The Baldwin Locomotive Works has devel oped the Mallet articulated design by placing driving-wheels under the tender, and thus uti lizing the weight of the latter for tractive pur poses. The first locomotive of this type was built in 1914, for the Erie Railroad. It runs on 28 wheels, 24 of which are used as driving wheels; develops a tractive force of 160.000 pounds and weighs in working order 853,000 pounds. At the time of its construction, it was the heaviest steam locomotive ever guilt.
It was a similar demand for more powerful locomotives, for engines not only capable of hauling heavy trains at sustained high speed, but also of accelerating speed rapidly after starting, that brought the new and heavier types of passenger locomotive into existence. The old American, or eight-wheel type, was wholly incapable of meeting such requirements. The ten-wheel engine, which had been used quite successfully on heavy grades, proved unsatisfactory in such an emergency, owing to its comparatively small driving wheels, its in adequate firebox, an adhesion considerably in excess of the requirements, and excessive re sistance within the machine itself. To over come these difficulties the Baldwin Locomotive Works constructed the Atlantic type of engine. It was substantially a ten-wheel locomotive, in which the rear pair of driving wheels was replaced by a pair of trailing wheels of smaller diameter, permitting the introduction of a deep firebox with ample grate area and volume suffi cient to admit of thorough combustion. Great boiler capacity was available in proportion to the adhesion; the driving wheels were closely coupled, and the total wheel base was suffi ciently long to give smoothness of motion at high speed, and, at the same time, sufficient flexibility.
A development of the Atlantic type is found in the Pacific type, which has three pairs of driving-w-heels with a four-wheeled leading and a two-wheeled trailing truck. Since the intro duction of steel passenger cars, which fre quently weight 75 tons or more apiece, Pacific type locomotives have come into extensive use, because of their great starting power and their ability to run at sustained high speed when hauling heavy trains.
The prices of these heavy locomotives are materially higher than those of the engines built in 1897; and large locomotives for road service now frequently cost $25,000 or more.
The fuel cost is the largest single item of expense in the operation of locomotives, and various devices have been tried, from time to time, with a view to promoting fuel economy. Compound locomotives, which were first intro duced in Europe, were built with this end in view ; and were at one time extensively used in the United States. These locomotives were successful as fuel savers; but, under the methods of operation prevailing on American railways, they proved difficult to maintain; and they are no longer being built, except in the case of the Mallet articulated compound loco motive, which is practically two single expan sion engines placed under one boiler. The cylinders of the two engines are of different sizes, the larger, or low-pressure, receiving the exhaust from the smaller, or high-pressure; but the various features which proved unde sirable in other types of compound locomotives are eliminated in the Mallet design.
The most effective fuel saving device at present in use on locomotives is the super heater. When saturated steam is used in the cylinders of a locomotive, a considerable por tion of it — sometimes as much as 25 or 30 per cent — is condensed into water, and does no useful work. This loss can be avoided by heating the steam, before it enters the cylinders, to such an extent that its temperature will not be lowered to the point of condensation. In a locomotive using superheated steam, a number of the small boiler tubes are replaced by con siderably larger tubes, and in these are placed groups of pipes through which the steam cir culates before it enters the cylinders. The steam absorbs heat from the furnace gases passing through the large tubes, and its tem perature is raised to such an extent that cylinder condensation is avoided. The increase in temperature depends upon the design of the superheater and the conditions under which the engine is working. It frequently amounts to 200° F. or more, and the fuel saving effected per horse power developed, as compared with a similar locomotive using saturated steam, averages about 25 per cent.