June 7, 2005
Source: Audi AG
240 kW (326 bhp): with this extremely impressive power output, the new Audi A8 4.2 TDI quattro puts itself ahead of all the other diesel-engined cars on the international market. The entirely redeveloped 4.2-litre, eight-cylinder biturbo engine accelerates the lightweight saloon with its aluminium body from 0 to 100 km/h in just 5.9 seconds. The top speed is electronically governed to 250 km/h. In addition to these dynamic qualities, the new power plant displays the kind of refinement that sets standards even in the top segment of the motoring world, and ennobles the A8 4.2 TDI quattro, turning it into the perfect long-distance touring saloon.
The new engine is also exemplary in terms of economy. Combined as standard with quattro permanent four-wheel drive and 6-speed tiptronic transmission, it uses on average just 9.4 litres of diesel fuel for every 100 kilometres. This means it will cover more than 950 kilometres on a full tank without a stop. The Audi A8 4.2 TDI quattro is equipped as standard with a diesel particulate filter, which works without additives and consequently lasts the lifetime of the vehicle without servicing.
The A8 4.2 quattro has a chassis with air suspension and adaptive damping – otherwise known as Audi adaptive air suspension. It allows the vehicle to be driven either in a decidedly sporty fashion or with an extremely comfortable ride, just as the driver wishes.
Like the other A8 models, the new TDI has the exclusive front end with single-frame radiator grille. The luxury saloon can be ordered from the beginning of June either with the standard wheelbase or in its long-wheelbase version, which is 130 millimetres longer. First deliveries will take place in the third quarter of 2005. The basic price of the A8 4.2 TDI quattro in Germany is €81,000.
The sheer power and the characteristics 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, not least, economical diesel engines.
Entirely redesigned engine
The predecessor of the new A8 4.2 TDI had already scored top marks among the luxury-class diesel saloons with its 4-litre, eight-cylinder engine. Thanks to its 275 bhp and 650 Nm of torque, it was the most powerful V8 diesel-engined car for a long period following its debut in the summer of 2003. Its fuel consumption figures of just 9.6 litres per 100 km also earned it a top place among its competitors. With these features the A8 4.0 TDI succeeded in becoming the European number one among the V8 diesel luxury saloons, with a market share of 71 %.
In the light of such statistics, the development goals for the new TDI represented a daunting challenge:
- The most powerful car diesel engine on the market.
- Top figures in terms of driving dynamics, with low fuel consumption
- Optimum engine acoustics
- Low weight
- Compact, short design
In order to implement these specifications, Audi’s TDI development team designed an entirely new engine. Just like the 3.0 and 2.7 TDI before it, the 4.2-litre V8 TDI is a member of Audi’s new family of V-format engines, and has chain-drive for the camshaft and ancillaries.
The cylinder spacing in the new engine block is now 90 millimetres. This gives the engine a compact overall length of just 520 millimetres. One current competitor V8 with the same engine capacity measures all of 140 millimetres more.
The engine block weighing 62 kg altogether is made of vermicular graphite cast iron (GGV) – an important factor in achieving the low engine weight of only 255 kilograms. This is all of 15 kilograms less than its predecessor, the 4.0 TDI.
The 4.2 TDI is thus one of the lightest V8 diesels ever. This benefits the power-to-weight ratio and the weight distribution in equal measure – two characteristics that are of central importance for the driving dynamics of the new A8 4.2 TDI quattro.
With an engine capacity of 4,134 cc, the combustion chambers of the new engine are almost exactly 0.2 litres bigger than in the case of the 4.0 TDI. The bore has been enlarged by 2 mm compared with its predecessor, to 83 millimetres, and the stroke of 95.5 millimetres is identical with that of the 4.0 TDI. The compression ratio is 16.5:1.
The surfaces of the cylinder bores are treated using UV laser exposure. This special honing process ensures significantly reduced oil consumption in the TDI engine, and, as a result, a significant reduction in particulate formation during combustion.
The forged crankshaft made of chrome molybdenum steel is offset in such a way that forces and moments of the first and second order are avoided – a decisive bonus delivering extremely low levels of vibration.
Chain drive for camshaft and ancillaries
The cylinder head is made of aluminium, and two camshafts are used for valve control in each. They are driven via a maintenance-free chain drive, as are the oil, water and power-assisted steering pumps.
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. This low-friction drive technique plays a decisive part in reducing both the fuel consumption and the emissions of the 4.2-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 reduction in mechanical noise from the valve gear and contribute to the 4.2 TDI’s outstandingly quiet running.
The rigid construction of the engine block and the unit’s effective encapsulation also help to lessen the sound emitted by the eight-cylinder engine.
Biturbo with two intercoolers
A concept that has already proven its worth on the 4.0 TDI engine: two turbochargers with variable turbine geometry develop high torque even at low engine speeds, in conjunction with a high power output. Variable turbine geometry means that the flow of exhaust gas to the turbine rotor can be optimised depending on load and engine speed. By means of air mass metering, the engine management ensures that both turbochargers run constantly at the same speed and therefore achieve the same delivery rate. Maximum boost pressure is 2.5 bar; the turbochargers rotate at up to 226,000 revolutions per minute.
The variable turbine geometry is supported by electrically operated actuators on the new 4.2 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 an excellent response.
The 4.2 TDI has two 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 extremely 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 between the right and left cylinder banks. This feature boosts cylinder charging and consequently enhances the engine’s output.
There are continuously variable swirl flaps integrated into the intake tract. These permit the air movement to be adjusted to suit the engine speed and load at the time. High tumble at low engine load results in optimised combustion, just as low tumble does at high engine load. In practice, this means that a closed swirl duct at low load increases swirl, while open ducts at high load boost cylinder charging.
