Working Principles of a Steam Locomotive

Published in

RailPub

6 min readJan 26, 2021

Introduction

When one pictures a scene from a vintage railway, or even when the word “train” pops up in a conversation, there’s a good chance that the first image that comes to mind is that of a steam locomotive puffing through the countryside. Steam locomotives were the dominant method of propulsion on the railway up until the 1960s in Britain and much of the world, playing a pivotal role in the development of the railway and earning an immortal spot in the hearts of many enthusiasts. Remarkably, the basic principles behind steam locomotion have hardly changed since the Rainhill trials up to the modern day, which this article hopes to detail and explain, yet countless variations and continual improvement of performance in locomotives over 150 years is testament to the ingenuity and talent of engineers seeking to constantly optimise and innovate.

The Boiler

The boiler’s chief function is to heat water and convert it into gaseous form. Fig.2 shows a typical boiler system, with the red portion being the fire chamber and blue areas indicating water. Water is automatically fed from the tender into the boiler, whilst fuel for combustion is manually fed into the firebox from the tender by the fireman.

Coal is usually used a fuel due to its higher energy content, with bituminous coal possessing a specific energy (energy per unit mass) of 6667–9722 Wh/kg, far greater than that for wood of 4500 Wh/kg [3], leading to higher heat energy output from combustion. However, most locomotives can use a variety of fuels, making them more versatile than other forms of locomotion. At the base of the firebox is a grate made from cast iron firebars to admit sufficient oxygen for combustion.

The hot fumes produced from combustion of fuel rise through the firebox and into the boiler tubes, causing the tubes to heat up and subsequently heat the water surrounding them. A large number of tubes are used to maximise the total surface area of heated tubing in contact with water, hence increasing the rate of heat transfer and the effectiveness of boiler gases.

Steam Control and Optimisation devices

As water is converted into high-pressured steam in the boiler, steam rises and is collected in the steam dome at the top of the boiler, where a regulator valve operated by the train driver controls the passage of steam from the dome into the superheater, and consequently into the cylinders.

*Figure 3: Regulator and superheating systems* **[4].**

The superheater is a device designed to superheat steam as to maximise its effectiveness. Steam produced through heating of water in the boiler, being in direct contact with liquid water, is a mixture of gas and water vapour, leading to a high moisture content. This is known as saturated steam. Saturated steam is located in region A on Fig.4, where adding heat only leads to the formation of more steam by converting water vapour in the saturated steam into gaseous form, without causing a large increase in temperature.

Figure 4: An energy-temperature diagram showing different states of steam **[5]**.

However, as heating continues, point B is eventually reached where all the remaining moisture in the steam has been converted into gaseous form. This steam is known as dry steam. Consequently further heating of dry steam (producing superheated steam) will not lead to an increase in steam content, but will lead to an increase in temperature of the steam itself. In the context of steam locomotives (Figure 3), steam is superheated by passing it through superheater elements (yellow) located in especially large boiler tubes known as flue tubes, before passing it onto the main steam pipe towards the cylinders (red). Superheated steam brings several advantages for locomotive use, including avoiding condensation of water in pipes (necessitating draining, also reducing steam available to generate tractive effort), and a lack of water droplets in the steam meaning reduced corrosion. Superheated steam also occupies a larger volume than saturated steam leading to greater steam pressure and efficiency.

Driving Mechanism

After superheating, steam passes through the main steam pipe(s) towards the cylinder(s) located on either side of the locomotive. The reciprocal motion in the cylinders is created through timed actuations of the valve controlling steam admission, which drives the piston in turn driving the valve movement.

At A in Fig.5, fresh steam (red) is admitted into the cylinder through the admission port by the valve as the piston is at the end of its stroke. This high-pressure steam exerts a force on the piston forcing it to travel away from the admission port. The movement of the piston also forces low pressure steam from the previous stroke (yellow) out of the admission port, which has been opened on the other side.

At B the piston has reached its cut-off where the admission port is now obstructed by the valve, prohibiting the entry of additional high pressure steam into the cylinder. As soon as the piston reaches the cut-off point for the next stroke, high pressure steam is once again admitted into the other side of the cylinder at C for a repetitive cycle generating reciprocal motion, which is converted into rotational motion by a connecting rod attached to a driving wheel (being directly driven by the piston). A valve gear (Fig.6) controlled by the connecting rod is used to drive the movement of the valve controlling steam admission into the cylinder.

Exhaust

After the steam has been used and ejected from the cylinder(s), it is removed from the locomotive through an exhaust system known as a blastpipe, as illustrated in Fig.7. As steam is removed from the cylinder(s), it

*Figure 7: A simple Blastpipe arrangement* **[7]**

flows through the exhaust pipes as shown by the white arrows. The steam converges at the top of the exhaust pipes in a blast pipe, which produces a jet of steam exiting the smokebox through the chimney. This draft leaving the blastpipe also draws gases through the boiler pipes from the boiler, ensuring a continual flow of air through the boiler for combustion.

Numerous modifications have been made to the blastpipe to maximise its effectiveness. For example, narrowing the blastpipe nozzle will lead to an increase in steam speed for a stronger draught of air in the boiler tubes according to Bernoulli’s Principle. However, this may also lead to back pressure in the cylinder as the evicted steam cannot escape as quickly through the blastpipe, leading to reduced piston efficiency. The number of blastpipes in the exhaust system and chimney shape can also be altered to create a larger draft (more sources of steam) without compromising back pressure. Finally, newer locomotives are also usually fitted with blowers, which vent steam directly from the boiler into the firebox to ensure that suction of air through the boiler tubes continues even when the locomotive is stationary (no exhaust steam from cylinders and blast pipes) to maintain combustion.

Conclusion

Steam locomotives are operated on simple physical principles, with the elements required to operate the locomotive (fuel and water) being relatively abundant, consequently being reliable when maintained correctly. However, steam locomotives were extremely costly to maintain and operate due to labour intensive processes such as firebox cleaning and coaling in comparison with modern-day locomotives, reducing their cost effectiveness in the long term. Steam locomotives also generated large amounts of pollution with soot and combusted gases polluting trackside environments. Finally, steam locomotives would take up hours for a full startup and sufficient steam to be generated, whereas would be done with the push of a button on modern-day locomotives, making steam locomotives much less operationally flexible in contrast. All these factors combined led to the phasing out of steam power on railways by the 1950s, but nevertheless its dominance in rail transport for over a century before this has earned steam locomotion its place in transport history.

Thank you for reading this far (and if you haven’t you should). Stay tuned for more articles from ICRTS!