Electric Car

I

Introduction

Electric Car, automobile propelled by one or more electric motors, drawing power from an onboard source of electricity. Electric cars are mechanically simpler and more durable than gasoline-powered cars. They produce less pollution than do gasoline-powered cars.

II

How Electric Cars Work

An electric car stores its energy on board—typically in batteries, but alternatively with capacitors or flywheel storage devices. Or it may generate energy using a fuel cell or generator. A fuel cell is a specialized form of battery that combines hydrogen with oxygen in a chemical reaction that produces electricity and water vapor. Unlike an electric cell or battery, a fuel cell does not run down or require recharging; it operates as long as the fuel and an oxidizer are supplied continuously from outside the cell. Most current versions of electric cars use some combination of these energy sources. “Pure” electric cars, however, run only on batteries and need a charger to replenish the battery's power from an electrical outlet.

The hybrid electric vehicle (HEV) uses both an electric motor or motors and a gasoline or diesel engine to extend the car’s range and often to provide additional power. A conventional HEV, such as the Toyota Prius, uses battery power up to certain speeds and the gasoline engine for higher speeds, and can draw on both power sources if needed. The batteries are recharged by the gasoline engine, which acts as a generator, and in most models by the energy generated from braking. Another type of HEV, known as a plug-in hybrid electric vehicle (PHEV), uses an extra battery or batteries to extend the range of the vehicle. The PHEV can be plugged into a typical 120-volt electric outlet, like those found in most households, for a recharge. Regardless of the energy source, an electric car needs a controller, which is connected to the accelerator pedal, for directing the flow of electricity from the energy source to the motor.

Most electric cars have used lead-acid batteries, but new types of batteries, including zinc-chlorine, nickel metal hydride, and sodium-sulfur, have been developed. The conventional HEV has used nickel metal hydride. Some automakers have moved to using lithium-ion batteries. General Motors, for example, has chosen lithium-ion batteries for their HEV prototype known as the Volt. Tesla Motors’s all-electric Roadster, which went into production in 2008, also uses lithium-ion batteries. The motor of an electric car harnesses the battery's electrical energy by converting it to kinetic energy. The driver simply switches on the power, selects “Forward” or “Reverse” with another switch, and steps on the accelerator pedal.

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While the internal-combustion engine of a conventional car has many moving parts and must convert the linear motion of pistons and rods into rotary motion at the wheels, an electric motor has only a single rotating element. Like a gasoline-powered car, an electric car has a system (called a power train) of gears, shafts, and joints that transmit motion from the motor to the car wheels. Most electric cars do not have clutches or multispeed transmissions. In order to go backward, the flow of electricity through the motor is reversed, changing the rotation of the motor and causing the power train to make the wheels rotate in the other direction.

Most electric cars have a regenerative braking system—the braking system acts as a battery charger. When drivers ease up on the accelerator or step on a brake pedal, the drive motor acts as a generator and converts the vehicle’s momentum back into electricity and stores it in the battery. Converting the kinetic energy into electric energy slows the car. Electric cars also have a brake pedal and a traditional braking system, which uses friction to slow the vehicle for quick and emergency stopping. These friction brakes convert kinetic energy to heat. In gasoline-powered cars this energy is wasted, the heat being dissipated into the surrounding air. Energy conservation in electric cars, however, is so important that engineers found a way to recover the heat and use it—for example, by heating the passenger compartment.

III

Advantages and Disadvantages

Electric cars represent a cleaner way to convert fossil fuels—oil (see Petroleum), coal, and natural gas (see Gases, Fuel) produced from the remains of prehistoric plants and animals—to automotive power. The fossil fuels are burned at a power plant, or onboard in HEVs, to make electricity to recharge the battery. Substances that pollute the air can be controlled more easily at a power plant than at the tailpipes of millions of gasoline-burning cars, and in HEVs, electronic controls can be used to make the engines run only as needed and to do so more efficiently. The result is that air quality, especially in large cities, can be improved with electric cars or hybrid electric vehicles.

