How a Steam engine works

The earliest railways employed horses to draw carts along railed tracks. As the development of steam engines progressed through the 1700s, various attempts were made to apply them to road and railway use. The first attempts were made in Great Britain; the earliest steam rail locomotive was built in 1804 by Richard Trevithick and Andrew Vivian. It ran with mixed success on the narrow gauge "Penydarren tramroad" at Merthyr Tydfl in Wales.. Then followed the successful twin cylinder locomotive by Christopher Blackett's team built at Wylam in 1811, closely followed by Matthew Murrays rack locomotive for the edge railed Middleton Railway in 1812 ^[2]. These early efforts culminated in 1829 with the Rainhill Trials and the opening of the Liverpool and Manchester Railway a year later making exclusive use of steam power for both passenger and freight trains.

The United States started developing steam locomotives in 1829 with the Baltimore and Ohio Railroad's Tom Thumb. This was the first locomotive to run in America, although it was intended as a demonstration of the potential of steam traction, rather than as a revenue-earning locomotive. The first successful steam railway in the US was the South Carolina Railroad whose inaugural train ran in December 1830 hauled by the Best Friend of Charleston. Many of the earliest locomotives for American railroads were imported from England, including the Stourbridge Lion and the John Bull, but a domestic locomotive manufacturing industry was quickly established, with locomotives like the DeWitt Clinton being built in the 1830s.

Boiler

The typical steam locomotive employs a horizontal fire-tube boiler partially filled with water. A firebox, its walls and roof constantly surrounded by water, is incorporated generally to the rear of the boiler when the locomotive is traveling chimney-first; this is where a combustible is burnt, the heat generated thereby being transferred to the water in the boiler in order to make the steam that constitutes working medium. The combustion gases flow from the firebox into a bundle of parallel tubes, also surrounded by water, which continue to transfer heat to the water. At the front of the boiler is the smokebox, a chamber where the combustion gases are mixed with the jet of exhaust steam, the whole being ejected into a chimney (US: "smoke stack") voiding into the outside air.

Steam circuit

The generated steam is stored in the steam space above the water in the partially-filled boiler. Its working pressure is limited by spring-loaded safety valves. It is then collected either in a perforated tube fitted above the water level or from a dome that often houses the regulator valve, or throttle, the purpose of which is to control the amount of steam leaving the boiler. The steam then either travels directly along and down a steam pipe to the engine unit or may have first to pass into the wet header of a superheater, the role of the latter being to eliminate water droplets suspended in the "saturated steam", the state in which it leaves the boiler. On leaving the superheater, the "dried" steam exits the dry header of the superheater and passing down a steam pipe enters the steam chests adjacent to the cylinders of a reciprocating engine. Inside each steam chest is a sliding valve that distributes the steam via ports that connect the steam chest to the ends of the cylinder space. The role of the valves is twofold: admission of each fresh dose of steam and exhaust of the used steam once it has done its work.

The cylinders are double acting, with steam admitted to each side of the piston in turn. In a two-cylinder locomotive, one cylinder is located on each side of the locomotive. The cranks are set 90� out of phase with each other. During a full rotation of the driving wheel, steam provides four power strokes per revolution; that is to say each cylinder receives two injections of steam. The first stroke is to the front of the piston and the second stroke to the rear of the piston; hence two working strokes. Consequently two deliveries of steam onto each piston face in two cylinders generates a full revolution of the driving wheel. The driving wheels are connected on each side by coupling rods (US: "connecting rods") to transmit power from the main driver to the other wheels. At the two "dead centres", when the connecting rod is on the same axis as the crankpin on the driving wheel, it will be noted that no turning force can be applied. If the locomotive were to come to rest in this position it would be impossible for it to move off again, so the cylinders and crankpins are arranged such that the dead centres occur out of phase with each other. This precaution is unnecessary on most other reciprocating engines (such as an internal combustion engine) which are never expected to start from rest under their own power, and employ a flywheel to overcome the dead centres.

Each piston transmits power directly through a connecting rod (US: main rod) and a crankpin (US: wristpin) on the driving wheel (US "main driver) or to a crank on a driving axle. The movement of the valves in the steam chest is controlled through a set of rods and linkages called the valve gear, actuated from the driving axle or else from the crankpin; the valve gear includes devices that combine the roles of reversing the engine, adjusting valve travel and the timing of the admission and exhaust events. The cut-off point determines the moment when the valve obturates a steam port, "cutting off" admission steam and thus determining the proportion of the stroke, during which steam is admitted into the cylinder; for example a 50% cut-off admits steam for half the stroke of the piston. The remainder of the stroke is driven by the expansive force of the steam. Careful use of cut-off provides economical use of steam and, in turn, reduces fuel and water consumption. The reversing lever (US: Johnson bar), or screw-reverser, (if so equipped) which controls the cut-off therefore performs a similar function to a gearshift in an automobile.

