Audi R8 TDI Le Mans: The Drivetrain
Audi’s engineers were in an ideal position to use their experience building the racing engine to develop the road version. Like the other power units in this range, the distance between cylinder bore axes on the V12 TDI is 90 mm (3.54 in.). Yet its included angle is 60 degrees, not 90 degrees. This means no free inertial forces or mass moments of inertia can occur with the V12. The results are refined in every respect.
The 83.0 mm (3.27 in.) bore and 91.4 mm (3.60 in.) stroke produce a total displacement of 5,934 cc – just like the 3.0 TDI. At only 684 mm (26.93 in.) long, this large diesel engine is very compact and just 166 mm (6.54 in.) longer than the V8 TDI. This compact length is key to accommodating the V12 in the mid-engined Audi R8.
The V12 TDI crankcase is made from gray cast iron with vermicular graphite – a high-tech material referred to as GJV-450 that is already used on the V6 and V8 TDI engines. GJV-450, made by a patented casting process, is about 40 percent more rigid and 100 percent more fatigue-resistant than gray cast iron. This enabled the developers to make its walls thinner, cutting its weight by around
The two cylinder heads are each made from three main elements. These are a base section made from a high-strength aluminum alloy incorporating the intake and exhaust ports, an oil-bearing upper section, and a reinforcing ladder frame supporting the two camshafts.
The valves are actuated by low-friction roller cam followers; the compression ratio is 16.0:1. Map-controlled swirl variation of the combustion air has been adopted from the V6 and V8 TDI engines. This produces permanently optimized swirl concerning both emissions and high performance.
Ultramodern Injection Technology
As is the case in the Audi V engines, the no-maintenance chain drive is mounted at the back of the engine, where it occupies little space. Its layout has changed on the new V12 TDI. The camshafts’ sprocket engages in an intermediate gear via which two Simplex chains drive the camshafts. Two more chains drive the oil pump and the two high-pressure pumps actuate the common rail injection system.
The two new dual-piston high-pressure pumps form part of the common rail injection system supplied by specialty manufacturer Bosch. The two pumps build up a pressure of up to 2,000 bar in the rails. The piezo injectors with eight-hole nozzles have also been fundamentally revised.
The high pressure distributes the mixture optimally throughout the combustion chamber. The result is that the ignition process is faster, more homogeneous and more acoustically refined. The more efficient combustion process also increases power output, cuts consumption and reduces pollutant emissions.
The current generation of so-called inline injectors makes effective use of the piezo effect: piezo crystals expand in a fraction of a millisecond when an electrical voltage is applied. The number of injection processes per operating cycle can be varied across a wide range thanks to piezo technology – reaching as many as five fuel injection operations in the case of the V12 TDI.
As well as the main injection, pilot and post injections are possible. Pilot injections tone down the acoustic harshness of the combustion process. Retarded post injections are designed specifically to increase the temperature of the exhaust gas, promoting regeneration of the two standard particulate filters.
The two turbochargers are located on the outside of the engine’s V, each of them supplying one bank of cylinders. Thanks to their variable turbine geometry, the full flow of exhaust gas always passes through the turbine, so the chargers respond slickly – even at low engine speeds – and operate very efficiently.
The two turbochargers, which generate up to 2.6 bar of boost pressure, play a crucial role in producing the huge torque of 1,000 Nm (737.56 lb-ft) that the V12 TDI maintains from 1,750 rpm to 3,000 rpm. In developing 368 kW (500 hp), the diesel achieves a specific output of 62.0 kW (84.3 hp) per liter displacement.
Two large intercoolers reduce the temperature of the compressed air. The V12 has a twin-pipe exhaust system with two particulate filters. The intake system is similar in structure with one air cleaner per cylinder bank, with an airflow meter behind it. Two control units, sharing the workload in a master/slave principle, manage events in the engine.
The Audi R8 TDI Le Mans already fulfills the Euro 6 emissions standard that is likely to take effect in 2014 and calls for sharply reduced nitrogen oxides. By also designing in ultra-precise fuel metering by the common rail system, Audi’s engineers have made full use of current clean diesel technology.
The heart of the system is a special catalytic converter downstream of the oxidizing catalyst and the particulate filter. The second component in the system is an additional tank containing an aqueous urea solution. Small quantities of the solution, known as “AdBlue,” are injected into the exhaust system. The hot exhaust gases break the solution down to form ammonia that splits the nitric oxides into nitrogen and water. The system remains effective for the entire service life of the vehicle.
The dynamic character of a sports car depends not just on its performance and torque; the transmission ratios have to be right too. In keeping with the character of a high-performance sports car with unbeatable torque potential, the transmission in the R8 TDI Le Mans has six manually operated gears.
The manual transmission is very compact in design. Together with the small-diameter double-plate clutch, this means it can be installed low down. The manual transmission has very short shift travel and utterly precise guiding of the shifter into the open gear lever gate. It is made from polished aluminum, has an agreeable feel and exquisite sports car looks.
Such a high-performance Audi also has quattro permanent all-wheel drive. In the case of this mid-engine sports car, power is distributed variably between the front and rear wheels from a starting ratio of 40:60 to optimize the handling.