The steam locomotive, therefore, must not only produce super heated steam for the development of draw-bar pull, but it must also supply saturated and superheated steam to various auxiliaries and for train operation. In the case of a passenger locomotive, even greater demands for power are required in connection with the train lighting, heating, air-conditioning and ventilating equip ment; hot and cold water and refrigeration systems and other devices. The most modern conventional type will produce at its best, from 6 to 8% thermal efficiency at the tender draw bar in terms of the heat value in the fuel fired. In average road service this percentage will be reduced to from 4 to 6% and is being obtained in the basic design by the use of from 200 to 250 lb. boiler pressure, in combination with an average amount of superheat in from two to four single expansion cylinders, or in duplex multiple expansion types of cylinders. In the handling of heavy freight tonnage, the question of locomotives of great power as opposed to grade reduction involves, as a primary con sideration, the operating cost per train mile, the train load and the resulting cost per ton mile for various kinds and capacities of locomotives on different gradients. For secondary consideration there are the factors of maintenance of way and structures and of the mechanical facilities required for the handling and upkeep. (See RAILWAYS for a discussion of problem.) Speed, curvature and grade are factors that largely control the loading of locomotives, as well as the cost for their operation. In view of engine and train crew wages and fuel being the governing fac tors in train operating expenses, it is believed that on the basis of an eight-hour day, per ioo m. run, freight train running speeds of 15, 20 and 25 m. per hour are more economical, from a trans portation standpoint, than speeds of io, 3o and 35 m. per hour. In general, freight locomotives handling low class tonnage should be loaded to haul trains at an average schedule speed between terminals, including road delays, of from 12 to 15 m. per hour on low grade, and of from io to 12 m. per hour on high grade lines, which is as fast as economy will allow.
During the past ten (I o) years, in order for the railroads to meet the intensive competition of the highway trucks and buses, freight train speeds have been materially increased for the pur pose of moving higher class freight on shorter schedules. This has made necessary a substantial increase in the sustained hauling capacity of steam locomotives, with the result that boiler capaci ties have been increased by greater evaporation surface, boiler water storage and higher steam pressures, in conjunction with increased total temperature of the steam admitted to the valve chests and cylinders by a combination of the higher pressure and higher superheat. (Where from 200 to 250 lb. steam pressure was formerly used, from 275 to 325 lb., is now provided in the conventional designs of locomotive boilers, and from 35o to 500 lb. in the cylindrical-drum water-tube type firebox boilers, in combination with superheated steam total temperatures of as high as 75o° F., and poppet valves actuated by outside rotary cam type of valve gear.) Boiler Pressures.—Until 1895, steam locomotive boiler pres sures were generally limited to 15o or 16o lb. gauge pressure. Then there was an advance to 200 lb. and in 1903-05 the Balti
more and Ohio went to 235 lb. in its first Mallet articulated com pound locomotive, Baltimore and Ohio No. 2,400; and in to 225 lb. in its first 35 Pacific type passenger locomotives. Dur ing the next 20 years there was practically no increase, the general tendency being to use 200 lb. as a maximum, due largely to the adherence to the conventional type of locomotive boiler with its flat and radial sheet stay-bolts, water-legs and other non-self supporting surfaces. During the past five years, on account of the public service central power station boilers being installed with pressures ranging from 35o to 65o lb., and marine boiler pressures going to 35o and 400 lb., there has been a tendency to raise the locomotive steam pressures and temperatures on account of the ability to increase locomotive capacity within the established clearance and weight limitations and the possible fuel and water savings. The conventional radial and flat sheet stayed water-leg type of fire-box is in some cases now being superseded by self-sustaining cylindrical-drum water-tube type of fire-boxes. A conventional type of boiler is now carrying steam pressures as high as 325 lb., but for higher pressures, as in the case of the Delaware and Hudson Railroad 35o to 500 lb. pressure, multiple expansion, and the Baltimore and Ohio Railroad 35o lb. pressure, single expansion locomotives, the cylindrical-drum water-tube type of fire-box is being substituted.
The "Horatio Allen" (named after the engineer who ran the first locomotive in the Western Hemisphere), high-powered locomotive No. 1,400 completed in 1924, was an epochal event in steam locomotive development. It has a total loaded weight of engine and tender in working order of 273 tons, was designed by the Delaware and Hudson Company and built at the Schenectady works of the American Locomotive Company. Instead of having the usual water-leg fire-box with its undesirable flat sheets and staybolts and the sluggish circulation of water around these sheets, the fire-box is built up of self-supporting cylindrical struc tures in the form of drums and tubes disposed horizontally and vertically, requiring no stays, and are directly exposed to the radiant heat of combustion and split the boiler water into small streams to provide for its rapid circulation, quick absorption of heat and release of the steam bubbles. The "Horatio Allen," when operating on a o•5% ascending grade in tonnage freight service, with 3,217 actual gross tons in the train, at an average speed of 16.5 m. per hour, with coal of about 13,500 B.T.U., can produce an equivalent evaporation of 1135 lb. of water per pound of coal at an average high-pressure cylinder cut-off of 64% ; on the basis of indicated horse-power per hour, including auxiliaries, the dry coal consumption was 2.15 lb., and the steam consumption was 17.5 lb. ; on the basis of draw-bar pull, the dry coal consumption was 2.14 pounds. During this performance the efficiency of the boiler was 80.6%, the machine efficiency was 9386%, the thermal efficiency at the tender in terms of the heat value of the coal was 8.72%. An important advantage of the higher steam pressure on the "Horatio Allen" and the "John B. Jervis" locomotives has been found to be in the ability to increase the capacity of a loco motive within the same roadway clearance and weight limitations.