Mechanical Engineer Meets Steam Engine - Learn All About Steam Locomotives

Mechanical Engineer Meets Steam Engine - Learn All About Steam Locomotives

  • Katarina Knafelj Jakovac

    November 2, 2023

Steam engines laid the foundation for the rapid development of mechanical parts and machinery mechanisms and were the precursors of numerous sophisticated machines still in use today.

Concurrently, the development of the railway and telegraph networks began, leading to faster information, people, and goods flow, boosting the economy, and creating new values as never before in human history. The steam engine provided a significant impetus to the industrial revolution in Europe.

Why was the steam engine invented?

The beginning of the idea of a device using the power of steam to replace human labor is attributed to Heron, who constructed the first primitive machine. Throughout history, many people worked on developing this idea, and the first recorded successful application was achieved by Thomas Savery in 1698 when he created a steam pump.

When the steam engine was invented, the production of goods at that time largely operated on the principles of manufacturing and small family crafts. The main function of the initial steam engines was to power pumps that drained water from mines.

Coal mining and the extraction of various metals were enormously important economic sectors in 18th-century Great Britain.

The steam engine was constructed in 1712 by Thomas Newcomen to power water pumps in coal mines.

He aimed to harness the machine's power to replace horses that were used to operate primitive water pump mechanisms in mines. Horses were expensive, and coal was cheap.

Newcomen's steam engine was fairly simple and consisted of a cylinder with a piston. The image shows a cross-section of Newcomen's simple steam engine.

Thomas Newcomen's Steam Engine

Image: Thomas Newcomen's Steam Engine (Source)

Atmospheric pressure would push the piston downward after water vapor condensed in the cylinder, creating a vacuum. The power obtained in this way was transmitted further through a mechanism.

A significant amount of coal was required for heating the water and generating steam to power the piston in Newcomen's steam engine.

As mines grew larger, more and more water needed to be pumped out. Newcomen's steam engine was not up to the task in terms of its working capacity.

Maintenance of machines and equipment was an unknown concept in the 18th century.

If water constantly flooded the mines, the steam engine would stop working, causing a halt in mining operations.

James Watt, by 1764, was already known for his skill in crafting and repairing mechanical devices such as compasses, scales, quadrants, small mechanical toys, and musical instruments.

James Watt was also very frustrated and annoyed by the frequent need to repair damaged Newcomen steam engines brought to him by mine owners in northern England.

After each repair of a Newcomen steam engine, like any conscientious mechanic, James Watt would run the machine for a test to check its functionality.

The steam engine barely worked.

How did James Watt improve the steam engine?

Extremely frustrated that all his efforts to repair existing steam engines resulted in meager progress, James Watt began experimenting.

In the process, he discovered that roughly 75% of thermal energy was spent heating the cylinder during each working cycle.

Thermal energy was wasted because cold water was injected later in the cycle to condense the steam, reducing pressure.

By repeatedly heating and cooling the cylinder, the steam engine had low efficiency because most of the thermal energy was spent on heating rather than converting into usable mechanical work.

He realized that the biggest drawback of Newcomen's steam engine was the amount of latent heat lost during the change of water's phase from liquid to steam.

Therefore, water condensation had to be separated from the steam phase in a separate chamber connected to the steam cylinder. James Watt aimed to improve the existing steam engine and came up with the idea of creating a separate condenser. The cylinder's dimensions were 127 cm, and the entire machine's height was 7 meters.

James Watt's Steam Engine

Image: James Watt's Steam Engine (Source)

Due to the constant threat of a steam boiler explosion, which was very primitively regulated, and frequent malfunctions such as leakage and scale buildup, James Watt restricted his machine to operate only using atmospheric-pressure steam.

In addition, he added numerous other improvements: a rotating shaft, a pressure-regulating valve, and a speed governor.

He obtained a patent in 1769 but did not make further efforts to commercialize his invention.

Now enters the scene Matthew Boulton, a wealthy manufacturer and owner of a factory producing fasteners, buckles, and simple tools.

The machines in his factory were powered by hydropower, which posed a particular problem during the summer when the river's water level decreased.

As a result, production had to adapt to the available energy supply.

Matthew Boulton realized that applying the steam engine for water pumping or powering machines would provide the necessary production capacity and continuous operation.

James Watt and Matthew Boulton became business partners in a joint venture, with Watt responsible for the technical aspects and steam engine development, and Boulton for finances.

They hired master founder John Wilkinson, who was highly successful in casting cannons, to make a steam cylinder.

It was essential for James Watt that the cylinder casting was free of impurities and porosity to achieve optimal heat transfer and minimize the risk of an explosion.

