Audi Q7 V12 TDI
A time of 5.5 seconds for the sprint from a standstill to 100 km/h and an electronically governed top speed of 250 km/h put the performance SUV from Audi on a par with high-calibre sports cars. The engine limits its consumption to an average of just 11.9 litres/100 km.
The new V12 TDI is the crowning glory of an engine line-up brimming with dynamic performance – each and every one of the diesel engines from Audi generates a high output and tremendous pulling power for a sporty drive. This character has emerged over and over again in the brand's models, not least in the form of the six- and eight-cylinder 2.7 TDI, 3.0 TDI and 4.2 TDI models.
The pioneering role that Audi assumes in the field of diesel technology stems from the peerless expertise it has amassed over a period of decades. Since the 1970s, the Audi engineers have been responsible for one vital breakthrough after another, the most important being the world's first ever TDI engine to be fitted in a passenger car which made its debut in 1989. And in 2008, Audi intends to start making inroads into the diesel market with Bluetec technology: thanks to Bluetec, TDI models bearing the four-ring emblem are not just sporty and economical, they can also run so cleanly that they comply with the emissions legislation in all 50 states.
A year ago, Audi added yet another new chapter to the ongoing diesel success story on race tracks around the world. Developing over 650 bhp, the V12 TDI fitted in the R10 TDI sports prototype left its petrol-powered rivals trailing in all of the races it participated in, including the most challenging of them all, the classic Le Mans 24 Hours. The R10 TDI also triumphed in the LMP1 category of the American Le Mans Series with an impressive string of victories.
The V12 TDI – the new flagship V engine
When working on the road-going version of the V12 TDI, which will be built in its Györ plant in , Audi drew on all of the know-how that underpins the racing engine. The range-topping diesel is the new flagship model in the Audi range of V engines, spanning petrol and diesel units with 6, 8 and 10 cylinders.
The V12 TDI's cylinders actually sweep a greater volume than the racing engine, which has a capacity of 5.5 litres. Its 5,934 cc are produced by a bore of
83.0 millimetres 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 millimetres. Measuring just 684 mm long, the V12 diesel has an extremely compact design. Indeed, its compact size was one of the key prerequisites for integrating the V12 into the Audi 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. Consequently, the engine's smoothness is flawless in every regard.
Lightweight construction: crankcase made from vermicular graphite
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 percent more rigid than cast iron and twice as resistant to fatigue.
This enabled the development team to make the walls thinner, paving the way for a potential weight-saving of around 15 percent 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.
Perfect air ducting: flaps in the intake ducts
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.
2,000 bars of pressure: ground-breaking common-rail technology
The injection technology featured in the V12 TDI is every bit as revolutionary. The high-pressure pumps both form part of the common-rail system supplied by system specialist Bosch. The twin plunger pumps are capable of building up pressures as high as 2,000 bar in the rails, considerably more than the previous usual maximum of, at most, 1,600 bar.
The piezoelectric injectors with their eight-hole nozzles have also undergone a thorough reworking. The tiniest quantities of diesel fuel are injected into the combustion chambers through their eight-hole nozzles, whose diameter has now been reduced to a mere 0.12 millimetres. 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. What's more, the improved combustion efficiency increases power output whilst reducing both fuel consumption and pollutant emissions.
Ultra-fast and ultra-precise: the piezoelectric injectors
The latest generation of inline injectors employs the piezo effect: piezo crystals expand slightly when an electrical voltage is applied to them. Over 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 – by comparison, a wink of the eye lasts 100 ms.
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.
Supercharging: a turbocharger for each bank of cylinders
Attached to the outside of the engine's V are the two turbochargers which each supply compressed air to one bank of cylinders. Their 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 therefore have a vital role to play in achieving the formidable peak torque of 1,000 Nm which the new V12 TDI constantly has on tap between 1,750 rpm and 3,000 rpm. The diesel's 368 kW/500 bhp equates to a specific output of 62.0 kW/84.3 bhp per litre.
Cooling effect: two intercoolers to reduce the temperature of the air
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 Audi Q7 V12 TDI already complies with the Euro 5 emissions standard, which is due to come into force in mid-2010 and will make especially tough demands in terms of reducing nitrogen oxides. It was with this in mind that the Audi engineers devoted particular attention not only to the new common-rail system and its high-precision fuel metering, but also to the exhaust gas recirculation system.
At partial throttle, up to 50 percent of the exhaust gases are fed back into the intake air in order to reduce NOx emissions. The extensive recirculation system incorporates a water cooler which reduces the temperature of the exhaust gases sharply.
The awesome V12 TDI directs its power as standard to a fast-action, smooth-shifting new six-speed tiptronic transmission. From here, the forces flow to a quattro driveline, which 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, exceptional agility and exemplary steering precision.
To ensure that its phenomenal performance can be safely harnessed, the Audi Q7 V12 TDI is equipped with powerful, ventilated disc brakes. The ESP dynamic handling control system features a hill descent assist as well as a special off-road mode. The safety specification is rounded off by the pairs of front, side and head airbags in the interior.
An elaborate double-wishbone independent suspension has been used for each of the four wheels, while the majority of the axle components are made from aluminium. The adaptive air suspension, incorporating an electronically controlled damper system, comes as standard, creating a perfect synthesis of sporty handling on the one hand and silky ride comfort on the other. The body's ride height can be adjusted from its starting position of 180 mm above the ground up to the lift mode offering a whole 240 mm of ground clearance. The Audi Q7 V12 TDI study rides on 20-inch cast aluminium wheels.
Design: the best of both worlds
The Audi Q7 V12 TDI is just as fascinating to look at as it is to drive. The principal feature which distinguishes it from its lesser-powered siblings is the contrasting paintwork of the underfloor panels at the front and rear. The striking, almost vertical single-frame radiator grille has a chromed finish and echoes the dynamic Audi S6 and Audi S8 models, which both head their respective ranges powered by V10 FSI engines. Flanking the grille are wide headlight units with visible light tubes.