The range and charging problems of electric cars are solved by combining an electric motor with a gasoline motor. The most successful of these hybrids has been the Toyota Prius, which approximately doubles the mileage of a normal car. The way it does this, however, is not what most people would imagine. Instead of carrying a large battery, the Prius carries a battery so small that it can be hidden. When we drive, we subconsciously vary the pressure on the gas pedal every second or so as the road curves or rises and falls a little, or because of traffic. Each time the car coasts, its kinetic energy charges the battery, and this energy is re-used in the next few seconds when the gas pedal is pressed to maintain speed. At a stop light, the braking energy is stored and used for startup when the light turns green. Just by saving these small, instantaneous bits of energy, the car can greatly reduce its gas consumption. A dashboard display shows a red symbol every time 50 Wh of energy has been saved and re-used by the car. Fifty watt-hours sounds like a piddling amount of energy. A TV or computer draws 5W when it is off, so 50 W-hrs can power only 10 such devices in a home for one hour out of 24. However, as shown in Box 3.7, 50 W-hrs is equivalent to 241 horsepower-seconds, or almost 50 horse­power for five seconds. This allows the car to have fast pickup after a stop. Fifty kilowatt-hours (67 HP-hrs) would be more normal for a car that didn’t have instan­taneous response to small accelerations and decelerations. Indeed, hackers who have modified the Prius by adding a large battery have increased its mileage from 45 mpg (5.2 liters/100 km) to 100 mpg (2.4 liters/100 km), but at great expense. More on the hardware in the Prius is given in footnote 52.

Hybrid cars incorporate many other improvements to decrease fuel consumption. A continuously variable transmission is more efficient than a 4-speed automatic or a 5-speed manual. A switch available in some models turns off the gas motor altogether so that the car runs on electric alone until the battery gets low. When the energy used to climb hills is recovered and the braking energy is stored for use in starting again, an electric car is very efficient in city traffic. In traffic jams when normal cars are burning gas without moving, electric hybrids can get surprisingly high mileage. Driving at high speeds is another matter; the car has to push its way through the soup we call air. In perfect streamlining, the front of the vehicle slices the air apart. The air streams above and below then rejoin each other at the back of the car, pushing the car forward. But there is friction, and heat is lost in the windshield; and there is turbu­lence, so the stream at the rear is not smooth. There are also protuberances: windows, door handles, tires, and, above all, the rear view mirrors. Sticking your hand out the window at autobahn speeds will show how much energy is needed to push through the atmosphere. Wind drag accounts for 60% of energy use; tire friction, 10%; and engine and transmission line losses account for the rest. In the Prius, sticking to the speed limit can save 10% in gasoline, but over-inflating the tires can save only 1%. Retuning the electronic fuel injection can save 10%. Effective streamlining is mea­sured by the drag coefficient Cd, on which more information is given in footnote 53.

In both hybrids and normal cars, gasoline is used inefficiently when the car is cold. A car rated at 30 miles/gallon (mpg) may get only 12 mpg when it first starts. A Prius which gets 45 mpg when warm drops to 30-35 mpg until the engine and catalytic converter warm up. This loss is avoided when running on electric alone. In hybrids, battery power can be used to heat up the catalytic converter more rapidly. Both motors in a hybrid depend on rare, precious metals. A catalytic converter contains about 5 g of platinum worth about $500. On the other hand, electric motors use permanent magnets made with neodymium. Their batteries may contain more than 10 kg of lanthanum. These materials, however, can be recycled. Many rare-earth elements are used in hybrids, and the concern is that China has a near monopoly on the supply of these elements.