The Audi Q7 V12 TDI - the world’s most powerful diesel passenger car
By Mike Hanlon
September 14, 2006
September 15, 2006 Audi is set to debut the first twelve-cylinder diesel engine in a series passenger car. A six-litre V12 will be available in the Audi Q7 that is reminiscent of the engine featured in the all-conquering R10 Le Mans racing car and it catapults the driving performance of the Audi Q7 SUV into the echelons of top-class sports cars. The V12 TDI engine produces a colossal 500 PS and 1,000 Nm of torque giving the car a 0 to 100 km/h time of just 5.5 seconds, and an electronically governed top speed of 250 km/h. Maximum torque is available from just 1,750 rpm. The high-tech diesel engine, with the world’s first 2,000 bar Bosch Common Rail system, provides the high-performance Audi SUV with superior power reserves in any situation. The impressive sound of the new Audi is provided by the ultra-modern piezo-injection system. The V12 TDI transmits this power through a new six-speed tiptronic gearbox to the quattro permanent four-wheel drive system.
Although the V12 TDI belongs to the Audi V-engine family, this model features a special cylinder angle of 60° – perfect for this engine type. Apart from the light and highly durable crankcase, a further highlight is the new chain drive assembly. This includes the two newly developed high-pressure pumps in the common rail injection system.
Up to 2,000 bar of pressure is kept within the twelve piezo injectors – a further technological breakthrough. Both performance and efficiency are optimised by this high injection pressure, and the running smoothness also remains excellent. Exhaust emissions are also low. The twin-turbo V12 TDI conforms to the Euro 5 emissions class which should come into force in 2010. Fuel consumption is only 11.9 litres per 100 kilometres. With the high-tech twelve-cylinder engine using TDI technology, the diesel pioneers at Audi have opened new horizons in motoring.
The new six-speed tiptronic gearbox, which features extensive reinforcements, shifts quickly and comfortably. It transfers the enormous power to the quattro permanent four-wheel drive system. The new Audi Q7 V12 TDI has an average fuel consumption of just 11.9 litres per 100 kilometres.
Audi rewrote the diesel record books earlier this year on the racetrack. Following its debut in Sebring, Florida, the premium R10 prototype sports car won every long-distance race it entered. These included the Le Mans 24 Hours road race, where the V12 race engine in the R10 squeezed 650 bhp from its 5.5-litre engine capacity, reaching a top speed of 330 km/h. What completely surprised fans and experts alike however, was the sound of the engine: unlike most racing engines, the powerful Audi diesel engine runs whisper-quiet.
Transferring race car technology into series production is an Audi tradition. The FSI engine in the predecessor to the R10, the R8, notched up five victories in Le Mans with its petrol direct injection. Audi has already transferred FSI performance to the production line. The most recent examples of this are the high-revving V8 in the RS 4 and the V10 FSI in the S6 and S8.
The new series-production V12 TDI, assembled in Audi’s plant in Györ, Hungary, reaches a still outstanding level of 500 bhp. When designing the engine, it was kept in mind that the six-litre engine would be a derivative of the current family of Audi V-engines, which up to now were limited to six, eight and ten cylinders. The basic findings from the race car were of course transferred to the series-production engine. The V12 TDI aggregate has a cylinder clearance of 90 mm, which is standard for engines of this type. Nonetheless, the engine has a cylinder angle of 60° instead of 90°, leading to an elimination of inertia in this type of V12 design. Driving comfort is thus ensured in every situation.
The total engine capacity of 5,934 cm3 is derived from a bore measurement of 83 mm and a stroke of 91.4 mm, exactly the same as that of the 3.0 TDI. The large diesel engine is extremely compact. Measuring 684 mm in length, it is only 166 mm longer than the V8 TDI. These dimensions are the primary requirement for integrating the V12 into the Audi Q7.
The crankcase of the V12 TDI is constructed from cast-iron with vermicular graphite. This high-tech material, known as GJV-450, has already been used in the V6 and V8 models.
