by Chris Woodford. Last updated: October 4, 2016.
Think back 100 years to a world where
people generally got around by
walking or riding horses. What changed things? The invention of the car.
Wheels may be 5500 years old, but the cars we
drive round in today made their debut only in 1885. That was
when German engineer Karl Benz (1844–1929) fastened a small gasoline
(petrol) engine to a three-wheeled cart and made the first primitive,
gas-powered car. Although Benz developed the automobile, another German
engineer, Nikolaus Otto (1832–1891), was arguably even more
important—for he was the man who'd invented the gasoline engine in the
first place, about two decades earlier. It's a testament to Otto's
genius that virtually every car engine made ever since has been
inspired by his "four-stroke" design. Let's take a look at how it works!
Photo: Car engines turn energy locked in liquid fuel into
heat and kinetic energy.
They're full of pipes and cylinders because they work like mini chemical plants.
This is the powerful V12 engine on a gloriously restored Jaguar XJS sports car from the late 1970s.
What is a car?
Photo: The restored (and nicely polished!) engine in a classic car from the early 1970s.
That's not quite such an obvious question as it seems. A car is a
metal box with wheels at the corners that gets you from A to B, yes,
but it's more than that. In scientific terms, a car is an
energy converter: a machine that releases the energy locked in a fuel like
gasoline (petrol) or diesel and turns it into mechanical energy in
moving wheels and gears. When the wheels power the car, the
mechanical energy becomes kinetic energy: the energy that the
car and its occupants have as they go along.
How do we get power from petroleum?
Cars, trucks, trains, ships, and planes—all these things are powered
by fuels made from petroleum. Also known as
petroleum is the thick, black, energy-rich liquid buried deep
underground that became the world's most important source of energy
during the 20th century. After being pumped to the surface,
petroleum is shipped or piped to a refinery and separated into
gasoline, kerosene, and diesel fuels, and a whole host of other
petrochemicals—used to make everything from paints to plastics.
Photo: Petroleum can be extracted from the ground
by "nodding donkey" pumps like this one.
Picture courtesy of US Department of Energy.
Petroleum fuels are made from hydrocarbons:
inside consist mostly of carbon and hydrogen atoms (with a fewer other
elements, such as oxygen, attached for good measure).
Wood, paper, and
coal also contain hydrocarbons. We can turn hydrocarbons into useful
energy simply by burning them. When you burn hydrocarbons in air, their
molecules split apart. The carbon and hydrogen combine with oxygen from
the air to make carbon dioxide gas and water, while the energy that
held the molecules together is released as heat. This process, which is
called combustion, releases huge amounts of
energy. When you sit round
a camp fire, warming yourself near the flames, you're really soaking up
energy produced by billions of molecules cracking open and splitting
People have been burning hydrocarbons to make energy for over a
million years—that's why fire was invented. But ordinary fires are
usually quite inefficient. When you cook sausages on a camp fire, you
waste a huge amount of energy. Heat shoots off in all directions;
hardly any goes into the cooking pot—and even less into the food. Car
engines are much more efficient: they waste less energy and put more of
it to work. What's so clever about them is that they burn fuel in
closed containers, capturing most of the heat energy the fuel releases,
and turning it into mechanical energy that can drive the car along.
Photo: Why does the world use so much oil? There are now about a billion petroleum-powered
cars on the planet and, as this chart shows, even the most energy-efficient models burn through at least 10 barrels
(420 gallons) of petroleum in a year. Drawn using energy impact scores for 2016 models shown on the US Department of Energy's Fuel Economy website.
What are the main parts of a car engine?
Car engines are built around a set of "cooking pots" called cylinders
(usually anything from two to twelve of them, but typically four, six,
or eight) inside which the fuel burns. The cylinders are made of
super-strong metal and sealed shut, but at one end they open and close
like bicycle pumps: they have
tight-fitting pistons (plungers) that can
slide up and down inside them. At the top of each cylinder, there are
(essentially "gates" letting things in or out that can be opened and
closed very quickly). The inlet valve allows
fuel and air to enter the cylinder from a
carburetor or electronic fuel-injector; the outlet valve lets
the exhaust gases
escape. At the top of the cylinder, there is also a sparking
(or spark plug), an electrically controlled device that makes a spark
to set fire to the fuel. At the bottom of the cylinder, the piston is
attached to a constantly turning axle called a
The crankshaft powers the car's gearbox which, in turn, drives the wheels.
How many cylinders does an engine need?
One problem with the four-stroke design is that the crankshaft is being
by the cylinder for only one stage out of four. That's why cars
typically have at least four cylinders, arranged so they fire out of
step with one another. At any moment, one cylinder is always going
through each one of the four stages—so there is always one cylinder
powering the crankshaft and there's no loss of power. With a
12-cylinder engine, there are at least
three cylinders powering the crankshaft at any time—and that's why
those engines are used in fast and powerful cars.
