Compound Mallet Question

[Anonymous]

Old Timer,

I think some eastern steam fans, including the Ed King article in Classic Trains don't really understand what the Western railroads were doing. It's true that the Northerns, Challengers, Big Boys, and Yellowstones were designed to haul a certain sized train over a particular ruling grade at a certain speed without double-heading, but there was a lot more involved in how these locos were used than just dragging tonnage up a short hill.

An important factor was greatly extended engine runs, which meant the loco had to get over all the intermediate sized grades without helpers at timetable speeds. Many of the Western roads have somewhat rollercoaster profiles with long gentle grades both uphill and down. On the Northern Pacific for example, the modern roller-bearing Northerns reduced engine change points from 7 with the old Pacifics to 1 between St Paul and Livingston. At Livingston, they were sent back after a 4 hour turn-around. Challengers on the U.P ran all the way across Wyoming. Others ran on the line from Green River to Portland. They did get helpers or were double-headed on the ruling grade but handled the train the rest of the way.

Because the western grades were often so long, dealing with slow moving helpers like the Y-classes was a problem because what goes up must come down. They either had to be run all the way across the division or run them light back a long way downhill. During WW-II U.P. acquired some compound & non-compound articulateds from the Eastern roads. They tried them as helpers on the Wyoming grades and they ended up tying up the line getting back down. Big Boys and Challengers did run at high speed downhill, the mallets couldn't do that without tearing up the track and themselves. Even with diesels the Western roads try to avoid helpers.

U.P. had tried mallet compound 2-8-8-0''s earlier but they ended up in helper service on the Oregon grades after being simpled and had the original 57" driver tire size increased from 3 to 5 inches. That experiment lead to the 4-12-2 which had 3 cylinders, 67" drivers, 4300 hp, 96,000 TE, and could pull the same trrain as the mallets at double the speed. Of course that engine would be no more suitable on an Eastern coal-drag as the mallets were on the western grades. [:)] [:)]

Also consider that the western roads were hauling a lot more perishables than coal, so getting the trains over the road was more important than dragging every freight car an engine could pull out of the yard. And most of them were quite profitable [:)]

[Anonymous]

GP40-2 says that the N&W Y-6b was a dog above 30 MPH . . .
Sorry, forty, but the Y-6b's horsepower curve didn't drop off THAT rapidly. But the N&W didn't design the Y-6b (and the Y-6b was just the last class built; the Y-5 and Y-6 and Y-6a classes were just as potent as the Y-6b, incorporating all the newer features as they were developed) to produce high speed horsepower. They had the Class A 2-6-6-4 for those applications.

But if a railroad had a topography like N&W's, with the grades and sharp curves N&W had, the Y-6 was the answer. The Y-6 produced DBHP equivalent to the Big Boy and Allegheny at 25 MPH (both those went on to produce more DBHP at higher speeds, of course), but for N&W's grades and curves it was perfect.

The compounding that everyone else, and you, disdain meant that this locomotive could do what it did with a boiler the size of a big 4-8-4.

Now, my question is - that if compounding could work so well for N&W (don't forget that N&W vied to be at the top of the statistical heap as far as gross ton miles per train hour, and gross income carried over to net - with grades and curvature against loaded coal traffic far worse than C&O's) why would some form of compound not have worked as well for C&O and, of course UP? I'm talking about an engine with maybe 63" instead of the Y-6's 58" drivers . . .

On the subject of front end hunting - this needn't have been a problem unless you were going to operate the engine for long distances at speeds in excess of, say, 70 MPH. It wasn't a problem for the N&W 2-6-6-4, and its designers took heart from the high-speed success of the Seaboard 2-6-6-4, which appeared in 1935 and quickly proved itself capable of 60+MPH operation.

But here's another comparison, recently in print, that makes the N&W/C&O/UP deal more telling:

C&O 2-6-6-6 had 67% of its weight on drivers.
Big Boy had 71% of its weight on drivers.
N&W Y-6 had 89.7% of its weight on drivers.
EMD F-unit had 100.0% of its weight on drivers.

This means that the Allegheny's vaunted 6-wheel trailing truck that made it possible to have almost 7500DBHP at 40+MPH, and Big Boy's 4-wheel leading and trailing trucks that helped make it capable of 80MPH speeds, and made it so pretty, were, in mountain service, a liability. Dead weight that had to be dragged up the hill every trip.

So history tells us that if you want super horsepower, or super speed, go with the C&O and UP engines.

If you want to simply make money, maybe follow N&W's lead.

Too bad more didn't. Those old dog Y-6s helped N&W pay out almost two billion dollars in common stock dividends over the 86 years of its existence, they and their predecessor Ys were there for 43 years of it.

Old Timer

[rrnut282]

Don't forget to think about how the slide valve works. Very little of the time is the valve open to allow the lower pressure steam exhausted by the HP cylinder to push back against the HP cylinder. Thermodynamics says that high (temp/pressure/energy) state steam will naturally seek to go a low(er) state. So, as long as the exhausted steam has had some of its energy expended moving the piston and is lower in temp or pressure, it will seek another outlet (the LP cylinder) rather than work against the higher pressure "raw" steam. (in theory, anyway)

[jchnhtfd]

In principle (and I won't repeat Overmod up there!) compounding can make more complete and efficient use of higher pressure steam; the high pressure cylinders dropping the pressure from the top to some intermediate level, the next set stepping down, and so on. Each set of cylinders is no more (nor less) efficient than it would be if it exhausted at atmospheric and started at atmospheric plus whatever. Things are somewhat complicated by the variable cut-off in the throttle, however. Properly used with dry (superheated) steam, a steam engine running with a partial cut-off is every bit as efficient as a compound running with no or less cut-off.

