You are here: Home page > Engineering > Car engines

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

Car engines

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.

Sponsored links


  1. What is a car?
  2. How do we get power from petroleum?
  3. What are the main parts of a car engine?
  4. How does a four-stroke engine make power?
  5. How many cylinders does an engine need?
  6. How big do the cylinders need to be?
  7. How can we make cleaner engines?
  8. Find out more

What is a car?

Inside a classic car engine

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. The challenge of building a car engine is to get as much energy out of each drop of fuel as possible—to make the car go as far and as fast as it can.

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 "crude oil", 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.

Nodding donkey oil pump

Photo: Petroleum can be extracted from the ground by "nodding donkey" pumps like this one. Photo by Carol M. Highsmith courtesy of The Lyda Hill Texas Collection of Photographs in Carol M. Highsmith's America Project, Library of Congress, Prints and Photographs Division.

Petroleum fuels are made from hydrocarbons: the molecules 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 apart!

Chart comparing typical fuel consumption in barrels per year of various cars.

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 model here burns through at least 6.1 barrels (256 US gallons) of petroleum in a year. Drawn using energy impact scores for 2024 models shown on the US Department of Energy's Fuel Economy website.

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.

Sponsored links

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 two valves (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 plug (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 crankshaft. The crankshaft powers the car's gearbox which, in turn, drives the wheels.

How does a four-stroke engine make power?

How the cylinder in a car engine makes power

Watch this animation and you'll see that a car engine makes its power by endlessly repeating a series of four steps (called strokes):

  1. Intake: The piston (green) is pulled down inside the cylinder (gray) by the momentum of the crankshaft (gray wheel at the bottom). Most of the time the car is moving along, so the crankshaft is always turning. The inlet valve (left) opens, letting a mixture of fuel and air (blue cloud) into the cylinder through the purple pipe.
  2. Compression: The inlet valve closes. The piston moves back up the cylinder and compresses (squeezes) the fuel-air mixture, which makes it much more flammable. When the piston reaches the top of the cylinder, the sparking plug (yellow) fires.
  3. Power: The spark ignites the fuel-air mixture causing a mini explosion. The fuel burns immediately, giving off hot gas that pushes the piston back down. The energy released by the fuel is now powering the crankshaft.
  4. Exhaust: The outlet valve (right) opens. As the crankshaft continues to turn, the piston is forced back up the cylinder for a second time. It forces the exhaust gases (produced when the fuel burned) out through the exhaust outlet (blue pipe).

The whole cycle then repeats itself.

How many cylinders does an engine need?

One problem with the four-stroke design is that the crankshaft is being powered 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.

Morris Minor 4 cylinder engineJaguar XJS V12 engine

Photo: More cylinders mean more power. 1) White: 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). 2) Red: 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). Apart from the number of cylinders, the engines also differ in their cylinder designs. The Morris engine is "undersquare," which means it has quite narrow (small-bore) cylinders that push the piston a long distance (stroke). The Jaguar engine is "oversquare," with wider cylinders whose pistons don't push so far. These terms are explained more fully below.

Sponsored links

How big do the cylinders need to be?

It's not just how many cylinders a car has that's important but how much power each one can make as it pushes out its piston. That depends on the size of the cylinder, which, in turn, depends on two key measurements: the diameter of the cylinder (called its bore) and how far the piston moves out (its stroke). The area of a circle is π × radius2, and since the bore is twice the radius, the useful volume of a car cylinder is (π/4) × bore × bore × stroke. In physics terms, the volume of the cylinder is related to how much work the fuel does as it expands, how much energy it transfers to the piston, and (if we consider how often this happens), how much power the car makes. So the bore and stroke are very important—and that's why they're often quoted in technical specifications for car engines along with the number of cylinders. You'll often see these measurements written in the form bore × stroke (so, for example, 90 × 86mm means a bore of 90mm and a stroke of 86mm).

How the bore, stroke, and displacement of car cylinder are measured

Artwork: How the bore, stroke, and displacement of one cylinder are measured. The bore is the diameter of the cylinder, the stroke is the distance the piston moves, and the displacement is the effective volume.

