How Steam Engines Work

By: Marshall Brain & Yara Simón  | 
The Jacobite Express crosses a curved viaduct at Scotland's Glenfinnan.
The Jacobite Express crosses a curved viaduct at Scotland's Glenfinnan. David Cation / Getty Images

Steam engines played a pivotal role in shaping the modern world. They were the first engine type to see widespread use. First invented by Thomas Newcomen in 1705, steam engines saw big improvements in 1769, courtesy of James Watt (who we remember each time we talk about "60-watt light bulbs").

Steam engines ­powered all early locomotives, steam boats and factories, and therefore, acted as the foundation of the Industrial Revolution.


Early Steam History

The origins of steam technology trace back to around A.D. 75, with Hero, a renowned mathematician, writing extensively about the mechanics and properties of air and presenting plans for a rudimentary steam engine. His design featured a hollow sphere with bent tubes protruding from either side. By filling the sphere with water and placing it above a fire, the heat would vaporize the water; the resulting steam would escape through the tubes, causing the sphere to rotate. This invention served as the basis for future developments in steam technology, but the contributions of many others made steam power a possibility.

In the 17th century, Italian scholar Giovanni Battista della Porta observed steam’s role in creating a vacuum. He theorized that when water converted to steam inside a closed container, it resulted in increased pressure, while the condensation of steam back into water would yield decreased pressure.


In 1679, French scientist Denis Papin transformed della Porta’s theory into reality through a project, a pressure cooker-like device called the “Digest or Engine for Softening Bones.” The sealed cooking pot became the first practical application of steam pressure. And Papin further developed the concept by using a sliding piston on top of a closed cylinder filled with water. As the water became hot, the steam expanded and pushed the piston upward. Upon cooling and condensation, the resulting vacuum drew the piston back down.

These early experiments and innovations marked significant milestones in the journey toward harnessing steam power and paved way for future inventors and engineers to refine and expand upon these concepts.

The Birth of Steam Engines

In 1698, military engineer Thomas Savery secured a patent for his innovative steam pump, dubbed “Miner’s Friend.” Savery’s steam pump comprised a boiling chamber that directed steam into a separate container. Inside, the container had a pipe equipped with a non-return valve extended into the water that required extraction. The cold water that was poured onto the steam-filled contained caused the steam to condense back into a liquid state. As this transformation occurred, a vacuum drew water from below through the pipe.

Drawing from Savery’s separation of the boiler and steam cylinder and Papin’s steam-driven piston, English inventor Thomas Newcomen was one of the key pioneers in the development of steam power. He introduced the atmospheric engine in the early 18th century. This practical steam engine pumped water out of mines and operated by using steam to create a partial vacuum in a cylinder, causing atmospheric pressure to push a piston downward.


James Watt further improved the design and efficiency of steam engines. In the late 18th century, he developed the Watt atmospheric engine, which incorporated a separate condenser and the ability to harness the expansive force of steam. His steam engine’s efficiency made them suitable for transportation, which helped connect distant regions and facilitated the movement of goods.

The birth of steam power and its subsequent advancements not only propelled industrialization but also transformed society. Steam power became the driving force behind the industrial revolution.

Steam Engine Operation

This diagram shows the major components of a piston steam engine. This sort of engine would be typical in a steam locomotive.
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The engine shown is a double-acting steam engine because the valve allows high-pressure steam to act alternately on both faces of the piston. The following animation shows the engine in action.

­You can see that the slide valve is in charge of letting the high-pressure steam into either side of the cylinder. The control rod for the valve is usually hooked into a linkage attached to the cross-head so that the motion of the cross-head slides the valve as well. (On a steam locomotive, this linkage also allows the engineer to put the train into reverse.)


You can see in this diagram that the exhaust steam simply vents out into the air. This fact explains two things about steam locomotives:

  • It explains why they have to take on water at the station — the water is constantly being lost through the steam exhaust.
  • It explains where the "choo-choo" sound comes from. When the valve opens the cylinder to release its steam exhaust, the steam escapes under a great deal of pressure and makes a "choo!" sound as it exits. When the train is first starting, the piston is moving very slowly, but then as the train starts rolling the piston gains speed. The effect of this is the "Choo ... choo ... choo ... choo choo-choo-choo" that we hear when it starts moving.

On a steam locomotive, the cross-head normally links to a drive rod, and from there to coupling rods that drive the locomotive's wheels. The cross-head is connected to a drive rod that connects to one of three drive wheels for the train. The three wheels are connected via coupling rods so they turn in unison.

Stationary Steam Engines

A stationary steam engine is a fixed engine not intended for mobility. Unlike steam engines used in locomotives or ships, stationary steam engines stay in place and power factories, mills, pumping stations or electricity generators. They typically consist of a large steam boiler, a reciprocating or rotary steam engine and varius auxiliary components to control and regular the steam flow and power output.


5 Types of Steam Engines

We have seen several types of steam-powered engines throughout history. Here are a few notable ones:

  1. Reciprocating steam engine: This common type of steam engine features the reciprocating motion of pistons. Reciprocating steam engines uses steam pressure to move a piston back and forth in a cylinder, converting linear motion into rotational motion.
  2. Vertical steam engine: In a vertical steam engine, the cylinder is in a vertical position and the pistons move up and down.
  3. Horizontal steam engine: These types of engine have a cylinder positioned horizontally. Here, the pistons move back and forth.
  4. Uniflow steam engine: These engines are characterized by steam flowing through the cylinder in a single direction.
  5. Condensing steam engine: This type uses a separate condenser to improve efficiency. This steam is condensed back into water after performing work in the cylinder, creating a vacuum that increases the engine’s power and efficiency.



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The high-pressure steam for a steam engine comes from a boiler. The boiler's job is to apply heat to water to create steam. There are two approaches: fire tube and water tube.

A fire-tube boiler was more common in the 1800s. It consists of a tank of water perforated with pipes. The hot gases from a coal or wood fire run through the pipes to heat the water in the tank.


In a fire-tube boiler, the entire tank is under pressure, so if the tank bursts it creates a major explosion.

More common today are water-tube boilers, in which water runs through a rack of tubes that are positioned in the hot gases from the fire. The following simplified diagram shows you a typical layout for a water-tube boiler:

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In a real boiler, things would be much more complicated because the goal of the boiler is to extract every possible bit of heat from the burning fuel to improve efficiency.

For more information on steam engines and all sorts of other engines, check out the links on the next page.