Common rail technology with piezo injectors
The Bosch EDC16 CP system, with fuel delivery, trigger-start, boost pressure and exhaust-gas recirculation control, is responsible for engine management on the V8 TDI. As on the A8 with a petrol engine, this electronic system now means that it is possible to start the engine automatically at the push of a button.
Mixture preparation is by means of a latest-generation common rail system. It has a high-pressure pump located inside the V of the engine – and one pressurized pipe or “rail” for each cylinder bank. This increases the maximum injection pressure to 1,600 bar, 250 bar more than with former common rail systems – a value that corresponds with the weight of a medium-sized saloon on the surface area of a fingernail.
This high injection pressure results in even finer atomisation of the fuel and therefore in better mixture preparation and more efficient combustion. The consequence is not merely extra power and torque, but also reduced fuel consumption and emissions.
The piezo inline injectors are undoubtedly the most important innovation of the new common rail system. Electrically excitable piezo crystals undertake the triggering of the injectors. As soon as a voltage is applied to the crystals, they contract and, assisted by a hydraulic element, mechanically trigger the opening of the injector needle.
The piezo injectors offer multiple advantages compared with conventional solenoid valves. The moving masses on the injector needles can, for instance, be reduced by no less than 75 percent, from sixteen to four grams in each case.
This enables considerably smaller and more precisely metered injection quantities. At the same time, piezo technology permits a higher injector needle speed. With a value of 1.3 metres per second, the piezo system responds twice as fast as other common rail injectors.
The number of injection processes per combustion cycle can thus be varied almost at will. Audi’s TDI development engineers opted for up to four injection processes for the 4.2 V8: in addition to the main injection process, double pilot injection takes place at the lower end of the speed range and single pilot injection in the medium speed range.
This strategy produces a smoother combustion process, the benefits of which are most immediately apparent in the engine’s acoustic behaviour. The 4.2 TDI engine has once again become significantly quieter and smoother-running, even compared to its already highly cultivated predecessor, and without doubt it sets the benchmark for the class in this discipline.
The fuel is injected into the combustion chamber via eight-hole nozzles. They too play a major part in making the combustion process ultra-efficient, and thus improving the quality of emissions, by distributing the fuel homogeneously throughout the combustion chamber.
Highly effective exhaust emission control
The 4.2 TDI has a twin-pipe exhaust system with two particulate filters as standard. In each exhaust pipe the cleaning of the exhaust gases is undertaken by an oxidising catalytic converter close to the engine and a catalytically coated particulate filter.
The catalytic converters close to the engine ensure adherence to the EU4 limits by rapidly converting hydrocarbons and carbon monoxide.
The particulate filter, made of silicon carbide with thin-wall technology, is located in the under-floor area. The combination of thin-wall technology and catalytic coating enables controlled regeneration from temperatures of approx. 580 °C, with simultaneously low exhaust back pressure.
This is reflected in only marginally increased fuel consumption and enhanced regeneration efficiency.
The A8 4.2 TDI can cover distances of up to 2,000 kilometres before the filter reaches its maximum charge level. The charge status of the filter is continuously checked by means of monitoring of the exhaust back pressure and parallel simulation calculation for both exhaust pipes separately.
When regeneration becomes necessary, the engine management system generates the required increase in temperature by means of measures taken within the engine and exothermic conversion of unburned hydrocarbons in the oxidising catalytic converter. Up to 5 injections are also possible with the common-rail injection system used at Audi.
Top performance and impressive fuel economy
The 4.2 TDI engine is currently the most powerful diesel engine to be found in any production saloon car. 240 kW (326 bhp) at 3,750 rpm are the new benchmark – an increase of no less than 51 bhp over its 275 bhp predecessor.
The A8 4.2 TDI quattro has the performance of a top athlete. This lightweight luxury saloon with its aluminium body takes only 5.9 seconds to sprint from a standstill to 100 km/h – making it quicker than any other diesel saloon on the international market. It is capable of a governed top speed of 250 km/h.
The torque gives the driver a feeling of pulling power in every speed range that is otherwise only to be experienced in a sports car. 650 Newton metres between 1600 and 3500 rpm make the A8 4.0 TDI quattro the highest-torque Audi in existence
Despite the 51 bhp increase in engine output, the new Audi A8 4.2 TDI quattro consumes no more fuel than its predecessor – quite the opposite, in fact: average consumption of just 9.4 litres / 100 km means a cut in fuel consumption of 0.2 litres – a figure that is nothing less than sensational!
Audi A8 4.2 TDI quattro: supreme sportiness
All Audi A8 V8 TDI models are equipped with 6-speed tiptronic transmission with DSP Dynamic Shift Programme and additional sports programme. 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.
quattro permanent four-wheel drive converts the engine’s performance into superbly effective propulsion and cornering stability, effortlessly and safely converting its enormous torque onto the road. The aluminium Audi Space Frame (ASF) body provides the high-strength basis for low weight, dynamic performance and excellent handling.
The Audi A8 has an aluminium chassis with four-link front suspension and self-tracking trapezoidal-link rear suspension. Adaptive air suspension – an air suspension system with continuous damper control – provides the driving characteristics suitable for every driving situation. It simultaneously reduces body movement, thus optimizing road holding and comfort.
With its combination of a powerful TDI, highly developed lightweight construction and the dynamic qualities of its suspension, the Audi A8 4.2 TDI presents itself as a luxury saloon offering a new level of sporty dynamism and superiority.