Heat engines
by Chris Woodford. Last updated: March 16, 2022.
In our age of fuel cells and electric cars, steam locomotives (and even gasoline-powered cars) might seem like horribly old technology. But take a broader view of history and you'll see that even the oldest steam engine is a very modern invention indeed. Humans have been using tools to multiply their muscle power for something like 2.5 million years, but only in the last 300 years or so have we perfected the art of making "muscles"— engine-powered machines—that work all by themselves. Put it another way: humans have been without engines for over 99.9 percent of our existence on Earth!
Now we have engines, of course, we couldn't possibly do without them. Who could imagine life without cars, trucks, ships, or planes—all of them propelled by powerful engines. And engines don't just move us around the world, they help us radically reshape it. From bridges and tunnels to skyscrapers and dams, virtually every major building and structure people have made in the last couple of centuries has been built with the help of engines—cranes, diggers, dumper trucks, and bulldozers among them. Engines have also fueled the modern agricultural revolution: a vast proportion of all our food is now harvested or transported using engine power. Engines don't make the world go round, but they're involved in virtually everything else that happens on our planet. Let's take a closer look at what they are and how they work!
Photo: A steam locomotive "in steam" (actually working) at the Mid-Continent Railway Museum in North Freedom, Wisconsin. Photo courtesy of the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
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Contents
What is a heat engine?
An engine is a machine that turns the energy locked in fuel into force and motion. Coal is no obvious use to anyone: it's dirty, old, rocky stuff buried underground. Burn it in an engine, however, and you can release the energy it contains to power factory machines, cars, boats, or locomotives. The same is true of other fuels such as natural gas, gasoline, wood, and peat. Since engines work by burning fuels to release heat, they're sometimes called heat engines. The process of burning fuel involves a chemical reaction called combustion where the fuel burns in oxygen in the air to make carbon dioxide and steam. (Generally, engines make air pollution as well because the fuel isn't always 100 percent pure and doesn't burn perfectly cleanly.)
“Everyone knows that heat can produce motion. That it possesses vast motive-power no one can doubt...”
Nicolas Sadi Carnot, 1824
There are two main types of heat engines: external combustion and internal combustion:
- In an external combustion engine, the fuel burns outside and away from the main bit of the engine where the force and motion are produced. A steam engine is a good example: there's a coal fire at one end that heats water to make steam. The steam is piped into a strong metal cylinder where it moves a tight-fitting plunger called a piston back and forth. The moving piston powers whatever the engine is attached to (maybe a factory machine or the wheels of a locomotive). This is an external combustion engine because the coal is burning outside and some distance from the cylinder and piston.
- In an internal combustion engine, the fuel burns inside the cylinder. In a typical car engine, for example, there are something like four to six separate cylinders inside which gasoline is constantly burning with oxygen to release heat energy. The cylinders "fire" alternately to ensure the engine produces a steady supply of power that drives the car's wheels.
Internal combustion engines are generally far more efficient than external combustion engines because no energy is wasted transmitting heat from a fire and boiler to the cylinder; everything happens in one place.
Artwork: In an external combustion engine (such as a steam engine), fuel burns outside the cylinder and the heat (typically in the form of hot steam) has to be piped some distance. In an internal combustion engine (such as a car engine), the fuel burns right inside the cylinders, which is much more efficient.
Photo: A steam engine is an external combustion engine because the coal burns in the firebox (where the driver stands) some distance from the cylinder where the actual power is produced.
How does an engine power a machine?
Artwork: The basic concept of a heat engine: a machine that converts heat energy into work by shuttling back and forth between a high temperature and a lower one. A typical heat engine is powered by burning fuel (lower left) and uses an expanding-contracting piston (upper center) to carry the fuel's energy to a spinning wheel (lower right).
Engines use pistons and cylinders, so the power they produce is a continual back-and-forth, push-and-pull, or reciprocating motion. Trouble is, many machines (and virtually all vehicles) rely on wheels that turn round and round—in other words, rotational motion. There are various different ways of turning reciprocating motion into rotational motion (or vice-versa). If you've ever watched a steam engine chuffing along, you'll have noticed how the wheels are driven by a crank and connecting rod: a simple lever-linkage that connects one side of a wheel to a piston so the wheel turns around as the piston pumps back and forth.
An alternative way to convert reciprocating into rotational motion is to use gears. This is what brilliant Scottish engineer James Watt (1736–1819) decided to do in 1781 when he discovered the crank mechanism he needed to use in his improved design of steam engine was, in fact, already protected by a patent. Watt's design is known as a sun and planet gear) and consists of two or more gear wheels, one of which (the planet) is pushed up and down by the piston rod, moving around the other gear (the Sun), and causing it to rotate.
Photo: Two ways of converting reciprocating motion into rotary motion: First photo: A sun and planet gear. When the piston moves up and down, the gears go round and round. Second photo: The problem of converting up-and-down to round-and-round motion is simply solved in this foot-powered lathe. When you press up and down on the treadle (the foot peddle), you make the string rise and fall. That makes the shaft the string is attached to rotate at speed, powering the lathe and a drill or other tool attached to it. Both photos taken at Think Tank, the science museum in Birmingham, England.
