What are the advanatges of one over the other? Union Pacific Big Boy and Challenger locomotives were simple expansion with the same steam pressure applied to all four cylinders. The Norfolk and Western opted for true Mallets..compound expansion locomotives where steam was first used in the hind two cylinders and then routed for further expansion in the two oversized front cylinders. One might think that the Mallet might be more efficient as the same steam is used twice before being dissipated to the atmosphere..is that correct?
Well, yes and no. Yes, the double use of steam from high pressure to low pressure cylinders is a bit more efficient, but there is a penalty paid in road speed. The Big Boys and Challengers were designed for 70mph running so compounding just wouldn't work. The Norfolk and Western's Y6 Mallets could get up to 50mph but I don't think they were run that fast very often. The Class A simple articulated was a greyhound though, 60-plus mile per hour running was nothing for them.
Most compound Mallets were used in drag freight and pusher service, so speed wasn't a factor.
Looking forward to hearing from others on this one!
Interesting response...thanks Firelock!
You're welcome, Ulrich!
Since compounding in the United States was mostly in the form of Mallet locomotives, with their ponderous low pressure cylinders and low drivers, there is a general impression that compounds were inherently slow.
`T`aint necessarily so.
Some of the Cole and Vauclain compounds built around the turn of the last century were high-drivered 2-4-2 and 4-4-2 types, and were quite capable of a good turn of speed. More recently, there were compounds built on the far side of the Atlantic that could FLY!
Back in the early '70s, Bill Withuhn designed a 4-cylinder triple expansion compound engine that used internally connected drivers to balance and all but eliminate dynamic augment. If built, it would have been capable of speeds comparable to a N&W J.
One key difference between the Mallet and a four cylinder triple expansion loco is that three of the four cylinders are meant be the same size - greatly simplifying the balancing required.
So, are such engines practical? Innumerable ocean-going ships had four cylinder triple expansion engines, including all the WWII Liberty ships. They were chosen for the latter because they were simple, reliable and didn't require reduction gears like a steam turbine.
There was one such loco built in the US - the Delaware and Hudson's Leonor F. Loree. It was a low-drivered 4-8-0, basically a super-consolidation, not designed for speed. The limiting factor was driver size, not compounding.
Chuck
There's a big difference between a marine VTE engine, which operates at relatively low speeds, and a Vauclain or de Glehn compound, which are high speed designs. Also, most of the early compound designs were withdrawn with the advent of superheating.
The various D&H experimentals in the 1400 series were complex high-maintenance designs. It was said that they could pull well but you had to send half of the shop force out with them.
As I recall, the boiler design of the D&H engines was really strange to me--and it may well have contributed to the high maintenance requirements. I can picture one or two of these engines in my mind (they really looked strange to me, back in the fifties, when Trains had an article on these engines).
Paul, was it the Vauclain that had a high pressure cylinder on one side and a low pressure cylinder on the other? Or did it have a high and low pressure cylinder on each side? I confess that I had not heard the name "de Glehn;" what was the cylinder arrangement?
Johnny
tomikawaTT Since compounding in the United States was mostly in the form of Mallet locomotives, with their ponderous low pressure cylinders and low drivers, there is a general impression that compounds were inherently slow. `T`aint necessarily so. Some of the Cole and Vauclain compounds built around the turn of the last century were high-drivered 2-4-2 and 4-4-2 types, and were quite capable of a good turn of speed. More recently, there were compounds built on the far side of the Atlantic that could FLY! Back in the early '70s, Bill Withuhn designed a 4-cylinder triple expansion compound engine that used internally connected drivers to balance and all but eliminate dynamic augment. If built, it would have been capable of speeds comparable to a N&W J. One key difference between the Mallet and a four cylinder triple expansion loco is that three of the four cylinders are meant be the same size - greatly simplifying the balancing required. So, are such engines practical? Innumerable ocean-going ships had four cylinder triple expansion engines, including all the WWII Liberty ships. They were chosen for the latter because they were simple, reliable and didn't require reduction gears like a steam turbine. There was one such loco built in the US - the Delaware and Hudson's Leonor F. Loree. It was a low-drivered 4-8-0, basically a super-consolidation, not designed for speed. The limiting factor was driver size, not compounding. Chuck
IIRC, the Ross Rowland's ACE3000 project proposed using the Withuhn system;an improvement on the "Duplex Drive" system used on the PRR's T1 and Q1/Q2 series of locomotives. I recall reading that it caused some dissagreements within the design team and later versions of the design may have abandoned it...
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
Deggesty Paul, was it the Vauclain that had a high pressure cylinder on one side and a low pressure cylinder on the other? Or did it have a high and low pressure cylinder on each side? I confess that I had not heard the name "de Glehn;" what was the cylinder arrangement?
You're thinking of a cross compound. Vauclain compounds had a high-pressure and low-pressure cylinder on each side (four total), two cylinders on each side one above the other on one crosshead. The de Glehn compound is a French design, I don't know the particulars.
The de Glehn system allowed for independent cut off of each of the high pressure and low pressure cylinders, thus allowing for a series of various working combinations. I have yet to see any indepth readings as to how this worked but apparently it was quite a system.
