Electric cars
by Chris Woodford. Last updated: August 16, 2011.
Count the cars on Earth and you'll
eventually reach a number
bigger than 500 million! Cars will never outnumber people—there
are over 6 billion (6000 million) of us on the planet—but
in some places, at least, the numbers are getting close. In the
United States, the world's most car-crazy nation, there are now about
500 cars for every 1000 people.
Cars give us a tremendous amount of freedom, but they bring problems too. Since they burn fossil fuels like oil, they make smog that chokes our cities and the carbon dioxide their engines produce is a major contributor to global warming. Sometime in the not-too-distant future, all this will change. Most people will have fuel-efficient electric cars that are cleaner and quieter than the gas guzzlers they drive today. Let's get ready for the future—and take a closer look at how electric cars work!
Photo: The energy that an electric car uses has
to come from somewhere—and it isn't necessarily "clean and green": it might be produced in a conventional
power plant from coal, oil, or natural gas.
It's far better for the environment to generate electricity for cars from sunlight using solar panels,
like those shown in the background, but even electric cars fueled by coal are cleaner
than cars running on petrol. Picture by Mike Linenberger,
courtesy of US National
Renewable Energy Laboratory/Department of Energy (NREL/DoE).
What is a car?
Photo: Right: Electric future or electric past? The GM Sunraycer experimental solar car looks amazingly futuristic—but is actually quite old now. The whole of the back section (everything to the right of the white cockpit) is covered in thousands of silicon and gallium arsenide solar cells, while the driver sits in the pod at the front. Sunraycer won the 2000-mile World Solar Challenge race in the late 1980s. Picture by Warren Gretz courtesy of US DOE/NREL.
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 turns the energy locked in a fuel like gasoline (petrol) or diesel into mechanical energy: 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.
Cars convert energy in their engines, which usually have anything from two to twelve fuel-burning compartments called cylinders. Inside the cylinders, gasoline mixes with air, burns, and gives off hot exhaust gas. This pushes a piston down the cylinder that turns turns a rotating axle called a crankshaft. A car's pistons push up and down its cylinders hundreds of times each minute. With the help of a gearbox, which helps the engine to produce more force (for going uphill) or more speed (for driving on the straight), the crankshaft drives the wheels.
Now you might be thinking "Hang on! A gasoline engine has a battery in it, so doesn't it use some electricity?" Yes and no! Although electricity is used to start the engine initially (through the battery) and electric sparking plugs ignite the fuel, a gasoline engine is essentially a chemical and mechanical way of producing power—electricity plays only a small part in how it works.
In summary, then, a traditional energy-converting, gasoline-fueled car has four interesting parts:
- The fuel itself (the gas or diesel). This is the source of all the car's energy.
- Something that stores the fuel (the gas tank).
- An engine that converts the fuel into mechanical power (the cylinders and pistons).
- A transmission system that takes the mechanical power from the engine and uses it to drive the car along (the crankshaft, gearbox, wheels).
How is an electric car different to a normal one?
In an electric car, these things work differently. First, and most obviously, an electric car is powered by electricity so there is no gas tank. Instead, there are usually several large batteries that store the power. The batteries are charged up overnight on a domestic electricity outlet (mains power socket) and then gradually run down as the car drives along. Instead of an engine, electric cars have one or more electric motors to drive the gearbox and wheels. An electric motor is a tightly wrapped coil of wire that can spin around freely inside an outer casing of powerful magnets. When electricity is fed into the coil, it generates a magnetic field that makes the coil rotate very quickly inside the magnets. The spinning coil is fastened to an axle (central shaft) that can be used to drive the car's wheels. Some cars have four electric motors, one built into each wheel; these are known as hub motors.
Electric cars don't all work the same way. Some have fuel cells instead of batteries. These are a bit like batteries that never run down. They take in hydrogen gas, from a tank, and convert it into a steady supply of electricity. Solar cars work differently again: they have large solar panels on their roofs or bodies and use this to charge up their batteries as they go along.
Photo: You don't have to pump gasoline into electric cars. Instead, you plug them into the wall to recharge their batteries! Picture by Keith Wipke, courtesy of US National Renewable Energy Laboratory/Department of Energy (NREL/DoE).
What are hybrid cars?
Hybrid cars combine the best of both worlds. They have a small gasoline engine, just like the one in a normal car. This is typically used to drive the car at high speeds on freeways (motorways) or open country roads. They also have a small electric motor and batteries, which drives the car quietly, cleanly, and efficiently in cities. On a city street, the batteries power the electric motor and drive the car. But when the car is freewheeling downhill or braking, the opposite happens: the wheels, turned by the car's momentum, power the electric motor and generate electricity, which helps to recharge the batteries. This is known as regenerative braking and it saves a great deal of energy that would otherwise be wasted. That's one of the things that makes hybrid cars, such as the Toyota Prius, extremely energy efficient. Plug-in hybrids allow you to recharge their batteries by plugging them into one of your household electricity outlets (typically overnight).
Photo: It's not rocket science—or is it? Left: NASA had to use electric car technology to drive its Apollo Lunar Roving Vehicle (sometimes called the Moon Rover) because there's no air on the Moon to power an internal combustion engine. The Lunar Rover was driven by four electric motors, one for each wheel, all powered by two 36-volt batteries. It's pictured here in 1972, during the Apollo 17 mission, being driven by astronaut Gene Cernan. Read more about the technology inside the Apollo Lunar Roving Vehicle. Picture courtesy of Great Images in NASA.
Right: Batteries have always been the problem in making successful electric cars because they carry energy less effectively than gasoline. Here, gigantic nickel-zinc batteries are being loaded inside a prototype electric car by NASA engineers in 1977. Picture courtesy of NASA Glenn Research Center (NASA-GRC).
