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.
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