been assurance, particularly on the chain-grate 600psi B&W boiler type used for Jawn Henry, that no direct oversight of the fiery end of the steam generation system would be necessary 'most of the time'.
In practice of course any visibility problem, whether on a Garratt or Meyer or on a long arrangement with bunkers or other structure in the way, would be handled in part via either a periscope arrangement or cameras. A suitable 'enough' FM-TV system was developed for Air Force guided weapons by the end of WWII and while this might not be a marvel of precision video it would certainly serve to show most of the situation ahead and to the sides of the advancing locomotive.
Likely the correct answer to firing, where it could be adopted, was the continuous chain grate, which could not be 'hand bombed' except in a relatively crude sense. Presumably most issues with mine-run dirt, slag, tramp iron or other indigestible inclusions, or excessive lumps could be solved right at the coal gate. Ash arrangements are also fairly simple in principle, and can be made enclosed enough to satisfy reasonable early EPA-style regulation there. Of course there are other fun things that can happen to a large chain-grate firebox, especially if reversing the grate becomes advisable in a quick stop, but those issues can be at least addressed in good detail design. Note that as on a Meyer the actual drop of the chain grate can be remarkably deep; it is instructive to look at the primitive ash-dumping arrangements on the N&W TE-1 which are nearly right on the railhead at the bottoms of the door arrangement and consider what a more modern arrangement there might require.
While we are on the subject of fuel bunkering and firebox proximity, I'd be remiss not to mention the Belgian Franco-Crosti locomotive. But don't expect me to describe it in detail. To those who know that locomotive, no explanation is necessary. To those who don't know it, no explanation is possible.
A somewhat obvious solution to the bunker-size issue is to use Top Gon or skip type transfer loading from some other coal pile. This treats the "bunker" like the feed hopper in a pulverized-coal utility boiler, and it can then be fairly tall and convoluted and still be perfectly 'capacious' enough. There is a problem, of course, with arranging skip loading the length of a 'trailing' boiler to reach those front-mounted-bunker doors and clear the combustion exhaust... I leave the picturing of some suitable arrangements to the reader.
I have found the idea of a coal-dust fired steam locomotive interesting for a very long time, probably because of my tendency to prefer oil-firing for steam locomotives.
Here is a brief article on the general subject:
https://en.wikipedia.org/wiki/Pulverized_coal-fired_boiler
And here, to my surprise, is an article referring to a batch of steam locomotives using the process, apparently reliably, for at least 20 years:
https://en.wikipedia.org/wiki/Prussian_G_12
Note: not ALL of them, but at least 43.
Keep in mind that there is very little in common between typical utility firing and systems on locomotives. In particular, flameholding with highly variable demand following is no fun, even less than it is on light oil.
With normal stoking, keeping heat distribution in the firebox structure where it should be is relatively easy, whether distributing with jets of Elvin/Detroit flingers. PC has all the fixed-nozzle joy of oil in this regard; you will not be taking guns on and off the line for turnup and turndown as you would on a big once-through watertube boiler. There is also close to zero chance you will capture significant ash from the gas stream, so whatever doesn't glass out on the elements will blow out into the environment, very likely with the irritating characteristics noted for the TE-1 boiler exhaust but now in much greater amount per ton fired.
The real thing that caused the trouble, though, was not the moisture or the jamming or the variable firing or the flameouts. It was a combination of jointed rail and relatively primitive suspension. Sooner or later every mobile system using pre-pulverized coal has a levitated-dust explosion, and the sequelae from one of these usually cost more than years of whatever you were supposed to be saving or realizing as an advantage from the more direct heat release.
A G12 is a cute locomotive, but it has nowhere near the firing rate of what an American locomotive would need to justify the capital expenses associated with PC. I am trying to remember if there were trials of the StuG system here; we can I think leave the Fuller-Lehigh experiments with low-rank out of the discussion as oh Lord were they the answer to questions no railroaders would ask!
Something perhaps interesting was the idea of using SRC rather than just PC. That is a more expensive 'engineered fuel' in some of the same sense #2 diesel with additives is, but it has no ash problem at all and the trace elements that are causing so much viewing-with-alarm at present (see the thread with the arsenic levels) can be relatively easily separated during processing. The GM coal-burning turbine Eldorados ran on a variant of this, and it would be technically possible to run a combined-cycle plant with the larger space available on a locomotive, with steam bottoming at sufficient scale. This of course solves most of the difficulties with primary firing in a Stephenson boiler, but if anything makes the explosion hazard more extreme...
In 1938 on the London, Midland and Scottish Railway (LMS) the new Duchess Class locomotives did not ave smoke deflectors on and the engine drivers complained the smoke 'got in the way of their view'. Smoke deflectors were quickly fitted.
Here is 60163 Tornado with smoke deflectors in July 2019.
David
To the world you are someone. To someone you are the world
I cannot afford the luxury of a negative thought
One thing that tends to get glossed over and overlooked in many aspects of railroading is the prevailing winds in areas where railraods operate - thus the 'wrong side of the tracks'. Even where population doesn't exist the winds still blow and blew back in the steam era - prevailing winds in some areas may have benefited steam engines with wind deflectors - some didn't.
Overmod, didn't the German O5 003 run on pulverised coal? And weren't all the SP cab-forwards oil burners? I've seen pictures of Garratts on the East African Railway with the coal bunker forward and always wondered how the coal got into the firebox.
54light15 And weren't all the SP cab-forwards oil burners?
And weren't all the SP cab-forwards oil burners?
You are correct. The cab-forwards would not have been possible if they were burning normal coal. The North Pacific Coast engine that may have inspired the cab-forwards was also an oil burner.
54light15Overmod, didn't the German 05 003 run on pulverised coal?
One of the chief difficulties with PC on locomotives is turndown. The expected economical range is far outside powerplant range, and you have the somewhat Hobsonian choice of having the burners where good oil systems have them, firing backward under the arch for long flame travel, but where they are inaccessible if a wide number of troubles befall them, or putting several bends in a long pipe full of levitated pulverized steam coal to get it to where it is usually blown in near-explosive mixture in the desired pattern to most 'fill' the radiant volume. There is no heat reserve (as is possible for example with chain grates or building a 'heel') and many of the same severe thermal-distortion issues as with oil firing now complicated with flameholding issues.
The late Victorian Railways setup was supposed to be pretty good, but it was a system that used pre-ground coal to save the trouble and expense of on-board mills and classifiers. Sooner or later this always results in a BOOM! that discourages much of the competitive feeling re not adopting diesel-electrics...
I've seen pictures of Garratts on the East African Railway with the coal bunker forward and always wondered how the coal got into the firebox.
Note the existence of the Union-Garratts, which put the feed bunker on an extension of the boiler cradle rather than articulating it on one of the engines. This was specifically done to simplify using an auger-fed stoker without hinge or joint issues -- I have never heard that this did not provide the advantages expected.
Note that transfer from a larger bunker further out into the relatively small bunker built onto the frame extension was possible using less expensive methods of transfer than those requiring a coal pusher and plates to work effectively...
BTW the 'correct' way to run a Garratt was with the cab leading the boiler, to keep smoke effects from being a problem at all. This is not what most people think of when they imaging a Garratt running...
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