Hi ShroomZed
You asked why the piston valves are (must be) so enormous on
these engines - well, that's about like asking why are the arms
of Aleesha Young so enormous. And the designer of the NORD
railway in France, Marc de Caso, has long been taken from us.
The engine on your picture is preserved at the Musée de Chemin
de Fer in France. She is a four cylinder compound and what you
comment on are the HP cylinders, i.e. you have fresh steam pipes
coming down to the steam chest of the outside cylinders (middle)
then steam is being transferred to the inside LP cylinders and you
want to have capacity in the intermediate receiver in order to keep
steam pressure from fluctuating too hefty with each exhaust HP
and intake LP. These were the outer conducts besides the fresh
steam pipe - and all of it is being covered by one combined cladding.
What exactly Aleesha can do with her arms you might want to ask
herself, but the 150.C locomotives this way could better keep up pulling
force at rising speed due to better steam flow, in other words produce
higher hp outputs.
About the same - yet somewhat more elegantly - can be seen on the
'Little Wolf' the infatigable 141.P engines - Mikados of average
(European) size that in spite of their 65" wheels were used to run
heavy express trains of 16, 18 or more bogie cars at up to 120 km/h
(~75 mph) over ondulating profiles developing up to 4000 ihp (PLM
double exhaust) or 4400 ihp (double Kylchap exhaust) for the later
series
Well, it takes ample inner volume and careful inner streamlining to
design a compound properly - that's why I don't see how it could
have been adapted to the heavier US steam locomotives without
a hefty boost in live steam pressure and temperature to balance it.
The Norfolk & Western Y-6 gives you an impression where
dimensions would go to - and then create another problem:
how to mechanically and thermally proper handle steam in
and out these enormous cylinders.
Much more to be written about this ..
generally it can be said that when a compound did not
live up to expectations, it was not the system of double
expansion - it was the lack of quality of design, where simple
'good enough' engineering did not suffice.
More maybe another time, guys!
Welcome *) back my friends
to the show that never ends!
Juniatha
(BTW in 1993 I designed a de Glehn four cylinder compound HP outsides)
myself to German profile and axle load specifications as a free lance effort -
I tell you, I know what I'm talking about ..)
*me)
On those steam chests ...
Ehm - I tried to post a photo of Aleesha Young - no. Yes
Ok, just take a look here No
US body builder Aleesha Young
will it stay?
I can't say!
Gee, hey!
The 151A still has the inside rods and cranks, and the arrangement of rear cylinders is not that strong. It certainly succeeds at reducing augment by dividing the drive, but keeping the engines in phase.
In general you want the biggest valve area (port area) you can arrange, subject to minimizing the effective dead space in the port volume between valve and piston and the physical resistance including internal weight of the valve. In exhaust valves at the end of long expansion you have the need to accommodate large volume, which is where a Willoteaux valve becomes useful. I do not have a fabrication diagram to show you, but think of a Trick valve made like two piston valves with internal passages arranged in series, made of stamped or formed metal for light weight. (If I remember correctly the infamous 152P design had these and they show up in the longitudinal section drawings)
https://i2.wp.com/www.advanced-steam.org/wp-content/uploads/2017/05/JD-letter-image-1.jpg?ssl=1
Now I was fooled early into thinking that Willoteaux valves could accelerate port opening dramatically by rotating on their axis as well as sliding, which by careful design of the steam edge in a long-lap valve can give extremely good port opening while maintaining short effective cutoff, something a purely sliding valve would need longer longitudinal lap and longer travel to produce (as the valve would have to acquire considerable speed before crossing the admission edge even with flow-streamlined passages from near-circumferential ports). I actually designed these, cribbing a bit from Bulleid's Leader, before being apprised that this was not, in fact, what a Willoteaux valve did -- I still don't consider the principle novel as I was 'told' it would be of use somehow or somewhere ...
If you combine double-ported Willoteaux valve travel with long-lap long-travel design precepts -- and the postwar French for travel up above 16" on one de Caso design, as I recall -- you will not be surprised to see the physical valve bore become extended in length. You would also see passages corresponding to the elongated ports in the liners, and the admission and inlet passages might extend quite far along the elongated valve bore at each end of the cylinder, above it or inboard from it... as I think visible on the 151 shown.
