About This 2004 Audi A8 4.0 TDI quattro
Posted: 01 Mar 2003
275 bhp, 650 Newton metres
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 dÃ©but 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.
Biturbo with two intercoolers
A concept that has already proven its worth on the 3.3 TDI engine: two turbochargers with variable turbine geometry develop high torque even at low engine speeds, in conjunction with a high power output. By means of air mass metering, the engine management ensures that both superchargers run constantly at the same speed and therefore achieve the same delivery rate.
The variable turbine geometry is supported by electrically operated actuators on the new 4.0 TDI. These actuators adjust the vanes faster and with greater exactitude. Particularly at low engine speeds, this builds up the boost pressure faster and thus achieves a significantly better response. The dreaded problem of 'turbo lag' is an utterly alien concept on this engine.
Another new feature on the 4.0 TDI is that there are two air-to-air intercoolers, arranged on the right and left in front of the engine. They are ideally positioned in the airflow beneath the headlights, and are therefore capable of operating highly efficiently.
Each of the two turbochargers serves one bank of cylinders. The two intake manifolds are interconnected by means of a 'feedthrough' system, to equalise the pressure in the two cylinder banks. This feature boosts cylinder charging and consequently enhances the engine's output.
There are variable, two-position swirl flaps integrated into the intake tract. They can be used to adjust the air movement according to the prevailing engine speed and load. High swirl at a low engine load optimises the combustion process as, conversely, does low swirl at a high engine load.
Highly effective exhaust emission control
To minimise thermal losses from the exhaust gas before it reaches the supercharger, the exhaust manifold has air-gap insulation.
Exhaust gas recirculation, on the other hand, requires efficient cooling, in particular in order to reduce NOx and particle emissions effectively.
The gas is cooled in two water-type radiators inside the engine's 'vee', an arrangement which contributes towards the engine's efficient use of space.
The exhaust gas is purified by two close-coupled primary catalytic converters - located close to the turbochargers - and the main converters under the floor. As a result of all these measures, the A8 4.0 TDI - equipped with quattro drive and 6?speed automatic transmission - easily undercuts the limit values of the EU III standard.
Consumption significantly reduced
One feature of the new engine that is undoubtedly as welcome as its ecological efficiency is its impressive fuel economy. In remarkable contrast to the shift in its power and torque curves, its fuel consumption has actually been reduced. It averages a mere 9.6 litres of diesel fuel per 100 kilometres - a figure that is even below the already economical value for the old A8 3.3 TDI, despite the fact that its power output is 50 bhp higher and this vehicle weighs more overall.
It therefore comes as no surprise to learn that the new A8 4.0 TDI quattro has an operating range that singles out this luxury saloon as the perfect long-distance vehicle: it can cover almost 940 kilometres on a single tank of fuel.
A8 4.0 TDI quattro: supreme sportiness in the luxury segment
The new 6-speed tiptronic with Dynamic Shift Program DSP and additional Sport program is available for the first time in conjunction with a TDI 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 by 30 percent - a vital requirement of greater dynamism on the one hand, and optimum economy on the other.
As an option, it is also possible to change gear via shift paddles mounted on the steering wheel, as well as via the selector lever. The ergonomic advantages of this concept have already been demonstrated on Audi's Le Mans champion model, the R8: drivers can also change gear when cornering, without having to take their hands off the steering wheel.
In conjunction with the low weight of the aluminium body built using the Audi Space Frame principle, the engine and transmission turn in an excellent performance coupled with exemplary economy.
quattro permanent four-wheel drive continues to lead the way in the luxury segment. It translates the engine's performance into superbly effective propulsion and cornering stability, and effortlessly translates the immense torque into locomotion.
The air suspension for the new A8's aluminium running gear is a unique standard feature, providing the right characteristic in every driving situation.
By means of the continuously variable damper settings, the adaptive air suspension reduces body movements, while optimising roadholding and ride comfort.
The driver can choose between four predefined settings, for a suspension characteristic ranging from overtly sporty to ultra-comfortable.
The market for luxury-class diesel vehicles
For all the scepticism that this concept used to generate, diesel vehicles have now become successful, mainstream members of the luxury car class. Out of around 105,000 German-built luxury saloon cars sold each year, 22,600 are diesel-powered. This figure has risen remarkably, and is more than double the 1999 figure, when sales of diesel models in the luxury class reached a mere 9,000.
It remains the case that diesel vehicles are almost exclusive to Europe. The German market accounts for 47 percent of the total, with 52 percent being exported to other European countries. Only one percent is shipped overseas or to the Far East.
The 4.0 TDI quattro is expected to capture a share of approximately 30 percent of total A8 sales in Germany, and exceed a share of over 20 percent in the rest of Europe.