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Artwork illustrating the basic concept of a heat engine that converts heat energy into mechanical work.

Heat engines

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by Chris Woodford. Last updated: June 22, 2018.

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!

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).

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.)

There are two main types of heat engines: external combustion and internal combustion:

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.

External and internal combustion engines compared

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.

How does an engine power a machine?

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.

Planetary epicyclic gear. Foot-powered lathe.
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.

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

An Easton Amos gas pumping steam engine

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

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. Steam was pumped into the cylinder forcing the piston to rise and the beam to move down. Then water was squirted into the cylinder, cooling the steam, creating a partial vacuum, and making the beam tilt back the other way. Beam engines were an important technological advance, but they were much too large, slow, and inefficient to power factory machines and trains.

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.

Various engines on display at Birmingham Think Tank science museum in England.

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.

A red Jaguar XJS sports car with the bonnet/hood open

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.

Engines in theory

The steam engine Manston with tender

Photo: Steam engines are inherently inefficient. Carnot's work tells us that, for maximum efficiency, the steam in an engine like this needs to be superheated (so it's above its usual boiling point of 100°C) and then allowed to expand and cool down as much as possible in the cylinders so it gives up as much energy as it can to the pistons.

The pioneers of engines were engineers, not scientists. Newcomen and Watt were hands-on, practical "doers" rather than head-scratching, theoretical thinkers. It wasn't until Frenchman Nicolas Sadi Carnot (1796–1832) came along in 1824—well over a century after Newcomen built his first steam engine—that any attempt was made to understand the theory of how engines worked and how they could be improved from a truly scientific perspective. Carnot was interested in figuring out how engines could be made more efficient (in other words, how more energy could be obtained from the same amount of fuel). Instead of tinkering with a real steam engine and trying to improve it by trial and error (the kind of approach Watt had taken with Newcomen's engine), he made himself a theoretical engine—on paper—and played around with math instead.

The Carnot cycle

The Carnot heat engine is a fairly simple mathematical model of how the best possible piston and cylinder engine could operate in theory, by endlessly repeating four steps now called the Carnot cycle. We're not going to go into the detailed theory here, or the math (if you're interested, see NASA's Carnot Cycle page and the excellent Heat Engines: the Carnot Cycle page by Michael Fowler, which has a superb flash animation).

A basic Carnot engine consists of a gas trapped in a cylinder with a piston. The gas takes energy from a heat source, expands, cools, and pushes a piston out. As the piston returns into the cylinder, it compresses and heats the gas, so the gas ends the cycle at exactly the same pressure, volume, and temperature it started off with. A Carnot engine doesn't lose any energy to friction or its surroundings. It's completely reversible—a theoretically perfect and perfectly theoretical model of how engines work. But it tells us a lot about real engines too.

How efficient is an engine?

What is worth noting is the conclusion Carnot reached: the efficiency of an engine (real or theoretical) depends on the maximum and minimum temperatures between which it operates. In math terms, the efficiency of a Carnot engine operating between Tmax (its maximum temperature) and Tmin (its minimum temperature) is:

(Tmax−Tmin) / Tmax

where both temperatures are measured in kelvin (K). Making the temperature of the fluid inside the cylinder higher at the start of the cycle makes it more efficient; making the temperature lower at the opposite extreme of the cycle also makes it more efficient. In other words, a really efficient heat engine operates between the greatest possible temperature difference. In other words, we want Tmax to be as high as possible and Tmin as low as possible. That's why things like steam turbines in power plants have to use cooling towers to cool their steam down as much as possible: that's how they can get the most energy from the steam and produce the most electricity. In the real world, moving vehicles like cars and planes obviously can't have anything like cooling towers, and it's hard to achieve low Tmin temperatures, so raising Tmax is the thing we usually focus on there instead. Real engines—in cars, trucks, jet planes, and space rockets—work at enormously high temperatures (so they have to be built from high-temperature materials such as alloys and ceramics).

What's the maximum efficiency of an engine?

Is there a limit to how efficient a heat engine can be? Yes! Tmin can never be less than zero (at absolute zero), so, according to our equation above, no engine can be more efficient than Tmax/Tmax = 1, which is the same as 100 percent efficient—and most real engines don't get anywhere near that. If you had a steam engine operating between 50°C and 100°C, it would be about 13 percent efficient. To get that to 100 percent efficiency, you'd have to cool your steam to absolute zero (−273°C or 0K), which is obviously impossible. Even if you could cool it to freezing (0°C or 273K), you would still only manage 27 percent efficiency.

Line chart showing how the efficiency of a steam engine or turbine increases when it operates at higher temperatures.

Chart: Heat engines are more efficient when they work between bigger temperature differences. Assuming a constant icy minimum temperature (0°C or 273K), the efficiency slowly climbs as we raise the maximum temperature. But note that we're getting diminishing returns: for each 50°C temperaure rise, the efficiency is rising less each time. In other words, we can never get to 100 percent efficient just by raising the maximum temperature.

This also helps us understand why later steam engines (pioneered by engineers such as Richard Trevithick and Oliver Evans) used much higher pressures of steam than the ones produced by people like Thomas Newcomen. Higher-pressure engines were smaller, lighter, and easier to mount on moving vehicles, but they were also much more efficient: at higher pressures, water boils at higher temperatures, and that gives us greater efficiency. At twice atmospheric pressure, water boils at about 120°C (393K), giving an efficiency of 30 percent with a minimum temperature of 0°C; at four times atmospheric pressure, the boiling temperature is 143°C (417K), and the efficiency close to 35 percent. That's a big improvement, but still a long way from 100 percent. Steam turbines in power plants use really high pressures (over 200 times atmospheric pressure is typical). At 200 atmospheres, water boils at about 365°C (~640K), giving a maximum, theoretical efficiency of about 56 percent if we can also cool the water right down to freezing (and if there are no other heat losses or inefficiencies). Even under those extreme and ideal conditions, we're still a very long way from 100 percent efficient; real turbines are more likely to achieve 35–45 percent. Making efficient heat engines is much harder than it looks!

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