The de Glehn compounds had four cylinders, two HP cylinders driving cranked axles inside the frames, two LP driving conventionally outside.
The inside drive was to the first driver and those cylinders were far forward in relation to the frame. The outside drive was to the second driver and those cylinders were mounted pretty far back, typically over the rear of the pilot truck instead of the center. The idea was that the inside and outside rods were the same length to keep things in balance mechanically.
The British "Castle class" locomotives had the de Glehn mechanical arrangement thought to be easy on the tracks owing to the mechanical balance, but they were simple expansion.
Part of what makes compounding complicated is the simple/compound transition valve. Compound locomotives often have an arrangement to operate in simple mode, perhaps admitting steam to the larger low-pressure (LP) cylinders through some pressure reducing valve. This helps with starting. Based on discussion on another thread on this forum, it was explained to me that this "simpling valve" is not just some turn of a handle kind of valve, but is this largish thing, usually tucked between the frames, to reroute steam and exhaust in the different modes and can be "servo driven" (power operated by applying steam to move the valve into the correct position).
Compounding can achieve greater expansion of the steam, but it is not clear how this is better than simply using a shorter cutoff with simple expansion. Another problem with compounding is that you have to extract exhaust steam through valves on the HP cylinders, port that steam into a receiver, and then supply that steam through another set of intack valves into the LP cylinders. The indicator diagrams I have seen show a substantial gap between the pressure-volume loop of the HP and LP cylinders representing the valve losses I speak of.
But compounding may reduce the thermal losses by separating the expansion into stages. There may also be lowering of friction loss because the larger, LP cylinders don't need to maintain as tight a seal as the smaller HP cylinders exposed to higher steam pressure.
I am thinking the reason that compounding went out when superheat came along is that for the same boost in fuel economy, the railroads would rather maintain a superheater rather than deal with the more complex machinery of a compound. In France where coal was in short supply, the did both.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Why couldn't the steam, once its done its work in the cylinders, be routed back to the boiler for reheating and reuse? Instead of spending so much energy in turning water to steam wouldn't reheating the steam that's been used already (and adding a little water as needed) be more efficient?
Deggesty As I recall, the boiler design of the D&H engines was really strange to me--and it may well have contributed to the high maintenance requirements. I can picture one or two of these engines in my mind (they really looked strange to me, back in the fifties, when Trains had an article on these engines).
It was the June 1967 issue - which happened to be the first issue that I bought...
- Erik
The D&H high-pressure experimentals had water tube boilers - and water tube boilers have had an unhappy record when used on rails. Unlike the maritime environment, rail-borne boilers are subject to routine jolts and bounces that only happen to ships in combat.
Another problem for the Leonor F. Loree was that both cylinders on each side drove the same crankpin. That must have put a LOT of stress on that crankpin - with no way to use tandem rods or any other technique to spread the stress to a second axle box.
Finally, the design was a one-off, with all the disadvantages of non-standardization that status implied. It was a brainchild and pet project of its namesake. When Mr. Loree was no longer there to cheer it on, it was quietly scrapped in favor of conventional 4-8-4s and simple-expansion Challengers.
Ulrich Why couldn't the steam, once its done its work in the cylinders, be routed back to the boiler for reheating and reuse? Instead of spending so much energy in turning water to steam wouldn't reheating the steam that's been used already (and adding a little water as needed) be more efficient?
South African Railways was famous for its condensing 4-8-4's, which were built to support steam operation through a desert where water was unavailable. The condensing gear, which was mounted in the tender, had its own maintenance needs above and beyond that of the rest of the locomotive. SAR also had otherwise identical conventional 4-8-4's for service elsewhere, which implies that the condensing gear existed only for a special situation.
Why does the steam need to be condensed first? The purpose of the fire and that huge boiler is to boil water to make steam...
Once the steam is boiled it does its work in the cylinders and then LEAVES the cylinders as steam and is disipated to the atmosphere. Why not take that used steam and route it back to the boiler for reheating? After all...reheating used steam would involve less energy output than heating water to produce steam. I understand about the condensor...the condensor takes the used steam and turns it back to water which is then routed back to the boiler for reuse. Why not take the condensor out of the loop and just reheat the used steam in the boiler?
It's relatively easy to get water into the boiler by way of an injector or pumps. Getting low-pressure steam back into a high-pressure boiler is impossible.
The exhaust steam from the cylinder is too low pressure to reenter the boiler directly, and too hot for an injector to work. The exhaust steam could be used to preheat boiler water using a feedwater heater, so as to not waste the energy available in the exhaust steam, but you also need the steam pressure from the exhaust steam to make the boiler drafting work.
As I recall, the exhaust steam has lubricating oil, from the cylinders, mixed with it, and this should be removed before the condensed steam is reintroduced into the boiler (though this detail did not seem to bother the designers of the triplexes, as the exhaust from the cylinders under the tender went into the tank); was the equipment necessary to take care of this a part of the complex condensation system on the SAR condensing engines (which had a blower to maintain the draft)?
