Audi A8 4.0 TDI quattro
Audi is extending the range of engines available for the new A8 with a supremely powerful TDI version. The A8 4.0 TDI quattro, which has an output of 202 kW (275 bhp), is making its début at the Geneva Motor Show. The newly developed eight-cylinder biturbo engine with two intercoolers not only has a higher swept volume and power output than its predecessor, the 3.3-litre 165 kW (225 bhp) engine; it is the first TDI version in Audi's new range of V engines and has a chain drive for camshafts and ancillaries.
The A8 4.0 TDI quattro definitively elevates the performance of diesel-powered cars to sports status. This fleet-footed sports saloon with aluminium body sprints from 0 to 100 km/h in just 6.7 seconds and achieves a governed top speed of 250 km/h. Its overwhelming torque of 650 Newton metres offers a quality of traction that can otherwise only be experienced in sports cars, across the entire speed range.
650 Newton metres not only make the A8 4.0 TDI quattro the highest-torque Audi in existence, but put it right at the top of its class. The 4.0 TDI engine is currently the highest-powered, highest-torque V8 diesel engine in any production saloon car. Its impressive refinement moreover sets a new benchmark in this class.
The sheer power and the specific characteristic of this eight-cylinder power unit once again reaffirm the credentials of the Audi A8 as the sportiest saloon car in the luxury segment. And thanks to its top-of-the-range TDI, Audi once again demonstrates its expertise and dominance in the development of such powerful, refined and notably economical diesel engines.
Compact power plant
Following in the footsteps of the 4.2-litre petrol engine which powers the Audi S4, the 4.0 TDI is now the second member of Audi's new family of V-engines. The TDI, too, is remarkable for its very compact dimensions: the engine measures just 516 millimetres in length. One particular new V8 engine built by a competitor is almost 150 millimetres longer, even though its displacement is the same.
The 90-degree engine block of the V8 is made from vermicular graphite cast iron, a material that exhibits almost twice the strength of conventional grey cast iron yet weighs around 15 percent less. This is one of the vital clues to the engine's low overall weight of just 270 kilograms, which makes the 4.0 TDI one of the lightest diesel V8s around.
Both the bore (now 81.0 mm) and the stroke (now 95.5 mm) are larger than on the predecessor version. This results in an overall displacement of 3936 cc. The cylinder spacing is 88 millimetres, and the compression ratio 17.5:1.
There are four valves per cylinder, for optimum charging of the combustion chamber. The valves on the new V8 are operated by roller cam followers with hydraulic valve clearance compensation, whereas the earlier version still used sliding cam followers. This low-friction drive technique plays a decisive part in reducing both the fuel consumption and the emissions of the 4.0-litre engine.
The use of roller cam followers is likewise beneficial to the engine's acoustics. In conjunction with the twisted-type and therefore virtually play-free camshaft drive, these followers are instrumental to the 4.0 TDI's smooth running.
New common rail technology
The newly developed Bosch EDC16 system, with fuel delivery, injection-start, boost pressure and exhaust-gas recirculation control, is responsible for engine management in the V8 TDI.
As on the A8 with V8 petrol engine, this electronic system now means that it is possible to start the engine automatically at the push of a button. The preheating system ensures that it takes no more than a few moments for the engine to spring into action, even at very low winter temperatures.
As is typical of a V8 TDI, mixture preparation is performed by a common rail system with one high-pressure pump and one rail per cylinder bank. On the new generation of engines, the injection pressure has been raised from 1350 bar to 1600 bar. This results in even finer atomisation of the fuel, in other words in more effective mixture preparation. The consequence is not merely extra power and torque, but also reduced fuel consumption and emissions.
The second-generation solenoid-valve injectors with 7-hole nozzles are also new. They, too, play a major part in making the combustion process ultra-efficient, and thus improving the quality of emissions, by distributing the fuel very homogeneously throughout the combustion chamber. The result is that even though the Audi A8 4.0 TDI quattro - unlike its challengers - has permanent four-wheel drive, it clearly undercuts the limit values specified in the EU III standard.
What is more, the new common rail system is capable of injecting fuel several times per combustion cycle. Single and double pilot injection produces a smoother combustion process; the benefits of this are most immediately apparent in the engine's acoustic behaviour. The 4.0 TDI engine is not only much quieter and smoother-running than its predecessor, it now unquestionably serves as the benchmark in its class.
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