There are parallels, with of course very different physical geometry, for poppets of the different types and for Cossart drop valves, which are like a hybrid of piston and poppet valves actuated at right angles. In some modes of operation (e.g. flow streamlining becomes more and more critical at higher speeds with shorter events), sometimes what look like exaggeratedly excessive steam space and highly-superheatable volume close to the valves can become important.
If you do not use circulated boiler water through tracer lines, you will need somewhat exaggerated jacket volume, including if you want to try Chapelon's idea from 160 A1 in actual daily North American practice (where, for example, starting losses or freezing are very real concerns). Wardale famously says that the gains from jackets are not worth the disadvantages over good insulation if you use his approach to valves (articulated heads, multiple diesel-style rings, vastly better supplied tribology, and steam-cooled liners for a start) and I adopted my method in part because other systems on the engine facilitate it, and it can reduce the water rate meaningfully at times, not that it is or isn't more thermodynamically superior in itself.
So you're suggesting a system similar to the French PLM 151A which is something I've given some thought myself. I've always been rather impressed with them all things considered.
Speaking of French locomotives again, can someone explain to me what the heck is going on with the cylinders on these Nord/SNCF decapods?
I've never understood why the piston valves are so enormous on these engines. There's also an enormous structure emerging from the top of the steam chests, although I'm guessing those are just huge steam pipes. I've never seen a cylinder arrangement like this on any other engines.
ShroomZedSo you're suggesting using a fraction of the exhaust steam from the HP cylinders for further expansion? The steam is still at a lower pressure so it seems that the piston thrusts of the LP cylinders due to reduced size will be much lower than of the HP cylinders either way.
Since it is increasingly difficult to scoop water above 80mph the reduction of water rate becomes more important if sustained high speed is desired ... not a problem actually faced in American practice but surely threatened at a number of times. I thought this was a reasonable balance (no pun intended) between the running characteristics of a 4-cylinder engine and the lower water rate of a compound.
There is of course applicability of the approach to a conjugated duplex -- the ACE3000 (assuming you could construct the frame and inside rods of a Withuhn conjugated duplex properly, which I do think possible) is an example. Using Deem-style (geared) conjugation between the engines, the mismatch that has to be accommodated in the conjugation is limited the better the 'load following' on the LP engine can be. In my opinion at least a proper Deem, while it will be detented to phase to 135-degree alignment when running, would be capable of 'slip' between engines in operation (via the Ferguson clutch) and this would affect how the IP injection would be modulated.
(Incidentally, in general i think a high-speed reciprocating engine needs its valve gear to 'fail safe' at speed, which in part means no purely 'electric' or solenoid-actuated valve gear that might easily lock in a fully-admitting or fully-blocked position at random, possibly at times of greatest stress -- the results with Timken narrow-width rods, in particular, being near-immediately catastrophic. So all the special high-speed accommodations are made in, say, the follower arrangement in RC poppet gear, or in Wagner throttles at the individual HP inlets (as on the ACE 3000 for a different purpose) or in boosted IP modulation; if they fail the mechanical valves will keep the engine 'in time' to give it a better chance to get down through the critical speeds under control ... or a fighting chance to do so, anyway. This safety is a primary reason it is unwise to use direct-connected reciprocating locomotives in true high speed. If you have access to Carpenter's translation of La Licomotive a Vapeur, he included Chapelon's "150mph" design; see if you think safety is adequate at the rod loads involved ... and Chapelon is the one who deduced the required large deflection in Timken rods during high-speed running!
[/quote]Is there an inherent disadvantage to applying the LP cylinders on the outside?[/quote]Augment mass7, and less relevant for North American clearances with inside frame on standard gauge, loading-gage clearances. This was and is a major factor in general British design. There is a valid preference for inside connection for high speed (see the Belgian 4-4-2s) and lower augment (Woodard's Central Machinery Support) just as there is for keeping some of the balance weight of heavy rods in bobweights inboard instead of arranged in a driver rim (as in at least one Burlington ten-coupled locomotive circa WWI). In modern practice the piston mass is important enough that very long stroke (up to 34"!) was preferable to larger bore in achieving low(er) augment. On that basis alone LP inboard would make better sense; the shorter and more direct passages from exhaust to front end are an additional advantage. I personally want the jacketed HP cylinders where I can inspect and work on them easily; that would NOT be true of inside cylinders in a cast (or welded composite) engine bed...
