Car engines
by Chris Woodford. Last updated: March 11, 2013.
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?
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.
Photo: The restored (and nicely polished!) engine in a classic car from the early 1970s.
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.
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!
Photo: Petroleum can be extracted from the ground
by "nodding donkey" pumps like this one.
Picture courtesy of US Department of Energy.
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.
