You are over thinking this.
Just consider a conventional loco for a minute - there are effectively four intake ports that produce power. At least one is always open to accept steam, otherwise the loco may not be able to start.
Now, on the triplex same is true on the low pressure cylinders. Since the low pressure steam only comes from one high pressure cylinder but feeds two low pressure cylinders, the low pressure cylinders are effectively twice as big and the high pressure cylinders.
So, even if at the exact moment the steam is exhausted from the high pressure cylinder, only one of the two low pressure cylinders its feeds has an open intake port, the steam will have some place to go, and shortly there after the other low pressure cylinder with be on one of its two intake strokes and consume all the available steam, without any regard for the normal quartering of the complete low pressure engine.
Again the important thing about the triplex, one 3617 cu in high pressure cylinder exhausts in to a total of 7234 cu inches of low pressure cylinders. The fact the the larger low pressure cylinder volume is two separate cylinders makes no difference - it is still twice the volume to take advantage of the low pressure steam just like any other compound.
Sheldon
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A review of Hendersons Baldwin patent for the triplex shows the center cylinder of a given side divided equally between the front and rear cylinder. Patent diagram showing center cylinder “D” with pipe “e” going to forward truck and pipe “f” going to the tender truck.
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This is why I said the ‘correct’ expansion ratio for the dimensions involved was about 2.2 to 2.4, not the precise 2:1 that using all equal size and stroke would provide. There were some compounds (de Glehn/du Bousquet in particular) that used different HP and LP stroke to tinker with expansion ratio.
All three engines on the Triplexes were 90-degree- quartered DA, so as to give maximum self-starting with the intercepting valve open. Note that the ‘receiver’ for the forward engine comes into equilibrium with both steam chests nearly equally, so there is little actual phasing concern (at least at starting, and to be honest at practical road speed that one if these things would reach), and the LP engine runs just like a large-cylindered engine with lower throttle/admission pressure.
Where the horror occurs is at higher cyclic, where the expansion is greater and nucleares condensation drops the peak pressure and hence effective MEP much more quickly than expected expansion of steam ‘as a gas’ would. This is also where the practical benefit of resuperheat, if it can be cost-effectively assured, would be tremendous. Chapelon on 160 A1 tried an interesting method of preheating the HP cylinder metal, and resuperheating the LP steam (he did it in short Schmidt-type elements in flues down at the bottom of the boiler barrel) – notably, neither experiment was ‘proceeded with’ for very long, and abandoned well before “the fix was in” respecting good steam when powers that be wanted to electrify…
A moment’s reflection on the disaster in steam distribution that was the Toleman locomotive will tell you why there will be concerns with ‘sending steam both ways’ as pictured, rather than using the approach on the Triplexes as built. Remember that the ‘exhaust backpressure’ during the exhaust release is at or somewhat above receiver pressure – and you now have double the length of pipe, but worse, you have sharp angles at a two-way fitting. I won’t use the fine old term Angus Sinclair did, but mass flow in that arrangement is going to suck, and so is HP compression when the valves close to exhaust but still have travel and lap to dead center…
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Found on an obscure website regarding the Triplex:
“The locomotives were also unique in that both the
high- and low-pressure pistons were the same size, using different
valve sizes to accommodate the changes in pressure. “
According to this, there were different valve sizes although the cylinder sizes were the same. I’m assuming they’re talking about the shuttle valve sizes.
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Yes, the valves were larger to give less port restriction for the steam mass flow. You can think of it as the difference of lower-pressure steam needing to move the expansive mass into ‘double the volume’ in the same time per stroke. Larger diameter can also allow shorter valve stroke.
It was my general understanding that the reciprocating mass of the larger LP valve spools was not as significant to the ‘nosing’ problem as larger pistons (e.g. the Erie 0-8-8-0s or the Virginian 2-10-10-2s). I have not done any calculations to confirm or disprove that, however.
(A secondary calculation of potential interest would be to see if the Alco (and N&W) high-speed method of hinging the front engine – removing the vertical hinging of the forward engine – would have helped any with the nosing-while-drifting concern.)
Without getting into thermodynamics and all the math that goes with the science of steam -
If the boiler pressure is 300 psi it will move from the boiler to the first set of cylinders at nominally 300 psi and perform work and use up some of that 300 psi energy - with the work energy extracted the steam will leave the primary cylinder at 300 psi minus the work. I would feature that the ‘speed’ of the steam would be less as it departs the cylinder after the work is completed. In going to the smokebox on a simple engine the steam gets exhausted to atmosphere. If the engine were compound, the exhaust steam from the simple engine would be going to the low pressure cylinders of that engine - the speed of the steam going to the low pressure engine is slower than the speed of the steam going to the high pressure engine - because of its relatively slower speed the low pressure engine needs to have different valve timing constraints than the high pressure engine.
