Like its predecessor, the R15 TDI uses a turbocharged diesel engine, although the R15's V10 engine is physically smaller than the R10's V12. The smaller engine is pushed further toward the middle of the car than in the R10, resulting in a more neutral weight balance that gives the car better agility around the corners than its predecessor.
The car was tested for the first time in December 2008, and made its competition debut at the 2009 12 Hours of Sebring race, 21 March 2009. The R15 got off to a perfect start by winning the 12 Hours of Sebring, setting a new race record in the process. 
Three R15 TDIs participated in the 24 Hours of Le Mans in June 2009, under the control of Joest Racing. Peugeot, its rival, with its 908 HDi FAP, took the top two spots in the 24-hour race, ending Audi's five-win streak that lasted back to 2004 with the gasoline-powered R8.
The R15 TDI features a 5.5 litres (336 cu in) Turbocharged Direct Injection (TDI) turbodiesel V10 engine, rated at over 600 PS (440 kW; 590 bhp) and 1,050 newton-metres (774 lbf⋅ft) torque. The electrical system uses a lithium-ion battery, a first for Audi sports prototypes, as well as LED headlights, and a unique system of LED rear lights that are mounted on the rear wing endplate.
In the week running up to the 2009 24 Hours of Le Mans, rivals Peugeot lodged a protest against the R15, claiming that its bodywork did not comply with regulations stating that parts of the bodywork cannot be fitted with the sole purpose of generating downforce. However, after the Wednesday free practice session, the ACO rejected Peugeot's protest. At the 2009 Le Mans, Audi was unable to continue its winning streak that dated back to 2004. The No. 3 R15 ran off at Indianapolis corner in the beginning of the race but eventually finished 17th. The No. 2, driven by Luhr, crashed and retired. In the evening, the No. 1 Audi lost a lap to the leading Peugeot which was faster, and further technical issues dropped it a full 7 laps down the order. The No. 1 Audi clinched a podium finish, finishing in third place.
Audi announced on 25 August 2009 that two R15s would race at the 2009 Petit Le Mans. Both Audis led for approximately 90% of the race but a late spin during the final rain-soaked caution handed the victory to one of the Peugeot 908 HDi FAPs entered by Team Peugeot Total. This loss was Audi's first since competing in Petit Le Mans since their initial attempt at Road Atlanta back in 2000.
R15 TDI Plus (2010)
In response to losing the 24 Hours of Le Mans and the Petit Le Mans to the Peugeot 908s, Audi updated their R15 for the 2010 season, creating the R15 plus. 2010 Regulations reduced the size of diesel LMP1 air restrictors and turbocharger boost pressure. However, despite this, Audi's engineering team led by Ulrich Baretzky were able to achieve engine power and performance that was comparable to the 2009 powerplant. Aerodynamic efficiency was also a major area of focus for 2010. The frontal area of the car was redesigned to reveal the raw crash structures. The front fenders were also lowered in an effort to reduce drag while the concept of air running through the car was abandoned for a more conventional design. The air channel that exited to the side of the car was redesigned resulting in a more conventional configuration, and a new headlight concept was introduced. Audi stated that they had improved the fuel tank and cooling system as well.
The new vehicle was a success at the 2010 8 Hours of Le Castellet, winning five laps ahead of the next competitor Aston Martin. The Oreca Peugeot, which was supposedly its rival, dropped a full eight laps down the order because the airjacks failed to come off. Audi continue to go flat out and in the end finished ten laps ahead of the Peugeot. Audi had now been able to achieve both the speed and reliability combination that they considered sufficient to match the Peugeot challenge for Le Mans. A full squad of three cars was entered for the next race which was the 2010 Spa 1000 km, finishing third (#7, behind the two Peugeots – No. 3 took first, No. 2 took second), fifth (#9) and twelfth (#8) respectively. Audi treated the race as a setup exercise for car configuration at Spa that would then be used for the 2010 24 Hours of Le Mans.
