A 21st Century Vision of Steam Traction

The Main Components of the Steam Engine

by Muhammad Fadhli Mustaffa & Ahmad Lutfi Mohayiddin

The steam engine of the locomotive consists of different segments, each with its own important function. We will now be looking at the main working components as found in most steam locomotives.


The Firebox

The firebox is the compartment in the locomotive which houses the fire. It is designed to burn fuel, usually coal, efficiently and to produce sufficient heat to boil water and create steam. Fireboxes consist of two enclosures, the outer firebox and the inner firebox. The former is usually made of steel while the latter is either made of copper or steel. They are connected by stays, which are bolts that keep the inner firebox rigid within the outer firebox.

The firebox

Boiler water surrounds the inner firebox to allow maximum benefit from the fire for heating and it also prevents the inner box plates melting due to the intense heat from the fire. The outer firebox is really an extension to the boiler. When the boiler is filled, water will enter the outer firebox legs and cover the roof of the inner firebox.

Primary air is admitted through the grate at the base of the firebox and drawn to the underside of the fire. This is controlled by damper doors in the ashpan. Secondary air is admitted through the firehole and guided towards the fire by the deflector plates. In some cases, secondary air also flows through tubes or hollow stays through the side water spaces.

Inside the firebox, a firebrick arch is positioned over the fire so that the heat from the fire is deflected towards the back of the firebox to ensure the hot gases are distributed towards the tubes more evenly. The hot gases deflected by the brick arch combines with the secondary air to ensure complete combustion. Moreover, the brick arch prevents the cool secondary air from reaching the tubes in the boiler.


The Boiler

The boiler is the enclosure on a locomotive where steam is produced. It must be filled with water almost to the top. Hot gases from the boiler pass through hollow tubes running the length of the boiler, thus heating the water. When the water boils, the steam it generates forms in the space between the top of the water and the top of the boiler. Steam will tend to rise to the highest point, which is the steam dome. The amount of steam in the steam dome released to the main steam pipe is controlled by the regulator valve. If more power is needed, more steam is released.

The traditional form of boiler had the same diameter throughout its length and was known as the parallel boiler. A later type was the tapered boiler, which had a narrower front compared to the rear. This allowed more of the water to be at the rear where the greater heat from the firebox was available.

Boilers are insulated and then covered with a thin steel covering. This is done to reduce heat loss. Wood, and then asbestos, was used as the insulating material for many years. In recent years, various forms of natural or man-made insulating materials have been used.


The Regulator

Once the boiler has generated sufficient steam, it can be used for useful work. A lever is used to control the amount of steam entering the locomotive cylinders. This lever is called the regulator. A regulator valve fitted on top of the boiler and housed in the dome is opened and closed by means of a long shaft connected to a lever accessed from the driver’s position in the locomotive cab. The shaft passes through the boiler steam space.

Normally in the United Kingdom, the regulator is mounted on the top centre of the firebox backplate so that it is moved clockwise or anticlockwise to open or close the regulator valve. If more steam is allowed into the cylinders, more work can be done on the pistons. As a result, more power is produced at the wheels, causing the locomotive to accelerate or be able to pull a larger load.



The steam generated in a boiler is called saturated steam because it is in contact with water. Using the pressure of saturated steam to move the pistons in the cylinder is very inefficient since water will be condensed during the expansion. Condensed steam produces little power.

The process of superheating the steam

Superheated steam is steam which has been reheated after its production in the boiler. It has less water vapour and will therefore not condense as rapidly as saturated steam, enabling the power output of a locomotive to be increased up to 25%, thereby increasing the efficiency. It also leads to a 25% saving in coal and 30% saving of water consumption. Basically, superheating increases the heat content and volume of the steam, making it more fluid and using less for a given work output.

Saturated steam from the boiler travels via the main steam pipe to the superheater header, which is a box divided into two separate spaces; one for saturated steam and the other for superheated steam. The saturated steam flows through superheater element pipes situated inside large flue tubes in the boiler where it is heated up. It now becomes superheated steam and its temperature is usually around 320°C to 370°C. The steam then flows through the steam pipes to the cylinders.


The Cylinders, Pistons and Cylinder Valves

A steam locomotive usually has two, three of four cylinders depending on the design and power requirements. Next to each cylinder is the steam chest containing a valve which controls the flow of steam into and out from the cylinder. Usually the cylinder and the steam chest are designed on the same block as in the figure below.

Inside one of the cylinders

A cylinder typically has a port at each end and the valve covers and uncovers these ports so that at one time one port acts as the admission port while the other as the exhaustion port. The fresh steam from the boiler stores a large amount of energy and tends to expand immediately after being injected into the cylinder. The energy from the high pressure steam is converted into mechanical work by pushing against the piston in the cylinder.


The Blastpipe

The pipe which carries the exhaust steam from the cylinders to the centre of the smokebox is called the blastpipe. It is vertical and placed at the bottom of the smokebox facing the chimney exit. Other then letting exhaust steam to pass through, the smokebox also draws gases from the firebox to exit through the chimney.

The smokebox contains the blastpipe

When passing through the smokebox, the exhaust steam creates a draught which draws the smoke and gases along the boiler tubes and pulls fresh air through the grate in the firebox. If more steam is used, there will be a bigger draught and consequently a hotter fire, thus creating a faster steam production.

If the nozzle is narrowed, the steam velocity increases and the draught will be stronger. Although a high velocity steam jet may create a strong draught, it is uneven. Usually, lumps of half-burned coal will be drawn out with the smoke from the fire when the draught is uneven. Combustion will then be very inefficient as the fuel is not burned properly. A constricting nozzle increases steam velocity, but it also creates back pressure in the cylinder as the used steam cannot escape fast enough. An ideal steam blast is strong but slow and steady, and provides a fast passage for the steam leaving the cylinders.

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