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Main parts of a steam locomotive. 1 - chimney; 2 - dome; 3 - smokebox;
4 - boiler; 5 - firebox; 6 - cab; 7 - tender; 8 - cylinder; 9 - driven
wheels; 10 - non-driven wheels; 11 - connecting rods and valvegear
The firebox
The firebox is where the fire is built and maintained. Most British steam locos were designed to burn coal, but wood and oil were also used. As well as providing power to drive the locomotive, the fire ultimately powers everything else on the train: the brakes, the carriage heating, the water injectors that fill the boiler when it is under pressure, the lubrication system, and even the whistle. The fire can also be used to keep your tea warm and, if you're brave, cook your dinner. It is the job of the fireman to keep the fire at the right height, and to maintain the correct pressure in the boiler. If the fire gets too low, then there may not be enough pressure in the boiler to drive the locomotive, especially on hills. If it gets too high, then the boiler may have to exhaust steam through its safety valves, which is noisy and inefficient.As all the power of the engine ultimately comes from the fire, a heavy-duty loco will need a significantly larger firebox than smaller one. The largest fireboxes are found on the passenger express trains
The boiler
The boiler in a steam locomotive is essentially a big water tank through which run tubes that carry the hot gasses from the fire. Because the fire would otherwise be hot enough to melt the firebox, the water in the boiler completely encloses the fire chamber as well (figure~\ref{figure:flow}). As the host gasses run through the boiler, they give up their heat to the water, which eventually boils and liberates steam. For reasons which are explained in physic books, the steam liberated by boiling water cannot be much hotter than the boiling point of water itself, that is, 100 Celcius. More modern locos therefore used superheaters (see below) to heat the steam more directly.Because the steam liberated by boiling cannot easily escape from the boiler (it has to do some work first), the boiler tends to run at a higher pressure than the surroundings. In the largest engines, pressures of over 300 psi are common; that's about ten times the pressure in fully-inflated car tyre. Because the boiler walls are rigid, there is little for them to do if the pressure gets too high other than to crack, which would be a Bad Thing. So there are normally safety pressure release valves in the top of the boiler to vent steam safely in the event of over-pressure. An over-pressure condition is most likely to occur where the fire has been built up too high for the work the engine is to do. Typically this will happen when the locomotive is bought to rest in a station with the fire still high from the journey. While it does no real harm to have the boiler blow off steam this way, it is noisy and damp, and therefore unpleasant for people nearby. As Gordon says in the Thomas the Tank Engine books: ``It isn't wrong, but we just don't do it!''. A good fireman is expected to know the route, and allow the fire to die down sufficiently when coming to an extended halt.
The smokebox
Hot fumes from the firebox are drawn through the boiler tubes and are collected in the smokebox, from whence they are exhausted by the chimney.The smokebox commonly contains the superheater tubes and the blast pipes (see below).
Superheater tubes are used to raise the temperature of the steam coming out of the boiler to above the boiling temperature of water, i.e., 100 Celcius. This is very dificult to do simply by heating the water itself - as the boiler does - and the hotter the steam, the more efficient the locomotive.
After heavy use the smokebox tends to accumulate dust and ash, so the front can usually be swung open for access.
Simplified view of the arrangment of firebox, boiler, and smokebox.
Note that the firebox and boiler
are continuous, and that the fire chamber is completely surrounded
by water. The hot gasses from the firebox are drawn along metal
tubes in the boiler, eventually to escape through the firebox
and chimney. Air to feed the fire is drawn from the dampers under
the firebox, and through the firebox door in the cab
Chimney
The chimney serves as an exhaust for the fumes of combustion; obviously they have to go somewhere. However, the chimney is more than just a hole in the top of the firebox, and chimney design was the subject of a great deal of research. This is because a roaring fire in the firebox will need a good supply of oxygen for combustion, so the chimney has to allow a good flow of air through the firebox and bolier tubes. At the same time, it has to prevent sparks from firebox from being swept out of the firebox and causing fires near the line.As well as combustion products, the chimney will also carry away the exhaust steam from the cylinders. Exhausting steam from the cylinders this way is not only convenient, it also improves combustion. This is because the rush of steam up the chimney draws air up into the firebox and thus through the fire.
This use of the exhaust steam to fan the fire is only possible when the locomotive is moving; when it is stationary no steam is exhausted and there is there fire no blast effect. So all but the earliest locomotives had blast pipes whose job was to direct a blast of steam from the boiler up the chimney. Although this is potentially wasteful of steam, it does have the effect of drawing a more foreceful stream of air through the firebox, and can be applied whenever there is steam in the boiler. The valve that admits steam from the boiler to the blast pipe is called the blower.
