Spokane Portland & Seattle Z-6 and Z-8 Challengers

[L-105]

The large Alco built Z-6, Z-7 and Z-8 4-6-6-4s of the Northern Pacific seem to be relatively unknown / rarely mentioned when it comes to dicussions of the 'ultimate American steam locomotive' compared to say the UP 4-8-8-4s, N&W As, C&O H-8s. Now I can understand that the Northern Pacific use of ~ 8750 BTU/lb low grade Rosebud coal would have reduced heat flux relative to more energy rich higher grades of coal and hence the larger fireboxes to compensate.

However it turns out that the Spokane Portland and Seattle railroad, jointly owned by Northern Pacific and Great Northern, had their own 4-6-6-4s (Z-6, Z-8) built essentially to same dimensions as those of the Northern Pacific, except they burned oil. 

Now considering the ~ 17000 - 20000 BTU/lb of oil, and the fact that they had same large firebox and heating surfaces as those of the Northern Pacific engines (including some 839 sq ft of direct heating surface for Z-6 and 756 sq ft for the Z-8), these engines must have been mighty, at least in terms of evaporative capacity. Would be intersted to know if the SP&S Z-6 and Z-8 challengers were regarded as particularly free-steaming engines.

[kgbw49]

The Great Northern actually owned SP&S Z-6 903 and 904 for the better part of a decade, numbering them 4000 and 4001. These were 69-inch-drivered machines with roller bearings on all axles and operated at 250 PSI with a tractive effort of 104,266 lbs.

Page 362 of the book "Steam Locomotives of the Great Northern Railway" has a chart showing them rated at 4,050 HP and being able to pull 10,500 tons on a 0.2% ascending grade.

[kgbw49]

Here is a link to some additional information:

https://www.steamlocomotive.com/locobase.php?country=USA&wheel=4-6-6-4&railroad=sps

[L-105]

kgbw49

has a chart showing them rated at 4,050 HP

Is this data derived from the oil burning engines or the Rosebud coal burning Northern Pacific engines?

Im sure I've seen a similar figure touted elsewhere for the Northern Pacific challengers, which I can understand when they're burning lower specific energy Rosebud coal, but a rating of 4050 hp seems a bit low for an oil burning late-era superpower steam locomotive that had a larger firebox and more direct heating surface than a UP 4-8-8-4, a boiler pressure that equalled or exceeded the SP AC-9, DM&IR M3/M4, B&O EM-1 yellowstones (Z-6=250psi, AC-9=250 psi, M3/M4=240psi, EM-1=235psi), and a superheater surface of over 2100 sq ft.

kgbw49

and being able to pull 10,500 tons on a 0.2% ascending grade

At what speed would this have been?

 

[kgbw49]

L-105, the data in the table is based on the two oil-burning Z-6 Challengers 4000 and 4001 owned by the GN during the 1940s.

The table shows horsepower and then tonnage on percent grades all the way up to 2.2%.

Here are the HP and 0.2% grade numbers:

Z-6 4,050 HP and 10,500 tons (4-6-6-4)

N-3 4,200 HP and 11,413 tons (2-8-8-0)

R-1a 4,224 HP and 13,500 tons (2-8-8-2)

R-1b 4,370 HP and 14,470 tons (2-8-8-2)

R-2 5,193 HP and 15,540 tons (2-8-8-2)

The Z-6 had 69-inch drivers and all the others had 63-inch drivers.

All the locomotives in the table were oil-burners.

I hope this is useful info for you!

 

[kgbw49]

Good-looking machines:

https://www.steamlocomotive.com/whyte/4-6-6-4/USA/photos/sps900-humphreys.jpg

https://www.steamlocomotive.com/whyte/4-6-6-4/USA/photos/np5125-eilenburger.jpg

https://digital.denverlibrary.org/digital/collection/p15330coll22/id/67065/

http://www.athearn.com/newsletter/042216/01_Genesis_Z8_update_042216.pdf

[L-105]

That’s very useful thanks kgbw49. Would be interesting to know at what speed that tonnage was being hauled, as the Z-6 with its 69 inch drivers would have been optimized for fast freight service rather than slow slogs.

The Z-6/Z-7/Z-8 Challengers were large impressive looking engines, as were the Baldwin built D&RGW L-105 Challengers, which had more than 800 sq ft of direct heating surface and 136.5 sq ft grate. I believe the Great Northern also had a design for a large 4-6-6-4 with 73 inch drivers and some 133 sq ft grate, which sound as if they could have been a contender with the Northern Pacific engines for being the largest Challengers. Additionally I believe there was a design for a 2-8-8-4 for the Great Northern with 69 inch drivers and 180 sq ft grate, which could have surpassed the Northern Pacific Z-5 in size, although I don’t know what the heating surfaces were.