Produced in a patented casting procedure, GJV-450 is around 40 % more rigid and 100 % more fatigue-resistant than grey cast iron. This allowed the developers to reduce the wall thickness. The weight potential compared to conventional grey cast iron is around 15 %.
The crankshaft, forged from a chrome and molybdenum steel alloy, is held by an extremely rigid main bearing bracket made from cast iron reinforced by nodular graphite. The forged piston rods are cracked, whilst the forged pistons are made from aluminium.
Both cylinder heads are composed of three main elements. The lower section is made from a highly durable and light aluminium alloy where the intake and escape channels are integrated. The upper section guides the flow of engine oil, whilst both camshafts are stored within a reinforcing ladder frame.
The valves are actuated using low-friction roller cam followers with a compression ratio of 16:1. The engine characteristic swirl variations of combustion air were taken from the V6 and V8 TDI engines. The optimum swirl with respect to emissions and simultaneous high performance can therefore be set.
Like most Audi V-engines, the maintenance-free chain drive is located on the back of the engine in a space-saving arrangement. A new layout is used in the new V12 TDI. The chain wheel of the crankshaft locks into the gearwheel. From here two simplex chains drive the camshaft. Two further chains drive the oil pump and both high-pressure pumps in the common rail injection system.
Both new double stamped high-pressure pumps are part of the new common rail injection system developed by the specialists at Bosch. Both pumps amass up to 2,000 bar of pressure in the rails; only 1,600 bar was usually achieved up to now.
The piezo injectors, with their eight-hole jets, have also been radically updated.
An optimal spray is created in the combustion chamber through the high pressure. This in turn allows the ignition process to take place quicker and more homogenously, leading to a much better sound. The efficient combustion also improves performance whilst reducing emissions and fuel consumption.
The current generation of inline injectors use the piezo effect. When an electric current is created, piezo crystals expand in milliseconds. This expansion is then directly transported (inline) by the V12 TDI injectors – containing over one hundred piezo plates – to the injection nozzle needles without the use of mechanisms in between.
The number of injection processes per power stroke can be widely varied using piezo technology, in the case of the V12 TDI up to five injections.
In addition to the main injection, pre and post-injections are possible. Pre-injections reduce the acoustic hardness of the combustion procedure. Post-injections serve to increase the temperature of the emissions, helping regeneration of both standard-fit particulate filters.
Both turbochargers can be found on the exterior of the V-engine, each supplied with a cylinder bank. Thanks to its adjustable guide vane geometry, the complete gas emission stream is always fed through the turbines. The chargers respond even at low levels of revolution and reach high levels of efficiency.
Both turbochargers amass a charge pressure of up to 2.6 bar and are a key aspect of the exceptional torque levels of 1,000 Nm that are supplied to the V12 TDI constantly between 1,750 and 3,000 rpm. With its 368 kW (500 bhp), the diesel reaches a specific performance of 62 kW (84.3 bhp) per litre of engine capacity.
Two large charge air coolers reduce the temperature of the compressed air. The V12 comes with a double-flow exhaust system with two particulate filters. The suction unit is built in a similar way – each cylinder bank contains an air filter behind which an air mass meter is stored. Two control units manage what happens within the engine according to the master-slave concept.
The Audi Q7 V12 TDI will conform to the Euro 5 emissions class which will come into force in 2010 and demand a reduction of nitrogen oxide emissions. Aside from the precise analysis of the fuel injection system through the common rail system, the Audi engineers have focused especially on exhaust gas recirculation. At partial load, up to 50 % of emissions are recirculated through the air intake in order to reduce nitrogen oxide emissions. The large-dimension recirculation system is stored in the upper section of the V cylinder and includes a water cooler which reduces emission temperature significantly.
The Audi Q7 V12 TDI is impressive not only with regard to technology but also in terms of appearance. The most striking visual changes to the Audi Q7 are the underbody panels at the front and rear. The chrome-plated single-frame radiator grille evokes the tradition of the Audi S6 and S8. These two cars, with their V10 FSI engines, are the top sports models in their class. The V12 TDI features a powerful braking system which is every bit its equal.