Steam locomotive basics 

Instead to reinvent the hot water in writing by myself this chapter which be could be useful for some visitors according to some mail received, I've decided to place here a carbon copy of a chapter called "How an engine works?", extracted from a nice and well documented British book dedicated to the children (The Wonder Book of Railways for boys and girls) apparently belonging to a series of "Wonder Books", educational publications for the youngest. There is no date of printing on it, but I believe, seeing the introduced locomotives as well as the visual background of the photos that it was made in the early thirties.

It should be completed and made more attractive in a next update by the addition of some animations of the most common valve gears....

Click the figure for an enlarged view

... It is now necessary to consider the steam, or working, part of the engine.  The largest and most noticeable feature is, of course, the boiler.  Here there is a furnace, the heat of which turns the water into steam; but we must first of all understand what is meant by pressure.

If we boil water in an open saucepan it is converted into steam, but as the steam can get away it has no pressure and is of little, if any, use.  But in a saucepan with a tight lid the steam cannot escape though more of it is being made all the time, so that it gradually gets force, or pressure, until it is strong enough to lift the lid.

For a locomotive engine the steam must have considerable pressure, and the boiler needs to be very strong to withstand the enormous power of the steam within.  How great this strain is, can be realized if you like to work out how many tons of pressure, at the rate of 200 pounds on every square inch, a boiler with a plate surface of about 30,000 square inches has to stand.

The boiler is in the form of a cylinder of steel plates riveted together, with a diameter of about 5 feet, and a length of 12 feet or so. At the front it is closed in by a steel plate called the front tube plate, and at the back end it is joined to the outer firebox.  This outer firebox is only a casing, but inside it is a box of copper plates in which is the furnace, and this is the inner firebox, or firebox proper.

The inner firebox is connected with the outer firebox only at the bottom directly, but it is also attached by some hundreds of copper stay bolts all round the back and sides.  The front plate is pierced with two or three hundred holes, and in these the ends of long tubes, about 2 inches in diameter, are fitted. These tubes extend right through the barrel to the plate which closes the front end of the boiler barrel, and which receives their other ends.  This plate is therefore called the front tube plate, while the tube plate of the inner firebox is called the back tube plate.

In the firebox is the grate for the fire, and an opening at the back is the fire-hole, closed by a fire-door. Here the fireman supplies coal to keep fire going, usually at such a rate that on a journey of a hundred miles he will have to fire about two tons of coal, if not more, so that It does not have an easy time. The grate itself may be 8 feet long and 4 feet wide, so that he has hard work to throw the coal to the more distant points of the fire. From the grate the flames and smoke pass through the tubes, so that the water in the boiler is heated not only from the inner firebox itself, which has water all round it and on top, but also by the tubes as these pass through the water. In front of the barrel is the smokebox; on which is the chimney, and here the smoke from the fire escapes after it has done its work in the boiler.

The top part of the boiler barrel is empty, though the water has to be high enough to cover all the tubes and the top of the inner firebox by six or seven inches at least, and in this space the steam collects. But as the water is boiling furiously, and there is a lot of splashing, in most engines a steam dome is fitted on top of the barrel so that the steam can rise further from the water. Inside the dome is a pipe carried up nearly to the top, and this pipe collects steam as dry as possible and then passes through the barrel into the smokebox, where it divides to lead steam away to the cylinders where the actual work is done. A valve in the steam dome is worked by the driver's regulator handle, and allows steam to be turned on or shut off as required.

In most engines nowadays steam is superheated before it is used. That is, it is heated again after it has left the boiler, so that particles of water in it are turned into steam, and the steam is made hotter than it otherwise would be. This process of "superheating," or "over-heating," is of great value, but need not be gone into now. We must, however, consider how the superheating is effected. The top three rows of tubes in the boiler are made of larger size than the others, about 5 inches in diameter, and in these some small steam tubes are fitted so that the steam inside these smaller tubes can pass to and fro several times.  Smoke and flame from the firebox are, of course, passing through the large tubes, to heat the water outside them, but they at the same time heat the steam in the small inner tubes and so cause the steam to be superheated.

