"What’s most impressive about the 919 is the two ways Porsche recovers energy to juice up its hybrid system. The first system uses an electric generator with a turbine driven by the exhaust. A second system gathers kinetic energy during braking just like a Toyota Prius. All that energy is stored in a water-cooled, lithium-ion battery pack (Porsche isn’t saying how big it is) that powers an electric motor (Porsche won’t say how powerful) that drives the front wheels, giving the 919 on-demand all wheel drive.
Porsche is competing in the Premiere Class, which gives it the maximum eight megajoules to play with. Although that means the hybrid system is heavier and more complex–which means there are more things that can go wrong–it also means less fuel consumption. Greater efficiency means less time spent refueling, which means greater distance covered, and that’s the entire point of endurance racing. Porsche says the 919′s engine will be driven at full load 75 percent of the 8.48-mile circuit, and use just 1.23 gallons of fuel in the process. Drivers adjust, on the fly, when and how the electric boost comes on, allowing them to account for, say, track conditions or a dogfight in traffic."