Walschaerts valve gear in a steam locomotive. In this animation, the red colour represents live steam entering the cylinder, blue represents expanded (spent) steam being exhausted from the cylinder. Note that the cylinder receives two steam injections during each full rotation; the same occurs in the cylinder on the other side of the engine.

Exhaust steam is directed upwards to the atmosphere through the chimney, by way of a nozzle called a blastpipe that gives rise to the familiar "chuffing" sound of the steam locomotive. The blastpipe is placed at a strategic point inside the smokebox that is at the same time traversed by the combustion gases drawn through the boiler and grate by the action of the blast. The combining of the two streams is crucial to the efficiency of any steam locomotive and the internal profiles of the chimney, (or more strictly speaking, the ejector) require careful design and adjustment. This has been the object of intensive studies by a number engineers (and almost totally ignored by others with sometimes catastrophic effect). The fact that the draught depends on the exhaust pressure means that power delivery and power generation are automatically self-adjusting and among other issues, a balance has to be struck between obtaining sufficient draught for combustion whilst giving the gases and particles sufficient time to be consumed. In the past, fierce draught could lift the fire off the grate, or cause the ejecting of unburnt particles leading to the dirt and pollution for which steam locomotives had an unenviable reputation in the past. Moreover, the pumping action of the exhaust has the counter effect of exerting back pressure on the side of the piston receiving steam, thus somewhat reducing cylinder power. Designing the exhaust ejector has become a specific science in which Chapelon, Giesl and Porta were successive masters, and was largely responsible for spectacular improvements in thermal efficiency but drastic reduction in maintenance time and pollution.

Chassis

With European locomotives, the chassis is the principal structure onto which the boiler is mounted and which incorporates the various elements of the running gear.The chassis consists of two mainframes kept apart and square by spacers and buffer beams. For many years, in America practice , the boiler was the main structural element, with built-up bar frames, smokebox saddle/cylinder structure and drag beam integrated therein; but from the late 1920s with the introduction of superpower, the cast-steel locomotive bed became the norm, incorporating frames, spring hangers, motion brackets, smokebox saddle and cylinder blocks incorporated into a single complex, sturdy but heavy casting. Andr Chapelon developed a similar structure but of welded construction with around 30% saving in weight for the still-born 2-10-4 locomotives the construction of which was begun then abandoned in 1946.

Running gear

This includes the brake gear, wheel sets, axleboxes, springing and the "motion" that includes connecting rods and valve gear. The transmission of the power from the pistons to the rails and the behaviour of the locomotive as a vehicle, able to negotiate curves, points and irregularities in the track is of paramount importance. Because reciprocating power has to be directly applied to the rail from 0 rpm upwards, this poses unique problems of adhesion of the driving wheels to the smooth rail surface. Adhesive weight is the portion of the locomotives weight bearing on the driving wheels. This is made more effective if a pair of driving wheels is able to make the most of its axle load i.e. its individual share of the adhesive weight. Locomotives with compensating levers connecting the ends of plate springs have often been deemed a complication but locomotives fitted with them have usually been less prone to loss of traction due to wheel-slip.

Locomotives with total adhesion, i.e. where all the wheels are coupled together, generally lack stability at speed. This makes desirable the inclusion of unpowered carrying wheels mounted on two-wheeled trucks or 4-wheeled bogies centred by springs that help to guide the locomotive through curves. These usually take the weight of the cylinders in front or of the firebox at the rear end when the width of this exceeds that of the mainframes. For multiple coupled wheels on a rigid chassis a variety of systems for controlled side-play exist.Generally, the largest locomotives are permanently coupled to a tender that carries the water and fuel. Alternatively, locomotives working shorter distances carry the fuel in a bunker, and the water in tanks mounted on the engine, the latter placed either alongside the boiler or on top of it; these are called tank engines.

