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

Car engines

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by Chris Woodford. Last updated: March 13, 2017.

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?

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.

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. Picture courtesy of US Department of Energy.

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 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.

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.

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 (grey 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 engine Jaguar XJS V12 engine
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!

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.

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