|The Audi V12 TDI. Click to enlarge.|
Leveraging the work it did with its V12 TDI R10 racing car (earlier post), Audi has applied a 6.0-liter, Euro-5 compliant 12-cylinder diesel TDI engine—the first V12 passenger car engine—in a concept version of the Audi Q7. The Audi Q7 V12 TDI study delivers 368 kW (493 hp) and a massive 1,000 Nm (737 lb-ft) of torque.
The power of the turbocharged V12 TDI takes the SUV from 0 to 100 kph in 5.5 seconds, with fuel consumption of 11.9 liters/100km (20 mpg US).
|Performance of the engine. Click to enlarge.|
The Q7 V12 TDI’s cylinders actually sweep a greater volume than the racing engine, which has a capacity of 5.5 liters. Its 5,934 cc are produced by a bore of 83.0 millimeters and a stroke of 91.4 mm, the same dimensions as the six-cylinder 3.0 TDI.
The two banks of cylinders are positioned opposite one another with an offset of 17 millimeters. Measuring just 684 mm long, the V12 diesel has an extremely compact design that enables it to fit into the Q7.
The V12 TDI features the same 90 mm spacing between cylinders as the other models in the V engine series. However, the cylinder banks are placed at an angle of 60 degrees to one another instead of the customary 90 degrees. This particular V12 configuration suppresses all of the inertial forces and mass moments which would otherwise cause vibrations.
The V12 TDI’s crankcase is made from a mixture of cast iron and vermicular graphite. Audi already uses this high-tech material, known as GJV-450, in the manufacture of the V6 TDI and V8 TDI. GJV-450, which is produced using a patented casting method, is around 40% more rigid than cast iron and twice as resistant to fatigue.
Use of this material enabled the development team to make the walls thinner, paving the way for a potential weight-saving of around 15% compared to conventional cast iron.
The crankshaft is forged from chrome-molybdenum alloy steel and is mounted with the aid of an extremely rigid main bearing bridge made from nodular cast iron. The forged connecting rods are cracked, and the pistons are made from aluminium.
The two cylinder heads each comprise three main elements. The bottom section, incorporating the intake and exhaust ports and through which the coolant flows, is made from a low-pressure die-cast aluminium alloy that has a high-strength, lightweight construction. The top section, which conducts the oil, is pressure-cast. The cylinder head is crowned by a reinforcing ladder-type frame which holds the two camshafts.
|Piston and valve system for the V12. Click to enlarge.|
The valves are actuated by four camshafts acting via low-friction roller-type cam followers, the compression ratio is 16.0:1. The map-controlled swirl variation of the combustion air deployed in the V6 and V8 TDI models has been retained. With this system, one of the intake ports for each cylinder is either opened or closed by means of an electrically operated flap, depending on the engine’s operating point. Intermediate flap positions are also possible. This allows the swirl to be adjusted for optimum exhaust emissions combined with high power delivery.
As is customary on V engines from Audi, the maintenance-free chain drive is fitted in a space-saving location on the rear face of the engine. The developers devised a new layout for the V12 TDI, however. The crankshaft’s sprocket wheel meshes with an intermediate gear which in turn drives the camshafts by means of two simplex chains. Two additional chains drive the oil pump as well as both high-pressure pumps for the common-rail injection system.
Bosch supplies a 2,000-bar injection system for the V12. The hole diameter of the eight-hole injector nozzles has now been reduced to a mere 0.12 millimeters. The high pressure produces an optimum spray pattern inside the combustion chamber, which makes for a faster, more homogeneous and, as a result, more acoustically refined ignition process. The improved combustion efficiency increases power output while reducing both fuel consumption and emissions.
More than 300 piezo discs are packed into each of the V12 TDI injectors, which transmit the minimal expansion to the valve needle directly (inline) without the need for any form of transmission mechanism between the two. Each actuation takes just a few milliseconds.
The fuel pressure and quantity can be adjusted according to requirements; the number of injection phases per power stroke can be varied up to a maximum of five with the piezoelectric technology in the V12 TDI. Main injection can be accompanied by both pilot and post-injection phases. Pilot injection lessens the harshness of the combustion sound, which is particularly noticeable at low engine loads.
The delayed post-injection of fuel serves to increase the temperature of the exhaust gas. This allows any particulate residue to be burned off in order to regenerate the two particulate filters which are included as standard.
Two variable vane geometry turbochargers supply compressed air—each to one bank of cylinders. The variable vane geometry ensures that the full exhaust flow is directed through the turbine at all times. The turbochargers offer fast response even at low rev speeds and attain a high degree of efficiency.
Both turbochargers generate up to 2.6 bar of absolute boost pressure and are key in achieving the peak torque of 1,000 Nm.
The temperature of the compressed air is reduced by two large intercoolers. The V12 comes with a double-flow exhaust system featuring two particulate filters. The intake system is similarly structured—for each bank of cylinders there is an air cleaner followed immediately by a hot-film air mass meter. Two control units operating based on the master-slave principle orchestrate proceedings inside the engine.
|The Q7 V12 TDI study.|
The Audi Q7 V12 TDI is Euro-5 compliant. At partial throttle, up to 50% of the exhaust gases are fed back into the intake air via the Exhaust Gas Recirculation system to reduce NOx emissions. The extensive recirculation system incorporates a water cooler which reduces the temperature of the exhaust gases sharply.
The V12 TDI is coupled with a new six-speed tiptronic transmission. The quattro driveline splits the drive power 40:60 between the front and rear wheels under normal driving conditions. This results in slightly tail-heavy, sporty handling characteristics.