The Audi TDI – Tech Workshop 2014
The racing engines
Audi considers racing to be the ideal test bench for production, and the toughest testing lab of all is the 24 Hours of Le Mans. 2006 was the first time that Audi entered a car powered by a TDI engine. Audi has celebrated 13 overall victories in 15 starts, eight of them with TDI engines. This includes a 1-2 finish on June 15. The same requirements apply to the sport prototypes at Le Mans and in the FIA World Endurance Championship (WEC) as to production cars: to always get more and more out of every drop of fuel, to increase efficiency and at the same time continuously reduce consumption even further.
Over the years the rules at Le Mans have placed increasingly restrictive limits on TDI engines. For example, the diameter of the air restrictor has decreased by 34 percent since 2006, displacement by nearly 33 percent. Absolute output declined as a result by approximately 25 percent from more than 478 kW (650 hp) in 2006 to around 360 kW (490 hp) in 2013.
At the same time, Audi rigorously downsized and vastly improved the specific output. This increased from 87 kW (118 hp) per liter of displacement in 2006 to 107 kW (146 hp) in 2011 – an increase of nearly 24 percent. The piston area output – the output delivered by each individual cylinder – grew during this period of time from 40 kW (54 hp) to 66 kW (90 hp), in other words by 65 percent. While the driven speed has increased further, Audi dramatically reduced fuel consumption during race operations at Le Mans.
2006 – 2008: The V12 TDI in the Audi R10 TDI
With the R10 TDI and its twelve-cylinder TDI engine, Audi opened a new chapter in racing. Following its debut, the diesel race car has completed an overwhelming series of victories. With 1,100 Nm (811.3 lb-ft), the 5.5-liter TDI engine vastly exceeded that of the gasoline engines. At rated speed, the very quiet-running twin-turbo produced roughly 480 kW (more than 650 hp). Drivers shifted into second gear at just 5,000 rpm. Two particulate filters cleaned the exhaust gas, and a sequential five-speed transmission transferred the power to the rear axle.
The comparatively low fuel consumption and the long range of the R10 TDI were the key to success at the 24-Hours of Le Mans in 2006. Frank Biela, Emanuele Pirro and Marco Werner brought their car into the pits just 27 times. The same team won again with the Audi R10 TDI in 2007, despite difficult weather conditions and the fact that the race organizers reduced the permissible fuel tank volume by ten percent. In 2008, Rinaldo Capello, Allan McNish and Tom Kristensen completed the hat trick for the Audi R10 TDI.
2009/2010: The V10 TDI in the Audi R15 TDI
With the R15 TDI, Audi spread the 5.5 liters of displacement across two fewer cylinders. The V10 TDI had roughly 440 kW (approximately 600 hp) and more than 1,050 Nm (774.4 lb-ft) of torque. It was shorter and lighter than the twelve-cylinder, which benefited the agility of the new sport prototype significantly.
Audi celebrated a dominating 1-2-3 finish with the open sport prototype in 2010. Timo Bernhard, Romain Dumas and Mike Rockenfeller improved the distance record, which Porsche had established 39 years before, by five laps or 75.4 kilometers (46.9 mi) to 5,410.713 kilometers (3,362.1 mi).
Although the Le Mans rules had once again reduced boost pressure and air flow, the performance of the ten-cylinder TDI remained virtually unchanged. Audi used turbochargers with variable turbine geometry (VTG) for the first time in the V10 TDI, which improved throttle response substantially. Exhaust gas temperatures of up to 1,050 degrees Celsius were extremely demanding on the material. Steel pistons, which had previously been tested in the V12, were also used for the first time in the V10 TDI. This enable even higher pressures to be used, thus providing for even greater efficiency.
2011 – 2013: The V6 TDI in the Audi R18 TDI, R18 ultra and R18 e-tron quattro
In 2011, Audi took to the track at the 24-hour race in the R18 TDI – the brand’s first closed sport prototype since the R8C in 1999. The new rules required drastic downsizing of the engine to a displacement of 3.7 liters. Newly designed from the ground up, the lightweight and compact V6 TDI with a cylinder bank angle of 120 degrees produced over 397 kW (540 hp) and more than 900 Nm (663.8 lb-ft) of torque, which was transferred to the wheels via the likewise new six-speed transmission. The common-rail system generated up to 2,600 bar of pressure.
Audi also broke new ground with respect to the layout and cooling of the cylinder heads. From now on the intake side was on the outside, the hot exhaust side on the inside. The mono-turbocharger was located inside the V and drew its fresh air from the scoop on the roof. The large VTG turbocharger, which developed up to 2.0 bar of relative boost pressure (2011: 2,960 mbar absolute; 2012 – 2013: 2,800 mbar absolute), featured an innovative double-flow design and opposing intakes for the exhaust flows, plus two outlets on the compressor side. The compressed air flowed via separate intercoolers into two exhaust manifolds. The race at Le Mans was very dramatic. Marcel Fässler, André Lotterer and Benoît Tréluyer took the checkered flag in the remaining Audi R18 TDI just 13.854 seconds ahead of four Peugeots.
With the motor-generator unit (MGU) on the front axle, which depending on the amount of energy can temporarily produce up to 170 kW of power, the Audi R18 e-tron quattro had temporary all-wheel drive. Fässler, Lotterer and Tréluyer celebrated the first victory of a hybrid race car at Le Mans with an Audi 1-2-3 finish in 2012. Tom Kristensen, Loïc Duval and Allan McNish won the following year.
2014: The new V6 TDI in the Audi R18 e-tron quattro
The new R18 e-tron quattro that Audi entered at Le Mans in June 2014 is powered by a completely redesigned V6 TDI with a displacement of 4.0 liters. Its performance data: a good 395 kW (537 hp) and more than 800 Nm (590.0 lb-ft) of torque. Injection pressure is more than 2,800 bar. Thanks to very intensive detail work, the engine is easily the lightest and at the same time most efficient racing diesel engine from Audi. Fuel consumption decreased by more than 25 percent compared with the 3.7-liter engine. The hybrid system – the motor-generator unit at the front of the car and the flywheel storage system next to the driver – delivers more than 170 kW. Equipped with this technology package, Audi entered the 24-hour race in the energy class up to two megajoules of recuperative energy. The new rules limited the amount of available energy per lap, but left many other parameters freely configurable. In a dramatic race marked by numerous lead changes, the No. 2 Audi R18 e-tron quattro claimed the overall victory. Marcel Fässler/André Lotterer/Benoît Tréluyer completed 379 laps. Tom Kristensen/ Luca di Grassi/Marc Gené took second place in the No. 1 car to complete the Audi triumph. The winning car consumed 22 percent less fuel than its predecessor from 2013. Since the start of the TDI era (2006), Audi has reduced fuel consumption at the 24 Hours of Le Mans by 38 percent.