There is a Swedish locomotive (the class F I think) with HP inside and LP outside that used two inside steam chests for the sets of cylinders with outside Walscherts, but all the cylinders are in line.
A number of multicylinder simples, for example post-WWII in Poland or some of the UP Nines, took to using full sets of outside radial valve gear to get around the ... let's call them little idiosyncracies ... of Gresley conjugating levers. Some of this can be interesting when an inside cylinder was arranged to be actuated from the pilot deck and now has to be remachined or modified for inside access. In general anything 'inboard' in typical North American capitalist practice involved too much work or special attention to justify its retention; note the king, long history of all those Alco three-cylinder-simple "answers" to increased augment on larger engines before 1928, almost all of which were expensively rebuilt, and the exceptions frequently cursed (see the amusing quote about the SP 4-10-2s that it was like 'having an expensive mistress at every division point')
In the presence of the post-N&W J and Niagara revolution in lightweight-rod practice (and the UP 800 and now NYC L-3 and L-4 Mohawk practice with lightweight bushed plain-bearing rods) it really makes less sense to retain balanced-compound inside cylinders of either kind if external (e.g. conjugated duplex) is an option. That is particularly true in a world -- and North American PSR is rapidly returning to one like it in practice -- where peak train speed is 45-50mph.
I am not a steam 'maven'.
It seems to me with each further use of 'exhaust' steam that the pressure and thus the 'speed' of movement of such steam slows down thus slowing down the mechanical operation of the machine down to the speed of the lowest pressure 'engine' is able to operate at.
Annimation of the triple expansion engines of the Titanic.
https://www.youtube.com/watch?v=ptDFqY-0Do8
By the time the steam reaches the turbine is it below the boiling temperature of water??????
Never too old to have a happy childhood!
Overmod These are not enormous LP cylinders, as they might be on a straight de Glehn-du Bousquet -- think of this as a four-cylinder 135-degree locomotive that uses a proportion of exhaust steam in somewhat larger cylinders to conserve water. The tunnel cranks allow the inside mains to run close to the inside face of the bearing box, permitting a larger bore than an inside crank would normally allow.
These are not enormous LP cylinders, as they might be on a straight de Glehn-du Bousquet -- think of this as a four-cylinder 135-degree locomotive that uses a proportion of exhaust steam in somewhat larger cylinders to conserve water. The tunnel cranks allow the inside mains to run close to the inside face of the bearing box, permitting a larger bore than an inside crank would normally allow.
So you're suggesting using a fraction of the exhaust steam from the HP cylinders for further expansion? The steam is still at a lower pressure so it seems that the piston thrusts of the LP cylinders due to reduced size will be much lower than of the HP cylinders either way.
Is there an inherent disadvantage to applying the LP cylinders on the outside? I know many of the later French four cylinders did this (such as the Chapelon 240P).
All a compound does is divide the expansion of steam into more manageable ranges, instead of trying to do it all with (theoretically) modulated admission and early cutoff/long compression and so forth. It does not 'use steam twice' -- it uses it only once per cylinder but over a different range of pressure. This becomes significant if you have caught the 'lowest possible backpressure' bug, as so many of us have at one time or another. An unmodulated compound has a certain design HP back pressure (roughly equivalent to what the admission pressure on a 2-cylinder locm=omotive with the LP cylinder dimensions would be) and to get reasonable expansion thrust over the required stroke, that pressure may be surprisingly high. It is the differential between high HP admission pressure and lower HP exhaust pressure that determines the thrust from a HP cylinder.
In the case of the 160 A1, the steam admitted to the HP first circulated through jackets surrounding as much of the cylinder as possible, and the mass flow of 'heating' steam was determined by admission. Presumably a good insulating lagging was supplied outside the steam plenum space. The steam in the plenum is hotter than that going into the cylinders, and it acts to keep the whole cylinder structure hot 'from the outside' to lessen the cyclic effect of wall condensation.