Not knowingly having seen a compound steam engine in operation I can only surmise that compounding limited the speed with which it could handle its steam and thus limited the operating speed of the locomotive.
My historical ‘observations’ stem from several books on B&O motive power where in the early part of the 20th Century a number of compound articulated engines were purchased and after several years of operation they were rebuilt as simple machine with all cylinders getting steam pressure from the boiler.
In the marine world they took ‘compounding’ as step further and used the exhaust steam another time with their triple expansion engines - the Titanic power plant was a triple expansion steam engine.
Yeah, and how did that work out?
I know, triple expansion steam engines were very common, and reliable in the marine world,
Jus messin with ya,
Doug
It did hit the iceberg at speed - so I guess the triple expansion steam engine worked fine in propelling the vessel - just not so good in avoiding the iceberg.
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The engines on Titanic worked fine, if a bit noisier than the engines on Mauretania/Lusitania that cost far more.
The point there was that a quadruple-expansion engine would have been cumbersome to run, much noisier in probable operation, and of dubious mechanical value – whereas exhausting LP at a reasonable backpressure and expansing the exhaust from the two main engines through an entirely subatmospheric ‘reaction turbine’ contributed substantial forward shp for essentially zero main-engine losses. (Since the prompt condensation for low backpressure was lavishly available in the cold North Atlantic, there was little problem with accommodating required mass flow).
You could probably figure out Titanic’s maximum speed with just two triple-expansion engines, but it wouldn’t be anywhere near 25 knots; you can also figure out the speed with three triple-expansion engines and the extra boilers to run them, on a ship that wasn’t intended to be a speed queen.
I was taught that you treat the HP engine of a compound locomotive that will ultimately exhaust to atmosphere entirely as admitting at throttle pressure (and superheat conditiions) and exhausting at an artificually-high pressure that is determined by desired receiver pressure. The speed and mass flow of exhaust at HP release is factored into achieving that receiver pressure, and the valves sized and timed accordingly, with the added ‘fun’ that on a radial valve gear like Stephenson or Walschaerts, the exhaust timing is “the same” as the admission timing.
The receiver pressure gets ‘smoothed out’ some by the time of subsequent LP admissiton… or so was the theory. You then designed for admission being at receiver pressure and superheat, and exhaust… well, kinda looking after itself, with the assumption it would generate sufficient draft under automatic-action conditions. This was the place that most of the ‘balanced compounds’ came to such grief in the pre-Schmidt superheater era. Any superheat that the HP admission valve lubrication would tolerate was pretty much gone by the time the steam worked its way through the LP valves, and the mass flow might not allow sufficient expansion to keep the final pressure… or even the MEP in some wet designs … at anywhere near a pressure that would actually generate piston thrust rather than just facilitate nucleate condensation. If you had two complete sets of cutoff, as on a de Glehn/duBousquet engine, you could twiddle with longer HP cutoff to get more steam to allow balanced LP cutoff… but you were ALWAYS twiddling, and the results fairly dire it you twiddled wrong, including far less than the “big savings” that the owners were expecting as the only reason for the complex and balky idea.
When compounding became more practical (in France) it was with skilled ‘mecaniciens’ doing the twiddling, and higher boiler pressure and superheat (see Chapelon on poppet valves circa 1926 as mentioned in the other thread, and the steam-circuit path on the origina 160 A1 HP cylinders). France was an unusual case doe to the Government-imposed speed limit of 100 km/h, which meant French locomotives didn’t need much top speed but needed power to accelerate a train quickly to that speed after any check. Economically, coal being ridiculously scarce.
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The late 1930’s GE Turbomotive was an attempt to use power plant style of boiler and turbo-electric drive to get good performance with a closed steam cycle. With a relative small turbine, one could the same expansion as a multiple compound steam engine. The downfall was the condenser, air cooling didn’t work as well as water cooling.
Exhausting at above atmospheric pressure throws away a lot of the enthalpy.
N&W’s Y class mallets apparently worked nicely, but the N&W used the simple A’s for high speed freight. The D&H high pressure compounds also worked, but were a maintenance headache.
Steamotive, not Turbomotive (that was the British 4-6-2 with the Guy turbine and mechanical gear drive): originally 1200psi, then 1500psi. Had to use fully demineralized water at that pressure (not even silica) so obligate full condensing, which they did very competently for steam-to-air without some form of active recompression (look at the size of the turbine vs. the exhaust plenum expansion!)
What killed GE 1 and 2 (aside from running out of condenser) was that each unit represented enormous complexity and cost for only 2500 horsepower, with all the weight of a cast underframe and the losses and fragility of DC generation and traction motors. To me this was like a modern version of the Heilmann ‘second Rocket’ in decidedly NOT being the technology to replace mainline steam… and what was adequate hp in the era of the 201A became rapidly inadequate with 12- and then 16-567s. The Baldwin Essl locomotive would make 6000hp out of a little more than one-half the ‘practical’ UP consist, with a damn sight more flexibility in service if you actually built and controlled it right, and nobody wanted to adopt that.
