Pennsylvania 2-10-0 "Decapod" Locomotives in the USA

Class I1s Type A (Locobase 5153)

Data from "Pennsylvania Locomotive of the Decapod Type", Railway Age, Volume 62, No 24 (15 June 1917), pp. 1241-1243; "PRR 'Decapod Type Locomotive, Railway Mechanical Engineer, Volume 91, No 7 (July 1917), pp 369-372; "New 'Decapod' Freight Locomotive, Pennsylvania Railroad", Locomotive Magazine, Volume XXIV [24], No 305 (15 January 1918), pp5-6; "2-10-0 Type Locomotive; Pennsylvania Railroad", Engineering, Volume 110, (22 October 1920), pp.538-539 and Plate LXIV [64] following p 542; James Partington, "Avoidable Waste in Locomotive Operation as Affected by Design", N-o. 1816, December 1921 in Transactions of the American Society of - Mechanical Engineers, Volume 43 (New York: The Society, 1922), 987-1011. (Thanks to Randal O'Toole for his 28 August 2022 email noting an incorrect citation for Partington's paper.).

Works numbers were 3165 in December 1916.

3540-3554 in October 1918, 3555-3566 in October, 3567 in November, 3568-3579 in December

3580-3591 in January 1919, 3592-3601 in February, 3602-3612 in March, 3613-3623 in April, 3624-3634 in May, 3635-3644 in June, 3645-3651 in July, 3652--3660 in August, 3661-3662 in September.

Pennsy built the first 123 at their Juniata shops in 1916 and 1918-1919. Never very stylish, these were brute-force engines serving wherever there was a long train and a steep grade.

Most observers of this new design contended that it was and then broke the rules of conventional practice by an idea expressed emphatically by RME as "one of the widest departures from the well know well-worn groove of conventional practice which has ever been undertaken."

Two features in particular stood out to most of these analysts. One was the very high boiler pressure (250 psi), which was higher than almost any other locomotive then in use. The other was the choice to use a maximum valve cutoff of only 50% --half the length of the cylinder stroke . It was clear, RME observed, that "operating at full stroke is accompanied by a waste of steam as compared with the expansive use of the steam at higher speeds."

Using a maximum cutoff of 50% in combination with larger cylinders to maintain the same starting tractive effort of 90,024 lb (40,834 kg or 400.44 kN) was"the most promising field for the conservation of the steam supply which may be used to stay bolt increased horsepower power for a given boiler size." 50% cutoff and to the use of 2 inch (51 mm) lap in the valve travel, quite a bit longer than most at that time, each valve had small auxiliary ports that allowed some steam to begin moving the piston until the cylinder's main port opened.

RME pointed out the potential difficulty with such a short maximum cutoff, which lay in increased stresses on the running gear "…owing to the higher initial piston load necessary to produce a given mean effective piston pressure as compared with ad from the full stroke." Although RME supposed that the higher initial piston loads would require somewhat heavier reciprocating parts, rods, crank pins and "may be expected to increase the wear and tear throughout the running gear." But the advantages seemed considerable as well , RME suggested, such is a more uniform starting torque and therefore a decreased tendency toward slipping."

James Partington underlined the benefits a few years later benefits when he commented that the secret to the I-1's great success lay in setting the proportions to allow for limited cutoff operation. Continuous high tractive effort levels on the long uphill runs were achieved by using a long stroke and large cylinders, but cutting off the steam at 50% of the stroke rather than the more usual 90%. Limiting the cutoff may allowed the railroad to use 12" (305 mm) piston valves.

"The expected increase in economy of coal and water," Partington observed, "...has been fully realized. Not only has the engine shown remarkable efficiency, but the economy under wide ranges of load is especially remarkable [sic]."

As a reminder of what "efficiency" consisted of in the steam era, note that the I-1 achieved a maximum of 8.1% thermal efficiency (generating 1,777 ihp), and averaged over 7%. Maximum IHP came to 3,080 (at 40% cutoff and 2.9 lb of coal per IHP hour.).

Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.

See Locobase 15291 for the 475 Baldwins of 1922-1923, which introduced the Type E superheater.

Class I1s Type E - 1922 (Locobase 15921)

Data from DeGolyer, Volume 68, pp. 388+ . Works numbers were:

1922

November 55725-55730, 55777-55785

December 55817-55855, 55943, 55945

1923

January 55946-55989, 56069-56076

March 56164-56194

April 56410-56415, 56432-56452

May 56489-56502, 56531-56535, 56546-56565, 56615-56629

June 56643-56682

July 56747-56758, 56776-56803

August 56869-56895, 56945-56967

September 57037-57061, 57100-57104, 57125-57170

October 57229-57231, 57272-57317

William D. Edson (1974) recorded that these engines also had "[a] Belpaire firebox, Worthington [BL-2] feed water heater, [mechanical] stoker, power reverse gear, heat-treated steel reciprocating parts, and underhung crossheads." Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. A combustion chamber measuring 42.25" added 90 sq ft (8.35 sq m), which together with the 31 sq ft (2.9 sq m) of fire brick tubes, completed the supplementary direct heating surface. Also, the Pennsy had adopted an unusually "square" tube and flue arrangement (usually engines of that era fitted with Type E superheaters had many more flues than tubes). See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox..

As Locobase 67 relates, tests on the I-1s in 1923 led to changes in the boiler.

Class I1s Type E - 1929 (Locobase 67)

Data from table in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also "Tests of A Class I 1s Freight Locomotive Equipped with a Feed Water Heater and Type E Superheater", Locomotive Testing Plant Bulletin No. 32 (1924). (Many thanks to Chris Hohl for his 22 September 2017 email reporting unlikely boiler pressure values for 177 entries. A Locobase macro caused the error .)

Baldwin delivered these 475 decapods in 1922-1923; see Locobase 15921

Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.

The dimensions shown in the specifications come from the Pennsylvania's diagrams, which were dated from the late 1920s. The changes apparently resulted from the Pennsylvania's tests of an I-1s in February 1923. Engineer of Tests F M Waring stated that the "substitution of the Type E for the Type A superheater and the resultant large increase in heating surface has not noticeably increased the evaporative capacity or efficiency of the boiler." He concludes that the evaporative heating surface area in this design limited any possible gains regardless of the size of the superheater.

Waring also noted that use of the Worthington BL-2 feed water heater resulted in a 14% savings in coal use.

Although no recommendations for modifications to the I-1's boiler appear in the report and the Belpaire firebox remained unchanged, differences found in diagrams prepared just a few years later suggest that the Pennsy's motive power heads decided to adjust the balance of the tube-flue area. Given the Waring's conclusion regarding the boiler's EHS, it's odd that both tube and flue counts both dropped, but the boiler lost more than twice as many 3 1/2" flues than 2 1/4" tubes. Also, tube and flues were all trimmed by 6" (152 mm).

In fact, both the M-1 4-8-2 and the I-1 2-10-0 used boilers with many more small tubes than were found in most boilers with Type E superheaters.

Beginning in 1930, almost all of the I1s engines were redesignated I1sa, which indicated a modification to the valves to permit running at cutoffs of up to 78%, equated to a starting tractive effort of 96,026 lb (43,557 kg or 427.14 kN). By 1944, 479 had been converted with 119 more still designated I1s.