You'll also see the total volume of a car's cylinders quoted in a measurement called the displacement, which is the volume of a car's cylinders multiplied by how many of them there are. (In other words, it's π/4 × bore × bore × stroke × number of cylinders.) So when you hear a car described as having a "two-liter engine," that usually means it has four cylinders of 0.5 liters or six cylinders of 0.33 liters. The displacement is a rough guide to how much power a car engine can make and you'll usually see it quoted in either liters or cc (cubic centimeters); 1 liter is the same as 1000 cc.

Six coffee mugs lined up to represent the displacement of a two-liter car engine

Photo: Make your own two-liter engine! If you find it hard to visualize a two-liter engine, try this. An average coffee mug holds about 0.3 liters and is roughly the same dimensions as a typical car cylinder. Six of these mugs lined up give you a volume of about two liters—the total displacement of the engine in a large family saloon.

Typical bore and stroke sizes are 70–100mm (roughly 3–4 in). You might think making a more powerful engine is simply a matter of choosing a bigger bore and stroke, but there's much more to it than that, and there clearly have to be compromises (for example, you can't make small cars with enormous cylinders). In practice, the bore and stroke affect a number of different things, not just how powerful and efficient the engine is overall, but how much power it makes at different speeds: whether it's optimized for high power at high speed (as in a race car) or high power and fuel economy at lower speeds (as in a long-distance truck). If the bore and stroke measurement is more or less the same, the engine is described as square. A bigger bore and a shorter stroke gives us what's called an oversquare (short-stroke) engine. It has bigger valves for shifting more gas through the cylinders at higher speeds, so it can can make high power at higher rpm, and it's a good arrangement for a race car or a superbike (powerful motorbike). A smaller bore and a longer stroke, in what's called an undersquare (long-stroke) engine, gives us more power at lower revs, which is great for a slow-moving, heavy truck or a heavier motorbike.

All of this is a bit of a generalization, because it's easy to find examples of all types of cars that use square, oversquare, and undersquare engines, as the following table of engines past and present clearly shows.

Bore (mm) Stroke (mm) Disp (cc) Cyls
Subaru BRZ 86 86 1998 4
Nissan Qashqai 72.2 73.2 1199 4
Lincoln Town Car 90.2 90 4601 8
Bugatti Chiron 86 86 7993 16
Under square (long stroke)
Harley Davidson (bike) 99.9 111 1746 2
Morris Minor 57 90 918 4
Ford Model T 95 101 2878 4
Lamborghini Huracan 84.5 92.8 5204 10
Over square (short stroke)
Ducatti Panigale (bike) 116 60.8 1285 4
Saab 9000 90 78 1985 4
Land Rover Defender 88.9 71.1 3528 8
Jaguar XJS 90 70 5343 12

See how there are fast sports cars, everyday cars, and utility vehicles in all three categories? In other words, you can't draw simple conclusions from the size of a car's cylinders alone. A super-speedy Porsche 911 from the 1980s had cylinder measurements of 91mm × 76.4mm, but a sedate Saab 9000 from the same era used pretty much the same (90mm × 78mm). Unless you're designing car engines, you don't really need to worry about the detailed nitty-gritty. All you need to remember is the bottom line—the basic science from the law of conservation of energy: you can't get more energy out of a machine than you put into it. If you want to get more power from a car engine, you'll either need more cylinders or the same number of cylinders making more power (which you can achieve in various different ways according to when and how you want that power to be delivered).

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!

Energy density of petroleum fuels, wood, and batteries compared on a bar chart.

Chart: 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!

Pie chart showing how a car wastes up to 85 percent of its energy in drivetrain, parasitic, and engine losses.

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. [Last retrieved: October 2021.]

Sponsored links

Find out more

On this website

You might like these other articles on our site covering related topics:


For older readers

For younger readers


Please do NOT copy our articles onto blogs and other websites

Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.

Text copyright © Chris Woodford 2007, 2020. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Tell your friends

If you've enjoyed this website, please kindly tell your friends about us on your favorite social sites.

Press CTRL + D to bookmark this page for later, or email the link to a friend.

Cite this page

Woodford, Chris. (2007/2020) Car Engines. Retrieved from [Accessed (Insert date here)]


@misc{woodford_2FA, author = "Woodford, Chris", title = "Car Engines", publisher = "Explain that Stuff", year = "2007", url = "", urldate = "2024-02-26" }

Can't find what you want? Search our site below

More to explore on our website...

Back to top