Smoke box size and locomotive performance

[Ulrich]

I recently saw a photo of a steam locomotive with a caption that drew attention to the engine's unusually long smoke box. How does the smoke box size influence the operating characteristics of a locomotive?

[Overmod]

Ulrich

I recently saw a photo of a steam locomotive with a caption that drew attention to the engine's unusually long smoke box. How does the smoke box size influence the operating characteristics of a locomotive?

In a wide variety of respects.  If you have access to a '20s or '30s Locomotive Cyclopedia, look at the discussion of 'front end' design, and you will get some idea of the variables and how they are related.  (A more specific reference, if you can find it, is Koopmans' "The Fire Burns Much Brighter," or you might Google up Goss' "Locomotive Sparks.")   Note in particular the route by which the combustion gas gets to the smokebox and then into the area around the exhaust steam jets.  Excessive volume in the smokebox is not useful in providing the required flow patterns.  

I suspect the highly extended smokeboxes were sometimes used as an attempt to provide adequate space for cinders trapped by the internal netting, or decrease the pulsating effect of low-speed exhaust on the fire.   They're not particularly effective at that, and after the proportions of the Master Mechanic front end were developed, you see comparatively little about them.

[Firelock76]

I read recently that the Strasburg Railroads'  Number 90, the ex-Great Western decapod was originally equipped with an extra-long smokebox because of the poor-grade western lignite coal it was fueled with.  The article didn't specify the reasons why.  Cut down on sparks?  More efficiency at the firebox end?  I don't know.  At any rate it now has a smokebox compatable with eastern bituminous and has had for some time.

[friend611]

I do know that in the N&W Y6, the waffle-iron exhaust nozzle, etc. was slanted forward so that the smokebox on the engine would not have to be too long. This also resulted in a slanted front to the stack.

lois

 

[BigJim]

I think you will actually find that the smoke stack was slanted forward in order to clear the front end throttle.

[Overmod]

I thought the stack was slanted forward to get some additional entrainment length in the 'chimney'.  One of the converted Australian Victorian R class 4-6-4s had double stacks that were tilted forward and backward respectively for this purpose -- somewhat strange-looking but fitting clearances.  There is a drawing of a front end with forward-slanted stack -- I want to say done for PRR in the late '30s -- in the '47 Cyc.  

There were other ways to achieve the effect: ATSF for instance had a number of classes of modern power equipped with pop-up stack extensions.

[BigJim]

That may be true for the Aussie R class, but, the following drawing of the Y6 with multiple valve front end throttle should be self explanatory:

http://www.nwhs.org/archivesdb/detail.php?ID=48461

Now, compare that to the Y5 with the Bradford front end throttle:

http://www.nwhs.org/archivesdb/detail.php?ID=26522

 

[M636C]

The two modified R class had an arrangement based on David Wardale's South African Class 26 4-8-4.

The 26 had an Elesco feed water heater between the two stacks and the arrangement was dictated by the original blast pipe location.

There may have been some advantage found during the extensive testing of the Class 26 in the longer nozzle permitted by the angled stack.

I recall that Porta suggested a pair of forward angled nozzles for the new built "Tornado", taking advantage of its already long smokebox and forward located stack, which would have been indistinguishable from the outside.

I'm not really convinced that you would get better entrainment of gases from a longer exposure to the same exhaust steam in a more complex multiple nozzle..... Stewart Cox clearly didn't think so, following testing of the Giesl ejector on the BR 9F.

Both R class have had standard stacks replaced, but 766 was converted to standard gauge and hasn't run since modification.

The two R class had modified valves, and if they were retained, they might have kept the reported improved economy.

M636C