It was pointed out to me that a typical road locomotive at 'diameter speed' is seeing the wall temperature fluctuation due to falling steam temperature in only something like .007" of the metal adjacent to the bore. The higher the temperature of the metal immediately adjacent to this cycling, the less likely the low-temperature end of the cycling will be to induce condensation in the steam contacting it.
The efficacy of steam jacketing on high-pressure steam-engine efficiency was well recognized by 1874; I suspect much more would have been made of this had 'steam carriages' been allowed to flourish in England. I go so far as to advocate the circulation of overcritical boiler water through tracer lines (with a Lamont-style low relative pressure drop and hence little pumping energy requirement)to keep relative parts of the cylinder structure hot inside the kind of insulation Wardale calls for. While I suspect the jet pump in a Cunningham circulator can provide adequate flow for this as well as redirecting downcoming convective circulation through the water legs, it may be that a separate pump arrangement for cylinder tracing ought to be used.
I have an issue with "no superheat" in HP admission. Not all the phase change occurs in steam due to wall effects; there is also a certain amount of 'nucleate condensation' that essentially results from the work being done by the steam, which abstracts heat energy from the mass of steam confined in the cylinder to the point where phase change in the volume of steam, not just at the contact with engine structure, occurs. The effect on the steam's behavior as a gas, though, is the same in both cases: expansion can produce more pressure drop in the later stages than expected if steam were closer to a perfect gas. To this end, superheat is added to keep either form of condensation from affecting much of the mass of steam in the cylinder -- and I do think a certain amount, enough to 'make it' past the jacketing space when the cylinders have come to running equilibrium, ought to be added to produce the performance expected of a locomotive given a Schmidt superheater.
It can also be observed that adding superheat is NOT a good way to increase the performance of the heat engine as a prime mover. Relatively small added heat greatly increases the pressure, and the various stretching and deforming effects to be associated with pressure, but that same relatively small amount of steam extracted as work makes the pressure fall just as dramatically ... not a particularly good tradeoff when the heavy boiler construction, greater need for safety devices, etc. needed for static high pressure is considered.
Now, reheat going to the LP side is much more important in principle, as the phase change to liquid occurs at lower and lower temperature as pressure increases. So adding heat to preclude this phase change while active thrust is being harvested is generally a good idea, particularly if it can be done in a way that does not greatly impair mass flow or produce long TOF from HP exhaust to LP inlet. (Chapelon arranged greater-than-usual gas flow to 'belly flues' in the boiler and mounted hairpin exchange elements in them, to help obtain high (but not excessive or runaway) resuperheat with low interference with flow.)
Those familiar with the 'art' of superheater dampers will realize that balancing the added reheat flow complicates things -- perhaps more than saving the initial cost of a HP superheater arrangement will do.
Note that perhaps the greatest potential saving is the adoption of the 'asynchronous compound' -- in which there are no piston rings and indeed may be no contact of any kind between the piston periphery and the adjacent bore; these work with the constant equivalent of 'slip' during admission and exhaust pressure and volume are accordingly larger (in proportion to what would be observed if the 'passed' steam were leaking). The higher-pressure, higher-volume exhaust is handled through an appropriate plenum and admitted to a turbine of complementary design, which completes the stages of compound expansion down to whatever relatively low pressure is deemed most cost-effective. Much applicable oilless-engine construction can be applied to one of the asynchronous-compound designs, including the use of unlubricated bushings and glands to keep oil of any kind out of the steam path or condensate. Traditionally this wasn't used much as it had all the expense of a turbine and electric final drive, combined with the maintenance intensiveness (etc.) of a conventionally reciprocating setup. But for energy recovery, and flexibility of 'boosting', and perhaps adapting a form of turbocompounding, the idea can be attractive.
ShroomZedFirst thing is, can you tell me how the von Borries system works? I know there is a two cylinder form and a four cylinder; I’m assuming you’re talking about the four cylinder.
Key to smooth running with this system is that the LP admission has to be proportioned and timed so that the corresponding piston thrust over the stroke resembles that in the HP. Historically this was done 'well enough' that some engines, including the original PRR T class with British styling and 84" drivers, could reach considerable speed with a heavy (for the day) consist.
