The workings of a steam locomotive are fairly well known, a sort of boiler on wheels most would say. However, not many know of the exact science that goes into designing such an engine. This section on the basics of a steam locomotive takes you from the names and functions of each component, the features of their design, and ending with the importance of the mineral composition of a lump of coal or the water in the tender.
Beginning with the heart of a steam locomotive - its boiler - it is the most compact of all types of boiler in relation to the amount of steam that it produces. Suitable for pressures up to about 300 pounds per square inch, it is usually fired by coal although oil has also been widely used. In some countries, the fuel can also be wood, sugar cane waste or even peat.
The fuel is burned on the grate of the inner firebox. This firebox is surrounded by water in the outer firebox shell and so absorbing radiant heat from the fire. The gap between the inner and outer fireboxes is maintained by hundreds of rigid stays.
To support combustion, air is admitted into two areas:
1, Primary air enters via damper doors in the ashpan and is drawn through the firebed. This makes the fuel incandescent but it is not sufficient to burn all of the elements of the coal.
2, Secondary air is drawn through the firehole door or small flaps.
The brick arch within the inner firebox is constructed of firebrick or refractory concrete and serves three purposes. As its material is incandescent, it encourages combustion of gas distilled from the firebed; it lengthens the path of those gases to give additional time for combustion and it prevents cool air reaching the fire tubes as it enters the firehole door.
The hot gases are drawn through long tubes surrounded by water in the boiler barrel, to the smokebox at the front of the engine. On later locomotive boilers, these tubes are of two types, small ones of about 1¾ - 2¼ inches in diameter and large flues of 5 - 5½ inches diameter.
The saturated steam that is generated collects above the water in the boiler. Its journey to the cylinders is controlled by the regulator valve operated from within the cab by the regulator handle. It travels through the main steam pipe to the superheater header, which is divided into two separate areas. The saturated steam at a temperature of about 200ºC passes through the superheater elements and increasing its temperature to about 350ºC. Returning to the other side of the superheater header, the superheated steam flows via steam pipes to the valves and then on to the cylinders.
The gases from the fire, now much cooler after giving up much of their heat to the water and steam, are ejected through the chimney. This is greatly assisted by the exhaust steam from the cylinders passing through the reduced orifice of the blastpipe at high speed, and capturing the gases on the way. By this method the smokebox maintains a partial vacuum that provides a draw on the fire.
Replacement water is forced into the boiler by injectors or pumps, maintaining a safe level above the inner firebox crown. Safety valves on top of the boiler release steam when the pressure within the boiler rises above a predetermined level.