Engines
Last updated: August 27, 2009.
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 tunnels and bridges 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 fuelled 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: The engine hall at Think Tank (the science museum in Birmingham, England) is an amazing collection of 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!
What is an 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:
- 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 some
distance 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.
Photo: A steam engine's cylinder and piston.
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 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 an epicyclic
gear (or planetary or
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: Left: A planetary gear. When the piston moves up and down, the gears go round and round. Right: 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
There are half-a-dozen or so main types of engines that make power by burning fuel:
External combustion 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, the science museum in Birmingham, England.
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—eventually—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.
Photo: Stirling engines don't necessarily involve combustion,
though they're always powered by an external heat source. Here the heat is coming from
a giant array of mirrors that gather solar energy and
feed it to the engine. Photo by Warren Gretz courtesy of US DOE/NREL
(US Department of Energy/National Renewable Energy Laboratory).
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. A hi-res version of this image is available from our photo library.
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.
Today, diesel engines are still the machines of choice for driving
such things 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 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 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 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).
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.
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 end of the cycle
also makes it more efficient. In other words, a really efficient heat engine operates between
the greatest possible temperature difference.
That's why real engines—in cars, trucks, jet planes, and space rockets—work
at such enormously high temperatures (and why they have to be built from high-temperature
materials such as alloys and ceramics). It's also
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.
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. This engine is Bulleid Pacific No. 34070 "Manston" on the Swanage Railway.
