Compact power plant
Here is an engine beyond compare: the twelve-cylinder power unit in the form of a double "V", or "W", for the Audi A8 L 6.0 quattro is quite unparalleled by any other modern-day unit. This engine made its successful début in the top version of the previous generation. There, the W12 developed 420 bhp and made the luxury saloon the most highly powered representative of its class at the time of its unveiling.
The "W" design principle has one inestimable advantage: despite the larger number of cylinder units, and despite a clearly higher displacement, the W12 is as compact as a V8 and thus has much smaller dimensions than a conventional V12 engine.
The scope for pairing of this engine with quattro four-wheel drive is thanks solely to its unusually short length of 513 millimetres and its width of 690 millimetres. Other twelve-cylinder engines installed at the front of the car can only drive the rear wheels - in this case there is not enough space under the bonnet for additional front-wheel drive components.
Another advantage of this compact design - together with the use of the light metals aluminium and magnesium - is the very low weight of the twelve-cylinder engine. And this in turn helps to produce an unbeatably low total vehicle weight of 1995 kilograms, lighter than any of its competitors. One horsepower on the Audi A8 L 6.0 quattro has to accelerate just 4.4 kilograms - a figure that is on a par with a high-performance sports car.
The W principle
The Audi twelve-cylinder engine is also unparalleled in the history of engine technology - despite the fact that there have already been examples of this W-engine design in automotive history. The legendary 1920s Rumpler "tear-drop" car, for example, for which its designer developed a six-cylinder engine in 3x2 formation. Or, just a few years later, the world record car driven by Campbell, Seagrave and Cobb: this was powered by a W12 aircraft engine with a capacity of 24 litres and developing 502 bhp.
Audi, too, has already made use of a W12 engine in the past: the legendary Avus super sports car study, presented in 1991, had an output of 509 bhp.
A direct comparison with these engines clearly reveals the special feature of the Audi engine. Unlike its historical predecessors, it is not a W-engine in the classic sense with three separate cylinder banks, where three connecting rods are mounted on a single crankpin. The new twelve-cylinder engine, which the Audi design engineers have developed together with their Volkswagen colleagues, is different: in this case, there are two connecting rods per crank pin.
A glance at the engine's cylinder blocks shows that two ultra-compact inline V6 units have been combined into one. Each of the two cylinder blocks contains six cylinders arranged in pairs at an angle of 15 degrees. This offset configuration makes an ideal synthesis of the very compact width - compared with the classic V6 - and the very short length of a four-cylinder engine.
Joining these two units at an angle of 72 degrees to produce a twelve-cylinder engine has a third advantage: the shallow height of the engine means that the aerodynamically beneficial, flat silhouette of the A8's front end can remain unchanged.
A further advantage is that the unmistakable sporty aura of the Audi A8 design has been retained in the top-of-the-range version, where perhaps it is most expected.
Another benefit of this design is the high rigidity of the engine as a whole. This means extremely low levels of vibration and translates into a standard of vibrational comfort which surpasses even the high level expected of a conventional twelve-cylinder engine.
The heart of the W12 engine is the crankshaft running in seven bearings and weighing just 21.2 kilograms. The crankpins are offset to achieve a constant firing order as on a V6 engine.
Optimum charging, low emissions
Double overhead camshafts per cylinder bank are responsible for the valve gear. Driven by a single chain, they operate two inlet and two exhaust valves per cylinder via low-friction roller cam followers, a principle which helps not least to reduce fuel consumption noticeably.
The camshafts can be adjusted variably - by 52 degrees at the inlet valves and 22 degrees at the exhaust valves - which means that the optimum valve timing can be realised across the entire engine-speed range. It is thus possible to achieve good idle-speed quality, low crude emissions, low fuel consumption and ample torque across the speed range.
Mixture preparation is performed by the Bosch Motronic ME 7.1.1 with electronic sequential injection. This system includes electronic throttle control, hot-film air-mass sensing, cylinder-selective knock control via four sensors and permanent lambda control.
Exhaust emission control is performed by four close-coupled ceramic catalytic converters. Eight heated oxygen sensors continually monitor the exhaust gas. As a result, even such a powerful engine as this easily satisfies the emission limits according to EU 4.
Greater power, lower consumption
The latest generation of the twelve-cylinder engine has been substantially uprated. Its extra 30 bhp mean that its power output has increased by fourteen percent. It can call upon 560 Newton-metres of torque across a broad speed range from 2,300 to 5,300 rpm. This gain - along with the 20 Nm increase in peak torque to 580 Nm - is the outcome of painstaking improvements to many different areas.
The new 6.0 W12 for instance has lighter pistons and connecting rods - the lower weights are a vital condition of this large-capacity engine's stability at high operating speeds. At the same time the exhaust system's pressure losses in the intake zone and the counterpressure have been reduced. This means that the power unit is able to breathe more freely, that the charge cycle proceeds more swiftly and consequently that the engine is more responsive to operation of the accelerator pedal.
Another outcome of the engine's enhanced efficiency is its lower fuel consumption - even compared with its predecessor, which developed 30 bhp less. Together with the particularly effective modifications to the new 6-speed tiptronic, the detailed engine optimisation measures have cut fuel consumption by 0.8 to 13.8 litres of Super Plus fuel over 100 km - a reduction of over five percent.
Inimitable twelve-cylinder sound
There's no doubt that a successful twelve-cylinder engine owes a great deal to its very special sound. Audi thus also placed a great deal of emphasis during development on these specific acoustic qualities.
This means an extremely low-vibration idle-speed quality, which is also free from low-frequency components thanks to the incorporation of an exhaust flap. The W12 in the A8 moreover achieves a particularly low noise level during partial-load acceleration and at constant vehicle speeds.
At the same time, Audi's acoustics engineers have composed an extremely sporty, powerful sound pattern, albeit with a touch of refinement, for when this engine is accelerating at full load.
This unmistakable sound design combines the demand for maximum acoustic quality with the emotional and dynamic component, which ultimately also typifies the overall character of the vehicle in conjunction with its excellent performance.
These acoustic strengths are based on the meticulous tuning of the vehicle as a whole, for which several components are responsible: alongside dynamically optimising the crankshaft and camshaft drives, the focus was placed on fine-tuning the unit mountings, the exhaust and air intake system and the body silencing and damping concept.
Efficient power transmission
The new 6-speed tiptronic with Dynamic Shift Program DSP and additional Sport program is available for the first time in conjunction with an Audi twelve-cylinder engine. Thanks to the extra gear, the engine's output and torque are always unleashed with such precision that the conflicting requirements of spontaneous power flow, high performance and perfectly refined running are all achieved.
The additional sixth speed moreover means that the spread between the lowest and highest ratios has been increased to 6.04 - a vital requirement of greater dynamism on the one hand, and optimum economy on the other.
As an option, it is possible to change gear either using the selector lever in the tiptronic gate or via shift paddles mounted on the steering wheel. The ergonomic advantages of this concept have already been demonstrated on the Le Mans champion model, the Audi R8: drivers can also change gear when cornering, without having to take their hands off the steering wheel.
quattro permanent four-wheel drive continues to lead the way in the luxury segment. It converts the engine's performance with superb efficiency into propulsion and cornering stability, and effortlessly translates the immense torque into locomotion.