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Audi has announced that it plans to enter a pair of diesel hybrid Audi R18 prototypes in the FIA World Endurance Championship this year, including the 24 Hours of Le Mans. The pair of hybrid entries will be designated as R18H, and will compete along with a pair of non-hybrid diesel powered R18s.
Most of you think of the Toyota Prius or the Honda Accord or Insight, when you think of a hybrid automobile. Those are street driven gasoline hybrids that store regenerated energy in a “battery”. The Audis are racing hybrids, powered by a 3.7L V-6 Audi TDI diesel engine. The hybrid system currently used in racing is known as a “flywheel hybrid” – essentially storing energy generated during braking in the form of inertia, into a flywheel to be released as needed. Flywheel hybrids are fairly new to modern automobile racing. But they are not new to the automotive or transportation world. GM experimented in the 1970s with flywheel hybrid systems, but none were made available to the public. In 2010, Porsche debuted the 911GT3R racing hybrid to evaluate a “through-the-road” hybrid system. The GT3R was a flywheel hybrid, in which the flywheel and housing sat where the passenger seat would normally be. If you think this is just an exercise in “being green”, think again. The energy stored in the Porsche’s flywheel system could release an additional 163 horsepower for up to six seconds at a time. What race driver couldn’t use that?
It works like this. In the case of the Porsche, the front wheels were connected to electric motors/generators. Under braking, the motors sent a charge to a carbon fiber flywheel, sitting inside a housing mounted in the passenger compartment of the car. This flywheel had magnets embedded in it, and would act like a starter motor – spinning at speeds up to 40,000 rpm. Simply put, reversing the flow of electricity allowed the flywheel to act like an alternator, and power the motors attached to the wheels. For the Audi and other FIA approved applications, the flywheel will be driven (energy stored) and energy released using one of two methods. Either a CVT (Continuously Variable Transmission) or a CFT (Clutched Flywheel Transmission) will connect the Flywheel energy to the drive train. These systems provide multiple gear ratios, allowing the drive train of the car to spin the flywheel faster (even during deceleration), storing energy, and to slow the flywheel down, releasing energy during moments of acceleration.
A final question might be, “why flywheels, instead of batteries or capacitors”? In racing the driver accelerates and brakes harder and more often than one does in street driving. Chemical batteries are limited in their rate of absorption and release of energy. Charging and discharging at such a rapid rate would lead to overheating and failure of a typical storage battery. This is one reason that street hybrids are limited to about 0.3G’s of deceleration during regenerative braking, while a racing hybrid can decelerate at over 1G.