A 21st Century Vision of Steam Traction

How the Steam Engine of the Locomotive Works

by Muhammad Fadhli Mustaffa & Ahmad Lutfi Mohayiddin

In this section, we will take a look at the working principles of the steam engine used in locomotives. Basically, solid fuel such as coal is burned to heat up water. When it gets hot enough, water will condense into steam. The pressure of the steam pushes the pistons which consequently move the gears and the wheels, thus moving the locomotive.


The Flow of the Steam and Gases

 Solid fuel is burned on the grate inside the firebox. The primary air is admitted below the grate and is drawn to the firebed while the secondary air is admitted through the firehole door. The firebrick arch lengthens the path of the hot gases from the burning of the fuel to ensure complete combustion. The hot gases are then drawn through long tubes in the boiler to the smokebox and out of the locomotive from the chimney.

The steam engine

The heat from the firebox heats up the water in the boiler. Water is also heated by the heat from the hot gases going through the long tubes. As water becomes hotter, it turns into saturated steam which collects above the water. The regulator valve, which controls the passage of the steam to the cylinders, is situated in the dome. There are also safety valves on top of the boiler to release steam if the pressure tends to rise to a dangerous level.

The saturated steam flows through the main steam pipe to the superheater header. It then travels through superheater element pipes in the boiler where it is heated up. After coming out of these pipes through the superheater header, it will have become superheated steam. The extremely hot steam then flows through steam pipes to the cylinders where its pressure moves the pistons which move the wheels of the locomotive.

In the smokebox, exhaust steam passes through the blastpipe to the chimney at high speed due to the confined vent of the blastpipe. This creates a partial vacuum in the smokebox which provides the draw of the air to the firebox and ensures that the hot gases are drawn out of the firebox via the tubes in the boiler.


Valve and Piston Working

In a steam engine, the movement of the valve ensures that steam is admitted to and exhausted from the cylinder at the right moment. For a typical cylinder that has two ports, the function of the valve is to admit superheated steam at one end while allowing used or exhaust steam to escape at the other. As a result of covering and uncovering these ports in sequence, the piston is pushed forward and backward by the high pressure steam from the boiler. To regulate the movement of the valve, a mechanical valve gear system is used and this is discussed further in the following subsections.

To know how the valve affects the speed of the locomotive, we have to understand a few terms which are common among steam locomotives operators and enthusiasts. Lap refers to the amount of overlap between the valve and the port. In slow moving locomotives, the long lap on the exhaust port gives time for the steam trapped in the cylinder to expand fully to push the piston. On the other hand, on higher speed locomotives the exhaust port is made to open early (short lap) when the valve is in mid-position thus allowing the steam to escape faster. Furthermore, higher speed locomotives also have long lead which means that the admission port is already open when the piston is at the end of its movement so there is a sufficient steam pressure that will immediately pushes the piston back to begin its next movement.

Cut-off denotes the position of the piston, at the moment the valve is closing the admission port. When the engine is working hard and slowly, long cut-off admits steam for most of the stroke of the piston. On fast running locomotives this will cause back pressure to the boiler. To avoid unnecessary back pressure, cut-off is reduced so that steam is admitted for only 20% of the piston stroke and the remainder of the stroke is due to the expansion of the high pressure steam.

An indicator diagram for a steam engine

The indicator diagram such as the one above was used by steam locomotive engineers during the steam era to estimate the locomotive’s efficiency in converting the steam’s energy into useful power at various speeds and cut-offs. The horizontal line OA shows the pressure as the steam enters the cylinder. At cut-off, the pressure drops as the steam expands and does work to push against the piston. After the exhaust port opens, the line reverses (CD) to indicate the start of the return stroke of the piston. It shows the low pressure as the steam is exhausted. The line DE at the end of the return stroke registers a pressure rise due to the compression of the remaining steam after the exhaust port has closed. As fresh steam is admitted into the cylinder, the pressure rises back to point O and the cycle repeats.

The working cycle of the valve and piston


The Walschaert System

The Walschaert system

The locomotive valve gear enables the driver to choose the cut-off of the steam admission and to reverse the locomotives. One of the most common valve gear systems found on UK built locomotives is the Walschaert system, which was first patented in 1844 by Egide Walschaerts, a Belgian engineer. It first appeared on a British railway in 1878. It did not become popular in Britain until the twentieth century but it is now generally regarded as the best valve gear design due to the easy maintenance.

In this system, the fore-and-aft movement of the valve spindle depends on the combined movement of the combination lever and the expansion link. The combination lever motion is worked by the crosshead at the end of the piston rod. It is connected to the expansion link by the radius rod. The expansion link movement is obtained from its connection to the eccentric road. The other end of the eccentric road that is attached to the crank axle caused the pendulum-like motion of the expansion link.

By adjusting the position of the radius rod in the expansion link, we can adjust the length of travel of the valve spindle. This can be done by lifting or lowering the reversing rod from the cab. To obtain the maximum valve travel (longest cut-off and maximum steam admission), the radius rod is positioned furthest from the centre of the expansion link. On the other hand, moving the radius rod up and down from one half of the expansion link reverses the movement of the locomotive.

Cycle of operation of the Walscheart system


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