The 2010 24 Hours of Le Mans race got off to a tightly contested start with Peugeot occupying the top spots (as they did for qualifying) and for a large part of the race. However, it was apparent throughout the race that Audi were unable to match the pace with the Peugeots, but were working according to a different race strategy. By pushing the French manufacturer to the limit, three of the Peugeot cars experienced engine problems due to connecting rod failure towards the latter part of the race. With engine troubles for three of the Peugeots that forced the cars to retire before the end of the race, and an early exit by the No. 3 due to a suspension failure, the three Audis would finish 1–2–3 (the No. 9 took first, No. 8 took second and No. 7 took third), with all cars exceeding the previous distance record of 5,335.313 km (3,315.210 mi) set in the 1971 race by Dr Helmut Marko and Gijs van Lennep: the winning No. 9 car, led by Mike Rockenfeller and two Porsche factory drivers Timo Bernhard and Romain Dumas, set not only a record-tying number of laps around Le Mans of 397 laps, but eclipsed the distance record outright at a distance of 5,410.7 km (3,362.1 mi).
Post-Le Mans Audi did not collect any more victories and lost all three remaining races to the Peugeot 908, which happened to be part of the ILMC championship. Audi fielded two cars into the Silverstone 1000 km but the No. 7 crashed and retired. The sole remaining No. 8 finished third. At Petit Le Mans two cars were entered but both lost to a Peugeot 1–2 victory. At the R15's last race, two cars were entered into the Zhuhai 1000 km. Both cars were leading the 908s by thirty seconds after Sebastian Bourdais make contact with a GT2 Porsche. A late safety car session saw the Audi's lead shrink to thirteen seconds. Afterwards a controversial team-work by Peugeot saw the No. 1 908 slowed down the No. 7 Audi enough to let the No. two Peugeot come out of the pits just two seconds ahead. The race ended with Peugeot No. 2 winning four seconds ahead of the Audi. Audi did not win the LMP1 Le Mans Series team championship nor the ILMC LMP1 championship.
The R15's final race was at the 2011 Sebring 12 Hours. Both cars ran in 2010 configuration with new restrictors. The No. 1 Audi finished fifth overall after two consecutive tyre punctures and rear bodywork damage in the hands of Mike Rockenfeller. Later in the race the No. 1 had to serve a penalty for speeding in pitlane. The No. 2 ran at the front exchanging the lead with Peugeot until a shunt by one of the Peugeots at turn 17 put Capello out of contention with a suspension failure. The No. 2 finished fourth overall, albeit five laps down from the winning Oreca Peugeot. Throughout its career the R15 won three of the ten races it entered; Sebring in 2009, Paul Ricard and Le Mans in 2010.
With a change in Le Mans Prototype engine regulations planned for 2011, Audi developed the closed-top R18 to succeed the R15. Wolfgang Ullrich cited favourable changes to pitstop regulations and aerodynamic efficiency as the reasons for adopting a closed-cockpit design. The R18 was Audi's first-ever coupe since the failed Audi R8C in 1999, although the Bentley Speed 8, which won in 2003 with an Audi-developed engine, was also a closed-cockpit car.
The R15 TDI's final start came at the 2011 12 Hours of Sebring, where it competed as an upgraded R15++, as the R18 was still being developed at the time.
|Audi R15 TDI|
|Design||Le Mans Prototype (LMP1)|
|Motor type||V10-Diesel motor with turbocharger|
|Engine displacement||5500 cc|
|Power||over 440 kW (590 HP)|
|Torque||1050 Nm (774 lb ft)|
|Vmax||over 330 km/h (205 mph)|
|Drive wheels||Rear wheels|
|Transmission||sequential, pneumatic actuated 5-gear|
|Brakes||2-circuit hydraulic braking system, carbon fibre disc brakes front and back|
|Length||4,650 mm (183.1 in)|
|Width||2,000 mm (78.7 in)|
|Height||1,030 mm (40.6 in)|
|Fuel tank capacity||81 L (21 US gal; 18 imp gal)|
|Weight||min. 900 kg (1,984 lb)|
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