Some engines and machines need to turn rotary motion into reciprocating motion. For that, you need something that works in the opposite way to a crankshaft—namely a cam. A cam is a non-circular (typically egg-shaped) wheel, which has something like a bar resting on top of it. As the axle turns the wheel, the wheel makes the bar rise up and down. Can't picture that? Try imagining a car whose wheels are egg-shaped. As it drives along, the wheels (cams) turn round as usual but the car body bounces up and down at the same time—so rotational motion produces reciprocating motion (bouncing) in the passengers!
Cams are at work in all kinds of machines. There's a cam in an electric toothbrush that makes the brush move back and forth as an electric motor inside spins around.
Types of engines
Photo: External combustion: This stationary steam engine was used to pump natural gas to people's homes from 1864. Photo taken at Think Tank.
There are half-a-dozen or so main types of engines that make power by burning fuel:
External combustion engines
Beam engines (atmospheric engines)
The earliest steam engines were giant machines that filled entire buildings and they were typically used for pumping water from flooded mines. Pioneered by Englishman Thomas Newcomen (1663/4–1729) in the early 18th century, they had a single cylinder and a piston attached to a large beam that rocked back and forth. The heavy beam was normally tilted down so that the piston was high in the cylinder. Steam was pumped into the cylinder, then water was squirted in afterward, cooling the steam, creating a partial vacuum, and making the beam tilt back the other way, before the process was repeated. Beam engines were an important technological advance, but they were much too large, slow, and inefficient to power factory machines and trains.
Artwork: How an atmospheric (beam) engine works (simplified). The engine consists of a heavy beam (gray), mounted on a tower (black), which can swing up and down. Normally the beam is tilted down and to the right under the weight of the pump equipment attached to it. A hot-water boiler (1) fires steam (2) up into the cylinder (3). When the cylinder is full, cold water is squirted in from a tank (4). This condenses the steam, creating lower pressure in the cylinder. Because the atmospheric pressure (air) above the piston is higher than the pressure below it, the piston is pushed down, the whole beam tilts to the left, and the pump is pulled upward, drawing water out from the mine (5).
Steam engines
In the 1760s, James Watt greatly improved Newcomen's steam engine, making it smaller, more efficient, and more powerful—and effectively turning steam engines into more practical and affordable machines. Watt's work led to stationary steam engines that could be used in factories and compact, moving engines that could power steam locomotives. Read more in our article on steam engines.
Stirling engines
Not all external combustion engines are huge and inefficient. Scottish clergyman Robert Stirling (1790–1878) invented a very clever engine that has two cylinders with pistons powering two cranks driving a single wheel. One cylinder is kept permanently hot (heated by an external energy source that can be anything from a coal fire to a geothermal energy supply) while the other is kept permanently cold. The engine works by shuttling the same volume of gas (permanently sealed inside the engine) back and forth between the cylinders through a device called a regenerator, which helps to retain energy and greatly increases the engine's efficiency. Stirling engines don't necessarily involve combustion, though they're always powered by an external heat source. Find out more in our main article on Stirling engines.
Photo: The engine hall at Think Tank (the science museum in Birmingham, England) is an amazing collection of energetic machines dating back to the 18th century. Exhibits include the enormous Smethwick steam engine, the oldest working engine in the world. It's not shown in this picture, largely because it was too big to photograph!
Internal combustion engines
Gasoline (petrol) engines
In the mid-19th century, several European engineers including Frenchman Joseph Étienne Lenoir (1822–1900) and German Nikolaus Otto (1832–1891) perfected internal combustion engines that burned gasoline. It was a short step for Karl Benz (1844–1929) to hook up one of these engines to a three-wheeled carriage and make the world's first gas-powered automobile. Read more in our article on car engines.
Photo: A powerful gasoline-powered, internal-combustion engine from a Jaguar sports car.
Diesel engines
Later in the 19th century, another German engineer, Rudolf Diesel (1858–1913), realized he could make a much more powerful internal combustion engine that could run off all kinds of different fuels. Unlike gasoline engines, diesel engines compress fuel much more so it spontaneously bursts into flames and releases the heat energy locked inside it. Today, diesel engines are still the machines of choice for driving heavy vehicles such as trucks, ships, and construction machines, as well as many cars. Read more in our article on diesel engines.
Rotary engines
One of the drawbacks of internal combustion engines is that they need cylinders, pistons, and a spinning crankshaft to harness their power: the cylinders are stationary while the pistons and crankshaft are constantly moving. A rotary engine is a radically different design of internal combustion engine in which the "cylinders" (which aren't always cylinder shaped) rotate around what is effectively a stationary crankshaft. Although rotary engines date back to the 19th century, perhaps the best-known design is the relatively modern Wankel rotary engine, notably used in some Japanese Mazda cars. Wikipedia's article on the Wankel rotary engine is a good introduction with a brilliant little animation.
