Electric cars
Last updated: March 21, 2010.
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 take a closer look at how
electric cars work.
Photo: The energy that an electric car uses has
to come from somewhere. It might be produced in a conventional power plant,
but it's far better for the environment to generate electricity from
sunlight using solar panels
like those shown in the background. Picture by Mike Linenberger,
courtesy of US National
Renewable Energy Laboratory/Department of Energy (NREL/DoE).
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 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.
Photo: The large, 12-cylinder gasoline engine in this Jaguar sports car is effectively
a compact chemical plant. Although electricity is used to start the engine (through
the battery) and electric sparking plugs ignite the fuel, what we have here is essentially
a mechanical way of producing power—electricity plays a relatively small part in
conventional engines like this.
Simply speaking, then, our energy-converting 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).
Photo: The past and future of electric vehicles.
Left: Electric past: An electric milk float from the 1960s. This one, now "retired," is pictured at a classic car
rally, but you still sometimes see vehicles like this on the road.
Right: Electric future? The GM Sunraycer experimental solar car. The back end 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.
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 a wheel.
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 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).
Hybrid cars combine the best of both worlds. They have a small
petrol 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.
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).