Showing how the tubes for the steam pass to and
fro inside large fire tubes in the boiler. (Click to enlarge).

In the smokebox is a header, a steel box divided into two sections. Steam from the steam dome enters one section, goes thence through the small tubes referred to, and comes back to the other section of the header.  From this it passes to the cylinders.

Most engines have two cylinders; some have three or four.  These cylinders may be described as closed boxes, and in each there is a sliding block, or piston.  Steam is admitted at one end of the cylinder, and by its pressure forces the piston to the other end.  This steam is then allowed to escape, while a fresh lot is admitted at the other end of the cylinder to force the piston back again.  This goes on in each cylinder all the time, so that the piston is moving to and fro, and this causes a piston rod connected with it and passing through one end of the cylinder to move in the same way.  Jointed to the other end of the piston rod is a connecting rod, and this is pivoted either to the outside of a driving-wheel-if the cylinders are outside the frames, as in many engines-or is connected with a crank formed in the axle of the driving wheels.  The action is, in fact, somewhat similar to that of a cyclist's feet acting upon the pedals of his machine; and so the to-and-fro movements of the piston cause the wheels to rotate.

In an engine with two cylinders these are arranged so that when one piston is at one end or the other of its movement, or stroke, the other piston is half-way along and therefore exerting its greatest power.

For admitting steam to the cylinders and cutting it off and allowing used steam to escape, a slide valve, or piston valve, working in a steam chest alongside the cylinder is used.  Steam from the boiler or from the superheater header enters the steam chest, from which openings lead to the ends of the cylinder, and also through a passage away to the smokebox for escape by the chimney, and the slide or piston valve is so moved to and fro that it opens or closes these openings or passages to admit steam to one end or the other of the cylinder or to allow used steam to get away (in most cases, the live steam coming from the boiler is fed to the middle of the piston valve assembly, i.e. between the two halves of the piston, the extremities of the device being connected to the blast pipe - Thierry's note).

A pedagogic version of a Walshaerts’ valve gear used to drive a piston valve in this example (click the photo for a larger version).

For working these valves a valve gear is employed. In some engines this is inside the frames, but in most big modern locomotives it is fitted outside the wheels. This valve gear has to provide for causing the engine to work either forward or backward (among another very important use that is beyond the purpose of this text written for novices... T.S.), and is therefore governed by a reversing gear worked by a reversing lever, or by a reversing wheel in the driver's cab. Sometimes this lever or wheel is worked by a small steam engine of its own, for the parts to be moved on a big engine are heavy, and place a great strain on the driver.

Steam having done its work in the cylinders passes into the blast pipe in the smokebox directly under the chimney. It still has considerable force left in it, and this is very important, as the steam is passed into the chimney with a strong blast and so provides a draught upon the fire. This draught is, in fact, strong enough to draw lumps of burning coal from the fire right through the long tubes and even to throw it in the form of "sparks" high above the chimney. By means of the draught the fire is kept burning fiercely, as it needs to be. You may look into the firebox while an engine is standing in a station, and the fire does not appear to be anything very wonderful. But see that same fire while travelling and when the blast is on; It is then one blaze of intense whiteness, far too bright for more than & passing glance.

There are, of course, many other parts of an engine: the cab where the driver and the fireman stand, and where all the various levers and valves and gauges are placed; the tender which carries the spare supplies of water and coal; the brake gear, lubricators, and various other fittings. But we have said enough to give an idea of how a steam locomotive engine is made, and why and how it works.

At last, as the site deals (mainly) with compound engines, I must add that the text above explains what happen in  a Single Expansion engine and that the it also applies to dual expansion locomotives with the difference that in a compound the steam, after to have worked in a first cylinder, enters in a second cylinder where it expands again. The total expansion is so shared between two set of cylinders whose capacity



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