Photo: More cylinders mean more power. Left: A 4-cylinder, 48hp
Morris Minor engine from the 1960s. This engine is so incredibly tiny, it really looks like there's something missing—but it
can still manage a top speed close to 125 km/h (80mph). Right: A huge V12
Jaguar XJS sports car engine from the mid/late 1970s gives a top speed of about 240 km/h (140 mph). It's something like 300hp (about six times more powerful than the Morris engine).
How can we make cleaner engines?
There's no doubt that Otto's gasoline engine was an invention of
genius—but it's now
a victim of its own success. With around a billion cars on the
planet, the pollution produced
by vehicles is a serious—and still growing—problem. The carbon dioxide
released when fuels are burned is also a major cause of global warming.
The solution could be electric cars that get their
energy from cleaner sources of power or hybrid cars that use a combination of
electricity and gasoline power.
So why do we still use gasoline?
There's a very good reason why the overwhelming majority of cars, trucks, and other vehicles on the
planet are still powered by oil-based fuels such as gasoline and diesel: as the chart here shows very clearly, they pack more energy into each kilogram (or liter) than virtually any other substance. Batteries sound great in theory,
but kilogram for kilogram, petroleum fuels carry much more energy!
Chart: Left: Why we still use petroleum-based fuels: a kilogram of gasoline, diesel, or kerosene contains about 100 times as much energy as a kilogram of batteries. Scientists say it has a higher "energy density" (packs more energy per unit volume); in simple terms, it takes you further down the road.
That's not to say that cars (and their engines) are perfect—or anything like. There are lots of steps and stages in between the cylinders (where energy is released) and the wheels (where power is applied to the road) and, at each stage, some energy is wasted. For that reason, in the worst cases, as little as 15 percent or so of the energy that was originally in the fuel you burn actually moves you down the road. Or, to put it another way, for every dollar you put in your gas tank, 85 cents are wasted in various ways!
Chart: Cars waste most of the energy we feed them in fuel. Left: In stop-start city driving, only about 17 percent of the energy in gasoline (green slice) provides useful power to move you down the road. The other 83 percent is wasted (red slices) in the engine, in parasitic losses (in things like the alternator, which makes electricity), and in the drivetrain (between the engine and the wheels). Right: Things are a bit better on the highway, where useful power can nudge up to 25 percent or slightly more. Even so,
the bulk of the energy is still wasted.
Source: Fuel Economy: Where the Energy Goes, US Department of Energy Office of Energy Efficiency & Renewable Energy.
Find out more
On this website
You might like these other articles on our site covering related topics:
For older readers
- American Horsepower by Mike Mueller. MotorBooks International, 2006. Great photos and detailed descriptions of some of the greatest car engines of the last 100 years.
- Legendary Car Engines by John Simister and Tim Andrew. MotorBooks International, 2004. A detailed look at 20 classic engines.
- The People's Tycoon: Henry Ford and the American Century by Steven Watts. Vintage, 2006. A huge and wonderfully readable account of the world's greatest car maker (and his often contrary thoughts and ways).
For younger readers
- Car Science by Richard Hammond. Dorling Kindersley, 2007. Cars can teach you about science and science can teach you about cars! I was one of the consultants and contributors to this very colorful, lavishly illustrated book. Most suitable for ages 9–12—and a great read for car-mad "youngsters" of all ages.
- Eyewitness Car by Richard Sutton. Dorling Kindersley, 2005. A classic DK Eyewitness book that combines science, technology, and history. Various different editions available since the 1990s. Also for ages 9–12.
- Chinese Automaker Adopts New Efficient Engine Design by Martin LaMonica. IEEE Spectrum. March 5, 2014. Fewer cylinders, extra pistons, electric clutches—there are many ways to improve the efficiency of traditional internal combustion engines.
- The lab pushing petrol car engines to new extremes by Jon Stewart. BBC News, October 2, 2013. How Argonne National Laboratory is pushing traditional technology to make cleaner, more efficient engines.
- Packing 123 Horsepower Into 3 Cylinders by Lindsay Brooke. The New York Times. March 8, 2013. How and why auto makers have developed a new generation of "triples" (three-cylinder engines).
- Smaller engines drive petrol revival
by Jorn Madslien. BBC News, May 20, 2012. Small-engined gasoline cars are achieving fuel savings of 15–20 percent.
- One Step Closer to the No-Iron Car by Don Sherman. The New York Times. October 22, 2009. Are plastics and composites the future for lighter, more fuel efficient engines?
- Tomorrow's world: future petrol engine tech news by Tim Pollard. Car Magazine, September 28, 2009. Downsizing, turbocharging, and other key trends in gas engine technology explored.