Pennsy built the first 123 at their Juniata shops in 1916 and 1918-1919; Baldwin delivered 475 more in 1922-1923. Never very stylish, these were brute-force engines serving wherever there was a long train and a steep grade.
Alco Estimating Engineer James Partington (in "Avoidable Waste in Locomotive Operation as Affected by Design", Railway Age, Volume 95, No. 11 (5 November 1921), pp. 673-677) comments 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.)
Baldwin delivered these 475 in 1922-1923.
November 55725-55730, 55777-55785
December 55817-55855, 55943, 55945
January 55946-55989, 56069-56076
April 56410-56415, 56432-56452
May 56489-56502, 56531-56535, 56546-56565, 56615-56629
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 boiler, [mechanical] stoker, Worthington feedwater heater, 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. See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.
Baldwin's specs show the class as delivered in 1922-1923. At that time, their boilers contained 114 tubes of 2 1/4" and 200 flues measuring 3 1/4", each of which measured 19 feet 1 inch (1.75 sq m) long. The specs also show a combustion chamber measuring 42.25" and adding 90 sq ft (8.35 sq m). 31 sq ft (2.9 sq m) of fire brick tubes completed the supplementary direct heating surface. Total evaporative heating surface in the spec was 4,818 sq ft (447.6 sq m). Superheating surface was calculated as 1,986 sq ft (184.5 sq m).
The dimensions shown in the specifications come from the Pennsylvania's diagrams, which were dated from the late 1920s. The unusually "square" tube and flue arrangment (usually engines of that era fitted with Type E superheaters had many more flues than tubes). 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 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, the changes shown in diagrams prepared just a few years later suggest that the Pennsy's motive power heads decided to balance the tube-flue area.
|Specifications by Steve Llanso|
|Class||I1s Type A||I1s Type E|
|Railroad||Pennsylvania (PRR)||Pennsylvania (PRR)|
|Locomotive Length and Weight|
|Ratio of driving wheelbase to overall engine wheebase||0.70||0.70|
|Overall Wheelbase (engine & tender)||73.04'||73.37'|
|Axle Loading (Maximum Weight per Axle)||72600 lbs|
|Weight on Drivers||334500 lbs||352500 lbs|
|Engine Weight||366500 lbs||386100 lbs|
|Tender Light Weight||182000 lbs||204700 lbs|
|Total Engine and Tender Weight||548500 lbs||590800 lbs|
|Tender Water Capacity||9000 gals||10300 gals|
|Tender Fuel Capacity (oil/coal)||17.5 tons||18.7 tons|
|Minimum weight of rail (calculated) on which locomotive could run||112 lb/yard||118 lb/yard|
|Geometry Relating to Tractive Effort|
|Boiler Pressure||250 psi||250 psi|
|Cylinders (dia x stroke)||30.5" x 32"||30.5" x 32"|
|Tractive Effort||102027 lbs||102027 lbs|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||3.28||3.45|
|Firebox Area||272 sq. ft||322 sq. ft|
|Grate Area||70 sq. ft||69.90 sq. ft|
|Evaporative Heating Surface||4315 sq. ft||4590 sq. ft|
|Superheating Surface||1360 sq. ft||1634 sq. ft|
|Combined Heating Surface||5675 sq. ft||6224 sq. ft|
|Evaporative Heating Surface/Cylinder Volume||159.46||169.62|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||17500||17475|
|Same as above plus superheater percentage||21700||22019|
|Same as above but substitute firebox area for grate area||84320||101430|