A modern engine will use a combination of HP exhaust and boiler steam to produce the desired pressure in the LP cylinder matching HP expansion. It's very similar to one-half a de Glehn-du Bousquet arrangement, giving the same four DA impulses per revolution of a 2-cylinder simple. No inside cranks or cylinders, no weird rods and bent axles to clear inside mains, cannon boxes and roller bearings no problem.
One principal "reason" is to reduce the effective water rate of what would otherwise be a 2-cylinder locomotive. It is theoretically possible to perform a reheat pass a la 160 A1 in addition to the modulated steam injection, but careful proportioning and design of the steam circuit is necessary.
Something that catches my eye though is your choice to put the low pressure cylinders within the frame; I have a suspicion that American-size low pressure cylinders wouldn’t be able to fit within the frame, even if we’re talking about pressures of 300 p.s.i. or higher being implemented.
The 'automatic adjustment' looks at a couple of parameters of thrust from the four cylinders, and tinkers with the modulated admission so the LP matches the HP. Fine 'tuning' can be made if the system latency is not quick or there are feedback lags; I was using a method of spring actuation of valves 'wound up' by crosshead motion, so some of the fine adjustment could be done with mechanism that did not have to transmit high power.
As I recall, the Midland Compounds used the same Smith system as Baldwin 60000: the single HP cylinder fed somewhere around 2.3x to 2.7x the volume of 2 LP cylinders phased 120 degrees apart -- the LP volume chosen to be the 'natural' expansion of the volume of steam from the HP event. This was by far the most successful of the three-cylinder compounds, and with a reasonable degree of initial superheat you could get reasonable expansion out of the LP before wall or nucleate condensation started to bite... and to an extent you could compensate for these with an 'excess' of steam in proportion. Balance of these things is not the easiest job in the world, though, especially if you intend high sustained horsepower but will run the engine across a wide range of speeds and loads. Here again a small amount of throttled steam in each of the LP admission ports can smooth out some of the running. I am ignorant of the specific arrangement on the Midland Compounds for balance across the range of loads -- I'm sure there are people like Bill Hall who have carefully analyzed the arrangement.
I was also wondering what you think of the Maffei four cylinder system (used on Bavarian express engines) and how it compares to other four cylinder systems.
Certainly the Maffei system performed well, I expect on a par with other well-engineered balanced-compound arrangements. That is something I could NOT say of the Plancher System -- which almost has to be seen to be appreciated. I confess I still don't quite understand the point of it, but the Italians had cab-forward locomotives using it, so it has to have worked on some level...
I heard that compound steam didn't live up to its promise of efficiency owing to ghastly condensation by the time steam reached the low-pressure cylinders.
This was how Andre Chapelon got such phenomenal boosts in performance, by figuring out what was going on in the the French de Glehn compound and what the steam conditions were?
I consider Chapelon's greatest work to be not is high-performance passenger locomotive the 242-A1 (a 4-8-4 by the usual counting of wheels instead of axles). Rather, it was the 160-A1 6-cylinder and 12-driving-wheel drag-freight locomotive. The Advanced Steam Traction people in recent years had a technical conference, where one of the papers addressed the experiments done on that locomotive offering cylinder jacketing along with the capability (in the shop) to change whether the HP cylinders are fed superheated steam or the LP cylinders receive reheated steam. The paper addressed which combination gave the best outcome, which was to feed the HP cylinders with saturated steam and rely on the cylinder jacket to prevent condensation and to supply the LP cylinders with reheated steam, obviating the need for their cylinder jacket. Don't know if I got this right, but for superheating only the HP feed, there has to be gobs of superheat to make it all the way to the LP exhaust without power and coal-robbing condensation -- what I suggest here may have been the optimal trade between efficiency and valve lubrication problems?
The 160-A1 was said to be remarkable in that its thermal efficiency increased at lower speeds. Don't know if, count them, 6 cylinders along with superheat to the HP set, reheat to the LP set and cylinder jacketing them all is economically feasible, but the locomotive was a test bed into what it would take to get an efficient hill-climbing freight locomotive instead of those picturesque photos of climbing the ruling grade with sparks and cinders blasting up from the stack like a scale model of Stromboli.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Quite interesting stuff, gives me a good amount to think about.