The machine was four times more powerful compared to Newcomen's.

The company bought machine parts from numerous suppliers, and then employees assembled steam engines under the supervision of engineers.

The profit was made based on a nearly 30% difference in the amount of coal used by the less efficient Newcomen steam engine compared to the more efficient Watt steam engine.

Buyers of steam engines were mine owners who needed to continually pump water and owners of textile manufacturing factories.

The versatility of steam engines proved so great that Matthew Boulton later used them to power coin minting machines.

In the period 1783-1800, over 500 steam engines were produced and used in Great Britain.

How do steam locomotives work, and what are their main components?

William Murdoch was an employee of the Watt-Boulton company, responsible for installing and repairing steam engines in factories and mines of customers.

Continuously working on steam engine mechanisms, he made certain improvements that so impressed James Watt and Matthew Boulton that they took him as a third partner in the company.

William Murdoch came up with the idea of using the steam engine for transportation and thus created the prototype of the first steam locomotive. Many others later improved and developed this invention.

During the 19th century, steam engines were widely used to power locomotives for transporting goods and passengers over longer distances.

Although steam locomotives are now museum pieces, they can still teach us and remind us of the basics of mechanical devices.

During a visit to the technical museum, I saw and studied one of the largest exhibits, a steam locomotive with an exposed steam boiler due to the removed casing and open cylinders and auxiliary systems.

This is a Prussian locomotive model S 10, which was produced between 1910 and 1914 when a total of 202 locomotives were built. The next image shows a locomotive model S with the outer casing removed

Screenshot 2024-02-03 at 20.08.35.png

Image: Steam locomotive with removed casing, cross-section of the steam engine (Source: Author's Archive)

Below the casing or steel cylindrical shell, you can see the cross-section of the steam boiler and pipes, the steam cylinder with piston, the front part of the boiler for exhaust gases, and the transmission mechanism.

In the next picture, you can see the locomotive from the other side with the complete casing, and part of the drive mechanism connected to three pairs of driven wheels.

Parna lokomotiva sa cjelovitom oplatom

Image: Steam locomotive with complete casing (Source: Author's Archive)
The steam locomotive model P8 was produced in 1919 for hauling freight and express passenger trains and was used until the 1970s.

In the following picture, the portal or cover of the P8 model locomotive is open.

The front part of the smoke gas boiler is visible, where the heat exchanger tubes are located.

Prednja strana lokomotive

Image: Front view of the locomotive (Source: Author's Archive)
Over the past 200 years, the principle of steam engine operation and energy conversion for locomotive propulsion has remained unchanged, although the locomotive's design has undergone numerous changes.

Every steam locomotive fundamentally consists of two parts: the steam boiler (firebox, tubes, preheater, valves) and the power unit (cylinders with pistons, crossheads, transmission mechanism, wheels).

The simplified cross-section of the locomotive and its main components are shown in the illustration.

The basic operation of every steam locomotive is for steam under pressure, ranging from 14 bar to 22 bar, to enter the cylinder space with a piston, expand, and push the piston.


Image: Parts of a steam locomotive (Source)

The piston connected to the crosshead translates the translational motion to the crosshead of the drive mechanism connected to the drive wheels.

This leads to the transfer of translational motion to rotational, or the locomotive's wheels start moving.

Steam expands in the cylinder until it reaches atmospheric pressure.

At the bottom of the steam boiler's firebox, there is a grate where fuel combustion occurs. Hot combustion gases rise to the upper part of the firebox, the combustion chamber.

Fireboxes where coal is burned have ash pans at the bottom to collect ash, which can be shaken down using a lever.

Smoke gases leave the combustion chamber with turbulent flow through a system of pipes in a water-filled boiler.

The heat of the smoke gases heats the water until it evaporates.

Steam is created, and under pressure, it rises towards the dome and then goes to the steam cylinders through a system of parallel pipes.

The steam quantity is regulated by a control valve located in the dome.

Another set of pipes transports the steam to a superheater, where the steam is further heated to a higher temperature before entering the steam cylinders.

Using superheated steam, as opposed to saturated steam, increases the efficiency of a steam locomotive's operation by 25% to 30% (refer to the Mollier h-s diagram for water vapor).

When we examine the boiler with its pipes more closely, does it remind you of the precursor to today's modern shell-and-tube heat exchangers?

Furthermore, the steam locomotive's boiler is essentially a pressure vessel that needs careful regulation.

Safety valves are designed and adjusted to automatically open and release steam if the pressure in the steam boiler exceeds the allowed limit.

The space above the firebox must always be filled with water. If the water level falls below the height of the firebox's crown sheet (right side of the image), the boiler overheats, and the risk of a boiler explosion increases.

