Everett Lueck Wrote:
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>
... I have been told that since the thing had
> all 3 cylinders the same size that high pres-
> sure steam went to the front two sets and the
> third set was fed low pressure steam from the
> middle set.
Whoever told you that didn't understand the physics of steam expansion. Basically after the steam has doubled in volume and lost half its pressure in the primary cylinders, there's enough steam to feed cylinders of twice the volume on the second pass (minus losses due to friction, additional cooling in transit, etc.). In a Y6b, for example (assuming, for simplicity that exactly half the pressure is used up in the high-pressure cylinders) then in order to get the same thrust from the low-pressure cylinders the low-pressure pistons must have twice the surface area - i.e. the diameter of the low-pressure pistons must equal √2.0 times that of the high-pressure cylinders, or - if the same diameter - there must be twice as many of them .
> That, of course made the third set somewhat less
> powerful than the front two. Whether or not there
> was a distributing valve to allow high pressure
> steam to the rear set I do not know, but if so, it
> would have caused a severe back pressure prob-
> lem for the middle set, since it exhausted to the
> rear set.
>
> I think that the thing might have worked better if
> you could have supplied high pressure steam to
> the middle and rear sets and had a large set of
> front low pressure cylinders like a standard Mallet
> compound, but I really don't know.
As noted above, there should be enough low-pressure exhaust from the middle high-pressure cylinders to power two sets of same-size pistons in front and behind. Since the middle set is working against the back-pressure from the others, the difference in pressure on the two sides of the high-pressure piston - high vs low - should be about the same as the difference on the two sides of the low-pressure piston - low vs atmospheric (plus nozzle back-pressure, etc.), so the power of all three sets should be about the same. The designers can use fluid dynamics to take into account the losses due to friction and heat loss as the steam moves from the middle cylinders to the front and rear sets, and adjust the valve-gear settings appropriately to balance the power more evenly.
> The results with the Erie and Virginian triplexes
> seemed to be that the demands of the three
> cylinders for steam at the moment when peak
> tractive effort was needed outstripped the boiler's
> ability to make steam and so the engine basically
> fell down.
>
> When better steels for higher pressure boilers
> were developed, train speeds had increased also
> and the emphasis was on horsepower not tractive
> effort, and no one attempted to revisit the
> concept.
>
> Think of what an N&W Y-6b boiler which could
> provide the requisite amount of steam, along with
> its driving system with the huge low pressure
> compound cylinders could have done with an extra
> high pressure engine under the tender ...
But Everett -
As noted by others previously, that third engine - like the third engine under the tender in Chris' triplex, above - would lose traction as fuel and water were consumed. IMHO, the N&W Y6b's were about as powerful and efficient as a drag-freight engine could be, and the N&W and U.P. x-6-6-4's as good a multi-purpose engine as possible. OTOH, the triplex would have the advantage over the Y6b of not having such large low-pressure pistons, which limited speed due to the large amount of mass involved in their reciprocating motion.
- Russ