First thing is, can you tell me how the von Borries system works? I know there is a two cylinder form and a four cylinder; I’m assuming you’re talking about the four cylinder. Suitable information about it is elusive to me with a lack of suitable images.
Using a tunnel crank is certainly a distinctive way to actuate the inner axle and is certainly a way to handle the higher forces. Something that catches my eye though is your choice to put the low pressure cylinders within the frame; I have a suspicion that American-size low pressure cylinders wouldn’t be able to fit within the frame, even if we’re talking about pressures of 300 p.s.i. or higher being implemented. There must be simething there I’m missing.
I‘d appreciate your method of the automatic adjustments on the De Glehn as well; would this be similar in principle to the arrangement used on the Midland compounds or would it be something else?
There are some fun answers to this.
Of course, the obvious contender is the extension of the N&W booster-valve system to full proportional IP modulation -- something that even with relay-logic controls of the '50s could be done effectively. This would produce both equivalent horsepower and full balance out of both engines of a Mallet compound, like a Y-class locomotive, and allow it free running without the usual losses to a speed typical of most North American manifest freight, while maintaining the Rankine-cycle benefits, the enhanced slip recovery, and more importantly the water-rate savings for large power over straight single expansion. No more steering refinement than that used on, say, the latter AMC locomotives would be required on single-axle lead and trailing trucks, were the suspension to be arranged as on the A-class and the Challengers to take vertical compliance entirely in the equalization.
I am not certain that the maintenance advantages of full roller and needle-bearing rodwork would entirely justify its use on such a locomotive, considering the sizable additional weight over a more conventional floating-bushing setup like that on the UP 800s and appropriate alloy steel to keep rod mass low. This would be one place that the Chinese experiments into automobile-style pressure lubrication of rod bearings through cross-drilling and seals might be of real worth.
The same modulation approach can easily be applied to a von Borries locomotive, even at higher nominal cyclic than a Mallet. Since cross-balancing concerns are the principal issue with these, it becomes attractive to have compounding with only two main pins... and with proper Chapelon-style steam jacketing and some other applicable technique, a comparatively simple 'self-starting' locomotive can be provided and worked. I suspect that an augmented version of the modulation system could substitute for a starting valve in these engines, relieving a considerable amount of constructional and maybe operational complexity in starting arrangements.
I was partial to the balanced de Glehn style of compounding (on what were designed as very large 4-8-4s) in my teens; these got around the cranked-axle difficulty with the same solution I proposed for the Withuhn conjugated duplex: tunnel cranks (as in some of the Mercedes locomotive diesels) where the inner race of the bearing also carries the crankpin seat. Construction and maintenance of these should be no more complicated than the original bearings in the N&W As -- the catch being, as with all inside bearings, that rollers are not particularly easy to implement as they can't be functionally split at reciprocating-steam pressures and duty cycles. I divided the modulated drive between the leading and second axles, with the LP cylinders in the center as on classical de Glehn-du Bousquet locomotives, but the pressure adjustments were handled automatically rather than using two complete sets of independent gear. Etc. etc. etc.
It might have been interesting to see a true North American Garratt with eight-coupled engines, something even Beyer Peacock did not propose. Theoretically all that would be required within the practical water-rate criteria would be double-sixes, probably with not more than 69"-70" wheels, as things get ridiculously long ridiculously fast, and pivots were not a particular Beyer-Peacock design specialty even with nominally cast engine beds...
Finally, a perfectly good Meyer can be constructed under a double-Belpaire, either with six- or eight-coupled engines, and with modulated IP that can be a compound exhausting at the 'stack end'.
Hullo, it’s certainly been a while since I’ve said anything. Crazy crazy year, but I suspect everyone here already knows enough about that.
Something I’ve been looking at a lot lately are the varieties of compound locomotives used worldwide, including the various late nineteenth century and early twentieth century methods attempted in the United States and with my layman knowledge trying to think about what form of compounding works best in the United States (besides the matter of the mallet of course, think of this as a discussion pertaining exculsively with rigid frame locomotives).
