R18 e-tron quattro (Germany, 2012)

The Audi R18 e-tron Quattro is an all-wheel drive, diesel sports car prototype that will see its track debut at the 6 Hours of Spa on May 5. It is also launching a second car, the Audi R18 Ultra, that is identical to the R18 e-tron but without the hybrid powertrain.  It will compete alongside the R18 e-tron Quattro at the 24 Hours of Le Mans. This two-pronged approach allows Audi to hedge its bets by running four cars at Le Mans – two hybrid and two conventional diesels. 

The R18 e-tron Quattro achieves its all-wheel drive system by having the rear axle powered by the conventional diesel engine, while the front axle is powered by two electric motors. Energy is recovered from the front axle while braking and stored in a flywheel, a la Formula 1's KERS system. The power generated can be discharged to the front wheels at speeds above 120km/h. While this is happening, the 375kW (510hp) V6 diesel engine continues to operate on the rear wheels. 

Audi developed the hybrid system in the R18 e-tron in just 18 months from concept to final test. 

At the same time, Audi developed the R18 Ultra to race along side the e-tron. The Ultra is identical to the e-tron, except that it lacks the front axle-based hybrid system. Audi claims that the Ultra is the lightest prototype it has ever built. Audi had to compensate from the hybrid system's weight when building the e-tron. These weight saving measures were carried over into the Ultra as well. One of these is a new carbon fiber housing for the transmission. 

There are several advantages to Audi's strategy. First, it can share nearly all the parts between the two cars if something were to go wrong. Second, if Audi got something wrong with the hybrid, it means that it will still have a competitive car in sports car racing. 

 "Audi has always consciously selected championships and categories in racing that have a close relationship to production and therefore have technical relevance for the Audi customers,” said Head of Audi Motorsport Dr. Wolfgang Ullrich 

Audi Sport Team Joest will campaign two R18 e-trons and two R18 Ultras at Le Mans. The hybrid cars will be driven by Marcel Fässler, André Lotterer and Benoît Tréluyer in one car and Dindo Capello, Tom Kristensen and Allan McNish in the other. The Ultras will be driven by Loïc Duval, Timo Bernhard and Romain Dumas in one and Marco Bonanomi, Oliver Jarvis and Mike Rockenfeller in the other. All of the same drivers except for Rockenfeller will race the cars at the 6 hours of Spa. One each of the e-tron and Ultra will race at the rest of the rounds of the WEC, except for the first round at Sebring where Audi will field last year's car. 

It is intriguing to compare the R18 e-tron Quattro to the Toyota TS030 that it will be competing against. Where Audi has decided to use a flywheel to store the energy from braking, Toyota has chosen to use capacitors. The Toyota can run purely electrically whenever it wants, it has even touted the fact that the driver can choose to run on just electric power at low speeds. Audi's system on the other hand, can only be activated above 120km/h.