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!

What is a car?

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?

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?

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.

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!

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!