The fuel source used depends on what is economically available locally to the railway. In the UK and parts of Europe, a plentiful supply of coal made this the obvious choice from the earliest days of the steam engine. German, Russian, Australian and British railways experimented using coal dust to fire locomotives. Up to around 1850 in the U.S.A the vast majority of locomotives burnt wood until most of the Eastern forests were cleared; from that time on coal burning became more widespread and wood burners were restricted to rural and logging districts. In Europe, this lasted well into the 20th century. Bagasse, a waste by-product of the refining process, was burned in sugar cane farming operations. In the USA, the ready availability of oil made this a popular steam locomotive fuel; the Southern Pacific, for example, went directly from wood to oil. equipment. In Victoria, Australia after World War II, many steam locomotives were converted to heavy oil firing.

A number of tourist lines and heritage locomotives in Switzerland, Argentina and Australia have been using light diesel-type oil.

Water was supplied at stopping places and locomotive depots from a dedicated water tower connected to water cranes or gantries. In the UK, the USA and France, water troughs (US track pans) were provided on some main lines to allow locomotives to replenish their water supply without stopping. This was achieved by using a 'water scoop' fitted under the tender or the rear water tank in the case of a large tank engine; the fireman remotely lowered the scoop into the trough, the speed of the engine forced the water up into the tank, and the scoop was raised again once it was full.

Water is an essential element in the operation of a steam locomotive; because as Swengel argued:

it has the highest specific heat of any common substance; that is more thermal energy is stored by heating water to a given temperature than would be stored by heating an equal mass of steel or copper to the same temperature. In addition, the property of vapourising (forming steam) stores additional energy without increasing the temperature...water is a very satisfactory medium for converting thermal energy of fuel into mechanical energy

Swengel went on to note that "at low temperature and relatively low boiler outputs" good water and regular boiler washout was an acceptable practise, even though such maintenance was high. As steam pressures increased, however, a problem of "foaming" or "priming" developed in the boiler, wherein dissolved solids in the water formed "tough-skinned bubbles" inside the boiler, which in turn were carried into the steam pipes and could blow off the cylinder heads. To overcome the problem, hot mineral concentrated water was deliberately wasted (blowing down) from the boiler from time to time. Higher steam pressures required more blowing down of water out of the boiler. Oxygen generated by boiling water attacks the boiler and with increased steam pressures the rate of rust (iron oxide) generated inside the boiler increases. One way to help overcome the problem was water treatment. Swengel suggested that the problems around water, contributed to the interest in electrification of railways.

In the 1970s L.D. Porta developed a sophisticated heavy duty chemical water treatment that not only keeps the inside of the boiler clean and prevents corrosion, but modifies the foam in such a way as to form a compact "blanket" on the water surface that filters the steam as it is produced, keeping it pure and preventing carry-over into the cylinders of water and suspended abrasive matter.

Crew [See Engineer/ Brakeman/Fireman]

A locomotive is controlled from the backhead of the firebox and the crew is usually protected by a cab. A crew of at least two people is normally required to operate a steam locomotive. One, the (US: engineer), is responsible for controlling the locomotive and the fireman is responsible for the fire, steam pressure, and water.Wood-burners emit large quantities of flying sparks which necessitate an efficient spark arresting device generally mostly housed in the smokestack. Many types were fittedthe most common early type being the Bonnet stack that incorporated a cone-shaped deflector placed before the mouth of the chimney pipe plus a wire screen covering the wide stack exit; more efficient was the Radley and Hunter centrifugal type patented in 1850, (generally known as the diamond stack) incorporating baffles so orientated as to induce a swirl effect in the chamber that encouraged the embers to burn out and fall to the bottom as ash. In the self-cleaning smokebox the opposite effect was achieved: by allowing the flue gasses to strike a series of deflector plates, angled in such a way that the blast was not impaired, the larger particles were broken into small pieces that would be ejected with the blast, rather than settle in the bottom of the smokebox to be removed by hand at the end of the run. As with the arrestor, a screen was incorporated to retain any large embers.

Research resources:

Fisher and Williams, Pocket Edition of Locomotive Engineering (Chicago, 1911) T. A. Annis, Modern Locomotives (Adrian Michigan, 1912) C. E. Allen, Modern Locomotive (Cambridge, England, 1912) W. G. Knight, Practical Questions on Locomotive Operating (Boston, 1913) G. R. Henderson, Recent Development of the Locomotive (Philadelphia, 1913) Wright and Swift (editors) Locomotive Dictionary (third edition, Philadelphia, 1913) Roberts and Smith, Practical Locomotive Operating (Philadelphia, 1913) E. Prothero, Railways of the World (New York, 1914) M. M. Kirkman, The Locomotive (Chicago, 1914) C. L. Dickerson, The Locomotive and Things You Should Know About it (Clinton, Illinois, 1914) other materials listed used under common license or free documentation license. National Archieves