Hi Robert
Diesel locos as any fully bogie suspended loco concept have
full-adhesion wheel arrangements (w/a) such as B-B / C-C / D-D or BB-BB or B-B-B / C-C-C not to mention the odd w/a's possible.
That is: they have no carrying axles. That is their great advantage in starting t.e. over classic concept steam.
To build a steam loco of the power output of a Challenger, you have to give it a certain size boiler. This has a certain - large - masse. This in turn demands a chassis that is substantial enough to support it and hold the forces induced by the cylinders. So the chassis by itself will become of substantial masse by itself. This, in respect of the given axle load limitation (!) will demand a certain number of axles in total. Of this total only so many axles can be driven because the number of coupled axles in one drive set is again limited by considerations of curving, of mechanical stress by drive forces etc.
At the time the Challenger was developed, the first of challenges went to design. A two times six-coupled design of a comparatively fast running engine was something that first asked to be accomplished at that time.
Care was taken, the frames and chassis structure would be strong enough to stand daily hard working. It became a story of success. Building upon it and experience gained with it the Big Boy design was developed to get a two times
eight-coupled design and still keep the fast running qualities.
In order to have good tracking characteristics, classic concept steam locos need carrying axles in trucks or bogies to guide the loco at front. And they need further carrying axles to support the firebox end of the boiler. It is thus not that carrying axles make an engine weaker but they make it stronger at speed because they allow for a bigger boiler. Having those axles you have to allow a certain minimum load so to protect them from flanges climbing rail in switches or adverse rail conditions. The load admitted on the front bogie of the Challengers met these requirements. It is my personal view that a four wheel bogie was not really necessary, a two wheel Zara or Krauss-Helmholtz truck would have done equally well, would have saved some weight and would have allowed for fully identical drive design of front and rear sets. But as things were, UP never used such trucks and they found no problem in using a four wheel design. As concerns the rear delta truck it was loaded as necessary to support boiler masse, i.e.: after having distributed all the admissible weight to drivers, which was done, the rest goes to these trucks. There is no other way.
I think you should perhaps not compare steam to diesel one-to-one in view of tractive effort. If diesels have one indisputable advantage over steam it is their formidable starting effort. But starting a train is one thing - accelerating it to revenue earning speed is another. That latter point is where boiler capacity joins the game: the more steaming the more power at speed and the higher the sustained running speed with a given load. That is where a 4-6-6-4 was superior to a 2-6-6-2 for instance, and in fact even to a 2-8-8-2 of same overall engine weight because of the specific output characteristics of the two engine types at speed.
A third cylinder:
Yes, placed in the middle between frames and usually near the transversal plane of the outer cylinders. Three cylinder machines were used in 4-8-2, in 4-10-2 and in one 4-12-2 design, the latter unique of the Union Pacific which had a daring and capable loco design department that developed the design in cooperation with the ALCO as far back as the 1920s.
Many European engine designs used a three cylinder machine, mostly for fast running engines but in cases also for freight. The hill climbing capacity of the DR 44 class three cylinder Decapod was legendary (for European conditions of axle loads, engine size and train loads; DR = Deutsche Reichsbahn).
I hope it will not come puzzling - but there were
four cylinder designs also, and those were often realized as
compound machines, having two high pressure and two low pressure cylinders, one set outsides one set insides the frames.
No, this is not the end of the story, there were design studies put up for
five cylinder compound machines for 2-10-4 and 4-8-4 engines and there was one engine built that had a
six cylinder compound machine to drive a twelve coupled wheel set, the 160.A.1 locomotive.
There were Mallet designs put up with two cylinder on the high pressure set and three or four cylinders on the low pressure set in order to cure the throttling effects caused by excessive cylinder volume of ordinary two low pressure cylinder designs. And there were three engines built that had six cylinders distributed over
three drive sets, although these were all arranged outsides as in ordinary two cylinder engines but set in compound mode with the central engine as high pressure, the front and the tender engine as low pressure units. These were
Henderson's Triplex designs, an early effort towards to your idea of full adhesion or nearly ...
They gave me a highball, have to leave
-bye-
Juniatha
edit: signature & eagle (picture by Ronnie Hedge)
