The essential elements of a steam engine have changed little since 1830 and if they were placed one next to each other, the Fussée by Stephenson and one of the gigantic Big Boys of the Union Pacific, it would be clear that both machines belong to the same glorious race.
A steam locomotive is composed of two combined elements: one generating the steam and another one using it as driving force. The first element is formed by the firebox, the boiler and the smokebox, as well as the tender, where the coal and water are stored. The second element consists of the tubular network and the mechanism to distribute the steam, the cylinders, the transmission gear and the driving wheels, that make the locomotive to move. The water storage of the tender is supplied, either by means of a hose while the machine is stopped, either while in march by means of a sort of "spoon" that descends to a narrow canal between the rails.
Water passes to the boiler by means of injectors actuated by the steam that the locomotive produces. The fuel of the tender is thrown (firewood, coal) or injected (liquid fuel) on the grill, where it is burned; the air needed for the combustion arrives through regulation valves placed underneath the grill. Flames and smoke pass through the tubular network in the cylindrical boiler, heating the water to the point of ebullition and, then, they escape by the smokebox and the smokestack. The steam accumulates in a dome above the boiler and, from there, by means of a system of valves and ducts, it is introduced in the superheater, which is a system of small elbow tubes, placed in the interior of the large smoke tubes. The overheating increases the temperature of the steam and, consequently, its pressure.
This high-pressure steam pass then by the valves which, alternatively, open in one and another end of the cylinder, applying energy to the piston in one or another direction. The system of valves is disposed in such a way that, thanks to a series of rods in the distributor system, the steam can only pass during one part of the piston movement, to maximize the expansive force of the steam, economizing at the same time fuel and water. Instead of being simply discarded, the steam that has been used is injected again in the smokebox, where it circulates creating a flux of air through ducts, which increases the draft and the vaporization power of the machine.
The machinist manages two essential control instruments: the regulator, which allows to dosify the amount of steam arriving to the cylinders, and the valve control, which controls the duration of the steam admission time. Albeit this initial procedure was not substantially modified, however, over time numerous improvements were introduced that allowed to increase more and more the power and performance of the locomotives. New types of fireboxes were designed, adapted to the different fuels. The relation between the area of the grill and the volume of the boiler was modified to increase the capacity of vaporization and different types of valves - sliding, rotatory or butterfly - were experimented, as well as exhaust systems.
1: Regulator. 2: Injectors levers. 3: Firebox hatch. 4: Admission lever. 5: Boiler steam pressure manometer. 6: Cylinders steam pressure manometer. 7: Firebox hatch lever. 8: Brakes manometers. 9: Parking brake. 10: Brakes lever.
One of the best ways of exploiting to the maximum the expansion force of the steam is the system established by the French engineer Alfred de Glehn, by means of which the high-pressure steam actuates, firstly, in a set of cylinders of relatively small diameter; after exiting from these cylinders, already at low pressure, the steam actuates on a second set of cylinders of larger diameter. In the locomotives of this type, the small cylinders, at high pressure, were generally placed in the interior of the frame and connected by means of an elbow axis to a pair of driving wheels, whereas the larger cylinders, at low pressure, were placed in the exterior of the frame and actuated in another pair of driving wheels by means of an ordinary rod.
These compound locomotives were used during many time in Europe, with exception of Great Britain, where, like in United States, soon they were discarded due to their complexity and high maintenance cost. The maximum of energy in steam traction was reached when another French engineer, André Chapelon, doubled the performance of an old Pacific locomotive, by means of an internal reform and a widening of the admission intakes in the cylinders. He also modified the exhaust system crated by the Finnish engineer Kylala, managing to increase traction power; this device was denominated Kylchap.