by Chris Woodford. Last updated: September 23, 2019.
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
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
- 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.
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.
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).
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.
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.
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.
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
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.
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!
Find out more
On this website
On other sites
One of the best ways to understand engines is to watch animations of them at work.
Here are two good very sites that explore a wide variety of different engines:
- Animated engines: This great site covers virtually every kind of engine you can think of with easy to understand animations and very clear written descriptions.
- See the engines at work: A collection of very nicely drawn animations of real-life engines from the London Science Museum. (Archived via the Wayback Machine.)
- Six Easy Pieces by Richard P. Feynman. Penguin, 1998. Chapter 4 is a very original explanation of the conservation of energy, including quite a simple account of why no engine or machine is more efficient than a perfectly reversible (ideal) one.
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