...O-M-G !!
That's a complex story - lots has been written about it , lots of it is plain rubbish .
Even professional engineers of steam locomotive builders and railroads often seem to have had but rather limited understanding of what really can be obtained by compounding , what it's for and what it takes to make it work . Occasionally , even today there are examples of people springing up exposing but fantastic ignorance when happily offering what seemingly they believe should be new relevations .
No insult intended .
With regards
Juniatha
Maybe you could help me with this...why can't the steam, once its done its work in the cylinder, be routed back to the boiler for reheating instead of being disipated to the atmosphere? This would be a closed loop where the steam would be reheated and reused. So far I'm told that it would be impossible to get the low pressure used steam back into the boiler where the pressure is higher.
Marine steam engines (on ocean-going vessels anyway) operated with condensers almost from the very beginning, salt water being absolutely the last thing you want in a boiler. Exhaust steam was condensed, then routed back into the boilers. Boiler pressure is just way too high to get the used steam back in in an uncondensed state.
By the way, anyone ever hear the old firemans saying? "If you wouldn't drink it, don't put it in the tender!"
Oh, and so no-one gets the wrong impression, I never said compounding and speed were mutually exclusive! I'm well aware of Vauclains, DeGlehns, and Chapelons compounds. A compound MALLET wasn't a speedster on account of the difficulty of balancing those massive cylinders and assorted running gear behind the pilot.
Ulrich Maybe you could help me with this...why can't the steam, once its done its work in the cylinder, be routed back to the boiler for reheating instead of being disipated to the atmosphere? This would be a closed loop where the steam would be reheated and reused. So far I'm told that it would be impossible to get the low pressure used steam back into the boiler where the pressure is higher.
Firelock76 By the way, anyone ever hear the old firemans saying? "If you wouldn't drink it, don't put it in the tender!"
So a bottle of Jack Daniel's into the tender if the Trainmaster is walking towards the cab would be OK?
erikem Ulrich Maybe you could help me with this...why can't the steam, once its done its work in the cylinder, be routed back to the boiler for reheating instead of being disipated to the atmosphere? This would be a closed loop where the steam would be reheated and reused. So far I'm told that it would be impossible to get the low pressure used steam back into the boiler where the pressure is higher. What you are describing is similar closed cycle gas turbine using steam as the working fluid instead of air or some other gas. The spent steam would have to be compressed before it could heated again and that compression takes a lot of energy - typical figures for a gas turbine is that ~75% of the power produced by the turbine is used in the compressor. Condensed steam (liquid water) is much less energy intensive to pump up to high pressure than a gas (it's very dense and almost incompressible). The downside is that you do throw away a lot of energy in condensation. - Erik
I would also add that the gas turbine (which operates using the Brayton cycle), steam power plants (Rankine cycle), steam engines (Rankine cycle), Stirling engines (Stirling cycle), etc. are heat engines. All heat engines must reject heat to a low temperature reservoir. Thus, heat rejection to the atmosphere is necessary regardless of the working fluid, including pure steam. Note that heat rejection for open gas turbine cycles is accomplished by dumping the working fluid into the atmosphere.
Anthony V.
beaulieu Firelock76 By the way, anyone ever hear the old firemans saying? "If you wouldn't drink it, don't put it in the tender!" So a bottle of Jack Daniel's into the tender if the Trainmaster is walking towards the cab would be OK?
Why not? Steam engines wanna feel good too, ya know!
Firelock76 A compound MALLET wasn't a speedster on account of the difficulty of balancing those massive cylinders and assorted running gear behind the pilot.
But back to Firelock76's remark; Why other roads didn't persue developement of the compound mallet like the N&W did with their Y class, I have no idea. However as a former N&W steam engineer related to me, "You didn't want to run the Y6's much over 63mph. After that they started getting a little shakey". Now, is that "balancing those massive cylinders and assorted running gear" enough?
.
I know we've had speed discussions in the past, and what it really boils down to is application to the circumstances.
Certainly not every train was expected to run near the 100mph mark, very few were in fact, it gets down to the application. If a Y6 on the N&W wasn't meant to run any faster than say, 40 to 50 miles per hour, or frequently slower, that was OK, coal isn't going to spoil if it doesn't get to market yesterday. On the other hand, the Erie grabbed a lot of perishable freight business when they started running Berkshires that were comfortable running over the 50 mph mark. See where I'm going with this?
And passenger use? A passenger wants to get where he's going quickly, that was the original attraction of the railroads to begin with, so for passenger use you're going to need a, well for lack of a better term, "speedster". It was true in 1840 and it's true now.
And OK, if the N&W designers were good enough to get the Y6 to run at up to 63mph reliably, well good for them! It only confirms what geniuses they were. If other 'roads didn't want to follow N&W's lead on this I'm sure they had their reasons, some good, maybe some not so good. Personally, I think a lot of locomotive builders were secretly jealous, maybe insanely so, that those "hillbillies" down in Roanoke were better at the craft than they were!
OK, coal isn't going to spoil if it doesn't get to market yesterday.
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