The gasoline engine is a marvelous piece of engineering. Honed over hundreds of generations of models, it fires an explosion thousands of times a second, and yet we can hardly hear it as it smoothly pushes the car through the air. What is wrong with it? It uses gasoline very inefficiently, and it emits carbon at a rate equivalent to throwing a charcoal briquette out the window every quarter mile.

Electric cars are even quieter… so silent that it has been proposed to put a noise generator in them to warn pedestrians. Electric cars have no emissions, but they get their electricity from power plants that emit GHGs. However, power plants burn fossil fuels much more efficiently than cars do, so the total emissions are lower. It is because power plants run at much higher temperatures than cars can, and the Carnot efficiency (see Chap. 2) is much higher. There is a big difference between 40 and 15% efficiency, and most people do not realize this. The main problem with electric cars is the battery. There is no type of battery of reasonable size and weight that can take a car 300 miles on one charge, and it takes many hours to recharge the battery. If you run out of “gas” in an electric car, you would have to stay in a motel with a plug. But electric cars have great advantages. We will consider these next and hybrids later.

Efficiencies of Gas and Electric Cars

A normal car can use only about 15% of the energy in gasoline, though some say it could be 30%. The breakdown is shown in Fig. 3.52. Most of the energy is lost in heat, 30% in the radiator and 30% in the exhaust from the muffler. A few percent more is lost in the engine and in the transmission line between the motor and the wheels. Fully 17% is used in idling while the car is not moving, such as at a red light. The motor has to be running so that it can start again rapidly. Accessories such as lights and radio take only 2%. That leaves only 12.6% for propulsion of the car.

PROPULSION

12.6%

About half of this is lost as heat in the brakes to stop the car. The rest, 6.8%, is all there is left to move the car!

Electric cars store energy in a battery bank and use that to drive a motor that drives the wheels. The battery may get a little warm, but the heat energy lost is trivial compared with the 60% in normal cars. The stand-by energy is saved since the motor simply turns off when the car is coasting or stopped. The braking energy is recovered into the battery, though the brakes will get a little hot, and that energy will be lost. The accessories, including the lights, the radio, and the computer, will take a few percent, and so will the transmission, but all the rest is available to move the car. Electric cars can convert about 75% of the energy stored its battery into useful power. The battery is charged with electricity from the grid, and the environmental impact of that process depends on the location. In most places, coal or natural gas is used to generate electricity, and GHGs are emitted. However, this is better than burning oil products in cars for several reasons. Power plants can be 40% efficient, three times better than cars. So less fuel is consumed and less CO2 is emitted. Furthermore, power plants can be located some distance from cities, thus sparing them from pollution. Electric vehicles emit only water. In locations where hydroelectric or nuclear power is available, the air is even cleaner. Even noise pollution is abated.

Vehicles running totally on electricity are being used successfully in service vehicles and golf carts, which do not have far to travel. The Tesla Roadster has shown that electric cars can have sports-car performance at a price. The big buga­boo is transportable energy. There is no known type of battery that will carry a car 300 miles and recharge in 5 min, as we can get from gasoline. Meanwhile, we can save on gasoline by using hybrids. These will be discussed next, followed by battery prospects.