Remember that N&W designed and almost built the real 2-8-8-2 for “fast” freight: the Y7, which looked nothing like the compound Y classes and was only given up because the Government was messing with allowable train lengths in the late Thirties. As I’ve noted, you could likely have gotten the necessary speed out of a rebuilt Y compound, but it would be a lot harder than a simple engine with proper valves, valve gear, and cylinder compression relief. It would be interesting to see what there is in the NWHS records about why the Y7 was never resumed after 1938… exactly the time period we’re considering.
I don’t know! To my way of thinking compounding slowed down the movement steam from the throttle valve to the final exhaust out the stack. The first use in the high pressure cylinders takes energy and speed away from the steam as it get funneled into the low pressure cylinders which causes the steam to lose even more speed and pressure as gets exhausted out through the smoke box and the stack. The engine can only go as fast as the low pressure engine will let it.
Compounding’s main objective was to extract as much tractive effort from the expansion of steam as opposed to speed. Speed is likely not a consideration when performing a long haul at any kind of grade.
You use larger passages as the steam loses pressure; from before the era of James Mulhfeld it was understood that mass flow was the important determinant of extractable expansion horsepower. The most extreme illustration of this was the exhaust plenum on the 1500psi Steamotive plant (which used full “compound” expansion through one fairly compact turbine to full condensation to ‘vacuum’, at substantial mass flow).
One of the issues with using a Giesl ejector on a compound engine was the relative size of jets in the ‘fan’ required for high mass flow at low exhaust pressure. Another was recompression of the ‘clearance’ volume in larger ports and passages at higher cyclic – the reason some Mallets had to fit sizable steam-admitting drifting valves, even though carefully enormous ‘vacuum relief’ sniffing valves were fitted to big Mallet LO engines as early as 1906-1909.
The N&W assiduously worked on a curious fairy story from the early 1950s on: that the Y6b was the be-all and end-all of fast modern steam power, quicker than the As. For some reason this went hand-in-hand with chronically-clanking rods and periodic “unraveling” (to use Ed King’s term) valve gear components when you actually looked at the locomotives in service.
Part of this was the idea that their ‘booster valve’, which put some reheat into the LP steam, actually got the timing and duration of admission plus the profile of expansion ‘equal enough’ in the LP cylinders for the engine to operate in balance above 40mph, which was minimum ‘time freight’ speed at the time. It was by no means clear to me that it was, or that the concerns (including Chapelon’s) about what was desirable to modulate to get HP and LP trust to balance at high speed.
Which is by no means to say it could’ve be done practically, and even cost-efficiently with '50s technology.
As a kid, when steam was in action on the B&O, I was never in a position to be able to watch steam handle trains on serious grades. My only observation of such operation has been on recent clips showing up on YouTube.
TO ME - it would appear that bulk commodity trains handled by steam in heavy grade territory is moving in the 5 -6 MPH range - which is roughly half what the minimum continuous speed of the first generation diesel-electrics and the diesels did not have to stop for water during their trips between crew changes.
All the articulated steam you’d likely have seen would be simple articulateds (EM-1s and the transplanted SAL 2-6-6-4s?)
As I recall, one ‘traditional’ approach to calculating train factor for a given locomotive was to assume it would ‘just’ be able to operate at some minimum speed when the train was on the ruling section. That would have some ‘cushion’ for bad rail, poor steaming, etc. but fundamental greed might say ‘if it stalls, just have them double’…
Diesels are constant-horsepower right down to generator and traction-motor limitations on a grade. You then have a command decision to make: do you want helping, or snapping? Helping is adding more horsepower to the train so it slows only to your 5 to 6mph allowance on the ‘worst’ parts; ‘snapping’ is more horsepower to give a shorter time/faster speed over the section. That was the era of Time-Savers, so I’d expect the speed of ‘time freight’ over the grade would be arranged to be higher, but regular trains might still economically slog… unless you need to start working the grade more as a one-speed operation (for example if there is congestion).
This is all more than familiar to you, of course.
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First Gen road diesels had a minimum continuous speed in the range of 11 MPH and could be operated into Short Time ratings at the amperage listed for the time listed.
My observation of YouTube videos of trains operating in the B&O’s ruling main line grades makes it seem that 5 to 6 MPH was the speed steam was able to maintain on the grades. Operationally water stops would have to be figured into the run times as working at max loading goes through a lot of water fast. N&W added canteen’s with additional water to their main line (not helper) steam.
B&O used head end helpers on their passenger trains over the main line mountain grades to Chicago and St. Louis. The E units B&O used in passenger service had a minimum continuous speed in the neighborhood of 22 MPH, with large trains the E units had trouble pulling the train on the grades at speeds above MCS - With helper - steam or diesel attaining and maintaining MCS was easy.
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