Audi Q7 V12 TDI in Depth

Press Release

The Engine

The new twelve-cylinder TDI, produced at the Györ plant in Hungary, is a member of the current family of V diesel engines from Audi, which up to now was restricted to 6 and 8-cylinder units. As with other inline engines, it has a spacing of 90 millimeters (3.54 inches) between cylinder centers. Yet its included angle is 60 degrees, not 90 – the reason is that on this layout of V12, no free inertial forces or mass moments of inertia can then occur. The results are refined in every respect.

The 83.0-millimeter bore (3.27 inches) and 91.4-millimeter stroke (3.60 inches) of the twelve-cylinder are the same as the 3.0 TDI, producing a total displacement of 5,934 cc. The two cylinder banks are positioned opposite one another with an offset of 17 millimeters (0.67 inch). Measuring 684 millimeters (26.93 inches) in length the powerful engine is extremely compact, only 166 millimeters (6.54 inches) longer than the V8 TDI. This dimension was the key prerequisite for integrating the V12 into the Audi Q7.

The V12 TDI crankcase is made from 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 15 percent compared with conventional gray cast iron. The crankshaft forged from chrome-molybdenum alloy steel is mounted with the aid of an extremely rigid main bearing bridge made of nodular cast iron. The forged connecting rods are cracked and the pistons are made of aluminum.

The two cylinder heads are each made from three main elements. The base section is made from a high-strength, lightweight aluminum alloy, through which the coolant flows and where the intake and exhaust ports are incorporated. Above this are an oil-bearing upper section and a reinforcing ladder-type frame supporting the two camshafts. The valves are actuated by low-friction roller cam followers. The compression ratio is 16.0:1.

As usual on the Audi V engines, the no-maintenance chain drive is mounted on the reverse of the engine, where it occupies little space. However, it has a new layout: the camshaft’s sprocket engages in an intermediate gear via which two simplex chains drive the camshafts. Two additional chains drive the oil pump and the high-pressure pumps for the common rail injection system – an important innovation for the V12 TDI.

2,000 bar: common rail technology with a new level of performance

The two twin-plunger high-pressure pumps have been completely redeveloped. They can create pressures as high as 2,000 bar in the rails – up to now 1,800 bar was the norm. The piezo injectors with their eight-hole nozzles, each with a diameter measuring only 0.12 millimeters (0.005 inch), have also undergone a fundamental reworking. The high pressure creates an extremely precise spray pattern in the combustion chambers that permits a faster, more homogeneous and therefore more acoustically refined ignition process. The highly efficient combustion process also increases power output, cuts consumption and reduces pollutant emissions.

The current generation of so-called inline injectors utilizes the piezo effect: piezo crystals expand in a fraction of a millisecond when an electrical voltage is applied. Over 300 piezo disks are packed into the V12 TDI injectors, which use the expansion to initiate the injection process directly. Each actuation takes just a few milliseconds – at 100 milliseconds, the wink of an eye lasts much longer.

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. One to two pilot injections ensure smooth engine operation in all load and speed ranges. Delayed post-injection serves to increase the temperature of the exhaust gas in order to regenerate the two particulate filters included as standard.

The two turbochargers are located on the outside of the engine’s vee, each of them supplying one bank of cylinders. Thanks to their variable vane geometry the full exhaust flow is directed through the turbines at all times – the turbochargers respond even at low engine speeds and achieve a high level of efficiency.

2.7 bar: high boost pressure

Both turbocharges generate up to 2.7 bar of absolute boost pressure. Two large intercoolers reduce the temperature of the compressed air and thus increase the level of oxygen required for combustion. The V12 TDI features a dual-branch intake system. For each bank of cylinders there is an air filter, behind which an air mass meter is located.

Two ultra-fast control units manage events in the engine, sharing the workload according to a master/slave principle. The six-liter has adopted map-controlled swirl variation of the combustion air from its smaller TDI siblings. The electrically controlled flaps can be adjusted to ensure optimum swirl with respect to high performance and low emissions.

The exhaust system also has a dual-branch design; here Audi engineers focused their attention on exhaust gas recirculation. At partial throttle, up to 50 percent of the exhaust is fed back into the intake air in order to reduce NOx emissions. The large-dimension recirculation system, located at the top of the inner vee, incorporates a water cooler that uses map-based control to lower the temperature of the exhaust gas.

With its 368 kW (500 hp) the diesel generates a specific power output of 62.0 kW (84.3 hp) per liter of capacity – worthy of a sports car. Even more impressive is its maximum torque: between 1,750 rpm and 3,250 rpm the V12 TDI has a constant 1,000 Nm (737.56 lb-ft) on tap – this amounts to 168 Nm (123.91 lb-ft) per liter of capacity. The 11.3 liters of fuel that the potent TDI requires on average per 100 km (20.82 US mpg) is a reflection of its superior efficiency. This figure is even more astounding when compared with gasoline engines offered by the competition – the fuel consumption of these units is much higher all-round.