Water level gauges or sight glasses are installed to monitor the water level.

Steam passes through steam pipes and enters the steam cylinders, where it expands and moves the pistons.

Once the steam has transferred its energy and done useful work by moving the pistons, the valve system allows the subcooled lower pressure steam to escape through an exhaust pipe located in the boiler filled with smoke gases.

The locomotive's wheels start moving as a result of the motion transfer from the pistons through the crosshead and crosshead guide, and the motion of the power mechanism is controlled by a lever located in the locomotive's cab at the rear.

The same lever is used to control the direction of locomotive movement, as well as to accelerate or decelerate.

Once the steam has driven the piston to the end of the cylinder, the crosshead, crosshead guide, and the connecting rod's pivot system convert the piston's translational motion (back-and-forth) into the circular rotation of the wheels.

Counterweights or counterbalance weights positioned at the opposite ends of the connecting rod and wheel joint allow for maintaining a balanced position during movement.

Early locomotives had one pair of driven wheels, and more complex mechanisms with a larger number of wheels were developed later, with the largest number of wheels driven from a single cylinder assembly being six (6) pairs.

Due to the large diameter and the need for flexibility in movement, numerous locomotives featured two steam engines and two sets of driven wheels.

The arrangement, number, and construction of wheels and the drive mechanism depended on the locomotive's purpose.

After the steam has transferred its energy in the cylinders, it exits through an exhaust pipe and mixes with smoke gases.

This creates a draft that further pulls the smoke gases through pipes into the front part of the boiler.

Fresh air enters the firebox through openings or ports on the front and bottom of the firebox.

The mixed exhaust steam and smoke gases exit through the locomotive's chimney with fast, turbulent flow.

The flow of the resulting mixture enters the chimney pipes, leading to a sudden decrease in speed, creating the well-known sound of the locomotive.

Since the amount of exhaust gas depends on the consumed steam leaving the cylinders, it must be anticipated when the engineer closes the valve.

For this purpose, a group of small nozzles, known as steam jets or blast pipes for directing steam, is placed at the front of the smoke gas boiler.

In the smoke gas boiler, partially burned coal particles from the firebox collect in the mixture of smoke gases.

When a sufficient amount of particles accumulates to impede gas flow, vortexing of the particles and their expulsion in the form of cinders occurs through the chimney, along with a mixture of steam and smoke gases.

Which parts of a steam locomotive are most susceptible to failures?

A steam boiler can explode if the steam exceeds the maximum allowed pressure.

Steam boilers were riveted together until the discovery of welding and its widespread use in joining metal materials.

Riveted joints have lower strength compared to well-executed welds, so as steam pressure increased, the load on riveted joints also increased.

Furthermore, pressure tests as a method of testing pressure vessels only became widespread in the 20th century, so it's questionable how thoroughly the original steam boilers were tested before being put into operation.

Fires in the firebox are another potential danger, occurring when overheating and a lack of water in the space above the firebox would take place.

Early locomotives had no fire protection systems.

Breakage of parts in the transmission mechanism could occur due to poor manufacturing or the use of low-quality materials for making the driving axles, crossheads, piston rods, or connecting rods.

Tube ruptures through which smoke gases pass occurred due to material fatigue in the tubes or the presence of excessively hot gases.

Failure of safety valves or loss of functionality is another potential failure, given that there were no legal regulations for regular service and testing of safety valves at that time.

Other potential problems encountered by locomotive engineers included reduced combustion efficiency due to poor-quality coal and lower efficiency, significant mechanical losses, heat losses due to inadequate steam utilization, insufficient steam superheating, excessively stiff or ruptured springs, water leakage from the steam boiler, clogged steam pipes, and reduced heat transfer due to scale buildup on pipe surfaces.


The invention of the steam engine was revolutionary for industrial production and led to fundamental changes in all aspects of society.

The steam locomotive is an excellent practical example of how the steam engine improved and developed the transportation of people and goods, with numerous mechanical mechanisms finding application in various industrial sectors.

Great Britain paid tribute to its two giants, Watt and Boulton, who laid the foundations of modern industry by featuring their portraits on the reverse of the £50 note.


Slika: Back of the £50 note with portraits of Matthew Boulton and James Watt (Source)

Underneath their portraits, the following quotes appear:

Sir, I sell here what all the world desires to have – POWER (Boulton, on the left side) I can think of nothing else but this MACHINE (Watt, on the right side)

The SI unit for power is named Watt in honor of James Watt and is defined as the work of 1 Joule done in 1 second. It is equivalent to 1/746 horsepower (hp) for determining mechanical and electrical power.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
November 2, 2023

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.