Decades of dedicated effort to improve engines and power trains have reduced to only about 80-85 percent the portion of cars' fuel energy that is lost before it gets to the wheels. (About 95 percent of the resulting wheelpower hauls the car itself, so that less than two percent of the fuel energy actually ends up hauling the driver.)
This appalling waste has a simple main cause: cars are made of steel, and steel is heavy, so powerful engines are required to accelerate them. Only about one sixth of the average engine's power is typically needed for highway driving, and only about one twentieth for city driving. Such gross oversizing halves the engine's average efficiency and complicates efforts to cut pollution. And the problem is getting worse: half the efficiency gains since 1985 have been squandered on making engines even more powerful.
Every year auto makers add more gadgets to compensate a bit more for the huge driveline losses inherent in propelling steel behemoths. But a really efficient car can't be made of steel, for the same reason that a successful airplane can't be made of cast iron. We need to design cars less like tanks and more like airplanes. When we do, magical things start to happen, thanks to the basic physics of cars.
Because about five to seven units of fuel are needed to deliver one unit of energy to the wheels, saving energy at the wheels offers immensely amplified savings in fuel. Wheelpower is lost in three ways. In city driving on level roads about a third of the wheelpower is used to accelerate the car, and hence ends up heating the brakes when the car stops. Another third (rising to 60-70 percent at highway speeds) heats the air the car pushes aside. The last third heats the tires and the road.
The key to a superefficient car is to cut all three losses by making the car very light and aerodynamically slippery, and then recovering most of its braking energy. Such a design could:
* cut weight (hence the force required for acceleration) by 65-75 percent through the use of advanced materials, chiefly synthetic composites, while improving safety through greater strength and sophisticated design
*cut aerodynamic drag by 60-80 percent through sleeker streamlining and more compact packaging
*cut tire and road energy loss by 65-80 percent through the combination of better tires and lighter weight.
Once this "ultralight strategy" has largely eliminated the losses of energy that can't be recovered, the only other place the wheelpower can go is into braking. And if the wheels are driven by special electric motors that can also operate as electronic brakes, they can convert unwanted motion back into useful electricity.
However, a hypercar isn't an ordinary electric car, running on batteries that are recharged by being plugged into utility power. Despite impressive recent progress, such cars still can't carry very much or go very far without needing heavy batteries that suffer from relatively high cost and short life. Since gasoline and other liquid fuels store a hundred times as much useful energy per pound as batteries do, a long driving range is best achieved by carrying energy in the form of fuel, not batteries, and then burning that fuel as needed in a tiny onboard engine to make the electricity to run the wheel motors. A few batteries (or, soon, a carbon-fiber "superflywheel") can temporarily store the braking energy recovered from those wheel motors and reuse at least 70 percent of it for hill climbing and acceleration. With its power so augmented, the engine needs to handle only the average load, not the peak load, so it can shrink to about a tenth the current normal size. It will run at or very near its optimal point, doubling efficiency, and turn off whenever it's not needed.
This arrangement is called a "hybrid-electric drive," because it uses electric wheel motors but makes the electricity onboard from fuel. Such a propulsion system weighs only about a fourth as much as that of a battery-electric car, which must haul a half ton of batteries down to the store to buy a six-pack. Hybrids thus offer the advantages of electric propulsion without the disadvantages of batteries.