The dome
The dome acts as the collector of steam from the boiler. It provides a volume of hot, dry steam away from the water in the boiler itself. As such it is usually built at, or close to, the highest point on the boiler. The use of a dome reduces the likelihood that water, or water vapour, will be fed into the cyliners instead of steam, which would have catastrophic consequences. Typically the driver's regulator handle (see later) operates a valve in the dome, so the driver can control the amount of steam fed into the cylinders and, therefore, the power generated by the locomotive.The cylinders
Hot steam under pressure is fed from the boiler into the cylinder, where it pushes a piston which eventually turns the wheels. The flow of steam into the cylinder is governed by the steam chest, a valve assembly mounted directly above the cylinder. The valves can admit steam either behind the piston, which pushes it forward, or in from of the piston, which pushes it back. The ability to apply steam to both sides of the piston is important because, unlike a car, locos don't have gearboxes. So the only way to switch a locomotive between forward and reverse motion is by controlling the times at which steam is injected onto each side of the piston.The opening and closing of the valves in the steam chest is controlled by the motion of the wheels themselves, adjusted by the driver by means of the reversing gear in the cab (see below). The wheel motion is coupled onto the steam chest by means of valvegear.
Although the steam that is admitted to the cylinder is hot and dry, it very soon cools and, as it cools, water condenses out. Water is not very compressible, and if enough of it accumulates in the cylinder it may prevent full piston travel. If full-pressure steam is admitted to a cylinder whose piston cannot move properly, there is an excellent chance that this will blow the end of the cylinder off. So cylinders invariably have either safety valves or drain cocks. Drain cocks allow water that has condensed to run out, and are usually operated from the cab by the driver. As most drivers keep the drain cocks open whenever the train is stationary, you'll see a blast of steam from the cylinders as the train comes to a halt, and another blast of steam when it pulls away. Normal practice is to keep the drain cocks open for about six wheel revolutions.
The steamchest and cylinder
The valves in the steam chest control the admission of steam onto the front or back of the piston. However, the steam chest alone cannot synchronize the valve action to the movement of the locomotive: this is the job of the valve gear. Typically the steamchest will either be an integral part of the cylinder assembly (as in the photo below), or will be mounted close to the cylinder assembly and coupled to it using thick pipework. The valves in the steamchest are actuated by a pushrod that is coupled to the valvegear. By controlling the timing of the valve movements with respect to the wheel movements, the locomotive can be driven forward or backwards, and with varying degrees of effort.
In this photograph, the steamchest cover has been removed,
showing the steamchest valves. In this example, the steamchest
and the cylinder form a single assembly: you can see the piston itself
at the bottom of the photo. Normally, steam would be admitted
to the top of the steamchest, and the valves would admit it
to either the front or the back of the piston. The opening
and closing of the valves is controlled by the rod that enters
the steamchest just below the cover: as the rod is pushed
back and forward by the valvegear, steam is admitted to either
side of the piston.
The valvegear
The purpose of the valvegear is to open and close the valves in the steamchest at the appropriate points. Like everything else on a locomotive, the valvegear derives its motive force from the movement of the piston in the cylinder. The steamchest pushrod typically requires a full movement of about 4 inches to swich from full forward pressure on the piston to full reverse pressure. However, the piston itself may move through several feet during its working cycle. Clearly the steamchest cannot be directly coupled to the piston. In practice, some fraction of the piston's motion is coupled to the steamchest pushrod. That is, for each foot of motion of the piston, the steamchest pushrod may move up to an inch. In fact, the amount it moves, and the direction, will be controlled by the reversing gear in the cab. For example, when the steamchest pushrod is moving in the same direction as the piston, the loco is moving forward. When they are moving in the opposite direction, the loco is moving backwards. Between these two extremes the loco will not be powered at all.
Connecting rods
The pistons move in a reciproocating, front-to-back fashion but wheels, of course, have to rotate. The connecting rods connect the pushrods of the pistons to the wheels, with a rotating coupling at each end. To prevent the piston being subjected to a stressful bending action, the rotating coupling at the pushrod end takes the form of a crosshead which slides back and forth along slidebars
The connecting rod runs back and forth along the slidebar, which
prevents the cylinder being damaged by twisting stresses on
the piston. The crosshead connects the connecting rod to the
pushrod, which in turn is coupled to the wheel
The tender
A large locomotive consumes a good deal of coal and water, and it is more efficient if the loco carries its own supplies, rather than having to stop to refuel and rewater every few miles. Small engines would carry as much water as could be accomodated in their tanks, and as much coal as would fit in a hopper behind the cab. These supplies were not adequate for large locos, so such locos would typically pull a tender. The tender was nothing more than a big hopper full of coal and water, towed along behind the engine.
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