Today's electric cars are more efficient than gasoline-powered cars. They are considered an easy and effective way to harness existing energy sources because any energy source can be converted into electricity. Pure electric cars do not require new ways of delivering fuel because electricity is already distributed to virtually every home and business. However, pure electric cars require charging stations, special equipment that can recharge an electric car battery quickly and efficiently. This special equipment can be installed in a home garage or in the trunk of the car. To extend the range of an electric car, charging stations would need to be placed strategically throughout a city.

IV

Hybrid Gas-Electric Vehicles

The weaknesses of batteries led to the marketplace failures of some pioneering electric cars such as General Motors’ EV1 and Honda’s EV Plus in the late 1990s. Customers refused to accept the limited range and power of these vehicles, and the manufacturers halted production. Development of such cars, however, pushed forward the technology for electric power trains, resulting in more efficient methods of converting battery power between the direct current (DC) used in most batteries and the alternating current (AC) required by the motors best suited to automotive use. Although developed for cars using batteries, this electric power train technology will still be useful in cars that get their electricity from fuel cells. Most automakers appear to be looking toward a future in which fuel cells will convert hydrogen, methanol, or some other fuel into electricity to drive the cars of the future. Many experts believe cars powered by fuel cells will go on sale as early as 2010.

Until cars with fuel cell technology come to market, automotive experts believe that car manufacturers will offer hybrid electric vehicles (HEVs) in wide variety. These HEVs represent an interim step, taking advantage of clean electric power, but also of proven internal combustion engine technology. These systems are similar to the diesel-electric locomotives that replaced steam engines on railroad trains. (In trains, diesel engines charge batteries that drive motors in the wheels.) Electronic components enable internal combustion engines, powered either by gasoline or diesel, to charge a car’s batteries, propel it down the road, or shut down entirely. An HEV at a stoplight typically sits silent, burning no fuel and making no pollution, if the batteries are in a sufficient state of charge. If driven slowly, as in heavy traffic, the vehicle might move only on electric power. Only when more power is demanded for acceleration or to move a heavy load, does the gasoline or diesel engine come into play.

The earliest examples of HEVs, such as the Honda Insight and Toyota Prius, use electric motors to supplement engines that are smaller and more fuel-efficient than they would be if they had to provide all the power alone. These cars are complex, however, because the engines are used not only as generators to charge batteries, but also to move the car. Using both gasoline and electricity requires more complicated transmissions and other systems that add weight. Some systems even use the gasoline engine to drive the rear wheels and electric power at the front wheels for a form of four-wheel drive.

Currently, there are three types of drivetrains that operate HEVs. A drivetrain is the system of components that transfer power to the axle that turns, or drives, the wheels of the vehicle. The three types are the series drivetrain, the parallel drivetrain, and the series/parallel drivetrain. In the series type only the electric motor turns the wheels. The motor receives power from the batteries or from a generator that is powered by a gasoline engine. The series type performs at its best in stop-and-go city driving. General Motors’ prototype Volt HEV uses a series drivetrain. In the parallel type both the gasoline engine and the electric motor generate power that turns the wheels. The Honda Motor Company uses the parallel type in its Insight, Civic, and Accord HEVs. The series/parallel type combines the designs of both and is used in the Toyota Prius and the Ford Escape Hybrid. This type of drivetrain operates as a series type at low speeds and as a parallel type at high speeds. It requires a generator, a larger battery pack than in other types, and more complex computers.

Because HEVs make less pollution and save fuel, they are expected to become more commonplace despite the added complexity. Current HEVs get about 77 km (48 mi) per gallon in city driving and 72 km (45 mi) per gallon in highway driving, according to efficiency ratings issued by the Environmental Protection Agency (EPA) in February 2007. However, PHEVs hold greater promise for fuel efficiency and lower greenhouse gas emissions, such as the carbon dioxide released by internal-combustion engines. That is because the PHEV can operate on the battery alone if the driver’s commute only requires use of the electric motor, and the electric motor is recharged from a household outlet. The PHEV may never require use of the gasoline engine. Power plants that supply household electricity may use only hydro or nuclear power or make more efficient use of fossil fuels.

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