The main problem with compound locomotives is that while they give the advantage of better thermal efficiency and reduce fuel costs, their arrangements often result in high-maintenance costs from relatively high numbers of components and complicated steam pipe networks, pain-in-the-rear end accessibility for certain arrangements, and in some cases the system was mechanically problematic from the outset (like the four cylinder Vauclain), to the point that either no profit was gained or indeed lost from experiments which were gambles.
From what I’ve reviewed, the more mechanically sound and efficient the system was, the more complicated it was overall and inaccessibility increased dramatically, along with the chances for something to go wrong. The main examples here would be the four cylinder balanced compounds with inside and outside cylinders, which if properly run (more that could be expected from the average United States driver) could give astounding fuel savings and at the same time were phenomenally smooth runners. The main problem comes from the complexity of the system, with two sets of valve gear inside the frames. Crank axles were required as well which spelled bad news in the U.S.. Problem is crank axle assemblies from what I know simply often did not have the strength to withstand the forces distributed through an American locomotive. These were consequently most popular in continental Europe where operating conditions suited them to a greater extent and had an extremely brief period in the U.S..
The United States tried to compromise the advantages of compounding with flexibility and simplicity, which had mixed results at best. The Vauclain outside four cylinders had only two sets of valve gear being actuated by cross heads working from the high-and-low pressure cylinders in harmony. The differential thrust forces from the different pressure cylinders caused torquing effects on the cross head which was a large problem. The tandem four cylinder was even worse, with the asemblies being extremely cumbersome and the differential forces of the cylinders once again causing extreme mechanical strain. The two cylinder compound is the simplest and has the least amount of parts to go wrong, but the uneven size and weight of the cylinders, outside the frame at that, will cause yawing no matter what and that causes extensive damage to the frames of the loco and the rails.
One of the better systems I‘ve come across is one that was used on the Midland Railway in the late nineteenth century on a few 4-4-0 classes and some neighbouring railways; they were three cylinder compounds with one high pressure cylinder between the frames and the two low pressure on the outside. They were designed in mind with flexibility as one of their strong suits and consequently had a quite ingenious steam admission system which allowed multiple different steam arrangements and automatically switched from simple to compound with a selected pressure by the driver by a series of valves and spindles (in the starting valve, if the optimum pressure is achieved, an opening to that allows steam to go straight to the low pressure cylinders is closed and normal compounding begins). It still is a rather complicated system but did have success in the U.K., which otherwise had little to no true success with compounding designs (the infamous Webb designs for example).
Essentially the lesson learned is that like with many components of the steam locomotive, a compromise had to be made between mechanical flexibility and robustness and efficiency of the design. The Americans for obvious reasons slid towards simplicity as it suited their operating needs the most, abd complexity was only later increasingly sought as the I.C.C. imposed larger and larger wage restrictions and efficiency was the optimal way to gain a profit as simply and solely enlarging designs wasn’t going to cut it anymore. The superheater, although acceptance was relatively sluggish in the U.S. compared to Europe gave fuel savings comparable to or better than compounding with much more simplicity which quickly won the Americans over and compounding was seen as unesessary and a hassle. The British world follow a similar path although they werent as averse to complicated arrangements (although much of that is likely a response to extremely constrictive loading gauge, large cylinders and heavy Walschaerts valve gear with lateral flexibility arrangements had nowhere to go in nine-foot wide locomotives).
Even central Europe eventually gave up compounding and France was the main user of compounds up to the end of steam.
So the question of this topic is let’s say hypothetically the United States wasn’t as willing to drop compounding and at least some tried to advance it, or perhaps compounding was returned to in the thirties or forties, what would be the optimal rigid frame compounding arrangement for United States use, with the knowledge gained towards the end of steam and the massive advancement in assembly and construction? I’m sliding towards a three cylinder arrangement, but I am gravely aware of Baldwin 60000 and the general failure of three cylinder constructions in the U.S. past the 1910s, with the maintenance horror stories of the western 4-10-2s and 4-12-2s. Im curious to see what some of you have to suggest for compounding for ‘modern‘ locomotives in United States operating conditions.
Discuss away.
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