There were a whole bunch of problems with compounds, though. Overmod mentions the hunting of the front engine on Mallets. This was pretty bad, but later designs of simple articulateds (e.g. Challengers) showed that this could be overcome. Much more serious, however, was the problem on simple engines in getting all the various piston and rod thrusts balanced. The Delaware & Hudson had some fascinating compounds (including a triple expansion) which were highly efficient -- and went through bearings like there was no tomorrow. Maintenance nightmares.

As Csshegewisch notes, the French, who seem to love complicated machinery, were willing to live with the TLC these beasts required -- but remember that some very simple US built Mikes were the engines which saw steam out in France. In the marine setting (some ships went so far as to have quadruple expansion) the maintenance and TLC were available. Still... as soon as turbines became less costly, only the lowest tramps stuck with pistons.

[CSSHEGEWISCH]

Robert La Massena's speculative article in TRAINS in 1973 or 1974 addressed some of the problems of compounding. His proposed design for a rigid-frame high-speed compound reduced some of the back-pressure problem.

The French were masters of high-speed compound designs although they were willing to accept higher maintenance down time then any American mechanical officer. French locomotives were tailored more to a specific service than any American design and usually had a pretty low availability rate.

[Anonymous]

I'm not sure what it did, but Compounds had something called an Interceptor valve. Some could be run simple for starting, maybe that's what it was for???

Triple expansion steam engines were very common in marine use so you may find a lot more technical information by seraching the web for them. Titanic had 2 of them and the left-over steam had enough pressure left to drive a steam turbine. Most WWII era freighters also used triple expansion steam engines.

[Overmod]

Keep in mind that compounding can be beneficial, even desirable, as steam pressure rises and full expansion in a single cylinder becomes physically impractical. This was a focus of some American design theory in the mid-20s, producing Baldwin's 60000 demonstrator (now at the Franklin Institute in Philadelphia) as an example.

The 'back pressure' in a steam cylinder is no more a "limitation" on smoothness or hp output than the compression in a diesel is. You size the hp bore and stroke appropriately for the pressure drop (for power) and to give appropriate steam mass flow and expansion to produce comparable power at the LP cylinders.

Modern compounds could be given intermediate balance via a rather simple (pun not intended) method -- by valving an appropriate amount of boiler steam into the LP flow, via an intermediate 'receiver', it was possible to balance the HP and LP piston efforts very precisely, as well as in an equivalent simple-admission engine. I have always associated this with Andre Chapelon, but N&W applied a very similar principle to the later Y6bs (the 'booster valve' discussed by Newton) and AFAIK locomotives so equipped were quite happy running at 40+mph. One can only speculate on whether future development of 'high-speed compounds' would have produced locomotives on which the fuel savings balanced out the higher capital and maintenance costs of the boosting and equalization systems -- certainly low-mass high-strength materials in the running gear would have allowed Mallets to run as fast as freight power normally could on most railroads...

Mallet problems at high speed (as opposed to simple articulateds) usually involved the mass and size of the LP pistons, which worse yet were usually on the hinged forward engine, right where the two-wheel Bissell lead truck acted as a dual-pivot pendulum. "A-hunting we will go!" This, and suspension, are to me much more significant problems than any introduced by differential steam pressure...

[M636C]

In the early years of the last century, before the Schmidt superheater was generally adopted, a number of compound locomotives, particularly those built for Santa Fe, included a reheater which was a separate section of the boiler between the normal barrel and the smokebox. This had larger diameter tubes to reduce the gas flow resistance. In some articulated locomotives with jointed boilers, the whole forward section in front of the joint was a reheater (conveniently close to the forward cylinders).
When the Schmidt superheater was introduced, the extra energy in the steam allowed the elimination of the reheater, since the risk of condensation and cylinder damage was geatly reduced.

Peter

[GP40-2]

Short answer: The steam exiting the high pressure cylinders still has some latent energy. Allowing it to expand in volume again (that's why the low pressure cylinders are so large) allows the steam to release this energy.

Sounds good in theory, but didn't work as well as expected in practice. Most compound locomotive were either scrapped of converted into simple expansion locomotives.

Most famous compound was N&W Y6b. Had good low speed tractive effort but was a dog above 30mph. Started losing HP faster than a rock falling off a cliff.

I don't think there is a way to compensate for the backpressure. Compound mallets were used in slow speed service anyway.

[Anonymous]

Here is an interesting question I haven't been able to come up with an answer for. Perhaps one of the older railheads can help me out.

In a compound mallet, steam first enters the rear smaller chamber to power the rear set of drivers. As it is exhausted, it is passed, under reduced temperature and pressure (some of the energy having been used to move the piston) into the larger forward cylinders, where it is used and then expelled.

Now my question, how does this work? The first part is easy; however once the steam exits the first set of cylinders, it must exert a back pressure on the first cylinder as it enters the front cylinders, reducing the efficiency of the movement of the first piston. Once the steam is in the front cylinder, it pushes the larger cylinder to drive the front drivers, then is expelled against what is minimal pressure.

So simply, at least as I see it, the steam entering the back cylinders moves the drivers and forces the ejected steam into the front cylinder. This seems to be a big waste of energy reducing the efficiency of the locomotive.

So how do they compensate for the back pressure? Is there somekind of reheating of the steam for the second use and a one way valve. Can anyone help me with this one?