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!
Just look at the Services the N&W used their Articulateds in the Y classes where the Coal Haulers they dragged the Coal over the Mountains. The A Class hauled the Speed Ball Freights that had to get there plus in WW2 hauled Troop trains at times. The Js and their Moutains handled the Passenger trains. They did what was needed and if not for a Leadership change I think would have stayed Steam until the mid 60’s
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.
Taint 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?
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…
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 b
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?
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.
Chuck
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)?