The Locomotive Of Today

56 The Boiler : Fuels. some roads necessitates the adoption of a special class of coal as for instance the running of engines inside the suburban distncts of large cities, where a smokeless fuel is a desider- atum. Briquettes are largely used on Continental railways, and have been experimented with here. They are made up of coal dust mixed with pitch and other ingredients, and com- pressed into blocks. Welsh coal, being more liable to crumble and be broken during transportation than the harder varieties supplies a large percentage of the dust made up into briquettes’ Liquid fuel has been adopted to a large extent on some of our railways, and is the exclusive fuel in some countries, notably Southern Russia, South America, and the far East, where the supply of suitable oil fuel is practically unlimitgd. The English method, or Holden system, is to spray it into the firebox by means ofa steam jet, and intermix it with the air for combus- tion above a base of incandescent solid fuel or material laid over the firebars, whilst the Russian process is to similarly spray the oil fuel by steam into a brickwork furnace built in the firebox, the coal-burning arrangements having been entirely removed. In the English system steam can be readily raised with a wood and coal fire as usual, but with the Russian apparatus the oil burners must be started and worked from some independent source until steam is made in the boiler. Ihe first method has the advantage of heating the plates ot i e nrebox more gradually, which are consequently strained less than when an intense heat is maintained in a compara- tively cool firebox. Before considering the principles of combustion involved m the firing of our locomotive boiler we must first ascertain the composition of the chief fuel. A good coal is made up of a large percentage of carbon, combined with hydrogen oxygen, and other gases and mineral ash. The quantities of these vary considerably in different grades, an almost endless vanety of qualities of coal being obtainable in different parts of the world, but a fair sample has the following- analysis : Carbon Oxygen Hydrogen Nitrogen Sulphur Ash . . 7'2 i 375 0-41 O'IO 2*2 ioctoo The specific gravity is 1-3. The carbon is most relied on to produce the heat in cow